Mechanisms and significance of brain glucose signaling in energy balance, glucose homeostasis, and food-induced reward

Sex-Dimorphic Effects of Hypoglycemia on Metabolic Sensor mRNA Expression in Ventromedial Hypothalamic Nucleus-Dorsomedial Division (VMNdm) Growth Hormone-Releasing Hormone Neurons

Growth hormone-releasing hormone (Ghrh) neurons in the dorsomedial ventromedial hypothalamic nucleus (VMNdm) express the metabolic transcription factor steroidogenic factor-1 and hypoglycemia-sensitive neurochemicals of diverse chemical structures, transmission modes, and temporal signaling profiles. Ghrh imposes neuromodulatory control of coexpressed transmitters. Multiple metabolic sensory mechanisms are employed in the brain, including screening of the critical nutrient glucose or the energy currency ATP. Here, combinatory laser-catapult-microdissection/single-cell multiplex qPCR tools were used to investigate whether these neurons possess molecular machinery for monitoring cellular metabolic status and if these biomarkers exhibit sex-specific sensitivity to insulin-induced hypoglycemia. Data show that hypoglycemia up- (male) or downregulated (female) Ghrh neuron glucokinase (Gck) mRNA; Ghrh gene silencing decreased baseline and hypoglycemic patterns of Gck gene expression in each sex. Ghrh neuron glucokinase regulatory protein (Gckr) transcript levels were respectively diminished or augmented in hypoglycemic male vs female rats; this mRNA profile was decreased by Ghrh siRNA in both sexes. Gene transcripts encoding catalytic alpha subunits of the energy monitor 5-AMP-activated protein kinase (AMPK), i.e., Prkaa1 and 2, were increased by hypoglycemia in males, yet only the former mRNA was hypoglycemia-sensitive in females. Ghrh siRNA downregulated baseline and hypoglycemia-associated Prkaa subunit mRNAs in males but elicited divergent changes in Prkaa2 transcripts in eu- vs hypoglycemic females. Results provide unique evidence that VMNdm Ghrh neurons express the characterized metabolic sensor biomarkers glucokinase and AMPK and that the corresponding gene profiles exhibit distinctive sex-dimorphic transcriptional responses to hypoglycemia. Data further document Ghrh neuromodulation of baseline and hypoglycemic transcription patterns of these metabolic gene profiles.

Research Advances on the Damage Mechanism of Skin Glycation and Related Inhibitors

Our skin is an organ with the largest contact area between the human body and the external environment. Skin aging is affected directly by both endogenous factors and exogenous factors (e.g., UV exposure). Skin saccharification, a non-enzymatic reaction between proteins, e.g., dermal collagen and naturally occurring reducing sugars, is one of the basic root causes of endogenous skin aging. During the reaction, a series of complicated glycation products produced at different reaction stages and pathways are usually collectively referred to as advanced glycation end products (AGEs). AGEs cause cellular dysfunction through the modification of intracellular molecules and accumulate in tissues with aging. AGEs are also associated with a variety of age-related diseases, such as diabetes, cardiovascular disease, renal failure (uremia), and Alzheimer's disease. AGEs accumulate in the skin with age and are amplified through exogenous factors, e.g., ultraviolet radiation, resulting in wrinkles, loss of elasticity, dull yellowing, and other skin problems. This article focuses on the damage mechanism of glucose and its glycation products on the skin by summarizing the biochemical characteristics, compositions, as well as processes of the production and elimination of AGEs. One of the important parts of this article would be to summarize the current AGEs inhibitors to gain insight into the anti-glycation mechanism of the skin and the development of promising natural products with anti-glycation effects.

Carbohydrate and sleep: An evaluation of putative mechanisms

Sleep problems are extremely common in industrialized countries and the possibility that diet might be used to improve sleep has been considered. The topic has been reviewed many times, resulting in the frequent suggestion that carbohydrate increases the uptake of tryptophan by the brain, where it is metabolized into serotonin and melatonin, with the suggestion that this improves sleep. An alternative mechanism was proposed based on animal literature that has been largely ignored by those considering diet and sleep. The hypothesis was that, as in the hypothalamus there are glucose-sensing neurons associated with the sleep-wake cycle, we should consider the impact of carbohydrate-induced changes in the level of blood glucose. A meta-analysis found that after consuming a lower amount of carbohydrate, more time was spent in slow-wave sleep (SWS) and less in rapid-eye-movement sleep. As the credibility of alternative mechanisms has tended not to have been critically evaluated, they were considered by examining their biochemical, nutritional, and pharmacological plausibility. Although high carbohydrate consumption can increase the uptake of tryptophan by the brain, it only occurs with such low levels of protein that the mechanism is not relevant to a normal diet. After entering the brain tryptophan is converted to serotonin, a neurotransmitter known to influence so many different aspects of sleep and wakefulness, that it is not reasonable to expect a uniform improvement in sleep. Some serotonin is converted to melatonin, although the exogenous dose of melatonin needed to influence sleep cannot be credibly provided by the diet. This review was registered in the International Prospective Register of Systematic Reviews (CRD42020223560).

