Fibrinolysis is essential for fracture repair and prevention of heterotopic ossification

RNA therapeutics to control fibrinolysis: review on applications in biology and medicine

Regulation of fibrinolysis, the process that degrades blood clots, is pivotal in maintaining hemostasis. Dysregulation leads to thrombosis or excessive bleeding. Proteins in the fibrinolysis system include fibrinogen, coagulation factor XIII, plasminogen, tissue plasminogen activator, urokinase plasminogen activator, α2-antiplasmin, thrombin-activatable fibrinolysis inhibitor, plasminogen activator inhibitor-1, α2-macroglobulin, and others. While each of these is a potential therapeutic target for diseases, they lack effective or long-acting inhibitors. Rapid advances in RNA-based technologies are creating powerful tools to control the expression of proteins. RNA agents can be long-acting and tailored to either decrease or increase production of a specific protein. Advances in nucleic acid delivery, such as by lipid nanoparticles, have enabled the delivery of RNA to the liver, where most proteins of coagulation and fibrinolysis are produced. This review will summarize the classes of RNA that induce 1) inhibition of protein synthesis, including small interfering RNA and antisense oligonucleotides; 2) protein expression, including messenger RNA and self-amplifying RNA; and 3) gene editing for gene knockdown and precise editing. It will review specific examples of RNA therapies targeting proteins in the coagulation and fibrinolysis systems and comment on the wide range of opportunities for controlling fibrinolysis for biological applications and future therapeutics using state-of-the-art RNA therapies.

Human versus Rat PRF on Collagen Membranes: A Pilot Study of Mineralization in Rat Calvaria Defect Model

Platelet-rich fibrin, the coagulated plasma fraction of blood, is commonly used to support natural healing in clinical applications. The rat calvaria defect is a standardized model to study bone regeneration. It remains, however, unclear if the rat calvaria defect is appropriate to investigate the impact of human PRF (Platelet-Rich Fibrin) on bone regeneration. To this end, we soaked Bio-Gide® collagen membranes in human or rat liquid concentrated PRF before placing them onto 5 mm calvarial defects in Sprague Dawley rats. Three weeks later, histology and micro-computed tomography (μCT) were performed. We observed that the collagen membranes soaked with rat PRF show the characteristic features of new bone and areas of mineralized collagen matrix, indicated by a median mineralized volume of 1.5 mm3 (range: 0.9; 5.3 mm3). Histology revealed new bone growing underneath the membrane and hybrid bone where collagen fibers are embedded in the new bone. Moreover, areas of passive mineralization were observed. The collagen membranes soaked with human PRF, however, were devoid of histological features of new bone formation in the center of the defect; only occasionally, new bone formed at the defect margins. Human PRF (h-PRF) caused a median bone volume of 0.9 mm3 (range: 0.3-3.3 mm3), which was significantly lower than what was observed with rat PRF (r-PRF), with a BV median of 1.2 mm3 (range: 0.3-5.9 mm3). Our findings indicate that the rat calvaria defect model is suitable for assessing the effects of rat PRF on bone formation, but caution is warranted when extrapolating conclusions regarding the efficacy of human PRF.

Platelets: A Potential Factor that Offers Strategies for Promoting Bone Regeneration

Bone defects represent a prevalent category of clinical injuries, causing significant pain and escalating health care burdens. Effectively addressing bone defects is thus of paramount importance. Platelets, formed from megakaryocyte lysis, have emerged as pivotal players in bone tissue repair, inflammatory responses, and angiogenesis. Their intracellular storage of various growth factors, cytokines, and membrane protein receptors contributes to these crucial functions. This article provides a comprehensive overview of platelets' roles in hematoma structure, inflammatory responses, and angiogenesis throughout the process of fracture healing. Beyond their application in conjunction with artificial bone substitute materials for treating bone defects, we propose the potential future use of anticoagulants such as heparin in combination with these materials to regulate platelet number and function, thereby promoting bone healing. Ultimately, we contemplate whether manipulating platelet function to modulate bone healing could offer innovative ideas and directions for the clinical treatment of bone defects.

How to explain the beneficial effects of platelet-rich plasma

Platelet-rich plasma (PRP) is the platelet and leukocyte-containing plasmatic fraction of anticoagulated autologous blood. While evidence supporting the clinical use of PRP in dentistry is low, PRP is widely used in sports medicine, orthopedics, and dermatology. Its beneficial activity is commonly attributed to the growth factors released from platelets accumulating in PRP; however, evidence is indirect and not comprehensive. There is thus a demand to revisit PRP with respect to basic and translational science. This review is to (i) recapitulate protocols and tools to prepare PRP; (ii) to discuss the cellular and molecular composition of PRP with a focus on platelets, leukocytes, and the fibrin-rich extracellular matrix of coagulated plasma; and finally (iii) to discuss potential beneficial effects of PRP on a cellular and molecular level with an outlook on its current use in dentistry and other medical fields.

Implications of siRNA Therapy in Bone Health: Silencing Communicates

The global statistics of bone disorders, skeletal defects, and fractures are frightening. Several therapeutic strategies are being used to fix them; however, RNAi-based siRNA therapy is starting to prove to be a promising approach for the prevention of bone disorders because of its advanced capabilities to deliver siRNA or siRNA drug conjugate to the target tissue. Despite its 'bench-to-bedside' usefulness and approval by food and drug administration for five siRNA-based therapeutic medicines: Patisiran, Vutrisiran, Inclisiran, Lumasiran, and Givosiran, its use for the other diseases still remains to be resolved. By correcting the complications and complexities involved in siRNA delivery for its sustained release, better absorption, and toxicity-free activity, siRNA therapy can be harnessed as an experimental tool for the prevention of complex and undruggable diseases with a personalized medicine approach. The present review summarizes the findings of notable research to address the implications of siRNA in bone health for the restoration of bone mass, recovery of bone loss, and recuperation of bone fractures.

Lymphatic platelet thrombosis limits bone repair by precluding lymphatic transporting DAMPs

Lymphatic vessels (LVs) interdigitated with blood vessels, travel and form an extensive transport network in the musculoskeletal system. Blood vessels in bone regulate osteogenesis and hematopoiesis, however, whether LVs in bone affect fracture healing is unclear. Here, by near infrared indocyanine green lymphatic imaging (NIR-ICG), we examined lymphatic draining function at the tibial fracture sites and found lymphatic drainage insufficiency (LDI) occurred as early as two weeks after fracture. Sufficient lymphatic drainage facilitates fracture healing. In addition, we identified that lymphatic platelet thrombosis (LPT) blocks the draining lymphoid sinus and LVs, caused LDI and then inhibited fracture healing, which can be rescued by a pharmacological approach. Moreover, unblocked lymphatic drainage decreased neutrophils and increased M2-like macrophages of hematoma niche to support osteoblast (OB) survival and bone marrow-derived mesenchymal stem cell (BMSC) proliferation via transporting damage-associated molecular patterns (DAMPs). These findings demonstrate that LPT limits bone regeneration by blocking lymphatic drainage from transporting DAMPs. Together, these findings represent a novel way forward in the treatment of bone repair.

Maintaining the balance: the critical role of plasmin activity in orthopedic surgery injury response

The musculoskeletal system plays vital roles in the body, facilitating movement, protecting vital structures, and regulating hematopoiesis and mineral metabolism. Injuries to this system are common and can cause chronic pain, loss of range of motion, and disability. The acute phase response (APR) is a complex process necessary for surviving and repairing injured musculoskeletal tissue. To conceptualize the APR, it is useful to divide it into 2 distinct phases, survival and repair. During the survival-APR, a "damage matrix" primarily composed of fibrin, via thrombin activity, is produced to contain the zone of injury. Once containment is achieved, the APR transitions to the repair phase, where reparative inflammatory cells use plasmin to systematically remove the damage matrix and replace it with new permanent matrices produced by differentiated mesenchymal stem cells. The timing of thrombin and plasmin activation during their respective APR phases is crucial for appropriate regulation of the damage matrix. This review focuses on evidence indicating that inappropriate exuberant activation of plasmin during the survival-APR can result in an overactive APR, leading to an "immunocoagulopathy" that may cause "immunothrombosis" and death. Conversely, preclinical data suggest that too little plasmin activity during the repair-APR may contribute to failed tissue repair, such as a fracture nonunion, and chronic inflammatory degenerative diseases like osteoporosis. Future clinical studies are required to affirm these findings. Therefore, the temporal-spatial functions of plasmin in response to musculoskeletal injury and its pharmacologic manipulation are intriguing new targets for improving orthopedic care.

