The Cl-/H+ antiporter ClC-7 is the primary chloride permeation pathway in lysosomes

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 osteopetrosis(3). 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 channels(4), 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.