Alterations in cerebellar protein synthesis in propylthiouracil-induced hypothyroid rats

Thyroid hormones play an important role in the regulation of gene expression and cerebellar development in the rat. The drug propylthiouracil was used to disrupt thyroid hormone synthesis. This permitted investigation of gene expression under three different states of hormonal deprivation: early in development (PTU 1), late in development (PTU 2), and continual deprivation (PTU 3). The effects of these deprivations on some aspects of the synthesis of two proteins known to be affected by T$\backslash sb3$, apolipoprotein E (apo E) and transferrin (T$\backslash sbf$), were investigated.

Thyroid hormone's permissive role on growth and development, morphological features as well as body and organ weights were analyzed and found to be significantly reduced in the hypothyroid state. The development of cerebellar cortical strata and cellular morphology was altered by prenatal, as well as postnatal PTU administration.

To establish cerebellar synthesis of apo E and T$\backslash sbf$, total RNA was extracted, poly A$\backslash sp+$ RNA isolated and translated in an in vitro reticulocyte lysate system. The specific proteins were immunoprecipitated and analyzed by SDS-PAGE. To correlate these results with the amounts of mRNA's for apo E and T$\backslash sbf$, northern hybridization was performed.

Both apo E and T$\backslash sbf$ levels synthesized in vitro with RNA from control animals increased as development proceeded, with apo E produced in greater quantity. While hepatic apo E synthesis increases in the hypothyroid state (presumably as a result of regulation at the transcriptional level), cerebellar apo E translatable mRNA levels were not affected by PTU treatment. Like hepatic T$\backslash sbf$, cerebellar T$\backslash sbf$ levels were substantially reduced in the hypothyroid state, as compared to control values. Developmentally, apo E mRNA levels declined, while T$\backslash sbf$ mRNA levels and T$\backslash sbf$ synthesized in vitro were parallel in control animals. PTU 1 treated animals, in which free T$\backslash sb3$ in the circulation increased after PN day 20, demonstrated increases in both apo E and T$\backslash sbf$ mRNA levels at the same time point.

It appears that the regulation of apo E and T$\backslash sbf$ mRNA levels differ from each other within the cerebellum and may be regulated differently from those of the liver. The increase in in vitro protein synthesis and mRNA levels in PTU 1 treated animals may be a result of developmental recovery, rather than specific T$\backslash sb3$ mediated gene regulation.