Bone microstructure and its associated genetic variability in 12 inbred mouse strains: microCT study and in silico genome scan

A Model of Immediate Implant Placement to Evaluate Early Osseointegration in 129/Sv Diabetic Mice

PURPOSE

To analyze the process of early oral osseointegration of titanium (Ti) implants in diabetic 129/Sv mice through microCT and histologic and immunohistochemical analysis.

MATERIALS AND METHODS

A group of 30 male 129/Sv mice was equally subdivided into two groups: (1) nondiabetic (ND), in which mice did not undergo systemic alterations and received a standard diet, and (2) diabetic (D), in which mice were provided a high-fat diet from the age of 6 weeks until the conclusion of the study and received two intraperitoneal (IP) injections of streptozotocin (STZ) at a concentration of 100 mg/Kg each. Each mouse underwent extraction of a maxillary first molar, and customized Ti screws (0.50 mm diameter, 1.5 mm length) were placed in the residual alveolar sockets of the palatal roots. At 7 and 21 days after implant placement, the animals were euthanized for maxilla and pancreas collection. Maxillae containing Ti implants were analyzed with microCT, histology, and immunohistochemistry for cells that were positive for F4/80, CD146, runt-related transcription factor 2 (Runx2), and proliferating cell nuclear antigen (PCNA). Pancreata were histologically analyzed. Quantitative data were statistically analyzed with a significance level at 5% (P < .05).

RESULTS

ND mice presented successful healing and osseointegration, with a significantly higher fraction of bone volume compared to D mice, both at the alveolar sockets (53.39 ± 5.93 and 46.08 ± 3.18, respectively) and at the implant sites (68.88 ± 7.07 and 44.40 ± 6.98, respectively) 21 days after implant placement. Histologic evaluation revealed that the ND mice showed a significant decrease in inflammatory infiltrate and a significant increase in newly formed bone matrix at 21 days, whereas peri-implant sites in the D mice were predominantly encapsulated by fibrous tissue and chronic inflammatory infiltrate. Immunohistochemical characterization revealed higher Runx2 osteoblast differentiation and higher cell proliferation activity in the ND mice at 7 days, while higher amounts of macrophages were present in D mice at 7 and 21 days. Interestingly, no differences were found in CD146-positive cells when comparing ND and D mice.

CONCLUSIONS

This study evaluated the effects of immediate dental implant placement in 129/Sv diabetic mice by using specific healing markers to identify changes in cellular events involved in early oral osseointegration. This approach may serve as tool to evaluate new materials and surface coatings to improve osseointegration in diabetic patients.

Influence of age and gender on alveolar bone healing post tooth extraction in 129 Sv mice: a microtomographic, histological, and biochemical characterization

OBJECTIVES

To analyze the effect of biological sex and aging on craniofacial bone features in 129 Sv mice and their influence on dental socket healing post tooth extraction.

MATERIALS AND METHODS

A total of 52 129 Sv mice were used, of which 28 were young (3-4 months) and 24 were aged (17-18 months), equally distributed according to biological sex. After an upper right incisor extraction, mice specimens were collected at 7, 14, and 21-days post-surgery for microtomographic (microCT) and comprehensive histological analysis. Mandible, skull bones, and maxillae at 21 days were analyzed by microCT, while blood plasma samples were collected for the detection of key bone turnover markers (P1NP and CTX-1) by enzyme-linked immunosorbent (ELISA) assay.

RESULTS

Aged females depicted significantly decreased mineralized bone content in alveolar sockets in comparison to young females and aged males at day 7, and aged males at day 14. Mandible RCA and Ma.AR of aged females were also significantly decreased in comparison with young females. Histological evaluation revealed that all alveolar sockets healed at 21 days with inflammation resolution and deposition of new bone. Immunohistochemistry for TRAP revealed increased area density for osteoclasts in alveolar sockets of aged females when compared to young females at 21 days. While a significant increase in CTX-1 levels was detected in blood plasma of aged females when compared to young females, P1NP levels did not significantly change between young and older females. No significant changes were observed for males.

CONCLUSIONS

Age and gender can significantly affect craniofacial bones of 129 Sv mice, especially maxilla and mandible in females. Considering the altered bone resorption parameters and delayed alveolar bone healing in older females, careful deliberation is necessary during development of pre-clinical models for craniofacial research.

CLINICAL RELEVANCE

Aging can be a contributing factor to slower bone healing in craniofacial bones. However, there are no sufficient experimental studies that have addressed this phenomenon along with biological sex taken into consideration.

