The Neurofascins orchestrate assembly and maintenance of axonal domains in the central nervous system

Close interaction between oligodendrocytes and axons is essential to initiate myelination and to form specialised domains along myelinated fibres. These domains are characterised by the assembly of protein complexes at the axon-glia interface and key components of these complexes are the Neurofascins. Neurofascins are transmembrane glycoproteins belonging to the L1 subgroup of the Immunoglobulin (Ig) superfamily of cell adhesion molecules. The Neurofascin (Nfasc) gene is subject to extensive alternative splicing. Two of the best characterised isoforms are Nfasc155 and Nfasc186, which are expressed in glia and neurons respectively. In myelinated fibres, Nfasc186 is the predominant isoform expressed at nodes of Ranvier and axon initial segments (AIS) in both the central and peripheral nervous system (CNS and PNS), whereas Nfasc155 resides on the glial side of the paranodal axoglial junction. The Neurofascin gene has been inactivated by homologous recombination and Neurofascin-null mice die within the first week of postnatal life. The main focus of this work was to investigate the role of the Neurofascins in the developing CNS. Similarly to what has been previously observed in the PNS, this study shows that in myelinated fibres of the spinal cord, nodal and paranodal markers are mislocalised and axoglial junctions do not form in the absence of the Neurofascins. In contrast to the PNS, where ensheathment of axons is unaffected, myelin proteins in the CNS are greatly reduced in the mutant. This appears to be due to the reduced ability of oligodendrocyte myelinating processes to extend along axons. This work also shows that the role of Nfasc186 is to maintain the long term stability of the AIS rather than its assembly. In the PNS, Nfasc186 was found to play an essential role in node assembly. However, PNS and CNS nodes are likely to assemble by different mechanisms. To investigate the relative contribution of the Neurofascin isoforms in CNS node assembly, this work made use of transgenic lines in which either neuronal Nfasc186 or glial Nfasc155 was expressed on a Neurofascin null background. Expression of either isoform was found to independently rescue the nodal complex and a model of how the Neurofascins cooperate in the assembly of the CNS node of Ranvier is proposed

Synaptic membrane rafts:traffic lights for local neurotrophin signaling?

Lipid rafts, cholesterol and lipid rich microdomains, are believed to play important roles as platforms for the partitioning of transmembrane and synaptic proteins involved in synaptic signaling, plasticity, and maintenance. There is increasing evidence of a physical interaction between post-synaptic densities and post-synaptic lipid rafts. Localization of proteins within lipid rafts is highly regulated, and therefore lipid rafts may function as traffic lights modulating and fine-tuning neuronal signaling. The tyrosine kinase neurotrophin receptors (Trk) and the low-affinity p75 neurotrophin receptor (p75(NTR)) are enriched in neuronal lipid rafts together with the intermediates of downstream signaling pathways, suggesting a possible role of rafts in neurotrophin signaling. Moreover, neurotrophins and their receptors are involved in the regulation of cholesterol metabolism. Cholesterol is an important component of lipid rafts and its depletion leads to gradual loss of synapses, underscoring the importance of lipid rafts for proper neuronal function. Here, we review and discuss the idea that translocation of neurotrophin receptors in synaptic rafts may account for the selectivity of their transduced signals

Glial and neuronal isoforms of Neurofascin have distinct roles in the assembly of nodes of Ranvier in the central nervous system

Rapid nerve impulse conduction in myelinated axons requires the concentration of voltage-gated sodium channels at nodes of Ranvier. Myelin-forming oligodendrocytes in the central nervous system (CNS) induce the clustering of sodium channels into nodal complexes flanked by paranodal axoglial junctions. However, the molecular mechanisms for nodal complex assembly in the CNS are unknown. Two isoforms of Neurofascin, neuronal Nfasc186 and glial Nfasc155, are components of the nodal and paranodal complexes, respectively. Neurofascin-null mice have disrupted nodal and paranodal complexes. We show that transgenic Nfasc186 can rescue the nodal complex when expressed in Nfasc−/− mice in the absence of the Nfasc155–Caspr–Contactin adhesion complex. Reconstitution of the axoglial adhesion complex by expressing transgenic Nfasc155 in oligodendrocytes also rescues the nodal complex independently of Nfasc186. Furthermore, the Nfasc155 adhesion complex has an additional function in promoting the migration of myelinating processes along CNS axons. We propose that glial and neuronal Neurofascins have distinct functions in the assembly of the CNS node of Ranvier

Room Temperature Chemoresistive Gas Sensor Based on Organic-Functionalized Graphene Oxide

In the wide palette of chemoresistive sensing materials, hybrid nanocomposites have quickly gained [...

