A neuronal MCT2 knockdown in the rat somatosensory cortex reduces both the NMR lactate signal and the BOLD response during whisker stimulation

Abstract

Although several in vitro and ex vivo evidence support the existence of lactate exchange between astrocytes and neurons, a direct demonstration in vivo is still lacking. In the present study, a lentiviral vector carrying a short hairpin RNA (shRNA) was used to downregulate the expression of the monocarboxylate transporter type 2 (MCT2) in neurons of the rat somatosensory cortex (called S1BF) by ~ 25%. After one hour of whisker stimulation, HRMAS 1H-NMR spectroscopy analysis of S1BF perchloric acid extracts showed that while an increase in lactate content is observed in both uninjected and shRNA-control injected extracts, such an effect was abrogated in shMCT2 injected rats. A 13C-incorporation analysis following [1-13C]glucose infusion during the stimulation confirmed that the elevated lactate observed during activation originates from newly synthesized [3-13C]lactate, with blood-derived [1-13C]glucose being the precursor. Moreover, the analysis of the 13C-labeling of glutamate in position C3 and C4 indicates that upon activation, there is an increase in TCA cycle velocity for control rats while a decrease is observed for MCT2 knockdown animals. Using in vivo localized 1H-NMR spectroscopy, an increase in lactate levels is observed in the S1BF area upon whisker stimulation for shRNA-control injected rats but not for MCT2 knockdown animals. Finally, while a robust BOLD fMRI response was evidenced in control rats, it was absent in MCT2 knockdown rats. These data not only demonstrate that glucose-derived lactate is locally produced following neuronal activation but also suggest that its use by neurons via MCT2 is probably essential to maintain synaptic activity within the barrel cortex.

Neurons rely on glucose rather than astrocytic lactate during stimulation

“…These results (and others in Díaz‐García et al, ) argue strongly that neuronal NADH CYT signals are produced not by import of astrocytic lactate, but rather by stimulation of neuronal aerobic glycolysis, both in stimulated dentate granule neurons of the hippocampus and in L2/3 neurons of somatosensory barrel cortex (activated by whisker stimulation). Further, these results are consistent with the experimental results of Mazuel et al () measuring lactate increases in stimulated barrel cortex, with or without knockdown of neuronal MCT2. These experiments showed that prolonged stimulation increased the lactate content of barrel cortex, but that this effect was absent when neuronal MCT2 was knocked down.…”

“…These experiments showed that prolonged 5 Barros and Weber (2018) have remarked on the fact that the lactate transient was not increased with MCT inhibition, and taken this as evidence that neurons do not contribute to the accumulation of lactate. However, the reduced dips in glucose with MCT inhibition indicate that the increased NADH CYT exerted back-pressure on glycolysis (as discussed below; this is also the implication of the Mazuel et al (2017) study discussed below). In addition, the signal-to-noise ratio for the lactate determination is smaller than for NADH CYT , and lactate is a less sensitive measure than NADH CYT .…”

“…An additional question in need of further study is whether there is shuttling of lactate from astrocytes to neurons at rest, when neurons are not stimulated. Biosensor measurements of NADH CYT at 6 Note, however, that this was not the final conclusion of the Mazuel et al (2017) paper.…”

CrossTalk proposal: an important astrocyte‐to‐neuron lactate shuttle couples neuronal activity to glucose utilisation in the brain

“…; Mazuel et al . ). Significantly, the deletion of MCTs in glial cells but not in neurons could be rescued by lactate, meaning that maintaining function requires neurons to have access to extracellular lactate.…”

Metabolic compartmentalization between astroglia and neurons in physiological and pathophysiological conditions of the neurovascular unit

“…Irrespective of accumulating evidence supporting the ANLSH, the fate of lactate, if it is really produced by astroglia via a glutamate signal from activated neurons, has remained a matter of controversy for more than 25 years . The original ANLSH followed by numerous reports proposed that activated neurons consume lactate produced by astroglia . A strong argument for this is based on a kinetic property of MCT2 expressed in neurons, which has a low kilometer value.…”