Metabolic coupling between glia and neurons

Traumatic Brain Injury: A Comprehensive Review of Biomechanics and Molecular Pathophysiology

Traumatic brain injury (TBI) is a critical public health concern with profound consequences for affected individuals. This comprehensive literature review delves into TBI intricacies, encompassing primary injury biomechanics and the molecular pathophysiology of the secondary injury cascade. Primary TBI involves a complex interplay of forces, including impact loading, blast overpressure, and impulsive loading, leading to diverse injury patterns. These forces can be categorized into inertial (e.g., rotational acceleration causing focal and diffuse injuries) and contact forces (primarily causing focal injuries like skull fractures). Understanding their interactions is crucial for effective injury management. The secondary injury cascade in TBI comprises multifaceted molecular and cellular responses, including altered ion concentrations, dysfunctional neurotransmitter networks, oxidative stress, and cellular energy disturbances. These disruptions impair synaptic function, neurotransmission, and neuroplasticity, resulting in cognitive and behavioral deficits. Moreover, neuroinflammatory responses play a pivotal role in exacerbating damage. As we endeavor to bridge the knowledge gap between biomechanics and molecular pathophysiology, further research is imperative to unravel the nuanced interplay between mechanical forces and their consequences at the molecular and cellular levels, ultimately guiding the development of targeted therapeutic strategies to mitigate the debilitating effects of TBI. In this study, we aim to provide a concise review of the bridge between biomechanical processes causing primary injury and the ensuing molecular pathophysiology of secondary injury, while detailing the subsequent clinical course for this patient population. This knowledge is crucial for advancing our understanding of TBI and developing effective interventions to improve outcomes for those affected.

Butyrate reduction and HDAC4 increase underlie maternal high fructose-induced metabolic dysfunction in hippocampal astrocytes in female rats

Maternal nutrient intake influences the health of the offspring via microenvironmental systems in digestion and absorption. Maternal high fructose diet (HFD) impairs hippocampus-dependent memory in adult female rat offspring. However, the underlying mechanisms remain largely unclear. Maternal HFD causes microbiota dysbiosis. In this study, we find that the plasma level of butyrate, a major metabolite of microbiota, is significantly decreased in the adult female maternal HFD offspring. In these rats, GPR43, a butyrate receptor was downregulated in the hippocampus. Moreover, the expressions of mitochondrial transcription factor A (TFAM), and peroxisome proliferator-activated receptor gamma coactivator 1α (PGC-1α) were downregulated in the hippocampus. The decreases of these functional proteins were reversed by fructooligosaccharides (FOS, a probiotic) treatment in adulthood. Astrocytes are critical for energy metabolism in the brain. Primary astrocyte culture from female maternal HFD offspring indicated that GPR43 and the mitochondrial biogenesis were significantly suppressed, which was reversed by supplemental butyrate incubation. The oxygen consumption rate (OCR) was reduced in the HFD group and rescued by butyrate. Intriguingly, the nuclear histone deacetylase 4 (HDAC4) was enhanced in the HFD group, suggesting an inhibitory role of butyrate on histone deacetylase activity. Inhibition of HDAC4 effectively restored the OCR, bioenergetics, and biogenesis of mitochondria. Together, these results suggested that the impaired butyrate signaling by maternal HFD could underlie the reduced mitochondrial functions in the hippocampus via HDAC4-mediated epigenetic changes.

Astrocyte-derived lactate in stress disorders

Stress disorders are psychiatric disorders arising following stressful or traumatic events. They could deleteriously affect an individual's health because they often co-occur with mental illnesses. Considerable attention has been focused on neurons when considering the neurobiology of stress disorders. However, like other mental health conditions, recent studies have highlighted the importance of astrocytes in the pathophysiology of stress-related disorders. In addition to their structural and homeostatic support role, astrocytes actively serve several functions in regulating synaptic transmission and plasticity, protecting neurons from toxic compounds, and providing metabolic support for neurons. The astrocyte-neuron lactate shuttle model sets forth the importance of astrocytes in providing lactate for the metabolic supply of neurons under intense activity. Lactate also plays a role as a signaling molecule and has been recently studied regarding its antidepressant activity. This review discusses the involvement of astrocytes and brain energy metabolism in stress and further reflects on the importance of lactate as an energy supply in the brain and its emerging antidepressant role in stress-related disorders.

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.

Short-term consumption of highly processed diets varying in macronutrient content impair the sense of smell and brain metabolism in mice

OBJECTIVE

Food processing greatly contributed to increased food safety, diversity, and accessibility. However, the prevalence of highly palatable and highly processed food in our modern diet has exacerbated obesity rates and contributed to a global health crisis. While accumulating evidence suggests that chronic consumption of such foods is detrimental to sensory and neural physiology, it is unclear whether its short-term intake has adverse effects. Here, we assessed how short-term consumption (<2 months) of three diets varying in composition and macronutrient content influence olfaction and brain metabolism in mice.

METHODS

The diets tested included a grain-based standard chow diet (CHOW; 54% carbohydrate, 32% protein, 14% fat; #8604 Teklad Rodent diet , Envigo Inc.), a highly processed control diet (hpCTR; 70% carbohydrate, 20% protein, 10% fat; #D12450B, Research Diets Inc.), and a highly processed high-fat diet (hpHFD; 20% carbohydrate, 20% protein, 60% fat; #D12492, Research Diets Inc.). We performed behavioral and metabolic phenotyping, electro-olfactogram (EOG) recordings, brain glucose metabolism imaging, and mitochondrial respirometry in different brain regions. We also performed RNA-sequencing (RNA-seq) in the nose and across several brain regions, and conducted differential expression analysis, gene ontology, and network analysis.

RESULTS

We show that short-term consumption of the two highly processed diets, but not the grain-based diet, regardless of macronutrient content, adversely affects odor-guided behaviors, physiological responses to odorants, transcriptional profiles in the olfactory mucosa and brain regions, and brain glucose metabolism and mitochondrial respiration.

CONCLUSIONS

Even short periods of highly processed food consumption are sufficient to cause early olfactory and brain abnormalities, which has the potential to alter food choices and influence the risk of developing metabolic disease.

Carotid body contribution to the physio-pathological consequences of intermittent hypoxia: role of nitro-oxidative stress and inflammation

Obstructive sleep apnoea (OSA), characterized by chronic intermittent hypoxia (CIH), is considered to be an independent risk for hypertension. The pathological cardiorespiratory consequences of OSA have been attributed to systemic oxidative stress, inflammation and sympathetic overflow induced by CIH, but an emerging body of evidence indicates that a nitro-oxidative and pro-inflammatory milieu within the carotid body (CB) is involved in the potentiation of CB chemosensory responses to hypoxia, which contribute to enhance the sympathetic activity. Accordingly, autonomic and cardiovascular alterations induced by CIH are critically dependent on an abnormally heightened CB chemosensory input to the nucleus of tractus solitarius (NTS), where second-order neurons project onto the rostral ventrolateral medulla (RVLM), activating pre-sympathetic neurons that control pre-ganglionic sympathetic neurons. CIH produces oxidative stress and neuroinflammation in the NTS and RVLM, which may contribute to the long-term irreversibility of the CIH-induced alterations. This brief review is mainly focused on the contribution of nitro-oxidative stress and pro-inflammatory molecules on the hyperactivation of the hypoxic chemoreflex pathway including the CB and the brainstem centres, and whether the persistence of autonomic and cardiorespiratory alterations may depend on the glial-related neuroinflammation induced by the enhanced CB chemosensory afferent input.

