Microglial Lipid Biology in the Hypothalamic Regulation of Metabolic Homeostasis

Rethinking the role of microglia in obesity

Microglia are the macrophages of the central nervous system (CNS), implying their role in maintaining brain homeostasis. To achieve this, these cells are sensitive to a plethora of endogenous and exogenous signals, such as neuronal activity, cellular debris, hormones, and pathological patterns, among many others. More recent research suggests that microglia are highly responsive to nutrients and dietary variations. In this context, numerous studies have demonstrated their significant role in the development of obesity under calorie surfeit. Because many reviews already exist on this topic, we have chosen to present the state of our reflections on various concepts put forth in the literature, bringing a new perspective whenever possible. Our literature review focuses on studies conducted in the arcuate nucleus of the hypothalamus, a key structure in the control of food intake. Specifically, we present the recent data available on the modifications of microglial energy metabolism following the consumption of an obesogenic diet and their consequences on hypothalamic neuron activity. We also highlight the studies unraveling the mechanisms underlying obesity-related sexual dimorphism. The review concludes with a list of questions that remain to be addressed in the field to achieve a comprehensive understanding of the role of microglia in the regulation of body energy metabolism. This article is part of the Special Issue on "Microglia".

Brain lipid sensing and the neural control of energy balance

The central nervous system continuously detects circulating concentrations of lipids such as fatty acids and troglycerides. Once information has been detected, the central nervous system can in turn participate in the control of energy balance and blood sugar levels and in particular regulate the secretion and action of insulin. Neurons capable of detecting circulating lipid variations are located in the hypothalamus and in other regions such as the nucleus accumbens, the striatum or the hippocampus. An excess of lipids will have deleterious effects and may induce central lipotoxicity, in particular following local production of ceramides and the appearance of neuroinflammation which may lead to metabolic diseases such as obesity and type 2 diabetes.

Effects of paternal arachidonic acid supplementation on offspring behavior and hypothalamus inflammation markers in the mouse

Arachidonic acid (AA) is involved in inflammation and plays a role in growth and brain development in infants. We previously showed that exposure of mouse sires to AA for three consecutive generations induces a cumulative change in fatty acid (FA) involved in inflammation and an increase in body and liver weight in the offspring. Here, we tested the hypothesis that paternal AA exposure changes the progeny's behavioral response to a proinflammatory insult, and asked whether tissue-specific FA are associated with that response. Male BALB/c mice were supplemented daily with three doses of AA for 10 days and crossed to non-supplemented females (n = 3/dose). Two-month-old unsupplemented male and female offspring (n = 6/paternal AA dose) were exposed to Gram-negative bacteria-derived lipopolysaccharides (LPS) or saline control two hours prior to open field test (OFT) behavioral analysis and subsequent sacrifice. We probed for significant effects of paternal AA exposure on: OFT behaviors; individual FA content of blood, hypothalamus and hypothalamus-free brain; hypothalamic expression profile of genes related to inflammation (Tnfa, Il1b, Cox1, Cox2) and FA synthesis (Scd1, Elovl6). All parameters were affected by paternal AA supplementation in a sex-specific manner. Paternal AA primed the progeny for behavior associated with increased anxiety, with a marked sex dimorphism: high AA doses acted as surrogate of LPS in males, realigning a number of OFT behaviors that in females were differential between saline and LPS groups. Progeny hypothalamic Scd1, a FA metabolism enzyme with documented pro-inflammatory activity, showed a similar pattern of differential expression between saline and LPS groups at high paternal AA dose in females, that was blunted in males. Progeny FA generally were not affected by LPS, but displayed non-linear associations with paternal AA doses. In conclusion, we document that paternal exposure to AA exerts long-term behavioral and biochemical effects in the progeny in a sex-specific manner.

Correlation analysis of hypertension, traditional Chinese medicine constitution, and LPL gene polymorphism in the elderly in communities in Shanghai

BACKGROUND

Research on the genetic mechanisms of hypertension has been a hot topic in the cardiovascular field.

OBJECTIVE

To study the correlation between senile hypertension and traditional Chinese medicine (TCM) constitution and lipoprotein lipase (LPL) gene polymorphism and to provide the theoretical basis for TCM prevention and treatment of hypertension.

METHODS

The elderly population in communities in Shanghai (hypertensive: 264 cases; non-hypertensive: 159 cases) was taken as the research object. Essential data and information on TCM constitution were collected. The LPL gene mutation was detected using the second-generation sequencing method. Statistical analysis was performed to clarify the relationship between hypertension and senile hypertension. The correlation of TCM constitution with risk factors and LPL gene polymorphisms was studied.

