THE MACROMOLECULAR ORGANIZATION OF DENTINE MATRIX COLLAGEN. II. PERIODATE DEGRADATION AND CARBOHYDRATE CROSS-LINKING

The unique biomineralization transcriptome and proteome of Lytechinus variegatus teeth

BACKGROUND

Matrix-regulated biomineralization involves the specific nucleation and growth of mineral phases within or upon preformed structured organic matrices. We hypothesized that there might be a general mechanism whereby anionic, phosphorylated mineral ion-binding proteins assist in specifically locating the mineral ions with respect to the mineralizing structural organic matrix. Here we extended these studies to invertebrate mineralization in Lytechinus variegatus (Lv) teeth.

MATERIALS AND METHODS

The tooth proteins were extracted and the phosphoproteins occluded in the mineral were enriched by passage through a ProQ Diamond phosphoprotein enrichment column, and subjected to MS/MS analysis. A Lv RNA-seq derived transcriptome database was generated. The MS/MS data found 25 proteins previously classified as "Predicted uncharacterized proteins" and many of the spicule matrix proteins. As these 25 proteins were also identified with the transcriptome analysis, and were thus no longer "hypothetical" but real proteins in the Lv tooth. Each protein was analyzed for the presence of a signal peptide, an acidic pI≤4, and the ability to be phosphorylated.

RESULTS

Four new Lv tooth specific Pro-Ala-rich proteins were found, representing a new class of proteins.

CONCLUSION

The tooth is different from the spicules and other urchin skeletal elements in that only the tooth contains both "high" and "very high" magnesium calcite, [Ca(1-X) Mg(X) CO3], where X is the mole fraction of Mg. We speculate that our newly discovered proline-alanine rich proteins, also containing sequences of acidic amino acids, may be involved in the formation of high magnesium and very high magnesium calcite.

Biomineralization mechanisms: a new paradigm for crystal nucleation in organic matrices

There is substantial practical interest in the mechanism by which the carbonated apatite of bone mineral can be initiated specifically in a matrix. The current literature is replete with studies aimed at mimicking the properties of vertebrate bone, teeth, and other hard tissues by creating organic matrices that can be mineralized in vitro and either functionally substitute for bone on a permanent basis or serve as a temporary structure that can be replaced by normal remodeling processes. A key element in this is mineralization of an implant with the matrix and mineral arranged in the proper orientations and relationships. This review examines the pathway to crystallization from a supersaturated calcium phosphate solution in vitro, focusing on the basic mechanistic questions concerning mineral nucleation and growth. Since bone and dentin mineral forms within collagenous matrices, we consider how the in vitro crystallization mechanisms might or might not be applicable to understanding the in vivo processes of biomineralization in bone and dentin. We propose that the pathway to crystallization from the calcium phosphate-supersaturated tissue fluids involves the formation of a dense liquid phase of first-layer bound-water hydrated calcium and phosphate ions in which the crystallization is nucleated. SIBLING proteins and their in vitro analogs, such as polyaspartic acids, have similar dense liquid first-layer bound-water surfaces which interact with the dense liquid calcium phosphate nucleation clusters and modulate the rate of crystallization within the bone and dentin collagen fibril matrix.

Use of a specific MMP-inhibitor (galardin) for preservation of hybrid layer

OBJECTIVE

Dentinal MMPs have been claimed to contribute to the auto-degradation of collagen fibrils within incompletely resin-infiltrated hybrid layers and their inhibition may, therefore, slow the degradation of hybrid layer. This study aimed to determine the contribution of a synthetic MMPs inhibitor (galardin) to the proteolytic activity of dentinal MMPs and to the morphological and mechanical features of hybrid layers after aging.

METHODS

Dentin powder obtained from human molars was treated with galardin or chlorhexidine digluconate and zymographically analyzed. Microtensile bond strength was also evaluated in extracted human teeth. Exposed dentin was etched with 35% phosphoric acid and specimens were assigned to (1) pre-treatment with galardin as additional primer for 30s and (2) no pre-treatment. A two-step etch-and-rinse adhesive (Adper Scotchbond 1XT, 3M ESPE) was then applied in accordance with manufacturer's instructions and resin composite build-ups were created. Specimens were immediately tested for their microtensile bond strength or stored in artificial saliva for 12 months prior to being tested. Data were evaluated by two-way ANOVA and Tukey's tests (alpha=0.05). Additional specimens were prepared for interfacial nanoleakage analysis under light microscopy and TEM, quantified by two independent observers and statistically analyzed (chi(2) test, alpha=0.05).

