Regulation of calcium signaling by polycystin-

【摘要】  Autosomal dominant PKD (ADPKD) is a common lethal genetic disorder characterized by progressive development of fluid-filled cysts in the kidney and other target organs. ADPKD is caused by mutations in the PKD1 and PKD2 genes, encoding the transmembrane proteins polycystin-1 (PC1) and polycystin-2 (PC2), respectively. Although the function and putative interacting ligands of PC1 are largely unknown, recent evidence indicates that PC2 behaves as a TRP-type Ca 2+ -permeable nonselective cation channel. The PC2 channel is implicated in the transient increase in cytosolic Ca 2+ in renal epithelial cells and may be linked to the activation of subsequent signaling pathways. Recent studies also indicate that PC1 functionally interacts with PC2 such that the PC1-PC2 channel complex is an obligatory novel signaling pathway implicated in the transduction of environmental signals into cellular events. The present review purposely avoids issues of regulation of PC2 expression and trafficking and focuses instead on the evidence for the TRP-type cation channel function of PC2. How its role as a cation channel may unmask mechanisms that trigger Ca 2+ transport and regulation is the focus of attention. PC2 channel function may be essential in renal cell function and kidney development. Nonrenal-targeted expression of PC2 and related proteins, including the cardiovascular system, also suggests previously unforeseeable roles in signal transduction.

【关键词】  kidney autosomal dominant polycystic kidney disease polycystin transient receptor potential channels nonselective cation channels calcium channels

THIS REVIEW PURPOSELY AVOIDS detailing regulatory aspects of polycystin-2 (PC2) trafficking and developmental expression and cellular location of the channel protein, including cell biological issues of autosomal dominant polycystic kidney disease (ADPKD), for which highly comprehensive reviews can be found ( 49, 84, 137 ). This review focuses instead on the description of PC2 as a channel, its structural and functional similarities with other related transient receptor potential (TRP) channels, to which family it is a recent addition, and potential roles of PC2 in Ca 2+ signaling. Educated guesses can be drawn insofar as PC2 function, regulation, and/or interaction with other proteins are concerned, in particular new information on the molecular interaction with PC1 and expected regulation found in other TRP channels.

PKD1 AND PKD2 GENE PRODUCTS AND POLYCYSTIC KIDNEY DISEASE-RELATED GENES

Fig. 1. Topological features of the autosomal dominant polycystic kidney disease (ADPKD) gene products polycystin-1 (PC1) and polycystin-2 (PC2). Top : large glycosylated transmembrane protein PC1 ( left ), of unknown function, can be topologically defined as containing 3 domains, a large extracellular region containing domains likely involved in cell matrix and cell-to-cell interactions (see text and Ref. 136 for details). The transmembrane domains are still of unknown function. However, partial homology with PC2 may imply receptor properties and/or parts of a functional channel complex. The short cytoplasmic tail of PC1 contains a coiled-coil region that couples with the COOH-terminal region of PC2. This region is also a target for potential regulation by several intracellular pathways (see text and Ref. 137 ). Polycystin-2 ( right ) is a newly added member of the transient receptor potential (TRP) superfamily of Ca 2+ -permeable cation channels, which is likely involved in cell signaling events and/or Ca 2+ transport in target tissues. Bottom : reported and expected signaling mechanisms affecting and effecting PC1 ( left ) or PC2 ( right ). Downward-pointing arrows indicate systems effected by either protein. Upward-pointing arrows indicate regulatory mechanisms that target either protein. Bidirectional arrows indicate putative regulatory pathways where effector and effected components of the regulatory pathways may communicate in either direction. This is most obviously evident in such regulatory pathways as those implicating elements of the actin cytoskeleton. A number of regulatory proteins are implicated or can be presumed of regulatory nature on PC1 function. These include kinases, G proteins, and lipid byproducts, still to be identified. Some of the evidence is unfortunately drawn from studies where the cytoplasmic tail, instead of the complete protein, has been assessed. Protein overexpression is still controversial as to its validity to assess normal cell function. Nonetheless, the evidence may provide a primer for further studies targeting direct regulatory pathways. See text for details and references.

PC1 has been linked to a number of signaling events and regulatory effects in various cell functions ( Fig. 1 ). PC1 was shown to regulate the cell cycle through the JAK-STAT signal transducing transcription pathway ( 6 ). Madin-Darby canine kidney (MDCK) cells stably expressing human PC1 have a decreased proliferation rate and growth arrest in G 0 /G 1, an effect that may be correlated with the cell substratum surface onto which the cells grow ( 6 ). This phenomenon, which apparently requires the presence of PC2 ( 6 ), was associated with the PC1-induced inhibition of CD2-type kinases and activation of STAT transcription factors ( 6 ), thus regulating cell signaling mechanisms. The expression of the COOH-terminal end of PC1, for example, is also associated with the regulation of renal tubulogenesis, by stimulating TCF-dependent gene transcription and the Wnt signaling pathway in embryonic kidney cells ( 53 ). This may be partially associated with PC1 interactions with intermediate filaments ( 144 ) and activation of the PKC signaling pathway ( 86 ) ( Fig. 5 ). Another potentially relevant signaling pathway associated with PC1 function entails G protein regulation ( 24 ). The COOH-terminal tail of PC1 activates G 0 /G i -type G proteins in brain lysates in vitro ( 93 ) ( Fig. 1 ). PC1 activation of G proteins may be regulated by PC2 ( 24 ), although the molecular steps of this interaction will require further understanding. Overexpression of the PC1 COOH-terminal tail has also been linked to activation of endogenous cation channels in Xenopus oocytes ( 124 ) and purinergic receptor regulation of Ca 2+ transport ( 1 ). Of particular relevance to an understanding of the onset and potential pathophysiological implications in ADPKD is demonstrated by the structural/functional interactions between PC1 and PC2 ( 123, 143 ). The coiled-coil domain of PC1 interacts with the COOH-terminal end of PC2 ( Fig. 1 ) in a process that implies functional changes in the PC2 protein. This evidence may suggest, however, that when interacting, both proteins are required as a novel signaling pathway to work properly ( 85 ).

Fig. 5. Hypothetical model of Ca 2+ entry steps associated with expression of TRP channels and interacting proteins. In this hypothetical cell, combining evidence from epithelial and nonepithelial cells, Ca 2+ influx is largely mediated by members of the TRP channel superfamily. TRP channels may coassemble such that the expected channel properties and regulatory pathways regulating their function can be numerous and serve a variety of cell functions. The PC1-PC2 channel elicits an environmentally triggered Ca 2+ entry, which, in turn, activates and/or is potentiated by intracellular Ca 2+ stores. PC1 can be coupled to G protein receptors by exchanging and/or activating various G proteins, which, in turn, may be regulated by PC2 in certain cells. Other TRP channels may be associated with Ca 2+ influx steps in epithelial cells, in particular CaT1 (TrpV6). TRP channels are also regulated by the G protein-induced activation of the PLC pathway and the production of diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP 3 ). This latter second messenger activates IP 3 receptor (IP 3 R) from endoplasmic reticulum (ER)/sarcoplasmic reticulum (SR) compartments. Other intracellular channels include the Ca 2+ -dependent ryonadine receptor (RyR) and PC2 itself. These channels can also regulate, by apposition, plasma membrane TRP channels. This is complementary or in parallel with their expected role in the release of Ca 2+ from intracellular stores. Specific TRP channels may have regulatory/activating pathways not shared by other members of the same family. TrpV1 (VR-1), the capsaicin-sensitive vanilloid receptor, is likely regulated by arachidonic acid (AA) and its metabolites such as HPETE. This may require coupling to receptor activation by DAG lipases and other DAG-related enzymes. The presence of PC1 multiprotein complexes in cell-cell and cell-matrix regions (see Ref. 137 for details) may still offer another means of regulation based on interactions with cytoskeletal proteins, which may bind to TRP channels, and/or PC2 itself.

THE PC2 PROTEIN

In 1996, Mochizuki et al. ( 77 ) discovered the second locus responsible for ADPKD ( Fig. 1 ). Positional cloning determined the presence of the PKD2 gene. This predicted a 968-amino acid peptide, PC2, for the gene product containing the paradigmatic six putative transmembrane (6TM) domains of TRP channels ( Fig. 2 ), with homology with Na + and Ca 2+ voltage-gated channels ( 77 ). Homology between PC2 and other TRP channels centers in a 270-amino acid stretch from TM2 through TM6 and intervening loops of PC2 particularly similar to the voltage-activated E -subunit of Ca 2+ channels. It is expected, but yet to be proven, that PC2 as do other TRP channels ( 66 ) functionally behaves as a homo- or hetero-tetramer ( Fig. 2 ). The -subunit of Ca 2+ -permeable cation channels of excitable tissues is topologically assembled as four domains each with a 6TM topology ( 16 ). The similarity also extended to the COOH-terminal end of PC2 ( Fig. 2 ), including the EF-hand domain present in some TRP channels, and the -subunit of the L-type Ca 2+ channel ( 77 ). This is particularly relevant in light of the fact that the EF-hand provides a regulatory region of voltage-gated channel inactivation ( 107 ). The above homology led to the original hypothesis that PC2 may behave as a cation channel ( 77 ). PC2 has been identified in several renal and nonrenal tissues ( 12, 18, 28 ), including pancreas, liver, lung, bowel, brain, thymus, reproductive organs, and placenta ( 77, 91 ). In the kidney, PC2 was originally found in the plasma membrane of thick ascending limb of Henle and the distal convoluted tubule. Weaker labeling has been found in the proximal tubule and collecting ducts ( 71 ). Conflicting results have been reported, however, for the presence of PC2 in the proximal tubule ( 28 ). PC2 has also been detected in vascular tissues and the mammalian heart, where it may play an important role in nonrenal manifestations of ADPKD. Contrary to PC1, whose expression seems highest during fetal development, expression of PC2 is similar in both fetal and adult tissues. This suggests that the two proteins may not always be associated with and/or functional in the same complex ( 18 ). Early evidence whereby PC2 location was assessed in cells heterologously expressing the protein ( 12 ) would indicate that PC2-overexpressing cells retain the protein in the endoplasmic reticulum (ER). While confirming PC2 targeting to the ER, a recent study showed that inner medullary collecting duct (IMCD) and 293T cells indeed express endogenous amounts of functional plasma membrane PC2 ( 69 ). However, LLC-PK 1 cells have endogenous intracellular PC2 ( 69 ). This would be in agreement with a predicted ER-retention motif in the PC2 protein sequence (amino acids 787-820). This region also contains two putative phosphorylation sites with consensus motifs for PKA and PKC ( 77 ) ( Fig. 2 ). However, the role of PC2 phosphorylation in the trafficking and regulation of the protein is still unknown. Truncated PC2 at, or before Arg742, thus missing most of the COOH tail, can more easily reach the plasma membrane ( 19, 58 ). The COOH-terminal tail of PC2 is also potentially relevant for putative interactions with cytoskeletal proteins that bind to the channel, including troponin I ( 65 ) and tropomyosin-1 ( 62 ).

Fig. 2. Topology of the PC2 channel. A : topology of the PC2 channel conforms to that of all TRP channels, including 6 transmembrane (TM) domains and cytoplasmic NH 2 - and COOH-terminal tails. The expected pore region of PC2, as that of other TRP channels, is expected in between TM5 and TM6, with the P-loop in the center, which contains the conductance pore of the channel. TM1-4 contain the expected "sensor" region of the channel ( 59 ). This remains to be proven (as with other parts of the channel protein). B : topology of the functional channel may be tetrameric as expected for other TRP channels ( 66 ), where the TM5-6 region of each monomer contributes to the channel pore. Heterotetrameric channel structures are also possible and expected for PC2, as PC2/TrpC1 have been shown to interact with each other ( 122 ). C : COOH-terminal tail of PC2 is rich in putative regulatory interacting pathways, including consensus sites for specific phosphorylation, binding regions for Ca 2+ and calmodulin, and cytoskeletal proteins such as tropomyosin-1 and troponin I (see text for details). The position of ADPKD causing mutation R742X, missing regulation by PC1, is indicated as it is indicated in the region that seems to help retain the wild-type protein in the endoplasmic reticulum (ER). Trafficking of PC2 may also be potentially relevant as a regulatory mechanism of PC2 in cell function.

