Myelin lipids as nervous system energy reserves

Neuronal functions and impulse propagation depend on the continuous supply of glucose1,2. Surprisingly, the mammalian brain has no obvious energy stores, except for astroglial glycogen granules3. Oligodendrocytes make myelin for rapid axonal impulse conduction4 and also support axons metabolically with lactate5-7. Here, we show that myelin itself, a lipid-rich membrane compartment, becomes a local energy reserve when glucose is lacking. In the mouse optic nerve, a model white matter tract, oligodendrocytes survive glucose deprivation far better than astrocytes, by utilizing myelin lipids which requires oxygen and fatty acid beta-oxidation. Importantly, fatty acid oxidation also contributes to axonal ATP and basic conductivity. This metabolic support by fatty acids is an oligodendrocyte function, involving mitochondria and myelin-associated peroxisomes, as shown with mice lacking Mfp2. To study reduced glucose availability in vivo without physically starving mice, we deleted the Slc2a1 gene from mature oligodendrocytes. This caused a significant decline of the glucose transporter GLUT1 from the myelin compartment leading to myelin sheath thinning. We suggest a model in which myelin turnover under low glucose conditions can transiently buffer axonal energy metabolism. This model may explain the gradual loss of myelin in a range of neurodegenerative diseases8 with underlying hypometabolism9.