Astrocytes and microglia in the coordination of CNS development and homeostasis

Glia have emerged as important architects of central nervous system (CNS) development and maintenance. While traditionally glial contributions to CNS development and maintenance have been studied independently, there is growing evidence that either suggests or documents that glia may act in coordinated manners to effect developmental patterning and homeostatic functions in the CNS. In this review, we focus on astrocytes, the most abundant glia in the CNS, and microglia, the earliest glia to colonize the CNS highlighting research that documents either suggestive or established coordinated actions by these glial cells in various CNS processes including cell and/or debris clearance, neuronal survival and morphogenesis, synaptic maturation, and circuit function, angio-/vasculogenesis, myelination, and neurotransmission. Some molecular mechanisms underlying these processes that have been identified are also described. Throughout, we categorize the available evidence as either suggestive or established interactions between microglia and astrocytes in the regulation of the respective process and raise possible avenues for further research. We conclude indicating that a better understanding of coordinated astrocyte-microglial interactions in the developing and mature brain holds promise for developing effective therapies for brain pathologies where these processes are perturbed.

Astrocyte metabolism and signaling pathways in the CNS

Astrocytes comprise half of the cells in the central nervous system and play a critical role in maintaining metabolic homeostasis. Metabolic dysfunction in astrocytes has been indicated as the primary cause of neurological diseases, such as depression, Alzheimer's disease, and epilepsy. Although the metabolic functionalities of astrocytes are well known, their relationship to neurological disorders is poorly understood. The ways in which astrocytes regulate the metabolism of glucose, amino acids, and lipids have all been implicated in neurological diseases. Metabolism in astrocytes has also exhibited a significant influence on neuron functionality and the brain's neuro-network. In this review, we focused on metabolic processes present in astrocytes, most notably the glucose metabolic pathway, the fatty acid metabolic pathway, and the amino-acid metabolic pathway. For glucose metabolism, we focused on the glycolysis pathway, pentose-phosphate pathway, and oxidative phosphorylation pathway. In fatty acid metabolism, we followed fatty acid oxidation, ketone body metabolism, and sphingolipid metabolism. For amino acid metabolism, we summarized neurotransmitter metabolism and the serine and kynurenine metabolic pathways. This review will provide an overview of functional changes in astrocyte metabolism and provide an overall perspective of current treatment and therapy for neurological disorders.

The crosstalking of lactate-Histone lactylation and tumor

Lactate was once considered to be a by-product of energy metabolism, but its unique biological value was only gradually explored with the advent of the Warburg effect. As an end product of glycolysis, lactate can act as a substrate for energy metabolism, a signal transduction molecule, a regulator of the tumor microenvironment and immune cells, and a regulator of the deubiquitination of specific enzymes, and is involved in various biological aspects of tumor regulation, including energy shuttling, growth and invasion, angiogenesis and immune escape. Furthermore, we describe a novel lactate-dependent epigenetic modification, namely histone lactylation modification, and review the progress of its study in tumors, mainly involving the reprogramming of tumor phenotypes, regulation of related gene expression, mediation of the glycolytic process in tumor stem cells (CSCs) and influence on the tumor immune microenvironment. The study of epigenetic regulation of tumor genes by histone modification is still in its infancy, and we expect that by summarizing the effects of lactate and histone modification on tumor and related gene regulation, we will clarify the scientific significance of future histone modification studies and the problems to be solved, and open up new fields for targeted tumor therapy.

Amyloid-β Pathology-Specific Cytokine Secretion Suppresses Neuronal Mitochondrial Metabolism

Introduction

Neuroinflammation and metabolic dysfunction are early alterations in Alzheimer's disease (AD) brain that are thought to contribute to disease onset and progression. Glial activation due to protein deposition results in cytokine secretion and shifts in brain metabolism, which have been observed in AD patients. However, the mechanism by which this immunometabolic feedback loop can injure neurons and cause neurodegeneration remains unclear.

Methods

We used Luminex XMAP technology to quantify hippocampal cytokine concentrations in the 5xFAD mouse model of AD at milestone timepoints in disease development. We used partial least squares regression to build cytokine signatures predictive of disease progression, as compared to healthy aging in wild-type littermates. We applied the disease-defining cytokine signature to wild-type primary neuron cultures and measured downstream changes in gene expression using the NanoString nCounter system and mitochondrial function using the Seahorse Extracellular Flux live-cell analyzer.

Results

We identified a pattern of up-regulated IFNγ, IP-10/CXCL10, and IL-9 as predictive of advanced disease. When healthy neurons were exposed to these cytokines in proportions found in diseased brain, gene expression of mitochondrial electron transport chain complexes, including ATP synthase, was suppressed. In live cells, basal and maximal mitochondrial respiration were impaired following cytokine stimulation.

Conclusions

We identify a pattern of cytokine secretion predictive of progressing amyloid-β pathology in the 5xFAD mouse model of AD that reduces expression of mitochondrial electron transport complexes and impairs mitochondrial respiration in healthy neurons. We establish a mechanistic link between disease-specific immune cues and impaired neuronal metabolism, potentially causing neuronal vulnerability and susceptibility to degeneration in AD.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-023-00782-y.

Astrocytes as a target for therapeutic strategies in epilepsy: current insights

Astrocytes are specialized non-neuronal glial cells of the central nervous system, contributing to neuronal excitability and synaptic transmission (gliotransmission). Astrocytes play a key roles in epileptogenesis and seizure generation. Epilepsy, as a chronic disorder characterized by neuronal hyperexcitation and hypersynchronization, is accompanied by substantial disturbances of glial cells and impairment of astrocytic functions and neuronal signaling. Anti-seizure drugs that provide symptomatic control of seizures primarily target neural activity. In epileptic patients with inadequate control of seizures with available anti-seizure drugs, novel therapeutic candidates are needed. These candidates should treat epilepsy with anti-epileptogenic and disease-modifying effects. Evidence from human and animal studies shows that astrocytes have value for developing new anti-seizure and anti-epileptogenic drugs. In this review, we present the key functions of astrocytes contributing to neuronal hyperexcitability and synaptic activity following an etiology-based approach. We analyze the role of astrocytes in both development (epileptogenesis) and generation of seizures (ictogenesis). Several promising new strategies that attempted to modify astroglial functions for treating epilepsy are being developed: (1) selective targeting of glia-related molecular mechanisms of glutamate transport; (2) modulation of tonic GABA release from astrocytes; (3) gliotransmission; (4) targeting the astrocytic Kir4.1-BDNF system; (5) astrocytic Na+/K+/ATPase activity; (6) targeting DNA hypo- or hypermethylation of candidate genes in astrocytes; (7) targeting astrocytic gap junction regulators; (8) targeting astrocytic adenosine kinase (the major adenosine-metabolizing enzyme); and (9) targeting microglia-astrocyte communication and inflammatory pathways. Novel disease-modifying therapeutic strategies have now been developed, such as astroglia-targeted gene therapy with a broad spectrum of genetic constructs to target astroglial cells.

Role of astrocytes in sleep deprivation: accomplices, resisters, or bystanders?

