A murine skeletal adaptation that significantly increases cortical bone mechanical properties. Implications for human skeletal fragility

Exploration of the skeletal phenotype of the Col1a1 +/Mov13 mouse model for haploinsufficient osteogenesis imperfecta type 1

INTRODUCTION

Osteogenesis Imperfecta is a rare genetic connective tissue disorder, characterized by skeletal dysplasia and fragile bones. Currently only two mouse models have been reported for haploinsufficient (HI) mild Osteogenesis Imperfecta (OI); the Col1a1 ^+/Mov13 (Mov13) and the Col1a1 ^+/-365 mouse model. The Mov13 mice were created by random insertion of the Mouse Moloney leukemia virus in the first intron of the Col1a1 gene, preventing the initiation of transcription. Since the development of the Mov13 mice almost four decades ago and its basic phenotypic characterization in the 90s, there have not been many further studies. We aimed to extensively characterize the Mov13 mouse model in order to critically evaluate its possible use for preclinical studies of HI OI.

METHODS

Bone tissue from ten heterozygous Mov13 and ten wild-type littermates (WT) C57BL/6J mice (50% males per group) was analyzed at eight weeks of age with bone histomorphometry, micro computed tomography (microCT), 3-point bending, gene expression of different collagens, as well as serum markers of bone turnover.

RESULTS

The Mov13 mouse presented a lower bone strength and impaired material properties based on our results of 3-point bending and microCT analysis respectively. In contrast, no significant differences were found for all histomorphometric parameters. In addition, no significant differences in Col1a1 bone expression were present, but there was a significant lower P1NP concentration, a bone formation marker, measured in serum. Furthermore, bone tissue of Mov13 mice presented significantly higher expression of collagens (Col1a2, Col5a1 and Col5a2), and bone metabolism markers (Bglap, Fgf23, Smad7, Edn1 and Eln) compared to WT. Finally, we measured a significantly lower Col1a1 expression in heart and skin tissue and also determined a higher expression of other collagens in the heart tissue.

CONCLUSION

Although we did not detect a significant reduction in Col1a1 expression in the bone tissue, a change in bone structure and reduction in bone strength was noted. Regrettably, the variability of the bone phenotype and the appearance of severe lymphoma in adult Mov13 mice, does not favor their use for the testing of new long-term drug studies. As such, a new HI OI type 1 mouse model is urgently needed.

Cortical bone relationships are maintained regardless of sex and diet in a large population of LGXSM advanced intercross mice

Introduction

Knowledge of bone structure-function relationships in mice has been based on relatively small sample sets that limit generalizability. We sought to investigate structure-function relationships of long bones from a large population of genetically diverse mice. Therefore, we analyzed previously published data from the femur and radius of male and female mice from the F34 generation of the Large-by-Small advanced intercross line (LGXSM AI), which have over a two-fold continuous spread of bone and body sizes (Silva et al. 2019 JBMR).

Methods

Morphological traits, mechanical properties, and estimated material properties were collected from the femur and radius from 1113 LGXSM AI adult mice (avg. age 25 wks). Males and females fed a low-fat or high-fat diet were evaluated to increase population variation. The data were analyzed using principal component analysis (PCA), Pearson's correlation, and multivariate linear regression.

Results

Using PCA groupings and hierarchical clustering, we identified a reduced set of traits that span the population variation and are relatively independent of each other. These include three morphometry parameters (cortical area, medullary area, and length), two mechanical properties (ultimate force and post-yield displacement), and one material property (ultimate stress). When comparing traits of the femur to the radius, morphological traits are moderately well correlated (r²: 0.18-0.44) and independent of sex and diet. However, mechanical and material properties are weakly correlated or uncorrelated between the long bones. Ultimate force can be predicted from morphology with moderate accuracy for both long bones independent of variations due to genetics, sex, or diet; however, predictions miss up to 50 % of the variation in the population. Estimated material properties in the femur are moderately to strongly correlated with bone size parameters, while these correlations are very weak in the radius.

Discussion

Our results indicate that variation in cortical bone phenotype in the F34 LGXSM AI mouse population can be adequately described by a reduced set of bone traits. These traits include cortical area, medullary area, bone length, ultimate force, post-yield displacement, and ultimate stress. The weak correlation of mechanical and material properties between the femur and radius indicates that the results from routine three-point bending tests of one long bone (e.g., femur) may not be generalizable to another long bone (e.g., radius). Additionally, these properties could not be fully predicted from bone morphology alone, confirming the importance of mechanical testing. Finally, material properties of the femur estimated based on beam theory equations showed a strong dependence on geometry that was not seen in the radius, suggesting that differences in femur size within a study may confound interpretation of estimated material properties.

An Update on Animal Models of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a heterogeneous disorder characterized by bone fragility, multiple fractures, bone deformity, and short stature. In recent years, the application of next generation sequencing has triggered the discovery of many new genetic causes for OI. Until now, more than 25 genetic causes of OI and closely related disorders have been identified. However, the mechanisms of many genes on skeletal fragility in OI are not entirely clear. Animal models of OI could help to understand the cellular, signaling, and metabolic mechanisms contributing to the disease, and how targeting these pathways can provide therapeutic targets. To date, a lot of animal models, mainly mice and zebrafish, have been described with defects in 19 OI-associated genes. In this review, we summarize the known genetic causes and animal models that recapitulate OI with a main focus on engineered mouse and zebrafish models. Additionally, we briefly discuss domestic animals with naturally occurring OI phenotypes. Knowledge of the specific molecular basis of OI will advance clinical diagnosis and potentially stimulate targeted therapeutic approaches.

Type-I collagen produced by distinct fibroblast lineages reveals specific function during embryogenesis and Osteogenesis Imperfecta

Type I collagen (Col1) is the most abundant protein in mammals. Col1 contributes to 90% of the total organic component of bone matrix. However, the precise cellular origin and functional contribution of Col1 in embryogenesis and bone formation remain unknown. Single-cell RNA-sequencing analysis identifies Fap⁺ cells and Fsp1⁺ cells as the major contributors of Col1 in the bone. We generate transgenic mouse models to genetically delete Col1 in various cell lineages. Complete, whole-body Col1 deletion leads to failed gastrulation and early embryonic lethality. Specific Col1 deletion in Fap⁺ cells causes severe skeletal defects, with hemorrhage, edema, and prenatal lethality. Specific Col1 deletion in Fsp1⁺ cells results in Osteogenesis Imperfecta-like phenotypes in adult mice, with spontaneous fractures and compromised bone healing. This study demonstrates specific contributions of mesenchymal cell lineages to Col1 production in organogenesis, skeletal development, and bone formation/repair, with potential insights into cell-based therapy for patients with Osteogenesis Imperfecta.

Serum PTH, PTH1R/ATF4 pathway, and the sRANKL/OPG system in bone as a new link between bone growth, cross-sectional geometry, and strength in young rats with experimental chronic kidney disease

The progression of chronic kidney disease (CKD) in children is associated with deregulated parathyroid hormone (PTH), growth retardation, and low bone accrual. PTH can cause both catabolic and anabolic impact on bone, and the activating transcription factor 4 (ATF4), a downstream target gene of PTH, is related to its anabolic effect. Osteoprotegerin (OPG) and receptor activator of NF-κB ligand (RANKL) are PTH-dependent cytokines, which may play an important role in the regulation of bone remodeling. This study aimed to evaluate the impact of endogenous PTH and the bone RANKL/OPG system on bone growth, cross-sectional geometry and strength utilizing young, nephrectomized rats. The parameters of cross-sectional geometry were significantly elevated in rats with CKD during the three-month experimental period compared with the controls, and they were strongly associated with serum PTH levels and the expression of parathyroid hormone 1 receptor (PTH1R)/ATF4 genes in bone. Low bone soluble RANKL (sRANKL) levels and sRANKL/OPG ratios were also positively correlated with cross-sectional bone geometry and femoral length. Moreover, the analyzed geometric parameters were strongly related to the biomechanical properties of femoral diaphysis. In summary, the mild increase in endogenous PTH, its anabolic PTH1R/ATF4 axis and PTH-dependent alterations in the bone RANKL/OPG system may be one of the possible mechanisms responsible for the favorable impact on bone growth, cross-sectional geometry and strength in young rats with experimental CKD.

