Quantifying osteoblast and osteocyte apoptosis: challenges and rewards

Efferocytosis in inflammatory bone disorders

Efferocytosis, the clearance of apoptotic cells (ACs) by phagocytes, is crucial for bone homeostasis and immune balance. This tightly regulated process depends on molecular markers such as phosphatidylserine on ACs and MERTK on phagocytes. In the bone microenvironment, multiple cell types participate in efferocytosis, including osteal macrophages, mesenchymal stem cells, osteoblasts, and osteoclasts, directly influencing bone remodeling and immune responses. Impaired efferocytosis disrupts bone turnover, exacerbates inflammation, and contributes to inflammatory bone diseases. Despite its recognized importance, the precise mechanisms regulating efferocytosis in osteoimmunology remain underexplored, including specific signaling pathways, cell-specific interactions, and therapeutic applications. Recent advances highlight the therapeutic potential of targeting efferocytosis using modalities and biomaterial-based strategies. This review systematically examines the role of efferocytosis in osteoimmunology, discusses key challenges in its therapeutic translation, and explores emerging strategies to optimize efferocytosis-based interventions for inflammatory bone disorders.

The Mdm2-p53 axis links cementocyte survival to cellular cementum volume

Cementocytes are terminally differentiated cells embedded in cellular cementum, an important hard tissue covering the apical regions of tooth roots. However, the roles of cementocytes in cellular cementum remain enigmatic. Here, we show that Murine Double Minute 2 (Mdm2), an E3 ubiquitin ligase that plays vital roles in regulating cell proliferation, apoptosis, and differentiation to influence tissue or organ development, is highly expressed in the cementocytes of mice. To investigate the role of cementocyte-expressed Mdm2, Dmp1-Cre;Mdm2flox/flox (Mdm2 cKO)mice were obtained to inactivate Mdm2 in cementocytes. The results showed that Mdm2 was successfully ablated and Mdm2 cKO mice display increased cementocyte apoptosis and reduced cellular cementum volume. p53, the canonical substrate of Mdm2, was accumulated and hyperactivated in the cementocytes of Mdm2 cKO mice and in cultured IDG-CM6 cells (a cementocyte cell line) treated with Nutlin3a, an inhibitor of Mdm2. Further experiments showed that inactivation of 1 allele of p53 significantly rescued the increased cementocyte apoptosis and the decreased cellular cementum volume in Mdm2 cKO mice. Therefore, p53 is targeted by Mdm2 for degradation and mediates the role of Mdm2 in cementocyte survival and cellular cementum volume. Notably, Mdm2 cKO mice exhibited decreased differentiation of cementoblasts (the cell type primarily responsible for cementum deposition) and reduced rate of cellular cementum deposition. Meanwhile, OCCM-30 cells (a cementoblast cell line) showed diminished migration, proliferation, differentiation, and mineralization ability after culture with conditioned medium (CM) from Nutlin3a-pretreated IDG-CM6 cells. Intriguingly, Mdm2 cKO mice displayed significantly increased osteoclast formation and cementum resorption. Meanwhile, in vitro experiments verified that CM from Nutlin3a-pretreated IDG-CM6 cells induced osteoclast differentiation of bone marrow macrophages. Collectively, these results demonstrate that Mdm2-mediated degradation of p53 promotes cementocyte survival, and that cementocytes affect the cell behaviors of cementoblasts and osteoclasts through a paracrine mode to modulate cellular cementum volume.

Dietary carotenoid intake and fracture risk based on NHANES 2013-2018 data: a propensity score matching

INTRODUCTION

Several epidemiological studies have reported inconsistent findings on the association between carotenoid intake and fracture risk. This study aimed to determine the association between individual carotenoid intake and fracture risk.

METHODS

A cross-sectional study based on data from the National Health and Nutrition Examination Survey (NHANES), 2013-2018. This study identified elderly individuals with valid and complete data on carotenoid intake and fracture risk. The average dietary intakes of α-carotene, β-carotene, β-cryptoxanthin, lycopene, and lutein + zeaxanthin was taken based on the two 24-hour recall interviews. Matching was done based on age, sex, and body mass index (BMI). Logistic regression models were used to test the associations between carotenoids and fracture risk. All analyses were performed by using R (version 3.4.3; R Foundation for Statistical Computing, Vienna, Austria).

RESULTS

A total of 5491 (1140 cases and 4351 control) subjects were included in this study. The average age of the subjects was 55.62 ± 14.84 years old. In the adjusted model, the risk of osteoporotic fracture was decreased by 6.2% (odd ratio (OR): 0.938; 95% confidence interval (CI): 0.699 to 0.989) and 1.4% (OR 0.986; 95% CI: 0.975 to 0.997) for dietary intake of beta-carotene and lycopene, respectively.

CONCLUSIONS

Dietary intakes of beta-carotene and lycopene have significantly reduced the risk of osteoporotic fracture among the elderly population in the United States of America.

Bioactive hydrogel formulations for regeneration of pathological bone defects

Bone defects caused by osteoporosis, infection, diabetes, post-tumor resection, and nonunion often cause severe pain and markedly increase morbidity and mortality, which remain a significant challenge for orthopedic surgeons. The precise local treatments for these pathological complications are essential to avoid poor or failed bone repair. Hydrogel formulations serve as injectable innovative platforms that overcome microenvironmental obstacles and as delivery systems for controlled release of various bioactive substances to bone defects in a targeted manner. Additionally, hydrogel formulations can be tailored for specific mechanical strengths and degradation profiles by adjusting their physical and chemical properties, which are crucial for prolonged drug retention and effective bone repair. This review summarizes recent advances in bioactive hydrogel formulations as three-dimensional scaffolds that support cell proliferation and differentiation. It also highlights their role as smart drug-delivery systems with capable of continuously releasing antibacterial agents, anti-inflammatory drugs, chemotherapeutic agents, and osteogenesis-related factors to enhance bone regeneration in pathological areas. Furthermore, the limitations of hydrogel formulations in pathological bone repair are discussed, and future development directions are proposed, which is expected to pave the way for the repair of pathological bone defects.

Association between dietary carotenoid intake and vertebral fracture in people aged 50 years and older: a study based on the National Health and Nutrition Examination Survey

This study discussed the association between dietary intake of carotenoids and vertebral fractures by analyzing the target data, concluding a result of negative association and providing valuable information on vertebral fracture.

OBJECTIVE

This study is to explore the association between dietary intake of carotenoids and vertebral fractures.

METHODS

Data of individuals aged ≥ 50 years from the 2013-2014 National Health and Nutrition Examination Survey (NHANES) database were used in this cross-sectional study. Information on dietary carotenoid intake was obtained from the first 24-h dietary recall interview. Vertebral fractures were assessed using dual-energy x-ray absorptiometry (DXA). The weighted multivariable logistic regression model was established to assess the association between dietary carotenoid intake and risk of vertebral fracture. Subgroup analysis of fracture history and menopausal status was performed for further analysis of this relationship.

RESULTS

Of the 2053 eligible study subjects, there were 1021 men and 1032 women. Increased β-carotene intake was associated with decreased odds of vertebral fracture in women (odds ratio (OR) = 0.77, 95% confidence interval (CI) 0.60-0.99, P = 0.047), after adjusting for covariates. In men without a history of fracture, lutein and zeaxanthin intake was negatively associated with increased odds of vertebral fracture (OR = 0.70, 95% CI 0.50-0.99, P = 0.048). In women without a history of fracture, increased β-carotene intake was associated with decreased odds of vertebral fracture (OR = 0.78, 95% CI 0.61-0.99, P = 0.047). In postmenopausal women, β-carotene intake was also negatively associated with increased odds of vertebral fracture (OR = 0.77, 95% CI 0.60-0.99, P = 0.048).

CONCLUSION

This research concludes a negative association between dietary carotenoid intake, especially β-carotene, and vertebral fractures in women, revealing a potential dietary prevention tactic for vertebral fractures in the future.

[Research progress on the regulatory cell death of osteoblasts in periodontitis]

Periodontitis is a chronic inflammatory disease characterized by progressive destruction of alveolar bone. The most critical mechanism underlying alveolar bone destruction is the imbalance of bone homeostasis, where osteoblast-mediated bone matrix synthesis plays an important role in regulating bone homeostasis. Regulated cell death is instrumental in both the inflammatory microenvironment and the regulation of bone homeostasis. Chronic inflammation, oxidative stress, and other factors can be directly involved in mitochondrial and death receptor-mediated signaling pathways, modulating B-cell lymphoma 2 family proteins and cysteine aspartic acid specific protease (caspase) activity, thereby affecting osteoblast apoptosis and alveolar bone homeostasis. Chronic inflammation and cellular damage induce osteoblast necroptosis via the RIPK1/RIPK3/MLKL signaling pathway, exacerbating the inflammatory response and accelerating alveolar bone destruction. Stimuli such as pathogenic microorganisms and cellular injury may also activate caspase-1-dependent or independent signaling pathways and gasdermin D family proteins, promoting osteoblast pyroptosis and releasing pro-inflammatory cytokines to mediate alveolar bone damage. Iron overload and lipid peroxidation in periodontitis can trigger ferroptosis in osteoblasts, impacting their survival and function, ultimately leading to bone homeostasis imbalance. This article focuses on the mechanism of periodontal disease affecting bone homeostasis through regulatory cell death, aiming to provide research evidence for the treatment of periodontitis and alveolar bone homeostasis imbalance.

