Polycystin-1 interacts with inositol 1,4,5-trisphosphate receptor to modulate intracellular Ca2+ signaling with implications for polycystic kidney disease

Calcium signalling and transport in the kidney

The kidney plays a pivotal role in regulating calcium levels within the body. Approximately 98% of the filtered calcium is reabsorbed in the nephron, and this process is tightly controlled to maintain calcium homeostasis, which is required to facilitate optimal bone mineralization, preserve serum calcium levels within a narrow range, and support intracellular signalling mechanisms. The maintenance of these functions is attributed to a delicate balance achieved by various calcium channels, transporters, and calcium-binding proteins in renal cells. Perturbation of this balance due to deficiency or dysfunction of calcium channels and calcium-binding proteins can lead to severe complications. For example, polycystic kidney disease is linked to aberrant calcium transport and signalling. Furthermore, dysregulation of calcium levels can promote the formation of kidney stones. This Review provides an updated description of the key aspects of calcium handling in the kidney, focusing on the function of various calcium channels and the physiological stimuli that control these channels or are communicated through them. A discussion of the role of calcium as an intracellular second messenger and the pathophysiology of renal calcium dysregulation, as well as a summary of gaps in knowledge and future prospects, are also included.

Calcium and Neural Stem Cell Proliferation

Intracellular calcium plays a pivotal role in central nervous system (CNS) development by regulating various processes such as cell proliferation, migration, differentiation, and maturation. However, understanding the involvement of calcium (Ca2+) in these processes during CNS development is challenging due to the dynamic nature of this cation and the evolving cell populations during development. While Ca2+ transient patterns have been observed in specific cell processes and molecules responsible for Ca2+ homeostasis have been identified in excitable and non-excitable cells, further research into Ca2+ dynamics and the underlying mechanisms in neural stem cells (NSCs) is required. This review focuses on molecules involved in Ca2+ entrance expressed in NSCs in vivo and in vitro, which are crucial for Ca2+ dynamics and signaling. It also discusses how these molecules might play a key role in balancing cell proliferation for self-renewal or promoting differentiation. These processes are finely regulated in a time-dependent manner throughout brain development, influenced by extrinsic and intrinsic factors that directly or indirectly modulate Ca2+ dynamics. Furthermore, this review addresses the potential implications of understanding Ca2+ dynamics in NSCs for treating neurological disorders. Despite significant progress in this field, unraveling the elements contributing to Ca2+ intracellular dynamics in cell proliferation remains a challenging puzzle that requires further investigation.

Autosomal Dominant Polycystic Kidney Disease: From Pathophysiology of Cystogenesis to Advances in the Treatment

Autosomal dominant polycystic kidney disease (ADPKD) is the most common genetic renal disease, with an estimated prevalence between 1:1000 and 1:2500. It is mostly caused by mutations of the PKD1 and PKD2 genes encoding polycystin 1 (PC1) and polycystin 2 (PC2) that regulate cellular processes such as fluid transport, differentiation, proliferation, apoptosis and cell adhesion. Reduction of calcium ions and induction of cyclic adenosine monophosphate (sAMP) promote cyst enlargement by transepithelial fluid secretion and cell proliferation. Abnormal activation of MAPK/ERK pathway, dysregulated signaling of heterotrimeric G proteins, mTOR, phosphoinositide 3-kinase, AMPK, JAK/STAT activator of transcription and nuclear factor kB (NF-kB) are involved in cystogenesis. Another feature of cystic tissue is increased extracellular production and recruitment of inflammatory cells and abnormal connections among cells. Moreover, metabolic alterations in cystic cells including defective glucose metabolism, impaired beta-oxidation and abnormal mitochondrial activity were shown to be associated with cyst expansion. Although tolvaptan has been recently approved as a drug that slows ADPKD progression, some patients do not tolerate tolvaptan because of frequent aquaretic. The advances in the knowledge of multiple molecular pathways involved in cystogenesis led to the development of animal and cellular studies, followed by the development of several ongoing randomized controlled trials with promising drugs. Our review is aimed at pathophysiological mechanisms in cystogenesis in connection with the most promising drugs in animal and clinical studies.

The Mitochondrial Ca2+ import complex is altered in ADPKD

Mutations in either of the polycystic kidney disease genes, PKD1 or PKD2, engender the growth of cysts, altering renal function. Cystic growth is supported by major changes in cellular metabolism, some of which involve the mitochondrion, a major storage site for Ca²⁺ and a key organelle in cellular Ca²⁺ signaling. The goal here was to understand the role of components of the mitochondrial Ca²⁺ uptake complex in PC1-mutant cells in autosomal dominant polycystic kidney disease (ADPKD). We found that the mitochondrial Ca²⁺ uniporter (MCU) and voltage-dependent anion channels 1& 3 (VDAC) were down-regulated in different mouse and cell models of ADPKD along with the Ca²⁺-dependent enzyme, pyruvate dehydrogenase phosphatase (PDHX). The release of Ca²⁺ from the endoplasmic reticulum, and Ca²⁺ uptake by the mitochondria were upregulated in PC1(polycystin)-null cells. We also observed an enhanced staining with MitoTracker Red CMXRos in PC1-null cultured cells than in PC1-containing cells and a substantially higher increase in response to ER Ca²⁺ release. Increased colocalization of the Ca²⁺ sensitive dye, rhodamine2, with MitoTracker Green suggested an increase Ca²⁺ entry into the mitochondria in PC1 null cells subsequent to Ca²⁺ release from the ER or from Ca²⁺ entry from the extracellular solution. These data clearly demonstrate abnormal release of Ca²⁺ by the ER and corresponding alterations in Ca²⁺ uptake by the mitochondria in PC1^-null cells. Importantly, inhibiting mitochondrial Ca²⁺ uptake with the specific inhibitor Ru360 inhibited cyst growth and altered both apoptosis and cell proliferation. We further show that the decrease in mitochondrial proteins and abnormally high Ca²⁺ signaling can be reversed by application of the cystic fibrosis (CFTR) corrector, VX-809. We conclude that enhanced Ca²⁺ signaling and alterations in proteins association with the mitochondrial Ca²⁺ uptake complex are associated with malfunction of PC1. Finally, our results identify novel therapeutic targets for treating ADPKD.

