Neurons define non-myelinated axon segments by the regulation of galectin-4-containing axon membrane domains

Natalia Díez-Revuelta,Alonso M Higuero,José Abad-Rodríguez,Hans-Joachim Gabius,María Peñas-De-La-Iglesia,Silvia Velasco

doi:10.1038/s41598-017-12295-6

Abstract

The mechanism underlying selective myelination of axons versus dendrites or neuronal somata relies on the expression of somatodendritic membrane myelination inhibitors (i.e. JAM2). However, axons still present long unmyelinated segments proposed to contribute to axonal plasticity and higher order brain functions. Why these segments remain unmyelinated is still an unresolved issue. The bifunctional lectin galectin-4 (Gal-4) organizes the transport of axon glycoproteins by binding to N-acetyllactosamine (LacNac) termini of N-glycans. We have shown that Gal-4 is sorted to segmental domains (G4Ds) along the axon surface, reminiscent of these long unmyelinated axon segments in cortical neurons. We report here that oligodendrocytes (OLGs) do not deposit myelin on Gal-4 covered surfaces or myelinate axonal G4Ds. In addition, Gal-4 interacts and co-localizes in G4Ds with contactin-1, a marker of another type of non-myelinated segments, the nodes of Ranvier. Neither Gal-4 expression nor G4D dimensions are affected by myelin extracts or myelinating OLGs, but are reduced with neuron maturation. As in vitro, Gal-4 is consistently segregated from myelinated structures in the brain. Our data shape the novel concept that neurons establish axon membrane domains expressing Gal-4, the first inhibitor of myelination identified in axons, whose regulated boundaries delineate myelination-incompetent axon segments along development.

Highlights

Axons are selectively myelinated while the somatodendritic membrane remains free of myelin insulation
Gal-4 is enriched at the membrane of the nascent axon[28], and is secreted by non-classical mechanisms to the extracellular milieu, where it keeps oligodendrocyte precursors (OLPs) in their undifferentiated proliferative state, unable to synthesize myelin[27]
When Gal-4-HA expression is induced in other cells of the nervous system that express very low levels of endogenous Gal-4, such as astrocytes, non-permeabilized immunostaining for HA is observed as membrane “patches” (Fig. 1c, arrowheads)

Summary

Introduction

Axons are selectively myelinated while the somatodendritic membrane remains free of myelin insulation. Myelination does not require local stimulation or special geometric conditions, as surfaces of any shape can be myelinated in the absence of molecular inductors[1,2] An explanation for this selective axon myelination has been recently reported, whereby the somatodendritic membrane expression of the Junction Adhesion Molecule 2 (JAM2) inhibits local myelination[3]. Gal-4 is early sorted to the axon membrane, and it is expressed in discrete segments along axon surface of differentiated neurons[28] The role of these membrane Gal-4-containing domains (G4Ds) is unknown, though their disposition in axons, reminiscent of non-myelinated segments of cortical neurons[4,29], and the fact that soluble Gal-4 retards oligodendrocyte maturation[27], prompted our hypothesis that Gal-4 in G4Ds could produce localized myelination impairment of functional consequences. We identify Gal-4 as the first inhibitor of local myelination in axons, and propose that its expression in axon membrane domains defines myelination-incompetent axon segments regulated along neuronal development

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Results

Conclusion