Monocarboxylate transporter expression in mouse brain

Ingested Ketone Ester Leads to a Rapid Rise of Acetyl-CoA and Competes with Glucose Metabolism in the Brain of Non-Fasted Mice

The role of ketone bodies in the cerebral energy homeostasis of neurological diseases has begun to attract recent attention particularly in acute neurological diseases. In ketogenic therapies, ketosis is achieved by either a ketogenic diet or by the administration of exogenous ketone bodies. The oral ingestion of the ketone ester (KE), (R)-3-hydroxybutyl (R)-3-hydroxybutyrate, is a new method to generate rapid and significant ketosis (i.e., above 6 mmol/L) in humans. KE is hydrolyzed into β-hydroxybutyrate (βHB) and its precursor 1,3-butanediol. Here, we investigate the effect of oral KE administration (3 mg KE/g of body weight) on brain metabolism of non-fasted mice using liquid chromatography in tandem with mass spectrometry. Ketosis (Cmax = 6.83 ± 0.19 mmol/L) was obtained at Tmax = 30 min after oral KE-gavage. We found that βHB uptake into the brain strongly correlated with the plasma βHB concentration and was preferentially distributed in the neocortex. We showed for the first time that oral KE led to an increase of acetyl-CoA and citric cycle intermediates in the brain of non-fasted mice. Furthermore, we found that the increased level of acetyl-CoA inhibited glycolysis by a feedback mechanism and thus competed with glucose under physiological conditions. The brain pharmacodynamics of this oral KE strongly suggest that this agent should be considered for acute neurological diseases.

Mechanism of Action of Ketogenic Diet Treatment: Impact of Decanoic Acid and Beta-Hydroxybutyrate on Sirtuins and Energy Metabolism in Hippocampal Murine Neurons

The ketogenic diet (KD), a high-lipid and low-carbohydrate diet, has been used in the treatment of epilepsy, neurodegenerative disorders, inborn errors of metabolism and cancer; however, the exact mechanism/s of its therapeutic effect is not completely known. We hypothesized that sirtuins (SIRT)—a group of seven NAD-dependent enzymes and important regulators of energy metabolism may be altered under KD treatment. HT22 hippocampal murine neurons were incubated with two important KD metabolites–beta-hydroxybutyrate (BHB) (the predominant ketone body) and decanoic acid (C10), both accumulating under KD. Enzyme activity, protein, and gene expressions of SIRT 1-4, enzyme capacities of the mitochondrial respiratory chain complexes (MRC), citrate synthase (CS) and gene expression of monocarboxylate transporters were measured in control (untreated) and KD-treated cells. Incubation with both–BHB and C10 resulted in significant elevation of SIRT1 enzyme activity and an overall upregulation of the MRC. C10 incubation showed prominent increases in maximal activities of complexes I + III and complex IV of the MRC and ratios of their activities to that of CS, pointing towards a more efficient functioning of the mitochondria in C10-treated cells.

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

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

The human central nervous system discharges carbon dioxide and lactic acid into the cerebrospinal fluid

BACKGROUND

The central nervous system was previously thought to draw oxygen and nutrition from the arteries and discharge carbon dioxide and other metabolic wastes into the venous system. At present, the functional role of cerebrospinal fluid in brain metabolism is not fully known.

METHODS

In this prospective observational study, we performed gas analysis on venous blood and cerebrospinal fluid simultaneously acquired from 16 consecutive preoperative patients without any known neurological disorders.

RESULTS

The carbon dioxide partial pressure (pCO₂) (p < 0.0001) and lactic acid level (p < 0.001) in the cerebrospinal fluid were significantly higher than those in the peripheral venous blood, suggesting that a considerable proportion of metabolic carbon dioxide and lactic acid is discharged from the central nervous system into the cerebrospinal fluid. The oxygen partial pressure (pO₂) was much higher in the cerebrospinal fluid than in the venous blood, corroborating the conventional theory of cerebrospinal fluid circulatory dynamics. The pCO₂ of the cerebrospinal fluid showed a strong negative correlation with age (R = - 0.65, p = 0.0065), but the other studied variables did not show significant correlation with age.

CONCLUSION

Carbon dioxide and lactic acid are discharged into the circulating cerebrospinal fluid, as well as into the venules. The level of carbon dioxide in the cerebrospinal fluid significantly decreased with age.

Altered lactate metabolism in Huntington's disease is dependent on GLUT3 expression

AIMS

Huntington's disease (HD) is a neurodegenerative disorder characterized by progressive abnormalities in cognitive function, mental state, and motor control. HD is characterized by a failure in brain energy metabolism. It has been proposed that monocarboxylates, such as lactate, support brain activity. During neuronal synaptic activity, ascorbic acid released from glial cells stimulates lactate and inhibits glucose transport. The aim of this study was to evaluate the expression and function of monocarboxylate transporters (MCTs) in two HD models.

METHODS

Using immunofluorescence, qPCR, and Western blot analyses, we explored mRNA and protein levels of MCTs in the striatum of R6/2 animals and HdhQ7/111 cells. We also evaluated MCT function in HdhQ7/111 cells using radioactive tracers and the fluorescent lactate sensor Laconic.

RESULTS

We found no significant differences in the mRNA or protein levels of neuronal MCTs. Functional analyses revealed that neuronal MCT2 had a high catalytic efficiency in HD cells. Ascorbic acid did not stimulate lactate uptake in HD cells. Ascorbic acid was also unable to inhibit glucose transport in HD cells because they exhibit decreased expression of the neuronal glucose transporter GLUT3.

CONCLUSION

We demonstrate that stimulation of lactate uptake by ascorbic acid is a consequence of inhibiting glucose transport. Supporting this, lactate transport stimulation by ascorbic acid in HD cells was completely restored by overexpressing GLUT3. Therefore, alterations in GLUT3 expression could be responsible for inefficient use of lactate in HD neurons, contributing to the metabolic failure observed in HD.

Physiology and molecular biology of barrier mechanisms in the fetal and neonatal brain

Properties of the local internal environment of the adult brain are tightly controlled providing a stable milieu essential for its normal function. The mechanisms involved in this complex control are structural, molecular and physiological (influx and efflux transporters) frequently referred to as the 'blood-brain barrier'. These mechanisms include regulation of ion levels in brain interstitial fluid essential for normal neuronal function, supply of nutrients, removal of metabolic products, and prevention of entry or elimination of toxic agents. A key feature is cerebrospinal fluid secretion and turnover. This is much less during development, allowing greater accumulation of permeating molecules. The overall effect of these mechanisms is to tightly control the exchange of molecules into and out of the brain. This review presents experimental evidence currently available on the status of these mechanisms in developing brain. It has been frequently stated for over nearly a century that the blood-brain barrier is not present or at least is functionally deficient in the embryo, fetus and newborn. We suggest the alternative hypothesis that the barrier mechanisms in developing brain are likely to be appropriately matched to each stage of its development. The contributions of different barrier mechanisms, such as changes in constituents of cerebrospinal fluid in relation to specific features of brain development, for example neurogenesis, are only beginning to be studied. The evidence on this previously neglected aspect of brain barrier function is outlined. We also suggest future directions this field could follow with special emphasis on potential applications in a clinical setting.

