Cellular activation triggered by the autosomal dominant polycystic kidney disease gene product PKD2

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.

Metformin improves relevant disease parameters in an autosomal dominant polycystic kidney disease mouse model

Autosomal dominant polycystic kidney disease (ADPKD), caused by mutations in the polycystin 1 (PKD1) or polycystin 2 genes, presents with progressive development of kidney cysts and eventual end-stage kidney disease with limited treatment options. Previous work has shown that metformin reduces cyst growth in rapid ADPKD mouse models via inhibition of cystic fibrosis transmembrane conductance regulator-mediated fluid secretion, mammalian target of rapamycin, and cAMP pathways. The present study importantly tested the effectiveness of metformin as a therapy for ADPKD in a more clinically relevant Pkd1RC/RC mouse model, homozygous for the R3277C knockin point mutation in the Pkd1 gene. This mutation causes ADPKD in humans. Pkd1RC/RC male and female mice, which have a slow progression to end-stage kidney disease, received metformin (300 mg/kg/day in drinking water vs. water alone) from 3 to 9 or 12 mo of age. As previously reported, Pkd1RC/RC females had a more severe disease phenotype as compared with males. Metformin treatment reduced the ratio of total kidney weight-to-body weight relative to age-matched and sex-matched untreated controls at both 9 and 12 mo and reduced the cystic index in females at 9 mo. Metformin also increased glomerular filtration rate, lowered systolic blood pressure, improved anemia, and lowered blood urea nitrogen levels relative to controls in both sexes. Moreover, metformin reduced gene expression of key inflammatory markers and both gene and protein expression of kidney injury marker-1 and cyclin-dependent kinase-1 versus untreated controls. Altogether, these findings suggest several beneficial effects of metformin in this highly relevant slowly progressive ADPKD mouse model, which may help inform new ADPKD therapies in patients.NEW & NOTEWORTHY Metformin treatment improved ADPKD disease severity in a relevant, slowly progressive ADPKD mouse model that recapitulates a PKD-associated PKD1 mutation. Relative to controls, metformin reduced kidney weight/body weight, cystic index and BUN levels, while improving GFR, blood pressure and anemia. Metformin also reduced key inflammatory and injury markers, along with cell proliferation markers. These findings suggest several beneficial effects of metformin in this ADPKD mouse model, which may help inform new ADPKD therapies in patients.

c-Jun N-terminal kinase (JNK) signaling contributes to cystic burden in polycystic kidney disease

Polycystic kidney disease is an inherited degenerative disease in which the uriniferous tubules are replaced by expanding fluid-filled cysts that ultimately destroy organ function. Autosomal dominant polycystic kidney disease (ADPKD) is the most common form, afflicting approximately 1 in 1,000 people. It primarily is caused by mutations in the transmembrane proteins polycystin-1 (Pkd1) and polycystin-2 (Pkd2). The most proximal effects of Pkd mutations leading to cyst formation are not known, but pro-proliferative signaling must be involved for the tubule epithelial cells to increase in number over time. The c-Jun N-terminal kinase (JNK) pathway promotes proliferation and is activated in acute and chronic kidney diseases. Using a mouse model of cystic kidney disease caused by Pkd2 loss, we observe JNK activation in cystic kidneys and observe increased nuclear phospho c-Jun in cystic epithelium. Genetic removal of Jnk1 and Jnk2 suppresses the nuclear accumulation of phospho c-Jun, reduces proliferation and reduces the severity of cystic disease. While Jnk1 and Jnk2 are thought to have largely overlapping functions, we find that Jnk1 loss is nearly as effective as the double loss of Jnk1 and Jnk2. Jnk pathway inhibitors are in development for neurodegeneration, cancer, and fibrotic diseases. Our work suggests that the JNK pathway should be explored as a therapeutic target for ADPKD.

Autosomal dominant polycystic kidney disease is a leading cause of end stage renal disease requiring dialysis or kidney transplant. During disease development, the cells lining the kidney tubules proliferate. This proliferation transforms normally small diameter tubules into fluid-filled cysts that enlarge with time, eventually destroying all kidney function. Despite decades of research, polycystic kidney disease remains incurable. Furthermore, the precise signaling events involved in cyst initiation and growth remain unclear. The c-Jun N-terminal kinase (JNK), is a major pathway regulating cellular proliferation and differentiation but its importance to polycystic kidney disease was not known. We show that JNK activity is elevated in cystic kidneys and that reducing JNK activity decreases cyst growth pointing to JNK inhibition as a therapeutic strategy for treating polycystic kidney disease.

Polycystin-1 regulates bone development through an interaction with the transcriptional coactivator TAZ

Polycystin-1 (PC1), encoded by the PKD1 gene that is mutated in the autosomal dominant polycystic kidney disease, regulates a number of processes including bone development. Activity of the transcription factor RunX2, which controls osteoblast differentiation, is reduced in Pkd1 mutant mice but the mechanism governing PC1 activation of RunX2 is unclear. PC1 undergoes regulated cleavage that releases its C-terminal tail (CTT), which translocates to the nucleus to modulate transcriptional pathways involved in proliferation and apoptosis. We find that the cleaved CTT of PC1 (PC1-CTT) stimulates the transcriptional coactivator TAZ (Wwtr1), an essential coactivator of RunX2. PC1-CTT physically interacts with TAZ, stimulating RunX2 transcriptional activity in pre-osteoblast cells in a TAZ-dependent manner. The PC1-CTT increases the interaction between TAZ and RunX2 and enhances the recruitment of the p300 transcriptional co-regulatory protein to the TAZ/RunX2/PC1-CTT complex. Zebrafish injected with morpholinos directed against pkd1 manifest severe bone calcification defects and a curly tail phenotype. Injection of messenger RNA (mRNA) encoding the PC1-CTT into pkd1-morphant fish restores bone mineralization and reduces the severity of the curly tail phenotype. These effects are abolished by co-injection of morpholinos directed against TAZ. Injection of mRNA encoding a dominant-active TAZ construct is sufficient to rescue both the curly tail phenotype and the skeletal defects observed in pkd1-morpholino treated fish. Thus, TAZ constitutes a key mechanistic link through which PC1 mediates its physiological functions.

