Specific association of the gene product of PKD2 with the TRPC1 channel

Intracellular Ca2+ storage in health and disease: a dynamic equilibrium

Homeostatic control of the endoplasmic reticulum (ER) both as the site for protein handling (synthesis, folding, trafficking, disaggregation and degradation) and as a Ca2+ store is of crucial importance for correct functioning of the cell. Disturbance of the homeostatic control mechanisms leads to a vast array of severe pathologies. The Ca2+ content of the ER is a dynamic equilibrium between active uptake via Ca2+ pumps and Ca2+ release by a number of highly regulated Ca2+-release channels. Regulation of the Ca2+-release channels is very complex and several mechanisms are still poorly understood or controversial. There is increasing evidence that a number of unrelated proteins, either by themselves or in association with other Ca2+ channels, can provide additional Ca2+-leak pathways. The ER is a dynamic organelle and changes in its size and components have been described, either as a result of (de)differentiation processes affecting the secretory capacity of cells, or as a result of adaptation mechanisms to diverse stress conditions such as the unfolded protein response and autophagy. In this review we want to give an overview of the current knowledge of the (short-term) regulatory mechanisms that affect Ca2+-release and Ca2+-leak pathways and of the (long-term) adaptations in ER size and capacity. Understanding of the consequences of these mechanisms for cellular Ca2+ signaling could provide a huge therapeutic potential.

Molecular advances in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic disease (ADPKD) is the most common form of inherited kidney disease that results in renal failure. The understanding of the pathogenesis of ADPKD has advanced significantly since the discovery of the 2 causative genes, PKD1 and PKD2. Dominantly inherited gene mutations followed by somatic second-hit mutations inactivating the normal copy of the respective gene result in renal tubular cyst formation that deforms the kidney and eventually impairs its function. The respective gene products, polycystin-1 and polycystin-2, work together in a common cellular pathway. Polycystin-1, a large receptor molecule, forms a receptor-channel complex with polycystin-2, which is a cation channel belonging to the TRP family. Both polycystin proteins have been localized to the primary cilium, a nonmotile microtubule-based structure that extends from the apical membrane of tubular cells into the lumen. Here we discuss recent insights in the pathogenesis of ADPKD including the genetics of ADPKD, the properties of the respective polycystin proteins, the role of cilia, and some cell-signaling pathways that have been implicated in the pathways related to PKD1 and PKD2.

The transient receptor potential channels TRPP2 and TRPC1 form a heterotetramer with a 2:2 stoichiometry and an alternating subunit arrangement

There is functional evidence that polycystin-2 (TRPP2) interacts with other members of the transient receptor potential family, including TRPC1 and TRPV4. Here we have used atomic force microscopy to study the structure of the TRPP2 homomer and the interaction between TRPP2 and TRPC1. The molecular volumes of both Myc-tagged TRPP2 and V5-tagged TRPC1 isolated from singly transfected tsA 201 cells indicated that they assembled as homotetramers. The molecular volume of the protein isolated from cells expressing both TRPP2 and TRPC1 was intermediate between the volumes of the two homomers, suggesting that a heteromer was being formed. The distribution of angles between pairs of anti-Myc antibodies bound to TRPP2 particles had a large peak close to 90 degrees and a smaller peak close to 180 degrees , consistent with the assembly of TRPP2 as a homotetramer. In contrast, the corresponding angle distributions for decoration of the TRPP2-TRPC1 heteromer by either anti-Myc or anti-V5 antibodies had predominant peaks close to 180 degrees . This decoration pattern indicates a TRPP2:TRPC1 subunit stoichiometry of 2:2 and an alternating subunit arrangement.

Polycystin-1 and -2 dosage regulates pressure sensing

Autosomal-dominant polycystic kidney disease, the most frequent monogenic cause of kidney failure, is induced by mutations in the PKD1 or PKD2 genes, encoding polycystins TRPP1 and TRPP2, respectively. Polycystins are proposed to form a flow-sensitive ion channel complex in the primary cilium of both epithelial and endothelial cells. However, how polycystins contribute to cellular mechanosensitivity remains obscure. Here, we show that TRPP2 inhibits stretch-activated ion channels (SACs). This specific effect is reversed by coexpression with TRPP1, indicating that the TRPP1/TRPP2 ratio regulates pressure sensing. Moreover, deletion of TRPP1 in smooth muscle cells reduces SAC activity and the arterial myogenic tone. Inversely, depletion of TRPP2 in TRPP1-deficient arteries rescues both SAC opening and the myogenic response. Finally, we show that TRPP2 interacts with filamin A and demonstrate that this actin crosslinking protein is critical for SAC regulation. This work uncovers a role for polycystins in regulating pressure sensing.

A pathogenic C terminus-truncated polycystin-2 mutant enhances receptor-activated Ca2+ entry via association with TRPC3 and TRPC7

Mutations in PKD2 gene result in autosomal dominant polycystic kidney disease (ADPKD). PKD2 encodes polycystin-2 (TRPP2), which is a homologue of transient receptor potential (TRP) cation channel proteins. Here we identify a novel PKD2 mutation that generates a C-terminal tail-truncated TRPP2 mutant 697fsX with a frameshift resulting in an aberrant 17-amino acid addition after glutamic acid residue 697 from a family showing mild ADPKD symptoms. When recombinantly expressed in HEK293 cells, wild-type (WT) TRPP2 localized at the endoplasmic reticulum (ER) membrane significantly enhanced Ca(2+) release from the ER upon muscarinic acetylcholine receptor (mAChR) stimulation. In contrast, 697fsX, which showed a predominant plasma membrane localization characteristic of TRPP2 mutants with C terminus deletion, prominently increased mAChR-activated Ca(2+) influx in cells expressing TRPC3 or TRPC7. Coimmunoprecipitation, pulldown assay, and cross-linking experiments revealed a physical association between 697fsX and TRPC3 or TRPC7. 697fsX but not WT TRPP2 elicited a depolarizing shift of reversal potentials and an enhancement of single-channel conductance indicative of altered ion-permeating pore properties of mAChR-activated currents. Importantly, in kidney epithelial LLC-PK1 cells the recombinant 679fsX construct was codistributed with native TRPC3 proteins at the apical membrane area, but the WT construct was distributed in the basolateral membrane and adjacent intracellular areas. Our results suggest that heteromeric cation channels comprised of the TRPP2 mutant and the TRPC3 or TRPC7 protein induce enhanced receptor-activated Ca(2+) influx that may lead to dysregulated cell growth in ADPKD.

Structural and molecular basis of the assembly of the TRPP2/PKD1 complex

Mutations in PKD1 and TRPP2 account for nearly all cases of autosomal dominant polycystic kidney disease (ADPKD). These 2 proteins form a receptor/ion channel complex on the cell surface. Using a combination of biochemistry, crystallography, and a single-molecule method to determine the subunit composition of proteins in the plasma membrane of live cells, we find that this complex contains 3 TRPP2 and 1 PKD1. A newly identified coiled-coil domain in the C terminus of TRPP2 is critical for the formation of this complex. This coiled-coil domain forms a homotrimer, in both solution and crystal structure, and binds to a single coiled-coil domain in the C terminus of PKD1. Mutations that disrupt the TRPP2 coiled-coil domain trimer abolish the assembly of both the full-length TRPP2 trimer and the TRPP2/PKD1 complex and diminish the surface expression of both proteins. These results have significant implications for the assembly, regulation, and function of the TRPP2/PKD1 complex and the pathogenic mechanism of some ADPKD-producing mutations.

