RIN3 mutations impairing binding of the Alzheimer's disease-associated protein BIN1 lead to RAB5 hyperactivation and endosomal pathology.

Alzheimer's disease (AD) risk is strongly influenced by genetic variants that converge on pathways regulating endosomal homeostasis. Among these, BIN1 and RIN3 have emerged as susceptibility genes, yet their functional relationship in AD remains largely unknown. Here, we investigated how BIN1 and RIN3 interaction regulates RAB5 activity and endosomal pathology. RIN3 has been shown to bind BIN1, and we previously reported that this interaction modulates amyloid-β (Aβ) precursor protein (APP) trafficking and Aβ generation in vitro. To extend these findings, we used Rin3 constitutive knockout (Rin3-CKO) mice and CRISPR-Cas9-edited human induced pluripotent stem cell-derived neurons carrying either BIN1 knockout or rare familial AD RIN3 missense mutations within the BIN1-binding domain. We found that disruption of BIN1-RIN3 binding, through either genetic deletion or pathogenic RIN3 variants, resulted in RIN3-mediated RAB5 hyperactivation and enlargement of neuronal endosomes, a hallmark of early AD pathology. Transcriptomic profiling further revealed dysregulated expression of AD-related genes. Together, these findings establish BIN1 as a critical regulator of RIN3-driven RAB5 activation and neuronal endosomal homeostasis.