Diana McKinnon

Tendons are mechanical bridges that connect bone to muscle and transform the force generate by muscle contraction. Tendon injuries are a common pathology accounting for 30% of orthopedic surgery consultations. Currently, there is limited therapies available for tendon healing, the mainstay of treatment now is pain control and physical therapy¹. One especially important aspect of tendon healing involves fibrosis formation. Fibrosis leads to the development of adhesions between the tendon and synovial sheath ultimately limiting gliding and producing stiffness post injury. Key cells involved in tendon repair post injury include macrophages and tenocytes. Macrophages induce transforming growth factor beta⁶ (TGF-B) which in turn activates a Smad signaling pathway⁴.  Smads activated by TGF-B will translocate to the nucleus where Smad complexes regulate transcription of target genes through their interaction with various transcription factors. The pathway between macrophages and profibrotic proteins post tendon injury is being illuminated.  Studies are revealing the mechanism behind how macrophages influence fibrosis formation on a molecular level. This creates future targets of therapy that could widen the now limited treatment options available. Methods: Macrophage depleted mice were compared to macrophage carrying mice to assess differences in fibrosis formation.  It was confirmed that macrophages secreted exosomes using PCR and western blot analysis, and further that exosomes secreted microRNA -21-5p, which is a known mediator of fibrosis³, using labeling techniques and Transwell plates. Using RT-PCR and WB analysis gene expression was assessed between microRNA-21-5p exposed tenocytes and fibroblasts and control groups. Immunofluorescence staining were used to confirm uptake of exosomes and histology was used to assess fibrosis development post injury.  Results: Depletion of macrophages resulted in less fibrosis formation as assessed by histology and range of motion, which were found to be significant differences. Furthermore, macrophages secrete microRNA carrying exosomes, confirmed by electron microscopy and nanoparticle-tracking analysis, which are taken up by tenocytes and fibroblasts at injury site^2,5. Ultimately, micro-RNA 21-5p results in the upregulation of profibrotic genes (COL I, COL III, alpha-SMA, and TFGB1) and the degradation of the inhibitory Smad7 protein’s mRNA, which leads to more fibrosis formation post injury². Conclusions: Collectively, these findings showed that macrophages secrete exosomal miR-21-5p that directly targets Smad7, leading to the activation of fibrogenesis in tendon cells. These results demonstrate that intercellular crosstalk between macrophages and tendon cells is mediated by macrophage-derived miR-21-5p-containing exosomes that control the fibrotic healing response². This pathway introduces varies molecular components which could become targets for future therapies in tendon repair.

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2. Cui, H., He, Y., Chen, S., Zhang, D., Yu, Y. and Fan, C., 2019. Macrophage-Derived miRNA-Containing Exosomes Induce Peritendinous Fibrosis after Tendon Injury through the miR-21-5p/Smad7 Pathway. Molecular Therapy – Nucleic Acids, 14, pp.114-130.
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