Diet-Induced Obesity Elicits Macrophage Infiltration and Reduction in Spine Density in the Hypothalami of Male but Not Female Mice

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".

Food Cravings and Obesity in Women with Polycystic Ovary Syndrome: Pathophysiological and Therapeutic Considerations

Polycystic ovary syndrome (PCOS), the most common endocrine disorder in women of reproductive age, constitutes a metabolic disorder frequently associated with obesity and insulin resistance (IR). Furthermore, women with PCOS often suffer from excessive anxiety and depression, elicited by low self-esteem due to obesity, acne, and hirsutism. These mood disorders are commonly associated with food cravings and binge eating. Hypothalamic signaling regulates appetite and satiety, deteriorating excessive food consumption. However, the hypothalamic function is incapable of compensating for surplus food in women with PCOS, leading to the aggravation of obesity and a vicious circle. Hyperandrogenism, IR, the reduced secretion of cholecystokinin postprandially, and leptin resistance defined by leptin receptors' knockout in the hypothalamus have been implicated in the pathogenesis of hypothalamic dysfunction and appetite dysregulation. Diet modifications, exercise, and psychological and medical interventions have been applied to alleviate food disorders, interrupting the vicious circle. Cognitive-behavioral intervention seems to be the mainstay of treatment, while the role of medical agents, such as GLP-1 analogs and naltrexone/bupropion, has emerged.

Obesity-Related Hypogonadism in Women

Obesity-related hypogonadotropic hypogonadism is a well-characterized condition in men (termed male obesity-related secondary hypogonadism; MOSH); however, an equivalent condition has not been as clearly described in women. The prevalence of polycystic ovary syndrome (PCOS) is known to increase with obesity, but PCOS is more typically characterized by increased gonadotropin-releasing hormone (GnRH) (and by proxy luteinizing hormone; LH) pulsatility, rather than by the reduced gonadotropin levels observed in MOSH. Notably, LH levels and LH pulse amplitude are reduced with obesity, both in women with and without PCOS, suggesting that an obesity-related secondary hypogonadism may also exist in women akin to MOSH in men. Herein, we examine the evidence for the existence of a putative non-PCOS "female obesity-related secondary hypogonadism" (FOSH). We précis possible underlying mechanisms for the occurrence of hypogonadism in this context and consider how such mechanisms differ from MOSH in men, and from PCOS in women without obesity. In this review, we consider relevant etiological factors that are altered in obesity and that could impact on GnRH pulsatility to ascertain whether they could contribute to obesity-related secondary hypogonadism including: anti-Müllerian hormone, androgen, insulin, fatty acid, adiponectin, and leptin. More precise phenotyping of hypogonadism in women with obesity could provide further validation for non-PCOS FOSH and preface the ability to define/investigate such a condition.

Sex differences in systemic inflammation and immune function in diet-induced obesity rodent models: A systematic review

Understanding sex differences in immunological responses in the context of obesity is important to improve health outcomes. This systematic review aimed to investigate sex differences in systemic inflammation, immune cell phenotype, and function in diet-induced obesity (DIO) animal models. A systematic search in Medline, Embase, and CINAHL from inception to April 2023 was conducted, using a combination of the following concepts: sex, obesity, cytokines, and immune cell phenotypes/function. Forty-one publications reporting on systemic inflammation (61%), cell phenotype (44%), and/or function (7%) were included. Females had lower systemic inflammation compared with males in response to DIO intervention and a higher proportion of macrophage (M)2-like cells compared with males that had a higher proportion of M1-like in adipose tissue. Although there were no clear sex differences in immune function, high-fat DIO intervention remains an important factor in the development of immune dysfunction in both males and females, including disturbances in cytokine production, proliferation, and migration of immune cells. Yet, the mechanistic links between diet and obesity on such immune dysfunction remain unclear. Future studies should investigate the role of diet and obesity in the functionality of immune cells and employ adequate methods for a high-quality investigation of sex differences in this context.

Diet-Induced Obesity Increases Monocyte/Macrophage Proliferation during Skin Wound Healing in Mice

Obesity is associated with low-grade chronic inflammation and impaired glucose metabolism, both of which are detrimental to wound healing. C-C motif chemokine receptor 2 (CCR2) plays an important role in cell recruitment during healing, and our recent studies revealed the significance of CCR2-CCL2 signaling in promoting the proliferation of pro-inflammatory monocytes/macrophages in wounds. Therefore, we sought to determine whether diet-induced obesity increases monocyte/macrophage proliferation and their accumulation in skin wounds. We first confirmed that wound closure was delayed in obese CCR2RFP/+ mice fed with a high-fat diet (HFD) compared to mice fed with a normal diet (ND). Using in vivo imaging and flow cytometry analysis, we found that HFD mice had significantly increased accumulation of CCR2+ monocytes/macrophages, particularly pro-inflammatory CCR2+Ly6C+ cells in wounds compared to their ND counterparts. Importantly, HFD mice exhibited an increased proliferation of wound CCR2+Ly6C+ compared to ND mice. Together, our data suggest that obesity leads to an increased proliferation and accumulation of pro-inflammatory CCR2+Ly6C+ monocytes/macrophages in skin wounds, which may contribute to delayed healing.

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.

Single-Cell Transcriptomics Identifies Pituitary Gland Changes in Diet-Induced Obesity in Male Mice

Obesity is a chronic disease with increasing prevalence worldwide. Obesity leads to an increased risk of heart disease, stroke, and diabetes, as well as endocrine alterations, reproductive disorders, changes in basal metabolism, and stress hormone production, all of which are regulated by the pituitary. In this study, we performed single-cell RNA sequencing of pituitary glands from male mice fed control and high-fat diet (HFD) to determine obesity-mediated changes in pituitary cell populations and gene expression. We determined that HFD exposure is associated with dramatic changes in somatotrope and lactotrope populations, by increasing the proportion of somatotropes and decreasing the proportion of lactotropes. Fractions of other hormone-producing cell populations remained unaffected. Gene expression changes demonstrated that in HFD, somatotropes became more metabolically active, with increased expression of genes associated with cellular respiration, and downregulation of genes and pathways associated with cholesterol biosynthesis. Despite a lack of changes in gonadotrope fraction, genes important in the regulation of gonadotropin hormone production were significantly downregulated. Corticotropes and thyrotropes were the least affected in HFD, while melanotropes exhibited reduced proportion. Lastly, we determined that changes in plasticity and gene expression were associated with changes in hormone levels. Serum prolactin was decreased corresponding to reduced lactotrope fraction, while lower luteinizing hormone and follicle-stimulating hormone in the serum corresponded to a decrease in transcription and translation. Taken together, our study highlights diet-mediated changes in pituitary gland populations and gene expression that play a role in altered hormone levels in obesity.

Peripheral and central macrophages in obesity

Obesity is associated with chronic, low-grade inflammation. Excessive nutrient intake causes adipose tissue expansion, which may in turn cause cellular stress that triggers infiltration of pro-inflammatory immune cells from the circulation as well as activation of cells that are residing in the adipose tissue. In particular, the adipose tissue macrophages (ATMs) are important in the pathogenesis of obesity. A pro-inflammatory activation is also found in other organs which are important for energy metabolism, such as the liver, muscle and the pancreas, which may stimulate the development of obesity-related co-morbidities, including insulin resistance, type 2 diabetes (T2D), cardiovascular disease (CVD) and non-alcoholic fatty liver disease (NAFLD). Interestingly, it is now clear that obesity-induced pro-inflammatory signaling also occurs in the central nervous system (CNS), and that pro-inflammatory activation of immune cells in the brain may be involved in appetite dysregulation and metabolic disturbances in obesity. More recently, it has become evident that microglia, the resident macrophages of the CNS that drive neuroinflammation, may also be activated in obesity and can be relevant for regulation of hypothalamic feeding circuits. In this review, we focus on the action of peripheral and central macrophages and their potential roles in metabolic disease, and how macrophages interact with other immune cells to promote inflammation during obesity.

Prostaglandin E2 activates melanin-concentrating hormone neurons to drive diet-induced obesity

Hypothalamic inflammation reduces appetite and body weight during inflammatory diseases, while promoting weight gain when induced by high-fat diet (HFD). How hypothalamic inflammation can induce opposite energy balance outcomes remains unclear. We found that prostaglandin E2 (PGE2), a key hypothalamic inflammatory mediator of sickness, also mediates diet-induced obesity (DIO) by activating appetite-promoting melanin-concentrating hormone (MCH) neurons in the hypothalamus in rats and mice. The effect of PGE2 on MCH neurons is excitatory at low concentrations while inhibitory at high concentrations, indicating that these neurons can bidirectionally respond to varying levels of inflammation. During prolonged HFD, endogenous PGE2 depolarizes MCH neurons through an EP2 receptor-mediated inhibition of the electrogenic Na+/K+-ATPase. Disrupting this mechanism by genetic deletion of EP2 receptors on MCH neurons is protective against DIO and liver steatosis in male and female mice. Thus, an inflammatory mediator can directly stimulate appetite-promoting neurons to exacerbate DIO and fatty liver.

