Macrophages in bone fracture healing: Their essential role in endochondral ossification

A biodegradable magnesium alloy promotes subperiosteal osteogenesis via interleukin-10-dependent macrophage immunomodulation

In situ bone regeneration and vertical bone augmentation have been huge problems in clinical practice, always imposing a significant economic burden and causing patient suffering. Herein, MgZnYNd magnesium alloy rod implantation in mouse femur resulted in substantial subperiosteal new bone formation, with osteoimmunomodulation playing a pivotal role. Abundant macrophages were attracted to the subperiosteal new bone region and proved to be the most important regulation cells for bone regeneration. Periosteum stripping, macrophage depletion, and interleukin-10 (IL-10) blockade effectively diminished the MgZnYNd alloy-induced subperiosteal osteogenesis. Mechanistically, the degradation products of MgZnYNd alloy promoted M2 macrophage polarization and the secretion of anti-inflammatory cytokine IL-10, which enhanced periosteum-derived stem cells (PDSCs) osteogenesis through the JAK1-STAT3 pathway. An anti-IL-10 neutralizing antibody or STAT3 inhibitor significantly inhibited M2 macrophage-mediated osteogenic differentiation of PDSCs. Transcriptomics and proteomics revealed that periostin is the core regulator of PDSCs osteogenic differentiation. Furthermore, a novel clinical translation application of Mg-induced subperiosteal osteogenesis was developed, demonstrating its ability to preserve the height and width of the alveolar crest in rats and rabbits following tooth extraction. Collectively, these findings unveil a previously undefined role for Mg alloy-induced subperiosteal osteogenesis via macrophage-mediated osteoimmunomodulation, suggesting the therapeutic potential of magnesium alloy in bone regeneration and bone augmentation.

A novel strategy for bone defect repair: Stromal cell-derived factor 1α sustained-release acellular fish scale scaffolds combined with injection of bone marrow mesenchymal stem cells promote bone regeneration

Patients with bone defects often have weak cell vitality and differentiation ability of endogenous bone marrow mesenchymal stem cells (BMSCs), which makes bone regeneration face challenges. At present, the bone tissue engineering strategies are mainly to build grafts by loading cells on scaffolds in vitro. These strategies face many difficulties that limit their clinical application. To this end, we developed a new strategy for bone defect repair, namely chemotactic cell-free scaffolds combined with BMSCs injection. We first prepared a polydopamine-functionalized acellular fish scale scaffold that can continuously release stromal cell-derived factor 1α (SDF-1α) (termed as SDF-1α/PAFS) in vivo for at least 10 days. The study results showed that the scaffold not only has excellent mechanical properties and good biocompatibility but also has reactive oxygen scavenging activity, immunomodulation, angiogenesis, and osteogenesis. More importantly, SDF-1α/PAFS can recruit postoperatively injected BMSCs into bone defects for bone repair. We constructed the mouse cranial bone defect model, and in vivo experimental results confirmed that the strategy of combining SDF-1α/PAFS with BMSCs injection can effectively promote bone defect repair. Overall, this study provides a promising strategy for bone defect repair, with better clinical convenience and operability.

Biomaterial-based strategies for bone cement: modulating the bone microenvironment and promoting regeneration

Osteoporotic bone defect and fracture healing remain significant challenges in clinical practice. While traditional therapeutic approaches provide some regulation of bone homeostasis, they often present limitations and adverse effects. In orthopedic procedures, bone cement serves as a crucial material for stabilizing osteoporotic bone and securing implants. However, with the exception of magnesium phosphate cement, most cement variants lack substantial bone regenerative properties. Recent developments in biomaterial science have opened new avenues for enhancing bone cement functionality through innovative modifications. These advanced materials demonstrate promising capabilities in modulating the bone microenvironment through their distinct physicochemical properties. This review provides a systematic analysis of contemporary biomaterial-based modifications of bone cement, focusing on their influence on the bone healing microenvironment. The discussion begins with an examination of bone microenvironment pathology, followed by an evaluation of various biomaterial modifications and their effects on cement properties. The review then explores regulatory strategies targeting specific microenvironmental elements, including inflammatory response, oxidative stress, osteoblast-osteoclast homeostasis, vascular network formation, and osteocyte-mediated processes. The concluding section addresses current technical challenges and emerging research directions, providing insights for the development of next-generation biomaterials with enhanced functionality and therapeutic potential.

Endogenous sulfur dioxide deficiency impairs bone regeneration through abolishing S-sulfenylating p38 at cysteine 211

Bone defects result in substantial medical expenses and a diminished quality of life for patients. Macrophage polarization is crucial in the bone regeneration process mediated by bone marrow-derived mesenchymal stem cells (BMSCs). macrophage-derived sulfur dioxide (SO2), the fourth endogenous gas signaling molecule, following hydrogen sulfide (H2S), has been shown to regulate macrophage chemotaxis and inflammatory responses. Nevertheless, the specific regulatory effects and mechanisms of macrophage-derived SO2 on bone regeneration are not yet fully understood. This study reveals for the first time that the absence of macrophage-derived SO2 promotes M1 macrophage polarization, whereas the administration of exogenous SO2 donors inhibits M1 polarization. The deficiency of macrophage-derived SO2 results in impaired osteogenic differentiation of BMSCs, whereas the administration of SO2 donors enhances this differentiation process. Further investigations have elucidated that p38α MAPK (p38) is crucial in mediating SO2's effects on M1 macrophage polarization and BMSCs osteogenic differentiation. Mechanistically, SO2 induces S-sulfenylation of p38 in macrophages, an effect that can be reversed by the thiol reductant dithiothreitol. Additionally, the C211S mutation in p38 abrogates the SO2-induced S-sulfenylation of p38, thereby preventing the inhibition of p38 activation and subsequently disrupting the regulation of M1 macrophage polarization and BMSCs osteogenic differentiation. In a model of mouse calvarial bone defects, we consistently observed that inhibiting SO2 production using the SO2-generating enzyme inhibitor HDX impaired bone regeneration capacity in mice, whereas the administration of an SO2 donor enhanced this capacity. In summary, macrophage-derived SO2 S-sulfenylates p38 at cysteine 211, thereby suppressing p38 activation, which inhibits M1 polarization and subsequently maintains the osteogenic differentiation of BMSCs. This study is the first to elucidate the role and mechanism of SO2 in sustaining osteogenesis, offering a novel strategy for addressing bone defect-related disorders.

Pathophysiology of bone remodelling cycle: Role of immune system and lipids

Osteoporosis is the most common skeletal disease worldwide, characterized by low bone mineral density, resulting in weaker bones, and an increased risk of fragility fractures. The maintenance of bone mass relies on the precise balance between bone synthesis and resorption. The close relationship between the immune and skeletal systems, called "osteoimmunology", was coined to identify these overlapping "scientific worlds", and its function resides in the evaluation of the mutual effects of the skeletal and immune systems at the molecular and cellular levels, in both physiological and pathological states. Lipids play an essential role in skeletal metabolism and bone health. Indeed, bone marrow and its skeletal components demand a dramatic amount of daily energy to control hematopoietic turnover, acquire and maintain bone mass, and actively being involved in whole-body metabolism. Statins, the main therapeutic agents in lowering plasma cholesterol levels, are able to promote osteoblastogenesis and inhibit osteoclastogenesis. This review is meant to provide an updated overview of the pathophysiology of bone remodelling cycle, focusing on the interplay between bone, immune system and lipids. Novel therapeutic strategies for the management of osteoporosis are also discussed.

Non-contact electrical stimulation via a Vector-potential transformer promotes bone healing in drill-hole injury model

INTRODUCTION

We investigated the effects of non-contact electrical stimulation via a Vector-potential (VP) transformer, a novel physical therapy device, on bone healing in drill-hole injury models.

MATERIALS AND METHODS

Six-week-old male Wistar rats, after a one-week acclimation period, were divided into three groups: the control group (CO), the bone injury group (BI), in which a drill-hole injury was created, and the VP stimulation group (VP), which received non-contact electrical stimulation via a VP transformer after bone injury. In the VP group, rats underwent stimulation at 200 kHz for 30 minutes per day, seven days per week.

RESULTS

The VP group exhibited increased bone formation as early as day 7 post-injury, with significantly higher bone volume than the BI group at all time points (day 7: p = 0.0003; day 14: p = 0.0024; day 21: p = 0.0001). By day 21, the VP group showed lighter toluidine blue staining and reduced biglycan immunoreactivity compared to the BI group. Bone mineral density also increased (p = 0.0008). Osteoblasts in the VP group displayed abundant cytoplasm and a high capacity for osteocalcin synthesis. Additionally, the VP group demonstrated increased expression of Bglap (day 5: p = 0.0068; day 7: p = 0.0096) and Ctsk (day 7: p = 0.0329; day 14: p = 0.0171), along with a higher number of TRAP-positive osteoclasts (day 21: p = 0.0159) compared to the BI group.

