Investigations on myelinogenesis in vitro: regulation of 5'-nucleotidase activity by thyroid hormone in cultures of dissociated cells from embryonic mouse brain.

The developmental pattern of the myelin-associated 5'-nucleotidase and its regulation by L-3,3',5,-triiodothyronine (T3) have been demonstrated in a culture system of cells dissociated from embryonic mouse brain. Hypothyroid calf serum containing low levels of T3 (31 ng/100 ml), and thyroxine, T4 (less than 1 microgram/ml), was used in the culture medium in place of normal calf serum (T3, 103 ng/100ml; T4, 5.7 micrograms/ml) to render the cultures responsive to exogenously added T3. By means of T3 supplementation, the lower levels of enzyme activity observed in the cultures grown in the presence of hypothyroid calf-serum containing medium could be restored to a considerable extent although not completely to normal values. Half-maximal stimulatory effect was obtained at 3.9 X 10(-8)M T3 concentration. Among the various substrates tested, 5'-AMP, 5'-UMP and 5'-CMP were equally good, while 5'-GMP yielded approximately half the activity.