Sexual dimorphism in obesity is governed by RELMα regulation of adipose macrophages and eosinophils

Obesity incidence is increasing worldwide with the urgent need to identify new therapeutics. Sex differences in immune cell activation drive obesity-mediated pathologies where males are more susceptible to obesity comorbidities and exacerbated inflammation. Here, we demonstrate that the macrophage-secreted protein RELMα critically protects females against high-fat diet (HFD)-induced obesity. Compared to male mice, serum RELMα levels were higher in both control and HFD-fed females and correlated with frequency of adipose macrophages and eosinophils. RELMα-deficient females gained more weight and had proinflammatory macrophage accumulation and eosinophil loss in the adipose stromal vascular fraction (SVF), while RELMα treatment or eosinophil transfer rescued this phenotype. Single-cell RNA-sequencing of the adipose SVF was performed and identified sex and RELMα-dependent changes. Genes involved in oxygen sensing and iron homeostasis, including hemoglobin and lncRNA Gm47283/Gm21887, correlated with increased obesity, while eosinophil chemotaxis and response to amyloid-beta were protective. Monocyte-to-macrophage transition was also dysregulated in RELMα-deficient animals. Collectively, these studies implicate a RELMα-macrophage-eosinophil axis in sex-specific protection against obesity and uncover new therapeutic targets for obesity.

[18F]GE-180-PET and Post Mortem Marker Characteristics of Long-Term High-Fat-Diet-Induced Chronic Neuroinflammation in Mice

Obesity is characterized by immoderate fat accumulation leading to an elevated risk of neurodegenerative disorders, along with a host of metabolic disturbances. Chronic neuroinflammation is a main factor linking obesity and the propensity for neurodegenerative disorders. To determine the cerebrometabolic effects of diet-induced obesity (DIO) in female mice fed a long-term (24 weeks) high-fat diet (HFD, 60% fat) compared to a group on a control diet (CD, 20% fat), we used in vivo PET imaging with the radiotracer [¹⁸F]FDG as a marker for brain glucose metabolism. In addition, we determined the effects of DIO on cerebral neuroinflammation using translocator protein 18 kDa (TSPO)-sensitive PET imaging with [¹⁸F]GE-180. Finally, we performed complementary post mortem histological and biochemical analyses of TSPO and further microglial (Iba1, TMEM119) and astroglial (GFAP) markers as well as cerebral expression analyses of cytokines (e.g., Interleukin (IL)-1β). We showed the development of a peripheral DIO phenotype, characterized by increased body weight, visceral fat, free triglycerides and leptin in plasma, as well as increased fasted blood glucose levels. Furthermore, we found obesity-associated hypermetabolic changes in brain glucose metabolism in the HFD group. Our main findings with respect to neuroinflammation were that neither [¹⁸F]GE-180 PET nor histological analyses of brain samples seem fit to detect the predicted cerebral inflammation response, despite clear evidence of perturbed brain metabolism along with elevated IL-1β expression. These results could be interpreted as a metabolically activated state in brain-resident immune cells due to a long-term HFD.

Is adiposity associated with white matter microstructural health and intelligence differently in males and females?

OBJECTIVE

The role of vascular risk factors in age-related brain degeneration has long been the subject of intense study, but the role of obesity remains understudied. Given known sex differences in fat storage and usage, this study investigates sex differences in the association between adiposity and white matter microstructural integrity, an important early marker of brain degeneration.

METHODS

This study assesses the associations between adiposity (abdominal fat ratio and liver proton density fat fraction) and brain health (measures of intelligence and white matter microstructure using diffusion-tensor imaging [DTI]) in a group of UK Biobank participants.

RESULTS

This study finds that intelligence and DTI metrics are indeed associated with adiposity differently in males and females. These sex differences are distinct from those in the associations of DTI metrics with age and blood pressure.

CONCLUSIONS

Taken together, these findings suggest that there are inherent sex-driven differences in how brain health is associated with obesity.

Sensory neuron LKB1 mediates ovarian and reproductive function

Treatments for reproductive disorders in women primarily consist of hormone replacement therapy, which can have negative health impacts. Bidirectional communication between sensory neurons and innervated organs is an emerging area of interest in tissue physiology with potential relevance for reproductive disorders. Indeed, the metabolic activity of sensory neurons can have profound effects on reproductive phenotypes. To investigate this phenomenon, we utilized a murine model with conditional deletion in sensory neurons of liver kinase B1 (LKB1), a serine/threonine kinase that regulates cellular metabolism. Female mice with this LKB1 deletion (Nav1.8cre;LKB1fl/fl) had significantly more pups per litter compared to wild-type females. Interestingly, the LKB1 genotype of male breeders had no effect on fertility outcomes, thus indicating a female-specific role of sensory neuron metabolism in fertility. LKB1 deletion in sensory neurons resulted in reduced ovarian innervation from dorsal root ganglia neurons and increased follicular turnover compared to littermate controls. In summary, LKB1 expression in peripheral sensory neurons plays an important role in modulating fertility of female mice via ovarian sensory innervation.

Altered GnRH neuron and ovarian innervation characterize reproductive dysfunction linked to the Fragile X messenger ribonucleoprotein (Fmr1) gene mutation

Introduction

Mutations in the Fragile X Messenger Ribonucleoprotein 1 (FMR1) gene cause Fragile X Syndrome, the most common monogenic cause of intellectual disability. Mutations of FMR1 are also associated with reproductive disorders, such as early cessation of reproductive function in females. While progress has been made in understanding the mechanisms of mental impairment, the causes of reproductive disorders are not clear. FMR1-associated reproductive disorders were studied exclusively from the endocrine perspective, while the FMR1 role in neurons that control reproduction was not addressed.

Results

Here, we demonstrate that similar to women with FMR1 mutations, female Fmr1 null mice stop reproducing early. However, young null females display larger litters, more corpora lutea in the ovaries, increased inhibin, progesterone, testosterone, and gonadotropin hormones in the circulation. Ovariectomy reveals both hypothalamic and ovarian contribution to elevated gonadotropins. Altered mRNA and protein levels of several synaptic molecules in the hypothalamus are identified, indicating reasons for hypothalamic dysregulation. Increased vascularization of corpora lutea, higher sympathetic innervation of growing follicles in the ovaries of Fmr1 nulls, and higher numbers of synaptic GABAA receptors in GnRH neurons, which are excitatory for GnRH neurons, contribute to increased FSH and LH, respectively. Unmodified and ovariectomized Fmr1 nulls have increased LH pulse frequency, suggesting that Fmr1 nulls exhibit hyperactive GnRH neurons, regardless of the ovarian feedback.

Conclusion

These results reveal Fmr1 function in the regulation of GnRH neuron secretion, and point to the role of GnRH neurons, in addition to the ovarian innervation, in the etiology of Fmr1-mediated reproductive disorders.

Sex differences in hearing impairment due to diet-induced obesity in CBA/Ca mice

BACKGROUND

Obesity is an independent risk factor for hearing loss. Although attention has focused on major obesity comorbidities such as cardiovascular disease, stroke, and type 2 diabetes, the impact of obesity on sensorineural organs, including the auditory system, is unclear. Using a high-fat diet (HFD)-induced obese mouse model, we investigated the impact of diet-induced obesity on sexual dimorphism in metabolic alterations and hearing sensitivity.

METHODS

Male and female CBA/Ca mice were randomly assigned to three diet groups and fed, from weaning (at 28 days) to 14 weeks of age, a sucrose-matched control diet (10 kcal% fat content diet), or one of two HFDs (45 or 60 kcal% fat content diets). Auditory sensitivity was evaluated based on the auditory brainstem response (ABR), distortion product otoacoustic emission (DPOAE), and ABR wave 1 amplitude at 14 weeks of age, followed by biochemical analyses.

