Osteomacs and Bone Regeneration

Advances in biomimetic hydrogel for articular cartilage defect repair: Enabling immunomodulation and chondrogenesis

Articular cartilage defects, as a core pathologic feature in the progression of osteoarthritis, and their irreversible degenerative changes lead to functional impairment and socioeconomic burden for tens of millions of patients worldwide. Hydrogels have become key biomaterials in cartilage regeneration with the three-dimensional network structure, programmable mechanical properties, and cell-adaptive microenvironment of biomimetic extracellular matrix. In recent years, several hydrogel systems with in vitro/in vivo repair potential have been developed by modulating the material topology, dynamic mechanical response, and delivery of bioactive factors, and some of them have entered the clinical translation stage. This review systematically explains the biomimetic design principles of hydrogels. It analyzes the immunomodulation and chondrogenic mechanisms mediated by hydrogels, providing a theoretical framework for the development of next-generation smart cartilage repair materials.

Age-related alterations of angiogenesis, inflammation and bone microarchitecture during fracture healing in mice

The surgical treatment of geriatric patients represents a major challenge in traumatology. It is well known that aging affects fracture healing. However, the exact pathophysiology of age-related changes in angiogenesis, inflammation and bone remodeling remains still elusive. Therefore, we herein studied the differences of femoral fracture healing in young adult (3-4 months) and aged (16-18 months) CD-1 mice by using a stable closed femoral fracture model with intramedullary screw fixation. The callus tissue was analyzed by means of X-ray, micro-computed tomography (µCT), histology and immunohistochemistry. We found a deteriorated trabecular architecture and a reduced bone formation within the callus tissue of aged mice. Moreover, aged animals showed an increased number of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts at an early healing time point, whereas the fraction of mature α-smooth muscle actin (SMA)-positive microvessels was significantly reduced. Furthermore, the numbers of macrophages and granulocytes were higher in the callus tissue of aged animals at the end of the healing process. Taken together, these results demonstrate a delayed femoral fracture healing in aged CD-1 mice. This is most likely caused by an early overshooting osteoclast response, a decelerated maturation of the callus microvasculature and a late increased recruitment of pro-inflammatory cells. Targeting these alterations may contribute to the development of novel treatment approaches for the stimulation of bone regeneration in geriatric patients.

The inhibitory effect of salidroside on RANKL-induced osteoclast formation via NFκB suppression

Bone fractures are a prevalent clinical issue, and recent studies highlighted the promising potential of natural bone healing agents in enhancing fracture repair and regeneration. The regulatory interaction mechanism between osteoblasts and osteoclasts is crucial for bone cell biology and bone disease. In Mongolian medicine, people have used the Rhodiola rosea (R. rosea) extract to accelerate bone healing in bone fractures. Salidroside is a bioactive compound of R. rosea. Salidroside is known to regulate bone metabolism and inhibit the activation of osteoclast cells, but how it affects the differentiation of osteoclasts is unknown. We examined the effect of R. rosea extract and its bioactive compound salidroside on the RANKL-induced osteoclast formation in RAW 264.7 cells. The present study observed that salidroside directly inhibits RANKL-induced TRAP-positive osteoclast formation. Immunoblotting analysis revealed that salidroside inhibited the expression of c-Fos and NFATc1, osteoclastogenic key transcription factors, by suppressing late activation of p65 NFκB. Further, the ethanol extracts of R. rosea significantly reduced the RANKL-induced osteoclasts in a dose-dependent manner. In conclusion, salidroside inhibits RANKL-induced osteoclast formation via suppressing the NFκB/c-Fos/NFATc1 signalling pathway. R. rosea, a primary source of salidroside, is helpful for bone healing via its inhibitory effect on osteoclast formation.

Current status of nano-embedded growth factors and stem cells delivery to bone for targeted repair and regeneration

Bone-related diseases like osteoarthritis and osteoporosis impact millions globally, affecting quality of life. Osteoporosis considerably enhances the probability of bone fractures of the wrist, hip, and spine. Enhancement and acceleration of functional bone development can be achieved through the sustained delivery of growth factors (GFs) and cells in biomaterial carriers. The delivery of bioactive compounds in a targeted, spatiotemporal way that most closely resembles the natural defect repair process can be achieved by designing the carrier system with established release kinetics. Furthermore, the carrier can serve as a substrate that mimics the extracellular matrix, facilitating osteoprogenitor cell infiltration and growth for integrative tissue healing. In this report, we explore the significance of GFs within the realm of bone and cartilage tissue engineering, encompassing their encapsulation and delivery methodologies, the kinetics of release, and their amalgamation with biomaterials and stem cells (SCs) to facilitate the mending of bone fractures. Moreover, the significance of GFs in evaluating the microenvironment of bone tissue through reciprocal signaling with cells and biomaterial scaffolds is emphasized which will serve as the foundation for prospective advances in bone and cartilage tissue engineering as well as therapeutic equipment. Nanoparticles are being used in regenerative medicine to promote bone regeneration and repair by delivering osteoinductive growth factors like BMP-2, VEGF, TGF-β. These nanocarriers allow controlled release, minimizing adverse effects and ensuring growth factors are concentrated at the injury site. They are also mixed with mesenchymal stem cells (MSCs) to improve their engraftment, differentiation, and survival. This approach is a key step in developing multi-model systems that more efficiently facilitate bone regeneration. Researchers are exploring smart nanoparticles with immunomodulatory qualities to improve bonre regeneration and reduce inflammation in injury site. Despite promising preclinical results, challenges include cost management, regulatory approval, and long term safety. However, incorporating stem cell transport and growth factors in nanoparticles could revolutionize bone regeneration and offer more personalized therapies for complex bone disorders and accidents.

The translational potential of this article

Stem cell transport and growth factors encapsulated in nanoparticles are becoming revolutionary methods for bone regeneration and repair. By encouraging stem cells to develop into osteoblasts, osteoinductive GFs like BMP-2, VEGF, and TGF-β can be delivered under control due to nanomaterials like nanoparticles, nanofibers, and nanotubes. By ensuring sustained release, these nanocarriers lessen adverse effects and enhance therapeutic results. In order to prove their survival and development, MCSs, which are essential for bone regeneration, are mixed with nanoparticles, frequently using scaffolds that resemble the ECM of bone. Furthermore, by adjusting to the injured environment and lowering inflammation, immunomodulatory nanostructures and stimuli-responsive nanomaterials can further maximize. While there are still shotcomings to overcome, including managing expenses, negotiating regulatory processes, and guaranteeing long-term safety, this method promises to outperform traditional bone grafting by providing quicker, more individualized, and more efficient treatments. Nano-embedded growth factors and stem cell technologies have the potential to revolutionize orthopedic therapy and significantly enhance patient outcomes with further research.

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.

Aging Influences Fracture Healing on the Cellular Level and Alters Systemic RANKL and OPG Concentrations in a Murine Model

Clinical complications frequently follow polytrauma and bleeding fractures, increasing the risk of delayed fracture healing and nonunions, especially in aged patients. Therefore, this study examines age's impact on fracture repair with and without severe bleeding in mice. Young (17-26 weeks) and aged (64-72 weeks) male C57BL/6J mice (n = 72 in total, n = 6 per group) are allocated into 3 groups: the fracture group (Fx) undergoes femur osteotomy stabilized via external fixator, the combined trauma group (THFx) additionally receives pressure-controlled trauma hemorrhage (TH) and Sham animals are implanted with catheter and fixator without blood loss or osteotomy. Femoral bones are evaluated histologically 24 h and 3 weeks post-trauma, while RANKL/OPG and β-CTx are measured systemically via ELISA after 3 weeks. Aging results in less mineralized bone and fewer osteoclasts within the fracture of aged mice in contrast to young groups after three weeks. Systemically, aged animals exhibit increased RANKL and OPG levels after fracture compared to their young counterparts. The RANKL/OPG ratio rises in aged Fx animals compared to young mice, with a similar trend in THFx groups. In conclusion, age has an effect during the later course of fracture healing on the cellular and systemic levels.

