Polycystin-2 associates with tropomyosin-1, an actin microfilament component

Activation of PIEZO1 Attenuates Kidney Cystogenesis In Vitro and Ex Vivo

Key Points

Background

The disruption of calcium signaling associated with polycystin deficiency is a key factor in abnormal epithelial growth in autosomal dominant polycystic kidney disease. Calcium homeostasis can be influenced by mechanotransduction. The mechanosensitive cation channel PIEZO1 has been implicated in sensing intrarenal pressure and regulating urinary osmoregulation, but its role in kidney cystogenesis is unclear.

Methods

We hypothesized that altered mechanotransduction contributes to cystogenesis in autosomal dominant polycystic kidney disease and that activation of mechanosensitive cation channels could be a therapeutic strategy.

Results

We demonstrate that Yoda1, a PIEZO1 activator, increases intracellular calcium and reduces forskolin-induced cAMP levels in mouse inner medullary collecting duct (mIMCD3) cells. Notably, knockout of polycystin-2 attenuated the efficacy of Yoda1 in reducing cAMP levels in mIMCD3 cells. Yoda1 also reduced forskolin-induced mIMCD3 cyst surface area in vitro and cystic index in mouse metanephros ex vivo in a dose-dependent manner. However, collecting duct–specific PIEZO1 knockout neither induced cystogenesis in wild-type mice nor altered cystogenesis in the Pkd1RC/RC mouse model.

Conclusions

These findings support the potential role of PIEZO1 agonists in mitigating cystogenesis by increasing intracellular calcium and reducing cAMP levels, but the unaltered in vivo cystic phenotype after PIEZO1 knockout in the collecting duct suggests possible redundancy in mechanotransductive pathways.

Activation of Piezo1 Inhibits Kidney Cystogenesis

The disruption of calcium signaling associated with polycystin deficiency has been proposed as the primary event underlying the increased abnormally patterned epithelial cell growth characteristic of Polycystic Kidney Disease. Calcium can be regulated through mechanotransduction, and the mechanosensitive cation channel Piezo1 has been implicated in sensing of intrarenal pressure and in urinary osmoregulation. However, a possible role for PIEZO1 in kidney cystogenesis remains undefined. We hypothesized that cystogenesis in ADPKD reflects altered mechanotransduction, suggesting activation of mechanosensitive cation channels as a therapeutic strategy for ADPKD. Here, we show that Yoda-1 activation of PIEZO1 increases intracellular Ca 2+ and reduces forskolin-induced cAMP levels in mIMCD3 cells. Yoda-1 reduced forskolin-induced IMCD cyst surface area in vitro and in mouse metanephros ex vivo in a dose-dependent manner. Knockout of polycystin-2 dampened the efficacy of PIEZO1 activation in reducing both cAMP levels and cyst surface area in IMCD3 cells. However, collecting duct-specific Piezo1 knockout neither induced cystogenesis in wild-type mice nor affected cystogenesis in the Pkd1 RC/RC model of ADPKD. Our study suggests that polycystin-2 and PIEZO1 play a role in mechanotransduction during cystogenesis in vitro , and ex vivo , but that in vivo cyst expansion may require inactivation or repression of additional suppressors of cystogenesis and/or growth. Our study provides a preliminary proof of concept for PIEZO1 activation as a possible component of combination chemotherapy to retard or halt cystogenesis and/or cyst growth.

TRPP2 ion channels: The roles in various subcellular locations

TRPP2 (PC2, PKD2 or Polycytin-2), encoded by PKD2 gene, belongs to the nonselective cation channel TRP family. Recently, the three-dimensional structure of TRPP2 was constructed. TRPP2 mainly functions in three subcellular compartments: endoplasmic reticulum, plasma membrane and primary cilia. TRPP2 can act as a calcium-activated intracellular calcium release channel on the endoplasmic reticulum. TRPP2 also interacts with other Ca²⁺ release channels to regulate calcium release, like IP3R and RyR2. TRPP2 acts as an ion channel regulated by epidermal growth factor through activation of downstream factors in the plasma membrane. TRPP2 binding to TRPC1 in the plasma membrane or endoplasmic reticulum is associated with mechanosensitivity. In cilium, TRPP2 was found to combine with PKD1 and TRPV4 to form a complex related to mechanosensitivity. Because TRPP2 is involved in regulating intracellular ion concentration, TRPP2 mutations often lead to autosomal dominant polycystic kidney disease, which may also be associated with cardiovascular disease. In this paper, we review the molecular structure of TRPP2, the subcellular localization of TRPP2, the related functions and mechanisms of TRPP2 at different sites, and the diseases related to TRPP2.

Polycystin-2 (TRPP2): Ion channel properties and regulation

Polycystin-2 (TRPP2, PKD2, PC2) is the product of the PKD2 gene, whose mutations cause Autosomal Dominant Polycystic Kidney Disease (ADPKD). PC2 belongs to the superfamily of TRP (Transient Receptor Potential) proteins that generally function as Ca²⁺-permeable nonselective cation channels implicated in Ca²⁺ signaling. PC2 localizes to various cell domains with distinct functions that likely depend on interactions with specific channel partners. Functions include receptor-operated, nonselective cation channel activity in the plasma membrane, intracellular Ca²⁺ release channel activity in the endoplasmic reticulum (ER), and mechanosensitive channel activity in the primary cilium of renal epithelial cells. Here we summarize our current understanding of the properties of PC2 and how other transmembrane and cytosolic proteins modulate this activity, providing functional diversity and selective regulatory mechanisms to its role in the control of cellular Ca²⁺ homeostasis.

[Cardiovascular disorders in autosomal dominant polycystic kidney disease]

Autosomal dominant polycystic kidney disease is the most frequent genetic kidney disease. Cardiovascular disorders associated with autosomal dominant polycystic kidney disease are multiple and may occur early in life. In autosomal dominant polycystic kidney disease cardiovascular morbidity and mortality are related both to the nonspecific consequences of chronic kidney disease and to the particular phenotype of autosomal dominant polycystic kidney disease. Compared to the general population, patients with autosomal dominant polycystic kidney disease present an increased prevalence of hypertension, left ventricular hypertrophy, atrial fibrillation, valvular diseases, aneurisms and arterial dissections. This review article provides an update on cardiovascular disorders associated with autosomal dominant polycystic kidney disease and recent pathophysiological developments.

