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.

Myelin is of vital importance to the central nervous system and its disruption is related to a large number of both neurodevelopmental and neurodegenerative diseases. The differences observed between human and rodent oligodendrocytes make animals inadequate for modeling these diseases. Although developing human in vitro models for oligodendrocytes and myelinated axons has been a great challenge, 3D cell cultures derived from iPSC are now available and able to partially reproduce the myelination process. We have previously developed a human iPSC-derived 3D brain organoid model (also called BrainSpheres) that contains a high percentage of myelinated axons and is highly reproducible. Here, we have further refined this technology by applying multiple readouts to study myelination disruption. Myelin was assessed by quantifying immunostaining/confocal microscopy of co-localized myelin basic protein (MBP) with neurofilament proteins as well as proteolipid protein 1 (PLP1). Levels of PLP1 were also assessed by Western blot. We identified compounds capable of inducing developmental neurotoxicity by disrupting myelin in a systematic review to evaluate the relevance of our BrainSphere model for the study of the myelination/demyelination processes. Results demonstrated that the positive reference compound (cuprizone) and two of the three potential myelin disruptors tested (Bisphenol A, Tris(1,3-dichloro-2-propyl) phosphate, but not methyl mercury) decreased myelination, while ibuprofen (negative control) had no effect. Here, we define a methodology that allows quantification of myelin disruption and provides reference compounds for chemical-induced myelin disruption.

Hepatic steatosis is the first step leading to non-alcoholic fatty liver disease, which represents a major complication of obesity. Here, we show that MCT1 haploinsufficient mice resist to hepatic steatosis development when fed a high fat diet. They exhibit a reduced hepatic capacity to metabolize monocarboxylates such as lactate compared to wildtype mice. To understand how this resistance to steatosis develops, we used HFD fed wildtype mice with hepatic steatosis and MCT1 haploinsufficient mice to study hepatic metabolism. AMPK is constitutively activated in the liver of MCT1 haploinsufficient mice, leading to an inactivation of SREBP1. Therefore, expression of key transcription factors for lipid metabolism, such as PPARα and γ, CHREB, or SREBP1 itself, as well as several enzymes including FAS and CPT1, was not upregulated in these mice when fed a high fat diet. It is proposed that reduced hepatic lactate metabolism is responsible for the protection against hepatic steatosis in MCT1 haploinsufficient mice via a constitutive activation of AMPK and repression of several major elements involved in hepatic lipid metabolism. Our results support a role of increased lactate uptake in hepatocytes during HFD that, in turn, induce a metabolic shift stimulating SREBP1 activity and lipid accumulation.

The effect of a cellular prion protein (PrP^c) deficiency on neuroenergetics was primarily analyzed via surveying the expression of genes specifically involved in lactate/pyruvate metabolism, such as monocarboxylate transporters (MCT1, MCT2, MCT4). The aim of the present study was to elucidate a potential involvement of PrP^c in the regulation of energy metabolism in different brain regions. By using quantitative real-time polymerase chain reaction (qRT-PCR), we observed a marked reduction in MCT1 mRNA expression in the cortex of symptomatic Zürich I Prnp^−/− mice, as compared to their wild-type (WT) counterparts. MCT1 downregulation in the cortex was accompanied with significantly decreased expression of the MCT1 functional interplayer, the Na⁺/K⁺ ATPase α2 subunit. Conversely, the MCT1 mRNA level was significantly raised in the cerebellum of Prnp^−/− vs. WT control group, without a substantial change in the Na⁺/K⁺ ATPase α2 subunit expression. To validate the observed mRNA findings, we confirmed the observed change in MCT1 mRNA expression level in the cortex at the protein level. MCT4, highly expressed in tissues that rely on glycolysis as an energy source, exhibited a significant reduction in the hippocampus of Prnp^−/− vs. WT mice. The present study demonstrates that a lack of PrP^c leads to altered MCT1 and MCT4 mRNA/protein expression in different brain regions of Prnp^−/− vs. WT mice. Our findings provide evidence that PrP^c might affect the monocarboxylate intercellular transport, which needs to be confirmed in further studies.

In the hypothalamus, detection of energy substrates such as glucose is essential to regulate food intake and peripheral energy homeostasis. Metabolic interactions between astrocytes and neurons via lactate exchange have been proposed as a hypothalamic glucose‐sensing mechanism, but the molecular basis remains uncertain. Mouse hypothalamic astrocytes in vitro were found to exhibit a stronger glycolytic phenotype in basal conditions than cortical astrocytes. It was associated with higher protein expression levels of the Pyruvate Kinase Isoform M2 (Pkm2) and its more prominent nuclear localization. In parallel, hypothalamic astrocytes also expressed higher levels of the monocarboxylate transporter Slc16a3 (Mct4), which were dependent on Pkm2 expression. The stronger Mct4 expression in hypothalamic versus cortical astrocytes is an intrinsic characteristic, as it was also present after their direct isolation from adult mouse tissue. The high lactate release capacity of hypothalamic astrocytes was demonstrated to depend on the expression of Mct4, but not Mct1. Unlike cortical astrocytes, hypothalamic astrocytes in culture do not respond to glutamate with enhanced glycolysis, but instead, they modulate their lactate production according to glucose concentrations in an AMPK‐dependent manner, an effect observed in both mouse and human hypothalamic astrocytes in vitro. Our study shows that hypothalamic and cortical astrocytes are geared to have distinct glycolytic responses to glucose and glutamate, respectively. These results reveal a metabolic specialization of astrocytes in order to fulfill distinct area‐specific functions: glucose‐sensing in the hypothalamus versus activity‐dependent neuronal energetic supply in cortical regions.