Ashrafi G, Wu Z, Farrell RJ, Ryan TA. GLUT4 Mobilization Supports Energetic Demands of Active Synapses. Neuron 2017; 93: 606-615. e3. [PMID: 28111082 DOI: 10.1016/j.neuron.2016.12.020]

AMPK as a Therapeutic Target: Advancing Epilepsy Management Through Metabolic Modulation

Epilepsy is often marked by paroxysmal seizures that disrupt the brain's sensory, motor, and psychosocial functions. The underlying pathology is generally believed to involve an imbalance between excitatory and inhibitory neurotransmission. However, a less explored but significant contributor to epilepsy is the collapse of the brain's metabolic and bioenergetic systems. The breakdown of the brain's bioenergetic system leads to the activation of various detrimental downstream signaling cascades that ultimately result in oxidative stress, neuroinflammation, and reduced autophagic flux, all of which impair neuronal-glial communication and precipitate epileptic attacks. This highlights the pressing need for a therapeutic agent to address these complex challenges. Researchers have identified adenosine monophosphate kinase (AMPK) as a potential solution. AMPK acts as the body's primary stress sensor, activated in response to the deficiency of growth factors and nutrient starvation to restore energy homeostasis. AMPK activation also maintains the intricate communication between neurons and glial cells, preserving synaptic plasticity integrity, mitigating mitochondrial damage, and dampening inflammatory signaling cascades. Despite demonstrating significant efficacy in managing a range of peripheral and neurological disorders, the role of AMPK in neurotransmission and epilepsy remains unexplored. This review explores the multifaceted molecular roles of AMPK beyond its traditional metabolic regulatory functions, suggesting that targeting AMPK could provide a novel avenue for drug development in epilepsy treatment.

Regulation of synaptic mitochondria by extracellular vesicles and its implications for neuronal metabolism and synaptic plasticity

Mitochondrial metabolism in neurons is necessary for energetically costly processes like synaptic transmission and plasticity. As post-mitotic cells, neurons are therefore faced with the challenge of maintaining healthy functioning mitochondria throughout lifetime. The precise mechanisms of mitochondrial maintenance in neurons, and particularly in morphologically complex dendrites and axons, are not fully understood. Evidence from several biological systems suggests the regulation of cellular metabolism by extracellular vesicles (EVs), secretory lipid-enclosed vesicles that have emerged as important mediators of cell communication. In the nervous system, neuronal and glial EVs were shown to regulate neuronal circuit development and function, at least in part via the transfer of protein and RNA cargo. Interestingly, EVs have been implicated in diseases characterized by altered metabolism, such as cancer and neurodegenerative diseases. Furthermore, nervous system EVs were shown to contain proteins related to metabolic processes, mitochondrial proteins and even intact mitochondria. Here, we present the current knowledge of the mechanisms underlying neuronal mitochondrial maintenance, and highlight recent evidence suggesting the regulation of synaptic mitochondria by neuronal and glial cell EVs. We further discuss the potential implications of EV-mediated regulation of mitochondrial maintenance and function in neuronal circuit development and synaptic plasticity.

A budget for brain metabolic water production by glucose catabolism during rest, rises in activity and sleep

Maintaining brain fluid homeostasis is of critical importance for creating a stable environment conducive to optimal neuronal functioning, nutrient distribution, and waste product removal. In this study, we employed previously published data on brain oxygen and glucose consumption in awake rodents or humans to quantify the metabolic water production associated with distinct pathways of glucose metabolism. It is predicted that neuronal mitochondria are the primary source of metabolic water at rest, resulting in a continuous efflux into the cytosol, interstitial fluid, and cerebrospinal fluid. Net metabolic water production is predicted to be reduced by increases in activity due to a shift in metabolism from glucose oxidation to include glycolysis in neurons and ATP hydrolysis by the major cation pumps, which involves water consumption (ATP + H2O → ADP + Pi). In comparison, glycogenolysis, which occurs concurrently with the activation of astrocytes, potentially represents a major but previously unidentified contributor to metabolic water. Metabolic water production is dependent on the state of the brain, with a reduction of 30-40% occurring during deep sleep. Our estimates indicate that metabolic water functions as a conduit for interstitial fluid production within the brain, enabling flexible and efficient distribution of fluid that flows seamlessly from the parenchyma to the subarachnoid space and lymphatic vessels to facilitate the removal of brain waste, independent of the glymphatic system.

Sleep loss is a metabolic disorder

Sleep loss dysregulates cellular metabolism and energy homeostasis. Highly metabolically active cells, such as neurons, enter a catabolic state during periods of sleep loss, which consequently disrupts physiological functioning. Specific to the central nervous system, sleep loss results in impaired synaptogenesis and long-term memory, effects that are also characteristic of neurodegenerative diseases. In this review, we describe how sleep deprivation increases resting energy expenditure, leading to the development of a negative energy balance-a state with insufficient metabolic resources to support energy expenditure-in highly active cells like neurons. This disruption of energetic homeostasis alters the balance of metabolites, including adenosine, lactate, and lipid peroxides, such that energetically costly processes, such as synapse formation, are attenuated. During sleep loss, metabolically active cells shunt energetic resources away from those processes that are not acutely essential, like memory formation, to support cell survival. Ultimately, these findings characterize sleep loss as a metabolic disorder.

Multi-target approach to Alzheimer's disease prevention and treatment: antioxidant, anti-inflammatory, and amyloid- modulating mechanisms

Alzheimer's disease (AD) is characterized by amyloid-β (Aβ) plaque accumulation, neurofibrillary tangles, neuroinflammation, and progressive cognitive decline, posing a significant global health challenge. Growing evidence suggests that dietary polyphenols may reduce the risk and progression of AD through multifaceted neuroprotective mechanisms. Polyphenols regulate amyloid proteostasis by inhibiting β/γ-secretase activity, preventing Aβ aggregation, and enhancing clearance pathways. Their strong antioxidant properties neutralize reactive oxygen species, chelate redox-active metals, and activate cytoprotective enzymes via Nrf2 signaling. This review examines the potential therapeutic targets, signaling pathways, and molecular mechanisms by which dietary polyphenols exert neuroprotective effects in AD, focusing on their roles in modulating amyloid proteostasis, oxidative stress, neuroinflammation, and cerebrovascular health. Polyphenols mitigate neuroinflammation by suppressing NF-κB signaling and upregulating brain-derived neurotrophic factor, supporting neuroplasticity and neurogenesis. They also enhance cerebrovascular health by improving cerebral blood flow, maintaining blood-brain barrier integrity, and modulating angiogenesis. This review examines the molecular and cellular pathways through which polyphenols exert neuroprotective effects, focusing on their antioxidant, anti-inflammatory, and amyloid-modulating roles. We also discuss their influence on key AD pathologies, including Aβ deposition, tau hyperphosphorylation, oxidative stress, and neuroinflammation. Insights from clinical and preclinical studies highlight the potential of polyphenols in preventing or slowing AD progression. Future research should explore personalized dietary strategies that integrate genetic and lifestyle factors to optimize the neuroprotective effects of polyphenols.

