Brain Glucose Metabolism: Integration of Energetics with Function

Gender differences in cell volume fraction (CVF): a structural parameter reflecting the energy efficiency of maintaining the resting membrane potential

The cell volume fraction (CVF) of the human brain is high (~82%) and is preserved across healthy aging while the brain declines in volume. These two observations, supported by several independent techniques, suggest that CVF is an important structural parameter. A new biophysical model is presented that incorporates CVF into the Goldman equation of classical membrane electrophysiology. The Goldman equation contains few structural constraints beyond two compartments separated by a semipermeable membrane supporting ion gradients. As potassium is the most permeable ion in the resting state, the resting membrane potential is determined by the potassium ion gradient. This biophysical model indicates that the sodium-potassium ion pumps use less energy at high CVF to maintain the resting membrane potential, explaining the high value of CVF and its conservation with healthy aging. CVF is measured to be statistically significantly higher in the brains of males compared with females, suggesting a structural requirement for higher energy efficiency in the larger male brain to support the greater number of neurons and synapses. As CVF can be measured in humans using quantitative sodium MRI and has potential implications for brain health, CVF may be a quantitative parameter that is useful for assessment of brain health, especially in patients with diseases such as dementia and psychiatric disease that do not have anatomical correlates detectable by clinical proton MRI.

Mitochondrial heterogeneity and homeostasis through the lens of a neuron

Mitochondria are vital organelles with distinct morphological features and functional properties. The dynamic network of mitochondria undergoes structural and functional adaptations in response to cell-type-specific metabolic demands. Even within the same cell, mitochondria can display wide diversity and separate into functionally distinct subpopulations. Mitochondrial heterogeneity supports unique subcellular functions and is crucial to polarized cells, such as neurons. The spatiotemporal metabolic burden within the complex shape of a neuron requires precisely localized mitochondria. By travelling great lengths throughout neurons and experiencing bouts of immobility, mitochondria meet distant local fuel demands. Understanding mitochondrial heterogeneity and homeostasis mechanisms in neurons provides a framework to probe their significance to many other cell types. Here, we put forth an outline of the multifaceted role of mitochondria in regulating neuronal physiology and cellular functions more broadly.

Lactylation may be a Novel Posttranslational Modification in Inflammation in Neonatal Hypoxic-Ischemic Encephalopathy

Perinatal hypoxia-ischemia remains the most common cause of acute neonatal brain injury and is associated with a high death rate and long-term neurological abnormalities such as memory and cognitive deficits and dyskinesia. Hypoxia-ischemia triggers an inflammatory cascade in the brain that is amplified by the activation of immune cells and the influx of peripheral immune cells into the brain parenchyma in response to cellular injury. Thus, acute cerebral hypoxic-ischemic inflammation is a major contributor to the pathogenesis of newborn hypoxic-ischemic brain injury. Lactate is a glycolysis end product that can regulate inflammation through histone lactylation, a unique posttranslational modification that was identified in recent studies. The purpose of this review is to outline the recent improvements in our understanding of microglia-mediated hypoxic-ischemic inflammation and to further discuss how histone lactylation regulates inflammation by affecting macrophage activation. These findings may suggest that epigenetic reprogramming-associated lactate input is linked to disease outcomes such as acute neonatal brain injury pathogenesis and the therapeutic effects of drugs and other strategies in relieving neonatal hypoxic-ischemic brain injury. Therefore, improving our knowledge of the reciprocal relationships between histone lactylation and inflammation could lead to the development of new immunomodulatory therapies for brain damage in newborns.

BrainPhys Neuronal Media Support Physiological Function of Mitochondria in Mouse Primary Neuronal Cultures

In vitro neuronal cultures are extensively used in the field of neurosciences as they represent an accessible experimental tool for neuronal genetic manipulation, time-lapse imaging, and drug screening. Optimizing the cultivation of rodent primary neuronal cultures led to the development of defined media that support the growth and maintenance of different neuronal types. Recently, a new neuronal medium, BrainPhys (BP), was formulated envisioning the mimicry of brain physiological conditions and suitability for cultured human iPSC-derived neurons and rat primary neurons. However, its advantages in mouse primary neuronal cultures and its effects in neuronal bioenergetics are yet to be demonstrated. In this study, we validated the beneficial use of BP in mouse primary neuronal cultures based on the observation that neuronal cultures in BP media showed enhanced ATP levels, which increased throughout neuronal maturation, a finding that correlates with higher mitochondrial activity and ATP production at later maturation stages, as well as an increased glycolysis response on mitochondrial inhibition and increased mitochondrial fuel flexibility. Taken together, our data demonstrate that BP medium promotes mitochondrial activity along with neuronal maturation of in vitro cultures.

Nurr1 Modulation Mediates Neuroprotective Effects of Statins

The ligand-sensing transcription factor Nurr1 emerges as a promising therapeutic target for neurodegenerative pathologies but Nurr1 ligands for functional studies and therapeutic validation are lacking. Here pronounced Nurr1 modulation by statins for which clinically relevant neuroprotective effects are demonstrated, is reported. Several statins directly affect Nurr1 activity in cellular and cell-free settings with low micromolar to sub-micromolar potencies. Simvastatin as example exhibits anti-inflammatory effects in astrocytes, which are abrogated by Nurr1 knockdown. Differential gene expression analysis in native and Nurr1-silenced cells reveals strong proinflammatory effects of Nurr1 knockdown while simvastatin treatment induces several neuroprotective mechanisms via Nurr1 involving changes in inflammatory, metabolic and cell cycle gene expression. Further in vitro evaluation confirms reduced inflammatory response, improved glucose metabolism, and cell cycle inhibition of simvastatin-treated neuronal cells. These findings suggest Nurr1 involvement in the well-documented but mechanistically elusive neuroprotection by statins.

Metabolic changes favor the activity and heterogeneity of reactive astrocytes

Reactive astrocytes undergo morphological, molecular, metabolic, and functional remodeling in response to central nervous system (CNS) damage. However, we still know very little about how the metabolic switching of astrocytes influences, or is influenced by, reactive astrocytes in response to neurological diseases. In this review, we initially cover a brief introduction into reactive astrocyte function under pathological conditions. Subsequently, we summarize the emerging roles of glucose and lipid metabolism in reactive astrocytes in the context of CNS injury to provide a new insight into metabolic mechanisms of reactive astrocyte-mediated neuroprotection or damage. Finally, we propose that deciphering the mechanistic link between astrocyte heterogeneity metabolism and improved methods is an emerging frontier for the therapeutic investigation of CNS injury and disease.

Therapeutic implications of glucose transporters (GLUT) in cerebral ischemia

Cerebral ischemia is a leading cause of death in the globe, with a large societal cost. Deprivation of blood flow, together with consequent glucose and oxygen shortage, activates a variety of pathways that result in permanent brain damage. As a result, ischemia raises energy demand, which is linked to significant alterations in brain energy metabolism. Even at the low glucose levels reported in plasma during ischemia, glucose transport activity may adjust to assure the supply of glucose to maintain normal cellular function. Glucose transporters in the brain are divided into two groups: sodium-independent glucose transporters (GLUTs) and sodium-dependent glucose cotransporters (SGLTs).This review assess the GLUT structure, expression, regulation, pathobiology of GLUT in cerebral ischemia and regulators of GLUT and it also provides the synopsis of the literature exploring the relationship between GLUT and the various downstream signalling pathways for e.g., AMP-activated protein kinase (AMPK), CREB (cAMP response element-binding protein), Hypoxia-inducible factor 1 (HIF)-1, Phosphatidylinositol 3-kinase (PI3-K), Mitogen-activated protein kinase (MAPK) and adenylate-uridylate-rich elements (AREs). Therefore, the aim of the present review was to elaborate the therapeutic implications of GLUT in the cerebral ischemia.

