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.