Roles of Fatty Acids in Microglial Polarization: Evidence from In Vitro and In Vivo Studies on Neurodegenerative Diseases

Microglial-specific knockdown of iron import gene, Slc11a2, blunts LPS-induced neuroinflammatory responses in a sex-specific manner

Neuroinflammation and microglial iron load are significant hallmarks found in several neurodegenerative diseases. In in vitro systems, microglia preferentially upregulate the iron importer, divalent metal transporter 1 (DMT1, gene name Slc11a2) in response to inflammatory stimuli, and it has been shown that iron can augment cellular inflammation, suggesting a feed-forward loop between mechanisms involved in iron import and inflammatory signaling. However, it is not understood how microglial iron import mechanisms contribute to inflammation in vivo, or whether altering a microglial iron-related gene affects the inflammatory response. These studies aimed to determine the effect of knocking down microglial iron import gene Slc11a2 on the inflammatory response in vivo. We generated a novel model of tamoxifen-inducible, microglial-specific Slc11a2 knockdown using Cx3cr1Cre-ERT2 mice. Transgenic male and female mice were administered intraperitoneal saline or lipopolysaccharide (LPS) and assessed for sickness behavior post-injection. Plasma cytokines and microglial bulk RNA sequencing (RNASeq) analyses were performed at 4 h post-LPS, and microglia were collected for gene expression analysis after 24 h. A subset of mice was assessed in a behavioral test battery following LPS-induced sickness recovery. Control male, but not female, mice significantly upregulated microglial Slc11a2 at 4 and 24 h following LPS. In Slc11a2 knockdown mice, we observed an improvement in the acute behavioral sickness response post-LPS in male, but not female, animals. Microglia from male, but not female, knockdown animals exhibited a significant decrease in LPS-provoked pro-inflammatory cytokine expression after 24 h. RNASeq data from male knockdown microglia 4 h post-LPS revealed a robust downregulation in inflammatory genes including Il6, Tnfα, and Il1β, and an increase in anti-inflammatory and homeostatic markers (e.g., Tgfbr1, Cx3cr1, and Trem2). This corresponded with a profound decrease in plasma pro-inflammatory cytokines 4 h post-LPS. At 4 h, male knockdown microglia also upregulated expression of markers of iron export, iron recycling, and iron homeostasis and decreased iron storage and import genes, along with pro-oxidant markers such as Cybb, Nos2, and Hif1α. Overall, this work elucidates how manipulating a specific gene involved in iron import in microglia alters acute inflammatory signaling and overall cell activation state in male mice. These data highlight a sex-specific link between a microglial iron import gene and the pro-inflammatory response to LPS in vivo, providing further insight into the mechanisms driving neuroinflammatory disease.

Omega fatty acid ratios and neurodegeneration in a healthy environment

Neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, Multiple Sclerosis pose substantial public health challenges. While genetics play a primary role, recent research emphasizes the impact of environmental factors, particularly diet and lifestyle. This study investigates the initiating effects of Omega (ω)- 3 and Omega (ω)- 6 fatty acids on neuroinflammation, potentially contributing to these diseases. Using BV-2 microglial cells, we explored the influence of different fatty acid compositions and ratios on cell viability, cytokine production, morphological changes, and lipid peroxidation. Notably, a 2/1 ω-6:ω-3 ratio led to decreased cell viability. Fatty acid compositions influenced cytokine secretion, with reduced TNF-α suggesting anti-inflammatory effects. IL-17 increased, while IL-4 and IL-10 decreased in the 15/1 ω-6:ω-3 ratio, indicating complex cytokine interactions. This study found that polyunsaturated fatty acids interventions induced microglial activation, altering cell morphology even without immunostimulants. These findings demonstrate the intricate nature of fatty acid interactions with microglial cells and their potential implications for neuroinflammation. Further research is needed to clarify mechanisms and their relevance to neurodegenerative diseases, informing possible therapeutic strategies.