RESULTS

We found significant sexual dimorphism in HFD-induced metabolic alterations and obesity-related hearing loss. Male mice exhibited greater weight gain, hyperglycemia, increased ABR thresholds at low frequencies, elevated DPOAE, and lower ABR wave 1 amplitude compared to female mice. The hair cell (HC) ribbon synapse (CtBP2) puncta showed significant sex differences. The serum concentration of adiponectin, an otoprotective adipokine, was significantly higher in female than in male mice; cochlear adiponectin levels were elevated by HFD in female but not male mice. Adiponectin receptor 1 (AdipoR1) was widely expressed in the inner ear, and cochlear AdipoR1 protein levels were increased by HFD, in female but not male mice. Stress granules (G3BP1) were significantly induced by the HFD in both sexes; conversely, inflammatory (IL-1β) responses were observed only in the male liver and cochlea, consistent with phenotype HFD-induced obesity.

CONCLUSIONS

Female mice are more resistant to the negative effects of an HFD on body weight, metabolism, and hearing. Females showed increased peripheral and intra-cochlear adiponectin and AdipoR1 levels, and HC ribbon synapses. These changes may mediate resistance to HFD-induced hearing loss seen in female mice.

Sexual dimorphism in obesity is governed by RELMα regulation of adipose macrophages and eosinophils

Obesity incidence is increasing worldwide with the urgent need to identify new therapeutics. Sex differences in immune cell activation drive obesity-mediated pathologies where males are more susceptible to obesity co-morbidities and exacerbated inflammation. Here, we demonstrate that the macrophage-secreted protein RELMα critically protects females against high fat diet-induced obesity. Compared to male mice, RELMα levels were elevated in both control and high fat dietfed females and correlated with adipose macrophages and eosinophils. RELMα-deficient females gained more weight and had pro-inflammatory macrophage accumulation and eosinophil loss, while both RELMα treatment and eosinophil transfer rescued this phenotype. Single cell RNA-sequencing of the adipose stromal vascular fraction was performed and identified sex and RELMα-dependent changes. Genes involved in oxygen sensing and iron homeostasis, including hemoglobin and lncRNA Gm47283, correlated with increased obesity, while eosinophil chemotaxis and response to amyloid-beta were protective. Monocyte-to-macrophage transition was also dysregulated in RELMα-deficient animals. Collectively, these studies implicate a RELMα-macrophage-eosinophil axis in sex-specific protection against obesity and uncover new therapeutic targets for obesity.

Sodium-glucose cotransporter 2 inhibitor ameliorates high fat diet-induced hypothalamic-pituitary-ovarian axis disorders

High-fat diet (HFD) consumption is known to be associated with ovulatory disorders among women of reproductive age. Previous studies in animal models suggest that HFD-induced microglia activation contributes to hypothalamic inflammation. This causes the dysfunction of the hypothalamic-pituitary-ovarian (HPO) axis, leading to subfertility. Sodium-glucose cotransporter 2 (SGLT2) inhibitors are a novel class of lipid-soluble antidiabetic drugs that target primarily the early proximal tubules in kidney. Recent evidence revealed an additional expression site of SGLT2 in the central nervous system (CNS), indicating a promising role of SGLT2 inhibitors in the CNS. In type 2 diabetes patients and rodent models, SGLT2 inhibitors exhibit neuroprotective properties through reduction of oxidative stress, alleviation of cerebral atherosclerosis and suppression of microglia-induced neuroinflammation. Furthermore, clinical observations in patients with polycystic ovary syndrome (PCOS) demonstrated that SGLT2 inhibitors ameliorated patient anthropometric parameters, body composition and insulin resistance. Therefore, it is of importance to explore the central mechanism of SGLT2 inhibitors in the recovery of reproductive function in patients with PCOS and obesity. Here, we review the hypothalamic inflammatory mechanisms of HFD-induced microglial activation, with a focus on the clinical utility and possible mechanism of SGLT2 inhibitors in promoting reproductive fitness.

Identification of hub genes and candidate herbal treatment in obesity through integrated bioinformatic analysis and reverse network pharmacology

Obesity is a global epidemic elevating the risk of various metabolic disorders. As there is a lack of effective drugs to treat obesity, we combined bioinformatics and reverse network pharmacology in this study to identify effective herbs to treat obesity. We identified 1011 differentially expressed genes (DEGs) of adipose tissue after weight loss by analyzing five expression profiles (GSE103766, GSE35411, GSE112307, GSE43471, and GSE35710) from the Gene Expression Omnibus (GEO) database. We identified 27 hub genes from the protein-protein interaction (PPI) network by performing MCODE using the Search Tool for the Retrieval of Interacting Genes (STRING) database. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses revealed that these hub genes have roles in the extracellular matrix-receptor interaction, cholesterol metabolism, PI3K-Akt signaling pathway, etc. Ten herbs (Aloe, Portulacae Herba, Mori Follum, Silybum Marianum, Phyllanthi Fructus, Pollen Typhae, Ginkgo Semen, Leonuri Herba, Eriobotryae Folium, and Litseae Fructus) targeting the nine hub genes (COL1A1, MMP2, MMP9, SPP1, DNMT3B, MMP7, CETP, COL1A2, and MUC1) using six ingredients were identified as the key herbs. Quercetin and (-)-epigallocatechin-3-gallate were determined to be the key ingredients. Lastly, Ingredients-Targets, Herbs-Ingredients-Targets, and Herbs-Taste-Meridian Tropism networks were constructed using Cytoscape to elucidate this complex relationship. This study could help identify promising therapeutic targets and drugs to treat obesity.

Glial-neuron crosstalk in health and disease: A focus on metabolism, obesity, and cognitive impairment

Dementia is a complex set of disorders affecting normal cognitive function. Recently, several clinical studies have shown that diabetes, obesity, and components of the metabolic syndrome (MetS) are associated with cognitive impairment, including dementias such as Alzheimer's disease. Maintaining normal cognitive function is an intricate process involving coordination of neuron function with multiple brain glia. Well-orchestrated bioenergetics is a central requirement of neurons, which need large amounts of energy but lack significant energy storage capacity. Thus, one of the most important glial functions is to provide metabolic support and ensure an adequate energy supply for neurons. Obesity and metabolic disease dysregulate glial function, leading to a failure to respond to neuron energy demands, which results in neuronal damage. In this review, we outline evidence for links between diabetes, obesity, and MetS components to cognitive impairment. Next, we focus on the metabolic crosstalk between the three major glial cell types, oligodendrocytes, astrocytes, and microglia, with neurons under physiological conditions. Finally, we outline how diabetes, obesity, and MetS components can disrupt glial function, and how this disruption might impair glia-neuron metabolic crosstalk and ultimately promote cognitive impairment.

Overnutrition Induced Cognitive Impairment: Insulin Resistance, Gut-Brain Axis, and Neuroinflammation

Overnutrition-related obesity has become a worldwide epidemic, and its prevalence is expected to steadily rise in the future. It is widely recognized that obesity exerts negative impacts on metabolic disorders such as type 2 diabetes mellitus (T2DM) and cardiovascular diseases. However, relatively fewer reports exist on the impairment of brain structure and function, in the form of memory and executive dysfunction, as well as neurogenerative diseases. Emerging evidence indicates that besides obesity, overnutrition diets independently induce cognitive impairments via multiple mechanisms. In this study, we reviewed the clinical and preclinical literature about the detrimental effects of obesity or high-nutrition diets on cognitive performance and cerebral structure. We mainly focused on the role of brain insulin resistance (IR), microbiota-gut-brain axis, and neuroinflammation. We concluded that before the onset of obesity, short-term exposure to high-nutrition diets already blunted central responses to insulin, altered gut microbiome composition, and activated inflammatory mediators. Overnutrition is linked with the changes in protein expression in brain insulin signaling, leading to pathological features in the brain. Microbiome alteration, bacterial endotoxin release, and gut barrier hyperpermeability also occur to trigger mental and neuronal diseases. In addition, obesity or high-nutrition diets cause chronic and low-grade systematic inflammation, which eventually spreads from the peripheral tissue to the central nervous system (CNS). Altogether, a large number of unknown but potential routes interact and contribute to obesity or diet-induced cognitive impairment. The challenge for future research is to identify effective interventions involving dietary shifts and personalized therapy targeting the underlying mechanisms to prevent and improve cognition deficits.

Enriched Marine Oil Supplement Increases Specific Plasma Specialized Pro-Resolving Mediators in Adults with Obesity

BACKGROUND

Specialized pro-resolving mediators (SPMs), synthesized from PUFAs, resolve inflammation and return damaged tissue to homeostasis. Thus, increasing metabolites of the SPM biosynthetic pathway may have potential health benefits for select clinical populations, such as subjects with obesity who display dysregulation of SPM metabolism. However, the concentrations of SPMs and their metabolic intermediates in humans with obesity remains unclear.

