MERIT, a cellular system coordinating lysosomal repair, removal and replacement.

Membrane integrity is essential for cellular survival and function. The spectrum of mechanisms protecting cellular and intracellular membranes is not fully known. Our recent work has uncovered a cellular system termed MERIT for lysosomalmembranerepair, removal and replacement. Specifically, lysosomal membrane damage induces, in succession, ESCRT-dependent membrane repair, macroautophagy/autophagy-dominant removal of damaged lysosomes, and initiation of lysosomal biogenesis via transcriptional programs. The MERIT system is governed by galectins, a family of cytosolically synthesized lectins recognizing beta-galactoside glycans. We found in this study that LGALS3 (galectin 3) detects membrane damage by detecting exposed lumenal glycosyl groups, recruits and organizes ESCRT components PDCD6IP/ALIX, CHMP4A, and CHMPB at damaged sites on the lysosomes, and facilitates ESCRT-driven repair of lysosomal membrane. At later stages, LGALS3 cooperates with TRIM16, an autophagy receptor-regulator, to engage autophagy machinery in removal of excessively damaged lysosomes. In the absence of LGALS3, repair and autophagy are less efficient, whereas TFEB nuclear translocation increases to compensate lysosomal deficiency via de novo lysosomal biogenesis. The MERIT system protects endomembrane integrity against a broad spectrum of agents damaging the endolysosomal network including lysosomotropic drugs,Mycobacterium tuberculosis, or neurotoxic MAPT/tau.