Signalling pathways that control vertebrate haematopoietic stem cell specification

Abstract

Haematopoietic stem cells (HSCs) are tissue-specific stem cells that replenish all mature blood lineages during the lifetime of an individual. Clinically, HSCs form the foundation of transplantation-based therapies for leukaemias and congenital blood disorders. Researchers have long been interested in understanding the normal signalling mechanisms that specify HSCs in the embryo, in part because recapitulating these requirements in vitro might provide a means to generate immune-compatible HSCs for transplantation. Recent embryological work has demonstrated the existence of previously unknown signalling requirements. Moreover, it is now clear that gene expression in the nearby somite is integrally involved in regulating the transition of the embryonic endothelium to a haemogenic fate. Here, we review current knowledge of the intraembryonic signals required for the specification of HSCs in vertebrates.

Figure 1. Haematopoietic stem cells

a | Haematopoietic stem cell (HSC) specification is preceded by earlier waves of embryonic haematopoiesis. Primitive macrophages and erythrocytes arise first. A transient definitive erythromyeloid progenitor (EMP) population also precedes HSCs. b | Primitive blood in mouse derives from extra-embryonic tissue of the yolk sac in blood islands. In zebrafish, primitive red blood cells derive from mesoderm of the intermediate cell mass and primitive myeloid cells derive from anterior lateral plate mesoderm. EMPs are specified in the mouse yolk sac and zebrafish caudal haematopoietic tissue. c,d | HSCs (red) appear at embryonic day (E)10.5 in mouse and 32 hours post fertilization in zebrafish from haemogenic endothelium of the dorsal aorta, from where they bud into the arterial circulation (mouse; c) or the venous circulation (zebrafish; d).

Figure 2. Early signalling regulating HSC specification

Haematopoietic stem cells (HSCs) derive from haemogenic endothelium of the dorsal aorta, which is of mesodermal origin (pink). a | In mouse, Nodal, WNT3 and bone morphogenetic protein 4 (BMP4) are required for primitive streak formation (not shown) and mesoderm specification. In zebrafish, Nodal is required for mesoderm specification. b | HSCs derive specifically from ventroposterior mesoderm, specification of which requires WNT ligands, BMPs and fibroblast growth factors (FGFs). WNT and BMP signalling are actively suppressed in the anterior region by secreted antagonists, whereas WNT, BMP and FGF signalling cooperate to specify ventroposterior mesoderm. c | HSCs derive from lateral plate mesoderm. Mesoderm is lateralized by WNT and BMPs, which regulate the expression of caudal-type homeobox 1 (CDX1), CDX2 and CDX4.

Figure 3. Processes regulated by Sonic hedgehog

Sonic hedgehog (Shh) and vascular endothelial growth factor A (Vegfa) function as attractive cues to haemangioblasts converging to the midline in zebrafish. Shh induces Notch _|- dependent proliferation of haemangioblasts. Vegfa is required for the expression of Notch receptors in haemogenic endothelium. Shh regulates vegfa expression directly and through calcitonin receptor-like receptor (Crlr). The Ets transcription factor Etv6 (also known as Tel1) also regulates vegfa expression in Xenopus laevis. HSC, haematopoietic stem cell.

Figure 4. Processes regulated by Notch

Arteriovenous specification in zebrafish. a | Sonic hedgehog (Shh) produced by the notochord induces somite expression of vascular endothelial growth factor A (Vegfa), which, in turn, through binding its receptor Vegfr2, induces expression of notch in haemogenic endothelial cells. b | Notch regulates arteriovenous specification by regulating ephrin B2 (efnb2) expression. Arterial cells (light green) express Efnb2 and segregate from venous cells (dark green), which express the Efnb2 receptor Eph receptor b4 (Ephb4). c | Notch ligands including Jagged 1 expressed by neighbouring cells activate Notch receptors to specify haematopoietic stem cells (HSCs) by activating gata2 expression, which in turn activates runx1 expression. The relevant runx1 enhancer does not contain Notch-responsive binding elements, which indicates that Notch does not directly regulate runx1 expression.

Figure 5. Model for indirect Notch-mediated haematopoietic stem cell specification in zebrafish

a | Schematic of a model for Wnt16-mediated regulation of haematopoietic stem cells (HSCs) through Notch patterning of the ventromedial compartment of the somite, the sclerotome. Wnt16 regulates somitic expression of the Delta family ligand genes dlc and dld. Both Wnt16 and the combined actions of Dlc and Dld are required for the specification of sclerotome, which contains vascular smooth muscle cell precursors. These precursors emigrate from the somite (b), sheathe the dorsal aorta (c) and signal to haemogenic endothelium to trigger HSC specification (d).