Fundamental Neurochemistry Review: Microglial immunometabolism in traumatic brain injury

Abstract

Traumatic brain injury (TBI) is a devastating neurological disorder caused by a physical impact to the brain that promotes diffuse damage and chronic neurodegeneration. Key mechanisms believed to support secondary brain injury include mitochondrial dysfunction and chronic neuroinflammation. Microglia and brain-infiltrating macrophages are responsible for neuroinflammatory cytokine and reactive oxygen species (ROS) production after TBI. Their production is associated with loss of homeostatic microglial functions such as immunosurveillance, phagocytosis, and immune resolution. Beyond providing energy support, mitochondrial metabolic pathways reprogram the pro- and anti-inflammatory machinery in immune cells, providing a critical immunometabolic axis capable of regulating immunologic response to noxious stimuli. In the brain, the capacity to adapt to different environmental stimuli derives, in part, from microglia's ability to recognize and respond to changes in extracellular and intracellular metabolite levels. This capacity is met by an equally plastic metabolism, capable of altering immune function. Microglial pro-inflammatory activation is associated with decreased mitochondrial respiration, whereas anti-inflammatory microglial polarization is supported by increased oxidative metabolism. These metabolic adaptations contribute to neuroimmune responses, placing mitochondria as a central regulator of post-traumatic neuroinflammation. Although it is established that profound neurometabolic changes occur following TBI, key questions related to metabolic shifts in microglia remain unresolved. These include (a) the nature of microglial mitochondrial dysfunction after TBI, (b) the hierarchical positions of different metabolic pathways such as glycolysis, pentose phosphate pathway, glutaminolysis, and lipid oxidation during secondary injury and recovery, and (c) how immunometabolism alters microglial phenotypes, culminating in chronic non-resolving neuroinflammation. In this basic neurochemistry review article, we describe the contributions of immunometabolism to TBI, detail primary evidence of mitochondrial dysfunction and metabolic impairments in microglia and macrophages, discuss how major metabolic pathways contribute to post-traumatic neuroinflammation, and set out future directions toward advancing immunometabolic phenotyping in TBI.

Keywords: metabolism; microglia; mitochondria; neuroimmunology; traumatic brain injury.

Figure 1.. Glucose metabolism in microglia following pro- and anti-inflammatory activation

During pro-inflammatory microglial activation glucose flux through the glycolytic and pentose-phosphate pathways is increased (pink arrows). Many of these biochemical features are mirrored in microglia following TBI (yellow arrows). Anti-inflammatory microglial activation results in reduced glucose uptake and hexokinase activity (dashed green arrows), which are indicative of reduced glycolytic metabolism. Two-way half arrows indicate reversible reactions, and increased size of one-way half arrow indicates shifts in the reaction equilibrium. Abbreviations: Glucose (GLU); Glucose-6-Phosphate (G6P); Fructose-6-Phosphate (F6P); Fructose-1,6-biphosphate (F1,6P); Dihydroxyacetone-phosphate (DHAP); Glyceraldehyde-3-Phosphate (GA3P); Phosphoenolpyruvate (PEP); 6-Phosphogluconate (6PG); Ribulose-5-phosphate (Ru5P); Ribose-5-phosphate (R5P), Xylulose-5-Phosphate (X5P); Sedoheptulose-7-phosphate (S7P), Erythrose-4-phosphate (E4P); Phosphofructokinase-1 (PFK1); Pyruvate-kinase M2 (PKM2); Lactate Dehydrogenase (LDH); NADPH Oxidase 2 (NOX2). Oxidative Pentose Phosphate Pathway (Ox. PPP); Non-oxidative Pentose Phosphate Pathway (Non-ox PPP); Superoxide (O₂⁻). Created with BioRender.com.

Figure 2:. The destination of glucose across the spectrum of microglial functional phenotypes

Microglial plasticity and functional responses are accompanied by changes in metabolism. Glucose flux through either glycolysis or pentose phosphate pathway is tailored to support metabolic requirements of microglia. This metabolic flexibility allows microglial metabolism to be skewed from the full conversion of glucose into pyruvate, supporting aerobic metabolism in phagocytic/surveillant microglia, to a cyclic pentose phosphate pathway that provides increased NADPH yield in preference of pyruvate production to support NADPH oxidase activity in inflammatory microglia. In chronic TBI or aging, chronically activated microglia display features such as increased levels of NOX2 and lipid accumulation, which suggest that glucose metabolism in microglia under these conditions may be irreversibly shifted towards NADPH production. Abbreviations: Reactive oxygen species (ROS), Glucose (Glu), Glucose-6-phosphate (G6P); Fructose-6-phosphate (F6P); Ribulose-5-phosphate (R5P); Dihydroxyacetone phosphate (DHAP); Glyceraldehyde-3-phosphate (GA3P); Damage associated molecular patterns (DAMPs). Created with BioRender.com.

