Astacin Metalloproteinase <i>Pp</i> MC5 of Mesenchyme Cells Regulates Epithelial Cell Proliferation and Survival to Promote Starfish Larval Development

The simple, triploblastic, epithelial body structure of the Patiria pectinifera starfish larva contains one mesenchyme cell type distributed across the blastocoel. In this study, we elucidated the role of a novel astacin metalloproteinase, PpMC5, tethered to the membrane of mesenchyme cells. PpMC5 knockdown by injection of morpholino oligo (MO) into the oocyte inhibited normal growth throughout the bipinnaria stage and caused a dwarfed phenotype consisting of reduced cell number and size, developmental delay, and cell debris in the blastocoel. When transforming growth factor β, P. pectinifera-specific TGFb gene (PpTGFb) preincubated with dissociated PpMC5 control larval cells was injected into the blastocoel of PpMC5 morphants, their dwarfism was significantly rescued. PpTGFb-MO oocyte injection alone only decreased the cell number. Transmission electron microscopy revealed thickening of the basal lamina, oncosis, and apoptosis in PpMC5 morphants. Overall, these results suggest that PpMC5 activates PpTGFb, participates in basal lamina remodeling, regulates epithelial cell proliferation and survival, and promotes larval development. Thus, PpMC5 modulates both cellular and developmental processes.