Several PKD2 homologous genes have been identified to date (reviewed in Refs. 49 and 112 ). Polycystin-like (PCL), encoded by Pkd2L ( 89 ), was the first member of the "polycystin" TRPP subfamily of TRP channels to be identified as a cation channel ( 20 ). Not itself a cause of ADPKD, PCL is highly expressed in fetal kidney and liver and is deleted in Krd mice expressing renal and eye defects ( 89 ). A functional role of PCL in these defects still requires experimental proof. PCL shares significant sequence homology (50% identity and 71% similarity) and similar topology and domain organization with PC2. Chen et al. ( 20 ) demonstrated that expression of PCL in Xenopus oocytes was associated with a novel large-conductance (137 pS) Ca 2+ -permeable nonselective cation channel. PKD2 homologs can now be considered a subfamily of transmembrane proteins with potential channel-like properties. Both PCL and PC2 have been found to display highly similar channel properties ( 20, 34 ). Other highly homologous PKD2 gene homologs have been identified, for which there is no apparent link between their expression and ADPKD ( 89, 126 ). However, it is possible that their homology to the PKD2 and PKDL genes reflects sensory functions based on their intrinsic channel properties, yet to be determined. Mutations in either the PKD1 or the PKD2 genes present rather similar clinical manifestations of ADPKD. Similar pathophysiological features are also manifested in animal models carrying mutations in either gene ( 10, 68, 141 ). Thus the current evidence suggests that PC1 and PC2 are both an integral part of an obligatory functional complex implicated in signaling events linked to one or more cell functions. Considering the systemic nature of the disease, potential roles for the PC1-PC2 partnership include the regulation of ion transport, activation of kinase cascades, and/or other regulatory cell functions. Recent evidence shows that PC2 and homologs act as Ca 2+ -permeable cation channels ( 20, 34, 125 ). Thus PC1-PC2 interactions may regulate Ca 2+ transport and cell signaling ( 85 ) ( Fig. 3 ), renal epithelial cell growth, and normal tubular morphogenesis ( 86 ). This suggests that mutations in either gene likely result in a common dysfunctional Ca 2+ signaling mechanism, which controls various cell functions. Several of the functional features and potential physiological roles of PC2 arise from structural/functional homology with TRP channels.

Fig. 3. Effect of PC1 COOH tail on wild-type PC2 channel activity. A : in vitro translated PC2 was reconstituted in a lipid bilayer system, where single-channel activity was observed at the beginning of the experiment ( top trace, left ). Inactivation occurred either spontaneously (shown at the beginning of the trace) or after holding of the membrane to negative potentials (data not shown). Addition of PC1 COOH-terminal tail (P1CC) restored PC2 channel activity ( top trace, right ). Representative expanded traces are shown below for each condition, including spontaneous channel activity ( a ), spontaneous inactivation ( b ), and 2 subsequent channel reactivation levels after the addition of P1CC ( c and d ). Data are reproduced from Ref. 143 with permission. B : effect of various PC1 tail constructs and negative controls on wild-type PC2. Addition of the GST prepared as a control for the glutathione S -transferase (GST)-PC1 chimeras was without effect, as expected. Neither addition of the NH 2 -terminal portion of the COOH tail of PC1 (P1CN) missing the coiled-coil domain nor the coiled-coil containing tail mutated in Q4215P had an effect. Addition of P1CC, the wild-type tail, reestablished wild-type PC2 channel activity. C : model of the regulation by the PC1 tail of PC2. Neither mutated PC1 nor truncated PC2 interacts with each other such that a functional channel complex is elicited.

PC2 and the TRP Channel Superfamily

The TRP paradigm was initially observed in spontaneous Drosophila mutants with a unique photoreceptor phenotype ( 76 ). The TRP mutant has a transient rather than a prolonged response during light stimulation. This basic defect implied the reduced activity of a Ca 2+ -permeable light-sensitive conductance, which is required for a sustained response ( 75 ). Interestingly, addition of micromolar concentrations of La 3+ to wild-type, but not TRP mutant, photoreceptors mimicked the TRP phenotype ( 75, 118 ). The above findings gave rise to the original hypothesis that the TRP mutant implicates a novel Ca 2+ -permeable cation channel. The cloning of TRP and mammalian gene homologs ( 80, 97, 134, 150 ) has led to rapid progress in the identification and characterization of a number of TRP channel family members ( 7, 23, 75, 79 ). The TRP superfamily 20 proteins with variable homologies (reviewed in Refs. 75 and 78 ). TRP channel activity may represent ubiquitous mechanisms for Ca 2+ entry and subsequent contribution to the rise in intracellular Ca 2+. TRP proteins are found in a variety of organs, including the brain, heart, kidney, testis, lung, liver, spleen, ovaries, intestine, prostate, placenta, uterus, and vascular tissues, and numerous cell types (reviewed in Ref. 75 ).

TRP proteins contain 6TM, S1 - S6, with a characteristic pore region between transmembrane segments S5 and S6, which is typical of voltage-gated channels ( 7, 97 ). This sequence is the most conserved region in all members of the TRP superfamily. TRP proteins display homology with the extended family of voltage- and cyclic nucleotide-gated channels, in particular the transmembrane domains of the pore region ( 97 ). However, the voltage sensor present in the S4 domain of voltage-gated cation channels is missing in TRP channels (reviewed in Refs. 75 and 78 ). A 25-amino acid "TRP domain" of unknown function is also present in the COOH terminus of most, but not all, TRP channels. Variations in the either short or long cytoplasmic tails may provide further specificity. The NH 2 -terminal cytoplasmic domain of TRPC and TRPV channels contains several ankyrin repeats, whereas the TRPC and TRPM COOH termini contain proline-rich motifs. Interestingly, TRP channels also display distinct sequences for putative interactions with calmodulin (CaM), dystrophin, and the PDZ-scaffolding protein ENAD in their COOH-terminal ends (reviewed in Ref. 75 ). These sequences may be relevant in the regulation of channel function by such mechanisms as Ca 2+ -induced channel inactivation and cytoskeletal control of channel function. Evidence is mounting to suggest that TRP channels oligomerize with homolog or heterologous partners, enabling channel complexes with distinct functional features ( 7, 67, 122, 145 ). The suggestion that a tetramer of TRP subunits underlies the prototypical channel structure ( 7 ) may also be in reasonable agreement with the partial homology of TRP channels with voltage-gated cation channels of excitable tissues. The -subunit of the L-type Ca 2+ channel indeed has four consecutive 6TM-containing structural domains ( 16 ).

Based on their functional properties, TRP proteins are nonselective cation channels with diverse cation permselectivity properties, including a high Ca 2+ selectivity in some, but not all, TRP channels ( 75 ). The two light-sensitive Drosophila TRP channels were originally identified, based on their high ( P Ca : P Na 100:1) or low ( P Ca : P Na 4:1) Ca 2+ permeability. The low-Ca 2+ -selective TRP conductance is encoded at least in part by the TRP-like (TRPL) gene, 40% identical to TRP ( 87, 97 ). TRP channels are linked to sensory stimuli, including cold and heat, osmotic challenges, and other receptor stimulatory responses ( 78, 130 ). The TRP family was initially classified ( 41 ) into short (TRPC), osm-9-related (TRPV, vanilloid), and long (melstatin-related, TRPM) channels, including the sensory channels involved in phototransduction, and cold and hot sensations ( 21 ). TRP channels are now classified in a newly encompassing and comprehensive nomenclature that has divided this family into at least six subgroups ( 78, 79 ) linked to activating and/or regulatory mechanisms. The TRPC (canonical) subfamily is the closest to Drosophila TRP. The other two groups include TRP channels associated with other sensory responses, including TRPV and TRPM receptors. Thus three original subfamilies of TRP channels were designated, namely, as short, long, and osm-9-like, which respond to a nomenclature based on the length, and thus potential regulation of their cytoplasmic tails ( 41 ). The osm-9-like gene encodes a TRP protein associated with osmotic responses in the C. elegans ( 113 ). The TRP superfamily has now been extended to 6TM proteins with weaker homology but potentially similar topological features and regulatory roles in cell function. New members of the TRP channel superfamily include the recently discovered epithelial Ca 2+ channels CaT1 and ECaC, now renamed TRPV6 and TRPV5, respectively ( 45, 96, 149 ). CaT1 and ECaC are highly homologous ( 75% sequence identity) ( 95 ) and may represent a major contributing factor to the apical Ca 2+ absorption step in transporting epithelia. CaT1 is preferentially expressed in intestinal tissues, whereas ECaC is expressed in the apical membrane of epithelial cells lining the rabbit, mouse, and rat distal nephron ( 44 ). Both CaT1 and ECaC channels are permeable to Ca 2+ ( 45, 95 ). However, channel activity is inversely proportional to intracellular Ca 2+ concentrations, and single-channel conductance in the presence of external physiological concentrations of the ion deems the conductance undetectable ( 95, 128 ). More recently, the Ca 2+ -permeable nonselective cation channels PC2 and PCL ( 20, 77 ) have been incorporated in the TRPP subfamily of TRP proteins ( 79 ). Further information on the growing number and physiological properties of these channels is the basis of this review. The PC2 topologically similar protein mucolipin-1 (TRPML subfamily), which is genetically linked to mucolipidosis type IV, is also speculated to preserve the cation channel structure of PC2 ( 117 ). Mucolipin homologs have been recently implicated in sensory functions that also imply potential ion channel activity ( 25 ). At least one recent study reports that ML1 expression in Xenopus oocytes is associated with a novel Ca 2+ -permeable cation conductance ( 60 ). TRP proteins, of which PC2 and homologs are newly joined members, are ubiquitous channel proteins likely involved in transient Ca 2+ influx and subsequent transduction signals. The conserved TM domain topology among members of the TRP superfamily of channels contrasts with the wide diversity of tissue and cell expression and the variety of signaling mechanisms in which they are involved. It is likely that TRP channels are regulated by a number of specific intracellular ligands, mostly associated with their cytoplasmic domains. These include putative PKC (TrpC, TrpV) and other other kinases PKA [TrpP1-polycystin-2, TrpV1-2, 5-6, CaT1, endothelial Ca 2+ channel (ECaC)1], and phosphatidylinositol 3-kinase (TRpC2-3, 5-7, TrpV1, 4, 6), inositol 1,4,5-trisphosphate (IP 3 ) receptor (IP 3 R) and calmodulin interactions (all TrpC), and PDZ domains (Trp, TrpC4, 5), including INAD and NINAC myosin III (reviewed in Refs. 66 and 78 ). Other putative ligands for intracellular regulation include cytoskeletal proteins, such as dystrophin-like motifs (TrpC1), and interactions with troponin I and tropomyosin-1 (PC2 or TrpP1). Other expected ligands for intracellular regulation may directly or indirectly involve trimeric and small G proteins, ATP, IP 3, and diacylglycerol (DAG), and arachidonic acid (AA) ligands such as AA itself. AA byproducts of potential DAG kinases and lipases reactions include anandamides (TrpV1) and HPETE (TrpV), which are linked to regulation of the vanilloid receptors (reviewed in Ref. 4 ).

