Treadmill running and targeted tibial loading differentially improve bone mass in mice

Effect of different running protocols on bone morphology and microarchitecture of the forelimbs in a male Wistar rat model

BACKGROUND

It is accepted that the metabolic response of bone tissue depends on the intensity of the mechanical loads, but also on the type and frequency of stress applied to it. Physical exercise such as running involves stresses which, under certain conditions, have been shown to have the best osteogenic effects. However, at high intensity, it can be deleterious for bone tissue. Consequently, there is no clear consensus as to which running modality would have the best osteogenic effects.

AIM

Our objective was to compare the effects of three running modalities on morphological and micro-architectural parameters on forelimb bones.

METHODS

Forty male Wistar rats were randomly divided into four groups: high intensity interval training (HIIT), continuous running, combined running ((alternating HIIT and continuous modalities) and sedentary (control). The morphometry, trabecular microarchitecture and cortical porosity of the ulna, radius and humerus were analyzed using micro-tomography.

RESULTS

All three running modalities resulted in bone adaptation, with an increase in the diaphyseal diameter of all three bones. The combined running protocol had positive effects on the trabecular thickness in the distal ulna. The HIIT protocol resulted in an increase in both medio-lateral diameter and cortical bone area over total area (Ct.Ar/Tt.Ar) at the ulnar shaft compared with sedentary condition. Moreover, the HIIT protocol decreased the mean surface area of the medulla (Ma.Ar) according to sedentary condition at the ulnar shaft.

CONCLUSION

This study has shown that HIIT resulted in a decrease in trabecular bone fraction in favor of cortical bone area at the ulna.

Photobiomodulation Effect of Diode Laser on Differentiation of Osteoprogenitor Cells in Rat Bone Marrow

BACKGROUND/AIM

Bone marrow cells contain nonhematopoietic cells with the ability to differentiate into osteogenic, chondrogenic, and adipogenic lineages. Mechanical stress influences osteoblast differentiation of bone marrow cells into osteogenic, chondrogenic, and adipogenic lineages, measurable as the abundance of alkaline phosphatase-positive (ALP+) colony-forming unit-fibroblasts (CFU-F); however, the effect of diode laser irradiation on osteoblast differentiation is unknown. The aim of this study was to analyze the effects of photobiomodulation on the osteogenic differentiation of mesenchymal stem cells in the bone marrow, using the CFU-F assay.

MATERIALS AND METHODS

Bone marrow cells isolated from rat tibiae were cultured and irradiated with a diode laser (wavelength 808 nm) at a total energy of 0 J (control), 50 J, and 150 J.

RESULTS

On day 7 after irradiation, ALP+ CFU-F were most abundant in the 50 J group and the least abundant in the 150 J group. Mineralized nodule formation was observed after long-term culture (21 days). Compared with the control group, there were significantly more nodules in the 50 J group and significantly fewer nodules in the 150 J group. Osteocalcin mRNA expression was highest in the 50 J group, and there was no difference between the control and 150 J groups.

CONCLUSION

Irradiation with 50 J was effective in stimulating osteogenesis in bone marrow stem cells. These findings suggest that diode laser irradiation can induce osteogenesis in rat bone marrow cells in an energy-dependent manner, and appears suitable for application in bone regeneration therapy.

Mechanical Stress Via Muscle Contractile Exercise Suppresses Atrophic Alterations of Bone-microstructure in Immobilized Rat Femurs

OBJECTIVES

This study aimed to determine whether mechanical stress via muscle contractile exercise with belt electrode-skeletal muscle electrical stimulation (B-SES) device effectively prevents immobilization-induced bone atrophy.

METHODS

Wistar rats were randomly divided into the control (CON) group, immobilization (IM) group (immobilized treatment only), HES and LES groups (immobilized treatment and high or low-intensity electrical muscular stimulation through B-SES device). Bilateral femurs were used for X-ray micro-CT and biomechanical tests.

