MCT2 is a major neuronal monocarboxylate transporter in the adult mouse brain

From energy metabolism to mood regulation: The rise of lactate as a therapeutic target

BACKGROUND

Disruption of cerebral energy metabolism is increasingly recognized as a key factor in the pathophysiology of mood disorders. Lactate, beyond its role as a metabolic byproduct, is now understood to be a critical player in brain energy homeostasis and a modulator of neuronal function. Recent advances in understanding lactate shuttling between astrocytes and neurons have opened new avenues for exploring its multifaceted roles in mood regulation. Exercise, known to modulate brain lactate levels, further underscores the potential of lactate as a therapeutic target in mood disorders.

AIM OF REVIEW

This review delves into the alterations in cerebral lactate associated with mood disorders, emphasizing their implications for brain energy dynamics and signaling pathways. Additionally, we discuss the therapeutic potential of lactate in mood disorders, particularly through its capacity to remodel cerebral function. We conclude by assessing the promise of exercise-induced lactate production as a novel strategy for mood disorder treatment.

KEY SCIENTIFIC CONCEPTS OF THE REVIEW

Alterations in brain lactate may contribute to the pathogenesis of mood disorders. In several studies, lactate is not only a substrate for brain energy metabolism, but also a molecule that triggers signaling cascades. Specifically, lactate is involved in the regulation of neurogenesis, neuroplasticity, endothelial cell function, and microglia lysosomal acidification, therefore improving mood disorders. Meanwhile, exercise as a low-risk intervention strategy can improve mood disorders through lactate regulation. Thus, the evidence from this review supports that lactate could be a potential therapeutic target for mood disorder, contributing to a deeper understanding of mood disorder pathogenesis and intervention.

Intermittent fasting and neurodegenerative diseases: Molecular mechanisms and therapeutic potential

Neurodegenerative disorders are straining public health worldwide. During neurodegenerative disease progression, aberrant neuronal network activity, bioenergetic impairment, adaptive neural plasticity impairment, dysregulation of neuronal Ca2+ homeostasis, oxidative stress, and immune inflammation manifest as characteristic pathological changes in the cellular milieu of the brain. There is no drug for the treatment of neurodegenerative disorders, and therefore, strategies/treatments for the prevention or treatment of neurodegenerative disorders are urgently needed. Intermittent fasting (IF) is characterized as an eating pattern that alternates between periods of fasting and eating, requiring fasting durations that vary depending on the specific protocol implemented. During IF, depletion of liver glycogen stores leads to the production of ketone bodies from fatty acids derived from adipocytes, thereby inducing an altered metabolic state accompanied by cellular and molecular adaptive responses within neural networks in the brain. At the cellular level, adaptive responses can promote the generation of synapses and neurons. At the molecular level, IF triggers the activation of associated transcription factors, thereby eliciting the expression of protective proteins. Consequently, this regulatory process governs central and peripheral metabolism, oxidative stress, inflammation, mitochondrial function, autophagy, and the gut microbiota, all of which contribute to the amelioration of neurodegenerative disorders. Emerging evidence suggests that weight regulation significantly contributes to the neuroprotective effects of IF. By alleviating obesity-related factors such as blood-brain barrier dysfunction, neuroinflammation, and β-amyloid accumulation, IF enhances metabolic flexibility and insulin sensitivity, further supporting its potential in mitigating neurodegenerative disorders. The present review summarizes animal and human studies investigating the role and underlying mechanisms of IF in physiology and pathology, with an emphasis on its therapeutic potential. Furthermore, we provide an overview of the cellular and molecular mechanisms involved in regulating brain energy metabolism through IF, highlighting its potential applications in neurodegenerative disorders. Ultimately, our findings offer novel insights into the preventive and therapeutic applications of IF for neurodegenerative disorders.

Distinct roles of ascorbic acid in extracellular vesicles and free form: Implications for metabolism and oxidative stress in presymptomatic Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a CAG trinucleotide repeat expansion in the first exon of the huntingtin gene. The huntingtin protein (Htt) is ubiquitously expressed and localized in several organelles, including endosomes, where it plays an essential role in intracellular trafficking. Presymptomatic HD is associated with a failure in energy metabolism and oxidative stress. Ascorbic acid is a potent antioxidant that plays a key role in modulating neuronal metabolism and is highly concentrated in the brain. During synaptic activity, neurons take up ascorbic acid released by glial cells; however, this process is disrupted in HD. In this study, we aim to elucidate the molecular and cellular mechanisms underlying this dysfunction. Using an electrophysiological approach in presymptomatic YAC128 HD slices, we observed decreased ascorbic acid flux from astrocytes to neurons, which altered neuronal metabolic substrate preferences. Ascorbic acid efflux and recycling were also decreased in cultured astrocytes from YAC128 HD mice. We confirmed our findings using GFAP-HD160Q, an HD mice model expressing mutant N-terminal Htt mainly in astrocytes. For the first time, we demonstrated that ascorbic acid is released from astrocytes via extracellular vesicles (EVs). Decreased number of particles and exosomal markers were observed in EV fractions from cultured YAC128 HD astrocytes and Htt-KD cells. We observed reduced number of multivesicular bodies (MVBs) in YAC128 HD striatum via electron microscopy, suggesting mutant Htt alters MVB biogenesis. EVs containing ascorbic acid effectively reduced reactive oxygen species, whereas "free" ascorbic acid played a role in modulating neuronal metabolic substrate preferences. These findings suggest that the early redox imbalance observed in HD arises from a reduced release of ascorbic acid-containing EVs by astrocytes. Meanwhile, a decrease in "free" ascorbic acid likely contributes to presymptomatic metabolic impairment.

Particle-based MR modeling with diffusion, microstructure, and enzymatic reactions

PURPOSE

To develop a novel particle-based in silico MR model and demonstrate applications of this model to signal mechanisms which are affected by the spatial organization of particles, including metabolic reaction kinetics, microstructural effects on diffusion, and radiofrequency (RF) refocusing effects in gradient-echo sequences.

METHODS

The model was developed by integrating a forward solution of the Bloch equations with a Brownian dynamics simulator. Simulation configurations were then designed to model MR signal dynamics of interest, with a primary focus on hyperpolarized 13C MRI methods. Phantom scans and spectrophotometric assays were conducted to validate model results in vitro.

RESULTS

The model accurately reproduced the reaction kinetics of enzyme-mediated conversion of pyruvate to lactate. When varying proportions of restrictive structure were added to the reaction volume, nonlinear changes in the reaction rate measured in vitro were replicated in silico. Modeling of RF refocusing effects characterized the degree of diffusion-weighted contribution from preserved residual magnetization in nonspoiled gradient-echo sequences.

CONCLUSIONS

These results show accurate reproduction of a range of MR signal mechanisms, establishing the model's capability to investigate the multifactorial signal dynamics such as those underlying hyperpolarized 13C MRI data.

