The subcellular localization of TRPP2 modulates its function

Unstructural Biology of TRP Ion Channels: The Role of Intrinsically Disordered Regions in Channel Function and Regulation

The first genuine high-resolution single particle cryo-electron microscopy structure of a membrane protein determined was a transient receptor potential (TRP) ion channel, TRPV1, in 2013. This methodical breakthrough opened up a whole new world for structural biology and ion channel aficionados alike. TRP channels capture the imagination due to the sheer endless number of tasks they carry out in all aspects of animal physiology. To date, structures of at least one representative member of each of the six mammalian TRP channel subfamilies as well as of a few non-mammalian families have been determined. These structures were instrumental for a better understanding of TRP channel function and regulation. However, all of the TRP channel structures solved so far are incomplete since they miss important information about highly flexible regions found mostly in the channel N- and C-termini. These intrinsically disordered regions (IDRs) can represent between a quarter to almost half of the entire protein sequence and act as important recruitment hubs for lipids and regulatory proteins. Here, we analyze the currently available TRP channel structures with regard to the extent of these "missing" regions and compare these findings to disorder predictions. We discuss select examples of intra- and intermolecular crosstalk of TRP channel IDRs with proteins and lipids as well as the effect of splicing and post-translational modifications, to illuminate their importance for channel function and to complement the prevalently discussed structural biology of these versatile and fascinating proteins with their equally relevant 'unstructural' biology.

An Overview of In Vivo and In Vitro Models for Autosomal Dominant Polycystic Kidney Disease: A Journey from 3D-Cysts to Mini-Pigs

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inheritable cause of end stage renal disease and, as of today, only a single moderately effective treatment is available for patients. Even though ADPKD research has made huge progress over the last decades, the precise disease mechanisms remain elusive. However, a wide variety of cellular and animal models have been developed to decipher the pathophysiological mechanisms and related pathways underlying the disease. As none of these models perfectly recapitulates the complexity of the human disease, the aim of this review is to give an overview of the main tools currently available to ADPKD researchers, as well as their main advantages and limitations.

Post-translational modifications of the polycystin proteins

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited cause of kidney failure and affects up to 12 million people worldwide. Germline mutations in two genes, PKD1 or PKD2, account for almost all patients with ADPKD. The ADPKD proteins, polycystin-1 (PC1) and polycystin-2 (PC2), are regulated by post-translational modifications (PTM), with phosphorylation, glycosylation and proteolytic cleavage being the best described changes. A few PTMs have been shown to regulate polycystin trafficking, signalling, localisation or stability and thus their physiological function. A key challenge for the future will be to elucidate the functional significance of all the individual PTMs reported to date. Finally, it is possible that site-specific mutations that disrupt PTM could contribute to cystogenesis although in the majority of cases, confirmatory evidence is awaited.

Mass Spectrometry-Based Analysis of TRPP2 Phosphorylation

Differential phosphorylation of proteins is a key regulatory mechanism in biology. Immunoprecipitation-coupled mass spectrometry facilitates the targeted analysis of transient receptor ion potential channel polycystin-2 (TRPP2) phosphorylation. However, empirical testing is required to optimize experimental conditions for immunoprecipitation and mass spectrometry. Here, we present a detailed workflow for the reliable analysis of endogenous TRPP2 phosphorylation in differentiated renal epithelial cells.

Non-compaction of ventricular myocardium with polycystic kidney disease with cardiogenic cerebral embolism

Non-compaction of ventricular myocardium is a rare cardiomyopathy involving an early arrest of normal compaction of myocardium during fetal ontogenesis. Autosomal dominant polycystic kidney disease (ADPKD) is a hereditary nephropathy characterised by multiple renal cysts replacing the renal parenchyma and extrarenal manifestations. Here, we report a case of 65-year-old man, chronic smoker, presented with sudden onset right brachial monoparesis, exertional dyspnoea, orthopnoea, bipedal swelling and diagnosed as a case of ADPKD with left ventricular non-compaction cardiomyopathy with acute left ventricular failure and cardiogenic cerebral embolism (no evidence of atrial fibrillation); based on characteristic appearance on two-dimensional echocardiography and cardiac magnetic resonance. The patient was managed with guideline-directed pharmacotherapy for heart failure and anticoagulation as a secondary stroke prevention measure. Through this case report, we try to discuss the association between two rare entities and individualisation of treatment options available as a case-based approach, as no standard treatment guidelines are available.

