BMP signaling pathway and origin of hematopoiesis in lateral mesoderm
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The bone morphogenetic protein (BMP) signaling pathway is essential during gastrulation for the generation of ventral mesoderm, which makes it a challenge to define functions for this pathway at later stages of development. We have established an approach to disrupt BMP signaling specifically in lateral mesoderm during somitogenesis, by targeting a dominant-negative BMP receptor to Lmo2 positive cells in developing zebrafish embryos. This results in expansion of hematopoietic and endothelial cells, while restricting the expression domain of the pronephric marker pax2.1. Expression of a constitutively active receptor and transplantation experiments were used to confirm that BMP signaling in lateral mesoderm restricts subsequent hemato-vascular development. These results show that the BMP signaling pathway continues to function after cells are committed to a lateral mesoderm fate (blood, vasculature) and influences subsequent lineage decisions by restricting hemato-vascular fate in favor of pronephric development. We further wanted to understand how diversity of functions arise in the BMP signaling pathway. Smad1 and Smad5 are the major transcription factors that are phosphorylated upon BMP ligand activation of BMP type I receptors and in turn activate downstream genes. As smad1 and smad5 are essential genes and have overlapping expression patterns it is difficult to delineate their individual functions during somitogenesis. A global transcriptional profiling approach was utilized to understand functional differences between smad1 and smad5. Microarray studies with smad1 and smad5 knockdown indicate that during the somitogenesis stage smad1 and smad5 regulate different sets of genes. A differential pattern of gene regulation emerges between smad1 and smad5 where smad5 preferentially regulates the BMP synexpression group. While smad1 regulates more downstream genes involved in organogenesis, data indicates smad5 is involved in an intricate pattern of positive and negative feedback loops within the BMP signaling pathway. The results suggests that smad5 may be involved in establishing timing and intensity of the BMP pathway while smad1 acts as a regulator of the downstream pathway genes.