Central Nervous System Control of Glucose Homeostasis: A Therapeutic Target for Type 2 Diabetes?

Scaffold-free endocrine tissue engineering: role of islet organization and implications in type 1 diabetes

Type 1 diabetes (T1D) is a chronic hyperglycemia disorder emerging from beta-cell (insulin secreting cells of the pancreas) targeted autoimmunity. As the blood glucose levels significantly increase and the insulin secretion is gradually lost, the entire body suffers from the complications. Although various advances in the insulin analogs, blood glucose monitoring and insulin application practices have been achieved in the last few decades, a cure for the disease is not obtained. Alternatively, pancreas/islet transplantation is an attractive therapeutic approach based on the patient prognosis, yet this treatment is also limited mainly by donor shortage, life-long use of immunosuppressive drugs and risk of disease transmission. In research and clinics, such drawbacks are addressed by the endocrine tissue engineering of the pancreas. One arm of this engineering is scaffold-free models which often utilize highly developed cell-cell junctions, soluble factors and 3D arrangement of islets with the cellular heterogeneity to prepare the transplant formulations. In this review, taking T1D as a model autoimmune disease, techniques to produce so-called pseudoislets and their applications are studied in detail with the aim of understanding the role of mimicry and pointing out the promising efforts which can be translated from benchside to bedside to achieve exogenous insulin-free patient treatment. Likewise, these developments in the pseudoislet formation are tools for the research to elucidate underlying mechanisms in pancreas (patho)biology, as platforms to screen drugs and to introduce immunoisolation barrier-based hybrid strategies.

Brain glucose metabolism: Role of nitric oxide

One possible reason for failure in achieving optimal glycemic control in patients with type 2 diabetes (T2D) is that less attention has been paid to the brain, a fundamental player in glucose homeostasis, that consumes about 25% of total glucose utilization. In addition, animal and human studies indicate that nitric oxide (NO) is a critical player in glucose metabolism. NO synthesis from L-arginine is lower in patients with T2D, and endothelial NO synthase (eNOS)-derived NO bioavailability is lower in T2D. NO in the nervous system plays a role in neurovascular coupling (NVC) and the hypothalamic control of glucose sensing and energy homeostasis, influencing glucose utilization. This review explores NO's role in the brain's glucose metabolism. Literature indicates that glucose metabolism is different between neurons and astrocytes. Unlike neurons, astrocytes have a higher rate of glycolysis and a greater ability for lactate production. Astrocytes produce a greater amount of NO than neurons. NO inhibits mitochondrial respiration in both neurons and astrocytes and decreases intracellular ATP. NO-induced inhibition of mitochondrial respiration in neurons is not accompanied by compensatory glycolysis because phosphofructokinase 2.3 (PFK2.3), the most potent activator of PFK1 and thus glycolysis, is subjected to ubiquitylation and proteasomal degradation by cadherin-1 (Cdh1)-activated anaphase-promoting complex/cyclosome (APC/C), which leads to a low glycolytic rate in neurons. In astrocytes, NO inhibits mitochondrial respiration, but astrocytes display compensatory glycolysis by activating the adenosine monophosphate (AMP)-activated protein kinase (AMPK) pathway.

Machine learning and molecular dynamics simulations predict potential TGR5 agonists for type 2 diabetes treatment

Introduction

Treatment of type 2 diabetes (T2D) remains a significant challenge because of its multifactorial nature and complex metabolic pathways. There is growing interest in finding new therapeutic targets that could lead to safer and more effective treatment options. Takeda G protein-coupled receptor 5 (TGR5) is a promising antidiabetic target that plays a key role in metabolic regulation, especially in glucose homeostasis and energy expenditure. TGR5 agonists are attractive candidates for T2D therapy because of their ability to improve glycemic control. This study used machine learning-based models (ML), molecular docking (MD), and molecular dynamics simulations (MDS) to explore novel small molecules as potential TGR5 agonists.

Methods

Bioactivity data for known TGR5 agonists were obtained from the ChEMBL database. The dataset was cleaned and molecular descriptors based on Lipinski's rule of five were selected as input features for the ML model, which was built using the Random Forest algorithm. The optimized ML model was used to screen the COCONUT database and predict potential TGR5 agonists based on their molecular features. 6,656 compounds predicted from the COCONUT database were docked within the active site of TGR5 to calculate their binding energies. The four top-scoring compounds with the lowest binding energies were selected and their activities were compared to those of the co-crystallized ligand. A 100 ns MDS was used to assess the binding stability of the compounds to TGR5.

Results

Molecular docking results showed that the lead compounds had a stronger affinity for TGR5 than the cocrystallized ligand. MDS revealed that the lead compounds were stable within the TGR5 binding pocket.

Discussion

The combination of ML, MD, and MDS provides a powerful approach for predicting new TGR5 agonists that can be optimised for T2D treatment.

