We have a major interest in studying the genetic and cell biological basis of convergent extension during vertebrate gastrulation. The basic body plan of vertebrate embryos is established during gastrulation by a series of co-ordinated movements of cells that give rise to the three germ layers and overtly shape the embryonic body axis. Convergent extension is one such movement, and is required for the antero-posterior extension of dorsal tissues such as the notochord and neural plate. What are the mechanisms driving cells to move in appropriate directions within the embryo? How do such large population of cells communicate one another to achieve this process? We are trying to find answers to these questions by using a combination of genetic, molecular, embryological and in vivo imaging approaches, using zebrafish and Xenopus embryos model systems.

Recently, we have focused on the secreted signalling molecule Wnt11 (see Figure 1). We demonstrated that the zebrafish silberblick (slb) locus encodes Wnt11 and that slb is required for proper convergent extension. Also, we suggested that Slb/Wnt11 regulates convergent extension via a non-canonical Wnt pathway similar to the one involved in establishing planar cell polarity in Drosophila (see references below). To understand how Slb/Wnt11 regulates convergent extension movements within the mesoderm and ectoderm we are currently dissecting the genetic components of the Slb/Wnt11 pathway and are also analysing the cell behaviours that are affected in the absence of activity of this protein.

Figure 1
In situ hybridisation of zebrafish late gastrula embryos showing expression of wnt11 (in purple) and papc (in blue).

In vivo analyses of cell behaviour during convergent extension.
(Miguel Concha, Steve Wilson and Masa Tada) Zebrafish embryos are amenable to in vivo imaging studies because of the size and transparency of the embryo (see attached movie). To elucidate the cell behaviours involved in convergence extension, we are performing time-lapse microscopy of large populations of cells. By using DIC optics, we are able to track the movements and shape changes of individual cells that take place during this phase of morphogenesis and analyse them by means of sophisticated computer software in collaboration with Richard Adams (University of Bath). The comparison of cell behaviours between wild-type and mutant embryos will help us to further unravel the complex events that constitute convergent extension.

As an alternative method to address how Slb/Wnt11 controls cell behaviours during convergent extension, we are developing wnt11-GFP transgenic lines. Hopefully such lines will help us to dissect the role of wnt11 within the three domains of expression in the zebrafish gastrula: lateral neuroectoderm, anterior paraxial mesoderm and germ ring.

Isolation of genes that regulate convergent extension.

We are searching for molecules that impart directionality to the cell movements along the medio-lateral and antero-posterior axes during convergent extension. A) Genetic screens in zebrafish To dissect the genetic components involved in the regulation of convergent extension, we have focused on the secreted signalling molecule Silberblick (Slb)/Wnt11. Taking advantage of the zebrafish as an established vertebrate genetic system, we have begun a pilot mutant screen to search for dominant enhancers and suppressors of slb function, in collaboration with Carl-Philipp Heisenberg’s group (MPI, Dresden). The screen is being carried out by looking at relative positions of prechordal plate, notochord and the anterior edge of neural plate in tail-bud stage embryos. One candidate gene potentially isolated from this screen is knypek (kny), which encodes a member of the glypican family of heparan sulphate proteoglycans, previously identified by Lila Solnica-Krezel’s group (see Mol. Cell 1, 251-264, 2001). You can find out more about this and other screens on the Genetic Screens page. B) Functional screen in Xenopus Xenopus embryos are amenable to functional screens aimed at isolating cDNAs encoding activities of interest. In combination with explant assays it is possible to isolate genes that function in specific developmental processes. For example, treatment of animal caps (prospective ectodermal tissue) with mesoderm inducers such as Activin causes cells to undergo convergent extension. This Activin-induced elongation, however, can be blocked by inhibition of the Wnt11 pathway (see Figure 2). To identify factors mediating Slb/Wnt11 signaling, we will conduct a functional screen to isolate genes that modify the Slb/Wnt11 modulation of convergent extension in Activin-treated animal cap explants.

Figure 2
Animal pole cells of Xenopus blastura embryos forms spheres in control medium (left panel) but undergo convergent extension when treated with Activin (middle panel). Interfering with Wnt11 function blocks the Activin-induced elongation of animal pole cells (right panel).