RESULTS

The inhibitory effect of galardin on dentinal MMPs was confirmed by zymographic analysis, as complete inhibition of both MMP-2 and -9 was observed. The use of galardin had no effect on immediate bond strength, while it significantly decreased bond degradation after 1 year (p<0.05). Interfacial nanoleakage expression after aging revealed reduced silver deposits in galardin-treated specimens compared to controls (p<0.05).

CONCLUSIONS

This study confirmed that the proteolytic activity of dentinal MMPs was inhibited by the use of galardin in a therapeutic primer. Galardin also partially preserved the mechanical integrity of the hybrid layer created by a two-step etch-and-rinse adhesive after artificial aging.

Host-derived loss of dentin matrix stiffness associated with solubilization of collagen

Matrix metalloproteinases (MMPs) bound to dentin matrices are activated during adhesive bonding procedures and are thought to contribute to the progressive degradation of resin-dentin bonds over time. The purpose of this study was to evaluate the changes in mechanical, biochemical, and structural properties of demineralized dentin treated with or without chlorhexidine (CHX), a known MMP-inhibitor. After demineralizing dentin beams in EDTA or phosphoric acid (PA), the baseline modulus of elasticity (E) of each beam was measured by three-point flexure. Specimens were pretreated with water (control) or with 2% CHX (experimental) and then incubated in artificial saliva (AS) at 37 degrees C for 4 weeks. The E of each specimen was remeasured weekly and, the media was analyzed for solubilized dentin collagen at first and fourth week of incubation. Some specimens were processed for electron microscopy (TEM) immediately after demineralization and after 4 weeks of incubation. In EDTA and PA-demineralized specimens, the E of the control specimens fell (p < 0.05) after incubation in AS, whereas there were no changes in E of the CHX-pretreated specimens over time. More collagen was solubilized from PA-demineralized controls (p < 0.05) than from EDTA-demineralized matrices after 1 or 4 weeks. Less collagen (p < 0.05) was solubilized from CHX-pretreated specimens demineralized in EDTA compared with PA. TEM examination of control beams revealed that prolonged demineralization of dentin in 10% PA (12 h) did not denature the collagen fibrils.

Molecular cloning and characterization of lustrin A, a matrix protein from shell and pearl nacre of Haliotis rufescens

A specialized extracellular matrix of proteins and polysaccharides controls the morphology and packing of calcium carbonate crystals and becomes occluded within the mineralized composite during formation of the molluscan shell and pearl. We have cloned and characterized the cDNA coding for Lustrin A, a newly described matrix protein from the nacreous layer of the shell and pearl produced by the abalone, Haliotis rufescens, a marine gastropod mollusc. The full-length cDNA is 4,439 base pairs (bp) long and contains an open reading frame coding for 1,428 amino acids. The deduced amino acid sequence reveals a highly modular structure with a high proportion of Ser (16%), Pro (14%), Gly (13%), and Cys (9%). The protein contains ten highly conserved cysteine-rich domains interspersed by eight proline-rich domains; a glycine- and serine-rich domain lies between the two cysteine-rich domains nearest the C terminus, and these are followed by a basic domain and a C-terminal domain that is highly similar to known protease inhibitors. The glycine- and serine-rich domain and at least one of the proline-rich domains show sequence similarity to proteins of two extracellular matrix superfamilies (one of which also is involved in the mineralized matrixes of bone, dentin, and avian eggshell). The arrangement of alternating cysteine-rich domains and proline-rich domains is strikingly similar to that found in frustulins, the proteins that are integral to the silicified cell wall of diatoms. Its modular structure suggests that Lustrin A is a multifunctional protein, whereas the occurrence of related sequences suggest it is a member of a multiprotein family.

Phosphorylation of the proteins of the extracellular matrix of mineralized tissues by casein kinase-like activity

