Obesity-associated microglial inflammatory activation paradoxically improves glucose tolerance.

Hypothalamic gliosis associated with high-fat diet (HFD) feeding increases susceptibility to hyperphagia and weight gain. However, the body-weight-independent contribution of microglia to glucose regulation has not been determined. Here, we show that reducing microglial nuclear factor κB (NF-κB) signaling via cell-specific IKKß deletion exacerbates HFD-induced glucose intolerance despite reducing body weight and adiposity. Conversely, two genetic approaches to increase microglial pro-inflammatory signaling (deletion of an NF-κB pathway inhibitor and chemogenetic activation through a modified Gq-coupled muscarinic receptor) improved glucose tolerance independently of diet in both lean and obese rodents. Microglial regulation of glucose homeostasis involves a tumor necrosis factor alpha (TNF-α)-dependent mechanism that increases activation of pro-opiomelanocortin (POMC) and other hypothalamic glucose-sensing neurons, ultimately leading to a marked amplification of first-phase insulin secretion via a parasympathetic pathway. Overall, these data indicate that microglia regulate glucose homeostasis in a body-weight-independent manner, an unexpected mechanism that limits the deterioration of glucose tolerance associated with obesity.

Central androgen action reverses hypothalamic astrogliosis and atherogenic risk factors induced by orchiectomy and high-fat diet feeding in male mice.

Hypogonadism in males confers elevated cardiovascular disease (CVD) risk by unknown mechanisms. Recent radiological evidence suggests that low testosterone (T) is associated with mediobasal hypothalamic (MBH) gliosis, a central nervous system (CNS) cellular response linked to metabolic dysfunction. To address mechanisms linking CNS androgen action to CVD risk, we generated a hypogonadal, hyperlipidemic mouse model with orchiectomy (ORX) combined with hepatic PCSK9 overexpression. After 4 wk of high-fat, high-sucrose diet (HFHS) consumption, despite equal body weights and glucose tolerance, androgen-deficient ORX mice had a more atherogenic lipid profile and increased liver and leukocyte inflammatory signaling compared with sham-operated control mice. Along with these early CVD risk indicators, ORX markedly amplified HFHS-induced astrogliosis in the MBH. Transcriptomic analysis further revealed that ORX and high-fat diet feeding induced upregulation of inflammatory pathways and downregulation of metabolic pathways in hypothalamic astrocytes. To interrogate the role of sex steroid signaling in the CNS in cardiometabolic risk and MBH inflammation, central infusion of T and dihydrotestosterone (DHT) was performed on ORX mice. Central DHT prevented MBH astrogliosis and reduced the liver inflammatory signaling and monocytosis induced by HFHS and ORX; T had a partial protective effect. Finally, a cross-sectional study in 41 adult men demonstrated a positive correlation between radiological evidence of MBH gliosis and plasma lipids. These findings demonstrate that T deficiency in combination with a Western-style diet promotes hypothalamic gliosis concomitant with increased atherogenic risk factors and provide supportive evidence for regulation of lipid metabolism and cardiometabolic risk determinants by the CNS action of sex steroids.NEW & NOTEWORTHY This study provides evidence that hypothalamic gliosis is a key early event through which androgen deficiency in combination with a Western-style diet might lead to cardiometabolic dysregulation in males. Furthermore, this work provides the first evidence in humans of a positive association between hypothalamic gliosis and LDL-cholesterol, advancing our knowledge of CNS influences on CVD risk progression.

Prostaglandin PGE2 Receptor EP4 Regulates Microglial Phagocytosis and Increases Susceptibility to Diet-Induced Obesity.

In rodents, susceptibility to diet-induced obesity requires microglial activation, but the molecular components of this pathway remain incompletely defined. Prostaglandin PGE2 levels increase in the mediobasal hypothalamus during high-fat-diet (HFD) feeding, and the PGE2 receptor EP4 regulates microglial activation state and phagocytic activity, suggesting a potential role for microglial EP4 signaling in obesity pathogenesis. To test the role of microglial EP4 in energy balance regulation, we analyzed the metabolic phenotype in a microglia-specific EP4 knockout (MG-EP4 KO) mouse model. Microglial EP4 deletion markedly reduced weight gain and food intake in response to HFD feeding. Corresponding with this lean phenotype, insulin sensitivity was also improved in HFD-fed MG-EP4 KO mice, though glucose tolerance remained surprisingly unaffected. Mechanistically, EP4-deficient microglia showed an attenuated phagocytic state marked by reduced CD68 expression and fewer contacts with pro-opiomelanocortin (POMC) neuron processes. These cellular changes observed in the MG-EP4 KO mice corresponded with an increased density of POMC neurites extending into the paraventricular nucleus (PVN). These findings reveal that microglial EP4 signaling promotes body weight gain and insulin resistance during HFD feeding. Furthermore, the data suggest that curbing microglial phagocytic function may preserve POMC cytoarchitecture and PVN input to limit overconsumption during diet-induced obesity.