Monocarboxylate Transporter 1 May Benefit Cerebral Ischemia via Facilitating Lactate Transport From Glial Cells to Neurons

Monocarboxylate transporter 1 (MCT1) is expressed in glial cells and some populations of neurons. MCT1 facilitates astrocytes or oligodendrocytes (OLs) in the energy supplement of neurons, which is crucial for maintaining the neuronal activity and axonal function. It is suggested that MCT1 upregulation in cerebral ischemia is protective to ischemia/reperfusion (I/R) injury. Otherwise, its underlying mechanism has not been clearly discussed. In this review, it provides a novel insight that MCT1 may protect brain from I/R injury via facilitating lactate transport from glial cells (such as, astrocytes and OLs) to neurons. It extensively discusses (1) the structure and localization of MCT1; (2) the regulation of MCT1 in lactate transport among astrocytes, OLs, and neurons; and (3) the regulation of MCT1 in the cellular response of lactate accumulation under ischemic attack. At last, this review concludes that MCT1, in cerebral ischemia, may improve lactate transport from glial cells to neurons, which subsequently alleviates cellular damage induced by lactate accumulation (mostly in glial cells), and meets the energy metabolism of neurons.

A Ca2+-Dependent Mechanism Boosting Glycolysis and OXPHOS by Activating Aralar-Malate-Aspartate Shuttle, upon Neuronal Stimulation

Calcium is an important second messenger regulating a bioenergetic response to the workloads triggered by neuronal activation. In embryonic mouse cortical neurons using glucose as only fuel, activation by NMDA elicits a strong workload (ATP demand)-dependent on Na+ and Ca2+ entry, and stimulates glucose uptake, glycolysis, pyruvate and lactate production, and oxidative phosphorylation (OXPHOS) in a Ca2+-dependent way. We find that Ca2+ upregulation of glycolysis, pyruvate levels, and respiration, but not glucose uptake, all depend on Aralar/AGC1/Slc25a12, the mitochondrial aspartate-glutamate carrier, component of the malate-aspartate shuttle (MAS). MAS activation increases glycolysis, pyruvate production, and respiration, a process inhibited in the presence of BAPTA-AM, suggesting that the Ca2+ binding motifs in Aralar may be involved in the activation. Mitochondrial calcium uniporter (MCU) silencing had no effect, indicating that none of these processes required MCU-dependent mitochondrial Ca2+ uptake. The neuronal respiratory response to carbachol was also dependent on Aralar, but not on MCU. We find that mouse cortical neurons are endowed with a constitutive ER-to-mitochondria Ca2+ flow maintaining basal cell bioenergetics in which ryanodine receptors, RyR2, rather than InsP3R, are responsible for Ca2+ release, and in which MCU does not participate. The results reveal that, in neurons using glucose, MCU does not participate in OXPHOS regulation under basal or stimulated conditions, while Aralar-MAS appears as the major Ca2+-dependent pathway tuning simultaneously glycolysis and OXPHOS to neuronal activation.SIGNIFICANCE STATEMENT Neuronal activation increases cell workload to restore ion gradients altered by activation. Ca2+ is involved in matching increased workload with ATP production, but the mechanisms are still unknown. We find that glycolysis, pyruvate production, and neuronal respiration are stimulated on neuronal activation in a Ca2+-dependent way, independently of effects of Ca2+ as workload inducer. Mitochondrial calcium uniporter (MCU) does not play a relevant role in Ca2+ stimulated pyruvate production and oxygen consumption as both are unchanged in MCU silenced neurons. However, Ca2+ stimulation is blunt in the absence of Aralar, a Ca2+-binding mitochondrial carrier component of Malate-Aspartate Shuttle (MAS). The results suggest that Ca2+-regulated Aralar-MAS activation upregulates glycolysis and pyruvate production, which fuels mitochondrial respiration, through regulation of cytosolic NAD+/NADH ratio.

Transcriptomes Suggest That Pinniped and Cetacean Brains Have a High Capacity for Aerobic Metabolism While Reducing Energy-Intensive Processes Such as Synaptic Transmission

The mammalian brain is characterized by high energy expenditure and small energy reserves, making it dependent on continuous vascular oxygen and nutritional supply. The brain is therefore extremely vulnerable to hypoxia. While neurons of most terrestrial mammals suffer from irreversible damage after only short periods of hypoxia, neurons of the deep-diving hooded seal (Cystophora cristata) show a remarkable hypoxia-tolerance. To identify the molecular mechanisms underlying the intrinsic hypoxia-tolerance, we excised neurons from the visual cortices of hooded seals and mice (Mus musculus) by laser capture microdissection. A comparison of the neuronal transcriptomes suggests that, compared to mice, hooded seal neurons are endowed with an enhanced aerobic metabolic capacity, a reduced synaptic transmission and an elevated antioxidant defense. Publicly available whole-tissue brain transcriptomes of the bowhead whale (Balaena mysticetus), long-finned pilot whale (Globicephala melas), minke whale (Balaenoptera acutorostrata) and killer whale (Orcinus orca), supplemented with 2 newly sequenced long-finned pilot whales, suggest that, compared to cattle (Bos taurus), the cetacean brain also displays elevated aerobic capacity and reduced synaptic transmission. We conclude that the brain energy balance of diving mammals is preserved during diving, due to reduced synaptic transmission that limits energy expenditure, while the elevated aerobic capacity allows efficient use of oxygen to restore energy balance during surfacing between dives.

High-resolution relaxometry-based calibrated fMRI in murine brain: Metabolic differences between awake and anesthetized states

Functional magnetic resonance imaging (fMRI) techniques using the blood-oxygen level-dependent (BOLD) signal have shown great potential as clinical biomarkers of disease. Thus, using these techniques in preclinical rodent models is an urgent need. Calibrated fMRI is a promising technique that can provide high-resolution mapping of cerebral oxygen metabolism (CMR_O2). However, calibrated fMRI is difficult to use in rodent models for several reasons: rodents are anesthetized, stimulation-induced changes are small, and gas challenges induce noisy CMR_O2 predictions. We used, in mice, a relaxometry-based calibrated fMRI method which uses cerebral blood flow (CBF) and the BOLD-sensitive magnetic relaxation component, R₂', the same parameter derived in the deoxyhemoglobin-dilution model of calibrated fMRI. This method does not use any gas challenges, which we tested on mice in both awake and anesthetized states. As anesthesia induces a whole-brain change, our protocol allowed us to overcome the former limitations of rodent studies using calibrated fMRI. We revealed 1.5-2 times higher CMR_O2, dependent upon brain region, in the awake state versus the anesthetized state. Our results agree with alternative measurements of whole-brain CMR_O2 in the same mice and previous human anesthesia studies. The use of calibrated fMRI in rodents has much potential for preclinical fMRI.

Unveiling the anti-senescence effects and senescence-associated secretory phenotype (SASP) inhibitory mechanisms of Scutellaria baicalensis Georgi in low glucose-induced astrocytes based on boolean network

BACKGROUND

Astrocytes senescence has been demonstrated in the aging brain and Alzheimer's disease (AD). Moreover, lower glucose metabolism has been confirmed in the early stage of AD. However, whether low glucose could induce astrocytes senescence remain ambiguous. Studies have shown that the ethanol extracts of Scutellaria baicalensis Georgi (SGE) exert neuroprotective and anti-aging effects, while whether SGE could delay astrocytes senescence was unclear.

