Neuronal Glucose Research Articles

Misprogramming of glucose metabolism impairs recovery of hippocampal slices from neuronal GLT-1 knockout mice and contributes to excitotoxic injury through mitochondrial superoxide production.

We have previously reported a failure of recovery of synaptic function in the CA1 region of acute hippocampal slices from mice with a conditional neuronal knockout (KO) of GLT-1 (EAAT2, Slc1A2) driven by synapsin-Cre (synGLT-1 KO). The failure of recovery of synaptic function is due to excitotoxic injury. We hypothesized that changes in mitochondrial metabolism contribute to the heightened vulnerability to excitotoxicity in the synGLT-1 KO mice. We found impaired flux of carbon from 13C-glucose into the tricarboxylic acid cycle in synGLT-1 KO cortical and hippocampal slices compared with wild-type (WT) slices. In addition, we found downregulation of the neuronal glucose transporter GLUT3 in both genotypes. Flux of carbon from [1,2-13C]acetate, thought to be astrocyte-specific, was increased in the synGLT-KO hippocampal slices but not cortical slices. Glycogen stores, predominantly localized to astrocytes, are rapidly depleted in slices after cutting, and are replenished during exvivo incubation. In the synGLT-1 KO, replenishment of glycogen stores during exvivo incubation was compromised. These results suggest both neuronal and astrocytic metabolic perturbations in the synGLT-1 KO slices. Supplementing incubation medium during recovery with 20 mM D-glucose normalized glycogen replenishment but had no effect on recovery of synaptic function. In contrast, 20 mM non-metabolizable L-glucose substantially improved recovery of synaptic function, suggesting that D-glucose metabolism contributes to the excitotoxic injury in the synGLT-1 KO slices. L-lactate substitution for D-glucose did not promote recovery of synaptic function, implicating mitochondrial metabolism. Consistent with this hypothesis, phosphorylation of pyruvate dehydrogenase, which decreases enzyme activity, was increased in WT slices during the recovery period, but not in synGLT-1 KO slices. Since metabolism of glucose by the mitochondrial electron transport chain is associated with superoxide production, we tested the effect of drugs that scavenge and prevent superoxide production. The superoxide dismutase/catalase mimic EUK-134 conferred complete protection and full recovery of synaptic function. A site-specific inhibitor of complex III superoxide production, S3QEL-2, was also protective, but inhibitors of NADPH oxidase were not. In summary, we find that the failure of recovery of synaptic function in hippocampal slices from the synGLT-1 KO mouse, previously shown to be due to excitotoxic injury, is caused by production of superoxide by mitochondrial metabolism.

Restoring hippocampal glucose metabolism rescues cognition across Alzheimer's disease pathologies.

Impaired cerebral glucose metabolism is a pathologic feature of Alzheimer Disease (AD), and recent proteomic studies highlight a disruption of glial carbohydrate metabolism with disease progression. Here, we report that inhibition of indoleamine-2,3-dioxygenase 1 (IDO1), which metabolizes tryptophan to kynurenine (KYN) in the first step of the kynurenine pathway, rescues hippocampal memory function and plasticity in preclinical models of amyloid and tau pathology by restoring astrocytic metabolic support of neurons. Activation of IDO1 in astrocytes by amyloid-beta 42 and tau oligomers, two major pathological effectors in AD, increases KYN and suppresses glycolysis in an AhR-dependent manner. Conversely, pharmacological IDO1 inhibition restores glycolysis and lactate production. In amyloid-producing APP Swe -PS1 ΔE9 and 5XFAD mice and in tau-producing P301S mice, IDO1 inhibition restores spatial memory and improves hippocampal glucose metabolism by metabolomic and MALDI-MS analyses. IDO1 blockade also rescues hippocampal long-term potentiation (LTP) in a monocarboxylate transporter (MCT)-dependent manner, suggesting that IDO1 activity disrupts astrocytic metabolic support of neurons. Indeed, in vitro mass-labeling of human astrocytes demonstrates that IDO1 regulates astrocyte generation of lactate that is then taken up by human neurons. In co-cultures of astrocytes and neurons derived from AD subjects, deficient astrocyte lactate transfer to neurons was corrected by IDO1 inhibition, resulting in improved neuronal glucose metabolism. Thus, IDO1 activity disrupts astrocytic metabolic support of neurons across both amyloid and tau pathologies and in a model of AD iPSC-derived neurons. These findings also suggest that IDO1 inhibitors developed for adjunctive therapy in cancer could be repurposed for treatment of amyloid- and tau-mediated neurodegenerative diseases.

