Effect of thyroid hormone on the synthesis of sialosyl galactosylceramide (GM4) in myelinogenic cultures of cells dissociated from embryonic mouse brain.

A sialyltransferase, which catalyzes the biosynthesis of the myelin-associated sialosyl galactosylceramide (GM4) from galactocerebroside and cytidine-5'-monophospho-N-acetylneuraminic acid, has been detected in primary reaggregating, surface adhering cultures of cells dissociated from embryonic mouse brain. The ontogenetic profile of this enzyme in culture mimics its in vivo developmental pattern in that its activity could be detected only after 28 days in vitro and reached peak values around 48 days in vitro. Between 48 to 75 days in culture (oldest age studied) only a very slow increase in activity is observed. Unlike other myelin marker enzymes whose activities appear at an earlier time in development, the gene expression of the sialyltransferase responds relatively slowly to stimulation by triiodothyronine. However, if exposed to hypothyroid conditions at an early developmental age before the enzyme activity is expressed, little or no activity appears in latter stages of development.