Bound phosphoproteins enhance mineralization of alkaline phosphatase-collagen complexes in vivo

Bioinorganics and biomaterials: bone repair

The field of bioinorganics is well established in the development of a variety of therapies. However, their application to bone regeneration, specifically by way of localized delivery from functional implants, is in its infancy and is the topic of this review. The toxicity of inorganics is species, dose and duration specific. Little is known about how inorganic ions are effective therapeutically since their use is often the result of serendipity, observations from nutritional deficiency or excess and genetic disorders. Many researchers point to early work demonstrating a role for their element of interest as a micronutrient critical to or able to alter bone growth, often during skeletal development, as a basis for localized delivery. While one can appreciate how a deficiency can cause disruption of healing, it is difficult to explain how a locally delivered excess in a preclinical model or patient, which is presumably of normal nutritional status, can evoke more bone or faster healing. The review illustrates that inorganics can positively affect bone healing but various factors make literature comparisons difficult. Bioinorganics have the potential to have just as big an impact on bone regeneration as recombinant proteins without some of the safety concerns and high costs.

Glucuronic acid and phosphoserine act as mineralization mediators of collagen I based biomimetic substrates

Glucuronic acid (GlcA) and phosphoserine (pS) carrying acidic functional groups were used as model molecules for glycosaminoglycans and phosphoproteins, respectively to mimic effects of native biomolecules and influence the mineralization behaviour of collagen I. Collagen substrates modified with GlcA showed a stable interaction between GlcA and collagen fibrils. Substrates were mineralized using the electrochemically assisted deposition (ECAD) in a Ca(2+)/H( x )PO (4) ((3-x)) electrolyte at physiological pH and temperature. During mineralization of collagen-GlcA matrices, crystalline hydroxyapatite (HA) formed earlier with increasing GlcA content of the collagen matrix, while the addition of pS to the electrolyte succeeded in inhibiting the transformation of preformed amorphous calcium phosphate (ACP) to HA. The lower density of the resulting mineralization and the coalesced aggregates formed at a certain pS concentration suggest an interaction between calcium and the phosphate groups of pS involving the formation of complexes. Combining GlcA-modified collagen and pS-modified electrolyte showed dose-dependent cooperative effects.

Enzyme Directed Templating of Artificial Bone Mineral

Bone is one of Nature's most remarkable materials, not only for its mechanical properties but also for its ability to repair fractures and remodel its microstructure in response to stress. At the nanoscale bone is a supramolecular matrix of collagen fibers reinforced by hydroxyapatite crystals with a high degree of order. Emulating elements of the biological synthesis of this composite could help develop strategies for advanced materials. Previous work has demonstrated the use of functionalized peptide amphiphile nanofibers in a two-dimensional system to emulate hydroxyapatite mineralization in natural bone. We describe here an artificial, in vitro biomineralization process that allows a similar process to occur in three dimensions. The system employs the natural enzyme alkaline phosphatase and a phosphorylated, anionic nanofiber gel matrix to template hydroxyapatite nanocrystals with size, shape, and crystallographic orientation resembling natural bone mineral. The formation of this biomimetic mineral in three dimensions results from the synergy of fiber-induced nucleation and the temporal control of phosphate ion harvesting by the enzyme. Gradual enzymatic harvesting of ions for crystal growth and the strong nucleating ability of the phosphorylated fibers suppresses uncontrolled precipitation of mineral. The strategy could lead to biomimetic materials to promote bone regeneration or the synthesis of hybrid materials with crystallographically defined structures.

Alkaline phosphatase-induced mineral deposition to anchor collagen fibrils to a solid surface

Reconstruction of tendon and ligament tissues requires proper attachment of the tissue-engineered construct to surrounding tissues. A problem of reconstructing collagen-rich tissues is that an in vitro engineered collagenous network containing fibroblasts will contract and detach from a solid surface. In vivo anchorage of soft connective tissues to mineralized tissues like bones and teeth is accomplished by embedding collagen fibrils into mineralized layers. Mineralization is partially the result of local activity of the enzyme alkaline phosphatase (ALP). In this study, we tested whether ALP-induced mineral deposition at the interface between a collagen gel and a polystyrene or polyetheretherketone (PEEK) surface could prevent gel detachment from the surface. Coating of culture wells with intestinal ALP prevented detachment of gels harbored with human periodontal ligament (PDL) fibroblasts in the presence of its substrate beta-glycerophosphate. Mineral deposition was observed predominantly at the interface of collagen gel and well surface. The contractile properties of fibroblasts were not influenced by either ALP, beta-glycerophosphate or both. The presence of ALP on a solid surface and providing its substrate to allow mineral deposition can prevent detachment of collagen matrices. Our findings provide a tool to induce attachment of fibrillar collagen to a solid surface; an approach that seems useful for reconstruction of load-bearing tissues and attachment of ligaments to implants.