Sleep plays an essential role in all studied animals with a nervous system. However, sleep deprivation leads to various pathological changes and neurobehavioral problems. Astrocytes are the most abundant cells in the brain and are involved in various important functions, including neurotransmitter and ion homeostasis, synaptic and neuronal modulation, and blood-brain barrier maintenance; furthermore, they are associated with numerous neurodegenerative diseases, pain, and mood disorders. Moreover, astrocytes are increasingly being recognized as vital contributors to the regulation of sleep-wake cycles, both locally and in specific neural circuits. In this review, we begin by describing the role of astrocytes in regulating sleep and circadian rhythms, focusing on: (i) neuronal activity; (ii) metabolism; (iii) the glymphatic system; (iv) neuroinflammation; and (v) astrocyte-microglia cross-talk. Moreover, we review the role of astrocytes in sleep deprivation comorbidities and sleep deprivation-related brain disorders. Finally, we discuss potential interventions targeting astrocytes to prevent or treat sleep deprivation-related brain disorders. Pursuing these questions would pave the way for a deeper understanding of the cellular and neural mechanisms underlying sleep deprivation-comorbid brain disorders.

Using stable isotope tracing to unravel the metabolic components of neurodegeneration: Focus on neuron-glia metabolic interactions

Disrupted brain metabolism is a critical component of several neurodegenerative diseases. Energy metabolism of both neurons and astrocytes is closely connected to neurotransmitter recycling via the glutamate/GABA-glutamine cycle. Neurons and astrocytes hereby work in close metabolic collaboration which is essential to sustain neurotransmission. Elucidating the mechanistic involvement of altered brain metabolism in disease progression has been aided by the advance of techniques to monitor cellular metabolism, in particular by mapping metabolism of substrates containing stable isotopes, a technique known as isotope tracing. Here we review key aspects of isotope tracing including advantages, drawbacks and applications to different cerebral preparations. In addition, we narrate how isotope tracing has facilitated the discovery of central metabolic features in neurodegeneration with a focus on the metabolic cooperation between neurons and astrocytes.

Atrophic astrocytes in aged marmosets present tau hyperphosphorylation, RNA oxidation, and DNA fragmentation

Astrocytes perform multiple essential functions in the brain showing morphological changes. Hypertrophic astrocytes are commonly observed in cognitively healthy aged animals, implying a functional defense mechanism without losing neuronal support. In neurodegenerative diseases, astrocytes show morphological alterations, such as decreased process length and reduced number of branch points, known as astroglial atrophy, with detrimental effects on neuronal cells. The common marmoset (Callithrix jacchus) is a non-human primate that, with age, develops several features that resemble neurodegeneration. In this study, we characterize the morphological alterations in astrocytes of adolescent (mean 1.75 y), adult (mean 5.33 y), old (mean 11.25 y), and aged (mean 16.83 y) male marmosets. We observed a significantly reduced arborization in astrocytes of aged marmosets compared to younger animals in the hippocampus and entorhinal cortex. These astrocytes also show oxidative damage to RNA and increased nuclear plaques in the cortex and tau hyperphosphorylation (AT100). Astrocytes lacking S100A10 protein show a more severe atrophy and DNA fragmentation. Our results demonstrate the presence of atrophic astrocytes in the brains of aged marmosets.

Loss of neuron network coherence induced by virus-infected astrocytes: a model study

Coherent activations of brain neuron networks underlie many physiological functions associated with various behavioral states. These synchronous fluctuations in the electrical activity of the brain are also referred to as brain rhythms. At the cellular level, rhythmicity can be induced by various mechanisms of intrinsic oscillations in neurons or the network circulation of excitation between synaptically coupled neurons. One specific mechanism concerns the activity of brain astrocytes that accompany neurons and can coherently modulate synaptic contacts of neighboring neurons, synchronizing their activity. Recent studies have shown that coronavirus infection (Covid-19), which enters the central nervous system and infects astrocytes, can cause various metabolic disorders. Specifically, Covid-19 can depress the synthesis of astrocytic glutamate and gamma-aminobutyric acid. It is also known that in the post-Covid state, patients may suffer from symptoms of anxiety and impaired cognitive functions. We propose a mathematical model of a spiking neuron network accompanied by astrocytes capable of generating quasi-synchronous rhythmic bursting discharges. The model predicts that if the release of glutamate is depressed, normal burst rhythmicity will suffer dramatically. Interestingly, in some cases, the failure of network coherence may be intermittent, with intervals of normal rhythmicity, or the synchronization can disappear.

Alzheimer's disease-specific cytokine secretion suppresses neuronal mitochondrial metabolism

Introduction

Neuroinflammation and metabolic dysfunction are early alterations in Alzheimer's disease brain that are thought to contribute to disease onset and progression. Glial activation due to protein deposition results in cytokine secretion and shifts in brain metabolism, which have been observed in Alzheimer's disease patients. However, the mechanism by which this immunometabolic feedback loop can injure neurons and cause neurodegeneration remains unclear.

Methods

We used Luminex XMAP technology to quantify hippocampal cytokine concentrations in the 5xFAD mouse model of Alzheimer's disease at milestone timepoints in disease development. We used partial least squares regression to build cytokine signatures predictive of disease progression, as compared to healthy aging in wild-type littermates. We applied the disease-defining cytokine signature to wild-type primary neuron cultures and measured downstream changes in gene expression using the NanoString nCounter system and mitochondrial function using the Seahorse Extracellular Flux live-cell analyzer.

Results

We identified a pattern of up-regulated IFNγ, IP-10, and IL-9 as predictive of advanced disease. When healthy neurons were exposed to these cytokines in proportions found in diseased brain, gene expression of mitochondrial electron transport chain complexes, including ATP synthase, was suppressed. In live cells, basal and maximal mitochondrial respiration were impaired following cytokine stimulation.

Conclusions

An Alzheimer's disease-specific pattern of cytokine secretion reduces expression of mitochondrial electron transport complexes and impairs mitochondrial respiration in healthy neurons. We establish a mechanistic link between disease-specific immune cues and impaired neuronal metabolism, potentially causing neuronal vulnerability and susceptibility to degeneration in Alzheimer's disease.

The emerging science of Glioception: Contribution of glia in sensing, transduction, circuit integration of interoception

Interoception is the process by which the nervous system regulates internal functions to achieve homeostasis. The role of neurons in interoception has received considerable recent attention, but glial cells also contribute. Glial cells can sense and transduce signals including osmotic, chemical, and mechanical status of extracellular milieu. Their ability to dynamically communicate "listening" and "talking" to neurons is necessary to monitor and regulate homeostasis and information integration in the nervous system. This review introduces the concept of "Glioception" and focuses on the process by which glial cells sense, interpret and integrate information about the inner state of the organism. Glial cells are ideally positioned to act as sensors and integrators of diverse interoceptive signals and can trigger regulatory responses via modulation of the activity of neuronal networks, both in physiological and pathological conditions. We believe that understanding and manipulating glioceptive processes and underlying molecular mechanisms provide a key path to develop new therapies for the prevention and alleviation of devastating interoceptive dysfunctions, among which pain is emphasized here with more focused details.

