Polycystin-2 expression is regulated by a PC2-binding domain in the intracellular portion of fibrocystin.

Autosomal dominant (ADPKD) and autosomal recessive (ARPKD) polycystic kidney disease are caused by mutations in Pkd1/Pkd2 and Pkhd1, which encode polycystins (PCs) and fibrocystin/polyductin (FPC). Our recent study reported that a deficiency in FPC increases the severity of cystic disease in Pkd2 mutants and down-regulates PC2 in vivo, but the precise molecular mechanism of these effects is unknown (Kim, I., Fu, Y., Hui, K., Moeckel, G., Mai, W., Li, C., Liang, D., Zhao, P., Ma, J., Chen, X.-Z., George, A. L., Jr., Coffey, R. J., Feng, Z. P., and Wu, G. (2008) J. Am. Soc. Nephrol. 19, 455-468). In this study, through the use of deletion and mutagenesis strategies, we identified a PC2-binding domain in the intracellular C terminus of FPC and an FPC-binding domain in the intracellular N terminus of PC2. These binding domains provide a molecular basis for the physical interaction between PC2 and FPC. In addition, we also found that physical interaction between the binding domains of PC2 and FPC is able to prevent down-regulation of PC2 induced by loss of FPC. In vivo, we generated a mouse model of ADPKD with hypomorphic Pkd2 alleles (Pkd2nf3/nf3) and show that PC2 down-regulation is accompanied by a phenotype similar to that of Pkhd1(-/-) mice. These findings demonstrate a common mechanism underlying cystogenesis in ADPKD and ARPKD and provide insight into the molecular relationship between PC2 and FPC.