Polycystin-1, the gene product of PKD1, induces resistance to apoptosis and spontaneous tubulogenesis in MDCK cells

Do polycystins function as cation channels?

PURPOSE OF REVIEW

During the past 2 years growing evidence has emerged that polycystins (polycystin-1 and polycystin-2) are ion channels or regulators of ion channels. This suggests that autosomal-dominant polycystic kidney disease (ADPKD), which arises from mutations in polycystins, is a form of ion-channel disease (channelopathy). The present review addresses the properties and the mode of action of polycystin channels; it also discusses how polycystin channel signaling may be involved in cyst formation in ADPKD.

RECENT FINDINGS

The precise functions of polycystin-1 and polycystin-2 are unclear. However, recent work has revealed that polycystin-1 may induce or modulate ion channels, including polycystin-2 channels, and that polycystin-2 functions as a calcium-regulated, calcium-permeable cation channel on the endoplasmic reticulum or on the plasma membrane with polycystin-1. These data suggest that ion-channel signaling mediated by polycystins is important for tubule formation in kidney and that disrupted signaling results in cyst formation.

SUMMARY

ADPKD is a systemic hereditary disease that is characterized by renal and hepatic cysts, and results in end-stage renal failure in 50% of affected individuals. Most cases (>95%) are caused by genetic mutations in either the PKD1 or the PKD2 gene, or both, which encode polycystin-1 and polycystin-2, respectively. The present review provides a hint of how malfunction of polycystins may give rise to cysts, based on recent observations concerning polycystin channels. Polycystin channel signaling may prove to be an important new target for therapy of ADPKD.

Canine PKD1 is a single-copy gene: genomic organization and comparative analysis

Most cases of autosomal dominant polycystic kidney disease are caused by mutations in the gene PKD1, encoding polycystin-1. To gain insight into the role of polycystin-1 in tubulogenesis and cystogenesis using the well-characterized canine kidney epithelial cell line MDCK, we have now cloned and characterized the exon/intron structure of the canine gene PKD1. FISH analysis showed that the dog genome lacks the multiple PKD1 homologs present in human. Intron 21 of dog PKD1 lacked the polypyrimidine tract characteristic of the human gene, whereas pyrimidine-rich elements were identified in canine intron 30. Canine polycystin-1 showed a higher degree of homology with the human counterpart and lower homology with mouse and rat. A striking degree of conservation (97% identity) was determined for the leucine-rich repeat domain between dog and human. Also, the homology analysis indicated that 4 of 16 Ig-like repeats (IgIII, IgVII, IgX, and IgXV) are likely to be functionally significant. This is particularly important in light of our recent findings demonstrating that Iglike domains form strong homophilic interactions and can mediate cell-cell adhesion. These data enable detailed analysis of the role of polycystin-1 in cystogenesis and tubulogenesis using the canine MDCK cell line.

Identification, characterization, and localization of a novel kidney polycystin-1-polycystin-2 complex

The functions of the two proteins defective in autosomal dominant polycystic kidney disease, polycystin-1 and polycystin-2, have not been fully clarified, but it has been hypothesized that they may heterodimerize to form a "polycystin complex" involved in cell adhesion. In this paper, we demonstrate for the first time the existence of a native polycystin complex in mouse kidney tubular cells transgenic for PKD1, non-transgenic kidney cells, and normal adult human kidney. Polycystin-1 is heavily N-glycosylated, and several glycosylated forms of polycystin-1 differing in their sensitivity to endoglycosidase H (Endo H) were found; in contrast, native polycystin-2 was fully Endo H-sensitive. Using highly specific antibodies to both proteins, we show that polycystin-2 associates selectively with two species of full-length polycystin-1, one Endo H-sensitive and the other Endo H-resistant; importantly, the latter could be further enriched in plasma membrane fractions and co-immunoprecipitated with polycystin-2. Finally, a subpopulation of this complex co-localized to the lateral cell borders of PKD1 transgenic kidney cells. These results demonstrate that polycystin-1 and polycystin-2 interact in vivo to form a stable heterodimeric complex and suggest that disruption of this complex is likely to be of primary relevance to the pathogenesis of cyst formation in autosomal dominant polycystic kidney disease.

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.

