Investigating the relationship between the AD risk gene SORL1 and TDP-43 pathology in Alzheimer's Disease - Summary TDP-43 pathology occurs in the majority of individuals with high Alzheimer's disease neuropathologic change (ADNC). This accumulation of cytoplasmic hyperphosphorylated aggregates of TDP-43 (pTDP-43) in neurons has been termed limbic predominant age-related TDP-43 encephalopathy neuropathologic change (LATE-NC). LATE-NC occurs in similar brain regions that are affected by ADNC, however the underlying mechanisms of polyproteinopathies in the context of AD, including how they develop, if and how they interact, and the involvement of the diverse cell types of the CNS, are not well understood. We recently described a family with a pathogenic variant in the AD-associated gene SORL1 where several variant carriers underwent autopsy at the University of Washington Alzheimer's Disease Research Center. This variant, SORL1 R953C, segregated with high ADNC and a TDP-43 pathology that was characteristic for LATE- NC, but occurred in cases with much younger ages of onset. SORL1 has defined roles in endosomal trafficking and regulation of amyloid precursor protein (APP) processing, but how SORL1 in particular, and endosomal dysfunction in general, may contribute to polyproteinopathy in neurodegeneration remains to be explored. In this study we will use human induced pluripotent stem cell (hiPSC)-derived neural cells generated from SORL1 variant carriers and controls to investigate how dysfunction in endosomal trafficking and cellular stress may contribute to the accumulation, mis-localization, and phosphorylation of TDP-43. We will also test whether cells that harbor pathogenic SORL1 variants are more susceptible to modulation of TDP-43 expression. Because pathologic TDP-43 has been described in both neurons and glia, we will generate cortical neurons and astrocytes from hiPSCs for these experiments. We will perform a comprehensive characterization of endosomal pathology in post-mortem brain tissue from donors with ADNC+LATE-NC vs. ADNC or LATE-NC only. We will also analyze endosomal pathology from post-mortem samples of SORL1 variant carriers. For these studies we will use our newly established pipeline for high-resolution imaging of endosomal morphology in post-mortem tissue. Our goal in this exploratory R21 proposal is to test the hypothesis that endosomal dysfunction is a driver of TDP-43 co- pathology in AD and to develop a model of LATE-NC in a tractable, human in vitro system. Our studies will elucidate the molecular mechanisms of how dysfunction in SORL1 and endosomal pathways may lead to TDP- 43 pathology and provide a comprehensive analysis of endosomal pathology in brains of subjects with LATE-NC which, if successful, could open novel therapeutic avenues.
