Active repression of RAR signaling is required for head formation.

Area of Science:

• Developmental Biology
• Molecular Biology
• Genetics

Background:

• Retinoic acid receptors (RARs) regulate gene transcription via coactivator/corepressor recruitment.
• While corepressor mechanisms are understood, in vivo evidence for RAR-mediated repression's necessity is lacking.
• RAR signaling is crucial for anteroposterior axis patterning, with absence of retinoic acid (RA) needed for anterior patterning.

Purpose of the Study:

• To investigate the role of RAR-mediated repression in anterior patterning and head formation.
• To test the hypothesis that unliganded RARs mediate repression important for anterior development.

Main Methods:

• Utilized dominant-negative corepressors to reduce RAR-mediated repression.
• Employed an RAR inverse agonist to enhance target gene repression.
• Performed morpholino-based RARalpha loss-of-function studies.
• Used microinjection for rescue experiments with wild-type or dominant-negative RARalpha.
• Assessed effects on anterior structures, gene expression, and response to head-inducing factors.

Main Results:

• Reducing RAR-mediated repression mimicked RA treatment effects, causing reduced anterior structures and gene expression.
• Enhancing RAR-mediated repression led to up-regulated anterior neural markers and expanded anterior structures.
• RARalpha loss-of-function phenocopied repression reduction, with rescue confirming RAR's repressive role.
• Increased RAR-mediated repression potentiated head induction by cerberus; derepression blocked it.

Conclusions:

• RAR-mediated repression is critical for vertebrate head formation.
• This study provides in vivo evidence for a necessary role of RAR-mediated repression.
• Establishes a novel function for RARs in development and highlights the biological importance of active repression by nuclear hormone receptors.