High Fluoride Ingestion Impairs Bone Fracture Healing by Attenuating M2 Macrophage Differentiation

Zebrafish larvae as a model for investigating dual effects of fluoride on bone development

Long-term excessive ingestion of fluoride is a severe public health threat globally. Skeletal fluorosis, a significant manifestation of prolonged fluoride exposure, is characterized by aberrant bone structure and alterations in bone function. However, there is currently a shortage of an efficient, fast, and easy-to-operate biological model for application in the field of fluorosis research. Zebrafish larvae, with human - like skeletal traits, high reproduction, rapid development, and transparency, are commonly used in bone disease studies. This study evaluates the potential of zebrafish larvae as a novel model for fluoride-induced bone impairment. Results showed dose-dependent differences in cranial and spinal bone mineralization in zebrafish larvae exposed to sodium fluoride (NaF). The detection results of bone formation-related indicators indicated a considerable increase in alkaline phosphatase (ALP) activity in zebrafish larvae at doses of 0.5 and 1 mg/L. Simultaneously, the expression of critical bone formation proteins (BMP2, and β-catenin) was elevated in the 1 and 4 mg/L groups, which is largely consistent with the results of cranial bone mineralization. Fluoride - exposed zebrafish also showed abnormal bone metabolism markers. The total phosphorus (TP) content in the zebrafish larvae of the 100 mg/L group was markedly reduced. The total calcium (TCa) content in the zebrafish of the NaF group zebrafish was slightly decreased, although the tartrate-resistant acid phosphatase (StrACP) activity increased. In conclusion, different fluoride doses cause osteoporosis and osteosclerosis in zebrafish larvae, linked to enhanced osteogenic and osteoclastic activities and abnormal key bone - forming protein expression.

Low Fluoride Regulates Macrophage Polarization Through Mitochondrial Autophagy Mediated by PINK1/Parkin Axis

Fluoride exposure has been shown to affect immune cell subsets and immune function, but its impact on macrophage polarization remains unclear. This study investigates the effects of low fluoride exposure on macrophage polarization and its underlying mechanisms through epidemiological surveys, animal experiments, and in vitro cell experiments. In the population-based epidemiological survey, we used mass cytometry to assess the impact of low fluoride exposure (0.570-2.027 mg/L) in the environment on human immune cell populations following the current water improvement and fluoride reduction measures. A rat fluorosis model was established by treating rats with sodium fluoride (NaF) in drinking water at concentrations of 0 mg/L, 5 mg/L, 10 mg/L, 25 mg/L, and 50 mg/L for 90 days., and morphological changes were assessed by hematoxylin-eosin (H&E) staining and transmission electron microscopy in the spleen of rats. Flow cytometry was used to analyze the proportion of macrophage subtypes in the spleen, while Western blot and immunofluorescence were performed to detect the expression of mitochondrial autophagy-related proteins. An M1 macrophage model was constructed in vitro by inducing THP-1 cells, and the effects of fluoride on macrophage-related cell markers and cytokines were assessed using flow cytometry and ELISA, respectively, following intervention with an autophagy inhibitor. Mitochondrial membrane potential and mitochondrial-lysosomal colocalization are analyzed through flow cytometry and confocal microscopy. The study aims to investigate the role of mitophagy in sodium fluoride-induced macrophage polarization. Epidemiological investigations revealed that low fluoride increases the proportion of blood monocytes, as well as the expression levels of CD68 (a macrophage surface marker), CD86 (an M1 macrophage marker), and the inflammatory cytokine IFN-γ in peripheral blood mononuclear cells (PBMCs). In the rats of NaF-treated groups, splenic tissues exhibited inflammatory infiltration, mitochondrial swelling, and increased autophagosome formation. Moreover, low fluoride activated the PINK1/Parkin-mediated mitophagy pathway, promoting an increase in the M2/M1 macrophage ratio. In vitro experiments further confirmed that autophagy inhibitors reversed the NaF-induced increase in the M2/M1 macrophage ratio. This study demonstrates that low fluoride induces inflammatory responses in the body and drives M1 macrophage polarization toward M2 macrophages via mitophagy. These findings highlight the potential immunological risks associated with low fluoride and provide mechanistic insights into the interplay among fluoride, mitophagy, and macrophage polarization.