Metabolic Reprogramming of Astrocytes in Pathological Conditions: Implications for Neurodegenerative Diseases

Astrocytes play a pivotal role in maintaining brain energy homeostasis, supporting neuronal function through glycolysis and lipid metabolism. This review explores the metabolic intricacies of astrocytes in both physiological and pathological conditions, highlighting their adaptive plasticity and diverse functions. Under normal conditions, astrocytes modulate synaptic activity, recycle neurotransmitters, and maintain the blood-brain barrier, ensuring a balanced energy supply and protection against oxidative stress. However, in response to central nervous system pathologies such as neurotrauma, stroke, infections, and neurodegenerative diseases like Alzheimer's and Huntington's disease, astrocytes undergo significant morphological, molecular, and metabolic changes. Reactive astrocytes upregulate glycolysis and fatty acid oxidation to meet increased energy demands, which can be protective in acute settings but may exacerbate chronic inflammation and disease progression. This review emphasizes the need for advanced molecular, genetic, and physiological tools to further understand astrocyte heterogeneity and their metabolic reprogramming in disease states.

Exploring the ketogenic diet's potential in reducing neuroinflammation and modulating immune responses

The ketogenic diet (KD) is marked by a substantial decrease in carbohydrate intake and an elevated consumption of fats and proteins, leading to a metabolic state referred to as "ketosis," where fats become the primary source of energy. Recent research has underscored the potential advantages of the KD in mitigating the risk of various illnesses, including type 2 diabetes, hyperlipidemia, heart disease, and cancer. The macronutrient distribution in the KD typically entails high lipid intake, moderate protein consumption, and low carbohydrate intake. Restricting carbohydrates to below 50 g/day induces a catabolic state, prompting metabolic alterations such as gluconeogenesis and ketogenesis. Ketogenesis diminishes fat and glucose accumulation as energy reserves, stimulating the production of fatty acids. Neurodegenerative diseases, encompassing Alzheimer's disease, Parkinson's disease are hallmarked by persistent neuroinflammation. Evolving evidence indicates that immune activation and neuroinflammation play a significant role in the pathogenesis of these diseases. The protective effects of the KD are linked to the generation of ketone bodies (KB), which play a pivotal role in this dietary protocol. Considering these findings, this narrative review seeks to delve into the potential effects of the KD in neuroinflammation by modulating the immune response. Grasping the immunomodulatory effects of the KD on the central nervous system could offer valuable insights into innovative therapeutic approaches for these incapacitating conditions.

Astrocytic metabolic control of orexinergic activity in the lateral hypothalamus regulates sleep and wake architecture

Neuronal activity undergoes significant changes during vigilance states, accompanied by an accommodation of energy demands. While the astrocyte-neuron lactate shuttle has shown that lactate is the primary energy substrate for sustaining neuronal activity in multiple brain regions, its role in regulating sleep/wake architecture is not fully understood. Here we investigated the involvement of astrocytic lactate supply in maintaining consolidated wakefulness by downregulating, in a cell-specific manner, the expression of monocarboxylate transporters (MCTs) in the lateral hypothalamus of transgenic mice. Our results demonstrate that reduced expression of MCT4 in astrocytes disrupts lactate supply to wake-promoting orexin neurons, impairing wakefulness stability. Additionally, we show that MCT2-mediated lactate uptake is necessary for maintaining tonic firing of orexin neurons and stabilizing wakefulness. Our findings provide both in vivo and in vitro evidence supporting the role of astrocyte-to-orexinergic neuron lactate shuttle in regulating proper sleep/wake stability.

Fibroblast growth factor 21 enhances learning and memory performance in mice by regulating hippocampal L-lactate homeostasis

Fibroblast growth factor 21 (FGF21) is one endogenous metabolic molecule that functions as a regulator in glucose and lipid homeostasis. However, the effect of FGF21 on L-lactate homeostasis and its mechanism remains unclear until now. Forty-five Six-week-old male C57BL/6 mice were divided into three groups: control, L-lactate, and FGF21 (1.5 mg/kg) groups. At the end of the treatment, nuclear magnetic resonance-based metabolomics, and key proteins related to L-lactate homeostasis were determined respectively to evaluate the efficacy of FGF21 and its mechanisms. The results showed that, compared to the vehicle group, the L-lactate-treated mice displayed learning and memory performance impairments, as well as reduced hippocampal ATP and NADH levels, but increased oxidative stress, mitochondrial dysfunction, and apoptosis, which suggesting inhibited L-lactate-pyruvate conversion in the brain. Conversely, FGF21 treatment ameliorated the L-lactate accumulation state, accompanied by restoration of the learning and memory defects, indicating enhanced L-lactate uptake and utilization in hippocampal neurons. We demonstrated that maintaining constant L-lactate-pyruvate flux is essential for preserving neuronal bioenergetic and redox levels. FGF21 contributed to preparing the brain for situations of high availability of L-lactate, thus preventing neuronal vulnerability in metabolic reprogramming.

Extracellular lactate as an alternative energy source for retinal bipolar cells

Retinal bipolar and amacrine cells receive visual information from photoreceptors and participate in the first steps of image processing in the retina. Several studies have suggested the operation of aerobic glycolysis and a lactate shuttle system in the retina due to the high production of this metabolite under aerobic conditions. However, whether bipolar cells form part of this metabolic circuit remains unclear. Here, we show that the monocarboxylate transporter 2 is expressed and functional in inner retinal neurons. Additionally, we used genetically encoded FRET nanosensors to demonstrate the ability of inner retinal neurons to consume extracellular lactate as an alternative to glucose. In rod bipolar cells, lactate consumption allowed cells to maintain the homeostasis of ions and electrical responses. We also found that lactate synthesis and transporter inhibition caused functional alterations and an increased rate of cell death. Overall, our data shed light on a notable but still poorly understood aspect of retinal metabolism.

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.

Differential Effects of Chronic Ethanol Use on Mouse Neuronal and Astroglial Metabolic Activity

Chronic alcohol use disorder, a major risk factor for the development of neuropsychiatric disorders including addiction to other substances, is associated with several neuropathology including perturbed neuronal and glial activities in the brain. It affects carbon metabolism in specific brain regions, and perturbs neuro-metabolite homeostasis in neuronal and glial cells. Alcohol induced changes in the brain neurochemical profile accompany the negative emotional state associated with dysregulated reward and sensitized stress response to withdrawal. However, the underlying alterations in neuro-astroglial activities and neurochemical dysregulations in brain regions after chronic alcohol use are poorly understood. This study evaluates the impact of chronic ethanol use on the regional neuro-astroglial metabolic activity using ¹H-[¹³C]-NMR spectroscopy in conjunction with infusion of [1,6-¹³C₂]glucose and sodium [2-¹³C]acetate, respectively, after 48 h of abstinence. Besides establishing detailed ¹³C labeling of neuro-metabolites in a brain region-specific manner, our results show chronic ethanol induced-cognitive deficits along with a reduction in total glucose oxidation rates in the hippocampus and striatum. Furthermore, using [2-¹³C]acetate infusion, we showed an alcohol-induced increase in astroglial metabolic activity in the hippocampus and prefrontal cortex. Interestingly, increased astroglia activity in the hippocampus and prefrontal cortex was associated with a differential expression of monocarboxylic acid transporters that are regulating acetate uptake and metabolism in the brain.