TRPP2 ion channels: Critical regulators of organ morphogenesis in health and disease

Ion channels control the membrane potential and mediate transport of ions across membranes. Archetypical physiological functions of ion channels include processes such as regulation of neuronal excitability, muscle contraction, or transepithelial ion transport. In that regard, transient receptor potential ion channel polycystin 2 (TRPP2) is remarkable, because it controls complex morphogenetic processes such as the establishment of properly shaped epithelial tubules and left-right-asymmetry of organs. The fascinating question of how an ion channel regulates morphogenesis has since captivated the attention of scientists in different disciplines. Four loosely connected key insights on different levels of biological complexity ranging from protein to whole organism have framed our understanding of TRPP2 physiology: 1) TRPP2 is a non-selective cation channel; 2) TRPP2 is part of a receptor-ion channel complex; 3) TRPP2 localizes to primary cilia; and 4) TRPP2 is required for organ morphogenesis. In this review, we will discuss the current knowledge in these key areas and highlight some of the challenges ahead.

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.

New Insights into the Molecular Mechanisms Targeting Tubular Channels/Transporters in PKD Development

BACKGROUND

Autosomal dominant polycystic kidney disease (PKD) or autosomal recessive PKD is caused by a mutation in the PKD1, PKD2 or PKHD1 gene, which encodes polycystin-1, polycystin-2 or fibrocystin, respectively. Embryonic and postnatal mutation studies show that transport or channel function is dysregulated before the initiation of cystogenesis, suggesting that the abnormality of transport or channel function plays a critical role in the pathology of PKD.

SUMMARY

Polycystin-2 by itself is a calcium-permeable cation channel, and its channel function can be regulated by polycystin-1 or fibrocystin. In this paper, we reviewed the current knowledge about calcium transports and cyclic adenosine monophosphate (cAMP)-driven chloride transports in PKD. In addition, the function and the underlining mechanism of glucose transporters, phosphate transporters and water channels in PKD are also discussed.

KEY MESSAGES

Abnormalities in calcium handling and exuberant cAMP-dependent cystic fibrosis transmembrane conductance regulator-mediated fluid secretion in the collecting duct are the most important issues in the pathogenesis of PKD.

The functions of TRPP2 in the vascular system

TRPP2 (polycystin-2, PC2 or PKD2), encoded by the PKD2 gene, is a non-selective cation channel with a large single channel conductance and high Ca(2+) permeability. In cell membrane, TRPP2, along with polycystin-1, TRPV4 and TRPC1, functions as a mechanotransduction channel. In the endoplasmic reticulum, TRPP2 modulates intracellular Ca(2+) release associated with IP3 receptors and the ryanodine receptors. Noteworthily, TRPP2 is widely expressed in vascular endothelial and smooth muscle cells of all major vascular beds, and contributes to the regulation of vessel function. The mutation of the PKD2 gene is a major cause of autosomal dominant polycystic kidney disease (ADPKD), which is not only a common genetic disease of the kidney but also a systemic disorder associated with abnormalities in the vasculature; cardiovascular complications are the leading cause of mortality and morbidity in ADPKD patients. This review provides an overview of the current knowledge regarding the TRPP2 protein and its possible role in cardiovascular function and related diseases.

The zebrafish Kupffer's vesicle as a model system for the molecular mechanisms by which the lack of Polycystin-2 leads to stimulation of CFTR

In autosomal dominant polycystic kidney disease (ADPKD), cyst inflation and continuous enlargement are associated with marked transepithelial ion and fluid secretion into the cyst lumen via cystic fibrosis transmembrane conductance regulator (CFTR). Indeed, the inhibition or degradation of CFTR prevents the fluid accumulation within cysts. The in vivo mechanisms by which the lack of Polycystin-2 leads to CFTR stimulation are an outstanding challenge in ADPKD research and may bring important biomarkers for the disease. However, hampering their study, the available ADPKD in vitro cellular models lack the three-dimensional architecture of renal cysts and the ADPKD mouse models offer limited access for live-imaging experiments in embryonic kidneys. Here, we tested the zebrafish Kupffer's vesicle (KV) as an alternative model-organ. KV is a fluid-filled vesicular organ, lined by epithelial cells that express both CFTR and Polycystin-2 endogenously, being each of them easily knocked-down. Our data on the intracellular distribution of Polycystin-2 support its involvement in the KV fluid-flow induced Ca²⁺-signalling. Mirroring kidney cysts, the KV lumen inflation is dependent on CFTR activity and, as we clearly show, the knockdown of Polycystin-2 results in larger KV lumens through overstimulation of CFTR. In conclusion, we propose the zebrafish KV as a model organ to study the renal cyst inflation. Favouring its use, KV volume can be easily determined by in vivo imaging offering a live readout for screening compounds and genes that may prevent cyst enlargement through CFTR inhibition.

