Sensory neurons expressing calcitonin gene-related peptide α regulate adaptive thermogenesis and diet-induced obesity

Topical menthol, a pharmacological cold mimic, induces cold sensitivity, adaptive thermogenesis and brown adipose tissue activation in mice

AIM

Brown adipose tissue (BAT) thermogenesis has profound energy-expanding potential, which makes it an attractive target tissue to combat ever-increasing obesity and its other associated metabolic complications. Although it is fairly accepted that cold is a potent inducer of BAT activation and function, there are limited studies on the mechanisms of pharmacological cold-mimicking agents, such as the TRPM8 agonist, menthol, on BAT thermogenesis and activation.

METHODS

Herein, we sought to determine the effect of topical application of menthol (10% w/v [4 g/kg] cream formulation/day for 15 days) on temperature sensitivity behaviour (thermal gradient assay, nesting behaviour), adaptive thermogenesis (infrared thermography, core body temperature), BAT sympathetic innervation (tyrosine hydroxylase immunohistochemistry) and activation (18F-FDG PET-CT analysis, Uncoupling Protein 1 immunohistochemistry and BAT gene expression), whole-body energy expenditure (indirect calorimetry) and other metabolic variables in male C57BL/6N mice.

RESULTS

We show that male C57BL/6N mice: (a) develop a warm-seeking and cold-avoiding thermal preference phenotype; (b) display increased locomotor activity and adaptive thermogenesis; (c) show augmented sympathetic innervation in BAT and its activation; (d) exhibit enhanced gluconeogenic capacity (increased glucose excursion in response to pyruvate) and insulin sensitivity; and (e) show enhanced whole-body energy expenditure and induced lipid-utilizing phenotype after topical menthol application.

CONCLUSIONS

Taken together, our findings highlight that pharmacological cold mimicking using topical menthol application presents a potential therapeutic strategy to counter weight gain and related complications.

Wiring the Brain for Wellness: Sensory Integration in Feeding and Thermogenesis: A Report on Research Supported by Pathway to Stop Diabetes

The recognition of sensory signals from within the body (interoceptive) and from the external environment (exteroceptive), along with the integration of these cues by the central nervous system, plays a crucial role in maintaining metabolic balance. This orchestration is vital for regulating processes related to both food intake and energy expenditure. Animal model studies indicate that manipulating specific populations of neurons in the central nervous system which influence these processes can effectively modify energy balance. This body of work presents an opportunity for the development of innovative weight loss therapies for the treatment of obesity and type 2 diabetes. In this overview, we delve into the sensory cues and the neuronal populations responsible for their integration, exploring their potential in the development of weight loss treatments for obesity and type 2 diabetes. This article is the first in a series of Perspectives that report on research funded by the American Diabetes Association Pathway to Stop Diabetes program.

ARTICLE HIGHLIGHTS

The Afferent Function of Adipose Innervation

Adipose tissue innervation is critical for regulating metabolic and energy homeostasis. While the sympathetic efferent innervation of fat is well characterized, the role of sensory or afferent innervation remains less explored. This article reviews previous work on adipose innervation and recent advances in the study of sensory innervation of adipose tissues. We discuss key open questions, including the physiological implications of adipose afferents in homeostasis as well as potential cross talk with sympathetic neurons, the immune system, and hormonal pathways. We also outline the general technical challenges of studying dorsal root ganglia innervating fat, along with emerging technologies that may overcome these barriers. Finally, we highlight areas for further research to deepen our understanding of the afferent function of adipose innervation.

Sensory nerve and neuropeptide diversity in adipose tissues

Both brown and white adipose tissues (BAT/WAT) are innervated by the peripheral nervous system, including efferent sympathetic nerves that communicate from the brain/central nervous system out to the tissue, and afferent sensory nerves that communicate from the tissue back to the brain and locally release neuropeptides to the tissue upon stimulation. This bidirectional neural communication is important for energy balance and metabolic control, as well as maintaining adipose tissue health through processes like browning (development of metabolically healthy brown adipocytes in WAT), thermogenesis, lipolysis, and adipogenesis. Decades of sensory nerve denervation studies have demonstrated the particular importance of adipose sensory nerves for brown adipose tissue and WAT functions, but far less is known about the tissue's sensory innervation compared to the better-studied sympathetic nerves and their neurotransmitter norepinephrine. In this review, we cover what is known and not yet known about sensory nerve activities in adipose, focusing on their effector neuropeptide actions in the tissue.