CONCLUSION

Non-contact electrical stimulation via a VP transformer promotes bone healing in drill-hole injury models.

Polydopamine-functionalized acellular fish scale scaffolds for accelerated bone tissue regeneration

The complex microenvironment changes in the bone defect site are the main reason for its refractory treatment, including the significant increase in the level of reactive oxygen species (ROS) and inflammatory dysregulation. There is an urgent need to develop some bioactive materials that can regulate the microenvironment and promote bone regeneration. This study proposed a new strategy for designing bone tissue engineering scaffolds based on fish scales and developed a polydopamine-functionalized acellular fish scale scaffold (PDA-AFS). The results showed that PDA-AFS had excellent mechanical properties, special three-dimensional surface topography, and biodegradability. In vitro results showed that PDA-AFS had good biocompatibility and cell adhesion ability, could effectively reduce ROS levels, and had immunomodulatory activity. More importantly, PDA-AFS can enhance osteogenic differentiation of bone marrow mesenchymal stem cells and promote endogenous bone regeneration in critical-sized calvarial bone defects. In addition, transcriptome analysis also provided clues to its possible osteogenic mechanism. Overall, we provide a new strategy for designing bone tissue engineering scaffolds based on fish scales for bone regeneration treatment of bone defects.

Functional tendon regeneration is driven by regulatory T cells and IL-33 signaling

Tendon injuries heal by scar, leading to poor function. To date, the role of immune cells remains underexplored. Using a neonatal mouse model of functional tendon healing compared to adult scar-mediated healing, we identified a regenerative immune profile that is associated with type 1 inflammation followed by rapid polarization to type 2, driven by macrophages and regulatory T cells (Treg cells). Single-cell and bulk RNA sequencing also revealed neonatal Treg cells with an immunomodulatory signature distinct from adult. Neonatal Treg cell ablation resulted in a dysregulated immune response, failed tenocyte recruitment, and impaired regeneration. Adoptive transfer further confirmed the unique capacity of neonatal Treg cells to rescue functional regeneration. We showed that neonatal Treg cells mitigate interleukin-33 (IL-33) to enable tenocyte recruitment and structural restoration, and that adult IL-33 deletion improves functional healing. Collectively, these findings demonstrate that Treg cells and IL-33 immune dysfunction are critical components of failed tendon healing and identify potential targets to drive tendon regeneration.

Tailoring osteo-immunomodulatory micro-environments via a bioactive 3D PLA scaffold to potentiate regenerative healing

The burden of long bone fractures impacts society profoundly. Polylactic acid (PLA) and PLA-based scaffolds have been widely used for bone tissue engineering due to its biodegradable and non-toxic properties. However, PLA scaffolds possess no biological and immunoregulatory activity. In addition, M2 phenotypic macrophages and M2 macrophage inducer have shown pro-healing effect. Therefore, in this study, we sought to harness the potent osteo-immunomodulatory properties of anti-inflammatory M2 macrophages and encapsulated bioactive interleukin-4 microdroplets (MDs) into the PLA scaffold matrix in order to enhance the biological activity and osteo-immunomodulatory properties of PLA scaffold. The MTT assay, live and dead staining, and phalloidin/DAPI dual staining were used to evaluate biocompatibility in human bone marrow mesenchymal stem cells (hBMSCs). Unstimulated macrophages and inflammatory macrophages were further used to test its immunoregulatory role. The immunoregulatory role of MDs-IL4/PLA hybrid scaffolds on hBMSCs differentiation was further investigated in vitro. The MDs-IL4/PLA hybrid scaffolds showed high biocompatibility in hBMSCs. In addition, MDs-IL4/PLA hybrid scaffolds demonstrated good immunoregulatory role in unstimulated and inflammatory macrophages, as evidenced by marked morphological changes, significantly increased M2 phenotypic markers, and reduction of pro-inflammatory marker expression, etc. Furthermore, our results indicate that the MDs-IL4/PLA hybrid scaffolds could significantly enhance the osteogenic differentiation of hBMSCs via its potential immunomodulatory properties. The subcutaneous implantation of MDs-IL4/PLA scaffold demonstrated significant increases in IL-4, IL-10, CCL2, CCL20, beta-NGF, and TIMP-1 expressions, indicating its notable in vivo immunomodulatory effects. By harnessing the synergistic effects of PLA and MDs-IL4, this novel scaffold holds promise for advancing regenerative medicine approaches, particularly in bone tissue engineering.

A review on innovations in hydroxyapatite: advancing sustainable and multifunctional dental implants

The increasing relevance of biomaterials in medical curements and the care of aging populations has led to significant advancements in the development and modification of these materials. Hydroxyapatite (HAp), a biocompatible ceramic that mimics the composition of bone mineral, stands out for its remarkable abilities. Its stability in body fluids and ability to integrate with bone without causing toxicity or inflammation made it a prime candidate for biomedical utilization, particularly in odontology and orthopedics. Dental implants, which necessitate a strong interface with the jawbone to effectively support prosthetic devices, are enhanced by coatings of calcium phosphate-based materials like HAp. This enhances osseointegration, ensuring a strong bond and longevity of the implant. HAp may be synthesized both synthetically and from natural sources like mammalian bones, marine shells, and plants, each offering unique trace elements that improve its bioactivity. The synthesis of HAp involves differential technique, including chemical precipitation, and hydrothermal techniques, each impacting the final abilities of the material. The use of natural sources is especially promising, providing a sustainable, cost-effective alternative that retains essential biocompatibility. Hence, this article aims to explore the synthesis, properties, and biomedical applications of hydroxyapatite (HAp), with a special emphasis on its role in improving the performance and durability of dental implants. It also addresses the challenges in manufacturing and biocompatibility, offering insights into future advancements in this field. By addressing current challenges in manufacturing and biocompatibility, HAp paves the way for more effective and long-lasting dental treatments.

CircAars-Engineered ADSCs Facilitate Maxillofacial Bone Defects Repair Via Synergistic Capability of Osteogenic Differentiation, Macrophage Polarization and Angiogenesis

Adipose-derived stem cells (ADSCs) hold significant promise in bone tissue engineering due to their self-renewal capacity and easy accessibility. However, their limited osteogenic potential remains a critical challenge for clinical application in bone repair. Emerging evidence suggests that circular RNAs (circRNAs) play a key role in regulating stem cell fate and osteogenesis. Despite this, the specific mechanisms by which circRNAs influence ADSCs in the context of bone tissue engineering are largely unexplored. This study introduces a novel strategy utilizing circAars, a specific circRNA, to modify ADSCs, which are then incorporated into gelatin methacryloyl (GelMA) hydrogels for the repair of critical-sized maxillofacial bone defects. The findings reveal that circAars predominantly localizes in the cytoplasm of ADSCs, where it acts as a competitive sponge for miR-128-3p, enhancing the osteogenic differentiation and migration capabilities of ADSCs. Furthermore, circAars-engineered ADSCs facilitate macrophage polarization from the M1 to M2 phenotype and enhance endothelial cell (EC) angiogenic potential through a paracrine mechanism. Additionally, GelMA scaffolds loaded with circAars-engineered ADSCs accelerate the repair of critical-sized maxillofacial bone defects by synergistically promoting osteogenesis, macrophage M2 polarization, and angiogenesis. This approach offers a promising therapeutic strategy for the treatment of critical-sized maxillofacial defects.

Coculture of mesenchymal stem cells and macrophage: A narrative review

Stem cell transplantation is a promising treatment for repairing damaged tissues, but challenges like immune rejection and ethical concerns remain. Mesenchymal stem cells (MSCs) offer high differentiation potential and immune regulatory activity, showing promise in treating diseases such as gynecological, neurological, and kidney disorders. With scientific progress, MSC applications are overcoming traditional treatment limitations. In MSCs-macrophage coculture, MSCs transform macrophages into anti-inflammatory M2 macrophages, reducing inflammation, whereas macrophages enhance MSCs osteogenic differentiation. This coculture is vital for immune modulation and tissue repair, with models varying by contact type and dimensional arrangements. Factors such as coculture techniques and cell ratios influence outcomes. Benefits include improved heart function, wound healing, reduced lung inflammation, and accelerated bone repair. Challenges include optimizing coculture conditions. This study reviews the methodologies, factors, and mechanisms of MSC-macrophage coculture, providing a foundation for tissue engineering applications. SIGNIFICANCE STATEMENT: This review underlines the significant role of mesenchymal stem cell-macrophage coculture, providing a foundation for tissue engineering application.