EPO has multiple positive effects on astrocytes in an experimental model of ischemia

Erythropoietin (EPO) has neuroprotective effects in central nervous system injury models. In clinical trials EPO has shown beneficial effects in traumatic brain injury (TBI) as well as in ischemic stroke. We have previously shown that EPO has short-term effects on astrocyte glutamatergic signaling in vitro and that administration of EPO after experimental TBI decreases early cytotoxic brain edema and preserves structural and functional properties of the blood-brain barrier. These effects have been attributed to preserved or restored astrocyte function. Here we explored the effects of EPO on astrocytes undergoing oxygen-glucose-deprivation, an in vitro model of ischemia. Measurements of glutamate uptake, intracellular pH, intrinsic NADH fluorescence, Na,K-ATPase activity, and lactate release were performed. We found that EPO within minutes caused a Na,K-ATPase-dependent increase in glutamate uptake, restored intracellular acidification caused by glutamate and increased lactate release. The effects on intracellular pH were dependent on the sodium/hydrogen exchanger NHE. In neuron-astrocyte co-cultures, EPO increased NADH production both in astrocytes and neurons, however the increase was greater in astrocytes. We suggest that EPO preserves astrocyte function under ischemic conditions and thus may contribute to neuroprotection in ischemic stroke and brain ischemia secondary to TBI.

APOE4 drives transcriptional heterogeneity and maladaptive immunometabolic responses of astrocytes

Apolipoprotein E4 (APOE4) is the strongest risk allele associated with the development of late onset Alzheimer's disease (AD). Across the CNS, astrocytes are the predominant expressor of APOE while also being critical mediators of neuroinflammation and cerebral metabolism. APOE4 has been consistently linked with dysfunctional inflammation and metabolic processes, yet insights into the molecular constituents driving these responses remain unclear. Utilizing complementary approaches across humanized APOE mice and isogenic human iPSC astrocytes, we demonstrate that ApoE4 alters the astrocyte immunometabolic response to pro-inflammatory stimuli. Our findings show that ApoE4-expressing astrocytes acquire distinct transcriptional repertoires at single-cell and spatially-resolved domains, which are driven in-part by preferential utilization of the cRel transcription factor. Further, inhibiting cRel translocation in ApoE4 astrocytes abrogates inflammatory-induced glycolytic shifts and in tandem mitigates production of multiple pro-inflammatory cytokines. Altogether, our findings elucidate novel cellular underpinnings by which ApoE4 drives maladaptive immunometabolic responses of astrocytes.

Interplay between biochemical processes and network properties generates neuronal up and down states at the tripartite synapse

Neuronal up and down states have long been known to exist both in vitro and in vivo. A variety of functions and mechanisms have been proposed for their generation, but there has not been a clear connection between the functions and mechanisms. We explore the potential contribution of cellular-level biochemistry to the network-level mechanisms thought to underlie the generation of up and down states. We develop a neurochemical model of a single tripartite synapse, assumed to be within a network of similar tripartite synapses, to investigate possible function-mechanism links for the appearance of up and down states. We characterize the behavior of our model in different regions of parameter space and show that resource limitation at the tripartite synapse affects its ability to faithfully transmit input signals, leading to extinction-down states. Recovery of resources allows for "reignition" into up states. The tripartite synapse exhibits distinctive "regimes" of operation depending on whether ATP, neurotransmitter (glutamate), both, or neither, is limiting. Our model qualitatively matches the behavior of six disparate experimental systems, including both in vitro and in vivo models, without changing any model parameters except those related to the experimental conditions. We also explore the effects of varying different critical parameters within the model. Here we show that availability of energy, represented by ATP, and glutamate for neurotransmission at the cellular level are intimately related, and are capable of promoting state transitions at the network level as ignition and extinction phenomena. Our model is complementary to existing models of neuronal up and down states in that it focuses on cellular-level dynamics while still retaining essential network-level processes. Our model predicts the existence of a "final common pathway" of behavior at the tripartite synapse arising from scarcity of resources and may explain use dependence in the phenomenon of "local sleep." Ultimately, sleeplike behavior may be a fundamental property of networks of tripartite synapses.

Metabolic Heterogeneity of Cerebral Cortical and Cerebellar Astrocytes

Astrocytes play critical roles in regulating neuronal synaptogenesis, maintaining blood-brain barrier integrity, and recycling neurotransmitters. Increasing numbers of studies have suggested astrocyte heterogeneity in morphology, gene profile, and function. However, metabolic phenotype of astrocytes in different brain regions have not been explored. In this paper, we investigated the metabolic signature of cortical and cerebellar astrocytes using primary astrocyte cultures. We observed that cortical astrocytes were larger than cerebellar astrocytes, whereas cerebellar astrocytes had more and longer processes than cortical astrocytes. Using a Seahorse extracellular flux analyzer, we demonstrated that cortical astrocytes had higher mitochondrial respiration and glycolysis than cerebellar astrocytes. Cerebellar astrocytes have lower spare capacity of mitochondrial respiration and glycolysis as compared with cortical astrocytes. Consistently, cortical astrocytes have higher mitochondrial oxidation and glycolysis-derived ATP content than cerebellar astrocytes. In addition, cerebellar astrocytes have a fuel preference for glutamine and fatty acid, whereas cortical astrocytes were more dependent on glucose to meet energy demands. Our study indicated that cortical and cerebellar astrocytes display distinct metabolic phenotypes. Future studies on astrocyte metabolic heterogeneity and brain function in aging and neurodegeneration may lead to better understanding of the role of astrocyte in brain aging and neurodegenerative disorders.

Digoxin Induces Human Astrocyte Reaction In Vitro

Astrocyte reaction is a complex cellular process involving astrocytes in response to various types of CNS injury and a marker of neurotoxicity. It has been abundantly studied in rodents but relatively poorly in human cells due to limited access to the brain. Astrocytes play important roles in cerebral energy metabolism and are also key players in neuroinflammation. Astroglial metabolic and inflammatory changes have been reported with age, leading to the hypothesis that mitochondrial metabolism and inflammatory responses are interconnected. However, the relationship between energy metabolism and astrocyte reactivity in the context of neurotoxicity is not known. We hypothesized that changes in energy metabolism of astrocytes will be coupled to their activation by xenobiotics. Astrocyte reaction and associated energy metabolic changes were assessed by immunostaining, gene expression, proteomics, metabolomics, and extracellular flux analyses after 24 h of exposure of human ReN-derived astrocytes to digoxin (1-10 µM) or TNFα (30 ng/ml) used as a positive control. Strong astrocytic reaction was observed, accompanied by increased glycolysis at low concentrations of digoxin (0.1 and 0.5 µM) and after TNFα exposure, suggesting that increased glycolysis may be a common feature of reactive astrocytes, independent of the triggering molecule. In conclusion, whether astrocyte activation is triggered by cytokines or a xenobiotic, it is strongly tied to energy metabolism in human ReN-derived astrocytes. Increased glycolysis might be considered as an endpoint to detect astrocyte activation by potentially neurotoxic compounds in vitro. Finally, ReN-derived astrocytes may help to decipher mechanisms of neurotoxicity in ascertaining the ability of chemicals to directly target astrocytes.

Comparative analysis of astrocytes in the prefrontal cortex of primates: Insights into the evolution of human brain energetics

Astrocytes are the main homeostatic cell of the brain involved in many processes related to cognition, immune response, and energy expenditure. It has been suggested that the distribution of astrocytes is associated with brain size, and that they are specialized in humans. To evaluate these, we quantified astrocyte density, soma volume, and total glia density in layer I and white matter in Brodmann's area 9 of humans, chimpanzees, baboons, and macaques. We found that layer I astrocyte density, soma volume, and ratio of astrocytes to total glia cells were highest in humans and increased with brain size. Overall glia density in layer I and white matter were relatively invariant across brain sizes, potentially due to their important metabolic functions on a per volume basis. We also quantified two transporters involved in metabolism through the astrocyte-neuron lactate shuttle, excitatory amino acid transporter 2 (EAAT2) and glucose transporter 1 (GLUT1). We expected these transporters would be increased in human brains due to their high rate of metabolic consumption and associated gene activity. While humans have higher EAAT2 cell density, GLUT1 vessel volume, and GLUT1 area fraction compared to baboons and chimpanzees, they did not differ from macaques. Therefore, EAAT2 and GLUT1 are not related to increased energetic demands of the human brain. Taken together, these data provide evidence that astrocytes play a unique role in both brain expansion and evolution among primates, with an emphasis on layer I astrocytes having a potentially significant role in human-specific metabolic processing and cognition.