Light modulates glucose and lipid homeostasis via the sympathetic nervous system

Light is an important environmental factor for vision and for diverse physiological and psychological functions. Light can also modulate glucose metabolism. Here, we show that in mice, light is critical for glucose and lipid homeostasis by regulating the sympathetic nervous system, independent of circadian disruption. Light deprivation from birth elicits insulin hypersecretion, glucagon hyposecretion, lower gluconeogenesis, and reduced lipolysis by 6 to 8 weeks in male, but not female, mice. These metabolic defects are consistent with blunted sympathetic activity, and indeed, sympathetic responses to a cold stimulus are substantially attenuated in dark-reared mice. Further, long-term dark rearing leads to body weight gain, insulin resistance, and glucose intolerance. Notably, metabolic dysfunction can be partially alleviated by 5 weeks exposure to a regular light-dark cycle. These studies provide insight into circadian-independent mechanisms by which light directly influences whole-body physiology and better understanding of metabolic disorders linked to aberrant environmental light conditions.

Brain Defense of Glycemia in Health and Diabetes

The brain coordinates the homeostatic defense of multiple metabolic variables, including blood glucose levels, in the context of ever-changing external and internal environments. The biologically defended level of glycemia (BDLG) is the net result of brain modulation of insulin-dependent mechanisms in cooperation with the islet, and insulin-independent mechanisms through direct innervation and neuroendocrine control of glucose effector tissues. In this article, we highlight evidence from animal and human studies to develop a framework for the brain's core homeostatic functions-sensory/afferent, integration/processing, and motor/efferent-that contribute to the normal BDLG in health and its elevation in diabetes.

ARTICLE HIGHLIGHTS

Cardiac corin and atrial natriuretic peptide regulate liver glycogen metabolism and glucose homeostasis

BACKGROUND

Cardiovascular function and metabolic homeostasis are closely linked, but the underlying mechanisms are not fully understood. Corin is a protease that activates atrial natriuretic peptide (ANP), an essential hormone for normal blood pressure and cardiac function. The goal of this study is to investigate a potential corin and ANP function in regulating liver glycogen metabolism and glucose homeostasis.

METHODS

Liver glycogen and blood glucose levels were analyzed in Corin or Nppa (encoding ANP) knockout (KO) mice. ANP signaling was examined in livers from Corin and Nppa KO mice and in cultured human and mouse hepatocytes by western blotting.

RESULTS

We found that Corin and Nppa KO mice had reduced liver glycogen contents and increased blood glucose levels. By analyzing conditional KO mice lacking either cardiac or renal Corin, we showed that cardiac corin and ANP act in an endocrine manner to enhance cGMP-protein kinase G (PKG)-AKT-GSK3 signaling in hepatocytes. In cultured hepatocytes, ANP treatment stimulated PKG signaling, glucose uptake, and glycogen production, which could be blocked by small molecule PKG and AKT inhibitors.

CONCLUSIONS

Our results indicate that corin and ANP are important regulators in liver glycogen metabolism and glucose homeostasis, suggesting that defects in the corin and ANP pathway may contribute to both cardiovascular and metabolic diseases.

Light modulates glucose and lipid homeostasis via the sympathetic nervous system

Light is an important environmental factor for vision, and for diverse physiological and psychological functions. Light can also modulate glucose metabolism. Here, we show that in mice, light is critical for glucose and lipid homeostasis by regulating the sympathetic nervous system, independent of circadian disruption. Light deprivation from birth elicits insulin hypersecretion, glucagon hyposecretion, lower gluconeogenesis, and reduced lipolysis by 6-8 weeks, in male, but not, female mice. These metabolic defects are consistent with blunted sympathetic activity, and indeed, sympathetic responses to a cold stimulus are significantly attenuated in dark-reared mice. Further, long-term dark rearing leads to body weight gain, insulin resistance, and glucose intolerance. Notably, metabolic dysfunction can be partially alleviated by 5 weeks exposure to a regular light-dark cycle. These studies provide insight into circadian-independent mechanisms by which light directly influences whole-body physiology and inform new approaches for understanding metabolic disorders linked to aberrant environmental light conditions.

Navigating the Intersection of Glycemic Control and Fertility: A Network Perspective

The rising incidence of metabolic diseases is linked to elevated blood glucose levels, contributing to conditions such as diabetes and promoting the accumulation of advanced glycation end products (AGEs). AGEs, formed by non-enzymatic reactions between sugars and proteins, build up in tissues and are implicated in various diseases. This article explores the relationship between glycemic control and AGE accumulation, focusing on fertility implications. A computational model using network theory was developed, featuring a molecular database and a network with 145 nodes and 262 links, categorized as a Barabasi-Albert scale-free network. Three main subsets of nodes emerged, centered on glycemic control, fertility, and immunity, with AGEs playing a critical role. The transient receptor potential vanilloid 1 (TRPV1), a receptor expressed in several tissues including sperm, was identified as a key hub, suggesting that the modulation of TRPV1 in sperm by AGEs may influence fertility. Additionally, a novel link between glycemic control and immunity was found, indicating that immune cells may play a role in endocytosing specific AGEs. This discovery underscores the complex interplay between glycemic control and immune function, with significant implications for metabolic, immune health, and fertility.