OBJECTIVES

The primary objective of this study was to determine if a marine oil supplement increased specific metabolites of the SPM biosynthetic pathway in adults with obesity. The second objective was to determine if the supplement changed the relative abundance of key immune cell populations. Finally, given the critical role of antibodies in inflammation, we determined if ex vivo CD19 + B-cell antibody production was modified by marine oil intervention.

METHODS

Twenty-three subjects [median age: 56 y; BMI (in kg/m2): 33.1] consumed 2 g/d of a marine oil supplement for 28-30 d. The supplement was particularly enriched with 18-hydroxyeicosapentaenoic (HEPE), 14-hydroxydocosahexaenoic acid (14-HDHA), and 17-HDHA. Blood was collected pre- and postsupplementation for plasma mass spectrometry oxylipin and fatty acid analyses, flow cytometry, and B-cell isolation. Paired t-tests and Wilcoxon tests were used for statistical analyses.

RESULTS

Relative to preintervention, the supplement increased 6 different HEPEs and HDHAs accompanied by changes in plasma PUFAs. Resolvin E1 and docosapentaenoic acid-derived maresin 1 concentrations were increased 3.5- and 4.7-fold upon intervention, respectively. The supplement did not increase the concentration of D-series resolvins and had no effect on the abundance of immune cells. Ex vivo B-cell IgG but not IgM concentrations were lowered postsupplementation.

CONCLUSIONS

A marine oil supplement increased select SPMs and their metabolic intermediates in adults with obesity. Additional studies are needed to determine if increased concentrations of specific SPMs control the resolution of inflammation in humans with obesity. This trial was registered at clinicaltrials.gov as NCT04701138.

Near-infrared light reduces glia activation and modulates neuroinflammation in the brains of diet-induced obese mice

Neuroinflammation is a key event in neurodegenerative conditions such as Alzheimer's disease (AD) and characterizes metabolic pathologies like obesity and type 2 diabetes (T2D). Growing evidence in humans shows that obesity increases the risk of developing AD by threefold. Hippocampal neuroinflammation in rodents correlates with poor memory performance, suggesting that it contributes to cognitive decline. Here we propose that reducing obesity/T2D-driven neuroinflammation may prevent the progression of cognitive decline associated with AD-like neurodegenerative states. Near-infrared light (NIR) has attracted increasing attention as it was shown to improve learning and memory in both humans and animal models. We previously reported that transcranial NIR delivery reduced amyloid beta and Tau pathology and improved memory function in mouse models of AD. Here, we report the effects of NIR in preventing obesity-induced neuroinflammation in a diet-induced obese mouse model. Five-week-old wild-type mice were fed a high-fat diet (HFD) for 13 weeks to induce obesity prior to transcranial delivery of NIR for 4 weeks during 90-s sessions given 5 days a week. After sacrifice, brain slices were subjected to free-floating immunofluorescence for microglia and astrocyte markers to evaluate glial activation and quantitative real-time polymerase chain reaction (PCR) to evaluate expression levels of inflammatory cytokines and brain-derived neurotrophic factor (BDNF). The hippocampal and cortical regions of the HFD group had increased expression of the activated microglial marker CD68 and the astrocytic marker glial fibrillary acidic protein. NIR-treated HFD groups showed decreased levels of these markers. PCR revealed that hippocampal tissue from the HFD group had increased levels of pro-inflammatory interleukin (IL)-1β and tumor necrosis factor-α. Interestingly, the same samples showed increased levels of the anti-inflammatory IL-10. All these changes were attenuated by NIR treatment. Lastly, hippocampal levels of the neurotrophic factor BDNF were increased in NIR-treated HFD mice, compared to untreated HFD mice. The marked reductions in glial activation and pro-inflammatory cytokines along with elevated BDNF provide insights into how NIR could reduce neuroinflammation. These results support the use of NIR as a potential non-invasive and preventive therapeutic approach against chronic obesity-induced deficits that are known to occur with AD neuropathology.

Sex differences in the fecal microbiome and hippocampal glial morphology following diet and antibiotic treatment

Rising obesity rates have become a major public health concern within the United States. Understanding the systemic and neural effects of obesity is crucial in designing preventive and therapeutic measures. In previous studies, administration of a high fat diet has induced significant weight gain for mouse models of obesity. Interestingly, sex differences in high-fat diet-induced weight gain have been observed, with female mice gaining significantly less weight compared to male mice on the same high-fat diet. It has also been observed that consumption of a high-fat diet can increase neurogliosis, but the mechanism by which this occurs is still not fully understood. Recent research has suggested that the gut microbiome may mediate diet-induced glial activation. The current study aimed to (1) analyze changes to the gut microbiome following consumption of a high fat (HF) diet as well as antibiotic treatment, (2) evaluate hippocampal microgliosis and astrogliosis, and (3) identify sex differences within these responses. We administered a low fat (Research Diets D12450 K) or high fat diet (Research Diets D12451) to male and female C57Bl/6 mice for sixteen weeks. Mice received an antibiotic cocktail containing 0.5g/L of vancomycin, 1.0 g/L ampicillin, 1.0 g/L neomycin, and 1.0 g/L metronidazole in their drinking water during the last six weeks of the study and were compared to control mice receiving normal drinking water throughout the study. We observed a significant reduction in gut microbiome diversity for groups that received the antibiotic cocktail, as determined by Illumina next-generation sequencing. Male mice fed the HF diet (± antibiotics) had significantly greater body weights compared to all other groups. And, female mice fed the low fat (LF) diet and administered antibiotics revealed significantly decreased microgliosis and astrogliosis in the hippocampus compared to LF-fed females without antibiotics. Interestingly, male mice fed the LF diet and administered antibiotics revealed significantly increased microgliosis, but decreased astrogliosis, compared to LF-fed males without antibiotics. The observed sex differences in LF-fed mice given antibiotics brings forward questions about sex differences in nutrient metabolism, gut microbiome composition, and response to antibiotics.

Palmitic Acid and Oleic Acid Differently Modulate TLR2-Mediated Inflammatory Responses in Microglia and Macrophages

The relationship between systemic immunity and neuroinflammation is widely recognised. Infiltration of peripheral immune cells to the CNS during certain chronic inflammatory states contributes significantly to neuropathology. Obesity and its co-morbidities are primary risk factors for neuroinflammatory and neurodegenerative conditions, including Alzheimer’s disease (AD). Dietary fats are among the most proinflammatory components of the obesogenic diet and play a prominent role in the low-grade systemic inflammation associated with the obese state. Saturated fatty acid (SFA) is largely implicated in the negative consequences of obesity, while the health benefits of monounsaturated fatty acid (MUFA) are widely acknowledged. The current study sought to explore whether SFA and MUFA differently modulate inflammatory responses in the brain, compared with peripheral immune cells. Moreover, we assessed the neuroinflammatory impact of high-fat-induced obesity and hypothesised that a MUFA-rich diet might mitigate inflammation despite obesogenic conditions. Toll-like receptor (TLR)2 mediates the inflammation associated with both obesity and AD. Using the TLR2 agonist lipoteichoic acid (LTA), we report that pre-exposure to either palmitic acid (PA) or oleic acid (OA) attenuated cytokine secretion from microglia, but heightened sensitivity to nitric oxide (NO) production. The reduction in cytokine secretion was mirrored in LTA-stimulated macrophages following exposure to PA only, while effects on NO were restricted to OA, highlighting important cell-specific differences. An obesogenic diet over 12 weeks did not induce prominent inflammatory changes in either cortex or hippocampus, irrespective of fat composition. However, we reveal a clear disparity in the effects of MUFA under obesogenic and non-obesogenic conditions.

Disruption of Pituitary Gonadotrope Activity in Male Rats After Short- or Long-Term High-Fat Diets Is Not Associated With Pituitary Inflammation

Overnutrition is associated with the activation of inflammatory pathways in metabolically linked organs and an early hypothalamic inflammation is now known to disrupt the central control of metabolic function. Because we demonstrated that fatty acids (FA) target the pituitary and affect gonadotropin synthesis, we asked whether overnutrition induces pituitary inflammation that may contribute to obesity-associated disorders in the control of reproduction. We analyzed pituitary inflammation and hypothalamic-pituitary-testicular axis in male rats fed a short- (4 weeks) or long-term (20 weeks) high-fat diet. The effect of diet enrichment with the ω3 polyunsaturated FA, DHA, was also analyzed. After only 4 weeks and before weight gain of rats, high-fat diet caused a significant decrease in pituitary gonadotropin and hypothalamic GnRH transcript levels despite unchanged testosterone and inhibin B levels. Contrasting with the hypothalamus, there was no concomitant increases in gene expression of pituitary inflammatory mediators and even a reduction of prototypical cytokines such as interleukin-1β and TNF-α. No inflammation was still detected in the pituitary after 20 weeks although gonadotropin transcripts and circulating levels were still altered. Gonadotropins were the only pituitary hormones remaining affected at this stage of the regimen, underlying a differential susceptibility of pituitary lineages to metabolic disorders. DHA enrichment of the diet did not prevent alterations of gonadotrope activity due to either a long- or a short-term high-fat diet although it blocked early hypothalamic inflammation and attenuated several metabolic effects. Taken together, our findings suggest that high-fat diet-induced defects in gonadotrope activity in male rats occurred despite a lack of pituitary inflammation.