Summary: Here, we tested the zebrafish Kupffer's vesicle (KV) as a model organ to study, through in vivo imaging of KV volume, the stimulation of cystic fibrosis transmembrane conductance regulator (CFTR) in autosomal dominant polycystic kidney disease ADPKD.

Modulation of the secretory pathway rescues zebrafish polycystic kidney disease pathology

Mutations in polycystin 1 and polycystin 2 are responsible for autosomal dominant polycystic kidney disease, the most common heritable human disease. Polycystins function as calcium ion channels, but their impact on cell physiology is not fully known. Recent findings suggest that polycystins could function in the maintenance of extracellular matrix integrity. In zebrafish, polycystin 2 knockdown induces kidney cysts, hydrocephalus, left/right asymmetry defects, and strong dorsal axis curvature. Here, we show that increased notochord sheath collagen deposition in polycystin 2-deficient embryos is directly linked to axis defects. Increased collagen II protein accumulation did not associate with increased col2a1 mRNA or a decrease in matrix metalloproteinase activity but, instead, it associated with increased expression of the endoplasmic reticulum/Golgi transport coat protein complex II Sec proteins. sec24D knockdown prevented dorsal axis curvature and kidney cystogenesis in polycystin 2 morphants. Nontoxic doses of brefeldin A also prevented the dorsal axis curvature formation in polycystin 2 morphants and curly up polycystin 2 mutants. Brefeldin A treatment after the onset of polycystin deficiency phenotypes reversed the curved axis phenotype but not kidney cyst progression. Our results suggest that polycystin 2 deficiency causes increased collagen II synthesis with upregulation of secretory pathway coat protein complex II components. Restoration of normal rates of secretory protein synthesis and secretion may be a new target in the treatment of autosomal dominant polycystic kidney disease.

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.

A complex of BBS1 and NPHP7 is required for cilia motility in zebrafish

Bardet-Biedl syndrome (BBS) and nephronophthisis (NPH) are hereditary autosomal recessive disorders, encoded by two families of diverse genes. BBS and NPH display several overlapping phenotypes including cystic kidney disease, retinitis pigmentosa, liver fibrosis, situs inversus and cerebellar defects. Since most of the BBS and NPH proteins localize to cilia and/or their appendages, BBS and NPH are considered ciliopathies. In this study, we characterized the function of the transcription factor Nphp7 in zebrafish, and addressed the molecular connection between BBS and NPH. The knockdown of zebrafish bbs1 and nphp7.2 caused similar phenotypic changes including convergent extension defects, curvature of the body axis, hydrocephalus, abnormal heart looping and cystic pronephros, all consistent with an altered ciliary function. Immunoprecipitation assays revealed a physical interaction between BBS1 and NPHP7, and the simultaneous knockdown of zbbs1 and znphp7.2 enhanced the cystic pronephros phenotype synergistically, suggesting a genetic interaction between zbbs1 and znphp7.2 in vivo. Deletion of zBbs1 or zNphp7.2 did not compromise cilia formation, but disrupted cilia motility. Although NPHP7 has been shown to act as transcriptional repressor, our studies suggest a crosstalk between BBS1 and NPHP7 in regulating normal function of the cilium.

