The human brain weighs approximately 2% of the body; however, it consumes about 20% of a person's total energy intake. Cellular bioenergetics in the central nervous system involves a delicate balance between biochemical processes engaged in energy conversion and those responsible for respiration. Neurons have high energy demands, which rely on metabolic coupling with glia, such as with oligodendrocytes and astrocytes. It has been well established that astrocytes recycle and transport glutamine to neurons to make the essential neurotransmitters, glutamate and GABA, as well as shuttle lactate to support energy synthesis in neurons. However, the metabolic role of oligodendrocytes in the central nervous system is less clear. In this review, we discuss the energetic demands of oligodendrocytes in their survival and maturation, the impact of altered oligodendrocyte energetics on disease pathology, and the role of energetic metabolites, taurine, creatine, N-acetylaspartate, and biotin, in regulating oligodendrocyte function.

Animal models of central nervous system (CNS) demyelination, including toxin-induced focal demyelination and immune-mediated demyelination through experimental autoimmune encephalomyelitis (EAE), have provided valuable insights into the mechanisms of neuroinflammation and CNS remyelination. However, the ability to track changes in transcripts, proteins, and metabolites, as well as cellular populations during the evolution of a focal lesion, has remained challenging. Here, we developed a method to label CNS demyelinating lesions by the intraperitoneal injection of a vital dye, neutral red (NR), into mice before killing. We demonstrate that NR-labeled lesions can be easily identified on the intact spinal cord in both lysolecithin- and EAE-mediated demyelination models. Using fluorescence microscopy, we detected NR in activated macrophages/microglia and astrocytes, but not in oligodendrocytes present in lesions. Importantly, we successfully performed RT-qPCR, Western blot, flow cytometry, and mass spectrometry analysis of precisely dissected NR-labeled lesions at 5, 10, and 20 d postlesion (dpl) and found differential changes in transcripts, proteins, cell populations, and metabolites in lesions over the course of remyelination. Therefore, NR administration is a simple and powerful method to track and analyze the detailed molecular, cellular, and metabolic changes that occur within the lesion microenvironment over time following CNS injury. Furthermore, this method can be used to identify molecular and metabolic pathways that regulate neuroinflammation and remyelination and facilitate the development of therapies to promote repair in demyelinating disorders such as multiple sclerosis.

Cerebral creatine deficiency syndrome (CCDS) is an inborn error of metabolism characterized by intellectual delays, seizures, and autistic-like behavior. However, the role of endogenously synthesized creatine on central nervous system (CNS) development and function remains poorly understood. Here, magnetic resonance spectroscopy (MRS) of adult mouse brains from both sexes revealed creatine synthesis is dependent on the expression of the enzyme, guanidinoacetate methyltransferase (GAMT). To identifyGamt-expressed cells, and howGamtaffects postnatal CNS development, we generated a mouse line by knocking-in a green fluorescent protein (GFP) which is expressed upon excision ofGamt. We found thatGamtis expressed in mature oligodendrocytes during active myelination in the developing postnatal CNS. Homozygous deletion ofGamtresulted in significantly reduced mature oligodendrocytes and delayed myelination in the corpus callosum. Moreover, the absence of endogenous creatine resulted in altered AMPK signaling in the brain, reduced brain creatine kinase expression in cortical neurons, and signs of axonal damage. Experimental demyelination in mice after tamoxifen induced conditional deletion ofGamtin oligodendrocyte lineage cells resulted in delayed maturation of oligodendrocytes and myelin coverage in lesions. Moreover, creatine and cyclocreatine supplementation can enhance remyelination after demyelination. Our results suggest endogenously synthesized creatine controls the bioenergetic demand required for the timely maturation of oligodendrocytes during postnatal CNS development, and that delayed myelination and altered CNS energetics through the disruption of creatine synthesis might contribute to conditions such as CCDS.SIGNIFICANCE STATEMENT:Cerebral creatine deficiency syndrome (CCDS) is a rare disease of inborn errors in metabolism, which is characterized by intellectual delays, seizures, and autism-like behavior. We found that oligodendrocytes are the main source of endogenously synthesized creatine in the adult CNS, and the loss of endogenous creatine synthesis led to delayed myelination. Our study suggests impaired cerebral creatine synthesis affects the timing of myelination and may impact brain bioenergetics.

Myelination of the CNS relies on the production and differentiation of oligodendrocyte (OL) precursor cells (OPCs) into mature OLs. During the first month of postnatal life, OPCs that populate the corpus callosum (CC) arise from neural stem cells (NSCs) in the subcallosal subventricular zone (SVZ), and then differentiate to generate myelinating OLs. However, the signals that regulate these processes are not fully understood. In this study, we show that endogenous expression of the retinoic acid (RA)-synthesizing enzyme retinaldehyde dehydrogenase 2 (RALDH2) is required for OPC generation and differentiation in the postnatal subcortical white matter. In male and female pups, conditional deletion of Raldh2 reduced OPC numbers and differentiation. Moreover, decreased OPC numbers coincided with reductions in NSC survival and expression of the sonic hedgehog (SHH) signaling effector protein Gli1 in the SVZ. Additionally, GFAP expression in the CC was decreased, and cortical neuron numbers were altered. Our work suggests a role for endogenous RALDH2-dependent RA synthesis in OPC production and differentiation in the CC, as well as in the development of other cell types derived from NSCs in the embryonic ventricular zone (VZ) and SVZ, as well as the postnatal subcallosal SVZ.

OBJECTIVES/GOALS: Oligodendrocytes (OL) are glial cells of the central nervous system (CNS) responsible for the energy demanding task of generating myelin sheaths during development and remyelination after demyelinating injury. One metabolite shown to significantly increase ATP production in OL is the nitrogenous organic acid, creatine. Creatine plays an essential role in ATP buffering within tissues with highly fluctuating energy demands such as brain and muscle. Interestingly, mature OL, which are the cells capable of myelin production, are the main cells in the CNS expressing the rate-limiting enzyme for creatine synthesis, guanidinoacetate methyltransferase (Gamt). Patients with mutations in Gamt display intellectual disabilities, impaired myelination and seizures. Therefore, we hypothesize that creatine may be essential for developmental myelination and improve remyelination. METHODS/STUDY POPULATION: To investigate these hypotheses, we developed a new transgenic mouse model with LoxP sites flanking exons 2-6 of the Gamt gene where excision leads to expression of a green fluorescent tag allowing us to track the cells normally expressing Gamt. RESULTS/ANTICIPATED RESULTS: In this mouse model, we show a 95% (±0.47%, n = 3) co-localization of Gamt within mature OL during postnatal (P) day P14. Next, we show that knocking out Gamt leads to a significant reduction in OL in the major CNS white matter tract, the corpus callosum, at P14 and P21 (P14: 0.007, n = 3; P21: 0.04, n = 3). Here, we also investigate whether dietary creatine can enhance remyelination in the cuprizone model of toxic demyelination. DISCUSSION/SIGNIFICANCE OF IMPACT: These studies highlight the important role creatine plays in developmental myelination and investigate whether creatine can provide a therapeutic value during a CNS demyelinating insult.