Upregulation of Blood and Endothelial Markers by Fli1-VP16

WISH of zebrafish 10 s stage embryos injected with Fli1 or Fli1-VP16. In whole-mounted embryos, head to top, focusing on PLM (G–M). In flat-mounted embryos, head to top (A–F, M–R) or to left (S, T). Red arrows indicate expanded expression.

Fli1 May Act in Parallel with Cloche but Upstream of Scl and Etsrp

(A–J) WISH on wild-type or Cloche mutant zebrafish embryos injected with combinations of Fli1-VP16 mRNA and/or Scl/Etsrp MOs. 10 s stage, dorsal view, head to top.

(K) A genetic regulatory network linking Bmp to Fli1 at the top of the hemangioblast hierarchy, drawn according to Davidson et al. [29]. Solid lines, direct regulation; dashed lines, no evidence for direct or indirect. Evidence for links quoted in text, apart from Gata2 as a direct Bmp target [32].

Fli1 expression in the PLM is independent of Cloche at an early stage.
(A,B) Whole mount in situ hybridisation showing Fli1 expression in wild type (A) and Cloche (B) embryos at 6s. Note that PLM expression of Fli1 is retained in Cloche (black arrows), whereas ALM expression is absent (red arrows); (C,D) Erg expression in wild type and Clo embryos; (E,F) Gata2 expression in wild type and Clo embryos; (G,H) Scl expression in wild type and Clo embryos; (I,J) Etsrp expression in wt and Clo embryos. Note that both ALM and PLM expression of Erg (D, red arrows), Scl (H, red arrows) and Etsrp (J, red arrows), and PLM expression of Gata2 (F, red arrows) are absent in Clo embryos.

Loss- and gain-of function experiments reveal differential control of Fli1 expression by BMP signalling.
Zebrafish embryos transgenic for a heat shock inducible, dominant negative Bmp receptor were heat shocked at 30% epiboly and processed for WISH at 10s (A-F) or 6s (I,J). (G) Expanded WISH signals for Fli1 (red arrows) in dino mutants. (H) No expansion of Nkx2.5 and Krox20 in dino mutants. Dorsal views.