Cdc42 deficiency causes ciliary abnormalities and cystic kidneys

Ciliogenesis and cystogenesis require the exocyst, a conserved eight-protein trafficking complex that traffics ciliary proteins. In culture, the small GTPase Cdc42 co-localizes with the exocyst at primary cilia and interacts with the exocyst component Sec10. The role of Cdc42 in vivo, however, is not well understood. Here, knockdown of cdc42 in zebrafish produced a phenotype similar to sec10 knockdown, including tail curvature, glomerular expansion, and mitogen-activated protein kinase (MAPK) activation, suggesting that cdc42 and sec10 cooperate in ciliogenesis. In addition, cdc42 knockdown led to hydrocephalus and loss of photoreceptor cilia. Furthermore, there was a synergistic genetic interaction between zebrafish cdc42 and sec10, suggesting that cdc42 and sec10 function in the same pathway. Mice lacking Cdc42 specifically in kidney tubular epithelial cells died of renal failure within weeks of birth. Histology revealed cystogenesis in distal tubules and collecting ducts, decreased ciliogenesis in cyst cells, increased tubular cell proliferation, increased apoptosis, increased fibrosis, and led to MAPK activation, all of which are features of polycystic kidney disease, especially nephronophthisis. Taken together, these results suggest that Cdc42 localizes the exocyst to primary cilia, whereupon the exocyst targets and docks vesicles carrying ciliary proteins. Abnormalities in this pathway result in deranged ciliogenesis and polycystic kidney 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.

Polycystins and cellular Ca2+ signaling

The cystic phenotype in autosomal dominant polycystic kidney disease is characterized by a profound dysfunction of many cellular signaling patterns, ultimately leading to an increase in both cell proliferation and apoptotic cell death. Disturbance of normal cellular Ca²⁺ signaling seems to be a primary event and is clearly involved in many pathways that may lead to both types of cellular responses. In this review, we summarize the current knowledge about the molecular and functional interactions between polycystins and multiple components of the cellular Ca²⁺-signaling machinery. In addition, we discuss the relevant downstream responses of the changed Ca²⁺ signaling that ultimately lead to increased proliferation and increased apoptosis as observed in many cystic cell types.

Calcium signaling in renal tubular cells

The kidney handles calcium by filtration and reabsorption. About 60% of the plasma calcium is filterable, and 99% is reabsorbed in the tubule. In the proximal tubule, the reabsorption is passive and paracellular, but in the distal tubule is active and transcellular. Thus, renal tubular cells are exposed to very high concentrations of calcium in both, the extracellular and the intracellular compartments. Extracellular calcium signaling is transmitted by the calcium sensing receptor, located both in the luminal and basolateral sides of tubular cells. This receptor is able to control levels of extracellular calcium and acts in consequence to maintain calcium homeostasis. Furthermore, renal tubular cells possess several calcium channels that regulate some of the cell functions. Among those, voltage gated calcium channels, transient receptor potential channels and N-methyl-D-aspartate receptor channels have been reported to control several functions. Those functions include survival, apoptosis, differentiation, epithelial-mesenchymal transition, and active vitamin D and renin synthesis.

Zebrafish kidney development: basic science to translational research

The zebrafish has become a significant model system for studying renal organogenesis and disease, as well as for the quest for new therapeutics, because of the structural and functional simplicity of the embryonic kidney. Inroads to the nature and disease states of kidney-related ciliopathies and acute kidney injury (AKI) have been advanced by zebrafish studies. This model organism has been instrumental in the analysis of mutant gene function for human disease with respect to ciliopathies. Additionally, in the AKI field, recent work in the zebrafish has identified a bona fide adult zebrafish renal progenitor (stem) cell that is required for neo-nephrogenesis, both during the normal lifespan and in response to renal injury. Taken together, these studies solidify the zebrafish as a successful model system for studying the broad spectrum of ciliopathies and AKI that affect millions of humans worldwide, and point to a very promising future of zebrafish drug discovery. The emphasis of this review will be on the role of the zebrafish as a model for human kidney-related ciliopathies and AKI, and how our understanding of these complex pathologies is being furthered by this tiny teleost.

Nephrocystin-4 is required for pronephric duct-dependent cloaca formation in zebrafish

NPHP4 mutations cause nephronophthisis, an autosomal recessive cystic kidney disease associated with renal fibrosis and kidney failure. The NPHP4 gene product nephrocystin-4 interacts with other nephrocystins, cytoskeletal and ciliary proteins; however, the molecular and cellular functions of nephrocystin-4 have remained elusive. Here we demonstrate that nephrocystin-4 is required for normal cloaca formation during zebrafish embryogenesis. Time-lapse imaging of the developing zebrafish pronephros revealed that tubular epithelial cells at the distal pronephros actively migrate between the yolk sac extension and the blood island towards the ventral fin fold to join the proctodeum and to form the cloaca. Nphp4-deficient pronephric duct cells failed to connect with their ectodermal counterparts, and instead formed a vesicle at the obstructed end of the pronephric duct. Nephrocystin-4 interacts with nephrocystin-1 and Par6. Depletion of zebrafish NPHP1 (nphp1) increased the incidence of cyst formation and randomization of the normal body axis, but did not augment cloaca malformation in nphp4-deficient zebrafish embryos. However, simultaneous depletion of zebrafish Par6 (pard6) aggravated cloaca formation defects in nphp4-depleted embryos, suggesting that nphp4 orchestrates directed cell migration and cloaca formation through interaction with the Par protein complex.

