Comparison of bone tissue properties in mouse models with collagenous and non-collagenous genetic mutations using FTIRI

Association of trabecular bone score and bone mineral apparent density with the severity of bone fragility in children and adolescents with osteogenesis imperfecta: A cross-sectional study

Osteogenesis imperfecta (OI) is a hereditary skeletal disease characterized by bone fragility. Areal bone mineral density (BMD), evaluated by dual-energy X-ray absorptiometry (DXA), is used to assess bone brittleness. The height-adjusted BMD Z-score (BMDHAZ) is calculated in children and adolescents with OI to reduce the confounding factor of short stature. However, even with the BMDHAZ, severity evaluation in children and adolescents with OI is challenging because certain abnormalities in bone quality cannot be accurately assessed by BMD analysis. The trabecular bone scores (TBS) and bone mineral apparent density (BMAD), which represent the structural integrity of bone and bone-size-associated BMD, respectively, are associated with fracture risk. Recently, age- and sex-specific reference ranges have been reported, enabling the calculation of Z-scores for children. To evaluate which density measurements show the highest correlation with fracture risk, we analyzed the associations between the Z-scores of TBS, BMAD, and BMDHAZ, fracture rate, and genetic variants. We retrospectively reviewed 42 participants with OI aged 5 to 20 years who underwent DXA. COL1A1/2 pathogenic variants were detected in 41 of the 42 participants. In participants with nonsense and frameshift variants (n = 17) resulting in haploinsufficiency and mild phenotype, the TBS Z-score was negatively correlated with fracture rate (FR) (r = -0.50, p = 0.042). In participants with glycine substitution (n = 9) causing the severe phenotype, the BMAD Z-scores were negatively correlated with FR (r = -0.74, p = 0.022). No correlation between the BMDHAZ and FR was observed in both groups. These findings suggest that the TBS and BMAD are useful in assessing children and adolescents with OI with specific genetic variants.

Alterations of bone material properties in growing Ifitm5/BRIL p.S42 knock-in mice, a new model for atypical type VI osteogenesis imperfecta

INTRODUCTION

Osteogenesis imperfecta (OI) is a heterogenous group of heritable connective tissue disorders characterized by high bone fragility due to low bone mass and impaired bone material properties. Atypical type VI OI is an extremely rare and severe form of bone dysplasia resulting from a loss-of-function mutation (p.S40L) in IFITM5/BRIL,the causative gene of OI type V and decreased osteoblast secretion of pigment epithelium-derived factor (PEDF), as in OI type VI. It is not yet known which alterations at the material level might lead to such a severe phenotype. We therefore characterized bone tissue at the micrometer level in a novel heterozygous Ifitm5/BRIL p.S42L knock-in murine model at 4 and 8 weeks of age.

METHODS

We evaluated in female mice, total body size, femoral and lumbar bone mineral density (BMD) by dual-energy X-ray absorptiometry. In the femoral bone we examined osteoid deposition by light microscopy, assessed bone histomorphometry and mineralization density distribution by quantitative backscattered electron imaging (qBEI). Osteocyte lacunae were examined by qBEI and the osteocyte lacuno-canalicular network by confocal laser scanning microscopy. Vasculature was examined indirectly by qBEI as 2D porosity in cortex, and as 3D porosity by micro-CT in third trochanter. Collagen orientation was examined by second harmonic generation microscopy. Two-way ANOVA was used to discriminate the effect of age and genotype.

RESULTS

Ifitm5/BRIL p.S42L female mice are viable, do not differ in body size, fat and lean mass from wild type (WT) littermates but have lower whole-body, lumbar and femoral BMD and multiple fractures. The average and most frequent calcium concentration, CaMean and CaPeak, increased with age in metaphyseal and cortical bone in both genotypes and were always higher in Ifitm5/BRIL p.S42L than in WT, except CaMean in metaphysis at 4 weeks of age. The fraction of highly mineralized bone area, CaHigh, was also increased in Ifitm5/BRIL p.S42L metaphyseal bone at 8 weeks of age and at both ages in cortical bone. The fraction of lowly mineralized bone area, CaLow, decreased with age and was not higher in Ifitm5/BRIL p.S42L, consistent with lack of hyperosteoidosis on histological sections by visual exam. Osteocyte lacunae density was higher in Ifitm5/BRIL p.S42L than WT, whereas canalicular density was decreased. Indirect measurements of vascularity revealed a higher pore density at 4 weeks in cortical bone of Ifitm5/BRIL p.S42L than in WT and at both ages in the third trochanter. Importantly, the proportion of bone area with disordered collagen fibrils was highly increased in Ifitm5/BRIL p.S42L at both ages.

CONCLUSIONS

Despite normal skeletal growth and the lack of a collagen gene mutation, the Ifitm5/BRIL p.S42L mouse shows major OI-related bone tissue alterations such as hypermineralization of the matrix and elevated osteocyte porosity. Together with the disordered lacuno-canalicular network and the disordered collagen fibril orientation, these abnormalities likely contribute to overall bone fragility.

