Osteocyte-driven bone remodeling

Different impact of short-term and long-term hindlimb disuse on bone homeostasis

Disuse osteoporosis is one of the major problems of bone health which commonly occurs in astronauts during long-term spaceflight and bedridden patients. However, the mechanisms underlying such mechanical unloading induced bone loss have not been fully understood. In this study, we employed hindlimb-unloading mice models with different length of tail suspension to investigate if the bone loss was regulated by distinct factors under different duration of disuse. Our micro-CT results showed more significant decrease of bone mass in 6W (6-week) tail-suspension mice compared to the 1W (1-week) tail-suspension ones, as indicated by greater reduction of BV/TV, Tb.N, B.Ar/T.Ar and Ct.Th. RNA-sequencing results showed significant effects of hindlimb disuse on cell locomotion and immune system process which could cause bone loss.Real-time quantitative PCR results indicated a greater number of bone formation related genes that were downregulated in short-term tail-suspension mice compared to the long-term ones. It is, thus, suggested while sustained hindlimb unloading continuously contributes to bone loss, molecular regulation of bone homeostasis tends to reach a balance during this process.

Chromium nanoparticles improve bone turnover regulation in rats fed a high-fat, low-fibre diet

The aim of the study was to investigate the effect of returning to a balanced diet combined with chromium picolinate (CrPic) or chromium nanoparticles (CrNPs) supplementation at a pharmacologically relevant dose of 0.3 mg/kg body weight on the expression level of selected genes and bone turnover markers in the blood and bones of rats fed an obese diet. The results of the study showed that chronic intake of a high-fat obesogenic diet negatively affects bone turnover by impairing processes of both synthesis and degradation of bones. The switch to a healthy diet proved insufficient to regulate bone metabolism disorders induced by an obesogenic diet, even when it was supplemented with chromium, irrespective of its form. Supplementation with CrPic with no change in diet stimulated bone metabolism only at the molecular level, towards increased osteoclastogenesis (bone resorption). In contrast, CrNPs added to the high-fat diet effectively regulated bone turnover by increasing both osteoblastogenesis and osteoclastogenesis, with these changes directed more towards bone formation. The results of the study suggest that unfavourable changes in bone metabolism induced by chronic intake of a high-fat diet can be mitigated by supplementation with CrNPs, whereas a change in eating habits fails to achieve a similar effect.

A detailed methodology for the long-term in vitro culture and analysis of three-dimensional, self-structuring bone models generated from cell lines or primary osteoblastic cell populations

Background

There are insufficient in vitro bone models that accommodate long-term culture of osteoblasts and support their differentiation to osteocytes. The increased demand for effective therapies for bone diseases, and the ethical requirement to replace animals in research, warrants the development of such models.Here we present an in-depth protocol to prepare, create and maintain three-dimensional, in vitro, self-structuring bone models that support osteocytogenesis and long-term osteoblast survival (>1 year).

Methods

Osteoblastic cells are seeded on a fibrin hydrogel, cast between two beta-tricalcium phosphate anchors. Analytical methods optimised for these self-structuring bone model (SSBM) constructs, including RT-qPCR, immunofluorescence staining and XRF, are described in detail.

Results

Over time, the cells restructure and replace the initial matrix with a collagen-rich, mineralising one; and demonstrate differentiation towards osteocytes within 12 weeks of culture.

Conclusions

Whilst optimised using a secondary human cell line (hFOB 1.19), this protocol readily accommodates osteoblasts from other species (rat and mouse) and origins (primary and secondary). This simple, straightforward method creates reproducible in vitro bone models that are responsive to exogenous stimuli, offering a versatile platform for conducting preclinical translatable research studies.

The role of photobiomodulation in accelerating bone repair

Bone repair is faced with obstacles such as slow repair rates and limited bone regeneration capacity. Delayed healing even nonunion could occur in bone defects, influencing the life quality of patients severely. Photobiomodulation (PBM) utilizes different light sources to derive beneficial therapeutic effects with the advantage of being non-invasive and painless, providing a promising strategy for accelerating bone repair. In this review, we summarize the parameters, mechanisms, and effects of PBM regulating bone repair, and further conclude the current clinical application of PBM devices in bone repair. The wavelength of 635-980 nm, the output power of 40-100 mW, and the energy density of less than 100 J/cm2 are the most commonly used parameters. New technologies, including needle systems and biocompatible and implantable optical fibers, offer references to realize an efficient and safe strategy for bone repair. Further research is required to establish the reliability of outcomes from in vivo and in vitro studies and to standardize clinical trial protocols.

Musculoskeletal crosstalk in chronic obstructive pulmonary disease and comorbidities: Emerging roles and therapeutic potentials

Chronic Obstructive Pulmonary Disease (COPD) is a multifaceted respiratory disorder characterized by progressive airflow limitation and systemic implications. It has become increasingly apparent that COPD exerts its influence far beyond the respiratory system, extending its impact to various organ systems. Among these, the musculoskeletal system emerges as a central player in both the pathogenesis and management of COPD and its associated comorbidities. Muscle dysfunction and osteoporosis are prevalent musculoskeletal disorders in COPD patients, leading to a substantial decline in exercise capacity and overall health. These manifestations are influenced by systemic inflammation, oxidative stress, and hormonal imbalances, all hallmarks of COPD. Recent research has uncovered an intricate interplay between COPD and musculoskeletal comorbidities, suggesting that muscle and bone tissues may cross-communicate through the release of signalling molecules, known as "myokines" and "osteokines". We explored this dynamic relationship, with a particular focus on the role of the immune system in mediating the cross-communication between muscle and bone in COPD. Moreover, we delved into existing and emerging therapeutic strategies for managing musculoskeletal disorders in COPD. It underscores the development of personalized treatment approaches that target both the respiratory and musculoskeletal aspects of COPD, offering the promise of improved well-being and quality of life for individuals grappling with this complex condition. This comprehensive review underscores the significance of recognizing the profound impact of COPD on the musculoskeletal system and its comorbidities. By unravelling the intricate connections between these systems and exploring innovative treatment avenues, we can aspire to enhance the overall care and outcomes for COPD patients, ultimately offering hope for improved health and well-being.

Nanoparticle and Nanotopography-Induced Activation of the Wnt Pathway in Bone Regeneration

Background and Aims: Recent research has focused on developing nanoparticle and nanotopography-based technologies for bone regeneration. The Wingless-related integration site (Wnt) signaling pathway has been shown to play a vital role in this process, in particular in osteogenic differentiation and proliferation. The exact mechanisms by which nanoparticles and nanotopographies activate the Wnt signaling pathway, however, are not fully understood. This review aimed to elucidate the mechanisms by which nanoscale technologies activate the Wnt signaling pathway during bone regeneration. Methods: The terms "Wnt," "bone," and "nano*" were searched on PubMed and Ovid with no date limit. Only original research articles related to Wnt signaling and bone regeneration in the context of nanotopographies, nanoparticles, or scaffolds with nanotopographies/nanoparticles were reviewed. Results: The primary mechanism by which nanoparticles activated the Wnt pathway was by internalization through the endocytic pathway or diffusion through the cell membrane, leading to accumulation of nonphosphorylated β-catenin in the cytoplasm and subsequently downstream osteogenic signaling (e.g., upregulation of runt-related transcription factor 2 [RUNX2]). The specific size of the nanoparticles and the process of endocytosis itself has been shown to modulate the Wnt-β-catenin pathway. Nanotopographies were shown to directly activate frizzled receptors, initiating Wnt/β-catenin signaling. Additional studies showed nanotopographies to activate the Wnt/calcium (Wnt/Ca2+)-dependent and Wnt/planar cell polarity pathways through nuclear factor of activated T cells, and α5β1 integrin stimulation. Finally, scaffolds containing nanotopographies/nanoparticles were found to induce Wnt signaling through a combination of ion release (e.g., lithium, boron, lanthanum, and icariin), which inhibited glycogen synthase kinase 3 beta (GSK-3β) activity, and through similar mechanisms to the nanotopographies. Conclusion: This review concludes that nanoparticles and nanotopographies cause Wnt activation through several different mechanisms, specific to the size, shape, and structure of the nanoparticles or nanotopographies. Endocytosis-related mechanisms, integrin signaling and ion release were the major mechanisms identified across nanoparticles, nanotopographies, and scaffolds, respectively. Knowledge of these mechanisms will help develop more effective targeted nanoscale technologies for bone regeneration. Impact statement Nanoparticles and nanotopographies can activate the Wingless-related integration site (Wnt) signaling pathway, which is essential for bone regeneration. This review has identified that activation is due to endocytosis, integrin signaling and ion release, depending on the size, shape, and structure of the nanoparticles or nanotopographies. By identifying and further understanding these mechanisms, more effective nanoscale technologies that target the Wnt signaling pathway can be developed. These technologies can be used for the treatment of nonunion bone fractures, a major clinical challenge, with the potential to improve the quality of life of millions of patients around the world.

