Molecular signaling in bone cells: Regulation of cell differentiation and survival

Insights and implications of sexual dimorphism in osteoporosis

Osteoporosis, a metabolic bone disease characterized by low bone mineral density and deterioration of bone microarchitecture, has led to a high risk of fatal osteoporotic fractures worldwide. Accumulating evidence has revealed that sexual dimorphism is a notable feature of osteoporosis, with sex-specific differences in epidemiology and pathogenesis. Specifically, females are more susceptible than males to osteoporosis, while males are more prone to disability or death from the disease. To date, sex chromosome abnormalities and steroid hormones have been proven to contribute greatly to sexual dimorphism in osteoporosis by regulating the functions of bone cells. Understanding the sex-specific differences in osteoporosis and its related complications is essential for improving treatment strategies tailored to women and men. This literature review focuses on the mechanisms underlying sexual dimorphism in osteoporosis, mainly in a population of aging patients, chronic glucocorticoid administration, and diabetes. Moreover, we highlight the implications of sexual dimorphism for developing therapeutics and preventive strategies and screening approaches tailored to women and men. Additionally, the challenges in translating bench research to bedside treatments and future directions to overcome these obstacles will be discussed.

Explorating the mechanism of Epimedii folium-Rhizoma drynariae herbal pair promoted bone defects healing through network pharmacology and experimental studies

ETHNOPHARMACOLOGICAL RELEVANCE

Bone defects are difficult to treat and have a high incidence of nonunion. The Epimedii folium-Rhizoma drynariae herbal pair (EDP) is a traditional Chinese medicine (TCM) used for treating bone diseases. However, the mechanisms by which EDP promotes osteogenesis or bone formation remain largely unclear.

AIM OF THE STUDY

This study aimed to investigate the mechanism of EDP promoted bone formation in bone defects using network pharmacology and experiments.

MATERIALS AND METHODS

The chemical components of EDP were analyzed by UHPLC-MS. The hub target and pathway enrichment analysis was conducted using molecular docking or network pharmacology. The pharmacological actions of EDP were determined by μCT and histopathology examination using a bone defect rat model. The effects of EDP on the mRNA expression of Bmp2, Smad2/5, Runx2, and Alp genes were measured by RT-PCR, while changes in the protein expressions of BMP2, COL1A1, SPP1, ALP, and RUNX2in the tibia tissues of the rats in response to EDP were analyzed by immunohistochemical staining or Western blot. We also performed cell viability assays, Alizarin Red and ALP staining assays, and RT-PCR to better understand how EDP affected osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs).

RESULTS

Identified 14 key compounds and 47 hub targets of EDP that may be involved in promoting osteogenesis to repair bone defects. And the BMP/Smad/Runx2 pathway was likely the key pathway through which EDP promoted bone defects repairing. The results of in vivo rat experiments indicated that EDP effectively promoted tibia repair in the model rats and activated the BMP/Smad/Runx2 pathway in the tibia tissue, with upregulating Bmp2, Bmpr1α, Smad2/5, Runx2, and Alp genes, and increased the protein expression of BMP2, COL1A1, RUNX2, and ALP. In vitro, EDP was found to increase the proliferation, differentiation, and mineralization in BMSCs- and also up-regulated the expression of key genes in the BMP/Smad/Runx2 pathway.

CONCLUSION

This study highlighted the ability of EDP to promote the osteogenic differentiation to enable bone repair by activating the BMP/Smad/Runx2 pathway.

Exploring the Cellular and Molecular Mechanism of Discoidin Domain Receptors (DDR1 and DDR2) in Bone Formation, Regeneration, and Its Associated Disease Conditions

The tyrosine kinase family receptor of discoidin domain receptors (DDR1 and DDR2) is known to be activated by extracellular matrix collagen catalytic binding protein receptors. They play a remarkable role in cell proliferation, differentiation, migration, and cell survival. DDR1 of the DDR family regulates matrix-metalloproteinase, which causes extracellular matrix (ECM) remodeling and reconstruction during unbalanced homeostasis. Collagenous-rich DDR1 triggers the ECM of cartilage to regenerate the cartilage tissue in osteoarthritis (OA) and temporomandibular disorder (TMD). Moreover, DDR2 is prominently present in the fibroblasts, smooth muscle cells, myofibroblasts, and chondrocytes. It is crucial in generating and breaking collagen vital cellular activities like proliferation, differentiation, and adhesion mechanisms. However, the deficiency of DDR1 rather than DDR2 was detrimental in cases of OA and TMDs. DDR1 stimulated the ECM cartilage and improved bone regeneration. Based on the above information, we made an effort to outline the advancement of the utmost promising DDR1 and DDR2 regulation in bone and cartilage, also summarizing their structural, biological activity, and selectivity.

Insulin-like growth factor receptor / mTOR signaling elevates global translation to accelerate zebrafish fin regenerative outgrowth

Zebrafish robustly regenerate fins, including their characteristic bony ray skeleton. Amputation activates intra-ray fibroblasts and dedifferentiates osteoblasts that migrate under a wound epidermis to establish an organized blastema. Coordinated proliferation and re-differentiation across lineages then sustains progressive outgrowth. We generate a single cell transcriptome dataset to characterize regenerative outgrowth and explore coordinated cell behaviors. We computationally identify sub-clusters representing most regenerative fin cell lineages, and define markers of osteoblasts, intra- and inter-ray fibroblasts and growth-promoting distal blastema cells. A pseudotemporal trajectory and in vivo photoconvertible lineage tracing indicate distal blastemal mesenchyme restores both intra- and inter-ray fibroblasts. Gene expression profiles across this trajectory suggest elevated protein production in the blastemal mesenchyme state. O-propargyl-puromycin incorporation and small molecule inhibition identify insulin growth factor receptor (IGFR)/mechanistic target of rapamycin kinase (mTOR)-dependent elevated bulk translation in blastemal mesenchyme and differentiating osteoblasts. We test candidate cooperating differentiation factors identified from the osteoblast trajectory, finding IGFR/mTOR signaling expedites glucocorticoid-promoted osteoblast differentiation in vitro. Concordantly, mTOR inhibition slows but does not prevent fin regenerative outgrowth in vivo. IGFR/mTOR may elevate translation in both fibroblast- and osteoblast-lineage cells during the outgrowth phase as a tempo-coordinating rheostat.