WNKs regulate mouse behavior and alter central nervous system glucose uptake and insulin signaling.

Certain areas of the brain involved in episodic memory and behavior, such as the hippocampus, express high levels of insulin receptors and glucose transporter-4 (GLUT4) and are responsive to insulin. Insulin and neuronal glucose metabolism improve cognitive functions and regulate mood in humans. Insulin-dependent GLUT4 trafficking has been extensively studied in muscle and adipose tissue, but little work has demonstrated either how it is controlled in insulin-responsive brain regions or its mechanistic connection to cognitive functions. In this study, we demonstrate that inhibition of WNK (With-No-lysine (K)) kinases improves learning and memory in mice. Neuronal inhibition of WNK enhances in vivo hippocampal glucose uptake. Inhibition of WNK enhances insulin signaling output and insulin-dependent GLUT4 trafficking to the plasma membrane in mice primary neuronal cultures and hippocampal slices. Therefore, we propose that the extent of neuronal WNK kinase activity has an important influence on learning, memory and anxiety-related behaviors, in part, by modulation of neuronal insulin signaling.

Tonic excitation by astrocytic GABA causes neuropathic pain by augmenting neuronal activity and glucose metabolism

Neuropathic pain is a debilitating condition caused by the hyperexcitability of spinal dorsal horn neurons and is often characterized by allodynia. Although neuron-independent mechanisms of hyperexcitability have been investigated, the contribution of astrocyte-neuron interactions remains unclear. Here, we show evidence of reactive astrocytes and their excessive GABA release in the spinal dorsal horn, which paradoxically leads to the tonic excitation of neighboring neurons in a neuropathic pain model. Using multiple electrophysiological methods, we demonstrated that neuronal hyperexcitability is attributed to both increased astrocytic GABA synthesis via monoamine oxidase B (MAOB) and the depolarized reversal potential of GABA-mediated currents (EGABA) via the downregulation of the neuronal K+/Cl− cotransporter KCC2. Furthermore, longitudinal 2-deoxy-2-[18F]-fluoro-D-glucose microPET imaging demonstrated increased regional glucose metabolism in the ipsilateral dorsal horn, reflecting neuronal hyperexcitability. Importantly, inhibiting MAOB restored the entire astrocytic GABA-mediated cascade and abrogated the increased glucose metabolism and mechanical allodynia. Overall, astrocytic GABA-mediated tonic excitation is critical for neuronal hyperexcitability, leading to mechanical allodynia and neuropathic pain.

Neuronal glucose sensing mechanisms and circuits in the control of insulin and glucagon secretion.

Glucose homeostasis is mainly under the control of the pancreatic islet hormones insulin and glucagon, which, respectively, stimulate glucose uptake and utilization by liver, fat, and muscle and glucose production by the liver. The balance between the secretions of these hormones is under the control of blood glucose concentrations. Indeed, pancreatic islet β-cells and α-cells can sense variations in glycemia and respond by an appropriate secretory response. However, the secretory activity of these cells is also under multiple additional metabolic, hormonal, and neuronal signals that combine to ensure the perfect control of glycemia over a lifetime. The central nervous system (CNS), which has an almost absolute requirement for glucose as a source of metabolic energy and thus a vital interest in ensuring that glycemic levels never fall below ∼5 mM, is equipped with populations of neurons responsive to changes in glucose concentrations. These neurons control pancreatic islet cell secretion activity in multiple ways: through both branches of the autonomic nervous system, through the hypothalamic-pituitary-adrenal axis, and by secreting vasopressin (AVP) in the blood at the level of the posterior pituitary. Here, we present the autonomic innervation of the pancreatic islets; the mechanisms of neuron activation by a rise or a fall in glucose concentration; how current viral tracing, chemogenetic, and optogenetic techniques allow integration of specific glucose sensing neurons in defined neuronal circuits that control endocrine pancreas function; and, finally, how genetic screens in mice can untangle the diversity of the hypothalamic mechanisms controlling the response to hypoglycemia.

