New Animal Models Reveal Stage-Specific Hematopoietic Functions for the BMP Signaling Pathway.

Genetic analyses in mouse and zebrafish demonstrate that a conserved BMP signaling pathway is essential for normal development of the embryonic vascular and hematopoietic systems. However, it has not been clear which aspects of hemato-vascular development are regulated by the pathway beyond the initial specification of embryonic mesoderm. We present new animal models for studying BMP signaling at specific stages of both embryonic and adult hematopoiesis. First, we developed approaches to modulate BMP signaling conditionally at specific stages of development in transgenic zebrafish. For this purpose we adapted the Crelox system to express a dominant-negative BMP receptor in cells derived from specific progenitor populations, with subsequent analysis of hematopoietic and vascular development using GFP reporter strains of fish. For example, we find that BMP signaling is required in LMO2+ cells for the normal development of both hematopoietic and vascular systems. Yolk sac vasculature and trunk angiogenesis are both disrupted, and primitive erythropoiesis is also impaired, when these cells express the mutant receptor. In contrast, BMP signaling is apparently no longer required in primitive erythroid cells once they have activated expression of the GATA-1 gene. In a second approach, we studied hematopoiesis in zebrafish mutants for Smad5, which is an intracellular mediator of BMP signaling. For example, homozygote piggytail (pgy) mutant embryos live until day 3–5, yet the fish have little to no blood and no circulation. Surprisingly, we find that key hematopoietic regulatory genes including GATA-1, GATA-2, SCL, and LMO2 are expressed in the pgy mutant fish, but that their patterns of expression are altered. Thus, the progenitor cells initiate a hematopoietic program but fail to differentiate. We also established adult pgy fish that carry a GATA1::GFP transgene, to facilitate the analysis of adult-stage blood cell profiles using flow cytometry. Blood cell populations were monitored under normal conditions and following phenylhydrazine-induced hemolytic anemia in wild-type and pgy/+ fish. Preliminary results indicate that pgy/+ adult fish are able to mount an erythropoietic response to hemolytic anemia. However, the kinetics are altered, specifically in the transition from the proerythroblast to the polychromatophilic erythroblast stage. In summary, we present data using novel animal models disrupted for normal BMP signaling, and demonstrate roles for this pathway at specific stages of differentiation for both embryonic and adult hematopoiesis.