Astrocytic glycogen-derived lactate fuels the brain during exhaustive exercise to maintain endurance capacity

Brain glycogen stored in astrocytes provides lactate as an energy source to neurons through monocarboxylate transporters (MCTs) to maintain neuronal functions such as hippocampus-regulated memory formation. Although prolonged exhaustive exercise decreases brain glycogen, the role of this decrease and lactate transport in the exercising brain remains less clear. Because muscle glycogen fuels exercising muscles, we hypothesized that astrocytic glycogen plays an energetic role in the prolonged-exercising brain to maintain endurance capacity through lactate transport. To test this hypothesis, we used a rat model of exhaustive exercise and capillary electrophoresis-mass spectrometry-based metabolomics to observe comprehensive energetics of the brain (cortex and hippocampus) and muscle (plantaris). At exhaustion, muscle glycogen was depleted but brain glycogen was only decreased. The levels of MCT2, which takes up lactate in neurons, increased in the brain, as did muscle MCTs. Metabolomics revealed that brain, but not muscle, ATP was maintained with lactate and other glycogenolytic/glycolytic sources. Intracerebroventricular injection of the glycogen phosphorylase inhibitor 1,4-dideoxy-1,4-imino-d-arabinitol did not affect peripheral glycemic conditions but suppressed brain lactate production and decreased hippocampal ATP levels at exhaustion. An MCT2 inhibitor, α-cyano-4-hydroxy-cinnamate, triggered a similar response that resulted in lower endurance capacity. These findings provide direct evidence for the energetic role of astrocytic glycogen-derived lactate in the exhaustive-exercising brain, implicating the significance of brain glycogen level in endurance capacity. Glycogen-maintained ATP in the brain is a possible defense mechanism for neurons in the exhausted brain.

Metabolic Vulnerability in the Neurodegenerative Disease Glaucoma

Axons can be several orders of magnitude longer than neural somas, presenting logistical difficulties in cargo trafficking and structural maintenance. Keeping the axon compartment well supplied with energy also presents a considerable challenge; even seemingly subtle modifications of metabolism can result in functional deficits and degeneration. Axons require a great deal of energy, up to 70% of all energy used by a neuron, just to maintain the resting membrane potential. Axonal energy, in the form of ATP, is generated primarily through oxidative phosphorylation in the mitochondria. In addition, glial cells contribute metabolic intermediates to axons at moments of high activity or according to need. Recent evidence suggests energy disruption is an early contributor to pathology in a wide variety of neurodegenerative disorders characterized by axonopathy. However, the degree to which the energy disruption is intrinsic to the axon vs. associated glia is not clear. This paper will review the role of energy availability and utilization in axon degeneration in glaucoma, a chronic axonopathy of the retinal projection.

Potential protective effects of autophagy activated in MPP+ treated astrocytes

Astrocytes, which have various important functions, have previously been associated with Parkinsons disease (PD), particularly in 1-methyl-4-phenylpyridinium (MPP+) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) models of PD. MPP+ is the toxic metabolite of MPTP and is generated by the enzymatic activity of monoamine oxidase B, which is predominantly located in astrocytes. MPP+ acts as a mitochondrial complex I inhibitor. Autophagy is an evolutionarily conserved self-digestion pathway in eukaryotic cells, which occurs in response to various types of stress, including starvation and oxidative stress. Lithium treatment has previously been shown to induce autophagy in astrocytes by inhibiting the enzyme inositol monophosphatase, which may aid in the treatment of neurodegenerative diseases, including Huntington's disease, in which the toxic protein is an autophagy substrate. Therefore, using western blotting and MTT assay, the present study aimed to investigate the protective effects of lithium-induced autophagy against astrocyte injury caused by MPP+ treatment, as well as the potential underlying mechanisms. The results of the present study suggested that lithium was able to induce autophagy in astrocytes treated with MPP+, and this likely occurred via activation of the phosphoinositide 3-kinase/AKT pathway.

The genetic variation in monocarboxylic acid transporter 2 (MCT2) has functional and clinical relevance with male infertility

Monocarboxylic acid transporter 2 (MCT2) transports pyruvate and lactate outside and inside of sperms, mainly as energy sources and plays roles in the regulation of spermatogenesis. We investigated the association among genetic variations in the MCT2 gene, male infertility and MCT2 expression levels in sperm. The functional and genetic significance of the intron 2 (+28201A > G, rs10506398) and 3’ untranslated region (UTR) single nucleotide polymorphism (SNP) (+2626G > A, rs10506399) of MCT2 variants were investigated. Two MCT2 polymorphisms were associated with male infertility (n = 471, P < 0.05). In particular, the MCT2-3’ UTR SNP (+2626 G > A) had a strong association with the oligoasthenoteratozoospermia (OAT) group. The +2626GG type had an almost 2.4-fold higher sperm count than that of the +2626AA type (+2626GG; 66 × 10⁶vs +2626AA; 27 × 10⁶, P < 0.0001). The MCT2-3’ UTR SNP may be important for expression, as it is located at the MCT2 3’ UTR. The average MCT2 protein amount in sperm of the +2626GG type was about two times higher than that of the +2626AA type. The results suggest that genetic variation in MCT2 has functional and clinical relevance with male infertility.

Influx mechanisms in the embryonic and adult rat choroid plexus: a transcriptome study

The transcriptome of embryonic and adult rat lateral ventricular choroid plexus, using a combination of RNA-Sequencing and microarray data, was analyzed by functional groups of influx transporters, particularly solute carrier (SLC) transporters. RNA-Seq was performed at embryonic day (E) 15 and adult with additional data obtained at intermediate ages from microarray analysis. The largest represented functional group in the embryo was amino acid transporters (twelve) with expression levels 2–98 times greater than in the adult. In contrast, in the adult only six amino acid transporters were up-regulated compared to the embryo and at more modest enrichment levels (<5-fold enrichment above E15). In E15 plexus five glucose transporters, in particular Glut-1, and only one monocarboxylate transporter were enriched compared to the adult, whereas only two glucose transporters but six monocarboxylate transporters in the adult plexus were expressed at higher levels than in embryos. These results are compared with earlier published physiological studies of amino acid and monocarboxylate transport in developing rodents. This comparison shows correlation of high expression of some transporters in the developing brain with higher amino acid transport activity reported previously. Data for divalent metal transporters are also considered. Immunohistochemistry of several transporters (e.g., Slc16a10, a thyroid hormone transporter) gene products was carried out to confirm translational activity and to define cellular distribution of the proteins. Overall the results show that there is substantial expression of numerous influx transporters in the embryonic choroid plexus, many at higher levels than in the adult. This, together with immunohistochemical evidence and data from published physiological transport studies suggests that the choroid plexus in embryonic brain plays a major role in supplying the developing brain with essential nutrients.

Lactate transport and signaling in the brain: potential therapeutic targets and roles in body-brain interaction

Lactate acts as a 'buffer' between glycolysis and oxidative metabolism. In addition to being exchanged as a fuel by the monocarboxylate transporters (MCTs) between cells and tissues with different glycolytic and oxidative rates, lactate may be a 'volume transmitter' of brain signals. According to some, lactate is a preferred fuel for brain metabolism. Immediately after brain activation, the rate of glycolysis exceeds oxidation, leading to net production of lactate. At physical rest, there is a net efflux of lactate from the brain into the blood stream. But when blood lactate levels rise, such as in physical exercise, there is net influx of lactate from blood to brain, where the lactate is used for energy production and myelin formation. Lactate binds to the lactate receptor GPR81 aka hydroxycarboxylic acid receptor (HCAR1) on brain cells and cerebral blood vessels, and regulates the levels of cAMP. The localization and function of HCAR1 and the three MCTs (MCT1, MCT2, and MCT4) expressed in brain constitute the focus of this review. They are possible targets for new therapeutic drugs and interventions. The author proposes that lactate actions in the brain through MCTs and the lactate receptor underlie part of the favorable effects on the brain resulting from physical exercise.

Transporters at CNS barrier sites: obstacles or opportunities for drug delivery?