Lithium: A Promising Anticancer Agent

Lithium is a therapeutic cation used to treat bipolar disorders but also has some important features as an anti-cancer agent. In this review, we provide a general overview of lithium, from its transport into cells, to its innovative administration forms, and based on genomic, transcriptomic, and proteomic data. Lithium formulations such as lithium acetoacetate (LiAcAc), lithium chloride (LiCl), lithium citrate (Li3C6H5O7), and lithium carbonate (Li2CO3) induce apoptosis, autophagy, and inhibition of tumor growth and also participate in the regulation of tumor proliferation, tumor invasion, and metastasis and cell cycle arrest. Moreover, lithium is synergistic with standard cancer therapies, enhancing their anti-tumor effects. In addition, lithium has a neuroprotective role in cancer patients, by improving their quality of life. Interestingly, nano-sized lithium enhances its anti-tumor activities and protects vital organs from the damage caused by lipid peroxidation during tumor development. However, these potential therapeutic activities of lithium depend on various factors, such as the nature and aggressiveness of the tumor, the type of lithium salt, and its form of administration and dosage. Since lithium has been used to treat bipolar disorder, the current study provides an overview of its role in medicine and how this has changed. This review also highlights the importance of this repurposed drug, which appears to have therapeutic cancer potential, and underlines its molecular mechanisms.

Imaging of lactate metabolism in retinal Müller cells with a FRET nanosensor

Müller cells, the glial cells of the retina, provide metabolic support for photoreceptors and inner retinal neurons, and have been proposed as source of the significant lactate production of this tissue. To better understand the role of lactate in retinal metabolism, we expressed a lactate and a glucose nanosensor in organotypic mouse retinal explants cultured for 14 days, and used FRET imaging in acute vibratome sections of the explants to study metabolite flux in real time. Pharmacological manipulation with specific monocarboxylate transporter (MCT) inhibitors and immunohistochemistry revealed the functional expression of MCT1, MCT2 and MCT4 in Müller cells of retinal explants. The introduction of FRET nanosensors to measure key metabolites at the cellular level may contribute to a better understanding of heretofore poorly understood issues in retinal metabolism.

Activation of lactate receptor HCAR1 down-modulates neuronal activity in rodent and human brain tissue

Lactate can be used by neurons as an energy substrate to support their activity. Evidence suggests that lactate also acts on a metabotropic receptor called HCAR1, first described in the adipose tissue. Whether HCAR1 also modulates neuronal circuits remains unclear. In this study, using qRT-PCR, we show that HCAR1 is present in the human brain of epileptic patients who underwent resective surgery. In brain slices from these patients, pharmacological HCAR1 activation using a non-metabolized agonist decreased the frequency of both spontaneous neuronal Ca²⁺ spiking and excitatory post-synaptic currents (sEPSCs). In mouse brains, we found HCAR1 expression in different regions using a fluorescent reporter mouse line and in situ hybridization. In the dentate gyrus, HCAR1 is mainly present in mossy cells, key players in the hippocampal excitatory circuitry and known to be involved in temporal lobe epilepsy. By using whole-cell patch clamp recordings in mouse and rat slices, we found that HCAR1 activation causes a decrease in excitability, sEPSCs, and miniature EPSCs frequency of granule cells, the main output of mossy cells. Overall, we propose that lactate can be considered a neuromodulator decreasing synaptic activity in human and rodent brains, which makes HCAR1 an attractive target for the treatment of epilepsy.

Whole-brain neuronal MCT2 lactate transporter expression links metabolism to human brain structure and function

Brain activity is constrained by local availability of chemical energy, which is generated through compartmentalized metabolic processes. By analyzing data of whole human brain gene expression, we characterize the spatial distribution of seven glucose and monocarboxylate membrane transporters that mediate astrocyte–neuron lactate shuttle transfer of energy. We found that the gene coding for neuronal MCT2 is the only gene enriched in cerebral cortex where its abundance is inversely correlated with cortical thickness. Coexpression network analysis revealed that MCT2 was the only gene participating in an organized gene cluster enriched in K+ dynamics. Indeed, the expression of KATP subunits, which mediate lactate increases with spiking activity, is spatially coupled to MCT2 distribution. Notably, MCT2 expression correlated with fluorodeoxyglucose positron emission tomography task-dependent glucose utilization. Finally, the MCT2 messenger RNA gradient closely overlaps with functional MRI brain regions associated with attention, arousal, and stress. Our results highlight neuronal MCT2 lactate transporter as a key component of the cross-talk between astrocytes and neurons and a link between metabolism, cortical structure, and state-dependent brain function.

Effects of Fibroblast Growth Factor 21 on Lactate Uptake and Usage in Mice with Diabetes-Associated Cognitive Decline

Fibroblast growth factor 21 (FGF21) is an endocrine hormone that exerts beneficial effects on glucose and lipid metabolic homeostasis. However, the impact of FGF21 on type 1 diabetes-associated cognitive decline (DACD) and its mechanisms of action remain unclear. In this study, we aimed to evaluate the effects of FGF21 on lactate uptake and usage in a mouse model of streptozotocin-induced DACD. Six-week-old male C57BL/6 mice were divided into the control, diabetic, and FGF21 (which received 2 mg/kg recombinant human FGF21) groups. At the end of the treatment period, learning and memory performance, nuclear magnetic resonance-based metabonomics, and expressions of various hippocampal protein were analyzed to determine the efficacy of FGF21. The results showed that compared to the control mice, the diabetic mice had reduced long-term memory performance after the hyperglycemic insult; decreased hippocampal levels of lactate dehydrogenase-B (LDH-B) activity, bioenergy metabolites, and monocarboxylate transporter 2 (MCT2); and increased lactate levels. Impaired phosphoinositide 3-kinase (PI3K) signaling was also observed in the diabetic mice. However, FGF21 treatment improved LDH-B activity, β-nicotinamide adenine dinucleotide, and ATP levels, and increased MCT2 expression and PI3K signaling pathway, which in turn improved the learning and memory defects. These findings demonstrated that the effects of FGF21 on DACD were associated with its ability to improve LDH-B-mediated lactate usage and MCT2-dependent lactate uptake. Further, these beneficial effects of FGF21 in the hippocampus were mediated by the PI3K signaling pathways.

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

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

Environmental Enrichment and Estrogen Upregulate Beta-Hydroxybutyrate Underlying Functional Improvement

Environmental enrichment (EE) is a promising therapeutic strategy in improving metabolic and neuronal responses, especially due to its non-invasive nature. However, the exact mechanism underlying the sex-differential effects remains unclear. The aim of the current study was to investigate the effects of EE on metabolism, body composition, and behavioral phenotype based on sex. Long-term exposure to EE for 8 weeks induced metabolic changes and fat reduction. In response to the change in metabolism, the level of βHB were influenced by sex and EE possibly in accordance to the phases of estrogen cycle. The expression of β-hydroxybutyrate (βHB)-related genes and proteins such as monocarboxylate transporters, histone deacetylases (HDAC), and brain-derived neurotrophic factor (BDNF) were significantly regulated. In cerebral cortex and hippocampus, EE resulted in a significant increase in the level of βHB and a significant reduction in HDAC, consequently enhancing BDNF expression. Moreover, EE exerted significant effects on motor and cognitive behaviors, indicating a significant functional improvement in female mice under the condition that asserts the influence of estrogen cycle. Using an ovariectomized mice model, the effects of EE and estrogen treatment proved the hypothesis that EE upregulates β-hydroxybutyrate and BDNF underlying functional improvement in female mice. The above findings demonstrate that long-term exposure to EE can possibly alter metabolism by increasing the level of βHB, regulate the expression of βHB-related proteins, and improve behavioral function as reflected by motor and cognitive presentation following the changes in estrogen level. This finding may lead to a marked improvement in metabolism and neuroplasticity by EE and estrogen level.

