Recent Advances in Studies on Nacreous Layer Biomineralization. Molecular and Cellular Aspects

Recent advances in biotechnology promoted evolution in the general study of life sciences, and more specifically, in the study of biomineralization. Here I review information from recently developed studies on molecular and cellular aspects of molluscan shell nacreous layer formation. First, I summarize the primary and tertiary structures of the organic matrix (OM) proteins recently identified on the basis of cDNA cloning and classify them into several groups. Further discussion is focused on the possible function of the OM proteins involved in the regulation of CaCO3 polymorphism by in vitro examination of crystallization. In addition, several new findings from in vitro and in vivo studies of gene expression and cell culture are discussed in relation to biomineralization. In this manner, a current overview of the process of molluscan shell nacreous layer formation on the molecular level is presented. INTRODUCTION Hard tissue formation by organisms is commonly regulated by many kinds of macromolecules constituting an OM. The OM of molluscan shells is one of the best studied of all CaCO3 biomineral structures, and considerable information has been accumulated regarding its morphology, structure, and histochemical properties (Wada, 1964; Grégoire, 1972; Weiner & Hood, 1975; Weiner, 1979; Nakahara et al., 1980, 1982; Samata, 1988). An OM is roughly classified into a soluble matrix (SM) and an insoluble matrix (ISM) based on its solubility in solutions. Some confusion has arisen over the names of OM-fractions based on their solubility in solutions and no definitive nomenclature has yet been established. The general types of soluble proteins referred to in the field of biomineralization are: 1) proteins soluble in the solution used for decalcification such as EDTA and dilute acid; 2) soluble protein in water used for dialysis after decalcification, such as water-soluble protein; and 3) solubilized proteins incorporated into insoluble proteins by using dense urea solution or detergents. It is worth noting that it is not clear whether the various methods of decalcification release the same molecules. In fact, soluble matrix in Mili-Q water without any demineralization process contained components with completely different amino acid compositions with (1) Laboratory of Cell Biology, Faculty of Environmental Health, Azabu University 1-17-71 Fuchinobe, Sagamihara 229-8501 Japan e-mail: [email protected] Thalassas, 2004, 20 (1): 25-44 An International Journal of Marine Sciences