Molecular Mechanism of Central Nervous System Myelinogenesis: In Vitro Self-Assembly of Myelin Membrane Lipid and Protein Structures

Abstract

The myelin sheath is an insulating, compacted, multilamellar biological membrane that facilitates efficient propagation of action potentials down neuronal axons, and is critical for proper physiological function. Recently, EM imaging has provided compelling in vivo data that puts to question the long-established mechanism for the formation of central nervous system (CNS) myelin. The new data have led to the proposal of a new model involving (1) the preformation of myelin membrane tubules that are trafficked to the neuronal axon, where they (2) undergo a transition from tubular to lamellar form and thus form the final compact myelin sheath [Szuchet et al. J. Struct. Biol. 190, 56-72 (2015)]. To investigate this mechanism, we designed in vitro experiments to probe the interactions of myelin lipids as they (1) self-assemble into tubules and (2) transition into lamellar form. Using fluid-cell AFM, TEM, and DLS, we have investigated the self-assembly of lipidic tubules, their transition into the multilamellar structure of mature myelin, and how this process is modulated by lipid composition and the presence of myelin basic protein. Our data support a galactosylceramide (GalCer) concentration-dependent response in lipid morphology that drives a transition from stable tubules to nonspecific aggregates with decreasing GalCer concentration. Our in vitro findings at high GalCer concentrations align with the in vivo tubules observed in ovine embryonic oligodendrocytes, suggesting that these structures of major myelin lipids can be stable precursors for myelination.