Contemporary perspectives on heterotopic ossification

Discoidin domain receptor 2 is an important modulator of BMP signaling during heterotopic bone formation

Bone morphogenetic proteins are essential for bone regeneration/fracture healing but can also induce heterotopic ossification (HO). Understanding accessory factors modulating BMP signaling would provide both a means of enhancing BMP-dependent regeneration while preventing HO. This study focuses on the ability of the collagen receptor, discoidin domain receptor 2 (DDR2), to regulate BMP activity. As will be shown, induction of bone formation by subcutaneous BMP2 implants is severely compromised in Ddr2-deficient mice. In addition, Ddr2 deficiency attenuates HO in mice expressing the ACVR1 mutation associated with human fibrodysplasia ossificans progressiva. In cells migrating into BMP2 implants, DDR2 is co-expressed with GLI1, a skeletal stem cell marker, and DDR2/GLI1-positive cells participate in BMP2-induced bone formation where they contribute to chondrogenic and osteogenic lineages. Consistent with this distribution, conditional knockout of Ddr2 in Gli1-expressing cells inhibited bone formation to the same extent seen in globally Ddr2-deficient animals. This response was explained by selective inhibition of Gli1+ cell proliferation without changes in apoptosis. The basis for this DDR2 requirement was explored further using bone marrow stromal cells. Although Ddr2 deficiency inhibited BMP2-dependent chondrocyte and osteoblast differentiation and in vivo, bone formation, early BMP responses including SMAD phosphorylation remained largely intact. Instead, Ddr2 deficiency reduced the nuclear/cytoplasmic ratio of the Hippo pathway intermediates, YAP and TAZ. This suggests that DDR2 regulates Hippo pathway-mediated responses to the collagen matrix, which subsequently affect BMP responsiveness. In summary, DDR2 is an important modulator of BMP signaling and a potential therapeutic target both for enhancing regeneration and treating HO.

Denervation‑induced NRG3 aggravates muscle heterotopic ossification via the ErbB4/PI3K/Akt signaling pathway

Peripheral nerve injury exacerbates progression of muscle heterotopic ossification (HO) and induces changes in expression of local cytokines in muscle tissue. The objective of the present study was to assess the impact of peripheral nerve injury on muscle HO development and the mechanism of cytokine modulation. A mouse model of gastrocnemius muscle HO was established and the sciatic nerve cut to simulate peripheral nerve injury. To evaluate the underlying factors contributing to the exacerbation of muscle HO resulting from denervation, fresh muscle tissue was collected and micro‑computed tomography, histochemical staining, RNA‑sequencing, reverse transcription‑quantitative PCR, Western blot, muscle tissue chip array were performed to analyze the molecular mechanisms. Sciatic nerve injury exacerbated HO in the gastrocnemius muscle of mice. Moreover the osteogenic differentiation of nerve‑injured muscle tissue‑derived fibro‑adipogenic progenitors (FAPs) increased in vitro. The expression of neuregulin 3 (NRG3) was demonstrated to be increased after nerve injury by muscle tissue chip array. Subsequent transcriptome sequencing analysis of muscle tissue revealed an enrichment of the PI3K/Akt pathway following nerve injury and an inhibitor of the PI3K/Akt pathway reduced the osteogenic differentiation of FAPs. Mechanistically, in vitro, peripheral nerve injury increased secretion of NRG3, which, following binding to ErbB4 on the cell surface of FAPs, promoted expression of osteogenesis‑associated genes via the PI3K/Akt signaling pathway, thus contributing to osteogenic differentiation of FAPs. In vivo, inhibition of the PI3K/Akt pathway effectively protected against muscle HO induced by peripheral nerve injury in mice. The present study demonstrated that the regulatory roles of NRG3 and the PI3K/Akt pathway in peripheral nerve injury exacerbated muscle HO and highlights a potential therapeutic intervention for treatment of peripheral nerve injury‑induced muscle HO.

Exploring the role of peripheral nerves in trauma-induced heterotopic ossification

Recent studies have linked pain and the resultant nociception-induced neural inflammation (NINI) to trauma-induced heterotopic ossification (THO). It is postulated that nociception at the injury site stimulates the transient receptor potential vanilloid-1 (the transient receptor potential cation channel subfamily V member 1) receptors on sensory nerves within the injured tissues resulting in the expression of neuroinflammatory peptides, substance P (SP), and calcitonin gene-related peptide (CGRP). Additionally, BMP-2 released from fractured bones and soft tissue injury also selectively activates TRVP1 receptors, resulting in the release of SP and CGRP and causing neuroinflammation and degranulation of mast cells causing the breakdown the blood-nerve barrier (BNB), leading to release of neural crest derived progenitor cells (NCDPCs) into the injured tissue. Parallel to this process BMP-2 initiates the NCDPCs toward osteogenic differentiation. CGRP has direct osteogenic effects on osteoprogenitor cells/mesenchymal stem cells, by activating BMP-2 via canonical Wnt/β-catenin signaling and cAMP-cAMP-response element binding protein signaling. BMP-2 binds to TGF-βRI and activates TGF-β-activated kinase 1 (TAK1) leading to phosphorylation of SMAD1/5/8, which binds to the co-activator SMAD4 and translocates to the nucleus to serve as transcription factor for BMP responsive genes critical in osteogenesis such as Runx2 and others. Thus, NINI phenotypes, and specifically CGRP induction, play a crucial role in THO initiation and progression through the activation of the BMP pathway, breakdown of the BNB, leading to the escape of NCDPCs, and the osteogenic differentiation of the latter.

The influence of nerve surgical techniques at time of amputation on the prevalence of heterotopic ossification in transtibial amputees

BACKGROUND

Heterotopic ossification (HO) can form after amputation and may cause pain and functional impairment. We aimed to describe the prevalence of HO in a civilian population of transtibial amputees. We hypothesized that the decreased rate of symptomatic neuroma following active nerve surgery (Targeted Muscle Reinnervation (TMR) or Regenerative Peripheral Nerve Interface (RPNI)) may subsequently lead to a lower prevalence of HO compared to passive nerve surgery (i.e. traction neurectomy) performed at the time of amputation.