PCL and PC2 as Cation Channels

The gene product of Pkd2L (PCL) was the first PC2-like protein to be determined as a functional cation channel ( 20 ). PCL permeates a number of mono- and divalent cations, including Na +, K +, Ba 2+, and Ca 2+, respectively. PCL has a 4:1 divalent-monovalent permselectivity ratio, and the channel displays slight outward rectification in the presence of asymmetrical Na + /K + ( 20 ). Channel activity by PCL is regulated by Ca 2+, whose channel activity can be substantially increased when either extracellular or intracellular Ca 2+ is raised ( 20 ). PCL-expressing oocytes preincubated in a Ca 2+ -free solution evoke large transient (<30 s) currents after addition of external Ca 2+ (5 mM) ( 20 ). A second addition of Ca 2+, however, inhibits the currents. Thus external Ca 2+ induces both activation and subsequent inhibition of PCL. This suggests a Ca 2+ -induced inactivation process somewhat reminiscent of that observed in voltage-gated cation channels ( 11, 152 ) and some members of the TRP family ( 88 ). This may be in contrast with TRP, TRPL, and TRPV6, which are blocked by varying concentrations of Mg 2+ ( 39, 129 ), but whose function has little dependence on extracellular Ca 2+. The Mg 2+ block in some TRP channels, however, is modulated by extracellular Ca 2+ ( 75 ). Recently, Li et al. ( 64 ) entertained the possibility that the putative Ca 2+ binding EF-hand region in the COOH-terminal tail of PCL may be implicated in its Ca 2+ regulation. Spliced versions of PCL from liver and testis, containing and lacking this region, respectively, showed the same extent of channel inactivation by external Ca 2+, indicating functional differences for Ca 2+ regulation between PCL and voltage-gated cation channels. The two PCL splice variants have different activating currents, however ( 64 ), suggesting that the role of this region will require further study. PC2, which contains the EF-hand domain, may also be regulated by Ca 2+ by a yet unknown mechanism.

Evidence for the cation channel properties of PC2 was obtained as an attempt to identify the molecular nature of the most abundant nonselective cation channel in the outer membrane of term human placenta, the syncytiotrophoblast (hST) ( 34 ). Interestingly, the placenta is one of the three most important organs responsible for Ca 2+ homeostasis, the other two being the kidney and intestine ( 95 ). The intrinsic properties of the PC2 cation channel activity were later confirmed by expression of human protein in Sf9 insect cells ( 34 ) and by reconstitution of the in vitro translated gene product ( 34 ). The spontaneously active cation-selective channel ascribed to PC2 functions in hST has a single 177-pS channel conductance in a K + gradient. A 134-pS nonrectifying single-channel conductance is observed in symmetrical K + ( 34 ). Channel activity shows at least four subconductance states of 30 pS each ( 34 ). Placental PC2 function has a 1:1 Na + /K + permselectivity with a slightly increased permeability to Ca 2+ (1.3:1) compared with monovalent cations. Similar results were found for reconstituted human PC2 ( 34 ). Interestingly, Ca 2+ permeation through the PC2 channel inhibited its own channel activity ( 34 ), suggesting a Ca 2+ -induced Ca 2+ inactivation effect. This inhibition takes two forms, including fast flickering of the large-conductance state and a slow decrease in subconductance states of the channel ( 34 ). Inhibition by Ca 2+ has only been observed by exposure of the channel to high extracellular Ca 2+. Thus it is unlikely that physiological extracellular Ca 2+ may resemble the inhibition observed in other Ca 2+ -permeable TRP channels such as CaT1 (TrpV6) and ECaC (TrpV5) ( 46, 88 ). Whether the EF-hand present in the COOH terminus of the protein, PC2 regulatory proteins, or the pore conductance itself plays a role in this inhibition is yet to be determined. Reported data by Vassilev et al. ( 125 ) showed different permselectivity ratios and rectification patterns in Xenopus -expressed PC2. Nevertheless, a large single-channel conductance, multiple substates, Ca 2+ regulation, and a slight divalent cation preference all seem to be intrinsic properties of the PC2 cation channel. While PC2 cation channel activity is not intrinsically outwardly rectifying, its open probability displays evident voltage dependence ( 33 ). This may be an intrinsically relevant mechanism of PC2 channel regulation. Voltage dependence of PC2's open probability has been confirmed in the ER-localized PC2 ( 58 ) and more recently in IMCD and MDCK cells expressing the endogenous protein ( 69 ). Like other nonselective cation channels and in particular some TRP family members, PC2 channel function is inhibited by the nonselective cation channel blockers La 3+, Gd 3+, and the diuretic amiloride, also a known inhibitor of epithelial Na + -permeable channels ( 34 ).

Channel activity by PC2 has been determined in various cells and expression systems. A nonselective cation channel was recently observed in the plasma membrane of IMCD and MDCK renal epithelial cells, which was consistent with the presence of endogenous PC2 ( 69 ). This was actually suggested by immunocytochemical and Western blot analyses, where plasma membrane PC2 was found in cells expressing endogenous, but not the overexpressed protein ( 69 ). The 170-pS single-channel conductance found in K + is similar to that previously observed. However, the high (7:1) K + /Na + permselectivity ratio differs from reported data in Sf9 cells and the endogenous hST reconstituted channel ( 34 ). Channel activity was increased in epithelial cells overexpressing PC2, suggesting that both endogenous and heterologously expressed PC2 behave similarly ( 69 ). Confirmation that this channel activity is indeed mediated by PC2 alone is still lacking. Nevertheless, Vassilev et al. ( 125 ) had previously reported that PKD2 -cRNA injection in Xenopus oocytes is associated with the expression of a highly similar cation-selective, Ca 2+ -permeable ion channel. In another study, Hanaoka et al. ( 37 ) showed that heterologous coexpression of PC2 and PC1 in Chinese hamster ovary (CHO) cells, but neither one alone produced a novel, Ca 2+ -permeable cation conductance not previously observed in untransfected cells. In that report, the cation conductance induced by PC1-PC2 coexpression was prevented by removing the COOH-terminal end of either protein. However, this is at odds with the fact that the truncated PC2 should traffic more freely and reach the plasma membrane ( 19 ). Interestingly, the coexpressed PC1-PC2 whole cell data in CHO cells showed outward rectification and a sixfold higher permeability to Ca 2+ compared with monovalent cations ( 37 ). The available information albeit with minor discrepancies is overall consistent with PC2's being a nonselective cation channel with weak preferential cation permselectivity for Ca 2+, over Na + and K + (compared with canonical TRP channels). PC2 likely represents a relevant molecular pathway for Ca 2+ entry into epithelial tissues.

The discrepancies among the various reports may fall within different explanations, ranging from simple technical differences to specific cell models and expression systems. However, more complicated scenarios are also plausible, including heterooligomerization of endogenous proteins and PC2-like channels, which display distinct molecular and functional properties. TRP channels can change their phenotypic characteristics depending on whether each protein oligomerizes with itself or other TRP channel isoforms. Xu et al. ( 145 ) showed that coexpression of TRP and TRPL in 293T cells led to outwardly rectifying currents distinct from expression of either protein alone. TRPC1 interacts with TRPC5 to enable novel cation channel activity in the brain ( 115 ). Most interestingly, however, PC2 has been reported to interact with TRPC1 but not TRPC3 ( 122 ). This interaction, which awaits functional proof, was expected by the interacting COOH-terminal ends of either protein. However, TRPC1-PC2 structural interaction was found to be at least partially mediated by their respective S2 - S4 interacting TM domains in both proteins. Thus relevant TRP-PC2 topological interactions may render channel complexes of functional properties different from those observed with PC2 alone (i.e., purified reconstituted protein) or by such complexes as the PC1-PC2 partnership ( 85 ).

Cells overexpressing PC2 have been found to accumulate the protein in intracellular compartments, in particular the ER ( 12, 28, 58, 69 ). In a recent paper, Koulen et al. ( 58 ) assessed the role of wild-type and mutated PC2 in intracellular compartments. LLC-PK 1 renal epithelial cells were transfected with wild-type and mutated PC2, including the COOH-terminal truncated L703X-PC2 and a missense mutation of PC2 (D511V-PC2). The protein was almost exclusively found in pre-medial Golgi and the ER, by cell gradient analysis. While full-length and D511V PC2 were confined to pre-medial Golgi membranes, the L703X PC2 partially translocated to the plasma membrane as expected from the loss of the ER retention signal ( 77 ). Reconstituted ER membranes from LLC-PK 1 cells overexpressing wild-type PC2 but not naïve membranes displayed inward Ba 2+ currents in a Ba 2+ chemical gradient ( 58 ). This was tantamount to the presence of a functional PC2 channel in the ER. The ER-located PC2 channel had a slope conductance of 114 pS in Ba 2+ and slightly smaller conductances in Ca 2+ and Mg 2+, respectively. The truncated L703X PC2, however, had a reduced single-channel conductance of 28 pS in Ba 2+. This is in contrast to data showing that reconstituted L742X PC2 has similar functional properties compared with wild-type PC2 ( 143 ). Higher negative potentials increased reconstituted PC2 channel activity, suggesting a voltage-dependent channel activity. This was considered a further suggestion of a functional PC2, as IP 3 receptors present in the ER are not voltage dependent ( 27, 133 ). The possibility that the currents of the PC2-overexpressing cells were mediated by either IP 3 R, which activates IP 3 R, or ryanodine (RyR) receptors was further excluded by the absence of IP 3 and addition of the RyR inhibitor ruthenium red, respectively. The L703X mutant PC2 required larger negative membrane potentials for channel activation compared with the wild-type protein. Furthermore, the open probability of wild-type but not L703X PC2 channel increased over a physiological range of Ca 2+ concentrations, whereas higher cytoplasmic Ca 2+ lowered PC2 channel activity. However, negative potentials that fully activated wild-type PC2 channels had no effect on the L703X PC2. The differences between L703X PC2 and other rather similar truncations (L742X) and wild-type PC2 may suggest a relevant role of this missing domain of PC2 in channel activation. No currents were observed in ER vesicles containing the D511V PC2, suggesting a loss of function in this mutation. Although this evidence does not necessarily make a case for a functional role of intracellular PC2 under physiological conditions, there is no compelling evidence why PC2 may not participate as a Ca 2+ transport mechanism in more than one cellular location.

PC2 CHANNEL REGULATION

Regulation of PC2 channel function is essential to our understanding not only of the potential mechanisms of interaction with PC1 and the onset of ADKPD but also its physiological response and functional role in cell function. Two aspects of PC2 channel regulation are herein summarized, including those ascribed to the changes in the intrinsic properties of the channel itself and those potentially linked to putative interactions with associated proteins. This evidence is only emerging from recent studies, and further experimentation will be required to unequivocally ascribe particular roles to the various potential partners of the PC2 channel. It is evident that the COOH-terminal end of PC2, which is rich in putative interacting domains, may play an important role in regulating its channel function. Regulatory target sites include the domain that interacts with the coiled-coil region in the cytoplasmic tail of PC1 ( 37, 143 ). Other regulatory sites may be relevant in either trafficking and/or direct interactions with the membrane-associated protein. These are the putative ER-retention site, an EF-hand and calmodulin-binding domain, and putative phosphorylation sites in the COOH-terminal tail of PC2 ( 77 ).