Bioceramics/Electrospun Polymeric Nanofibrous and Carbon Nanofibrous Scaffolds for Bone Tissue Engineering Applications

Bone tissue engineering integrates biomaterials, cells, and bioactive agents to propose sophisticated treatment options over conventional choices. Scaffolds have central roles in this scenario, and precisely designed and fabricated structures with the highest similarity to bone tissue have shown promising outcomes. On the other hand, using nanotechnology and nanomaterials as the enabling options confers fascinating properties to the scaffolds, such as precisely tailoring the physicochemical features and better interactions with cells and surrounding tissues. Among different nanomaterials, polymeric nanofibers and carbon nanofibers have attracted significant attention due to their similarity to bone extracellular matrix (ECM) and high surface-to-volume ratio. Moreover, bone ECM is a biocomposite of collagen fibers and hydroxyapatite crystals; accordingly, researchers have tried to mimic this biocomposite using the mineralization of various polymeric and carbon nanofibers and have shown that the mineralized nanofibers are promising structures to augment the bone healing process in the tissue engineering scenario. In this paper, we reviewed the bone structure, bone defects/fracture healing process, and various structures/cells/growth factors applicable to bone tissue engineering applications. Then, we highlighted the mineralized polymeric and carbon nanofibers and their fabrication methods.

Reambulation following hindlimb unloading attenuates disuse-induced changes in murine fracture healing

Patients with bone and muscle loss from prolonged disuse have higher risk of falls and subsequent fragility fractures. In addition, fracture patients with continued disuse and/or delayed physical rehabilitation have worse clinical outcomes compared to individuals with immediate weight-bearing activity following diaphyseal fracture. However, the effects of prior disuse followed by physical reambulation on fracture healing cellular processes and adjacent bone and skeletal muscle recovery post-injury remains poorly defined. To bridge this knowledge gap and inform future treatment and rehabilitation strategies for fractures, a preclinical model of fracture healing with a history of prior unloading with and without reambulation was employed. First, skeletally mature male and female C57BL/6J mice (18 weeks) underwent hindlimb unloading by tail suspension (HLU) for 3 weeks to induce significant bone and muscle loss modeling enhanced bone fragility. Next, mice had their right femur fractured by open surgical dissection (stabilized with 24-gauge pin). The, mice were randomly assigned to continued HLU or allowed normal weight-bearing reambulation (HLU + R). Mice given normal cage activity throughout the experiment served as healthy age-matched controls. All mice were sacrificed 4-days (DPF4) or 14-days (DPF14) following fracture to assess healing and uninjured hindlimb musculoskeletal properties (6-10 mice per treatment/biological sex). We found that continued disuse following fracture lead to severely diminished uninjured hindlimb skeletal muscle mass (gastrocnemius and soleus) and femoral bone volume adjacent to the fracture site compared to healthy age-matched controls across mouse sexes. Furthermore, HLU led to significantly decreased periosteal expansion (DPF4) and osteochondral tissue formation by DPF14, and trends in increased osteoclastogenesis (DPF14) and decreased woven bone vascular area (DPF14). In contrast, immediate reambulation for 2 weeks after fracture, even following a period of prolonged disuse, was able to increase hindlimb skeletal tissue mass and increase osteochondral tissue formation, albeit not to healthy control levels, in both mouse sexes. Furthermore, reambulation attenuated osteoclast formation seen in woven bone tissue undergoing disuse. Our results suggest that weight-bearing skeletal loading in both sexes immediately following fracture may improve callus healing and prevent further fall risk by stimulating skeletal muscle anabolism and decreasing callus resorption compared to minimal or delayed rehabilitation regimens.

Fibrinolysis Regulation: A Promising Approach to Promote Osteogenesis

Soon after bone fracture, the initiation of the coagulation cascade results in the formation of a blood clot, which acts as a natural material to facilitate cell migration and osteogenic differentiation at the fracture site. The existence of hematoma is important in early stage of bone healing, but the persistence of hematoma is considered harmful for bone regeneration. Fibrinolysis is recently regarded as a period of critical transition in angiogenic-osteogenic coupling, it thereby is vital for the complete healing of the bone. Moreover, the enhanced fibrinolysis is proposed to boost bone regeneration through promoting the formation of blood vessels, and fibrinolysis system as well as the products of fibrinolysis also play crucial roles in the bone healing process. Therefore, the purpose of this review is to elucidate the fibrinolysis-derived effects on osteogenesis and summarize the potential approaches-improving bone healing by regulating fibrinolysis, with the purpose to further understand the integral roles of fibrinolysis in bone regeneration and to provide theoretical knowledge for potential fibrinolysis-related osteogenesis strategies. Impact statement Fibrinolysis emerging as a new and viable therapeutic intervention to be contained within osteogenesis strategies, however to now, there have been no review articles which collates the information between fibrinolysis and osteogenesis. This review, therefore, focusses on the effects that fibrinolysis exerts on bone healing, with a purpose to provide theoretical reference to develop new strategies to modulate fibrinolysis to accelerate fibrinolysis thus enhancing bone healing.

The role of Meteorin-like in skeletal development and bone fracture healing

Meteorin-like protein (Metrnl), homologous to the initially identified neurotrophic factor Meteorin, is a secreted, multifunctional protein. Here we used mouse models to investigate Metrnl's role in skeletal development and bone fracture healing. During development Metrnl was expressed in the perichondrium and primary ossification center. In neonates, single cell RNA-seq of diaphyseal bone demonstrated strongest expression of Metrnl transcript by osteoblasts. In vitro, Metrnl was osteoinductive, increasing osteoblast differentiation and mineralization in tissue culture models. In vivo, loss of Metrnl expression resulted in no change in skeletal metrics in utero, at birth, or during postnatal growth. Six-week-old Metrnl-null mice displayed similar body length, body weight, tibial length, femoral length, BV/TV, trabecular number, trabecular thickness, and cortical thickness as littermate controls. In 4-month-old mice, lack of Metrnl expression did not change structural stiffness, ultimate force, or energy to fracture of femora under 3-point-bending. Last, we investigated the role of Metrnl in bone fracture healing. Metrnl expression increased in response to tibial injury, however, loss of Metrnl expression did not affect the amount of bone deposited within the healing tissue nor did it change the structural parameters of healing tissue. This work identifies Metrnl as a dispensable molecule for skeletal development. However, the osteoinductive capabilities of Metrnl may be utilized to modulate osteoblast differentiation in cell-based orthopedic therapies.

Osteocyte-specific dentin matrix protein 1 : the role of mineralization regulation in low-magnitude high-frequency vibration enhanced osteoporotic fracture healing

Aims

There is an increasing concern of osteoporotic fractures in the ageing population. Low-magnitude high-frequency vibration (LMHFV) was shown to significantly enhance osteoporotic fracture healing through alteration of osteocyte lacuno-canalicular network (LCN). Dentin matrix protein 1 (DMP1) in osteocytes is known to be responsible for maintaining the LCN and mineralization. This study aimed to investigate the role of osteocyte-specific DMP1 during osteoporotic fracture healing augmented by LMHFV.

Methods

A metaphyseal fracture was created in the distal femur of ovariectomy-induced osteoporotic Sprague Dawley rats. Rats were randomized to five different groups: 1) DMP1 knockdown (KD), 2) DMP1 KD + vibration (VT), 3) Scramble + VT, 4) VT, and 5) control (CT), where KD was performed by injection of short hairpin RNA (shRNA) into marrow cavity; vibration treatment was conducted at 35 Hz, 0.3 g; 20 minutes/day, five days/week). Assessments included radiography, micro-CT, dynamic histomorphometry and immunohistochemistry on DMP1, sclerostin, E11, and fibroblast growth factor 23 (FGF23). In vitro, murine long bone osteocyte-Y4 (MLO-Y4) osteocyte-like cells were randomized as in vivo groupings. DMP1 KD was performed by transfecting cells with shRNA plasmid. Assessments included immunocytochemistry on osteocyte-specific markers as above, and mineralized nodule staining.

Results

Healing capacities in DMP1 KD groups were impaired. Results showed that DMP1 KD significantly abolished vibration-enhanced fracture healing at week 6. DMP1 KD significantly altered the expression of osteocyte-specific markers. The lower mineralization rate in DMP1 KD groups indicated that DMP1 knockdown was associated with poor fracture healing process.

Conclusion

The blockage of DMP1 would impair healing outcomes and negate LMHFV-induced enhancement on fracture healing. These findings reveal the importance of DMP1 in response to the mechanical signal during osteoporotic fracture healing.

Cite this article: Bone Joint Res 2022;11(7):465–476.

Variable Response to Antifibrinolytics Correlates with Blood-loss and Transfusion in Posterior Spinal Fusion

PURPOSE

Posterior spinal fusion (PSF) activates the fibrinolytic protease plasmin, which is implicated in blood loss and transfusion. While antifibrinolytic drugs have improved blood loss and reduced transfusion, variable blood loss has been observed in similar PSF procedures treated with the same dose of antifibrinolytics. However, both the cause of this and the appropriate measures to determine antifibrinolytic efficacy during high-blood-loss spine surgery are unknown, making clinical trials to optimize antifibrinolytic dosing in PSF difficult. We hypothesized that patients undergoing PSF respond differently to antifibrinolytic dosing, resulting in variable blood loss, and that specific diagnostic markers of plasmin activity will accurately measure the efficacy of antifibrinolytics in PSF.

METHODS

A prospective study of 17 patients undergoing elective PSF with the same dosing regimen of TXA was conducted. Surgery-induced plasmin activity was exhaustively analyzed in perioperative blood samples and correlated to measures of inflammation, bleeding, and transfusion.