METHODS

Adult patients undergoing transtibial amputation at a tertiary care center between 2000 and 2023 were included. Patient data were collected through chart review. The most recent post-amputation X-ray of the residual limb was assessed for HO presence, according to the Walter Reed classification. A random subset of X-rays (10.0 %) was independently assessed by five clinicians and two radiologists, and inter-rater reliability (IRR) was calculated using Cohen's kappa (κ). Multivariable logistic regression was conducted to identify factors associated with HO presence.

RESULTS

In total, 665 limbs of 632 patients were included. The median time between amputation and X-ray was 1.7 years (IQR: 0.3-6.2). HO was identified in 326 X-rays (49.0 %) and was commonly present on the distal residual tibia (68.1 %) and fibula (69.0 %). Traditional amputations (i.e. those without TMR or RPNI (OR: 2.0, p = 0.014)), and the presence of a symptomatic neuroma (OR: 2.3, p < 0.001), were independently associated with a higher prevalence of HO. The IRR of the two radiologists was κ = 0.99, the overall IRR of all evaluators was κ = 0.92.

CONCLUSIONS

HO is a common finding in transtibial amputees. Peripheral nerve surgerical techniques that actively address amputated nerve endings to reduce symptomatic neuroma formation may decrease the prevalence of HO.

Spatiotemporal Analysis of Mesenchymal Stem Cells Fate Determination by Inflammatory Niche Following Soft Tissue Injury at a Single-Cell Level

Heterotopic ossification (HO), often arising in response to traumatic challenges, results from the aberrant osteochondral differentiation of mesenchymal stem cells (MSCs). Nevertheless, the impact of trauma-induced inflammatory exposure on MSC fate determination remains ambiguous. In this study, the cellular diversity within inflammatory lesions is elucidated, comprising MSCs and several innate and adaptive immune cells. It is observed that quiescent MSCs transition into cycling MSCs, subsequently giving rise to chondrogenic (cMSC) and/or osteogenic (oMSC) lineages within the inflammatory microenvironment following muscle or tendon injuries, as revealed through single-cell RNA sequencing (scRNA-seq), spatial transcriptome and lineage tracing analysis. Moreover, these investigations demonstrate that neutrophils and natural killer (NK) cells enhance transition of quiescent MSCs into cycling MSCs, which is also controlled by M1 macrophages, a subpopulation of macrophages can also stimulate cMSC and oMSC production from cycling MSCs. Additionally, M2 macrophages, CD4+ and CD8+ T lymphocytes are found to promote chondrogenesis. Further analysis demonstrates that immune cells promotes the activation of signaling transducers and activators of transcription (STAT) pathway and phosphoinositide 3 (PI3K)/protein kinase B (AKT) pathway in MSC proliferation and osteochondral progenitors' production, respectively. These findings highlight the dynamics of MSC fate within the inflammatory lesion and unveil the molecular landscape of osteoimmunological interactions, which holds promise for advancing HO treatment.

Suppression of TNF-α activity by immobilization rescues Mkx expression and attenuates tendon ossification in a mouse Achilles tenotomy model

Traumatic heterotopic ossification is a condition in which extraskeletal bone formation occurs in soft tissues after injury. It most commonly occurs in patients who had major orthopedic surgery and in those with severe extremity injuries. The lesion causes local pain and can impair motor function of the affected limb, but there is currently no established prophylaxis or treatment for this condition. In this study, we show that immobilization at an early stage of the inflammatory response after injury can attenuate ossification formation in a murine Achilles tenotomy model. Gene expression analysis revealed a decrease in the expression of Tnf and an increase in the expression of Mkx, which encodes one of the master regulators of tendon differentiation, Mohawk. Notably, we found that TNF-α suppressed the expression of Mkx transcripts and accelerated the osteogenic differentiation of tendon-derived mesenchymal stem cells (MSCs), suggesting that TNF-α acts as a negative regulator of Mkx transcription. Consistent with these findings, pharmaceutical inhibition of TNF-α increased the expression of Mkx transcripts and suppressed bone formation in this mouse model. These findings reveal the previously unrecognized involvement of TNF-α in regulating tendon MSC fate through suppression of Mkx expression and suggest that TNF-α is a potential target for preventing traumatic heterotopic ossification.

IRE1α inhibits osteogenic differentiation of mouse embryonic fibroblasts by limiting Shh signaling

OBJECTIVES

This study aimed to investigate the effect of endoplasmic reticulum (ER) stress sensor inositol-requiring enzyme 1α (IRE1α) on the sonic hedgehog N-terminus (N-Shh)-enhanced-osteogenic differentiation process in mouse embryonic fibroblasts (MEFs).

MATERIALS AND METHODS

Osteogenesis of MEFs was observed by alkaline phosphatase (ALP) staining, alizarin red staining, and Von Kossa staining assays. Activation of unfolded protein response and Shh signaling were examined using real-time quantitative PCR and western blot assays. IRE1α-deficient MEFs were used to explore the effect of IRE1α on N-Shh-driven osteogenesis.

RESULTS

N-Shh increased ALP activity, matrix mineralization, and the expression of Alp and Col-I in MEFs under osteogenic conditions; notably, this was reversed when combined with the ER stress activator Tm treatment. Interestingly, the administration of N-Shh decreased the expression of IRE1α. Abrogation of IRE1α increased the expression of Shh pathway factors in osteogenesis-induced MEFs, contributing to the osteogenic effect of N-Shh. Moreover, IRE1α-deficient MEFs exhibited elevated levels of osteogenic markers.

CONCLUSIONS

Our findings suggest that the IRE1α-mediated unfolded protein response may alleviate the ossification of MEFs by attenuating Shh signaling. Our research has identified a strategy to inhibit excessive ossification, which may have clinical significance in preventing temporomandibular joint bony ankylosis.

