Aging, Osteocytes, and Mechanotransduction

Prkd1 regulates the formation and repair of plasma membrane disruptions (PMD) in osteocytes

We and others have seen that osteocytes sense high-impact osteogenic mechanical loading via transient plasma membrane disruptions (PMDs) which initiate downstream mechanotransduction. However, a PMD must be repaired for the cell to survive this wounding event. Previous work suggested that the protein Prkd1 (also known as PKCμ) may be a critical component of this PMD repair process, but the specific role of Prkd1 in osteocyte mechanobiology had not yet been tested. We treated MLO-Y4 osteocytes with Prkd1 inhibitors (Go6976, kbNB 142-70, staurosporine) and generated an osteocyte-targeted (Dmp1-Cre) Prkd1 conditional knockout (CKO) mouse. PMD repair rate was measured via laser wounding and FM1-43 dye uptake, PMD formation and post-wounding survival were assessed via fluid flow shear stress (50 dyn/cm2), and in vitro osteocyte mechanotransduction was assessed via measurement of calcium signaling. To test the role of osteocyte Prkd1 in vivo, Prkd1 CKO and their wildtype (WT) littermates were subjected to 2 weeks of unilateral axial tibial loading and loading-induced changes in cortical bone mineral density, geometry, and formation were measured. Prkd1 inhibition or genetic deletion slowed osteocyte PMD repair rate and impaired post-wounding cell survival. These effects could largely be rescued by treating osteocytes with the FDA-approved synthetic copolymer Poloxamer 188 (P188), which was previously shown to facilitate membrane resealing and improve efficiency in the repair rate of PMD in skeletal muscle myocytes. In vivo, while both WT and Prkd1 CKO mice demonstrated anabolic responses to tibial loading, the magnitude of loading-induced increases in tibial BMD, cortical thickness, and periosteal mineralizing surface were blunted in Prkd1 CKO as compared to WT mice. Prkd1 CKO mice also tended to show a smaller relative difference in the number of osteocyte PMD in loaded limbs and showed greater lacunar vacancy, suggestive of impaired post-wounding osteocyte survival. While P188 treatment rescued loading-induced increases in BMD in the Prkd1 CKO mice, it surprisingly further suppressed loading-induced increases in cortical bone thickness and cortical bone formation. Taken together, these data suggest that Prkd1 may play a pivotal role in the regulation and repair of the PMD response in osteocytes and support the idea that PMD repair processes can be pharmacologically targeted to modulate downstream responses, but suggest limited utility of PMD repair-promoting P188 in improving bone anabolic responses to loading.

Aging decreases osteocyte peri-lacunar-canalicular system turnover in female C57BL/6JN mice

Osteocytes engage in bone resorption and mineralization surrounding their expansive lacunar-canalicular system (LCS) through peri-LCS turnover. However, fundamental questions persist about where, when, and how often osteocytes engage in peri-LCS turnover and how these processes change with aging. Furthermore, whether peri-LCS turnover is associated with natural variation in cortical tissue strain remains unexplored. To address these questions, we utilized confocal scanning microscopy, immunohistochemistry, and scanning electron microscopy to characterize osteocyte peri-LCS turnover in the cortical (mid-diaphysis) and cancellous (metaphysis) regions of femurs from young adult (5 mo) and early-old-age (22 mo) female C57BL/6JN mice. LCS bone mineralization was measured by the presence of perilacunar fluorochrome labels. LCS bone resorption was measured by immunohistochemical marker of bone resorption. The dynamics of peri-LCS turnover were estimated from serial fluorochrome labeling, where each mouse was administered two labels between 2 and 16 days before euthanasia. Osteocyte participation in mineralizing their surroundings is highly abundant in both cortical and cancellous bone of young adult mice but significantly decreases with aging. LCS bone resorption also decreases with aging. Aging has a greater impact on peri-LCS turnover dynamics in cancellous bone than in cortical bone. Lacunae with recent peri-LCS turnover are larger in both age groups. While peri-LCS turnover is associated with variation in tissue strain between cortical quadrants and intracortical location for 22 mo mice, these associations were not seen for 5 mo mice. The impact of aging on decreasing peri-LCS turnover may have significant implications for bone quality and mechanosensation.

Propelling Minimally Invasive Tissue Regeneration With Next-Era Injectable Pre-Formed Scaffolds

The growing aging population, with its associated chronic diseases, underscores the urgency for effective tissue regeneration strategies. Biomaterials play a pivotal role in the realm of tissue reconstruction and regeneration, with a distinct shift toward minimally invasive (MI) treatments. This transition, fueled by engineered biomaterials, steers away from invasive surgical procedures to embrace approaches offering reduced trauma, accelerated recovery, and cost-effectiveness. In the realm of MI tissue repair and cargo delivery, various techniques are explored. While in situ polymerization is prominent, it is not without its challenges. This narrative review explores diverse biomaterials, fabrication methods, and biofunctionalization for injectable pre-formed scaffolds, focusing on their unique advantages. The injectable pre-formed scaffolds, exhibiting compressibility, controlled injection, and maintained mechanical integrity, emerge as promising alternative solutions to in situ polymerization challenges. The conclusion of this review emphasizes the importance of interdisciplinary design facilitated by synergizing fields of materials science, advanced 3D biomanufacturing, mechanobiological studies, and innovative approaches for effective MI tissue regeneration.

Gut microbial alterations in arginine metabolism determine bone mechanical adaptation

Although mechanical loading is essential for maintaining bone health and combating osteoporosis, its practical application is limited to a large extent by the high variability in bone mechanoresponsiveness. Here, we found that gut microbial depletion promoted a significant reduction in skeletal adaptation to mechanical loading. Among experimental mice, we observed differences between those with high and low responses to exercise with respect to the gut microbial composition, in which the differential abundance of Lachnospiraceae contributed to the differences in bone mechanoresponsiveness. Microbial production of L-citrulline and its conversion into L-arginine were identified as key regulators of bone mechanoadaptation, and administration of these metabolites enhanced bone mechanoresponsiveness in normal, aged, and ovariectomized mice. Mechanistically, L-arginine-mediated enhancement of bone mechanoadaptation was primarily attributable to the activation of a nitric-oxide-calcium positive feedback loop in osteocytes. This study identifies a promising anti-osteoporotic strategy for maximizing mechanical loading-induced skeletal benefits via the microbiota-metabolite axis.

Eldecalcitol protected osteocytes against ferroptosis of D-gal-induced senescent MLO-Y4 cells and ovariectomized mice

BACKGROUND

Active vitamin D analog eldecalcitol is clinically applied in treatment of postmenopausal osteoporosis. This study aims to determine the role of eldecalcitol in the protection of osteocytes from senescence and the associated ferroptosis.

METHODS

The MLO-Y4 osteocytes were exposed to D-gal inducing senescence. The ovariectomized (OVX) mice treated with D-gal using as an aging inducer were intraperitoneally injected with eldecalcitol. The multiplexed confocal imaging, fluorescence in situ hybridization and transmission electron microscopy were applied in assessing osteocytic properties. Immunochemical staining and immunoblotting were carried out to detect abundance and expression of molecules.

RESULTS

The ablation of vitamin D receptor led to a reduction in amounts of osteocytes, a loss of dendrites, an increase in mRNA expression of SASP factors and in protein expression of senescent factors, as well as changes in mRNA expression of ferroptosis-related genes (PTGS2 & RGS4). Eldecalcitol reversed senescent phenotypes of MLO-Y4 cells shown by improving cell morphology and density, decreasing β-gal-positive cell accumulation, and down-regulating protein expression (P16, P21 & P53). Eldecalcitol reduced intracellular ROS and MDA productions, elevated JC-1 aggregates, and up-regulated expression of Nrf2 and GPX4. Eldecalcitol exhibited osteopreserve effects in D-gal-induced aging OVX mice. The confocal imaging displayed its improvement on osteocytic network organization. Eldecalcitol decreased the numbers of senescent osteocytes at tibial diaphysis by SADS assay and attenuated mRNA expression of SASP factors as well as down-regulated protein expression of senescence-related factors and restored levels of ferroptotic biomarkers in osteocytes-enriched bone fraction. It reduced 4-HNE staining area, stimulated Nrf2-positive staining, and promoted nuclear translocation of Nrf2 in osteocytes of mice as well as inhibited and promoted protein expression of 4-HNE and Nrf2, respectively, in osteocytes-enriched bone fraction.

CONCLUSIONS

The present study revealed the ameliorative effects of eldecalcitol on senescence and the associated ferroptosis of osteocytes, contributing to its preservation against osteoporosis of D-gal-induced senescent ovariectomized mice.

