Myelin-associated glycoprotein

MAG
Identifiers
Aliases	MAG, GMA, S-SIGLEC-4A, SIGLEC4A, SPG75, myelin associated glycoprotein
External IDs	OMIM: 159460 MGI: 96912 HomoloGene: 1771 GeneCards: MAG
Gene location (Human) Chr. Chromosome 19 (human)[1] Band 19q13.12 Start 35,292,125 bp[1] End 35,313,807 bp[1]
Gene location (Mouse) Chr. Chromosome 7 (mouse)[2] Band 7 B1|7 19.26 cM Start 30,598,601 bp[2] End 30,614,298 bp[2]
RNA expression pattern Bgee Human Mouse (ortholog) Top expressed in inferior ganglion of vagus nerve middle frontal gyrus corpus callosum pons superior vestibular nucleus subthalamic nucleus external globus pallidus ventral tegmental area putamen Brodmann area 10 Top expressed in pontine nuclei lateral geniculate nucleus globus pallidus dorsal tegmental nucleus ventral tegmental area medulla oblongata medial geniculate nucleus superior colliculus cerebellar cortex inferior colliculus More reference expression data BioGPS n/a
Gene ontology Molecular function signaling receptor binding lipid binding protein kinase binding carbohydrate binding sialic acid binding protein homodimerization activity ganglioside GT1b binding Cellular component cytoplasm plasma membrane integral component of plasma membrane membrane integral component of membrane paranode region of axon myelin sheath adaxonal region myelin sheath compact myelin Schmidt-Lanterman incisure membrane raft mesaxon Biological process cell adhesion nervous system development negative regulation of neuron projection development substantia nigra development central nervous system myelination negative regulation of axon extension positive regulation of myelination negative regulation of neuron apoptotic process negative regulation of neuron differentiation positive regulation of astrocyte differentiation negative regulation of axonogenesis leukocyte migration cellular response to mechanical stimulus cell-cell adhesion via plasma-membrane adhesion molecules transmission of nerve impulse axon regeneration Sources:Amigo / QuickGO
Orthologs Species Human Mouse Entrez 4099 17136 Ensembl ENSG00000105695 ENSMUSG00000036634 UniProt P20916 P20917 RefSeq (mRNA) NM_080600 NM_001199216 NM_002361 NM_010758 NM_001346084 NM_001346085 NM_001346086 NM_001346087 NM_001346088 NM_001346089 NM_001369262 RefSeq (protein) NP_001186145 NP_002352 NP_542167 NP_001333013 NP_001333014 NP_001333015 NP_001333016 NP_001333017 NP_001333018 NP_034888 NP_001356191 Location (UCSC) Chr 19: 35.29 – 35.31 Mb Chr 7: 30.6 – 30.61 Mb PubMed search [3] [4]
Wikidata
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Myelin-associated glycoprotein (MAG, Siglec-4) is a type 1 transmembrane protein glycoprotein localized in periaxonal Schwann cell and oligodendrocyte membranes, where it plays a role in glial-axonal interactions. MAG is a member of the SIGLEC family of proteins and is a functional ligand of the NOGO-66 receptor, NgR.^[5] MAG is believed to be involved in myelination during nerve regeneration in the PNS^[6] and is vital for the long-term survival of the myelinated axons following myelinogenesis.^[7] In the CNS MAG is one of three main myelin-associated inhibitors of axonal regeneration after injury,^[8] making it an important protein for future research on neurogenesis in the CNS.

Structure

MAG is a 100 kDA glycoprotein.^[9] Uncleaved MAG is a complete transmembrane form, which acts as a signaling and adhesion molecule.^[10] MAG can also act as a signaling molecule as a soluble protein after it has been proteolytically shed. This form of the protein is called dMAG.^[11]

Adhesion

MAG has an extended conformation of five immunoglobulin (Ig) domains and a homodimeric arrangement involving membrane-proximal domains Ig4 and Ig5. MAG-oligosaccharide complex structures and biophysical assays show how MAG engages axonal gangliosides at domain Ig1.^[12]

Function

Myelin-axon interactions

MAG is a critical protein in the formation and maintenance of myelin sheaths. MAG is localized on the inner membrane of the myelin sheath and interacts with axonal membrane proteins to attach the myelin sheath to the axon.^[13].\ Mutations to the MAG gene are implicated in demyelination diseases such as multiple sclerosis.^[14]

Inhibition of nerve regeneration

Axons in the central nervous system do not regenerate after injury the same way that axons in the peripheral nervous system do. ^[15] The mechanism responsible for inhibited neuroregeneration is regulated by three main proteins, one of which is MAG.^{[16] [17]} The exact mechanism through which MAG inhibits neuroregeneration appears to be through binding of NgR. This receptor is also bound by Nogo protein, suggesting that the mechanism of myelin-associated inhibition of axon regeneration through NgR the has redundant ligands^{[clarification needed]}, furthering the inhibition.^[18] MAG binds with high affinity to NgR, suggesting that it is equally as responsible for inhibition of axon regeneration as Nogo.^[19]

Rho kinase pathway

Once MAG (or Nogo) has bound to NgR, NgR activates the rho kinase (ROCK) pathway. The activation of the rho kinase pathway leads to the phosphorylation of proteins which inhibit neurite outgrowth.^[20]

See also

• Myelin
• Myelinogenesis
• NgR
• Myelin oligodendrocyte glycoprotein
• Anti-MAG peripheral neuropathy

External links

• Myelin-associated+glycoprotein at the U.S. National Library of Medicine Medical Subject Headings (MeSH)

References

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