Irisin as an agent for protecting against osteoporosis: A review of the current mechanisms and pathways

BACKGROUND

Osteoporosis is recognized as a skeletal disorder characterized by diminished bone tissue quality and density. Regular physical exercise is widely acknowledged to preserve and enhance bone health, but the detailed molecular mechanisms involved remain unclear. Irisin, a factor derived from muscle during exercise, influences bone and muscle. Since its discovery in 2012, irisin has been found to promote bone growth and reduce bone resorption, establishing a tangible link between muscle exertion and bone health. Consequently, the mechanism by which irisin prevents osteoporosis have attracted significant scientific interest.

AIM OF THE REVIEW

This study aims to elucidate the multifaceted relationship between exercise, irisin, and bone health. Focusing on irisin, a muscle-derived factor released during exercise, we seek to understand its role in promoting bone growth and inhibiting resorption. Through a review of current research article on irisin in osteoporosis, Our review provides a deep dive into existing research on influence of irisin in osteoporosis, exploring its interaction with pivotal signaling pathways and its impact on various cell death mechanisms and inflammation. We aim to uncover the molecular underpinnings of how irisin, secreted during exercise, can serve as a therapeutic strategy for osteoporosis.

KEY SCIENTIFIC CONCEPTS OF THE REVIEW

Irisin, secreted during exercise, plays a vital role in bridging muscle function to bone health. It not only promotes bone growth but also inhibits bone resorption. Specifically, Irisin fosters osteoblast proliferation, differentiation, and mineralization predominantly through the ERK, p38, and AMPK signaling pathways. Concurrently, it regulates osteoclast differentiation and maturation via the JNK, Wnt/β-catenin and RANKL/RANK/OPG signaling pathways. This review further delves into the profound significance of irisin in osteoporosis and its involvement in diverse cellular death mechanisms, including apoptosis, autophagy, ferroptosis, and pyroptosis.

Pharmacological effects of methysticin and L-sulforaphane through the Nrf2/ARE signaling pathway in MLO-Y4 osteocytes: in vitro study

BACKGROUND

Oxidative stress plays a crucial role in the pathogenesis of many skeletal diseases by inducing osteocyte death. The transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) is a master regulator of various antioxidant gene expressions through antioxidant response element (ARE) against cellular oxidative stress and can be induced by various stimulants, including the phytochemicals methysticin (MET) and L-sulforaphane (SFN). This study aimed to establish an osteocyte in vitro model to investigate the pharmacological effects of MET and SFN on the Nrf2/ARE pathway.

METHODS

MLO-Y4 murine osteocytes and the stably transduced MLO-Y4-SIN-lenti-ARE reporter gene cell line were used. MET and SFN were used as Nrf2 inducers. The cytotoxicity of MET, SFN, and hydrogen peroxide (H2O2) was evaluated using the CytoTox-Glo™ Assay. Time- and dose-dependent ARE induction was examined by Monoluciferase Assay. The mRNA and protein expressions of Nrf2 target markers, such as heme-oxygenase 1 (Ho-1), NADPH quinone dehydrogenase 1 (Nqo1), and thioredoxin reductase 1 (Txnrd1), were detected by RT-qPCR, Western Blot, and immunofluorescence staining, respectively. Osteogenesis markers, osteopontin, and osteocalcin were compared with and without treatment by immunofluorescence staining.

RESULTS

The experimental data showed that MET and SFN induced ARE activity in a time- and dose-dependent manner and increased the mRNA and protein expression of antioxidant markers compared to vehicle-treated controls. The protein expression of osteopontin and osteocalcin in the samples treated with SFN were significantly higher than without treatment, and the number of cell death treated with SFN was significantly lower than without treatment under H2O2-induced stress conditions.

CONCLUSIONS

Nrf2 inducers MET and SFN increased the mRNA expression of antioxidant genes through the Nrf2/ARE pathway in osteocytes. Notably, SFN increased the protein expression of osteocyte-associated osteogenic markers and suppressed cell death under H2O2-induced stress condition. Thus, Nrf2 stimulators can exert stress-relieving and osteogenic effects on osteocytes.

Targeting SAT1 prevents osteoporosis through promoting osteoclast apoptosis

Osteoporosis is a systemic bone disease characterized by decreased bone mass that is tightly regulated by the coordinated actions of osteoclasts and osteoblasts. Apoptosis as a precise programmed cell death involves a cascade of gene expression events which are mechanistically linked to the regulation of bone metabolism. Nevertheless, the critical biomolecules involved in regulating cell apoptosis in osteoporosis remain unknown. To gain a deeper insight into the relationship between apoptosis and osteoporosis, this study integrated the sequencing results of human samples and using a machine learning workflow to overcome the limitations of a single study. Among all immune cell populations, we assessed the apoptotic level and portrayed the distinct subtypes and lineage differentiation of monocytic cells in osteoporotic tissues. Osteoclasts expressed a higher level of Spermidine/spermine-N1-Acetyltransferase1 (SAT1) during osteoclastogenesis which prevented osteoclasts apoptosis and facilitate osteoporosis progression. In addition, Berenil, one potent SAT1 inhibitor, increased osteoclast apoptosis and reversed the bone loss in the femurs of a murine ovariectomy model. In summary, Berenil promotes osteoclast apoptosis, inhibits the bone resorption and improves the abnormal bone structure in vitro and in vivo models by targeting SAT1, demonstrating its potential as a precise therapeutic strategy for clinical osteoporosis treatment.

Noncoding RNAs: the crucial role of programmed cell death in osteoporosis

Osteoporosis is the most common skeletal disease characterized by an imbalance between bone resorption and bone remodeling. Osteoporosis can lead to bone loss and bone microstructural deterioration. This increases the risk of bone fragility and fracture, severely reducing patients' mobility and quality of life. However, the specific molecular mechanisms involved in the development of osteoporosis remain unclear. Increasing evidence suggests that multiple noncoding RNAs show differential expression in the osteoporosis state. Meanwhile, noncoding RNAs have been associated with an increased risk of osteoporosis and fracture. Noncoding RNAs are an important class of factors at the level of gene regulation and are mainly involved in cell proliferation, cell differentiation, and cell death. Programmed cell death is a genetically-regulated form of cell death involved in regulating the homeostasis of the internal environment. Noncoding RNA plays an important role in the programmed cell death process. The exploration of the noncoding RNA-programmed cell death axis has become an interesting area of research and has been shown to play a role in many diseases such as osteoporosis. In this review, we summarize the latest findings on the mechanism of noncoding RNA-mediated programmed cell death on bone homeostasis imbalance leading to osteoporosis. And we provide a deeper understanding of the role played by the noncoding RNA-programmed cell death axis at the gene regulatory level of osteoporosis. We hope to provide a unique opportunity to develop novel diagnostic and therapeutic approaches for osteoporosis.

Altered Osteoblast Metabolism with Aging Results in Lipid Accumulation and Oxidative Stress Mediated Bone Loss

Cellular aging is associated with dysfunction of numerous tissues affecting multiple organ systems. A striking example of this is related to age-related bone loss, or osteoporosis, increasing fracture incidence. Interestingly, the two compartments of bone, cortical and cancellous or trabecular, rely on different mechanisms for development and maintenance during 'normal' aging. At a cellular level, the aging process disturbs a multitude of intracellular pathways. In particular, alterations in cellular metabolic functions thereby impacting cellular bioenergetics have been implicated in multiple tissues. Therefore, this study aimed to characterize how metabolic processes were altered in bone forming osteoblasts in aged mice compared to young mice. Metabolic flux analyses demonstrated both stromal cells and mature, matrix secreting osteoblasts from aged mice exhibited mitochondrial dysfunction. This was also accompanied by a lack of adaptability or metabolic flexibility to utilize exogenous substrates compared to osteoblasts cultured from young mice. Additionally, lipid droplets accumulated in both early stromal cells and mature osteoblasts from aged mice, which was further depicted as increased lipid content within the bone cortex of aged mice. Global transcriptomic analysis of the bone further supported these metabolic data as enhanced oxidative stress genes were up-regulated in aged mice, while osteoblast-related genes were down-regulated when compared to the young mice. Collectively, these data suggest that aging results in altered osteoblast metabolic handling of both exogenous and endogenous substrates which could contribute to age-related osteoporosis.

