Regulation of Energy Expenditure by Brainstem GABA Neurons.

Homeostatic control of core body temperature is essential for survival. Temperature is sensed by specific neurons, in turn eliciting both behavioral (i.e., locomotion) and physiologic (i.e., thermogenesis, vasodilatation) responses. Here, we report that a population of GABAergic (Vgat-expressing) neurons in the dorsolateral portion of the dorsal raphe nucleus (DRN), hereafter DRNVgat neurons, are activated by ambient heat and bidirectionally regulate energy expenditure through changes in both thermogenesis and locomotion. We find that DRNVgat neurons innervate brown fat via a descending projection to the raphe pallidus (RPa). These neurons also densely innervate ascending targets implicated in the central regulation of energy expenditure, including the hypothalamus and extended amygdala. Optogenetic stimulation of different projection targets reveals that DRNVgat neurons are capable of regulating thermogenesis through both a "direct" descending pathway through the RPa and multiple "indirect" ascending pathways. This work establishes a key regulatory role for DRNVgat neurons in controlling energy expenditure.

Identification of a Brainstem Circuit Controlling Feeding.

Hunger, driven by negative energy balance, elicits the search for and consumption of food. While this response is in part mediated by neurons in the hypothalamus, the role of specific cell types in other brain regions is less well defined. Here, we show that neurons in the dorsal raphe nucleus, expressing vesicular transporters for GABA or glutamate (hereafter, DRNVgat and DRNVGLUT3 neurons), are reciprocally activated by changes in energy balance and that modulating their activity has opposite effects on feeding-DRNVgat neurons increase, whereas DRNVGLUT3 neurons suppress, food intake. Furthermore, modulation of these neurons in obese (ob/ob) mice suppresses food intake and body weight and normalizes locomotor activity. Finally, using molecular profiling, we identify druggable targets in these neurons and show that local infusion of agonists for specific receptors on these neurons has potent effects on feeding. These data establish the DRN as an important node controlling energy balance. PAPERCLIP.

Critical roles of transcriptional coactivator MED1 in the formation and function of mouse adipose tissues.

The MED1 subunit has been shown to mediate ligand-dependent binding of the Mediator coactivator complex to multiple nuclear receptors, including the adipogenic PPARγ, and to play an essential role in ectopic PPARγ-induced adipogenesis of mouse embryonic fibroblasts. However, the precise roles of MED1, and its various domains, at various stages of adipogenesis and in adipose tissue have been unclear. Here, after establishing requirements for MED1, including specific domains, for differentiation of 3T3L1 cells and both primary white and brown preadipocytes, we used multiple genetic approaches to assess requirements for MED1 in adipocyte formation, maintenance, and function in mice. We show that MED1 is indeed essential for the differentiation and/or function of both brown and white adipocytes, as its absence in these cells leads to, respectively, defective brown fat function and lipodystrophy. This work establishes MED1 as an essential transcriptional coactivator that ensures homeostatic functions of adipocytes.

Mitofusin 2 in POMC neurons connects ER stress with leptin resistance and energy imbalance.

Mitofusin 2 (MFN2) plays critical roles in both mitochondrial fusion and the establishment of mitochondria-endoplasmic reticulum (ER) interactions. Hypothalamic ER stress has emerged as a causative factor for the development of leptin resistance, but the underlying mechanisms are largely unknown. Here, we show that mitochondria-ER contacts in anorexigenic pro-opiomelanocortin (POMC) neurons in the hypothalamus are decreased in diet-induced obesity. POMC-specific ablation of Mfn2 resulted in loss of mitochondria-ER contacts, defective POMC processing, ER stress-induced leptin resistance, hyperphagia, reduced energy expenditure, and obesity. Pharmacological relieve of hypothalamic ER stress reversed these metabolic alterations. Our data establish MFN2 in POMC neurons as an essential regulator of systemic energy balance by fine-tuning the mitochondrial-ER axis homeostasis and function. This previously unrecognized role for MFN2 argues for a crucial involvement in mediating ER stress-induced leptin resistance.

