Nodal-dependent mesendoderm specification requires the combinatorial activities of FoxH1 and Eomesodermin

Area of Science:

• Developmental Biology
• Molecular Biology
• Genetics

Background:

• Vertebrate mesendoderm patterning relies on the Nodal signaling pathway and its transcription factor FoxH1.
• Complete loss of FoxH1 function does not always replicate all Nodal signaling loss phenotypes, suggesting alternative downstream factors.
• Understanding these alternative pathways is crucial for comprehending early vertebrate development.

Purpose of the Study:

• To investigate the FoxH1-dependent and -independent roles of Nodal signaling in mesendoderm patterning.
• To characterize a novel zebrafish FoxH1 mutant, 'midway', for studying Nodal signaling dynamics.
• To elucidate the contribution of transcription factors like Eomesodermin in Nodal pathway output.

Main Methods:

• Utilized a novel recessive zebrafish FoxH1 mutation ('midway') affecting the Smad-interaction domain.
• Employed gel shift assays, Nodal overexpression experiments, and genetic epistasis analyses.
• Assessed gene expression of mesendoderm markers and manipulated Eomesodermin activity.

Main Results:

• The 'midway' mutant phenotypically represents a more complete loss of FoxH1-dependent Nodal signaling than previous mutants.
• Midway mutants lack notochords but largely retain endoderm and non-axial mesoderm (paraxial, intermediate, blood precursors) specification.
• Eomesodermin activity is essential for specifying these non-notochord tissues in midway embryos, with its inhibition phenocopying complete Nodal loss.

Conclusions:

• Nodal signaling employs distinct combinations of transcription factors, including FoxH1 and Eomesodermin, to specify different mesendoderm tissues.
• FoxH1-independent Nodal signaling, mediated by factors like Eomesodermin, plays a critical role in patterning non-axial mesoderm and endoderm.
• These findings provide insights into Nodal pathway evolution, notochord development, and chordate evolution.