Polycystin-1 is required for insulin-like growth factor 1-induced cardiomyocyte hypertrophy

Cardiac hypertrophy is the result of responses to various physiological or pathological stimuli. Recently, we showed that polycystin-1 participates in cardiomyocyte hypertrophy elicited by pressure overload and mechanical stress. Interestingly, polycystin-1 knockdown does not affect phenylephrine-induced cardiomyocyte hypertrophy, suggesting that the effects of polycystin-1 are stimulus-dependent. In this study, we aimed to identify the role of polycystin-1 in insulin-like growth factor-1 (IGF-1) signaling in cardiomyocytes. Polycystin-1 knockdown completely blunted IGF-1-induced cardiomyocyte hypertrophy. We then investigated the molecular mechanism underlying this result. We found that polycystin-1 silencing impaired the activation of the IGF-1 receptor, Akt, and ERK1/2 elicited by IGF-1. Remarkably, IGF-1-induced IGF-1 receptor, Akt, and ERK1/2 phosphorylations were restored when protein tyrosine phosphatase 1B was inhibited, suggesting that polycystin-1 knockdown deregulates this phosphatase in cardiomyocytes. Moreover, protein tyrosine phosphatase 1B inhibition also restored IGF-1-dependent cardiomyocyte hypertrophy in polycystin-1-deficient cells. Our findings provide the first evidence that polycystin-1 regulates IGF-1-induced cardiomyocyte hypertrophy through a mechanism involving protein tyrosine phosphatase 1B.

More Than Just Simple Interaction between STIM and Orai Proteins: CRAC Channel Function Enabled by a Network of Interactions with Regulatory Proteins

The calcium-release-activated calcium (CRAC) channel, activated by the release of Ca2+ from the endoplasmic reticulum (ER), is critical for Ca2+ homeostasis and active signal transduction in a plethora of cell types. Spurred by the long-sought decryption of the molecular nature of the CRAC channel, considerable scientific effort has been devoted to gaining insights into functional and structural mechanisms underlying this signalling cascade. Key players in CRAC channel function are the Stromal interaction molecule 1 (STIM1) and Orai1. STIM1 proteins span through the membrane of the ER, are competent in sensing luminal Ca2+ concentration, and in turn, are responsible for relaying the signal of Ca2+ store-depletion to pore-forming Orai1 proteins in the plasma membrane. A direct interaction of STIM1 and Orai1 allows for the re-entry of Ca2+ from the extracellular space. Although much is already known about the structure, function, and interaction of STIM1 and Orai1, there is growing evidence that CRAC under physiological conditions is dependent on additional proteins to function properly. Several auxiliary proteins have been shown to regulate CRAC channel activity by means of direct interactions with STIM1 and/or Orai1, promoting or hindering Ca2+ influx in a mechanistically diverse manner. Various proteins have also been identified to exert a modulatory role on the CRAC signalling cascade although inherently lacking an affinity for both STIM1 and Orai1. Apart from ubiquitously expressed representatives, a subset of such regulatory mechanisms seems to allow for a cell-type-specific control of CRAC channel function, considering the rather restricted expression patterns of the specific proteins. Given the high functional and clinical relevance of both generic and cell-type-specific interacting networks, the following review shall provide a comprehensive summary of regulators of the multilayered CRAC channel signalling cascade. It also includes proteins expressed in a narrow spectrum of cells and tissues that are often disregarded in other reviews of similar topics.

Modulation of polycystic kidney disease by G-protein coupled receptors and cyclic AMP signaling

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a systemic disorder associated with polycystic liver disease (PLD) and other extrarenal manifestations, the most common monogenic cause of end-stage kidney disease, and a major burden for public health. Many studies have shown that alterations in G-protein and cAMP signaling play a central role in its pathogenesis. As for many other diseases (35% of all approved drugs target G-protein coupled receptors (GPCRs) or proteins functioning upstream or downstream from GPCRs), treatments targeting GPCR have shown effectiveness in slowing the rate of progression of ADPKD. Tolvaptan, a vasopressin V2 receptor antagonist is the first drug approved by regulatory agencies to treat rapidly progressive ADPKD. Long-acting somatostatin analogs have also been effective in slowing the rates of growth of polycystic kidneys and liver. Although no treatment has so far been able to prevent the development or stop the progression of the disease, these encouraging advances point to G-protein and cAMP signaling as a promising avenue of investigation that may lead to more effective and safe treatments. This will require a better understanding of the relevant GPCRs, G-proteins, cAMP effectors, and of the enzymes and A-kinase anchoring proteins controlling the compartmentalization of cAMP signaling. The purpose of this review is to provide an overview of general GPCR signaling; the function of polycystin-1 (PC1) as a putative atypical adhesion GPCR (aGPCR); the roles of PC1, polycystin-2 (PC2) and the PC1-PC2 complex in the regulation of calcium and cAMP signaling; the cross-talk of calcium and cAMP signaling in PKD; and GPCRs, adenylyl cyclases, cyclic nucleotide phosphodiesterases, and protein kinase A as therapeutic targets in ADPKD.

Adhesion GPCRs as a paradigm for understanding polycystin-1 G protein regulation

Polycystin-1, whose mutation is the most frequent cause of autosomal dominant polycystic kidney disease, is an extremely large and multi-faceted membrane protein whose primary or proximal cyst-preventing function remains undetermined. Accumulating evidence supports the idea that modulation of cellular signaling by heterotrimeric G proteins is a critical function of polycystin-1. The presence of a cis-autocatalyzed, G protein-coupled receptor (GPCR) proteolytic cleavage site, or GPS, in its extracellular N-terminal domain immediately preceding the first transmembrane domain is one of the notable conserved features of the polycystin-1-like protein family, and also of the family of cell adhesion GPCRs. Adhesion GPCRs are one of five families within the GPCR superfamily and are distinguished by a large N-terminal extracellular region consisting of multiple adhesion modules with a GPS-containing GAIN domain and bimodal functions in cell adhesion and signal transduction. Recent advances from studies of adhesion GPCRs provide a new paradigm for unraveling the mechanisms by which polycystin-1-associated G protein signaling contributes to the pathogenesis of polycystic kidney disease. This review highlights the structural and functional features shared by polycystin-1 and the adhesion GPCRs and discusses the implications of such similarities for our further understanding of the functions of this complicated protein.

