Effect of abnormal mineralization on the mechanical behavior of X-linked hypophosphatemic mice femora

Understanding the structural biology of osteomalacia through multiscale 3D X-ray and electron tomographic imaging: a review of X-linked hypophosphatemia, the Hyp mouse model, and imaging methods

Biomineralization in bones and teeth is a highly regulated extracellular event. In the skeleton, mineralization at the tissue level is controlled within the collagenous extracellular matrix by both circulating and local factors. While systemic regulation of mineral ion homeostasis has been well-studied over many decades, much less is known about the regulation of mineralization at the local level directly within the extracellular matrix. Some local regulators have been identified, such as tissue-nonspecific alkaline phosphatase (TNAP), phosphate-regulating endopeptidase homolog X-linked (PHEX), pyrophosphate, and osteopontin, and others are currently under investigation. Dysregulation of the actions of enzyme-inhibitor substrate pairs engaged in mineralization (as we describe by the Stenciling Principle for extracellular matrix mineralization) leads to osteomalacic "soft bone" diseases, such as hypophosphatasia (HPP) and X-linked hypophosphatemia (XLH). This review addresses how advances in 3D imaging tools and software now allow contextual and correlative viewing and interpretation of mineralized tissue structure across most length scales. Contextualized and integrated 3D multiscale data obtained from these imaging modalities have afforded an unprecedented structural biology view of bone from the macroscale to the nanoscale. Such correlated volume imaging data is highly quantitative, providing not only an integrated view of the skeleton in health, but also a means to observe alterations that occur in disease. In the context of the many hierarchical levels of skeletal organization, here we summarize structural features of bone over multiple length scales, with a focus on nano- and microscale features as viewed by X-ray and electron tomography imaging methods (submicron μCT and FIB-SEM). We additionally summarize structural changes observed after dysregulation of the mineralization pathway, focusing here on the Hyp mouse model for XLH. More specifically, we summarize how mineral patterns/packs at the microscale (3D crossfibrillar mineral tessellation), and how this is defective in Hyp mouse bone and Hyp enthesis fibrocartilage.

OMIBONE: Omics-driven computer model of bone regeneration for personalized treatment

Treatment of bone fractures are standardized according to the AO classification, which mainly refers to the mechanical stabilization required in a given situation but neglect individual differences due to patient's healing potential or accompanying diseases. Specially in elderly or immune-compromised patients, the complexity of individual constrains on a biological as well as mechanical level are hard to account for. Here, we introduce a novel framework that allows to predict bone regeneration outcome using combined proteomic and mechanical analyses in a computer model. The framework uses Ingenuity Pathway Analysis (IPA) software to link protein changes to alterations in biological processes and integrates these in an Agent-Based Model (ABM) of bone regeneration. This combined framework allows to predict bone formation and the potential of an individual to heal a given fracture setting. The performance of the framework was evaluated by replicating the experimental setup of a mouse femur fracture stabilized with an intramedullary pin. The model was informed by serum derived proteomics data. The tissue formation patterns were compared against experimental data based on x-ray and histology images. The results indicate the framework potential in predicting an individual's bone formation potential and hold promise as a concept to enable personalized bone healing predictions for a chosen fracture fixation.

Osteoblast-lineage calcium/calmodulin-dependent kinase 2 delta and gamma regulates bone mass and quality

Bone regulates its mass and quality in response to diverse mechanical, hormonal, and local signals. The bone anabolic or catabolic responses to these signals are often received by osteocytes, which then coordinate the activity of osteoblasts and osteoclasts on bone surfaces. We previously established that calcium/calmodulin-dependent kinase 2 (CaMKII) is required for osteocytes to respond to some bone anabolic cues in vitro. However, a role for CaMKII in bone physiology in vivo is largely undescribed. Here, we show that conditional codeletion of the most abundant isoforms of CaMKII (delta and gamma) in mature osteoblasts and osteocytes [Ocn-cre:Camk2d/Camk2g double-knockout (dCKO)] caused severe osteopenia in both cortical and trabecular compartments by 8 wk of age. In addition to having less bone mass, dCKO bones are of worse quality, with significant deficits in mechanical properties, and a propensity to fracture. This striking skeletal phenotype is multifactorial, including diminished osteoblast activity, increased osteoclast activity, and altered phosphate homeostasis both systemically and locally. These dCKO mice exhibited decreased circulating phosphate (hypophosphatemia) and increased expression of the phosphate-regulating hormone fibroblast growth factor 23. Additionally, dCKO mice expressed less bone-derived tissue nonspecific alkaline phosphatase protein than control mice. Consistent with altered phosphate homeostasis, we observed that dCKO bones were hypo-mineralized with prominent osteoid seams, analogous to the phenotypes of mice with hypophosphatemia. Altogether, these data reveal a fundamental role for osteocyte CaMKIIδ and CaMKIIγ in the maintenance of bone mass and bone quality and link osteoblast/osteocyte CaMKII to phosphate homeostasis.

Sclerostin antibody improves phosphate metabolism hormones, bone formation rates, and bone mass in adult Hyp mice

X-linked hypophosphatemia (XLH) is caused by a loss-of-function mutation in the phosphate regulating gene with homology to endopeptidase located on the X chromosome (PHEX). Loss of functional PHEX results in elevated fibroblast growth factor 23 (FGF23), impaired phosphate reabsorption, and inhibited skeletal mineralization. Sclerostin, a protein produced primarily by osteocytes, suppresses bone formation by antagonizing canonical Wnt-signaling and is reported to be elevated in XLH patients. Our previous study reported that a monoclonal antibody to sclerostin (Scl-Ab) decreases FGF23 and increases phosphate and bone mass in growing Hyp mice (XLH murine model). In the current study, we investigated the efficacy of Scl-Ab in treating XLH pathophysiology in adult Hyp mice that are past the period of rapid skeletal growth (12 and 20-weeks old). We hypothesized that Scl-Ab would not only increase bone formation, bone strength and bone mass, but would also normalize phosphate regulating hormones, FGF23, parathyroid hormone (PTH), and vitamin 1,25(OH)2D. Scl-Ab treatment increased cortical area, trabecular bone volume fraction, trabecular bone formation rate, and the bending moment in both sexes of both age groups. Scl-Ab treatment suppressed circulating levels of intact FGF23 and c-term FGF23 in treated male and female wild-type and Hyp mice of both age groups and improved both vitamin 1,25(OH)2D and PTH. Scl-Ab treated Hyp mice also showed evidence of increased renal expression of the sodium-phosphate co-transporter, NPT2a, specifically in the female Hyp mice. Our study suggests that Scl-Ab treatment can improve several skeletal and metabolic pathologies associated with XLH, further establishes the role of sclerostin in the regulation of FGF23 and provides evidence that Scl-Ab can improve phosphate regulation by targeting the bone-renal axis.