Metabolic profiling of adult and pediatric gliomas reveals enriched glucose availability in pediatric gliomas and increased fatty acid oxidation in adult gliomas

Gliomas are the most common primary brain tumors and a major source of mortality and morbidity in adults and children. Recent genomic studies have identified multiple molecular subtypes; however metabolic characterization of these tumors has thus far been limited. We performed metabolic profiling of 114 adult and pediatric primary gliomas and integrated metabolomic data with transcriptomics and DNA methylation classes. We identified that pediatric tumors have higher levels of glucose and reduced lactate compared to adult tumors regardless of underlying genetics or grade, suggesting differences in availability of glucose and/or utilization of glucose for downstream pathways. Differences in glucose utilization in pediatric gliomas may be facilitated through overexpression of SLC2A4, which encodes the insulin-stimulated glucose transporter GLUT4. Transcriptomic comparison of adult and pediatric tumors suggests that adult tumors may have limited access to glucose and experience more hypoxia, which is supported by enrichment of lactate, 2-hydroxyglutarate (2-HG), even in isocitrate dehydrogenase (IDH) wild-type tumors, and 3-hydroxybutyrate, a ketone body that is produced by oxidation of fatty acids and ketogenic amino acids during periods of glucose scarcity. Our data support adult tumors relying more on fatty acid oxidation, as they have an abundance of acyl carnitines compared to pediatric tumors and have significant enrichment of transcripts needed for oxidative phosphorylation. Our findings suggest striking differences exist in the metabolism of pediatric and adult gliomas, which can provide new insight into metabolic vulnerabilities for therapy.

Brain aging shows nonlinear transitions, suggesting a midlife "critical window" for metabolic intervention

Understanding the key drivers of brain aging is essential for effective prevention and treatment of neurodegenerative diseases. Here, we integrate human brain and physiological data to investigate underlying mechanisms. Functional MRI analyses across four large datasets (totaling 19,300 participants) show that brain networks not only destabilize throughout the lifetime but do so along a nonlinear trajectory, with consistent temporal "landmarks" of brain aging starting in midlife (40s). Comparison of metabolic, vascular, and inflammatory biomarkers implicate dysregulated glucose homeostasis as the driver mechanism for these transitions. Correlation between the brain's regionally heterogeneous patterns of aging and gene expression further supports these findings, selectively implicating GLUT4 (insulin-dependent glucose transporter) and APOE (lipid transport protein). Notably, MCT2 (a neuronal, but not glial, ketone transporter) emerges as a potential counteracting factor by facilitating neurons' energy uptake independently of insulin. Consistent with these results, an interventional study of 101 participants shows that ketones exhibit robust effects in restabilizing brain networks, maximized from ages 40 to 60, suggesting a midlife "critical window" for early metabolic intervention.

Autophagy regulator ATG5 preserves cerebellar function by safeguarding its glycolytic activity

Dysfunctions in autophagy, a cellular mechanism for breaking down components within lysosomes, often lead to neurodegeneration. The specific mechanisms underlying neuronal vulnerability due to autophagy dysfunction remain elusive. Here we show that autophagy contributes to cerebellar Purkinje cell (PC) survival by safeguarding their glycolytic activity. Outside the conventional housekeeping role, autophagy is also involved in the ATG5-mediated regulation of glucose transporter 2 (GLUT2) levels during cerebellar maturation. Autophagy-deficient PCs exhibit GLUT2 accumulation on the plasma membrane, along with increased glucose uptake and alterations in glycolysis. We identify lysophosphatidic acid and serine as glycolytic intermediates that trigger PC death and demonstrate that the deletion of GLUT2 in ATG5-deficient mice mitigates PC neurodegeneration and rescues their ataxic gait. Taken together, this work reveals a mechanism for regulating GLUT2 levels in neurons and provides insights into the neuroprotective role of autophagy by controlling glucose homeostasis in the brain.

Hippocampal PDHA1 gene knockout inhibits the Warburg effect leading to cognitive dysfunction and attenuates the beneficial effects of ZiBuPiYin recipe on cognition

ETHNOPHARMACOLOGICAL RELEVANCE

The attenuation of the Warburg effect is an important pathological feature of cognitive dysfunction, and enhancing the Warburg effect is conducive to improving cognitive function. However, the pathogenic mechanisms underlying cognitive dysfunction remain incompletely elucidated. ZiBuPiYin Recipe (ZBPYR) is a traditional Chinese herbal compound used clinically for the treatment of cognitive dysfunction with significant efficacy. Nonetheless, the molecular mechanism underlying its beneficial effects remains elusive.

AIM OF THE STUDY

The objective of this study is to investigate whether the attenuation of the Warburg effect exists in a mouse model of cognitive dysfunction induced by knockout of the pyruvate dehydrogenase E1 component subunit alpha (PDHA1) gene in the hippocampus, as well as the interventional effect of ZBPYR.

MATERIALS AND METHODS

Using mice with PDHA1 gene knockout in the hippocampus and their littermate control mice as study subjects, behavioral experiments were conducted to assess the impact of PDHA1 gene knockout on cognitive function and the interventional effect of ZBPYR. We detected the expression of the Warburg effect-associated rate-limiting enzymes and PI3K/AKT pathway-related proteins. Subsequently, in PC12 cells, we explored the effect of the Warburg effect on cell apoptosis as well as the role of PDHA1 in the regulation of the PI3K/AKT-Warburg effect and the potential mechanism of ZBPYR in improving cognitive function.

RESULTS

Mice with knockout of the PDHA1 gene in the hippocampus exhibited cognitive dysfunction, inhibition of the PI3K/AKT pathway, reduction of the Warburg effect, and neuronal damage. In vitro experiments indicated that silencing of PDHA1 in the hippocampus inhibited the PI3K/AKT-Warburg effect, leading to cell apoptosis and mediated the effect of ZBPYR in improving cognitive function.

CONCLUSION

Our data not only suggest that the hippocampal PDHA1-PI3K/AKT-Warburg effect may be involved in the pathogenesis of cognitive dysfunction, but also demonstrate that PDHA1 knockout can abolish the beneficial effects of ZBPYR on cognition. This research aids in unraveling the cause of cognitive dysfunction and, therefore, offers a promising and innovative therapeutic target for these patients.

Intracellular Lactate Dynamics in Drosophila Glutamatergic Neurons

Rates of lactate production and consumption reflect the metabolic state of many cell types, including neurons. Here, we investigate the effects of nutrient deprivation on lactate dynamics in Drosophila glutamatergic neurons by leveraging the limiting effects of the diffusion barrier surrounding cells in culture. We found that neurons constitutively consume lactate when availability of trehalose, the glucose disaccharide preferred by insects, is limited by the diffusion barrier. Acute mechanical disruption of the barrier reduced this reliance on lactate. Through kinetic modeling and experimental validation, we demonstrate that neuronal lactate consumption rates correlate inversely with their mitochondrial density. Further, we found that lactate levels in neurons exhibited temporal correlations that allowed prediction of cytosolic lactate dynamics after the disruption of the diffusion barrier from pre-perturbation lactate fluctuations. Collectively, our findings reveal the influence of diffusion barriers on neuronal metabolic preferences, and demonstrate the existence of temporal correlations between lactate dynamics under conditions of nutrient deprivation and those evoked by the subsequent restoration of nutrient availability.

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 ex vivo incubation. In the synGLT-1 KO, replenishment of glycogen stores during ex vivo 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.

Insulin resistance compromises midbrain organoid neuronal activity and metabolic efficiency predisposing to Parkinson's disease pathology

Growing evidence indicates that type 2 diabetes (T2D) is associated with an increased risk of developing Parkinson's disease (PD) through shared disease mechanisms. Studies show that insulin resistance, which is the driving pathophysiological mechanism of T2D plays a major role in neurodegeneration by impairing neuronal functionality, metabolism and survival. To investigate insulin resistance caused pathological changes in the human midbrain, which could predispose a healthy midbrain to PD development, we exposed iPSC-derived human midbrain organoids from healthy individuals to either high insulin concentration, promoting insulin resistance, or to more physiological insulin concentration restoring insulin signalling function. We combined experimental methods with metabolic modelling to identify the most insulin resistance-dependent pathogenic processes. We demonstrate that insulin resistance compromises organoid metabolic efficiency, leading to increased levels of oxidative stress. Additionally, insulin-resistant midbrain organoids showed decreased neuronal activity and reduced amount of dopaminergic neurons, highlighting insulin resistance as a significant target in PD prevention.