A single amino acid residue constitutes the third dimerization domain essential for the assembly and function of the tetrameric polycystin-2 (TRPP2) channel

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 nonselective calcium channel. PC2 has been found to form oligomers in native tissues, suggesting that similar to other TRP channels, it may form functional homo- or heterotetramers with other TRP subunits. We have recently demonstrated that the homodimerization of PC2 is mediated by both N-terminal and C-terminal domains, and it is known that PC2 can heterodimerize with PC1, TRPC1, and TRPV4. In this paper, we report that a single cysteine residue, Cys⁶³², mutated in a known PKD2 pedigree, constitutes the third dimerization domain for PC2. PC2 truncation mutants lacking both N and C termini could still dimerize under nonreducing conditions. Mutation of Cys⁶³² alone abolished dimerization in these mutants, indicating that it was the critical residue mediating disulfide bond formation between PC2 monomers. Co-expression of C632A PC2 mutants with wild-type PC2 channels reduced ATP-sensitive endoplasmic reticulum Ca²⁺ release in HEK293 cells. The combination of C632A and mutations disrupting the C-terminal coiled-coil domain (Val⁸⁴⁶, Ile⁸⁵³, Ile⁸⁶⁰, Leu⁸⁶⁷ or 4M) nearly abolished dimer formation and ATP-dependent Ca²⁺ release. However, unlike the 4M PC2 mutant, a C632A mutant could still heterodimerize with polycystin-1 (PC1). Our results indicate that PC2 homodimerization is regulated by three distinct domains and that these events regulate formation of the tetrameric PC2 channel.

The exocyst protein Sec10 interacts with Polycystin-2 and knockdown causes PKD-phenotypes

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by formation of renal cysts that destroy the kidney. Mutations in PKD1 and PKD2, encoding polycystins-1 and -2, cause ADPKD. Polycystins are thought to function in primary cilia, but it is not well understood how these and other proteins are targeted to cilia. Here, we provide the first genetic and biochemical link between polycystins and the exocyst, a highly-conserved eight-protein membrane trafficking complex. We show that knockdown of exocyst component Sec10 yields cellular phenotypes associated with ADPKD, including loss of flow-generated calcium increases, hyperproliferation, and abnormal activation of MAPK. Sec10 knockdown in zebrafish phenocopies many aspects of polycystin-2 knockdown-including curly tail up, left-right patterning defects, glomerular expansion, and MAPK activation-suggesting that the exocyst is required for pkd2 function in vivo. We observe a synergistic genetic interaction between zebrafish sec10 and pkd2 for many of these cilia-related phenotypes. Importantly, we demonstrate a biochemical interaction between Sec10 and the ciliary proteins polycystin-2, IFT88, and IFT20 and co-localization of the exocyst and polycystin-2 at the primary cilium. Our work supports a model in which the exocyst is required for the ciliary localization of polycystin-2, thus allowing for polycystin-2 function in cellular processes.

Fluid transport and cystogenesis in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most frequent inherited nephropathy. The development and enlargement of cysts in ADPKD requires tubular cell proliferation, abnormalities in the extracellular matrix and transepithelial fluid secretion. Multiple studies have suggested that fluid secretion across ADPKD cyst-lining cells is driven by the transepithelial secretion of chloride, mediated by the apical CFTR channel and specific basolateral transporters. The whole secretory process is stimulated by increased levels of cAMP in the cells, probably reflecting modifications in the intracellular calcium homeostasis and abnormal stimulation of the vasopressin V2 receptor. This review will focus on the pathophysiology of fluid secretion in ADPKD cysts, starting with classic, morphological and physiological studies that were followed by investigations of the molecular mechanisms involved and therapeutic trials targeting these pathways in cellular and animal models and ADPKD patients. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Receptor protein tyrosine phosphatases are novel components of a polycystin complex