Effect of pH on PC2 Channel Function

Changes in pH, a byproduct of cell metabolic activity, is known to affect several cellular functions, including ion transport, enzymatic activity, and gene expression. Early studies in excitable tissues indicated that pH changes modify voltage-gated Na + channel function. Neural Na + currents are inhibited by a rise in H + ions ( 140 ). Similar findings were observed for L-type Ca 2+ channels. Hess and collaborators ( 43, 80 ) originally determined that the L-type Ca 2+ channel, which shares homology with PC2 ( 77 ), is also sensitive to changes in pH. The voltage dependence and H + accessibility suggested that the protonation site of L-type Ca 2+ channels is external to the conductance pore ( 43 ). Changes in pH have been found to regulate a number of PC2-related TRP channels. The PC2 homolog PCL, for example, is blocked by a reduction in cytoplasmic pH ( 20 ), suggesting homologous topologies between PC2 and PCL. However, pH regulation of TRP channel members of the sensory-related TRPV subfamily, such as TrpV1, seems to be stimulatory ( 15 ). Thus external acidic pH increases the TrpV1 currents otherwise stimulated by capsaicin and similar ligands. This regulation may take place from the extracellular domain of the protein; however, the channel is also modulated by intracellular PtdIn( 4, 5 )P2 ( 15 ). Recently, we observed that PC2 channel function is voltage dependent and highly sensitive to changes in pH ( 33 ). The protonation site in PC2 with a p K a of 6.4 regulates channel activity by controlling the voltage dependence of the single-channel kinetics ( 33 ). A reduction in intracellular pH decreased the channel's open probability but not the single-channel conductance. The data placed the protonation site within the conductance pore of PC2. This was confirmed by the pH effect on the in vitro translated material ( 33 ). Furthermore, the R742X PC2, missing most of its cytoplasmic domain and potential interactions with regulatory proteins, preserves the inhibition by changes in intracellular pH ( 19 ). The internal pH-sensitive site in PC2 may be a relevant feature of members of the "polycystin" TRPP subfamily of TRP channels and may play an important role in metabolic acidosis. pH regulation has also been determined in the TRP-type CaT1 and ECaC channels, both epithelial Ca 2+ channels in renal and intestinal epithelia. However, ECaC is inhibited by extracellular pH ( 127 ), a property that seems to be shared by CaT1 ( 95 ). Future experiments will be required to assess whether the regulation by pH of the various members of the TRP family of channels is a common functional mechanism ascribed to topologically similar domains of the channel proteins.

Effect of Ca 2+ on PC2 Channel Function

The evidence indicates that PC2 is a nonselective cation channel with a weak preferential permeability to divalent cations, including Ca 2+. This brings forth the current hypothesis that PC2 functionally behaves as a Ca 2+ -permeable channel. Other members of the TRP superfamily are indeed Ca 2+ channels, and the original TRP phenotype was described based on a light-induced Ca 2+ signaling event ( 40 ). CaT1 and ECaC are TRP-related channels thought to play an important functional role in Ca 2+ permeation, despite the fact that their intrinsic divalent currents are often undetectable. Thus Ca 2+ itself may be a major contributor to the transient Ca 2+ signal as its rise in the intracellular compartment often constitutes a negative feedback mechanism in the regulation of Ca 2+ influx. Brehm and Eckert ( 11 ) originally observed that while voltage-activated L-type-mediated Ca 2+ currents normally inactivate, Ba 2+ currents through the same channels did not. Thus it was originally suggested that Ca 2+ itself is required for L-type Ca 2+ channel inactivation. The Ca 2+ -dependent inactivation is present in a number of channels. The nature of the inhibition would be such that the Ca 2+ delivered in the cytoplasmic vicinity of the channel is the effector of a self-inhibitory cascade, including potential regulatory sites in the channels themselves or in their surroundings. Ca 2+ -induced L-type Ca 2+ channel inactivation may be elicited by a COOH-terminal region encompassing at least three distinct domains. This region in L-type Ca 2+ channels contains the putative Ca 2+ binding EF-hand motif, two hydrophilic residues (asparagine and glutamic acid) downstream of the EF-hand motif, and a putative IQ CaM binding motif ( 16, 107, 152 ). The EF-hand and IQ CaM putative Ca 2+ binding motifs have been observed in a number of channels. These include voltage-gated Ca 2+ channels ( 152 ), cyclic nucleotide gated (CNG) channels ( 120 ), TRP family members ( 97, 111 ), and NMDA ( 26 ) and RyRs ( 105 ). The putative Ca 2+ binding motifs are usually present in the COOH-terminal end of the channel proteins, where they are most likely implicated in Ca 2+ binding and/or the selective Ca 2+ -CaM regulation (reviewed in Ref. 107 ). CaM binding and regulation have been demonstrated in the TRPC1 ( 111 ) and TRPL channels ( 110 ). Both PC2 ( 77 ) and PCL ( 64 ) also contain putative Ca 2+ binding and CaM-interacting domains, with the expectation that this domain may play an important functional role in their regulation.

Regulation of TRPC1 by Ca 2+ and CaM interactions may also bear relevance in potential regulatory mechanisms of PC2 function, as PC2 binds and interacts with various domains of TRPC1 in a TRP homolog-specific manner ( 107 ). Extracellular Ca 2+ -induced regulation of polycystin channels may thus be an intrinsic mechanism of channel function. Little is known about Ca 2+ binding to PC2. However, Ca 2+ seems to regulate and inhibit its channel function ( 34, 125 ). A recent study by Li at al. ( 64 ), however, assessed the role of the EF-hand in the PC2 homolog PCL, where the presence of Ca 2+ induced both channel activation and inactivation. PCL splice variants from liver [(PCL-LV) lacking the EF-hand] and testis [(PCL-TS) lacking only 45 amino acids of the COOH tail] were used in that study. Both PCL splice variants exhibited spontaneous cation channel activity and Ca 2+ -induced channel activation. However, PCL-LV exhibited threefold increased activation compared with PCL-TS. Ca 2+ transport through the PC2-related ECaC and CaT1 (ECaC2) was also shown to present an inhibition of channel activity, consistent with a self-induced inhibition. However, in these two Ca 2+ -permeable channels, with different inhibitory kinetics, the domains implicated in the Ca 2+ -induced inhibition seem to reside outside of both the NH 2 - and COOH-terminal tails of the proteins ( 88 ). The fact that Ca 2+ transport through PC2 modifies the residence time for the various subconductance states of the channel ( 34 ) may provide a hint as to the presence of a novel Ca 2+ -induced inactivating mechanism. At least one recent report indicates that the truncated L703X PC2 has a much higher threshold for voltage activation than the wild-type protein ( 58 ). This further suggests a role of the COOH-terminal end of PC2 in the regulation of channel function.

Interactions Between PC1 and PC2

Aside from potential regulatory mechanisms of PC2 channel function that relate to its intrinsic electrophysiological properties, including voltage dependence and charge carrier permselectivity properties, little is yet known about how PC2 channel function is regulated. As for other TRP-related channels, however, it is expected that potentially interacting proteins, in particular those that may have an affinity for the COOH-terminal end of the protein, may play a relevant role in channel regulation. In the case of PC2, a potential interaction between both PKD gene products was suspected from the similar clinical manifestations of the disease, regardless of which gene was affected. A structural interaction between PC1 and PC2 was originally described by the cytoplasmic binding of their respective COOH-terminal tails ( 123 ). This, in turn, raised the early hypothesis that a PC1-PC2 functional partnership is a potentially relevant physiological regulatory mechanism for what we now know is the affected PC2 channel function. Hanaoka et al. ( 37 ) originally observed that at least in transfected CHO cells overexpressing both proteins, only PC1-PC2 coexpression renders a functional channel complex. Elimination of the cytoplasmic domain of either protein was sufficient to obliterate the PC1-PC2-induced whole cell conductance in CHO cells ( 37 ). Truncated R742X PC2, missing the ER retention site, showed no differences from untransfected CHO cells ( 37 ). This contrasts with at least one report showing that R742X PC2 reaches the plasma membrane with better ease than its wild-type counterpart ( 19 ). Actually, R742X PC2 channel function is remarkably similar to the wild-type channel ( 19, 143 ). Thus no auxiliary proteins seem to be required for PC2 to enable channel activity ( 34 ). It is likely, however, that while interacting with PC2, PC1 plays a seemingly important regulatory role in its channel function ( 37, 85 ).

Colocalization and a structural interaction between PC2 and PC1 would suggest ( 123 ) that modulation of PC2 channel activity by PC1 requires the COOH terminus of PC1 and its binding counterpart, the cytoplasmic tail of PC2 ( 143 ). To substantiate the importance of PC1-PC2 functional interactions on the channel activity of PC2, Xu et al. ( 143 ) reconstituted wild-type PC2 in a bilayer system ( Fig. 3 ). The isolated PC2 channel inactivated both spontaneously and by hyperpolarizing holding potentials. Addition of the coiled-coil-containing domain of the PC1 cytoplasmic tail (P1CC; Fig. 3 ) reversed the inhibition of wild-type PC2 and increased the activity of already functional channels. The addition of a PC1 tail containing the Q4215P substitution, a mutation expected to disrupt the coiled-coil domain and cause the disease when present in the human gene, however, was unable to elicit a similar activation ( 143 ). Thus the COOH tail of PC1 is an agonist that helps stabilize PC2 channel function. Conversely, truncated R742X PC2, itself an active channel, but missing most of the COOH-terminal end of the channel, failed to respond to the normal P1CC tail after channel inactivation ( 143 ). Thus modulation of PC2 channel activity by PC1 may be an important component of biological functions ascribed to this molecular complex. This is also potentially relevant as PC1, whose function as a whole protein is yet unknown, may be involved in more than one regulatory function. As a signaling mechanism, physical interactions of the cytoplasmic domain of PC1 with proteins other than PC2 may provide a more flexible regulatory role depending on the effector channels or signaling proteins. Expression of the cytoplasmic tail of PC1 in Xenopus oocytes, for example, was found to activate an endogenous, nonselective cation channel in this cell model ( 124 ), representing a structure likely distinct from PC2. This suggests PC1 regulation of more than one target channel protein. Aguiari et al. ( 1 ) recently reported that expression of the PC1 tail potentiates purinergic receptor-induced intracellular Ca 2+ release in human renal HEK-293 cells. The PC1 cytoplasmic tail has also been found to stabilize endogenous -catenin and stimulate TCF-dependent gene transcription in HEK-293 cells ( 53 ). These disparate findings point to a flexible regulatory role of the cytoplasmic domain of PC1 in diverse signaling pathways, including those effected by changes in channel function.

Polycystins and the Ciliary Connection

Evidence for a physiological interaction between PC1 and the PC2 channel is more clearly established in at least one recent report providing interesting new understanding of the role PC1 plays in transducing environmental signals to PC2. Nauli et al. ( 85 ) recently found that wild-type PC1 colocalizes with PC2 and ciliary proteins of the microtubular machinery in the stem and basal body of the primary cilium of mouse embryonic renal epithelial cells. The nonmotile primary cilium of renal epithelial cells, until recently only thought to be a vestigial appendage, has drawn renewed attention because of its potential role in sensory signaling ( 98, 99, 109 ). Praetorius and Spring ( 98 ) determined that flow-induced stress in MDCK cells is associated with cilium bending and a rise in intracellular Ca 2+ elicited by both a transient Ca 2+ influx and the subsequent Ca 2+ release from intracellular stores. Chemical excision of the primary cilium eliminated this signal ( 99 ). Similarly, in highly differentiated embryonic mouse renal epithelial cells with a well-developed primary cilium, Nauli et al. ( 85 ) observed that shear flow induced a PC2-mediated Ca 2+ influx. In this study, PC1 was localized in the primary cilium of cultured embryonic collecting duct renal epithelial cells from wild-type mice and mice homozygous for the PKD1 exon 34-targeted deletion ( Pkd1 del34/del34 ). This mutation causes the expression of truncated PC1, which leads to the formation of renal cysts at embryonic days 14-15 ( 68 ). PC1 colocalized with PC2 in primary cilia from wild-type but not Pkd1 del34/del34 renal epithelial cells. PC1-deficient cells did not respond to shear stress with a rapid rise in intracellular Ca 2+ as did the wild-type cells ( 85 ) ( Fig. 4 ). This change in intracellular Ca 2+ was elicited by the PC2-mediated and extracellular Ca 2+ -dependent Ca 2+ influx and the activation of the store-operated intracellular Ca 2+ release ( Fig. 5 ). The evidence suggests a functional link between a normal and not a dysfunctional PC1-PC2 channel complex in the transduction of environmental signals. Nevertheless, other compensatory mechanisms such as receptor-induced Ca 2+ increase may remain largely intact in the absence of a functional PC1. Inhibition of the PC2-mediated Ca 2+ rise in wild-type cells to flow stimulation ( 85 ) by added antibodies against either PC1 or PC2 confirmed the requirement of both proteins in the mechanotransduction response of renal epithelial cells. This finding further suggests that functional blockade of either protein obliterated the entire signaling event. It is thus likely that PC1-PC2 interaction is obligatory in that only a normal channel complex between the two proteins enables a functional response. The response to thrombin was actually higher in the Pkd1 del34/del34 cells compared with those expressing wild-type PC1. Although unknown at present, it is still possible that PC1-independent activation of PC2 may be elicited by receptor-mediated events. Conversely, TRP-related channel events expected from Ca 2+ -permeable channels such as CaT1 (TrpV6) may also play an either compensatory and/or complementary role in Ca 2+ influx steps in renal epithelial cells.