Adsorption and interactions of dentine phosphoprotein with hydroxyapatite and collagen

Dentine phosphoprotein (DPP) has been proposed to both promote and inhibit mineral deposition during dentinogenesis. The present study aimed to investigate the molecular interactions of DPP and dephosphorylated DPP (DPP-p) with hydroxyapatite (HAP). Bovine DPP was purified and dephosphorylated by alkaline phosphatase to obtain DPP-p. DPP and DPP-p adsorption to HAP was determined along with their ability, when free in solution or bound to collagen, to influence HAP-induced crystal growth. Absorption isotherms suggested that lower DPP concentrations (1.5-6.25 microg ml(-1)) demonstrated a reduced affinity for HAP compared with higher protein concentrations (12.5-50.0 microg ml(-1)). Dephosphorylated DPP had a much reduced affinity for HAP compared with DPP. Dentine phosphoprotein inhibited seeded HAP crystal growth, in a dose-dependent manner, whilst removal of the phosphate groups reduced this inhibition. When bound to collagen fibrils, DPP significantly promoted the rate of HAP crystal growth over 0-8 min. Conversely, DPP-p and collagen significantly decreased the rate of crystal growth over 0-18 min. These results indicate a major role for the phosphate groups present on DPP in HAP crystal growth. In addition, concentration-dependent conformational changes to DPP, and the interaction with other matrix components, such as collagen, are important in predicting its dual role in the mineralization of dentine.

In vitro differentiation and mineralization of human dental pulp cells induced by dentin extract

In this study, the progenitor cells isolated from the human dental pulp were used to study the effects of ethylenediaminetetraacetic acid-soluble dentin extract (DE) on their differentiation and mineralization to better understand tissue injury and repair in the tooth. Mineralization of the matrix was increasingly evident at 14, 21, and 28 d after treatment with a mineralization supplement (MS) (ascorbic acid [AA], beta-glycerophosphate [beta-GP]) and MS + DE. Real-time polymerase chain reaction results showed type I collagen upregulation after the addition of MS + DE at 7 d. Alkaline phosphatase was downregulated after the mineralization became obvious at 14 d. Bone sialoprotein was shown to be upregulated in the mineralized cell groups at all time points and dentin sialophosphoprotein after 7 d. Core binding factor a 1 was upregulated by the treatment of MS and DE at 7, 14, and 21 d. These results indicated that the MS of AA, beta-GP, and DE synergistically induced cell differentiation of pulp progenitor cells into odontoblast-like cells and induced in vitro mineralization.

Fluoride alters casein kinase II and alkaline phosphatase activity in vitro with potential implications for dentine mineralization

Dentine phosphoprotein (DPP), a major non-collagenous acidic protein of dentine, undergoes altered phosphorylation in vivo in the presence of high fluoride concentrations. This has major implications for the altered mineralization patterns found during fluorosis. In dentine, casein kinase II is involved in phosphorylating DPP, and alkaline phosphatase (ALP) is ascribed roles in the dephosphorylation of DPP, increasing the inorganic phosphate at the mineralization front and the removal of pyrophosphate. Here the influence of fluoride in vitro on the activity of purified casein kinase II and ALP and its relation to altered patterns of mineralization were examined. Kinetic analysis showed that casein kinase II activity was completely inhibited at 0.04 M NaF. Vmax when compared to the control assay was significantly decreased (P < 0.0001) between concentrations 4 x 10(-4)-4 x 10(-8) M NaF. Significant changes to the Km (P < 0.0001) were also observed. ALP activity was inhibited by NaF (0.09-9 x 10(-8) M), with Vmax significantly decreased (P < 0.0001) at 0.09 M NaF. Alterations in the activity of these enzymes in the presence of fluoride may in part explain the decreased phosphorylation observed in DPP isolated from fluorotic dentine and may aid understanding of the altered matrix mediated mineralization patterns found during fluorosis.

Blood circulation as source for osteopontin in acellular extrinsic fiber cementum and other mineralizing tissues

Osteopontin (OPN) is one of the major non-collagenous proteins in root cementum and other mineralized tissues. Although most of this mineral-seeking protein is thought to be produced by local tissue cells, some of it might enter the mineralizing matrix from the blood. To test this hypothesis, we followed the distribution of a single dose of purified porcine or rat 125I-labeled OPN injected i.v. in rats, in mineralizing and non-mineralizing tissues and in subcutaneously implanted collagenous implants. The animals were killed 30 or 48 hrs after injection. Tissues (calvaria, tibia, lower and upper jaws) were harvested and processed for radioautography and biochemical analysis. Tissues as well as calcifying collagenous implants proved to have taken up radiolabel. In EDTA extracts of long bones, the majority of the radiolabel was demonstrated to be associated with intact OPN. The iodinated protein was also found in the acellular extrinsic fiber cementum (acellular cementum) layer investing the continuously growing incisors, in laminae limitantes, cement lines, and in forming bone near the mineralization front. Further, the label was present in the circumpulpal dentin of the incisors, and some of it appeared to have been incorporated into developing enamel. It is concluded that OPN in acellular cementum and other mineralizing tissues may-at least partially-originate from sources outside the direct environment following its transportation via serum.

