Specificity of Smad signaling in hematopoiesis

Abstract

The BMP signaling pathway has been shown to be important for dorso-ventral patterning and hematopoiesis in the embryo, and proposed to be important for the regulation of adult hematopoiesis. Defects in BMP signaling components, are embryonic lethal complicating the analysis of BMP functions in the adult, and in determining the individual roles of these molecules. Activation of the BMP receptors by ligand binding activates Smads 1, 5 and 8. In this study the effect of knockdown of BMP signaling was tested by manipulating Smad1 and Smad5 in embryos and adults.;Loss of function approaches were used in the zebrafish model system, in which embryos are not dependent on extra-embryonic tissue, to distinguish the functions of Smad1 and Smad5 in embryogenesis. I used Smad1 and Smad5 morpholinos, and the Smad5 mutant, piggytaildty40, to study the effect of gene knockdown. I show that Smad1 knockdown causes a unique phenotype. Smad1 morphants have defects in brain, heart, gut and hematopoietic system, including an increase in erythrocytes and lack of mature macrophages. This is unlike the Smad5 mutants that have a dorsalized phenotype, and are anemic with macrophage normal numbers. Loss of either Smad, in the embryo, causes a lack of definitive hematopoiesis. I show that Smad1 is able to substitute for Smad5 during embryogenesis, while the converse is not true.;In the adult, I show that BMP signaling continues to be important for hematopoiesis. Using heterozygote Smad5 mutants, I show that adult fish with the stronger mutant Smad5 allele, somitabundtc24, are anemic. Smad5 mutant fish also respond to hemolytic anemia with altered kinetics compared to wildtype. This indicates that Smad function is relevant to adult hematopoiesis, and identifies specific functions in steady state and stress response erythropoiesis.;I have shown that Smad1 and Smad5 have different functions during embryogenesis, in particular, during hematopoiesis. BMP signaling is known to function at multiple places and times during development. How this pathway is able to control different biological functions is only partially understood. This study has shown one way in which this diversity is initiated at the signaling level through differential activation of Smad1 and Smad5.