Cell-specific expression pattern of monocarboxylate transporters in astrocytes and neurons observed in different mouse brain cortical cell cultures

Immunocytochemical expression of monocarboxylate transporters in the human visual cortex at midgestation

Lactate and the other monocarboxylates are a major energy source for the developing brain. We investigated the immunocytochemical expression of two monocarboxylate transporters, MCT1 and MCT2, in the human visual cortex between 13 and 26 post-ovulatory weeks. We used immunoperoxidase and immunofluorescence techniques to determine whether these transporters co-localized with markers for blood vessels (CD34), neurons (microtubule-associated protein 2 [MAP2], SMI 311), radial glia (vimentin), or astrocytes (glial fibrillary acidic protein [GFAP], S100beta protein). MCT1 immunoreactivity was visible in blood vessel walls as early as the 13th week of gestation mainly in the cortical plate and subplate. At this stage, less than 10% of vessels in the ventricular layer expressed MCT1, whereas all blood vessels walls showed this immunoreactivity at the 26th gestational week. Starting at the 19th week of gestation, sparse MCT1 positive cell bodies were detected, some of them co-localized with MAP2 immunoreactivity. MCT2 immunoreactivity was noted in astrocytic cell bodies from week 19 and spread subsequently to the astrocyte end-feet in contact with blood vessels. MCTs immunoreactivities were most marked in the subplate and deep cortical plate, where the most differentiated neurons were located. Our findings suggest that monocarboxylate trafficking between vessels (MCT1), astrocytes (MCT2) and some postmitotic neurons (MCT1) could develop gradually toward 20 gestational weeks (g.w.). These data suggest that lactate or other monocarboxylates could represent a significant energy source for the human visual cortex at this early stage.

Cellular and subcellular distribution of monocarboxylate transporters in cultured brain cells and in the adult brain

Monocarboxylate transporters (MCTs) are involved in the uptake and release of lactate, pyruvate, and ketone bodies. Studies of their distribution at both the mRNA and protein levels have highlighted the specific expression of MCT1, MCT2, and more recently MCT4 in the central nervous system. MCT1 was found strongly expressed by cortical astrocytes both in vitro and in vivo. It was also found at high levels on blood vessels, ependymocytes, and glia limitans. A subset of neurons in vitro exhibited a weak but significant MCT1 expression. In contrast, it was determined that MCT2 represents the predominant neuronal MCT on cultured neurons as well as on neurons throughout the brain parenchyma. At the subcellular level, part of MCT2 is located in postsynaptic densities. Specific populations of astrocytes in the white matter also exhibited MCT2 expression in the rat, but not in the mouse brain. MCT4 was found exclusively in astrocytes in several areas including the cortex, the hippocampus, and the cerebellum. MCT2 expression increased in cultured neurons with days in vitro commensurate with increased synapse formation. Moreover, a significant increase in MCT2 expression was observed in cultured neurons exposed to noradrenaline, an effect involving a regulation at the translational level. The description of MCTs on different cell types in the central nervous system together with clear evidence for regulation of their expression further emphasize the important role that monocarboxylates, and particularly lactate, might play in brain energy metabolism not only during development but also in the adult.

Early acquisition of typical metabolic features upon differentiation of mouse neural stem cells into astrocytes

Specific metabolic features, such as glutamate reuptake, have been associated with normal functions of mature astrocytes. In this study, we examined whether these characteristics are acquired together with classical phenotypic markers of differentiated astrocytes. Differentiation of E14 mouse neurospheres into astrocytes was induced by the addition of fetal bovine serum (FBS). Degree of differentiation was assessed by reverse transcription-polymerase chain reaction (RT-PCR) and immunofluorescence for both GFAP and nestin. Neural stem cells expressed nestin but not GFAP, while differentiated astrocytes were immunopositive for GFAP but displayed low levels of nestin expression. A strong increase in the expression of the glutamate transporter GLAST and the monocarboxylate transporter MCT1 accompanied phenotypic changes. In addition, active glutamate transport appeared in differentiated astrocytes, as well as their capacity to increase aerobic glycolysis in response to glutamate. Leukemia inhibitory factor (LIF) and ciliary neurotrophic factor, but not interleukin-6, triggered the expression of phenotypic and morphological characteristics of astrocytes. In addition, exposure to LIF led to the appearance of metabolic features typically associated with astrocytes. Altogether, our results show that acquisition of some specific metabolic features by astrocytes occurs early in their differentiation process and that LIF represents a candidate signal to induce their expression.

Lactate transport and transporters: General principles and functional roles in brain cells

