Troponin I binds polycystin-L and inhibits its calcium-induced channel activation

Discovery and Characteristics of a Novel Antitumor Cyclopeptide Derived from Shark

Peptides pose a challenge in drug development due to their short half-lives in vivo. In this study, we conducted in vitro degradation experiments on SAIF, which is a shark-derived peptide that we previously studied. The degradation fragments were sequenced and a truncated peptide sequence was identified. The truncated peptide was then cloned and expressed via the E. coli system with traceless cloning to form a novel cyclic peptide in vitro oxidation condition via the formation of a disulfide bond between the N- and C-termini, which was named ctSAIF. ctSAIF exhibited high anti-HCC activity and enhanced enzymatic stability in vitro, and retained antitumor activity and good biocompatibility in systemic circulation in a HCC xenograft model. Our study discovered and characterized a novel shark-derived cyclic peptide with antitumor activity, laying a foundation for its further development as an antitumor drug candidate. The study also provided a new solution for peptide drug development.

Inhibition of TRPP3 by calmodulin through Ca2+/calmodulin-dependent protein kinase II

Transient receptor potential (TRP) polycystin-3 (TRPP3) is a non-selective cation channel activated by Ca2+ and protons and is involved in regulating ciliary Ca2+ concentration, hedgehog signaling and sour tasting. The TRPP3 channel function and regulation are still not well understood. Here we investigated regulation of TRPP3 by calmodulin (CaM) by means of electrophysiology and Xenopus oocytes as an expression model. We found that TRPP3 channel function is enhanced by calmidazolium, a CaM antagonist, and inhibited by CaM through binding of the CaM N-lobe to a TRPP3 C-terminal domain not overlapped with the EF-hand. We further revealed that the TRPP3/CaM interaction promotes phosphorylation of TRPP3 at threonine 591 by Ca2+/CaM-dependent protein kinase II, which mediates the inhibition of TRPP3 by CaM.

Acid-induced off-response of PKD2L1 channel in Xenopus oocytes and its regulation by Ca(2.)

Polycystic kidney disease (PKD) protein 2 Like 1 (PKD2L1), also called transient receptor potential polycystin-3 (TRPP3), regulates Ca(2+)-dependent hedgehog signalling in primary cilia, intestinal development and sour tasting but with an unclear mechanism. PKD2L1 is a Ca(2+)-permeable cation channel that is activated by extracellular Ca(2+) (on-response) in Xenopus oocytes. PKD2L1 co-expressed with PKD protein 1 Like 3 (PKD1L3) exhibits extracellular acid-induced activation (off-response, i.e., activation following acid removal) but whether PKD1L3 participates in acid sensing remains unclear. Here we used the two-microelectrode voltage-clamp, site directed mutagenesis, Western blotting, reverse transcriptase-polymerase chain reaction (RT-PCR) and immunofluorescence, and showed that PKD2L1 expressed in oocytes exhibits sustained off-response currents in the absence of PKD1L3. PKD1L3 co-expression augmented the PKD2L1 plasma membrane localization but did not alter the observed properties of the off-response. PKD2L1 off-response was inhibited by an increase in intracellular Ca(2+). We also identified two intra-membrane residues aspartic acid 349 (D349) and glutamic acid 356 (E356) in the third transmembrane domain that are critical for PKD2L1 channel function. Our study suggests that PKD2L1 may itself sense acids and defines off-response properties in the absence of PKD1L3.

The TRPP subfamily and polycystin-1 proteins

It has been exciting times since the identification of polycystic kidney disease 1 (PKD1) and PKD2 as the genes mutated in autosomal dominant polycystic kidney disease (ADPKD). Biological roles of the encoded proteins polycystin-1 and TRPP2 have been deduced from phenotypes in ADPKD patients, but recent insights from vertebrate and invertebrate model organisms have significantly expanded our understanding of the physiological functions of these proteins. The identification of additional TRPP (TRPP3 and TRPP5) and polycystin-1-like proteins (PKD1L1, PKD1L2, PKD1L3, and PKDREJ) has added yet another layer of complexity to these fascinating cellular signalling units. TRPP proteins assemble with polycystin-1 family members to form receptor-channel complexes. These protein modules have important biological roles ranging from tubular morphogenesis to determination of left-right asymmetry. The founding members of the polycystin family, TRPP2 and polycystin-1, are a prime example of how studying human disease genes can provide insights into fundamental biological mechanisms using a so-called "reverse translational" approach (from bedside to bench). Here, we discuss the current literature on TRPP ion channels and polycystin-1 family proteins including expression, structure, physical interactions, physiology, and lessons from animal model systems and human disease.

