Chloride and the endosomal-lysosomal pathway: emerging roles of CLC chloride transporters.
Several members of the CLC family of Cl(-) channels and transporters are expressed in vesicles of the endocytotic-lysosomal pathway, all of which are acidified by V-type proton pumps. These CLC proteins are thought to facilitate vesicular acidification by neutralizing the electric current of the H(+)-ATPase. Indeed, the disruption of ClC-5 impaired the acidification of endosomes, and the knock-out (KO) of ClC-3 that of endosomes and synaptic vesicles. KO mice are available for all vesicular CLCs ( ClC-3 to ClC-7), and ClC-5 and ClC-7, as well as its beta-subunit Ostm1, are mutated in human disease. The associated mouse and human pathologies, ranging from impaired endocytosis and nephrolithiasis ( ClC-5) to neurodegeneration ( ClC-3), lysosomal storage disease ( ClC-6, ClC-7/ Ostm1) and osteopetrosis ( ClC-7/ Ostm1), were crucial in identifying the physiological roles of vesicular CLCs. Whereas the intracellular localization of ClC-6 and ClC-7/ Ostm1 precluded biophysical studies, the partial expression of ClC-4 and -5 at the cell surface allowed the detection of strongly outwardly rectifying currents that depended on anions and pH. Surprisingly, ClC-4 and ClC-5 (and probably ClC-3) do not function as Cl(-) channels, but rather as electrogenic Cl(-)-H(+) exchangers. This hints at an important role for luminal chloride in the endosomal-lysosomal system.[1]References
- Chloride and the endosomal-lysosomal pathway: emerging roles of CLC chloride transporters. Jentsch, T.J. J. Physiol. (Lond.) (2007) [Pubmed]
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