Functional evaluation of Dent's disease-causing mutations:: implications for ClC-5 channel trafficking and internalization

ClC- 5 is a member of the ClC family of voltage-gated chloride channels. Loss- of- function mutations of its corresponding gene ( CLCN5) cause Dent's disease, an X- linked kidney disorder, characterized by low-molecular weight proteinuria, hypercalciuria, nephrocalcinosis/ nephrolithiasis, and progressive renal failure. Here, we examined the effect of different mutations on function and cellular trafficking of the recombinant protein. Mutant CLCN5 cDNAs were generated by site directed mutagenesis for two premature stop codon variants ( R347X and M517I fsX528), and several missense mutations ( C221R, L324R, G462 V, and R516 W). We also tested L521R ( instead of L521R fsX526 observed) and mutants G506E and R648X ( previously reported by others). After heterologous expression in Xenopus oocytes, ClC- 5 channel activity and surface expression were determined by two- electrode voltage- clamp analysis and ClC- 5 surface ELISA, respectively. Except for the R516 W and R648X variants, none of the mutated proteins induced functional chloride currents or reached the plasma membrane. This is readily understandable for the truncation mutations. Yet, the tested missense mutations are distributed over different transmembrane regions, implying that correct channel structure and orientation in the membrane is not only a prerequisite for proper ClC- 5 function but also for Golgi exit. Interestingly, the R648X mutant although functionally compromised, displayed a significant increase in surface expression. This. finding might be explained by the deletion of a ClC- 5 carboxy- terminal PY- like internalization signal, which in turn impairs channel removal from the membrane. Our observations further imply that recruitment of ClC- 5 to alternative routes ( plasma membrane or early endosomes) in the trans- Golgi network is mediated via different signal sequences.