Endogenous cannabinoids are required for MC4R-mediated control of energy homeostasis

Chronic intermittent ethanol produces nociception through endocannabinoid-independent mechanisms in mice

Alcohol use disorder (AUD) affects millions of people and represents a significant health and economic burden. Pain is a frequently under-treated aspect of hyperkatifeia during alcohol withdrawal, yet to date no drugs have received FDA approval for the treatment of this indication in AUD patients. This study aims to evaluate the potential of targeting bioactive lipid signaling pathways as a therapeutic approach for treating alcohol withdrawal-related pain hypersensitivity. We utilized a chronic intermittent ethanol (CIE) vapor exposure model in C57BL/6J mice of both sexes to establish alcohol dependence and demonstrated that CIE produced robust tactile allodynia and thermal hyperalgesia during withdrawal that was independent of prior blood alcohol levels. Next, we evaluated four drugs for their efficacy in reversing tactile allodynia during abstinence from CIE using a cross-over treatment design that included FDA-approved naltrexone as well as commercially available inhibitors targeting the inflammatory lipid signaling enzymes fatty acid amide hydrolase (FAAH), monoacylglycerol lipase (MAGL), and 15-Lipoxygenase (LOX). None of these compounds produced significant therapeutic benefit in reversing established CIE-induced tactile allodynia, despite attenuating pain-like behaviors at these doses in other chronic pain models. Additionally, we assessed plasma endocannabinoid levels in both sexes during withdrawal. We found that there was an inherent sex difference in the endogenous anti-inflammatory endocannabinoid tone in naive mice and that CIE treatment affected endocannabinoids levels in female mice only. These findings underscore the need to better understand the underlying causes of AUD-induced allodynia and to develop novel therapeutic approaches to mitigate pain hypersensitivity in AUD patients.

Deficiency of neuronal LGR4 increases energy expenditure and inhibits food intake via hypothalamic leptin signaling

The metabolic effects of leucine-rich repeat-containing G protein-coupled receptor 4 (LGR4) remain largely unknown. Here, we showed that knockdown of Lgr4 in nestin progenitor or Sp1 mature neurons reduced high fat diet (HFD)-induced obesity by increasing energy expenditure and inhibiting food intake. Deficiency of LGR4 in AgRP neurons increased energy expenditure, and inhibited food intake, leading to alterations in glucose and lipid metabolism. Knock-down of Lgr4 in Sf1 neurons enhanced energy expenditure, reduced adiposity, and improved glucose and lipid metabolism. The metabolic benefits of neuronal LGR4 occurred via improvement of leptin signaling in AgRP and Sf1 neurons. Knockdown of Lgr4 in nestin, Sp1, AgRP or Sf1 neurons decreased hypothalamic levels of SOCS-3, and increased phosphorylation of STAT3. These alterations were associated with a significant reduction in the hypothalamic levels of β-catenin. Inhibition of β-catenin signaling by Dkk1 significantly attenuated the decrement of phospho-STAT3 and concurrent increase of SOCS-3 induced by Rspondin 3, an endogenous ligand for LGR4. Our results thus demonstrate that hypothalamic LGR4 may promote energy conversation by increasing food intake and decreasing energy expenditure. Deficiency of neuronal LGR4 improves hypothalamic leptin sensitivity via suppression of β-catenin signaling.

GPR139, an Ancient Receptor and an Emerging Target for Neuropsychiatric and Behavioral Disorders

GPR139 is an orphan G-protein-coupled receptor that is predominantly expressed in several midbrain regions, e.g., the habenula, striatum, and hypothalamus. GPR139 gene is highly conserved across vertebrate phylogenetic taxa, suggesting its fundamental importance in neurophysiology. Evidence from both animal studies and human genetic association studies has demonstrated that dysregulation of GPR139 expression and function is linked to aberrant behaviors, cognitive deficits, alterations in sleep and alertness, and substance abuse and withdrawal. Animal knockout models suggest that GPR139 plays an anti-opioid role by modulating the signaling activity of the μ-opioid receptor (MOR), as well as the intensity of withdrawal symptoms and nociception in behavioral paradigms. Modulation of GPR139 activity by surrogate agonists such as TAK-041 and JNJ-63533054 has shown promising results in experimental models; however, the use of TAK-041 in clinical trials has produced heterogeneous effects and has not met the intended primary endpoint. Here, we highlight current in vitro and in vivo studies of GPR139, its potential physiological roles, and therapeutic potential in the pathophysiology of neuropsychiatric and behavioral disorders. This review aims to focus on the current knowledge gaps to facilitate future studies that will contribute to the understanding of GPR139 as a therapeutic target for neuropsychiatric and behavioral disorders.

