Brain innate immunity regulates hypothalamic arcuate neuronal activity and feeding behavior

A high-fat diet influences neural stem and progenitor cell environment in the medulla of adult mice

It has been widely established that neural stem cells (NSCs) exist in the adult mammalian brain. The area postrema (AP) and the ependymal cell layer of the central canal (CC) in the medulla were recently identified as NSC niches. There are two types of NSCs: astrocyte-like cells in the AP and tanycyte-like cells in the CC. However, limited information is currently available on the characteristics and functional significance of these NSCs and their progeny in the AP and CC. The AP is a part of the dorsal vagal complex (DVC), together with the nucleus of the solitary tract (Sol) and the dorsal motor nucleus of the vagus (10 N). DVC is the primary site for the integration of visceral neuronal and hormonal cues that act to inhibit food intake. Therefore, we examined the effects of high-fat diet (HFD) on NSCs and progenitor cells in the AP and CC. Eight-week-old male mice were fed HFD for short (1 week) and long periods (4 weeks). To detect proliferating cells, mice consecutively received intraperitoneal injections of BrdU for 7 days. Brain sections were processed with immunohistochemistry using various cell markers and BrdU antibodies. Our data demonstrated that adult NSCs and neural progenitor cells (NPCs) in the medulla responded more strongly to short-term HFD than to long-term HFD. HFD increased astrocyte density in the Sol and 10 N, and increased microglial/macrophage density in the AP and Sol. Furthermore, long-term HFD induced mild inflammation in the medulla, suggesting that it affected the proliferation of NSCs and NPCs.

Synergism Between Hypothalamic Astrocytes and Neurons in Metabolic Control

Astrocytes are no longer considered as passive support cells. In the hypothalamus, these glial cells actively participate in the control of appetite, energy expenditure, and the processes leading to obesity and its secondary complications. Here we briefly review studies supporting this conclusion and the advances made in understanding the underlying mechanisms.

Microglia/macrophage-specific deletion of TLR-4 protects against neural effects of diet-induced obesity

Obesity is associated with numerous adverse neural effects, including reduced neurogenesis, cognitive impairment, and increased risks for developing Alzheimer's disease (AD) and vascular dementia. Obesity is also characterized by chronic, low-grade inflammation that is implicated in mediating negative consequences body-wide. Toll-like receptor 4 (TLR4) signaling from peripheral macrophages is implicated as an essential regulator of the systemic inflammatory effects of obesity. In the brain, obesity drives chronic neuroinflammation that involves microglial activation, however the contributions of microglia-derived TLR4 signaling to the consequences of obesity are poorly understood. To investigate this issue, we first generated mice that carry an inducible, microglia/macrophage-specific deletion of TLR4 that yields long-term TLR4 knockout only in brain indicating microglial specificity. Next, we analyzed the effects of microglial TLR4 deletion on systemic and neural effects of a 16-week of exposure to control versus obesogenic high-fat diets. In male mice, TLR4 deletion generally yielded limited effects on diet-induced systemic metabolic dysfunction but significantly reduced neuroinflammation and impairments in neurogenesis and cognitive performance. In female mice maintained on obesogenic diet, TLR4 deletion partially protected against weight gain, adiposity, and metabolic impairments. Compared to males, females showed milder diet-induced neural consequences, against which TLR4 deletion was protective. Collectively, these findings demonstrate a central role of microglial TLR4 signaling in mediating the neural effects of obesogenic diet and highlight sexual dimorphic responses to both diet and TLR4.

Perinatal Western-style diet exposure associated with decreased microglial counts throughout the arcuate nucleus of the hypothalamus in Japanese macaques

Perinatal exposure to a high-fat, high-sugar Western-style diet (WSD) is associated with altered neural circuitry in the melanocortin system. This association may have an underlying inflammatory component, as consumption of a WSD during pregnancy can lead to an elevated inflammatory environment. Our group previously demonstrated that prenatal WSD exposure was associated with increased markers of inflammation in the placenta and fetal hypothalamus in Japanese macaques. In this follow-up study, we sought to determine whether this heightened inflammatory state persisted into the postnatal period, as prenatal exposure to inflammation has been shown to reprogram offspring immune function and long-term neuroinflammation would present a potential means for prolonged disruptions to microglia-mediated neuronal circuit formation. Neuroinflammation was approximated in 1-yr-old offspring by counting resident microglia and peripherally derived macrophages in the region of the hypothalamus examined in the fetal study, the arcuate nucleus (ARC). Microglia and macrophages were immunofluorescently stained with their shared marker, ionized calcium-binding adapter molecule 1 (Iba1), and quantified in 11 regions along the rostral-caudal axis of the ARC. A mixed-effects model revealed main effects of perinatal diet (P = 0.011) and spatial location (P = 0.003) on Iba1-stained cell count. Perinatal WSD exposure was associated with a slight decrease in the number of Iba1-stained cells, and cells were more densely located in the center of the ARC. These findings suggest that the heightened inflammatory state experienced in utero does not persist postnatally. This inflammatory response trajectory could have important implications for understanding how neurodevelopmental disorders progress.NEW & NOTEWORTHY Prenatal Western-style diet exposure is associated with increased microglial activity in utero. However, we found a potentially neuroprotective reduction in microglia count during early postnatal development. This trajectory could inform the timing of disruptions to microglia-mediated neuronal circuit formation. Additionally, this is the first study in juvenile macaques to characterize the distribution of microglia along the rostral-caudal axis of the arcuate nucleus of the hypothalamus. Nearby neuronal populations may be greater targets during inflammatory insults.

Microgliosis: a double-edged sword in the control of food intake

Maintaining energy balance is essential for survival and health. This physiological function is controlled by the brain, which adapts food intake to energy needs. Indeed, the brain constantly receives a multitude of biological signals that are derived from digested foods or that originate from the gastrointestinal tract, energy stores (liver and adipose tissues) and other metabolically active organs (muscles). These signals, which include circulating nutrients, hormones and neuronal inputs from the periphery, collectively provide information on the overall energy status of the body. In the brain, several neuronal populations can specifically detect these signals. Nutrient-sensing neurons are found in discrete brain areas and are highly enriched in the hypothalamus. In turn, specialized brain circuits coordinate homeostatic responses acting mainly on appetite, peripheral metabolism, activity and arousal. Accumulating evidence shows that hypothalamic microglial cells located at the vicinity of these circuits can influence the brain control of energy balance. However, microglial cells could have opposite effects on energy balance, that is homeostatic or detrimental, and the conditions for this shift are not totally understood yet. One hypothesis relies on the extent of microglial activation, and nutritional lipids can considerably change it.

Microglia Sirt6 modulates the transcriptional activity of NRF2 to ameliorate high-fat diet-induced obesity

BACKGROUND

Microglia play a pivotal role in neuroinflammation, while obesity triggers hypothalamic microglia activation and inflammation. Sirt6 is an important regulator of energy metabolism in many peripheral tissues and hypothalamic anorexic neurons. However, the exact mechanism for microglia Sirt6 in controlling high-fat diet-induced obesity remain unknown.

METHODS

Microglia Sirt6 expression levels under various nutritional conditions were measured in the hypothalamus of mice. Also, microglia Sirt6-deficient mice were provided various diets to monitor metabolic changes and hypothalamic inflammatory response. Besides, RNA-seq and Co-IP of microglia with Sirt6 alterations were conducted to further investigate the detailed mechanism by which Sirt6 modulated microglia activity.

RESULTS

We found that Sirt6 was downregulated in hypothalamic microglia in mice given a high-fat diet (HFD). Additionally, knockout of microglia Sirt6 exacerbated high-fat diet-induced hypothalamic microglial activation and inflammation. As a result, mice were more prone to obesity, exhibiting a decrease in energy expenditure, impaired glucose tolerance, insulin and leptin resistance, and increased food intake. In vitro, Sirt6 overexpression in BV2 cells displayed protective effects against oleic acid and palmitic acid treatment-derived inflammatory response. Mechanically, Sirt6 deacetylated and stabilised NRF2 to increase the expression of anti-oxidative genes and defend against reactive oxygen species overload. Pharmacological inhibition of NRF2 eliminated the beneficial modulating effects of Sirt6 on microglial activity.