Pronounced cortical porosity and sex-specific patterns of increased bone and osteocyte lacunar mineralization characterize the human distal fibula with aging

The high occurrence of distal fibula fractures among older women suggests a potential link to impaired bone health. Here we used a multiscale imaging approach to investigate the microarchitecture, mineralization, and biomechanics of the human distal fibula in relation to age and sex. Micro-computed tomography was performed to analyze the local volumetric bone mineral density and various microarchitectural parameters of the trabecular and the cortical compartment. Bone mineral density distribution and osteocyte lacunar parameters were quantified using quantitative backscattered electron imaging in periosteal, endocortical, and trabecular regions. Additionally, cortical hardness and Young's modulus were assessed by nanoindentation. While cortical porosity strongly increased with age independent of sex, trabecular microarchitecture remained stable. Notably, nearly half of the specimens showed non-bony hypermineralized tissue located at the periosteum, similar to that previously detected in the femoral neck, with no consistent association with advanced age. Independent of this finding, cortical and trabecular mineralization, i.e., mean calcium content, as well as endocortical tissue hardness increased with age in males but not females. Importantly, we also observed mineralized osteocyte lacunae that increased with age specifically in females. In conclusion, our results indicate that skeletal aging of the distal fibula is signified not only by pronounced cortical porosity but also by an increase in mineralized osteocyte lacunae in females. These findings may provide an explanation for the increased occurrence of ankle fractures in older women.

Guiding bone cell network formation in 3D via photosensitized two-photon ablation

A long-standing challenge in skeletal tissue engineering is to reconstruct a three-dimensionally (3D) interconnected bone cell network in vitro that mimics the native bone microarchitecture. While conventional hydrogels are extensively used in studying bone cell behavior in vitro, current techniques lack the precision to manipulate the complex pericellular environment found in bone. The goal of this study is to guide single bone cells to form a 3D network in vitro via photosensitized two-photon ablation of microchannels in gelatin methacryloyl (GelMA) hydrogels. A water-soluble two-photon photosensitizer (P2CK) was added to soft GelMA hydrogels to enhance the ablation efficiency. Remarkably, adding 0.5 mM P2CK reduced the energy dosage threshold five-fold compared to untreated controls, enabling more cell-compatible ablation. By employing low-energy ablation (100 J/cm2) with a grid pattern of 1 µm wide and 30 µm deep microchannels, we induced dendritic outgrowth in human mesenchymal stem cells (hMSC). After 7 days, the cells successfully utilized the microchannels and formed a 3D network. Our findings reveal that cellular viability after low-energy ablation was comparable to unablated controls, whereas high-energy ablation (500 J/cm2) resulted in 42 % cell death. Low-energy grid ablation significantly promoted network formation and >40 µm long protrusion outgrowth. While the broad-spectrum matrix metalloproteinase inhibitor (GM6001) reduced cell spreading by inhibiting matrix degradation, cells invaded the microchannel grid with long protrusions. Collectively, these results emphasize the potential of photosensitized two-photon hydrogel ablation as a high-precision tool for laser-guided biofabrication of 3D cellular networks in vitro. STATEMENT OF SIGNIFICANCE: The inaccessible nature of osteocyte networks in bones renders fundamental research on skeletal biology a major challenge. This limit is partly due to the lack of high-resolution tools that can manipulate the pericellular environment in 3D cultures in vitro. To create bone-like cellular networks, we employ a two-photon laser in combination with a two-photon sensitizer to erode microchannels with low laser dosages into GelMA hydrogels. By providing a grid of microchannels, the cells self-organized into a 3D interconnected network within days. Laser-guided formation of 3D networks from single cells at micron-scale resolution is demonstrated for the first time. In future, we envisage in vitro generation of bone cell networks with user-dictated morphologies for both fundamental and translational bone research.

Imaging of joint response to exercise with MRI and PET

Imaging of the joint in response to loading stress may provide additional measures of joint structure and function beyond conventional, static imaging studies. Exercise such as running, stair climbing, and squatting allows evaluation of the joint response to larger loading forces than during weight bearing. Quantitative MRI (qMRI) may assess properties of cartilage and meniscus hydration and organization in vivo that have been investigated to assess the functional response of these tissues to physiological stress. [18F]sodium fluoride ([18F]NaF) interrogates areas of newly mineralizing bone and provides an opportunity to study bone physiology, including perfusion and mineralization rate, as a measure of joint loading stress. In this review article, methods utilizing quantitative MRI, PET, and hybrid PET-MRI systems for assessment of the joint response to loading from exercise in vivo are examined. Both methodology and results of various studies performed are outlined and discussed. Lastly, the technical considerations, challenges, and future opportunities for these approaches are addressed.

Effects of Moderate- to High-Impact Exercise Training on Bone Structure Across the Lifespan: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

Moderate- to high-impact exercise improves bone mineral density (BMD) across the lifespan, but its effects on bone structure, which predicts fracture independent of areal BMD, are unclear. This systematic review and meta-analysis investigated effects of impact exercise on volumetric BMD (vBMD) and bone structure. Four databases (PubMed, Embase, SPORTDiscus, Web of Science) were searched up to March 2022 for randomized controlled trials (RCTs) investigating the effects of impact exercise, with ground reaction forces equal to or greater than running, compared with sham or habitual activity, on bone vBMD and structure. Bone variables were measured by quantitative computed tomography or magnetic resonance imaging at the tibia, radius, lumbar spine, and femur. Percentage changes in bone variables were compared among groups using mean differences (MD) and 95% confidence intervals (CI) calculated via random effects meta-analyses. Subgroup analyses were performed in children/adolescents (<18 years), adults (18-50 years), postmenopausal women, and older men. Twenty-eight RCTs (n = 2985) were included. Across all studies, impact exercise improved trabecular vBMD at the distal tibia (MD = 0.54% [95% CI 0.17, 0.90%]), total vBMD at the proximal femur (3.11% [1.07, 5.14%]), and cortical thickness at the mid/proximal radius (1.78% [0.21, 3.36%]). There was no effect on vBMD and bone structure at the distal radius, femoral shaft, or lumbar spine across all studies or in any subgroup. In adults, impact exercise decreased mid/proximal tibia cortical vBMD (-0.20% [-0.24, -0.15%]). In postmenopausal women, impact exercise improved distal tibia trabecular vBMD (0.79% [0.32, 1.25%]). There was no effect on bone parameters in children/adolescents in overall analyses, and there were insufficient studies in older men to perform meta-analyses. Impact exercise may have beneficial effects on bone structure and vBMD at various skeletal sites, but additional high-quality RCTs in different age and sex subgroups are needed to identify optimal exercise protocols for improving bone health across the lifespan. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Microvascular Disease Associates with Larger Osteocyte Lacunae in Cortical Bone in Type 2 Diabetes Mellitus

Clinical studies indicate that microvascular disease (MVD) affects bone microstructure and decreases bone strength in type 2 diabetes mellitus (T2D). Osteocytes are housed in small voids within the bone matrix and lacunae and act as sensors of mechanical forces in bone. These cells regulate osteoclastic bone resorption and osteoblastic bone formation as well as osteocytic perilacunar remodeling. We hypothesized that MVD changes morphometric osteocyte lacunar parameters in individuals with T2D. We collected iliac crest bone biopsies from 35 individuals (10 female, 25 male) with T2D with MVD (15%) or without MVD (21%) with a median age of 67 years (interquartile range [IQR] 62-72 years). The participants were included based on c-peptide levels >700 pmol L-1, absence of anti-GAD65 antibodies, and glycated hemoglobin (HbA1c) levels between 40 and 82 mmol mol-1 or 5.8% and 9.7%, respectively. We assessed osteocyte lacunar morphometric parameters in trabecular and cortical bone regions using micro-computed tomography (micro-CT) at a nominal resolution of 1.2 μm voxel size. The cortical osteocyte lacunar volume (Lc.V) was 7.7% larger (p = 0.05) and more spherical (Lc.Sr, p < 0.01) in the T2D + MVD group. Using linear regression, we found that lacunar density (Lc.N/BV) in trabecular but not cortical bone was associated with HbA1c (p < 0.05, R 2 = 0.067) independently of MVD. Furthermore, Lc.V was larger and Lc.Sr higher in the center than in the periphery of the trabecular and cortical bone regions (p < 0.05). In conclusion, these data imply that MVD may impair skeletal integrity, possibly contributing to increased skeletal fragility in T2D complicated by MVD. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Large scale analysis of osteocyte lacunae in klotho hypomorphic mice using high-resolution micro-computed tomography

BACKGROUND

Osteocytes are the most abundant cell type in adult bone, and the morphological characteristics of osteocytes and their lacunae appear to influence bone mass and fragility. Although conventional computed tomography (CT) has contributed greatly to advances in bone morphometry, capturing details of the entire hierarchical assembly, e.g., osteocyte lacuna parameters, has been limited by the analytical performance of CT (> 1 µm resolution).

METHODS

We used high-resolution (700 nm) micro-CT to evaluate and compare the osteocyte lacuna parameters over a large scale, i.e., in a maximum of about 45,700 lacunae (average), in tibial metaphyseal cortical bones of wild-type (WT) and αKlotho-hypomorphic (kl/kl) mice, the latter a model that exhibits osteopenia and aberrant osteocytes.

