Unexpected timely fracture union in matrix metalloproteinase 9 deficient mice

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.

Irisin promotes fracture healing by improving osteogenesis and angiogenesis

Background

Osteogenesis and angiogenesis are important for bone fracture healing. Irisin is a muscle-derived monokine that is associated with bone formation.

Methods

To demonstrate the effect of irisin on bone fracture healing, closed mid-diaphyseal femur fractures were produced in 8-week-old C57BL/6 mice. Irisin was administrated intraperitoneally every other day after surgery, fracture healing was assessed by using X-rays. Bone morphometry of the fracture callus were assessed by using micro-computed tomography. Femurs of mice from each group were assessed by the three-point bending testing. Effect of irisin on osteogenic differentiation in mesenchymal stem cells in vitro was evaluated by quantitative real-time polymerase chain reaction (qRT-PCR), alkaline phosphatase staining and alizarin red staining. Angiogenesis of human umbilical vein endothelial cells (HUVECs) were evaluated by qRT-PCR, migration tests, and tube formation assays.

Results

Increased callus formation, mineralization and tougher fracture healing were observed in the irisin-treated group than in the control group, indicating the better fracture callus healing due to Irisin treatment. The vessel surface and vessel volume fraction of the callus also increased in the irisin-treated group. The expression of BMP2, CD31, and VEGF in callus were enhanced in the irisin-treated group. In mouse bone mesenchymal stem cells, irisin promoted ALP expression and mineralization, and increased the expression of osteogenic genes, including OSX, Runx2, OPG, ALP, OCN and BMP2. Irisin also promoted HUVEC migration and tube formation. Expression of angiogenic genes, including ANGPT1, ANGPT2, VEGFb, CD31, FGF2, and PDGFRB in HUVECs were increased by irisin.

Conclusion

All the results indicate irisin can promote fracture healing through osteogenesis and angiogenesis. These findings help in the understanding of muscle–bone interactions during fracture healing.

The Translational Potential of this Article

Irisin was one of the most important monokine secreted by skeletal muscle. Studies have found that irisin have anabolic effect one bone remodeling through affecting osteocyte and osteoblast. Based on our study, irisin could promote bone fracture healing by increasing bone mass and vascularization, which provide a potential usage of irisin to promote fracture healing and improve clinical outcomes.

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.

The Deleterious Effects of Impaired Fibrinolysis on Skeletal Development Are Dependent on Fibrin(ogen), but Independent of Interlukin-6

Chronic diseases in growing children, such as autoimmune disorders, obesity, and cancer, are hallmarked by musculoskeletal growth disturbances and osteoporosis. Many of the skeletal changes in these children are thought to be secondary to chronic inflammation. Recent studies have likewise suggested that changes in coagulation and fibrinolysis may contribute to musculoskeletal growth disturbances. In prior work, we demonstrated that mice deficient in plasminogen, the principal protease of degrading and clearing fibrin matrices, suffer from inflammation-driven systemic osteoporosis and that elimination of fibrinogen resulted in normalization of IL-6 levels and complete rescue of the skeletal phenotype. Given the intimate link between coagulation, fibrinolysis, and inflammation, here we determined if persistent fibrin deposition, elevated IL-6, or both contribute to early skeletal aging and physeal disruption in chronic inflammatory conditions. Skeletal growth as well as bone quality, physeal development, and vascularity were analyzed in C57BL6/J mice with plasminogen deficiency with and without deficiencies of either fibrinogen or IL-6. Elimination of fibrinogen, but not IL-6, rescued the skeletal phenotype and growth disturbances in this model of chronic disease. Furthermore, the skeletal phenotypes directly correlated with both systemic and local vascular changes in the skeletal environment. In conclusion, these results suggest that fibrinolysis through plasmin is essential for skeletal growth and maintenance, and is multifactorial by limiting inflammation and preserving vasculature.

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.