The onset of amelogenin nanosphere aggregation studied by small-angle X-ray scattering and dynamic light scattering

Proteins with predominantly hydrophobic character called amelogenins play a key role in the formation of the highly organized enamel tissue by forming nanospheres that interact with hydroxyapatite crystals. In the present investigation, we have studied the temperature and pH-dependent self-assembly of two recombinant mouse amelogenins, rM179 and rM166, the latter being an engineered version of the protein that lacks a 13 amino acid hydrophilic C-terminus. It has been postulated that this hydrophilic domain plays an important role in controlling the self-assembly behavior of rM179. By small-angle X-ray and neutron scattering, as well as by dynamic light scattering, we observed the onset of an aggregation of the rM179 protein nanospheres at pH 8. This behavior of the full-length recombinant protein is best explained by a core-shell model for the nanospheres, where hydrophilic and negatively charged side chains prevent the agglomeration of hydrophobic cores of the protein nanospheres at lower temperatures, while clusters consisting of several nanospheres start to form at elevated temperatures. In contrast, while capable of forming nanospheres, rM166 shows a very different aggregation behavior resulting in the formation of larger precipitates just above room temperature. These results, together with recent observations that rM179, unlike rM166, can regulate mineral organization in vitro, suggest that the aggregation of nanospheres of the full-length amelogenin rM179 is an important step in the self-assembly of the enamel matrix.

Association of COLIA1 Sp1 alleles with defective bone nodule formation in vitro and abnormal bone mineralization in vivo

Previous work identified a G/T polymorphism affecting a Sp1 binding site in a regulatory region of the COLIA1 gene that predisposes to osteoporotic fractures by affecting bone strength through mechanisms that are partly independent of differences in bone mineral density (BMD). To clarify the mechanisms by which COLIA1 Sp1 alleles influence bone strength we used quantitative backscattered electron imaging (qBEI) to characterize bone mineralization in biopsy samples from subjects of different COLIA1 genotype and studied the ability of osteoblast-like cells cultured from subjects of different genotypes to form mineralized bone nodules. The qBEI analysis showed a significant (P = 0.014) reduction in mineralization in bone biopsies from G/T heterozygotes (n = 6) compared with G/G homozygotes (n = 7) and a significant increase in heterogeneity of mineralization (P = 0.017). The in vitro studies showed that osteoblasts derived from G/T heterozygotes (n = 5) were significantly less able to produce mineralized bone nodules than G/G homozygotes (n = 10) at all time-points examined (P < 0.0001). We conclude that carriage of the COLIA1 Sp1 "T" allele is associated with an impaired ability of osteoblast-like cells to form mineralized bone nodules in vitro and with abnormalities of bone mineralization in vivo. This suggests that the increased bone fragility in carriers of the COLIA1 Sp1 allele may result in part from defects in bone mineralization.

Combined treatment with PTH (1-34) and OPG increases bone volume and uniformity of mineralization in aged ovariectomized rats

The combination of PTH with OPG has been proposed as a potential therapy in patients with severe osteoporosis. In the present study, we examined the bone material of aged ovariectomized (OVX) rats treated either with PTH (1-34) or OPG alone or in combination of both. The micro- and nanostructural characteristics of the mineralized bone were evaluated using quantitative backscattered electron imaging (qBEI) and small-angle X-ray scattering (SAXS). Rats (n=68) were either sham-operated or ovariectomized (OVX) at the age of 3 months, and 15 months later, OVX animals were treated either with vehicle, OPG (10 mg/kg), PTH (80 microg/kg) or a combination of both during 5.5 months. All treatments were by subcutaneous injection, 3 days per week. Secondary metaphyseal spongiosa from distal femora was assessed for mineralized bone volume (BV/TV), for the mean Ca-concentration (Camean), the width of the bone mineralization density distribution (Cawidth), as well as the average mineral particle thickness parameter (T) and the degree of alignment of the mineral particles (rho). A remarkable increase of BV/TV up to 139% (P<0.001) was observed in the PTH-treated groups independently of OPG. Camean was slightly increased (+1.7%, P<0.05) in the OPG-treated group. Cawidth was reduced (-6.4%, P<0.01, and -8.9%, P<0.001) in animals treated with OPG and PTH+OPG, respectively. In contrast, Cawidth in sham-operated rats was 16.0% (P<0.001) higher than in OVX. The T parameter was not altered in the trabecular bone within the group of treated and untreated OVX rats. However, the non-ovariectomized animals exhibited a significantly lower T value (-7.1%, P<0.01) with respect to OVX. In conclusion, qBEI and SAXS data of OVX rats suggest that PTH alone was responsible for increase of bone volume, whereas OPG positively influenced the homogeneity and density of mineralization without affecting the nanostructure of the bone material.

