Testicular regulation of neuronal glucose and monocarboxylate transporter gene expression profiles in CNS metabolic sensing sites during acute and recurrent insulin-induced hypoglycemia

MicroRNA-7a overexpression in VMH restores the sympathoadrenal response to hypoglycemia

It is proposed that the impaired sympathoadrenal response to hypoglycemia induced by recurrent insulin-induced hypoglycemia (RH) is an adaptive phenomenon induced by specific changes in microRNA expression in the ventromedial hypothalamus (VMH). To test this hypothesis, genome-wide microRNAomic profiling of the VMH by RNA-sequencing was performed in control rats and rats treated for RH. Differential expression analysis identified microRNA-7a-5p and microRNA-665 as potential mediators of this phenomenon. To further test this hypothesis, experiments were conducted consisting of targeted lentiviral-mediated overexpression of microRNA-7a-5p and downregulation of microRNA-665 in the VMH. Hyperinsulinemic hypoglycemic clamp experiments demonstrated that targeted overexpression of microRNA-7a-5p (but not downregulation of microRNA-665) in the VMH of RH rats restored the epinephrine response to hypoglycemia. This restored response to hypoglycemia was associated with a restoration of GABAA receptor gene expression. Finally, a direct interaction of microRNA-7a-5p with the 3'-UTR of GABAA receptor α1-subunit (Gabra1) gene was demonstrated in a luciferase assay. These findings indicate that (a) the impaired sympathoadrenal response RH induces is associated with changes in VMH microRNA expression and (b) microRNA-7a-5p, possibly via direct downregulation of GABA receptor gene expression, may serve as a mediator of the altered sympathoadrenal response to hypoglycemia.

Hindbrain 5'-Adenosine Monophosphate-activated Protein Kinase Mediates Short-term Food Deprivation Inhibition of the Gonadotropin-releasing Hormone-Luteinizing Hormone Axis: Role of Nitric Oxide

Hindbrain-derived stimuli restrain the gonadotropin-releasing hormone (GnRH)-pituitary luteinizing hormone (LH) reproductive neuroendocrine axis during energy insufficiency. Interruption of food intake, planned or unplanned, is emblematic of modern life. This study investigated the premise that the hindbrain energy sensor 5'-adenosine monophosphate-activated protein kinase (AMPK) inhibits reproductive neuroendocrine function in short term, e.g. 18-h food-deprived (FD) estradiol (E)-implanted ovariectomized female rats. Intra-caudal fourth ventricular administration of the AMPK inhibitor Compound C (Cc) reversed FD-induced inhibition of rostral preoptic (rPO) GnRH protein expression and LH release in animals given E to replicate proestrus (high-E dose-, but not metestrus (low-E dose)-stage plasma steroid levels. FD caused Cc-reversible augmentation or diminution of preoptic norepinephrine (NE) activity in high- versus low-E rats, respectively, and AMPK-independent reductions in hypothalamic NE accumulation in the latter. Nitric oxide (NO) and kisspeptin are key stimulatory signals for the preovulatory LH surge. Here, FD inhibited rPO neuronal nitric oxide synthase protein expression in high-, but not low-E-dosed animals. Lateral ventricular delivery of the NO donor 3-morpholinosydnonimine (SIN-1) reversed inhibitory GnRH and LH responses to FD in high-E rats, and normalized rPO Vglut2, anteroventral periventricular KiSS1, and dorsomedial hypothalamic RFRP-3 mRNA and/or protein profiles. Data show that FD curtails reproductive neuroendocrine outflow by hindbrain AMPK-dependent mechanisms in the presence of peak estrous cycle E levels. Results indicate that neural networks linking this sensor to GnRH neurons likely involve NO signaling, which may function upstream of one or more neurotransmitters identified here by SIN-1-reversible inhibitory responses to FD.

Acidosis mediates recurrent hypoglycemia-induced increase in ischemic brain injury in treated diabetic rats

OBJECTIVES

Cerebral ischemia is a serious possible manifestation of diabetic vascular disease. Recurrent hypoglycemia (RH) enhances ischemic brain injury in insulin-treated diabetic (ITD) rats. In the present study, we determined the role of ischemic acidosis in enhanced ischemic brain damage in RH-exposed ITD rats.