Yin-Yang control of energy balance by lipids in the hypothalamus: The endocannabinoids vs bile acids case

Obesity is a chronic and debilitating disorder that originates from alterations in energy-sensing brain circuits controlling body weight gain and food intake. The dysregulated syntheses and actions of lipid mediators in the hypothalamus induce weight gain and overfeeding, but the molecular and cellular underpinnings of these alterations remain elusive. In response to changes in the nutritional status, different lipid sensing pathways in the hypothalamus direct body energy needs in a Yin-Yang model. Endocannabinoids orchestrate the crosstalk between hypothalamic circuits and the sympathetic nervous system to promote food intake and energy accumulation during fasting, whereas bile acids act on the same top-down axis to reduce energy intake and possibly storage after the meal. In obesity, the bioavailability and downstream cellular actions of endocannabinoids and bile acids are altered in hypothalamic neurons involved in body weight and metabolic control. Thus, the onset and progression of this disease might result from an imbalance in hypothalamic sensing of multiple lipid signals, which are possibly integrated by common molecular nodes. In this viewpoint, we discuss a possible model that explains how bile acids and endocannabinoids may exert their effects on energy balance regulation via interconnected mechanisms at the level of the hypothalamic neuronal circuits. Therefore, we propose a new conceptual framework for understanding and treating central mechanisms of maladaptive lipid action in obesity.

Regulation of Energy and Glucose Homeostasis by the Nucleus of the Solitary Tract and the Area Postrema

The central nervous system regulates feeding, weight and glucose homeostasis in rodents and humans, but the site-specific mechanisms remain unclear. The dorsal vagal complex in the brainstem that contains the nucleus of the solitary tract (NTS) and area postrema (AP) emerges as a regulatory center that impacts energy and glucose balance by monitoring hormonal and nutrient changes. However, the specific mechanistic metabolic roles of the NTS and AP remain elusive. This mini-review highlights methods to study their distinct roles and recent findings on their metabolic differences and similarities of growth differentiation factor 15 (GDF15) action and glucose sensing in the NTS and AP. In summary, future research aims to characterize hormonal and glucose sensing mechanisms in the AP and/or NTS carries potential to unveil novel targets that lower weight and glucose levels in obesity and diabetes.

Diabetes mellitus-Progress and opportunities in the evolving epidemic

Diabetes, a complex multisystem metabolic disorder characterized by hyperglycemia, leads to complications that reduce quality of life and increase mortality. Diabetes pathophysiology includes dysfunction of beta cells, adipose tissue, skeletal muscle, and liver. Type 1 diabetes (T1D) results from immune-mediated beta cell destruction. The more prevalent type 2 diabetes (T2D) is a heterogeneous disorder characterized by varying degrees of beta cell dysfunction in concert with insulin resistance. The strong association between obesity and T2D involves pathways regulated by the central nervous system governing food intake and energy expenditure, integrating inputs from peripheral organs and the environment. The risk of developing diabetes or its complications represents interactions between genetic susceptibility and environmental factors, including the availability of nutritious food and other social determinants of health. This perspective reviews recent advances in understanding the pathophysiology and treatment of diabetes and its complications, which could alter the course of this prevalent disorder.

Gut microbiota and type 2 diabetes mellitus: a focus on the gut-brain axis

Type 2 diabetes mellitus (T2DM) has become one of the most serious public healthcare challenges, contributing to increased mortality and disability. In the past decades, significant progress has been made in understanding the pathogenesis of T2DM. Mounting evidence suggested that gut microbiota (GM) plays a significant role in the development of T2DM. Communication between the GM and the brain is a complex bidirectional connection, known as the "gut-brain axis," via the nervous, neuroendocrine, and immune systems. Gut-brain axis has an essential impact on various physiological processes, including glucose metabolism, food intake, gut motility, etc. In this review, we provide an outline of the gut-brain axis. We also highlight how the dysbiosis of the gut-brain axis affects glucose homeostasis and even results in T2DM.

Global, neuronal or β cell-specific deletion of inceptor improves glucose homeostasis in male mice with diet-induced obesity

Insulin resistance is an early complication of diet-induced obesity (DIO)1, potentially leading to hyperglycaemia and hyperinsulinaemia, accompanied by adaptive β cell hypertrophy and development of type 2 diabetes2. Insulin not only signals via the insulin receptor (INSR), but also promotes β cell survival, growth and function via the insulin-like growth factor 1 receptor (IGF1R)3-6. We recently identified the insulin inhibitory receptor (inceptor) as the key mediator of IGF1R and INSR desensitization7. But, although β cell-specific loss of inceptor improves β cell function in lean mice7, it warrants clarification whether inceptor signal inhibition also improves glycaemia under conditions of obesity. We assessed the glucometabolic effects of targeted inceptor deletion in either the brain or the pancreatic β cells under conditions of DIO in male mice. In the present study, we show that global and neuronal deletion of inceptor, as well as its adult-onset deletion in the β cells, improves glucose homeostasis by enhancing β cell health and function. Moreover, we demonstrate that inceptor-mediated improvement in glucose control does not depend on inceptor function in agouti-related protein-expressing or pro-opiomelanocortin neurons. Our data demonstrate that inceptor inhibition improves glucose homeostasis in mice with DIO, hence corroborating that inceptor is a crucial regulator of INSR and IGF1R signalling.