The role of extracellular matrix phosphorylation on energy dissipation in bone

Protein phosphorylation, critical for cellular regulatory mechanisms, is implicated in various diseases. However, it remains unknown whether heterogeneity in phosphorylation of key structural proteins alters tissue integrity and organ function. Here, osteopontin phosphorylation level declined in hypo- and hyper- phosphatemia mouse models exhibiting skeletal deformities. Phosphorylation increased cohesion between osteopontin polymers, and adhesion of osteopontin to hydroxyapatite, enhancing energy dissipation. Fracture toughness, a measure of bone’s mechanical competence, increased with ex-vivo phosphorylation of wildtype mouse bones and declined with ex-vivo dephosphorylation. In osteopontin-deficient mice, global matrix phosphorylation level was not associated with toughness. Our findings suggest that phosphorylated osteopontin promotes fracture toughness in a dose-dependent manner through increased interfacial bond formation. In the absence of osteopontin, phosphorylation increases electrostatic repulsion, and likely protein alignment and interfilament distance leading to decreased fracture resistance. These mechanisms may be of importance in other connective tissues, and the key to unraveling cell–matrix interactions in diseases.

Nanoscale mechanisms in age-related hip-fractures

Nanoscale mineralized collagen fibrils may be important determinants of whole-bone mechanical properties and contribute to the risk of age-related fractures. In a cross-sectional study nano- and tissue-level mechanics were compared across trabecular sections from the proximal femora of three groups (n = 10 each): ageing non-fractured donors (Controls); untreated fracture patients (Fx-Untreated); bisphosphonate-treated fracture patients (Fx-BisTreated). Collagen fibril, mineral and tissue mechanics were measured using synchrotron X-Ray diffraction of bone sections under load. Mechanical data were compared across groups, and tissue-level data were regressed against nano. Compared to controls fracture patients exhibited significantly lower critical tissue strain, max strain and normalized strength, with lower peak fibril and mineral strain. Bisphosphonate-treated exhibited the lowest properties. In all three groups, peak mineral strain coincided with maximum tissue strength (i.e. ultimate stress), whilst peak fibril strain occurred afterwards (i.e. higher tissue strain). Tissue strain and strength were positively and strongly correlated with peak fibril and mineral strains. Age-related fractures were associated with lower peak fibril and mineral strain irrespective of treatment. Indicating earlier mineral disengagement and the subsequent onset of fibril sliding is one of the key mechanisms leading to fracture. Treatments for fragility should target collagen-mineral interactions to restore nano-scale strain to that of healthy bone.

Investigation of bone matrix composition, architecture and mechanical properties reflect structure-function relationship of cortical bone in glucocorticoid induced osteoporosis

Glucocorticoid induced osteoporosis (GIOP) is the most common negative consequence of long-term glucocorticoid treatment, leading to increased fracture risk followed by loss of mobility and high mortality risk. These biologically induced changes in bone quality at molecular level lead to changes both in bone matrix architecture and bone matrix composition. However, the quantitative details of changes in bone quality - and especially their link to reduced macroscale mechanical properties are still largely missing. In this study, a mouse model for glucocorticoid-induced osteoporosis (GIOP) was used to investigate mechanical and material alterations in bone cortex (natural nanocomposite) at different scale. By combining quantitative backscattered electron (qBSE) imaging, nanoindentation and high brilliance synchrotron X-ray nanomechanical imaging on a genetically modified mouse model of GIOP, we were able to quantify the local indentation modulus, mineralization distribution and the alterations of nanoscale structures and deformation mechanisms in the mid-diaphysis of femur, and relate them to the macroscopic mechanical changes. Our results showed clear and significant changes in terms of material quality of bone at nanoscale and microscale, which manifests itself in development of spatial heterogeneities in mineralization and indentation moduli across the bone organ, with potential implications for increased fracture risk.

Correlative vibrational spectroscopy and 2D X-ray diffraction to probe the mineralization of bone in phosphate-deficient mice

Bone crystallite chemistry and structure change during bone maturation. However, these properties of bone can also be affected by limited uptake of the chemical constituents of the mineral by the animal. This makes probing the effect of bone-mineralization-related diseases a complicated task. Here it is shown that the combination of vibrational spectroscopy with two-dimensional X-ray diffraction can provide unparalleled information on the changes in bone chemistry and structure associated with different bone pathologies (phosphate deficiency) and/or health conditions (pregnancy, lactation). Using a synergistic analytical approach, it was possible to trace the effect that changes in the remodelling regime have on the bone mineral chemistry and structure in normal and mineral-deficient (hypophosphatemic) mice. The results indicate that hypophosphatemic mice have increased bone remodelling, increased carbonate content and decreased crystallinity of the bone mineral, as well as increased misalignment of crystallites within the bone tissue. Pregnant and lactating mice that are normal and hypophosphatemic showed changes in the chemistry and misalignment of the apatite crystals that can be related to changes in remodelling rates associated with different calcium demand during pregnancy and lactation.

An in vitro study on dentin demineralization and remineralization: Collagen rearrangements and influence on the enucleated phase

Dentin remineralization is of clinical relevance in the therapy of caries and dentin hypersensitivity. This study is aimed at gaining more insights on a molecular scale, through IR spectroscopy, into dentin demineralization and remineralization. The dentin demineralization by ethylenediaminetetraacetic acid, EDTA (17%, 2 h) significantly altered the secondary structure distribution of collagen, upon loss of interaction with calcium ions. To investigate dentin remineralization, previously demineralized human dentin slices were soaked in Dulbecco's Phosphate Buffered Saline (DPBS) or Hank's Balanced Salt Solution HBSS, in close contact with three commercial cements used as sustained releasing sources of Ca²⁺ and OH^- ions (i.e. calcium hydroxide- and calcium silicate-based cements). IR spectroscopy showed the occurrence of remineralization under these conditions. Collagen did not lose its ability to chelate Ca²⁺, and these interactions allowed collagen to rearrange into a conformation similar to that of sound dentin. This process appeared slower in HBSS than DPBS, as also shown by the lower degree of maturation of the inorganic phase enucleated in the former medium (amorphous calcium phosphate versus B-type carbonated apatite). Collagen appeared to act as a spatial constraint to crystal deposition, affecting crystallinity and carbonate content of the enucleated phase. Remineralization was found to strongly depend on the calcium releasing ability of the cements. The fast formation of a rough apatite biocoating may represent a favorable clinical condition in the context of mineralized tissue regeneration.