RESULTS

The maximum load value was significantly lower in the IM and HES groups than in the CON group and significantly higher in the LES group than in the IM group. The maximum crushing load was significantly lower in the IM, HES, and LES groups than in the CON group, and significantly higher in the HES and LES groups than that in the IM group. In micro-CT, the mechanical stress by B-SES device did not affect degenerative microstructural changes in the cortical bone, but prevented those changes in the cancellous bone.

CONCLUSIONS

Applying mechanical stress via B-SES device suppressed the loss of cancellous bone density and degenerative microstructural changes caused by immobilization, which in turn suppressed the reduction of bone strength. From these findings, muscle contractile exercise may be effective in preventing immobilization-induced bone atrophy.

Effects of different running intensities on the micro-level failure strain of rat femoral cortical bone structures: a finite element investigation

BACKGROUND

Running with the appropriate intensity may produce a positive influence on the mechanical properties of cortical bone structure. However, few studies have discussed the effects of different running intensities on the mechanical properties at different levels, especially at the micro-level, because the micromechanical parameters are difficult to measure experimentally.

METHODS

An approach that combines finite element analysis and experimental data was proposed to predict a micromechanical parameter in the rat femoral cortical bone structure, namely, the micro-level failure strain. Based on the previous three-point bending experimental information, fracture simulations were performed on the femur finite element models to predict their failure process under the same bending load, and the micro-level failure strains in tension and compression of these models were back-calculated by fitting the experimental load-displacement curves. Then, the effects of different running intensities on the micro-level failure strain of rat femoral cortical bone structure were investigated.

RESULTS

The micro-level failure strains of the cortical bone structures expressed statistical variations under different running intensities, which indicated that different mechanical stimuli of running had significant influences on the micromechanical properties. The greatest failure strain occurred in the cortical bone structure under low-intensity running, and the lowest failure strain occurred in the structure under high-intensity running.

CONCLUSIONS

Moderate and low-intensity running were effective in enhancing the micromechanical properties, whereas high-intensity running led to the weakening of the micromechanical properties of cortical bone. Based on these, the changing trends in the micromechanical properties were exhibited, and the effects of different running intensities on the fracture performance of rat cortical bone structures could be discussed in combination with the known mechanical parameters at the macro- and nano-levels, which provided the theoretical basis for reducing fracture incidence through running exercise.

Piezo1-mediated M2 macrophage mechanotransduction enhances bone formation through secretion and activation of transforming growth factor-β1

Macrophages are multifunctional immune system cells that are essential for the mechanical stimulation-induced control of metabolism. Piezo1 is a non-selective calcium channel expressed in multifarious tissues to convey mechanical signals. Here, a cellular model of tension was used to study the effect of mechanical stretch on the phenotypic transformation of macrophages and its mechanism. An indirect co-culture system was used to explore the effect of macrophage activation on bone marrow mesenchymal stem cells (BMSCs), and a treadmill running model was used to validate the mechanism in vivo for in vitro studies. p53 was acetylated and deacetylated by macrophages as a result of mechanical strain being detected by Piezo1. This process is able to polarize macrophages towards M2 and secretes transforming growth factor-beta (TGF-β1), which subsequently stimulates BMSCs migration, proliferation and osteogenic differentiation. Knockdown of Piezo1 inhibits the conversion of macrophages to the reparative phenotype, thereby affecting bone remodelling. Blockade of TGF-β I, II receptors and Piezo1 significantly reduced exercise-increased bone mass in mice. In conclusion, we showed that mechanical tension causes calcium influx, p53 deacetylation, macrophage polarization towards M2 and TGF-β1 release through Piezo1. These events support BMSC osteogenesis.