PURPOSE

This study investigated the anti-senescence effect of SGE in low glucose-induced T98G cells and primary astrocytes, and explored the possible mechanisms based on boolean network.

METHODS

The neuroprotective effects of SGE in low glucose-induced T98G cells were evaluated by measurement of cell viability, LDH, ROS and ATP. The anti-senescence effects of SGE were investigated by detection of senescence-associated β-galactosidase (SA-β-Gal), senescence-associated secretory phenotype (SASP), cell cycle and senescence-related markers. The possible mechanisms of SGE in delaying astrocytes senescence were discovered through integrating transcriptomics with boolean network, and validation experiments were further performed.

RESULTS

Our results revealed that low glucose could induce astrocytes senescence, and SGE could delay astrocytes senescence by decreasing the staining rate of SA-β-gal, reducing secretions of SASP factors (IL-6, CXCL1, MMP-1), alleviating cell cycle arrest in G0/G1 phase, decreasing the formation of punctate DNA foci and down-regulating the expression of p16INK4A, p21 and γH2A.X. Transcriptomics and further verification results showed that SGE could markedly inhibit the mRNA expression levels of SASP factors (CXCL10, CXCL2, CCL2, IL-6, CXCR4, CCR7). Moreover, C-X-C motif chemokine 10 (CXCL10) was predicted to be the key SASP factor affecting the network stability by using boolean network. Further experiments validated that SGE could markedly reduce CXCL10 level, decrease the secretion of IL-6 and inhibit cell migration in CXCL10 induced primary astrocytes.

CONCLUSION

In summary, our research unmasks that the anti-senescence effects of SGE were highly correlated with the suppression of SASP secretions, and CXCL10 mediated the SASP inhibition effect of SGE in low glucose-induced astrocytes. Our study highlights that the delay of astrocytes senescence and the inhibition of SASP might be a new mechanism of SGE for alleviating neurodegenerative diseases such as AD.

Glucose sparing by glycogenolysis (GSG) determines the relationship between brain metabolism and neurotransmission

Over the last two decades, it has been established that glucose metabolic fluxes in neurons and astrocytes are proportional to the rates of the glutamate/GABA-glutamine neurotransmitter cycles in close to 1:1 stoichiometries across a wide range of functional energy demands. However, there is presently no mechanistic explanation for these relationships. We present here a theoretical meta-analysis that tests whether the brain's unique compartmentation of glycogen metabolism in the astrocyte and the requirement for neuronal glucose homeostasis lead to the observed stoichiometries. We found that blood-brain barrier glucose transport can be limiting during activation and that the energy demand could only be met if glycogenolysis supports neuronal glucose metabolism by replacing the glucose consumed by astrocytes, a mechanism we call Glucose Sparing by Glycogenolysis (GSG). The predictions of the GSG model are in excellent agreement with a wide range of experimental results from rats, mice, tree shrews, and humans, which were previously unexplained. Glycogenolysis and glucose sparing dictate the energy available to support neuronal activity, thus playing a fundamental role in brain function in health and disease.

Aerobic glycolysis imaging of epileptic foci during the inter-ictal period

BACKGROUND

In drug-resistant epilepsy, surgical resection of the epileptic focus can end seizures. However, success is dependent on the ability to identify foci locations and, unfortunately, current methods like electrophysiology and positron emission tomography can give contradictory results. During seizures, glucose is metabolized at epileptic foci through aerobic glycolysis, which can be imaged through the oxygen-glucose index (OGI) biomarker. However, inter-ictal (between seizures) OGI changes have not been studied, which has limited its application.

METHODS

18 healthy controls and 24 inter-ictal, temporal lobe epilepsy patients underwent simultaneous positron emission tomography (PET) and magnetic resonance imaging (MRI) scans. We used [18F]fluorodeoxyglucose-PET (FDG-PET) to detect cerebral glucose metabolism, and calibrated functional MRI to acquire relative oxygen consumption. With these data, we calculated relative OGI maps.

FINDINGS

While bilaterally symmetrical in healthy controls, we observed, in patients during the inter-ictal period, higher OGI ipsilateral to the epileptic focus than contralateral. While traditional FDG-PET results and temporal lobe OGI results usually both agreed with invasive electrophysiology, in cases where FDG-PET disagreed with electrophysiology, temporal lobe OGI agreed with electrophysiology, and vice-versa.

INTERPRETATION

As either our novel epilepsy biomarker or traditional approaches located foci in every case, our work provides promising insights into metabolic changes in epilepsy. Our method allows single-session OGI measurement which can be useful in other diseases.

FUNDING

This work was supported by ShanghaiTech University, the Shanghai Municipal Government, the National Natural Science Foundation of China Grant (No. 81950410637) and Shanghai Municipal Key Clinical Specialty (No. shslczdzk03403). F. H. and P. H. were supported by USA National Institute of Health grants (R01 NS-100106, R01 MH-067528).Z. W. was supported by the Key-Area Research and Development Program of Guangdong Province (2019B030335001), National Natural Science Foundation of China (No. 82151303), and National Key R&D Program of China (No. 2021ZD0204002).

Biallelic variants in SLC38A3 encoding a glutamine transporter cause epileptic encephalopathy

The solute carrier (SLC) superfamily encompasses >400 transmembrane transporters involved in the exchange of amino acids, nutrients, ions, metals, neurotransmitters and metabolites across biological membranes. SLCs are highly expressed in the mammalian brain; defects in nearly 100 unique SLC-encoding genes (OMIM: https://www.omim.org) are associated with rare Mendelian disorders including developmental and epileptic encephalopathy and severe neurodevelopmental disorders. Exome sequencing and family-based rare variant analyses on a cohort with neurodevelopmental disorders identified two siblings with developmental and epileptic encephalopathy and a shared deleterious homozygous splicing variant in SLC38A3. The gene encodes SNAT3, a sodium-coupled neutral amino acid transporter and a principal transporter of the amino acids asparagine, histidine, and glutamine, the latter being the precursor for the neurotransmitters GABA and glutamate. Additional subjects with a similar developmental and epileptic encephalopathy phenotype and biallelic predicted-damaging SLC38A3 variants were ascertained through GeneMatcher and collaborations with research and clinical molecular diagnostic laboratories. Untargeted metabolomic analysis was performed to identify novel metabolic biomarkers. Ten individuals from seven unrelated families from six different countries with deleterious biallelic variants in SLC38A3 were identified. Global developmental delay, intellectual disability, hypotonia, and absent speech were common features while microcephaly, epilepsy, and visual impairment were present in the majority. Epilepsy was drug-resistant in half. Metabolomic analysis revealed perturbations of glutamate, histidine, and nitrogen metabolism in plasma, urine, and CSF of selected subjects, potentially representing biomarkers of disease. Our data support the contention that SLC38A3 is a novel disease gene for developmental and epileptic encephalopathy and illuminate the likely pathophysiology of the disease as perturbations in glutamine homeostasis.

Current Insights Into Oligodendrocyte Metabolism and Its Power to Sculpt the Myelin Landscape

Once believed to be part of the nervenkitt or “nerve glue” network in the central nervous system (CNS), oligodendroglial cells now have established roles in key neurological functions such as myelination, neuroprotection, and motor learning. More recently, oligodendroglia has become the subject of intense investigations aimed at understanding the contributions of its energetics to CNS physiology and pathology. In this review, we discuss the current understanding of oligodendroglial metabolism in regulating key stages of oligodendroglial development and health, its role in providing energy to neighboring cells such as neurons, as well as how alterations in oligodendroglial bioenergetics contribute to disease states. Importantly, we highlight how certain inputs can regulate oligodendroglial metabolism, including extrinsic and intrinsic mediators of cellular signaling, pharmacological compounds, and even dietary interventions. Lastly, we discuss emerging studies aimed at discovering the therapeutic potential of targeting components within oligodendroglial bioenergetic pathways.

