Chronic kidney disease and aging differentially diminish bone material and microarchitecture in C57Bl/6 mice

Multiscale effects of the calcimimetic drug, etelcalcetide on bone health of rats with secondary hyperparathyroidism induced by chronic kidney disease

Chronic kidney disease-induced secondary hyperparathyroidism (CKD-SHPT) heightens fracture risk through impaired mineral homeostasis and elevated levels of uremic toxins (UTs), which in turn enhance bone remodeling. Etelcalcetide (Etel), a calcium-sensing receptor (CaSR) agonist, suppresses parathyroid hormone (PTH) in hyperparathyroidism to reduce excessive bone resorption, leading to increased bone mass. However, Etel's effect on bone quality, chemical composition, and strength is not well understood. To address these gaps, we established a CKD-SHPT rat model and administered Etel at a human-equivalent dose concurrently with disease induction. The effects on bone and mineral homeostasis were compared with a CKD-SHPT (vehicle-treated group) and a control group (rats without SHPT). Compared with vehicle-treated CKD-SHPT rats, Etel treatment improved renal function, reduced circulating UT levels, improved mineral homeostasis parameters, decreased PTH levels, and prevented mineralization defects. The upregulation of mineralization-promoting genes by Etel in CKD-SHPT rats might explain its ability to prevent mineralization defects. Etel preserved both trabecular and cortical bones with attendant suppression of osteoclast function, besides increasing mineralization. Etel maintained the number of viable osteocytes to the control level, which could also contribute to its beneficial effects on bone. CKD-SHPT rats displayed increased carbonate substitution of matrix and mineral, decreased crystallinity, mineral-to-matrix ratio, and collagen maturity, and these changes were mitigated by Etel. Further, Etel treatment prevented CKD-SHPT-induced deterioration in bone strength and mechanical behavior. Based on these findings, we conclude that in CKD-SHPT rats, Etel has multiscale beneficial effects on bone that involve remodeling suppression, mineralization gene upregulation, and preservation of osteocytes.

NFATc1 Is Required for Vitamin D- and Phosphate-Mediated Regulation of Osteocyte Lacuno-Canalicular Remodeling

Osteocytes are embedded in lacunae and connected by canaliculi (lacuno-canalicular network, LCN). Bones from mice with X-linked hypophosphatemia (Hyp), which have impaired production of 1,25 dihydroxyvitamin D (1,25D) and hypophosphatemia, have abnormal LCN structure that is improved by treatment with 1,25D or an anti-FGF23 targeting antibody, supporting roles for 1,25D and phosphate in regulating LCN remodeling. Bones from mice lacking the vitamin D receptor (VDR) in osteocytes (Vdrf/f;Dmp1Cre+) and mice lacking the sodium phosphate transporter 2a (Npt2aKO), which have low serum phosphate with high serum 1,25D, have impaired LCN organization, demonstrating that osteocyte-specific actions of 1,25D and hypophosphatemia regulate LCN remodeling. In osteoclasts, nuclear factor of activated T cells cytoplasmic 1 (NFATc1) is critical for stimulating bone resorption. Since osteocytes also resorb matrix, we hypothesize that NFATc1 plays a role in 1,25D and phosphate-mediated LCN remodeling. Consistent with this, 1,25D and phosphate suppress Nfatc1 mRNA expression in IDG-SW3 osteocytes, and knockdown of Nfatc1 expression in IDG-SW3 cells blocks 1,25D- and phosphate-mediated suppression of matrix resorption gene expression and 1,25D- and phosphate-mediated suppression of RANKL-induced acidification of the osteocyte microenvironment. To determine the role of NFATc1 in 1,25D- and phosphate-mediated LCN remodeling in vivo, histomorphometric analyses of tibiae from mice lacking osteocyte-specific Nfatc1 in Vdrf/f;Dmp1Cre+ and Npt2aKO mice were performed, demonstrating that bones from these mice have decreased lacunar size and expression of matrix resorption genes, and improved canalicular structure compared to Vdrf/f;Dmp1Cre+ and Npt2aKO control. This study demonstrates that NFATc1 is necessary for 1,25D- and phosphate-mediated regulation of LCN remodeling.

Multi-scale cortical bone traits vary in females and males from two mouse models of genetic diversity

Understanding the genetic basis of cortical bone traits can allow for the discovery of novel genes or biological pathways regulating bone health. Mice are the most widely used mammalian model for skeletal biology and allow for the quantification of traits that cannot easily be evaluated in humans, such as osteocyte lacunar morphology. The goal of our study was to investigate the effect of genetic diversity on multi-scale cortical bone traits of 3 long bones in skeletally-mature mice. We measured bone morphology, mechanical properties, material properties, lacunar morphology, and mineral composition of mouse bones from 2 populations of genetic diversity. Additionally, we compared how intrabone relationships varied in the 2 populations. Our first population of genetic diversity included 72 females and 72 males from the 8 inbred founder strains used to create the Diversity Outbred (DO) population. These 8 strains together span almost 90% of the genetic diversity found in mice (Mus musculus). Our second population of genetic diversity included 25 genetically unique, outbred females and 25 males from the DO population. We show that multi-scale cortical bone traits vary significantly with genetic background; heritability values range from 21% to 99% indicating genetic control of bone traits across length scales. We show for the first time that lacunar shape and number are highly heritable. Comparing the 2 populations of genetic diversity, we show that each DO mouse does not resemble a single inbred founder, but instead the outbred mice display hybrid phenotypes with the elimination of extreme values. Additionally, intrabone relationships (eg, ultimate force vs. cortical area) were mainly conserved in our 2 populations. Overall, this work supports future use of these genetically diverse populations to discover novel genes contributing to cortical bone traits, especially at the lacunar length scale.

Novel insights into osteocyte and inter-organ/tissue crosstalk

Osteocyte, a cell type living within the mineralized bone matrix and connected to each other by means of numerous dendrites, appears to play a major role in body homeostasis. Benefiting from the maturation of osteocyte extraction and culture technique, many cross-sectional studies have been conducted as a subject of intense research in recent years, illustrating the osteocyte-organ/tissue communication not only mechanically but also biochemically. The present review comprehensively evaluates the new research work on the possible crosstalk between osteocyte and closely situated or remote vital organs/tissues. We aim to bring together recent key advances and discuss the mutual effect of osteocyte and brain, kidney, vascular calcification, muscle, liver, adipose tissue, and tumor metastasis and elucidate the therapeutic potential of osteocyte.

