Regulation of calcium signaling by the PKD2 gene product
Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common genetic diseases. It affects 1 in 1000 Americans with the development of epithelial cysts in the kidney, liver, and pancreas. Naturally occurring mutations in two separate genes, PKD1 and PKD2, are responsible for the vast majority (~99%) of all cases of ADPKD. PKD1 encodes a large plasma membrane protein with a long extracellular domain, while PKD2 encodes an ion channel of the TRP superfamily (currently named, TRPP2). We and others have shown that PKD1 physically interacts with TRPP2 to form an ion channel complex (PKD1/TRPP2) that links extracellular stimuli to Ca2+ influx. However, the molecular identity of these extracellular stimuli remains elusive. We have shown that TRPP2 functions as a receptor-operated ion channel activated by cell surface receptors coupled to phosphoinositide metabolism and the small GTPase, RhoA. We have recently identified a subset of the Wnt secreted glycoproteins as potential ligands for the PKD1/TRP2 channel complex. Interestingly, these ligands signal through RhoA. Specifically, Wnt4/5a/9b can directly bind to the extracellular domain of PKD1 and induce a Ca2+ influx in kidney epithelial cells that is dependent on PKD1 or TRPP2. Conversely, ectopic expression of PKD1 and TRPP2 leads to Wnt-induced Ca2+ influx. This novel Wnt-induced Ca2+ influx pathway appears not to involve the Wnt cognate Frizzled receptors. This new pathway is physiologically relevant as mice lacking Wnt9b, pkd1, or pkd2 develop kidney cysts. These data lead us to propose that PKD1 and TRPP2 form a receptor-channel complex which is activated by Wnt proteins, and that the loss of this pathway leads to the development of epithelial cysts. We will test this hypothesis by asking: 1) Can pathogenic mutations in PKD1 or PKD2 disrupt Wnt-induced Ca2+ entry? 2) What is the molecular basis of the activation process of the PKD1/TRPP2 complex by Wnt proteins? 3) How is PKD1/TRPP2-mediated Ca2+ influx regulated by the canonical Wnt pathway? And 4) Does Wnt9b activate PKD1/TRPP2 in vivo and can this activation be modified by known interacting proteins of TRPP2 in vivo? These questions will be addressed by complementary approaches in cell culture, zebrafish embryos, and the mouse. The proposed studies will help us understand fundamental properties of PKD1 and TRPP2 and their roles in cystogenesis. As the pathophysiological basis of ADPKD is unknown, these experiments will set the stage for the development of therapeutic strategies.