Exercise-Linked Skeletal Irisin Ameliorates Diabetes-Associated Osteoporosis by Inhibiting the Oxidative Damage-Dependent miR-150-FNDC5/Pyroptosis Axis

Recent evidence suggests that physical exercise (EX) promotes skeletal development. However, the impact of EX on the progression of bone loss and deterioration of mechanical strength in mice with type 2 diabetic mellitus (T2DM) remains unexplored. In the current study, we investigated the effect of EX on bone mass and mechanical quality using a diabetic mouse model. The T2DM mouse model was established with a high-fat diet with two streptozotocin injections (50 mg/kg/body wt) in C57BL/6 female mice. The diabetic mice underwent treadmill exercises (5 days/week at 7-11 m/min for 60 min/day) for 8 weeks. The data showed that diabetes upregulated miR-150 expression through oxidative stress and suppressed FNDC5/Irisin by binding to its 3'-untranslated region. The decreased level of irisin further triggers the pyroptosis response in diabetic bone tissue. EX or N-acetyl cysteine or anti-miRNA-150 transfection in T2DM mice restored FNDC5/Irisin expression and bone formation. Furthermore, EX or recombinant irisin administration prevented T2DM-Induced hyperglycemia and improved glucose intolerance in diabetic mice. Furthermore, osteoblastic knockdown of Nlrp3 silencing (si-Nlrp3) or pyroptosis inhibitor (Ac-YVADCMK [AYC]) treatment restores bone mineralization in diabetic mice. Micro-computed tomography scans and mechanical testing revealed that trabecular bone microarchitecture and bone mechanical properties were improved after EX in diabetic mice. Irisin, either induced by skeleton or daily EX or directly administered, prevents bone loss by mitigating inflammasome-associated pyroptosis signaling in diabetic mice. This study demonstrates that EX-induced skeletal irisin ameliorates diabetes-associated glucose intolerance and bone loss and possibly provides a mechanism of its effects on metabolic osteoporosis.

To do one and to get more: Part I. Diabetes and bone

Type 2 diabetes mellitus (T2DM), is a chronic metabolic disease, characterized by the presence of hyperglycemia and insulin resistance. The key treatment strategies for T2DM include modification of lifestyle, medications, and continuous glucose monitoring. DM patients often have DM-associated morbidities and comorbidities; however, disorders of musculoskeletal system are often neglected, compared to other major systems in DM patients. Based on sharing similar pathophysiology of DM and osteoporosis, it is supposed that the use of antidiabetic agents (ADAs) may not only provide the lowering glucose level effect and the maintenance of the sugar homeostasis to directly delay the tissue damage secondary to hyperglycemia but also offer the benefits, such as the prevention of developing osteoporosis and fractures. Based on the current review, evidence shows the positive correlation between DM and osteoporosis or fracture, but the effectiveness of using ADA in the prevention of osteoporosis and subsequent reduction of fracture seems to be inconclusive. Although the benefits of ADA on bone health are uncertain, the potential value of "To do one and to get more" therapeutic strategy should be always persuaded. At least, one of the key treatment strategies as an establishment of healthy lifestyle may work, because it improves the status of insulin resistance and subsequently helps DM control, prevents the DM-related micro- and macrovascular injury, and possibly strengthens the general performance of musculoskeletal system. With stronger musculoskeletal system support, the risk of "fall" may be decreased, because it is associated with fracture. Although the ADA available in the market does not satisfy the policy of "To do one and to get more" yet, we are looking forward to seeing the continuously advanced technology of drug development on diabetic control, and hope to see their extra-sugar-lowering effects.

A cholinergic neuroskeletal interface promotes bone formation during postnatal growth and exercise

The autonomic nervous system is a master regulator of homeostatic processes and stress responses. Sympathetic noradrenergic nerve fibers decrease bone mass, but the role of cholinergic signaling in bone has remained largely unknown. Here, we describe that early postnatally, a subset of sympathetic nerve fibers undergoes an interleukin-6 (IL-6)-induced cholinergic switch upon contacting the bone. A neurotrophic dependency mediated through GDNF-family receptor-α2 (GFRα2) and its ligand, neurturin (NRTN), is established between sympathetic cholinergic fibers and bone-embedded osteocytes, which require cholinergic innervation for their survival and connectivity. Bone-lining osteoprogenitors amplify and propagate cholinergic signals in the bone marrow (BM). Moderate exercise augments trabecular bone partly through an IL-6-dependent expansion of sympathetic cholinergic nerve fibers. Consequently, loss of cholinergic skeletal innervation reduces osteocyte survival and function, causing osteopenia and impaired skeletal adaptation to moderate exercise. These results uncover a cholinergic neuro-osteocyte interface that regulates skeletogenesis and skeletal turnover through bone-anabolic effects.