Macrophage variants in laboratory research: most are well done, but some are RAW

Macrophages play a pivotal role in the innate immune response. While their most characteristic function is phagocytosis, it is important not to solely characterize macrophages by this activity. Their crucial roles in body development, homeostasis, repair, and immune responses against pathogens necessitate a broader understanding. Macrophages exhibit remarkable plasticity, allowing them to modify their functional characteristics in response to the tissue microenvironment (tissue type, presence of pathogens or inflammation, and specific signals from neighboring cells) swiftly. While there is no single defined "macrophage" entity, there is a diverse array of macrophage types because macrophage ontogeny involves the differentiation of progenitor cells into tissue-resident macrophages, as well as the recruitment and differentiation of circulating monocytes in response to tissue-specific cues. In addition, macrophages continuously sense and respond to environmental cues and tissue conditions, adjusting their functional and metabolic states accordingly. Consequently, it is of paramount importance to comprehend the heterogeneous origins and functions of macrophages employed in in vitro studies, as each available in vitro macrophage model is associated with specific sets of strengths and limitations. This review centers its attention on a comprehensive comparison between immortalized mouse macrophage cell lines and primary mouse macrophages. It provides a detailed analysis of the strengths and weaknesses inherent in these in vitro models. Finally, it explores the subtle distinctions between diverse macrophage cell lines, offering insights into numerous factors beyond the model type that can profoundly influence macrophage function.

Efferocytosis and Bone Dynamics

PURPOSE OF REVIEW

This review summarizes the recently published scientific evidence regarding the role of efferocytosis in bone dynamics and skeletal health.

RECENT FINDINGS

Several types of efferocytes have been identified within the skeleton, with macrophages being the most extensively studied. Efferocytosis is not merely a 'clean-up' process vital for maintaining skeletal homeostasis; it also plays a crucial role in promoting resolution pathways and orchestrating bone dynamics, such as osteoblast-osteoclast coupling during bone remodeling. Impaired efferocytosis has been associated with aging-related bone loss and various skeletal pathologies, including osteoporosis, osteoarthritis, rheumatoid arthritis, and metastatic bone diseases. Accordingly, emerging evidence suggests that targeting efferocytic mechanisms has the potential to alleviate these conditions. While efferocytosis remains underexplored in the skeleton, recent discoveries have shed light on its pivotal role in bone dynamics, with important implications for skeletal health and pathology. However, there are several knowledge gaps and persisting technical limitations that must be addressed to fully unveil the contributions of efferocytosis in bone.

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.

Amorphous TiO2nano-coating on stainless steel to improve its biological response

This study delves into the potential of amorphous titanium oxide (aTiO2) nano-coating to enhance various critical aspects of non-Ti-based metallic orthopedic implants. These implants, such as medical-grade stainless steel (SS), are widely used for orthopedic devices that demand high strength and durability. The aTiO2nano-coating, deposited via magnetron sputtering, is a unique attempt to improve the osteogenesis, the inflammatory response, and to reduce bacterial colonization on SS substrates. The study characterized the nanocoated surfaces (SS-a TiO2) in topography, roughness, wettability, and chemical composition. Comparative samples included uncoated SS and sandblasted/acid-etched Ti substrates (Ti). The biological effects were assessed using human mesenchymal stem cells (MSCs) and primary murine macrophages. Bacterial tests were carried out with two aerobic pathogens (S. aureusandS. epidermidis) and an anaerobic bacterial consortium representing an oral dental biofilm. Results from this study provide strong evidence of the positive effects of the aTiO2nano-coating on SS surfaces. The coating enhanced MSC osteoblastic differentiation and exhibited a response similar to that observed on Ti surfaces. Macrophages cultured on aTiO2nano-coating and Ti surfaces showed comparable anti-inflammatory phenotypes. Most significantly, a reduction in bacterial colonization across tested species was observed compared to uncoated SS substrates, further supporting the potential of aTiO2nano-coating in biomedical applications. The findings underscore the potential of magnetron-sputtering deposition of aTiO2nano-coating on non-Ti metallic surfaces such as medical-grade SS as a viable strategy to enhance osteoinductive factors and decrease pathogenic bacterial adhesion. This could significantly improve the performance of metallic-based biomedical devices beyond titanium.

Exploring the Role of Hormones and Cytokines in Osteoporosis Development

The disease of osteoporosis is characterized by impaired bone structure and an increased risk of fractures. There is a significant impact of cytokines and hormones on bone homeostasis and the diagnosis of osteoporosis. As defined by the World Health Organization (WHO), osteoporosis is defined as having a bone mineral density (BMD) that is 2.5 standard deviations (SD) or more below the average for young and healthy women (T score < -2.5 SD). Cytokines and hormones, particularly in the remodeling of bone between osteoclasts and osteoblasts, control the differentiation and activation of bone cells through cytokine networks and signaling pathways like the nuclear factor kappa-B ligand (RANKL)/the receptor of RANKL (RANK)/osteoprotegerin (OPG) axis, while estrogen, parathyroid hormones, testosterone, and calcitonin influence bone density and play significant roles in the treatment of osteoporosis. This review aims to examine the roles of cytokines and hormones in the pathophysiology of osteoporosis, evaluating current diagnostic methods, and highlighting new technologies that could help for early detection and treatment of osteoporosis.

Induction of osteoblast apoptosis stimulates macrophage efferocytosis and paradoxical bone formation

Apoptosis is crucial for tissue homeostasis and organ development. In bone, apoptosis is recognized to be a main fate of osteoblasts, yet the relevance of this process remains underexplored. Using our murine model with inducible Caspase 9, the enzyme that initiates intrinsic apoptosis, we triggered apoptosis in a proportion of mature osteocalcin (OCN+) osteoblasts and investigated the impact on postnatal bone development. Osteoblast apoptosis stimulated efferocytosis by osteal macrophages. A five-week stimulation of OCN+ osteoblast apoptosis in 3-week-old male and female mice significantly enhanced vertebral bone formation while increasing osteoblast precursors. A similar treatment regimen to stimulate osterix+ cell apoptosis had no impact on bone volume or density. The vertebral bone accrual following stimulation of OCN+ osteoblast apoptosis did not translate in improved mechanical strength due to disruption of the lacunocanalicular network. The observed bone phenotype was not influenced by changes in osteoclasts but was associated with stimulation of macrophage efferocytosis and vasculature formation. Phenotyping of efferocytic macrophages revealed a unique transcriptomic signature and expression of factors including VEGFA. To examine whether macrophages participated in the osteoblast precursor increase following osteoblast apoptosis, macrophage depletion models were employed. Depletion of macrophages via clodronate-liposomes and the CD169-diphtheria toxin receptor mouse model resulted in marked reduction in leptin receptor+ and osterix+ osteoblast precursors. Collectively, this work demonstrates the significance of osteoblast turnover via apoptosis and efferocytosis in postnatal bone formation. Importantly, it exposes the potential of targeting this mechanism to promote bone anabolism in the clinical setting.