Vibrational spectroscopic analysis of hydroxyapatite in HYP mice and individuals with X-linked hypophosphatemia

BACKGROUND

X-linked hypophosphatemia (XLH) is the most common form of familial phosphate-wasting disorders, due to an inactivating mutation in the phosphate-regulating neutral endopeptidase, X-linked gene. Persistent osteomalacia, enthesophytes, osteophytes, degenerative arthritis and dental abscesses/periodontal disease dominate the adult disorder. However, the impact of insufficient phosphate on hydroxyapatite composition, the major inorganic component of bone and teeth, is unknown in individuals with XLH.

METHODS

Using Raman spectroscopy, the carbonate (CO3 2-) to phosphate (PO4 3-) ion ratio was measured in HYP and wild-type mice and in primary and permanent teeth from XLH individuals and unaffected controls.

RESULTS

There was a significant difference in carbonate ion substitution between the HYP and wild-type femoral cortical bone (0.36 ± 0.08 versus 0.24 ± 0.04; p < 0.001). Carbonate ion substitution levels were also higher in permanent XLH teeth compared with unaffected individuals (0.39 ± 0.12 versus 0.23 ± 0.04; p < 0.001), but not in primary teeth (0.29 ± 0.11 versus 0.26 ± 0.02; p = 0.29). Complementary Fourier transform infrared analyses demonstrated higher relative intensities of the four major vibrational bands originating from the carbonate anion in XLH teeth compared with unaffected controls.

CONCLUSION

Ionic substitution within the crystal lattice is a common feature of hydroxyapatite and one that confers the physiological properties of bone that impact mechanical strength and the process of bone remodeling. Our data demonstrating anionic substitution in human dentin from individuals with XLH validate the use of dentin as a proxy for bone and to better understand the molecular adaptations that occur in the biochemical milieu of XLH.

Symmetrically reduced stiffness and increased extensibility in compression and tension at the mineralized fibrillar level in rachitic bone

In metabolic bone diseases, the alterations in fibrillar level bone-material quality affecting macroscopic mechanical competence are not well-understood quantitatively. Here, we quantify the fibrillar level deformation in cantilever bending in a mouse model for hereditary rickets (Hpr). Microfocus in-situ synchrotron small-angle X-ray scattering (SAXS) combined with cantilever bending was used to resolve nanoscale fibril strain in tensile- and compressive tissue regions separately, with quantitative backscattered scanning electron microscopy used to measure microscale mineralization. Tissue-level flexural moduli for Hpr mice were significantly (p<0.01) smaller compared to wild-type (~5 to 10-fold reduction). At the fibrillar level, the fibril moduli within the tensile and compressive zones were significantly (p<0.05) lower by ~3- to 5-fold in Hpr mice compared to wild-type mice. Hpr mice have a lower mineral content (24.2±2.1Cawt.% versus 27.4±3.3Ca wt.%) and its distribution was more heterogeneous compared to wild-type animals. However, the average effective fibril modulus did not differ significantly (p>0.05) over ages (4, 7 and 10weeks) between tensile and compressive zones. Our results indicate that incompletely mineralized fibrils in Hpr mice have greater deformability and lower moduli in both compression and tension, and those compressive and tensile zones have similar moduli at the fibrillar level.

Bone and cartilage demonstrate changes localized to bone marrow edema-like lesions within osteoarthritic knees

OBJECTIVE

Our objective is to understand the biological and mechanical pathways linking cartilage, bone, and marrow changes in the progression of osteoarthritis (OA). The aim of the present study was to evaluate bone structure and composition within bone marrow edema-like lesion (BMEL) regions associated with knee OA.

METHODS

Tibial plateau specimens (n = 18) were collected from 10 subjects with knee OA during total knee arthroplasty (TKA). Magnetic resonance (MR) imaging was used to identify BMEL and quantify metrics of cartilage composition. Micro-computed tomography (μCT) and high-resolution peripheral quantitative computed tomography (HR-pQCT) were used to quantify density and microstructure of the subchondral trabecular bone. Fourier transform infrared (FTIR) spectroscopy was used to quantify tissue composition.

RESULTS

Trabecular bone within BMEL was higher in volume fraction, with more and thicker trabeculae that were more plate-like in structure compared to unaffected regions. BMEL trabecular tissue composition had decreased phosphate and carbonate content. Marrow infiltration by a fibrous collagen network and evidence of increased bone remodeling were present. Structural and compositional changes were specifically localized to regions underlying cartilage degradation.

CONCLUSION

These results support the paradigm of focal interactions among bone, marrow, and cartilage in the progression of knee OA. Quantitative evaluation of tissue changes and interactions may aid in the understanding of disease pathophysiology and provide imaging markers for disease progression.