TRPC1 Associates With BK Ca Channel to Form a Signal Complex in Vascular Smooth Muscle Cells

TRPC1 (transient receptor potential canonical 1) is a Ca 2+ -permeable cation channel involved in diverse physiological function. TRPC1 may associate with other proteins to form a signaling complex, which is crucial for channel function. In the present study, we investigated the interaction between TRPC1 and large conductance Ca 2+ -sensitive K + channel (BK Ca ). With the use of potentiometric fluorescence dye DiBAC 4 (3), we found that store-operated Ca 2+ influx resulted in membrane hyperpolarization of vascular smooth muscle cells (VSMCs). The hyperpolarization was inhibited by an anti-TRPC1 blocking antibody T1E3 and 2 BK Ca channel blockers, charybdotoxin and iberiotoxin. These data were confirmed by sharp microelectrode measurement of membrane potential in VSMCs of intact arteries. Furthermore, T1E3 treatment markedly enhanced the membrane depolarization and contraction of VSMCs in response to several contractile agonists including phenylephrine, endothelin-1, and U-46619. In coimmunoprecipitation experiments, an antibody against BK Ca α-subunit [BK Ca (α)] could pull down TRPC1, and moreover an anti-TRPC1 antibody could reciprocally pull down BK Ca (α). Double-labeling immunocytochemistry showed that TRPC1 and BK Ca were colocalized in the same subcellular regions, mainly on the plasma membrane, in VSMCs. These data suggest that, TRPC1 physically associates with BK Ca in VSMCs and that Ca 2+ influx through TRPC1 activates BK Ca to induce membrane hyperpolarization. The hyperpolarizing effect of TRPC1-BK Ca coupling could serve to reduce agonist-induced membrane depolarization, thereby preventing excessive contraction of VSMCs to contractile agonists.

Function and regulation of TRPP2 at the plasma membrane

The vast majority (approximately 99%) of all known cases of autosomal dominant polycystic kidney disease (ADPKD) are caused by naturally occurring mutations in two separate, but genetically interacting, loci, pkd1 and pkd2. pkd1 encodes a large multispanning membrane protein (PKD1) of unknown function, while pkd2 encodes a protein (TRPP2, polycystin-2, or PKD2) of the transient receptor potential (TRP) superfamily of ion channels. Biochemical, functional, and genetic studies support a model in which PKD1 physically interacts with TRPP2 to form an ion channel complex that conveys extracellular stimuli to ionic currents. However, the molecular identity of these extracellular stimuli remains elusive. Functional studies in cell culture show that TRPP2 can be activated in response to mechanical cues (fluid shear stress) and/or receptor tyrosine kinase (RTK) and G protein-coupled receptor (GPCR) activation at the cell surface. Recent genetic studies in Chlamydomonas reinhardtii show that CrPKD2 functions in a pathway linking cell-cell adhesion and Ca(2+) signaling. The mode of activation depends on protein-protein interactions with other channel subunits and auxiliary proteins. Therefore, understanding the mechanisms underlying the molecular makeup of TRPP2-containing complexes is critical in delineating the mechanisms of TRPP2 activation and, most importantly, the mechanisms by which naturally occurring mutations in pkd1 or pkd2 lead not only to ADPKD, but also to other defects reported in model organisms lacking functional TRPP2. This review focuses on the molecular assembly, function, and regulation of TRPP2 as a cell surface cation channel and discusses its potential role in Ca(2+) signaling and ADPKD pathophysiology.

The multimeric structure of polycystin-2 (TRPP2): structural-functional correlates of homo- and hetero-multimers with TRPC1

Polycystin-2 (PC2, TRPP2), the gene product of PKD2, whose mutations cause autosomal dominant polycystic kidney disease (ADPKD), belongs to the superfamily of TRP channels. PC2 is a non-selective cation channel, with multiple subconductance states. In this report, we explored structural and functional properties of PC2 and whether the conductance substates represent monomeric contributions to the channel complex. A kinetic analysis of spontaneous channel currents of PC2 showed that four intrinsic, non-stochastic subconductance states, which followed a staircase behavior, were both pH- and voltage-dependent. To confirm the oligomeric contributions to PC2 channel function, heteromeric PC2/TRPC1 channel complexes were also functionally assessed by single channel current analysis. Low pH inhibited the PC2 currents in PC2 homomeric complexes, but failed to affect PC2 currents in PC2/TRPC1 heteromeric complexes. Amiloride, in contrast, abolished PC2 currents in both the homomeric PC2 complexes and the heteromeric PC2/TRPC1 complexes, thus PC2/TRPC1 complexes have distinct functional properties from the homomeric complexes. The topological features of the homomeric PC2-, TRPC1- and heteromeric PC2/TRPC1 channel complexes, assessed by atomic force microscopy, were consistent with structural tetramers. TRPC1 homomeric channels had different average diameter and protruding height when compared with the PC2 homomers. The contribution of individual monomers to the PC2/TRPC1 hetero-complexes was easily distinguishable. The data support tetrameric models of both the PC2 and TRPC1 channels, where the overall conductance of a particular channel will depend on the contribution of the various functional monomers in the complex.

Cyst formation in kidney via B-Raf signaling in the PKD2 transgenic mice

The pathogenic mechanisms of human autosomal dominant polycystic kidney disease (ADPKD) have been well known to include the mutational inactivation of PKD2. Although haploinsufficiency and loss of heterozygosity at the Pkd2 locus can cause cyst formation in mice, polycystin-2 is frequently expressed in the renal cyst of human ADPKD, raising the possibility that deregulated activation of PKD2 may be associated with the cystogenesis of human ADPKD. To determine whether increased PKD2 expression is physiologically pathogenic, we generated PKD2-overexpressing transgenic mice. These mice developed typical renal cysts and an increase of proliferation and apoptosis, which are reflective of the human ADPKD phenotype. These manifestations were first observed at six months, and progressed with age. In addition, we found that ERK activation was induced by PKD2 overexpression via B-Raf signaling, providing a possible molecular mechanism of cystogenesis. In PKD2 transgenic mice, B-Raf/MEK/ERK sequential signaling was up-regulated. Additionally, the transgenic human polycystin-2 partially rescues the lethality of Pkd2 knock-out mice and therefore demonstrates that the transgene generated a functional product. Functional strengthening or deregulated activation of PKD2 may be a direct cause of ADPKD. The present study provides evidence for an in vivo role of overexpressed PKD2 in cyst formation. This transgenic mouse model should provide new insights into the pathogenic mechanism of human ADPKD.