CONCLUSION

Collectively, our results revealed that microglial Sirt6 was a primary contributor of microglial activation in the central regulation of obesity. Thus, microglial Sirt6 may be an important therapeutic target for obesity.

Reduction of oxytocin-containing neurons and enhanced glymphatic activity in the hypothalamic paraventricular nucleus of patients with type 2 diabetes mellitus

Evidence from animal experiments has shown that the hypothalamic paraventricular nucleus (PVN) plays a key role in regulating body weight and blood glucose levels. However, it is unclear whether neuron populations in the human PVN are involved in the development of type 2 diabetes mellitus (T2DM). To address this, we investigated the neuronal and glial populations in the PVN of 26 T2DM patients and 20 matched controls. Our findings revealed a significant reduction in oxytocin (Oxt) neuron density in the PVN of T2DM patients compared to controls, while other neuronal populations remained unchanged. This suggests that Oxt neurons may play a specific role in the pathophysiology of T2DM. Interestingly, the reduction in Oxt neurons was accompanied by a decreased melanocortinergic input in to the PVN as reflected by a reduction in alpha-MSH immunoreactivity. We also analysed two glial cell populations, as they are important for maintaining a healthy neural microenvironment. We found that microglial density, phagocytic capacity, and their proximity to neurons were not altered in T2DM patients, indicating that the loss of Oxt neurons is independent of changes in microglial immunity. However, we did observe a reduction in the number of astrocytes, which are crucial for providing trophic support to local neurons. Moreover, a specific subpopulation of astrocytes characterized by aquaporin 4 expression was overrepresented in T2DM patients. Since this subset of astrocytes is linked to the glymphatic system, their overrepresentation might point to alterations in the hypothalamic waste clearance system in T2DM. Our study shows selective loss of Oxt neurons in the PVN of T2DM individuals in association with astrocytic reduction and gliovascular remodelling. Therefore, hypothalamic Oxt neurons may represent a potential target for T2DM treatment modalities.

Hypothalamic astrocytes control systemic glucose metabolism and energy balance

The hypothalamus is key in the control of energy balance. However, strategies targeting hypothalamic neurons have failed to provide viable options to treat most metabolic diseases. Conversely, the role of astrocytes in systemic metabolic control has remained largely unexplored. Here, we show that obesity promotes anatomically restricted remodeling of hypothalamic astrocyte activity. In the paraventricular nucleus (PVN) of the hypothalamus, chemogenetic manipulation of astrocytes results in bidirectional control of neighboring neuron activity, autonomic outflow, glucose metabolism, and energy balance. This process recruits a mechanism involving the astrocytic control of ambient glutamate levels, which becomes defective in obesity. Positive or negative chemogenetic manipulation of PVN astrocyte Ca2+ signals, respectively, worsens or improves metabolic status of diet-induced obese mice. Collectively, these findings highlight a yet unappreciated role for astrocytes in the direct control of systemic metabolism and suggest potential targets for anti-obesity strategy.

The role of hypothalamic endoplasmic reticulum stress in schizophrenia and antipsychotic-induced weight gain: A narrative review

Schizophrenia (SCZ) is a serious mental illness that affects 1% of people worldwide. SCZ is associated with a higher risk of developing metabolic disorders such as obesity. Antipsychotics are the main treatment for SCZ, but their side effects include significant weight gain/obesity. Despite extensive research, the underlying mechanisms by which SCZ and antipsychotic treatment induce weight gain/obesity remain unclear. Hypothalamic endoplasmic reticulum (ER) stress is one of the most important pathways that modulates inflammation, neuronal function, and energy balance. This review aimed to investigate the role of hypothalamic ER stress in SCZ and antipsychotic-induced weight gain/obesity. Preliminary evidence indicates that SCZ is associated with reduced dopamine D2 receptor (DRD2) signaling, which significantly regulates the ER stress pathway, suggesting the importance of ER stress in SCZ and its related metabolic disorders. Antipsychotics such as olanzapine activate ER stress in hypothalamic neurons. These effects may induce decreased proopiomelanocortin (POMC) processing, increased neuropeptide Y (NPY) and agouti-related protein (AgRP) expression, autophagy, and leptin and insulin resistance, resulting in hyperphagia, decreased energy expenditure, and central inflammation, thereby causing weight gain. By activating ER stress, antipsychotics such as olanzapine activate hypothalamic astrocytes and Toll-like receptor 4 signaling, thereby causing inflammation and weight gain/obesity. Moreover, evidence suggests that antipsychotic-induced ER stress may be related to their antagonistic effects on neurotransmitter receptors such as DRD2 and the histamine H1 receptor. Taken together, ER stress inhibitors could be a potential effective intervention against SCZ and antipsychotic-induced weight gain and inflammation.

The limitations of investigating appetite through circuit manipulations: are we biting off more than we can chew?

Disordered eating can underpin a number of debilitating and prevalent chronic diseases, such as obesity. Broader advances in psychopharmacology and biology have motivated some neuroscientists to address diet-induced obesity through reductionist, pre-clinical eating investigations on the rodent brain. Specifically, chemogenetic and optogenetic methods developed in the 21st century allow neuroscientists to perform in vivo, region-specific/projection-specific/promoter-specific circuit manipulations and immediately assess the impact of these manipulations on rodent feeding. These studies are able to rigorously conclude whether a specific neuronal population regulates feeding behaviour in the hope of eventually developing a mechanistic neuroanatomical map of appetite regulation. However, an artificially stimulated/inhibited rodent neuronal population that changes feeding behaviour does not necessarily represent a pharmacological target for treating eating disorders in humans. Chemogenetic/optogenetic findings must therefore be triangulated with the array of theories that contribute to our understanding of appetite. The objective of this review is to provide a wide-ranging discussion of the limitations of chemogenetic/optogenetic circuit manipulation experiments in rodents that are used to investigate appetite. Stepping into and outside of medical science epistemologies, this paper draws on philosophy of science, nutrition, addiction biology and neurophilosophy to prompt more integrative, transdisciplinary interpretations of chemogenetic/optogenetic appetite data. Through discussing the various technical and epistemological limitations of these data, we provide both an overview of chemogenetics and optogenetics accessible to non-neuroscientist obesity researchers, as well as a resource for neuroscientists to expand the number of lenses through which they interpret their circuit manipulation findings.

Long-term priming of hypothalamic microglia is associated with energy balance disturbances under diet-induced obesity

Exposure of microglia to an inflammatory environment may lead to their priming and exacerbated response to future inflammatory stimuli. Here we aimed to explore hypothalamic microglia priming and its consequences on energy balance regulation. A model of intracerebroventricular administration of neuraminidase (NA, which is present in various pathogens such as influenza virus) was used to induce acute neuroinflammation. Evidences of primed microglia were observed 3 months after NA injection, namely (1) a heightened response of microglia located in the hypothalamic arcuate nucleus after an in vivo inflammatory challenge (high fat diet [HFD] feeding for 10 days), and (2) an enhanced response of microglia isolated from NA-treated mice and challenged in vitro to LPS. On the other hand, the consequences of a previous NA-induced neuroinflammation were further evaluated in an alternative inflammatory and hypercaloric scenario, such as the obesity generated by continued HDF feeding. Compared with sham-injected mice, NA-treated mice showed increased food intake and, surprisingly, reduced body weight. Besides, NA-treated mice had enhanced microgliosis (evidenced by increased number and reactive morphology of microglia) and a reduced population of POMC neurons in the basal hypothalamus. Thus, a single acute neuroinflammatory event may elicit a sustained state of priming in microglial cells, and in particular those located in the hypothalamus, with consequences in hypothalamic cytoarchitecture and its regulatory function upon nutritional challenges.