Polycystins as components of large multiprotein complexes of polycystin interactors

Naturally occurring mutations in two separate genes, PKD1 and PKD2, are responsible for the vast majority of all cases of autosomal dominant polycystic kidney disease (ADPKD), one of the most common genetic diseases affecting 1 in 1000 Americans. The hallmark of ADPKD is the development of epithelial cysts in the kidney, liver, and pancreas. PKD1 encodes a large plasma membrane protein (PKD1, PC1, or Polycystin-1) with a long extracellular domain and has been speculated to function as an atypical G protein coupled receptor. PKD2 encodes an ion channel of the Transient Receptor Potential superfamily (TRPP2, PKD2, PC2, or Polycystin-2). Despite the identification of these genes more than 20 years ago, the molecular function of their encoded proteins and the mechanism(s) by which mutations in PKD1 and PKD2 cause ADPKD remain elusive. Genetic, biochemical, and functional evidence suggests they form a multiprotein complex present in multiple locations in the cell, including the plasma membrane, endoplasmic reticulum, and the primary cilium. Over the years, numerous interacting proteins have been identified using directed and unbiased approaches, and shown to modulate function, cellular localization, and protein stability and turnover of Polycystins. Delineation of the molecular composition of the Polycystin complex can have a significant impact on understanding their cellular function in health and disease states and on the identification of more specific and effective therapeutic targets.

iTRAQ-based quantitative proteomics reveals the neuroprotection of rhubarb in experimental intracerebral hemorrhage

ETHNOPHARMACOLOGICAL RELEVANCE

Rhubarb is a traditional Chinese medicine(TCM), that possesses neuroprotective, anti-inflammatory, antibacterial, antioxidative, purgative and anticancer properties, and has been used to treat intracerebral hemorrhage (ICH) and many other diseases.

AIMS OF THE STUDY

This study aimed to investigate the changes of brain protein in ICH rats treated with rhubarb and to explore the multi-target mechanism of rhubarb in the treatment of ICH via bioinformatics analysis of differentially expressed proteins (DEPs).

MATERIALS AND METHODS

Rats were subjected to collagenase-induced ICH and then treated orally with 3 or 12 g/kg rhubarb daily for 2 days following ICH. After sacrifice, total protein of brain tissue was extracted, and isobaric tag for relative and absolute quantification (iTRAQ)-based liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis was employed to quantitatively identify of the DEPs in two treatment groups compared with the vehicle group. The DEPs were analyzed by Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) and STRING databases. Bioinformatics Analysis Tool for Molecular mechanism of TCM (BATMAN-TCM) was used to predict the target of rhubarb and western blotting was used for verification.

RESULTS

In total, 1356 proteins were identified with a 1% false discovery rate (FDR). Among them, 55 DEPs were significantly altered in the sham, vehicle, low dose rhubarb group (LDR, 3 g/kg), and high dose rhubarb group (HDR, 12 g/kg). Enrichment analysis of GO annotations indicated that rhubarb mainly regulated expression of some neuron projection proteins involved in the response to drug and nervous system development. The dopaminergic synapse pathway was found to be the most significant DEP in the combined analysis of the KEGG and BATMAN-TCM databases. Based on the results of the STRING analysis, oxidative stress (OS), calcium binding protein regulation, vascularization, and energy metabolism were important in the rhubarb therapeutic process.

CONCLUSION

Rhubarb achieves its effects mainly through the dopaminergic synapse pathway in ICH treatment. The ICH-treating mechanisms of rhubarb may also involve anti-OS, calcium binding protein regulation, angiogenic regulation, and energy metabolism improvement. This study adds new evidence to clinical applications of rhubarb for ICH.

Autosomal Dominant Polycystic Kidney Patients May Be Predisposed to Various Cardiomyopathies

Introduction

Mutations in PKD1 and PKD2 cause autosomal dominant polycystic kidney disease (ADPKD). Experimental evidence suggests an important role of the polycystins in cardiac development and myocardial function. To determine whether ADPKD may predispose to the development of cardiomyopathy, we have evaluated the coexistence of diagnoses of ADPKD and primary cardiomyopathy in our patients.

Methods

Clinical data were retrieved from medical records for patients with a coexisting diagnosis of ADPKD and cardiomyopathies evaluated at the Mayo Clinic (1984-2015).

Results

Among the 58 of 667 patients with available echocardiography data, 39 (5.8%) had idiopathic dilated cardiomyopathy (IDCM), 17 (2.5%) had hypertrophic obstructive cardiomyopathy, and 2 (0.3%) had left ventricular noncompaction. Genetic data were available for 19, 8, and 2 cases of IDCM, hypertrophic obstructive cardiomyopathy, and left ventricular noncompaction, respectively. PKD1 mutations were detected in 42.1%, 62.5%, and 100% of IDCM, hypertrophic obstructive cardiomyopathy, and left ventricular noncompaction cases, respectively. PKD2 mutations were detected only in IDCM cases and were overrepresented (36.8%) relative to the expected frequency in ADPKD (15%). In at least 1 patient from 3 IDMC families and 1 patient from a hypertrophic obstructive cardiomyopathy family, the cardiomyopathy did not segregate with ADPKD, suggesting that the PKD mutations may be predisposing factors rather than solely responsible for the development of cardiomyopathy.

Discussion

Coexistence of ADPKD and cardiomyopathy in our tertiary referral center cohort appears to be higher than expected by chance. We suggest that PKD1 and PKD2 mutations may predispose to primary cardiomyopathies and that genetic interactions may account for the observed coexistence of ADPKD and cardiomyopathies.