Effects of Weight Bearing on Marrow Adipose Tissue and Trabecular Bone after Anterior Cruciate Ligament Reconstruction in the Rat Proximal Tibial Epiphysis

The effects of mechanical unloading after anterior cruciate ligament (ACL) reconstruction on bone and marrow adipose tissue (MAT) are unclear. We investigated weight bearing effects on bone and MAT after ACL reconstruction. Rats underwent unilateral knee ACL transection and reconstruction, followed by hindlimb unloading (non-weight bearing), no intervention (low-weight bearing, the hindlimb standing time ratio (STR; operated/contralateral) during treadmill locomotion ranging from 0.55 to 0.91), or sustained morphine administration (moderate-weight bearing, STR ranging from 0.80 to 0.95). Untreated rats were used as controls. At 7 or 14 days after surgery, changes in trabecular bone and MAT in the proximal tibial were assessed histologically. Histological assessments at 7 or 14 days after surgery showed that ACL reconstruction without post-operative intervention did not significantly change trabecular bone and MAT areas. Hindlimb unloading after ACL reconstruction induced MAT accumulation with adipocyte hyperplasia and hypertrophy within 14 days, but did not significantly affect trabecular bone area. Increased weight bearing through morphine administration did not affect trabecular bone and MAT parameters. Our results suggest that early weight bearing after ACL reconstruction is important in reducing MAT accumulation, and that reduction in weight bearing alone is not sufficient to induce bone loss early after ACL reconstruction.

Osteoinduction and osteoimmunology: Emerging concepts

The recognition and importance of immune cells during bone regeneration, including around bone biomaterials, has led to the development of an entire field termed "osteoimmunology," which focuses on the connection and interplay between the skeletal system and immune cells. Most studies have focused on the "osteogenic" capacity of various types of bone biomaterials, and much less focus has been placed on immune cells despite being the first cell type in contact with implantable devices. Thus, the amount of literature generated to date on this topic makes it challenging to extract needed information. This review article serves as a guide highlighting advancements made in the field of osteoimmunology emphasizing the role of the osteoimmunomodulatory properties of biomaterials and their impact on osteoinduction. First, the various immune cell types involved in bone biomaterial integration are discussed, including the prominent role of osteal macrophages (OsteoMacs) during bone regeneration. Thereafter, key biomaterial properties, including topography, wettability, surface charge, and adsorption of cytokines, growth factors, ions, and other bioactive molecules, are discussed in terms of their impact on immune responses. These findings highlight and recognize the importance of the immune system and osteoimmunology, leading to a shift in the traditional models used to understand and evaluate biomaterials for bone regeneration.

Calcium - a scoping review for Nordic Nutrition Recommendations 2023

The aim of this scoping review was to conduct evidence-based documentations between calcium (Ca) intake and health outcomes for updating dietary reference values (DRVs) and food-based dietary guidelines (FBDGs) in the sixth edition of Nordic Nutrient Recommendations (NNR2023). The systematic literature search was limited to reviews on human data published between 2011 and June 2021. Systematic reviews (SRs) and original publications of relevance for this scoping review were included. A common practice of designing studies on health outcomes related to Ca supplement intake is to examine combined Ca and vitamin D, and therefore, a combination of Ca with vitamin D (CaD) was included in this review. In total, 27 studies addressing the association between dietary or supplemental Ca on bone health, bone mineral density (BMD), pregnancy-related outcomes, cardiovascular diseases (CVD), cancers, obesity, and mortality were reviewed. SRs showed that both dietary and supplemental Ca intakes were positively associated with BMD, but evidence did not support the benefit in fracture prevention. Current evidence did not support that Ca or CaD supplementation increases risk of coronary heart disease or all-cause mortality in older adults, but that Ca may be beneficial for hypertension, especially in young people. Increasing Ca intake may be beneficial during pregnancy, especially for those at high risk of pre-eclampsia due to ethnicity, age, high BMI, and those with low baseline Ca intake. The associations between high Ca intake and cancers were varied, with strong evidence that high consumption of dairy products is protective against colorectal cancer and limited-suggestive evidence that dairy products and diets high in Ca might also be protective against breast cancer. Moreover, there is limited-suggestive evidence that dairy products and diets high in Ca increase the risk of prostate cancer. Based on current evidence, Ca intake is beneficial or neutral in relation to most of the outcomes evaluated in this review. Data from the Nordic countries show that average Ca intake is around the same as previously recommended by NNR. However, the average Ca intake in the Baltic countries is below the recommendations.

Impaired Efferocytosis Enables Apoptotic Osteoblasts to Escape Osteoimmune Surveillance During Aging

Macrophage efferocytosis of apoptotic osteoblasts (apoOBs) is a key osteoimmune process for bone homeostasis. However, apoOBs frequently accumulate in aged bone marrow, where they may mount proinflammatory responses and progressive bone loss. The reason why apoOBs are not cleared during aging remains unclear. In this study, it is demonstrated that aged apoOBs upregulate the immune checkpoint molecule CD47, which is controlled by SIRT6-regulated transcriptional pausing, to evade clearance by macrophages. Using osteoblast- and myeloid-specific gene knockout mice, SIRT6 is further revealed to be a critical modulator for apoOBs clearance via targeting CD47-SIRPα checkpoint. Moreover, apoOBs activate SIRT6-mediated chemotaxis to recruit macrophages by releasing apoptotic vesicles. Two targeting delivery strategies are developed to enhance SIRT6 activity, resulting in rejuvenated apoOBs clearance and delayed age-related bone loss. Collectively, the findings reveal a previously unknown linkage between immune surveillance and bone homeostasis and targeting the SIRT6-regulated mechanism can be a promising therapeutic strategy for age-related bone diseases.

Beneficial effects of Apelin-13 on metabolic diseases and exercise

Apelin, a novel endogenous ligand of the G-protein-coupled receptor APJ, is encoded by the APLN gene and can be hydrolyzed into multiple subtypes, with Apelin-13 being one of the most active subtypes of the Apelin family. Recent studies have revealed that Apelin-13 functions as an adipokine that participates in the regulation of different biological processes, such as oxidative stress, inflammation, apoptosis, and energy metabolism, thereby playing an important role in the prevention and treatment of various metabolic diseases. However, the results of recent studies on the association between Apelin-13 and various metabolic states remain controversial. Furthermore, Apelin-13 is regulated or influenced by various forms of exercise and could therefore be categorized as a new type of exercise-sensitive factor that attenuates metabolic diseases. Thus, in this review, our purpose was to focus on the relationship between Apelin-13 and related metabolic diseases and the regulation of response movements, with particular reference to the establishment of a theoretical basis for improving and treating metabolic diseases.

The admission pH is a risk factor of preoperative deep vein thrombosis in geriatric hip fracture: a retrospective cohort study

This study evaluated the association between body pH value and preoperative deep vein thrombosis (DVT) in geriatric hip fractures. Older adult patients with hip fractures were screened between January 2015 and September 2019. The demographic and clinical characteristics of the patients were collected. Multivariate binary logistic regression and generalized additive models were used to identify the linear and nonlinear associations between pH value and preoperative DVT. Analyses were performed using EmpowerStats and R software. A total of 1465 patients were included in the study. DVT occurred in 476 (32.6%) of these admitted older adults. We observed a nonlinear association between the serum pH value and preoperative DVT in geriatric patients with hip fractures. A pH value of 7.39 was the inflection point in the curve, with pH highly correlated with DVT at pH < 7.39 (odds ratio [OR] 19.47; 95% confidence interval [CI] 1.45-260.91; P = 0.0249). Patients with lower pH had a lower chance of preoperative DVT formation, and the risk of DVT increased 18.47-fold for every 0.1 unit change in pH. Although at pH > 7.39, pH was not correlated with DVT (OR 1.26; 95% CI 0.85-1.86; P = 0.2561), the odds of DVT did not vary with pH, and the highest risk of thrombosis was reached. The body pH value is nonlinearly associated with preoperative DVT in geriatric patients with hip fractures, and it could be considered a predictor of the risk of DVT.Registered information This study is registered in the website of Chinese Clinical Trial Registry (ChiCTR: ChiCTR2200057323).