The Role of Obesity and Diabetes in Dementia

Chronic conditions such as obesity, diabetes, and dementia are increasing in the United States (US) population. Knowledge of these chronic conditions, preventative measures, and proper management tactics is important and critical to preventing disease. The overlap between obesity, diabetes, and dementia is becoming further elucidated. These conditions share a similar origin through the components of increasing age, gender, genetic and epigenetic predispositions, depression, and a high-fat Western diet (WD) that all contribute to the inflammatory state associated with the development of obesity, diabetes, and dementia. This inflammatory state leads to the dysregulation of food intake and insulin resistance. Obesity is often the cornerstone that leads to the development of diabetes and, subsequently, in the case of type 2 diabetes mellitus (T2DM), progression to “type 3 diabetes mellitus (T3DM)”. Obesity and depression are closely associated with diabetes. However, dementia can be avoided with lifestyle modifications, by switching to a plant-based diet (e.g., a Mediterranean diet (MD)), and increasing physical activity. Diet and exercise are not the only treatment options. There are several surgical and pharmacological interventions available for prevention. Current and future research within each of these fields is warranted and offers the chance for new treatment options and a better understanding of the pathogenesis of each condition.

Microbial neuraminidase induces TLR4-dependent long-term immune priming in the brain

Innate immune memory explains the plasticity of immune responses after repeated immune stimulation, leading to either enhanced or suppressed immune responses. This process has been extensively reported in peripheral immune cells and also, although modestly, in the brain. Here we explored two relevant aspects of brain immune priming: its persistence over time and its dependence on TLR receptors. For this purpose, we used an experimental paradigm consisting in applying two inflammatory stimuli three months apart. Wild type, toll-like receptor (TLR) 4 and TLR2 mutant strains were used. The priming stimulus was the intracerebroventricular injection of neuraminidase (an enzyme that is present in various pathogens able to provoke brain infections), which triggers an acute inflammatory process in the brain. The second stimulus was the intraperitoneal injection of lipopolysaccharide (a TLR4 ligand) or Pam3CSK4 (a TLR2 ligand). One day after the second inflammatory challenge the immune response in the brain was examined. In wild type mice, microglial and astroglial density, as well as the expression of 4 out of 5 pro-inflammatory genes studied (TNFα, IL1β, Gal-3, and NLRP3), were increased in mice that received the double stimulus compared to those exposed only to the second one, which were initially injected with saline instead of neuraminidase. Such enhanced response suggests immune training in the brain, which lasts at least 3 months. On the other hand, TLR2 mutants under the same experimental design displayed an enhanced immune response quite similar to that of wild type mice. However, in TLR4 mutant mice the response after the second immune challenge was largely dampened, indicating the pivotal role of this receptor in the establishment of immune priming. Our results demonstrate that neuraminidase-induced inflammation primes an enhanced immune response in the brain to a subsequent immune challenge, immune training that endures and that is largely dependent on TLR4 receptor.

CX3CL1 Action on Microglia Protects from Diet-Induced Obesity by Restoring POMC Neuronal Excitability and Melanocortin System Activity Impaired by High-Fat Diet Feeding

Both hypothalamic microglial inflammation and melanocortin pathway dysfunction contribute to diet-induced obesity (DIO) pathogenesis. Previous studies involving models of altered microglial signaling demonstrate altered DIO susceptibility with corresponding POMC neuron cytological changes, suggesting a link between microglia and the melanocortin system. We addressed this hypothesis using the specific microglial silencing molecule, CX3CL1 (fractalkine), to determine whether reducing hypothalamic microglial activation can restore POMC/melanocortin signaling to protect against DIO. We performed metabolic analyses in high fat diet (HFD)-fed mice with targeted viral overexpression of CX3CL1 in the hypothalamus. Electrophysiologic recording in hypothalamic slices from POMC-MAPT-GFP mice was used to determine the effects of HFD feeding and microglial silencing via minocycline or CX3CL1 on GFP-labeled POMC neurons. Finally, mice with hypothalamic overexpression of CX3CL1 received central treatment with the melanocortin receptor antagonist SHU9119 to determine whether melanocortin signaling is required for the metabolic benefits of CX3CL1. Hypothalamic overexpression of CX3CL1 increased leptin sensitivity and POMC gene expression, while reducing weight gain in animals fed an HFD. In electrophysiological recordings from hypothalamic slice preparations, HFD feeding was associated with reduced POMC neuron excitability and increased amplitude of inhibitory postsynaptic currents. Microglial silencing using minocycline or CX3CL1 treatment reversed these HFD-induced changes in POMC neuron electrophysiologic properties. Correspondingly, blockade of melanocortin receptor signaling in vivo prevented both the acute and chronic reduction in food intake and body weight mediated by CX3CL1. Our results show that suppressing microglial activation during HFD feeding reduces DIO susceptibility via a mechanism involving increased POMC neuron excitability and melanocortin signaling.

The central nervous system control of energy homeostasis: high fat diet induced hypothalamic microinflammation and obesity

Obesity is believed to arise through the imbalance of energy homeostasis controlled by the central nervous system, where the hypothalamus plays the fundamental role in energy metabolism. In this review, we will provide an overview regarding the functions of POMC neurons and AgRP neurons in acute nucleus of the hypothalamus which mediated the energy metabolism, highlighting their interactions with peripheral organs derived hormones in control of energy homeostasis. Furthermore, the role of high fat diet induced hypothalamic microinflammation in the pathogenesis of obesity will be discussed. We hope this review could help researchers to understand the mechanism of hypothalamus in control of energy metabolism, and design related drugs to block the pathways involving in the impaired metabolism in obese patients.

Metabolic factors in the regulation of hypothalamic innate immune responses in obesity

The hypothalamus is a central regulator of body weight and energy homeostasis. There is increasing evidence that innate immune activation in the mediobasal hypothalamus (MBH) is a key element in the pathogenesis of diet-induced obesity. Microglia, the resident immune cells in the brain parenchyma, have been shown to play roles in diverse aspects of brain function, including circuit refinement and synaptic pruning. As such, microglia have also been implicated in the development and progression of neurological diseases. Microglia express receptors for and are responsive to a wide variety of nutritional, hormonal, and immunological signals that modulate their distinct functions across different brain regions. We showed that microglia within the MBH sense and respond to a high-fat diet and regulate the function of hypothalamic neurons to promote food intake and obesity. Neurons, glia, and immune cells within the MBH are positioned to sense and respond to circulating signals that regulate their capacity to coordinate aspects of systemic energy metabolism. Here, we review the current knowledge of how these peripheral signals modulate the innate immune response in the MBH and enable microglia to regulate metabolic control.

Microglia Regulate Neuronal Circuits in Homeostatic and High-Fat Diet-Induced Inflammatory Conditions

Microglia are brain resident macrophages, which actively survey the surrounding microenvironment and promote tissue homeostasis under physiological conditions. During this process, microglia participate in synaptic remodeling, neurogenesis, elimination of unwanted neurons and cellular debris. The complex interplay between microglia and neurons drives the formation of functional neuronal connections and maintains an optimal neural network. However, activation of microglia induced by chronic inflammation increases synaptic phagocytosis and leads to neuronal impairment or death. Microglial dysfunction is implicated in almost all brain diseases and leads to long-lasting functional deficiency, such as hippocampus-related cognitive decline and hypothalamus-associated energy imbalance (i.e., obesity). High-fat diet (HFD) consumption triggers mediobasal hypothalamic microglial activation and inflammation. Moreover, HFD-induced inflammation results in cognitive deficits by triggering hippocampal microglial activation. Here, we have summarized the current knowledge of microglial characteristics and biological functions and also reviewed the molecular mechanism of microglia in shaping neural circuitries mainly related to cognition and energy balance in homeostatic and diet-induced inflammatory conditions.

