Characterisation and expression of the PKD-1 protein, polycystin, in renal and extrarenal tissues

Studies of the nature and expression of the PKD-1 gene product, polycystin, have been complicated by duplication of the PKD-1 gene, the low expression of the PKD-1 gene in adult tissues and the lack of antibodies to epitopes in the duplicated region of polycystin. Using five monoclonal and polyclonal antibodies to epitopes encompassing the whole of the polycystin molecule, we have studied the biochemical and cellular expression of polycystin, and sought to identify homologues and alternative splice forms of polycystin. We find that polycystin exists as a 400-500 kDa protein in normal kidney and in a range of renal epithelial cell lines by immunoblotting, using antibodies to four different epitopes. No evidence of translated products from the genes (HG) homologous to PKD-1 nor any major splice forms of polycystin was found. In renal cells, polycystin could be detected as a cell surface protein but significant intracellular concentrations were also found by cellular fractionation and immunofluorescence. In normal and PKD-1 fetal tissues, immunoreactive polycystin was detected in many different cell types outside the kidney including vascular smooth muscle cells, endothelium, pancreatic, biliary and respiratory ductal epithelia, thyroidal epithelium, endocardium, myocardium, oocytes and Leydig cells. In the developing mouse kidney, polycystin expression is seen in all nephron segments but expression becomes restricted to mature distal tubules and collecting ducts in adult life. These observations clarify the nature of the PKD-1 protein, provide a molecular basis for understanding the systemic nature of PKD-1 and may explain the known phenotypic difference in the nature of renal cysts between 'early-onset' cases and typical adult-onset disease.