Cell type-specific roles of lysosomal chloride/proton exchange

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ClC-7 is a Cl-/H+ exchanger that, together with its beta-subunit Ostm1, localizes to lysosomes and to the osteoclast ruffled border. Dysfunction of ClC-7/Ostm1 leads to osteopetrosis and a neurodegenerative lysosomal storage disease in mice and patients. The osteopetrosis may in part be caused by the underdevelopment of the ruffled border of osteoclasts, which is normally formed by lysosomal exocytosis. Indeed, ClC-7-deficient fibroblasts and mast cells showed significantly reduced lysosomal exocytosis or degranulation, respectively. The lysosomal dysfunction, which involves a slowed protein degradation and the accumulation of autophagic material, is likely due to alterations of the lysosomal ion composition. Using cells derived from our various mouse models with different mutations in the Clcn7 gene, we found that lysosomes were acidified to a normally low pH. However, we found a drastic decrease in the lysosomal Cl- concentration and further alterations of the lysosomal ion homeostasis upon ClC-7 deficiency or its conversion into a pure Cl- channel. We recently generated a novel mouse model carrying a human osteopetrosis-causing mutation that accelerates the gating of ClC-7. Surprisingly, these osteopetrotic mice did not show any signs of lysosomal dysfunction or neurodegeneration. In contrast, recently reported patients and another mouse model with a gain-of-function mutation in ClC-7 displayed a severe lysosomal disease but lacked bone pathology. These differential effects by ClC-7 alteration suggest cell type-specific roles for ClC-7. This project addresses these cell type-specific functions and the involvement of ClC-7 in lysosomal ion homeostasis in general. One focus lies on the differential roles of ClC-7 in bone resorption and basic lysosomal function. We will study and compare the effects of various ClC-7 mutations on the ion concentrations, morphology and function of lysosomes in different cell types, including neurons and osteoclasts, from our respective mouse models. Another aspect is the function of ClC-7 in the immune system. Using cell type-specific ClC-7 KO mice, we will investigate the roles of ClC-7 in the exocytosis of lysosome-related organelles, phagocytosis and micropinocytosis by specialized immune cells on a cellular and organismal level. Lastly, we plan to gain mechanistic insight into ClC-7 activity and the alterations of lysosomal ion concentrations by ClC-7 dysfunction. We aim at developing an optical sensor to monitor ClC-7 activity on lysosomes. Moreover, we will measure further ion species and we will investigate the reason for alterations thereof with the diverse disease-causing alterations of ClC-7.