Lactate modulates the activity of primary cortical neurons through a receptor-mediated pathway

Glycolysis: A multifaceted metabolic pathway and signaling hub

Glycolysis is a highly conserved metabolic pathway responsible for the anaerobic production of adenosine triphosphate (ATP) from the breakdown of glucose molecules. While serving as a primary metabolic pathway in prokaryotes, glycolysis is also utilized by respiring eukaryotic cells, providing pyruvate to fuel oxidative metabolism. Furthermore, glycolysis is the primary source of ATP production in multiple cellular states (e.g., hypoxia) and is particularly important in maintaining bioenergetic homeostasis in the most abundant cell type in the human body, the erythrocyte. Beyond its role in ATP production, glycolysis also functions as a signaling hub, producing several metabolic intermediates which serve roles in both signaling and metabolic processes. These signals emanating from the glycolytic pathway can profoundly impact cell function, phenotype, and fate and have previously been overlooked. In this review, we will discuss the role of the glycolytic pathway as a source of signaling molecules in eukaryotic cells, emphasizing the newfound recognition of glycolysis' multifaceted nature and its importance in maintaining cellular homeostasis, beyond its traditional role in ATP synthesis.

GLUT1 and cerebral glucose hypometabolism in human focal cortical dysplasia is associated with hypermethylation of key glucose regulatory genes

Focal cortical dysplasia (FCD) is recognized as a significant etiological factor in pharmacoresistant intractable epilepsy, linked with disturbances in neurovascular metabolism. Our study investigated regulation of glucose-transporter1 (GLUT1) and cerebral hypometabolism within FCD subtypes. Surgically excised human brain specimens underwent histopathological categorization. A subset of samples (paired with matching blood) was assessed for DNA methylation changes of glucose metabolism-related genes. We evaluated GLUT1, VEGFα, MCT2, and mTOR expression by western blot analysis, measured glucose-lactate concentrations, and established correlations with patients' demographic and clinical profiles. Furthermore, we investigated the impact of DNA methylation inhibitor decitabine and hypometabolic condition on the uptake of [3H]-2-deoxyglucose and ATPase in epileptic brain endothelial cells (EPI-EC). We observed hypermethylation of GLUT1 and glucose metabolic genes in FCD brain/blood samples and could distinguish FCDIIa/b from mMCD, MOGHE and non-lesional types in brain. Low GLUT1 and glucose-lactate ratios corresponded to elevated VEGFα and MCT2 in FCDIIa/b vs non-lesional tissues, independent of age, gender, seizure-onset, or duration of epilepsy. Increased mTOR signaling in FCDIIa/b tissues was evident. Decitabine stimulation increased GLUT1, decreased VEGFα expression, restored glucose uptake and ATPase activity in EPI-ECs and reduced mTOR and MCT2 levels in HEK cells. We demonstrated: 1) hypermethylation of glucose regulatory genes distinguish FCDIIa/b from mMCD, MOGHE and non-lesional types, 2) glucose uptake reduction is due to GLUT1 suppression mediated possibly by a GLUT1-mTOR mechanism; and 3) DNA methylation regulates cellular glucose update and metabolism. Together, these studies may lead to GLUT1-mediated biomarkers, glucose metabolism and identify early intervention strategies in FCD.

Impact of Bifidobacterium longum Subspecies infantis on Pediatric Gut Health and Nutrition: Current Evidence and Future Directions

Background: the intestinal microbiota, a complex community vital to human health, is shaped by microbial competition and host-driven selective pressures. Among these microbes, Bifidobacterium plays a crucial role in early gut colonization during neonatal stages, where Bifidobacterium longum subspecies infantis (B. infantis) predominates and is particularly prevalent in healthy breastfed infants. Objectives: as we embark on a new era in nutrition of the pediatric population, this study seeks to examine the existing understanding regarding B. infantis, encompassing both preclinical insights and clinical evidence. Methods: through a narrative disceptation of the current literature, we focus on its genetic capacity to break down various substances that support its survival and dominance in the intestine. Results: using "omics" technologies, researchers have identified beneficial mechanisms of B. infantis, including the production of short-chain fatty acids, serine protease inhibitors, and polysaccharides. While B. infantis declines with age and in various diseases, it remains a widely used probiotic with documented benefits for infant and child health in numerous studies. Conclusions: the current scientific evidence underscores the importance for ongoing research and clinical trials for a deeper understanding of B. infantis's role in promoting long-term health.

Schizophrenia, a disease of impaired dynamic metabolic flexibility: A new mechanistic framework

Schizophrenia is a chronic, neurodevelopmental disorder with unknown aetiology and pathophysiology that emphasises the role of neurotransmitter imbalance and abnormalities in synaptic plasticity. The currently used pharmacological approach, the antipsychotic drugs, which have limited efficacy and an array of side-effects, have been developed based on the neurotransmitter hypothesis. Recent research has uncovered systemic and brain abnormalities in glucose and energy metabolism, focusing on altered glycolysis and mitochondrial oxidative phosphorylation. These findings call for a re-conceptualisation of schizophrenia pathophysiology as a progressing bioenergetics failure. In this review, we provide an overview of the fundamentals of brain bioenergetics and the changes identified in schizophrenia. We then propose a new explanatory framework positing that schizophrenia is a disease of impaired dynamic metabolic flexibility, which also reconciles findings of abnormal glucose and energy metabolism in the periphery and in the brain along the course of the disease. This evidence-based framework and testable hypothesis has the potential to transform the way we conceptualise this debilitating condition and to develop novel treatment approaches.

Lactate/Hydroxycarboxylic Acid Receptor 1 in Alzheimer's Disease: Mechanisms and Therapeutic Implications-Exercise Perspective

Lactate has a novel function different from previously known functions despite its traditional association with hypoxia in skeletal muscle. It plays various direct and indirect physiological functions. It is a vital energy source within the central nervous system (CNS) and a signal transmitter regulating crucial processes, such as angiogenesis and inflammation. Activating lactate and its associated receptors elicits effects like synaptic plasticity and angiogenesis alterations. These effects can significantly influence the astrocyte-neuron lactate shuttle, potentially impacting cognitive performance. Decreased cognitive function relates to different neurodegenerative conditions, including Alzheimer's disease (AD), ischemic brain injury, and frontotemporal dementia. Therefore, lactic acid has significant potential for treating neurodegenerative disorders. Exercise is a method that induces the production of lactic acid, which is similar to the effect of lactate injections. It is a harmless and natural way to achieve comparable results. Animal experiments demonstrate that high-intensity intermittent exercise can increase vascular endothelial growth factor (VEGF) levels, thus promoting angiogenesis. In vivo, lactate receptor-hydroxycarboxylic acid receptor 1 (HCAR1) activation can occur by various stimuli, including variations in ion concentrations, cyclic adenosine monophosphate (cAMP) level elevations, and fluctuations in the availability of energy substrates. While several articles have been published on the benefits of physical activity on developing Alzheimer's disease in the CNS, could lactic acid act as a bridge? Understanding how HCAR1 responds to these signals and initiates associated pathways remains incomplete. This review comprehensively analyzes lactate-induced signaling pathways, investigating their influence on neuroinflammation, neurodegeneration, and cognitive decline. Consequently, this study describes the unique role of lactate in the progression of Alzheimer's disease.