The extracellular matrix of the connective tissue contains non-collagenous proteins (NCP) which are acidic in character. The NCP of mineralizing systems (bone, dentin) differ from those of the non-mineralizing systems (skin, tendon) in that the mineralized tissue NCP are frequently phosphorylated. The phosphorylated proteins have been implicated in various aspects of the mineralization process. Thus, it is of interest to consider the mechanism and regulation of phosphorylation of the major matrix NCP. The majority of the phosphorylation takes place at Ser or Thr residues embedded within acidic sequences, and therefore are targets for casein kinase I (CK1) or casein kinase II (CK2)-like kinases. CK1 and CK2 are distantly related members of the protein kinase family. They are ubiquitous, constitutively active, second-messenger-independent kinases. CK1 is found in a variety of isoforms, all homologous to the alpha-subunit of the protein kinase family. It acts as a monomer. The active form of CK2 is a tetrameric holoenzyme, with 2 alpha catalytic subunits and 2 beta regulatory subunits. The CK2 alpha has activity alone, but the holoenzyme is four- to five-fold that activity. CK2 can use either ATP or GTP as the phosphate donor, but CK1 can use only ATP. The CK2 activity which phosphorylates the mineralized tissue NCP appears to be localized to membrane-associated cell fractions, and is present in the endoplasmic reticulum and Golgi compartments in osteoblasts, where phosphorylation of the secreted proteins appears to take place as co- and post-translational processes. Data indicate that both alpha and beta subunits of the membrane-associated CK2 are isoforms of the cytosolic CK2 in the same cells. The CK1 has not been specifically localized. Studies of dephosphorylated NCP such as phosphophoryn (PP) have shown that CK1 will not phosphorylate dephosphorylated dPP unless prior phosphorylation with CK2 has been carried out. In turn, CK2 activity may be initiated only after an initial phosphorylation of one of the messenger-dependent kinases. Thus, the phosphorylation reactions in mineralized tissues may be a tightly regulated hierarchical or sequential cascade of intracellular phosphorylation events.

Mechanism of calcification: role of collagen fibrils and collagen-phosphoprotein complexes in vitro and in vivo

Samples of decalcified chicken bone together with varying concentrations of phosphoproteins from bone or egg yolk (phosvitin) were used in vitro as heterogenous nucleators for the induction of Ca-P apatite crystals. The lag time between exposure of the collagen-phosphoprotein complexes and the time nucleation of crystals occurred decreased as the concentration of Ser(P) and Thr(P) increased. Enzymatic cleavage of the phosphate groups by wheat germ and phosphatase reversed this effort, indicating that the phosphate group per se principally facilitated the nucleation of Ca-P crystals by the phosphoprotein complex and collagen.

On the problem of covalent linkages between phosphoproteins and collagen in bovine dentin and bone

The majority of phosphoproteins in bovine bone and dentin are insoluble in EDTA and guanidine hydrochloride (Gu.HCl) at 2 degrees C. After removal of EDTA and Gu.HCl-soluble proteins at 2 degrees C, collagen alpha-chains and alpha-chain polymers were extracted from bovine bone and dentin in Gu.HCl at elevated temperatures and purified by several chromatographic techniques and SDS-PAGE. Small amounts of O-phosphoserine were found in all collagen components. In contrast, O-phosphoserine was not detected in the purified collagen components soluble in EDTA or Gu.HCl at 2 degrees C nor was hydroxyproline detected in the EDTA-soluble phosphoproteins. In contrast, although the vast majority of EDTA-insoluble collagen and phosphoprotein molecules can be readily dissociated by a variety of molecular sieving and ion-exchange chromatographic procedures, a small number are very strongly associated or covalently cross-linked. These results are consistent with the findings that both hydroxyproline and hydroxylysine are present in purified phosphoprotein components released from the EDTA-insoluble tissue by bacterial collagenase. The hydroxylysine/100 hydroxyproline ratios in the phosphoprotein-collagen complexes are much higher than those in dentin or bone collagens.

Phosphoprotein modulation of apatite crystallization

Several phosphoprotein preparations (phosvitin, rat incisor and fetal calf molar dentin phosphoproteins) all inhibit apatite growth/replication from pre-existing crystal seeds in metastable solutions. Two stages of the crystal growth process were inhibited by these phosphoproteins. First an initial lag period was induced, probably associated with seed surface phenomena. This period was prolonged indefinitely when a combination of phosphoprotein precoated seeds was used together with soluble phosphoproteins in the crystal growth reaction. Second, the phosphoproteins prolonged that stage of the reaction where octacalcium phosphate is the predominant mineral phase present prior to its conversion to the final apatite product. Pre-treatment of the phosphoproteins with calcium diminished their inhibitory activity to seeded crystal growth as well as towards de novo apatite formation in synthetic extracellular fluids. The presence of collagen diminished the inhibitory activity of the phosphoproteins towards de novo precipitation but had no effect on phosphoprotein-modulated apatite crystal growth in the seeded systems. These results suggest a potential regulatory role for phosphoproteins in dentin mineralization.

