Neurotrophin-3 acts on the endothelial-mesenchymal transition of heterotopic ossification in rats

The Role of Neuromodulation and Potential Mechanism in Regulating Heterotopic Ossification

Heterotopic ossification (HO) is a pathological process characterized by the aberrant formation of bone in muscles and soft tissues. It is commonly triggered by traumatic brain injury, spinal cord injury, and burns. Despite a wide range of evidence underscoring the significance of neurogenic signals in proper bone remodeling, a clear understanding of HO induced by nerve injury remains rudimentary. Recent studies suggest that injury to the nervous system can activate various signaling pathways, such as TGF-β, leading to neurogenic HO through the release of neurotrophins. These pathophysiological changes lay a robust groundwork for the prevention and treatment of HO. In this review, we collected evidence to elucidate the mechanisms underlying the pathogenesis of HO related to nerve injury, aiming to enhance our understanding of how neurological repair processes can culminate in HO.

Roles of Plasminogen Activator Inhibitor-1 in Heterotopic Ossification Induced by Achilles Tenotomy in Thermal Injured Mice

Heterotopic ossification (HO) is the process by which ectopic bone forms at an extraskeletal site. Inflammatory conditions induce plasminogen activator inhibitor 1 (PAI-1), an inhibitor of fibrinolysis, which regulates osteogenesis. In the present study, we investigated the roles of PAI-1 in the pathophysiology of HO induced by trauma/burn treatment using PAI-1-deficient mice. PAI-1 deficiency significantly promoted HO and increased the number of alkaline phosphatase (ALP)-positive cells in Achilles tendons after trauma/burn treatment. The mRNA levels of inflammation markers were elevated in Achilles tendons of both wild-type and PAI-1-deficient mice after trauma/burn treatment and PAI-1 mRNA levels were elevated in Achilles tendons of wild-type mice. PAI-1 deficiency significantly up-regulated the expression of Runx2, Osterix, and type 1 collagen in Achilles tendons 9 weeks after trauma/burn treatment in mice. In in vitro experiments, PAI-1 deficiency significantly increased ALP activity and mineralization in mouse osteoblasts. Moreover, PAI-1 deficiency significantly increased ALP activity and up-regulated osteocalcin expression during osteoblastic differentiation from mouse adipose-tissue-derived stem cells, but suppressed the chondrogenic differentiation of these cells. In conclusion, the present study showed that PAI-1 deficiency promoted HO in Achilles tendons after trauma/burn treatment partly by enhancing osteoblast differentiation and ALP activity in mice. Endogenous PAI-1 may play protective roles against HO after injury and inflammation.

Neurovascular Interactions in the Development of the Vasculature

Vertebrates have developed a network of blood vessels and nerves throughout the body that enables them to perform complex higher-order functions and maintain homeostasis. The 16th-century anatomical text 'De humani corporis fabrica' describes the networks of blood vessels and nerves as having a branching pattern in which they are closely aligned and run parallel one to another. This close interaction between adjacent blood vessels and nerves is essential not only for organogenesis during development and repair at the time of tissue damage but also for homeostasis and functional expression of blood vessels and nerves. Furthermore, it is now evident that disruptions in neurovascular interactions contribute to the progression of various diseases including cancer. Therefore, we highlight recent advances in vascular biology research, with a particular emphasis on neurovascular interactions.

mTORC1 coordinates NF-κB signaling pathway to promote chondrogenic differentiation of tendon cells in heterotopic ossification

