2025 Theses Doctoral

The Role of Lipid Metabolism in Microglia Immune Responses and the Link to ALS

Kabra, Khushbu

The role of cholesterol metabolism and lipid homeostasis in regulating immune cell function has been well-studied in the context of specific pathways, such as TLR signaling and interleukin signaling, particularly in cardio-metabolic diseases where inflammation plays a crucial role. We now know that lipid dysregulation is also a common feature of several neurodegenerative diseases, including Alzheimer’s disease (AD), Amyotrophic lateral sclerosis (ALS), and Parkinson’s disease (PD). However, the exact mechanisms by which alterations in specific lipid species cause immune dysfunction and neuroinflammation are not well understood. Current studies to understand and develop a lipid ‘signature’ for neurodegenerative diseases have used patient plasma or CSF samples, and there is still a large gap in our understanding of these lipid alterations at a cellular level.

To address this gap, we first sought to understand how lipid homeostasis, specifically cholesterol metabolism, is regulated by the uptake of different lipoproteins. We then investigated cholesterol-mediated regulation of immune responses in the brain, focusing on microglia-like cells. We found that cholesterol internalization mediated by HDL induces an increase in IL-1β secretion by driving cholesterol uptake and accumulation of free cholesterol. LDL, on the other hand, does so at higher concentrations and drives an increase in cellular triglyceride levels. Inhibiting the HDL-specific lipoprotein receptor SR-B1 was sufficient to reduce cholesterol uptake and cytokine expression in microglia stimulated with LPS.

Finally, we investigated the role of cholesterol homeostasis in ALS, a neurodegenerative disease characterized by neuroinflammation and lipid dysregulation, and attempted to identify converging lipid pathways that may be driving pathology across different ALS mutations. We used shRNA knockdown to reduce the expression of six genes – three of these are well-known ALS-risk genes (TARDBP, FUS, and SOD1), and the other three are lipid genes that have been linked to cases of ALS (ERLIN2, SPTLC1, and TREM2). There were several overlapping alterations in lipid metabolism genes, specifically, we found cholesterol biosynthesis pathways to be downregulated in FUS, TARDBP, and ERLIN2 knockdowns. TARDBP knockdown had the most differentially expressed lipid metabolism genes in microglia-like cells.

To understand the functional consequences of this, we measured lipid droplets, cholesterol uptake, triglyceride levels, and alterations in phagocytosis and cytokine expression in both TARDBP and FUS knockdowns. Our data showed significant accumulation of lipid droplets in both, driven by triglyceride accumulation. We further characterized the TARDBP knockdown and showed that nuclear depletion of TDP-43 results in increased fatty acid uptake and triglyceride synthesis. Inhibiting triglyceride synthesis using DGAT enzyme inhibitors in TARDBP knockdown MDMi rescued most of the observed phenotypes, suggesting these alterations were driven by triglyceride accumulation.