TMEM16A drives renal cyst growth by augmenting Ca2+ signaling in M1 cells

Abstract

Polycystic kidney disease (PKD) leads to continuous decline of renal function by growth of renal cysts. Enhanced proliferation and transepithelial chloride secretion through cystic fibrosis transmembrane conductance regulator (CFTR) and Ca²⁺-activated TMEM16A Cl⁻ channels is thought to cause an increase in cyst volume. Recent work shows the pro-proliferative role of the Ca²⁺ activated Cl⁻ channel TMEM16A (anoctamin 1), and demonstrates the essential contribution of TMEM16A to CFTR-dependent Cl⁻ secretion. The present data demonstrate an increase in intracellular Ca²⁺ ([Ca²⁺]i) signals and Cl⁻ secretion by TMEM16A, in renal collecting duct principle cells from dog (MDCK) and mouse (M1) as well as primary tubular epithelial cells from PKD1−/− knockout mice. M1 organoids proliferated, increased expression of TMEM16A, and secreted Cl⁻ upon knockdown of endogenous polycystin 1 or 2 (PKD1,2), by retroviral transfection with shPKD1 and shPKD2, respectively. Knockdown of PKD1 or PKD2 increased basal intracellular Ca²⁺ levels and enhanced purinergic Ca²⁺ release from endoplasmic reticulum. In contrast, ryanodine receptors were found not to be expressed in mouse renal epithelial cells and caffeine had no effects on [Ca²⁺]i. Ca²⁺ signals, proliferation, and Cl⁻ secretion were largely reduced by knockdown or blockade of TMEM16A. TMEM16A may be therefore important for enhanced Ca²⁺ release from IP₃-sensitive Ca²⁺ stores in polycystic kidney disease.

Key messages

• ADPKD leads to continuous decline of renal function by growth of renal cysts.

• Knockdown of PKD1 or PKD2 increases TMEM16A expression.

• TMEM16A enhanced intracellular Ca²⁺ signals, Cl⁻ secretion, and proliferation.

• TMEM16A contributes to cyst growth in ADPKD.

Electronic supplementary material

The online version of this article (10.1007/s00109-020-01894-y) contains supplementary material, which is available to authorized users.

New emerging roles of Polycystin-2 in the regulation of autophagy

Polycystin-2 (PC2) is a calcium channel that can be found in the endoplasmic reticulum, the plasmatic membrane, and the primary cilium. The structure of PC2 is characterized by a highly ordered C-terminal tail with an EF-motif (calcium-binding domain) and a canonical coiled-coil domain (CCD; interaction domain), and its activity is regulated by interacting partners and post-translational modifications. Calcium mobilization into the cytosol by PC2 has been mainly associated with cell growth and differentiation, and therefore mutations or dysfunction of PC2 lead to renal and cardiac consequences. Interestingly, PC2-related pathologies are usually treated with rapamycin, an autophagy stimulator. Autophagy is an intracellular degradation process where recycling material is sequestered into autophagosomes and then hydrolyzed by fusion with a lysosome. Interestingly, several studies have provided evidence that PC2 may be required for autophagy, suggesting that PC2 maintains a physiologic catabolic state.

Structure and function of polycystins: insights into polycystic kidney disease

Mutations in the polycystins PC1 or PC2 cause autosomal dominant polycystic kidney disease (ADPKD), which is characterized by the formation of fluid-filled renal cysts that disrupt renal architecture and function, ultimately leading to kidney failure in the majority of patients. Although the genetic basis of ADPKD is now well established, the physiological function of polycystins remains obscure and a matter of intense debate. The structural determination of both the homomeric PC2 and heteromeric PC1-PC2 complexes, as well as the electrophysiological characterization of PC2 in the primary cilium of renal epithelial cells, provided new valuable insights into the mechanisms of ADPKD pathogenesis. Current findings indicate that PC2 can function independently of PC1 in the primary cilium of renal collecting duct epithelial cells to form a channel that is mainly permeant to monovalent cations and is activated by both membrane depolarization and an increase in intraciliary calcium. In addition, PC2 functions as a calcium-activated calcium release channel at the endoplasmic reticulum membrane. Structural studies indicate that the heteromeric PC1-PC2 complex comprises one PC1 and three PC2 channel subunits. Surprisingly, several positively charged residues from PC1 occlude the ionic pore of the PC1-PC2 complex, suggesting that pathogenic polycystin mutations might cause ADPKD independently of an effect on channel permeation. Emerging reports of novel structural and functional findings on polycystins will continue to elucidate the molecular basis of ADPKD.

Fundamental insights into autosomal dominant polycystic kidney disease from human-based cell models

Several animal- and human-derived models are used in autosomal dominant polycystic kidney disease (ADPKD) research to gain insight in the disease mechanism. However, a consistent correlation between animal and human ADPKD models is lacking. Therefore, established human-derived models are relevant to affirm research results and translate findings into a clinical set-up. In this review, we give an extensive overview of the existing human-based cell models. We discuss their source (urine, nephrectomy and stem cell), immortalisation procedures, genetic engineering, kidney segmental origin and characterisation with nephron segment markers. We summarise the most studied pathways and lessons learned from these different ADPKD models. Finally, we issue recommendations for the derivation of human-derived cell lines and for experimental set-ups with these cell lines.

TMEM33 regulates intracellular calcium homeostasis in renal tubular epithelial cells

Mutations in the polycystins cause autosomal dominant polycystic kidney disease (ADPKD). Here we show that transmembrane protein 33 (TMEM33) interacts with the ion channel polycystin-2 (PC2) at the endoplasmic reticulum (ER) membrane, enhancing its opening over the whole physiological calcium range in ER liposomes fused to planar bilayers. Consequently, TMEM33 reduces intracellular calcium content in a PC2-dependent manner, impairs lysosomal calcium refilling, causes cathepsins translocation, inhibition of autophagic flux upon ER stress, as well as sensitization to apoptosis. Invalidation of TMEM33 in the mouse exerts a potent protection against renal ER stress. By contrast, TMEM33 does not influence pkd2-dependent renal cystogenesis in the zebrafish. Together, our results identify a key role for TMEM33 in the regulation of intracellular calcium homeostasis of renal proximal convoluted tubule cells and establish a causal link between TMEM33 and acute kidney injury.