Abnormal Bone Tissue Organization and Osteocyte Lacunocanalicular Network in Early-Onset Osteoporosis Due to SGMS2 Mutations

Pathological variants in SGMS2, encoding sphingomyelin synthase 2 (SMS2), result in a rare autosomal dominant skeletal disorder with cranial doughnut lesions. The disease manifests as early-onset osteoporosis or a more severe skeletal dysplasia with low bone mineral density, frequent fractures, long-bone deformities, and multiple sclerotic cranial lesions. The exact underlying molecular features and skeletal consequences, however, remain elusive. This study investigated bone tissue characteristics in two adult males with a heterozygous SGMS2 mutation p.Arg50* and significant bone fragility. Transiliac bone biopsy samples from both (patient 1: 61 years; patient 2: 29 years) were analyzed by bone histomorphometry, confocal laser scanning microscopy, and quantitative backscattered electron imaging (qBEI). Bone histomorphometry portrayed largely normal values for structural and turnover parameters, but in both patient 1 and patient 2, respectively, osteoid thickness (-1.80 SD, -1.37 SD) and mineralizing surface (-1.03 SD, -2.73 SD) were reduced and osteoid surface increased (+9.03 SD, +0.98 SD), leading to elevated mineralization lag time (+8.16 SD, +4.10 SD). qBEI showed low and heterogeneous matrix mineralization (CaPeak -2.41 SD, -3.72 SD; CaWidth +7.47 SD, +4.41 SD) with a chaotic arrangement of collagenous fibrils under polarized light. Last, osteocyte lacunae appeared abnormally large and round in shape and the canalicular network severely disturbed with short-spanned canaliculi lacking any orderliness or continuity. Taken together, these data underline a central role for functional SMS2 in bone matrix organization and mineralization, lacunocanalicular network, and in maintaining skeletal strength and integrity. These data bring new knowledge on changes in bone histology resulting from abnormal sphingomyelin metabolism and aid en route to better understanding of sphingolipid-related skeletal disorders. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Intrafibrillar mineralization deficiency and osteogenesis imperfecta mouse bone fragility

Osteogenesis imperfecta (OI), a brittle bone disease, is known to result in severe bone fragility. However, its ultrastructural origins are still poorly understood. In this study, we hypothesized that deficient intrafibrillar mineralization is a key contributor to the OI induced bone brittleness. To test this hypothesis, we explored the mechanical and ultrastructural changes in OI bone using the osteogenesis imperfecta murine (oim) model. Synchrotron X-ray scattering experiments indicated that oim bone had much less intrafibrillar mineralization than wild type bone, thus verifying that the loss of mineral crystals indeed primarily occurred in the intrafibrillar space of oim bone. It was also found that the mineral crystals were organized from preferentially in longitudinal axis in wild type bone to more randomly in oim bone. Moreover, it revealed that the deformation of mineral crystals was more coordinated with collagen fibrils in wild type than in oim bone, suggesting that the load transfer deteriorated between the two phases in oim bone. The micropillar test revealed that the compression work to fracture of oim bone (8.2 ± 0.9 MJ/m³) was significantly smaller (p < 0.05) than that of wild type bone (13.9 ± 2.7 MJ/m³), while the bone strength was not statistically different (p > 0.05) between the two genotype groups. In contrast, the uniaxial tensile test showed that the ultimate strength of wild type bone (50 ± 4.5 MPa) was significantly greater (p < 0.05) than that of oim bone (38 ± 5.3 MPa). Furthermore, the nanoscratch test showed that the toughness of oim bone was much less than that of wild type bone (6.6 ± 2.2 GJ/m³ vs. 12.6 ± 1.4 GJ/m³). Finally, in silico simulations using a finite element model of sub-lamellar bone confirmed the links between the reduced intrafibrillar mineralization and the observed changes in the mechanical behavior of OI bone. Taken together, these results provide important mechanistic insights into the underlying cause of poor mechanical quality of OI bone, thus pave the way toward future treatments of this brittle bone disease.

Raman and Fourier transform infrared imaging for characterization of bone material properties

As the application of Raman spectroscopy to study bone has grown over the past decade, making it a peer technology to FTIR spectroscopy, it has become critical to understand their complimentary roles. Recent technological advancements have allowed these techniques to collect grids of spectra in a spatially resolved fashion to generate compositional images. The advantage of imaging with these techniques is that it allows the heterogenous bone tissue composition to be resolved and quantified. In this review we compare, for non-experts in the field of vibrational spectroscopy, the instrumentation and underlying physical principles of FTIR imaging (FTIRI) and Raman imaging. Additionally, we discuss the strengths and limitations of FTIR and Raman spectroscopy, address sample preparation, and discuss outcomes to provide researchers insight into which techniques are best suited for a given research question. We then briefly discuss previous applications of FTIRI and Raman imaging to characterize bone tissue composition and relationships of compositional outcomes with mechanical performance. Finally, we discuss emerging technical developments in FTIRI and Raman imaging which provide new opportunities to identify changes in bone tissue composition with disease, age, and drug treatment.

Effects of teriparatide on bone in autochthonous transgenic model mice for diabetes mellitus (Akita mice)

OBJECTIVES

The purpose of this study is to evaluate the effects of teriparatide (TPTD) on bone mineral density (BMD), bone strength, and bone quality in Akita mouse models of diabetes mellitus.

METHODS

Twelve-week-old female Akita mice and control mice (C57/BL/6NCrSlc) were divided into 4 groups: control mice treated with vehicle (n = 7) or TPTD (n = 6); and Akita mice treated with vehicle (n = 6) or TPTD (n = 7). TPTD or vehicle was administered subcutaneously 3 times a week for 8 weeks. Blood glucose, serum sclerostin, total tibial BMD, femoral shaft bone strength, and bone quality using Fourier-transform infrared spectroscopy imaging were evaluated.

RESULTS

No significant differences in serum sclerostin levels were evident among these groups after 8 weeks of treatment. TPTD significantly increased BMD in control mice (+12.7%, P = 0.02) and Akita mice (+29.2%, P = 0.001) compared with vehicle. Maximum load and stiffness were significantly higher in Akita mice treated with TPTD than in Akita mice treated with vehicle (+56.6%, P = 0.03 and + 90.5%, P = 0.02, respectively). On Fourier-transform infrared spectroscopy imaging, the mineral/matrix ratio was significantly lower in Akita mice treated with vehicle than in control mice (-12.2%, P = 0.02), and TPTD treatment significantly increased the mineral/matrix ratio (P = 0.003).