Inhibition of M2 macrophage-mediated mesenchymal stem cell migration: Boldine attenuates elbow heterotopic ossification

BACKGROUND

Heterotopic ossification (HO) is characterized by abnormal bone formation in soft tissues, often following trauma or surgery. Transforming growth factor-beta (TGF-β) signaling and M2 macrophage polarization play critical roles in the recruitment and differentiation of mesenchymal stromal/progenitor cells (MSPCs), promoting HO.

METHODS

An elbow joint trauma-induced HO mouse model was established, where model mice were treated with dichloromethylene-bisphosphonate (Cl2MBP) liposomes or PBS liposomes to deplete macrophages. In addition, boldine was administered to evaluate its therapeutic effect on HO formation. Bone marrow mesenchymal stem cells (BMSCs) were also extracted for in vitro experiments. Quantitative real-time PCR (qRT-PCR) and Western blot were conducted to assess gene and protein expression. In vivo methods included Micro-Computed Tomography (Micro-CT) to assess bone formation, histological staining to evaluate tissue changes, immunohistochemistry (IHC) and immunofluorescence to analyze macrophage, CD73+ and CD105+ cells infiltration. In vitro, BMSCs were identified by flow cytometry and treated with interleukin-10 (IL-10) and/or boldine, and assays such as cell viability (Cell Counting Kit 8 (CCK8)), migration (Transwell), immunofluorescence, ALP staining, and Alizarin Red S staining, were conducted to assess osteogenic differentiation.

RESULTS

Boldine treatment significantly reduced HO formation, decreased collagen deposition, and inhibited M2 macrophage infiltration (P < 0.05). In vitro, boldine reduced IL-10-induced cell activity, migration, and osteogenic differentiation of BMSCs and inhibited TGF-β and pSmad2/3/Smad2/3 protein (P < 0.05).

CONCLUSION

Boldine attenuates HO by inhibiting M2 macrophage-mediated MSPC migration and might involve the TGF-β signaling, suggesting its potential as a therapeutic approach for managing HO.

Fluoride induces immune-inflammatory disorder in the kidneys via histone lysine crotonylation in vivo

Fluoride is an essential trace element for human. Adequate levels of fluoride are crucial for maintaining skeletal growth, but excessive fluoride exposure entering the body can cause renal damage, including damaged renal tubules and impaired renal function. However, the mechanism on fluoride-induced kidney injury remains unclear. This study aimed to explore the immune-inflammatory imbalance induced by fluoride and its possible mechanism in the kidneys. Mice were exposed to sodium fluoride (NaF) (0, 25, 50 and 100 mg/L) for five months. The results showed that NaF increased the renal weight and renal index. The NaF-treated groups exhibited higher serum creatinine (Cre), blood urea nitrogen (BUN), albumin (ALB) total protein (TP) levels. Further, NaF increased reactive oxygen species (ROS) levels, lipid peroxidation (LPO) levels and malondialdehyde (MDA) level. Superoxide dismutase (SOD) activity was reduced and glutathione (GSH) activities were reduced in fluoride-treated group. NaF treatment also downregulated the nuclear factor E2-related factor (Nrf2) protein and its downstream enzymes heme oxygenase-1 (HO-1) and NAD(P)H: Quinone Oxidoreductase 1(NQO1) in the kidneys. Further, NaF shifted Th1/Th2 balance toward Th1 bias. Similarly, NaF exhibited increased macrophages and augmented M1 differentiation but suppressed M2 differentiation. The renal inflammatory response was also induced by fluoride via activation of the NOD-like receptor family pyrin domain containing 3 (NLRP3) inflammasome and increase of the pro-inflammatory factors tumour necrosis factor-α (TNF-α), transforming growth factor-β (TGF-β), interleukin-6 (IL-6) and interleukin-18 (IL-18). In addition, NaF treatment reduced the expression of the histone 2B lysine 12 crotonylation (H2BK12cr) and H4K8cr proteins as well as decreased the histone acetyltransferase P300 protein. NaF incresed the protein expression of histone decrotonylation enzyme sirtuin1 (sirt1) and histone deacetylase 3 (HDAC3) and upregulated HDAC2 protein. These findings demonstrate that fluoride exposure induces renal dysfunction and oxidative injury, affects M1/M2 polarization and Th1/Th2 differentiation, and promotes the inflammatory response via histone lysine crotonylation, ultimately resulting in nephrotoxicity.