Immunoengineering Biomaterials for Musculoskeletal Tissue Repair across Lifespan

Musculoskeletal diseases and injuries are among the leading causes of pain and morbidity worldwide. Broad efforts have focused on developing pro-regenerative biomaterials to treat musculoskeletal conditions; however, these approaches have yet to make a significant clinical impact. Recent studies have demonstrated that the immune system is central in orchestrating tissue repair and that targeting pro-regenerative immune responses can improve biomaterial therapeutic outcomes. However, aging is a critical factor negatively affecting musculoskeletal tissue repair and immune function. Hence, understanding how age affects the response to biomaterials is essential for improving musculoskeletal biomaterial therapies. This review focuses on the intersection of the immune system and aging in response to biomaterials for musculoskeletal tissue repair. The article introduces the general impacts of aging on tissue physiology, the immune system, and the response to biomaterials. Then, it explains how the adaptive immune system guides the response to injury and biomaterial implants in cartilage, muscle, and bone and discusses how aging impacts these processes in each tissue type. The review concludes by highlighting future directions for the development and translation of personalized immunomodulatory biomaterials for musculoskeletal tissue repair.

OsteoMac: A new player on the bone biology scene

The knowledge of bone biology has undergone major advances in recent decades. In bone, resorbing osteoclasts have classically been described as tissue-resident macrophages, however, it is currently known that a new subtype of macrophages, called OsteoMacs, are specialised bone-resident macrophages, which, depending on certain conditions, may play an important role not only in bone homeostasis, but also in promoting pro-anabolic functions or in creating an inflammatory environment. There is growing evidence that these osteal macrophages may influence the development of bone-loss diseases. It is essential to understand the biological bases underlying bone physiological processes to search for new therapeutic targets for bone-loss diseases, such as osteoporosis, rheumatoid arthritis, or even periodontal disease. This narrative review provides an update on the origin, characterisation, and possible roles of osteoMacs in bone biology. Finally, the potential clinical applications of this new cell in bone-loss disorders are discussed.

Differences in macrophage expression in induced membranes by fixation method - Masquelet technique using a mouse's femur critical-sized bone defect model

INTRODUCTION

Masquelet's induced membrane technique (MIMT) is an emerging method for reconstructing critical-sized bone defects. However, an incomplete understanding of the underlying biological and physical processes hinders further optimization. This study investigated the effect of different bone-defect fixation methods on macrophage expression in an induced membrane using a novel mouse plate-fixed Masquelet model.

METHODS

Mice were divided into Plate-fixed Masquelet (P-M), Intramedullary-fixed Masquelet (IM-M), Plate-fixed Control (P-C), and Back subfascial (B) groups. In the P-M and IM-M groups, a polymethylmethacrylate (PMMA) spacer was implanted into a 3 mm bone defect, while the defect in the P-C group remained unfilled. In group B, a spacer was inserted under the back fascia to examine membrane formation caused by a simple foreign body reaction. Tissues were collected at 1, 2, and 4 weeks postoperatively. Hematoxylin and eosin (H&E) staining and immunohistochemistry (CD68 and CD163: macrophage markers) were performed to assess macrophage expression within the membrane. qPCR was performed to measure the expression of CD68, CD163, and fibroblast growth factor 2 (FGF2).

RESULTS

Four weeks post-operation, the P-M group presented with minimal callus growth, whereas the IM-M group exhibited vigorous growth. The P-M and IM-M groups displayed a tri-layered membrane structure, which is consistent with the results of previous studies. The IM-M group had significantly thicker membranes, whereas the P-M group exhibited higher expression levels of CD68, CD163, and FGF2. Group P-C showed no osteogenesis, whereas group B maintained a thin, cell-dense membrane structure. The P-M group consistently showed higher gene expression levels than the P-C and P-B groups.

CONCLUSION

This study introduced a mouse plate fixation model for MIMT. The induced membranes could be adequately evaluated in this model. Induced membranes are formed by foreign body reactions to PMMA spacers; however, their properties are clearly different from those of simple foreign body reaction capsules and granulation tissues that infiltrate bone defects, suggesting that they are more complex tissues. The characteristics and expression of macrophages within these induced membranes varied according to the bone defect fixation method.

Interaction of Macrophages with Bone Healing Microenvironment: Mechanism and Biomaterials

Extensive bone fractures, which can seriously impact both health and quality of life, cannot easily heal naturally, especially if the patient has an underlying medical condition or is aging. The most promising approach to addressing such fractures is bone regeneration through bone tissue engineering. Bone regeneration is a complex process that consists of three distinct phases: inflammation, repair, and remodeling. Macrophages play a bridging role between the various cells involved in each stage of bone regeneration, interacting with different microenvironments and advancing the bone healing process. Although the origin and function of macrophages have been extensively studied, the mechanisms underlying their interaction with the bone healing microenvironment remain unexplored, including the association of microenvironmental changes with macrophage reprogramming and the role of macrophages in cells in the microenvironment. This review summarizes the bone regeneration process and recent advances in research on interactions between macrophages and the bone healing microenvironment and discusses novel biological strategies to promote bone regeneration by modulating macrophages for the treatment of bone injury and loss.

The impact of age and sex on the inflammatory response during bone fracture healing

Inflammation is thought to be dysregulated with age leading to impaired bone fracture healing. However, broad analyses of inflammatory processes during homeostatic bone aging and during repair are lacking. Here, we assessed changes in inflammatory cell and cytokine profiles in circulation and in bone tissue to identify age- and sex-dependent differences during homeostasis and repair. During homeostatic aging, male mice demonstrated accumulation of CD4+ helper T cells and CD8+ cytotoxic T cells within bone while both pro-inflammatory "M1" and anti-inflammatory "M2" macrophage numbers decreased. Female mice saw no age-associated changes in immune-cell population in homeostatic bone. Concentrations of IL-1β, IL-9, IFNγ, and CCL3/MIP-1α increased with age in both male and female mice, whereas concentrations of IL-2, TNFα, TNFR1, IL-4, and IL-10 increased only in female mice - thus we termed these "age-accumulated" cytokines. There were no notable changes in immune cell populations nor cytokines within circulation during aging. Sex-dependent analysis demonstrated slight changes in immune cell and cytokine levels within bone and circulation, which were lost upon fracture injury. Fracture in young male mice caused a sharp decrease in number of M1 macrophages; however, this was not seen in aged male mice nor in female mice of any age. Injury itself induced a decrease in the number of CD8+ T cells within the local tissue of aged male and of female mice but not of young mice. Cytokine analysis of fractured mice revealed that age-accumulated cytokines quickly dissipated after fracture injury, and did not re-accumulate in newly regenerated tissue. Conversely, CXCL1/KC-GRO, CXCL2/MIP-2, IL-6, and CCL2/MCP-1 acted as "fracture response" cytokines: increasing sharply after fracture, eventually returning to baseline. Collectively, we classify measured cytokines into three groups: (1) age-accumulated cytokines, (2) female-specific age-accumulated cytokines, and (3) fracture response cytokines. These inflammatory molecules represent potential points of intervention to improve fracture healing outcome.

Osteomacs promote maintenance of murine hematopoiesis through megakaryocyte-induced upregulation of Embigin and CD166

Maintenance of hematopoietic stem cell (HSC) function in the niche is an orchestrated event. Osteomacs (OM) are key cellular components of the niche. Previously, we documented that osteoblasts, OM, and megakaryocytes interact to promote hematopoiesis. Here, we further characterize OM and identify megakaryocyte-induced mediators that augment the role of OM in the niche. Single-cell mRNA-seq, mass spectrometry, and CyTOF examination of megakaryocyte-stimulated OM suggested that upregulation of CD166 and Embigin on OM augment their hematopoiesis maintenance function. CD166 knockout OM or shRNA-Embigin knockdown OM confirmed that the loss of these molecules significantly reduced the ability of OM to augment the osteoblast-mediated hematopoietic-enhancing activity. Recombinant CD166 and Embigin partially substituted for OM function, characterizing both proteins as critical mediators of OM hematopoietic function. Our data identify Embigin and CD166 as OM-regulated critical components of HSC function in the niche and potential participants in various in vitro manipulations of stem cells.