The potential therapeutic effects of Panax notoginseng in osteoporosis: A comprehensive review

BACKGROUND

Accumulating evidence shows that Panax notoginseng, a well-known medicinal herb, has an ideal effect on prevention and treatment of skeletal diseases. In this study, we reviewed clinical applications of clinical application as well as phytochemistry, pharmacokinetics, pharmacology in improving bone quality and toxicity of Panax notoginseng.

PURPOSE

Review the phytochemistry, pharmacokinetics, pharmacology involved in the improving bone metabolism and toxicity of Panax notoginseng and evaluate its potential as a traditional Chinese herbal medicine for osteoporosis.

METHODS

Several databases were consulted, including PubMed, China National Knowledge Infrastructure, National Science and Technology Library and Web of Science. The following words or phrases were used alone or in combinations in the titles and/or abstracts: "","Panax notoginseng", "Sanqi", "osteoporosis", "bone", "osteoblast", "osteoclast", "phytochemistry", "pharmacology" and "pharmacokinetics". Altogether 160 papers were cited.

RESULTS

8 clinical trials of Panax notoginseng alone for the treatment of osteoporosis were identified, most of which used traditional Chinese patent medicines to treat osteoporosis fractures. In these clinical trials, Panax notoginseng preparations have achieved relatively good therapeutic effects. However, more rigorous large-scale experiments are expected to prove their efficacy. Phytochemistry study showed that saponins, flavonoids, polysaccharides are the main active ingredients extracted from Panax notoginseng and the transformation of saponins during the processing explains the different effects of raw and cooked Panax notoginseng. The pharmacokinetics data reveals that protopanaxdiol-type (ppd-type) saponins possesses higher bioavailability than protopanaxtriol-type(ppt-type) saponins and ppd-type saponins such as ginsenoside Ra3, Rb1, and Rd can represent suitable pharmacokinetic markers for Panax notoginseng extracts. The data from animal experiments demonstrates that Panax notoginseng can improve bone quality in ovariectomized, diabetic, hyperlipidemia, radiation-induced, and arthritis rats through the regulation of anti-adipogenesis, anti-inflammation, anti-oxidation, angiogenesis and estrogenic effects. In vitro experiments, the activities of improving bone quality of Panax notoginseng and its ingredients may be attributed to the regulation of multiple signaling pathways, including Wnt/β-catenin, BMP/BMP-R, AMPK/mTOR, GPER/PI3K/AKT, etc. Acute and chronic toxicity as well as genotoxicity studies show that Panax notoginseng is well tolerated while long term use may lead to liver and kidney toxicity.

CONCLUSIONS

Panax notoginseng is a superior medicinal herb that contains multiple active ingredients and could play a potential role in the prevention and treatment of osteoporosis. Further studies should concentrate on developing Panax notoginseng products with higher curative effect and bioavailability.

PDMS biointerfaces featuring honeycomb-like well microtextures designed for a pro-healing environment

Even today, the reduction of complications following breast implant surgery together with the enhancement of implant integration and performance through the modulation of the foreign body response (FBR), remains a fundamental challenge in the field of plastic surgery. Therefore, tailoring the material's physical characteristics to modulate FBR can represent an effective approach in implantology. While polydimethylsiloxane (PDMS) patterning on 2D substrates is a relatively established and available procedure, micropatterning multiscaled biointerfaces on a controlled large area has been more challenging. Therefore, in the present work, a specific designed honeycomb-like well biointerface was designed and obtained by replication in PDMS at large scale and its effectiveness towards creating a pro-healing environment was investigated. The grayscale masks assisted laser-based 3D texturing method was used for creating the required moulds in Polycarbonate for large area replication. By comparison to the smooth substrate, the honeycomb topography altered the fibroblasts' behaviour in terms of adhesion and morphology and reduced the macrophages' inflammatory response. Additionally, the microstructured surface effectively inhibited macrophage fusion, significantly limiting the colonization of both Gram-positive and Gram-negative microbial strains on the tested surfaces. Overall, this study introduces an innovative approach to mitigate the in vitro FBR to silicone, achieved through the creation of a honeycomb-inspired topography for prosthetic interfaces.

It is not waste if it is therapy: cellular, secretory and functional properties of reamer-irrigator-aspirator (RIA)-derived autologous bone grafts

BACKGROUND

Large bone defects resulting from trauma, disease, or resection often exceed the intrinsic capacity of bones to heal. The current gold standard addressing these defects is autologous bone grafting (ABG). Procedures such as reamer-irrigator-aspirator (RIA) and conventional bone grafting from the iliac crest are widely recognized as highly effective interventions for critical-size bone defects. The early phase of fracture healing is particularly crucial, as it can determine whether a complete bony union occurs, or if delayed healing or non-unions develop. The initial composition of the bone marrow (BM)-rich ABG transplant, with its unique cellular (e.g., leukocytes, monocytes, and granulocytes) and acellular (e.g., growth factors and extracellular proteins) components, plays a key role in this process. However, despite many successful case reports, the role of ABG cells, growth factors, and their precise contributions to bone healing remain largely elusive.

MATERIALS AND METHODS

We characterized the native cellularity of both solid and liquid RIA-derived ABG by analyzing primary, minimally manipulated populations of monocytes, macrophages, and T cells, as well as hematopoietic, endothelial, and mesenchymal progenitor cells by flow cytometry. Growth factor and cytokine contents were assessed through antibody arrays. Possible functional and immunomodulatory properties of RIA liquid were evaluated in functional in vitro assays.

RESULTS

Growth factor and protein arrays revealed a plethora of soluble factors that can be linked to specific immunomodulatory and angiogenic properties, which were evaluated for their potency using functional in vitro assays. We could demonstrate a strong M2-macrophage phenotype inducing the effect of RIA liquid on macrophages. Additionally, we observed an increase in anti-inflammatory T cell subsets generated from peripheral blood mononuclear cells and BM mononuclear cells upon stimulation with RIA liquid . Finally, in vitro endothelial tube formation assays revealed highly significant angiogenic properties of RIA liquid, even at further dilutions.

CONCLUSION

The cytokine and protein content of RIA liquid exhibits potent immunomodulatory and angiogenic properties. These findings suggest significant therapeutic potential for RIA liquid in modulating immune responses and promoting angiogenesis. Anti-inflammatory and angiogenic properties demonstrated in this study might also help to further define and understand its particular mode of action while also providing explanations to the excellent bone-healing properties of ABG in general.

LEVEL OF EVIDENCE

Case-series (Level 4).

Selective promotion of sensory innervation-mediated immunoregulation for tissue repair

Sensory innervation triggers the regenerative response after injury. However, dysfunction and impairment of sensory nerves, accompanied by excessive inflammation impede tissue regeneration. Consequently, specific induction of sensory innervation to mediate immunoregulation becomes a promising therapeutic approach. Herein, we developed a cell/drug-free strategy to selectively boost endogenous sensory innervation to harness immune responses for promoting tissue rehabilitation. Specifically, a dual-functional phage was constructed with a sensory nerve-homing peptide and a β-subunit of nerve growth factor (β-NGF)-binding peptide. These double-displayed phages captured endogenic β-NGF and localized to sensory nerves to promote sensory innervation. Furthermore, regarding bone regeneration, phage-loaded hydrogels achieved rapid sensory nerve ingrowth in bone defect areas. Mechanistically, sensory neurotization facilitated M2 polarization of macrophages through the Sema3A/XIAP/PAX6 pathway, thus decreasing the M1/M2 ratio to induce the dissipation of local inflammation. Collectively, these findings highlight the essential role of sensory innervation in manipulating inflammation and provide a conceptual framework based on neuroimmune interactions for promoting tissue regeneration.

Advancements in 3D gel culture systems for enhanced angiogenesis in bone tissue engineering

Angiogenesis-osteogenesis coupling is a crucial process in bone tissue engineering, requiring a suitable material structure for vessel growth. Recently, the 3D culture system has gained significant attention due to its benefits in cell growth, proliferation and tissue regeneration. Its most notable advantage is its ECM-like function, which supports endothelial cell adhesion and facilitates the formation of vascular-like networks-crucial for angiogenesis-osteogenesis coupling. Hydrogels, with their highly hydrophilic polymer network resembling the extracellular matrix, make the 3D gel culture system an ideal approach for angiogenesis due to its cellular integrity and adjustable properties. This article reviews the current use of 3D gel culture systems in bone tissue engineering, covering substrates, characteristics and processing technologies, thereby offering readers profound insights into these systems.