Brain-body communication in metabolic control

A thorough understanding of the mechanisms controlling energy homeostasis is needed to prevent and treat metabolic morbidities. While the contribution of organs such as the liver, muscle, adipose tissue, and pancreas to the regulation of energy has received wide attention, less is known about the interplay with the nervous system. Here, we highlight the role of the nervous systems in regulating metabolism beyond the classic hypothalamic endocrine signaling models and discuss the contribution of circadian rhythms, higher brain regions, and sociodemographic variables in the energy equation. We infer that interdisciplinary approaches are key to conceptually advancing the current research frontier and devising innovative therapies to prevent and treat metabolic disease.

Sensory spinal interoceptive pathways and energy balance regulation

Interoception plays an important role in homeostatic regulation of energy intake and metabolism. Major interoceptive pathways include gut-to-brain and adipose tissue-to brain signaling via vagal sensory nerves and hormones, such as leptin. However, signaling via spinal sensory neurons is rapidly emerging as an additional important signaling pathway. Here we provide an in-depth review of the known anatomy and functions of spinal sensory pathways and discuss potential mechanisms relevant for energy balance homeostasis in health and disease. Because sensory innervation by dorsal root ganglia (DRG) neurons goes far beyond vagally innervated viscera and includes adipose tissue, skeletal muscle, and skin, it is in a position to provide much more complete metabolic information to the brain. Molecular and anatomical identification of function specific DRG neurons will be important steps in designing pharmacological and neuromodulation approaches to affect energy balance regulation in disease states such as obesity, diabetes, and cancer.

Brown adipose tissue but not tibia exhibits a dramatic response to acute reduction in environmental temperature in growing male mice

Mice are typically housed at room temperature (∼22 °C), which is well below their thermoneutral zone and results in cold stress. Chronic cold stress leads to increased adaptive thermogenesis and reductions in cancellous bone volume and bone marrow adipose tissue mass in long bones of growing mice. There is strong evidence that increased neuronal activity initiates the metabolic response of intrascapular brown adipose tissue (BAT) to cold stress, but it is less clear whether bone is regulated through a similar mechanism. Therefore, we compared the short-term response of BAT and whole tibia to a reduction in environmental temperature. To accomplish this, we transferred a group of 6-week-old male mice from 32 °C to 22 °C housing and sacrificed the mice 24 h later. Age-matched controls were maintained at 32 °C. We then evaluated expression levels of a panel of genes related to adipocyte differentiation and fat metabolism in BAT and tibia, and a panel of genes related to bone metabolism in tibia. The decrease in housing temperature resulted in changes in expression levels for 47/86 genes related to adipocyte differentiation and fat metabolism in BAT, including 9-fold and 17-fold increases in Ucp1 and Dio2, respectively. In contrast, only 1/86 genes related to adipocyte differentiation and fat metabolism and 4/84 genes related to bone metabolism were differentially expressed in tibia. These findings suggest that bone, although innervated with sensory and sympathetic neurons, does not respond as rapidly as BAT to changes in environmental temperature.

Myeloid cell ACE shapes cellular metabolism and function in PCSK-9 induced atherosclerosis

