MAG

Official Full Name

myelin associated glycoprotein

Organism

Homo sapiens

GeneID

4099

Background

The protein encoded by this gene is a type I membrane protein and member of the immunoglobulin superfamily. It is thought to be involved in the process of myelination. It is a lectin that binds to sialylated glycoconjugates and mediates certain myelin-neuron cell-cell interactions. Three alternatively spliced transcripts encoding different isoforms have been described for this gene. [provided by RefSeq, Nov 2010]

Synonyms

GMA; S-MAG; SPG75; SIGLEC4; SIGLEC4A; SIGLEC-4A;

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Detailed Information

Myelin associated glycoprotein (MAG) is a transmembrane glycoprotein located on Schwann cells and oligodendrocytes adjacent to the myelin axon and acts between the colloid and axon. It belongs to the subgroup of sialic acid in the immunoglobulin superfamily and contains five immunoglobulin-like regions. MAG is divided into two subtypes, large and small, which have different expressions at different stages of myelination and maintenance. MAG plays a major role in the regeneration of peripheral nerves.

MAG Signal Transduction

The signal of MAG is not yet clear, but it was found to be mainly through the Rho /Rho kinase pathway. The Rho family is a class of guanosine triphosphate ase (GTP ase). Rho mediated activation of downstream effectors such as Rho-coupled kinase (ROCK), which in turn induces actin polymerization, ultimately leading to collapse of growth cones and inhibition of axon growth. The study also found that Rho inactivation or elevated cAMP levels in neurons, MAG-mediated inhibition will be blocked. Therefore, it is considered that the signal transduction pathway involved in Rho and cAMP is also required for MAG to exert an inhibitory effect, and the effects of the two are consistent with the MAG-mediated inhibitory effect.

Figure 1. Myelin inhibitors of axon regeneration. (Vajn, et al. 2013)

Development of MAG and PNS

In the peripheral nervous system (PNS) and the central nervous system (CNS), myelin sheath is composed of Schwann cells and some protein components, such as P0 protein, myelin basic egg white, peripheral myelin protein, myelin lipid protein and MAG. MAG is initially expressed in myelin sheath formation very early. MAG has already played its role in the initial stage of oligodendrocyte processes and axon interactions as well as in the stage of myelin sheath formation around axons through oligodendrocyte membrane and Schwann membrane. Evidence for MAG-promoting PNS myelin growth stems from studies of dorsal root ganglion (DRG) neurons and embryonic retinal neurons, and the results confirm that MAG promotes DRG neuron growth.

The Role of MAG after Peripheral Nerve Injury

Peripheral nerve injury can regenerate to a certain extent, but such regeneration occurs after Wallerian degeneration, that is, myelin is removed, Schwann cells return to the state of non-myelin and myelin protein expression decreases. The Rho /Rho kinase pathway is the signal transduction pathway of MAG, and Wallerian degeneration after nerve injury is related to the Rho /Rho kinase pathway to a certain extent. MAG accelerated axonal degeneration after the activation of Rho /Rho kinase in vitro experiments in the spinal cord. in vivo experiments, Wallerian denaturation can be blocked by RHO kinase inhibitors, while inhibiting Rho kinase activity in vivo and in vitro can delay Wallerian denaturation. However, RhoA activity was not detected in the injured sciatic axons, and the surface Rho/Rho kinase pathway in PNS was only one of the factors that accelerated Wallerian denaturation. Recent studies have found that topical application of MAG after sciatic nerve cutting can reduce the recovery of damage-promoting.

Studies have shown that chronic compression injury can stimulate the sprouting of axons. It also shows that in the chronic nerve compression injury of the nerve, the local MAG down-regulation is a major signal to the axon sprouting response. MAG has a positive regulation effect on the diameter of myelinated axons. The diameter of the axons surrounded by myelin in PNS of MAG-deficient mice is significantly reduced, which may be related to the decrease of neurofilament spacing and neurofilament phosphorylation, suggesting that mutations in the myelin gene may cause secondary changes in nerve cells.

References:

Vajn, K. , Plunkett, J. A. , Tapanescastillo, A. , & Oudega, M. . (2013). Axonal regeneration after spinal cord injury in zebrafish and mammals: differences, similarities, translation. Neuroscience Bulletin, 29(4), 402-410.
Chaudhry, N. , Bachelin, C. , Zujovic, V. , Hilaire, M. , Baldwin, K. T. , & Follis, R. M. , et al. (2017). Myelin-associated glycoprotein inhibits schwann cell migration and induces their death. Journal of Neuroscience the Official Journal of the Society for Neuroscience, 37(24), 5885.
Pronker, M. F. , Lemstra, S. , Snijder, J. , Heck, A. J. R. , Thies-Weesie, D. M. E. , & Pasterkamp, R. J. , et al. (2016). Structural basis of myelin-associated glycoprotein adhesion and signalling. Nature Communications, 7, 13584.

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