Vasopressin antagonists in polycystic kidney disease

Increased cell proliferation and fluid secretion, probably driven by alterations in intracellular calcium homeostasis and cyclic adenosine 3,5-phosphate, play an important role in the development and progression of polycystic kidney disease. Hormone receptors that affect cyclic adenosine monophosphate and are preferentially expressed in affected tissues are logical treatment targets. There is a sound rationale for considering the arginine vasopressin V2 receptor as a target. The arginine vasopressin V2 receptor antagonists OPC-31260 and tolvaptan inhibit the development of polycystic kidney disease in cpk mice and in three animal orthologs to human autosomal recessive polycystic kidney disease (PCK rat), autosomal dominant polycystic kidney disease (Pkd2/WS25 mice), and nephronophthisis (pcy mouse). PCK rats that are homozygous for an arginine vasopressin mutation and lack circulating vasopressin are markedly protected. Administration of V2 receptor agonist 1-deamino-8-D-arginine vasopressin to these animals completely recovers the cystic phenotype. Administration of 1-deamino-8-D-arginine vasopressin to PCK rats with normal arginine vasopressin aggravates the disease. Suppression of arginine vasopressin release by high water intake is protective. V2 receptor antagonists may have additional beneficial effects on hypertension and chronic kidney disease progression. A number of clinical studies in polycystic kidney disease have been performed or are currently active. The results of phase 2 and phase 2-3 clinical trials suggest that tolvaptan is safe and well tolerated in autosomal dominant polycystic kidney disease. A phase 3, placebo-controlled, double-blind study in 18- to 50-yr-old patients with autosomal dominant polycystic kidney disease and preserved renal function but relatively rapid progression, as indicated by a total kidney volume >750 ml, has been initiated and will determine whether tolvaptan is effective in slowing down the progression of this disease.

Domain mapping of the polycystin-2 C-terminal tail using de novo molecular modeling and biophysical analysis

In polycystic kidney disease (PKD), polycystin-2 (PC2) is frequently mutated or truncated in the C-terminal cytoplasmic tail (PC2-C). The currently accepted model of PC2-C consists of an EF-hand motif overlapping with a short coiled coil; however, this model fails to explain the mechanisms by which PC2 truncations C-terminal to this region lead to PKD. Moreover, direct PC2 binding to inositol 1,4,5-trisphosphate receptor, KIF3A, and TRPC1 requires residues in PC2-C outside this region. To address these discrepancies and investigate the role of PC2-C in PC2 function, we performed de novo molecular modeling and biophysical analysis. De novo molecular modeling of PC2-C using the ROBETTA server predicts two domains as follows: an EF-hand motif (PC2-EF) connected by a linker to a previously unidentified C-terminal coiled coil (PC2-CC). This model differs substantially from the current model and correlates with limited proteolysis, matrix-assisted laser desorption/ionization mass spectroscopy, N-terminal sequencing, and improved coiled coil prediction algorithms. PC2-C is elongated and oligomerizes through PC2-CC, as measured by analytical ultracentrifugation and size exclusion chromatography, whereas PC2-EF is globular and monomeric. We show that PC2-C and PC2-EF have micromolar affinity for calcium (Ca2+) by isothermal titration calorimetry and undergo Ca2+-induced conformational changes by circular dichroism. Mutation of predicted EF-hand loop residues in PC2 to alanine abolishes Ca2+ binding. Our results suggest that PC2-CC is involved in PC2 oligomerization, and PC2-EF is a Ca2+-sensitive switch. PKD-associated PC2 mutations are located in regions that may disrupt these functions, providing structural insight into how PC2 mutations lead to disease.

TRPP2 and TRPV4 form a polymodal sensory channel complex

The primary cilium has evolved as a multifunctional cellular compartment that decorates most vertebrate cells. Cilia sense mechanical stimuli in various organs, but the molecular mechanisms that convert the deflection of cilia into intracellular calcium transients have remained elusive. Polycystin-2 (TRPP2), an ion channel mutated in polycystic kidney disease, is required for cilia-mediated calcium transients but lacks mechanosensitive properties. We find here that TRPP2 utilizes TRPV4 to form a mechano- and thermosensitive molecular sensor in the cilium. Depletion of TRPV4 in renal epithelial cells abolishes flow-induced calcium transients, demonstrating that TRPV4, like TRPP2, is an essential component of the ciliary mechanosensor. Because TRPV4-deficient zebrafish and mice lack renal cysts, our findings challenge the concept that defective ciliary flow sensing constitutes the fundamental mechanism of cystogenesis.

Cystic disease of the kidney

This review focuses on the mechanisms that underlie the development of human renal cystic diseases. A pathological, clinical, and pathophysiological overview is given. Initial analysis of the cell biology of inappropriate hyperproliferation accompanied by fluid secretion of cyst-lining epithelia has been followed by the elucidation of fundamental defects in epithelial polarity, cell-matrix and cell-cell interactions, and apoptosis, all of which are discussed. Identification of the genes and proteins responsible for several renal cystic diseases has led to a more complete understanding of defects in renal developmental programming, differentiation, and morphogenesis, all of which underlie cystic diseases of the kidney.

Identification and functional characterization of an N-terminal oligomerization domain for polycystin-2

Autosomal dominant polycystic kidney disease (ADPKD), the most common inherited cause of kidney failure, is caused by mutations in either PKD1 (85%) or PKD2 (15%). The PKD2 protein, polycystin-2 (PC2 or TRPP2), is a member of the transient receptor potential (TRP) superfamily and functions as a non-selective calcium channel. PC2 has been found to form oligomers in native tissues suggesting that it may form functional homo- or heterotetramers with other subunits, similar to other TRP channels. Our experiments unexpectedly revealed that PC2 mutant proteins lacking the known C-terminal dimerization domain were still able to form oligomers and co-immunoprecipitate full-length PC2, implying the possible existence of a proximal dimerization domain. Using yeast two-hybrid and biochemical assays, we have mapped an alternative dimerization domain to the N terminus of PC2 (NT2-1-223, L224X). Functional characterization of this domain demonstrated that it was sufficient to induce cyst formation in zebrafish embryos and inhibit PC2 surface currents in mIMCD3 cells probably by a dominant-negative mechanism. In summary, we propose a model for PC2 assembly as a functional tetramer which depends on both C- and N-terminal dimerization domains. These results have significant implications for our understanding of PC2 function and disease pathogenesis in ADPKD and provide a new strategy for studying PC2 function.

PKD1 haploinsufficiency is associated with altered vascular reactivity and abnormal calcium signaling in the mouse aorta

Mutations in PKD1 are associated with autosomal dominant polycystic kidney disease (ADPKD), which leads to major cardiovascular complications. We used mice with a heterozygous deletion of Pkd1 (Pkd1+/-) and wild-type (Pkd1+/+) littermates to test whether Pkd1 haploinsufficiency is associated with a vascular phenotype in different age groups. Systolic blood pressure measured by the tail-cuff method was similar up to 20 weeks of age, but significantly higher in 30-week-old Pkd1+/- compared to Pkd1+/+. By contrast, similar telemetric recordings were obtained in unrestrained Pkd1+/- and Pkd1+/+ mice. The contractile responses evoked by KCl or phenylephrine were similar in young animals but increased in abdominal aortas of 30-week-old Pkd1+/- mice, and acetylcholine-evoked relaxation was depressed. Basal cytosolic calcium, KCl, and phenylephrine-evoked calcium signals were significantly lower in the Pkd1+/- aortas, whereas calcium release evoked by caffeine or thapsigargin was significantly larger. These changes were paralleled with a significant change in the mRNA expression of Pkd2, Trpc1, Orai1, and Serca2a in the aortas from Pkd1+/- vs. Pkd1+/+. These results are the first to indicate that haploinsufficiency in Pkd1 is associated with altered intracellular calcium homeostasis and increased vascular reactivity in the aorta with compensatory changes in transport proteins involved in the calcium signaling network.