Impact of glycemic treatment and blood glucose monitoring on outcomes in patients with acute ischemic stroke without prior diabetes: a longitudinal cohort study

OBJECTIVES

To explore the short- and long-term effects of glycemic management-through glycemic treatment and blood glucose monitoring (BGM)-on stroke recurrence and mortality specifically in patients experiencing a first-ever ischemic stroke (FIS) with hyperglycemia (FISHG) who have not previously been diagnosed with diabetes mellitus (DM).

METHODS

We gathered data on patients who were registered on Taiwan's National Health Insurance Research Database from 2000 to 2015. We one-fold propensity-score-matched (by sex, age, and index date) 207,054 patients into 3 cohorts: those with FIS (1) without hyperglycemia, (2) hyperglycemia without glycemic treatment, and (3) hyperglycemia with glycemic treatment. We used Cox proportional hazard regression to evaluate the short- (within 1 year after FIS) and long-term (9.3 ± 8.6 years after FIS) prognostic effects of glycemic management on stroke recurrence and mortality of FISHG.

RESULTS

Stroke recurrence and mortality were significantly more likely in the patients with FISHG than their counterparts without hyperglycemia (p < 0.05). Under glycemic treatment, patients with FISHG demonstrated lower risk of mortality at every follow-up than those without (p < 0.001) but were not less likely to have stroke recurrence (p > 0.05). Integrating BGM with glycemic treatment in the FISHG cohort significantly reduced the risk of stroke recurrence compared to patients receiving only glycemic treatment at 1-month, 3-month, 6-month, and 1-year post-stroke follow-ups (adjusted hazard ratios = 0.84, 0.90, 0.88, and 0.92, respectively); additionally, this approach significantly decreased mortality risk at each post-stroke follow-up period (p < 0.05).

CONCLUSIONS

BGM combined with glycemic treatment significantly improves prognosis in patients with FISHG who have not been previously diagnosed with DM, reducing the risks of stroke recurrence and mortality.

Mitochondrial plasticity: An emergent concept in neuronal plasticity and memory

Mitochondria are classically viewed as 'on demand' energy suppliers to neurons in support of their activity. In order to adapt to a wide range of demands, mitochondria need to be highly dynamic and capable of adjusting their metabolic activity, shape, and localization. Although these plastic properties give them a central support role in basal neuronal physiology, recent lines of evidence point toward a role for mitochondria in the regulation of high-order cognitive functions such as memory formation. In this review, we discuss the interplay between mitochondrial function and neural plasticity in sustaining memory formation at the molecular and cellular levels. First, we explore the global significance of mitochondria in memory formation. Then, we will detail the memory-relevant cellular and molecular mechanisms of mitochondrial plasticity. Finally, we focus on those mitochondrial functions, including but not limited to ATP production, that give mitochondria their pivotal role in memory formation. Altogether, this review highlights the central role of mitochondrial structural and functional plasticity in supporting and regulating neuronal plasticity and memory.

Does Metabolic Manager Show Encouraging Outcomes in Alzheimer's?: Challenges and Opportunity for Protein Tyrosine Phosphatase 1b Inhibitors

Protein tyrosine phosphatase 1b (PTP1b) is a member of the protein tyrosine phosphatase (PTP) enzyme group and encoded as PTP1N gene. Studies have evidenced an overexpression of the PTP1b enzyme in metabolic syndrome, anxiety, schizophrenia, neurodegeneration, and neuroinflammation. PTP1b inhibitor negatively regulates insulin and leptin pathways and has been explored as an antidiabetic agent in various clinical trials. Notably, the preclinical studies have shown that recuperating metabolic dysfunction and dyshomeostasis can reverse cognition and could be a possible approach to mitigate multifaceted Alzheimer's disease (AD). PTP1b inhibitor thus has attracted attention in neuroscience, though the development is limited to the preclinical stage, and its exploration in large clinical trials is warranted. This review provides an insight on the development of PTP1b inhibitors from different sources in diabesity. The crosstalk between metabolic dysfunction and insulin insensitivity in AD and type-2 diabetes has also been highlighted. Furthermore, this review presents the significance of PTP1b inhibition in AD based on pathophysiological facets, and recent evidences from preclinical and clinical studies.

Mitochondrial pyruvate transport regulates presynaptic metabolism and neurotransmission

Glucose has long been considered the primary fuel source for the brain. However, glucose levels fluctuate in the brain during sleep or circuit activity, posing major metabolic stress. Here, we demonstrate that the mammalian brain uses pyruvate as a fuel source, and pyruvate can support neuronal viability in the absence of glucose. Nerve terminals are sites of metabolic vulnerability, and we show that mitochondrial pyruvate uptake is a critical step in oxidative ATP production in hippocampal terminals. We find that the mitochondrial pyruvate carrier is post-translationally modified by lysine acetylation, which, in turn, modulates mitochondrial pyruvate uptake. Our data reveal that the mitochondrial pyruvate carrier regulates distinct steps in neurotransmission, namely, the spatiotemporal pattern of synaptic vesicle release and the efficiency of vesicle retrieval-functions that have profound implications for synaptic plasticity. In summary, we identify pyruvate as a potent neuronal fuel and mitochondrial pyruvate uptake as a critical node for the metabolic control of neurotransmission in hippocampal terminals.

Astrocytic GLUT1 reduction paradoxically improves central and peripheral glucose homeostasis

Astrocytes are considered an essential source of blood-borne glucose or its metabolites to neurons. Nonetheless, the necessity of the main astrocyte glucose transporter, i.e., GLUT1, for brain glucose metabolism has not been defined. Unexpectedly, we found that brain glucose metabolism was paradoxically augmented in mice with astrocytic GLUT1 reduction (GLUT1ΔGFAP mice). These mice also exhibited improved peripheral glucose metabolism especially in obesity, rendering them metabolically healthier. Mechanistically, we observed that GLUT1-deficient astrocytes exhibited increased insulin receptor-dependent ATP release, and that both astrocyte insulin signaling and brain purinergic signaling are essential for improved brain function and systemic glucose metabolism. Collectively, we demonstrate that astrocytic GLUT1 is central to the regulation of brain energetics, yet its depletion triggers a reprogramming of brain metabolism sufficient to sustain energy requirements, peripheral glucose homeostasis, and cognitive function.

Monitoring of activity-driven trafficking of endogenous synaptic proteins through proximity labeling

To enable transmission of information in the brain, synaptic vesicles fuse to presynaptic membranes, liberating their content and exposing transiently a myriad of vesicular transmembrane proteins. However, versatile methods for quantifying the synaptic translocation of endogenous proteins during neuronal activity remain unavailable, as the fast dynamics of synaptic vesicle cycling difficult specific isolation of trafficking proteins during such a transient surface exposure. Here, we developed a novel approach using synaptic cleft proximity labeling to capture and quantify activity-driven trafficking of endogenous synaptic proteins at the synapse. We show that accelerating cleft biotinylation times to match the fast dynamics of vesicle exocytosis allows capturing endogenous proteins transiently exposed at the synaptic surface during neural activity, enabling for the first time the study of the translocation of nearly every endogenous synaptic protein. As proof-of-concept, we further applied this technology to obtain direct evidence of the surface translocation of noncanonical trafficking proteins, such as ATG9A and NPTX1, which had been proposed to traffic during activity but for which direct proof had not yet been shown. The technological advancement presented here will facilitate future studies dissecting the molecular identity of proteins exocytosed at the synapse during activity, helping to define the molecular machinery that sustains neurotransmission in the mammalian brain.

