ClC-5: a chloride channel with multiple roles in renal tubular albumin uptake.

ClC-5 is a chloride (Cl(-)) channel expressed in renal tubules and is critical for normal tubular function. Loss of function nonsense or missense mutations in ClC-5 are associated with Dent's disease, a condition in which patients present with low molecular weight (LMW) proteinuria (including albuminuria), hypercalciuria and nephrolithiasis. Several key studies in ClC-5 knockout mice have shown that the proteinuria results from defective tubular reabsorption of proteins. ClC-5 is typically regarded as an intracellular Cl(-) channel and thus the defect in this receptor-mediated uptake pathway was initially attributed to the failure of the early endosomes to acidify correctly. ClC-5 was postulated to play a key role in transporting the Cl(-) ions required to compensate for the movement of H(+) during endosomal acidification. However, more recent studies suggest additional roles for ClC-5 in the endocytosis of albumin. ClC-5 is now known to be expressed at low levels at the cell surface and appears to be a key component in the assembly of the macromolecular complex involved in protein endocytosis. Furthermore, mutations in ClC-5 affect the trafficking of v-H(+)-ATPase and result in decreased expression of the albumin receptor megalin/cubulin. Thus, the expression of ClC-5 at the cell surface as well as its presence in endosomes appears to be essential for normal protein uptake by the renal proximal tubule.