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.

Effects of acidosis on the structure, composition, and function of adult murine femurs

Physiologic pH is maintained in a narrow range through multiple systemic buffering systems. Metabolic Acidosis (MA) is an acid-base disorder clinically characterized by a decrease in systemic pH and bicarbonate (HCO₃^-) levels. Acidosis affects millions annually, resulting in decreased bone mineral density and bone volume and an increased rate of fracture. We developed an adult murine model of diet-induced metabolic acidosis via graded NH₄Cl administration that successfully decreased systemic pH over a 14 day period to elucidate the effects of acidosis on the skeletal system. Blood gas analyses measured an increase in blood calcium and sodium levels indicating a skeletal response to 14 days of acidosis. MA also significantly decreased femur ultimate strength, likely due to modifications in bone morphology as determined from decreased microcomputed tomography values of centroid distance and area moment of inertia. These structural changes may be caused by aberrant remodeling based on histological data evidencing altered OCL activity in acidosis. Additionally, we found that acidosis significantly decreased bone CO₃ content in a site-specific manner similar to the bone phenotype observed in human MA. We determined that MA decreased bone strength thus increasing fracture risk, which is likely caused by alterations in bone shape and compounded by changes in bone composition. Additionally, we suggest the temporal regulation of cell-mediated remodeling in MA is more complex than current literature suggests. We conclude that our model reliably induces MA and has deleterious effects on skeletal form and function, presenting similarly to the MA bone phenotype in humans.

A novel transgenic murine model with persistently brittle bones simulating osteogenesis imperfecta type I

Osteogenesis imperfecta (OI) type I caused by the null allele of COL1A1 gene is in the majority in clinical OI cases. Currently, heterozygous Mov-13 mice generated by virus insertion in the first intron of col1a1 is the exclusive model to modulate OI type I, in spite of the gradually recovered bone mineral and mechanical properties. A newly designed heterozygous col1a1^±365 OI mouse was produced in the present study by partial exons knockout (exon 2-exon 5, 365 nt of mRNA) using CRISPR/Cas9 system. The deletion resulted in generally large decrease in type I collagen synthesis due to frameshift mutation and premature chain termination, closely mimicking the pathogenic mechanism in affected individuals. And the strain possessed significantly sparse mineral scaffolds, bone loss, lowered mechanical strength and broken bone metabolism by 8 and 20 weeks compared to their littermates, suggesting a sustained skeletal weakness. Notably, the remarkable down-regulation of Yes-associated protein (YAP), one of the key coactivator in Hippo signaling pathway, was first found both in the femur and adipose derived mesenchymal stem cells (ADSCs) under osteogenic differentiation of col1a1^±365 mice, which might be responsible for the reduced osteogenic potential and brittle bones. Still, further research was needed in order to illuminate the underlying mechanism.

Genetic causes and mechanisms of Osteogenesis Imperfecta

Osteogenesis Imperfecta (OI) is a genetic disorder characterized by various clinical features including bone deformities, low bone mass, brittle bones, and connective tissue manifestations. The predominant cause of OI is due to mutations in the two genes that encode type I collagen. However, recent advances in sequencing technology has led to the discovery of novel genes that are implicated in recessive and dominant OI. These include genes that regulate the post-translational modification, secretion and processing of type I collagen as well as those required for osteoblast differentiation and bone mineralization. As such, OI has become a spectrum of genetic disorders informing about the determinants of both bone quantity and quality. Here we summarize the known genetic causes of OI, animal models that recapitulate the human disease and mechanisms that underlie disease pathogenesis. Additionally, we discuss the effects of disrupted collagen networks on extracellular matrix signaling and its impact on disease progression.

Enhanced Wnt signaling improves bone mass and strength, but not brittleness, in the Col1a1(+/mov13) mouse model of type I Osteogenesis Imperfecta

Osteogenesis Imperfecta (OI) comprises a group of genetic skeletal fragility disorders. The mildest form of OI, Osteogenesis Imperfecta type I, is frequently caused by haploinsufficiency mutations in COL1A1, the gene encoding the α1(I) chain of type 1 collagen. Children with OI type I have a 95-fold higher fracture rate compared to unaffected children. Therapies for OI type I in the pediatric population are limited to anti-catabolic agents. In adults with osteoporosis, anabolic therapies that enhance Wnt signaling in bone improve bone mass, and ongoing clinical trials are determining if these therapies also reduce fracture risk. We performed a proof-of-principle experiment in mice to determine whether enhancing Wnt signaling in bone could benefit children with OI type I. We crossed a mouse model of OI type I (Col1a1(+/Mov13)) with a high bone mass (HBM) mouse (Lrp5(+/p.A214V)) that has increased bone strength from enhanced Wnt signaling. Offspring that inherited the OI and HBM alleles had higher bone mass and strength than mice that inherited the OI allele alone. However, OI+HBM and OI mice still had bones with lower ductility compared to wild-type mice. We conclude that enhancing Wnt signaling does not make OI bone normal, but does improve bone properties that could reduce fracture risk. Therefore, agents that enhance Wnt signaling are likely to benefit children and adults with OI type 1.

Establishing biomechanical mechanisms in mouse models: practical guidelines for systematically evaluating phenotypic changes in the diaphyses of long bones

Mice are widely used in studies of skeletal biology, and assessment of their bones by mechanical testing is a critical step when evaluating the functional effects of an experimental perturbation. For example, a gene knockout may target a pathway important in bone formation and result in a "low bone mass" phenotype. But how well does the skeleton bear functional loads; eg, how much do bones deform during loading and how resistant are bones to fracture? By systematic evaluation of bone morphological, densitometric, and mechanical properties, investigators can establish the "biomechanical mechanisms" whereby an experimental perturbation alters whole-bone mechanical function. The goal of this review is to clarify these biomechanical mechanisms and to make recommendations for systematically evaluating phenotypic changes in mouse bones, with a focus on long-bone diaphyses and cortical bone. Further, minimum reportable standards for testing conditions and outcome variables are suggested that will improve the comparison of data across studies. Basic biomechanical principles are reviewed, followed by a description of the cross-sectional morphological properties that best inform the net cellular effects of a given experimental perturbation and are most relevant to biomechanical function. Although morphology is critical, whole-bone mechanical properties can only be determined accurately by a mechanical test. The functional importance of stiffness, maximum load, postyield displacement, and work-to-fracture are reviewed. Because bone and body size are often strongly related, strategies to adjust whole-bone properties for body mass are detailed. Finally, a comprehensive framework is presented using real data, and several examples from the literature are reviewed to illustrate how to synthesize morphological, tissue-level, and whole-bone mechanical properties of mouse long bones.

Advances in Skeletal Dysplasia Genetics

Skeletal dysplasias result from disruptions in normal skeletal growth and development and are a major contributor to severe short stature. They occur in approximately 1/5,000 births, and some are lethal. Since the most recent publication of the Nosology and Classification of Genetic Skeletal Disorders, genetic causes of 56 skeletal disorders have been uncovered. This remarkable rate of discovery is largely due to the expanded use of high-throughput genomic technologies. In this review, we discuss these recent discoveries and our understanding of the molecular mechanisms behind these skeletal dysplasia phenotypes. We also cover potential therapies, unusual genetic mechanisms, and novel skeletal syndromes both with and without known genetic causes. The acceleration of skeletal dysplasia genetics is truly spectacular, and these advances hold great promise for diagnostics, risk prediction, and therapeutic design.

The use of nano-computed tomography to enhance musculoskeletal research

Advances in computed tomography (CT) imaging are opening new avenues toward more precise characterization and quantification of connective tissue microarchitecture. In the last two decades, micro-computed tomography (microCT) has significantly augmented destructive methods for the 3D micro-analysis of tissue structure, primarily in the bone research field. Recently, microCT has been employed in combination with contrast agents to generate contrast-enhanced images of soft tissues that are otherwise difficult to visualize due to their native radiodensity. More recent advances in CT technology have enabled ultra-high resolution imaging by utilizing a more powerful nano-focused X-ray source, such as that found in nano-computed tomography (nanoCT) systems. NanoCT imaging has facilitated the expansion of musculoskeletal research by reducing acquisition time and significantly expanding the range of samples that can be imaged in terms of size, age and tissue-type (bone, muscle, tendon, cartilage, vessels and adipose tissue). We present the application and early results of nanoCT imaging in various tissue types and how this ultra-high resolution imaging modality is capable of characterizing microstructures at levels of details previously not possible. Contrast-enhanced imaging techniques to enable soft-tissue visualization and characterization are also outlined.

