A distinct hypothalamus-to-β cell circuit modulates insulin secretion

Parallel labeled-line organization of sympathetic outflow for selective organ regulation in mice

The sympathetic nervous system is crucial for responding to environmental changes. This regulation is coordinated by the spinal sympathetic preganglionic neurons (SPNs), innervating both postganglionic neurons and the adrenal gland. Despite decades of research supporting the concept of selective control within this system, the neural circuit organization responsible for the output specificity remains poorly understood. Here, by combining recent single-cell transcriptome data with viral-genetic toolkits in mice, we identify two subtypes of SPNs in the lower thoracic spinal cord, defined at the molecular level, exhibiting nonoverlapping patterns of innervation: one specifically projecting to the celiac/superior mesenteric ganglia, and the other targeting the adrenal grand. Chemogenetic manipulations on these distinct SPN subtypes revealed selective impacts on the motility of the gastrointestinal tracts or glucose metabolism mediated by the adrenal gland, respectively. This molecularly delineated parallel labeled-line organization in sympathetic outflows presents a potential avenue for selectively manipulating organ functions.

TRPV1: A novel target for the therapy of diabetes and diabetic complications

BACKGROUND

Diabetes mellitus is a chronic metabolic disease characterized by abnormally elevated blood glucose levels. Type II diabetes accounts for approximately 90% of all cases. Several drugs are available for hyperglycemia treatment. However, the current therapies for managing high blood glucose do not prevent or reverse the disease progression, which may result in complications and adverse effects, including diabetic neuropathy, retinopathy, and nephropathy. Hence, developing safer and more effective methods for lowering blood glucose levels is imperative. Transient receptor potential vanilloid-1 (TRPV1) is a significant member of the transient receptor potential family. It is present in numerous body tissues and organs and performs vital physiological functions.

PURPOSE

This review aimed to develop new targeted TRPV1 hypoglycemic drugs by systematically summarizing the mechanism of action of the TRPV1-based signaling pathway in preventing and treating diabetes and its complications.

METHODS

Literature searches were performed in the PubMed, Web of Science, Google Scholar, Medline, and Scopus databases for 10 years from 2013 to 2023. The search terms included "diabetes," "TRPV1," "diabetic complications," and "capsaicin."

RESULTS

TRPV1 is an essential potential target for treating diabetes mellitus and its complications. It reduces hepatic glucose production and food intake and promotes thermogenesis, metabolism, and insulin secretion. Activation of TRPV1 ameliorates diabetic nephropathy, retinopathy, myocardial infarction, vascular endothelial dysfunction, gastroparesis, and bladder dysfunction. Suppression of TRPV1 improves diabetes-related osteoporosis. However, the therapeutic effects of activating or suppressing TRPV1 may vary when treating diabetic neuropathy and periodontitis.

CONCLUSION

This review demonstrates that TRPV1 is a potential therapeutic target for diabetes and its complications. Additionally, it provides a theoretical basis for developing new hypoglycemic drugs that target TRPV1.

Plasma oxytocin levels in response to glucagon in patients with arginine vasopressin deficiency (central diabetes insipidus) and healthy controls

PURPOSE

We recently demonstrated an additional oxytocin (OT) deficiency in patients with arginine vasopressin (AVP) deficiency (central diabetes insipidus) by using 3,4-methylenedioxy-methamphetamine (MDMA) as a novel provocation test. However, the implication of the MDMA provocation test in clinical practice might be challenging. Glucagon effectively stimulates vasopressinergic neurons with a strong increase in plasma copeptin. We therefore hypothesized that this provocation test might also stimulate OT.

METHODS

This is a predefined secondary analysis of a prospective double-blind, randomised, placebo-controlled cross-over trial involving ten patients with AVP deficiency and ten sex- and body-mass index-matched healthy participants at the University Hospital Basel, Switzerland. Each participant underwent the glucagon test (s.c. injection of 1 mg glucagon) and placebo test (s.c. injection of 0.9% normal saline). Plasma OT levels were measured at baseline, 60, 120 and 180 min after injection. The primary objective was to determine whether glucagon stimulates OT and whether OT levels differ between patients with AVP deficiency and healthy participants. The primary outcome (maximum change in OT within 180 min) was compared between groups and conditions using a linear mixed effects model.

RESULTS

In healthy participants, the median OT at baseline was 82.7 pg/ml [62.3-94.3] and slightly increased to a maximum of 93.3 pg/ml [87.2-121.1] after injection of glucagon, resulting in a change increase of 24.9 pg/ml [5.1-27.8]. Similarly, in patients with AVP deficiency, the median OT at baseline was 73.9 pg/ml [65.3-81.6] and slightly increased after glucagon injection to 114.9 pg/ml [70.9-140.9], resulting in a change increase of 36.8 pg/ml [-2.2 to 51.2]. The results from the mixed model showed no effect between glucagon compared to placebo on OT (difference: -0.5 pg/ml; 95%-CI [-25, 24]; p = 0.97) and no significant treatment-by-group interaction effect between patients compared to healthy participants (interaction: 28 pg/ml; 95%-CI [-7, 62]; p = 0.13).

CONCLUSION

We found no effect of glucagon on plasma OT levels and no difference between patients with AVP deficiency and healthy participants.

The area postrema: a critical mediator of brain-body interactions

The dorsal vagal complex contains three structures: the area postrema, the nucleus tractus solitarii, and the dorsal motor nucleus of the vagus. These structures are tightly linked, both anatomically and functionally, and have important yet distinct roles in not only conveying peripheral bodily signals to the rest of the brain but in the generation of behavioral and physiological responses. Reports on the new discoveries in these structures were highlights of the symposium. In this outlook, we focus on the roles of the area postrema in mediating brain-body interactions and its potential utility as a therapeutic target, especially in cancer cachexia.