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutation of PKD1 and PKD2 that encode polycystin-1 and polycystin-2. Polycystin-1 is tyrosine phosphorylated and modulates multiple signaling pathways including AP-1, and the identity of the phosphatases regulating polycystin-1 are previously uncharacterized. Here we identify members of the LAR protein tyrosine phosphatase (RPTP) superfamily as members of the polycystin-1complex mediated through extra- and intracellular interactions. The first extracellular PKD1 domain of polycystin-1 interacts with the first Ig domain of RPTPσ, while the polycystin-1 C-terminus of polycystin-1 interacts with the regulatory D2 phosphatase domain of RPTPγ. Additional homo- and heterotypic interactions between RPTPs recruit RPTPδ. The multimeric polycystin protein complex is found localised in cilia. RPTPσ and RPTPδ are also part of a polycystin-1/E-cadherin complex known to be important for early events in adherens junction stabilisation. The interaction between polycystin-1 and RPTPγ is disrupted in ADPKD cells, while RPTPσ and RPTPδ remain closely associated with E-cadherin, largely in an intracellular location. The polycystin-1 C-terminus is an in vitro substrate of RPTPγ, which dephosphorylates the c-Src phosphorylated Y4237 residue and activates AP1-mediated transcription. The data identify RPTPs as novel interacting partners of the polycystins both in cilia and at adhesion complexes and demonstrate RPTPγ phosphatase activity is central to the molecular mechanisms governing polycystin-dependent signaling. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Polycystin-2 activity is controlled by transcriptional coactivator with PDZ binding motif and PALS1-associated tight junction protein

Autosomal dominant polycystic kidney disease (ADPKD) is the most frequent monogenic cause of kidney failure, characterized by the development of renal cysts. ADPKD is caused by mutations of the polycystin-1 (PC1) or polycystin-2 (PC2) genes. PC2 encodes a Ca(2+)-permeable cation channel, and its dysfunction has been implicated in cyst development. The transcriptional coactivator with PDZ binding motif (TAZ) is required for the integrity of renal cilia. Its absence results in the development of renal cysts in a knock-out mouse model. TAZ directly interacts with PC2, and it has been suggested that another yet unidentified PDZ domain protein may be involved in the TAZ/PC2 interaction. Here we describe a novel interaction of TAZ with the multi-PDZ-containing PALS1-associated tight junction protein (PATJ). TAZ interacts with both the N-terminal PDZ domains 1-3 and the C-terminal PDZ domains 8-10 of PATJ, suggesting two distinct TAZ binding domains. We also show that the C terminus of PC2 strongly interacts with PDZ domains 8-10 and to a weaker extent with PDZ domains 1-3 of PATJ. Finally, we demonstrate that both TAZ and PATJ impair PC2 channel activity when co-expressed with PC2 in oocytes of Xenopus laevis. These results implicate TAZ and PATJ as novel regulatory elements of the PC2 channel and might thus be involved in ADPKD pathology.

The activation of membrane targeted CaMK-II in the zebrafish Kupffer's vesicle is required for left-right asymmetry

Intracellular calcium ion (Ca2+) elevation on the left side of the mouse embryonic node or zebrafish Kupffer's vesicle (KV) is the earliest asymmetric molecular event that is functionally linked to lateral organ placement in these species. In this study, Ca2+/CaM-dependent protein kinase (CaMK-II) is identified as a necessary target of this Ca2+ elevation in zebrafish embryos. CaMK-II is transiently activated in approximately four interconnected cells along the anterior left wall of the KV between the six- and 12-somite stages, which is coincident with known left-sided Ca2+ elevations. Within these cells, activated CaMK-II is observed at the surface and in clusters, which appear at the base of some KV cilia. Although seven genes encode catalytically active CaMK-II in early zebrafish embryos, one of these genes also encodes a truncated inactive variant (αKAP) that can hetero-oligomerize with and target active enzyme to membranes. αKAP, β2 CaMK-II and γ1 CaMK-II antisense morpholino oligonucleotides, as well as KV-targeted dominant negative CaMK-II, randomize organ laterality and southpaw (spaw) expression in lateral plate mesoderm (LPM). Left-sided CaMK-II activation was most dependent on an intact KV, the PKD2 Ca2+ channel and γ1 CaMK-II; however, αKAP, β2 CaMK-II and the RyR3 ryanodine receptor were also necessary for full CaMK-II activation. This is the first report to identify a direct Ca2+-sensitive target in left-right asymmetry and supports a model in which membrane targeted CaMK-II hetero-oligomers in nodal cells transduce the left-sided PKD2-dependent Ca2+ signals to the LPM.

