P2.58 Thursday, Jan. 5 Programmed cell fusion of PMCs during skeletal development in the sea urchin Lytechinus variegatus LYONS, Deirdre/C*; MCCLAY, David/R; Duke University, Durham NC firstname.lastname@example.org
Skeletogenesis in sea urchin embryos occurs after the development of a multinucleate syncytium that will surround the larval skeleton. Syncytium formation requires a cell-cell fusion event within the primary mesenchyme cell (PMC) population. We are using a transplant-based fusion assay to study the events of PMC fusion in vivo. The PMC lineage undergoes multiple specification-state transitions, some of which are described by the PMC gene regulatory network (GRN). The necessary fusogenic kernel remains to be defined. Previously, the transcription factors FoxN2/3, Alx1 and Twist were shown to be necessary for PMC fusion using a transplant assay with control and morpholino-knockdown micromeres, the PMC progenitors. This assay is now being used on other genes in the network to identify the top of the PMC fusion kernel. These fusion morphants provide an entry point for studying the cell biological mechanisms controlling fusion in the sea urchin. Studies of fusion in other systems—such as in mammalian trophoblast formation and muscle formation in fly and mammals —provide a list of fusion-associated candidate genes that might be conserved in urchin PMC fusion. These include the membrane protein tetraspanin, the metalloprotease ADAM, the guanine nucleotide exchange factor Dock180 and FGF-like receptor 1 (FGFRL1). The expression pattern of sea urchin homologs of these genes has been examined in cleavege, gastrula, and larval stages. To test if these genes are involved in PMC fusion, we are examining their expression levels in FoxN2/3-, Alx- and Twist-knockdown embryos, in which fusion is blocked. Identifying the network that drives fusion in sea urchins, and studying this process in vivo, will contribute to our general understanding of syncytium formation in metazoans.