N2-Polarized Neutrophils Guide Bone Mesenchymal Stem Cell Recruitment and Initiate Bone Regeneration: A Missing Piece of the Bone Regeneration Puzzle

Regulation of osteoimmune microenvironment via functional dynamic hydrogel for diabetic bone regeneration

Bone regeneration and repair face formidable challenges under diabetic conditions, primarily due to the disruption of macrophage polarization induced by diabetes and the inflammatory imbalance within the bone microenvironment. We have developed a novel dynamic hydrogel system (AG-CD@LINA), constructed through the coordination crosslinking of thiolated gelatin (SH-Gelatin) and gold ions (Au3+), followed by grafting with cyclodextrin to load the ligand linagliptin. This hydrogel effectively inhibits the formation of M1 macrophages and the expression of pro-inflammatory cytokines by gradually releasing linagliptin. Simultaneously, it promotes the formation of M2 macrophages and the expression of anti-inflammatory cytokines, thus improving the inflammatory microenvironment of diabetic bone defects. Consequently, it facilitates the migration of mesenchymal stem cells and angiogenic cells, augments osteogenic activity, and promotes vascularization, collectively accelerating the regeneration of diabetic bone tissue. Mechanistically, polarization occurs through the TLR3-NF-κB signaling pathway. In vivo experiments demonstrate that the in-situ injection of the hydrogel enhances the regeneration of bone tissue and the restoration of bone structure in diabetic bone defects, effectively modulating local inflammation and promoting vascular formation. This study suggests that functionalized dynamic hydrogels can improve the inflammatory microenvironment by regulating in situ macrophage polarization, thereby facilitating the reconstruction of bone microstructure. This approach represents a promising novel therapeutic strategy for diabetic bone defects.

Exosome-capturing scaffold promotes endogenous bone regeneration through neutrophil-derived exosomes by enhancing fast vascularization

Exosomes (Exos), extracellular vesicles of endosomal origin, are a promising therapeutic platform for tissue regeneration. In the current study, an exosome-capturing scaffold (ECS) was designed to attract and anchor exosomes via electrostatic adherence followed by lipophilic interactions. Our findings demonstrate that local enrichment of exosomes in the ECS implanted into critical mandibular defects could significantly accelerate endogenous bone regeneration by enhancing vascularization at the defect site. Notably, neutrophil (PMN)-derived exosomes (PMN-Exos) were identified as the predominant exosome subtype among all captured exosomes. During endogenous bone regeneration, PMN-Exos promoted endogenous vascularization primarily by stimulating the proliferation of endothelial progenitor cells (EPCs), which play a pivotal role in the vasculogenesis of new blood vessels. Mechanistically, vascularization involved PMN-Exo-derived miR455-3p, which promotes EPC proliferation by targeting the Smad4 pathway. In conclusion, this study offers an ECS with broad application prospects for enhancing tissue regeneration by accelerating vascularization. The elucidation of underlying mechanisms paves the way for developing novel strategies to regenerate various tissues and organs.

Revealing the molecular mechanism of Buzhong Yiqi Decoction for tendon bone healing on the basis of network pharmacology, molecular docking and experimental validation

ETHNOPHARMACOLOGICAL RELEVANCE

Buzhong Yiqi Decoction (BD), a traditional formula in Chinese medicine, is clinically and historically recognized for its effectiveness in reducing physical fatigue and promoting strength, as well as enhancing bone remodeling. Nevertheless, its specific molecular mechanisms related to bone formation have yet to be thoroughly characterized.

AIM OF THE STUDY

This study aims to investigate the effects and mechanisms of BD on osteogenesis in bone marrow mesenchymal stem cells and in a model of tendon-bone junction injury in mice.

MATERIALS AND METHODS

By establishing a mouse model of tendon-bone junction injury, the pathological morphology of the tendon-bone junction in mice was observed. Determining the mechanism of action of BD in regulating osteogenic differentiation through network pharmacology and molecular docking. Flow analysis and osteogenic induction assay were utilized to verify the effect of BD in promoting BMSCs osteogenic differentiation in vitro. In vivo experiments were performed to validate the impact of BD in improving healing after tendon-bone junction injury in mice by promoting osteogenic differentiation.

RESULTS

Bone loss at the heel bone end is an essential pathophysiologic process in the natural healing process after injury to the tendon-bone junction. Using network pharmacology and molecular docking, we identified the PI3K-Akt signaling pathway as a critical mediator of BD-induced osteogenic differentiation. In vitro experiments demonstrated that BD promoted BMSC osteogenesis, while in vivo assays confirmed the enhancement of tendon-bone healing in mice models.

CONCLUSION

These results suggest that BD can effectively promote tendon-bone repair, with the PI3K-Akt pathway playing a crucial role in its therapeutic effects, positioning BD as a promising candidate for improving musculoskeletal injury recovery.

COX-2 inhibition as a therapeutic strategy for bone loss in Staphylococcus aureus osteomyelitis

Bone loss in Staphylococcus aureus (S. aureus) osteomyelitis poses a serious challenge to orthopedic treatment, but the underlying mechanism of systemic osteoporosis caused by chronic infection is not completely clear. In this study, γ-irradiation-killed S. aureus (IKSA) was applied to simulate the inflammation and explore the mechanism of systemic bone loss caused by it. In this study, we found that the systemic application of IKSA caused bone loss in mice through increasing osteoclasts and decreasing osteoblasts. An immune response profile with up-regulated COX-2 is identified based on our transcriptional data from IKSA mice bone marrow cells. COX-2 expression is widely up-regulated in bone marrow immune cells, such as myeloid-derived suppressor cells (MDSCs), neutrophils and macrophages in the IKSA-treated mice. Mechanistically, COX-2 stimulated the increasing proportion of MDSCs and neutrophils and the inflammatory response of the bone marrow immune cells, that may regulate bone metabolism. Importantly, COX-2 inhibitor, celecoxib could rescue the bone loss induced by IKSA, which may reason from decrease of inflammatory gene expression in MDSCs, neutrophils and macrophages. Excitingly, COX-2 expression is also increased in bone marrow from mice and patients with S. aureus osteomyelitis. These findings suggested a therapeutic potential for inhibiting COX-2 in combating bone loss in S. aureus osteomyelitis.

Functionalized Biomimetic Nanoparticles Targeting the IL-10/IL-10Rα/Glycolytic Axis in Synovial Macrophages Alleviate Cartilage Degeneration in Osteoarthritis

Osteoarthritis (OA) is a low-grade inflammatory disease that is highly associated with severe hyperplasia of the synovial membrane and the degeneration of cartilage. Interleukin-10 (IL-10), has been extensively studied, while its receptor, IL-10Rα, has not been widely mentioned in the context of OA. A significant difference is found in the expression of IL-10Rα in synovial macrophages from normal and OA patients, along with a marked increase in the glycolytic activity of synovial macrophages. In IL-10RαLysm OA mice, the specific deficiency of IL-10Rα exacerbated the progression of OA. Mechanistically, hypoxia-inducible factor-1α (HIF-1α) is identified as a key transcription factor, and its inhibition significantly weakened the glycolytic process. Additionally, differences in ferroptosis of chondrocytes are observed. After co-culturing the two types of cells in vitro, a significant connection is found between the glycolytic state of synovial macrophages and the ferroptosis of chondrocytes. To achieve targeted therapy, MI@UN, a biomimetic nanoparticle encapsulating NO-prednisolone in UIO-66-NH2, surface-modified with IL-10, and coated with macrophage membranes (MM), is developed. It significantly slows osteoarthritis progression in mice. This offers new insights into OA pathogenesis, highlighting IL-10Rα as a therapeutic target and supporting MI@UN's translational use for OA treatment.

Neutrophils in Tissue Injury and Repair: Molecular Mechanisms and Therapeutic Targets

Tissue repair represents a highly intricate and ordered dynamic process, critically reliant on the orchestration of immune cells. Among these, neutrophils, the most abundant leukocytes in the body, emerge as the initial immune responders at injury sites. Traditionally recognized for their antimicrobial functions in innate immunity, neutrophils now garner attention for their indispensable roles in tissue repair. This review delves into their novel functions during the early stages of tissue injury. We elucidate the mechanisms underlying neutrophil recruitment and activation following tissue damage and explore their contributions to vascular network formation. Furthermore, we investigate the pivotal role of neutrophils during the initial phase of repair across different tissue types. Of particular interest is the investigation into how the fate of neutrophils influences overall tissue healing outcomes. By shedding light on these emerging aspects of neutrophil function in tissue repair, this review aims to pave the way for novel strategies and approaches in future organ defect repair, regeneration studies, and advancements in tissue engineering. The insights provided here have the potential to significantly impact the field of tissue repair and regeneration.

