Localization of glycosylated matrix proteins in secretory porcine enamel and their possible functional roles in enamel mineralization

Abstract

The present study was undertaken to investigate glycosylation of porcine enamel proteins secreted in the secretory stage of amelogenesis and to gain insight into functional roles of glycosylated proteins in enamel mineralization. Enamel proteins, isolated from various zones of the secretory enamel, were separated by SDS-PAGE and then transferred on to a nitrocellulose membrane. The transblotted proteins were visualized with either antibodies against porcine amelogenins or various biotin-conjugated lectins. The lectins used were Con-A, GS-II, STA, WGA, s-WGA, GS-I, MPA, VVA, PNA, RCA-I, DBA, SJA, UEA-I, Lotus-A and LPA. The results of the immuno- and lectin blottings revealed that most of the lectins did not bind to porcine amelogenins, while a large number of non-amelogenins having various molecular masses were stained strongly with the conjugated WGA, Con A and MPA lectins. On the basis of the binding specificity with the lectins, porcine non-amelogenins were classified into two groups: WGA (and Con A)-binding moieties at 60–90 kDa (WGA-HMW); and MPA-binding moieties at 13–17 kDa (MPA-LMW). These two groups of non-amelogenins differed distinctly in terms of their localization and stability in the secretory tissue and their adsorption properties onto hydroxyapatite. The WGA-HMW were concentrated in the outer region adjacent to the ameloblasts and disappeared (due to degradation) in the underlying inner secretory enamel. In contrast, the MPA-LMW were found in all zones of the secretory enamel and their quantity remained relatively constant. Histochemical studies using FITC-conjugated WGA and MPA showed that the fluorescence-labelling of WGA was localized in the core region of prism rods, while the fluorescence-labelling of MPA was locally limited at the rim of prism rods or at the prism sheath. In separate adsorption studies, it was found that the WGA-HMW, as well as the intact amelogenins, displayed a high adsorption affinity on to apatite crystals, whereas the MPA-LMW showed only marginal adsorption on to apatitic surfaces. The overall results indicate that part of the heterogeneity found in porcine enamel proteins can be ascribed to variations of carbohydrate moieties attached to non-amelogenins. As to possible functional roles of porcine enamel proteins, the results strongly suggest that the WGA-HMW non-amelogenins and the intact amelogenins participate directly in regulation of the precipitation and growth of enamel crystals taking place in the vicinity of the ameloblasts, while the MPA-LMW non-amelogenins may contribute to development of a well-organized enamel structure by supporting a framework of prism rods.