PM2.5 Exposure of Mice during Spermatogenesis: A Role of Inhibitor κB Kinase 2 in Pro-Opiomelanocortin Neurons

BACKGROUND

Epidemiological studies have shown that exposure to ambient fine particulate matter with aerodynamic diameter less than or equal to 2.5 μm (PM2.5) correlates with a decrease in sperm count, but the biological mechanism remains elusive.

OBJECTIVES

This study tested whether hypothalamic inflammation, an emerging pathophysiological mediator, mediates the development of lower epididymal sperm count due to PM2.5 exposure.

METHODS

Inhibitor κB kinase 2 (IKK2) was conditionally knocked out either in all neurons or subtypes of hypothalamic neurons of mice. Effects of concentrated ambient PM2.5 (CAP) exposure on hypothalamic inflammation, the hypothalamic-pituitary-gonadal (HPG) axis, and epididymal sperm count of these mouse models were then assessed. Furthermore, to test whether hypothalamic inflammation is sufficient to decrease sperm production, we overexpressed constitutively active IKK2 (IKK2ca) either in all neurons or subtypes of hypothalamic neurons and assessed hypothalamic inflammation, the HPG axis, and sperm production of these overexpression mouse models.

RESULTS

CAP-exposed wild-type control mice vs. filtered air (FA)-exposed wild-type control mice had a higher expression of hypothalamic inflammatory markers, lower functional indexes of the HPG axis, and a lower epididymal sperm count. In contrast, all these measurements for CAP- vs. FA-exposed mice deficient of IKK2 in all neurons were comparable. We also found that overexpression of IKK2ca in either all neurons or pro-opiomelanocortin (POMC) neurons only, but not in Agouti-related protein (AgRP) neurons only, resulted in lower functional indexes of the HPG axis and a lower epididymal sperm count. Moreover, we showed that CAP- vs. FA-exposed mice deficient of IKK2 in POMC neurons had a comparable expression of hypothalamic inflammatory markers, comparable functional indexes of the HPG axis, and a comparable epididymal sperm count.

DISCUSSION

This mouse model study shows a causal role of IKK2 of POMC neurons in the development of lower epididymal sperm count due to PM2.5 exposure, providing a mechanistic insight into this emerging pathogenesis. https://doi.org/10.1289/EHP8868.

Mechanistic insight into high-fat diet-induced metabolic inflammation in the arcuate nucleus of the hypothalamus

A high-fat diet (HFD) is linked with cytokines production by non-neuronal cells within the hypothalamus, which mediates metabolic inflammation. These cytokines then activate different inflammatory mediators in the arcuate nucleus of the hypothalamus (ARC), a primary hypothalamic area accommodating proopiomelanocortin (POMC) and agouti-related peptide (AGRP) neurons, first-order neurons that sense and integrate peripheral metabolic signals and then respond accordingly. These mediators, such as inhibitor of κB kinase-β (IKKβ), suppression of cytokine signaling 3 (SOCS3), c-Jun N-terminal kinases (JNKs), protein kinase C (PKC), etc., cause insulin and leptin resistance in POMC and AGRP neurons and support obesity and related metabolic complications. On the other hand, inhibition of these mediators has been shown to counteract the impaired metabolism. Therefore, it is important to discuss the contribution of neuronal and non-neuronal cells in HFD-induced hypothalamic inflammation. Furthermore, understanding few other questions, such as the diets causing hypothalamic inflammation, the gender disparity in response to HFD feeding, and how hypothalamic inflammation affects ARC neurons to cause impaired metabolism, will be helpful for the development of therapeutic approaches to prevent or treat HFD-induced obesity.

C1q/TNF-related protein 4 restores leptin sensitivity by downregulating NF-κB signaling and microglial activation

Objective

C1qTNF-related protein 4 (CTRP4) acts in the hypothalamus to modulate food intake in diet-induced obese mice and has been shown to exert an anti-inflammatory effect on macrophages. Since high-fat diet-induced microglial activation and hypothalamic inflammation impair leptin signaling and increase food intake, we aimed to explore the potential connection between the anorexigenic effect of CTRP4 and the suppression of hypothalamic inflammation in mice with DIO.

Methods

Using an adenovirus-mediated hypothalamic CTRP4 overexpression model, we investigated the impact of CTRP4 on food intake and the hypothalamic leptin signaling pathway in diet-induced obese mice. Furthermore, central and plasma proinflammatory cytokines, including TNF-α and IL-6, were measured by Western blotting and ELISA. Changes in the hypothalamic NF-κB signaling cascade and microglial activation were also examined in vivo. In addition, NF-κB signaling and proinflammatory factors were investigated in BV-2 cells after CTRP4 intervention.

Results

We found that food intake was decreased, while leptin signaling was significantly improved in mice with DIO after CTRP4 overexpression. Central and peripheral TNF-α and IL-6 levels were reduced by central Ad-CTRP4 administration. Hypothalamic NF-κB signaling and microglial activation were also significantly suppressed in vivo. In addition, NF-κB signaling was inhibited in BV-2 cells following CTRP4 intervention, which was consistent with the decreased production of TNF-α and IL-6.

Conclusions

Our data indicate that CTRP4 reverses leptin resistance by inhibiting NF-κB-dependent microglial activation and hypothalamic inflammation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02167-2.

Hypothalamic neuropeptides and neurocircuitries in Prader Willi syndrome

Prader-Willi Syndrome (PWS) is a rare and incurable congenital neurodevelopmental disorder, resulting from the absence of expression of a group of genes on the paternally acquired chromosome 15q11-q13. Phenotypical characteristics of PWS include infantile hypotonia, short stature, incomplete pubertal development, hyperphagia and morbid obesity. Hypothalamic dysfunction in controlling body weight and food intake is a hallmark of PWS. Neuroimaging studies have demonstrated that PWS subjects have abnormal neurocircuitry engaged in the hedonic and physiological control of feeding behavior. This is translated into diminished production of hypothalamic effector peptides which are responsible for the coordination of energy homeostasis and satiety. So far, studies with animal models for PWS and with human post-mortem hypothalamic specimens demonstrated changes particularly in the infundibular and the paraventricular nuclei of the hypothalamus, both in orexigenic and anorexigenic neural populations. Moreover, many PWS patients have a severe endocrine dysfunction, e.g. central hypogonadism and/or growth hormone deficiency, which may contribute to the development of increased fat mass, especially if left untreated. Additionally, the role of non-neuronal cells, such as astrocytes and microglia in the hypothalamic dysregulation in PWS is yet to be determined. Notably, microglial activation is persistently present in non-genetic obesity. To what extent microglia, and other glial cells, are affected in PWS is poorly understood. The elucidation of the hypothalamic dysfunction in PWS could prove to be a key feature of rational therapeutic management in this syndrome. This review aims to examine the evidence for hypothalamic dysfunction, both at the neuropeptidergic and circuitry levels, and its correlation with the pathophysiology of PWS.

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

Diet-induced obesity can originate from the dysregulated activity of hypothalamic neuronal circuits, which are critical for the regulation of body weight and food intake. The exact mechanisms underlying such neuronal defects are not yet fully understood, but a maladaptive cross-talk between neurons and surrounding microglial is likely to be a contributing factor. Functional and anatomical connections between microglia and hypothalamic neuronal cells are at the core of how the brain orchestrates changes in the body's metabolic needs. However, such a melodious interaction may become maladaptive in response to prolonged diet-induced metabolic stress, thereby causing overfeeding, body weight gain, and systemic metabolic perturbations. From this perspective, we critically discuss emerging molecular and cellular underpinnings of microglia-neuron communication in the hypothalamic neuronal circuits implicated in energy balance regulation. We explore whether changes in this intercellular dialogue induced by metabolic stress may serve as a protective neuronal mechanism or contribute to disease establishment and progression. Our analysis provides a framework for future mechanistic studies that will facilitate progress into both the etiology and treatments of metabolic disorders.