A Focus on Astrocyte Contribution to Parkinson's Disease Etiology

Parkinson's disease (PD) is an incurable neurodegenerative disease of high prevalence, characterized by the prominent death of dopaminergic neurons in the substantia nigra pars compacta, which produces dopamine deficiency, leading to classic motor symptoms. Although PD has traditionally been considered as a neuronal cell autonomous pathology, in which the damage of vulnerable neurons is responsible for the disease, growing evidence strongly suggests that astrocytes might have an active role in the neurodegeneration observed. In the present review, we discuss several studies evidencing astrocyte implications in PD, highlighting the consequences of both the loss of normal homeostatic functions and the gain in toxic functions for the wellbeing of dopaminergic neurons. The revised information provides significant evidence that allows astrocytes to be positioned as crucial players in PD etiology, a factor that needs to be taken into account when considering therapeutic targets for the treatment of the disease.

Citrate shuttling in astrocytes is required for processing cocaine-induced neuron-derived excess peroxidated fatty acids

Disturbances in lipid metabolism in the CNS contribute to neurodegeneration and cognitive impairments. Through tight metabolic coupling, astrocytes provide energy to neurons by delivering lactate and cholesterol and by taking up and processing neuron-derived peroxidated fatty acids (pFA). Disruption of CNS lipid homeostasis is observed in people who use cocaine and in several neurodegenerative disorders, including HIV. The brain's main source of energy is aerobic glycolysis, but numerous studies report a switch to β-oxidation of FAs in response to cocaine. Unlike astrocytes, in response to cocaine, neurons cannot efficiently consume excess pFAs for energy. Accumulation of pFA in neurons induces autophagy and release of pFA. Astrocytes endocytose the pFA for oxidation as an energy source. Our data show that blocking mitochondrial/cytosolic citrate transport reduces the neurotrophic capacity of astrocytes, leading to decreased neuronal fitness.

Implications of glial metabolic dysregulation in the pathophysiology of neurodegenerative diseases

Glial cells are the most abundant cells of the brain, outnumbering neurons. These multifunctional cells are crucial for maintaining brain homeostasis by providing trophic and nutritional support to neurons, sculpting synapses, and providing an immune defense. Glia are highly plastic and undergo both structural and functional alterations in response to changes in the brain microenvironment. Glial phenotypes are intimately regulated by underlying metabolic machinery, which dictates the effector functions of these cells. Altered brain energy metabolism and chronic neuroinflammation are common features of several neurodegenerative diseases. Microglia and astrocytes are the major glial cells fueling the ongoing neuroinflammatory process, exacerbating neurodegeneration. Distinct metabolic perturbations in microglia and astrocytes, including altered carbohydrate, lipid, and amino acid metabolism have been documented in neurodegenerative diseases. These disturbances aggravate the neurodegenerative process by potentiating the inflammatory activation of glial cells. This review covers the recent advances in the molecular aspects of glial metabolic changes in the pathophysiology of neurodegenerative diseases. Finally, we discuss studies exploiting glial metabolism as a potential therapeutic avenue in neurodegenerative diseases.

Should We Void Lactate in the Pathophysiology of Delayed Onset Muscle Soreness? Not So Fast! Let’s See a Neurocentric View!

The pathophysiology of delayed onset muscle soreness is not entirely known. It seems to be a simple, exercise-induced delayed pain condition, but has remained a mystery for over 120 years. The buildup of lactic acid used to be blamed for muscle fatigue and delayed onset muscle soreness; however, studies in the 1980s largely refuted the role of lactate in delayed onset muscle soreness. Regardless, this belief is widely held even today, not only in the general public, but within the medical and scientific community as well. Current opinion is highlighting lactate’s role in delayed onset muscle soreness, if neural dimension and neuro-energetics are not overlooked. By doing so, lactate seems to have an essential role in the initiation of the primary damage phase of delayed onset muscle soreness within the intrafusal space. Unaccustomed or strenuous eccentric contractions are suggested to facilitate lactate nourishment of proprioceptive sensory neurons in the muscle spindle under hyperexcitation. However, excessive acidosis and lactate could eventually contribute to impaired proprioception and increased nociception under pathological condition. Furthermore, lactate could also contribute to the secondary damage phase of delayed onset muscle soreness in the extrafusal space, primarily by potentiating the role of bradykinin. After all, neural interpretation may help us to dispel a 40-year-old controversy about lactate’s role in the pathophysiology of delayed onset muscle soreness.

Energy supply per neuron is constrained by capillary density in the mouse brain

Neuronal densities vary enormously across sites within a brain. Does the density of the capillary bed vary accompanying the presumably larger energy requirement of sites with more neurons, or with larger neurons, or is energy supply constrained by a mostly homogeneous capillary bed? Here we find evidence for the latter, with a capillary bed that represents typically between 0.7 and 1.5% of the volume of the parenchyma across various sites in the mouse brain, whereas neuronal densities vary by at least 100-fold. As a result, the ratio of capillary cells per neuron decreases uniformly with increasing neuronal density and therefore with smaller average neuronal size across sites. Thus, given the relatively constant capillary density compared to neuronal density in the brain, blood and energy availability per neuron is presumably dependent on how many neurons compete for the limited supply provided by a mostly homogeneous capillary bed. Additionally, we find that local capillary density is not correlated with local synapse densities, although there is a small but significant correlation between lower neuronal density (and therefore larger neuronal size) and more synapses per neuron within the restricted range of 6,500–9,500 across cortical sites. Further, local variations in the glial/neuron ratio are not correlated with local variations in the number of synapses per neuron or local synaptic densities. These findings suggest that it is not that larger neurons, neurons with more synapses, or even sites with more synapses demand more energy, but simply that larger neurons (in low density sites) have more energy available per cell and for the totality of its synapses than smaller neurons (in high density sites) due to competition for limited resources supplied by a capillary bed of fairly homogeneous density throughout the brain.

The F1Fo-ATPase inhibitor protein IF1 in pathophysiology

The endogenous inhibitor of ATP synthase is a protein of about 10 kDa, known as IF1 which binds to the catalytic domain of the enzyme during ATP hydrolysis. The main role of IF1 consists of limiting ATP dissipation under condition of severe oxygen deprivation or in the presence of dysfunctions of mitochondrial respiratory complexes, causing a collapse in mitochondrial membrane potential and therefore ATP hydrolysis. New roles of IF1 are emerging in the fields of cancer and neurodegeneration. Its high expression levels in tumor tissues have been associated with different roles favouring tumor formation, progression and evasion. Since discordant mechanisms of action have been proposed for IF1 in tumors, it is of the utmost importance to clarify them in the prospective of defining novel approaches for cancer therapy. Other IF1 functions, including its involvement in mitophagy, may be protective for neurodegenerative and aging-related diseases. In the present review we aim to clarify and discuss the emerging mechanisms in which IF1 is involved, providing a critical view of the discordant findings in the literature.