Effects of a Diabetic Microenvironment on Neurodegeneration: Special Focus on Neurological Cells.

Diabetes is a chronic metabolic condition associated with high levels of blood glucose which leads to serious damage to the heart, kidney, eyes, and nerves. Elevated blood glucose levels damage brain function and cognitive abilities. They also lead to various neurological and neuropsychiatric disorders, including chronic neurodegeneration and cognitive decline. High neuronal glucose levels can cause drastic neuronal damage due to glucose neurotoxicity. Astrocytes, a type of glial cell, play a vital role in maintaining brain glucose levels through neuron-astrocyte coupling. Hyperglycemia leads to progressive decline in neuronal networks and cognitive impairment, contributing to neuronal dysfunction and fostering a neurodegenerative environment. In this review, we summarize the various connections, functions, and impairments of glial cells due to metabolic dysfunction in the diabetic brain. We also summarize the effects of hyperglycemia on various neuronal functions in the diabetic brain.

Daily fluctuations in blood glucose with normal aging are inversely related to hippocampal synaptic mitochondrial proteins

Defective brain glucose utilization is a hallmark of Alzheimer’s disease (AD) while Type II diabetes and elevated blood glucose escalate the risk for AD in later life. Isolating contributions of normal aging from coincident metabolic or brain diseases could lead to refined approaches to manage specific health risks and optimize treatments targeted to susceptible older individuals. We evaluated metabolic, neuroendocrine, and neurobiological differences between young adult (6 months) and aged (24 months) male rats. Compared to young adults, blood glucose was significantly greater in aged rats at the start of the dark phase of the day but not during the light phase. When challenged with physical restraint, a potent stressor, aged rats effected no change in blood glucose whereas blood glucose increased in young adults. Tissues were evaluated for markers of oxidative phosphorylation (OXPHOS), neuronal glucose transport, and synapses. Outright differences in protein levels between age groups were not evident, but circadian blood glucose was inversely related to OXPHOS proteins in hippocampal synaptosomes, independent of age. The neuronal glucose transporter, GLUT3, was positively associated with circadian blood glucose in young adults whereas aged rats tended to show the opposite trend. Our data demonstrate aging increases daily fluctuations in blood glucose and, at the level of individual differences, negatively associates with proteins related to synaptic OXPHOS. Our findings imply that glucose dyshomeostasis may exacerbate metabolic aspects of synaptic dysfunction that contribute to risk for age-related brain disorders.

Frontal and anterior temporal hypometabolism post chemoradiation in head and neck cancer: A real-world PET study.

Adverse neurological effects after cancer therapy are common, but biomarkers to diagnose, monitor, or risk stratify patients are still not validated or used clinically. An accessible imaging method, such as fluorodeoxyglucose positron emission tomography (FDG PET) of the brain, could meet this gap and serve as a biomarker for functional brain changes. We utilized FDG PET to evaluate which brain regions are most susceptible to altered glucose metabolism after chemoradiation in patients with head and neck cancer (HNCa). Real-world FDG PET images were acquired as standard of care before and after chemoradiation for HNCa in 68 patients. Linear mixed-effects voxelwise models assessed changes after chemoradiation in cerebral glucose metabolism quantified with standardized uptake value ratio (SUVR), covarying for follow-up time and patient demographics. Voxelwise analysis revealed two large clusters of decreased glucose metabolism in the medial frontal and polar temporal cortices following chemoradiation, with decreases of approximately 5% SUVR after therapy. These findings provide evidence that standard chemoradiation for HNCa can lead to decreased neuronal glucose metabolism, contributing to literature emphasizing the vulnerability of the frontal and anterior temporal lobes, especially in HNCa, where these areas may be particularly vulnerable to indirect radiation-induced injury. FDG PET shows promise as a sensitive biomarker for assessing these changes.