RESULTS

The primary TCM constitutions in the hypertension group were phlegm-dampness constitution (51.52%), yin-deficiency constitution (17.42%), balanced constitution (15.53%), and yin-deficiency (9.43%). Logistic regression analysis showed that the phlegm-dampness constitution (P< 0.05, OR = 2.587) and yin-deficiency constitution (P< 0.01, OR = 2.693) were the risk constitutions of hypertension in the elderly. A total of 37 LPL gene mutation loci (SNP: 22; new discovery: 15) were detected in the LPL gene, and the mutation rates of rs254, rs255, rs3208305, rs316, rs11570891, rs328, rs11570893, and rs13702 were relatively high, which were 26.24%, 26.24%, 16.08%, 14.66%, 13.24%, 12.06%, and 10.64%. In the phlegm-dampness group, the proportion of rs254 CC type, rs255 TT type, and rs13702 TT type in the hypertensive group (77.21%, 77.21%, and 93.38%) was higher than that in the non-hypertensive group (56.41%, 56.41%, and 82.05%), The difference was statistically significant (P< 0.05).

CONCLUSION

The phlegm-dampness constitution and yin-deficiency constitution are the risk factors of hypertension in the elderly; in the phlegm-dampness population, rs254 CC type, rs255 TT type, and rs13702 TT type are the risk factors for elderly hypertension.

The immunometabolic reprogramming of microglia in Alzheimer's disease

Alzheimer's disease (AD) is an age-related neurodegenerative disorder (NDD). In the central nervous system (CNS), immune cells like microglia could reprogram intracellular metabolism to alter or exert cellular immune functions in response to environmental stimuli. In AD, microglia could be activated and differentiated into pro-inflammatory or anti-inflammatory phenotypes, and these differences in cellular phenotypes resulted in variance in cellular energy metabolism. Considering the enormous energy requirement of microglia for immune functions, the changes in mitochondria-centered energy metabolism and substrates of microglia are crucial for the cellular regulation of immune responses. Here we reviewed the mechanisms of microglial metabolic reprogramming by analyzing their flexible metabolic patterns and changes that occurred in their metabolism during the development of AD. Further, we summarized the role of drugs in modulating immunometabolic reprogramming to prevent neuroinflammation, which may shed light on a new research direction for AD treatment.

CD1d-dependent neuroinflammation impairs tissue repair and functional recovery following a spinal cord injury

Tissue damage resulting from a spinal cord injury (SCI) is primarily driven by a robust neuroimmune/neuroinflammatory response. This intricate process is mainly governed by a multitude of cytokines and cell surface proteins in the central nervous system (CNS). However, the critical components of the neuroimmune/neuroinflammatory response during SCI are still not well-defined. In this study, we investigated the impact of CD1d, an MHC class I-like molecule mostly known for presenting lipid antigens to natural killer T (NKT) cells and regulating immune/inflammatory responses, on neuroimmune/neuroinflammatory responses induced by SCI. We observed an increased expression of CD1d on various cell types within the spinal cord, including microglia/macrophages, oligodendrocytes (ODCs), and endothelial cells (DCs), but not on neurons or astrocytes post-SCI. In comparison to wildtype (WT) mice, a T10 contusive SCI in CD1d knockout (CD1dKO or Cd1d -/- ) mice resulted in markedly reduced proinflammatory cytokine release, microglia/macrophage activation and proliferation. Following SCI, the levels of inflammatory cytokines and activation/proliferation of microglia/macrophages were dramatically reduced, while anti-inflammatory cytokines such as IL-4 and growth factors like VEGF were substantially increased in the spinal cord tissues of CD1dKO mice when compared to WT mice. In the post-acute phase of SCI (day 7 post-SCI), CD1dKO mice had a significantly higher frequency of tissue-repairing macrophages, but not other types of immune cells, in the injured spinal cord tissues compared to WT mice. Moreover, CD1d-deficiency protected spinal cord neuronal cells and tissue, promoting functional recovery after a SCI. However, the neuroinflammation in WT mouse spinal cords was independent of the canonical CD1d/NKT cell axis. Finally, treatment of injured mice with a CD1d-specific monoclonal antibody significantly enhanced neuroprotection and improved functional recovery. Therefore, CD1d promotes the proinflammatory response following a SCI and represents a potential therapeutic target for spinal cord repair.