Bone Mass and Osteoblast Activity Are Sex-Dependent in Mice Lacking the Estrogen Receptor α in Chondrocytes and Osteoblast Progenitor Cells

While estrogen receptor alpha (ERα) is known to be important for bone development and homeostasis, its exact function during osteoblast differentiation remains unclear. Conditional deletion of ERα during specific stages of osteoblast differentiation revealed different bone phenotypes, which were also shown to be sex-dependent. Since hypertrophic chondrocytes can transdifferentiate into osteoblasts and substantially contribute to long-bone development, we aimed to investigate the effects of ERα deletion in both osteoblast and chondrocytes on bone development and structure. Therefore, we generated mice in which the ERα gene was inactivated via a Runx2-driven cyclic recombinase (ERαfl/fl; Runx2Cre). We analyzed the bones of 3-month-old ERαfl/fl; Runx2Cre mice by biomechanical testing, micro-computed tomography, and cellular parameters by histology. Male ERαfl/fl; Runx2Cre mice displayed a significantly increased cortical bone mass and flexural rigidity of the femurs compared to age-matched controls with no active Cre-transgene (ERαfl/fl). By contrast, female ERαfl/fl; Runx2Cre mice exhibited significant trabecular bone loss, whereas in cortical bone periosteal and endosteal diameters were reduced. Our results indicate that the ERα in osteoblast progenitors and hypertrophic chondrocytes differentially contributes to bone mass regulation in male and female mice and improves our understanding of ERα signaling in bone cells in vivo.

In Vitro Radiosensitivity of Murine Marrow Stromal Cells Varies Across Donor Strains

Mouse models are widely used in the study of musculoskeletal radiobiology both in vivo and in vitro. Two of the most commonly used mouse strains are C57BL/6 and BALB/c. However, little is known about their equivalence in response to ionizing radiation. In this study we compare the responses of marrow stromal cells derived from both of these strains to X rays in vitro at passages 0 and 2. Colony-forming efficiency was significantly higher in BALB/c marrow stromal cells at passage 0. Radiation-induced decreases in colony-forming unit (CFU) formation at passage 0 were comparable across both strains at 0-2 Gy, but BALB/c stromal cells were more radiosensitive than C57BL/6 stromal cells at 3-7 Gy. Osteogenic differentiation at passage 2 was not affected by radiation for either strain. This work demonstrates that commonly used inbred mouse strains differ in their early-passage marrow stromal cell responses to X rays, including self-renewal and differentiation potential. This variability is an important point to consider when selecting an animal model for in vivo or in vitro study.

Comparison of inbred mouse strains shows diverse phenotypic outcomes of intervertebral disc aging

Intervertebral disc degeneration presents a wide spectrum of clinically degenerative disc phenotypes; however, the contribution of genetic background to the degenerative outcomes has not been established. We characterized the spinal phenotype of 3 mouse strains with varying cartilage-regenerative potential at 6 and 23 months: C57BL/6, LG/J and SM/J. All strains showed different aging phenotypes. Importantly, LG/J mice showed an increased prevalence of dystrophic disc calcification in caudal discs with aging. Quantitative-histological analyses of LG/J and SM/J caudal discs evidenced accelerated degeneration compared to BL6, with cellular disorganization and cell loss together with fibrosis of the NP, respectively. Along with the higher grades of disc degeneration, SM/J, at 6M, also differed the most in terms of NP gene expression compared to other strains. Moreover, although we found common DEGs between BL6 and LG/J aging, most of them were divergent between the strains. Noteworthy, the common DEGs altered in both LG/J and BL6 aging were associated with inflammatory processes, response to stress, cell differentiation, cell metabolism and cell division. Results suggested that disc calcification in LG/J resulted from a dystrophic calcification process likely aggravated by cell death, matrix remodelling, changes in calcium/phosphate homeostasis and cell transformation. Lastly, we report 7 distinct phenotypes of human disc degeneration based on transcriptomic profiles, that presented similar pathways and DEGs found in aging mouse strains. Together, our results suggest that disc aging and degeneration depends on the genetic background and involves changes in various molecular pathways, which might help to explain the diverse phenotypes seen during disc disease.

Inhibition of myostatin prevents microgravity-induced loss of skeletal muscle mass and strength

The microgravity conditions of prolonged spaceflight are known to result in skeletal muscle atrophy that leads to diminished functional performance. To assess if inhibition of the growth factor myostatin has potential to reverse these effects, mice were treated with a myostatin antibody while housed on the International Space Station. Grip strength of ground control mice increased 3.1% compared to baseline values over the 6 weeks of the study, whereas grip strength measured for the first time in space showed flight animals to be -7.8% decreased in strength compared to baseline values. Control mice in space exhibited, compared to ground-based controls, a smaller increase in DEXA-measured muscle mass (+3.9% vs +5.6% respectively) although the difference was not significant. All individual flight limb muscles analyzed (except for the EDL) weighed significantly less than their ground counterparts at the study end (range -4.4% to -28.4%). Treatment with myostatin antibody YN41 was able to prevent many of these space-induced muscle changes. YN41 was able to block the reduction in muscle grip strength caused by spaceflight and was able to significantly increase the weight of all muscles of flight mice (apart from the EDL). Muscles of YN41-treated flight mice weighed as much as muscles from Ground IgG mice, with the exception of the soleus, demonstrating the ability to prevent spaceflight-induced atrophy. Muscle gene expression analysis demonstrated significant effects of microgravity and myostatin inhibition on many genes. Gamt and Actc1 gene expression was modulated by microgravity and YN41 in opposing directions. Myostatin inhibition did not overcome the significant reduction of microgravity on femoral BMD nor did it increase femoral or vertebral BMD in ground control mice. In summary, myostatin inhibition may be an effective countermeasure to detrimental consequences of skeletal muscle under microgravity conditions.