Polycystin and calcium signaling in cell death and survival

Mutations in polycystin-1 (PC1) and polycystin-2 (PC2) result in a commonly occurring genetic disorder, called Autosomal Dominant Polycystic Kidney Disease (ADPKD), that is characterized by the formation and development of kidney cysts. Epithelial cells with loss-of-function of PC1 or PC2 show higher rates of proliferation and apoptosis and reduced autophagy. PC1 is a large multifunctional transmembrane protein that serves as a sensor that is usually found in complex with PC2, a calcium (Ca²⁺)-permeable cation channel. In addition to decreased Ca²⁺ signaling, several other cell fate-related pathways are de-regulated in ADPKD, including cAMP, MAPK, Wnt, JAK-STAT, Hippo, Src, and mTOR. In this review we discuss how polycystins regulate cell death and survival, highlighting the complexity of molecular cascades that are involved in ADPKD.

Identification and Expression Analysis of the Complete Family of Zebrafish pkd Genes

Polycystic kidney disease (PKD) proteins are trans-membrane proteins that have crucial roles in many aspects of vertebrate development and physiology, including the development of many organs as well as left–right patterning and taste. They can be divided into structurally-distinct PKD1-like and PKD2-like proteins and usually one PKD1-like protein forms a heteromeric polycystin complex with a PKD2-like protein. For example, PKD1 forms a complex with PKD2 and mutations in either of these proteins cause Autosomal Dominant Polycystic Kidney Disease (ADPKD), which is the most frequent potentially-lethal single-gene disorder in humans. Here, we identify the complete family of pkd genes in zebrafish and other teleosts. We describe the genomic locations and sequences of all seven genes: pkd1, pkd1b, pkd1l1, pkd1l2a, pkd1l2b, pkd2, and pkd2l1. pkd1l2a/pkd1l2b are likely to be ohnologs of pkd1l2, preserved from the whole genome duplication that occurred at the base of the teleosts. However, in contrast to mammals and cartilaginous and holostei fish, teleosts lack pkd2l2, and pkdrej genes, suggesting that these have been lost in the teleost lineage. In addition, teleost, and holostei fish have only a partial pkd1l3 sequence, suggesting that this gene may be in the process of being lost in the ray-finned fish lineage. We also provide the first comprehensive description of the expression of zebrafish pkd genes during development. In most structures we detect expression of one pkd1-like gene and one pkd2-like gene, consistent with these genes encoding a heteromeric protein complex. For example, we found that pkd2 and pkd1l1 are expressed in Kupffer's vesicle and pkd1 and pkd2 are expressed in the developing pronephros. In the spinal cord, we show that pkd1l2a and pkd2l1 are co-expressed in KA cells. We also identify potential co-expression of pkd1b and pkd2 in the floor-plate. Interestingly, and in contrast to mouse, we observe expression of all seven pkd genes in regions that may correspond to taste receptors. Taken together, these results provide a crucial catalog of pkd genes in an important model system for elucidating cell and developmental processes and modeling human diseases and the most comprehensive analysis of embryonic pkd gene expression in any vertebrate.

Next Generation Sequencing and Linkage Analysis for the Molecular Diagnosis of a Novel Overlapping Syndrome Characterized by Hypertrophic Cardiomyopathy and Typical Electrical Instability of Brugada Syndrome

BACKGROUND

Familial hypertrophic cardiomyopathy (HCM) is an autosomal dominant inherited disorder; mutations in at least 20 genes have been associated. Brugada syndrome (BrS) is an autosomal dominant inherited disorder caused by mutations mainly in theSCN5Agene. A new clinical entity that consists of HCM, typical electrical instability of BrS and sudden death (SD), is described.

METHODS AND RESULTS

The family was constituted by 7 members, 4 of who presented clinical features of HCM and electrical instability of BrS. The clinical presentation of proband was ventricular fibrillation. All members were clinically evaluated by physical examination, 12-lead electrocardiography, 2-dimensional echocardiography, stress test, electrocardiogram Holter, flecainide test, and electrophysiological study. An integrated linkage analysis and next generation sequencing (NGS) approach was used to identify the causative mutation. Linkage with the α-tropomyosin (TPM1) gene on chromosome 15q22 was identified. The NGS study identified a missense mutation within theTPM1gene (c.574G>A; p.E192K), exactly located in a binding domain with polycystin-2 protein. No other pathogenic mutations were identified.

CONCLUSIONS

This is the first report of an association between HCM and BrS, and the first to use a combined approach of linkage and NGS to identify a causative mutation in SD. The present study expands the clinical spectrum of disorders associated with theTPM1gene and may be useful to report novel mechanisms of electrical instability in HCM and BrS.

Polycystin-2 (TRPP2) Regulation by Ca(2+) Is Effected and Diversified by Actin-Binding Proteins

Calcium regulation of Ca(2+)-permeable ion channels is an important mechanism in the control of cell function. Polycystin-2 (PC2, TRPP2), a member of the transient receptor potential superfamily, is a nonselective cation channel with Ca(2+) permeability. The molecular mechanisms associated with PC2 regulation by Ca(2+) remain ill-defined. We recently demonstrated that PC2 from human syncytiotrophoblast (PC2hst) but not the in vitro translated protein (PC2(iv)), functionally responds to changes in intracellular (cis) Ca(2+). In this study we determined the regulatory effect(s) of Ca(2+)-sensitive and -insensitive actin-binding proteins (ABPs) on PC2(iv) channel function in a lipid bilayer system. The actin-bundling protein α-actinin increased PC2(iv) channel function in the presence of cis Ca(2+), although instead was inhibitory in its absence. Conversely, filamin that shares actin-binding domains with α-actinin had a strong inhibitory effect on PC2(iv) channel function in the presence, but no effect in the absence of cis Ca(2+). Gelsolin stimulated PC2(iv) channel function in the presence, but not the absence of cis Ca(2+). In contrast, profilin that shares actin-binding domains with gelsolin, significantly increased PC2(iv) channel function both in the presence and absence of Ca(2+). The distinct effect(s) of the ABPs on PC2(iv) channel function demonstrate that Ca(2+) regulation of PC2 is actually mediated by direct interaction(s) with structural elements of the actin cytoskeleton. These data indicate that specific ABP-PC2 complexes would confer distinct Ca(2+)-sensitive properties to the channel providing functional diversity to the cytoskeletal control of transient receptor potential channel regulation.

