Actin-binding protein tropomodulin-1 is necessary for basal constriction formation and bud detachment in Hydra vulgaris

Tropomodulins (TMODs) are a group of proteins that stabilize the actin filament network by capping the slow-growing end of F-actin. Although their roles in tissue morphogenesis have been extensively studied in triploblastic organisms, their function in diploblastic <i>Hydra</i> remained unexplored. This study identified that in <i>Hydra</i>, <i>TMOD1</i> expression is strongly upregulated in ectodermal cells at the parent-offspring junction during basal constriction formation and bud detachment. Notably, siRNA-mediated <i>TMOD1</i> knockdown inhibits basal constriction formation, a pre-requisite morphological event for bud detachment. <i>TMOD1</i> knockdown polyps show fragmented stress fibers and altered cell morphology adjacent to the basal constriction site. Finally, pharmacological studies showed that FGFR signaling is necessary for <i>TMOD1</i> expression. Collectively, our results establish that Hydra <i>TMOD1</i> is an FGFR-ERK2 signaling-regulated gene that is exclusively expressed in the epitheliomuscular ectodermal cells at the parent-offspring junction and is essential for bud separation from the parent <i>Hydra</i> polyp by promoting F-actin polymerization/stabilization.