Compartment-specific inhibition of cytosolic Cathepsin B mitigates microglial senescence in the aging brain.

Microglial senescence is a hallmark of brain aging, but how lysosomal dysfunction fuels their pro-aging activity remains unresolved. Here we identify cytosolic Cathepsin B (CatB) as a pivotal driver of microglial senescence. In aged mice and senescence-induced models, lysosomal membrane permeabilization releases CatB into the cytosol, where it remains enzymatically active at neutral pH. Cytosolic CatB promotes senescence, and its inhibition mitigates this process, whereas cytosolic delivery of recombinant CatB accelerates it. Mechanistically, cytosolic CatB binds and degrades small nuclear ribonucleoprotein polypeptide E (SNRPE), a spliceosome component, inducing senescence-associated phenotypes without triggering cell death. Notably, conventional CatB inhibitors active under acidic conditions are ineffective, while neutral pH-active inhibitors block microglial senescence both in vitro and in aged mouse brains. These findings uncover cytosolic CatB as a spliceosome-targeting mediator of microglial senescence and suggest SNRPE stabilization or compartment-specific CatB inhibition as potential therapeutic strategies to counter brain aging and neurodegeneration.