A potential strategy for reducing cysts in autosomal dominant polycystic kidney disease with a CFTR corrector

Autosomal dominant polycystic kidney disease (ADPKD) is associated with progressive enlargement of cysts, leading to a decline in function and renal failure that cannot be prevented by current treatments. Mutations in pkd1 and pkd2, encoding the polycystin 1 and 2 proteins, induce growth-related pathways, including heat shock proteins, as occurs in some cancers, raising the prospect that pharmacological interventions that target these pathways might alleviate or prevent ADPKD. Here, we demonstrate a role for VX-809, a corrector of cystic fibrosis transmembrane conductance regulator (CFTR), conventionally used to manage cystic fibrosis in reducing renal cyst growth. VX-809 reduced cyst growth in Pkd1-knockout mice and in proximal, tubule-derived, cultured Pkd1 knockout cells. VX-809 reduced both basal and forskolin-activated cAMP levels and also decreased the expression of the adenylyl cyclase AC3 but not of AC6. VX-809 also decreased resting levels of intracellular Ca²⁺ but did not affect ATP-stimulated Ca²⁺ release. Notably, VX-809 dramatically decreased thapsigargin-induced release of Ca²⁺ from the endoplasmic reticulum (ER). VX-809 also reduced the levels of heat shock proteins Hsp27, Hsp70, and Hsp90 in mice cystic kidneys, consistent with the restoration of cellular proteostasis. Moreover, VX-809 strongly decreased an ER stress marker, the GADD153 protein, and cell proliferation but had only a small effect on apoptosis. Given that administration of VX-809 is safe, this drug potentially offers a new way to treat patients with ADPKD.

Activation of Calcium-Sensing Receptor increases intracellular calcium and decreases cAMP and mTOR in PKD1 deficient cells

Clinical and fundamental research suggest that altered calcium and cAMP signaling might be the most proximal events in ADPKD pathogenesis. Cells from ADPKD cysts have a reduced resting cytosolic calcium [Ca²⁺]_i and increased cAMP levels. CaSR plays an essential role in regulating calcium homeostasis. Its activation is associated with [Ca²⁺]_i increase and cAMP decrease, making CaSR a possible therapeutic target. Human conditionally immortalized Proximal Tubular Epithelial cells (ciPTEC) with stable knockdown of PKD1 (ciPTEC-PC1KD) and ciPTEC generated from an ADPKD1 patient (ciPTEC-PC1Pt) were used as experimental tools. CaSR functional expression was confirmed by studies showing that the calcimimetic NPS-R568 induced a significant increase in [Ca²⁺]_i in ciPTEC-PC1KD and ciPTEC-PC1Pt. Resting [Ca²⁺]_i were significantly lower in ciPTEC-PC1KD with respect to ciPTECwt, confirming calcium dysregulation. As in native cyst cells, significantly higher cAMP levels and mTOR activity were found in ciPTEC-PC1KD compared to ciPTECwt. Of note, NPS-R568 treatment significantly reduced intracellular cAMP and mTOR activity in ciPTEC-PC1KD and ciPTEC-PC1Pt. To conclude, we demonstrated that selective CaSR activation in human ciPTEC carrying PKD1 mutation increases [Ca²⁺]_i, reduces intracellular cAMP and mTOR activity, reversing the principal dysregulations considered the most proximal events in ADPKD pathogenesis, making CaSR a possible candidate as therapeutic target.

Ryanodine Receptors in Autophagy: Implications for Neurodegenerative Diseases?

Intracellular Ca²⁺ signaling is important in the regulation of several cellular processes including autophagy. The endoplasmic reticulum (ER) is the main and largest intracellular Ca²⁺ store. At the ER two protein families of Ca²⁺ release channels, inositol 1,4,5-trisphosphate receptors (IP₃Rs) and ryanodine receptors (RyRs), are expressed. Several studies have reported roles in the regulation of autophagy for the ubiquitously expressed IP₃R. For instance, IP₃R-mediated Ca²⁺ release supresses basal autophagic flux by promoting mitochondrial metabolism, while also promoting the rapid initial increase in autophagic flux in response to nutrient starvation. Insights into the contribution of RyRs in autophagy have been lagging significantly compared to the advances made for IP₃Rs. This is rather surprising considering that RyRs are predominantly expressed in long-lived cells with specialized metabolic needs, such as neurons and muscle cells, in which autophagy plays important roles. In this review article, recent studies revealing roles for RyRs in the regulation of autophagy will be discussed. Several RyR-interacting proteins that have been established to modulate both RyR function and autophagy will also be highlighted. Finally, the involvement of RyRs in neurodegenerative diseases will be addressed. Inhibition of RyR channels has not only been shown to be beneficial for treating several of these diseases but also regulates autophagy.

The Sorting Nexin 3 Retromer Pathway Regulates the Cell Surface Localization and Activity of a Wnt-Activated Polycystin Channel Complex

Autosomal dominant polycystic kidney disease (ADPKD) is caused by inactivating mutations in PKD1 (85%) or PKD2 (15%). The ADPKD proteins encoded by these genes, polycystin-1 (PC1) and polycystin-2 (PC2), form a plasma membrane receptor-ion channel complex. However, the mechanisms controlling the subcellular localization of PC1 and PC2 are poorly understood. Here, we investigated the involvement of the retromer complex, an ancient protein module initially discovered in yeast that regulates the retrieval, sorting, and retrograde transport of membrane receptors. Using yeast two-hybrid, biochemical, and cellular assays, we determined that PC2 binds two isoforms of the retromer-associated protein sorting nexin 3 (SNX3), including a novel isoform that binds PC2 in a direct manner. Knockdown of SNX3 or the core retromer protein VPS35 increased the surface expression of endogenous PC1 and PC2 in vitro and in vivo and increased Wnt-activated PC2-dependent whole-cell currents. These findings indicate that an SNX3-retromer complex regulates the surface expression and function of PC1 and PC2. Molecular targeting of proteins involved in the endosomal sorting of PC1 and PC2 could lead to new therapeutic approaches in ADPKD.