Magnetic nanocomplexes coupled with an external magnetic field modulate macrophage phenotype - a non-invasive strategy for bone regeneration

Chronic inflammation is a major cause for the pathogenesis of musculoskeletal diseases such as fragility fracture, and nonunion. Studies have shown that modulating the immune phenotype of macrophages from proinflammatory to prohealing mode can heal recalcitrant bone defects. Current therapeutic strategies predominantly apply biochemical cues, which often lack target specificity and controlling their release kinetics in vivo is challenging spatially and temporally. We show a magnetic iron-oxide nanocomplexes (MNC)-based strategy to resolve chronic inflammation in the context of promoting fracture healing. MNC internalized pro-inflammatory macrophages, when coupled with an external magnetic field, exert an intracellular magnetic force on the cytoskeleton, which promotes a prohealing phenotype switch. Mechanistically, the intracellular magnetic force perturbs actin polymerization, thereby significantly reducing nuclear to cytoplasm redistribution of MRTF-A and HDAC3, major drivers of inflammatory and osteogenic gene expressions. This significantly reduces Nos2 gene expression and subsequently downregulates the inflammatory response, as confirmed by quantitative PCR analysis. These findings are a proof of concept to develop MNC-based resolution-centric therapeutic intervention to direct macrophage phenotype and function towards healing and can be translated either to supplement or replace the currently used anti-inflammatory therapies for fracture healing.

Increased PHOSPHO1 and alkaline phosphatase expression during the anabolic bone response to intermittent parathyroid hormone delivery

The administration of intermittent parathyroid hormone (iPTH) is anabolic to the skeleton. Recent studies with cultured osteoblasts have revealed that the expression of PHOSPHO1, a bone-specific phosphatase essential for the initiation of mineralisation, is regulated by PTH. Therefore, this study sought to determine whether the bone anabolic response to iPTH involves modulation of expression of Phospho1 and of other enzymes critical for bone matrix mineralisation. To mimic iPTH treatment, primary murine osteoblasts were challenged with 50 nM PTH for 6 h in every 48 h period for 8 days (4 cycles), 14 days (7 cycles) and 20 days (10 cycles) in total. The expression of both Phospho1 and Smpd3 was almost completely inhibited after 4 cycles, whereas 10 cycles were required to stimulate a similar response in Alpl expression. To explore the in vivo role of PHOSPHO1 in PTH-mediated osteogenesis, the effects of 14- and 28-day iPTH (80 µg/kg/day) administration was assessed in male wild-type (WT) and Phospho1-/- mice. The expression of Phospho1, Alpl, Smpd3, Enpp1, Runx2 and Trps1 expression was enhanced in the femora of WT mice following iPTH administration but remained unchanged in the femora of Phospho1-/- mice. After 28 days of iPTH administration, the anabolic response in the femora of WT was greater than that noted in Phospho1-/- mice. Specifically, cortical and trabecular bone volume/total volume, as well as cortical thickness, were increased in femora of iPTH-treated WT but not in iPTH-treated Phospho1-/- mice. Trabecular bone osteoblast number was also increased in iPTH-treated WT mice but not in iPTH-treated Phospho1-/-  mice. The increased levels of Phospho1, Alpl, Enpp1 and Smpd3 in WT mice in response to iPTH administration is consistent with their contribution to the potent anabolic properties of iPTH in bone. Furthermore, as the anabolic response to iPTH was attenuated in mice deficient in PHOSPHO1, this suggests that the osteoanabolic effects of iPTH are at least partly mediated via bone mineralisation processes.

Effects of joint immobilization and treadmill exercise on marrow adipose tissue and trabecular bone after anterior cruciate ligament reconstruction in the rat proximal tibial epiphysis

Marrow adipose tissue (MAT) adversely affects bone metabolism under certain conditions. Although mechanical stress is an important factor in regulating MAT and bone mass, how stress from different rehabilitation protocols after anterior cruciate ligament (ACL) reconstruction affects trabecular bone and MAT is unclear. We aimed to examine the effects of joint immobilization and treadmill exercise on trabecular bone and MAT after ACL reconstruction. Rats received unilateral knee ACL transection and reconstruction surgery. After surgery, rats were reared without intervention, with joint immobilization, or with treadmill exercise (12 m/min, 60 min/day, six days/week), with untreated rats as controls. At two or four weeks after starting experiments, we examined histological changes in trabecular bone and MAT in the proximal tibial epiphysis. After ACL reconstruction, there were no significant changes in trabecular bone area and MAT area at both time points. Joint immobilization after ACL reconstruction resulted in reduced trabecular bone area and MAT accumulation due to adipocyte hyperplasia and hypertrophy within four weeks. Treadmill exercise after ACL reconstruction did not affect any parameters in trabecular bone and MAT. We detected a moderate negative correlation between trabecular bone area and MAT area. Therefore, MAT accumulation induced by joint immobilization may contribute, at least in part, to reducing trabecular bone area. To minimize trabecular bone loss and MAT accumulation, joint immobilization after ACL reconstruction should be minimized. Exercise after ACL reconstruction did not alter trabecular bone and MAT.

Cell death regulation: A new way for natural products to treat osteoporosis

Osteoporosis is a common metabolic bone disease that results from the imbalance of homeostasis within the bone. Intra-bone homeostasis is dependent on a precise dynamic balance between bone resorption by osteoclasts and bone formation by mesenchymal lineage osteoblasts, which comprises a series of complex and highly standardized steps. Programmed cell death (PCD) (e.g., apoptosis, autophagy, ferroptosis, pyroptosis, and necroptosis) is a cell death process that involves a cascade of gene expression events with tight structures. These events play a certain role in regulating bone metabolism by determining the fate of bone cells. Moreover, existing research has suggested that natural products derived from a wide variety of dietary components and medicinal plants modulate the PCDs based on different mechanisms, which show great potential for the prevention and treatment of osteoporosis, thus revealing the emergence of more acceptable complementary and alternative drugs with lower costs, fewer side effects and more long-term application. Accordingly, this review summarizes the common types of PCDs in the field of osteoporosis. Moreover, from the perspective of targeting PCDs, this review also discussed the roles of currently reported natural products in the treatment of osteoporosis and the involved mechanisms. Based on this, this review provides more insights into new molecular mechanisms of osteoporosis and provides a reference for developing more natural anti-osteoporosis drugs in the future.

Exercise for osteoporosis: A literature review of pathology and mechanism

Osteoporosis (OP) is a disease that weakens bones and has a high morbidity rate worldwide, which is prevalent among the elderly, particularly, women of postmenopausal age. The dynamic balance between bone formation and resorption is necessary for normal bone metabolism. Many factors, including aging, estrogen deficiency, and prolonged immobilization, disrupt normal apoptosis, autophagy, and inflammation, leading to abnormal activation of osteoclasts, which gradually overwhelm bone formation by bone resorption. Moderate exercise as an effective non-drug treatment helps increase bone formation and helps relieve OP. The possible mechanisms are that exercise affects apoptosis and autophagy through the release of exercise-stimulated myohormone and the secretion of anti-inflammatory cytokines via mechanical force. In addition, exercise may also have an impact on the epigenetic processes involved in bone metabolism. Mechanical stimulation promotes bone marrow mesenchymal stem cells (BMSCs) to osteogenic differentiation by altering the expression of non-coding RNAs. Besides, by reducing DNA methylation, the mechanical stimulus can also alter the epigenetic status of osteogenic genes and show associated increased expression. In this review, we reviewed the possible pathological mechanisms of OP and summarized the effects of exercise on bone metabolism, and the mechanisms by which exercise alleviates the progression of OP, to provide a reference for the prevention and treatment of OP.

Skeletal Aging

Aging represents the single greatest risk factor for chronic diseases, including osteoporosis, a skeletal fragility syndrome that increases fracture risk. Optimizing bone strength throughout life reduces fracture risk. Factors critical for bone strength include nutrition, physical activity, and vitamin D status, whereas unhealthy lifestyles, illnesses, and certain medications (eg, glucocorticoids) are detrimental. Hormonal status is another important determinant of skeletal health, with sex steroid concentrations, particularly estrogen, having major effects on bone remodeling. Aging exacerbates bone loss in both sexes and results in imbalanced bone resorption relative to formation; it is associated with increased marrow adiposity, osteoblast/osteocyte apoptosis, and accumulation of senescent cells. The mechanisms underlying skeletal aging are as diverse as the factors that determine the strength (and thus fragility) of bone. This review updates our current understanding of the epidemiology, pathophysiology, and treatment of osteoporosis and provides an overview of the underlying hallmark mechanisms that drive skeletal aging.