Lactate is transported across cell membranes by diffusional, saturable cotransport with protons, mediated by monocarboxylate transporters (MCTs). This transport is bidirectional and in the absence of a transcellular H(+) gradient, it can increase the intracellular concentration of lactate up to but not beyond the extracellular level (or vice versa). If extra- and intracellular pH differ, however, the equilibrium level is determined by the gradients of both lactate anions and protons. Rates of lactate uptake are determined most often by measuring uptake of labeled lactate, e.g., [U-14C]lactate. In the case of lactate and other compounds that are metabolized, errors are introduced easily because continuing inwardly directed diffusional net transport of label can be achieved by intracellular metabolism, reducing the intracellular level of the nonmetabolized lactate and thus maintaining a concentration gradient between extra- and intracellular concentrations of the nonmetabolized compound (metabolism-driven uptake). For measurement of facilitated diffusion kinetics, it is essential that the period during which the uptake is measured is short enough that little or no metabolism-driven uptake contributes to the measured uptake (or that first-order regression analysis is carried out to obtain initial uptake rates from nonlinear traces). To achieve initial uptake rates, incubation periods well below 1 min are generally required. Lactate uptake is fast in astrocytes, which express powerful, low-affinity MCTs, i.e., MCT1 and MCT4. Due to the low affinity of these transporters, they respond to increased lactate gradients with enhanced transporter activity. The predominant MCT in neurons is the high-affinity MCT2, which can only increase its activity to a limited extent in the face of an increased lactate gradient. This is reflected by a high-affinity lactate uptake, although most investigators also have demonstrated a component of lactate uptake with lower affinity. In both neurons and astrocytes, however, facilitated diffusion is fast enough that under most conditions lactate fluxes will be determined mainly by the rate of metabolism-driven uptake, and MCT-mediated transport only will be rate-limiting after establishment of large transmembrane gradients.

Lactate as a pivotal element in neuron-glia metabolic cooperation

Lactate has been considered for a long time as a metabolic waste and/or a sign of hypoxia in the central nervous system. Nevertheless, clear evidence that lactate can constitute an adequate energy substrate for brain tissue has been provided as early as in the 1950s with the pioneering work of McIlwain in brain slices. Over the years, several studies using different approaches have confirmed that lactate is efficiently oxidized by brain cells in vitro. Moreover, lactate has been shown under certain circumstances to have a neuroprotective effect and support neuronal activity. Similar confirmation of lactate utilization in vivo as well as putative neuroprotection in various excitotoxic models has been provided. Lactate was even shown to restore cognitive performance upon an hypoglycemic episode in humans. More recently, it was proposed that lactate could be produced by astrocytes and released in the extracellular space to form a pool readily available for neurons in case of high energy demands. Several elements support the concept of a lactate shuttle between astrocytes and neurons in the central nervous system. Among them, the description of specific monocarboxylate transporters found on both astrocytes and neurons is an important observation consistent with this concept. Interestingly, lactate shuttles between different cell types within the same organ have been described outside the central nervous system, notably in muscle and testis. Thus, lactate is emerging as a valuable intercellular exchange molecule in different systems including the brain where it might be an essential element of neuron-glia metabolic interactions.

Noradrenaline enhances monocarboxylate transporter 2 expression in cultured mouse cortical neurons via a translational regulation

Regulation of the expression of MCT1 and MCT2, two isoforms of the monocarboxylate transporter (MCT) family, was investigated in primary cultures of mouse cortical neurons. Under basal conditions, both MCT immunoreactivities (IR) were found in the cell soma and dendrites, although IR for MCT1 appeared less bright than for MCT2. Treatment of cultured cortical neurons with 100 microm noradrenaline (NA) led, after a few hours, to a striking enhancement in fluorescence intensity associated with MCT2 IR in the cell soma as well as in dendrites. In contrast, MCT1 IR was not altered by NA treatment. Western blot experiments performed on cultured neurons treated with NA confirmed that MCT2 protein expression was increased. Forskolin and dBcAMP also enhanced MCT2 expression, suggesting the implication of a cAMP-mediated pathway in the effect of NA. Surprisingly, neither NA, dBcAMP nor forskolin affected MCT2 mRNA expression. Application of cycloheximide, a protein synthesis inhibitor, prevented the enhancement of MCT2 IR, while the mRNA synthesis inhibitor actinomycin D also blocked the effect of NA on MCT2 IR levels. These results suggest that regulation of MCT2 expression in neurons by NA occurs at the translational level despite the requirement for an as yet unknown transcriptional step.

Metabolic coupling between glia and neurons

The coupling between synaptic activity and glucose utilization(neurometabolic coupling) is a central physiological principle of brain function that has provided the basis for 2-deoxyglucose-based functional imaging with positron emission tomography (PET). Astrocytes play a central role in neurometabolic coupling, and the basic mechanism involves glutamate-stimulated aerobic glycolysis; the sodium-coupled reuptake of glutamate by astrocytes and the ensuing activation of the Na-K-ATPase triggers glucose uptake and processing via glycolysis, resulting in the release of lactate from astrocytes. Lactate can then contribute to the activity-dependent fuelling of the neuronal energy demands associated with synaptic transmission. An operational model, the `astrocyte–neuron lactate shuttle', is supported experimentally by a large body of evidence,which provides a molecular and cellular basis for interpreting data obtained from functional brain imaging studies. In addition, this neuron–glia metabolic coupling undergoes plastic adaptations in parallel with adaptive mechanisms that characterize synaptic plasticity. Thus, distinct subregions of the hippocampus are metabolically active at different time points during spatial learning tasks, suggesting that a type of metabolic plasticity,involving by definition neuron–glia coupling, occurs during learning. In addition, marked variations in the expression of genes involved in glial glycogen metabolism are observed during the sleep–wake cycle, with in particular a marked induction of expression of the gene encoding for protein targeting to glycogen (PTG) following sleep deprivation. These data suggest that glial metabolic plasticity is likely to be concomitant with synaptic plasticity.