Significance Statement

The cell surface molecule, CD1d, is known to be recognized by cells of the immune system. To our knowledge, this is the first observation that the CD1d molecule significantly contributes to neuroinflammation following a spinal cord injury (SCI) in a manner independent of the CD1d/NKT cell axis. This is important, because this work reveals CD1d as a potential therapeutic target following an acute SCI for which there are currently no effective treatments.

TREM2: Potential therapeutic targeting of microglia for Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease, resulting in the loss of cognitive ability and memory. However, there is no specific treatment to mechanistically inhibit the progression of Alzheimer's disease, and most drugs only provide symptom relief and do not fundamentally reverse AD. Current studies show that triggering receptor expressed on myeloid cells 2 (TREM2) is predominantly expressed in microglia of the central nervous system (CNS) and is involved in microglia proliferation, survival, migration and phagocytosis. The current academic view suggests that TREM2 and its ligands have CNS protective effects in AD. Specifically, TREM2 acts by regulating the function of microglia and promoting the clearance of neuronal toxic substances and abnormal proteins by microglia. In addition, TREM2 is also involved in regulating inflammatory response and cell signaling pathways, affecting the immune response and regulatory role of microglia. Although the relationship between TREM2 and Alzheimer's disease has been extensively studied, its specific mechanism of action is not fully understood. The purpose of this review is to provide a comprehensive analysis of the research of TREM2, including its regulation of the inflammatory response, lipid metabolism and phagocytosis in microglia of CNS in AD, and to explore the potential application prospects as well as limitations of targeting TREM2 for the treatment of AD.

High-Fat Diet Consumption Induces Neurobehavioral Abnormalities and Neuronal Morphological Alterations Accompanied by Excessive Microglial Activation in the Medial Prefrontal Cortex in Adolescent Mice

The association between a high-fat diet (HFD) consumption and emotional/cognitive disorders is widely documented. One distinctive feature of the prefrontal cortex (PFC), a kernel emotion- and cognition-related brain region, is its protracted adolescent maturation, which makes it highly vulnerable to the detrimental effects of environmental factors during adolescence. Disruption of the PFC structure and function is linked to emotional/cognitive disorders, especially those that emerge in late adolescence. A HFD consumption is common among adolescents, yet its potential effects on PFC-related neurobehavior in late adolescence and any related underlying mechanisms are yet to be established. In the present study, adolescent (postnatal days 28-56) male C57BL/6J mice were fed a control diet (CD) or a HFD and underwent behavioral tests in addition to Golgi staining and immunofluorescence targeting of the medial PFC (mPFC). The HFD-fed adolescent mice exhibited anxiety- and depression-like behavior and abnormal mPFC pyramidal neuronal morphology accompanied by alterations in microglial morphology indicative of a heightened state of activation and increased microglial PSD95⁺ inclusions signifying excessive phagocytosis of the synaptic material in the mPFC. These findings offer novel insights into the neurobehavioral effects due to adolescent HFD consumption and suggest a contributing role in microglial dysfunction and prefrontal neuroplasticity deficits for HFD-associated mood disorders in adolescents.

The Significance of Hypothalamic Inflammation and Gliosis for the Pathogenesis of Obesity in Humans

Accumulated preclinical literature demonstrates that hypothalamic inflammation and gliosis are underlying causal components of diet-induced obesity in rodent models. This review summarizes and synthesizes available translational data to better understand the applicability of preclinical findings to human obesity and its comorbidities. The published literature in humans includes histopathologic analyses performed postmortem and in vivo neuroimaging studies measuring indirect markers of hypothalamic tissue microstructure. Both support the presence of hypothalamic inflammation and gliosis in children and adults with obesity. Findings predominantly point to tissue changes in the region of the arcuate nucleus of the hypothalamus, although findings of altered tissue characteristics in whole hypothalamus or other hypothalamic regions also emerged. Moreover, the severity of hypothalamic inflammation and gliosis has been related to comorbid conditions, including glucose intolerance, insulin resistance, type 2 diabetes, and low testosterone levels in men, independent of elevated body adiposity. Cross-sectional findings are augmented by a small number of prospective studies suggesting that a greater degree of hypothalamic inflammation and gliosis may predict adiposity gain and worsening insulin sensitivity in susceptible individuals. In conclusion, existing human studies corroborate a large preclinical literature demonstrating that hypothalamic neuroinflammatory responses play a role in obesity pathogenesis. Extensive or permanent hypothalamic tissue remodeling may negatively affect the function of neuroendocrine regulatory circuits and promote the development and maintenance of elevated body weight in obesity and/or comorbid endocrine disorders.