Age- and Strain-Related Differences in Bone Microstructure and Body Composition During Development in Inbred Male Mouse Strains

We explored age- and strain-related differences in bone microstructure and body composition in male C57BL/6J, DBA/2JRj and C3H/J mice. Bone microstructure of the femur, tibia and L4 was assessed by μCT at the age of 8, 16 and 24 weeks. The weight of several muscles and fat depots were measured at the same time points. At all timepoints, C3H/J mice had the thickest cortices followed by DBA/2JRj and C57BL/6J mice. Nevertheless, C57BL/6J mice had higher Tb.BV/TV and Tb.N, and lower Tb.Sp than DBA/2JRj and C3H/J mice at least at 24 weeks of age. Skeletal development patterns differed among strains. C57BL/6J and DBA/2JRj mice, but not C3H/J mice, experienced significant increases in the sum of the masses of 6 individual muscles by 24 weeks of age. In C57BL/6J and DBA/2JRj mice, the mass of selected fat depots reached highest values at 24 weeks, whist, in C3H/J mice, the highest values of fat depots masses were achieved at 16 weeks. Early strain differences in muscle and fat masses were largely diminished by 24 weeks of age. C3H/J and C57BL/6J mice displayed the most favorable cortical and trabecular bone parameters, respectively. Strain differences in body composition were less overt than strain specificity in bone microstructure, however, they possibly influenced aspects of skeletal development.

Protective Effects of Controlled Mechanical Loading of Bone in C57BL6/J Mice Subject to Disuse

Prolonged reduction in weightbearing causes bone loss. Disuse of bone is associated with recovery from common musculoskeletal injury and trauma, bed rest resulting from various medical conditions, and spaceflight. The hindlimb‐suspension rodent model is popular for simulating unloading and disuse. We hypothesized that controlled mechanical loading of the tibia would protect against bone loss occurring from concurrent disuse. Additionally, we hypothesized that areas of high mechanical peak strains (midshaft) would provide more protection than areas of lower strain (distal shaft). Adult C57BL6/J mice were suspended for 3 weeks, with one limb subjected to tibial compression four times per week. μCT imaging was completed at days 0, 11, and 21, in addition to serum analysis. Significant bone loss caused by hindlimb suspension was detected in trabecular bone by day 11 and worsened by day 21 (p < 0.05). Bone loss was also detected in cortical thickness and area fraction by day 21. However, four short bouts per week of compressive loading protected the loaded limb from much of this bone loss. At day 21, we observed a 50% loss in trabecular bone volume/total volume and a 6% loss in midshaft cortical thickness in unloaded limbs, but only 15% and 2% corresponding losses in contralateral loaded limbs (p = 0.001 and p = 0.02). Many bone geometry parameters of the loaded limbs of suspended animals did not significantly differ from non‐suspended control limbs. Conversely, this protective effect of loading was not detected in cortical bone at the lower‐strained distal shaft. Analysis of bone metabolism markers suggested that the benefits of loading occurred through increased formation instead of decreased resorption. This study uniquely isolates the role of externally applied mechanical loading of the mouse tibia, in the absence of muscle stimulation, in protecting bone from concurrent disuse‐related loss, and demonstrates that limited bouts of loading may be highly effective during prolonged disuse. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Dietary calcium intake and genetics have site-specific effects on peak trabecular bone mass and microarchitecture in male mice

Trabecular bone (Tb) is used for rapid exchange of calcium (Ca) in times of physiologic need and the site-specific characteristics of Tb may explain why certain sites are more vulnerable to osteoporosis. We hypothesized that peak trabecular bone mass (PTBM) and Tb microarchitecture are differentially regulated by dietary Ca intake, genetics, or Gene-by-Diet (GxD) interactions at the distal femur and the fifth lumbar (L5) vertebra. Male mice from 62 genetically distinct lines were fed basal (0.5%) or low (0.25%) Ca diets from 4 to 12 wks of age. Afterwards, the right femur and L5 vertebra were removed and trabecular bone was analyzed by μCT. In mice fed the basal diet, bone volume fraction (BV/TV), trabecular number (Tb.N), and connectivity density (Conn.D) were significantly higher in the L5 vertebra than femur. Femur Tb had a weaker, more rod-like structure than the L5 vertebrae while mice fed the low Ca diet developed rod-like structures at both sites. Dietary Ca restriction also caused a greater relative reduction of Tb.N and Conn.D in the femur than L5 vertebra, i.e. it was more harmful to the integrity of Tb microarchitecture in femur. Genetics was a major determinant of Tb at both sites, e.g. heritability of BV/TV on the basal diet = 0.65 (femur) and 0.68 (L5 vertebra). However, while GxD interactions altered the impact of dietary Ca restriction on Tb parameters at both sites, the effect was not uniform, e.g. some lines had site-specific responses to Ca restriction. The significance of our work is that there are site-specific effects of dietary Ca restriction and genetics that work independently and interactively to influence the attainment of PTBM and Tb microarchitecture.

