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Genetic or pharmacological depletion of senescent cells or inhibition of their function reduces bone loss in aged mice. Aging is associated with increased cellular senescence, which is hypothesized to drive the eventual development of multiple comorbidities 1 . Here we investigate a role for senescent cells in age-related bone loss through multiple approaches. In particular, we used either genetic (i.e., the INK-ATTAC 'suicide' transgene encoding an inducible caspase 8 expressed specifically in senescent cells 2 , 3 , 4 ) or pharmacological (i.e., 'senolytic' compounds 5 , 6 ) means to eliminate senescent cells. We also inhibited the production of the proinflammatory secretome of senescent cells using a JAK inhibitor (JAKi) 3 , 7 . In aged (20- to 22-month-old) mice with established bone loss, activation of the INK-ATTAC caspase 8 in senescent cells or treatment with senolytics or the JAKi for 2–4 months resulted in higher bone mass and strength and better bone microarchitecture than in vehicle-treated mice. The beneficial effects of targeting senescent cells were due to lower bone resorption with either maintained (trabecular) or higher (cortical) bone formation as compared to vehicle-treated mice. In vitro studies demonstrated that senescent-cell conditioned medium impaired osteoblast mineralization and enhanced osteoclast-progenitor survival, leading to increased osteoclastogenesis. Collectively, these data establish a causal role for senescent cells in bone loss with aging, and demonstrate that targeting these cells has both anti-resorptive and anabolic effects on bone. Given that eliminating senescent cells and/or inhibiting their proinflammatory secretome also improves cardiovascular function 4 , enhances insulin sensitivity 3 , and reduces frailty 7 , targeting this fundamental mechanism to prevent age-related bone loss suggests a novel treatment strategy not only for osteoporosis, but also for multiple age-related comorbidities.

Cortical bone structure is a crucial determinant of bone strength, yet for many years studies of novel genes and cell signalling pathways regulating bone strength have focused on the control of trabecular bone mass. Here we focus on mechanisms responsible for cortical bone development, growth, and degeneration, and describe some recently described genetic-driven modifications in humans and mice that reveal how these processes may be controlled. We start with embryonic osteogenesis of preliminary bone structures preceding the cortex and describe how this structure consolidates then matures to a dense, vascularised cortex containing an increasing proportion of lamellar bone. These processes include modelling-induced, and load-dependent, asymmetric cortical expansion, which enables the cortex’s transition from a highly porous woven structure to a consolidated and thickened highly mineralised lamellar bone structure, infiltrated by vascular channels. Sex-specific differences emerge during this process. With aging, the process of consolidation reverses: cortical pores enlarge, leading to greater cortical porosity, trabecularisation and loss of bone strength. Each process requires co-ordination between bone formation, bone mineralisation, vascularisation, and bone resorption, with a need for locational-, spatial- and cell-specific signalling pathways to mediate this co-ordination. We will discuss these processes, and a number of cell-signalling pathways identified in both murine and human genetic studies to regulate cortical bone mass, including signalling through gp130, STAT3, PTHR1, WNT16, NOTCH, NOTUM and sFRP4.

Bisphosphonates are a common treatment to reduce osteoporotic fractures. This treatment induces osseous structural and compositional changes accompanied by positive effects on osteoblasts and osteocytes. Here, we test the hypothesis that restored osseous cell behavior, which resembles characteristics of younger, healthy cortical bone, leads to improved bone quality. Microarchitecture and mechanical properties of young, treatment-naïve osteoporosis and bisphosphonate-treated cases were investigated in femoral cortices. Tissue strength was measured using three-point bending. Collagen fibril-level deformation was assessed in non-traumatic and traumatic fracture states using synchrotron small-angle x-ray scattering (SAXS) at low and high strain rates. The lower modulus, strength and fibril deformation measured at low strain rates reflects susceptibility for osteoporotic low-energy fragility fractures. Independent of age, disease and treatment status, SAXS revealed reduced fibril plasticity at high strain rates, characteristic of traumatic fracture. The significantly reduced mechanical integrity in osteoporosis may originate from porosity and alterations to the intra/extrafibrillar structure, while the fibril deformation under treatment indicates improved nano-scale characteristics. In conclusion, losses in strength and fibril deformation at low strain rates correlate with the occurrence of fragility fractures in osteoporosis, while improvements in structural and mechanical properties following bisphosphonate treatment may foster resistance to fracture during physiological strain rates.