Ganoderma lucidum spore powder alleviates rheumatoid arthritis-associated pain hypersensitivity through inhibiting accumulation, N1 polarization, and ROS production of neutrophils in mice

Introduction

Rheumatoid arthritis (RA) is a chronic condition characterized by joint pain that significantly impairs patients' work and daily lives. The limited understanding of the pathological mechanisms underlying RA-related pain poses challenges for effective clinical pain management. Ganoderma lucidum spore powder (GLSP) has demonstrated therapeutic benefits in various diseases, with no reported toxicity or adverse effects.

Methods

This study investigates the role of neutrophils in the pathological mechanisms of RA-related pain using collagen-induced arthritis (CIA) mice and an ex vivo neutrophil model. A combination of techniques, including animal models, flow cytometry, behavioral testing, cell adoptive transfer, and network pharmacology analysis, was employed to evaluate the effects and targets of GLSP on pain symptoms and neutrophil activity in CIA mice.

Results

Flow cytometric analysis revealed the accumulation and activation of neutrophils in the paws of CIA mice. Furthermore, the levels of pro-inflammatory CD95+ neutrophil subpopulations (N1 state) and ROS+ cells in the affected paws were positively correlated with the severity of mechanical allodynia and heat hyperalgesia observed in these mice. Our findings indicate that oral administration of GLSP significantly alleviates joint destruction, paw swelling, and pain hypersensitivity in CIA mice. Notably, GLSP reversed CIA-induced neutrophil accumulation, N1 polarization, and reactive oxygen species (ROS) production. Both network pharmacology target prediction and in vivo/in vitro experimental validation indicated that GLSP inhibits N1 polarization and ROS production in neutrophils by modulating the TNF-α signaling pathway, thus exerting RA-specific analgesic effects.

Discussion

In summary, this study offers new insights into the pathological mechanisms of RA-related pain and demonstrates that neutrophil accumulation, N1 polarization, and ROS production contribute to RA-related pain. GLSP alleviates RA-related pain by inhibiting the pro-inflammatory phenotype of neutrophils, highlighting its potential for clinical translation in the treatment of RA.

M2 Macrophage-Derived Extracellular Vesicles Reprogram Immature Neutrophils into Anxa1hi Neutrophils to Enhance Inflamed Bone Regeneration

Periodontitis is a microbiome-related inflammation that can lead to irreversible bone reduction and even tooth loss. This study reveals that macrophage polarization states significantly influence periodontal homeostasis, with M2 macrophage-derived extracellular vesicles (M2-EVs) playing a pivotal role in mitigating periodontitis-induced bone loss. Single-cell RNA sequencing of periodontal tissues treated with M2-EVs uncovered a unique Anxa1hi neutrophil subpopulation exhibiting pro-reparative properties. This subpopulation is characterized by immaturity and demonstrated osteogenic and angiogenic capabilities in vivo, partially mediated through the secretion of oncostatin M (OSM) signals. The findings suggest that this functional heterogeneity arises from M2-EVs disrupting the neutrophil maturation trajectory, with pivotal reprogramming genes, such as Acvrl1 and Fpr2, driving the differentiation of the Anxa1hi reparative subpopulation. This work underscores the potential of targeting M2 macrophage-neutrophil interactions to promote the regeneration of inflamed bone tissues.

The Nexus Between Traditional Chinese Medicine and Immunoporosis: Implications in the Treatment and Management of Osteoporosis

Osteoporosis (OP) is a globally prevalent bone disease characterized by reduced bone mass and heightened fracture risk, posing a significant health and economic challenge to aging societies worldwide. Osteoimmunology-an emerging field of study-investigates the intricate relationship between the skeletal and the immune systems, providing insights into the immune system's impact on bone health and disease progression. Recent research has demonstrated the essential roles played by various immune cells (T cells, B cells, macrophages, dendritic cells, mast cells, granulocytes, and innate lymphoid cells) in regulating bone metabolism, homeostasis, formation, and remodeling through interactions with osteoclasts (OC) and osteoblasts (OB). These findings underscore that osteoimmunology provides an essential theoretical framework for understanding the pathogenesis of various skeletal disorders, including OP. Traditional Chinese medicine (TCM) and its active ingredients have significant clinical value in OP treatment. Unfortunately, despite their striking multieffect pathways in the pharmacological field, current research has not yet summarized them in a comprehensive and detailed manner with respect to their interventional roles in immune bone diseases, especially OP. Consequently, this review addresses recent studies on the mechanisms by which immune cells and their communication molecules contribute to OP development. Additionally, it explores the potential therapeutic benefits of TCM and its active components in treating OP from the perspective of osteoimmunology. The objective is to provide a comprehensive framework that enhances the understanding of the therapeutic mechanisms of TCM in treating immune-related bone diseases and to facilitate the development of novel therapeutic strategies.

M2 macrophages-derived exosomes for osteonecrosis of femoral head treatment: modulating neutrophil extracellular traps formation and endothelial phenotype transition

Exosomes have shown good potential in ischemic injury disease treatments. However, evidence about their effect and molecular mechanisms in osteonecrosis of femoral head (ONFH) treatment is still limited. Here, we revealed the cell biology characters of ONFH osteonecrosis area bone tissue in single cell scale and thus identified a novel ONFH treatment approach based on M2 macrophages-derived exosomes (M2-Exos). We further show that M2-Exos are highly effective in the treatment of ONFH by modulating the phenotypes communication between neutrophil and endothelium including neutrophil extracellular traps formation and endothelial phenotype transition. Additionally, we identified that M2-Exos' therapeutic effect is attributed to the high content of miR-93-5p and constructed miR-93-5p overexpression model in vitro and in vivo based on lentivirus and adeno-associated virus respectively. Then we found miR-93-5p can not only reduce neutrophil extracellular traps formation but also improve angiogenic ability of endothelial cells. These results provided a new theoretical basis for the clinical application of ONFH therapeutic exosomes.

Immunomodulation in Bone Tissue Engineering: Recent Advancements in Scaffold Design and Biological Modifications for Enhanced Regeneration

Bone defects, whether caused by trauma, cancer, infectious diseases, or surgery, can significantly impair people's quality of life. Although autografts are the gold standard for treating bone defects, they often fall short in adequately forming bone tissue. The field of bone tissue engineering has made strides in using scaffolds with various biomaterials, stem cells, and growth factors to enhance bone healing. However, some biological structures do not yield satisfactory therapeutic outcomes for new bone formation. Recent studies have shed light on the crucial role of immunomodulation, specifically the interaction between the implanted scaffold and host immune systems, in bone regeneration. Immune cells, particularly macrophages, are pivotal in the inflammatory response, angiogenesis, and osteogenesis. This review delves into the immune system's mechanism toward foreign bodies and the recent advancements in scaffolds' physical and biological properties that foster bone regeneration by modulating macrophage polarization to an anti-inflammatory phenotype and enhancing the osteoimmune microenvironment.

Injectable photosensitive bone cement enhancing angiogenesis and osteogenic differentiation for the treatment of bone nonunion

Nonunion fractures present a significant clinical challenge because of their complex microenvironment, which includes poor vascularization, insufficient osteogenesis, infection, and separation of fracture ends. The current clinical treatments have certain limitations. Inspired by this phenomenon, sandcastle worms secrete adhesive proteins that bind sand grains, shell fragments, and mineral particles, thereby constructing their "castles." In this study, we developed an injectable bone cement using methacryloyl chitosan (CSMA) combined with a specific concentration of oyster shell nanoparticles (OS-np) to treat nonunion fractures. Oyster shells are composed primarily of calcium carbonate, which releases ions that promote angiogenesis and osteogenesis. The in vivo results at 8 weeks showed that the expression of BMP2, RUNX2, and VEGF in the OS-np/CSMA group was increased by 5.47, 4.38, and 3.54 times, respectively, compared to the control group, significantly enhancing vascularization and bone repair in the bone nonunion model. The injectability of the OS-np/CSMA bone cement ensures that it can adapt well to the complex structures of nonunion sites, providing a supportive matrix for new bone formation. Both in vivo and in vitro osteogenesis experiments demonstrated that the OS-np/CSMA bone cement significantly enhanced vascularization and bone repair in nonunion models, which was because of the synergistic effects of ion release and the bioactive properties of the oyster shell nanoparticles. This study highlights the potential of OS-np/CSMA injectable bone cement as a promising treatment strategy for complex nonunion fractures that effectively promotes angiogenesis and osteogenesis.

