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Nuclear receptor corepressor recruitment by unliganded thyroid hormone receptor in gene repression during Xenopus
Laurent M Sachs1, Peter L Jones, Emmanuelle Havis
1Unit on Molecular Morphogenesis, Laboratory of Gene Regulation and Development, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892-5431, USA. sachs@mnhn.fr
Abstract:
Thyroid hormone receptors (TR) act as activators of transcription in the presence of the thyroid hormone (T(3)) and as repressors in its absence. While many in vitro approaches have been used to study the molecular mechanisms of TR action, their physiological relevance has not been addressed. Here we investigate how TR regulates gene expression during vertebrate postembryonic development by using T(3)-dependent amphibian metamorphosis as a model. Earlier studies suggest that TR acts as a repressor during premetamorphosis when T(3) is absent. We hypothesize that corepressor complexes containing the nuclear receptor corepressor (N-CoR) are key factors in this TR-dependent gene repression, which is important for premetamorphic tadpole growth. To test this hypothesis, we isolated Xenopus laevis N-CoR (xN-CoR) and showed that it was present in pre- and metamorphic tadpoles. Using a chromatin immunoprecipitation assay, we demonstrated that xN-CoR was recruited to the promoters of T(3) response genes during premetamorphosis and released upon T(3) treatment, accompanied by a local increase in histone acetylation. Furthermore, overexpression of a dominant-negative N-CoR in tadpole tail muscle led to increased transcription from a T(3)-dependent promoter. Our data indicate that N-CoR is recruited by unliganded TR to repress target gene expression during premetamorphic animal growth, an important process that prepares the tadpole for metamorphosis.
Insights
Nuclear receptor corepressor (N-CoR) represses gene expression in the absence of thyroid hormone (T(3)) during tadpole development. This N-CoR recruitment by thyroid hormone receptors (TR) is crucial for premetamorphic growth.
Area of Science:
- Molecular Endocrinology
- Developmental Biology
- Gene Regulation
Background:
- Thyroid hormone receptors (TR) modulate gene transcription, acting as activators with thyroid hormone (T(3)) and repressors without it.
- In vitro studies elucidate TR mechanisms, but physiological relevance in development remains less understood.
- Amphibian metamorphosis provides a T(3)-dependent model to study TR's role in postembryonic development.
Purpose of the Study:
- To investigate the physiological role of TR-mediated gene repression during vertebrate postembryonic development.
- To test the hypothesis that nuclear receptor corepressor (N-CoR) complexes mediate TR-dependent gene repression crucial for premetamorphic tadpole growth.
Main Methods:
- Isolation of Xenopus laevis N-CoR (xN-CoR) and confirmation of its presence in tadpoles.
- Chromatin immunoprecipitation (ChIP) assay to assess xN-CoR recruitment to T(3) response gene promoters.
- Histone acetylation analysis and dominant-negative N-CoR overexpression in tadpole muscle.
Main Results:
- xN-CoR was recruited to T(3) response gene promoters during premetamorphosis (low T(3)) and released upon T(3) treatment.
- T(3) treatment led to increased histone acetylation at these promoters.
- Overexpression of dominant-negative xN-CoR increased transcription from a T(3)-dependent promoter.
Conclusions:
- Nuclear receptor corepressor (N-CoR) is recruited by unliganded thyroid hormone receptors (TR) to repress target gene expression.
- This N-CoR-mediated repression is vital for premetamorphic tadpole growth and development.
- The findings highlight a key molecular mechanism preparing tadpoles for metamorphosis.