Kinetic Parameters and Lactate Dehydrogenase Isozyme Activities Support Possible Lactate Utilization by Neurons

Lactate regulates respiratory efficiency and mitochondrial dynamics in primary rat podocytes

Podocytes are crucial for regulating glomerular permeability. They have foot processes that are integral to the renal filtration barrier. Understanding their energy metabolism could shed light on the pathogenesis of filtration barrier injury. Lactate has been increasingly recognized as more than a waste product and has emerged as a significant metabolic fuel and reserve. The recent identification of lactate transporters in podocytes, the expression of which is modulated by glucose levels and lactate, highlights lactate's relevance. The present study investigated the impact of lactate on podocyte respiratory efficiency and mitochondrial dynamics. We confirmed lactate oxidation in podocytes, suggesting its role in cellular energy production. Under conditions of glucose deprivation or lactate supplementation, a significant shift was seen toward oxidative phosphorylation, reflected by an increase in the oxygen consumption rate/extracellular acidification rate ratio. Notably, lactate dehydrogenase A (LDHA) and lactate dehydrogenase B (LDHB) isoforms, which are involved in lactate conversion to pyruvate, were detected in podocytes for the first time. The presence of lactate led to higher intracellular pyruvate levels, greater LDH activity, and higher LDHB expression. Furthermore, lactate exposure increased mitochondrial DNA-to-nuclear DNA ratios and resulted in upregulation of the mitochondrial biogenesis markers peroxisome proliferator-activated receptor coactivator-1α and transcription factor A mitochondrial, regardless of glucose availability. Changes in mitochondrial size and shape were observed in lactate-exposed podocytes. These findings suggest that lactate is a pivotal energy source for podocytes, especially during energy fluctuations. Understanding lactate's role in podocyte metabolism could offer insights into renal function and pathologies that involve podocyte injury.

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.

PARP-1 inhibitor alleviates cerebral ischemia/reperfusion injury by reducing PARylation of HK-1 and LDH in mice

Poly (ADP-ribose) polymerase-1 (PARP-1) activity significantly increases during cerebral ischemia/reperfusion. PARP-1 is an NAD+-consumption enzyme. PARP-1 hyperactivity causes intracellular NAD+ deficiency and bioenergetic collapse, contributing to neuronal death. Besides, the powerful trigger of PARP-1 causes the catalyzation of poly (ADP-ribosyl)ation (PARylation), a posttranslational modification of proteins. Here, we found that PARP-1 was activated in the ischemic brain tissue during middle-cerebral-artery occlusion and reperfusion (MCAO/R) for 24 h, and PAR accumulated in the neurons in mice. Using immunoprecipitation, Western blotting, liquid chromatography-mass spectrometry, and 3D-modeling analysis, we revealed that the activation of PARP-1 caused PARylation of hexokinase-1 and lactate dehydrogenase-B, which, therefore, caused the inhibition of these enzyme activities and the resulting cell energy metabolism collapse. PARP-1 inhibition significantly reversed the activity of hexokinase and lactate dehydrogenase, decreased infarct volume, and improved neuronal deficiency. PARP-1 inhibitor combined with pyruvate further alleviated MCAO/R-induced ischemic brain injury in mice. As such, we conclude that PARP-1 inhibitor alleviates neuronal death partly by inhibiting the PARylation of metabolic-related enzymes and reversing metabolism reprogramming during cerebral ischemia/reperfusion injury in mice. PARP-1 inhibitor combined with pyruvate might be a promising therapeutic approach against brain ischemia/reperfusion injury.

Lactate's behavioral switch in the brain: An in-silico model

Emerging evidence emphasizes lactate's involvement in both physiological processes (energy metabolism, memory, etc.) and disease (traumatic brain injury, epilepsy, etc.). Furthermore, the usefulness of mathematical modeling in deciphering underlying dynamics of the brain to investigate lactate roles and mechanisms of action has been well established. Here, we analyze a novel mathematical model of brain lactate exchanges between four compartments: neurons, astrocytes, capillaries, and extracellular space. A system of four ordinary differential equations is proposed to explain interactions between these compartments. We first optimize and analyze the model's parameters under normal, resting state conditions, and then use it to simulate changes linked to elevated arterial lactate. Our results show that even though increased arterial lactate results in increased uptake by astrocytes and release to the extracellular space, it cannot strongly recover the initial drop in neuronal lactate concentration. Also, we show that the direction of lactate transport between the compartments is influenced by the maximum astrocyte production rate and the transport rate between astrocytes and extracellular space.

Chronic neuronal activation leads to elevated lactate dehydrogenase A through the AMP-activated protein kinase/hypoxia-inducible factor-1α hypoxia pathway

Recent studies suggest that changes in neuronal metabolism are associated with epilepsy. High rates of ATP depletion, lactate dehydrogenase A and lactate production have all been found in epilepsy patients, animal and tissue culture models. As such, it can be hypothesized that chronic seizures lead to continuing elevations in neuronal energy demand which may lead to an adapted metabolic response and elevations of lactate dehydrogenase A. In this study, we examine elevations in the lactate dehydrogenase A protein as a long-term cellular adaptation to elevated metabolic demand from chronic neuronal activation. We investigate this cellular adaptation in human tissue samples and explore the mechanisms of lactate dehydrogenase A upregulation using cultured neurones treated with low Mg²⁺, a manipulation that leads to NMDA-mediated neuronal activation. We demonstrate that human epileptic tissue preferentially upregulates neuronal lactate dehydrogenase A, and that in neuronal cultures chronic and repeated elevations in neural activity lead to upregulation of neuronal lactate dehydrogenase A. Similar to states of hypoxia, this metabolic change occurs through the AMP-activated protein kinase/hypoxia-inducible factor-1α pathway. Our data therefore reveal a novel long-term bioenergetic adaptation that occurs in chronically activated neurones and provide a basis for understanding the interplay between metabolism and neural activity during epilepsy.

Stretch stress propels glutamine dependency and glycolysis in optic nerve head astrocytes

Glaucoma is an optic neuropathy that leads to irreversible blindness, the most common subtype of which is typified by a chronic increase in intraocular pressure that promotes a stretch injury to the optic nerve head. In rodents, the predominant glial cell in this region is the optic nerve head astrocyte that provides axons with metabolic support, likely by releasing lactate produced through astrocytic glycolysis. Our primary hypothesis is that stretching of the optic nerve head astrocytes alters their metabolic activity, thereby advancing glaucoma-associated degeneration by compromising the metabolic support that the astrocytes provide to the axons in the optic nerve head. Metabolic changes in optic nerve head astrocytes were investigated by subjecting them to 24 h of 12% biaxial stretch at 1 Hz then measuring the cells’ bioenergetics using a Seahorse XFe24 Analyzer. We observed significant glycolytic and respiratory activity differences between control and stretched cells, including greater extracellular acidification and lower ATP-linked respiration, yet higher maximal respiration and spare capacity in stretched optic nerve head astrocytes. We also determined that both control and stretched optic nerve head astrocytes displayed a dependency for glutamine over pyruvate or long-chain fatty acids for fuel. The increased use of glycolysis as indicated by the extracellular acidification rate, concomitant with a dependency on glutamine, suggests the need to replenish NAD + for continued glycolysis and provision of carbon for TCA cycle intermediates. Stretch alters optic nerve astrocyte bioenergetics to support an increased demand for internal and external energy.