Medication-related osteonecrosis of the jaws after tooth extraction in senescent female mice treated with zoledronic acid: Microtomographic, histological and immunohistochemical characterization

Treatment with cumulative dosages of zoledronic acid (ZA) in elderly patients is a risk factor for the development of medication-related osteonecrosis of the jaws (MRONJ), mainly related to surgical triggers such as tooth extraction. However, animal models for the investigation and understanding of MRONJ pathophysiology in senescent and postmenopausal stages remains to be developed and characterized. The aim of this study was to analyze MRONJ development in senescent female mice treated with cumulative dosages of ZA. For this purpose, twenty 129/Sv female mice, 64 weeks old, were treated with 0.9% saline solution as control group (n = 10), and with ZA at 250μg/Kg (n = 10), once a week, starting 4 weeks before the upper right incisor extraction and until the end of the experimental time points (7 days and 21 days). At 7 and 21 days post-surgery, specimens were harvested for microCT, histological, birefringence and immunohistochemical analysis. Clinically, an incomplete epithelialization was observed in ZA group at 7 days and a delayed bone matrix mineralization and collagen maturation at 7 and 21 days compared to the controls. Controls revealed sockets filled with mature bone at 21 days as observed by microCT and birefringence, while ZA group presented delayed bone deposition at 7 and 21 days, as well increased leukocyte infiltration and blood clot at 7 days, and increased bone sequestrum and empty osteocyte lacunae at 21 days (p<0.05). Also, ZA group presented decreased quantity of TGFb+ and Runx-2+ cells at 7 days, and decreased quantity of TRAP+ osteoclasts compared to the control at 21 days (p<0.05). Altogether, these data demonstrate the usefulness of this model to understanding the pathophysiology of MRONJ.

High Fidelity of Mouse Models Mimicking Human Genetic Skeletal Disorders

The 2019 International Skeletal Dysplasia Society nosology update lists 441 genes for which mutations result in rare human skeletal disorders. These genes code for enzymes (33%), scaffolding proteins (18%), signal transduction proteins (16%), transcription factors (14%), cilia proteins (8%), extracellular matrix proteins (5%), and membrane transporters (4%). Skeletal disorders include aggrecanopathies, channelopathies, ciliopathies, cohesinopathies, laminopathies, linkeropathies, lysosomal storage diseases, protein-folding and RNA splicing defects, and ribosomopathies. With the goal of evaluating the ability of mouse models to mimic these human genetic skeletal disorders, a PubMed literature search identified 260 genes for which mutant mice were examined for skeletal phenotypes. These mouse models included spontaneous and ENU-induced mutants, global and conditional gene knockouts, and transgenic mice with gene over-expression or specific base-pair substitutions. The human X-linked gene ARSE and small nuclear RNA U4ATAC, a component of the minor spliceosome, do not have mouse homologs. Mouse skeletal phenotypes mimicking human skeletal disorders were observed in 249 of the 260 genes (96%) for which comparisons are possible. A supplemental table in spreadsheet format provides PubMed weblinks to representative publications of mutant mouse skeletal phenotypes. Mutations in 11 mouse genes (Ccn6, Cyp2r1, Flna, Galns, Gna13, Lemd3, Manba, Mnx1, Nsd1, Plod1, Smarcal1) do not result in similar skeletal phenotypes observed with mutations of the homologous human genes. These discrepancies can result from failure of mouse models to mimic the exact human gene mutations. There are no obvious commonalities among these 11 genes. Body BMD and/or radiologic dysmorphology phenotypes were successfully identified for 28 genes by the International Mouse Phenotyping Consortium (IMPC). Forward genetics using ENU mouse mutagenesis successfully identified 37 nosology gene phenotypes. Since many human genetic disorders involve hypomorphic, gain-of-function, dominant-negative and intronic mutations, future studies will undoubtedly utilize CRISPR/Cas9 technology to examine transgenic mice having genes modified to exactly mimic variant human sequences. Mutant mice will increasingly be employed for drug development studies designed to treat human genetic skeletal disorders.

SIGNIFICANCE

Great progress is being made identifying mutant genes responsible for human rare genetic skeletal disorders and mouse models for genes affecting bone mass, architecture, mineralization and strength. This review organizes data for 441 human genetic bone disorders with regard to heredity, gene function, molecular pathways, and fidelity of relevant mouse models to mimic the human skeletal disorders. PubMed weblinks to citations of 249 successful mouse models are provided.

