Etude du processus de biominéralisation chez les mollusques à travers trois exemples (la réparation coquillière (Haliotis tuberculata), la formation des perles (Pinctada magnaritifera) et la composition de la matrice coquillière (Helix aspersa maxima))

Les mécanismes sous-jacents à la formation des coquilles chez les mollusques, et particulièrement chez les bivalves et les gastéropodes, font l'objet d'un nombre croissant d'études ces dernières années. Ces travaux ont notamment permis de mettre en évidence l'existence d'une plasticité morphologique intra-spécifique de la calcification chez plusieurs espèces de mollusques. Au cours de cette thèse, nous avons choisi d'aborder cette notion de plasticité du processus de biominéralisation à travers 3 cas particuliers : la réparation coquillière chez le gastéropode marin Haliotis tuberculata, la greffe d'un tissu minéralisant chez l'huître perlière Pinctada margaritifera, et la composition de la coquille d'un gastéropode terrestre (Helix aspersa maxima). Chez H. tuberculata et P. margaritifera, les résultats obtenus soulignent l'existence d'un réagencement, plus ou moins important, des tissus minéralisants suite au stress (lésion de la coquille/greffe), allant d'une simple variation de la vitesse de minéralisation (réparation), à une modification structurelle importante du tissu minéralisant (greffe). Concernant Helix aspersa maxima, les composantes matricielles de la coquille chez cette espèce présentent des caractéristiques "atypiques" : interaction très faible (voir inexistante) avec le CaCO3 in vitro. Par ailleurs, les séquences partielles des protéines matricielles obtenues au cours de ce travail ne présentent pas d'homologies avec les séquences présentes dans les bases de données actuelles. Le changement drastique d'environnement pourrait être à l'origine de cette disparité entre les matrices d'organismes marins et celle de notre modèle (Helix aspersa maxima).The mechanisms underlying the formation of shells in molluscs, especially bivalves and gastropods, are the subject of a growing number of studies in recent years. These works had enabled to highlight the existence of an intra-specific morphological plasticity of calcification in several species of molluscs. In this thesis, we chose to approach this concept of plasticity of the biomineralization process through three specific cases: the shell repair in the marine gastropod Haliotis tuberculata, transplantation of a mineralizing tissue in the pearl oyster Pinctada margaritifera, and the composition of the shell of a land snail (Helix aspersa maxima). In H. tuberculata and P. margaritifera, the results emphasized the existence of a rearrangement, more or less important of mineralizing tissues following stress (shell lesion / transplant), ranging from a simple change in the rate of mineralization (repair), to an important structural change of the mineralizing tissue (transplant). On Helix aspersa maxima, the matrix components of the shell in this species have "atypical" characteristics: very weak interaction (or not) with CaCO3 in vitro. In addition, partial sequences of matrix proteins obtained in this work showed no homology with sequences in current databases. The drastic change of environment could be the cause of this disparity between the matrices of marine organisms and that of our model (Helix aspersa maxima).CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF

The shell matrix of the pulmonate land snail Helix aspersa maxima.

12 pagesInternational audienceIn mollusks, the shell mineralization process is controlled by an array of proteins, glycoproteins and polysaccharides that collectively constitute the shell matrix. In spite of numerous researches, the shell protein content of a limited number of model species has been investigated. This paper presents biochemical data on the common edible land snail Helix aspersa maxima, a model organism for ecotoxicological purposes, which has however been poorly investigated from a biomineralization viewpoint. The shell matrix of this species was extracted and analyzed biochemically for functional in vitro inhibition assay, for amino acid and monosaccharides compositions. The matrix was further analyzed on 1 and 2D gels and short partial protein sequences were obtained from 2D gel spots. Serological comparisons were established with a set of heterologous antibodies, two of which were subsequently used for subsequent immunogold localization of matrix components. Our data suggest that the shell matrix of Helix aspersa maxima may differ widely from the shell secretory repertoire of the marine mollusks studied so far, such as the gastropod Haliotis or the pearl oyster Pinctada. In particular, most of the biochemical properties generally attributed to soluble shell matrices, such as calcium-binding capability, or the capacity to interfere in vitro with the precipitation of calcium carbonate or to inhibit the precipitation of calcium carbonate, were not recorded with this matrix. This drastic change in the biochemical properties of the landsnail shell matrix puts into question the existence of a unique molecular model for molluscan shell formation, and may be related to terrestrialisation