Hypothalamic GABAergic neurons: their roles in health and metabolic diseases

Hypothalamic GABAergic neurons are important in regulating metabolic homeostasis and energy balance. Serving as critical integrators of catabolic and anabolic processes, these neurons orchestrate a broad spectrum of metabolic functions, including feeding, nutrient metabolism, fluid homeostasis, basal metabolism, thermoregulation, and circadian rhythms. Recent advances in neuroscience have facilitated a deeper exploration of the role of hypothalamic GABAergic neurons in metabolic regulation. Emerging research has uncovered key mechanisms through which these neurons modulate energy balance and maintain metabolic balance. These findings not only enhance our understanding of obesity and related metabolic disorders but also underscore the link between hypothalamic dysfunction and prevalent metabolic diseases such as obesity and type 2 diabetes. This review summarizes the latest advancements in our understanding of the role of hypothalamic GABAergic neurons in metabolic regulation. It aims to elucidate the neural and molecular mechanisms underlying hypothalamic control of metabolism, offering new perspectives for the diagnosis and treatment of metabolic disorders.

Nmnat2 deficiency in the arcuate nucleus or paraventricular nucleus induces Sarm1-independent neuron loss and liraglutide-reversible obesity

Nicotinamide mononucleotide adenylyltransferase 2 (Nmnat2) plays an important role in maintaining axon integrity, and the arcuate nucleus (ARC), and paraventricular nucleus (PVN) are crucial nuclei in the control of energy balance. However, the effect of Nmnat2 deficiency in ARC and PVN is still unclear. Nmnat2 loxP/loxP or Nmnat2 loxP/loxP , Sarm1 -/- mice were bilaterally injected with AAV-CMV-GFP-Cre once into the ARC, PVN, or lateral parabrachial nucleus (LPBN) to obtain Nmnat2 ARC-/- , Nmnat2 PVN-/- , Nmnat2 LPBN-/- , Nmnat2 ARC-/- , SKO, Nmnat2 PVN-/- , SKO, or Nmnat2 LPBN-/- , SKO mice. Syn1-Cre mice were bilaterally injected with AAV-EF1a-flex-taCasp3-TEVp once into the ARC or PVN to specifically induce neuron loss. Metabolic changes were measured in the mice intraperitoneally injected with or without liraglutide, a glucagon-like peptide-1 (GLP-1) analog. Neuron loss and neuron activation were monitored by immunofluorescence. Deletion of Nmnat2 in ARC or PVN of mice leads to neuron loss, increased food intake, and obesity in a Sarm1-independent manner. Intraperitoneal injection of liraglutide activates neurons in PVN and LPBN, and attenuates hyperphagia and obesity induced by Nmnat2 deletion or apoptosis of Syn1-positive neurons in ARC or PVN, but has no significant effect on neuron loss. Nmnat2 deficiency in LPBN leads to death within 2 weeks, which can be markedly rescued by Sarm1 deficiency. These data show that deletion of Nmnat2 in ARC or PVN in adult mice leads to Sarm1-independent neuron loss, and liraglutide-reversible hyperphagia and obesity. These findings also elucidate the integrated role of ARC or PVN for downregulating food intake, the requirement of LPBN for survival, and the ARC- or PVN-independent effect of GLP-1 on food intake.

Brain endocannabinoid control of metabolic and non-metabolic feeding behaviors

The central endocannabinoid (eCB) system in brain shows a crucial role in the regulation of feeding behaviors, influencing both metabolic and non-metabolic mechanisms of appetite control, which has been paid much attention. Although there are already many review articles discussing eCB modulation of feeding behaviors, our paper attempts to summarize the recent advancements through synapses, circuits, and network in brain. Our focus is on the dual role of eCB signalling in regulating metabolic energy balance and hedonic reward-related feeding. In the context of metabolic regulation of feeding behaviors, eCBs affect the hypothalamic circuits that balance hunger and satiety through signal integration related to energy status and nutrient availability. Dysregulation of this system can contribute to metabolic disorders such as obesity and anorexia. In non-metabolic feeding, the eCB system influences the hedonic aspects of eating by modulating reward pathways, including the mesolimbic system and the olfactory bulb, critical for motivating food intake and processing sensory cues. This review also explores therapeutic strategies targeting the eCB system, including cannabinoid receptor antagonists and eCB hydrolase enzyme inhibitors, which hold promise for treating conditions associated with appetite dysregulation and eating disorders. By synthesizing recent findings, we aim to highlight the intricate mechanisms through which the eCB system affects feeding behavior and to propose future directions for research and therapeutic intervention in the realm of appetite control and eating disorders.