Two different correlations between nanoindentation modulus and mineral content in the bone–cartilage interface

The biomechanical properties of the zone of calcified cartilage (ZCC) in articulating joints are of clinical relevance due to the role ZCC plays in load transfer from cartilage to bone. To determine the micron-level mechanical properties and their correlation to mineral concentration in the ZCC, we combined nanoindentation (for micrometer level stiffness E(r) and hardness H) and quantitative back-scattered electron imaging or qBEI (for micrometer level mean calcium concentration Ca(Mean)) to study the ZCC-subchondral bone junction in 3 embedded human patellae. Nanoindentation line scans were correlated to qBEI analysis in the ZCC. The correlation between local stiffness and local mineral content was different in calcified cartilage compared to bone. The stiffness and hardness of calcified cartilage was typically lower than subchondral bone for the same mineral content. ZCC showed a wider range of variation in calcium content (1-28 wt %) compared to subchondral bone (16-26 wt %). 2D material property maps of the ZCC were generated from the mechanical-mineral correlation, showing that bands of high and low stiffness were found between the bone and tidemark, and between the ZCC and the unmineralized cartilage.

Synchrotron diffraction study of deformation mechanisms in mineralized tendon

The high stiffness and toughness of biomineralized tissues are related to the material deformation mechanisms at different levels of organization, from trabeculae and osteons at the micrometer level to the mineralized collagen fibrils at the nanometer length scale. Quantitatively little is known about the sub-micrometer deformation mechanisms under applied load. Using a parallel-fibred mineralized tissue from the turkey leg tendon as a model for the mineralized collagen fibrils, we used in situ tensile testing with synchrotron x-ray diffraction to measure the average fibril deformation with applied external strain. Diffraction peak splitting occurred at large strains, implying an inhomogeneous elongation of collagen fibrils. Scanning electron microscopy measurements lead us to conclude that the inhomogeneous mineralization in mineralized tendon is at the origin of the high fracture strain.

Normal mineralization and nanostructure of sclerotic bone in mice overexpressing Fra-1

Increased bone mass due to elevated number of active osteoblasts has been reported for transgenic mice overexpressing the transcription factor Fra-1. To explore the potential of the anabolic action of Fra-1 in treatment of osteoporosis, we examined the integrity of bone matrix generated in Fra-1 transgenic mice. Femora from Fra-1 transgenic (Fra-1 tg) and wild-type littermates were analyzed for bone mineralization density distribution (BMDD) and nanostructure using quantitative backscattered electron imaging (qBEI) and scanning small angle X-ray scattering (scanning-SAXS), respectively. For comparison, we studied mice lacking c-Fos (Fos-/-), which develop osteopetrosis because of the absence of osteoclasts. Morphometrical analysis of metaphyseal spongiosa revealed an up to 5-fold increase in bone volume for Fra-1 transgenic compared to wild type. BMDD indicated a transient lower mineralization of bone for Fra-1 transgenic at 5 and 8 weeks, which became comparable to that of wild-type mice by 8 months. The homogeneity of mineralization was not altered in the Fra-1 transgenic mice at any ages examined. However, it was strikingly reduced in Fos-/- due to an abundance of hypermineralized cartilage. The bone nanostructure did not show abnormalities in Fra-1 transgenic or Fos-/-. These results provide a rationale for the development of therapeutic applications involving Fra-1-induced bone formation.

