Dlk1 is a negative regulator of emerging hematopoietic stem and progenitor cells

Abstract

The first mouse adult-repopulating hematopoietic stem cells emerge in the aorta-gonad-mesonephros region at embryonic day (E) 10.5. Their numbers in this region increase thereafter and begin to decline at E12.5, thus pointing to the possible existence of both positive and negative regulators of emerging hematopoietic stem cells. Our recent expression analysis of the aorta-gonad-mesonephros region showed that the Delta-like homologue 1 (Dlk1) gene is up-regulated in the region of the aorta-gonad-mesonephros where hematopoietic stem cells are preferentially located. To analyze its function, we studied Dlk1 expression in wild-type and hematopoietic stem cell-deficient embryos and determined hematopoietic stem and progenitor cell activity in Dlk1 knockout and overexpressing mice. Its role in hematopoietic support was studied in co-culture experiments using stromal cell lines that express varying levels of Dlk1. We show here that Dlk1 is expressed in the smooth muscle layer of the dorsal aorta and the ventral sub-aortic mesenchyme, where its expression is dependent on the hematopoietic transcription factor Runx1. We further demonstrate that Dlk1 has a negative impact on hematopoietic stem and progenitor cell activity in the aorta-gonad-mesonephros region in vivo, which is recapitulated in co-cultures of hematopoietic stem cells on stromal cells that express varying levels of Dlk1. This negative effect of Dlk1 on hematopoietic stem and progenitor cell activity requires the membrane-bound form of the protein and cannot be recapitulated by soluble Dlk1. Together, these data suggest that Dlk1 expression by cells of the aorta-gonad-mesonephros hematopoietic microenvironment limits hematopoietic stem cell expansion and is, to our knowledge, the first description of such a negative regulator in this tissue.

Endothelial Smad4 restrains the transition to hematopoietic progenitors via suppression of ERK activation

“…2,3,42 Currently, increasing studies have disclosed the molecular identities of the niche for regulating AGM hematopoiesis by affecting multiple cellular behaviors of hematopoietic cells. 14,32,34,43 Most recently, the migration of the lateral plate mesoderm to the midline has been proven to be required for endothelial Runx1 expression to initiate aortic hematopoietic cluster generation in chick embryos. 33 Nevertheless, how the endothelial-hematopoietic transition supporting microenvironment is physiologically modulated is unknown.…”

A short history of hemogenic endothelium

“…In the absence of Runx1 there is a compaction of the cell layers ventral to the dorsal aorta [36,48], suggesting that Runx1 plays a role in the migration/organization of the smooth muscle cell layer. Interestingly, Runx1 affects the expression of a negative regulator of hematopoiesis, Dlk1 in the AGM smooth muscle cells [75]. The full contribution of smooth muscle cells to AGM hematopoiesis remains to be elucidated.…”

Concise Review: From Greenhouse to Garden: The Changing Soil of the Hematopoietic Stem Cell Microenvironment During Development

“…The gene is upregulated in the HSCrich middle third of the dorsal aorta compared with the rostral and caudal thirds [24]. It has been shown to promote FL hematopoiesis [51,52] and examination of the AGM at E11 shows that it is expressed in sympathoadrenal cells, where it is downstream of Gata3, as well as the smooth muscle layer of the dorsal aorta and the ventral mesenchyme, where it is downstream of Runx1 [50]. Paradoxically, functional studies support its role as a negative regulator of AGM hematopoiesis: AGMs from Dlk1 transgenic embryos that overexpress Dlk1 have reduced HSC repopulating ability compared with wild-type embryos, while AGMs from Dlk1 knockout embryos give rise to increased numbers of progenitors.…”