Association of matrix acid and alkaline phosphatases with mineralization of cartilage and endochondral bone

The activities of acid and alkaline phosphatases were localized by enzyme histochemistry in the chondroepiphyses of 5 week old rabbits. Using paraformaldehyde-lysine-periodate as fixative, the activity of acid phosphatase was particularly well preserved and could be demonstrated not only in osteoclasts, but also in chondrocytes as well as in the cartilage and early endochondral matrices. The acid phosphatase in the chondrocytes and the matrix was tartrate-resistant, but inhibited by 2 mM sodium fluoride, whereas for osteoclasts 50-100 mM sodium fluoride were required for inhibition. Simultaneous localisation of both acid and alkaline phosphatase activities was possible in tissue that had been fixed in 85% ethanol and processed immediately. In the growth plates of the secondary ossification centre and the physis, there was a sequential localisation of the two phosphatases associated with chondrocyte maturation. The matrix surrounding immature epiphyseal chondrocytes or resting/proliferating growth plate chondrocytes contained weak acid phosphatase activity. Maturing chondrocytes were positive for alkaline phosphatase which spread to the matrix in the pre-mineralizing zone, in a pattern that was consistent with the known location of matrix vesicles. The region of strong alkaline phosphatase activity was the precise region where acid phosphatase activity was reduced. With the onset of cartilage calcification, alkaline phosphatase activity disappeared, but strong acid phosphatase activity was found in close association with the early mineral deposition. Acid phosphatase activity was also present in the matrix of the endochondral bone, but was only found in early spicules which had recently mineralised. The results suggest that alkaline phosphatase activity is required in preparation of mineralization, whereas acid phosphatase activity might have a contributory role during the early progression of mineral formation.

Mineralization of alkaline phosphatase-complexed collagen implants in the rat in relation to serum inorganic phosphate

The present study was designed to determine the relationship between mineralization of collagenous matrices and serum levels of calcium and inorganic phosphate. Collagen slices were prepared from bovine dentin or cortical bone and complexed with varying amounts of intestinal alkaline phosphatase (ALP). The enzyme was added to induce de novo mineralization. The ALP-complexed slices were implanted subcutaneously over the skull and in the dorsolateral aspect of the abdominal wall in female Wistar rats of various ages (5-, 10-, 20-, or 35-week-old) and in young male rats fed on a low-P diet. After 1-4 weeks, the implants were removed and analyzed for calcium and phosphate content. In addition, serum levels of calcium and phosphate (total and inorganic) were determined. It was shown that the highest mineral influx occurred in the younger rats (which were also highest in serum P(i)), whereas almost no mineral uptake occurred in the older ones. Also in rats fed on a low-P diet (which were low in serum P(i), a strongly decreased mineral influx was noted. In all animal groups a positive correlation was found between the degree of mineralization and serum P(i). No distinct relationship was found between serum Ca/organic phosphate levels and mineral influx in the implants. In vitro incubation of ALP-collagen conjugates in serum from younger and older rats confirmed our view that serum P(i), besides local levels of ALP, is important in de novo mineral deposition. For accretion of mineral in partially remineralized collagenous carriers, ALP activity was not required.

Mineralization of alkaline phosphatase-complexed collagenous implants in the rat: relation with age, sex, and site of implantation

This study was designed to determine the effects of age, sex, and site of implantation on the extent of alkaline phosphatase-complexed collagen sheets mineralization in the animal body. Collagen sheets were prepared from bovine dentin and cortical bone and complexed with varying amounts of intestinal alkaline phosphatase (ALP). Controls were without enzyme or with heat-inactivated enzyme. Sheets were implanted subcutaneously over the skull and in the dorsolateral abdominal wall in 5- or 20-week-old male and female Wistar rats. After 2-3 weeks the implants were removed and analyzed for phosphate and calcium content. Our findings have shown that alkaline phosphatase-induced mineralization of collagenous implants is influenced to a considerable extent by age, sex, and site of implantation. Highest mineral influx was seen in the younger males. Implants in younger females and older males contained less mineral, whereas those installed in the older females were almost free of calcium phosphate deposits. Dentinal implants in the skull region contained more mineral than those in the abdominal wall.