Receptor for activated C kinase 1 (RACK1) inhibits function of transient receptor potential (TRP)-type channel Pkd2L1 through physical interaction

Pkd2L1 (also called TRPP3) is a non-selective cation channel permeable to Ca(2+), Na(+), and K(+) and is activated by Ca(2+). It is also part of an acid-triggered off-response cation channel complex. We previously reported roles of the Pkd2L1 C-terminal fragments in its channel function, but the role of the N terminus remains unclear. Using a yeast two-hybrid screening, we found that the Pkd2L1 N terminus interacts with the receptor for activated C kinase 1 (RACK1), a scaffolding/anchoring protein implicated in various cellular functions. This interaction requires the last two Trp-Asp (WD) motifs of RACK1 and fragment Ala(19)-Pro(45) of Pkd2L1. The interaction was confirmed by GST pulldown, blot overlay, and co-immunoprecipitation assays. By (45)Ca tracer uptake and two-microelectrode voltage clamp electrophysiology, we found that in Xenopus oocytes with RACK1 overexpression Pkd2L1 channel activity is abolished or substantially reduced. Combining with oocyte surface biotinylation experiments, we demonstrated that RACK1 inhibits the function of Pkd2L1 channel on the plasma membrane in addition to reducing its total and plasma membrane expression. Overexpressing Pkd2L1 N- or C-terminal fragments as potential blocking peptides for the Pkd2L1-RACK1 interaction, we found that Pkd2L1 N-terminal fragment Met(1)-Pro(45), but not Ile(40)-Ile(97) or C-terminal fragments, abolishes the inhibition of Pkd2L1 channel by overexpressed and oocyte-native RACK1 likely through disrupting the Pkd2L1-RACK1 association. Taken together, our study demonstrated that RACK1 inhibits Pkd2L1 channel function through binding to domain Met(1)-Pro(45) of Pkd2L1. Thus, Pkd2L1 is a novel target channel whose function is regulated by the versatile scaffolding protein RACK1.

A polypeptide from shark troponin I can inhibit angiogenesis and tumor growth

The shark troponin I gene (TnI) was found for the first time in this study to inhibit endothelial cell proliferation and angiogenesis. This shark TnI had 68.9% amino acid homology with human TnI, whereas the polypeptide from Lys91 to Leu123, which is thought to be the active site of TnI, had 78.8% homology with the corresponding fragment of human TnI. However, the polypeptide of shark had higher activity to inhibit the proliferation of HUVEC and tumor cell lines than that of human TnI. To investigate the anti-angiogenesis and anti-tumor effect of the shark TnI polypeptide, the DNA sequence of polypeptide (Lys91-Leu123) of white-spot catshark TnI(psTnI) was cloned and fused with the His-SUMO cDNA, followed by expression in Escherichia coli. After its purification by Ni(2+) affinity chromatography, the fusion His-SUMO-psTnI protein was digested with the SUMO enzyme to release psTnI. The inhibitory ability of this recombinant shark TnI polypeptide for angiogenesis was confirmed by chicken embryo allantoic membrane (CAM) test and IHC analysis. It was also found by breast carcinoma xenograft study in Balb/c mice that this polypeptide could inhibit tumor growth in vivo.

TRPP channels and polycystins

The founding member of the TRPP family, TRPP2, was identified as one of the disease genes causing autosomal dominant polycystic kidney disease (ADPKD). ADPKD is the most prevalent, potentially lethal, monogenic disorder in humans, with an average incidence of one in 400 to one in 1,000 individuals worldwide. Here we give an overview of TRPP ion channels and Polycystin-1 receptor proteins focusing on more recent studies. We include the Polycystin-1 family since these proteins are functionally linked to TRPP channels.