Targeted overexpression of vitamin D receptor in osteoblasts increases calcium concentration without affecting structural properties of bone mineral crystals

Increased cross-sectional area and strength of long bones has been observed in transgenic mice with 2-fold (OSV9) and 3-fold (OSV3) elevation of osteoblast vitamin D receptor (VDR) levels. In the present study, mineralization density distributions, including typical calcium content (Ca(Peak)) and homogeneity of mineralization (Ca(Width)) of femoral bone and growth plate cartilage, were determined by quantitative backscattered electron imaging (qBEI). Fourier-transform infrared (FTIR) microspectroscopy was used to examine mineral content, collagen and crystal maturation, and scanning small angle X-ray scattering (scanning-SAXS) for studying mineral particle thickness and alignment. In addition, X-ray diffraction (XRD) of distal tibiae revealed mineral particle c-axis size. In trabecular bone, the increase in Ca(Peak) was significant for both OSV9 (+ 3.14%, P = 0.03) and OSV3 (+ 3.43%, P = 0.02) versus controls with 23.61 +/- 0.45 S.D. wt% Ca baseline values. In cortical bone, Ca(Peak) was enhanced for the OSV3 mice (+ 1.84%, P = 0.02) versus controls with 26.61 +/- 0.28 S.D. wt% Ca, and OSV9 having intermediate values. Additionally, there was significantly increased homogeneity of mineralization as denoted by a reduction of Ca(Width) (-8.4%, P = 0.01) in primary spongiosa. FTIR microspectroscopy, with the exception of an increased collagen maturity in OSV3 trabecular bone (+ 9.9%, P = 0.02), XRD, and scanning-SAXS indicated no alterations in the nanostructure of transgenic bone. These findings indicate that elevation of osteoblastic vitamin D response led to formation of normal bone with higher calcium content. These material properties, together with indications of decreased bone resorption in secondary spongiosa and increased cortical periosteal bone formation, appear to contribute to the improved mechanical properties of their long bones and suggest an important physiological role of the vitamin D-endocrine system in normal bone mineralization.

Mineralized microstructure of calcified avian tendons: a scanning small angle X-ray scattering study

The micrometer level spatial distribution of the size, shape, and orientation of mineral crystallites in the calcifying matrix of tendons near the edge of the mineralizing front was investigated by scanning small angle X-ray scattering using synchrotron X-ray radiation. Using a special microbeam arrangement enabling 20 microm beam resolution and short measurement times, linear diffraction scans were made on sections from the normally calcifying tendons (tibialis cranialis) from the domestic turkey, which calcify in the distal to proximal direction. A change in shape and arrangement of mineral crystals was observed within the first 200 microm of the mineralization front, and the mineral crystal distribution was highly anisotropic with crystals aligned parallel to the fiber axis. In a cross-section of the tendon cut at right angles to the fiber axis, the orientation distribution of crystals was not azimuthally symmetric, and showed a small but nonzero anisotropy and a continuous change in mean orientation angle across the width of the tendon cross-section.

Characteristics of mineral particles in the human bone/cartilage interface

Bone and cartilage consist of different organic matrices, which can both be mineralized by the deposition of nano-sized calcium phosphate particles. We have studied these mineral particles in the mineralized cartilage layer between bone and different types of cartilage (bone/articular cartilage, bone/intervertebral disk, and bone/growth cartilage) of individuals aged 54 years, 12 years, and 6 months. Quantitative backscattered electron imaging and scanning small-angle X-ray scattering at a synchrotron radiation source were combined with light microscopy to determine calcium content, mineral particle size and alignment, and collagen orientation, respectively. Mineralized cartilage revealed a higher calcium content than the adjacent bone (p<0.05 for all samples), whereas the highest values were found in growth cartilage. Surprisingly, we found the mineral platelet width similar for bone and mineralized cartilage, with the exception of the growth cartilage sample. The most striking result, however, was the abrupt change of mineral particle orientation at the interface between the two tissues. While the particles were aligned perpendicular to the interface in cartilage, they were oriented parallel to it in bone, reflecting the morphology of the underlying organic matrices. The tight bonding of mineralized cartilage to bone suggests a mechanical role for the interface of the two elastically different tissues, bone and cartilage.