Fig. 4. Effect of PC1 presence on the mechanical flow and receptor-mediated increase in intracellular Ca 2+. Changes in intracellular Ca 2+ ([Ca 2+ ] ex ) were followed in embryonic mouse renal epithelial cells loaded with the Ca 2+ -sensitive dye fura 2. Representative changes in intracellular Ca 2+ to mechanical flow in wild-type (wt) and Pkd1 del34/del34 cells ( top left ) are shown ( C ). Cells expressing a truncated PC1 ( Pkd1 del34/del34 ) failed to respond to volume flow with an increase in intracellular Ca 2+. Mechanotransduction-induced Ca 2+ rise depended on the presence of extracellular Ca 2+ ( top right ). This response reached a peak 10 s after the flow was applied. Addition of thrombin (1 mM, bottom ), in contrast, increased intracellular Ca 2+ in both wt and Pkd1 del34/del34 cells in the presence ( bottom left ) or absence ( bottom right ) of extracellular Ca 2+. Interestingly, Pkd1 del34/del34 cells had a greater response to thrombin compared with wt cells in both the presence or absence of extracellular Ca 2+. Data are reproduced from Ref. 85 with permission from the authors and Nature Genet. A : PC1-PC2 channel complexes are likely present in more than one cellular location in epithelial cells. Its recent description in the nonmotile cilia ( B ) may entail their involvement in signaling mechanisms where these proteins are linked to other ciliary structures, including axonemal microtubules and their associated proteins ( C ).

Connections between ciliary structures and PKD originally arose from mouse models of autosomal recessive polycystic kidney disease (ARPKD), which are now genetically linked to deficiencies in ciliary proteins and not ADPKD-related genes. The Tg737 gene affected in the orpk Pkd mouse encodes a novel protein, polaris, which localizes to the ciliary basal body and axoneme ( 147 ). Orpk mice show shortened cilia and left-right symmetry defects ( 83 ) in addition to PKD. Orpk mice also show increased ciliary PC2 ( 94 ). Homozygous inv mice lacking a functional inversin protein exhibit situs inversus and severe renal and pancreatic cystic disease ( 82 ). Inversin has recently been located to primary cilia of renal epithelia ( 81 ). Although inversin function is unknown, cytoskeletal interactions and Ca 2+ regulation by CaM binding may be expected to regulate this protein ( 82, 90 ). The congenital polycystic kidney ( cpk ) mouse model also provided early clues to a sensory role of ciliary function in PKD. The cpk gene encodes a novel protein, cystin ( 47 ), which is expressed and colocalizes with polaris in cilia in renal epithelial cells. Yoder et al. ( 146 ) recently reported the colocalization of PC1 and PC2 with cystin, polaris, and ciliary tubulin in cilia of renal epithelial cells. Thus the encompassed evidence suggests novel mechanotransduction signaling mechanisms linking the sensory function mediated by cilia bending with the activation of the PC1-PC2 channel complex ( Fig. 4 ). How this channel complex senses environmental responses and/or couples to ciliary proteins is yet unknown. However, evidence is mounting for all these proteins to comprise novel signaling pathways where all components are required for a normal response. This hypothesis is evidenced by novel TRP channel roles in sensory function in organisms such as C. elegans, where various TRP homologs have been implicated in touch and other sensory responses ( 35 ). The C. elegans homologs of PC1 ( Lov-1 ), PC2 ( Pkd2 ), and the polaris homolog osm-5 ( 42 ) were recently located in ciliated sensory neurons of this organism ( 3 ). Worms expressing mutations in any of the genes exhibit similar sensory defects, suggesting a common deficient cilia-mediated sensory pathway ( 42 ). As indicated above, PKD -unrelated mutations induce cystic renal disease. It is thus tempting to postulate that novel signaling pathways, involving microtubular and other cytoskeletal proteins, and the channels reviewed in this report may all play a role in transducing environmental information into cellular events. Trp and TrpL, for example, are distributed along the ciliary appendages of the photoreceptors in adult Drosophila ( 23 ). This phenomenon, which resembles the one observed with the PC1-PC2 channel complex in renal epithelial cells, raises the interesting question as to their functional connection with the Ca 2+ storage compartments, with which they are associated ( 13 ).

Cytoskeletal Connection to PC2

Interestingly, signals associated with shear stress, including cell stretch and anisoosmotic cell volume changes, cell-matrix adhesion, and motility are all linked to, and/or elicited by, changes in cytoskeletal structures. It is thus tempting to postulate cytoskeletal interactions with PC2 (and likely PC1) as potential regulatory pathways of its channel function. This hypothesis requires experimental evidence. Nevertheless, as expected from its putative interaction with the extracellular matrix, potential role in cell-cell interactions, and localization in focal adhesion cell domains, PC1 has been found to interact with several focal adhesion proteins, including pp125FAK, pp60src, p130Cas, and paxillin ( 31 ). Focal adhesion complexes are also linked to regulatory actin-binding proteins, including vinculin, talin, and -actinin, as well as the cell adherens junction proteins E-cadherin and - and -catenins, which coprecipitate with PC1 ( 31 ). Functional correlates for these interactions in the context of PC1-PC2 channel complexes are still missing. However, Geng et al. ( 31 ) have shown that PC1-containing multiprotein complexes could be disrupted by cytochalasin D but not the microtubular disrupter colchicine ( 31 ). This puts forward the interesting possibility for the actin cytoskeleton to be implicated in the formation and function of PC1 multiprotein complexes, including those that elicit PC2 channel regulation. Furthermore, potential interactions of these multiprotein complexes with PC2 are also possible, as the PC2 protein is also associated with actin cytoskeletal components. Hax-1, a binding partner of cortactin, which connects and modifies actin filamental structures, for example, was found to interact with PC2 ( 30 ). The functional role of this interaction is still unknown. Nevertheless, Li and collaborators have recently found that the cytoskeletal proteins troponin I ( 65 ) and tropomyosin-1 ( 62 ) bind directly to PC2, further strengthening a link between cytoskeletal dynamics and the PC2 channel. Recent evidence by the same group provided the first experimental proof for troponin I regulation of PCL channel function ( 63 ). Thus a close association between ion channels such as PC2 and environmental signals may be linked through cytoskeletal structures as has been established for other channel proteins (reviewed in Refs. 14 and 52 ). Interactions between PC2 and homologs with cytoskeletal proteins such as troponin I and tropomyosin-1 further stress the relevant role of the actin cytoskeleton in the control of PC2 channel function. This is especially relevant in the regulation of PC2 in extrarenal locations such as the cardiovascular system, also a target of ADPKD.

PC2 in the Cardiovascular System

The true systemic nature of ADPKD is perhaps manifested by ever-growing evidence for the importance of PKD1 and PKD2 mutations in extrarenal abnormalities. Extrarenal ADPKD is most commonly associated with premature death and disability by the presence of vascular phenotypes most often associated with ruptured intracranial aneurysms ( 17, 108 ), resulting in subarachnoid hemorrhage ( 108 ). ADPKD accounts for 9% of familial intracranial aneurysms ( 106 ). Common vascular manifestations of ADPKD include saccular intracranial aneurysms, dolichoectasias, aortic root dilatation, dissection of the thoracic aorta and cervicocephalic arteries, and coronary artery aneurysms ( 29, 108 ). Both PKD1 and PKD2 mutations have been associated with cranial aneurysms. Moreover, it has been shown that PKD2, PKDL, and PKD2L2 are expressed in the heart ( 2, 36, 89 ). Therefore, putative channel function by either one of these proteins may play a yet unrecognized role in the cardiovascular system. This is supported by the expression of polycystins in vascular smooth muscle cells ( 104 ) and cardiac myocytes ( 132 ). Vascular leakage and multiple focal hemorrhages in mouse embryos homozygous for Pkd1 or Pkd2 null mutations further support a direct role for the polycystins in ADPKD-associated vascular disease ( 54, 142 ). A reduced life expectancy has recently been reported in heterozygous Pkd2+/ - mice ( 104 ). This is accompanied by an early onset hypertension even in the absence of cystic renal disease. Thus haploinsufficiency may play a role in some extrarenal manifestations of ADPKD ( 104 ). Qian et al. ( 104 ) recently found decreased PC2 expression in Pkd2+/ - vascular smooth muscle cells and enhanced intracranial vascular abnormalities in Pkd2+/ - mice induced to develop hypertension ( 104 ). Freshly dissociated Pkd2+/ - vascular smooth muscle cells were found to have lower basal intracellular Ca 2+ compared with wild-type cells. Furthermore, caffeine plus thapsigargin-induced Ca 2+ release from intracellular stores was also decreased in these cells. Interestingly, this was also associated with decreased store-operated Ca 2+ channel activity. Thus inactivation of just one Pkd2 allele is sufficient to significantly reduce PC2 expression and alter intracellular Ca 2+ homeostasis in vascular smooth muscle cells. Based on these findings, the authors proposed that the abnormal intracellular Ca 2+ regulation linked to Pkd2 haploinsufficiency is directly related to the vascular phenotype. Another report, however, points to the possibility that cardiac expression of Pkd2 homolog genes may have a functional role in cardiac function. Rat left ventricular myocytes display nonselective cation channel activity with characteristics similar to those observed for PC2 and its homologs ( 132 ), suggesting that PC2 and/or homologs may underlie this channel function in ventricular myocytes. Addition of extracellular caffeine, by increasing intracellular Ca 2+, led to cation channel activation in these cells. Single-channel characterization of these currents showed channel currents with a conductance of 300 pS, roughly twice as large as that of PC2, and similar permeability for monovalent cations, including Na +, K +, Li +, and Cs + ( 132 ). The channel was also permeable to divalent cations, including Ca 2+ and Ba 2+. Pharmacological features included inhibition by amiloride with an affinity similar to that reported for PC2, and trivalent cations Gd 3+ and La 3+, also in a fashion similar to those observed for PC2 in human placenta ( 34 ). Single-cell RT-PCR analysis of individual rat left ventricular myocytes revealed the expression of Pkd2 and Pkd2L2 genes but neither PkdL nor Pkd1 ( 132 ). Thus a PC2-like channel is present in cardiac ventricular myocytes, whose function is independent of PC1 regulation. Whether this channel phenotype is the functional fingerprint of PC2 alone, and/or in association with the PKD2L2 (or other) gene product, will require future experimentation. A functional PC2 channel may have widespread implications in Ca 2+ signaling events.

