Microglia-Neuron Crosstalk in Obesity: Melodious Interaction or Kiss of Death?

Microglia in physiological conditions and the importance of understanding their homeostatic functions in the arcuate nucleus

Microglia are highly dynamic cells that have been mainly studied under pathological conditions. The present review discusses the possible implication of microglia as modulators of neuronal electrical responses in physiological conditions and hypothesizes how these cells might modulate hypothalamic circuits in health and during obesity. Microglial cells studied under physiological conditions are highly diverse, depending on the developmental stage and brain region. The evidence also suggests that neuronal electrical activity modulates microglial motility to control neuronal excitability. Additionally, we show that the expression of genes associated with neuron-microglia interaction is down-regulated in obese mice compared to control-fed mice, suggesting an alteration in the contact-dependent mechanisms that sustain hypothalamic arcuate-median eminence neuronal function. We also discuss the possible implication of microglial-derived signals for the excitability of hypothalamic neurons during homeostasis and obesity. This review emphasizes the importance of studying the physiological interplay between microglia and neurons to maintain proper neuronal circuit function. It aims to elucidate how disruptions in the normal activities of microglia can adversely affect neuronal health.

The Impact of the Endocrine and Immunological Function of Adipose Tissue on Reproduction in Women with Obesity

Obesity, which leads to metabolic dysregulation and body function impairment, emerges as one of the pressing health challenges worldwide. Excessive body fat deposits comprise a dynamic and biologically active organ possessing its own endocrine function. One of the mechanisms underlying the pathophysiology of obesity is low-grade systemic inflammation mediated by pro-inflammatory factors such as free fatty acids, lipopolysaccharides, adipokines (including leptin, resistin and visfatin) and cytokines (TNF-α, IL-1β, Il-6), which are secreted by adipose tissue. Together with obesity-induced insulin resistance and hyperandrogenism, the exacerbated immune response has a negative impact on the hypothalamic-pituitary-gonadal axis at all levels and directly affects reproduction. In women, it results in disrupted ovarian function, irregular menstrual cycles and anovulation, contributing to infertility. This review focuses on the abnormal intracellular communication, altered gene expression and signaling pathways activated in obesity, underscoring its multifactorial character and consequences at a molecular level. Extensive presentation of the complex interplay between adipokines, cytokines, immune cells and neurons may serve as a foundation for future studies in search of potential sites for more targeted treatment of reproductive disorders related to obesity.

Alisol A Exerts Neuroprotective Effects Against HFD-Induced Pathological Brain Aging via the SIRT3-NF-κB/MAPK Pathway

Chronic consumption of a high-fat diet (HFD) has profound effects on brain aging, which is mainly characterized by cognitive decline, inflammatory responses, and neurovascular damage. Alisol A (AA) is a triterpenoid with therapeutic potential for metabolic diseases, but whether it has a neuroprotective effect against brain aging caused by a HFD has not been investigated. Six-month-old male C57BL6/J mice were exposed to a HFD with or without AA treatment for 12 weeks. Behavioral tasks were used to assess the cognitive abilities of the mice. Neuroinflammation and changes in neurovascular structure in the brains were examined. We further assessed the mechanism by which AA exerts neuroprotective effects against HFD-induced pathological brain aging in vitro and in vivo. Behavioral tests showed that cognitive function was improved in AA-treated animals. AA treatment reduced microglia activation and inflammatory cytokine release induced by a HFD. Furthermore, AA treatment increased the number of hippocampal neurons, the density of dendritic spines, and the expression of tight junction proteins. We also demonstrated that AA attenuated microglial activation by targeting the SIRT3-NF-κB/MAPK pathway and ameliorated microglial activation-induced tight junction degeneration in endothelial cells and apoptosis in hippocampal neurons. The results of this study show that AA may be a promising agent for the treatment of HFD-induced brain aging.