Histone deacetylase 6 inhibition reduces cysts by decreasing cAMP and Ca2+ in knock-out mouse models of polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is associated with progressive enlargement of multiple renal cysts, often leading to renal failure that cannot be prevented by a current treatment. Two proteins encoded by two genes are associated with ADPKD: PC1 (pkd1), primarily a signaling molecule, and PC2 (pkd2), a Ca²⁺ channel. Dysregulation of cAMP signaling is central to ADPKD, but the molecular mechanism is unresolved. Here, we studied the role of histone deacetylase 6 (HDAC6) in regulating cyst growth to test the possibility that inhibiting HDAC6 might help manage ADPKD. Chemical inhibition of HDAC6 reduced cyst growth in PC1-knock-out mice. In proximal tubule-derived, PC1-knock-out cells, adenylyl cyclase 6 and 3 (AC6 and -3) are both expressed. AC6 protein expression was higher in cells lacking PC1, compared with control cells containing PC1. Intracellular Ca²⁺ was higher in PC1-knock-out cells than in control cells. HDAC inhibition caused a drop in intracellular Ca²⁺ and increased ATP-simulated Ca²⁺ release. HDAC6 inhibition reduced the release of Ca²⁺ from the endoplasmic reticulum induced by thapsigargin, an inhibitor of endoplasmic reticulum Ca²⁺-ATPase. HDAC6 inhibition and treatment of cells with the intracellular Ca²⁺ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) reduced cAMP levels in PC1-knock-out cells. Finally, the calmodulin inhibitors W-7 and W-13 reduced cAMP levels, and W-7 reduced cyst growth, suggesting that AC3 is involved in cyst growth regulated by HDAC6. We conclude that HDAC6 inhibition reduces cell growth primarily by reducing intracellular cAMP and Ca²⁺ levels. Our results provide potential therapeutic targets that may be useful as treatments for ADPKD.

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.

Regulation of Polycystin-1 Function by Calmodulin Binding

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a common genetic disease that leads to progressive renal cyst growth and loss of renal function, and is caused by mutations in the genes encoding polycystin-1 (PC1) and polycystin-2 (PC2), respectively. The PC1/PC2 complex localizes to primary cilia and can act as a flow-dependent calcium channel in addition to numerous other signaling functions. The exact functions of the polycystins, their regulation and the purpose of the PC1/PC2 channel are still poorly understood. PC1 is an integral membrane protein with a large extracytoplasmic N-terminal domain and a short, ~200 amino acid C-terminal cytoplasmic tail. Most proteins that interact with PC1 have been found to bind via the cytoplasmic tail. Here we report that the PC1 tail has homology to the regulatory domain of myosin heavy chain including a conserved calmodulin-binding motif. This motif binds to CaM in a calcium-dependent manner. Disruption of the CaM-binding motif in PC1 does not affect PC2 binding, cilia targeting, or signaling via heterotrimeric G-proteins or STAT3. However, disruption of CaM binding inhibits the PC1/PC2 calcium channel activity and the flow-dependent calcium response in kidney epithelial cells. Furthermore, expression of CaM-binding mutant PC1 disrupts cellular energy metabolism. These results suggest that critical functions of PC1 are regulated by its ability to sense cytosolic calcium levels via binding to CaM.

STIM1fl/fl Ksp-Cre Mouse has Impaired Renal Water Balance

BACKGROUND/AIM

STIM1 is as an essential component in store operated Ca2+ entry. However give the paucity of information on the role of STIM1 in kidney, the aim was to study the function of STIM1 in the medulla of the kidney.

METHODS

we crossed a Ksp-cre mouse with another mouse containing two loxP sites flanking Exon 6 of STIM1. The Ksp-cre mouse is based upon the Ksp-cadherin gene promoter which expresses cre recombinase in developing nephrons, collecting ducts (SD) and thick ascending limbs (TAL) of the loop of Henle.

RESULTS

The offspring of these mice are viable without gross morphological changes, however, we noticed that the STIM1 Ksp-cre knockout mice produced more urine compared to control. To examine this more carefully, we fed mice low (LP) and high protein (HP) diets respectively. When mice were fed HP diet STIM1 ko mice had significantly increased urinary volume and lower specific gravity compared to wt mice. In STIM1 ko mice fed HP diet urine creatinine and urea were significantly lower compared to wt mice fed HP diet, however the fractional excretion was the same.

CONCLUSION

These data support the idea that STIM1 ko mice have impaired urinary concentrating ability when challenged with HP diet is most likely caused by impaired Ca2+-dependent signal transduction through the vasopressin receptor cascade.

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 Role of G-Protein-Coupled Receptor Proteolysis Site Cleavage of Polycystin-1 in Renal Physiology and Polycystic Kidney Disease

Polycystin-1 (PC1) plays an essential role in renal tubular morphogenesis, and PC1 dysfunction causes human autosomal dominant polycystic kidney disease. A fundamental characteristic of PC1 is post-translational modification via cleavage at the juxtamembrane GPCR proteolysis site (GPS) motif that is part of the larger GAIN domain. Given the considerable biochemical complexity of PC1 molecules generated in vivo by this process, GPS cleavage has several profound implications on the intracellular trafficking and localization in association with their particular function. The critical nature of GPS cleavage is further emphasized by the increasing numbers of PKD1 mutations that significantly affect this cleavage process. The GAIN domain with the GPS motif therefore represents the key structural element with fundamental importance for PC1 and might be polycystic kidney disease’s (PKD) Achilles’ heel in a large spectrum of PKD1 missense mutations. We highlight the central roles of PC1 cleavage for the regulation of its biogenesis, intracellular trafficking and function, as well as its significance in polycystic kidney disease.

