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Fig. 9. Diagram of the proposed interactions between Notch2, Hey proteins and
Tbx2 in the regulation of Bmp in the chick heart. Hey1 and Hey2 are
proposed to function cell autonomously within prospective chamber myocardium
to inhibit Bmp2 expression (Bmp4 in zebrafish), and thereby
suppress AVC and IC identity. Our data indicate that Notch regulates
Hey1 but not Hey2, suggesting a Notch-independent input into
Hey2. Although loss of ventricular Hey2 expression is seen in
Tbx5-deficient mouse embryos
(Bruneau et al., 2001), Tbx5 is
unable to induce Hey2 (see Fig.
7), suggesting that any influence on Hey2 is indirect (as
indicated by `?'). An important finding from the present study is that Tbx2
feeds back to suppress Hey1 and Hey2 in the AVC and IC
region (Fig. 7), and this
mechanism is postulated to sharpen the border with prospective chamber
myocardium.
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