Brain Metabolic Alterations in Alzheimer's Disease

The brain is one of the most energy-consuming organs in the body. Satisfying such energy demand requires compartmentalized, cell-specific metabolic processes, known to be complementary and intimately coupled. Thus, the brain relies on thoroughly orchestrated energy-obtaining agents, processes and molecular features, such as the neurovascular unit, the astrocyte-neuron metabolic coupling, and the cellular distribution of energy substrate transporters. Importantly, early features of the aging process are determined by the progressive perturbation of certain processes responsible for adequate brain energy supply, resulting in brain hypometabolism. These age-related brain energy alterations are further worsened during the prodromal stages of neurodegenerative diseases, namely Alzheimer's disease (AD), preceding the onset of clinical symptoms, and are anatomically and functionally associated with the loss of cognitive abilities. Here, we focus on concrete neuroenergetic features such as the brain's fueling by glucose and lactate, the transporters and vascular system guaranteeing its supply, and the metabolic interactions between astrocytes and neurons, and on its neurodegenerative-related disruption. We sought to review the principles underlying the metabolic dimension of healthy and AD brains, and suggest that the integration of these concepts in the preventive, diagnostic and treatment strategies for AD is key to improving the precision of these interventions.

Lactate transporters in the rat barrel cortex sustain whisker-dependent BOLD fMRI signal and behavioral performance

Lactate is an efficient neuronal energy source, even in presence of glucose. However, the importance of lactate shuttling between astrocytes and neurons for brain activation and function remains to be established. For this purpose, metabolic and hemodynamic responses to sensory stimulation have been measured by functional magnetic resonance spectroscopy and blood oxygen level-dependent (BOLD) fMRI after down-regulation of either neuronal MCT2 or astroglial MCT4 in the rat barrel cortex. Results show that the lactate rise in the barrel cortex upon whisker stimulation is abolished when either transporter is down-regulated. Under the same paradigm, the BOLD response is prevented in all MCT2 down-regulated rats, while about half of the MCT4 down-regulated rats exhibited a loss of the BOLD response. Interestingly, MCT4 down-regulated animals showing no BOLD response were rescued by peripheral lactate infusion, while this treatment had no effect on MCT2 down-regulated rats. When animals were tested in a novel object recognition task, MCT2 down-regulated animals were impaired in the textured but not in the visual version of the task. For MCT4 down-regulated animals, while all animal succeeded in the visual task, half of them exhibited a deficit in the textured task, a similar segregation into two groups as observed for BOLD experiments. Our data demonstrate that lactate shuttling between astrocytes and neurons is essential to give rise to both neurometabolic and neurovascular couplings, which form the basis for the detection of brain activation by functional brain imaging techniques. Moreover, our results establish that this metabolic cooperation is required to sustain behavioral performance based on cortical activation.

Visual analysis on the research of monocarboxylate transporters based on CiteSpace

BACKGROUND

Monocarboxylate transports (MCTs), a family of solute carrier protein, play an important role in maintenance of cellular stability in tumor cells by mediating lactate exchange across membranes. The objective of this paper is to evaluate the knowledge structure, development trend, and research hotspot of MCTs research field systematically and comprehensively.

METHODS

Based on the 1526 publications from 2010 to 2020 retrieved from "Web of Science Core Collection" (WoSCC), we visually analyzed the MCTs research in terms of subject category, scientific collaboration network, keywords, and high-frequency literature using CiteSpace.

RESULTS

The number of publications exhibits an upward trend from 2010 to 2020 and the top 5 countries in the MCTs research were the United States, China, Japan, Germany, and England. Visser TJ was the most prolific author, while Halestrap AP was the most influential author with the highest citations. Analysis of the 7 cluster units from the co-cited references and keywords revealed that high expression of MCTs induced by oxidative stress and glycolysis was the pivotal point in the MCTs research field, while regulation of metabolism in tumor microenvironment, prognostic markers of cancer, and targeted inhibitors are the top 3 research frontiers topics.

CONCLUSION

This study will help the new researcher to understand the MCTs related field, master the research frontier, and obtain valuable scientific information, thus providing directions for follow-up research.

Oxidative Stress and the Neurovascular Unit

The neurovascular unit (NVU) is a relatively recent concept that clearly describes the relationship between brain cells and their blood vessels. The components of the NVU, comprising different types of cells, are so interrelated and associated with each other that they are considered as a single functioning unit. For this reason, even slight disturbances in the NVU could severely affect brain homeostasis and health. In this review, we aim to describe the current state of knowledge concerning the role of oxidative stress on the neurovascular unit and the role of a single cell type in the NVU crosstalk.

Molecular mechanisms of brain water transport

Our brains consist of 80% water, which is continuously shifted between different compartments and cell types during physiological and pathophysiological processes. Disturbances in brain water homeostasis occur with pathologies such as brain oedema and hydrocephalus, in which fluid accumulation leads to elevated intracranial pressure. Targeted pharmacological treatments do not exist for these conditions owing to our incomplete understanding of the molecular mechanisms governing brain water transport. Historically, the transmembrane movement of brain water was assumed to occur as passive movement of water along the osmotic gradient, greatly accelerated by water channels termed aquaporins. Although aquaporins govern the majority of fluid handling in the kidney, they do not suffice to explain the overall brain water movement: either they are not present in the membranes across which water flows or they appear not to be required for the observed flow of water. Notably, brain fluid can be secreted against an osmotic gradient, suggesting that conventional osmotic water flow may not describe all transmembrane fluid transport in the brain. The cotransport of water is an unconventional molecular mechanism that is introduced in this Review as a missing link to bridge the gap in our understanding of cellular and barrier brain water transport.

Neuroprotective role of lactate in rat neonatal hypoxia-ischemia

Hypoxic-ischemic (HI) encephalopathy remains a major cause of perinatal mortality and chronic disability in newborns worldwide (1-6 for 1000 births). The only current clinical treatment is hypothermia, which is efficient for less than 60% of babies. Mainly considered as a waste product in the past, lactate, in addition to glucose, is increasingly admitted as a supplementary fuel for neurons and, more recently, as a signaling molecule in the brain. Our aim was to investigate the neuroprotective effect of lactate in a neonatal (seven day old) rat model of hypoxia-ischemia. Pups received intra-peritoneal injection(s) of lactate (40 μmol). Size and apparent diffusion coefficients of brain lesions were assessed by magnetic resonance diffusion-weighted imaging. Oxiblot analyses and long-term behavioral studies were also conducted. A single lactate injection induced a 30% reduction in brain lesion volume, indicating a rapid and efficient neuroprotective effect. When oxamate, a lactate dehydrogenase inhibitor, was co-injected with lactate, the neuroprotection was completely abolished, highlighting the role of lactate metabolism in this protection. After three lactate injections (one per day), pups presented the smallest brain lesion volume and a complete recovery of neurological reflexes, sensorimotor capacities and long-term memory, demonstrating that lactate administration is a promising therapy for neonatal HI insult.

