THE ROLE OF CREATINE IN PROMOTING OLIGODENDROCYTE SURVIVAL AND MODULATING AXONAL MITOCHONDRIA IN THE CNS

Abstract

Oligodendrocytes maintain neuronal integrity in the central nervous system (CNS). Chronic oligodendrocyte loss, a feature of the demyelinating disorder multiple sclerosis (MS), contributes to axonal dysfunction. Current therapies reduce MS severity, but do not prevent disease progression, characterized by accumulated chronic demyelination and neurodegeneration. Pharmacological compounds that promote oligodendrocyte survival and maintain neuronal integrity would be beneficial for MS. One such candidate is creatine, a cytoprotective organic acid involved in ATP buffering. As the creatine-synthesizing enzyme guanidinoacetate-methyltransferase (Gamt) is most highly expressed in oligodendrocytes in the CNS, these cells are likely the primary source of creatine in the brain. Intriguingly, human creatine deficiencies are characterized by severe neurological deficits, indicating that neurons are also heavily reliant on creatine. Therefore, the overarching goals of this dissertation were to investigate how creatine-mediated modulation of mitochondrial function affects 1) oligodendrocyte lineage cell proliferation, progression, differentiation, and survival under basal and injury conditions and 2) neuronal morphology and mitochondrial dynamics.

In this study, creatine directly increased mitochondrial ATP production in purified mouse oligodendrocyte lineage cell cultures, and exerted robust protection of oligodendrocytes by preventing cell death in both naïve and lipopolysaccharide (LPS)-treated mixed glia. Moreover, lysolecithin-mediated demyelination in Gamt-deficient mice did not affect oligodendrocyte precursor cell (OPC) recruitment, but resulted in exacerbated apoptosis of regenerated oligodendrocytes in CNS lesions. Remarkably, creatine administration into mice with demyelinating injury reduced oligodendrocyte apoptosis, thereby increasing oligodendrocyte density and myelin basic protein (MBP) staining in CNS lesions. Creatine did not affect the recruitment of macrophages/microglia into lesions, suggesting that creatine affects oligodendrocyte survival independent of inflammation. These results demonstrate a novel function for creatine in promoting oligodendrocyte viability during CNS remyelination.

This work also demonstrates a crucial role for creatine in modulating neuronal mitochondria. Gamt-deficient neurons exhibited aberrant neuronal respiration and axonal mitochondrial dynamics, which likely contributed to their reduced morphological complexity. Alternatively, addition of creatine or oligodendrocyte-derived secreted factors lowered the density of stationary axonal mitochondria, suggesting reduced energetic burden in treated neurons. Overall, this work demonstrates that creatine treatment may be beneficial for normalizing mitochondrial function in order to protect CNS cells in neurological disease.