Monitoring Neurochemistry in Traumatic Brain Injury Patients Using Microdialysis Integrated with Biosensors: A Review

In a traumatically injured brain, the cerebral microdialysis technique allows continuous sampling of fluid from the brain’s extracellular space. The retrieved brain fluid contains useful metabolites that indicate the brain’s energy state. Assessment of these metabolites along with other parameters, such as intracranial pressure, brain tissue oxygenation, and cerebral perfusion pressure, may help inform clinical decision making, guide medical treatments, and aid in the prognostication of patient outcomes. Currently, brain metabolites are assayed on bedside analysers and results can only be achieved hourly. This is a major drawback because critical information within each hour is lost. To address this, recent advances have focussed on developing biosensing techniques for integration with microdialysis to achieve continuous online monitoring. In this review, we discuss progress in this field, focusing on various types of sensing devices and their ability to quantify specific cerebral metabolites at clinically relevant concentrations. Important points that require further investigation are highlighted, and comments on future perspectives are provided.

Toxic Effects of Glyphosate on the Nervous System: A Systematic Review

Glyphosate, a non-selective systemic biocide with broad-spectrum activity, is the most widely used herbicide in the world. It can persist in the environment for days or months, and its intensive and large-scale use can constitute a major environmental and health problem. In this systematic review, we investigate the current state of our knowledge related to the effects of this pesticide on the nervous system of various animal species and humans. The information provided indicates that exposure to glyphosate or its commercial formulations induces several neurotoxic effects. It has been shown that exposure to this pesticide during the early stages of life can seriously affect normal cell development by deregulating some of the signaling pathways involved in this process, leading to alterations in differentiation, neuronal growth, and myelination. Glyphosate also seems to exert a significant toxic effect on neurotransmission and to induce oxidative stress, neuroinflammation and mitochondrial dysfunction, processes that lead to neuronal death due to autophagy, necrosis, or apoptosis, as well as the appearance of behavioral and motor disorders. The doses of glyphosate that produce these neurotoxic effects vary widely but are lower than the limits set by regulatory agencies. Although there are important discrepancies between the analyzed findings, it is unequivocal that exposure to glyphosate produces important alterations in the structure and function of the nervous system of humans, rodents, fish, and invertebrates.

Cognitive disorder and dementia in type 2 diabetes mellitus

Insulin, a key pleiotropic hormone, regulates metabolism through several signaling pathways in target tissues including skeletal muscle, liver, and brain. In the brain, insulin modulates learning and memory, and impaired insulin signaling is associated with metabolic dysregulation and neurodegenerative diseases. At the receptor level, in aging and Alzheimer’s disease (AD) models, the amount of insulin receptors and their functions are decreased. Clinical and animal model studies suggest that memory improvements are due to changes in insulin levels. Furthermore, diabetes mellitus (DM) and insulin resistance are associated with age-related cognitive decline, increased levels of β-amyloid peptide, phosphorylation of tau protein; oxidative stress, pro-inflammatory cytokine production, and dyslipidemia. Recent evidence shows that deleting brain insulin receptors leads to mild obesity and insulin resistance without influencing brain size and apoptosis development. Conversely, deleting insulin-like growth factor 1 receptor (IGF-1R) affects brain size and development, and contributes to behavior changes. Insulin is synthesized locally in the brain and is released from the neurons. Here, we reviewed proposed pathophysiological hypotheses to explain increased risk of dementia in the presence of DM. Regardless of the exact sequence of events leading to neurodegeneration, there is strong evidence that mitochondrial dysfunction plays a key role in AD and DM. A triple transgenic mouse model of AD showed mitochondrial dysfunction, oxidative stress, and loss of synaptic integrity. These alterations are comparable to those induced in wild-type mice treated with sucrose, which is consistent with the proposal that mitochondrial alterations are associated with DM and contribute to AD development. Alterations in insulin/IGF-1 signaling in DM could lead to mitochondrial dysfunction and low antioxidant capacity of the cell. Thus, insulin/IGF-1 signaling is important for increased neural processing and systemic metabolism, and could be a specific target for therapeutic strategies to decrease alterations associated with age-related cognitive decline.

Glucose Metabolism, Neural Cell Senescence and Alzheimer’s Disease

Alzheimer’s disease (AD), an elderly neurodegenerative disorder with a high incidence and progressive memory decline, is one of the most expensive, lethal, and burdening diseases. To date, the pathogenesis of AD has not been fully illustrated. Emerging studies have revealed that cellular senescence and abnormal glucose metabolism in the brain are the early hallmarks of AD. Moreover, cellular senescence and glucose metabolism disturbance in the brain of AD patients may precede amyloid-β deposition or Tau protein phosphorylation. Thus, metabolic reprogramming targeting senescent microglia and astrocytes may be a novel strategy for AD intervention and treatment. Here, we recapitulate the relationships between neural cell senescence and abnormal glucose metabolism (e.g., insulin signaling, glucose and lactate metabolism) in AD. We then discuss the potential perspective of metabolic reprogramming towards an AD intervention, providing a theoretical basis for the further exploration of the pathogenesis of and therapeutic approach toward AD.

Harpephyllum caffrum fruit (wild plum) facilitates glucose uptake and modulates metabolic activities linked to neurodegeneration in isolated rat brain: An in vitro and in silico approach

Alteration in brain glucose metabolism due to glucose uptake reduction has been described in the onset of certain neurodegenerative disorders. This study determined Harpephyllum caffrum fruit's potential ability to improve glucose uptake and its modulatory effects on intrinsic antioxidant, glucogenic, cholinergic, and nucleotide-hydrolyzing enzyme activities in isolated rat brain. Consequently, the bioactive compounds of the fruits were identified with LC-MS. The fruit significantly improved brain glucose uptake following coincubation with glucose and brain tissue. The fruit extract also elevated GSH level, SOD, catalase, glycogen phosphorylase, and ENTPDase activities while simultaneously suppressing NO and malonaldehyde levels and fructose-1,6-bisphosphatase, ATPase, acetylcholinesterase and butyrylcholinesterase activities. LC-MS analysis revealed S-methylcysteine sulfoxide, dihydroquercetin, 3,4-dimethyl-2,5-bis(3,4,5-trimethoxyphenyl) tetrahydrofuran (MTHF), nobiletin, puerarin, quercetin 3-rutinoside, 8-D-glucosyl-4',5,7-trihydroxyflavone, asperulosidic acid, 1,2,4,6-tetragalloylglucose, and phellamurin. This study suggests the neuroprotective effects of H. caffrum fruit due to its ability to enhance glucose uptake, attenuate glucose-induced oxidative stress while modulating glucogenic, cholinergic, and nucleotide-hydrolyzing enzyme activities in normal brain tissues. PRACTICAL APPLICATIONS: Available scientific evidence describes oxidative stress as one of the physiological processes contributing to aging-associated neurodegeneration in humans. In this regard, commonly consumed natural products from plants have attracted much interest due to their ability to mitigate redox imbalance-related pathologies that affect various organs in the body such as the brain. Harpephyllum caffrum or bush mango is an evergreen plant native to the South African vegetation. The fruit from the plant is consumed locally as food or specifically for improving the nutritional quality of meals as deserts or condiments. While previous findings described the high antioxidant properties of the fruits, this study reported possible mechanisms via which the plant may exhibit ameliorative effects against oxidative stress-related neurological disorders in the brain. Hence, findings from the current work present another justification for the significance of fruits as a safer nutraceutical alternative for therapy in neurological disease management.