From Obesity to Hippocampal Neurodegeneration: Pathogenesis and Non-Pharmacological Interventions

High-caloric diet and physical inactivity predispose individuals to obesity and diabetes, which are risk factors of hippocampal neurodegeneration and cognitive deficits. Along with the adipose-hippocampus crosstalk, chronically inflamed adipose tissue secretes inflammatory cytokine could trigger neuroinflammatory responses in the hippocampus, and in turn, impairs hippocampal neuroplasticity under obese and diabetic conditions. Hence, caloric restriction and physical exercise are critical non-pharmacological interventions to halt the pathogenesis from obesity to hippocampal neurodegeneration. In response to physical exercise, peripheral organs, including the adipose tissue, skeletal muscles, and liver, can secret numerous exerkines, which bring beneficial effects to metabolic and brain health. In this review, we summarized how chronic inflammation in adipose tissue could trigger neuroinflammation and hippocampal impairment, which potentially contribute to cognitive deficits in obese and diabetic conditions. We also discussed the potential mechanisms underlying the neurotrophic and neuroprotective effects of caloric restriction and physical exercise by counteracting neuroinflammation, plasticity deficits, and cognitive impairments. This review provides timely insights into how chronic metabolic disorders, like obesity, could impair brain health and cognitive functions in later life.

Effects of Ketone Bodies on Brain Metabolism and Function in Neurodegenerative Diseases

Under normal physiological conditions the brain primarily utilizes glucose for ATP generation. However, in situations where glucose is sparse, e.g., during prolonged fasting, ketone bodies become an important energy source for the brain. The brain’s utilization of ketones seems to depend mainly on the concentration in the blood, thus many dietary approaches such as ketogenic diets, ingestion of ketogenic medium-chain fatty acids or exogenous ketones, facilitate significant changes in the brain’s metabolism. Therefore, these approaches may ameliorate the energy crisis in neurodegenerative diseases, which are characterized by a deterioration of the brain’s glucose metabolism, providing a therapeutic advantage in these diseases. Most clinical studies examining the neuroprotective role of ketone bodies have been conducted in patients with Alzheimer’s disease, where brain imaging studies support the notion of enhancing brain energy metabolism with ketones. Likewise, a few studies show modest functional improvements in patients with Parkinson’s disease and cognitive benefits in patients with—or at risk of—Alzheimer’s disease after ketogenic interventions. Here, we summarize current knowledge on how ketogenic interventions support brain metabolism and discuss the therapeutic role of ketones in neurodegenerative disease, emphasizing clinical data.

Urinary ketone body loss leads to degeneration of brain white matter in elderly SLC5A8-deficient mice

SLC5A8 is a sodium-coupled monocarboxylate and ketone transporter expressed in various epithelial cells. A putative role of SLC5A8 in neuroenergetics has been also hypothesized. To clarify this issue, we studied the cerebral phenotype of SLC5A8-deficient mice during aging. Elderly SLC5A8-deficient mice presented diffuse leukoencephalopathy characterized by intramyelinic oedema without demyelination suggesting chronic energetic crisis. Hypo-metabolism in the white matter of elderly SLC5A8-deficient mice was found using ^99mTc-hexamethylpropyleneamine oxime (HMPAO) single-photon emission CT (SPECT). Since the SLC5A8 protein could not be detected in the mouse brain, it was hypothesized that the leukoencephalopathy of aging SLC5A8-deficient mice was caused by the absence of slc5a8 expression in a peripheral organ, i.e. the kidney, where SLC5A8 is strongly expressed. A hyper-excretion of the ketone β-hydroxybutyrate (BHB) in the urine of SLC5A8-deficient mice was observed and showed that SLC5A8-deficient mice suffered a cerebral BHB insufficiency. Elderly SLC5A8-deficient mice also presented altered glucose metabolism. We propose that the continuous renal loss of BHB leads to a chronic energetic deficiency in the brain of elderly SLC5A8-deficient mice who are unable to counterbalance their glucose deficit. This study highlights the importance of alternative energetic substrates in neuroenergetics especially under conditions of restricted glucose availability.

Disrupted function of lactate transporter MCT1, but not MCT4, in Schwann cells affects the maintenance of motor end-plate innervation

Recent studies in neuron-glial metabolic coupling have shown that, in the CNS, astrocytes and oligodendrocytes support neurons with energy-rich lactate/pyruvate via monocarboxylate transporters (MCTs). The presence of such transporters in the PNS, in both Schwann cells and neurons, has prompted us to question if a similar interaction may be present. Here we describe the generation and characterization of conditional knockout mouse models where MCT1 or MCT4 is specifically deleted in Schwann cells (named MCT1 and MCT4 cKO). We show that MCT1 cKO and MCT4 cKO mice develop normally and that myelin in the PNS is preserved. However, MCT1 expressed by Schwann cells is necessary for long-term maintenance of motor end-plate integrity as revealed by disrupted neuromuscular innervation in mutant mice, while MCT4 appears largely dispensable for the support of motor neurons. Concomitant to detected structural alterations, lumbar motor neurons from MCT1 cKO mice show transcriptional changes affecting cytoskeletal components, transcriptional regulators, and mitochondria related transcripts, among others. Together, our data indicate that MCT1 plays a role in Schwann cell-mediated maintenance of motor end-plate innervation thus providing further insight into the emerging picture of the biology of the axon-glia metabolic crosstalk.

Glucose metabolism links astroglial mitochondria to cannabinoid effects

Astrocytes take up glucose from the bloodstream to provide energy to the brain, thereby allowing neuronal activity and behavioural responses^1-5. By contrast, astrocytes are under neuronal control through specific neurotransmitter receptors^5-7. However, whether the activation of astroglial receptors can directly regulate cellular glucose metabolism to eventually modulate behavioural responses is unclear. Here we show that activation of mouse astroglial type-1 cannabinoid receptors associated with mitochondrial membranes (mtCB₁) hampers the metabolism of glucose and the production of lactate in the brain, resulting in altered neuronal functions and, in turn, impaired behavioural responses in social interaction assays. Specifically, activation of astroglial mtCB₁ receptors reduces the phosphorylation of the mitochondrial complex I subunit NDUFS4, which decreases the stability and activity of complex I. This leads to a reduction in the generation of reactive oxygen species by astrocytes and affects the glycolytic production of lactate through the hypoxia-inducible factor 1 pathway, eventually resulting in neuronal redox stress and impairment of behavioural responses in social interaction assays. Genetic and pharmacological correction of each of these effects abolishes the effect of cannabinoid treatment on the observed behaviour. These findings suggest that mtCB₁ receptor signalling can directly regulate astroglial glucose metabolism to fine-tune neuronal activity and behaviour in mice.