Effects of selective breeding for voluntary exercise, chronic exercise, and their interaction on muscle attachment site morphology in house mice

Skeletal muscles attach to bone at their origins and insertions, and the interface where tendon meets bone is termed the attachment site or enthesis. Mechanical stresses at the muscle/tendon-bone interface are proportional to the surface area of the bony attachment sites, such that a larger attachment site will distribute loads over a wider area. Muscles that are frequently active and/or are of larger size should cause attachment sites to hypertrophy (training effect); however, experimental studies of animals subjected to exercise have provided mixed results. To enhance our ability to detect training effects (a type of phenotypic plasticity), we studied a mouse model in which 4 replicate lines of High Runner (HR) mice have been selectively bred for 57 generations. Selection is based on the average number of wheel revolutions on days 5 & 6 of a 6-day period of wheel access as young adults (6-8 weeks old). Four additional lines are bred without regard to running and serve as non-selected controls (C). On average, mice from HR lines voluntarily run ~3 times more than C mice on a daily basis. For this study, we housed 50 females (half HR, half C) with wheels (Active group) and 50 (half HR, half C) without wheels (Sedentary group) for 12 weeks starting at weaning (~3 weeks old). We tested for evolved differences in muscle attachment site surface area between HR and C mice, plastic changes resulting from chronic exercise, and their interaction. We used a precise, highly repeatable method for quantifying the three-dimensional (3D) surface area of four muscle attachment sites: the humerus deltoid tuberosity (the insertion point for the spinodeltoideus, superficial pectoralis, and acromiodeltoideus), the femoral third trochanter (the insertion point for the quadratus femoris), the femoral lesser trochanter (the insertion point for the iliacus muscle), and the femoral greater trochanter (insertion point for the middle gluteal muscles). In univariate analyses, with body mass as a covariate, mice in the Active group had significantly larger humerus deltoid tuberosities than Sedentary mice, with no significant difference between HR and C mice and no interaction between exercise treatment and linetype. These differences between Active and Sedentary mice were also apparent in the multivariate analyses. Surface areas of the femoral third trochanter, femoral lesser trochanter, and femoral greater trochanter were unaffected by either chronic wheel access or selective breeding. Our results, which used robust measurement protocols and relatively large sample sizes, demonstrate that muscle attachment site morphology can be (but is not always) affected by chronic exercise experienced during ontogeny. However, contrary to previous results for other aspects of long bone morphology, we did not find evidence for evolutionary coadaptation of muscle attachments with voluntary exercise behavior in the HR mice.

Effects of Raloxifene and tibial loading on bone mass and mechanics in male and female mice

Purpose: Raloxifene (RAL) is a selective estrogen receptor modulator (SERM) that has previously been shown to cause acellular benefits to bone tissue. Due to these improvements, RAL was combined with targeted tibial loading to assess if RAL treatment during periods of active bone formation would allow for further mechanical enhancements.Methods: Structural, mechanical, and microstructural effects were assessed in bone from C57BL/6 mice that were treated with RAL (0.5 mg/kg), tibial loading, or both for 6 weeks, beginning at 10 weeks of age.Results:Ex vivo microcomputed tomography (CT) images indicated RAL and loading work together to improve bone mass and architecture, especially within the cancellous region of males. Increases in cancellous bone volume fraction were heavily driven by increases in trabecular thickness, though there were some effects on trabecular spacing and number. In the cortical regions, RAL and loading both increased cross-sectional area, cortical area, and cortical thickness. Whole-bone mechanical testing primarily indicated the effects of loading. Further characterization through Raman spectroscopy and nanoindentation showed load-based changes in mineralization and micromechanics, while both loading and RAL caused changes in the secondary collagen structure. In contrast to males, in females, there were large load-based effects in the cancellous and cortical regions, resulting in increased whole-bone mechanical properties. RAL had less of an effect on cancellous and cortical architecture, though some effects were still present.Conclusion: RAL and loading work together to impact bone architecture and mechanical integrity, leading to greater improvements than either treatment individually.

