Beschreibung:
<jats:p>
<jats:bold>
<jats:italic>
<jats:underline>Rationale</jats:underline>
:
</jats:italic>
</jats:bold>
T-box transcription factors play critical roles in the coordinated formation of the working chambers and the atrioventricular canal (AVC). Tbx2 patterns embryonic myocardial cells to form the AVC and suppresses their differentiation into chamber myocardium.
<jats:italic>Tbx20</jats:italic>
-deficient embryos, which fail to form chambers, ectopically express
<jats:italic>Tbx2</jats:italic>
throughout the entire heart tube, providing a potential mechanism for the function of Tbx20 in chamber differentiation.
</jats:p>
<jats:p>
<jats:bold>
<jats:italic>
<jats:underline>Objective</jats:underline>
:
</jats:italic>
</jats:bold>
To identify the mechanism of
<jats:italic>Tbx2</jats:italic>
suppression by Tbx20 and to investigate the involvement of Tbx2 in Tbx20-mediated chamber formation.
</jats:p>
<jats:p>
<jats:bold>
<jats:italic>
<jats:underline>Methods and Results</jats:underline>
:
</jats:italic>
</jats:bold>
We generated
<jats:italic>Tbx20</jats:italic>
and
<jats:italic>Tbx2</jats:italic>
single and double knockout embryos and observed that loss of
<jats:italic>Tbx2</jats:italic>
did not rescue the
<jats:italic>Tbx20</jats:italic>
-deficient heart from failure to form chambers. However, Tbx20 is required to suppress
<jats:italic>Tbx2</jats:italic>
in the developing chambers, a prerequisite to localize its strong differentiation-inhibiting activity to the AVC. We identified a bone morphogenetic protein (Bmp)/Smad-dependent
<jats:italic>Tbx2</jats:italic>
enhancer conferring AVC-restricted expression and Tbx20-dependent chamber suppression of
<jats:italic>Tbx2</jats:italic>
in vivo. Unexpectedly, we found in transfection and localization studies in vitro that both Tbx20 and mutant isoforms of Tbx20 unable to bind DNA attenuate Bmp/Smad-dependent activation of
<jats:italic>Tbx2</jats:italic>
by binding Smad1 and Smad5 and sequestering them from Smad4.
</jats:p>
<jats:p>
<jats:bold>
<jats:italic>
<jats:underline>Conclusions</jats:underline>
:
</jats:italic>
</jats:bold>
Our data suggest that Tbx20 directly interferes with Bmp/Smad signaling to suppress
<jats:italic>Tbx2</jats:italic>
expression in the chambers, thereby confining
<jats:italic>Tbx2</jats:italic>
expression to the prospective AVC region.
</jats:p>