Identification of the structural motif responsible for trimeric assembly of the C-terminal regulatory domains of polycystin channels PKD2L1 and PKD2

Polycystin 2-type cation channels PKD2 and PKD2L1 interact with polycystin 1-type proteins PKD1 and PKD1L3 respectively, to form receptor-cation-channel complexes. The PKD2L1-PKD1L3 complex perceives sour taste, whereas disruption of the PKD2-PKD1 complex, responsible for mechanosensation, leads to development of ADPKD (autosomal-dominant polycystic kidney disease). Besides modulating channel activity and related signalling events, the CRDs (C-terminal regulatory domains) of PKD2 and PKD2L1 play a central role in channel oligomerization. The present study investigates the aggregation state of purified full-length PKD2L1-CRD as well as truncations of CRDs from PKD2 channels. Far- and near-UV CD spectroscopy show that the full-length PKD2L1 CRD (PKD2L1-198) and the truncated PKD2 CRD (PKD2-244) are alpha-helical with no beta-sheet, the alpha-helix content agrees with sequence-based predictions, and some of its aromatic residues are in an asymmetric environment created at least by partially structured regions. Additionally, the CRD truncations exhibit an expected biochemical function by binding Ca2+ in a physiologically relevant range with Kd values of 2.8 muM for PKD2-244 and 0.51 muM for PKD2L1-198. Complimentary biophysical and biochemical techniques establish that truncations of the PKD2 and PKD2L1 CRDs are elongated molecules that assemble as trimers, and the trimeric aggregation state is independent of Ca2+ binding. Finally, we show that a common coiled-coil motif is sufficient and necessary to drive oligomerization of the PKD2 and PKD2L1 CRD truncations under study. Despite the moderate sequence identity (39%) between CRDs of PKD2 and PKD2L1, they both form trimers, implying that trimeric organization of CRDs may be true of all polycystin channels.

Analysis of the cytoplasmic interaction between polycystin-1 and polycystin-2

Autosomal dominant polycystic kidney disease (ADPKD) arises following mutations of either Pkd1 or Pkd2. The proteins these genes encode, polycystin-1 (PC1) and polycystin-2 (PC2), form a signaling complex using direct intermolecular interactions. Two distinct domains in the C-terminal tail of PC2 have recently been identified, an EF-hand and a coiled-coil domain. Here, we show that the PC2 coiled-coil domain interacts with the C-terminal tail of PC1, but that the PC2 EF-hand domain does not. We measured the K0.5 of the interaction between the C-terminal tails of PC1 and PC2 and showed that the direct interaction of these proteins is abrogated by a PC1 point mutation that was identified in ADPKD patients. Finally, we showed that overexpression of the PC1 C-terminal tail in MDCK cells alters the Ca2+ response, but that overexpression of the PC1 C-terminal tail containing the disease mutation does not. These results allow a more detailed understanding of the mechanism of pathogenic mutations in the cytoplasmic regions of PC1 and PC2.

Isoform-specific variation in the intrinsic disorder of troponin I

Various intrinsic disorder (ID) prediction algorithms were applied to the three tissue isoforms of troponin I (TnI). The results were interpreted in terms of the known structure and dynamics of troponin. In line with previous results, all isoforms of TnI were predicted to have large stretches of ID. The predictions show that the C-termini of all isoforms are extensively disordered as is the N-terminal extension of the cardiac isoform. Cardiac TnI likely belongs to the group of intrinsically disordered signalling hub proteins. For a given portion of the protein sequence, most ID prediction approaches indicate isoform-dependent variations in the probability of disorder. Comparison of machine learning and physically based approaches suggests the ID variations are only partially attributable to local variations in the ratio of charged to hydrophobic residues. The VSL2B algorithm predicts the largest variations in ID across the isoforms, with the cardiac isoform having the highest probability of structured regions, and the fast-skeletal isoform having no intrinsic structure. The region corresponding to residues 57-95 of the fast-skeletal isoform, known to form a coiled coil substructure with troponin T, was highly variable between isoforms. The isoform-specific ID variations may have mechanistic significance, modulating the extent to which conformational fluctuations in tropomyosin are communicated to the troponin complex. We discuss structural mechanisms for this communication. Overall, the results motivate the development of predictors designed to address relative levels of disorder between highly similar proteins.

