1. Membrane Transport Biophysics Unit, and,
2. Light Microscopy Facility, Porter Neuroscience Research Center, National Institute of Neurological Disorders and Stroke, National Institutes of Health, 35 Convent Drive, Building 35, MSC 3701, Bethesda, Maryland 20892, USA

Correspondence to: Joseph A. Mindell¹ Correspondence and requests for materials should be addressed to J.A.M. (Email: mindellj@ninds.nih.gov).

Abstract

Lysosomes are the stomachs of the cell—terminal organelles on the endocytic pathway where internalized macromolecules are degraded. Containing a wide range of hydrolytic enzymes, lysosomes depend on maintaining acidic luminal pH values for efficient function. Although acidification is mediated by a V-type proton ATPase, a parallel anion pathway is essential to allow bulk proton transport^{1, 2}. The molecular identity of this anion transporter remains unknown. Recent results of knockout experiments raise the possibility that ClC-7, a member of the CLC family of anion channels and transporters, is a contributor to this pathway in an osteoclast lysosome-like compartment, with loss of ClC-7 function causing osteopetrosis³. Several mammalian members of the CLC family have been characterized in detail; some (including ClC-0, ClC-1 and ClC-2) function as Cl^--conducting ion channels⁴, whereas others act as Cl^-/H⁺antiporters (ClC-4 and ClC-5)^{5, 6}. However, previous attempts at heterologous expression of ClC-7 have failed to yield evidence of functional protein, so it is unclear whether ClC-7 has an important function in lysosomal biology, and also whether this protein functions as a Cl^- channel, a Cl^-/H⁺ antiporter, or as something else entirely. Here we directly demonstrate an anion transport pathway in lysosomes that has the defining characteristics of a CLC Cl^-/H⁺ antiporter and show that this transporter is the predominant route for Cl^- through the lysosomal membrane. Furthermore, knockdown of ClC-7 expression by short interfering RNA can essentially ablate this lysosomal Cl^-/H⁺ antiport activity and can strongly diminish the ability of lysosomes to acidify in vivo, demonstrating that ClC-7 is a Cl^-/H⁺ antiporter, that it constitutes the major Cl^- permeability of lysosomes, and that it is important in lysosomal acidification.

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