The blood-brain barrier (BBB) and blood-cerebrospinal fluid (BCSF) barriers are critical determinants of CNS homeostasis. Additionally, the BBB and BCSF barriers are formidable obstacles to effective CNS drug delivery. These brain barrier sites express putative influx and efflux transporters that precisely control permeation of circulating solutes including drugs. The study of transporters has enabled a shift away from "brute force" approaches to delivering drugs by physically circumventing brain barriers towards chemical approaches that can target specific compounds of the BBB and/or BCSF barrier. However, our understanding of transporters at the BBB and BCSF barriers has primarily focused on understanding efflux transporters that efficiently prevent drugs from attaining therapeutic concentrations in the CNS. Recently, through the characterization of multiple endogenously expressed uptake transporters, this paradigm has shifted to the study of brain transporter targets that can facilitate drug delivery (i.e., influx transporters). Additionally, signaling pathways and trafficking mechanisms have been identified for several endogenous BBB/BCSF transporters, thereby offering even more opportunities to understand how transporters can be exploited for optimization of CNS drug delivery. This review presents an overview of the BBB and BCSF barrier as well as the many families of transporters functionally expressed at these barrier sites. Furthermore, we present an overview of various strategies that have been designed and utilized to deliver therapeutic agents to the brain with a particular emphasis on those approaches that directly target endogenous BBB/BCSF barrier transporters.

Functional interaction between bicarbonate transporters and carbonic anhydrase modulates lactate uptake into mouse cardiomyocytes

Blood-derived lactate is a precious energy substrate for the heart muscle. Lactate is transported into cardiomyocytes via monocarboxylate transporters (MCTs) together with H(+), which couples lactate uptake to cellular pH regulation. In this study, we have investigated how the interplay between different acid/base transporters and carbonic anhydrases (CA), which catalyze the reversible hydration of CO2, modulates the uptake of lactate into isolated mouse cardiomyocytes. Lactate transport was estimated both as lactate-induced acidification and as changes in intracellular lactate levels measured with a newly developed Förster resonance energy transfer (FRET) nanosensor. Recordings of intracellular pH showed an increase in the rate of lactate-induced acidification when CA was inhibited by 6-ethoxy-2-benzothiazolesulfonamide (EZA), while direct measurements of lactate flux demonstrated a decrease in MCT transport activity, when CA was inhibited. The data indicate that catalytic activity of extracellular CA increases lactate uptake and counteracts intracellular lactate-induced acidification. We propose a hypothetical model, in which HCO3 (-), formed from cell-derived CO2 at the outer surface of the cardiomyocyte plasma membrane by membrane-anchored, extracellular CA, is transported into the cell via Na(+)/HCO3 (-) cotransport to counteract intracellular acidification, while the remaining H(+) stabilizes extracellular pH at the surface of the plasma membrane during MCT activity to enhance lactate influx into cardiomyocytes.

The neuroplastin adhesion molecules are accessory proteins that chaperone the monocarboxylate transporter MCT2 to the neuronal cell surface

BACKGROUND

The neuroplastins np65 and np55 are two synapse-enriched immunoglobulin (Ig) superfamily adhesion molecules that contain 3 and 2 Ig domains respectively. Np65 is implicated in long term, activity dependent synaptic plasticity, including LTP. Np65 regulates the surface expression of GluR1 receptor subunits and the localisation of GABA(A) receptor subtypes in hippocampal neurones. The brain is dependent not only on glucose but on monocarboxylates as sources of energy. The. monocarboxylate transporters (MCTs) 1-4 are responsible for the rapid proton-linked translocation of monocarboxylates including pyruvate and lactate across the plasma membrane and require association with either embigin or basigin, proteins closely related to neuroplastin, for plasma membrane expression and activity. MCT2 plays a key role in providing lactate as an energy source to neurons.

METHODOLOGY/FINDINGS

Here we use co-transfection of neuroplastins and monocarboxylate transporters into COS-7 cells to demonstrate that neuroplastins can act as ancillary proteins for MCT2. We also show that Xenopus laevis oocytes contain endogenous neuroplastin and its knockdown with antisense RNA reduces the surface expression of MCT2 and associated lactate transport. Immunocytochemical studies show that MCT2 and the neuroplastins are co-localised in rat cerebellum. Strikingly neuroplastin and MCT2 are enriched in the same parasagittal zebrin II-negative stripes.

CONCLUSIONS

These data strongly suggest that neuroplastins act as key ancillary proteins for MCT2 cell surface localisation and activity in some neuronal populations, thus playing an important role in facilitating the uptake of lactate for use as a respiratory fuel.

Metabolite regulation of nucleo-cytosolic trafficking of carbohydrate response element-binding protein (ChREBP): role of ketone bodies

The carbohydrate response element-binding protein (ChREBP) is a glucose-responsive transcription factor that plays a critical role in converting excess carbohydrate to storage fat in liver. In response to changing glucose levels, ChREBP activity is regulated by nucleo-cytoplasmic shuttling of ChREBP via interactions with 14-3-3 proteins and importins. The nuclear/cytosol trafficking is regulated partly by phosphorylation/dephosphorylation of serine 196 mediated by cAMP-dependent protein kinase and protein phosphatase. We show here that protein-free extracts of starved and high fat-fed livers contain metabolites that activate interaction of ChREBP·14-3-3 and inhibit the ChREBP/importin α interaction, resulting in cytosolic localization. These metabolites were identified as β-hydroxybutyrate and acetoacetate. Nuclear localization of GFP-ChREBP is rapidly inhibited in hepatocytes incubated in β-hydroxybutyrate or fatty acids, and the observed inhibition is closely correlated with the production of ketone bodies. These observations show that ketone bodies play an important role in the regulation of ChREBP activity by restricting ChREBP localization to the cytoplasm, thus inhibiting fat synthesis during periods of ketosis.

Role of monocarboxylate transporters in drug delivery to the brain

Monocarboxylate transporters (MCTs) are known to mediate the transport of short chain monocarboxylates such as lactate, pyruvate and butyrate. Currently, fourteen members of this transporter family have been identified by sequence homology, of which only the first four members (MCT1- MCT4) have been shown to mediate the proton-linked transport of monocarboxylates. Another transporter family involved in the transport of endogenous monocarboxylates is the sodium coupled MCTs (SMCTs). These act as a symporter and are dependent on a sodium gradient for their functional activity. MCT1 is the predominant transporter among the MCT isoforms and is present in almost all tissues including kidney, intestine, liver, heart, skeletal muscle and brain. The various isoforms differ in terms of their substrate specificity and tissue localization. Due to the expression of these transporters in the kidney, intestine, and brain, they may play an important role in influencing drug disposition. Apart from endogenous short chain monocarboxylates, they also mediate the transport of exogenous drugs such as salicylic acid, valproic acid, and simvastatin acid. The influence of MCTs on drug pharmacokinetics has been extensively studied for γ-hydroxybutyrate (GHB) including distribution of this drug of abuse into the brain and the results will be summarized in this review. The physiological role of these transporters in the brain and their specific cellular localization within the brain will also be discussed. This review will also focus on utilization of MCTs as potential targets for drug delivery into the brain including their role in the treatment of malignant brain tumors.

Oligodendroglia metabolically support axons and contribute to neurodegeneration

Oligodendroglia support axon survival and function through mechanisms independent of myelination, and their dysfunction leads to axon degeneration in several diseases. The cause of this degeneration has not been determined, but lack of energy metabolites such as glucose or lactate has been proposed. Lactate is transported exclusively by monocarboxylate transporters, and changes to these transporters alter lactate production and use. Here we show that the most abundant lactate transporter in the central nervous system, monocarboxylate transporter 1 (MCT1, also known as SLC16A1), is highly enriched within oligodendroglia and that disruption of this transporter produces axon damage and neuron loss in animal and cell culture models. In addition, this same transporter is reduced in patients with, and in mouse models of, amyotrophic lateral sclerosis, suggesting a role for oligodendroglial MCT1 in pathogenesis. The role of oligodendroglia in axon function and neuron survival has been elusive; this study defines a new fundamental mechanism by which oligodendroglia support neurons and axons.