Gut Microbiome, Inflammation, and Cerebrovascular Function: Link Between Obesity and Cognition

Obesity affects 13% of the adult population worldwide and this number is only expected to increase. Obesity is known to have a negative impact on cardiovascular and metabolic health, but it also impacts brain structure and function; it is associated with both gray and white matter integrity loss, as well as decreased cognitive function, including the domains of executive function, memory, inhibition, and language. Especially midlife obesity is associated with both cognitive impairment and an increased risk of developing dementia at later age. However, underlying mechanisms are not yet fully revealed. Here, we review recent literature (published between 2010 and March 2021) and discuss the effects of obesity on brain structure and cognition, with a main focus on the contributions of the gut microbiome, white adipose tissue (WAT), inflammation, and cerebrovascular function. Obesity-associated changes in gut microbiota composition may cause increased gut permeability and inflammation, therewith affecting cognitive function. Moreover, excess of WAT in obesity produces pro-inflammatory adipokines, leading to a low grade systemic peripheral inflammation, which is associated with decreased cognition. The blood-brain barrier also shows increased permeability, allowing among others, peripheral pro-inflammatory markers to access the brain, leading to neuroinflammation, especially in the hypothalamus, hippocampus and amygdala. Altogether, the interaction between the gut microbiota, WAT inflammation, and cerebrovascular integrity plays a significant role in the link between obesity and cognition. Future research should focus more on the interplay between gut microbiota, WAT, inflammation and cerebrovascular function to obtain a better understanding about the complex link between obesity and cognitive function in order to develop preventatives and personalized treatments.

Hypothalamic inflammation in metabolic disorders and aging

The hypothalamus is a critical brain region for the regulation of energy homeostasis. Over the years, studies on energy metabolism primarily focused on the neuronal component of the hypothalamus. Studies have recently uncovered the vital role of glial cells as an additional player in energy balance regulation. However, their inflammatory activation under metabolic stress condition contributes to various metabolic diseases. The recruitment of monocytes and macrophages in the hypothalamus helps sustain such inflammation and worsens the disease state. Neurons were found to actively participate in hypothalamic inflammatory response by transmitting signals to the surrounding non-neuronal cells. This activation of different cell types in the hypothalamus leads to chronic, low-grade inflammation, impairing energy balance and contributing to defective feeding habits, thermogenesis, and insulin and leptin signaling, eventually leading to metabolic disorders (i.e., diabetes, obesity, and hypertension). The hypothalamus is also responsible for the causation of systemic aging under metabolic stress. A better understanding of the multiple factors contributing to hypothalamic inflammation, the role of the different hypothalamic cells, and their crosstalks may help identify new therapeutic targets. In this review, we focus on the role of glial cells in establishing a cause-effect relationship between hypothalamic inflammation and the development of metabolic diseases. We also cover the role of other cell types and discuss the possibilities and challenges of targeting hypothalamic inflammation as a valid therapeutic approach.

Metabolic syndrome and male fertility disorders: Is there a causal link?

Infertility is a global health problem affecting 10-15% of couples in reproductive age. Recent studies have provided growing evidence supporting that lifestyle factors can affect male fertility through alterations in endocrine profiles, spermatogenesis and/or sperm function. One of these critical factors could be the change in the food intake behavior in modern societies that produces metabolic alterations. Regarding this, metabolic syndrome (MetS) prevalence has increased in epidemic in the last 40-50 years. Although MetS is associated with advanced age, changes in lifestyles have accelerated the appearance of symptoms in the reproductive age. We review herein the current understanding of the relationship between MetS and the male reproductive status. For this purpose, in this narrative review a comprehensive literature search was made in both animal models and men, allowing us to evaluate such relationship. This analysis showed a high variability in the reproductive phenotypes observed in patients and mice suffering MetS, including sperm parameters, fertility and offspring health. In view of this, we proposed that the reproductive effects, which are diverse and not robust, observed among MetS-affected males, might depend on additional factors not associated with the metabolic condition and contributed not only by the affected male but also by his partner. With this perspective, this review provides a more accurate insight of this syndrome critical for the identification of specific diagnostic indicators and treatment of MetS-induced fertility disorders.

Mechanisms Driving Palmitate-Mediated Neuronal Dysregulation in the Hypothalamus

The hypothalamus maintains whole-body homeostasis by integrating information from circulating hormones, nutrients and signaling molecules. Distinct neuronal subpopulations that express and secrete unique neuropeptides execute the individual functions of the hypothalamus, including, but not limited to, the regulation of energy homeostasis, reproduction and circadian rhythms. Alterations at the hypothalamic level can lead to a myriad of diseases, such as type 2 diabetes mellitus, obesity, and infertility. The excessive consumption of saturated fatty acids can induce neuroinflammation, endoplasmic reticulum stress, and resistance to peripheral signals, ultimately leading to hyperphagia, obesity, impaired reproductive function and disturbed circadian rhythms. This review focuses on the how the changes in the underlying molecular mechanisms caused by palmitate exposure, the most commonly consumed saturated fatty acid, and the potential involvement of microRNAs, a class of non-coding RNA molecules that regulate gene expression post-transcriptionally, can result in detrimental alterations in protein expression and content. Studying the involvement of microRNAs in hypothalamic function holds immense potential, as these molecular markers are quickly proving to be valuable tools in the diagnosis and treatment of metabolic disease.

Spexin: Its role, regulation, and therapeutic potential in the hypothalamus

Spexin is the most recently discovered member of the galanin/kisspeptin/spexin family of peptides. This 14-amino acid peptide is highly conserved and is implicated in homeostatic functions including, but not limited to, metabolism, energy homeostasis, and reproduction. Spexin is expressed by neurons in the hypothalamus, which coordinate energy homeostasis and reproduction. Critically, levels of spexin appear to be altered in disorders related to energy homeostasis and reproduction, such as obesity, diabetes, and polycystic ovarian syndrome. In this review, we discuss the evidence for the involvement of spexin in the hypothalamic control of energy homeostasis and reproduction. The anorexigenic properties of spexin have been attributed to its effects on the energy-regulating neuropeptide Y/agouti-related peptide neurons and proopiomelanocortin neurons. While the role of spexin in reproduction remains unclear, there is evidence that gonadotropin-releasing hormone expressing neurons may produce and respond to spexin. Furthermore, we discuss the disorders and concomitant treatments, which have been reported to alter spexin expression, as well as the underlying signaling mechanisms that may be involved. Finally, we discuss the biochemical basis of spexin, its interaction with its cognate receptors, and how this information can be adapted to develop therapeutics for disorders related to the alteration of energy homeostasis and reproduction.

The role of Western diets and obesity in peripheral immune cell recruitment and inflammation in the central nervous system

As the prevalence of obesity and chronic disease increases, the role of nutrition is taking center stage as a potential root cause of not just metabolic-related illnesses, but also of disorders of the central nervous system (CNS). Consumption of a modern, westernized diet, such as a high fat diet (HFD) that contains excess saturated fatty acids (SFAs), refined carbohydrates, and ultra-processed ingredients has been shown to induce neuroinflammation in multiple brain regions important for energy homeostasis, cognitive function, and mood regulation in rodents, non-human primates, and humans. This review article summarizes the literature showing Western diets, via SFA increases, can increase the reactivity and alter the function of multiple types of immune cells from both the innate and adaptive branches of the immune system, with a specific focus on microglia, macrophages, dendritic cells, and T-cells. These changes in immune and neuroimmune signaling have important implications for neuroinflammation and brain health and will be an important factor in future psychoneuroimmunology research.

