BMPs and STATs: From mesoderm to blood

Abstract

Primitive erythrocytes are the first red blood cells to form in the embryo. The lineage is transient and produces nucleated cells that express embryonic globins distinct from adult (definitive) erythrocytes. The molecular requirements for primitive erythropoiesis are not well understood. This work focuses on better defining the role of Bone Morphogenetic Protein (BMP) signaling in primitive erythropoiesis and on determining the function of Signal Transducer and Activator of Transcription (STAT) signaling in this lineage. In order to clarify the role of BMP signaling during primitive erythropoiesis, I developed an inducible system to block BMP signaling in a controlled fashion during embryogenesis of the African clawed frog, Xenopus laevis. For this purpose, an inhibitory Smad, xSmad6, was fused to the estrogen receptor ligand binding domain. I show that ER-xSmad6 is inducible by estradiol and when activated during blastula stages causes the same dorsalized phenotype as caused by overexpression of native xSmad6. When ER-xSmad6 is induced after gastrulation, there is a specific defect in primitive erythropoiesis without any effect on dorsal/ventral axis patterning. This indicates that continued BMP signaling is required following early patterning events for proper differentiation of the primitive erythroid lineage. I next considered if the BMP pathway might induce a transition to regulation of erythropoiesis by the erythropoietin receptor-STAT pathway. In order to understand the function of STAT signaling in primitive erythropoiesis, hyperactive and dominant negative forms of xSTAT5 were overexpressed in Xenopus embryos followed by in situ hybridization with various markers. These studies reveal that, contrary to the initial hypothesis, activated xSTAT5 inhibits primitive erythropoiesis by preventing the proper differentiation of nascent ventral mesoderm. Similarly, it is known that Fibroblast Growth Factor (FGF) signaling inhibits primitive erythrocyte differentiation when hyperactivated due to its posteriorizing activity. This work further shows that xSTAT5 functions downstream of embryonic FGF (eFGF) to inhibit primitive erythropoiesis. I propose that this eFGF-STAT5 pathway may serve to ensure the correct size and proper timing of development of the ventral blood island to regulate primitive erythropoiesis.