Dexamethasone Induced Osteocyte Apoptosis in Steroid-Induced Femoral Head Osteonecrosis through ROS-Mediated Oxidative Stress

OBJECTIVE

Glucocorticoid (GC) overuse is strongly associated with steroid-induced osteonecrosis of the femoral head (SINFH). However, the underlying mechanism of SINFH remains unclear. This study aims to investigate the effect of dexamethasone (Dex)-induced oxidative stress on osteocyte apoptosis and the underlying mechanisms.

METHODS

Ten patients with SINFH and 10 patients with developmental dysplasia of the hips (DDH) were enrolled in our study. Sixty rats were randomly assigned to the Control, Dex, Dex + N-Acetyl-L-cysteine (NAC), Dex + Dibenziodolium chloride (DPI), NAC, and DPI groups. Magnetic resonance imaging (MRI) was used to examine edema in the femoral head of rats. Histopathological staining was performed to assess osteonecrosis. Immunofluorescence staining with TUNEL and 8-OHdG was conducted to evaluate osteocyte apoptosis and oxidative damage. Immunohistochemical staining was carried out to detect the expression of NOX1, NOX2, and NOX4. Viability and apoptosis of MLO-Y4 cells were measured using the CCK-8 assay and TUNEL staining. 8-OHdG staining was conducted to detect oxidative stress. 2',7'-Dichlorodihydrofluorescein diacetate (DCFH-DA) staining was performed to measure reactive oxygen species (ROS). The expression of NOX1, NOX2, and NOX4 in MLO-Y4 cells was analyzed by Western blotting. Multiple comparisons were performed using one-way analysis of variance (ANOVA).

RESULTS

In patients and the rat model, hematoxylin-eosin (HE) staining revealed a significantly higher rate of empty lacunae in the SINFH group than in the DDH group. Immunofluorescence staining indicated a significant increase in TUNEL-positive cells and 8-OHdG-positive cells in the SINFH group compared to the DDH group. Immunohistochemical staining demonstrated a significant increase in the expression of NOX1, NOX2, and NOX4 proteins in SINFH patients compared to DDH patients. Moreover, immunohistochemical staining showed a significant increase in the proportion of NOX2-positive cells compared to the Control group in the femoral head of rats. In vitro, Dex significantly inhibited the viability of osteocyte cells and induced apoptosis. After Dex treatment, the intracellular ROS level increased. However, Dex treatment did not alter the expression of NOX proteins in vitro. Additionally, NAC and DPI inhibited the generation of intracellular ROS and partially alleviated osteocyte apoptosis in vivo and in vitro.

CONCLUSION

This study demonstrates that GC promotes apoptosis of osteocyte cells through ROS-induced oxidative stress. Furthermore, we found that the increased expression of NOXs induced by GC serves as an important source of ROS generation.

From the Mind to the Spine: The Intersecting World of Alzheimer's and Osteoporosis

PURPOSE OF REVIEW

This comprehensive review delves into the intricate interplay between Alzheimer's disease (AD) and osteoporosis, two prevalent conditions with significant implications for individuals' quality of life. The purpose is to explore their bidirectional association, underpinned by common pathological processes such as aging, genetic factors, inflammation, and estrogen deficiency.

RECENT FINDINGS

Recent advances have shown promise in treating both Alzheimer's disease (AD) and osteoporosis by targeting disease-specific proteins and bone metabolism regulators. Monoclonal antibodies against beta-amyloid and tau for AD, as well as RANKL and sclerostin for osteoporosis, have displayed therapeutic potential. Additionally, ongoing research has identified neuroinflammatory genes shared between AD and osteoporosis, offering insight into the interconnected inflammatory mechanisms. This knowledge opens avenues for innovative dual-purpose therapies that could address both conditions, potentially revolutionizing treatment approaches for AD and osteoporosis simultaneously. This review underscores the potential for groundbreaking advancements in early diagnosis and treatment by unraveling the intricate connection between AD and bone health. It advocates for a holistic, patient-centered approach to medical care that considers both cognitive and bone health, ultimately aiming to enhance the overall well-being of individuals affected by these conditions. This review article is part of a series of multiple manuscripts designed to determine the utility of using artificial intelligence for writing scientific reviews.

Mind Gaps and Bone Snaps: Exploring the Connection Between Alzheimer's Disease and Osteoporosis

PURPOSE OF REVIEW

This comprehensive review discusses the complex relationship between Alzheimer's disease (AD) and osteoporosis, two conditions that are prevalent in the aging population and result in adverse complications on quality of life. The purpose of this review is to succinctly elucidate the many commonalities between the two conditions, including shared pathways, inflammatory and oxidative mechanisms, and hormonal deficiencies.

RECENT FINDINGS

AD and osteoporosis share many aspects of their respective disease-defining pathophysiology. These commonalities include amyloid beta deposition, the Wnt/β-catenin signaling pathway, and estrogen deficiency. The shared mechanisms and risk factors associated with AD and osteoporosis result in a large percentage of patients that develop both diseases. Previous literature has established that the progression of AD increases the risk of sustaining a fracture. Recent findings demonstrate that the reverse may also be true, suggesting that a fracture early in the life course can predispose one to developing AD due to the activation of these shared mechanisms. The discovery of these commonalities further guides the development of novel therapeutics in which both conditions are targeted. This detailed review delves into the commonalities between AD and osteoporosis to uncover the shared players that bring these two seemingly unrelated conditions together. The discussion throughout this review ultimately posits that the occurrence of fractures and the mechanism behind fracture healing can predispose one to developing AD later on in life, similar to how AD patients are at an increased risk of developing fractures. By focusing on the shared mechanisms between AD and osteoporosis, one can better understand the conditions individually and as a unit, thus informing therapeutic approaches and further research. This review article is part of a series of multiple manuscripts designed to determine the utility of using artificial intelligence for writing scientific reviews.

Functional and analytical recapitulation of osteoclast biology on demineralized bone paper

Osteoclasts are the primary target for osteoporosis drug development. Recent animal studies revealed the crucial roles of osteoblasts in regulating osteoclastogenesis and the longer lifespans of osteoclasts than previously thought with fission and recycling. However, existing culture platforms are limited to replicating these newly identified cellular processes. We report a demineralized bone paper (DBP)-based osteoblast culture and osteoclast assay platform that replicates osteoclast fusion, fission, resorption, and apoptosis with high fidelity and analytical power. An osteoid-inspired DBP supports rapid and structural mineral deposition by osteoblasts. Coculture osteoblasts and bone marrow monocytes under biochemical stimulation recapitulate osteoclast differentiation and function. The DBP-based bone model allows longitudinal quantitative fluorescent monitoring of osteoclast responses to bisphosphonate drug, substantiating significantly reducing their number and lifespan. Finally, we demonstrate the feasibility of humanizing the bone model. The DBP-based osteo assay platforms are expected to advance bone remodeling-targeting drug development with improved prediction of clinical outcomes.