Polycystins and mechanotransduction: From physiology to disease

Polycystins are key mechanosensor proteins able to respond to mechanical forces of external or internal origin. They are widely expressed in primary cilium and plasma membrane of several cell types including kidney, vascular endothelial and smooth muscle cells, osteoblasts and cardiac myocytes modulating their physiology. Interaction of polycystins with diverse ion channels, cell-cell and cell-extracellular matrix junctional proteins implicates them in the regulation of cell structure, mechanical force transmission and mechanotransduction. Their intracellular localization in endoplasmic reticulum further regulates subcellular trafficking and calcium homeostasis, finely-tuning overall cellular mechanosensitivity. Aberrant expression or genetic alterations of polycystins lead to severe structural and mechanosensing abnormalities including cyst formation, deregulated flow sensing, aneurysms, defective bone development and cancer progression, highlighting their vital role in human physiology.

Expression of polycystins and fibrocystin on primary cilia of lung cells

Mutations in polycystin-1, polycystin-2, or fibrocystin account for autosomal dominant or recessive polycystic kidney disease. Renal cystogenesis is linked to abnormal localization and function of these cystoproteins in renal primary cilia. They are also expressed in extrarenal tissues in which their functions are unclear. Here we found that human type-II alveolar epithelial A549, airway submucosal Calu-3 cells, and rat bronchioles contain primary or multiple cilia in which we detected these cystoproteins. At sub-confluency, polycystin-1 was expressed on plasma membrane, while polycystin-2 was localized to the ER of resting cells. Both polycystins were detected on the spindle and mid-body of mitotic cells, while fibrocystin was on centrosome throughout cell cycle. Polycystins and fibrocystin may participate in regulating mucociliary sensing and transport within pulmonary airways.

Polycystins and partners: proposed role in mechanosensitivity

Mutations of the two polycystins, PC1 and PC2, lead to polycystic kidney disease. Polycystins are able to form complexes with numerous families of proteins that have been suggested to participate in mechanical sensing. The proposed role of polycystins and their partners in the kidney primary cilium is to sense urine flow. A role for polycystins in mechanosensing has also been shown in other cell types such as vascular smooth muscle cells and cardiac myocytes. At the plasma membrane, polycystins interact with diverse ion channels of the TRP family and with stretch-activated channels (Piezos, TREKs). The actin cytoskeleton and its interacting proteins, such as filamin A, have been shown to be essential for these interactions. Numerous proteins involved in cell-cell and cell-extracellular matrix junctions interact with PC1 and/or PC2. These multimeric protein complexes are important for cell structure integrity, the transmission of force, as well as for mechanosensing and mechanotransduction. A group of polycystin partners are also involved in subcellular trafficking mechanisms. Finally, PC1 and especially PC2 interact with elements of the endoplasmic reticulum and are essential components of calcium homeostasis. In conclusion, we propose that both PC1 and PC2 act as conductors to tune the overall cellular mechanosensitivity.

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.

Polycystin-2 mutations lead to impaired calcium cycling in the heart and predispose to dilated cardiomyopathy

Mutations in PKD1 and PKD2, the genes encoding the proteins polycystin-1 (PC1) and polycystin-2 (PC2), cause autosomal dominant polycystic kidney disease (ADPKD). Although the leading cause of mortality in ADPKD is cardiovascular disease, the relationship between these conditions remains poorly understood. PC2 is an intracellular calcium channel expressed in renal epithelial cells and in cardiomyocytes, and is thus hypothesized to modulate intracellular calcium signaling and affect cardiac function. Our first aim was to study cardiac function in a zebrafish model lacking PC2 (pkd2 mutants). Next, we aimed to explore the relevance of this zebrafish model to human ADPKD by examining the Mayo Clinic's ADPKD database for an association between ADPKD and idiopathic dilated cardiomyopathy (IDCM). Pkd2 mutant zebrafish showed low cardiac output and atrioventricular block. Isolated pkd2 mutant hearts displayed impaired intracellular calcium cycling and calcium alternans. These results indicate heart failure in the pkd2 mutants. In human ADPKD patients, we found IDCM to coexist frequently with ADPKD. This association was strongest in patients with PKD2 mutations. Our results demonstrate that PC2 modulates intracellular calcium cycling, contributing to the development of heart failure. In human subjects we found an association between ADPKD and IDCM and suggest that PKD mutations contribute to the development of heart failure.

Structural interaction and functional regulation of polycystin-2 by filamin

Filamins are important actin cross-linking proteins implicated in scaffolding, membrane stabilization and signal transduction, through interaction with ion channels, receptors and signaling proteins. Here we report the physical and functional interaction between filamins and polycystin-2, a TRP-type cation channel mutated in 10–15% patients with autosomal dominant polycystic kidney disease. Yeast two-hybrid and GST pull-down experiments demonstrated that the C-termini of filamin isoforms A, B and C directly bind to both the intracellular N- and C-termini of polycystin-2. Reciprocal co-immunoprecipitation experiments showed that endogenous polycystin-2 and filamins are in the same complexes in renal epithelial cells and human melanoma A7 cells. We then examined the effect of filamin on polycystin-2 channel function by electrophysiology studies with a lipid bilayer reconstitution system and found that filamin-A substantially inhibits polycystin-2 channel activity. Our study indicates that filamins are important regulators of polycystin-2 channel function, and further links actin cytoskeletal dynamics to the regulation of this channel protein.

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.

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.