Ciliary Neurotrophic Factor Derived From Astrocytes Protects Retinal Ganglion Cells Through PI3K/AKT, JAK/STAT, and MAPK/ERK Pathways

Purpose

The purpose of this study was to investigate the roles of ciliary neurotrophic factor (CNTF) on the protective effects of astrocytes on retinal ganglion cells (RGCs).

Methods

Primary RGCs were isolated from neonatal rats. Oxidative stress was induced, and the effects of co-culture with astrocytes and CNTF treatment on RGCs were evaluated. The pathways commonly altered by astrocytes and CNTF were investigated. Effects of each pathway were investigated using pathway inhibitors against PI3K/AKT, JAK/STAT, and MAPK/ERK. RNA sequencing was performed to identify the genes upregulated and downregulated by CNTF treatment.

Results

Astrocytes improved the viability and increased β3-tubulin expression in RGCs. The concentration of CNTF increased in the RGC-astrocyte co-culture medium. The protective effects of astrocytes were abolished by neutralization with the anti-CNTF antibody; thus, CNTF may play an important role in the effects mediated by astrocytes. Furthermore, CNTF treatment alone enhanced the viability and β3-tubulin expression of RGCs and increased the population of viable RGCs under oxidative stress. The PI3K/AKT pathway was associated with both RGC viability and β3-tubulin expression. However, the JAK/STAT pathway increased the viability of RGCs, whereas the MAPK/ERK pathway was associated with β3-tubulin expression. RNA sequencing revealed the CNTF-upregulated genes associated with response to DNA damage and downregulated genes associated with photoreceptor cell differentiation.

Conclusions

Our data revealed protective effects of astrocyte-derived CNTF on RGCs. In addition, we showed that multiple pathways exert these protective effects and identified the novel genes involved. These results may be helpful in developing treatments for RGC injury.

Hippocampus-sensitive and striatum-sensitive learning one month after morphine or cocaine exposure in male rats

These experiments examined whether morphine and cocaine alter the balance between hippocampal and striatal memory systems measured long after drug exposure. Male rats received injections of morphine (5 mg/kg), cocaine (20 mg/kg), or saline for five consecutive days. One month later, rats were trained to find food on a hippocampus-sensitive place task or a striatum-sensitive response task. Relative to saline controls, morphine-treated rats exhibited impaired place learning but enhanced response learning; prior cocaine exposure did not significantly alter learning on either task. Another set of rats was trained on a dual-solution T-maze that can be solved with either place or response strategies. While a majority (67%) of control rats used place solutions, morphine treatment one month prior resulted in the exclusive use of response solutions (100%). Prior cocaine treatment did not significantly alter strategy selection. Molecular markers related to learning and drug abuse were measured in the hippocampus and striatum one month after drug exposure in behaviorally untested rats. Protein levels of glial-fibrillary acidic protein (GFAP), an intermediate filament specific to astrocytes, increased significantly in the hippocampus after morphine exposure, but not after cocaine exposure. Exposure to morphine or cocaine did not significantly change levels of brain-derived neurotrophic factor (BDNF) or a downstream target of BDNF signaling, glycogen synthase kinase 3β (GSK3β), in the hippocampus or striatum. Thus, exposure to morphine resulted in a long-lasting shift from hippocampal toward striatal dominance during learning, an effect that may be associated with lasting alterations in hippocampal astrocytes. Cocaine produced changes in the same direction, suggesting that use of a higher dose or longer duration of exposure might produce effects comparable to those seen with morphine.

Evaluation of p21 expression and related autism-like behavior in Bisphenol-A exposed offspring of Wistar albino rats

BACKGROUND

Bisphenol A (BPA), an endocrine disruptor, may be involved in the etiology of autism spectrum disorders (ASD); however, the mechanism of neuronal and astrocytic damage remains ambiguous. A possible role of altered expression of p21 in autistic-like behavior in rat offspring was examined with prenatal and postnatal BPA exposure.

METHODS

Wistar albino dams were exposed to BPA (5 mg/kg) intraperitoneally throughout pregnancy and until the third postnatal day (PND). Pups were examined on 21st PND for behavioral test. Blood samples were collected for serum lactate levels and pups were sacrificed. Right frontal cortices were dissected out and processed for H&E, immunohistochemical analysis, and gene expression.

RESULTS

Anxiety like behavior and thigmotaxis along with reduction in serum lactate concentrations were observed in pups exposed to BPA. Decline in neuronal number and decreased astrocytic population with reduced dendritic spines were revealed by H&E and immunohistochemical analysis, respectively, in right frontal cortices. Over expression of p21 was also detected in BPA-exposed offspring.

CONCLUSIONS

Over expression of p21 may be associated with autistic behavior. Further studies are recommended to explore the structural alterations in other white matter pathways in frontal cortices.

Alteration of the Individual Metabolic Network of the Brain Based on Jensen-Shannon Divergence Similarity Estimation in Elderly Patients With Type 2 Diabetes Mellitus

The aim of this study was to investigate the interactive effect between aging and type 2 diabetes mellitus (T2DM) on brain glucose metabolism, individual metabolic connectivity, and network properties. Using a 2 × 2 factorial design, 83 patients with T2DM (40 elderly and 43 middle-aged) and 69 sex-matched healthy control subjects (HCs) (34 elderly and 35 middle-aged) underwent 18F-fluorodeoxyglucose positron emission tomography/magnetic resonance scanning. Jensen-Shannon divergence was applied to construct individual metabolic connectivity and networks. The topological properties of the networks were quantified using graph theoretical analysis. The general linear model was used to mainly estimate the interaction effect between aging and T2DM on glucose metabolism, metabolic connectivity, and network. There was an interaction effect between aging and T2DM on glucose metabolism, metabolic connectivity, and regional metabolic network properties (all P < 0.05). The post hoc analyses showed that compared with elderly HCs and middle-aged patients with T2DM, elderly patients with T2DM had decreased glucose metabolism, increased metabolic connectivity, and regional metabolic network properties in cognition-related brain regions (all P < 0.05). Age and fasting plasma glucose had negative correlations with glucose metabolism and positive correlations with metabolic connectivity. Elderly patients with T2DM had glucose hypometabolism, strengthened functional integration, and increased efficiency of information communication mainly located in cognition-related brain regions. Metabolic connectivity pattern changes might be compensatory changes for glucose hypometabolism.

Lactate Supply from Astrocytes to Neurons and its Role in Ischemic Stroke-induced Neurodegeneration

Glucose transported to the brain is metabolized to lactate in astrocytes and supplied to neuronal cells via a monocarboxylic acid transporter (MCT). Lactate is used in neuronal cells for various functions, including learning and memory formation. Furthermore, lactate can block stroke-induced neurodegeneration. We aimed to clarify the effect of astrocyte-produced lactate on stroke-induced neurodegeneration. Previously published in vivo and in vitro animal and cell studies, respectively, were searched in PubMed, ScienceDirect, and Web of Science. Under physiological conditions, lactate production and release by astrocytes are regulated by changes in lactate dehydrogenase (LDH) and MCT expression. Moreover, considering stroke, lactate production and supply are regulated through hypoxia-inducible factor (HIF)-1α expression, especially with hypoxic stimulation, which may promote neuronal apoptosis; contrastingly, neuronal survival may be promoted via HIF-1α. Stroke stimulation could prevent neurodegeneration through the strong enhancement of lactate production, as well as upregulation of MCT4 expression to accelerate lactate supply. However, studies using astrocytes derived from animal stroke models revealed significantly reduced lactate production and MCT expression. These findings suggest that the lack of lactate supply may strongly contribute to hypoxia-induced neurodegeneration. Furthermore, diminished lactate supply from astrocytes could facilitate stroke-induced neurodegeneration. Therefore, astrocyte-derived lactate may contribute to stroke prevention.