CONCLUSIONS

TPTD thus improved BMD and bone strength in both control mice and Akita mice, with improvements in the mineral/matrix ratio among Akita mice.

Impact of vitamin C on teriparatide treatment in the improvement of bone mineral density, strength, and quality in vitamin C-deficient rats

Age-related decreases in serum levels of vitamin C (VC) may negatively affect the efficacy of anti-osteoporotic pharmacotherapy. The purpose of this study was to evaluate the effects of VC and teriparatide (TPTD) on bone mineral density (BMD), strength, and quality in VC-deficient osteogenic disorder Shionogi (ODS) rats. Six-month-old female ODS rats were divided into an untreated ODS control group, a VC group, a TPTD group, and a VC + TPTD group, based on the administration of VC and TPTD (n = 10 each). VC was given as 2.0 mg/ml supplemented water. TPTD was administered subcutaneously once a week at 30 µg/kg body weight. After 12 weeks of treatment, BMDs of the femur and lumbar spine, bone strengths of the femoral diaphysis and metaphysis, and cancellous bone quality of proximal tibiae as estimated by Fourier transform infrared spectroscopy (FTIR) were compared between groups. Compared to the ODS control group, the VC group showed significantly higher total femoral BMD, but the TPTD group showed significantly higher femoral and lumbar spinal BMD, maximum load of femoral metaphysis, and hydroxyapatite (HA) crystallinity by FTIR (p < 0.05). In addition to the increases shown in the TPTD group, the VC + TPTD group also showed significantly higher stiffness of the femoral diaphysis and breaking energy of the femoral metaphysis compared to the ODS control group (p < 0.05). These results indicated that TPTD alone increased cancellous/cortical BMD and cancellous bone strength with improvement of HA crystallinity in ODS rats, but addition of VC supplementation further improved cortical bone strength.

Role of SMPD3 during Bone Fracture Healing and Regulation of Its Expression

Sphingomyelin phosphodiesterase 3 (SMPD3), a lipid-metabolizing enzyme present in bone and cartilage, has important roles in the developing skeleton. We previously showed that SMPD3 deficiency results in delayed extracellular matrix (ECM) mineralization and severe skeletal deformities in an inducible knockout mouse model, Smpd3^flox/flox ; Osx-Cre mice, in which Smpd3 was ablated in Osx-expressing chondrocytes and osteoblasts during early skeletogenesis. However, as shown in the current study, ablation of Smpd3 postnatally in 3-month-old Smpd3^flox/flox ; Osx-Cre mice resulted in only a mild bone mineralization defect. Interestingly, though, there was a marked increase of unmineralized osteoid in the fractured tibiae of 3-month-old Smpd3^flox/flox ; Osx-Cre mice. As was the case in the embryonic bones, we also observed impaired chondrocyte apoptosis at the fracture sites of Smpd3^flox/flox ; Osx-Cre mice. We further examined how Smpd3 expression is regulated in ATDC5 chondrogenic cells by two major regulators of chondrogenesis, bone morphogenetic protein 2 (BMP-2) and PTHrP. Our data show that BMP-2 positively regulates Smpd3 expression via p38 mitogen-activated protein kinase. Taken together, our findings show that SMPD3 plays a significant role in ECM mineralization and chondrocyte apoptosis during fracture healing. Furthermore, our gene expression analyses suggest that BMP-2 and PTHrP exert opposing effects on the regulation of Smpd3 expression in chondrocytes.

Polarized infrared spectroscopy imaging applied to structural analysis of bilirubin aggregate at liquid-liquid interface

Fourier transform infrared spectroscopy imaging (FTIRSI) combined with spectral analysis and polarization approach was creatively used to investigate both structures of bilirubin (BR) precipitate and BR aggregate at liquid-liquid interface. It was found by spectral analysis that the internal hydrogen bonds of BR molecules all broke and the dihedral angles increased during the formation of BR aggregate at liquid-liquid interface. And the BR molecule might be of layer assembly along the long axis direction of CD half-group to form J-type aggregates, which could be parallel to the direction of the transition dipole moment of BR aggregate. The further study of polarized imaging/anisotropy revealed that the absorbance of 1570 and 1703 cm^-1 bands of BR aggregate changed periodically at intervals of 90°, which were not shown in BR precipitate case, indicating that the CC of the corresponding lactam ring and the CO of the adjacent carboxyl groups formed ordered arrangement in BR aggregate. It also suggested that the two positions might be the active sites which J-type aggregates assembled on. The combined technique was firstly applied in interfacial aggregate research, which was helpful for further understanding and controlling the aggregation as well as structural transformation of BR molecules so as to decrease physiological hazard and facilitate the wide spread application in biomedicine.

Bone quality changes associated with aging and disease: a review

Bone quality encompasses all the characteristics of bone that, in addition to density, contribute to its resistance to fracture. In this review, we consider changes in architecture, porosity, and composition, including collagen structure, mineral composition, and crystal size. These factors all are known to vary with tissue and animal ages, and health status. Bone morphology and presence of microcracks, which also contribute to bone quality, will not be discussed in this review. Correlations with mechanical performance for collagen cross-linking, crystallinity, and carbonate content are contrasted with mineral content. Age-dependent changes in humans and rodents are discussed in relation to rodent models of disease. Examples are osteoporosis, osteomalacia, osteogenesis imperfecta (OI), and osteopetrosis in both humans and animal models. Each of these conditions, along with aging, is associated with increased fracture risk for distinct reasons.

