Bone

BackgroundPalliative radiation therapy (RT) for metastatic spine disease significantly increases the risk of vertebral fractures. However, the temporal mechanisms underlying radiation-induced vertebral bone fragility remain poorly understood. ObjectiveTo evaluate the longitudinal effects of a single high-dose irradiation, simulating palliative RT, on vertebral bone mechanical, architectural, and compositional properties in a healthy, skeletally mature rat model. MethodsThirty-one male Sprague Dawley rats received a single 15 Gy lumbar spine irradiation (IR). L4 vertebrae were assessed across all groups (irradiation: 7, 14, and 28 days post-IR, controls: at 0 and 28 days post-IR) for compressive strength and stiffness, micro-CT-derived bone composition and trabecular indices, serum bone turnover markers (NTX and BAP) and advanced glycation endproducts (AGEs). ResultsIrradiation induced progressive deterioration of vertebral bone mechanical properties, with strength decreasing up to 44% and stiffness up to 38% by 28 days post-IR, compared to 0- day controls. Trabecular bone exhibited reduced BMD, BV/TV, and Tb.N with increased Tb.Sp, a shift toward a more rod-like structure. Early post-IR changes suggested disrupted bone remodeling, characterized by elevated NTX and AGEs, but decreased BAP. Multivariable regression demonstrated that Tb.Th and AGEs were independent predictors of stiffness, collectively explaining 61% of its variance. DiscussionHigh-dose irradiation induces sustained temporal degradation of vertebral mechanical properties driven by both trabecular architectural deterioration and alterations in bone matrix quality. Measures of bone composition and non-enzymatic bone turnover suggest this early damage was driven by disruption of bone cellular homeostasis, favoring increased resorption over formation. These findings support that radiation impairs both structural integrity and pre-yield mechanical behavior, providing mechanistic insight into the elevated fracture risk observed clinically after irradiation for metastatic spine disease. Lay summaryThis study used a rat model to mimic palliative radiation therapy for cancer that has spread to the spine and evaluated the changes in bone quality up to 28 days post-therapy. We found that irradiation progressively weakened the structural integrity and composition of the bones in the spine and disrupted the normal balance of bone breakdown and repair, leading to greater bone loss and fragility. Our findings provide insight into the increased risk of fractures observed in patients receiving radiation therapy to the spine and may support efforts to better protect bone health during treatment.

RNF43 and ZNRF3 are transmembrane E3 ubiquitin ligases that negatively regulate Wnt signaling by promoting ubiquitination and degradation of Frizzled receptors. Loss of either gene enhances Wnt/{beta}-catenin signaling and has been linked to tumorigenesis. Wnt signaling is a key regulator of skeletal development and bone homeostasis, and pharmacologic activation of this pathway is an established therapy for osteoporosis. In Xenopus laevis, simultaneous disruption of rnf43 and znrf3 results in supernumerary limb formation; however, their roles in mammalian limb development and skeletal maintenance remain unclear. We demonstrate that mice homozygous for null alleles of both Rnf43 and Znrf3 do not develop supernumerary limbs. Because activation of Wnt/{beta}-catenin signaling in osteoblasts increases bone mass, we hypothesized that osteoblast-specific deletion of Rnf43 and/or Znrf3 would produce a high-bone-mass phenotype. Instead, osteoblast-specific loss of Znrf3 resulted in age-and sex-dependent reductions in trabecular bone mass, characterized by decreased bone mineral density and bone volume fraction, reduced trabecular number, and increased trabecular separation. Cortical bone exhibited increased cross-sectional size with reduced cortical area fraction and altered structural properties, while tissue mineral density was unchanged. In contrast, deletion of Rnf43 had minimal skeletal effects, and combined deletion of both genes did not exacerbate the phenotype observed with loss of Znrf3 alone. These findings identify Znrf3 as the dominant functional paralog regulating bone architecture in mature osteoblasts and underscore the importance of evaluating skeletal geometry when modulating upstream Wnt regulators.

