A long journey to oyster stem cell markers. CgSox2 and CgPOU2 synergic DNA binding and NODE proteins

Summary statementOyster Sox2 and POU bind in synergy to human Sox-Oct DNA motif, which suggests a role in stem cell maintenance. Sole among the oyster NODE proteins to alter chromatin SalL assembles in large hollow structures that exclude chromatin. Precursor or stem cells have been identified in mollusk, but determination of differentiation stages is still hampered by lack of markers, notably Oct4, that does not exist among invertebrates. To address this thorny question, oyster Nanog Oct4 DEacetylase proteins were studied, particularly POU proteins, a family that includes vertebrate Oct4. First, we showed that essential residues of 2 major vertebrate stem cell proteins, Sox2 and Oct4, are conserved through most metazoans which led to experimentally show in vitro that recombinant Sox2 protein specifically binds to WT, the mammalian Sox2-Oct4 consensus DNA motif. Furthermore, oyster Sox2 and POU2-3 synergistically bound to WT thus mimicking Sox2 and Oct4 synergy. Altogether, these data suggest that invertebrate POU2 or 3 are Oct4 orthologs. Expression in mammalian cells showed that oyster NODE proteins do not significantly alter chromatin structure except SalL, the homolog of SalL4. Confocal images revealed barrel-shaped hollow structures, orthogonal to cell plane and constituted of aggregates. These bodies were able to fuse or to disassemble/reassemble in fewer but much bigger hollow structures that excluded chromatin.