Role of calcium in polycystic kidney disease: From signaling to pathology

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited monogenic kidney disease. Characterized by the development and growth of cysts that cause progressive kidney enlargement, it ultimately leads to end-stage renal disease. Approximately 85% of ADPKD cases are caused by mutations in the PKD1 gene, while mutations in the PKD2 gene account for the remaining 15% of cases. The PKD1 gene encodes for polycystin-1 (PC1), a large multi-functional membrane receptor protein able to regulate ion channel complexes, whereas polycystin-2 (PC2), encoded by the PKD2 gene, is an integral membrane protein that functions as a calcium-permeable cation channel, located mainly in the endoplasmic reticulum (ER). In the primary cilia of the epithelial cells, PC1 interacts with PC2 to form a polycystin complex that acts as a mechanosensor, regulating signaling pathways involved in the differentiation of kidney tubular epithelial cells. Despite progress in understanding the function of these proteins, the molecular mechanisms associated with the pathogenesis of ADPKD remain unclear. In this review we discuss how an imbalance between functional PC1 and PC2 proteins may disrupt calcium channel activities in the cilium, plasma membrane and ER, thereby altering intracellular calcium signaling and leading to the aberrant cell proliferation and apoptosis associated with the development and growth of renal cysts. Research in this field could lead to the discovery of new molecules able to rebalance intracellular calcium, thereby normalizing cell proliferation and reducing kidney cyst progression.

Genetic Mechanisms of ADPKD

Autosomal dominant polycystic kidney disease is caused by mutation of PKD1 (polycystic kidney disease-1) or PKD2 (polycystic kidney disease-2). PKD1 and PKD2 encode PC1 (polycystin-1) and PC2 (polycystin-2), respectively. In addition, the mutation of cilia-associated proteins is also a recognized major factor of pathogenesis, since PC1 and PC2 are located in primary cilium. Abnormalities of PC1 or PC2 lead to aberrant signaling through downstream pathways, such as the negative growth regulation, G protein activation, and canonical and non-canonical Wnt pathways. According to the "second hit" model, an additional somatic mutation results in the expansion of cyst growth. In this chapter we discuss the genetic mechanisms and signaling pathways involved in ADPKD.

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.

Vasopressin and disruption of calcium signalling in polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common monogenic kidney disease and is responsible for 5-10% of cases of end-stage renal disease worldwide. ADPKD is characterized by the relentless development and growth of cysts, which cause progressive kidney enlargement associated with hypertension, pain, reduced quality of life and eventual kidney failure. Mutations in the PKD1 or PKD2 genes, which encode polycystin-1 (PC1) and polycystin-2 (PC2), respectively, cause ADPKD. However, neither the functions of these proteins nor the molecular mechanisms of ADPKD pathogenesis are well understood. Here, we review the literature that examines how reduced levels of functional PC1 or PC2 at the primary cilia and/or the endoplasmic reticulum directly disrupts intracellular calcium signalling and indirectly disrupts calcium-regulated cAMP and purinergic signalling. We propose a hypothetical model in which dysregulated metabolism of cAMP and purinergic signalling increases the sensitivity of principal cells in collecting ducts and of tubular epithelial cells in the distal nephron to the constant tonic action of vasopressin. The resulting magnified response to vasopressin further enhances the disruption of calcium signalling that is initiated by mutations in PC1 or PC2, and activates downstream signalling pathways that cause impaired tubulogenesis, increased cell proliferation, increased fluid secretion and interstitial inflammation.

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.

Strategies targeting cAMP signaling in the treatment of polycystic kidney disease

Polycystic kidney disease (PKD) is a leading cause of ESRD worldwide. In PKD, excessive cell proliferation and fluid secretion, pathogenic interactions of mutated epithelial cells with an abnormal extracellular matrix and alternatively activated interstitial macrophages, and the disruption of mechanisms controlling tubular diameter contribute to cyst formation. Studies with animal models suggest that several diverse pathophysiologic mechanisms, including dysregulation of intracellular calcium levels and cAMP signaling, mediate these cystogenic mechanisms. This article reviews the evidence implicating calcium and cAMP as central players in a network of signaling pathways underlying the pathogenesis of PKD and considers the therapeutic relevance of treatment strategies targeting cAMP signaling.

Hyperphosphorylation of polycystin-2 at a critical residue in disease reveals an essential role for polycystin-1-regulated dephosphorylation

Mutations in PKD1 (85%) or PKD2 (15%) account for almost all cases of autosomal dominant polycystic kidney disease (ADPKD). The ADPKD proteins, termed as polycystin-1 (PC1) and polycystin-2 (PC2), interact via their C-termini to form a receptor-ion channel complex whose function and regulation are not fully understood. Here, we report the first phosphorylated residue (Ser(829)) in PC2, whose dephosphorylation is mediated by PC1 binding through the recruitment of protein phosphatase-1 alpha (PP1α). Using a new phosphospecific antibody (pPC2) to this site, we demonstrate that Ser(829) is phosphorylated by Protein kinase A (PKA) but remains constitutively phosphorylated in cells and tissues lacking PC1. cAMP increased pSer(829) basolateral localization in MDCK cells in a time dependent manner and was essential for pronephric development in Xenopus embryos. When constitutively expressed, a complex phenotype associated with enhanced ATP-dependent ER Ca(2+) release and loss of growth suppression was observed in cycling cells. These results reveal a reciprocal functional link between PC1 and PC2 which is critically dependent on their interaction. Unopposed cAMP stimulated hyperphosphorylation of PC2 in the absence of functional PC1 could contribute to cyst initiation in PKD1 patients and represents a new molecular paradigm in understanding ADPKD pathogenesis.

The Roles of Primary cilia in Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD) is an inherited genetic disorder that results in progressive renal cyst formation with ultimate loss of renal function and other systemic disorders. These systemic disorders include abnormalities in cardiovascular, portal, pancreatic and gastrointestinal systems. ADPKD is considered to be among the ciliopathy diseases due to the association with abnormal primary cilia function. In order to understand the full course of primary cilia and its association with ADPKD, the structure, functions and role of primary cilia have been meticulously investigated. As a result, the focus on primary cilia has emerged to support the vital roles of primary cilia in ADPKD. The primary cilia have been shown to have not only a mechanosensory function but also a chemosensory function. Both structural and functional defects in primary cilia result in cystic kidney disease and vascular hypertension. Thus, the mechanosenory and chemosensory functions will be analyzed in regards to ADPKD.

