Thyroid hormone receptor/c-erbA: control of commitment and differentiation in the neuronal/chromaffin progenitor line PC12

Thyroid hormone regulates reelin and dab1 expression during brain development

The reelin and dab1 genes are necessary for appropriate neuronal migration and lamination during brain development. Since these processes are controlled by thyroid hormone, we studied the effect of thyroid hormone deprivation and administration on the expression of reelin and dab1. As shown by Northern analysis, in situ hybridization, and immunohistochemistry studies, hypothyroid rats expressed decreased levels of reelin RNA and protein during the perinatal period [embryonic day 18 (E18) and postnatal day 0 (P0)]. The effect was evident in Cajal-Retzius cells of cortex layer I, as well as in layers V/VI, hippocampus, and granular neurons of the cerebellum. At later ages, however, Reelin was more abundant in the cortex, hippocampus, cerebellum, and olfactory bulb of hypothyroid rats (P5), and no differences were detected at P15. Conversely, Dab1 levels were higher at P0, and lower at P5 in hypothyroid animals. In line with these results, reelin RNA and protein levels were higher in cultured hippocampal slices from P0 control rats compared to those from hypothyroid animals. Significantly, thyroid-dependent regulation of reelin and dab1 was confirmed in vivo and in vitro by hormone treatment of hypothyroid rats and organotypic cultures, respectively. In both cases, thyroid hormone led to an increase in reelin expression. Our data suggest that the effects of thyroid hormone on neuronal migration may be in part mediated through the control of reelin and dab1 expression during brain ontogenesis.

Beta receptor isoforms are not essential for thyroid hormone-dependent acceleration of PCP-2 and myelin basic protein gene expression in the developing brains of neonatal mice

In rat pups, thyroid hormone dependent brain development coincides with the appearance of the thyroid hormone receptor (TR)beta1 isoform. This finding led to the suggestion that TRbeta1 plays an essential role in brain development. The recent availability of a mouse TRbeta knockout strain allowed us to test this possibility by determining whether TRbeta is essential for the normal developmental pattern of expression of two thyroid hormone regulated brain genes, myelin basic protein (MBP), and Purkinje cell protein 2 (Pcp-2). Northern analysis of total mRNA from the brains of wild-type mice established that, as in the rat pup, the initial rate of rise of the MBP and Pcp-2 mRNA is slowed in the hypothyroid state. Supporting the effectiveness of TRbeta gene deletion was the finding that the thiiodothyronine (T3) nuclear binding capacity in the livers and brains of knockout animals was consistent with the fractional contribution of TRbeta1 to total binding capacity in the wild-type tissues. Further, no TRbeta1 could be detected by isoform-specific immunoprecipitation of nuclear receptor extracts. However, deletion of the functional TRbeta in the TRbeta knockout mice did not affect the normal ontogeny of expression of the Pcp-2 and MBP genes in the postnatal pup. We conclude that TRbeta is not essential for the normal developmental expression of these T3 dependent brain genes.

Impact of PCBs on thyroid hormone directed brain development

Thyroid hormones regulate neuronal proliferation, migration, process outgrowth, synaptic development, and myelin formation in specific brain regions. Because brain development occurs during discrete windows of time, inappropriate levels of thyroid hormones in definitive periods can produce permanent damage, the nature of which depends upon the timing and magnitude of the insult. Thyroid hormones cross the placenta and enter the brain primarily as thyroxine (T4); therefore, conditions selectively lowering serum T4 levels alter brain hormone availability. Triiodothyronine (T3) is the predominant form of the hormone that binds to the receptor. T3 is produced from T4 in the brain by the enzyme type II, 5'-deiodinase. Polychlorinated biphenyls (PCBs) are synthetic environmental toxicants that bear a striking structural resemblance to the active thyroid hormones and can, depending upon the species, dosage, and congener used, act as agonists, antagonists, and partial agonists to thyroid hormones.

The adult rat hippocampus contains primordial neural stem cells

Adult-derived hippocampal progenitors generate neurons, astrocytes, and oligodendrocytes in vitro and following grafting into the adult brain. Although these progenitors have a considerable capacity for in vitro self renewal, it is not known if each lineage is generated by separate committed precursors or by multipotent stem cells. By genetic marking, we have followed individual cells through the process of proliferative expansion, commitment, and differentiation. All three lineages are generated by single marked cells and the relative proportions of each lineage can be strongly influenced by environmental cues. Differentiation is accompanied by a characteristic progression of lineage-specific markers and can be potentiated by retinoic acid, elevated cyclic AMP, or neurotrophic factors. The ability to genetically mark and clone normal diploid hippocampal progenitors provides the first definitive evidence that multipotent neural stem cells exist outside of the adult striatal subventricular zone and supports the hypothesis that FGF-2-responsive neural stem cells may be broadly distributed in the adult brain.

Thyroid hormone promotes BCL-2 expression and prevents apoptosis of early differentiating cerebellar granule neurons

Programmed cell death is a basic cellular process that has aroused much interest in recent years. Like immune cells, cultures of cerebellar granule neurons are very homogeneous and provide a unique opportunity for quantifying by flow cytometry one form of programmed cell death in the CNS, the apoptosis, and for studying its regulation by neurotrophic factors. We found that thyroid hormone promoted postmitotic survival by preventing the apoptosis of newly formed and early differentiated granule neurons in a dose-dependent manner. This regulation could be through the protein bcl-2, which is known to prevent cell death. This protein was present at all stages of granule neuron differentiation and appeared to be developmentally regulated. It was underexpressed in apoptotic granule neurons. The protein content of the cerebellum in hypothyroid rats was drastically reduced. In contrast, thyroid hormone caused a marked dose-dependent increase in the amounts of this protein in granule neuron cultures. The possibility that thyroid hormone may be directly or indirectly required to promote cell survival is discussed, in terms of the hormone control of the local delivery of neurotrophins, such as NGF and NT-3, as well as the expression of their low affinity receptors, gp75. We suggest that thyroid hormone has a permissive action on the developing CNS.

Expression of neurotrophins and the trk family of neurotrophin receptors in normal and hypothyroid rat brain

Thyroid hormone deficiency has dramatic effects on rat brain maturation. The expression of genes encoding neurotrophins and the trk family of neurotrophin receptors has been evaluated in several brain regions of normal and of neonatal or adult hypothyroid rats to analyze whether they are subject to thyroid hormone action. We found that hypothyroidism decreased trk mRNA levels in its major site of expression, the striatum, on postnatal days 5 (P5; 45%) and 15 (P15; 25%) and also in adults (35%). In contrast, no differences in trkB or trkC mRNAs levels were observed in any brain region at studied ages. According to previous reports, p75LNGFR mRNA was elevated in hypothyroid cerebellum as compared to age-matched controls on P5 and P15. We have also observed a distinct pattern for neurotrophin genes. The level of NGF mRNA was 20-50% lower in the cortex, hippocampus, and cerebellum of hypothyroid rats on neonatal hypothyroid rats on P15 and also after adult-onset hypothyroidism. Treatment of neonatally-induced hypothyroid rats with a single injection of triiodothyronine led to the recovery of hippocampal but not cortex NGF mRNA levels to that of control animals. On the contrary, no differences in the relatively high expression of the two mRNAs encoding BDNF were observed in any brain area. In contrast to a recent report, we did not find a reduction in brain NT-3 mRNA levels in hypothyroid animals. If any, the effect of thyroid deficiency in the hippocampus and cortex seems to be an early upregulation of NT-3 expression.(ABSTRACT TRUNCATED AT 250 WORDS)