March 1, 2000;
Transient cardiac expression of the tinman-family homeobox gene, XNkx2-10.
In Drosophila, the tinman
homeobox gene is absolutely required for heart
development. In the vertebrates, a small family of tinman
-related genes, the cardiac NK-2 genes, appear to play a similar role in the formation of the vertebrate heart
. However, targeted gene ablation of one of these genes, Nkx2-5, results in defects in only the late stages of cardiac development suggesting the presence of a rescuing gene function early in development. Here, we report the characterization of a novel tinman
-related gene, XNkx2-10, which is expressed during early heart
development in Xenopus. Using in vitro assays, we show that XNkx2-10 is capable of transactivating expression from promoters previously shown to be activated by other tinman
-related genes, including Nkx2-5. Furthermore, Xenopus Nkx2-10 can synergize with the GATA-4 and SRF
transcription factors to activate reporter gene expression.
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Fig. 2. Expression pattern of XNkx2-10 by wholemount in situ hybridization. (A) Anterior view of a late neurula stage embryo showing XNkx2-10 expression in endodermal and mesodermal cells (arrow) adjacent to the neurual tube. (B) Lateral view of an early tailbud embryo showing XNkx2-10 transcripts in the pharyngeal endoderm and cardiac primordia (arrow). (C) Ventral view of the embryo in (B) showing both cardiac (black arrows) and pharyngeal (white arrowheads) staining. (D) Mid tailbud embryo (stage 32) stained for both XHex (purple), which marks the developing liver (Newman et al., 1997) and XNkx2-10 (blue). The heart (arrow), directly anterior to the liver (small arrowhead) is negative for XNkx2-10 at this stage. Note the extensive pharyngeal staining, extending anteriorly past the forming thyroid (large arrowhead). (E) Mid tailbud embryo (stage 32) stained for both XHex (purple) and XNkx2-5 (blue). Compare with (D) and note the strong cardiac staining (arrow). (F–H) Sections of embryos stained for XNkx2-10. (I–K) Sections of embryos stained for XNkx2-5. (F and I) Parasagittal section through an early tailbud stage (stage 22) embryo showing expression of both XNkx2-10 and XNkx2-5 in the endoderm and the mesoderm. (G and J) Cross-section through a tailbud stage embryo (stage 26) showing transcripts in both the pharyngeal endoderm (large arrowhead) and cardiac mesoderm (small arrowhead). (H and K) Cross-section through a late tailbud stage embryo. Note the lack of XNkx2-10 expression in the folded heart (small arrowhead) and the lack of XNkx2-5 expression in the pharyngeal endoderm (large arrowhead). Arrows highlight expression of both XNkx2-5 and XNkx2-10 in the mesoderm immediately dorsal to the heart. In situ analysis was performed as described by Harland (1991).