Detection of Brain‐derived Neurotrophic Factor Expression in Insulin‐Resistant SH‐SY5Y Neuroblastoma Cells

AbstractBackgroundMore researchers are coining Alzheimer’s disease (AD) “type 3 diabetes” due to its significant comorbidity and shared pathology with type 2 diabetes mellitus (T2DM). A bi‐directional relationship between T2DM and AD exists, by which 50‐75% of individuals with T2DM have a greater risk of developing AD compared to non‐diabetes and that AD symptoms progress with impaired glucose utilization. Interestingly, brain‐derived neurotrophic factor (BDNF), a small protein that maintains neuron health and glucose homeostasis, has been implicated in both AD and IR. As such, this experiment investigates if insulin resistance (IR) alters BDNF expression in SH‐SY5Y neuroblastoma cells.MethodObjective 1: create a model of insulin resistance at 24, 48, 72 and 96 hours (hr) and determine expression of pAkt (S473), tAkt, insulin receptor b (INSRB) and pINSR (T1362) on Western blots. (n = 5)Objective 2: Detect mRNA and protein BDNF via Rt‐PCR and Western blotting. (n = 5)Objectives 3: Determine alterations to BDNF expression via Western blotting and qPCR. (n = 5)ResultLevels of pAkt expression were significantly decreased in insulin‐pretreated and activated samples at each timepoint of 24, 48, 72 and 96 hr; although, with each timepoint, pAkt does appear to steadily increase. pro‐BDNF and mature BDNF were detected at their respective sizes, 27kDa and 19kDa, on a Western blot and BDNF mRNA was detected on an agarose gel after PCR.ConclusionIt appears that insulin resistance is occurring at 24, 48, 72 and 96 hr solely based off Akt expression. This result will be further confirmed with the the completion of INSR‐B and p‐INSR blots. Ongoing experiments include western blots and qPCRs of BDNF to determine if there is any alteration to expression.

Understanding the progression of Alzheimer’s Disease in 5XFAD transgenic female mice using 1H‐[13C]‐NMR Spectroscopy

AbstractBackground5XFAD female mice have been reported to exhibit severe plaque loading at an early age. However, brain glucose metabolism is poorly understood in this model. The objective of the current study was to understand neurometabolism and its role in disease progression and pathophysiology in female transgenic mice at various age points.MethodThe current study uses a tracer‐based approach, which involves infusion of [1,6]13C2 labeled glucose into AD mouse model 5XFAD, to observe the changes in the neuronal glucose oxidation in this pathological condition with the progression of age.ResultThere was a significant memory loss in the 6 months‐old (6M) transgenic females compared to the 15 months‐old (15M) animals through Novel Object Recognition and Y‐Maze tests. The neurochemical profile showed higher myo‐inositol levels in the cerebral cortex in the 15M mice wrt. the 6M ones, which implies that there is increased gliosis and neuroinflammation in the former. There is also an increased level of glutamine in the 15M mice, which is known to act as an osmolyte and cause brain edema. However, the cerebral metabolic rates of glucose oxidation in Glutamatergic and GABAergic neurons did not show a significant difference in 5xFAD female mice when compared with wild‐type mice of the same age.ConclusionThe imperturbation in the female mice might be linked to some active underlying neuroprotective mechanism and needs to be explored further. Also, to get a clear picture of the gender‐dependent AD progression, studies need to be conducted in the age‐matched male mice as well.

Glucose Transport and Utilization in the Hippocampus: From Neurophysiology to Diabetes-Related Development of Dementia.

The association of diabetes with cognitive dysfunction has at least 60 years of history, which started with the observation that children with type 1 diabetes mellitus (T1D), who had recurrent episodes of hypoglycemia and consequently low glucose supply to the brain, showed a deficit of cognitive capacity. Later, the growing incidence of type 2 diabetes mellitus (T2D) and dementia in aged populations revealed their high association, in which a reduced neuronal glucose supply has also been considered as a key mechanism, despite hyperglycemia. Here, we discuss the role of glucose in neuronal functioning/preservation, and how peripheral blood glucose accesses the neuronal intracellular compartment, including the exquisite glucose flux across the blood-brain barrier (BBB) and the complex network of glucose transporters, in dementia-related areas such as the hippocampus. In addition, insulin resistance-induced abnormalities in the hippocampus of obese/T2D patients, such as inflammatory stress, oxidative stress, and mitochondrial stress, increased generation of advanced glycated end products and BBB dysfunction, as well as their association with dementia/Alzheimer's disease, are addressed. Finally, we discuss how these abnormalities are accompained by the reduction in the expression and translocation of the high capacity insulin-sensitive glucose transporter GLUT4 in hippocampal neurons, which leads to neurocytoglycopenia and eventually to cognitive dysfunction. This knowledge should further encourage investigations into the beneficial effects of promising therapeutic approaches which could improve central insulin sensitivity and GLUT4 expression, to fight diabetes-related cognitive dysfunctions.