Deficiency of Tristetraprolin Triggers Hyperthermia through Enhancing Hypothalamic Inflammation

Tristetraprolin (TTP), an RNA-binding protein, controls the stability of RNA by capturing AU-rich elements on their target genes. It has recently been identified that TTP serves as an anti-inflammatory protein by guiding the unstable mRNAs of pro-inflammatory proteins in multiple cells. However, it has not yet been investigated whether TTP affects the inflammatory responses in the hypothalamus. Since hypothalamic inflammation is tightly coupled to the disturbance of energy homeostasis, we designed the current study to investigate whether TTP regulates hypothalamic inflammation and thereby affects energy metabolism by utilizing TTP-deficient mice. We observed that deficiency of TTP led to enhanced hypothalamic inflammation via stimulation of a variety of pro-inflammatory genes. In addition, microglial activation occurred in the hypothalamus, which was accompanied by an enhanced inflammatory response. In line with these molecular and cellular observations, we finally confirmed that deficiency of TTP results in elevated core body temperature and energy expenditure. Taken together, our findings unmask novel roles of hypothalamic TTP on energy metabolism, which is linked to inflammatory responses in hypothalamic microglial cells.

Astrocyte Clocks and Glucose Homeostasis

The endogenous timekeeping system evolved to anticipate the time of the day through the 24 hours cycle of the Earth's rotation. In mammals, the circadian clock governs rhythmic physiological and behavioral processes, including the daily oscillation in glucose metabolism, food intake, energy expenditure, and whole-body insulin sensitivity. The results from a series of studies have demonstrated that environmental or genetic alterations of the circadian cycle in humans and rodents are strongly associated with metabolic diseases such as obesity and type 2 diabetes. Emerging evidence suggests that astrocyte clocks have a crucial role in regulating molecular, physiological, and behavioral circadian rhythms such as glucose metabolism and insulin sensitivity. Given the concurrent high prevalence of type 2 diabetes and circadian disruption, understanding the mechanisms underlying glucose homeostasis regulation by the circadian clock and its dysregulation may improve glycemic control. In this review, we summarize the current knowledge on the tight interconnection between the timekeeping system, glucose homeostasis, and insulin sensitivity. We focus specifically on the involvement of astrocyte clocks, at the organism, cellular, and molecular levels, in the regulation of glucose metabolism.

The Role of the Gut Microbiome, Immunity, and Neuroinflammation in the Pathophysiology of Eating Disorders

There is a growing recognition that both the gut microbiome and the immune system are involved in a number of psychiatric illnesses, including eating disorders. This should come as no surprise, given the important roles of diet composition, eating patterns, and daily caloric intake in modulating both biological systems. Here, we review the evidence that alterations in the gut microbiome and immune system may serve not only to maintain and exacerbate dysregulated eating behavior, characterized by caloric restriction in anorexia nervosa and binge eating in bulimia nervosa and binge eating disorder, but may also serve as biomarkers of increased risk for developing an eating disorder. We focus on studies examining gut dysbiosis, peripheral inflammation, and neuroinflammation in each of these eating disorders, and explore the available data from preclinical rodent models of anorexia and binge-like eating that may be useful in providing a better understanding of the biological mechanisms underlying eating disorders. Such knowledge is critical to developing novel, highly effective treatments for these often intractable and unremitting eating disorders.

Astrocytic Regulation of Neural Circuits Underlying Behaviors

Astrocytes, characterized by a satellite-like morphology, are the most abundant type of glia in the central nervous system. Their main functions have been thought to be limited to providing homeostatic support for neurons, but recent studies have revealed that astrocytes actually actively interact with local neural circuits and play a crucial role in information processing and generating physiological and behavioral responses. Here, we review the emerging roles of astrocytes in many brain regions, particularly by focusing on intracellular changes in astrocytes and their interactions with neurons at the molecular and neural circuit levels.

Long-term bisphenol A exposure exacerbates diet-induced prediabetes via TLR4-dependent hypothalamic inflammation

Bisphenol A (BPA), an environmental endocrine-disrupting compound, has been revealed associated with metabolic disorders such as obesity, prediabetes, and type 2 diabetes (T2D). However, its underlying mechanisms are still not fully understood. Here, we provide new evidence that BPA is a risk factor for T2D from a case-control study. To explore the detailed mechanisms, we used two types of diet models, standard diet (SD) and high-fat diet (HFD), to study the effects of long-term BPA exposure on prediabetes in 4-week-old mice. We found that BPA exposure for 12 weeks exacerbated HFD-induced prediabetic symptoms. Female mice showed increased body mass, serum insulin level, and impaired glucose tolerance, while male mice only exhibited impaired glucose tolerance. No change was found in SD-fed mice. Besides, BPA exposure enhanced astrocyte-dependent hypothalamic inflammation in both male and female mice, which impaired proopiomelanocortin (POMC) neuron functions. Moreover, eliminating inflammation by toll-like receptor 4 (TLR4) knockout significantly abolished the effects of BPA on the hypothalamus and diet-induced prediabetes. Taken together, our data establish a key role for TLR4-dependent hypothalamic inflammation in regulating the effects of BPA on prediabetes.

The infundibular peptidergic neurons and glia cells in overeating, obesity, and diabetes

Dysfunctional regulation of energy homeostasis results in increased bodyweight and obesity, eventually leading to type 2 diabetes mellitus. The infundibular nucleus (IFN) of the hypothalamus is the main regulator of energy homeostasis. The peptidergic neurons and glia cells of the IFN receive metabolic cues concerning energy state of the body from the circulation. The IFN can monitor hormones like insulin and leptin and nutrients like glucose and fatty acids. All these metabolic cues are integrated into an output signal regulating energy homeostasis through the release of neuropeptides. These neuropeptides are released in several inter- and extrahypothalamic brain regions involved in regulation of energy homeostasis. This review will give an overview of the peripheral signals involved in the regulation of energy homeostasis, the peptidergic neurons and glial cells of the IFN, and will highlight the main intra-hypothalamic projection sites of the IFN.

Molecular mechanisms and the vital roles of resistin, TLR 4, and NF-κB in treating type 2 diabetic complications

Background

Type 2 diabetes in obese (≥ 25 and ≥ 30 kg/m2) patients is the foremost cause of cardiovascular complications like stroke, osteoarthritis, cancers (endometrial, breast, ovarian, liver, kidney, colon, and prostate), and vascular complications like diabetic neuropathy, diabetic and retinopathy, and diabetic nephropathy. It is recognized as a global burden disorder with high prevalence in middle-income nations which might lead to a double burden on health care professionals. Hence, this review emphasizes on understanding the complexity and vital signaling tracts involved in diabetic complications for effective treatment.

Main body

Type 2 diabetes in overweight patients induces the creation of specific ROS that further leads to changes in cellular proliferation, hypothalamus, and fringe. The resistin, TLR4, and NF-κB signalings are mainly involved in the progression of central and fringe changes such as insulin resistance and inflammation in diabetic patients. The overexpression of these signals might lead to the rapid progression of diabetic vascular complications induced by the release of proinflammatory cytokines, chemokines, interleukins, and cyclooxygenase-mediated chemicals. Until now, there has been no curative treatment for diabetes. Therefore, to effectively treat complications of type 2 diabetes, the researchers need to concentrate on the molecular mechanisms and important signaling tracts involved.

Conclusion

In this review, we suggested the molecular mechanism of STZ-HFD induced type 2 diabetes and the vital roles of resistin, TLR4, and NF-κB signalings in central, fringe changes, and development diabetic complications for its effective treatment.

