C9orf72/SMCR8 complex maintains microglial homeostasis via RAB8A-ESCRT-mediated lysosomal repair

Microglia are critical regulators of neuroinflammation and neurodegeneration. Haploinsufficiency of C9orf72, the most common genetic cause of ALS and FTD, has been mainly linked to autophagy-lysosomal pathway defects, but its precise role in microglial lysosomal function remains unclear. Here we identify the C9orf72/SMCR8 complex as a key regulator of microglial homeostasis through lysosomal repair. Loss of C9orf72 and SMCR8 in mice causes age-dependent neuroinflammation and microgliosis, with microglia adopting a disease-associated state. In aged CNS tissue, microglia display lysosomal damage, marked by galectin-3 upregulation and its accumulation on lysosomes. To model this process, we applied the lysosomotropic agent LLOMe to microglia, which recapitulated lysosomal damage and revealed defective recruitment of phosphorylated RAB8A and the ESCRT machinery in C9orf72/SMCR8-deficient cells. Notably, mutant microglia accumulate GTP-bound RAB8A, which becomes aberrantly hyperphosphorylated and mislocalized to non-lysosomal vesicles. We further show that the GAP activity of the C9orf72/SMCR8 complex is essential for lysosomal repair. These findings uncover a previously unrecognized role for the C9orf72/SMCR8 complex in coordinating RAB8A-ESCRT-mediated lysosomal repair, thereby safeguarding microglial homeostasis and limiting neuroinflammation.