Cell-specific modulation of monocarboxylate transporter expression contributes to the metabolic reprograming taking place following cerebral ischemia.

Monocarboxylate transporters (MCTs) are involved in lactate trafficking and utilization by brain cells. As lactate is not only overproduced during ischemia but its utilization was shown to be essential upon recovery, we analyzed the expression of the main cerebral MCTs at 1 and 24h after an ischemic insult induced by a transient occlusion of the left middle cerebral artery (MCAO) in CD1 mice (n=5, 7 and 10 for control, 1 and 24h groups, respectively). After 1h of reperfusion, an upregulation of the three MCTs was observed in the striatum (MCT1 ipsilateral 2.73 ± 0.2 and contralateral 2.01 ± 0.4; MCT2 ipsilateral 2.1 ± 0.1; MCT4 ipsilateral 1.65 ± 0.1) and in the surrounding cortex of both the ipsilateral (MCT1 2.4 ± 0.4; MCT2 1.62 ± 0.2; MCT4 1.31 ± 0.1) and contralateral (MCT1 2.78 ± 0.4; MCT2 1.76 ± 0.2) hemispheres, compared to the corresponding sham hemispheres. An increase of MCT1 (ipsilateral 2.1 ± 0.2) and MCT2 (contralateral 1.9 ± 0.1) expression was also observed in the hippocampus, while no effect was observed for MCT4. At 24h of reperfusion, total MCT2 and MCT4 expressions were decreased in the striatum (MCT2 ipsilateral 0.32 ± 0.1 and contralateral 0.63 ± 0.1; MCT4 ipsilateral 0.59 ± 0.1) and the surrounding cortex (MCT4 ipsilateral 0.67 ± 0.1), compared to the sham. At the cellular level, neurons which usually express only MCT2 strongly expressed MCT1 at both time points. Surprisingly, staining for MCT4 appeared on neurons and was strong at 24h post-insult, in the striatum and the cortex of both hemispheres. A similar expression pattern was observed also in the ipsilateral hemisphere of the sham operated animals at 24h. Overall, our study indicates that cell-specific changes in MCT expression induced by an ischemic insult may participate to the metabolic adaptations taking place in the brain after a transient ischemic episode

A probable dual mode of action for both L- and D-lactate neuroprotection in cerebral ischemia.

Lactate has been shown to offer neuroprotection in several pathologic conditions. This beneficial effect has been attributed to its use as an alternative energy substrate. However, recent description of the expression of the HCA1 receptor for lactate in the central nervous system calls for reassessment of the mechanism by which lactate exerts its neuroprotective effects. Here, we show that HCA1 receptor expression is enhanced 24 hours after reperfusion in an middle cerebral artery occlusion stroke model, in the ischemic cortex. Interestingly, intravenous injection of L-lactate at reperfusion led to further enhancement of HCA1 receptor expression in the cortex and striatum. Using an in vitro oxygen-glucose deprivation model, we show that the HCA1 receptor agonist 3,5-dihydroxybenzoic acid reduces cell death. We also observed that D-lactate, a reputedly non-metabolizable substrate but partial HCA1 receptor agonist, also provided neuroprotection in both in vitro and in vivo ischemia models. Quite unexpectedly, we show D-lactate to be partly extracted and oxidized by the rodent brain. Finally, pyruvate offered neuroprotection in vitro whereas acetate was ineffective. Our data suggest that L- and D-lactate offer neuroprotection in ischemia most likely by acting as both an HCA1 receptor agonist for non-astrocytic (most likely neuronal) cells as well as an energy substrate

Distribution of monocarboxylate transporters in the peripheral nervous system suggests putative roles in lactate shuttling and myelination.

Lactate, a product of glycolysis, has been shown to play a key role in the metabolic support of neurons/axons in the CNS by both astrocytes and oligodendrocytes through monocarboxylate transporters (MCTs). Despite such importance in the CNS, little is known about MCT expression and lactate function in the PNS. Here we show that mouse MCT1, MCT2, and MCT4 are expressed in the PNS. While DRG neurons express MCT1, myelinating Schwann cells (SCs) coexpress MCT1 and MCT4 in a domain-specific fashion, mainly in regions of noncompact myelin. Interestingly, SC-specific downregulation of MCT1 expression in rat neuron/SC cocultures led to increased myelination, while its downregulation in neurons resulted in a decreased amount of neurofilament. Finally, pure rat SCs grown in the presence of lactate exhibited an increase in the level of expression of the main myelin regulator gene Krox20/Egr2 and the myelin gene P0. These data indicate that lactate homeostasis participates in the regulation of the SC myelination program and reveal that similar to CNS, PNS axon-glial metabolic interactions are most likely mediated by MCTs

