Fig. 1

Schematic representation of biogenesis of ILVs. ILVs are the precursors of exosomes created by the inward budding of microdomains and their fission. Assembly of ESCRT machinery starts with the localization of ESCRT-0 on EEs. PI3P aids in recruiting early ESCRT protein Hrs. Hrs then binds with Tsg101 to involve ESCRT-I in the process. ESCRT-I, in turn, binds with ESCRT-II. Subsequent binding of CHMP6 of ESCRT-III with ESCRT-II activates CHMP4 to the endosomal membrane. CHMP4, along with ubiquitinated protein play a pivot in the inward budding of the membrane to form ILVs containing proteins, DNA and miRs. Polymerization of CHMP4 forming spiral coils store potential energy, on elastic compression, this energy gets released giving rise to negative curvature within the membrane As part of the endocytic mechanism, exosome precursors are discharged into MVBs. Also, ESCRT-independent mechanisms involving ceramide, tetraspanins (CD63), and Rabs (Rab31), respectively, have been proposed. In the nSMase2-ceramide pathway, FAN upregulates ceramide production from sphingomyelin. Ceramide, self-associates to form raft structures within the cell membrane to initiate the formation of curvature and subsequent budding. Caveolin-1 and flotilin play vital roles in raft formation and sorting activities. After the sorting procedures are finished, MVBs actively bypass lysosomal fusion. Rabs ensure to prevent MVBs from degradation before fusing with the plasma membrane. Sphingomyelin tends to reorganize the plasma membrane into lipid raft microdomains which subsequently trigger negative curvature in the membrane. Arrows indicate downstream cellular events. EE, early endosome; ESCRT, an endosomal sorting complex required for transport; ILVs, intraluminal vesicles; miR, microRNA; MVB, multivesicular body