RESULTS

While markers of in vivo plasmin activation (PAP and D-dimer) suggested significant breakthrough plasmin activation and fibrinolysis (P < 0.01), in vitro plasmin assays, including TEG, did not detect plasmin activation. In vivo measures of breakthrough plasmin activation correlated with blood loss (R2 = 0.400, 0.264; P < 0.01), transfusions (R2 = 0.388; P < 0.01), and complement activation (R2 = 0.346, P < 0.05).

CONCLUSIONS

Despite all patients receiving a high dose of TXA, its efficacy among patients was variable, indicated by notable intra-operative plasmin activity. Markers of in vivo plasmin activation best correlated with clinical outcomes. These findings suggest that the efficacy of antifibrinolytic therapy to inhibit plasmin in PSF surgery should be determined by markers of in vivo plasmin activation in future studies.

LEVEL OF EVIDENCE

Level II-diagnostic.

Determining the pharmacologic window of bisphosphonates that mitigates severe injury-induced osteoporosis and muscle calcification, while preserving fracture repair

Following severe injury, biomineralization is disrupted and limited therapeutic options exist to correct these pathologic changes. This study utilized a clinically relevant murine model of polytrauma including a severe injury with concomitant musculoskeletal injuries to identify when bisphosphonate administration can prevent the paradoxical decrease of biomineralization in bone and increased biomineralization in soft tissues, yet not interfere with musculoskeletal repair.

INTRODUCTION

Systemic and intrinsic mechanisms in bone and soft tissues help promote biomineralization to the skeleton, while preventing it in soft tissues. However, severe injury can disrupt this homeostatic biomineralization tropism, leading to adverse patient outcomes due to a paradoxical decrease of biomineralization in bone and increased biomineralization in soft tissues. There remains a need for therapeutics that restore the natural tropism of biomineralization in severely injured patients. Bisphosphonates can elicit potent effects on biomineralization, though with variable impact on musculoskeletal repair. Thus, a critical clinical question remains as to the optimal time to initiate bisphosphonate therapy in patients following a polytrauma, in which bone and muscle are injured in combination with a severe injury, such as a burn.

METHODS

To test the hypothesis that the dichotomous effects of bisphosphonates are dependent upon the time of administration relative to the ongoing biomineralization in reparative bone and soft tissues, this study utilized murine models of isolated injury or polytrauma with a severe injury, in conjunction with sensitive, longitudinal measure of musculoskeletal repair.

RESULTS

This study demonstrated that if administered at the time of injury, bisphosphonates prevented severe injury-induced bone loss and soft tissue calcification, but did not interfere with bone repair or remodeling. However, if administered between 7 and 21 days post-injury, bisphosphonates temporally and spatially localized to sites of active biomineralization, leading to impaired fracture callus remodeling and permanence of soft tissue calcification.

CONCLUSION

There is a specific pharmacologic window following polytrauma that bisphosphonates can prevent the consequences of dysregulated biomineralization, yet not impair musculoskeletal regeneration.

Effect of Local Tranexamic Acid on the Quality of Bone Healing in a Rat Spinal Fusion Model

Introduction

The use of the antifibrinolytic agent tranexamic acid has positive effects on bleeding control, but our knowledge is still limited regarding how fibrinolysis suppression changes the process of bone formation and the quality of bone. Because of the several side effects of systemic tranexamic acid, topical usage has been established in several procedures. This study aimed to investigate the effect of local tranexamic acid on vertebral fusion by using macroscopic, radiologic, and microscopic techniques. We also attempted to determine the safe dose range in case some doses had negative effects on fusion.

Methods

Twenty-eight Wistar albino rats underwent intertransverse fusion. All rats were randomized into four groups: groups treated with local tranexamic acid doses of 1 mg/kg (D1), 10 mg/kg (D10), and 100 mg/kg (D100) and the control group with no drug (D0). At the end of the eighth week, all rats were sacrificed for evaluation in terms of palpation, mammography, and histopathologic analysis.

Results

The manual palpation results presented with lower fusion rates in D10 and D100 groups than in the control group. Radiological examination results were significantly higher in the control group. The histopathologic examination revealed no significant differences between groups in the percent of new bone formation.

Conclusions

Our results showed that local administration of tranexamic acid reduced the quality and stability of fusion without a delay in bone formation. However, doses of 1 mg/kg did not reduce the stability in the palpation test. Our findings suggest that 1 mg/kg dose is a critical threshold above which tranexamic acid reduced the bone healing process of fusion and that surgeons should consider the doses of local tranexamic acid during surgery.

Suppression of fibrin(ogen)-driven pathologies in disease models through controlled knockdown by lipid nanoparticle delivery of siRNA

Fibrinogen plays a pathologic role in multiple diseases. It contributes to thrombosis and modifies inflammatory and immune responses, supported by studies in mice expressing fibrinogen variants with altered function or with a germline fibrinogen deficiency. However, therapeutic strategies to safely and effectively tailor plasma fibrinogen concentration are lacking. Here, we developed a strategy to tune fibrinogen expression by administering lipid nanoparticle (LNP)-encapsulated small interfering RNA (siRNA) targeting the fibrinogen α chain (siFga). Three distinct LNP-siFga reagents reduced both hepatic Fga messenger RNA and fibrinogen levels in platelets and plasma, with plasma levels decreased to 42%, 16%, and 4% of normal within 1 week of administration. Using the most potent siFga, circulating fibrinogen was controllably decreased to 32%, 14%, and 5% of baseline with 0.5, 1.0, and 2.0 mg/kg doses, respectively. Whole blood from mice treated with siFga formed clots with significantly decreased clot strength ex vivo, but siFga treatment did not compromise hemostasis following saphenous vein puncture or tail transection. In an endotoxemia model, siFga suppressed the acute phase response and decreased plasma fibrinogen, D-dimer, and proinflammatory cytokine levels. In a sterile peritonitis model, siFga restored normal macrophage migration in plasminogen-deficient mice. Finally, treatment of mice with siFga decreased the metastatic potential of tumor cells in a manner comparable to that observed in fibrinogen-deficient mice. The results indicate that siFga causes robust and controllable depletion of fibrinogen and provides the proof-of-concept that this strategy can modulate the pleiotropic effects of fibrinogen in relevant disease models.

Parameters of Hemostasis in Sheep Implanted with Composite Scaffold Settled by Stimulated Mesenchymal Stem Cells-Evaluation of the Animal Model

Implantation of composite scaffolds could be potentially associated with the risk of hemostatic disturbances in a recipient. However, there is a lack of information on possible alterations in clotting mechanisms resulting from such a procedure. The aim of the present work was to investigate changes in hemostatic parameters in sheep implanted with a scaffold composed of poly(ε-caprolactone) and hydroxyapatite and tricalcium phosphate (9:4.5:4.5), settled previously with mesenchymal stem cells stimulated by fibroblast growth factor-2 and bone morphogenetic protein-2. Nine Merino sheep were examined for 7 days, and measurements of clotting times (PT, aPTT), activities of antithrombin, protein C and clotting factors II-XII, and concentrations of fibrinogen and D-dimer were carried out before and 1 h, 24 h, 3 days and 7 days after scaffold implantation. The introduction of scaffold initially resulted in a slowdown of the clotting processes (most evident 24 h after surgery); PT and aPTT increased to 14.8 s and 33.9 s, respectively. From the third day onwards, most of these alterations began to return to normal values. The concentration of fibrinogen rose throughout the observation period (up to 8.4 g/L), mirroring the ongoing inflammatory reaction. However, no signals of significant disturbances in hemostatic processes were detected in the sheep tested.

Plasminogen Regulates Fracture Repair by Promoting the Functions of Periosteal Mesenchymal Progenitors

Defective or insufficient bone repair and regeneration are common in patients as a result of major trauma or severe disease. Cell therapy with periosteal mesenchymal progenitors, which can be limited in severe injury, serves as a promising approach; however, its efficacy is limited due to a repair-hostile ischemic tissue microenvironment after traumatic fracture. Here we report that plasminogen (Plg), a factor that is upregulated in these environments, is critical for fracture healing. Plg knockout mice had impaired trabecular and cortical bone structure and exhibited delayed and incomplete fracture healing. Interestingly, Plg deficiency greatly reduced the thickness of expanded periosteum, suggesting a role of Plg in periosteal mesenchymal progenitor-mediated bone repair. In culture, Plg increased cell proliferation and migration in periosteal mesenchymal progenitors and inhibited cell death under ischemic conditions. Mechanistically, we revealed that Plg cleaved and activated Cyr61 to regulate periosteal progenitor function. Thus, our study uncovers a cellular mechanism underlying fracture healing, by which Plg activates Cyr61 to promote periosteal progenitor proliferation, survival, and migration and improves bone repair after fracture. Targeting Plg may offer a rational and effective therapeutic opportunity for improving fracture healing. © 2021 American Society for Bone and Mineral Research (ASBMR).