A polycystin-2 (TRPP2) dimerization domain essential for the function of heteromeric polycystin complexes

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in two genes, PKD1 and PKD2, which encode polycystin-1 (PC1) and polycystin-2 (PC2), respectively. Earlier work has shown that PC1 and PC2 assemble into a polycystin complex implicated in kidney morphogenesis. PC2 also assembles into homomers of uncertain functional significance. However, little is known about the molecular mechanisms that direct polycystin complex assembly and specify its functions. We have identified a coiled coil in the C-terminus of PC2 that functions as a homodimerization domain essential for PC1 binding but not for its self-oligomerization. Dimerization-defective PC2 mutants were unable to reconstitute PC1/PC2 complexes either at the plasma membrane (PM) or at PM-endoplasmic reticulum (ER) junctions but could still function as ER Ca(2+)-release channels. Expression of dimerization-defective PC2 mutants in zebrafish resulted in a cystic phenotype but had lesser effects on organ laterality. We conclude that C-terminal dimerization of PC2 specifies the formation of polycystin complexes but not formation of ER-localized PC2 channels. Mutations that affect PC2 C-terminal homo- and heteromerization are the likely molecular basis of cyst formation in ADPKD.

The ADPKD genes pkd1a/b and pkd2 regulate extracellular matrix formation

Mutations in polycystin1 (PKD1) account for the majority of autosomal dominant polycystic kidney disease (ADPKD). PKD1 mutations are also associated with vascular aneurysm and abdominal wall hernia, suggesting a role for polycystin1 in extracellular matrix (ECM) integrity. In zebrafish, combined knockdown of the PKD1 paralogs pkd1a and pkd1b resulted in dorsal axis curvature, hydrocephalus, cartilage and craniofacial defects, and pronephric cyst formation at low frequency (10-15%). Dorsal axis curvature was identical to the axis defects observed in pkd2 knockdown embryos. Combined pkd1a/b, pkd2 knockdown demonstrated that these genes interact in axial morphogenesis. Dorsal axis curvature was linked to notochord collagen overexpression and could be reversed by knockdown of col2a1 mRNA or chemical inhibition of collagen crosslinking. pkd1a/b- and pkd2-deficient embryos exhibited ectopic, persistent expression of multiple collagen mRNAs, suggesting a loss of negative feedback signaling that normally limits collagen gene expression. Knockdown of pkd1a/b also dramatically sensitized embryos to low doses of collagen-crosslinking inhibitors, implicating polycystins directly in the modulation of collagen expression or assembly. Embryos treated with wortmannin or LY-29400 also exhibited dysregulation of col2a1 expression, implicating phosphoinositide 3-kinase (PI3K) in the negative feedback signaling pathway controlling matrix gene expression. Our results suggest that pkd1a/b and pkd2 interact to regulate ECM secretion or assembly, and that altered matrix integrity may be a primary defect underlying ADPKD tissue pathologies.

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.

Down-regulation of Pkd2 by siRNAs suppresses cell-cell adhesion in the mouse melanoma cells

The Pkd2 gene encodes an integral protein (~130 kDa), named polycystin-2 (PC-2). PC-2 is mainly involved in autosomal dominant polycystic kidney disease. Recently, polycystin-1/polycystin-2 complex has been shown to act as an adhesion complex mediating or regulating cell-cell or cell-matrix adhesion, suggesting that PC-2 may play a role in cell-cell/cell-matrix interactions. Here, we knocked down the expression of Pkd2 gene with small interfering RNAs (siRNAs) in the mouse melanoma cells (B16 cells), indicating that the cells transfected with the targeted siRNAs significantly suppressed cell-cell adhesion, but not cell-matrix adhesion, compared to the cells transfected with non-targeted control (NC) siRNA. This study provides the first directly functional evidence that PC-2 mediates cell-cell adhesion. Furthermore, we demonstrated that PC-2 modulated cell-cell adhesion may be, at least partially, associated with E-cadherin. Collectively, these findings for the first time showed that PC-2 may mediate cell-cell adhesion, at least partially, through E-cadherin.