LDHB Deficiency Promotes Mitochondrial Dysfunction Mediated Oxidative Stress and Neurodegeneration in Adult Mouse Brain

Age-related decline in mitochondrial function and oxidative stress plays a critical role in neurodegeneration. Lactate dehydrogenase-B (LDHB) is a glycolytic enzyme that catalyzes the conversion of lactate, an important brain energy substrate, into pyruvate. It has been reported that the LDHB pattern changes in the brain during ageing. Yet very little is known about the effect of LDHB deficiency on brain pathology. Here, we have used Ldhb knockout (Ldhb^−/−) mice to test the hypothesis that LDHB deficiency plays an important role in oxidative stress-mediated neuroinflammation and neurodegeneration. LDHB knockout (Ldhb^−/−) mice were generated by the ablation of the Ldhb gene using the Cre/loxP-recombination system in the C57BL/6 genetic background. The Ldhb^−/− mice were treated with either osmotin (15 μg/g of the body; intraperitoneally) or vehicle twice a week for 5-weeks. After behavior assessments, the mice were sacrificed, and the cortical and hippocampal brain regions were analyzed through biochemical and morphological analysis. Ldhb^−/− mice displayed enhanced reactive oxygen species (ROS) and lipid peroxidation (LPO) production, and they revealed depleted stores of cellular ATP, GSH:GSSG enzyme ratio, and downregulated expression of Nrf2 and HO-1 proteins, when compared to WT littermates. Importantly, the Ldhb^−/− mice showed upregulated expression of apoptosis mediators (Bax, Cytochrome C, and caspase-3), and revealed impaired p-AMPK/SIRT1/PGC-1alpha signaling. Moreover, LDHB deficiency-induced gliosis increased the production of inflammatory mediators (TNF-α, Nf-ĸB, and NOS2), and revealed cognitive deficits. Treatment with osmotin, an adipoR1 natural agonist, significantly increased cellular ATP production by increasing mitochondrial function and attenuated oxidative stress, neuroinflammation, and neuronal apoptosis, probably, by upregulating p-AMPK/SIRT1/PGC-1alpha signaling in Ldhb^−/− mice. In brief, LDHB deficiency may lead to brain oxidative stress-mediated progression of neurodegeneration via regulating p-AMPK/SIRT1/PGC-1alpha signaling, while osmotin could improve mitochondrial functions, abrogate oxidative stress and alleviate neuroinflammation and neurodegeneration in adult Ldhb^−/− mice.

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

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

Murburn precepts for lactic-acidosis, Cori cycle, and Warburg effect: Interactive dynamics of dehydrogenases, protons, and oxygen

It is unresolved why lactate is transported to the liver for further utilization within the physiological purview of Cori cycle, when muscles have more lactate dehydrogenase (LDH) than liver. We point out that the answer lies in thermodynamics/equilibriums. While the utilization of NADH for the reduction of pyruvate to lactate can be mediated via the classical mechanism, the oxidation of lactate (with/without the uphill reduction of NAD⁺ ) necessitates alternative physiological approaches. The latter pathway occurs via interactive equilibriums involving the enzyme, protons and oxygen or diffusible reactive oxygen species (DROS). Since liver has high DROS, the murburn activity at LDH would enable the cellular system to tide over the unfavorable energy barriers of the forward reaction (~476 kJ/mol; earlier miscalculated as ~26 kJ/mole). Further, the new mechanism does not necessitate any "smart decision-making" or sophisticated control by/of proteins. The DROS-based murburn theory explains the invariant active-site structure of LDH isozymes and their multimeric nature. The theoretical insights, in silico evidence and analyses of literature herein also enrich our understanding of the underpinnings of "lactic acidosis" (lowering of physiological pH accompanied by lactate production), Warburg effect (increased lactate production at high pO₂ by cancer cells) and approach for cancer therapy.

Mitochondrial Transport in Glycolysis and Gluconeogenesis: Achievements and Perspectives

Some metabolic pathways involve two different cell components, for instance, cytosol and mitochondria, with metabolites traffic occurring from cytosol to mitochondria and vice versa, as seen in both glycolysis and gluconeogenesis. However, the knowledge on the role of mitochondrial transport within these two glucose metabolic pathways remains poorly understood, due to controversial information available in published literature. In what follows, we discuss achievements, knowledge gaps, and perspectives on the role of mitochondrial transport in glycolysis and gluconeogenesis. We firstly describe the experimental approaches for quick and easy investigation of mitochondrial transport, with respect to cell metabolic diversity. In addition, we depict the mitochondrial shuttles by which NADH formed in glycolysis is oxidized, the mitochondrial transport of phosphoenolpyruvate in the light of the occurrence of the mitochondrial pyruvate kinase, and the mitochondrial transport and metabolism of L-lactate due to the L-lactate translocators and to the mitochondrial L-lactate dehydrogenase located in the inner mitochondrial compartment.

FiCoS: A fine-grained and coarse-grained GPU-powered deterministic simulator for biochemical networks

Mathematical models of biochemical networks can largely facilitate the comprehension of the mechanisms at the basis of cellular processes, as well as the formulation of hypotheses that can be tested by means of targeted laboratory experiments. However, two issues might hamper the achievement of fruitful outcomes. On the one hand, detailed mechanistic models can involve hundreds or thousands of molecular species and their intermediate complexes, as well as hundreds or thousands of chemical reactions, a situation generally occurring in rule-based modeling. On the other hand, the computational analysis of a model typically requires the execution of a large number of simulations for its calibration, or to test the effect of perturbations. As a consequence, the computational capabilities of modern Central Processing Units can be easily overtaken, possibly making the modeling of biochemical networks a worthless or ineffective effort. To the aim of overcoming the limitations of the current state-of-the-art simulation approaches, we present in this paper FiCoS, a novel "black-box" deterministic simulator that effectively realizes both a fine-grained and a coarse-grained parallelization on Graphics Processing Units. In particular, FiCoS exploits two different integration methods, namely, the Dormand-Prince and the Radau IIA, to efficiently solve both non-stiff and stiff systems of coupled Ordinary Differential Equations. We tested the performance of FiCoS against different deterministic simulators, by considering models of increasing size and by running analyses with increasing computational demands. FiCoS was able to dramatically speedup the computations up to 855×, showing to be a promising solution for the simulation and analysis of large-scale models of complex biological processes.

A Metabolic Landscape for Maintaining Retina Integrity and Function

Neurons have high metabolic demands that are almost exclusively met by glucose supplied from the bloodstream. Glucose is utilized in complex metabolic interactions between neurons and glia cells, described by the astrocyte-neuron lactate shuttle (ANLS) hypothesis. The neural retina faces similar energy demands to the rest of the brain, with additional high anabolic needs to support continuous renewal of photoreceptor outer segments. This demand is met by a fascinating variation of the ANLS in which photoreceptors are the central part of a metabolic landscape, using glucose and supplying surrounding cells with metabolic intermediates. In this review we summarize recent evidence on how neurons, in particular photoreceptors, meet their energy and biosynthetic requirements by comprising a metabolic landscape of interdependent cells.