A leptin-BDNF pathway regulating sympathetic innervation of adipose tissue.

Mutations in the leptin gene (ob) result in a metabolic disorder that includes severe obesity1, and defects in thermogenesis2 and lipolysis3, both of which are adipose tissue functions regulated by the sympathetic nervous system. However, the basis of these sympathetic-associated abnormalities remains unclear. Furthermore, chronic leptin administration reverses these abnormalities in adipose tissue, but the underlying mechanism remains to be discovered. Here we report that ob/ob mice, as well as leptin-resistant diet-induced obese mice, show significant reductions of sympathetic innervation of subcutaneous white and brown adipose tissue. Chronic leptin treatment of ob/ob mice restores adipose tissue sympathetic innervation, which in turn is necessary to correct the associated functional defects. The effects of leptin on innervation are mediated via agouti-related peptide and pro-opiomelanocortin neurons in the hypothalamic arcuate nucleus. Deletion of the gene encoding the leptin receptor in either population leads to reduced innervation in fat. These agouti-related peptide and pro-opiomelanocortin neurons act via brain-derived neurotropic factor-expressing neurons in the paraventricular nucleus of the hypothalamus (BDNFPVH). Deletion of BDNFPVH blunts the effects of leptin on innervation. These data show that leptin signalling regulates the plasticity of sympathetic architecture of adipose tissue via a top-down neural pathway that is crucial for energy homeostasis.

Author Correction: Microbiota modulate sympathetic neurons via a gut-brain circuit.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Microbiota modulate sympathetic neurons via a gut-brain circuit.

Connections between the gut and brain monitor the intestinal tissue and its microbial and dietary content1, regulating both physiological intestinal functions such as nutrient absorption and motility2,3, and brain-wired feeding behaviour2. It is therefore plausible that circuits exist to detect gut microorganisms and relay this information to areas of the central nervous system that, in turn, regulate gut physiology4. Here we characterize the influence of the microbiota on enteric-associated neurons by combining gnotobiotic mouse models with transcriptomics, circuit-tracing methods and functional manipulations. We find that the gut microbiome modulates gut-extrinsic sympathetic neurons: microbiota depletion leads to increased expression of the neuronal transcription factor cFos, and colonization of germ-free mice with bacteria that produce short-chain fatty acids suppresses cFos expression in the gut sympathetic ganglia. Chemogenetic manipulations, translational profiling and anterograde tracing identify a subset of distal intestine-projecting vagal neurons that are positioned to have an afferent role in microbiota-mediated modulation of gut sympathetic neurons. Retrograde polysynaptic neuronal tracing from the intestinal wall identifies brainstem sensory nuclei that are activated during microbial depletion, as well as efferent sympathetic premotor glutamatergic neurons that regulate gastrointestinal transit. These results reveal microbiota-dependent control of gut-extrinsic sympathetic activation through a gut-brain circuit.

Burning Question: How Does Our Brain Process Positive and Negative Cues Associated with Thermosensation?

Whether it is the dramatic suffocating sensation from a heat wave in the summer or the positive reinforcement arising from a hot drink on a cold day; we can certainly agree that our thermal environment underlies our daily rhythms of sensation. Extensive research has focused on deciphering the central circuits responsible for conveying the impact of thermogenesis on mammalian behavior. Here, we revise the recent literature responsible for defining the behavioral correlates that arise from thermogenic fluctuations in mammals. We transition from the physiological significance of thermosensation to the circuitry responsible for the autonomic or behavioral responses associated with it. Subsequently, we delve into the positive and negative valence encoded by thermoregulatory processes. Importantly, we emphasize the crucial junctures where reward, pain, and thermoregulation intersect, unveiling a complex interplay within these neural circuits. Finally, we briefly outline fundamental questions that are pending to be addressed in the field. Fully deciphering the thermoregulatory circuitry in mammals will have far-reaching medical implications. For instance, it may lead to the identification of novel targets to overcome thermal pain or allow the maintenance of our core temperature in prolonged surgeries.

