C9orf72repeat expansion-carrying iPSC-microglia from FTD patients show increased phagocytic activity concomitantly with decreased number of autophagosomal-lysosomal vesicles

Abstract

AbstractC9orf72hexanucleotide repeat expansion (HRE) is a major genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. The role of microglia in theseC9orf72HRE-associated diseases is understudied. To elucidate effects ofC9orf72HRE on microglia, we have characterized human induced pluripotent stem cell-derived microglia (iMG) from behavioral variant frontotemporal dementia (bvFTD) patients carrying theC9orf72HRE.C9orf72HRE iMG were compared to iMG from healthy controls and sporadic bvFTD patients. The phenotypes of iMG were analyzed using bulk RNA sequencing, biochemical and immunofluorescence analyses, and live cell imaging.C9orf72HRE-carrying iMG showed nuclear RNA foci and poly-GP dipeptide repeat proteins but no decreased C9orf72 mRNA or protein expression. TDP-43 pathology was absent from all bvFTD iMG. As compared to healthy control iMG, quantitative immunofluorescence analyses indicated that all bvFTD iMG had reduced number, size, and intensity of LAMP2-A-positive vesicles.C9orf72HRE-carrying iMG additionally showed decreased number, size, and intensity of p62/SQSTM1-positive vesicles. These changes were accompanied by increased phagocytic activity of theC9orf72HRE-carrying iMG. Serum starvation increased phagocytic activity also in the iMG of sporadic bvFTD patients. RNA sequencing revealed that iMG ofC9orf72HRE-carrying bvFTD patients as compared to the iMG of sporadic bvFTD patients showed differential gene expression in pathways related to RNA and protein regulation and mitochondrial metabolism. Our data suggest potential alterations in the autophagosomal/lysosomal pathways in bvFTD patient iMG, which are further reinforced by theC9orf72HRE and functionally manifest as increased phagocytic activity.