The monocarboxylate transporter MCT4 : mechanism of regulation in astrocytes and its putative role in cerebral ischemia

Despite its small fraction of the total body weight (2%), the brain contributes for 20% and 25% respectively of the total oxygen and glucose consumption of the whole body. Indeed, glucose has been considered the energy substrate par excellence for the brain. However, evidence accumulated over the last half century revealed an important role for the monocarboxylate lactate in fulfilling the energy needs of neurons. This is particularly true during physiological neuronal activation and in pathological conditions. Lactate transport into and out of the cell is mediated by a family of proton-linked transporters called monocarboxylate transporters (MCTs). In the central nervous system, only three of them have been well characterized: MCT2 is the predominant neuronal isoform, while the other non¬neuronal cell types of the brain express the ubiquitous isoform MCT1. Quite recently, the MCT4 isoform has been described in astrocytes. Due to its high transport capacity compared to the other two isoforms, MCT4 is particularly adapted for glycolytic cells. Because of its recent discovery in the brain, nothing was known about its regulation in the central nervous system. Here we show that MCT4 is regulated by oxygen levels in primary cultures of astrocytes in a time- and concentration-dependent manner via the hypoxia inducible factor-la (HIF-la). Moreover, we showed that MCT4 expression is essential for astrocyte survival under low oxygen conditions. In parallel, we investigated the possible implication of the pyruvate kinase isoform Pkm2, a strong enhancer of glycolysis, in its regulation. Then we showed that MCT4 expression, as well as the expression of the other two MCT isoforms, is altered in a murine model of stroke. Surprisingly, neurons started to express MCT4, as well as MCT1, under such conditions. Altogether, these data suggest that MCT4, due to its high transport capacity for lactate, may be the isoform that enables cells to operate a major metabolic adaptation in response to pathological situations that alter metabolic homeostasis of the brain. -- Le cerveau représente 2% du poids corporel total, mais il contribue pour 20% de la consommation totale d'oxygène et 25% de celle de glucose au repos. Le glucose est considéré comme le substrat énergétique par excellence pour le cerveau. Néanmoins, depuis un demi- siècle maintenant, de plus en plus de travaux ont démontré que le lactate joue un rôle majeur dans le métabolisme cérébral et est capable du subvenir aux besoins énergétiques des neurones. Le lactate est tout particulièrement nécessaire pendant l'activation neuronale ainsi qu'en situation pathologique. Le transport du lactate à travers la barrière hématoencéphalique ainsi qu'à travers les membranes cellulaires est assuré par la famille des transporteurs aux monocarboxylates (MCTs). Dans le système nerveux central, uniquement trois d'entre eux ont été décrits: MCT2 est considéré comme le transporteur neuronal, alors que les autres types cellulaires qui constituent le cerveau expriment l'isoforme ubiquitaire MCT1. Récemment, l'isoforme MCT4 a été rapportée sur les astrocytes. Dû à sa grande capacité de transport pour le lactate, MCT4 est tout particulièrement adapté pour soutenir le métabolisme des cellules hautement glycolytiques, comme les astrocytes. En raison de sa toute récente découverte, les aspects comprenant sa régulation et son rôle dans le cerveau sont pour l'instant méconnus. Les résultats exposés dans ce travail démontrent dans un premier temps que l'expression de MCT4 est régulée par les niveaux d'oxygène dans les cultures d'astrocytes corticaux par le biais du facteur de transcription HIF-la. De plus, nous avons démontré que l'expression de MCT4 est essentielle à la survie des astrocytes quand le niveau d'oxygénation baisse. En parallèle, des résultats préliminaires suggèrent que l'isoforme 2 de la pyruvate kinase, un puissant régulateur de la glycolyse, pourrait jouer un rôle dans la régulation de MCT4. Dans la deuxième partie du travail nous avons démontré que l'expression de MCT4, ainsi que celle de MCT1 et MCT2, est altérée dans un modèle murin d'ischémie cérébrale. De façon surprenante, les neurones expriment MCT4 dans cette condition, alors que ce n'est pas le cas en condition physiologique. En tenant compte de ces résultats, nous suggérons que MCT4, dû à sa particulièrement grande capacité de transport pour le lactate, représente le MCT qui permet aux cellules du système nerveux central, notamment les astrocytes et les neurones, de s'adapter à de très fortes perturbations de l'homéostasie métabolique du cerveau qui surviennent en condition pathologique

Agriculture in imperial Italy

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