Regulation of protein-coding gene and long noncoding RNA pairs in liver of conventional and germ-free mice following oral PBDE exposure

Gut microbiome communicates with the host liver to modify hepatic xenobiotic biotransformation and nutrient homeostasis. Polybrominated diphenyl ethers (PBDEs) are persistent environmental contaminants that are detected in fatty food, household dust, and human breast milk at worrisome levels. Recently, long noncoding RNAs (lncRNAs) have been recognized as novel biomarkers for toxicological responses and may regulate the transcriptional/translational output of protein-coding genes (PCGs). However, very little is known regarding to what extent the interactions between PBDEs and gut microbiome modulate hepatic lncRNAs and PCGs, and what critical signaling pathways are impacted at the transcriptomic scale. In this study, we performed RNA-Seq in livers of nine-week-old male conventional (CV) and germ-free (GF) mice orally exposed to the most prevalent PBDE congeners BDE-47 and BDE-99 (100 μmol/kg once daily for 4-days; vehicle: corn oil, 10 ml/kg), and unveiled key molecular pathways and PCG-lncRNA pairs targeted by PBDE-gut microbiome interactions. Lack of gut microbiome profoundly altered the PBDE-mediated transcriptomic response in liver, with the most prominent effect observed in BDE-99-exposed GF mice. The top pathways up-regulated by PBDEs were related to xenobiotic metabolism, whereas the top pathways down-regulated by PBDEs were in lipid metabolism and protein synthesis in both enterotypes. Genomic annotation of the differentially regulated lncRNAs revealed that majority of these lncRNAs overlapped with introns and 3'-UTRs of PCGs. Lack of gut microbiome profoundly increased the percentage of PBDE-regulated lncRNAs mapped to the 3'-UTRs of PCGs, suggesting the potential involvement of lncRNAs in increasing the translational efficiency of PCGs by preventing miRNA-3'-UTR binding, as a compensatory mechanism following toxic exposure to PBDEs. Pathway analysis of PCGs paired with lncRNAs revealed that in CV mice, BDE-47 regulated nucleic acid and retinol metabolism, as well as circadian rhythm; whereas BDE-99 regulated fatty acid metabolism. In GF mice, BDE-47 differentially regulated 19 lncRNA-PCG pairs that were associated with glutathione conjugation and transcriptional regulation. In contrast, BDE-99 up-regulated the xenobiotic-metabolizing Cyp3a genes, but down-regulated the fatty acid-metabolizing Cyp4 genes. Taken together, the present study reveals common and unique lncRNAs and PCG targets of PBDEs in mouse liver, and is among the first to show that lack of gut microbiome sensitizes the liver to toxic exposure of BDE-99 but not BDE-47. Therefore, lncRNAs may serve as specific biomarkers that differentiate various PBDE congeners as well as environmental chemical-mediated dysbiosis. Coordinate regulation of PCG-lncRNA pairs may serve as a more efficient molecular mechanism to combat against xenobiotic insult, and especially during dysbiosis-induced increase in the internal dose of toxicants.

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.

Polycystin-1 cleavage and the regulation of transcriptional pathways

Autosomal dominant polycystic kidney disease (ADPKD) is the most common genetic cause of end-stage renal disease, affecting approximately 1 in 1,000 people. The disease is characterized by the development of numerous large fluid-filled renal cysts over the course of decades. These cysts compress the surrounding renal parenchyma and impair its function. Mutations in two genes are responsible for ADPKD. The protein products of both of these genes, polycystin-1 and polycystin-2, localize to the primary cilium and participate in a wide variety of signaling pathways. Polycystin-1 undergoes several proteolytic cleavages that produce fragments which manifest biological activities. Recent results suggest that the production of polycystin-1 cleavage fragments is necessary and sufficient to account for at least some, although certainly not all, of the physiological functions of the parent protein.

Fluid shear stress induces renal epithelial gene expression through polycystin-2-dependent trafficking of extracellular regulated kinase

BACKGROUND

The cilium and cilial proteins have emerged as principal mechanosensors of renal epithelial cells responsible for translating mechanical forces into intracellular signals. Polycystin-2 (PC-2), a cilial protein, regulates flow/shear-induced changes in intracellular Ca(2+) ([Ca(2+)](i)) and recently has been implicated in the regulation of mitogen-activated protein (MAP) kinases. We hypothesize that fluid shear stress (FSS) activates PC-2 which regulates MAP kinase and, in turn, induces MAP kinase-dependent gene expression, specifically, monocyte chemoattractant protein-1 (MCP-1).

METHODS

To test this, PC-2 expression was constitutively reduced in a murine inner medullary collecting duct (IMCD3) cell line, and the expression of FSS-induced MCP-1 expression and MAP kinase signaling compared between the parental (PC-2-expressing) and PC-2-deficient IMCD3 cells.

RESULTS

FSS induces MAP kinase signaling and downstream MCP-1 mRNA expression in wild-type IMCD3 cells, while inhibitors of MAP kinase prevented the FSS-induced MCP-1 mRNA response. In contradistinction, FSS did not induce MCP-1 mRNA expression in PC-2-deficient cells, but did increase activation of the upstream MAP kinases. Wild-type cells exposed to FSS augmented the nuclear abundance of activated MAP kinase while PC-2-deficient cells did not.

CONCLUSIONS

PC-2 regulates FSS-induced MAP kinase trafficking into the nucleus of CD cells.

Polycystin-1 C-terminal cleavage is modulated by polycystin-2 expression

Autosomal dominant polycystic kidney disease is caused by mutations in the genes encoding polycystin-1 (PC-1) and polycystin-2 (PC-2). PC-1 cleavage releases its cytoplasmic C-terminal tail (CTT), which enters the nucleus. To determine whether PC-1 CTT cleavage is influenced by PC-2, a quantitative cleavage assay was utilized, in which the DNA binding and activation domains of Gal4 and VP16, respectively, were appended to PC-1 downstream of its CTT domain (PKDgalvp). Cells cotransfected with the resultant PKDgalvp fusion protein and PC-2 showed an increase in luciferase activity and in CTT expression, indicating that the C-terminal tail of PC-1 is cleaved and enters the nucleus. To assess whether CTT cleavage depends upon Ca2+ signaling, cells transfected with PKDgalvp alone or together with PC-2 were incubated with several agents that alter intracellular Ca2+ concentrations. PC-2 enhancement of luciferase activity was not altered by any of these treatments. Using a series of PC-2 C-terminal truncated mutations, we identified a portion of the PC-2 protein that is required to stimulate PC-1 CTT accumulation. These data demonstrate that release of the CTT from PC-1 is influenced and stabilized by PC-2. This effect is independent of Ca2+ but is regulated by sequences contained within the PC-2 C-terminal tail, suggesting a mechanism through which PC-1 and PC-2 may modulate a novel signaling pathway.

TRPP2 and TRPV4 form a polymodal sensory channel complex

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

Mutant polycystin-2 induces proliferation in primary rat tubular epithelial cells in a STAT-1/p21-independent fashion accompanied instead by alterations in expression of p57KIP2 and Cdk2

Background

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is characterized by the formation of multiple fluid-filled cysts that destroy the kidney architecture resulting in end-stage renal failure. Mutations in genes PKD1 and PKD2 account for nearly all cases of ADPKD. Increased cell proliferation is one of the key features of the disease. Several studies indicated that polycystin-1 regulates cellular proliferation through various signaling pathways, but little is known about the role played by polycystin-2, the product of PKD2. Recently, it was reported that as with polycystin-1, polycystin-2 can act as a negative regulator of cell growth by modulating the levels of the cyclin-dependent kinase inhibitor, p21 and the activity of the cyclin-dependent kinase 2, Cdk2.

