ClC transporters: discoveries and challenges in defining the mechanisms underlying function and regulation of ClC-5

Abstract

The involvement of several members of the chloride channel (ClC) family of membrane proteins in human disease highlights the need to define the mechanisms underlying their function and the consequences of disease-causing mutations. Despite the utility of high-resolution structural models, our understanding of the molecular basis for function of the chloride channels and transporters in the family remains incomplete. In this review, we focus on recent discoveries regarding molecular mechanisms underlying the regulated chloride:proton antiporter activity of ClC-5, the protein mutated in the Dent's disease-a kidney disease presenting with proteinuria and renal failure in severe cases. We discuss the putative role of ClC-5 in receptor-mediated endocytosis and protein uptake by the proximal renal tubule and the possible molecular and cellular consequences of disease-causing mutations. However, validation of these models will require future study of the intrinsic function of this transporter, in situ, in the membranes of recycling endosomes in proximal tubule epithelial cells.

Ion flux and the function of endosomes and lysosomes: pH is just the start

“…Nevertheless, the universality of this mechanism has been hard to reconcile with all of the published data. Both the ClC-3 and ClC-4 knock-out mice do not have gross endosomal acidification defects [15] while the requirement of ClC-7 as an electric shunt during lysosomal acidification has been disputed [6,18,19], suggesting that the role of Cl À /H þ exchange in endo-lysosome acidification is more complicated than originally proposed. Whether these discrepancies reflect the compensatory and/or overlapping activities of family members, experimental, or tissue-type differences will require further study.…”

“…Promptly, the remainder of the family was identified, but it was not until much later that the intracellular family members (ClC-3-7) were found to actually function as Cl À /H þ antiporters, due in part to the inherent difficulties of working with intracellular ion transporters [15]. Furthermore, these studies identified a single glutamate residue in the pore region which mediates the interchange from antiport exchanger to pure anion channel [13,14].…”

“…Notably, although the first characterized members of the family (ClC-1,ClC-2, Ka, Kb) were found to function as ClCs at the plasma membrane, those localized to intracellular compartments (ClC-3-7) function as electrogenic ion exchangers, catalyzing the transport of two Cl À anions for one H þ ion [13,14]. Various ClCs have been localized throughout the endocytic pathway (Table 1) from the early and recycling endosomes to the lysosome and have been proposed to be a primary determinant of endolysosomal pH [15].…”

Role of the ubiquitin system in regulating ion transport

“…The ClC family comprises both Cl − channels and Cl − /H + antiporters [94,215]. ClC5 is a Cl − /H + antiporter that is expressed primarily in endosomal membranes in the proximal tubules and is responsible for endosomal acidification, a role it carries out in collaboration with V-ATPase.…”

Novel diuretic targets

“…About half of all CLCs are secondary active transporters, catalyzing the stoichiometric exchange of two Cl Ϫ ions for one proton, while the other half are passive channels, catalyzing rapid downhill movement of Cl Ϫ (1, 109, 120). Both types play important roles in kidney tubular function (23,26,30,86,88,110,151). In the channel branch of the family, the two kidney-specific homologs, ClC-Ka and ClCKb, have central roles in tubular Cl Ϫ reabsorption that make them important targets for novel therapeutic interventions (36,41,61,63,65,75).…”