MCT2 overexpression rescues metabolic vulnerability and protects retinal ganglion cells in two models of glaucoma

Improving cellular access to energy substrates is one strategy to overcome observed declines in energy production and utilization in the aged and pathologic central nervous system. Monocarboxylate transporters (MCTs), the movers of lactate, pyruvate, and ketone bodies into or out of a cell, are significantly decreased in the DBA/2 J mouse model of glaucoma. In order to confirm MCT decreases are disease-associated, we decreased MCT2 in the retinas of MCT2^fl/+ mice using an injection of AAV2-cre, observing significant decline in ATP production and visual evoked potential. Restoring MCT2 levels in retinal ganglion cells (RGCs) via intraocular injection of AAV2-GFP-MCT2 in two models of glaucoma, the DBA/2 J (D2), and a magnetic bead model of ocular hypertension (OHT), preserved RGCs and their function. Viral-mediated overexpression of MCT2 increased RGC density and axon number, reduced energy imbalance, and increased mitochondrial function as measured by cytochrome c oxidase and succinate dehydrogenase activity in both models of glaucoma. Ocular hypertensive mice injected with AAV2:MCT2 had significantly greater P1 amplitude as measured by pattern electroretinogram than mice with OHT alone. These findings indicate overexpression of MCT2 improves energy homeostasis in the glaucomatous visual system, suggesting that expanding energy input options for cells is a viable option to combat neurodegeneration.

Astrocytes and neurons communicate via a monocarboxylic acid shuttle

Since formulation of the Astrocyte-Neuron Lactate Shuttle (ANLS) hypothesis in 1994, the hypothesis has provoked criticism and debate. Our review does not criticise, but rather integrates experimental data characterizing proton-linked monocarboxylate transporters (MCTs) into the ANLS. MCTs have wide substrate specificity and are discussed to be in protein complex with a proton donor (PD). We particularly focus on the proton-driven transfer of l-lactic acid (l-lacH) and pyruvic acid (pyrH), were PDs link MCTs to a flow of energy. The precise nature of the PD predicts the activity and catalytic direction of MCTs. By doing so, we postulate that the MCT4·phosphoglycerate kinase complex exports and at the same time in the same astrocyte, MCT1·carbonic anhydrase II complex imports monocarboxylic acids. Similarly, neuronal MCT2 preferentially imports pyrH. The repertoire of MCTs in astrocytes and neurons allows them to communicate via monocarboxylic acids. A change in imported pyrH/l-lacH ratio in favour of l-lacH encodes signals stabilizing the transit of glucose from astrocytes to neurons. The presented astrocyte neuron communication hypothesis has the potential to unite the community by suggesting that the exchange of monocarboxylic acids paves the path of glucose provision.

Lactate and BDNF: Key Mediators of Exercise Induced Neuroplasticity?

Accumulating evidence from animal and human studies supports the notion that physical exercise can enhance neuroplasticity and thus reduce the risk of several neurodegenerative diseases (e.g., dementia). However, the underlying neurobiological mechanisms of exercise induced neuroplasticity are still largely unknown. One potential mediator of exercise effects is the neurotrophin BDNF, which enhances neuroplasticity via different pathways (e.g., synaptogenesis, neurogenesis, long-term potentiation). Current research has shown that (i) increased peripheral lactate levels (following high intensity exercise) are associated with increased peripheral BDNF levels, (ii) lactate infusion at rest can increase peripheral and central BDNF levels and (iii) lactate plays a very complex role in the brain’s metabolism. In this review, we summarize the role and relationship of lactate and BDNF in exercise induced neuroplasticity.

Metabolic compartmentalization between astroglia and neurons in physiological and pathophysiological conditions of the neurovascular unit

Astroglia or astrocytes, the most abundant cells in the brain, are interposed between neuronal synapses and microvasculature in the brain gray matter. They play a pivotal role in brain metabolism as well as in the regulation of cerebral blood flow, taking advantage of their unique anatomical location. In particular, the astroglial cellular metabolic compartment exerts supportive roles in dedicating neurons to the generation of action potentials and protects them against oxidative stress associated with their high energy consumption. An impairment of normal astroglial function, therefore, can lead to numerous neurological disorders including stroke, neurodegenerative diseases, and neuroimmunological diseases, in which metabolic derangements accelerate neuronal damage. The neurovascular unit (NVU), the major components of which include neurons, microvessels, and astroglia, is a conceptual framework that was originally used to better understand the pathophysiology of cerebral ischemia. At present, the NVU is a tool for understanding normal brain physiology as well as the pathophysiology of numerous neurological disorders. The metabolic responses of astroglia in the NVU can be either protective or deleterious. This review focuses on three major metabolic compartments: (i) glucose and lactate; (ii) fatty acid and ketone bodies; and (iii) D- and L-serine. Both the beneficial and the detrimental roles of compartmentalization between neurons and astroglia will be discussed. A better understanding of the astroglial metabolic response in the NVU is expected to lead to the development of novel therapeutic strategies for diverse neurological diseases.

Identification of novel common variants associated with chronic pain using conditional false discovery rate analysis with major depressive disorder and assessment of pleiotropic effects of LRFN5

Chronic pain is a complex trait that is moderately heritable and genetically, as well as phenotypically, correlated with major depressive disorder (MDD). Use of the conditional false discovery rate (cFDR) approach, which leverages pleiotropy identified from existing GWAS outputs, has been successful in discovering novel associated variants in related phenotypes. Here, genome-wide association study outputs for both von Korff chronic pain grade and for MDD were used to identify variants meeting a cFDR threshold for each outcome phenotype separately, as well as a conjunctional cFDR (ccFDR) threshold for both phenotypes together. Using a moderately conservative threshold, we identified a total of 11 novel single nucleotide polymorphisms (SNPs), six of which were associated with chronic pain grade and nine of which were associated with MDD. Four SNPs on chromosome 14 were associated with both chronic pain grade and MDD. SNPs associated only with chronic pain grade were located within SLC16A7 on chromosome 12. SNPs associated only with MDD were located either in a gene-dense region on chromosome 1 harbouring LINC01360, LRRIQ3, FPGT and FPGT-TNNI3K, or within/close to LRFN5 on chromosome 14. The SNPs associated with both outcomes were also located within LRFN5. Several of the SNPs on chromosomes 1 and 14 were identified as being associated with expression levels of nearby genes in the brain and central nervous system. Overall, using the cFDR approach, we identified several novel genetic loci associated with chronic pain and we describe likely pleiotropic effects of a recently identified MDD locus on chronic pain.

The distribution and density of monocarboxylate transporter 2 in cerebral cortex, hippocampus and cerebellum of wild-type mice

Monocarboxylates cannot cross the blood-brain barrier freely to participate in brain energy metabolism. Specific monocarboxylate transporters (MCTs) are needed to cross cellular membranes. Monocarboxylate transporter 2 (MCT2) is a major monocarboxylate transporter encoded by the SLC16A7 gene. Recent studies reported that neurodegenerative diseases of the CNS, such as Alzheimer's disease (AD) and Parkinson's disease (PD), were related to energy metabolic impairment. MCT2 also plays an important role in energy metabolism in the CNS. To provide experimental evidence for future research on the role of MCT2 in the pathological process of CNS degenerative diseases, the distribution and density of MCT2 in different subregions of wild-type mouse brain was examined using immunohistochemistry, western blot and immunogold post-embedding electron microscopic techniques. The amount of MCT2 was higher in cerebellum than in cortex and hippocampus on western blots, and there was no statistical difference between cortex and hippocampus. Immunohistochemistry assay revealed the highest density of MCT2 in the CA3 of the hippocampus. The granular cell layer of the cerebellum contained more MCT2 than the molecular layer. The MCT2 density on the end feet of astrocytes of molecular layer was lower than in hippocampus, but the postsynaptic densities (PSDs) of asymmetric synapses in the molecular layer exhibited a high density using immunogold post-embedding electron microscopic techniques.