Stretched Vascular Endothelial Cells Polarized Neutrophils to N2 Type via TRPC1-IL13-STAT3 Axis

OBJECTIVE

VECs play a crucial role in regulating the function of neutrophils, which is essential for immune responses and inflammation. As stretch-sensitive cells, VECs sense mechanical stretch through surface mechanoreceptors, converting external mechanical stimuli into biochemical signals. This study aimed to explore the molecular mechanisms underlying the regulation of neutrophil behavior by stretched VECs.

MATERIALS AND METHODS

The key cytokine-inducing neutrophil N2 polarization in the conditioned medium from stretched vascular endothelial cells (CM-stretch) was validated through multifactorial matrix and flow cytometry. Additionally, the molecular mechanism underlying the response of vascular endothelial cells to stretch was systematically verified through layer-by-layer analysis using WB.

RESULTS

IL13, not IL4, was ultimately identified as a key cytokine-inducing neutrophil N2 polarization in CM-stretch. Inhibition of the transient receptor potential channel (TRPC1) and siRNA-mediated knockdown of TRPC1 both significantly decreased IL13 production. Furthermore, neutralizing IL13 in the CM-stretch or inhibiting STAT3 phosphorylation inhibited neutrophil N2 polarization, as evidenced by reduced CD206 and VEGFA expression.

CONCLUSIONS

These results demonstrate that stretched VECs initiate a signaling cascade that induces neutrophil N2 polarization through the TRPC1-IL13-STAT3 axis, suggesting that mechanical stretching of VECs could shift neutrophil function from a pro-inflammatory to a more regulatory and healing role.

Enhancing the Implant Osteointegration via Supramolecular Co-Assembly Coating with Early Immunomodulation and Cell Colonization

Osteointegration, the effective coupling between an implant and bone tissue, is a highly intricate biological process. The initial stages of bone-related immunomodulation and cellular colonization play crucial roles, but have received limited attention. Herein, a novel supramolecular co-assembled coating of strontium (Sr)-doped metal polyphenol networks (MPN) modified with c(RGDfc) is developed and well-characterized, for eliciting an early immunomodulation and cellular colonization. The results showed that the (Sr-MPN)@RGD coating significantly regulated the polarization of macrophages to the M2 phenotype by controllable release of Sr, and promote the initial adhesion of bone marrow mesenchymal stem cells (BMSCs) by RGD presented on MPN. Notably, the (Sr-MPN)@RGD attenuated osteoclast differentiation and oxidative stress as well as enhanced osteoblast differentiation and angiogenesis due to macrophage polarization toward M2 phenotype, which in turn has a profound effect on neighboring cells through paracrine signaling. In vivo results showed that the (Sr-MPN)@RGD coating manifested superior osseointegration and bone maturation to the bare Ti-rod or Ti-rod coated with MPN and Sr-MPN. This work contributed to the design of multifunctional implant coatings that address the complex biological process of osteointegration from the perspective of orchestrating stem cell recruitment with immunomodulatory strategies.

Injectable acellular matrix microgel assembly with stem cell recruitment and chondrogenic differentiation functions promotes microfracture-based articular cartilage regeneration

Articular cartilage repair and regeneration is still a significant challenge despite years of research. Although microfracture techniques are commonly used in clinical practice, the newborn cartilage is usually fibrocartilage rather than hyaline cartilage, which is mainly attributed to the inadequate microenvironment for effectively recruiting, anchoring, and inducing bone marrow mesenchymal stem cells (BMSCs) to differentiate into hyaline cartilage. This paper introduces a novel cartilage acellular matrix (CACM) microgel assembly with excellent microporosity, injectability, tissue adhesion, BMSCs recruitment and chondrogenic differentiation capabilities to improve the microfracture-based articular cartilage regeneration. Specifically, the sustained release of simvastatin (SIM) from the SIM@CACM microgel assembly efficiently recruits BMSCs in the early stage of cartilage regeneration, while the abundant interconnected micropores and high specific area assure the quick adhesion, proliferation and infiltration of BMSCs. Additionally, the active factors within the CACM matrix, appropriate mechanical properties of the microgel assembly, and excellent tissue adhesion provide a conductive environment for the continuous chondrogenic differentiation of BMSCs into hyaline cartilage. Owing to the synergistic effect of the above-mentioned factors, good articular cartilage repair and regeneration is achieved.

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.

Advanced Bioresponsive Drug Delivery Systems for Promoting Diabetic Vascularized Bone Regeneration

The treatment of bone defects in diabetes mellitus (DM) patients remains a major challenge since the diabetic microenvironments significantly impede bone regeneration. Many abnormal factors including hyperglycemia, elevated oxidative stress, increased inflammation, imbalanced osteoimmune, and impaired vascular system in the diabetic microenvironment will result in a high rate of impaired, delayed, or even nonhealing events of bone tissue. Stimuli-responsive biomaterials that can respond to endogenous biochemical signals have emerged as effective therapeutic systems to treat diabetic bone defects via the combination of microenvironmental regulation and enhanced osteogenic capacity. Following the natural bone healing processes, coupling of angiogenesis and osteogenesis by advanced bioresponsive drug delivery systems has proved to be of significant approach for promoting bone repair in DM. In this Review, we have systematically summarized the mechanisms and therapeutic strategies of DM-induced impaired bone healing, outlined the bioresponsive design for drug delivery systems, and highlighted the vascularization strategies for promoting bone regeneration. Accordingly, we then overview the recent advances in developing bioresponsive drug delivery systems to facilitate diabetic vascularized bone regeneration by remodeling the microenvironment and modulating multiple regenerative cues. Furthermore, we discuss the development of adaptable drug delivery systems with unique features for guiding DM-associated bone regeneration in the future.

Biomimetic Mineralized Collagen Scaffolds for Bone Tissue Engineering: Strategies on Elaborate Fabrication for Bioactivity Improvement

Biomimetic mineralized collagen (BMC) scaffolds represent an innovative class of bone-repair biomaterials inspired by the natural biomineralization process in bone tissue. Owing to their favorable biocompatibility and mechanical properties, BMC scaffolds have garnered significant attention in bone tissue engineering. However, most studies have overlooked the importance of bioactivity, resulting in collagen scaffolds with suboptimal osteogenic potential. In this review, the composition of the mineralized extracellular matrix (ECM) in bone tissue is discussed to provide guidance for biomimetic collagen mineralization. Subsequently, according to the detailed fabrication procedure of BMC scaffolds, the substances that can regulate both the fabrication process and biological activities is summarized. Furthermore, a potential strategy for developing BMC scaffolds with superior mechanical properties and biological activities for bone tissue engineering is proposed.

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.

Comprehensive Overview of Interface Strategies in Implant Osseointegration

With the improvement of implant design and the expansion of application scenarios, orthopedic implants have become a common surgical option for treating fractures and end‐stage osteoarthritis. Their common goal is rapidly forming and long‐term stable osseointegration. However, this fixation effect is limited by implant surface characteristics and peri‐implant bone tissue activity. Therefore, this review summarizes the strategies of interface engineering (osteogenic peptides, growth factors, and metal ions) and treatment methods (porous nanotubes, hydrogel embedding, and other load‐release systems) through research on its biological mechanism, paving the way to achieve the adaptation of both and coordination between different strategies. With the transition of the osseointegration stage, interface engineering strategies have demonstrated varying therapeutic effects. Especially, the activity of osteoblasts runs almost through the entire process of osseointegration, and their physiological activities play a dominant role in bone formation. Furthermore, diseases impacting bone metabolism exacerbate the difficulty of achieving osseointegration. This review aims to assist future research on osseointegration engineering strategies to improve implant‐bone fixation, promote fracture healing, and enhance post‐implantation recovery.