Maternal Metformin Treatment during Gestation and Lactation Improves Skeletal Muscle Development in Offspring of Rat Dams Fed High-Fat Diet

Maternal high-fat (HF) diet is associated with offspring metabolic disorder. This study intended to determine whether maternal metformin (MT) administration during gestation and lactation prevents the effect of maternal HF diet on offspring’s skeletal muscle (SM) development and metabolism. Pregnant Sprague-Dawley rats were divided into four groups according to maternal diet {CHOW (11.8% fat) or HF (60% fat)} and MT administration {control (CT) or MT (300 mg/kg/day)} during gestation and lactation: CH-CT, CH-MT, HF-CT, HF-MT. All offspring were weaned on CHOW diet. SM was collected at weaning and 18 weeks in offspring. Maternal metformin reduced plasma insulin, leptin, triglyceride and cholesterol levels in male and female offspring. Maternal metformin increased MyoD expression but decreased Ppargc1a, Drp1 and Mfn2 expression in SM of adult male and female offspring. Decreased MRF4 expression in SM, muscle dysfunction and mitochondrial vacuolization were observed in weaned HF-CT males, while maternal metformin normalized them. Maternal metformin increased AMPK phosphorylation and decreased 4E-BP1 phosphorylation in SM of male and female offspring. Our data demonstrate that maternal metformin during gestation and lactation can potentially overcome the negative effects of perinatal exposure to HF diet in offspring, by altering their myogenesis, mitochondrial biogenesis and dynamics through AMPK/mTOR pathways in SM.

Effect of Advanced Glycation End-Products and Excessive Calorie Intake on Diet-Induced Chronic Low-Grade Inflammation Biomarkers in Murine Models

Chronic Low-Grade Inflammation (CLGI) is a non-overt inflammatory state characterized by a continuous activation of inflammation mediators associated with metabolic diseases. It has been linked to the overconsumption of Advanced Glycation End-Products (AGEs), and/or macronutrients which lead to an increase in local and systemic pro-inflammatory biomarkers in humans and animal models. This review provides a summary of research into biomarkers of diet-induced CLGI in murine models, with a focus on AGEs and obesogenic diets, and presents the physiological effects described in the literature. Diet-induced CLGI is associated with metabolic endotoxemia, and/or gut microbiota remodeling in rodents. The mechanisms identified so far are centered on pro-inflammatory axes such as the interaction between AGEs and their main receptor AGEs (RAGE) or increased levels of lipopolysaccharide. The use of murine models has helped to elucidate the local and systemic expression of CLGI mediators. These models have enabled significant advances in identification of diet-induced CLGI biomarkers and resultant physiological effects. Some limitations on the translational (murine → humans) use of biomarkers may arise, but murine models have greatly facilitated the testing of specific dietary components. However, there remains a lack of information at the whole-organism level of organization, as well as a lack of consensus on the best biomarker for use in CLGI studies and recommendations as to future research conclude this review.

Effect of diet-induced obesity on kisspeptin-neurokinin B-dynorphin A neurons in the arcuate nucleus and luteinizing hormone secretion in sex hormone-primed male and female rats

Metabolic stress resulting from either lack or excess of nutrients often causes infertility in both sexes. Kisspeptin-neurokinin B-dynorphin A (KNDy) neurons in the arcuate nucleus (ARC) has been suggested to be a key players in reproduction via direct stimulation of the pulsatile gonadotropin-releasing hormone (GnRH) and subsequent gonadotropin release in mammalian species. In this study, we investigated the effect of high-fat diet (HFD) on hypothalamic KNDy gene expression to examine the pathogenic mechanism underlying obesity-induced infertility in male and female rats. Male and female rats at 7 weeks of age were fed with either a standard or HFD for 4 months. In the male rats, the HFD caused a significant suppression of ARC Kiss1 and Pdyn gene expressions, but did not affect the plasma luteinizing hormone (LH) levels and sizes of the morphology of the testis and epididymis. In the female rats, 58% of the HFD-fed female rats exhibited irregular estrous cycles, whereas the remaining rats showed regular cycles. Two of the 10 rats that showed HFD-induced irregular estrous cycles showed profound suppression of LH pulse frequency and the number of ARC Kiss1-expressing cells, whereas the other females showed normal LH pulses and ARC Kiss1 expression. Our finding shows that suppression of ARC Kiss1 expression might be the initial pathological change of hypogonadotropic hypogonadism in HFD-fed male rats, while the obese-related infertility in the female rats may be mainly induced by KNDy-independent pathways. Taken together, ARC kisspeptin neurons in male rats may be susceptible to HFD-induced obesity compared with those in female rats.

Visceral adipose tissue imparts peripheral macrophage influx into the hypothalamus

BACKGROUND

Obesity is characterized by a systemic inflammation and hypothalamic neuroinflammation. Systemic inflammation is caused by macrophages that infiltrate obese adipose tissues. We previously demonstrated that high-fat diet (HFD)-fed male mice exhibited peripheral macrophage infiltration into the hypothalamus, in addition to activation of resident microglia. Since this infiltration contributes to neuroinflammation and neuronal impairment, herein we characterize the phenotype and origin of these hypothalamic macrophages in HFD mice.

METHODS

C57BL/6J mice were fed HFD (60% kcal from fat) or control diet with matching sucrose levels, for 12-16 weeks. Males and females were analyzed separately to determine sex-specific responses to HFD. Differences in hypothalamic gene expression in HFD-fed male and female mice, compared to their lean controls, in two different areas of the hypothalamus, were determined using the NanoString neuroinflammation panel. Phenotypic changes in macrophages that infiltrated the hypothalamus in HFD-fed mice were determined by analyzing cell surface markers using flow cytometry and compared to changes in macrophages from the adipose tissue and peritoneal cavity. Adipose tissue transplantation was performed to determine the source of hypothalamic macrophages.

RESULTS

We determined that hypothalamic gene expression profiles demonstrate sex-specific and region-specific diet-induced changes. Sex-specific changes included larger changes in males, while region-specific changes included larger changes in the area surrounding the median eminence. Several genes were identified that may provide partial protection to female mice. We also identified diet-induced changes in macrophage migration into the hypothalamus, adipose tissue, and peritoneal cavity, specifically in males. Further, we determined that hypothalamus-infiltrating macrophages express pro-inflammatory markers and markers of metabolically activated macrophages that were identical to markers of adipose tissue macrophages in HFD-fed mice. Employing adipose tissue transplant, we demonstrate that hypothalamic macrophages can originate from the visceral adipose tissue.

CONCLUSION

HFD-fed males experience higher neuroinflammation than females, likely because they accumulate more visceral fat, which provides a source of pro-inflammatory macrophages that migrate to other tissues, including the hypothalamus. Our findings may explain the male bias for neuroinflammation and the metabolic syndrome. Together, our results demonstrate a new connection between the adipose tissue and the hypothalamus in obesity that contributes to neuroinflammation and hypothalamic pathologies.

Chronic High Fat Diet Intake Impairs Hepatic Metabolic Parameters in Ovariectomized Sirt3 KO Mice

High fat diet (HFD) is an important factor in the development of metabolic diseases, with liver as metabolic center being highly exposed to its influence. However, the effect of HFD-induced metabolic stress with respect to ovary hormone depletion and sirtuin 3 (Sirt3) is not clear. Here we investigated the effect of Sirt3 in liver of ovariectomized and sham female mice upon 10 weeks of feeding with standard-fat diet (SFD) or HFD. Liver was examined by Folch, gas chromatography and lipid hydroperoxide analysis, histology and oil red staining, RT-PCR, Western blot, antioxidative enzyme and oxygen consumption analyses. In SFD-fed WT mice, ovariectomy increased Sirt3 and fatty acids synthesis, maintained mitochondrial function, and decreased levels of lipid hydroperoxides. Combination of ovariectomy and Sirt3 depletion reduced pparα, Scd-1 ratio, MUFA proportions, CII-driven respiration, and increased lipid damage. HFD compromised CII-driven respiration and activated peroxisomal ROS scavenging enzyme catalase in sham mice, whereas in combination with ovariectomy and Sirt3 depletion, increased body weight gain, expression of NAFLD- and oxidative stress-inducing genes, and impaired response of antioxidative system. Overall, this study provides evidence that protection against harmful effects of HFD in female mice is attributed to the combined effect of female sex hormones and Sirt3, thus contributing to preclinical research on possible sex-related therapeutic agents for metabolic syndrome and associated diseases.