Resolution of inflammation in bone regeneration: From understandings to therapeutic applications

Impaired bone healing occurs in 5-10% of cases following injury, leading to a significant economic and clinical impact. While an inflammatory response upon injury is necessary to facilitate healing, its resolution is critical for bone tissue repair as elevated acute or chronic inflammation is associated with impaired healing in patients and animal models. This process is governed by important crosstalk between immune cells through mediators that contribute to resolution of inflammation in the local healing environment. Approaches modulating the initial inflammatory phase followed by its resolution leads to a pro-regenerative environment for bone regeneration. In this review, we discuss the role of inflammation in bone repair, the negative impact of dysregulated inflammation on bone tissue regeneration, and how timely resolution of inflammation is necessary to achieve normal healing. We will discuss applications of biomaterials to treat large bone defects with a specific focus on resolution of inflammation to modulate the immune environment following bone injury, and their observed functional benefits. We conclude the review by discussing future strategies that could lead to the realization of anti-inflammatory therapeutics for bone tissue repair.

Insights into the Cellular and Molecular Mechanisms That Govern the Fracture-Healing Process: A Narrative Review

Fracture-healing is a complex multi-stage process that usually progresses flawlessly, resulting in restoration of bone architecture and function. Regrettably, however, a considerable number of fractures fail to heal, resulting in delayed unions or non-unions. This may significantly impact several aspects of a patient’s life. Not surprisingly, in the past few years, a substantial amount of research and number of clinical studies have been designed, aiming at shedding light into the cellular and molecular mechanisms that regulate fracture-healing. Herein, we present the current knowledge on the pathobiology of the fracture-healing process. In addition, the role of skeletal cells and the impact of marrow adipose tissue on bone repair is discussed. Unveiling the pathogenetic mechanisms that govern the fracture-healing process may lead to the development of novel, smarter, and more effective therapeutic strategies for the treatment of fractures, especially of those with large bone defects.

The Cellular Choreography of Osteoblast Angiotropism in Bone Development and Homeostasis

Interaction between endothelial cells and osteoblasts is essential for bone development and homeostasis. This process is mediated in large part by osteoblast angiotropism, the migration of osteoblasts alongside blood vessels, which is crucial for the homing of osteoblasts to sites of bone formation during embryogenesis and in mature bones during remodeling and repair. Specialized bone endothelial cells that form "type H" capillaries have emerged as key interaction partners of osteoblasts, regulating osteoblast differentiation and maturation and ensuring their migration towards newly forming trabecular bone areas. Recent revolutions in high-resolution imaging methodologies for bone as well as single cell and RNA sequencing technologies have enabled the identification of some of the signaling pathways and molecular interactions that underpin this regulatory relationship. Similarly, the intercellular cross talk between endothelial cells and entombed osteocytes that is essential for bone formation, repair, and maintenance are beginning to be uncovered. This is a relatively new area of research that has, until recently, been hampered by a lack of appropriate analysis tools. Now that these tools are available, greater understanding of the molecular relationships between these key cell types is expected to facilitate identification of new drug targets for diseases of bone formation and remodeling.

H3K36 methyltransferase NSD1 regulates chondrocyte differentiation for skeletal development and fracture repair

Chondrocyte differentiation is a critical process for endochondral ossification, which is responsible for long bone development and fracture repair. Considerable progress has been made in understanding the transcriptional control of chondrocyte differentiation; however, epigenetic regulation of chondrocyte differentiation remains to be further studied. NSD1 is a H3K36 (histone H3 at lysine 36) methyltransferase. Here, we showed that mice with Nsd1 deficiency in Prx1⁺ mesenchymal progenitors but not in Col2⁺ chondrocytes showed impaired skeletal growth and fracture healing accompanied by decreased chondrogenic differentiation. Via combined RNA sequencing (RNA-seq) and chromatin immunoprecipitation sequencing (ChIP-seq) analysis, we identified sex determining region Y box 9 (Sox9), the key transcription factor of chondrogenic differentiation, as a functional target gene of NSD1. Mechanistically, NSD1 regulates Sox9 expression by modulating H3K36me1 and H3K36me2 levels in the Sox9 promoter region, constituting a novel epigenetic regulatory mechanism of chondrogenesis. Moreover, we found that NSD1 can directly activate the expression of hypoxia-inducible factor 1α (HIF1α), which plays a vital role in chondrogenic differentiation through its regulation of Sox9 expression. Collectively, the results of our study reveal crucial roles of NSD1 in regulating chondrogenic differentiation, skeletal growth, and fracture repair and expand our understanding of the function of epigenetic regulation in chondrogenesis and skeletal biology.

Plasma D-dimer and FDP are promising biomarkers to predict perioperative fibrinolysis and bleeding following primary total joint arthroplasty: A STROBE compliant article

Perioperative bleeding is associated with postoperative hyperfibrinolysis caused by surgical trauma in the setting of total hip and knee arthroplasty (THA/ TKA). The study aimed to clarify the dynamics of postoperative fibrinolytic activity and the values of fibrin degeneration products and thromboelastography (TEG) to guide precisive antifibrinolytic therapy.Forty three patients undergoing primary unilateral THAs and 40 TKAs were included to the prospective observational cohort study. Venous blood sample at different time points (preoperative, intraoperative, postoperative 6 hours, 12 hours, 24 hours, 48 hours) were drawn to test D-dimer, fibrin (-ogen) degradation products (FDP) and TEG.The TEG parameters associated with coagulation (R, K, α, MA, and CI) and fibrinolysis (estimate percent lysis and Ly30) were all in normal range although had a higher level than preoperative time (P < .05). The postoperative levels of D-dimer and FDP were higher than preoperative level (P < .05). The dynamics of D-dimer and FDP presented a bimodal pattern, which peaked at 6 hours postoperatively, then remained and decreased until 24 hours, but would rebound at 48 hours postoperatively with smaller amplitude. Moreover, FDP6h (P = .028), D-Dimer6h (P = .044), FDP12h (P = .009), D-dimer12h (P = .007), and FDP48h (P = .016) were all correlated with total blood loss on POD3.FDP and D-dimer were effective and practical markers for prediction of acute postoperative fibrinolytic activity, which peaked at 6 hours after end of surgery and would maintain for at least 24 hours.

Fibrinolysis as a target to enhance osteoporotic fracture healing by vibration therapy in a metaphyseal fracture model

Aims

Fibrinolysis plays a key transition step from haematoma formation to angiogenesis and fracture healing. Low-magnitude high-frequency vibration (LMHFV) is a non-invasive biophysical modality proven to enhance fibrinolytic factors. This study investigates the effect of LMHFV on fibrinolysis in a clinically relevant animal model to accelerate osteoporotic fracture healing.

Methods

A total of 144 rats were randomized to four groups: sham control; sham and LMHFV; ovariectomized (OVX); and ovariectomized and LMHFV (OVX-VT). Fibrinolytic potential was evaluated by quantifying fibrin, tissue plasminogen activator (tPA), and plasminogen activator inhibitor-1 (PAI-1) along with healing outcomes at three days, one week, two weeks, and six weeks post-fracture.

Results

All rats achieved healing, and x-ray relative radiopacity for OVX-VT was significantly higher compared to OVX at week 2. Martius Scarlet Blue (MSB) staining revealed a significant decrease of fibrin content in the callus in OVX-VT compared with OVX on day 3 (p = 0.020). Mean tPA from muscle was significantly higher for OVX-VT compared to OVX (p = 0.020) on day 3. Mechanical testing revealed the mean energy to failure was significantly higher for OVX-VT at 37.6 N mm (SD 8.4) and 71.9 N mm (SD 30.7) compared with OVX at 5.76 N mm (SD 7.1) (p = 0.010) and 17.7 N mm (SD 11.5) (p = 0.030) at week 2 and week 6, respectively.

Conclusion

Metaphyseal fracture healing is enhanced by LMHFV, and one of the important molecular pathways it acts on is fibrinolysis. LMHFV is a promising intervention for osteoporotic metaphyseal fracture healing. The improved mechanical properties, acceleration of fracture healing, and safety justify its role into translation to future clinical studies.

Cite this article: Bone Joint Res 2021;10(1):41–50.

Sustained depletion of FXIII-A by inducing acquired FXIII-B deficiency

The activated form of coagulation factor XIII (FXIII-A2B2), FXIII-A*, is a hemostatic enzyme essential for inhibiting fibrinolysis by irreversibly crosslinking fibrin and antifibrinolytic proteins. Despite its importance, there are no modulatory therapeutics. Guided by the observation that humans deficient in FXIII-B have reduced FXIII-A without severe bleeding, we hypothesized that a suitable small interfering RNA (siRNA) targeting hepatic FXIII-B could safely decrease FXIII-A. Here we show that knockdown of FXIII-B with siRNA in mice and rabbits using lipid nanoparticles resulted in a sustained and controlled decrease in FXIII-A. The concentration of FXIII-A in plasma was reduced by 90% for weeks after a single injection and for more than 5 months with repeated injections, whereas the concentration of FXIII-A in platelets was unchanged. Ex vivo, crosslinking of α2-antiplasmin and fibrin was impaired and fibrinolysis was enhanced. In vivo, reperfusion of carotid artery thrombotic occlusion was also enhanced. Re-bleeding events were increased after challenge, but blood loss was not significantly increased. This approach, which mimics congenital FXIII-B deficiency, provides a potential pharmacologic and experimental tool to modulate FXIII-A2B2 activity.

