Acceleration of conduction velocity linked to clustering of nodal components precedes myelination

• Published in: Proceedings of the National Academy of Sciences - PNAS 2015-01, Vol.112 (3), p.E321-E328
• Main Authors: Freeman, Sean A., Desmazières, Anne, Simonnet, Jean, Gatta, Marie, Pfeiffer, Friederike, Aigrot, Marie Stéphane, Rappeneau, Quentin, Guerreiro, Serge, Michel, Patrick Pierre, Yanagawa, Yuchio, Barbin, Gilles, Brophy, Peter J., Fricker, Desdemona, Lubetzki, Catherine, Sol-Foulon, Nathalie
• Format: Article
• Language: English
• Subjects: Animals; Biological Sciences; Cell adhesion & migration; Hippocampus - metabolism; Mice; Myelin Sheath - metabolism; Neurons; PNAS Plus; Proteins; Rats; Sodium; Velocity
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1419099112

Significance Cellular and molecular mechanisms underlying the assembly of nodes of Ranvier of myelinated axons in the CNS are still only partly understood. Our study shows the influence of intrinsic cues and glial extrinsic factors for nodal protein clustering before myelination on specific hippocampal neuronal subpopulations and extends to electrophysiological understandings and in vivo relevance. Although conduction velocity along axons has long been thought to mostly rely on the insulating properties of myelin, we here show that nodal protein aggregation can increase this speed in the absence of such insulation. These results highlight the role of nodal clusters per se on conduction velocity by uncoupling it from myelination. High-density accumulation of voltage-gated sodium (Na ᵥ) channels at nodes of Ranvier ensures rapid saltatory conduction along myelinated axons. To gain insight into mechanisms of node assembly in the CNS, we focused on early steps of nodal protein clustering. We show in hippocampal cultures that prenodes (i.e., clusters of Na ᵥ channels colocalizing with the scaffold protein ankyrinG and nodal cell adhesion molecules) are detected before myelin deposition along axons. These clusters can be induced on purified neurons by addition of oligodendroglial-secreted factor(s), whereas ankyrinG silencing prevents their formation. The Na ᵥ isoforms Na ᵥ1.1, Na ᵥ1.2, and Na ᵥ1.6 are detected at prenodes, with Na ᵥ1.6 progressively replacing Na ᵥ1.2 over time in hippocampal neurons cultured with oligodendrocytes and astrocytes. However, the oligodendrocyte-secreted factor(s) can induce the clustering of Na ᵥ1.1 and Na ᵥ1.2 but not of Na ᵥ1.6 on purified neurons. We observed that prenodes are restricted to GABAergic neurons, whereas clustering of nodal proteins only occurs concomitantly with myelin ensheathment on pyramidal neurons, implying separate mechanisms of assembly among different neuronal subpopulations. To address the functional significance of these early clusters, we used single-axon electrophysiological recordings in vitro and showed that prenode formation is sufficient to accelerate the speed of axonal conduction before myelination. Finally, we provide evidence that prenodal clusters are also detected in vivo before myelination, further strengthening their physiological relevance.