Brain energetics and glucose transport in metabolic diseases: role in neurodegeneration

OBJECTIVES

Neurons and glial cells are the main functional and structural elements of the brain, and the former depends on the latter for their nutritional, functional and structural organization, as well as for their energy maintenance.

METHODS

Glucose is the main metabolic source that fulfills energetic demands, either by direct anaplerosis or through its conversion to metabolic intermediates. Development of some neurodegenerative diseases have been related with modifications in the expression and/or function of glial glucose transporters, which might cause physiological and/or pathological disturbances of brain metabolism. In the present contribution, we summarized the experimental findings that describe the exquisite adjustment in expression and function of glial glucose transporters from physiologic to pathologic metabolism, and its relevance to neurodegenerative diseases.

RESULTS

A exhaustive literature review was done in order to gain insight into the role of brain energetics in neurodegenerative disease. This study made evident a critical involvement of glucose transporters and thus brain energetics in the development of neurodegenerative diseases.

DISCUSSION

An exquisite adjustment in the expression and function of glial glucose transporters from physiologic to pathologic metabolism is a biochemical signature of neurodegenerative diseases.

Glucose metabolism impairment as a hallmark of progressive myoclonus epilepsies: a focus on neuronal ceroid lipofuscinoses

Glucose is the brain's main fuel source, used in both energy and molecular production. Impaired glucose metabolism is associated with adult and pediatric neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), GLUT1 deficiency syndrome, and progressive myoclonus epilepsies (PMEs). PMEs, a group of neurological disorders typical of childhood and adolescence, account for 1% of all epileptic diseases in this population worldwide. Diffuse glucose hypometabolism is observed in the brains of patients affected by PMEs such as Lafora disease (LD), dentatorubral-pallidoluysian (DRPLA) atrophy, Unverricht-Lundborg disease (ULD), and myoclonus epilepsy with ragged red fibers (MERRFs). PMEs also include neuronal ceroid lipofuscinoses (NCLs), a subgroup in which lysosomal and autophagy dysfunction leads to progressive loss of vision, brain atrophy, and cognitive decline. We examine the role of impaired glucose metabolism in neurodegenerative diseases, particularly in the NCLs. Our literature review, which includes findings from case reports and animal studies, reveals that glucose hypometabolism is still poorly characterized both in vitro and in vivo in the different NCLs. Better identification of the glucose metabolism pathway impaired in the NCLs may open new avenues for evaluating the therapeutic potential of anti-diabetic agents in this population and thus raise the prospect of a therapeutic approach able to delay or even halt disease progression.

Development of a novel glucose-dendrimer based therapeutic targeting hyperexcitable neurons in neurological disorders

Neuronal hyperexcitability and excitotoxicity lies at the core of debilitating brain disorders such as epilepsy and traumatic brain injury, culminating in neuronal death and compromised brain function. Overcoming this challenge requires a unique approach that selectively restores normal neuronal activity and rescues neurons from impending damage. However, delivering drugs selectively to hyperexcitable neurons has been a challenge, even upon local administration. Here, we demonstrate the remarkable ability of a novel, scalable, generation-two glucose-dendrimer (GD2) made primarily of glucose and ethylene glycol building blocks, to specifically target hyperexcitable neurons in primary culture, ex vivo acute brain slices, and in vivo mouse models of acute seizures. Pharmacology experiments in ex vivo brain slices suggest GD2 uptake in neurons is mediated through glucose transporters (GLUT and SGLT). Inspired by these findings, we conjugated GD2 with a potent anti-epileptic drug, valproic acid (GD2-VPA), for efficacy studies in the pilocarpine-mouse model of seizure. When delivered intranasally, GD2-VPA significantly decreased the seizure-severity. In summary, our findings demonstrate the unique selectivity of glucose dendrimers in targeting hyperexcitable neurons, even upon intranasal delivery, laying the foundation for neuron-specific therapies for the precise protection and restoration of neuronal function, for targeted neuroprotection.

Organization of a functional glycolytic metabolon on mitochondria for metabolic efficiency

Glucose, the primary cellular energy source, is metabolized through glycolysis initiated by the rate-limiting enzyme hexokinase (HK). In energy-demanding tissues like the brain, HK1 is the dominant isoform, primarily localized on mitochondria, and is crucial for efficient glycolysis-oxidative phosphorylation coupling and optimal energy generation. This study unveils a unique mechanism regulating HK1 activity, glycolysis and the dynamics of mitochondrial coupling, mediated by the metabolic sensor enzyme O-GlcNAc transferase (OGT). OGT catalyses reversible O-GlcNAcylation, a post-translational modification influenced by glucose flux. Elevated OGT activity induces dynamic O-GlcNAcylation of the regulatory domain of HK1, subsequently promoting the assembly of the glycolytic metabolon on the outer mitochondrial membrane. This modification enhances the mitochondrial association with HK1, orchestrating glycolytic and mitochondrial ATP production. Mutation in HK1's O-GlcNAcylation site reduces ATP generation in multiple cell types, specifically affecting metabolic efficiency in neurons. This study reveals a previously unappreciated pathway that links neuronal metabolism and mitochondrial function through OGT and the formation of the glycolytic metabolon, providing potential strategies for tackling metabolic and neurological disorders.

Pyruvate dehydrogenase complex E1 subunit α crotonylation modulates cocaine-associated memory through hippocampal neuron activation

Neuronal activation is required for the formation of drug-associated memory, which is critical for the development, persistence, and relapse of drug addiction. Nevertheless, the metabolic mechanisms underlying energy production for neuronal activation remain poorly understood. In the study, a large-scale proteomics analysis of lysine crotonylation (Kcr), a type of protein posttranslational modification (PTM), reveals that cocaine promoted protein Kcr in the hippocampal dorsal dentate gyrus (dDG). We find that Kcr is predominantly discovered in a few enzymes critical for mitochondrial energy metabolism; in particular, pyruvate dehydrogenase (PDH) complex E1 subunit α (PDHA1) is crotonylated at the lysine 39 (K39) residue through P300 catalysis. Crotonylated PDHA1 promotes pyruvate metabolism by activating PDH to increase ATP production, thus providing energy for hippocampal neuronal activation and promoting cocaine-associated memory recall. Our findings identify Kcr of PDHA1 as a PTM that promotes pyruvate metabolism to enhance neuronal activity for cocaine-associated memory.

Phosphoglycerate kinase is a central leverage point in Parkinson's disease-driven neuronal metabolic deficits

Although certain drivers of familial Parkinson's disease (PD) compromise mitochondrial integrity, whether metabolic deficits underly other idiopathic or genetic origins of PD is unclear. Here, we demonstrate that phosphoglycerate kinase 1 (PGK1), a gene in the PARK12 susceptibility locus, is rate limiting in neuronal glycolysis and that modestly increasing PGK1 expression boosts neuronal adenosine 5'-triphosphate production kinetics that is sufficient to suppress PARK20-driven synaptic dysfunction. We found that this activity enhancement depends on the molecular chaperone PARK7/DJ-1, whose loss of function significantly disrupts axonal bioenergetics. In vivo, viral expression of PGK1 confers protection of striatal dopamine axons against metabolic lesions. These data support the notion that bioenergetic deficits may underpin PD-associated pathologies and point to improving neuronal adenosine 5'-triphosphate production kinetics as a promising path forward in PD therapeutics.