Bone and Joint-on-Chip Platforms: Construction Strategies and Applications

Organ-on-a-chip, also known as "tissue chip," is an advanced platform based on microfluidic systems for constructing miniature organ models in vitro. They can replicate the complex physiological and pathological responses of human organs. In recent years, the development of bone and joint-on-chip platforms aims to simulate the complex physiological and pathological processes occurring in human bones and joints, including cell-cell interactions, the interplay of various biochemical factors, the effects of mechanical stimuli, and the intricate connections between multiple organs. In the future, bone and joint-on-chip platforms will integrate the advantages of multiple disciplines, bringing more possibilities for exploring disease mechanisms, drug screening, and personalized medicine. This review explores the construction and application of Organ-on-a-chip technology in bone and joint disease research, proposes a modular construction concept, and discusses the new opportunities and future challenges in the construction and application of bone and joint-on-chip platforms.

Multimodal analyses of immune cells during bone repair identify macrophages as a therapeutic target in musculoskeletal trauma

Musculoskeletal traumatic injuries (MTI) involve soft tissue lesions adjacent to a bone fracture leading to fibrous nonunion. The impact of MTI on the inflammatory response to fracture and on the immunomodulation of skeletal stem/progenitor cells (SSPCs) remains unknown. Here, we used single-nucleus transcriptomic analyses to describe the immune cell dynamics after bone fracture and identified distinct macrophage subsets with successive pro-inflammatory, pro-repair and anti-inflammatory profiles. Concurrently, SSPCs transition via a pro- and anti-inflammatory fibrogenic phase of differentiation prior to osteochondrogenic differentiation. In a preclinical MTI mouse model, the injury response of immune cells and SSPCs is disrupted leading to a prolonged pro-inflammatory phase and delayed resolution of inflammation. Macrophage depletion improves bone regeneration in MTI demonstrating macrophage involvement in fibrous nonunion. Finally, pharmacological inhibition of macrophages using the CSF1R inhibitor Pexidartinib ameliorates healing. These findings reveal the coordinated immune response of macrophages and skeletal stem/progenitor cells as a driver of bone healing and as a primary target for the treatment of trauma-associated fibrosis.

Fluoride Exposure from Drinking Water Increases the Risk of Stroke: An Ecological Study in Changwu Town, China

BACKGROUND

Stroke is a major cause of death globally and the leading cause in China. Excessive fluoride exposure has been linked to cardiovascular conditions related to stroke risk factors such as hypertension, atherosclerosis, dyslipidemia, and cardiomyopathy. However, evidence supporting the association between fluoride exposure and stroke risk is limited.

METHODS

We constructed an ecological study in Changwu Town, Heilongjiang Province, China, a typical endemic fluorosis area caused by excessive fluoride exposure from drinking water. We collected demographic data, stroke prevalence, and mortality information from 2017 to 2021. Fluoride exposure data were obtained from the national monitoring project on endemic fluorosis. Water fluoride concentrations were measured using the standardized methods. Trend changes in stroke rates were assessed using annual percentage change (APC). Differences in stroke rates among fluoride exposure groups were analyzed using chi-square tests.

RESULTS

From 2017 to 2021, the all-ages and age-standardized stroke prevalence rates of permanent residents in Changwu Town increased year by year, while the all-ages and age-standardized mortality rates did not change significantly. The prevalence rates of stroke were significantly higher in endemic fluorosis areas compared to non-endemic areas (p < 0.001). Stratifying the population into tertile groups based on the water fluoride cumulative exposure index (WFCEI) revealed statistically significant differences in stroke prevalence rates (p < 0.001), showing a dose-response relationship with the WFCEI. However, the all-ages and age-standardized mortality rates of stroke were not found to be related to fluoride exposure.

CONCLUSIONS

Long-term excessive fluoride exposure from drinking water may increase the risk of stroke prevalence, indicating fluoride overexposure as a potential risk factor for stroke.

Progress in research on the role of fluoride in immune damage

Excessive fluoride intake from residential environments may affect multiple tissues and organs; however, the specific pathogenic mechanisms are unclear. Researchers have recently focused on the damaging effects of fluoride on the immune system. Damage to immune function seriously affects the quality of life of fluoride-exposed populations and increases the incidence of infections and malignant tumors. Probing the mechanism of damage to immune function caused by fluoride helps identify effective drugs and methods to prevent and treat fluorosis and improve people's living standards in fluorosis-affected areas. Here, the recent literature on the effects of fluoride on the immune system is reviewed, and research on fluoride damage to the immune system is summarized in terms of three perspectives: immune organs, immune cells, and immune-active substances. We reviewed that excessive fluoride can damage immune organs, lead to immune cells dysfunction and interfere with the expression of immune-active substances. This review aimed to provide a potential direction for future fluorosis research from the perspective of fluoride-induced immune function impairment. In order to seek the key regulatory indicators of fluoride on immune homeostasis in the future.