Sex hormones are essential regulators of skeletal maintenance, but the cell-specific mechanisms by which osteocytes mediate the skeletal consequences of sex hormone deficiency remain incompletely understood. Osteocyte estrogen receptor {beta} (Ot-ER{beta}) has been implicated in sex-specific regulation of bone mass, particularly in male mice, yet its role in coordinating bone morphology and mechanical competence following sex hormone withdrawal is unclear. In this study, male and female mice with osteocyte-targeted ER{beta} deletion (ER{beta}-dOT) and littermate controls were subjected to orchiectomy (ORX), ovariectomy (OVX), or sham surgery at 20 weeks of age. Four weeks later, vertebral and tibial bone morphology were assessed by micro-computed tomography, and tibial mechanical behavior was evaluated using strain gauge-calibrated, microCT-based finite element modeling. ORX induced substantial cancellous bone loss in the lumbar vertebra and proximal tibia of male mice regardless of genotype. In cortical bone, however, ORX reduced tibial cortical area and minimum moment of inertia in male littermate controls, whereas these cortical deficits were attenuated in male ER{beta}-dOT mice. Consistent with these morphological changes, ORX increased finite element-predicted peak tensile and compressive strains in tibial cortical and cancellous compartments and reduced whole-bone stiffness in male controls, but these mechanical deteriorations were largely prevented by Ot-ER{beta} deletion. In contrast, OVX produced modest changes in female tibial cortical geometry and increased cancellous bone strains, but these responses were not strongly dependent on Ot-ER{beta}. Together, these findings reveal that Ot-ER{beta} mediates the skeletal response to sex hormone withdrawal in a sex- and compartment-dependent manner. Specifically, Ot-ER{beta} contributes to ORX-induced deterioration of tibial cortical morphology and mechanical competence in male mice, whereas it is largely dispensable for OVX-induced skeletal changes in female mice. This work highlights osteocyte ER{beta} as a sex-specific regulator linking hormonal status, bone morphology, and load-induced strain environments.

Introduction Denosumab increases bone mineral density and reduces fracture risk in patients with osteoporosis. However, whether BMD response to denosumab differs by age, particularly during longer term treatment, remains unclear. This study investigated the association between baseline age and BMD gain during 3 years of denosumab treatment in patients with osteoporosis. Methods This retrospective study included patients with osteoporosis who were treated with denosumab. DXA-based BMD and bone turnover markers were followed for up to 3 years. Percent BMD gain from baseline, defined as %BMD gain, was evaluated. The longitudinal association between baseline age and %BMD gain was assessed using multivariable linear mixed-effects models for the lumbar spine and total hip. Analyses were performed in the treatment naive cohort and the overall cohort according to prior osteoporosis treatment status. Results A total of 255 patients were included in the analysis, of whom 110 had not received prior osteoporosis treatment. In multivariable linear mixed-effects models, older baseline age was associated with smaller lumbar spine %BMD gain in the treatment naive cohort at both 1 and 3 years. Each 1-year increase in age was associated with a 0.187 percentage-point lower lumbar spine %BMD gain at 1 year and a 0.293 percentage-point lower gain at 3 years (1 year: {beta} = -0.187, p = 0.006, 3 years: {beta} = -0.293, p = 0.031). In contrast, baseline age was not significantly associated with total hip %BMD gain in the treatment naive cohort (1 year: {beta} = -0.011, p = 0.826; 3 years: {beta} = 0.028, p = 0.727). In the overall cohort, baseline age was not significantly associated with %BMD gain at either the lumbar spine or total hip at 1 or 3 years (all p > 0.05). Conclusion Older baseline age was associated with a modestly smaller lumbar spine BMD gain in treatment naive patients, whereas no significant age-related association was observed at the total hip. In the overall cohort, age was not significantly associated with BMD gain at either site. These findings suggest that age may have a limited, site specific influence on BMD response to denosumab, particularly in treatment naive patients, and may support more individualized treatment planning in patients with osteoporosis.

The porous bone cortex comprises an interconnected network of intracortical vascular canals and osteocyte lacunae, embedded within the bone mineral. Increases in cortical porosity reduce bone strength and increase fracture risk. To date, our understanding of mechanisms coupling the arrangements of the vascular: lacunar network in the bone cortex is poorly understood yet it could be key in establishing regulation of cortical porosity evident with age. Using synchrotron radiation-based computed tomography we develop automated tools to characterise the 3D spatial organisation and morphology of osteocyte lacunae, and the bone vasculature at the tibiofibular junction (TFJ), defining posterior, medial, lateral, and anterior regions in male C57BL/6 mice (n = 3). We also investigate the role of osteoblast-derived VEGF in regulating the 3D spatial arrangement by conditional disruption of VEGF in osteocalcin-expressing cells (OcnVEGFKO versus WT, n = 3). Regional lacunar phenotypes were assessed by 3D distance mapping of lacunar organisation surrounding the vascular compartments, including endosteal and periosteal surfaces, or intracortical canals. Surface-associated lacunae were indistinct in size across posterior, medial, lateral and anterior regions. However, lacunae associated with intracortical canals were significantly larger exclusively within the posterior region. In the absence of VEGF, the increased lacunar volume associated with posterior intracortical canals was lost. Our results suggest that the influence of intracortical canals on lacunar volumes is spatially regulated and sensitive to locally produced growth factors such as osteoblast-derived VEGF.

