Mouse Mix gene is activated early during differentiation of ES and F9 stem cells and induces endoderm in frog embryos.

In frog and zebrafish, the Mix/Bix family of paired type homeodomain proteins play key roles in specification and differentiation of mesendoderm. However, in mouse, only a single Mix gene (mMix) has been identified to date and its function is unknown. We have analyzed the expression of mouse Mix RNA and protein in embryos, embryoid bodies formed from embryonic stem cells and F9 teratocarcinoma cells, as well as several differentiated cell types. Expression in embryoid bodies in culture mirrors that in embryos, where Mix is transcribed transiently in primitive (visceral) endoderm (VE) and in nascent mesoderm. In F9 cells induced by retinoic acid to differentiate to VE, mMix is coordinately expressed with three other endodermal transcription factors, well before AFP, and its protein product is localized to the nucleus. In a subpopulation of nascent mesodermal cells from embryonic stem cell embryoid bodies, mMix is coexpressed with Brachyury. Intriguingly, mMix mRNA is detected in a population (T+Flk1+) of cells which may contain hemangioblasts, before the onset of hematopoiesis and activation of hematopoietic markers. In vitro and in vivo, mMix expression in nascent mesoderm is rapidly down-regulated and becomes undetectable in differentiated cell types. In the region of the developing gut, mMix expression is confined to the mesoderm of mid- and hindgut but is absent from definitive endoderm. Injection of mouse mMix RNA into early frog embryos results in axial truncation of developing tadpoles and, in animal cap assays, mMix alone is sufficient to activate expression of several endodermal (but not mesodermal) markers. Although these observations do not exclude a possible cell-autonomous function for mMix in mesendodermal progenitor cells, they do suggest an additional, non-cell autonomous role in nascent mesoderm in the formation and/or patterning of adjacent definitive endoderm.

Structure, upstream promoter region, and functional domains of a mouse and human Mix paired-like homeobox gene.

Mix/Bix proteins represent a vertebrate subgroup of paired-like homeodomain proteins which are known to function around the time of gastrulation. Here we report the structures of the genomic and upstream promoter regions of a mouse and human Mix-like gene. Both genes map to syntenic regions of chromosome 1 and contain two coding exons, with the paired-type homeodomain split between the exons within helix 3. Differentiating mouse embryonic stem cells transcribe a messenger RNA of approximately 2.6 kb. The first exon encodes the translation initiation codon and a 5' untranslated region of approximately 90 bp. Sequence analysis of the 960 bp upstream of the transcription start site of the mouse Mix gene revealed the presence of a putative initiator region and TATA box as well as potential Smad, FoxH1/FAST, T-box, COUP-TF, C/EBP, GATA, HNF3 binding sites and retinoic acid response elements. A number of these sites are conserved in the human Mix promoter. We find that most paired-related homeodomain proteins, including mouse and human Mix, contain a proline-rich region within their amino termini which may interact with other proteins. Mouse and human Mix proteins contain highly conserved carboxy-terminal polar/acidic regions with the potential to form an amphipathic helix and the ability to activate transcription in yeast. Mouse Mix expressed in COS cells or in vitro binds a DNA consensus sequence identified previously for paired class homeodomain proteins. These studies suggest that a number of features of paired-like protein structure and function are conserved across diverse species and provide a useful framework for studying the function and regulation of the mouse Mix gene.