CTSL and neurodegenerative disease: Indeed, activated microglia secrete the lysosome protease CTSL6,28,43, and CTSL has been found to be involved in some in vitro and in vivo models of several neurodegenerative diseases, including PD44; however, few studies have explored the specific mechanism of CTSL secretion by microglia and clarified whether secreted CTSL contributes to the pathogenesis of PD.