At the crossroads of homoeostasis and disease: roles of the PACS proteins in membrane traffic and apoptosis

The endomembrane system in mammalian cells has evolved over the past two billion years from a simple endocytic pathway in a single-celled primordial ancestor to complex networks supporting multicellular structures that form metazoan tissue and organ systems. The increased organellar complexity of metazoan cells requires additional trafficking machinery absent in yeast or other unicellular organisms to maintain organ homoeostasis and to process the signals that control proliferation, differentiation or the execution of cell death programmes. The PACS (phosphofurin acidic cluster sorting) proteins are one such family of multifunctional membrane traffic regulators that mediate organ homoeostasis and have important roles in diverse pathologies and disease states. This review summarizes our current knowledge of the PACS proteins, including their structure and regulation in cargo binding, their genetics, their roles in secretory and endocytic pathway traffic, interorganellar communication and how cell-death signals reprogramme the PACS proteins to regulate apoptosis. We also summarize our current understanding of how PACS genes are dysregulated in cancer and how viral pathogens ranging from HIV-1 to herpesviruses have evolved to usurp the PACS sorting machinery to promote virus assembly, viral spread and immunoevasion.

Autosomal dominant polycystic kidney disease: the last 3 years

Autosomal dominant polycystic kidney disease is the most prevalent, potentially lethal monogenic disorder. It has large inter- and intra-familial variability explained to a large extent by its genetic heterogeneity and modifier genes. An increased understanding of its 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 therapies. The purpose of this review is to update the core of knowledge in this area with recent publications that have appeared during 2006-2009.

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.

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.

Increased expression of secreted frizzled-related protein 4 in polycystic kidneys

Autosomal dominant polycystic kidney disease (ADPKD) is a common hereditary disease associated with progressive renal failure. Although cyst growth and compression of surrounding tissue may account for some loss of renal tissue, the other factors contributing to the progressive renal failure in patients with ADPKD are incompletely understood. Here, we report that secreted frizzled-related protein 4 (sFRP4) is upregulated in human ADPKD and in four different animal models of PKD, suggesting that sFRP4 expression is triggered by a common mechanism that underlies cyst formation. Cyst fluid from ADPKD kidneys activated the sFRP4 promoter and induced production of sFRP4 protein in renal tubular epithelial cell lines. Antagonism of the vasopressin 2 receptor blocked both promoter activity and tubular sFRP4 expression. In addition, sFRP4 selectively influenced members of the canonical Wnt signaling cascade and promoted cystogenesis of the zebrafish pronephros. sFRP4 was detected in the urine of both patients and animals with PKD, suggesting that sFRP4 may be a potential biomarker for monitoring the progression of ADPKD. Taken together, these observations suggest a potential role for SFRP4 in the pathogenesis of ADPKD.

Genetic and physical interaction between the NPHP5 and NPHP6 gene products

Nephronophthisis (NPHP) is an autosomal recessive cystic kidney disease, caused by mutations of at least nine different genes. Several extrarenal manifestations characterize this disorder, including cerebellar defects, situs inversus and retinitis pigmentosa. While the clinical manifestations vary significantly in NPHP, mutations of NPHP5 and NPHP6 are always associated with progressive blindness. This clinical finding suggests that the gene products, nephrocystin-5 and nephrocystin-6, participate in overlapping signaling pathways to maintain photoreceptor homeostasis. To analyze the genetic interaction between these two proteins in more detail, we studied zebrafish embryos after depletion of NPHP5 and NPHP6. Knockdown of zebrafish zNPHP5 and zNPHP6 produced similar phenotypes, and synergistic effects were observed after the combined knockdown of zNPHP5 and zNPHP6. The N-terminal domain of nephrocystin-6-bound nephrocystin-5, and mapping studies delineated the interacting site from amino acid 696 to 896 of NPHP6. In Xenopus laevis, knockdown of NPHP5 caused substantial neural tube closure defects. This phenotype was copied by expression of the nephrocystin-5-binding fragment of nephrocystin-6, and rescued by co-expression of nephrocystin-5, supporting a physical interaction between both gene products in vivo. Since the N- and C-terminal fragments of nephrocystin-6 engage in the formation of homo- and heteromeric protein complexes, conformational changes seem to regulate the interaction of nephrocystin-6 with its binding partners.