Critical role for the lactate transporter, monocarboxylate transporter 1 (mct1), in the regeneration of peripheral nerves

Axons, and particularly regenerating axons, have high metabolic needs in order to maintain critical functions such as axon transport and membrane depolarization. Though some of the required energy likely comes form extracellular glucose and ATP generated in the soma, we and others hypothesize that some of the energy may be supplied by lactate. Unlike glucose that requires glycolytic enzymes to produce pyruvate, lactate can be converted directly to pyruvate by lactate dehydrogenase and transported into mitochondria for oxidative metabolism. In order to be transported into or out of cells, lactate requires specific monocarboxylate transporters (MCTs), the most abundant of which is MCT1. If MCT1 and lactate are critical for nerve function and regeneration, we hypothesize that MCT1 heterozygote null mice, which appear phenotypically normal despite having approximately 40% MCT1 as compared to wildtype littermate mice, would have reduced capacity for repair following nerve injury. To investigate this, adult MCT1 heterozygote null mice or wild-type mice underwent unilateral sciatic nerve crush in the proximal thigh. We found that regeneration of the sciatic nerve, as measured by recovery of compound muscle action potentials (CMAP) in the lateral plantar muscles following proximal sciatic nerve stimulation, was delayed from a median of 21 days in wildtype mice to 38.5 days in MCT1 heterozygote mice. In fact, half of the MCT1 heterozygote null mice had no recovery of CMAP by the endpoint of the study at 42 days, while all of the wild-type mice had recovered. In addition, the maximal amplitude of CMAP recovery in MCT1 heterozygote mull mice was reduced from a mean of 3 mV to 0.5 mV. As would be expected, the denervated gastrocnemius muscle of MCT1 heterozygote null mice remained atrophic at 42 days compared to wild-type mice. Our experiments show that lactate supplied through MCT1 is necessary for nerve regeneration. Experiments are underway to determine whether loss of MCT1 prevents nerve regrowth directly due to reduced energy supply to axons or indirectly by dysfunctional Schwann cells normally dependent on lactate supply through MCT1.