Effects of Different Types of Mechanical Loading on Trabecular Bone Microarchitecture in Rats

Mechanical loading is generally considered to have a positive impact on the skeleton; however, not all types of mechanical loading have the same beneficial effect. Many researchers have investigated which types of mechanical loading are more effective for improving bone mass and strength. Among the various mechanical loads, high-impact loading, such as jumping, appears to be more beneficial for bones than low-impact loadings such as walking, running, or swimming. Therefore, the different forms of mechanical loading exerted by running, swimming, and jumping exercises may have different effects on bone adaptations. However, little is known about the relationships between the types of mechanical loading and their effects on trabecular bone structure. The purpose of this article is to review the recent reports on the effects of treadmill running, jumping, and swimming on the trabecular bone microarchitecture in small animals. The effects of loading on trabecular bone architecture appear to differ among these different exercises, as several reports have shown that jumping increases the trabecular bone mass by thickening the trabeculae, whereas treadmill running and swimming add to the trabecular bone mass by increasing the trabecular number, rather than the thickness. This suggests that different types of exercise promote gains in trabecular bone mass through different architectural patterns in small animals.

Moderate tibial loading and treadmill running, but not overloading, protect adult murine bone from destruction by metastasized breast cancer

Osteolytic bone lesions, which develop in many metastatic breast cancer patients, impair bone integrity and lead to adverse skeletal related events that are difficult to treat and sometimes fatal. Moderate mechanical loading has been shown to suppress osteolysis in young mice with breast cancer. In this study, we aimed to investigate the dose-dependent effects of mechanical loading on protecting the integrity of adult skeletons with breast cancer. Localized tibial loading and aerobic treadmill running with three levels of varying intensity were tested in a syngeneic mammary tumor bone metastasis model. Adult C57BL/6J female mice (14-week-old, N = 88 mice) received intra-tibial injections of Py8119 triple-negative murine breast cancer cells or PBS and underwent 4 to 5 weeks of exercise or acted as sedentary/non-loaded controls. The bone structure was monitored longitudinally with weekly in vivo micro-computed tomography imaging, while the cellular responses in bone and marrow were examined using immunohistochemistry. Moderate treadmill running (16 m/min, 50 min/day, 5 days/week, and 5 weeks) and tibial loading (4.5 N, 630 με, 4 Hz, 300 cycles/day, 5 days/week, and 4 weeks) suppressed tumor-induced bone destruction, as evaluated by full-thickness perforation of tibial cortex and the volume of osteolytic lesions in the cortex. In contrast, tibial loading at higher magnitude (8 N, 1100 με) induced woven bone and accelerated bone destruction, compared with the non-loaded controls. The three exercise regimens differentially affected osteocyte apoptosis, osteocyte hypoxia, osteoclast activity, bone marrow vasculature, and tumor proliferation. In conclusion, the relationship between exercise intensity and the risk of breast cancer-induced osteolysis was found to follow a J-shaped curve in a preclinical model, suggesting the need to optimize exercise parameters in order to harness the skeletal benefits of exercise in metastatic breast cancers.