Heterotopic ossification (HO) is a pathological bone formation based on endochondral ossification distinguished by ossification within muscles, tendons, or other soft tissues. There has been growing studies focusing on the treatment with rapamycin to inhibit HO, but the mechanism of mTORC1 on HO remains unclear. Tendon cells (TDs) are the first cells to form during tendon heterotopic ossification. Here, we used an in vivo model of HO and an in vitro model of chondrogenesis induction to elucidate the effect and underlying mechanism of mTORC1 in HO. The current study highlights the effect of rapamycin on murine Achilles tenotomy-induced HO and the role of mTORC1 signaling pathway on TDs. Our result showed that mTORC1 was activation in the early stage of HO, whereas the mTORC1 maintained low expression in the mature ectopic cartilage tissue and the ectopic bone formation sites. The use of mTORC1-specific inhibitor (rapamycin) immediately after Achilles tendon injury could suppress the formation of HO; once ectopic cartilage and bone had formed, treatment with rapamycin could not significantly inhibit the progression of HO. Mechanistically, mTORC1 stimulation by silencing of TSC1 promoted the expression of the chondrogenic markers in TDs. In TDs, treated with mTORC1 stimulation by silencing of TSC1, mTORC1 increased the activation of the NF-κB signaling pathway. NF-κB selective inhibitor BAY11-7082 significantly suppressed the chondrogenesis of TDs that treated with mTORC1 stimulation by silencing of TSC1. Together, our findings demonstrated that mTORC1 promoted HO by regulating TDs chondrogenesis partly through the NF-κB signaling pathway; and rapamycin could be a viable HO therapeutic regimen.

Human osteoprogenitor cells obtained from traumatic heterotopic ossification samples showed enhanced osteogenic differentiation potential and ERK/Hedgehog signaling than that from normal bone

Traumatic heterotopic ossification (HO) refers to the abnormal ectopic osteogenesis following trauma, causing limb dysfunction and seriously lowering the life quality of patients. Aberrant osteogenic behavior of progenitor cells that ectopically accumulated within the soft tissues are believed to be responsible for HO formation. However, the detailed mechanism still remained to be clarified. Here in this study, we successfully isolated osteoprogenitors from human heterotopic ossification tissues (HO-ops) and identified their stemness and multi-directional differentiation potential. Using alkaline phosphatase staining together with alizarin red staining, we confirmed that the HO-ops in the heterotopic ossified tissues gained greater osteogenic potential than the normal human bone marrow mesenchymal stem cells (HBMSCs). RT-qPCR also indicated that HO-ops obtained more gene transcriptions of critical osteogenic determinators than HBMSCs. In addition, through Western blot, we proved that ERK signaling pathway and Hedgehog signaling pathway were significantly activated in the HO-ops. When U0126 and cyclopamine were used to inhibit ERK and hedgehog signaling respectively, the osteogenic potential of HO-ops decreased significantly. The hedgehog signaling and ERK signaling also showed cross-talk in HO-ops during osteogenic differentiation in HO-ops during osteogenic differentiation. The elevated ERK and hedgehog signaling was further confirmed in the human traumatic HO sample sections by immunohistochemical staining. In sum, our results showed that the activation of ERK and Hedgehog signaling pathway jointly enhanced the osteogenic potential of HO-ops to induce the formation of traumatic HO, which provides novel insights into the molecular basis of HO formation and offers promising targets for future therapeutic strategy. This article is protected by copyright. All rights reserved.

Nervous System-Driven Osseointegration

Implants are essential therapeutic tools for treating bone fractures and joint replacements. Despite the in-depth study of osseointegration for more than fifty years, poor osseointegration caused by aseptic loosening remains one of the leading causes of late implant failures. Osseointegration is a highly sophisticated and spatiotemporal process in vivo involving the immune response, angiogenesis, and osteogenesis. It has been unraveled that the nervous system plays a pivotal role in skeletal health via manipulating neurotrophins, neuropeptides, and nerve cells. Herein, the research related to nervous system-driven osseointegration was systematically analyzed and reviewed, aiming to demonstrate the prominent role of neuromodulation in osseointegration. Additionally, it is indicated that the implant design considering the role of neuromodulation might be a promising way to prevent aseptic loosening.