Role of macrophages and their exosomes in orthopedic diseases

Exosomes are vesicles with a lipid bilayer structure that carry various active substances, such as proteins, DNA, non-coding RNA, and nucleic acids; these participate in the immune response, tissue formation, and cell communication. Owing to their low immunogenicity, exosomes play a key role in regulating the skeletal immune environment. Macrophages are important immune cells that swallow various cellular and tissue fragments. M1-like and M2-like macrophages differentiate to play pro-inflammatory, anti-inflammatory, and repair roles following stimulation. In recent years, the increase in the population base and the aging of the population have led to a gradual rise in orthopedic diseases, placing a heavy burden on the social medical system and making it urgent to find effective solutions. Macrophages and their exosomes have been demonstrated to be closely associated with the pathogenesis and prognosis of orthopedic diseases. An in-depth understanding of their mechanisms of action and the interaction between them will be helpful for the future clinical treatment of orthopedic diseases. This review focuses on the mechanisms of action, diagnosis, and treatment of orthopedic diseases involving macrophages and their exosomes, including fracture healing, diabetic bone damage, osteosarcoma, and rheumatoid arthritis. In addition, we discuss the prospects and major challenges faced by macrophages and their exosomes in clinical practice.

Isolation of Murine Neonatal and Adult Osteomacs to Examine Their Role in the Hematopoietic Niche

Maintenance of hematopoietic stem cell (HSC) function is an orchestrated event between multiple cell types, and crosstalk between these cell types is an essential part of HSC regulation. Among the cell groups of the niche involved in this process are a group of bone-resident macrophages known as osteomacs (OM). Previously, it was demonstrated that OM and osteoblasts contained within neonatal calvarial cells are critical to maintain hematopoietic function. Additionally, interactions between neonatal calvarial cells and megakaryocytes further enhance this hematopoietic activity. In this chapter, we explore one such interaction involving OM and osteoblasts in the hematopoietic niche. We describe a protocol to isolate OM from both neonatal and adult mice, and subsequently use colony-forming assays to demonstrate their interaction with osteoblasts in maintaining HSC function.

Importance of Metalloproteinase Enzyme Group in Selected Skeletal System Diseases

Bone tissue is a dynamic structure that is involved in maintaining the homeostasis of the body due to its multidirectional functions, such as its protective, endocrine, or immunological role. Specialized cells and the extracellular matrix (ECM) are responsible for the remodeling of specific bone structures, which alters the biomechanical properties of the tissue. Imbalances in bone-forming elements lead to the formation and progression of bone diseases. The most important family of enzymes responsible for bone ECM remodeling are matrix metalloproteinases (MMPs)-enzymes physiologically present in the body's tissues and cells. The activity of MMPs is maintained in a state of balance; disruption of their activity is associated with the progression of many groups of diseases, including those of the skeletal system. This review summarizes the current understanding of the role of MMPs in bone physiology and the pathophysiology of bone tissue and describes their role in specific skeletal disorders. Additionally, this work collects data on the potential of MMPs as bio-markers for specific skeletal diseases.

Inflammatory Processes Affecting Bone Health and Repair

PURPOSE OF REVIEW

The purpose of this article is to review the current understanding of inflammatory processes on bone, including direct impacts of inflammatory factors on bone cells, the effect of senescence on inflamed bone, and the critical role of inflammation in bone pain and healing.

RECENT FINDINGS

Advances in osteoimmunology have provided new perspectives on inflammatory bone loss in recent years. Characterization of so-called inflammatory osteoclasts has revealed insights into physiological and pathological bone loss. The identification of inflammation-associated senescent markers in bone cells indicates that therapies that reduce senescent cell burden may reverse bone loss caused by inflammatory processes. Finally, novel studies have refined the role of inflammation in bone healing, including cross talk between nerves and bone cells. Except for the initial stages of fracture healing, inflammation has predominately negative effects on bone and increases fracture risk. Eliminating senescent cells, priming the osteo-immune axis in bone cells, and alleviating pro-inflammatory cytokine burden may ameliorate the negative effects of inflammation on bone.

M2 macrophage-derived exosomes promote diabetic fracture healing by acting as an immunomodulator

Diabetes mellitus is a chronically inflamed disease that predisposes to delayed fracture healing. Macrophages play a key role in the process of fracture healing by undergoing polarization into either M1 or M2 subtypes, which respectively exhibit pro-inflammatory or anti-inflammatory functions. Therefore, modulation of macrophage polarization to the M2 subtype is beneficial for fracture healing. Exosomes perform an important role in improving the osteoimmune microenvironment due to their extremely low immunogenicity and high bioactivity. In this study, we extracted the M2-exosomes and used them to intervene the bone repair in diabetic fractures. The results showed that M2-exosomes significantly modulate the osteoimmune microenvironment by decreasing the proportion of M1 macrophages, thereby accelerating diabetic fracture healing. We further confirmed that M2-exosomes induced the conversion of M1 macrophages into M2 macrophages by stimulating the PI3K/AKT pathway. Our study offers a fresh perspective and a potential therapeutic approach for M2-exosomes to improve diabetic fracture healing.

Application of stem cells in regeneration medicine

Regeneration is a complex process affected by many elements independent or combined, including inflammation, proliferation, and tissue remodeling. Stem cells is a class of primitive cells with the potentiality of differentiation, regenerate with self-replication, multidirectional differentiation, and immunomodulatory functions. Stem cells and their cytokines not only inextricably linked to the regeneration of ectodermal and skin tissues, but also can be used for the treatment of a variety of chronic wounds. Stem cells can produce exosomes in a paracrine manner. Stem cell exosomes play an important role in tissue regeneration, repair, and accelerated wound healing, the biological properties of which are similar with stem cells, while stem cell exosomes are safer and more effective. Skin and bone tissues are critical organs in the body, which are essential for sustaining life activities. The weak repairing ability leads a pronounced impact on the quality of life of patients, which could be alleviated by stem cell exosomes treatment. However, there are obstacles that stem cells and stem cells exosomes trough skin for improved bioavailability. This paper summarizes the applications and mechanisms of stem cells and stem cells exosomes for skin and bone healing. We also propose new ways of utilizing stem cells and their exosomes through different nanoformulations, liposomes and nanoliposomes, polymer micelles, microspheres, hydrogels, and scaffold microneedles, to improve their use in tissue healing and regeneration.

From Synthesis to Clinical Trial: Novel Bioinductive Calcium Deficient HA/β-TCP Bone Grafting Nanomaterial

Maxillary sinus augmentation is a commonly used procedure for the placement of dental implants. However, the use of natural and synthetic materials in this procedure has resulted in postoperative complications ranging from 12% to 38%. To address this issue, we developed a novel calcium deficient HA/β-TCP bone grafting nanomaterial using a two-step synthesis method with appropriate structural and chemical parameters for sinus lifting applications. We demonstrated that our nanomaterial exhibits high biocompatibility, enhances cell proliferation, and stimulates collagen expression. Furthermore, the degradation of β-TCP in our nanomaterial promotes blood clot formation, which supports cell aggregation and new bone growth. In a clinical trial involving eight cases, we observed the formation of compact bone tissue 8 months after the operation, allowing for the successful installation of dental implants without any early postoperative complications. Our results suggest that our novel bone grafting nanomaterial has the potential to improve the success rate of maxillary sinus augmentation procedures.