METHODS

Diabetic rats were treated with insulin and mild/moderate RH was induced for 5 days. Three sets of experiments were performed. The first set evaluated the effects of RH exposure on global cerebral ischemia-induced acidosis in ITD rats. The second set evaluated the effect of an alkalizing agent (Tris-(hydroxymethyl)-aminomethane: THAM) on ischemic acidosis-induced brain injury in RH-exposed ITD rats. The third experiment evaluated the effect of the glucose transporter (GLUT) inhibitor on ischemic acidosis-induced brain injury in RH-exposed ITD rats. Hippocampal pH and lactate were measured during ischemia and early reperfusion for all three experiments. Neuronal survival in Cornu Ammonis 1 (CA1) hippocampus served as a measure of ischemic brain injury.

FINDINGS

Prior RH exposure increases lactate concentration and decreases pH during ischemia and early reperfusion when compared to controls. THAM and GLUT inhibitor treatments attenuated RH-induced increase in ischemic acidosis. GLUT inhibitor treatment reduced the RH-induced increase in lactate levels. Both THAM and GLUT inhibitor treatments significantly decreased ischemic damage in RH-exposed ITD rats.

CONCLUSIONS

Ischemia causes increased acidosis in RH-exposed ITD rats via a GLUT-sensitive mechanism. Exploring downstream pathways may help understand mechanisms by which prior exposure to RH increases cerebral ischemic damage.

Sex-Specific Life Course Changes in the Neuro-Metabolic Phenotype of Glut3 Null Heterozygous Mice: Ketogenic Diet Ameliorates Electroencephalographic Seizures and Improves Sociability

We tested the hypothesis that exposure of glut3+/- mice to a ketogenic diet ameliorates autism-like features, which include aberrant behavior and electrographic seizures. We first investigated the life course sex-specific changes in basal plasma-cerebrospinal fluid (CSF)-brain metabolic profile, brain glucose transport/uptake, glucose and monocarboxylate transporter proteins, and adenosine triphosphate (ATP) in the presence or absence of systemic insulin administration. Glut3+/- male but not female mice (5 months of age) displayed reduced CSF glucose/lactate concentrations with no change in brain Glut1, Mct2, glucose uptake or ATP. Exogenous insulin-induced hypoglycemia increased brain glucose uptake in glut3+/- males alone. Higher plasma-CSF ketones (β-hydroxybutyrate) and lower brain Glut3 in females vs males proved protective in the former while enhancing vulnerability in the latter. As a consequence, increased synaptic proteins (neuroligin4 and SAPAP1) with spontaneous excitatory postsynaptic activity subsequently reduced hippocampal glucose content and increased brain amyloid β1-40 deposition in an age-dependent manner in glut3+/- males but not females (4 to 24 months of age). We then explored the protective effect of a ketogenic diet on ultrasonic vocalization, sociability, spatial learning and memory, and electroencephalogram seizures in male mice (7 days to 6 to 8 months of age) alone. A ketogenic diet partially restored sociability without affecting perturbed vocalization, spatial learning and memory, and reduced seizure events. We conclude that (1) sex-specific and age-dependent perturbations underlie the phenotype of glut3+/- mice, and (2) a ketogenic diet ameliorates seizures caused by increased cortical excitation and improves sociability, but fails to rescue vocalization and cognitive deficits in glut3+/- male mice.

Monocarboxylate transporters in the brain and in cancer

Monocarboxylate transporters (MCTs) constitute a family of 14 members among which MCT1-4 facilitate the passive transport of monocarboxylates such as lactate, pyruvate and ketone bodies together with protons across cell membranes. Their anchorage and activity at the plasma membrane requires interaction with chaperon protein such as basigin/CD147 and embigin/gp70. MCT1-4 are expressed in different tissues where they play important roles in physiological and pathological processes. This review focuses on the brain and on cancer. In the brain, MCTs control the delivery of lactate, produced by astrocytes, to neurons, where it is used as an oxidative fuel. Consequently, MCT dysfunctions are associated with pathologies of the central nervous system encompassing neurodegeneration and cognitive defects, epilepsy and metabolic disorders. In tumors, MCTs control the exchange of lactate and other monocarboxylates between glycolytic and oxidative cancer cells, between stromal and cancer cells and between glycolytic cells and endothelial cells. Lactate is not only a metabolic waste for glycolytic cells and a metabolic fuel for oxidative cells, but it also behaves as a signaling agent that promotes angiogenesis and as an immunosuppressive metabolite. Because MCTs gate the activities of lactate, drugs targeting these transporters have been developed that could constitute new anticancer treatments. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