Graphical abstract

Immunometabolic Changes in Glia - A Potential Role in the Pathophysiology of Obesity and Diabetes

Chronic low-grade inflammation is a feature of the pathophysiology of obesity and diabetes in the CNS as well as peripheral tissues. Glial cells are critical mediators of the response to inflammation in the brain. Key features of glia include their metabolic flexibility, sensitivity to changes in the CNS microenvironment, and ability to rapidly adapt their function accordingly. They are specialised cells which cooperate to promote and preserve neuronal health, playing important roles in regulating the activity of neuronal networks across the brain during different life stages. Increasing evidence points to a role of glia, most notably astrocytes and microglia, in the systemic regulation of energy and glucose homeostasis in the course of normal physiological control and during disease. Inflammation is an energetically expensive process that requires adaptive changes in cellular metabolism and, in turn, metabolic intermediates can also have immunomodulatory actions. Such "immunometabolic" changes in peripheral immune cells have been implicated in contributing to disease pathology in obesity and diabetes. This review will discuss the evidence for a role of immunometabolic changes in glial cells in the systemic regulation of energy and glucose homeostasis, and how this changes in the context of obesity and diabetes.

Glial endozepines and energy balance: Old peptides with new tricks

The contribution of neuroglial interactions to the regulation of energy balance has gained increasing acceptance in recent years. In this context, endozepines, endogenous analogs of benzodiazepine derived from diazepam-binding inhibitor, are now emerging as major players. Produced by glial cells (astrocytes and tanycytes), endozepines have been known for two decades to exert potent anorexigenic effects by acting at the hypothalamic level. However, it is only recently that their modes of action, including the mechanisms by which they modulate energy metabolism, have begun to be elucidated. The data available today are abundant, significant, and sometimes contradictory, revealing a much more complex regulation than initially expected. Several mechanisms of action of endozepines seem to coexist at the central level, particularly in the hypothalamus. The brainstem has also recently emerged as a potential site of action for endozepines. In addition to their central anorexigenic effects, endozepines may also display peripheral effects promoting orexigenic actions, adding to their complexity and raising yet more questions. In this review, we attempt to provide an overview of our current knowledge in this rapidly evolving field and to pinpoint questions that remain unanswered.

Neuropeptide Y Is an Immunomodulatory Factor: Direct and Indirect

Neuropeptide Y (NPY), which is widely distributed in the nervous system, is involved in regulating a variety of biological processes, including food intake, energy metabolism, and emotional expression. However, emerging evidence points to NPY also as a critical transmitter between the nervous system and immune system, as well as a mediator produced and released by immune cells. In vivo and in vitro studies based on gene-editing techniques and specific NPY receptor agonists and antagonists have demonstrated that NPY is responsible for multifarious direct modulations on immune cells by acting on NPY receptors. Moreover, via the central or peripheral nervous system, NPY is closely connected to body temperature regulation, obesity development, glucose metabolism, and emotional expression, which are all immunomodulatory factors for the immune system. In this review, we focus on the direct role of NPY in immune cells and particularly discuss its indirect impact on the immune response.

Role of nutraceuticals during the transition period of dairy cows: a review

The transition period of dairy cattle is characterized by a number of metabolic, endocrine, physiologic, and immune adaptations, including the occurrence of negative energy balance, hypocalcemia, liver dysfunction, overt systemic inflammatory response, and oxidative stress status. The degree and length of time during which these systems remain out of balance could render cows more susceptible to disease, poor reproductive outcomes, and less efficient for milk production and quality. Studies on both monogastrics and ruminants have reported the health benefits of nutraceuticals (e.g. probiotics, prebiotics, dietary lipids, functional peptides, phytoextracts) beyond nutritional value, interacting at different levels of the animal’s physiology. From a physiological standpoint, it seems unrealistic to disregard any systemic inflammatory processes. However, an alternate approach is to modulate the inflammatory process per se and to resolve the systemic response as quickly as possible.

To this aim, a growing body of literature underscores the efficacy of nutraceuticals (active compounds) during the critical phase of the transition period. Supplementation of essential fatty acids throughout a 2-month period (i.e. a month before and a month after calving) successfully attenuates the inflammatory status with a quicker resolution of phenomenon. In this context, the inflammatory and immune response scenario has been recognized to be targeted by the beneficial effect of methyl donors, such as methionine and choline, directly and indirectly modulating such response with the increase of antioxidants GSH and taurine. Indirectly by the establishment of a healthy gastrointestinal tract, yeast and yeast-based products showed to modulate the immune response, mitigating negative effects associated with parturition stress and consequent disorders.

The use of phytoproducts has garnered high interest because of their wide range of actions on multiple tissue targets encompassing a series of antimicrobial, antiviral, antioxidant, immune-stimulating, rumen fermentation, and microbial modulation effects. In this review, we provide perspectives on investigations of regulating the immune responses and metabolism using several nutraceuticals in the periparturient cow.

Review: Pro-inflammatory cytokines and hypothalamic inflammation: implications for insufficient feed intake of transition dairy cows

Improvements in feed intake of dairy cows entering the early lactation period potentially decrease the risk of metabolic disorders, but before developing approaches targeting the intake level, mechanisms controlling and dysregulating energy balance and feed intake need to be understood. This review focuses on different inflammatory pathways interfering with the neuroendocrine system regulating feed intake of periparturient dairy cows. Subacute inflammation in various peripheral organs often occurs shortly before or after calving and is associated with increased pro-inflammatory cytokine levels. These cytokines are released into the circulation and sensed by neurons located in the hypothalamus, the key brain region regulating energy balance, to signal reduction in feed intake. Besides these peripheral humoral signals, glia cells in the brain may produce pro-inflammatory cytokines independent of peripheral inflammation. Preliminary results show intensive microglia activation in early lactation, suggesting their involvement in hypothalamic inflammation and the control of feed intake of dairy cows. On the other hand, pro-inflammatory cytokine-induced activation of the vagus nerve transmits signalling to the brain, but this pathway seems not exclusively necessary to signal feed intake reduction. Yet, less studied in dairy cows so far, the endocannabinoid system links inflammation and the hypothalamic control of feed intake. Distinct endocannabinoids exert anti-inflammatory action but also stimulate the posttranslational cleavage of neuronal proopiomelanocortin towards β-endorphin, an orexigen promoting feed intake. Plasma endocannabinoid concentrations and hypothalamic β-endorphin levels increase from late pregnancy to early lactation, but less is known about the regulation of the hypothalamic endocannabinoid system during the periparturient period of dairy cows. Dietary fatty acids may modulate the formation of endocannabinoids, which opens new avenues to improve metabolic health and immune status of dairy cows.

Microglial regulation of satiety and cognition

Microglia have been known for decades as key immune cells that shape the central nervous system (CNS) during development and respond to brain pathogens and injury in adult life. Recent findings now suggest that these cells also play a highly complex role in several other functions of the CNS. In this review, we provide a brief overview of the established microglial functions in development and disease. We also discuss emerging research suggesting that microglia are important for both cognitive function and the regulation of food intake. With respect to cognitive function, current data suggest microglia are not indispensable for neurogenesis, synaptogenesis or cognition in the healthy young adult, although they crucially modulate and support these functions. In doing so, they are likely important in supporting the balance between apoptosis and survival of newborn neurones and in orchestrating appropriate synaptic remodelling in response to a learning stimulus. We also explore the possibility of a role for microglia in feeding and satiety. Microglia have been implicated in both appetite suppression with sickness and obesity and in promoting feeding under some conditions and we discuss these findings here, highlighting the contribution of these cells to healthy brain function.

Toll-like Receptor 4 Signaling in Neurons Enhances Calcium-Permeable α-Amino-3-Hydroxy-5-Methyl-4-Isoxazolepropionic Acid Receptor Currents and Drives Post-Traumatic Epileptogenesis

OBJECTIVE

Traumatic brain injury is a major risk factor for acquired epilepsies, and understanding the mechanisms underlying the early pathophysiology could yield viable therapeutic targets. Growing evidence indicates a role for inflammatory signaling in modifying neuronal excitability and promoting epileptogenesis. Here we examined the effect of innate immune receptor Toll-like receptor 4 (TLR4) on excitability of the hippocampal dentate gyrus and epileptogenesis after brain injury.

METHODS

Slice and in vivo electrophysiology and Western blots were conducted in rats subject to fluid percussion brain injury or sham injury.

