Functionally distinct roles for T and Tbx6 during mouse development

Area of Science:

• Developmental Biology
• Molecular Genetics
• Gene Regulation

Background:

• T-box transcription factors T and Tbx6 are co-expressed in mouse primitive streak.
• T is crucial for notochord development, while Tbx6 is essential for presomitic mesoderm development.
• Both factors share a DNA-binding T-domain, suggesting potential functional overlap.

Purpose of the Study:

• To investigate the functional similarities and differences in DNA binding and transcriptional activity of T and Tbx6.
• To analyze the genetic interactions and competition between T and Tbx6 in vivo.
• To determine if Tbx6 can functionally substitute for T.

Main Methods:

• In vitro DNA binding and transcriptional assays using T-domains of T and Tbx6.
• Generation of mouse embryos with altered Tbx6 expression levels (overexpression, heterozygous).
• Genetic analysis using dominant-negative TWis allele and Tbx6 hypomorphic allele (rib-vertebrae).
• Tbx6 knockin at the T locus in embryonic stem cells for chimeric embryo analysis.

Main Results:

• T and Tbx6 exhibit different affinities for DNA binding sites at the Dll1 enhancer.
• Tbx6 shows tenfold higher transcriptional activation than T in vitro.
• Competition between T and Tbx6 for target gene enhancers requires functional DNA binding domains.
• Tbx6 overexpression leads to short tails, mimicking T heterozygous phenotype.
• Tbx6+/- TWis/+ embryos display rib fusions and vertebral malformations, similar to Tbx6 hypomorphic phenotypes.
• Tbx6 knockin at the T locus results in T null-like phenotypes in chimeric embryos.

Conclusions:

• Differences in DNA binding affinity and transcriptional activity between T and Tbx6 provide a molecular basis for their distinct roles.
• Tbx6 cannot functionally replace T due to these molecular differences.
• Competition between T and Tbx6 in vivo may contribute to observed developmental phenotypes.