RESULTS

Of osteocyte lacuna parameters, lacunar surface per lacunar volume were significantly lower and lacuna diameter were significantly larger in kl/kl mice compared to WT mice. By analysis of individual osteocyte lacunae, we found that lacunar sphericity in kl/kl mice was higher than that in WT mice, and the diameters in the major and the minor axes were respectively lower and higher in kl/kl mice, especially at the proximal site of the region of interest.

CONCLUSION

We successfully assessed osteocyte lacuna parameters on the largest scale in mice reported to date and found that the shape of osteocyte lacunae of kl/kl mice are significantly different from those of WT mice. Although the mechanisms underlying the lacunar shape differences observed are not yet clear, changes in lacunar geometry are known to affect the transitions of strains to the osteocyte microenvironment and likely local osteocyte response(s). Thus, the fact that the differences are limited to the mesial region near the primary spongiosa suggests the likelihood of site-specific anomalies in mechanosensitive effects in kl/kl osteocytes with consequent site-specific effects bone metabolism and function.

Drug Delivery and Therapy Strategies for Osteoporosis Intervention

With the advent of the aging society, osteoporosis (OP) risk increases yearly. Currently, the clinical usage of anti-OP drugs is challenged by recurrent side effects and poor patient compliance, regardless of oral, intravenous, or subcutaneous administration. Properly using a drug delivery system or formulation strategy can achieve targeted drug delivery to the bone, diminish side effects, improve bioavailability, and prolong the in vivo residence time, thus effectively curing osteoporosis. This review expounds on the pathogenesis of OP and the clinical medicaments used for OP intervention, proposes the design approach for anti-OP drug delivery, emphatically discusses emerging novel anti-OP drug delivery systems, and enumerates anti-OP preparations under clinical investigation. Our findings may contribute to engineering anti-OP drug delivery and OP-targeting therapy.

Telomere length as a marker of changes in body composition and fractures-an analysis of data from the NHANES 2001-2002

Purpose

There has been an association between changes in body composition, fracture incidence, and age in previous studies. Telomere length (TL) has been proposed as a biomarker of aging. However, the relationship between body composition, fractures, and TL has rarely been studied. Therefore, this study aimed to investigate the correlation between TL and body composition and fractures.Patients and methods: 20950 participants from the 2001-2002 National Health and Nutrition Examination Survey (NHANES) were included in the final analysis. In NHANES, body compositions were measured with DXA, and TL was determined with quantitative PCR. Correlation analysis of TL and body composition was conducted using multivariate weighted linear regression and logistic regression models.

Results

The results showed that TL positively correlated with bone mineral density (BMD) and bone mineral content (BMC) in most body parts. However, BMD and BMC were negatively connected with TL in the upper limbs and skull. Fat content was negatively associated with TL, while muscle content was positively linked to TL. In addition, TL's trend analysis results were consistent with the regression model when transformed from a continuous to a classified variable. An increase in TL was associated with a higher incidence of wrist fractures, while a decrease in spine fractures. The above correlation also has a certain degree of sex specificity.

Conclusion

Our study indicate that TL is associated with body composition as well as fractures, but further research is needed to confirm these contrasting associations in the skull, upper limbs, and wrists.

The influence of different shaped osteocyte lacunae on microcrack initiation and propagation

BACKGROUND

The morphology of osteocyte lacunae varies in bones of different ages and bone pathologies. Osteocyte lacunae can cause stress concentration and initiate microcracks. However, the influence of changes in osteocyte lacunar shape on microcrack is unknown. Therefore, the aim of this study was to determine the effects of osteocyte lacunae with different shapes on microcrack initiation and propagation.

METHODS

Osteon models containing osteocyte lacunae with different shapes were established. The progressive damage analysis method, based on computer simulations, was used to study the evolution of microdamage within the osteon, including the processes of intralaminar and interlaminar microdamage.

FINDINGS

Models with larger DoE values can effectively delay or prevent the formation of linear microcracks, which ensures high fracture toughness of cortical bone. It is subjected to stronger mechanical stimulation, making it more sensitive to loads. Models with smaller DoE values increase the load threshold for microdamage generation and reduces its impact on bone mechanical performance, making it less susceptible to microdamage than models with larger DoE values.

INTERPRETATION

These findings enhance the limited knowledge of the influence of the lacunar shape on microdamage and contribute to a better understanding of bone biomechanics.

Radiation Induces Bone Microenvironment Disruption by Activating the STING-TBK1 Pathway

Background and Objectives: Damage to normal bone tissue following therapeutic irradiation (IR) represents a significant concern, as IR-induced bone microenvironment disruption can cause bone loss and create a more favorable environment for tumor metastases. The aim of the present study was to explore the cellular regulatory mechanism of IR-induced bone microenvironment disruption to effectively prevent radiotherapy-associated adverse effects in the future. Materials and Methods: In this study, a mouse model of local IR was established via local irradiation of the left hind limb of BALB/c mice with 12 Gy X-rays, and an in vitro osteocyte (OCY) model was established by exposing osteocyte-like MLO-Y4 cells to 2, 4, and 8 Gy irradiation to analyze multicellular biological injuries and cellular senescence. Small interfering RNA (siRNA) transfection at the cellular level and a selective antagonist intervention C-176 at the animal level were used to explore the potential role of the stimulator of interferon genes (STING) on IR-induced bone microenvironment disruption. Results: The results showed that 12 Gy local IR induces multicellular dysfunction, manifested as ascension of OCYs exfoliation, activation of osteoclastogenesis, degeneration of osteogenesis and fate conversion of adipogenesis, as well as cellular senescence and altered senescence-associated secretory phenotype (SASP) secretion. Furthermore, the expression of STING was significantly elevated, both in the primary OCYs harvested from locally irradiated mice and in vitro irradiated MLO-Y4 cells, accompanied by the markedly upregulated levels of phosphorylated TANK-binding kinase 1 (P-TBK1), RANKL and sclerostin (SOST). STING-siRNA transfection in vitro restored IR-induced upregulated protein expression of P-TBK1 and RANKL, as well as the mRNA expression levels of inflammatory cytokines, such as IL-1α, IL-6 and NF-κB, accompanied by the alleviation of excessive osteoclastogenesis. Finally, administration of the STING inhibitor C-176 mitigated IR-induced activation of osteoclastogenesis and restraint of osteogenesis, ameliorating the IR-induced biological damage of OCYs, consistent with the inhibition of P-TBK1, RANKL and SOST. Conclusions: The STING-P-TBK1 signaling pathway plays a crucial role in the regulation of the secretion of inflammatory cytokines and osteoclastogenesis potential in IR-induced bone microenvironment disruption. The selective STING antagonist can be used to intervene to block the STING pathway and, thereby, repair IR-induced multicellular biological damage and mitigate the imbalance between osteoclastogenesis and osteoblastgenesis.

Hypochondroplasia gain-of-function mutation in FGFR3 causes defective bone mineralization in mice

Hypochondroplasia (HCH) is a mild dwarfism caused by missense mutations in fibroblast growth factor receptor 3 (FGFR3), with the majority of cases resulting from a heterozygous p.Asn540Lys gain-of-function mutation. Here, we report the generation and characterization of the first mouse model (Fgfr3Asn534Lys/+) of HCH to our knowledge. Fgfr3Asn534Lys/+ mice exhibited progressive dwarfism and impairment of the synchondroses of the cranial base, resulting in defective formation of the foramen magnum. The appendicular and axial skeletons were both severely affected and we demonstrated an important role of FGFR3 in regulation of cortical and trabecular bone structure. Trabecular bone mineral density (BMD) of long bones and vertebral bodies was decreased, but cortical BMD increased with age in both tibiae and femurs. These results demonstrate that bones in Fgfr3Asn534Lys/+ mice, due to FGFR3 activation, exhibit some characteristics of osteoporosis. The present findings emphasize the detrimental effect of gain-of-function mutations in the Fgfr3 gene on long bone modeling during both developmental and aging processes, with potential implications for the management of elderly patients with hypochondroplasia and osteoporosis.

CT-based radiomics can identify physiological modifications of bone structure related to subjects' age and sex

PURPOSE

Radiomics of vertebral bone structure is a promising technique for identification of osteoporosis. We aimed at assessing the accuracy of machine learning in identifying physiological changes related to subjects' sex and age through analysis of radiomics features from CT images of lumbar vertebrae, and define its generalizability across different scanners.

MATERIALS AND METHODS

We annotated spherical volumes-of-interest (VOIs) in the center of the vertebral body for each lumbar vertebra in 233 subjects who had undergone lumbar CT for back pain on 3 different scanners, and we evaluated radiomics features from each VOI. Subjects with history of bone metabolism disorders, cancer, and vertebral fractures were excluded. We performed machine learning classification and regression models to identify subjects' sex and age respectively, and we computed a voting model which combined predictions.