Constant mineralization density distribution in cancellous human bone

The degree of mineralization of bone matrix is an important factor in determining the mechanical competence of bone. The remodeling and modeling activities of bone cells together with the time course of mineralization of newly formed bone matrix generate a characteristic bone mineralization density distribution (BMDD). In this study we investigated the biological variance of the BMDD at the micrometer level, applying a quantitative backscattered electron imaging (qBEI) method. We used the mean calcium concentration (Ca(Mean)), the most frequent calcium concentration (Ca(Peak)), and full width at half maximum (Ca(Width)) to characterize the BMDD. In none of the BMDD parameters were statistically significant differences found due to ethnicity (15 African-American vs. 27 Caucasian premenopausal women), skeletal site variance (20 ilium, 24 vertebral body, 13 patella, 13 femoral neck, and 13 femoral head), age (25 to 95 years), or gender. Additionally, the interindividual variance of Ca(Mean) and Ca(Peak), irrespective of biological factors, was found to be remarkably small (SD < 2.1% of means). However, there are significant changes in the BMDD in the case of bone diseases (e.g., osteomalacia) or following clinical treatment (e.g., alendronate). From the lack of intraindividual changes among different skeletal sites we conclude that diagnostic transiliac biopsies can be used to determine the BMDD variables of cancellous bone for the entire skeleton of the patient. In order to quantify deviations from normal mineralization, a reference BMDD for adult humans was calculated using bone samples from 52 individuals. Because we find the BMDD to be essentially constant in healthy adult humans, qBEI provides a sensitive means to detect even small changes in mineralization due to bone disease or therapeutic intervention.

Orientation of mineral crystallites and mineral density during skeletal development in mice deficient in tissue nonspecific alkaline phosphatase

Tissue nonspecific alkaline phosphatase (TNALP) is thought to play an important role in mineralization processes, although its exact working mechanism is not known. In the present investigation we have studied mineral crystal characteristics in the developing skeleton of TNALP-deficient mice. Null mutants (n = 7) and their wild-type littermates (n = 7) were bred and killed between 8 and 22 days after birth. Skeletal tissues were processed to assess mineral characteristics (small angle X-ray scattering, quantitative backscattered electron imaging), and to analyze bone by light microscopy and immunolabeling. The results showed a reduced longitudinal growth and a strongly delayed epiphyseal ossification in the null mutants. This was accompanied by disturbances in mineralization pattern, in that crystallites were not orderly aligned with respect to the longitudinal axis of the cortical bone. Among the null mutants, a great variability in the mineralization parameters was noticed. Also, immunolabeling of osteopontin (OPN) revealed an abnormal distribution pattern of the protein within the bone matrix. Whereas in the wild-type animals OPN was predominantly observed in cement and reversal lines, in the null mutants, OPN was also randomly dispersed throughout the nonmineralized matrix, with focal densities. In contrast, the distribution pattern of osteocalcin (OC) was comparable in both types of animals. It is concluded that ablation of TNALP results not only in hypomineralization of the skeleton, but also in a severe disorder of the mineral crystal alignment pattern in the corticalis of growing long bone in association with a disordered matrix architecture, presumably as a result of impaired bone remodeling and maturation.

Viscoelastic properties of collagen: synchrotron radiation investigations and structural model

Collagen type I is the most abundant structural protein in tendon, skin and bone, and largely determines the mechanical behaviour of these connective tissues. To obtain a better understanding of the relationship between structure and mechanical properties, tensile tests and synchrotron X-ray scattering have been carried out simultaneously, correlating the mechanical behaviour with changes in the microstructure. Because intermolecular cross-links are thought to have a great influence on the mechanical behaviour of collagen, we also carried out experiments using cross-link-deficient tail-tendon collagen from rats fed with beta-APN, in addition to normal controls. The load-elongation curve of tendon collagen has a characteristic shape with, initially, an increasing slope, corresponding to an increasing stiffness, followed by yielding and then fracture. Cross-link-deficient collagen produces a quite different curve with a marked plateau appearing in some cases, where the length of the tendon increases at constant stress. With the use of in situ X-ray diffraction, it was possible to measure simultaneously the elongation of the collagen fibrils inside the tendon and of the tendon as a whole. The overall strain of the tendon was always larger than the strain in the individual fibrils, which demonstrates that some deformation is taking place in the matrix between fibrils. Moreover, the ratio of fibril strain to tendon strain was dependent on the applied strain rate. When the speed of deformation was increased, this ratio increased in normal collagen but generally decreased in cross-link-deficient collagen, correlating to the appearance of a plateau in the force-elongation curve indicating creep. We proposed a simple structural model, which describes the tendon at a hierarchical level, where fibrils and interfibrillar matrix act as coupled viscoelastic systems. All qualitative features of the strain-rate dependence of both normal and cross-link-deficient collagen can be reproduced within this model. This complements earlier models that considered the next smallest level of hierarchy, describing the deformation of collagen fibrils in terms of changes in their molecular packing.