Structural pharmacology of PTH and PTHrP

Parathyroid hormone (PTH) and PTH-related peptide (PTHrP) regulate extracellular phosphate and calcium homeostasis as well as bone remodeling. PTH is a classic endocrine peptide hormone whose synthesis and negative feedback by multiple factors control release from the parathyroid glands. PTHrP is ubiquitously expressed (pre- and postnatally) and acts in an autocrine/paracrine manner. This review considers the structural pharmacology and actions of PTH and PTHrP, biological consequences of inherited mutations, engineered analogs that illuminate similarities and differences in physiologic actions, and targeted therapeutic opportunities.

Laser-mediated osteoblast ablation triggers a pro-osteogenic inflammatory response regulated by reactive oxygen species and glucocorticoid signaling in zebrafish

In zebrafish, transgenic labeling approaches, robust regenerative responses and excellent in vivo imaging conditions enable precise characterization of immune cell behavior in response to injury. Here, we monitored osteoblast-immune cell interactions in bone, a tissue which is particularly difficult to in vivo image in tetrapod species. Ablation of individual osteoblasts leads to recruitment of neutrophils and macrophages in varying numbers, depending on the extent of the initial insult, and initiates generation of cathepsin K+ osteoclasts from macrophages. Osteoblast ablation triggers the production of pro-inflammatory cytokines and reactive oxygen species, which are needed for successful macrophage recruitment. Excess glucocorticoid signaling as it occurs during the stress response inhibits macrophage recruitment, maximum speed and changes the macrophage phenotype. Although osteoblast loss is compensated for within a day by contribution of committed osteoblasts, macrophages continue to populate the region. Their presence is required for osteoblasts to fill the lesion site. Our model enables visualization of bone repair after microlesions at single-cell resolution and demonstrates a pro-osteogenic function of tissue-resident macrophages in non-mammalian vertebrates.

BMSC-Derived ApoEVs Promote Craniofacial Bone Repair via ROS/JNK Signaling

Bone defect caused by trauma, neoplasia, congenital defects, or periodontal disease is a major cause of disability and physical limitation. The transplantation of bone marrow mesenchymal stem cells (BMSCs) promotes bone repair and regeneration. However, it has been shown that most BMSCs die within a short period after transplantation. During apoptosis, BMSCs generate a large number of apoptotic cell-derived extracellular vesicles (ApoEVs). This study aims to understand the potential role of ApoEVs in craniofacial bone defect repair and regeneration. First, we confirmed that BMSCs undergo apoptosis within 2 d after transplantation into the defect of the cranium. Abundant ApoEVs were generated from apoptotic BMSCs. Uptake of ApoEVs efficiently promoted the proliferation, migration, and osteogenic differentiation of recipient BMSCs in vitro. ApoEVs from cells in the middle stage of apoptosis were the most efficient to enhance the regenerative capacity of BMSCs. Moreover, a critical size bone defect model in rats was used to evaluate the osteogenic property of ApoEVs in vivo. Local transplantation of ApoEVs promoted bone regeneration in the calvarial defect. Mechanistically, ApoEVs promoted new bone formation by increasing intracellular reactive oxygen species to activate JNK signaling. This study reveals a previously unknown role of the dying transplanted BMSCs in promoting the viability of endogenous BMSCs and repairing the calvarial defects. Since it could avoid several adverse effects and limits of BMSC cytotherapy, treatment of ApoEVs might be a promising strategy in craniofacial bone repair and regeneration.

A Highly Selective GSK-3β Inhibitor CHIR99021 Promotes Osteogenesis by Activating Canonical and Autophagy-Mediated Wnt Signaling

The discovery and application of small molecules is one of the practical strategies of safe osteogenic drugs. The small molecule CHIR99021 (C91) is a highly specific, safe, and most effective GSK-3β Inhibitor. This study found that it efficiently activates the canonical Wnt signaling of bone marrow stromal cell ST2 and promotes osteoblast differentiation and mineralization. C91 increases the production and biochemical activity of osteoblast marker alkaline phosphatase, the expression of osteoblast marker genes Alpl, Bglap, Runx2, and Sp7, and the formation of bone nodules. Triptonide is a transcription inhibitor of Wnt target gene, which diminishes C91-induced osteoblast differentiation in a dose-dependent manner. Meanwhile, C91 also induces autophagy through autophagosome formation and conversion of autophagy biomarker LC-3I into LC-3II. Autophagy inhibitor 3MA partially reduces C91-induced osteoblast differentiation and mineralization; autophagy inducer Rapamycin increases the expression of β-catenin to promote osteogenic differentiation, but cannot alleviate the inhibition of Triptonide on C91-induced osteogenic differentiation, indicating the crosstalk of canonical Wnt signaling and autophagy regulates C91-induced osteoblast differentiation. Furthermore, in order to simulate the in vivo detection of C91 in osteogenesis process, we made a C91 slow-release hydrogel with our newly established polycaprolactone and cell-integrated 3D printing system (PCCI3D module). The sustained release C91 promotes the differentiation and mineralization of ST2 cells. C91 can also enhance the proliferative activity of ST2 cells. The release rate of C91 from hydrogel gradually decreases within 7 days. During this period, the C91 is released by 83.0% and the cell viability maintained at 96.4%. Therefore, the small molecule Wnt agonist C91 promotes osteogenesis through caonical and autophagy-mediated Wnt signaling pathway with an option for translational application.

The periosteum: a simple tissue with many faces, with special reference to the antler-lineage periostea

Periosteum is a thin membrane covering bone surfaces and consists of two layers: outer fibrous layer and inner cambium layer. Simple appearance of periosteum has belied its own complexity as a composite structure for physical bone protection, mechano-sensor for sensing mechanical loading, reservoir of biochemical molecules for initiating cascade signaling, niche of osteogenic cells for bone formation and repair, and "umbilical cord" for nourishing bone tissue. Periosteum-derived cells (PDCs) have stem cell attributes: self-renewal (no signs of senescence until 80 population doublings) and multipotency (differentiate into fibroblasts, osteoblasts, chondrocytes, adipocytes and skeletal myocytes). In this review, we summarized the currently available knowledge about periosteum and with special references to antler-lineage periostea, and demonstrated that although periosteum is a type of simple tissue in appearance, with multiple faces in functions; antler-lineage periostea add another dimension to the properties of somatic periostea: capable of initiation of ectopic organ formation upon transplantation and full mammalian organ regeneration when interacted with the covering skin. Very recently, we have translated this finding into other mammals, i.e. successfully induced partial regeneration of the amputated rat legs. We believe further refinement along this line would greatly benefit human health.

The diverse origin of bone-forming osteoblasts

Osteoblasts are the only cells that can give rise to bones in vertebrates. Thus, one of the most important functions of these metabolically active cells is mineralized matrix production. Because osteoblasts have a limited lifespan, they must be constantly replenished by preosteoblasts, their immediate precursors. Because disruption of the regulation of bone-forming osteoblasts results in a variety of bone diseases, a better understanding of the origin of these cells by defining the mechanisms of bone development, remodeling, and regeneration is central to the development of novel therapeutic approaches. In recent years, substantial new insights into the origin of osteoblasts-largely owing to rapid technological advances in murine lineage-tracing approaches and other single-cell technologies-have been obtained. Collectively, these findings indicate that osteoblasts involved in bone formation under various physiological, pathological, and therapeutic conditions can be obtained from numerous sources. The origins of osteoblasts include, but are not limited to, chondrocytes in the growth plate, stromal cells in the bone marrow, quiescent bone-lining cells on the bone surface, and specialized fibroblasts in the craniofacial structures, such as sutures and periodontal ligaments. Because osteoblasts can be generated from local cellular sources, bones can flexibly respond to regenerative and anabolic cues. However, whether osteoblasts derived from different cellular sources have distinct functions remains to be investigated. Currently, we are at the initial stage to aptly unravel the incredible diversity of the origins of bone-forming osteoblasts. © 2021 American Society for Bone and Mineral Research (ASBMR).

Molecular Interactions between Dietary Lipids and Bone Tissue during Aging

Age-related bone disorders such as osteoporosis or osteoarthritis are a major public health problem due to the functional disability for millions of people worldwide. Furthermore, fractures are associated with a higher degree of morbidity and mortality in the long term, which generates greater financial and health costs. As the world population becomes older, the incidence of this type of disease increases and this effect seems notably greater in those countries that present a more westernized lifestyle. Thus, increased efforts are directed toward reducing risks that need to focus not only on the prevention of bone diseases, but also on the treatment of persons already afflicted. Evidence is accumulating that dietary lipids play an important role in bone health which results relevant to develop effective interventions for prevent bone diseases or alterations, especially in the elderly segment of the population. This review focuses on evidence about the effects of dietary lipids on bone health and describes possible mechanisms to explain how lipids act on bone metabolism during aging. Little work, however, has been accomplished in humans, so this is a challenge for future research.

