Myelin-associated oligodendrocytic basic protein modulates the arrangement of radial growth of the axon and the radial component of myelin

Myelin-associated oligodendrocytic basic protein (MOBP) is abundantly expressed in central nervous system myelin, and shares several characteristics with myelin basic protein (MBP), in terms of regional distribution and function. MOBP causes experimental allergic encephalomyelitis (EAE) and is associated with multiple sclerosis. MOBP has several isoforms, and the carboxy termini that are unique to each isoform contain a cluster of positively charged amino acids residues. These residues are considered to facilitate myelin sheath compaction, as does MBP. MOBP-deficient mice had compact myelin of a normal appearance. The myelin periodicity in MOBP-deficient mice was equal to that in wild-type mice. When mice were exposed to hexachlorophene, a known dysmyelinating agent, however, the myelin showed widening of the major dense lines. By morphometric analysis of the optic nerve in MOBP-deficient and MBP/MOBP-double-deficient mice, we demonstrated that MOBP played a role in controlling axonal diameter. After glutaraldehyde and tannic acid fixation, a radial series of rod-like electron enhancements in the intraperiod line in the CNS myelin, the so-called "radial component", was found arrayed in a straight radial direction in MOBP-deficient mice, while these components were scattered in an oblique zigzag direction in wild-type mice. Thus, it can be concluded that MOBP is essential for normal arrangement of the radial component.