Altered voltage-dependence of slowly activating chloride-proton antiport by late endosomal ClC-6 explains distinct neurological disorders.

ABSTRACT

ClC-6 is an intracellularly localised member of the CLC family of chloride transport proteins. It presumably functions in the endolysosomal compartment as a chloride-proton antiporter, despite a paucity of biophysical studies in direct support. Observations of lysosomal storage disease, as well as neurodegenerative disorders, emerge with its disruption by knockout or mutation, respectively. An incomplete understanding of wild-type ClC-6 function obscures clear mechanistic insight into disease aetiology. Here, high-resolution recording protocols that incorporate extreme voltage pulses permit detailed biophysical measurement and analysis of transient capacitive, as well as ionic transport currents. This approach reveals that wild-type ClC-6 activation and transport require depolarisation to voltages beyond 140 mV. Mutant Y553C associated with early-onset neurodegeneration exerts gain-of-function by shifting the half-maximal voltage for activation to less depolarised voltages. Moreover, we show that the E267A proton glutamate mutant conserves transport currents, albeit reduced. Lastly, the positive shift in activation voltage shown by V580M, a mutant identified in a patient with late-onset lysosomal storage disease, can explain loss-of-function leading to disease. KEY POINTS Ionic composition and pH within intracellular compartments, such as endolysosomes, rely on the activity of chloride/proton transporters including ClC-6. Distinct CLCN6 mutations were previously found in individuals with neurodegenerative disease, and also putatively associated with neuronal ceroid lipofuscinosis. Limited knowledge of wild-type ClC-6 transport function impedes understanding of mechanisms underlying these conditions. We resolved transient and transport currents that permit measurement of voltage- and pH-dependences, as well as kinetics, for wild-type and disease-associated mutant ClC-6s. These findings define wild-type ClC-6 function robustly, and reveal how alterations of the slow activation gating of the transporter cause different kinds of neurological diseases.