AMP-activated protein kinase in the amygdala and hippocampus contributes to enhanced fear memory in diabetic mice

BACKGROUND AND PURPOSE

Diabetic patients have an increased risk of psychiatric disorders. Because hyperglycaemia increases L-lactate in the brain and L-lactate inhibits AMP-activated protein kinase (AMPK), this study investigated the role of L-lactate and AMPK in strengthened fear memory, a model for human psychiatric disorders, in diabetic mice.

EXPERIMENTAL APPROACH

The diabetic model was mice injected with streptozotocin. Fear memory was measured using the conditioned fear test with low (0.45 mA) or high (0.50 mA) foot shock to cause low and high freezing, respectively. Protein levels of AMPK and phosphorylated AMPK (pAMPK) were measured by western blotting and immunohistochemistry.

KEY RESULTS

At 0.45 mA, the AMPK inhibitor dorsomorphin increased freezing, which was inhibited by the AMPK activator acadesine. In contrast, at 0.50 mA, acadesine decreased freezing, which was inhibited by dorsomorphin. In diabetic mice, pAMPK was decreased in the amygdala and hippocampus. Diabetic mice showed increased freezing at 0.45 mA, which was inhibited by acadesine. In the amygdala and hippocampus, L-lactate was increased in diabetic mice and injection of L-lactate into non-diabetic mice increased freezing at 0.45 mA. In addition, L-lactate decreased pAMPK in the hippocampus, but not the amygdala, and increase in freezing induced by L-lactate was inhibited by acadesine. Dorsomorphin-induced increase in freezing was inhibited by the AMPA receptor antagonist NBQX.

CONCLUSIONS AND INTERPRETATION

In diabetic mice, L-lactate is increased in the amygdala and hippocampus, possibly through hyperglycaemia, which strengthens fear memory through inhibition of AMPK and activation of glutamatergic function.

A lactate-dependent shift of glycolysis mediates synaptic and cognitive processes in male mice

Astrocytes control brain activity via both metabolic processes and gliotransmission, but the physiological links between these functions are scantly known. Here we show that endogenous activation of astrocyte type-1 cannabinoid (CB1) receptors determines a shift of glycolysis towards the lactate-dependent production of D-serine, thereby gating synaptic and cognitive functions in male mice. Mutant mice lacking the CB1 receptor gene in astrocytes (GFAP-CB1-KO) are impaired in novel object recognition (NOR) memory. This phenotype is rescued by the gliotransmitter D-serine, by its precursor L-serine, and also by lactate and 3,5-DHBA, an agonist of the lactate receptor HCAR1. Such lactate-dependent effect is abolished when the astrocyte-specific phosphorylated-pathway (PP), which diverts glycolysis towards L-serine synthesis, is blocked. Consistently, lactate and 3,5-DHBA promoted the co-agonist binding site occupancy of CA1 post-synaptic NMDA receptors in hippocampal slices in a PP-dependent manner. Thus, a tight cross-talk between astrocytic energy metabolism and gliotransmission determines synaptic and cognitive processes.

Altered neuronal lactate dehydrogenase A expression affects cognition in a sex- and age-dependent manner

The astrocyte-neuron lactate shuttle (ANLS) model posits that astrocyte-generated lactate is transported to neurons to fuel memory processes. However, neurons express high levels of lactate dehydrogenase A (LDHA), the rate-limiting enzyme of lactate production, suggesting a cognitive role for neuronally generated lactate. It was hypothesized that lactate metabolism in neurons is critical for learning and memory. Here transgenic mice were generated to conditionally induce or knockout (KO) the Ldha gene in CNS neurons of adult mice. High pattern separation memory was enhanced by neuronal Ldha induction in young females, and by neuronal Ldha KO in aged females. In older mice, Ldha induction caused cognitive deficits whereas Ldha KO caused cognitive improvements. Genotype-associated cognitive changes were often only observed in one sex or oppositely in males and females. Thus, neuronal-generated lactate has sex-specific cognitive effects, is largely indispensable at young age, and may be detrimental to learning and memory with aging.

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.

The lactate receptor HCAR1: A key modulator of epileptic seizure activity

Epilepsy affects millions globally with a significant portion exhibiting pharmacoresistance. Abnormal neuronal activity elevates brain lactate levels, which prompted the exploration of its receptor, the hydroxycarboxylic acid receptor 1 (HCAR1) known to downmodulate neuronal activity in physiological conditions. This study revealed that HCAR1-deficient mice (HCAR1-KO) exhibited lowered seizure thresholds, and increased severity and duration compared to wild-type mice. Hippocampal and whole-brain electrographic seizure analyses revealed increased seizure severity in HCAR1-KO mice, supported by time-frequency analysis. The absence of HCAR1 led to uncontrolled inter-ictal activity in acute hippocampal slices, replicated by lactate dehydrogenase A inhibition indicating that the activation of HCAR1 is closely associated with glycolytic output. However, synthetic HCAR1 agonist administration in an in vivo epilepsy model did not modulate seizures, likely due to endogenous lactate competition. These findings underscore the crucial roles of lactate and HCAR1 in regulating circuit excitability to prevent unregulated neuronal activity and terminate epileptic events.

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

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

Brain energy metabolism: A roadmap for future research

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

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

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

High lactic acid levels in the brain contribute to the generation of focal slowing on the electroencephalogram

Focal slowing on the EEG is often associated with structural pathology of the brain. Despite the clinical significance of focal slowing, the actual electrochemical mechanisms underlying this EEG phenomenon are still poorly understood. This paper briefly reviews the role of lactate in the pathogenesis of brain disorders that are primarily related to focal EEG slowing. An attempt is made to trace the hypothetical link between this EEG pattern and focal cerebral tissue lactacidosis.

Lactylation Modification in Cardiometabolic Disorders: Function and Mechanism

Cardiovascular disease (CVD) is recognized as the primary cause of mortality and morbidity on a global scale, and developing a clear treatment is an important tool for improving it. Cardiometabolic disorder (CMD) is a syndrome resulting from the combination of cardiovascular, endocrine, pro-thrombotic, and inflammatory health hazards. Due to their complex pathological mechanisms, there is a lack of effective diagnostic and treatment methods for cardiac metabolic disorders. Lactylation is a type of post-translational modification (PTM) that plays a regulatory role in various cellular physiological processes by inducing changes in the spatial conformation of proteins. Numerous studies have reported that lactylation modification plays a crucial role in post-translational modifications and is closely related to cardiac metabolic diseases. This article discusses the molecular biology of lactylation modifications and outlines the roles and mechanisms of lactylation modifications in cardiometabolic disorders, offering valuable insights for the diagnosis and treatment of such conditions.