Deep learning models to map osteocyte networks can successfully distinguish between young and aged bone

Osteocytes, the most abundant and mechanosensitive cells in bone tissue, play a pivotal role in bone homeostasis and mechano-responsiveness, orchestrating the intricate balance between bone formation and resorption under daily activity. Studying osteocyte connectivity and understanding their intricate arrangement within the lacunar canalicular network (LCN) is essential for unraveling bone physiology. This is particularly true as our bones age, which is associated with decreased integrity of the osteocyte network, disrupted mass transport, and lower sensitivity to the mechanical stimuli that allow the skeleton to adapt to changing demands. Much work has been carried out to investigate this relationship, often involving high resolution microscopy of discrete fragments of this network, alongside advanced computational modelling of individual cells. However, traditional methods of segmenting and measuring osteocyte connectomics are time-consuming and labour-intensive, often hindered by human subjectivity and limited throughput. In this study, we explore the application of deep learning and computer vision techniques to automate the segmentation and measurement of osteocyte connectomics, enabling more efficient and accurate analysis. We compare several state-of-the-art computer vision models (U-Nets and Vision Transformers) to successfully segment the LCN, finding that an Attention U-Net model can accurately segment and measure 81.8% of osteocytes and 42.1% of dendritic processes, when compared to manual labelling. While further development is required, we demonstrate that this degree of accuracy is already sufficient to distinguish between bones of young (2 month old) and aged (36 month old) mice, as well as capturing the degeneration induced by genetic modification of osteocytes. By harnessing the power of these advanced technologies, further developments can unravel the complexities of osteocyte networks in unprecedented detail, revolutionising our understanding of bone health and disease.

Osteocytes and Primary Cilia

PURPOSE OF REVIEW

The purpose of this review is to provide a background on osteocytes and the primary cilium, discussing the role it plays in osteocyte mechanosensing.

RECENT FINDINGS

Osteocytes are thought to be the primary mechanosensing cells in bone tissue, regulating bone adaptation in response to exercise, with the primary cilium suggested to be a key mechanosensing mechanism in bone. More recent work has suggested that, rather than being direct mechanosensors themselves, primary cilia in bone may instead form a key chemo-signalling nexus for processing mechanoregulated signalling pathways. Recent evidence suggests that pharmacologically induced lengthening of the primary cilium in osteocytes may potentiate greater mechanotransduction, rather than greater mechanosensing. While more research is required to delineate the specific osteocyte mechanobiological molecular mechanisms governed by the primary cilium, it is clear from the literature that the primary cilium has significant potential as a therapeutic target to treat mechanoregulated bone diseases, such as osteoporosis.

Inositol 1,4,5-trisphosphate receptor type 2 is associated with the bone-vessel axis in chronic kidney disease-mineral bone disorder

Objective: The pathogenesis of renal osteopathy and cardiovascular disease suggests the disordered bone-vessel axis in chronic kidney disease-mineral bone disorder (CKD-MBD). However, the mechanism of the bone-vessel axis in CKD-MBD remains unclear.Methods: We established a CKD-MBD rat model to observe the pathophysiological phenotype of the bone-vessel axis and performed RNA sequencing of aortas to identify novel targets of the bone-vessel axis in CKD-MBD.Results: The microarchitecture of the femoral trabecular bone deteriorated and alveolar bone loss was aggravated in CKD-MBD rats. The intact parathyroid hormone and alkaline phosphatase levels increased, 1,25-dihydroxyvitamin D3 levels decreased, and intact fibroblast growth factor-23 levels did not increase in CKD-MBD rats at 16 weeks; other bone metabolic parameters in the serum demonstrated dynamic characteristics. With calcium deposition in the abdominal aortas of CKD-MBD rats, RNA sequencing of the aortas revealed a significant decrease in inositol 1,4,5-trisphosphate receptor type 2 (ITPR2) gene levels in CKD-MBD rats. A similar trend was observed in rat aortic smooth muscle cells. As a secretory protein, ITPR2 serum levels decreased at 4 weeks and slightly increased without statistical differences at 16 weeks in CKD-MBD rats. ITPR2 serum levels were significantly increased in patients with vascular calcification, negatively correlated with blood urea nitrogen levels, and positively correlated with serum tartrate-resistant acid phosphatase 5b levels.Conclusion: These findings provide preliminary insights into the role of ITPR2 in the bone-vessel axis in CKD-MBD. Thus, ITPR2 may be a potential target of the bone-vessel axis in CKD-MBD.

Multi-Scale Cortical Bone Traits Vary in Two Mouse Models of Genetic Diversity

Understanding the genetic basis of cortical bone traits can allow for the discovery of novel genes or biological pathways regulating bone health. Mice are the most widely used mammalian model for skeletal biology and allow for the quantification of traits that can't easily be evaluated in humans, such as osteocyte lacunar morphology. The goal of our study was to investigate the effect of genetic diversity on multi-scale cortical bone traits of three long bones in skeletally-mature mice. We measured bone morphology, mechanical properties, material properties, lacunar morphology, and mineral composition of mouse bones from two populations of genetic diversity. Additionally, we compared how intra-bone relationships varied in the two populations. Our first population of genetic diversity included 72 females and 72 males from the eight Inbred Founder strains used to create the Diversity Outbred (DO) population. These eight strains together span almost 90% of the genetic diversity found in mice (Mus musculus). Our second population of genetic diversity included 25 genetically unique, outbred females and 25 males from the DO population. We show that multi-scale cortical bone traits vary significantly with genetic background; heritability values range from 21% to 99% indicating genetic control of bone traits across length scales. We show for the first time that lacunar shape and number are highly heritable. Comparing the two populations of genetic diversity, we show each DO mouse does not resemble a single Inbred Founder but instead the outbred mice display hybrid phenotypes with the elimination of extreme values. Additionally, intra-bone relationships (e.g., ultimate force vs. cortical area) were mainly conserved in our two populations. Overall, this work supports future use of these genetically diverse populations to discover novel genes contributing to cortical bone traits, especially at the lacunar length scale.

Treatment with fibroblast growth factor 19 increases skeletal muscle fiber size, ameliorates metabolic perturbations and hepatic inflammation in 5/6 nephrectomized mice

Chronic kidney disease (CKD) is associated with osteosarcopenia, and because a physical decline in patients correlates with an increased risk of morbidity, an improvement of the musculoskeletal system is expected to improve morbi-mortality. We recently uncovered that the intestinal hormone Fibroblast Growth Factor 19 (FGF19) is able to promote skeletal muscle mass and strength in rodent models, in addition to its capacity to improve glucose homeostasis. Here, we tested the effects of a treatment with recombinant human FGF19 in a CKD mouse model, which associates sarcopenia and metabolic disorders. In 5/6 nephrectomized (5/6Nx) mice, subcutaneous FGF19 injection (0.1 mg/kg) during 18 days increased skeletal muscle fiber size independently of food intake and weight gain, associated with decreased gene expression of myostatin. Furthermore, FGF19 treatment attenuated glucose intolerance and reduced hepatic expression of gluconeogenic genes in uremic mice. Importantly, the treatment also decreased gene expression of liver inflammatory markers in CKD mice. Therefore, our results suggest that FGF19 may represent a novel interesting therapeutic strategy for a global improvement of sarcopenia and metabolic complications in CKD.