Obesity as a Neuroendocrine Disorder

Obesity is one of the most prevalent diseases in the world. Based on hundreds of clinical and basic investigations, its etiopathogenesis goes beyond the simple imbalance between energy intake and expenditure. The center of the regulation of appetite and satiety lies in the nuclei of the hypothalamus where peripheral signals derived from adipose tissue (e.g., leptin), the gastrointestinal tract, the pancreas, and other brain structures, arrive. These signals are part of the homeostatic control system (eating to survive). Additionally, a hedonic or reward system (eating for pleasure) is integrated into the regulation of appetite. This reward system consists of a dopaminergic circuit that affects eating-related behaviors influencing food preferences, food desires, gratification when eating, and impulse control to avoid compulsions. These systems are not separate. Indeed, many of the hormones that participate in the homeostatic system also participate in the regulation of the hedonic system. In addition, factors such as genetic and epigenetic changes, certain environmental and sociocultural elements, the microbiota, and neuronal proinflammatory effects of high-energy diets also contribute to the development of obesity. Therefore, obesity can be considered a complex neuroendocrine disease, and all of the aforementioned components should be considered for the management of obesity.

Is human obesity an inflammatory disease of the hypothalamus?

Obesity and its comorbidities are long-standing, challenging global health problems. Lack of exercise, overnutrition, and especially the consumption of fat-rich foods are some of the most important factors leading to an increase in prevalence in modern society. The pathophysiology of obesity as a metabolic inflammatory disease has moved into focus since new therapeutic approaches are required. The hypothalamus, a brain area responsible for energy homeostasis, has recently received special attention in this regard. Hypothalamic inflammation was identified to be associated with diet-induced obesity and new evidence suggests that it may be, beyond that, a pathological mechanism of the disease. This inflammation impairs the local signaling of insulin and leptin leading to dysfunction of the regulation of energy balance and thus, weight gain. After a high-fat diet consumption, activation of inflammatory mediators such as the nuclear factor κB or c-Jun N-terminal kinase pathway can be observed, accompanied by elevated secretion of pro-inflammatory interleukins and cytokines. Brain resident glia cells, especially microglia and astrocytes, initiate this release in response to the flux of fatty acids. The gliosis occurs rapidly before the actual weight gain. Dysregulated hypothalamic circuits change the interaction between neuronal and non-neuronal cells, contributing to the establishment of inflammatory processes. Several studies have reported reactive gliosis in obese humans. Although there is evidence for a causative role of hypothalamic inflammation in the obesity development, data on underlying molecular pathways in humans are limited. This review discusses the current state of knowledge on the relationship between hypothalamic inflammation and obesity in humans.

Liraglutide Counteracts Endoplasmic Reticulum Stress in Palmitate-Treated Hypothalamic Neurons without Restoring Mitochondrial Homeostasis

One feature of high-fat diet-induced neurodegeneration in the hypothalamus is an increased level of palmitate, which is associated with endoplasmic reticulum (ER) stress, loss of CoxIV, mitochondrial fragmentation, and decreased abundance of MC4R. To determine whether antidiabetic drugs protect against ER and/or mitochondrial dysfunction by lipid stress, hypothalamic neurons derived from pre-adult mice and neuronal Neuro2A cells were exposed to elevated palmitate. In the hypothalamic neurons, palmitate exposure increased expression of ER resident proteins, including that of SERCA2, indicating ER stress. Liraglutide reverted such altered ER proteostasis, while metformin only normalized SERCA2 expression. In Neuro2A cells liraglutide, but not metformin, also blunted dilation of the ER induced by palmitate treatment, and enhanced abundance and expression of MC4R at the cell surface. Thus, liraglutide counteracts, more effectively than metformin, altered ER proteostasis, morphology, and folding capacity in neurons exposed to fat. In palmitate-treated hypothalamic neurons, mitochondrial fragmentation took place together with loss of CoxIV and decreased mitochondrial membrane potential (MMP). Metformin, but not liraglutide, reverted mitochondrial fragmentation, and both liraglutide and metformin did not protect against either loss of CoxIV abundance or MMP. Thus, ER recovery from lipid stress can take place in hypothalamic neurons in the absence of recovered mitochondrial homeostasis.

Alterations of the expression levels of glucose, inflammation, and iron metabolism related miRNAs and their target genes in the hypothalamus of STZ-induced rat diabetes model

BACKGROUND

The hypothalamus of the central nervous system is implicated in the development of diabetes due to its glucose-sensing function. Dysregulation of the hypothalamic glucose-sensing neurons leads to abnormal glucose metabolism. It has been described that fractalkine (FKN) is involved in the development of hypothalamic inflammation, which may be one of the underlying causes of a diabetic condition. Moreover, iron may play a role in the pathogenesis of diabetes via the regulation of hepcidin, the iron regulatory hormone synthesis. MicroRNAs (miRNAs) are short non-coding molecules working as key regulators of gene expression, usually by inhibiting translation. Hypothalamic miRNAs are supposed to have a role in the control of energy balance by acting as regulators of hypothalamic glucose metabolism via influencing translation.