A novel mouse model of intervertebral disc degeneration shows altered cell fate and matrix homeostasis

Intervertebral disc degeneration and associated low back and neck pain is a ubiquitous health condition that affects millions of people world-wide, and causes high incidence of disability and enormous medical/societal costs. However, lack of appropriate small animal models with spontaneous disease onset has impeded our ability to understand the pathogenetic mechanisms that characterize and drive the degenerative process. We report, for the first time, early onset spontaneous disc degeneration in SM/J mice known for their poor regenerative capacities compared to "super-healer" LG/J mice. In SM/J mice, degenerative process was marked by decreased nucleus pulposus (NP) cellularity and changes in matrix composition at P7, 4, and 8 weeks with increased severity by 17 weeks. Distinctions between NP and annulus fibrosus (AF) or endplate cartilage were lost, and NP and AF of SM/J mice showed higher histological grades. There was increased NP cell death in SM/J mice with decreased phenotypic marker expression. Polarized microscopy and FTIR spectroscopy demonstrated replacement of glycosaminoglycan-rich NP matrix with collagenous fibrous tissue. The levels of ARGxx were increased in, indicating higher aggrecan turnover. Furthermore, an aberrant expression of collagen X and MMP13 was observed in the NP of SM/J mice, along with elevated expression of Col10a1, Ctgf, and Runx2, markers of chondrocyte hypertrophy. Likewise, expression of Enpp1 as well as Alpl was higher, suggesting NP cells of SM/J mice promote dystrophic mineralization. There was also a decrease in several pathways necessary for NP cell survival and function including Wnt and VEGF signaling. Importantly, SM/J discs were stiffer, had decreased height, and poor vertebral bone quality, suggesting compromised motion segment mechanical functionality. Taken together, our results clearly demonstrate that SM/J mouse strain recapitulates many salient features of human disc degeneration, and serves as a novel small animal model.

Age-dependent variations of cancellous bone in response to ovariectomy in C57BL/6J mice

The ovariectomized (OVX) mouse model has been widely accepted to be suitable for the study of postmenopausal osteoporosis. However, whether C57BL/6J mice, a commonly used genetic background mouse strain, is an appropriate model for postmenopausal osteoporosis remains controversial. The present study investigated the effect of the OVX model on alterations in bone density and microarchitecture in C57BL/6J female mice of different ages. C57BL/6J mice were divided into 8-, 12- and 16-week-old groups (OVX₈, OVX₁₂ and OVX₁₆) from the beginning of OVX. At 8 weeks post-surgery, the mice were anesthetized and micro-computed tomography was used to analyze the bone density and microarchitecture. The results revealed that OVX-induced loss of cancellous bone was greatest in OVX₈, moderate in OVX₁₂, and only a weak bone loss was observed in the OVX₁₆ group when compared with the SHAM₁₆ control group. In addition, the effect of genetic backgrounds in response to the OVX model were examined. Several other strains of mice, including inbred (BALB/c) and outbred (ICR and Kunming), were used in the present study, all of which were subjected to OVX at 8 weeks of age. The present findings revealed that the highest rate of bone loss was detected in C57BL/6J female mice. In addition, treatment with estrogen (17β-estradiol, 30 µg/kg five times per week) led to a significant increase in bone density in C57BL/6J mice compared with the other strains of mice. Therefore, these results may provide novel insights into the age- and strain-associated effect of OVX on regulating turnover of bone in female mice. The present findings also suggest 8-week-old C57BL/6J mice as an animal model for postmenopausal osteoporosis and preclinical testing of potential therapies for this disease.

Novel Genetic Variants Associated With Increased Vertebral Volumetric BMD, Reduced Vertebral Fracture Risk, and Increased Expression of SLC1A3 and EPHB2

Genome-wide association studies (GWASs) have revealed numerous loci for areal bone mineral density (aBMD). We completed the first GWAS meta-analysis (n = 15,275) of lumbar spine volumetric BMD (vBMD) measured by quantitative computed tomography (QCT), allowing for examination of the trabecular bone compartment. SNPs that were significantly associated with vBMD were also examined in two GWAS meta-analyses to determine associations with morphometric vertebral fracture (n = 21,701) and clinical vertebral fracture (n = 5893). Expression quantitative trait locus (eQTL) analyses of iliac crest biopsies were performed in 84 postmenopausal women, and murine osteoblast expression of genes implicated by eQTL or by proximity to vBMD-associated SNPs was examined. We identified significant vBMD associations with five loci, including: 1p36.12, containing WNT4 and ZBTB40; 8q24, containing TNFRSF11B; and 13q14, containing AKAP11 and TNFSF11. Two loci (5p13 and 1p36.12) also contained associations with radiographic and clinical vertebral fracture, respectively. In 5p13, rs2468531 (minor allele frequency [MAF] = 3%) was associated with higher vBMD (β = 0.22, p = 1.9 × 10-8 ) and decreased risk of radiographic vertebral fracture (odds ratio [OR] = 0.75; false discovery rate [FDR] p = 0.01). In 1p36.12, rs12742784 (MAF = 21%) was associated with higher vBMD (β = 0.09, p = 1.2 × 10-10 ) and decreased risk of clinical vertebral fracture (OR = 0.82; FDR p = 7.4 × 10-4 ). Both SNPs are noncoding and were associated with increased mRNA expression levels in human bone biopsies: rs2468531 with SLC1A3 (β = 0.28, FDR p = 0.01, involved in glutamate signaling and osteogenic response to mechanical loading) and rs12742784 with EPHB2 (β = 0.12, FDR p = 1.7 × 10-3 , functions in bone-related ephrin signaling). Both genes are expressed in murine osteoblasts. This is the first study to link SLC1A3 and EPHB2 to clinically relevant vertebral osteoporosis phenotypes. These results may help elucidate vertebral bone biology and novel approaches to reducing vertebral fracture incidence. © 2016 American Society for Bone and Mineral Research.