Tissue macrophages: origin, heterogenity, biological functions, diseases and therapeutic targets

Macrophages are immune cells belonging to the mononuclear phagocyte system. They play crucial roles in immune defense, surveillance, and homeostasis. This review systematically discusses the types of hematopoietic progenitors that give rise to macrophages, including primitive hematopoietic progenitors, erythro-myeloid progenitors, and hematopoietic stem cells. These progenitors have distinct genetic backgrounds and developmental processes. Accordingly, macrophages exhibit complex and diverse functions in the body, including phagocytosis and clearance of cellular debris, antigen presentation, and immune response, regulation of inflammation and cytokine production, tissue remodeling and repair, and multi-level regulatory signaling pathways/crosstalk involved in homeostasis and physiology. Besides, tumor-associated macrophages are a key component of the TME, exhibiting both anti-tumor and pro-tumor properties. Furthermore, the functional status of macrophages is closely linked to the development of various diseases, including cancer, autoimmune disorders, cardiovascular disease, neurodegenerative diseases, metabolic conditions, and trauma. Targeting macrophages has emerged as a promising therapeutic strategy in these contexts. Clinical trials of macrophage-based targeted drugs, macrophage-based immunotherapies, and nanoparticle-based therapy were comprehensively summarized. Potential challenges and future directions in targeting macrophages have also been discussed. Overall, our review highlights the significance of this versatile immune cell in human health and disease, which is expected to inform future research and clinical practice.

Remineralizing capacity of zinc oxide eugenol sealer following the addition of nanohydroxyapatite-tyrosine amino acid: An in vivo animal study

OBJECTIVE

Although several sealers have been developed, the zinc oxide eugenol (ZOE) sealer is still used in many private dental clinics. This study aimed to compare the biocompatibility and remineralizing capacity οf ZOE sealer with the addition of nanohydroxyapatite-tyrosine amino acid.

METHODS

This study was conducted on Twenty rabbits. The rabbits were divided into four groups based on the test observation period (3, 7, 21, and 28 days) following surgical implantation. General anesthesia was administered to each rabbit, and a subcutaneous incision of approximately 1 ± 0.5 cm was made along the symphyseal area of the mandible of each rabbit. Four bone cavities were generated in the interdental space of the lower jaw between the central and molar teeth of each rabbit, with one longitudinal subcutaneous incision. The ZOE sealers were mixed according to the manufacturer's guidelines and directly inserted within the cavities generated at the end of each test period. The animals were sacrificed, and bone biopsy was carried out at the site of testing. The biopsy samples were obtained and subjected to histological analysis using a low-power light microscope (Olympus C ⅹ 21, Japan) and immunohistochemistry using Ki67 antibody.

RESULTS

The collected information was examined by parametric statistical tests using SPSS software version ''22." One-way ANOVA and post-hoc Duncan's tests were used to measure the significance among various groups, with statistical significance set, when "P ≤ 0.01".

CONCLUSION

The 20% mixed endodontic sealer displayed excellent outcomes compared to other experimental groups, as identified by higher new bone formation at every evaluation period.

Engineering next-generation oxygen-generating scaffolds to enhance bone regeneration

In bone, an adequate oxygen (O2) supply is crucial during development, homeostasis, and healing. Oxygen-generating scaffolds (OGS) have demonstrated significant potential to enhance bone regeneration. However, the complexity of O2 delivery and signaling in vivo makes it challenging to tailor the design of OGS to precisely meet this biological requirement. We review recent advances in OGS and analyze persisting engineering and translational hurdles. We also discuss the potential of computational and machine learning (ML) models to facilitate the integration of novel imaging data with biological readouts and advanced biomanufacturing technologies. By elucidating how to tackle current challenges using cutting-edge technologies, we provide insights for transitioning from traditional to next-generation OGS to improve bone regeneration in patients.

Polytrauma impairs fracture healing accompanied by increased persistence of innate inflammatory stimuli and reduced adaptive response

The field of bone regeneration has primarily focused on investigating fracture healing and nonunion in isolated musculoskeletal injuries. Compared to isolated fractures, which frequently heal well, fractures in patients with multiple bodily injuries (polytrauma) may exhibit impaired healing. While some papers have reported the overall cytokine response to polytrauma conditions, significant gaps in our understanding remain in how fractures heal differently in polytrauma patients. We aimed to characterize fracture healing and the temporal local and systemic immune responses to polytrauma in a murine model of polytrauma composed of a femur fracture combined with isolated chest trauma. We collected serum, bone marrow from the uninjured limb, femur fracture tissue, and lung tissue over 3 weeks to study the local and systemic immune responses and cytokine expression after injury. Immune cell distribution was assessed by flow cytometry. Fracture healing was characterized using microcomputed tomography (microCT), histological staining, immunohistochemistry, mechanical testing, and small angle X-ray scattering. We detected more innate immune cells in the polytrauma group, both locally at the fracture site and systemically, compared to other groups. The percentage of B and T cells was dramatically reduced in the polytrauma group 6 h after injury and remained low throughout the study duration. Fracture healing in the polytrauma group was impaired, evidenced by the formation of a poorly mineralized and dysregulated fracture callus. Our data confirm the early, dysregulated inflammatory state in polytrauma that correlates with disorganized and impaired fracture healing.

PTX3-assembled pericellular hyaluronan matrix enhances endochondral ossification during fracture healing and heterotopic ossification

Endochondral ossification (EO) is a pivotal process during fracture healing and traumatic heterotopic ossification (HO), involving the cartilaginous matrix synthesis and mineralization. Unlike the extracellular matrix, the hyaluronan (HA)-rich pericellular matrix (PCM) directly envelops chondrocytes, serving as the frontline for extracellular signal reception and undergoing dynamic remodeling. Pentraxin 3 (PTX3), a secreted glycoprotein, facilitates HA matrix assembly and remodeling. However, it remains unclear whether PTX3 affects EO by regulating HA-rich PCM assembly of chondrocytes, thereby impacting fracture healing and traumatic HO. This study demonstrates that PTX3 deficiency impairs fracture healing and inhibits traumatic HO, but dose not affect growth plate development in mice. PTX3 expression is up-regulated during chondrocyte matrix synthesis and maturation and is localized in the HA-rich PCM. PTX3 promotes the assembly of HA-rich PCM in a serum- and TSG6-dependent manner, fostering CD44 receptor clustering, activating the FAK/AKT signaling pathway, and promoting chondrocyte matrix synthesis and maturation. Local injection of PTX3/TSG6 matrix protein mixture effectively promotes fracture healing in mice. In conclusion, PTX3-assembled HA-rich PCM promotes chondrocyte matrix synthesis and maturation via CD44/FAK/AKT signaling. This mechanism facilitates EO during fracture healing and traumatic HO in mice.

T cell related osteoimmunology in fracture healing: Potential targets for augmenting bone regeneration

Last decade has witnessed increasing evidence which highlights the roles of immune cells in bone regeneration. Numerous immune cell types, including macrophages, T cells, and neutrophils are involved in fracture healing by orchestrating a series of events that modulate bone formation and remodeling. In this review, the role of T cell immunity in fracture healing has been summarized, and the modulatory effects of T cell immunity in inflammation, bone formation and remodeling have been highlighted. The review also summarizes the specific roles of different T cell subsets, including CD4+ T cells, CD8+ T cells, regulatory T cells, T helper 17 cells, and γδ T cells in modulating fracture healing. The current therapeutics targeting T cell immunity to enhance fracture healing have also been reviewed, aiming to provide insights from a translational standpoint. Overall, this work discusses recent advances and challenges in the interdisciplinary research field of T cell related osteoimmunology and its implications in fracture healing.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

Delayed unions or non-unions of bone fractures remain a challenge in clinical practice. Developing a deep understanding of the roles of immune cells, including T cells, in fracture healing will facilitate the advancement of novel therapeutics of fracture nonunion. This review summarizes the current understanding of different T cell subsets involved in various phases of fracture healing, providing insights for targeting T cells as an alternative strategy to enhance bone regeneration.

Communication between endothelial cells and osteoblasts in regulation of bone homeostasis: Notch players

Endothelial cells coat blood vessels and release molecular signals to affect the fate of other cells. Endothelial cells can adjust their behavior in response to the changing microenvironmental conditions. During bone regeneration, bone tissue cells release factors that promote blood vessel growth. Notch is a key signaling that regulates cell fate decisions in many tissues and plays an important role in bone tissue development and homeostasis. Understanding the interplay between angiogenesis and osteogenesis is currently a focus of research efforts in order to facilitate and improve osteogenesis when needed. Our review explores the cellular and molecular mechanisms including Notch-dependent endothelial-MSC communication that drive osteogenesis-angiogenesis processes and their effects on bone remodeling and repair.