Akkermansia muciniphila promotes type H vessel formation and bone fracture healing by reducing gut permeability and inflammation

Improving revascularization is one of the major measures in fracture treatment. Moderate local inflammation triggers angiogenesis, whereas systemic inflammation hampers angiogenesis. Previous studies showed that Akkermansia muciniphila, a gut probiotic, ameliorates systemic inflammation by tightening the intestinal barrier. In this study, fractured mice intragastrically administrated with A. muciniphila were found to display better fracture healing than mice treated with vehicle. Notably, more preosteclasts positive for platelet-derived growth factor-BB (PDGF-BB) were induced by A. muciniphila at 2 weeks post fracture, coinciding with increased formation of type H vessels, a specific vessel subtype that couples angiogenesis and osteogenesis, and can be stimulated by PDGF-BB. Moreover, A. muciniphila treatment significantly reduced gut permeability and inflammation at the early stage. Dextran sulfate sodium (DSS) was used to disrupt the gut barrier to determine its role in fracture healing and whether A. muciniphila still can stimulate bone fracture healing. As expected, A. muciniphila evidently improved gut barrier, reduced inflammation and restored the impaired bone healing and angiogenesis in DSS-treated mice. Our results suggest that A. muciniphila reduces intestinal permeability and alleviates inflammation, which probably induces more PDGF-BB+ preosteoclasts and type H vessel formation in callus, thereby promoting fracture healing. This study provides the evidence for the involvement of type H vessels in fracture healing and suggests the potential of A. muciniphila as a promising strategy for bone healing.This article has an associated First Person interview with the first author of the paper.

Effects of fibrinogen synthesis inhibition on vascular cognitive impairment during traumatic brain injury in mice

Traumatic brain injury (TBI) is associated with increased blood content of fibrinogen (Fg), called hyperfibrinogenemia (HFg), which results in enhanced cerebrovascular permeability and leads to short-term memory (STM) reduction. Previously, we showed that extravasated Fg was deposited in the vasculo-astrocyte interface and was co-localized with cellular prion protein (PrP^C) during mild-to-moderate TBI in mice. These effects were accompanied by neurodegeneration and STM reduction. However, there was no evidence presented that the described effects were the direct result of the HFg during TBI. We now present data indicating that inhibition of Fg synthesis can ameliorate TBI-induced cerebrovascular permeability and STM reduction. Cortical contusion injury (CCI) was induced in C57BL/6J mice. Then mice were treated with either Fg antisense oligonucleotide (Fg-ASO) or with control-ASO for two weeks. Cerebrovascular permeability to fluorescently labeled bovine serum albumin was assessed in cortical venules following evaluation of STM with memory assessement tests. Separately, brain samples were collected in order to define the expression of PrP^C via Western blotting while deposition and co-localization of Fg and PrP^C, as well as gene expression of inflammatory marker activating transcription factor 3 (ATF3), were characterized with real-time PCR. Results showed that inhibition of Fg synthesis with Fg-ASO reduced overexpression of AFT3, ameliorated enhanced cerebrovascular permeability, decreased expression of PrP^C and Fg deposition, decreased formation of Fg-PrP^C complexes in brain, and improved STM. These data provide direct evidence that a CCI-induced inflammation-mediated HFg could be a triggering mechanism involved in vascular cognitive impairment seen previously in our studies during mild-to-moderate TBI.

Tooth-Supporting Hard Tissue Regeneration Using Biopolymeric Material Fabrication Strategies

The mineralized tissues (alveolar bone and cementum) are the major components of periodontal tissues and play a critical role to anchor periodontal ligament (PDL) to tooth-root surfaces. The integrated multiple tissues could generate biological or physiological responses to transmitted biomechanical forces by mastication or occlusion. However, due to periodontitis or traumatic injuries, affect destruction or progressive damage of periodontal hard tissues including PDL could be affected and consequently lead to tooth loss. Conventional tissue engineering approaches have been developed to regenerate or repair periodontium but, engineered periodontal tissue formation is still challenging because there are still limitations to control spatial compartmentalization for individual tissues and provide optimal 3D constructs for tooth-supporting tissue regeneration and maturation. Here, we present the recently developed strategies to induce osteogenesis and cementogenesis by the fabrication of 3D architectures or the chemical modifications of biopolymeric materials. These techniques in tooth-supporting hard tissue engineering are highly promising to promote the periodontal regeneration and advance the interfacial tissue formation for tissue integrations of PDL fibrous connective tissue bundles (alveolar bone-to-PDL or PDL-to-cementum) for functioning restorations of the periodontal complex.

Roles of fibrinolytic factors in the alterations in bone marrow hematopoietic stem/progenitor cells during bone repair

In bone tissues, metabolic turnover through bone resorption by osteoclasts and bone formation by osteoblasts, termed bone remodeling, is strictly controlled and maintains homeostasis. Fibrinolytic factors are expressed in osteoclasts and osteoblasts, and are involved in bone remodeling through bone resorption and formation. The repair/regeneration process after bone injury is divided into the acute inflammatory, repair, and remodeling stages. Osteoblasts, osteoclasts, chondrocytes, and macrophages involved in the bone repair process originate from hematopoietic stem/progenitor cells (HSPCs) and mesenchymal stem cells (MSCs) in the bone marrow. Therefore, stem cells in the bone marrow may be strongly influenced by bone injury. The urokinase-type PA (u-PA)/plasminogen (Plg) system functions in macrophage accumulation/phagocytosis through chemokines in the acute inflammatory stage, and Plg increases blood vessel-related growth factor expression, being involved in vascularization in mice. Plasminogen activator inhivitor-1 (PAI-1) causes bone loss and delayed bone repair through the inhibition of osteoblast differentiation in a drug-induced diabetes model in mice. Plg is considered to induce transforming growth factor-β (TGF-β) production in macrophages in the bone repair process, TGF-β release from the extracellular matrix through the activation of matrix metalloproteinase-9 (MMP-9), and stromal cell-derived factor-1 (SDF-1) expression in endosteal preosteoblasts, leading to the induction of bone marrow HSPCs in mice. Based on the above, establishment of a fibrinolytic factor-targeting method efficiently promoting bone repair/regeneration and fracture healing, and development of a new osteoporosis treatment method and diagnostic marker are awaited.

Fibrinogen and Neuroinflammation During Traumatic Brain Injury

Many neurodegenerative diseases such as Alzheimer's disease (AD), multiple sclerosis, and traumatic brain injury (TBI) are associated with systemic inflammation. Inflammation itself results in increased blood content of fibrinogen (Fg), called hyperfibrinogenemia (HFg). Fg is not only considered an acute phase protein and a marker of inflammation, but has been shown that it can cause inflammatory responses. Fibrin deposits have been associated with memory reduction in neuroinflammatory diseases such as AD and TBI. Reduction in short-term memory has been seen during the most common form of TBI, mild-to-moderate TBI. Fibrin deposits have been found in brains of patients with mild-to-moderate TBI. The vast majority of the literature emphasizes the role of fibrin-activated microglia as the mediator in the neuroinflammation pathway. However, the recent discovery that astrocytes, which constitute approximately 30% of the cells in the mammalian central nervous system, manifest different reactive states warrants further investigations in the causative role of HFg in astrocyte-mediated neuroinflammation. Our previous study showed that Fg deposited in the vasculo-astrocyte interface-activated astrocytes. However, little is known of how Fg directly affects astrocytes and neurons. In this review, we summarize studies that show the effect of Fg on different types of cells in the vasculo-neuronal unit. We will also discuss the possible mechanism of HFg-induced neuroinflammation during TBI.

Proteomic characterization of a trauma-based rat model of heterotopic ossification identifies interactive signaling networks as potential therapeutic targets

Heterotopic ossification (HO) is the formation of ectopic bone in soft tissues observed in patients following blast injuries, orthopedic or head trauma, burns, or in the context of inborn mutations of genes involved in osteogenesis. There is no universally accepted therapy for HO. This study has used global unbiased mass spectrometry proteomic approaches, validated by western immunoblots, to interrogate skeletal muscle tissues obtained from a highly reproducible rat model of trauma induced HO. During early the phase of HO development, statistically significant modulation of proteins within the following pathways was identified: coagulation, cyclic AMP, extracellular matrix, immunity/inflammation, NADH metabolism, TGFβ. These metabolic proteins and pathways have the potential to serve as diagnostic, prognostic, and therapeutic targets for this devastating orthopedic condition that has considerable impact on the patient's quality of life. Furthermore, the findings confirm and extend previous in vitro stromal/stem cell and clinical studies from the field. SIGNIFICANCE: This study confirms and extends the field's understanding of the protein pathways that are modulated in a rat model of trauma induced heterotopic ossification. The identification of specific proteins such as the AP1 transcription factor as well as protein families such as the complement/coagulation pathway and serine protease inhibitors as biomarkers have potential clinical translational value. These outcomes have relevance to the physiological and pathological mineralization processes contributing to the recovery of orthopedic trauma patients.

