Germ layer patterning via morphogen crosstalk

Abstract

During early embryonic development, the naïve undifferentiated cells of the early blastula must be specified correctly in accordance with the organism’s body plan. This process is mediated in part by secreted signaling molecules called ‘morphogens’, which diffuse from a localized source and form a concentration gradient across the tissue. This concentration gradient generates a signaling gradient, and cells then respond differently to different levels of the graded signal Nodal and BMP are two such morphogens studied in this dissertation. Nodal plays a key role in germ layer patterning, while BMP controls dorsal-ventral patterning. Together, their mutual interaction triggers the signaling pathways needed to build an embryonic axes. In order to address how these signals orchestrate patterning, I first developed several new tools and methods. I developed a transplantation device to transplant cells from zebrafish embryos to generate localized sources of morphogens, as well as a double staining method to visualize Nodal and BMP signaling. I also generated an in-vitro tool for benchmarking fluorescence recovery after photobleaching protocols. Having established these experimental methods, I measured the diffusion coefficient of BMP and its inhibitor, Chordin, and found evidence which indicates that BMP interacts with Chordin via a source-sink model. Next, I transplanted sources of fluorescently tagged Nodal and BMP into zebrafish embryos, robustly inducing the formation of secondary axes. Nodal and BMP signal cell non-autonomously and form similar protein gradients in this context, but the signaling range of Nodal (pSmad2) is shorter than the BMP range (pSmad5). This yields a localized region of pSmad2 activity around the Nodal source, overlapping with a broad domain of pSmad5 activity across the embryo. Cell fates induced in various regions stereotypically correlate with pSmad2:pSmad5 ratios and can even be induced BMP/Nodal-independently with different ratios of constitutively active Smad2 and Smad5. Strikingly, I found that Smad2 and Smad5 antagonize each other for specific cell fates, providing a mechanism for how cells integrate and discriminate between overlapping signals during development.