Tropomyosin is an interaction partner of the Drosophila coiled coil protein yuri gagarin

The Drosophila gene yuri gagarin is a complex locus encoding three protein isoform classes that are ubiquitously expressed in the organism. Mutations to the gene affect processes as diverse as gravitactic behavior and spermatogenesis. The larger Yuri isoforms contain extensive coiled-coil regions. Our previous studies indicate that one of the large isoform classes (Yuri-65) is required for formation of specialized F-actin-containing structures generated during spermatogenesis, including the so-called actin "cones" that mediate spermatid individualization. We used the tandem affinity purification of a tagged version of Yuri-65 (the TAP-tagging technique) to identify proteins associated with Yuri-65 in the intact organism. Tropomyosin, primarily as the 284-residue isoform derived from the ubiquitously expressed Tropomyosin 1 gene was thus identified as a major Yuri interaction partner. Co-immunoprecipitation experiments confirmed this interaction. We have established that the stable F-actin cones of spermatogenesis contain Tropomyosin 1 (Tm1) and that in mutant yuri(F64), failure of F-actin cone formation is associated with failure of Tm1 to accumulate at the cone initiation sites. In investigating possible interactions of Tm1 and Yuri in other tissues, we discovered that Tm1 and Yuri frequently colocalize with the endoplasmic reticulum. Tropomyosin has been implicated in actin-mediated membrane trafficking activity in other systems. Our findings suggest that Yuri-Tm1 complexes participate in related functions.

Protein kinase D-mediated phosphorylation of polycystin-2 (TRPP2) is essential for its effects on cell growth and calcium channel activity

PKD2 is mutated in 15% of patients with autosomal dominant polycystic kidney disease. The PKD2 protein, polycystin-2 or TRPP2, is a nonselective Ca2+-permeable cation channel that has been shown to function at several locations, including primary cilia, basolateral membrane, and at the endoplasmic reticulum (ER). Nevertheless, the factors that regulate the channel activity of polycystin-2 are not well understood. Polycystin-2 has been shown previously to be regulated by phosphorylation at two serine residues (Ser812 and Ser76) with distinct functional consequences. Here, we report the identification of a previously unrecognized phosphorylation site within the polycystin-2 C terminus (Ser801), and we demonstrate that it is phosphorylated by protein kinase D. Phosphorylation at this site was significantly increased in response to serum and epidermal growth factor stimulation. In nonciliated Madin-Darby canine kidney I cells, inducible expression of polycystin-2 inhibited cell proliferation compared with wild-type cells. Mutagenesis at Ser801 abolished these effects and reduced ATP-stimulated Ca2+ release from ER stores. Finally, we show that a pathogenic mutation (S804N) within the consensus kinase recognition sequence abolished Ser801 phosphorylation. These results suggest that growth factor-stimulated, protein kinase D-mediated phosphorylation of polycystin-2 is essential for its ER channel function and links extracellular stimuli to its effects on cell growth and intracellular calcium regulation.

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.

Polycystin-dependent fluid flow sensing targets histone deacetylase 5 to prevent the development of renal cysts

Polycystin 1 and polycystin 2 are large transmembrane proteins, which, when mutated, cause autosomal dominant polycystic kidney disease (ADPKD), a highly prevalent human genetic disease. The polycystins are thought to form a receptor-calcium channel complex in the plasma membrane of renal epithelial cells and elicit a calcium influx in response to mechanical stimulation, such as fluid flow across the apical surface of renal epithelial cells. The functional role of the polycystins in mechanosensation remains largely unknown. Here, we found that myocyte enhancer factor 2C (MEF2C) and histone deacetylase 5 (HDAC5), two key regulators of cardiac hypertrophy, are targets of polycystin-dependent fluid stress sensing in renal epithelial cells in mice. We show that fluid flow stimulation of polarized epithelial monolayers induced phosphorylation and nuclear export of HDAC5, which are crucial events in the activation of MEF2C-based transcription. Kidney-specific knockout of Mef2c, or genetrap-inactivation of a MEF2C transcriptional target, MIM, resulted in extensive renal tubule dilation and cysts, whereas Hdac5 heterozygosity or treatment with TSA, an HDAC inhibitor, reduced cyst formation in Pkd2(-/-) mouse embryos. These findings suggest a common signaling motif between myocardial hypertrophy and maintenance of renal epithelial architecture, and a potential therapeutic approach to treat ADPKD.

A short carboxy-terminal domain of polycystin-1 reorganizes the microtubular network and the endoplasmic reticulum

Mutations of PKD1 cause autosomal dominant polycystic kidney disease (ADPKD), a syndrome characterized by kidney cysts and progressive renal failure. Polycystin-1, the protein encoded by PKD1, is a large integral membrane protein with a short carboxy-terminal cytoplasmic domain that appears to initiate multiple cellular programs. We report now that this polycystin-1 domain contains a novel motif responsible for rearrangements of intermediate filaments, microtubules and the endoplasmic reticulum (ER). This motif reveals homology to CLIMP-63, a microtubule-binding protein that rearranges the ER. Our findings suggest that polycystin-1 influences the shape and localization of both the microtubular network and the ER.

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.

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.

Reciprocal interaction with G-actin and tropomyosin is essential for aquaporin-2 trafficking

Trafficking of water channel aquaporin-2 (AQP2) to the apical membrane and its vasopressin and protein kinase A (PKA)–dependent regulation in renal collecting ducts is critical for body water homeostasis. We previously identified an AQP2 binding protein complex including actin and tropomyosin-5b (TM5b). We show that dynamic interactions between AQP2 and the actin cytoskeleton are critical for initiating AQP2 apical targeting. Specific binding of AQP2 to G-actin in reconstituted liposomes is negatively regulated by PKA phosphorylation. Dual color fluorescence cross-correlation spectroscopy reveals local AQP2 interaction with G-actin in live epithelial cells at single-molecule resolution. Cyclic adenosine monophosphate signaling and AQP2 phosphorylation release AQP2 from G-actin. In turn, AQP2 phosphorylation increases its affinity to TM5b, resulting in reduction of TM5b bound to F-actin, subsequently inducing F-actin destabilization. RNA interference–mediated knockdown and overexpression of TM5b confirm its inhibitory role in apical trafficking of AQP2. These findings indicate a novel mechanism of channel protein trafficking, in which the channel protein itself critically regulates local actin reorganization to initiate its movement.

