Neuroinflammation causes changes to the nodes of Ranvier in Multiple Sclerosis normal-appearing white matter.

Abstract

Background: In addition to the focal demyelinating lesions in multiple sclerosis (MS), both imaging and neuropathological analyses have demonstrated the presence of a more diffuse pathology in both the white and grey matter, including changes to the structure of nodes of Ranvier in the normal-appearing white matter (NAWM). Objective: We have examined the expression of the paranodal axonal protein Caspr1, the voltage-gated channels Nav and Kv1.2 at nodes and juxtaparanodes respectively, and SMI32+ (dephosphorylated neurofilament) axons in NAWM areas from post-mortem progressive MS brains compared to controls. This axo-geometrical data on nodal changes was then integrated into a computational model of an axon developed with NEURON. To test our hypothesis, rats were injected into the cerebral subarachnoid space with lentiviral vectors for lymphotoxin-α and interferon-γ, and structural changes were examined 3 months later. Furthermore, a cerebellar tissue culture model was used to induce nodal pathology by the activation of microglia with TNF, interferon-γ, conditioned microglial medium and glutamate administration. Results: The paranodal domain in MS NAWM tissue was longer on average than in control and Kv1.2 channels appeared dislocated towards the paranode. These changes were associated with stressed axons and activation of microglia. When these changes were inserted into the computational model, a rapid decrease in velocity was observed as the paranodal peri-axonal space was increased, reaching conduction failure when the axons were less than 1mm of diameter. The same structural changes were observed in the corpus callosum of our rat model and were associated with microglia/astrocyte activation. TNF, interferon-γ, conditioned microglial medium and glutamate also generated paranodal elongation in the cerebellar cultures axons and was reversed/halted by an NMDA blocker. Conclusion: Microglia activated by pro-inflammatory cytokines may release high levels of glutamate, which triggers paranodal pathology in MS NAWM, contributing to axonal damage and subsequent conduction deficits.