Pharmacologic or genetic targeting of peripheral nerves prevents peri-articular traumatic heterotopic ossification

Heterotopic ossification (HO) is a pathological process that commonly arises following severe polytrauma, characterized by the anomalous differentiation of mesenchymal progenitor cells and resulting in the formation of ectopic bone in non-skeletal tissues. This abnormal bone growth contributes to pain and reduced mobility, especially when adjacent to a joint. Our prior observations suggested an essential role of NGF (Nerve Growth Factor)-responsive TrkA (Tropomyosin Receptor Kinase A)-expressing peripheral nerves in regulating abnormal osteochondral differentiation following tendon injury. Here, we utilized a recently developed mouse model of hip arthroplasty-induced HO to further validate the role of peripheral nerve regulation of traumatic HO. Nerve ingrowth was either modulated using a knockin transgenic animals with point mutation in TrkA, or local treatment with an FDA-approved formulation of long acting Bupivacaine which prevents peripheral nerve growth. Results demonstrate exuberant sensory and sympathetic nerve growth within the peri-articular HO site, and that both methods to reduce local innervation significantly reduced heterotopic bone formation. TrkA inhibition led to a 34% reduction in bone volume, while bupivacaine treatment resulted in a 50% decrease. Mechanistically, alterations in TGFβ and FGF signaling activation accompanied both methods of local denervation, and a shift in macrophages from M1 to M2 phenotypes was observed. In sum, these studies reinforce the observations that peripheral nerves play a role in the etiopathogenesis of HO, and that targeting local nerves represents a potential therapeutic approach for disease prevention.

Involvement of the Wnt/β-catenin signalling pathway in heterotopic ossification and ossification-related diseases

Heterotopic ossification (HO) is a pathological condition characterized by the formation of bone within soft tissues. The development of HO is a result of abnormal activation of the bone formation programs, where multiple signalling pathways, including Wnt/β-catenin, BMP and hedgehog signalling, are involved. The Wnt/β-catenin signalling pathway, a conserved pathway essential for various fundamental activities, has been found to play a significant role in pathological bone formation processes. It regulates angiogenesis, chondrocyte hypertrophy and osteoblast differentiation during the development of HO. More importantly, the crosstalk between Wnt signalling and other factors including BMP, Hedgehog signalling, YAP may contribute in a HO-favourable manner. Moreover, several miRNAs may also be involved in HO formation via the regulation of Wnt signalling. This review aims to summarize the role of Wnt/β-catenin signalling in the pathogenesis of HO, its interactions with related molecules, and potential preventive and therapeutic measures targeting Wnt/β-catenin signalling.

Upper Urothelial Tract Extraosseous Bone Formation: An Unexpected Finding and Differential Diagnostic Considerations

Extraosseous bone formation of the upper urothelial tract is an unusual phenomenon with limited documentation in the uropathology literature, reported in only 2 clinical series of patients undergoing percutaneous nephrolithotomy for the management of renal stones. While speculations regarding the pathogenesis of this occurrence have been published, heterotopic ossification is still poorly understood. We report the finding of extraosseous bone formation in the renal pelvis of a 30-year-old male patient with a history of kidney stones. Histologic sections of the ureter and renal pelvis showed submucosal nodules of woven bone. Ancillary fluorescence in-situ hybridization studies were negative for MDM2 amplification and USP6 rearrangement.

Dual and opposing role of retinoic acid receptor signaling in mesenchymal stem cells for tendon ossification in mice

Heterotopic ossification is abnormal bone formation in soft tissues that occurs primarily after injury and major surgery. This condition often causes local pain and limits joint motion in the affected limb. Currently, there is no effective treatment or prophylaxis for this condition other than surgical removal of the lesion. Recent studies suggest that retinoic acid receptor (RAR) agonists are effective in suppressing heterotopic ossification in patients with Fibrodysplasia Ossificans Progressiva, a congenital disorder characterized by progressive ossification of soft tissue, by suppressing the aberrant differentiation of mesenchymal stem cells in muscle. In this study, we aimed to elucidate the potential use of RAR agonists in suppressing injury-induced ectopic tendon ossification using a mouse Achilles tenotomy model. Contrary to our initial hypothesis, administration of RAR agonists throughout the experimental period (5 weeks) accelerated ectopic tendon ossification in our model. Of note, in vitro differentiation experiments using tendon-derived mesenchymal stem cells revealed that RAR agonists play opposing roles in osteogenic and chondrogenic differentiation, promoting the former and suppressing the latter. Indeed, we found that RAR agonists suppressed tendon ossification when administered before cartilage nodule formation, but promoted it when administered after. These results suggest that RAR agonists have a dual and opposing effect on tendon ectopic ossification, depending on the duration and timing of their administration. Our data may provide a basis for further investigation of the potential use of RAR agonists in the treatment of injury-induced heterotopic ossification.

Downregulation of TGF-β1 in fibro-adipogenic progenitors initiates muscle ectopic mineralization

In previous studies, we have demonstrated that stress response-induced high glucocorticoid levels could be the underlying cause of traumatic heterotopic ossification (HO), and we have developed a glucocorticoid-induced ectopic mineralization (EM) mouse model by systemic administration of a high dose of dexamethasone (DEX) to animals with muscle injury induced by cardiotoxin injection. In this model, dystrophic calcification (DC) developed into HO in a cell autonomous manner. However, it is not clear how DC is formed after DEX treatment. Therefore, in this study, we aimed to explore how glucocorticoids initiate muscle EM at a cellular and molecular level. We showed that DEX treatment inhibited inflammatory cell infiltration into injured muscle but inflammatory cytokine production in the muscle was significantly increased, suggesting that other non-inflammatory muscle cell types may regulate the inflammatory response and the muscle repair process. Accompanying this phenotype, transforming growth factor β1 (TGF-β1) expression in fibro-adipogenic progenitors (FAPs) was greatly downregulated. Since TGF-β1 is a strong immune suppressor and FAP's regulatory role has a large impact on muscle repair, we hypothesized that downregulation of TGF-β1 in FAPs after DEX treatment resulted in this hyperinflammatory state and subsequent failed muscle repair and EM formation. To test our hypothesis, we utilized a transgenic mouse model to specifically knockout Tgfb1 gene in PDGFRα-positive FAPs to investigate if the transgenic mice could recapitulate the phenotype that was induced by DEX treatment. Our results showed that the transgenic mice completely phenocopied this hyperinflammatory state and spontaneously developed EM following muscle injury. On the contrary, therapeutics that enhanced TGF-β1 signaling in FAPs inhibited the inflammatory response and attenuated muscle EM. In summary, these results indicate that FAPs-derived TGF-β1 is a key molecule in regulating muscle inflammatory response and subsequent EM, and that glucocorticoids exert their effect via downregulating TGF-β1 in FAPs.