Neuronal glucose sensing mechanisms and circuits in the control of insulin and glucagon secretion

Glucose homeostasis is mainly under the control of the pancreatic islet hormones insulin and glucagon, which, respectively, stimulate glucose uptake and utilization by liver, fat, and muscle and glucose production by the liver. The balance between the secretions of these hormones is under the control of blood glucose concentrations. Indeed, pancreatic islet β-cells and α-cells can sense variations in glycemia and respond by an appropriate secretory response. However, the secretory activity of these cells is also under multiple additional metabolic, hormonal, and neuronal signals that combine to ensure the perfect control of glycemia over a lifetime. The central nervous system (CNS), which has an almost absolute requirement for glucose as a source of metabolic energy and thus a vital interest in ensuring that glycemic levels never fall below ∼5 mM, is equipped with populations of neurons responsive to changes in glucose concentrations. These neurons control pancreatic islet cell secretion activity in multiple ways: through both branches of the autonomic nervous system, through the hypothalamic-pituitary-adrenal axis, and by secreting vasopressin (AVP) in the blood at the level of the posterior pituitary. Here, we present the autonomic innervation of the pancreatic islets; the mechanisms of neuron activation by a rise or a fall in glucose concentration; how current viral tracing, chemogenetic, and optogenetic techniques allow integration of specific glucose sensing neurons in defined neuronal circuits that control endocrine pancreas function; and, finally, how genetic screens in mice can untangle the diversity of the hypothalamic mechanisms controlling the response to hypoglycemia.

Oxytocin does not acutely improve glucose tolerance in men with type 2 diabetes

AIM

To assess oxytocin's acute glucoregulatory impact in men with type 2 diabetes in the context of our previous findings that oxytocin improves β-cell responsivity in healthy men.

METHODS

In a double-blind, crossover comparison, intranasal oxytocin (24 IU) and placebo, respectively, were administered to 25 fasted men with non-insulin-treated type 2 diabetes (age ± standard error of the mean, 63.40 ± 1.36 years; body mass index, 27.77 ± 0.66 kg/m2; HbA1c, 6.86% ± 0.08%; Homeostatic Model Assessment of Insulin Resistance (HOMA-IR, 3.44 ± 0.39) 60 minutes before an oral glucose tolerance test (oGTT). Key outcomes were compared with previous results in men with normal weight or obesity.

RESULTS

Oxytocin compared with placebo increased plasma oxytocin concentrations and reduced the heart rate, but did not alter glucose metabolism in the 3 hours after oGTT onset (area under the curve, glucose, 2240 ± 80.5 vs. 2190 ± 69.5 mmol/L × min; insulin, 45 663 ± 4538 vs. 44 343 ± 4269 pmol/L × min; C-peptide, 235 ± 5.1 vs. 231 ± 15.9 nmol/L × min).

CONCLUSIONS

This outcome contrasts with the oxytocin-induced attenuation of early postprandial glucose excursions in normal-weight individuals, but is in line with the absence of respective effects in men with obesity. We conclude that insulin resistance in type 2 diabetes is associated with decreased sensitivity to the acute glucoregulatory effect of oxytocin in male individuals.

Research advances in understanding crosstalk between organs and pancreatic β-cell dysfunction

Obesity has increased dramatically worldwide. Being overweight or obese can lead to various conditions, including dyslipidaemia, hypertension, glucose intolerance and metabolic syndrome (MetS), which may further lead to type 2 diabetes mellitus (T2DM). Previous studies have identified a link between β-cell dysfunction and the severity of MetS, with multiple organs and tissues affected. Identifying the associations between pancreatic β-cell dysfunction and organs is critical. Research has focused on the interaction between the liver, gut and pancreatic β-cells. However, the mechanisms and related core targets are still not perfectly elucidated. The aims of this review were to summarize the mechanisms of β-cell dysfunction and to explore the potential pathogenic pathways and targets that connect the liver, gut, adipose tissue, muscle, and brain to pancreatic β-cell dysfunction.

Acetylome Analyses Provide New Insights into the Effect of Chronic Intermittent Hypoxia on Hypothalamus-Dependent Endocrine Metabolism Impairment

Paediatric obstructive sleep apnoea (OSA) is a highly prevalent sleep disorder resulting in chronic intermittent hypoxia (CIH) that has been linked to metabolism and endocrine impairment. Protein acetylation, which is a frequently occurring posttranslational modification, plays pivotal roles in the regulation of hypothalamic processes. However, the effects of CIH-induced global protein acetylation on hypothalamic function and endocrine metabolism remain poorly understood. To bridge this knowledge gap, we conducted a study utilizing liquid chromatography-mass spectrometry to analyse the lysine acetylome and proteome of the hypothalamus in healthy infantile mice exposed to 3 weeks of intermittent hypoxia (as a CIH model) compared to normoxic mice (as controls). Our analysis identified and quantified 2699 Kac sites in 2453 proteins. These acetylated proteins exhibited disruptions primarily in endocrine metabolism, the citrate cycle (TCA cycle), synapse function, and circadian entrainment. Additionally, we observed significant down-regulation of proteins that are known to be involved in endocrine hormone secretion. This study aimed to elucidate the molecular mechanisms underlying CIH-induced alterations in protein acetylation within the hypothalamus. By providing valuable insights into the pathophysiological processes associated with CIH and their impacts on hypothalamic function, our findings contribute to a deeper understanding of the consequences stemming from CIH-induced changes in protein acetylation within the hypothalamus as well as its potential role in endocrine impairment.