Bone material properties in premenopausal women with idiopathic osteoporosis

Idiopathic osteoporosis (IOP) in premenopausal women is characterized by fragility fractures at low or normal bone mineral density (BMD) in otherwise healthy women with normal gonadal function. Histomorphometric analysis of transiliac bone biopsy samples has revealed microarchitectural deterioration of cancellous bone and thinner cortices. To examine bone material quality, we measured the bone mineralization density distribution (BMDD) in biopsy samples by quantitative backscattered electron imaging (qBEI), and mineral/matrix ratio, mineral crystallinity/maturity, relative proteoglycan content, and collagen cross-link ratio at actively bone forming trabecular surfaces by Raman microspectroscopy and Fourier transform infrared microspectroscopy (FTIRM) techniques. The study groups included: premenopausal women with idiopathic fractures (IOP, n = 45), or idiopathic low BMD (Z-score ≤ -2.0 at spine and/or hip) but no fractures (ILBMD, n = 19), and healthy controls (CONTROL, n = 38). BMDD of cancellous bone showed slightly lower mineral content in IOP (both the average degree of mineralization of cancellous bone [Cn.Ca(Mean) ] and mode calcium concentration [Cn.Ca(Peak) ] are 1.4% lower) and in ILBMD (both are 1.6% lower, p < 0.05) versus CONTROL, but no difference between IOP and ILBMD. Similar differences were found when affected groups were combined versus CONTROL. The differences remained significant after adjustment for cancellous mineralizing surface (MS/BS), suggesting that the reduced mineralization of bone matrix cannot be completely accounted for by differences in bone turnover. Raman microspectroscopy and FTIRM analysis at forming bone surfaces showed no differences between combined IOP/ILBMD groups versus CONTROL, with the exceptions of increased proteoglycan content per mineral content and increased collagen cross-link ratio. When the two affected subgroups were considered individually, mineral/matrix ratio and collagen cross-link ratio were higher in IOP than ILBMD. In conclusion, our findings suggest that bone material properties differ between premenopausal women with IOP/ILBMD and normal controls. In particular, the altered collagen properties at sites of active bone formation support the hypothesis that affected women have osteoblast dysfunction that may play a role in bone fragility.

Hypophosphatemic rickets is associated with disruption of mineral orientation at the nanoscale in the flat scapula bones of rachitic mice with development

Metabolic bone disorders such as rickets are associated with altered in vivo muscular force distributions on the skeletal system. During development, these altered forces can potentially affect the spatial and temporal dynamics of mineralised tissue formation, but the exact mechanisms are not known. Here we have used a murine model of hypophosphatemic rickets (Hpr) to study the development of the mineralised nanostructure in the intramembranously ossifying scapulae (shoulder bone). Using position-resolved scanning small angle X-ray scattering (SAXS), we quantified the degree and direction of mineral nanocrystallite alignment over the width of the scapulae, from the load bearing lateral border (LB) regions to the intermediate infraspinous fossa (IF) tissue. These measurements revealed a significant (p<0.05) increase in mineral nanocrystallite alignment in the LB when compared to the IF region, with increased tissue maturation in wild-type mice; this was absent in mice with rickets. The crystallites were more closely aligned to the macroscopic bone boundary in the LB when compared to the IF region in both wild type and Hpr mice, but the degree of alignment was reduced in Hpr mice. These findings are consistent with a correlation between the nanocrystallites within fibrils and in vivo muscular forces. Thus our results indicate a relevant mechanism for the observed increased macroscopic deformability in rickets, via a significant alteration in the mineral particle alignment, which is mediated by an altered spatial distribution of muscle forces.

Significant deterioration in nanomechanical quality occurs through incomplete extrafibrillar mineralization in rachitic bone: evidence from in-situ synchrotron X-ray scattering and backscattered electron imaging

Bone diseases such as rickets and osteoporosis cause significant reduction in bone quantity and quality, which leads to mechanical abnormalities. However, the precise ultrastructural mechanism by which altered bone quality affects mechanical properties is not clearly understood. Here we demonstrate the functional link between altered bone quality (reduced mineralization) and abnormal fibrillar-level mechanics using a novel, real-time synchrotron X-ray nanomechanical imaging method to study a mouse model with rickets due to reduced extrafibrillar mineralization. A previously unreported N-ethyl-N-nitrosourea (ENU) mouse model for hypophosphatemic rickets (Hpr), as a result of missense Trp314Arg mutation of the phosphate regulating gene with homologies to endopeptidase on the X chromosome (Phex) and with features consistent with X-linked hypophosphatemic rickets (XLHR) in man, was investigated using in situ synchrotron small angle X-ray scattering to measure real-time changes in axial periodicity of the nanoscale mineralized fibrils in bone during tensile loading. These determine nanomechanical parameters including fibril elastic modulus and maximum fibril strain. Mineral content was estimated using backscattered electron imaging. A significant reduction of effective fibril modulus and enhancement of maximum fibril strain was found in Hpr mice. Effective fibril modulus and maximum fibril strain in the elastic region increased consistently with age in Hpr and wild-type mice. However, the mean mineral content was ∼21% lower in Hpr mice and was more heterogeneous in its distribution. Our results are consistent with a nanostructural mechanism in which incompletely mineralized fibrils show greater extensibility and lower stiffness, leading to macroscopic outcomes such as greater bone flexibility. Our study demonstrates the value of in situ X-ray nanomechanical imaging in linking the alterations in bone nanostructure to nanoscale mechanical deterioration in a metabolic bone disease.

Lathyrism-induced alterations in collagen cross-links influence the mechanical properties of bone material without affecting the mineral

In the present study a rat animal model of lathyrism was employed to decipher whether anatomically confined alterations in collagen cross-links are sufficient to influence the mechanical properties of whole bone. Animal experiments were performed under an ethics committee approved protocol. Sixty-four female (47 day old) rats of equivalent weights were divided into four groups (16 per group): Controls were fed a semi-synthetic diet containing 0.6% calcium and 0.6% phosphorus for 2 or 4 weeks and β-APN treated animals were fed additionally with β-aminopropionitrile (0.1% dry weight). At the end of this period the rats in the four groups were sacrificed, and L2-L6 vertebra were collected. Collagen cross-links were determined by both biochemical and spectroscopic (Fourier transform infrared imaging (FTIRI)) analyses. Mineral content and distribution (BMDD) were determined by quantitative backscattered electron imaging (qBEI), and mineral maturity/crystallinity by FTIRI techniques. Micro-CT was used to describe the architectural properties. Mechanical performance of whole bone as well as of bone matrix material was tested by vertebral compression tests and by nano-indentation, respectively. The data of the present study indicate that β-APN treatment changed whole vertebra properties compared to non-treated rats, including collagen cross-links pattern, trabecular bone volume to tissue ratio and trabecular thickness, which were all decreased (p<0.05). Further, compression tests revealed a significant negative impact of β-APN treatment on maximal force to failure and energy to failure, while stiffness was not influenced. Bone mineral density distribution (BMDD) was not altered either. At the material level, β-APN treated rats exhibited increased Pyd/Divalent cross-link ratios in areas confined to a newly formed bone. Moreover, nano-indentation experiments showed that the E-modulus and hardness were reduced only in newly formed bone areas under the influence of β-APN, despite a similar mineral content. In conclusion the results emphasize the pivotal role of collagen cross-links in the determination of bone quality and mechanical integrity. However, in this rat animal model of lathyrism, the coupled alterations of tissue structural properties make it difficult to weigh the contribution of the anatomically confined material changes to the overall mechanical performance of whole bone. Interestingly, the collagen cross-link ratio in bone forming areas had the same profile as seen in actively bone forming trabecular surfaces in human iliac crest biopsies of osteoporotic patients.