The Roles and Mechanisms of Actions of Vitamin C in Bone: New Developments

Vitamin C is an important antioxidant and cofactor that is involved in the regulation of development, function, and maintenance of several cell types in the body. Deficiencies in vitamin C can lead to conditions such as scurvy, which, among other ailments, causes gingivia, bone pain, and impaired wound healing. This review examines the functional importance of vitamin C as it relates to the development and maintenance of bone tissues. Analysis of several epidemiological studies and genetic mouse models regarding the effect of vitamin C shows a positive effect on bone health. Overall, vitamin C exerts a positive effect on trabecular bone formation by influencing expression of bone matrix genes in osteoblasts. Recent studies on the molecular pathway for vitamin C actions that include direct effects of vitamin C on transcriptional regulation of target genes by influencing the activity of transcription factors and by epigenetic modification of key genes involved in skeletal development and maintenance are discussed. With an understanding of mechanisms involved in the uptake and metabolism of vitamin C and knowledge of precise molecular pathways for vitamin C actions in bone cells, it is possible that novel therapeutic strategies can be developed or existing therapies can be modified for the treatment of osteoporotic fractures.

Immobilization induced osteopenia is strain specific in mice

Immobilization causes rapid and massive bone loss. By comparing Botulinum Toxin A (BTX)-induced bone loss in mouse strains with different genetic backgrounds we investigated whether the genetic background had an influence on the severity of the osteopenia. Secondly, we investigated whether BTX had systemic effects on bone. Female mice from four inbred mouse strains (BALB/cJ, C57BL/6 J, DBA/2 J, and C3H/HeN) were injected unilaterally with BTX (n = 10/group) or unilaterally with saline (n = 10/group). Mice were euthanized after 21 days, and the bone properties evaluated using μCT, DXA, bone histomorphometry, and mechanical testing. BTX resulted in substantially lower trabecular bone volume fraction (BV/TV) and trabecular thickness in all mouse strains. The deterioration of BV/TV was significantly greater in C57BL/6 J (− 57%) and DBA/2 J (− 60%) than in BALB/cJ (− 45%) and C3H/HeN (− 34%) mice. The loss of femoral neck fracture strength was significantly greater in C57BL/6 J (− 47%) and DBA/2 J (− 45%) than in C3H (− 25%) mice and likewise the loss of mid-femoral fracture strength was greater in C57BL/6 J (− 17%), DBA/2 J (− 12%), and BALB/cJ (− 9%) than in C3H/HeN (− 1%) mice, which were unaffected. Using high resolution μCT we found no evidence of a systemic effect on any of the microstructural parameters of the contralateral limb. Likewise, there was no evidence of a systemic effect on the bone strength in any mouse strain. We did, however, find a small systemic effect on aBMD in DBA/2 J and C3H/HeN mice. The present study shows that BTX-induced immobilization causes the greatest loss of cortical and trabecular bone in C57BL/6 J and DBA/2 J mice. A smaller loss of bone microstructure and fracture strength was seen in BALB/cJ mice, while the bone microstructure and fracture strength of C3H/HeN mice were markedly less affected. This indicates that BTX-induced loss of bone is mouse strain dependent. We found only minimal systemic effects of BTX.

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Botulinum Toxin A (Botox) causes only minimal systemic effects in mice.

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The degree of immobilization induced osteopenia is highly strain specific in mice.

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The greatest degree of bone loss was observed with C57BL/6 J and DBA/2 J mice followed by BALB/cJ mice after Botox-injection.

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C3H/HeN mice had the smallest bone loss following Botox-injection.

In vivo axial loading of the mouse tibia

Noninvasive methods to apply controlled, cyclic loads to the living skeleton are used as anabolic procedures to stimulate new bone formation in adults and enhance bone mass accrual in growing animals. These methods are also invaluable for understanding bone signaling pathways. Our focus here is on a particular loading model: in vivo axial compression of the mouse tibia. An advantage of loading the tibia is that changes are present in both the cancellous envelope of the proximal tibia and the cortical bone of the tibial diaphysis. To load the tibia of the mouse axially in vivo, a cyclic compressive load is applied up to five times a week to a single tibia per mouse for a duration lasting from 1 day to 6 weeks. With the contralateral limb as an internal control, the anabolic response of the skeleton to mechanical stimuli can be studied in a pairwise experimental design. Here, we describe the key parameters that must be considered before beginning an in vivo mouse tibial loading experiment, including methods for in vivo strain gauging of the tibial midshaft, and then we describe general methods for loading the mouse tibia for an experiment lasting multiple days.

