Identification of a fibronectin interaction site in the extracellular matrix protein ameloblastin |
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Authors: | Michael Beyeler Roman Lutz Beat Trueb |
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Institution: | a Department of Clinical Research, University of Bern, Murtenstrasse 35, 3010 Bern, Switzerland b Friedrich Miescher Institute for Biomedical Research, Novartis Research Foundation, Maulbeerstrasse 66, 4058 Basel, Switzerland c Department of Orthodontics and Dentofacial Orthopedics, University of Bern, 3010 Bern, Switzerland d Department of Rheumatology, University Hospital, 3010 Bern, Switzerland |
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Abstract: | Mammalian teeth are composed of hydroxyapatite crystals that are embedded in a rich extracellular matrix. This matrix is produced by only two cell types, the mesenchymal odontoblasts and the ectodermal ameloblasts. Ameloblasts secrete the enamel proteins amelogenin, ameloblastin, enamelin and amelotin. Odontoblasts secrete collagen type I and several calcium-binding phosphoproteins including dentin sialophosphoprotein, dentin matrix protein, bone sialoprotein and osteopontin. The latter four proteins have recently been grouped in the family of the SIBLINGs (small integrin-binding ligand, N-linked glycoproteins) because they display similar gene structures and because they contain an RGD tripeptide sequence that binds to integrin receptors and thus mediates cell adhesion.We have prepared all the other tooth-specific proteins in recombinant form and examined whether they might also promote cell adhesion similar to the SIBLINGs. We found that only ameloblastin consistently mediated adhesion of osteoblastic and fibroblastic cells to plastic or titanium surfaces. The activity was dependent on the intact three-dimensional structure of ameloblastin and required de novo protein synthesis of the adhering cells. By deletion analysis and in vitro mutagenesis, the active site could be narrowed down to a sequence of 13 amino acid residues (VPIMDFADPQFPT) derived from exon 7 of the rat ameloblastin gene or exons 7-9 of the human gene. Kinetic studies and RNA interference experiments further demonstrated that this sequence does not directly bind to a cell surface receptor but that it interacts with cellular fibronectin, which in turn binds to integrin receptors.The identification of a fibronectin-binding domain in ameloblastin might permit interesting applications for dental implantology. Implants could be coated with peptides containing the active sequence, which in turn would recruit fibronectin from the patient's blood. The recruited fibronectin should then promote cell adhesion on the implant surface, thereby accelerating osseointegration of the implant. |
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Keywords: | AMBN ameloblastin AMELX amelogenin AMTN amelotin BSA bovine serum albumin BSP bone sialoprotein DMEM Dulbecco's modified Eagle's medium DMP1 dentin matrix protein DPP dentin phosphoprotein DSP dentin sialoprotein DSPP dentin sialophosphoprotein ECM extracellular matrix ENAM enamelin FN fibronectin OPN osteopontin PBS phosphate buffered saline SIBLING small integrin-binding ligand N-linked glycoprotein |
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