MC4R Variants Modulate α-MSH and Setmelanotide Induced Cellular Signaling at Multiple Levels

CONTEXT

The melanocortin-4 receptor (MC4R) plays an important role in body weight regulation. Pathogenic MC4R variants are the most common cause of monogenic obesity.

OBJECTIVE

We have identified 17 MC4R variants in adult and pediatric patients with obesity. Here we aimed to functionally characterize these variants by analyzing 4 different aspects of MC4R signaling. In addition, we aimed to analyze the effect of setmelanotide, a potent MC4R agonist, on these MC4R variants.

MATERIALS AND METHODS

Cell surface expression and α-melanocyte stimulating hormone (α-MSH)- or setmelanotide-induced cAMP response, β-arrestin-2 recruitment, and ERK activation were measured in cells expressing either wild type or variant MC4R.

RESULTS

We found a large heterogeneity in the function of these variants. We identified variants with a loss of response for all studied MC4R signaling, variants with no cAMP accumulation or ERK activation but normal β-arrestin-2 recruitment, and variants with normal cAMP accumulation and ERK activation but decreased β-arrestin-2 recruitment, indicating disrupted desensitization and signaling mechanisms. Setmelanotide displayed a greater potency and similar efficacy as α-MSH and induced significantly increased maximal cAMP responses of several variants compared to α-MSH. Despite the heterogeneity in functional response, there was no apparent difference in the obesity phenotype in our patients.

CONCLUSION

We show that these obesity-associated MC4R variants affect MC4R signaling differently yet lead to a comparable clinical phenotype. Our results demonstrate the clinical importance of assessing the effect of MC4R variants on a range of molecular signaling mechanisms to determine their association with obesity, which may aid in improving personalized treatment.

Activating MC4R Promotes Functional Recovery by Repressing Oxidative Stress-Mediated AIM2 Activation Post-spinal Cord Injury

Spinal cord injury (SCI) is a destructive neurological trauma that induces permanent sensory and motor impairment as well as a deficit in autonomic physiological function. Melanocortin receptor 4 (MC4R) is a G protein-linked receptor that is extensively expressed in the neural system and contributes to inhibiting inflammation, regulating mitochondrial function, and inducing programmed cell death. However, the effect of MC4R in the modulation of oxidative stress and whether this mechanism is related to the role of absent in melanoma 2 (AIM2) in SCI are not confirmed yet. In the current study, we demonstrated that MC4R is significantly increased in the neurons of spinal cords after trauma and oxidative stimulation of cells. Further, activation of MC4R by RO27-3225 effectively improved functional recovery, inhibited AIM2 activation, maintained mitochondrial homeostasis, repressed oxidative stress, and prevented Drp1 translocation to the mitochondria. Meanwhile, treating Drp1 inhibitors would be beneficial in reducing AIM2 activation, and activating AIM2 could abolish the protective effect of MC4R on neuron homeostasis. In conclusion, we demonstrated that MC4R protects against neural injury through a novel process by inhibiting mitochondrial dysfunction, oxidative stress, as well as AIM2 activation, which may serve as an available candidate for SCI therapy.

Energy deficiency promotes rhythmic foraging behavior by activating neurons in paraventricular hypothalamic nucleus

Background

Dysregulation of feeding behavior leads to a variety of pathological manifestations ranging from obesity to anorexia. The foraging behavior of animals affected by food deficiency is not fully understood.

Methods

Home-Cage system was used to monitor the behaviors. Immunohistochemical staining was used to monitor the trend of neuronal activity. Chemogenetic approach was used to modify neuronal activity.