Hindlimb unloading in C57BL/6J mice induces bone loss at thermoneutrality without change in osteocyte and lacuno-canalicular network

Impaired mechanical stimuli during hindlimb unloading (HLU) are believed to exacerbate osteocyte paracrine regulation of osteoclasts. We hypothesized that bone loss and deterioration of the osteocyte lacuno-canalicular network are attenuated in HLU mice housed at thermoneutrality (28 °C) compared with those housed at ambient temperature (22 °C). Following acclimatization, 20-week-old male C57BL/6J mice were submitted to HLU or kept in pair-fed control cages (CONT), for 5 days (5d) or 14d, at 22 °C or 28 °C. In the femur distal metaphysis, thermoneutral CONT mice had higher bone volume (p = 0.0007, BV/TV, in vivo μCT, vs. 14dCONT22) whilst osteoclastic surfaces of CONT and HLU were greater at 22 °C (5dCONT22 + 53 %, 5dHLU22 + 50 %, 14dCONT22 + 186 %, 14dHLU22 + 104 %, vs matching 28 °C group). In the femur diaphysis and at both temperatures, 14dHLU exhibited thinner cortices distally or proximally compared to controls; the mid-diaphysis being thicker at 28 °C than at 22 °C in all groups. Expression of cortical genes for proteolytic enzyme (Mmp13), markers for osteoclastogenic differentiation (MCSF, RANKL), and activity (TRAP, Ctsk) were increased following 22 °C HLU, whereas only Ctsk expression was increased following 28 °C HLU. Expression of cortical genes for apoptosis, senescence, and autophagy were not elevated following HLU at any temperature. Osteocyte density at the posterior mid-diaphysis was similar between groups, as was the proportion of empty lacunae (<0.5 %). However, analysis of the lacuno-canalicular network (LCN, fluorescein staining) revealed unstained areas in the 14dHLU22 group only, suggesting disrupted LCN flow in this group alone. In conclusion, 28 °C housing influences the HLU bone response but does not prevent bone loss. Furthermore, our results do not show osteocyte senescence or death, and at thermoneutrality, HLU-induced bone resorption is not triggered by osteoclastic activators RANKL and MCSF.

Instrumented nanoindentation in musculoskeletal research

Musculoskeletal tissues, such as bone, cartilage, and muscle, are natural composite materials that are constructed with a hierarchical structure ranging from the cell to tissue level. The component differences and structural complexity, together, require comprehensive multiscale mechanical characterization. In this review, we focus on nanoindentation testing, which is used for nanometer to sub-micrometer length scale mechanical characterization. In the following context, we will summarize studies of nanoindentation in musculoskeletal research, examine the critical factors that affect nanoindentation testing results, and briefly summarize other commonly used techniques that can be conjoined with nanoindentation for synchronized imaging and colocalized characterization.

Collagen cross-link profiles and mineral are different between the mandible and femur with site specific response to perturbed collagen

Compromises to collagen and mineral lead to a decrease in whole bone quantity and quality in a variety of systemic diseases, yet, clinically, disease manifestations differ between craniofacial and long bones. Collagen alterations can occur through post-translational modification via lysyl oxidase (LOX), which catalyzes enzymatic collagen cross-link formation, as well as through non-enzymatic advanced glycation end products (AGEs) such as pentosidine and carboxymethyl-lysine (CML). Characterization of the cross-links and AGEs, and comparison of the mineral and collagen modifications in craniofacial and long bones represent a critical gap in knowledge. However, alterations to either the mineral or collagen in bone may contribute to disease progression and, subsequently, the anatomical site dependence of a variety of diseases. Therefore, we hypothesized that collagen cross-links and AGEs differ between craniofacial and long bones and that altered collagen cross-linking reduces mineral quality in an anatomic location dependent. To study the effects of cross-link inhibition on mineralization between anatomical sites, beta-aminoproprionitrile (BAPN) was administered to rapidly growing, 5-8 week-old male mice. BAPN is a dose-dependent inhibitor of LOX that pharmacologically alters enzymatic cross-link formation. Long bones (femora) and craniofacial bones (mandibles) were compared for mineral quantity and quality, collagen cross-link and AGE profiles, and tissue level mechanics, as well as the response to altered cross-links via BAPN. A highly sensitive liquid chromatography/mass spectrometry (LC-MS) method was developed which allowed for quantification of site-dependent accumulation of the advanced glycation end-product, carboxymethyl-lysine (CML). CML was ∼8.3× higher in the mandible than the femur. The mandible had significantly higher collagen maturation, mineral crystallinity, and Young's modulus, but lower carbonation, than the femur. BAPN also had anatomic specific effects, leading to significant decreases in mature cross-links in the mandible, and an increase in mineral carbonation in the femur. This differential response of both the mineral and collagen composition to BAPN between the mandible and femur highlights the need to further understand how inherent compositional differences in collagen and mineral contribute to anatomic-site specific manifestations of disease in both craniofacial and long bones.

A pilot study on the nanoscale properties of bone tissue near lacunae in fracturing women

The goal of this study is to investigate the causes of osteoporosis-related skeletal fragility in postmenopausal women. We hypothesize that bone fragility in these individuals is largely due to mineral, and/or intrinsic material properties in the osteocyte lacunar/peri-lacunar regions of bone tissue. Innovative measurements with nanoscale resolution, including scanning electron microscope (SEM), an atomic force microscope that is integrated with infrared spectroscopy (AFM-IR), and nanoindentation, were used to characterize osteocyte lacunar and peri-lacunar properties in bone biopsies from fracturing (Cases) and matched (Age, BMD), non-fracturing (Controls) postmenopausal healthy women. In the peri-lacunar space, the nanoindentation results show that the modulus and hardness of the Controls are lower than the Cases. The AFM-IR results conclusively show that the mineral matrix, maturity (peak) (except in outer/far regions in Controls) were greater in Controls than in Cases. Furthermore, these results indicate that while mineral-to-matrix area ratio tend to be greater, the mineral maturity and crystallinity peak ratio "near" lacunae is greater than at regions "far" or more distance from lacunae in the Controls only. Due to the heterogeneity of bone structure, additional measurements are needed to provide more convincing evidence of altered lacunar characteristics and changes in the peri-lacunar bone as mechanisms related to postmenopausal women and fragility. Such findings would motivate new osteocyte-targeted treatments to reduce fragility fracture risks in these groups.

