ISGylation controls exosome secretion by promoting lysosomal degradation of MVB proteins

TLDR

Exosomes are vesicles released by fusion of multivesicular bodies with the plasma membrane and mediate cell‑to‑cell communication, with post‑translational modifications influencing protein sorting into these vesicles. The study aims to determine whether ISGylation, a ubiquitin‑like modification, controls exosome release. Using ISG15‑knockout mice and mice expressing a catalytically inactive USP18, the authors show that ISG15 conjugation regulates exosome secretion both in vitro and in vivo. ISGylation decreases MVB numbers, promotes their lysosomal co‑localization and degradation of proteins such as TSG101, impairs exosome secretion, and this effect is reversed by lysosomal or autophagy inhibition, establishing ISGylation as a novel regulator of exosome production.

Abstract

Abstract Exosomes are vesicles secreted to the extracellular environment through fusion with the plasma membrane of specific endosomes called multivesicular bodies (MVB) and mediate cell-to-cell communication in many biological processes. Posttranslational modifications are involved in the sorting of specific proteins into exosomes. Here we identify ISGylation as a ubiquitin-like modification that controls exosome release. ISGylation induction decreases MVB numbers and impairs exosome secretion. Using ISG15-knockout mice and mice expressing the enzymatically inactive form of the de-ISGylase USP18, we demonstrate in vitro and in vivo that ISG15 conjugation regulates exosome secretion. ISG15 conjugation triggers MVB co-localization with lysosomes and promotes the aggregation and degradation of MVB proteins. Accordingly, inhibition of lysosomal function or autophagy restores exosome secretion. Specifically, ISGylation of the MVB protein TSG101 induces its aggregation and degradation, being sufficient to impair exosome secretion. These results identify ISGylation as a novel ubiquitin-like modifier in the control of exosome production.