Mechanical Loading Attenuates Radiation-Induced Bone Loss in Bone Marrow Transplanted Mice

Relations Between Bone Quantity, Microarchitecture, and Collagen Cross-links on Mechanics Following In Vivo Irradiation in Mice

Humans are exposed to ionizing radiation via spaceflight or cancer radiotherapy, and exposure from radiotherapy is known to increase risk of skeletal fractures. Although irradiation can reduce trabecular bone mass, alter trabecular microarchitecture, and increase collagen cross‐linking, the relative contributions of these effects to any loss of mechanical integrity remain unclear. To provide insight, while addressing both the monotonic strength and cyclic‐loading fatigue life, we conducted total‐body, acute, gamma‐irradiation experiments on skeletally mature (17‐week‐old) C57BL/6J male mice (n = 84). Mice were administered doses of either 0 Gy (sham), 1 Gy (motivated by cumulative exposures from a Mars mission), or 5 Gy (motivated by clinical therapy regimens) with retrieval of the lumbar vertebrae at either a short‐term (11‐day) or long‐term (12‐week) time point after exposure. Micro‐computed tomography was used to assess trabecular and cortical quantity and architecture, biochemical composition assays were used to assess collagen quality, and mechanical testing was performed to evaluate vertebral compressive strength and fatigue life. At 11 days post‐exposure, 5 Gy irradiation significantly reduced trabecular mass (p < 0.001), altered microarchitecture (eg, connectivity density p < 0.001), and increased collagen cross‐links (p < 0.001). Despite these changes, vertebral strength (p = 0.745) and fatigue life (p = 0.332) remained unaltered. At 12 weeks after 5 Gy exposure, the trends in trabecular bone persisted; in addition, regardless of irradiation, cortical thickness (p < 0.01) and fatigue life (p < 0.01) decreased. These results demonstrate that the highly significant effects of 5 Gy total‐body irradiation on the trabecular bone morphology and collagen cross‐links did not translate into detectable effects on vertebral mechanics. The only mechanical deficits observed were associated with aging. Together, these vertebral results suggest that for spaceflight, irradiation alone will likely not alter failure properties, and for radiotherapy, more investigations that include post‐exposure time as a positive control and testing of both failure modalities are needed to determine the cause of increased fracture risk. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Evaluation of high-linearity bone radiation detectors exposed to gamma-rays via FTIR measurements

In radiation physics, the study of new alternative dosimeters is of interest to the growing branch of dosimetric characterization for radiotherapy applications. The goal of this work was to expose bone samples to high doses and evaluate their linearity response to gamma rays. The Fourier Transform Infrared (FTIR) spectrophotometry technique was employed as the evaluation technique, and based on the spectrophotometry absorbance profiles the linearity was assessed based on the following methods: Area Under the Curve (AUC), Wavenumber Method (WM), Partial Component Regression (PCR) and Partial Least-Square Regression (PLSR) methods. The bone samples were irradiated with absorbed doses of 10 Gy up to 500 Gy using a ⁶⁰Co Gamma Cell-220 system. The results showed, for the calibration curves of the system, adequate linearity on all methods. In conclusion, the results indicate a good linear response and therefore an interesting potential radiation detector.

Mechanical loading prevents bone destruction and exerts anti-tumor effects in the MOPC315.BM.Luc model of myeloma bone disease