Correlations Between Bone Mechanical Properties and Bone Composition Parameters in Mouse Models of Dominant and Recessive Osteogenesis Imperfecta and the Response to Anti-TGF-β Treatment

Osteogenesis imperfecta (OI) is a group of genetic disorders characterized by brittle bones that are prone to fracture. Although previous studies in animal models investigated the mechanical properties and material composition of OI bone, little work has been conducted to statistically correlate these parameters to identify key compositional contributors to the impaired bone mechanical behaviors in OI. Further, although increased TGF-β signaling has been demonstrated as a contributing mechanism to the bone pathology in OI models, the relationship between mechanical properties and bone composition after anti-TGF-β treatment in OI has not been studied. Here, we performed follow-up analyses of femurs collected in an earlier study from OI mice with and without anti-TGF-β treatment from both recessive (Crtap^-/- ) and dominant (Col1a2^+/P.G610C ) OI mouse models and WT mice. Mechanical properties were determined using three-point bending tests and evaluated for statistical correlation with molecular composition in bone tissue assessed by Raman spectroscopy. Statistical regression analysis was conducted to determine significant compositional determinants of mechanical integrity. Interestingly, we found differences in the relationships between bone composition and mechanical properties and in the response to anti-TGF-β treatment. Femurs of both OI models exhibited increased brittleness, which was associated with reduced collagen content and carbonate substitution. In the Col1a2^+/P.G610C femurs, reduced hydroxyapatite crystallinity was also found to be associated with increased brittleness, and increased mineral-to-collagen ratio was correlated with increased ultimate strength, elastic modulus, and bone brittleness. In both models of OI, regression analysis demonstrated that collagen content was an important predictor of the increased brittleness. In summary, this work provides new insights into the relationships between bone composition and material properties in models of OI, identifies key bone compositional parameters that correlate with the impaired mechanical integrity of OI bone, and explores the effects of anti-TGF-β treatment on bone-quality parameters in these models. © 2016 American Society for Bone and Mineral Research.

Decreasing maternal myostatin programs adult offspring bone strength in a mouse model of osteogenesis imperfecta

During fetal development, the uterine environment can have effects on offspring bone architecture and integrity that persist into adulthood; however, the biochemical and molecular mechanisms remain unknown. Myostatin is a negative regulator of muscle mass. Parental myostatin deficiency (Mstn^tm1Sjl/+) increases muscle mass in wild-type offspring, suggesting an intrauterine programming effect. Here, we hypothesized that Mstn^tm1Sjl/+ dams would also confer increased bone strength. In wild-type offspring, maternal myostatin deficiency altered fetal growth and calvarial collagen content of newborn mice and conferred a lasting impact on bone geometry and biomechanical integrity of offspring at 4 mo of age, the age of peak bone mass. Second, we sought to apply maternal myostatin deficiency to a mouse model with osteogenesis imperfecta (Col1a2^oim), a heritable connective tissue disorder caused by abnormalities in the structure and/or synthesis of type I collagen. Femora of male Col1a2^oim/+ offspring from natural mating of Mstn^tm1Sjl/+ dams to Col1a2^oim/+sires had a 15% increase in torsional ultimate strength, a 29% increase in tensile strength, and a 24% increase in energy to failure compared with age, sex, and genotype-matched offspring from natural mating of Col1a2^oim/+ dams to Col1a2^oim/+ sires. Finally, increased bone biomechanical strength of Col1a2^oim/+ offspring that had been transferred into Mstn^tm1Sjl/+ dams as blastocysts demonstrated that the effects of maternal myostatin deficiency were conferred by the postimplantation environment. Thus, targeting the gestational environment, and specifically prenatal myostatin pathways, provides a potential therapeutic window and an approach for treating osteogenesis imperfecta.

Smpd3 Expression in both Chondrocytes and Osteoblasts Is Required for Normal Endochondral Bone Development

Sphingomyelin phosphodiesterase 3 (SMPD3), a lipid-metabolizing enzyme present in bone and cartilage, has been identified to be a key regulator of skeletal development. A homozygous loss-of-function mutation called fragilitas ossium (fro) in the Smpd3 gene causes poor bone and cartilage mineralization resulting in severe congenital skeletal deformities. Here we show that Smpd3 expression in ATDC5 chondrogenic cells is downregulated by parathyroid hormone-related peptide through transcription factor SOX9. Furthermore, we show that transgenic expression of Smpd3 in the chondrocytes of fro/fro mice corrects the cartilage but not the bone abnormalities. Additionally, we report the generation of Smpd3(flox/flox) mice for the tissue-specific inactivation of Smpd3 using the Cre-loxP system. We found that the skeletal phenotype in Smpd3(flox/flox); Osx-Cre mice, in which the Smpd3 gene is ablated in both late-stage chondrocytes and osteoblasts, closely mimics the skeletal phenotype in fro/fro mice. On the other hand, Smpd3(flox/flox); Col2a1-Cre mice, in which the Smpd3 gene is knocked out in chondrocytes only, recapitulate the fro/fro mouse cartilage phenotype. This work demonstrates that Smpd3 expression in both chondrocytes and osteoblasts is required for normal endochondral bone development.

Bone mineral properties in growing Col1a2(+/G610C) mice, an animal model of osteogenesis imperfecta