Identification of phosphoproteins in kidney tissues from patients with autosomal dominant polycystic kidney disease

Protein phosphorylation is a very important PTM. Phosphorylation/dephosphorylation of a protein can alter its behavior in almost every conceivable way. Previous studies indicate that abnormal phosphorylation is involved in the pathogenesis of autosomal dominant polycystic kidney disease (ADPKD). However, large-scale proteomic analysis of altered phosphoproteins in ADPKD has not been reported. In this study, total proteins from ADPKD cystic kidney tissues (n = 5) and normal kidney tissues (n = 5) were extracted and phosphoproteins were enriched by phosphate metal affinity chromatography, then separated by 2-DE and identified by LC-MS/MS. Between the two groups, 48 protein spots showing more than a twofold difference were detected. Among them, 28 spots were up-regulated and 20 down-regulated in ADPKD kidney tissues. Of these, 38 different proteins were identified including cell signaling proteins, cytoskeleton proteins, mitochondria metabolic enzymes, antioxidant proteins, molecular chaperones, transcription factors and regulators. Two differential phosphoproteins, annexin II and tropomyosin, were further confirmed by immunoprecipitation and Western blot analysis. The results show that there are many kinds of abnormal phosphoproteins in ADPKD cystic kidney tissues. More studies on the functions of the differential phosphoproteins may provide us new clues for ADPKD pathogenesis and treatment.

Interplay between TRP channels and the cytoskeleton in health and disease

Transient receptor potential (TRP) channels are a family of cation channels that play a key role in ion homeostasis and cell volume regulation. In addition, TRP channels are considered universal integrators of sensory information required for taste, vision, hearing, touch, temperature, and the detection of mechanical force. Seminal investigations exploring the molecular mechanisms of phototransduction in Drosophila have demonstrated that TRP channels operate within macromolecular complexes closely associated with the cytoskeleton. More recent evidence shows that mammalian TRP channels similarly connect to the cytoskeleton to affect cytoskeletal organization and cell adhesion via ion-transport-dependent and -independent mechanisms. In this review, we discuss new insights into the interplay between TRP channels and the cytoskeleton and provide recent examples of such interactions in different physiological systems.

Human tropomyosin isoforms in the regulation of cytoskeleton functions

Over the past two decades, extensive molecular studies have identified multiple tropomyosin isoforms existing in all mammalian cells and tissues. In humans, tropomyosins are encoded by TPM1 (alpha-Tm, 15q22.1), TPM2 (beta-Tm, 9p13.2-p13.1), TPM3 (gamma-Tm, 1q21.2) and TPM4 (delta-Tm, 19p13.1) genes. Through the use of different promoters, alternatively spliced exons and different sites of poly(A) addition signals, at least 22 different tropomyosin cDNAs with full-length open reading frame have been cloned. Compelling evidence suggests that these isoforms play important determinants for actin cytoskeleton functions, such as intracellular vesicle movement, cell migration, cytokinesis, cell proliferation and apoptosis. In vitro biochemical studies and in vivo localization studies suggest that different tropomyosin isoforms have differences in their actin-binding properties and their effects on other actin-binding protein functions and thus, in their specification ofactin microfilaments. In this chapter, we will review what has been learned from experimental studies on human tropomyosin isoforms about the mechanisms for differential localization and functions of tropomyosin. First, we summarize current information concerning human tropomyosin isoforms and relate this to the functions of structural homologues in rodents. We will discuss general strategies for differential localization oftropomyosin isoforms, particularly focusing on differential protein turnover and differential isoform effects on other actin binding protein functions. We will then review tropomyosin functions in regulating cell motility and in modulating the anti-angiogenic activity of cleaved high molecular weight kininogen (HKa) and discuss future directions in this area.

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.

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.

Polycystin-2 cation channel function in the human syncytiotrophoblast is regulated by microtubular structures

Polycystin-2 (PC2), encoded by PKD2, which is one of the genes whose mutations cause polycystic kidney disease, is abundantly produced in the apical domain of the syncytiotrophoblast (hST) of term human placenta. PC2, a TRP-type (TRPP2) non-selective cation channel, is present in primary cilia of renal epithelial cells, a microtubule-based ancillary structure with sensory function. The hST has abundant cytoskeletal structures, and actin filament dynamics regulate PC2 channel function in this epithelium. However, it is expected that the apical hST excludes microtubular structures. Here, we demonstrated by Western blot and immunocytochemical analyses that hST apical vesicles indeed contain microtubule structural components, including tubulin isoforms, acetylated alpha-tubulin, and the kinesin motor proteins KIF3A and KIF3B. PC2 and tubulin were substantially colocalized in hST vesicles. Treatment of hST vesicles with either the microtubular disrupter colchicine (15 microM) or the microtubular stabilizer paclitaxel (taxol, 15 microM) resulted in distinct patterns of microtubular re-organization and PC2 redistribution. We also observed that changes in microtubular dynamics regulate PC2 channel function. Addition of colchicine rapidly inhibited PC2 channel activity in lipid-bilayer reconstituted hST membranes. Addition of either tubulin and GTP, or taxol, however, stimulated PC2 channel activity in control hST membranes. Interestingly, we found that the kinesin motor protein KIF3A was capable of increasing PC2 channel activity in hST. We believe that the data are the first to provide a direct demonstration of a microtubular interaction with PC2 in the hST. This interaction thus plays an important regulatory role in the control of ion transport in the human placenta.

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.

