Flk-1, a Receptor for Vascular Endothelial Growth Factor (VEGF), Is Expressed by Retinal Progenitor Cells

Throughout development of the vertebrate retina, progenitor cells are multipotential, producing a variety of distinctive cell types. Little is known of the molecular mechanisms directing the determination of cell fate. We have examined retinal progenitor cells for expression of receptor tyrosine kinases in an attempt to define receptors that could allow a progenitor to respond to its environment. We found that the receptor tyrosine kinase Flk-1, previously shown to be expressed in endothelial cells, is also expressed in neural progenitor cells of the mouse retina. Flk-1 RNA expression in the retinal progenitors commences with the onset of neuronal differentiation and persists throughout retinal neurogenesis. Flk-1 RNA and protein levels in the retina vary temporally during development, as shown by in situ hybridization and Western blot analysis. Patterns of beta-galactosidase expression in mice containing the lacZ gene in place of the Flk-1 gene are consistent with Flk-1 being expressed in retinal progenitors. In addition, we show that the ligand of Flk-1, vascular endothelial growth factor (VEGF), is expressed in the developing retina by differentiated cells and that a chimeric ligand of VEGF fused to alkaline phosphatase binds to proliferating retinal progenitors. Furthermore, the neural retina-derived Flk-1 protein kinase is activated by VEGF in vitro. Thus, the Flk-1 receptor protein kinase is expressed on the surface of neural progenitors in mouse retina and may play a critical role in neurogenesis as well as in vasculogenesis.