Contribution of Alzheimer's disease risk genes in the maintenance of endolysosomal transport regulation.

Abstract

Alzheimer's disease (AD) is the most common form of dementia of which the incidence increases with age. AD has a strong genetic component, for which further study is essential to identify pathways that can be targeted therapeutically, as no cure is available yet. While most studies have focused over the years on the early onset risk factors (i.e. APP, PSEN1 and PSEN2), those represent only a minor fraction of all cases. GWAS studies have identified ≈30 late-onset AD (LOAD) susceptibility loci that, when functionally clustered, are directly or indirectly linked to cellular trafficking routes and functioning. These include genes linked to cholesterol metabolism, endocytic transport regulation and lysosomal processes. The genetic findings concur with the appearance of endolysosomal abnormalities at preclinical stages, before the appearance of senile plaques and tangles. Using a CRISPR-Cas9 based approach, knock-out (KO) of various risk factors from early (PSEN1) or late (PICALM, PLD3) onset risk factors were realized in fibroblasts and neuronal SH-SY5Y cells. Analysis of the endolysosomal pathway and lysosomal functions were carried out using biochemical assays, lipidomic analysis of isolated organelles, confocal imaging as well as electron microscopy. Both the deficiency of presenilins and late risk factors led to endolysosomal dysfunctions, as characterized by an enlargement of late endosomes/lysosomes that accumulate diverse types of lipids such as cholesterol or glycerophospholipids. Further focusing on PSENs, their depletion led to a delay in the endosomal maturation, resulting in an accumulation of endosomal cargoes and adapters such as the retromer subunit VPS35. Re-expression of either catalytic or inactive PSEN1 was sufficient to correct these aberrant accumulations. Whereas the endolysosomal abnormalities could be connected to the accumulation of APP-CTFs, the nature of γ-secretase-independent contribution remains enigmatic. Activation of recycling significantly alleviated endolysosomal defects observed in PSENdKO cells. Altogether, our results underscore that different upstream mechanisms could lead to similar endolysosomal dysfunctions. Our observations are also shedding light on the potential of modulating/restoring endosomal recycling regulation as a strategy to alleviate observed early endolysosomal dysfunctions at preclinical stages of AD pathogenesis.