Monocarboxylate transporter 2 and stroke severity in a rodent model of sleep apnea

Stroke is not only more prevalent but is also associated with more severe adverse functional outcomes among patients with sleep apnea. Monocarboxylate transporters (MCT) are important regulators of cellular bioenergetics, have been implicated in brain susceptibility to acute severe hypoxia (ASH), and could underlie the unfavorable prognosis of cerebrovascular accidents in sleep apnea patients. Rodents were exposed to either intermittent hypoxia (IH) during sleep, a characteristic feature of sleep apnea, or to sustained hypoxia (SH), and expression of MCT1 and MCT2 was assessed. In addition, the functional recovery to middle cerebral artery occlusion (MCAO) in rats and hMCT2 transgenic mice and of hippocampal slices subjected to ASH was assessed, as well as the effects of MCT blocker and MCT2 antisense oligonucleotides and siRNAs. IH, but not SH, induced significant reductions in MCT2 expression over time at both the mRNA and protein levels and in the functional recovery of hippocampal slices subjected to ASH. Similarly, MCAO-induced infarcts were significantly greater in IH-exposed rats and mice, and overexpression of hMCT2 in mice markedly attenuated the adverse effects of IH. Exogenous pyruvate treatment reduced infarct volumes in normoxic rats but not in IH-exposed rats. Administration of the MCT2 blocker 4CN, but not the MCT1 antagonist p-chloromercuribenzene sulfonate, increased infarct size. Thus, prolonged exposures to IH mimicking sleep apnea are associated with increased CNS vulnerability to ischemia that is mediated, at least in part, by concomitant decreases in the expression and function of MCT2. Efforts to develop agonists of MCT2 should provide opportunities to ameliorate the overall outcome of stroke.

Temporal changes in mRNA expression of the brain nutrient transporters in the lithium-pilocarpine model of epilepsy in the immature and adult rat

The lithium-pilocarpine model mimics most features of human temporal lobe epilepsy. Following our prior studies of cerebral metabolic changes, here we explored the expression of transporters for glucose (GLUT1 and GLUT3) and monocarboxylates (MCT1 and MCT2) during and after status epilepticus (SE) induced by lithium-pilocarpine in PN10, PN21, and adult rats. In situ hybridization was used to study the expression of transporter mRNAs during the acute phase (1, 4, 12 and 24h of SE), the latent phase, and the early and late chronic phases. During SE, GLUT1 expression was increased throughout the brain between 1 and 12h of SE, more strongly in adult rats; GLUT3 increased only transiently, at 1 and 4h of SE and mainly in PN10 rats; MCT1 was increased at all ages but 5-10-fold more in adult than in immature rats; MCT2 expression increased mainly in adult rats. At all ages, MCT1 and MCT2 up-regulation was limited to the circuit of seizures while GLUT1 and GLUT3 changes were more widespread. During the latent and chronic phases, the expression of nutrient transporters was normal in PN10 rats. In PN21 rats, GLUT1 was up-regulated in all brain regions. In contrast, in adult rats GLUT1 expression was down-regulated in the piriform cortex, hilus and CA1 as a result of extensive neuronal death. The changes in nutrient transporter expression reported here further support previous findings in other experimental models demonstrating rapid transcriptional responses to marked changes in cerebral energetic/glucose demand.

Altered expression of brain monocarboxylate transporter 1 in models of temporal lobe epilepsy

Monocarboxylate transporter 1 (MCT1) facilitates the transport of monocarboxylate fuels (lactate, pyruvate and ketone bodies) and acidic drugs, such as valproic acid, across cell membranes. We recently reported that MCT1 is deficient on microvessels in the epileptogenic hippocampal formation in patients with medication-refractory temporal lobe epilepsy (TLE). To further define the role of MCT1 in the pathophysiology of TLE, we used immunohistochemistry and stereological analysis to localize and quantify the transporter in the hippocampal formation in three novel and highly relevant rat models of TLE and in nonepileptic control animals. One model utilizes methionine sulfoximine to induce brain glutamine synthetase deficiency and recurrent limbic seizures, while two models employ an episode of perforant pathway stimulation to cause epilepsy. MCT1 was lost on microvessels and upregulated on astrocytes in the hippocampal formation in all models of TLE. Notably, the loss of MCT1 on microvessels was not due to a reduction in microvessel density. The similarities in MCT1 expression among human subjects with TLE and several animal models of the disease strongly suggest a critical role of this molecule in the pathogenesis of TLE. We hypothesize that the downregulation of MCT1 may promote seizures via impaired uptake of ketone bodies and antiepileptic drugs by the epileptogenic brain. We also propose that the overexpression of MCT1 on astrocytes may lead to increased uptake or release of monocarboxylates by these cells, with important implications for brain metabolism and excitability. These hypotheses can now be rigorously tested in several animal models that replicate key features of human TLE.

Regulation of oligodendrocyte development and myelination by glucose and lactate

In the gray matter of the brain, astrocytes have been suggested to export lactate (derived from glucose or glycogen) to neurons to power their mitochondria. In the white matter, lactate can support axon function in conditions of energy deprivation, but it is not known whether lactate acts by preserving energy levels in axons or in oligodendrocytes, the myelinating processes of which are damaged rapidly in low energy conditions. Studies of cultured cells suggest that oligodendrocytes are the cell type in the brain that consumes lactate at the highest rate, in part to produce membrane lipids presumably for myelin. Here, we use pH imaging to show that oligodendrocytes in the white matter of the rat cerebellum and corpus callosum take up lactate via monocarboxylate transporters (MCTs), which we identify as MCT1 by confocal immunofluorescence and electron microscopy. Using cultured slices of developing cerebral cortex from mice in which oligodendrocyte lineage cells express GFP (green fluorescent protein) under the control of the Sox10 promoter, we show that a low glucose concentration reduces the number of oligodendrocyte lineage cells and myelination. Myelination is rescued when exogenous l-lactate is supplied. Thus, lactate can support oligodendrocyte development and myelination. In CNS diseases involving energy deprivation at times of myelination or remyelination, such as periventricular leukomalacia leading to cerebral palsy, stroke, and secondary ischemia after spinal cord injury, lactate transporters in oligodendrocytes may play an important role in minimizing the inhibition of myelination that occurs.

MCT expression and lactate influx/efflux in tanycytes involved in glia-neuron metabolic interaction

Metabolic interaction via lactate between glial cells and neurons has been proposed as one of the mechanisms involved in hypothalamic glucosensing. We have postulated that hypothalamic glial cells, also known as tanycytes, produce lactate by glycolytic metabolism of glucose. Transfer of lactate to neighboring neurons stimulates ATP synthesis and thus contributes to their activation. Because destruction of third ventricle (III-V) tanycytes is sufficient to alter blood glucose levels and food intake in rats, it is hypothesized that tanycytes are involved in the hypothalamic glucose sensing mechanism. Here, we demonstrate the presence and function of monocarboxylate transporters (MCTs) in tanycytes. Specifically, MCT1 and MCT4 expression as well as their distribution were analyzed in Sprague Dawley rat brain, and we demonstrate that both transporters are expressed in tanycytes. Using primary tanycyte cultures, kinetic analyses and sensitivity to inhibitors were undertaken to confirm that MCT1 and MCT4 were functional for lactate influx. Additionally, physiological concentrations of glucose induced lactate efflux in cultured tanycytes, which was inhibited by classical MCT inhibitors. Because the expression of both MCT1 and MCT4 has been linked to lactate efflux, we propose that tanycytes participate in glucose sensing based on a metabolic interaction with neurons of the arcuate nucleus, which are stimulated by lactate released from MCT1 and MCT4-expressing tanycytes.