Neuro-Adipokine Crosstalk in Alzheimer's Disease.

The connection between body weight alterations and Alzheimer's disease highlights the intricate relationship between the brain and adipose tissue in the context of neurological disorders. During midlife, weight gain increases the risk of cognitive decline and dementia, whereas in late life, weight gain becomes a protective factor. Despite their substantial impact on metabolism, the role of adipokines in the transition from healthy aging to neurological disorders remains largely unexplored. We aim to investigate how the adipose tissue milieu and the secreted adipokines are involved in the transition between biological and pathological aging, highlighting the bidirectional relationship between the brain and systemic metabolism. Understanding the function of these adipokines will allow us to identify biomarkers for early detection of Alzheimer's disease and uncover novel therapeutic options.

Functional analysis of distinct populations of subthalamic nucleus neurons on Parkinson's disease and OCD-like behaviors in mice.

The subthalamic nucleus (STN) is a component of the basal ganglia and plays a key role to control movement and limbic-associative functions. STN modulation with deep brain stimulation (DBS) improves the symptoms of Parkinson's disease (PD) and obsessive-compulsive disorder (OCD) patients. However, DBS does not allow for cell-type-specific modulation of the STN. While extensive work has focused on elucidating STN functionality, the understanding of the role of specific cell types is limited. Here, we first performed an anatomical characterization of molecular markers for specific STN neurons. These studies revealed that most STN neurons express Pitx2, and that different overlapping subsets express Gabrr3, Ndnf, or Nos1. Next, we used optogenetics to define their roles in regulating locomotor and limbic functions in mice. Specifically, we showed that optogenetic photoactivation of STN neurons in Pitx2-Cre mice or of the Gabrr3-expressing subpopulation induces locomotor changes, and improves locomotion in a PD mouse model. In addition, photoactivation of Pitx2 and Gabrr3 cells induced repetitive grooming, a phenotype associated with OCD. Repeated stimulation prompted a persistent increase in grooming that could be reversed by fluoxetine treatment, a first-line drug therapy for OCD. Conversely, repeated inhibition of STNGabrr3 neurons suppressed grooming in Sapap3 KO mice, a model for OCD. Finally, circuit and functional mapping of STNGabrr3 neurons showed that these effects are mediated via projections to the globus pallidus/entopeduncular nucleus and substantia nigra reticulata. Altogether, these data identify Gabrr3 neurons as a key population in mediating the beneficial effects of STN modulation thus providing potential cellular targets for PD and OCD drug discovery.

Central myelin dysfunction bridges obesity and neurological diseases.

The central nervous system (CNS) relies on myelin for proper functioning. Myelin remodeling is a risk factor for neurometabolic and endocrine malfunction, resulting in cognitive decline and heightened susceptibility to neurological diseases. The plasticity of myelin upon nutrient shifts may lead to dietary and hormonal interventions for preventing and treating neural complications.

Dynamic rewiring of neurovasculature in health and disease.

Brain vasculature is chiefly considered a support network responsible for delivering signaling molecules and nutrients to neural cells. Several central disorders exhibit disruptions in functional and structural plasticity of this network. Considering this vasculature as structurally dynamic, it challenges the field's view and may be important for brain-directed therapeutic strategies.

Maternal nutritional programming shapes the cerebral landscape.

The escalating prevalence of maternal obesity raises concerns about its influence on offspring health. Exposure to obesogenic environments during early development leads to persistent alterations in brain function contributing to neurological disorders. Nutritional programming emerges as a promising avenue to counteract the deleterious effects of maternal obesity on offspring neurodevelopment.

FGF21 counteracts alcohol intoxication by activating the noradrenergic nervous system.