Phlpp1 Expression in Osteoblasts Plays a Modest Role in Bone Homeostasis

Prior work demonstrated that Phlpp1 deficiency alters limb length and bone mass, but the cell types involved and requirement of Phlpp1 for this effect were unclear. To understand the function of Phlpp1 within bone-forming osteoblasts, we crossed Phlpp1 floxed mice with mice harboring type 1 collagen (Col1a12.3kb)-Cre. Mineralization of bone marrow stromal cell cultures derived from Phlpp1 cKOCol1a1 was unchanged, but levels of inflammatory genes (eg, Ifng, Il6, Ccl8) and receptor activator of NF-κB ligand/osteoprotegerin (RANKL/OPG) ratios were enhanced by either Phlpp1 ablation or chemical inhibition. Micro-computed tomography of the distal femur and L5 vertebral body of 12-week-old mice revealed no alteration in bone volume per total volume, but compromised femoral bone microarchitecture within Phlpp1 cKOCol1a1 conditional knockout females. Bone histomorphometry of the proximal tibia documented no changes in osteoblast or osteoclast number per bone surface but slight reductions in osteoclast surface per bone surface. Overall, our data show that deletion of Phlpp1 in type 1 collagen-expressing cells does not significantly alter attainment of peak bone mass of either males or females, but may enhance inflammatory gene expression and the ratio of RANKL/OPG. Future studies examining the role of Phlpp1 within models of advanced age, inflammation, or osteocytes, as well as functional redundancy with the related Phlpp2 isoform are warranted. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The impacts of oral and gut microbiota on alveolar bone loss in periodontitis

Periodontitis, a chronic infectious disease, primarily arises from infections and the invasion of periodontal pathogens. This condition is typified by alveolar bone loss resulting from host immune responses and inflammatory reactions. Periodontal pathogens trigger aberrant inflammatory reactions within periodontal tissues, thereby exacerbating the progression of periodontitis. Simultaneously, these pathogens and metabolites stimulate osteoclast differentiation, which leads to alveolar bone resorption. Moreover, a range of systemic diseases, including diabetes, postmenopausal osteoporosis, obesity and inflammatory bowel disease, can contribute to the development and progression of periodontitis. Many studies have underscored the pivotal role of gut microbiota in bone health through the gut-alveolar bone axis. The circulation may facilitate the transfer of gut pathogens or metabolites to distant alveolar bone, which in turn regulates bone homeostasis. Additionally, gut pathogens can elicit gut immune responses and direct immune cells to remote organs, potentially exacerbating periodontitis. This review summarizes the influence of oral microbiota on the development of periodontitis as well as the association between gut microbiota and periodontitis. By uncovering potential mechanisms of the gut-bone axis, this analysis provides novel insights for the targeted treatment of pathogenic bacteria in periodontitis.

Cortical microarchitecture and remodeling-associated gene expression related to oral cancer prognosis

The objective of this study was to assess the remodeling-associated gene expression in the mandible of patients diagnosed with oral squamous cell carcinoma (OSCC), investigating the cortical microarchitecture, and their influence on disease-free survival (DFS) and overall survival (OS) rates. A total of twenty-four patients who underwent mandibulectomy for OSCC treatment had two bone fragments harvested from the mandible for gene expression (RANK, RANKL, OPG, and SOST), and microarchitecture analysis, including bone volume, surface, mineral density, degree of anisotropy, and fractal dimension. The prognosis of the patients was assessed. The results revealed that RANK, RANKL, and SOST were predominantly downregulated, while OPG was completely downregulated. Tumors located adjacent to the posterior region of the mandible (p = 0.02), with a bone mineral density below 1.03 g/cm3 HA (p = 0.001), and a bone volume less than 86.47% (p = 0.03) were associated with poor outcomes. In conclusion, bone-remodeling-associated genes exhibited downregulation in the cortex of the mandible in OSCC patients. Additionally, the tumor's location within the mandible, bone volume, and cortical bone mineral density were identified as factors impacting DFS.

Comparative study in estrogen-depleted mice identifies skeletal and osteocyte transcriptomic responses to abaloparatide and teriparatide

Osteocytes express parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptors and respond to the PTHrP analog abaloparatide (ABL) and to the PTH 1-34 fragment teriparatide (TPTD), which are used to treat osteoporosis. Several studies indicate overlapping but distinct skeletal responses to ABL or TPTD, but their effects on cortical bone may differ. Little is known about their differential effects on osteocytes. We compared cortical osteocyte and skeletal responses to ABL and TPTD in sham-operated and ovariectomized mice. Administered 7 weeks after ovariectomy for 4 weeks at a dose of 40 μg/kg/d, TPTD and ABL had similar effects on trabecular bone, but ABL showed stronger effects in cortical bone. In cortical osteocytes, both treatments decreased lacunar area, reflecting altered peri-lacunar remodeling favoring matrix accumulation. Osteocyte RNA-Seq revealed that several genes and pathways were altered by ovariectomy and affected similarly by TPTD and ABL. Notwithstanding, several signaling pathways were uniquely regulated by ABL. Thus, in mice, TPTD and ABL induced a positive osteocyte peri-lacunar remodeling balance, but ABL induced stronger cortical responses and affected the osteocyte transcriptome differently. We concluded that ABL affected the cortical osteocyte transcriptome in a manner subtly different from TPTD, resulting in more beneficial remodeling/modeling changes and homeostasis of the cortex.

Bone Involvement in Rheumatoid Arthritis and Spondyloartritis: An Updated Review

Several rheumatologic diseases are primarily distinguished by their involvement of bone tissue, which not only serves as a mere target of the condition but often plays a pivotal role in its pathogenesis. This scenario is particularly prominent in chronic inflammatory arthritis such as rheumatoid arthritis (RA) and spondyloarthritis (SpA). Given the immunological and systemic nature of these diseases, in this review, we report an overview of the pathogenic mechanisms underlying specific bone involvement, focusing on the complex interactions that occur between bone tissue's own cells and the molecular and cellular actors of the immune system, a recent and fascinating field of interest defined as osteoimmunology. Specifically, we comprehensively elaborate on the distinct pathogenic mechanisms of bone erosion seen in both rheumatoid arthritis and spondyloarthritis, as well as the characteristic process of aberrant bone formation observed in spondyloarthritis. Lastly, chronic inflammatory arthritis leads to systemic bone involvement, resulting in systemic bone loss and consequent osteoporosis, along with increased skeletal fragility.

Connexin 43 and cell culture substrate differentially regulate OCY454 osteocytic differentiation and signaling to primary bone cells

Connexin 43 (Cx43), the predominate gap junction protein in bone, is essential for intercellular communication and skeletal homeostasis. Previous work suggests that osteocyte-specific deletion of Cx43 leads to increased bone formation and resorption; however, the cell-autonomous role of osteocytic Cx43 in promoting increased bone remodeling is unknown. Recent studies using three-dimensional (3D) culture substrates in OCY454 cells suggest that 3D cultures may offer increased bone remodeling factor expression and secretion, such as sclerostin and receptor activator of nuclear factor-κB ligand (RANKL). In this study, we compared culturing OCY454 osteocytes on 3D Alvetex scaffolds with traditional 2D tissue culture, both with [wild-type (WT)] and without Cx43 (Cx43 KO). Conditioned media from OCY454 cell cultures were used to determine soluble signaling to differentiate primary bone marrow cells into osteoblasts and osteoclasts. OCY454 cells cultured on 3D portrayed a mature osteocytic phenotype, relative to cells on 2D, shown by increased osteocytic gene expression and reduced cell proliferation. In contrast, OCY454 differentiation based on these same markers was not affected by Cx43 deficiency in 3D. Interestingly, increased sclerostin secretion was found in 3D cultured WT cells compared with that of Cx43 KO cells. Conditioned media from Cx43 KO cells promoted increased osteoblastogenesis and osteoclastogenesis, with maximal effects from 3D cultured Cx43 KO cells. These results suggest that Cx43 deficiency promotes increased bone remodeling in a cell-autonomous manner with minimal changes in osteocyte differentiation. Finally, 3D cultures appear better suited to study mechanisms from Cx43-deficient OCY454 osteocytes in vitro due to their ability to promote osteocyte differentiation, limit proliferation, and increase bone remodeling factor secretion.NEW & NOTEWORTHY 3D cell culture of OCY454 cells promoted increased differentiation compared with traditional 2D culture. Although Cx43 deficiency did not affect OCY454 differentiation, it resulted in increased signaling, promoting osteoblastogenesis and osteoclastogenesis. Our results suggest that Cx43 deficiency promotes increased bone remodeling in a cell-autonomous manner with minimal changes in osteocyte differentiation. Also, 3D cultures appear better suited to study mechanisms in Cx43-deficient OCY454 osteocytes.

