Many of the important events that define the body plan in the early mouse embryo unfold in one day of gestation, between e7.5 and e8.5 (Fig. 1)
Figure 1. The mouse embryo grows and changes morphology dramatically between embryonic day 7.5 and 8.5.
We study mutations that affect the morphogenetic events that take place at this stage to learn how intercellular signals are coupled to cell behavior during the establishment of the body plan. Two examples of mutations that have profound effect on early morphogenesis are khlo and lulu.
The khlo mutation disrupts both morphogenesis and cell fate determination in the early embryo (Rakeman and Anderson, 2006). khlo mutants show a variety of morphogenetic defects - the most dramatic is cardia bifida, failure of the 2 lateral heart primordial to move to the midline and form a single heart tube. In addition, about 25% of khlo mutants show partial or complete duplication of the body axis (Figure 2).
Figure 2. Most Nap1khlo embryos show a single axis marked by the expression of Brachyury, as in wild type but about 25 percent show partial (middle) or complete axis duplication (left).
We found that the khlo mutations inactivates Nap1, a regulator of the WAVE complex that controls cell migration, and that the twinned-embryo phenotype is caused by defective migration of the anterior visceral endoderm, an population of extraembryonic organizer cells. We are currently studying how Nap 1 and other genes, including Rac1, Pten and cofilin, work together to respond to intercellular signals and control cell migrations in the early embryo.
The lulu mutation causes a different set of defects in early morphogenesis of the mesoderm and neural plate (Fig.3).
Figure 3. 3D confocal reconstruction of e8.5 embryos stained for expression of the neural marker Sox2. The neural plate of lulu embryos is completely open and highly convoluted, in contrast to the closing neural tube of the wild type embryo.
During gastrulation, cells in lulu mutants are trapped in the primitive streak at an intermediate stage of the epithelial-mesenchymal transition that generates the mesodermal layer of the embryo; as a result, the embryos have very little paraxial mesoderm. Epithelial layers of the later lulu embryo are also disrupted: definitive endoderm is specified but does not form a gut tube, and the neural plate is broad and forms ectopic folds rather than closing to make the neural tube. The lulu mutation inactivates a FERM-domain protein, Epb4.1l5. Our data indicate that, like other FERM-domain proteins, Lulu is important for linking the actin cytoskeleton to the plasma membrane. We are currently testing the hypothesis that Lulu also is required for the activity of specific signaling pathways in the embryo.