Neuroprotective role of lactate in a human in vitro model of the ischemic penumbra

In patients suffering from cerebral ischemic stroke, there is an urgent need for treatments to protect stressed yet viable brain cells. Recently, treatment strategies that induce neuronal activity have been shown to be neuroprotective. Here, we hypothesized that neuronal activation might maintain or trigger the astrocyte-to-neuron lactate shuttle (ANLS), whereby lactate is released from astrocytes to support the energy requirements of ATP-starved hypoxic neurons, and this leads to the observed neuroprotection. We tested this by using a human cell based in vitro model of the ischemic penumbra and investigating whether lactate might be neuroprotective in this setting. We found that lactate transporters are involved in the neuroprotective effect mediated by neuronal activation. Furthermore, we showed that lactate exogenously administered before hypoxia correlated with neuroprotection in our cellular model. In addition, stimulation of astrocyte with consequent endogenous production of lactate resulted in neuroprotection. To conclude, here we presented evidence that lactate transport into neurons contributes to neuroprotection during hypoxia providing a potential basis for therapeutic approaches in ischemic stroke.

AMPK role in epilepsy: a promising therapeutic target?

Epilepsy is a complex and multifaceted neurological disorder characterized by spontaneous and recurring seizures. It poses significant therapeutic challenges due to its diverse etiology and often-refractory nature. This comprehensive review highlights the pivotal role of AMP-activated protein kinase (AMPK), a key metabolic regulator involved in cellular energy homeostasis, which may be a promising therapeutic target for epilepsy. Current therapeutic strategies such as antiseizure medication (ASMs) can alleviate seizures (up to 70%). However, 30% of epileptic patients may develop refractory epilepsy. Due to the complicated nature of refractory epilepsy, other treatment options such as ketogenic dieting, adjunctive therapy, and in limited cases, surgical interventions are employed. These therapy options are only suitable for a select group of patients and have limitations of their own. Current treatment options for epilepsy need to be improved. Emerging evidence underscores a potential association between impaired AMPK functionality in the brain and the onset of epilepsy, prompting an in-depth examination of AMPK's influence on neural excitability and ion channel regulation, both critical factors implicated in epileptic seizures. AMPK activation through agents such as metformin has shown promising antiepileptic effects in various preclinical and clinical settings. These effects are primarily mediated through the inhibition of the mTOR signaling pathway, activation of the AMPK-PI3K-c-Jun pathway, and stimulation of the PGC-1α pathway. Despite the potential of AMPK-targeted therapies, several aspects warrant further exploration, including the detailed mechanisms of AMPK's role in different brain regions, the impact of AMPK under various conditional circumstances such as neural injury and zinc toxicity, the long-term safety and efficacy of chronic metformin use in epilepsy treatment, and the potential benefits of combination therapy involving AMPK activators. Moreover, the efficacy of AMPK activators in refractory epilepsy remains an open question. This review sets the stage for further research with the aim of enhancing our understanding of the role of AMPK in epilepsy, potentially leading to the development of more effective, AMPK-targeted therapeutic strategies for this challenging and debilitating disorder.

Cerebral Lactate Participates in Hypoxia-induced Anapyrexia Through its Receptor G Protein-coupled Receptor 81

Hypoxia-induced anapyrexia is thought to be a regulated decrease in body core temperature (Tcore), but the underlying mechanism remains unclear. Recent evidence suggests that lactate, a glycolysis product, could modulate neuronal excitability through the G protein-coupled receptor 81 (GPR81). The present study aims to elucidate the role of central lactate and GPR81 in a rat model of hypoxia-induced anapyrexia. The findings revealed that hypoxia (11.1% O2, 2 h) led to an increase in lactate in cerebrospinal fluid (CSF) and a decrease in Tcore. Injection of dichloroacetate (DCA, 5 mg/kg, 1 μL), a lactate production inhibitor, to the third ventricle (3 V), alleviated the increase in CSF lactate and the decrease in Tcore under hypoxia. Immunofluorescence staining showed GPR81 was expressed in the preoptic area of hypothalamus (PO/AH), the physiological thermoregulation integration center. Under normoxia, injection of GPR81 agonist 3-chloro-5-hydroxybenzoic acid (CHBA, 0.05 mg/kg, 1 μL) to the 3 V, reduced Tcore significantly. In addition, hypoxia led to a dramatic increase in tail skin temperature and a decrease in interscapular brown adipose tissue skin temperature. The number of c-Fos+ cells in the PO/AH increased after exposure to 11.1% O2 for 2 h, but administration of DCA to the 3 V blunted this response. Injection of CHBA to the 3 V also increased the number of c-Fos+ cells in the PO/AH under normoxia. In light of these, our research has uncovered the pivotal role of central lactate-GPR81 signaling in anapyrexia, thereby providing novel insights into the mechanism of hypoxia-induced anapyrexia.

Versatile lactate signaling via HCAR1: a multifaceted GPCR involved in many biological processes

G-coupled protein receptors (GPCRs) are the ultimate refuge of pharmacology and medicine as more than 40% of all marketed drugs are directly targeting these receptors. Through cell surface expression, they are at the forefront of cellular communication with the outside world. Metabolites among the conveyors of this communication are becoming more prominent with the recognition of them as ligands for GPCRs. HCAR1 is a GPCR conveyor of lactate. It is a class A GPCR coupled to Gαi which reduces cellular cAMP along with the downstream Gβγ signaling. It was first found to inhibit lipolysis, and lately has been implicated in diverse cellular processes, including neural activities, angiogenesis, inflammation, vision, cardiovascular function, stem cell proliferation, and involved in promoting pathogenesis for different conditions, such as cancer. Other than signaling from the plasma membrane, HCAR1 shows nuclear localization with different location-biased activities therein. Although different functions for HCAR1 are being discovered, its cell and molecular mechanisms are yet ill understood. Here, we provide a comprehensive review on HCAR1, which covers the literature on the subject, and discusses its importance and relevance in various biological phenomena.

Astrocyte and L-lactate in the anterior cingulate cortex modulate schema memory and neuronal mitochondrial biogenesis

Astrocyte-derived L-lactate was shown to confer beneficial effects on synaptic plasticity and cognitive functions. However, how astrocytic Gi signaling in the anterior cingulate cortex (ACC) modulates L-lactate levels and schema memory is not clear. Here, using chemogenetic approach and well-established behavioral paradigm, we demonstrate that astrocytic Gi pathway activation in the ACC causes significant impairments in flavor-place paired associates (PAs) learning, schema formation, and PA memory retrieval in rats. It also impairs new PA learning even if a prior associative schema exists. These impairments are mediated by decreased L-lactate in the ACC due to astrocytic Gi activation. Concurrent exogenous L-lactate administration bilaterally into the ACC rescues these impairments. Furthermore, we show that the impaired schema memory formation is associated with a decreased neuronal mitochondrial biogenesis caused by decreased L-lactate level in the ACC upon astrocytic Gi activation. Our study also reveals that L-lactate-mediated mitochondrial biogenesis is dependent on monocarboxylate transporter 2 (MCT2) and NMDA receptor activity - discovering a previously unrecognized signaling role of L-lactate. These findings expand our understanding of the role of astrocytes and L-lactate in the brain functions.

