Role of Counterions in Acidification in Mouse Liver Lysosomes

Abstract

The lysosome is a membrane-bound compartment that digests macromolecules and cellular debris. To function properly, lysosomes must maintain an acidic environment (pH 4.5-5.0), generated by the proton-pumping action of a v-type H+ ATPase. However, since the ATPase is highly electrogenic, the voltage generated by its action must be dissipated by another ion movement, known as the ‘counterion pathway.’ The mechanism of counterion movement is controversial, with proposals that it involves Cl- ions entering the lysosome or cations exiting. The ClC-7 Cl-/H+ antiporter, aCLC gene family member which is the primary pathway for Cl- movement across the membranes of rat liver lysosomes, has been proposed as an important contributor to the counterion pathway. However, several studies suggested that lysosomal pH is unaffected in cultured cells from ClC-7 knockout mice, hinting that other ion conductances contribute to the counterion pathway. Here, we sought to comprehensively analyze the role of ClC-7 and the Cl- movement it mediates in a single well-characterized system: liver lysosomes isolated from mice with tissue specific knockouts of the transporter. We loaded lysosomes with the pH-sensitive fluorescent probe FITC-dextran by intraperitoneal injection of mice, then isolated lysosomes or liver cells for further characterization. Acidification in mouse liver lysosomes is strongly dependent on the presence of Cl- in the bathing buffer, and is minimally affected by varying buffer concentrations of K+, suggesting that Cl- indeed serves as the counterion. As expected, knocking out the ClC-7 gene in these cells abolishes the previously described H+-coupled Cl- transport. Lysosomes from the knockout mouse liver are strongly reduced in ATP-driven acidification, consistent with ClC-7 acting as the primary counterion pathway in this system.