The dual role of CCND1 in heterotopic ossification: A Non-canonical Pathway for Celecoxib treatment

Objective

To explore the effective targets of Celecoxib in the treatment of heterotopic ossification using network pharmacology methods.

Methods

Potential molecules related to heterotopic ossification were obtained by retrieving the GEO and CTD databases and intersecting them. Potential binding targets of Celecoxib were acquired from the STITCH database. A protein-protein interaction network was constructed between potential binding targets of Celecoxib and potential related molecules of heterotopic ossification using the STRING database. Molecules in the protein-protein interaction network were further analyzed using GO and KEGG enrichment analysis in R software, followed by enrichment analysis of active molecules in the Celecoxib-heterotopic ossification target dataset. Hub genes were selected based on the "degree" value and enrichment within the protein-protein interaction network. The binding affinity of hub genes to Celecoxib was observed using molecular docking techniques. Finally, in vitro experiments were conducted to validate the effectiveness of hub genes and explore their regulatory role in the progression of heterotopic ossification. Additionally, the therapeutic effect of Celecoxib, which modulates the expression of the hub genes, was investigated in the treatment of heterotopic ossification.

Results

568 potential molecules related to heterotopic ossification and 76 potential binding targets of Celecoxib were identified. After intersection, 13 potential functional molecules in Celecoxib's treatment of heterotopic ossification were obtained. KEGG analysis suggested pathways such as Rheumatoid arthritis, NF-kappa B signaling pathway, Pathways in cancer, Antifolate resistance, MicroRNAs in cancer play a role in the treatment of heterotopic ossification by Celecoxib. Further enrichment analysis of the 13 potential functional molecules identified 5 hub genes: IL6, CCND1, PTGS2, IGFBP3, CDH1. Molecular docking results indicated that Celecoxib displayed excellent binding affinity with CCND1 among the 5 hub genes. Experimental validation found that CCND1 is highly expressed in the progression of heterotopic ossification, promoting heterotopic ossification in the early stages and inhibiting it in the later stages, with Celecoxib's treatment of heterotopic ossification depending on CCND1.

Conclusion

In the process of treating heterotopic ossification with Celecoxib, immune and inflammatory signaling pathways play a significant role. The therapeutic effect of Celecoxib on heterotopic ossification depends on the hub gene CCND1, which plays different roles at different stages of the progression of heterotopic ossification, ultimately inhibiting the occurrence of heterotopic ossification.

Safety and effectiveness of the three-dimensional-printed guide plate-assisted rotation axis positioning of a hinged external fixator for the elbow

PURPOSE

We aimed to evaluate the safety and effectiveness of three-dimensional (3D)-printed guide plates for assisting in the positioning of the rotation axis of an elbow-hinged external fixator.

METHODS

Terrible triad (TT) patients, who were screened using the predefined inclusion and exclusion criteria, underwent installation of a hinged external fixator on the basis of internal fixation; 3D-printed guide plates, generated from the patient's imaging data, assisted in positioning the rotation axis. All patients received the same peri-operative management and were followed up at six, 12, 24, and 48 weeks postoperatively. The duration of positioning pin placement, the number of fluoroscopies, pin placement success rate, types and incidence of post-operative complications, and the Mayo elbow performance score (MEPS) of the diseased elbow and range of motion (ROM) of both elbows were assessed.

RESULTS

In 25 patients who completed the follow-up, the average time required for positioning pin placement was 329.32 ± 42.38 s (263-443 s), the average number of fluoroscopies was 2.32 ± 0.48 times (2-3 times), and the pin placement success rate was 100%. At the last follow-up, the mean MEPS of the diseased elbow was 97.50 ± 6.92 (75-100), with an excellent and good rate of 100%, and all patients demonstrated stable concentric reduction. The average range of flexion and extension was 135.08° ± 17.10° (77-146°), while the average range of rotation was 169.21° ± 18.14° (108-180°). No significant difference was observed in the average ROM between the both elbows (P > 0.05). Eight (32%) patients developed post-operative complications, including elbow stiffness due to heterotopic ossification in three (12%) patients, all of whom did not require secondary intervention.

CONCLUSION

Utilizing 3D-printed guide plates for positioning the rotation axis of an elbow-hinged external fixator significantly reduced intra-operative positioning pin placement time and the number of fluoroscopies with excellent positioning results. Satisfactory results were also obtained in terms of post-operative complications, elbow ROM, and functional scores.

From inflammation to bone formation: the intricate role of neutrophils in skeletal muscle injury and traumatic heterotopic ossification

Neutrophils are emerging as an important player in skeletal muscle injury and repair. Neutrophils accumulate in injured tissue, thus releasing inflammatory factors, proteases and neutrophil extracellular traps (NETs) to clear muscle debris and pathogens when skeletal muscle is damaged. During the process of muscle repair, neutrophils can promote self-renewal and angiogenesis in satellite cells. When neutrophils are abnormally overactivated, neutrophils cause collagen deposition, functional impairment of satellite cells, and damage to the skeletal muscle vascular endothelium. Heterotopic ossification (HO) refers to abnormal bone formation in soft tissue. Skeletal muscle injury is one of the main causes of traumatic HO (tHO). Neutrophils play a pivotal role in activating BMPs and TGF-β signals, thus promoting the differentiation of mesenchymal stem cells and progenitor cells into osteoblasts or osteoclasts to facilitate HO. Furthermore, NETs are specifically localized at the site of HO, thereby accelerating the formation of HO. Additionally, the overactivation of neutrophils contributes to the disruption of immune homeostasis to trigger HO. An understanding of the diverse roles of neutrophils will not only provide more information on the pathogenesis of skeletal muscle injury for repair and HO but also provides a foundation for the development of more efficacious treatment modalities for HO.

Heterotopic Ossification Secondary to Motor Vehicle Collision Trauma Leading to Femoroacetabular Impingement Syndrome: A Case Report

Introduction

A patient presented for recalcitrant right hip pain secondary to femoroacetabular impingement (FAI) after blunt motor vehicle trauma and following the development of a 12 cm heterotopic ossification (HO). FAI is an increasingly recognized diagnosis where the hip joint is exposed to repeated femoral microtrauma from high-level physical activity or trauma, often causing labral ossification, and perhaps underlying a similar biological mechanism to HO.