Effects of Interval Exercise Training on Serum Biochemistry and Bone Mineral Density in Dogs

Simple Summary

In this study, six male beagle dogs underwent 12 weeks of interval exercise following the Frequency, Intensity, Time/duration, Type, Volume, and Progression (FITT-VP) training principle. The heart rate (HR) response was measured during the entire exercise period, and changes in bone mineral density (BMD), muscle volume (MV), and hematology and serum biomarkers were evaluated at the pre-exercise training period and post-exercise training period. We showed that exercise training increased BMD in the femur and serum total alkaline phosphatase (TALP), aspartate aminotransferase, and creatine kinase levels. In addition, our data suggest a positive correlation between BMD and TALP, demonstrating that increased TALP might be an important contributing factor for enhancing BMD with physical training in dogs.

Abstract

Exercise has been suggested as a powerful intervention for health care and fitness management in humans; however, few studies have demonstrated the benefits of exercise training in dogs. The purpose of this study was to examine the effects of exercise training on heart rate (HR), bone mineral density (BMD), muscle volume (MV), and hematological and serum biomarkers in dogs. Six healthy beagles completed the interval treadmill exercise, developed on the basis of the FITT principle, two times a week for 12 weeks. To evaluate the physiological parameters, the HR values were analyzed using the Polar H10 system during the entire exercise period. At pre-and post-exercise, quantitative computed tomography and hematological and serum biochemical parameters were analyzed. The interval exercise resulted in a normal HR response and no adverse behavioral or physiological effects on the dogs. We showed that exercise improved BMD in the femur (541.6 ± 16.7 vs. 610.2 ± 27.8 HA, p < 0.01) and increased serum total alkaline phosphatase (TALP; 68.6 ± 9.2 vs. 81.3 ± 17.2, p < 0.01), aspartate aminotransferase (23.5 ± 1.0 vs. 33.5 ± 1.6, p < 0.01), and creatine kinase (114.8 ± 5.3 vs. 214.0 ± 20.8, p < 0.01) levels. There was a positive relationship between BMD and TALP (femur: r = 0.760, p = 0.004; vertebrae: r = 0.637; p = 0.025). Our findings suggest that interval exercise training is beneficial to increase BMD in the femur, and an increased TALP level would be a concomitant mechanism for enhancing BMD with exercise in dogs.

Limited impacts of thermoneutral housing on bone morphology and mechanical properties in growing female mice exposed to external loading and raloxifene treatment

Thermoregulation is an important factor that could have physiological consequences on pre-clinical research outcomes. Simply housing mice at thermoneutral temperature has been shown to prevent the well-established loss of cancellous bone that is typical in growing mice. In this study, active tissue formation was induced by non-invasive tibial loading in female mice and combined with raloxifene treatment to assess whether temperature could enhance their combined effects on bone morphology and mechanical properties. It was hypothesized that by removing the cold stress under which normal lab mice are housed, a metabolic boost would allow for further architectural and mechanical improvements in mice exposed to a combination of tibial loading and raloxifene. Ten-week old female C57BL/6J mice were treated with raloxifene, underwent tibial loading to a maximum tensile stress of 2050 με, and were housed in thermoneutral conditions (32 °C) for 6 weeks. We investigated bone morphology through microcomputed tomography (μCT), mechanical properties via four-point bending, and fracture toughness testing. Results confirmed previous work showing a combined effect of external loading and raloxifene which led to greater improvements in most properties than either individual treatment. Counter to the hypothesis, temperature had modest effects on body weight, overall bone size, and trabecular architecture, and most effects were detrimental. Thermoneutrality had no impact on mechanical integrity or fracture toughness. In most cases, the magnitude of temperature-based effects were less robust than either RAL treatment or loading.

The Effect of Single Versus Group μCT on the Detection of Trabecular and Cortical Disease Phenotypes in Mouse Bones

