Modulating SORL1 expression affects endolysosomal trafficking function in hiPSC‐derived models of Alzheimer's disease

Background The SORL1 gene encodes for the protein SorLA, a sorting receptor involved in retromer‐related endosomal traffic. Many SORL1 genetic variants increase Alzheimer’s disease (AD) risk, and rare loss‐of‐function truncation mutations have been found to be causal of AD. SORL1 is expressed in neurons and glia of the central nervous system and loss of SORL1 has been reported AD tissue. Method To model the causal loss‐of‐function mutations, we used CRISPR/Cas9 technology to deplete SORL1 in human induced pluripotent stem cells (hiPSCs) to test the hypothesis that loss of SORL1 ( SORL1 KO) contributes to AD pathogenesis by leading to dysfunction in endo‐lysosomal trafficking. In contrast, certain SORL1 variants may be acting in a protective capacity to help prevent AD pathogenesis. To model protective variants, we used a PiggyBac Transposon system to test the hypothesis that overexpression of SORL1 ( SORL1 OE) protects against AD pathogenesis and endo‐lysosomal trafficking dysfunction. Results We report that loss of SORL1 in hiPSC‐derived neurons leads to early endosome enlargement, a cellular phenotype that is indicative of ‘traffic jams’ and is now considered a hallmark cytopathology AD. We validate defects in endo‐lysosomal traffic in SORL1 KO neurons by showing decreased localization of amyloid‐precursor protein (APP) to the degradative pathway. We confirm decreased function of the degradative pathway in SORL1 KO neurons through experiments using pH‐sensitive pHrodo dyes. Conversely, in SORL1 OE neurons we show nominal early endosome morphology, reduced localization of APP to the degradative pathway, and increased functionality of the degradative pathway. Finally, we determined that defects in endo‐lysosomal trafficking caused by loss of SORL1 affect synapse function by analyzing SORL1 KO neurons on multielectrode arrays. Conclusion Collectively, these findings clarify where and how SORL1 links to AD. Moreover, our data, together with recent findings, underscores how sporadic AD pathways that regulate endosomal trafficking, and autosomal‐dominant AD pathways that regulate APP cleavage, independently converge on AD’s defining cytopathology.