Genetic reduction of cilium length by targeting intraflagellar transport 88 protein impedes kidney and liver cyst formation in mouse models of autosomal polycystic kidney disease.

Polycystin-1 (PC1) and -2 (PC2), products of the PKD1 and PKD2 genes, are mutated in autosomal dominant polycystic kidney disease (ADPKD). They localize to the primary cilia; however, their ciliary function is in dispute. Loss of either the primary cilia or PC1 or PC2 causes cyst formation. However, loss of both cilia and PC1 or PC2 inhibits cyst growth via an unknown pathway. To help define a pathway, we studied cilium length in human and mouse kidneys. We found cilia are elongated in kidneys from patients with ADPKD and from both Pkd1 and Pkd2 knockout mice. Cilia elongate following polycystin inactivation. The role of intraflagellar transport proteins in Pkd1-deficient mice is also unknown. We found that inactivation of Ift88 (a gene expressing a core component of intraflagellar transport) in Pkd1 knockout mice, as well as in a new Pkd2 knockout mouse, shortened the elongated cilia, impeded kidney and liver cystogenesis, and reduced cell proliferation. Multi-stage in vivo analysis of signaling pathways revealed ß-catenin activation as a prominent, early, and sustained event in disease onset and progression in Pkd2 single knockout but not in Pkd2.Ift88 double knockout mouse kidneys. Additionally, AMPK, mTOR and ERK pathways were altered in Pkd2 single knockout mice but only AMPK and mTOR pathway alteration were rescued in Pkd2.Ift88 double knockout mice. Thus, our findings advocate an essential role of polycystins in the structure and function of the primary cilia and implicate ß-catenin as a key inducer of cystogenesis downstream of the primary cilia. Our data suggest that modulating cilium length and/or its associated signaling events may offer novel therapeutic approaches for ADPKD.

A cAMP and CREB-mediated feed-forward mechanism regulates GSK3ß in polycystic kidney disease.

Glycogen synthase kinase 3ß (GSK3ß), a serine/threonine protein kinase, is commonly known to be regulated at the level of its activity. However, in some diseases including polycystic kidney disease (PKD), GSK3ß expression is increased and plays a pathophysiological role. The current studies aimed to determine the mechanism for the increased GSK3ß expression in PKD and its significance to disease progression. In mouse models of PKD, increases in renal GSK3ß corresponded with increases in renal cAMP levels and disease progression. In vivo and in vitro studies revealed that GSK3ß is a cAMP-responsive gene, and elevated cAMP levels, as seen in PKD, can increase GSK3ß expression. In normal mice, vasopressin signaling induced by water deprivation increased GSK3ß expression, which decreased following rehydration. Examination of the GSK3ß promoter revealed five potential binding sites for the transcription factor, cAMP response element binding protein (CREB). CREB was found to bind to GSK3ß promoter and essential for cAMP-mediated regulation of GSK3ß. Importantly, this regulation was demonstrated to be part of a feed-forward loop in which cAMP through CREB regulates GSK3ß expression, and GSK3ß in turn positively regulates cAMP generation. GSK3ß or CREB inhibition reduced transepithelial fluid secretion and cyst expansion in vitro Thus, disruption at any point of this destructive cycle may be therapeutically useful to reduce cyst expansion and preserve renal function in PKD.