The pathogenesis of atherosclerosis is defined by impaired lipid handling by macrophages which increases intracellular lipid accumulation. This dysregulation of macrophages triggers the accumulation of apoptotic cells and chronic inflammation which contributes to disease progression. We previously reported that mice with increased macrophage-specific angiotensin-converting enzyme, termed ACE10/10 mice, resist atherosclerosis in an adeno-associated virus-proprotein convertase subtilisin/kexin type 9 (AAV-PCSK9)-induced model. This is due to increased lipid metabolism by macrophages which contributes to plaque resolution. However, the importance of ACE in peripheral blood monocytes, which are the primary precursors of lesional-infiltrating macrophages, is still unknown in atherosclerosis. Here, we show that the ACE-mediated metabolic phenotype is already triggered in peripheral blood circulating monocytes and that this functional modification is directly transferred to differentiated macrophages in ACE10/10 mice. We found that Ly-6Clo monocytes were increased in atherosclerotic ACE10/10 mice. The monocytes isolated from atherosclerotic ACE10/10 mice showed enhanced lipid metabolism, elevated mitochondrial activity, and increased adenosine triphosphate (ATP) levels which implies that ACE overexpression is already altered in atherosclerosis. Furthermore, we observed increased oxygen consumption (VO2), respiratory exchange ratio (RER), and spontaneous physical activity in ACE10/10 mice compared to WT mice in atherosclerotic conditions, indicating enhanced systemic energy consumption. Thus, ACE overexpression in myeloid lineage cells modifies the metabolic function of peripheral blood circulating monocytes which differentiate to macrophages and protect against atherosclerotic lesion progression due to better lipid metabolism.

Transcriptome and secretome profiling of sensory neurons reveals sex differences in pathways relevant to insulin sensing and insulin secretion

Sensory neurons in the dorsal root ganglia (DRG) convey somatosensory and metabolic cues to the central nervous system and release substances from stimulated terminal endings in peripheral organs. Sex-biased variations driven by the sex chromosome complement (XX and XY) have been implicated in the sensory-islet crosstalk. However, the molecular underpinnings of these male-female differences are not known. Here, we aim to characterize the molecular repertoire and the secretome profile of the lower thoracic spinal sensory neurons and to identify molecules with sex-biased insulin sensing- and/or insulin secretion-modulating activity that are encoded independently of circulating gonadal sex hormones. We used transcriptomics and proteomics to uncover differentially expressed genes and secreted molecules in lower thoracic T5-12 DRG sensory neurons derived from sexually immature 3-week-old male and female C57BL/6J mice. Comparative transcriptome and proteome analyses revealed differential gene expression and protein secretion in DRG neurons in males and females. The transcriptome analysis identified, among others, higher insulin signaling/sensing capabilities in female DRG neurons; secretome screening uncovered several sex-specific candidate molecules with potential regulatory functions in pancreatic β cells. Together, these data suggest a putative role of sensory interoception of insulin in the DRG-islet crosstalk with implications in sensory feedback loops in the regulation of β-cell activity in a sex-biased manner. Finally, we provide a valuable resource of molecular and secretory targets that can be leveraged for understanding insulin interoception and insulin secretion and inform the development of novel studies/approaches to fathom the role of the sensory-islet axis in the regulation of energy balance in males and females.

The metabolic and functional roles of sensory nerves in adipose tissues

Homeostatic regulation of adipose tissue is critical for the maintenance of energy balance and whole-body metabolism. The peripheral nervous system provides bidirectional neural communication between the brain and adipose tissue, thereby providing homeostatic control. Most research on adipose innervation and nerve functions has been limited to the sympathetic nerves and their neurotransmitter norepinephrine. In recent years, more work has focused on adipose sensory nerves, but the contributions of subsets of sensory nerves to metabolism and the specific roles contributed by sensory neuropeptides are still understudied. Advances in imaging of adipose innervation and newer tissue denervation techniques have confirmed that sensory nerves contribute to the regulation of adipose functions, including lipolysis and browning. Here, we summarize the historical and latest findings on the regulation, function and plasticity of adipose tissue sensory nerves that contribute to metabolically important processes such as lipolysis, vascular control and sympathetic axis cross-talk.

Somatosensory innervation of adipose tissues

The increasing prevalence of obesity and type 2 diabetes has led to a greater interest in adipose tissue physiology. Adipose tissue is now understood as an organ with endocrine and thermogenic capacities in addition to its role in fat storage. It plays a critical role in systemic metabolism and energy regulation, and its activity is tightly regulated by the nervous system. Fat is now recognized to receive sympathetic innervation, which transmits information from the brain, as well as sensory innervation, which sends information into the brain. The role of sympathetic innervation in adipose tissue has been extensively studied. However, the extent and the functional significance of sensory innervation have long been unclear. Recent studies have started to reveal that sensory neurons robustly innervate adipose tissue and play an important role in regulating fat activity. This brief review will discuss both historical evidence and recent advances, as well as important remaining questions about the sensory innervation of adipose tissue.