Microglia mediate the early-life programming of adult glucose control

Mammalian glucose homeostasis is, in part, nutritionally programmed during early neonatal life, a critical window for the formation of synapses between hypothalamic glucoregulatory centers. Although microglia are known to prune synapses throughout the brain, their specific role in refining hypothalamic glucoregulatory circuits remains unknown. Here, we show that microglia in the mediobasal hypothalamus (MBH) of mice actively engage in synaptic pruning during early life. Microglial phagocytic activity is induced following birth, regresses upon weaning from maternal milk, and is exacerbated by feeding dams a high-fat diet while lactating. In particular, we show that microglia refine perineuronal nets (PNNs) within the neonatal MBH. Indeed, transiently depleting microglia before weaning (P6-16), but not afterward (P21-31), remarkably increased PNN abundance in the MBH. Furthermore, mice lacking microglia only from P6-16 had glucose intolerance due to impaired glucose-responsive pancreatic insulin secretion in adulthood, a phenotype not seen if microglial depletion occurred after weaning. Viral retrograde tracing revealed that this impairment is linked to a reduction in the number of neurons in specific hypothalamic glucoregulatory centers that synaptically connect to the pancreatic β-cell compartment. These findings show that microglia facilitate synaptic plasticity in the MBH during early life through a process that includes PNN refinement, to establish hypothalamic circuits that regulate adult glucose homeostasis.

Integrative Functions of the Hypothalamus: Linking Cognition, Emotion and Physiology for Well-being and Adaptability

Background

The hypothalamus, a small yet crucial neuroanatomical structure, integrates external (e.g., environmental) and internal (e.g., physiological/hormonal) stimuli. This integration governs various physiological processes and influences cognitive, emotional, and behavioral outcomes. It serves as a functional bridge between the nervous and endocrine systems, maintaining homeostasis and coordinating bodily functions.

Summary

Recent advancements in the neurobiology of the hypothalamus have elucidated its functional map, establishing a causal relationship between its responses-such as respiration, sleep, and stress-and various physiological processes. The hypothalamus facilitates and coordinates these complex processes by processing diverse stimuli, enabling the body to maintain internal balance and respond effectively to external demands. This review delves into the hypothalamus's intricate connections with cognition, emotion, and physiology, exploring how these interactions promote overall well-being and adaptability.

Key Message

Targeted external stimuli can modulate hypothalamic neuronal activities, impacting the physiological, cognitive, and emotional landscape. The review highlights non-invasive techniques, such as controlled breathing exercises, optimized sleep architecture, and stress management, as potential methods to enhance hypothalamic function. Ultimately, this comprehensive review underscores the multifaceted role of the hypothalamus in integrating signals, maintaining homeostasis, and influencing cognition, emotion, and physiology.

Could Insulin Be a Better Regulator of Appetite/Satiety Balance and Body Weight Maintenance in Response to Glucose Exposure Compared to Sucrose Substitutes? Unraveling Current Knowledge and Searching for More Appropriate Choices

BACKGROUND

Insulin exerts a crucial impact on glucose control, cellular growing, function, and metabolism. It is partially modulated by nutrients, especially as a response to the intake of foods, including carbohydrates. Moreover, insulin can exert an anorexigenic effect when inserted into the hypothalamus of the brain, in which a complex network of an appetite/hunger control system occurs. The current literature review aims at thoroughly summarizing and scrutinizing whether insulin release in response to glucose exposure may be a better choice to control body weight gain and related diseases compared to the use of sucrose substitutes (SSs) in combination with a long-term, well-balanced diet.

METHODS

This is a comprehensive literature review, which was performed through searching in-depth for the most accurate scientific databases and applying effective and relevant keywords.

RESULTS

The insulin action can be inserted into the hypothalamic orexigenic/anorexigenic complex system, activating several anorexigenic peptides, increasing the hedonic aspect of food intake, and effectively controlling the human body weight. In contrast, SSs appear not to affect the orexigenic/anorexigenic complex system, resulting in more cases of uncontrolled body weight maintenance while also increasing the risk of developing related diseases.

CONCLUSIONS

Most evidence, mainly derived from in vitro and in vivo animal studies, has reinforced the insulin anorexigenic action in the hypothalamus of the brain. Simultaneously, most available clinical studies showed that SSs during a well-balanced diet either maintain or even increase body weight, which may indirectly be ascribed to the fact that they cannot cover the hedonic aspect of food intake. However, there is a strong demand for long-term longitudinal surveys to effectively specify the impact of SSs on human metabolic health.

Area postrema neurons mediate interleukin-6 function in cancer cachexia

Interleukin-6 (IL-6) has been long considered a key player in cancer cachexia. It is believed that sustained elevation of IL-6 production during cancer progression causes brain dysfunctions, which ultimately result in cachexia. However, how peripheral IL-6 influences the brain remains poorly understood. Here we show that neurons in the area postrema (AP), a circumventricular structure in the hindbrain, is a critical mediator of IL-6 function in cancer cachexia in male mice. We find that circulating IL-6 can rapidly enter the AP and activate neurons in the AP and its associated network. Peripheral tumor, known to increase circulating IL-6, leads to elevated IL-6 in the AP, and causes potentiated excitatory synaptic transmission onto AP neurons and AP network hyperactivity. Remarkably, neutralization of IL-6 in the brain of tumor-bearing mice with an anti-IL-6 antibody attenuates cachexia and the hyperactivity in the AP network, and markedly prolongs lifespan. Furthermore, suppression of Il6ra, the gene encoding IL-6 receptor, specifically in AP neurons with CRISPR/dCas9 interference achieves similar effects. Silencing Gfral-expressing AP neurons also attenuates cancer cachectic phenotypes and AP network hyperactivity. Our study identifies a central mechanism underlying the function of peripheral IL-6, which may serve as a target for treating cancer cachexia.