RESULTS

The studies identify that TLR4 signaling in neurons augments dentate granule cell calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (CP-AMPAR) currents after brain injury. Blocking TLR4 signaling in vivo shortly after brain injury reduced dentate network excitability and seizure susceptibility. When blocking of TLR4 signaling after injury was delayed, however, this treatment failed to reduce postinjury seizure susceptibility. Furthermore, TLR4 signal blocking was less efficacious in limiting seizure susceptibility when AMPAR currents, downstream targets of TLR4 signaling, were transiently enhanced. Paradoxically, blocking TLR4 signaling augmented both network excitability and seizure susceptibility in uninjured controls. Despite the differential effect on seizure susceptibility, TLR4 antagonism suppressed cellular inflammatory responses after injury without impacting sham controls.

INTERPRETATION

These findings demonstrate that independently of glia, the immune receptor TLR4 directly regulates post-traumatic neuronal excitability. Moreover, the TLR4-dependent early increase in dentate excitability is causally associated with epileptogenesis. Identification and selective targeting of the mechanisms underlying the aberrant TLR4-mediated increase in CP-AMPAR signaling after injury may prevent epileptogenesis after brain trauma. ANN NEUROL 2020;87:497-515.

Interleukin-1 reduces food intake and body weight in rat by acting in the arcuate hypothalamus

A reduction in food intake is commonly observed after bacterial infection, a phenomenon that can be reproduced by peripheral administration of Gram-negative bacterial lipopolysaccharide (LPS) or interleukin-1beta (IL-1β), a pro-inflammatory cytokine released by LPS-activated macrophages. The arcuate nucleus of the hypothalamus (ARH) plays a major role in food intake regulation and expresses IL-1 type 1 receptor (IL-1R1) mRNA. In the present work, we tested the hypothesis that IL-1R1 expressing cells in the ARH mediate IL-1β and/or LPS-induced hypophagia in the rat. To do so, we developed an IL-1β-saporin conjugate, which eliminated IL-R1-expressing neurons in the hippocampus, and micro-injected it into the ARH prior to systemic IL-1β and LPS administration. ARH IL-1β-saporin injection resulted in loss of neuropeptide Y-containing cells and attenuated hypophagia and weight loss after intraperitoneal IL-1β, but not LPS, administration. In conclusion, the present study shows that ARH NPY-containing neurons express functional IL-1R1s that mediate peripheral IL-1β-, but not LPS-, induced hypophagia. Our present and previous findings indicate that the reduction of food intake after IL-1β and LPS are mediated by different neural pathways.

Emerging roles for hypothalamic microglia as regulators of physiological homeostasis

The hypothalamus is a crucial brain region that responds to external stressors and functions to maintain physiological homeostatic processes, such as core body temperature and energy balance. The hypothalamus regulates homeostasis by producing hormones that thereby influence the production of other hormones that then control the internal milieu of the body. Microglia are resident macrophages and phagocytic immune cells of the central nervous system (CNS), classically known for surveying the brain's environment, responding to neural insults, and disposing of cellular debris. Recent evidence has shown that microglia are also responsive to external stressors and can influence both the development and function of the hypothalamus in a sex-dependent manner. This emerging microglia-hypothalamic interaction raises the intriguing notion that microglia might play an unappreciated role in hypothalamic control of physiological homeostasis. In this review, we briefly outline how the hypothalamus regulates physiological homeostasis and then describe how this literature overlaps with our understanding of microglia's role in the CNS. We also outline the current literature demonstrating how microglia loss or activation affects the hypothalamus, and ultimately homeostasis. We conclude by proposing how microglia could be key regulators of homeostatic processes by sensing cues external to the CNS and transmitting them through the hypothalamus.

The melanocortin pathway and control of appetite-progress and therapeutic implications

The initial discovery that ob/ob mice become obese because of a recessive mutation of the leptin gene has been crucial to discover the melanocortin pathway to control appetite. In the melanocortin pathway, the fed state is signaled by abundance of circulating hormones such as leptin and insulin, which bind to receptors expressed at the surface of pro-opiomelanocortin (POMC) neurons to promote processing of POMC to the mature hormone α-melanocyte-stimulating hormone (α-MSH). The α-MSH released by POMC neurons then signals to decrease energy intake by binding to melanocortin-4 receptor (MC4R) expressed by MC4R neurons to the paraventricular nucleus (PVN). Conversely, in the 'starved state' activity of agouti-related neuropeptide (AgRP) and of neuropeptide Y (NPY)-expressing neurons is increased by decreased levels of circulating leptin and insulin and by the orexigenic hormone ghrelin to promote food intake. This initial understanding of the melanocortin pathway has recently been implemented by the description of the complex neuronal circuit that controls the activity of POMC, AgRP/NPY and MC4R neurons and downstream signaling by these neurons. This review summarizes the progress done on the melanocortin pathway and describes how obesity alters this pathway to disrupt energy homeostasis. We also describe progress on how leptin and insulin receptors signal in POMC neurons, how MC4R signals and how altered expression and traffic of MC4R change the acute signaling and desensitization properties of the receptor. We also describe how the discovery of the melanocortin pathway has led to the use of melanocortin agonists to treat obesity derived from genetic disorders.

Conditional microglial depletion in rats leads to reversible anorexia and weight loss by disrupting gustatory circuitry

Microglia are highly sensitive to dietary influence, becoming activated acutely and long-term by high fat diet. However, their role in regulating satiety and feeding in healthy individuals remains unclear. Here we show that microglia are essential for the normal regulation of satiety and metabolism in rats. Short-term microglial depletion in a Cx3cr1-Dtr rat led to a dramatic weight loss that was largely accounted for by an acute reduction in food intake. This weight loss and anorexia were not likely due to a sickness response since the rats did not display peripheral or central inflammation, withdrawal, anxiety-like behavior, or nausea-associated pica. Hormonal and hypothalamic anatomical changes were largely compensatory to the suppressed food intake, which occurred in association with disruption of the gustatory circuitry at the paraventricular nucleus of the thalamus. Thus, microglia are important in supporting normal feeding behaviors and weight, and regulating preference for palatable food. Inhibiting this circuitry is able to over-ride strong compensatory drives to eat, providing a potential target for satiety control.

Molecular Mechanisms Underlying Obesity-Induced Hypothalamic Inflammation and Insulin Resistance: Pivotal Role of Resistin/TLR4 Pathways

Low-grade inflammation and insulin resistance are among the clinical features of obesity that are thought to promote the progressive onset of type 2 diabetes. However, the underlying mechanisms linking these disorders remain not fully understood. Recent reports pointed out hypothalamic inflammation as a major step in the onset of obesity-induced insulin resistance. In light of the increasing prevalence of obesity and T2D, two worldwide public health concerns, deciphering mechanisms implicated in hypothalamic inflammation constitutes a major challenge in the field of insulin-resistance/obesity. Several clinical and experimental studies have identified resistin as a key hormone linking insulin-resistance to obesity, notably through the activation of Toll Like Receptor (TLR) 4 signaling pathways. In this review, we present an overview of the molecular mechanisms underlying obesity-induced hypothalamic inflammation and insulin resistance with peculiar focus on the role of resistin/TLR4 signaling pathway.

Cell and molecular mechanisms behind diet-induced hypothalamic inflammation and obesity

Diet-induced obesity (DIO) is associated with chronic, low-grade inflammation in the hypothalamus, a key regulator of energy homeostasis. Current studies have revealed the involvement of different cell types, as well as cell and molecular mechanisms, that contribute to diet-induced hypothalamic inflammation (DIHI) and DIO. Subsequent to the discovery that high-fat diet and saturated fatty acids increase the expression of hypothalamic cytokines prior to weight gain, research has focused on understanding the cellular and molecular mechanisms underlying these changes, in addition to the role of inflammation in the pathogenesis of obesity. Recent studies have proposed that the inhibition of pro-inflammatory pathways in microglia and astrocytes is sufficient to protect against DIHI and prevent obesity. In addition, impairment of intracellular and epigenetic mechanisms, such as hypothalamic autophagy and changes in the methylation pattern of certain genes, have been implicated in susceptibility to DIHI and DIO. Interestingly, a sexual dimorphism has been found during DIO in hypothalamic inflammation, glial activation and metabolic diseases, and recent data support an important role of sex steroids in DIHI. These new exciting findings uncover novel obesity pathogenic mechanisms and provide targets to develop therapeutic approaches.