Glucocorticoid-induced autophagy and apoptosis in bone

Glucocorticoids are widely prescribed to treat various allergic and autoimmune diseases; however, long-term use results in glucocorticoid-induced osteoporosis, characterized by consistent changes in bone remodeling with decreased bone formation as well as increased bone resorption. Not only bone mass but also bone quality decrease, resulting in an increased incidence of fractures. The primary role of autophagy is to clear up damaged cellular components such as long-lived proteins and organelles, thus participating in the conservation of different cells. Apoptosis is the physiological death of cells, and plays a crucial role in the stability of the environment inside a tissue. Available basic and clinical studies indicate that autophagy and apoptosis induced by glucocorticoids can regulate bone metabolism through complex mechanisms. In this review, we summarize the relationship between apoptosis, autophagy and bone metabolism related to glucocorticoids, providing a theoretical basis for therapeutic targets to rescue bone mass and bone quality in glucocorticoid-induced osteoporosis.

Long bone fractures in Cerdocyon thous: macroscopic and microstructural evaluation

Abstract The aim of the present study was to perform the macroscopic and microstructural morphological classification of long bone fractures of Cerdocyon thous. Eighteen cadavers of the species were necropsied, and subjected to radiographic and microscopical evaluation when long bone fractures were detected. Among the 18 cadavers, eight (44%) had fractures equally distributed (33.33%) in the femur, humerus, or tibia. More frequently (61.54%), the fractures were simple and affected the diaphysis, and in smaller proportions (23.08%) reached the physeal line. In diaphyseal and metaphyseal fractures, microscopical evaluation revealed cortical bone tissue, with longitudinal osteons that contained longitudinal and intermediate collagen fibres and lamellae with a delamination aspect. On the other hand, in epiphyseal fractures, trabecular bone tissue was more frequently observed, consisting of trabeculae with disorganised collagen fibres and absence of osteons. In both cases low activity, osteocytes, and low coverage of osteoblasts on the bone surface were noted. It was concluded that the frequency of fractures in the long bones of C. thous was 44%, with females being more predisposed. The findings support the hypothesis that fractures in such animals are caused by being run over by automobiles. The present study contributes significantly in alerting clinicians and surgeons to the types of fractures that C. thous is more predisposed to, its places of greatest occurrence, and its microstructure. Thus, there is a need for joint actions aimed at reducing the number of cases of wild animals being run over by automobiles.

Histone demethylase Jmjd3 modulates osteoblast apoptosis induced by tumor necrosis factor-alpha through directly targeting RASSF5

Purpose: Regulation of gene expression is fine-tuned by a dynamic equilibrium between repressive modifications and transcriptional activation of histone tails. Jumonji domain-containing 3 (Jmjd3), also known as KDM6B, is a specific histone demethylase for trimethylation on histone H3 lysine 27 (H3K27me3) that specifically removes the methylation of H3K27me3 and promotes gene expression. Our previous study showed that Jmjd3 inhibits serum deprivation-induced osteoblast apoptosis. In this study, we clarified the role of Jmjd3 in tumor necrosis factor-alpha (TNF-α)-induced osteoblast apoptosis. Materials and Methods: Jmjd3 activity was inhibited by GSK-J4. Transfection of osteoblastic murine MC3T3-E1 cells with short hairpin RNA (shRNA) was used to establish stable Jmjd3 knockdown cells. Osteoblast apoptosis was detected using Annexin V-APC/PI staining, cysteinyl aspartate specific protease-3 (caspase-3) activity assays, and Western blot. Real-time polymerase chain reaction (PCR) and chromatin immunoprecipitation (ChIP) assays were performed to clarify the mechanism responsible for Jmjd3-regulated osteoblast apoptosis induced by TNF-α. Results: Based on Annexin V-APC/PI staining, caspase-3 activation, and poly ADP-ribose polymerase (PARP) cleavage, pretreatment with GSK-J4 and knockdown of Jmjd3 by shRNA transfection each inhibited osteoblast apoptosis. Furthermore, knockdown of Jmjd3 decreased the expression of Ras association domain family 5 (RASSF5), which is a pro-apoptotic gene of the Ras associated domain family. H3K27me3 levels in the promoter region of RASSF5 were up-regulated in the Jmjd3 knockdown cells. Conclusions: Jmjd3 regulated TNF-α-induced osteoblast apoptosis by targeting RASSF5.

Effects of β-carotene intake on the risk of fracture: a Bayesian meta-analysis

BACKGROUND

Epidemiological studies examining the association between β-carotene intake and risk of fracture have reported inconsistent findings. We conducted a meta-analysis to investigate the association between β-carotene intake and risk of fracture.

METHODS

We systematically searched PubMed, EMBASE and Cochrane library databases for relevant articles that were published until December 2019. We also identified studies from reference lists of articles identified from the clinical databases. The frequentist and Bayesian random-effects model was used to synthesize data.

RESULTS

Nine studies with a total of 190,545 men and women, with an average age of 59.8 years, were included in this meta-analysis. For β-carotene intake (1.76-14.30 mg/day), the pooled risk ratio (RR) of any fracture was 0.67 (95% Credible Interval (CrI): 0.51-0.82; heterogeneity: P = 0.66, I2 = 0.00%) and 0.63 (95%CrI: 0.44-0. 82) for hip fracture. By study design, the pooled RRs were 0.55 (95% CrI: 0.14-0.96) for case-control studies and 0.82 (95% CrI: 0.58-0.99) for cohort studies. By geographic region, the pooled RRs were 0.58 (95% CrI: 0.28-0.89), 0.86 (95% CrI: 0.35-0.1.37), and 0.91(95% CrI: 0.75-1.00) for studies conducted in China, the United States, and Europe, respectively. By sex, the pooled RRs were 0.88 (95% CrI: 0.73-0.99) for males and 0.76 (95% CrI, 0.44-1.07) for females. There was a 95% probability that β-carotene intake reduces risk of hip fracture and any type of fracture by more than 20%.

CONCLUSIONS

The present meta-analysis suggests that β-carotene intake was inversely associated with fracture risk, which was consistently observed for case-control and cohort studies. Randomized controlled trials are warranted to confirm this relationship.

Intracellular Accumulation of Advanced Glycation End Products Induces Osteoblast Apoptosis Via Endoplasmic Reticulum Stress

Osteoporosis is an aging-associated disease that is attributed to excessive osteoblast apoptosis. It is known that the accumulation of advanced glycation end products (AGEs) in bone extracellular matrix deteriorates osteoblast functions. However, little is known about the interaction between intracellular AGE accumulation and the induction of osteoblast apoptosis. In this study, we investigated the effect of intracellular AGE accumulation on osteoblast apoptosis in vitro and in vivo. In vitro, murine osteoblastic MC3T3-E1 cells were treated with glycolaldehyde (GA), an AGE precursor. GA-induced intracellular AGE accumulation progressed in time- and dose-dependent manners, followed by apoptosis induction. Intracellular AGE formation also activated endoplasmic reticulum (ER) stress-related proteins (such as glucose-regulated protein 78, inositol-requiring protein-1α (IRE1α), and c-Jun N-terminal kinase) and induced apoptosis. In agreement, treatment with the ER stress inhibitor 4-phenylbutyric acid and knocking down IRE1α expression ameliorated osteoblast apoptosis. Furthermore, the ratio between AGE- and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL)-positive osteoblasts in human vertebral bodies was significantly higher in an elderly group than in a younger group. A positive linear correlation between the ratio of AGE-positive and TUNEL-positive osteoblasts (r = 0.72) was also observed. Collectively, these results indicate that AGEs accumulated in osteoblasts with age and that intracellular AGE accumulation induces apoptosis via ER stress. These findings offer new insight into the mechanisms of osteoblast apoptosis and age-related osteoporosis. © 2020 American Society for Bone and Mineral Research.

