Parker B Antin

The homeobox gene Hex is expressed in multiple cell types during embryogenesis and is required for liver and monocyte development. Hex is expressed in the foregut region of late gastrula avian and mammalian embryos in a pattern that overlaps with expression of bone morphogenetic proteins (BMPs). Here we investigate the relationship between BMP signaling and Hex gene expression. We find that Hex expression in avian anterior lateral endoderm is regulated by autocrine BMP signaling. Characterization of the mouse Hex gene promoter identified a 71-nucleotide BMP-responsive element (BRE) that is required for up-regulation of Hex by an activated BMP signaling pathway. The Hex BRE binds Smad4 and Smad1-Smad4 complexes in vitro, and in transfection assays, it is responsive to Smad1 and Smad4 but not to Smad2 and Smad4 or Smad3 and Smad4. The BRE contains two copies of a GCCGnCGC-like motif that in Drosophila is the binding site for Mad and Madea followed by two CAGAG boxes that are similar to sequences required for transforming growth factor-beta/activin responsiveness of several vertebrate genes. Mutation of the GC elements, but not the two CAGAG boxes, abolishes Smads responsiveness in the intact Hex promoter, whereas mutations in both the GC elements and CAGAG boxes show that they act cooperatively to confer Smads responsiveness to the Hex promoter. The Hex BRE can confer Smads responsiveness to a heterologous promoter, and in this context, both the GC-rich elements and the CAGAG boxes are required for Smads-dependent promoter activity. An element almost identical to the Hex BRE is present within the BMP-responsive Nkx2-5 gene promoter, suggesting that the Hex BRE represents a common response element for genes regulated by BMP signaling in the foregut region of the embryo.

PMID: 3047142;PMCID: PMC2115209;Abstract:

The chicken gene encoding cardiac troponin T (cTNT) is expressed in both cardiac and skeletal muscle during early embryonic development, but is specifically repressed in skeletal muscle during fetal development. To determine if the cis-acting sequences governing transcription of a single gene in these two related cell types are the same, we have transfected promoter/upstream segments of the cTNT gene coupled to the bacterial chloramphenicol acetyltransferase gene into primary cultures of early embryonic cardiac and skeletal muscle cells. Using this assay system, chloramphenicol acetyltransferase activity directed by the cTNT promoter/upstream region was between two and three orders of magnitude higher in cardiac or skeletal muscle cells than in fibroblast cells, indicating that cis elements responsible for cell-specific expression reside in this region of the cTNT gene. Deletion experiments showed that a 67-nucleotide DNA segment residing between 268 and 201 nucleotides upstream of cTNT transcription initiation site is required for the cTNT promoter activity in embryonic cardiac cells. This region is not required in embryonic skeletal muscle cells because promoter construction containing only 129 upstream nucleotides is transcriptionally active in these cells. These results demonstrate that different cis-acting sequences are required for cTNT expression in early embryonic cardiac and skeletal muscle cells. Nonessential regions residing farther upstream, on the other hand, affected the level of expression of these minimum regions in a similar manner in both cell types. The data from these experiments indicate, therefore, that transcription of the cTNT promoter in early embryonic cardiac and skeletal muscle cells is governed both by common and divergent regulatory elements in cis and in trans.

The forkhead transcription factor gene E1 (FOXE1) plays an important role in regulation of thyroid development, palate formation and hair morphogenesis in mammals. However, avian FOXE1 genes have not been characterized and as such, codon evolution of FOXE1 orthologs in a broader evolutionary context of mammals and birds is not known.