Abstract
Expression of the neurotrophin receptor trkB is regulated by thyroid hormone (T3) during development of the rat brain. trkB mRNA levels, coding for the full-length and the truncated isoforms, are increased in the cerebral cortex of neonatal experimental hypothyroid animals. Run-on transcription assays with nuclei from postnatal day 15, hypothyroid, and control cerebral cortices demonstrated that an increase in the transcription rate of the trkB gene accounts for the observed effect. Transient transfection experiments using a reporter plasmid containing a 7-kilobase pair DNA fragment upstream of the mouse trkB gene showed that unliganded thyroid hormone receptor (T3R) increases promoter activity, whereas addition of T3 reverses that activity below basal levels. Deletion analysis shows that the T3-dependent repression requires binding of the T3R to a specific region located downstream of the transcription start site. This region, at nucleotide position -465/-432, contains an array of thyroid hormone response half-sites that bind preferentially T3R as heterodimers with retinoid X receptor and whose deletion causes loss of the T3-dependent repression. These half-sites are able to confer negative regulation by T3 to a heterologous promoter, thus indicating the functionality of these sequences. These results demonstrate that, in the developing rat brain, T3 down-regulates the expression of the trkB gene through the active repression of a novel negative response element located downstream of its transcription initiation site.
Highlights
Development of the nervous system follows a sequence of specific events, including proliferation of precursors, differentiation, survival of different cell types, and the establishment of
This region, at nucleotide position ؊465/؊432, contains an array of thyroid hormone response halfsites that bind preferentially T3R as heterodimers with retinoid X receptor and whose deletion causes loss of the T3-dependent repression. These half-sites are able to confer negative regulation by T3 to a heterologous promoter, indicating the functionality of these sequences. These results demonstrate that, in the developing rat brain, T3 down-regulates the expression of the trkB gene through the active repression of a novel negative response element located downstream of its transcription initiation site
¶ Supported by a contract from Programa de Contratacion Temporal de Investigadores del Consejo Superior Investigaciones Cientificas
Summary
T3, triiodothyronine; T3R(s), thyroid hormone nuclear receptor(s); T3RE(s), thyroid hormone response element(s); RXR, retinoid X receptor; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; CAT, chloramphenicol acetyltransferase; bp, base pair(s); kb, kilobase pair(s); EMSA, electrophoretic mobility shift assay; LUC, luciferase; N2a, neuro-2a; RSV, Rous sarcoma virus; GST, glutathione S-transferase; P, postnatal day. We find that the sequences required for the action of T3R as a ligand-dependent repressor are located downstream of the transcriptional initiation site These sequences do not contain a consensus T3RE but a series of imperfect widely spaced half-sites within the Ϫ437/Ϫ342 fragment, whose deletion causes loss of the T3-dependent repression. This sequence binds T3R/RXR heterodimers and confers repression by T3 to a heterologous promoter when it is inserted downstream of the transcription initiation site. These results indicate that the T3-dependent repression of trkB gene expression in the developing brain could be involved in the deleterious effects found in the neonatal hypothyroidism syndrome
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