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.

Evolutionary Advantages of Stimulus-Driven EEG Phase Transitions in the Upper Cortical Layers

Spatio-temporal brain activity monitored by EEG recordings in humans and other mammals has identified beta/gamma oscillations (20-80 Hz), which are self-organized into spatio-temporal structures recurring at theta/alpha rates (4-12 Hz). These structures have statistically significant correlations with sensory stimuli and reinforcement contingencies perceived by the subject. The repeated collapse of self-organized structures at theta/alpha rates generates laterally propagating phase gradients (phase cones), ignited at some specific location of the cortical sheet. Phase cones have been interpreted as neural signatures of transient perceptual experiences according to the cinematic theory of brain dynamics. The rapid expansion of essentially isotropic phase cones is consistent with the propagation of perceptual broadcasts postulated by Global Workspace Theory (GWT). What is the evolutionary advantage of brains operating with repeatedly collapsing dynamics? This question is answered using thermodynamic concepts. According to neuropercolation theory, waking brains are described as non-equilibrium thermodynamic systems operating at the edge of criticality, undergoing repeated phase transitions. This work analyzes the role of long-range axonal connections and metabolic processes in the regulation of critical brain dynamics. Historically, the near 10 Hz domain has been associated with conscious sensory integration, cortical "ignitions" linked to conscious visual perception, and conscious experiences. We can therefore combine a very large body of experimental evidence and theory, including graph theory, neuropercolation, and GWT. This cortical operating style may optimize a tradeoff between rapid adaptation to novelty vs. stable and widespread self-organization, therefore resulting in significant Darwinian benefits.

Astrocytic Changes in Mitochondrial Oxidative Phosphorylation Protein Levels in Parkinson's Disease

BACKGROUND

Mitochondrial dysfunction within neurons, particularly those of the substantia nigra, has been well characterized in Parkinson's disease and is considered to be related to the pathogenesis of this disorder. Dysfunction within this important organelle has been suggested to impair neuronal communication and survival; however, the reliance of astrocytes on mitochondria and the impact of their dysfunction on this essential cell type are less well characterized.

OBJECTIVE

This study aimed to uncover whether astrocytes harbor oxidative phosphorylation (OXPHOS) deficiencies in Parkinson's disease and whether these deficiencies are more likely to occur in astrocytes closely associated with neurons or those more distant from them.

METHODS

Postmortem human brain sections from patients with Parkinson's disease were subjected to imaging mass cytometry for individual astrocyte analysis of key OXPHOS proteins across all five complexes.

RESULTS

We show the variability in the astrocytic expression of mitochondrial proteins between individuals. In addition, we found that there is evidence of deficiencies in respiratory chain subunit expression within these important glia and changes, particularly in mitochondrial mass, associated with Parkinson's disease and that are not simply a consequence of advancing age.

CONCLUSION

Our data show that astrocytes, like neurons, are susceptible to mitochondrial defects and that these could have an impact on their reactivity and ability to support neurons in Parkinson's disease.

Time-dependent changes in hippocampal and striatal glycogen long after maze training in male rats

Long-lasting biological changes reflecting past experience have been studied in and typically attributed to neurons in the brain. Astrocytes, which are also present in large number in the brain, have recently been found to contribute critically to learning and memory processing. In the brain, glycogen is primarily found in astrocytes and is metabolized to lactate, which can be released from astrocytes. Here we report that astrocytes themselves have intrinsic neurochemical plasticity that alters the availability and provision of metabolic substrates long after an experience. Rats were trained to find food on one of two versions of a 4-arm maze: a hippocampus-sensitive place task and a striatum-sensitive response task. Remarkably, hippocampal glycogen content increased while striatal levels decreased during the 30 days after rats were trained to find food in the place version, but not the response version, of the maze tasks. A long-term consequence of the durable changes in glycogen stores was seen in task-by-site differences in extracellular lactate responses activated by testing on a working memory task administered 30 days after initial training, the time when differences in glycogen content were most robust. These results suggest that astrocytic plasticity initiated by a single experience may augment future availability of energy reserves, perhaps priming brain areas to process learning of subsequent experiences more effectively.

Activation of Astrocytes in the Persistence of Post-hypoxic Respiratory Augmentation

Acute hypoxia increases ventilation. After cessation of hypoxia loading, ventilation decreases but remains above the pre-exposure baseline level for a time. However, the mechanism of this post-hypoxic persistent respiratory augmentation (PHRA), which is a short-term potentiation of breathing, has not been elucidated. We aimed to test the hypothesis that astrocytes are involved in PHRA. To this end, we investigated hypoxic ventilatory responses by whole-body plethysmography in unanesthetized adult mice. The animals breathed room air, hypoxic gas mixture (7% O₂, 93% N₂) for 2min, and again room air for 10min before and after i.p. administration of low (100mg/kg) and high (300mg/kg) doses of arundic acid (AA), an astrocyte inhibitor. AA suppressed PHRA, with the high dose decreasing ventilation below the pre-hypoxic level. Further, we investigated the role of the astrocytic TRPA1 channel, a putative ventilatory hypoxia sensor, in PHRA using astrocyte-specific Trpa1 knockout (asTrpa1^−/−) and floxed Trpa1 (Trpa1^f/f) mice. In both Trpa1^f/f and asTrpa1^−/− mice, PHRA was noticeable, indicating that the astrocyte TRPA1 channel was not directly involved in PHRA. Taken together, these results indicate that astrocytes mediate the PHRA by mechanisms other than TRPA1 channels that are engaged in hypoxia sensing.

Blood-Brain Barrier Dysfunction in CNS Disorders and Putative Therapeutic Targets: An Overview

The blood-brain barrier (BBB) is a fundamental component of the central nervous system (CNS). Its functional and structural integrity is vital to maintain the homeostasis of the brain microenvironment by controlling the passage of substances and regulating the trafficking of immune cells between the blood and the brain. The BBB is primarily composed of highly specialized microvascular endothelial cells. These cells’ special features and physiological properties are acquired and maintained through the concerted effort of hemodynamic and cellular cues from the surrounding environment. This complex multicellular system, comprising endothelial cells, astrocytes, pericytes, and neurons, is known as the neurovascular unit (NVU). The BBB strictly controls the transport of nutrients and metabolites into brain parenchyma through a tightly regulated transport system while limiting the access of potentially harmful substances via efflux transcytosis and metabolic mechanisms. Not surprisingly, a disruption of the BBB has been associated with the onset and/or progression of major neurological disorders. Although the association between disease and BBB disruption is clear, its nature is not always evident, specifically with regard to whether an impaired BBB function results from the pathological condition or whether the BBB damage is the primary pathogenic factor prodromal to the onset of the disease. In either case, repairing the barrier could be a viable option for treating and/or reducing the effects of CNS disorders. In this review, we describe the fundamental structure and function of the BBB in both healthy and altered/diseased conditions. Additionally, we provide an overview of the potential therapeutic targets that could be leveraged to restore the integrity of the BBB concomitant to the treatment of these brain disorders.