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.

Malglycemia in the critical care setting. Part II: Relative and absolute hypoglycemia

INTRODUCTION

The relationship between critical care mortality and hypoglycemia, both relative (>30% below average preadmission glycemia) and absolute (blood glucose (BG) <70 mg/dL (<10 mmol/L)) requires further definition.

METHODS

We assessed the risk-adjusted relationship between hospital mortality with relative hypoglycemia using the Glycemic Ratio (GR), and with absolute hypoglycemia using BG in a retrospective cohort investigation (n = 4790).

RESULTS

Relative hypoglycemia excursions below GR 0.7 with a of 24-h non-exposure period between excursions in those with HbA1c ≥ 8% were independently associated with mortality (n = 373, OR 2.49, 95% CI 1.54-4.04, p = 0.0002) but not those with HbA1c < 8% (n = 4417, OR 0.98 95% CI 0.89-1.08, p = 0.70). Hours below GR 0.7 (1.0037, 0.9995-1.0080, 0.0846) or minimum GR (0.0896, 0.0030-2.6600, 0.1632) were not independently associated with outcome. Absolute hypoglycemia occurred across the HbA1c spectrum in a U-shaped pattern. There was no difference in mortality associated with exposure to BG < 70 mg/dL for HbA1c ≥ 6.5% vs <6.5% (29.7% vs 24.3%, p = 0.77). Hours below 70 mg/dL demonstrated strongest association with outcome, while minimum BG, and excursions below 70 mg/dL were also independently associated.

CONCLUSIONS

Relative hypoglycemia represented by excursions below GR 0.7 in those with HbA1c ≥ 8% occurred commonly and was independently associated with mortality. Absolute hypoglycemia had similar association with mortality regardless of HbA1c.

Regulation of the Cortisol Axis, Glucagon, and Growth Hormone by Glucose Is Altered in Prediabetes and Type 2 Diabetes

CONTEXT

Insulin-antagonistic, counter-regulatory hormones have been implicated in the development of type 2 diabetes (T2D).

OBJECTIVE

In this cross-sectional study, we investigated whether glucose-dependent regulation of such hormones differ in individuals with T2D, prediabetes (PD), and normoglycemia (NG).

METHODS

Fifty-four individuals with or without T2D underwent one hyperinsulinemic-normoglycemic-hypoglycemic and one hyperglycemic clamp with repeated hormonal measurements. Participants with T2D (n = 19) were compared with a group-matched (age, sex, BMI) subset of participants without diabetes (ND, n = 17), and also with participants with PD (n = 18) and NG (n = 17).

RESULTS

In T2D vs ND, glucagon levels were higher and less suppressed during the hyperglycemic clamp whereas growth hormone (GH) levels were lower during hypoglycemia (P < .05). Augmented ACTH response to hypoglycemia was present in PD vs NG (P < .05), with no further elevation in T2D. In contrast, glucagon and GH alterations were more marked in T2D vs PD (P < .05).In the full cohort (n = 54), augmented responses of glucagon, cortisol, and ACTH and attenuated responses of GH correlated with adiposity, dysglycemia, and insulin resistance. In multilinear regressions, insulin resistance was the strongest predictor of elevated hypoglycemic responses of glucagon, cortisol, and ACTH. Conversely, fasting glucose and HbA1c were the strongest predictors of low GH levels during hypoglycemia and elevated, i.e. less suppressed glucagon levels during hyperglycemia, respectively. Notably, adiposity measures were also strongly associated with the responses above.

CONCLUSIONS

Altered counter-regulatory hormonal responses to glucose variations are observed at different stages of T2D development and may contribute to its progression by promoting insulin resistance and dysglycemia.

Inter-organ crosstalk during development and progression of type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is characterized by tissue-specific insulin resistance and pancreatic β-cell dysfunction, which result from the interplay of local abnormalities within different tissues and systemic dysregulation of tissue crosstalk. The main local mechanisms comprise metabolic (lipid) signalling, altered mitochondrial metabolism with oxidative stress, endoplasmic reticulum stress and local inflammation. While the role of endocrine dysregulation in T2DM pathogenesis is well established, other forms of inter-organ crosstalk deserve closer investigation to better understand the multifactorial transition from normoglycaemia to hyperglycaemia. This narrative Review addresses the impact of certain tissue-specific messenger systems, such as metabolites, peptides and proteins and microRNAs, their secretion patterns and possible alternative transport mechanisms, such as extracellular vesicles (exosomes). The focus is on the effects of these messengers on distant organs during the development of T2DM and progression to its complications. Starting from the adipose tissue as a major organ relevant to T2DM pathophysiology, the discussion is expanded to other key tissues, such as skeletal muscle, liver, the endocrine pancreas and the intestine. Subsequently, this Review also sheds light on the potential of multimarker panels derived from these biomarkers and related multi-omics for the prediction of risk and progression of T2DM, novel diabetes mellitus subtypes and/or endotypes and T2DM-related complications.

