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The VEGF receptor, FLK1, is essential for differentiation of the endothelial lineage and for embryonic vascular development. Using comparative genomics, we have identified conserved ETS and Krüppel-like factor (KLF) binding sites within the Flk1 enhancer. In transgenic studies, mutation of either site results in dramatic reduction of Flk1 reporter expression. Overexpression of KLF2 or the ETS transcription factor ERG is sufficient to induce ectopic Flk1 expression in the Xenopus embryo. Inhibition of KLF2 function in the Xenopus embryo results in a dramatic reduction in Flk1 transcript levels. Furthermore, we show that KLF2 and ERG associate in a physical complex and that the two proteins synergistically activate transcription of Flk1. Since the ETS and KLF protein families have independently been recognized as important regulators of endothelial gene expression, cooperation between the two families has broad implications for gene regulation during development, normal physiology and vascular disease.
Fig. 3. Inhibition of KLF2 function results in reduced Flk1 expression in the Xenopus embryo. (A-C) Whole-mount in situ hybridization analysis of Flk1, Erg and Klf2 expression in Xenopus embryos (stage 34, lateral view). For each gene, expression is observed in the endothelial cells of the major developing vessels, including the posterior cardinal vein (PCV), intersomitic vessels (IS), aortic arches (AA) and in the forming plexus on the flank of the embryo (PL). (D,E) Klf2 MO effectively blocks translation of a control Klf2 transcript. (D) Bright-field and fluorescent images of embryos injected with a control transcript in which the 5′ UTR of Klf2 was fused to the coding sequences of GFP (Klf2-GFP). (E) Bright-field and fluorescent images of embryos injected with Klf2-GFP transcript plus Klf2 MO (25 ng). Note that GFP reporter fluorescence is greatly inhibited by Klf2 MO treatment. (F) Embryo injected with 50 ng of a control MO and assayed for expression of Flk1 transcripts. The inset is a higher magnification view, centered on the developing posterior cardinal vein. (G,H) Two different embryos injected with 50 ng of Klf2 MO and assayed for expression of Flk1. Klf2 MO-injected embryos show a dramatic reduction of Flk1 expression. (I) Embryo injected with 500 pg of GFP mRNA and assayed for Flk1 transcripts. (J,K) Two different embryos injected with 250 pg of mRNA encoding a dominant-repressor form of KLF2 (DR-KLF2) and assayed for Flk1 transcripts. Embryos expressing the DR-KLF2 construction show a dramatic reduction of Flk1 transcripts. (L) qRT-PCR analysis reveals significant reduction in Flk1 transcript levels in Klf2 MO-treated embryos. Results shown are the average of three separate embryos for each MO treatment. (M-M′) Klf2 MO treatment eliminates vascular tubes. Histological section through a stage 42 embryo injected with Klf2 MO. Somites (s) and notochord (nc) are indicated. Scale bar: 100 μm. Injected side is to the right (arrowhead). M′ and M′ show enlargements of the region of the posterior cardinal vein on the untreated and treated sides, respectively. Although the pronephric duct (pnd) is visible on both sides, no tube corresponding to the posterior cardinal vein (pcv) is visible on the treated side.
Fig. 7. KLF2 is necessary and sufficient for expression of other vascular marker genes. In situ hybridization analysis of X-msr and VE-cadherin expression in injected Xenopus embryos (stage 34, lateral view). (A-D) Klf2 MO blocks expression of the vascular markers X-msr and VE-cadherin, as compared with control MO-injected embryos. The insets are higher magnification views, centered on the developing posterior cardinal vein. (E-H) Injection of Klf2 mRNA (500 pg) induces ectopic expression of X-msr and VE-cadherin (arrows).
Klf2 MO blocks expression of the vascular markers X-msr and VE-cadherin, as compared with control MO-injected embryos. The insets are higher magnification views, centered on the developing posterior cardinal vein.
The developing circulatory system in a Xenopus laevis embryo, visualized through gene expression of aplnr (apelin receptor). The inset is a higher magnification view, centered on the developing posterior cardinal vein, and branching intersomitic vessels.
Klf2 MO blocks expression of the vascular markers X-msr and VE-cadherin, as compared with control MO-injected embryos. The insets are higher magnification views, centered on the developing posterior cardinal vein.
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