In adult mice, new bone accrual following mechanical load is mediated by the neurotrophin nerve growth factor (NGF) that is expressed by osteoblasts on the bone surface. NGF can bind to its high affinity receptor, neurotrophic tyrosine kinase receptor type 1 (TrkA), on peripheral sensory nerves resident in bone and support new bone formation. However, the osteoanabolic therapeutic potential of NGF-TrkA signaling to repair bone is limited due to the long-lasting thermal and mechanical hyperalgesia induced by administration of NGF in mice and humans. Here, we investigated whether 1) mature osteoblasts are the primary source of NGF required for bone accrual following loading, and 2) a small molecule TrkA receptor agonist - gambogic amide - can harness the downstream osteoanabolic potential of NGF-TrkA signaling in the absence of endogenous NGF. Loss of Ngf transcription in mature osteoblasts did not appear to affect bone structure or bone mass in adulthood. However, Ngf knockout mice significantly reduced periosteal bone accrual and osteogenic Wnt transcription in response to loading compared to wildtype mice. Intraperitoneal injection of gambogic amide prior to loading was unable to produce its osteoanabolic effects in Ngf knockout mice, suggesting that gambogic amide primarily functions in collaboration with endogenous NGF in bone. In total, our study reveals an important role for osteoblastic NGF in the skeletal adaptation of bone to mechanical forces.

The Linker of Nucleoskeleton and Cytoskeleton (LINC) complex is a crucial connective component between the nuclear envelope and the cytoskeleton involving various cellular processes including nuclear positioning, nuclear architecture, and mechanotransduction. How LINC complexes regulate bone formation in vivo, however, is not well understood. To start bridging this gap, here we created a LINC disruption murine model using transgenic mice expressing Cre recombinase enzyme under the control of the Osterix (Osx-Cre) which is primarily active in pre-osteoblasts and floxed Tg(CAG-LacZ/EGFP-KASH2) mice. Tg(CAG-LacZ/EGFP-KASH2) mice contain a lox-STOP-lox flanked LacZ gene which is deleted upon cre recombination allowing for the overexpression of an EGFP-KASH2 fusion protein. This overexpressed protein disrupts endogenous Nesprin-Sun binding leading to disruption of LINC complexes. Thus, crossing these two lines results in a Osx-driven LINC disruption (ODLD) specific to pre-osteoblasts. In this study, we investigated how this LINC disruption affects exercise induced bone accrual. ODLD cells had decreased osteogenic and adipogenic potential in vitro compared to non-disrupted controls and sedentary ODLD mice showed decreased bone quality at 8-weeks. Upon access to a voluntary running wheel ODLD animals showed increased running time and distance; however, our 6-week exercise intervention did not significantly affect bone microarchitecture and bone mechanical properties.

Osteoclasts derived from hematopoietic stem cells control bone resorption. Identifying novel molecules that can epigenetically regulate osteoclastogenesis has been an important basic and clinical issue. The polycomb group (PcG) protein enhancer of zeste homologue 2 (Ezh2), a histone lysine methyltransferase is associated with epigenetic regulation of numerous cellular processes, it is not yet clear on its involvement in bone cell development and homeostasis. Here, we crossed LysM-Cre mice with Ezh2flox/flox mice to delete Ezh2 in myeloid cell lineage mature macrophages. Conditional deletion of Ezh2 in macrophages resulted in significant increases in postnatal bone growth in the first 6 months of life, but tibia length and body weight gains were not different in knockout mice compared with their wild type controls. Significantly decreased osteoclastogenesis but increased bone mass without osteopetrosis were found in Ezh2 conditional knockout (CKO) mice. In contrast to female mice, one floxed Ezh2 gene copy recombinant with LysM-Cre+ (Ezh2flox/+LysM-Cre+) produced increased bone mass in young adult male mice compared with control mice (Ezh2flox/flox, LysM-Cre+ and wild type). Inflammatory milieu in bone was significantly lower in both male and female CKO mice compared with their respective controls. Deletion of Ezh2 in macrophages triggered increased gene expression of osteoclast suppressors, IRF8, MafB and Arg1 due to decreased Ezh2-induced trimethylation of H3K27me3. Conversely, NFATc1 and Cathepsin k expression were decreased. These findings suggest that pre-osteoclastic cell differentiation is under epigenetic control of osteoclast suppressive gene expression via an Ezh2-dependent mechanisms.