Animals that consume fermenting fruit and nectar are at risk of exposure to ethanol and the detrimental effects of inebriation. In this report, we show that the hormone FGF21, which is strongly induced by ethanol in murine and human liver, stimulates arousal from intoxication without changing ethanol catabolism. Mice lacking FGF21 take longer than wild-type littermates to recover their righting reflex and balance following ethanol exposure. Conversely, pharmacologic FGF21 administration reduces the time needed for mice to recover from ethanol-induced unconsciousness and ataxia. FGF21 did not counteract sedation caused by ketamine, diazepam, or pentobarbital, indicating specificity for ethanol. FGF21 mediates its anti-intoxicant effects by directly activating noradrenergic neurons in the locus coeruleus region, which regulates arousal and alertness. These results suggest that this FGF21 liver-brain pathway evolved to protect against ethanol-induced intoxication and that it might be targeted pharmaceutically for treating acute alcohol poisoning.

Pharmacological targeting of glutamatergic neurons within the brainstem for weight reduction.

Food intake and body weight are tightly regulated by neurons within specific brain regions, including the brainstem, where acute activation of dorsal raphe nucleus (DRN) glutamatergic neurons expressing the glutamate transporter Vglut3 (DRNVglut3) drive a robust suppression of food intake and enhance locomotion. Activating Vglut3 neurons in DRN suppresses food intake and increases locomotion, suggesting that modulating the activity of these neurons might alter body weight. Here, we show that DRNVglut3 neurons project to the lateral hypothalamus (LHA), a canonical feeding center that also reduces food intake. Moreover, chronic DRNVglut3 activation reduces weight in both leptin-deficient (ob/ob) and leptin-resistant diet-induced obese (DIO) male mice. Molecular profiling revealed that the orexin 1 receptor (Hcrtr1) is highly enriched in DRN Vglut3 neurons, with limited expression elsewhere in the brain. Finally, an orally bioavailable, highly selective Hcrtr1 antagonist (CVN45502) significantly reduces feeding and body weight in DIO. Hcrtr1 is also co-expressed with Vglut3 in the human DRN, suggesting that there might be a similar effect in human. These results identify a potential therapy for obesity by targeting DRNVglut3 neurons while also establishing a general strategy for developing drugs for central nervous system disorders.

Remission of obesity and insulin resistance is not sufficient to restore mitochondrial homeostasis in visceral adipose tissue.

Metabolic plasticity is the ability of a biological system to adapt its metabolic phenotype to different environmental stressors. We used a whole-body and tissue-specific phenotypic, functional, proteomic, metabolomic and transcriptomic approach to systematically assess metabolic plasticity in diet-induced obese mice after a combined nutritional and exercise intervention. Although most obesity and overnutrition-related pathological features were successfully reverted, we observed a high degree of metabolic dysfunction in visceral white adipose tissue, characterized by abnormal mitochondrial morphology and functionality. Despite two sequential therapeutic interventions and an apparent global healthy phenotype, obesity triggered a cascade of events in visceral adipose tissue progressing from mitochondrial metabolic and proteostatic alterations to widespread cellular stress, which compromises its biosynthetic and recycling capacity. In humans, weight loss after bariatric surgery showed a transcriptional signature in visceral adipose tissue similar to our mouse model of obesity reversion. Overall, our data indicate that obesity prompts a lasting metabolic fingerprint that leads to a progressive breakdown of metabolic plasticity in visceral adipose tissue.

Multifaceted actions of melanin-concentrating hormone on mammalian energy homeostasis.

Melanin-concentrating hormone (MCH) is a small cyclic peptide expressed in all mammals, mainly in the hypothalamus. MCH acts as a robust integrator of several physiological functions and has crucial roles in the regulation of sleep-wake rhythms, feeding behaviour and metabolism. MCH signalling has a very broad endocrine context and is involved in physiological functions and emotional states associated with metabolism, such as reproduction, anxiety, depression, sleep and circadian rhythms. MCH mediates its functions through two receptors (MCHR1 and MCHR2), of which only MCHR1 is common to all mammals. Owing to the wide variety of MCH downstream signalling pathways, MCHR1 agonists and antagonists have great potential as tools for the directed management of energy balance disorders and associated metabolic complications, and translational strategies using these compounds hold promise for the development of novel treatments for obesity. This Review provides an overview of the numerous roles of MCH in energy and glucose homeostasis, as well as in regulation of the mesolimbic dopaminergic circuits that encode the hedonic component of food intake.

