Repeated hypoglycemia remodels neural inputs and disrupts mitochondrial function to blunt glucose-inhibited GHRH neuron responsiveness

Growth hormone-releasing hormone and its analogues in health and disease

Growth hormone-releasing hormone (GHRH) and its ability to stimulate the production and release of growth hormone from the pituitary were discovered more than four decades ago. Since then, this hormone has been studied extensively and research into its functions is still ongoing. GHRH has multifaceted roles beyond the originally identified functions that encompass a variety of direct extrapituitary effects. In this Review, we illustrate the different biological activities of GHRH, covering the effects of GHRH agonists and antagonists in physiological and pathological contexts, along with the underlying mechanisms. GHRH and GHRH analogues have been implicated in cell growth, wound healing, cell death, inflammation, immune functions, mood disorders, feeding behaviour, neuroprotection, diabetes mellitus and obesity, as well as cardiovascular, lung and neurodegenerative diseases and some cancers. The positive effects observed in preclinical models in vitro and in vivo strongly support the potential use of GHRH agonists and antagonists as clinical therapeutics.

GHRH in diabetes and metabolism

Despite over a century of insulin therapy and recent advances in glucose monitoring, diabetes and its complications remain a significant burden. Current medications are not durable, with symptoms often returning after treatment ends, and responses vary between patients. Additionally, the effectiveness of many medications diminishes over time, highlighting the need for alternative approaches. Maintaining β-cell mass and promoting β-cell regeneration offer more curable treatments, while cell replacement therapies could be an option if regeneration is not feasible. For both strategies, enhancing β-cell survival is crucial. Growth hormone-releasing hormone (GHRH) was originally discovered for its ability to stimulate the production and release of growth hormone (GH) from the pituitary. Beyond the hypothalamus, GHRH is produced in peripheral tissues, with its receptor, GHRHR, expressed in tissues such as the pituitary, pancreas, adipose tissue, intestine, and liver. Several studies have shown that GHRH and its analogs enhance the survival of insulin-producing pancreatic β-cells both in vitro and in animal models. These beneficial effects strongly support the potential of GHRH agonists and antagonists for the clinical treatment of human metabolic diseases or for enhancing β-cell survival in cells used for transplantation. In the current review, we will discuss the roles of hypothalamic and extrahypothalamic GHRH in metabolism in physiological and pathological contexts, along with the underlying mechanisms. Furthermore, we will discuss the potential beneficial effects of GHRH analogs for the treatment of metabolic diseases.

Homoplantaginin alleviates high glucose-induced vascular endothelial senescence by inhibiting mtDNA-cGAS-STING pathway via blunting DRP1-mitochondrial fission-VDAC1 axis

Vascular endothelial senescence is a major risk factor for diabetic vascular complications. Abnormal mitochondrial fission by dynamically related protein 1 (DRP1) accelerates vascular endothelial cell senescence. Homoplantaginin (Hom) is a flavonoid in Salvia plebeia R. Br. with protecting mitochondrial and repairing vascular properties. However, the relevant mechanism of Hom against diabetic vascular endothelial cell senescence remains unclear. Here, we used db/db mice and high glucose (HG)-treated human umbilical vein endothelial cells (HUVECs) to assess the anti-vascular endothelial cell senescence of Hom. We found that Hom inhibited senescence-associated β-galactosidase activity, decreased the levels of senescence markers, and senescence-associated secretory phenotype factors. Additionally, Hom inhibited the expression of cGAS-STING pathway and downstream inflammatory factors. STING inhibitor H-151 delayed endothelial senescence, whereas STING overexpression attenuated the anti-endothelial senescence effect of Hom. Furthermore, we observed that Hom reduced mitochondrial fragmentation and inhibited abnormal mitochondrial fission using transmission electron microscopy. Importantly, Hom has a stronger effect on mitochondrial fission protein than mitochondrial fusion protein, especially downregulated the expression of DRP1. DRP1 inhibitor Mdivi-1 suppressed cGAS-STING pathway and vascular endothelial senescence, yet DRP1 agonist FCCP attenuated the effect of Hom. Surprisingly, Hom blunted abnormal mitochondrial fission mediated by DRP1 mitochondrial localization, suppressed interaction of DRP1 with VDAC1 and prevented VDAC1 oligomerization, which was necessary for mtDNA escape and subsequent cGAS-STING pathway activation. These results revealed a previously unrecognized mechanism that Hom alleviated vascular endothelial senescence by inhibited mtDNA-cGAS-STING signaling pathway via blunting DRP1-mitochondrial fission-VDAC1 axis.

Characterization and Regulation of the Neonatal Growth Hormone Surge

High neonatal growth hormone (GH) secretion has been described in several species. However, the neuroendocrine mechanisms behind this surge remain unknown. Thus, the pattern of postnatal GH secretion was investigated in mice and rats. Blood GH levels were very high on postnatal day (P)1 and progressively decreased until near zero by P17 in C57BL/6 mice without sex differences. This pattern was similar to that observed in rats, except that female rats showed higher GH levels on P1 than males. In comparison, follicle-stimulating hormone exhibited higher secretion in females during the first 3 weeks of life. Hypothalamic Sst mRNA and somatostatin neuroendocrine terminals in the median eminence were higher in P20/P21 mice than in newborns. Knockout mice for GH-releasing hormone (GHRH) receptor showed no GH surge, whereas knockdown mice for the Sst gene displayed increased neonatal GH peak. Leptin deficiency caused only minor effects on early-life GH secretion. GH receptor ablation in neurons or the entire body did not affect neonatal GH secretion, but the subsequent reduction in blood GH levels was attenuated or prevented by these genetic manipulations, respectively. This phenotype was also observed in knockout mice for the insulin-like growth factor-1 (IGF-1) receptor in GHRH neurons. Moreover, glucose-induced hyperglycemia overstimulated GH secretion in neonatal mice. In conclusion, GH surge in the first days of life is not regulated by negative feedback loops. However, neonatal GH secretion requires GHRH receptor, and is modulated by somatostatin and blood glucose levels, suggesting that this surge is controlled by hypothalamic-pituitary communication.