A preliminary study exploring the mechanical properties of normal and Mgp-deficient mouse femurs during early growth

Matrix Gla protein (MGP) is mostly known to be a calcification inhibitor, as its absence leads to ectopic calcification of different tissues such as cartilage or arteries. MGP deficiency also leads to low bone mass and delayed bone growth. In the present contribution, we investigate the effect of MGP deficiency on the structural and material mechanical bone properties by focusing on the elastic response of femurs undergoing three-points bending. To this aim, biomechanical tests are performed on femurs issued from Mgp-deficient mice at 14, 21, 28, and 35 days of postnatal life and compared to healthy control femurs. µCT acquisitions enable to reconstruct bone geometries and are used to construct subject-specific finite element models avoiding some of the reported limitations concerning the use of beam-like assumptions for small bone samples. Our results indicate that MGP deficiency may be associated to differences in both structural and material properties of femurs during early stages of development. MGP deficiency appears to be related to a decrease in bone dimensions, compensated by higher material properties resulting in similar structural bone properties at P35. The search for a unique density-elasticity relationship based on calibrated bone mineral density (BMD) indicates that MGP deficiency may affect bone tissue in several ways, that may not be represented uniquely from the quantification of BMD. Despite of its limitation to elastic response, the present preliminary study reports for the very first time the mechanical skeletal properties of Mgp-deficient mice at early stages of development.

An inducible model for unraveling the effects of advanced glycation end-product accumulation in aging connective tissues

PURPOSE

In connective tissues there is a clear link between increasing age and degeneration. Advanced glycation end-products (AGEs) are believed to play a central role. AGEs are sugar-induced non-enzymatic crosslinks which accumulate in collagen with age and diabetes, altering tissue mechanics and cellular function. Despite ample correlative evidence linking collagen glycation to tissue degeneration, little is known how AGEs impact cell-matrix interactions, limiting therapeutic options. One reason for this limited understanding is that AGEs are typically induced using high concentrations of ribose which decrease cell viability, making it impossible to investigate cell-matrix interactions. The objective of this study was to develop a system to trigger AGE accumulation while maintaining cell viability.

MATERIALS AND METHODS

Using cell-seeded high density collagen gels, we investigated the effect of two systems for AGE induction, ribose at low concentrations (30, 100, and 200 mM) over 15 days of culture and riboflavin (0.25 and 0.75 mM) induced with blue light for 40 seconds (riboflavin-465 nm).

RESULTS

We found ribose and riboflavin-465 nm treatment produces fluorescent AGE quantities which match and/or exceed human fluorescent AGE levels for various tissues, ages, and diseases, without affecting cell viability or metabolism. Interestingly, a 40 second treatment of riboflavin-465 nm produced similar levels of fluorescent AGEs as 3 days of 100 mM ribose treatment.

CONCLUSIONS

Riboflavin-465 nm is a promising method to trigger AGEs on demand in vivo or in vitro without impacting cell viability and offers potential for unraveling the mechanism of AGEs in age and diabetes related tissue damage.

Calcimimetics Alter Periosteal and Perilacunar Bone Matrix Composition and Material Properties in Early Chronic Kidney Disease

Chronic kidney disease (CKD) affects 15% of Americans and greatly increases fracture risk due to elevated parathyroid hormone, cortical porosity, and reduced bone material quality. Calcimimetic drugs are used to lower parathyroid hormone (PTH) in CKD patients, but their impact on bone matrix properties remains unknown. We hypothesized that tissue-level bone quality is altered in early CKD and that calcimimetic treatment will prevent these alterations. To test this hypothesis, we treated Cy/+ rats, a model of spontaneous and progressive CKD-mineral and bone disorder (CKD-MBD), with KP-2326, a preclinical analogue of etelcalcetide, early in the CKD disease course. To measure tissue-level bone matrix composition and material properties, we performed colocalized Raman spectroscopy and nanoindentation on new periosteal bone and perilacunar bone using hydrated femur sections. We found that CKD and KP treatment lowered mineral type B carbonate substitution whereas KP treatment increased mineral crystallinity in new periosteal bone. Reduced elastic modulus was lower in CKD but was not different in KP-treated rats versus CTRL. In perilacunar bone, KP treatment lowered type B carbonate substitution, increased crystallinity, and increased mineral-to-matrix ratio in a spatially dependent manner. KP treatment also increased reduced elastic modulus and hardness in a spatially dependent manner. Taken together, these data suggest that KP treatment improves material properties on the tissue level through a combination of lowering carbonate substitution, increasing mineral crystallinity, and increasing relative mineralization of the bone early in CKD. As a result, the mechanical properties were improved, and in some regions, were the same as control animals. Therefore, calcimimetics may help prevent CKD-induced bone deterioration by improving bone quality in new periosteal bone and in bone tissue near osteocyte lacunae. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Perilacunar bone tissue exhibits sub-micrometer modulus gradation which depends on the recency of osteocyte bone formation in both young adult and early-old-age female C57Bl/6 mice

Osteocytes resorb and replace bone local to the lacunar-canalicular system (LCS). However, whether osteocyte remodeling impacts bone quality adjacent to the LCS is not understood. Further, while aging is well-established to decrease osteocyte viability and truncate LCS geometry, it is unclear if aging also decreases perilacunar bone quality. To address these questions, we employed atomic force microscopy (AFM) to generate nanoscale-resolution modulus maps for cortical femur osteocyte lacunae from young (5-month) and early-old-age (22-month) female C57Bl/6 mice. AFM-mapped lacunae were also imaged with confocal laser scanning microscopy to determine which osteocytes recently deposited bone as determined by the presence of fluorochrome labels administered 2d and 8d before euthanasia. Modulus gradation with distance from the lacunar wall was compared for labeled (i.e., bone forming) and non-labeled lacunae in both young and aged mice. All mapped lacunae showed sub-microscale modulus gradation, with peak modulus values 200-400 nm from the lacunar wall. Perilacunar modulus gradations depended on the recency of osteocyte bone formation (i.e., the presence of labels). For both ages, 2d-labeled perilacunar bone had lower peak and bulk modulus compared to non-labeled perilacunar bone. Lacunar length reduced with age, but lacunar shape and size were not strong predictors of modulus gradation. Our findings demonstrate for the first time that osteocyte perilacunar remodeling impacts bone tissue modulus, one contributor to bone quality. Given the immense scale of the LCS, differences in perilacunar modulus resulting from osteocyte remodeling activity may affect the quality of a substantial amount of bone tissue.