Inflammation and gut dysbiosis as drivers of CKD-MBD

Two decades ago, Kidney Disease: Improving Global Outcomes coined the term chronic kidney disease-mineral and bone disorder (CKD-MBD) to describe the syndrome of biochemical, bone and extra-skeletal calcification abnormalities that occur in patients with CKD. CKD-MBD is a prevalent complication and contributes to the excessively high burden of fractures and cardiovascular disease, loss of quality of life and premature mortality in patients with CKD. Thus far, therapy has focused primarily on phosphate retention, abnormal vitamin D metabolism and parathyroid hormone disturbances, but these strategies have largely proved unsuccessful, thus calling for paradigm-shifting concepts and innovative therapeutic approaches. Interorgan crosstalk is increasingly acknowledged to have an important role in health and disease. Accordingly, mounting evidence suggests a role for both the immune system and the gut microbiome in bone and vascular biology. Gut dysbiosis, compromised gut epithelial barrier and immune cell dysfunction are prominent features of the uraemic milieu. These alterations might contribute to the inflammatory state observed in CKD and could have a central role in the pathogenesis of CKD-MBD. The emerging fields of osteoimmunology and osteomicrobiology add another level of complexity to the pathogenesis of CKD-MBD, but also create novel therapeutic opportunities.

Mechanical properties of trabeculae and osteocyte morphology change significantly in different areas of the necrotic femoral head

Background: Osteonecrosis of the femoral head is a complex hip ailment. The precise changes in bone tissue during the disease's onset remain unclear. It is vital to assess both the quantity and quality of the trabecular state in a necrotic femoral head. Aim: This study aims to identify and compare the ultrastructural changes in osteocyte morphology and nanomechanical characteristics within various regions of necrotic femoral heads. Methods: Between December 2016 and May 2023, we gathered ten necrotic femoral heads from patients and five femoral heads from cadavers. The samples from the necrotic femoral heads were categorized into three areas: necrotic, sclerotic, and normal. Our assessment methods encompassed hematoxylin and eosin staining, sclerostin (SOST) immunohistochemistry, micro-computed tomography, nanoindentation, and acid-etched scanning electron microscopy. These techniques enabled us to examine the SOST expression, trabecular microstructure, micromechanical properties of trabeculae, and modifications in osteocyte morphology at the ultrastructural level. Results: The protein level of SOST was found to be lower in the sclerotic area. In the necrotic area, decreased values of bone volume fraction, trabecular thickness, and trabecular number and an increased value of trabecular separation were found. Conversely, in the sclerotic area, higher mean values of bone volume fraction, trabecular number, and trabecular thickness and lower trabecular separation indicated significant changes in the structural characteristics of trabeculae. Compared with the healthy area, the elastic modulus and hardness in the sclerotic area were significantly higher than those in the necrotic, normal, and control areas, while those in necrotic areas were significantly lower than those in the healthy area. The number of osteocytes tended to increase in the sclerotic area with more canalicular cells compared to the healthy area and control group. Conclusion: These results imply that the stress distribution within the sclerotic area could potentially lead to enhanced trabecular quality and quantity. This effect is also reflected in the increased count of osteocytes and their canaliculars. It is plausible that the sclerotic trabecular bone plays a pivotal role in the repair of necrotic femoral heads.

The role of cells and signal pathways in subchondral bone in osteoarthritis

Osteoarthritis (OA) is mainly caused by ageing, strain, trauma, and congenital joint abnormalities, resulting in articular cartilage degeneration. During the pathogenesis of OA, the changes in subchondral bone (SB) are not only secondary manifestations of OA, but also an active part of the disease, and are closely associated with the severity of OA. In different stages of OA, there were microstructural changes in SB. Osteocytes, osteoblasts, and osteoclasts in SB are important in the pathogenesis of OA. The signal transduction mechanism in SB is necessary to maintain the balance of a stable phenotype, extracellular matrix (ECM) synthesis, and bone remodelling between articular cartilage and SB. An imbalance in signal transduction can lead to reduced cartilage quality and SB thickening, which leads to the progression of OA. By understanding changes in SB in OA, researchers are exploring drugs that can regulate these changes, which will help to provide new ideas for the treatment of OA.

Interoceptive regulation of skeletal tissue homeostasis and repair

Recent studies have determined that the nervous system can sense and respond to signals from skeletal tissue, a process known as skeletal interoception, which is crucial for maintaining bone homeostasis. The hypothalamus, located in the central nervous system (CNS), plays a key role in processing interoceptive signals and regulating bone homeostasis through the autonomic nervous system, neuropeptide release, and neuroendocrine mechanisms. These mechanisms control the differentiation of mesenchymal stem cells into osteoblasts (OBs), the activation of osteoclasts (OCs), and the functional activities of bone cells. Sensory nerves extensively innervate skeletal tissues, facilitating the transmission of interoceptive signals to the CNS. This review provides a comprehensive overview of current research on the generation and coordination of skeletal interoceptive signals by the CNS to maintain bone homeostasis and their potential role in pathological conditions. The findings expand our understanding of intersystem communication in bone biology and may have implications for developing novel therapeutic strategies for bone diseases.

Engineering Antioxidant Surfaces for Titanium-Based Metallic Biomaterials

Prolonged inflammation induced by orthopedic metallic implants can critically affect the success rates, which can even lead to aseptic loosening and consequent implant failure. In the case of adverse clinical conditions involving osteoporosis, orthopedic trauma and implant corrosion-wear in peri-implant region, the reactive oxygen species (ROS) activity is enhanced which leads to increased oxidative stress. Metallic implant materials (such as titanium and its alloys) can induce increased amount of ROS, thereby critically influencing the healing process. This will consequently affect the bone remodeling process and increase healing time. The current review explores the ROS generation aspects associated with Ti-based metallic biomaterials and the various surface modification strategies developed specifically to improve antioxidant aspects of Ti surfaces. The initial part of this review explores the ROS generation associated with Ti implant materials and the associated ROS metabolism resulting in the formation of superoxide anion, hydroxyl radical and hydrogen peroxide radicals. This is followed by a comprehensive overview of various organic and inorganic coatings/materials for effective antioxidant surfaces and outlook in this research direction. Overall, this review highlights the critical need to consider the aspects of ROS generation as well as oxidative stress while designing an implant material and its effective surface engineering.

Connexin 43 and Cell Culture Substrate Differentially Regulate OCY454 Osteocytic Differentiation and Signaling to Primary Bone Cells

Connexin 43 (Cx43), the predominate gap junction protein in bone, is essential for intercellular communication and skeletal homeostasis. Previous work suggests osteocyte-specific deletion of Cx43 leads to increased bone formation and resorption, however the cell-autonomous role of osteocytic Cx43 in promoting increased bone remodeling is unknown. Recent studies using 3D culture substrates in OCY454 cells suggest 3D cultures may offer increased bone remodeling factor expression and secretion, such as sclerostin and RANKL. In this study, we compared culturing OCY454 osteocytes on 3D Alvetex scaffolds to traditional 2D tissue culture, both with (WT) and without Cx43 (Cx43 KO). Conditioned media from OCY454 cell cultures was used to determine soluble signaling to differentiate primary bone marrow stromal cells into osteoblasts and osteoclasts. OCY454 cells cultured on 3D portrayed a mature osteocytic phenotype, relative to cells on 2D, shown by increased osteocytic gene expression and reduced cell proliferation. In contrast, OCY454 differentiation based on these same markers was not affected by Cx43 deficiency in 3D. Interestingly, increased sclerostin secretion was found in 3D cultured WT cells compared to Cx43 KO cells. Conditioned media from Cx43 KO cells promoted increased osteoblastogenesis and increased osteoclastogenesis, with maximal effects from 3D cultured Cx43 KO cells. These results suggest Cx43 deficiency promotes increased bone remodeling in a cell autonomous manner with minimal changes in osteocyte differentiation. Finally, 3D cultures appear better suited to study mechanisms from Cx43-deficient OCY454 osteocytes in vitro due to their ability to promote osteocyte differentiation, limit proliferation, and increase bone remodeling factor secretion.

New and Noteworthy

3D cell culture of OCY454 cells promoted increased differentiation compared to traditional 2D culture. While Cx43 deficiency did not affect OCY454 differentiation, it resulted in increased signaling, promoting osteoblastogenesis and osteoclastogenesis. Our results suggest Cx43 deficiency promotes increased bone remodeling in a cell autonomous manner with minimal changes in osteocyte differentiation. Also, 3D cultures appear better suited to study mechanisms in Cx43-deficient OCY454 osteocytes.