Research Progress of Macrophages in Bone Regeneration

Bone tissue regeneration plays an increasingly important role in contemporary clinical treatment. The reconstruction of bone defects remains a huge challenge for clinicians. Bone regeneration is regulated by the immune system, in which inflammation is an important regulating factor in bone formation and remodeling. As the main cells involved in inflammation, macrophages play a key role in osteogenesis by polarizing into different phenotypes during different stages of bone regeneration. Considering this, this review mainly summarizes the function of macrophage in bone regeneration based on mesenchymal stem cells (MSCs), osteoblasts, osteoclasts, and vascular cells. In conclusion, anti-inflammatory macrophages (M2) have a greater potentiality to promote bone regeneration than M0 and classically activated proinflammatory macrophages (M1). In the fracture and bone defect models, tissue engineering materials can induce the transition from M1 to M2, alter the bone microenvironment, and promote bone regeneration through interactions with bone-related cells and blood vessels. The review provides a further understanding of macrophage polarization behavior in the evolving field of bone immunology.

The Cell-Specific Role of SHP2 in Regulating Bone Homeostasis and Regeneration Niches

Src homology-2 containing protein tyrosine phosphatase (SHP2), encoded by PTPN11, has been proven to participate in bone-related diseases, such as Noonan syndrome (NS), metachondromatosis and osteoarthritis. However, the mechanisms of SHP2 in bone remodeling and homeostasis maintenance are complex and undemonstrated. The abnormal expression of SHP2 can influence the differentiation and maturation of osteoblasts, osteoclasts and chondrocytes. Meanwhile, SHP2 mutations can act on the immune system, vasculature and nervous system, which in turn affect bone development and remodeling. Signaling pathways regulated by SHP2, such as mitogen-activated protein kinase (MAPK), Indian hedgehog (IHH) and phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/protein kinase B (AKT), are also involved in the proliferation, differentiation and migration of bone functioning cells. This review summarizes the recent advances of SHP2 on osteogenesis-related cells and niche cells in the bone marrow microenvironment. The phenotypic features of SHP2 conditional knockout mice and underlying mechanisms are discussed. The prospective applications of the current agonists or inhibitors that target SHP2 in bone-related diseases are also described. Full clarification of the role of SHP2 in bone remodeling will shed new light on potential treatment for bone related diseases.

Spatiotemporal Regulation of the Bone Immune Microenvironment via Dam-Like Biphasic Bionic Periosteum for Bone Regeneration

The bone immune microenvironment (BIM) regulates bone regeneration and affects the prognosis of fractures. However, there is currently no effective strategy that can precisely modulate macrophage polarization to improve BIM for bone regeneration. Herein, a hybridized biphasic bionic periosteum, inspired by the BIM and functional structure of the natural periosteum, is presented. The gel phase is composed of genipin-crosslinked carboxymethyl chitosan and collagen self-assembled hybrid hydrogels, which act as the "dam" to intercept IL-4 released during the initial burst from the bionic periosteum fiber phase, thus maintaining the moderate inflammatory response of M1 macrophages for mesenchymal stem cell recruitment and vascular sprouting at the acute fracture. With the degradation of the gel phase, released IL-4 cooperates with collagen to promote the polarization towards M2 macrophages, which reconfigure the local microenvironment by secreting PDGF-BB and BMP-2 to improve vascular maturation and osteogenesis twofold. In rat cranial defect models, the controlled regulation of the BIM is validated with the temporal transition of the inflammatory/anti-inflammatory process to achieve faster and better bone defect repair. This strategy provides a drug delivery system that constructs a coordinated BIM, so as to break through the predicament of the contradiction between immune response and bone tissue regeneration.

Osteogenic capacity of octacalcium phosphate involving macrophage polarization

Previous research has found that octacalcium phosphate (OCP) increases macrophage accumulation and alters the initial inflammatory response. However, the role of the immune response induced by OCP in osteogenesis remains unknown. This study investigated the behavior of macrophages and bone regeneration capacity during the early inflammatory stage of OCP-mediated osteogenesis. To assess the change in macrophage polarization and osteogenic capacity, we used a standardized rat defect model filled with OCP or calcium-deficient hydroxyapatite (CDHA)-a material obtained through the hydrolysis of the original OCP. OCP or CDHA granules were incubated with RAW264 cells for 5 days to investigate the effect of physicochemical characteristics on macrophage cytokine/chemokine expression in vitro. Our in vivo results show that due to the OCP implantation, macrophages in the rat tibial defect area tend to polarize to the M2 phenotype (anti-inflammatory) and inhibit the formation of the M1 phenotype (pro-inflammatory). In comparison to CDHA, OCP exhibited superior bone regeneration potential due to its rapid promotion of cortical bone healing and stimulation of macrophage-related growth factors. Furthermore, our in vitro results have shown that OCP regulates the expression of macrophage chemokines over time. Compared to incubation with CDHA, incubation with OCP caused changes in the ionic microenvironment. These findings suggest that the OCP-mediated macrophage polarization and secretion profile not only regulate immune function but also positively affect osteogenesis.

Performance of Polydioxanone-Based Membrane in Association with 3D-Printed Bioceramic Scaffolds in Bone Regeneration

This study evaluated the bioactivity of 3D-printed β-tricalcium phosphate (β-TCP) scaffolds or hydroxyapatite (HA) scaffolds associated with polydioxanone (PDO) membrane (Plenum^® Guide) for guided bone regeneration in rats. Fifty-four rats were divided into three groups (n = 18 animals): autogenous bone + PDO membrane (Auto/PG); 3D-printed β-TCP + PDO membrane (TCP/PG); and 3D-printed HA + PDO membrane (HA/PG). A surgical defect in the parietal bone was made and filled with the respective scaffolds and PDO membrane. The animals were euthanized 7, 30, and 60 days after the surgical procedure for micro-CT, histomorphometric, and immunolabeling analyses. Micro-CT showed an increase in trabecular thickness and a decrease in trabecular separation, even with similar bone volume percentages between TCP/PG and HA/PG vs. Auto/PG. Histometric analysis showed increased bone formation at 30 days in the groups compared to 7 days postoperatively. Immunolabeling analysis showed an increase in proteins related to bone formation at 30 days, and both groups showed a similar immunolabeling pattern. This study concludes that 3D-printed scaffolds associated with PDO membrane (Plenum^® Guide) present similar results to autogenous bone for bone regeneration.

Autogenous bone-guided induced membrane technique in closed/small-sized open high-energy fractures in benign inflammatory environment: a case series

OBJECTIVE

Infection and nonunion are the two most challenging issues for high-energy fractures. This study aimed to explore the clinical effect of benign inflammation-cultivated bone growth activity in the treatment of closed/small-sized open and high-energy fractures.

METHODS

This study is a case series of closed/small-sized open and high-energy fractures of the lower limbs treated at our hospital from April 2009 to February 2017. All patients underwent debridement and external fixation in the early stage, followed by internal fixation in the second stage. After the operation, fracture healing was monitored by X-ray, and early-stage knee function training was initiated. Also, bone grafting was performed to stimulate the healing reaction, eliminating the atrophic nonunion factors.

RESULTS

The operation in all 75 cases was carried out after the inflammatory responses completely subsided, leading to secondary wound healing. Bony union appeared in 71 patients who did not suffer from any pain and could stand up and walk without any restriction. Among them, 68 patients could flex their knee > 100°, and three patients had knee flexion ranging from 80 to 100°. No infections occurred after the second operation.

CONCLUSION

This two-stage treatment for high-energy fractures could avoid the damage caused by excessive inflammatory responses that occurred following early-stage one-time internal fixation. This method protected benign inflammatory-callus reactions induced by the primary injury and utilized the advantages of closed reduction in AO fixation with open reduction, thereby avoiding potential infection and nonunion caused by one-time fixation during the early stage.