Methods

Here we utilized different kidney cell-lines expressing wild-type and mutant PKD2 as well as primary tubular epithelial cells isolated from a PKD transgenic rat to further explore the contribution of the p21/Cdk2 pathway in ADPKD proliferation.

Results

Surprisingly, over-expression of wild-type PKD2 in renal cell lines failed to inactivate Cdk2 and consequently had no effect on cell proliferation. On the other hand, expression of mutated PKD2 augmented proliferation only in the primary tubular epithelial cells of a rat model but this was independent of the STAT-1/p21 pathway. On the contrary, multiple approaches revealed unequivocally that expression of the cyclin-dependent kinase inhibitor, p57^KIP2, is downregulated, while p21 remains unchanged. This p57 reduction is accompanied by an increase in Cdk2 levels.

Conclusion

Our results indicate the probable involvement of p57^KIP2 on epithelial cell proliferation in ADPKD implicating a new mechanism for mutant polycystin-2 induced proliferation. Most importantly, contrary to previous studies, abnormal proliferation in cells expressing mutant polycystin-2 appears to be independent of STAT-1/p21.

Advances in management of polycystic liver disease

The focus of this review is polycystic liver disease, a genetic disorder characterized by multiple macroscopic liver cysts that initially bud from biliary epithelium but subsequently lack communication with the biliary tree. There are two main clinical presentations: polycystic liver associated with autosomal dominant polycystic kidney disease and isolated polycystic liver disease. Both of these forms of polycystic liver disease exhibit an autosomal dominant pattern of inheritance. Clinical manifestations of polycystic liver disease are related to either mass effect of the volume of hepatic cysts or to complications arising within the cysts. Polycystic liver disease rarely progresses to hepatic failure or clinical complications of portal hypertension. Management is directed at counseling patients and families, treating complications and reducing cyst load by surgical techniques: cyst fenestration, hepatic resection or, rarely, hepatic transplantation. Recent research suggests that blockade of cyst secretion or inhibition of epithelial cells might be useful in halting progression of disease--these observations are discussed in this review.

Potential pharmacological interventions in polycystic kidney disease

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

Fish and frogs: models for vertebrate cilia signaling

The presence of cilia in many vertebrate cell types and its function has been ignored for many years. Only in the past few years has its importance been rediscovered. In part, this was triggered by the realization that many gene products mutated in polycystic kidney diseases are localized to cilia and dysfunctional cilia result in kidney disease. Another breakthrough was the observation that the establishment of the left-right body axis is dependent on cilia function. Since then, many other developmental paradigms have been shown to rely on cilia-dependent signaling. In addition to mouse and Chlamydomonas, lower vertebrate model systems such as zebrafish, medaka and Xenopus have provided important new insights into cilia signaling and its role during embryonic development. This review will summarize those studies. We will also illustrate how these lower vertebrates are promising model systems for future studies defining the physiological function of cilia during organogenesis and disease pathophysiology.

Polycystic kidney disease and renal injury repair: common pathways, fluid flow, and the function of polycystin-1

The root cause for most cases of autosomal-dominant polycystic kidney disease (ADPKD) is mutations in the polycystin-1 (PC1) gene. While PC1 has been implicated in a perplexing variety of protein interactions and signaling pathways, what its normal function is and why its disruption leads to the proliferation of renal epithelial cells are unknown. Recent results suggest that PC1 is involved in mechanotransduction by primary cilia measuring the degree of luminal fluid flow. PC1 has also recently been shown to regulate the mTOR and signal transducers and activators of transcription (STAT) 6 pathways. These two pathways are normally dormant in the healthy kidney but are activated in response to injury and appear to drive a proliferative repair response. This review develops the idea that a critical function of PC1 and primary cilia in the adult kidney may be to sense renal injury by detecting changes in luminal fluid flow and to trigger proliferation. Constitutive activation of these pathways in ADPKD would lead to the futile attempt to repair a nonexisting injury, resulting in cyst growth. The existence of many known cellular and molecular similarities between renal repair and ADPKD supports this model.

The isolated polycystin-1 COOH-terminal can activate or block polycystin-1 signaling

Much of what is known of the activities of polycystin-1 has been inferred from the effects of the isolated cytoplasmic COOH-terminal domain, but it is not clear whether the truncation acts like polycystin-1, as a dominant negative, or in unrelated pathways. To address this question, we have examined functional interactions between the intact and truncated forms of polycystin-1 in one cell system. In cells expressing only native polycystin-1, introduction of the truncation replicated the activity of the full-length protein. Conversely, when background levels of polycystin-1 were modestly elevated, the truncation acted as a dominant negative. Hence, the truncation acts in the polycystin pathway, but with effects that depend upon the background level of polycystin-1 expression. Our data raise the possibility that the cytoplasmic carboxyl terminus, either through cleavage products or intramolecular interactions, might feed back to modulate the activity of parent or intact polycystin-1.

TRPP2 and autosomal dominant polycystic kidney disease

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

The polycystin 1-C-terminal fragment stimulates ERK-dependent spreading of renal epithelial cells

Polycystin 1, the product of the PKD1 gene, is mutated in autosomal dominant polycystic kidney disease, a disease characterized by renal cyst formation and progressive renal failure. We show that expression of the C-terminal domain of human polycystin-1 (PKD1-CT) triggers spreading of isolated inner medullary collecting duct cells, a process mediated by Erk. As inner medullary collecting duct cells spread, PKD1-CT localizes to cell-extracellular matrix contacts, interacts with focal adhesion proteins Fak and paxillin, and stimulates Fak phosphorylation, paxillin phosphorylation, Fak-paxillin association, and formation of small focal complexes. PKD1-CT-mediated spreading requires membrane localization and the integrity of the C-terminal protein binding sites. We additionally show that Pkd1 null proximal tubule cells generated from Pkd1(flox/-):TSLargeT mice by in vitro Cre recombinase transfection demonstrate diminished spreading when compared with Pkd(flox/-) heterozygous parental cells. These findings suggest that membrane-bound PC1 has a central role in regulating morphogenic protein signaling at cell-matrix interfaces in non-confluent cells.

