title: Conditional requirement for the Flk-1 receptor in the in vitro generation of early hematopoietic cells.
abstract: Genetic studies in mice have previously demonstrated an intrinsic requirement for the vascular endothelial growth factor (VEGF) receptor Flk-1 in the early development of both the hematopoietic and endothelial cell lineages. In this study, embryonic stem (ES) cells homozygous for a targeted null mutation in flk-1 (flk-1 (-/-)) were examined for their hematopoietic potential in vitro during embryoid body (EB) formation or when cultured on the stromal cell line OP9. Surprisingly, in EB cultures flk-1 (-/-) ES cells were able to differentiate into all myeloid-erythroid lineages, albeit at half the frequency of heterozygous lines. In contrast, although flk-1 (-/-) ES cells formed mesodermal-like colonies on OP9 monolayers, they failed to generate hematopoietic clusters even in the presence of exogenous cytokines. However, flk-1 (-/-) OP9 cultures did contain myeloid precursors, albeit at greatly reduced percentages. This defect was rescued by first allowing flk-1 (-/-) ES cells to differentiate into EBs and then passaging these cells onto OP9 stroma. Thus, the requirement for Flk-1 in early hematopoietic development can be abrogated by alterations in the microenvironment. This finding is consistent with a role for Flk-1 in regulating the migration of early mesodermally derived precursors into a microenvironment that is permissive for hematopoiesis.
authors.affiliation: Program in Molecular Biology and Cancer, Samuel Lunenfeld Research Institute, Mount Sinai Hospital, 600 University Avenue, Toronto, ON, M5G 1X5, Canada.
authors.forename: M
authors.initials: M
authors.lastname: Hidaka
authors.forename: W L
authors.initials: WL
authors.lastname: Stanford
authors.forename: A
authors.initials: A
authors.lastname: Bernstein
mesh_terms: D000818:Animals; D002454:Cell Differentiation; D002460:Cell Line; D019070:Cell Lineage / Q000502:physiology*; D016228:Endothelial Growth Factors / Q000502:physiology; D004730:Endothelium, Vascular / Q000166:cytology / Q000196:embryology / Q000502:physiology*; D006410:Hematopoiesis; D006412:Hematopoietic Stem Cells / Q000166:cytology / Q000502:physiology*; D008222:Lymphokines / Q000502:physiology; D051379:Mice; D020794:Receptor Protein-Tyrosine Kinases / Q000502:physiology*; D017978:Receptors, Growth Factor / Q000502:physiology*; D011975:Receptors, Mitogen / Q000502:physiology; D040262:Receptors, Vascular Endothelial Growth Factor; D042461:Vascular Endothelial Growth Factor A; D042442:Vascular Endothelial Growth Factors
chemical_list: D016228:Endothelial Growth Factors; D008222:Lymphokines; D017978:Receptors, Growth Factor; D011975:Receptors, Mitogen; D042461:Vascular Endothelial Growth Factor A; D042442:Vascular Endothelial Growth Factors; D020794:Receptor Protein-Tyrosine Kinases; D040262:Receptors, Vascular Endothelial Growth Factor