Elucidating the role of the AD risk gene SORL1 in hiPSC‐derived microglial cells

Abstract

AbstractBackgroundhiPSC models, from which all cell types of the CNS can be generated, have provided key insights into cell‐type specific processes implicated by AD genetics. Endo‐lysosomal network (ELN) trafficking is a biological pathway implicated in AD and the endosomal receptor SORL1 is strongly associated with AD risk, however studies of SORL1 function in glial cells are lacking. Because ELN trafficking is highly related to neuroimmune function, we modeled SORL1 deficiency in hiPSC‐derived microglia and tested whether pharmacologic or genetic rescue with components of retromer improve cellular phenotypes.MethodWe used an established isogenic series of hiPSC lines that harbor deficiencies in SORL1 (SORL1 KO or SORL1 +/‐) or missense variants in SORL1 (SORL1Var) associated with AD. We differentiated microglial‐like cells (hiPSC‐MGLs) and used cellular assays that examined the function of the ELN. In hiPSC‐MGLs we measured phagocytosis, lysosome function, and pro‐inflammatory cytokine release. For rescue experiments, we treated cells with TPT‐260, a pharmacologic chaperone that stabilizes retromer, and lentiviral vectors expressing retromer subunits.ResultIn SORL1 deficient and SORL1 variant hiPSC‐MGLs, we document ELN phenotypes that converge on the lysosome, including lysosomal swelling and impaired degradation. Interestingly, we also document decreased lysosomal exocytosis, a calcium‐regulated process by which immune cells release lysosomal enzymes and undegraded substrates to the extracellular space to aid in cellular homeostasis, immune‐related signaling and tissue remodeling. In SORL1 deficient cells, lysosomal impairments were observed with abnormal neuroimmune function including differential expression and secretion of cytokines, in response to pro‐inflammatory stimuli Our preliminary data suggests that enhancement of retromer function, either genetically or pharmacologically, can improve these phenotypes.ConclusionUsing an hiPSC‐based model, we show that we can uncover dynamic phenotypes that are linked to AD risk genes. Mutations in SORL1 are now frequently discovered in early‐onset AD patients and the SORL1 locus has a strong genome‐wide signal for late‐onset AD. Together, this suggests that therapeutic strategies targeting the SORL1‐retromer pathway are greatly needed. In this study, we elucidate a role for the SORL1‐retromer pathway in neuroimmune function and provide validation that enhancing this pathway can improve cell‐type specific AD phenotypes in a human model.