Fyn tyrosine kinase participates in the compact myelin sheath formation in the central nervous system.

The cellular mechanisms for spiral wrapping and compaction of myelin sheaths by oligodendrocytes are not known yet. In this study, we examined the role of fyn tyrosine kinase, which could be responsible for molecular events during the stage of myelination in the CNS. Western blot and immunohistochemical analyses revealed that fyn-deficient mice have significantly lower levels of myelin basic protein (MBP), which is required for intracellular membrane adhesion parts so-called major dense line (MDL) and thought to be essential for the stability of myelin sheath. Electron microscopy verified that the myelin ultrastructure could be used to distinguish fyn-deficient mice from wild-type mice, showing a thin and redundant myelin sheath in the corpus callosum. Further, the electron-dense 'major' line in myelin from the purified myelin fractions remained condensed, and myelin compaction was split opened in fyn-deficient mice. To determine whether there was a change in the microheterogeneity of MBP due to a post-translational event we first investigated peptidylarginine deiminase (PAD), which is an enzyme that converts arginine residues in peptides to citrulline residues. PAD immunoreactivity was observed both in the myelin from fyn-deficient and wild-type mice. By Western blot analysis we found an increase of the citrullined form of MBP. In addition, MBP from fyn-deficient mice did weakly induce vesicle aggregation properties of MBP-mediated adhesion. We concluded that although oligodendrocytes from fyn-deficient mice are able to wrap around the axon, they are unable to form compact myelin due to decreased MBP level and the presence of increased citrullinated MBP.