Neuronal activity-driven O-GlcNAcylation promotes mitochondrial plasticity

Neuronal activity is an energy-intensive process that is largely sustained by instantaneous fuel utilization and ATP synthesis. However, how neurons couple ATP synthesis rate to fuel availability is largely unknown. Here, we demonstrate that the metabolic sensor enzyme O-linked N-acetyl glucosamine (O-GlcNAc) transferase regulates neuronal activity-driven mitochondrial bioenergetics in hippocampal and cortical neurons. We show that neuronal activity upregulates O-GlcNAcylation in mitochondria. Mitochondrial O-GlcNAcylation is promoted by activity-driven glucose consumption, which allows neurons to compensate for high energy expenditure based on fuel availability. To determine the proteins that are responsible for these adjustments, we mapped the mitochondrial O-GlcNAcome of neurons. Finally, we determine that neurons fail to meet activity-driven metabolic demand when O-GlcNAcylation dynamics are prevented. Our findings suggest that O-GlcNAcylation provides a fuel-dependent feedforward control mechanism in neurons to optimize mitochondrial performance based on neuronal activity. This mechanism thereby couples neuronal metabolism to mitochondrial bioenergetics and plays a key role in sustaining energy homeostasis.

Parkinson's disease gene, Synaptojanin1, dysregulates the surface maintenance of the dopamine transporter

Missense mutations of PARK20/SYNJ1 (synaptojanin1/Synj1) were found in complex forms of familial Parkinsonism. However, the Synj1-regulated molecular and cellular changes associated with dopaminergic dysfunction remain unknown. We now report a fast depletion of evoked dopamine and impaired maintenance of the axonal dopamine transporter (DAT) in the Synj1 haploinsufficient (Synj1+/-) neurons. While Synj1 has been traditionally known to facilitate the endocytosis of synaptic vesicles, we provide in vitro and in vivo evidence demonstrating that Synj1 haploinsufficiency results in an increase of total DAT but a reduction of the surface DAT. Synj1+/- neurons exhibit maladaptive DAT trafficking, which could contribute to the altered DA release. We showed that the loss of surface DAT is associated with the impaired 5'-phosphatase activity and the hyperactive PI(4,5)P2-PKCβ pathway downstream of Synj1 deficiency. Thus, our findings provided important mechanistic insight for Synj1-regulated DAT trafficking integral to dysfunctional DA signaling, which might be relevant to early Parkinsonism.

1-Deoxynojirimycin attenuates pathological markers of Alzheimer's disease in the in vitro model of neuronal insulin resistance

Insulin resistance, the hallmark of type 2 diabetes mellitus (T2DM), has emerged as a pathological feature in Alzheimer's disease (AD). Given the shared role of insulin resistance in T2DM and AD, repurposing peripheral insulin sensitizers is a promising strategy to preserve neuronal insulin sensitivity and prevent AD. 1-Deoxynojirimycin (DNJ), a bioactive iminosugar, exhibited insulin-sensitizing effects in metabolic tissues and was detected in brain tissue post-oral intake. However, its impact on brain and neuronal insulin signaling has not been described. Here, we investigated the effect of DNJ treatment on insulin signaling and AD markers in insulin-resistant human SK-N-SH neuroblastoma, a cellular model of neuronal insulin resistance. Our findings show that DNJ increased the expression of insulin signaling genes and the phosphorylation status of key molecules implicated in insulin resistance (Y1146-pIRβ, S473-pAKT, S9-GSK3B) while also elevating the expression of glucose transporters Glut3 and Glut4, resulting in higher glucose uptake upon insulin stimuli. DNJ appeared to mitigate the insulin resistance-driven increase in phosphorylated tau and Aβ1-42 levels by promoting insulin-induced phosphorylation of GSK3B (a major tau kinase) and enhancing mRNA expression of the insulin-degrading enzyme (IDE) pivotal for insulin and Aβ clearance. Overall, our study unveils probable mechanisms underlying the potential benefits of DNJ for AD, wherein DNJ attenuates tau and amyloid pathologies by reversing neuronal insulin resistance. This provides a scientific basis for expanding the use of DNJ-containing products for neuroprotective purposes and prompts further research into compounds with similar mechanisms of action.

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.

Characterization of β-Hydroxybutyrate as a Cell Autonomous Fuel for Active Excitatory and Inhibitory Neurons: β-Hydroxybutyrate as a Fuel for Active Neurons

The ketogenic diet is an effective treatment for drug-resistant epilepsy, but the therapeutic mechanisms are poorly understood. Although ketones are able to fuel the brain, it is not known whether ketones are directly metabolized by neurons on a time scale sufficiently rapid to fuel the bioenergetic demands of sustained synaptic transmission. Here, we show that nerve terminals can use the ketone β-hydroxybutyrate in a cell- autonomous fashion to support neurotransmission in both excitatory and inhibitory nerve terminals and that this flexibility relies on Ca2+ dependent upregulation of mitochondrial metabolism. Using a genetically encoded ATP sensor, we show that inhibitory axons fueled by ketones sustain much higher ATP levels under steady state conditions than excitatory axons, but that the kinetics of ATP production following activity are slower when using ketones as fuel compared to lactate/pyruvate for both excitatory and inhibitory neurons.

Metabolic regulation of single synaptic vesicle exo- and endocytosis in hippocampal synapses

Glucose has long been considered a primary energy source for synaptic function. However, it remains unclear to what extent alternative fuels, such as lactate/pyruvate, contribute to powering synaptic transmission. By detecting individual release events in hippocampal synapses, we find that mitochondrial ATP production regulates basal vesicle release probability and release location within the active zone (AZ), evoked by single action potentials. Mitochondrial inhibition shifts vesicle release closer to the AZ center and alters the efficiency of vesicle retrieval by increasing the occurrence of ultrafast endocytosis. Furthermore, we uncover that terminals can use oxidative fuels to maintain the vesicle cycle during trains of activity. Mitochondria are sparsely distributed along hippocampal axons, and we find that terminals containing mitochondria display enhanced vesicle release and reuptake during high-frequency trains. Our findings suggest that mitochondria not only regulate several fundamental features of synaptic transmission but may also contribute to modulation of short-term synaptic plasticity.

Activity-driven synaptic translocation of LGI1 controls excitatory neurotransmission

The fine control of synaptic function requires robust trans-synaptic molecular interactions. However, it remains poorly understood how trans-synaptic bridges change to reflect the functional states of the synapse. Here, we develop optical tools to visualize in firing synapses the molecular behavior of two trans-synaptic proteins, LGI1 and ADAM23, and find that neuronal activity acutely rearranges their abundance at the synaptic cleft. Surprisingly, synaptic LGI1 is primarily not secreted, as described elsewhere, but exo- and endocytosed through its interaction with ADAM23. Activity-driven translocation of LGI1 facilitates the formation of trans-synaptic connections proportionally to the history of activity of the synapse, adjusting excitatory transmission to synaptic firing rates. Accordingly, we find that patient-derived autoantibodies against LGI1 reduce its surface fraction and cause increased glutamate release. Our findings suggest that LGI1 abundance at the synaptic cleft can be acutely remodeled and serves as a critical control point for synaptic function.