Lactate in the tumour microenvironment: From immune modulation to therapy

Disordered metabolic states, which are characterised by hypoxia and elevated levels of metabolites, particularly lactate, contribute to the immunosuppression in the tumour microenvironment (TME). Excessive lactate secreted by metabolism-reprogrammed cancer cells regulates immune responses via causing extracellular acidification, acting as an energy source by shuttling between different cell populations, and inhibiting the mechanistic (previously ‘mammalian’) target of rapamycin (mTOR) pathway in immune cells. This review focuses on recent advances in the regulation of immune responses by lactate, as well as therapeutic strategies targeting lactate anabolism and transport in the TME, such as those involving glycolytic enzymes and monocarboxylate transporter inhibitors. Considering the multifaceted roles of lactate in cancer metabolism, a comprehensive understanding of how lactate and lactate-targeting therapies regulate immune responses in the TME will provide insights into the complex relationships between metabolism and antitumour immunity.

Metabolic Transporters in the Peripheral Nerve-What, Where, and Why?

Cellular metabolism is critical not only for cell survival, but also for cell fate, function, and intercellular communication. There are several different metabolic transporters expressed in the peripheral nervous system, and they each play important roles in maintaining cellular energy. The major source of energy in the peripheral nervous system is glucose, and glucose transporters 1 and 3 are expressed and allow blood glucose to be imported and utilized by peripheral nerves. There is also increasing evidence that other sources of energy, particularly monocarboxylates such as lactate that are transported primarily by monocarboxylate transporters 1 and 2 in peripheral nerves, can be efficiently utilized by peripheral nerves. Finally, emerging evidence supports an important role for connexins and possibly pannexins in the supply and regulation of metabolic energy. In this review, we will first define these critical metabolic transporter subtypes and then examine their localization in the peripheral nervous system. We will subsequently discuss the evidence, which comes both from experiments in animal models and observations from human diseases, supporting critical roles played by these metabolic transporters in the peripheral nervous system. Despite progress made in understanding the function of these transporters, many questions and some discrepancies remain, and these will also be addressed throughout this review. Peripheral nerve metabolism is fundamentally important and renewed interest in these pathways should help to answer many of these questions and potentially provide new treatments for neurologic diseases that are partly, or completely, caused by disruption of metabolism.

Assessment of astrocytes as a mediator of memory and learning in rodents

The classical view of astrocytes is that they provide supportive functions for neurons, transporting metabolites and maintaining the homeostasis of the extracellular milieu. This view is gradually changing with the advent of molecular genetics and optical methods allowing interrogation of selected cell types in live experimental animals. An emerging view that astrocytes additionally act as a mediator of synaptic plasticity and contribute to learning processes has gained in vitro and in vivo experimental support. Here we focus on the literature published in the past two decades to review the roles of astrocytes in brain plasticity in rodents, whereby the roles of neurotransmitters and neuromodulators are considered to be comparable to those in humans. We outline established inputs and outputs of astrocytes and discuss how manipulations of astrocytes have impacted the behavior in various learning paradigms. Multiple studies suggest that the contribution of astrocytes has a considerably longer time course than neuronal activation, indicating metabolic roles of astrocytes. We advocate that exploring upstream and downstream mechanisms of astrocytic activation will further provide insight into brain plasticity and memory/learning impairment.

Olfactory bulb astrocytes mediate sensory circuit processing through Sox9 in the mouse brain

The role of transcription factors during astrocyte development and their subsequent effects on neuronal development has been well studied. Less is known about astrocytes contributions towards circuits and behavior in the adult brain. Astrocytes play important roles in synaptic development and modulation, however their contributions towards neuronal sensory function and maintenance of neuronal circuit architecture remain unclear. Here, we show that loss of the transcription factor Sox9 results in both anatomical and functional changes in adult mouse olfactory bulb (OB) astrocytes, affecting sensory processing. Indeed, astrocyte-specific deletion of Sox9 in the OB results in decreased odor detection thresholds and discrimination and it is associated with aberrant neuronal sensory response maps. At functional level, loss of astrocytic Sox9 impairs the electrophysiological properties of mitral and tufted neurons. RNA-sequencing analysis reveals widespread changes in the gene expression profiles of OB astrocytes. In particular, we observe reduced GLT-1 expression and consequential alterations in glutamate transport. Our findings reveal that astrocytes are required for physiological sensory processing and we identify astrocytic Sox9 as an essential transcriptional regulator of mature astrocyte function in the mouse OB.

l-Lactate: Food for Thoughts, Memory and Behavior

More and more evidence shows how brain energy metabolism is the linkage between physiological and morphological synaptic plasticity and memory consolidation. Different types of memory are associated with differential inputs, each with specific inputs that are upstream diverse molecular cascades depending on the receptor activity. No matter how heterogeneous the response is, energy availability represents the lowest common denominator since all these mechanisms are energy consuming and the brain networks adapt their performance accordingly. Astrocytes exert a primary role in this sense by acting as an energy buffer; glycogen granules, a mechanism to store glucose, are redistributed at glance and conveyed to neurons via the Astrocyte–Neuron Lactate Shuttle (ANLS). Here, we review how different types of memory relate to the mechanisms of energy delivery in the brain.

Astrocyte-to-neuron transportation of enhanced green fluorescent protein in cerebral cortex requires F-actin dependent tunneling nanotubes

Tunneling nanotube (TNT), a dynamic cell–cell contact, is dependent on actin polymerization. TNTs are efficient in transporting ions, proteins and organelles intercellularly, which are important mechanisms in physiological and pathological processes. Reported studies on the existence and function of TNTs among neural cells focus on cultured cell for the convenience in detecting TNTs’ ultrastructure. In this study, the adeno-associated virus (AAV-GFAP-EGFP-p2A-cre) was injected into the cerebral cortex of knock-in mice ROSA26 GNZ. GFAP promoter initiated the expression of enhanced green fluorescent protein (EGFP) in infected astrocytes. At 10 days post injection (10 DPI), EGFP transferred from astrocytes in layer I–III to neurons in layer V. The dissemination of EGFP was not through endocytosis or exosome. Applying microscopes, we found that the intercellular transportation of EGFP through contact connection was F-actin dependent. Therefore, we concluded that EGFP transported from astrocytes to neurons in cortex via F-actin dependent TNTs. This study first proved that proteins transported intercellularly via TNTs in brain.

Morphometric analysis of astrocytes in vocal production circuits of common marmoset (Callithrix jacchus)

Astrocytes, the star-shaped glial cells, are the most abundant non-neuronal cell population in the central nervous system. They play a key role in modulating activities of neural networks, including those involved in complex motor behaviors. Common marmosets (Callithrix jacchus), the most vocal non-human primate (NHP), have been used to study the physiology of vocalization and social vocal production. However, the neural circuitry involved in vocal production is not fully understood. In addition, even less is known about the involvement of astrocytes in this circuit. To understand the role, that astrocytes may play in the complex behavior of vocalization, the initial step may be to study their structural properties in the cortical and subcortical regions that are known to be involved in vocalization. Here, in the common marmoset, we identify all astrocytic subtypes seen in other primate's brains, including intralaminar astrocytes. In addition, we reveal detailed structural characteristics of astrocytes and perform morphometric analysis of astrocytes residing in the cortex and midbrain regions that are associated with vocal production. We found that cortical astrocytes in these regions illustrate a higher level of complexity when compared to those in the midbrain. We hypothesize that this complexity that is expressed in cortical astrocytes may reflect their functions to meet the metabolic/structural needs of these regions.