Underlying purinergic signaling important for monocilium-dependent signaling in ductal epithelia : implications for polycystic kidney disease

by (Michael Brian) Hovater, Michael

This thesis concerns purinergic signaling in renal epithelial cells of normal and polycystic kidneys. The first section discusses first principles of “purinergic signaling” as they relate to the nephron and the urinary bladder. Remodeled and encapsulated cysts in autosomal dominant PKD (ADPKD) and remodeled “pseudocysts” in autosomal recessive polycystic kidney disease (ARPKD) of the renal collecting duct create an ideal microenvironment for purinergic signaling. Once “trapped” in these microenvironments in a “closed system,” purinergic signaling becomes chronic and plays a significant epigenetic and detrimental role in the progression of ADPKD in particular, once the remodeling of the renal tissue has begun. In the PKD “cystic” microenvironments, we argue that normal purinergic signaling within the lumen of the nephron provides detrimental acceleration of ADPKD once remodeling is complete. The second section presents data that suggest apical monocilia are sensory organelles. Renal epithelial cells release ATP constitutively under basal conditions and release higher quantities of the purine nucleotide in response to different stimuli. In autosomal recessive polycystic kidney disease (ARPKD) mice and humans, collecting duct epithelial cells lack an apical central monocilium or express dysfunctional proteins within that monocilium. Collecting duct principal cells derived from an Oak Ridge polycystic kidney (orpkTg737) mouse model of autosomal recessive polycystic kidney iii disease (ARPKD) lack a well-formed apical central cilium. We compared these cells grown as polarized cell monolayers on permeable supports to the same cells where the apical monocilium was genetically rescued with the wild-type Tg737 gene that encodes Polaris, a protein essential to cilia formation. These data suggest that apical monocilia are sensory organelles, and that their presence in the apical membrane facilitates the formation of a mature ATP secretion apparatus in response to chemical, osmotic and mechanical stimuli. The cilium and autocrine ATP signaling appear to work in concert to control cell Ca 2+. Loss of a cilium-regulated autocrine purinergic signaling system may be a critical underlying etiology for ARPKD and may lead to dysinhibition and/or upregulation of multiple Na+ absorptive mechanisms and a resultant severe hypertensive phenotype in autosomal recessive PKD. iv