Genetic deletion of zinc transporter ZnT3 induces progressive cognitive deficits in mice by impairing dendritic spine plasticity and glucose metabolism

Zinc transporter 3 (ZnT3) is abundantly expressed in the brain, residing in synaptic vesicles, where it plays important roles in controlling the luminal zinc levels. In this study, we found that ZnT3 knockout in mice decreased zinc levels in the hippocampus and cortex, and was associated with progressive cognitive impairments, assessed at 2, 6, and 9-month of age. The results of Golgi-Cox staining demonstrated that ZnT3 deficiency was associated with an increase in dendritic complexity and a decrease in the density of mature dendritic spines, indicating potential synaptic plasticity deficit. Since ZnT3 deficiency was previously linked to glucose metabolism abnormalities, we tested the expression levels of genes related to insulin signaling pathway in the hippocampus and cortex. We found that the Expression of glucose transporters, GLUT3, GLUT4, and the insulin receptor in the whole tissue and synaptosome fraction of the hippocampus of the ZnT3 knockout mice were significantly reduced, as compared to wild-type controls. Expression of AKT (A serine/threonine protein kinase) and insulin-induced AKT phosphorylation was also reduced in the hippocampus of ZnT3 knockout mice. We hypothesize that the ZnT3 deficiency and reduced brain zinc levels may cause cognitive impairment by negatively affecting glycose metabolism via decreased expression of key components of insulin signaling, as well as via changes in synaptic plasticity. These finding may provide new therapeutic target for treatments of neurodegenerative disorders.

D-β-hydroxybutyrate stabilizes hippocampal CA3-CA1 circuit during acute insulin resistance

The brain primarily relies on glycolysis for mitochondrial respiration but switches to alternative fuels such as ketone bodies (KBs) when less glucose is available. Neuronal KB uptake, which does not rely on glucose transporter 4 (GLUT4) or insulin, has shown promising clinical applicability in alleviating the neurological and cognitive effects of disorders with hypometabolic components. However, the specific mechanisms by which such interventions affect neuronal functions are poorly understood. In this study, we pharmacologically blocked GLUT4 to investigate the effects of exogenous KB D-β-hydroxybutyrate (D-βHb) on mouse brain metabolism during acute insulin resistance (AIR). We found that both AIR and D-βHb had distinct impacts across neuronal compartments: AIR decreased synaptic activity and long-term potentiation (LTP) and impaired axonal conduction, synchronization, and action potential (AP) properties, while D-βHb rescued neuronal functions associated with axonal conduction, synchronization, and LTP.

In vivo calibration of genetically encoded metabolite biosensors must account for metabolite metabolism during calibration and cellular volume

Isotopic assays of brain glucose utilization rates have been used for more than four decades to establish relationships between energetics, functional activity, and neurotransmitter cycling. Limitations of these methods include the relatively long time (1-60 min) for the determination of labeled metabolite levels and the lack of cellular resolution. Identification and quantification of fuels for neurons and astrocytes that support activation and higher brain functions are a major, unresolved issues. Glycolysis is preferentially up-regulated during activation even though oxygen level and supply are adequate, causing lactate concentrations to quickly rise during alerting, sensory processing, cognitive tasks, and memory consolidation. However, the fate of lactate (rapid release from brain or cell-cell shuttling coupled with local oxidation) is long disputed. Genetically encoded biosensors can determine intracellular metabolite concentrations and report real-time lactate level responses to sensory, behavioral, and biochemical challenges at the cellular level. Kinetics and time courses of cellular lactate concentration changes are informative, but accurate biosensor calibration is required for quantitative comparisons of lactate levels in astrocytes and neurons. An in vivo calibration procedure for the Laconic lactate biosensor involves intracellular lactate depletion by intravenous pyruvate-mediated trans-acceleration of lactate efflux followed by sensor saturation by intravenous infusion of high doses of lactate plus ammonium chloride. In the present paper, the validity of this procedure is questioned because rapid lactate-pyruvate interconversion in blood, preferential neuronal oxidation of both monocarboxylates, on-going glycolytic metabolism, and cellular volumes were not taken into account. Calibration pitfalls for the Laconic lactate biosensor also apply to other metabolite biosensors that are standardized in vivo by infusion of substrates that can be metabolized in peripheral tissues. We discuss how technical shortcomings negate the conclusion that Laconic sensor calibrations support the existence of an in vivo astrocyte-neuron lactate concentration gradient linked to lactate shuttling from astrocytes to neurons to fuel neuronal activity.

Brain energy metabolism: A roadmap for future research

Although we have learned much about how the brain fuels its functions over the last decades, there remains much still to discover in an organ that is so complex. This article lays out major gaps in our knowledge of interrelationships between brain metabolism and brain function, including biochemical, cellular, and subcellular aspects of functional metabolism and its imaging in adult brain, as well as during development, aging, and disease. The focus is on unknowns in metabolism of major brain substrates and associated transporters, the roles of insulin and of lipid droplets, the emerging role of metabolism in microglia, mysteries about the major brain cofactor and signaling molecule NAD+, as well as unsolved problems underlying brain metabolism in pathologies such as traumatic brain injury, epilepsy, and metabolic downregulation during hibernation. It describes our current level of understanding of these facets of brain energy metabolism as well as a roadmap for future research.

Mechanistic stoichiometric relationship between the rates of neurotransmission and neuronal glucose oxidation: Reevaluation of and alternatives to the pseudo-malate-aspartate shuttle model

The ~1:1 stoichiometry between the rates of neuronal glucose oxidation (CMRglc-ox-N) and glutamate (Glu)/γ-aminobutyric acid (GABA)-glutamine (Gln) neurotransmitter (NT) cycling between neurons and astrocytes (VNTcycle) has been firmly established. However, the mechanistic basis for this relationship is not fully understood, and this knowledge is critical for the interpretation of metabolic and brain imaging studies in normal and diseased brain. The pseudo-malate-aspartate shuttle (pseudo-MAS) model established the requirement for glycolytic metabolism in cultured glutamatergic neurons to produce NADH that is shuttled into mitochondria to support conversion of extracellular Gln (i.e., astrocyte-derived Gln in vivo) into vesicular neurotransmitter Glu. The evaluation of this model revealed that it could explain half of the 1:1 stoichiometry and it has limitations. Modifications of the pseudo-MAS model were, therefore, devised to address major knowledge gaps, that is, submitochondrial glutaminase location, identities of mitochondrial carriers for Gln and other model components, alternative mechanisms to transaminate α-ketoglutarate to form Glu and shuttle glutamine-derived ammonia while maintaining mass balance. All modified models had a similar 0.5 to 1.0 predicted mechanistic stoichiometry between VNTcycle and the rate of glucose oxidation. Based on studies of brain β-hydroxybutyrate oxidation, about half of CMRglc-ox-N may be linked to glutamatergic neurotransmission and localized in pre-synaptic structures that use pseudo-MAS type mechanisms for Glu-Gln cycling. In contrast, neuronal compartments that do not participate in transmitter cycling may use the MAS to sustain glucose oxidation. The evaluation of subcellular compartmentation of neuronal glucose metabolism in vivo is a critically important topic for future studies to understand glutamatergic and GABAergic neurotransmission.

