To understand how TGF-β superfamily ligands function in vivo we use early zebrafish embryos as a model system. We are particularly interested in how these ligands are regulated, how they function in a dose-dependent manner and how they contribute to tissue specification.
A key tool for this work has been a transgenic zebrafish line that expresses an mRFP reporter driven by BMP responsive elements. We have determined the mechanism whereby the ventral to dorsal BMP gradient is set up in early zebrafish embryos and show that it occurs as a result of graded transcription of the ligands, with little or no diffusion of the ligands (Ramel and Hill, 2013;Dev Biol. 378: 170-82). This is in contrast to well known mechanisms of BMP gradient formation determined in Drosophilawhich involve either extensive diffusion, or shuttling of the BMP proteins from their site of production.
We have determined how BMP signalling is remodelled post-gastrulation and have shown that the ventral to dorsal gradient evolves into two distinct stripes at the neural plate border: one coinciding with the neural crest, and the other abutting the epidermis. In between is a region devoid of BMP activity, which is specified as preplacodal ectoderm (PPE). We have identified the BMP ligands involved and determined the underlying mechanism (Figure 1).
We have demonstrated a key role for the secreted BMP regulator, Crossveinless 2, as a factor essential to concentrate BMP activity in the stripes, and show that the cell autonomous BMP inhibitor, BAMBI-b is unregulated in the PPE under the control of the Distalless transcription factors Dlx3b/4b, and is an excellent candidate to inhibit BMP signalling in this domain (Reichert et al., 2013; Development. 140: 4435-44).
We are now extending this work to understand how BMP signalling at the anterior neural plate is involved in determining the specification of telencephalon versus eye field.
Figure 1: Tissue patterning at the neural plate border in zebrafish embryos. Upper panels: Double fluorescent in situs (DFISH) showing the expression pattern in four somite stage zebrafish embryos of cvl2, dlx3b and n-cadherin (n-cad). cvl2 marks the BMP activity domains, the inner of which is the neural crest; dlx3b marks the PPE; n-cad marks the neural plate. ncad and dlx3b have been labelled with the same colour to allow visualisation of three different mRNAs. The view is dorsal, with the anterior to the top right. Lower panels: DFISH for cvl2 and dlx3b showing how the expression domains evolve over time from late gastrulation (95% epiboly) to the one somite stage. Lateral views with dorsal to the right. V, ventral; D, dorsal; A, anterior; P, posterior; PPE, preplacodal ectoderm; NP, neural plate (Click to view larger image)