ECM-mimicking composite hydrogel for accelerated vascularized bone regeneration

Bioactive hydrogel materials have great potential for applications in bone tissue engineering. However, fabrication of functional hydrogels that mimic the natural bone extracellular matrix (ECM) remains a challenge, because they need to provide mechanical support and embody physiological cues for angiogenesis and osteogenesis. Inspired by the features of ECM, we constructed a dual-component composite hydrogel comprising interpenetrating polymer networks of gelatin methacryloyl (GelMA) and deoxyribonucleic acid (DNA). Within the composite hydrogel, the GelMA network serves as the backbone for mechanical and biological stability, whereas the DNA network realizes dynamic capabilities (e.g., stress relaxation), thereby promoting cell proliferation and osteogenic differentiation. Furthermore, functional aptamers (Apt19S and AptV) are readily attached to the DNA network to recruit bone marrow mesenchymal stem cells (BMSCs) and achieve sustained release of loaded vascular endothelial growth factor towards angiogenesis. Our results showed that the composite hydrogel could facilitate the adhesion of BMSCs, promote osteogenic differentiation by activating focal adhesion kinase (FAK)/phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt)/β-Catenin signaling pathway, and eventually enhance vascularized bone regeneration. This study shows that the multifunctional composite hydrogel of GelMA and DNA can successfully simulate the biological functions of natural bone ECM and has great potential for repairing bone defects.

Progranulin-dependent repair function of regulatory T cells drives bone-fracture healing

Local immunoinflammatory events instruct skeletal stem cells (SSCs) to repair/regenerate bone after injury, but mechanisms are incompletely understood. We hypothesized that specialized Tregs are necessary for bone repair and interact directly with SSCs through organ-specific messages. Both in human patients with bone fracture and a mouse model of bone injury, we identified a bone injury-responding Treg subpopulation with bone-repair capacity marked by CCR8. Local production of CCL1 induced a massive migration of CCR8+ Tregs from periphery to the injury site. Depending on secretion of progranulin (PGRN), a protein encoded by the granulin (Grn) gene, CCR8+ Tregs supported the accumulation and osteogenic differentiation of SSCs and thereby bone repair. Mechanistically, we revealed that CCL1 enhanced expression levels of basic leucine zipper ATF-like transcription factor (BATF) in CCR8+ Tregs, which bound to the Grn promoter and increased Grn translational output and then PGRN secretion. Together, our work provides a new perspective in osteoimmunology and highlights possible ways of manipulating Treg signaling to enhance bone repair and regeneration.

Nanoarchitecture-Integrated Hydrogel Boosts Angiogenesis-Osteogenesis-Neurogenesis Tripling for Infected Bone Fracture Healing

Infected fracture healing is a complicated process that includes intricate interactions at the cellular and molecular levels. In addition to angiogenesis and osteogenesis, the significance of neurogenesis in fracture healing has also been recognized in recent years. Here, a nanocomposite hydrogel containing pH-responsive zinc-gallium-humic acids (HAs) nanoparticles is developed. Through the timed release of Zn2+, Ga3+, and HAs, the hydrogel exhibits potent antibacterial effects and promotes angiogenesis, osteogenesis, and neurogenesis. The enhanced neurogenesis further promotes angiogenesis and osteogenesis, forming a mutually supportive angiogenesis-osteogenesis-neurogenesis cycle at the fracture site. The hydrogel achieves rapid infected fracture healing and improves tissue regeneration in mice. This study proposes a comprehensive treatment approach that combines antibacterial effects with the regulation of tissue regeneration to improve infected fracture healing.

Progress in Dentin-Derived Bone Graft Materials: A New Xenogeneic Dentin-Derived Material with Retained Organic Component Allows for Broader and Easier Application

The optimal repair of rigid mineralized tissues, such as bone, in cases of fracture, surgical resection, or prosthetic placement, is a complex process often necessitating the use of bone graft materials. Autogenous bone from the patient is generally the gold standard in terms of outcomes but also has disadvantages, which have resulted in extensive research in the field of tissue engineering to develop better and more convenient alternatives. In the dental field, several initiatives have demonstrated that the dentin material derived from extracted teeth produces excellent results in terms of repairing bone defects and supporting dental implants. Dentin is acellular and thus, in contrast to autogenous bone, cannot provide osteoblasts or other cellular elements to the grafted region, but it does contain growth and differentiation factors, and has other properties that make it an impressive material for bone repair. In this review, the beneficial properties of dentin and the ways it interacts with the host bone are described in the context of bone graft materials. Autogenous tooth material has limitations, particularly in terms of the need for tooth extraction and the limited amount available, which currently restrict its use to particular dental procedures. The development of a xenograft dentin-derived material, which retains the properties of autogenous dentin, is described. Such a material could potentially enable the use of dentin-derived material more widely, particularly in orthopedic indications where its properties may be advantageous.

Neutrophils are involved in early bone formation during midpalatal expansion

OBJECTIVE

Midpalatal expansion (MPE) is routinely employed to treat transverse maxillary arch deficiency. Neutrophils are indispensable for recruiting bone marrow stromal cells (BMSCs) at the initial stage of bone regeneration. This study aimed to explore whether neutrophils participate in MPE and how they function during bone formation under mechanical stretching.

MATERIALS AND METHODS

The presence and phenotype of neutrophils in the midpalatal suture during expansion were detected by flow cytometry and immunofluorescence staining. The possible mechanism of neutrophil recruitment and polarization was explored in vitro by exposing vascular endothelial cells (VECs) to cyclic tensile strain.

RESULTS

The number of neutrophils in the distracted suture peaked on Day 3, and N2-type neutrophils significantly increased on Day 5 after force application. The depletion of circulatory neutrophils reduced bone volume by 43.6% after 7-day expansion. The stretched VECs recruited neutrophils via a CXCR2 mechanism in vitro, which then promoted BMSC osteogenic differentiation through the VEGFA/VEGFR2 axis. Consistently, these neutrophils showed higher expression of canonical N2 phenotype genes, including CD206 and Arg1.

CONCLUSIONS

These results suggested that neutrophils participated in early bone formation during MPE. Based on these findings, we propose that stretched VECs recruited and polarized neutrophils, which, in turn, induced BMSC osteogenic differentiation.

Impact of SDF-1 and AMD3100 on Hair Follicle Dynamics in a Chronic Stress Model

Chronic stress is a common cause of hair loss, involving inflammatory responses and changes in cellular signaling pathways. This study explores the mechanism of action of the SDF-1/CXCR4 signaling axis in chronic stress-induced hair loss. The research indicates that SDF-1 promotes hair follicle growth through the PI3K/Akt and JAK/STAT signaling pathways. Transcriptome sequencing analysis was conducted to identify differentially expressed genes in the skin of normal and stressed mice, with key genes SDF-1/CXCR4 selected through machine learning and a protein-protein interaction network established. A chronic stress mouse model was created, with injections of SDF-1 and AMD3100 administered to observe hair growth, weight changes, and behavioral alterations and validate hair follicle activity. Skin SDF-1 concentrations were measured, differentially expressed genes were screened, and pathways were enriched. Activation of the PI3K/Akt and JAK/STAT signaling pathways was assessed, and siRNA technology was used in vitro to inhibit the expression of SDF-1 or CXCR4. SDF-1 promoted hair follicle activity, with the combined injection of SDF-1 and AMD3100 weakening this effect. The activation of the PI3K/Akt and JAK/STAT signaling pathways was observed in the SDF-1 injection group, confirmed by Western blot and immunofluorescence. Silencing SDF-1 through siRNA-mediated inhibition reduced cell proliferation and migration abilities. SDF-1 promotes hair growth in chronic stress mice by activating the PI3K/Akt and JAK/STAT pathways, an effect reversible by AMD3100. The SDF-1/CXCR4 axis may serve as a potential therapeutic target for stress-induced hair loss.

A bio-functional cryogel with antioxidant activity for potential application in bone tissue repairing

Antioxidant and free radical resistance has been a key concern of tissue engineering. In this study, Hydroxyapatite (HAp) with osteogenic activity and Oligomeric Proantho Cyanidins (OPC) with antioxidant activity were chemically grafted to prepare gelatin-based biofunctional aerogel (GHPOS). SEM results confirmed that these aerogels exhibited obvious macroporous structure and could provide a suitable microenvironment for bone cell growth. The addition of HAP-PEI-OPC made it have good antioxidant activity, and the cell results proved that the aerogel prepared in this study had good cytocompatibility and did not produce cytotoxicity. The addition of nanoparticles played an important role in the activity of 3T3-E1. The results showed that these bioactive aerogel scaffolds have potential applications in bone tissue engineering.