Acetyl Tributyl Citrate Exposure at Seemingly Safe Concentrations Induces Adverse Effects in Different Genders of Type 2 Diabetes Mice, Especially Brain Tissue

Acetyl tributyl citrate (ATBC) is a widely used phthalate substitute. Although ATBC is considered to be with a safe dosage of up to 1000 mg/kg/day, studies on its effects in some sensitive populations, such as diabetic patients, are relatively rare. Epidemiological studies have shown that there is a link between diabetes and nervous system diseases. However, toxicological studies have not fully confirmed this yet. In this study, glycolipid metabolism, cognitive deficits, brain tissue damage, levels of neurotransmitters, beta-amyloid plaques (Aβ), hyperphosphorylated tau protein (p-Tau), oxidative stress and inflammation, as well as glial cell homeostatic levels in the brain tissue of type 2 diabetes (T2DM) mice, were determined after ATBC exposure (0, 2, 20, and 200 mg/kg/day) for 90 days. The results confirmed that ATBC exposure aggravated the disorder of glycolipid metabolism and caused cognitive deficits in T2DM mice; induced histopathological alterations and Aβ and p-Tau accumulation, and reduced the levels of 5-hydroxytryptamine and acetylcholine in T2DM mouse brains; oxidative stress and glial cell homeostatic levels in T2DM mouse brains were also changed. Some of the adverse effects were gender-dependent. These findings support the theory that T2DM mice, especially males, are more sensitive to ATBC exposure. Although the safe dose of ATBC is high, prolonged exposure at seemingly safe concentrations has the potential to aggravate diabetes symptoms and cause brain tissue damage in T2DM mice.

Obesity-associated microglial inflammatory activation paradoxically improves glucose tolerance

Hypothalamic gliosis associated with high-fat diet (HFD) feeding increases susceptibility to hyperphagia and weight gain. However, the body-weight-independent contribution of microglia to glucose regulation has not been determined. Here, we show that reducing microglial nuclear factor κB (NF-κB) signaling via cell-specific IKKβ deletion exacerbates HFD-induced glucose intolerance despite reducing body weight and adiposity. Conversely, two genetic approaches to increase microglial pro-inflammatory signaling (deletion of an NF-κB pathway inhibitor and chemogenetic activation through a modified Gq-coupled muscarinic receptor) improved glucose tolerance independently of diet in both lean and obese rodents. Microglial regulation of glucose homeostasis involves a tumor necrosis factor alpha (TNF-α)-dependent mechanism that increases activation of pro-opiomelanocortin (POMC) and other hypothalamic glucose-sensing neurons, ultimately leading to a marked amplification of first-phase insulin secretion via a parasympathetic pathway. Overall, these data indicate that microglia regulate glucose homeostasis in a body-weight-independent manner, an unexpected mechanism that limits the deterioration of glucose tolerance associated with obesity.

Identification of Hypothalamic Glucoregulatory Neurons That Sense and Respond to Changes in Glycemia

To investigate whether glucoregulatory neurons in the hypothalamus can sense and respond to physiological variation in the blood glucose (BG) level, we combined continuous arterial glucose monitoring with continuous measures of the activity of a specific subset of neurons located in the hypothalamic ventromedial nucleus that express pituitary adenylate cyclase activating peptide (VMNPACAP neurons) obtained using fiber photometry. Data were collected in conscious, free-living mice during a 1-h baseline monitoring period and a subsequent 2-h intervention period during which the BG level was raised either by consuming a chow or a high-sucrose meal or by intraperitoneal glucose injection. Cross-correlation analysis revealed that, following a 60- to 90-s delay, interventions that raise the BG level reliably associate with reduced VMNPACAP neuron activity (P < 0.01). In addition, a strong positive correlation between BG and spontaneous VMNPACAP neuron activity was observed under basal conditions but with a much longer (∼25 min) temporal offset, consistent with published evidence that VMNPACAP neuron activation raises the BG level. Together, these findings are suggestive of a closed-loop system whereby VMNPACAP neuron activation increases the BG level; detection of a rising BG level, in turn, feeds back to inhibit these neurons. To our knowledge, these findings constitute the first evidence of a role in glucose homeostasis for glucoregulatory neurocircuits that, like pancreatic β-cells, sense and respond to physiological variation in glycemia.

ARTICLE HIGHLIGHTS

By combining continuous arterial glucose monitoring with fiber photometry, studies investigated whether neurons in the murine ventromedial nucleus that express pituitary adenylate cyclase activating peptide (VMNPACAP neurons) detect and respond to changes in glycemia in vivo. VMNPACAP neuron activity rapidly decreases (within <2 min) when the blood glucose level is raised by either food consumption or glucose administration. Spontaneous VMNPACAP neuron activity also correlates positively with glycemia, but with a longer temporal offset, consistent with reports that hyperglycemia is induced by experimental activation of these neurons. Like pancreatic β-cells, neurons in the hypothalamic ventromedial nucleus appear to sense and respond to physiological variation in glycemia.