Control of Physiologic Glucose Homeostasis via the Hypothalamic Modulation of Gluconeogenic Substrate Availability

The brain augments glucose production during fasting, but the mechanisms are poorly understood. Here, we show that Cckbr-expressing neurons in the ventromedial hypothalamic nucleus (VMNCckbr cells) prevent low blood glucose during fasting through sympathetic nervous system (SNS)-mediated augmentation of adipose tissue lipolysis and substrate release. Activating VMNCckbr neurons mobilized gluconeogenic substrates without altering glycogenolysis or gluconeogenic enzyme expression. Silencing these cells (CckbrTetTox animals) reduced fasting blood glucose, impaired lipolysis, and decreased circulating glycerol (but not other gluconeogenic substrates) despite normal insulin, counterregulatory hormones, liver glycogen, and liver gluconeogenic gene expression. Furthermore, β3-adrenergic adipose tissue stimulation in CckbrTetTox animals restored lipolysis and blood glucose. Hence, VMNCckbr neurons impact blood glucose not by controlling islet or liver physiology, but rather by mobilizing gluconeogenic substrates. These findings establish a central role for hypothalamic and SNS signaling during normal glucose homeostasis and highlight the importance of gluconeogenic substrate mobilization during physiologic fasting.

Investigating the role of an immediate early gene FOS as a potential regulator of autophagic response to hypoglycemia in embryonic hypothalamic neurons

Hypoglycemia-associated autonomic failure (HAAF) is a well-established complication of diabetes. Although HAAF has serious outcomes such as recurrent morbidity, coma, and death, the mechanisms of HAAF and its pathological components are largely unknown. Our previous studies have revealed that hypoglycemia is associated with the upregulation of an immediate early gene - FOS. In addition, it is documented that glucose deprivation activates neuronal autophagic activities. Therefore, the present study aimed to identify the role of FOS and one of the core components of the autophagy pathway, Beclin-1 (encoded by the BECN1 gene), in the regulation of autophagic mechanisms in embryonic hypothalamic neurons in response to hypoglycemic conditions. Embryonic Mouse Hypothalamic Cell Line N39 (mHypoE-N39 or N39) was cultured in reduced concentrations of glucose (2000, 900, 500, and 200 mg/L). Gene and protein expression, as well as immunofluorescence studies on autophagy were conducted under different reduced glucose concentrations in N39 hypothalamic neurons with and without FOS and BECN1 gene knockdowns (KD). The outcomes of the present study have demonstrated a significant increase in autophagosome formation and subsequent lysosomal degradation in the hypothalamic neurons in response to reduced glucose concentrations. This hypoglycemic response appears to be lowered to a similar extent in the FOS KD and BECN1 KD cells, albeit insignificantly from the negative control, is indicative of the involvement of FOS in the autophagic response of hypothalamic neurons to hypoglycemia. Moreover, the KD cells exhibited a change in morphology and reduced cell viability compared with the control cells. Our findings suggest that reduced FOS expression could potentially be associated with impaired autophagic activities that are dependent on BECN1, which could lead to decreased or blunted hypothalamic activation in response to hypoglycemia, and this, in turn, may contribute to the development of HAAF.

Souza L, Worker CJ, Reyes Mendez ME, Gayban AJB, Cooper SG, Sanchez Solano A, Bergman RN, Stefanovski D, Morton GJ, Schwartz MW, Feng Earley Y. (Pro)renin receptor signaling in hypothalamic tyrosine hydroxylase neurons is required for obesity-associated glucose metabolic impairment

Glucose homeostasis is achieved via complex interactions between the endocrine pancreas and other peripheral tissues and glucoregulatory neurocircuits in the brain that remain incompletely defined. Within the brain, neurons in the hypothalamus appear to play a particularly important role. Consistent with this notion, we report evidence that (pro)renin receptor (PRR) signaling within a subset of tyrosine hydroxylase (TH) neurons located in the hypothalamic paraventricular nucleus (PVNTH neurons) is a physiological determinant of the defended blood glucose level. Specifically, we demonstrate that PRR deletion from PVNTH neurons restores normal glucose homeostasis in mice with diet-induced obesity (DIO). Conversely, chemogenetic inhibition of PVNTH neurons mimics the deleterious effect of DIO on glucose. Combined with our finding that PRR activation inhibits PVNTH neurons, these findings suggest that, in mice, (a) PVNTH neurons play a physiological role in glucose homeostasis, (b) PRR activation impairs glucose homeostasis by inhibiting these neurons, and (c) this mechanism plays a causal role in obesity-associated metabolic impairment.

Region-specific transcriptomic responses to obesity and diabetes in macaque hypothalamus

The hypothalamus plays a crucial role in the progression of obesity and diabetes; however, its structural complexity and cellular heterogeneity impede targeted treatments. Here, we profiled the single-cell and spatial transcriptome of the hypothalamus in obese and sporadic type 2 diabetic macaques, revealing primate-specific distributions of clusters and genes as well as spatial region, cell-type-, and gene-feature-specific changes. The infundibular (INF) and paraventricular nuclei (PVN) are most susceptible to metabolic disruption, with the PVN being more sensitive to diabetes. In the INF, obesity results in reduced synaptic plasticity and energy sensing capability, whereas diabetes involves molecular reprogramming associated with impaired tanycytic barriers, activated microglia, and neuronal inflammatory response. In the PVN, cellular metabolism and neural activity are suppressed in diabetic macaques. Spatial transcriptomic data reveal microglia's preference for the parenchyma over the third ventricle in diabetes. Our findings provide a comprehensive view of molecular changes associated with obesity and diabetes.

Attention to Innate Circadian Rhythm and the Impact of Its Disruption on Diabetes

Novel strategies are required to reduce the risk of developing diabetes and/or clinical outcomes and complications of diabetes. In this regard, the role of the circadian system may be a potential candidate for the prevention of diabetes. We reviewed evidence from animal, clinical, and epidemiological studies linking the circadian system to various aspects of the pathophysiology and clinical outcomes of diabetes. The circadian clock governs genetic, metabolic, hormonal, and behavioral signals in anticipation of cyclic 24-hour events through interactions between a "central clock" in the suprachiasmatic nucleus and "peripheral clocks" in the whole body. Currently, circadian rhythmicity in humans can be subjectively or objectively assessed by measuring melatonin and glucocorticoid levels, core body temperature, peripheral blood, oral mucosa, hair follicles, rest-activity cycles, sleep diaries, and circadian chronotypes. In this review, we summarized various circadian misalignments, such as altered light-dark, sleep-wake, rest-activity, fasting-feeding, shift work, evening chronotype, and social jetlag, as well as mutations in clock genes that could contribute to the development of diabetes and poor glycemic status in patients with diabetes. Targeting critical components of the circadian system could deliver potential candidates for the treatment and prevention of type 2 diabetes mellitus in the future.