Contemporary perspectives on heterotopic ossification

Heterotopic ossification (HO) is the formation of ectopic bone that is primarily genetically driven (fibrodysplasia ossificans progressiva [FOP]) or acquired in the setting of trauma (tHO). HO has undergone intense investigation, especially over the last 50 years, as awareness has increased around improving clinical technologies and incidence, such as with ongoing wartime conflicts. Current treatments for tHO and FOP remain prophylactic and include NSAIDs and glucocorticoids, respectively, whereas other proposed therapeutic modalities exhibit prohibitive risk profiles. Contemporary studies have elucidated mechanisms behind tHO and FOP and have described new distinct niches independent of inflammation that regulate ectopic bone formation. These investigations have propagated a paradigm shift in the approach to treatment and management of a historically difficult surgical problem, with ongoing clinical trials and promising new targets.

Capacitive interdigitated system of high osteoinductive/conductive performance for personalized acting-sensing implants

Replacement orthopedic surgeries are among the most common surgeries worldwide, but clinically used passive implants cannot prevent failure rates and inherent revision arthroplasties. Optimized non-instrumented implants, resorting to preclinically tested bioactive coatings, improve initial osseointegration but lack long-term personalized actuation on the bone-implant interface. Novel bioelectronic devices comprising biophysical stimulators and sensing systems are thus emerging, aiming for long-term control of peri-implant bone growth through biointerface monitoring. These acting-sensing dual systems require high frequency (HF) operations able to stimulate osteoinduction/osteoconduction, including matrix maturation and mineralization. A sensing-compatible capacitive stimulator of thin interdigitated electrodes and delivering an electrical 60 kHz HF stimulation, 30 min/day, is here shown to promote osteoconduction in pre-osteoblasts and osteoinduction in human adipose-derived mesenchymal stem cells (hASCs). HF stimulation through this capacitive interdigitated system had significant effects on osteoblasts' collagen-I synthesis, matrix, and mineral deposition. A proteomic analysis of microvesicles released from electrically-stimulated osteoblasts revealed regulation of osteodifferentiation and mineralization-related proteins (e.g. Tgfb3, Ttyh3, Itih1, Aldh1a1). Proteomics data are available via ProteomeXchange with the identifier PXD028551. Further, under HF stimulation, hASCs exhibited higher osteogenic commitment and enhanced hydroxyapatite deposition. These promising osteoinductive/conductive capacitive stimulators will integrate novel bioelectronic implants able to monitor the bone-implant interface and deliver personalized stimulation to peri-implant tissues.

Macrophages in heterotopic ossification: from mechanisms to therapy

Heterotopic ossification (HO) is the formation of extraskeletal bone in non-osseous tissues. It is caused by an injury that stimulates abnormal tissue healing and regeneration, and inflammation is involved in this process. It is worth noting that macrophages are crucial mediators of inflammation. In this regard, abundant macrophages are recruited to the HO site and contribute to HO progression. Macrophages can acquire different functional phenotypes and promote mesenchymal stem cell (MSC) osteogenic differentiation, chondrogenic differentiation, and angiogenesis by expressing cytokines and other factors such as the transforming growth factor-β1 (TGF-β1), bone morphogenetic protein (BMP), activin A (Act A), oncostatin M (OSM), substance P (SP), neurotrophin-3 (NT-3), and vascular endothelial growth factor (VEGF). In addition, macrophages significantly contribute to the hypoxic microenvironment, which primarily drives HO progression. Thus, these have led to an interest in the role of macrophages in HO by exploring whether HO is a "butterfly effect" event. Heterogeneous macrophages are regarded as the "butterflies" that drive a sequence of events and ultimately promote HO. In this review, we discuss how the recruitment of macrophages contributes to HO progression. In particular, we review the molecular mechanisms through which macrophages participate in MSC osteogenic differentiation, angiogenesis, and the hypoxic microenvironment. Understanding the diverse role of macrophages may unveil potential targets for the prevention and treatment of HO.

