Investigations on Myelination In Vitro: Regulation of 2′,3′‐Cyclic Nucleotide 3′‐Phosphohydrolase by Thyroid Hormone in Cultures of Dissociated Brain Cells from Embryonic Mice

Abstract
The direct influence of l‐3,3′,5‐triiodothyronine (T3) on the development of 2′,3′‐cyclic nucleotide 3′‐phosphohydrolase (EC 3.1.4.37, CNPase) is demonstrated by using an in vitro culture system of dissociated embryonic mouse brain cells. Serum from a thyroidectomized calf, which contained low levels of T3 (31 ng/100 ml), and thyroxine, T4 (3, 103 ng/100 ml; T4, 5.7 μg/ml) to render the culture responsive to exogenously added T3. The lower levels of enzyme activity observed in the presence of such a deficient medium could be restored to normal values by T3 supplementation. Half‐maximal effect was obtained with 2.5 ± 10−9m‐T3. Three days of hormone treatment resulted in the maximal stimulation of CNPase. T4 was less effective in inducing CNPase activity and the inactive analog of the hormone, reverse T3 (3,3′,5′‐T3) was ineffective. The morphological appearance of the cells was characterized by deformed (smaller size and less in number) reaggregates in the cultures, lacking hormone.

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