Visualizing Monocarboxylates and Other Relevant Metabolites in the Ex Vivo Drosophila Larval Brain Using Genetically Encoded Sensors.

The high energy requirements of brains due to electrical activity are one of their most distinguishing features. These requirements are met by the production of ATP from glucose and its metabolites, such as the monocarboxylates lactate and pyruvate. It is still unclear how this process is regulated or who the key players are, particularly in Drosophila. Using genetically encoded Förster resonance energy transfer-based sensors, we present a simple method for measuring the transport of monocarboxylates and glucose in glial cells and neurons in an ex-vivo Drosophila larval brain preparation. The protocol describes how to dissect and adhere a larval brain expressing one of the sensors to a glass coverslip. We present the results of an entire experiment in which lactate transport was measured in larval brains by knocking down previously identified monocarboxylate transporters in glial cells. Furthermore, we demonstrate how to rapidly increase neuronal activity and track metabolite changes in the active brain. The described method, which provides all necessary information, can be used to analyze other Drosophila living tissues.

Epileptiform Discharges Reduce Neuronal ATP Production by Inhibiting F0F1-ATP Synthase Activity via A Zinc-α2-Glycoprotein-Dependent Mechanism.

Neuronal energy metabolism dysfunction, especially adenosine triphosphate (ATP) supply decrease, is observed in epilepsy and associated with epileptogenesis and prognosis. Zinc-α2-glycoprotein (ZAG) is known as an important modulator of energy metabolism and involved in neuronal glucose metabolism, fatty acid metabolism, and ketogenesis impairment in seizures, but its effect on neuronal ATP synthesis in seizures and the specific mechanism are unclear. In this study, we verified the localization of ZAG in primary cultured neuronal mitochondria by using double-labeling immunofluorescence, immune electron microscopy, and western blot. ZAG level in neuronal mitochondria was modulated by lentiviruses and detected by western blot. The F0F1-ATP synthase activity, ATP level, and acetyl-CoA level were measured. The binding between ZAG and F0F1-ATP synthase was determined by coimmunoprecipitation. We found that both ZAG and F0F1-ATP synthase existed in neuronal mitochondria, and there was mutual binding between them. Epileptiform discharge-induced decrease of mitochondrial ZAG level was reversed by ZAG overexpression. Epileptiform discharge or ZAG knockdown decreased F0F1-ATP synthase activity and ATP level in neurons, which were reversed by ZAG overexpression, while overexpression of ZAG along only increased F0F1-ATP synthase activity but not increased ATP level. Meanwhile, neither epileptiform discharges nor changes of ZAG level can alter the acetyl-CoA level. Moreover, epileptiform discharge did not alter F0F1-ATP synthase level. In conclusion, epileptiform discharge-induced ZAG decrease in neuronal mitochondria is correlated to F0F1-ATP synthase activity inhibition, which may possibly lead to ATP supply impairments. ZAG may be a potential therapeutic target for treating neuronal energy metabolism dysfunction in seizures with further researches.

Aquaporin-4 Deficiency is Associated with Cognitive Impairment and Alterations in astrocyte-neuron Lactate Shuttle.

Cognitive impairment refers to notable declines in cognitive abilities including memory, language, and emotional stability leading to the inability to accomplish essential activities of daily living. Astrocytes play an important role in cognitive function, and homeostasis of the astrocyte-neuron lactate shuttle (ANLS) system is essential for maintaining cognitive functions. Aquaporin-4 (AQP-4) is a water channel expressed in astrocytes and has been shown to be associated with various brain disorders, but the direct relationship between learning, memory, and AQP-4 is unclear. We examined the relationship between AQP-4 and cognitive functions related to learning and memory. Mice with genetic deletion of AQP-4 showed significant behavioral and emotional changes including hyperactivity and instability, and impaired cognitive functions such as spatial learning and memory retention. 18F-FDG PET imaging showed significant metabolic changes in the brains of AQP-4 knockout mice such as reductions in glucose absorption. Such metabolic changes in the brain seemed to be the direct results of changes in the expression of metabolite transporters, as the mRNA levels of multiple glucose and lactate transporters in astrocytes and neurons were significantly decreased in the cortex and hippocampus of AQP-4 knockout mice. Indeed, AQP-4 knockout mice showed significantly higher accumulation of both glucose and lactate in their brains compared with wild-type mice. Our results show that the deficiency of AQP-4 can cause problems in the metabolic function of astrocytes and lead to cognitive impairment, and that the deficiency of AQP4 in astrocyte endfeet can cause abnormalities in the ANLS system.