Ghrelin Regulates Glucose and Glutamate Transporters in Hypothalamic Astrocytes

Hypothalamic astrocytes can respond to metabolic signals, such as leptin and insulin, to modulate adjacent neuronal circuits and systemic metabolism. Ghrelin regulates appetite, adiposity and glucose metabolism, but little is known regarding the response of astrocytes to this orexigenic hormone. We have used both in vivo and in vitro approaches to demonstrate that acylated ghrelin (acyl-ghrelin) rapidly stimulates glutamate transporter expression and glutamate uptake by astrocytes. Moreover, acyl-ghrelin rapidly reduces glucose transporter (GLUT) 2 levels and glucose uptake by these glial cells. Glutamine synthetase and lactate dehydrogenase decrease, while glycogen phosphorylase and lactate transporters increase in response to acyl-ghrelin, suggesting a change in glutamate and glucose metabolism, as well as glycogen storage by astrocytes. These effects are partially mediated through ghrelin receptor 1A (GHSR-1A) as astrocytes do not respond equally to desacyl-ghrelin, an isoform that does not activate GHSR-1A. Moreover, primary astrocyte cultures from GHSR-1A knock-out mice do not change glutamate transporter or GLUT2 levels in response to acyl-ghrelin. Our results indicate that acyl-ghrelin may mediate part of its metabolic actions through modulation of hypothalamic astrocytes and that this effect could involve astrocyte mediated changes in local glucose and glutamate metabolism that alter the signals/nutrients reaching neighboring neurons.

Estradiol regulates effects of hindbrain activator 5-aminoimidazole-4-carboxamide-riboside administration on hypothalamic adenosine 5'-monophosphate-activated protein kinase activity and metabolic neurotransmitter mRNA and protein expression

Hindbrain adenosine 5'-monophosphate-activated protein kinase (AMPK) activation alters hypothalamic neuronal genomic activity in an estradiol (E)-dependent manner. This study examines the premise that E regulates metabolic effector neuron reactivity to hindbrain AMPK. Paraventricular (PVH), arcuate (ARH), and ventromedial (VMH) nuclei were micropunched from brains of E- or oil (O)-implanted ovariectomized female rats that had been injected, into the fourth ventricle, with the AMPK activator 5-aminoimidazole-4-carboxamide-riboside (AICAR; A) or saline (S) and analyzed by quantitative polymerase chain reaction and Western blotting for neurotransmitter mRNA and protein expression. PVH corticotrophin-releasing hormone gene and protein profiles were decreased in O/A and E/A animals. ARH pro-opiomelanocortin (POMC) mRNA and protein were both elevated in O/A but were diminished or unchanged, respectively, in E/A animals; ARH neuropeptide Y (NPY) transcription was inhibited in O/A and E/A animals, but neuropeptide content was augmented in E/A only. VMH SF-1 mRNA and protein were reduced in O and E animals. AICAR did not alter AMPK protein in any structure but elevated PVH (↑E), did not alter ARH, and decreased VMH (↓O,↓E) pAMPK. Results demonstrate hypothalamic metabolic neurotransmitter and AMPK reactivity to hindbrain AMPK activation, including E-dependent adjustments in POMC and NPY transcription and protein expression. Dissimilar POMC (↑O vs. ↔E) and NPY (↓O vs. ↑E) neuropeptide responses to caudal fourth ventricle AICAR indicate E regulation of hindbrain AMPK signaling and/or target receptivity, implying that ARH-controlled metabolic responses may differ in the presence vs. absence of E. Evidence for variable changes in hypothalamic AMPK activity resulting from hindbrain sensor manipulation suggests that individual (or region-based groups of) AMPK-expressing neuron populations are uniquely impacted by hindbrain AMPK.