Genetic perturbations that impair functional trait interactions lead to reduced bone strength and increased fragility in mice

Functional adaptation may complicate the choice of phenotype used in genetic studies that seek to identify genes contributing to fracture susceptibility. Often, genetic variants affecting one trait are compensated by coordinated changes in other traits. Bone fracture is a prototypic example because mechanical function of long bones (stiffness and strength) depends on how the system coordinately adjusts the amount (cortical area) and quality (tissue-mineral density, TMD) of bone tissue to mechanically offset the natural variation in bone robustness (total area/length). We propose that efforts aimed at identifying genes regulating fracture resistance will benefit from better understanding how functional adaptation contributes to the genotype-phenotype relationship. We analyzed the femurs of C57BL/6J-Chr(A/J)/NaJ Chromosome Substitution Strains (CSSs) to systemically interrogate the mouse genome for chromosomes harboring genes that regulate mechanical function. These CSSs (CSS-i, i=the substituted chromosome) showed changes in mechanical function on the order of -26.6 to +11.5% relative to the B6 reference strain after adjusting for body size. Seven substitutions showed altered robustness, cortical area, or TMD, but no effect on mechanical function (CSS-4, 5, 8, 9, 17, 18, 19); six substitutions showed altered robustness, cortical area, or TMD, and reduced mechanical function (CSS-1, 2, 6, 10, 12, 15); and one substitution also showed reduced mechanical function but exhibited no significant changes in the three physical traits analyzed in this study (CSS-3). A key feature that distinguished CSSs that maintained function from those with reduced function was whether the system adjusted cortical area and TMD to the levels needed to compensate for the natural variation in bone robustness. These results provide a novel biomechanical mechanism linking genotype with phenotype, indicating that genes control function not only by regulating individual traits, but also by regulating how the system coordinately adjusts multiple traits to establish function.

Autoimmunity contributes to nociceptive sensitization in a mouse model of complex regional pain syndrome

Complex regional pain syndrome (CRPS) is a painful, disabling, chronic condition whose etiology remains poorly understood. The recent suggestion that immunological mechanisms may underlie CRPS provides an entirely novel framework in which to study the condition and consider new approaches to treatment. Using a murine fracture/cast model of CRPS, we studied the effects of B-cell depletion using anti-CD20 antibodies or by performing experiments in genetically B-cell-deficient (μMT) mice. We observed that mice treated with anti-CD20 developed attenuated vascular and nociceptive CRPS-like changes after tibial fracture and 3 weeks of cast immobilization. In mice with established CRPS-like changes, the depletion of CD-20+ cells slowly reversed nociceptive sensitization. Correspondingly, μMT mice, deficient in producing immunoglobulin M (IgM), failed to fully develop CRPS-like changes after fracture and casting. Depletion of CD20+ cells had no detectable effects on nociceptive sensitization in a model of postoperative incisional pain, however. Immunohistochemical experiments showed that CD20+ cells accumulate near the healing fracture but few such cells collect in skin or sciatic nerves. On the other hand, IgM-containing immune complexes were deposited in skin and sciatic nerve after fracture in wild-type, but not in μMT fracture/cast, mice. Additional experiments demonstrated that complement system activation and deposition of membrane attack complexes were partially blocked by anti-CD20+ treatment. Collectively, our results suggest that CD20-positive B cells produce antibodies that ultimately support the CRPS-like changes in the murine fracture/cast model. Therapies directed at reducing B-cell activity may be of use in treating patients with CRPS.

Adenylyl cyclase 6 mediates loading-induced bone adaptation in vivo

Primary cilia are single, nonmotile, antenna-like structures extending from the apical membrane of most mammalian cells. They may mediate mechanotransduction, the conversion of external mechanical stimuli into biochemical intracellular signals. Previously we demonstrated that adenylyl cyclase 6 (AC6), a membrane-bound enzyme enriched in primary cilia of MLO-Y4 osteocyte-like cells, may play a role in a primary cilium-dependent mechanism of osteocyte mechanotransduction in vitro. In this study, we determined whether AC6 deletion impairs loading-induced bone formation in vivo. Skeletally mature mice with a global knockout of AC6 exhibited normal bone morphology and responded to osteogenic chemical stimuli similar to wild-type mice. Following ulnar loading over 3 consecutive days, bone formation parameters were assessed using dynamic histomorphometry. Mice lacking AC6 formed significantly less bone than control animals (41% lower bone formation rate). Furthermore, there was an attenuated flow-induced increase in COX-2 mRNA expression levels in primary bone cells isolated from AC6 knockout mice compared to controls (1.3±0.1- vs. 2.6±0.2-fold increase). Collectively, these data indicate that AC6 plays a role in loading-induced bone adaptation, and these findings are consistent with our previous studies implicating primary cilia and AC6 in a novel mechanism of osteocyte mechanotransduction.