The pathogenesis of autosomal dominant polycystic kidney disease

In individuals with autosomal dominant polycystic kidney disease (ADPKD), renal function deteriorates as the kidneys become replaced by multitudes of fluid-filled cysts. Although the PKD genes were identified a decade ago, the pathway(s) leading from mutation to disease remain the subject of intense investigation. As a result of this work, it has become apparent that the polycystins are multifunctional proteins that, in the broadest sense, appear to be involved in the transduction of a number of environmental cues into appropriate cellular responses. It is likely that the central pathogenetic pathway for cystogenesis stems from de-differentiation of tubular epithelial cells. Available evidence indicates that loss of polycystin activity leads to subtle derangements of cell calcium regulation through several possible pathways. Abnormal cell calcium homeostasis might then lead to altered differentiation in affected cells. The study of the polycystins has revealed some entirely novel insights into fundamental cell biology but these have not yet been satisfactorily integrated into a verified pathogenetic pathway for the development of ADPKD.

Extracellular signal-regulated kinase inhibition slows disease progression in mice with polycystic kidney disease

The expression of mitogen-activated protein kinases (MAPK) in DBA/2-pcy/pcy (pcy) mice, a murine model of polycystic kidney disease was investigated. Proliferating cell nuclear antigen-positive cells were recognized in cyst epithelium from embryonic day 14.5 to 25 wk of age. Extracellular signal-regulated kinase (ERK) was expressed in the renal tubules of control and pcy mice, but stronger immunostaining was observed in cyst epithelium. Phosphorylated ERK was detected only in pcy mice and was localized predominantly in the cysts. p38 MAPK (p38) was no longer expressed after birth in controls but was detected in the cyst epithelium and in occasional tubular cells of pcy mice at all stages examined. c-Jun N-terminal kinase (JNK) was expressed in all tubular segments of controls after neonatal day 7, whereas in pcy kidneys, tubules became positive for JNK after 8 wk, and the cysts expressed little JNK. Administration of an oral MAP/ERK kinase inhibitor, PD184352, 400 mg/kg per d, to 10-wk-old pcy mice daily for the first week and then every third day for 6 additional weeks significantly decreased BP, kidney weight, serum creatinine level, and water intake and significantly increased urine osmolality. The cystic index and expression of phosphorylated ERK and ERK were significantly lower in PD184352-treated pcy mice. These results demonstrate that the expression of MAPK is dysregulated in cyst epithelium and that inhibition of ERK slowed the progression of renal disease in pcy mice.

The mTOR pathway is regulated by polycystin-1, and its inhibition reverses renal cystogenesis in polycystic kidney disease

Autosomal-dominant polycystic kidney disease (ADPKD) is a common genetic disorder that frequently leads to renal failure. Mutations in polycystin-1 (PC1) underlie most cases of ADPKD, but the function of PC1 has remained poorly understood. No preventive treatment for this disease is available. Here, we show that the cytoplasmic tail of PC1 interacts with tuberin, and the mTOR pathway is inappropriately activated in cyst-lining epithelial cells in human ADPKD patients and mouse models. Rapamycin, an inhibitor of mTOR, is highly effective in reducing renal cystogenesis in two independent mouse models of PKD. Treatment of human ADPKD transplant-recipient patients with rapamycin results in a significant reduction in native polycystic kidney size. These results indicate that PC1 has an important function in the regulation of the mTOR pathway and that this pathway provides a target for medical therapy of ADPKD.

Polycystin-2 regulates proliferation and branching morphogenesis in kidney epithelial cells

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by the formation of multiple fluid-filled cysts that expand over time and destroy the renal architecture. Loss or mutation of polycystin-1 or polycystin-2, the respective proteins encoded by the ADPKD genes PKD1 and PKD2, is associated with most cases of ADPKD. Thus, the polycystin proteins likely play a role in cell proliferation and morphogenesis. Recent studies indicate that polycystin-1 is involved in these processes, but little is known about the role played by polycystin-2. To address this question, we created a number of related cell lines variable in their expression of polycystin-2. We show that the basal and epidermal growth factor-stimulated rate of cell proliferation is higher in cells that do not express polycystin-2 versus those that do, indicating that polycystin-2 acts as a negative regulator of cell growth. In addition, cells not expressing polycystin-2 exhibit significantly more branching morphogenesis and multicellular tubule formation under basal and hepatocyte growth factor-stimulated conditions than their polycystin-2-expressing counterparts, suggesting that polycystin-2 may also play an important role in the regulation of tubulogenesis. Cells expressing a channel mutant of polycystin-2 proliferated faster than those expressing the wild-type protein, but exhibited blunted tubule formation. Thus, the channel activity of polycystin-2 may be an important component of its regulatory machinery. Finally, we show that polycystin-2 regulation of cell proliferation appears to be dependent on its ability to prevent phosphorylated extracellular-related kinase from entering the nucleus. Our results indicate that polycystin-2 is necessary for the proper growth and differentiation of kidney epithelial cells and suggest a possible mechanism for the cyst formation seen in ADPKD2.

Molecular pathogenesis of ADPKD: the polycystin complex gets complex

Autosomal-dominant polycystic kidney disease (ADPKD) is one of the most common human monogenic diseases with an incidence of 1:400 to 1:1000. It is characterized by the progressive development and enlargement of focal cysts in both kidneys, typically resulting in end-stage renal disease (ESRD) by the fifth decade. The cystogenic process is highly complex with a cellular phenotype consistent with "dedifferentiation" (i.e., a high proliferative rate, increased apoptosis, altered protein sorting, changed secretory characteristics, and disorganization of the extracellular matrix). Although cystic renal disease is the major cause of morbidity, the occurrence of nonrenal cysts, most notably in the liver (occasionally resulting in clinically significant polycystic liver disease) and the increased prevalence of other abnormalities including intracranial aneurysms, indicate that ADPKD is a systemic disorder. Following the identification of the first ADPKD gene, PKD1, 10 years ago and PKD2 2 years later, considerable progress has been made in defining the etiology and understanding the pathogenesis of this disorder, knowledge that is now leading to the development of several promising new therapies. The purpose of this review is to summarize our current state of knowledge as to the structure and function of the PKD1 and PKD2 proteins, polycystin-1 and -2, respectively, and explore how mutation at these loci results in the spectrum of changes seen in ADPKD.