A dahlia flower extract has antidiabetic properties by improving insulin function in the brain

Butein, a rare chalcone found in the toxic plant Toxicodendron vernicifluum, has been shown to regulate glucose homeostasis via inhibition of the nuclear factor kappa-B kinase subunit beta (IKKβ)/nuclear factor kappa B (NF-κB) pathway in the brain. Here, we investigated whether the nonpoisonous plant Dahlia pinnata could be a source of butein as a potential treatment for type 2 diabetes (T2D). In mice fed a high-fat diet (HFD) to induce glucose intolerance, an oral D. pinnata petal extract improved glucose tolerance at doses of 3.3 mg/kg body weight and 10 mg/kg body weight. Surprisingly, this effect was not mediated by butein alone but by butein combined with the closely related flavonoids, sulfuretin and/or isoliquiritigenin. Mechanistically, the extract improved systemic insulin tolerance. Inhibition of phosphatidylinositol 3-kinase to block insulin signaling in the brain abrogated the glucoregulatory effect of the orally administered extract. The extract reinstated central insulin signaling and normalized astrogliosis in the hypothalamus of HFD-fed mice. Using NF-κB reporter zebrafish to determine IKKβ/NF-κB activity, a potent anti-inflammatory action of the extract was found. A randomized controlled crossover clinical trial on participants with prediabetes or T2D confirmed the safety and efficacy of the extract in humans. In conclusion, we identified an extract from the flower petals of D. pinnata as a novel treatment option for T2D, potentially targeting the central regulation of glucose homeostasis as a root cause of the disease.

Anoctamin 4 channel currents activate glucose-inhibited neurons in the mouse ventromedial hypothalamus during hypoglycemia

Glucose is the basic fuel essential for maintenance of viability and functionality of all cells. However, some neurons - namely, glucose-inhibited (GI) neurons - paradoxically increase their firing activity in low-glucose conditions and decrease that activity in high-glucose conditions. The ionic mechanisms mediating electric responses of GI neurons to glucose fluctuations remain unclear. Here, we showed that currents mediated by the anoctamin 4 (Ano4) channel are only detected in GI neurons in the ventromedial hypothalamic nucleus (VMH) and are functionally required for their activation in response to low glucose. Genetic disruption of the Ano4 gene in VMH neurons reduced blood glucose and impaired counterregulatory responses during hypoglycemia in mice. Activation of VMHAno4 neurons increased food intake and blood glucose, while chronic inhibition of VMHAno4 neurons ameliorated hyperglycemia in a type 1 diabetic mouse model. Finally, we showed that VMHAno4 neurons represent a unique orexigenic VMH population and transmit a positive valence, while stimulation of neurons that do not express Ano4 in the VMH (VMHnon-Ano4) suppress feeding and transmit a negative valence. Together, our results indicate that the Ano4 channel and VMHAno4 neurons are potential therapeutic targets for human diseases with abnormal feeding behavior or glucose imbalance.

Cognition from the Body-Brain Partnership: Exaptation of Memory

Examination of cognition has historically been approached from language and introspection. However, human language-dependent definitions ignore the evolutionary roots of brain mechanisms and constrain their study in experimental animals. We promote an alternative view, namely that cognition, including memory, can be explained by exaptation and expansion of the circuits and algorithms serving bodily functions. Regulation and protection of metabolic and energetic processes require time-evolving brain computations enabling the organism to prepare for altered future states. Exaptation of such circuits was likely exploited for exploration of the organism's niche. We illustrate that exploration gives rise to a cognitive map, and in turn, environment-disengaged computation allows for mental travel into the past (memory) and the future (planning). Such brain-body interactions not only occur during waking but also persist during sleep. These exaptation steps are illustrated by the dual, endocrine-homeostatic and memory, contributions of the hippocampal system, particularly during hippocampal sharp-wave ripples.

Spectro-spatial features in distributed human intracranial activity proactively encode peripheral metabolic activity

Mounting evidence demonstrates that the central nervous system (CNS) orchestrates glucose homeostasis by sensing glucose and modulating peripheral metabolism. Glucose responsive neuronal populations have been identified in the hypothalamus and several corticolimbic regions. However, how these CNS gluco-regulatory regions modulate peripheral glucose levels is not well understood. To better understand this process, we simultaneously measured interstitial glucose concentrations and local field potentials in 3 human subjects from cortical and subcortical regions, including the hypothalamus in one subject. Correlations between high frequency activity (HFA, 70-170 Hz) and peripheral glucose levels are found across multiple brain regions, notably in the hypothalamus, with correlation magnitude modulated by sleep-wake cycles, circadian coupling, and hypothalamic connectivity. Correlations are further present between non-circadian (ultradian) HFA and glucose levels which are higher during awake periods. Spectro-spatial features of neural activity enable decoding of peripheral glucose levels both in the present and up to hours in the future. Our findings demonstrate proactive encoding of homeostatic glucose dynamics by the CNS.