Thermostability of Lactate Dehydrogenase in Rat Brain under Conditions of Short-Term Moderate Hypothermia

Thermostability of rat brain lactate dehydrogenase (LDH) was studied in intact animals and animals subjected to moderate short-term hypothermia. Two exponential stages, rapid and slow, were distinguished in the thermodenaturation kinetics. The contribution of the rapid phase to the lactate dehydrogenase denaturation kinetics was more significant: the energy of activation for this phase was 2.33 times lower than that for the slow phase. Moderate shortterm hypothermia led to a significant decrease of lactate dehydrogenase thermostability: thermodenaturation rate constants for the rapid (k₁) and slow (k₂) phases increased. Significant changes in parameters a and b reflecting the initial proportion of the two native forms of the enzyme developed only at 40°C. As hypothermia caused no appreciable changes in the energy of activation of lactate dehydrogenase denaturation, a significant contribution of the entropic factor to the decrease of free energy of enzyme denaturation was hypothesized. The data indicated significant labilization of lactate dehydrogenase structure under conditions of moderate hypothermia.

Castanopsis lamontii Water Extract Shows Potential in Suppressing Pathogens, Lipopolysaccharide-Induced Inflammation and Oxidative Stress-Induced Cell Injury

Castanopsis lamontii is traditionally used to prevent inflammatory diseases such as periodontitis and pharyngitis by residents in southwest China. However, little scientific evidence has been found to support this. In this research, the antibacterial activities of Castanopsis lamontii water extract (CLE) were assessed using the micro-dilution method. The anti-inflammatory and antioxidant activities of CLE were investigated in RAW264.7 cells. Key bioactive compounds in CLE were also explored. Results showed that CLE was capable of inhibiting the periodontitis pathogen Porphyromonas gingivalis and the pharyngitis pathogen β-hemolytic Streptococcus. It suppressed lipopolysaccharide-induced inflammation in RAW 264.7 cells via inactivating the TLR4/NF-κB pathway. Besides, it reduced oxidative stress-induced cell injury via scavenging reactive oxygen species. Chemical composition analysis revealed that CLE was rich in epicatechin and procyanidin B2. Further studies confirmed that epicatechin predominantly contributed to the antibacterial activities of CLE, while procyanidin B2 was mainly responsible for the anti-inflammatory activities of CLE. Both compounds contributed to the antioxidant activities of CLE. Acute oral toxicity tests proved that CLE was practically non-toxic. These results provide experimental evidences of the health-beneficial effects of CLE and may help promote the application of CLE in the food and health industries.

The age-related changes and differences in energy metabolism and glutamate-glutamine recycling in the d-gal-induced and naturally occurring senescent astrocytes in vitro

Previously, we successfully established a d-galactose (d-gal)-induced astrocyte aging model in vitro. However, whether the changes in the aged astrocytes induced by d-gal are similar to those occurred in naturally are unknown. Therefore, in current study, we simultaneously established d-gal-induced and naturally aged astrocyte aging models in vitro to explore the age-related changes and to compare the differences in these two astrocyte aging models. The Seahorse Extracellular Flux Analyzer was used to examine the mitochondrial metabolism and glycolysis activities of the young and senescent astrocytes. The results showed that the mitochondrial ATP-linked oxygen consumption rates (OCRs) were decreased markedly both in the d-gal-induced and naturally occurring senescent astrocytes. The basal glycolysis activity was increased in the naturally occurring senescent astrocytes, whereas it was decreased in the d-gal-induced senescent astrocytes. Western blot analysis showed that isocitrate dehydrogenase 3 (IDH3), succinate dehydrogenase (SDH) and malate dehydrogenase 2 (MDH2) were markedly decreased both in these two aging models, whereas the iron‑sulfur cluster assembly enzyme (ISCU) was up-regulated in the naturally occurring senescent astrocytes but was down-regulated in the d-gal-induced senescent astrocytes. The expression levels of glial glutamate transporter-1 (GLT-1), glutamine synthetase (GS) and γ-aminobutyric acid type B receptor subunit 2 (GABA_BR2) were also markedly decreased in these two aging models. In addition, the PI3K/AKT signaling pathway was to be inactivated both in the d-gal-induced and naturally occurring senescent astrocytes. These results indicate that the age-related changes in d-gal-induced senescent astrocytes are not fully consistent with those in naturally occurring senescent astrocytes, and it may be not suitable to use d-gal-induced senescent astrocytes to replace the naturally occurring senescent astrocytes to explore the aging mechanisms under some circumstances.

Fundamentals of CNS energy metabolism and alterations in lysosomal storage diseases

The brain has a very high requirement for energy. Adult brain relies on glucose as an energy substrate, whereas developing brain can utilize alternative substrates as well as glucose for energy and for the biosynthesis of lipids and proteins required for brain development. Metabolism provides the energy required to support all cellular functions and brain development and building blocks for macromolecules. Lysosomes are organelles involved in breakdown of biological compounds including proteins and complex lipids in the body and brain. Recent studies suggest that lysosomal dysfunction can damage neurons and/or alter neurotransmitter homeostasis. Several studies also implicate mitochondrial dysfunction in the pathophysiology of brain damage in lysosomal storage diseases. This manuscript provides a brief review of energy metabolism and the key pathways involved in metabolism in brain. Roles of lysosomes related to metabolism and neurotransmission are discussed, and evidence for mitochondrial dysfunction in several lysosomal storage diseases is presented. This article is part of the Special Issue "Lysosomal Storage Disorders".

An auditory brainstem nucleus as a model system for neuronal metabolic demands

The correlation between neuronal activity and metabolism is essential for coding, plasticity, neurological disorders and the interpretation of functional neuroimaging data. Most likely, metabolic requirements depend upon neuron type, and macroscopic energy demands vary with brain region. However, specific needs of individual neuron types are enigmatic. Therefore, we monitored metabolic activity in the lateral superior olive (LSO), an auditory brainstem nucleus containing only one neuron type. LSO neurons exhibit extreme but well-described biophysics with firing rates of several hundred hertz and low input resistances of a few megaohms. We recorded changes in NADH and flavin adenine dinucleotide (FAD) autofluorescence and O₂ concentration in acute brainstem slices of Mongolian gerbils (Meriones unguiculatus) following electrical stimulation. The LSO shows the typical biphasic NADH/FAD response up to a physiologically relevant frequency of 400 Hz. In the same animal, we compared the LSO with the hippocampal CA1 region and the cerebral cortex. The rate of NADH/FADH₂ consumption and regeneration was slowest in LSO. However, frequency dependence was only similar during the consumption phase but varied during regeneration within the three brain regions. Changes in NADH, FAD and O₂ levels and blocking metabolic reactions indicate a pronounced contribution of mitochondrial oxidative phosphorylation in the LSO which is known for the other brain regions as well. Lactate transport and interconversion are involved in LSO metabolism as we found in immunohistochemical and pharmacological experiments. Our findings show that the LSO represents an apt, biophysically distinct model for brain metabolism and that neuronal properties determine metabolic needs.

Brain activity-induced neuronal glucose uptake/glycolysis: Is the lactate shuttle not required?

The astrocyte-neuron lactate shuttle (ANLS) hypothesis posits that during neuronal activation, astrocytic glycolysis consumes glucose and generates lactate, with the latter then imported by neurons as a preferred fuel. The hypothesis has been controversial, with multiple theoretical postulates for and against, and with empirical evidence that were either supportive or otherwise. Recent findings using direct in vivo imaging of lactate and glucose uptake as well as associated metabolic changes in neurons have now placed important constraints on the hypothesis. Here, I review these recent findings and discuss their implications on neuronal energetics.

