Loss of milk fat globule-epidermal growth factor 8 (MFG-E8) in mice leads to low bone mass and accelerates ovariectomy-associated bone loss by increasing osteoclastogenesis

Efferocytosis in inflammatory bone disorders

Efferocytosis, the clearance of apoptotic cells (ACs) by phagocytes, is crucial for bone homeostasis and immune balance. This tightly regulated process depends on molecular markers such as phosphatidylserine on ACs and MERTK on phagocytes. In the bone microenvironment, multiple cell types participate in efferocytosis, including osteal macrophages, mesenchymal stem cells, osteoblasts, and osteoclasts, directly influencing bone remodeling and immune responses. Impaired efferocytosis disrupts bone turnover, exacerbates inflammation, and contributes to inflammatory bone diseases. Despite its recognized importance, the precise mechanisms regulating efferocytosis in osteoimmunology remain underexplored, including specific signaling pathways, cell-specific interactions, and therapeutic applications. Recent advances highlight the therapeutic potential of targeting efferocytosis using modalities and biomaterial-based strategies. This review systematically examines the role of efferocytosis in osteoimmunology, discusses key challenges in its therapeutic translation, and explores emerging strategies to optimize efferocytosis-based interventions for inflammatory bone disorders.

Efferocytosis at the frontline of homeostasis: Shaping the bone microenvironment and therapeutic implications in related diseases

Bone is a dynamic tissue that constantly undergoes remodeling processes throughout life to maintain its structure and integrity. During this process, physiological bone turnover, which is shaped by apoptosis, occurs in cells in the bone microenvironment. The clearance of these apoptotic cells (ACs) is executed by phagocytes through a process called efferocytosis, which simply means taking to the grave "burial." Efferocytosis is a multistage process involving the recognition, binding, internalization, and digestion of ACs, culminating in the resolution of inflammation. Critically, aberrations in efferocytosis lead to the accumulation of apoptotic corpses, impairing tissue homeostasis and contributing to various pathologies as well as bone-related diseases. Emerging evidence suggests that modulating/activating efferocytosis at any stage represents a promising therapeutic strategy for managing bone-related diseases, especially those associated with aging and inflammation. This review discusses the current understanding of the cellular and molecular mechanisms of efferocytosis, its roles within the bone microenvironment, and potential therapeutic interventions targeting efferocytosis in age-related bone diseases.

Efferocytosis: Unveiling its potential in autoimmune disease and treatment strategies

Efferocytosis is a crucial process whereby phagocytes engulf and eliminate apoptotic cells (ACs). This intricate process can be categorized into four steps: (1) ACs release "find me" signals to attract phagocytes, (2) phagocytosis is directed by "eat me" signals emitted by ACs, (3) phagocytes engulf and internalize ACs, and (4) degradation of ACs occurs. Maintaining immune homeostasis heavily relies on the efficient clearance of ACs, which eliminates self-antigens and facilitates the generation of anti-inflammatory and immunosuppressive signals that maintain immune tolerance. However, any disruptions occurring at any of the efferocytosis steps during apoptosis can lead to a diminished efficacy in removing apoptotic cells. Factors contributing to this inefficiency encompass dysregulation in the release and recognition of "find me" or "eat me" signals, defects in phagocyte surface receptors, bridging molecules, and other signaling pathways. The inadequate clearance of ACs can result in their rupture and subsequent release of self-antigens, thereby promoting immune responses and precipitating the onset of autoimmune diseases such as systemic lupus erythematosus, rheumatoid arthritis, type 1 diabetes, and multiple sclerosis. A comprehensive understanding of the efferocytosis process and its implications can provide valuable insights for developing novel therapeutic strategies that target this process to prevent or treat autoimmune diseases.

MFG-E8 promotes osteogenic differentiation of human bone marrow mesenchymal stem cells through GSK3β/β-catenin signaling pathway