Cortical bone, prominently found in the diaphyseal region of long bones, can resist higher ultimate stresses than trabecular bone and serves as the primary load-bearing compartment of the skeleton. The importance of cortical bone in determining mechanical strength and assessing fracture risk has been highlighted in both experimental and computational studies, motivating the need for better understanding through large-scale analysis. To support large data processing, an automated technique for measuring cortical thickness from clinical CT scans with sub-millimetre accuracy was introduced by Treece et al. (2012). However, this method struggles to reconstruct and calculate cortical bone thickness across diverse long bone morphologies. In this study, we present an adapted version of the technique with improved robustness. When the 2012 published method is evaluated on 240 long bones across six types, it resulted in failures across all test cases, with mean failure rates of 2.9%, 8.0%, 10.5%, 13.7%, 17.9%, and 24.8% in humerus, femur, radius, ulna, tibia and fibula, respectively. In contrast, the proposed new method eliminated failures in all bones except for the fibula, where 9 out of 40 test cases failed with a reduced mean failure rate of 1.9%. These results demonstrate that the new method broadens the applicability of the previous approach by robustly handling morphological variation, making it more suitable for large-scale studies. We anticipate the proposed workflow will serve as a valuable resource for analysing datasets with population-level variability and improving our understanding of osteogenic phenomena in clinically meaningful contexts.

Abstract Context Testosterone treatment increases bone mineral density (BMD) in hypogonadal men. Effects on bone microarchitecture, a determinant of fracture risk, are unknown. Objective We aimed to determine the effect of testosterone treatment on bone microarchitecture using high resolution–peripheral quantitative computed tomography (HR-pQCT). Methods Men ≥ 50 years of age were recruited from 6 Australian centers and were randomized to receive injectable testosterone undecanoate or placebo over 2 years on the background of a community-based lifestyle program. The primary endpoint was cortical volumetric BMD (vBMD) at the distal tibia, measured using HR-pQCT in 177 men (1 center). Secondary endpoints included other HR-pQCT parameters and bone remodeling markers. Areal BMD (aBMD) was measured by dual-energy x-ray absorptiometry (DXA) in 601 men (5 centers). Using a linear mixed model for repeated measures, the mean adjusted differences (95% CI) at 12 and 24 months between groups are reported as treatment effect. Results Over 24 months, testosterone treatment, versus placebo, increased tibial cortical vBMD, 9.33 mg hydroxyapatite (HA)/cm3) (3.96, 14.71), P < 0.001 or 3.1% (1.2, 5.0); radial cortical vBMD, 8.96 mg HA/cm3 (3.30, 14.62), P = 0.005 or 2.9% (1.0, 4.9); total tibial vBMD, 4.16 mg HA/cm3 (2.14, 6.19), P < 0.001 or 1.3% (0.6, 1.9); and total radial vBMD, 4.42 mg HA/cm3 (1.67, 7.16), P = 0.002 or 1.8% (0.4, 2.0). Testosterone also significantly increased cortical area and thickness at both sites. Effects on trabecular architecture were minor. Testosterone reduced bone remodeling markers CTX, −48.1 ng/L [−81.1, −15.1], P < 0.001 and P1NP, −6.8 μg/L[−10.9, −2.7], P < 0.001. Testosterone significantly increased aBMD at the lumbar spine, 0.04 g/cm2 (0.03, 0.05), P < 0.001 and the total hip, 0.01 g/cm2 (0.01, 0.02), P < 0.001. Conclusion In men ≥ 50 years of age, testosterone treatment for 2 years increased volumetric bone density, predominantly via effects on cortical bone. Implications for fracture risk reduction require further study.

In this study, bovine cortical bone was investigated under plasma treatment process to investigate the potential for improvements in their structural characteristics. The bone specimens were treatment with low pressure argon cold plasma at different treatment times; 15, 30, and 45 min. Various techniques such as X-ray diffraction, scanning electron microscopy, surface roughness testing and automatic LCR Bridge were utilized to study the plasma-induced modifications on the structural and dielectric properties of the bone. SEM images revealed the elimination of some outer atoms from the bone surface during the 30-minute plasma ablation process, leading to more noticeable grain size of hydroxyapatite. XRD measurements confirmed the obtained results as mentoring the changes in crystallite size and strain parameters. As the treatment time approached 45 min, crystallite size increased, along with surface roughness parameters and relaxation time. These findings contribute to a better understanding of the microstructural and morphological changes occurring on the bone surface during cold argon plasma treatment.