Bone continually adapts to changing external loading conditions via (re)modeling (modeling and remodeling) processes. While physical activity is known to beneficially enhance bone mass in healthy individuals, little is known in how physical stimuli affect osteolytic bone destruction in patients suffering from multiple myeloma bone disease. Multiple myeloma (MM) is caused by malignant plasma cells in the bone marrow, shifting the balance in bone remodeling towards massive resorption. We hypothesized that in vivo tibial mechanical loading has anabolic effects in mice with locally injected MOPC315.BM.Luc cells. Conventional microCT analysis revealed enhanced cortical bone mass and microstructure in loaded compared to nonloaded mice. State-of-the-art time-lapse microCT based image analysis demonstrated bone (re)modeling processes at the endosteal and periosteal surfaces as the underlying causes of increased bone mass. Loading prevented the progression and development of osteolytic destruction. Physical stimuli also diminished local MM cell growth and dissemination evidenced by quantification of MM cell-specific immunoglobulin A levels in the serum of mice and by bioluminescence analysis. These data indicate that mechanical loading not only rescues the bone phenotype, but also exerts cell-extrinsic anti-myeloma effects in the MOPC315.BM.Luc model. In conclusion, the use of physical stimuli should be further investigated as an anabolic treatment for osteolytic bone destruction in patients with MM.

Radiation-induced changes to bone composition extend beyond periosteal bone

BACKGROUND

Cancer patients receiving radiotherapy for soft tissue sarcomas are often at risk of post-irradiation (post-RTx) bone fragility fractures, but our understanding of factors controlling radiation-induced bone injury is limited. Previous studies have evaluated post-RTx changes to cortical bone composition in the periosteum of irradiated tibiae, but have not evaluated effects of irradiation in deeper tissues, such as endosteal or mid-cortical bone, and whether there are differential spatial effects of irradiation. In this study, we hypothesize that post-RTx changes to cortical bone composition are greater in endosteal compared to mid-cortical or periosteal bone.

METHODS

A pre-clinical mouse model of limited field hindlimb irradiation was used to evaluate spatial and temporal post-RTx changes to the metaphyseal cortex of irradiated tibiae. Irradiation was delivered unilaterally to the hindlimbs of 12-wk old female BALB/cJ mice as 4 consecutive daily doses of 5 Gy each. RTx and non-RTx tibiae were obtained at 0, 2, 4, 8, and 12 wks post-RTx (n = 9 mice/group/time). Raman spectroscopy was used to evaluate spatial and temporal post-RTx changes to cortical bone composition in age-matched RTx and non-RTx groups.

RESULTS

Significant early spatial differences in mineral/matrix and collagen crosslink ratios were found between endosteal and periosteal or mid-cortical bone at 2-wks post-RTx. Although spatial differences were transient, mineral/matrix ratios significantly decreased and collagen crosslink ratios significantly increased with post-RTx time throughout the entire tibial metaphyseal cortex.

CONCLUSIONS

Irradiation negatively impacts the composition of cortical bone in a spatially-dependent manner starting as early as 2-wks post-RTx. Long-term progressive post-RTx changes across all cortical bone sites may eventually contribute to the increased risk of post-RTx bone fragility fractures.

Bench to Bedside: Animal Models of Radiation Induced Musculoskeletal Toxicity

Ionizing radiation is a critical aspect of current cancer therapy. While classically mature bone was thought to be relatively radio-resistant, more recent data have shown this to not be the case. Radiation therapy (RT)-induced bone loss leading to fracture is a source of substantial morbidity. The mechanisms of RT likely involve multiple pathways, including changes in angiogenesis and bone vasculature, osteoblast damage/suppression, and increased osteoclast activity. The majority of bone loss appears to occur rapidly after exposure to ionizing RT, with significant changes in cortical thickness being detectable on computed tomography (CT) within three to four months. Additionally, there is a dose–response relationship. Cortical thinning is especially notable in areas of bone that receive >40 gray (Gy). Methods to mitigate toxicity due to RT-induced bone loss is an area of active investigation. There is an accruing clinical trial investigating the use of risderonate, a bisphosphonate, to prevent rib bone loss in patients undergoing lung stereotactic body radiation therapy (SBRT). Additionally, several other promising therapeutic/preventative approaches are being explored in preclinical studies, including parathyroid hormone (PTH), amifostine, and mechanical loading of irradiated bones.