Brain glycogen content is increased in the acute and interictal chronic stages of the mouse pilocarpine model of epilepsy

Glucose is the main brain fuel in fed conditions, while astrocytic glycogen is used as supplemental fuel when the brain is stimulated. Brain glycogen levels are decreased shortly after induced seizures in rodents, but little is known about how glycogen levels are affected interictally in chronic models of epilepsy. Reduced glutamine synthetase activity has been suggested to lead to increased brain glycogen levels in humans with chronic epilepsy. Here, we used the mouse pilocarpine model of epilepsy to investigate whether brain glycogen levels are altered, both acutely and in the chronic stage of the model. One day after pilocarpine‐induced convulsive status epilepticus (CSE), glycogen levels were higher in the hippocampal formation, cerebral cortex, and cerebellum. Opposite to expected, this was accompanied by elevated glutamine synthetase activity in the hippocampus but not the cortex. Increased interictal glycogen amounts were seen in the hippocampal formation and cerebral cortex in the chronic stage of the model (21 days post‐CSE), suggesting long‐lasting alterations in glycogen metabolism. Glycogen solubility in the cerebral cortex was unaltered in this epilepsy mouse model. Glycogen synthase kinase 3 beta (Gsk3b) mRNA levels were reduced in the hippocampal formations of mice in the chronic stage, which may underlie the elevated brain glycogen content in this model. This is the first report of elevated interictal glycogen levels in a chronic epilepsy model. Increased glycogen amounts in the brain may influence seizure susceptibility in this model, and this warrants further investigation.

The metabolic basis of epilepsy

The brain is a highly energy-demanding organ and requires bioenergetic adaptability to balance normal activity with pathophysiological fuelling of spontaneous recurrent seizures, the hallmark feature of the epilepsies. Recurrent or prolonged seizures have long been known to permanently alter neuronal circuitry and to cause excitotoxic injury and aberrant inflammation. Furthermore, pathological changes in bioenergetics and metabolism are considered downstream consequences of epileptic seizures that begin at the synaptic level. However, as we highlight in this Review, evidence is also emerging that primary derangements in cellular or mitochondrial metabolism can result in seizure genesis and lead to spontaneous recurrent seizures. Basic and translational research indicates that the relationships between brain metabolism and epileptic seizures are complex and bidirectional, producing a vicious cycle that compounds the deleterious consequences of seizures. Metabolism-based treatments such as the high-fat, antiseizure ketogenic diet have become mainstream, and metabolic substrates and enzymes have become attractive molecular targets for seizure prevention and recovery. Moreover, given that metabolism is crucial for epigenetic as well as inflammatory changes, the idea that epileptogenesis can be both negatively and positively influenced by metabolic changes is rapidly gaining ground. Here, we review evidence that supports both pathophysiological and therapeutic roles for brain metabolism in epilepsy.

The Activation of GPR27 Increases Cytosolic L-Lactate in 3T3 Embryonic Cells and Astrocytes

G-protein-coupled receptors (GPCRs) represent a family with over 800 members in humans, and one-third of these are targets for approved drugs. A large number of GPCRs have unknown physiologic roles. Here, we investigated GPR27, an orphan GPCR belonging to the family of super conserved receptor expressed in the brain, with unknown functions. Cytosolic levels of L-lactate ([lactate]i), the end product of aerobic glycolysis, were measured with the Laconic fluorescence resonance energy transfer nanosensor. In single 3T3 wild-type (WT) embryonic cells, the application of 8535 (1 µM), a surrogate agonist known to activate GPR27, resulted in an increase in [lactate]i. Similarly, an increase was recorded in primary rat astrocytes, a type of neuroglial cell abundant in the brain, which contain glycogen and express enzymes of aerobic glycolysis. In CRISPR-Cas9 GPR27 knocked out 3T3 cells, the 8535-induced increase in [lactate]i was reduced compared with WT controls. Transfection of the GPR27-carrying plasmid into the 3T3KOGPR27 cells rescued the 8535-induced increase in [lactate]i. These results indicate that stimulation of GPR27 enhances aerobic glycolysis and L-lactate production in 3T3 cells and astrocytes. Interestingly, in the absence of GPR27 in 3T3 cells, resting [lactate]i was increased in comparison with controls, further supporting the view that GPR27 regulates L-lactate homeostasis.

Hormone-sensitive lipase is localized at synapses and is necessary for normal memory functioning in mice

Hormone-sensitive lipase (HSL) is mainly present in adipose tissue where it hydrolyses diacylglycerol. Although expression of HSL has also been reported in the brain, its presence in different cellular compartments is uncertain, and its role in regulating brain lipid metabolism remains hitherto unexplored. We hypothesized HSL might play a role in regulating the availability of bioactive lipids necessary for neuronal function, and therefore investigated whether dampening HSL activity could lead to brain dysfunction. In mice, we found HSL protein and enzymatic activity throughout the brain, both localized within neurons and enriched in synapses. HSL-null mice were then analyzed using a battery of behavioral tests. Relative to wild-type littermates, HSL-null mice showed impaired short- and long-term memory, yet preserved exploratory behaviurs. Molecular analysis of the cortex and hippocampus showed increased expression of genes involved in glucose utilization in the hippocampus, but not cortex, of HSL-null mice compared to controls. Furthermore, lipidomics analyses indicated an impact of HSL deletion on the profile of bioactive lipids, including a decrease in endocannabinoids and eicosanoids that are known to modulate neuronal activity, cerebral blood flow, and inflammation processes. Accordingly, mild increases in the expression of pro-inflammatory cytokines in HSL mice compared to littermates were suggestive of low-grade inflammation. We conclude that HSL has a homeostatic role in maintaining pools of lipids required for normal brain function. It remains to be tested, however, whether the recruitment of HSL for the synthesis of these lipids occurs during increased neuronal activity, or whether HSL participates in neuroinflammatory responses.

Neurons undergo pathogenic metabolic reprogramming in models of familial ALS

Objectives

Methods

Results

Conclusions

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Our work is the first to perform a comprehensive and quantitative analysis of intermediary metabolism in neurons in the setting of fALS causing gene products.

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Because the cardinal feature of ALS is death of motor neurons, these new studies are directly relevant to the pathogenesis of ALS.

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Our functional interrogations begin to unpack how metabolic re-wiring is induced by fALS genes and it will be very interesting, in the future, to gain insight in amino acid fueling of the TCA cycle.

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We suspect pleiotropic effects of amino acid fueling, and this may lead to very targeted therapeutic interventions.

Microglial metabolic flexibility: emerging roles for lactate

Microglia, the resident macrophages of the central nervous system (CNS), play important functions in the healthy and diseased brain. In the emerging field of immunometabolism, progress has been made in understanding how cellular metabolism can orchestrate the key responses of tissue macrophages, such as phagocytosis and inflammation. However, very little is known about the metabolic control of microglia. Lactate, now recognized as a crucial metabolite and a central substrate in metabolic flexibility, is emerging not only as a novel bioenergetic fuel for microglial metabolism but also as a potential modulator of cellular function. Parallels with macrophages will help in understanding how microglial lactate metabolism is implicated in brain physiology and pathology, and how it could be targeted for therapeutic purposes.