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

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

Modulating macrophage polarization for the enhancement of fracture healing, a systematic review

Background

All fracture repairs start with the innate immune system with the inflammatory response known as the inflammatory stage guided and driven by the secretion of chemokine by the ruptured tissue, followed by the sequential recruitment of neutrophils, monocytes and macrophages. These innate immune cells would infiltrate the fracture site and secrete inflammatory cytokines to stimulate recruitment of more immune cells to arrive at the fracture site coordinating subsequent stages of the repair process. In which, subsidence of pro-inflammatory M1 macrophage and transformation to anti-inflammatory M2 macrophages promotes osteogenesis that marks the start of the anabolic endochondral stage.

Methods

Literature search was performed on Pubmed, Embase, and Web of Science databases (last accessed 15th April 2021) using “macrophage AND fracture”. Review was performed in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline.

Results

Eleven pre-clinical animal studies out of 429 articles were included in this systematic review according to our inclusion and exclusion criteria. All of which investigated interventions targeting to modulate the acute inflammatory response and macrophage polarization as evident by various markers in association with fracture healing outcomes.

Conclusion

This systematic review summarizes attempts to modulate the innate immune response with focuses on promoting macrophage polarization from M1 to M2 phenotype targeting the enhancement of fracture injury repair. Methods used to achieve the goal may include applications of damage-associated molecular pattern (DAMP), pathogen-associated molecular pattern (PAMP) or mechanical stimulation that hold high translational potentials for clinical application in the near future.

Cells-Micropatterning Biomaterials for Immune Activation and Bone Regeneration

Natural tissues are composed of ordered architectural organizations of multiple tissue cells. The spatial distribution of cells is crucial for directing cellular behavior and maintaining tissue homeostasis and function. Herein, an artificial bone bioceramic scaffold with star-, Tai Chi-, or interlacing-shaped multicellular patterns is constructed. The "cross-talk" between mesenchymal stem cells (MSCs) and macrophages can be effectively manipulated by altering the spatial distribution of two kinds of cells in the scaffolds, thus achieving controllable modulation of the scaffold-mediated osteo-immune responses. Compared with other multicellular patterns, the Tai Chi pattern with a 2:1 ratio of MSCs to macrophages is more effective in activating anti-inflammatory M2 macrophages, improving MSCs osteogenic differentiation, and accelerating new bone formation in vivo. In brief, the Tai Chi pattern generates a more favorable osteo-immune environment for bone regeneration, exhibiting enhanced immunomodulation and osteogenesis, which may be associated with the activation of BMP-Smad, Oncostatin M (OSM), and Wnt/β-catenin signaling pathways in MSCs mediated by macrophage-derived paracrine signaling mediators. The study suggests that the manipulation of cell distribution to improve tissue formation is a feasible approach that can offer new insights for the design of tissue-engineered bone substitutes with multicellular interactions.

Systemic and local innate immune responses to surgical co-transplantation of mesenchymal stromal cells and biphasic calcium phosphate for bone regeneration

Bone regeneration from mesenchymal stromal cells (MSC) is attributed to comprehensive immune modulation mediated by the MSC. However, the temporal and spatial regulation of these immune responses has not yet been described. The aim of the present study was to assess the local and systemic innate immune responses to implantation of biphasic calcium phosphate biomaterial (BCP) alone, or with bone marrow derived MSC (BCP+MSC), in critical-sized calvarial bone defects of Lewis rats. Four weeks after implantation, flow cytometry analysis of innate immune cells revealed increased numbers of circulating classical monocyte-macrophages (MM) and decreased non-classical MM in the BCP+MSC group. At week 8, this differential systemic MM response was associated with an increased presence of local tissue anti-inflammatory macrophages expressing CD68 and CD163 markers (M2-like). In the BCP group without MSC, NK cells increased at weeks 1 and 4, and neutrophils increased in circulation at weeks 2 and 8. At week 8, the increase in number of neutrophils in circulation was associated with decreased local tissue neutrophils, in the BCP+MSC group. Gene expression analysis of tissue biopsies from defects implanted with BCP+MSC, in comparison to BCP alone, revealed upregulated expression of early osteogenesis genes along with macrophage differentiation-related genes at weeks 1 and 8 and neutrophil chemotaxis-related genes at week 1. This study is the first to demonstrate that surgical implantation of BCP or BCP+MSC grafts differentially regulate both systemic and local tissue innate immune responses which enhance bone formation. The results provide new insights into immune mechanisms underlying MSC-mediated bone regeneration. STATEMENT OF SIGNIFICANCE: The suitability of biphasic calcium phosphate and mesenchymal stromal cell construct (BCP+MSC) transplantation is evident from their progress in clinical trials for treating challenging maxillofacial bone defects. But less is known about the overall immune response generated by this surgical process and how it later impacts the bone formation. To this end, it is crucial to understand for both clinicians and researchers, the systemic immune response to transplanting MSC in patients for ensuring both the safety and efficacy of cell therapies. In this study, we used rat calvarial bone defect model and showed that both systemic and local innate immunes responses (monocyte-macrophages and neutrophils) are favorably directed towards enhanced bone formation in BCP+MSC implanted defects, as compared to BCP alone.

Sequential Delivery of Different MicroRNA Nanocarriers Facilitates the M1-to-M2 Transition of Macrophages

The early-stage repair of bone injuries dominated by the inflammatory phase is significant for successful bone healing, and the phenotypic transition of macrophages in the inflammatory phase plays indispensable roles during the bone healing process. The goal of this paper is to design a microRNA delivery nanocarrier for strictly temporal guidance of the polarization of macrophages by the sequential delivery of different microRNAs. The results showed that microRNA nanocarriers, synthesized through free radical polymerization, could be internalized by macrophages with about a cellular uptake efficiency of 80%, and the sequential delivery of microRNA-155 nanocarriers and microRNA-21 nanocarriers proved, for the first time, that it could promote an efficient and timely switch from the M1 to the M2 phenotype along the time point of bone tissue repair. The strategy proposed in this paper holds potential for controlling sequential M1-to-M2 polarization of macrophages, which provides another perspective for the treatment of bone tissue regeneration.

Enhanced Bone Remodeling After Fracture Priming

The immune system is an active component of bone repair. Mast cells influence the recruitment of macrophages, osteoclasts and blood vessels into the repair tissue. We hypothesized that if mast cells and other immune cells are sensitized to recognize broken bone, they will mount an increased response to subsequent fractures that may be translated into enhanced healing. To test this, we created a bone defect on the left leg of anesthetized mice and 2 weeks later, a second one on the right leg. Bone repair in the right legs was then compared to control mice that underwent the creation of bilateral window bone defects at the same time. Mice were euthanized at 14 and 56 days. Mineralized tissue quantity and morphometric parameters were assessed using micro-CT and histology. The activity of osteoblasts, osteoclasts, vascular endothelial cells, mast cells, and macrophages was evaluated using histochemistry. Our main findings were (1) no significant differences in the amount of bone produced at 14- or 56 days post-operative between groups; (2) mice exposed to subsequent fractures showed significantly better bone morphometric parameters after 56 days post-operative; and (3) significant increases in the content of blood vessels, osteoclasts, and the number of macrophages in the subsequent fracture group. Our results provide strong evidence that a transient increase in the inflammatory state of a healing injury promotes faster bone remodelling and increased neo-angiogenesis. This phenomenon is also characterized by changes in mast cell and macrophage content that translate into more active recruitment of mesenchymal stromal cells.

