Comparative studies of phosphoprotein preparations from rat incisor dentin

Protocols for Studying Formation and Mineralization of Dental Tissues In Vivo: Extraction Protocol for Isolating Dentin Matrix Proteins from Developing Teeth

The organic material in developing dentin is 90% type I collagen and 10% non-collagenous proteins. The key to understanding dentin biomineralization is to study how these proteins collectively precipitate and organize hydroxyapatite crystals. The first step in characterizing the proteins within a mineralizing matrix is to efficiently extract and isolate the essential molecular participants and elucidate their structural and biochemical properties. In this study, we expanded previous approaches to develop an improved strategy for the extraction of extracellular matrix proteins from the dentin of developing teeth. Proteins in dentin powder were sequentially extracted in the order Tris-guanidine buffer, HCl-formic acid solution, acetic acid-NaCl solution, Tris-NaCl buffer, and a second Tris-guanidine buffer. Individual fractions were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), by gelatin or casein zymography, and by Western blot analysis using dentin sialoprotein (DSP)- or dentin glycoprotein (DGP)-specific antibodies. This approach was used to purify assorted porcine dentin non-collagenous proteins.

Characteristics of phosphorylated and non-phosphorylated dentine phosphoprotein

Heterogeneity among the odontoblast-specific, highly phosphorylated acidic protein dentine phosphoprotein (DPP) obtained from different species has been reported by several investigators. In the present study, the apparent molecular-mass variations in rabbit and mouse DPP were investigated. Extracellular matrix (ECM) DPPs were isolated and characterized. Primary gene products, before post-translational phosphorylation, were analysed based upon translation products produced in a rabbit reticulocyte lysate cell-free system using a polyclonal mouse anti-DPP antibody. Nascent non-phosphorylated DPPs were also identified from intracellular protein extracts. Mouse and rabbit ECM phosphoproteins exhibited a 10 kDa difference in size. However, nascent intracellular or translation products from both species showed the same lower molecular mass (approx. 45 kDa). Furthermore, Northern-blot analysis showed a single mRNA of the same size in both species (approx. 1.6 kb) which contains information for a protein no larger than 50 kDa. Our results indicate that the difference in molecular mass (or electrophoretic behaviour) among DPPs from different species is due to post-translational modifications, in this case phosphorylation.

Separation of non-collagenous proteins of bone formed in titanium implants: experimental study in the rabbit tibia, using a bone harvest chamber

The non-collagenous proteins of mature cortical rabbit bone and 3 wk old bone specimens formed in titanium implant bone harvest chambers, were separated by means of fine pressure liquid chromatography. It was found that a sufficient amount of bone could be obtained from five bone harvest chambers in five rabbits during a period of 15 wk. A different protein chromatographic pattern was found in bone grown in bone harvest chambers as compared to normal cortical bone from the same region. This indicates a difference in bone quality and shows that this implant design can be used for biochemical analysis of bone formed adjacent to a biomaterial.

Production and characterization of antibodies against murine dentine phosphoprotein

Experiments were designed to produce and characterize a polyclonal antibody directed against mouse dentine phosphoprotein, the major non-collagenous protein of the dentine extracellular matrix. Dental extracellular matrix proteins from 2-day-postnatal Swiss-Webster-mouse tooth organs were extracted with 0.5 M-acetic acid, followed by 4 M-guanidinium chloride/0.5 M-EDTA. Mouse dentine phosphoprotein yields were further increased by precipitation with 1 M-CaCl2. Final purification was achieved by excising and eluting dentine phosphoprotein polypeptide bands from preparative sodium dodecyl sulphate/urea/polyacrylamide gels. Mouse dentine phosphoprotein is a single component of approx. 72 kDa and has a characteristic amino acid composition of 33% aspartic acid and 55% serine/phosphoserine. A polyclonal antibody was raised in rabbits against purified mouse dentine phosphoprotein and was shown to be monospecific by enzyme-linked immunoabsorbent, dot-immunobinding and 'Western transfer' assays. This antibody was used to detect the expression and localization of dentine phosphoprotein in 1-day-postnatal mouse tooth organs. This antigen was localized intracellularly within the monolayer of odontoblasts, which line the perimeter of the dental papilla mesenchyme, and within the odontoblastic cell processes, which traverse the predentine matrix. Newly forming mineralized dentine matrix was also cross-reactive with the dentine phosphoprotein specific antibody. The non-mineralized predentine matrix did not contain any detectable cross-reactive antigens.

Disagreement in molecular weight determinations of dentin phosphoprotein

The molecular weight of highly phosphorylated phosphoprotein from rat incisor dentin was estimated by three different methods: sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, gel chromatography in 6 M guanidinium chloride (GdnHCl) and analytical ultracentrifugation at high salt concentration. SDS-polyacrylamide gel electrophoresis revealed an abnormal migration of the phosphoprotein and its enzymatically dephosphorylated derivative in comparison to standard proteins. Depending on the acrylamide concentration in the gel (7.5-20%), the apparent molecular weight of the phosphoprotein varied from 90,000 to 27,000. Similar results were obtained for the dephosphorylated protein. In gel chromatography using 6 M GdnHCl as eluent the phosphoprotein had an elution volume corresponding to that of a standard protein with a molecular weight of 67,000, while the enzymatically dephosphorylated phosphoprotein showed an apparent molecular weight of 30,000. The phosphoprotein behaved non-ideally in equilibrium sedimentation runs. The apparent molecular weight was strongly concentration-dependent and the extrapolation to zero concentration was uncertain. However, after enzymatic dephosphorylation the concentration dependence disappeared and a molecular weight of 28,000 could be calculated. Since the phosphate groups represent 26% of the phosphoprotein by weight, the true molecular weight of the highly phosphorylated phosphoprotein components from rat incisor dentin should be 38,000. The study shows that it is not possible directly to estimate the molecular weight of this type of protein by standard methods elaborated for normal globular proteins.

Non-collagenous proteins of rat dentin. Evidence that phosphoprotein is not covalently bound to collagen

The non-collagenous proteins of rat dentin that remain firmly bound to the matrix after demineralization were studied in order to ascertain if they are covalently linked to insoluble dentin collagen. After solubilization with CNBr or with bacterial collagenase, unusually small amounts of dentin phosphoprotein were detected in the matrix. The phosphoprotein obtained by CNBr digestion of the matrix was separated from collagen peptides using two chromatographic steps. Thus even this small quantity of phosphoprotein found in decalcified rat dentin matrix was not covalently bound to collagen.