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Molecular Landscapes by David S. Goodsell

Myelin, 2020

Acknowledgement: Illustration by David S. Goodsell, RCSB Protein Data Bank

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:

  1. Proteolipid protein, myelin basic protein, and zig-zag stripes of claudin-11 glue the layers of myelin together.
  2. 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.
  3. Myelin-oligodendrocyte glycoprotein on the outer surface of the oligodendrocyte makes a speculative interaction with proteoglycans.
  4. 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.
  5. A braided ring of actin supports the axon membrane, and successive rings are connected by spectrin.
  6. Neurofilaments and microtubules form a cytoskeleton inside the axon. Long intrinsically-disordered segments control the spacing between them.
  7. Mitochondria are transported by molecular motors like dynein along microtubules.
  8. 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.
  9. Monocarboxylic acid transporters may deliver lactic acid from the oligodendrocyte to the axon.


Myelin-specific proteins
A. PLP (proteolipid protein)
B. MBP (myelin basic protein)
C. MOG (myelin oligodendrocyte glycoprotein)
D. Claudin-11 (based on claudin-15)
E. MAG (myelin-associated glycoprotein)

Other Oligodendrocyte Proteins
F. Integrin
G. Dystroglycan complex
H. Signaling complex
I. Tyrosine-protein kinase Fyn (based on src)
J. Septin
K. Anillin
L. MCT1 (monocarboxylate transporter 1)

Axon
M. Voltage-gated sodium channel
N. Voltage-gated potassium channel
O. MCT2 (monocarboxylate transporter 2)
P. Actin
Q. Adducin
R. Spectrin
S. Neurofilament
T. Microtubule
U. Dynein-dynactin
V. Syntaphilin

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.