Case Report

In this case report, we have an otherwise healthy 49-year-old male who was involved in a high-speed motor vehicle collision who was diagnosed with right hip FAI secondary to HO (Brooker's Class IV) and indicated for surgical excision of the HO anterior to the right proximal femur. The care team and patient initially trialed non-operative conservative treatment with non-steroidal anti-inflammatories drugs (NSAIDs) and hypothesized therapeutic success using a non-surgical approach. Surgical resection was pursued with the patient after a failure of conservative measures. The patient reported a zero out of ten on a ten-point numerical rating scale for pain, he also stated improved quality of life, satisfaction with the procedure, and subsequent rehabilitation at 1-month post-operative follow-up.

Conclusion

HO with near complete ankylosis of the hip joint may be causative of FAI when untreated. Although this case demonstrates a rarely studied traumatic etiology of impingement secondary to HO, initial standard conservative anti-inflammatory treatment can still be pursued. By analyzing the periarticular impact of HO secondary to non-surgical trauma, we can utilize and make inferential correlations from the literature, studying HO and impingement in the setting of prior hip surgery to guide treatment and prognosis in those presenting with FAI symptoms secondary to blunt force trauma.

Trauma patient heterotopic ossification diagnosis is associated with increased hospital length of stay

BACKGROUND

Traumatic heterotopic ossification (tHO) refers to the development of extra-skeletal bone in muscle and soft tissues following tissue insult secondary to surgery or trauma. This presents a persistent clinical concern associated with significant patient morbidity and expense to diagnose and treat. Traumatic HO is a substantial barrier to rehabilitation for trauma-injured patients. As such, the development of tHO after burn and other trauma is hypothesised to prolong inpatient length of stay (LOS) and thus increase health care costs.

OBJECTIVE

To investigate the association between an inpatient tHO diagnosis and hospital LOS in trauma patients.

METHODS

A retrospective audit of trauma patients over a 14-year period was completed using data from four WA hospitals. Burn and neurological trauma patients diagnosed with tHO as an inpatient (tHO+) and control subjects (tHO-), matched (1:3) by age, gender, and injury severity factors, were identified using medical diagnostic codes. Data relating to patient and injury-related determinants of LOS from tHO+ and tHO- subjects were analysed to model the association of tHO on total hospital length of stay.

RESULTS

188 identified patients were hospitalised due to traumatic injury; 47 patients with tHO following burn injury (n = 17), spinal cord injury (n = 13) and traumatic brain injury (n = 17), and 141 control patients. Those who developed tHO during hospitalisation had a significantly higher median LOS than matched trauma patients who did not develop tHO (142 days vs. 61 days). Multivariate regression analyses identified the following independent predictive factors of a prolonged hospital LOS: tHO diagnosis, mechanical ventilation hours, injury to the hip region and thigh area, other ossification disorder, pressure injury, admission to intensive care unit and deep vein thrombosis. Trauma patients diagnosed with tHO during their hospital admission stayed 1.6 times longer than trauma patients matched for injury severity without a tHO diagnosis (IRR 1.56, 95% CI 1.35-1.79, p<0.001).

CONCLUSION

Traumatic heterotopic ossification is an independent explanatory factor for increased hospital LOS in patients following burns, spinal cord, and traumatic brain injury. Early diagnosis may assist in reducing the impact of tHO on acute hospital stay after trauma.

iMSC-mediated delivery of ACVR2B-Fc fusion protein reduces heterotopic ossification in a mouse model of fibrodysplasia ossificans progressiva

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease caused by a gain-of-function mutation in ACVR1, which is a bone morphogenetic protein (BMP) type I receptor. Moreover, it causes progressive heterotopic ossification (HO) in connective tissues. Using FOP patient-derived induced pluripotent stem cells (FOP-iPSCs) and mouse models, we elucidated the underlying mechanisms of FOP pathogenesis and identified a candidate drug for FOP.

METHODS

In the current study, healthy mesenchymal stem/stromal cells derived from iPSCs (iMSCs) expressing ACVR2B-Fc (iMSCACVR2B-Fc), which is a neutralizing receptobody, were constructed. Furthermore, patient-derived iMSCs and FOP mouse model (ACVR1R206H, female) were used to confirm the inhibitory function of ACVR2B-Fc fusion protein secreted by iMSCACVR2B-Fc on BMP signaling pathways and HO development, respectively.

RESULTS

We found that secreted ACVR2B-Fc attenuated BMP signaling initiated by Activin-A and BMP-9 in both iMSCs and FOP-iMSCs in vitro. Transplantation of ACVR2B-Fc-expressing iMSCs reduced primary HO in a transgenic mouse model of FOP. Notably, a local injection of ACVR2B-Fc-expressing iMSCs and not an intraperitoneal injection improved the treadmill performance, suggesting compound effects of ACVR2B-Fc and iMSCs.

CONCLUSIONS

These results offer a new perspective for treating FOP through stem cell therapy.

Intersections of Fibrodysplasia Ossificans Progressiva and Traumatic Heterotopic Ossification

Heterotopic ossification (HO) is a debilitating pathology where ectopic bone develops in areas of soft tissue. HO can develop as a consequence of traumatic insult or as a result of dysregulated osteogenic signaling, as in the case of the orphan disease fibrodysplasia ossificans progressiva (FOP). Traumatic HO (tHO) formation is mediated by the complex interplay of signaling between progenitor, inflammatory, and nerve cells, among others, making it a challenging process to understand. Research into the pathogenesis of genetically mediated HO (gHO) in FOP has established a pathway involving uninhibited activin-like kinase 2 receptor (ALK2) signaling that leads to downstream osteogenesis. Current methods of diagnosis and treatment lag behind pre-mature HO detection and progressive HO accumulation, resulting in irreversible decreases in range of motion and chronic pain for patients. As such, it is necessary to draw on advancements made in the study of tHO and gHO to better diagnose, comprehend, prevent, and treat both.