Molecular biology of the blood-brain and the blood-cerebrospinal fluid barriers: similarities and differences

Efficient processing of information by the central nervous system (CNS) represents an important evolutionary advantage. Thus, homeostatic mechanisms have developed that provide appropriate circumstances for neuronal signaling, including a highly controlled and stable microenvironment. To provide such a milieu for neurons, extracellular fluids of the CNS are separated from the changeable environment of blood at three major interfaces: at the brain capillaries by the blood-brain barrier (BBB), which is localized at the level of the endothelial cells and separates brain interstitial fluid (ISF) from blood; at the epithelial layer of four choroid plexuses, the blood-cerebrospinal fluid (CSF) barrier (BCSFB), which separates CSF from the CP ISF, and at the arachnoid barrier. The two barriers that represent the largest interface between blood and brain extracellular fluids, the BBB and the BCSFB, prevent the free paracellular diffusion of polar molecules by complex morphological features, including tight junctions (TJs) that interconnect the endothelial and epithelial cells, respectively. The first part of this review focuses on the molecular biology of TJs and adherens junctions in the brain capillary endothelial cells and in the CP epithelial cells. However, normal function of the CNS depends on a constant supply of essential molecules, like glucose and amino acids from the blood, exchange of electrolytes between brain extracellular fluids and blood, as well as on efficient removal of metabolic waste products and excess neurotransmitters from the brain ISF. Therefore, a number of specific transport proteins are expressed in brain capillary endothelial cells and CP epithelial cells that provide transport of nutrients and ions into the CNS and removal of waste products and ions from the CSF. The second part of this review concentrates on the molecular biology of various solute carrier (SLC) transport proteins at those two barriers and underlines differences in their expression between the two barriers. Also, many blood-borne molecules and xenobiotics can diffuse into brain ISF and then into neuronal membranes due to their physicochemical properties. Entry of these compounds could be detrimental for neural transmission and signalling. Thus, BBB and BCSFB express transport proteins that actively restrict entry of lipophilic and amphipathic substances from blood and/or remove those molecules from the brain extracellular fluids. The third part of this review concentrates on the molecular biology of ATP-binding cassette (ABC)-transporters and those SLC transporters that are involved in efflux transport of xenobiotics, their expression at the BBB and BCSFB and differences in expression in the two major blood-brain interfaces. In addition, transport and diffusion of ions by the BBB and CP epithelium are involved in the formation of fluid, the ISF and CSF, respectively, so the last part of this review discusses molecular biology of ion transporters/exchangers and ion channels in the brain endothelial and CP epithelial cells.

Monocarboxylate transporter 1 is deficient on microvessels in the human epileptogenic hippocampus

Monocarboxylate transporter 1 (MCT1) facilitates the transport of important metabolic fuels (lactate, pyruvate and ketone bodies) and possibly also acidic drugs such as valproic acid across the blood-brain barrier. Because an impaired brain energy metabolism and resistance to antiepileptic drugs are common features of temporal lobe epilepsy (TLE), we sought to study the expression of MCT1 in the brain of patients with this disease. Immunohistochemistry and immunogold electron microscopy were used to assess the distribution of MCT1 in brain specimens from patients with TLE and concomitant hippocampal sclerosis (referred to as mesial TLE or MTLE (n=15)), patients with TLE and no hippocampal sclerosis (non-MTLE, n=13) and neurologically normal autopsy subjects (n=8). MCT1 was present on an extensive network of microvessels throughout the hippocampal formation in autopsy controls and to a lesser degree in non-MTLE. Patients with MTLE were markedly deficient in MCT1 on microvessels in several areas of the hippocampal formation, especially CA1, which exhibited a 37% to 48% loss of MCT1 on the plasma membrane of endothelial cells when compared with non-MTLE. These findings suggest that the uptake of blood-derived monocarboxylate fuels and possibly also acidic drugs, such as valproic acid, is perturbed in the epileptogenic hippocampus, particularly in MTLE. We hypothesize that the loss of MCT1 on brain microvessels is mechanistically involved in the pathophysiology of drug-resistant TLE, and propose that re-expression of MCT1 may represent a novel therapeutic approach for this disease.

Overview of the proton-coupled MCT (SLC16A) family of transporters: characterization, function and role in the transport of the drug of abuse gamma-hydroxybutyric acid

The transport of monocarboxylates, such as lactate and pyruvate, is mediated by the SLC16A family of proton-linked membrane transport proteins known as monocarboxylate transporters (MCTs). Fourteen MCT-related genes have been identified in mammals and of these seven MCTs have been functionally characterized. Despite their sequence homology, only MCT1-4 have been demonstrated to be proton-dependent transporters of monocarboxylic acids. MCT6, MCT8 and MCT10 have been demonstrated to transport diuretics, thyroid hormones and aromatic amino acids, respectively. MCT1-4 vary in their regulation, tissue distribution and substrate/inhibitor specificity with MCT1 being the most extensively characterized isoform. Emerging evidence suggests that in addition to endogenous substrates, MCTs are involved in the transport of pharmaceutical agents, including gamma-hydroxybuytrate (GHB), 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase inhibitors (statins), salicylic acid, and bumetanide. MCTs are expressed in a wide range of tissues including the liver, intestine, kidney and brain, and as such they have the potential to impact a number of processes contributing to the disposition of xenobiotic substrates. GHB has been extensively studied as a pharmaceutical substrate of MCTs; the renal clearance of GHB is dose-dependent with saturation of MCT-mediated reabsorption at high doses. Concomitant administration of GHB and L: -lactate to rats results in an approximately two-fold increase in GHB renal clearance suggesting that inhibition of MCT1-mediated reabsorption of GHB may be an effective strategy for increasing renal and total GHB elimination in overdose situations. Further studies are required to more clearly define the role of MCTs on drug disposition and the potential for MCT-mediated detoxification strategies in GHB overdose.

Ascorbic acid participates in a general mechanism for concerted glucose transport inhibition and lactate transport stimulation

In this paper, we present a novel function for ascorbic acid. Ascorbic acid is an important water-soluble antioxidant and cofactor in various enzyme systems. We have previously demonstrated that an increase in neuronal intracellular ascorbic acid is able to inhibit glucose transport in cortical and hippocampal neurons. Because of the presence of sodium-dependent vitamin C transporters, ascorbic acid is highly concentrated in brain, testis, lung, and adrenal glands. In this work, we explored how ascorbic acid affects glucose and lactate uptake in neuronal and non-neuronal cells. Using immunofluorescence and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis, the expression of glucose and ascorbic acid transporters in non-neuronal cells was studied. Like neurons, HEK293 cells expressed GLUT1, GLUT3, and SVCT2. With radioisotope-based methods, only intracellular ascorbic acid, but not extracellular, inhibits 2-deoxyglucose transport in HEK293 cells. As monocarboxylates such as pyruvate and lactate, are important metabolic sources, we analyzed the ascorbic acid effect on lactate transport in cultured neurons and HEK293 cells. Intracellular ascorbic acid was able to stimulate lactate transport in both cell types. Extracellular ascorbic acid did not affect this transport. Our data show that ascorbic acid inhibits glucose transport and stimulates lactate transport in neuronal and non-neuronal cells. Mammalian cells frequently present functional glucose and monocarboxylate transporters, and we describe here a general effect in which ascorbic acid functions like a glucose/monocarboxylate uptake switch in tissues expressing ascorbic acid transporters.

Supply and demand in cerebral energy metabolism: the role of nutrient transporters

Glucose is the obligate energetic fuel for the mammalian brain, and most studies of cerebral energy metabolism assume that the majority of cerebral glucose utilization fuels neuronal activity via oxidative metabolism, both in the basal and activated state. Glucose transporter (GLUT) proteins deliver glucose from the circulation to the brain: GLUT1 in the microvascular endothelial cells of the blood-brain barrier (BBB) and glia; GLUT3 in neurons. Lactate, the glycolytic product of glucose metabolism, is transported into and out of neural cells by the monocarboxylate transporters (MCT): MCT1 in the BBB and astrocytes and MCT2 in neurons. The proposal of the astrocyte-neuron lactate shuttle hypothesis suggested that astrocytes play the primary role in cerebral glucose utilization and generate lactate for neuronal energetics, especially during activation. Since the identification of the GLUTs and MCTs in brain, much has been learned about their transport properties, that is capacity and affinity for substrate, which must be considered in any model of cerebral glucose uptake and utilization. Using concentrations and kinetic parameters of GLUT1 and -3 in BBB endothelial cells, astrocytes, and neurons, along with the corresponding kinetic properties of the MCTs, we have successfully modeled brain glucose and lactate levels as well as lactate transients in response to neuronal stimulation. Simulations based on these parameters suggest that glucose readily diffuses through the basal lamina and interstitium to neurons, which are primarily responsible for glucose uptake, metabolism, and the generation of the lactate transients observed on neuronal activation.

