Transcriptional regulation by the cleaved polycystin-1 tail

Mutations in the gene encoding polycystin-1 (Pkd1) cause Autosomal Dominant Polycystic Kidney Disease, a genetic disorder characterized by the progressive accumulation of fluid-filled renal cysts that disrupts the normal kidney architecture, leading to renal failure. Many reports in the literature highlight the importance of proliferation and apoptosis in the pathogenesis of cyst formation. Previous data from my lab and others have demonstrated a cleavage that liberates the soluble cytoplasmic C-terminal tail of Polycystin-1 (PC1-CTT), which then translocates into the nucleus. Given the specific localization of the PC1-CTT in the nucleus, I set out to investigate the hypothesis that the PC1-CTT might be involved in the regulation of transcriptional pathways related to cellular proliferation and apoptosis.;I show that the carboxy-terminal tail of polycystin-1 is released by a gamma-secretase-mediated cleavage and that it regulates the Wnt and CHOP signaling pathways. Loss of polycytin-1 expression results in increased proliferation and apoptosis, while reintroduction of its C-terminal tail fragment into Pkd1 null cells is sufficient to reestablish normal growth rate, suppress apoptosis, and prevent cyst formation in three dimensional culture. Furthermore, expression of the PC1-CTT in zebrafish is sufficient to rescue the dorsal body curvature phenotype that is produced by morpholino-induced knockdown of PC1. Inhibition of gamma-secretase activity impairs the ability of polycystin-1 to suppress growth and apoptosis in cultured renal epithelial cells, and leads to cyst formation in three dimensional culture. In addition, gamma-secretase inhibition produces a tail curvature phenotype in zebrafish that is partially rescued by expression of the PC1CTT. The released polycystin-1 C-terminal tail fragment interacts with the transcription factors TCF and CHOP, which regulate proliferation and apoptosis, respectively. The polycystin-1 tail acts through a convergent mechanistic pathway to regulate TCF and CHOP, disrupting these protein's interactions with their common transcriptional co-activator, p300. Thus, loss of the negative regulation of Tcf and CHOP by the cleaved polycystin-1 C-terminal tail fragment may contribute to the increased proliferation and apoptosis observed in Autosomal Dominant Polycystic Kidney Disease.;Polycystin-1 plays an important regulatory role in bone growth and development, as evidenced by decreased bone density in the Pkd1 mutant mouse. The activity of the transcription factor RunX2, the master control gene for osteoblast differentiation, is downregulated in Pkd1 mutant mice, suggesting a relationship between PC1 and RunX2. However, a direct interaction between RunX2 and PC1 had not been established, and there was little evidence concerning the mechanism of PC1 regulation of RunX2. Using a novel 'Co-Activator Trap' screen consisting of a library of Gal-4 tagged transcription factors, the activity of which can be monitored by luciferase production, I identified the transcriptional co-activator TAZ (Wwtr1) to be highly stimulated by the PC1-CTT. TAZ is an essential co-activator of RunX2 activity, and may thus represent the missing mechanistic link between PC1 and RunX2. I found that the PC1-CTT strongly stimulates RunX2 activity in a TAZ-dependent manner, and that this stimulatory effect requires p300. Furthermore, I show that the PC1-CTT enhances the ability of TAZ/RunX2 to recruit p300, providing further evidence in support of the model that p300 serves as a common mechanistic link for transcriptional regulation by the PC1-CTT.