Role of Lipids in Regulation of Neuroglial Interactions

Lipids comprise an extremely heterogeneous group of compounds that perform a wide variety of biological functions. Traditional view of lipids as important structural components of the cell and compounds playing a trophic role is currently being supplemented by information on the possible participation of lipids in signaling, not only intracellular, but also intercellular. The review article discusses current data on the role of lipids and their metabolites formed in glial cells (astrocytes, oligodendrocytes, microglia) in communication of these cells with neurons. In addition to metabolic transformations of lipids in each type of glial cells, special attention is paid to the lipid signal molecules (phosphatidic acid, arachidonic acid and its metabolites, cholesterol, etc.) and the possibility of their participation in realization of synaptic plasticity, as well as in other possible mechanisms associated with neuroplasticity. All these new data can significantly expand our knowledge about the regulatory functions of lipids in neuroglial relationships.

The Pathological Activation of Microglia Is Modulated by Sexually Dimorphic Pathways

Microglia are the primary immunocompetent cells of the central nervous system (CNS). Their ability to survey, assess and respond to perturbations in their local environment is critical in their role of maintaining CNS homeostasis in health and disease. Microglia also have the capability of functioning in a heterogeneous manner depending on the nature of their local cues, as they can become activated on a spectrum from pro-inflammatory neurotoxic responses to anti-inflammatory protective responses. This review seeks to define the developmental and environmental cues that support microglial polarization towards these phenotypes, as well as discuss sexually dimorphic factors that can influence this process. Further, we describe a variety of CNS disorders including autoimmune disease, infection, and cancer that demonstrate disparities in disease severity or diagnosis rates between males and females, and posit that microglial sexual dimorphism underlies these differences. Understanding the mechanism behind differential CNS disease outcomes between men and women is crucial in the development of more effective targeted therapies.

Chronic high-fat diet induces overeating and impairs synaptic transmission in feeding-related brain regions

Obesity is linked to overeating, which can exacerbate unhealthy weight gain. However, the mechanisms for mediating such linkages are elusive. In the current study, we hypothesized that synaptic remodeling occurs in feeding-related brain regions of obese mice. To investigate this, we established a high-fat diet (HFD)-induced obese mouse model and observed that these mice consumed excessive calories. The effect of chronic HFD feeding on lipid droplet accumulation in different brain structures was also investigated. We found that lipid droplets accumulated on the ependyma of the third ventricle (3V), which is surrounded by key areas of the hypothalamus that are involved in feeding. Then, the spontaneous synaptic activity of miniature excitatory postsynaptic current (mEPSC) and miniature inhibitory postsynaptic current (mIPSC) was recorded in these hypothalamic areas. HFD induced a decreased amplitude of mEPSC in the arcuate nucleus (ARC) and the ventromedial hypothalamus (VMH), meanwhile, increased the frequency in the VMH. In addition, HFD reduced the frequency of mIPSC in the lateral hypothalamus (LH) and increased the amplitude of mIPSC in the paraventricular nucleus of the hypothalamus (PVH). Subsequently, we also measured the synaptic activity of nucleus accumbens (NAc) neurons, which play a vital role in the hedonic aspect of eating, and discovered that HFD diminished the frequency of both mEPSC and mIPSC in the NAc. These findings suggest that chronic HFD feeding leads to lipid accumulation and synaptic dysfunction in specific brain regions, which are associated with energy homeostasis and reward regulation, and these impairments may lead to the overeating of obesity.

Systemic Beta-Hydroxybutyrate Affects BDNF and Autophagy into the Retina of Diabetic Mice

Background: Diabetic retinopathy (DR) is a neurovascular disease, characterized by a deficiency of brain-derived neurotrophic factor (BDNF), a regulator of autophagy. Beta-hydroxybutyrate (BHB), previously reported as a protective agent in DR, has been associated with BDNF promotion. Here, we investigated whether systemic BHB affects the retinal levels of BDNF and local autophagy in diabetic mice with retinopathy; Methods: C57BL/6J mice were administered with intraperitoneal (i.p.) streptozotocin (STZ) (75 mg/kg) injection to develop diabetes. After 2 weeks, they received i.p. injections of BHB (25−50−100 mg/kg) twice a week for 10 weeks. Retinal samples were collected in order to perform immunofluorescence, Western blotting, and ELISA analysis; Results: BHB 50 mg/kg and 100 mg/kg significantly improved retinal BDNF levels (p < 0.01) in diabetic mice. This improvement was negatively associated with autophagosome−lysosome formations (marked by LC3B and ATG14) and to higher levels of connexin 43 (p < 0.01), a marker of cell integrity. Moreover, BHB administration significantly reduced M1 microglial activation and autophagy (p < 0.01); Conclusions: The systemic administration of BHB in mice with DR improves the retinal levels of BDNF, with the consequent reduction of the abnormal microglial autophagy. This leads to retinal cell safety through connexin 43 restoration.