Early growth response gene 1 regulates bone properties in mice

Transcriptional regulation of the postnatal skeleton is incompletely understood. Here, we determined the consequence of loss of early growth response gene 1 (EGR-1) on bone properties. Analyses were performed on both the microscopic and molecular levels utilizing micro-computed tomography (micro-CT) and Fourier transform infrared imaging (FTIRI), respectively. Mice deficient in EGR-1 (Egr-1 (-/-)) were studied and compared to sex- and age-matched wild-type (wt) control animals. Femoral trabecular bone in male Egr-1 (-/-) mice demonstrated osteopenic characteristics marked by reductions in both bone volume fraction (BV/TV) and bone mineral density (BMD). Morphological analysis revealed fewer trabeculae in these animals. In contrast, female Egr-1 (-/-) animals had thinner trabeculae, but BV/TV and BMD were not significantly reduced. Analysis of femoral cortical bone at the mid-diaphysis did not show significant osteopenic characteristics but detected changes in cross-sectional geometry in both male and female Egr-1 (-/-) animals. Functionally, this resulted in decreased resistance to three-point bending as indicated by a reduction in maximum load, failure load, and stiffness. Assessment of compositional bone properties, including mineral-to-matrix ratio, carbonate-to-phosphate ratio, crystallinity, and cross-linking, in femurs by FTIRI did not show any significant differences or an appreciable trend between Egr-1 (-/-) and wt mice of either sex. Unexpectedly, rib bone from Egr-1 (-/-) animals displayed distinct osteopenic traits that were particularly pronounced in female mice. This study provides genetic evidence that both sex and skeletal site are critical determinants of EGR-1 activity in vivo and that its site-specific action may contribute to the mechanical properties of bone.

Mineral composition is altered by osteoblast expression of an engineered G(s)-coupled receptor

Activation of the G(s) G protein-coupled receptor Rs1 in osteoblasts increases bone mineral density by 5- to 15-fold in mice and recapitulates histologic aspects of fibrous dysplasia of the bone. However, the effects of constitutive G(s) signaling on bone tissue quality are not known. The goal of this study was to determine bone tissue quality in mice resulting from osteoblast-specific constitutive G(s) activation, by the complementary techniques of FTIR spectroscopy and synchrotron radiation micro-computed tomography (SRμCT). Col1(2.3)-tTA/TetO-Rs1 double transgenic (DT) mice, which showed osteoblast-specific constitutive G(s) signaling activity by the Rs1 receptor, were created. Femora and calvariae of DT and wild-type (WT) mice (6 and 15 weeks old) were analyzed by FTIR spectroscopy. WT and DT femora (3 and 9 weeks old) were imaged by SRμCT. Mineral-to-matrix ratio was 25% lower (P = 0.010), carbonate-to-phosphate ratio was 20% higher (P = 0.025), crystallinity was 4% lower (P = 0.004), and cross-link ratio was 11% lower (P = 0.025) in 6-week DT bone. Differences persisted in 15-week animals. Quantitative SRμCT analysis revealed substantial differences in mean values and heterogeneity of tissue mineral density (TMD). TMD values were 1,156 ± 100 and 711 ± 251 mg/cm(3) (mean ± SD) in WT and DT femoral diaphyses, respectively, at 3 weeks. Similar differences were found in 9-week animals. These results demonstrate that continuous G(s) activation in murine osteoblasts leads to deposition of immature bone tissue with reduced mineralization. Our findings suggest that bone tissue quality may be an important contributor to increased fracture risk in fibrous dysplasia patients.

Survey of the enthesopathy of X-linked hypophosphatemia and its characterization in Hyp mice

X-linked hypophosphatemia (XLH) is characterized by rickets and osteomalacia as a result of an inactivating mutation of the PHEX (phosphate-regulating gene with homology to endopeptidases on the X chromosome) gene. PHEX encodes an endopeptidase that, when inactivated, results in elevated circulating levels of FGF-23, a novel phosphate-regulating hormone (a phosphatonin), thereby resulting in increased phosphate excretion and impaired bone mineralization. A generalized and severe mineralizing enthesopathy in patients with XLH was first reported in 1985; we likewise report a survey in which we found evidence of enthesopathy in fibrocartilaginous insertion sites, as well as osteophyte formation, in the majority of patients. Nonetheless, there has been very little focus on the progression and pathogenesis underlying the paradoxical heterotopic calcification of tendon and ligament insertion sites. Such studies have been hampered by lack of a model of mineralizing enthesopathy. We therefore characterized the involvement of the most frequently targeted fibrocartilaginous tendon insertion sites in Hyp mice, a murine model of the XLH mutation that phenocopies the human syndrome in every detail including hypophosphatemia and elevated FGF-23. Histological examination of the affected entheses revealed that mineralizing insertion sites, while thought to involve bone spur formation, were not due to bone-forming osteoblasts but instead to a significant expansion of mineralizing fibrocartilage. Our finding that enthesis fibrocartilage cells specifically express fibroblast growth factor receptor 3 (FGFR3)/Klotho suggests that the high circulating levels of FGF-23, characteristic of XLH and Hyp mice, may be part of the biochemical milieu that underlies the expansion of mineralizing enthesis fibrocartilage.

The PHEX transgene corrects mineralization defects in 9-month-old hypophosphatemic mice

Hypophosphatemia is an X-linked dominant disorder resulting from a mutation in the PHEX gene. While osteoblast-specific expression of the PHEX transgene has been reported to decrease the phosphate wasting associated with the disease in male hypophosphatemic (HYP) mice, there are reports that the mineralization defect is only partially corrected in young animals. To test the hypothesis that osteoblast-specific expression of the PHEX gene for a longer time would correct the mineralization defect, this study examined the bones of 9-month-old male and female HYP mice and their wild-type controls with or without expression of the transgene under a collagen type I promoter. Serum phosphate levels, alkaline phosphatase activity, and FGF23 levels were also measured. Mineral analyses based on wide-angle X-ray diffraction, Fourier transform-infrared (FT-IR) spectroscopy, and FT-IR imaging confirmed the decreased mineral content and increased mineral crystal size in male HYP humerii compared to wild-type males and females with or without the transgene and in female HYP mice with or without the transgene. There was a significant increase in mineral content and a decrease in crystallinity in the HYP males' bones with the transgene, compared to those without. Of interest, expression of the transgene in wild-type animals significantly increased the mineral content in both males and females without having a detectable effect on crystallinity or carbonate content. In contrast to the bones, based on micro-computed tomography and FT-IR imaging, at 9 months there were no significant differences between the HYP and the WT teeth, precluding analysis of the effect of the transgene.

