The sympathetic nervous system in the 21st century: Neuroimmune interactions in metabolic homeostasis and obesity

GPCR-mediated regulation of beige adipocyte formation: Implications for obesity and metabolic health

Obesity and its associated complications pose a significant burden on health. The non-shivering thermogenesis (NST) and metabolic capacity properties of brown adipose tissue (BAT), which are distinct from those of white adipose tissue (WAT), in combating obesity and its related metabolic diseases has been well documented. However, beige adipose tissue, the third and relatively novel type of adipose tissue, which emerges in extensive presence of WAT and shares similar favorable metabolic properties with BAT, has garnered considerable attention in recent years. In this review, we focused on the role of G protein-coupled receptors (GPCRs), the largest receptor family and the most successful class of drug targets in humans, in the induction of beige adipocytes. More importantly, we highlight researchers' clinical treatment attempts to ameliorate obesity and other related metabolic diseases through the formation and activation of beige adipose tissue. In summary, this review provides valuable insights into the formation of beige adipose tissue and the involvement of GPCRs, based on the latest advancements in scientific research.

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.

The Sympathetic-Immune Milieu in Metabolic Health and Diseases: Insights from Pancreas, Liver, Intestine, and Adipose Tissues

Sympathetic innervation plays a crucial role in maintaining energy balance and contributes to metabolic pathophysiology. Recent evidence has begun to uncover the innervation landscape of sympathetic projections and sheds light on their important functions in metabolic activities. Additionally, the immune system has long been studied for its essential roles in metabolic health and diseases. In this review, the aim is to provide an overview of the current research progress on the sympathetic regulation of key metabolic organs, including the pancreas, liver, intestine, and adipose tissues. In particular, efforts are made to highlight the critical roles of the peripheral nervous system and its potential interplay with immune components. Overall, it is hoped to underscore the importance of studying metabolic organs from a comprehensive and interconnected perspective, which will provide valuable insights into the complex mechanisms underlying metabolic regulation and may lead to novel therapeutic strategies for metabolic diseases.

Immunomodulatory leptin receptor+ sympathetic perineurial barrier cells protect against obesity by facilitating brown adipose tissue thermogenesis

Adipose tissues (ATs) are innervated by sympathetic nerves, which drive reduction of fat mass via lipolysis and thermogenesis. Here, we report a population of immunomodulatory leptin receptor-positive (LepR+) sympathetic perineurial barrier cells (SPCs) present in mice and humans, which uniquely co-express Lepr and interleukin-33 (Il33) and ensheath AT sympathetic axon bundles. Brown ATs (BATs) of mice lacking IL-33 in SPCs (SPCΔIl33) had fewer regulatory T (Treg) cells and eosinophils, resulting in increased BAT inflammation. SPCΔIl33 mice were more susceptible to diet-induced obesity, independently of food intake. Furthermore, SPCΔIl33 mice had impaired adaptive thermogenesis and were unresponsive to leptin-induced rescue of metabolic adaptation. We therefore identify LepR+ SPCs as a source of IL-33, which orchestrate an anti-inflammatory BAT environment, preserving sympathetic-mediated thermogenesis and body weight homeostasis. LepR+IL-33+ SPCs provide a cellular link between leptin and immune regulation of body weight, unifying neuroendocrinology and immunometabolism as previously disconnected fields of obesity research.

Three-dimensional Co-culture Model for Live Imaging of Pancreatic Islets, Immune Cells, and Neurons in Agarose Gel

During the onset of autoimmune diabetes, nerve-immune cell interactions seem to play an important role; however, there are currently no models to follow and interfere with these interactions over time in vivo or in vitro. Two-dimensional in vitro models provide insufficient information and microfluidics or organs on a chip are usually challenging to work with. We present here what we believe to be the first simple model that provides the opportunity to co-culture pancreatic islets with sympathetic nerves and immune cells. This model is based on our stamping device that can be 3D printed (STL file provided). Due to the imprint in the agarose gel, sympathetic neurons, pancreatic islets, and macrophages can be seeded in specific locations at a level that allows for confocal live-cell imaging. In this protocol, we provide the instructions to construct and perform live cell imaging experiments in our co-culture model, including: 1) design for the stamping device to make the imprint in the gel, 2) isolation of sympathetic neurons, pancreatic islets, and macrophages, 3) co-culture conditions, 4) how this can be used for live cell imaging, and 5) possibilities for wider use of the model. In summary, we developed an easy-to-use co-culture model that allows manipulation and imaging of interactions between sympathetic nerves, pancreatic islets, and macrophages. This new co-culture model is useful to study nerve-immune cell-islet interactions and will help to identify the functional relevance of neuro-immune interactions in the pancreas. Key features • A novel device that allows for 3D co-culture of sympathetic neurons, pancreatic islets, and immune cells • The device allows the capture of live interactions between mouse sympathetic neurons, pancreatic islets, and immune cells in a controlled environment after six days of co-culturing. • This protocol uses cultured sympathetic neurons isolated from the superior cervical ganglia using a previously established method (Jackson and Tourtellotte, 2014) in a 3D co-culture. • This method requires 3D printing of our own designed gel-stamping device (STL print file provided on SciLifeLab FigShare DOI: 10.17044/scilifelab.24073062).