BDNF and Lactate as Modulators of Hippocampal CA3 Network Physiology

Growing evidence supports the notion that brain-derived neurotrophic factor (BDNF) and lactate are potent modulators of mammalian brain function. The modulatory actions of those biomolecules influence a wide range of neuronal responses, from the shaping of neuronal excitability to the induction and expression of structural and synaptic plasticity. The biological actions of BDNF and lactate are mediated by their cognate receptors and specific transporters located in the neuronal membrane. Canonical functions of BDNF occur via the tropomyosin-related kinase B receptor (TrkB), whereas lactate acts via monocarboxylate transporters or the hydroxycarboxylic acid receptor 1 (HCAR1). Both receptors are highly expressed in the central nervous system, and some of their physiological actions are particularly well characterized in the hippocampus, a brain structure involved in the neurophysiology of learning and memory. The multifarious neuronal circuitry between the axons of the dentate gyrus granule cells, mossy fibers (MF), and pyramidal neurons of area CA3 is of great interest given its role in specific mnemonic processes and involvement in a growing number of brain disorders. Whereas the modulation exerted by BDNF via TrkB has been extensively studied, the influence of lactate via HCAR1 on the properties of the MF-CA3 circuit is an emerging field. In this review, we discuss the role of both systems in the modulation of brain physiology, with emphasis on the hippocampal CA3 network. We complement this review with original data that suggest cross-modulation is exerted by these two independent neuromodulatory systems.

Loss of microglial MCT4 leads to defective synaptic pruning and anxiety-like behavior in mice

Microglia, the innate immune cells of the central nervous system, actively participate in brain development by supporting neuronal maturation and refining synaptic connections. These cells are emerging as highly metabolically flexible, able to oxidize different energetic substrates to meet their energy demand. Lactate is particularly abundant in the brain, but whether microglia use it as a metabolic fuel has been poorly explored. Here we show that microglia can import lactate, and this is coupled with increased lysosomal acidification. In vitro, loss of the monocarboxylate transporter MCT4 in microglia prevents lactate-induced lysosomal modulation and leads to defective cargo degradation. Microglial depletion of MCT4 in vivo leads to impaired synaptic pruning, associated with increased excitation in hippocampal neurons, enhanced AMPA/GABA ratio, vulnerability to seizures and anxiety-like phenotype. Overall, these findings show that selective disruption of the MCT4 transporter in microglia is sufficient to alter synapse refinement and to induce defects in mouse brain development and adult behavior.

History and Function of the Lactate Receptor GPR81/HCAR1 in the Brain: A Putative Therapeutic Target for the Treatment of Cerebral Ischemia

GPR81 is a G-protein coupled receptor (GPCR) discovered in 2001, but deorphanized only 7 years later, when its affinity for lactate as an endogenous ligand was demonstrated. More recently, GPR81 expression and distribution in the brain were also confirmed and the function of lactate as a volume transmitter has been suggested since then. These findings shed light on a new function of lactate acting as a signaling molecule in the central nervous system, in addition to its well-known role as a metabolic fuel for neurons. GPR81 seems to act as a metabolic sensor, coupling energy metabolism, synaptic activity, and blood flow. Activation of this receptor leads to Gi-mediated downregulation of adenylyl cyclase and subsequent reduction in cAMP levels, regulating several downstream pathways. Recent studies have also suggested the potential role of lactate as a neuroprotective agent, mainly under brain ischemic conditions. This effect is usually attributed to the metabolic role of lactate, but the underlying mechanisms need further investigation and could be related to lactate signaling via GPR81. The activation of GPR81 showed promising results for neuroprotection: it modulates many processes involved in the pathophysiology of ischemia. In this review, we summarize the history of GPR81, starting with its deorphanization; then, we discuss GPR81 expression and distribution, signaling transduction cascades, and neuroprotective roles. Lastly, we propose GPR81 as a potential target for the treatment of cerebral ischemia.

Progressive myoclonic epilepsy type 1 (EPM1) patients present with abnormal 1H MRS brain metabolic profiles associated with cognitive function

PURPOSE

Progressive myoclonic epilepsy, type 1A (EPM1, Unverricht-Lundborg disease), is a rare neurodegenerative autosomal recessive disorder characterized by stimulus-sensitive and action myoclonus and tonic-clonic epileptic seizures. Patients develop neurological symptoms, including ataxia, intention tremor, and dysarthria, over time, with relatively limited and nonspecific MRI atrophy findings. The effects of the disease on brain metabolism are largely unknown.

METHOD

Eighteen EPM1 patients (9 M, 9F) underwent clinical evaluation and neuropsychological testing, which included the assessment of intellectual ability, verbal memory, and psychomotor and executive functions. Magnetic resonance spectroscopy (MRS) and imaging (MRI) were performed on a 1.5 T MRI system. 2D MRS chemical shift imaging (CSI) maps (TE = 270) were obtained from the following regions of the brain: basal ganglia, thalamus, insula, splenium, and occipital white and gray matter, and N-acetyl-aspartate (NAA)-, choline (Cho)-, and lactate (Lac)-to-creatine (Cr) ratios were analyzed. Ten healthy age-and sex-matched subjects (5M, 5F) were used as controls for MRS.

RESULTS

We found significant brain metabolic changes involving lactate, NAA, and choline, which are widespread in the basal ganglia, thalamic nuclei, insula, and occipital areas of EPM1 patients. Changes, especially in the right insula, basal ganglia, and thalamus, were associated with intellectual abilities and impairment of the psychomotor and executive functions of EPM1 patients.

CONCLUSION

Multiple brain metabolic alterations suggest the presence of neurodegeneration associated with EPM1 progression. The changes in metabolite ratios are associated with the neurocognitive dysfunction caused by the disease. However, the role of MRS findings in understanding pathophysiology of EPM1 warrants further studies.