Role of Delta/Notch-like EGF-related receptor in blood glucose homeostasis

Cell-cell interactions are necessary for optimal endocrine functions in the pancreas. β-cells, characterized by the expression and secretion of the hormone insulin, are a major constituent of functional micro-organs in the pancreas known as islets of Langerhans. Cell-cell contacts between β-cells are required to regulate insulin production and glucose-stimulated insulin secretion, which are key determinants of blood glucose homeostasis. Contact-dependent interactions between β-cells are mediated by gap junctions and cell adhesion molecules such as E-cadherin and N-CAM. Recent genome-wide studies have implicated Delta/Notch-like EGF-related receptor (Dner) as a potential susceptibility locus for Type 2 Diabetes in humans. DNER is a transmembrane protein and a proposed Notch ligand. DNER has been implicated in neuron-glia development and cell-cell interactions. Studies herein demonstrate that DNER is expressed in β-cells with an onset during early postnatal life and sustained throughout adulthood in mice. DNER loss in adult β-cells in mice (β-Dner cKO mice) disrupted islet architecture and decreased the expression of N-CAM and E-cadherin. β-Dner cKO mice also exhibited impaired glucose tolerance, defects in glucose- and KCl-induced insulin secretion, and decreased insulin sensitivity. Together, these studies suggest that DNER plays a crucial role in mediating islet cell-cell interactions and glucose homeostasis.

Distinct Roles for Brain and Pancreas in Basal and Postprandial Glucose Homeostasis

The glucose homeostasis system ensures that the circulating glucose level is maintained within narrow physiological limits both in the fasting (or basal) state and following a nutrient challenge. Although glucose homeostasis is traditionally conceptualized as a single overarching system, evidence reviewed here suggests that basal glycemia and glucose tolerance are governed by distinct control systems. Specifically, whereas glucose tolerance appears to be determined largely by interactions between insulin secretion and insulin sensitivity, basal-state glucose homeostasis is predominated by insulin-independent mechanisms governed largely by the brain. In addition to a new perspective on how glucose homeostasis is achieved, this "dual control system" hypothesis offers a feasible and testable explanation for observations that are otherwise difficult to reconcile and sheds new light on the integration of central and peripheral metabolic control mechanisms. The implications of this model for the pathogenesis and treatment of impaired fasting glucose, impaired glucose tolerance, and type 2 diabetes are also discussed.

Brain Glucose Sensing and the Problem of Relative Hypoglycemia

"Relative hypoglycemia" is an often-overlooked complication of diabetes characterized by an increase in the glycemic threshold for detecting and responding to hypoglycemia. The clinical relevance of this problem is linked to growing evidence that among patients with critical illness, higher blood glucose in the intensive care unit is associated with higher mortality among patients without diabetes but lower mortality in patients with preexisting diabetes and an elevated prehospitalization HbA1c. Although additional studies are needed, the cardiovascular stress associated with hypoglycemia perception, which can occur at normal or even elevated glucose levels in patients with diabetes, offers a plausible explanation for this difference in outcomes. Little is known, however, regarding how hypoglycemia is normally detected by the brain, much less how relative hypoglycemia develops in patients with diabetes. In this article, we explore the role in hypoglycemia detection played by glucose-responsive sensory neurons supplying peripheral vascular beds and/or circumventricular organs. These observations support a model wherein relative hypoglycemia results from diabetes-associated impairment of this neuronal glucose-sensing process. By raising the glycemic threshold for hypoglycemia perception, this impairment may contribute to the increased mortality risk associated with standard glycemic management of critically ill patients with diabetes.

Therapeutic effects of icariin and icariside II on diabetes mellitus and its complications

Diabetes mellitus (DM) is a global health issue in the twenty-first century, and there are numerous challenges in preventing and alleviating its chronic complications. The herb Epimedium has beneficial therapeutic effects on various human diseases, including DM. Its major flavonoid component, icariin, has significant anti-DM activity and may help improve pancreatic β-cell dysfunction and insulin resistance. Furthermore, preclinical evidence has shown that icariin and its in vivo bioactive form, icariside II, have preventive and therapeutic effects on several diabetic complications, including diabetic cardiomyopathy, diabetic vascular endothelial disorder, diabetic nephropathy, and diabetic erectile dysfunction. In this review, we present the general and toxicological information concerning icariin and icariside II and review the anti-DM effects of icariin from a molecular perspective. Additionally, we discuss the potential benefits of icariin and icariside II on the important pathological mechanisms of various diabetic complications. Despite positive preclinical evidence, additional investigations are needed before relevant clinical studies can be conducted. Therefore, we conclude with suggestions for future research. Hopefully, this review will provide a comprehensive molecular perspective for future research and product development related to icariin and icariside II in treating DM and diabetic complications.