Role of tandospirone, a 5-HT1A receptor partial agonist, in the treatment of central nervous system disorders and the underlying mechanisms

5-hydroxytryptamine (5-HT, serotonin) is an important neurotransmitter in the modulation of the cognitive, behavioral and psychological functions in animals and humans. Among the fourteen subtypes of 5-HT receptor, 5-HT1A receptor has been extensively studied. Tandospirone, an azapirone derivative with strong and selective agonist effect on 5-HT1A receptor, has been used for the treatment of anxiety disorders especially generalized anxiety disorder for decades. Recently, tandospirone showed the efficacy in relieving the syndromes of social anxiety disorder and post-traumatic stress disorder as well as in potentiating the effect of antidepressants in the treatment of depression in both preclinical and clinical studies. More impressively, the beneficial effect of tandospirone has been revealed on improvement of motor dysfunction of Parkinson's disease and cognitive deficits of schizophrenia either in monotherapy or in combination with other drugs. This review discusses the superiority of tandospirone in the treatment of the disorders and associated mechanisms in central nervous system from the literature.

Divergent lactate dehydrogenase isoenzyme profile in cellular compartments of primate forebrain structures

The compartmentalization and association of lactate dehydrogenase (LDH) with specific cellular structures (e.g., synaptosomal, sarcoplasmic or mitochondrial) may play an important role in brain energy metabolism. Our previous research revealed that LDH in the synaptosomal fraction shifts toward the aerobic isoforms (LDH-B) among the large-brained haplorhine primates compared to strepsirrhines. Here, we further analyzed the subcellular localization of LDH in primate forebrain structures using quantitative Western blotting and ELISA. We show that, in cytosolic and mitochondrial subfractions, LDH-B expression level was relatively elevated and LDH-A declined in haplorhines compared to strepsirrhines. LDH-B expression in mitochondrial fractions of the neocortex was preferentially increased, showing a particularly significant rise in the ratio of LDH-B to LDH-A in chimpanzees and humans. We also found a significant correlation between the protein levels of LDH-B in mitochondrial fractions from haplorhine neocortex and the synaptosomal LDH-B that suggests LDH isoforms shift from a predominance of A-subunits toward B-subunits as part of a system that spatially buffers dynamic energy requirements of brain cells. Our results indicate that there is differential subcellular compartmentalization of LDH isoenzymes that evolved among different primate lineages to meet the energy requirements in neocortical and striatal cells.

A disease with a sweet tooth: exploring the Warburg effect in Alzheimer's disease

After more than 80 years from the revolutionary discoveries of Otto Warburg, who observed high glucose dependency, with increased glycolysis and lactate production regardless of oxygen availability in most cancer cells, the 'Warburg effect' returns to the fore in neuronal cells affected by Alzheimer's disease (AD). Indeed, it seems that, in the mild phase of AD, neuronal cells "prefer" to use the energetically inefficient method of burning glucose by glycolysis, as in cancer, proving to become resistant to β-amyloid (Aβ)-dependent apoptosis. However, in the late phase, while most AD brain cells die in response to Aβ toxicity, only small populations of neurons, exhibiting increased glucose uptake and glycolytic flux, are able to survive as they are resistant to Aβ. Here we draw an overview on the metabolic shift for glucose utilization from oxidative phosphorylation to glycolysis, focusing on the hypothesis that, as extreme attempt to oppose the impending death, mitochondria-whose dysfunction and central role in Aβ toxicity is an AD hallmark-are sent into quiescence, this likely contributing to activate mechanisms of resistance to Aβ-dependent apoptosis. Finally, the attempt turns out fruitless since the loss of the adaptive advantage afforded by elevated aerobic glycolysis exacerbates the pathophysiological processes associated with AD, making the brain susceptible to Aβ-induced neurotoxicity and leading to cell death and dementia. The understanding of how certain nerve cells become resistant to Aβ toxicity, while the majority dies, is an attractive challenge toward the identification of novel possible targets for AD therapy.

A mechanistic kinetic description of lactate dehydrogenase elucidating cancer diagnosis and inhibitor evaluation

As a key enzyme for glycolysis, lactate dehydrogenase (LDH) remains as a topic of great interest in cancer study. Though a number of kinetic models have been applied to describe the dynamic behavior of LDH, few can reflect its actual mechanism, making it difficult to explain the observed substrate and competitor inhibitions at wide concentration ranges. A novel mechanistic kinetic model is developed based on the enzymatic processes and the interactive properties of LDH. Better kinetic simulation as well as new enzyme interactivity information and kinetic properties extracted from published articles via the novel model was presented. Case studies were presented to a comprehensive understanding of the effect of temperature, substrate, and inhibitor on LDH kinetic activities for promising application in cancer diagnosis, inhibitor evaluation, and adequate drug dosage prediction.

Comparison of Glutamate Turnover in Nerve Terminals and Brain Tissue During [1,6-13C2]Glucose Metabolism in Anesthetized Rats

The ¹³C turnover of neurotransmitter amino acids (glutamate, GABA and aspartate) were determined from extracts of forebrain nerve terminals and brain homogenate, and fronto-parietal cortex from anesthetized rats undergoing timed infusions of [1,6-¹³C₂]glucose or [2-¹³C]acetate. Nerve terminal ¹³C fractional labeling of glutamate and aspartate was lower than those in whole cortical tissue at all times measured (up to 120 min), suggesting either the presence of a constant dilution flux from an unlabeled substrate or an unlabeled (effectively non-communicating on the measurement timescale) glutamate pool in the nerve terminals. Half times of ¹³C labeling from [1,6-¹³C₂]glucose, as estimated by least squares exponential fitting to the time course data, were longer for nerve terminals (Glu_C4, 21.8 min; GABA_C2 21.0 min) compared to cortical tissue (Glu_C4, 12.4 min; GABA_C2, 14.5 min), except for Asp_C3, which was similar (26.5 vs. 27.0 min). The slower turnover of glutamate in the nerve terminals (but not GABA) compared to the cortex may reflect selective effects of anesthesia on activity-dependent glucose use, which might be more pronounced in the terminals. The ¹³C labeling ratio for glutamate-C4 from [2-¹³C]acetate over that of ¹³C-glucose was twice as large in nerve terminals compared to cortex, suggesting that astroglial glutamine under the ¹³C glucose infusion was the likely source of much of the nerve terminal dilution. The net replenishment of most of the nerve terminal amino acid pools occurs directly via trafficking of astroglial glutamine.

The interplay between metabolic homeostasis and neurodegeneration: insights into the neurometabolic nature of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal, neurodegenerative disease that is characterized by the selective degeneration of upper motor neurons and lower spinal motor neurons, resulting in the progressive paralysis of all voluntary muscles. Approximately 10 % of ALS cases are linked to known genetic mutations, with the remaining 90 % of cases being sporadic. While the primary pathology in ALS is the selective death of upper and lower motor neurons, numerous studies indicate that an imbalance in whole body and/or cellular metabolism influences the rate of progression of disease. This review summarizes current research surrounding the impact of impaired metabolic physiology in ALS. We extend ideas to consider prospects that lie ahead in terms of how metabolic alterations may impact the selective degeneration of neurons in ALS and how targeting of adenosine triphosphate-sensitive potassium (K_ATP) channels may represent a promising approach for obtaining neuroprotection in ALS.