Fourier transform infrared imaging microspectroscopy and tissue-level mechanical testing reveal intraspecies variation in mouse bone mineral and matrix composition

Fracture susceptibility is heritable and dependent upon bone morphology and quality. However, studies of bone quality are typically overshadowed by emphasis on bone geometry and bone mineral density. Given that differences in mineral and matrix composition exist in a variety of species, we hypothesized that genetic variation in bone quality and tissue-level mechanical properties would also exist within species. Sixteen-week-old female A/J, C57BL/6J (B6), and C3H/HeJ (C3H) inbred mouse femora were analyzed using Fourier transform infrared imaging and tissue-level mechanical testing for variation in mineral composition, mineral maturity, collagen cross-link ratio, and tissue-level mechanical properties. A/J femora had an increased mineral-to-matrix ratio compared to B6. The C3H mineral-to-matrix ratio was intermediate of A/J and B6. C3H femora had reduced acid phosphate and carbonate levels and an increased collagen cross-link ratio compared to A/J and B6. Modulus values paralleled mineral-to-matrix values, with A/J femora being the most stiff, B6 being the least stiff, and C3H having intermediate stiffness. In addition, work-to-failure varied among the strains, with the highly mineralized and brittle A/J femora performing the least amount of work-to-failure. Inbred mice are therefore able to differentially modulate the composition of their bone mineral and the maturity of their bone matrix in conjunction with tissue-level mechanical properties. These results suggest that specific combinations of bone quality and morphological traits are genetically regulated such that mechanically functional bones can be constructed in different ways.

Accretion of bone quantity and quality in the developing mouse skeleton

In this work, we found that bone mineral formation proceeded very rapidly in mice by 1 day of age, where the degree of mineralization, the tissue mineral density, and the mineral crystallinity reached 36%, 51%, and 87% of the adult values, respectively. However, even though significant mineralization had occurred, the elastic modulus of 1-day-old bone was only 14% of its adult value, indicating that the intrinsic stiffening of the bone lags considerably behind the initial mineral formation.

INTRODUCTION

To meet the mechanical challenges during early development, the skeleton requires the rapid accretion of bone quality and bone quantity. Here, we describe early bone development in the mouse skeleton and test the hypothesis that specific compositional properties determine the stiffness of the tissue.

MATERIALS AND METHODS

Tibias of female BALB mice were harvested at eight time-points (n = 4 each) distributed between 1 and 40 days of age and subjected to morphometric (muCT), chemical (Fourier transform infrared microspectroscopy), and mechanical (nanoindentation) analyses. Tibias of 450-day-old mice served as fully mineralized control specimens.

RESULTS

Bone growth proceeded very rapidly; at 1 day of age, the degree of mineralization (phosphate/protein ratio), the density of mineralized bone (TMD), and mineral crystallinity had reached 36%, 51%, and 87% of the adult (450 days) values, respectively. Spatially, the variability in mineralization across the mid-diaphysis was very high for the early time-points and declined over time. In contrast to the notable changes in mineralization, carbonate substitution into the mineral lattice (carbonate/phosphate ratio) and collagen cross-linking did not show any significant changes over this time period. Even though significant mineralization had occurred, the elastic modulus of 1-day-old bone was only 14% of the adult value and increased to 89% (of its adult value) after 40 days. Between samples of different time-points, significant positive correlations were observed between the elastic modulus and TMD (r(2) = 0.84), phosphate/protein ratio (r(2) = 0.59), and crystallinity (r(2) = 0.23), whereas collagen cross-linking showed a small but significant negative correlation (r(2) = 0.15).

CONCLUSIONS

These data indicate that specific chemical and morphometric properties modulate bone's stiffness during early growth. The intrinsic stiffening of the bone, however, lags considerably behind the initial mineral formation, emphasizing the importance of bone mineral quality for optimizing matrix integrity.

Insights into material and structural basis of bone fragility from diseases associated with fractures: how determinants of the biomechanical properties of bone are compromised by disease

Minimal trauma fractures in bone diseases are the result of bone fragility. Rather than considering bone fragility as being the result of a reduced amount of bone, we recognize that bone fragility is the result of changes in the material and structural properties of bone. A better understanding of the contribution of each component of the material composition and structure and how these interact to maintain whole bone strength is obtained by the study of metabolic bone diseases. Disorders of collagen (osteogenesis imperfecta and Paget's disease of bone), mineral content, composition and distribution (fluorosis and osteomalacia); diseases of high remodeling (postmenopausal osteoporosis, hyperparathyroidism, and hyperthyroidism) and low remodeling (osteopetrosis, pycnodysostosis); and other diseases (idiopathic male osteoporosis, corticosteroid-induced osteoporosis) produce abnormalities in the material composition and structure that lead to bone fragility. Observations in patients and in animal models provide insights on the biomechanical consequences of these illnesses and the nature of the qualities of bone that determine its strength.

FT-IR imaging of native and tissue-engineered bone and cartilage

Fourier transform infrared (FT-IR) imaging and microspectroscopy have been extensively applied to the analyses of tissues in health and disease. Spatially resolved mid-IR data has provided insights into molecular changes that occur in diseases of connective or collagen-based tissues, including, osteoporosis, osteogenesis imperfecta, osteopetrosis and pathologic calcifications. These techniques have also been used to probe chemical changes associated with load, disuse, and micro-damage in bone, and with degradation and repair in cartilage. This review summarizes the applications of FT-IR microscopy and imaging for analyses of bone and cartilage in healthy and diseased tissues, and illustrates the application of these techniques for the characterization of tissue-engineered bone and cartilage.