Pharmacokinetics and tissue distribution of Ramulus Mori (Sangzhi) alkaloids in rats and its effects on liver enzyme activity

Ramulus Mori (Sangzhi) alkaloids (SZ-A) derived from twigs of mulberry (Morus alba L., genus Morus in the Moraceae family) was approved by the National Medical Products Administration in 2020 for the treatment of type 2 diabetes mellitus. In addition to excellent hypoglycemic effect, increasing evidence has confirmed that SZ-A exerts multiple pharmacological effects, such as protecting pancreatic ß-cell function, stimulating adiponectin expression, and alleviating hepatic steatosis. Importantly, a specific distribution of SZ-A in target tissues following oral absorption into the blood is essential for the induction of multiple pharmacological effects. However, there is a lack of studies thoroughly exploring the pharmacokinetic profiles and tissue distribution of SZ-A following oral absorption into the blood, particularly dose-linear pharmacokinetics and target tissue distribution associated with glycolipid metabolic diseases. In the present study, we systematically investigated the pharmacokinetics and tissue distribution of SZ-A and its metabolites in human and rat liver microsomes, and rat plasma, as well as its effects on the activity of hepatic cytochrome P450 enzymes (CYP450s). The results revealed that SZ-A was rapidly absorbed into the blood, exhibited linear pharmacokinetic characteristics in the dose range of 25-200 mg/kg, and was broadly distributed in glycolipid metabolism-related tissues. The highest SZ-A concentrations were observed in the kidney, liver, and aortic vessels, followed by the brown and subcutaneous adipose tissues, and the heart, spleen, lung, muscle, pancreas, and brain. Except for the trace oxidation products produced by fagomine, other phase I or phase II metabolites were not detected. SZ-A had no inhibitory or activating effects on major CYP450s. Conclusively, SZ-A is rapidly and widely distributed in target tissues, with good metabolic stability and a low risk of triggering drug-drug interactions. This study provides a framework for deciphering the material basis of the multiple pharmacological functions of SZ-A, its rational clinical use, and the expansion of its indications.

Effects of adrenergic-stimulated lipolysis and cytokine production on in vitro mouse adipose tissue-islet interactions

Inflammatory cytokines and non-esterified fatty acids (NEFAs) are obesity-linked factors that disturb insulin secretion. The aim of this study was to investigate whether pancreatic adipose tissue (pWAT) is able to generate a NEFA/cytokine overload within the pancreatic environment and as consequence to impact on insulin secretion. Pancreatic fat is a minor fat depot, therefore we used high-fat diet (HFD) feeding to induce pancreatic steatosis in mice. Relative Adipoq and Lep mRNA levels were higher in pWAT of HFD compared to chow diet mice. Regardless of HFD, Adipoq and Lep mRNA levels of pWAT were at least 10-times lower than those of epididymal fat (eWAT). Lipolysis stimulating receptors Adrb3 and Npr1 were expressed in pWAT and eWAT, and HFD reduced their expression in eWAT only. In accordance, HFD impaired lipolysis in eWAT but not in pWAT. Despite expression of Npr mRNA, lipolysis was stimulated solely by the adrenergic agonists, isoproterenol and adrenaline. Short term co-incubation of islets with CD/HFD pWAT did not alter insulin secretion. In the presence of CD/HFD eWAT, glucose stimulated insulin secretion only upon isoproterenol-induced lipolysis, i.e. in the presence of elevated NEFA. Isoproterenol augmented Il1b and Il6 mRNA levels both in pWAT and eWAT. These results suggest that an increased sympathetic activity enhances NEFA and cytokine load of the adipose microenvironment, including that of pancreatic fat, and by doing so it may alter beta-cell function.

New Insights into the Role of Insulin and Hypothalamic-Pituitary-Adrenal (HPA) Axis in the Metabolic Syndrome

Recent data suggests that (pre)diabetes onset is preceded by a period of hyperinsulinemia. Consumption of the "modern" Western diet, over-nutrition, genetic background, decreased hepatic insulin clearance, and fetal/metabolic programming may increase insulin secretion, thereby causing chronic hyperinsulinemia. Hyperinsulinemia is an important etiological factor in the development of metabolic syndrome, type 2 diabetes, cardiovascular disease, polycystic ovarian syndrome, and Alzheimer's disease. Recent data suggests that the onset of prediabetes and diabetes are preceded by a variable period of hyperinsulinemia. Emerging data suggest that chromic hyperinsulinemia is also a driving force for increased activation of the hypothalamic-adrenal-pituitary (HPA) axis in subjects with the metabolic syndrome, leading to a state of "functional hypercortisolism". This "functional hypercortisolism" by antagonizing insulin actions may prevent hypoglycemia. It also disturbs energy balance by shifting energy fluxes away from muscles toward abdominal fat stores. Synergistic effects of hyperinsulinemia and "functional hypercortisolism" promote abdominal visceral obesity and insulin resistance which are core pathophysiological components of the metabolic syndrome. It is hypothesized that hyperinsulinemia-induced increased activation of the HPA axis plays an important etiological role in the development of the metabolic syndrome and its consequences. Numerous studies have demonstrated reversibility of hyperinsulinemia with lifestyle, surgical, and pharmaceutical-based therapies. Longitudinal studies should be performed to investigate whether strategies that reduce hyperinsulinemia at an early stage are successfully in preventing increased activation of the HPA axis and the metabolic syndrome.