6533b7d0fe1ef96bd125b535

RESEARCH PRODUCT

Biominéralisation chez les Céphalopodes (Mollusca) : processus moléculaires et évolution

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Molluscs constitute one of the most diversified phyla within metazoans, known for their ability to mineralize a shell. The shell biomineralization is a genetically controlled process that is performed extracellularly via a calcifying organic matrix. This latter, which remains finally occluded (at least, partially) into the shell, is a mixture of proteins, glycoproteins, lipids and polysaccharides, that are collectively secreted by the external calcifying epithelium of the mantle. Today, the shell matrix is usually considered as the ‘molecular toolbox’ for constructing the shell. Since its discovery, it has been the focus of numerous studies, but mainly on bivalves and gastropods leaving cephalopods in the shadow.Cephalopods are an important class of molluscs, from which only part of the living representatives possess a calcified shell (internal or external). The macroevolutionary history of this clade suggests – from conserved forms to the most derived ones – a general tendency to shell reduction, internalization, up to its complete disappearance. In spite of rather well established phylogenetic relationships between living shell-bearing cephalopods, molecular mechanisms responsible for the shell formation remain almost entirely unknown. Thus, one question arises: do all of these mineralizing cephalopods share a common “molecular toolbox” to elaborate their shell? This project proposes to answer this puzzling question by exploring the shell biomineralization of three living cephalopods via biochemical and proteomic approaches on their organic matrices, combined to a microstructural investigation of their shells.The first organism is the Ram’s horn squid Spirula spirula (Spirulidae), a small pelagic cephalopod whose biology and life cycle remain still poorly documented. Solid-state nuclear magnetic resonance and infra-red analyses suggest that polysaccharides constitute a major part of the organic matrix. Proteomics and in silico searches on heterologous metazoans models and on the spirule’s transcriptome revealed several peptides, many of them not matching with already known molluscan or cephalopod shell proteins. These observations suggest that the Ram’s horn squid possesses a unique set of proteins and does not contain a phylogenetic signal.The second organism,the winged argonaut Argonauta hians (Argonautidae), possesses an external shell, not homologous to that of other cephalopods/molluscs since it is secreted by the female’s first dorsal arm pair. The shell matrix contains an important quantity of acid-soluble proteinaceous matrix. Sugars, which are in low proportion,, appear mostly as sulphated glycosaminoglycans. Proteomics identified several peptide sequences that match with a number of proteins, not shared with other molluscs. This finding may suggest the recruitment of unique molecular tools for the shell mineralization.The third model, the common cuttlefish Sepia officinalis (Sepiidae), is the subject of a an ongoing research. The first results reveal the high proportion of the acid-insoluble matrix, which is predominantly chitinous. Histological investigations of fresh specimens and SEM observations of the cuttlebone microstructures plead in favour of a close contact between both organic and mineralized tissues, and suggest in addition a bi-directional process of mineralization, starting from the inner prismatic layer of the dorsal shield.Our results on the molecular / structural aspects of the biomineralization of our three models are discussed in an evolutionary perspective.