Overexpression of Fam20C in osteoblast in vivo leads to increased cortical bone formation and osteoclastic bone resorption

Fam20C, which phosphorylates many secretory proteins with S-x-E/pS motifs, is highly expressed in bone and tooth tissues, implying that Fam20C-mediated phosphorylation is critical for regulation of these mineralized tissues. Previous studies of Fam20C-deficient mice revealed that Fam20C plays important roles in bone formation and mineralization. However, Fam20C-deficient mice develop hypophosphatemia, a systemic factor that masks the local effect of Fam20C in the bone tissue; consequently, the local role of Fam20C remains unknown. To elucidate the local function of Fam20C in bone tissue, we studied osteoblast-specific Fam20C transgenic (Fam20C-Tg) mice, which have no alteration in serum calcium and phosphate levels. Fam20C-Tg mice had more highly phosphorylated proteins in bone tissue than wild-type mice. In cortical bone of Fam20C-Tg mice, bone volume, mineralization surface (MS/BS), and mineral apposition rate (MAR) were elevated; in addition, the transgenic mice had an elevated number of vascular canals, resulting in an increased cortical porosity. Osteocyte number was elevated in the transgenics, but osteoblast number was unchanged. The microstructure of bone matrix characterized by the preferential orientation of collagen and apatite, was degraded and thus the mechanical function of bone material was deteriorated. In trabecular bone of Fam20C-Tg mice, bone volume was reduced, whereas MS/BS and MAR were unchanged. Osteoclast number was elevated and eroded surface area was non-significantly elevated with an increased serum CTX-I level, whereas osteoblast number was unchanged. These findings indicated that Fam20C overexpression in osteoblasts promotes cortical bone formation by increasing MS/BS and MAR and promoting osteocyte differentiation, but does not affect trabecular bone formation. Furthermore, Fam20C overexpression indirectly promotes osteoclastic bone resorption in cortical and trabecular bones. Our findings show that osteoblastic Fam20C-mediated phosphorylation in bone tissue regulates bone formation and resorption, and bone material quality.

Vitamin K2 stimulates MC3T3‑E1 osteoblast differentiation and mineralization through autophagy induction

Vitamin K2 likely exerts its protective effects during osteoporosis by promoting osteoblast differentiation and mineralization. However, the precise mechanism remains to be fully elucidated. Autophagy maintains cell homeostasis by breaking down and eliminating damaged proteins and organelles. Increasing evidence in recent years has implicated autophagy in the development of osteoporosis. The aim of the present study was to verify whether vitamin K2 (VK2) can induce autophagy during the differentiation and mineralization of osteoblasts. In the present study, MC3T3-E1 osteoblasts were treated with various doses of VK2 (10⁻⁸−10⁻³ M) for 1–5 days. The results revealed no cytotoxicity at concentrations below 10⁻⁵ M, but cell viability was reduced in a dose-dependent manner at concentrations above 10⁻⁵ M. Furthermore, MC3T3-E1 osteoblasts were seeded in 6-well plates in complete medium supplemented with dexamethasone, β-glycerophosphate and vitamin C (VC) for osteogenic differentiation. MC3T3-E1 osteoblasts treated with different concentrations (10⁻⁵, 10⁻⁶ and 10⁻⁷ M) of VK2 for 24 h on days 1, 3, 5 and 7 of the differentiation protocol. It was confirmed that VK2 promoted osteoblast differentiation and mineralization by using alkaline phosphatase (ALP) and alizarin red staining. Using western blotting, immunofluorescence, monodansylcadaverine staining and reverse transcription-quantitative polymerase chain reaction, it was observed that VK2 induced autophagy in osteoblasts. The results revealed that VK2 (1 µM) significantly increased ALP activity and the conversion of microtubule associated protein 1 light chain 3-α (LC3)II to LC3I in MC3T3-E1 osteoblasts (P<0.05) at every time point. The number of fluorescent bodies and the intensity increased with VK2, and decreased following treatment with 3-MA+VK2. There was an increase in the mRNA expression levels of ALP, osteocalcin (OCN) and Runt-related transcription factor 2 in VK2-treated cells (P<0.01). The present study further confirmed the association between autophagy and osteoblast differentiation and mineralization through treatment with an autophagy inhibitor [3-methyladenine (3-MA)]. Osteoblasts treated with 3-MA exhibited significant inhibition of ALP activity and osteogenic differentiation (both P<0.05). In addition, ALP activity and osteogenesis in the VK2+3-MA group was lower compared with VK2-treated cells (P<0.05 for both). The present study confirmed that VK2 stimulated autophagy in MC3T3 cells to promote differentiation and mineralization, which may be a potential therapeutic target for osteoporosis.

Bone remodeling: A tissue-level process emerging from cell-level molecular algorithms

The human skeleton undergoes constant remodeling throughout the lifetime. Processes occurring on microscopic and molecular scales degrade bone and replace it with new, fully functional tissue. Multiple bone remodeling events occur simultaneously, continuously and independently throughout the body, so that the entire skeleton is completely renewed about every ten years.Bone remodeling is performed by groups of cells called Bone Multicellular Units (BMU). BMUs consist of different cell types, some specialized in the resorption of old bone, others encharged with producing new bone to replace the former. These processes are tightly regulated so that the amount of new bone produced is in perfect equilibrium with that of old bone removed, thus maintaining bone microscopic structure.To date, many regulatory molecules involved in bone remodeling have been identified, but the precise mechanism of BMU operation remains to be fully elucidated. Given the complexity of the signaling pathways already known, one may question whether such complexity is an inherent requirement of the process or whether some subset of the multiple constituents could fulfill the essential role, leaving functional redundancy to serve an alternative safety role. We propose in this work a minimal model of BMU function that involves a limited number of signals able to account for fully functional BMU operation. Our main assumptions were i) at any given time, any cell within a BMU can select only one among a limited choice of decisions, i.e. divide, die, migrate or differentiate, ii) this decision is irreversibly determined by depletion of an appropriate internal inhibitor and iii) the dynamics of any such inhibitor are coupled to that of specific external mediators, such as hormones, cytokines, growth factors. It was thus shown that efficient BMU operation manifests as an emergent process, which results from the individual and collective decisions taken by cells within the BMU unit in the absence of any external planning.

Role of nutritional vitamin D in osteoporosis treatment

Osteoporosis is a systemic skeletal disorder characterized by a decrease in bone mass and microarchitectural deterioration of bone tissue. The World Health Organization has defined osteoporosis as a decrease in bone mass (50%) and bony quality (50%). Vitamin D, a steroid hormone, is crucial for skeletal health and in mineral metabolism. Its direct action on osteoblasts and osteoclasts and interaction with nonskeletal tissues help in maintaining a balance between bone turnover and bone growth. Vitamin D affects the activity of osteoblasts, osteoclasts, and osteocytes, suggesting that it affects bone formation, bone resorption, and bone quality. At physiological concentrations, active vitamin D maintains a normal rate of bone resorption and formation through the RANKL/OPG signal. However, active vitamin D at pharmacological concentration inhibits bone resorption at a higher rate than that of bone formation, which influences the bone quality and quantity. Nutritional vitamin D rather than active vitamin D activates osteoblasts and maintains serum 25(OH)D3 concentration. Despite many unanswered questions, much data support nutritional vitamin D use in osteoporosis patients. This article emphasizes the role of nutritional vitamin D replacement in different turnover status (high or low bone turnover disorders) of osteoporosis together with either anti-resorptive (Bisphosphonate, Denosumab et.) or anabolic (Teriparatide) agents when osteoporosis persists.

Advanced oxidation protein products induce pre-osteoblast apoptosis through a nicotinamide adenine dinucleotide phosphate oxidase-dependent, mitogen-activated protein kinases-mediated intrinsic apoptosis pathway

Osteoblast apoptosis contributes to age‐related bone loss. Advanced oxidation protein products (AOPPs) are recognized as the markers of oxidative stress and potent inducers of apoptosis. We have demonstrated that AOPP accumulation was correlated with age‐related bone loss. However, the effect of AOPPs on the osteoblast apoptosis still remains unknown. Exposure of osteoblastic MC3T3‐E1 cells to AOPPs caused the excessive generation of reactive oxygen species (ROS) by activating nicotinamide adenine dinucleotide phosphate (NADPH) oxidases. Increased ROS induced phosphorylation of mitogen‐activated protein kinases (MAPKs), which subsequently triggered intrinsic apoptosis pathway by inducing mitochondrial dysfunction, endoplasmic reticulum stress, and Ca²⁺ overload and eventually leads to apoptosis. Chronic AOPP loading in aged Sprague‐Dawley rats induced osteoblast apoptosis and activated NADPH oxidase signaling cascade, in combination with accelerated bone loss and deteriorated bone microstructure. Our study suggests that AOPPs induce osteoblast apoptosis by the NADPH oxidase‐dependent, MAPK‐mediated intrinsic apoptosis pathway.

Age-related mechanical strength evolution of trabecular bone under fatigue damage for both genders: Fracture risk evaluation

Bone tissue is a living composite material, providing mechanical and homeostatic functions, and able to constantly adapt its microstructure to changes in long term loading. This adaptation is conducted by a physiological process, known as "bone remodeling". This latter is manifested by interactions between osteoclasts and osteoblasts, and can be influenced by many local factors, via effects on bone cell differentiation and proliferation. In the current work, age and gender effects on damage rate evolution, throughout life, have been investigated using a mechanobiological finite element modeling. To achieve the aim, a mathematical model has been developed, coupling both cell activities and mechanical behavior of trabecular bone, under cyclic loadings. A series of computational simulations (ABAQUS/UMAT) has been performed on a 3D human proximal femur, allowing to investigate the effects of mechanical and biological parameters on mechanical strength of trabecular bone, in order to evaluate the fracture risk resulting from fatigue damage. The obtained results revealed that mechanical stimulus amplitude affects bone resorption and formation rates, and indicated that age and gender are major factors in bone response to the applied loadings.