Localization and loss-of-function implicates ciliary proteins in early, cytoplasmic roles in left-right asymmetry

Left-right asymmetry is a crucial feature of the vertebrate body plan. While much molecular detail of this patterning pathway has been uncovered, the embryonic mechanisms of the initiation of asymmetry, and their evolutionary conservation among species, are still not understood. A popular recent model based on data from mouse embryos suggests extracellular movement of determinants by ciliary motion at the gastrulating node as the initial step. An alternative model, driven by findings in the frog and chick embryo, focuses instead on cytoplasmic roles of motor proteins. To begin to test the latter hypothesis, we analyzed the very early embryonic localization of ciliary targets implicated in mouse LR asymmetry. Immunohistochemistry was performed on frog and chick embryos using antibodies that have (KIF3B, Polaris, Polycystin-2, acetylated alpha-tubulin) or have not (LRD, INV, detyrosinated alpha-tubulin) been shown to detect in frog embryos only the target that they detect in mammalian tissue. Immunohistochemistry revealed localization signals for all targets in the cytoplasm of cleavage-stage Xenopus embryos, and in the base of the primitive streak in chick embryos at streak initiation. Importantly, several left-right asymmetries were detected in both species, and the localization signals were dependent on microtubule and actin cytoskeletal organization. Moreover, loss-of-function experiments implicated very early intracellular microtubule-dependent motor protein function as an obligate aspect of oriented LR asymmetry in Xenopus embryos. These data are consistent with cytoplasmic roles for motor proteins in patterning the left-right axis that do not involve ciliary motion.

RIP-ed and ready to dance: new mechanisms for polycystin-1 signaling

Polycystin-1, the protein encoded by the principal gene involved in autosomal dominant polycystic kidney disease, has been implicated in extracellular sensing as well as in cell-cell and cell-matrix interactions. However, the precise mechanisms involved in polycystin-1 signaling are not well defined. A report in this issue of the JCI demonstrates that the C-terminal tail of polycystin-1 is cleaved from the membrane through regulated intramembrane proteolysis (RIP) and that this domain translocates to the nucleus, where it activates the AP-1 transcription pathway. This translocation appears to be modulated by polycystin-2, with which polycystin-1 is thought to interact. These findings provide what we believe to be the first evidence that polycystin-1 can signal directly to the nucleus and that polycystin-1-polycystin-2 interactions do not require colocalization of these proteins in the same membrane compartment.

PKD2 Interacts and Co-localizes with mDia1 to Mitotic Spindles of Dividing Cells

Mutations in pkd2 result in the type 2 form of autosomal dominant polycystic kidney disease, which accounts for approximately 15% of all cases of the disease. PKD2, the protein product of pkd2, belongs to the transient receptor potential superfamily of cation channels, and it can function as a mechanosensitive channel in the primary cilium of kidney cells, an intracellular Ca(2+) release channel in the endoplasmic reticulum, and/or a nonselective cation channel in the plasma membrane. We have identified mDia1/Drf1 (mammalian Diaphanous or Diaphanous-related formin 1 protein) as a PKD2-interacting protein by yeast two-hybrid screen. mDia1 is a member of the RhoA GTPase-binding formin homology protein family that participates in cytoskeletal organization, cytokinesis, and signal transduction. We show that mDia1 and PKD2 interact in native and in transfected cells, and binding is mediated by the cytoplasmic C terminus of PKD2 binding to the mDia1 N terminus. The interaction is more prevalent in dividing cells in which endogenous PKD2 and mDia1 co-localize to the mitotic spindles. RNA interference experiments reveal that endogenous mDia1 knockdown in HeLa cells results in the loss of PKD2 from mitotic spindles and alters intracellular Ca(2+) release. Our results suggest that mDia1 facilitates the movement of PKD2 to a centralized position during cell division and has a positive effect on intracellular Ca(2+) release during mitosis. This may be important to ensure equal segregation of PKD2 to the daughter cell to maintain a necessary level of channel activity. Alternatively, PKD2 channel activity may be important in the cell division process or in cell fate decisions after division.

Altered distribution and co-localization of polycystin-2 with polycystin-1 in MDCK cells after wounding stress

Polycystin-1 and -2 are integral membrane glycoproteins defective in autosomal dominant polycystic kidney disease (ADPKD). Recent studies showed a coupled polycystin-1 and -2 action in cell signaling and channel activation suggesting an important biological role for the two proteins at the plasma membrane. To gain a better understanding about the (co)-distribution and dynamics of the polycystin-1 and -2 complex under stress conditions, we used a wound-healing model of Madine Darby canine kidney (MDCK) renal epithelial cells. In this model, cells near the wound edge undergo a process of reorganization to active migration, while cells further from the edge are unaffected and remain confluent. For the first time, endogenous polycystin-1 and -2 were found to partly co-localize in the plasma membrane of confluent monolayers, and both proteins co-localized in the primary cilium. Upon wound healing, the association of polycystin-2 to the membrane was greatly reduced at the wound edge and the submarginal cells. Polycystin-1 remained incorporated to the membrane at the edge of the cell sheet at all time points, although strongly reduced in lamellipodia-forming cells. Adherens junctions and desmosomes, and respective connected actin and keratin cytoskeleton were also disturbed in lamellipodia-forming cells. We propose that altered subcellular localization of polycystin-1 and -2 as a result of stress will affect signaling and other cellular processes mediated by these proteins.

Polycystin-1 distribution is modulated by polycystin-2 expression in mammalian cells

Mutations in PKD1 and PKD2, the genes that encode polycystin-1 and polycystin-2 respectively, account for almost all cases of autosomal dominant polycystic kidney disease. Although the polycystins are believed to interact in vivo, the two proteins often display dissimilar patterns and gradients of expression during development. In an effort to understand this apparent discrepancy, we investigated how changes in polycystin-2 expression can affect the subcellular localization of polycystin-1. We show that, when polycystin-1 is expressed alone in a PKD2 null cell line, it localizes to the cell surface, as well as to the endoplasmic reticulum. When co-expressed with polycystin-2, however, polycystin-1 is not seen at the cell surface and co-localizes completely with polycystin-2 in the endoplasmic reticulum. The localization of a polycystin-1 fusion protein was similarly affected by changes in its level of expression relative to that of polycystin-2. This phenomenon was observed in populations as well as in individual COS-7 cells. Our data suggest that the localization of polycystin-1 can be regulated via the relative expression level of polycystin-2 in mammalian cells. This mechanism may help to explain the divergent patterns and levels of expression observed for the polycystins, and may provide clues as to how the function of these two proteins are regulated during development.

Renal Abnormalities in Mice Caused by Insufficiency of p38α

p38MAP kinase (p38) is activated by hypertonicity and has been implicated to play a pivotal role in the renal system in survival under hypertonic conditions, both in vitro and in vivo. Although there are many aspects of the molecular events via the p38 pathway, its contribution to renal physiology and pathophysiology remains unclear. To elucidate the physiological relevance of p38 in renal function, we performed histochemical and biochemical characterization of p38alpha+/- mice. Although p38alpha+/- mice appeared normal, they showed 24% higher water intake (P < 0.05) and 16% higher kidney weight to total body weight ratio (P < 0.01) at 21 weeks of age. Histological examination of the kidney showed abnormalities such as dilation of proximal convoluted tubules, vacuolar degeneration, focal interstitial fibrosis, and inflammation and enlargement of Bowman's capsule with advancing age. Taken together, these results suggest that p38alpha plays an important role in the structural and functional maintenance of the normal kidney and its insufficiency causes renal abnormalities.