The Col1a2(+/G610C) knock-in mouse, models osteogenesis imperfecta in a large old order Amish family (OOA) with type IV OI, caused by a G-to-T transversion at nucleotide 2098, which alters the gly-610 codon in the triple-helical domain of the α2(I) chain of type I collagen. Mineral and matrix properties of the long bones and vertebrae of male Col1a2(+/G610C) and their wild-type controls (Col1a2(+/+)), were characterized to gain insight into the role of α2-chain collagen mutations in mineralization. Additionally, we examined the rescuability of the composition by sclerostin inhibition initiated by crossing Col1a2(+/G610C) with an LRP(+/A214V) high bone mass allele. At age 10-days, vertebrae and tibia showed few alterations by micro-CT or Fourier transform infrared imaging (FTIRI). At 2-months-of-age, Col1a2(+/G610C) tibias had 13% fewer secondary trabeculae than Col1a2(+/+), these were thinner (11%) and more widely spaced (20%) than those of Col1a2(+/+) mice. Vertebrae of Col1a2(+/G610C) mice at 2-months also had lower bone volume fraction (38%), trabecular number (13%), thickness (13%) and connectivity density (32%) compared to Col1(a2+/+). The cortical bone of Col1a2(+/G610C) tibias at 2-months had 3% higher tissue mineral density compared to Col1a2(+/+); Col1a2(+/G610C) vertebrae had lower cortical thickness (29%), bone area (37%) and polar moment of inertia (38%) relative to Col1a2(+/+). FTIRI analysis, which provides information on bone chemical composition at ~7μm-spatial resolution, showed tibias at 10-days did not differ between genotypes. Comparing identical bone types in Col1a2(+/G610C) to Col1a2(+/+) at 2-months-of-age, tibias showed higher mineral-to-matrix ratio in trabeculae (17%) and cortices (31%). and in vertebral cortices (28%). Collagen maturity was 42% higher at 10-days-of-age in Col1a2(+/G610C) vertebral trabeculae and in 2-month tibial cortices (12%), vertebral trabeculae (42%) and vertebral cortices (12%). Higher acid-phosphate substitution was noted in 10-day-old trabecular bone in vertebrae (31%) and in 2-month old trabecular bone in both tibia (31%) and vertebrae (4%). There was also a 16% lower carbonate-to-phosphate ratio in vertebral trabeculae and a correspondingly higher (22%) carbonate-to-phosphate ratio in 2month-old vertebral cortices. At age 3-months-of-age, male femurs with both a Col1a2(+/G610C) allele and a Lrp5 high bone mass allele (Lrp5+/A214V) showed an improvement in bone composition, presenting higher trabecular carbonate-to-phosphate ratio (18%) and lower trabecular and cortical acid-phosphate substitutions (8% and 18%, respectively). Together, these results indicate that mutant collagen α2(I) chain affects both bone quantity and composition, and the usefulness of this model for studies of potential OI therapies such as anti-sclerostin treatments.

Bone matrix hypermineralization in prolyl-3 hydroxylase 1 deficient mice

Lack of prolyl 3-hydroxylase 1 (P3H1) due to mutations in P3H1 results in severe forms of recessive osteogenesis imperfecta. In the present study, we investigated the bone tissue characteristics of P3H1 null mice. Histomorphometric analyses of cancellous bone in the proximal tibia and lumbar vertebra in 1-month and 3-month old mice demonstrated that P3H1 deficient mice had low trabecular bone volume and low mineral apposition rate, but normal osteoid maturation time and normal osteoblast and osteoclast surfaces. Quantitative backscattered electron imaging revealed that the bone mineralization density distribution was shifted towards higher values, indicating hypermineralization of bone matrix. It thus appears that P3H1 deficiency leads to decreased deposition of extracellular matrix by osteoblasts and increased incorporation of mineral into the matrix.

Makings of a brittle bone: Unexpected lessons from a low protein diet study of a mouse OI model

Glycine substitutions in type I collagen appear to cause osteogenesis imperfecta (OI) by disrupting folding of the triple helix, the structure of which requires Gly in every third position. It is less clear, however, whether the resulting bone malformations and fragility are caused by effects of intracellular accumulation of misfolded collagen on differentiation and function of osteoblasts, effects of secreted misfolded collagen on the function of bone matrix, or both. Here we describe a study originally conceived for testing how reducing intracellular accumulation of misfolded collagen would affect mice with a Gly610 to Cys substitution in the triple helical region of the α2(I) chain. To stimulate degradation of misfolded collagen by autophagy, we utilized a low protein diet. The diet had beneficial effects on osteoblast differentiation and bone matrix mineralization, but also affected bone modeling and suppressed overall animal growth. Our more important observations, however, were not related to the diet. They revealed how altered osteoblast function and deficient bone formation by each cell caused by the G610C mutation combined with increased osteoblastogenesis might make the bone more brittle, all of which are common OI features. In G610C mice, increased bone formation surface compensated for reduced mineral apposition rate, resulting in normal cortical area and thickness at the cost of altering cortical modeling process, retaining woven bone, and reducing the ability of bone to absorb energy through plastic deformation. Reduced collagen and increased mineral density in extracellular matrix of lamellar bone compounded the problem, further reducing bone toughness. The latter observations might have particularly important implications for understanding OI pathophysiology and designing more effective therapeutic interventions.

Myostatin deficiency partially rescues the bone phenotype of osteogenesis imperfecta model mice

Mice with osteogenesis imperfecta (+/oim), a disorder of bone fragility, were bred to mice with muscle over growth to test whether increasing muscle mass genetically would improve bone quality and strength. The results demonstrate that femora from mice carrying both mutations have greater mechanical integrity than their +/oim littermates.

INTRODUCTION

Osteogenesis imperfecta is a heritable connective tissue disorder due primarily to mutations in the type I collagen genes resulting in skeletal deformity and fragility. Currently, there is no cure, and therapeutic strategies encompass the use of antiresorptive pharmaceuticals and surgical bracing, with limited success and significant potential for adverse effects. Bone, a mechanosensing organ, can respond to high mechanical loads by increasing new bone formation and altering bone geometry to withstand increased forces. Skeletal muscle is a major source of physiological loading on bone, and bone strength is proportional to muscle mass.

METHODS

To test the hypothesis that congenic increases in muscle mass in the osteogenesis imperfecta murine model mouse (oim) will improve their compromised bone quality and strength, heterozygous (+/oim) mice were bred to mice deficient in myostatin (+/mstn), a negative regulator of muscle growth. The resulting adult offspring were evaluated for hindlimb muscle mass, and bone microarchitecture, physiochemistry, and biomechanical integrity.

