The Central Proposition: Oligodendrocytes don't just insulate axons they feed them. When metabolic support fails, axons starve from the outside in, explaining distal degeneration in ALS and MS.

The Mechanism:

Monocarboxylate Transporter 1 (MCT1): Oligodendrocytes express high levels of MCT1 on their membranes, particularly in myelinated regions. This transporter exports lactate generated from glucose or glycogen into the periaxonal space.

Axonal Lactate Uptake: Axons express MCT2 on their surface, importing lactate and converting it to pyruvate for mitochondrial oxidative phosphorylation. Active axons prefer lactate over glucose.

Metabolic Coupling: This creates symbiotic unit oligodendrocyte glycolysis fuels axonal oxidative metabolism. One oligodendrocyte supports up to 50 axons metabolically, independent of myelination.

MCT1 Decline: In ALS, aging, and MS, oligodendrocyte MCT1 expression decreases by 40-60%. Oxidative stress, inflammation, or genetic vulnerability drives this downregulation.

Energy Failure: Distal axons farthest from cell body mitochondria depend most on local lactate. Without oligodendrocyte fuel, they cannot maintain ion gradients, transport vesicles, or sustain firing.

Dying-Back Degeneration: Energy-starved axons degenerate first at distal terminals, progressing proximally. Node of Ranvier integrity fails, conduction blocks, and eventual Wallerian degeneration follows.

The ALS Connection:

SOD1 mice show oligodendrocyte MCT1 loss before symptoms

Human ALS spinal cord shows reduced MCT1 expression

Oligodendrocyte progenitor dysfunction precedes motor neuron death

The MS Connection:

Inflammatory demyelination damages oligodendrocyte processes

Remyelination failure leaves axons metabolically unsupported

Normal-appearing white matter shows MCT1 reduction

Therapeutic Implications:

MCT1 upregulators (HDAC inhibitors, PPAR agonists) enhancing lactate export

Exogenous lactate or ketones bypassing transporter failure

Oligodendrocyte protection preserving metabolic function

Axonal MCT2 enhancers increasing import capacity

This reframes neurodegeneration as glial starvation axons dying from lack of local nutrition.