Heterogeneity of astrocyte and NG2 cell insertion at the node of ranvier

Abstract

The node of Ranvier is a functionally important site on the myelinated axon where sodium channels are clustered and regeneration of action potentials occurs, allowing fast saltatory conduction of action potentials. Early ultrastructural studies have revealed the presence of “glia” or “astrocytes” at the nodes. NG2 cells, also known as oligodendrocyte precursor cells or polydendrocytes, which are a resident glial cell population in the mature mammalian central nervous system that is distinct from astrocytes, have also been shown to extend processes that contact the nodes. However, the prevalence of the two types of glia at the node has remained unknown. We have used specific cell surface markers to examine the association of NG2 cells and astrocytes with the nodes of Ranvier in the optic nerve, corpus callosum, and spinal cord of young adult mice or rats. We show that more than 95% of the nodes in all three regions contained astrocyte processes, while 33–49% of nodes contained NG2 cell processes. NG2 cell processes were associated more frequently with larger nodes. A few nodes were devoid of glial apposition. Electron microscopy and STED (stimulated emission depletion) super resolution microscopy confirmed the presence of dual glial insertion at some nodes and further revealed that NG2 cell processes contacted the nodal membrane at discrete points, while astrocytes had broader processes that surrounded the nodes. The study provides the first systematic quantitative analysis of glial cell insertions at central nodes of Ranvier.

NG2 Glia: Novel Roles beyond Re-/Myelination

“…In line with this view, confocal and electron microscopy studies showed that NG2 glia establish physical contacts with functionally relevant neuronal domains, including dendrites [41,42], somata [14,41,43,44], nodes of Ranvier [45,46] and synapses [47,48] (Figure 1C). About 30-50% of the nodes are contacted by both NG2 glia and astrocyte processes in the optic nerve, corpus callosum, and spinal cord of young adult rodents.…”

“…Notably, while NG2 glia are never coupled via gap-junctions [59,60], some of them express connexin 32 [61] and partly couple to mature OLs [62]. Further, electron microscopy analysis showed that NG2 glia processes often contact the paranodal loops of myelin [46]. These data suggest a privileged communication between NG2 glia and other elements in the oligodendroglial lineage.…”

“…However, since NG2 glia processes have been observed in close apposition to neuronal dendrites and somata or interdigitating within the synaptic cleft of neuron-to-neuron synapses, this enrichment may be also related to a role of NG2 glia in neuronal synaptogenesis and synaptic plasticity. Similarly, ECM molecules expressed by NG2 glia (i.e., brevican, versican, tenascin R) importantly contribute to the assembly, maintenance and function of the nodes of Ranvier [46], thereby suggesting a specific NG2 glia contribution to neuronal maturation and action potential transmission.…”

“…About 30-50% of the nodes are contacted by both NG2 glia and astrocyte processes in the optic nerve, corpus callosum, and spinal cord of young adult rodents. Ultrastructural analyses revealed that NG2 glia processes extend fine, finger-like projections that contact the nodal membrane at discrete points, while astrocytes had broader processes that surround the entire nodes [46] (Figure 1C). Similarly, NG2 glia and astrocytes often contact the same axon terminals, with NG2 glia thin processes interdigitating between the pre-and post-synaptic elements and large astrocytic processes ensheating the entire synapses [41,47] ( Figure 1C).…”

“…In such perivascular niche, NG2 glia establish direct interactions through their processes or somata with both endothelial cells and pericytes. Beside these perivascular NG2 glia, a large fraction of parenchymal NG2 cells, whose somata are located away from blood vessels, extend some processes to physically interact with pericytes and microvessels [41,46] (Figure 1C). Such a distribution suggests some form of crosstalk among NG2 glia, endothelial cells and pericytes (see below).…”