Technical feasibility of multimodal imaging in neonatal hypoxic-ischemic encephalopathy from an ovine model to a human case series

Hypoxic-ischemic encephalopathy (HIE) secondary to perinatal asphyxia occurs when the brain does not receive enough oxygen and blood. A surrogate marker for "intact survival" is necessary for the successful management of HIE. The severity of HIE can be classified based on clinical presentation, including the presence of seizures, using a clinical classification scale called Sarnat staging; however, Sarnat staging is subjective, and the score changes over time. Furthermore, seizures are difficult to detect clinically and are associated with a poor prognosis. Therefore, a tool for continuous monitoring on the cot side is necessary, for example, an electroencephalogram (EEG) that noninvasively measures the electrical activity of the brain from the scalp. Then, multimodal brain imaging, when combined with functional near-infrared spectroscopy (fNIRS), can capture the neurovascular coupling (NVC) status. In this study, we first tested the feasibility of a low-cost EEG-fNIRS imaging system to differentiate between normal, hypoxic, and ictal states in a perinatal ovine hypoxia model. Here, the objective was to evaluate a portable cot-side device and perform autoregressive with extra input (ARX) modeling to capture the perinatal ovine brain states during a simulated HIE injury. So, ARX parameters were tested with a linear classifier using a single differential channel EEG, with varying states of tissue oxygenation detected using fNIRS, to label simulated HIE states in the ovine model. Then, we showed the technical feasibility of the low-cost EEG-fNIRS device and ARX modeling with support vector machine classification for a human HIE case series with and without sepsis. The classifier trained with the ovine hypoxia data labeled ten severe HIE human cases (with and without sepsis) as the "hypoxia" group and the four moderate HIE human cases as the "control" group. Furthermore, we showed the feasibility of experimental modal analysis (EMA) based on the ARX model to investigate the NVC dynamics using EEG-fNIRS joint-imaging data that differentiated six severe HIE human cases without sepsis from four severe HIE human cases with sepsis. In conclusion, our study showed the technical feasibility of EEG-fNIRS imaging, ARX modeling of NVC for HIE classification, and EMA that may provide a biomarker of sepsis effects on the NVC in HIE.

Lactate as a determinant of neuronal excitability, neuroenergetics and beyond

Over the last decades, lactate has emerged as important energy substrate for the brain fueling of neurons. A growing body of evidence now indicates that it is also a signaling molecule modulating neuronal excitability and activity as well as brain functions. In this review, we will briefly summarize how different cell types produce and release lactate. We will further describe different signaling mechanisms allowing lactate to fine-tune neuronal excitability and activity, and will finally discuss how these mechanisms could cooperate to modulate neuroenergetics and higher order brain functions both in physiological and pathological conditions.

Metabolomics in aging research: aging markers from organs

Metabolism plays an important role in regulating aging at several levels, and metabolic reprogramming is the main driving force of aging. Due to the different metabolic needs of different tissues, the change trend of metabolites during aging in different organs and the influence of different levels of metabolites on organ function are also different, which makes the relationship between the change of metabolite level and aging more complex. However, not all of these changes lead to aging. The development of metabonomics research has opened a door for people to understand the overall changes in the metabolic level in the aging process of organisms. The omics-based "aging clock" of organisms has been established at the level of gene, protein and epigenetic modifications, but there is still no systematic summary at the level of metabolism. Here, we reviewed the relevant research published in the last decade on aging and organ metabolomic changes, discussed several metabolites with high repetition rate, and explained their role in vivo, hoping to find a group of metabolites that can be used as metabolic markers of aging. This information should provide valuable information for future diagnosis or clinical intervention of aging and age-related diseases.

Running from stress: a perspective on the potential benefits of exercise-induced small extracellular vesicles for individuals with major depressive disorder

Aerobic exercise promotes beneficial effects in the brain including increased synaptic plasticity and neurogenesis and regulates neuroinflammation and stress response via the hypothalamic-pituitary-adrenal axis. Exercise can have therapeutic effects for numerous brain-related pathologies, including major depressive disorder (MDD). Beneficial effects of aerobic exercise are thought to be mediated through the release of "exerkines" including metabolites, proteins, nucleic acids, and hormones that communicate between the brain and periphery. While the specific mechanisms underlying the positive effects of aerobic exercise on MDD have not been fully elucidated, the evidence suggests that exercise may exert a direct or indirect influence on the brain via small extracellular vesicles which have been shown to transport signaling molecules including "exerkines" between cells and across the blood-brain barrier (BBB). sEVs are released by most cell types, found in numerous biofluids, and capable of crossing the BBB. sEVs have been associated with numerous brain-related functions including neuronal stress response, cell-cell communication, as well as those affected by exercise like synaptic plasticity and neurogenesis. In addition to known exerkines, they are loaded with other modulatory cargo such as microRNA (miRNA), an epigenetic regulator that regulates gene expression levels. How exercise-induced sEVs mediate exercise dependent improvements in MDD is unknown. Here, we perform a thorough survey of the current literature to elucidate the potential role of sEVs in the context of neurobiological changes seen with exercise and depression by summarizing studies on exercise and MDD, exercise and sEVs, and finally, sEVs as they relate to MDD. Moreover, we describe the links between peripheral sEV levels and their potential for infiltration into the brain. While literature suggests that aerobic exercise is protective against the development of mood disorders, there remains a scarcity of data on the therapeutic effects of exercise. Recent studies have shown that aerobic exercise does not appear to influence sEV size, but rather influence their concentration and cargo. These molecules have been independently implicated in numerous neuropsychiatric disorders. Taken together, these studies suggest that concentration of sEVs are increased post exercise, and they may contain specifically packaged protective cargo representing a novel therapeutic for MDD.

Whole grain metabolite 3,5-dihydroxybenzoic acid is a beneficial nutritional molecule with the feature of a double-edged sword in human health: a critical review and dietary considerations

Nonprocessed foodstuffs of plant origin, especially whole-grain cereals, are considered to be health-promoting components of the human diet. While most of their well-studied effects derive from their high fiber content and low glycemic index, the presence of underrated phenolic phytonutrients has recently been brought to the attention of nutritionists. In this review, we report and discuss findings on the sources and bioactivities of 3,5-dihydroxybenzoic acid (3,5-DHBA), which is both a direct dietary component (found, e.g., in apples) and, more importantly, a crucial metabolite of whole-grain cereal-derived alkylresorcinols (ARs). 3,5-DHBA is a recently described exogenous agonist of the HCAR1/GPR81 receptor. We concentrate on the HCAR1-mediated effects of 3,5-DHBA in the nervous system, on the maintenance of cell stemness, regulation of carcinogenesis, and response to anticancer therapy. Unexpectedly, malignant tumors take advantage of HCAR1 expression to sense 3,5-DHBA to support their growth. Thus, there is an urgent need to fully identify the role of whole-grain-derived 3,5-DHBA during anticancer therapy and its contribution in the regulation of vital organs of the body via its specific HCAR1 receptor. We discuss here in detail the possible consequences of the modulatory capabilities of 3,5-DHBA in physiological and pathological conditions in humans.