The Role of Central Oxytocin in Autonomic Regulation

Oxytocin (OXT), a neuropeptide originating from the hypothalamus and traditionally associated with peripheral functions in parturition and lactation, has emerged as a pivotal player in the central regulation of the autonomic nervous system (ANS). This comprehensive ANS, comprising sympathetic, parasympathetic, and enteric components, intricately combines sympathetic and parasympathetic influences to provide unified control. The central oversight of sympathetic and parasympathetic outputs involves a network of interconnected regions spanning the neuroaxis, playing a pivotal role in the real-time regulation of visceral function, homeostasis, and adaptation to challenges. This review unveils the significant involvement of the central OXT system in modulating autonomic functions, shedding light on diverse subpopulations of OXT neurons within the paraventricular nucleus of the hypothalamus and their intricate projections. The narrative progresses from the basics of central ANS regulation to a detailed discussion of the central controls of sympathetic and parasympathetic outflows. The subsequent segment focuses specifically on the central OXT system, providing a foundation for exploring the central role of OXT in ANS regulation. This review synthesizes current knowledge, paving the way for future research endeavors to unravel the full scope of autonomic control and understand multifaceted impact of OXT on physiological outcomes.

Virus-Based Neural Circuit Tracing

Neural circuits provide an anatomical basis for functional networks. Therefore, dissecting the structure of neural circuits is an indispensable prerequisite to understanding how the brain functions. Knowing how the neural circuits organize and function under physiological conditions and their progressive alterations under pathophysiological conditions are key to understanding the underlying circuit mechanism of diseases, thus finding cures for the diseases. Recombinant neurotropic viruses are important tools for neural circuit tracing with many advantages over non-viral tracers: they allow for anterograde, retrograde, and trans-synaptic delivery of tracers in a cell-type-specific, circuit-selective manner. We herein summarize the recent developments in the viral tools for neural circuit tracing, discuss the key principles of using viral tools in neuroscience research, and highlight innovations for developing and optimizing viral tools for neural circuit tracing across diverse animal species, including nonhuman primates.

Analysis of the management and therapeutic performance of diabetes mellitus employing special target

Diabetes mellitus (DM) is a chronic metabolic condition characterized predominantly by hyperglycemia. The most common causes contributing to the pathophysiology of diabetes are insufficient insulin secretion, resistance to insulin's tissue-acting effects, or a combination of both. Over the last 30 years, the global prevalence of diabetes increased from 4% to 6.4%. If no better treatment or cure is found, this amount might climb to 430 million in the coming years. The major factors of the disease's deterioration include age, obesity, and a sedentary lifestyle. Finding new therapies to manage diabetes safely and effectively without jeopardizing patient compliance has always been essential. Among the medications available to manage DM on this journey are glucagon-like peptide-1 agonists, thiazolidinediones, sulphonyl urease, glinides, biguanides, and insulin-targeting receptors discovered more than 10 years ago. Despite the extensive preliminary studies, a few clinical observations suggest this process is still in its early stages. The present review focuses on targets that contribute to insulin regulation and may be employed as targets in treating diabetes since they may be more efficient and secure than current and traditional treatments.

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.

Neurotransmitters regulate β cells insulin secretion: A neglected factor

β cells are the main cells responsible for the hypoglycemic function of pancreatic islets, and the insulin secreted by these cells is the only hormone that lowers blood glucose levels in the human body. β cells are regulated by various factors, among which neurotransmitters make an important contribution. This paper discusses the effects of neurotransmitters secreted by various sympathetic and parasympathetic nerves on β cells and summarizes the mechanisms by which various neurotransmitters regulate insulin secretion. Many neurotransmitters do not have a single source and are not only released from nerve terminals but also synthesized by β cells themselves, allowing them to synergistically regulate insulin secretion. Almost all of these neurotransmitters depend on the presence of glucose to function, and their actions are mostly related to the Ca2+ and cAMP concentrations. Although neurotransmitters have been extensively studied, many of their mechanisms remain unclear and require further exploration by researchers.

Neurochemical Basis of Inter-Organ Crosstalk in Health and Obesity: Focus on the Hypothalamus and the Brainstem

Precise neural regulation is required for maintenance of energy homeostasis. Essential to this are the hypothalamic and brainstem nuclei which are located adjacent and supra-adjacent to the circumventricular organs. They comprise multiple distinct neuronal populations which receive inputs not only from other brain regions, but also from circulating signals such as hormones, nutrients, metabolites and postprandial signals. Hence, they are ideally placed to exert a multi-tier control over metabolism. The neuronal sub-populations present in these key metabolically relevant nuclei regulate various facets of energy balance which includes appetite/satiety control, substrate utilization by peripheral organs and glucose homeostasis. In situations of heightened energy demand or excess, they maintain energy homeostasis by restoring the balance between energy intake and expenditure. While research on the metabolic role of the central nervous system has progressed rapidly, the neural circuitry and molecular mechanisms involved in regulating distinct metabolic functions have only gained traction in the last few decades. The focus of this review is to provide an updated summary of the mechanisms by which the various neuronal subpopulations, mainly located in the hypothalamus and the brainstem, regulate key metabolic functions.

