Conditional requirement for the Flk-1 receptor in the in vitro generation of early hematopoietic cells

Genetic studies in mice have previously demonstrated an intrinsic requireme nt 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 mut ation in flk-1 (flk-1 (-/-)) were examined for their hematopoietic potentia l in vitro during embryoid body (EB) formation or when cultured on the stro mal cell line OP9. Surprisingly, in EB cultures flk-1 (-/-) ES cells H-ere able to differentiate into all myeloid-erythroid lineages, albeit at half t he frequency of heterozygous lines. In contrast, although flk-1 (-/-) ES ce lls formed mesodermal-like colonies on OP9 monolayers, they failed to gener ate hematopoietic clusters even in the presence of exogenous cytokines, How ever, flk-1 (-/-) OP9 cultures did contain myeloid precursors, albeit at gr eatly reduced percentages. This defect was rescued by first allowing flk-1 (-/-) ES cells to differentiate into EBs and then passaging these cells ont o OP9 stroma, Thus, the requirement for Flk-1 in early hematopoietic develo pment can be abrogated by alterations in the microenvironment, This finding is consistent with a role for Flk-1 in regulating the migration of early m esodermally derived precursors into a microenvironment that is permissive f or hematopoiesis.