The nature of covalent complexes of phosphoproteins with collagen in the bovine dentin matrix

The bovine dentin matrix still contains some noncollagenous proteins after thorough extraction and decalcification. These have been obtained following digestion of the matrix by cyanogen bromide. Peptides containing non-collagenous portions were isolated by chromatography on diethylaminoethyl cellulose columns and fractionated on hydroxyapatite columns. Several fractions were obtained. The principal component was a complex between a highly-phosphorylated serine-aspartic acid-rich protein and a collagen peptide. These collagenous and non-collagenous moieties could not be separated from each other even under highly dissociative solvent conditions. After digestion with collagenase, the resulting phosphoprotein fraction still contained a few residues of hydroxyproline and hydroxylysine, and an enhanced content of proline, compared to the equivalent directly extractable phosphophoryn of the matrix. These data were interpreted as indicating that the phosphophoryn which is not extractable in 0.5M ethylenediaminetetraacetic acid is in fact covalently bound to some specific section of the matrix collagen. The covalent modification of the collagen matrix with highly acidic phosphoproteins may have an important role in the mineralization process.

Dentin proteins: chemistry, structure and biosynthesis

One of the most abundant noncollagenous proteins of dentin is a phosphoprotein rich in aspartic acid and phosphoserine. This protein occurs in soluble and inextractable forms, the latter being associated with the insoluble collagenous matrix. This protein is capable of tightly binding a relatively high level of calcium. Biosynthetic and radioautographic data suggest that shortly after its biosynthesis, the phosphoprotein is transported and bound to the collagen at the predentin-dentin junction. This event is probably central to the mineralization process, though other glycoproteins may be involved.

Type-I trimer and type-I collagen in neutral-salt-soluble lathyritic-rat dentine

Triple-helical collagen molecules have been obtained from EDTA-demineralized lathyritic rat incisors by neutral buffer extraction. Component alpha chains, isolated by sequential ion-exchange and gel-filtration chromatography, were shown to be alpha1 I and alpha2 chains by cyanogen bromide peptide analysis. The alpha1 I:alpha2 chain ratio was approximately 3:1, which is greater than expected for type I collagen. The excess of alpha1 I chains over that required for type I collagen was due to the presence of type I trimer molecules. Fractional salt precipitation separated type I collagen from type I trimer. It is not known at present if type I trimer synthesis also occurs in normal rat tissues.

The identity of a cyanogen bromide fragment of bovine dentin collagen containing the site of an intermolecular cross-link

A peptide fraction isolated from a cyanogen bromide digest of bovine dentin collagen had a molecular weight of 46000. Its size and amino acid composition indicated that it could not consist of peptides derived from the cleavage of a single alpha chain. On reduction with tritiated sodium borohydride, radioactivity was incorporated primarily into 5, 5'-dihydroxylysinonorleucine without degradation at the peptide backbone. Periodate cleavage of the reduced or nonreduced peptide fraction generated one fragment of molecular weight 28000 and one of 18000 completely accounting for the size of the parent peptide. On amino acid analysis the constituent single-chain peptides were determined to be alpha2CB4 and alpha1CB6. Both peptides isolated after periodate oxidation of the tritiated borohydride reduced cross-link peptide were found to contain (3H)hydroxynorvaline. These data show that some hydroxylysine of alpha2CB4, a helical region peptide, was present in aldehyde form and could act as the aldehyde donor icross-link, Schiff's base formation. The only cross-linkage of this alpha2CB4 acting as an aldehyde donor peptide to alpha1CB6 would be a helical region to helical region bond, perhaps accounting for the unusual stability and low solubility of dentin collagen.

The extraction of phosphoproteins from bovine dentin

The phosphoprotein obtained by the neutral pH tris buffer extraction of acetic acid demineralized bovine dentin has been compared with the phosphoprotein extracted directly during the neutral pH EDTA demineralization process. The phosphoproteins isolated by DEAE-cellulose chromatography from the neutral pH EDTA demineralization extract are not identical to those isolated by the same procedure from the dentin which had been subjected to acid demineralization. The two demineralization procedures yield phosphoproteins different in amino acid content and in presence of 260 nm UV absorbing moiety. Even after sequential acid demineralization, trisbuffer extraction and EDTA extraction, the residual dentin contains phosphoprotein. A peptide fragment containing both collagen and phosphoprotein moieties has been isolated following digestion and cleavage of the insoluble dentin collagen with cyanogen bromide. The acid demineralization process appears to be accompanied by degradation which removes both protein and non-protein components from the phosphoprotein.

Peptide mapping of partial acid hydrolysates of human dentin and bovine skin collagens by thin-layer methods

Human dentin and bovine skin collagens, partially hydrolyzed with acid, were compared by peptide mapping on thin-layer plates. The patterns, which showed 20 to 25 spots, were similar for the two types of collagen. No major differences in the structural characteristics of dentin and skin collagens can be predicted from these results.