Direct TGF-ß signaling via alk4/5/7 pathway is involved in gut bending in sea urchin embryos

Background: Precise gastrulation is essential for formation of functional bodies in cnidarians and bilaterians. Previously, by using an alk4/5/7-specific inhibitor, we showed that transforming growth factor-beta (TGF-ß)-alk4/5/7 signaling pathway is important for correct gut bending in sea urchin embryos. However, it is still unclear where functional TGF-ß signals are received in embryos for correct gut bending because details of the spatiotemporal expression pattern of alk4/5/7 have not been reported. Results: We revealed that alk4/5/7 are expressed from the 2-cell to early pluteus stage throughout the entire body, including the invaginating gut. To investigate whether TGF-ß signals directly received in endoderm are required for correct gut bending, we made chimeras in which alk4/5/7 translation was inhibited only in endomesoderm lineage. As a result, the gut of the chimeric embryos did not bend precisely, in contrast to the control chimeras. Conclusion: We conclude that direct TGF-ß signaling to the endoderm via alk4/5/7 pathway regulates correct gut bending. However, TGF-ß-alk4/5/7 pathway is not related to mouth opening because the mouth is formed without TGF-ß signaling to the endoderm. This research contributes to understanding the mechanisms leading to the proper positioning of the end of the archenteron for forming a through-gut, which is commonly needed for bilaterians.