BMECs Ameliorate High Glucose-Induced Morphological Aberrations and Synaptic Dysfunction via VEGF-Mediated Modulation of Glucose Uptake in Cortical Neurons.

It has been demonstrated that diabetes cause neurite degeneration in the brain and cognitive impairment and neurovascular interactions are crucial for maintaining brain function. However, the role of vascular endothelial cells in neurite outgrowth and synaptic formation in diabetic brain is still unclear. Therefore, present study investigated effects of brain microvascular endothelial cells (BMECs) on high glucose (HG)-induced neuritic dystrophy using a coculture model of BMECs with neurons. Multiple immunofluorescence labelling and western blot analysis were used to detect neurite outgrowth and synapsis formation, and living cell imaging was used to detect uptake function of neuronal glucose transporters. We found cocultured with BMECs significantly reduced HG-induced inhibition of neurites outgrowth (including length and branch formation) and delayed presynaptic and postsynaptic development, as well as reduction of neuronal glucose uptake capacity, which was prevented by pre-treatment with SU1498, a vascular endothelial growth factor (VEGF) receptor antagonist. To analyse the possible mechanism, we collected BMECs cultured condition medium (B-CM) to treat the neurons under HG culture condition. The results showed that B-CM showed the same effects as BMEC on HG-treated neurons. Furthermore, we observed VEGF administration could ameliorate HG-induced neuronal morphology aberrations. Putting together, present results suggest that cerebral microvascular endothelial cells protect against hyperglycaemia-induced neuritic dystrophy and restorate neuronal glucose uptake capacity by activation of VEGF receptors and endothelial VEGF release. This result help us to understand important roles of neurovascular coupling in pathogenesis of diabetic brain, providing a new strategy to study therapy or prevention for diabetic dementia. Hyperglycaemia induced inhibition of neuronal glucose uptake and impaired to neuritic outgrowth and synaptogenesis. Cocultured with BMECs/B-CM and VEGF treatment protected HG-induced inhibition of glucose uptake and neuritic outgrowth and synaptogenesis, which was antagonized by blockade of VEGF receptors. Reduction of glucose uptake may further deteriorate impairment of neurites outgrowth and synaptogenesis.

Effect of eIF2α in Neuronal Injury Induced by High Glucose and the Protective Mechanism of Resveratrol.

Diabetes mellitus (DM) is a type of metabolic disease characterized by chronic hyperglycemia, which can lead to different degrees of cognitive decline. Therefore, it is crucial to explore the molecular biological mechanisms of neuronal injury. In this study, we investigated the effect of high glucose on eIF2α expression and the mechanism of neuronal injury, and on this basis, the protective mechanism of resveratrol is explored. Treatment with 50 mM high glucose in cortical neurons increased the levels of eIF2α phosphorylation; the expressions of ATF4 and CHOP increased. ISRIB alleviated high glucose-induced neuronal injury by reducing eIF2α phosphorylation when neurons were pretreated with ISRIB before high glucose treatment. Compared with the high glucose-treated group, resveratrol pretreatment reduced eIF2α phosphorylation, the levels of its downstream molecules ATF4 and CHOP, and LDH release. Resveratrol reduced the level of cortical eIF2α phosphorylation and the expression of its downstream molecules in DM mice and improved the ability of spatial memory and learning in DM mice without affecting anxiety and motor performance. Meanwhile, resveratrol modulated the expression of Bcl-2 protein and also effectively decreased the DM-induced up-regulation of Bax, caspase-3, p53, p21, and p16. Taken together, these results suggested that high glucose caused neuronal injury through the eIF2α/ATF4/CHOP pathway which was inhibited by ISRIB and resveratrol. The present study indicates that eIF2α is the new target for the treatment of high glucose-induced neuronal injury, and resveratrol is a potential new medicine to treat diabetes encephalopathy.

Lactate as a supplemental fuel for synaptic transmission and neuronal network oscillations: Potentials and limitations.