Structural development of the mineralized tissue in the human L4 vertebral body

Knowledge of the structural development of the human vertebrae from non-weight-bearing before birth to weight-bearing after birth is still poor. We studied the mineralized tissue of the developing lumbar L4 vertebral body at ages 15 weeks postconception to 97 years from the tissue level (trabecular architecture) to the material level (micro- and nanostructure). Trabecular architecture was investigated by 2D histomorphometry and the material level was examined by quantitative backscattered electron imaging (for typical calcium content, CaMaxFreq) and scanning small-angle X-ray scattering (for mean mineral particle thickness). During early development, the trabecular orientation changed from a radial to a vertical/horizontal pattern. For bone area per tissue area and trabecular width in postnatal cancellous bone, the maximum was reached at adolescence (20 years), while for trabecular number the maximum was reached at childhood (approximately 1 year). CaMaxFreq was lower in early bone (approximately 21 wt%) than in mineralized cartilage (approximately 29 wt%) and adolescent bone (approximately 23 wt%). In conclusion, the changes at the tissue level were observed to continue throughout life while the development of bone at the material level (CaMaxFreq, mineral particle thickness and orientation) is essentially complete after the first years of life. CaMaxFreq and mean particle thickness increase rapidly during the first years and reach saturation. Remarkably, when these parameters are plotted versus logarithm of age, they appear linear.

Age- and genotype-dependence of bone material properties in the osteogenesis imperfecta murine model (oim)

Cortical mineralization of long bones was studied in collagen alpha2(I)-deficient mice (oim) used as a model for human osteogenesis imperfecta. Aspects of the age development of the mice were characterized by combining nanometer- to micrometer-scale structural analysis with microhardness measurements. Bone structure was determined from homozygous (oim/oim) and heterozygous (oim/+) mice and their normal (+/+) littermates as a function of animal age by small-angle X-ray scattering (SAXS) and quantitative backscattered electron imaging (qBEI) measurements. SAXS studies found anomalies in the size and arrangement of bone mineral crystals in both homozygous and heterozygous mice aged 1-14 months. Generally, the crystals were smaller in thickness and less well aligned in these mice compared with control animals. An increase in the mean crystal thickness of the bone was found within all three genotypes up to an age of 3 months. Vicker's hardness measurements were significantly enhanced for oim bone (homozygotes and heterozygotes) compared with controls. The microhardness values were correlated directly with increased mineral content of homozygous and heterozygous compared with control bone, as determined by qBEI analysis. There was also a significant increase of mineral content with age. Two possibilities for collagen-mineral association are discussed for explaining the increased hardness and mineral content of oim/oim bone, together with its decreased toughness and thinner mineral crystals. As a consequence of the present measurements, one model for oim bone could incorporate small and densely packed mineral crystals. A second model for possible collagen-mineral association in oim material would consist of two families of mineral crystals, one being smaller and the other being much larger than the crystals found in normal mouse long bones.

Graded microstructure and mechanical properties of human crown dentin

The mineralized tissue of intertubular dentin is a collagen-mineral composite with considerable local variations of mechanical properties. Area scans of human coronal dentin were made by complementary methods to investigate correlations between local mechanical properties and the density, size, and crystallinity of the mineral particles. Scanning images from the same specimen were collected with Fourier-transform infrared microspectroscopy in reflectance mode (FTIR-RM), small angle X-ray scattering (SAXS), quantitative backscattered electron imaging (qBEI), and Nanoindentation in an atomic force microscope. The mineral content of dentin was found to decrease and the thickness of mineral crystals to increase towards the dentin-enamel junction (DEJ). Hardness and elastic modulus both decreased towards the DEJ. In a correlation analysis, the mineral content and, even more, the thickness of mineral crystals were found as the best predictors of hardness. The dentin layer close to the DEJ corresponds to a local minimum in hardness and elastic modulus, a configuration known to be an effective obstacle for crack propagation. Hence, the observed variations of mechanical and structural properties in an area between 0 and 1.5 mm below the DEJ define crown dentin as a gradient material optimized for its mechanical function.