Advances in the application and research of biomaterials in promoting bone repair and regeneration through immune modulation

With the ongoing development of osteoimmunology, increasing evidence indicates that the local immune microenvironment plays a critical role in various stages of bone formation. Consequently, modulating the immune inflammatory response triggered by biomaterials to foster a more favorable immune microenvironment for bone regeneration has emerged as a novel strategy in bone tissue engineering. This review first examines the roles of various immune cells in bone tissue injury and repair. Then, the contributions of different biomaterials, including metals, bioceramics, and polymers, in promoting osteogenesis through immune regulation, as well as their future development directions, are discussed. Finally, various design strategies, such as modifying the physicochemical properties of biomaterials and integrating bioactive substances, to optimize material design and create an immune environment conducive to bone formation, are explored. In summary, this review comprehensively covers strategies and approaches for promoting bone tissue regeneration through immune modulation. It offers a thorough understanding of current research trends in biomaterial-based immune regulation, serving as a theoretical reference for the further development and clinical application of biomaterials in bone tissue engineering.

Dual release scaffolds as a promising strategy for enhancing bone regeneration: an updated review

Advancements in tissue regeneration, particularly bone regeneration is key area of research due to potential of novel therapeutic approaches. Efforts to reduce reliance on autologous and allogeneic bone grafts have led to the development of biomaterials that promote synchronized and controlled bone healing. However, the use of growth factors is limited by their short half-life, slow tissue penetration, large molecular size and potential toxicity. These factors suggest that traditional delivery methods may be inadequate hence, to address these challenges, new strategies are being explored. These novel approaches include the use of bioactive substances within advanced delivery systems that enable precise spatiotemporal control. Dual-release composite scaffolds offer a promising solution by reducing the need for multiple surgical interventions and simplifying the treatment process. These scaffolds allow for sustained and controlled drug release, enhancing bone repair while minimizing the drawbacks of conventional methods. This review explores various dual-drug release systems, discussing their modes of action, types of drugs used and release mechanisms to improve bone regeneration.

Role of macrophage in intervertebral disc degeneration

Intervertebral disc degeneration is a degenerative disease where inflammation and immune responses play significant roles. Macrophages, as key immune cells, critically regulate inflammation through polarization into different phenotypes. In recent years, the role of macrophages in inflammation-related degenerative diseases, such as intervertebral disc degeneration, has been increasingly recognized. Macrophages construct the inflammatory microenvironment of the intervertebral disc and are involved in regulating intervertebral disc cell activities, extracellular matrix metabolism, intervertebral disc vascularization, and innervation, profoundly influencing the progression of disc degeneration. To gain a deeper understanding of the inflammatory microenvironment of intervertebral disc degeneration, this review will summarize the role of macrophages in the pathological process of intervertebral disc degeneration, analyze the regulatory mechanisms involving macrophages, and review therapeutic strategies targeting macrophage modulation for the treatment of intervertebral disc degeneration. These insights will be valuable for the treatment and research directions of intervertebral disc degeneration.

Inhibition of MEK1/2 Signaling Pathway Limits M2 Macrophage Polarization and Interferes in the Dental Socket Repair Process in Mice

Dental socket repair theoretically involves a constructive inflammatory immune response, which evolves from an initial M1 prevalence to a subsequent M2 dominance. In this scenario, the MEK1/2 signaling pathway is allegedly involved in M2 polarization. This study aimed to evaluate the impact of MEK1/2 pharmacological inhibition in the local host response and repair outcome. C57Bl/6-WT 8-week-old male mice were submitted to the extraction of the right upper incisor and treated (or not, control group) with MEK1/2 inhibitor PD0325901 (10 mg/kg/24 h/IP, MEK1/2i group) and analyzed at 0, 3, 7, and 14 days using microcomputed tomography, histomorphometry, birefringence, immunohistochemistry, and PCR array analysis. The results demonstrate that MEK1/2 inhibition limits the development of M2 response over time, being associated with lower expression of M2, MSCs, and bone markers, lower levels of growth and osteogenic factors, along with a higher expression of iNOS, IL-1b, IL-6, and TNF-α, as well inflammatory chemokines, indicating a predominantly M1 pro-inflammatory environment. This modulation of local inflammatory immune response is associated with impaired bone formation as demonstrated by microtomographic and histomorphometric data. The results show that MEK1/2 inhibition delays bone repair after tooth extraction, supporting the concept that M2 macrophages are essential elements for host response regulation and proper repair.

No bones about it: regulatory T cells promote fracture healing

Regulatory T cells (Tregs) are increasingly being recognized for their role in promoting tissue repair. In this issue of the JCI, Chen et al. found that Tregs at the site of bone injury contribute to bone repair. The CCL1/CCR8 chemokine system promoted the accumulation of Tregs at the site of bone injury, where Tregs supported skeletal stem cell (SSC) accumulation and osteogenic differentiation. CCL1 increased the transcription factor basic leucine zipper ATF-like transcription factor (BATF) in CCR8+ Tregs, which induced the secretion of progranulin that promoted SSC osteogenic function and new bone formation. This study highlights the ever-expanding role of Tregs in tissue repair by demonstrating their ability to expand stem cells at a site of injury.

Local erythropoiesis directs oxygen availability in bone fracture repair

Bone fracture ruptures blood vessels and disrupts the bone marrow, the site of new red blood cell production (erythropoiesis). Current dogma holds that bone fracture causes severe hypoxia at the fracture site, due to vascular rupture, and that this hypoxia must be overcome for regeneration. Here, we show that the early fracture site is not hypoxic, but instead exhibits high oxygen tension (> 55 mmHg, or 8%), similar to the red blood cell reservoir, the spleen. This elevated oxygen stems not from angiogenesis but from activated erythropoiesis in the adjacent bone marrow. Fracture-activated erythroid progenitor cells concentrate oxygen through haemoglobin formation. Blocking transferrin receptor 1 (CD71)-mediated iron uptake prevents oxygen binding by these cells, induces fracture site hypoxia, and enhances bone repair through increased angiogenesis and osteogenesis. These findings upend our current understanding of the early phase of bone fracture repair, provide a mechanism for high oxygen tension in the bone marrow after injury, and reveal an unexpected and targetable role of erythroid progenitors in fracture repair.

Bioinspired adhesive polydopamine-metal-organic framework functionalized 3D customized scaffolds with enhanced angiogenesis, immunomodulation, and osteogenesis for orbital bone reconstruction

Critical-sized orbital bone defects can lead to significant maxillofacial deformities and even eye movement disorders. The challenges associated with these defects, including their complicated structure, inadequate blood supply, and limited availability of progenitor cells that hinder successful repair. To overcome these issues, we developed a novel approach using computer numerical control (CNC) material reduction manufacturing technology to produce a customized polyetheretherketone (PEEK) scaffold that conforms to the specific shape of orbital bone defects. Deferoxamine (DFO) was in situ encapsulated into polydopamine-hybridized zeolitic imidazolate framework-8 (pZIF8-DFO) nanoparticles, which was subsequently adhered to the sulfonated PEEK (sPEEK) scaffold through polydopamine modification. This functionalization enhanced drug loading efficiency and imparted anti-inflammatory properties to the nanoparticle system. Our in vitro findings demonstrated that the sustained release of DFO from the sPEEK/pZIF8-DFO scaffolds extended over 14 days and significantly promoted angiogenesis and progenitor cell recruitment, as evidenced by increased expression of HIF-1α, VEGF, and SDF-1α expression in human umbilical vein endothelial cells (HUVECs). Moreover, sPEEK/pZIF8-DFO scaffolds exhibited superior immunomodulation and osteogenic differentiation capabilities on Raw 264.7 cells and rabbit bone marrow mesenchymal stem cells (rBMSCs), respectively. Most notably, our in vivo rabbit orbital bone defects revealed that sPEEK/pZIF8-DFO scaffolds resulted in a greater volume of new bone formation than on sPEEK and sPEEK/pZIF8 scaffolds, with partial bone connection to the sPEEK/pZIF8-DFO scaffolds. In summary, we develop a novel PEEK scaffold that combines enhanced angiogenesis, stem cell recruitment, immunomodulation, and osteogenic differentiation, showcasing its promising potential for orbital bone reconstruction.

