CHICKEN NKX-2.8 - A NOVEL HOMEOBOX GENE EXPRESSED IN EARLY HEART PROGENITOR CELLS AND PHARYNGEAL POUCH-2 AND POUCH-3 ENDODERM

Members of the MK family of homeobox transcription factors regulate cr itical steps of organogenesis during vertebrate development. In the st udies described in this report, we have isolated and functionally char acterized the chicken Nkx-2.8 (cNkx-2.8) cDNA and protein and defined the temporal and spatial pattern of cNkx-2.8 gene expression during ch icken development. cNkx-2.8 transcripts are first detectable at HH sta ge 7 in the splanchnopleura. At stage 10(+), the cNkx-2.8 gene is expr essed in the linear heart tube and the dorsal half of the vitelline ve in. However, after looping, HH stage 13, cNkx-2.8 is no longer express ed in the looped heart tube, but is expressed in the ventral pharyngea l endoderm. At stage 15, in addition to the pharyngeal expression patt ern, cNkx-2.8 is expressed in the ectoderm of the pharyngeal arches an d the aortic sac. By HH Stage 17, cNkx-2.8 expression is detectable in lateral endoderm of the second and third pharyngeal pouches, the post erior portion of the aortic sac, and the sinus venosus. cNkx-2.8 binds to previously characterized Nkx2-1 and Nkx2-5 DNA-binding sites and o verexpression of cNkx-2.8 transactivates a minimal promoter which cont ains multimerized Nkx-2 DNA-binding sites. In addition, cNkx-2.8 and s erum response factor can coactivate a minimal cardiac alpha-actin prom oter. These data are consistent with a model in which cNkx-2.8 perform s a unique temporally and spatially restricted function in the develop ing embryonic heart and pharyngeal region. Moreover, the coexpression of cNkx-2.5 and -2.8 raises the possibility that cNkx-2.8 may have a r edundant role with cNkx-2.5 in the coalescing heart tube and may play an important role in the transcriptional program(s) that underlies thy mus formation. The existence of multiple NK-2 family members and their partially overlapping patterns of expression are discussed Within the framework of a ''Nkx code.'' (C) 1997 Academic Press.