The HIF-1α/PLOD2 axis integrates extracellular matrix organization and cell metabolism leading to aberrant musculoskeletal repair

While hypoxic signaling has been shown to play a role in many cellular processes, its role in metabolism-linked extracellular matrix (ECM) organization and downstream processes of cell fate after musculoskeletal injury remains to be determined. Heterotopic ossification (HO) is a debilitating condition where abnormal bone formation occurs within extra-skeletal tissues. Hypoxia and hypoxia-inducible factor 1α (HIF-1α) activation have been shown to promote HO. However, the underlying molecular mechanisms by which the HIF-1α pathway in mesenchymal progenitor cells (MPCs) contributes to pathologic bone formation remain to be elucidated. Here, we used a proven mouse injury-induced HO model to investigate the role of HIF-1α on aberrant cell fate. Using single-cell RNA sequencing (scRNA-seq) and spatial transcriptomics analyses of the HO site, we found that collagen ECM organization is the most highly up-regulated biological process in MPCs. Zeugopod mesenchymal cell-specific deletion of Hif1α (Hoxa11-CreERT2; Hif1afl/fl) significantly mitigated HO in vivo. ScRNA-seq analysis of these Hoxa11-CreERT2; Hif1afl/fl mice identified the PLOD2/LOX pathway for collagen cross-linking as downstream of the HIF-1α regulation of HO. Importantly, our scRNA-seq data and mechanistic studies further uncovered that glucose metabolism in MPCs is most highly impacted by HIF-1α deletion. From a translational aspect, a pan-LOX inhibitor significantly decreased HO. A newly screened compound revealed that the inhibition of PLOD2 activity in MPCs significantly decreased osteogenic differentiation and glycolytic metabolism. This suggests that the HIF-1α/PLOD2/LOX axis linked to metabolism regulates HO-forming MPC fate. These results suggest that the HIF-1α/PLOD2/LOX pathway represents a promising strategy to mitigate HO formation.

Role of the NF-kB signalling pathway in heterotopic ossification: biological and therapeutic significance

Heterotopic ossification (HO) is a pathological process in which ectopic bone develops in soft tissues within the skeletal system. Endochondral ossification can be divided into the following types of acquired and inherited ossification: traumatic HO (tHO) and fibrodysplasia ossificans progressiva (FOP). Nuclear transcription factor kappa B (NF-κB) signalling is essential during HO. NF-κB signalling can drive initial inflammation through interactions with the NOD-like receptor protein 3 (NLRP3) inflammasome, Sirtuin 1 (SIRT1) and AMP-activated protein kinase (AMPK). In the chondrogenesis stage, NF-κB signalling can promote chondrogenesis through interactions with mechanistic target of rapamycin (mTOR), phosphatidylinositol-3-kinase (PI3K)/AKT (protein kinase B, PKB) and other molecules, including R-spondin 2 (Rspo2) and SRY-box 9 (Sox9). NF-κB expression can modulate osteoblast differentiation by upregulating secreted protein acidic and rich in cysteine (SPARC) and interacting with mTOR signalling, bone morphogenetic protein (BMP) signalling or integrin-mediated signalling under stretch stimulation in the final osteogenic stage. In FOP, mutated ACVR1-induced NF-κB signalling exacerbates inflammation in macrophages and can promote chondrogenesis and osteogenesis in mesenchymal stem cells (MSCs) through interactions with smad signalling and mTOR signalling. This review summarizes the molecular mechanism of NF-κB signalling during HO and highlights potential therapeutics for treating HO.

Heterotopic mineralization (ossification or calcification) in aged musculoskeletal soft tissues: A new candidate marker for aging

Aging can lead to various disorders in organisms and with the escalating impact of population aging, the incidence of age-related diseases is steadily increasing. As a major risk factor for chronic illnesses in humans, the prevention and postponement of aging have become focal points of research among numerous scientists. Aging biomarkers, which mirror molecular alterations at diverse levels in organs, tissues, and cells, can be used to monitor and evaluate biological changes associated with aging. Currently, aging biomarkers are primarily categorized into physiological traits, imaging characteristics, histological features, cellular-level alterations, and molecular-level changes that encompass the secretion of aging-related factors. However, in the context of the musculoskeletal soft tissue system, aging-related biological indicators primarily involve microscopic parameters at the cellular and molecular levels, resulting in inconvenience and uncertainty in the assessment of musculoskeletal soft tissue aging. To identify convenient and effective indicators, we conducted a comprehensive literature review to investigate the correlation between ectopic mineralization and age-related changes in the musculoskeletal soft tissue system. Here, we introduce the concept of ectopic mineralization as a macroscopic, reliable, and convenient biomarker for musculoskeletal soft tissue aging and present novel targets and strategies for the future management of age-related musculoskeletal soft tissue disorders.

ANGPTL4 binds to the leptin receptor to regulate ectopic bone formation

Leptin protein was thought to be unique to leptin receptor (LepR), but the phenotypes of mice with mutation in LepR [db/db (diabetes)] and leptin [ob/ob (obese)] are not identical, and the cause remains unclear. Here, we show that db/db, but not ob/ob, mice had defect in tenotomy-induced heterotopic ossification (HO), implicating alternative ligand(s) for LepR might be involved. Ligand screening revealed that ANGPTL4 (angiopoietin-like protein 4), a stress and fasting-induced factor, was elicited from brown adipose tissue after tenotomy, bound to LepR on PRRX1+ mesenchymal cells at the HO site, thus promotes chondrogenesis and HO development. Disruption of LepR in PRRX1+ cells, or lineage ablation of LepR+ cells, or deletion of ANGPTL4 impeded chondrogenesis and HO in mice. Together, these findings identify ANGPTL4 as a ligand for LepR to regulate the formation of acquired HO.

Enhanced BMP signaling in Cathepsin K-positive tendon progenitors induces heterotopic ossification

Heterotopic ossification (HO) is abnormal bone growth in soft tissues that results from injury, trauma, and rare genetic disorders. Bone morphogenetic proteins (BMPs) are critical osteogenic regulators which are involved in HO. However, it remains unclear how BMP signaling interacts with other extracellular stimuli to form HO. To address this question, using the Cre-loxP recombination system in mice, we conditionally expressed the constitutively activated BMP type I receptor ALK2 with a Q207D mutation (Ca-ALK2) in Cathepsin K-Cre labeled tendon progenitors (hereafter "Ca-Alk2:Ctsk-Cre"). Ca-Alk2:Ctsk-Cre mice were viable but they formed spontaneous HO in the Achilles tendon. Histological and molecular marker analysis revealed that HO is formed via endochondral ossification. Ectopic chondrogenesis coincided with enhanced GLI1 production, suggesting that elevated Hedgehog (Hh) signaling is involved in the pathogenesis of HO. Interestingly, focal adhesion kinase, a critical mediator for the mechanotransduction pathway, was also activated in Ca-Alk2:Ctsk-Cre mice. Our findings suggest that enhanced BMP signaling may elevate Hh and mechanotransduction pathways, thereby causing HO in the regions of the Achilles tendon.

