Expression and function of CD105 during the onset of hematopoiesis from Flk1+ precursors

Abstract

During ontogeny, the hematopoietic system is established from mesodermderived precursors; however, molecular events regulating the onset of hematopoiesis are not well characterized. Several members of the transforming growth factor ␤ (TGF-␤) superfamily have been implicated as playing a role during mesoderm specification and hematopoiesis. CD105 (endoglin) is an accessory receptor for members of the TGF-␤ superfamily. Here it is reported that during the differentiation of murine embryonic stem (ES) cells in vitro, hematopoietic commitment within Flk1 ؉ mesodermal precursor populations is characterized by CD105 expression. In particular, CD105 is expressed during the progression from the Flk1 ؉ CD45 ؊ to Flk1 ؊ CD45 ؉ stage. The developmentally regulated expression of CD105 suggests that it may play a role during early hematopoiesis from Flk1 ؉ precursors. To determine whether CD105 plays a functional role during early hematopoietic development, the potential of CD105-deficient ES cells to differentiate into various hematopoietic lineages in vitro was assessed. In the absence of CD105, myelopoiesis and definitive erythropoiesis were severely impaired. In contrast, lymphopoiesis appeared to be only mildly affected. Thus, these findings suggest that the regulated expression of CD105 functions to support lineage-specific hematopoietic development from Flk1 ؉ precursors. IntroductionDuring vertebrate embryogenesis, the onset of hematopoiesis and vasculogenesis occurs in the extra-embryonic yolk sac with the formation of blood islands from aggregates of mesodermal precursors. Cells within these clusters differentiate into primitive erythrocytes while those at the periphery differentiate into endothelial cells. The temporal and spatial coupling in the appearance of hematopoietic and endothelial cells led to the hypothesis that these lineages are derived from a common progenitor. 1 Recent studies have shown that the fms-like receptor tyrosine kinase Flk1 (also known as vascular endothelial growth factor receptor-2), which is expressed on subsets of mesoderm, 2 is critical for the normal development of both hematopoietic and endothelial lineages. Flk1-deficient (flk1 Ϫ/Ϫ ) mice die in utero at embryonic day 8.5 (E8.5) owing to defects in blood island and vasculature formation. 3 Further investigations revealed that flk1 Ϫ/Ϫ embryonic stem (ES) cells failed to contribute to hematopoietic or endothelial cells in chimeric mice, although residual hematopoietic and endothelial activities were observed during differentiation in vitro. [4][5][6] Moreover, in vitro clonal assays provided direct evidence that hematopoietic and endothelial cell lineages are derived from a common precursor 2,7 that expresses Flk1. 2 Thus, Flk1 expression may serve to identify the hemangioblast, a putative bipotent progenitor for the hematopoietic and endothelial lineages. The molecular events contributing to hematopoietic and endothelial development immediately following the Flk1 ϩ stage remain to be elucidated.Members of the transforming growth ...

Endoglin is required for hemangioblast and early hematopoietic development

“…Our results suggest that anemia might result from a direct effect of endoglin on hematopoiesis. This assumption is supported by in vitro studies using day-9 and -12 Eng -/-ES/OP9 co-cultures, which show a reduction in myelo-erythropoietic progenitors in the absence of endoglin (Cho et al, 2001). Interestingly, in our studies, haploinsufficiency was observed for primitive erythropoiesis.…”

“…Wild-type mouse E14 and CCE as well as Eng +/-and Eng -/-ES cells (generously provided by J. C. Zuniga-Pflucker and M. Letarte from the University of Toronto, ON, Canada) (Cho et al, 2001) were used in this study. Eng -/-ES cells were generated from heterozygous Eng +/-ES cells following selection in a high concentration of G418 (Bourdeau et al, 1999).…”

“…By differentiating embryonic stem (ES) cells using the OP9 coculture system, Cho and colleagues (Cho et al, 2001) have shown that myelopoiesis and definitive erythropoiesis is impaired in the absence of endoglin. However primitive erythropoiesis, reflective of yolk sac hematopoiesis, was not investigated in this study.…”

Dynamic expression of Endoglin, a TGF-beta co-receptor, during pre-circulation vascular development in chick

“…In vitro differentiation assays showed that Endoglin -/-mESCs have reduced myelopoiesis and erythropoiesis, whereas the formation of lymphoid and vascular precursors appeared unaffected (Cho et al, 2001). Anemia observed in patients with HHT1 and in Endoglin -/-mice is another indicator for a possible role of Endoglin in erythropoiesis.…”

“…Furthermore, it has been suggested that Endoglin is involved in early hematopoietic development, with Endoglin being expressed in mouse embryonic stem cell (mESC)-derived mesodermal FLK1 + precursors (Cho et al, 2001) and blast colony-forming cells (BL-CFCs) (Perlingeiro, 2007). In vitro differentiation assays showed that Endoglin -/-mESCs have reduced myelopoiesis and erythropoiesis, whereas the formation of lymphoid and vascular precursors appeared unaffected (Cho et al, 2001).…”