Adrenergic regulation of astroglial aerobic glycolysis and lipid metabolism: Towards a noradrenergic hypothesis of neurodegeneration

Ageing is a key factor in the development of cognitive decline and dementia, an increasing and challenging problem of the modern world. The most commonly diagnosed cognitive decline is related to Alzheimer's disease (AD), the pathophysiology of which is poorly understood. Several hypotheses have been proposed. The cholinergic hypothesis is the oldest, however, recently the noradrenergic system has been considered to have a role as well. The aim of this review is to provide evidence that supports the view that an impaired noradrenergic system is causally linked to AD. Although dementia is associated with neurodegeneration and loss of neurons, this likely develops due to a primary failure of homeostatic cells, astrocytes, abundant and heterogeneous neuroglial cells in the central nervous system (CNS). The many functions that astrocytes provide to maintain the viability of neural networks include the control of ionic balance, neurotransmitter turnover, synaptic connectivity and energy balance. This latter function is regulated by noradrenaline, released from the axon varicosities of neurons arising from the locus coeruleus (LC), the primary site of noradrenaline release in the CNS. The demise of the LC is linked to AD, whereby a hypometabolic CNS state is observed clinically. This is likely due to impaired release of noradrenaline in the AD brain during states of arousal, attention and awareness. These functions controlled by the LC are needed for learning and memory formation and require activation of the energy metabolism. In this review, we address first the process of neurodegeneration and cognitive decline, highlighting the function of astrocytes. Cholinergic and/or noradrenergic deficits lead to impaired astroglial function. Then, we focus on adrenergic control of astroglial aerobic glycolysis and lipid droplet metabolism, which play a protective role but also promote neurodegeneration under some circumstances, supporting the noradrenergic hypothesis of cognitive decline. We conclude that targeting astroglial metabolism, glycolysis and/or mitochondrial processes may lead to important new developments in the future when searching for medicines to prevent or even halt cognitive decline.

Metabolic Recruitment in Brain Tissue

Information processing imposes urgent metabolic demands on neurons, which have negligible energy stores and restricted access to fuel. Here, we discuss metabolic recruitment, the tissue-level phenomenon whereby active neurons harvest resources from their surroundings. The primary event is the neuronal release of K⁺ that mirrors workload. Astrocytes sense K⁺ in exquisite fashion thanks to their unique coexpression of NBCe1 and α2β2 Na⁺/K⁺ ATPase, and within seconds switch to Crabtree metabolism, involving GLUT1, aerobic glycolysis, transient suppression of mitochondrial respiration, and lactate export. The lactate surge serves as a secondary recruiter by inhibiting glucose consumption in distant cells. Additional recruiters are glutamate, nitric oxide, and ammonium, which signal over different spatiotemporal domains. The net outcome of these events is that more glucose, lactate, and oxygen are made available. Metabolic recruitment works alongside neurovascular coupling and various averaging strategies to support the inordinate dynamic range of individual neurons.

Efficacy of Bifidobacterium longum alone or in multi-strain probiotic formulations during early life and beyond

The significance of Bifidobacterium to human health can be appreciated from its early colonization of the neonatal gut, where Bifidobacterium longum represents the most abundant species. While its relative abundance declines with age, it is further reduced in several diseases. Research into the beneficial properties of B. longum has unveiled a range of mechanisms, including the production of bioactive molecules, such as short-chain fatty acids, polysaccharides, and serine protease inhibitors. From its intestinal niche, B. longum can have far-reaching effects in the body influencing immune responses in the lungs and even skin, as well as influencing brain activity. In this review, we present the biological and clinical impacts of this species on a range of human conditions beginning in neonatal life and beyond. The available scientific evidence reveals a strong rationale for continued research and further clinical trials that investigate the ability of B. longum to treat or prevent a range of diseases across the human lifespan.

Metabolic contributions to neuronal deficits caused by genomic disruption of schizophrenia risk gene SETD1A

Regulation of neuronal metabolism during early brain development is crucial for directing synaptic plasticity and proper circuit formation. Alterations in neuronal glycolysis or mitochondrial function are associated with several neuropsychiatric disorders, including schizophrenia. Recently, loss-of-function mutations in SETD1A, a histone methyltransferase, have been linked to increased schizophrenia risk and global developmental delay. Here, we show that heterozygous disruption of SETD1A in human induced pluripotent stem cell (hiPSC)-derived neurons results in reduced neurite outgrowth and spontaneous activity, two phenotypes commonly associated with schizophrenia, as well as alterations in metabolic capacity. Furthermore, supplementing culture media with metabolic intermediates ameliorated changes in neurite outgrowth and spontaneous activity, suggesting that metabolic dysfunction contributes to neuronal phenotypes caused by SETD1A haploinsufficiency. These findings highlight a previously unknown connection between SETD1A function, metabolic regulation, and neuron development, and identifies alternative avenues for therapeutic development.

Lactate-Mediated Signaling in the Brain-An Update

Lactate is a universal metabolite produced and released by all cells in the body. Traditionally it was viewed as energy currency that is generated from pyruvate at the end of the glycolytic pathway and sent into the extracellular space for other cells to take up and consume. In the brain, such a mechanism was postulated to operate between astrocytes and neurons many years ago. Later, the discovery of lactate receptors opened yet another chapter in the quest to understand lactate actions. Other ideas, such as modulation of NMDA receptors were also proposed. Up to this day, we still do not have a consensus view on the relevance of any of these mechanisms to brain functions or their contribution to human or animal physiology. While the field develops new ideas, in this brief review we analyze some recently published studies in order to focus on some unresolved controversies and highlight the limitations that need to be addressed in future work. Clearly, only by using similar and overlapping methods, cross-referencing experiments, and perhaps collaborative efforts, we can finally understand what the role of lactate in the brain is and why this ubiquitous molecule is so important.

Lactate drives cellular DNA repair capacity: Role of lactate and related short-chain fatty acids in cervical cancer chemoresistance and viral infection

The characteristic feature of a cancer microenvironment is the presence of a highly elevated concentration of L-lactate in the tumor niche. The lactate-rich environment is also maintained by commensal mucosal microbiota, which has immense potential for affecting cancer cells through its receptoric and epigenetic modes of action. Some of these lactate activities might be associated with the failure of anticancer therapy as a consequence of the drug resistance acquired by cancer cells. Upregulation of cellular DNA repair capacity and enhanced drug efflux are the most important cellular mechanisms that account for ineffective radiotherapy and drug-based therapies. Here, we present the recent scientific knowledge on the role of the HCA1 receptor for lactate and lactate intrinsic activity as an HDAC inhibitor in the development of an anticancer therapy-resistant tumor phenotype, with special focus on cervical cancer cells. In addition, a recent study highlighted the viable role of interactions between mammalian cells and microorganisms in the female reproductive tract and demonstrated an interesting mechanism regulating the efficacy of retroviral transduction through lactate-driven modulation of DNA-PKcs cellular localization. To date, very few studies have focused on the mechanisms of lactate-driven enhancement of DNA repair and upregulation of particular multidrug-resistance proteins in cancer cells with respect to their intracellular regulatory mechanisms triggered by lactate. This review presents the main achievements in the field of lactate impact on cell biology that may promote undesirable alterations in cancer physiology and mitigate retroviral infections.

Understanding lactate sensing and signalling

Metabolites generated from cellular and tissue metabolism have been rediscovered in recent years as signalling molecules. They may act as cofactor of enzymes or be linked to proteins as post-translational modifiers. They also act as ligands for specific receptors, highlighting that their neglected functions have, in fact, a long standing in evolution. Lactate is one such metabolite that has been considered for long time a waste product of metabolism devoid of any biological function. However, in the past 10 years, lactate has gained much attention in several physio-pathological processes. Mechanisms of sensing and signalling have been discovered and implicated in a broad range of diseases, from cancer to inflammation and fibrosis, providing opportunities for novel therapeutic avenues. Here, we review some of the most recently discovered mechanisms of lactate sensing and signalling.

