Calcium binding of enamel proteins and their derivatives with emphasis on the calcium-binding domain of porcine sheathlin

AMBN (ameloblastin)is an enamel matrix protein that regulates cell adhesion, proliferation, and differentiation of ameloblasts. In AMBN-deficient mice, ameloblasts are detached from the enamel matrix, continue to proliferate, and form a multiple cell layer; often, odontogenic tumors develop in the maxilla with age. However, the mechanism of AMBN functions in these biological processes remains unclear. By using recombinant AMBN proteins, we found that AMBN had heparin binding domains at the C-terminal half and that these domains were critical for AMBN binding to dental epithelial cells. Overexpression of fulllength AMBN protein inhibited proliferation of human ameloblastoma AM-1 cells, but overexpression of heparin binding domain-deficient AMBN protein had no inhibitory effect. In full-length AMBN-overexpressing AM-1 cells, the expression of Msx2, which is involved in the dental epithelial progenitor phenotype, was decreased, whereas the expression of cell proliferation inhibitors p21 and p27 was increased. We also found that the expression of enamelin, a marker of differentiated ameloblasts, was induced, suggesting that AMBN promotes odontogenic tumor differentiation. Thus, our results suggest that AMBN promotes cell binding through the heparin binding sites and plays an important role in preventing odontogenic tumor development by suppressing cell proliferation and maintaining differentiation phenotype through Msx2, p21, and p27.The extracellular matrix provides structural support for cells and regulates cell proliferation, migration, differentiation, and apoptosis for tissue development and homeostasis (1). The extracellular matrix also plays a crucial role in pathological processes and diseases, such as wound healing, tumorigenesis, and cancer development (2, 3). AMBN (ameloblastin), also known as amelin and sheathlin, is a tooth-specific extracellular matrix and the most abundant non-amelogenin enamel matrix protein (4 -6). AMBN is expressed primarily by ameloblasts, which are differentiated from the oral ectoderm and form a polarized single cell layer underlying the enamel matrix. In a previous study, we created Ambn-null mice and demonstrated that AMBN is required for cell attachment and polarization and for maintaining the differentiation state of ameloblasts and is essential for enamel formation (3). Overexpression of Ambn in transgenic mice causes abnormal enamel crystallite formation and enamel rod morphology (7). These results suggest that enamel formation and rod morphology are influenced by temporal and spatial expressions of AMBN and imply that the AMBN gene locus may be involved in the etiology of a number of cases of undiagnosed hereditary amelogenesis imperfecta (8). Further, it was reported that recombinant AMBN enhances pulpal wound healing and reparative dentine formation following pulpotomy procedures, suggesting that it functions as a signal molecule in epithelial-mesenchymal interactions (9).We previously reported that about 20% of Ambn-null mice developed an odontogenic tumor of dental ep...

Section: Discussionmentioning

confidence: 65%

Critical Role of Heparin Binding Domains of Ameloblastin for Dental Epithelium Cell Adhesion and Ameloblastoma Proliferation

Sonoda

Iwamoto

Nakamura

et al. 2009

“…It has previously been observed that ameloblastin isolated from porcine enamel harbors several post-translational modifications, including hydroxylation of Pro-11 and Pro-324, phosphorylation of Ser-17, and O-linked glycosylation of Ser-86 and Thr-361, respectively (63,64). Because ameloblastin produced in a prokaryotic expression system is not post-translationally modified, the here-reported results would suggest that the described or as yet unidentified post-translational modifications of AMBN are not essential for the capacity to self-associate into supramolecular structures.…”

Section: Discussionmentioning

confidence: 99%

Intrinsically Disordered Enamel Matrix Protein Ameloblastin Forms Ribbon-like Supramolecular Structures via an N-terminal Segment Encoded by Exon 5

Wald

Osičková

Šulc

et al. 2013

Background: Ameloblastin plays a key role in the complex biomineralization process that forms tooth enamel, the hardest tissue of the body. Results: Ameloblastin self-associates into ribbon-like supramolecular structures via a short segment encoded by exon 5. Conclusion: Ameloblastin self-association may be essential for correct structural organization and mineralization of the enamel in vivo. Significance: The results provide molecular insight into biology of tooth enamel formation.

“…This 106-amino acid domain contains two phosphoserines and three N-linked glycosylations (68,69). The 32-kDa enamelin does not engage in protein-protein interactions with amelogenin (70) but shows relatively strong affinity for binding to hydroxyapatite (71). This part of enamelin accumulates to relatively high levels because its extensive post-translational modifications make it resistant to further degradation by MMP-20 (72), which is the predominant proteolytic activity in secretory stage enamel (73).…”

Section: Discussionmentioning

confidence: 99%

Enamel Defects and Ameloblast-specific Expression in Enam Knock-out/lacZ Knock-in Mice

Smith

et al. 2008

151

193

Enamelin is critical for proper dental enamel formation, and defects in the human enamelin gene cause autosomal dominant amelogenesis imperfecta. We used gene targeting to generate a knock-in mouse carrying a null allele of enamelin (Enam) that has a lacZ reporter gene replacing the Enam translation initiation site and gene sequences through exon 7. Correct targeting of the transgene was confirmed by Southern blotting and PCR analyses. No enamelin protein could be detected by Western blotting in the Enam-null mice. Histochemical 5-bromo-4-chloro-3-indolyl-␤-D-galactopyranoside (X-gal) staining demonstrated ameloblast-specific expression of enamelin. The enamel of the Enam ؉/؊ mice was nearly normal in the maxillary incisors, but the mandibular incisors were discolored and tended to wear rapidly where they contacted the maxillary incisors. The Enam ؊/؊ mice showed no true enamel. Radiography, microcomputed tomography, and light and scanning electron microscopy were used to document changes in the enamel of Enam ؊/؊ mice but did not discern any perturbations of bone, dentin, or any other tissue besides the enamel layer. Although a thick layer of enamel proteins covered normal-appearing dentin of unerupted teeth, von Kossa staining revealed almost a complete absence of mineral formation in this protein layer. However, a thin, highly irregular, mineralized crust covered the dentin on erupted teeth, apparently arising from the formation and fusion of small mineralization foci (calcospherites) in the deeper part of the accumulated enamel protein layer. These results demonstrate ameloblast-specific expression of enamelin and reveal that enamelin is essential for proper enamel matrix organization and mineralization.