Lactate shuttled from the blood circulation, astrocytes, oligodendrocytes or even activated microglia (resident macrophages) to neurons has been hypothesized to represent a major source of pyruvate compared to what is normally produced endogenously by neuronal glucose metabolism. However, the role of lactate oxidation in fueling neuronal signaling associated with complex cortex function, such as perception, motor activity, and memory formation, is widely unclear. This issue has been experimentally addressed using electrophysiology in hippocampal slice preparations (ex vivo) that permit the induction of different neural network activation states by electrical stimulation, optogenetic tools or receptor ligand application. Collectively, these studies suggest that lactate in the absence of glucose (lactate only) impairs gamma (30-70 Hz) and theta-gamma oscillations, which feature high energy demand revealed by the cerebral metabolic rate of oxygen (CMRO2, set to 100%). The impairment comprises oscillation attenuation or moderate neural bursts (excitation-inhibition imbalance). The bursting is suppressed by elevating the glucose fraction in energy substrate supply. By contrast, lactate can retain certain electric stimulus-induced neural population responses and intermittent sharp wave-ripple activity that features lower energy expenditure (CMRO2 of about 65%). Lactate utilization increases the oxygen consumption by about 9% during sharp wave-ripples reflecting enhanced adenosine-5'-triphosphate (ATP) synthesis by oxidative phosphorylation in mitochondria. Moreover, lactate attenuates neurotransmission in glutamatergic pyramidal cells and fast-spiking, γ-aminobutyric acid (GABA)ergic interneurons by reducing neurotransmitter release from presynaptic terminals. By contrast, the generation and propagation of action potentials in the axon is regular. In conclusion, lactate is less effective than glucose and potentially detrimental during neural network rhythms featuring high energetic costs, likely through the lack of some obligatory ATP synthesis by aerobic glycolysis at excitatory and inhibitory synapses. High lactate/glucose ratios might contribute to central fatigue, cognitive impairment, and epileptic seizures partially seen, for instance, during exhaustive physical exercise, hypoglycemia and neuroinflammation.

Hypothalamic PVN astrocytes fail to properly set glutamate tone in obese mice contributing to impaired glucose metabolism in this condition

The hypothalamus is key in the control of energy balance, but strategies targeting hypothalamic neurons have failed to provide viable options to treat most metabolic diseases. Conversely, the role of astrocytes in systemic metabolic control has remained largely unexplored. Here, we show that obesity promotes anatomically restricted exacerbated Ca 2+ activity in hypothalamic paraventricular nucleus (PVN) astrocytes. Bidirectional chemogenetic manipulation of astrocyte Ca 2+ activity in lean mice stimulated or inhibited the activity of neighboring parvocellular neurons. Importantly, these effects translated into bidirectional regulation of sympathetic activity, glucose metabolism, and energy balance. We found that the astrocyte-mediated effects on PVN neurons were blocked by the glutamate receptor blocker kynurenic acid, and the astrocyte glutamate transport blocker TBOA, suggesting that the astrocyte-neuron signaling and the effects on glucose metabolism involved the astrocytic control of ambient glutamate levels. Notably, we found that TBOA alone stimulated neurons in lean mice but failed to evoke a response in obese mice. This suggests that the astrocyte glutamate transporter activity is impaired in obese mice. Finally, we found that chemogenetic inhibition of astrocyte Ca 2+ activity in obese mice improved glucose metabolism in this condition. Together, our results indicate that exacerbated astrocyte Ca 2+ activity leads to impaired glutamate transporter activity, standing as a likely mechanism contributing to altered PVN neuronal activity and glucose homeostasis during this condition. Collectively, these findings highlight a yet unappreciated role for astrocytes in the direct control of systemic metabolism and suggest potential targets for anti-obesity strategy. NHLBI F32 HL158172 (MKK) NHLBI HL090948 (JES) NINDS NS094640 (JES) This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Postmortem Brains from Subjects with Diabetes Mellitus Display Reduced GLUT4 Expression and Soma Area in Hippocampal Neurons: Potential Involvement of Inflammation.