Emerging Evidence of Pathological Roles of Very-Low-Density Lipoprotein (VLDL)

Embraced with apolipoproteins (Apo) B and Apo E, triglyceride-enriched very-low-density lipoprotein (VLDL) is secreted by the liver into circulation, mainly during post-meal hours. Here, we present a brief review of the physiological role of VLDL and a systemic review of the emerging evidence supporting its pathological roles. VLDL promotes atherosclerosis in metabolic syndrome (MetS). VLDL isolated from subjects with MetS exhibits cytotoxicity to atrial myocytes, induces atrial myopathy, and promotes vulnerability to atrial fibrillation. VLDL levels are affected by a number of endocrinological disorders and can be increased by therapeutic supplementation with cortisol, growth hormone, progesterone, and estrogen. VLDL promotes aldosterone secretion, which contributes to hypertension. VLDL induces neuroinflammation, leading to cognitive dysfunction. VLDL levels are also correlated with chronic kidney disease, autoimmune disorders, and some dermatological diseases. The extra-hepatic secretion of VLDL derived from intestinal dysbiosis is suggested to be harmful. Emerging evidence suggests disturbed VLDL metabolism in sleep disorders and in cancer development and progression. In addition to VLDL, the VLDL receptor (VLDLR) may affect both VLDL metabolism and carcinogenesis. Overall, emerging evidence supports the pathological roles of VLDL in multi-organ diseases. To better understand the fundamental mechanisms of how VLDL promotes disease development, elucidation of the quality control of VLDL and of the regulation and signaling of VLDLR should be indispensable. With this, successful VLDL-targeted therapies can be discovered in the future.

Dietary fat: a potent microglial influencer

Poor nutrition, lack of exercise, and genetic predisposition all contribute to the growing epidemic of obesity. Overweight/obesity create an environment of chronic inflammation that leads to negative physiological and neurological outcomes, such as diabetes, cardiovascular disease, and anxiety/depression. While the whole body contributes to metabolic homeostasis, the neuroimmune system has recently emerged as a key regulator of metabolism. Microglia, the resident immune cells of the brain, respond both directly and indirectly to dietary fat, and the environment in which microglia develop contributes to their responsiveness later in life. Thus, high maternal weight during pregnancy may have consequences for microglial function in offspring. Here, we discuss the most recent findings on microglia signaling in overweight/obesity with a focus on perinatal programming.

Profiling Microglia in a Mouse Model of Machado–Joseph Disease

Microglia have been increasingly implicated in neurodegenerative diseases (NDs), and specific disease associated microglia (DAM) profiles have been defined for several of these NDs. Yet, the microglial profile in Machado–Joseph disease (MJD) remains unexplored. Here, we characterized the profile of microglia in the CMVMJD135 mouse model of MJD. This characterization was performed using primary microglial cultures and microglial cells obtained from disease-relevant brain regions of neonatal and adult CMVMJD135 mice, respectively. Machine learning models were implemented to identify potential clusters of microglia based on their morphological features, and an RNA-sequencing analysis was performed to identify molecular perturbations and potential therapeutic targets. Our findings reveal morphological alterations that point to an increased activation state of microglia in CMVMJD135 mice and a disease-specific transcriptional profile of MJD microglia, encompassing a total of 101 differentially expressed genes, with enrichment in molecular pathways related to oxidative stress, immune response, cell proliferation, cell death, and lipid metabolism. Overall, these results allowed us to define the cellular and molecular profile of MJD-associated microglia and to identify genes and pathways that might represent potential therapeutic targets for this disorder.

Synapses, Microglia, and Lipids in Alzheimer's Disease

Alzheimer's disease (AD) is characterised by synaptic dysfunction accompanied by the microscopically visible accumulation of pathological protein deposits and cellular dystrophy involving both neurons and glia. Late-stage AD shows pronounced loss of synapses and neurons across several differentially affected brain regions. Recent studies of advanced AD using post-mortem brain samples have demonstrated the direct involvement of microglia in synaptic changes. Variants of the Apolipoprotein E and Triggering Receptors Expressed on Myeloid Cells gene represent important determinants of microglial activity but also of lipid metabolism in cells of the central nervous system. Here we review evidence that may help to explain how abnormal lipid metabolism, microglial activation, and synaptic pathophysiology are inter-related in AD.