Polycystin-1 activation of c-Jun N-terminal kinase and AP-1 is mediated by heterotrimeric G proteins

Functional analysis of polycystin-1, the product of the gene most frequently mutated in autosomal dominant polycystic kidney disease, has revealed that this protein is involved in the regulation of diverse signaling pathways such as the activation of the transcription factor AP-1 and modulation of Wnt signaling. However, the initial steps involved in the activation of such cascades have remained unclear. We demonstrated previously that the C-terminal cytosolic tail of polycystin-1 binds and activates heterotrimeric G proteins in vitro. To test if polycystin-1 can activate cellular signaling cascades via heterotrimeric G protein subunits, polycystin-1 C-terminal tail-mediated c-Jun N-terminal kinase (JNK) and AP-1 activities were assayed in transiently transfected 293T cells in the presence of dominant-negative, G protein inhibiting constructs, and in the presence of cotransfected Galpha subunits. The results showed that polycystin-1-mediated JNK/AP-1 activation is mediated by Galpha and Gbetagamma subunits. Polycystin-1-mediated AP-1 activity could be significantly augmented by cotransfected Galpha(i), Galpha(q), and Galpha(12/13) subunits, suggesting that polycystin-1 can couple with and activate several heterotrimeric G protein families.

Cation channel regulation by COOH-terminal cytoplasmic tail of polycystin-1: mutational and functional analysis

Polycystin-1 (PKD1) mutations account for approximately 85% of autosomal dominant polycystic kidney disease (ADPKD). We have shown previously that oocyte surface expression of a transmembrane fusion protein encoding part of the cytoplasmic COOH terminus of PKD1 increases activity of a Ca2+-permeable cation channel. We show here that human ADPKD mutations incorporated into this fusion protein attenuated or abolished encoded cation currents. Point mutations and truncations showed that cation current expression requires integrity of a region encompassing the putative coiled coil domain of the PKD1 cytoplasmic tail. Whereas these loss-of-function mutants did not exhibit dominant negative phenotypes, coexpression of a fusion protein expressing the interacting COOH-terminal cytoplasmic tail of PKD2 did suppress cation current. Liganding of the ectodomain of the PKD1 fusion protein moderately activated cation current. The divalent cation permeability and pharmacological profile of the current has been extended. Inducible expression of the PKD1 fusion in EcR-293 cells was also associated with activation of cation channels and increased Ca2+ entry.

The polycystin-1 C-terminal fragment triggers branching morphogenesis and migration of tubular kidney epithelial cells

Mutations of either PKD1 or PKD2 cause autosomal dominant polycystic kidney disease, a syndrome characterized by extensive formation of renal cysts and progressive renal failure. Homozygous deletion of Pkd1 or Pkd2, the genes encoding polycystin-1 and polycystin-2, disrupt normal renal tubular differentiation in mice but do not affect the early steps of renal development. Here, we show that expression of the C-terminal 112 amino acids of human polycystin-1 triggers branching morphogenesis and migration of inner medullary collecting duct (IMCD) cells, and support in vitro tubule formation. The integrity of the polycystin-2-binding region is necessary but not sufficient to induce branching of IMCD cells. The C-terminal domain of polycystin-1 stimulated protein kinase C-alpha (PKC-alpha), but not the extracellular signal-regulated kinases ERK1 or ERK2. Accordingly, inhibition of PKC, but not ERK, prevented polycystin-1-mediated IMCD cell morphogenesis. In contrast, HGF-mediated morphogenesis required ERK activation but was not dependent on PKC. Our findings demonstrate that the C-terminal domain of polycystin-1, acting in a ligand-independent fashion, triggers unique signaling pathways for morphogenesis, and likely plays a central role in polycystin-1 function.

Mutations of the human polycystic kidney disease 2 (PKD2) gene

Autosomal dominant polycystic kidney disease (ADPKD) is an inherited nephropathy, usually of late onset (onset between third to seventh decade), primarily characterized by the formation of fluid-filled cysts in the kidneys. It is one of the most frequent inherited conditions affecting approximately 1:1,000 Caucasians. Two major genes have been identified and characterized in detail: PKD1 and PKD2, mapping on chromosomes 16p13.3 and 4q21-23, respectively. A third gene, PKD3, has been implicated in selected families. Polycystic kidney disease of types 1 or 2 follows a very similar course of symptoms, both being multisystem pleiotropic disorders of indistinguishable picture on clinical grounds. The only difference is that patients with PKD2 mutations run a milder course compared to PKD1 carriers, with an average 10-20 years later age of onset and lower probability to reach end-stage-renal failure. The proteins polycystin-1 and -2 are trans-membranous glycoproteins hypothesized to participate in a common signaling pathway, interacting with each other and with other proteins, and coordinately expressed in normal and cystic tissue. Renal cysts most probably arise after a second somatic event, which inactivates the inherited healthy allele of the same gene, or perhaps one of the alleles of the other gene counterpart, generating a trans-heterozygous state. This article reviews the reported mutations in PKD2. Mutations of all kinds have been reported over the entire sequence of the PKD2 gene, with no apparent significant clustering and with some evidence of genotype/phenotype correlation. Most families harbor their own private mutations but a few recurrent events have been reported in unrelated families.

Interaction with podocin facilitates nephrin signaling

Mutations of NPHS1 or NPHS2, the genes encoding for the glomerular podocyte proteins nephrin and podocin, cause steroid-resistant proteinuria. In addition, mice lacking CD2-associated protein (CD2AP) develop a nephrotic syndrome that resembles NPHS mutations suggesting that all three proteins are essential for the integrity of glomerular podocytes. Although the precise glomerular function of either protein remains unknown, it has been suggested that nephrin forms zipper-like interactions to maintain the structure of podocyte foot processes. We demonstrate now that nephrin is a signaling molecule, which stimulates mitogen-activated protein kinases. Nephrin-induced signaling is greatly enhanced by podocin, which binds to the cytoplasmic tail of nephrin. Mutational analysis suggests that abnormal or inefficient signaling through the nephrin-podocin complex contributes to the development of podocyte dysfunction and proteinuria.

Polycystic kidney disease: from the bedside to the gene and back

Collated in this highly personal commentary are the most important research findings of the past 10 years that deal primarily with the renal manifestations of inherited polycystic kidney diseases. Progress in understanding these complex disorders has followed two major concurrent and convergent lines of investigation: genes and genetic mechanisms, and pathogenesis and progression. The field has moved from descriptive pathobiology to the elucidation of molecular mechanisms consequent to genetic and epigenetic events. Doubtless, the favorite works of some who have labored diligently in this field have not been fully exalted, and for this I apologize. Were I the editor, this entire celebratory volume would be used to extol the thrilling growth of knowledge during the tenure of this polycystic kidney disease watcher.