Current diagnostic tools are neither informative of, nor concurrent with the underlying biological processes of fracture healing. We examined if there would be protein markers in the plasma of non-operatively fixed, isolated, non-articular, humeral fractures that would be informative of these biological processes. Five follow-up visits over a six-month period were examined in 24 patients that healed (union) and 7 non-healers (nonunion). Using the first or last clinic visit as the reference, 118 proteins out of [~]7000 proteins that were assayed, showed differential expression at log2 fold change [≥]1.3 fold and a significance of FDR [≤] 0.5. Multiple proteins related to endochondral bone development showed significant increased plasma levels during the first 12-16 weeks. Proteins associated with acute phase response, inflammation and lipid storage showed decreased levels after their initial induction. Using a time point to time point comparison for the first three follow-up visits (up to 9 weeks after injury) between healed and nonunion patients, identified 41 proteins with differential levels at p [≤] 0.05 significance, with ten overlapping the 118 proteins that tracked normal healing. The differential group associated with the nonunion patient s included specific extracellular matrix and acute phase proteins that showed decreased levels relative to normal healers. These data establish the progression of endochondral bone formation and place them in context to radiological and functional outcomes in human fracture healing. The identification of differentially expressed proteins in nonunion plasma suggests the potential for the development of diagnostic markers for the early intervention of nonunion. ClinicalTrials.gov#NCT05143476 Sponsor/Funding SourceDOD W81XWH-19-1-0796

To evaluate how recent advances in deep learning can improve the construction of quantitative phenotypes for genome-wide association studies (GWAS), we focused on the context of osteoporosis and bone mineral density (BMD) measurements. We applied image classifiers and transformer models to three distinct tasks. First, we developed quantitative estimates of osteoporosis severity using bone X-ray images. Second, we compared standard approaches for handling confounding variables with a multi-factor strategy based on transformer models trained on UK Biobank data. Third, we investigated whether image-based models could predict how single nucleotide polymorphisms (SNPs) associated with BMD influence bone structure. While our results were promising, application of deep learning methods did not yield substantial improvements over established approaches. Nonetheless, our findings highlight the potential of integrating imaging and machine learning techniques to refine phenotype definitions in genetic studies.

Schwann cells (SCs) provide support for nerves throughout the body. Despite importance for nerve function and repair, the morphology and distribution of SCs in bone remains largely undefined. In this study we used a "Schwann Cell Mapper" mouse (Mpz-Cre+/-;TdT+/+;Ngfr-eGFP+/+) and a "p75 Lineage Tracer" mouse (Ngfr-CreERT2+/-;ZsGreen1+/+) to study SC localization and morphology within the adult mouse calvaria, limb, and vertebrae. We found that all nerves in bone were covered by mature myelinating or non-myelinating SCs, labeled by MPZ and p75-NGFR, respectively. Mature SCs populated the periosteum and entered the bone marrow through transcortical canals. Non-myelinating SCs outnumbered myelinating SCs in bone, with a ratio of [~]2:1 by length density. Non-myelinating SCs in periosteum had more branching and increased size relative to myelinating SCs. In addition, we identified two candidate populations of MPZ lineage+ and p75-NGFR+ immature SCs (iSCs) that were distributed throughout the calvarial periosteum. Similar to mature SCs, p75-NGFR+ candidate iSCs were more prevalent at a ratio of [~]3:1. Overall, neural crest-derived calvarial bone had evidence of increased SC maturity relative to mesoderm-derived sites, identifying niche-level differences in SC maturation. Lastly, lineage tracing revealed that mesenchymal lineages in bone were largely negative for both MPZ and p75-NGFR (<0.1% labeling). These findings provide a framework of SC organization in bone and highlight previously unrecognized diversity across skeletal compartments. By defining distribution and morphology, this work lays the foundation for future studies investigating how SCs contribute to bone biology, including roles in repair, pain, and homeostasis.