RESULTS

The model was trained on 173 subjects and tested on an internal validation dataset of 60. Radiomics was able to identify subjects' sex within single CT scanner (ROC AUC: up to 0.9714), with lower performance on the combined dataset of the 3 scanners (ROC AUC: 0.5545). Higher consistency among different scanners was found in identification of subjects' age (R2 0.568 on all scanners, MAD 7.232 years), with highest results on a single CT scanner (R2 0.667, MAD 3.296 years).

CONCLUSION

Radiomics features are able to extract biometric data from lumbar trabecular bone, and determine bone modifications related to subjects' sex and age with great accuracy. However, acquisition from different CT scanners reduces the accuracy of the analysis.

Insights into the Molecular and Hormonal Regulation of Complications of X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is characterized by mutations in the PHEX gene, leading to elevated serum levels of FGF23, decreased production of 1,25 dihydroxyvitamin D3 (1,25D), and hypophosphatemia. Those affected with XLH manifest impaired growth and skeletal and dentoalveolar mineralization as well as increased mineralization of the tendon–bone attachment site (enthesopathy), all of which lead to decreased quality of life. Many molecular and murine studies have detailed the role of mineral ions and hormones in regulating complications of XLH, including how they modulate growth and growth plate maturation, bone mineralization and structure, osteocyte-mediated mineral matrix resorption and canalicular organization, and enthesopathy development. While these studies have provided insight into the molecular underpinnings of these skeletal processes, current therapies available for XLH do not fully prevent or treat these complications. Therefore, further investigations are needed to determine the molecular pathophysiology underlying the complications of XLH.

Molecular Basis beyond Interrelated Bone Resorption/Regeneration in Periodontal Diseases: A Concise Review

Periodontitis is the sixth most common chronic inflammatory disease, destroying the tissues supporting the teeth. There are three distinct stages in periodontitis: infection, inflammation, and tissue destruction, where each stage has its own characteristics and hence its line of treatment. Illuminating the underlying mechanisms of alveolar bone loss is vital in the treatment of periodontitis to allow for subsequent reconstruction of the periodontium. Bone cells, including osteoclasts, osteoblasts, and bone marrow stromal cells, classically were thought to control bone destruction in periodontitis. Lately, osteocytes were found to assist in inflammation-related bone remodeling besides being able to initiate physiological bone remodeling. Furthermore, mesenchymal stem cells (MSCs) either transplanted or homed exhibit highly immunosuppressive properties, such as preventing monocytes/hematopoietic precursor differentiation and downregulating excessive release of inflammatory cytokines. In the early stages of bone regeneration, an acute inflammatory response is critical for the recruitment of MSCs, controlling their migration, and their differentiation. Later during bone remodeling, the interaction and balance between proinflammatory and anti-inflammatory cytokines could regulate MSC properties, resulting in either bone formation or bone resorption. This narrative review elaborates on the important interactions between inflammatory stimuli during periodontal diseases, bone cells, MSCs, and subsequent bone regeneration or bone resorption. Understanding these concepts will open up new possibilities for promoting bone regeneration and hindering bone loss caused by periodontal diseases.

Cellular and Molecular Mechanisms Associating Obesity to Bone Loss

Obesity is an alarming disease that favors the upset of other illnesses and enhances mortality. It is spreading fast worldwide may affect more than 1 billion people by 2030. The imbalance between excessive food ingestion and less energy expenditure leads to pathological adipose tissue expansion, characterized by increased production of proinflammatory mediators with harmful interferences in the whole organism. Bone tissue is one of those target tissues in obesity. Bone is a mineralized connective tissue that is constantly renewed to maintain its mechanical properties. Osteoblasts are responsible for extracellular matrix synthesis, while osteoclasts resorb damaged bone, and the osteocytes have a regulatory role in this process, releasing growth factors and other proteins. A balanced activity among these actors is necessary for healthy bone remodeling. In obesity, several mechanisms may trigger incorrect remodeling, increasing bone resorption to the detriment of bone formation rates. Thus, excessive weight gain may represent higher bone fragility and fracture risk. This review highlights recent insights on the central mechanisms related to obesity-associated abnormal bone. Publications from the last ten years have shown that the main molecular mechanisms associated with obesity and bone loss involve: proinflammatory adipokines and osteokines production, oxidative stress, non-coding RNA interference, insulin resistance, and changes in gut microbiota. The data collection unveils new targets for prevention and putative therapeutic tools against unbalancing bone metabolism during obesity.

Histomorphometric analysis of osteocyte density and trabecular structure of 92 vertebral bodies of different ages and genders

BACKGROUND

Knowledge of the histomorphometric structure of the vertebral body and factors influencing the structure is essential for a fundamental understanding of osteoporosis and osteoporotic fractures. The present study is focused on osteocyte density - a parameter seldom investigated so far - and trabecular width as well as bone area over tissue area in human vertebral bodies.

METHODS

Ninety-two vertebral body specimens (C5, C6, Th8, Th12, L1, L2) from 12 males and seven females were studied (Ethics Application Number A 2017-0072). The prepared vertebral specimens were extracted from the ventral aspect with a Jamshidi needle®. The punches were decalcified and subsequently H&E stained. Using the Fiji/Image J program (version 1.53 f, Wayne Resband, National Institute of Mental Health, USA), osteocyte numbers were counted per calcified bone surface, and the trabecular width and bone area of trabecular bone were measured. The collected data were analyzed using the statistical software package SPSS, version 23.0 (SPSS Inc., Chicago, USA). Pearson's correlation coefficient was used for correlation analyses. Multiple linear regression analyses were also performed.

RESULTS

Osteocyte density did not differ significantly in comparisons based on gender and age (≤65 years; ≥66 years). Men had wider trabeculae (p < 0.001) and a higher bone area over tissue area (BA/TA, %) (p = 0.025) than women. Individuals over 65 years of age had thinner trabeculae (p < 0.001) and a smaller BA/TA (%) (p < 0.001) than younger individuals. Multiple linear regression analyses were performed to determine the influence of 'gender' and 'age' on trabecular width and bone area over tissue area. The R² was 0.388 for trabecular width and 0.227 for BA/TA (%). Per year of life, trabecular width decreases by 0.368 µm (β < 0.001) and BA/TA (%) by 0.001% (β = 0.001). Men have on average 8.2 µm wider trabeculae than women (β = 0.035). A negative correlation (r = -0.275) was observed between trabecular width and osteocyte density. The wider the trabeculae, the fewer osteocytes per mm² (p = 0.008).

CONCLUSIONS

Surprisingly, we found no difference in osteocyte density with reference to age or gender. However, we did register significant age- and gender-related differences in bone area over tissue area and trabecular thickness. The age-related differences were more pronounced, implying that age-dependent loss of bone structure may be more important than differences between genders.

Bone Cells Metabolic Changes Induced by Ageing

Bone is a living organ that exhibits active metabolic processes, presenting constant bone formation and resorption. The bone cells that maintain local homeostasis are osteoblasts, osteoclasts, osteocytes and bone marrow stem cells, their progenitor cells. Osteoblasts are the main cells that govern bone formation, osteoclasts are involved in bone resorption, and osteocytes, the most abundant bone cells, also participate in bone remodeling. All these cells have active metabolic activities, are interconnected and influence each other, having both autocrine and paracrine effects. Ageing is associated with multiple and complex bone metabolic changes, some of which are currently incompletely elucidated. Ageing causes important functional changes in bone metabolism, influencing all resident cells, including the mineralization process of the extracellular matrix. With advancing age, a decrease in bone mass, the appearance of specific changes in the local microarchitecture, a reduction in mineralized components and in load-bearing capacity, as well as the appearance of an abnormal response to different humoral molecules have been observed. The present review points out the most important data regarding the formation, activation, functioning, and interconnection of these bone cells, as well as data on the metabolic changes that occur due to ageing.

Changes in adipose bone marrow and bone morphology in X-linked hypophosphatemic rickets

INTRODUCTION

X-linked hypophosphatemic (XLH) rickets causes significant bone deformities in the lower limbs resulting from a bone mineralization defect. According to Frost's Mechanostat theory, compensatory modeling of the bones takes place during increased mechanical loads. In addition, mechanical stimuli modulate the differentiation of mesenchymal stem cells; common precursors to bone marrow adipocytes and osteoblasts.

HYPOTHESIS

Bone deformities of the lower limbs lead to increased femoral bone mass and decreased fatty infiltration of the bone marrow (FIBM) in children with XLH rickets compared to a control group.

PATIENTS AND METHODS

Eleven children (10.3years [6-17]) with XLH rickets and 22 healthy children (10.2years [5-15.5]) underwent lower limb Magnetic Resonance Imaging. A calculation of FIBM was performed at the mid-femur, as well as a calculation of the total bone cross-sectional area (CSA), the cortical CSA, the anteroposterior and mediolateral axes of the femur, bone marrow and the thickness of the femoral cortices.