Differential Stability of PNS and CNS Nodal Complexes When Neuronal Neurofascin Is Lost

Fast, saltatory conduction in myelinated nerves requires the clustering of voltage-gated sodium channels (Nav) at nodes of Ranvier in a nodal complex. The Neurofascin (Nfasc) gene encodes neuronal Neurofascin 186 (Nfasc186) at the node and glial Neurofascin 155 at the paranode, and these proteins play a key role in node assembly. However, their role in the maintenance and stability of the node is less well understood. Here we show that by inducible ablation of Nfasc in neurons in adult mice, Nfasc186 expression is reduced by >99% and 94% at PNS and CNS nodes, respectively. Gliomedin and NrCAM at PNS and brevican at CNS nodes are largely lost with neuronal neurofascin; however, Nav at nodes of Ranvier persist, albeit with ∼40% reduction in expression levels. βIV Spectrin, ankyrin G, and, to a lesser extent, the β1 subunit of the sodium channel, are less affected at the PNS node than in the CNS. Nevertheless, there is a 38% reduction in PNS conduction velocity. Loss of Nfasc186 provokes CNS paranodal disorganization, but this does not contribute to loss of Nav. These results show that Nav at PNS nodes are still maintained in a nodal complex when neuronal neurofascin is depleted, whereas the retention of nodal Nav in the CNS, despite more extensive dissolution of the complex, suggests a supportive role for the partially disrupted paranodal axoglial junction in selectively maintaining Nav at the CNS node

Array-Comparative Genomic Hybridization Analysis in Fetuses with Major Congenital Malformations Reveals that 24% of Cases Have Pathogenic Deletions/Duplications

Karyotyping and aCGH are routinely used to identify genetic determinants of major congenital malformations (MCMs) in fetal deaths or terminations of pregnancy after prenatal diagnosis. Pathogenic rearrangements are found with a variable rate of 9-39% for aCGH. We collected 33 fetuses, 9 with a single MCM and 24 with MCMs involving 2-4 organ systems. aCGH revealed copy number variants in 14 out of 33 cases (42%). Eight were classified as pathogenic which account for a detection rate of 24% (8/33) considering fetuses with 1 or more MCMs and 33% (8/24) taking into account fetuses with multiple malformations only. Three of the pathogenic variants were known microdeletion syndromes (22q11.21 deletion, central chromosome 22q11.21 deletion, and TAR syndrome) and 5 were large rearrangements, adding up to >11 Mb per subject and comprising strong phenotype-related genes. One of those was a de novo complex rearrangement, and the remaining 4 duplications and 2 deletions were 130-900 kb in size, containing 1-7 genes, and were classified as variants of unknown clinical significance. Our study confirms aCGH as a powerful technique to ascertain the genetic etiology of fetal major congenital malformations

Neurofascin 140 is an embryonic neuronal neurofascin isoform that promotes the assembly of the node of ranvier

Rapid nerve conduction in myelinated nerves requires the clustering of voltage-gated sodium channels at nodes of Ranvier. The Neurofascin (Nfasc) gene has a unique role in node formation because it encodes glial and neuronal isoforms of neurofascin (Nfasc155 and Nfasc186, respectively) with key functions in assembling the nodal macromolecular complex. A third neurofascin, Nfasc140, has also been described; however, neither the cellular origin nor function of this isoform was known. Here we show that Nfasc140 is a neuronal protein strongly expressed during mouse embryonic development. Expression of Nfasc140 persists but declines during the initial stages of node formation, in contrast to Nfasc155 and Nfasc186, which increase. Nevertheless, Nfasc140, like Nfasc186, can cluster voltage-gated sodium channels (Nav) at the developing node of Ranvier and can restore electrophysiological function independently of Nfasc155 and Nfasc186. This suggests that Nfasc140 complements the function of Nfasc155 and Nfasc186 in initial stages of the assembly and stabilization of the nodal complex. Further, Nfasc140 is reexpressed in demyelinated white matter lesions of postmortem brain tissue from human subjects with multiple sclerosis. This expands the critical role of the Nfasc gene in the function of myelinated axons and reveals further redundancy in the mechanisms required for the formation of this crucial structure in the vertebrate nervous system