Inhibition of Ezh2 redistributes bivalent domains within transcriptional regulators associated with WNT and Hedgehog pathways in osteoblasts

Bivalent epigenomic regulatory domains containing both activating histone 3 lysine 4 (H3K4me3) and repressive lysine 27 (H3K27me3) trimethylation are associated with key developmental genes. These bivalent domains repress transcription in the absence of differentiation signals but maintain regulatory genes in a poised state to allow for timely activation. Previous studies demonstrated that enhancer of zeste homolog 2 (Ezh2), a histone 3 lysine 27 (H3K27) methyltransferase, suppresses osteogenic differentiation and that inhibition of Ezh2 enhances commitment of osteoblast progenitors in vitro and bone formation in vivo. Here, we examined the mechanistic effects of Tazemetostat (EPZ6438), an Food and Drug Administration approved Ezh2 inhibitor for epithelioid sarcoma treatment, because this drug could potentially be repurposed to stimulate osteogenesis for clinical indications. We find that Tazemetostat reduces H3K27me3 marks in bivalent domains in enhancers required for bone formation and stimulates maturation of MC3T3 preosteoblasts. Furthermore, Tazemetostat activates bivalent genes associated with the Wingless/integrated (WNT), adenylyl cyclase (cAMP), and Hedgehog (Hh) signaling pathways based on transcriptomic (RNA-seq) and epigenomic (chromatin immunoprecipitation [ChIP]-seq) data. Functional analyses using selective pathway inhibitors and silencing RNAs demonstrate that the WNT and Hh pathways modulate osteogenic differentiation after Ezh2 inhibition. Strikingly, we show that loss of the Hh-responsive transcriptional regulator Gli1, but not Gli2, synergizes with Tazemetostat to accelerate osteoblast differentiation. These studies establish epigenetic cooperativity of Ezh2, Hh-Gli1 signaling, and bivalent regulatory genes in suppressing osteogenesis. Our findings may have important translational ramifications for anabolic applications requiring bone mass accrual and/or reversal of bone loss.

Role of Cx43 on the Bone Cell Generation, Function, and Survival

The presence of gap junction intercellular communication structures in bone cells has been known since the early 1970s, further confirmed by Doty and Marotti at the structural level in the 1980-1990s. Work by Civitelli, Donahue, and others showed the expression of Cx43 at the mRNA and protein levels in all bone cell types: osteoclasts (bone resorbing cells), osteoblasts (bone forming cells), and osteocytes (mature osteoblasts embedded in the bone matrix that regulate the function of both osteoclasts and osteoblasts). While Cx45, Cx46, and Cx37 were also shown to be expressed in bone cells, most studies have focused on Cx43, the most abundant member of the connexin (Cx) family of proteins expressed in bone. The role of Cx43 has been shown to be related to the formation of gap junction intercellular channels, to unopposed hemichannels, and to channel independent functions of the molecule. Cx43 participates in the response of bone cells to pharmacological, hormonal, and mechanical stimuli, and it is involved in the skeletal phenotype with old age. Human and murine studies have shown that mutations of Cx43 lead to oculodentodigital dysplasia and craniometaphyseal dysplasia, both conditions associated with abnormalities in the skeleton. However, whereas substantial advances have been made on the skeletal role of Cx43, further research is needed to understand the basis for the effects of mutated Cx43 and potential ways to prevent the effects of these mutations on bone.

Effect of periostin on bone metabolic and autophagy factors during tooth eruption in mice

This study aimed to investigate the effect of periostin (PN) on the expression of receptor activator of nuclear factor-κB ligand (RANKL), osteoprotegerin (OPG), microtubule-associated protein1 light chain 3B (LC3B), and Beclin1 in mouse alveolar bone specimens and cultured osteoblasts in vitro, to preliminarily explore the role of PN and autophagy in remodeling bone metabolism during tooth eruption. Mice at 5 days of age were injected with 75 ng/mL recombinant PN protein under the periosteum for 3 consecutive days according to the standard of 1 mL/100 g/day. Then, their mandibles were removed, and the expression of bone metabolic and autophagy factors was detected by immunohistochemistry. Mouse osteoblast-like cells cultured in vitro were treated with recombinant PN at a concentration of 75 ng/mL. The changes in the aforementioned indicators were compared again by immunofluorescence and western blotting 72 h after dosing. The results of the mouse samples showed that the protein expression of RANKL, LC3B, and Beclin1 decreased, accompanied by the decrease in RANKL/OPG ratio. However, OPG protein expression increased in the dosing group. Immunofluorescence and western blotting results of osteoblasts cultured in vitro showed that the protein expression of RANKL, LC3B, Beclin1, and the RANKL/OPG ratio in the experimental group decreased, but OPG expression increased. PN may regulate alveolar bone metabolism during tooth eruption by inhibiting the RANKL/OPG ratio and autophagy, which will provide a new research perspective for further exploration of the mechanisms during tooth eruption.