Fracture nonunion and bone defects are challenging for orthopedic surgeons. Milk fat globule-epidermal growth factor 8 (MFG-E8), a glycoprotein possibly secreted by macrophages in a fracture hematoma, participates in bone development. However, the role of MFG-E8 in the osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) is unclear. We investigated the osteogenic effect of MFG-E8 in vitro and in vivo. The CCK-8 assay was used to assess the effect of recombinant human MFG-E8 (rhMFG-E8) on the viability of hBMSCs. Osteogenesis was investigated using RT-PCR, Western blotting, and immunofluorescence. Alkaline phosphatase (ALP) and Alizarin red staining were used to evaluate ALP activity and mineralization, respectively. An enzyme-linked immunosorbent assay was conducted to evaluate the secretory MFG-E8 concentration. Knockdown and overexpression of MFG-E8 in hBMSCs were established via siRNA and lentivirus vector transfection, respectively. Exogenous rhMFG-E8 was used to verify the in vivo therapeutic effect in a tibia bone defect model based on radiographic analysis and histological evaluation. Endogenous and secretory MFG-E8 levels increased significantly during the early osteogenic differentiation of hBMSCs. Knockdown of MFG-E8 inhibited the osteogenic differentiation of hBMSCs. Overexpression of MFG-E8 and rhMFG-E8 protein increased the expression of osteogenesis-related genes and proteins and enhanced calcium deposition. The active β-catenin to total β-catenin ratio and the p-GSK3β protein level were increased by MFG-E8. The MFG-E8-induced enhanced osteogenic differentiation of hBMSCs was partially attenuated by a GSK3β/β-catenin signaling inhibitor. Recombinant MFG-E8 accelerated bone healing in a rat tibial-defect model. In conclusion, MFG-E8 promotes the osteogenic differentiation of hBMSCs by regulating the GSK3β/β-catenin signaling pathway and so, is a potential therapeutic target.

Insights into the Differential Preservation of Bone Proteomes in Inhumed and Entombed Cadavers from Italian Forensic Caseworks

Bone is a hard biological tissue and a precious reservoir of information in forensic investigations as it retains key biomolecules commonly used for identification purposes. Bone proteins have recently attracted significant interest for their potential in estimating post-mortem interval (PMI) and age at death (AAD). However, the preservation of such proteins is highly dependent on intrinsic and extrinsic factors that can hinder the potential application of molecular techniques to forensic sciences. The present study aims at investigating the effects that two commonly used types of burial practices (entombment and inhumation) have on bone protein survival. The sample consists of 14 exhumed individuals from cemeteries in Southern Italy with different AADs (29-85 years) and PMIs (1-37 years). LC-MS/MS analyses show that 16 proteins are better preserved under the entombed conditions and 4 proteins are better preserved under the inhumed conditions, whereas no clear differences are detected for post-translational protein modifications. Furthermore, several potential "stable" protein markers (i.e., proteins not affected by the burial environment) are identified for PMI and AAD estimation. Overall, these results show that the two burial environments play a role in the differential preservation of noncollagenous proteins, confirming the potential of LC-MS/MS-based proteomics in forensic sciences.

MFG-E8 promotes osteogenic transdifferentiation of smooth muscle cells and vascular calcification by regulating TGF-β1 signaling

Vascular calcification occurs in arterial aging, atherosclerosis, diabetes mellitus, and chronic kidney disease. Transforming growth factor-β1 (TGF-β1) is a key modulator driving the osteogenic transdifferentiation of vascular smooth muscle cells (VSMCs), leading to vascular calcification. We hypothesize that milk fat globule–epidermal growth factor 8 (MFG-E8), a glycoprotein expressed in VSMCs, promotes the osteogenic transdifferentiation of VSMCs through the activation of TGF-β1-mediated signaling. We observe that the genetic deletion of MFG-E8 prevents calcium chloride-induced vascular calcification in common carotid arteries (CCAs). The exogenous application of MFG-E8 to aged CCAs promotes arterial wall calcification. MFG-E8-deficient cultured VSMCs exhibit decreased biomineralization and phenotypic transformation to osteoblast-like cells in response to osteogenic medium. MFG-E8 promotes β1 integrin–dependent MMP2 expression, causing TGF-β1 activation and subsequent VSMC osteogenic transdifferentiation and biomineralization. Thus, the established molecular link between MFG-E8 and vascular calcification suggests that MFG-E8 can be therapeutically targeted to mitigate vascular calcification.

A molecular link between the milk fat globule–epidermal growth factor 8 (MFG-E8), activation of vascular calcification driver TGF-β1 and osteogenic differentiation of vascular smooth muscle cells suggests that MFG-E8 could be a therapeutic target for vascular calcification.