Innate immune response to bone fracture healing

The field of osteoimmunology has primarily focused on fracture healing in isolated musculoskeletal injuries. The innate immune system is the initial response to fracture, with inflammatory macrophages, cytokines, and neutrophils arriving first at the fracture hematoma, followed by an anti-inflammatory phase to begin the process of new bone formation. This review aims to first discuss the current literature and knowledge gaps on the immune responses governing single fracture healing by encompassing the individual role of macrophages, neutrophils, cytokines, mesenchymal stem cells, bone cells, and other immune cells. This paper discusses the interactive effects of these cellular responses underscoring the field of osteoimmunology. The critical role of the metabolic environment in guiding the immune system properties will be highlighted along with some effective therapeutics for fracture healing in the context of osteoimmunology. However, compared to isolated fractures, which frequently heal well, long bone fractures in over 30 % of polytrauma patients exhibit impaired healing. Clinical evidence suggests there may be distinct physiologic and inflammatory pathways altered in polytrauma resulting in nonunion. Nonunion is associated with worse patient outcomes and increased societal healthcare costs. The dysregulated immunomodulatory/inflammatory response seen in polytrauma may lead to this increased nonunion rate. This paper will investigate the differences in immune response between isolated and polytrauma fractures. Finally, future directions for fracture studies are explored with consideration of the emerging roles of newly discovered immune cell functions in fracture healing, the existing challenges and conflicting results in the field, the translational potential of these studies in clinic, and the more complex nature of polytrauma fractures that can alter cell functions in different tissues.

3D cryo-printed hierarchical porous scaffolds provide immobilization of surface-functionalized sleep-inspired small extracellular vesicles: synergistic therapeutic strategies for vascularized bone regeneration based on macrophage phenotype modulation and angiogenesis-osteogenesis coupling

Bone defect healing is a multi-factorial process involving the inflammatory microenvironment, bone regeneration and the formation of blood vessels, and remains a great challenge in clinical practice. Combined use of three-dimensional (3D)-printed scaffolds and bioactive factors is an emerging strategy for the treatment of bone defects. Scaffolds can be printed using 3D cryogenic printing technology to create a microarchitecture similar to trabecular bone. Melatonin (MT) has attracted attention in recent years as an excellent factor for promoting cell viability and tissue repair. In this study, porous scaffolds were prepared by cryogenic printing with poly(lactic-co-glycolic acid) and ultralong hydroxyapatite nanowires. The hierarchical pore size distribution of the scaffolds was evaluated by scanning electron microscopy (SEM) and micro-computed tomography (micro-CT). Sleep-inspired small extracellular vesicles (MT-sEVs) were then obtained from MT-stimulated cells and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-poly(ethylene glycol)-inorganic pyrophosphate (DSPE-PEG-PPi) was used to modify the membrane of MT-sEVs to obtain PPi-MT-sEVs. RNA sequencing was performed to explore the potential mechanisms. The results demonstrated that PPi-MT-sEVs not only enhanced cell proliferation, migration and angiogenesis, but also regulated the osteogenic/adipogenic fate determination and M1/M2 macrophage polarization switch in vitro. PPi-MT-sEVs were used to coat scaffolds, enabled by the capacity of PPi to bind to hydroxyapatite, and computational simulations were used to analyze the interfacial bonding of PPi and hydroxyapatite. The macrophage phenotype-modulating and osteogenesis-angiogenesis coupling effects were evaluated in vivo. In summary, this study suggests that the combination of hierarchical porous scaffolds and PPi-MT-sEVs could be a promising candidate for the clinical treatment of bone defects.

Macrophages at Low-Inflammatory Status Improved Osteogenesis via Autophagy Regulation

Accumulating evidence indicates that the interaction between immune and skeletal systems is vital in bone homeostasis. However, the detailed mechanisms between macrophage polarization and osteogenic differentiation of mesenchymal stromal cells (bone marrow-derived stromal cells [BMSCs]) remain largely unknown. We observed enhanced macrophage infiltration along with bone formation in vivo, which showed a transition from early-stage M1 phenotype to later stage M2 phenotype, cells at the transitional stage expressed both M1 and M2 markers that actively participated in osteogenesis, which was mimicked by stimulating macrophages with lower inflammatory stimulus (compared with typical M1). Using conditioned medium (CM) from M0, typical M1, low-inflammatory M1 (M1semi), and M2 macrophages, it was found that BMSCs treated with M1semi CM showed significantly induced migration, osteogenic differentiation, and mineralization, compared with others. Along with the induced osteogenesis, the autophagy level was the highest in M1semi CM-treated BMSCs, which was responsible for BMSC migration and osteogenic differentiation, as autophagy interruption significantly abolished this effect. This study indicated that low-inflammatory macrophages could activate autophagy in BMSCs to improve osteogenesis.

CCL2/CCR2 Signalling in Mesenchymal Stem/Progenitor Cell Recruitment and Fracture Healing in Mice

Macrophage efferocytosis (clearance of apoptotic cells) is crucial for tissue homeostasis and wound repair, where macrophages secrete factors that promote resolution of inflammation and regenerative signalling. This study examined the role of efferocytic macrophage-associated CCL2 secretion, its influence on mesenchymal stem/progenitor cell (MSPC) chemotaxis, and in vivo cell recruitment using Ccr2-/- (KO) mice with disrupted CCL2 receptor signalling in two regenerative models: ossicle implants and ulnar stress fractures. Single cell RNA sequencing and PCR validation indicated that efferocytosis of various apoptotic cells at bone injury sites (osteoblasts, pre-osteoblasts, MSPC) upregulated CCL2. CCL2 gradients enhanced MSPC migration through type I collagen matrices. In vivo, MSPC (LepR+) infiltration was significantly reduced while macrophage (F4/80+) infiltration increased in KO ossicle implants versus WT. In ulnar stress fractures, micro-CT revealed increased mineralized callus incidence in CCR2 KO male mice 5 days post injury (dpi) versus WT. By 7-dpi callus fractional bone volume, trabecular thickness, and bone mineral density were increased versus WT. Immunohistochemistry of mice 5-dpi confirmed an increase in callus area (including soft tissue); however, the percent of osteoprogenitors (%Osx+) within the callus was not different. These findings suggest that CCL2 differentially impacts MSPC recruitment depending on bone wound healing model.

The Immune-Centric Revolution Translated into Clinical Application: Peripheral Blood Mononuclear Cell (PBMNC) Therapy in Diabetic Patients with No-Option Critical Limb-Threatening Ischemia (NO-CLTI)-Rationale and Meta-Analysis of Observational Studies

Chronic limb-threatening ischemia (CLTI), the most advanced form of peripheral arterial disease (PAD), is the comorbidity primarily responsible for major lower-limb amputations, particularly for diabetic patients. Autologous cell therapy has been the focus of efforts over the past 20 years to create non-interventional therapeutic options for no-option CLTI to improve limb perfusion and wound healing. Among the different available techniques, peripheral blood mononuclear cells (PBMNC) appear to be the most promising autologous cell therapy due to physio-pathological considerations and clinical evidence, which will be discussed in this review. A meta-analysis of six clinical studies, including 256 diabetic patients treated with naive, fresh PBMNC produced via a selective filtration point-of-care device, was conducted. PBMNC was associated with a mean yearly amputation rate of 15.7%, a mean healing rate of 62%, and a time to healing of 208.6 ± 136.5 days. Moreover, an increase in TcPO2 and a reduction in pain were observed. All-cause mortality, with a mean rate of 22.2% and a yearly mortality rate of 18.8%, was reported. No serious adverse events were reported. Finally, some practical and financial considerations are provided, which point to the therapy's recommendation as the first line of treatment for this particular and crucial patient group.

Zfp260 choreographs the early stage osteo-lineage commitment of skeletal stem cells

The initial fine-tuning processes are crucial for successful bone regeneration, as they guide skeletal stem cells through progenitor differentiation toward osteo- or chondrogenic fate. While fate determination processes are well-documented, the mechanisms preceding progenitor commitment remain poorly understood. Here, we identified a transcription factor, Zfp260, as pivotal for stem cell maturation into progenitors and directing osteogenic differentiation. Zfp260 is markedly up-regulated as cells transition from stem to progenitor stages; its dysfunction causes lineage arrest at the progenitor stage, impairing bone repair. Zfp260 is required for maintaining chromatin accessibility and regulates Runx2 expression by forming super-enhancer complexes. Furthermore, the PKCα kinase phosphorylates Zfp260 at residues Y173, S182, and S197, which are essential for its functional activity. Mutations at these residues significantly impair its functionality. These findings position Zfp260 as a vital factor bridging stem cell activation with progenitor cell fate determination, unveiling a element fundamental to successful bone regeneration.

