HYPERHOMOCYSTEINEMIA SUPPRESSES MICROGLIAL AMYLOID BETA PHAGOCYTOSIS AND EXACERBATES ALZHEIMER'S DISEASE VIA INSULIN-LIKE GROWTH FACTOR-1 REDUCTION

Abstract

The pathological hallmark of Alzheimer's disease (AD) is the accumulation of amyloid β (Aβ) plaques, present in the majority of clinically diagnosed cases. Treatment options, such as the FDA-approved Lecanemab, target early-stage AD by promoting Aβ clearance via microglial (MG) phagocytosis, but no effective therapy exists for late-stage AD. Hyperhomocysteinemia (HHcy), prevalent in the elderly, is an established risk factor for AD, with previous studies linking it to various pathologies but not to Aβ dynamics or MG function. This study examined the hypothesis that HHcy-induced epigenetic changes impair MG clearance of Aβ, potentially exacerbating AD. It also investigated the role of IGF-1 (Insulin-like growth factor 1) in MG phagocytosis, considering the established influence of hypomethylation on both IGF-1 expression and AD progression. In this study, we probed the interplay among HHcy, Aβ accumulation, and MG phagocytosis in AD using Cbs-/- mice as a model. Our approach delineated an HHcy-induced hypomethylation status in both plasma and brain cortex. HHcy increased Aβ deposition without a corresponding rise in Aβ production in the 5xFAD x Cbs-/- mouse brains. This was characterized by elevated levels of soluble Aβ40 and insoluble Aβ42, alongside unaltered the β-secretase and s-APPβ expression. Single-nucleus multiomic profiling unveiled five distinct MG subclusters (MG_0 to MG_4) in the hippocampi of both Cbs-/- and control mice. MG_0, specific to control mice, involved activated MG phagocytosis. Subclusters MG_2 and MG_3, predominantly identified in Cbs-/- mice, exhibited decreased migration and phagocytosis. Subcluster MG_4, unique to Cbs-/- mice, displayed impaired cytoskeleton organization and dendritic development. HHcy suppressed MG Aβ phagocytosis activity in the mouse MG cell line SIM-A9 (ex vivo phagocytosis), freshly isolated MG from HHcy mice with Cbs-/- or a high-methionine (HM) diet (ex vivo phagocytosis), and in 3xTg-AD mice with an HM diet (in vivo phagocytosis). Through meta-analysis over transcriptomes of Cbs-/- mouse MG, human and mouse AD MG, Aβ phagocytosis model, and human AD methylome, we identified 5 differentially expressed genes (DEGs) (Flt1, Calponin 3, Igf1, Cacna2d4, and Celsr) in HHcy-suppressed phagocytic AD MG. All of them have been reported to regulate the migration and Aβ phagocytosis of MG and macrophages (MΦ). IGF-1 expression changes are the most significant between liver-specific Ahcy-/- (model of hypomethylation) and HHcy-altered phagocytic AD MG. In both AD and non-AD mice, the HHcy group exhibited significantly lower IGF-1 levels in the brain, plasma, and MG than the non-HHcy group. Additionally, a negative correlation was observed between Igf1 levels and MG Aβ phagocytosis. Our study underscored the critical role of HHcy in AD progression, particularly through its detrimental impact on MG Aβ phagocytosis and altered MG subclusters, leading to reduced Aβ clearance. We identified key genes, such as IGF-1, essential for MG function. These findings indicated that HHcy may exacerbate AD and potentially diminish the efficacy of treatments like Lecanemab, highlighting the importance of HHcy-targeted strategies in AD therapy.