MFG-E8 regulated by miR-99b-5p protects against osteoarthritis by targeting chondrocyte senescence and macrophage reprogramming via the NF-κB pathway

Milk fat globule-epidermal growth factor (EGF) factor 8 (MFG-E8), as a necessary bridging molecule between apoptotic cells and phagocytic cells, has been widely studied in various organs and diseases, while the effect of MFG-E8 in osteoarthritis (OA) remains unclear. Here, we identified MFG-E8 as a key factor mediating chondrocyte senescence and macrophage polarization and revealed its role in the pathology of OA. We found that MFG-E8 expression was downregulated both locally and systemically as OA advanced in patients with OA and in mice after destabilization of the medial meniscus surgery (DMM) to induce OA. MFG-E8 loss caused striking progressive articular cartilage damage, synovial hyperplasia, and massive osteophyte formation in OA mice, which was relieved by intra-articular administration of recombinant mouse MFG-E8 (rmMFG-E8). Moreover, MFG-E8 restored chondrocyte homeostasis, deferred chondrocyte senescence and reprogrammed macrophages to the M2 subtype to alleviate OA. Further studies showed that MFG-E8 was inhibited by miR-99b-5p, expression of which was significantly upregulated in OA cartilage, leading to exacerbation of experimental OA partially through activation of NF-κB signaling in chondrocytes. Our findings established an essential role of MFG-E8 in chondrocyte senescence and macrophage reprogramming during OA, and identified intra-articular injection of MFG-E8 as a potential therapeutic target for OA prevention and treatment.

Inflammatory bone loss associated with MFG-E8 deficiency is rescued by teriparatide

A prolonged increase in proinflammatory cytokines is associated with osteoporotic and autoimmune bone loss and, conversely, anti-inflammatory pathways are associated with protection against bone loss. Milk fat globule-epidermal growth factor (MFG-E)-8 is a glycoprotein that is proresolving, regulates apoptotic cell clearance, and has been linked to autoimmune disease and skeletal homeostasis. The role of MFG-E8 in the young vs. adult skeleton was determined in mice deficient in MFG-E8 (KO). In vivo, trabecular bone was similar in MFG-E8KO and wild-type (WT) mice at 6 and 16 wk, whereas 22 wk adult MFG-E8KO mice displayed significantly reduced trabecular BV/TV. The number of osteoclasts per bone surface was increased in 22-wk MFG-E8 KO vs. WT mice, and recombinant murine MFG-E8 decreased the number and size of osteoclasts in vitro. Adult MFG-E8KO spleen weight:body weight was increased compared with WT, and flow cytometric analysis showed significantly increased myeloid-derived suppressor cells (CD11b^hiGR-1⁺) and neutrophils (CD11b^hiLy6G⁺) in MFG-E8KO bone marrow, suggesting an inflammatory phenotype. PTH-treated MFG-E8KO mice showed a greater anabolic response (+124% BV/TV) than observed in PTH-treated WT mice (+64% BV/TV). These data give insight into the role of MFG-E8 in the adult skeleton and suggest that anabolic PTH may be a valuable therapeutic approach for autoimmune-associated skeletal disease.-Michalski, M. N., Seydel, A. L., Siismets, E. M., Zweifler, L. E., Koh, A. J., Sinder, B. P., Aguirre, J. I., Atabai, K., Roca, H., McCauley, L. K. Inflammatory bone loss associated with MFG-E8 deficiency is rescued by teriparatide.

Alterations in expression of endometrial milk fat globule-EGF factor 8 (MFG-E8) and leukemia inhibitory factor (LIF) in patients with infertility and endometriosis

OBJECTIVE

The aim of this study was to compare the endometrial expression of milk fat globule-EGF factor 8 (MFG-E8), its receptor integrin αvβ3, and leukemia inhibitory factor (LIF) in patients with endometriosis and infertility and in healthy fertile patients during the window of implantation.

METHODS

Five patients with peritoneal endometriosis and infertility (case group) and four healthy fertile patients (control group) were recruited. All patients were either diagnosed with or ruled out for endometriosis by laparoscopic surgery; the case group underwent surgery for infertility investigation and the control group for tubal ligation. Endometrial biopsies were performed in all patients during the window of implantation (LH+8 to LH+10), and then the samples were analyzed by immunochemistry for MFG-E8, integrin αvβ3, and LIF.

RESULTS

In patients with endometriosis and infertility, expression of MFG-E8 was significantly increased in the glandular epithelium when compared to healthy fertile patients (p<0.001). Moreover, LIF expression was lower in patients with endometriosis and infertility (p<0.05). Nevertheless, we found no difference in integrin αvβ3 expression between the groups (p=0.084).

CONCLUSION

This study showed for the first time that MFG-E8 expression is impaired in the endometrium of patients with endometriosis and infertility during the window of implantation. Moreover, LIF is also diminished in the endometrium of these patients as shown before.