Mesenchymal stromal cells restrain the Th17 cell response via L-amino-acid oxidase within lymph nodes

Mesenchymal stromal/stem cells (MSC) have emerged as a promising therapeutic avenue for treating autoimmune diseases, eliciting considerable interest and discussion regarding their underlying mechanisms. This study revealed the distinctive ability of human umbilical cord MSC to aggregate within the lymph nodes of mice afflicted with autoimmune diseases, but this phenomenon was not observed in healthy mice. The specific distribution is driven by the heightened expression of the CCL21-CCR7 axis in mice with autoimmune diseases, facilitating the targeted homing of MSC to the lymph nodes. Within the lymph nodes, MSC exhibit a remarkable capacity to modulate Th17 cell function, exerting a pronounced anti-inflammatory effect. Transplanted MSC stimulates the secretion of L-amino-acid oxidase (LAAO), a response triggered by elevated levels of tumor necrosis factor-α (TNF-α) in mice with autoimmune diseases through the NF-κB pathway. The presence of LAAO is indispensable for the efficacy of MSC, as it significantly contributes to the inhibition of Th17 cells. Furthermore, LAAO-derived indole-3-pyruvic acid (I3P) serves as a potent suppressor of Th17 cells by activating the aryl hydrocarbon receptor (AHR) pathway. These findings advance our understanding of the global immunomodulatory effects exerted by MSC, providing valuable information for optimizing therapeutic outcomes.

Targeting micromotion for mimicking natural bone healing by using NIPAM/Nb2C hydrogel

Natural fracture healing is most efficient when the fine-tuned mechanical force and proper micromotion are applied. To mimick this micromotion at the fracture gap, a near-infrared-II (NIR-II)-activated hydrogel was fabricated by integrating two-dimensional (2D) monolayer Nb2C nanosheets into a thermally responsive poly(N-isopropylacrylamide) (NIPAM) hydrogel system. NIR-II-triggered deformation of the NIPAM/Nb2C hydrogel was designed to generate precise micromotion for co-culturing cells. It was validated that micromotion at 1/300 Hz, triggering a 2.37-fold change in the cell length/diameter ratio, is the most favorable condition for the osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs). Moreover, mRNA sequencing and verification revealed that micromotion-induced augmentation was mediated by Piezo1 activation. Suppression of Piezo1 interrupts the mechano-sensitivity and abrogates osteogenic differentiation. Calvarial and femoral shaft defect models were established to explore the biocompatibility and osteoinductivity of the Micromotion Biomaterial. A series of research methods, including radiography, micro-CT scanning, and immunohistochemical staining have been performed to evaluate biosafety and osteogenic efficacy. The in vivo results revealed that tunable micromotion strengthens the natural fracture healing process through the sequential activation of endochondral ossification, promotion of neovascularization, initiation of mineral deposition, and combinatory acceleration of full-thickness osseous regeneration. This study demonstrated that Micromotion Biomaterials with controllable mechanophysical characteristics could promote the osteogenic differentiation of BMSCs and facilitate full osseous regeneration. The design of NIPAM/Nb2C hydrogel with highly efficient photothermal conversion, specific features of precisely controlled micromotion, and bionic-mimicking bone-repair capabilities could spark a new era in the field of regenerative medicine.

Microenvironment-sensitive nanozymes for tissue regeneration

Tissue defect is one of the significant challenges encountered in clinical practice. Nanomaterials, including nanoparticles, nanofibers, and metal-organic frameworks, have demonstrated an extensive potential in tissue regeneration, offering a promising avenue for future clinical applications. Nonetheless, the intricate landscape of the inflammatory tissue microenvironment has engendered challenges to the efficacy of nanomaterial-based therapies. This quandary has spurred researchers to pivot towards advanced nanotechnological remedies for overcoming these therapeutic constraints. Among these solutions, microenvironment-sensitive nanozymes have emerged as a compelling instrument with the capacity to reshape the tissue microenvironment and enhance the intricate process of tissue regeneration. In this review, we summarize the microenvironmental characteristics of damaged tissues, offer insights into the rationale guiding the design and engineering of microenvironment-sensitive nanozymes, and explore the underlying mechanisms that underpin these nanozymes' responsiveness. This analysis includes their roles in orchestrating cellular signaling, modulating immune responses, and promoting the delicate process of tissue remodeling. Furthermore, we discuss the diverse applications of microenvironment-sensitive nanozymes in tissue regeneration, including bone, soft tissue, and cartilage regeneration. Finally, we shed our sights on envisioning the forthcoming milestones in this field, prospecting a future where microenvironment-sensitive nanozymes contribute significantly to the development of tissue regeneration and improved clinical outcomes.

Xenogenous implanted dental follicle stem cells promote periodontal regeneration through inducing the N2 phenotype of neutrophils

BACKGROUND

Periodontal tissue loss is the main reason for tooth mobility and loss caused by periodontal disease. Dental follicle stem cells (DFSCs) have significant therapeutic potential in periodontal regeneration, which maybe mainly depends on their potent immunomodulatory capacity. Consequently, this study aims to elucidate the impact of implanted xenogenous DFSCs on innate immune responses during early and late stages in the periodontal defect repair period.

METHODS

To trace and investigate the immunomodulation mechanisms of DFSCs in vivo, DFSCs were engineered (E-DFSCs) using lentiviral vectors expressing CD63-enhanced green fluorescent protein (CD63-EGFP) and β-Actin-mCherry protein (ACTB-mCherry) to exhibit green and red fluorescence. The biological characteristics and functions of E-DFSCs were verified by proliferation, differentiation, and co-culture experiments in vitro. In vivo, the periodontal regeneration capacity of E-DFSCs was detected by implantation of murine periodontal defect model, and the response of innate immune cells was detected at the 1st, 3rd, and 5th days (early stage) and 4th week (late stage) after implantation.

RESULTS

In vitro assessments showed that E-DFSCs retain similar properties to their non-engineered counterparts but exhibit enhanced macrophage immunomodulation capability. In mice models, four-week micro-CT and histological evaluations indicated that E-DFSCs have equivalent efficiency to DFSCs in periodontal defect regeneration. At the early stage of repair in mice periodontal defect, fluorescence tracking showed that implanted E-DFSCs might primarily activate endogenous cells through direct contact and indirect actions, and most of these cells are myeloperoxidase-positive neutrophils. Additionally, compared with the control group, the neutrophilic infiltration and conversion of N2-type were significantly increased in the E-DFSC group. At the late stage of defect regeneration, more M2-type macrophages, fewer TRAP + osteoclasts, and an upregulated OPG/RANKL ratio were detected in the E-DFSC group compared to the control group, which indicated that immune balance tilts towards healing and bone formation.

CONCLUSION

The xenogenous implanted DFSCs can induce the N2 phenotype of neutrophils in the early stage, which can activate the innate immune mechanism of the host to promote periodontal tissue regeneration.

Spatiotemporal controlled released hydrogels for multi-system regulated bone regeneration

Bone defect is one of the urgent problems to be solved in clinics, and it is very important to construct efficient scaffold materials to facilitate bone tissue regeneration. Hydrogels, characterized by their unique three-dimensional network structure, serve as excellent biological scaffold materials. Their internal pores are capable of loading osteogenic drugs to expedite bone formation. The rate and quality of new bone formation are intimately linked with immune regulation and vascular remodeling. The strategic sequential release of drugs to balance inflammation and regulate vascular remodeling is crucial for initiating the osteogenic process. Through the design of hydrogel microstructures, it is possible to achieve sequential drug release and the drug action time can be prolonged, thereby catering to the multi-systemic collaborative regulation needs of osteosynthesis. The drug release rate within the hydrogel is governed by swelling control systems, physical control systems, chemical control systems, and environmental control systems. Utilizing these control systems to design hydrogel materials capable of multi-drug delivery optimizes the construction of the bone microenvironment. Consequently, this facilitates the spatiotemporal controlled released of drugs, promoting bone tissue regeneration. This paper reviews the principles of the controlled release system of various sustained-release hydrogels and the advancements in research on hydrogel multi-drug delivery systems for bone tissue regeneration.

Simultaneously Modulating HIF-1α and HIF-2α and Optimizing Macrophage Polarization through the Biomimetic Gene Vector toward the Treatment of Osteoarthritis

In osteoarthritis (OA), articular cartilage is continuously submerged in a hypoxic environment throughout life, and hypoxia-inducible factors (HIFs) play a crucial role in OA progression. Among the various HIF phenotypes, HIF-1α positively contributes to maintaining the stability of the articular cartilage matrix. In contrast, HIF-2α has a detrimental effect, leading to chondrocyte apoptosis and exacerbating inflammation. Notably, there is currently no simultaneous regulation of HIF-1α and HIF-2α for OA treatment. Thus, the biomimetic gene vector (MENP) was developed for co-delivery of siHIF-2α and Mg2+ to the inflamed regions in OA joints, comprising an inner core consisting of siHIF-2α and Mg2+ and an outer M2 macrophage membrane. In vitro and in vivo studies demonstrate that MENP effectively targets inflamed areas, efficiently silences HIF-2α, and facilitates HIF-1α-mediated cartilage restoration through Mg2+. Furthermore, it indirectly promotes the polarization of macrophages toward an anti-inflammatory M2 phenotype through its action on inflamed synoviocytes. Overall, MENP is an efficient biomimetic vehicle for alleviating inflammation and promoting cartilage repair, representing an appealing approach for OA treatment.

