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Loss of primary cilia results in deregulated and unabated apical calcium entry in ARPKD collecting duct cells

Departments of ¹Physiology, ²Cell Biology, ³Medicine, and ⁴Genetics and Translational Medicine, University of Alabama at Birmingham, Birmingham, Alabama

Submitted 23 November 2005 ; accepted in final form 30 December 2005

Recent genetic analysis has identified a pivotal role of primary cilia in the pathogenesis of polycystic kidney disease (PKD). However, little is known regarding how cilia loss/dysfunction contributes to cyst development. In epithelial cells, changes in apical fluid flow induce cilia-mediated Ca²⁺ entry via polycystin-2 (PC2), a cation channel. The Oak Ridge Polycystic Kidney (orpk) mouse contains a mutated Tg737 gene that disrupts expression of polaris, a protein required for ciliogenesis. These studies examine the effect of cilia malformation on Ca²⁺ entry in orpk cilia(–) collecting duct principal cells, and in orpk cells in which wild-type Tg737 was reintroduced, orpk cilia(+). [Ca²⁺]_i was monitored in confluent cell monolayers using fluorescence microscopy. Intrinsic apical Ca²⁺ entry was measured by Mn²⁺ quenching and Ca²⁺ depletion/readdition under flow conditions below the threshold for stimulation. We found that unstimulated apical Ca²⁺ entry was markedly increased in cilia(–) cells and was sensitive to Gd³⁺, an inhibitor of PC2. Electrophysiological measurements demonstrate increased abundance of an apical channel, consistent with PC2, in cilia(–) cells. Immunofluorescence studies revealed that PC2, normally expressed on and at the base of cilia in orpk cilia(+) cells, was observed throughout the apical membrane in cilia(–) cells. Furthermore, cilia(–) cells displayed elevated subapical Ca²⁺ levels measured with the near-membrane Ca²⁺ indicator FFP-18. We propose that cilia exert a tonic regulatory influence on apical Ca²⁺ entry, and absence of cilia results in loss of spatial organization of PC2, causing unregulated Ca²⁺ entry and elevations in subapical [Ca²⁺], a factor which may contribute to cyst formation.

Oak Ridge Polycystic Kidney; autosomal recessive polycystic kidney disease; polycystin-2; Ca²⁺ permeability

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