Lactate metabolism in anoxic turtles: an integrative review

Painted turtles can accumulate lactic acid to extremely high concentrations during long-term anoxic submergence, with plasma lactate exceeding 200 mmol l(-1). The aims of this review are twofold: (1) To summarize aspects of lactate metabolism in anoxic turtles that have not been reviewed previously and (2) To identify gaps in our knowledge of turtle lactate metabolism by comparing it with lactate metabolism during and after exercise in other vertebrates. The topics reviewed include analyses of lactate's fate during recovery, the effects of temperature on lactate accumulation and clearance, the interaction of activity and recovery metabolism, fuel utilization during recovery, stress hormone responses during and following anoxia, and cellular lactate transport mechanisms. An analysis of lactate metabolism in anoxic turtles in the context of the 'lactate shuttle' hypothesis is also presented.

Alternative promoter usage and alternative splicing contribute to mRNA heterogeneity of mouse monocarboxylate transporter 2

Expression patterns of monocarboxylate transporter 2 (MCT2) display mRNA diversity in a tissue-specific fashion. We cloned and characterized multiple mct2 5'-cDNA ends from the mouse and determined the structural organization of the mct2 gene. We found that transcription of this gene was initiated from five independent genomic regions that spanned >80 kb on chromosome 10, resulting in five unique exon 1 variants (exons 1a, 1b, 1c, 1d, and 1e) that were then spliced to the common exon 2. Alternative splicing of four internal exons (exons AS1, AS2, AS3, and exon 3) greatly increased the complexity of mRNA diversity. While exon 1c was relatively commonly used for transcription initiation in various tissues, other exon 1 variants were used in a tissue-specific fashion, especially exons 1b and 1d that were used exclusively for testis-specific expression. Sequence analysis of 5'-flanking regions upstream of exons 1a, 1b, and 1c revealed the presence of numerous potential binding sites for ubiquitous transcription factors in all three regions and for transcription factors implicated in testis-specific or hypoxia-induced gene expression in the 1b region. Transient transfection assays demonstrated that each of the three regions contained a functional promoter and that the in vitro, cell type-specific activities of these promoters were consistent with the tissue-specific expression pattern of the mct2 gene in vivo. These results indicate that tissue-specific expression of the mct2 gene is controlled by multiple alternative promoters and that both alternative promoter usage and alternative splicing contribute to the remarkable mRNA diversity of the gene.

Induction of Monocarboxylate Transporter 2 Expression and Ketone Transport following Traumatic Brain Injury in Juvenile and Adult Rats

Based on recent work demonstrating age-dependent ketogenic neuroprotection after traumatic brain injury (TBI), it was hypothesized that the neuroprotection among early post-weaned animals was related to induced cerebral transport of ketones after injury. Regional changes in monocarboxylate transporter 2 (MCT2) were acutely examined with immunohistochemistry after sham surgery or controlled cortical impact injury among postnatal day 35 and adult rats. Both ages showed elevated MCT2 expression in the ipsilateral cerebral vasculature after TBI. Using Western blotting, MCT2 expression was 80-88% greater in microvessels isolated from postnatal day 35 rats at all time points relative to adults. The increased MCT2 expression was temporally correlated with an age-related increase in cerebral uptake of ketones, when ketones were made available after injury.

Mitoenergetic failure in Alzheimer disease

Brain cells are highly energy dependent for maintaining ion homeostasis during high metabolic activity. During active periods, full mitochondrial function is essential to generate ATP from electrons that originate with the oxidation of NADH. Decreasing brain metabolism is a significant cause of cognitive abnormalities of Alzheimer disease (AD), but it remains uncertain whether this is the cause of further pathology or whether synaptic loss results in a lower energy demand. Synapses are the first to show pathological symptoms in AD before the onset of clinical symptoms. Because synaptic function has high energy demands, interruption in mitochondrial energy supply could be the major factor in synaptic failure in AD. A newly discovered age-related decline in neuronal NADH and redox ratio may jeopardize this function. Mitochondrial dehydrogenases and several mutations affecting energy transfer are frequently altered in aging and AD. Thus, with the accumulation of genetic defects in mitochondria at the level of energy transfer, the issue of neuronal susceptibility to damage as a function of age and age-related disease becomes important. In an aging rat neuron model, mitochondria are both chronically depolarized and produce more reactive oxygen species with age. These concepts suggest that multiple treatment targets may be needed to reverse this multifactorial disease. This review summarizes new insights based on the interaction of mitoenergetic failure, glutamate excitotoxicity, and amyloid toxicity in the exacerbation of AD.

Targeting energy metabolism in brain cancer: review and hypothesis

Malignant brain tumors are a significant health problem in children and adults and are often unmanageable. As a metabolic disorder involving the dysregulation of glycolysis and respiration, malignant brain cancer is potentially manageable through changes in metabolic environment. A radically different approach to brain cancer management is proposed that combines metabolic control analysis with the evolutionarily conserved capacity of normal cells to survive extreme shifts in physiological environment. In contrast to malignant brain tumors that are largely dependent on glycolysis for energy, normal neurons and glia readily transition to ketone bodies (beta-hydroxybutyrate) for energy in vivo when glucose levels are reduced. The bioenergetic transition from glucose to ketone bodies metabolically targets brain tumors through integrated anti-inflammatory, anti-angiogenic, and pro-apoptotic mechanisms. The approach focuses more on the genomic flexibility of normal cells than on the genomic defects of tumor cells and is supported from recent studies in orthotopic mouse brain tumor models and in human pediatric astrocytoma treated with dietary energy restriction and the ketogenic diet.

High intensity exercise decreases global brain glucose uptake in humans

Physiological activation increases glucose uptake locally in the brain. However, it is not known how high intensity exercise affects regional and global brain glucose uptake. The effect of exercise intensity and exercise capacity on brain glucose uptake was directly measured using positron emission tomography (PET) and [18F]fluoro-deoxy-glucose ([18F]FDG). Fourteen healthy, right-handed men were studied after 35 min of bicycle exercise at exercise intensities corresponding to 30, 55 and 75% of on three separate days. [18F]FDG was injected 10 min after the start of the exercise. Thereafter exercise was continued for another 25 min. PET scanning of the brain was conducted after completion of the exercise. Regional glucose metabolic rate (rGMR) decreased in all measured cortical regions as exercise intensity increased. The mean decrease between the highest and lowest exercise intensity was 32% globally in the brain (38.6+/-4.6 versus 26.1+/-5.0 micromol (100 g)-1 min-1, P<0.001). Lactate availability during exercise tended to correlate negatively with the observed brain glucose uptake. In addition, the decrease in glucose uptake in the dorsal part of the anterior cingulate cortex (37% versus 20%, P<0.05 between 30% and 75% of VO2max) was significantly more pronounced in subjects with higher exercise capacity. These results demonstrate that brain glucose uptake decreases with increase in exercise intensity. Therefore substrates other than glucose, most likely lactate, are utilized by the brain in order to compensate the increased energy needed to maintain neuronal activity during high intensity exercise. Moreover, it seems that exercise training could be related to adaptive metabolic changes locally in the frontal cortical regions.