The combination of aging and chronic kidney disease leads to an exacerbated cortical porosity phenotype

PURPOSE

Chronic kidney disease (CKD) and aging are each independently associated with higher fracture risk. Although CKD is highly prevalent in the aging population, the interaction between these two conditions with respect to bone structure and mechanics is not well understood. The purpose of this study was to examine cortical porosity and mechanical properties in skeletally mature young and aging mice with CKD.

METHODS

CKD was induced by feeding 16-week and 78-week male mice 0.2% adenine (AD) for six weeks followed by two weeks of maintenance on a control diet for a total study duration of eight weeks of CKD; control (CON) animals of each age were fed a standard diet. Serum biochemistries, μCT imaging, and mechanical properties via four-point bending were assessed at the endpoint.

RESULTS

Phosphorus, parathyroid hormone, and blood urea nitrogen were elevated in both ages of AD mice compared to age-matched CON; aging AD mice had PTH and BUN values higher than all other groups. Femoral cortical porosity was more than four-fold higher in aging AD mice compared to young AD mice and more than two-fold higher compared to age-matched controls. Structural and estimated material mechanical properties were both lower in aging mice, but there were no significant interactions between AD treatment and age.

CONCLUSION

These data show an interaction between CKD and aging that produces a more severe biochemical and cortical bone phenotype. This highlights the importance of studying mechanisms and potential interventions in both young and aged animals to translate to a broader spectrum of CKD patients.

Mechanobiology of Bone Metastatic Cancer

PURPOSE OF REVIEW

In this review, we provide an overview of what is currently known about the impacts of mechanical stimuli on metastatic tumor-induced bone disease (TIBD). Further, we focus on the role of the osteocyte, the skeleton's primary mechanosensory cell, which is central to the skeleton's mechanoresponse, sensing and integrating local mechanical stimuli, and then controlling the downstream remodeling balance as appropriate.

RECENT FINDINGS

Exercise and controlled mechanical loading have anabolic effects on bone tissue in models of bone metastasis. They also have anti-tumorigenic properties, in part due to offsetting the vicious cycle of osteolytic bone loss as well as regulating inflammatory signals. The impacts of metastatic cancer on the mechanosensory function of osteocytes remains unclear. Increased mechanical stimuli are a potential method for mitigating TIBD.

Bone biomechanical properties and tissue-scale bone quality in a genetic mouse model of familial dysautonomia

PURPOSE

Familial dysautonomia (FD) is associated with a high prevalence of bone fractures, but the impacts of the disease on bone mass and quality are unclear. The purpose of this study was to evaluate tissue through whole-bone scale bone quality in a mouse model of FD.

METHODS

Femurs from mature adult Tuba1a-Cre; Elp1^LoxP/LoxP conditional knockouts (CKO) (F = 7, M = 4) and controls (F = 5, M = 6) were evaluated for whole-bone flexural material properties, trabecular microarchitecture and cortical geometry, and areal bone mineral density (BMD). Adjacent maps spanning the thickness of femur midshaft cortical bone assessed tissue-scale modulus (nanoindentation), bone mineralization, mineral maturity, and collagen secondary structure (Raman spectroscopy).

RESULTS

Consistent with prior studies on this mouse model, the Elp1 CKO mouse model recapitulated several key hallmarks of human FD, with one difference being the male mice tended to have a more severe phenotype than females. Deletion of Elp1 in neurons (using the neuronal-specific Tuba1a-cre) led to a significantly reduced whole-bone toughness but not strength or modulus. Elp1 CKO female mice had reduced trabecular microarchitecture (BV/TV, Tb.Th, Conn.D.) but not cortical geometry. The mutant mice also had a small but significant reduction in cortical bone nanoindentation modulus. While bone tissue mineralization and mineral maturity were not impaired, FD mice may have altered collagen secondary structure. Changes in collagen secondary structure were inversely correlated with bone toughness. BMD from dual-energy x-ray absorptiometry (DXA) was unchanged with FD.

CONCLUSION

The deletion of Elp1 in neurons is sufficient to generate a mouse line which demonstrates loss of whole-bone toughness, consistent with the poor bone quality suspected in the clinical setting. The Elp1 CKO model, as with human FD, impacts the nervous system, gut, kidney function, mobility, gait, and posture. The bone quality phenotype of Elp1 CKO mice, which includes altered microarchitecture and tissue-scale material properties, is complex and likely influenced by these multisystemic changes. This mouse model may provide a useful platform to not only investigate the mechanisms responsible for bone fragility in FD, but also a powerful model system with which to evaluate potential therapeutic interventions for bone fragility in FD patients.

Poor bone matrix quality: What can be done about it?

PURPOSE OF THE REVIEW

Bone's ability to withstand load resisting fracture and adapting to it highly depends on the quality of its matrix and its regulators. This review focuses on the contribution of bone quality to fracture resistance and possible therapeutic targets for skeletal fragility in aging and disease.

RECENT FINDINGS

The highly organized, hierarchical composite structure of bone extracellular matrix together with its (re)modeling mechanisms and microdamage dynamics determines its stiffness, strength, and toughness. Aging and disease affect the biological processes regulating bone quality, thus resulting in defective extracellular matrix and bone fragility. Targeted therapies are being developed to restore bone's mechanical integrity. However, their current limitations include low tissue selectivity and adverse side effects. Biological and mechanical insights into the mechanisms controlling bone quality, together with advances in drug delivery and studies in animal models, will accelerate the development and translation to clinical application of effective targeted-therapeutics for bone fragility.

The impact of advanced glycation end products on bone properties in chronic kidney disease

PURPOSE OF REVIEW

Chronic kidney disease (CKD) affects over 15% of Americans and results in an increased risk of skeletal fractures and fracture-related mortality. However, there remain great challenges in estimating fracture risk in CKD patients, as conventional metrics such as bone density assess bone quantity without accounting for the material quality of the bone tissue. The purpose of this review is to highlight the detrimental effects of advanced glycation end products (AGEs) on the structural and mechanical properties of bone, and to demonstrate the importance of including bone quality when assessing fracture risk in CKD patients.

RECENT FINDINGS

Increased oxidative stress and inflammation drive the production of AGEs in CKD patients that form nonenzymatic crosslinks between type I collagen fibrils in the bone matrix. Nonenzymatic crosslinks stiffen and embrittle the bone, reducing its ability to absorb energy and resist fracture. Clinical measurement of AGEs is typically indirect and fails to distinguish the identity and properties of the various AGEs.

