Altered Central Nutrient Sensing in Male Mice Lacking Insulin Receptors in Glut4-Expressing Neurons

Stimulation of the hepatoportal nerve plexus with focused ultrasound restores glucose homoeostasis in diabetic mice, rats and swine

Peripheral neurons that sense glucose relay signals of glucose availability to integrative clusters of neurons in the brain. However, the roles of such signalling pathways in the maintenance of glucose homoeostasis and their contribution to disease are unknown. Here we show that the selective activation of the nerve plexus of the hepatic portal system via peripheral focused ultrasound stimulation (pFUS) improves glucose homoeostasis in mice and rats with insulin-resistant diabetes and in swine subject to hyperinsulinemic-euglycaemic clamps. pFUS modulated the activity of sensory projections to the hypothalamus, altered the concentrations of metabolism-regulating neurotransmitters, and enhanced glucose tolerance and utilization in the three species, whereas physical transection or chemical blocking of the liver-brain nerve pathway abolished the effect of pFUS on glucose tolerance. Longitudinal multi-omic profiling of metabolic tissues from the treated animals confirmed pFUS-induced modifications of key metabolic functions in liver, pancreas, muscle, adipose, kidney and intestinal tissues. Non-invasive ultrasound activation of afferent autonomic nerves may represent a non-pharmacologic therapy for the restoration of glucose homoeostasis in type-2 diabetes and other metabolic diseases.

Schizophrenia: a disorder of broken brain bioenergetics

A substantial and diverse body of literature suggests that the pathophysiology of schizophrenia is related to deficits of bioenergetic function. While antipsychotics are an effective therapy for the management of positive psychotic symptoms, they are not efficacious for the complete schizophrenia symptom profile, such as the negative and cognitive symptoms. In this review, we discuss the relationship between dysfunction of various metabolic pathways across different brain regions in relation to schizophrenia. We contend that several bioenergetic subprocesses are affected across the brain and such deficits are a core feature of the illness. We provide an overview of central perturbations of insulin signaling, glycolysis, pentose-phosphate pathway, tricarboxylic acid cycle, and oxidative phosphorylation in schizophrenia. Importantly, we discuss pharmacologic and nonpharmacologic interventions that target these pathways and how such interventions may be exploited to improve the symptoms of schizophrenia.

Diet-Induced Metabolic Dysfunction of Hypothalamic Nutrient Sensing in Rodents

A sedentary lifestyle and excessive nutrient intake resulting from the consumption of high-fat and calorie-rich diets are environmental factors contributing to the rapid growth of the current pandemic of type 2 diabetes mellitus (DM2). Fasting hyperglycemia, an established hallmark of DM2, is caused by excessive production of glucose by the liver, resulting in the inability of insulin to suppress endogenous glucose production. To prevent inappropriate elevations of circulating glucose resulting from changes in nutrient availability, mammals rely on complex mechanisms for continuously detecting these changes and to respond to them with metabolic adaptations designed to modulate glucose output. The mediobasal hypothalamus (MBH) is the key center where nutritional cues are detected and appropriate modulatory responses are integrated. However, certain environmental factors may have a negative impact on these adaptive responses. For example, consumption of a diet enriched in saturated fat in rodents resulted in the development of a metabolic defect that attenuated these nutrient sensing mechanisms, rendering the animals prone to developing hyperglycemia. Thus, high-fat feeding leads to a state of “metabolic disability” in which animals’ glucoregulatory responses fail. We postulate that the chronic faltering of the hypothalamic glucoregulatory mechanisms contributes to the development of metabolic disease.

Neural Circuits of Interoception

The present paper considers recent progress in our understanding of the afferent/ascending neural pathways and neural circuits of interoception. Of particular note is the extensive role of rostral neural systems, including cortical systems, in the recognition of internal body states, and the reciprocal role of efferent/descending systems in the regulation of those states. Together these reciprocal interacting networks entail interoceptive circuits that play an important role in a broad range of functions beyond the homeostatic maintenance of physiological steady-states. These include the regulation of behavioral, cognitive, and affective processes across conscious and nonconscious levels of processing. We highlight recent advances and knowledge gaps that are important for accelerating progress in the study of interoception.

Insulin action in the brain regulates both central and peripheral functions

The brain has been traditionally thought to be insensitive to insulin, primarily because insulin does not stimulate glucose uptake/metabolism in the brain (as it does in classic insulin-sensitive tissues such as muscle, liver, and fat). However, over the past 20 years, research in this field has identified unique actions of insulin in the brain. There is accumulating evidence that insulin crosses into the brain and regulates central nervous system functions such as feeding, depression, and cognitive behavior. In addition, insulin acts in the brain to regulate systemic functions such as hepatic glucose production, lipolysis, lipogenesis, reproductive competence, and the sympathoadrenal response to hypoglycemia. Decrements in brain insulin action (or brain insulin resistance) can be observed in obesity, type 2 diabetes (T2DM), aging, and Alzheimer's disease (AD), indicating a possible link between metabolic and cognitive health. Here, we describe recent findings on the pleiotropic actions of insulin in the brain and highlight the precise sites, specific neuronal population, and roles for supportive astrocytic cells through which insulin acts in the brain. In addition, we also discuss how boosting brain insulin action could be a therapeutic option for people at an increased risk of developing metabolic and cognitive diseases such as AD and T2DM. Overall, this perspective article serves to highlight some of these key scientific findings, identify unresolved issues, and indicate future directions of research in this field that would serve to improve the lives of people with metabolic and cognitive dysfunctions.