Diabetes mellitus (DM) is an important risk factor for dementia, which is a common neurodegenerative disorder. DM is known to activate inflammation, oxidative stress, and advanced glycation end products (AGEs) generation, all capable of inducing neuronal dysfunctions, thus participating in the neurodegeneration progress. In that process, disturbed neuronal glucose supply plays a key role, which in hippocampal neurons is controlled by the insulin-sensitive glucose transporter type 4 (GLUT4). We investigated the expression of GLUT4, nuclear factor NF-kappa B subunit p65 [NFKB (p65)], carboxymethyllysine and synapsin1 (immunohistochemistry), and soma area in human postmortem hippocampal samples from control, obese, and obese+DM subjects (41 subjects). Moreover, in human SH-SY5Y neurons, tumor necrosis factor (TNF) and glycated albumin (GA) effects were investigated in GLUT4, synapsin-1 (SYN1), tyrosine hydroxylase (TH), synaptophysin (SYP) proteins, and respective genes; NFKB binding activity in the SLC2A4 promoter; effects of increased histone acetylation grade by histone deacetylase 3 (HDAC3) inhibition. Hippocampal neurons (CA4 area) of obese+DM subjects displayed reduced GLUT4 expression and neuronal soma area, associated with increased expression of NFKB (p65). Challenges with TNF and GA decreased the SLC2A4/GLUT4 expression in SH-SY5Y neurons. TNF decreased SYN1, TH, and SYP mRNAs and respective proteins, and increased NFKB binding activity in the SLC2A4 promoter. Inhibition of HDAC3 increased the SLC2A4 expression and the total neuronal content of CRE-binding proteins (CREB/ICER), and also counterbalanced the repressor effect of TNF upon these parameters. This study revealed reduced postmortem human hippocampal GLUT4 content and neuronal soma area accompanied by increased proinflammatory activity in the brains of DM subjects. In isolated human neurons, inflammatory activation by TNF reduced not only the SLC2A4/GLUT4 expression but also the expression of some genes related to neuronal function (SYN1, TH, SYP). These effects may be related to epigenetic regulations (H3Kac and H4Kac status) since they can be counterbalanced by inhibiting HDAC3. These results uncover the improvement in GLUT4 expression and/or the inhibition of HDAC3 as promising therapeutic targets to fight DM-related neurodegeneration.

Visualizing reactive astrocyte-neuron interaction in Alzheimer's disease using 11C-acetate and 18F-FDG.

Reactive astrogliosis is a hallmark of Alzheimer's disease (AD). However, a clinically validated neuroimaging probe to visualize the reactive astrogliosis is yet to be discovered. Here, we show that PET imaging with 11C-acetate and 18F-fluorodeoxyglucose (18F-FDG) functionally visualizes the reactive astrocyte-mediated neuronal hypometabolism in the brains with neuroinflammation and AD. To investigate the alterations of acetate and glucose metabolism in the diseased brains and their impact on the AD pathology, we adopted multifaceted approaches including microPET imaging, autoradiography, immunohistochemistry, metabolomics, and electrophysiology. Two AD rodent models, APP/PS1 and 5xFAD transgenic mice, one adenovirus-induced rat model of reactive astrogliosis, and post-mortem human brain tissues were used in this study. We further curated a proof-of-concept human study that included 11C-acetate and 18F-FDG PET imaging analyses along with neuropsychological assessments from 11 AD patients and 10 healthy control subjects. We demonstrate that reactive astrocytes excessively absorb acetate through elevated monocarboxylate transporter-1 (MCT1) in rodent models of both reactive astrogliosis and AD. The elevated acetate uptake is associated with reactive astrogliosis and boosts the aberrant astrocytic GABA synthesis when amyloid-β is present. The excessive astrocytic GABA subsequently suppresses neuronal activity, which could lead to glucose uptake through decreased glucose transporter-3 in the diseased brains. We further demonstrate that 11C-acetate uptake was significantly increased in the entorhinal cortex, hippocampus and temporo-parietal neocortex of the AD patients compared to the healthy controls, while 18F-FDG uptake was significantly reduced in the same regions. Additionally, we discover a strong correlation between the patients' cognitive function and the PET signals of both 11C-acetate and 18F-FDG. We demonstrate the potential value of PET imaging with 11C-acetate and 18F-FDG by visualizing reactive astrogliosis and the associated neuronal glucose hypometablosim for AD patients. Our findings further suggest that the acetate-boosted reactive astrocyte-neuron interaction could contribute to the cognitive decline in AD.