Relationship between skeletal muscle MCT1 and accumulated exercise during voluntary wheel running

We examined whether the quantity of exercise performed influences the expression of monocarboxylate transporter (MCT) 1 and MCT4 in mouse skeletal muscles (plantaris, tibialis anterior, soleus) and heart. Wheel running exercise (1, 3, and 6 wk) was used, which results in marked variations in self-selected running activity. Differences in muscle MCT1 and MCT4 among animals, before the initiation of running, were not related to the quantity of exercise performed on the first day of wheel running. No changes in MCT4 were observed over the course of the study (P > 0.05). After 6 wk of running, were there significant increases in heart (50%; P < 0.05) and muscle MCT1 (31-60%; P < 0.05) but not after 1 and 3 wk (P > 0.05). Because skeletal muscle MCT1 and running distances varied considerably, we examined the relationship between these two parameters. Within the first week of training, MCT1 was negatively correlated with the accumulated running distance (r = -0.70, P < 0.05). On further analysis, it appears that, in the first week, excessive running (>20 km/wk) represses MCT1 (-16.1%; P < 0.05), whereas more modest amounts of running (<20 km/wk) increase MCT1 (+37%; P < 0.05). After 3 wk of running, a positive relationship was observed between MCT1 and running distance (r = +0.76), although there is a threshold that must be exceeded before an increase over the control animals occurs. Finally, in week 6, when MCT1 was increased in the tibialis anterior and plantaris muscles, there were no correlations with the accumulated running distances. These studies have shown that mild exercise training fails to increase MCT4 and that changes in MCT1 are complex, depending not only the accumulated exercise but also on the stage of training.

Neuroenergetics: calling upon astrocytes to satisfy hungry neurons

Classical neuroenergetics states that glucose is the exclusive energy substrate of brain cells and its full oxidation provides all the necessary energy to support brain function. Recent data have revealed a more intricate picture in which astrocytes play a key role in supplying lactate as an additional energy substrate in register with glutamatergic activity.

Pyruvate shuttle in muscle cells: high-affinity pyruvate transport sites insensitive to trans-lactate efflux

The specificity of the transport mechanisms for pyruvate and lactate and their sensitivity to inhibitors were studied in L6 skeletal muscle cells. Trans- and cis-lactate effects on pyruvate transport kinetic parameters were examined. Pyruvate and lactate were transported by a multisite carrier system, i.e., by two families of sites, one with low affinity and high capacity (type I sites) and the other with high affinity and low capacity (type II). The multisite character of transport kinetics was not modified by either hydroxycinnamic acid (CIN) or p-chloromercuribenzylsulfonic acid (PCMBS), which exert different types of inhibition. The transport efficiency (TE) ratios of maximal velocity to the trans-activation dissociation constant (Kt) showed that lactate and pyruvate were preferentially transported by types I and II sites, respectively. The cis-lactate effect was observed with high Ki values for both sites. The trans-lactate effect on pyruvate transport occurred only on type I sites and exhibited an asymmetric interaction pattern (Kt of inward lactate > Kt of outward lactate). The inability of lactate to trans-stimulate type II sites suggests that intracellular lactate cannot recruit these sites. The high-affinity type II sites act as a specific pyruvate shuttle and constitute an essential relay for the intracellular lactate shuttle.

A reduced cerebral metabolic ratio in exercise reflects metabolism and not accumulation of lactate within the human brain

During maximal exercise lactate taken up by the human brain contributes to reduce the cerebral metabolic ratio, O(2)/(glucose + 1/2 lactate), but it is not known whether the lactate is metabolized or if it accumulates in a distribution volume. In one experiment the cerebral arterio-venous differences (AV) for O(2), glucose (glc) and lactate (lac) were evaluated in nine healthy subjects at rest and during and after exercise to exhaustion. The cerebrospinal fluid (CSF) was drained through a lumbar puncture immediately after exercise, while control values were obtained from six other healthy young subjects. In a second experiment magnetic resonance spectroscopy ((1)H-MRS) was performed after exhaustive exercise to assess lactate levels in the brain (n = 5). Exercise increased the AV(O2) from 3.2 +/- 0.1 at rest to 3.5 +/- 0.2 mM (mean +/-s.e.m.; P < 0.05) and the AV(glc) from 0.6 +/- 0.0 to 0.9 +/- 0.1 mM (P < 0.01). Notably, the AV(lac) increased from 0.0 +/- 0.0 to 1.3 +/- 0.2 mm at the point of exhaustion (P < 0.01). Thus, maximal exercise reduced the cerebral metabolic ratio from 6.0 +/- 0.3 to 2.8 +/- 0.2 (P < 0.05) and it remained low during the early recovery. Despite this, the CSF concentration of lactate postexercise (1.2 +/- 0.1 mM; n= 7) was not different from baseline (1.4 +/- 0.1 mM; n= 6). Also, the (1)H-MRS signal from lactate obtained after exercise was smaller than the estimated detection limit of approximately 1.5 mM. The finding that an increase in lactate could not be detected in the CSF or within the brain rules out accumulation in a distribution volume and indicates that the lactate taken up by the brain is metabolized.

Developmental switch in brain nutrient transporter expression in the rat

Normal development of both human and rat brain is associated with a switch in metabolic fuel from a combination of glucose and ketone bodies in the immature brain to a nearly total reliance on glucose in the adult. The delivery of glucose, lactate, and ketone bodies from the blood to the brain requires specific transporter proteins, glucose and monocarboxylic acid transporter proteins (GLUTs and MCTs), respectively. Developmental expression of the GLUTs in rat brain, i.e., 55-kDa GLUT1 in the blood-brain barrier (BBB), 45-kDa GLUT1 and GLUT3 in vascular-free brain, corresponds to maturational increases in cerebral glucose uptake and utilization. It has been suggested that MCT expression peaks during suckling and sharply declines thereafter, although a comparable detailed study has not been done. This study investigated the temporal and regional expression of MCT1 and MCT2 mRNA and protein in the BBB and the nonvascular brain during postnatal development in the rat. The results confirmed maximal MCT1 mRNA and protein expression in the BBB during suckling and a decline with maturation, coincident with the switch to glucose as the predominant cerebral fuel. However, nonvascular MCT1 and MCT2 levels do not reflect changes in cerebral energy metabolism, suggesting a more complex regulation. Although MCT1 assumes a predominantly glial expression in postweanling brain, the concentration remains fairly constant, as does that of MCT2 in neurons. The maintenance of nonvascular MCT levels in the adult brain implies a major role for these proteins, in concert with the GLUTs in both neurons and astrocytes, to transfer glycolytic intermediates during cerebral energy metabolism.

Characterization of insulin signaling in rat retina in vivo and ex vivo

Insulin receptor (IR) signaling cascades have been studied in many tissues, but retinal insulin action has received little attention. Retinal IR signaling and activity were investigated in vivo in rats that were freely fed, fasted, or injected with insulin by phosphotyrosine immunoblotting and by measuring kinase activity. A retina explant system was utilized to investigate the IR signaling cascade, and immunohistochemistry was used to determine which retinal cell layers respond to insulin. Basal IR activity in the retina was equivalent to that in brain and significantly greater than that of liver, and it remained constant between freely fed and fasted rats. Furthermore, IR signaling increased in the retina after portal vein administration of supraphysiological doses of insulin. Ex vivo retinas responded to 10 nM insulin with IR beta-subunit (IRbeta) and IR substrate-2 (IRS-2) tyrosine phosphorylation and AktSer473 phosphorylation. The retina expresses mRNA for all three Akt isoforms as determined by in situ hybridization, and insulin specifically increases Akt-1 kinase activity. Phospho-AktSer473 immunoreactivity increases in retinal nuclear cell layers with insulin treatment. These results demonstrate that the retinal IR signaling cascade to Akt-1 possesses constitutive activity, and that exogenous insulin further stimulates this prosurvival pathway. These findings may have implications in understanding normal and dysfunctional retinal physiology.