Dynamic Changes in Cytosolic ATP Levels in Cultured Glutamatergic Neurons During NMDA-Induced Synaptic Activity Supported by Glucose or Lactate

We have previously shown that synaptic transmission fails in cultured neurons in the presence of lactate as the sole substrate. Thus, to test the hypothesis that the failure of synaptic transmission is a consequence of insufficient energy supply, ATP levels were monitored employing the ATP biosensor Ateam1.03YEMK. While inducing synaptic activity by subjecting cultured neurons to two 30 s pulses of NMDA (30 µM) with a 4 min interval, changes in relative ATP levels were measured in the presence of lactate (1 mM), glucose (2.5 mM) or the combination of the two. ATP levels reversibly declined following NMDA-induced neurotransmission activity, as indicated by a reversible 10-20 % decrease in the response of the biosensor. The responses were absent when the NMDA receptor antagonist memantine was present. In the presence of lactate alone, the ATP response dropped significantly more than in the presence of glucose following the 2nd pulse of NMDA (approx. 10 vs. 20 %). Further, cytosolic Ca(2+) homeostasis during NMDA-induced synaptic transmission is partially inhibited by verapamil indicating that voltage-gated Ca(2+) channels are activated. Lastly, we showed that cytosolic Ca(2+) homeostasis is supported equally well by both glucose and lactate, and that a pulse of NMDA causes accumulation of Ca(2+) in the mitochondrial matrix. In summary, we have shown that ATP homeostasis during neurotransmission activity in cultured neurons is supported by both glucose and lactate. However, ATP homeostasis seems to be negatively affected by the presence of lactate alone, suggesting that glucose is needed to support neuronal energy metabolism during activation.

Synaptosomal lactate dehydrogenase isoenzyme composition is shifted toward aerobic forms in primate brain evolution

With the evolution of a relatively large brain size in haplorhine primates (i.e. tarsiers, monkeys, apes, and humans), there have been associated changes in the molecular machinery that delivers energy to the neocortex. Here we investigated variation in lactate dehydrogenase (LDH) expression and isoenzyme composition of the neocortex and striatum in primates using quantitative Western blotting and isoenzyme analysis of total homogenates and synaptosomal fractions. Analysis of isoform expression revealed that LDH in synaptosomal fractions from both forebrain regions shifted towards a predominance of the heart-type, aerobic isoform LDH-B among haplorhines as compared to strepsirrhines (i.e. lorises and lemurs), while in the total homogenate of the neocortex and striatum there was no significant difference in LDH isoenzyme composition between the primate suborders. The largest increase occurred in synapse-associated LDH-B expression in the neocortex, with an especially remarkable elevation in the ratio of LDH-B/LDH-A in humans. The phylogenetic variation in the ratio of LDH-B/LDH-A was correlated with species-typical brain mass but not the encephalization quotient. A significant LDH-B increase in the subneuronal fraction from haplorhine neocortex and striatum suggests a relatively higher rate of aerobic glycolysis that is linked to synaptosomal mitochondrial metabolism. Our results indicate that there is a differential composition of LDH isoenzymes and metabolism in synaptic terminals that evolved in primates to meet increased energy requirements in association with brain enlargement.

Role of energy metabolic deficits and oxidative stress in excitotoxic spinal motor neuron degeneration in vivo

MN (motor neuron) death in amyotrophic lateral sclerosis may be mediated by glutamatergic excitotoxicity. Previously, our group showed that the microdialysis perfusion of AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionate) in the rat lumbar spinal cord induced MN death and permanent paralysis within 12 h after the experiment. Here, we studied the involvement of energy metabolic deficiencies and of oxidative stress in this MN degeneration, by testing the neuroprotective effect of various energy metabolic substrates and antioxidants. Pyruvate, lactate, β-hydroxybutyrate, α-ketobutyrate and creatine reduced MN loss by 50–65%, preserved motor function and completely prevented the paralysis. Ascorbate, glutathione and glutathione ethyl ester weakly protected against motor deficits and reduced MN death by only 30–40%. Reactive oxygen species formation and 3-nitrotyrosine immunoreactivity were studied 1.5–2 h after AMPA perfusion, during the initial MN degenerating process, and no changes were observed. We conclude that mitochondrial energy deficiency plays a crucial role in this excitotoxic spinal MN degeneration, whereas oxidative stress seems a less relevant mechanism. Interestingly, we observed a clear correlation between the alterations of motor function and the number of damaged MNs, suggesting that there is a threshold of about 50% in the number of healthy MNs necessary to preserve motor function.

Mitochondrial energy substrates protect against in vivo excitotoxic spinal motor neuron degeneration and the consequent paralysis, whereas antioxidants are less efficient. These results allowed to establish a minimal threshold number of spinal motor neurons necessary to preserve motor function.

Early deficits in glycolysis are specific to striatal neurons from a rat model of huntington disease

In Huntington disease (HD), there is increasing evidence for a link between mutant huntingtin expression, mitochondrial dysfunction, energetic deficits and neurodegeneration but the precise nature, causes and order of these events remain to be determined. In this work, our objective was to evaluate mitochondrial respiratory function in intact, non-permeabilized, neurons derived from a transgenic rat model for HD compared to their wild type littermates by measuring oxygen consumption rates and extracellular acidification rates. Although HD striatal neurons had similar respiratory capacity as those from their wild-type littermates when they were incubated in rich medium containing a supra-physiological glucose concentration (25 mM), pyruvate and amino acids, respiratory defects emerged when cells were incubated in media containing only a physiological cerebral level of glucose (2.5 mM). According to the concept that glucose is not the sole substrate used by the brain for neuronal energy production, we provide evidence that primary neurons can use lactate as well as pyruvate to fuel the mitochondrial respiratory chain. In contrast to glucose, we found no major deficits in HD striatal neurons’ capacity to use pyruvate as a respiratory substrate compared to wild type littermates. Additionally, we used extracellular acidification rates to confirm a reduction in anaerobic glycolysis in the same cells. Interestingly, the metabolic disturbances observed in striatal neurons were not seen in primary cortical neurons, a brain region affected in later stages of HD. In conclusion, our results argue for a dysfunction in glycolysis, which might precede any defects in the respiratory chain itself, and these are early events in the onset of disease.

Comparison of primary and secondary rat astrocyte cultures regarding glucose and glutathione metabolism and the accumulation of iron oxide nanoparticles

Astrocyte-rich primary cultures (APCs) are frequently used as a model system for the investigation of properties of brain astrocytes. However, as APCs contain a substantial number of microglial and oligodendroglial cells, biochemical parameters determined for such cultures may at least in part reflect also the presence of the contaminating cell types. To lower the potential contributions of microglial and oligodendroglial cells on properties of the astrocytes in APCs we prepared rat astrocyte-rich secondary cultures (ASCs) by subculturing of APCs and compared these ASCs with APCs regarding basal metabolic parameters, specific enzyme activities and the accumulation of iron oxide nanoparticles. Immunocytochemical characterization revealed that ASCs contained only minute amounts of microglial and oligodendroglial cells. ASCs and APCs did not significantly differ in their specific glucose consumption and lactate production rates, in their specific iron and glutathione contents, in their specific activities of various enzymes involved in glucose and glutathione metabolism nor in their accumulation of iron oxide nanoparticles. Thus, the absence or presence of some contaminating microglial and oligodendroglial cells appears not to substantially modulate the investigated metabolic parameters of astrocyte cultures.