Hippo Signaling Pathway Involvement in Osteopotential Regulation of Murine Bone Marrow Cells Under Simulated Microgravity

The development of osteopenia is one of the most noticeable manifestations of the adverse effects of space factors on crew members. The Hippo signaling pathway has been shown to play a central role in regulating the functional activity of cells through their response to mechanical stimuli. In the present study, the components of the Hippo pathway and the protective properties of osteodifferentiation inducers were investigated under simulated microgravity (smg) using a heterotypic bone marrow cell culture model, which allows for the maintenance of the close interaction between the stromal and hematopoietic compartments, present in vivo and of great importance for both the fate of osteoprogenitors and hematopoiesis. After 14 days of smg, the osteopotential and osteodifferentiation of bone marrow stromal progenitor cells, the expression of Hippo cascade genes and the immunocytochemical status of the adherent fraction of bone marrow cells, as well as the paracrine profile in the conditioned medium and the localization of Yap1 and Runx2 in mechanosensitive cells of the bone marrow were obtained. Simulated microgravity negatively affects stromal and hematopoietic cells when interacting in a heterotypic murine bone marrow cell culture. This is evidenced by the decrease in cell proliferation and osteopotential. Changes in the production of pleiotropic cytokines IL-6, GROβ and MCP-1 were revealed. Fourteen days of simulated microgravity induced a decrease in the nuclear translocation of Yap1 and the transcription factor Runx2 in the stromal cells of the intact group. Exposure to osteogenic induction conditions partially compensated for the negative effect of simulated microgravity. The data obtained will be crucial for understanding the effects of spaceflight on osteoprogenitor cell growth and differentiation via Hippo-Yap signaling.

A Janus Microsphere Delivery System Orchestrates Immunomodulation and Osteoinduction by Fine-tuning Release Profiles

Bone regeneration is a well-orchestrated process synergistically involving inflammation, angiogenesis, and osteogenesis. Therefore, an effective bone graft should be designed to target multiple molecular events and biological demands during the bone healing process. In this study, a biodegradable gelatin methacryloyl (GelMA)-based Janus microsphere delivery system containing calcium phosphate oligomer (CPO) and bone morphogenetic protein-2 (BMP-2) is developed based on natural biological events. The exceptional adjustability of GelMA facilitates the controlled release and on-demand application of biomolecules, and optimized delivery profiles of CPO and BMP-2 are explored. The sustained release of CPO during the initial healing stages contributes to early immunomodulation and promotes mineralization in the late stage. Meanwhile, the administration of BMP-2 at a relatively high concentration within the therapeutic range enhances the osteoinductive property. This delivery system, with fine-tuned release patterns, induces M2 macrophage polarization and creates a conducive immuno-microenvironment, which in turn facilitates effective bone regeneration in vivo. Collectively, this study proposes a bottom-up concept, aiming to develop a user-friendly and easily controlled delivery system targeting individual biological events, which may offer a new perspective on developing function-optimized biomaterials for clinical use.

An immunomodulatory and osteogenic bacterial cellulose scaffold for bone regeneration via regulating the immune microenvironment

Creating a bone homeostasis microenvironment that balances osteogenesis and immunity is a substantial challenge for bone regeneration. Here, we prepared an immunomodulatory and osteogenic bacterial cellulose scaffold (FOBS) via a facile one-pot approach. The aldehyde groups were generated via selective oxidation of the hydroxyl groups of bacterial cellulose, offering the bonding sites for dopamine through a Schiff base reaction. At the same time, the deposition of Ca2+ and PO43- was promoted on the aldehyde cellulose scaffold because of the high affinity of the catechol moiety for Ca2+. Compared with that of the unmodified scaffold, the hydroxyapatite content of FOBS increased by 47.1 % according to the ICP results. Interestingly, FOBS regulated the immune microenvironment to accelerate the conversion of M1 to M2 macrophages. The expressions of ARG-1 and Dectin-1 (M2) in the FOBS group increased by >100 %. The expression of osteogenic differentiation of BMSCs was also upregulated. In a rat cranial defect model, the BV/TV of FOBS was significantly increased. Further immunohistochemical analysis revealed that an improved immune microenvironment promoted the osteogenic differentiation of stem cells in vivo. This work provides an effective and easy-to-operate strategy for the development of the bone tissue engineering scaffolds.

Increased neutrophil extracellular traps caused by diet-induced obesity delay fracture healing

Obesity, recognized as a risk factor for nonunion, detrimentally impacts bone health, with significant physical and economic repercussions for affected individuals. Nevertheless, the precise pathomechanisms by which obesity impairs fracture healing remain insufficiently understood. Multiple studies have identified neutrophil granulocytes as key players in the systemic immune response, being the predominant immune cells in early fracture hematomas. This study identified a previously unreported critical period for neutrophil infiltration into the callus. In vivo experiments demonstrated that diet-induced obesity (DIO) mice showed earlier neutrophil infiltration, along with increased formation of neutrophil extracellular traps (NETs), compared to control mice during the endochondral phase of fracture repair. Furthermore, Padi4 knockout was found to reduce NET formation and mitigate the fracture healing delays caused by high-fat diets. Mechanistically, in vitro analyses revealed that NETs, by activating NLRP3 inflammasomes, inhibited the osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) and concurrently promoted M1-like macrophage polarization. These findings establish a connection between NET formation during the endochondral phase and delayed fracture healing, suggesting that targeting NETs could serve as a promising therapeutic approach for addressing obesity-induced delays in fracture recovery.

Macrophage-derived amphiregulin promoted the osteogenic differentiation of chondrocytes through EGFR/Yap axis and TGF-β activation

Endochondral ossification represents a crucial biological process in skeletal development and bone defect repair. Macrophages, recognized as key players in the immune system, are now acknowledged for their substantial role in promoting endochondral ossification within cartilage. Concurrently, the epidermal growth factor receptor (EGFR) ligand amphiregulin (Areg) has been documented for its contributory role in restoring bone tissue homeostasis post-injury. However, the mechanism by which macrophage-secreted Areg facilitates bone repair remains elusive. In this study, the induction of macrophage depletion through in vivo administration of clodronate liposomes was employed in a standard open tibial fracture mouse model to assess bone healing using micro-computed tomography (micro-CT) analysis, histomorphology, and ELISA serum evaluations. The investigation revealed sustained expression of Areg during the fracture healing period in wild-type mice. Macrophage depletion significantly reduced the number of macrophages on the local bone surface and vital organs. This reduction led to diminished Areg secretion, decreased collagen production, and delayed fracture healing. However, histological and micro-CT assessments at 7 and 21 days post-local Areg treatment exhibited a marked improvement of bone healing compared to the vehicle control. In vitro studies demonstrated an increase of Areg secretion by the Raw264.7 cells upon ATP stimulation. Indirect co-culture of Raw264.7 and ATDC5 cells indicated that Areg overexpression enhanced the osteogenic potential of chondrocytes, and vice versa. This osteogenic promotion was attributed to Areg's activation of the membrane receptor EGFR in the ATDC5 cell line, the enhanced phosphorylation of transcription factor Yap, and the facilitation of the expression of bioactive TGF-β by chondrocytes. Collectively, this research elucidates the direct mechanistic effects of macrophage-secreted Areg in promoting bone homeostasis following bone injury.

Relative contributions of osteal macrophages and osteoclasts to postnatal bone development in CSF1R-deficient rats and phenotype rescue following wild-type bone marrow cell transfer

Macrophage and osteoclast proliferation, differentiation and survival are regulated by colony-stimulating factor 1 receptor (CSF1R) signaling. Osteopetrosis associated with Csf1 and Csf1r mutations has been attributed to the loss of osteoclasts and deficiency in bone resorption. Here, we demonstrate that homozygous Csf1r mutation in rat leads to delayed postnatal skeletal ossification associated with substantial loss of osteal macrophages in addition to osteoclasts. Osteosclerosis and site-specific skeletal abnormalities were reversed by intraperitoneal transfer of wild-type bone marrow cells (bone marrow cell transfer, BMT) at weaning. Following BMT, IBA1+ macrophages were detected before TRAP+ osteoclasts at sites of ossification restoration. These observations extend evidence that osteal macrophages independently contribute to bone anabolism and are required for normal postnatal bone growth and morphogenesis.

M2 macrophage-derived exosomes for bone regeneration: A systematic review

OBJECTIVE

This systematic review aims to evaluate existing evidence to investigate the therapeutic efficacy of M2 macrophage-derived exosomes in bone regeneration.

DESIGN

A comprehensive search between 2020 and 2024 across PubMed, Web of Science, and Scopus was conducted using a defined search strategy to identify relevant studies regarding the following question: "What is the impact of M2 macrophage-derived exosomes on bone regeneration?". Controlled in vitro and in vivo studies were included in this study. The SYRCLE tool was used to evaluate the risk of bias in the included animal studies.

RESULTS

This review included 20 studies published. Seven studies were selected for only in vitro analysis, whereas 13 studies underwent both in vitro and in vivo analyses. The in vivo studies employed animal models, including 163 C57BL6 mice and 73 Sprague-Dawley rats. Exosomes derived from M2 macrophages were discovered to be efficacious in promoting bone regeneration and vascularization in animal models of bone defects. These effects were primarily confirmed through morphological and histological assessments. This remarkable outcome is attributed to the regulation of multiple signaling pathways, as evidenced by the findings of 11 studies investigating the involvement of miRNAs in this intricate process. In addition, in vitro studies observed positive effects on cell proliferation, migration, osteogenesis, and angiogenesis. Heterogeneity in study methods hinders direct comparison of results across studies.