RISING STARS: Targeting G protein-coupled receptors to regulate energy homeostasis

G protein-coupled receptors (GPCRs) have a critical role in energy homeostasis, contributing to food intake, energy expenditure and glycaemic control. Dysregulation of energy expenditure can lead to metabolic syndrome (abdominal obesity, elevated plasma triglyceride, LDL cholesterol and glucose, and high blood pressure), which is associated with an increased risk of developing obesity, diabetes mellitus, non-alcoholic fatty liver disease and cardiovascular complications. As the prevalence of these chronic diseases continues to rise worldwide, there is an increased need to understand the molecular mechanisms by which energy expenditure is regulated to facilitate the development of effective therapeutic strategies to treat and prevent these conditions. In recent years, drugs targeting GPCRs have been the focus of efforts to improve treatments for type-2 diabetes and obesity, with GLP-1R agonists a particular success. In this review, we focus on nine GPCRs with roles in energy homeostasis that are current and emerging targets to treat obesity and diabetes. We discuss findings from pre-clinical models and clinical trials of drugs targeting these receptors and challenges that must be overcome before these drugs can be routinely used in clinics. We also describe new insights into how these receptors signal, including how accessory proteins, biased signalling, and complex spatial signalling could provide unique opportunities to develop more efficacious therapies with fewer side effects. Finally, we describe how combined therapies, in which multiple GPCRs are targeted, may improve clinical outcomes and reduce off-target effects.

CGRP physiology, pharmacology, and therapeutic targets: migraine and beyond

Calcitonin gene-related peptide (CGRP) is a neuropeptide with diverse physiological functions. Its two isoforms (α and β) are widely expressed throughout the body in sensory neurons as well as in other cell types, such as motor neurons and neuroendocrine cells. CGRP acts via at least two G protein-coupled receptors that form unusual complexes with receptor activity-modifying proteins. These are the CGRP receptor and the AMY1 receptor; in rodents, additional receptors come into play. Although CGRP is known to produce many effects, the precise molecular identity of the receptor(s) that mediates CGRP effects is seldom clear. Despite the many enigmas still in CGRP biology, therapeutics that target the CGRP axis to treat or prevent migraine are a bench-to-bedside success story. This review provides a contextual background on the regulation and sites of CGRP expression and CGRP receptor pharmacology. The physiological actions of CGRP in the nervous system are discussed, along with updates on CGRP actions in the cardiovascular, pulmonary, gastrointestinal, immune, hematopoietic, and reproductive systems and metabolic effects of CGRP in muscle and adipose tissues. We cover how CGRP in these systems is associated with disease states, most notably migraine. In this context, we discuss how CGRP actions in both the peripheral and central nervous systems provide a basis for therapeutic targeting of CGRP in migraine. Finally, we highlight potentially fertile ground for the development of additional therapeutics and combinatorial strategies that could be designed to modulate CGRP signaling for migraine and other diseases.

Diet-induced loss of adipose hexokinase 2 correlates with hyperglycemia

Chronically high blood glucose (hyperglycemia) leads to diabetes and fatty liver disease. Obesity is a major risk factor for hyperglycemia, but the underlying mechanism is unknown. Here, we show that a high-fat diet (HFD) in mice causes early loss of expression of the glycolytic enzyme Hexokinase 2 (HK2) specifically in adipose tissue. Adipose-specific knockout of Hk2 reduced glucose disposal and lipogenesis and enhanced fatty acid release in adipose tissue. In a non-cell-autonomous manner, Hk2 knockout also promoted glucose production in liver. Furthermore, we observed reduced hexokinase activity in adipose tissue of obese and diabetic patients, and identified a loss-of-function mutation in the hk2 gene of naturally hyperglycemic Mexican cavefish. Mechanistically, HFD in mice led to loss of HK2 by inhibiting translation of Hk2 mRNA. Our findings identify adipose HK2 as a critical mediator of local and systemic glucose homeostasis, and suggest that obesity-induced loss of adipose HK2 is an evolutionarily conserved mechanism for the development of selective insulin resistance and thereby hyperglycemia.