Osteocytes: New Kids on the Block for Cancer in Bone Therapy

The tumor microenvironment plays a central role in the onset and progression of cancer in the bone. Cancer cells, either from tumors originating in the bone or from metastatic cancer cells from other body systems, are located in specialized niches where they interact with different cells of the bone marrow. These interactions transform the bone into an ideal niche for cancer cell migration, proliferation, and survival and cause an imbalance in bone homeostasis that severely affects the integrity of the skeleton. During the last decade, preclinical studies have identified new cellular mechanisms responsible for the dependency between cancer cells and bone cells. In this review, we focus on osteocytes, long-lived cells residing in the mineral matrix that have recently been identified as key players in the spread of cancer in bone. We highlight the most recent discoveries on how osteocytes support tumor growth and promote bone disease. Additionally, we discuss how the reciprocal crosstalk between osteocytes and cancer cells provides the opportunity to develop new therapeutic strategies to treat cancer in the bone.

Bevacizumab, a vascular endothelial growth factor inhibitor, promotes orthodontic tooth movement in an experimental rat model

Objective

This study aimed to evaluate the impact of bevacizumab on orthodontic tooth movement (OTM) in Wistar rats.

Materials and methods

The OTM model was constructed by placing an orthodontic coil spring between the maxillary first molar and anterior tooth. Bevacizumab (Avastin®; 10 mg/kg twice per week) was started one week before the OTM and continued for 3 weeks. After 1 and 2 weeks, OTM distance and anterior tooth mobility were measured. Thereafter, the maxilla was dissected for micro-CT microarchitectural analysis, followed by histological analysis, and tartrate-resistant acid phosphatase (TRAP) staining. Moreover, the distributions of collagen fibers type-I and -III (Col-I and Col-III) were evaluated using Picro-Sirius red staining.

Results

Orthodontic force prompted bone resorption and formation on the pressure and tension sides, respectively. Bevacizumab therapy resulted in a 42% increase of OTM, particularly after 2 weeks. Furthermore, bevacizumab disturbed the morphometric structure at both pressure and tension sites. The histological evaluation indicated about 35-44% fewer osteoblasts in the bevacizumab group, especially at the tension side, whereas the proportion of TRAP-positive osteoclasts at the pressure side was 34-37% higher than the control. The mature Col-I was reduced at the tension site by 33%, whereas the Col-III/Col-I ratio was enhanced by 20-44% at pressure and tension sites, after 2 weeks, in the bevacizumab group.

Conclusion

Anti-vascular bevacizumab therapy accentuates OTM in rat model, possibly through the enhancement of bone resorption, at the pressure side, and the reduction of bone formation, at the tension side as well as dysregulation of collagen fibers distribution.

Investigation of Surface Layers on Biological and Synthetic Hydroxyapatites Based on Bone Mineralization Process

In this review, the current status of the influence of added ions (i.e., SiO₄^4-, CO₃^2-, etc.) and surface states (i.e., hydrated and non-apatite layers) on the biocompatibility nature of hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) is discussed. It is well known that HA is a type of calcium phosphate with high biocompatibility that is present in biological hard tissues such as bones and enamel. This biomedical material has been extensively studied due to its osteogenic properties. The chemical composition and crystalline structure of HA change depending on the synthetic method and the addition of other ions, thereby affecting the surface properties related to biocompatibility. This review illustrates the structural and surface properties of HA substituted with ions such as silicate, carbonate, and other elemental ions. The importance of the surface characteristics of HA and its components, the hydration layers, and the non-apatite layers for the effective control of biomedical function, as well as their relationship at the interface to improve biocompatibility, has been highlighted. Since the interfacial properties will affect protein adsorption and cell adhesion, the analysis of their properties may provide ideas for effective bone formation and regeneration mechanisms.

Insight into the effect of biomaterials on osteogenic differentiation of mesenchymal stem cells: A review from a mitochondrial perspective

Bone damage may be triggered by a variety of factors, and the damaged area often requires a bone graft. Bone tissue engineering can serve as an alternative strategy for repairing large bone defects. Mesenchymal stem cells (MSCs), the progenitor cells of connective tissue, have become an important tool for tissue engineering due to their ability to differentiate into a variety of cell types. The precise regulation of the growth and differentiation of the stem cells used for bone regeneration significantly affects the efficiency of this type of tissue engineering. During the process of osteogenic induction, the dynamics and function of localized mitochondria are altered. These changes may also alter the microenvironment of the therapeutic stem cells and result in mitochondria transfer. Mitochondrial regulation not only affects the induction/rate of differentiation, but also influences its direction, determining the final identity of the differentiated cell. To date, bone tissue engineering research has mainly focused on the influence of biomaterials on phenotype and nuclear genotype, with few studies investigating the role of mitochondria. In this review, we provide a comprehensive summary of researches into the role of mitochondria in MSCs differentiation and critical analysis regarding smart biomaterials that are able to "programme" mitochondria modulation was proposed. STATEMENT OF SIGNIFICANCE: : • This review proposed the precise regulation of the growth and differentiation of the stem cells used to seed bone regeneration. • This review addressed the dynamics and function of localized mitochondria during the process of osteogenic induction and the effect of mitochondria on the microenvironment of stem cells. • This review summarized biomaterials which affect the induction/rate of differentiation, but also influences its direction, determining the final identity of the differentiated cell through the regulation of mitochondria.

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

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

Exposure of suckling rats to hexavalent chromium (CrVI) alters bone formation at the base of the alveolus causing a delay in tooth eruption

OBJECTIVES

Hexavalent chromium (CrVI)-exposure of suckling rats has been shown to delay tooth eruption. However, the effects of CrVI-exposure on bone formation at the base of the alveolus, which provides a motive force for tooth eruption in the early stages of the process, remain unknown. The present work sought to evaluate the effect of CrVI-exposure on bone formation at the base of the alveolus during the intraosseous stage of tooth eruption in suckling rats, using histomorphometric and immunohistochemical studies.

METHODS

Experimental animals received 12.5 mg/kg-bw/day of potassium dichromate dissolved in saline solution by gavage starting on day 4 of the experiment; controls similarly received an equivalent volume of saline. All the animals were euthanized at the age of 9 days. The base of the developing alveolus at the level of the mesial root of the first lower molar was analyzed histomorphometrically and immunohistochemically. Data were statistically analyzed using student's t-test, with statistical significance set at p <0.05.

RESULTS

CrVI-exposed animals showed lower bone volume and height at the base of the alveolus, a significant preponderance of bone rest on the surface adjacent to the dental follicle, and a significantly lower percentage of positive Runt-related transcription factor-2 (RUNX2+) osteoblasts and a significantly higher number of mesenchymal-like RUNX2+ cells at the latter site.

CONCLUSION

CrVI-exposure during lactation affects bone formation at the base of the developing alveolus, delaying tooth eruption. These findings underscore the importance of controlling drinking water levels of toxic substances since their effects can alter the growth and development of individuals exposed during early childhood.

Oxidative Stress and Natural Antioxidants in Osteoporosis: Novel Preventive and Therapeutic Approaches

This review reports in detail the cellular and molecular mechanisms which regulate the bone remodeling process in relation to oxidative stress (OS), inflammatory factors, and estrogen deficiency. OS is considered an important pathogenic factor of osteoporosis, inducing osteocyte apoptosis and varying levels of specific factors, such as receptor activator κB ligand (RANKL), sclerostin, and, according to recent evidence, fibroblast growth factor 23, with consequent impairment of bone remodeling and high bone resorption. Bone loss increases the risk of fragility fractures, and the most commonly used treatments are antiresorptive drugs, followed by anabolic drugs or those with a double effect. In addition, recent data show that natural antioxidants contained in the diet are efficient in preventing and reducing the negative effects of OS on bone remodeling and osteocytes through the involvement of sirtuin type 1 enzyme. Indeed, osteocytes and some of their molecular factors are considered potential biological targets on which antioxidants can act to prevent and reduce bone loss, as well as to promote bone anabolic and regenerative processes by restoring physiological bone remodeling. Several data suggest including antioxidants in novel therapeutic approaches to develop better management strategies for the prevention and treatment of osteoporosis and OS-related bone diseases. In particular, anthocyanins, as well as resveratrol, lycopene, oleuropein, some vitamins, and thiol antioxidants, could have protective and therapeutic anti-osteoporotic effects.