Regulatory T cell-derived exosome mediated macrophages polarization for osteogenic differentiation in fracture repair

Refractory fracture presents an intractable challenge in trauma treatment. Selective polarization of macrophages as well as the recruitment of osteogenic precursor cells play key roles in osteogenic differentiation during fracture healing. Here we constructed regulatory T cell (Treg)-derived exosomes (Treg-Exo) for the treatment of fracture. The obtained exosomes displayed a spheroid shape with a hydrated particle size of approximately 130 nm. With further purification using CD39 and CD73 antibody-modified microfluidic chips, CD39 and CD73 specifically expressing exosomes were obtained. This kind of Treg-Exo utilized the ectonucleotidases of CD39 and CD73 to catalyze the high level of ATP in the fracture area into adenosine. The generated adenosine further promoted the selective polarization of macrophages. When interacting with mesenchymal stem cells (MSCs, osteogenic precursor cells), both Treg-Exo and Treg-Exo primed macrophages facilitated the proliferation and differentiation of MSCs. After administration in vivo, Treg-Exo effectively promoted fracture healing compared with conventional T cell-derived exosome. To further improve the delivery efficacy of exosomes and integrate multiple biological processes of fracture healing, an injectable hydrogel was fabricated to co-deliver Treg-Exo and stromal cell-derived factor 1 alpha (SDF-1α). With the dual effect of Treg-Exo for macrophage polarization and SDF-1α for MSC recruitment, the multifunctional hydrogel exerted a synergistic effect on fracture repair acceleration. This study provided a promising therapeutic candidate and synergistic strategy for the clinical treatment of fracture.

Multimodal analyses of immune cells during bone repair identify macrophages as a therapeutic target in musculoskeletal trauma

Musculoskeletal traumatic injuries (MTI) involve soft tissue lesions adjacent to a bone fracture leading to fibrous nonunion. The impact of MTI on the inflammatory response to fracture and on the immunomodulation of skeletal stem/progenitor cells (SSPCs) remains unknown. Here, we used single cell transcriptomic analyses to describe the immune cell dynamics after bone fracture and identified distinct macrophage subsets with successive pro-inflammatory, pro-repair and anti-inflammatory profiles. Concurrently, SSPCs transition via a pro- and anti-inflammatory fibrogenic phase of differentiation prior to osteochondrogenic differentiation. In a preclinical MTI mouse model, the injury response of immune cells and SSPCs is disrupted leading to a prolonged pro-inflammatory phase and delayed resolution of inflammation. Macrophage depletion improves bone regeneration in MTI demonstrating macrophage involvement in fibrous nonunion. Finally, pharmacological inhibition of macrophages using the CSF1R inhibitor Pexidartinib ameliorates healing. These findings reveal the coordinated immune response of macrophages and skeletal stem/progenitor cells as driver of bone healing and as a primary target for the treatment of trauma-associated fibrosis.

Summary

Hachemi et al. report the immune cell atlas of bone repair revealing macrophages as pro-fibrotic regulators and a therapeutic target for musculoskeletal regeneration. Genetic depletion or pharmacological inhibition of macrophages improves bone healing in musculoskeletal trauma.

Mineral coated microparticles doped with fluoride and complexed with mRNA prolong transfection in fracture healing