Direct binding of alpha-actinin enhances TRPP3 channel activity

Transient receptor potential (TRP) polycystin 2 and 3 (TRPP2 and 3) are homologous members of the TRP superfamily of cation channels but have different physiological functions. TRPP2 is part of a flow sensor, and is defective in autosomal dominant polycystic kidney disease and implicated in left-right asymmetry development. TRPP3 is reported to implicate in sour tasting in bipolar cells of taste buds of the tongue and in the regulation of pH-sensitive action potential in neurons surrounding the central canal of spinal cord. TRPP3 is present in both excitable and non-excitable cells in various tissues, such as retina, brain, heart, testis, and kidney, but its common and cell type-specific functional characteristics remain largely unknown. In this study, we investigated physical and functional interactions between TRPP3 and alpha-actinin, an actin-bundling protein known to regulate several types of ion channels. We employed planer lipid bilayer electrophysiology system to study the function of TRPP3 channel that was affinity-purified from Madin-Darby canine kidney cells. Upon reconstitution in bilayer, TRPP3 exhibited cation channel activities that were substantially augmented by alpha-actinin. The TRPP3-alpha-actinin association was documented by co-immunoprecipitation using native cells and tissues, yeast two-hybrid, and in vitro binding assays. Further, TRPP3 was abundantly present in mouse brain where it associates with alpha-actinin-2. Taken together, alpha-actinin not only attaches TRPP3 to the cytoskeleton but also up-regulates TRPP3 channel function. It remains to be determined whether the TRPP3-alpha-actinin interaction is relevant to acid sensing and other functions in neuronal and non-neuronal cells.

Inhibition of TRPP3 Channel by Amiloride and Analogs

TRPP3, a member of the transient receptor potential (TRP) superfamily of cation channels, is a Ca2+-activated channel permeable to Ca2+, Na+, and K+. TRPP3 has been implicated in sour tasting in bipolar cells of tongue and in regulation of pH-sensitive action potential in spinal cord neurons. TRPP3 is also present in excitable and nonexcitable cells of other tissues, including retina, brain, heart, testis, and kidney, with unknown functions. In this study, we examined the functional modulation of TRPP3 channel by amiloride and its analogs, known to inhibit several ion channels and transporters and respond to all taste stimuli, using Xenopus laevis oocyte expression, electrophysiology, and radiotracer measurements. We found that amiloride and its analogs inhibit TRPP3 channel activities with different affinities. Radiolabeled (45)Ca2+ uptake showed that TRPP3-mediated Ca2+ transport was inhibited by amiloride, phenamil, benzamil, and 5-(N-ethyl-N-isopropyl)amiloride (EIPA). Two-microelectrode voltage clamp experiments revealed that TRPP3-mediated Ca2+-activated currents are substantially inhibited by amiloride analogs, in an order of potency of phenamil > benzamil > EIPA > amiloride, with IC50 values of 0.14, 1.1, 10.5, and 143 microM, respectively. The inhibition potency positively correlated with the size of inhibitors. Using cell-attached patch clamping, we showed that the amiloride analogs decrease the open probability and mean open time but have no effect on single-channel conductance. Study of inhibition by phenamil in the presence of previously reported inhibitor tetrapentylammonium indicates that amiloride and organic cation inhibitors compete for binding the same site on TRPP3. TRPP3 may contribute to previously reported in vivo amiloride-sensitive cation transport.

Identification of 9 uterine genes that are regulated during mouse pregnancy and exhibit abnormal levels in the cyclooxygenase-1 knockout mouse

BACKGROUND

Preterm birth is the leading cause of all infant mortality. In 2004, 12.5% of all births were preterm. In order to understand preterm labor, we must first understand normal labor. Since many of the myometrial changes that occur during pregnancy are similar in mice and humans and mouse gestation is short, we have studied the uterine genes that change in the mouse during pregnancy. Here, we used microarray analysis to identify uterine genes in the gravid mouse that are differentially regulated in the cyclooxygenase-1 knockout mouse model of delayed parturition.

