An impaired routing of wild-type aquaporin-2 after tetramerization with anaquaporin-2 mutant explains dominant nephrogenic diabetes insipidus

Autosomal recessive and dominant nephrogenic diabetes insipidus (NDI), a di sease in which the kidney is unable to concentrate urine in response to vas opressin, are caused by mutations in the aquaporin-2 (AQP2) gene. Missense AQP2 proteins in recessive NDI have been shown to be retarded in the endopl asmic reticulum, whereas AQP2-E258K, an AQP2 mutant in dominant NDI, was re tained in the Golgi complex. In this study, we identified the molecular mec hanisms underlying recessive and dominant NDI, Sucrose gradient centrifugat ion of rat and human kidney proteins and subsequent immunoblotting revealed that AQP2 forms homotetramers. When expressed in oocytes, wild-type AQP2 a nd AQP2-E258K also formed homotetramers, whereas AQP2-R187C, a mutant in re cessive NDI, was expressed as a monomer. Upon co-injection, AQP2-E258K, but not AQP2-R187C, was able to heterotetramerize with wild-type AQP2, Since a n AQP monomer is the functional unit and AQP2-E258K is a functional but mis routed water channel, heterotetramerization of AQP2-E258K with wild-type AQ P2 and inhibition of further routing of this complex to the plasma membrane is the cause of dominant NDI, This case of NDI is the first example of a d ominant disease in which the 'loss-of-function' phenotype is caused by an i mpaired routing rather than impaired function of the wild-type protein.