The 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 frequently mutated gene in amyotrophic lateral sclerosis and frontotemporal dementia, has been linked to autophagy-lysosomal pathway defects, but the role of C9orf72 in microglia remains unclear. Here, we identify the C9orf72/SMCR8 complex as a key regulator of microglial homeostasis through promoting lysosomal membrane repair. Loss of C9orf72 and SMCR8 in mice causes age‑dependent neuroinflammation and microgliosis, with microglia adopting a disease-associated state. In aged brain and spinal cord tissue, microglia display lysosomal damage marked by galectin‑3 accumulation. Using a lysosomotropic agent to induce lysosomal damage in microglia, we find that C9orf72/SMCR8-deficient cells accumulate damaged lysosomes and show defective recruitment of phosphorylated RAB8A and the Endosomal Sorting Complexes Required for Transport (ESCRT) machinery to damaged lysosomes. Notably, mutant microglia accumulate GTP‑bound RAB8A, which becomes hyperphosphorylated and mislocalized to RAB7-positive, LAMP1-negative vesicles. The GTPase-activating activity of the C9orf72/SMCR8 complex is essential for lysosomal repair. Our findings reveal that the C9orf72/SMCR8 complex coordinates RAB8A-ESCRT-mediated lysosomal repair to safeguard microglial homeostasis and limit neuroinflammation.