Skeletal maturity leads to a reduction in the strain magnitudes induced within the bone: a murine tibia study

Bone adapts to changes in the local mechanical environment (e.g. strains) through formation and resorption processes. However, the bone adaptation response is significantly reduced with increasing age. The mechanical strains induced within the bone by external loading are determined by bone morphology and tissue material properties. Although it is known that changes in bone mass, architecture and bone tissue quality occur with age, to what extent they contribute to the altered bone adaptation response remains to be determined. This study investigated alterations in strains induced in the tibia of different aged female C57Bl/6J mice (young, 10-week-old; adult, 26-week-old; and elderly, 78-week-old) subjected to in vivo compressive loading. Using a combined in vivo/in silico approach, the strains in the bones were assessed by both strain gauging and finite element modeling experiments. In cortical bone, strain magnitudes induced at the mid-diaphysis decreased by 20% from young to adult mice and by 15% from adult to elderly mice. In the cancellous bone (at the proximal metaphysis), induced strains were 70% higher in young compared with adult and elderly mice. Taking into account previous studies showing a reduced bone adaptation response to mechanical loading in adulthood, these results suggest that the diminished adaptive response is in part due to a reduction in the strains induced within the bone.

First mouse model for combined osteogenesis imperfecta and Ehlers-Danlos syndrome

By using a genome-wide N-ethyl-N-nitrosourea (ENU)-induced dominant mutagenesis screen in mice, a founder with low bone mineral density (BMD) was identified. Mapping and sequencing revealed a T to C transition in a splice donor of the collagen alpha1 type I (Col1a1) gene, resulting in the skipping of exon 9 and a predicted 18-amino acid deletion within the N-terminal region of the triple helical domain of Col1a1. Col1a1(Jrt) /+ mice were smaller in size, had lower BMD associated with decreased bone volume/tissue volume (BV/TV) and reduced trabecular number, and furthermore exhibited mechanically weak, brittle, fracture-prone bones, a hallmark of osteogenesis imperfecta (OI). Several markers of osteoblast differentiation were upregulated in mutant bone, and histomorphometry showed that the proportion of trabecular bone surfaces covered by activated osteoblasts (Ob.S/BS and N.Ob/BS) was elevated, but bone surfaces undergoing resorption (Oc.S/BS and N.Oc/BS) were not. The number of bone marrow stromal osteoprogenitors (CFU-ALP) was unaffected, but mineralization was decreased in cultures from young Col1a1(Jrt) /+ versus +/+ mice. Total collagen and type I collagen content of matrices deposited by Col1a1(Jrt) /+ dermal fibroblasts in culture was ∼40% and 30%, respectively, that of +/+ cells, suggesting that mutant collagen chains exerted a dominant negative effect on type I collagen biosynthesis. Mutant collagen fibrils were also markedly smaller in diameter than +/+ fibrils in bone, tendon, and extracellular matrices deposited by dermal fibroblasts in vitro. Col1a1(Jrt) /+ mice also exhibited traits associated with Ehlers-Danlos syndrome (EDS): Their skin had reduced tensile properties, tail tendon appeared more frayed, and a third of the young adult mice had noticeable curvature of the spine. Col1a1(Jrt) /+ is the first reported model of combined OI/EDS and will be useful for exploring aspects of OI and EDS pathophysiology and treatment.

Systematic evaluation of skeletal mechanical function

Many genetic and environmental perturbations lead to measurable changes in bone morphology, matrix composition, and matrix organization. The skeletal system is highly adaptive, such that genetic and environmental perturbations affecting one trait are often accompanied by compensatory changes in other traits. This adaptation should be considered when attempting to draw meaningful conclusions about the role of a gene, for example. The challenge is to figure out which alterations can be attributed to the perturbation and which result from adaptive changes associated with establishing mechanical function. The goal of this chapter is to describe straight-forward biomechanical methods that can be used to determine whether a genetic or environmental perturbation affected bone strength. We introduce a systematic method for evaluating how bone strength was altered in the context of morphology and tissue-level mechanical properties, which are determined in large part from matrix composition, matrix organization, and porosity. We present this work as a first step toward screening mice for a phenotypic effect and for establishing the associated biomechanical mechanism by which function was altered. The outcome of these analyses generally provides insight into the next set of experiments required to further connect the cellular perturbation with the functional changes. The protocols were written to enable researchers without a background in engineering to conduct the assays or to enable researchers to better understand the outcomes of similar assays conducted by colleagues knowledgeable in engineering.

Variation in osteocyte lacunar morphology and density in the human femur--a synchrotron radiation micro-CT study

In recent years there has been growing interest in the spatial properties of osteocytes (including density and morphology) and how these potentially relate to adaptation, disease and aging. This interest has, in part, arisen from the availability of increasingly high-resolution 3D imaging modalities such as synchrotron radiation (SR) micro-CT. As resolution increases, field of view generally decreases. Thus, while increasingly detailed spatial information is obtained, it is unclear how representative this information is of the skeleton or even the isolated bone. The purpose of this research was to describe the variation in osteocyte lacunar density, morphology and orientation within the femur from a healthy young male human. Multiple anterior, posterior, medial and lateral blocks (2 mm × 2 mm) were prepared from the proximal femoral shaft and SR micro-CT imaged at the Advanced Photon Source. Average lacunar densities (± standard deviation) from the anterior, posterior, medial and lateral regions were 27,169 ± 1935, 26,3643 ± 1262, 37,521 ± 6416 and 33,972 ± 2513 lacunae per mm(3) of bone tissue, respectively. These values were significantly different between the medial and both the anterior and posterior regions (p<0.05). The density of the combined anterior and posterior regions was also significantly lower (p=0.001) than the density of the combined medial and lateral regions. Although no difference was found in predominant orientation, shape differences were found; with the combined anterior and posterior regions having more elongated (p=0.004) and flattened (p=0.045) lacunae, than those of the medial and lateral regions. This study reveals variation in osteocyte lacunar density and morphology within the cross-section of a single bone and that this variation can be considerable (up to 30% difference in density between regions). The underlying functional significance of the observed variation in lacunar density likely relates to localized variations in loading conditions as the pattern corresponds well with mechanical axes. Lower density and more elongate shapes being associated with the antero-posterior oriented neutral axis. Our findings demonstrate that the functional and pathological interpretations that are increasingly being drawn from high resolution imaging of osteocyte lacunae need to be better situated within the broader context of normal variation, including that which occurs even within a single skeletal element.

Low bone strength is a manifestation of phenylketonuria in mice and is attenuated by a glycomacropeptide diet

Purpose

Phenylketonuria (PKU), caused by phenylalanine (phe) hydroxylase loss of function mutations, requires a low-phe diet plus amino acid (AA) formula to prevent cognitive impairment. Glycomacropeptide (GMP), a low-phe whey protein, provides a palatable alternative to AA formula. Skeletal fragility is a poorly understood chronic complication of PKU. We sought to characterize the impact of the PKU genotype and dietary protein source on bone biomechanics.

Procedures

Wild type (WT; Pah^+/+) and PKU (Pah^enu2/enu2) mice on a C57BL/6J background were fed high-phe casein, low-phe AA, and low-phe GMP diets between 3 to 23 weeks of age. Following euthanasia, femur biomechanics were assessed by 3-point bending and femoral diaphyseal structure was determined. Femoral ex vivo bone mineral density (BMD) was assessed by dual-enengy x-ray absorptiometry. Whole bone parameters were used in prinicipal component analysis. Data were analyzed by 3-way ANCOVA with genotype, sex, and diet as the main factors.

