Motor Exit Point (MEP) Glia: Novel Myelinating Glia That Bridge CNS and PNS Myelin

Abstract

Oligodendrocytes (OLs) and Schwann cells (SCs) have traditionally been thought of as the exclusive myelinating glial cells of the central and peripheral nervous systems (CNS and PNS), respectively, for a little over a century. However, recent studies demonstrate the existence of a novel, centrally-derived peripheral glial population called motor exit point (MEP) glia, which myelinate spinal motor root axons in the periphery. Until recently, the boundaries that exist between the CNS and PNS, and the cells permitted to cross them, were mostly described based on fixed histological collections and static lineage tracing. Recent work in zebrafish using in vivo, time-lapse imaging has shed light on glial cell interactions at the MEP transition zone and reveals a more complex picture of myelination both centrally and peripherally.

Keywords: boundary cap cell; motor exit point glia; myelin; oligodendrocyte; schwann cell; zebrafish.

Figure 1

Centrally-derived motor exit point (MEP) glia myelinate motor nerve roots. (A) Lateral view of a olig2:egfp;foxd3:mcherry zebrafish trunk showing olig2⁺/foxd3⁺ MEP glia (arrows) and olig2⁻/foxd3⁺ SCs (arrowheads) along a motor nerve at 55 hour post-fertilization (hpf). (B) Diagram representing central and peripheral myelinating glial markers. (C) Pseudo-colored z-stack of a lateral view of a mbp:egfp-caax trunk at 5 day post-fertilization (dpf). MEP glial myelin sheaths (arrowheads) originate within the spinal cord and project laterally along motor nerve axons, coming close to schwann cells (SC) myelin (arrows). (D) SC myelin (bracket) but not MEP glial myelin (arrows) is absent in G-protein coupled receptor 126 (gpr126) mutants at 5 days post fertilization (dpf). (E) In situ hybridization showing the presence of Wnt-inhibitory factor 1 (wif1⁺) MEP glia (arrowheads) along motor nerve roots in 3 dpf control and gpr126 mutant larvae. Scale bar, (A–D) 50 μm, (E) 20 μm.

Figure 2

MEP glial myelin is flanked by nodes of ranvier. (A) Lateral view of a 5 dpf zebrafish trunk stained with antibodies specific to pan Na⁺ channels (clone k58/35) and acetylated tubulin shows that MEP glial territory is flanked by sodium channels clustered in Nodes of Ranvier (arrowheads) along motor nerve roots. (B) Diagram showing MBP⁺ MEP glial myelin (blue) delineated by nodes of Ranvier (green) along motor nerve root axons (magenta). (C) Immunostaining showing MBP⁺ MEP glial myelin (arrowhead) and nodes of Ranvier (arrows) along motor nerve root axons (magenta). Asterisks point to dorsal root ganglion (DRG). (D) Schematic of zebrafish MEP transition zones (TZs) and the diverse populations of glial cells orchestrating the CNS/PNS boundary. CNS-derived MEP glia (orange) that reside along motor neuron (mn; black) axons restrict the oligodendrocyte lineage (OL; green) to the spinal cord. Radial glia (teal green) cover the surface of the spinal cord and prevent peripheral glia such as MEP glia and SC (blue) from entering the CNS. (E) Schematic of a mammalian neural tube showing neural crest (NC)-derived boundary cap (BC) cells (pink) sitting at the dorsal root entry zone (DREZ) and MEP TZs. BC cells prevent mn (black) cell bodies from transgressing the spinal cord boundary. Scale bar, (A) 50 μm, (C) 25 μm.