Micro-computed tomography ({micro}CT) is widely used to assess trabecular bone microarchitecture, with trabecular separation (Tb.Sp) among the core parameters recommended for reporting. Tb.Sp is typically expressed as a single volume-weighted mean derived from maximal sphere fitting, although the underlying distribution of local separation values is rarely examined. Here, we show that Tb.Sp distributions in metaphyseal trabecular bone are frequently non-Gaussian and bimodal or multimodal. Using {micro}CT datasets from three established models of osteoporosis, spinal cord injury (SCI), ovariectomy (OVX), and ageing, we demonstrate that this behaviour is most evident in metaphyseal trabecular bone and is less apparent in epiphyseal trabecular bone or trabecular thickness distributions. We further show that multimodal metaphyseal Tb.Sp distributions correspond to two spatially distinct contributions within the marrow space: lower-diameter local separation within the residual trabecular network, and higher-diameter regions associated with larger contiguous marrow cavities. Based on this observation, we introduce a simple extension to standard morphometric analysis in which Tb.Sp is decomposed into local trabecular separation (Tb.SpL) and marrow cavity separation (Tb.SpM). Tb.Sp decomposition revealed model-specific patterns of trabecular deterioration. SCI was characterised predominantly by increased Tb.SpM, consistent with expansion of larger marrow cavities, whereas OVX showed a more subtle or distributed alteration. Ageing showed changes in both Tb.SpL and Tb.SpM, with the higher-diameter component becoming most prominent in older animals. Together, these findings demonstrate that mean Tb.Sp can mask structurally distinct forms of metaphyseal marrow-space organisation and support reporting distributional descriptors, and where appropriate Tb.SpL and Tb.SpM, alongside conventional Tb.Sp.

IntroductionTrabecular bone exhibits brittle behaviour governed by microscale deformation and damage processes, yet quantitative characterisation of crack progression remains challenging because classical fracture mechanics approaches do not apply to architecturally discontinuous porous tissues. This study evaluates whether synchrotron X-ray computed tomography (XCT) combined with digital volume correlation (DVC) can provide a practical experimental approach for quantifying crack opening behaviour in human trabecular bone. MethodSemicylindrical specimens harvested from femoral heads of hip-fracture donors (n = 5) and non-fracture controls (n = 5) underwent stepwise three-point-bending during XCT imaging. Full-field displacement maps enabled direct measurement of crack mouth opening displacement (CMOD), crack length (a), and their ratio, CMOD/a, used here as a geometry-normalised comparative descriptor of brittle response. Automated crack segmentation using phase-congruency crack detection (PCCD) was compared against manual measurements. ResultsXCT-DVC successfully resolved three-dimensional displacement discontinuities during crack initiation and propagation in all specimens. Hip-fracture donors exhibited significantly lower critical crack-opening ratios (CMOD/a)* than Controls (0.31 vs 0.47; p = 0.008) and reached mechanical instability at lower applied loads, consistent with a more brittle structural response under this test configuration. Despite these differences, total crack extension ({Delta}a*) was similar between groups. Automated crack tracking using phase-congruency-based segmentation showed excellent agreement with manual measurements (r{superscript 2} = 0.98), confirming reliable extraction of crack geometry from DVC displacement fields. DiscussionThese results indicate that XCT-DVC can provide a practical approach for quantifying crack-opening behaviour in trabecular bone when classical fracture-mechanics parameters are not applicable in anatomically constrained specimens. The reduced critical crack-opening ratios and earlier instability observed in Hip-fracture donors are consistent with a more brittle comparative mechanical response that is not captured by crack extension alone. The strong agreement between automated and manual crack measurements further supports displacement-based descriptors as reliable comparative indicators of brittle behaviour in porous, architecturally discontinuous tissues. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=76 SRC="FIGDIR/small/714043v1_ufig1.gif" ALT="Figure 1"> View larger version (28K): org.highwire.dtl.DTLVardef@31c5d7org.highwire.dtl.DTLVardef@1b3d9a4org.highwire.dtl.DTLVardef@95df7borg.highwire.dtl.DTLVardef@1834216_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOGraphical abstractC_FLOATNO C_FIG