Introduction: Impaired fracture healing, specifically non-union, has been found to occur up to 14% in tibial shaft fractures. The current standard of care to treat non-union often requires additional surgeries which can result in long recovery times. Injectable-based therapies to accelerate fracture healing have the potential to mitigate the need for additional surgeries. Gene therapies have recently undergone significant advancements due to developments in nanotechnology, which improve mRNA stability while reducing immunogenicity. Methods: In this study, we tested the efficacy of mineral coated microparticles (MCM) and fluoride-doped MCM (FMCM) to effectively deliver firefly luciferase (FLuc) mRNA lipoplexes (LPX) to the fracture site. Here, adult mice underwent a tibia fracture and stabilization method and all treatments were locally injected into the fracture. Level of osteogenesis and amount of bone formation were assessed using gene expression and histomorphometry respectively. Localized and systemic inflammation were measured through gene expression, histopathology scoring and measuring C-reactive protein (CRP) in the serum. Lastly, daily IVIS images were taken to track and measure transfection over time. Results: MCM-LPX-FLuc and FMCM-LPX-FLuc were not found to cause any cytotoxic effects when tested in vitro. When measuring the osteogenic potential of each mineral composition, FMCM-LPX-FLuc trended higher in osteogenic markers through qRT-PCR than the other groups tested in a murine fracture and stabilization model. Despite FMCM-LPX-FLuc showing slightly elevated il-1β and il-4 levels in the fracture callus, inflammation scoring of the fracture callus did not result in any differences. Additionally, an acute systemic inflammatory response was not observed in any of the samples tested. The concentration of MCM-LPX-FLuc and FMCM-LPX-FLuc that was used in the murine fracture model did not stimulate bone when analyzed through stereological principles. Transfection efficacy and kinetics of delivery platforms revealed that FMCM-LPX-FLuc prolongs the luciferase signal both in vitro and in vivo. Discussion: These data together reveal that FMCM-LPX-FLuc could serve as a promising mRNA delivery platform for fracture healing applications.

Reduced angiogenesis and delayed endochondral ossification in CD163-/- mice highlights a role of M2 macrophages during bone fracture repair

While recent studies showed that macrophages are critical for bone fracture healing, and lack of M2 macrophages have been implicated in models of delayed union, functional roles for specific M2 receptors have yet to be defined. Moreover, the M2 scavenger receptor CD163 has been identified as a target to inhibit sepsis following implant-associated osteomyelitis, but potential adverse effects on bone healing during blockage therapy have yet to be explored. Thus, we investigated fracture healing in C57BL/6 versus CD163-/- mice using a well-established closed, stabilized, mid-diaphyseal femur fracture model. While gross fracture healing in CD163-/- mice was similar to that of C57BL/6, plain radiographs revealed persistent fracture gaps in the mutant mice on Day 14, which resolved by Day 21. Consistently, 3D vascular micro-CT demonstrated delayed union on Day 21, with reduced bone volume (74%, 61%, and 49%) and vasculature (40%, 40%, and 18%) compared to C57BL/6 on Days 10, 14, and 21 postfracture, respectively (p < 0.01). Histology confirmed large amounts of persistent cartilage in CD163-/- versus C57BL/6 fracture callus on Days 7 and 10 that resolves over time, and immunohistochemistry demonstrated deficiencies in CD206+ M2 macrophages. Torsion testing of the fractures confirmed the delayed early union in CD163-/- femurs, which display decreased yield torque on Day 21, and a decreased rigidity with a commensurate increase in rotation at yield on Day 28 (p < 0.01). Collectively, these results demonstrate that CD163 is required for normal angiogenesis, callus formation, and bone remodeling during fracture healing, and raise potential concerns about CD163 blockade therapy.

The role of macrophages in fracture healing: a narrative review of the recent updates and therapeutic perspectives

Objective

This review addresses the latest advances in research on the role of macrophages in fracture healing, exploring their relationship with failures in bone consolidation and the perspectives for the development of advanced and innovative therapies to promote bone regeneration.

Background

The bone can fully restore its form and function after a fracture. However, the regenerative process of fracture healing is complex and is influenced by several factors, including macrophage activity. These cells have been found in the fracture site at all stages of bone regeneration, and their general depletion or the knockdown of receptors that mediate their differentiation, polarization, and/or function result in impaired fracture healing.

Methods

The literature search was carried out in the PubMed database, using combinations of the keywords "macrophage", "fracture healing, "bone regeneration", and "bone repair". Articles published within the last years (2017-2022) reporting evidence from in vivo long bone fracture healing experiments were included.

Conclusions

Studies published in the last five years on the role of macrophages in fracture healing strengthened the idea that what appears to be essential when it comes to a successful consolidation is the right balance between the M1/M2 populations, which have different but complementary roles in the process. These findings opened promising new avenues for the development of several macrophage-targeted therapies, including the administration of molecules and/or biomaterials intended to regulate macrophage differentiation and polarization, the local transplantation of macrophage precursors, and the use of exosomes to deliver signaling molecules that influence macrophage activities. However, more research is still warranted to better understand the diversity of macrophage phenotypes and their specific roles in each step of fracture healing and to decipher the key molecular mechanisms involved in the in vivo crosstalk between macrophages and other microenvironmental cell types, such as endothelial and skeletal stem/progenitor cells.