Inflammation-induced lysosomal dysfunction in human iPSC-derived microglia is exacerbated by APOE 4/4 genotype

Background. The ε4 isoform of apolipoprotein E (ApoE) is the most significant genetic risk factor for Alzheimer’s disease. Glial cells are the main source of ApoE in the brain, and in microglia, the ε4 isoform of ApoE has been shown to impair mitochondrial metabolism and the uptake of lipids and Aβ42. However, whether the ε4 isoform alters autophagy or lysosomal activity in microglia in basal and inflammatory conditions is unknown. Methods. Altogether, microglia-like cells (iMGs) from eight APOE 3/3 and six APOE 4/4 human induced pluripotent stem cell (iPSC) lines were used in this study. The responses of iMGs to Aβ42, LPS and IFNγ were studied by metabolomics, proteomics, and functional assays. Results. Here, we demonstrate that iMGs with the APOE 4/4 genotype exhibit reduced basal level pinocytosis and an overall downregulation of lysosomal proteins compared to APOE 3/3 iMGs. Inflammatory stimulation with a combination of LPS and IFNγ or Aβ42 induced PI3K/AKT/mTORC signaling pathway, increased pinocytosis, and blocked autophagic flux, leading to the accumulation of sequestosome 1 in both APOE 4/4 and APOE 3/3 iMGs. Exposure to Aβ42 furthermore caused lysosomal membrane permeabilization, which was significantly stronger in APOE 4/4 iMGs and positively correlated with the secretion of the proinflammatory chemokine IL-8. Metabolomics analysis indicated a dysregulation in amino acid metabolism, primarily L-glutamine, in APOE 4/4 iMGs. Conclusions. Overall, our results suggest that inflammation-induced metabolic reprogramming places lysosomes under substantial stress. Lysosomal stress is more detrimental in APOE 4/4 microglia, which exhibit defects in lysosomal biogenesis.