Metabolic determinants of Alzheimer's disease: A focus on thermoregulation

Alzheimer's disease (AD) is a complex age-related neurodegenerative disease, associated with central and peripheral metabolic anomalies, such as impaired glucose utilization and insulin resistance. These observations led to a considerable interest not only in lifestyle-related interventions, but also in repurposing insulin and other anti-diabetic drugs to prevent or treat dementia. Body temperature is the oldest known metabolic readout and mechanisms underlying its maintenance fail in the elderly, when the incidence of AD rises. This raises the possibility that an age-associated thermoregulatory deficit contributes to energy failure underlying AD pathogenesis. Brown adipose tissue (BAT) plays a central role in thermogenesis and maintenance of body temperature. In recent years, the modulation of BAT activity has been increasingly demonstrated to regulate energy expenditure, insulin sensitivity and glucose utilization, which could also provide benefits for AD. Here, we review the evidence linking thermoregulation, BAT and insulin-related metabolic defects with AD, and we propose mechanisms through which correcting thermoregulatory impairments could slow the progression and delay the onset of AD.

Metabolomic signature of the postprandial experience

BACKGROUND

Ingestion of a meal up to maximal tolerance induces unpleasant fullness sensation and changes in circulating metabolites. Our aim was to evaluate the relation between postprandial sensations and the metabolomic responses to a comfort meal.

METHODS

In 32 non-obese healthy men, homeostatic sensations (hunger/satiety, fullness), hedonic sensations (digestive well-being, mood), and the metabolomic profile in plasma (low-molecular weight metabolites and lipoprotein profiles) were measured before and 20 minutes after a comfort meal (warm ham and cheese sandwich and juice; total 300 mL; 425 kcal). Perception was measured on 10 cm scales and the metabolomic response by nuclear magnetic resonance spectroscopy.

KEY RESULTS

The comfort meal induced homeostatic sensations (satiety and fullness) associated with a positive hedonic reward (enhanced digestive well-being and mood) and a clear change in the metabolomic profile with a sharp discrimination between the pre and postprandial state by a non-supervised principal component analysis. The change in circulating metabolites correlated with the postprandial sensations: the increase in alanine correlated with the increase in fullness (R = 0.50; P = 0.004) and well-being (R = 0.50; P = 0.004); the increase in glucose correlated with the sensation of fullness (R = 0.40; P = 0.023) and enhanced mood (R = 0.41; P = 0.020).

CONCLUSION AND INFERENCES

Metabolomic changes in the response to a meal may provide an objective index of the postprandial experience, which may have clinical implications in the management of patients with poor meal tolerance or meal-related symptoms.

Lactate in the brain: from metabolic end-product to signalling molecule

Lactate in the brain has long been associated with ischaemia; however, more recent evidence shows that it can be found there under physiological conditions. In the brain, lactate is formed predominantly in astrocytes from glucose or glycogen in response to neuronal activity signals. Thus, neurons and astrocytes show tight metabolic coupling. Lactate is transferred from astrocytes to neurons to match the neuronal energetic needs, and to provide signals that modulate neuronal functions, including excitability, plasticity and memory consolidation. In addition, lactate affects several homeostatic functions. Overall, lactate ensures adequate energy supply, modulates neuronal excitability levels and regulates adaptive functions in order to set the 'homeostatic tone' of the nervous system.

Glucosensing capacity of rainbow trout telencephalon

To assess the hypothesis of glucosensing systems present in fish telencephalon, we first demonstrated in rainbow trout, by in situ hybridisation, the presence of glucokinase (GK). Then, we assessed the response of glucosensing markers in rainbow trout telencephalon 6 hours after i.c.v. treatment with glucose or 2-deoxyglucose (inducing glucoprivation). We evaluated the response of parameters related to the mechanisms dependent on GK, liver X receptor (LXR), mitochondrial activity, sweet taste receptor and sodium-glucose linked transporter 1 (SGLT-1). We also assessed mRNA abundance of neuropeptides involved in the metabolic control of food intake (agouti-related protein, neuropeptide Y, pro-opiomelanocortin, and cocaine- and amphetamine-related transcript), as well as the abundance and phosphorylation status of proteins possibly involved in linking glucosensing with neuropeptide expression, such as protein kinase B (AkT), AMP-activated protein kinase (AMPK), mechanistic target of rapamycin and cAMP response element-binding protein (CREB). The responses obtained support the presence in the telencephalon of a glucosensing mechanism based on GK and maybe one based on LXR, although they do not support the presence of mechanisms dependent on mitochondrial activity and SGLT-1. The mechanism based on sweet taste receptor responded to glucose but in a converse way to that characterised previously in the hypothalamus. In general, systems responded only to glucose but not to glucoprivation. Neuropeptides did not respond to glucose or glucoprivation. By contrast, the presence of glucose activates Akt and inhibits AMPK, CREB and forkhead box01. This is the first study in any vertebrate species in which the response to glucose of putative glucosensing mechanisms is demonstrated in the telencephalon. Their role might relate to processes other than homeostatic control of food intake, such as the hedonic and reward system.