Human-in-the-Loop Optimization of Transcranial Electrical Stimulation at the Point of Care: A Computational Perspective

Individual differences in the responsiveness of the brain to transcranial electrical stimulation (tES) are increasingly demonstrated by the large variability in the effects of tES. Anatomically detailed computational brain models have been developed to address this variability; however, static brain models are not “realistic” in accounting for the dynamic state of the brain. Therefore, human-in-the-loop optimization at the point of care is proposed in this perspective article based on systems analysis of the neurovascular effects of tES. First, modal analysis was conducted using a physiologically detailed neurovascular model that found stable modes in the 0 Hz to 0.05 Hz range for the pathway for vessel response through the smooth muscle cells, measured with functional near-infrared spectroscopy (fNIRS). During tES, the transient sensations can have arousal effects on the hemodynamics, so we present a healthy case series for black-box modeling of fNIRS−pupillometry of short-duration tDCS effects. The block exogeneity test rejected the claim that tDCS is not a one-step Granger cause of the fNIRS total hemoglobin changes (HbT) and pupil dilation changes (p < 0.05). Moreover, grey-box modeling using fNIRS of the tDCS effects in chronic stroke showed the HbT response to be significantly different (paired-samples t-test, p < 0.05) between the ipsilesional and contralesional hemispheres for primary motor cortex tDCS and cerebellar tDCS, which was subserved by the smooth muscle cells. Here, our opinion is that various physiological pathways subserving the effects of tES can lead to state−trait variability, which can be challenging for clinical translation. Therefore, we conducted a case study on human-in-the-loop optimization using our reduced-dimensions model and a stochastic, derivative-free covariance matrix adaptation evolution strategy. We conclude from our computational analysis that human-in-the-loop optimization of the effects of tES at the point of care merits investigation in future studies for reducing inter-subject and intra-subject variability in neuromodulation.

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.

Lactic Acid Regulation: A Potential Therapeutic Option in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic, persistent autoimmune disease that causes severe joint tissue damage and irreversible disability. Cumulative evidence suggests that patients suffering from RA for long durations are at risk of functional damage to cardiovascular, kidney, lung, and other tissues. This seriously affects the quality of work and life of patients. To date, no clear etiology of RA has been found. Recent studies have revealed that the massive proliferation of synoviocytes and immune cells requires a large amount of energy supply. Rapid energy supply depends on the anaerobic glucose metabolic pathway in both RA animal models and clinical patients. Anaerobic glycolysis can increase intracellular lactic acid (LA) content. LA induces the overexpression of monocarboxylate transporters (MCTs) in cell membranes. MCTs rapidly transport LA from the intracellular to the intercellular or articular cavity. Hence, a relatively high accumulation of LA could be formed in the intercellular and articular cavities of inflammatory joints. Moreover, LA contributes to the migration and activation of immune cells. Immune cells proliferate and secrete interleukins (IL) including IL-1, IL-2, IL-13, IL-17, and other inflammatory factors. These inflammatory factors enhance the immune inflammatory response of the body and aggravate the condition of RA patients. In this paper, the effects of LA on RA pathogenesis will be summarized from the perspective of the production, transport, and metabolism of synoviocytes and immune cells. Additionally, the drugs involved in the production, transport, and metabolism of LA are highlighted.

Lactate Neuroprotection against Transient Ischemic Brain Injury in Mice Appears Independent of HCAR1 Activation

Lactate can protect against damage caused by acute brain injuries both in rodents and in human patients. Besides its role as a metabolic support and alleged preferred neuronal fuel in stressful situations, an additional signaling mechanism mediated by the hydroxycarboxylic acid receptor 1 (HCAR1) was proposed to account for lactate’s beneficial effects. However, the administration of HCAR1 agonists to mice subjected to middle cerebral artery occlusion (MCAO) at reperfusion did not appear to exert any relevant protective effect. To further evaluate the involvement of HCAR1 in the protection against ischemic damage, we looked at the effect of HCAR1 absence. We subjected wild-type and HCAR1 KO mice to transient MCAO followed by treatment with either vehicle or lactate. In the absence of HCAR1, the ischemic damage inflicted by MCAO was less pronounced, with smaller lesions and a better behavioral outcome than in wild-type mice. The lower susceptibility of HCAR1 KO mice to ischemic injury suggests that lactate-mediated protection is not achieved or enhanced by HCAR1 activation, but rather attributable to its metabolic effects or related to other signaling pathways. Additionally, in light of these results, we would disregard HCAR1 activation as an interesting therapeutic strategy for stroke patients.

Lactate Is Answerable for Brain Function and Treating Brain Diseases: Energy Substrates and Signal Molecule

Research to date has provided novel insights into lactate's positive role in multiple brain functions and several brain diseases. Although notable controversies and discrepancies remain, the neurobiological role and the metabolic mechanisms of brain lactate have now been described. A theoretical framework on the relevance between lactate and brain function and brain diseases is presented. This review begins with the source and route of lactate formation in the brain and food; goes on to uncover the regulatory effect of lactate on brain function; and progresses to gathering the application and concentration variation of lactate in several brain diseases (diabetic encephalopathy, Alzheimer's disease, stroke, traumatic brain injury, and epilepsy) treatment. Finally, the dual role of lactate in the brain is discussed. This review highlights the biological effect of lactate, especially L-lactate, in brain function and disease studies and amplifies our understanding of past research.

Evaluation of Hydroxycarboxylic Acid Receptor 1 (HCAR1) as a Building Block for Genetically Encoded Extracellular Lactate Biosensors

The status of lactate has evolved from being considered a waste product of cellular metabolism to a useful metabolic substrate and, more recently, to a signaling molecule. The fluctuations of lactate levels within biological tissues, in particular in the interstitial space, are crucial to assess with high spatial and temporal resolution, and this is best achieved using cellular imaging approaches. In this study, we evaluated the suitability of the lactate receptor, hydroxycarboxylic acid receptor 1 (HCAR1, formerly named GPR81), as a basis for the development of a genetically encoded fluorescent lactate biosensor. We used a biosensor strategy that was successfully applied to molecules such as dopamine, serotonin, and norepinephrine, based on their respective G-protein-coupled receptors. In this study, a set of intensiometric sensors was constructed and expressed in living cells. They showed selective expression at the plasma membrane and responded to physiological concentrations of lactate. However, these sensors lost the original ability of HCAR1 to selectively respond to lactate versus other related small carboxylic acid molecules. Therefore, while representing a promising building block for a lactate biosensor, HCAR1 was found to be sensitive to perturbations of its structure, affecting its ability to distinguish between related carboxylic molecules.