Infrared imaging microscopy of bone: illustrations from a mouse model of Fabry disease

Bone is a complex tissue whose composition and properties vary with age, sex, diet, tissue type, health and disease. In this review, we demonstrate how infrared spectroscopy and infrared spectroscopic imaging can be applied to the study of these variations. A specific example of mice with Fabry disease (a lipid storage disease) is presented in which it is demonstrated that the bones of these young animals, while showing typical spatial variation in mineral content, mineral crystal size, and collagen maturity, do not differ from the bones of age- and sex-matched wild type animals.

Deep-ultraviolet Raman spectroscopy study of the effect of aging on human cortical bone

The age-related deterioration in bone quality and consequent increase in fracture incidence is an obvious health concern that is becoming increasingly significant as the population ages. Raman spectroscopy with deep-ultraviolet excitation (244 nm) is used to measure vibrational spectra from human cortical bone obtained from donors over a wide age range (34-99 years). The UV Raman technique avoids the fluorescence background usually found with visible and near-infrared excitation and, due to resonance Raman effects, is particularly sensitive to the organic component of bone. Spectral changes in the amide I band at 1640 cm(-1) are found to correlate with both donor age and with previously reported fracture toughness data obtained from the same specimens. These results are discussed in the context of possible changes in collagen cross-linking chemistry as a function of age, and are deemed important to further our understanding of the changes in the organic component of the bone matrix with aging.

Nanomechanical properties of facial sutures and sutural mineralization front

The mechanical properties of craniofacial sutures have rarely been investigated. Three facial sutures-the pre-maxillomaxillary (PMS), the nasofrontal (NFS), and the zygomaticotemporal (ZTS)-and their corresponding sutural mineralization fronts in 8 young New Zealand White rabbits were subjected to nano-indentation with atomic force microscopy as a test of the hypothesis that they have different mechanical properties. The average elastic modulus of the PMS was 1.46 +/- 0.24 MPa (mean +/- SD), significantly higher than both the ZTS (1.20 +/- 0.20) and NFS (1.16 +/- 0.18). The average elastic moduli of sutural mineralization fronts 30 micro m away were significantly higher than their corresponding sutures and had the same distribution pattern: the PMS (2.07 +/- 0.24 MPa) significantly higher than both the ZTS (1.56 +/- 0.29) and NFS (1.71 +/- 0.22). Analysis of these data suggests that facial sutures and their immediately adjacent sutural mineralization fronts have different capacities for mechanical deformation. The elastic properties of sutures and sutural mineralization fronts are potentially useful for improving our understanding of their roles in development.

Bone strength and related traits in HcB/Dem recombinant congenic mice

Fracture susceptibility depends jointly on bone mineral content (BMC), gross bone anatomy, and bone microarchitecture and quality. Overall, it has been estimated that 50-70% of bone strength is determined genetically. Because of the difficulty of performing studies of the genetics of bone strength in humans, we have used the HcB/Dem series of recombinant congenic (RC) mice to investigate this phenotype. We performed a comprehensive phenotypic analysis of the HcB/Dem strains including morphological analysis of long bones, measurement of ash percentage, and biomechanical testing. Body mass, ash percentage, and moment of inertia each correlated moderately but imperfectly with biomechanical performance. Several chromosome regions, on chromosomes 1, 2, 8, 10, 11, and 12, show sufficient evidence of linkage to warrant closer examination in further crosses. These studies support the view that mineral content, diaphyseal diameter, and additional nonmineral material properties contributing to overall bone strength are controlled by distinct sets of genes. Moreover, the mapping data are consistent with the existence of pleiotropic loci for bone strength-related phenotypes. These findings show the importance of factors other than mineral content in determining skeletal performance and that these factors can be dissected genetically.

FTIR microscopic imaging of collagen and proteoglycan in bovine cartilage

Articular cartilage, a connective tissue that provides resistance to compressive forces during joint movements, has not been examined in detail by conventional Fourier transform infrared (FTIR) spectroscopy, microspectroscopy (FTIRM), or imaging (FTIRI). The current study reports FTIRM and FTIRI analyses of normal bovine cartilage and identifies the specific molecular components of cartilage that contribute to its IR spectrum. FTIRM data acquired through the superficial, middle, and deep zones of thin sections of bovine articular cartilage showed a variation in intensities of the absorbance bands that arise from the primary nonaqueous components of cartilage, collagen, and proteoglycan (primarily aggrecan) and thus reflected the differences in quantity of these specific components. The spectra of mixtures of model compounds, which had varying proportions of type II collagen and aggrecan, were analyzed to identify spectral markers that could be used to quantitatively analyze these components in cartilage. Collagen and aggrecan were then imaged by FTIRI based on markers found in the model compounds. Polarization experiments were also performed to determine the spatial distribution of the collagen orientation in the different zones of cartilage. This study provides a framework in which complex pathological changes in this heterogeneous tissue can be assessed by IR microscopic imaging.

A physical, chemical, and mechanical study of lumbar vertebrae from normal, ovariectomized, and nandrolone decanoate-treated cynomolgus monkeys (Macaca fascicularis)

Ovariectomized cynomolgus monkeys have previously been investigated as a nonhuman primate model of postmenopausal osteoporosis (Jerome et al., Bone Miner 9:527-540; 1994). In the present study, Fourier transform infrared microspectroscopy (FTIRM) was used to verify that differences in bone mineral quality and quantity in the vertebrae of mature intact (INT) and ovariectomized (ovx) monkeys were analogous to those seen in osteoporotic and nondiseased human bones. FTIRM spectra were acquired from 15 trabeculae per vertebra from three ovx and three INT adult monkeys (mean age 8 years). These spectra were compared with those of both trabecular and previously reported osteonal bone obtained from 3 "normal" and 11 postmenopausal osteoporotic human subjects. While variations in the mineral:matrix ratio (mineral content), carbonate:phosphate ratio, and crystallinity are typical for trabecular bone from iliac crests of normal human subjects, the values of these parameters were relatively static for trabecular bone from postmenopausal osteoporotic human subjects. In general, trabecular bone from postmenopausal osteoporotic human subjects exhibited decreased mineral content (1.0 +/- 0.5 vs. 2.9 +/- 0.6), increased crystallinity, and increased carbonate:phosphate relative to controls. Similarly, trabecular bone from ovariectomized monkeys exhibited lower mineral content (5.8 +/- 0.2) compared with the INT group (6.2 +/- 0.2; p </= 0.05) and contained larger/more perfect apatite crystals (increased crystallinity) with increased carbonate:phosphate ratios. Variations in absolute values were attributable to site differences (ilium vs. vertebrae). To appreciate the importance of mineral properties on mechanical properties, compression testing was performed using cores of monkey L-3 and L-4 vertebral bodies from a separate group of monkeys. Treating monkeys with the anabolic steroid nandrolone decanoate (ND) immediately after ovariectomy and for the next 24 months (ND group), or beginning 12 months after ovariectomy (dND group), increased the ultimate stress compared with an ovx treatment group, despite large interanimal variations in bone architecture and mechanical properties. These data support the hypothesis that ovariectomized adult monkeys are an excellent model for postmenopausal osteoporosis, and can be used for the evaluation of therapeutic modalities.

