Aberrant phase separation from a rare ABI3 mutation drives microglial dysfunction and Alzheimer's risk.

INTRODUCTION: Although the ABI3 S209F variant is a recognized genetic risk for Alzheimer's disease (AD), its pathogenic mechanism remains elusive. METHODS: Using AD mouse models (amyloid precursor protein/presenilin 1 [APP/PS1] transgenic mice, 5×familial Alzheimer's disease (5×FAD) transgenic mice) with microglia-specific knockdown of Abelson-interactor family member 3 (ABI3) or expression of the wild-type or S209F mutant, we assessed disease pathology. Primary mouse microglia and HMC3 cells were used to examine ABI3 function and S209F effects. Liquid-liquid phase separation (LLPS) properties were characterized in cellular systems and with recombinant proteins. RESULTS: ABI3 was highly expressed in AD microglia. ABI3 enhanced microglial clustering around amyloid beta (Aβ) plaques, promoted Aβ clearance, and ameliorated cognitive decline, whereas S209F abolished these effects. Mechanistically, ABI3 undergoes LLPS essential for microglial migration and phagocytosis. The S209 residue lies within an intrinsic disordered region, and S209F disrupts phosphorylation-dependent LLPS, thereby impairing microglial functions. DISCUSSION: The AD-associated ABI3 S209F variant drives pathogenesis by disrupting LLPS, causing microglial dysfunction. Enhancing ABI3 phase separation represents a potential therapeutic strategy to boost microglial activity against Aβ pathology.