GYY4137 Promotes Mice Feeding Behavior via Arcuate Nucleus Sulfur-Sulfhydrylation and AMPK Activation

Hydrogen sulfide (H₂S) is an endogenous gaseous molecule and plays important biological and neurochemical roles in many processes such as the neural activity and immunity. The arcuate nucleus (ARC) of hypothalamus is a control center for appetite and energy metabolism. AMPK is a gage kinase in the monitoring of energy status and regulation of energy metabolism, and it can be activated by H₂S via CaMKKβ/AMPK pathway. But the role of H₂S in ARC and appetite has not been reported. Here we studied the orexigenic effect of H₂S and the mechanisms by means of GYY4137, a water soluble and slow-releasing donor of H₂S, and protein sulfur-sulfhydrylation analysis. We demonstrated that GYY4137-derived H₂S increased food intake of mice, augmented the production of neuropeptide Y (NPY), and elevated the protein sulfur-sulfhydrylation level and the activation of AMPK and CaMKKβ in ARC. Blocking sulfur-sulfhydrylation with DTT eliminated GYY4137-induced activation of AMPK and CaMKKβ. DTT and preventing AMPK activation in ARC with Compound C and Ara-A could both attenuate the orexigenic effect of GYY4137. These findings suggest that H₂S enhances appetite through protein sulfur-sulfhydrylation and the activation of AMPK and NPY function in ARC.

A high nutrient dense diet alters hypothalamic gene expressions to influence energy intake in pigs born with low birth weight

The low birth weight (LBW) individual had greater risk of developing metabolic dysfunction in adulthood. The aim of this study was to test whether the LBW individual is more prone to glucose intolerance on a high nutrient dense (HND) diet, and to investigate the associated hypothalamic gene expressions using pigs as model. The intake of digestible energy intake, if calculated on a body weight basis, was greater in LBW pigs than that of normal birth weight (NBW) pigs. The LBW pigs fed the HND diet had greater digestible energy intake than those fed the NND diet at adulthood, which did not occur for NBW pigs. Notably, up-regulated hypothalamic toll-like receptor 4, interleukin 6 and phospho-NFκB p65 expressions, and the altered expressions of hypothalamic leptin receptor, suppressor of cytokine signaling 3, agouti-related protein and proopiomelanocortin predicted the overconsumption of energy intake and development of glucose intolerance in LBW pigs fed the HND diet. Collectively, pigs born with LBW had a distinct hypothalamic leptin signaling to a high nutrient dense diet, which contributed to greater energy intake and glucose intolerance.

Knockdown of Tlr4 in the Arcuate Nucleus Improves Obesity Related Metabolic Disorders

High-fat diet-induced hypothalamic metabolic inflammation is emerging as a cause for the development of obesity. It is acknowledged that Toll-like receptor4 (TLR4) signaling plays a crucial role in triggering of the hypothalamic metabolic inflammation during the course of diet-induced obesity. Whether hypothalamic arcuate nucleus (ARC)-restricted TLR4 knockdown improves obesity-related metabolic disorders remains unexplored. In this study, we used TLR4 shRNA lentiviral particles to suppress the TLR4 expression in the hypothalamic ARC of diet-induced obese rat model by stereotaxic injection. Our results demonstrate that ARC-restricted TLR4 knockdown protects obese rats from diet-induced weight gain and energy intake, from diet-induced impaired glucose homeostasis and peripheral insulin resistance, and from high-fat diet-induced hepatic steatosis and adipocyte hypertrophy. Thus, we define ARC-restricted TLR4 knockdown as a potential strategy to combat metabolic disorders associated with obesity.

Toll-Like Receptor 2 Is a Regulator of Circadian Active and Inactive State Consolidation in C57BL/6 Mice

Regulatory systems required to maintain behavioral arousal remain incompletely understood. We describe a previously unappreciated role that toll-like receptor 2 (Tlr2, a membrane bound pattern recognition receptor that recognizes specific bacterial, viral, and fungal peptides), contributes toward regulation of behavioral arousal. In 4–4.5 month old mice with constitutive loss of Tlr2 function (Tlr2^−/− mice), we note a marked consolidation in the circadian pattern of both active and inactive states. Specifically, Tlr2^−/− mice demonstrated significantly fewer but longer duration active states during the circadian dark cycle, and significantly fewer but longer duration inactive states during the circadian light cycle. Tlr2^−/− mice also consumed less food and water, and moved less during the circadian light cycle. Analysis of circadian rhythms further suggested that Tlr2^−/− mice demonstrated less day-to-day variability in feeding, drinking, and movement behaviors. Reevaluation of this same mouse cohort at age 8–8.5 months revealed a clear blunting of these differences. However, Tlr2^−/− mice were still noted to have fewer short-duration active states during the circadian dark cycle, and continued to demonstrate significantly less day-to-day variability in feeding, drinking, and movement behaviors. These results suggest that Tlr2 function may have a role in promoting transitions between active and inactive states. Prior studies have demonstrated that Tlr2 regulates sickness behaviors including hypophagia, hyperthermia, and decreased activity. Our work suggests that Tlr2 function also evokes behavioral fragmentation, another aspect of sickness behavior and a clinically significant problem of older adults.

Decreased food anticipatory activity of obese mice relates to hypothalamic c-Fos expression

During daily Food Restriction (FR), obese Neotomodon alstoni mice present decreased Food Anticipatory Activity (FAA) compared to lean mice. Here, we investigated whether FOS expression in hypothalamic nuclei involved in food synchronization and anticipation parallels decreased FAA during daily FR of obese N. alstoni. Locomotor activity of lean and obese mice in ad libitum feeding conditions was monitored for at least two weeks. Then, a gradual restriction of food access was followed to establish a 5h period of daily food access. FR was maintained during at least two weeks before sacrifice of mice at the starting point of the feeding period. Obese mice subjected to FR displayed an overall reduction of FOS-positive (FOS+) hypothalamic neurons, while lean mice in a similar protocol exhibited an increase in FOS+ neurons within the arcuate and dorsomedial hypothalamic nuclei. These results are consistent with decreased FAA displayed by obese mice in comparison to lean mice. Furthermore, limbic system areas of lean mice, such as the cingulate cortex and the hippocampus, showed an increase in FOS during FR, while no responses were observed in obese mice. The daily food intake of obese mice was severely reduced during FR, compared to the ad libitum condition, whereas food intake in lean mice was not affected by FR. Current data suggests that decreased hypothalamic and limbic neuronal activation may contribute to the reduction of FAA in obese N. alstoni mice.

Microglia activation due to obesity programs metabolic failure leading to type two diabetes

Obesity is an energy metabolism disorder that increases susceptibility to the development of metabolic diseases. Recently, it has been described that obese subjects have a phenotype of chronic inflammation in organs that are metabolically relevant for glucose homeostasis and energy. Altered expression of immune system molecules such as interleukins IL-1, IL-6, IL-18, tumor necrosis factor alpha (TNF-α), serum amyloid A (SAA), and plasminogen activator inhibitor-1 (PAI-1), among others, has been associated with the development of chronic inflammation in obesity. Chronic inflammation modulates the development of metabolic-related comorbidities like metabolic syndrome (insulin resistance, glucose tolerance, hypertension and hyperlipidemia). Recent evidence suggests that microglia activation in the central nervous system (CNS) is a priority in the deregulation of energy homeostasis and promotes increased glucose levels. This review will cover the most significant advances that explore the molecular signals during microglia activation and inflammatory stage in the brain in the context of obesity, and its influence on the development of metabolic syndrome and type two diabetes.