| Back ground There are millions of people who get fractured every year for different reasons. Most of them can be recovered, but there is still 5-10% sufferring from un-union or delayed union [1]. The impaired fracture healing can cause disability. Under such an urgent situation like this, we need to dig the mechanism of fracture healing deeply, and try our best to find efficient ways to induce fracture healing in both of biochemical and clinical methods.Fracture healing is a complex event that involves the coordination of variety different processes, including the ex- and in-cellular osteogenic cells signaling pathways. Till now, the mechanism of fracture healing is not totally understood. Molecular mechanisms known to regulate skeletal tissue formation during embryological development are repeated during the fracture healing process [2].Besides the molecular biology factors, the progress of fracture healing is infected by the mechanical factors as well. Direct fracture healing occurs when rigid internal fixation anatomically reduces the fracture fragments. However, the majority of fractures heal by indirect fracture healing, which involves callus formation through endochondral ossification. Matrinlin-1(MATN1) is one of the many kinds of excellular matrix proteins, which are involved in this bone repair presses.MATN1 is a cartilage specific extracellular matrix protein which is critical for the transition from cartilage to bone during endochondral ossification. MATN1 is associated with collagen type II fibrils and proteoglycans, and forms collagen-independent and collagen-dependent filamentous networks in cartilage[3] [4]. But its function in vivo has not been studied yet. Since, MATN1 and fracture healing all have connections with the collagen; our work thereby is to determine whether MATN1 plays a role in fracture healing. Method A knockout mouse is a genetically engineered mouse in which one or more genes have been turned off through a targeted mutation.Nowadays, knockout mice are important animal models for studying the role of genes whose functions have not been identified. In this study, MATN1-/- and WT mice were used to investigate the effect of MATN1 in fracture healing. The classical mouse closed tibial fracture with intramedullary needle fixation model was used, which has some advantages, such as the easy manipulation, low infection rate and less requiry for the equipments. The methods used to value the fracture healing are as follows:the fracture healing process was followed by X-ray radiography, fluorescence molecular tomography (FMT) was used to analysis the vascularity and bone formation,micro-CT was also used to quantitifiy the fracture callus ,biological mechanical torsional test was used to value mechanical quality of the callus, Safranin-O/Fast-green staining histological analysis was included to value the callus as well, real time PCR was used to quantitify the target genes expression in the callus. There are three of them deserved to be introduced briefly. FMT technology provides non-invasive, whole body, deep tissue imaging in small animal models information-rich results. Now across the US, these systems are used for research in oncology as well as inflammatory, pulmonary, cardiovascular and skeletal disease. Biological targets and pathways can be monitored and quantified in real time - giving a deeper understanding of the biology underlying disease mechanisms and therapeutic response. The presented study is the first one in which FMT system is applied to study the fracture healing in the world wide. Micro-CT determined the newly formed bone, and quantified the new bone and unmineralized cavities [total volume (TV) in mm3] and volume of mineralized bone tissue [bone volume (BV) in mm3]. Biological mechanical torsional test is recognized as golden standard to value the fracture healing mechanical function, with determining the peak torque.Results In this study, we observed that MATN1-/- mice healed fast than WT mice. The X-ray images and histology staining results showed that MATN1-/- mice had more bone fromation in callus.From Safranin-O/Fast-green staining results, MATN1-/- mice stepped in mineralization stage ahead of WT mice. According to the real time in vivo imagining (FMT) and micro-CT analysis, MATN1-/- mice showed significant more vascular and bone formation than WT mice. The real time PCR results showed that MATN1 -/- mice expressed significant more angiogenesis genes on days 14, such as vascular endothelial growth factor (VEGF) and significant more osteogenic genes on days 21, such as bone morphogenetic protein (BMP). Besides that, the stromal cell-derived factor 1(SDF-1) is highly expressed in MATN1-/- mice than in WT mice.In summary, all the data we got in this study demonstrate for the first time that MATN1 inhibit bone repair in fracture healing.The molecular mechanism regulating the fracture healing by MATN1 is not clear at present. However, there are some reasons to support this phenomenon. The first possible reason is the inhibition angiogenesis caused by MATN1.The blood supply to the fracture site is vital important to the fracture healing. The second possible reason is that MATN1 may inhale the fracture healing through inhibiting the SDF-1 function.SDF-1 has been determined to have the ability to induce bone formation. So, the fracture healing progress should be infected by the down regulation to SDF-1. For the future, there will be lots of work to reveal the mechanism of matrilin-1's function in fracture healing.
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