TRP channels: an overview

The TRP ("transient receptor potential") family of ion channels now comprises more than 30 cation channels, most of which are permeable for Ca2+, and some also for Mg2+. On the basis of sequence homology, the TRP family can be divided in seven main subfamilies: the TRPC ('Canonical') family, the TRPV ('Vanilloid') family, the TRPM ('Melastatin') family, the TRPP ('Polycystin') family, the TRPML ('Mucolipin') family, the TRPA ('Ankyrin') family, and the TRPN ('NOMPC') family. The cloning and characterization of members of this cation channel family has exploded during recent years, leading to a plethora of data on the roles of TRPs in a variety of tissues and species, including mammals, insects, and yeast. The present review summarizes the most pertinent recent evidence regarding the structural and functional properties of TRP channels, focusing on the regulation and physiology of mammalian TRPs.

2007 Young Investigator Award: TRP'ing into a new era for glomerular disease

FSGS is a pathologic lesion that frequently causes the nephrotic syndrome and ensuing renal failure. The cause remains unknown in the majority of individuals; however, in the past two decades, rare familial forms have been identified. It has been suggested that known genetic causes of the hereditary form of this disease account for upwards of 18% of cases. Mutations in five genes have been found to cause inherited nephrotic syndromes and FSGS. In this article, I discuss the phenotypic characteristics of hereditary FSGS and the transient receptor potential cation channel 6 (TRPC6) protein, which is the genetic impetus for an autosomal dominant form of FSGS. The TRP channels have been implicated in varied biologic functions such as mechanosensation, ion homeostasis, cell growth, and phospholipase C-dependent calcium entry into cells. The mutated ion channel causes an increase in calcium transients. Current evidence also suggests that blocking TRPC6 channels may be of therapeutic benefit in idiopathic FSGS, a disease with a generally poor prognosis. Preliminary experiments reveal that the commonly used immunosuppressive agent FK-506 can inhibit TRPC6 activity in vivo. This creates the intriguing possibility that blocking TRPC6 channels within the podocyte may translate into long-lasting clinical benefits in patients with FSGS.

Role of vasopressin antagonists

Alterations in intracellular calcium homeostasis and cyclic adenosine 3',5'-phosphate likely underlie the increased cell proliferation and fluid secretion in polycystic kidney disease. Hormone receptors that affect cyclic adenosine 3',5'-phosphate and are preferentially expressed in affected tissues are logical treatment targets. There is a sound rationale for considering the arginine vasopressin V2 receptor as a target. The arginine vasopressin V2 receptor antagonists OPC-31260 and tolvaptan inhibit the development of polycystic kidney disease in cpk mice and in three animal orthologs to human autosomal recessive polycystic kidney disease (PCK rat), autosomal dominant polycystic kidney disease (Pkd2-/WS25 mice), and nephronophthisis(pcy mouse). PCK rats that are homozygous for an arginine vasopressin mutation and lack circulating vasopressin are markedly protected. Administration of V2 receptor agonist 1-deamino-8-D-arginine vasopressin to these animals completely recovers the cystic phenotype. Administration of 1-deamino-8-D-arginine vasopressin to PCK rats with normal arginine vasopressin aggravates the disease. Suppression of arginine vasopressin release by high water intake is protective. V2 receptor antagonists may have additional beneficial effects on hypertension and chronic kidney disease progression. A number of clinical studies in polycystic kidney disease have been performed or are currently active. The results of phase 2 and 2-3 studies indicate that tolvaptan seems to be safe and well tolerated in autosomal dominant polycystic kidney disease. A phase 3,placebo-controlled, double-blind study in 18- to 50-yr-old patients with autosomal dominant polycystic kidney disease and preserved renal function but relatively rapid progression, as indicated by a total kidney volume >750 ml, has been initiated.

Exploring mechanisms involved in renal tubular sensing of mechanical stretch following ureteric obstruction

Tubular mechanical stretch is the key primary insult in obstructive nephropathy. This review addresses how the renal tubular epithelium senses and responds to mechanical stretch. Using data from renal and nonrenal systems, we describe how sensing of stretch initially occurs via the activation of ion channels and subsequent increases in intracellular calcium levels. Calcium influxes activate a number of adaptive and proinjury responses. Key among these are 1) the activation of Rho, consequent cytoskeletal rearrangements, and downstream increases in focal adhesion assembly; and 2) phospholipase activation and resultant mitogen-activated protein kinase activation. These early signaling events culminate in adaptive cellular coupling to the extracellular matrix, a process termed the cell strengthening response. Direct links can be made between increased expression of genes involved in the development of obstructive nephropathy and initial sensing of mechanical stretch. The review illustrates the repercussions of mechanical stretch as a renal stress stimulus, specific to ureteric obstruction, and provides an insight into how tubular responses to mechanical stretch are ultimately implicated in the development of obstructive nephropathy.

Activation of TRPP2 through mDia1-dependent voltage gating

The TRPP2 cation channel is directly responsible for approximately 15% of all cases of autosomal dominant polycystic kidney disease. However, the mechanisms underlying fundamental properties of TRPP2 regulation, such as channel gating and activation, are unknown. We have shown that TRPP2 was activated by EGF and physically interacted with the mammalian diaphanous-related formin 1 (mDia1), a downstream effector of RhoA. Now, we show that mDia1 regulates TRPP2 by specifically blocking its activity at negative but not positive potentials. The voltage-dependent unblock of TRPP2 by mDia1 at positive potentials is mediated through RhoA-induced molecular switching of mDia1 from its autoinhibited state at negative potentials to its activated state at positive potentials. Under physiological resting potentials, EGF activates TRPP2 by releasing the mDia1-dependent block through the activation of RhoA. Our data reveal a new role of mDia1 in the regulation of ion channels and suggest a molecular basis for the voltage-dependent gating of TRP channels.

HNF4 alpha and the Ca-channel TRPC1 are novel disease candidate genes in diabetic nephropathy

OBJECTIVE

The nuclear receptor hepatic nuclear factor 4 alpha (HNF 4 alpha) is a master regulatory protein and an essential player in the control of a wide range of metabolic processes. Dysfunction of HNF 4 alpha is associated with metabolic disorders including diabetes. We were particularly interested in investigating molecular causes associated with diabetic nephropathy.

RESEARCH DESIGN AND METHODS

Novel disease candidate genes were identified by the chromatin immunoprecipitation-cloning assay and by sequencing of immunoprecipitated DNA. Expression of candidate genes was analyzed in kidney and liver of Zucker diabetic fatty (ZDF) and of streptozotocin (STZ)-administered rats and after siRNA-mediated silencing of HNF 4 alpha.

RESULTS

We identified the calcium-permeable nonselective transient receptor potential cation channel, subfamily C, member 1 (TRPC1) as a novel HNF 4 alpha gene target. Strikingly, TRPC1 is localized on human chromosome 3q22-24, i.e., a region considered to be a hotspot for diabetic nephropathy. We observed a significant reduction of TRPC1 gene expression in kidney and liver of diabetic ZDF and of STZ-administered rats as a result of HNF 4 alpha dysfunction. We found HNF 4 alpha and TRPC1 protein expression to be repressed in kidneys of diabetic patients diagnosed with nodular glomerulosceloris as evidenced by immunohistochemistry. Finally, siRNA-mediated functional knock down of HNF 4 alpha repressed TRPC1 gene expression in cell culture experiments.

