THE ROLE OF THE MURINE HOMEOBOX GENE CUX-1 IN KIDNEY DEVELOPMENT AND POLYCYSTIC KIDNEY DISEASE

Abstract

The murine homeobox gene cux-1 is evolutionarily conserved in drosophila, mice and humans. Cux-1 contains four DNA binding domains (3 cut repeat domains and a homeodomain) and functions as a transcription factor that represses the expression of the cyclin kinase inhibitor p27 during S-phase of the cell-cycle. Cux-1 is highly expressed in proliferating cells within the nephrogenic zone of developing kidneys. The role of Cux-1 during kidney development and Polycystic Kidney Disease (PKD) is unclear. Cux-1 is a transcription factor that binds to DNA when it is dephosphorylated. Calcineurin A (CnA) is a phosphatase that might be involved in regulating Cux-1 as both are expressed during early kidney development. Previous studies demonstrated that CnA knockout (-/-) mice display renal hypoplasia associated with ectopic expression of p27 in the nephrogenic zone. The opposite phenotype was observed when Cux-1 is overexpressed. Therefore metanephric kidney cultures, overexpressing Cux-1 were grown in the presence of cyclosporine A to inhibit Calcineurin. Overexpression of Cux-1 rescued growth inhibition due to Calcineurin inhibition. Calcineurin inhibition resulted in increased phospho-Cux-1 levels suggesting that Calcineurin may regulate Cux-1 and thus revealing a new pathway in kidney development. The cpk mouse model is the most widely characterized model for PKD. A hallmark of PKD is increased cell proliferation. The mechanism of cell proliferation in PKD is unclear although deregulation of cyclin kinase inhibitors appears to be involved. Cux-1 is highly expressed in cpk kidneys, however it is unclear if Cux-1 is required for PKD. The results here demonstrate that a mutation of Cux-1 (cux-1∆CR1) which lacks a Cathepsin-L proteolytic cleavage site, results in severe PKD when crossed onto cpk mice. Upregulation of Cux-1∆CR1 was observed and correlated with attenuated levels of p27 within cpk kidneys which suggests a potential mechanism for the acceleration of PKD. Alteration of the PKD phenotype by Cux-1 suggests that Cux-1 may act as a candidate modifier gene of PKD. The collection of studies presented within this body of work has helped to elucidate the importance of Cux-1 regulation by post-translational modification which requires further investigation as a critical factor in kidney development and PKD.