Inverse association between bone mineral density and fibrinogen in menopausal women

OBJECTIVE

Inflammatory diseases are risk factors for osteoporosis. We aimed to explore whether fibrinogen, which is linked to chronic inflammation, is associated with bone mineral density (BMD) in menopausal women.

METHODS

In this cross-sectional study, we analyzed 339 menopausal women from Zhejiang Province between January 2016 and October 2019. Linear regression analysis was performed to assess the relationship between fibrinogen and BMD.

RESULTS

Significant inverse association was observed between the serum fibrinogen level and BMD in menopausal women. The mean BMD in each quartile of fibrinogen level was 0.901, 0.897, 0.892, and 0.855 g/cm², respectively (p = 0.027). After adjusting for age, body mass index, metabolic profiles, blood inflammatory factors, and serum levels of estradiol, calcium, phosphorus, and alkaline phosphatase, fibrinogen levels remained significantly associated with BMD (regression coefficients for quartiles 1-3 vs. quartile 4 were 0.046, 0.027, and 0.036, respectively; p for trend <0.05).

CONCLUSIONS

Higher fibrinogen levels were associated with lower BMD in menopausal women, which was independent of age, body mass index, estradiol, and other factors. Therefore, serum fibrinogen can be used as a new predictor of reduced BMD in menopausal women.

Bushenhuoxue formula accelerates fracture healing via upregulation of TGF-β/Smad2 signaling in mesenchymal progenitor cells

BACKGROUND

Although Bushenhuoxue formula (BSHXF) is successfully used as a non-traumatic therapy in treating bone fracture in China, the molecular mechanism underlying its effects remains poorly understood.

PURPOSE

The present study aims to explore the therapeutic effects of BSHXF on fracture healing in mice and the underlying mechanism.

METHODS

We performed unilateral open transverse tibial fracture procedure in C57BL/6 mice which were treated with or without BSHXF. Fracture callus tissues were collected and analyzed by X-ray, micro-CT, biomechanical testing, histopathology and quantitative gene expression analysis. Tibial fracture procedure was also performed in Cre-negative and Gli1-CreER; Tgfbr2flox/flox conditional knockout (KO) mice (Tgfbr2Gli1ER) to determine if BSHXF enhances fracture healing in a TGF-β-dependent manner. In addition, scratch-wound assay and cell counting kit-8 (CCK-8) assay were used to evaluate the effect of BSHXF on cell migration and cell proliferation in C3H10T1/2 mesenchymal stem cells, respectively.

RESULTS

BSHXF promoted endochondral ossification and enhanced bone strength in wild-type (WT) or Cre- control mice. In contrast, BSHXF failed to promote bone fracture healing in Tgfbr2Gli1ER conditional KO mice. In the mice receiving BSHXF treatment, TGF-β/Smad2 signaling was significantly activated. Moreover, BSHXF enhanced cell migration and cell proliferation in C3H10T1/2 cells, which was strongly attenuated by the small molecule inhibitor SB525334 against TGF-β type I receptor.

CONCLUSION

These data demonstrated that BSHXF promotes fracture healing by activating TGF-β/Smad2 signaling. BSHXF may be used as a type of alternative medicine for the treatment of bone fracture healing.

Bone regeneration in rabbit calvarial defects using PRGF and adipose-derived stem cells: histomorphometrical analysis

Aim: The aim of this study was to evaluate the osteogenic potential of adipose-derived stem cells (ADSCs) and to assess the influence of plasma rich in growth factors (PRGF) on bone regeneration using ADSCs. Materials & methods: Bone defects were randomly allocated to the five treatment modalities: spontaneous healing, natural bovine bone mineral (BBM), BBM loaded with PRGF, BBM loaded with ADSCs and BBM loaded with a combination of ADSCs and PRGF. Results: The PRGF significantly enhanced the biomaterial-to-bone contact. Defects treated with ADSCs and PRGF or a combination of both showed the greatest bone regeneration. Conclusion: Combining PRGF and ADSCs boosts the bone graft regenerative potential at the earliest period of healing.

Plasminogen activation in the musculoskeletal acute phase response: Injury, repair, and disease

The musculoskeletal system is critical for movement and the protection of organs. In addition to abrupt injuries, daily physical demands inflict minor injuries, necessitating a coordinated process of repair referred to as the acute-phase response (APR). Dysfunctional APRs caused by severe injuries or underlying chronic diseases are implicated in pathologic musculoskeletal repair, resulting in decreased mobility and chronic pain. The molecular mechanisms behind these phenomena are not well understood, hindering the development of clinical solutions. Recent studies indicate that, in addition to regulating intravascular clotting, the coagulation and fibrinolytic systems are also entrenched in tissue repair. Although plasmin and fibrin are considered antithetical to one another in the context of hemostasis, in a proper APR, they complement one another within a coordinated spatiotemporal framework. Once a wound is contained by fibrin, activation of plasmin promotes the removal of fibrin and stimulates angiogenesis, tissue remodeling, and tissue regeneration. Insufficient fibrin deposition or excessive plasmin-mediated fibrinolysis in early convalescence prevents injury containment, causing bleeding. Alternatively, excess fibrin deposition and/or inefficient plasmin activity later in convalescence impairs musculoskeletal repair, resulting in tissue fibrosis and osteoporosis, while inappropriate fibrin or plasmin activity in a synovial joint can cause arthritis. Together, these pathologic conditions lead to chronic pain, poor mobility, and diminished quality of life. In this review, we discuss both fibrin-dependent and -independent roles of plasminogen activation in the musculoskeletal APR, how dysregulation of these mechanisms promote musculoskeletal degeneration, and the possibility of therapeutically manipulating plasmin or fibrin to treat musculoskeletal disease.

Dnmt3b ablation impairs fracture repair through upregulation of Notch pathway

We previously established that DNA methyltransferase 3b (Dnmt3b) is the sole Dnmt responsive to fracture repair and that Dnmt3b expression is induced in progenitor cells during fracture repair. In the current study, we confirmed that Dnmt3b ablation in mesenchymal progenitor cells (MPCs) resulted in impaired endochondral ossification, delayed fracture repair, and reduced mechanical strength of the newly formed bone in Prx1-Cre;Dnmt3bf/f (Dnmt3bPrx1) mice. Mechanistically, deletion of Dnmt3b in MPCs led to reduced chondrogenic and osteogenic differentiation in vitro. We further identified Rbpjκ as a downstream target of Dnmt3b in MPCs. In fact, we located 2 Dnmt3b binding sites in the murine proximal Rbpjκ promoter and gene body and confirmed Dnmt3b interaction with the 2 binding sites by ChIP assays. Luciferase assays showed functional utilization of the Dnmt3b binding sites in murine C3H10T1/2 cells. Importantly, we showed that the MPC differentiation defect observed in Dnmt3b deficiency cells was due to the upregulation of Rbpjκ, evident by restored MPC differentiation upon Rbpjκ inhibition. Consistent with in vitro findings, Rbpjκ blockage via dual antiplatelet therapy reversed the differentiation defect and accelerated fracture repair in Dnmt3bPrx1 mice. Collectively, our data suggest that Dnmt3b suppresses Notch signaling during MPC differentiation and is necessary for normal fracture repair.

Prevention of Anastomotic Leak Via Local Application of Tranexamic Acid to Target Bacterial-mediated Plasminogen Activation: A Practical Solution to a Complex Problem

OBJECTIVE

To investigate the role of bacterial- mediated plasminogen (PLG) activation in the pathogenesis of anastomotic leak (AL) and its mitigation by tranexamic acid (TXA).

BACKGROUND

AL is the most feared complication of colorectal resections. The pathobiology of AL in the setting of a technically optimal procedure involves excessive submucosal collagen degradation by resident microbes. We hypothesized that activation of the host PLG system by pathogens is a central and targetable pathway in AL.

METHODS

We employed kinetic analysis of binding and activation of human PLG by microbes known to cause AL, and collagen degradation assays to test the impact of PLG on bacterial collagenolysis. Further, we measured the ability of the antifibrinolytic drug TXA to inhibit this process. Finally, using mouse models of pathogen-induced AL, we locally applied TXA via enema and measured its ability to prevent a clinically relevant AL.

RESULTS

PLG is deposited rapidly and specifically at the site of colorectal anastomoses. TXA inhibited PLG activation and downstream collagenolysis by pathogens known to have a causal role in AL. TXA enema reduced collagenolytic bacteria counts and PLG deposition at anastomotic sites. Postoperative PLG inhibition with TXA enema prevented clinically and pathologically apparent pathogen-mediated AL in mice.

CONCLUSIONS

Bacterial activation of host PLG is central to collagenolysis and pathogen-mediated AL. TXA inhibits this process both in vitro and in vivo. TXA enema represents a promising method to prevent AL in high-risk sites such as the colorectal anastomoses.

The Size of Intramedullary Fixation Affects Endochondral-Mediated Angiogenesis During Fracture Repair

OBJECTIVES

To explore the effect of intramedullary pin size on the biology of a healing fracture, specifically endochondral angiogenesis. We hypothesized that fracture fixation with a smaller pin would permit greater interfragmentary strain resulting in increased total amount of vascular endothelial growth factor within the callus and greater angiogenesis compared to fixation with a larger pin.