METHODS

Gestational d18.0 uteri (n = 4) were collected from pregnant wild-type and cyclooxygenase-1 knockout mice. Part of the uterus was used for frozen sections and RNA was isolated from the remainder. Microarray analysis was performed at the Indiana University School of Medicine Genomic Core and analyzed using the Microarray Data Portal. Northern analysis was performed to confirm microarray data and the genes localized in the gravid uterus by in situ hybridization.

RESULTS

We identified 277 genes that are abnormally expressed in the gravid d18.0 cyclooxygenase-1 knockout mouse. Nine of these genes are also regulated in the normal murine uterus during the last half of gestation. Many of these genes are involved in the immune response, consistent with an important role of the immune system in parturition. Expression of 4 of these genes; arginase I, IgJ, Tnfrsf9 and troponin; was confirmed by Northern analysis to be mis-regulated during pregnancy in the knockout mouse. In situ hybridization of these genes demonstrated a similar location in the gravid wild-type and Cox-1 knockout mouse uteri.

CONCLUSION

To our knowledge, this is the first work to demonstrate the uterine location of these 4 genes in the mouse during late pregnancy. There are several putative transcription factor binding sites that are shared by many of the 9 genes identified here including; estrogen and progesterone response elements and Ets binding sites. In summary, this work identifies 9 uterine murine genes that may play a role in parturition. The function of these genes is consistent with an important role of the immune system in parturition.

Inhibition of polycystin-L channel by the Chinese herb Sparganum stoloniferum Buch.-Ham

The Chinese herb Sparganum stoloniferum Buch.-Ham. (SBH) is frequently used to improve blood circulation and to rehabilitate vascular obstruction in traditional Chinese medicine. It was recently reported that SBH reduces the proliferation of renal epithelial cells stimulated by epidermal growth factor (EGF), and inhibits the phosphorylation of the EGF receptor. SBH has also been used as a trial drug to treat polycystic kidney disease (PKD) patients in China. The potential molecular actions of SBH on PKD remain unknown. Autosomal dominant PKD (ADPKD) is associated with mutations in polycystin-1 or polycystin-2 (PC2). PC2 and its homologue, polycystin-L (PCL), are nonselective cation channels permeable to potassium, sodium, and calcium. Here, we examine the effects of SBH on the human PCL channel expressed in Xenopus oocytes, using 2-microelectrode voltage-clamp electrophysiology and radiotracer uptake measurements. In PCL-expressing oocytes, with or without preincubation with SBH, the PCL channel was inhibited by SBH in a dose-dependent and reversible manner; a concentration of 2% SBH completely abolished the channel activation. The IC50 value for SBH was 0.48% ± 0.03%, with a 10-min preincubation period. SBH was also found to inhibit the PCL-mediated 45Ca tracer uptake in oocytes. Our study suggests that SBH contains 1 or more yet-to-be determined components that are inhibitors of PCL channel. The therapeutic potential of SBH for ADPKD and its chemical composition remain to be investigated.

Polycystin-2 cation channel function is under the control of microtubular structures in primary cilia of renal epithelial cells

Mutations in the gene encoding polycystin-2 (PC2) result in autosomal dominant polycystic kidney disease and defects in left-right asymmetry during embryogenesis. PC2 is a TRP-type Ca(2+)-permeable non-selective cation channel, which is expressed in kidney and other organs. PC2 is present and functional in microtubule-containing primary cilia of renal epithelial cells. However, no information is yet available as to whether PC2 interacts with microtubules. Here, we assessed the role of microtubular dynamics in regulating PC2 channel function in primary cilia. Isolated ciliary membranes from LLC-PK1 epithelial cells were reconstituted in a lipid bilayer system. The acute addition of the microtubular disrupter colchicine (15 mum) rapidly abolished, whereas the addition of the microtubular stabilizer paclitaxel (taxol, 15 mum) increased ciliary PC2 channel activity. The further addition of alpha-tubulin plus GTP also stimulated PC2 channel activity in ciliary membranes. However, alpha-tubulin and GTP had no effect on in vitro translated PC2. Using the yeast two-hybrid assay, we found that PC2 interacts with the microtubule-dependent motor kinesin-2 subunit KIF3A, a protein involved in polycystic kidney disease. The interaction occurred through the carboxyl termini domain of both proteins, which was further confirmed by in vitro glutathione S-transferase pull-down and dot blot overlay assays. Co-immunoprecipitation experiments showed that PC2 and KIF3A are in the same complex in native HEK293, Madin-Darby canine kidney cells (MDCK), and LLC-PK1 cells. Immunofluorescent staining also showed substantial PC2 and KIF3A co-localization in primary cilia of renal epithelial cells. The data indicate that microtubular organization regulates PC2 function, which may explain, among others, the regulatory role of PC2 in the sensory function of primary cilia.