TRPP2 and autosomal dominant polycystic kidney disease

Mutations in TRPP2 (polycystin-2) cause autosomal dominant polycystic kidney disease (ADPKD), a common genetic disorder characterized by progressive development of fluid-filled cysts in the kidney and other organs. TRPP2 is a Ca(2+)-permeable nonselective cation channel that displays an amazing functional versatility at the cellular level. It has been implicated in the regulation of diverse physiological functions including mechanosensation, cell proliferation, polarity, and apoptosis. TRPP2 localizes to different subcellular compartments, such as the endoplasmic reticulum (ER), the plasma membrane and the primary cilium. The channel appears to have distinct functions in different subcellular compartments. This functional compartmentalization is thought to contribute to the observed versatility and specificity of TRPP2-mediated Ca(2+) signaling. In the primary cilium, TRPP2 has been suggested to function as a mechanosensitive channel that detects fluid flow in the renal tubule lumen, supporting the proposed role of the primary cilium as the unifying pathogenic concept for cystic kidney disease. This review summarizes the known and emerging functions of TRPP2, focusing on the question of how channel function translates into complex morphogenetic programs regulating tubular structure.

TRPP2 channel regulation

Polycystin-2, or TRPP2 according to the TRP nomenclature, is encoded by PKD2, a gene mutated in patients with autosomal-dominant polycystic kidney disease. Its precise subcellular location and its intracellular trafficking are a matter of intense debate, although consensus has emerged that it is located in primary cilia, a long-neglected organelle possibly involved in sensory functions. Polycystin-2 has a calculated molecular mass of 110 kDa, and according to structural predictions it contains six membrane-spanning domains and a pore-forming region between the 5th and 6th membrane-spanning domain. This section irst introduces the reader to the field of cystic kidney diseases and to the PKD2 gene, before the ion channel properties of polycystin-2 are discussed in great detail.

TRPpathies

Many human diseases are caused by mutations in ion channels. Dissecting the pathogenesis of these 'channelopathies' has yielded important insights into the regulation of vital biological processes by ions and has become a productive tool of modern ion channel biology. One of the best examples of a synergism between the clinical and basic science aspects of a modern biological topic is cystic fibrosis. Not only did the identification of the ion channel mutated in cystic fibrosis pinpoint the root cause of this disease, but it also has significantly advanced our understanding of basic biological processes as diverse as protein folding and epithelial fluid and electrolyte secretion. The list of confirmed 'channelopathies' is growing and several members of the TRP family of ion channels have been implicated in human diseases such as mucolipidosis type IV (MLIV), autosomal dominant polycystic kidney disease (ADPKD), familial focal segmental glomerulosclerosis (FSG), hypomagnesemia with secondary hypocalcaemia (HSH), and several forms of cancer. Analysing pathogenesis of the diseases linked to TRP dysregulation provides an exciting means of identifying novel functions of TRP channels.

Polycystin-2 immunolocalization and function in zebrafish

Polycystin-2 functions as a cation-permeable transient receptor potential ion channel in kidney epithelial cells and when mutated results in human autosomal dominant polycystic kidney disease. For further exploration of the in vivo functions of Polycystin-2, this study examined its expression and function during zebrafish embryogenesis. pkd2 mRNA is ubiquitously expressed, and its presence in the larval kidney could be confirmed by reverse transcription-PCR on isolated pronephroi. Immunostaining with anti-zebrafish Polycystin-2 antibody revealed protein expression in motile kidney epithelial cell cilia and intracellular cell membranes. Intracellular localization was segment specific; in the proximal nephron segment, Polycystin-2 was localized to basolateral cell membranes, whereas in the caudal pronephric segment, Polycystin-2 was concentrated in subapical cytoplasmic vesicles. Polycystin-2 also was expressed in muscle cells and in a variety of sensory cells that are associated with mechanotransduction, including cells of the ear, the lateral line organ, and the olfactory placodes. Disruption of Polycystin-2 mRNA expression resulted in pronephric kidney cysts, body axis curvature, organ laterality defects, and hydrocephalus-defects that could be rescued by expression of a human PKD2 mRNA. In-frame deletions in the first extracellular loop and C-terminal phosphofurin acidic cluster sorting protein-1 (PACS-1) binding sites in the cytoplasmic tail caused Polycystin-2 mislocalization to the apical cell surface. Unlike zebrafish intraflagellar transport protein (IFT) mutants, cyst formation was not associated with cilia defects and instead correlated with reduced kidney fluid output, expansion of caudal duct apical cell membranes, and occlusion of the caudal pronephric nephron segment.

Kinesin-2 mediates physical and functional interactions between polycystin-2 and fibrocystin

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in PKD1, encoding polycystin-1 (PC1), or PKD2 (polycystin-2, PC2). Autosomal recessive PKD (ARPKD) is caused by mutations in PKHD1, encoding fibrocystin/polyductin (FPC). No molecular link between ADPKD and ARPKD has been determined. Here, we demonstrated, by yeast two-hybrid and biochemical assays, that KIF3B, a motor subunit of kinesin-2, associates with PC2 and FPC. Co-immunoprecipitation experiments using Madin-Darby canine kidney (MDCK) and inner medullary collecting duct (IMCD) cells and human kidney revealed that PC2 and KIF3B, FPC and KIF3B and, furthermore, PC2 and FPC are endogenously in the same complex(es), though no direct association between the PC2 and FPC intracellular termini was detected. In vitro binding and Far Western blot experiments demonstrated that PC2 and FPC are in the same complex only if KIF3B is present, presumably by forming a PC2-KIF3B-FPC complex. This was supported by our observation that altering KIF3B level in IMCD cells by over-expression or siRNA significantly affected complexing between PC2 and FPC. Immunofluorescence experiments showed that PC2, FPC and KIF3B partially co-localized in primary cilia of over-confluent and perinuclear regions of sub-confluent cells. Furthermore, KIF3B mediated functional modulation of purified PC2 channels by FPC in a planer lipid bilayer electrophysiology system. The FPC C-terminus substantially stimulated PC2 channel activity in the presence of KIF3B, whereas FPC or KIF3B alone had no effect. Taken together, we discovered that kinesin-2 is a linker between PC2 and FPC and mediates the regulation of PC2 channel function by FPC. Our study may be important for elucidating common molecular pathways for PKD of different genotypes.

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.

