Mutations in the PKD1 gene are responsible for >85% of autosomal dominant polycystic kidney disease (ADPKD). The protein product of PKD1, polycystin-1, is a large, modular membrane protein, with putative ligand-binding motifs in the extracelluar N-terminal portion, 9-11 transmembrane domains and an intracellular C-terminal portion with phosphorylation sites. A role for polycystin-1 as a cell surface receptor involved in cell-matrix and cell-cell interactions has been proposed. In this study, we have analyzed polycystin-1 and associated protein distribution in normal human epithelial cells and examined the role of cell-matrix versus cell-cell interactions in regulation of the assembly of polycystin-1 multiprotein complexes. Immunocytochemistry, sucrose density gradient sedimentation, co-immunoprecipitation analyses and in vitro binding assays have shown that polycystin-1 associates with the focal adhesion proteins talin, vinculin, p130Cas, FAK, alpha-actinin, paxillin and pp60c-src in subconfluent normal human fetal collecting tubule (HFCT) epithelia when cell-matrix interactions predominate. Polycystin-1 also forms higher S value complexes with the cell-cell adherens junction proteins E-cadherin, beta- and gamma-catenins in confluent cultures when cell-cell interactions are predominant. Polycystin-1 multiprotein complexes can be disrupted by cytochalasin D but not by colchicine, suggesting involvement of the actin cytoskeleton. Although inhibition of tyrosine phosphorylation by tyrphostin inhibits polycystin-1-FAK interactions, E-cadherin interactions are enhanced. High calcium treatment also increases polycystin-1-E-cadherin interactions.

The human protein kinase X gene (PRKX) is a member of an ancient family of cAMP-dependent serine͞threonine kinases here shown to be phylogenetically distinct from the classical PKA, PKB͞Akt, PKC, SGK, and PKG gene families. Renal expression of the PRKX gene is developmentally regulated and restricted to the ureteric bud epithelium of the fetal metanephric kidney. Aberrant adult kidney expression of PRKX was found in autosomal dominant polycystic kidney disease. PRKX kinase expression markedly activated migration of cultured renal epithelial cells in the presence of cAMP; this effect was blocked by cell treatment with the PKA inhibitor H89 and was not observed in PKA-transfected cells. In addition, expression of PRKX kinase activated branching morphogenesis of Madin-Darby canine kidney cells in collagen gels even in the absence of cAMP and͞or hepatocyte growth factor, an effect not seen with either PKA expression or expression of a mutant, kinase-inactivated PRKX. These results suggest that the PRKX kinase may regulate epithelial morphogenesis during mammalian kidney development. Because another member of the PRKX gene family (the Dictyostelium discoideum gene KAPC-DICDI) also plays a role in cellular migration, these studies suggest that regulation of morphogenesis may be a distinctive property of these genes that has been conserved in evolution that is not shared with PKA family genes.

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Autosomal dominant polycystic kidney disease (ADPKD) is a very common inherited disease caused by mutations in PKD1 or PKD2 genes characterized by progressive enlargement of fluid-filled cysts and loss of renal function [1]. Previous studies proposed a role for human polycystin-1 in renal morphogenesis acting as a matrix receptor in focal adhesions and for polycystin-2 as a putative calcium channel [2, 3]. The genome of Caenorhabditis elegans contains 2 new members of the polycystin family: lov-1, the homolog for PKD1; and pkd-2, the homolog for PKD2 [4; this paper]. Mutation analysis in C. elegans showed similarly compromised male mating behaviors in all single and double lov-1 and pkd-2 mutants, indicating their participation in a single genetic pathway. Expression analysis localized LOV-1 and PKD-2 to the ends of sensory neurons in male tails and to the tips of CEM neurons in the head, consistent with functions as chemo- or mechanosensors. Human and C. elegans PKD1 and PKD2 homologs, transfected into mammalian renal epithelial cells, co-localized with paxillin in focal adhesions suggesting function in a single biological pathway. Based on the role of polycystins in C. elegans sensory neuron function and the conservation of PKD pathways we suggest that polycystins act as sensors of the extracellular environment, initiating, via focal adhesion assembly, intracellular transduction events in neuronal or morphogenetic processes.