Development of Early PCLP1-Expressing Haematopoietic Cells within the Avian Dorsal Aorta

FGF signalling restricts haematopoietic stem cell specification via modulation of the BMP pathway

Haematopoietic stem cells (HSCs) are produced during embryogenesis from the floor of the dorsal aorta. The localization of HSCs is dependent on the presence of instructive signals on the ventral side of the vessel. The nature of the extrinsic molecular signals that control the aortic haematopoietic niche is currently poorly understood. Here we demonstrate a novel requirement for FGF signalling in the specification of aortic haemogenic endothelium. Our results demonstrate that FGF signalling normally acts to repress BMP activity in the subaortic mesenchyme through transcriptional inhibition of bmp4, as well as through activation of two BMP antagonists, noggin2 and gremlin1a. Taken together, these findings demonstrate a key role for FGF signalling in establishment of the developmental HSC niche via its regulation of BMP activity in the subaortic mesenchyme. These results should help inform strategies to recapitulate the development of HSCs in vitro from pluripotent precursors.

Hematopoietic stem cell emergence in the conceptus and the role of Runx1

Hematopoietic stem cells (HSCs) are functionally defined as cells that upon transplantation into irradiated or otherwise immunocompromised adult organisms provide long-term reconstitution of the entire hematopoietic system. They emerge in the vertebrate conceptus around midgestation. Genetic studies have identified a number of transcription factors and signaling molecules that act at the onset of hematopoiesis, and have begun to delineate the molecular mechanisms underlying the formation of HSCs. One molecule that has been a particularly useful marker of this developmental event in multiple species is Runx1 (also known as AML1, Pebp2alpha). Runx1 is a sequence-specific DNA-binding protein, that along with its homologues Runx2 and Runx3 and their shared non-DNA binding subunit CBFbeta, constitute a small family of transcription factors called core-binding factors (CBFs). Runx1 is famous for its role in HSC emergence, and notorious for its involvement in leukemia, as chromosomal rearrangements and inactivating mutations in the human RUNX1 gene are some of the most common events in de novo and therapy-related acute myelogenous leukemia, myelodysplastic syndrome and acute lymphocytic leukemia. Here we will review the role of Runx1 in HSC emergence in the mouse conceptus and describe some of the genetic pathways that operate upstream and downstream of this gene. Where relevant, we will include data obtained from other species and embryonic stem (ES) cell differentiation cultures.

Hedgehog and Bmp polarize hematopoietic stem cell emergence in the zebrafish dorsal aorta

Hematopoietic stem cells (HSCs) are first detected in the floor of the embryonic dorsal aorta (DA), and we investigate the signals that induce the HSC program there. We show that while continued Hedgehog (Hh) signaling from the overlying midline structures maintains the arterial program characteristic of the DA roof, a ventral Bmp4 signal induces the blood stem cell program in the DA floor. This patterning of the DA by Hh and Bmp is the mirror image of that in the neural tube, with Hh favoring dorsal rather than ventral cell types, and Bmp favoring ventral rather than dorsal. With the majority of current data supporting a model whereby HSCs derive from arterial endothelium, our data identify the signal driving this conversion. These findings are important for the study of the production of HSCs from embryonic stem cells and establish a paradigm for the development of adult stem cells.

The role of podocalyxin in health and disease

Podocalyxin, a sialomucin most closely related to CD34 and endoglycan, is expressed by kidney podocytes, hematopoietic progenitors, vascular endothelia, and a subset of neurons; aberrant expression has recently been implicated in a range of cancers. Through interactions with several intracellular proteins and at least one extracellular ligand, podocalyxin regulates both adhesion and cell morphology. In the developing kidney, podocalyxin plays an essential role in the formation and maintenance of podocyte foot processes, and its absence results in perinatal lethality. Podocalyxin expression in the hematopoietic system correlates with cell migration and the seeding of new hematopoietic tissues. In addition, it is abnormally expressed in subsets of breast, prostate, liver, pancreatic, and kidney cancer as well as leukemia. Strikingly, it is often associated with the most aggressive cases, and it is likely involved in metastasis. Thus, a thorough investigation of the normal activities of podocalyxin may facilitate the development of new cancer treatment strategies.