Unraveling the complex metabolic nature of astrocytes

Since the initial description of astrocytes by neuroanatomists of the nineteenth century, a critical metabolic role for these cells has been suggested in the central nervous system. Nonetheless, it took several technological and conceptual advances over many years before we could start to understand how they fulfill such a role. One of the important and early recognized metabolic function of astrocytes concerns the reuptake and recycling of the neurotransmitter glutamate. But the description of this initial property will be followed by several others including an implication in the supply of energetic substrates to neurons. Indeed, despite the fact that like most eukaryotic non-proliferative cells, astrocytes rely on oxidative metabolism for energy production, they exhibit a prominent aerobic glycolysis capacity. Moreover, this unusual metabolic feature was found to be modulated by glutamatergic activity constituting the initial step of the neurometabolic coupling mechanism. Several approaches, including biochemical measurements in cultured cells, genetic screening, dynamic cell imaging, nuclear magnetic resonance spectroscopy and mathematical modeling, have provided further insights into the intrinsic characteristics giving rise to these key features of astrocytes. This review will provide an account of the different results obtained over several decades that contributed to unravel the complex metabolic nature of astrocytes that make this cell type unique.

Exploiting metabolic differences in glioma therapy

Brain function depends upon complex metabolic interactions amongst only a few different cell types, with astrocytes providing critical support for neurons. Astrocyte functions include buffering the extracellular space, providing substrates to neurons, interchanging glutamate and glutamine for synaptic transmission with neurons, and facilitating access to blood vessels. Whereas neurons possess highly oxidative metabolism and easily succumb to ischemia, astrocytes rely more on glycolysis and metabolism associated with synthesis of critical intermediates, hence are less susceptible to lack of oxygen. Astrocytoma and higher grade glioma cells demonstrate both basic metabolic mechanisms of astrocytes as well as tumors in general, e.g. they show a high glycolytic rate, lactate extrusion, ability to proliferate even under hypoxia, and opportunistic use of mechanisms to enhance metabolism and blood vessel generation, and suppression of cell death pathways. There may be differences in metabolism between neurons, normal astrocytes and astrocytoma cells, providing therapeutic opportunities against astrocytomas, including a wide range of enzyme and transporter differences, regulation of hypoxia-inducible factor (HIF), glutamate uptake transporters and glutamine utilization, differential sensitivities of monocarboxylate transporters, presence of glycogen, high interlinking with gap junctions, use of NADPH for lipid synthesis, utilizing differential regulation of synthetic enzymes (e.g. isocitrate dehydrogenase, pyruvate carboxylase, pyruvate dehydrogenase, lactate dehydrogenase, malate-aspartate NADH shuttle) and different glucose uptake mechanisms. These unique metabolic susceptibilities may augment conventional therapeutic attacks based on cell division differences and surface receptors alone, and are starting to be implemented in clinical trials.

Purification and properties of a monomeric lactate dehydrogenase from yak Hypoderma sinense larva

The objective of the present study was to study the characteristics of lactate dehydrogenase (LDH) from Hypoderma sinense larva. H. sinense larvae were collected from yak (Bos grunniens) and identified by a PCR-RFLP method. Analysis of LDH activity showed that the total LDH activity in H. sinense larva was negatively correlated with the length of larva. Polyacrylamide gel electrophoresis of the extracts of H. sinense larvae revealed one band of LDH, which was then purified by affinity chromatography and gel filtration. This enzyme showed an approximately 36 kDa band on SDS-gel under both reducing and non-reducing conditions, in addition, size exclusion chromatography analysis showed that its molecular weight was smaller than bovine serum albumin (67 kDa), indicating that it contains only one subunit. Michaelis constants (Km) values assay revealed that LDH from H. sinense larva showed significantly lower Km for lactate than other animals. LDH of H. sinense larva was stable at 60 °C for 15 min, and also exhibited high catalytic efficiency in a wide range of pH. HgCl₂ at the concentration of 0.1mM significantly decreased the activity of LDH from H. sinense larva but not at the concentration of 0.01 mM. The results of the present study demonstrate that LDH from H. sinense larva is a thermal stable and pH insensitive enzyme suitable for catalyzing both forward and reverse reactions.

The neuroprotective effect of lactate is not due to improved glutamate uptake after controlled cortical impact in rats

For many years lactate was considered to be a waste product of glycolysis. Data are accumulating that suggest that lactate is an important energy substrate for neurons during activation. In severe traumatic brain injury (TBI) glutamate release and ischemic cerebral blood flow (CBF) are major factors for a mismatch between energy demand and supply and for neuronal cell death. Although ATP and behavior could be improved by lactate treatment after TBI, no histological correlate nor any linkage to better astrocytic glutamate uptake or CBF as possible mechanisms have been described. We subjected male rats to a controlled cortical impact (CCI; 5 m/sec, 2.5 mm). To study the effects of lactate treatment on lesion volume, glutamate release, and CBF, animals were infused with either NaCl or 100 mM lactate for up to 3 h. The role of endogenous lactate was investigated by inhibiting transport with α-cyano-4-hydroxy-cinnamic acid (4-CIN; 90 mg/kg). Lactate treatment 15 min post-CCI reduced lesion volume from 21.1±2.8 mm³ to 12.1±1.9 mm³ at day 2 after CCI. Contusion produced a significant three- to fourfold increase of glutamate in microdialysates, but there was no significant difference between treatments that began 30 min before CCI. In this experiment lesion volume was significantly reduced by lactate at day 7 post-CCI (23.7±4 to 9.3±1-2 mm³). CBF increased immediately after CCI and dropped thereafter below baseline in all animals. Lactate infusion 15 min post-CCI elevated CBF for 20 min in 7 of 10 animals, whereas 7 of 8 NaCl-treated animals showed a further CBF decline. Neuroprotection was achieved by lactate treatment following contusion injury, whereas blocking of endogenous lactate transport exerted no adverse effects. Neuroprotection was not achieved by improved glutamate uptake into astrocytes, but was supported by augmented CBF following CCI. Due to its neuroprotective property, lactate might be a beneficial pharmacological treatment for TBI patients.

Determinants of brain cell metabolic phenotypes and energy substrate utilization unraveled with a modeling approach

Although all brain cells bear in principle a comparable potential in terms of energetics, in reality they exhibit different metabolic profiles. The specific biochemical characteristics explaining such disparities and their relative importance are largely unknown. Using a modeling approach, we show that modifying the kinetic parameters of pyruvate dehydrogenase and mitochondrial NADH shuttling within a realistic interval can yield a striking switch in lactate flux direction. In this context, cells having essentially an oxidative profile exhibit pronounced extracellular lactate uptake and consumption. However, they can be turned into cells with prominent aerobic glycolysis by selectively reducing the aforementioned parameters. In the case of primarily oxidative cells, we also examined the role of glycolysis and lactate transport in providing pyruvate to mitochondria in order to sustain oxidative phosphorylation. The results show that changes in lactate transport capacity and extracellular lactate concentration within the range described experimentally can sustain enhanced oxidative metabolism upon activation. Such a demonstration provides key elements to understand why certain brain cell types constitutively adopt a particular metabolic profile and how specific features can be altered under different physiological and pathological conditions in order to face evolving energy demands.

In an environment with appropriate oxygen levels (normoxia), most eukaryotic cells produce energy by oxidizing glucose into carbon dioxide and water. In this process, glucose is transformed into pyruvate, which then fuels oxidative phosphorylation in the mitochondria. Interestingly, Otto Warburg reported back in the 1920's that some eukaryotic cells prominently process glucose-derived pyruvate into lactate, hence “avoiding" the mitochondrial oxidation despite adequate oxygen concentrations. This phenomenon was termed aerobic glycolysis and was first observed in cancer cells. Since then, it has also been described in several normal tissues including the central nervous system. The biochemical basis of aerobic glycolysis has remained elusive until now. Taking advantage of a modeling approach, we unraveled the main metabolic characteristics that determine whether a cell will be strictly oxidative or rather will exhibit aerobic glycolysis. When applied in the context of the central nervous system, our findings not only provide a theoretical demonstration of why neurons and astrocytes differ in terms of metabolic profile, but also suggest that such complementarity forms the basis for metabolic cooperation between the two cell types.