Reduction of oxytocin-containing neurons and enhanced glymphatic activity in the hypothalamic paraventricular nucleus of patients with type 2 diabetes mellitus

Evidence from animal experiments has shown that the hypothalamic paraventricular nucleus (PVN) plays a key role in regulating body weight and blood glucose levels. However, it is unclear whether neuron populations in the human PVN are involved in the development of type 2 diabetes mellitus (T2DM). To address this, we investigated the neuronal and glial populations in the PVN of 26 T2DM patients and 20 matched controls. Our findings revealed a significant reduction in oxytocin (Oxt) neuron density in the PVN of T2DM patients compared to controls, while other neuronal populations remained unchanged. This suggests that Oxt neurons may play a specific role in the pathophysiology of T2DM. Interestingly, the reduction in Oxt neurons was accompanied by a decreased melanocortinergic input in to the PVN as reflected by a reduction in alpha-MSH immunoreactivity. We also analysed two glial cell populations, as they are important for maintaining a healthy neural microenvironment. We found that microglial density, phagocytic capacity, and their proximity to neurons were not altered in T2DM patients, indicating that the loss of Oxt neurons is independent of changes in microglial immunity. However, we did observe a reduction in the number of astrocytes, which are crucial for providing trophic support to local neurons. Moreover, a specific subpopulation of astrocytes characterized by aquaporin 4 expression was overrepresented in T2DM patients. Since this subset of astrocytes is linked to the glymphatic system, their overrepresentation might point to alterations in the hypothalamic waste clearance system in T2DM. Our study shows selective loss of Oxt neurons in the PVN of T2DM individuals in association with astrocytic reduction and gliovascular remodelling. Therefore, hypothalamic Oxt neurons may represent a potential target for T2DM treatment modalities.

Different sensitivity to diet-induced hyperinsulinemia and hyperglycemia between mice with global or bone marrow-specific apoE receptor-2 deficiency

This study explored the role of apoE receptor-2 (apoER2), a unique member of the LDL receptor family proteins with a restricted tissue expression profile, in modulating diet-induced obesity and diabetes. Unlike wild-type mice and humans in which chronic feeding of a high-fat Western-type diet leads to obesity and the prediabetic state of hyperinsulinemia before hyperglycemia onset, the Lrp8-/- mice with global apoER2 deficiency displayed lower body weight and adiposity, slower development of hyperinsulinemia, but the accelerated onset of hyperglycemia. Despite their lower adiposity, adipose tissues in Western diet-fed Lrp8-/- mice were more inflamed compared with wild-type mice. Additional experiments revealed that the hyperglycemia observed in Western diet-fed Lrp8-/- mice was due to impaired glucose-induced insulin secretion, ultimately leading to hyperglycemia, adipocyte dysfunction, and inflammation upon chronic feeding of the Western diet. Interestingly, bone marrow-specific apoER2-deficient mice were not defective in insulin secretion, exhibiting increased adiposity and hyperinsulinemia compared with wild-type mice. Analysis of bone marrow-derived macrophages revealed that apoER2 deficiency impeded inflammation resolution with lower secretion of IFN-β and IL-10 in response to LPS stimulation of IL-4 primed cells. The apoER2-deficient macrophages also showed an increased level of disabled-2 (Dab2) as well as increased cell surface TLR4, suggesting that apoER2 participates in Dab2 regulation of TLR4 signaling. Taken together, these results showed that apoER2 deficiency in macrophages sustains diet-induced tissue inflammation and accelerates obesity and diabetes onset while apoER2 deficiency in other cell types contributes to hyperglycemia and inflammation via defective insulin secretion.

Prognostic significance of TMEM131L in glioma and establishment of oxidative stress prognostic model

Gliomas are the most aggressive of all brain tumors. In this study, it was found that there is a significant expression of transmembrane-like 131 (TMEM131L) in glioma tissues. The relevance of TMEM131L in the diagnosis and clinical prognosis of GBM and LGG was verified by additional clinical correlation and survival analysis. The area under the curve (AUC) of the receiver operating characteristic (ROC) curve reflected the diagnostic effect of TMEM131L on the clinicopathologic features of glioma. As a unique molecular marker for the poor prognosis of overall survival (OS), PFI, and DSS in patients with GCB and LGG, TMEM131L might be employed, according to time-dependent ROC curves and Kaplan–Meier survival analysis at 1, 3, and 5 years. The potential methylation sites of TMEM131L were selected by correlation analysis between TMEM131L and DNA methylation sites. Meanwhile, TMEM131L was significantly correlated with matrix, immunity, and estimated scores of GBM and LGG. The CIBERSORT analysis revealed a significant correlation between immune checkpoint and infiltration of 22 different kinds of immune cells. Coexpression genes of TMEM131L associated with oxidative stress phenotype were screened by the LASSO logistic regression analysis. Nomogram and calibration curves further confirmed that the prognostic model composed of SYT1, CREB3L3, ITPR1, RASGRF2, PDX1, and RASGRF1 has good stability and potential application value for poor prognosis in patients with glioma.

The elusive cephalic phase insulin response: triggers, mechanisms, and functions

The cephalic phase insulin response (CPIR) is classically defined as a head receptor-induced early release of insulin during eating that precedes a postabsorptive rise in blood glucose. Here we discuss, first, the various stimuli that elicit the CPIR and the sensory signaling pathways (sensory limb) involved; second, the efferent pathways that control the various endocrine events associated with eating (motor limb); and third, what is known about the central integrative processes linking the sensory and motor limbs. Fourth, in doing so, we identify open questions and problems with respect to the CPIR in general. Specifically, we consider test conditions that allow, or may not allow, the stimulus to reach the potentially relevant taste receptors and to trigger a CPIR. The possible significance of sweetness and palatability as crucial stimulus features and whether conditioning plays a role in the CPIR are also discussed. Moreover, we ponder the utility of the strict classical CPIR definition based on what is known about the effects of vagal motor neuron activation and thereby acetylcholine on the β-cells, together with the difficulties of the accurate assessment of insulin release. Finally, we weigh the evidence of the physiological and clinical relevance of the cephalic contribution to the release of insulin that occurs during and after a meal. These points are critical for the interpretation of the existing data, and they support a sharper focus on the role of head receptors in the overall insulin response to eating rather than relying solely on the classical CPIR definition.