Lactate ions induce synaptic plasticity to enhance output from the central respiratory network

Lactate ion sensing has emerged as a process that regulates ventilation during metabolic challenges. Most work has focused on peripheral sensing of lactate for the control of breathing. However, lactate also rises in the central nervous system (CNS) during disturbances to blood gas homeostasis and exercise. Using an amphibian model, we recently showed that lactate ions, independently of pH and pyruvate metabolism, act directly in the brainstem to increase respiratory-related motor outflow. This response had a long washout time and corresponded with potentiated excitatory synaptic strength of respiratory motoneurons. Thus, we tested the hypothesis that lactate ions enhance respiratory output using cellular mechanisms associated with long-term synaptic plasticity within motoneurons. In this study, we confirm that 2 mM sodium lactate, but not sodium pyruvate, increases respiratory motor output in brainstem-spinal cord preparations, persisting for 2 h upon the removal of lactate. Lactate also led to prolonged increases in the amplitude of AMPA-glutamate receptor (AMPAR) currents in individual motoneurons from brainstem slices. Both motor facilitation and AMPAR potentiation by lactate required classic effectors of synaptic plasticity, L-type Ca²⁺ channels and NMDA receptors, as part of the transduction process but did not correspond with increased expression of immediate-early genes often associated with activity-dependent neuronal plasticity. Altogether these results show that lactate ions enhance respiratory motor output by inducing conserved mechanisms of synaptic plasticity and suggest a new mechanism that may contribute to coupling ventilation to metabolic demands in vertebrates. KEY POINTS: Lactate ions, independently of pH and metabolism, induce long-term increases in respiratory-related motor outflow in American bullfrogs. Lactate triggers a persistent increase in strength of AMPA-glutamatergic synapses onto respiratory motor neurons. Long-term plasticity of motor output and synaptic strength by lactate involves L-type Ca²⁺ channels and NMDA-receptors as part of the transduction process. Enhanced AMPA receptor function in response to lactate in the intact network is causal for motor plasticity. In sum, well-conserved synaptic plasticity mechanisms couple the brainstem lactate ion concentration to respiratory motor drive in vertebrates.

Lactate is an energy substrate for rodent cortical neurons and enhances their firing activity

Glucose is the mandatory fuel for the brain, yet the relative contribution of glucose and lactate for neuronal energy metabolism is unclear. We found that increased lactate, but not glucose concentration, enhances the spiking activity of neurons of the cerebral cortex. Enhanced spiking was dependent on ATP-sensitive potassium (KATP) channels formed with KCNJ11 and ABCC8 subunits, which we show are functionally expressed in most neocortical neuronal types. We also demonstrate the ability of cortical neurons to take-up and metabolize lactate. We further reveal that ATP is produced by cortical neurons largely via oxidative phosphorylation and only modestly by glycolysis. Our data demonstrate that in active neurons, lactate is preferred to glucose as an energy substrate, and that lactate metabolism shapes neuronal activity in the neocortex through KATP channels. Our results highlight the importance of metabolic crosstalk between neurons and astrocytes for brain function.

Lactate activates hypothalamic POMC neurons by intercellular signaling

Previous studies indicate that the activity of hypothalamic POMC neurons can be regulated by glucose via intracellular mechanisms, but its regulation by lactate is poorly understood. In addition to its energetic role, lactate acts as a signaling molecule. In this study, we evaluated the function and location of the lactate receptor, hydroxycarboxylic acid receptor 1 (HCAR1). We used a conditional genetic approach to label POMC neurons and evaluated their sensitivity to lactate using patch-clamp recordings. l-Lactate and 3-chloro-5-hydroxybenzoic acid (3Cl-HBA), HCAR1 specific agonist depolarized POMC neurons and the increase in excitability was abolished by pertussis toxin (PTX), indicating the involvement of Gαi/o-protein-coupled receptors. In addition, the depolarization of a subset of POMC neurons was sensitive to α-cyano-4-hydroxycinnamate (4-CIN), a lactate transporter blocker, suggesting that the depolarization induced by l-lactate can also occur by direct intracellular action. Surprisingly, HCAR1 was not detected in POMC neurons, but instead localized in astrocytes. These results suggest a new lactate-mediated mechanism for astrocyte-neuron intercellular communication.

Lactate as an Astroglial Signal Augmenting Aerobic Glycolysis and Lipid Metabolism

Astrocytes, heterogeneous neuroglial cells, contribute to metabolic homeostasis in the brain by providing energy substrates to neurons. In contrast to predominantly oxidative neurons, astrocytes are considered primarily as glycolytic cells. They take up glucose from the circulation and in the process of aerobic glycolysis (despite the normal oxygen levels) produce L-lactate, which is then released into the extracellular space via lactate transporters and possibly channels. Astroglial L-lactate can enter neurons, where it is used as a metabolic substrate, or exit the brain via the circulation. Recently, L-lactate has also been considered to be a signaling molecule in the brain, but the mechanisms of L-lactate signaling and how it contributes to the brain function remain to be fully elucidated. Here, we provide an overview of L-lactate signaling mechanisms in the brain and present novel insights into the mechanisms of L-lactate signaling via G-protein coupled receptors (GPCRs) with the focus on astrocytes. We discuss how increased extracellular L-lactate upregulates cAMP production in astrocytes, most likely viaL-lactate-sensitive G_s-protein coupled GPCRs. This activates aerobic glycolysis, enhancing L-lactate production and accumulation of lipid droplets, suggesting that L-lactate augments its own production in astrocytes (i.e., metabolic excitability) to provide more L-lactate for neurons and that astrocytes in conditions of increased extracellular L-lactate switch to lipid metabolism.

Two Metabolic Fuels, Glucose and Lactate, Differentially Modulate Exocytotic Glutamate Release from Cultured Astrocytes

Astrocytes have a prominent role in metabolic homeostasis of the brain and can signal to adjacent neurons by releasing glutamate via a process of regulated exocytosis. Astrocytes synthesize glutamate de novo owing to the pyruvate entry to the citric/tricarboxylic acid cycle via pyruvate carboxylase, an astrocyte specific enzyme. Pyruvate can be sourced from two metabolic fuels, glucose and lactate. Thus, we investigated the role of these energy/carbon sources in exocytotic glutamate release from astrocytes. Purified astrocyte cultures were acutely incubated (1 h) in glucose and/or lactate-containing media. Astrocytes were mechanically stimulated, a procedure known to increase intracellular Ca2+ levels and cause exocytotic glutamate release, the dynamics of which were monitored using single cell fluorescence microscopy. Our data indicate that glucose, either taken-up from the extracellular space or mobilized from the intracellular glycogen storage, sustained glutamate release, while the availability of lactate significantly reduced the release of glutamate from astrocytes. Based on further pharmacological manipulation during imaging along with tandem mass spectrometry (proteomics) analysis, lactate alone, but not in the hybrid fuel, caused metabolic changes consistent with an increased synthesis of fatty acids. Proteomics analysis further unveiled complex changes in protein profiles, which were condition-dependent and generally included changes in levels of cytoskeletal proteins, proteins of secretory organelle/vesicle traffic and recycling at the plasma membrane in aglycemic, lactate or hybrid-fueled astrocytes. These findings support the notion that the availability of energy sources and metabolic milieu play a significant role in gliotransmission.