Ca 2+ Transport and Signal Transduction

Ca 2+ is a ubiquitous intracellular second messenger that controls numerous physiological events. Ca 2+ signals are originated by a sudden, sizable rise in cytoplasmic Ca 2+, which is essential for a variety of cellular responses including sensory transduction, cell activation, degranulation and ion secretion, regulation of cell contraction, cell proliferation, and apoptosis. The increase in intracellular Ca 2+ can be achieved by one or more consecutive events. These signals include channel-mediated Ca 2+ influx and/or receptor-induced Ca 2+ release from intracellular stores. This may or may not activate the so-called "capacitative" membrane channel response (reviewed in Refs. 7, 100, 101, and 151 ). One of the most prevalent pathways implicated in the Ca 2+ rise in nonexcitable cells is the triggering of PLC activation and the subsequent production of the ligands DAG and IP 3 ( 7, 38, 74, 75, 100, 101, 138 ) ( Fig. 5 ). These metabolites target intracellular Ca 2+ stores, which elicit the release of Ca 2+ in most cells. The release of Ca 2+ from intracellular stores is dominated by two different types of Ca 2+ -permeable channels, RyR, that is itself activated by cytosolic Ca 2+, and IP 3 R, which, in turn, is activated by IP 3. These channels can be either colocalized in the same ER/sarcoplasmic reticulum (SR) compartments or be in independent pools, providing separate venues for the release of

【参考文献】
  Aguiari G, Campanella M, Manzati E, Pinton P, Banzi M, Moretti S, Piva R, Rizzuto R, and del Senno L. Expression of polycystin-1 C-terminal fragment enhances the ATP-induced Ca 2+ release in human kidney disease. Biochem Biophys Res Commun 301: 657-664, 2003.

Arnaout MA. Molecular genetics and pathogenesis of autosomal dominant polycystic kidney disease. Annu Rev Med 52: 93-123, 2001.

Barr MM and Sternberg PW. A polycystic kidney-disease gene homologue required for male mating behaviour in C. elegans. Nature 401: 386-389, 1999.

Benham CD, Davis JB, and Randall AD. Vanilloid and TRP channels: a family of lipid-gated cation channels. Neuropharmacology 42: 873-888, 2002.

Berridge MJ. Capacitive calcium entry. Biochem J 312: 1-11, 1995.

Bhunia AK, Piontek K, Boletta A, Liu L, Qian F, Xu PN, Germino FJ, and Germino GG. PKD1 induces p21(waf1) and regulation of the cell cycle via direct activation of the JAK-STAT signaling pathway in a process requiring PKD2. Cell 19: 157-168, 2002.

Birnbaumer L, Zhu X, Jiang M, Boulay G, Peyton M, Vannier B, Brown D, Platano D, Sadeghi H, Stefani E, and Birnbaumer M. On the molecular basis and regulation of cellular capacitative calcium entry: roles for Trp proteins. Proc Natl Acad Sci USA 93: 15195-15202, 1996.

Boletta A, Qian F, Onuchoc LF, Bragonzi A, Cortese M, Deen PM, Courtoy PJ, Soria MR, Devuyst O, Monaco L, and Germino GG. Biochemical characterization of bona fide polycystin-1 in vitro and in vivo. Am J Kidney Dis 38: 1421-1429, 2001.

Bootman MD, Lipp P, and Berridge MJ. The organisation and functions of local Ca 2+ signals. J Cell Sci 114: 2213-2222, 2001.

Boulter C, Mulroy S, Webb S, Fleming S, Brindle K, and Sandford R. Cardiovascular, skeletal, and renal defects in mice with a targeted disruption of the Pkd1 gene. Proc Natl Acad Sci USA 98: 12174-12179, 2001.

Brehm P and Eckert R. Calcium entry leads to inactivation of calcium channel in Paramecium. Science 202: 1203-1206, 1978.

Cai Y, Maeda Y, Cedzich A, Torres VE, Wu G, Hayashi T, Mochizuki T, Park JH, Witzgall R, and Somlo S. Identification and characterization of polycystin-2, the PKD2 gene product. J Biol Chem 274: 28557-28565, 1999.

Cantiello HF. A tale of two tails: ciliary mechanotransduction in ADPKD. Trends Mol Med 9: 234-236, 2003.

Cantiello HF and Prat AG. Role of actin filament organization in ion channel activity and cell volume regulation. In: Membrane Protein-Cytoskeleton Interactions, edited by Nelson WJ. San Diego, CA: Academy, 1996, p. 373-396.

Caterina MJ and Julius D. The vanilloid receptor: a molecular gateway to the pain pathway. Annu Rev Neurosci 24: 487-517, 2001.

Catterall WA. Structure and regulation of voltage-gated Ca 2+ channels. Annu Rev Cell Dev Biol 16: 521-555, 2000.

Chauveau D, Pirson Y, Verellen-Dumoulin C, Macnicol A, Gonzalo A, and Grünfeld JP. Intracranial aneurysms in autosomal dominant polycystic kidney disease. Kidney Int 45: 1140-1146, 1994.

Chauvet V, Qian F, Boute N, Cai Y, Phakdeekitacharoen B, Onuchic LF, Attie-Bitach T, Guicharnaud L, Devuyst O, Germino GG, and Gubler MC. Expression of PKD1 and PKD2 transcripts and proteins in human embryo and during normal kidney development. Am J Pathol 160: 973-983, 2002.

Chen XZ, Segal Y, Basora N, Guo L, Peng JB, Babakhanlou H, Vassilev PM, Brown EM, Hediger MA, and Zhou J. Transport function of the naturally occurring pathogenic polycystin-2 mutant, R742X. Biochem Biophys Res Commun 282: 1251-1256, 2001.

Chen XZ, Vassilev PM, Basora N, Peng JB, Nomura H, Segal Y, Brown EM, Reeders ST, Hediger MA, and Zhou J. Polycystin-L is a calcium-regulated cation channel permeable to calcium ions. Nature 401: 383-386, 1999.

Clapham DE. Hot and cold TRP ion channels. Science 295: 2228-2229, 2002.

Clapham DE. Sorting out MIC, TRP, and CRAC ion channels. J Gen Physiol 120: 217-220, 2002.

Clapham DE, Runnels LW, and Strübing C. The TRP ion channel family. Nature (Neurosci Rev) 2: 387-396, 2001.

Delmas P, Nomura H, Li X, Lakkis M, Luo Y, Segal Y, Fernandez-Fernandez JM, Harris P, Frischauf AM, Brown DA, and Zhou J. Constitutive activation of G-proteins by polycystin-1 is antagonized by polycystin-2. J Biol Chem 277: 11276-11283, 2002.

Di Palma F, Belyantseva I, Kim HJ, Vogt TF, Kachar B, and Noben-Trauth K. Mutations in Mcoln3 associated with deafness and pigmentation defects in varitint-waddler (Va) mice. Proc Natl Acad Sci USA 99: 14994-14999, 2002.

Ehlers MD, Zhang S, Bernhardt JP, and Huganir RL. Inactivation of NMDA receptors by direct interaction of calmodulin with the NR1 subunit. Cell 84: 745-755, 1996.

Ehrlich BE and Watras J. Inositol 1,4,5-trisphosphate activates a channel from smooth muscle sarcoplasmic reticulum. Nature 336: 583-586, 1988.

Foggensteiner L, Bevan A, Thomas R, Coleman N, Boulter C, Bradley J, Ibraghimov-Beskrovnaya O, Klinger K, and Sandford R. Cellular and subcellular distribution of polycystin-2, the protein product of the PKD2 gene. J Am Soc Nephrol 11: 814-827, 2000.

Gabow PA. Autosomal dominant polycystic kidney disease. New Engl J Med 329: 332-342, 1993.

Gallagher AR, Cedzich A, Gretz N, Somlo S, and Witzgall R. The polycystic kidney disease protein PKD2 interacts with Hax-1, a protein associated with the actin cytoskeleton. Proc Natl Acad Sci USA 97: 4017-4022, 2000.

Geng L, Burrow CR, Li HP, and Wilson PD. Modification of the composition of polycystin-1 multiprotein complexes by calcium and tyrosine phosphorylation. Biochim Biophys Acta 1535: 21-35, 2000.

Golovina VA, Platoshyn O, Bailey CL, Wang J, Limsuwan A, Sweeney M, Rubin LJ, and Yuan JX. Upregulated TRP and enhanced capacitative Ca 2+ entry in human pulmonary artery myocytes during proliferation. Am J Physiol Heart Circ Physiol 280: H746-H755, 2001.

González-Perrett S, Batelli M, Kim K, Essafi M, Timpanaro G, Montalbetti N, Reisin IL, Arnaout MA, and Cantiello HF. Voltage dependence and pH regulation of human polycystin-2 mediated cation channel activity. J Biol Chem 277: 24959-24966, 2002.

González-Perrett S, Kim K, Ibarra C, Damiano AE, Zotta E, Batelli M, Harris PC, Reisin IL, Arnaout MA, and Cantiello HF. Polycystin-2, the protein mutated in autosomal dominant polycystic kidney disease (ADPKD), is a Ca 2+ -permeable nonselective cation channel. Proc Natl Acad Sci USA 98: 1182-1187, 2001.

Goodman MB and Schwarz EM. Transducing touch in Caenorhabditis elegans. Annu Rev Physiol 65: 429-452, 2003.

Guo L, Schreiber TH, Weremowicz S, Morton CC, Lee C, and Zhou J. Identification and characterization of a novel polycystin family member, polycystin-L2, in mouse and human: sequence, expression, alternative splicing, and chromosomal localization. Genomics 64: 241-251, 2000.

Hanaoka K, Qian F, Boletta A, Bhunia AK, Piontek K, Tsiokas L, Sukhatme VP, Guggino WB, and Germino GG. Co-assembly of polycystin-1 and -2 produces unique cation-permeable currents. Nature 408: 990-994, 2000.

Hardie RC. Regulation of trp channels via lipid second messengers. Annu Rev Physiol 65: 735-759, 2003.

Hardie RC and Mojet MH. Magnesium-dependent block of the light-activated and trp-dependent conductance in Drosophila photoreceptors. J Neurosci 74: 2590-2599, 1995.

Hardie RC, Peretz A, Pollock JA, and Minke B. Ca 2+ limits the development of the light response in Drosophila photoreceptors. Proc R Soc Lond B Biol Sci 252: 223-229, 1993.

Harteneck C, Plant TD, and Schultz G. From worm to man: three subfamilies of TRP channels. Trends Neurosci 23: 159-166, 2000.

Haycraft CJ, Swoboda P, Taulman PD, Thomas JH, and Yoder BK. The C. elegans homolog of the murine cystic kidney disease gene Tg737 functions in a ciliogenic pathway and is disrupted in osm-5 mutant worms. Development 128: 1493-1505, 2001.

Hess P, Prod'Hom B, and Pietrobon D. Mechanisms of interaction of permeant ions and protons with dihydropyridine-sensitive calcium channels. Ann NY Acad Sci 560: 80-93, 1989.

Hoenderop JG, Muller D, Kemp AW, Hartog A, Suzuki M, Ishibashi K, Imai M, Sweep F, Willems PH, Os CH, and Bindels RJ. Calcitriol controls the epithelial calcium channel in kidney. J Am Soc Nephrol 12: 1342-1349, 2001.

Hoenderop JG, Nilius B, and Bindels RJ. ECaC: the gatekeeper of transepithelial Ca 2+ transport. Biochim Biophys Acta 1600: 6-11, 2002.

Hoenderop JG, van der Kemp AW, Hartog A, van Os CH, Willems PH, and Bindels RJ. The epithelial calcium channel, ECaC, is activated by hyperpolarization and regulated by cytosolic calcium. Biochem Biophys Res Commun 261: 488-492, 1999.

Hou X, Mrug M, Yoder BK, Lefkowitz EJ, Kremmidiotis G, D'Eustachio P, Beier DR, and Guay-Woodford LM. Cystin, a novel cilia-associated protein, is disrupted in the cpk mouse model of polycystic kidney disease. J Clin Invest 109: 533-540, 2002.

Hughes J, Ward CJ, Peral B, Aspinwall R, Clark K, San-Millan JL, Gamble V, and Harris PC. The polycystic kidney disease 1 (PKD1) gene encodes a novel protein with multiple cell recognition domains. Nature Genet 10: 151-160, 1995.

Igarashi P and Somlo S. Genetics and pathogenesis of polycystic kidney disease. J Am Soc Nephrol 13: 2384-2398, 2002.