Perception is associated with the brain's metabolic response to sensory stimulation

Processing of incoming sensory stimulation triggers an increase of cerebral perfusion and blood oxygenation (neurovascular response) as well as an alteration of the metabolic neurochemical profile (neurometabolic response). Here we show in human primary visual cortex (V1) that perceived and unperceived isoluminant chromatic flickering stimuli designed to have similar neurovascular responses as measured by blood oxygenation level dependent functional MRI (BOLD-fMRI) have markedly different neurometabolic responses as measured by functional MRS. In particular, a significant regional buildup of lactate, an index of aerobic glycolysis, and glutamate, an index of malate-aspartate shuttle, occurred in V1 only when the flickering was perceived, without any relation with behavioral or physiological variables. Whereas the BOLD-fMRI signal in V1, a proxy for input to V1, was insensitive to flickering perception by design, the BOLD-fMRI signal in secondary visual areas was larger during perceived than unperceived flickering, indicating increased output from V1. These results demonstrate that the upregulation of energy metabolism induced by visual stimulation depends on the type of information processing taking place in V1, and that 1H-fMRS provides unique information about local input/output balance that is not measured by BOLD fMRI.

Metabolic Contribution and Cerebral Blood Flow Regulation by Astrocytes in the Neurovascular Unit

The neurovascular unit (NVU) is a conceptual framework that has been proposed to better explain the relationships between the neural cells and blood vessels in the human brain, focused mainly on the brain gray matter. The major components of the NVU are the neurons, astrocytes (astroglia), microvessels, pericytes, and microglia. In addition, we believe that oligodendrocytes should also be included as an indispensable component of the NVU in the white matter. Of all these components, astrocytes in particular have attracted the interest of researchers because of their unique anatomical location; these cells are interposed between the neurons and the microvessels of the brain. Their location suggests that astrocytes might regulate the cerebral blood flow (CBF) in response to neuronal activity, so as to ensure an adequate supply of glucose and oxygen to meet the metabolic demands of the neurons. In fact, the adult human brain, which accounts for only 2% of the entire body weight, consumes approximately 20–25% of the total amount of glucose and oxygen consumed by the whole body. The brain needs a continuous supply of these essential energy sources through the CBF, because there are practically no stores of glucose or oxygen in the brain; both acute and chronic cessation of CBF can adversely affect brain functions. In addition, another important putative function of the NVU is the elimination of heat and waste materials produced by neuronal activity. Recent evidence suggests that astrocytes play pivotal roles not only in supplying glucose, but also fatty acids and amino acids to neurons. Loss of astrocytic support can be expected to lead to malfunction of the NVU as a whole, which underlies numerous neurological disorders. In this review, we shall focus on historical and recent findings with regard to the metabolic contributions of astrocytes in the NVU.

Astrocytes protect dopaminergic neurons against aminochrome neurotoxicity

Astrocytes protect neurons by modulating neuronal function and survival. Astrocytes support neurons in several ways. They provide energy through the astrocyte-neuron lactate shuttle, protect neurons from excitotoxicity, and internalize neuronal lipid droplets to degrade fatty acids for neuronal metabolic and synaptic support, as well as by their high capacity for glutamate uptake and the conversion of glutamate to glutamine. A recent reported astrocyte system for protection of dopamine neurons against the neurotoxic products of dopamine, such as aminochrome and other o-quinones, were generated under neuromelanin synthesis by oxidizing dopamine catechol structure. Astrocytes secrete glutathione transferase M2-2 through exosomes that transport this enzyme into dopaminergic neurons to protect these neurons against aminochrome neurotoxicity. The role of this new astrocyte protective mechanism in Parkinson´s disease is discussed.

Clozapine induces astrocyte-dependent FDG-PET hypometabolism

PURPOSE

Advances in functional imaging allowed us to visualize brain glucose metabolism in vivo and non-invasively with [¹⁸F]fluoro-2-deoxyglucose (FDG) positron emission tomography (PET) imaging. In the past decades, FDG-PET has been instrumental in the understanding of brain function in health and disease. The source of the FDG-PET signal has been attributed to neuronal uptake, with hypometabolism being considered as a direct index of neuronal dysfunction or death. However, other brain cells are also metabolically active, including astrocytes. Based on the astrocyte-neuron lactate shuttle hypothesis, the activation of the glutamate transporter 1 (GLT-1) acts as a trigger for glucose uptake by astrocytes. With this in mind, we investigated glucose utilization changes after pharmacologically downregulating GLT-1 with clozapine (CLO), an anti-psychotic drug.

METHODS

Adult male Wistar rats (control, n = 14; CLO, n = 12) received CLO (25/35 mg kg^-1) for 6 weeks. CLO effects were evaluated in vivo with FDG-PET and cortical tissue was used to evaluate glutamate uptake and GLT-1 and GLAST levels. CLO treatment effects were also assessed in cortical astrocyte cultures (glucose and glutamate uptake, GLT-1 and GLAST levels) and in cortical neuronal cultures (glucose uptake).

RESULTS

CLO markedly reduced in vivo brain glucose metabolism in several brain areas, especially in the cortex. Ex vivo analyses demonstrated decreased cortical glutamate transport along with GLT-1 mRNA and protein downregulation. In astrocyte cultures, CLO decreased GLT-1 density as well as glutamate and glucose uptake. By contrast, in cortical neuronal cultures, CLO did not affect glucose uptake.

CONCLUSION

This work provides in vivo demonstration that GLT-1 downregulation induces astrocyte-dependent cortical FDG-PET hypometabolism-mimicking the hypometabolic signature seen in people developing dementia-and adds further evidence that astrocytes are key contributors of the FDG-PET signal.

Long-Lasting Pathological Mental Fatigue After Brain Injury–A Dysfunction in Glutamate Neurotransmission?

Long-lasting mental or cognitive fatigue may be a disabling symptom after physically recovered skull trauma, stroke, infection, or inflammation in the central nervous system (CNS). It is difficult to go back to work and participate in familiar social activities, as typically the person is only able to remain mentally active for short periods, and if mentally exhausted, the recovery time will be disproportionally long. Mental fatigue after traumatic brain injury correlates with brain information processing speed. Information processing is energy consuming and requires widespread and specific neural signaling. Glutamate signaling is essential for information processing, including learning and memory. Low levels and the fine-tuning of extracellular glutamate are necessary to maintain a high precision in information processing. The astroglial cells are responsible for the fine-tuning of the glutamate transmission, but this capacity is attenuated by substances or conditions associated with neuro-inflammation in brain pathology. In this paper, we extend our previously presented hypothesis on the cellular mechanisms underlying mental fatigue suggesting a dysfunction in the astroglial support of the glutamate transmission. Changes in other neurotransmitters such as dopamine, serotonin, norepinephrine, GABA, and acetylcholine after brain injury are also taken into consideration.