T3 increases lactate transport and the expression of MCT4, but not MCT1, in rat skeletal muscle

Triiodothyronine (T3) regulates the expression of genes involved in muscle metabolism. Therefore, we examined the effects of a 7-day T3 treatment on the monocarboxylate transporters (MCT)1 and MCT4 in heart and in red (RG) and white gastrocnemius muscle (WG). We also examined rates of lactate transport into giant sarcolemmal vesicles and the plasmalemmal MCT1 and MCT4 in these vesicles. Ingestion of T3 markedly increased circulating serum T3 (P < 0.05) and reduced weight gain (P < 0.05). T3 upregulated MCT1 mRNA (RG +77, WG +49, heart +114%, P < 0.05) and MCT4 mRNA (RG +300, WG +40%). However, only MCT4 protein expression was increased (RG +43, WG +49%), not MCT1 protein expression. No changes in MCT1 protein were observed in any tissue. T3 treatment doubled the rate of lactate transport when vesicles were exposed to 1 mM lactate (P < 0.05). However, plasmalemmal MCT4 was only modestly increased (+13%, P < 0.05). We conclude that T3 1) regulates MCT4, but not MCT1, protein expression and 2) increases lactate transport rates. This latter effect is difficult to explain by the modest changes in plasmalemmal MCT4. We speculate that either the activity of sarcolemmal MCTs has been altered or else other MCTs in muscle may have been upregulated.

Effects of insulin on long-term potentiation in hippocampal slices from diabetic rats

AIMS/HYPOTHESIS

Cognitive deficits occur commonly in diabetic patients. It is unclear whether these impairments result from hypoglycaemia during intensive insulin therapy, or from the diabetes itself. The aim of this study was to examine if impaired energy utilization resulting from insulin deficiency contributes to impaired long-term potentiation (reflecting impaired synaptic plasticity). As long-term potentiation is considered a candidate cellular mechanism underlying learning and memory, understanding how diabetes alters long-term potentiation may provide insight into mechanisms producing cognitive deficits in diabetes.

METHODS

Electrophysiologic recordings were used to study long-term potentiation in the CA1 region of hippocampal slices from healthy rats and rats with streptozotocin-induced diabetes.

RESULTS

Long-term potentiation was difficult to induce in slices from diabetic rats in standard recording buffer (contains 10 mmol/l glucose). In slices from diabetic rats, increasing extracellular glucose failed to recover long-term potentiation induction, but 10 mmol/l pyruvate added to standard buffer enabled long-term potentiation induction. Moreover, incubation of slices from diabetic rats with insulin enabled long-term potentiation induction in standard buffer. Acute administration of streptozotocin alone did not impair long-term potentiation in slices from healthy animals, and changing extracellular glucose concentrations over the range of 5 mmol/l to 30 mmol/l did not alter long-term potentiation in slices from control rats.

CONCLUSIONS/INTERPRETATION

These observations suggest that impaired energy utilization from insulin deficiency, rather than the accompanying hyperglycaemia, impair long-term potentiation in diabetes. Impaired hippocampal synaptic plasticity could contribute to learning and cognitive impairment in diabetic patients.

Immunogold cytochemistry identifies specialized membrane domains for monocarboxylate transport in the central nervous system

An efficient exchange of lactate between different cell types (such as astrocytes and neurones) would require that lactate transporters are expressed in contiguous parts of the respective plasma membranes. To settle this issue we explored the subcellular expression pattern of monocarboxylate transporters (MCTs) by use of selective antibodies and high resolution immunogold cytochemistry. We investigated whether the membrane domains containing MCT1, MCT2 and MCT4 are spatially related to each other and to other membrane domains, i.e. those containing glutamate receptors. We used retina and cerebellum as a model for our investigations. We found that MCT1 was localized in the apical membrane of pigment epithelial cells and in the photoreceptor inner segment membrane in the retina. In the brain MCT1 was present in endothelial cells. MCT2 was localized in the postsynaptic membrane of parallel fiber-Purkinje cell synapses and MCT4 was situated in the membrane of glial cells in the cerebellum.

Astrocytic glycogen influences axon function and survival during glucose deprivation in central white matter

We tested the hypothesis that astrocytic glycogen sustains axon function during and enhances axon survival after 60 min of glucose deprivation. Axon function in the rat optic nerve (RON), a CNS white matter tract, was monitored by measuring the area of the stimulus-evoked compound action potential (CAP). Switching to glucose-free artificial CSF (aCSF) had no effect on the CAP area for approximately 30 min, after which the CAP rapidly failed. Exposure to glucose-free aCSF for 60 min caused irreversible injury, which was measured as incomplete recovery of the CAP. Glycogen content of the RON fell to a low stable level 30 min after glucose withdrawal, compatible with rapid use in the absence of glucose. An increase of glycogen content induced by high-glucose pretreatment increased the latency to CAP failure and improved CAP recovery. Conversely, a decrease of glycogen content induced by norepinephrine pretreatment decreased the latency to CAP failure and reduced CAP recovery. To determine whether lactate represented the fuel derived from glycogen and shuttled to axons, we used the lactate transport blockers quercetin, alpha-cyano-4-hydroxycinnamic acid (4-CIN), and p-chloromercuribenzene sulfonic acid (pCMBS). All transport blockers, when applied during glucose withdrawal, decreased latency to CAP failure and decreased CAP recovery. The inhibitors 4-CIN and pCMBS, but not quercetin, blocked lactate uptake by axons. These results indicated that, in the absence of glucose, astrocytic glycogen was broken down to lactate, which was transferred to axons for fuel.

Inner medullary lactate production and accumulation: a vasa recta model

Since anaerobic glycolysis yields two lactates for each glucose consumed and since it is reported to be a major source of ATP for inner medullary (IM) cell maintenance, it is a likely source of "external" IM osmoles. It has long been known that such an osmole source could theoretically contribute to the "single-effect" of the urine concentrating mechanism, but there was previously no suggestion of a plausible source. I used numerical simulation to estimate axial gradients of lactate and glucose that might be accumulated by countercurrent recycling in IM vasa recta (IMVR). Based on measurements in other tissues, anaerobic glycolysis (assumed to be independent of diuretic state) was estimated to consume approximately 20% of the glucose delivered to the IM. IM tissue mass and axial distribution of loops and vasa recta were according to reported values for rat and other rodents. Lactate (P(LAC)) and glucose (P(GLU)) permeabilities were varied over a range of plausible values. The model results suggest that P(LAC) of 100 x 10(-5) cm/s (similar to measured permeabilities for other small solutes) is sufficiently high to ensure efficient lactate recycling. By contrast, it was necessary in the model to reduce P(GLU) to a small fraction of this value (1/25th) to avoid papillary glucose depletion by countercurrent shunting. The results predict that IM lactate production could suffice to build a significant steady-state axial lactate gradient in the IM interstitium. Other modeling studies (Jen JF and Stephenson JL. Bull Math Biol 56: 491-514, 1994; and Thomas SR and Wexler AS. Am J Physiol Renal Fluid Electrolyte Physiol 269: F159-F171, 1995) have shown that 20-100 mosmol/kgH(2)O of unspecified external, interstitial, osmolytes could greatly improve IM concentrating ability. The present study gives several plausible scenarios consistent with accumulation of metabolically produced lactate osmoles, although only to the lower end of this range. For example, if 20% of entering glucose is consumed, the model predicts that papillary lactate would attain about 15 mM assuming vasa recta outflow is increased 30% by fluid absorbed from the nephrons and collecting ducts and that this lactate gradient would double if IM blood flow were reduced by one-half, as may occur in antidiuresis. Several experimental tests of the hypothesis are indicated.

Role of plasma membrane transporters in muscle metabolism

Muscle plays a major role in metabolism. Thus it is a major glucose-utilizing tissue in the absorptive state, and changes in muscle insulin-stimulated glucose uptake alter whole-body glucose disposal. In some conditions, muscle preferentially uses lipid substrates, such as fatty acids or ketone bodies. Furthermore, muscle is the main reservoir of amino acids and protein. The activity of many different plasma membrane transporters, such as glucose carriers and transporters of carnitine, creatine and amino acids, play a crucial role in muscle metabolism by catalysing the influx or the efflux of substrates across the cell surface. In some cases, the membrane transport process is subjected to intense regulatory control and may become a potential pharmacological target, as is the case with the glucose transporter GLUT4. The goal of this review is the molecular characterization of muscle membrane transporter proteins, as well as the analysis of their possible regulatory role.