Glycemic Challenge Is Associated with the Rapid Cellular Activation of the Locus Ceruleus and Nucleus of Solitary Tract: Circumscribed Spatial Analysis of Phosphorylated MAP Kinase Immunoreactivity

Rodent studies indicate that impaired glucose utilization or hypoglycemia is associated with the cellular activation of neurons in the medulla (Winslow, 1733) (MY), believed to control feeding behavior and glucose counterregulation. However, such activation has been tracked primarily within hours of the challenge, rather than sooner, and has been poorly mapped within standardized brain atlases. Here, we report that, within 15 min of receiving 2-deoxy-d-glucose (2-DG; 250 mg/kg, i.v.), which can trigger glucoprivic feeding behavior, marked elevations were observed in the numbers of rhombic brain (His, 1893) (RB) neuronal cell profiles immunoreactive for the cellular activation marker(s), phosphorylated p44/42 MAP kinases (phospho-ERK1/2), and that some of these profiles were also catecholaminergic. We mapped their distributions within an open-access rat brain atlas and found that 2-DG-treated rats (compared to their saline-treated controls) displayed greater numbers of phospho-ERK1/2+ neurons in the locus ceruleus (Wenzel and Wenzel, 1812) (LC) and the nucleus of solitary tract (>1840) (NTS). Thus, the 2-DG-activation of certain RB neurons is more rapid than perhaps previously realized, engaging neurons that serve multiple functional systems and which are of varying cellular phenotypes. Mapping these populations within standardized brain atlas maps streamlines their targeting and/or comparable mapping in preclinical rodent models of disease.

Area postrema neurons mediate interleukin-6 function in cancer-associated cachexia

Interleukin-6 (IL-6) has been long considered a key player in cancer-associated cachexia 1-15 . It is believed that sustained elevation of IL-6 production during cancer progression causes brain dysfunctions, which ultimately result in cachexia 16-20 . However, how peripheral IL-6 influences the brain remains poorly understood. Here we show that neurons in the area postrema (AP), a circumventricular structure in the hindbrain, mediate the function of IL-6 in cancer-associated cachexia in mice. We found that circulating IL-6 can rapidly enter the AP and activate AP neurons. Peripheral tumor, known to increase circulating IL-6 1-5,15,18,21-23 , leads to elevated IL-6 and neuronal hyperactivity in the AP, and causes potentiated excitatory synaptic transmission onto AP neurons. Remarkably, neutralization of IL-6 in the brain of tumor-bearing mice with an IL-6 antibody prevents cachexia, reduces the hyperactivity in an AP network, and markedly prolongs lifespan. Furthermore, suppression of Il6ra , the gene encoding IL-6 receptor, specifically in AP neurons with CRISPR/dCas9 interference achieves similar effects. Silencing of Gfral-expressing AP neurons also ameliorates the cancer-associated cachectic phenotypes and AP network hyperactivity. Our study identifies a central mechanism underlying the function of peripheral IL-6, which may serve as a target for treating cancer-associated cachexia.

Hypothalamic Hnscr regulates glucose balance by mediating central inflammation and insulin signal

OBJECTIVES

Hypothalamic dysfunction leads to glucose metabolic imbalance; however, the mechanisms still need clarification. Our current study was to explore the role of hypothalamic Hnscr in glucose metabolism.

MATERIALS AND METHODS

Using Hnscr knockout or htNSC-specific Hnscr overexpression mice, we evaluated the effects of Hnscr on glucose metabolism through GTTs, ITTs, serum indicator measurements, etc. Immunofluorescence staining and Western blotting were performed to test inflammation levels and insulin signalling in hypothalamus. Conditioned medium intervene were used to investigate the effects of htNSCs on neuronal cell line. We also detected the glucose metabolism of mice with htNSCs implantation.

RESULTS

Hnscr expression decreased in the hypothalamus after high-fat diet feed. Hnscr-null mice displayed aggravated systematic insulin resistance, while mice with htNSC-specific Hnscr overexpression had the opposite phenotype. Notably, Hnscr-null mice had increased NF-κB signal in htNSCs, along with enhanced inflammation and damaged insulin signal in neurons located in arcuate nucleus of hypothalamus. The secretions, including sEVs, of Hnscr-deficient htNSCs mediated the detrimental effects on the CNS cell line. Locally implantation with Hnscr-depleted htNSCs disrupted glucose homeostasis.

CONCLUSIONS

This study demonstrated that decreased Hnscr in htNSCs led to systematic glucose imbalance through activating NF-κB signal and dampening insulin signal in hypothalamic neurons.

Paraventricular nucleus-Medullary interactions: How they help enable endocrine responses to metabolic stress

The paraventricular hypothalamic nucleus (PVH) organizes neuroendocrine and autonomic responses to rapidly and slowly developing metabolic stressors that limit their impact on energy balance. The PVH together with the lateral hypothalamic area, and the arcuate and dorsomedial nuclei form a network that is defined by its inputs from medullary catecholamine neurons. These medullary neurons convey important glycemia and glucocorticoid feedback information that is integrated by the PVH and the rest of this network to control a variety of responses to metabolic stressors that have rapid (hypoglycemia) or slow onsets (eating a high calorie diet). This review focuses on how the responses to these two challenges are enabled by these catecholamine neurons, and the integrative nature of the network into which they project.