A tribute to Leif Hertz: The historical context of his pioneering studies of the roles of astrocytes in brain energy metabolism, neurotransmission, cognitive functions, and pharmacology identifies important, unresolved topics for future studies

Leif Hertz, M.D., D.Sc. (honōris causā) (1930-2018), was one of the original and noteworthy participants in the International Conference on Brain Energy Metabolism (ICBEM) series since its inception in 1993. The biennial ICBEM conferences are organized by neuroscientists interested in energetics and metabolism underlying neural functions; they have had a high impact on conceptual and experimental advances in these fields and on promoting collaborative interactions among neuroscientists. Leif made major contributions to ICBEM discussions and understanding of metabolic and signaling characteristics of astrocytes and their roles in brain function. His studies ranged from uptake of K+ from extracellular fluid and its stimulation of astrocytic respiration, identification, and regulation of enzymes specifically or preferentially expressed in astrocytes in the glutamate-glutamine cycle of excitatory neurotransmission, a requirement for astrocytic glycogenolysis for fueling K+ uptake, involvement of glycogen in memory consolidation in the chick, and pharmacology of astrocytes. This tribute to Leif Hertz highlights his major discoveries, the high impact of his work on astrocyte-neuron interactions, and his unparalleled influence on understanding the cellular basis of brain energy metabolism. His work over six decades has helped integrate the roles of astrocytes into neurotransmission where oxidative and glycogenolytic metabolism during neurotransmitter glutamate turnover are key aspects of astrocytic energetics. Leif recognized that brain astrocytic metabolism is greatly underestimated unless the volume fraction of astrocytes is taken into account. Adjustment for pathway rates expressed per gram tissue for volume fraction indicates that astrocytes have much higher oxidative rates than neurons and astrocytic glycogen concentrations and glycogenolytic rates during sensory stimulation in vivo are similar to those in resting and exercising muscle, respectively. These novel insights are typical of Leif's astute contributions to the energy metabolism field, and his publications have identified unresolved topics that provide the neuroscience community with challenges and opportunities for future research.

D-ꞵ-hydroxybutyrate stabilizes hippocampal CA3-CA1 circuit during acute insulin resistance

The brain primarily relies on glycolysis for mitochondrial respiration but switches to alternative fuels such as ketone bodies (KBs) when less glucose is available. Neuronal KB uptake, which does not rely on glucose transporter 4 (GLUT4) or insulin, has shown promising clinical applicability in alleviating the neurological and cognitive effects of disorders with hypometabolic components. However, the specific mechanisms by which such interventions affect neuronal functions are poorly understood. In this study, we pharmacologically blocked GLUT4 to investigate the effects of exogenous KB D-ꞵ-hydroxybutyrate (D-ꞵHb) on mouse brain metabolism during acute insulin resistance (AIR). We found that both AIR and D-ꞵHb had distinct impacts across neuronal compartments: AIR decreased synaptic activity and long-term potentiation (LTP) and impaired axonal conduction, synchronization, and action potential properties, while D-ꞵHb rescued neuronal functions associated with axonal conduction, synchronization, and LTP.

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.

Diet-induced diabetes is associated with lower hippocampal glycogen and reduced glycogenolysis following local exogenous insulin

Brain fuel (specifically, glucose) supply and metabolism are well-established to be limiting factors for cognitive performance, with the largest body of data being for hippocampally mediated tasks. Consistent with this, disease states such as Alzheimer's disease and insulin-resistant diabetes, that impair cognitive metabolism, impair cognition with this being shown again most prominently for hippocampally mediated processes. In addition to glucose supplied from the blood, brain oxidative metabolism can use local glycogen stores (within astrocytes) as a fuel source via conversion to lactate; both lactate and glycogen have been shown to be important contributors to regulation of cognitive metabolism. Insulin has been shown to be a key regulator of hippocampal cognitive and metabolic processes; in the periphery, insulin facilitates glycogen synthesis and storage, but the impact on brain glycogen is unclear. Furthermore, the impact of diet-induced diabetes on hippocampal glycogen levels and/or metabolism is unknown. Here, we show that in rats with high-fat diet-induced diabetes, hippocampal glycogen is reduced and is less responsive to acute intrahippocampal administration of insulin, which significantly reduces glycogen in the hippocampi of control animals: Our data suggest that impaired fuel availability from glycogen may be a contributing factor to the cognitive impairment seen in disease states that include central insulin resistance.

Glial metabolism versatility regulates mushroom body-driven behavioral output in Drosophila

Providing metabolic support to neurons is now recognized as a major function of glial cells that is conserved from invertebrates to vertebrates. However, research in this field has focused for more than two decades on the relevance of lactate and glial glycolysis for neuronal energy metabolism, while overlooking many other facets of glial metabolism and their impact on neuronal physiology, circuit activity, and behavior. Here, we review recent work that has unveiled new features of glial metabolism, especially in Drosophila, in the modulation of behavioral traits involving the mushroom bodies (MBs). These recent findings reveal that spatially and biochemically distinct modes of glucose-derived neuronal fueling are implemented within the MB in a memory type-specific manner. In addition, cortex glia are endowed with several antioxidant functions, whereas astrocytes can serve as pro-oxidant agents that are beneficial to redox signaling underlying long-term memory. Finally, glial fatty acid oxidation seems to play a dual fail-safe role: first, as a mode of energy production upon glucose shortage, and, second, as a factor underlying the clearance of excessive oxidative load during sleep. Altogether, these integrated studies performed in Drosophila indicate that glial metabolism has a deterministic role on behavior.

Neuronal glycolysis: focus on developmental morphogenesis and localized subcellular functions

Glycolysis is a metabolic pathway that directly generates adenosine triphosphate (ATP), provides metabolic intermediates for anabolism, and supports mitochondrial oxidative phosphorylation. This review addresses recent advances in our understanding of the functions of neuronal glycolysis during the development of neuronal morphogenesis, focusing on the emergent concept that neuronal glycolysis serves local subcellular bioenergetic roles in maintaining neuronal function. The current evidence indicates that glycolysis is subcellularly targeted to specific organelles and molecular machinery to locally supply bioenergetic support for defined subcellular mechanisms underlying neuronal morphogenesis (i.e. axon extension, axon retraction and axonal transport). Thus, the concept of glycolysis as a "housekeeping" mechanism in neurons would benefit revision and future work aim to further define its subcellular functions at varied developmental stages.

The Contribution of Type 2 Diabetes to Parkinson's Disease Aetiology

Type 2 diabetes (T2D) and Parkinson's disease (PD) are chronic disorders that have a significant health impact on a global scale. Epidemiological, preclinical, and clinical research underpins the assumption that insulin resistance and chronic inflammation contribute to the overlapping aetiologies of T2D and PD. This narrative review summarises the recent evidence on the contribution of T2D to the initiation and progression of PD brain pathology. It also briefly discusses the rationale and potential of alternative pharmacological interventions for PD treatment.

Role of Histone Deacetylase Inhibitor in Diabetic Painful Neuropathy

Diabetic painful neuropathy (DPN) is one of the most detrimental complications of diabetes. Alterations in neuroinflammatory mediators play significant roles in the development of DPN. Infiltration of the neutrophils and monocyte/macrophages contributes substantial role in the degenerative process of the distal sciatic nerve by forming neutrophil extracellular traps (NETs) under diabetic condition. Citrullination of histones due to increase in protein arginine deiminase (PAD) enzyme activity under hyperglycemia may promote NET formation, which can further increase the cytokine production by activating macrophages and proliferation of neutrophils. This study reveals that the increase in histone deacetylases (HDAC) is crucial in DPN and inhibition of HDAC using HDAC inhibitor (HDACi) FK228 would suppress NETosis and alleviate diabetic nerve degeneration and pain. FK228, also known as romidepsin, is FDA approved for the treatment of cutaneous T-cell lymphoma yet the molecular mechanisms of this drug are not completely understood in DPN. In this study, type 2 diabetic (T2D) mice with pain were treated with HDACi, FK228 1 mg/kg; I.P. 2 × /week for 3 weeks. The results demonstrate that FK228 treatment can alleviate thermal hyperalgesia and mechanical allodynia significantly along with changes in the expression of HDACs in the dorsal root ganglia (DRG) and spinal cord dorsal horn neurons of diabetic animals. The results also indicate that FK228 treatment can alter the expression of neutrophil elastase (NE), extracellular or cell free DNA (cfDNA), citrullinated histone-3 (CitH3), PADI4, growth-associated protein (GAP)-43, and glucose transporter (GLUT)-4. Overall, this study suggests that FK228 could amend the expression of nerve regeneration markers and inflammatory mediators in diabetic animals and may offer an alternative treatment approach for DPN.