Findings

Regardless of diet and sex, PKU femora were more brittle, as manifested by lower post-yield displacement, weaker, as manifested by lower energy and yield and maximal loads, and showed reduced BMD compared with WT femora. Four principal components accounted for 87% of the variance and all differed significantly by genotype. Regardless of genotype and sex, the AA diet reduced femoral cross-sectional area and consequent maximal load compared with the GMP diet.

Conclusions

Skeletal fragility, as reflected in brittle and weak femora, is an inherent feature of PKU. This PKU bone phenotype is attenuated by a GMP diet compared with an AA diet.

The influence of follistatin on mechanical properties of bone tissue in growing mice with overexpression of follistatin

Mechanical competence of bones is mainly associated with tissue quality that depends on proper bone metabolism processes. An imbalance in the regulation of bone metabolism leads to pathological changes in bone tissue leading to susceptibility to bone fractures and bone deterioration processes. Bone metabolism is regulated to a large extent by the members of the transforming growth factor-β superfamily, i.e., activins and bone morphogenetic proteins. However, their function is regulated by a single-chain protein called follistatin (FS). The aim of this study was to test the hypothesis that overexpression of FS in growing mice results in impairments in bone morphology and mechanical properties. Moreover, we wanted to investigate how geometrical, structural and material properties of bone tissue change with age. The experiment was performed on growing C57BL/6 TgNK14-mFst/6J mice, overexpressing FS (F mice) versus C57BL/6J mice used as controls (C mice). To establish how overexpression of FS influences bone tissue quality, we studied mice femurs to determine geometrical, structural and material properties of the skeleton. To determine mechanical resistance of bone tissue, femurs were loaded to failure in a three-point bending test. Obtained results indicated that overexpression of FS negatively influences bone metabolism. It was found that mutation results with a significant decrease of all measured biomechanical strength variables in F mice in comparison to C mice. Overexpression of FS leads to decreased quality of skeleton, increasing susceptibility to bone fractures.

Functional interactions among morphologic and tissue quality traits define bone quality

BACKGROUND

Advances in diagnostic and treatment regimens that aim to reduce fracture incidence will benefit from a better understanding of how bone morphology and tissue quality define whole-bone mechanical properties.

QUESTIONS/PURPOSES

The goal of this article was to review what is known about the interactions among morphologic and tissue quality traits and how these interactions contribute to bone quality (ie, whole-bone mechanical function). Several questions were addressed. First, how do interactions among morphology and tissue quality traits relate to functional adaptation? Second, what are the emergent patterns of functionally adapted trait sets in long bones? Third, how effective is phenotypic integration at establishing function across a population? Fourth, what are the emergent patterns of functionally adapted trait sets in corticocancellous structures? Fifth, how do functional interactions change with aging?

METHODS

A literature review was conducted with papers identified primarily through citations listed in reference sections as well as general searches using Google Scholar and PubMed.

RESULTS

The interactions among adult traits or phenotypic integration are an emergent property of the compensatory mechanisms complex systems used to establish function or homeostasis. Traits are not regulated independently but vary simultaneously (ie, covary) in specific ways to establish function. This covariation results in individuals acquiring unique sets of traits to establish bone quality.

CONCLUSIONS AND CLINICAL RELEVANCE

Biologic constraints imposed on the skeletal system result in a population showing a pattern of trait sets that is predictable based on external bone size and that can be used to identify individuals with reduced bone quality relative to their bone size and body size.

Whole bone mechanics and bone quality

BACKGROUND

The skeleton plays a critical structural role in bearing functional loads, and failure to do so results in fracture. As we evaluate new therapeutics and consider treatments to prevent skeletal fractures, understanding the basic mechanics underlying whole bone testing and the key principles and characteristics contributing to the structural strength of a bone is critical.

QUESTIONS/PURPOSES

We therefore asked: (1) How are whole bone mechanical tests performed and what are the key outcomes measured? (2) How do the intrinsic characteristics of bone tissue contribute to the mechanical properties of a whole bone? (3) What are the effects of extrinsic characteristics on whole bone mechanical behavior? (4) Do environmental factors affect whole bone mechanical properties?

METHODS

We conducted a PubMed search using specific search terms and limiting our included articles to those related to in vitro testing of whole bones. Basic solid mechanics concepts are summarized in the context of whole bone testing and the determinants of whole bone behavior.

RESULTS

Whole bone mechanical tests measure structural stiffness and strength from load-deformation data. Whole bone stiffness and strength are a function of total bone mass and the tissue geometric distribution and material properties. Age, sex, genetics, diet, and activity contribute to bone structural performance and affect the incidence of skeletal fractures.

CONCLUSIONS

Understanding and preventing skeletal fractures is clinically important. Laboratory tests of whole bone strength are currently the only measures for in vivo fracture prediction. In the future, combined imaging and engineering models may be able to predict whole bone strength noninvasively.

New perspectives on osteogenesis imperfecta

A new paradigm has emerged for osteogenesis imperfecta as a collagen-related disorder. The more prevalent autosomal dominant forms of osteogenesis imperfecta are caused by primary defects in type I collagen, whereas autosomal recessive forms are caused by deficiency of proteins which interact with type I procollagen for post-translational modification and/or folding. Factors that contribute to the mechanism of dominant osteogenesis imperfecta include intracellular stress, disruption of interactions between collagen and noncollagenous proteins, compromised matrix structure, abnormal cell-cell and cell-matrix interactions and tissue mineralization. Recessive osteogenesis imperfecta is caused by deficiency of any of the three components of the collagen prolyl 3-hydroxylation complex. Absence of 3-hydroxylation is associated with increased modification of the collagen helix, consistent with delayed collagen folding. Other causes of recessive osteogenesis imperfecta include deficiency of the collagen chaperones FKBP10 or Serpin H1. Murine models are crucial to uncovering the common pathways in dominant and recessive osteogenesis imperfecta bone dysplasia. Clinical management of osteogenesis imperfecta is multidisciplinary, encompassing substantial progress in physical rehabilitation and surgical procedures, management of hearing, dental and pulmonary abnormalities, as well as drugs, such as bisphosphonates and recombinant human growth hormone. Novel treatments using cell therapy or new drug regimens hold promise for the future.

Material and mechanical properties of bones deficient for fibrillin-1 or fibrillin-2 microfibrils

The contribution of non-collagenous components of the extracellular matrix to bone strength is largely undefined. Here we report that deficiency of fibrillin-1 or fibrillin-2 microfibrils causes distinct changes in bone material and mechanical properties. Morphometric examination of mice with hypomorphic or null mutations in fibrillin-1 or fibrillin-2, respectively, revealed appreciable differences in the postnatal shaping and growth of long bones. Fourier transform infrared imaging spectroscopy indicated that fibrillin-1 plays a predominantly greater role than fibrillin-2 in determining the material properties of bones. Biomechanical tests demonstrated that fibrillin-2 exerts a greater positive influence on the mechanical properties of bone than fibrillin-1 assemblies. Published evidence indirectly supports the notion that the above findings are mostly, if not exclusively, related to the differential control of TGFβ family signaling by fibrillin proteins. Our study therefore advances our understanding of the role that extracellular microfibrils play in bone physiology and implicitly, in the pathogenesis of bone loss in human diseases caused by mutations in fibrillin-1 or -2.

Guidelines for assessment of bone microstructure in rodents using micro-computed tomography

Use of high-resolution micro-computed tomography (microCT) imaging to assess trabecular and cortical bone morphology has grown immensely. There are several commercially available microCT systems, each with different approaches to image acquisition, evaluation, and reporting of outcomes. This lack of consistency makes it difficult to interpret reported results and to compare findings across different studies. This article addresses this critical need for standardized terminology and consistent reporting of parameters related to image acquisition and analysis, and key outcome assessments, particularly with respect to ex vivo analysis of rodent specimens. Thus the guidelines herein provide recommendations regarding (1) standardized terminology and units, (2) information to be included in describing the methods for a given experiment, and (3) a minimal set of outcome variables that should be reported. Whereas the specific research objective will determine the experimental design, these guidelines are intended to ensure accurate and consistent reporting of microCT-derived bone morphometry and density measurements. In particular, the methods section for papers that present microCT-based outcomes must include details of the following scan aspects: (1) image acquisition, including the scanning medium, X-ray tube potential, and voxel size, as well as clear descriptions of the size and location of the volume of interest and the method used to delineate trabecular and cortical bone regions, and (2) image processing, including the algorithms used for image filtration and the approach used for image segmentation. Morphometric analyses should be based on 3D algorithms that do not rely on assumptions about the underlying structure whenever possible. When reporting microCT results, the minimal set of variables that should be used to describe trabecular bone morphometry includes bone volume fraction and trabecular number, thickness, and separation. The minimal set of variables that should be used to describe cortical bone morphometry includes total cross-sectional area, cortical bone area, cortical bone area fraction, and cortical thickness. Other variables also may be appropriate depending on the research question and technical quality of the scan. Standard nomenclature, outlined in this article, should be followed for reporting of results.