Osteocyte Remodeling of the Lacunar-Canalicular System: What's in a Name?

PURPOSE OF REVIEW

Osteocytes directly modify the bone surrounding the expansive lacunar-canalicular system (LCS) through both resorption and deposition. The existence of this phenomenon is now widely accepted, but is referred to as "osteocyte osteolysis," "LCS remodeling," and "perilacunar remodeling," among other names. The uncertainty in naming this physiological process reflects the many persistent questions about why and how osteocytes interact with local bone matrix. The goal of this review is to examine the purpose and nature of LCS remodeling and its impacts on multiscale bone quality.

RECENT FINDINGS

While LCS remodeling is clearly important for systemic calcium mobilization, this process may have additional potential drivers and may impact the ability of bone to resist fracture. There is abundant evidence that the osteocyte can resorb and replace bone mineral and does so outside of extreme challenges to mineral homeostasis. The impacts of the osteocyte on organic matrix are less certain, especially regarding whether osteocytes produce osteoid. Though multiple lines of evidence point towards osteocyte production of organic matrix, definitive work is needed. Recent high-resolution imaging studies demonstrate that LCS remodeling influences local material properties. The role of LCS remodeling in the maintenance and deterioration of bone matrix quality in aging and disease are active areas of research. In this review, we highlight current progress in understanding why and how the osteocyte removes and replaces bone tissue and the consequences of these activities to bone quality. We posit that answering these questions is essential for evaluating whether, how, when, and why LCS remodeling may be manipulated for therapeutic benefit in managing bone fragility.

Differential Expression of Dickkopf 1 and Periostin in Mouse Strains with High and Low Bone Mass

By expressing different genes and proteins that regulate osteoclast as well as osteoblast formation, osteocytes orchestrate bone metabolism. The aim of this project was the evaluation of the differences in the osteocytes’ secretory activity in the low bone mass mouse strain C57BL/6J and the high bone mass strain C3H/J. The femura of eight- and sixteen-week-old male C57BL/6J and C3H/J mice—six animals per group—were analyzed. Using immunohistochemistry, osteocytes expressing dickkopf 1, sclerostin, periostin, fibroblast growth factor 23 (FGF23), and osteoprotegerin were detected. By means of the OsteoMeasure-System, 92.173 osteocytes were counted. At the age of eight weeks, approximately twice as many cortical and trabecular osteocytes from the C57BL/6J mice compared to the C3H/J mice expressed dickkopf 1 (p < 0.005). The number of cortical osteocytes expressing sclerostin was also higher in the C57BL/6J mice (p < 0.05). In contrast, the cortical and trabecular osteocytes expressing periostin were twice as high in the C3H/J mice (p < 0.005). The dickkopf 1 expressing osteocytes of the C57BL/6J mice decreased with age and showed a strain-specific difference only in cortical bone by 16 weeks of age (p < 0.05). In the C3H/J mice, the amount of osteocytes expressing periostin tended to increase with age. Thus, strain-related differences were maintained in 16-week-old rodents (p < 0.005). No strain-specific differences in the expression of FGF23 or osteoprotegerin in the cortical compartment could be detected. This experimental study showed that the osteocytes’ protein expression reflects differences in bone characteristics and strain-related differences during skeletal maturation. Besides the osteocytes’ expression of sclerostin, their expression of dickkopf 1 and periostin seems to be important for bone properties as well.

Tuning the resorption-formation balance in an in vitro 3D osteoblast-osteoclast co-culture model of bone

The aim of the present study was to further improve an in vitro 3D osteoblast (OB) - osteoclast (OC) co-culture model of bone by tuning it towards states of formation, resorption, and equilibrium for their future applications in fundamental research, drug development and personalized medicine. This was achieved by varying culture medium composition and monocyte seeding density, the two external parameters that affect cell behavior the most. Monocytes were seeded at two seeding densities onto 3D silk-fibroin constructs pre-mineralized by MSC-derived OBs and were co-cultured in one of three different media (OC stimulating, Neutral and OB stimulating medium) for three weeks. Histology showed mineralized matrix after co-culture and OC markers in the OC medium group. Scanning Electron Microscopy showed large OC-like cells in the OC medium group. Micro-computed tomography showed increased formation in the OB medium group, equilibrium in the Neutral medium group and resorption in the OC medium group. Culture supernatant samples showed high early tartrate resistant acid phosphatase (TRAP) release in the OC medium group, a later and lower release in the Neutral medium group, and almost no release in the OB medium group. Increased monocyte seeding density showed a less-than-proportional increase in TRAP release and resorption in OC medium, while it proportionally increased TRAP release in Neutral medium without affecting net resorption. The 3D OB-OC co-culture model was effectively used to show an excess of mineral deposition using OB medium, resorption using OC medium, or an equilibrium using Neutral medium. All three media applied to the model may have their own distinct applications in fundamental research, drug development, and personalized medicine.

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

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

ERK Inhibition Increases RANKL-Induced Osteoclast Differentiation in RAW 264.7 Cells by Stimulating AMPK Activation and RANK Expression and Inhibiting Anti-Osteoclastogenic Factor Expression

Bone absorption is necessary for the maintenance of bone homeostasis. An osteoclast (OC) is a monocyte-macrophage lineage cell that absorbs bone tissue. Extracellular signal-regulated kinases (ERKs) are known to play important roles in regulating OC growth and differentiation. In this study, we examined specific downstream signal pathways affected by ERK inhibition during OC differentiation. Our results showed that the ERK inhibitors PD98059 and U0126 increased receptor activator of NF-κB ligand (RANKL)-induced OC differentiation in RAW 264.7 cells, implying a negative role in OC differentiation. This is supported by the effect of ERK2-specific small interfering RNA on increasing OC differentiation. In contrast to our findings regarding the RAW 264.7 cells, the ERK inhibitors attenuated the differentiation of bone marrow-derived cells into OCs. The ERK inhibitors significantly increased the phosphorylation of adenosine 5'-monophosphate-activated protein kinase (AMPK) but not the activation of p38 MAPK, Lyn, and mTOR. In addition, while the ERK inhibition increased the expression of the RANKL receptor RANK, it decreased the expression of negative mediators of OC differentiation, such as interferon regulatory factor-8, B-cell lymphoma 6, and interferon-γ. These dichotomous effects of ERK inhibition suggest that while ERKs may play positive roles in bone marrow-derived cells, ERKs may also play negative regulatory roles in RAW 264.7 cells. These data provide important information for drug development utilizing ERK inhibitors in OC-related disease treatment.

Iron overload induced osteocytes apoptosis and led to bone loss in Hepcidin-/- mice through increasing sclerostin and RANKL/OPG

BACKGROUND

Numerous studies have demonstrated that iron overload is a risk factor of osteoporosis. However, there has been no systematic and in-depth studies on the effect of iron overload on osteocytes and its role in iron overload-induced bone loss. Therefore, to address this problem, we carried out in vitro and in vivo studies using MLO-Y4 osteocyte-like cells and Hepcidin^-/- mice as iron overload models.

METHODS

(1) MLO-Y4 cells were treated with ferric ammonium citrate (FAC). Intracellular reactive oxygen species (ROS) levels and apoptosis of MLO-Y4 cells were determined by flow cytometry. Western blotting was performed to evaluate the effect of FAC on the expression of sclerostin and RANKL/OPG. (2) The conditioned medium of MLO-Y4 cells after treatment with FAC was collected and used to treat pre-osteoblasts and monocytes. Alkaline phosphatase (ALP) staining and alizarin red (AR) staining were used to evaluate osteogenic differentiation capacity, and tartrate-resistant acid phosphatase (TRAP) staining was performed to demonstrate osteoclast differentiation capacity. (3) In vivo studies included a wild type mouse, Hepcidin^-/- mice, Hepcidin^-/- mice + deferoxamine (DFO), and Hepcidin^-/- mice + N-actyl-l-cysteine (NAC) group. Micro-CT was performed to evaluate the bone mineral density (BMD), bone volume, and bone micro-architecture of the mice, and three bending tests were used to assess bone strength. Histological analysis was used to detect alterations in bone turnover. TUNEL staining and scanning electron microscopy (SEM) were performed to evaluate the apoptosis and morphology of osteocytes. Immunohistochemical staining and Western blotting were used to determine alterations in sclerostin and RANKL/OPG expression levels in mice.