In Situ Gene Expression in Native Cryofixed Bone Tissue

Bone is a very complex tissue that is constantly changing throughout the lifespan. The precise mechanism of bone regeneration remains poorly understood. Large bone defects can be caused by gunshot injury, trauma, accidents, congenital anomalies and tissue resection due to cancer. Therefore, understanding bone homeostasis and regeneration has considerable clinical and scientific importance in the development of bone therapy. Macrophages are well known innate immune cells secreting different combinations of cytokines and their role in bone regeneration during bone healing is essential. Here, we present a method to identify mRNA transcripts in cryosections of non-decalcified rat bone using in situ hybridization and hybridization chain reaction to explore gene expression in situ for better understanding the gene expression of the bone tissues.

CCR2+ Macrophages Promote Orthodontic Tooth Movement and Alveolar Bone Remodeling

During mechanical force-induced alveolar bone remodeling, macrophage-mediated local inflammation plays a critical role. Yet, the detailed heterogeneity of macrophages is still unknown. Single-cell RNA sequencing was used to study the transcriptome heterogeneity of macrophages during alveolar bone remodeling. We identified macrophage subclusters with specific gene expression profiles and functions. CellChat and trajectory analysis revealed a central role of the Ccr2 cluster during development, with the CCL signaling pathway playing a crucial role. We further demonstrated that the Ccr2 cluster modulated bone remodeling associated inflammation through an NF-κB dependent pathway. Blocking CCR2 could significantly reduce the Orthodontic tooth movement (OTM) progression. In addition, we confirmed the variation of CCR2+ macrophages in human periodontal tissues. Our findings reveal that mechanical force-induced functional shift of the Ccr2 macrophages cluster mediated by NF-κB pathway, leading to a pro-inflammatory response and bone remodeling. This macrophage cluster may represent a potential target for the manipulation of OTM.

Clinically relevant preclinical animal models for testing novel cranio-maxillofacial bone 3D-printed biomaterials

Bone tissue engineering is a rapidly developing field with potential for the regeneration of craniomaxillofacial (CMF) bones, with 3D printing being a suitable fabrication tool for patient-specific implants. The CMF region includes a variety of different bones with distinct functions. The clinical implementation of tissue engineering concepts is currently poor, likely due to multiple reasons including the complexity of the CMF anatomy and biology, and the limited relevance of the currently used preclinical models. The 'recapitulation of a human disease' is a core requisite of preclinical animal models, but this aspect is often neglected, with a vast majority of studies failing to identify the specific clinical indication they are targeting and/or the rationale for choosing one animal model over another. Currently, there are no suitable guidelines that propose the most appropriate animal model to address a specific CMF pathology and no standards are established to test the efficacy of biomaterials or tissue engineered constructs in the CMF field. This review reports the current clinical scenario of CMF reconstruction, then discusses the numerous limitations of currently used preclinical animal models employed for validating 3D-printed tissue engineered constructs and the need to reduce animal work that does not address a specific clinical question. We will highlight critical research aspects to consider, to pave a clinically driven path for the development of new tissue engineered materials for CMF reconstruction.

Thioacetamide promotes osteoclast transformation of bone marrow macrophages by influencing PI3K/AKT pathways

Background

Osteoclast cell increase is a major risk factor for osteoporosis and degenerative bone and joint diseases. At present, RANKL and M-CSF are commonly used to induce osteoclastogenesis. Thioacetamide (TAA) can lead to many types of liver and kidney damage, but less attention has been paid to the association of TAA with bone damage. In this work, we investigated the effects of TAA on the osteoclastogenesis and differentiation of bone marrow macrophages (BMMs).

Methods

BMMs of SD rat suckling mice were taken for primary culture. CCK-8 was used to detect the toxic effects of TAA on BMMs, and flow cytometry was used to detect the effects of TAA on the cell cycle, cell viability, apoptosis and intracytoplasmic Ca2+ concentration of BMMs. TRAP staining was used to detect the effect of RANKL and M-CSF and TAA on osteoclast differentiation of BMMs. Western Blot was used to detect the expression level of PI3K/AKT pathway and osteoclast-specific proteins (TRAP and cathepsin K).

Results

The results suggested that TAA inhibited the proliferation of BMMs, while enhancing osteoclastogenesis at 0.5 mg/mL and 1 mg/mL as assayed by TRAP staining. Exposed to TAA, BMMs could differentiate into osteoclast-like cells with overexpression of cathepsin K and TRAP proteins. Western blot results showed that TAA can activate the expression levels of P-PI3K, P-AKT, P-P38, and P-JNK, accompanied by apoptosis of BMMs and increase in intracellular Ca2+.

Conclusion

TAA may induce osteoclast formation in BMMs by activating the expression of PI3K/AKT pathway proteins, which is comparable to the classic osteoclast differentiation inducer RANKL and M-CSF. This suggests that we may find a cheap osteoclast inducer.

Macrophage Involvement in Medication-Related Osteonecrosis of the Jaw (MRONJ): A Comprehensive, Short Review

Simple Summary

Medication-related Osteonecrosis of the Jaw (MRONJ) is a significant complication mainly of antiresorptive medications used in the management of bone diseases. MRONJ development may be accompanied by pain, eating discomfort, self-consciousness, and other symptoms that overall disturb patients’ everyday life. Hence, MRONJ occurrence is of growing clinical concern and affects treatment decisions. Although MRONJ has been extensively studied since being first reported in 2003, the mechanisms of disease pathogenesis have not yet been determined and disease management is mostly empirical. Recent data investigate the effects of antiresorptive medications on immune system components including macrophages and introduce these cells as key players in MRONJ pathogenesis. Considering macrophage versatility, developmental plasticity, and its pivotal role in immune response, the current short review focused on the potential involvement of these multi-potential cells in MRONJ pathogenesis. Understanding the complex role of macrophages in MRONJ pathophysiology will add new valuable data on disease prevention and control.

Abstract

Antiresorptive agents such as bisphosphonates (BP) and denosumab are commonly prescribed for the management of primary bone malignancy, bone metastasis, osteoporosis, Paget disease, or other bone disorders. Medication-related osteonecrosis of the Jaws (MRONJ) is a rare but significant complication of antiresorptive medications. Duration, dose, and antiresorptive potency as well as concomitant diseases, additional medications, and local factors affect MRONJ incidence and severity. MRONJ pathophysiology is still poorly understood. Nevertheless, decreased bone resorption due to osteoclastic inhibition along with trauma, infection/inflammation, or blood supply inhibition are considered synergistic factors for disease development. In addition, previous data research examined the effects of antiresorptive medication on immune system components and introduced potential alterations on immune response as novel elements in MRONJ pathogenesis. Considering that macrophages are the first cells in the nonspecific immune response, it is not surprising that these multifaceted players attracted increased attention in MRONJ research recently. This current review attempted to elucidate the effects of antiresorptive medications on several aspects of macrophage activity in relation to the complex inflammatory microenvironment of MRONJ. Collectively, unravelling the mode of action and extent of macrophages’ potential contribution in MRONJ occurrence will provide novel insight in disease pathogenesis and potentially identify intrinsic therapeutic targets.

Construction of a drug-containing microenvironment for in situ bone regeneration

Bioactive glass-coated hierarchical porous tricalcium phosphate ceramics were constructed as both bone scaffolds and drug delivery devices to treat S. aureus-infected bone defects.

Icariin-loaded Sulfonated Polyetheretherketone with Osteogenesis Promotion and Osteoclastogenesis Inhibition Properties via Immunomodulation for Advanced Osseointegration

Preventing prosthesis loosening due to insufficient osseointegration is critical for patients with osteoporosis. Endowing implants with immunomodulatory function can effectively enhance osseointegration. In this work, we loaded icariin (ICA) onto...