Lactate Deficit in an Alzheimer Disease Mouse Model: The Relationship With Neuronal Damage

Cerebral energy metabolism in Alzheimer disease (AD) has recently been given increasing attention. This study focuses on the alterations of cerebral lactate metabolism in the double-transgenic amyloid precursor protein/presenilin 1 (APP/PS1) mouse model of AD. Immunofluorescence staining and Western blotting analysis were used to identify the alterations of lactate content and lactate transporters (MCT1, MCT2, MCT4) in APP/PS1 mouse brains, which display amyloid beta plaques, reduced amounts of neurons and oligodendrocytes, and increased quantity of astrocytes. We found that lactate content and expressions of cerebral MCT1, MCT2, and MCT4 were decreased in APP/PS1 mice. In particular, lactate dehydrogenase A (LDHA) and B (LDHB) were reduced in neurons with increased ratios of LDHA and LDHB. This study suggests that the decreases of cerebral lactate content and lactate transporters may lead to the blockage of lactate transport from glia to neurons, resulting in neuronal lactate deficit. The increased ratio of neuronal LDHA and LDHB may represent a reaction of neurons to lactate deficit, although it cannot reverse the energy deficiency in neurons.

Role of Astrocytic Mitochondria in Limiting Ischemic Brain Injury?

Until recently, astrocyte processes were thought to be too small to contain mitochondria. However, it is now clear that mitochondria are found throughout fine astrocyte processes and are mobile with neuronal activity resulting in positioning near synapses. In this review, we discuss evidence that astrocytic mitochondria confer selective resiliency to astrocytes during ischemic insults and the functional significance of these mitochondria for normal brain function.

Lack of evidence for synaptic high-affinity γ-hydroxybutyric acid (GHB) transport in rat brain synaptosomes and 11 Na+ -dependent SLC neurotransmitter transporters

γ-Hydroxybutyric acid (GHB) is an endogenous compound proposed to act as a neurotransmitter. Na⁺ -dependent, high-affinity GHB transport has long been considered important evidence supporting this hypothesis. However, the molecular identity of such a high-affinity transporter remains unknown. In this study, we sought to identify and characterize GHB synaptic transport through a series of studies using both native and recombinant systems with the ultimate aim of providing evidence to clarify the proposed role of GHB as a neurotransmitter in the mammalian brain. Native [³ H]GHB transport was studied in isolated rat brain synaptosomes and compared to synaptic membranes. As a targeted approach, GHB was also screened against a panel of Na⁺ -dependent SLC6 neurotransmitter transporters recombinantly expressed in Xenopus laevis oocytes or tsA201 cells. Finally, the low-affinity GHB transporters, MCT1/2 and SMCT1, were probed as GHB transporters in L-[¹⁴ C]lactate uptake assays in synaptosomes. We found no evidence of high-affinity [³ H]GHB transport in purified rat brain cortical or striatal synaptosomes or at any of the 11 SLC6 transporters tested. Instead, our results indicate the binding of [³ H]GHB to an unidentified membrane component, distinct from any of the known GHB targets. In accordance with others, we found that GHB and the analog 3-hydroxycyclopent-1-enecarboxylic acid (HOCPCA) can, in millimolar concentrations, inhibit L-[¹⁴ C]lactate uptake at MCT1 and/or MCT2 and that this also can occur in synaptosomes. In conclusion, through a variety of in vitro pharmacological studies, we were unsuccessful in identifying a specific synaptic high-affinity transporter for GHB. Our findings emphasize the need to reevaluate GHB's role as a potential neurotransmitter. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Neuroprotective effect of rLosac on supplement-deprived mouse cultured cortical neurons involves maintenance of monocarboxylate transporter MCT2 protein levels

The recombinant Lonomia obliqua Stuart-factor activator (rLosac) is a recombinant hemolin which belongs to the immunoglobulin superfamily of cell adhesion molecules. It is capable of inducing pro-survival activity in serum-deprived human umbilical vein endothelial cells (HUVECs) and fibroblasts by increasing mitochondrial metabolism. We hypothesize that it could promote neuronal survival by acting on neuroenergetics. Our study reveals that treatment of primary mouse cortical neurons cultured in neurobasal medium lacking B27 supplement with rLosac led to an enhancement of cell viability in a time- and concentration-dependent manner. In parallel, preserved or enhanced phosphorylation of Akt, p44, and p42 MAPK, as well as mTOR was observed following treatment with rLosac. During deprivation, as assessed by western blot and qRT-PCR, protein and mRNA expression of MCT2 (the predominant neuronal monocarboxylate transporter allowing lactate use as an alternative energy substrate) decreased significantly in B27 supplement-deprived cortical neurons and was hardly detected after 24 h of deprivation. Interestingly, rLosac maintained MCT2 protein expression after 24 h of deprivation including at the cell surface without preventing mRNA loss. MCT2 knockdown reduced rLosac-enhanced cell viability, confirming its involvement in rLosac effect. Enhanced uptake of lactate was detected following rLosac treatment and might contribute to rLosac-enhanced viability during deprivation. In the presence of both lactate and rLosac, cell viability was higher than in the presence of lactate alone. Our observations suggest that rLosac promotes cell viability in stressed (B27 supplement-deprived) neurons by facilitating the use of lactate as energy substrate via the preservation of MCT2 protein expression. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Lactic Acid: No Longer an Inert and End-Product of Glycolysis

For decades, lactic acid has been considered a dead-end product of glycolysis. Research in the last 20+ years has shown otherwise. Through its transporters (MCTs) and receptor (GPR81), lactic acid plays a key role in multiple cellular processes, including energy regulation, immune tolerance, memory formation, wound healing, ischemic tissue injury, and cancer growth and metastasis. We summarize key findings of lactic acid signaling, functions, and many remaining questions.

Current technical approaches to brain energy metabolism

Neuroscience is a technology-driven discipline and brain energy metabolism is no exception. Once satisfied with mapping metabolic pathways at organ level, we are now looking to learn what it is exactly that metabolic enzymes and transporters do and when, where do they reside, how are they regulated, and how do they relate to the specific functions of neurons, glial cells, and their subcellular domains and organelles, in different areas of the brain. Moreover, we aim to quantify the fluxes of metabolites within and between cells. Energy metabolism is not just a necessity for proper cell function and viability but plays specific roles in higher brain functions such as memory processing and behavior, whose mechanisms need to be understood at all hierarchical levels, from isolated proteins to whole subjects, in both health and disease. To this aim, the field takes advantage of diverse disciplines including anatomy, histology, physiology, biochemistry, bioenergetics, cellular biology, molecular biology, developmental biology, neurology, and mathematical modeling. This article presents a well-referenced synopsis of the technical side of brain energy metabolism research. Detail and jargon are avoided whenever possible and emphasis is given to comparative strengths, limitations, and weaknesses, information that is often not available in regular articles.

