Uncoupling endosomal CLC chloride/proton exchange causes severe neurodegeneration

CLC chloride/proton exchangers may support acidification of endolysosomes and raise their luminal Cl(−) concentration. Disruption of endosomal ClC‐3 causes severe neurodegeneration. To assess the importance of ClC‐3 Cl(−)/H(+) exchange, we now generate Clcn3 (unc/unc) mice in which ClC‐3 is converted into a Cl(−) channel. Unlike Clcn3 (−/−) mice, Clcn3 (unc/unc) mice appear normal owing to compensation by ClC‐4 with which ClC‐3 forms heteromers. ClC‐4 protein levels are strongly reduced in Clcn3 (−/−), but not in Clcn3 (unc/unc) mice because ClC‐3(unc) binds and stabilizes ClC‐4 like wild‐type ClC‐3. Although mice lacking ClC‐4 appear healthy, its absence in Clcn3 (unc/unc)/Clcn4 (−/−) mice entails even stronger neurodegeneration than observed in Clcn3 (−/−) mice. A fraction of ClC‐3 is found on synaptic vesicles, but miniature postsynaptic currents and synaptic vesicle acidification are not affected in Clcn3 (unc/unc) or Clcn3 (−/−) mice before neurodegeneration sets in. Both, Cl(−)/H(+)‐exchange activity and the stabilizing effect on ClC‐4, are central to the biological function of ClC‐3.