CONCLUSIONS

Taken collectively, results obtained from animal studies could be translated to human diabetic nephropathy; there is evidence for a common regulation of HNF 4 alpha and TRPC1 in human and rat kidney pathologies. We propose dysregulation of HNF 4 alpha and TRPC1 as a possible molecular rationale in diabetic nephropathy.

Formation of a new receptor-operated channel by heteromeric assembly of TRPP2 and TRPC1 subunits

Although several protein-protein interactions have been reported between transient receptor potential (TRP) channels, they are all known to occur exclusively between members of the same group. The only intergroup interaction described so far is that of TRPP2 and TRPC1; however, the significance of this interaction is unknown. Here, we show that TRPP2 and TRPC1 assemble to form a channel with a unique constellation of new and TRPP2/TRPC1-specific properties. TRPP2/TRPC1 is activated in response to G-protein-coupled receptor activation and shows a pattern of single-channel conductance, amiloride sensitivity and ion permeability distinct from that of TRPP2 or TRPC1 alone. Native TRPP2/TRPC1 activity is shown in kidney cells by complementary gain-of-function and loss-of-function experiments, and its existence under physiological conditions is supported by colocalization at the primary cilium and by co-immunoprecipitation from kidney membranes. Identification of the heteromultimeric TRPP2/TRPC1 channel has implications in mechanosensation and cilium-based Ca(2+) signalling.

Molecular basis of the mammalian pressure-sensitive ion channels: focus on vascular mechanotransduction

Mechano-gated ion channels are implicated in a variety of neurosensory functions ranging from touch sensitivity to hearing. In the heart, rhythm disturbance subsequent to mechanical effects is also associated with the activation of stretch-sensitive ion channels. Arterial autoregulation in response to hemodynamic stimuli, a vital process required for protection against hypertension-induced injury, is similarly dependent on the activity of force-sensitive ion channels. Seminal work in prokaryotes and invertebrates, including the nematode Caenorhabditis elegans and the fruit fly drosophila, greatly helped to identify the molecular basis of volume regulation, hearing and touch sensitivity. In mammals, more recent findings have indicated that members of several structural family of ion channels, namely the transient receptor potential (TRP) channels, the amiloride-sensitive ENaC/ASIC channels and the potassium channels K2P and Kir are involved in cellular mechanotransduction. In the present review, we will focus on the molecular and functional properties of these channel subunits and will emphasize on their role in the pressure-dependent arterial myogenic constriction and the flow-mediated vasodilation.

Mediation of angiotensin II-induced Ca2+ signaling by polycystin 2 in glomerular mesangial cells

Ca(+) influx across the plasma membrane is a major component of mesangial cell (MC) response to vasoconstrictors. Polycystin 2 (PC2), the protein product of the gene mutated in type 2 autosomal dominant polycystic kidney disease, has been shown to function as a nonselective cation channel in a variety of cell types. The present study was performed to test the hypothesis that PC2 and its binding partners constitute a Ca(2+)-permeable channel and contribute to ANG II-induced Ca(2+) signaling in MCs. Western blot and immunocytochemistry showed PC2 expression in cultured human MCs. The existence of PC2 in MCs was further confirmed by immunohistochemsitry in rat kidney sections. Coimmunoprecipitation displayed a selective interaction of PC2 with canonical transient receptor potential (TRPC) proteins TRPC1 and TRPC4. Cell-attached patch-clamp experiments revealed that ANG II-induced membrane currents were enhanced by overexpression of pkd2 but significantly inhibited by knock down of pkd2, 30 microM Gd(3+) (a PC2 channel blocker), and dominant-negative pkd2 mutant (pkd2-D511V). Corresponding to the increase in channel currents, ANG II stimulation increased expression of PC2 on the cell surface of MCs and interaction with TRPC1 and TRPC4. Furthermore, ANG II-induced MC contraction was significantly reduced in pkd2-knocked down MCs. These data suggest that PC2 selectively assembles with TRPC1 and TRPC4 to form channel complexes mediating ANG II-induced Ca(2+) responses in MCs.

TRP channels in disease

The transient receptor potential (TRP) channels are a large family of proteins with six main subfamilies termed the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), and TRPA (ankyrin) groups. The sheer number of different TRPs with distinct functions supports the statement that these channels are involved in a wide range of processes ranging from sensing of thermal and chemical signals to reloading intracellular stores after responding to an extracellular stimulus. Mutations in TRPs are linked to pathophysiology and specific diseases. An understanding of the role of TRPs in normal physiology is just beginning; the progression from mutations in TRPs to pathophysiology and disease will follow. In this review, we focus on two distinct aspects of TRP channel physiology, the role of TRP channels in intracellular Ca2+ homeostasis, and their role in the transduction of painful stimuli in sensory neurons.

Potential pharmacological interventions in polycystic kidney disease

Polycystic kidney diseases (autosomal dominant and autosomal recessive) are progressive renal tubular cystic diseases, which are characterised by cyst expansion and loss of normal kidney structure and function. Autosomal dominant polycystic kidney disease (ADPKD) is the most common life- threatening, hereditary disease. ADPKD is more prevalent than Huntington's disease, haemophilia, sickle cell disease, cystic fibrosis, myotonic dystrophy and Down's syndrome combined. Early diagnosis and treatment of hypertension with inhibitors of the renin-angiotensin-aldosterone system (RAAS) and its potential protective effect on left ventricular hypertrophy has been one of the major therapeutic goals to decrease cardiac complications and contribute to improved prognosis of the disease. Advances in the understanding of the genetics, molecular biology and pathophysiology of the disease are likely to facilitate the improvement of treatments for these diseases. Developments in describing the role of intracellular calcium ([Ca(2+)](i)) and its correlation with cellular signalling systems, Ras/Raf/mitogen extracellular kinase (MEK)/extracellular signal-regulated protein kinase (ERK), and interaction of these pathways with cyclic adenosine monophosphate (cAMP) levels, provide new insights on treatment strategies. Blocking the vasopressin V(2) receptor, a major adenylyl cyclase agonist, demonstrated significant improvements in inhibiting cytogenesis in animal models. Because of activation of the mammalian target of rapamycin (mTOR) pathway, the use of sirolimus (rapamycin) an mTOR inhibitor, markedly reduced cyst formation and decreased polycystic kidney size in several animal models. Caspase inhibitors have been shown to decrease cytogenesis and renal failure in rats with cystic disease. Cystic fluid secretion results in cyst enlargement and somatostatin analogues have been shown to decrease renal cyst progression in patients with ADPKD. The safety and efficacy of these classes of drugs provide potential interventions for experimental and clinical trials.