Structural and Functional Rescue of Chronic Metabolically Stressed Optic Nerves through Respiration

Axon degeneration can arise from metabolic stress, potentially a result of mitochondrial dysfunction or lack of appropriate substrate input. In this study, we investigated whether the metabolic vulnerability observed during optic neuropathy in the DBA/2J (D2) model of glaucoma is due to dysfunctional mitochondria or impaired substrate delivery to axons, the latter based on our observation of significantly decreased glucose and monocarboxylate transporters in D2 optic nerve (ON), human ON, and mice subjected to acute glaucoma injury. We placed both sexes of D2 mice destined to develop glaucoma and mice of a control strain, the DBA/2J-Gpnmb⁺, on a ketogenic diet to encourage mitochondrial function. Eight weeks of the diet generated mitochondria, improved energy availability by reversing monocarboxylate transporter decline, reduced glial hypertrophy, protected retinal ganglion cells and their axons from degeneration, and maintained physiological signaling to the brain. A robust antioxidant response also accompanied the response to the diet. These results suggest that energy compromise and subsequent axon degeneration in the D2 is due to low substrate availability secondary to transporter downregulation.SIGNIFICANCE STATEMENT We show axons in glaucomatous optic nerve are energy depleted and exhibit chronic metabolic stress. Underlying the metabolic stress are low levels of glucose and monocarboxylate transporters that compromise axon metabolism by limiting substrate availability. Axonal metabolic decline was reversed by upregulating monocarboxylate transporters as a result of placing the animals on a ketogenic diet. Optic nerve mitochondria responded capably to the oxidative phosphorylation necessitated by the diet and showed increased number. These findings indicate that the source of metabolic challenge can occur upstream of mitochondrial dysfunction. Importantly, the intervention was successful despite the animals being on the cusp of significant glaucoma progression.

Activity-dependent astrocyte swelling is mediated by pH-regulating mechanisms

During neuronal activity in the mammalian brain, the K⁺ released into the synaptic space is initially buffered by the astrocytic compartment. In parallel, the extracellular space (ECS) shrinks, presumably due to astrocytic cell swelling. With the Na⁺ /K⁺ /2Cl^- cotransporter and the Kir4.1/AQP4 complex not required for the astrocytic cell swelling in the hippocampus, the molecular mechanisms underlying the activity-dependent ECS shrinkage have remained unresolved. To identify these molecular mechanisms, we employed ion-sensitive microelectrodes to measure changes in ECS, [K⁺ ]_o and [H⁺ ]_o /pH_o during electrical stimulation of rat hippocampal slices. Transporters and receptors responding directly to the K⁺ and glutamate released into the extracellular space (the K⁺ /Cl^- cotransporter, KCC, glutamate transporters and G protein-coupled receptors) did not modulate the extracellular space dynamics. The HCO3--transporting mechanism, which in astrocytes mainly constitutes the electrogenic Na⁺ / HCO3- cotransporter 1 (NBCe1), is activated by the K⁺ -mediated depolarization of the astrocytic membrane. Inhibition of this transporter reduced the ECS shrinkage by ∼25% without affecting the K⁺ transients, pointing to NBCe1 as a key contributor to the stimulus-induced astrocytic cell swelling. Inhibition of the monocarboxylate cotransporters (MCT), like-wise, reduced the ECS shrinkage by ∼25% without compromising the K⁺ transients. Isosmotic reduction of extracellular Cl^- revealed a requirement for this ion in parts of the ECS shrinkage. Taken together, the stimulus-evoked astrocytic cell swelling does not appear to occur as a direct effect of the K⁺ clearance, as earlier proposed, but partly via the pH-regulating transport mechanisms activated by the K⁺ -induced astrocytic depolarization and the activity-dependent metabolism.

Reduction of epileptiform activity in ketogenic mice: The role of monocarboxylate transporters

Epilepsy is a chronic neurological disorder that affects approximately 50 million people worldwide. Ketogenic diet (KD) can be a very effective treatment for intractable epilepsy. Potential mechanisms of action for KD have been proposed, including the re-balance among excitatory and inhibitory neurotransmission and decrease in the glycolytic rate in brain cells. KD has been shown to have an effect on the expression pattern of monocarboxylate transporters (MCT), however, it is unknown whether MCT transport activity is affected by KD and linked to the reduction of seizures during KD. Therefore, we studied the influence of KD on MCT transport activity and the role of MCTs during epileptiform activity. Our results showed a decrease in the epileptiform activity in cortical slices from mice fed on KD and in the presence of beta-hydroxybutyrate. KD increased transport capacity for ketone bodies and lactate in cortical astrocytes by raising the MCT1 expression level. Inhibition of MCT1 and MCT2 in control conditions decreases epileptiform activity, while in KD it induced an increase in epileptiform activity. Our results suggest that MCTs not only play an important role in the transport of ketone bodies, but also in the modulation of brain energy metabolism under normal and ketogenic conditions.

Acute liver failure is associated with altered cerebral expression profiles of long non-coding RNAs

Hepatic encephalopathy (HE) represents a serious complication of acute liver failure (ALF) in which cerebral edema leading to brainstem herniation as a result of increased intracranial hypertension is a major consequence. Long non-coding RNAs (lncRNAs) play a significant role in coordinating gene expression, with recent studies indicating an influence in the pathogenesis of several diseases. To investigate their involvement in the cerebral pathophysiology of ALF, we profiled the expression of lncRNAs in the frontal cortex of mice at coma stage following treatment with the hepatotoxin azoxymethane. Of the 35,923 lncRNAs profiled using microarrays, 868 transcripts were found to be differentially expressed in the ALF-treated group compared to the sham control group. Of these, 382 lncRNAs were upregulated and 486 lncRNAs downregulated. Pathway analysis revealed these lncRNAs target a number of biological and molecular pathways that include cytokine-cytokine receptor interaction, the mitogen activated protein kinase signaling pathway, the insulin signaling pathway, and the nuclear factor-κB signaling pathway. False discovery rate adjustment identified 9 upregulated lncRNAs, 2 of which are associated with neuroepithelial transforming gene 1 (NET1) and the monocarboxylate transporter 2 (Slc16a7), potential contributors to astrocyte cytoskeletal disruption/swelling and lactate production, respectively. Our findings suggest an important role for lncRNAs in the brain in ALF in relation to inflammation, neuropathology, and in terms of the functional basis of HE. Further work on these non-coding RNAs may lead to new therapeutic approaches for the treatment and management of cerebral dysfunction resulting from this potentially life-threatening disorder.

Brain metabolism in health, aging, and neurodegeneration

Brain cells normally respond adaptively to bioenergetic challenges resulting from ongoing activity in neuronal circuits, and from environmental energetic stressors such as food deprivation and physical exertion. At the cellular level, such adaptive responses include the "strengthening" of existing synapses, the formation of new synapses, and the production of new neurons from stem cells. At the molecular level, bioenergetic challenges result in the activation of transcription factors that induce the expression of proteins that bolster the resistance of neurons to the kinds of metabolic, oxidative, excitotoxic, and proteotoxic stresses involved in the pathogenesis of brain disorders including stroke, and Alzheimer's and Parkinson's diseases. Emerging findings suggest that lifestyles that include intermittent bioenergetic challenges, most notably exercise and dietary energy restriction, can increase the likelihood that the brain will function optimally and in the absence of disease throughout life. Here, we provide an overview of cellular and molecular mechanisms that regulate brain energy metabolism, how such mechanisms are altered during aging and in neurodegenerative disorders, and the potential applications to brain health and disease of interventions that engage pathways involved in neuronal adaptations to metabolic stress.