Three-dimensional dual-energy X-ray absorptiometry (3D-DXA) reconstructs proximal femur models from standard scans to estimate cortical and trabecular bone parameters. The aim of this study was to evaluate 3D-DXA against quantitative computed tomography (QCT) across independent international cohorts. The study included 537 subjects from four cohorts: an adult population from Spain, a postmenopausal female population from the United States, an osteoarthrosis population and a young population, both from Japan. Subjects underwent both 3D-DXA and QCT imaging. Accuracy was assessed using linear regression and Bland-Altman analysis to evaluate systematic and random errors. 3D-DXA parameters strongly correlated with QCT across all datasets, with correlation coefficients between 0.82 and 0.97. Random errors were consistent across cohorts and ranged between 16.55 and 19.91 mg/cm3 for integral volumetric bone mineral density (vBMD), between 13.52 and 18.47 mg/cm3 for trabecular vBMD, and between 9.13 and 11.37 mg/cm2 for cortical surface bone mineral density (sBMD). Systematic errors ranged between -14.84 and 4.50 mg/cm3 for integral vBMD, between -8.31 and 14.41 mg/cm3 for trabecular vBMD, and between -5.58 and 3.21 mg/cm2 for cortical sBMD. The variations in systematic errors were likely attributable to differences in QCT acquisition protocols. Overall, these results demonstrate consistent agreement between 3D-DXA and QCT across sex, age, ethnicity, geographic regions, and clinical profiles. Taken together, these findings support the use of 3D-DXA as an accurate, non-invasive, and clinically accessible technology for advanced assessment of the cortical and trabecular compartments of the proximal femur.

Neutralizing sclerostin antibodies (Scl-Ab) mitigate bone loss and promote bone formation to address fracture risk in postmenopausal osteoporosis. Clinically, this treatment is administered monthly for women at high risk of fragility fractures, who are often years into menopause. Preclinical studies have demonstrated that dampening of bone formation occurs with continuous dosing at supraphysiological doses. Osteoporotic bone loss occurs rapidly during early menopause, followed by longer-term changes in bone mineralization and osteocyte activity. Whether earlier administration of lower-exposure Scl-Ab can mitigate bone loss and osteocyte-driven secondary mineralisation is unknown. The objective of this study was to evaluate the effects of early intermittent low-dose Scl-Abon: (1) osteoclastogenesis and bone resorption, (2) perilacunar remodelling, (3) secondary mineralization, and (4) osteocyte mechanosensitivity. Female retired breeder Wistar rats underwent bilateral ovariectomy and received monthly low-dose Scl-Ab injections (2 mg/kg/month) from 3 to 14 weeks post-OVX, while a control group remained untreated. Early intermittent low-dose Scl-Ab treatment increased bone formation and reduced osteoclastogenesis and catabolic gene expression ((Sost, Ctsk, Mmp9) compared to untreated rats. Treatment also decreased the percentage of empty lacunae and the number of MMP14+ osteocytes, accompanied by lower perilacunar mineral density and smaller lacunar size, indicating improved osteocyte survival and reduced perilacunar remodelling. Conversely, expression of osteocyte-mediated mineralization genes (DMP1, PHEX, OPN, ALP) and mechanotransduction-related genes (Vcl, integrins 5, V, {beta}1, CX43, Axin2, IFT88, Adcy6, Pkd1, Cav1) were reduced. Together, these findings suggest that early intermittent low-dose Scl-Ab therapy promotes surface bone formation while attenuating osteocyte-mediated secondary mineralization after initial bone loss. Mini AbstractEarly intermittent low-dose sclerostin antibody treatment reduced osteoclastogenesis, bone resorption, and perilacunar remodelling, while promoting bone formation, decreasing osteocyte apoptosis, and downregulating genes associated with secondary mineralization and mechanosensitivity in ovariectomized rats. These findings suggest early intervention with Scl-Ab enhances bone formation and limits osteocyte apoptosis and subsequent secondary mineralization.

Osteoarthritis (OA) is the most common joint disease globally. In OA, articular cartilage degradation is often accompanied with sclerosis of the subchondral bone. However, the association between OA and tissue mineralization at the nanostructural level is currently not understood. Especially, it is technically challenging to identify calcified cartilage, where relevant but poorly understood pathological processes like tidemark multiplication and advancement occur. Here, we used state-of-the-art micro-focus small-angle X-ray scattering with high 5{micro}m spatial resolution to determine mineral crystal thickness in human subchondral bone and calcified cartilage. Specimens with a wide spectrum of OA severities were acquired from the medial and lateral compartments of medial compartment knee OA patients (n=15) and cadaver knees (n=10). For the first time, we identified a well-defined layer of calcified cartilage associated with pathological tidemark multiplication, containing 0.32nm thicker crystals compared to the rest of calcified cartilage. In addition, we found 0.2nm thicker mineral crystals in both tissues of the lateral compartment in OA compared with healthy knees, indicating a loading-related disease process since the lateral compartment is typically less loaded in medial compartment knee OA. Furthermore, the crystal thickness of the subchondral bone was lower with increasing histopathological OA severity. In summary, we report novel changes in mineral crystal thickness during OA. Our data suggest that unloading in the knee is associated with the growth of mineral crystals, which is especially evident in the calcified cartilage. In the subchondral bone, mineral crystals become thinner with increasing OA severity, which indicates new bone formation with sclerosis. One Sentence SummaryMineral crystal thickness increases with osteoarthritis in the lateral condyle that is typically unloaded.