RESULTS

Total bone CSA, total cortical CSA and bone marrow CSA were higher in the XLH group than in the control group (p<0.05). The mid-lateral diameters of the femur and bone marrow were more elongated than those of the control group (p<0.001). Only the anterior cortex was thinned in the XLH group (p=0.001), while there was no difference with the control group for the posterior, medial and lateral cortices. The total percentage of FIBM was 72.81% [±3.95] and 77.4% [±5.52] for the XLH and control groups respectively (p<0.001).

DISCUSSION

The increase in bone mass in the XLH population reflects an adaptation of bone tissue to the bone deformities present in this pathology. The decrease in FIBM indicates a lower risk of osteoporosis in the XLH population and may constitute a new monitoring parameter in this pathology.

LEVEL OF STUDY

III; Case-control study.

Microfluidic Co-culture Platforms for Studying Osteocyte Regulation of Other Cell Types under Dynamic Mechanical Stimulation

PURPOSE OF REVIEW

Osteocytes are the most abundant cell type in bone. These unique cells act primarily as mechanosensors and play crucial roles in the functional adaptation of bone tissue. This review aims to summarize the recent microfluidic studies on mechanically stimulated osteocytes in regulating other cell types.

RECENT FINDINGS

Microfluidics is a powerful technology that has been widely employed in recent years. With the advantages of microfluidic platforms, researchers can mimic multicellular environments and integrate dynamic systems to study osteocyte regulation under mechanical stimulation. Microfluidic platforms have been developed to investigate mechanically stimulated osteocytes in the direct regulation of multiple cell types, including osteoclasts, osteoblasts, and cancer cells, and in the indirect regulation of cancer cells via endothelial cells. Overall, these microfluidic studies foster the development of treatment approaches targeting osteocytes under mechanical stimulation.

Fracture Healing in the Setting of Endocrine Diseases, Aging, and Cellular Senescence

More than 2.1 million age-related fractures occur in the United States annually, resulting in an immense socioeconomic burden. Importantly, the age-related deterioration of bone structure is associated with impaired bone healing. Fracture healing is a dynamic process which can be divided into four stages. While the initial hematoma generates an inflammatory environment in which mesenchymal stem cells and macrophages orchestrate the framework for repair, angiogenesis and cartilage formation mark the second healing period. In the central region, endochondral ossification favors soft callus development while next to the fractured bony ends, intramembranous ossification directly forms woven bone. The third stage is characterized by removal and calcification of the endochondral cartilage. Finally, the chronic remodeling phase concludes the healing process. Impaired fracture healing due to aging is related to detrimental changes at the cellular level. Macrophages, osteocytes, and chondrocytes express markers of senescence, leading to reduced self-renewal and proliferative capacity. A prolonged phase of "inflammaging" results in an extended remodeling phase, characterized by a senescent microenvironment and deteriorating healing capacity. Although there is evidence that in the setting of injury, at least in some tissues, senescent cells may play a beneficial role in facilitating tissue repair, recent data demonstrate that clearing senescent cells enhances fracture repair. In this review, we summarize the physiological as well as pathological processes during fracture healing in endocrine disease and aging in order to establish a broad understanding of the biomechanical as well as molecular mechanisms involved in bone repair.

Loureirin B downregulates osteoclast differentiation of bone marrow macrophages by targeting the MAPK signaling pathway

Excessive absorption of osteoclasts will break the balance between osteoclasts and osteoblasts, leading to bone loss, decreased bone density, and increased bone fragility. We have shown that Loureirin B (LrB) can inhibit osteoclasts. In this study, we demonstrated the targeting-inhibitory mechanism of LrB acting on osteoclast precursor. Using SPR, HPLC and MALDI-TOF-MS to capture and analyze the target protein of Loureirin B in bone marrow macrophages (BMMs), we used this method to detect all target proteins that LrB acts on BMMs, and analyzed the distribution and enrichment rate of the target protein by DAVID enrichment analysis. Ledock molecular docking was used to detect the binding of LrB. We used Western Blot for verification. The target proteins of LrB acting on BMMs were Serpine1, Atp6ap1, Dvl1, Rhd, Fzd2, MAPK1, MAP2K2, MAPK3 and so on. MAPK1, MAP2K2 and MAPK3 were the most relevant. LrB treatment attenuated the expression of phosphorylated JNK and p38 kinases of the MAPK signaling pathway. Our research further confirmed that LrB affects the MAPK signaling pathway in BMMs, thereby inhibiting the differentiation of BMMs into osteoclasts. This discovery can confirm the mechanism by which LrB acts on BMMs.

Modelling acute myeloid leukemia (AML): What's new? A transition from the classical to the modern

Acute myeloid leukemia (AML) is a heterogeneous malignancy affecting myeloid cells in the bone marrow (BM) but can spread giving rise to impaired hematopoiesis. AML incidence increases with age and is associated with poor prognostic outcomes. There has been a disconnect between the success of novel drug compounds observed in preclinical studies of hematological malignancy and less than exceptional therapeutic responses in clinical trials. This review aims to provide a state-of-the-art overview on the different preclinical models of AML available to expand insights into disease pathology and as preclinical screening tools. Deciphering the complex physiological and pathological processes and developing predictive preclinical models are key to understanding disease progression and fundamental in the development and testing of new effective drug treatments. Standard scaffold-free suspension models fail to recapitulate the complex environment where AML occurs. To this end, we review advances in scaffold/matrix-based 3D models and outline the most recent advances in on-chip technology. We also provide an overview of clinically relevant animal models and review the expanding use of patient-derived samples, which offer the prospect to create more "patient specific" screening tools either in the guise of 3D matrix models, microphysiological "organ-on-chip" tools or xenograft models and discuss representative examples.

Primary cilia and PTH1R interplay in the regulation of osteogenic actions

Primary cilia are subcellular structures specialized in sensing different stimuli in a diversity of cell types. In bone, the primary cilium is involved in mechanical sensing and transduction of signals that regulate the behavior of mesenchymal osteoprogenitors, osteoblasts and osteocytes. To perform its functions, the primary cilium modulates a plethora of molecules including those stimulated by the parathyroid hormone (PTH) receptor type I (PTH1R), a master regulator of osteogenesis. Binding of the agonists PTH or PTH-related protein (PTHrP) to the PTH1R or direct agonist-independent stimulation of the receptor activate PTH1R signaling pathways. In turn, activation of PTH1R leads to regulation of bone formation and remodeling. Herein, we describe the structure, function and molecular partners of primary cilia in the context of bone, playing special attention to those signaling pathways that are mediated directly or indirectly by PTH1R in association with primary cilia during the process of osteogenesis.

Osteocyte-specific dentin matrix protein 1 : the role of mineralization regulation in low-magnitude high-frequency vibration enhanced osteoporotic fracture healing

Aims

There is an increasing concern of osteoporotic fractures in the ageing population. Low-magnitude high-frequency vibration (LMHFV) was shown to significantly enhance osteoporotic fracture healing through alteration of osteocyte lacuno-canalicular network (LCN). Dentin matrix protein 1 (DMP1) in osteocytes is known to be responsible for maintaining the LCN and mineralization. This study aimed to investigate the role of osteocyte-specific DMP1 during osteoporotic fracture healing augmented by LMHFV.

Methods

A metaphyseal fracture was created in the distal femur of ovariectomy-induced osteoporotic Sprague Dawley rats. Rats were randomized to five different groups: 1) DMP1 knockdown (KD), 2) DMP1 KD + vibration (VT), 3) Scramble + VT, 4) VT, and 5) control (CT), where KD was performed by injection of short hairpin RNA (shRNA) into marrow cavity; vibration treatment was conducted at 35 Hz, 0.3 g; 20 minutes/day, five days/week). Assessments included radiography, micro-CT, dynamic histomorphometry and immunohistochemistry on DMP1, sclerostin, E11, and fibroblast growth factor 23 (FGF23). In vitro, murine long bone osteocyte-Y4 (MLO-Y4) osteocyte-like cells were randomized as in vivo groupings. DMP1 KD was performed by transfecting cells with shRNA plasmid. Assessments included immunocytochemistry on osteocyte-specific markers as above, and mineralized nodule staining.

Results

Healing capacities in DMP1 KD groups were impaired. Results showed that DMP1 KD significantly abolished vibration-enhanced fracture healing at week 6. DMP1 KD significantly altered the expression of osteocyte-specific markers. The lower mineralization rate in DMP1 KD groups indicated that DMP1 knockdown was associated with poor fracture healing process.

Conclusion

The blockage of DMP1 would impair healing outcomes and negate LMHFV-induced enhancement on fracture healing. These findings reveal the importance of DMP1 in response to the mechanical signal during osteoporotic fracture healing.

Cite this article: Bone Joint Res 2022;11(7):465–476.