METHODS

Transverse mid-shaft femur fractures in 8-week-old mice were fixed with either a 23-gauge (G) or 30-G pin. Differences in interfragmentary strain at the fracture site were estimated between cohorts. A combination of histology, gene expression, serial radiography, and microcomputed tomography with and without vascular contrast agent were used to assess fracture healing and vascularity for each cohort.

RESULTS

Larger soft-tissue callus formation increased vascular endothelial growth factor-A expression, and a corresponding increase in vascular volume was observed in the higher strain, 30-G cohort. Radiographic analysis demonstrated earlier hard callus formation with greater initial interfragmentary strain, similar rates of union between pin size cohorts, yet delayed callus remodeling in mice with the larger pin size.

CONCLUSIONS

These findings suggest that the stability conferred by an intramedullary nail influences endochondral angiogenesis at the fracture.

The Long Pentraxin PTX3 as a Link Between Innate Immunity, Tissue Remodeling, and Cancer

The innate immune system comprises a cellular and a humoral arm. Humoral pattern recognition molecules include complement components, collectins, ficolins, and pentraxins. These molecules are involved in innate immune responses by recognizing microbial moieties and damaged tissues, activating complement, exerting opsonic activity and facilitating phagocytosis, and regulating inflammation. The long pentraxin PTX3 is a prototypic humoral pattern recognition molecule that, in addition to providing defense against infectious agents, plays several functions in tissue repair and regulation of cancer-related inflammation. Characterization of the PTX3 molecular structure and biochemical properties, and insights into its interactome and multiple roles in tissue damage and remodeling support the view that microbial and matrix recognition are evolutionarily conserved functions of humoral innate immunity molecules.

The multifaceted role of fibrinogen in tissue injury and inflammation

The canonical role of the hemostatic and fibrinolytic systems is to maintain vascular integrity. Perturbations in either system can prompt primary pathological end points of hemorrhage or thrombosis with vessel occlusion. However, fibrin(ogen) and proteases controlling its deposition and clearance, including (pro)thrombin and plasmin(ogen), have powerful roles in driving acute and reparative inflammatory pathways that affect the spectrum of tissue injury, remodeling, and repair. Indeed, fibrin(ogen) deposits are a near-universal feature of tissue injury, regardless of the nature of the inciting event, including injuries driven by mechanical insult, infection, or immunological derangements. Fibrin can modify multiple aspects of inflammatory cell function by engaging leukocytes through a variety of cellular receptors and mechanisms. Studies on the role of coagulation system activation and fibrin(ogen) deposition in models of inflammatory disease and tissue injury have revealed points of commonality, as well as context-dependent contributions of coagulation and fibrinolytic factors. However, there remains a critical need to define the precise temporal and spatial mechanisms by which fibrinogen-directed inflammatory events may dictate the severity of tissue injury and coordinate the remodeling and repair events essential to restore normal organ function. Current research trends suggest that future studies will give way to the identification of novel hemostatic factor-targeted therapies for a range of tissue injuries and disease.

Bone Fracture Acute Phase Response-A Unifying Theory of Fracture Repair: Clinical and Scientific Implications

Bone fractures create five problems that must be resolved: bleeding, risk of infection, hypoxia, disproportionate strain, and inability to bear weight. There have been enormous advancements in our understanding of the molecular mechanisms that resolve these problems after fractures, and in best clinical practices of repairing fractures. We put forth a modern, comprehensive model of fracture repair that synthesizes the literature on the biology and biomechanics of fracture repair to address the primary problems of fractures. This updated model is a framework for both fracture management and future studies aimed at understanding and treating this complex process. This model is based upon the fracture acute phase response (APR), which encompasses the molecular mechanisms that respond to injury. The APR is divided into sequential stages of "survival" and "repair." Early in convalescence, during "survival," bleeding and infection are resolved by collaborative efforts of the hemostatic and inflammatory pathways. Later, in "repair," avascular and biomechanically insufficient bone is replaced by a variable combination of intramembranous and endochondral ossification. Progression to repair cannot occur until survival has been ensured. A disproportionate APR-either insufficient or exuberant-leads to complications of survival (hemorrhage, thrombosis, systemic inflammatory response syndrome, infection, death) and/or repair (delayed- or non-union). The type of ossification utilized for fracture repair is dependent on the relative amounts of strain and vascularity in the fracture microenvironment, but any failure along this process can disrupt or delay fracture healing and result in a similar non-union. Therefore, incomplete understanding of the principles herein can result in mismanagement of fracture care or application of hardware that interferes with fracture repair. This unifying model of fracture repair not only informs clinicians how their interventions fit within the framework of normal biological healing but also instructs investigators about the critical variables and outputs to assess during a study of fracture repair.

Novel preclinical murine model of trauma-induced elbow stiffness

BACKGROUND

Peri-articular injury may result in functional deficits and pain. In particular, post-traumatic elbow stiffness is a debilitating condition, precluding patients from performing activities of daily living. As such, clinicians and basic scientists alike, aim to develop novel therapeutic interventions to prevent and treat elbow stiffness; thereby reducing patient morbidity. Yet, there is a paucity of pre-clinical models of peri-articular stiffness, especially of the upper extremity, necessary to develop and test the efficacy of therapeutics. We set out to develop a pre-clinical murine model of elbow stiffness, resulting from soft tissue injury, with features characteristic of pathology observed in these patients.

METHODS

A soft tissue peri-elbow injury was inflicted in mice using cardiotoxin. Pathologic tissue repair was induced by creating an investigator-imposed deficiency of plasminogen, a protease essential for musculoskeletal tissue repair. Functional testing was conducted through analysis of grip strength and gait. Radiography, microcomputed tomography, and histological analyses were employed to quantify development of heterotopic ossification.

RESULTS

Animals with peri-elbow soft tissues injury in conjunction with an investigator-imposed plasminogen deficiency, developed a significant loss of elbow function measured by grip strength (2.387 ± 0.136 N vs 1.921 ± 0.157 N, ****, p < 0.0001) and gait analysis (35.05 ± 2.775 mm vs 29.87 ± 2.075 mm, ***, p < 0.0002). Additionally, plasminogen deficient animals developed capsule thickening, delayed skeletal muscle repair, fibrosis, chronic inflammation, and heterotopic ossification; all features characteristic of pathology observed in patients with trauma-induced elbow stiffness.

CONCLUSION

A soft tissue injury to the peri-elbow soft tissue with a concomitant deficiency in plasminogen, instigates elbow stiffness and pathologic features similar to those observed in humans. This pre-clinical model is valuable for translational studies designed to investigate the contributions of pathologic features to elbow stiffness or as a high-throughput model for testing therapeutic strategies designed to prevent and treat trauma-induced elbow stiffness.

Pediatric Musculoskeletal Infection: Hijacking the Acute-Phase Response

Tissue injury activates the acute-phase response mediated by the liver, which promotes coagulation, immunity, and tissue regeneration. To survive and disseminate, musculoskeletal pathogens express virulence factors that modulate and hijack this response. As the acute-phase reactants required by these pathogens are most abundant in damaged tissue, these infections are predisposed to occur in tissues following traumatic or surgical injury. Staphylococcus aureus expresses the virulence factors coagulase and von Willebrand binding protein to stimulate coagulation and to form a fibrin abscess that protects it from host immune-cell phagocytosis. After the staphylococcal abscess community reaches quorum, which is the colony density that enables cell-to-cell communication and coordinated gene expression, subsequent expression of staphylokinase stimulates activation of fibrinolysis, which ruptures the abscess wall and results in bacterial dissemination. Unlike Staphylococcus aureus, Streptococcus pyogenes expresses streptokinase and other virulence factors to activate fibrinolysis and to rapidly disseminate throughout the body, causing diseases such as necrotizing fasciitis. Understanding the virulence strategies of musculoskeletal pathogens will help to guide clinical diagnosis and decision-making through monitoring of acute-phase markers such as C-reactive protein, erythrocyte sedimentation rate, and fibrinogen.

Combined sustained release of BMP2 and MMP10 accelerates bone formation and mineralization of calvaria critical size defect in mice

The effect of dual delivery of bone morphogenetic protein-2 (BMP-2) and matrix metalloproteinase 10 (MMP10) on bone regeneration was investigated in a murine model of calvarial critical-size defect, hypothesizing that it would result in an enhanced bone formation. Critical-size calvarial defects (4 mm diameter) were created in mice and PLGA microspheres preloaded with either BMP-2, MMP10 or a microsphere combination of both were transplanted into defect sites at different doses. Empty microspheres were used as the negative control. Encapsulation efficiency was assessed and in vivo release kinetics of BMP-2 and MMP10 were examined over 14 days. Histological analyses were used to analyze bone formation after four and eight weeks. Combination with MMP10 (30 ng) significantly enhanced BMP-2 (600 ng)-mediated osteogenesis, as confirmed by the increase in percentage of bone fill (p < .05) at four weeks. Moreover, it also increased mineral apposition rate (p < .05), measured by double labeling with tetracycline and calceine. MMP10 accelerates bone repair by enhancing BMP-2-promoted bone healing and improving the mineralization rate. In conclusion combination of MMP10 and BMP-2 may become a promising strategy for repair and regeneration of bone defects.