Lactate-induced release of GABA in the ventromedial hypothalamus contributes to counterregulatory failure in recurrent hypoglycemia and diabetes

Suppression of GABAergic neurotransmission in the ventromedial hypothalamus (VMH) is crucial for full activation of counterregulatory responses to hypoglycemia, and increased γ-aminobutyric acid (GABA) output contributes to counterregulatory failure in recurrently hypoglycemic (RH) and diabetic rats. The goal of this study was to establish whether lactate contributes to raising VMH GABA levels in these two conditions. We used microdialysis to deliver artificial extracellular fluid or L-lactate into the VMH and sample for GABA. We then microinjected a GABAA receptor antagonist, an inhibitor of lactate transport (4CIN), or an inhibitor of lactate dehydrogenase, oxamate (OX), into the VMH prior to inducing hypoglycemia. To assess whether lactate contributes to raising GABA in RH and diabetes, we injected 4CIN or OX into the VMH of RH and diabetic rats before inducing hypoglycemia. L-lactate raised VMH GABA levels and suppressed counterregulatory responses to hypoglycemia. While blocking GABAA receptors did not prevent the lactate-induced rise in GABA, inhibition of lactate transport or utilization did, despite the presence of lactate. All three treatments restored the counterregulatory responses, suggesting that lactate suppresses these responses by enhancing GABA release. Both RH and diabetic rats had higher baseline GABA levels and were unable to reduce GABA levels sufficiently to fully activate counterregulatory responses during hypoglycemia. 4CIN or OX lowered VMH GABA levels in both RH and diabetic rats and restored the counterregulatory responses. Lactate likely contributes to counterregulatory failure in RH and diabetes by increasing VMH GABA levels.

In situ coexpression of glucose and monocarboxylate transporter mRNAs in metabolic-sensitive caudal dorsal vagal complex catecholaminergic neurons: transcriptional reactivity to insulin-induced hypoglycemia and caudal hindbrain glucose or lactate repletion during insulin-induced hypoglycemia

The neurochemical phenotype(s) of metabolic sensing neurons in the dorsal vagal complex (DVC) remains unclear. These studies utilized single-cell quantitative real-time RT-PCR, in conjunction with laser-catapult microdissection, to address the hypothesis that DVC A2 neurons express genes that encode the characterized metabolic transducers, e.g. glucokinase (GCK) and the energy-dependent potassium channel, K(ATP). Studies show that either glucose or lactate alters synaptic firing of DVC chemosensory neurons, and that delivery of the latter fuel into the caudal hindbrain amplifies insulin-induced hypoglycemia (IIH) and elevates neuronal glucose and monocarboxylate transporter, GCK, and sulfonylurea-1 mRNA in the DVC. We thus examined the additional premise that IIH modifies A2 substrate transporter and metabolic transducer gene profiles, and that such transcriptional responses may be reversed by exogenous lactate and/or glucose. Individual tyrosine hydroxylase (TH)-immunoreactive (-ir) A2 neurons were microdissected from the caudal DVC 2 h after injection of insulin or saline, and continuous caudal fourth ventricular (CV4) infusion of lactate, glucose, or artificial cerebrospinal fluid. The data show that IIH decreased MCT2, but elevated GLUT3, GLUT4, GCK, and SUR-1 transcripts in A2 neurons. Blood glucose levels in insulin-injected rats were further reduced by CV4 infusion of either lactate or glucose. Lactate plus insulin reversed hypoglycemic reductions in MCT2 mRNA and further augmented GLUT3 transcripts in A2 neurons, whereas glucose infusion in insulin-injected rats further increased GLUT3 and GCK gene profiles. The present results demonstrate that caudal DVC A2 neurons express molecular markers for metabolic sensing, and genes that encode glucose and monocarboxylate transporters. Evidence that IIH reduces A2 MCT2, but elevates GLUT3 and GLUT4 gene profiles suggests that glucose may be a primary energy source to these cells during hypoglycemia, while decreased lactate uptake, alone or relative to glucose uptake, may be a critical manifestation of systemic glucose deficiency at the cellular level. Findings that singular fuel repletion does not normalize hypoglycemic patterns of glucose transporter, GCK, or SUR-1 mRNA expression in A2 neurons imply that sufficient supply of both energy substrates is required for metabolic balance, and that cellular adaptation to the prevalence of either fuel may increase cellular dependence on glucose-specific metabolites or other products.

