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.

No abstract

The monocarboxylate transporter MCT4 is a high capacity carrier important for lactate release from highly glycolytic cells. In the central nervous system, MCT4 is predominantly expressed by astrocytes. Surprisingly, MCT4 expression in cultured astrocytes is low, suggesting that a physiological characteristic, not met in culture conditions, is necessary. Here we demonstrate that reducing oxygen concentration from 21% to either 1 or 0% restored in a concentration-dependent manner the expression of MCT4 at the mRNA and protein levels in cultured astrocytes. This effect was specific for MCT4 since the expression of MCT1, the other astrocytic monocarboxylate transporter present in vitro, was not altered in such conditions. MCT4 expression was shown to be controlled by the transcription factor hypoxia-inducible factor-1α (HIF-1α) since under low oxygen levels, transfecting astrocyte cultures with a siRNA targeting HIF-1α largely prevented MCT4 induction. Moreover, the prolyl hydroxylase inhibitor dimethyloxalylglycine (DMOG) induced MCT4 expression in astrocytes cultured in presence of 21% oxygen. In parallel, glycolytic activity was enhanced by exposure to 1% oxygen as demonstrated by the increased lactate release, an effect dependent on MCT4 expression. Finally, MCT4 expression was found to be necessary for astrocyte survival when exposed for a prolonged period to 1% oxygen. These data suggest that a major determinant of astrocyte MCT4 expression in vivo is likely the oxygen tension. This could be relevant in areas of high neuronal activity and oxygen consumption, favouring astrocytic lactate supply to neurons. Moreover, it could also play an important role for neuronal recovery after an ischemic episode.

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.