METHODS

Using a miRNA array, we analysed the expression of diabetes, inflammation, and iron metabolism related miRNAs in the hypothalamus of a streptozotocin-induced rat type 1 diabetes model. Determination of the effect of miRNAs altered by STZ treatment on the target genes was carried out at protein level.

RESULTS

We found 18 miRNAs with altered expression levels in the hypothalamus of the STZ-treated animals, which act as the regulators of mRNAs involved in glucose metabolism, pro-inflammatory cytokine synthesis, and iron homeostasis suggesting a link between these processes in diabetes. The alterations in the expression level of these miRNAs could modify hypothalamic glucose sensing, tolerance, uptake, and phosphorylation by affecting the stability of hexokinase-2, insulin receptor, leptin receptor, glucokinase, GLUT4, insulin-like growth factor receptor 1, and phosphoenolpyruvate carboxykinase mRNA molecules. Additional miRNAs were found to be altered resulting in the elevation of FKN protein. The miRNA, mRNA, and protein analyses of the diabetic hypothalamus revealed that the iron import, export, and iron storage were all influenced by miRNAs suggesting the disturbance of hypothalamic iron homeostasis.

CONCLUSION

It can be supposed that glucose metabolism, inflammation, and iron homeostasis of the hypothalamus are linked via the altered expression of common miRNAs as well as the increased expression of FKN, which contribute to the imbalance of energy homeostasis, the synthesis of pro-inflammatory cytokines, and the iron accumulation of the hypothalamus. The results raise the possibility that FKN could be a potential target of new therapies targeting both inflammation and iron disturbances in diabetic conditions.

TRIM67 Deficiency Exacerbates Hypothalamic Inflammation and Fat Accumulation in Obese Mice

Obesity has achieved the appearance of a global epidemic and is a serious cause for concern. The hypothalamus, as the central regulator of energy homeostasis, plays a critical role in regulating food intake and energy expenditure. In this study, we show that TRIM67 in the hypothalamus was responsive to body-energy homeostasis whilst a deficiency of TRIM67 exacerbated metabolic disorders in high-fat-diet-induced obese mice. We found exacerbated neuroinflammation and apoptosis in the hypothalamus of obese TRIM67 KO mice. We also found reduced BDNF in the hypothalamus, which affected the fat sympathetic nervous system innervation and contributed to lipid accumulation in adipose tissue under high-fat-diet exposure. In this study, we reveal potential implications between TRIM67 and the hypothalamic function responding to energy overuptake as well as a consideration for the therapeutic diagnosis of obesity.

The Properties and Functions of Glial Cell Types of the Hypothalamic Median Eminence

The median eminence (ME) is part of the neuroendocrine system (NES) that functions as a crucial interface between the hypothalamus and pituitary gland. The ME contains many non-neuronal cell types, including oligodendrocytes, oligodendrocyte precursor cells (OPCs), tanycytes, astrocytes, pericytes, microglia and other immune cells, which may be involved in the regulation of NES function. For example, in mice, ablation of tanycytes (a special class of ependymal glia with stem cell-like functions) results in weight gain, feeding, insulin insensitivity and increased visceral adipose, consistent with the demonstrated ability of these cells to sense and transport both glucose and leptin, and to differentiate into neurons that control feeding and metabolism in the hypothalamus. To give a further example, OPCs in the ME of mice have been shown to rapidly respond to dietary signals, in turn controlling composition of the extracellular matrix in the ME, derived from oligodendrocyte-lineage cells, which may contribute to the previously described role of these cells in actively maintaining leptin-receptor-expressing dendrites in the ME. In this review, we explore and discuss recent advances such as these, that have developed our understanding of how the various cell types of the ME contribute to its function in the NES as the interface between the hypothalamus and pituitary gland. We also highlight avenues of future research which promise to uncover additional functions of the ME and the glia, stem and progenitor cells it contains.