Maternal Lead Exposure Impairs Offspring Learning and Memory via Decreased GLUT4 Membrane Translocation

Lead (Pb) can cause a significant neurotoxicity in both adults and children, leading to the impairment to brain function. Pb exposure plays a key role in the impairment of learning and memory through synaptic neurotoxicity, resulting in the cognitive function. Researches have demonstrated that Pb exposure plays an important role in the etiology and pathogenesis of neurodegenerative diseases, such as Alzheimer's disease. However, the underlying mechanisms remain unclear. In the current study, a gestational Pb exposure (GLE) rat model was established to investigate the underlying mechanisms of Pb-induced cognitive impairment. We demonstrated that low-level gestational Pb exposure impaired spatial learning and memory as well as hippocampal synaptic plasticity at postnatal day 30 (PND 30) when the blood concentration of Pb had already recovered to normal levels. Pb exposure induced a decrease in hippocampal glucose metabolism by reducing glucose transporter 4 (GLUT4) levels in the cell membrane through the phosphatidylinositol 3 kinase-protein kinase B (PI3K-Akt) pathway. In vivo and in vitro GLUT4 over-expression increased the membrane translocation of GLUT4 and glucose uptake, and reversed the Pb-induced impairment to synaptic plasticity and cognition. These findings indicate that Pb exposure impairs synaptic plasticity by reducing the level of GLUT4 in the cell membrane as well as glucose uptake via the PI3K-Akt signaling pathway, demonstrating a novel mechanism for Pb exposure-induced neurotoxicity.

Current understanding of glucose transporter 4 expression and functional mechanisms

Glucose is used aerobically and anaerobically to generate energy for cells. Glucose transporters (GLUTs) are transmembrane proteins that transport glucose across the cell membrane. Insulin promotes glucose utilization in part through promoting glucose entry into the skeletal and adipose tissues. This has been thought to be achieved through insulin-induced GLUT4 translocation from intracellular compartments to the cell membrane, which increases the overall rate of glucose flux into a cell. The insulin-induced GLUT4 translocation has been investigated extensively. Recently, significant progress has been made in our understanding of GLUT4 expression and translocation. Here, we summarized the methods and reagents used to determine the expression levels of Slc2a4 mRNA and GLUT4 protein, and GLUT4 translocation in the skeletal muscle, adipose tissues, heart and brain. Overall, a variety of methods such real-time polymerase chain reaction, immunohistochemistry, fluorescence microscopy, fusion proteins, stable cell line and transgenic animals have been used to answer particular questions related to GLUT4 system and insulin action. It seems that insulin-induced GLUT4 translocation can be observed in the heart and brain in addition to the skeletal muscle and adipocytes. Hormones other than insulin can induce GLUT4 translocation. Clearly, more studies of GLUT4 are warranted in the future to advance of our understanding of glucose homeostasis.

Altered Central Nutrient Sensing in Male Mice Lacking Insulin Receptors in Glut4-Expressing Neurons

Insulin signaling in the central nervous system influences satiety, counterregulation, and peripheral insulin sensitivity. Neurons expressing the Glut4 glucose transporter influence peripheral insulin sensitivity. Here, we analyzed the effects of insulin receptor (IR) signaling in hypothalamic Glut4 neurons on glucose sensing as well as leptin and amino acid signaling. By measuring electrophysiological responses to low glucose conditions, we found that the majority of Glut4 neurons in the ventromedial hypothalamus (VMH) were glucose excitatory neurons. GLUT4-Cre-driven insulin receptor knockout mice with a combined ablation of IR in Glut4-expressing tissues showed increased counterregulatory response to either 2-deoxyglucose-induced neuroglycopenia or systemic insulin-induced hypoglycemia. The latter response was recapitulated in mice with decreased VMH IR expression, suggesting that the effects on the counterregulatory response are likely mediated through the deletion of IRs on Glut4 neurons in the VMH. Using immunohistochemistry in fluorescently labeled hypothalamic Glut4 neurons, we showed that IR signaling promoted hypothalamic cellular signaling responses to the rise of insulin, leptin, and amino acids associated with feeding. We concluded that hypothalamic Glut4 neurons modulated the glucagon counterregulatory response and that IR signaling in Glut4 neurons was required to integrate hormonal and nutritional cues for the regulation of glucose metabolism.