Macrophages and skeletal health

Bone is in a constant state of remodeling, a process which was once attributed solely to osteoblasts and osteoclasts. Decades of research has identified many other populations of cells in the bone that participate and mediate skeletal homeostasis. Recently, osteal macrophages emerged as vital participants in skeletal remodeling and osseous repair. The exact mechanistic roles of these tissue-resident macrophages are currently under investigation. Macrophages are highly plastic in response to their micro-environment and are typically classified as being pro- or anti-inflammatory (pro-resolving) in nature. Given that inflammatory states result in decreased bone mass, proinflammatory macrophages may be negative regulators of bone turnover. Pro-resolving macrophages have been shown to release anabolic factors and may present a target for therapeutic intervention in inflammation-induced bone loss and fracture healing. The process of apoptotic cell clearance, termed efferocytosis, is mediated by pro-resolving macrophages and may contribute to steady-state bone turnover as well as fracture healing and anabolic effects of osteoporosis therapies. Parathyroid hormone is an anabolic agent in bone that is more effective in the presence of mature phagocytic macrophages, further supporting the hypothesis that efferocytic macrophages are positive contributors to bone turnover. Therapies which alter macrophage plasticity in tissues other than bone should be explored for their potential to treat bone loss either alone or in conjunction with current bone therapeutics. A better understanding of the exact mechanisms by which macrophages mediate bone homeostasis will lead to an expansion of pharmacologic targets for the treatment of osteoporosis and inflammation-induced bone loss.

Milk fat globule epidermal growth factor 8 inhibits periodontitis in non-human primates and its gingival crevicular fluid levels can differentiate periodontal health from disease in humans

AIM

We have previously shown that the secreted glycoprotein milk fat globule epidermal growth factor 8 (MFG-E8) has anti-inflammatory and anti-osteoclastogenic properties. Our objective was to investigate the potential of MFG-E8 as a diagnostic or therapeutic agent in periodontitis.

MATERIALS AND METHODS

Periodontitis was induced in non-human primates (NHPs) by placing ligatures around posterior teeth on both halves of the mandible for a split-mouth design: one side was treated with MFG-E8-Fc and the other with Fc control. Disease was assessed by clinical periodontal examinations, radiographic analysis of bone loss, and analysis of cytokine mRNA expression in gingival biopsy samples. Gingival crevicular fluid (GCF) was collected from human healthy volunteers or subjects with gingivitis, chronic moderate periodontitis, or chronic severe periodontitis. Additionally, GCF was collected from a subset of severe periodontitis patients following scaling and root planing (SRP) and after pocket reduction surgery. GCF was analysed to quantify MFG-E8 and periodontitis-relevant cytokines using multiplex assays.

RESULTS

In NHPs, sites treated with MFG-E8-Fc exhibited significantly less ligature-induced periodontal inflammation and bone loss than Fc control-treated sites. In humans, the GCF levels of MFG-E8 were significantly higher in health than in periodontitis, whereas the reverse was true for the proinflammatory cytokines tested. Consistently, MFG-E8 was elevated in GCF after both non-surgical (SRP) and surgical periodontal treatment of periodontitis patients.

CONCLUSION

MFG-E8 is, in principle, a novel therapeutic agent and biomarker of periodontitis.

Role of milk fat globule-epidermal growth factor 8 in osteoimmunology

Milk fat globule-epidermal growth factor 8 (MFG-E8) is a glycoprotein that is abundantly expressed in various tissues and has a pivotal role in the phagocytic clearance of apoptotic cells. However, MFG-E8 has also gained significant attention because of its wide range of functions in autoimmunity, inflammation and tissue homeostasis. More recently, MFG-E8 has been identified as a critical regulator of bone homeostasis, being expressed in both, osteoblasts and osteoclasts. In addition, it was shown that MFG-E8 fulfils an active role in modulating inflammatory processes, suggesting an anti-inflammatory role of MFG-E8 and proposing it as a novel therapeutic target for inflammatory diseases. This concise review focusses on the expression and regulation of MFG-E8 in the context of inflammatory bone diseases, highlights its role in the pathophysiology of osteoimmune diseases and discusses the therapeutic potential of MFG-E8.