Similarities and Differences in Bone Mineral Density between Multiple Sites in the Same Individual: An Elderly Cadaveric Study

Bone mineral density (BMD) is known to vary based on various factors, and the degree of variation is site-specific. However, few studies have investigated the relationship between bone density at trabecular bone-rich and cortical bone-rich sites in the same individual. In this study, we attempted to measure BMD at multiple sites using whole-body computed tomography images taken immediately after death and to clarify the similarities and differences between skeletal sites. Additionally, we aimed to examine the factors that influence changes in BMD, such as the loading environment, bone microstructure, and the ossification process of each skeletal region. A 3D model containing BMD data of the skull, clavicle, lumbar vertebrae, and femur (neck and diaphysis) was created using computed tomography images taken immediately after the death of 60 individuals (28 men and 32 women, average age: 84.0 years) who consented to participate in the study before death. Arbitrary measurement sites were defined, and bone density was measured at each site. We found that the BMDs of all regions were negatively correlated with age, but this correlation was weaker in the skull than in other regions. The negative correlation was especially pronounced in areas with more trabecular bones in men and in areas with more cortical bones in women. Furthermore, these findings suggest that factors, such as the loading environment, bone microstructure, and the ossification process of the skeletal sites, affect the BMD. Furthermore, our results suggest that it is important to assess the BMD of cortical bone in older women.

Comparison of the 3D-Microstructure Between Alveolar and Iliac Bone for Enhanced Bioinspired Bone Graft Substitutes

In oral- and maxillofacial bone augmentation surgery, non-vascularized grafts from the iliac crest demonstrate better clinical performance than alveolar bone grafts. The underlying mechanisms are not fully understood but are essential for the enhancement of bone regeneration scaffolds. Synchrotron Radiation µ-CT at a pixel size of 2.3 μm was used to characterize the gross morphology and the vascular and osteocyte lacuna porosity of patient-matched iliac crest/alveolar bone samples. The results suggest a difference in the spatial distribution of the vascular pore system. Fluid simulations reveal the permeability tensor to be more homogeneous in the iliac crest, indicating a more unidirectional fluid flow in alveolar bone. The average distance between bone mineral and the closest vessel pore boundary was found to be higher in alveolar bone. At the same time, osteocyte lacunae density is higher in alveolar bone, potentially compensating for the longer average distance between the bone mineral and vessel pores. The present study comprehensively quantified and compared the 3D microarchitecture of intraindividual human alveolar and iliac bone. The identified difference in pore network architecture may allow a bone graft from the iliac crest to exhibit higher regeneration potential due to an increased capacity to connect with the surrounding pore network of the residual bone. The results may contribute to understanding the difference in clinical performance when used as bone grafts and are essential for optimization of future scaffold materials.

Serum sclerostin levels in osteoporotic fracture patients

PURPOSE

Sclerostin inhibits bone formation and stimulates bone resorption. Previous studies found a positive association between bone density and serum sclerostin, but literature on sclerostin levels in osteoporotic fracture patients is scarce. The aim of the present study was to compare the serum sclerostin levels in osteoporotic and non-osteoporotic fracture patients and to assess the correlation of the sclerostin levels with bone mineral density and vitamin D status.

METHODS

In this cross-sectional study, we included patients over 50 years, with an extremity fracture after low-energy trauma treated between 2012 and 2018, with biobank samples and available bone density measurements by Dual X-ray Absorption. Osteoporosis was diagnosed according the World Health Organisation criteria. Vitamin D deficiency was defined as a 25(OH)D concentration < 30 nmol/L. After defrosting biobank samples, serum sclerostin was measured using the human SOST (sclerostin) enzyme-linked immunosorbent assay kit. We prespecified a subgroup analysis including only female patients.

RESULTS

179 patients were included of whom 139(78%) were female. In 46 patients (25.7%), osteoporosis was diagnosed. Bone mineral density was positively associated with sclerostin levels (r = 0.17, p = 0.026) and patients with osteoporosis had a significantly lower serum sclerostin compared to non-osteoporotic fracture patients (mean 41.9 pmol/L vs 48.1 pmol/L; p = 0.03). This difference remained significant after correction for potential confounders. Similar results were found in the subgroup of female patients. No association between serum sclerostin and vitamin D deficiency was found.

CONCLUSION

Osteoporotic fracture patients had lower levels of sclerostin than non-osteoporotic fracture patients. Future research should focus on the use of sclerostin as biomarker for osteoporosis in fracture patients.

Effects of Osteocyte Shape on Fluid Flow and Fluid Shear Stress of the Loaded Bone

This study was conducted to better understand the specific behavior of the intraosseous fluid flow. We calculated the number and distribution of bone canaliculi around the osteocytes based on the varying shapes of osteocytes. We then used these calculated parameters and other bone microstructure data to estimate the anisotropy permeability of the lacunar-canalicular network. Poroelastic finite element models of the osteon were established, and the influence of the osteocyte shape on the fluid flow properties of osteons under an axial displacement load was analyzed. Two types of boundary conditions (BC) that might occur in physiological environments were considered on the cement line of the osteon. BC1 allows free fluid passage from the outer elastic restraint boundary, and BC2 is impermeable and allows no free fluid passage from outer displacement constrained boundary. They both have the same inner boundary conditions that allow fluid to pass through. Changes in the osteocyte shape altered the maximum value of pressure gradient (PG), pore pressure (PP), fluid velocity (FV), and fluid shear stress (FSS) relative to the reference model (spherical osteocytes). The maximum PG, PP, FV, and FSS in BC2 were nearly 100% larger than those in BC1, respectively. It is found that the BC1 was closer to the real physiological environment. The fluid flow along different directions in the elongated osteocyte model was more evident than that in other models, which may have been due to the large difference in permeability along different directions. Changes in osteocyte shape significantly affect the degrees of anisotropy of fluid flow and porous media of the osteon. The model presented in this study can accurately quantify fluid flow in the lacunar-canalicular network.

Interactive effects of various loading parameters on the fluid dynamics within the lacunar-canalicular system for a single osteocyte

The osteocyte lacunar-canalicular system (LCS) serves as a mechanotransductive core where external loading applied to the skeleton is transduced into mechanical signals (e.g., fluid shear) that can be sensed by mechanosensors (osteocytes). The fluid velocity and shear stress within the LCS are affected by various loading parameters. However, the interactive effect of distinct loading parameters on the velocity and shear stress in the LCS remains unclear. To address this issue, we developed a multiscale modeling approach, combining a poroelastic finite element (FE) model with a single osteocytic LCS unit model to calculate the flow velocity and shear stress within the LCS. Next, a sensitivity analysis was performed to investigate individual and interactive effects of strain magnitude, strain rate, number of cycles, and intervening short rests between loading cycles on the velocity and shear stress around the osteocyte. Lastly, we developed a relatively simple regression model to predict those outcomes. Our results demonstrated that the strain magnitude or rate alone were the main factors affecting the velocity and shear stress; however, the combination of these two was not directly additive, and addition of a short rest between cycles could enhance the combination of these two related factors. These results show highly interactive effects of distinct loading parameters on fluid velocity and shear stress in the LCS. Specifically, our results suggest that an enhanced fluid dynamics environment in the LCS can be achieved with a brief number of load cycles combined with short rest insertion and high strain magnitude and rate.

Osteocytes in bone aging: Advances, challenges, and future perspectives

Osteocytes play a critical role in maintaining bone homeostasis and in regulating skeletal response to hormones and mechanical loading. Substantial evidence have demonstrated that osteocytes and their lacunae exhibit morphological changes in aged bone, indicating the underlying involvement of osteocytes in bone aging. Notably, recent studies have deciphered aged osteocytes to have characteristics such as impaired mechanosensitivity, accumulated cellular senescence, dysfunctional perilacunar/canalicular remodeling, and degenerated lacuna-canalicular network. However, detailed molecular mechanisms of osteocytes remain unclear. Nonetheless, osteocyte transcriptomes analyzed via advanced RNA sequencing (RNA-seq) techniques have identified several bone aging-related genes and signaling pathways, such as Wnt, Bmp/TGF, and Jak-STAT. Moreover, inflammation, immune dysfunction, energy shortage, and impaired hormone responses possibly affect osteocytes in age-related bone deterioration. In this review, we summarize the hallmarks of aging bone and osteocytes and discuss osteocytic mechanisms in age-related bone loss and impaired bone quality. Furthermore, we provide insights into the challenges faced and their possible solutions when investigating osteocyte transcriptomes. We also highlight that single-cell RNA-seq can decode transcriptomic messages in aged osteocytes; therefore, this technique can promote novel single cell-based investigations in osteocytes once a well-established standardized protocol specific for osteocytes is developed. Interestingly, improved understanding of osteocytic mechanisms have helped identify promising targets and effective therapies for aging-related osteoporosis and fragile fractures.