RESULTS

+/oim mice deficient in myostatin (+/mstn +/oim) were generated and demonstrated that myostatin deficiency increased body weight, muscle mass, and biomechanical strength in +/mstn +/oim mice as compared to +/oim mice. Additionally, myostatin deficiency altered the physiochemical properties of the +/oim bone but did not alter bone remodeling.

CONCLUSIONS

Myostatin deficiency partially improved the reduced femoral bone biomechanical strength of adult +/oim mice by increasing muscle mass with concomitant improvements in bone microarchitecture and physiochemical properties.

Are Changes in Composition in Response to Treatment of a Mouse Model of Osteogenesis Imperfecta Sex-dependent?

BACKGROUND

Osteogenesis imperfecta (OI) is a genetic disease characterized by skeletal fragility and deformity. There is extensive debate regarding treatment options in adults with OI. Antiresorptive treatment reduces the number of fractures in growing oim/oim mice, an animal model that reproducibly mimics the moderate-to-severe form of OI in humans. Effects of long-term treatments with antiresorptive agents, considered for treatment of older patients with OI with similar presentation (moderate-to-severe OI) are, to date, unknown.

QUESTIONS/PURPOSES

Fourier transform infrared (FTIR) imaging, which produces a map of the spatial variation in chemical composition in thin sections of bone, was used to address the following questions: (1) do oim/oim mice show a sex dependence in compositional properties at 6.5 months of age; (2) is there a sex-dependent response to treatment with antiresorptive agents used in the treatment of OI in humans; and (3) are any compositional parameters in oim/oim mice corrected to wild-type (WT) values after treatment?

METHODS

FTIR imaging data were collected from femurs from four to five mice per sex per genotype per treatment. Treatments were 24 weeks of saline, alendronate, or RANK-Fc; and 12 weeks of saline+12 weeks RANK-Fc and 12 weeks of alendronate+RANK-Fc. FTIR imaging compositional parameters measured in cortical and cancellous bones were mineral-to-matrix ratio, carbonate-to-mineral ratio, crystal size/perfection, acid phosphate substitution, collagen maturity, and their respective distributions (heterogeneities). Because of the small sample size, nonparametric statistics (Mann-Whitney U- and Kruskal-Wallis tests with Bonferroni correction) were used to compare saline-treated male and female mice of different genotypes and treatment effects by sex and genotype, respectively. Statistical significance was defined as p<0.05.

RESULTS

At 6.5 months, saline-treated male cortical oim/oim bone had increased mineral-to-matrix ratio (p=0.016), increased acid phosphate substitution (p=0.032), and decreased carbonate-to-mineral ratio (p=0.016) relative to WT. Cancellous bone in male oim/oim also had increased mineral-to-matrix ratio (p=0.016) relative to male WT. Female oim/oim mouse bone composition for all cortical and cancellous bone parameters was comparable to WT (p>0.05). Only the female WT mice showed a response of mean compositional properties to treatment, increasing mineral-to-matrix after RANK-Fc treatment in cancellous bone (p=0.036) compared with saline-treated mice. Male oim/oim increased mineral-to-matrix cortical and cancellous bone heterogeneity in response to all long-term treatments except for saline+RANK-Fc (p<0.04); female oim/oim cortical mineral-to-matrix bone heterogeneity increased with ALN+RANK-Fc and all treatments increased cancellous female oim/oim bone acid phosphate substitution heterogeneity (p<0.04).

CONCLUSIONS

Both oim/oim and WT mice, which demonstrate sex-dependent differences in composition with saline treatment, showed few responses to long-term treatment with antiresorptive agents. Female WT mice appeared to be more responsive; male oim/oim mice showed more changes in compositional heterogeneity. Changes in bone composition caused by these agents may contribute to improved bone quality in oim/oim mice, because the treatments are known to reduce fracture incidence.

CLINICAL RELEVANCE

The optimal drug therapy for long-term treatment of patients with moderate-to-severe OI is unknown. Based on bone compositional changes in mice, antiresorptive treatments are useful for continued treatment in OI. There is a reported sexual dimorphism in fracture incidence in adults with OI, but to date, no one has reported differences in response to pharmaceutical intervention. This study suggests that such an investigation is warranted.

Biomodification of PCL Scaffolds with Matrigel, HA, and SR1 Enhances De Novo Ectopic Bone Marrow Formation Induced by rhBMP-2

The de novo formation of ectopic bone marrow was induced using 1.2-mm-thin polycaprolactone (PCL) scaffolds biomodified with several different biomaterials. In vivo investigations of de novo bone and bone marrow formation indicated that subcutaneous implantation of PCL scaffolds coated with recombinant human bone morphogenetic protein-2 (rhBMP-2) plus Matrigel, hydroxyapatite (HA), or StemRegenin 1 (SR1) improved formation of bone and hematopoietic bone marrow as determined by microcomputed tomography, and histological and hematopoietic characterizations. Our study provides evidence that thin PCL scaffolds biomodified with Matrigel, HA, and SR1 mimic the environments of real bone and bone marrow, thereby enhancing the de novo ectopic bone marrow formation induced by rhBMP-2. This ectopic bone marrow model will serve as a unique and essential tool for basic research and for clinical applications of postnatal tissue engineering and organ regeneration.

