Localized expression of Strim1, a novel member of the TRIM-containing family, guides the skeletal morphogenetic program of the sea urchin embryo

The building of the skeleton in the indirect developing sea urchin embryo is a complex morphogenetic process that is executed by the Primary Mesenchyme Cells or PMCs (Ettensohn et al, 1997; Wilt 2002). It is well known that the PMCs acquire most of the positional and temporal information from the overlying ectoderm for skeletal initiation and growth (Armstrong 1993; Cavalieri et al, 2003; Röttinger et al, 2008). In this study, we analyze the function of a novel gene, encoding for a tripartite motif-containing (TRIM) protein denoted strim1, that adds up to the list of genes constituting the epithelial-mesenchymal signaling network. We show that strim1 is expressed in ectoderm regions adjacent to the bilateral clusters of PMCs. Strim1 misexpression causes the number of PMCs to double and leads to skeletal abnormalities. By micromere transplantations, we establish that skeletal defects depend upon strim1 misexpression in ectoderm cells. Reciprocally, knock-down of strim1 function abrogates PMC migration and blocks skeletogenesis. Identical phenotype is shown by chimeras in which strim1 function is blocked selectively in the ectoderm. We also show that clonal expression of strim1 into ectoderm cells from knocked-down embryos restores the correct skeletogenic program. Finally, we report that strim1 triggers the expression of the ectodermspecific gene pax2/5/8, and the PMC marker sm30 (an ectoderm signaling dependent gene). We conclude that strim1 function is able to elicit specific gene expression both in ectoderm cells and PMCs to guide the biomineralization during morphogenesis. References Armstrong, N, Hardin, J and McClay, DR (1993). Cell-cell interactions regulate skeleton formation in the sea urchin embryo. Development 119, 833-40. Cavalieri, V, Spinelli, G and Di Bernardo, M (2003). Impairing Otp homeodomain function in oral ectoderm cells affects skeletogenesis in sea urchin embryos. Dev Biol 262, 107-18. Ettensohn, CA, Guss, KA, Hodor, PG, and Malinda, KM (1997). The morphogenesis of the skeletal system of the sea urchin embryo, in: Collier JR (Ed.), Reproductive Biology of Invertebrates, vol. VII: Progress in Developmental Biology, Oxford & IBH publishing Co. Pvt. Ltd. New Delhi, Calcutta. 225-265. Röttinger, E, Saudemont, A, Duboc, V, Besnardeau, L, McClay, D and Lepage, T (2008). FGF signals guide migration of mesenchymal cells, control skeletal morphogenesis and regulate gastrulation during sea urchin development. Development 135, 353-65. Wilt, FH (2002). Biomineralization of the spicules of sea urchin embryos. Zoolog Sci 19, 253-61.