Murine Axial Compression Tibial Loading Model to Study Bone Mechanobiology: Implementing the Model and Reporting Results

In vivo, tibial loading in mice is increasingly used to study bone adaptation and mechanotransduction. To achieve standardized and defined experimental conditions, loading parameters and animal-related factors must be considered when performing in vivo loading studies. In this review, we discuss these loading and animal-related experimental conditions, present methods to assess bone adaptation, and suggest reporting guidelines. This review originated from presentations by each of the authors at the workshop "Developing Best Practices for Mouse Models of In Vivo Loading" during the Preclinical Models Section at the Orthopaedic Research Society Annual Meeting, San Diego, CA, March 2017. Following the meeting, the authors engaged in detailed discussions with consideration of relevant literature. The guidelines and recommendations in this review are provided to help researchers perform in vivo loading experiments in mice, and thus further our knowledge of bone adaptation and the mechanisms involved in mechanotransduction. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:233-252, 2020.

Comparison of knee injury threshold during tibial compression based on limb orientation in mice

Our previous studies used tibial compression overload to induce anterior cruciate ligament (ACL) rupture in mice, while others have applied similar or greater compressive magnitudes without injury. The causes of these differences in injury threshold are not known. In this study, we compared knee injury thresholds using a "prone configuration" and a "supine configuration" that differed with respect to hip, knee, and ankle flexion, and utilized different fixtures to stabilize the knee. Right limbs of female and male C57BL/6 mice were loaded using the prone configuration, while left limbs were loaded using the supine configuration. Mice underwent progressive loading from 2 to 20 N, or cyclic loading at 9 N or 14 N (n = 9-11/sex/loading method). Progressive loading with the prone configuration resulted in ACL rupture at an average of 10.2 ± 0.9 N for females and 11.4 ± 0.7 N for males. In contrast, progressive loading with the supine configuration resulted in ACL rupture in only 36% of female mice and 50% of male mice. Cyclic loading with the prone configuration resulted in ACL rupture after 15 ± 8 cycles for females and 24 ± 27 cycles for males at 9 N, and always during the first cycle for both sexes at 14 N. In contrast, cyclic loading with the supine configuration was able to complete 1,200 cycles at 9 N without injury for both sexes, and an average of 45 ± 41 cycles for females and 49 ± 25 cycles for males at 14 N before ACL rupture. These results show that tibial compression configurations can strongly affect knee injury thresholds during loading.

Simulated space radiation sensitizes bone but not muscle to the catabolic effects of mechanical unloading

Deep space travel exposes astronauts to extended periods of space radiation and mechanical unloading, both of which may induce significant muscle and bone loss. Astronauts are exposed to space radiation from solar particle events (SPE) and background radiation referred to as galactic cosmic radiation (GCR). To explore interactions between skeletal muscle and bone under these conditions, we hypothesized that decreased mechanical load, as in the microgravity of space, would lead to increased susceptibility to space radiation-induced bone and muscle loss. We evaluated changes in bone and muscle of mice exposed to hind limb suspension (HLS) unloading alone or in addition to proton and high (H) atomic number (Z) and energy (E) (HZE) (16O) radiation. Adult male C57Bl/6J mice were randomly assigned to six groups: No radiation ± HLS, 50 cGy proton radiation ± HLS, and 50 cGy proton radiation + 10 cGy 16O radiation ± HLS. Radiation alone did not induce bone or muscle loss, whereas HLS alone resulted in both bone and muscle loss. Absolute trabecular and cortical bone volume fraction (BV/TV) was decreased 24% and 6% in HLS-no radiation vs the normally loaded no-radiation group. Trabecular thickness and mineral density also decreased with HLS. For some outcomes, such as BV/TV, trabecular number and tissue mineral density, additional bone loss was observed in the HLS+proton+HZE radiation group compared to HLS alone. In contrast, whereas HLS alone decreased muscle mass (19% gastrocnemius, 35% quadriceps), protein synthesis, and increased proteasome activity, radiation did not exacerbate these catabolic outcomes. Our results suggest that combining simulated space radiation with HLS results in additional bone loss that may not be experienced by muscle.