Developmental regulation of myelin basic protein in dispersed cultures

Abstract

The expression of myelin basic protein, a major component of the myelin membrane, was studied in the absence of myelin formation in a unique situation in which these 2 processes were uncoupled. The oligodendrocytes that contained myelin basic protein were identified by immunofluorescence in primary disperse cultures derived from 20 day old fetal rat brain. Their number increased 20-fold between 15-34 days in culture. Morphologically identifiable myelin was never observed. The oligodendrocytes elaborated a complex network of processes and membranous sheets resembling unfurled myelin. Myelin basic protein appeared concomitantly in the perikaryon, processes and membranous sheets of the oligodendrocytes and remained distributed in these compartments throughout the culture period. The oligodendrocytes synthesized the 4 forms of myelin basic protein found in rodent brain with MW of 21,500, 18,500, 17,000 and 14,000 and modulated their expression with time in culture. The onset of rapid myelin basic protein accumulation, as measured by radioimmunoassay, took place after 25 days in culture. Myelin basic protein accumulated at the rate of 0.2 fmol per oligodendrocyte per day and reached a level of 300 pmol/mg of protein by 34 days. By 60 days, the amount of myelin basic protein had declined to 100 pmol/mg of protein, a level maintained up until at least 120 days. When the amount of myelin basic protein was correlated with the number of oligodendrocytes, each induced cell contained on average 1 fmol of this protein at the 3 time points (15, 28 and 34 days in culture) at which cells were counted. Our results indicate that the accumulation, modulation of the molecular forms and insertion of myelin basic protein into the membrane can occur in the absence of myelin formation, but that continued metabolic stability, subcellular sequestration and fine control of the relative proportions of the different forms of myelin basic protein may be dependent on myelin morphogenesis.