Jaggar JH and Nelson MT. Differential regulation of Ca 2+ sparks and Ca 2+ waves by UTP in rat cerebral artery smooth muscle cells. Am J Physiol Cell Physiol 279: C1528-C1539, 2000.

Jaggar JH, Porter VA, Lederer WJ, and Nelson MT. Calcium sparks in smooth muscle. Am J Physiol Cell Physiol 278: C235-C256, 2000.

Janmey P. The cytoskeleton and cell signaling: component localization and mechanical coupling. Physiol Rev 78: 763-781, 1998.

Kim E, Arnould T, Sellin LK, Benzing T, Fan MJ, Gruning W, Sokol SY, Drummond I, and Walz G. The polycystic kidney disease 1 gene product modulates Wnt signaling. J Biol Chem 274: 4947-4953, 1999.

Kim K, Drummond I, Ibraghimov-Beskrovnaya O, Klinger K, and Arnaout MA. Polycystin 1 is required for the structural integrity of blood vessels. Proc Natl Acad Sci USA 97: 1731-1736, 2000.

Kiselyov KI, Mignery GA, Zhu MX, and Muallem S. The N-terminal domain of the IP 3 receptor gates store-operated HTRP3 channels. Mol Cell 4: 423-429, 1999.

Kiselyov KI, Shin DM, Wang Y, Pessah IN, Allen PD, and Muallem S. Gating of store-operated channels by conformational coupling to ryanodine receptors. Mol Cell 6: 421-431, 2000.

Kiselyov KI, Xu X, Mozhayeva G, Kuo T, Pessah I, Mignery GA, Zhu MX, Birnbaumer L, and Muallem S. Functional interaction between InsP3 receptors and store-operated HTRP3 channels. Nature 396: 478-482, 1998.

Koulen P, Cai Y, Geng L, Maeda Y, Nishimura S, Witzgall R, Ehrlich BE, and Somlo S. Polycystin-2 is an intracellular calcium release channel. Nature Cell Biol 4: 191-197, 2002.

Kumanovics A, Levin G, and Blount P. Family ties of gated pores: evolution of the sensor module. FASEB J 16: 1623-1629, 2002.

LaPlante JM, Falardeau J, Sun M, Kanazirska M, Brown EM, Slaugenhaupt SA, and Vassilev PM. Identification and characterization of the single channel function of human mucolipin-1 implicated in mucolipidosis type IV, a disorder affecting the lysosomal pathway. FEBS Lett 532: 183-187, 2002.

Li A, Tian X, Sung SW, and Somlo S. Identification of two novel polycystic kidney disease-1-like genes in human and mouse genomes. Genomics 81: 596-608, 2003.

Li Q, Dai Y, Guo L, Liu Y, Hao C, Wu G, Basora N, Michalak M, and Chen XZ. Polycystin-2 associates with tropomyosin-1, an actin microfilament component. J Mol Biol 325: 949-962, 2003.

Li Q, Liu Y, Shen PY, Dai XQ, Wang S, Smillie LB, Sandford R, and Chen XZ. Troponin I binds polycystin-L and inhibits its calcium-induced channel activation. Biochemistry 42: 7618-7625, 2003.

Li Q, Liu Y, Zhao W, and Chen XZ. The calcium-binding EF-hand in polycystin-L is not a domain for channel activation and ensuing inactivation. FEBS Lett 516: 270-278, 2002.

Li Q, Shen PY, Wu G, and Chen XZ. Polycystin-2 interacts with troponin I, an angiogenesis inhibitor. Biochemistry 42: 450-457, 2003.

Li SW, Westwick J, and Poll CT. Receptor-operated Ca 2+ influx channels in leukocytes: a therapeutic target? Trends Pharmacol Sci 23: 63-70, 2002.

Lintschinger B, Balzer-Geldesetzer M, Baskaran T, Graier WF, Romanin C, Zhu MX, and Groshner K. Coassembly of Trp1 and Trp3 proteins generates diacylglycerol- and Ca 2+ -sensitive cation channels. J Biol Chem 275: 27799-27805, 2000.

Lu W, Peissel B, Babakhanlou H, Pavlova A, Geng L, Fan X, Larson C, Brent G, and Zhou J. Perinatal lethality with kidney and pancreas defects in mice with a targeted Pkd1 mutation. Nature Genet 17: 179-181, 1997.

Luo Y, Vassilev PM, Li X, Kawanabe Y, and Zhou J. Native polycystin 2 functions as a plasma membrane Ca 2+ -permeable cation channel in renal epithelia. Mol Cell Biol 23: 2600-2607, 2003.

Malhas AN, Abuknesha RA, and Price RG. Interaction of the leucine-rich repeats of polycystin-1 with extracellular matrix proteins: possible role in cell proliferation. Am J Soc Nephrol 13: 19-26, 2002.

Markowitz GS, Cai Y, Li L, Wu G, Ward LC, Somlo S, and D'Agati VD. Polycystin-2 expression is developmentally regulated. Am J Physiol Renal Physiol 277: F17-F25, 1999.

Mengerink KJ, Moy GW, and Vacquier VD. suREJ3, a polycystin-1 protein, is cleaved at the GPS domain and localizes to the acrosomal region of sea urchin sperm. J Biol Chem 277: 943-948, 2002.

Mikoshiba K and Hattori M. IP 3 receptor-operated calcium entry. Science's STKE 51: PE1, 2000.

Minke B. The TRP channel and phospholipase C-mediated signaling. Cell Mol Neurobiol 21: 629-643, 2001.

Minke B and Cook B. TRP channel proteins and signal transduction. Physiol Rev 82: 429-472, 2002.

Minke B, Wu C, and Pak WL. Induction of photoreceptor voltage noise in the dark in Drosophila mutant. Nature 258: 84-87, 1975.

Mochizuki T, Wu G, Hayashi T, Xenophontos SL, Veldhuisen B, Saris JJ, Reynolds DM, Cai Y, Gabow PA, Pierides A, Kimberling WJ, Breuning MH, Deltas CC, Peters DJ, and Somlo S. PKD2, a gene for polycystic kidney disease that encodes an integral membrane protein. Science 272: 1339-1342, 1996.

Montell C. Physiology, phylogeny, and functions of the TRP superfamily of cation channels. Science STKE 90: RE1, 2001.

Montell C, Birnbaumer L, Flockerzi V, Bindels RJ, Bruford EA, Caterina MJ, Clapham DE, Harteneck C, Heller S, Julius D, Kojima I, Mori Y, Penner R, Prawitt D, Scharenberg AM, Schultz G, Shimizu N, and Zhu MX. A unified nomenclature for the superfamily of TRP cation channels. Mol Cell 9: 229-231, 2002.

Montell C and Rubin GM. Molecular characterization of the Drosophila trp locus: a putative integral membrane protein required for phototransduction. Neuron 2: 1313-1323, 1989.

Morgan D, Eley L, Sayer J, Strachan T, Yates LM, Craighead AS, and Goodship JA. Expression analyses and interaction with the anaphase promoting complex protein Apc2 suggest a role for inversin in primary cilia and involvement in the cell cycle. Hum Mol Genet 11: 3345-3350, 2002.

Morgan D, Turnpenny L, Goodship J, Dai W, Majumder K, Matthews L, Gardner A, Schuster G, Vien L, Harrison W, Elder FF, Penman-Splitt M, Overbeek P, and Strachan T. Inversin, a novel gene in the vertebrate left-right axis pathway, is partially deleted in the inv mouse. Nature Genet 20: 149-156, 1998.

Murcia NS, Richards WG, Yoder BK, Mucenski ML, Dunlap JR, and Woychik RP. The Oak Ridge Polycystic Kidney (orpk) disease gene is required for left-right axis determination. Development 127: 2347-2355, 2000.

Murcia NS, Sweeney WE Jr, and Avner ED. New insights into the molecular pathophysiology of polycystic kidney disease. Kidney Int 55: 1187-1197, 1999.

Nauli SM, Alenghat FJ, Luo Y, Williams E, Vassilev P, Li X, Elia AEH, Lu W, Brown EM, Quinn SJ, Ingber DE, and Zhou J. Polycystins 1 and 2 mediate mechanosensation in the primary cilium of kidney cells. Nature Genet 33: 129-137, 2003.

Nickel C, Benzing T, Sellin L, Gerke P, Karihaloo A, Liu ZX, Cantley LG, and Walz G. The polycystin-1 C-terminal fragment triggers branching morphogenesis and migration of tubular kidney epithelial cells. J Clin Invest 109: 481-489, 2002.

Niemeyer BA, Suzuki E, Scott K, Jalink K, and Zuker CS. The Drosophila light-activated conductance is composed of the two channels TRP and TRPL. Cell 85: 651-659, 1996.

Nilius B, Prenen J, Hoenderop JG, Vennekens R, Hoefs S, Weidema AF, Droogmans G, and Bindels RJ. Fast and slow inactivation kinetics of the Ca 2+ channels ECaC1 and ECaC2 (TRPV5 and TRPV6). Role of the intracellular loop located between transmembrane segments 2 and 3. J Biol Chem 277: 30852-30858, 2002.

Nomura H, Turco AE, Pei Y, Kalaydjieva L, Schiavello T, Weremowicz S, Ji W, Morton CC, Meisler M, Reeders ST, and Zhou J. Identification of PKDL, a novel polycystic kidney disease 2-like gene whose murine homologue is deleted in mice with kidney and retinal defects. J Biol Chem 273: 25967-25973, 1998.

Nurnberger J, Bacallao RL, and Phillips CL. Inversin forms a complex with catenins and N-cadherin in polarized epithelial cells. Mol Biol Cell 13: 3096-3106, 2002.

Ong AC, Ward CJ, Butler RJ, Biddolph S, Bowker C, Torra R, Pei Y, and Harris PC. Coordinate expression of the autosomal dominant polycystic kidney disease proteins, polycystin-2 and polycystin-1, in normal and cystic tissue. Am J Pathol 154: 1721-1729, 1999.

Parnell SC, Magenheimer BS, Maser RL, and Calvet JP. Identification of the major site of in vitro PKA phosphorylation in the polycystin-1 C-terminal cytosolic domain. Biochem Biophys Res Commun 259: 539-543, 1999.

Parnell SC, Magenheimer BS, Maser RL, Rankin CA, Smine A, Okamoto T, and Calvet JP. The polycystic kidney disease-1 protein, polycystin-1, binds and activates heterotrimeric G-proteins in vitro. Biochem Biophys Res Commun 251: 625-631, 1998.

Pazour GJ, San Agustin JT, Follit JA, Rosenbaum JL, and Witman GB. Polycystin-2 localizes to kidney cilia and the ciliary level is elevated in orpk mice with polycystic kidney disease. Curr Biol 12: R378-R380, 2002.

Peng JB, Brown EM, and Hediger MA. Apical entry channels in calcium-transporting epithelia. News Physiol Sci 18: 158-163, 2003.

Peng JB, Chen XZ, Berger UV, Vassilev M, Tsukaguchi H, Brown EM, and Hediger M. Molecular cloning and characterization of a channel-like transporter mediating intestinal calcium absorption. J Biol Chem 274: 22739-22746, 1999.

Phillips AM, Bull A, and Kelly LE. Identification of a Drosophila gene encoding a calmodulin-binding protein with homology to the trp phototransduction gene. Neuron 8: 631-642, 1992.

Praetorius HA and Spring KR. Bending the MDCK cell primary cilium increases intracellular calcium. J Membr Biol 184: 71-79, 2001.

Praetorius HA and Spring KR. Removal of the MDCK cell primary cilium abolishes flow sensing. J Membr Biol 191: 69-76, 2003.

Putney JW Jr, Broad LM, Braun FJ, Lievremont JP, and Bird GS. Mechanisms of capacitative calcium entry. J Cell Sci 114: 2223-2229, 2001.