TRP channels and mechanosensory transduction: insights into the arterial myogenic response

Mechano-gated ion channels are implicated in a variety of key physiological functions ranging from touch sensitivity to arterial pressure regulation. Seminal work in prokaryotes and invertebrates provided strong evidence for the role of specific ion channels in volume regulation, touch sensitivity, or hearing, specifically the mechanosensitive channel subunits of large and small conductances (MscL and MscS), the mechanosensory channel subunits (MEC) and the transient receptor potential channel subunits (TRP). In mammals, recent studies further indicate that members of the TRP channel family may also be considered as possible candidate mechanosensors responding to either tension, flow, or changes in cell volume. However, contradictory results have challenged whether these TRP channels, including TRPC1 and TRPC6, are directly activated by mechanical stimulation. In the present review, we will focus on the mechanosensory function of TRP channels, discuss whether a direct or indirect mechanism is at play, and focus on the proposed role for these channels in the arterial myogenic response to changes in intraluminal pressure.

Function and dynamics of PKD2 in Chlamydomonas reinhardtii flagella

To analyze the function of ciliary polycystic kidney disease 2 (PKD2) and its relationship to intraflagellar transport (IFT), we cloned the gene encoding Chlamydomonas reinhardtii PKD2 (CrPKD2), a protein with the characteristics of PKD2 family members. Three forms of this protein (210, 120, and 90 kD) were detected in whole cells; the two smaller forms are cleavage products of the 210-kD protein and were the predominant forms in flagella. In cells expressing CrPKD2-GFP, about 10% of flagellar CrPKD2-GFP was observed moving in the flagellar membrane. When IFT was blocked, fluorescence recovery after photobleaching of flagellar CrPKD2-GFP was attenuated and CrPKD2 accumulated in the flagella. Flagellar CrPKD2 increased fourfold during gametogenesis, and several CrPKD2 RNA interference strains showed defects in flagella-dependent mating. These results suggest that the CrPKD2 cation channel is involved in coupling flagellar adhesion at the beginning of mating to the increase in flagellar calcium required for subsequent steps in mating.

Distinct protein domains regulate ciliary targeting and function of C. elegans PKD-2

TRPP2 (transient receptor potential polycystin-2) channels function in a range of cells where they are localized to specific subcellular regions including the endoplasmic reticulum (ER) and primary cilium. In humans, TRPP2/PC-2 mutations severely compromise kidney function and cause autosomal dominant polycystic kidney disease (ADPKD). The Caenorhabditis elegans TRPP2 homolog, PKD-2, is restricted to the somatodendritic (cell body and dendrite) and ciliary compartments of male specific sensory neurons. Within these neurons PKD-2 function is required for sensation. To understand the mechanisms regulating TRPP2 subcellular distribution and activity, we performed in vivo structure-function-localization studies using C. elegans as a model system. Our data demonstrate that somatodendritic and ciliary targeting requires the transmembrane (TM) region of PKD-2 and that the PKD-2 cytosolic termini regulate subcellular distribution and function. Within neuronal cell bodies, PKD-2 colocalizes with the OSM-9 TRP vanilloid (TRPV) channel, suggesting that these TRPP and TRPV channels may function in a common process. When human TRPP2/PC-2 is heterologously expressed in transgenic C. elegans animals, PC-2 does not visibly localize to cilia but does partially rescue pkd-2 null mutant defects, suggesting that human PC-2 and PKD-2 are functional homologs.

Kidney injury molecule 1 (Kim1) is a novel ciliary molecule and interactor of polycystin 2

Inherited mutations in genes encoding for ciliary proteins lead to a broad spectrum of human diseases, such as polycystic kidney disease (PKD), situs inversus and retinitis pigmentosa. In the human kidney, autosomal dominant PKD (ADPKD) is caused by mutations in PKD1 (PC1), or PKD2 (TRPP2). Both are necessary for ciliary mechanotransduction, whereby bending of the cilium elicits a calcium response in the cell. We have previously shown that overexpression of mutated forms of the chemosensor kidney injury molecule 1 (Kim1) abolishes the flow response in ciliated MDCK cells. Here we identify Kim1 as an endogenous ciliary protein. Kim1 co-precipitates with TRPP2. Mutational analysis reveals that the interaction between Kim1 and TRPP2 requires the ciliary sorting motif in the N-terminus of TRPP2, and the presence of a highly conserved tyrosine in the intracellular tail of Kim1, which has previously been shown to play a role in ciliary flow sensing. These data support the notion that TRPP2 functionally interacts with ciliary chemosensors.

Calcium channel inhibition accelerates polycystic kidney disease progression in the Cy/+ rat

In polycystic kidney disease, abnormal epithelial cell proliferation is the main factor leading to cyst formation and kidney enlargement. Cyclic AMP (cAMP) is mitogenic in cystic but antimitogenic in normal human kidney cells, which is due to reduced steady-state intracellular calcium levels in cystic compared to the normal cells. Inhibition of intracellular calcium entry with channel blockers, such as verapamil, induced cAMP-dependent cell proliferation in normal renal cells. To determine if calcium channel blockers have a similar effect on cell proliferation in vivo, Cy/+ rats, a model of dominant polycystic kidney disease, were treated with verapamil. Kidney weight and cyst index were elevated in verapamil-treated Cy/+ rats. This was associated with increased cell proliferation and apoptosis, elevated expression, and phosphorylation of B-Raf with stimulation of the mitogen-activated protein kinase MEK/ERK (mitogen-activated protein kinase kinase/extracellular-regulated kinase) pathway. Verapamil had no effect on kidney morphology or B-Raf stimulation in wild-type rats. We conclude that treatment of Cy/+ rats with calcium channel blockers increases activity of the B-Raf/MEK/ERK pathway accelerating cyst growth in the presence of endogenous cAMP, thus exacerbating renal cystic disease.

Store-operated calcium entry in vascular smooth muscle

In non-excitable cells, activation of G-protein-coupled phospholipase C (PLC)-linked receptors causes the release of Ca(2+) from intracellular stores, which is followed by transmembrane Ca(2+) entry. This Ca(2+) entry underlies a small and sustained phase of the cellular [Ca(2+)](i) increases and is important for several cellular functions including gene expression, secretion and cell proliferation. This form of transmembrane Ca(2+) entry is supported by agonist-activated Ca(2+)-permeable ion channels that are activated by store depletion and is referred to as store-operated Ca(2+) entry (SOCE) and represents a major pathway for agonist-induced Ca(2+) entry. In excitable cells such as smooth muscle cells, Ca(2+) entry mechanisms responsible for sustained cellular activation are normally considered to be mediated via either voltage-operated or receptor-operated Ca(2+) channels. Although SOCE occurs following agonist activation of smooth muscle, this was thought to be more important in replenishing Ca(2+) stores rather than acting as a source of activator Ca(2+) for the contractile process. This review summarizes our current knowledge of SOCE as a regulator of vascular smooth muscle tone and discusses its possible role in the cardiovascular function and disease. We propose a possible hypothesis for its activation and suggest that SOCE may represent a novel target for pharmacological therapeutic intervention.

Mechanosensor Channels in Mammalian Somatosensory Neurons

Mechanoreceptive sensory neurons innervating the skin, skeletal muscles and viscera signal both innocuous and noxious information necessary for proprioception, touch and pain. These neurons are responsible for the transduction of mechanical stimuli into action potentials that propagate to the central nervous system. The ability of these cells to detect mechanical stimuli impinging on them relies on the presence of mechanosensitive channels that transduce the external mechanical forces into electrical and chemical signals. Although a great deal of information regarding the molecular and biophysical properties of mechanosensitive channels in prokaryotes has been accumulated over the past two decades, less is known about the mechanosensitive channels necessary for proprioception and the senses of touch and pain. This review summarizes the most pertinent data on mechanosensitive channels of mammalian somatosensory neurons, focusing on their properties, pharmacology and putative identity.