Tranexamic acid in patients with post-traumatic elbow stiffness: protocol for a randomised, double-blind, placebo-controlled trial investigating the effectiveness of tranexamic acid at reducing the recurrence of heterotopic ossification after open elbow arthrolysis

INTRODUCTION

Exaggerated inflammatory response is one of the main mechanisms underlying heterotopic ossification (HO). It has been suggested that the antifibrinolytic drug tranexamic acid (TXA) can exert a significant anti-inflammatory effect during orthopaedic surgery. However, no prospective studies have yet investigated the effects of TXA on HO recurrence in patients following open elbow arthrolysis (OEA).

METHODS AND ANALYSIS

Here, we present a protocol for a single-centre, randomised, double-blind, placebo-controlled trial to investigate the effectiveness of TXA on HO recurrence after OEA in a single hospital. A minimum sample size of 138 eligible and consenting participants randomised into treatment and control groups in a 1:1 manner will be included. Patients will receive 2 g of intravenous TXA (experimental group) or placebo (normal saline, control group) administered before skin incision. The primary outcome is HO recurrence rate within 12 months after surgery. The secondary outcomes are the serum immune-inflammatory cytokines including erythrocyte sedimentation rate, C reactive protein, interleukin (IL)-6, IL-1β, IL-13 at the first and third day postoperatively, and elbow range of motion and functional score at 1.5, 6, 9 and 12 months after surgery. After completion of the trial, the results will be reported in accordance with the extensions of the Consolidated Standards of Reporting Trials Statement for trials. The results of this study should determine whether TXA can reduce the rates of HO occurrence after OEA.

ETHICS AND DISSEMINATION

Ethical approval has been granted by the Medical Ethics Committee of the Shanghai Sixth People's Hospital Affiliated to Shanghai Jiao Tong University School of Medicine (reference number 2022-123-(1)). The results of this study will be disseminated through presentations at academic conferences and publication in peer-reviewed journals.

TRIAL REGISTRATION NUMBER

ChiCTR2300068106.

Macrophage-Derived Extracellular DNA Initiates Heterotopic Ossification

Heterotopic ossification (HO) severely affects people's lives; however, its pathological mechanism remains poorly understood. Although extracellular DNA (ecDNA) has been shown to play important roles in pathological calcification, its effects in HO development and progression remain unknown. The in vivo rat Achilles tendon injury model and in vitro collagen I calcification model were used to evaluate the effects of ecDNA in the ectopic calcifications and the main cell types involved in those pathological process. Histology, immunofluorescent staining, reverse transcriptase-polymerase chain reaction analysis and micro-computed tomography were used to identify the distribution of macrophage-derived ecDNA and elucidate their roles in HO. The results showed that the amount of ecDNA and ectopic calcification increased significantly and exhibited a strong correlation in the injured tendons of HO model compared with those of the controls, which was accompanied by a significantly increased number of M2 macrophages in the injured tendon. During in vitro co-culture experiments, M2 macrophages calcified the reconstituted type I collagen and ectopic bone collected from the injured tendons of HO rats, while those effects were inhibited by deoxyribonuclease. More importantly, deoxyribonuclease reversed the pathological calcification in the injured rat tendon HO model. The present study showed that ecDNA from M2 macrophages initiates pathological calcification in HO, and the elimination of ecDNA might be developed into a clinical strategy to prevent ectopic mineralization diseases. The use of deoxyribonuclease for the targeted degradation of ecDNA at affected tissue sites provides a potential solution to treat diseases associated with ectopic mineralization.

NIR-triggered photodynamic therapy of traumatic heterotopic ossification with a type II collagen-targeted photosensitizer

Traumatic heterotopic ossification (HO) represents an intractable sequela following trauma with no currently effective prophylaxis or treatment. Photodynamic therapy (PDT) is a non-invasive treatment for various proliferative diseases. However, the specific effects of PDT on HO development remain unclear. In this study, the therapeutic potential of a near-infrared (NIR) probe-WL-808, composed of type II collagen-binding peptide (WYRGRL) and a PDT photosensitizer (IR-808), was evaluated for the innovative HO-targeted PDT approach. In vitro studies indicated that WL-808 could induce chondrocyte apoptosis and inhibit cell viability through ROS generation under NIR excitation. In vivo, the efficacy of WL-808-mediated PDT was tested on the tenotomy HO model mice. WL-808 specifically targeted the type II collagen cartilaginous template of HO, promoting cell apoptosis and enhancing extracellular matrix (ECM) degradation under 808 nm NIR excitation, which inhibited the final ectopic bone formation. Moreover, no obvious toxicity or side effects were detected after treatment with WL-808. Taken together, WL-808-mediated PDT significantly diminished ectopic cartilage and subsequent bone formation, providing a new perspective for HO prophylaxis and treatment.

The genetic background determines material-induced bone formation through the macrophage-osteoclast axis

Osteoinductive materials are characterized by their ability to induce bone formation in ectopic sites. Thus, osteoinductive materials hold promising potential for repairing bone defects. However, the mechanism of material-induced bone formation remains unknown, which limits the design of highly potent osteoinductive materials. Here, we demonstrated a genetic background link among macrophage polarization, osteoclastogenesis and material-induced bone formation. The intramuscular implantation of an osteoinductive material in FVB/NCrl (FVB) mice resulted in more M2 macrophages at week 1, more osteoclasts at week 2 and increased bone formation after week 4 compared with the results obtained in C57BL/6JOlaHsd (C57) mice. Similarly, in vitro, with a greater potential to form M2 macrophages, monocytes derived from FVB mice formed more osteoclasts than those derived from C57 mice. A transcriptomic analysis identified Csf1, Cxcr4 and Tgfbr2 as the main genes controlling macrophage-osteoclast coupling, which were further confirmed by related inhibitors. With such coupling, macrophage polarization and osteoclast formation of monocytes in vitro successfully predicted in vivo bone formation in four other mouse strains. Considering material-induced bone formation as an example of acquired heterotopic bone formation, the current findings shed a light on precision medicine for both bone regeneration and the treatment of pathological heterotopic bone formation.