Wnt5a is a key target for the pro-osteogenic effects of iron chelation on osteoblast progenitors

Iron overload due to hemochromatosis or chronic blood transfusions has been associated with the development of osteoporosis. However, the impact of changes in iron homeostasis on osteoblast functions and the underlying mechanisms are poorly defined. Since Wnt signaling is a critical regulator of bone remodeling, we aimed to analyze the effects of iron overload and iron deficiency on osteoblast function, and further define the role of Wnt signaling in these processes. Therefore, bone marrow stromal cells were isolated from wild-type mice and differentiated towards osteoblasts. Exposure of the cells to iron dose-dependently attenuated osteoblast differentiation in terms of mineralization and osteogenic gene expression, whereas iron chelation with deferoxamine promoted osteogenic differentiation in a time- and dose-dependent manner up to 3-fold. Similar results were obtained for human bone marrow stromal cells. To elucidate whether the pro-osteogenic effect of deferoxamine is mediated via Wnt signaling, we performed a Wnt profiler array of deferoxamine-treated osteoblasts. Wnt5a was amongst the most highly induced genes. Further analysis revealed a time- and dose-dependent induction of Wnt5a being up-regulated 2-fold after 48 h at 50 μM deferoxamine. Pathway analysis using specific inhibitors revealed that deferoxamine utilized the phosphatidylinositol-3-kinase and nuclear factor of activated T cell pathways to induce Wnt5a expression. Finally, we confirmed the requirement of Wnt5a in the deferoxamine-mediated osteoblast-promoting effects by analyzing the matrix mineralization of Wnt5a-deficient cells. The promoting effect of deferoxamine on matrix mineralization in wild-type cells was completely abolished in Wnt5a^-/- cells. Thus, these data demonstrate that Wnt5a is critical for the pro-osteogenic effects of iron chelation using deferoxamine.

Functional Role of Milk Fat Globule-Epidermal Growth Factor VIII in Macrophage-Mediated Inflammatory Responses and Inflammatory/Autoimmune Diseases

Inflammation involves a series of complex biological processes mediated by innate immunity for host defense against pathogen infection. Chronic inflammation is considered to be one of the major causes of serious diseases, including a number of autoimmune/inflammatory diseases, cancers, cardiovascular diseases, and neurological diseases. Milk fat globule-epidermal growth factor 8 (MFG-E8) is a secreted protein found in vertebrates and was initially discovered as a critical component of the milk fat globule. Previously, a number of studies have reported that MFG-E8 contributes to various biological functions including the phagocytic removal of damaged and apoptotic cells from tissues, the induction of VEGF-mediated neovascularization, the maintenance of intestinal epithelial homeostasis, and the promotion of mucosal healing. Recently, emerging studies have reported that MFG-E8 plays a role in inflammatory responses and inflammatory/autoimmune diseases. This review describes the characteristics of MFG-E8-mediated signaling pathways, summarizes recent findings supporting the roles of MFG-E8 in inflammatory responses and inflammatory/autoimmune diseases, and discusses MFG-E8 targeting as a potential therapeutic strategy for the development of anti-inflammatory/autoimmune disease drugs.

Milk Fat Globule-Epidermal Growth Factor 8 (MFG-E8) Is a Novel Anti-inflammatory Factor in Rheumatoid Arthritis in Mice and Humans

Milk fat globule-epidermal growth factor 8 (MFG-E8) is an anti-inflammatory glycoprotein that mediates the clearance of apoptotic cells and is implicated in the pathogenesis of autoimmune and inflammatory diseases. Because MFG-E8 also controls bone metabolism, we investigated its role in rheumatoid arthritis (RA), focusing on inflammation and joint destruction. The regulation of MFG-E8 by inflammation was assessed in vitro using osteoblasts, in arthritic mice and in patients with RA. K/BxN serum transfer arthritis (STA) was applied to MFG-E8 knock-out mice to assess its role in the pathogenesis of arthritis. Stimulation of osteoblasts with lipopolysaccharide (LPS) and tumor necrosis factor (TNF)-α downregulated the expression of MFG-E8 by 30% to 35%. MFG-E8-deficient osteoblasts responded to LPS with a stronger production of pro-inflammatory cytokines. In vivo, MFG-E8 mRNA levels were 52% lower in the paws of collagen-induced arthritic (CIA) mice and 24% to 42% lower in the serum of arthritic mice using two different arthritis models (CIA and STA). Similarly, patients with RA (n = 93) had lower serum concentrations of MFG-E8 (-17%) compared with healthy controls (n = 140). In a subgroup of patients who had a moderate to high disease activity (n = 21), serum concentrations of MFG-E8 rose after complete or partial remission had been achieved (+67%). Finally, MFG-E8-deficient mice subjected to STA exhibited a stronger disease burden, an increased number of neutrophils in the joints, and a more extensive local and systemic bone loss. This was accompanied by an increased activation of osteoclasts and a suppression of osteoblast function in MFG-E8-deficient mice. Thus, MFG-E8 is a protective factor in the pathogenesis of RA and subsequent bone loss. Whether MFG-E8 qualifies as a novel biomarker or therapeutic target for the treatment of RA is worth addressing in further studies.