RANKL inhibition improves bone properties in a mouse model of osteogenesis imperfecta

Recently, a new class of agents targeting the receptor activator of nuclear factor-kappaB ligand (RANKL) pathway has been developed for the treatment of osteoporosis and other bone diseases. In the current study, inhibition of the RANKL pathway was evaluated to assess effects on "bone quality" and fracture incidence in an animal model of osteogenesis imperfect (OI), the oim/oim mouse. Juvenile oim/oim ( approximately 6 weeks old) and wildtype (+/+) mice were treated with either a RANKL inhibitor (RANK-Fc) or saline. After treatment, bone density increased significantly in the femurs of both genotypes. Femoral length decreased with RANK-Fc in +/+ mice. Geometric measurements at mid-diaphysis in the oim/oim groups showed increases in the ML periosteal and endosteal diameters and AP cortical thickness in the treated groups. Within +/+ groups, ML cortical thickness and ML femoral periosteal diameter were significantly increased with RANK-Fc. Biomechanical testing revealed increased stiffness in oim/oim and +/+ mice. Total strain was increased with treatment in the +/+ mice. Histologically, RANKL inhibition resulted in retained growth plate cartilage in both genotypes. The average number of fractures sustained by RANK-Fc-treated oim/oim mice was not significantly decreased compared to saline treated oim/oim mice. This preclinical study demonstrated that RANKL inhibition at the current dose improved density and some geometric and biomechanical properties of oim/oim bone, but it did not decrease fracture incidence. Further studies that address commencement of therapy at earlier time points are needed to determine whether this mode of therapy will be clinically useful in OI.

A delay in pubertal onset affects the covariation of body weight, estradiol, and bone size

The skeletal system functions as a locomotive organ and a mineral reservoir and combinations of genetic and environmental factors affect the skeletal system. Although delayed puberty is associated with compromised bone mass, suppression of estrogen should be beneficial to cortical strength. The purpose was to employ path analysis to study bone strength and delayed puberty. Forty-five female rats were randomly assigned to a control group (n = 15) and an experimental group (n = 30) that received injections of gonadotropin releasing hormone antagonist (GnRH-a). Causal models were constructed by specifying directed paths between bone traits. The first model tested the hypothesis that the functional relationships between bone traits and body weight were altered by a delay in pubertal onset. GnRH-a injections during puberty altered the covariation between body weight and bone size. The second model was constructed to test the hypothesis that variability in stiffness was causally related to variability in body weight. The model also tested the relationship between the periosteal and endocortical surfaces and their relationship to stiffness. There was no change in the relationship between the surfaces in the GnRH-a group. The third model determined the effect of estradiol on both total area and relative cortical area in both groups. The relationship between periosteal surface and serum estradiol levels was only significant during estrogen suppression. These data suggest that increases in body weight during or prior to puberty may not be protective of bone strength.

Cellular mechanism of decreased bone in Brtl mouse model of OI: imbalance of decreased osteoblast function and increased osteoclasts and their precursors

The Brtl mouse, a knock-in model for moderately severe osteogenesis imperfecta (OI), has a G349C substitution in half of type I collagen alpha1(I) chains. We studied the cellular contribution to Brtl bone properties. Brtl cortical and trabecular bone are reduced before and after puberty, with BV/TV decreased 40-45%. Brtl ObS/BS is comparable to wildtype, and Brtl and wildtype marrow generate equivalent number of colony-forming units (CFUs) at both ages. However, OcS/BS is increased in Brtl at both ages (36-45%), as are TRACP(+) cell numbers (57-47%). After puberty, Brtl ObS/BS decreases comparably to wildtype mice, but osteoblast matrix production (MAR) decreases to one half of wildtype values. In contrast, Brtl OcS falls only moderately (approximately 16%), and Brtl TRACP staining remains significantly elevated compared with wildtype. Consequently, Brtl BFR decreases from normal at 2 mo to one half of wildtype values at 6 mo. Immunohistochemistry and real-time RT-PCR show increased RANK, RANKL, and osteoprotegerin (OPG) levels in Brtl, although a normal RANKL/OPG ratio is maintained. TRACP(+) precursors are markedly elevated in Brtl marrow cultures and form more osteoclasts, suggesting that osteoclast increases arise from more RANK-expressing precursors. We conclude that osteoblasts and osteoclasts are unsynchronized in Brtl bone. This cellular imbalance results in declining BFR as Brtl ages, consistent with reduced femoral geometry. The disparity in cellular number and function results from poorly functioning osteoblasts in addition to increased RANK-expressing precursors that respond to normal RANKL/OPG ratios to generate more bone-resorbing osteoclasts. Interruption of the stimulus that increases osteoclast precursors may lead to novel OI therapies.

Role of genetic background in determining phenotypic severity throughout postnatal development and at peak bone mass in Col1a2 deficient mice (oim)

Osteogenesis imperfecta (OI) is a genetically and clinically heterogeneous disease characterized by extreme bone fragility. Although fracture numbers tend to decrease post-puberty, OI patients can exhibit significant variation in clinical outcome, even among related individuals harboring the same mutation. OI most frequently results from mutations in type I collagen genes, yet how genetic background impacts phenotypic outcome remains unclear. Therefore, we analyzed the phenotypic severity of a known proalpha2(I) collagen gene defect (oim) on two genetic backgrounds (congenic C57BL/6J and outbred B6C3Fe) throughout postnatal development to discern the phenotypic contributions of the Col1a2 locus relative to the contribution of the genetic background. To this end, femora and tibiae were isolated from wildtype (Wt) and homozygous (oim/oim) mice of each strain at 1, 2 and 4 months of age. Femoral geometry was determined via muCT prior to torsional loading to failure to assess bone structural and material biomechanical properties. Changes in mineral composition, collagen content and bone turnover were determined using neutron activation analyses, hydroxyproline content and serum pyridinoline crosslinks. muCT analysis demonstrated genotype-, strain- and age-associated changes in femoral geometry as well as a marked decrease in the amount of bone in oim/oim mice of both strains. Oim/oim mice of both strains, as well as C57BL/6J (B6) mice of all genotypes, had reduced femoral biomechanical strength properties compared to Wt at all ages, although they improved with age. Mineral levels of fluoride, magnesium and sodium were associated with biomechanical strength properties in both strains and all genotypes at all ages. Oim/oim animals also had reduced collagen content as compared to Wt at all ages. Serum pyridinoline crosslinks were highest at two months of age, regardless of strain or genotype. Strain differences in bone parameters exist throughout development, implicating a role for genetic background in determining biomechanical strength. Age-associated improvements indicate that oim/oim animals partially compensate for their weaker bone material, but never attain Wt levels. These studies indicate the importance of genetic background in determining phenotypic severity, but the presence of the proalpha2(I) collagen gene defect and age of the animal are the primary determinants of phenotypic severity.

Biological co-adaptation of morphological and composition traits contributes to mechanical functionality and skeletal fragility

A path analysis was conducted to determine whether functional interactions exist among morphological, compositional, and microstructural traits for young adult human tibias. Data provided evidence that bone traits are co-adapted during ontogeny so that the sets of traits together satisfy physiological loading demands. However, certain sets of traits are expected to perform poorly under extreme load conditions.

