Molecular Landscapes by David S. Goodsell

Myelin, 2020

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Acknowledgement: Illustration by David S. Goodsell, RCSB Protein Data Bank. doi: 10.2210/rcsb_pdb/goodsell-gallery-030

This painting shows a cross-section through the internode region of a myelinated axon in the central nervous system. The axon is shown at the bottom, with the membrane in green, the cytoplasm and cytoskeleton in blue, and a mitochondrion in purple and magenta. The oligodendrocyte wraps multiple times around the axon, shown with the membrane and membrane-bound proteins in yellow and the cytoplasm and cytoskeleton in orange. A small region of connective tissue is shown at the top in red.

Many structural and functional features are shown in the illustration, often requiring some artistic license:

• Proteolipid protein, myelin basic protein, and zig-zag stripes of claudin-11 glue the layers of myelin together.
• Myelin-associated glycoprotein on the inner surface of the oligodendrocyte reaches across and interacts with glycolipids on the axon membrane. More information on this protein is presented in a Molecule of the Month article.
• Myelin-oligodendrocyte glycoprotein on the outer surface of the oligodendrocyte makes a speculative interaction with proteoglycans.
• Voltage-gated sodium and potassium channels are shown in the axon membrane, although they are far more plentiful in the Nodes of Ranvier, rather than in the internode region shown here.
• A braided ring of actin supports the axon membrane, and successive rings are connected by spectrin.
• Neurofilaments and microtubules form a cytoskeleton inside the axon. Long intrinsically-disordered segments control the spacing between them.
• Mitochondria are transported by molecular motors like dynein along microtubules.
• A collection of soluble enzymes taken from a proteomic study fill the oligodendrocyte, including glycolytic enzymes, lactate dehydrogenase, fatty acid synthase and others. A similar collection is shown inside the axon.
• Monocarboxylic acid transporters may deliver lactic acid from the oligodendrocyte to the axon.

Selected References

• Djannatian M, et al. (2019) Two adhesive systems cooperatively regulate axon ensheathment and myelin grown in the CNS. Nat Commun 10: 4794
• Vassilopoulos S, et al. (2019) Ultrastructure of the axonal periodic scaffold reveals a braid-like organization of actin rings. Nat Commun 10: 5803
• Stassart RM, Mobius W, Nave KA, Edgar JM (2018) The axon-myelin unit in development and degenerative disease. Front Neurosci 12: 467
• Yuan A, Rao MV, Veeranna, Nixon RA (2017) Neurofilaments and neurofilament proteins in health and disease. Cold Spring Harb Perspect Biol 9: a018309
• Kirkcaldie MTK, Collins JM (2016) The axon as a physical structure in health and acute trauma. J Chem Neuroanatomy 76: 9-18
• Nave KA, Werner HB (2014) Myelination of the nervous systems: mechanisms and functions. Annu Rev Cell Dev Biol 30: 503-533
• Scherer SS, Arroyo EJ (2014) Myelin: molecular architecture of CNS and PNS myelin sheath. Reference Module in Biomedical Research doi:10.1016/B978-0-12-801238-3.04676-6
• Xu K, Zhong G, Zhuang X (2013) Actin, spectrin, and associated proteins from a periodic cytoskeleton structure in axons. Science 339: 452-456
• Jahn O, Tenzer S, Werner HB (2009) Myelin proteomics: molecular anatomy of an insulating sheath. Mol Neurobiol 40: 55-72.