Somite-Derived Retinoic Acid Regulates Zebrafish Hematopoietic Stem Cell Formation

Abstract

Hematopoietic stem cells (HSCs) are multipotent progenitors that generate all vertebrate adult blood lineages. Recent analyses have highlighted the importance of somite-derived signaling factors in regulating HSC specification and emergence from dorsal aorta hemogenic endothelium. However, these factors remain largely uncharacterized. We provide evidence that the vitamin A derivative retinoic acid (RA) functions as an essential regulator of zebrafish HSC formation. Temporal analyses indicate that RA is required for HSC gene expression prior to dorsal aorta formation, at a time when the predominant RA synthesis enzyme, aldh1a2, is strongly expressed within the paraxial mesoderm and somites. Previous research implicated the Cxcl12 chemokine and Notch signaling pathways in HSC formation. Consequently, to understand how RA regulates HSC gene expression, we surveyed the expression of components of these pathways in RA-depleted zebrafish embryos. During somitogenesis, RA-depleted embryos exhibit altered expression of jam1a and jam2a, which potentiate Notch signaling within nascent endothelial cells. RA-depleted embryos also exhibit a severe reduction in the expression of cxcr4a, the predominant Cxcl12b receptor. Furthermore, pharmacological inhibitors of RA synthesis and Cxcr4 signaling act in concert to reduce HSC formation. Our analyses demonstrate that somite-derived RA functions to regulate components of the Notch and Cxcl12 chemokine signaling pathways during HSC formation.

Location, Location, Location: How Vascular Specialization Influences Hematopoietic Fates During Development

“…Presumably, this function is cxcr4 dependent. Indeed another study in zebrafish showed that somite-derived retinoic acid signals induced cxcr4 expression on lateral plate mesodermderived HE during formation of the nascent aorta (Pillay et al, 2016). However, as CXCL12 can interact with other chemokine receptors and integrins (Balabanian et al, 2005;Burns et al, 2006), future studies will be required to determine the precise receptors CXCL12 interacts with to support HSC emergence from aortic HE.…”

Role of Vitamin A/Retinoic Acid in Regulation of Embryonic and Adult Hematopoiesis

“…RA signaling is required for HSC marker expression even before the formation of the aorta, when the main RA synthesis enzyme, RALDH2, is expressed in the paraxial mesoderm. This requirement is related with the expression of junctional adhesion molecules JAM1a and JAM2a (which are essential for proper transduction of Notch signals), the chemokine CXCL12 and its main receptor CXCR4 [ 50 ].…”

Developmental HSC Microenvironments: Lessons from Zebrafish

“…Recent work has demonstrated that this juxtaposition is critical for the early events of hemogenic endothelial and HSC specification. Specifically, several groups demonstrated that signals and cells emanating from the somite are required for proper HSC formation within the zebrafish embryo (Clements et al 2011; Kim et al 2014; Kobayashi et al 2014; Lee et al 2014; Nguyen et al 2014; Pillay et al 2016; Pouget et al 2014). …”

“…Important to the hematopoietic field is the discovery that the endotomal cells are not hemogenic endothelial cells, but rather form an endothelial niche that promotes HSC induction from adjacent endothelial cells in a Cxcl12 (CXC chemokine ligand 12)-dependent manner. As a complement to these findings, Pillay et al further characterized somite-derived signaling factors and found that retinoic acid (RA) is required prior to DA formation to regulate components of the Notch and Cxcl12 chemokine signaling pathways (Pillay et al 2016). …”