Melanocortin 4 receptor stimulation prevents antidepressant-associated weight gain in mice caused by long-term fluoxetine exposure.

Contrasting with the predicted anorexigenic effect of increasing brain serotonin signaling, long-term use of selective serotonin reuptake inhibitor (SSRI) antidepressants correlates with body weight (BW) gain. This adverse outcome increases the risk of transitioning to obesity and interferes with treatment compliance. Here, we show that orally administered fluoxetine (Flx), a widely prescribed SSRI, increased BW by enhancing food intake in healthy mice at 2 different time points and through 2 distinct mechanisms. Within hours, Flx decreased the activity of a subset of brainstem serotonergic neurons by triggering autoinhibitory signaling through 5-hydroxytryptamine receptor 1a (Htr1a). Following a longer treatment period, Flx blunted 5-hydroxytryptamine receptor 2c (Htr2c) expression and signaling, decreased the phosphorylation of cAMP response element-binding protein (CREB) and STAT3, and dampened the production of pro-opiomelanocortin (POMC, the precursor of α-melanocyte stimulating hormone [α-MSH]) in hypothalamic neurons, thereby increasing food intake. Accordingly, exogenous stimulation of the melanocortin 4 receptor (Mc4r) by cotreating mice with Flx and lipocalin 2, an anorexigenic hormone signaling through this receptor, normalized feeding and BW. Flx and other SSRIs also inhibited CREB and STAT3 phosphorylation in a human neuronal cell line, suggesting that these noncanonical effects could also occur in individuals treated long term with SSRIs. By defining the molecular basis of long-term SSRI-associated weight gain, we propose a therapeutic strategy to counter this effect.

Mitochondrial cristae-remodeling protein OPA1 in POMC neurons couples Ca2+ homeostasis with adipose tissue lipolysis.

Appropriate cristae remodeling is a determinant of mitochondrial function and bioenergetics and thus represents a crucial process for cellular metabolic adaptations. Here, we show that mitochondrial cristae architecture and expression of the master cristae-remodeling protein OPA1 in proopiomelanocortin (POMC) neurons, which are key metabolic sensors implicated in energy balance control, is affected by fluctuations in nutrient availability. Genetic inactivation of OPA1 in POMC neurons causes dramatic alterations in cristae topology, mitochondrial Ca2+ handling, reduction in alpha-melanocyte stimulating hormone (α-MSH) in target areas, hyperphagia, and attenuated white adipose tissue (WAT) lipolysis resulting in obesity. Pharmacological blockade of mitochondrial Ca2+ influx restores α-MSH and the lipolytic program, while improving the metabolic defects of mutant mice. Chemogenetic manipulation of POMC neurons confirms a role in lipolysis control. Our results unveil a novel axis that connects OPA1 in POMC neurons with mitochondrial cristae, Ca2+ homeostasis, and WAT lipolysis in the regulation of energy balance.

Microbiota-modulated CART+ enteric neurons autonomously regulate blood glucose.

The gut microbiota affects tissue physiology, metabolism, and function of both the immune and nervous systems. We found that intrinsic enteric-associated neurons (iEANs) in mice are functionally adapted to the intestinal segment they occupy; ileal and colonic neurons are more responsive to microbial colonization than duodenal neurons. Specifically, a microbially responsive subset of viscerofugal CART+ neurons, enriched in the ileum and colon, modulated feeding and glucose metabolism. These CART+ neurons send axons to the prevertebral ganglia and are polysynaptically connected to the liver and pancreas. Microbiota depletion led to NLRP6- and caspase 11-dependent loss of CART+ neurons and impaired glucose regulation. Hence, iEAN subsets appear to be capable of regulating blood glucose levels independently from the central nervous system.