Homeostatic Regulation of Estrus Cycle of Young Female Mice on Western Diet

The etiology of reproductive disorders correlates with weight gain in patients, but the link between reproduction, diet, and weight has been difficult to translate in rodents. As rates of childhood obesity and reproductive disorders increase, the need to study the effects of weight and diet on adolescent females is key. Previous studies show that female mice are resistant to high-fat diet-induced weight gain, but the mechanisms are unclear. Literature also suggests that ovarian function is essential to resistance in weight gain, as an ovariectomy leads to a weight-gaining phenotype similar to male mice on a high-fat diet. However, reproductive changes that occur in adolescent mice on high-fat diet have not been assessed. Here, we show that regulation of the estrus cycle via progesterone is critical to metabolic homeostasis in female mice on a high-fat diet. Female mice were put on high-fat diet or control diet for 12 weeks starting at 4 weeks of age. Every 4 weeks, their estrus cycle was tracked and fasting glucose was measured. We found that after 4 weeks on high-fat diet, there was no difference in weight between groups, but an increase in time spent in proestrus and estrus in mice on high-fat diet and an increase in serum progesterone during proestrus. These results show that intact females modulate their estrus cycle in response to a high-fat diet as a mechanism of homeostatic regulation of body weight, protecting them from metabolic abnormalities. Understanding the mechanisms behind this protection may yield therapeutic opportunities for treatment of reproductive disorders in adolescent female patients.

Hypothalamic Inflammation as a Potential Pathophysiologic Basis for the Heterogeneity of Clinical, Hormonal, and Metabolic Presentation in PCOS

Polycystic ovary syndrome (PCOS) is the most common endocrine disorder among women of reproductive age. It is a heterogeneous condition characterized by reproductive, endocrine, metabolic, and psychiatric abnormalities. More than one pathogenic mechanism is involved in its development. On the other hand, the hypothalamus plays a crucial role in many important functions of the body, including weight balance, food intake, and reproduction. A high-fat diet with a large amount of long-chain saturated fatty acids can induce inflammation in the hypothalamus. Hypothalamic neurons can sense extracellular glucose concentrations and participate, with a feedback mechanism, in the regulation of whole-body glucose homeostasis. When consumed nutrients are rich in fat and sugar, and these regulatory mechanisms can trigger inflammatory pathways resulting in hypothalamic inflammation. The latter has been correlated with metabolic diseases, obesity, and depression. In this review, we explore whether the pattern and the expansion of hypothalamic inflammation, as a result of a high-fat and -sugar diet, may contribute to the heterogeneity of the clinical, hormonal, and metabolic presentation in PCOS via pathophysiologic mechanisms affecting specific areas of the hypothalamus. These mechanisms could be potential targets for the development of effective therapies for the treatment of PCOS.

Sex Differences in Macrophage Responses to Obesity-Mediated Changes Determine Migratory and Inflammatory Traits

The mechanisms whereby obesity differentially affects males and females are unclear. Because macrophages are functionally the most important cells in obesity-induced inflammation, we sought to determine reasons for male-specific propensity in macrophage migration. We previously determined that male mice fed a high-fat diet exhibit macrophage infiltration into the hypothalamus, whereas females were protected irrespective of ovarian estrogen, in this study, we show that males accumulate more macrophages in adipose tissues that are also more inflammatory. Using bone marrow cells or macrophages differentiated in vitro from male and female mice fed control or high-fat diet, we demonstrated that macrophages derived from male mice are intrinsically more migratory. We determined that males have higher levels of leptin in serum and adipose tissue. Serum CCL2 levels, however, are the same in males and females, although they are increased in obese mice compared with lean mice of both sexes. Leptin receptor and free fatty acid (FFA) receptor, GPR120, are upregulated only in macrophages derived from male mice when cultured in the presence of FFA to mimic hyperlipidemia of obesity. Unless previously stimulated with LPS, CCL2 did not cause migration of macrophages. Leptin, however, elicited migration of macrophages from both sexes. Macrophages from male mice maintained migratory capacity when cultured with FFA, whereas female macrophages failed to migrate. Therefore, both hyperlipidemia and hyperleptinemia contribute to male macrophage-specific migration because increased FFA induce leptin receptors, whereas higher leptin causes migration. Our results may explain sex differences in obesity-mediated disorders caused by macrophage infiltration.

The role of non-neuronal cells in hypogonadotropic hypogonadism

The hypothalamic-pituitary-gonadal axis is controlled by gonadotropin-releasing hormone (GnRH) released by the hypothalamus. Disruption of this system leads to impaired reproductive maturation and function, a condition known as hypogonadotropic hypogonadism (HH). Most studies to date have focused on genetic causes of HH that impact neuronal development and function. However, variants may also impact the functioning of non-neuronal cells known as glia. Glial cells make up 50% of brain cells of humans, primates, and rodents. They include radial glial cells, microglia, astrocytes, tanycytes, oligodendrocytes, and oligodendrocyte precursor cells. Many of these cells influence the hypothalamic neuroendocrine system controlling fertility. Indeed, glia regulate GnRH neuronal activity and secretion, acting both at their cell bodies and their nerve endings. Recent work has also made clear that these interactions are an essential aspect of how the HPG axis integrates endocrine, metabolic, and environmental signals to control fertility. Recognition of the clinical importance of interactions between glia and the GnRH network may pave the way for the development of new treatment strategies for dysfunctions of puberty and adult fertility.

The sex-specific patterns of changes in hypothalamic-pituitary-gonadal axis during experimental autoimmune encephalomyelitis

Multiple sclerosis develops during reproductive years in a sex-specific manner. Various neuroendocrine changes have been described in this inflammatory, demyelinating, and debilitating disease. We here aimed to determine the extent and sex specificity of alterations in the hypothalamic-pituitary-gonadal axis in the rat model of multiple sclerosis named experimental autoimmune encephalomyelitis. During the disease course, the hypothalamic tissue showed transient upregulation of inflammatory marker genes Gfap, Cd68, Ccl2, and Il1b in both sexes, but accompanied by sex-specific downregulation of Kiss1 (in females only) and Gnrh1 (in males only) expression. In females, the expression of gonadotrope-specific genes Lhb, Cga, and Gnrhr was also inhibited, accompanied by decreased basal but not stimulated serum luteinizing hormone levels and a transient arrest of the estrous cycle. In contrast, Fshb expression and serum progesterone levels were transiently elevated, findings consistent with the maintenance of the corpora lutea, and elevated immunohistochemical labeling of ovarian StAR, a rate limiting protein in steroidogenic pathway. In males, downregulation of Gnrhr expression and basal and stimulated serum luteinizing hormone and testosterone levels were accompanied by inhibited testicular StAR protein expression. We propose that inflammation of hypothalamic tissue downregulates Kiss1 and Gnrh1 expression in females and males, respectively, leading to sex-specific changes downstream the axis.

Palmitic Acid Targets Human Testicular Peritubular Cells and Causes a Pro-Inflammatory Response

Palmitic acid (PA) is a major fatty acid, derived from diet and endogenous production, which is being linked to inflammation. While such actions of PA at the level of the testis remain difficult to examine, we reasoned that studies in human testicular cells may be instructive. Human testicular peritubular cells (HTPCs) can be isolated from men and cultured. They have contractile properties but also produce Interleukin 6 (IL6), express the inflammasome member NLRP3, and via glia cell line derived neurotrophic factor (GDNF), they contribute to the spermatogonial stem cell niche. We found that PA at 100 µM significantly increased the levels of IL6, while NLRP3 or the related Interleukin 1 beta (IL1beta) were not affected. The contractility marker calponin (CNN1) and the growth factor GDNF were likewise not affected. ELISA studies confirmed the stimulatory PA actions on IL6. Hence, PA derived from diet and/or endogenous sources may be able to foster a pro-inflammatory milieu in the testis. A possible link of these results to diet and high fat intake and obesity is indicated by the about 12-fold elevated testicular levels of IL6 in testes of obese rhesus monkeys (n = 3), fed with a Western Style diet. They had elevated 2–5-fold increased body fat and increased circulating triglyceride levels. Further consequences of PA and obesity for testicular functions remain to be evaluated.

