Slit-Robo signalling establishes a Sphingosine-1-phosphate gradient to polarise fin mesenchyme

Immigration of mesenchymal cells into the growing fin and limb buds drives distal outgrowth, with subsequent tensile forces between these cells essential for fin and limb morphogenesis. Morphogens derived from the apical domain of the fin, orientate limb mesenchyme cell polarity, migration, division and adhesion. The zebrafish mutant stomp displays defects in fin morphogenesis including blister formation and associated loss of orientation and adhesion of immigrating fin mesenchyme cells. Positional cloning of stomp identifies a mutation in the gene encoding the axon guidance ligand, Slit3. We provide evidence that Slit ligands derived from immigrating mesenchyme act via Robo receptors at the apical ectodermal ridge (AER) to promote release of sphingosine-1-phosphate (S1P). S1P subsequently diffuses back to the mesenchyme to promote their polarisation, orientation, positioning and adhesion to the interstitial matrix of the fin fold. We thus demonstrate the coordination of the Slit-Robo and S1P signalling pathways in fin fold morphogenesis. Our work introduces a mechanism regulating the orientation, positioning and adhesion of its constituent cells.

Automated summary

Mesenchymal cell immigration into growing fins and limb buds drives distal outgrowth, with tensile forces between these cells essential for fin and limb morphogenesis. Morphogens derived from the apical fin domain regulate cell polarity, migration, division, and adhesion of limb mesenchyme cells. The mutant stomp in zebrafish displays defects in fin morphogenesis, including blister formation and loss of orientation and adhesion of immigrating fin mesenchyme cells. The gene encoding Slit3, an axon guidance ligand, is found to be mutated in stomp. Slit ligands derived from immigrating mesenchyme act via Robo receptors at the apical ectodermal ridge (AER) to promote the release of sphingosine-1-phosphate (S1P), which then diffuses back to the mesenchyme to regulate their polarization, orientation, positioning, and adhesion to the interstitial matrix of the fin fold.