Macrophage membrane-reversibly camouflaged nanotherapeutics accelerate fracture healing by fostering MSCs recruitment and osteogenic differentiation

The fracture healing outcome is largely dependent on the quantities as well as osteogenic differentiation capacities of mesenchymal stem cells (MSCs) at the lesion site. Herein, macrophage membrane (MM)-reversibly cloaked nanocomplexes (NCs) are engineered for the lesion-targeted and hierarchical co-delivery of short stromal derived factor-1α peptide (sSDF-1α) and Ckip-1 small interfering RNA (Ckip-1 siRNA, siCkip-1) to promote bone repair by concurrently fostering recruitment and osteogenic differentiation of endogenous MSCs. To construct the NCs, a membrane-penetrating α-helical polypeptide first assembles with siCkip-1, and the cationic NCs are sequentially coated with catalase and an outer shell of sSDF-1α-anchored MM. Due to MM-assisted inflammation homing, intravenously injected NCs could efficiently accumulate at the fractured femur, where catalase decomposes the local hydrogen peroxide to generate oxygen bubbles that drives the shedding of sSDF-1α-anchored MM in the extracellular compartment. The exposed, cationic inner core thus enables robust trans-membrane delivery into MSCs to induce Ckip-1 silencing. Consequently, sSDF-1α-guided MSCs recruitment cooperates with siCkip-1-mediated osteogenic differentiation to facilitate bone formation and accelerate bone fracture healing. This study provides an enlightened strategy for the hierarchical co-delivery of macromolecular drugs into different cellular compartments, and it also renders a promising modality for the management of fracture healing.

Advances in biomaterials for oral-maxillofacial bone regeneration: spotlight on periodontal and alveolar bone strategies

The intricate nature of oral-maxillofacial structure and function, coupled with the dynamic oral bacterial environment, presents formidable obstacles in addressing the repair and regeneration of oral-maxillofacial bone defects. Numerous characteristics should be noticed in oral-maxillofacial bone repair, such as irregular morphology of bone defects, homeostasis between hosts and microorganisms in the oral cavity and complex periodontal structures that facilitate epithelial ingrowth. Therefore, oral-maxillofacial bone repair necessitates restoration materials that adhere to stringent and specific demands. This review starts with exploring these particular requirements by introducing the particular characteristics of oral-maxillofacial bones and then summarizes the classifications of current bone repair materials in respect of composition and structure. Additionally, we discuss the modifications in current bone repair materials including improving mechanical properties, optimizing surface topography and pore structure and adding bioactive components such as elements, compounds, cells and their derivatives. Ultimately, we organize a range of potential optimization strategies and future perspectives for enhancing oral-maxillofacial bone repair materials, including physical environment manipulation, oral microbial homeostasis modulation, osteo-immune regulation, smart stimuli-responsive strategies and multifaceted approach for poly-pathic treatment, in the hope of providing some insights for researchers in this field. In summary, this review analyzes the complex demands of oral-maxillofacial bone repair, especially for periodontal and alveolar bone, concludes multifaceted strategies for corresponding biomaterials and aims to inspire future research in the pursuit of more effective treatment outcomes.

A Genetically Engineered "Reinforced Concrete" Scaffold Regulates the N2 Neutrophil Innate Immune Cascade to Repair Bone Defects

The innate immune response is crucial to inflammation, but how neutrophils and macrophages act in bone repair and tissue engineering treatment strategies await clarification. In this study, it is found that N2 neutrophils release stronger "eat me" signals to induce macrophage phagocytosis and polarize into the M2 anti-inflammatory phenotype. Guided by this biological mechanism, a mesoporous bioactive glass scaffold (MBG) is filled with hyaluronic acid methacryloyl (HAMA) hydrogel loaded with Transforming growth factor-β1 (TGFβ1) adenovirus (Ad@H), constructing a genetically engineered composite scaffold (Ad@H/M). The scaffold not only has good hydrophilicity and biocompatibility, but also provides mechanical stress support for bone repair. Adenovirus infection quickly induces N2 neutrophils, upregulating NF-κB and MAPK signaling pathways through Toll-like receptor 4 (TLR4) to promote the inflammatory response and macrophage phagocytosis. Macrophages perform phagocytosis and polarize towards the M2 phenotype, mediating the inflammatory response by inhibiting the PI3K-AKT-NF-κB pathway, maintaining homeostasis of the osteogenic microenvironment. The role of the Ad@H/M scaffold in regulating early inflammation and promoting long-term bone regeneration is further validated in vivo. In brief, this study focuses on the cascade of reactions between neutrophils and macrophage subtypes, and reports a composite scaffold that coordinates the innate immune response to promote bone repair.

Regulation of metabolic microenvironment with a nanocomposite hydrogel for improved bone fracture healing

Bone nonunion poses an urgent clinical challenge that needs to be addressed. Recent studies have revealed that the metabolic microenvironment plays a vital role in fracture healing. Macrophages and bone marrow-derived mesenchymal stromal cells (BMSCs) are important targets for therapeutic interventions in bone fractures. Itaconate is a TCA cycle metabolite that has emerged as a potent macrophage immunomodulator that limits the inflammatory response. During osteogenic differentiation, BMSCs tend to undergo aerobic glycolysis and metabolize glucose to lactate. Copper ion (Cu2+) is an essential trace element that participates in glucose metabolism and may stimulate glycolysis in BMSCs and promote osteogenesis. In this study, we develop a 4-octyl itaconate (4-OI)@Cu@Gel nanocomposite hydrogel that can effectively deliver and release 4-OI and Cu2+ to modulate the metabolic microenvironment and improve the functions of cells involved in the fracture healing process. The findings reveal that burst release of 4-OI reduces the inflammatory response, promotes M2 macrophage polarization, and alleviates oxidative stress, while sustained release of Cu2+ stimulates BMSC glycolysis and osteogenic differentiation and enhances endothelial cell angiogenesis. Consequently, the 4-OI@Cu@Gel system achieves rapid fracture healing in mice. Thus, this study proposes a promising regenerative strategy to expedite bone fracture healing through metabolic reprogramming of macrophages and BMSCs.

Advances in electroactive biomaterials: Through the lens of electrical stimulation promoting bone regeneration strategy

The regenerative capacity of bone is indispensable for growth, given that accidental injury is almost inevitable. Bone regenerative capacity is relevant for the aging population globally and for the repair of large bone defects after osteotomy (e.g., following removal of malignant bone tumours). Among the many therapeutic modalities proposed to bone regeneration, electrical stimulation has attracted significant attention owing to its economic convenience and exceptional curative effects, and various electroactive biomaterials have emerged. This review summarizes the current knowledge and progress regarding electrical stimulation strategies for improving bone repair. Such strategies range from traditional methods of delivering electrical stimulation via electroconductive materials using external power sources to self-powered biomaterials, such as piezoelectric materials and nanogenerators. Electrical stimulation and osteogenesis are related via bone piezoelectricity. This review examines cell behaviour and the potential mechanisms of electrostimulation via electroactive biomaterials in bone healing, aiming to provide new insights regarding the mechanisms of bone regeneration using electroactive biomaterials.

The translational potential of this article

This review examines the roles of electroactive biomaterials in rehabilitating the electrical microenvironment to facilitate bone regeneration, addressing current progress in electrical biomaterials and the mechanisms whereby electrical cues mediate bone regeneration. Interactions between osteogenesis-related cells and electroactive biomaterials are summarized, leading to proposals regarding the use of electrical stimulation-based therapies to accelerate bone healing.

