A self-generated Toddler gradient guides mesodermal cell migration
The sculpting of germ layers during gastrulation relies on coordinated migration of progenitor cells, yet the cues controlling these long-range directed movements remain largely unknown. While directional migration often relies on a chemokine gradient generated from a localized source, we find that zebrafish ventrolateral mesoderm is guided by the uniformly expressed and secreted protein Toddler/ELABELA/Apela, acting as a self-generated gradient. We show that the Apelin receptor, which is specifically expressed in mesodermal cells, has a dual role during gastrulation, acting as a scavenger receptor to generate a Toddler gradient, and as a chemokine receptor to sense this guidance cue. Thus, we uncover a single receptor-based self-generated gradient as the enigmatic guidance cue that can robustly steer the directional migration of mesoderm through the complex and continuously changing environment of the gastrulating embryo. One sentence summaryAplnr has a dual role to self-generate and sense a Toddler gradient directing mesodermal cells during zebrafish gastrulation.