Prioritization of Genes Relevant to Bone Fragility Through the Unbiased Integration of Aging Mouse Bone Transcriptomics and Human GWAS Analyses

Identifying new genetic determinants of bone mineral density (BMD) and fracture promises to yield improved diagnostics and therapies for bone fragility. However, prioritizing candidate genes from genome-wide screens can be challenging. To overcome this challenge, we prioritized mouse genes that are differentially expressed in aging mouse bone based on whether their human homolog is associated with human BMD and/or fracture. Unbiased RNA-seq analysis of young and old male C57BL/6 mouse cortical bone identified 1499, 1685, and 5525 differentially expressed genes (DEGs) in 1, 2, and 2.5-year-old bone, relative to 2-month-old bone, respectively. Gene-based scores for heel ultrasound bone mineral density (eBMD) and fracture were estimated using published genome-wide association studies (GWAS) results of these traits in the UK Biobank. Enrichment analysis showed that mouse bone DEG sets for all three age groups, relative to young bone, are significantly enriched for eBMD, but only the oldest two DEG sets are enriched for fracture. Using gene-based scores, this approach prioritizes among thousands of DEGs by a factor of 5- to 100-fold, yielding 10 and 21 genes significantly associated with fracture in the two oldest groups of mouse DEGs. Though these genes were not the most differentially expressed, they included Sost, Lrp5, and others with well-established functions in bone. Several others have, as yet, unknown roles in the skeleton. Therefore, this study accelerates identification of new genetic determinants of bone fragility by prioritizing a clinically relevant and experimentally tractable number of candidate genes for functional analysis. Finally, we provide a website (www.mouse2human.org) to enable other researchers to easily apply our strategy. © 2022 American Society for Bone and Mineral Research (ASBMR).

Bone permeability and mechanotransduction: Some current insights into the function of the lacunar-canalicular network

Lacunar-canalicular (LC) permeability involves the passage of fluids, nutrients, oxygen, ions, and signalling molecules through bone tissue, facilitating the maintenance of bone vitality and function and responses to various physiological conditions and diseases. LC permeability and fluid flow-shear stress/drag force play important roles in mechanotransduction in bone tissue by inducing mechanical stimuli in osteocytes, modulating cellular functions, and determining bone adaptation. Alterations in LC structure may therefore influence the fluid flow pattern through the LC network, thereby affecting the ability of osteocytes to sense and translate mechanical signals and possibly contributing to bone remodelling. Several bone-health conditions are associated with changes in LC structure and function and may affect mechanotransduction and responses, although the mechanisms underlying these associations are still not fully understood. In this review, recent studies of LC networks, their formation and transfer mechanical stimuli, and changes in structure, functional permeability, and mechanotransduction that result from age, pathology, and mechanical loading are discussed. Additionally, applications of vibration and low-intensity pulsed ultrasound in bone healthcare and regeneration fields are also presented.

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.

Impaired bone quality in the superolateral femoral neck occurs independent of hip geometry and bone mineral density

Skeletal adaptation is substantially influenced by mechanical loads. Osteocytes and their lacuno-canalicular network have been identified as a key player in load sensation and bone quality regulation. In the femoral neck, one of the most common fracture sites, a complex loading pattern with lower habitual loading in the superolateral neck and higher compressive stresses in the inferomedial neck is present. Variations in the femoral neck-shaft angle (NSA), i.e., coxa vara or coxa valga, provide the opportunity to examine the influence of loading patterns on bone quality. We obtained femoral neck specimens of 28 osteoarthritic human subjects with coxa vara, coxa norma and coxa valga during total hip arthroplasty. Bone mineral density (BMD) was assessed preoperatively by dual energy X-ray absorptiometry (DXA). Cortical and trabecular microstructure and three-dimensional osteocyte lacunar characteristics were assessed in the superolateral and inferomedial neck using ex vivo high resolution micro-computed tomography. Additionally, BMD distribution and osteocyte lacunar characteristics were analyzed by quantitative backscattered electron imaging (qBEI). All groups presented thicker inferomedial than superolateral cortices. Furthermore, the superolateral site exhibited a lower osteocyte lacunar density along with lower lacunar sphericity than the inferomedial site, independent of NSA. Importantly, BMD and corresponding T-scores correlated with microstructural parameters at the inferomedial but not superolateral neck. In conclusion, we provide micromorphological evidence for fracture vulnerability of the superolateral neck, which is independent of NSA and BMD. The presented bone qualitative data provide an explanation why DXA may be insufficient to predict a substantial proportion of femoral neck fractures. STATEMENT OF SIGNIFICANCE: The femoral neck, one of the most common fracture sites, is subject to a complex loading pattern. Site-specific differences (i.e., superolateral vs. inferomedial) in bone quality influence fracture risk, but it is unclear how this relates to hip geometry and bone mineral density (BMD) measurements in vivo. Here, we examine femoral neck specimens using a variety of high-resolution imaging techniques and demonstrate impaired bone quality in the superolateral compared to the inferomedial neck. Specifically, we found impaired cortical and trabecular microarchitecture, mineralization, and osteocyte properties, regardless of neck-shaft angle. Since BMD correlated with bone quality of the inferomedial but not the superolateral neck, our results illustrate why bone densitometry may not predict a substantial proportion of femoral neck fractures.

The Mechanosensory Role of Osteocytes and Implications for Bone Health and Disease States

Bone homeostasis is a dynamic equilibrium between bone-forming osteoblasts and bone-resorbing osteoclasts. This process is primarily controlled by the most abundant and mechanosensitive bone cells, osteocytes, that reside individually, within chambers of porous hydroxyapatite bone matrix. Recent studies have unveiled additional functional roles for osteocytes in directly contributing to local matrix regulation as well as systemic roles through endocrine functions by communicating with distant organs such as the kidney. Osteocyte function is governed largely by both biochemical signaling and the mechanical stimuli exerted on bone. Mechanical stimulation is required to maintain bone health whilst aging and reduced level of loading are known to result in bone loss. To date, both in vivo and in vitro approaches have been established to answer important questions such as the effect of mechanical stimuli, the mechanosensors involved, and the mechanosensitive signaling pathways in osteocytes. However, our understanding of osteocyte mechanotransduction has been limited due to the technical challenges of working with these cells since they are individually embedded within the hard hydroxyapatite bone matrix. This review highlights the current knowledge of the osteocyte functional role in maintaining bone health and the key regulatory pathways of these mechanosensitive cells. Finally, we elaborate on the current therapeutic opportunities offered by existing treatments and the potential for targeting osteocyte-directed signaling.

Silk Fibroin Microparticle Scaffold for Use in Bone Void Filling: Safety and Efficacy Studies

Silk fibroin (SF) is a natural biocompatible protein polymer extracted from cocoons of silkworm Bombyx mori. SF can be processed into a variety of different forms and shapes that can be used as scaffolds to support bone regeneration. Three-dimensional (3D) SF scaffolds have shown promise in bone-void-filling applications. In in vitro studies, it has been demonstrated that a microparticle-based SF (M-RSF) scaffold promotes the differentiation of stem cells into an osteoblastic lineage. The expression of differentiation markers was also significantly higher for M-RSF scaffolds as compared to other SF scaffolds and commercial ceramic scaffolds. In this work, we have evaluated the in vitro and in vivo biocompatibility of M-RSF scaffolds as per the ISO 10993 guidelines in a Good Laboratory Practice (GLP)-certified facility. The cytotoxicity, immunogenicity, genotoxicity, systemic toxicity, and implantation studies confirmed that the M-RSF scaffold is biocompatible. Further, the performance of the M-RSF scaffold to support bone formation was evaluated in in vivo bone implantation studies in a rabbit model. Calcium sulfate (CaSO₄) scaffolds were chosen as reference material for this study as they are one of the preferred materials for bone-void-filling applications. M-RSF scaffold implantation sites showed a higher number of osteoblast and osteoclast cells as compared to CaSO₄ implantation sites indicating active bone remodeling. The number density of osteocytes was double for M-RSF scaffold implantation sites, and these M-RSF scaffold implantation sites were characterized by enhanced collagen deposition, pointing toward a finer quality of the new bone formed. Moreover, the M-RSF scaffold implantation sites had a negligible incidence of secondary fractures as compared to the CaSO₄ implantation sites (∼50% sites with secondary fracture), implying a reduction in postsurgical complications. Thus, the study demonstrates that the M-RSF scaffold is nontoxic for bone-void-filling applications and facilitates superior healing of fracture defects as compared to commercial calcium-based bone void fillers.