Micro‐computed tomography is a critical assessment tool for bone‐related preclinical research, especially in murine models. To expedite the scanning process, researchers often image multiple bones simultaneously; however, it is unknown if this impacts scan quality and alters the ability to detect differences between experimental groups. The purpose of this study was to assess the effect of multibone scanning on detecting disease‐induced changes in bone microarchitecture and mineral density by group scanning two murine models with known skeletal defects: the Col1a2^G610C/+ model of osteogenesis imperfecta and an adenine‐induced model of chronic kidney disease. Adult male femurs were scanned individually and in random groups of three and eight in a Bruker Skyscan 1172 and 1176, respectively, then assessed for standard trabecular and cortical bone measures. Although scanning methodology altered raw values, with trabecular microarchitecture values more affected than cortical properties, a disease phenotype was still detectable in both group and solo scans. However, tissue mineral density in both trabecular and cortical bone was significantly impacted by group versus solo scanning. Researchers may be able to use small groupings in a single μCT scan to expedite preclinical analyses when the overall bone phenotype is large to decrease costs and increase speed of discoveries; however the details of scanning (single or group) should always be reported. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Minimally invasive laser Doppler flowmetry is suitable for serial bone perfusion measurements in mice

In vivo laser Doppler flowmetry (LDF) has previously been used to quantify blood perfusion accurately at a single timepoint in the murine tibial metaphysis. However, this procedure entailed substantial disruption to soft tissues overlying the bone and caused notable localized inflammation for several weeks after the procedure, impeding serial measurements in the same mouse. In this study, we tested a less invasive technique to measure perfusion in the tibia with LDF and determined that it can be used serially in the same mouse without causing signs of inflammation or gait perturbations. Twenty 14-week-old C57Bl/6J mice were evenly divided into groups that either had daily treadmill exercise or remained sedentary. Within these activity groups, mice were evenly subdivided into groups that received LDF measurements either weekly or only once at the study endpoint. Bone perfusion was measured with LDF in the anteromedial region of the right tibial metaphysis. Serum concentrations of interleukin 6, incision site wound area, and interlimb coordination during gait were measured weekly for four weeks. Tibial perfusion did not differ significantly between exercise and sedentary groups within the weekly or endpoint-only LDF groups at any timepoint. Perfusion was significantly increased in the third week in the weekly LDF group relative to measurements in the second and fourth weeks. Ligation of the femoral artery caused consistent, rapid reductions in tibial perfusion, validating that LDF is sensitive to changes in tibial blood supply. Weekly LDF procedures did not adversely affect gait, as interlimb coordination during treadmill locomotion was similar between weekly and endpoint-only LDF groups at every timepoint. Images of the incision site show wound closure within one week, and serum concentrations of interleukin 6 were not significantly different between weekly and endpoint-only groups. Together, these findings demonstrate that our minimally invasive LDF technique is suitable for serial in vivo measurements of intraosseous blood perfusion without inducing localized inflammation or negatively affecting gait patterns in mice.

Stroke Prevents Exercise-induced Gains in Bone Microstructure But Not Composition in Mice

Ischemic stroke induces rapid loss in bone mineral density up to 13 times greater than during normal aging, leading to markedly increased risk of fracture. Little is known about skeletal changes following stroke beyond density loss. In this study we use a mild-moderate middle cerebral artery occlusion model to determine the effects of ischemic stroke without bedrest on bone microstructure, dynamic bone formation, and tissue composition. Twenty-seven 12-week-old male C57Bl/6J mice received either a stroke or sham surgery and then either received daily treadmill exercise or remained sedentary for four weeks. All mice were ambulatory immediately following stroke, and limb coordination during treadmill exercise was unaffected by stroke, indicating similar mechanical loading across limbs for surgery groups. Stroke did not directly detriment microstructure, but exercise only stimulated adaptation in sham group, not stroke group, with increased bone volume fraction and trabecular thickness in the sham distal femoral metaphysis. Stroke differentially decreased cortical area in the affected limb relative to the unaffected limb of the distal femoral metaphysis, and endosteal bone formation rate in the affected tibial diaphysis. Although exercise failed to improve bone microstructure following stroke, exercise increased mineral-to-matrix content in stroke but not sham. Together, these results show that stroke inhibits exercise-induced changes to femoral microstructure but not tibial composition, even without changes to gait. Similarly, affected-unaffected limb differences in cortical bone structure and bone formation rate in ambulatory mice show that stroke affects bone health even without bedrest.