Approximately 85-90% of individuals with Osteogenesis Imperfecta (OI) have dominant pathogenic variants in the COL1A1 or COL1A2 genes. This leads to decreased or abnormal Collagen type I production. Subsequently, bone formation is strongly reduced, causing bone fragility and liability to fractures throughout life. OI is clinically classified in 5 types with the severity ranging from mild to lethal depending on the gene and the type and location of the OI-causative variant and the subsequent effect on (pro) collagen type I synthesis. However, the specific effects on the phenotype and function of osteoblasts are not fully understood. To investigate this, the OI murine model was used, with the oim/oim (OIM) mice closest resembling severely deforming OI type 3 in humans. We showed that in OIM, COL1 mutation results in a multifactorial inhibition of the osteogenic differentiation and maturation as well as inhibition of osteoclastogenesis. The phenotype of differentiated OIM osteoblasts also differs from that of wild type mature osteoblasts, with upregulated oxidative cell stress and autophagy pathways, possibly in response to the intracellular accumulation of type I collagen mRNA. The extracellular accumulation of defective type I collagen fibres contributes to activation of the TGF-{beta} signalling pathway and activates the inflammatory pathway. These effects combine to destabilise the balance of bone turnover, increasing bone fragility. Together, these findings identify the complex mechanisms underlying OI bone fragility in the OIM model of severe OI and can potentially enable identification of clinically relevant endpoints to assess the efficacy of innovative pro-osteogenic treatment for patients with OI.

PurposeLamellar bone that forms in moderate and severe osteogenesis imperfecta (OI) is often composed of structurally irregular lamellae compared to those in normal bone. Polarization light microscopy (PLM) demonstrates lamellar bone well but has rarely been used for quantitative studies; information available on normal bone lamellae tends to be variable and studies specifically assessing OI bone lamellae have not been done. We report on PLM histomorphometry quantifying bright and dark lamellar thicknesses in normal and OI bone. Manual measurements of individual lamellar thicknesses have been made on histologic sections using the cellSens image analysis system; in an effort to augment the number of measurements we also developed a method of automated thickness averaging in quantifying regions of lamellae. MethodsFemoral and tibial cortical bone fragments from 5 individuals 5 - 26 years old (without molecular bone disorders) and 8 individuals 5 - 16 years old with progressively deforming (Sillence III) OI were obtained. The fragments were decalcified, infiltrated in JB4 solution, embedded in JB4 plastic, sectioned at 5 thickness and stained with 1% toluidine blue for light and polarizing microscopy. Manual measurements: Strict criteria for measurement, primarily to eliminate oblique lamellae, included accumulations of 16-20 bright and dark lamellae under PLM with a relatively narrow range of thicknesses, flattened elliptical osteocytes along the longitudinal axis of the lamellae and canaliculi passing from the walls of the osteocyte lacunae at right angles to the lamellae. Histomorphometric measurements of bright and dark lamellae by PLM were made at 20X magnification. Automated measurements: A script for automated measurement of average lamellar thicknesses from PLM images was developed in MATLAB (Mathworks, Natick, MA) to make measurement faster and less subjective. The script isolates a region from an image for measurement and marks each pixel as either bright or dark based on a local average intensity threshold. It then takes multiple pixel measurements along the length of the lamellae in the image and returns the average thickness of each in m. Results1. OI bone mean lamellar thickness values are always less than those in normal bone. The mean value for all OI bright and dark lamellae combined is 1.80 {+/-} 0.72 m and the value in normal bone is 2.54 {+/-} 0.92 m. 2. Mean value for the bright lamellae is less than that for the dark lamellae in both normal and OI bone. The mean value for bright lamellae in OI is 1.47 {+/-} 0.53 m and for dark lamellae 2.18 {+/-} 0.72 m; in normal bone the mean value for bright lamellae is 2.06 {+/-} 0.54 m and for dark lamellae 3.07 {+/-} 0.96 m. The differences are statistically significant: between groups of normal and OI lamellae (p<0.001), normal and OI light bands (p<0.001), and normal and OI dark bands (p<0.001). 3. Ratio of mean values for bright/dark lamellar thicknesses is the same in OI and normal bone. The ratio in OI bone is 0.67 (range: 0.54 - 0.83) and in normal bone 0.67 (range: 0.60 - 0.88). 4. Validation of automated vs. manual datasets: For each lamella in the validation dataset, the percent difference between the automated and manual measurements was calculated. The mean of the absolute values of these percent differences was 18.9%, a statistically non-significant difference (p = 0.0518). Discussion and conclusionsLamellar bone that forms in moderate and severe OI is composed of thinner and less regular lamellae than those in normal bone. i) PLM histomorphometry shows mean lamellar thicknesses (bright and dark merged) are statistically significantly decreased in OI compared to normal bone as are bright and dark lamellar thicknesses measured independently. ii) The automated method can be adapted readily to the assessment process for lamellar thicknesses and is, most likely, more accurate since it averages a greatly increased number of measurements per individual lamella. iii) Lamellar thickness measurements can be helpful in assessing the effect of specific collagen mutations on OI bone synthesis and warrant inclusion in both research and clinical histomorphometric assessments.