Unique micro- and nano-scale mineralization pattern of human osteogenesis imperfecta type VI bone

Osteogenesis imperfecta (OI) is a heterogeneous group of inheritable connective tissue disorders characterized by mutation in genes involved in collagen synthesis and leading to increased bone fragility, low bone mass, impaired bone material properties and abnormally high bone matrix mineralization. Recessive OI type VI is caused by mutation in SERPINF1 leading to a loss-of-function of pigment epithelium-derived factor (PEDF) a collagen-binding protein with potent antiangiogenic activity. Affected patients develop a severe OI phenotype with a striking histological characteristic, rare in other OI types, of an excess of osteoid tissue and prolonged mineralization lag time. To get insights into matrix mineralization, we evaluated biopsies from 9 affected children by quantitative and by high-resolution backscattered electron imaging and assessed bone mineralization density distribution. Thickness, shape and arrangement of mineral particles were measured in a subset of 4 patients by synchrotron small angle X-ray scattering. Typical calcium content in the bone matrix was found to be increased compared to controls, even exceeding values found previously in OI patients with collagen-gene mutations. A main characteristic however, is the coexistence of this highly mineralized bone matrix with seams showing abnormally low mineral content. Atypical collagen fibril organization was found in the perilacunar region of young osteocytes, suggesting a disturbance in the early steps of mineralization. These observations are consistent with the presence of a heterogeneous population of mineral particles with unusual size, shape and arrangement, especially in the region with lower mineral content. The majority of the particles in the highly mineralized bone areas were less disorganized, but smaller and more densely packed than in controls and in previously measured OI patients. These data suggest that the lack of PEDF impairs a proper osteoblast-osteocyte transition and consequently affects the early steps of mineralization, downstream collagen assembly making OI type VI different from "classical" OI with mutations in collagen-type I encoding genes, despite the typical hypermineralization of the bone matrix.

Sphingolipid metabolism and its role in the skeletal tissues

The regulators affecting skeletal tissue formation and its maintenance include a wide array of molecules with very diverse functions. More recently, sphingolipids have been added to this growing list of regulatory molecules in the skeletal tissues. Sphingolipids are integral parts of various lipid membranes present in the cells and organelles. For a long time, these macromolecules were considered as inert structural elements. This view, however, has radically changed in recent years as sphingolipids are now recognized as important second messengers for signal-transduction pathways that affect cell growth, differentiation, stress responses and programmed death. In the current review, we discuss the available data showing the roles of various sphingolipids in three different skeletal cell types-chondrocytes in cartilage and osteoblasts and osteoclasts in bone. We provide an overview of the biology of sphingomyelin phosphodiesterase 3 (SMPD3), an important regulator of sphingolipid metabolism in the skeleton. SMPD3 is localized in the plasma membrane and has been shown to cleave sphingomyelin to generate ceramide, a bioactive lipid second messenger, and phosphocholine, an essential nutrient. SMPD3 deficiency in mice impairs the mineralization in both cartilage and bone extracellular matrices leading to severe skeletal deformities. A detailed understanding of SMPD3 function may provide a novel insight on the role of sphingolipids in the skeletal tissues.

Microstructure and compressive mechanical properties of cortical bone in children with osteogenesis imperfecta treated with bisphosphonates compared with healthy children

Osteogenesis imperfecta (OI) is a genetic disorder characterized by a change in bone tissue quality, but little data are available to describe the factors involved at the macroscopic scale. To better understand the effect of microstructure alterations on the mechanical properties at the sample scale, we studied the structural and mechanical properties of six cortical bone samples from children with OI treated with bisphosphonates and compared them to the properties of three controls. Scanning electron microscopy, high resolution computed tomography and compression testing were used to assess these properties. More resorption cavities and a higher osteocyte lacunar density were observed in OI bone compared with controls. Moreover, a higher porosity was measured for OI bones along with lower macroscopic Young's modulus, yield stress and ultimate stress. The microstructure was impaired in OI bones; the higher porosity and osteocyte lacunar density negatively impacted the mechanical properties and made the bone more prone to fracture.

Multi-scale analysis of bone chemistry, morphology and mechanics in the oim model of osteogenesis imperfecta

Osteogenesis imperfecta is a congenital disease commonly characterized by brittle bones and caused by mutations in the genes encoding Type I collagen, the single most abundant protein produced by the body. The oim model has a natural collagen mutation, converting its heterotrimeric structure (two α1 and one α2 chains) into α1 homotrimers. This mutation in collagen may impact formation of the mineral, creating a brittle bone phenotype in animals. Femurs from male wild type (WT) and homozygous (oim/oim) mice, all at 12 weeks of age, were assessed using assays at multiple length scales with minimal sample processing to ensure a near-physiological state. Atomic force microscopy (AFM) demonstrated detectable differences in the organization of collagen at the nanoscale that may partially contribute to alterations in material and structural behavior obtained through mechanical testing and reference point indentation (RPI). Changes in geometric and chemical structure obtained from µ-Computed Tomography and Raman spectroscopy indicate a smaller bone with reduced trabecular architecture and altered chemical composition. Decreased tissue material properties in oim/oim mice are likely driven by changes in collagen fibril structure, decreasing space available for mineral nucleation and growth, as supported by a reduction in mineral crystallinity. Multi-scale analyses of this nature offer much in assessing how molecular changes compound to create a degraded, brittle bone phenotype.

Mechanical and mineral properties of osteogenesis imperfecta human bones at the tissue level

Osteogenesis imperfecta (OI) is a genetic disorder characterized by an increase in bone fragility on the macroscopic scale, but few data are available to describe the mechanisms involved on the tissue scale and the possible correlations between these scales. To better understand the effects of OI on the properties of human bone, we studied the mechanical and chemical properties of eight bone samples from children suffering from OI and compared them to the properties of three controls. High-resolution computed tomography, nanoindentation and Raman microspectroscopy were used to assess those properties. A higher tissue mineral density was found for OI bone (1.131 gHA/cm3 vs. 1.032 gHA/cm3, p=0.032), along with a lower Young's modulus (17.6 GPa vs. 20.5 GPa, p=0.024). Obviously, the mutation-induced collagen defects alter the collagen matrix, thereby affecting the mineralization. Raman spectroscopy showed that the mineral-to-matrix ratio was higher in the OI samples, while the crystallinity was lower, suggesting that the mineral crystals were smaller but more abundant in the case of OI. This change in crystal size, distribution and composition contributes to the observed decrease in mechanical strength.