Messages from the Mineral: How Bone Cells Communicate with Other Tissues

Bone is a highly dynamic tissue, and the constant actions of bone-forming and bone-resorbing cells are responsible for attaining peak bone mass, maintaining bone mass in the adults, and the subsequent bone loss with aging and menopause, as well as skeletal complications of diseases and drug side-effects. It is now accepted that the generation and activity of bone-forming osteoblasts and bone-resorbing osteoclasts is modulated by osteocytes, osteoblast-derived cells embedded in the bone matrix. The interaction among bone cells occurs through direct contact and via secreted molecules. In addition to the regulation of bone cell function, molecules released by these cells are also able to reach the circulation and have effects in other tissues and organs in healthy individuals. Moreover, bone cell products have also been associated with the establishment or progression of diseases, including cancer and muscle weakness. In this review, we will discuss the role of bone as an endocrine organ, and the effect of selected, osteoblast-, osteocyte-, and osteoclast-secreted molecules on other tissues.

Suppressing Chondrocyte Hypertrophy to Build Better Cartilage

Current clinical strategies for restoring cartilage defects do not adequately consider taking the necessary steps to prevent the formation of hypertrophic tissue at injury sites. Chondrocyte hypertrophy inevitably causes both macroscopic and microscopic level changes in cartilage, resulting in adverse long-term outcomes following attempted restoration. Repairing/restoring articular cartilage while minimizing the risk of hypertrophic neo tissue formation represents an unmet clinical challenge. Previous investigations have extensively identified and characterized the biological mechanisms that regulate cartilage hypertrophy with preclinical studies now beginning to leverage this knowledge to help build better cartilage. In this comprehensive article, we will provide a summary of these biological mechanisms and systematically review the most cutting-edge strategies for circumventing this pathological hallmark of osteoarthritis.

Osteogenic Differentiation Effect of Human Periodontal Ligament Stem-Cell Initial Cell Density on Autologous Cells and Human Bone Marrow Stromal Cells

Stem cells have differentiation and regulation functions. Here, we discussed the impact of cell culture density on stem cell proliferation, osteoblastogenesis, and regulation. To discuss the effect of the initial culture density of human periodontal ligament stem cells (hPDLSCs) on the osteogenic differentiation of autologous cells, we found that the hPDLSC proliferation rate decreased with an increase in the initial plating density (0.5-8 × 10⁴ cells/cm²) for the 48 h culture cycle. After hPDLSCs induced osteogenic differentiation for 14 days with different initial cell culture densities, the expression of osteoprotegerin (OPG) and runt-related transcription factor 2(RUNX2) and the OPG/ Receptor Activator of Nuclear Factor-κ B Ligand (RANKL) ratio were the highest in the hPDLSCs initially plated at a density of 2 × 10⁴ cells/cm², and the average cell calcium concentration was also the highest. To study hPDLSCs regulating the osteoblastic differentiation of other cells, we used 50 μg/mL of secreted exosomes derived from hPDLSCs cultured using different initial cell densities to induce human bone marrow stromal cell (hBMSC) osteogenesis. After 14 days, the results indicated that the gene expression of OPG, Osteocalcin(OCN,)RUNX2, and osterix and the OPG/RANKL ratio were the highest in the 2 × 10⁴ cells/cm² initial cell density group, and the average calcium concentration was also the highest. This provides a new idea for the clinical application of stem cell osteogenesis.

Increased Kremen2 predicts worse prognosis in colon cancer

Background: Colon cancer (CC) is the fifth most prevalent cancer around the globe and poses a major risk to human health. Even though Kremen2 serves as a prognostic indicator in individuals with malignant tumours, its role in evaluating the prognosis of individuals with colon cancer has not been confirmed. Methods: Here, we examined the protein expression of Kremen2 in CC tissues and paired adjacent normal tissues by immunohistochemistry (IHC), then analyzed the clinical and RNA-seq data presented in The Cancer Genome Atlas (TCGA) database to confirm the relationship between Kremen2 levels and CC. In addition, the associations between Kremen2 mRNA expression and infiltrating immune cells were examined. Results: The study showed that the mRNA expression and protein level of Kremen2 were increased in CC tissues compared with adjacent normal tissues. According to Kaplan-Meier analysis, high Kremen2 expression in CC was linked to poor overall survival and progression-free survival. Clinical correlation analysis highlighted that a high level of Kremen2 expression was strongly linked with tumour progression, particularly lymph node metastasis. Cox regression analysis highlighted that Kremen2 was an independent prognostic indicator for CC. Bioinformatic studies highlighted that Kremen2 might be associated with the immune status in CC. Conclusion: Increased Kremen2 could serve as a potential prognostic CC biomarker.

Aryl hydrocarbon receptor (AhR)-mediated signaling as a critical regulator of skeletal cell biology

The aryl hydrocarbon receptor (AhR) has been implicated in regulating skeletal progenitor cells and the activity of bone-forming osteoblasts and bone-resorbing osteoclasts, thereby impacting bone mass and the risk of skeletal fractures. The AhR also plays an important role in the immune system within the skeletal niche and in the differentiation of mesenchymal stem cells into other cell lineages including chondrocytes and adipocytes. This transcription factor responds to environmental pollutants which can act as AhR ligands, initiating or interfering with various signaling cascades to mediate downstream effects, and also responds to endogenous ligands including tryptophan metabolites. This review comprehensively describes the reported roles of the AhR in skeletal cell biology, focusing on mesenchymal stem cells, osteoblasts, and osteoclasts, and discusses how AhR exhibits sexually dimorphic effects in bone. The molecular mechanisms mediating AhR's downstream effects are highlighted to emphasize the potential importance of targeting this signaling cascade in skeletal disorders.

Research on the Mechanism of Liuwei Dihuang Decoction for Osteoporosis Based on Systematic Biological Strategies

Background

Osteoporosis is an important health problem worldwide. Liuwei Dihuang Decoction (LDD) and its main ingredients may have a good clinical effect on osteoporosis. Meanwhile, its mechanism for treating osteoporosis needs to be further revealed in order to provide a basis for future drug development.