SUMMARY

Accounting for the impact of AGEs on the skeleton in CKD patients may improve our estimation of overall bone quality, fracture risk, and treatments to improve both bone quantity and quality by reducing AGEs in patients with CKD merit investigation in order to improve our understanding of the etiology of increased fracture risk.

Disrupted osteocyte connectivity and pericellular fluid flow in bone with aging and defective TGF-β signaling

Skeletal fragility in the elderly does not simply result from a loss of bone mass. However, the mechanisms underlying the concurrent decline in bone mass, quality, and mechanosensitivity with age remain unclear. The important role of osteocytes in these processes and the age-related degeneration of the intricate lacunocanalicular network (LCN) in which osteocytes reside point to a primary role for osteocytes in bone aging. Since LCN complexity severely limits experimental dissection of these mechanisms in vivo, we used two in silico approaches to test the hypothesis that LCN degeneration, due to aging or an osteocyte-intrinsic defect in transforming growth factor beta (TGF-β) signaling (TβRIIocy-/-), is sufficient to compromise essential osteocyte responsibilities of mass transport and exposure to mechanical stimuli. Using reconstructed confocal images of bone with fluorescently labeled osteocytes, we found that osteocytes from aged and TβRIIocy-/- mice had 33 to 45% fewer, and more tortuous, canaliculi. Connectomic network analysis revealed that diminished canalicular density is sufficient to impair diffusion even with intact osteocyte numbers and overall LCN architecture. Computational fluid dynamics predicts that the corresponding drop in shear stress experienced by aged or TβRIIocy-/- osteocytes is highly sensitive to canalicular surface area but not tortuosity. Simulated expansion of the osteocyte pericellular space to mimic osteocyte perilacunar/canalicular remodeling restored predicted shear stress for aged osteocytes to young levels. Overall, these models show how loss of LCN volume through LCN pruning may lead to impaired fluid dynamics and osteocyte exposure to mechanostimulation. Furthermore, osteocytes emerge as targets of age-related therapeutic efforts to restore bone health and function.

Age and Sex Differences in Load-Induced Tibial Cortical Bone Surface Strain Maps

Bone adapts its architecture to the applied load; however, it is still unclear how bone mechano‐adaptation is coordinated and why potential for adaptation adjusts during the life course. Previous animal models have suggested strain as the mechanical stimulus for bone adaptation, but yet it is unknown how mouse cortical bone load‐related strains vary with age and sex. In this study, full‐field strain maps (at 1 N increments up to 12 N) on the bone surface were measured in young, adult, and old (aged 10, 22 weeks, and 20 months, respectively), male and female C57BL/6J mice with load applied using a noninvasive murine tibial model. Strain maps indicate a nonuniform strain field across the tibial surface, with axial compressive loads resulting in tension on the medial side of the tibia because of its curved shape. The load‐induced surface strain patterns and magnitudes show sexually dimorphic changes with aging. A comparison of the average and peak tensile strains indicates that the magnitude of strain at a given load generally increases during maturation, with tibias in female mice having higher strains than in males. The data further reveal that postmaturation aging is linked to sexually dimorphic changes in average and maximum strains. The strain maps reported here allow for loading male and female C57BL/6J mouse legs in vivo at the observed ages to create similar increases in bone surface average or peak strain to more accurately explore bone mechano‐adaptation differences with age and sex. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Lacunar-canalicular bone remodeling: Impacts on bone quality and tools for assessment

Osteocytes can resorb as well as replace bone adjacent to the expansive lacunar-canalicular system (LCS). Suppressed LCS remodeling decreases bone fracture toughness, but it is unclear how altered LCS remodeling impacts bone quality. The first goal of this review is to assess how LCS remodeling impacts LCS morphology as well as the composition and mechanical properties of surrounding bone tissue. The second goal is to compare tools available for the assessment of bone quality at length-scales that are physiologically-relevant to LCS remodeling. We find that changes to LCS morphology occur in response to a variety of physiological conditions and diseases and can be classified in two general phenotypes. In the 'aging phenotype', seen in aging and in some disuse models, the LCS is truncated and osteocytes apoptosis is increased. In the 'osteocytic osteolysis' phenotype, which is adaptive in some physiological settings and possibly maladaptive in others, the LCS enlarges and osteocytes generally maintain viability. Bone composition and mechanical properties vary near the osteocyte and change with at least some conditions that alter LCS morphology. However, few studies have evaluated bone composition and mechanical properties close to the LCS and so the impacts of LCS remodeling phenotypes on bone tissue quality are still undetermined. We summarize the current understanding of how LCS remodeling impacts LCS morphology, tissue-scale bone composition and mechanical properties, and whole-bone material properties. Tools are compared for assessing tissue-scale bone properties, as well as the resolution, advantages, and limitations of these techniques.

Age and gender related differences in load-strain response in C57Bl/6 mice

We examined the changes in mechanical strain response of male and female mouse tibia and ulna, using axial compression tests, to assess age-related changes in tibiae and ulnae by a non-contact strain measurement technique called the digital image correlation (DIC) and the standard strain gage. A unique aspect of the study was to compare bones from the same animal to study variations in behavior with aging. This study was conducted using male and female C57Bl/6 mice at 6, 12 and 22 months of age (N=6-7 per age and sex) using three load levels. The DIC technique was able to detect a greater number of statistically significant differences in comparison to the strain gaging method. Male ulna showed significantly higher DIC strains compared to strains captured from strain gage at all three levels of load at 6 months and in the lowest load at 12 months. DIC measurements revealed that the ulna becomes stiffer with aging for both males and females, which resulted in 0.4 to 0.8 times reduced strains in the 22-month group compared to the 6 month group. Male tibia showed three-fold increased strains in the 22 months group at 11.5 N load compared to 6 months group (p<.05).

Physical characterization of swine and human skin: Correlations between Raman spectroscopy, Tensile testing, Atomic force microscopy (AFM), Scanning electron microscopy (SEM), and Multiphoton microscopy (MPM)

BACKGROUND

Swine dorsum is commonly utilized as a model for studying skin wounds and assessment of dermatological and cosmetic medicaments. The human abdomen is a common location for dermatological intervention.

OBJECTIVE

This study provides a correlation between spectral, mechanical, and structural characterization techniques, utilized for evaluating human abdominal skin and swine dorsum.

METHODS

Raman spectroscopy (RS), tensile testing, ballistometry, AFM, SEM, and MPM were utilized to characterize and compare full-thickness skin properties in swine and human model.

