Role for Rab7 in maturation of late autophagic vacuoles

TLDR

Rab7 is a small GTPase involved in late endocytic trafficking and lysosome biogenesis, but its role in mammalian autophagy remains unclear. The authors inhibited Rab7 function by RNA interference and overexpression of a dominant‑negative Rab7 mutant. Rab7 is required for the final maturation of late autophagic vacuoles, as its inhibition prevents the formation of perinuclear LC3‑Rab7 aggregates and delays recruitment to vacuoles, whereas early autophagosome maturation proceeds independently of Rab7.

Abstract

The small GTP binding protein Rab7 has a role in the late endocytic pathway and lysosome biogenesis. The role of mammalian Rab7 in autophagy is, however, unknown. We have addressed this by inhibiting Rab7 function with RNA interference and overexpression of dominant negative Rab7. We show here that Rab7 was needed for the formation of preferably perinuclear, large aggregates, where the autophagosome marker LC3 colocalised with Rab7 and late endosomal and lysosomal markers. By electron microscopy we showed that these large aggregates corresponded to autophagic vacuoles surrounding late endosomal or lysosomal vesicles. Our experiments with quantitative electron microscopy showed that Rab7 was not needed for the initial maturation of early autophagosomes to late autophagic vacuoles, but that it participated in the final maturation of late autophagic vacuoles. Finally, we showed that the recruitment of Rab7 to autophagic vacuoles was retarded in cells deficient in the lysosomal membrane proteins Lamp1 and Lamp2, which we have recently shown to accumulate late autophagic vacuoles during starvation. In conclusion, our results showed a role for Rab7 in the final maturation of late autophagic vacuoles.