Brain-to-BAT - and Back?: Crosstalk between the Central Nervous System and Thermogenic Adipose Tissue in Development and Therapy of Obesity

The body of mammals harbors two distinct types of adipose tissue: while cells within the white adipose tissue (WAT) store surplus energy as lipids, brown adipose tissue (BAT) is nowadays recognized as the main tissue for transforming chemical energy into heat. This process, referred to as 'non-shivering thermogenesis', is facilitated by the uncoupling of the electron transport across mitochondrial membranes from ATP production. BAT-dependent thermogenesis acts as a safeguarding mechanism under reduced ambient temperature but also plays a critical role in metabolic and energy homeostasis in health and disease. In this review, we summarize the evolutionary structure, function and regulation of the BAT organ under neuronal and hormonal control and discuss its mutual interaction with the central nervous system. We conclude by conceptualizing how better understanding the multifaceted communicative links between the brain and BAT opens avenues for novel therapeutic approaches to treat obesity and related metabolic disorders.

The Single-Cell Revelation of Thermogenic Adipose Tissue

The past two decades have witnessed an upsurge in the appreciation of adipose tissue (AT) as an immuno-metabolic hub harbouring heterogeneous cell populations that collectively fine-tune systemic metabolic homeostasis. Technological advancements, especially single-cell transcriptomics, have offered an unprecedented opportunity for dissecting the sophisticated cellular networks and compositional dynamics underpinning AT remodelling. The "re-discovery" of functional brown adipose tissue dissipating heat energy in human adults has aroused tremendous interest in exploiting the mechanisms underpinning the engagement of AT thermogenesis for combating human obesity. In this review, we aim to summarise and evaluate the use of single-cell transcriptomics that contribute to a better appreciation of the cellular plasticity and intercellular crosstalk in thermogenic AT.

Transcriptome study digs out BMP2 involved in adipogenesis in sheep tails

Background

Hu sheep and Tibetan sheep in China are characterized by fat tails and thin tails, respectively. Several transcriptomes have been conducted in different sheep breeds to identify the differentially expressed genes (DEGs) underlying this trait. However, these studies identified different DEGs in different sheep breeds.

Results

Hence, RNA sequencing was performed on Hu sheep and Tibetan sheep. We obtained a total of 45.57 and 43.82 million sequencing reads, respectively. Two libraries mapped reads from 36.93 and 38.55 million reads after alignment to the reference sequences. 2108 DEGs were identified, including 1247 downregulated and 861 upregulated DEGs. GO and KEGG analyses of all DEGs demonstrated that pathways were enriched in the regulation of lipolysis in adipocytes and terms related to the chemokine signalling pathway, lysosomes, and glycosaminoglycan degradation. Eight genes were selected for validation by RT–qPCR. In addition, the transfection of BMP2 overexpression into preadipocytes resulted in increased PPAR-γ expression and expression. BMP2 potentially induces adipogenesis through LOX in preadipocytes. The number of lipid drops in BMP2 overexpression detected by oil red O staining was also greater than that in the negative control.

Conclusion

In summary, these results showed that significant genes (BMP2, HOXA11, PPP1CC and LPIN1) are involved in the regulation of adipogenesis metabolism and suggested novel insights into metabolic molecules in sheep fat tails.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08657-8.

Orexin receptor type 2 agonism inhibits thermogenesis in brown adipose tissue by attenuating afferent innervation

Orexin signaling has been associated with energy expenditure and brown adipose tissue (BAT) function. However, conflicting data exist in the field about how orexin signaling regulates BAT thermogenesis. In this study, we show that a specific orexin receptor type 2 (OX2R) agonist [Ala11, D-Leu15]-OxB (OB-Ala) inhibited intrascapular brown adipose tissue (iBAT) thermogenesis by reducing sympathetic output to iBAT. This effect is mediated by OX2Rs located on afferent nerve endings innervating iBAT instead of brown adipocyte itself. Microinjection of OB-Ala into iBAT inhibited iBAT thermogenesis in mice upon cold exposure and neuronal activity in the paraventricular nucleus. Findings suggest that OB-Ala could inhibit iBAT thermogenesis by attenuating sensory input thereby inhibiting the sympathetic-sensory iBAT feedback loop. Our study uncovers a novel primary action site of orexin in the regulation of energy balance.