Methods

A systematic biological methodology was utilized to construct and analyze the LDD-osteoporosis network. After that, the human transcription data of LDD intervention in patients with osteoporosis and protein arrays data of LDD intervention in osteoporosis rats were collected. The human transcription data analysis, protein arrays data analysis, and molecular docking were performed to validate the findings of the prediction network (LDD-osteoporosis PPI network). Finally, animal experiments were conducted to verify the prediction results of systematic pharmacology.

Results

(1) LDD-osteoporosis PPI network shows the potential compounds, potential targets (such as ALB, IGF1, SRC, and ESR1), clusters, biological processes (such as positive regulation of calmodulin 1-monooxygenase activity, estrogen metabolism, and endothelial cell proliferation), and signaling and Reactome pathways (such as JAK-STAT signaling pathway, osteoclast differentiation, and degradation of the extracellular matrix) of LDD intervention in osteoporosis. (2) Human transcriptomics data and protein arrays data validated the findings of the LDD-osteoporosis PPI network. (3) The animal experiments showed that LDD can improve bone mineral density (BMD), increase serum estradiol (E2) and alkaline phosphatase (ALP) levels, and upregulate Wnt3a and β-catenin mRNA expression (P < 0.05). (4) Molecular docking results showed that alisol A, dioscin, loganin, oleanolic acid, pachymic acid, and ursolic acid may stably bind to JAK2, ESR1, and CTNNB1.

Conclusion

LDD may have a therapeutic effect on osteoporosis through regulating the targets (such as ALB, IGF1, SRC, and ESR1), biological processes (such as positive regulation of calmodulin 1-monooxygenase activity, estrogen metabolism, and endothelial cell proliferation), and pathways (such as JAK-STAT signaling pathway, osteoclast differentiation, and degradation of the extracellular matrix) found in this research.

The hEag1 K+ Channel Inhibitor Astemizole Stimulates Ca2+ Deposition in SaOS-2 and MG-63 Osteosarcoma Cultures

The hEag1 (Kv10.1) K⁺ channel is normally found in the brain, but it is ectopically expressed in tumor cells, including osteosarcoma. Based on the pivotal role of ion channels in osteogenesis, we tested whether pharmacological modulation of hEag1 may affect osteogenic differentiation of osteosarcoma cell lines. Using molecular biology (RT-PCR), electrophysiology (patch-clamp) and pharmacology (astemizole sensitivity, IC₅₀ = 0.135 μM) we demonstrated that SaOS-2 osteosarcoma cells also express hEag1 channels. SaOS-2 cells also express to KCa1.1 K⁺ channels as shown by mRNA expression and paxilline sensitivity of the current. The inhibition of hEag1 (2 μM astemizole) or KCa1.1 (1 mM TEA) alone did not induce Ca²⁺ deposition in SaOS-2 cultures, however, these inhibitors, at identical concentrations, increased Ca²⁺ deposition evoked by the classical or pathological (inorganic phosphate, Pi) induction pathway without causing cytotoxicity, as reported by three completer assays (LDH release, MTT assay and SRB protein assay). We observed a similar effect of astemizole on Ca²⁺ deposition in MG-63 osteosarcoma cultures as well. We propose that the increase in the osteogenic stimuli-induced mineral matrix formation of osteosarcoma cell lines by inhibiting hEag1 may be a useful tool to drive terminal differentiation of osteosarcoma.

NEDD4 E3 Ligases: Functions and Mechanisms in Bone and Tooth

Protein ubiquitination is a precisely controlled enzymatic cascade reaction belonging to the post-translational modification of proteins. In this process, E3 ligases catalyze the binding of ubiquitin (Ub) to protein substrates and define specificity. The neuronally expressed developmentally down-regulated 4 (NEDD4) subfamily, belonging to the homology to E6APC terminus (HECT) class of E3 ligases, has recently emerged as an essential determinant of multiple cellular processes in different tissues, including bone and tooth. Here, we place special emphasis on the regulatory role of the NEDD4 subfamily in the molecular and cell biology of osteogenesis. We elucidate in detail the specific roles, downstream substrates, and upstream regulatory mechanisms of the NEDD4 subfamily. Further, we provide an overview of the involvement of E3 ligases and deubiquitinases in the development, repair, and regeneration of another mineralized tissue—tooth.

MicroRNA-101a enhances trabecular bone accrual in male mice

High-throughput microRNA sequencing was performed during differentiation of MC3T3-E1 osteoblasts to develop working hypotheses for specific microRNAs that control osteogenesis. The expression data show that miR-101a, which targets the mRNAs for the epigenetic enzyme Ezh2 and many other proteins, is highly upregulated during osteoblast differentiation and robustly expressed in mouse calvaria. Transient elevation of miR-101a suppresses Ezh2 levels, reduces tri-methylation of lysine 27 in histone 3 (H3K27me3; a heterochromatic mark catalyzed by Ezh2), and accelerates mineralization of MC3T3-E1 osteoblasts. We also examined skeletal phenotypes of an inducible miR-101a transgene under direct control of doxycycline administration. Experimental controls and mir-101a over-expressing mice were exposed to doxycycline in utero and postnatally (up to 8 weeks of age) to maximize penetrance of skeletal phenotypes. Male mice that over-express miR-101a have increased total body weight and longer femora. MicroCT analysis indicate that these mice have increased trabecular bone volume fraction, trabecular number and trabecular thickness with reduced trabecular spacing as compared to controls. Histomorphometric analysis demonstrates a significant reduction in osteoid volume to bone volume and osteoid surface to bone surface. Remarkably, while female mice also exhibit a significant increase in bone length, no significant changes were noted by microCT (trabecular bone parameters) and histomorphometry (osteoid parameters). Hence, miR-101a upregulation during osteoblast maturation and the concomitant reduction in Ezh2 mediated H3K27me3 levels may contribute to the enhanced trabecular bone parameters in male mice. However, the sex-specific effect of miR-101a indicates that more intricate epigenetic mechanisms mediate physiological control of bone formation and homeostasis.