RESULTS

(1) FAC increased intracellular ROS and apoptosis in MLO-Y4 cells, while FAC enhanced the expression of sclerostin and RANKL/OPG in MLO-Y4 cells. (2) Conditioned medium of MLO-Y4 cells inhibited the osteogenic capacity of osteoblasts while stimulating osteoclast differentiation. (3) By increasing oxidative stress, iron overload promotes the apoptosis of osteocytes and undermines the morphology of osteocytes in Hepcidin^-/- mice, further increasing the expression levels of sclerostin and RANKL/OPG in osteocytes, which is considered to be the causative factor for reduced bone formation and enhanced bone resorption. DFO administration reduced iron levels, and NAC treatment decreased oxidative stress in Hepcidin^-/- mice. Therefore, DFO or NAC treatment rescued the decrease in BMD, bone volume, and bone strength and attenuated the deterioration of bone architecture in Hepcidin^-/- mice by attenuating the effect of iron overload on osteocytes.

CONCLUSION

Osteocyte apoptosis due to increased ROS and resultant sclerostin and RANKL/OPG expression alteration was the main reason for bone loss in Hepcidin^-/- mice. Osteocytes are the main targets for the prevention and treatment of iron overload-induced osteoporosis.

PTH1R translocation to primary cilia in mechanically-stimulated ostecytes prevents osteoclast formation via regulation of CXCL5 and IL-6 secretion

Osteocytes respond to mechanical forces controlling osteoblast and osteoclast function. Mechanical stimulation decreases osteocyte apoptosis and promotes bone formation. Primary cilia have been described as potential mechanosensors in bone cells. Certain osteogenic responses induced by fluid flow (FF) in vitro are decreased by primary cilia inhibition in MLO-Y4 osteocytes. The parathyroid hormone (PTH) receptor type 1 (PTH1R) modulates osteoblast, osteoclast, and osteocyte effects upon activation by PTH or PTH-related protein (PTHrP) in osteoblastic cells. Moreover, some actions of PTH1R seem to be triggered directly by mechanical stimulation. We hypothesize that PTH1R forms a signaling complex in the primary cilium that is essential for mechanotransduction in osteocytes and affects osteocyte-osteoclast communication. MLO-Y4 osteocytes were stimulated by FF or PTHrP (1-37). PTH1R and primary cilia signaling were abrogated using PTH1R or primary cilia specific siRNAs or inhibitors, respectively. Conditioned media obtained from mechanically- or PTHrP-stimulated MLO-Y4 cells inhibited the migration of preosteoclastic cells and osteoclast differentiation. Redistribution of PTH1R along the entire cilium was observed in mechanically stimulated MLO-Y4 osteocytic cells. Preincubation of MLO-Y4 cells with the Gli-1 antagonist, the adenylate cyclase inhibitor (SQ22536), or with the phospholipase C inhibitor (U73122), affected the migration of osteoclast precursors and osteoclastogenesis. Proteomic analysis and neutralizing experiments showed that FF and PTH1R activation control osteoclast function through the modulation of C-X-C Motif Chemokine Ligand 5 (CXCL5) and interleukin-6 (IL-6) secretion in osteocytes. These novel findings indicate that both primary cilium and PTH1R are necessary in osteocytes for proper communication with osteoclasts and show that mechanical stimulation inhibits osteoclast recruitment and differentiation through CXCL5, while PTH1R activation regulate these processes via IL-6.

A matter of origin - identification of SEMA3A, BGLAP, SPP1 and PHEX as distinctive molecular features between bone site-specific human osteoblasts on transcription level

In oral and maxillofacial bone reconstruction, autografts from the iliac crest represent the gold standard due to their superior clinical performance, compared to autografts derived from other extraoral regions. Thus, the aim of our study was to identify putative differences between osteoblasts derived from alveolar (hOB-A) and iliac crest (hOB-IC) bone of the same donor (nine donors) by means of their molecular properties in 2D and 3D culture. We thereby focused on the gene expression of biomarkers involved in osteogenic differentiation, matrix formation and osteoclast modulation. Furthermore, we examined the transcriptional response to Vit.D3 in hOB-A and hOB-IC. Our results revealed different modulation modes of the biomarker expression in osteoblasts, namely cell origin/bone entity-dependent, and culture configuration- and/or time-dependent modulations. SEMA3A, SPP1, BGLAP and PHEX demonstrated the strongest dependence on cell origin. With respect to Vit.D3-effects, BGLAP, SPP1 and ALPL displayed the highest Vit.D3-responsiveness. In this context we demonstrated that the transcriptional Vit.D3-response concerning SPP1 and ALPL in human osteoblasts depended on the cell origin. The results indicate a higher bone remodeling activity of iliac crest than alveolar osteoblasts and support the growing evidence that a high osteoclast activity at the host-/donor bone interface may support graft integration.

WRN promotes bone development and growth by unwinding SHOX-G-quadruplexes via its helicase activity in Werner Syndrome

Werner Syndrome (WS) is an autosomal recessive disorder characterized by premature aging due to mutations of the WRN gene. A classical sign in WS patients is short stature, but the underlying mechanisms are not well understood. Here we report that WRN is indispensable for chondrogenesis, which is the engine driving the elongation of bones and determines height. Zebrafish lacking wrn exhibit impairment of bone growth and have shorter body stature. We pinpoint the function of WRN to its helicase domain. We identify short-stature homeobox (SHOX) as a crucial and direct target of WRN and find that the WRN helicase core regulates the transcriptional expression of SHOX via unwinding G-quadruplexes. Consistent with this, shox−/− zebrafish exhibit impaired bone growth, while genetic overexpression of SHOX or shox expression rescues the bone developmental deficiency induced in WRN/wrn-null mutants both in vitro and in vivo. Collectively, we have identified a previously unknown function of WRN in regulating bone development and growth through the transcriptional regulation of SHOX via the WRN helicase domain, thus illuminating a possible approach for new therapeutic strategies.

Cellular Senescence in Bone

Senescence is an irreversible cell-cycle arrest process induced by environmental, genetic, and epigenetic factors. An accumulation of senescent cells in bone results in age-related disorders, and one of the common problems is osteoporosis. Deciphering the basic mechanisms contributing to the chronic ailments of aging may uncover new avenues for targeted treatment. This review focuses on the mechanisms and the most relevant research advancements in skeletal cellular senescence. To identify new options for the treatment or prevention of age-related chronic diseases, researchers have targeted hallmarks of aging, including telomere attrition, genomic instability, cellular senescence, and epigenetic alterations. First, this chapter provides an overview of the fundamentals of bone tissue, the causes of skeletal involution, and the role of cellular senescence in bone and bone diseases such as osteoporosis. Next, this review will discuss the utilization of pharmacological interventions in aging tissues and, more specifically, highlight the role of senescent cells to identify the most effective and safe strategies.

Paradoxical role of reactive oxygen species in bone remodelling: implications in osteoporosis and possible nanotherapeutic interventions

Osteoporosis is a metabolic bone disorder that affects both sexes and is the most common cause of fractures. Osteoporosis therapies primarily inhibit osteoclast activity, and are seldom designed to trigger new bone growth thereby frequently causing severe systemic adverse effects. Physiologically, the intracellular redox state depends on the ratio of pro-oxidants, oxidizing agents (reactive oxygen species, ROS) and antioxidants. ROS is the key contributor to oxidative stress in osteoporosis as changes in redox state are responsible for dynamic bone remodeling and bone regeneration. Imbalances in ROS generation vs. antioxidant systems play a pivotal role in pathogenesis of osteoporosis, stimulating osteoblasts and osteocytes towards osteoclastogenesis. ROS prevents mineralization and osteogenesis, causing increased turnover of bone loss. Alternatively, antioxidants either directly or indirectly, contribute to activation of osteoblasts leading to differentiation and mineralization, thereby reducing osteoclastogenesis. Owing to the unpredictability of immune responsiveness and reported adverse effects, despite promising outcomes from drugs against oxidative stress, treatment in clinics targeting osteoclast has been limited. Nanotechnology-mediated interventions have gained remarkable superiority over other treatment modalities in regenerative medicine. Nanotherapeutic approaches exploit the antioxidant properties of nanoparticles for targeted drug delivery to trigger bone repair, by enhancing their osteogenic and anti-osteoclastogenic potentials to influence the biocompatibility, mechanical properties and osteoinductivity. Therefore, exploiting nanotherapeutics for maintaining the differentiation and proliferation of osteoblasts and osteoclasts is quintessential.