Imaging cerebral 2-ketoisocaproate metabolism with hyperpolarized (13)C magnetic resonance spectroscopic imaging

The branched chain amino acid transaminase (BCAT) has an important role in nitrogen shuttling and glutamate metabolism in the brain. The purpose of this study was to describe the cerebral distribution and metabolism of hyperpolarized 2-keto[1-(13)C]isocaproate (KIC) in the normal rat using magnetic resonance modalities. Hyperpolarized KIC is metabolized to [1-(13)C]leucine (leucine) by BCAT. The results show that KIC and its metabolic product, leucine, are present at imageable quantities 20 seconds after end of KIC administration throughout the brain. Further, significantly higher metabolism was observed in hippocampal regions compared with the muscle tissue. In conclusion, the cerebral metabolism of hyperpolarized KIC is imaged and hyperpolarized KIC may be a promising substrate for evaluation of cerebral BCAT activity in conjunction with neurodegenerative disease.

Sweet sixteen for ANLS

Since its introduction 16 years ago, the astrocyte-neuron lactate shuttle (ANLS) model has profoundly modified our understanding of neuroenergetics by bringing a cellular and molecular resolution. Praised or disputed, the concept has never ceased to attract attention, leading to critical advances and unexpected insights. Here, we summarize recent experimental evidence further supporting the main tenets of the model. Thus, evidence for distinct metabolic phenotypes between neurons (mainly oxidative) and astrocytes (mainly glycolytic) have been provided by genomics and classical metabolic approaches. Moreover, it has become clear that astrocytes act as a syncytium to distribute energy substrates such as lactate to active neurones. Glycogen, the main energy reserve located in astrocytes, is used as a lactate source to sustain glutamatergic neurotransmission and synaptic plasticity. Lactate is also emerging as a neuroprotective agent as well as a key signal to regulate blood flow. Characterization of monocarboxylate transporter regulation indicates a possible involvement in synaptic plasticity and memory. Finally, several modeling studies captured the implications of such findings for many brain functions. The ANLS model now represents a useful, experimentally based framework to better understand the coupling between neuronal activity and energetics as it relates to neuronal plasticity, neurodegeneration, and functional brain imaging.

Cytosolic Ca2+ regulates the energization of isolated brain mitochondria by formation of pyruvate through the malate–aspartate shuttle

The glutamate-dependent respiration of isolated BM (brain mitochondria) is regulated by Ca2+cyt (cytosolic Ca2+) (S0.5=225±22 nM) through its effects on aralar. We now also demonstrate that the α-glycerophosphate-dependent respiration is controlled by Ca2+cyt (S0.5=60±10 nM). At higher Ca2+cyt (>600 nM), BM accumulate Ca2+ which enhances the rate of intramitochondrial dehydrogenases. The Ca2+-induced increments of state 3 respiration decrease with substrate in the order glutamate>α-oxoglutarate>isocitrate>α-glycerophosphate>pyruvate. Whereas the oxidation of pyruvate is only slightly influenced by Ca2+cyt, we show that the formation of pyruvate is tightly controlled by Ca2+cyt. Through its common substrate couple NADH/NAD+, the formation of pyruvate by LDH (lactate dehydrogenase) is linked to the MAS (malate–aspartate shuttle) with aralar as a central component. A rise in Ca2+cyt in a reconstituted system consisting of BM, cytosolic enzymes of MAS and LDH causes an up to 5-fold enhancement of OXPHOS (oxidative phosphorylation) rates that is due to an increased substrate supply, acting in a manner similar to a ‘gas pedal’. In contrast, Ca2+mit (intramitochondrial Ca2+) regulates the oxidation rates of substrates which are present within the mitochondrial matrix. We postulate that Ca2+cyt is a key factor in adjusting the mitochondrial energization to the requirements of intact neurons.

Is lactate a volume transmitter of metabolic states of the brain?

We present the perspective that lactate is a volume transmitter of cellular signals in brain that acutely and chronically regulate the energy metabolism of large neuronal ensembles. From this perspective, we interpret recent evidence to mean that lactate transmission serves the maintenance of network metabolism by two different mechanisms, one by regulating the formation of cAMP via the lactate receptor GPR81, the other by adjusting the NADH/NAD(+) redox ratios, both linked to the maintenance of brain energy turnover and possibly cerebral blood flow. The role of lactate as mediator of metabolic information rather than metabolic substrate answers a number of questions raised by the controversial oxidativeness of astrocytic metabolism and its contribution to neuronal function.

Pathological alterations of astrocytes in stroke-prone spontaneously hypertensive rats under ischemic conditions

Stroke-prone spontaneously hypertensive rats (SHRSP/Izm) develop severe hypertension, and more than 95% of them die of cerebral stroke. We showed the vulnerability of neuronal cells of SHRSP/Izm rats. Furthermore, we analyzed the characteristics of SHRSP/Izm astrocytes during a stroke. It is known that the proliferating ability of SHRSP/Izm astrocytes is significantly enhanced compared with those in the normotensive Wistar Kyoto rats (WKY/Izm) strain. Conversely, the ability of SHRSP/Izm astrocytes to form tight junctions (TJ) was attenuated compared with astrocytes from WKY/Izm rats. During the stress of hypoxia and reoxygenation (H/R), lactate production, an energy source for neuronal cells, decreased in SHRSP/Izm astrocytes in comparison with the WKY/Izm strain. Moreover, during H/R, SHRSP/Izm astrocytes decreased their production of glial cell line-derived neurotrophic factor (GDNF) in comparison with WKY/Izm astrocytes. Furthermore, SHRSP/Izm rats decreased production of l-serine, compared with WKY/Izm rats following nitric oxide (NO) stimulation. Additionally, in H/R, astrocytes of SHRSP/Izm rats expressed adhesion molecules such as VCAM-1 at higher levels. It is possible that all of these differences between SHRSP/Izm and WKY/Izm astrocytes are not associated with the neurological disorders in SHRSP/Izm. However, attenuated production of lactate and reduced GDNF production in astrocytes may reduce required energy levels and weaken the nutritional status of SHRSP/Ism neuronal cells. We suggest that the attenuation of astrocytes' functions accelerates neuronal cell death during stroke, and may contribute to the development of strokes in SHRSP/Izm. In this review, we summarize the altered properties of SHRSP/Izm astrocytes during a stroke.

Pyruvate incubation enhances glycogen stores and sustains neuronal function during subsequent glucose deprivation

The use of energy substrates, such as lactate and pyruvate, has been shown to improve synaptic function when administered during glucose deprivation. In the present study, we investigated whether prolonged incubation with monocarboxylate (pyruvate or lactate) prior rather than during glucose deprivation can also sustain synaptic and metabolic function. Pyruvate pre-incubation(3-4h) significantly prolonged (>25 min) the tolerance of rat hippocampal slices to delayed glucose deprivation compared to control and lactate pre-incubated slices, as revealed by field excitatory post synaptic potentials (fEPSPs); pre-incubation with pyruvate also reduced the marked decrease in NAD(P)H fluorescence resulting from glucose deprivation. Moreover, pyruvate exposure led to the enhancement of glycogen stores with time, compared to glucose alone (12 μmol/g tissue at 4h vs. 3.5 μmol/g tissue). Prolonged resistance to glucose deprivation following exogenous pyruvate incubation was prevented by glycogenolysis inhibitors, suggesting that enhanced glycogen mediates the delay in synaptic activity failure. The application of an adenosine A1 receptor antagonist enhanced glycogen utilization and prolonged the time to synaptic failure, further confirming this hypothesis of the importance of glycogen. Moreover, tissue levels of ATP were also significantly maintained during glucose deprivation in pyruvate pretreated slices compared to control and lactate. In summary, these experiments indicate that pyruvate exposure prior to glucose deprivation significantly increased the energy buffering capacity of hippocampal slices, particularly by enhancing internal glycogen stores, delaying synaptic failure during glucose deprivation by maintaining ATP levels, and minimizing the decrease in the levels of NAD(P)H.

