A Hedgehog-FGF signaling axis patterns anterior mesoderm during gastrulation

Single cell technologies hold promise for resolving complex early developmental phenotypes. Here we define a novel Hedgehog (Hh)-Fibroblast Growth Factor (FGF) signaling axis for the formation of anterior mesoderm lineages during gastrulation. Single-cell transcriptome analysis of Hh-deficient mesoderm revealed selective deficits in anterior mesoderm populations—culminating in defects to anterior embryonic structures including the pharyngeal arches, heart, and anterior somites. Transcriptional profiling of Hh-deficient mesoderm during gastrulation revealed disruptions to both transcriptional patterning of the mesoderm and FGF signaling for mesoderm migration. Mesoderm-specific Fgf4/Fgf8 double mutants recapitulated anterior mesoderm defects and Hh-dependent GLI transcription factors modulated enhancers at FGF gene loci.  Cellular migration defects during gastrulation induced by Hh pathway antagonism were mitigated by FGF4 protein. These findings implicate a multicomponent signaling hierarchy activated by Hh ligands from the embryonic node and executed by FGF signals in nascent mesoderm to control anterior mesoderm patterning. Comparing the cellular composition of Mesp1Cre sorted E8.25 mesoderm between Wt embryos and embryos expressing a dominant negative Hh pathway transcriptional repressor (Gli3R).