Homocysteine disrupts lysosomal function by V-ATPase inhibition.

Lysosomes are degradation and signaling organelles central to metabolic homeostasis. It remains unclear whether and how harmful metabolites compromise lysosome function in the etiopathology of metabolic disorders. Combining Caenorhabditiselegans and mouse models, we demonstrate that homocysteine, an intermediate in methionine-cysteine metabolism and the cause of the life-threatening disease homocystinuria, disrupts lysosomal functions. In C. elegans, mutations in cystathionine β-synthase cause strong buildup of homocysteine and developmental arrest. We reveal that homocysteine binds to and homocysteinylates V-ATPase, causing its inhibition and consequently impairment of lysosomal degradative capacity. This leads to enormous enlargement of lysosomes with extensive cargo accumulation and lysosomal membrane damage in severe cases. Cbs-deficient mice similarly accumulate homocysteine, displaying abnormal or damaged lysosomes reminiscent of lysosomal storage diseases in multiple tissues. These findings not only uncover how a metabolite can damage lysosomes but also establish lysosomal impairment as a critical contributing factor to homocystinuria and homocysteine-related diseases.