3-dimensional imaging modalities for phenotyping genetically engineered mice

A variety of 3-dimensional (3D) digital imaging modalities are available for whole-body assessment of genetically engineered mice: magnetic resonance microscopy (MRM), X-ray microcomputed tomography (microCT), optical projection tomography (OPT), episcopic and cryoimaging, and ultrasound biomicroscopy (UBM). Embryo and adult mouse phenotyping can be accomplished at microscopy or near microscopy spatial resolutions using these modalities. MRM and microCT are particularly well-suited for evaluating structural information at the organ level, whereas episcopic and OPT imaging provide structural and functional information from molecular fluorescence imaging at the cellular level. UBM can be used to monitor embryonic development longitudinally in utero. Specimens are not significantly altered during preparation, and structures can be viewed in their native orientations. Technologies for rapid automated data acquisition and high-throughput phenotyping have been developed and continually improve as this exciting field evolves.

Integrative physiology of the aging bone: insights from animal and cellular models

Age-related bone loss is a common worldwide phenomenon in the aging population, placing them at an increased risk of fractures. Fortunately, basic and translational studies have been pivotal in providing us with a mechanistic understanding of the cellular and molecular pathophysiology of this condition. This review focuses on the current concepts and paradigms of age-related bone loss and how various animal and cellular models have broadened our understanding in this fascinating but complex area. Changes in hormonal, neuronal, and biochemical cues with age and their effect on bone have been discussed. This review also outlines recent studies on the relationship between bone and fat in the marrow, as well as the fate of the marrow mesenchymal stromal cell population, which can give rise to either bone-forming osteoblasts or fat-forming adipocytic cells as a function of age.

Postpubertal architectural developmental patterns differ between the L3 vertebra and proximal tibia in three inbred strains of mice

An understanding of normal microarchitectural bone development patterns of common murine models is needed. Longitudinal, structural, and mineralization trends were evaluated by in vivo microCT over 12 time points from 6-48 wk of age at the vertebra and tibia of C3H/HeN, C57BL/6, and BALB/C mice. Longitudinal growth occurred rapidly until 8-10 wk, slowed as the growth plate bridged, and fused at 8-10 mo. Structural augmentation occurred through formation of trabeculae at the growth plate and thickening of existing ones. In the vertebrae, BV/TV increased rapidly until 12 wk in all strains. Between 12 and 32 wk, the architecture was stable with BV/TV deviating <1.1%, 1.6%, and 3.4% for the C57BL/6, BALB/C, and C3H/HeN mice. In contrast, the tibial architecture changed continuously but more moderately for BV/TV and TbTh compared with the vertebra and with comparable or larger changes for TbN and TbSp. Age-related trabecular deterioration (decreased BV/TV and TbN; increased TbSp and structure model index) was evident at both sites at 32 wk. In all strains, the cortex continued to develop after trabecular values peaked. The temporal plateau of BMD was variable across mouse strains and site, whereas tissue mineral density was attained at approximately 6 mo for all sites and strains. Geometric changes at the tibial diaphysis occurred rapidly until 8-10 wk, providing the C57BL/6 mice and C3H/HeN mice with the highest torsional and compressive rigidity, respectively. In summary, key skeletal development milestones were identified, and architectural topology at the vertebra was found to be more stable than at the tibia.

High-resolution mapping of gene expression using association in an outbred mouse stock

Quantitative trait locus (QTL) analysis is a powerful tool for mapping genes for complex traits in mice, but its utility is limited by poor resolution. A promising mapping approach is association analysis in outbred stocks or different inbred strains. As a proof of concept for the association approach, we applied whole-genome association analysis to hepatic gene expression traits in an outbred mouse population, the MF1 stock, and replicated expression QTL (eQTL) identified in previous studies of F2 intercross mice. We found that the mapping resolution of these eQTL was significantly greater in the outbred population. Through an example, we also showed how this precise mapping can be used to resolve previously identified loci (in intercross studies), which affect many different transcript levels (known as eQTL “hotspots”), into distinct regions. Our results also highlight the importance of correcting for population structure in whole-genome association studies in the outbred stock.

In rodents, as in humans, traits such as obesity or diabetes are under the influence of many genes spread throughout the genome. Using linkage analysis, the locations of the major contributing genes can be mapped only to very large regions of chromosomes, usually encompassing hundreds of genes. This has made it difficult to identify the underlying genes and mutations. Another approach, analogous to genome-wide association in human populations, is to use association analyses among outbred stocks of mice. In this proof-of-principle article, we make use of common variations that locally perturb gene expression to demonstrate the greatly improved mapping resolution of association in mice. Our results indicate that association analyses in mice are a powerful approach to the dissection of complex traits and their underlying molecular networks.

Optimization of volumetric computed tomography for skeletal analysis of model genetic organisms

Forward and reverse genetics now allow researchers to understand embryonic and postnatal gene function in a broad range of species. Although some genetic mutations cause obvious morphological change, other mutations can be more subtle and, without adequate observation and quantification, might be overlooked. For the increasing number of genetic model organisms examined by the growing field of phenomics, standardized but sensitive methods for quantitative analysis need to be incorporated into routine practice to effectively acquire and analyze ever-increasing quantities of phenotypic data. In this study, we present platform-independent parameters for the use of microscopic x-ray computed tomography (microCT) for phenotyping species-specific skeletal morphology of a variety of different genetic model organisms. We show that microCT is suitable for phenotypic characterization for prenatal and postnatal specimens across multiple species.