Molecular basis of polycystic kidney disease: PKD1, PKD2 and PKHD1

Recent developments have helped elucidate the function of the autosomal dominant polycystic kidney disease proteins, polycystin-1 and polycystin-2, and have revealed the primary defect in autosomal recessive polycystic kidney disease, by positional cloning of the gene, PKHD1. Several studies demonstrating that polycystin-2 can act as a calcium-ion-permeable cation channel, and that polycystin-1 may be involved in regulating/localizing this channel, have provided compelling evidence of the function of these proteins. A role in regulating intracellular calcium levels seems likely, with the many cellular abnormalities associated with cystogenesis due to a disruption of calcium homeostasis. Improved mutation analysis in autosomal dominant polycystic kidney disease has led to the finding of genotype/phenotype correlations which could be related to possible cleavage of polycystin-1. A major recent breakthrough has revealed the primary defect in autosomal recessive polycystic kidney disease. Genetic analysis showed that the PCK rat model is orthologous to autosomal recessive polycystic kidney disease, and allowed the human gene, PKHD1, to be precisely localized and identified. PKHD1 is a large gene, encoding a protein, fibrocystin, of 4074 amino acids, which is predicted to have a large extracellular region, a single transmembrane domain and a short cytoplasmic tail. Fibrocystin may act as a receptor with critical roles in collecting-duct and biliary development.

PKD1 induces p21(waf1) and regulation of the cell cycle via direct activation of the JAK-STAT signaling pathway in a process requiring PKD2

Autosomal dominant polycystic kidney disease is characterized by cyst formation in the kidney and other organs and results from mutations of PKD1 or PKD2. Previous studies suggest that their gene products have an important role in growth regulation. We now show that expression of polycystin-1 activates the JAK-STAT pathway, thereby upregulating p21(waf1) and inducing cell cycle arrest in G0/G1. This process requires polycystin-2, a channel protein, as an essential cofactor. Mutations that disrupt polycystin-1/2 binding prevent activation of the pathway. Mouse embryos lacking Pkd1 have defective STAT1 phosphorylation and p21(waf1) induction. These results suggest that one function of the polycystin-1/2 complex is to regulate the JAK/STAT pathway and explain how mutations of either gene can result in dysregulated growth.

Expression of PKD1 and PKD2 transcripts and proteins in human embryo and during normal kidney development

Autosomal-dominant polycystic kidney disease, one of the most frequent human genetic disorders, is genetically heterogeneous. Most cases result from mutations of PKD1 or PKD2 encoding polycystin-1 or polycystin-2, respectively. Polycystin-1 is a large transmembrane protein containing several domains involved in cell-cell and/or cell-matrix interactions. Polycystin-2 is transmembrane glycoprotein sharing homology with some families of cation channels. Despite a large number of reports, the tissue distribution of these two proteins, especially of polycystin-1, is still debated. We investigated the expression pattern of PKD1 and PKD2 transcripts and proteins during human embryogenesis and kidney development, using Northern blot analysis, in situ hybridization, and immunohistochemical methods. For each gene, the expression pattern of transcripts and protein was concordant. In human 5- to 6-week-old embryos, both genes are widely expressed, mainly in neural tissue, cardiomyocytes, endodermal derivatives, and mesonephros. At this age, PKD2 but not PKD1 expression is observed in the ureteric bud and the uninduced metanephros. Thereafter, PKD2 is diffusely expressed at all stages of nephron development, whereas high PKD1 expression first appears in differentiated proximal tubules. Proximal tubule expression of both genes decreases from weeks 20 to 24 onwards. PKD1 transcripts, later restricted to distal tubules in fetal nephrogenesis, are no longer detected in adult kidneys, which nevertheless maintain a faint expression of polycystin-1, whereas persistent expression of PKD2 transcripts and protein is observed throughout nephrogenesis. Overall, contrary to previous observations, we found profound differences in the spatiotemporal expression of PKD1 and PKD2 during nephrogenesis, PKD2 being expressed earlier and more diffusely than PKD1. These data suggest that polycystins could interact with different partners, at least during kidney development.

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.

Polycystin-1 interacts with intermediate filaments

Polycystin-1, the protein defective in a majority of patients with autosomal dominant polycystic kidney disease, is a ubiquitously expressed multi-span transmembrane protein of unknown function. Subcellular localization studies found this protein to be a component of various cell junctional complexes and to be associated with the cytoskeleton, but the specificity and nature of such associations are not known. To identify proteins that interact with the polycystin-1 C-tail (P1CT), this segment was used as bait in a yeast two-hybrid screening of a kidney epithelial cell library. The intermediate filament (IF) protein vimentin was identified as a strong polycystin-1-interacting partner. Cytokeratins K8 and K18 and desmin were also found to interact with P1CT. These interactions were mediated by coiled-coil motifs in polycystin-1 and IF proteins. Vimentin, cytokeratins K8 and K18, and desmin also bound directly to P1CT in GST pull-down and in in vitro filament assembly assays. Two observations confirmed these interactions in vivo: (i) a cell membrane-anchored form of recombinant P1CT decorated the IF network and was found to associate with the cytoskeleton in detergent-solubilized cells and (ii) endogenous polycystin-1 distributed with IF at desmosomal junctions. Polycystin-1 may utilize this association for structural, storage, or signaling functions.