Neuronal-astrocyte metabolic interactions: understanding the transition into abnormal astrocytoma metabolism

Brain function depends on complex metabolic interactions among only a few different cell types, with astrocytes providing critical support for neurons. Astrocyte functions include buffering the extracellular space, providing substrates to neurons, interchanging glutamate and glutamine for synaptic transmission with neurons, and facilitating access to blood vessels. Whereas neurons possess highly oxidative metabolism and easily succumb to ischemia, astrocytes rely more on glycolytic metabolism and hence are less susceptible tolack of oxygen. Astrocytoma cells seem to retain basic metabolic mechanisms of astrocytes; for example, they show a high glycolytic rate, lactate extrusion, ability to flourish under hypoxia, and opportunistic use of mechanisms to enhance cell division and maintain growth. Differences in metabolism between neurons and astrocytes may also extend to astrocytoma cells, providing therapeutic opportunities against astrocytomas, including sensitivity to acetate, a high rate of glycolysis and lactate extrusion, glutamate uptake transporters, differential sensitivities of monocarboxylate transporters, presence of glycogen, high interlinking with gap junctions, use of nicotinamide adenine dinucleotide phosphate for lipid synthesis, using different isoforms of synthetic enzymes (e.g. isocitrate dehydrogenase, pyruvate carboxylase, pyruvate kinase, lactate dehydrogenase), and different glucose uptake mechanisms. These unique metabolic susceptibilities may augment conventional therapeutic attacks based on cell division differences and surface receptors alone.

Differential inhibition of mitochondrial respiratory complexes by inhalation of combustion smoke and carbon monoxide, in vivo, in the rat brain

Combustion smoke contains gases and particulates, which act via hypoxia and cytotoxicity producing mechanisms to injure cells and tissues. While carbon monoxide (CO) is the major toxicant in smoke, its toxicity is exacerbated in the presence of other compounds. Here, we examined modulations of mitochondrial and cytosolic energy metabolism by inhalation of combustion smoke versus CO, in vivo, in the rat brain. Measurements revealed reduced activities of respiratory chain (RC) complexes, with greater inhibition by smoke than equivalent CO in ambient air. In the case of RC complex IV, inhibition by CO and smoke was similar--suggesting that complex IV inhibition is primarily by the action of CO. In contrast, inhibition of complexes I and III was greater by smoke. Increases in cytosolic lactate dehydrogenase and pyruvate kinase activities accompanied inhibition of RC complexes, likely reflecting compensatory increases in cytosolic energy production. Together, the data provide new insights into the mechanisms of smoke inhalation-induced perturbations of brain energetics, which impact neuronal function and contribute to the development of neuropathologies in survivors of exposures to CO and combustion smoke.

Hyperglycaemia and diabetes impair gap junctional communication among astrocytes

Sensory and cognitive impairments have been documented in diabetic humans and animals, but the pathophysiology of diabetes in the central nervous system is poorly understood. Because a high glucose level disrupts gap junctional communication in various cell types and astrocytes are extensively coupled by gap junctions to form large syncytia, the influence of experimental diabetes on gap junction channel-mediated dye transfer was assessed in astrocytes in tissue culture and in brain slices from diabetic rats. Astrocytes grown in 15–25 mmol/l glucose had a slow-onset, poorly reversible decrement in gap junctional communication compared with those grown in 5.5 mmol/l glucose. Astrocytes in brain slices from adult STZ (streptozotocin)-treated rats at 20–24 weeks after the onset of diabetes also exhibited reduced dye transfer. In cultured astrocytes grown in high glucose, increased oxidative stress preceded the decrement in dye transfer by several days, and gap junctional impairment was prevented, but not rescued, after its manifestation by compounds that can block or reduce oxidative stress. In sharp contrast with these findings, chaperone molecules known to facilitate protein folding could prevent and rescue gap junctional impairment, even in the presence of elevated glucose level and oxidative stress. Immunostaining of Cx (connexin) 43 and 30, but not Cx26, was altered by growth in high glucose. Disruption of astrocytic trafficking of metabolites and signalling molecules may alter interactions among astrocytes, neurons and endothelial cells and contribute to changes in brain function in diabetes. Involvement of the microvasculature may contribute to diabetic complications in the brain, the cardiovascular system and other organs.

Coupling between the blood lactate-to-pyruvate ratio and MCA Vmean at the onset of exercise in humans

Activation-induced increase in cerebral blood flow is coupled to enhanced metabolic activity, maybe with brain tissue redox state and oxygen tension as key modulators. To evaluate this hypothesis at the onset of exercise in humans, blood was sampled at 0.1 to 0.2 Hz from the radial artery and right internal jugular vein, while middle cerebral artery mean flow velocity (MCA Vmean) was recorded. Both the arterial and venous lactate-to-pyruvate ratio increased after 10 s ( P < 0.05), and the arterial ratio remained slightly higher than the venous ( P < 0.05). The calculated average cerebral capillary oxygen tension decreased by 2.7 mmHg after 5 s ( P < 0.05), while MCA Vmean increased only after 30 s. Furthermore, there was an unaccounted cerebral carbohydrate uptake relative to the uptake of oxygen that became significant 50 s after the onset of exercise. These findings support brain tissue redox state and oxygenation as potential modulators of an increase in cerebral blood flow at the onset of exercise.

Blood lactate is an important energy source for the human brain

Lactate is a potential energy source for the brain. The aim of this study was to establish whether systemic lactate is a brain energy source. We measured in vivo cerebral lactate kinetics and oxidation rates in 6 healthy individuals at rest with and without 90 mins of intravenous lactate infusion (36 mumol per kg bw per min), and during 30 mins of cycling exercise at 75% of maximal oxygen uptake while the lactate infusion continued to establish arterial lactate concentrations of 0.89+/-0.08, 3.9+/-0.3, and 6.9+/-1.3 mmol/L, respectively. At rest, cerebral lactate utilization changed from a net lactate release of 0.06+/-0.01 to an uptake of 0.16+/-0.07 mmol/min during lactate infusion, with a concomitant decrease in the net glucose uptake. During exercise, the net cerebral lactate uptake was further increased to 0.28+/-0.16 mmol/min. Most (13)C-label from cerebral [1-(13)C]lactate uptake was released as (13)CO(2) with 100%+/-24%, 86%+/-15%, and 87%+/-30% at rest with and without lactate infusion and during exercise, respectively. The contribution of systemic lactate to cerebral energy expenditure was 8%+/-2%, 19%+/-4%, and 27%+/-4% for the respective conditions. In conclusion, systemic lactate is taken up and oxidized by the human brain and is an important substrate for the brain both under basal and hyperlactatemic conditions.