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.

Microglial Inflammatory Signaling Orchestrates the Hypothalamic Immune Response to Dietary Excess and Mediates Obesity Susceptibility.

Dietary excess triggers accumulation of pro-inflammatory microglia in the mediobasal hypothalamus (MBH), but the components of this microgliosis and its metabolic consequences remain uncertain. Here, we show that microglial inflammatory signaling determines the immunologic response of the MBH to dietary excess and regulates hypothalamic control of energy homeostasis in mice. Either pharmacologically depleting microglia or selectively restraining microglial NF-κB-dependent signaling sharply reduced microgliosis, an effect that includes prevention of MBH entry by bone-marrow-derived myeloid cells, and greatly limited diet-induced hyperphagia and weight gain. Conversely, forcing microglial activation through cell-specific deletion of the negative NF-κB regulator A20 induced spontaneous MBH microgliosis and cellular infiltration, reduced energy expenditure, and increased both food intake and weight gain even in absence of a dietary challenge. Thus, microglial inflammatory activation, stimulated by dietary excess, orchestrates a multicellular hypothalamic response that mediates obesity susceptibility, providing a mechanistic rationale for non-neuronal approaches to treat metabolic diseases.

Sex differences in microglial CX3CR1 signalling determine obesity susceptibility in mice.

Female mice are less susceptible to the negative metabolic consequences of high-fat diet feeding than male mice, for reasons that are incompletely understood. Here we identify sex-specific differences in hypothalamic microglial activation via the CX3CL1-CX3CR1 pathway that mediate the resistance of female mice to diet-induced obesity. Female mice fed a high-fat diet maintain CX3CL1-CX3CR1 levels while male mice show reductions in both ligand and receptor expression. Female Cx3cr1 knockout mice develop 'male-like' hypothalamic microglial accumulation and activation, accompanied by a marked increase in their susceptibility to diet-induced obesity. Conversely, increasing brain CX3CL1 levels in male mice through central pharmacological administration or virally mediated hypothalamic overexpression converts them to a 'female-like' metabolic phenotype with reduced microglial activation and body-weight gain. These data implicate sex differences in microglial activation in the modulation of energy homeostasis and identify CX3CR1 signalling as a potential therapeutic target for the treatment of obesity.

Correlations between Synaptic Initiation and Meiotic Recombination: A Study of Humans and Mice.

Meiotic recombination is initiated by programmed double strand breaks (DSBs), only a small subset of which are resolved into crossovers (COs). The mechanism determining the location of these COs is not well understood. Studies in plants, fungi, and insects indicate that the same genomic regions are involved in synaptic initiation and COs, suggesting that early homolog alignment is correlated with the eventual resolution of DSBs as COs. It is generally assumed that this relationship extends to mammals, but little effort has been made to test this idea. Accordingly, we conducted an analysis of synaptic initiation sites (SISs) and COs in human and mouse spermatocytes and oocytes. In contrast to our expectation, we observed remarkable sex- and species-specific differences, including pronounced differences between human males and females in both the number and chromosomal location of SISs. Further, the combined data from our studies in mice and humans suggest that the relationship between SISs and COs in mammals is a complex one that is not dictated by the sites of synaptic initiation as reported in other organisms, although it is clearly influenced by them.

ALDH Enzyme Expression Is Independent of the Spermatogenic Cycle, and Their Inhibition Causes Misregulation of Murine Spermatogenic Processes.

Perturbations in the vitamin A metabolism pathway could be a significant cause of male infertility, as well as a target toward the development of a male contraceptive, necessitating the need for a better understanding of how testicular retinoic acid (RA) concentrations are regulated. Quantitative analyses have recently demonstrated that RA is present in a pulsatile manner along testis tubules. However, it is unclear if the aldehyde dehydrogenase (ALDH) enzymes, which are responsible for RA synthesis, contribute to the regulation of these RA concentration gradients. Previous studies have alluded to fluctuations in ALDH enzymes across the spermatogenic cycle, but these inferences have been based primarily on qualitative transcript localization experiments. Here, we show via various quantitative methods that the three well-known ALDH enzymes (ALDH1A1, ALDH1A2, and ALDH1A3), and an ALDH enzyme previously unreported in the murine testis (ALDH8A1), are not expressed in a stage-specific manner in the adult testis, but do fluctuate throughout juvenile development in perfect agreement with the first appearance of each advancing germ cell type. We also show, via treatments with a known ALDH inhibitor, that lowered testicular RA levels result in an increase in blood-testis barrier permeability, meiotic recombination, and meiotic defects. Taken together, these data further our understanding of the complex regulatory actions of RA on various spermatogenic events and, in contrast with previous studies, also suggest that the ALDH enzymes are not responsible for regulating the recently measured RA pulse.