Inductive interactions mediated by interplay of asymmetric signalling underlie development of adult haematopoietic stem cells

Abstract

During embryonic development, adult haematopoietic stem cells (HSCs) emerge preferentially in the ventral domain of the aorta in the aorta–gonad–mesonephros (AGM) region. Several signalling pathways such as Notch, Wnt, Shh and RA are implicated in this process, yet how these interact to regulate the emergence of HSCs has not previously been described in mammals. Using a combination of ex vivo and in vivo approaches, we report here that stage-specific reciprocal dorso–ventral inductive interactions and lateral input from the urogenital ridges are required to drive HSC development in the aorta. Our study strongly suggests that these inductive interactions in the AGM region are mediated by the interplay between spatially polarized signalling pathways. Specifically, Shh produced in the dorsal region of the AGM, stem cell factor in the ventral and lateral regions, and BMP inhibitory signals in the ventral tissue are integral parts of the regulatory system involved in the development of HSCs.

Human haematopoietic stem cell development: from the embryo to the dish

“…Ventral polarisation in IAHC and HSC development in model organisms is at least partly driven by spatial asymmetry of molecular signalling in the AGM region (Wilkinson et al, 2009;Souilhol et al, 2016), suggesting similar mechanisms in human. From CS 13 (28 dpc), BMP4 expression is observed ventrally in a thin subendothelial mesenchymal layer of the dorsal aorta, which transiently expands and subsequently disappears by CS 16 (38 dpc).…”

“…BMP4 can upregulate KIT in various tissues and, through upregulation of KIT in human aortic endothelium, it may potentially facilitate HSC initiation (Marshall et al, 2007). However, further HSC development may require suppression of BMP4 signalling, as described in the mouse (Souilhol et al, 2016). FLT3, which marks mouse embryonic HSC precursors (Boyer et al, 2011), and its ligand are also expressed in IAHCs and surrounding endothelium, and may be involved in human HSC maturation (Marshall et al, 1999).…”

“…At this stage, functionally defined HSCs are still present in the AGM region . BMP4 is a negative regulator of HSC maturation: although it is involved in the formation of the HSC niche, its direct action on HSCs is likely limited by BMP antagonists (Souilhol et al, 2016). Notably, BMP4 can induce expression of tenascin C and fibronectin (Molloy et al, 2008), two extracellular matrix molecules that are also ventrally polarised in the human AGM region (Marshall et al, 1999).…”

“…Signalling through KIT is important for HSC maturation and its ligand, SCF (KITL in mouse; KITLG in human), is ventrally polarised in the mouse AGM region Souilhol et al, 2016). KIT is expressed in IAHCs and in the first functional HSCs emerging in the human embryo (Labastie et al, 1998;Ivanovs et al, 2014).…”

Dissecting BMP signaling input into the gene regulatory networks driving specification of the blood stem cell lineage

“…In addition, in chicken embryos, the DA and surrounding mesenchyme is already positive for phospho-Smad1/5 long before runx1 expression, suggesting that BMP signaling is already active at an earlier time point (55). Finally, the role of BMP signaling in the AGM has been revisited in a recent study in which the authors show that BMP signaling is down-regulated in the HSC lineage by BMP inhibitors and, unexpectedly, supplementing these precursors with BMP was inhibitory, whereas addition of noggin enhanced the production of HSCs (56). Thus, we conclude that, as in Xenopus, BMP signaling is likely to be only required for hemangioblast specification but not beyond.…”