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 mice in which ClC-3 is converted into a Cl channel.

A missense mutation accelerating the gating of the lysosomal Cl-/H+-exchanger ClC-7/Ostm1 causes osteopetrosis with gingival hamartomas in cattle.

Chloride-proton exchange by the lysosomal anion transporter ClC-7/Ostm1 is of pivotal importance for the physiology of lysosomes and bone resorption. Mice lacking either ClC-7 or Ostm1 develop a lysosomal storage disease and mutations in either protein have been found to underlie osteopetrosis in mice and humans. Some human disease-causing CLCN7 mutations accelerate the usually slow voltage-dependent gating of ClC-7/Ostm1.

CLCN7 and TCIRG1 mutations differentially affect bone matrix mineralization in osteopetrotic individuals.

Osteopetrosis is an inherited disorder of impaired bone resorption, with the most commonly affected genes being CLCN7 and TCIRG1, encoding the Cl(-) /H(+) exchanger CLC-7 and the a3 subunit of the vacuolar H(+) -ATPase, respectively. We and others have previously shown that the disease is frequently accompanied by osteomalacia, and that this additional pathology is also found in Tcirg1-deficient oc/oc mice. The remaining question was whether osteoid enrichment is specifically associated with TCIRG1 inactivation, or whether CLCN7 mutations would also cause skeletal mineralization defects.

Common gating of both CLC transporter subunits underlies voltage-dependent activation of the 2Cl-/1H+ exchanger ClC-7/Ostm1.

CLC anion transporters form dimers that function either as Cl(-) channels or as electrogenic Cl(-)/H(+) exchangers. CLC channels display two different types of "gates," "protopore" gates that open and close the two pores of a CLC dimer independently of each other and common gates that act on both pores simultaneously. ClC-7/Ostm1 is a lysosomal 2Cl(-)/1H(+) exchanger that is slowly activated by depolarization.

ClC-7 is a slowly voltage-gated 2Cl(-)/1H(+)-exchanger and requires Ostm1 for transport activity.

Mutations in the ClC-7/Ostm1 ion transporter lead to osteopetrosis and lysosomal storage disease. Its lysosomal localization hitherto precluded detailed functional characterization. Using a mutated ClC-7 that reaches the plasma membrane, we now show that both the aminoterminus and transmembrane span of the Ostm1 β-subunit are required for ClC-7 Cl(-)/H(+)-exchange, whereas the Ostm1 transmembrane domain suffices for its ClC-7-dependent trafficking to lysosomes.