Pressure-induced and store-operated cation influx in vascular smooth muscle cells is independent of TRPC1

Among the classical transient receptor potential (TRPC) subfamily, TRPC1 is described as a mechanosensitive and store-operated channel proposed to be activated by hypoosmotic cell swelling and positive pipette pressure as well as regulated by the filling status of intracellular Ca(2+) stores. However, evidence for a physiological role of TRPC1 may most compellingly be obtained by the analysis of a TRPC1-deficient mouse model. Therefore, we have developed and analyzed TRPC1(-/-) mice. Pressure-induced constriction of cerebral arteries was not impaired in TRPC1(-/-) mice. Smooth muscle cells from cerebral arteries activated by hypoosmotic swelling and positive pipette pressure showed no significant differences in cation currents compared to wild-type cells. Moreover, smooth muscle cells of TRPC1(-/-) mice isolated from thoracic aortas and cerebral arteries showed no change in store-operated cation influx induced by thapsigargin, inositol-1,4,5 trisphosphate, and cyclopiazonic acid compared to cells from wild-type mice. In contrast to these results, small interference RNAs decreasing the expression of stromal interaction molecule 1 (STIM1) inhibited thapsigargin-induced store-operated cation influx, demonstrating that STIM1 and TRPC1 are mutually independent. These findings also imply that, as opposed to current concepts, TRPC1 is not an obligatory component of store-operated and stretch-activated ion channel complexes in vascular smooth muscle cells.

TRP channels and kidney disease: lessons from polycystic kidney disease

Important insights in to the function of members of the TRP (transient receptor potential) channel superfamily have been gained from the identification of disease-related mutations. In particular the identification of mutations in the PKD2 gene in autosomal dominant polycystic kidney disease has revealed a link between TRP channel function, mechanosensation and the role of the primary cilium in renal cyst formation. The PKD2 gene encodes TRPP2 (transient receptor potential polycystin 2) that has significant homology to voltage-activated calcium and sodium TRP channels. It interacts with polycystin-1 to form a large membrane-associated complex that is localized to the renal primary cilium. Functional characterization of this polycystin complex reveals that it can respond to mechanical stimuli such as flow, resulting in influx of extracellular calcium and release of calcium from intracellular stores. TRPP2 is expressed in the endoplasmic reticulum/sarcoplasmic reticulum where it also regulates intracellular calcium signalling. Therefore TRPP2 modulates many cellular processes via intracellular calcium-dependent signalling pathways.

Genetic interaction studies link autosomal dominant and recessive polycystic kidney disease in a common pathway

Polycystic kidney disease (PKD) describes a heterogeneous collection of disorders that differ significantly with respect to their etiology and clinical presentation. They share, however, abnormal tubular morphology as a common feature, leading to the hypothesis that their respective gene products may function cooperatively in a common pathway to maintain tubular integrity. To study the pathobiology of one major form of human PKD, we generated a mouse line with a floxed allele of Pkhd1, the orthologue of the gene mutated in human autosomal recessive PKD. Cre-mediated excision of exons 3-4 results in a probable hypomorphic allele. Pkhd1(del3-4/del3-4) developed a range of phenotypes that recapitulate key features of the human disease. Like in humans, abnormalities of the biliary tract were an invariant finding. Most mice 6 months or older also developed renal cysts. Subsets of animals presented with either perinatal respiratory failure or exhibited growth retardation that was not due to the renal disease. We then tested for genetic interaction between Pkhd1 and Pkd1, the mouse orthologue of the gene most commonly linked to human autosomal dominant PKD. Pkd1(+/-); Pkhd1(del3-4/del3-4) mice had markedly more severe disease than Pkd1(+/+); Pkhd1(del3-4/del3-4) littermates. These studies are the first to show genetic interaction between the major loci responsible for human renal cystic disease in a common PKD pathway.

The Cytoskeletal Connection to Ion Channels as a Potential Mechanosensory Mechanism: Lessons from Polycystin-2 (TRPP2)

Mechanosensitivity of ion channels, or the ability to transfer mechanical forces into a gating mechanism of channel regulation, is split into two main working (not mutually exclusive) hypotheses. One is that elastic and/or structural changes in membrane properties act as a transducing mechanism of channel regulation. The other hypothesis involves tertiary elements, such as the cytoskeleton which, itself by dynamic interactions with the ion channel, may convey conformational changes, including those ascribed to mechanical forces. This hypothesis is supported by numerous instances of regulatory changes in channel behavior by alterations in cytoskeletal structures/interactions. However, only recently, the molecular nature of these interactions has slowly emerged. Recently, a surge of evidence has emerged to indicate that transient receptor potential (TRP) channels are key elements in the transduction of a variety of environmental signals. This chapter describes the molecular linkage and regulatory elements of polycystin-2 (PC2), a TRP-type (TRPP2) nonselective cation channel whose mutations cause autosomal dominant polycystic kidney disease (ADPKD). The chapter focuses on the involvement of cytoskeletal structures in the regulation of PC2 and discusses how these connections are the transducing mechanism of environmental signals to its channel function.

TRPCs as MS Channels

This chapter reviews recent evidence indicating that canonical or classical transient receptor potential (TRPC) channels are directly or indirectly mechanosensitive (MS) and can therefore be designated as mechano-operated channels (MOCs). The MS functions of TRPCs may be mechanistically related to their better known functions as store-operated and receptor-operated channels (SOCs and ROCs). Mechanical forces may be conveyed to TRPC channels through the "conformational coupling" mechanism that transmits information regarding the status of internal Ca(2+) stores. All TRPCs are regulated by receptors coupled to phospholipases that are themselves MS and can regulate channels via lipidic second messengers. Accordingly, there may be several nonexclusive mechanisms by which mechanical forces may regulate TRPC channels, including direct sensitivity to bilayer mechanics, physical coupling to internal membranes and/or cytoskeletal proteins, and sensitivity to lipidic second messengers generated by MS enzymes. Various strategies that can be used for separating out different MS-gating mechanisms and their possible role in specific TRPCs are discussed.

Autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease is the most prevalent, potentially lethal, monogenic disorder. It is associated with large interfamilial and intrafamilial variability, which can be explained to a large extent by its genetic heterogeneity and modifier genes. An increased understanding of the disorder's underlying genetic, molecular, and cellular mechanisms and a better appreciation of its progression and systemic manifestations have laid out the foundation for the development of clinical trials and potentially effective treatments.

