by Monya Baker
The protein Runx1 matters in most progenitors, but just for a while
A single haematopoietic stem cell can re-form an animal's entire blood system, but how these cells form in the embryo is a much-studied mystery. Now researchers at the University of Pennsylvania in Philadelphia piece together when and where an important gene in this process functions.
Haematopoietic stem (HS) cells are found at several sites in the early mouse conceptus (that is, the foetus and its extra-embryonic tissues). The transcription factor Runx1 is expressed at each of them. Runx1 is known to be necessary for HS cells to form in the vascular areas — the early aorta, yolk sac, placenta, umbilical vessels and vitelline arteries of the early digestive tract — but it's less clear which cells produce HSCs. Do blood cells come from the endothelial lining of blood vessels or from other progenitors?
Nancy Speck and her colleagues addressed that question. "Almost all HS cells in the adult bone marrow are born during a brief period in development from a small population of endothelial cells," she says. To investigate this, they used mice engineered with transgenic systems that would permanently delete the Runx1 gene after another gene was expressed. First they used a system that was activated by the gene coding for vascular endothelial cadherin (VEC). Other work had shown that, just over nine days post-conception, VEC is expressed throughout the vasculature. When the researchers used the system to label cells expressing VEC without excising Runx1, they found that 96% of bone marrow cells with the haematopoietic marker CD45 were labelled.
When Runx1 was excised in VEC-expressing cells, more than half of the foetuses died, and progenitor cells found at haematopoietic sites could not form colonies in in vitro culture as they usually do; the researchers were also unable to engraft bone marrow of other adult mice. Thus, Runx1 seems essential for endothelial cells to become HS cells.
Next, the researchers used a transgenic system that would excise Runx1 when another gene, Vav1, was expressed. Vav1 is expressed only in haematopoietic cells. Excising Runx1 after Vav1 was expressed did not cause higher death rates in foetuses or adults. Adult mice did have an array of blood problems, but progenitor cells were able to form colonies and could easily engraft. The results indicate that Runx1 is essential for VEC-expressing cells to become blood-forming stem cells.
But blood formation in the embryo is sure to be a continuing source of contention. "The authors have managed to pinpoint the critical haematopoietic function of Runx1 to a distinct development event," says Igor Samokhvalov, who studies blood formation in the embryo with Shin-Ichi Nishikawa at RIKEN Center for Developmental Biology in Kobe, Japan. However, he says, more precise genetic instruments could better tie results to VEC-expressing cells in the endothelium.
The paper provides "strong evidence that HSCs develop from an endothelial precursor," says Roger Patient, a developmental biologist at Oxford University in the United Kingdom. He notes another fate-tracing paper with similar results.
Speck says the next goal of her lab is to figure out when exactly Runx1 is required over the three days it takes for HS cells to emerge and to figure out, at the molecular level, how the transition to HS cells from special endothelial cells occurs. Understanding that, she believes, will help grow HS cells from embryonic stem cells, which could be a boon to patients needing bone marrow transplants and blood transfusions.
Source: Nature