Abstract Context Bone loss after bariatric surgery potentially could be mitigated by exercise. Objective To investigate the role of exercise training (ET) in attenuating bariatric surgery–induced bone loss. Design Randomized, controlled trial. Setting Referral center for bariatric surgery. Patients Seventy women with severe obesity, aged 25 to 55 years, who underwent Roux-en-Y gastric bypass (RYGB). Intervention Supervised, 6-month, ET program after RYGB vs. standard of care (RYGB only). Outcomes Areal bone mineral density (aBMD) was the primary outcome. Bone microarchitecture, bone turnover, and biochemical markers were secondary outcomes. Results Surgery significantly decreased femoral neck, total hip, distal radius, and whole body aBMD (P < 0.001); and increased bone turnover markers, including collagen type I C-telopeptide (CTX), procollagen type I N-propeptide (P1NP), sclerostin, and osteopontin (P < 0.05). Compared with RYGB only, exercise mitigated the percent loss of aBMD at femoral neck [estimated mean difference (EMD), −2.91%; P = 0.007;], total hip (EMD, −2.26%; P = 0.009), distal radius (EMD, −1.87%; P = 0.038), and cortical volumetric bone mineral density at distal radius (EMD, −2.09%; P = 0.024). Exercise also attenuated CTX (EMD, −0.20 ng/mL; P = 0.002), P1NP (EMD, −17.59 ng/mL; P = 0.024), and sclerostin levels (EMD, −610 pg/mL; P = 0.046) in comparison with RYGB. Exercise did not affect biochemical markers (e.g., 25(OH)D, calcium, intact PTH, phosphorus, and magnesium). Conclusion Exercise mitigated bariatric surgery–induced bone loss, possibly through mechanisms involving suppression in bone turnover and sclerostin. Exercise should be incorporated in postsurgery care to preserve bone mass. A 6-month, exercise training program significantly mitigated RYGB-induced bone mass loss at femoral neck, total hip and distal radius, and attenuated bone turnover markers and sclerostin levels.

Gadolinium-based contrast agents (GBCAs) are frequently used in patients undergoing magnetic resonance imaging. In GBCAs gadolinium (Gd) is present in a bound chelated form. Gadolinium is a rare-earth element, which is normally not present in human body. Though the blood elimination half-life of contrast agents is about 90 minutes, recent studies demonstrated that some tissues retain gadolinium, which might further pose a health threat due to toxic effects of free gadolinium. It is known that the bone tissue can serve as a gadolinium depot, but so far only bulk measurements were performed. Here we present a summary of experiments in which for the first time we mapped gadolinium in bone biopsy from a male patient with idiopathic osteoporosis (without indication of renal impairment), who received MRI 8 months prior to biopsy. In our studies performed by means of synchrotron radiation induced micro- and submicro-X-ray fluorescence spectroscopy (SR-XRF), gadolinium was detected in human cortical bone tissue. The distribution of gadolinium displays a specific accumulation pattern. Correlation of elemental maps obtained at ANKA synchrotron with qBEI images (quantitative backscattered electron imaging) allowed assignment of Gd structures to the histological bone structures. Follow-up beamtimes at ESRF and Diamond Light Source using submicro-SR-XRF allowed resolving thin Gd structures in cortical bone, as well as correlating them with calcium and zinc.

Although studies support disrupted bone remodelling within geriatric populations, mid‐life changes are understudied. To investigate this, we performed bone marrow transplantation assays using either 8‐ or 40‐week‐old mice. Micro‐CT analyses of lethally irradiated 8‐week‐old mice transplanted with 40‐week‐old bone marrow exhibited increased mid‐shaft femoral cortical bone mass and thickness. Intensive bone marrow regeneration mirrors hematopoietic development in that erythro‐myeloid progenitors (EMPs) first expand to support blood production before definitive hematopoietic stem cell (HSC) production. We hypothesized that reduced HSC capacity of 40‐week‐old bone marrow and compensatory expansion of EMPs may facilitate gains in cortical bone. Flow cytometry analyses revealed greater EMP to HSC ratios when mice were reconstituted with increasing percentages of middle‐aged bone marrow. To identify cell types mediating these effects, we performed comparative scRNA‐Seq analyses and identified CD11B+CD36+ myeloid cells exhibiting enriched expression of bone anabolic cytokines. Elevated levels of Wnt ligands, especially Wnt6, characterized these cells. In lineage tracing assays, CD11B+CD36+ cells were donor‐derived myeloid cells. In functional assays, we demonstrate that soluble factors produced by CD11B+CD36+ cells enhance osteogenesis. Moreover, CD11B/CD36/Wnt6 exquisitely mark anabolic macrophages within human bone marrow. These findings reveal a myeloid population present during midlife that enhances cortical bone.

Human cortical bone contains two types of tissue: osteonal and interstitial tissue. Growing bone is not well-known in terms of its intrinsic material properties. To date, distinctions between the mechanical properties of osteonal and interstitial regions have not been investigated in juvenile bone and compared to adult bone in a combined dataset. In this work, cortical bone samples obtained from fibulae of 13 juveniles patients (4 to 18 years old) during corrective surgery and from 17 adult donors (50 to 95 years old) were analyzed. Microindentation was used to assess the mechanical properties of the extracellular matrix, quantitative microradiography was used to measure the degree of bone mineralization (DMB), and Fourier transform infrared microspectroscopy was used to evaluate the physicochemical modifications of bone composition (organic versus mineral matrix). Juvenile and adult osteonal and interstitial regions were analyzed for DMB, crystallinity, mineral to organic matrix ratio, mineral maturity, collagen maturity, carbonation, indentation modulus, indicators of yield strain and tissue ductility using a mixed model. We found that the intrinsic properties of the juvenile bone were not all inferior to those of the adult bone. Mechanical properties were also differently explained in juvenile and adult groups. The study shows that different intrinsic properties should be used in case of juvenile bone investigation.