Effects of zinc and aprotinin on the healing of ulnar diaphyseal fractures in rabbits

The effects of zinc and aprotinin on fracture healing in experimentally induced fractures were investigated by means of histometric analyses and alkaline phosphatase histochemistry. Healthy 54 adult New Zealand White female rabbits were separated into three groups as control, zinc, and aprotinin treatment. The control animals did not receive any medicament; zinc sulphate was given orally to the rabbits in the Zn group for 15 days. Aprotinin was postoperatively infiltrated into the fracture area at the 3rdand 24thh following operation. Immobilization of fracture ends of all groups was similar throughout the experiment. The zinc administered group displayed the highest alkaline phosphatase positive cell level through the experiment. By day 30 after the operation, fibrocartilage and osseous tissues reached the highest levels in the zinc treated group. Based on the observation of augmented osseous tissue formation and increased alkaline phosphatase positive osteoblastic cell activity in the callus, it was conluded that Zn sulphate is a potent stimulator of bone formation by increasing mineralization in the fractured bone segments.

Unexpected timely fracture union in matrix metalloproteinase 9 deficient mice

Immediately following a fracture, a fibrin laden hematoma is formed to prevent bleeding and infection. Subsequently, the organized removal of fibrin, via the protease plasmin, is essential to permit fracture repair through angiogenesis and ossification. Yet, when plasmin activity is lost, the depletion of fibrin alone is insufficient to fully restore fracture repair, suggesting the existence of additional plasmin targets important for fracture repair. Previously, activated matrix metalloproteinase 9 (MMP-9) was demonstrated to function in fracture repair by promoting angiogenesis. Given that MMP-9 is a defined plasmin target, it was hypothesized that pro-MMP-9, following plasmin activation, promotes fracture repair. This hypothesis was tested in a fixed murine femur fracture model with serial assessment of fracture healing. Contrary to previous findings, a complete loss of MMP-9 failed to affect fracture healing and union through 28 days post injury. Therefore, these results demonstrated that MMP-9 is dispensable for timely fracture union and cartilage transition to bone in fixed femur fractures. Pro-MMP-9 is therefore not a significant target of plasmin in fracture repair and future studies assessing additional plasmin targets associated with angiogenesis are warranted.

Acquired and congenital forms of heterotopic ossification: new pathogenic insights and therapeutic opportunities

Heterotopic ossification (HO) involves the formation and accumulation of extraskeletal bone tissue at the expense of local tissues including muscles and connective tissues. There are common forms of HO that are triggered by extensive trauma, burns and other bodily insults, and there are also rare congenital severe forms of HO that occur in children with Fibrodysplasia Ossificans Progressiva or Progressive Osseous Heteroplasia. Given that HO is often preceded by inflammation, current treatments usually involve anti-inflammatory drugs alone or in combination with local irradiation, but are not very effective. Recent studies have provided novel insights into the pathogenesis of acquired and genetic forms of HO and have used the information to conceive and test new and more specific therapies in animal models. In this review, I provide salient examples of these exciting and promising advances that are undoubtedly paving the way toward resolution of this debilitating and at times fatal disease.

Trauma induced heterotopic ossification patient serum alters mitogen activated protein kinase signaling in adipose stem cells

Post-traumatic heterotopic ossification (HO) is the formation of ectopic bone in non-osseous structures following injury. The precise mechanism for bone development following trauma is unknown; however, early onset of HO may involve the production of pro-osteogenic serum factors. Here we evaluated serum from a cohort of civilian and military patients post trauma to determine early induction gene signatures in orthopaedic trauma induced HO. To test this, human adipose derived stromal/stem cells (hASCs) were stimulated with human serum from patients who developed HO following trauma and evaluated for a gene panel with qPCR. Pathway gene analysis ontology revealed that hASCs stimulated with serum from patients who developed HO had altered gene expression in the activator protein 1 (AP1) and AP1 transcriptional targets pathways. Notably, there was a significant repression in FOS gene expression in hASCs treated with serum from individuals with HO. Furthermore, the mitogen-activated protein kinase (MAPK) signaling pathway was activated in hASCs following serum exposure from individuals with HO. Serum from both military and civilian patients with trauma induced HO had elevated downstream genes associated with the MAPK pathways. Stimulation of hASCs with known regulators of osteogenesis (BMP2, IL6, Forskolin, and WNT3A) failed to recapitulate the gene signature observed in hASCs following serum stimulation, suggesting non-canonical mechanisms for gene regulation in trauma induced HO. These findings provide new insight for the development of HO and support ongoing work linking the systemic response to injury with wound specific outcomes.

Knockdown of Ggps1 in chondrocyte expedites fracture healing by accelerating the progression of endochondral ossification in mice

Bone fracture healing is achieved through the proliferation and differentiation of stem cells, while bone marrow stem cells (BMSCs) contribute to endochondral ossification. During fracture healing, mesenchymal progenitor cells first form a cartilaginous blastema that becomes vascularized to recruit precursor cells of osteoblasts through the bone morphogenetic protein 2 (Bmp2)/Smad-dependent Runx2 pathway. Statins deplete geranylgeranyl diphosphate (GGPP), which participates in the regulation of BMSCs differentiation, through the inhibition of 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase, leading to impaired protein geranylgeranylation, which strongly impacts the bone synthesis induced by Bmp2. Accordingly, we would like to investigate the role of geranylgeranyl diphosphate synthase 1 (Ggps1) in bone fracture via endochondral ossification in mice. We used a Cre-loxP system, namely the tamoxifen-inducible Collagen 2-CreER^T2 Ggps1 ^fl/fl, to eliminate specifically the Ggps1 activity in chondrocytes of 8-10-week-old mice. We found that the endochondral bone formation, calcification and vasculogenesis of the bony callus were accelerated in fractures in Ggps1^-/-mice. Together, the results of this study confirm that the specific deletion of Ggps1, using the Collagen 2-CreER^T2 mice, will accelerate the fracture healing process by activating the Bmp2/Smad-dependent Runx2 pathway. In addition, we managed to improve the fracture healing process by inhibiting the Ggps1 activity and its related products with statin drugs.

A new assay for global fibrinolysis capacity (GFC): Investigating a critical system regulating hemostasis and thrombosis and other extravascular functions

For many years, the importance of fibrinolysis has been recognized, first for its intravascular antithrombotic action, and more recently for its many extravascular activities, associated with matrix degradation and tissue remodeling. In the blood circulation system, fibrinolysis prevents thrombosis, and is associated with various biological and clinical situations: risk factors for cardio-vascular diseases in high risk clinical situations (type II diabetes, hypertension, triglycerides, high BMI, elevated glucose, etc.), probably resulting from a significant reduction of the fibrinolysis potential, and elevation of PAI-1. Noteworthy, t-PA is mainly present as an inactive complex with PAI-1, and its concentration in plasma tends to follow that of PAI-1, but in a lesser extent. Hypofibrinolysis can favor the occurrence of thrombotic events, and possibly other biological dysfunctions. Fibrinolysis activity is however difficult to evaluate as it has a delayed activity after clot formation, is initiated and regulated after fibrin generation, and conversely to clotting, its action is delayed (long lag phase) and slow, before being dramatically amplified leading to rapid clot dissolution. We have designed a new assay for evaluating the global fibrinolytic capacity (GFC) in the body. Reagents are used in association with a specific instrument, which can be connected to any computer, and dedicated software is used for analyzing clot lysis kinetics. The assay is performed in a micro-cuvette, introduced into one of the instrument wells at 37 °C, and light transmittance is continuously measured. Assayed plasma is first supplemented with a limited and constant amount of t-PA with silica and is then clotted with thrombin and calcium. Clot dissolution (measurement of turbidity change) is recorded over time using the dedicated instrument (Lysis Timer), and clot lysis kinetics are analyzed with the associated software: primary and secondary derivatives of the light transmission curve give information on kinetics and completion of clot dissolution. Total assay time is about 1 h (but in the presence of hypofibrinolysis it can be prolonged). The concentration of t-PA used for the assay has been adjusted (100 ng/ml) to obtain an optimal sensitivity to hypofibrinolysis within a short time interval, and clot dissolution occurs within about 45 min for normal individuals, with a broad range from 30 min to 60 min, with some samples presenting a clot dissolution time >60 min (hypofibrinolysis). This new assay is performed with the tested plasma intrinsic factors, especially its own fibrinogen, and only exogeneous t-PA is added. GFC is highly sensitive to PAI-1 activity, but other factors regulating fibrinolysis contribute to the clot dissolution kinetics. Freshly prepared or frozen and thawed citrated plasma can be used. The usefulness of this assay for clinical applications is under investigation. Although fibrinolysis is mainly initiated in the body upon stimulation or blood clotting, and rapidly diluted and inhibited in the circulation, evaluation of its "residual" activity in plasma is expected to reflect its global body potential.