Knockdown of BCL-3 Attenuates Inflammatory Response in Intracerebral Hemorrhage Through an rBMECs/MGs Microenvironment

Intracerebral hemorrhage (ICH) is a severe disease with high mortality. Recently, the role of BCL-3 in ICH has started to gain attention, but its mechanism remains unclear. A collagenase injection method was used to establish an ICH model in rats, and the expression of BCL-3 were detected. Rat brain microvascular endothelial cells (rBMECs) were isolated and induced with Hemin to establish an in vitro ICH model. The expression of BCL-3 was assessed, followed by detection of cell apoptosis. In the cell model, the recruitment, polarization, and pro-inflammatory features of the microglia (MGs) were assessed after co-cultured with rBMECs. Finally, in the ICH animal model, after knockdown of BCL-3, comprehensive evaluations of inflammatory responses in brain tissue, polarization and recruitment of microglia, and apoptosis were conducted. Results revealed an upregulated expression of BCL-3 in brain tissue of the ICH animal model. In Hemin-treated rBMECs, an upward trend in BCL-3 expression was observed, accompanied by an increase of cell apoptosis. After co-culturing with the in vitro model, microglia exhibited enhanced M1 polarization and intensified inflammatory responses. However, when BCL-3 expression was inhibited in the in vitro model, a reversal occurred in the polarization tendency and inflammatory responses of microglia. Additionally, after knockdown of BCL-3 in the animal model, notable improvements occurred in M1 polarization, infiltration of macrophages, and inflammatory reactions in the brain tissue. Therefore, BCL-3 modulates the inflammatory response after ICH occurrence through the BMECs/MGs microenvironment. Additionally, BCL-3 might be a potential therapeutic target for ICH management.

Soluble TREM2 triggers microglial dysfunction in neuromyelitis optica spectrum disorders

Microglia-mediated neuroinflammation contributes to acute demyelination in neuromyelitis optica spectrum disorders (NMOSD). Soluble triggering receptor expressed on myeloid cells 2 (sTREM2) in the CSF has been associated with microglial activation in several neurodegenerative diseases. However, the basis for this immune-mediated attack and the pathophysiological role of sTREM2 in NMOSD remain to be elucidated. Here, we performed Mendelian randomization analysis and identified a genetic association between increased CSF sTREM2 and NMOSD risk. CSF sTREM2 was elevated in patients with NMOSD and was positively correlated with neural injury and other neuroinflammation markers. Single-cell RNA sequencing of human macrophage/microglia-like cells in CSF, a proxy for microglia, showed that increased CSF sTREM2 was positively associated with microglial dysfunction in patients with NMOSD. Furthermore, we demonstrated that sTREM2 is a reliable biomarker of microglial activation in a mouse model of NMOSD. Using unbiased transcriptomic and lipidomic screens, we identified that excessive activation, overwhelmed phagocytosis of myelin debris, suppressed lipid metabolism and enhanced glycolysis underlie sTREM2-mediated microglial dysfunction, possibly through the nuclear factor kappa B (NF-κB) signalling pathway. These molecular and cellular findings provide a mechanistic explanation for the genetic association between CSF sTREM2 and NMOSD risk and indicate that sTREM2 could be a potential biomarker of NMOSD progression and a therapeutic target for microglia-mediated neuroinflammation.

Metabolic Control of Microglia

Microglia, immune sentinels of the central nervous system (CNS), play a critical role in maintaining its health and integrity. This chapter delves into the concept of immunometabolism, exploring how microglial metabolism shapes their diverse immune functions. It examines the impact of cell metabolism on microglia during various CNS states, including homeostasis, development, aging, and inflammation. Particularly in CNS inflammation, the chapter discusses how metabolic rewiring in microglia can initiate, resolve, or perpetuate inflammatory responses. The potential of targeting microglial metabolism as a therapeutic strategy for chronic CNS disorders with prominent innate immune cell activation is also explored.

Macrophage metabolism, phenotype, function, and therapy in hepatocellular carcinoma (HCC)

The pivotal role of the tumor microenvironment (TME) in the initiation and advancement of hepatocellular carcinoma (HCC) is widely acknowledged, as it fosters the proliferation and metastasis of HCC cells. Within the intricate TME of HCC, tumor-associated macrophages (TAMs) represent a significant constituent of non-malignant cells. TAMs engage in direct communication with cancer cells in HCC, while also exerting influence on other immune cells to adopt a tumor-supportive phenotype that facilitates tumor progression. Among the multifaceted mechanisms at play, the metabolic reprogramming of both tumor cells and macrophages leads to phenotypic alterations and functional modifications in macrophages. This comprehensive review elucidates the intricate interplay between cellular metabolism and macrophage phenotype/polarization, while also providing an overview of the associated signaling molecules and potential therapeutic strategies for HCC.