Exploring the dual character of metformin in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease, which results in dementia typically in the elderly. The disease is mainly characterized by the deposition of amyloid beta (Aβ) plaques and neurofibrillary tangles (NFTs) in the brain. However, only few drugs are available for AD because of its unknown pathological mechanism which limits the development of new drugs. Therefore, it is urgent to identify potential therapeutic strategies for AD. Moreover, research have showed that there is a significant association between Type 2 diabetes mellites (T2DM) and AD, suggesting that the two diseases may share common pathophysiological mechanisms. Such mechanisms include impaired insulin signaling, altered glucose metabolism, inflammation, oxidative stress, and premature aging, which strongly affect cognitive function and increased risk of dementia. Consequently, as a widely used drug for T2DM, metformin also has therapeutic potential for AD in vivo. It has been confirmed that metformin is beneficial on the brain of AD animal models. The mechanisms underlying the effects of metformin in Alzheimer's disease are complex and multifaceted. Metformin may work through mechanisms involving homeostasis of glucose metabolism, decrease of amyloid plaque deposition, normalization of tau protein phosphorylation and enhancement of autophagy. However, in clinical trials, metformin had little effects on patients with mild cognitive impairment or mild AD. Pathological effects and negative clinical results of metformin on AD make the current topic quite controversial. By reviewing the latest progress of related research, this paper summarizes the possible role of metformin in AD. The purpose of this study is not only to determine the potential treatment of AD, but also other related neurodegenerative diseases.

Neuroprotection and Disease Modification by Astrocytes and Microglia in Parkinson Disease

Oxidative stress and neuroinflammation are common bases for disease onset and progression in many neurodegenerative diseases. In Parkinson disease, which is characterized by the degeneration of dopaminergic neurons resulting in dopamine depletion, the pathogenesis differs between hereditary and solitary disease forms and is often unclear. In addition to the pathogenicity of alpha-synuclein as a pathological disease marker, the involvement of dopamine itself and its interactions with glial cells (astrocyte or microglia) have attracted attention. Pacemaking activity, which is a hallmark of dopaminergic neurons, is essential for the homeostatic maintenance of adequate dopamine concentrations in the synaptic cleft, but it imposes a burden on mitochondrial oxidative glucose metabolism, leading to reactive oxygen species production. Astrocytes provide endogenous neuroprotection to the brain by producing and releasing antioxidants in response to oxidative stress. Additionally, the protective function of astrocytes can be modified by microglia. Some types of microglia themselves are thought to exacerbate Parkinson disease by releasing pro-inflammatory factors (M1 microglia). Although these inflammatory microglia may further trigger the inflammatory conversion of astrocytes, microglia may induce astrocytic neuroprotective effects (A2 astrocytes) simultaneously. Interestingly, both astrocytes and microglia express dopamine receptors, which are upregulated in the presence of neuroinflammation. The anti-inflammatory effects of dopamine receptor stimulation are also attracting attention because the functions of astrocytes and microglia are greatly affected by both dopamine depletion and therapeutic dopamine replacement in Parkinson disease. In this review article, we will focus on the antioxidative and anti-inflammatory effects of astrocytes and their synergism with microglia and dopamine.

Ketogenic diet for human diseases: the underlying mechanisms and potential for clinical implementations

The ketogenic diet (KD) is a high-fat, adequate-protein, and very-low-carbohydrate diet regimen that mimics the metabolism of the fasting state to induce the production of ketone bodies. The KD has long been established as a remarkably successful dietary approach for the treatment of intractable epilepsy and has increasingly garnered research attention rapidly in the past decade, subject to emerging evidence of the promising therapeutic potential of the KD for various diseases, besides epilepsy, from obesity to malignancies. In this review, we summarize the experimental and/or clinical evidence of the efficacy and safety of the KD in different diseases, and discuss the possible mechanisms of action based on recent advances in understanding the influence of the KD at the cellular and molecular levels. We emphasize that the KD may function through multiple mechanisms, which remain to be further elucidated. The challenges and future directions for the clinical implementation of the KD in the treatment of a spectrum of diseases have been discussed. We suggest that, with encouraging evidence of therapeutic effects and increasing insights into the mechanisms of action, randomized controlled trials should be conducted to elucidate a foundation for the clinical use of the KD.

Voluntary Behavior and Training Conditions Modulate in vivo Extracellular Glucose and Lactate in the Mouse Primary Motor Cortex

Learning or performing new behaviors requires significant neuronal signaling and is metabolically demanding. The metabolic cost of performing a behavior is mitigated by exposure and practice which result in diminished signaling and metabolic requirements. We examined the impact of novel and habituated wheel running, as well as effortful behaviors on the modulation of extracellular glucose and lactate using biosensors inserted in the primary motor cortex of mice. We found that motor behaviors produce increases in extracellular lactate and decreases in extracellular glucose in the primary motor cortex. These effects were modulated by experience, novelty and intensity of the behavior. The increase in extracellular lactate appears to be strongly associated with novelty of a behavior as well as the difficulty of performing a behavior. Our observations are consistent with the view that a main function of aerobic glycolysis is not to fuel the current neuronal activity but to sustain new bio-infrastructure as learning changes neural networks, chiefly through the shuttling of glucose derived carbons into the pentose phosphate pathway for the biosynthesis of nucleotides.

OUP accepted manuscript

Metabolism regulates neuronal activity and modulates the occurrence of epileptic seizures. Here, using two rodent models of absence epilepsy, we show that hypoglycaemia increases the occurrence of spike-wave seizures. We then show that selectively disrupting glycolysis in the thalamus, a structure implicated in absence epilepsy, is sufficient to increase spike-wave seizures. We propose that activation of thalamic AMP-activated protein kinase, a sensor of cellular energetic stress and potentiator of metabotropic GABA_B-receptor function, is a significant driver of hypoglycaemia-induced spike-wave seizures. We show that AMP-activated protein kinase augments postsynaptic GABA_B-receptor-mediated currents in thalamocortical neurons and strengthens epileptiform network activity evoked in thalamic brain slices. Selective thalamic AMP-activated protein kinase activation also increases spike-wave seizures. Finally, systemic administration of metformin, an AMP-activated protein kinase agonist and common diabetes treatment, profoundly increased spike-wave seizures. These results advance the decades-old observation that glucose metabolism regulates thalamocortical circuit excitability by demonstrating that AMP-activated protein kinase and GABA_B-receptor cooperativity is sufficient to provoke spike-wave seizures.

Significance of Brain Glucose Hypometabolism, Altered Insulin Signal Transduction, and Insulin Resistance in Several Neurological Diseases

Several neurological diseases share pathological alterations, even though they differ in their etiology. Neuroinflammation, altered brain glucose metabolism, oxidative stress, mitochondrial dysfunction and amyloidosis are biological events found in those neurological disorders. Altered insulin-mediated signaling and brain glucose hypometabolism are characteristic signs observed in the brains of patients with certain neurological diseases, but also others such as type 2 diabetes mellitus and vascular diseases. Thus, significant reductions in insulin receptor autophosphorylation and Akt kinase activity, and increased GSK-3 activity and insulin resistance, have been reported in these neurological diseases as contributing to the decline in cognitive function. Supporting this relationship is the fact that nasal and hippocampal insulin administration has been found to improve cognitive function. Additionally, brain glucose hypometabolism precedes the unmistakable clinical manifestations of some of these diseases by years, which may become a useful early biomarker. Deficiencies in the major pathways of oxidative energy metabolism have been reported in patients with several of these neurological diseases, which supports the hypothesis of their metabolic background. This review remarks on the significance of insulin and brain glucose metabolism alterations as keystone common pathogenic substrates for certain neurological diseases, highlighting new potential targets.

Insulin resistance in ischemic stroke: Mechanisms and therapeutic approaches

The pathological condition of insulin resistance prevents the neuroprotective effects of insulin. Numerous studies have demonstrated that insulin resistance, as an independent risk factor for ischemic stroke, accelerates the formation of thrombosis and promotes the development of atherosclerosis, both of which are major mechanisms of ischemic stroke. Additionally, insulin resistance negatively affects the prognosis of patients with ischemic stroke regardless of whether the patient has diabetes, but the mechanisms are not well studied. We explored the association between insulin resistance and the primary mechanisms of brain injury in ischemic stroke (inflammation, oxidative stress, and neuronal damage), looking for potential causes of poor prognosis in patients with ischemic stroke due to insulin resistance. Furthermore, we summarize insulin resistance therapeutic approaches to propose new therapeutic directions for clinically improving prognosis in patients with ischemic stroke.