Berberine and aspirin prevent traumatic heterotopic ossification by inhibition of BMP signalling pathway and osteogenic differentiation

Heterotopic ossification (HO) is a pathological process that often occurs in soft tissues following severe trauma. There is no effective therapy for HO. The BMP signalling pathway plays an essential role in the pathogenesis of HO. Our previous study showed that AMPK negatively regulates the BMP signalling pathway and osteogenic differentiation. The present study aims to study the effect of two AMPK activators berberine and aspirin on osteogenic differentiation and HO induced by traumatic injury. The effects of two AMPK activators, berberine and aspirin, on BMP signalling and osteogenic differentiation were measured by western blot, ALP and Alizarin red S staining in C3H10T1/2 cells. A mouse model with Achilles tenotomy was employed to assess the effects of berberine and aspirin on HO using μCT and histological analysis. First, our study showed that berberine and aspirin inhibited phosphorylation of Smad1/5 induced by BMP6 and the inhibition was attributed to the down-regulation of ALK2 expression. Second, the combination of berberine and aspirin yielded more potent effects on BMP signalling. Third, we further found that there was an additive effect of berberine and aspirin combination on osteogenic differentiation. Finally, we found that berberine and aspirin blocked trauma-induced ectopic bone formation in mice, which may be through suppression of phosphorylation of Smad1/5 in injured tissues. Collectively, these findings indicate that berberine and aspirin inhibit osteogenic differentiation in C3H10T1/2 cells and traumatic HO in mice, possibly through the down-regulation of the BMP signalling pathway. Our study sheds a light on prevention and treatment of traumatic HO using AMPK pharmacological activators berberine and aspirin.

Genetic models for lineage tracing in musculoskeletal development, injury, and healing

Musculoskeletal development and later post-natal homeostasis are highly dynamic processes, marked by rapid structural and functional changes across very short periods of time. Adult anatomy and physiology are derived from pre-existing cellular and biochemical states. Consequently, these early developmental states guide and predict the future of the system as a whole. Tools have been developed to mark, trace, and follow specific cells and their progeny either from one developmental state to the next or between circumstances of health and disease. There are now many such technologies alongside a library of molecular markers which may be utilized in conjunction to allow for precise development of unique cell 'lineages'. In this review, we first describe the development of the musculoskeletal system beginning as an embryonic germ layer and at each of the key developmental stages that follow. We then discuss these structures in the context of adult tissues during homeostasis, injury, and repair. Special focus is given in each of these sections to the key genes involved which may serve as markers of lineage or later in post-natal tissues. We then finish with a technical assessment of lineage tracing and the techniques and technologies currently used to mark cells, tissues, and structures within the musculoskeletal system.

Verteporfin attenuates trauma-induced heterotopic ossification of Achilles tendon by inhibiting osteogenesis and angiogenesis involving YAP/β-catenin signaling

Heterotopic ossification occurs as a pathological ossification condition characterized by ectopic bone formation within soft tissues following trauma. Vascularization has long been established to fuel skeletal ossification during tissue development and regeneration. However, the feasibility of vascularization as a target of heterotopic ossification prevention remained to be further clarified. Here, we aimed to identify whether verteporfin as a widely used FDA-approved anti-vascularization drug could effectively inhibit trauma-induced heterotopic ossification formation. In the current study, we found that verteporfin not only dose dependently inhibited the angiogenic activity of human umbilical vein endothelial cells (HUVECs) but also the osteogenic differentiation of tendon stem cells (TDSCs). Moreover, YAP/β-catenin signaling axis was downregulated by the verteporfin. Application of lithium chloride, an agonist of β-catenin, recovered TDSCs osteogenesis and HUVECs angiogenesis that was inhibited by verteporfin. In vivo, verteporfin attenuated heterotopic ossification formation by decelerating osteogenesis and the vessels densely associated with osteoprogenitors formation, which could also be readily reversed by lithium chloride, as revealed by histological analysis and Micro-CT scan in a murine burn/tenotomy model. Collectively, this study confirmed the therapeutic effect of verteporfin on angiogenesis and osteogenesis in trauma-induced heterotopic ossification. Our study sheds light on the anti-vascularization strategy with verteporfin as a candidate treatment for heterotopic ossification prevention.

Hypoxia Drives Material-Induced Heterotopic Bone Formation by Enhancing Osteoclastogenesis via M2/Lipid-Loaded Macrophage Axis

Heterotopic ossification (HO) is a double-edged sword. Pathological HO presents as an undesired clinical complication, whereas controlled heterotopic bone formation by synthetic osteoinductive materials shows promising therapeutic potentials for bone regeneration. However, the mechanism of material-induced heterotopic bone formation remains largely unknown. Early acquired HO being usually accompanied by severe tissue hypoxia prompts the hypothesis that hypoxia caused by the implantation coordinates serial cellular events and ultimately induces heterotopic bone formation in osteoinductive materials. The data presented herein shows a link between hypoxia, macrophage polarization to M2, osteoclastogenesis, and material-induced bone formation. Hypoxia inducible factor-1α (HIF-1α), a crucial mediator of cellular responses to hypoxia, is highly expressed in an osteoinductive calcium phosphate ceramic (CaP) during the early phase of implantation, while pharmacological inhibition of HIF-1α significantly inhibits M2 macrophage, subsequent osteoclast, and material-induced bone formation. Similarly, in vitro, hypoxia enhances M2 macrophage and osteoclast formation. Osteoclast-conditioned medium enhances osteogenic differentiation of mesenchymal stem cells, such enhancement disappears with the presence of HIF-1α inhibitor. Furthermore, metabolomics analysis reveals that hypoxia enhances osteoclastogenesis via the axis of M2/lipid-loaded macrophages. The current findings shed new light on the mechanism of HO and favor the design of more potent osteoinductive materials for bone regeneration.