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.

Polycystin-1 and polycystin-2 are both required to amplify inositol-trisphosphate-induced Ca2+ release

Autosomal dominant polycystic kidney disease is caused by loss-of-function mutations in the PKD1 or PKD2 genes encoding respectively polycystin-1 and polycystin-2. Polycystin-2 stimulates the inositol trisphosphate (IP(3)) receptor (IP(3)R), a Ca(2+)-release channel in the endoplasmic reticulum (ER). The effect of ER-located polycystin-1 is less clear. Polycystin-1 has been reported both to stimulate and to inhibit the IP(3)R. We now studied the effect of polycystin-1 and of polycystin-2 on the IP(3)R activity under conditions where the cytosolic Ca(2+) concentration was kept constant and the reuptake of released Ca(2+) was prevented. We also studied the interdependence of the interaction of polycystin-1 and polycystin-2 with the IP(3)R. The experiments were done in conditionally immortalized human proximal-tubule epithelial cells in which one or both polycystins were knocked down using lentiviral vectors containing miRNA-based short hairpins. The Ca(2+) release was induced in plasma membrane-permeabilized cells by various IP(3) concentrations at a fixed Ca(2+) concentration under unidirectional (45)Ca(2+)-efflux conditions. We now report that knock down of polycystin-1 or of polycystin-2 inhibited the IP(3)-induced Ca(2+) release. The simultaneous presence of the two polycystins was required to fully amplify the IP(3)-induced Ca(2+) release, since the presence of polycystin-1 alone or of polycystin-2 alone did not result in an increased Ca(2+) release. These novel findings indicate that ER-located polycystin-1 and polycystin-2 operate as a functional complex. They are compatible with the view that loss-of-function mutations in PKD1 and in PKD2 both cause autosomal dominant polycystic kidney disease.

Calcium Channels in the Development, Maturation, and Function of Spermatozoa

A proper dialogue between spermatozoa and the egg is essential for conception of a new individual in sexually reproducing animals. Ca2+ is crucial in orchestrating this unique event leading to a new life. No wonder that nature has devised different Ca2+-permeable channels and located them at distinct sites in spermatozoa so that they can help fertilize the egg. New tools to study sperm ionic currents, and image intracellular Ca2+ with better spatial and temporal resolution even in swimming spermatozoa, are revealing how sperm ion channels participate in fertilization. This review critically examines the involvement of Ca2+ channels in multiple signaling processes needed for spermatozoa to mature, travel towards the egg, and fertilize it. Remarkably, these tiny specialized cells can express exclusive channels like CatSper for Ca2+ and SLO3 for K+, which are attractive targets for contraception and for the discovery of novel signaling complexes. Learning more about fertilization is a matter of capital importance; societies face growing pressure to counteract rising male infertility rates, provide safe male gamete-based contraceptives, and preserve biodiversity through improved captive breeding and assisted conception initiatives.

Polycystin-1, 2, and STIM1 interact with IP(3)R to modulate ER Ca release through the PI3K/Akt pathway

Dysregulation of Ca(2+) signaling and homeostasis has been linked to the development of ADPKD through aberrant functioning of the polycystins. In this study, we investigated the role of the polycystins in modulating Ca(2+) signaling. Expression of full-length PC1 in MDCK cells inhibited intracellular Ca(2+) release in response to ATP when compared to control cells. This phenotype correlated with reduced interaction of endogenous PC2 and IP(3)R in PC1-containing cells. We also found that endogenous STIM1 also interacted with the IP(3)R, and this interaction was enhanced by PC1 expression. Increased interaction between STIM1 and IP(3)R inhibited Ca(2+) release. PC1 regulates intracellular Ca(2+) release and the interaction of PC2-IP(3)R-STIM1 through the PI3K/Akt signaling pathway. Inhibition of the PI3K/Akt pathway in PC1 containing cells restored intracellular Ca(2+) release, increased the interaction between PC2 and IP(3)R and disrupted the STIM1-IP(3)R complex. Conversely, activation of the PI3K/Akt signaling pathway by HGF in control MDCK cells gave the reverse effects. It reduced the release of Ca(2+) to levels comparable to the PC1 cells, inhibited the association PC2 and IP(3)R, and increased the interaction between STIM and IP(3)R. Overall, our studies provide a potential mechanism for the modulation of intracellular Ca(2+) signaling by the polycystins.

Putative roles of cilia in polycystic kidney disease

The last 10 years has witnessed an explosion in research into roles of cilia in cystic renal disease. Cilia are membrane-enclosed finger-like projections from the cell, usually on the apical surface or facing into a lumen, duct or airway. Ten years ago, the major recognised functions related to classical "9+2" cilia in the respiratory and reproductive tracts, where co-ordinated beating clears secretions and assists fertilisation respectively. Primary cilia, which have a "9+0" arrangement lacking the central microtubules, were anatomical curiosities but several lines of evidence have implicated them in both true polycystic kidney disease and other cystic renal conditions: ranging from the homology between Caenorhabditis elegans proteins expressed on sensory cilia to mammalian polycystic kidney disease (PKD) 1 and 2 proteins, through the discovery that orpk cystic mice have structurally abnormal cilia to numerous recent studies wherein expression of nearly all cyst-associated proteins has been reported in the cilia or its basal body. Functional studies implicate primary cilia in mechanosensation, photoreception and chemosensation but it is the first of these which appears most important in polycystic kidney disease: in the simplest model, fluid flow across the apical surface of renal cells bends the cilia and induces calcium influx, and this is perturbed in polycystic kidney disease. Downstream effects include changes in cell differentiation and polarity. Pathways such as hedgehog and Wnt signalling may also be regulated by cilia. These data support important roles for cilia in the pathogenesis of cystic kidney diseases but one must not forget that the classic polycystic kidney disease proteins are expressed in several other locations where they may have equally important roles, such as in cell-cell and cell-matrix interactions, whilst it is not just aberrant cilia signalling that can lead to de-differentiation, loss of polarity and other characteristic features of polycystic kidney disease. Understanding how cilia fit into the other aspects of polycystic kidney disease biology is the challenge for the next decade. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Effect of PKD1 gene missense mutations on polycystin-1 membrane topogenesis