Biochemical characterization of bona fide polycystin-1 in vitro and in vivo

The most common form of autosomal dominant polycystic kidney disease (PKD) results from mutation of the PKD1 gene on chromosome 16p13.3. The gene encodes a 14-kb messenger RNA that is predicted to express a 462-kd membrane protein. The gene product, polycystin-1, has a large extracellular portion composed of a novel combination of protein-protein interacting domains and is postulated to be a plasma membrane receptor involved in cell-cell/matrix interactions. However, slow progress has been made in the characterization of polycystin-1 or the determination of its function. In fact, the protein is expressed at very low levels in tissues and cell lines and previous efforts directed at expression of recombinant protein had been largely unsuccessful. We have recently developed constructs of full-length human PKD1 complementary (cDNA) that can be expressed in both a stable and transient fashion in mammalian cells. We used these systems to characterize our antibodies and to track the protein in vivo. We report here the first biochemical characterization of recombinant polycystin-1 and show that the protein is a 520-kd glycosylated polypeptide with an unglycosylated core of 460 kd. Subcellular fractionation as well as biotinylation studies confirmed that the protein is plasma-membrane associated. Furthermore, we show that the recombinant protein localizes to cell-cell junctions in polarized madin darby canine kidney cells as revealed by indirect immunofluorescence. Our data represent the first characterization of polycystin-1 performed under highly controlled conditions.

Cardiovascular, skeletal, and renal defects in mice with a targeted disruption of the Pkd1 gene

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by cyst formation in the kidney, liver, and pancreas and is associated often with cardiovascular abnormalities such as hypertension, mitral valve prolapse, and intracranial aneurysms. It is caused by mutations in PKD1 or PKD2, encoding polycystin-1 and -2, which together form a cell surface nonselective cation ion channel. Pkd2-/- mice have cysts in the kidney and pancreas and defects in cardiac septation, whereas Pkd1(del34) -/- and Pkd1(L) -/- mice have cysts but no cardiac abnormalities, although vascular fragility was reported in the latter. Here we describe mice carrying a targeted mutation in Pkd1 (Pkd1(del17-21betageo)), which defines its expression pattern by using a lacZ reporter gene and may identify novel functions for polycystin-1. Although Pkd1(del17-21betageo) +/- adult mice develop renal and hepatic cysts, Pkd1(del17-21betageo) -/- embryos die at embryonic days 13.5-14.5 from a primary cardiovascular defect that includes double outflow right ventricle, disorganized myocardium, and abnormal atrio-ventricular septation. Skeletal development is also severely compromised. These abnormalities correlate with the major sites of Pkd1 expression. During nephrogenesis, Pkd1 is expressed in maturing tubular epithelial cells from embryonic day 15.5. This expression coincides with the onset of cyst formation in Pkd1(del34) -/-, Pkd1(L) -/-, and Pkd2-/- mice, supporting the hypothesis that polycystin-1 and polycystin-2 interact in vivo and that their failure to do so leads to abnormalities in tubule morphology and function.

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.

Human disease: Calcium signaling in polycystic kidney disease

Polycystic kidney disease results from loss of function of either of two novel proteins, polycystin-1 or polycystin-2. Recent studies show that intracellular calcium signaling is important in kidney development, and define defects in this signaling pathway as the basis of cyst formation in polycystic kidney disease.

Contribution of apoptotic cell death to renal injury

Cell number abnormalities are frequent in renal diseases, and range from the hypercellularity of postinfectious glomerulonephritis to the cell depletion of chronic renal atrophy. Recent research has shown that apoptosis and its regulatory mechanisms contribute to cell number regulation in the kidney. The role of apoptosis ranges from induction to repair and progression of renal injury. Death ligands and receptors, such as TNF and FasL, proapoptotic and antiapoptotic Bcl-2 family members and caspases have all been shown to participate in apoptosis regulation in the course of renal injury. These proteins represent potential therapeutic targets, which should be further explored.

Co-assembly of polycystin-1 and -2 produces unique cation-permeable currents

The human kidney is composed of roughly 1.2-million renal tubules that must maintain their tubular structure to function properly. In autosomal dominant polycystic kidney disease (ADPKD) cysts develop from renal tubules and enlarge independently, in a process that ultimately causes renal failure in 50% of affected individuals. Mutations in either PKD1 or PKD2 are associated with ADPKD but the function of these genes is unknown. PKD1 is thought to encode a membrane protein, polycystin-1, involved in cell-cell or cell-matrix interactions, whereas the PKD2 gene product, polycystin-2, is thought to be a channel protein. Here we show that polycystin-1 and -2 interact to produce new calcium-permeable non-selective cation currents. Neither polycystin-1 nor -2 alone is capable of producing currents. Moreover, disease-associated mutant forms of either polycystin protein that are incapable of heterodimerization do not result in new channel activity. We also show that polycystin-2 is localized in the cell in the absence of polycystin-1, but is translocated to the plasma membrane in its presence. Thus, polycystin-1 and -2 co-assemble at the plasma membrane to produce a new channel and to regulate renal tubular morphology and function.