How tough is brittle bone? Investigating osteogenesis imperfecta in mouse bone

The multiscale hierarchical structure of bone is naturally optimized to resist fractures. In osteogenesis imperfecta, or brittle bone disease, genetic mutations affect the quality and/or quantity of collagen, dramatically increasing bone fracture risk. Here we reveal how the collagen defect results in bone fragility in a mouse model of osteogenesis imperfecta (oim), which has homotrimeric α1(I) collagen. At the molecular level, we attribute the loss in toughness to a decrease in the stabilizing enzymatic cross-links and an increase in nonenzymatic cross-links, which may break prematurely, inhibiting plasticity. At the tissue level, high vascular canal density reduces the stable crack growth, and extensive woven bone limits the crack-deflection toughening during crack growth. This demonstrates how modifications at the bone molecular level have ramifications at larger length scales affecting the overall mechanical integrity of the bone; thus, treatment strategies have to address multiscale properties in order to regain bone toughness. In this regard, findings from the heterozygous oim bone, where defective as well as normal collagen are present, suggest that increasing the quantity of healthy collagen in these bones helps to recover toughness at the multiple length scales.

Mineral particle size in children with osteogenesis imperfecta type I is not increased independently of specific collagen mutations

Osteogenesis imperfecta (OI) type I represents the mildest form of OI and is usually caused by two classes of autosomal dominant mutations in collagen type I: haploinsufficiency leading to a reduced quantity of structurally normal collagen (quantitative mutation), or sequence abnormalities generating structurally aberrant collagen chains (qualitative mutation). An abnormally high bone matrix mineralization has been observed in all OI cases investigated so far, independently of mutation type. This raises the question whether the increased amount of mineral is due to mineral particles growing to larger sizes or to a higher number of more densely packed particles. For this reason, we revisit the problem by investigating the mineral particle size in cancellous bone from two subsets of the previously analyzed biopsies (patient's age: 2-4.2 and 7.6-11years) comparing OI quantitative mutations (n=5), OI qualitative mutations (n=5) and controls (n=6). We used a combined small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) setup with a beam diameter of 10μm of synchrotron radiation, which allows the determination of mineral particle characteristics in 10μm thick sections at the same positions where the matrix mineralization density was previously determined. The thickness parameter of mineral particles (T) was obtained from SAXS data and the mineral volume fraction was calculated from the mean calcium content of the bone matrix determined by quantitative back-scattered electron imaging (qBEI). The combination of these two quantities allowed calculating the true particle width (W) of the plate-like mineral crystals. T was larger in the older than in the younger age-group independently of genotype (p<0.004) and was larger in the controls than in each OI group. The qBEI results showed that the mineral volume fraction increased from 32.45wt.% in controls to 36.44wt.% in both OI groups (corresponding to a 12% increase in relative terms). Combining these data, we find that also W was larger in the older than in the younger age-group (p<0.002), but stayed equal or smaller in both OI genotypes (controls: 2.3nm±0.04, OI qualitative: 2.2±0.05; OI quantitative 2.3±0.04, mean±SEM). A linear regression analysis even suggests a slower increase of W in qualitative OI as compared to quantitative OI and controls, where the particle sizes stayed similar at all ages. We thus conclude that the high mineral density in human OI is not due to increased particle size but rather to increased particle packing density. The lack of an observed difference between the two classes of mutations suggests the occurrence of a bone cell defect downstream of the collagen mutation.

A mouse model for human osteogenesis imperfecta type VI

Osteogenesis imperfecta type VI (OI type VI) has recently be linked to a mutation in the SERPINF1 gene, which encodes pigment epithelium-derived factor (PEDF), a ubiquitously expressed protein originally described for its neurotrophic and antiangiogenic properties. In this study, we characterized the skeletal phenotype of a mouse with targeted disruption of Pedf. In normal mouse bone, Pedf was localized to osteoblasts and osteocytes. Micro-computed tomography (µCT) and quantitative bone histomorphometry in femurs of mature Pedf null mutants revealed reduced trabecular bone volume and the accumulation of unmineralized bone matrix. Fourier transform infrared microscopy (FTIR) indicated an increased mineral:matrix ratio in mutant bones, which were more brittle than controls. In vitro, osteoblasts from Pedf null mice exhibited enhanced mineral deposition as assessed by Alizarin Red staining and an increased mineral:matrix determined by FTIR analysis of calcified nodules. The findings in this mouse model mimic the principal structural and biochemical features of bone observed in humans with OI type VI and consequently provide a useful model with which to further investigate the role of PEDF in this bone disorder.

Gender-dependence of bone structure and properties in adult osteogenesis imperfecta murine model

Osteogenesis imperfecta (OI) is a dominant skeletal disorder characterized by bone fragility and deformities. Though the oim mouse model has been the most widely studied of the OI models, it has only recently been suggested to exhibit gender-dependent differences in bone mineralization. To characterize the impact of gender on the morphometry/ultra-structure, mechanical properties, and biochemical composition of oim bone on the congenic C57BL/J6 background, 4-month-old oim/oim, +/oim, and wild-type (wt) female and male tibiae were evaluated using micro-computed tomography, three-point bending, and Raman spectroscopy. Dramatic gender differences were evident in both cortical and trabecular bone morphological and geometric parameters. Male mice had inherently more bone and increased moment of inertia than genotype-matched female counterparts with corresponding increases in bone biomechanical strength. The primary influence of gender was structure/geometry in bone growth and mechanical properties, whereas the mineral/matrix composition and hydroxyproline content of bone were influenced primarily by the oim collagen mutation. This study provides evidence of the importance of gender in the evaluation and interpretation of potential therapeutic strategies when using mouse models of OI.