Expression of thyroid receptor isoforms in the human fetal central nervoussystem and the effects of intrauterine growth restriction

BACKGROUND Congenital hypothyroidism is known to be associated with mental retardation which, if recognized promptly, is largely prevented by thyroid hormone replacement. Intrauterine growth restriction (IUGR) is a major caus e of perinatal mortality and morbidity, and is also associated with neurode velopmental delay. Fetuses with IUGR have reduced circulating concentration s of free thyroxine (T4) and free triiodothyronine (T3), leading to the hyp othesis that a reduction in the tissue effects of thyroid hormones in the c entral nervous system (CNS) may contribute to neurodevelopmental morbidity. Since thyroid hormone effects are mediated through binding to specific nuc lear thyroid hormone receptors (TRs), we have defined the pattern of TR iso form expression in the CNS throughout normal human development and have com pared TR expression in the CNS of normal fetuses with those affected by IUG R. METHODS Samples of normal human fetal brain from first and second trimester s were obtained at surgical termination of pregnancy. Appropriately grown a nd third trimester fetuses affected by Intrauterine growth restriction (IUG R) were also investigated after unexplained stillbirth at post mortem exami nation. Reverse transcriptase polymerase chain reaction (RT-PCR) was used t o examine the expression of TR isoform mRNAs in frozen cerebral cortex from 10 to 16 weeks gestation. TR protein expression in human fetal brains (bot h cerebral hemispheres and cerebellum) was also examined in formalin fixed sections and expression of TR alpha 1, alpha 2, beta 1 and beta 2 isoforms being defined using semiquantiative immunocytochemistry. RESULTS RT-PCR revealed the presence of mRNAs encoding TR alpha 1, beta 1 a nd beta 2 isoforms and the nonfunctional TR alpha 2 variant in the fetal ce rebral cortex from week 10 of human pregnancy. Immunostaining of the fetal brain revealed TR alpha 1 and alpha 2 protein from week 11 of human gestati on. Expression of all TR isoform proteins was largely confined to the pyram idal neurones of the cerebral cortex and the Purkinje cells of the cerebell um with increasing receptor expression evident with gestational age. Semiqu antitative observer scoring showed that by the second trimester, there was a marked increase in the proportion of pyramidal and Purkinje cells express ing TR isoforms, while by the third trimester, all these cells immunostaine d. Comparison of TR immunostaining in the cerebral cortex and cerebellum fr om IUGR fetuses (n = 18) matched for gestational age to normal fetuses reve aled a lower intensity of expression of all the TR isoforms confirmed by ob server scoring and quantification using TR protein immunofluoresence (P < 0 .01). CONCLUSIONS Our findings indicate both pre- and post-translational expressi on of TR alpha and beta isoforms in the cerebral cortex of first trimester fetuses. These findings support the view that the transplacental passage of thyroid hormone in early gestation may be critical to neurological develop ment. Our finding that in severe IUGR the expression of TR isoforms in the human fetal cerebral cortex and cerebellum was significantly reduced, in as sociation with reduced circulating thyroid hormone concentrations indicate that changes in free ligand and receptors may affect CNS development. These findings should prompt further investigation of the potential therapeutic role of peripartum thyroid hormone treatment.