Endosome maturation, transport and functions

Efficient sorting of the material internalized by endocytosis is essential for key cellular functions and represents a, if not the, major trafficking pathway in mammalian cells. Incoming material – solutes, receptors and cargos, lipids and even pathogenic agents – are routed to various destinations within mammalian cells at two major sorting stations: the early and late endosome. The early endosome receives all manner of incoming material from the plasma membrane, as well as from the Golgi, and serves as an initial sorting nexus routing molecules back to the cell surface through recycling endosomes, to the trans-Golgi network by retrograde transport, or on to the late endosome/lysosome. The early endosome also regulates cell signaling, through the downregulation of internalized receptors, which are packaged into intralumenal vesicles that arise from inward invaginations of the limiting membrane. These multivesicular regions detach or mature from early endosomes and become free endocytic carrier vesicle/multivesicular body, which transports cargoes to late endosomes. The late endosome provides a central hub for incoming traffic from the endocytic, biosynthetic and autophagic pathways and outgoing traffic to the lysosomes, the Golgi complex or the plasma membrane. They also function as a key sensing/signaling platform that inform the cell about the nutrient situation. Herein we summarize the current understanding of the organization and functions of the endocytic pathway, differences across species, and the process of endosome maturation.