RESULTS

RS of both species' skin types revealed a similar assignment of vibrations in the fingerprint and the high wavenumber spectral regions. Structural imaging and mechanical characterization using ballistometry and tensile testing displayed differences in the inherent functional properties of human and swine skin. These differences correlated with variations in the Raman peak ratios, collagen intensity measured using SEM and MPM and collagen density measured using AFM.

CONCLUSION

A comprehensive evaluation of swine skin as a suitable substitute for human skin for mechanical and structural comparisons was performed. This data should be considered for better understanding the swine skin model for cutaneous drug delivery and wound applications. Additionally, correlation between RS, tensile testing, AFM, SEM, and MPM was performed as skin characterization tools.

Enhanced Klotho availability protects against cardiac dysfunction induced by uraemic cardiomyopathy by regulating Ca2+ handling

BACKGROUND AND PURPOSE

Klotho is a membrane-bound or soluble protein, originally identified as an age-suppressing factor and regulator of mineral metabolism. Klotho deficiency is associated with the development of renal disease, but its role in cardiac function in the context of uraemic cardiomyopathy is unknown.

EXPERIMENTAL APPROACH

We explored the effects of Klotho on cardiac Ca2+ cycling. We analysed Ca2+ handling in adult cardiomyocytes from Klotho-deficient (kl/kl) mice and from a murine model of 5/6 nephrectomy (Nfx). We also studied the effect of exogenous Klotho supplementation, by chronic recombinant Klotho treatment, or endogenous Klotho overexpression, using transgenic mice overexpressing Klotho (Tg-Kl), on uraemic cardiomyopathy. Hearts from Nfx mice were used to study Ca2+ sensitivity of ryanodine receptors and their phosphorylation state.

KEY RESULTS

Cardiomyocytes from kl/kl mice showed decreased amplitude of intracellular Ca2+ transients and cellular shortening together with an increase in pro-arrhythmic Ca2+ events compared with cells from wild-type mice. Cardiomyocytes from Nfx mice exhibited the same impairment in Ca2+ cycling as kl/kl mice. Changes in Nfx cardiomyocytes were explained by higher sensitivity of ryanodine receptors to Ca2+ and their increased phosphorylation at the calmodulin kinase type II and protein kinase A sites. Ca2+ mishandling in Nfx-treated mice was fully prevented by chronic recombinant Klotho administration or transgenic Klotho overexpression.

CONCLUSIONS AND IMPLICATIONS

Klotho emerges as an attractive therapeutic tool to improve cardiac Ca2+ mishandling observed in uraemic cardiomyopathy. Strategies that improve Klotho availability are good candidates to protect the heart from functional cardiac alterations in renal disease.

Bone turnover correlates with bone quantity but not bone microarchitecture in chronic hemodialysis

INTRODUCTION

In chronic hemodialysis, high-turnover bone disease was associated with decreased bone mineral density (BMD), poor bone quality (chemical and structural), and increased fracture risk. Our aim was to correlate bone turnover markers (BTMs) with bone microarchitecture measured by trabecular bone score (TBS) before and after correction for BMD.

MATERIALS AND METHODS

We measured lumbar spine (LS), femoral neck, and 1/3 radius BMD and LS TBS by dual X-ray absorptiometry in 81 patients on permanent hemodialysis. Bone turnover was assessed using serum parathyroid hormone, osteocalcin, C-terminal crosslaps of type 1 collagen, procollagen 1 N-terminal propeptide (P1NP), and alkaline phosphatase (ALP). No patient had any partial or total parathyroidectomy and no previous or current treatment with anti-osteoporotic drugs.

RESULTS

All BTMs correlated significantly with each other. Univariate regressions showed significant negative correlations between BTMs and BMD (best r = - 0.53, between P1NP and 1/3 radius Z-score) or BTMs and TBS (best r = - 0.27, p < 0.05 between ALP and TBS T-score). TBS correlated significantly with BMD at all three sites (best r = 0.5, between LS BMD and TBS T-score). Multivariate regression showed that TBS, crude or adjusted, correlated with LS BMD. No model retained any of the BTMs as independent variables due to the better prediction of BMD and multicollinearity.

CONCLUSION

We showed a progressively impaired bone microarchitecture with increasing bone turnover in chronic hemodialysis. However, this correlation is no longer present when controlling for bone mass. This suggests that impaired bone microarchitecture and increased fracture risk are dependent upon factors other than high bone turnover.

Age-related and sex-specific effects on architectural properties and biomechanical response of the C57BL/6N mouse femur, tibia and ulna

Aging is known to reduce bone quality and bone strength. We sought to determine how aging affects the biomechanical and architectural properties of various long bones, and if sex influences age related differences/changes. While researchers have extensively studied these changes in individual bones of mice, there is no comprehensive study of the changes in the bones from the same mice to study the changes with aging. We performed three point bending tests and microcomputed tomography (microCT) analysis on femurs, tibiae and ulnae. Three point bending tests were utilized to calculate biomechanical parameters and imaging was also performed using high resolution microCT to reveal both cortical and trabecular microarchitecture C57BL/6N mice were divided into three age groups: 6, 12 and 22 months. Each age and sex group consisted of 6–7 mice. The ultimate load to failure (UL), elastic stiffness (ES), modulus of elasticity (E) and the moment of inertia about bending axis (MOI) for each bone was calculated using three point bending test. MicroCT scans of all the bones were analyzed to determine cortical bone volume per tissue volume (C.BV/TV), trabecular bone volume per tissue volume (Tb.BV/TV), cortical bone area (B.Ar) using CTAn's microCT analysis and tested for correlation with the biomechanical parameters. Mean (standard error) values of UL in femur decreased from 19.8(0.6) N to 12.8(1.1) N (p < .01) and 17.9(0.6) N to 14.6(1.0) N (p = .02) from 6 to 22 months groups in males and females respectively. Similarly, UL in tibia decreased from 19.8(0.5) N to 14.3(0.2) N (p < .01) and 14.4(0.6) N to 9.5(1.0) N (p < .01) from 6 to 22 months group in males and females respectively. ES in femur decreased from 113.2(7) N/mm to 69.6(6.7) N/mm (p < .01) from 6 to 22 months in males only. ES in tibia decreased from 78.6(3.2) N/mm to 65.0(2.3) N/mm (p = .01) and 53.1(2.9) N/mm to 44.0(1.7) N/mm (p = .02) from 6 to 22 months in males and females respectively. Interestingly, ES in ulna increased from 8.2(0.8) N/mm to 10.9(1.0) N/mm (p = .051) from 6 to 22 months of age in females only. E in femur decreased from 4.0(0.4) GPa to 2.8(0.2) GPa (p = .01) and 6.7(0.5) GPa to 4.5(0.4) GPa (p = .01) from 6 to 22 months of age in males and females respectively while tibia showed no change. However, E in ulna increased from 7.0(0.8) GPa to 11.0(1.1) GPa (p = .01) from 6 to 22 months of age in females only. Changes in age and sex-related bone properties were more pronounced in the femur and tibia, while the ulna showed fewer overall differences. Most of the changes were observed in biomechanical compared to architectural properties and female bones are more severely affected by aging. In conclusion, our data demonstrate that care must be taken to describe bone site and sex-specific, rather than making broad generalizations when describing age-related changes on the biomechanical and architectural properties of the skeleton.

