RhoGEF12 regulates endosomal SORL1-retromer and its inhibition is therapeutic in human neuronal models of Alzheimer's disease.

The interaction of the endosomal sorting protein SORL1 with the retromer complex at endosomal membranes controls a recycling pathway whose dysfunction is pathogenic in Alzheimer's disease (AD) and is linked to other neurodegenerative disorders. To search for novel therapeutic targets, we hypothesize that endosomal SORL1-retromer might be regulated by SORL1's cytoplasmic tail. We begin by completing an in vitro analysis of the tail and show that its phosphorylation by ROCK2 (Rho-associated kinase 2) reduces SORL1's affinity to retromer. Since RhoGEF12 (Rho guanine nucleotide exchange factor 12) is an upstream activator of ROCK2 that is upregulated in AD, we used a RhoGEF12 pharmacological inhibitor to mechanistically and therapeutically validate the findings in neuronal cultures. First, in mouse neurons we confirm that the inhibitor increases endosomal SORL1-retromer. Next, we turned to human iPSC-derived neurons to show that the inhibitor reduces Aβ40 and Aβ42, an indicator of pathway upregulation, in a SORL1-dependent manner. Finally, we validate its therapeutic potential by applying the RhoGEF12 inhibitor to human iPSC-derived neurons expressing AD-associated mutations in either APP or SORL1. Collectively, our results identify a novel and therapeutically amenable mechanism that regulates endosomal SORL1-retromer and preclinically validate RhoGEF12 as a therapeutic target for AD and potentially other neurodegenerative disorders.