Bones adapt to external mechanical loads through a process known as mechanoadaptation. Osteocytes are the bone cells that sense the mechanical environment and initiate a biological response. Investigating the changes in osteocyte molecular expression following mechanical loading has been instrumental in characterizing the regulatory pathways involved in bone adaptation. However, current methods for examining osteocyte molecular expression do not preserve the three-dimensional structure of the bone, which plays a critical role in the mechanical stimuli sensed by the osteocytes and their spatially controlled biological responses. In this study, we used WISH-BONE to investigate the spatial distribution of Sost-mRNA transcripts and its encoded protein, sclerostin, in 3D mouse tibia midshaft following in vivo tibia loading. Our findings showed a decrease in the percentage of Sost-positive osteocytes predominantly at 25% and 37% of the bone length, and in the posterior-lateral side of the tibia after loading. Sclerostin-positive osteocytes in the loaded legs were found to be similar to the contralateral legs after 2 weeks of loading. This work is the first to provide a 3D analysis of Sost and sclerostin distribution in loaded versus contralateral mouse tibia midshafts. It also highlights the importance of the bone region analyzed and the method utilized when interpreting mechanoadaptation results. WISH-BONE represents a powerful tool for further characterization of mechanosensitive genes regulation in bone and holds potential for advancing the development of new treatments targeting mechanosensitivity-related bone disorders.

Tartrate-resistant acid phosphatase (TRAP/ACP5), primarily known as an osteoclast marker, has emerged as a critical regulator of skeletal integrity, regulating sex-specific bone growth, and bones response to mechanical load in young adult male mice. In this study, we investigated the sex-specific roles of TRAP in bone structure and response to mechanical stimuli in old (19-month-old) wild-type (WT) and TRAP-deficient (TRAP-/-) mice using micro-computed tomography, serum bone turnover markers, in vivo axial mechanical loading, and in vitro mechanotransduction assays. Our findings revealed that TRAP-/- mice of both sexes maintained shorter tibiae than WT mice independent of sex. Notably, male, but not female, TRAP-/- mice have increased trabecular bone volume fraction and cortical bone area compared to WT, indicative of disrupted bone remodelling processes in male mice. Interestingly, TRAP-deficiency substantially impaired the anabolic bone response to mechanical loading, affecting both trabecular and cortical compartments in both sexes, indicating that when challenged, TRAP is important for bone formation also in female mice. Mechanical stimulation in vitro of hematopoietic progenitor cells from WT and TRAP-/- mice revealed that the increased ATP-release in response to mechanical stimulation was only disrupted in male mice, while mechanically induced increase in osteoclast formation was inhibited in TRAP-/- mice of both sexes. These results highlight the importance of TRAP in maintaining trabecular architecture and cortical bone in male mice and underscore its critical function in mediating adaptive responses to mechanical loading of both sexes, during aging. Future investigations should focus on elucidation of TRAP-dependent pathways as potential therapeutic targets to counteract age-related deficits in bone adaptation and remodelling.