Transient receptor potential channels in cardiovascular function and disease

Sustained elevation in the intracellular Ca2+ concentration via Ca2+ influx, which is activated by a variety of mechanisms, plays a central regulatory role for cardiovascular functions. Recent molecular biological research has disclosed an unexpectedly diverse array of Ca(2+-entry channel molecules involved in this Ca2+ influx. These include more than ten transient receptor potential (TRP) superfamily members such as TRPC1, TRPC3-6, TRPV1, TRPV2, TRPV4, TRPM4, TRPM7, and polycystin (TRPP2). Most of them appear to be multimodally activated or modulated and show relevant features to both acute hemodynamic control and long-term remodeling of the cardiovascular system, and many of them have been found to respond not only to receptor stimulation but also to various forms of stimuli. There is good evidence to implicate TRPC1 in neointimal hyperplasia after vascular injury via store-depletion-operated Ca2+ entry. TRPC6 likely contributes to receptor-operated and mechanosensitive Ca2+ mobilizations, being involved in vasoconstrictor and myogenic responses and pulmonary arterial proliferation and its associated disease (idiopathic pulmonary arterial hypertension). Considerable evidence has also been accumulated for unique involvement of TRPV1 in blood flow/pressure regulation via sensory vasoactive neuropeptide release. New lines of evidence suggest that TRPV2 may act as a Ca2+-overloading pathway associated with dystrophic cardiomyopathy, TRPV4 as a mediator of endothelium-dependent hyperpolarization, TRPM7 as a proproliferative vascular Mg2+ entry channel, and TRPP2 as a Ca2+-entry channel requisite for vascular integrity. This review attempts to provide an overview of the current knowledge on TRP proteins and discuss their possible roles in cardiovascular functions and diseases.

Interaction of epithelial ion channels with the actin-based cytoskeleton

The interaction of ion channels with the actin-based cytoskeleton in epithelial cells not only maintains the polarized expression of ion channels within specific membrane domains, it also functions in the intracellular trafficking and regulation of channel activity. Initial evidence supporting an interaction between epithelial ion channels and the actin-based cytoskeleton came from patch-clamp studies examining the effects of cytochalasins on channel activity. Cytochalasins were shown to either activate or inactivate epithelial ion channels. An interaction between the actin-based cytoskeleton and epithelial ion channels was further supported by the fact that the addition of monomeric or filamentous actin to excised patches had an effect on channel activity comparable to that of cytochalasins. Through the recent application of molecular and proteomic approaches, we now know that the interactions between epithelial ion channels and actin can either be direct or indirect, the latter being mediated through scaffolding or actin-binding proteins that serve as links between the channels and the actin-based cytoskeleton. This review discusses recent advances in our understanding of the interactions between epithelial ion channels and the actin-based cytoskeleton, and the roles these interactions play in regulating the cell surface expression, activity, and intracellular trafficking of epithelial ion channels.

More than colocalizing with polycystin-1, polycystin-L is in the centrosome

Polycystin-1 and polycystin-2 are involved in autosomal dominant polycystic kidney disease by unknown mechanisms. These two proteins are located in primary cilia where they mediate mechanosensation, suggesting a link between cilia function and renal disease. In this study, we sought to characterize the subcellular localization of polycystin-L, a closely related member of polycystin-2, in epithelial renal cell lines. We have shown that endogenous polycystin-l subcellular distribution is different in proliferative and nonproliferative cultures. Polycystin-L is found mostly in the endoplasmic reticulum in subconfluent cell cultures, while in confluent cells it is redistributed to sites of cell-cell contact and to the primary cilium as is polycystin-1. Subcellular fractionation confirmed a common distribution of polycystin-L and polycystin-1 in the fractions corresponding to those containing the plasma membrane of postconfluent cells. Reciprocal coimmunoprecipitation experiments showed that polycystin-L was associated with polycystin-1 in a common complex in both subconfluent and confluent cell cultures. Interestingly, we also identified a novel site for a polycystin member (polycystin-L) in unciliated cells, the centrosome, which allowed us to reveal an involvement of polycystin-l in cell proliferation.