CONCLUSION

M2 macrophage-derived exosomes demonstrate remarkable potential for promoting bone regeneration. Further research optimizing their application and elucidating the underlying mechanisms can pave the way for clinical translation.

The Advantages and Shortcomings of Stem Cell Therapy for Enhanced Bone Healing

This review explores the regenerative potential of key progenitor cell types and therapeutic strategies to improve healing of complex fractures and bone defects. We define, summarize, and discuss the differentiation potential of totipotent, pluripotent, and multipotent stem cells, emphasizing the advantages and shortcomings of cell therapy for bone repair and regeneration. The fundamental role of mesenchymal stem cells is highlighted due to their multipotency to differentiate into the key lineage cells including osteoblasts, osteocytes, and chondrocytes, which are crucial for bone formation and remodeling. Hematopoietic stem cells (HSCs) also play a significant role; immune cells such as macrophages and T-cells modulate inflammation and tissue repair. Osteoclasts are multinucleated cells that are important to bone remodeling. Vascular progenitor (VP) cells are critical to oxygen and nutrient supply. The dynamic interplay among these lineages and their microenvironment is essential for effective bone restoration. Therapies involving cells that are more than "minimally manipulated" are controversial and include embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). ESCs, derived from early-stage embryos, possess pluripotent capabilities and have shown promise in preclinical studies for bone healing. iPSCs, reprogrammed from somatic cells, offer personalized medicine applications and can differentiate into various tissue-specific cell lines. Minimally manipulative cell therapy approaches such as the use of bone marrow aspirate concentrate (BMAC), exosomes, and various biomaterials for local delivery are explored for their effectiveness in bone regeneration. BMAC, which contains mostly immune cells but few mesenchymal and VPs, probably improves bone healing by facilitating paracrine-mediated intercellular communication. Exosome isolation harnesses the biological signals and cellular by-products that are a primary source for cell crosstalk and activation. Safe, efficacious, and cost-effective strategies to enhance bone healing using novel cellular therapies are part of a changing paradigm to modulate the inflammatory, repair, and regenerative pathways to achieve earlier more robust tissue healing and improved physical function. Impact Statement Stem cell therapy holds immense potential for bone healing due to its ability to regenerate damaged tissue. Nonmanipulated bone marrow aspirate contains mesenchymal stem cells that promote bone repair and reduce healing time. Induced pluripotent stem cells offer the advantage of creating patient-specific cells that can differentiate into osteoblasts, aiding in bone regeneration. Other delivery methods, such as scaffold-based techniques, enhance stem cell integration and function. Collectively, these approaches can improve treatment outcomes, reduce recovery periods, and advance our understanding of bone healing mechanisms, making them pivotal in orthopedic research and regenerative medicine.

The cell membrane as biofunctional material for accelerated bone repair

The cell (plasma) membrane is enriched with numerous receptors, ligands, enzymes, and phospholipids that play important roles in cell-cell and cell-extracellular matrix interactions governing, for instance, tissue development and repair. We previously showed that plasma membrane nanofragments (PMNFs) act as nucleation sites for bone formation in vivo, and induce in vitro mineralization within 1 day. In this study, we optimized the methods for generating, isolating, and applying PMNFs as a cell-free therapeutic to expedite bone defect repair. The PMNFs were isolated from different mouse cell lines (chondrocytes, osteoblasts, and fibroblasts), pre-conditioned, lyophilized, and subsequently transplanted into 2 mm critical-sized calvarial defects in mice (n = 75). The PMNFs from chondrocytes, following a 3-day pre-incubation, significantly accelerated bone repair within 2 weeks, through a coordinated attraction of macrophages, endothelial cells, and osteoblasts to the healing site. In vitro experiments confirmed that PMNFs enhanced cell adhesion. Comparison of the PMNF efficacy with phosphatidylserine, amorphous calcium phosphate (ACP), and living cells confirmed the unique ability of PMNFs to promote accelerated bone repair. Importantly, PMNFs promoted nearly complete integration of the regenerated bone with native tissue after 6 weeks (% non-integrated bone area = 15.02), contrasting with the partial integration (% non-integrated bone area = 56.10; p < 0.01, Student's test) with transplantation of ACP. Vickers microhardness tests demonstrated that the regenerated bone after 6 weeks (30.10 ± 1.75) exhibited hardness similar to native bone (31.07 ± 2.46). In conclusion, this is the first study to demonstrate that cell membrane can be a promising cell-free material with multifaceted biofunctional properties that promote accelerated bone repair. STATEMENT OF SIGNIFICANCE.

Endochondral Ossification for Spinal Fusion: A Novel Perspective from Biological Mechanisms to Clinical Applications

Degenerative scoliosis (DS), encompassing conditions like spondylolisthesis and spinal stenosis, is a common type of spinal deformity. Lumbar interbody fusion (LIF) stands as a conventional surgical intervention for this ailment, aiming at decompression, restoration of intervertebral height, and stabilization of motion segments. Despite its widespread use, the precise mechanism underlying spinal fusion remains elusive. In this review, our focus lies on endochondral ossification for spinal fusion, a process involving vertebral development and bone healing. Endochondral ossification is the key step for the successful vertebral fusion. Endochondral ossification can persist in hypoxic conditions and promote the parallel development of angiogenesis and osteogenesis, which corresponds to the fusion process of new bone formation in the hypoxic region between the vertebrae. The ideal material for interbody fusion cages should have the following characteristics: (1) Good biocompatibility; (2) Stable chemical properties; (3) Biomechanical properties similar to bone tissue; (4) Promotion of bone fusion; (5) Favorable for imaging observation; (6) Biodegradability. Utilizing cartilage-derived bone-like constructs holds promise in promoting bony fusion post-operation, thus warranting exploration in the context of spinal fusion procedures.

Engineering a microparticle-loaded rough membrane for guided bone regeneration modulating osteoblast response without inducing inflammation

Guided bone regeneration (GBR) is a widely used procedure that prevents the fast in-growth of soft tissues into bone defect. Among the different types of membranes, the use of collagen membranes is the gold standard. However, these membranes are implanted in tissue location where a severe acute inflammation will occur and can be negatively affected. The aim of this study was to develop a collagen-based membrane for GBR that incorporated alginate-hydroxyapatite microparticles. Membranes were manufactured using collagen type I and gelatin and alginate-hydroxyapatite microparticles. Membranes were assessed in terms of topography by scanning electron microscopy and confocal microscopy; stability by swelling after an overnight incubation in saline and enzymatic degradation against collagenase and mechanical properties by tensile tests. Furthermore, the biological response was assessed with SaOs-2 cells and THP-1 macrophages to determine alkaline phosphatase activity and inflammatory cytokine release. Our results showed that the incorporation of different percentages of these microparticles could induce changes in the surface topography. When the biological response was analyzed, either membranes were not cytotoxic to THP-1 macrophages or to SaOs-2 cells and they did not induce the release of pro-inflammatory cytokines. However, the different surface topographies did not induce changes in the macrophage morphology and the release of pro- and anti-inflammatory cytokines, suggesting that the effect of surface roughness on macrophage behavior could be dependent on other factors such as substrate stiffness and composition. Collagen-gelatin membranes with embedded alginate-hydroxyapatite microparticles increased ALP activity, suggesting a positive effect of them on bone regeneration, remaining unaffected the release of pro- and anti-inflammatory cytokines.

Harnessing Engineered Extracellular Vesicles from Mesenchymal Stem Cells as Therapeutic Scaffolds for Bone‐Related Diseases

Mesenchymal stem cells (MSCs) play a crucial role in maintaining bone homeostasis and are extensively explored for cell therapy in various bone‐related diseases. In addition to direct cell therapy, the secretion of extracellular vesicles (EVs) by MSCs has emerged as a promising alternative approach. MSC‐derived EVs (MSC‐EVs) offer equivalent therapeutic efficacy to MSCs while mitigating potential risks. These EVs possess unique properties that enable them to traverse biological barriers and deliver bioactive cargos to target cells. Furthermore, by employing modification and engineering strategies, the therapeutic effects and tissue targeting specificity of MSC‐EVs can be further enhanced to meet specific therapeutic needs. In this review, the mechanisms and advantages of MSC‐EV therapy in diseased bone tissues are highlighted. Through simple isolation and modification techniques, MSC‐EV‐based biomaterials have demonstrated great promise for bone regeneration. Finally, future perspectives on MSC‐EV therapy are presented, envisioning the development of next‐generation regenerative materials and bioactive agents for clinical translation in the field of bone regeneration.