Rapamycin prevents heterotopic ossification by inhibiting the mTOR pathway and oxidative stress

Rapamycin (RAPA), which was first described as an anti-fungal agent, is a potent immunosuppressant that suppresses tumors and inhibits the mTOR signaling pathway. Heterotopic ossification (HO) is abnormal bone formation outside the skeletal system (e.g., in muscles, tendons, articular capsules and other soft tissues), often due to trauma or injury. There are currently no drugs available to treat traumatic HO, largely due to limited understanding of the disease. In this study, we focused on the role of oxidative stress (OS) in the early stage of traumatic HO, and explored the underlying mechanism of traumatic HO by using RAPA to specifically inhibit the mTOR pathway, which is known to play a role in the pathogenesis of HO. To assess the effects of RAPA in traumatic HO, we used an NSE-BMP4 transgenic mouse model that develops ossification in response to traumatic injury and intramuscular injection of cardiotoxin to initiate injury. These mice were then treated with RAPA or vehicle intraperitoneally every other day for 2 weeks. Our results demonstrate that RAPA can inhibit HO through a number of different mechanisms. We show that OS and a strong inflammatory response contribute to the hypoxia associated with the early stages of HO, and that RAPA inhibits these responses. Furthermore, RAPA reduces the vascularization triggered by mTOR signaling that leads to HO formation. Therefore, we believe that RAPA could be an effective treatment for the early stages of HO.

Quercetin Attenuates Trauma-Induced Heterotopic Ossification by Tuning Immune Cell Infiltration and Related Inflammatory Insult

Heterotopic ossification (HO) is one of the most intractable disorders following musculoskeletal injury and is characterized by the ectopic presence of bone tissue in the soft tissue leading to severe loss of function in the extremities. Recent studies have indicated that immune cell infiltration and inflammation are involved in aberrant bone formation. In this study, we found increased monocyte/macrophage and mast cell accumulation during early HO progression. Macrophage depletion by clodronate liposomes and mast cell stabilization by cromolyn sodium significantly impeded HO formation. Therefore, we proposed that the dietary phytochemical quercetin could also suppress immune cell recruitment and related inflammatory responses to prevent HO. As expected, quercetin inhibited the monocyte-to-macrophage transition, macrophage polarization, and mast cell activation in vitro in a dose-dependent manner. Using a murine burn/tenotomy model, we also demonstrated that quercetin attenuated inflammatory responses and HO in vivo. Furthermore, elevated SIRT1 and decreased acetylated NFκB p65 expression were responsible for the mechanism of quercetin, and the beneficial effects of quercetin were reversed by the SIRT1 antagonist EX527 and mimicked by the SIRT agonist SRT1720. The findings in this study suggest that targeting monocyte/macrophage and mast cell activities may represent an attractive approach for therapeutic intervention of HO and that quercetin may serve as a promising therapeutic candidate for the treatment of trauma-induced HO by modulating SIRT1/NFκB signaling.

Melatonin Promotes Heterotopic Ossification Through Regulation of Endothelial-Mesenchymal Transition in Injured Achilles Tendons in Rats

Although heterotopic ossification (HO) has been reported to be a common complication of the posttraumatic healing process, the underlying mechanism remains unknown. Endothelial-mesenchymal transition (EndMT) is known to play a role in HO, and our recent study observed that neuroendocrine signals can promote HO by modulating EndMT. Melatonin, a neuroendocrine hormone secreted mainly by the pineal gland, has been documented to perform its function in the skeletal system. This study aimed at describing the expression of melatonin during the formation of HO in rat models of Achilles tendon injury and to further investigate its role in regulating EndMT in HO. Histological staining revealed the expression of melatonin throughout the formation of heterotopic bone in injured Achilles tendons, and the serum melatonin levels were increased after the initial injury. Double immunofluorescence showed that the MT2 melatonin receptor was notably expressed at the sites of injury. Micro-CT showed the enhancement of heterotopic bone volume and calcified areas in rats treated with melatonin. Additionally, our data showed that melatonin induced EndMT in primary rat aortic endothelial cells (RAOECs), which acquired traits including migratory function, invasive function and EndMT and MSC marker gene and protein expression. Furthermore, our data exhibited that melatonin promoted the osteogenic differentiation of RAOECs undergoing EndMT in vitro. Importantly, inhibition of the melatonin-MT2 pathway by using the MT2 selective inhibitor 4-P-PDOT inhibited melatonin-induced EndMT and osteogenesis both in vivo and in vitro. In conclusion, these findings demonstrated that melatonin promoted HO through the regulation of EndMT in injured Achilles tendons in rats, and these findings might provide additional directions for the management of HO.