Permeation and inhibition of polycystin-L channel by monovalent organic cations

Polycystin-L (PCL), homologous to polycystin-2 (71% similarity in protein sequence), is the third member of the polycystin family of proteins. Polycystin-1 and -2 are mutated in autosomal dominant polycystic kidney disease, but the physiological role of PCL has not been determined. PCL acts as a Ca-regulated non-selective cation channel permeable to mono- and divalent cations. To further understand the biophysical and pharmacological properties of PCL, we examined a series of organic cations for permeation and inhibition, using single-channel patch clamp and whole-cell two-microelectrode voltage clamp techniques in conjunction with Xenopus oocyte expression. We found that PCL is permeable to organic amines, methlyamine (MA, 3.8 A), dimethylamine (DMA, 4.6 A) and triethylamine (TriEA, 6 A), and to tetra-alkylammonium cation (TAA) tetra-methylammonium (TMA, 5.5-6.4 A). TAA compounds tetra-ethylammonium (TEA, 6.1-8.2 A) and tetra-propylammonium (TPA, 9.8 A) were impermeable through PCL and exhibited weak inhibition on PCL (IC50 values>13 mM). Larger TAA cations tetra-butylammonium (TBA, 11.6 A) and tetra-pentylammonium (TPeA, 13.2 A) were impermeable through PCL as well and showed strong inhibition (IC50 values of 2.7 mM and 1.3 microM, respectively). Inhibition by TBA was on decreasing the single-channel current amplitude and exhibited no effect on open probability (NPo) or mean open time (MOT), suggesting that it blocks the PCL permeation pathway. In contract, TEA, TPA and TPeA reduced NPo and MOT values but had no effect on the amplitude, suggesting their binding to a different site in PCL, which affects the channel gating. Taken together, our studies revealed that PCL is permeable to organic amines and TAA cation TMA, and that inhibition of PCL by large TAA cations exhibits two different mechanisms, presumably through binding either to the pore pathway to reduce permeant flux or to another site to regulate the channel gating. These data allow to estimate a channel pore size of approximately 7 A for PCL.

Cytoskeletal regulation of calcium-permeable cation channels in the human syncytiotrophoblast: role of gelsolin

The human syncytiotrophoblast (hST) is the most apical epithelial barrier that covers the villous tree of the human placenta. An intricate and highly organized network of cytoskeletal structures supports the hST. Recently, polycystin-2 (PC2), a TRP-type nonselective cation channel, was functionally observed in hST, where it may be an important player to Ca2+ transport. Little is known, however, about channel regulation in hST. In this report, the regulatory role of actin dynamics on PC2 channels reconstituted from hST apical membranes was explored. Acute addition of cytochalasin D (CD, 5 microg ml-1) to reconstituted hST apical membranes transiently increased K+ -permeable channel activity. The actin-binding proteins alpha-actinin and gelsolin, as well as PC2, were observed by Western blot and immunofluorescence analyses in hST vesicles. CD treatment of hST vesicles resulted in a re-distribution of actin filaments, in agreement with the effect of CD on K+ channel activity. In contrast, addition of exogenous monomeric actin, but not prepolymerized actin, induced a rapid inhibition of channel function in hST. This inhibition was obliterated by the presence of CD in the medium. The acute (<15 min) CD stimulation of K+ channel activity was mimicked by addition of the actin-severing protein gelsolin in the presence, but not in the absence, of micromolar Ca2+. Ca2+ transport through PC2 triggers a regulatory feedback mechanism, which is based on the severing and re-formation of filamentous actin near the channels. Cytoskeletal structures may thus be relevant to ion transport regulation in the human placenta.

Regulation of calcium signaling by polycystin-2

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 Ca2+-permeable nonselective cation channel. The PC2 channel is implicated in the transient increase in cytosolic Ca2+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 Ca2+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.