Emerging modulators for osteogenic differentiation: a combination of chemical and topographical cues for bone microenvironment engineering

Bone presents an intrinsic ability for self-regeneration and repair, however critical defects and large fractures require invasive and time-consuming clinical interventions. As an alternative to current therapy, bone tissue engineering (BTE) has primarily aimed to recreate the bone microenvironment by delivering key biomolecules and/or by modification of scaffolds to guide cell fate towards the osteogenic lineage or other phenotypes that may benefit the bone regeneration mechanism. Considering that bone cells communicate, in their native microenvironment, through biochemical and physical signals, most strategies fail when considering only chemical, geometrical or mechanical cues. This is not representative of the physiological conditions, where the cells are simultaneously in contact and stimulated by several cues. Therefore, this review explores the synergistic effect of biochemical/physical cues in regulating cellular events, namely cell adhesion, proliferation, osteogenic differentiation, and mineralization, highlighting the importance of the combined modifications for the development of innovative bone regenerative therapies.

Bone Regeneration and Oxidative Stress: An Updated Overview

Bone tissue engineering is a complex domain that requires further investigation and benefits from data obtained over past decades. The models are increasing in complexity as they reveal new data from co-culturing and microfluidics applications. The in vitro models now focus on the 3D medium co-culturing of osteoblasts, osteoclasts, and osteocytes utilizing collagen for separation; this type of research allows for controlled medium and in-depth data analysis. Oxidative stress takes a toll on the domain, being beneficial as well as destructive. Reactive oxygen species (ROS) are molecules that influence the differentiation of osteoclasts, but over time their increasing presence can affect patients and aid the appearance of diseases such as osteoporosis. Oxidative stress can be limited by using antioxidants such as vitamin K and N-acetyl cysteine (NAC). Scaffolds and biocompatible coatings such as hydroxyapatite and bioactive glass are required to isolate the implant, protect the zone from the metallic, ionic exchange, and enhance the bone regeneration by mimicking the composition and structure of the body, thus enhancing cell proliferation. The materials can be further functionalized with growth factors that create a better response and higher chances of success for clinical use. This review highlights the vast majority of newly obtained information regarding bone tissue engineering, such as new co-culturing models, implant coatings, scaffolds, biomolecules, and the techniques utilized to obtain them.

Porous 3D Scaffolds Enhance MSC Vitality and Reduce Osteoclast Activity

In the context of an aging population, unhealthy Western lifestyle, and the lack of an optimal surgical treatment, deep osteochondral defects pose a great challenge for the public health system. Biodegradable, biomimetic scaffolds seem to be a promising solution. In this study we investigated the biocompatibility of porous poly-((D,L)-lactide-ε-caprolactone)dimethacrylate (LCM) scaffolds in contrast to compact LCM scaffolds and blank cell culture plastic. Thus, morphology, cytotoxicity and metabolic activity of human mesenchymal stromal cells (MSC) seeded directly on the materials were analyzed after three and six days of culturing. Further, osteoclastogenesis and osteoclastic activity were assessed using reverse-transcriptase real-time PCR of osteoclast-specific genes, EIA and morphologic aspects after four, eight, and twelve days. LCM scaffolds did not display cytotoxic effects on MSC. After three days, metabolic activity of MSC was enhanced on 3D porous scaffolds (PS) compared to 2D compact scaffolds (CS). Osteoclast activity seemed to be reduced at PS compared to cell culture plastic at all time points, while no differences in osteoclastogenesis were detectable between the materials. These results indicate a good cytocompatibility of LCM scaffolds. Interestingly, porous 3D structure induced higher metabolic activity of MSC as well as reduced osteoclast activity.

Polyphenols-loaded electrospun nanofibers in bone tissue engineering and regeneration

Bone is a complex structure with unique cellular and molecular process in its formation. Bone tissue regeneration is a well-organized and routine process at the cellular and molecular level in humans through the activation of biochemical pathways and protein expression. Though many forms of biomaterials have been applied for bone tissue regeneration, electrospun nanofibrous scaffolds have attracted more attention among researchers with their physicochemical properties such as tensile strength, porosity, and biocompatibility. When drugs, antibiotics, or functional nanoparticles are taken as additives to the nanofiber, its efficacy towards the application gets increased. Polyphenol is a versatile green/phytochemical small molecule playing a vital role in several biomedical applications, including bone tissue regeneration. When polyphenols are incorporated as additives to the nanofibrous scaffold, their combined properties enhance cell attachment, proliferation, and differentiation in bone tissue defect. The present review describes bone biology encompassing the composition and function of bone tissue cells and exemplifies the series of biological processes associated with bone tissue regeneration. We have highlighted the molecular mechanism of bioactive polyphenols involved in bone tissue regeneration and specified the advantage of electrospun nanofiber as a wound healing scaffold. As the polyphenols contribute to wound healing with their antioxidant and antimicrobial properties, we have compiled a list of polyphenols studied, thus far, for bone tissue regeneration along with their in vitro and in vivo experimental biological results and salient observations. Finally, we have elaborated on the importance of polyphenol-loaded electrospun nanofiber in bone tissue regeneration and discussed the possible challenges and future directions in this field.