•

Assess age/gender related biomechanical property changes in femur, tibiae and ulnae.

•

C57Bl/6 mice were divided into three age groups: 6, 12 and 22 months.

•

Significant decline in the biomechanical response of femurs and tibiae at the age of 22 months.

•

Ulnae showed some improved properties with aging.

•

Aging affected biomechanical rather than microCT properties and female bones

Elevations in Cortical Porosity Occur Prior to Significant Rise in Serum Parathyroid Hormone in Young Female Mice with Adenine-Induced CKD

Chronic kidney disease (CKD) leads to significant bone loss primarily through the development of cortical porosity. In both patients and animal models of CKD, sustained elevations in serum parathyroid hormone (PTH) are associated with cortical porosity. In this study, we aimed to track the progression of cortical porosity and increased PTH utilizing the adenine-induced CKD model. Young female mice (8 weeks) were given 0.2% adenine to induce CKD. Tissues were collected from groups of adenine and age-matched control mice after 2, 6, and 10 weeks. Serum blood urea nitrogen was elevated at all time points in adenine mice, but serum PTH was only statistically elevated at the 10-week time point. Cortical porosity was sevenfold higher in 6-week adenine mice compared to age-matched controls and 14-fold higher in 10-week adenine mice vs. controls. Additionally, osteocyte receptor activator of nuclear factor κB ligand (RANKL) was elevated in adenine-fed mice, while annexin V, an early marker of cellular apoptosis, was mildly decreased in osteocytes in adenine-fed mice. Based on these results, we hypothesize high serum PTH signals to osteocytes prolonging their lifespan resulting in sustained RANKL which drives osteoclastic bone resorption in the cortex. In conclusion, our data show time-dependent elevations in serum PTH and cortical porosity in adenine-induced CKD mice and demonstrate changes in osteocyte RANKL and apoptosis which may contribute to the development of cortical pores.

Osteocytes' expression of the PTH/PTHrP receptor has differing effects on endocortical and periosteal bone formation during adenine-induced CKD

Osteocytes play a key role in the pathophysiology of chronic kidney disease (CKD). However, the extent to which osteocytes contribute to abnormalities in bone turnover due to excessive levels of parathyroid hormone (PTH) remains poorly understood. The purpose of this study was to determine the extent to which bone formation and tissue strength during the progression of CKD is modified through osteocytes' response to PTH. Conditional knockout mice targeting osteocytes' expression of the PTH/PTH-related protein type 1 receptor (PPR) were subjected to adenine-induced CKD. After 6-weeks of treatment, adenine-induced CKD was found to reduce bone formation at the periosteal and endocortical surfaces of the tibia. The loss in bone mass corresponded with a significant decrease in structural-level mechanical properties. In knockout mice, the loss of PPR expression in osteocytes further exacerbated the loss in bone formation at the endocortical surface, but inhibited bone loss at the periosteal surface. In general, the effects of adenine-induced CKD were not as extensive in female mice. Collectively, these findings demonstrate that osteocytes' response to PTH under adenine-induced CKD has a unique impact on bone turnover that is specific to the periosteal and endocortical surfaces.

Spaceflight-Induced Bone Tissue Changes that Affect Bone Quality and Increase Fracture Risk

PURPOSE OF REVIEW

Bone mineral density and systemic factors are used to assess skeletal health in astronauts. Yet, even in a general population, these measures fail to accurately predict when any individual will fracture. This review considers how long-duration human spaceflight requires evaluation of additional bone structural and material quality measures that contribute to microgravity-induced skeletal fragility.

RECENT FINDINGS

In both humans and small animal models following spaceflight, bone mass is compromised via reduced bone formation and elevated resorption levels. Concurrently, bone structural quality (e.g., trabecular microarchitecture) is diminished and the quality of bone material is reduced via impaired tissue mineralization, maturation, and maintenance (e.g., mediated by osteocytes). Bone structural and material quality are both affected by microgravity and may, together, jeopardize astronaut operational readiness and lead to increased fracture risk upon return to gravitational loading. Future studies need to directly evaluate how bone quality combines with diminished bone mass to influence bone strength and toughness (e.g., resistance to fracture). Bone quality assessment promises to identify novel biomarkers and therapeutic targets.

The age-related decrease in material properties of BALB/c mouse long bones involves alterations to the extracellular matrix

One possibility for the disproportionate increase in fracture risk with aging relative to the decrease in bone mass is an accumulation of changes to the bone matrix which deleteriously affect fracture resistance. In order to effectively develop new targets for osteoporosis, a preclinical model of the age-related loss in fracture resistance needs to be established beyond known age-related decreases in bone mineral density and bone volume fraction. To that end, we examined long bones of male and female BALB/c mice at 6-mo. and 20-mo. of age and assessed whether material and matrix properties of cortical bone significantly differed between the age groups. The second moment of area of the diaphysis (minimum and maximum principals for femur and radius, respectively) as measured by ex vivo micro-computed tomography (μCT) was higher at 20-mo. than at 6-mo. for both males and females, but ultimate moment as measured by three-point bending tests did not decrease with age. Cortical thickness was lower with age for males, but higher for old females. Partially accounting for differences in structure, material estimates of yield, ultimate stress, and toughness (left femur) were 12.6%, 11.1%, and 40.9% lower, respectively, with age for both sexes. The ability of the cortical bone to resist crack growth (right femur) was also 18.1% less for the old than for the young adult mice. These decreases in material properties were not due to changes in intracortical porosity as pore number decreased with age. Rather, age-related alterations in the matrix were observed for both sexes: enzymatic and non-enzymatic crosslinks by high performance liquid chromatography increased (femur), volume fraction of bound water by 1H-nuclear magnetic resonance relaxometry decreased (femur), cortical tissue mineral density by μCT increased (femur and radius), and an Amide I sub-peak ratio I1670/I1640 by Raman spectroscopy increased (tibia). Overall, there are multiple matrix changes to potentially target that could prevent the age-related decrease in fracture resistance observed in BALB/c mouse.