The cytoplasmic C-terminal fragment of polycystin-1 regulates a Ca2+-permeable cation channel

The cytoplasmic C-terminal portion of the polycystin-1 polypeptide (PKD1(1-226)) regulates several important cell signaling pathways, and its deletion suffices to cause autosomal dominant polycystic kidney disease. However, a functional link between PKD1 and the ion transport processes required to drive renal cyst enlargement has remained elusive. We report here that expression at the Xenopus oocyte surface of a transmembrane fusion protein encoding the C-terminal portion of the PKD1 cytoplasmic tail, PKD1(115-226), but not the N-terminal portion, induced a large, Ca(2+)-permeable cation current, which shifted oocyte reversal potential (E(rev)) by +33 mV. Whole cell currents were sensitive to inhibition by La(3+), Gd(3+), and Zn(2+), and partially inhibited by SKF96365 and amiloride. Currents were not activated by bath hypertonicity, but were inhibited by acid pH. Outside-out patches pulled from PKD1(115-226)-expressing oocytes exhibited a 5.1-fold increased NP(o) of endogenous 20-picosiemens cation channels of linear conductance. PKD1(115-226)-injected oocytes also exhibited elevated NP(o) of unitary calcium currents in outside-out and cell-attached patches, and elevated calcium permeability documented by fluorescence ratio and (45)Ca(2+) flux experiments. Both Ca(2+) conductance and influx were inhibited by La(3+). Mutation of candidate phosphorylation sites within PKD1(115-226) abolished the cation current. We conclude that the C-terminal cytoplasmic tail of PKD1 up-regulates inward current that includes a major contribution from Ca(2+)-permeable nonspecific cation channels. Dysregulation of these or similar channels in autosomal dominant polycystic kidney disease may contribute to cyst formation or expansion.

Exocyst is involved in cystogenesis and tubulogenesis and acts by modulating synthesis and delivery of basolateral plasma membrane and secretory proteins

Epithelial cyst and tubule formation are critical processes that involve transient, highly choreographed changes in cell polarity. Factors controlling these changes in polarity are largely unknown. One candidate factor is the highly conserved eight-member protein complex called the exocyst. We show that during tubulogenesis in an in vitro model system the exocyst relocalized along growing tubules consistent with changes in cell polarity. In yeast, the exocyst subunit Sec10p is a crucial component linking polarized exocytic vesicles with the rest of the exocyst complex and, ultimately, the plasma membrane. When the exocyst subunit human Sec10 was exogenously expressed in epithelial Madin-Darby canine kidney cells, there was a selective increase in the synthesis and delivery of apical and basolateral secretory proteins and a basolateral plasma membrane protein, but not an apical plasma membrane protein. Overexpression of human Sec10 resulted in more efficient and rapid cyst formation and increased tubule formation upon stimulation with hepatocyte growth factor. We conclude that the exocyst plays a central role in the development of epithelial cysts and tubules.

In vivo interaction of the adapter protein CD2-associated protein with the type 2 polycystic kidney disease protein, polycystin-2

We identified a developmentally regulated gene from mouse kidney whose expression is up-regulated in metanephrogenic mesenchyme cells when they are induced to differentiate to epithelial cells during kidney organogenesis. The deduced 70.5-kDa protein, originally named METS-1 (mesenchyme-to-epithelium transition protein with SH3 domains), has since been cloned as a CD2-associated protein (CD2AP). CD2AP is strongly expressed in glomerular podocytes, and the absence of CD2AP in mice results in congenital nephrotic syndrome. We have found that METS-1/CD2AP (hereafter referred to as CD2AP) is expressed at lower levels in renal tubular epithelial cells in the adult kidney, particularly in distal nephron segments. Independent yeast two-hybrid screens using the COOH-terminal region of either CD2AP or polycystin-2 as bait identified the COOH termini of polycystin-2 and CD2AP, respectively, as strong interacting partners. This interaction was confirmed in cultured cells by co-immunoprecipitation of endogenous polycystin-2 with endogenous CD2AP and vice versa. CD2AP shows a diffuse reticular cytoplasmic and perinuclear pattern of distribution, similar to polycystin-2, in cultured cells, and the two proteins co-localize by indirect double immunofluorescence microscopy. CD2AP is an adapter molecule that associates with a variety of membrane proteins to organize the cytoskeleton around a polarized site. Such a function fits well with that hypothesized for the polycystin proteins in renal tubular epithelial cells, and the present findings suggest that CD2AP has a role in polycystin-2 function.

MAPK signaling and the kidney

Following an overview of the biochemistry of mitogen-activated protein kinase (MAPK) pathways, the relevance of these signaling events to specific models of renal cell function and pathophysiology, both in vitro and in vivo, will be emphasized. In in vitro model systems, events activating the principal MAPK families [extracellular signal-regulated and c-Jun NH(2)-terminal kinase and p38] have been best characterized in mesangial and tubular epithelial cell culture systems and include peptide mitogens, cytokines, lipid mediators, and physical stressors. Several in vivo models of proliferative or toxic renal injury are also associated with aberrant MAPK regulation. It is anticipated that elucidation of downstream effector signaling mechanisms and a clearer understanding of the immediate and remote upstream activating pathways, when applied to these highly clinically relevant model systems, will ultimately provide much greater insight into the basis for specificity now seemingly absent from these signaling events.

IRAK-2 and PI 3-kinase synergistically activate NF-kappaB and AP-1

Antisense interleukin-1 (IL-1) receptor associated kinase-2 (IRAK-2) oligonucleotide (ODN) and antisense p110 PI 3-kinase ODN blocked IRAK-2 and p110 PI 3-kinase expression, respectively. As a result, antisense IRAK-2 ODN or antisense p110 PI 3-kinase ODN inhibited IL-1-induced NF-kappaB and AP-1 activation in HepG2 cells. The inhibition of NF-kappaB activation by antisense IRAK-2 ODN or antisense p110 PI 3-kinase ODN and the inhibition of AP-1 activation by antisense IRAK-2 ODN were incomplete, whereas AP-1 activation could be inhibited by antisense p110 PI 3-kinase ODN completely. These results indicate that IRAK-2 is necessary but insufficient to activate NF-kappaB and AP-1 completely and that although PI 3-kinase is not sufficient for NF-kappaB full activation, it is sufficient to activate AP-1 completely. The effects of IRAK-2 or PI 3-kinase on NF-kappaB and AP-1 activation were confirmed by the results that overexpression of IRAK-2 failed to fully activate NF-kappaB and AP-1 and that overexpression of p110 PI 3-kinase is insufficient for NF-kappaB full activation but sufficient for AP-1 activation. Cotransfection experiments showed that the combination of antisense IRAK-2 ODN and antisense p110 PI 3-kinase ODN resulted in additive inhibition of NF-kappaB as well as AP-1 activation. On the other hand, coexpression of IRAK-2 with p110 PI 3-kinase led to a synergistic activation of NF-kappaB and AP-1. These data suggest that IRAK-2 and PI 3-kinase cooperate to activate NF-kappaB and AP-1.