Effects of orchidectomy on adaptation of arcuate neuropeptide Y, proopiomelanocortin, and cocaine- and amphetamine-related transcript gene profiles to recurring insulin-induced hypoglycemia in the male rat

Our studies show that recurring insulin-induced hypoglycemia (RIIH) diminishes neuronal activation in several key components of the central metabolic regulatory circuitry, including the hypothalamic arcuate nucleus (ARH), and that orchidectomy (ORDX) modifies this impact of RIIH on Fos protein expression in this and other select neural structures. The testicular hormone, testosterone, regulates the expression of ARH neuropeptide genes of characterized metabolic relevance, including neuropeptide Y (NPY) and proopiomelanocortin (POMC). We investigated the hypothesis that acute hypoglycemia-associated patterns of ARH NPY, POMC, and cocaine and amphetamine-related transcript (CART) gene transcription, and potential RIIH-associated adaptive modifications in these expression profiles are regulated by testes-dependent mechanisms. ARH tissue was micropunched from serial frozen brains sections obtained from sham-operated (SHAM) or bilaterally ORDX male rats after sc injection of one or four doses of neutral protamine Hagedorn insulin, over as many days, or vehicle alone, and analyzed by quantitative real-time RT-PCR. In SHAM rats, acute hypoglycemia increased ARH NPY mRNA; precedent hypoglycemia elevated baseline gene expression in this group, but suppressed transcription during RIIH. In ORDX rats, ARH NPY mRNA was decreased during acute hypoglycemia and after multiple exposures; however, gene expression was not further suppressed by RIIH. ARH POMC gene transcription was not modified by acute or recurring hypoglycemia in the SHAM group. ORDX caused a reduction in both basal and acute hypoglycemic patterns of POMC transcription, relative to the SHAM controls, but enhanced baseline and RIIH-associated patterns of gene expression. ARH CART transcripts were not altered by acute or recurring hypoglycemia in SHAM rats, whereas ORDX animals exhibited elevated CART gene expression during RIIH. These data show that acute and recurring hypoglycemia exert opposite effects on ARH NPY gene expression in testes-intact male rats, and that these transcriptional responses are abolished by ORDX. Hypoglycemia had no impact on POMC nor CART mRNA profiles in the SHAM group, but both genes were upregulated during RIIH in ORDX rats. Collectively, these results demonstrate that in the male rat, testes-dependent mechanisms underlie patterns of acclimated or unvarying reactivity to RIIH of the specific ARH metabolic neuropeptide genes evaluated here.

Adaptation of glucokinase gene expression in the rat dorsal vagal complex in a model for recurrent intermediate insulin-induced hypoglycemia: impact of gender

Standard therapeutic management of insulin-dependent diabetes mellitus with intermediate-acting insulin poses a significant risk for iatrogenic hypoglycemia and associated hypoglycemia-associated autonomic failure. Gender differences in preservation of counterregulatory function during recurring hypoglycemia have been documented in the clinical setting. In rats, repeated induction of prolonged hypoglycemia by neutral protamine Hagadorn insulin (NPH) results in diminished neuronal transcriptional activation in several key metabolic loci in male, but not female rat brain, including the hindbrain dorsal vagal complex (DVC). Glucose is committed to glycolytic catabolism by hexokinase-mediated phosphorylation. The low-affinity, high K (m) hexokinase, glucokinase (GCK), monitors intracellular glucose levels in pancreatic beta cells, and is purported to fulfill a similar function in the CNS. GCK is expressed in the rat DVC, where mRNA is localized to neurons that exhibit electrophysiological sensitivity to glucose imbalance. The current study investigated the hypothesis that DVC GCK gene expression acclimates to RIIH in a gender-dependent manner. Quantitative real-time RT-PCR was used to evaluate GCK mRNA levels in microdissected DVC tissue obtained from male and female rats before or after one or serial doses of NPH. Basal DVC GCK transcripts were equal between ovariectomized (OVX) female rats implanted with estradiol benzoate (EB) or oil (O). Tissue mRNA levels were increased following a single NPH injection in both groups, but this response was greater in the presence of estrogen. Basal GCK gene expression was elevated by precedent insulin dosing in both O- and EB-implanted rats; however, transcripts were not modified relative to this adjusted baseline during subsequent NPH treatment in either group of females. While acute NPH treatment did not modify tissue transcript levels in orchidectomized (ORDX) or sham-ORDX rats, precedent NPH dosing increased basal GCK mRNA levels and further enhanced gene expression during ensuring hypoglycemia in sham males. These studies provide novel evidence for gender-dimorphic DVC GCK gene responses to single and serial intermediate-acting insulin administration. Further research is required to determine if and how stimulatory versus inhibitory transcriptional adaptation of this gene profile in male and female rats, respectively, impacts glucose-sensing functions in the DVC, and whether such adjustments may contribute to gender differences in magnitude of precedent hypoglycemic impairment of counterregulation.