Application of vibrational spectroscopy to the study of mineralized tissues (review)

The infrared and Raman spectroscopy of bone and teeth tissues are reviewed. Characteristic spectra are obtained for both the mineral and protein components of these tissues. Vibrational spectroscopy is used to study the mineralization process, to define the chemical structure changes accompanying bone diseases, and to characterize interactions between prosthetic implants and tissues. Microspectroscopy allows acquisition of spatially resolved spectra, with micron scale resolution. Recently developed imaging modalities allow tissue imaging with chemical composition contrast.

Elastic and physicochemical relationships within cortical bone

The purpose of this study was to examine the relationships that exist between the elastic properties and the physicochemical properties of cortical bone in two groups of experimental animals. The animal model was the immature mutant dwarf rat, and the groups consisted of rats treated and not treated with recombinant human growth hormone (rhGH). The objective was to establish and broaden the quantifiable link between the three-dimensional form and function of bone beyond the typical unidirectional measures. This study was based on previously reported work that refined the ultrasonic elasticity technique for use with small specimens (<1.0 mm) and determined that the administration of rhGH can counter the degenerative effects produced by hormone-suppressed downregulation on the elastic and physicochemical characteristics of cortical bone. Ultrasonic wave propagation and density measurements were used previously to determine the three-dimensional (orthotropic) material properties of rat femoral cortical bone. X-ray powder diffraction, microscopic, morphometric, and biochemical analysis techniques have been used to describe physicochemical properties, including mineral crystal size, cortical porosity, mineral and nonmineral content, and microstructural characteristics. In this study, mathematical relationships between the local physicochemical (independent variable) and elastic (dependent variable) properties were formulated via linear and nonlinear regression analyses. In general, apparent density was found to have the highest level of correlation with most of the longitudinal and shear moduli (R(2) = 0.300 to 0.800). Concomitantly, mineral crystal width and cortical porosity offered the best correlations with the Poisson's ratios (R(2) up to 0.600). Wilcoxon t tests verified a significant decrease in the elastic properties in dwarf rat cortical bone after rhGH treatments (p < 0.05). Physicochemical measures of bone quality (density, crystal size) generally decreased while measures of bone quantity (cortical area, moments of inertia) generally increased (p < 0.05) after rhGH treatments. Some mineral and nonmineral properties were unchanged. This study presents a quantifiable link between cortical bone elasticity and its composite construction as measured across two dramatically different experimental groups.

Calcitonin alters bone quality in beagle dogs

Because of its antiresorptive properties, calcitonin is widely used to prevent and treat osteoporosis. A stimulatory effect of calcitonin on osteoblasts has also been reported; however, a recent histologic study points to a negative effect of calcitonin on mineralization of cancellous bone. The present experiment was performed to determine whether the observed histological signs of alterations in mineralization are also observed in cortical bone and whether this results in changes in mechanical properties, mineral densities, or mineral properties of canine bone. Sixteen female adult beagle dogs were randomly allocated to receive either human calcitonin at a dose of 0.25 mg/dog (50 IU, n = 8) or vehicle (mannitol, n = 8) every other day for 16 weeks. At the end of the study, the dogs were euthanized. Both tibiae, L1 and L5 vertebrae, and iliac crest bone samples were excised and defleshed. Torsional mechanical properties of tibial diaphyses and compressive strengths of vertebrae were measured. Bone mineral densities (BMD) of tibiae and vertebrae were measured by using dual-energy X-ray absorptiometry. Ultrastructural mineral characteristics of iliac crest bone were determined by gravimetry and Fourier transform infrared spectroscopy (FTIR). Bone histomorphometry was performed in the cortical envelope of the iliac crest. Tibiae from dogs treated with calcitonin withstood significantly less maximum torque until failure, required less torsional energy to reach the maximum torque, and had less torsional stiffness than the tibiae from dogs treated with vehicle (p < 0.05). Cancellous cores of vertebrae from calcitonin-treated dogs withstood less compressive mechanical loading than did vertebral cores from vehicle-treated animals (p < 0.05). Dogs treated with calcitonin had less BMD of both tibiae and vertebrae than vehicle-treated animals (p < 0.05). Bones from calcitonin-treated dogs had significantly less ash content, which correlated with the lower phosphate-to-amide I (detected by FTIR) and greater carbonate-to-phosphate ratios than did bones from vehicle-treated dogs (p < 0.05). Calcitonin-treated dogs exhibited a decrease in bone formation and mineralization rates and an increase in mineralization lag time. These results point to a negative effect of calcitonin on bone quality. These findings are intriguing and call for further studies addressing whether the observed abnormalities are transient or permanent.

Mineral changes in a mouse model of osteogenesis imperfecta detected by Fourier transform infrared microscopy

Osteogenesis imperfecta (OI) is a heritable disease characterized by skeletal deformities and brittle bones. In the current study, the nature of the mineral in long bones of a mouse model of OI (oim/oim, a mutant which produces an alpha 1(I) collagen homotrimer) was examined by Fourier transform infrared microscopy. The mineral:matrix ratio of oim/oim cortical bone was greater than that of the heterozygous oim/+ and of the normal +/+ bones, probably as a result of reduced collagen content. The molecular environments of the apatitic phosphates differed among the oim/oim and the oim/+ and the +/+ bones. This was attributable to several factors, including dissimilar mineral-matrix interactions and differences in the chemical composition of the mineral. It was concluded from these data that the defective collagen matrix leads to abnormal mineral formation at the molecular level and thus results in tissues with reduced mechanical properties.