The mechanism of Epimedin B in treating osteoporosis as revealed by RNA sequencing-based analysis

With the ageing of populations, the management of osteoporosis is a priority of society in general. Epimedin B, a major ingredient of Herba Epimedii, which has the advantages of high content and hypotoxicity has been proved to be effective in preventing osteoporosis in vitro. However, the efficacy and mechanism of Epimedin B on osteoporosis in vivo have not been well elucidated yet. This study aimed to investigate the effects and the potential mechanisms of 8-week repeated oral administration of Epimedin B (10 and 20 mg/kg/day) on a mouse osteoporosis model. Effects of Epimedin B were evaluated by examinations of serum bone turnover markers, bone mineral density, bone microstructure parameters and histopathological section. Epimedin B significantly rose N-terminal propeptide of type I procollagen (P1NP) and dropped C-telopeptide of type I collagen (CTX1). Connectivity density (Conn.D) increased significantly while structure model index (DA) decreased significantly after treated by Epimedin B. Meanwhile, Epimedin B administration significantly increased the number of trabecular bones while significantly decreased the gap between them. Overall, Epimedin B showed beneficial effects on osteoporosis. Furthermore, RNA sequencing-based analysis revealed 5 significantly down-regulated transcripts and 107 significantly up-regulated transcripts between the Epimedin B administration group and the model group. These transcripts were mapped to 15 pathways by KEGG enrichment analysis, of which PI3K-Akt signalling pathway, MAPK signalling pathway and PPAR signalling pathway were most connected to osteoporosis. To conclude, Epimedin B is effective in treating osteoporosis in mice via regulating PI3K-Akt, MAPK and PPAR signalling pathway.

Impact of FasL Stimulation on Sclerostin Expression and Osteogenic Profile in IDG-SW3 Osteocytes

Simple Summary

FasL used to be considered as a classical ligand triggering cell death (apoptosis) via its receptor, Fas and thefollowing caspase cascade. As such, it is known to be involved in regulation within the bone. Recently, however, the knowledge has expanded about the non-apoptotic and caspase-independent engagement of the Fas/FasL pathway. The present investigation identified that stimulation of osteocytic IDG-SW3 cells by FasL leads to a dramatic decrease in expression of the major osteocytic marker, sclerostin. Additionally, other key components of the osteogenic pathways were impacted, notably in a caspase-independent manner. Such findings are of importance for basic biology as well as biomedical applications since osteocytes are the major population within adult bones and Fas signalling is one of therapeutical targets, e.g., in the anti-osteoporotic treatment.

Abstract

The Fas ligand (FasL) is known from programmed cell death, the immune system, and recently also from bone homeostasis. As such, Fas signalling is a potential target of anti-osteoporotic treatment based on the induction of osteoclastic cell death. Less attention has been paid to osteocytes, although they represent the majority of cells within the mature bone and are the key regulators. To determine the impact of FasL stimulation on osteocytes, differentiated IDG-SW3 cells were challenged by FasL, and their osteogenic expression profiles were evaluated by a pre-designed PCR array. Notably, the most downregulated gene was the one for sclerostin, which is the major marker of osteocytes and a negative regulator of bone formation. FasL stimulation also led to significant changes (over 10-fold) in the expression of other osteogenic markers: Gdf10, Gli1, Ihh, Mmp10, and Phex. To determine whether these alterations involved caspase-dependent or caspase-independent mechanisms, the IDG-SW3 cells were stimulated by FasL with and without a caspase inhibitor: Q-VD-OPh. The alterations were also detected in the samples treated by FasL along with Q-VD-OPh, pointing to the caspase-independent impact of FasL stimulation. These results contribute to an understanding of the recently emerging pleiotropic effects of Fas/FasL signalling and specify its functions in bone cells.

Triple Culture of Primary Human Osteoblasts, Osteoclasts and Osteocytes as an In Vitro Bone Model

In vitro evaluation of bone graft materials is generally performed by analyzing the interaction with osteoblasts or osteoblast precursors. In vitro bone models comprising different cell species can give specific first information on the performance of those materials. In the present study, a 3D co-culture model was established comprising primary human osteoblasts, osteoclasts and osteocytes. Osteocytes were differentiated from osteoblasts embedded in collagen gels and were cultivated with osteoblast and osteoclasts seeded in patterns on a porous membrane. This experimental setup allowed paracrine signaling as well as separation of the different cell types for final analysis. After 7 days of co-culture, the three cell species showed their typical morphology and gene expression of typical markers like ALPL, BSPII, BLGAP, E11, PHEX, MEPE, RANKL, ACP5, CAII and CTSK. Furthermore, relevant enzyme activities for osteoblasts (ALP) and osteoclasts (TRAP, CTSK, CAII) were detected. Osteoclasts in triple culture showed downregulated TRAP (ACP5) and CAII expression and decreased TRAP activity. ALP and BSPII expression of osteoblasts in triple culture were upregulated. The expression of the osteocyte marker E11 (PDPN) was unchanged; however, osteocalcin (BGLAP) expression was considerably downregulated both in osteoblasts and osteocytes in triple cultures compared to the respective single cultures.

Piezo Channels: Awesome Mechanosensitive Structures in Cellular Mechanotransduction and Their Role in Bone

Piezo channels are mechanosensitive ion channels located in the cell membrane and function as key cellular mechanotransducers for converting mechanical stimuli into electrochemical signals. Emerged as key molecular detectors of mechanical forces, Piezo channels' functions in bone have attracted more and more attention. Here, we summarize the current knowledge of Piezo channels and review the research advances of Piezo channels' function in bone by highlighting Piezo1's role in bone cells, including osteocyte, bone marrow mesenchymal stem cell (BM-MSC), osteoblast, osteoclast, and chondrocyte. Moreover, the role of Piezo channels in bone diseases is summarized.