Perspective of the GEMSTONE Consortium on Current and Future Approaches to Functional Validation for Skeletal Genetic Disease Using Cellular, Molecular and Animal-Modeling Techniques

The availability of large human datasets for genome-wide association studies (GWAS) and the advancement of sequencing technologies have boosted the identification of genetic variants in complex and rare diseases in the skeletal field. Yet, interpreting results from human association studies remains a challenge. To bridge the gap between genetic association and causality, a systematic functional investigation is necessary. Multiple unknowns exist for putative causal genes, including cellular localization of the molecular function. Intermediate traits ("endophenotypes"), e.g. molecular quantitative trait loci (molQTLs), are needed to identify mechanisms of underlying associations. Furthermore, index variants often reside in non-coding regions of the genome, therefore challenging for interpretation. Knowledge of non-coding variance (e.g. ncRNAs), repetitive sequences, and regulatory interactions between enhancers and their target genes is central for understanding causal genes in skeletal conditions. Animal models with deep skeletal phenotyping and cell culture models have already facilitated fine mapping of some association signals, elucidated gene mechanisms, and revealed disease-relevant biology. However, to accelerate research towards bridging the current gap between association and causality in skeletal diseases, alternative in vivo platforms need to be used and developed in parallel with the current -omics and traditional in vivo resources. Therefore, we argue that as a field we need to establish resource-sharing standards to collectively address complex research questions. These standards will promote data integration from various -omics technologies and functional dissection of human complex traits. In this mission statement, we review the current available resources and as a group propose a consensus to facilitate resource sharing using existing and future resources. Such coordination efforts will maximize the acquisition of knowledge from different approaches and thus reduce redundancy and duplication of resources. These measures will help to understand the pathogenesis of osteoporosis and other skeletal diseases towards defining new and more efficient therapeutic targets.

The influence of M-CSF on fracture healing in a mouse model

Macrophage colony-stimulating factor 1 (M-CSF) is known to play a critical role during fracture repair e.g. by recruiting stem cells to the fracture site and impacting hard callus formation by stimulating osteoclastogenesis. The aim of this experiment was to study the impact of systemic M-CSF application and its effect on bony healing in a mouse model of femoral osteotomy. Doing so, we studied 61 wild type (wt) mice (18-week-old female C57BL/6) which were divided into three groups: (1) femoral osteotomy, (2) femoral osteotomy + stabilization with external fixator and (3) femoral osteotomy + stabilization with external fixator + systemic M-CSF application. Further, 12 op/op mice underwent femoral osteotomy and served as proof of concept. After being sacrificed at 28 days bony bridging was evaluated ex vivo with µCT, histological and biomechanical testing. Systemic M-CSF application impacted osteoclasts numbers, which were almost as low as found in op/op mice. Regarding callus size, the application of M-CSF in wt mice resulted in significantly larger calluses compared to wt mice without systemic M-CSF treatment. We further observed an anabolic effect of M-CSF application resulting in increased trabecular thickness compared to wt animals without additional M-CSF application. Systemic M-CSF application did not alter biomechanical properties in WT mice. The impact of M-CSF application in a mouse model of femoral osteotomy was oppositional to what we were expecting. While M-CSF application had a distinct anabolic effect on callus size as well as trabecular thickness, this on bottom line did not improve biomechanical properties. We hypothesize that in addition to the well-recognized negative effects of M-CSF on osteoclast numbers this seems to further downstream cause a lack of feedback on osteoblasts. Ultimately, continuous M-CSF application in the absence of co-stimulatory signals (e.g. RANKL) might overstimulate the hematopoietic linage in favor of tissue macrophages instead of osteoclasts.

Cross Talk Between Macrophages and Cancer Cells in the Bone Metastatic Environment

The skeleton is a common site for cancer metastases with the bone microenvironment providing the appropriate conditions for cancer cell colonization. Once in bone, cancer cells effectively manipulate their microenvironment to support their growth and survival. Despite previous efforts to improve treatment modalities, skeletal metastases remain with poor prognoses. This warrants an improved understanding of the mechanisms leading to bone metastasis that will aid development of effective treatments. Macrophages in the tumor microenvironment are termed tumor associated macrophages (TAMs) and their crosstalk with cancer cells is critical in regulating tumorigenicity in multiple cancers. In bone metastases, this crosstalk is also being increasingly implicated but the specific signaling pathways remain incompletely understood. Here, we summarize the reported functions, interactions, and signaling of macrophages with cancer cells during the metastatic cascade to bone. Specifically, we review and discuss how these specific interactions impact macrophages and their profiles to promote tumor development. We also discuss the potential of targeting this crosstalk to inhibit disease progression. Finally, we identify the remaining knowledge gaps that will need to be addressed in order to fully consider therapeutic targeting to improve clinical outcomes in cancer patients.

Osteal macrophages support osteoclast-mediated resorption and contribute to bone pathology in a postmenopausal osteoporosis mouse model

Osteal macrophages (osteomacs) support osteoblast function and promote bone anabolism, but their contribution to osteoporosis has not been explored. Although mouse ovariectomy (OVX) models have been repeatedly used, variation in strain, experimental design and assessment modalities have contributed to no single model being confirmed as comprehensively replicating the full gamut of osteoporosis pathological manifestations. We validated an OVX model in adult C3H/HeJ mice and demonstrated that it presents with human postmenopausal osteoporosis features with reduced bone volume in axial and appendicular bone and bone loss in both trabecular and cortical bone including increased cortical porosity. Bone loss was associated with increased osteoclasts on trabecular and endocortical bone and decreased osteoblasts on trabecular bone. Importantly, this OVX model was characterized by delayed fracture healing. Using this validated model, we demonstrated that osteomacs are increased post-OVX on both trabecular and endocortical bone. Dual F4/80 (pan-macrophage marker) and tartrate-resistant acid phosphatase (TRAP) staining revealed osteomacs frequently located near TRAP⁺ osteoclasts and contained TRAP⁺ intracellular vesicles. Using an in vivo inducible macrophage depletion model that does not simultaneously deplete osteoclasts, we observed that osteomac loss was associated with elevated extracellular TRAP in bone marrow interstitium and increased serum TRAP. Using in vitro high-resolution confocal imaging of mixed osteoclast-macrophage cultures on bone substrate, we observed macrophages juxtaposed to osteoclast basolateral functional secretory domains scavenging degraded bone byproducts. These data demonstrate a role for osteomacs in supporting osteoclastic bone resorption through phagocytosis and sequestration of resorption byproducts. Overall, our data expose a novel role for osteomacs in supporting osteoclast function and provide the first evidence of their involvement in osteoporosis pathogenesis. © 2021 American Society for Bone and Mineral Research (ASBMR).

Roles of Non-Canonical Wnt Signalling Pathways in Bone Biology

The Wnt signalling pathway is one of the central signalling pathways in bone development, homeostasis and regulation of bone mineral density. It consists of numerous Wnt ligands, receptors and co-receptors, which ensure tight spatiotemporal regulation of Wnt signalling pathway activity and thus tight regulation of bone tissue homeostasis. This enables maintenance of optimal mineral density, tissue healing and adaptation to changes in bone loading. While the role of the canonical/β-catenin Wnt signalling pathway in bone homeostasis is relatively well researched, Wnt ligands can also activate several non-canonical, β-catenin independent signalling pathways with important effects on bone tissue. In this review, we will provide a thorough overview of the current knowledge on different non-canonical Wnt signalling pathways involved in bone biology, focusing especially on the pathways that affect bone cell differentiation, maturation and function, processes involved in bone tissue structure regulation. We will describe the role of the two most known non-canonical pathways (Wnt/planar cell polarity pathways and Wnt/Ca2+ pathway), as well as other signalling pathways with a strong role in bone biology that communicate with the Wnt signalling pathway through non-canonical Wnt signalling. Our goal is to bring additional attention to these still not well researched but important pathways in the regulation of bone biology in the hope of prompting additional research in the area of non-canonical Wnt signalling pathways.