The pathogenesis of autosomal dominant polycystic kidney disease: an update

The identification of PKD1 and PKD2, the two major genes responsible for autosomal dominant polycystic kidney disease, are the seminal discoveries upon which much of the current investigation into the pathogenesis of this common heritable disease is based. A major mechanistic insight was achieved with the discovery that autosomal dominant polycystic kidney disease occurs by a two-hit mechanism requiring somatic inactivation of the normal allele in individual polarized epithelial cells. Most recent advances are focused on the function of the respective protein products, polycystin-1 and polycystin-2. Indirect evidence supports an interaction between polycystin-1 and -2, albeit it is unlikely that they work in concert in all tissues and at all times. They associate in yeast two hybrid and cotransfection assays and there is a striking similarity in the renal and pancreatic cystic phenotypes of Pkd2-/- and Pkd1del34/del34 mice. Also, the respective homologues of both proteins are expressed in the same sensory neuronal cells in the nematode and the human disease phenotypes remain completely overlapping with the major difference being in relative severity. Mounting evidence supports the hypothesis that polycystin-1 is a cell surface receptor. A close homologue in the sea urchin sperm mediates the acrosome reaction in response to contact with egg-jelly, the nematode homologue functions in mechano- or chemosensation, and the solution structure of the repeated extracellular polycystic kidney disease domains reveals a beta-sandwich fold commonly found in surface receptor molecules. Indirect evidence also supports the initial hypothesis that polycystin-2 is a calcium channel subunit. Several closely related homologues retain the calcium channel signature motif but differ in their predicted interaction domains, and one of these homologues has been shown to be a calcium regulated cation channel. Several important distinctions in polcystin-1 and -2 function have also been discovered. Polycystin-2 has a role in cardiac development that polcystin-1 does not. High level polycystin-2 expression in renal epithelial cells coincides with maturation and elongation of tubules and, unlike polycystin-1, persists into adulthood. In cells in tissue culture, polycystin-2 is expressed exclusively in the endoplasmic reticulum whilst the cellular expression of polycystin-1 remains unknown. Overall, the difficult task of understanding the autosomal dominant polycystic disease process is proceeding apace.

Cellular and subcellular distribution of polycystin-2, the protein product of the PKD2 gene

Mutations in the PKD1 and PKD2 genes account for 85 and 15% of cases of autosomal dominant polycystic kidney disease, respectively. Polycystin-2, the product of the PKD2 gene, is predicted to be an integral membrane protein with homology to a family of voltage-activated Ca(2+) channels. In vitro studies suggest that it may interact with polycystin-1, the PKD1 gene product, via coiled-coil domains present in their C-terminal domains. In this study, the cellular and subcellular distribution of polycystin-2 is defined and compared with polycystin-1. A panel of rabbit polyclonal antisera was raised against polycystin-2 and shown to recognize a single band consistent with polycystin-2 in multiple tissues and cell lines by immunoprecipitation and Western blotting. Immunostaining of human and murine renal tissues demonstrated widespread and developmentally regulated expression of polycytin-2, with highest levels in the kidney in the thick ascending limbs of the loop of Henle and the distal convoluted tubule. In contrast, polycystin-1 expression, while localizing to the same tubular segments, was highest in the collecting ducts. Immunohistochemical staining and immunofluorescence microscopy localized polycystin-2 to the basolateral plasma membrane of kidney tubular epithelial cells compared with the junctional localization of polycystin-1. Differences in the developmental, cellular, and subcellular expression of polycystin-1 and polycystin-2 suggest that they may be able to function independently of each other in addition to a potential in vivo interaction via their C-termini.

The polycystic kidney disease protein PKD2 interacts with Hax-1, a protein associated with the actin cytoskeleton

Despite the recent positional cloning of the PKD1 and PKD2 genes, which are mutated in the great majority of patients with autosomal-dominant polycystic kidney disease (PKD), the pathogenic mechanism for cyst formation is still unclear. The finding, that the PKD1 and PKD2 proteins interact with each other through their COOH termini, suggests that both proteins are part of the same protein complex or signal transduction pathway. Using a yeast two-hybrid screen with the PKD2 protein, we isolated the PKD2-interacting protein Hax-1. The specificity of the interaction was demonstrated by the fact that PKD2L, a protein closely related to PKD2, failed to interact with Hax-1. Immunofluorescence experiments showed that in most cells PKD2 and Hax-1 colocalized in the cell body, but in some cells PKD2 and Hax-1 also were sorted into cellular processes and lamellipodia. Furthermore we demonstrated an association between Hax-1 and the F-actin-binding protein cortactin, which suggests a link between PKD2 and the actin cytoskeleton. We speculate that PKD2 is involved in the formation of cell-matrix contacts, which are dysfunctional without a wild-type PKD2 protein, thus leading to cystic enlargement of tubular structures in the kidney, liver, and pancreas.

Mutations in a NIMA-related kinase gene, Nek1, cause pleiotropic effects including a progressive polycystic kidney disease in mice

We previously have described a mouse model for polycystic kidney disease (PKD) caused by either of two mutations, kat or kat(2J), that map to the same locus on chromosome 8. The homozygous mutant animals have a latent onset, slowly progressing form of PKD with renal pathology similar to the human autosomal-dominant PKD. In addition, the mutant animals show pleiotropic effects that include facial dysmorphism, dwarfing, male sterility, anemia, and cystic choroid plexus. We previously fine-mapped the kat(2J) mutation to a genetic distance of 0.28 +/- 0.12 centimorgan between D8Mit128 and D8Mit129. To identify the underlying molecular defect in this locus, we constructed an integrated genetic and physical map of the critical region surrounding the kat(2J) mutation. Cloning and expression analysis of the transcribed sequences from this region identified Nek1, a NIMA (never in mitosis A)-related kinase as a candidate gene. Further analysis of the Nek1 gene from both kat/kat and kat(2J)/kat(2J) mutant animals identified a partial internal deletion and a single-base insertion as the molecular basis for these mutations. The complex pleiotropic phenotypes seen in the homozygous mutant animals suggest that the NEK1 protein participates in different signaling pathways to regulate diverse cellular processes. Our findings identify a previously unsuspected role for Nek1 in the kidney and open a new avenue for studying cystogenesis and identifying possible modes of therapy.