Bone remodeling stages under physiological conditions and glucocorticoid in excess: Focus on cellular and molecular mechanisms

This review provides a rationale for the cellular and molecular mechanisms of bone remodeling stages under physiological conditions and glucocorticoids (GCs) in excess. Remodeling is a synchronous process involving bone resorption and formation, proceeding through stages of: (1) resting bone, (2) activation, (3) bone resorption, (4) reversal, (5) formation, (6) termination. Bone remodeling is strictly controlled by local and systemic regulatory signaling molecules. This review presents current data on the interaction of osteoclasts, osteoblasts and osteocytes in bone remodeling and defines the role of osteoprogenitor cells located above the resorption area in the form of canopies and populating resorption cavities. The signaling pathways of proliferation, differentiation, viability, and cell death during remodeling are presented. The study of signaling pathways is critical to understanding bone remodeling under normal and pathological conditions. The main signaling pathways that control bone resorption and formation are RANK / RANKL / OPG; M-CSF – c-FMS; canonical and non-canonical signaling pathways Wnt; Notch; MARK; TGFβ / SMAD; ephrinB1/ephrinB2 – EphB4, TNFα – TNFβ, and Bim – Bax/Bak. Cytokines, growth factors, prostaglandins, parathyroid hormone, vitamin D, calcitonin, and estrogens also act as regulators of bone remodeling. The role of non-encoding microRNAs and long RNAs in the process of bone cell differentiation has been established. MicroRNAs affect many target genes, have both a repressive effect on bone formation and activate osteoblast differentiation in different ways. Excess of glucocorticoids negatively affects all stages of bone remodeling, disrupts molecular signaling, induces apoptosis of osteocytes and osteoblasts in different ways, and increases the life cycle of osteoclasts. Glucocorticoids disrupt the reversal stage, which is critical for the subsequent stages of remodeling. Negative effects of GCs on signaling molecules of the canonical Wingless (WNT)/β-catenin pathway and other signaling pathways impair osteoblastogenesis. Under the influence of excess glucocorticoids biosynthesis of biologically active growth factors is reduced, which leads to a decrease in the expression by osteoblasts of molecules that form the osteoid. Glucocorticoids stimulate the expression of mineralization inhibitor proteins, osteoid mineralization is delayed, which is accompanied by increased local matrix demineralization. Although many signaling pathways involved in bone resorption and formation have been discovered and described, the temporal and spatial mechanisms of their sequential turn-on and turn-off in cell proliferation and differentiation require additional research.

Osteon: Structure, Turnover, and Regeneration

Bone is composed of dense and solid cortical bone and honeycomb-like trabecular bone. Although cortical bone provides the majority of mechanical strength for a bone, there are few studies focusing on cortical bone repair or regeneration. Osteons (the Haversian system) form structural and functional units of cortical bone. In recent years, emerging evidences have shown that the osteon structure (including osteocytes, lamellae, lacunocanalicular network, and Haversian canals) plays critical roles in bone mechanics and turnover. Therefore, reconstruction of the osteon structure is crucial for cortical bone regeneration. This article provides a systematic summary of recent advances in osteons, including the structure, function, turnover, and regenerative strategies. First, the hierarchical structure of osteons is illustrated and the critical functions of osteons in bone dynamics are introduced. Next, the modeling and remodeling processes of osteons at a cellular level and the turnover of osteons in response to mechanical loading and aging are emphasized. Furthermore, several bioengineering approaches that were recently developed to recapitulate the osteon structure are highlighted. Impact statement This review provides a comprehensive summary of recent advances in osteons, especially the roles in bone formation, remodeling, and regeneration. Besides introducing the hierarchical structure and critical functions of osteons, we elucidate the modeling and remodeling of osteons at a cellular level. Specifically, we highlight the bioengineering approaches that were recently developed to mimic the hierarchical structure of osteons. We expect that this review will provide informative insights and attract increasing attentions in orthopedic community, shedding light on cortical bone regeneration in the future.

Bone anti-resorptive effects of coumarins on RANKL downstream cellular signaling: a systematic review of the literature

BACKGROUND

Members of the botanical families Apiaceae/Umbelliferae, Asteraceae, Fabaceae/Leguminosae, and Thymelaeaceae are rich in coumarins and have traditionally been used as ethnomedicines in many regions including Europe, Asia, and South America. Coumarins are a class of secondary metabolites that are widely present in plants, fungi, and bacteria and exhibit several pharmacological, biochemical, and therapeutic effects. Recently, many plants rich in coumarins and their derivatives were found to affect bone metabolism.

OBJECTIVE

To review scientific literature describing the mechanisms of action of coumarins in osteoclastogenesis and bone resorption.

MATERIALS AND METHODS

For this systematic review, the PubMed, Scopus, and Periodical Capes databases and portals were searched. We included in vitro research articles published between 2010 and 2020 that evaluated coumarins using osteoclastogenic markers.

RESULTS

Coumarins have been reported to downregulate RANKL-RANK signaling and various downstream signaling pathways required for osteoclast development, such as NF-κB, MAPK, Akt, and Ca²⁺ signaling, as well as pathways downstream of the nuclear factor of activated T-cells (NFATc1), including tartrate-resistant acid phosphatase (TRAP), cathepsin K (CTSK), and matrix metalloproteinase 9 (MMP-9).

CONCLUSIONS

Coumarins primarily inhibit osteoclast differentiation and activation by modulating different intracellular signaling pathways; therefore, they could serve as potential candidates for controlled randomized clinical trials aimed at improving human bone health.

Effect of periostin silencing on Runx2, RANKL and OPG expression in osteoblasts

OBJECTIVE

Normal tooth eruption is closely related to relevant genes and the dynamic balance between osteoblasts and osteoclasts. If secretion of RANKL and OPG by osteoblasts is disordered, relevant gene deficiencies or mutations will result in serious tooth eruption disturbances, e.g., cleidocranial dysplasia (CCD). Thus, we examined changes in Runx2, RANKL, and OPG protein expression in MC3T3-E1 cells after silencing the periostin gene, thus, providing an experimental basis to study tooth eruption mechanisms.

