Postnatal Loss of Neuronal and Glial Neurofascins Differentially Affects Node of Ranvier Maintenance and Myelinated Axon Function

Abstract

Intricate molecular interactions between neurons and glial cells underlie the creation of unique domains that are essential for saltatory conduction of action potentials by myelinated axons. Previously, the cell surface adhesion molecule Neurofascin (Nfasc) has been shown to have a dual-role in the establishment of axonal domains from both the glial and neuronal interface. While the neuron-specific isoform of Neurofascin (NF186) is indispensable for clustering of voltage-gated sodium channels at nodes of Ranvier; the glial-specific isoform of Neurofascin (NF155) is required for myelinating glial cells to organize the paranodal domain. Although many studies have addressed the individual roles of NF155 and NF186 in assembling paranodes and nodes, respectively; critical questions about their roles in the maintenance and long-term health of the myelinated axons remain, which we aimed to address in these studies. Here using spatiotemporal ablation of Neurofascin in neurons alone or together with myelinating glia, we report that loss of NF186 individually from postnatal mice leads to progressive nodal destabilization and axonal degeneration. While individual ablation of paranodal NF155 does not disrupt nodes of Ranvier; loss of NF186 combined with NF155 causes more accelerated nodal destabilization than loss of NF186 alone, providing strong evidence regarding a supporting role for paranodes in nodal maintenance. In both cases of NF186 loss, myelinating axons show ultrastructural changes and degeneration. Our studies reveal that long-term maintenance of nodes and ultimately the health of axons is correlated with the stability of NF186 within the nodal complex and the presence of auxiliary paranodes.

Pro-excitatory alterations in sodium channel activity facilitate subiculum neuron hyperexcitability in temporal lobe epilepsy

“…Subiculum bursting neurons were found to be in high density in the distal pyramidal layer just adjacent to the presubiculum, and WT neurons were distinguished by their propensity to produce a burst of APs at the onset of depolarization, followed by more tonic spiking (Figure 1A; WT) (Mattia et al, 1993; Petersen et al, 2017; Staff et al, 2000; Stewart and Wong, 1993; Taube, 1993). In contrast, subiculum neurons from TLE animals were hyperexcitable, firing more than a single burst of APs at lower current injection steps compared to WT (Figure 1A; TLE).…”

Ablation of cytoskeletal scaffolding proteins, Band 4.1B and Whirlin, leads to cerebellar purkinje axon pathology and motor dysfunction

“…Guinea pig anti-Caspr1 (Bhat et al, 2001), guinea pig anti-4.1B(Buttermore et al, 2011), guinea pig anti-Whrn (Green et al, 2013), rabbit anti-βIV Spectrin (βIV Spec) (Saifetiarova et al, 2017b), rat anti-pan Neurofascin (NFCt, recognizing the c-terminus) (Taylor et al, 2017) antibodies have been described previously. Other primary antibodies used for experiments are: a) mouse anti-potassium voltage-gated channel subfamily A member 2 (K V 1.2) (UC Davis/NIH NeuroMab Facility Cat# 75-008 RRID:AB_2296313); b) mouse anti-Neurofilament (Covance Research Products Inc Cat# SMI-312R RRID:AB_2315329); c) mouse anti-Calbindin (Calb) Sigma-Aldrich Cat# C9848, RRID:AB_476894); d) rabbit anti-Calb antibodies (Millipore Cat# AB1778, RRID:AB_2068336); e) mouse anti-β-Actin antibodies (Sigma-Aldrich Cat# A5441, RRID:AB_476744); f) rabbit anti-Caspr2 antibodies (Abeam Cat# ab33994, RRID:AB_2083506); and g) secondary antibodies for immunostaining (Alexa Fluor- 488, 568, 647) were purchased from Invitrogen (USA); e) IR-conjugated secondary antibodies for immunoblotting were purchased from LI-COR (USA).…”

Myelinated axon physiology and regulation of neural circuit function

“…Paranodes flank both sides of the node of Ranvier and contain septate‐like junctions that adhere the myelin sheath to the axon (Figure ) via a cell adhesion complex formed by glial‐Neurofascin binding to axonally‐expressed Contactin‐1 and Contactin‐associated protein 1 (Caspr1). Paranodes are essential for nodal stability (Amor et al, ; Desmazieres et al, ; Taylor, Saifetiarova, & Bhat, ; Zonta et al, ) and act as physical barriers that prevent the diffusion of nodal proteins including ion channels away from the node (Pillai et al, ). As noted above, very small changes in myelin sheath elongation or retraction could elicit relatively large changes with respect to node size by altering paranode position or dynamic activity.…”