David Wotton, Ph.D.
Assistant Professor of Biochemistry
and Molecular Genetics

Transcriptional Regulation by TGFb Signaling
and the Role of Transcriptional Corepressors

TGF beta signaling:

The gene responses activated by TGF beta family signaling underlie many developmental and proliferative events, including mesoderm induction, antiproliferative responses in mammalian cells and dorsalization in fly embryos. Mutations in components of the signal transduction pathway, resulting in a loss of TGF beta signaling can contribute to human cancer. From work with targeted disruptions in mice, it is clear that alterations in TGF beta signaling can result in severe developmental defects.

TGF beta family ligands bind to specific cell surface receptor serine/threonine kinases, resulting in receptor multimerization and activation. The response to TGF beta family ligands is dependent on the intracellular mediators of TGF beta signaling, the Smad proteins. Following TGF beta receptor-mediated phosphorylation and activation of pathway-specific Smad proteins, the activated Smad complexes move to the nucleus where they activate gene expression.

Smads and TGIF:

Transcriptional regulation by Smads is dependent on interactions with transcriptional coactivators and corepressors, including the homeodomain protein TGIF. Interaction of Smad proteins with TGIF precludes interaction with coactivators resulting in repression of transcription. TGIF can interact with several general corepressor, including histone deacetylases (HDAC) and can recruit HDACs to an activated Smad complex. The level of TGIF within the nucleus can thereby regulate the transcriptional outcome of a TGF beta response. Signaling inputs that regulate the level or activity of coactivators and corepressors with which Smads interact are likely to alter the balance between transcriptional activation and repression, thus modulating TGF beta-activated transcription.

Transcriptional repression by TGIF:

TGIF has intrinsic transcriptional repression functions, independent of TGF beta signaling. TGIF represses transcription when bound directly to DNA via its own homeodomain. Current work is aimed at identifying components of TGIF-containing transcriptional repression complexes and elucidating the role TGIF plays. We have demonstrated that TGIF interacts with HDAC and CtBP, which may repress transcription via interactions with polycomb group proteins. Thus, TGIF appears to target general transcriptional corepressors to specific gene responses, either via direct DNA binding, or by interaction with TGF beta-activated Smads.

TGIF is a homeodomain transcription factor, of the TALE superfamily. Members of this family of homeodomain proteins generally bind DNA in association with other DNA binding proteins. However, binding partners for TGIF are not yet known, and gene responses regulated by TGIF independent of TGF beta signaling remain to be identified. TGIF may play a role in regulating retinoid responsive transcription and it will be of interest to clarify the role of TGIF in this pathway and to identify other TGF beta-independent roles of TGIF. We are currently attempting to identify both TGIF target genes and other transcription factors with which TGIF interacts.

Holoprosencephaly:

We have recently demonstrated that the TGIF gene is mutated in holoprosencephaly (HPE), a human genetic disorder affecting craniofacial development. The primary defect in HPE is a failure of the brain to divide laterally. Our aim is now to gain a better understanding of the role of TGIF during development. Several components of the TGF beta signaling pathway have been demonstrated to affect brain development and have been implicated in HPE. The creation of specific mouse mutants targeting different functions of TGIF will determine whether it is loss of the TGF beta-dependent functions of TGIF that results in HPE and generate a better understanding of the developmental roles of TGIF.

TGIF targets general corepressor complexes to specific gene responses and appears to act in at least two separate transcriptional regulatory pathways:

It is now important to characterize DNA binding partners of TGIF and identify target genes. In the long term we must also generate an understanding of how TGIF regulates development and why loss of TGIF function results in HPE.