Naturally occurring mutations in two separate genes, PKD1 and PKD2, are responsible for the vast majority (~99%) of all cases of autosomal dominant polycystic kidney disease (ADPKD), one of the most common genetic diseases affecting 1 in 1000 Americans. The hallmark of ADPKD is the development of epithelial cysts in the kidney, liver, and pancreas. Currently, there is no effective treatment for ADPKD. PKD1 encodes a large plasma membrane protein (PKD1 or Polycystin 1) with a long extracellular domain and has been speculated that it can function as an atypical G protein coupled receptor. PKD2 encodes an ion channel of the Transient Receptor Potential superfamily (TRPP2, PKD2, or Polycystin 2). However, the molecular function of these proteins and the mechanism(s) by which mutations in PKD1 and PKD2 cause ADPKD have been elusive. We have shown recently that PKD1 and TRPP2 form a complex at the plasma membrane that is activated by secreted WNT ligands. WNT proteins bind directly to the extracellular domain of PKD1 and induce Ca2+ influx and whole cell currents that are dependent on TRPP2. The PKD1/TRPP2 complex contains Dishevelleds (DVLs), which are cytoplasmic proteins that mediate Wnt signaling. The PKD1/TRPP2 complex has an essential role in directed cell migration and chemotaxis in response to a WNT ligand. In frog embryos pkd1 works together with wnt9a and dvl2 to control kidney tubular diameter. Therefore, we hypothesize that PKD1 and TRPP2 mediate WNT-induced Ca2+ signaling that is essential for directed cell migration and contributes to the determination of kidney tubule diameter. In this proposal, we will determine the mechanism of WNT- induced activation of PKD1/TRPP2 (Specific Aim 1). Determine the step(s) in WNT-induced directed cell migration specifically affected by PKD1 and TRPP2 (Specific Aim 2). Determine whether DVLs alone or in association with other cytosolic proteins linked to Wnt signaling function downstream of PKD1 and TRPP2 in WNT-induced cell migration (Specific Aim 3). This proposal is expected to shed light onto the mechanisms of WNT-induced activation of the PKD1/TRPP2, the mechanisms by which these proteins regulate directed cell migration, and cellular pathways activated immediately downstream of WNT-induced PKD1/TRPP2-mediated Ca2+ signaling. Knowledge of these pathways can be used as the springboard for the discovery of new druggable targets for ADPKD.
The molecular and cellular functions of the Polycystins are unknown. The discovery that they function within the Wnt pathway, along with the known roles of this pathway in kidney development, now open a new avenue of investigation into the molecular and cellular pathophysiology of ADPKD and the development of new therapeutic approaches.
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