During early vertebrate embryo development, mesoderm tissue is patterned
along the dorsal-ventral (DV) axis by a complex set of signaling events. During early
Xenopus laevis (frog) development, antagonism of the Wnt and BMP signaling
pathways plays a crucial role for establishing mesoderm pattern along the DV axis.
FoxD1 is a member of the Forkhead box transcription factor family of
proteins. FoxD1 protein is expressed in the dorsal and dorsolateral region of the
developing Xenopus mesoderm, which gives rise to muscle and notochord fates. In
this study, we show that FoxD1 is necessary for the expression of both dorsal
(notochord) and dorsolateral (muscle) markers, since FoxD1 protein knock down
caused a significant loss in the expression of these markers in embryonic explants and
whole embryos. Additionally, ectopic expression of FoxD1 in ventral explants
sufficed to induce expression of muscle and notochord markers, indicating that FoxD1
protein has dorsalizing activity.
We have also examined FoxD1 protein interaction with two major signal
pathways that pattern the mesoderm, canonical Wnt and BMP, by overexpressing
their antagonists (Dkk and Noggin). Both BMP and Wnt antagonists are expressed in
the dorsal mesoderm and are crucial for the correct patterning of these regions during
development. Overexpression of either of these antagonists sufficed to hyperdorsalize either dorsolateral or ventral explants; the dorsolateral explants showed
elevated expression of dorsal markers with inhibition of the muscle markers.
Similarly, ventral explants exhibited significant reduction in the expression of the
ventral markers, with elevated expression of both dorsolateral (muscle) and dorsal
markers. Therefore, we observed a shift from ventral to more dorsal cell fates. When
combining these signals with overexpression and knock down of FoxD1, we observed
that these antagonists have an important role in regulating the activity of FoxD1. In
fact, we believe that the background created by various combinations and
concentrations of these patterning signals is what directs activity of FoxD1 in each
mesodermal region.