Neuromodulation of bone: Role of different peptides and their interactions (Review)

Our understanding of the skeletal system has been expanded upon the recognition of several neural pathways that serve important roles in bone metabolism and skeletal homeostasis, as bone tissue is richly innervated. Considerable evidence provided by in vitro, animal and human studies have further elucidated the importance of a host of hormones and local factors, including neurotransmitters, in modulating bone metabolism and osteo-chondrogenic differentiation, both peripherally and centrally. Various cells of the musculoskeletal system not only express receptors for these neurotransmitters, but also influence their endogenous levels in the skeleton. As with a number of physiological systems in nature, a neuronal pathway regulating bone turnover will be neutralized by another pathway exerting an opposite effect. These neuropeptides are also critically involved in articular cartilage homeostasis and pathogenesis of degenerative joint disorders, such as osteoarthritis. In the present Review, data on the role of several neuronal populations in nerve-dependent skeletal metabolism is examined, and the molecular events involved are explored, which may reveal broader relationships between two apparently unrelated organs.

Macrophage-derived neurotrophin-3 promotes heterotopic ossification in rats

Heterotopic ossification (HO) is a debilitating condition that results from traumatic injuries or genetic diseases, for which the underlying mechanisms remain unclear. Recently, we have demonstrated the expression of neurotrophin-3 (NT-3) and its role in promoting HO formation via mediating endothelial-mesenchymal transition (EndMT) of vascular endothelial cells. The current study investigated the role of NT-3 on the surrounding mesenchymal cells and its potential origin throughout HO formation at injured Achilles tendons in rats. We used an Achilles tenotomy to induce HO formation in vivo and cultured primary tendon-derived stem cells (TDSCs) to investigate the underlying mechanisms mediating the osteogenesis in vitro. Furthermore, RAW264.7 cells were employed to identify the origin of NT-3. The mRNA levels of NGF, BDNF, NT-3, and NT-4 and their tyrosine protein kinase (Trk) receptors as well as p75 receptor were elevated at injury sites. NT-3 and TrkC showed the highest induction. Neutralization of the NT-3-induced effects by the pan-Trk inhibitor GNF5837 reduced the expression of bone/cartilage-related genes while injection of NT-3 promoted HO formation with elevated mRNA of bone/cartilage-related markers at injured sites. In vitro, NT-3 accelerated osteogenic differentiation and mineralization of TDSCs through activation of the ERK1/2 and PI3K/Akt signaling pathways. Moreover, the colocalization of NT-3 and macrophages, including M1 and M2 macrophages, was observed in injured sites throughout HO formation, and in vitro studies demonstrated that activated macrophages mediated the secretion of NT-3. In addition, an increasing concentration of serum or supernatant NT-3 was observed both in vivo and in vitro. Depletion of macrophages with clodronate-loaded liposomes reduced HO formation as well as secretion and mRNA expression of NT-3. Our study suggests that macrophage-derived NT-3 may promote HO formation and osteogenesis of TDSCs via the ERK1/2 and PI3K/Akt signaling pathways, which may provide new insights for the therapeutic directions of HO in the future.

Endothelial to mesenchymal transition contributes to nicotine-induced atherosclerosis

