Numb positively regulates autophagic flux via regulating lysosomal function

Autophagy is a lysosome-dependent catabolic process involving in the degradation and recycling of unnecessary or damaged proteins and organelles. Emerging evidence indicates that autophagy dysfunction is closely related to various human diseases including cancer, aging, myopathies and neurodegenerative disorders. Here, using genetic knockdown, we uncover the role of Numb, an endocytic adaptor protein, in regulating the late steps of autophagy. We found that Numb depletion led to the accumulation of autophagic vacuole, as verified by RFP-LC3 staining combined with transmission electron microscopy. Further investigation indicated that Numb depletion impaired autophagic degradation through inhibiting the activities of lysosomal enzymes (Cathepsin D, β-glucuronidase and β-glucosidase). Moreover, Numb depletion induced elevation of lysosomal pH values and decrease of glycosylated lysosome-associated membrane proteins. We further observed that Rab7 activity was inhibited in Numb-depleted cells. Together, our findings revealed a novel function of Numb and its likely mechanism in regulation of autophagy events.