METHODS

Based on previous research, cells were divided into two groups according to the virus number: the contrast group (NC group; pFU-GW-016 PSC53349-1) and the experimental group (KD group; LVpFU-GW-016PSC66473-1). Cells were infected with the lentiviral vector (multiplicity of infection = 100) and assessed by image cytometry 72 h after infection. After screening cells for the strongest gene silencing effect, Runx2, RANKL and OPG protein expression were detected by western blotting.

RESULTS

Based on quantitative PCR, the periostin gene silencing efficiency in the KD group was over 90% (P < 0.01). After periostin gene silencing, compared with the control group, Runx2 and RANKL expression in the KD group was reduced (P < 0.01 and P < 0.05, respectively), but OPG protein expression showed no significant change (P > 0.05). The RANKL/OPG ratios in the KD group were lower than those in the NC group after periostin gene silencing (P < 0.05).

CONCLUSIONS

Silencing periostin may reduce the expression of Runx2, suggesting that there may be a synergistic relationship between periostin and Runx2 in their effects on osteoblast differentiation, while reducing RANKL expression obviously confirms that the NF-κB (nuclear factor κB) pathway plays an important role in this process and that periostin silencing changes the underlying tendency toward bone metabolism. This method could even provide an experimental basis for using exogenous periostin protein to treat some abnormal bone metabolism diseases, as it could be used as a supplement for the treatment of tooth eruption abnormalities caused by Runx2 gene deficiencies or mutations (CCD).

Injectable Hydrogels as Three-Dimensional Network Reservoirs for Osteoporosis Treatment

Despite tremendous progresses made in the field of tissue engineering over the past several decades, it remains a significant challenge for the treatment of osteoporosis (OP) due to the lack of appropriate carriers to improve the bioavailability of therapeutic agents and the unavailability of artificial bone matrix with desired properties for the replacement of damaged bone regions. Encouragingly, the development of injectable hydrogels for the treatment of OP has attracted increasing attention in recent years because they can serve either as a reservoir for various therapeutic species or as a perfect filler for bone injuries with irregular shapes. However, the relationship between the complicated pathological mechanism of OP and the properties of diverse polymeric materials lacks elucidation, which clearly hampers the clinical application of injectable hydrogels for the efficient treatment of OP. To clarify this relationship, this article summarized both localized and systematic treatment of OP using an injectable hydrogel-based strategy. Specifically, the pathogenesis of OP and the limitations of current treatment approaches were first analyzed. We further focused on the use of hydrogels loaded with various therapeutic substances following a classification standard of the encapsulated cargoes for OP treatment with an emphasis on the application and precautions of each category. A concluding remark on existing challenges and future directions of this rapidly developing research area was finally made. Impact statement Effective osteoporosis (OP) treatment remains a significant challenge due substantially to the unavailability of appropriate drug carriers and artificial matrices with desired properties to promote bone repair and replace damaged regions. For this purpose, this review focused on the development of diverse injectable hydrogel systems for the delivery of various therapeutic agents, including drugs, stem cells, and nucleic acids, for effective increase in bone mass and favorable osteogenesis. The summarized important guidelines are believed to promote clinical development and translation of hydrogels for the efficient treatment of OP and OP-related bone damages toward improved life quality of millions of patients.

Melatonin suppresses both osteoblast and osteoclast differentiation through repression of epidermal Erk signaling in the zebrafish scale

Melatonin has been implicated in the regulation of bone metabolism; however, the molecular mechanisms underlying its involvement in fracture healing are still obscure. We previously developed an in vivo fracture healing model using the scale of a double-transgenic zebrafish, trap:GFP; osterix:mCherry, which labels osteoclasts and osteoblasts with GFP and mCherry, respectively. Here we show using this model that melatonin inhibits both osteoblast and osteoclast differentiation under fracture stress through the repression of Erk signaling in epidermal cells of the scale. Melatonin treatment resulted in reduced numbers of both osteoblasts and osteoclasts in the fractured scale. Immunochemistry analysis revealed that Erk signals in epidermal cells, which express melatonin receptors, were greatly enhanced in response to fracture stress, but this enhancement was blocked by melatonin treatment. Moreover, inhibition of Erk signaling phenocopied the effects of melatonin treatment in the fractured scale. Collectively, these data suggest that the activation of epidermal Erk signaling is required for both osteoblast and osteoclast differentiation in the early stage of fracture healing, and melatonin suppresses epidermal Erk signaling, leading to impaired fracture healing.

Destroy to Rebuild: The Connection Between Bone Tissue Remodeling and Matrix Metalloproteinases

Bone is a dynamic organ that undergoes constant remodeling, an energetically costly process by which old bone is replaced and localized bone defects are repaired to renew the skeleton over time, thereby maintaining skeletal health. This review provides a general overview of bone’s main players (bone lining cells, osteocytes, osteoclasts, reversal cells, and osteoblasts) that participate in bone remodeling. Placing emphasis on the family of extracellular matrix metalloproteinases (MMPs), we describe how: (i) Convergence of multiple protease families (including MMPs and cysteine proteinases) ensures complexity and robustness of the bone remodeling process, (ii) Enzymatic activity of MMPs affects bone physiology at the molecular and cellular levels and (iii) Either overexpression or deficiency/insufficiency of individual MMPs impairs healthy bone remodeling and systemic metabolism. Today, it is generally accepted that proteolytic activity is required for the degradation of bone tissue in osteoarthritis and osteoporosis. However, it is increasingly evident that inactivating mutations in MMP genes can also lead to bone pathology including osteolysis and metabolic abnormalities such as delayed growth. We argue that there remains a need to rethink the role played by proteases in bone physiology and pathology.