Shear Flow-Assembled Janus Membrane with Bifunctional Osteogenic and Antibacterial Effects for Guided Bone Regeneration

Guided bone regeneration (GBR) membranes that reside at the interface between the bone and soft tissues for bone repair attract increasing attention, but currently developed GBR membranes suffer from relatively poor osteogenic and antibacterial effects as well as limited mechanical property and biodegradability. We present here the design and fabrication of a bifunctional Janus GBR membrane based on a shear flow-driven layer by a layer self-assembly approach. The Janus GBR membrane comprises a calcium phosphate-collagen/polyethylene glycol (CaP@COL/PEG) layer and a chitosan/poly(acrylic acid) (CHI/PAA) layer on different sides of a collagen membrane to form a sandwich structure. The membrane exhibits good mechanical stability and tailored biodegradability. It is found that the CaP@COL/PEG layer and CHI/PAA layer contribute to the osteogenic differentiation and antibacterial function, respectively. In comparison with the control group, the Janus GBR membrane displays a 2.52-time and 1.84-time enhancement in respective volume and density of newly generated bone. The greatly improved bone repair ability of the Janus GBR membrane is further confirmed through histological analysis, and it has great potential for practical applications in bone tissue engineering.

Extracellular Vesicles Derived from Neutrophils Accelerate Bone Regeneration by Promoting Osteogenic Differentiation of BMSCs

The reconstruction of bone defects has been associated with severe challenges worldwide. Nowadays, bone marrow mesenchymal stem cell (BMSC)-based cell sheets have rendered this approach a promising way to facilitate osteogenic regeneration in vivo. Extracellular vesicles (EVs) play an essential role in intercellular communication and execution of various biological functions and are often employed as an ideal natural endogenous nanomedicine for restoring the structure and functions of damaged tissues. The perception of polymorphonuclear leukocytes (neutrophils, PMNs) as indiscriminate killer cells is gradually changing, with new evidence suggesting a role for these cells in tissue repair and regeneration, particularly in the context of bone healing. However, the role of EVs derived from PMNs (PMN-EVs) in bone regeneration remains largely unknown, with limited research being conducted on this aspect. In the current study, we investigated the effects of PMN-EVs on BMSCs and the underlying molecular mechanisms as well as the potential application of PMN-EVs in bone regeneration. Toward this end, BMSC-based cell sheets with integrated PMN-EVs (BS@PMN-EVs) were developed for bone defect regeneration. PMN-EVs were found to significantly enhance the proliferation and osteogenic differentiation of BMSCs in vitro. Furthermore, BS@PMN-EVs were found to significantly accelerate bone regeneration in vivo by enhancing the maturation of the newly formed bone in rat calvarial defects; this is likely attributable to the effect of PMN-EVs in promoting the expression of key osteogenic proteins such as SOD2 and GJA1 in BMSCs. In conclusion, our findings demonstrate the crucial role of PMN-EVs in promoting the osteogenic differentiation of BMSCs during bone regeneration. Furthermore, this study proposes a novel strategy for enhancing bone repair and regeneration via the integration of PMN-EVs with BMSC-based cell sheets.

Osteoimmunology of Fracture Healing

PURPOSE OF REVIEW

The purpose of this review is to summarize what is known in the literature about the role inflammation plays during bone fracture healing. Bone fracture healing progresses through four distinct yet overlapping phases: formation of the hematoma, development of the cartilaginous callus, development of the bony callus, and finally remodeling of the fracture callus. Throughout this process, inflammation plays a critical role in robust bone fracture healing.

RECENT FINDINGS

At the onset of injury, vessel and matrix disruption lead to the generation of an inflammatory response: inflammatory cells are recruited to the injury site where they differentiate, activate, and/or polarize to secrete cytokines for the purposes of cell signaling and cell recruitment. This process is altered by age and by sex. Bone fracture healing is heavily influenced by the presence of inflammatory cells and cytokines within the healing tissue.

The effects of matrix stiffness on immune cells in bone biology

Bone and immune cells typically inhabit the same microenvironment and engage in mutual interactions to collectively execute the functions of the "osteoimmune system." Establishing a harmonized and enduring osteoimmune system significantly enhances bone regeneration, necessitating the maintenance of bone and immune homeostasis. Recently, mechanobiology has garnered increasing interest in bone tissue engineering, with matrix stiffness emerging as a crucial parameter that has been extensively investigated. The effect of matrix stiffness on bone homeostasis remains relatively clear. Soft substrates tend to significantly affect the chondrogenic differentiation of bone marrow mesenchymal stem cells, whereas increasing matrix stiffness is advantageous for osteogenic differentiation. Increased stiffness increases osteoclast differentiation and activity. Additionally, there is increasing emphasis on immune homeostasis, which necessitates dynamic communication between immune cells. Immune cells are crucial in initiating bone regeneration and driving early inflammatory responses. Functional changes induced by matrix stiffness are pivotal for determining the outcomes of engineered tissue mimics. However, inconsistent and incomparable findings regarding the responses of different immune cells to matrix stiffness can be perplexing owing to variations in the stiffness range, measurement methods, and other factors. Therefore, this study aimed to provide a comprehensive review of the specific effects of matrix stiffness on diverse immune cells, with a particular focus on its implications for bone regeneration, which would offer theoretical insights into the treatment of large segmental bony defects and assist in the clinical development of new engineering strategies.

Early-Responsive Immunoregulation Therapy Improved Microenvironment for Bone Regeneration Via Engineered Extracellular Vesicles

Overactivated inflammatory reactions hinder the bone regeneration process. Timely transformation of microenvironment from pro-inflammatory to anti-inflammatory after acute immune response is favorable for osteogenesis. Macrophages play an important role in the immune response to inflammation. Therefore, this study adopts TIM3 high expression extracellular vesicles (EVs) with immunosuppressive function to reshape the early immune microenvironment of bone injury, mainly by targeting macrophages. These EVs can be phagocytosed by macrophages, thereby increasing the infiltration of TIM3-positive macrophages (TIM3+ macrophages) and M2 subtypes. The TIM3+ macrophage group has some characteristics of M2 macrophages and secretes cytokines, such as IL-10 and TGF-β1 to regulate inflammation. TIM3, which is highly expressed in the engineered EVs, mediates the release of anti-inflammatory cytokines by inhibiting the p38/MAPK pathway and promotes osseointegration by activating the Bmp2 promoter to enhance macrophage BMP2 secretion. After evenly loading the engineered EVs into the hydrogel, the continuous and slow release of EVsTIM3OE recruits more anti-inflammatory macrophages during the early stages of bone defect repair, regulating the immune microenvironment and eliminating the adverse effects of excessive inflammation. In summary, this study provides a new strategy for the treatment of refractory wounds through early inflammation control.

Interaction between immuno-stem dual lineages in jaw bone formation and injury repair

The jawbone, a unique structure in the human body, undergoes faster remodeling than other bones due to the presence of stem cells and its distinct immune microenvironment. Long-term exposure of jawbones to an oral environment rich in microbes results in a complex immune balance, as shown by the higher proportion of activated macrophage in the jaw. Stem cells derived from the jawbone have a higher propensity to differentiate into osteoblasts than those derived from other bones. The unique immune microenvironment of the jaw also promotes osteogenic differentiation of jaw stem cells. Here, we summarize the various types of stem cells and immune cells involved in jawbone reconstruction. We describe the mechanism relationship between immune cells and stem cells, including through the production of inflammatory bodies, secretion of cytokines, activation of signaling pathways, etc. In addition, we also comb out cellular interaction of immune cells and stem cells within the jaw under jaw development, homeostasis maintenance and pathological conditions. This review aims to eclucidate the uniqueness of jawbone in the context of stem cell within immune microenvironment, hopefully advancing clinical regeneration of the jawbone.

Beyond host defense and tissue injury: the emerging role of neutrophils in tissue repair

Neutrophils, the most abundant immune cells in human blood, play a fundamental role in host defense against invading pathogens and tissue injury. Neutrophils carry potentially lethal weaponry to the affected site. Inadvertent and perpetual neutrophil activation could lead to nonresolving inflammation and tissue damage, a unifying mechanism of many common diseases. The prevailing view emphasizes the dichotomy of their function, host defense versus tissue damage. However, tissue injury may also persist during neutropenia, which is associated with disease severity and poor outcome. Numerous studies highlight neutrophil phenotypic heterogeneity and functional versatility, indicating that neutrophils play more complex roles than previously thought. Emerging evidence indicates that neutrophils actively orchestrate resolution of inflammation and tissue repair and facilitate return to homeostasis. Thus, neutrophils mobilize multiple mechanisms to limit the inflammatory reaction, assure debris removal, matrix remodeling, cytokine scavenging, macrophage reprogramming, and angiogenesis. In this review, we will summarize the homeostatic and tissue-reparative functions and mechanisms of neutrophils across organs. We will also discuss how the healing power of neutrophils might be harnessed to develop novel resolution and repair-promoting therapies while maintaining their defense functions.

