Cystin is required for maintaining fibrocystin (FPC) levels and safeguarding proteome integrity in mouse renal epithelial cells

Autosomal recessive polycystic kidney disease (ARPKD) is primarily caused by mutations in PKHD1, encoding fibrocystin (FPC). Recently, patients with CYS1 mutations, encoding cystin, have also been identified. While Pkhd1 mutant mice do not exhibit ARPKD-like phenotype, cystin deficient Cys1cpk/cpk (cpk) mice do. We explored potential functional interactions between FPC and cystin in mouse kidneys and cortical collecting duct (CCD) cell lines. FPC levels were markedly reduced in both cpk kidneys and CCD cells through selective autophagy. Re-expression of Cys1 in cpk kidneys restored and siRNA silencing of Cys1 in wild type-CCD cells reduced FPC levels. In contrast, FPC deficiency in Pkhd1 mutant kidneys did not diminish cystin levels. Consistent with the previously identified role of FPC in E3 ubiquitin ligase complexes (NEDD4), we observed reduced polyubiquitination with elevated epithelial sodium channel (NEDD4 target) levels and function in cpk-CCD cells. Our results imply that cystin stabilizes FPC in E3-ligase complexes and helps to maintain proteome integrity. In cpk mice, cystin loss causes increased MYC expression and also leads to severe FPC reduction and altered proteome. Together these perturbations promote cystogenesis. We speculate that in Pkhd1 mutants FPC loss alone is insufficient for kidney cyst formation, because cystin prevents c-Myc overexpression.