The mechanotransduction of MLO-Y4 cells is disrupted by the senescence-associated secretory phenotype of neighboring cells

Age-related bone loss is attributed to the accumulation of senescent cells and their increasing production of inflammatory cytokines as part of the senescence-associated secretory phenotype (SASP). In otherwise healthy individuals, osteocytes play a key role in maintaining bone mass through their primary function of responding to skeletal loading. Given that osteocytes' response to loading is known to steadily decline with age, we hypothesized that the increasing presence of senescent cells and their SASP inhibit osteocytes' response to loading. To test this hypothesis, we developed two in vitro models of senescent osteocytes and osteoblasts derived from MLO-Y4 and MC3T3 cell lines, respectively. The senescent phenotype was unique to each cell type based on distinct changes in cell cycle inhibitors and SASP profile. The SASP profile of senescent osteocytes was in part dependent on nuclear factor-κB signaling and presents a new potential mechanism to target the SASP in bone. Nonsenescent MLO-Y4 cells cultured with the SASP of each senescent cell type failed to exhibit changes in gene expression as well as ERK phosphorylation and prostaglandin E2 release. The SASP of senescent osteocytes had the largest effect and neutralizing interleukin-6 (IL-6) as part of the SASP restored osteocytes' response to loading. The loss in mechanotransduction due to IL-6 was attributed to a decrease in P2X7 expression and overall sensitivity to purinergic signaling. Altogether, these findings demonstrate that the SASP of senescent cells have a negative effect on the mechanotransduction of osteocytes and that IL-6 is a key SASP component that contributes to the loss in mechanotransduction.

The association between physical function and hyperkyphosis in older females: Protocol for a systematic review

Identifying factors that impact physical function in older populations is important for the maintenance of good health with aging. Age-related hyperkyphosis, an excessive curvature in the thoracic spine, affects up to 40% of the older adults and is more common in older females than males. An association of age-related hyperkyphosis with impaired physical function has been reported in numerous studies, however, other studies have reported that a greater magnitude of kyphosis did not associate with impaired physical function. Given the inconsistencies regarding the impact of hyperkyphosis on physical function, the purpose of our study is to perform a systematic review of the existing studies in order to better describe the association between hyperkyphosis and physical function. Prospective and retrospective cohort, case-control, and cross-sectional studies which measure physical function by valid functional tests and questionnaires in older females will be included. We will search Scopus, ISI Web of Science, Cochrane Library, PubMed, Cumulative Index to Nursing and Allied Health Literature (CINAHL) and PEDro databases. Studies will be searched and then selected by two independent reviewers based on quality assessment tools from the National Heart, Lung, and Blood Institute (NHLBI). A meta-analysis will be conducted if data reported for individual studies allow. Specifically, if two or more individual studies provide measures of central tendency and variability from any of the categories of physical function measures, data will be gathered for meta-analysis. If a meta-analysis is not possible, data will be synthesized and described in a narrative form by size and variability of effect, direction of effect, and association with hyperkyphosis.

Regulation of bone health through physical exercise: Mechanisms and types

Osteoporosis, characterized by bone mineral density reduction, bone mass loss, increased bone fragility, and propensity to fractures, is a common disease in older individuals and one of the most serious health problems worldwide. The imbalance between osteoblasts and osteoclasts results in the predominance of bone resorption and decreased bone formation. In recent years, it has been found that regular and proper exercise not only helps prevent the occurrence of osteoporosis but also adds benefits to osteoporosis therapy; accordingly, bone homeostasis is closely associated with mechanical stress and the intricate crosstalk between osteoblasts and osteoclasts. In this review, we summarize the mechanisms of exercise on osteoporosis and provide new proposals for the prevention and treatment of osteoporosis.

Aging, Osteo-Sarcopenia, and Musculoskeletal Mechano-Transduction

The decline in the mass and function of bone and muscle is an inevitable consequence of healthy aging with early onset and accelerated decline in those with chronic disease. Termed osteo-sarcopenia, this condition predisposes the decreased activity, falls, low-energy fractures, and increased risk of co-morbid disease that leads to musculoskeletal frailty. The biology of osteo-sarcopenia is most understood in the context of systemic neuro-endocrine and immune/inflammatory alterations that drive inflammation, oxidative stress, reduced autophagy, and cellular senescence in the bone and muscle. Here we integrate these concepts to our growing understanding of how bone and muscle senses, responds and adapts to mechanical load. We propose that age-related alterations in cytoskeletal mechanics alter load-sensing and mechano-transduction in bone osteocytes and muscle fibers which underscores osteo-sarcopenia. Lastly, we examine the evidence for exercise as an effective countermeasure to osteo-sarcopenia.

Osteocyte calcium signaling - A potential translator of mechanical load to mechanobiology

Osteocytes are embedded dendritic bone cells; by virtue of their position in bone tissue, ability to coordinate bone building osteoblasts and resorbing osteoclasts, and sensitivity to tissue level mechanical loading, they serve as the resident bone mechanosensor. The mechanisms osteocytes use to change mechanical loading into biological signals that drive tissue level changes has been well studied over the last 30 years, however the ways loading parameters are encoded at the cellular level are still not fully understood. Calcium signaling is a first messenger signal exhibited by osteocytes in response to mechanical forces. A body of work interrogating the mechanisms of osteocyte calcium signaling exists and is presently expanding, presenting the opportunity to better understand the relationship between calcium signaling characteristics and tuned osteocyte responses to tissue level strain features (e.g. magnitude, duration, frequency). This review covers the history of osteocyte load induced calcium signaling and highlights potential cellular mechanisms used by osteocytes to turn details about loading parameters into biological events.

Induction of somatopause in adult mice compromises bone morphology and exacerbates bone loss during aging

Somatopause refers to the gradual declines in growth hormone (GH) and insulin‐like growth factor‐1 throughout aging. To define how induced somatopause affects skeletal integrity, we used an inducible GH receptor knockout (iGHRKO) mouse model. Somatopause, induced globally at 6 months of age, resulted in significantly more slender bones in both male and female iGHRKO mice. In males, induced somatopause was associated with progressive expansion of the marrow cavity leading to significant thinning of the cortices, which compromised bone strength. We report progressive declines in osteocyte lacunar number, and increases in lacunar volume, in iGHRKO males, and reductions in lacunar number accompanied by ~20% loss of overall canalicular connectivity in iGHRKO females by 30 months of age. Induced somatopause did not affect mineral/matrix ratio assessed by Raman microspectroscopy. We found significant increases in bone marrow adiposity and high levels of sclerostin, a negative regulator of bone formation in iGHRKO mice. Surprisingly, however, despite compromised bone morphology, osteocyte senescence was reduced in the iGHRKO mice. In this study, we avoided the confounded effects of constitutive deficiency in the GH/IGF‐1 axis on the skeleton during growth, and specifically dissected its effects on the aging skeleton. We show here, for the first time, that induced somatopause compromises bone morphology and the bone marrow environment.

Biologic and pathologic aspects of osteocytes in the setting of medication-related osteonecrosis of the jaw (MRONJ)

Medication-related osteonecrosis of the jaw (MRONJ) is a potentially severe, debilitating condition affecting patients with cancer and patients with osteoporosis who have been treated with powerful antiresorptives (pARs) or angiogenesis inhibitors (AgIs). Oral risk factors associated with the development of MRONJ include tooth extraction and inflammatory dental disease (e.g., periodontitis, periapical infection). In bone tissues, osteocytes play a bidirectional role in which they not only act as the "receiver" of systemic signals from blood vessels, such as hormones and drugs, or local signals from the mineralized matrix as it is deformed, but they also play a critical role as "transmitter" of signals to the cells that execute bone modeling and remodeling (osteoclasts, osteoblasts and lining cells). When the survival capacity of osteocytes is overwhelmed, they can die. Osteocyte death has been associated with several pathological conditions. Whereas the causes and mechanisms of osteocyte death have been studied in conditions like osteonecrosis of the femoral head (ONFH), few studies of the causes and mechanisms of osteocyte death have been done in MRONJ. The three forms of cell death that affect most of the different cells in the body (apoptosis, autophagy, and necrosis) have been recognized in osteocytes. Notably, necroptosis, a form of regulated cell death with "a necrotic cell death phenotype," has also been identified as a form of cell death in osteocytes under certain pathologic conditions. Improving the understanding of osteocyte death in MRONJ may be critical for preventing disease and developing treatment approaches. In this review, we intend to provide insight into the biology of osteocytes, cell death, in general, and osteocyte death, in particular, and discuss hypothetical mechanisms involved in osteocyte death associated with MRONJ.

Tunnels in the rock: Dynamics of osteocyte morphogenesis

Osteocytes are dynamic, bone matrix-remodeling cells that form an intricate network of interconnected projections through the bone matrix, called the lacunar-canalicular system. Osteocytes are the dominant mechanosensory cells in bone and their mechanosensory and mechanotransductive functions follow their morphological form. During osteocytogenesis and development of the osteocyte lacunar-canalicular network, osteocytes must dramatically remodel both their cytoskeleton and their extracellular matrix. In this review, we summarize our current understanding of the mechanisms that govern osteocyte differentiation, cytoskeletal morphogenesis, mechanotransduction, and matrix remodeling. We postulate that the physiologic activation of matrix remodeling in adult osteocytes, known as perilacunar/canalicular remodeling (PLR) represents a re-activation of the developmental program by which the osteocyte network is first established. While much of osteocyte biology remains unclear, new tools and approaches make the present moment a particularly fruitful and exciting time to study the development of these remarkable cells.