SpADAM, an embryonic sea urchin ADAM related to vertebrate meltrins, is expressed primarily in mesenchyme, and functions in skeleton formation.
Date
2018-09-14
Authors
Rise, Matthew
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Abstract
The ADAM (A Disintegrin And Metalloproteinase) family of multidomain cell surface proteins has been shown to function in receptor-ligand processing, cell adhesion, cell fusion, and intracellular signaling. Several events in sea urchin embryogenesis involve proteolytic, adhesive, and fusogenic activities that could potentially be mediated by ADAMs. Here we describe an ADAM expressed during sea urchin development. Overlapping cDNA clones were used to identify a 3069 bp open reading frame. The deduced SpADAM protein is 1023 amino acids long and includes a signal peptide, a pro-domain, a metalloproteinase domain with a Zn-binding motif, a disintegrin domain, a cysteine-rich domain containing a putative fusion peptide, a transmembrane domain, and a proline-rich cytoplasmic domain. Phylogenetic analyses reveal that SpADAM is most closely related to mammalian ADAMs 12 and 19, and Xenopus ADAM 13. Southern blots indicate that SpADAM is a single copy gene and an alternatively spliced variant is identified. We have made a polyclonal antibody to a bacterially expressed fragment of the extracellular domain that recognizes 95 kDa and 72 kDa proteins. RNA in situ hybridizations and immunolocalizations show that SpADAM expression is dynamic throughout embryonic and larval development. In mesenchyme blastulae, SpADAM expression is highest in the vegetal plate, suggesting a potential role in ingression of primary mesenchyme cells. During gastrulation, the highest levels of SpADAM expression are seen in secondary mesenchyme cells (SMCs) as they release from the lengthening archenteron. Since SMCs are capable of homotypic fusion during this time, and SpADAM contains a putative fusogenic peptide, SpADAM could be involved in SMC-SMC fusion. In plutei, SpADAM is expressed in skeletogenic mesenchyme, muscle cells, pigment cells, and the preoral neural ectoderm. Primary mesenchyme cells express SpADAM weakly at ingression, and expression strengthens after syncytia are formed and skeletal elements are being secreted. Micromeres cultured in media containing anti-SpADAM antibodies or Fab fragments aggregate normally and form spicules, that are significantly longer than those in control cultures. We propose that SpADAM functions in skeletal morphogenesis, possibly in the removal of skeletal matrix. SpADAM has some of the properties of the types of molecules hypothesized to regulate skeleton morphogenesis through ectoderm signaling. SpADAM structure, expression, and function suggest that the group of meltrins; including SpADAM, ADAM 12, ADAM 13, and ADAM 19, are structurally and functionally conserved in deuterostomes.
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Keywords
Embryology, Ligands (Biochemistry), Sea urchins, Embryos