Examining the cardiac NK-2 genes in early heart development

Abstract

The cardiac NK-2 transcription factors are the vertebrate relatives of the Drosophila tinman gene. Without the Drosophila tinman gene, fruit flies fail to form their heart ("dorsal vessel"), and mutations or altered expression of cardiac NK-2 genes may lead to abnormal heart formation in vertebrates. Although the cardiac NK-2 gene NKX2-5 is recognized as an important factor in cases of human congenital heart disease and heart development in vertebrates, the roles of the other cardiac NK-2 genes are less clear. This report reviews what is known about the cardiac NK-2 genes in cardiac development, comparing studies in several different model systems.

Fig. 1

General domain structure of the cardiac NK-2 family of proteins. Four conserved motifs are found consistently in the NK-2 family of proteins. The domains and the amino acid sequence from human NKX2–5 corresponding to each domain starting from the amino terminus of the protein are the TN domain, the homeodomain, the NK-2 domain, and the sequence GIRAW. One of the distinguishing features of the homeodomain is the presence of a tyrosine (Y) at amino acid position 54

Fig. 2

The cardiac NK-2 factors can be sorted into three syntenic groups. Using the program Metazome (http://www.metazome.net/) and the genomic database at the Joint Genome Institute, a search for all chordate NK-2 genes was conducted, and a syntenic cluster analysis was performed. In each case, the cardiac NK-2 gene is indicated in black. Genes with a common identity are color coded, and the non-matching sequence is in white. Clustering based on NKX2–3, NKX2–5, and NKX 2–6 for human (Hsa), mouse (Mmu), rat (Rno), opossum (Mdo), chicken (Gga), frog (Xtr), and zebra fish (Dre) are included in the analysis

Fig. 3

Examination of cardiac Nk-2 gene expression in Xenopus laevis. Findings show mRNA isolated from: Xenopus stage 46 embryo eyes (e), heart (h), and embryo tail (t) or Xenopus adult eye (e), heart (h), liver (lv), gut (g), skin (s), skeletal muscle (m); and negative control (c). This figure is modified from Allen, Allen-Brady, and Weeks [3]

Fig. 4

Analysis of proteins bound to genomic regions of the cardiac NK-2 genes of Xenopus tropicalis. A ChIP-seq analysis of gastrulastage embryonic chromatin was performed using antibodies to H3K27me3, H3K4me3, and RNApolII. Analysis of each antibody is presented for nkx2–3, nkx2–5, and nkx2–10. Deep sequencing results of cDNA (RNA-seq) are indicated as well as known gene annotation and sequenced ESTs (bottom of each panel). Scaffold positions are across the top of the analysis for each gene. Direction or transcription is always from right to left in this figure