Overactive autophagy is a pathological mechanism underlying premature suture ossification in nonsyndromic craniosynostosis

Nonsyndromic craniosynostosis (NSC) is the most common craniosynostosis with the primary defect being one or more fused sutures. In contrast to syndromic craniosynostosis, the etiopathogenesis of NSC is largely unknown. Here we show that autophagy, a major catabolic process required for the maintenance of bone homeostasis and bone growth, is a pathological change associated with NSC. Using calvarial suture mesenchymal cells (SMCs) isolated from the fused and unfused sutures of NSC patients, we demonstrate that during SMC differentiation, the level of the autophagosomal marker LC3-II increases as osteogenic differentiation progresses, particularly at differentiation day 7, a stage concurrent with mineralization. In fused SMCs, autophagic induction was more robust than that in unfused SMCs, which consequently led to enhanced mineralized nodule formation. Perturbation of autophagy with rapamycin or wortmannin promoted or inhibited the ossification of SMCs, respectively. Our findings suggest that autophagy is essential for the osteogenic differentiation of SMCs and that overactive autophagy is a molecular abnormality underlying premature calvarial ossification in NSC.

Notch activation promotes osteoblast mineralization by inhibition of apoptosis

Notch activator Jagged1 (JAG1) plays a critical role in the regulation of osteoblast differentiation and bone metabolism. In this study, JAG1-induced osteoblast proliferation, differentiation, and mineralization has been analyzed in primary osteoblasts for up to 7 days. Alkaline phosphatase and Alizarin red staining showed an enhanced osteoblast maturation and mineralization in JAG1 treated cells, as well as higher mRNA levels of late osteoblast differentiation markers. In contrast, Notch inhibitor DAPT and deletion of Runx2 totally blocked JAG1 effects on osteoblast mineralization. Flow cytometry data further showed a significantly higher cell proliferation in early stages of culture at day 3, and lower levels of osteoblast apoptosis in late stages of culture at day 7. More importantly, activation of anti-apoptotic factor BCL-2 was enhanced, while pro-apoptotic factor Caspase3 was reduced in JAG1 treated osteoblasts. Therefore, we conclude that cell mineralization is enhanced via anti-apoptotic actions of Notch signaling within the osteoblast cells.

Liraglutide Inhibits the Apoptosis of MC3T3-E1 Cells Induced by Serum Deprivation through cAMP/PKA/β-Catenin and PI3K/AKT/GSK3β Signaling Pathways

In recent years, the interest towards the relationship between incretins and bone has been increasing. Previous studies have suggested that glucagon-like peptide-1 (GLP-1) and its receptor agonists exert beneficial anabolic influence on skeletal metabolism, such as promoting proliferation and differentiation of osteoblasts via entero-osseous-axis. However, little is known regarding the effects of GLP-1 on osteoblast apoptosis and the underlying mechanisms involved. Thus, in the present study, we investigated the effects of liraglutide, a glucagon-like peptide-1 receptor agonist, on apoptosis of murine MC3T3-E1 osteoblastic cells. We confirmed the presence of GLP-1 receptor (GLP-1R) in MC3T3-E1 cells. Our data demonstrated that liraglutide inhibited the apoptosis of osteoblastic MC3T3-E1 cells induced by serum deprivation, as detected by Annexin V/PI and Hoechst 33258 staining and ELISA assays. Moreover, liraglutide upregulated Bcl-2 expression and downregulated Bax expression and caspase-3 activity at intermediate concentration (100 nM) for maximum effect. Further study suggested that liraglutide stimulated the phosphorylation of AKT and enhanced cAMP level, along with decreased phosphorylation of GSK3β, increased β-catenin phosphorylation at Ser675 site and upregulated nuclear β-catenin content and transcriptional activity. Pretreatment of cells with the PI3K inhibitor LY294002, PKA inhibitor H89, and siRNAs GLP-1R, β-catenin abrogated the liraglutide-induced activation of cAMP, AKT, β-catenin, respectively. In conclusion, these findings illustrate that activation of GLP-1 receptor by liraglutide inhibits the apoptosis of osteoblastic MC3T3-E1 cells induced by serum deprivation through cAMP/PKA/β-catenin and PI3K/Akt/GSK3β signaling pathways.

Carotenoids and risk of fracture: a meta-analysis of observational studies

To quantify the association between dietary and circulating carotenoids and fracture risk, a meta-analysis was conducted by searching MEDLINE and EMBASE databases for eligible articles published before May 2016. Five prospective and 2 case-control studies with 140,265 participants and 4,324 cases were identified in our meta-analysis. Among which 5 studies assessed the association between dietary carotenoids levels and hip fracture risk, 2 studies focused on the association between circulating carotenoids levels and any fracture risk. A random-effects model was employed to summarize the risk estimations and their 95% confidence intervals (CIs). Hip fracture risk among participants with high dietary total carotenoids intake was 28% lower than that in participants with low dietary total carotenoids (OR: 0.72; 95% CI: 0.51, 1.01). A similar risk of hip fracture was found for β-carotene based on 5 studies, the summarized OR for high vs. low dietary β-carotene was 0.72 (95% CI: 0.54, 0.95). However, a significant between-study heterogeneity was found (total carotene: I2 = 59.4%, P = 0.06; β-carotene: I2 = 74.4%, P = 0.04). Other individual carotenoids did not show significant associations with hip fracture risk. Circulating carotene levels had no significant association with any fracture risk, the pooled OR (95% CI) was 0.83 (0.59, 1.17). Based on the evidence from observational studies, our meta-analysis supported the hypothesis that higher dietary total carotenoids or β-carotene intake might be potentially associated with a low risk of hip fracture, however, future well-designed prospective cohort studies and randomized controlled trials are warranted to specify the associations between carotenoids and fracture.

Macrophages and bone inflammation

Bone metabolism is tightly regulated by the immune system. Accelerated bone destruction is observed in many bone diseases, such as rheumatoid arthritis, fracture, and particle-induced osteolysis. These pathological conditions are associated with inflammatory responses, suggesting the contribution of inflammation to bone destruction. Macrophages are heterogeneous immune cells and are polarized into the proinflammatory M1 and antiinflammatory M2 phenotypes in different microenvironments. The cytokines produced by macrophages depend on the macrophage activation and polarization. Macrophages and macrophage-derived cytokines are important to bone loss in inflammatory bone disease. Recent studies have shown that macrophages can be detected in bone tissue and interact with bone cells. The interplay between macrophages and bone cells is critical to bone formation and repair. In this article, we focus on the role of macrophages in inflammatory bone diseases, as well as discuss the latest studies about macrophages and bone formation, which will provide new insights into the therapeutic strategy for bone disease.

Osteoblast-specific deletion of Hrpt2/Cdc73 results in high bone mass and increased bone turnover

Inactivating mutations that lead to loss of heterozygosity within the HRPT2/Cdc73 gene are directly linked to the development of primary hyperparathyroidism, parathyroid adenomas, and ossifying fibromas of the jaw (HPT-JT). The protein product of the Cdc73 gene, parafibromin, is a core member of the polymerase-associated factors (PAF) complex, which coordinates epigenetic modifiers and transcriptional machinery to control gene expression. We conditionally deleted Cdc73 within mesenchymal progenitors or within mature osteoblasts and osteocytes to determine the consequences of parafibromin loss within the mesenchymal lineage. Homozygous deletion of Cdc73 via the Dermo1-Cre driver resulted in embryos which lacked mesenchymal organ development of internal organs, including the heart and fetal liver. Immunohistochemical detection of cleaved caspase-3 revealed extensive apoptosis within the progenitor pools of developing organs. Unexpectedly, when Cdc73 was homozygously deleted within mature osteoblasts and osteocytes (via the Ocn-Cre driver), the mice had a normal life span but increased cortical and trabecular bone. OCN-Cre;Cdc73^flox/flox bones displayed large cortical pores actively undergoing bone remodeling. Additionally the cortical bone of OCN-Cre;Cdc73^flox/flox femurs contained osteocytes with marked amounts of cytoplasmic RNA and a high rate of apoptosis. Transcriptional analysis via RNA-seq within OCN-Cre;Cdc73^flox/flox osteoblasts showed that loss of Cdc73 led to a derepression of osteoblast-specific genes, specifically those for collagen and other bone matrix proteins. These results aid in our understanding of the role parafibromin plays within transcriptional regulation, terminal differentiation, and bone homeostasis.