SummaryThe first adult-repopulating hematopoietic stem cells (HSCs) emerge in the aorta-gonads-mesonephros (AGM) region of the embryo. We have recently identified the transcription factor Gata3 as being upregulated in this tissue specifically at the time of HSC emergence. We now demonstrate that the production of functional and phenotypic HSCs in the AGM is impaired in the absence of Gata3. Furthermore, we show that this effect on HSC generation is secondary to the role of Gata3 in the production of catecholamines, the mediators of the sympathetic nervous system (SNS), thus making these molecules key components of the AGM HSC niche. These findings demonstrate that the recently described functional interplay between the hematopoietic system and the SNS extends to the earliest stages of their codevelopment and highlight the fact that HSC development needs to be viewed in the context of the development of other organs.

Blood formation by hematopoietic stem cells (HSC) is regulated by a still incompletely defined network of general and HSC-specific regulators. In this study, we analyzed the role of G-protein coupled receptor 56 (Gpr56) as a candidate HSC regulator based on its differential expression in quiescent relative to proliferating HSCs and its common targeting by core HSC regulators. Detailed expression analysis revealed that Gpr56 is abundantly expressed by HSPCs during definitive hematopoiesis in the embryo and in the adult bone marrow, but its levels are reduced substantially as HSPCs differentiate. However, despite enriched expression in HSPCs, Gpr56-deficiency did not impair HSPC maintenance or function during steady-state or myeloablative stress-induced hematopoiesis. Gpr56-deficient HSCs also responded normally to physiological and pharmacological mobilization signals, despite the reported role of this GPCR as a regulator of cell adhesion and migration in neuronal cells. Moreover, Gpr56-deficient bone marrow engrafted with equivalent efficiency as wild-type HSCs in primary recipients; however, their reconstituting ability was reduced when subjected to serial transplantation. These data indicate that although GPR56 is abundantly and selectively expressed by primitive HSPCs, its high level expression is largely dispensable for steady-state and regenerative hematopoiesis.