Acute severe hypoglycemia alters mouse brain microvascular proteome

Hypoglycemia increases the risk related to stroke and neurodegenerative diseases, however, the underlying mechanisms are unclear. For the first time, we studied the effect of a single episode (acute) of severe (ASH) and mild (AMH) hypoglycemia on mouse brain microvascular proteome. After four-hour fasting, insulin was administered (i.p) to lower mean blood glucose in mice and induce ∼30 minutes of ASH (∼30 mg/dL) or AMH (∼75 mg/dL), whereas a similar volume of saline was given to control mice (∼130 mg/dL). Blood glucose was allowed to recover over 60 minutes either spontaneously or by 20% dextrose administration (i.p). Twenty-four hours later, the brain microvessels (BMVs) were isolated, and tandem mass tag (TMT)-based quantitative proteomics was performed using liquid chromatography-mass spectrometry (LC/MS). When compared to control, ASH significantly downregulated 13 proteins (p ≤ 0.05) whereas 23 proteins showed a strong trend toward decrease (p ≤ 0.10). When compared to AMH, ASH significantly induced the expression of 35 proteins with 13 proteins showing an increasing trend. AMH downregulated only 3 proteins. ASH-induced downregulated proteins are involved in actin cytoskeleton maintenance needed for cell shape and migration which are critical for blood-brain barrier maintenance and angiogenesis. In contrast, ASH-induced upregulated proteins are RNA-binding proteins involved in RNA splicing, transport, and stability. Thus, ASH alters BMV proteomics to impair cytoskeletal integrity and RNA processing which are critical for cerebrovascular function.

Disturbed hippocampal histidine metabolism contributes to cognitive impairment induced by recurrent nonsevere hypoglycemia in diabetes

Hypoglycemia is a common adverse reaction to glucose-lowering treatment. Diabetes mellitus (DM) combined with recurrent nonsevere hypoglycemia (RH) can accelerate cognitive decline. Currently, the metabolic pattern changes in cognition-related brain regions caused by this combined effect of DM and RH (DR) remain unclear. In this study, we first characterized the metabolic profiles of the hippocampus in mice exposed to DR using non-targeted metabolomic platforms. Our results showed that DR induced a unique metabolic pattern in the hippocampus, and several significant differences in metabolite levels belonging to the histidine metabolism pathway were discovered. Based on these findings, in the follow-up experiment, we found that histidine treatment could attenuate the cognitive impairment and rescue the neuronal and synaptic damage induced by DR in the hippocampus, which are closely related to ameliorated mitochondrial injury. These findings provide new insights into the metabolic mechanisms of the hippocampus in the progression of DR, and l-histidine supplementation may be a potential metabolic therapy in the future.

Inhibiting mitochondrial inflammation through Drp1/HK1/NLRP3 pathway: A mechanism of alpinetin attenuated aging-associated cognitive impairment

Mitochondrial inflammation triggered by abnormal mitochondrial division and regulated by the Drp1/HK1/NLRP3 pathway is correlated with the progression of aging-associated cognitive impairment (AACI). Alpinetin is a novel flavonoid derived from Zingiberaceae that has many bioactivities such as antiinflammation and anti-oxidation. However, whether alpinetin alleviates AACI by suppressing Drp1/HK1/NLRP3 pathway-inhibited mitochondrial inflammation is still unknown. In the present study, D-galactose (D-gal)-induced aging mice and BV-2 cells were used, and the effects of alpinetin on learning and memory function, neuroprotection and activation of the Drp1/HK1/NLRP3 pathway were investigated. Our data indicated that alpinetin significantly alleviated cognitive dysfunction and neuronal damage in the CA1 and CA3 regions of D-gal-treated mice. Moreover, D-gal-induced microglial activation was markedly reduced by alpinetin by inhibiting the Drp1/HK1/NLRP3 pathway-suppressed mitochondrial inflammation, down-regulating the levels of p-Drp1 (s616), VDAC, NLRP3, ASC, Cleaved-caspase 1, IL-18, and IL-1β, and up-regulating the expression of HK1. Furthermore, after Drp1 inhibition by Mdivi-1 in vitro, the inhibitory effect of alpinetin on Drp1/HK1/NLRP3 pathway was more evident. In summary, the current results implied that alpinetin attenuated aging-related cognitive deficits by inhibiting the Drp1/HK1/NLRP3 pathway and suppressing mitochondrial inflammation, suggesting that the inhibition of the Drp1/HK1/NLRP3 pathway is one of the mechanisms by which alpinetin attenuates AACI.

Recent advances in understanding hypothalamic control of defensive responses to hypoglycaemia

Maintenance of normal blood glucose is important for survival. In particular, brain function is dependent on circulating glucose. In health, a series of powerful counterregulatory defences operate to prevent/limit hypoglycaemia. These defences are altered to varying degrees in diabetes and in particular, a subset of people with diabetes can develop profound deficits in these defences placing them at increased risk of suffering episodes of severe hypoglycaemia. Brain is an important controller of glucose homeostasis and developments in molecular techniques have allowed the neurocircuitry of a number of important centrally-controlled homeostatic processes such as energy balance, thirst and thermoregulation to be defined. This review describes how some of these advances have allowed a better understanding of the neuronal/brain ensembles which help protect against hypoglycaemia.