The neutrophil-osteogenic cell axis promotes bone destruction in periodontitis

The immune-stromal cell interactions play a key role in health and diseases. In periodontitis, the most prevalent infectious disease in humans, immune cells accumulate in the oral mucosa and promote bone destruction by inducing receptor activator of nuclear factor-κB ligand (RANKL) expression in osteogenic cells such as osteoblasts and periodontal ligament cells. However, the detailed mechanism underlying immune-bone cell interactions in periodontitis is not fully understood. Here, we performed single-cell RNA-sequencing analysis on mouse periodontal lesions and showed that neutrophil-osteogenic cell crosstalk is involved in periodontitis-induced bone loss. The periodontal lesions displayed marked infiltration of neutrophils, and in silico analyses suggested that the neutrophils interacted with osteogenic cells through cytokine production. Among the cytokines expressed in the periodontal neutrophils, oncostatin M (OSM) potently induced RANKL expression in the primary osteoblasts, and deletion of the OSM receptor in osteogenic cells significantly ameliorated periodontitis-induced bone loss. Epigenomic data analyses identified the OSM-regulated RANKL enhancer region in osteogenic cells, and mice lacking this enhancer showed decreased periodontal bone loss while maintaining physiological bone metabolism. These findings shed light on the role of neutrophils in bone regulation during bacterial infection, highlighting the novel mechanism underlying osteoimmune crosstalk.

A xonotlite nanofiber bioactive 3D-printed hydrogel scaffold based on osteo-/angiogenesis and osteoimmune microenvironment remodeling accelerates vascularized bone regeneration

BACKGROUND

Coordination between osteo-/angiogenesis and the osteoimmune microenvironment is essential for effective bone repair with biomaterials. As a highly personalized and precise biomaterial suitable for repairing complex bone defects in clinical practice, it is essential to endow 3D-printed scaffold the above key capabilities.

RESULTS

Herein, by introducing xonotlite nanofiber (Ca6(Si6O17) (OH)2, CS) into the 3D-printed silk fibroin/gelatin basal scaffold, a novel bone repair system named SGC was fabricated. It was noted that the incorporation of CS could greatly enhance the chemical and mechanical properties of the scaffold to match the needs of bone regeneration. Besides, benefiting from the addition of CS, SGC scaffolds could accelerate osteo-/angiogenic differentiation of bone mesenchymal stem cells (BMSCs) and meanwhile reprogram macrophages to establish a favorable osteoimmune microenvironment. In vivo experiments further demonstrated that SGC scaffolds could efficiently stimulate bone repair and create a regeneration-friendly osteoimmune microenvironment. Mechanistically, we discovered that SGC scaffolds may achieve immune reprogramming in macrophages through a decrease in the expression of Smad6 and Smad7, both of which participate in the transforming growth factor-β (TGF-β) signaling pathway.

CONCLUSION

Overall, this study demonstrated the clinical potential of the SGC scaffold due to its favorable pro-osteo-/angiogenic and osteoimmunomodulatory properties. In addition, it is a promising strategy to develop novel bone repair biomaterials by taking osteoinduction and osteoimmune microenvironment remodeling functions into account.

Tricarboxylic Acid Cycle Metabolite-Coordinated Biohydrogels Augment Cranial Bone Regeneration Through Neutrophil-Stimulated Mesenchymal Stem Cell Recruitment and Histone Acetylation-Mediated Osteogenesis

Cranial bone defects remain a major clinical challenge, increasing patients' life burdens. Tricarboxylic acid (TCA) cycle metabolites play crucial roles in facilitating bone tissue regeneration. However, the development of TCA cycle metabolite-modified biomimetic grafts for skull bone regeneration still needs to be improved. The mechanism underlying the release of TCA cycle metabolites from biomaterials in regulating immune responses and mesenchymal stem cell (MSC) fate (migration and differentiation) remains unknown. Herein, this work constructs biomimetic hydrogels composed of gelatin and chitosan networks covalently cross-linked by genipin (CGG hydrogels). A series of TCA cycle metabolite-coordinated CGG hydrogels with strong mechanical and antiswelling performances are subsequently developed. Remarkably, the citrate (Na3Cit, Cit)-coordinated CGG hydrogels (CGG-Cit hydrogels) with the highest mechanical modulus and strength significantly promote skull bone regeneration in rat and murine cranial defects. Mechanistically, using a transgenic mouse model, bulk RNA sequencing, and single-cell RNA sequencing, this work demonstrates that CGG-Cit hydrogels promote Gli1+ MSC migration via neutrophil-secreted oncostatin M. Results also indicate that citrate improves osteogenesis via enhanced histone H3K9 acetylation on osteogenic master genes. Taken together, the immune microenvironment- and MSC fate-regulated CGG-Cit hydrogels represent a highly efficient and facile approach toward skull bone tissue regeneration with great potential for bench-to-bedside translation.

Low-Temperature Calcium Phosphate Ceramics Can Modulate Monocytes and Macrophages Inflammatory Response In Vitro

Creating bioactive materials for bone tissue regeneration and augmentation remains a pertinent challenge. One of the most promising and rapidly advancing approaches involves the use of low-temperature ceramics that closely mimic the natural composition of the extracellular matrix of native bone tissue, such as Hydroxyapatite (HAp) and its phase precursors (Dicalcium Phosphate Dihydrate-DCPD, Octacalcium Phosphate-OCP, etc.). However, despite significant scientific interest, the current knowledge and understanding remain limited regarding the impact of these ceramics not only on reparative histogenesis processes but also on the immunostimulation and initiation of local aseptic inflammation leading to material rejection. Using the stable cell models of monocyte-like (THP-1ATRA) and macrophage-like (THP-1PMA) cells under the conditions of LPS-induced model inflammation in vitro, the influence of DCPD, OCP, and HAp on cell viability, ROS and intracellular NO production, phagocytosis, and the secretion of pro-inflammatory cytokines was assessed. The results demonstrate that all investigated ceramic particles exhibit biological activity toward human macrophage and monocyte cells in vitro, potentially providing conditions necessary for bone tissue restoration/regeneration in the peri-implant environment in vivo. Among the studied ceramics, DCPD appears to be the most preferable for implantation in patients with latent inflammation or unpredictable immune status, as this ceramic had the most favorable overall impact on the investigated cellular models.

Evaluation of nanoscale versus hybrid micro/nano surface topographies for endosseous implants

We examined the effect of a nanoscale titanium surface topography (D) versus two hybrid micro/nanoscale topographies (B and OS) on adherent mesenchymal stem cells (MSCs) and bone marrow derived macrophages (BMMs) function in cell culture and in vivo. In the in vitro study, compared to OS and B surfaces, D surface induced earlier and greater cell spreading, and earlier and profound mRNA expression of RUNX2, Osterix and BMP2 in MSCs. D surface induced earlier and higher expression of RUNX2 and BMP2 and lower expression of inflammatory genes in implant adherent cells in vivo. Measurement of osteogenesis at implant surfaces showed greater bone-to-implant contact at D versus OS surfaces after 21 days. We explored the cell population on the D and OS implant surfaces 24 h after placement using single-cell RNA sequencing and identified distinct cell clusters including macrophages, neutrophils and B cells. D surface induced lower expression and earlier reduction of inflammatory genes expression in BMMs in vitro. BMMs on D, B and OS surfaces demonstrated a marked increase of BMP2 expression after 1 and 3 days, and this increase was significantly higher on D surface at day 3. Our data implicates a dynamic process that may be influenced by nanotopography at multiple stages of osseointegration including initial immunomodulation, recruitment of MSCs and later osteoblastic differentiation leading to bone matrix production and mineralization. The results suggest that a nanoscale topography (D) favorably modulates adherent macrophage polarization toward anti-inflammatory and regenerative phenotypes and promotes the osteoinductive phenotype of adherent mesenchymal stem cells. STATEMENT OF SIGNIFICANCE: Our manuscript contains original data developed to define effects of a novel nanotopography on the process of osseointegration at the cell and tissue level.  Few studies have compared the effects of a nanoscale surface versus the more typical hybrid micro/nano-scale surfaces used today. We have utilized single-cell RNA sequencing for the first time to identify earliest cell populations on implant surfaces in vivo. We provide data indicating that the nanoscale surface acts upon both osteoprogenitor and immune cell (macrophages) to alter the process of bone formation in a surface-specific manner. This work represents new observations regarding osseointegration and immunomodulation.