SCL/Tal-1 transcription factor acts downstream of cloche to specify hematopoietic and vascular progenitors in zebrafish

Abstract

SCL/Tal-1 is a transcription factor necessary for hematopoietic stem cell differentiation. Although SCL is also expressed in endothelial and neural progenitors, SCL function in these cells remains unknown. In the zebrafish mutant cloche (clo), SCL expression is nearly abolished in hematopoietic and vascular tissues. Correspondingly, it was shown previously that clo fails to differentiate blood and angioblasts. Genetic analysis demonstrates that the clo mutation is not linked to the SCL locus. Forced expression of SCL in clo embryos rescues the blood and vascular defects, suggesting that SCL acts downstream of clo to specify hematopoietic and vascular differentiation.

Figure 1

Zebrafish SCL peptide sequence compared with that of Xenopus, chick, mouse, and human (amino acids identical across species is in dark blue, with decreasing similarity in lighter shades).

Figure 2

SCL expression in vascular and hematopoietic progenitors during early development. SCL RNA in situ hybridization at 5-somites stage (A–C), 18-somites stage (D), 24 hpf (E–G), 26 hpf (H), 2 dpf (I), 3 dpf (J), 4 dpf (K,L), and 5 dpf (M). Transverse histological sections (6 μm) from 4 dpf embryos are shown in N–P, corresponding to labeled lines in L. All are in situs done with SCL, except K, which is with c-myb. All are viewed laterally, with anterior to the left and dorsal up, except A which is an anterior view, B and D, which are dorsal views, and C, which is a posterior view. (See text for details.)

Figure 3

The zebrafish clo mutation affects SCL expression in hematopoietic and vascular tissues. SCL RNA in situ hybridization at 5-somite stage (A), 24 hpf (B), and 2 dpf (C). SSLP is defined in the 3′ UTR of the SCL cDNA and is unlinked to clo (D). SCL maps to the centromere of linkage group 22 (E).

Figure 4

Forced expression of SCL rescues both the hematopoietic and vascular defects in clo. Microinjection rescue of clo^−/− embryos with cmv–SCLa2.1. Embryos are shown in lateral views in A–C, dorsal views in D, and lateral close-ups of posterior tail in E. (A) SCL in situ hybridizations of injected embryos at 24 and 48 hpf; (B) GATA-1 in situ hybridization; (C) o-dianisine staining; (D) flk-1 and (E) tie-1 in situ hybridization. Wild-type clo (wt), mutant CLO^−/− (mut), and rescued clo^−/− (rsc) are shown, all staged at 2 dpf. (See text for details.)

Figure 5

Proposed model of SCL function during hematopoiesis and vasculogenesis.