Mitochondrial pyruvate transport regulates presynaptic metabolism and neurotransmission

Glucose has long been considered the primary fuel source for the brain. However, glucose levels fluctuate in the brain during sleep, intense circuit activity, or dietary restrictions, posing significant metabolic stress. Here, we demonstrate that the mammalian brain utilizes pyruvate as a fuel source, and pyruvate can support neuronal viability in the absence of glucose. Nerve terminals are sites of metabolic vulnerability within a neuron and we show that mitochondrial pyruvate uptake is a critical step in oxidative ATP production in hippocampal terminals. We find that the mitochondrial pyruvate carrier is post-translationally modified by lysine acetylation which in turn modulates mitochondrial pyruvate uptake. Importantly, our data reveal that the mitochondrial pyruvate carrier regulates distinct steps in synaptic transmission, namely, the spatiotemporal pattern of synaptic vesicle release and the efficiency of vesicle retrieval, functions that have profound implications for synaptic plasticity. In summary, we identify pyruvate as a potent neuronal fuel and mitochondrial pyruvate uptake as a critical node for the metabolic control of synaptic transmission in hippocampal terminals.

Parkinson's disease gene, Synaptojanin1, dysregulates the surface maintenance of the dopamine transporter

Missense mutations of PARK20/SYNJ1 (synaptojanin1/Synj1) have been linked to complex forms of familial parkinsonism, however, the molecular and cellular changes associated with dopaminergic dysfunction remains unknown. We now report fast depletion of evoked dopamine (DA) and altered maintenance of the axonal dopamine transporter (DAT) in the Synj1+/- neurons. While Synj1 has been traditionally known to facilitate the endocytosis of synaptic vesicles, we demonstrated that axons of cultured Synj1+/- neurons exhibit an increase of total DAT but a reduction of the surface DAT, which could be exacerbated by neuronal activity. We revealed that the loss of surface DAT is specifically associated with the impaired 5'-phosphatase activity of Synj1 and the hyperactive downstream PI(4,5)P2-PKCβ pathway. Thus, our findings provided important mechanistic insight for Synj1-regulated DAT trafficking integral to dysfunctional DA signaling in early parkinsonism.

Insulin signalling regulates Pink1 mRNA localization via modulation of AMPK activity to support PINK1 function in neurons

Mitochondrial quality control failure is frequently observed in neurodegenerative diseases. The detection of damaged mitochondria by stabilization of PTEN-induced kinase 1 (PINK1) requires transport of Pink1 messenger RNA (mRNA) by tethering it to the mitochondrial surface. Here, we report that inhibition of AMP-activated protein kinase (AMPK) by activation of the insulin signalling cascade prevents Pink1 mRNA binding to mitochondria. Mechanistically, AMPK phosphorylates the RNA anchor complex subunit SYNJ2BP within its PDZ domain, a phosphorylation site that is necessary for its interaction with the RNA-binding protein SYNJ2. Notably, loss of mitochondrial Pink1 mRNA association upon insulin addition is required for PINK1 protein activation and its function as a ubiquitin kinase in the mitophagy pathway, thus placing PINK1 function under metabolic control. Induction of insulin resistance in vitro by the key genetic Alzheimer risk factor apolipoprotein E4 retains Pink1 mRNA at the mitochondria and prevents proper PINK1 activity, especially in neurites. Our results thus identify a metabolic switch controlling Pink1 mRNA localization and PINK1 activity via insulin and AMPK signalling in neurons and propose a mechanistic connection between insulin resistance and mitochondrial dysfunction.

Essential Role of Astrocytes in Learning and Memory

One of the most biologically relevant functions of astrocytes within the CNS is the regulation of synaptic transmission, i.e., the physiological basis for information transmission between neurons. Changes in the strength of synaptic connections are indeed thought to be the cellular basis of learning and memory. Importantly, astrocytes have been demonstrated to tightly regulate these processes via the release of several gliotransmitters linked to astrocytic calcium activity as well as astrocyte-neuron metabolic coupling. Therefore, astrocytes seem to be integrators of and actors upon learning- and memory-relevant information. In this review, we focus on the role of astrocytes in learning and memory processes. We delineate the recognized inputs and outputs of astrocytes and explore the influence of manipulating astrocytes on behaviour across diverse learning paradigms. We conclude that astrocytes influence learning and memory in various manners. Appropriate astrocytic Ca2+ dynamics are being increasingly identified as central contributors to memory formation and retrieval. In addition, astrocytes regulate brain rhythms essential for cognition, and astrocyte-neuron metabolic cooperation is required for memory consolidation.

Local and dynamic regulation of neuronal glycolysis in vivo

Energy metabolism supports neuronal function. While it is well established that changes in energy metabolism underpin brain plasticity and function, less is known about how individual neurons modulate their metabolic states to meet varying energy demands. This is because most approaches used to examine metabolism in living organisms lack the resolution to visualize energy metabolism within individual circuits, cells, or subcellular regions. Here, we adapted a biosensor for glycolysis, HYlight, for use in Caenorhabditis elegans to image dynamic changes in glycolysis within individual neurons and in vivo. We determined that neurons cell-autonomously perform glycolysis and modulate glycolytic states upon energy stress. By examining glycolysis in specific neurons, we documented a neuronal energy landscape comprising three general observations: 1) glycolytic states in neurons are diverse across individual cell types; 2) for a given condition, glycolytic states within individual neurons are reproducible across animals; and 3) for varying conditions of energy stress, glycolytic states are plastic and adapt to energy demands. Through genetic analyses, we uncovered roles for regulatory enzymes and mitochondrial localization in the cellular and subcellular dynamic regulation of glycolysis. Our study demonstrates the use of a single-cell glycolytic biosensor to examine how energy metabolism is distributed across cells and coupled to dynamic states of neuronal function and uncovers unique relationships between neuronal identities and metabolic landscapes in vivo.

Trem2 expression in microglia is required to maintain normal neuronal bioenergetics during development

Triggering receptor expressed on myeloid cells 2 (Trem2) is a myeloid cell-specific gene expressed in brain microglia, with variants that are associated with neurodegenerative diseases, including Alzheimer's disease. Trem2 is essential for microglia-mediated synaptic refinement, but whether Trem2 contributes to shaping neuronal development remains unclear. Here, we demonstrate that Trem2 plays a key role in controlling the bioenergetic profile of pyramidal neurons during development. In the absence of Trem2, developing neurons in the hippocampal cornus ammonis (CA)1 but not in CA3 subfield displayed compromised energetic metabolism, accompanied by reduced mitochondrial mass and abnormal organelle ultrastructure. This was paralleled by the transcriptional rearrangement of hippocampal pyramidal neurons at birth, with a pervasive alteration of metabolic, oxidative phosphorylation, and mitochondrial gene signatures, accompanied by a delay in the maturation of CA1 neurons. Our results unveil a role of Trem2 in controlling neuronal development by regulating the metabolic fitness of neurons in a region-specific manner.