Roles of BMP signaling in spatiotemporal coordination of embryonic axial patterning

Morphogen gradient signaling is a recurring theme in early embryonic patterning. A Bone Morphogenetic Protein (BMP) signaling gradient patterns cell fates along the dorsoventral (DV) axis in both vertebrates and invertebrates. By definition, a morphogen gradient specifies different tissue types with different levels of signal. For BMPs, high levels are required for ventral fate, intermediate levels for lateral fate, and low levels for dorsal fate. To more precisely define the mechanism of BMP signaling in patterning the DV axis, I have combined molecular, genetic and embryologic techniques in the zebrafish to investigate two aspects of BMP signaling that are related to its proposed graded activity: (1) the role of the gradient in neural crest specification, and (2) the temporal requirement for the activity gradient in patterning the DV axis. It is proposed that an intermediate level of BMP signaling is required for neural crest specification. Using morpholino antisense oligonucleotides, I demonstrate a dose-dependent response of neural crest to knockdown of the transcription factor Smad5, indicating that gradient interpretation occurs at or below the level of Smad5 activity. Using these embryos, I perform chimeric analysis to show that BMP signaling is required cell-autonomously by neural crest progenitor cells to determine their fate. This adds substantial support to the idea of BMPs acting as morphogens during DV axial patterning. The temporal response of cells to surrounding signals is an important aspect of embryonic patterning. To address the temporal response to BMP signaling, I used two heat-shock inducible transgenic zebrafish lines. By activating BMP signaling in strongly or weakly dorsalized zebrafish mutants, I determined that BMP signaling is required during early gastrulation to pattern the head and trunk DV axis, and during early somitogenesis to pattern the tail DV tissues. By inhibiting BMP signaling at various time points prior to and during gastrulation, I determined that cells along the anteroposterior (AP) axis progressively require BMP signaling over time, during early to mid-gastrulation. Based on this analysis, I propose a model whereby differential response to BMP signaling allows cells in both ectoderm and mesoderm to coordinately determine their DV and AP fates