INTRODUCTION

Previous data from inbred mouse strains suggested that biological processes within bone co-adapt morphological and compositional traits during ontogeny to satisfy physiological loading demands. Similar work in young adult humans showed that cortical tissue from slender tibias was stiffer, less ductile, and more susceptible to accumulating damage. Here we tested whether the relationships among morphology and tissue level mechanical properties were the result of biological processes that co-adapt physical traits, similar to those observed for the mouse skeleton.

MATERIALS AND METHODS

Cross-sectional morphology, bone slenderness (Tt.Ar/Le), and tissue level mechanical properties were measured from tibias from 14 female (22-46 yr old) and 17 male (17-46 yr old) donors. Physical bone traits measured included tissue density, ash content, water content, porosity, and the area fractions of osteonal, interstitial, and circumferential lamellar tissues. Bivariate relationships among traits were determined using linear regression analysis. A path analysis was conducted to test the hypothesis that Tt.Ar/Le is functionally related to mineralization (ash content) and the proportion of total area occupied by cortical bone.

RESULTS

Ash content correlated negatively with several traits including Tt.Ar/Le and marrow area, indicating that slender bones were constructed of tissue with higher mineralization. Path analysis revealed that slender tibias were compensated by higher mineralization and a greater area fraction of bone.

CONCLUSIONS

The results suggest that bone adapts by varying the relative amount of cortical bone within the diaphysis and by varying matrix composition. This co-adaptation is expected to lead to a particular set of traits that is sufficiently stiff and strong to support daily loads. However, increases in mineralization result in a more brittle and damageable material that would be expected to perform poorly under extreme load conditions. Therefore, focusing attention on sets of traits and the relationship among traits may advance our understanding of how genetic and environmental factors influence bone fragility.

Bone quality: the material and structural basis of bone strength

The material composition and structural design of bone determine its strength. Structure determines loads that can be tolerated but loads also determine structure. Bone modifies its material composition and structure to accommodate loads by adaptive modeling and remodeling. Adaptation is successful during growth but not aging because accumulating insults, including a reduction in the volume of bone formed in the basic multicellular unit (BMU), increased resorption in the BMU, increased remodeling rate in midlife in women and in some men because of sex hormone deficiency, and in both sexes in old age as a consequence of secondary hyperparathyroidism and reduced periosteal bone formation, all of which compromises the material composition of bone and its structure. An understanding of the mechanisms of adaptation and failed adaptation provides rational approaches to interventions that can prevent or restore bone fragility.

The use of fractography to supplement analysis of bone mechanical properties in different strains of mice

Fractography has not been fully developed as a useful technique in assessing failure mechanisms of bone. While fracture surfaces of osteonal bone have been explored, this may not apply to conventional mechanical testing of mouse bone. Thus, the focus of this work was to develop and evaluate the efficacy of a fractography protocol for use in supplementing the interpretation of failure mechanisms in mouse bone. Micro-computed tomography and three-point bending were performed on femora of two groups of 6-month-old mice (C57BL/6 and a mixed strain background of 129SV/C57BL6). SEM images of fracture surfaces were collected, and areas of "tension", "compression" and "transition" were identified. Percent areas of roughness were identified and estimated within areas of "tension" and "compression" and subsequently compared to surface roughness measurements generated from an optical profiler. Porosity parameters were determined on the tensile side. Linear regression analysis was performed to evaluate correlations between certain parameters. Results show that 129 mice exhibit significantly increased bone mineral density (BMD), number of "large" pores, failure strength, elastic modulus and energy to failure compared to B6 mice (p<0.001). Both 129 and B6 mice exhibit significantly (p<0.01) more percent areas of tension (49+/-1%, 42+/-2%; respectively) compared to compression (26+/-2%, 31+/-1%; respectively). In terms of "roughness", B6 mice exhibit significantly less "rough" areas (30+/-4%) compared to "smooth" areas (70+/-4%) on the tensile side only (p<0.001). Qualitatively, 129 mice demonstrate more evidence of bone toughening through fiber bridging and loosely connected fiber bundles. The number of large pores is positively correlated with failure strength (p=0.004), elastic modulus (p=0.002) and energy to failure (p=0.041). Percent area of tensile surfaces is positively correlated with failure strength (p<0.001), elastic modulus (p=0.016) and BMD (p=0.037). Percent area of rough compressive surfaces is positively correlated with energy to failure (p=0.039). Evaluation of fracture surfaces has helped to explain why 129 mice have increased mechanical properties compared to B6 mice, namely via toughening mechanisms on the compressive side of failure. Several correlations exist between fractography parameters and mechanical behavior, supporting the utility of fractography with skeletal mouse models.

Genetic randomization reveals functional relationships among morphologic and tissue-quality traits that contribute to bone strength and fragility

We examined femora from adult AXB/BXA recombinant inbred (RI) mouse strains to identify skeletal traits that are functionally related and to determine how functional interactions among these traits contribute to genetic variability in whole-bone stiffness, strength, and toughness. Randomization of A/J and C57BL/6J genomic regions resulted in each adult male and female RI strain building mechanically functional femora by assembling unique sets of morphologic and tissue-quality traits. A correlation analysis was conducted using the mean trait values for each RI strain. A third of the 66 correlations examined were significant, indicating that many bone traits covaried or were functionally related. Path analysis revealed important functional interactions among bone slenderness, cortical thickness, and tissue mineral density. The path coefficients describing these functional relations were similar for both sexes. The causal relationship among these three traits suggested that cellular processes during growth simultaneously regulate bone slenderness, cortical thickness, and tissue mineral density so that the combination of traits is sufficiently stiff and strong to satisfy daily loading demands. A disadvantage of these functional interactions was that increases in tissue mineral density also deleteriously affected tissue ductility. Consequently, slender bones with high mineral density may be stiff and strong but they are also brittle. Thus, genetically randomized mouse strains revealed a basic biological paradigm that allows for flexibility in building bones that are functional for daily activities but that creates preferred sets of traits under extreme loading conditions. Genetic or environmental perturbations that alter these functional interactions during growth would be expected to lead to loss of function and suboptimal adult bone quality.

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.

Procollagen C proteinase enhancer 1 genes are important determinants of the mechanical properties and geometry of bone and the ultrastructure of connective tissues

Procollagen C proteinases (pCPs) cleave type I to III procollagen C propeptides as a necessary step in assembling the major fibrous components of vertebrate extracellular matrix. The protein PCOLCE1 (procollagen C proteinase enhancer 1) is not a proteinase but can enhance the activity of pCPs approximately 10-fold in vitro and has reported roles in inhibiting other proteinases and in growth control. Here we have generated mice with null alleles of the PCOLCE1 gene, Pcolce, to ascertain in vivo roles. Although Pcolce-/- mice are viable and fertile, Pcolce-/- male, but not female, long bones are more massive and have altered geometries that increase resistance to loading, compared to wild type. Mechanical testing indicated inferior material properties of Pcolce-/- male long bone, apparently compensated for by the adaptive changes in bone geometry. Male and female Pcolce-/- vertebrae both appeared to compensate for inferior material properties with thickened and more numerous trabeculae and had a uniquely altered morphology in deposited mineral. Ultrastructurally, Pcolce-/- mice had profoundly abnormal collagen fibrils in both mineralized and nonmineralized tissues. In Pcolce-/- tendon, 100% of collagen fibrils had deranged morphologies, indicating marked functional effects in this tissue. Thus, PCOLCE1 is an important determinant of bone mechanical properties and geometry and of collagen fibril morphology in mammals, and the human PCOLCE1 gene is identified as a candidate for phenotypes with defects in such attributes in humans.

