Lipid in microglial biology - from material to mediator

Lipidomics Analysis of Human HMC3 Microglial Cells in an In Vitro Model of Metabolic Syndrome

Metabolic endotoxemia (ME) is associated with bacterial lipopolysaccharide (LPS, endotoxin) and increased levels of saturated fatty acids (SFAs) in the bloodstream, causing systemic inflammation. ME usually accompanies obesity and a diet rich in fats, especially SFAs. Numerous studies confirm the effect of ME-related endotoxin on microglial activation. Our study aimed to assess lipid metabolism and immune response in microglia pre-stimulated with TNFα (Tumor Necrosis Factor α) and then with endotoxin and palmitic acid (PA). Using ELISA, we determined cytokines IL-1β, IL-10, IL-13 (interleukin-1β, -10, -13, and TGFβ (Transforming Growth Factor β) in the culture medium from microglial cells stimulated for 24 h with TNFα and then treated with LPS (10 ng/mL) and PA (200 µM) for 24 h. HMC3 (Human Microglial Cells clone 3) cells produced negligible amounts of IL-1β, IL-10, and IL-13 after stimulation but secreted moderate levels of TGFβ. Changes in lipid metabolism accompanied changes in TREM2 (Triggering Receptor Expressed on Myeloid Cells 2) expression. HMC3 stimulation with endotoxin increased TREM2 expression, while PA treatment decreased it. Endotoxin increased ceramide levels, while PA increased triglyceride levels. These results indicated that pre-stimulation of microglia with TNFα significantly affects its interactions with LPS and PA and modulates lipid metabolism, which may lead to microglial activation silencing and neurodegeneration.

N-Glycan profile of the cell membrane as a probe for lipopolysaccharide-induced microglial neuroinflammation uncovers the effects of common fatty acid supplementation

Altered N-glycosylation of proteins on the cell membrane is associated with several neurodegenerative diseases. Microglia are an ideal model for studying glycosylation and neuroinflammation, but whether aberrant N-glycosylation in microglia can be restored by diet remains unknown. Herein, we profiled the N-glycome, proteome, and glycoproteome of the human microglia following lipopolysaccharide (LPS) induction to probe the impact of dietary and gut microbe-derived fatty acids-oleic acid, lauric acid, palmitic acid, valeric acid, butyric acid, isobutyric acid, and propionic acid-on neuroinflammation using liquid chromatography-tandem mass spectrometry. LPS changed N-glycosylation in the microglial glycocalyx altering high mannose and sialofucosylated N-glycans, suggesting the dysregulation of mannosidases, fucosyltransferases, and sialyltransferases. The results were consistent as we observed the restoration effect of the fatty acids, especially oleic acid, on the LPS-treated microglia, specifically on the high mannose and sialofucosylated glycoforms of translocon-associated proteins, SSRA and SSRB along with the cell surface proteins, CD63 and CD166. In addition, proteomic analysis and in silico modeling substantiated the potential of fatty acids in reverting the effects of LPS on microglial N-glycosylation. Our results showed that N-glycosylation is likely affected by diet by restoring alterations following LPS challenge, which may then influence the disease state.

Regulation of genes involved in the metabolic adaptation of murine microglial cells in response to elevated HIF-1α mediated activation

Microglia cells are activated in response to different stress signals. Several metabolic adaptations underlie microglia activation in the brain. Among these, in conditions like ischemic stroke and, hypoxic stress stimuli activate microglia cells. Hypoxic stress is mediated by HIF-1α. Although HIF-1α has been implicated in the alteration of metabolic pathways, changes in microglia lipid metabolism during M1 activation of microglia induced by elevated HIF-1α levels are yet to be understood. This can also merit interest in the development of novel targets to mitigate chronic inflammation. Our study aims to elucidate the transcriptional regulation of metabolic pathways in microglia cells during HIF-1α mediated activation. To study the adaptations in the metabolic pathways we induced microglia activation, by activating HIF-1α. Here, we show that microglia cells activated in response to elevated HIF-1α require ongoing lipogenesis and fatty acid breakdown. Notably, autophagy is activated during the initial stages of microglia activation. Inhibition of autophagy in activated microglia affects their viability and phagocytic activity. Collectively, our study expands the understanding of the molecular link between autophagy, lipid metabolism, and inflammation during HIF-1α mediated microglial activation that can lead to the development of promising strategies for controlling maladaptive activation states of microglia responsible for neuroinflammation. Together, our findings suggest that the role of HIF-1α in regulating metabolic pathways during hypoxia in microglia is beyond optimization of glucose utilization and distinctly regulates lipid metabolism during pro-inflammatory activation.

Towards a Unitary Hypothesis of Alzheimer's Disease Pathogenesis

The "amyloid cascade" hypothesis of Alzheimer's disease (AD) pathogenesis invokes the accumulation in the brain of plaques (containing the amyloid-β protein precursor [AβPP] cleavage product amyloid-β [Aβ]) and tangles (containing hyperphosphorylated tau) as drivers of pathogenesis. However, the poor track record of clinical trials based on this hypothesis suggests that the accumulation of these peptides is not the only cause of AD. Here, an alternative hypothesis is proposed in which the AβPP cleavage product C99, not Aβ, is the main culprit, via its role as a regulator of cholesterol metabolism. C99, which is a cholesterol sensor, promotes the formation of mitochondria-associated endoplasmic reticulum (ER) membranes (MAM), a cholesterol-rich lipid raft-like subdomain of the ER that communicates, both physically and biochemically, with mitochondria. We propose that in early-onset AD (EOAD), MAM-localized C99 is elevated above normal levels, resulting in increased transport of cholesterol from the plasma membrane to membranes of intracellular organelles, such as ER/endosomes, thereby upregulating MAM function and driving pathology. By the same token, late-onset AD (LOAD) is triggered by any genetic variant that increases the accumulation of intracellular cholesterol that, in turn, boosts the levels of C99 and again upregulates MAM function. Thus, the functional cause of AD is upregulated MAM function that, in turn, causes the hallmark disease phenotypes, including the plaques and tangles. Accordingly, the MAM hypothesis invokes two key interrelated elements, C99 and cholesterol, that converge at the MAM to drive AD pathogenesis. From this perspective, AD is, at bottom, a lipid disorder.