Regulation of ryanodine receptor-dependent calcium signaling by polycystin-2

Mutations in polycystin-2 (PC2) cause autosomal dominant polycystic kidney disease. A function for PC2 in the heart has not been described. Here, we show that PC2 coimmunoprecipitates with the cardiac ryanodine receptor (RyR2) from mouse heart. Biochemical assays showed that the N terminus of PC2 binds the RyR2, whereas the C terminus only binds to RyR2 in its open state. Lipid bilayer electrophysiological experiments indicated that the C terminus of PC2 functionally inhibited RyR2 channel activity in the presence of calcium (Ca(2+)). Pkd2(-/-) cardiomyocytes had a higher frequency of spontaneous Ca(2+) oscillations, reduced Ca(2+) release from the sarcoplasmic reticulum stores, and reduced Ca(2+) content compared with Pkd2(+/+) cardiomyocytes. In the presence of caffeine, Pkd2(-/-) cardiomyocytes exhibited decreased peak fluorescence, a slower rate of rise, and a longer duration of Ca(2+) transients compared with Pkd2(+/+). These data suggest that PC2 is important for regulation of RyR2 function and that loss of this regulation of RyR2, as occurs when PC2 is mutated, results in altered Ca(2+) signaling in the heart.

TRPC6 and FSGS: the latest TRP channelopathy

Focal and segmental glomerulosclerosis (FSGS) is a common cause of nephrotic syndrome in children and adults throughout the world. In the past 50 years, significant advances have been made in the identification and characterization of familial forms of nephrotic syndrome and FSGS. Resultant to these pursuits, several podocyte structural proteins such as nephrin, podocin, alpha-actinin 4 (ACTN4), and CD2-associated protein (CD2AP) have emerged to provide critical insight into the pathogenesis of hereditary nephrotic syndromes. The latest advance in familial FSGS has been the discovery of a mutant form of canonical transient receptor potential cation channel 6 (TRPC6), which causes an increase in calcium transients and essentially a gain of function in this cation channel located on the podocyte cell membrane. The TRP ion channel family is a diverse group of cation channels united by a common primary structure which contains six membrane-spanning domains, with both carboxy and amino termini located intracellularly. TRP channels are unique in their ability to activate independently of membrane depolarization. TRPC6 channels have been shown to be activated via phospholipase C stimulation. The mechanisms by which mutant TRPC6 causes an increase in intracellular calcium and leads to glomerulosclerosis are unknown. Mutant TRPC6 may affect critical interactions with the aforementioned podocyte structural proteins, leading to abnormalities in the slit diaphragm or podocyte foot processes. Mutant TRPC6 may also amplify injurious signals mediated by Ang II, a common final pathway of podocyte apoptosis in various mammalian species. Current evidence also suggests that blocking TRPC6 channels may be of therapeutic benefit in idiopathic FSGS, a disease with a generally poor prognosis. Preliminary experiments reveal the commonly used immunosuppressive agent FK-506 can inhibit TRPC6 activity in vivo. This creates the exciting possibility that blocking TRPC6 channels within the podocyte may translate into long-lasting clinical benefits in patients with FSGS.

Cell biology of polycystin-2

Naturally occurring mutations in two separate, but interacting loci, pkd1 and pkd2 are responsible for almost all cases of autosomal dominant polycystic kidney disease (ADPKD). ADPKD is one of the most common genetic diseases resulting primarily in the formation of large kidney, liver, and pancreatic cysts. Homozygous deletion of either pkd1 or pkd2 results in embryonic lethality in mice due to kidney and heart defects illustrating their indispensable roles in mammalian development. However, the mechanism by which mutations in these genes cause ADPKD and other developmental defects are unknown. Research in the past several years has revealed that PKD2 has multiple functions depending on its subcellular localization. It forms a receptor-operated, non-selective cation channel in the plasma membrane, a novel intracellular Ca2+ release channel in the endoplasmic reticulum (ER), and a mechanosensitive channel in the primary cilium. This review focuses on the functional compartmentalization of PKD2, its modes of activation, and PKD2-mediated signal transduction.

Cardiovascular polycystins: insights from autosomal dominant polycystic kidney disease and transgenic animal models

Mutations in the PKD1 and PKD2 polycystin genes are responsible for autosomal dominant polycystic kidney disease (ADPKD), one of the most prevalent genetic kidney disorders. ADPKD is a multisystem disease characterized by the formation of numerous fluid-filled cysts in the kidneys, the pancreas, and the liver. Moreover, major cardiovascular manifestations are common complications in ADPKD. Intracranial aneurysms and arterial hypertension are among the leading causes of mortality in this disease. In the present review, we summarize our current understanding of the role of polycystins in the development, maintenance, and function of the cardiovascular system.

TRPC1 Ca(2+)-permeable channels in animal cells

The full-length transient receptor (TRPC)1 polypeptide is composed of about 790 amino acids, and several splice variants are known. The predicted structure and topology is of an integral membrane protein composed of six transmembrane domains, and a cytoplasmic C- and N-terminal domain. The N-terminal domain includes three ankyrin repeat motifs. Antibodies which recognise TRPC1 have been developed, but it has been difficult to obtain antibodies which have high affinity and specificity for TRPC1. This has made studies of the cellular functions of TRPC1 somewhat difficult. The TRPC1 protein is widely expressed in different types of animal cells, and within a given cell is found at the plasma membrane and at intracellular sites. TRPC1 interacts with calmodulin, caveolin-1, the InsP3 receptor, Homer, phospholipase C and several other proteins. Investigations of the biological roles and mechanisms of action of TRPC1 have employed ectopic (over-expression or heterologous expression) of the polypeptide in addition to studies of endogenous TRPC1. Both approaches have encountered difficulties. TRPC1 forms heterotetramers with other TRPC polypeptides resulting in cation channels which are non-selective. TRPC1 may be: a component of the pore of store-operated Ca2+ channels (SOCs); a subsidiary protein in the pathway of activation of SOCs; activated by interaction with InsP3R; and/or activated by stretch. Further experiments are required to resolve the exact roles and mechanisms of activation of TRPC1. Cation entry through the TRPC1 channel is feed-back inhibited by Ca2+ through interaction with calmodulin, and is inhibited by Gd3+, La3+, SKF96365 and 2-APB, and by antibodies targeted to the external mouth of the TRPC1 pore. Activation of TRPC1 leads to the entry to the cytoplasmic space of substantial amounts of Na+ as well as Ca2+. A requirement for TRPC1 is implicated in numerous downstream cellular pathways. The most clearly described roles are in the regulation of growth cone turning in neurons. It is concluded that TRPC1 is a most interesting protein because of the apparent wide variety of its roles and functions and the challenges posed to those attempting to elucidate its primary intracellular functions and mechanisms of action.

TRP channels in platelet function

Ca2+ entry forms an essential component of platelet activation; however, the mechanisms associated with this process are not understood. Ca2+ entry upon receptor activation occurs as a consequence of intracellular store depletion (referred to as store-operated Ca2+ entry or SOCE), a direct action of second messengers on cation entry channels or the direct occupancy of a ligand-gated P2(Xi) receptor. The molecular identity of the SOCE channel has yet to be established. Transient receptor potential (TRP) proteins are candidate cation entry channels and are classified into a number of closely related subfamilies including TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin) and TRPML (mucolipins). From the TRPC family, platelets have been shown to express TRPC6 and TRPC1, and are likely to express other TRPC and other TRP members. TRPC6 is suggested to be involved with receptor-activated, diacyl-glycerol-mediated cation entry. TRPC1 has been suggested to be involved with SOCE, though many of the suggested mechanisms remain controversial. As no single TRP channel has the properties described for SOCE in platelets, it is likely that it is composed of a heteromeric association of TRP and related subunits, some of which may be present in intracellular compartments in the resting cell.