Caudal hindbrain lactate infusion alters glucokinase, SUR1, and neuronal substrate fuel transporter gene expression in the dorsal vagal complex, lateral hypothalamic area, and ventromedial nucleus hypothalamus of hypoglycemic male rats

While in vitro studies show that the oxidizable energy substrate, lactate, is a preferred fuel for CNS neurons during states of energy crisis, and that lactate may regulate neuronal glucose uptake under those conditions, its role in neuronal function in vivo remains controversial. Glucose-excited neurons in hindbrain dorsal vagal complex (DVC) monitor both glucose and lactate, and express both the glucose sensor, glucokinase (GK), and the SUR1 subunit of the plasma membrane energy transducer, K(ATP). Fourth ventricular lactate infusion exacerbates insulin-induced hypoglycemia (IIH) and IIH-associated patterns of DVC neuronal activation. We investigated the hypothesis that during glucoprivation, lactate regulates neuronal monocarboxylate and glucose transporter gene transcription in the DVC, and adjustments in these gene profiles are correlated with altered GK and SUR1 mRNA expression. We also examined whether caudal hindbrain lactate repletion alters the impact of hypoglycemia on substrate fuel uptake and metabolic sensing functions in other characterized metabolic monitoring sites, e.g., the ventromedial hypothalamic nucleus (VMH) and lateral hypothalamic area (LHA). qPCR was used to measure MCT2, GLUT3, GLUT4, GK, and SUR1 transcripts in the microdissected DVC, VMH, and LHA from groups of male rats treated by continuous infusion of aCSF or lactate into the caudal fourth ventricle (CV4), initiated prior to injection of Humulin R or saline. Blood glucose was decreased in response to insulin, a response that was significantly augmented by CV4 lactate infusion. IIH alone did not alter mean DVC MCT2, GLUT3, GLUT4, GK, or SUR1 mRNA levels, but these transcripts were increased in the lactate plus insulin group, relative to both euglycemic and aCSF-infused hypoglycemic rats. IIH decreased MCT2, GLUT3, and SUR1 gene profiles in the VMH; CV4 lactate infusion during IIH further diminished these transcripts, and suppressed GLUT4 and GK mRNA levels in this site. In LHA, IIH increased GLUT3 and SUR1 gene expression to an equal extent, with or without lactate, while GLUT4, MCT2, and GK mRNA levels were elevated only in response to lactate plus insulin. These studies show that caudal hindbrain-targeted delivery of exogenous lactate during IIH upregulates neuronal monocarboxylate and glucose transporter, GK, and SUR1 gene profiles in the DVC, and results in increased or decreased GLUT4 and GK mRNA in LHA and VMH, respectively. These data suggest that lactate and glucose utilization by DVC neurons may be enhanced in response to local lactate surfeit, alone or relative to glucose deficiency, and that increases in intracellular glucose and net energy yield may be correlated with elevated GK and SUR1 gene transcription, respectively, in local glucose sensing neurons. The results also imply that GLUT4- and GK-mediated glucose uptake and glucose sensing functions in the VMH and LHA may be reactive to DVC signaling of relative lactate abundance within the caudal hindbrain, and/or to physiological sequelae of this fuel augmentation, including amplified hypoglycemia.