Putney JW Jr and McKay RR. Capacitative calcium entry channels. Bioessays 21: 38-46, 1999. <a href="/cgi/external_ref?access_num=10.1002/(SICI)1521-1878(199901)21:1

Putney JW Jr, Takemura H, Hughes AR, Horstman DA, and Thastrup O. How do inositol phosphates regulate calcium signaling? FASEB J 3: 1899-1905, 1989.

Qian F, Boletta A, Bhunia AK, Xu H, Liu L, Ahrabi AK, Watnick TJ, Zhou F, and Germino GG. Cleavage of polycystin-1 requires the receptor for egg jelly domain and is disrupted by human autosomal-dominant polycystic kidney disease 1-associated mutations. Proc Natl Acad Sci USA 99: 16981-16986, 2002.

Qian Q, Li M, Cai Y, Ward CJ, Somlo S, Harris PC, and Torres VE. Analysis of the polycystins in aortic vascular smooth muscle cells. J Am Soc Nephrol 14: 2280-2287, 2003.

Rodney GG, Moore CP, Williams BY, Zhang JZ, Krol J, Pedersen SE, and Hamilton SL. Calcium binding to calmodulin leads to an N-terminal shift in its binding site on the ryanodine receptor. J Biol Chem 276: 2069-2074, 2001.

Ronkainen A, Hernesniemi J, Puranen M, Niemitukia L, Vanninen R, Ryynanen M, Kuivaniemi H, and Tromp G. Familial intracranial aneurysms. Lancet 349: 380-384, 1997.

Saimi Y and Kung C. Calmodulin as an ion channel subunit. Annu Rev Physiol 64: 289-311, 2002.

Schievink WI, Torres VE, Piepgras DG, and Wiebers DO. Saccular intracranial aneurysms in autosomal dominant polycystic kidney disease. J Am Soc Nephrol 3: 88-95, 1992.

Schwartz EA, Leonard ML, Bizios R, and Bowser SS. Analysis and modeling of the primary cilium bending response to fluid shear. Am J Physiol Renal Physiol 272: F132-F138, 1997.

Scott K, Sun Y, Beckingham K, and Zuker CS. Calmodulin regulation of Drosophila light-activated channels and receptor function mediates termination of the light response in vivo. Cell 91: 375-383, 1997.

Singh BB, Liu X, Tang J, Zhu MX, and Ambudkar IS. Calmodulin regulates Ca 2+ -dependent feedback inhibition of store-operated Ca 2+ influx by interaction with a site in the C terminus of TrpC1. Mol Cell 9: 739-750, 2002.

Stayner C and Zhou J. Polycystin channels and kidney disease. Trends Pharmacol Sci 22: 543-546, 2001.

Strotmann R, Harteneck C, Nunnenmacher K, Schultz G, and Plant TD. OTRPC4, a nonselective cation channel that confers sensitivity to extracellular osmolarity. Nature Cell Biol 2: 695-702, 2000.

Strubing C, Krapivinsky G, Krapivinsky L, and Clapham DE. Formation of novel TRPC channels by complex subunit interactions in embryonic brain. J Biol Chem 278: 39014-39019, 2003.

Strubing C, Krapivinsky G, Krapivinsky L, and Clapham DE. TRPC1 and TRPC5 form a novel cation channel in mammalian brain. Neuron 29: 645-655, 2001.

Sullivan LP, Wallace DP, and Grantham JJ. Epithelial transport in polycystic kidney disease. Physiol Rev 78: 1165-1191, 1998.

Sun M, Goldin E, Stahl S, Falardeau JL, Kennedy JC, Acierno JSJ, Bove C, Kaneski CR, Nagle J, Bromley MC, Colman M, Schiffmann R, and Slaugenhaupt SA. Mucolipidosis type IV is caused by mutations in a gene encoding a novel transient receptor potential channel. Hum Mol Genet 9: 2471-2478, 2000.

Suss-Toby E, Selinger Z, and Minke B. Lanthanum reduces the excitation efficiency in fly photoreceptors. J Gen Physiol 98: 849-868, 1991.

Taylor CW. Controlling calcium entry. Cell 111: 767-769, 2002.

Trudeau MC and Zagotta WN. Calcium/calmodulin modulation of olfactory and rod cyclic nucleotide-gated ion channels. J Biol Chem 278: 18705-18708, 2003.

Tsavaler L, Shapero MH, Morkowski S, and Laus R. Trp-p8, a novel prostate-specific gene, is up-regulated in prostate cancer and other malignancies and shares high homology with transient receptor potential calcium channel proteins. Cancer Res 61: 3760-3769, 2001.

Tsiokas L, Arnould T, Zhu C, Kim E, Walz G, and Sukhatme VP. Specific association of the gene product of PKD2 with the TRPC1 channel. Proc Natl Acad Sci USA 96: 3934-3939, 1999.

Tsiokas L, Kim E, Arnould T, Sukhatme YP, and Walz G. Homo- and hetero-dimeric interactions between the gene products of Pkd1 and Pkd2. Proc Natl Acad Sci USA 94: 6965-6970, 1997.

Vandorpe DH, Chernova MN, Jiang L, Sellin LK, Wilhelm S, Stuart-Tilley AK, Walz G, and Alper SL. The cytoplasmic C-terminal fragment of polycystin-1 regulates a Ca 2+ -permeable cation channel. J Biol Chem 276: 4093-4101, 2001.

Vassilev PM, Guo L, Chen XZ, Segal Y, Peng JB, Basora N, Babakhanlou H, Cruger G, Kanazirska M, Ye C, Brown EM, Hediger MA, and Zhou J. Polycystin-2 is a novel cation channel implicated in defective intracellular Ca 2+ homeostasis in polycystic kidney disease. Biochem Biophys Res Commun 282: 341-350, 2001.

Veldhuisen B, Spruit L, Dauwerse HG, Breuning MH, and Peters DJ. Genes homologous to the autosomal dominant polycystic kidney disease genes ( PKD1 and PKD2 ). Eur J Hum Genet 7: 860-872, 1999.

Vennekens R, Prenen J, Hoenderop JG, Bindels RJ, Droogmans G, and Nilius B. Modulation of the epithelial Ca 2+ channel ECaC by extracellular pH. Pflügers Arch 442: 237-242, 2001.

Vennekens R, Prenen J, Hoenderop JG, Bindels RJ, Droogmans G, and Nilius B. Pore properties and ionic block of the rabbit epithelial calcium channel expressed in HEK 293 cells. J Physiol 530: 183-191, 2001.

Voets T, Janssens A, Prenen J, Droogmans G, and Nilius B. Mg 2+ -dependent gating and strong inward rectification of the cation channel TRPV6. J Gen Physiol 121: 245-260, 2003.

Voets T and Nilius B. TRPs make sense. J Membr Biol 192: 1-8, 2003.

Voets T, Prenen J, Fleig A, Vennekens R, Watanabe H, Hoenderop JG, Bindels RJ, Droogmans G, Penner R, and Nilius B. CaT1 and the calcium release-activated calcium channel manifest distinct pore properties. J Biol Chem 276: 47767-47770, 2001.

Volk T, Schwoerer AP, Thiessen S, Schultz JH, and Ehmke H. A polycystin-2-like large conductance cation channel in rat left ventricular myocytes. Cardiovasc Res 58: 76-88, 2003.

Watras J, Bezprozvanny I, and Ehrlich BE. Inositol 1,4,5-trisphosphate-gated channels in cerebellum: presence of multiple conductance states. J Neurosci 11: 3239-3245, 1991.

Wes PD, Chevesich J, Jeromin A, Rosenberg C, Stetten G, and Montell C. Trpc1, a human homolog of a Drosophila store-operated channel. Proc Natl Acad Sci USA 92: 9652-9656, 1995.

West RJ, Sun AY, Church DL, and Lambie EJ. The C. elegans gon-2 gene encodes a putative TRP cation channel protein required for mitotic cell cycle progression. Gene 266: 103-110, 2001.

Weston BS, Malhas AN, and Price RG. Structure-function relationships of the extracellular domain of the autosomal dominant polycystic kidney disease-associated protein, polycystin-1. FEBS Lett 538: 8-13, 2003.

Wilson PD. Polycystin: new aspects of structure, function, and regulation. J Am Soc Nephrol 12: 834-845, 2001.

Wilson SM, Mason HS, Smith GD, Nicholson N, Johnston L, Janiak R, and Hume JR. Comparative capacitative calcium entry mechanisms in canine pulmonary and renal arterial smooth muscle cells. J Physiol 543: 917-931, 2002.

Wissenbach U, Niemeyer BA, Fixemer T, Schneidewind A, Trost C, Cavalie A, Reus K, Meese E, Bonkhoff H, and Flockerzi V. Expression of CaT-like, a novel calcium-selective channel, correlates with the malignancy of prostate cancer. J Biol Chem 276: 19461-19468, 2001.

Woodhull AM. Ion blockage of sodium channels in nerve. J Gen Physiol 61: 687-708, 1973.

Wu G, D'Agati V, Cai Y, Markowitz G, Park JH, Reynolds DM, Maeda Y, Le TC, Hou H Jr, Kucherlapati R, Edelmann W, and Somlo S. Somatic inactivation of Pkd2 results in polycystic kidney disease. Cell 93: 177-188, 1998.

Wu G, Markowitz GS, Li L, D'Agati VD, Factor SM, Geng L, Tibara S, Tuchman J, Cai Y, Park JH, van Adelsberg J, Hou HJ, Kucherlapati R, Edelmann W, and Somlo S. Cardiac defects and renal failure in mice with targeted mutations in Pkd2. Nature Genet 24: 75-78, 2000.

Xu GM, Gonzalez-Perrett S, Essafi M, Timpanaro GA, Montalbetti N, Arnaout MA, and Cantiello HF. Polycystin-1 activates and stabilizes the polycystin-2 channel. J Biol Chem 278: 1457-1462, 2003.

Xu GM, Sikaneta T, Sullivan BM, Zhang Q, Andreucci M, Stehle T, Drummond I, and Arnaout MA. Polycystin-1 interacts with intermediate filaments. J Biol Chem 276: 46544-46552, 2001.

Xu XZ, Li HS, Guggino WB, and Montell C. Coassembly of TRP and TRPL produces a distinct store-operated conductance. Cell 89: 1155-1164, 1997.

Yoder BK, Hou X, and Guay-Woodford LM. The polycystic kidney disease proteins, polycystin-1, polycystin-2, polaris, and cystin, are co-localized in renal cilia. J Am Soc Nephrol 13: 2508-2516, 2002.

Yoder BK, Tousson A, Millican L, Wu J, Bugg CE Jr, Schafer JA, and Balkovetz DF. Polaris, a protein disrupted in orpk mutant mice, is required for assembly of renal cilium. Am J Physiol Renal Physiol 282: F541-F552, 2002.

Yuasa T, Venugopal B, Weremowicz S, Morton CC, Guo L, and Zhou J. The sequence, expression, and chromosomal localization of a novel polycystic kidney disease 1-like gene, PKD1L1, in humans. Genomics 79: 376-386, 2002.

Yue L, Peng JB, Hediger MA, and Clapham DE. CaT1 manifests the pore properties of the calcium-release-activated calcium channel. Nature 410: 705-709, 2001.

Zhu X, Chu PB, Peyton M, and Birnbaumer L. Molecular cloning of a widely expressed human homologue for the Drosophila trp gene. FEBS Lett 373: 193-198, 1995.

Zitt C, Halaszovich CR, and Luckhoff A. The TRP family of cation channels: probing and advancing the concepts on receptor-activated calcium entry. Prog Neurobiol 66: 243-264, 2002.

Zuhlke RD and Reuter H. Ca 2+ -sensitive inactivation of L-type Ca 2+ channels depends on multiple cytoplasmic amino acid sequences of the 1C subunit. Proc Natl Acad Sci USA 95: 3287-3294, 1998.

作者单位：Renal Unit, Massachusetts General Hospital East, Charlestown, Massachusetts 02129