Figure 3.. TCA cycle and ETS changes during pro- and anti-inflammatory signaling in microglia, and following TBI

Pro-inflammatory microglial polarization (purple arrows) is accompanied by increased glutaminolysis and a reduction in Complex II (CII) activity. This reduces mitochondrial membrane potential (Δψm) and ATP production and causes succinate accumulation. Succinate accretion from CII inhibition and from GABA shunt may also stimulate glycolytic metabolism through increased HIF1α. TBI (yellow arrows) replicates many biochemical pathway changes observed in pro-inflammatory microglia, except for succinate, which undergoes increased succinate oxidation following TBI. Anti-inflammatory microglial polarization (green arrows) results in increased mitochondrial respiratory rates, and mitochondrial pyruvate and lactate oxidation. In addition, increased oxidative phosphorylation is accompanied by increased citrate, aconitate, isocitrate and itaconate synthesis. Broken arrows represent reduced flux of substrates. Abbreviations: Acetyl-CoA (AcCoA); Citrate (CIT); cis-Aconitate (ACO); Isocitrate (ISO); alpha-ketoglutarate (αKG); Succinate (SUC); Fumarate (FUM); Malate (MAL); 2-Oxalacetate (OAA); Itaconate (ITA); Glutamine (GLN); Glutamate (GLU); γ-aminobutyric-acid (GABA); Hypoxia-inducible factor 1 alpha (HIF1α); mitochondrial membrane potential (Δψm); Complex I (I); Complex II – Succinate dehydrogenase (II); Ubiquinol (Q); Complex III (III); Lactate (LAC); Pyruvate (PYR). Created with BioRender.com.

Figure 4.. Lipid metabolism alterations regulate immune response following TBI

Pro-inflammatory microglial polatization (purple arrows) is accompanied by increased expression in lipid biosynthesis and lipid accumulation. Following TBI (yellow arrows) increased lipid accumulation in microglia leads to increased mitochondrial production of reactive oxygen species. Also, the production of lipid metabolite docosahexaenoic acid (DHA) after TBI is associated with the expression of pro-inflammatory genes. During anti-inflammatory responses (green arrows), lipid metabolites arachidonic acid (AA) and 15-hydroxyeicosatetraenoic acid (15-HETE) activate microglial mitochondrial oxidative metabolism and boost phagocytic activity. N-3 lipid catabolism in microglia is associated with decreases in NLRP3 inflammasome activation. Activation of mitochondrial biogenesis regulator Pparg is associated with increased toll-like receptor 4 (TLR4) expression. Microglial metabolism of PUFAs is regulated by a negative feedforward mechanism orchestrated by STING. Abbreviations: 12/15-lipoxygenase (12/15-LOX); 15-Hydroxyeicosatetraenoic acid (15-HETE); Arachidonic acid (AA); Long-chain-fatty-acid-CoA ligase 1 (ACSL1); Docosahexaenoic acid (DHA); Electron transfer system (ETS); Fatty Acid-Coenzyme A (FA-CoA); Fatty Acid Desaturase 1 (FASD1); Fatty Acid Desaturase 2 (FASD2); Fatty acid synthase (FASN1); Lipase lipoprotein (LPL); Monounsaturated Fatty Acid (MUFA); NLR family pyrin domain containing 3 (NLRP3); Peroxisome proliferator- activated receptor gamma (Pparg); Polyunsaturated Fatty Acid (PUFA); Saturated Fatty Acid (SFA); Stimulator of interferon genes (STING); Toll-like receptor 4 (TLR4). Created with BioRender.com.

Figure 5.. Cholesterol inhibits lipid oxidation and stimulates glycolysis and lipid de novo biosynthesis in microglia

Despite not contributing to energy production, cholesterol promotes broad metabolic adaptations in microglia. Poor brain cholesterol clearance in the brain, observed in APOE4 carriers, is associated with increased lipid biosynthesis and accumulation of pro-inflammatory lipid-laden dysmorphic microglia (purple arrows). Loss-of-function mutations in cholesterol-related protein TREM2 recapitulate lipid metabolism alterations observed in APOE4 carriers. Cholesterol accumulation is associated with increases in glycolytic flux-limiting enzyme hexokinase 2 (HK2)and glycolytic regulator hypoxia-inducible factor alpha-1 (HIF-1α), and inhibits anti-inflammatory lipid oxidation pathway (green arrows) through regulatory enzyme lipoprotein lipase (LPL). Abbreviations: 15-Hydroxyeicosatetraenoic acid (15-HETE); Apolipoprotein Epsilon 4 (APOE4); Acetyl-CoA (Ac-CoA); Fatty Acid-Coenzyme A (FA-CoA); Hypoxia-Inducible Factor alpha-1 (HIF1α); Hexokinase 2 (HK2); Triggering Receptor Expressed On Myeloid Cells 2 (TREM2). Created with BioRender.com.

Figure 6.. Immunometabolic features of microglia following TBI

After TBI, microglia/macrophages display increased glucose metabolism through both glycolysis and PPP. The PPP increase feeds the NADPH pool, which serves as substrate to pro-inflammatory enzyme NADPH oxidase 2 (NOX2), a driver of chronic neuroinflammation after TBI. Increased glycolytic flux is associated with accumulation of lactate following injury, indicative of anaerobic glucose oxidation. Oxygen consumption and ATP production by the electron transfer system (ETS) are reduced, particularly that driven by Complex I substrates. Complex II contribution to ATP synthesis increases. Lipid catabolism is inhibited after TBI, and lipid biosynthesis is increased in microglia. This increase in lipid accumulation is sustained by the increase in NADPH production by PPP. Lipid accumulation aggravates the pro-inflammatory phenotype and could contribute to chronic neuroinflammation. Orange and blue arrows indicate pathways which are, respectively, decreased and increased following TBI. Abbreviations: Alpha-ketoglutarate (α-KG); Glucose transporter (GLUT1); Hexokinase (HK); Phosphofructokinase B3 (PFKB3); Pyruvate kinase M2 (PKM2); Pentose phosphate pathway (PPP); NADPH oxidase 2 (NOX2); Electron transfer system (ETS). Created with BioRender.com.