Similarities Between Disuse and Age-Induced Bone Loss

Disuse and aging are known risk factors associated with low bone mass and quality deterioration, resulting in increased fracture risk. Indeed, current and emerging evidence implicate a large number of shared skeletal manifestations between disuse and aging scenarios. This review provides a detailed overview of current preclinical models of musculoskeletal disuse and the clinical scenarios they seek to recapitulate. We also explore and summarize the major similarities between bone loss after extreme disuse and advanced aging at multiple length scales, including at the organ/tissue, cellular, and molecular level. Specifically, shared structural and material alterations of bone loss are presented between disuse and aging, including preferential loss of bone at cancellous sites, cortical thinning, and loss of bone strength due to enhanced fragility. At the cellular level bone loss is accompanied, during disuse and aging, by increased bone resorption, decreased formation, and enhanced adipogenesis due to altered gap junction intercellular communication, WNT/β-catenin and RANKL/OPG signaling. Major differences between extreme short-term disuse and aging are discussed, including anatomical specificity, differences in bone turnover rates, periosteal modeling, and the influence of subject sex and genetic variability. The examination also identifies potential shared mechanisms underlying bone loss in aging and disuse that warrant further study such as collagen cross-linking, advanced glycation end products/receptor for advanced glycation end products (AGE-RAGE) signaling, reactive oxygen species (ROS) and nuclear factor κB (NF-κB) signaling, cellular senescence, and altered lacunar-canalicular connectivity (mechanosensation). Understanding the shared structural alterations, changes in bone cell function, and molecular mechanisms common to both extreme disuse and aging are paramount to discovering therapies to combat both age-related and disuse-induced osteoporosis. © 2022 American Society for Bone and Mineral Research (ASBMR).

Bio-piezoelectricity: fundamentals and applications in tissue engineering and regenerative medicine

In recent years, smart materials have piqued the interest of scientists and physicians in the biomedical community owing to their ability to modify their properties in response to an external stimulation or changes in their surroundings. Biocompatible piezoelectric materials are an interesting group of smart materials due to their ability to produce electrical charges without an external power source. Electric signals produced by piezoelectric scaffolds can renew and regenerate tissues through special pathways like that found in the extracellular matrix. This review summarizes the piezoelectric phenomenon, piezoelectric effects generated within biological tissues, piezoelectric biomaterials, and their applications in tissue engineering and their use as biosensors.

Osteomyelitis, Oxidative Stress and Related Biomarkers

Bone is a very dynamic tissue, subject to continuous renewal to maintain homeostasis through bone remodeling, a process promoted by two cell types: osteoblasts, of mesenchymal derivation, are responsible for the deposition of new material, and osteoclasts, which are hematopoietic cells, responsible for bone resorption. Osteomyelitis (OM) is an invasive infectious process, with several etiological agents, the most common being Staphylococcus aureus, affecting bone or bone marrow, and severely impairing bone homeostasis, resulting in osteolysis. One of the characteristic features of OM is a strong state of oxidative stress (OS) with severe consequences on the delicate balance between osteoblastogenesis and osteoclastogenesis. Here we describe this, analyzing the effects of OS in bone remodeling and discussing the need for new, easy-to-measure and widely available OS biomarkers that will provide valid support in the management of the disease.

Osteoimmunology in Periodontitis: Local Proteins and Compounds to Alleviate Periodontitis

Periodontitis is one of the most common oral diseases resulting in gingival inflammation and tooth loss. Growing evidence indicates that it results from dysbiosis of the oral microbiome, which interferes with the host immune system, leading to bone destruction. Immune cells activate periodontal ligament cells to express the receptor activator of nuclear factor kappa-B (NF-κB) ligand (RANKL) and promote osteoclast activity. Osteocytes have active roles in periodontitis progression in the bone matrix. Local proteins are involved in bone regeneration through functional immunological plasticity. Here, we discuss the current knowledge of cellular and molecular mechanisms in periodontitis, the roles of local proteins, and promising synthetic compounds generating a periodontal regeneration effect. It is anticipated that this may lead to a better perception of periodontitis pathophysiology.

The Osteoclast Traces the Route to Bone Tumors and Metastases

Osteoclasts are highly specialized cells of the bone, with a unique apparatus responsible for resorption in the process of bone remodeling. They are derived from differentiation and fusion of hematopoietic precursors, committed to form mature osteoclasts in response to finely regulated stimuli produced by bone marrow–derived cells belonging to the stromal lineage. Despite a highly specific function confined to bone degradation, emerging evidence supports their relevant implication in bone tumors and metastases. In this review, we summarize the physiological role of osteoclasts and then focus our attention on their involvement in skeletal tumors, both primary and metastatic. We highlight how osteoclast-mediated bone erosion confers increased aggressiveness to primary tumors, even those with benign features. We also outline how breast and pancreas cancer cells promote osteoclastogenesis to fuel their metastatic process to the bone. Furthermore, we emphasize the role of osteoclasts in reactivating dormant cancer cells within the bone marrow niches for manifestation of overt metastases, even decades after homing of latent disseminated cells. Finally, we point out the importance of counteracting tumor progression and dissemination through pharmacological treatments based on a better understanding of molecular mechanisms underlying osteoclast lytic activity and their recruitment from cancer cells.

Extracellular acidification augments sclerostin and osteoprotegerin production by Ocy454 mouse osteocytes

Osteocytes sense the microenvironmental stimuli, including mechanical stress, and regulate bone resorption by osteoclasts and bone formation by osteoblasts. Diabetes and cancer metastasis to bone raise l-lactic acid in the bone tissue, causing acidification. Here, we investigated the effects of l-lactic acid and extracellular acidification on the function of mouse Ocy454 osteocytes. L- and d-lactic acid with low chiral selectivity and acidification of the medium raised the production of sclerostin and osteoprotegerin by Ocy454 cells. The mRNA expression of their genes increased after either treatment of L- and d-lactic acid or acidification of the medium. Furthermore, the conditioned medium of Ocy454 cells cultured in an acidic environment suppressed the induction of alkaline phosphatase activity in MC3T3-E1 cells, which was recovered by the anti-sclerostin antibody. While it is reported that HDAC5 inhibits the transcription of the sclerostin gene, extracellular acidification reduced the nuclear localization of HDAC5 in Ocy454 cells. While calmodulin kinase II (CaMKII) is known to phosphorylate and induce extranuclear translocation of HDAC5, KN-62, an inhibitor of CaMKII lowered the expression of the sclerostin gene in Ocy454 cells. Collectively, extracellular acidification is a microenvironmental factor that modulates osteocyte functions.

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

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

Osteocyte apoptosis, bone marrow adiposity and fibrosis in the irradiated human mandible

Purpose

To assess the effect of radiation therapy on osteocyte apoptosis, osteocyte death, and bone marrow adipocytes in the human mandible and its contribution to the pathophysiology of radiation damage to the mandibular bone.

Methods and Materials

Mandibular cancellous bone biopsies were taken from irradiated patients and nonirradiated controls. Immunohistochemical detection of cleaved caspase-3 was performed to visualize apoptotic osteocytes. The number of apoptotic osteocytes per bone area and per total amount of osteocytes, osteocytes per bone area, and empty lacunae per bone area were counted manually. The percentage fibrotic tissue and adipose tissue per bone marrow area, the percentage bone marrow of total area, and the mean adipocyte diameter (μm) was determined digitally from adjacent Goldner stained sections.

Results

Biopsies of 15 irradiated patients (12 men and 3 women) and 7 nonirradiated controls (5 men and 2 women) were assessed. In the study group a significant increase was seen in the number of empty lacunae, the percentage of adipose tissue of bone marrow area, and the adipocyte diameter. There was no significant difference in bone marrow fibrosis nor apoptotic osteocytes between the irradiated group and the controls.

Conclusions

Irradiation alone does not seem to induce excessive bone marrow fibrosis. The damage to bone mesenchymal stem cells leads to increased marrow adipogenesis and decreased osteoblastogenic potential. Early osteocyte death resulting in avital persisting bone matrix with severely impaired regenerative potential may contribute to the vulnerability of irradiated bone to infection and necrosis.