Viral Tools for Neural Circuit Tracing

Neural circuits provide an anatomical basis for functional networks. Therefore, dissecting the structure of neural circuits is essential to understanding how the brain works. Recombinant neurotropic viruses are important tools for neural circuit tracing with many advantages over non-viral tracers: they allow for anterograde, retrograde, and trans-synaptic delivery of tracers in a cell type-specific, circuit-selective manner. In this review, we summarize the recent developments in the viral tools for neural circuit tracing, discuss the key principles of using viral tools in neuroscience research, and highlight innovations for developing and optimizing viral tools for neural circuit tracing across diverse animal species, including nonhuman primates.

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

The sympathetic nervous system maintains metabolic homeostasis by orchestrating the activity of organs such as the pancreas, liver, and white and brown adipose tissues. From the first renderings by Thomas Willis to contemporary techniques for visualization, tracing, and functional probing of axonal arborizations within organs, our understanding of the sympathetic nervous system has started to grow beyond classical models. In the present review, we outline the evolution of these findings and provide updated neuroanatomical maps of sympathetic innervation. We offer an autonomic framework for the neuroendocrine loop of leptin action, and we discuss the role of immune cells in regulating sympathetic terminals and metabolism. We highlight potential anti-obesity therapeutic approaches that emerge from the modern appreciation of SNS as a neural network vis a vis the historical fear of sympathomimetic pharmacology, while shifting focus from post- to pre-synaptic targeting. Finally, we critically appraise the field and where it needs to go.

Hypothalamic astrocytes control systemic glucose metabolism and energy balance

The hypothalamus is key in the control of energy balance. However, strategies targeting hypothalamic neurons have failed to provide viable options to treat most metabolic diseases. Conversely, the role of astrocytes in systemic metabolic control has remained largely unexplored. Here, we show that obesity promotes anatomically restricted remodeling of hypothalamic astrocyte activity. In the paraventricular nucleus (PVN) of the hypothalamus, chemogenetic manipulation of astrocytes results in bidirectional control of neighboring neuron activity, autonomic outflow, glucose metabolism, and energy balance. This process recruits a mechanism involving the astrocytic control of ambient glutamate levels, which becomes defective in obesity. Positive or negative chemogenetic manipulation of PVN astrocyte Ca2+ signals, respectively, worsens or improves metabolic status of diet-induced obese mice. Collectively, these findings highlight a yet unappreciated role for astrocytes in the direct control of systemic metabolism and suggest potential targets for anti-obesity strategy.

Role of the paraventricular nucleus of the hypothalamus in sleep–wake regulation

The paraventricular nucleus of the hypothalamus (PVH) acts as a cohesive functional unit that regulates neuroendocrine and autonomic function, complex behavior, and negative emotions after stress. However, how the PVH integrates arousal with these biological functions has only recently been explored. Clinical reports, combined with neurotoxic lesioning, immunochemistry, neuronal activity recordings, and the polysomnographic analyses of genetically modified animals, have revealed that the PVH is important for the control of wakefulness. Here, we review emerging anatomical and neural mechanisms for sleep–wake regulation in the PVH to support its essential role in the promotion and maintenance of wakefulness.

Circadian clock, diurnal glucose metabolic rhythm, and dawn phenomenon

The circadian clock provides cue-independent anticipatory signals for diurnal rhythms of baseline glucose levels and glucose tolerance. The central circadian clock is located in the hypothalamic suprachiasmatic nucleus (SCN), which comprises primarily GABAergic neurons. The SCN clock regulates physiological diurnal rhythms of endogenous glucose production (EGP) and hepatic insulin sensitivity through neurohumoral mechanisms. Disruption of the molecular circadian clock is associated with the extended dawn phenomenon (DP) in type 2 diabetes (T2D), referring to hyperglycemia in the early morning without nocturnal hypoglycemia. The DP affects nearly half of patients with diabetes, with poorly defined etiology and a lack of targeted therapy. Here we review neural and secreted factors in physiological diurnal rhythms of glucose metabolism and their pathological implications for the DP.

The ventromedial hypothalamic nucleus: watchdog of whole-body glucose homeostasis

The brain, particularly the ventromedial hypothalamic nucleus (VMH), has been long known for its involvement in glucose sensing and whole-body glucose homeostasis. However, it is still not fully understood how the brain detects and responds to the changes in the circulating glucose levels, as well as brain-body coordinated control of glucose homeostasis. In this review, we address the growing evidence implicating the brain in glucose homeostasis, especially in the contexts of hypoglycemia and diabetes. In addition to neurons, we emphasize the potential roles played by non-neuronal cells, as well as extracellular matrix in the hypothalamus in whole-body glucose homeostasis. Further, we review the ionic mechanisms by which glucose-sensing neurons sense fluctuations of ambient glucose levels. We also introduce the significant implications of heterogeneous neurons in the VMH upon glucose sensing and whole-body glucose homeostasis, in which sex difference is also addressed. Meanwhile, research gaps have also been identified, which necessities further mechanistic studies in future.

The roles of cell-cell and organ-organ crosstalk in the type 2 diabetes mellitus associated inflammatory microenvironment

Type 2 diabetes mellitus (T2DM) is a classic metaflammatory disease, and the inflammatory states of the pancreatic islet and insulin target organs have been well confirmed. However, abundant evidence demonstrates that there are countless connections between these organs in the presence of a low degree of inflammation. In this review, we focus on cell-cell crosstalk among local cells in the islet and organ-organ crosstalk among insulin-related organs. In contrast to that in acute inflammation, macrophages are the dominant immune cells causing inflammation in the islets and insulin target organs in T2DM. In the inflammatory microenvironment (IME) of the islet, cell-cell crosstalk involving local macrophage polarization and proinflammatory cytokine production impair insulin secretion by β-cells. Furthermore, organ-organ crosstalk, including the gut-brain-pancreas axis and interactions among insulin-related organs during inflammation, reduces insulin sensitivity and induces endocrine dysfunction. Therefore, this crosstalk ultimately results in a cascade leading to β-cell dysfunction. These findings could have broad implications for therapies aimed at treating T2DM.