Brain Transcriptome Responses to Dexamethasone Depending on Dose and Sex Reveal Factors Contributing to Sex-Specific Vulnerability to Stress-Induced Disorders

BACKGROUND

Glucocorticoid (GC) receptor (GR) signaling in the hypothalamus (Hyp) and in the superordinate limbic structures, such as the hippocampus (Hip), conveys feedback regulation of the neuroendocrine stress response and acts upon other neurobiological functions that ultimately influence mental health. These responses are strongly influenced by sex, but the molecular causes are still largely unexplored.

METHODS

To investigate GR targets and their GC sensitivity in the Hyp and Hip, we treated juvenile male and female piglets with 10 (D10) or 60 (D60) µg/kg dexamethasone (DEX), a selective GR agonist, and analyzed transcriptome responses compared to a saline control group using RNA sequencing.

RESULTS

Both doses influenced similar biological functions, including cellular response to lipid and immune cell-related functions, but the transcriptional response to D10 was considerably weaker, particularly in the Hip. Weighted Gene Co-expression Network Analysis revealed a network of genes coordinately regulated by DEX in both structures, among which the alpha-arrestin ARRDC2 takes a central position. Distinct functional groups of genes were differentially regulated by DEX between sexes depending on the dose; at D10, these included particularly mitochondrial genes, whereas at D60 interferon signaling and lipid homeostasis genes were enriched. The general and sex-specific transcriptional responses to DEX highlight microglia as the prominent target. Several key marker genes of disease-associated microglia were regulated by DEX depending on sex, such as TREM2 and LPL.

CONCLUSION

The discovered expression signatures suggest that DEX induced a dysfunctional state of microglia in males, while in females microglia were primed, which could entail predisposition for different mental disorders.

Dysfunction of the oligodendrocytes in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by irreversible deterioration of upper and lower motor neurons (MNs). Previously, studies on the involvement of glial cells in the pathogenic process of ALS have mainly revolved around astrocytes and microglia. And oligodendrocytes (OLs) have only recently been highlighted. Grey matter demyelination within the motor cortex and proliferation of the oligodendrocyte precursor cells (OPCs) was observed in ALS patients. The selective ablation of mutant SOD1 (the dysfunctional superoxide dismutase) from the oligodendrocyte progenitors after birth significantly delayed disease onset and prolonged the overall survival in ALS mice model (SOD1^G37R). In this study, we review the several mechanisms of oligodendrocyte dysfunction involved in the pathological process of myelin damage and MNs death during ALS. Particularly, we examined the insufficient local energy supply from OLs to axons, impaired differentiation from OPCs into OLs mediated by oxidative stress damage, and inflammatory injury to the OLs. Since increasing evidence depicted that ALS is not a disease limited to MNs damage, exploring the mechanisms by which oligodendrocyte dysfunction is involved in MNs death would contribute to a more comprehensive understanding of ALS and identifying potential drug targets.

Astrocytic glycogen mobilization participates in salvianolic acid B-mediated neuroprotection against reperfusion injury after ischemic stroke

Astrocytic glycogen serves as an important glucose reserve, and its degradation provides extra support for neighboring neurons during energy deficiency. Salvianolic acid B (SAB) exerts a neuroprotective effect on reperfusion insult after cerebrovascular occlusion, but the effect of SAB on astrocytic glycogen and its relationship with neuroprotection are not completely understood. Here, we knocked down astrocyte-specific glycogen phosphorylase (GP, the rate-limiting enzyme in glycogenolysis) in vitro and in vivo and investigated the changes in key enzymes in glycogen metabolism by performing immunoblotting in vitro and immunofluorescence in vivo. Neurobehavioral and morphological assessments were conducted to uncover the outcomes during brain reperfusion. SAB accelerated astrocytic glycogenolysis by upregulating GP activity but not GP expression after reperfusion. Suppression of astrocytic glycogenolysis weakened SAB-mediated neuroprotection against the reperfusion insult. In addition, activation of glycogenolysis by SAB contributed to the survival of astrocytes and surrounding neurons by increasing antioxidant levels in astrocytes. Our data reveal that astrocytic GP represents an important metabolic target in SAB-induced protection against brain damage after cerebrovascular recanalization.

Role of ketone bodies in diabetes-induced dementia: sirtuins, insulin resistance, synaptic plasticity, mitochondrial dysfunction, and neurotransmitter

Patients with type 2 diabetes can have several neuropathologies, such as memory deficits. Recent studies have focused on the association between metabolic imbalance and neuropathological problems, and the associated molecular pathology. Diabetes triggers neuroinflammation, impaired synaptic plasticity, mitochondrial dysfunction, and insulin resistance in the brain. Glucose is a main energy substrate for neurons, but under certain conditions, such as fasting and starvation, ketone bodies can be used as an energy fuel for these cells. Recent evidence has shed new light on the role of ketone bodies in regulating several anti-inflammation cellular pathways and improving glucose metabolism, insulin action, and synaptic plasticity, thereby being neuroprotective. However, very high amount of ketone bodies can be toxic for the brain, such as in ketoacidosis, a dangerous complication that may occur in type 1 diabetes mellitus or alcoholism. Recent findings regarding the relationship between ketone bodies and neuropathogenesis in dementia are reviewed in this article. They suggest that the adequately low amount of ketone bodies can be a potential energy source for the treatment of diabetes-induced dementia neuropathology, considering the multifaceted effects of the ketone bodies in the central nervous system. This review can provide useful information for establishing the therapeutic guidelines of a ketogenic diet for diabetes-induced dementia.

Directly Reprogrammed Human Neurons to Understand Age-Related Energy Metabolism Impairment and Mitochondrial Dysfunction in Healthy Aging and Neurodegeneration

Brain aging is characterized by several molecular and cellular changes grouped as the hallmarks or pillars of aging, including organelle dysfunction, metabolic and nutrition-sensor changes, stem cell attrition, and macromolecular damages. Separately and collectively, these features degrade the most critical neuronal function: transmission of information in the brain. It is widely accepted that aging is the leading risk factor contributing to the onset of the most prevalent pathological conditions that affect brain functions, such as Alzheimer's, Parkinson's, and Huntington's disease. One of the limitations in understanding the molecular mechanisms involved in those diseases is the lack of an appropriate cellular model that recapitulates the “aged” context in human neurons. The advent of the cellular reprogramming of somatic cells, i.e., dermal fibroblasts, to obtain directly induced neurons (iNs) and induced pluripotent stem cell- (iPSC-) derived neurons is technical sound advances that could open the avenues to understand better the contribution of aging toward neurodegeneration. In this review, we will summarize the commonalities and singularities of these two approaches for the study of brain aging, with an emphasis on the role of mitochondrial dysfunction and redox biology. We will address the evidence showing that iNs retain age-related features in contrast to iPSC-derived neurons that lose the aging signatures during the reprogramming to pluripotency, rendering iNs a powerful strategy to deepen our knowledge of the processes driving normal cellular function decline and neurodegeneration in a human adult model. We will finally discuss the potential utilization of these novel technologies to understand the differential contribution of genetic and epigenetic factors toward neuronal aging, to identify and develop new drugs and therapeutic strategies.