The use of micro-CT to evaluate cortical bone geometry and strength in nude rats: correlation with mechanical testing, pQCT and DXA

In both clinical and experimental settings, access to quantitative methods enabling the objective evaluation of cortical bone mass, structure, geometry and strength are essential for the assessment of efficacy and safety of different treatments aimed to improve bone strength. The ability of non-invasive methodologies (DXA, pQCT and micro-CT) to assess and quantify cortical bone mass and geometry was tested in a nude rat model in which bone loss was induced by surgical castration. Treatment with a bone antiresorptive (alendronate) or a bone forming (PTH) drug was used to: (A) validate the nude rat model in terms of bone metabolism, (B) test the ability of each technology to detect change in cortical bone geometry and (C) correlate cortical bone geometry with bone strength data obtained by 3-point bending method. Our observations regarding effect of castration and treatment with PTH and alendronate on cortical bone parameters in nude rats is in general agreement with previously published data obtained in immunocompetent male rats under similar experimental conditions. Data presented here support the hypothesis that nude rats have similar bone physiology and response to known bone therapies to that observed in normal rats and therefore could be effectively used to predict skeletal response in humans. All three technologies deployed in this study (DXA, pQCT and micro-CT) proved useful in describing cancellous and/or cortical bone parameters and positive correlations were demonstrated between data obtained by different methods. The cross-sectional area of a bone structure is crucial for resisting loads in bending or torsion and is described as "areal moment of inertia" for bending, and as "polar moment of inertia" in torsion. Novel, three-dimensional micro-CT methodology used in this study to assess geometry of cortical bone provides data that accurately describes cortical bone geometry and parallels cortical bone strength results obtained by the 3-point bending method. Our micro-CT data meet the criteria of providing quick, reproducible and accurate answers regarding cortical bone geometry as a predictor of cortical bone strength.

Developmental roles of the BMP1/TLD metalloproteinases

The astacin family (M12A) of the metzincin subclan MA(M) of metalloproteinases has been detected in developing and mature individuals of species that range from hydra to humans. Functions of this family of metalloproteinase vary from digestive degradation of polypeptides, to biosynthetic processing of extracellular proteins, to activation of growth factors. This review will focus on a small subgroup of the astacin family; the bone morphogenetic protein 1 (BMP1)/Tolloid (TLD)-like metalloproteinases. In vertebrates, the BMP1/TLD-like metalloproteinases play key roles in regulating formation of the extracellular matrix (ECM) via biosynthetic processing of various precursor proteins into mature functional enzymes, structural proteins, and proteins involved in initiating mineralization of the ECM of hard tissues. Roles in ECM formation include: processing of the C-propeptides of procollagens types I-III, to yield the major fibrous components of vertebrate ECM; proteolytic activation of the enzyme lysyl oxidase, necessary to formation of covalent cross-links in collagen and elastic fibers; processing of NH2-terminal globular domains and C-propeptides of types V and XI procollagen chains to yield monomers that are incorporated into and control the diameters of collagen type I and II fibrils, respectively; processing of precursors for laminin 5 and collagen type VII, both of which are involved in securing epidermis to underlying dermis; and maturation of small leucine-rich proteoglycans. The BMP1/TLD-related metalloproteinases are also capable of activating the vertebrate transforming growth factor-beta (TGF-beta)-like "chalones" growth differentiation factor 8 (GDF8, also known as myostatin), and GDF11 (also known as BMP11), involved in negative feedback inhibition of muscle and neural tissue growth, respectively; by freeing them from noncovalent latent complexes with their cleaved prodomains. BMP1/TLD-like proteinases also liberate the vertebrate TGF-beta-like morphogens BMP2 and 4 and their invertebrate ortholog decapentaplegic, from latent complexes with the vertebrate extracellular antagonist chordin and its invertebrate ortholog short gastrulation (SOG), respectively. The result is formation of the BMP signaling gradients that form the dorsal-ventral axis in embryogenesis. Thus, BMP1/TLD-like proteinases appear to be key to regulating and orchestrating formation of the ECM and signaling by various TGF-beta-like proteins in morphogenetic and homeostatic events.

Determinants of skeletal fragility

Strategies to reduce fracture risk must be based on a sound understanding of the mechanisms that underline the increased incidence of fractures with age and with certain diseases. There is evidence that in addition to bone minerals density, other factors influence bone strength. The chapter reviews the biomechanical aspects of age-related fractures, including the interacting roles of traumatic loading and bone strength, and the factors that determine a bones resistances to fracture. Also discussed are the mechanisms by which anti-catabolic and anabolic therapies for osteoporosis may affect bone strength. Finally, several current and future methodologies for improving assessment of bone strength in patients are evaluated.

Genetic variation in structure-function relationships for the inbred mouse lumbar vertebral body

Structure-function relationships were determined for L(5) vertebral bodies from three inbred mouse strains. Genetic variability in whole bone mechanical properties could be explained by a combination of the traits specifying the amount, distribution, and quality of the cortical and trabecular bone tissue.

INTRODUCTION

Although phenotypically correlated with fracture, BMD may be disadvantageous to use in genetic and biomechanical analyses because BMD does not distinguish the contributions of the underlying morphological and compositional bone traits. Developing functional relationships between the underlying bone traits and whole bone mechanical properties should further our understanding of the genetics of bone fragility.

MATERIALS AND METHODS

Microarchitecture and composition of L(5) vertebral bodies (n = 10/strain) from A/J, C57BL/6J, and C3H/HeJ inbred mouse strains were determined using muCT with an isotropic voxel size of 16 mum(3). Failure load, stiffness, and total deformation as a measure of ductility were measured in compression using a noncontact strain extensometer imaging system. A correlation analysis related morphological and compositional bone traits to whole bone mechanical properties. A multivariate analysis identified structure-function relationships for each genotype.

RESULTS

No single bone trait accurately explained the genetic variation in mechanical properties. However, a combination of traits describing the amount, distribution, and quality of cortical and trabecular bone tissue explained >70% of the variation in vertebral mechanical properties. Importantly, structure-function relationships were unique among genotypes.

CONCLUSIONS

Different genetic backgrounds use different combinations of underlying bone traits to create mechanically functional structures. Using a single complex trait such as BMD or BV/TV as the sole phenotypic marker in genetic analyses may prove to be disadvantageous because of the complex relationship between mechanical properties and the underlying bone traits. Therefore, considering multiple bone traits and the interaction among these bone traits is necessary to understand the relationship between genetic background and complex whole bone mechanical properties.

Intracellular trafficking and degradation of unassociated proalpha2 chains of collagen type I

Procollagen I is a trimer consisting of two proalpha1(I) chains and one proalpha 2(I) chain. In certain cases of mild osteogenesis imperfecta, abnormal proalpha1(I) chains are degraded very soon after synthesis. As a consequence, the cells produce excess proalpha2(I) chains, which cannot form trimers and are not secreted. The objective of this work was to determine the intracellular fate of unassociated proalpha2(I) chains. Mov13 mouse fibroblasts, which do not synthesize proalpha1(I) mRNA, but do produce proalpha2(I) mRNA, were incubated with radioactive amino acids using pulse-chase protocols, and proteins were analyzed by gel electrophoresis, autoradiography, and Western blotting. Mov13 cells produced proalpha2(I) chains that were degraded intracellularly within 30 min. Degradation was inhibited when cells were treated with brefeldin-A, which blocks transit from endoplasmic reticulum to Golgi. Fixed cells exposed to various immunofluorescence markers and imaged by confocal laser scanning microscopy showed that proalpha2(I) chains colocalized with Golgi and lysosome markers. Degradation was inhibited and chains were secreted when cells were treated with wortmannin, which blocks trafficking to lysosomes. These results demonstrate that unassociated proalpha2(I) chains leave the endoplasmic reticulum, transit the Golgi, and enter lysosomes where they are degraded.

Brittle IV mouse model for osteogenesis imperfecta IV demonstrates postpubertal adaptations to improve whole bone strength

The Brtl mouse model for type IV osteogenesis imperfecta improves its whole bone strength and stiffness between 2 and 6 months of age. This adaptation is accomplished without a corresponding improvement in geometric resistance to bending, suggesting an improvement in matrix material properties.

INTRODUCTION

The Brittle IV (Brtl) mouse was developed as a knock-in model for osteogenesis imperfecta (OI) type IV. A Gly349Cys substitution was introduced into one col1a1 allele, resulting in a phenotype representative of the disease. In this study, we investigate the effect of the Brtl mutation on whole bone architecture, strength, and composition across a range of age groups.

MATERIALS AND METHODS

One-, 2-, 6-, and 12-month-old Brtl and wildtype (WT) mice were analyzed. Femurs were assessed at the central diaphysis for cortical geometric parameters using microCT and were subsequently mechanically tested to failure by four-point bending. Matrix material properties were predicted using microCT data to normalize data from mechanical tests. Raman spectroscopy and DXA were used to assess matrix composition.