Polycystin-1 (PC1), the product of the polycystic kidney disease-1 (PKD1) gene, has a number of reported missense mutations whose pathogenicity is indeterminate. Previously, we utilized N-linked glycosylation reporter tags along with membrane insertion and topology assays to define the 11 membrane-spanning domains (I-XI) of PC1. In this report, we utilize glycosylation assays to determine whether two reported human polymorphisms/missense mutations within transmembrane (TM) domains VI and X affect the membrane topology of PC1. M3677T within TM VI had no effect on the topology of this TM domain as shown by the ability of two native N-linked glycosylation sites within the extracellular loop following TM VI to be glycosylated. In contrast, G4031D, within TM X, decreased the glycosylation of TM X reporter constructs, demonstrating that the substitution affected the C-terminal translocating activity of TM X. Furthermore, G4031D reduced the membrane association of TM X and XI together. These results suggest that G4031D affects the membrane insertion and topology of the C-terminal portion of polycystin-1 and represents a bona fide pathogenic mutation.

The cell biology of polycystic kidney disease

Polycystic kidney disease is a common genetic disorder in which fluid-filled cysts displace normal renal tubules. Here we focus on autosomal dominant polycystic kidney disease, which is attributable to mutations in the PKD1 and PKD2 genes and which is characterized by perturbations of renal epithelial cell growth control, fluid transport, and morphogenesis. The mechanisms that connect the underlying genetic defects to disease pathogenesis are poorly understood, but their exploration is shedding new light on interesting cell biological processes and suggesting novel therapeutic targets.

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.

Calcium-mediated mechanisms of cystic expansion

In this review, we will discuss several well-accepted signaling pathways toward calcium-mediated mechanisms of cystic expansion. The second messenger calcium ion has contributed to a vast diversity of signal transduction pathways. We will dissect calcium signaling as a possible mechanism that contributes to renal cyst formation. Because cytosolic calcium also regulates an array of signaling pathways, we will first discuss cilia-induced calcium fluxes, followed by Wnt signaling that has attributed to much-discussed planar cell polarity. We will then look at the relationship between cytosolic calcium and cAMP as one of the most important aspects of cyst progression. The signaling of cAMP on MAPK and mTOR will also be discussed. We infer that while cilia-induced calcium fluxes may be the initial signaling messenger for various cellular pathways, no single signaling mediator or pathway is implicated exclusively in the progression of the cystic expansion. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Identification of a polycystin-1 cleavage product, P100, that regulates store operated Ca entry through interactions with STIM1

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a genetic disorder resulting in large kidney cysts and eventual kidney failure. Mutations in either the PKD1 or PKD2/TRPP2 genes and their respective protein products, polycystin-1 (PC1) and polycystin-2 (PC2) result in ADPKD. PC2 is known to function as a non-selective cation channel, but PC1's function and the function of PC1 cleavage products are not well understood. Here we identify an endogenous PC1 cleavage product, P100, a 100 kDa fragment found in both wild type and epitope tagged PKD1 knock-in mice. Expression of full length human PC1 (FL PC1) and the resulting P100 and C-Terminal Fragment (CTF) cleavage products in both MDCK and CHO cells significantly reduces the store operated Ca(2+) entry (SOCE) resulting from thapsigargin induced store depletion. Exploration into the roles of P100 and CTF in SOCE inhibition reveal that P100, when expressed in Xenopus laevis oocytes, directly inhibits the SOCE currents but CTF does not, nor does P100 when containing the disease causing R4227X mutation. Interestingly, we also found that in PC1 expressing MDCK cells, translocation of the ER Ca(2+) sensor protein STIM1 to the cell periphery was significantly altered. In addition, P100 Co-immunoprecipitates with STIM1 but CTF does not. The expression of P100 in CHO cells recapitulates the STIM1 translocation inhibition seen with FL PC1. These data describe a novel polycystin-1 cleavage product, P100, which functions to reduce SOCE via direct inhibition of STIM1 translocation; a function with consequences for ADPKD.

Polycystin-2 activation by inositol 1,4,5-trisphosphate-induced Ca2+ release requires its direct association with the inositol 1,4,5-trisphosphate receptor in a signaling microdomain

Autosomal dominant polycystic kidney disease is characterized by the loss-of-function of a signaling complex involving polycystin-1 and polycystin-2 (TRPP2, an ion channel of the TRP superfamily), resulting in a disturbance in intracellular Ca(2+) signaling. Here, we identified the molecular determinants of the interaction between TRPP2 and the inositol 1,4,5-trisphosphate receptor (IP(3)R), an intracellular Ca(2+) channel in the endoplasmic reticulum. Glutathione S-transferase pulldown experiments combined with mutational analysis led to the identification of an acidic cluster in the C-terminal cytoplasmic tail of TRPP2 and a cluster of positively charged residues in the N-terminal ligand-binding domain of the IP(3)R as directly responsible for the interaction. To investigate the functional relevance of TRPP2 in the endoplasmic reticulum, we re-introduced the protein in TRPP2(-/-) mouse renal epithelial cells using an adenoviral expression system. The presence of TRPP2 resulted in an increased agonist-induced intracellular Ca(2+) release in intact cells and IP(3)-induced Ca(2+) release in permeabilized cells. Using pathological mutants of TRPP2, R740X and D509V, and competing peptides, we demonstrated that TRPP2 amplified the Ca(2+) signal by a local Ca(2+)-induced Ca(2+)-release mechanism, which only occurred in the presence of the TRPP2-IP(3)R interaction, and not via altered IP(3)R channel activity. Moreover, our results indicate that this interaction was instrumental in the formation of Ca(2+) microdomains necessary for initiating Ca(2+)-induced Ca(2+) release. The data strongly suggest that defects in this mechanism may account for the altered Ca(2+) signaling associated with pathological TRPP2 mutations and therefore contribute to the development of autosomal dominant polycystic kidney disease.