New Challenges for Anatomists in the Era of Omics

Anatomic studies have traditionally relied on macroscopic, microscopic, and histological techniques to investigate the structure of tissues and organs. Anatomic studies are essential in many fields, including medicine, biology, and veterinary science. Advances in technology, such as imaging techniques and molecular biology, continue to provide new insights into the anatomy of living organisms. Therefore, anatomy remains an active and important area in the scientific field. The consolidation in recent years of some omics technologies such as genomics, transcriptomics, proteomics, and metabolomics allows for a more complete and detailed understanding of the structure and function of cells, tissues, and organs. These have been joined more recently by "omics" such as radiomics, pathomics, and connectomics, supported by computer-assisted technologies such as neural networks, 3D bioprinting, and artificial intelligence. All these new tools, although some are still in the early stages of development, have the potential to strongly contribute to the macroscopic and microscopic characterization in medicine. For anatomists, it is time to hitch a ride and get on board omics technologies to sail to new frontiers and to explore novel scenarios in anatomy.

Adipose 'neighborhoods' collaborate to maintain metabolic health

Body fat is stored in anatomically distinct adipose depots that vary in their cell composition and play specialized roles in systemic metabolic homeostasis via secreted products. Their local effects on nearby tissues (e.g. the gut and visceral adipose tissues) are increasingly recognized and this local crosstalk is being elucidated. The major subcutaneous fat depots, abdominal and gluteal-femoral, exert opposite effects on the risk of metabolic disease. The pace of research into developmental, sex, and genetic determinants of human adipose depot growth and function is rapidly accelerating, providing insight into the pathogenesis of metabolic dysfunction in persons with obesity.

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.

Pancreatic sympathetic innervation disturbance in type 1 diabetes

Pancreatic sympathetic innervation can directly affect the function of islet. The disorder of sympathetic innervation in islets during the occurrence of type 1 diabetes (T1D) has been reported to be controversial with the inducing factor unclarified. Several studies have uncovered the critical role that sympathetic signals play in controlling the local immune system. The survival and operation of endocrine cells can be regulated by immune cell infiltration in islets. In the current review, we focused on the impact of sympathetic signals working on islets cell regulation, and discussed the potential factors that can induce the sympathetic innervation disorder in the islets. We also summarized the effect of interference with the islet sympathetic signals on the T1D occurrence. Overall, complete understanding of the regulatory effect of sympathetic signals on islet cells and local immune system could facilitate to design better strategies to control inflammation and protect β cells in T1D therapy.

A critical update on the leptin-melanocortin system

The discovery of leptin in 1994 was an "eureka moment" in the field of neurometabolism that provided new opportunities to better understand the central control of energy balance and glucose metabolism. Rapidly, a prevalent model in the field emerged that pro-opiomelanocortin (POMC) neurons were key in promoting leptin's anorexigenic effects and that the arcuate nucleus of the hypothalamus (ARC) was a key region for the regulation of energy homeostasis. While this model inspired many important discoveries, a growing body of literature indicates that this model is now outdated. In this review, we re-evaluate the hypothalamic leptin-melanocortin model in light of recent advances that directly tackle previous assumptions, with a particular focus on the ARC. We discuss how segregated and heterogeneous these neurons are, and examine how the development of modern approaches allowing spatiotemporal, intersectional, and chemogenetic manipulations of melanocortin neurons has allowed a better definition of the complexity of the leptin-melanocortin system. We review the importance of leptin in regulating glucose homeostasis, but not food intake, through direct actions on ARC POMC neurons. We further highlight how non-POMC, GABAergic neurons mediate leptin's direct effects on energy balance and influence POMC neurons.

Intermittent Hypoxia and Weight Loss: Insights into the Etiology of the Sleep Apnea Phenotype

Sleep apnea (SA) is a major respiratory disorder with increased risk for hypertension and obesity; however, our understanding of the origins of this complex disorder remains limited. Because apneas lead to recurrent drops in O₂ during sleep, intermittent hypoxia (IH) is the main animal model to explore the pathophysiology of SA. Here, we assessed the impacts of IH on metabolic function and related signals. Adult male rats were exposed to 1 week of moderate IH (FiO₂ = 0.10-30 s, ten cycles/hour, 8 h/day). Using whole-body plethysmography, we measured respiratory variability and apnea index during sleep. Blood pressure and heart rate were measured by the tail-cuff method; blood samples were taken for multiplex assay. At rest, IH augmented arterial blood pressure, respiratory instability, but not apnea index. IH induced weight, fat, and fluid loss. IH also reduced food intake and plasma leptin, adrenocorticotropic hormone (ACTH), and testosterone levels but increased inflammatory cytokines. We conclude that IH does not replicate the metabolic clinical features of SA patient, thus raising our awareness of the limitations of the IH model. The fact that the risk for hypertension occurs before the appearance of apneas provides new insights into the progression of the disease.

Pain as a Protective Factor for Alzheimer Disease in Patients with Cancer

OBJECTIVE

Alzheimer disease (AD) and cancer have been reported to be inversely correlated in incidence, but the mechanism remains elusive.

METHODS

A case-control study was conducted, based on the SEER (Surveillance, Epidemiology, and End Results) Research Plus data, to evaluate 12 factors in patients with cancer.

RESULTS

Severe pain was related to reduced AD risk, while older age at cancer diagnosis, female, longer survival years after tumor diagnosis, more benign/borderline tumors, less cancer-directed surgery, and more chemotherapy were associated with higher AD risk. In addition, patients of different races or with different cancer sites were associated with different risks of getting AD. Cases had a higher prevalence of severe pain than controls in all race and cancer site subgroups, except for in digestive cancer, where the result was the opposite.

CONCLUSIONS

This study indicated pain as a novel protective factor for AD in patients with cancer. The mechanism behind it may provide new perspective on AD pathogenesis and AD-cancer association, which we discussed in our own hypothesis of the mechanism of pain action. In addition, digestive cancer pain had an opposite impact on AD risk from other cancer pains, which suggests the uniqueness of digestive system in interacting with the central nervous system.