Elucidating APOE4‐dependent cholesterol dysregulation in endolysosomes related to Alzheimer’s disease biology

Abstract

AbstractBackgroundAberrant cholesterol processing may be the missing link in the biology of AD, and in our understanding of AD etiology. APOE4 is a major genetic risk factor for AD, and a key player in cholesterol metabolism in the brain. Previous research from our group has shown that APOE4 is related to increased lysosomal cholesterol accumulation in human iPSC‐astrocytes, and altered endolysosomal morphology has been observed in the entorhinal cortex (EC) of aged APOE4 mice. However, it is unclear how endolysosomal cholesterol mistrafficking changes with age.MethodTherefore, we studied endolysosomal cholesterol accumulation in middle (15‐16month) and old age (25‐26month) old mice, enabling us to study these processes already in mid‐life. We used brain sections of humanized APOE mice, looking at different brain regions to study regional vulnerability and temporal progression. Cholesterol was labeled using the novel and highly specific D4H probe, enabling its quantification within the endolysosomal system using immunohistochemistry. Cholesterol and endolysosomal markers were quantified and colocalized using Cellprofiler.ResultOlder APOE4 mice showed an increase of total cholesterol in their brain, and specifically an increase of lysosomal cholesterol. Middle aged mice did not show a difference in total cholesterol, however, there was a specific decrease in early endosomal (EE) and late endosomal (LE) cholesterol only in the entorhinal cortex (EC) of female APOE3 mice, while lysosomal cholesterol was increased in this group (APOE4 and APOE2 showed similar levels). The EE and LE cholesterol differences were specific to neurons, but not the lysosomal cholesterol increase, indicating that non‐neuronal cells play a role. Another striking finding was the increase of neuronal EE cholesterol in the EC of middle aged APOE3 male vs female mice, whereas the reverse was seen in APOE4 brain tissue.ConclusionAltogether these results indicate that endolysosomal cholesterol is a dynamic process throughout aging, which may play and active role in AD biology. There is a noticeable shift from EE and EL cholesterol changes in midlife, to lysosomal cholesterol accumulation in older age, which are APOE‐genotype dependent. Notably, the EC specifically of female mice show the most variable endolysosomal cholesterol levels, and may be involved in increased vulnerability to pathology.