Results

We described here a unique mouse model of foraging behavior and unveiled that food deprivation significantly increases the general activities of mice with a daily rhythmic pattern, particularly foraging behavior. The increased foraging behavior is potentiated by food cues (mouthfeel, odor, size, and shape) and energy deficit, rather than macronutrient protein, carbohydrate, and fat. Notably, energy deficiency increases nocturnal neuronal activity in paraventricular hypothalamic nucleus (PVH), accompanying a similar change in rhythmic foraging behavior. Activating neuronal activity in PVH enhances the amplitude of foraging behavior in mice. Conversely, inactivating neuronal activity in PVH decreases the amplitude of foraging behavior and impairs the rhythm of foraging behavior.

Discussion

These results illustrate that energy status and food cues regulate the rhythmic foraging behavior via PVH neuronal activity. Understanding foraging behavior provides insights into the underlying mechanism of eating-related disorders.

Glutamatergic melanocortin-4 receptor neurons regulate body weight

The locus coeruleus (LC), enriched in vesicular glutamate transporter 2 (VGlut2) neurons, is a potential homeostasis-regulating hub. However, the identity of melanocortin-4 receptor (MC4R) neurons in the paraventricular nucleus (PVN) of the hypothalamus, PVNVGlut2::MC4R and LCVGlut2::MC4R regulation of body weight, and axonal projections of LCVGlut2 neurons remain unclear. Conditional knockout of MC4R in chimeric mice was used to confirm the effects of VGlut2. Interscapular brown adipose tissue was injected with pseudorabies virus to study the central nervous system projections. We mapped the LCVGlut2 circuitry. Based on the Cre-LoxP recombination system, specific knockdown of MC4R in VGlut2 neurons resulted in weight gain in chimeric mice. Adeno-associated virus-mediated knockdown of MC4R expression in the PVN and LC had potential superimposed effects on weight gain, demonstrating the importance of VGlut2 neurons. Unlike these wide-ranging efferent projections, the PVN, hypothalamic arcuate nucleus, supraoptic nucleus of the lateral olfactory tegmental nuclei, and nucleus tractus solitarius send excitatory projections to LCVGlut2 neurons. The PVN → LC glutamatergic MC4R long-term neural circuit positively affected weight management and could help treat obesity.

The Melanocortin System: A Promising Target for the Development of New Antidepressant Drugs

Major depression is one of the most prevalent mental disorders, causing significant human suffering and socioeconomic loss. Since conventional antidepressants are not sufficiently effective, there is an urgent need to develop new antidepressant medications. Despite marked advances in the neurobiology of depression, the etiology and pathophysiology of this disease remain poorly understood. Classical and newer hypotheses of depression suggest that an imbalance of brain monoamines, dysregulation of the hypothalamic-pituitary-adrenal axis (HPAA) and immune system, or impaired hippocampal neurogenesis and neurotrophic factors pathways are cause of depression. It is assumed that conventional antidepressants improve these closely related disturbances. The purpose of this review was to discuss the possibility of affecting these disturbances by targeting the melanocortin system, which includes adrenocorticotropic hormone-activated receptors and their peptide ligands (melanocortins). The melanocortin system is involved in the regulation of various processes in the brain and periphery. Melanocortins, including peripherally administered non-corticotropic agonists, regulate HPAA activity, exhibit anti-inflammatory effects, stimulate the levels of neurotrophic factors, and enhance hippocampal neurogenesis and neurotransmission. Therefore, endogenous melanocortins and their analogs are able to complexly affect the functioning of those body’s systems that are closely related to depression and the effects of antidepressants, thereby demonstrating a promising antidepressant potential.

Endocannabinoids at the synapse and beyond: implications for neuropsychiatric disease pathophysiology and treatment

Endocannabinoids (eCBs) are lipid neuromodulators that suppress neurotransmitter release, reduce postsynaptic excitability, activate astrocyte signaling, and control cellular respiration. Here, we describe canonical and emerging eCB signaling modes and aim to link adaptations in these signaling systems to pathological states. Adaptations in eCB signaling systems have been identified in a variety of biobehavioral and physiological process relevant to neuropsychiatric disease states including stress-related disorders, epilepsy, developmental disorders, obesity, and substance use disorders. These insights have enhanced our understanding of the pathophysiology of neurological and psychiatric disorders and are contributing to the ongoing development of eCB-targeting therapeutics. We suggest future studies aimed at illuminating how adaptations in canonical as well as emerging cellular and synaptic modes of eCB signaling contribute to disease pathophysiology or resilience could further advance these novel treatment approaches.