RESULTS

Our findings show a postpubertal adaptation in which Brtl femoral strength and stiffness increase through a mechanism independent of changes in whole bone geometry. These findings suggest an improvement in the material properties of the bone matrix itself, rather than improvements in whole bone geometry, as seen in previous mouse models of OI. Raman spectroscopic results suggest these findings may be caused by changes in mineral/matrix balance rather than improvements in mineral crystallinity.

CONCLUSIONS

Our findings parallel the currently unexplained clinical observation of decreased fractures in human OI patients after puberty. The Brtl mouse remains an important tool for investigating therapeutic interventions for OI.

Quantitative trait loci for femoral size and shape in a genetically heterogeneous mouse population

The aim of this study was to examine the genetic effects on cortical bone geometry. Genotypes from 487 mice were compared with geometric traits obtained from microCT. We found 14 genetic markers that associate with geometric traits, showing the complexity of genetic control over bone geometry.

INTRODUCTION

Previous studies have shown that genetic background affects bone characteristics, particularly bone mineral density, in both mouse and human populations. Much less is known, however, about the effects of polymorphic genes on bone size, shape, and mechanical integrity. In this study, we investigated the genetic determinants of geometric properties of cortical bone in mice.

MATERIALS AND METHODS

This study used a genetically heterogeneous mouse population, which is denoted UM-HET3 stock and is derived as the progeny of (BALB/cJ X C57BL/6J) F1 females and (C3H/HeJ X DBA/2J) F1 males. The experimental group consisted of 487 female UM-HET3 mice. Genotypic data from 99 polymorphic genetic loci was obtained from the mice at 4 weeks of age. At 18 months of age, the mice were humanely killed, and the right femurs were scanned with microcomputed tomography to assess geometric properties of cortical bone. A permutation-based test was used to detect significant associations between genetic markers and geometric traits. This test generates experiment-wise p values, which account for the effect of testing multiple hypotheses. An experiment-wise p < or = 0.05 was considered statistically significant.

RESULTS

Fourteen genetic markers were found to significantly associate with one or more geometric traits. Two markers (D3Mit62 and D4Mit155) were associated with traits describing bone size; 2 (D12Mit167 and D14Mit170) were linked with traits describing bone shape; and 10 (D1Nds2, D5Mit95, D6Mit216, D7Mit91, D8Mit51, D9Mit110, D11Mit83, D15Mit100, D15Mit171, and D17Mit46) were associated with both size and shape.

CONCLUSIONS

Our results indicate that the genetic control of cortical bone geometry is complex and that femoral size and shape may be influenced by different, although overlapping, groups of polymorphic loci.

Hierarchical structure of bone and micro-computed tomography

Bone is highly complex, with multiple hierarchical levels of structure. Micro-CT has been able to provide much information about the properties of bone at several of these levels at the mid-range of bone's hierarchical structure, and it will continue to provide a valuable tool for further characterizing bone in various conditions and explaining mechanisms of bone failure.

Bone brittleness varies with genetic background in A/J and C57BL/6J inbred mice

The contribution of genetic and environmental factors to variations in bone quality are understood poorly. We tested whether bone brittleness varies with genetic background using the A/J and C57BL/6J inbred mouse strains. Whole bone four-point bending tests revealed a 70% decrease in postyield deflection of A/J femurs compared with C57BL/6J, indicating that A/J femurs failed in a significantly more brittle manner. Cyclic loading studies indicated that A/J femurs accumulated damage differently than C57BL/6J femurs, consistent with their increased brittleness. Differences in matrix composition also were observed between the two mouse strains. A/J femurs had a 4.5% increase in ash content and an 11.8% decrease in collagen content. Interestingly, a reciprocal relationship was observed between femoral geometry and material stiffness; this relationship may have contributed to the brittle phenotype of A/J femurs. A/J femurs are more slender than those of C57BL/6J femurs; however, their 47% smaller moment of inertia appeared to be compensated by an increased tissue stiffness at the expense of altered tissue damageability. Importantly, these differences in whole bone mechanical properties between A/J and C57BL/6J femurs could not have been predicted from bone mass or density measures alone. The results indicated that bone brittleness is a genetically influenced trait and that it is associated with genetically determined differences in whole bone architecture, bone matrix composition, and mechanisms of cyclical damage accumulation.

Bone-directed expression of Col1a1 promoter-driven self-inactivating retroviral vector in bone marrow cells and transgenic mice

Gene therapy of bone would benefit from the availability of vectors that provide stable, osteoblast-specific expression. This would allow bone-specific expression of Col1a1 cDNAs for treatment of osteogenesis imperfecta. In addition, such a vector would restrict expression of secreted therapeutic proteins to the bone-synthesizing regions of the bone marrow after ex vivo transduction of marrow stromal cells and reintroduction of the cells into patients. Retrovirus vectors stably integrate into target cell genomes; however, long-term regulated expression from internal cellular promoters has not been consistently achieved. In some cases this is due to a stem cell-specific mechanism for transcriptional repression of retroviruses. We evaluated the ability of self-inactivating ROSA-derived vectors containing a bone-directed 2.3-kb rat Col1a1 promoter to display osteoblast-specific expression. In vitro expression was examined in bone marrow stromal cell cultures induced to undergo osteoblastic differentiation. In vivo expression was evaluated in chimeric mice derived from transduced embryonic stem cells. The results indicate that self-inactivating retrovirus vectors containing the Col1a1 promoter are not permanently inactivated in embryonic stem cells and are specifically expressed in osteoblasts in vivo and in vitro. Thus these vectors should be useful for bone-directed gene therapy.

Bone material elasticity in a murine model of osteogenesis imperfecta

To investigate the source of bone brittleness in the disease osteogenesis imperfecta (OI), biomechanical properties have been measured in the femurs from a homozygous (oim/oim) mutant mouse model of OI, its heterozygous littermates, and wild-type animals. The novel technique of ultrasound critical-angle reflectometry (UCR) was used to determine bone material elasticity matrix from measurements of the pressure and shear wave velocity at different orientations about selected points of the bone specimens. This nondestructive method is the only available means for obtaining measurements of this nature from a single surface. The ultrasound pressure wave velocity showed an increased isotropy in the homozygous compared to the wild-type specimens. This was reflected in a significant decrease in the principal elastic modulus measured along the length of the oim/oim bones (E33) while the modulus along the width (E11) did not change significantly, compared to wild-type specimens. The Poisson's ratio, v12, also had a significantly increased value in oim/oim bones. Measurements of these parameters in heterozygous animals generally fell between those from homozygous and control mice. The differences in the elasticity components in oim/oim bones indicate an altered stress distribution and a modified elastic response to loads, compared to normal bone.

Increased marrow-derived osteoprogenitor cells and endosteal bone formation in mice lacking thrombospondin 2

The phenotype of thrombospondin 2 (TSP2)-null mice includes abnormalities in collagen fibrils and increases in ligamentous laxity, vascular density, and bleeding time. In this study, analyses by computerized tomography (CT) revealed that cortical density was increased in long bones of TSP2-null mice. Histomorphometric analysis showed that the mid-diaphyseal endosteal bone formation rate (BFR) of TSP2-null mice was increased in comparison with that of wild-type (WT) animals. Although microgeometric analysis showed that periosteal and endosteal radii were reduced, the mechanical properties of femurs from TSP2-null mice were not significantly different from those of controls, presumably because of the concomitant increase in endosteal bone mass. Bone loss in ovariectomized mice was equivalent for WT and mutant mice, a finding that indicates that TSP2-null animals are capable of normal bone resorption. To further explore the cellular basis for the increased endosteal BFR in TSP2-null mice, marrow stromal cells (MSCs) were isolated and examined in vitro. These cells were found to be present in increased numbers in a colony forming unit (CFU) assay and showed an increased rate of proliferation in vitro. We conclude that TSP2 regulates the proliferation of osteoblast progenitors, directly or indirectly, and that in its absence endosteal bone formation is increased.

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.