A truncated polycystin-2 protein causes polycystic kidney disease and retinal degeneration in transgenic rats

The cloning of the PKD1 and PKD2 genes has led to promising new insight into the mechanisms that are responsible for cyst development in patients with autosomal dominant polycystic kidney disease. Although the dominant pattern of inheritance would argue for haploinsufficiency, a gain of function, or a dominant negative mechanism, there is good evidence that autosomal dominant polycystic kidney disease behaves like a recessive disease on a cellular level (two-hit mechanism of cystogenesis). For testing of whether other pathomechanisms in addition to the two-hit hypothesis can explain cyst formation, two transgenic rat lines that contain a truncated human polycystin-2 cDNA were generated. The protein product lacks almost the entire COOH-terminus and mimics mutations that frequently are found in patients. The transgene-encoded mRNA could be detected in multiple tissues of both transgenic lines, with the highest expression in the kidney. Both lines present with renal cysts that originate predominantly from the proximal tubule; in the tubular epithelial cells, the epitope-tagged mutant protein was detected in the brush border and in primary cilia. Further evidence of the involvement of primary cilia stems from the finding of retinal degeneration in the transgenic rats and from the fact that stably transfected LLC-PK(1) cells that inducibly produced the truncated polycystin-2 protein elaborated shorter cilia. Other experimental approaches, such as a knock-in strategy, will be necessary to validate these results, but this is the first preliminary evidence that cyst formation is due not only to somatic mutations.

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.

Identification of an N-terminal glycogen synthase kinase 3 phosphorylation site which regulates the functional localization of polycystin-2 in vivo and in vitro

PKD2 is mutated in 15% of patients with autosomal dominant polycystic kidney disease. Polycystin-2 (PC2), the PKD2 protein, is a non-selective Ca(2+)-permeable cation channel which may function at the cell surface and ER. Nevertheless, the factors that regulate the dynamic translocation of PC2 between the ER and other compartments are not well understood. Constitutive phosphorylation of PC2 at a single C-terminal site (Ser(812)) has been previously reported. As we were unable to abolish phospholabelling of PC2 in HEK293 cells by site-directed mutagenesis of Ser(812) or all five predicted phosphorylation sites in the C-terminus, we hypothesized that PC2 could also be phosphorylated at the N-terminus. In this paper, we report the identification of a new phosphorylation site for PC2 within its N-terminal domain (Ser(76)) and demonstrate that this residue is phosphorylated by glycogen synthase kinase 3 (GSK3). The consensus recognition sequence for GSK3 (Ser(76)/Ser(80)) is evolutionarily conserved down to lower vertebrates. In the presence of specific GSK3 inhibitors, the lateral plasma membrane pool of endogenous PC2 redistributes into an intracellular compartment in MDCK cells without any change in primary cilia localization. Finally, co-injection of wild-type but not a S76A/S80A mutant PKD2 capped mRNA could rescue the cystic phenotype induced by an antisense morpholino oligonucleotide to pkd2 in zebrafish pronephric kidney. We conclude that surface localization of PC2 is regulated by phosphorylation at a unique GSK3 site in its N-terminal domain in vivo and in vitro. This site is functionally significant for the maintenance of normal glomerular and tubular morphology.

Tight junction biology and kidney dysfunction

The epithelial tight junction (TJ) has three major functions. As a "gate," it serves as a regulatory barrier separating and maintaining biological fluid compartments of different composition. As a "fence," it generates and maintains the apicobasal polarity of cells that form the confluent epithelium. Finally, the TJ proteins form a trafficking and signaling platform that regulates cell growth, proliferation, differentiation, and dedifferentiation. Six examples are selected that illustrate the emerging link between TJ dysfunction and kidney disease. First, the glomerular slit diaphragm (GSD) is evolved, in part, from the TJ and, on maturation, exhibits all three functions of the TJ. GSD dysfunction leads to proteinuria and, in some instances, podocyte dedifferentiation and proliferation. Second, accumulating evidence supports epithelial-mesenchymal transformation (EMT) as a major player in renal fibrosis, the final common pathway that leads to end-stage renal failure. EMT is characterized by a loss of cell-cell contact and apicobasal polarity, which are hallmarks of TJ dysfunction. Third, in autosomal dominant polycystic kidney disease, mutations of the polycystins may disrupt their known interactions with the apical junction complex, of which the TJ is a major component. This can lead to disturbances in epithelial polarity regulation with consequent abnormal tubulogenesis and cyst formation. Fourth, evidence for epithelial barrier and polarity dysregulation in the pathogenesis of ischemic acute renal failure will be summarized. Fifth, the association between mutations of paracellin-1, the first TJ channel identified, and clinical disorders of magnesium and calcium wasting and bovine renal fibrosis will be used to highlight an integral TJ protein that can serve multiple TJ functions. Finally, the role of WNK4 protein kinase in shunting chloride across the TJ of the distal nephron will be addressed.

Protein interaction analysis of ST14 domains and their point and deletion mutants

ST14 (suppression of tumorigenicity 14) is a transmembrane serine protease that contains a serine protease catalytic (SP) domain, an SEA domain, two complement subcomponent C1r/s (CUB) domains, and four low density lipoprotein receptor class A domains. Glutathione S-transferase fusion proteins with SP, CUB, and low density lipoprotein receptor domains and their corresponding mutants were generated to analyze protein interactions with these domains. Modified glutathione S-transferase pull-down assays demonstrated the interaction between the SP domain and hepatocyte growth factor activator inhibitor-1. With the same method, a CUB domain-interacting protein was isolated and turned out to be the transmembrane protein with epidermal growth factor-like and two follistatin-like domains 1 (TMEFF1). Quantitative real time PCR revealed that the expression of the TMEFF1 gene was dependent on the transfection of the ST14 gene in the RKO cell line. Our results also suggested that ST14 and TMEFF1 were co-expressed in the human breast cancer cell line MCF7, human placenta, kidney, and liver tissues. Interestingly, these two genes were co-up-regulated in kidney tumors versus normal tissues, consistent with our results that showed the dependence of TMEFF1 expression on ST14 in RKO cells. Finally, homology modeling studies suggested that TMEFF1 might form a complex with ST14 by an interaction between epidermal growth factor and CUB domains.