Rationale: Nicotine exposure via cigarette smoking is strongly associated with atherosclerosis. However, the underlying mechanisms remain poorly understood. The current study aimed to identify whether endothelial to mesenchymal transition (EndMT) contributes to nicotine-induced atherosclerosis. Methods: ApoE-/- mice were administered nicotine in their drinking water for 12 weeks. The effects of nicotine on EndMT were determined by immunostaining on aortic root and RNA analysis in aortic intima. In vitro nicotine-treated cell model was established on human aortic endothelial cells (HAECs). The effects of nicotine on the expression of EndMT-related markers, ERK1/2 and Snail were quantified by real-time PCR, western blot and immunofluorescent staining. Results: Nicotine treatment resulted in larger atherosclerotic plaques in ApoE-/- mice. The vascular endothelial cells from nicotine-treated mice showed mesenchymal phenotype, indicating EndMT. Moreover, nicotine-induced EndMT process was accompanied by cytoskeleton reorganization and impaired barrier function. The α7 nicotine acetylcholine receptor (α7nAChR) was highly expressed in HAECs and its antagonist could effectively relieve nicotine-induced EndMT and atherosclerotic lesions in mice. Further experiments revealed that ERK1/2 signaling was activated by nicotine, which led to the upregulation of Snail. Blocking ERK1/2 with inhibitor or silencing Snail by small interfering RNA efficiently preserved endothelial phenotype upon nicotine stimulation. Conclusion: Our study provides evidence that EndMT contributes to the pro-atherosclerotic property of nicotine. Nicotine induces EndMT through α7nAChR-ERK1/2-Snail signaling in endothelial cells. EndMT may be a therapeutic target for smoking-related endothelial dysfunction and cardiovascular disease.

Severe Heterotopic Ossification in the Skeletal Muscle and Endothelial Cells Recruitment to Chondrogenesis Are Enhanced by Monocyte/Macrophage Depletion

Altered macrophage infiltration upon tissue damage results in inadequate healing due to inappropriate remodeling and stem cell recruitment and differentiation. We investigated in vivo whether cells of endothelial origin phenotypically change upon heterotopic ossification induction and whether infiltration of innate immunity cells influences their commitment and alters the ectopic bone formation. Liposome-encapsulated clodronate was used to assess macrophage impact on endothelial cells in the skeletal muscle upon acute damage in the ECs specific lineage-tracing Cdh5CreER^T2:R26REYFP/dtTomato transgenic mice. Macrophage depletion in the injured skeletal muscle partially shifts the fate of ECs toward endochondral differentiation. Upon ectopic stimulation of BMP signaling, monocyte depletion leads to an enhanced contribution of ECs chondrogenesis and to ectopic bone formation, with increased bone volume and density, that is reversed by ACVR1/SMAD pathway inhibitor dipyridamole. This suggests that macrophages contribute to preserve endothelial fate and to limit the bone lesion in a BMP/injury-induced mouse model of heterotopic ossification. Therefore, alterations of the macrophage-endothelial axis may represent a novel target for molecular intervention in heterotopic ossification.

Neurotrophin-3 acts on the endothelial-mesenchymal transition of heterotopic ossification in rats

Despite the fact that extensive studies have focused on heterotopic ossification (HO), its molecular mechanism remains unclear. The endothelial-mesenchymal transition (EndMT), which may be partially modulated by neuroendocrine cytokines is thought to play a major role in HO. Neurotrophin-3 (NT-3), which has neuroendocrine characteristics is believed to promote skeletal remodeling. Herein, we suggest that that NT-3 may promote HO formation through regulation of EndMT. Here, we used an in vivo model of HO and an in vitro model of EndMT induction to elucidate the effect and underlying mechanism of NT-3 on EndMT in HO. Our results showed that heterotopic bone and cartilage arose from EndMT and NT-3 promoted HO formation in vivo. Our in vitro results showed that NT-3 up-regulated mesenchymal markers (FSP-1, α-SMA and N-cadherin) and mesenchymal stem cell (MSC) markers (STRO-1, CD44 and CD90) and down-regulated endothelial markers (Tie-1, VE-cadherin and CD31). Moreover, NT-3 enhanced a chondrogenesis marker (Sox9) and osteogenesis markers (OCN and Runx2) via activation of EndMT. However, both EndMT specific inhibitor and tropomyosin-related kinase C (TrkC) specific inhibitor rescued NT-3-induced HO formation and EndMT induction in vivo and in vitro. In conclusion, our findings demonstrate that NT-3 promotes HO formation via modulation of EndMT both in vivo and in vitro, which offers a new potential target for the prevention and therapy of HO.