High-refined carbohydrate diet leads to polycystic ovary syndrome-like features and reduced ovarian reserve in female rats

Obesity is associated with several female reproductive complications, such as polycystic ovary syndrome (PCOS). The exact mechanism of this relationship remains unclear. Few previous studies using diet containing refined carbohydrate (HCD) leading to obesity have been performed and it is unclear if HCD is linked with reproductive dysfunctions. In this investigation, we assessed whether subchronic HCD exposure results in reproductive and other irregularities. Female rats were fed with HCD for 15 days and metabolic outcomes and reproductive tract morphophysiology were assessed. We further assessed reproductive tract inflammation, oxidative stress (OS) and fibrosis. HCD rats displayed metabolic impairments, such as an increase in body weight/adiposity, adipocyte hypertrophic, abnormal lipid profile, glucose tolerance and insulin resistance (IR) and hyperleptinemia. Improper functioning of the HCD reproductive tract was observed. Specifically, irregular estrous cyclicity, high LH levels and abnormal ovarian morphology coupled with reduction in primordial and primary follicle numbers was observed, suggesting ovarian reserve depletion. Improper follicular development and a reduction in antral follicles, corpora lutea and granulosa layer area together with an increase in cystic follicles were apparent. Uterine atrophy and reduction in endometrial gland (GE) number was observed in HCD rats. Reproductive tract inflammation, OS and fibrosis were seen in HCD rats. Further, strong positive correlations were observed between body weight/adiposity and IR with estrous cycle length, cystic follicles, ovarian reserve, GE and other abnormalities. Thus, these data suggest that the subchronic HCD exposure led to PCOS-like features, impaired ovarian reserve, GE number, and other reproductive abnormalities in female rats.

Obesity Drives Delayed Infarct Expansion, Inflammation, and Distinct Gene Networks in a Mouse Stroke Model

Obesity is associated with chronic peripheral inflammation, is a risk factor for stroke, and causes increased infarct sizes. To characterize how obesity increases infarct size, we fed a high-fat diet to wild-type C57BL/6J mice for either 6 weeks or 15 weeks and then induced distal middle cerebral artery strokes. We found that infarct expansion happened late after stroke. There were no differences in cortical neuroinflammation (astrogliosis, microgliosis, or pro-inflammatory cytokines) either prior to or 10 h after stroke, and also no differences in stroke size at 10 h. However, by 3 days after stroke, animals fed a high-fat diet had a dramatic increase in microgliosis and astrogliosis that was associated with larger strokes and worsened functional recovery. RNA sequencing revealed a dramatic increase in inflammatory genes in the high-fat diet-fed animals 3 days after stroke that were not present prior to stroke. Genetic pathways unique to diet-induced obesity were primarily related to adaptive immunity, extracellular matrix components, cell migration, and vasculogenesis. The late appearance of neuroinflammation and infarct expansion indicates that there may be a therapeutic window between 10 and 36 h after stroke where inflammation and obesity-specific transcriptional programs could be targeted to improve outcomes in people with obesity and stroke.

Sex differences in response to a high fat, high sucrose diet in both the gut microbiome and hypothalamic astrocytes and microglia

Objectives: Obesity is a major epidemic in our population and has emerged as a primary health concern. Consumption of a high fat, high sugar (HFHS) diet can specifically lead to gut dysbiosis, increased inflammation, and neuroinflammation. Interestingly, sex differences in the response to a HFHS diet are emerging. In this study, we investigated the effects of a HFHS diet compared to a low fat, low sugar (LFLS) diet in 8 week old male and female C57Bl/6 mice.Methods: The diet was administered for 14 weeks; body weights and food consumption were evaluated weekly.Results: Male and female mice fed the HFHS diet gained significantly more weight than LFLS-fed mice. However, in agreement with previous studies, males gained significantly more weight on the HFHS diet compared to females fed the same diet. Importantly, we determined significant sex and diet-induced differences to gut microbiome composition using next generation Illumina sequencing. We also observed significantly less astrocyte densitometry and no significant change to microglial morphology in the hypothalamus of Female HFHS compared to Female LFLS. On the other hand, Male HFHS revealed no change to hypothalamic astrogliosis, but increased microgliosis compared to Male LFLS.Discussion: In this study, we determined sex and diet-induced differences in both the gut and the brain, however, future studies will need to be performed in order to test the direct role of the gut microbiome to weight gain and neuroinflammation in male and female mice.

Estradiol treatment attenuates high fat diet-induced microgliosis in ovariectomized rats

Consumption of a high fat diet (HFD) increases circulating free fatty acids, which can enter the brain and promote a state of microgliosis, as defined by a change in microglia number and/or morphology. Most studies investigating diet-induced microgliosis have been conducted in male rodents despite well-documented sex differences in the neural control of food intake and neuroimmune signaling. This highlights the need to investigate how sex hormones may modulate the behavioral and cellular response to HFD consumption. Estradiol is of particular interest since it exerts a potent anorexigenic effect and has both anti-inflammatory and neuroprotective effects in the brain. As such, the aim of the current study was to investigate whether estradiol attenuates the development of HFD-induced microgliosis in female rats. Estradiol- and vehicle-treated ovariectomized rats were fed either a low-fat chow diet or a 60% HFD for 4 days, after which they were perfused and brain sections were processed via immunohistochemistry for microglia-specific Iba1 protein. Four days of HFD consumption promoted microgliosis, as measured via an increase in the number of microglia in the arcuate nucleus (ARC) of the hypothalamus and nucleus of the solitary tract (NTS), and a decrease in microglial branching in the ARC, NTS, lateral hypothalamus (LH), and ventromedial hypothalamus. Estradiol replacement attenuated the HFD-induced changes in microglia accumulation and morphology in the ARC, LH, and NTS. We conclude that estradiol has protective effects against HFD-induced microgliosis in a region-specific manner in hypothalamic and hindbrain areas implicated in the neural control of food intake.

Male hypogonadism: a review

This article contains a systematic review of the main developments that have occurred in the area of male hypogonadism between the publication of the Endocrine Society Guidelines of 2010 and 2018 and after 2018.

Effect of Inflammation on Female Gonadotropin-Releasing Hormone (GnRH) Neurons: Mechanisms and Consequences

: Inflammation has a well-known suppressive effect on fertility. The function of gonadotropin-releasing hormone (GnRH) neurons, the central regulator of fertility is substantially altered during inflammation in females. In our review we discuss the latest results on how the function of GnRH neurons is modified by inflammation in females. We first address the various effects of inflammation on GnRH neurons and their functional consequences. Second, we survey the possible mechanisms underlying the inflammation-induced actions on GnRH neurons. The role of several factors will be discerned in transmitting inflammatory signals to the GnRH neurons: cytokines, kisspeptin, RFamide-related peptides, estradiol and the anti-inflammatory cholinergic pathway. Since aging and obesity are both characterized by reproductive decline our review also focuses on the mechanisms and pathophysiological consequences of the impact of inflammation on GnRH neurons in aging and obesity.

Sexually Dimorphic Response of Increasing Dietary Intake of High Amylose Wheat on Metabolic and Reproductive Outcomes in Male and Female Mice

High amylose wheat (HAW) has a higher resistant starch content and lower glycaemic index than standard amylose wheat (SAW), which may be associated with health benefits. This study aimed to determine the effects of replacing SAW with HAW on metabolic and reproductive parameters in male and female mice. Male and female C57BL/6 mice were randomly divided into groups (n = 8/group/sex) and fed either a SAW65 (65% SAW w/w; control), HAW35 (35% HAW w/w), HAW50 (50% HAW w/w) or HAW65 (65% HAW w/w) diet for eight weeks. In male but not female, the HAW65 group had a lower abdominal circumference, relative total fat mass, relative gonadal fat mass and plasma leptin concentration compared to the HAW35 group. There were no differences in fasting blood glucose concentrations or plasma concentrations of cholesterol, triglycerides or non-esterified fatty acids between groups in either males or females. The HAW-fed males had a higher testicular weight and HAW-fed females spent less time in diestrus and a longer time in metestrus compared to the SAW-fed mice. Higher dietary intake of HAW appears to reduce abdominal fat deposition compared to the lower level of HAW in a sexually dimorphic manner. The impacts on reproductive parameters in the HAW-fed mice require further investigation.

Obesity, Neuroinflammation, and Reproductive Function

The increasing occurrence of obesity has become a significant public health concern. Individuals with obesity have higher prevalence of heart disease, stroke, osteoarthritis, diabetes, and reproductive disorders. Reproductive problems include menstrual irregularities, pregnancy complications, and infertility due to anovulation, in women, and lower testosterone and diminished sperm count, in men. In particular, women with obesity have reduced levels of both gonadotropin hormones, and, in obese men, lower testosterone is accompanied by diminished LH. Taken together, these findings indicate central dysregulation of the hypothalamic-pituitary-gonadal axis, specifically at the level of the GnRH neuron function, which is the final brain output for the regulation of reproduction. Obesity is a state of hyperinsulinemia, hyperlipidemia, hyperleptinemia, and chronic inflammation. Herein, we review recent advances in our understanding of how these metabolic and immune changes affect hypothalamic function and regulation of GnRH neurons. In the latter part, we focus on neuroinflammation as a major consequence of obesity and discuss findings that reveal that GnRH neurons are uniquely positioned to respond to inflammatory changes.