The ketone body β-hydroxybutyrate shifts microglial metabolism and suppresses amyloid-β oligomer-induced inflammation in human microglia

Fatty acids are metabolized by β-oxidation within the "mitochondrial ketogenic pathway" (MKP) to generate β-hydroxybutyrate (BHB), a ketone body. BHB can be generated by most cells but largely by hepatocytes following exercise, fasting, or ketogenic diet consumption. BHB has been shown to modulate systemic and brain inflammation; however, its direct effects on microglia have been little studied. We investigated the impact of BHB on Aβ oligomer (AβO)-stimulated human iPS-derived microglia (hiMG), a model relevant to the pathogenesis of Alzheimer's disease (AD). HiMG responded to AβO with proinflammatory activation, which was mitigated by BHB at physiological concentrations of 0.1-2 mM. AβO stimulated glycolytic transcripts, suppressed genes in the β-oxidation pathway, and induced over-expression of AD-relevant p46Shc, an endogenous inhibitor of thiolase, actions that are expected to suppress MKP. AβO also triggered mitochondrial Ca2+ increase, mitochondrial reactive oxygen species production, and activation of the mitochondrial permeability transition pore. BHB potently ameliorated all the above mitochondrial changes and rectified the MKP, resulting in reduced inflammasome activation and recovery of the phagocytotic function impaired by AβO. These results indicate that microglia MKP can be induced to modulate microglia immunometabolism, and that BHB can remedy "keto-deficiency" resulting from MKP suppression and shift microglia away from proinflammatory mitochondrial metabolism. These effects of BHB may contribute to the beneficial effects of ketogenic diet intervention in aged mice and in human subjects with mild AD.

Adverse effects of gestational ω-3 and ω-6 polyunsaturated fatty acid imbalance on the programming of fetal brain development

Obesity is a key medical challenge of our time. The increasing number of children born to overweight or obese women is alarming. During pregnancy, the circulation of the mother and her fetus interact to maintain the uninterrupted availability of essential nutrients for fetal organ development. In doing so, the mother's dietary preference determines the amount and composition of nutrients reaching the fetus. In particular, the availability of polyunsaturated fatty acids (PUFAs), chiefly their ω-3 and ω-6 subclasses, can change when pregnant women choose a specific diet. Here, we provide a succinct overview of PUFA biochemistry, including exchange routes between ω-3 and ω-6 PUFAs, the phenotypes, and probable neurodevelopmental disease associations of offspring born to mothers consuming specific PUFAs, and their mechanistic study in experimental models to typify signaling pathways, transcriptional, and epigenetic mechanisms by which PUFAs can imprint long-lasting modifications to brain structure and function. We emphasize that the ratio, rather than the amount of individual ω-3 or ω-6 PUFAs, might underpin physiologically correct cellular differentiation programs, be these for neurons or glia, during pregnancy. Thereupon, the PUFA-driven programming of the brain is contextualized for childhood obesity, metabolic, and endocrine illnesses.

The Relationship between Stress, Inflammation, and Depression

A narrative review about the relationship between stress, inflammation, and depression is made as follows: Chronic stress leads to various stress-related diseases such as depression. Although most human diseases are related to stress exposure, the common pathways between stress and pathophysiological processes of different disorders are still debatable. Chronic inflammation is a crucial component of chronic diseases, including depression. Both experimental and clinical studies have demonstrated that an increase in the levels of pro-inflammatory cytokines and stress hormones, such as glucocorticoids, substantially contributes to the behavioral alterations associated with depression. Evidence suggests that inflammation plays a key role in the pathology of stress-related diseases; however, this link has not yet been completely explored. In this study, we aimed to determine the role of inflammation in stress-induced diseases and whether a common pathway for depression exists. Recent studies support pharmacological and non-pharmacological treatment approaches significantly associated with ameliorating depression-related inflammation. In addition, major depression can be associated with an activated immune system, whereas antidepressants can exert immunomodulatory effects. Moreover, non-pharmacological treatments for major depression (i.e., exercise) may be mediated by anti-inflammatory actions. This narrative review highlights the mechanisms underlying inflammation and provides new insights into the prevention and treatment of stress-related diseases, particularly depression.