[Effects of adipose stem cell-derived exosomes on rat tendon healing and its impact on the periphery neuropeptides expression].

<b>Objective:</b> To investigate the effect of injectable adipose tissue stem cell-derived exosome-encapsulated hydrogel on the tendon healing in rats and to evaluate the temporal and spatial expressions of periphery neuropeptides at healing site. <b>Methods:</b> To generate the injectable exosome-encapsulated hydrogel, the methacryloylchloride solution (GelMA) and the photoinitiator were mixed first, then the exosome solution was added and oscillated together. Followed with exposing in the ultraviolet light in a wave length 405 nm for 30 seconds to form an injectable hydrogel. The Sprague Dawley (SD) rats were performed with full thickness transection and surgical repair administration of Achilles tendon to establish the animal model, including 4 groups: intact control (C group, selected the contralateral side of tendon transection with surgical repair group), tendon transection with surgical repair group (S group), tendon transection with surgical repair and hydrogel implantation group (H group) and tendon transection with surgical repair and exosome-encapsulated hydrogel group (E group). The samples were harvested on Day 7, 14 and 28 after the operation, respectively. Histopathological examination was performed with hematoxylin-eosin staining (HE staining) and immunohistochemistry staining of tenogenesis marker (Tenomodulin, TNMD), periphery neuropeptide markers (growth associated protein-43, GAP43; S100 calcium-binding protein B, S100B; neuropeptide Y, NPY; calcitonin gene-related peptide, CGRP; Substance P, SP). Morphological property was characterized with tendon length, cross-sectional area (CSA), and gastrocnemius weight ratio. Biomechanical testing was measured with maximum failure load, stiffness and tensile modulus. <b>Results:</b> The results of the quantitative polymerase chain reaction (qPCR) showed that the expression level of the TNMD gene in the E group was 2.12±0.43, which was significantly higher than that in the H group (1.26±0.28) and the S group (1.21±0.39) (both <i>P</i><0.05). Based on the immunohistochemistry staining, the expression of GAP43 can be detected with an significant enhancement in the E group (11.20%±0.53% positive area) relative to H group (7.25%±0.22% positive area) and S group (8.68%±0.45% positive area) (both <i>P</i><0.001) on day 28 post-surgery; whilst the expression of CGRP exhibited a depressed variation with a positive area of 7.62%±0.50% in E group relative to a positive area of 11.16%±1.33% in H group and a positive area of 10.16%±0.22% in S group on day 28 post-surgery, respectively (both <i>P</i><0.001). Besides, the E group showed that the morphological characterization with a superior restoration of tendon length to (11.67±0.58) mm and CSA to (5.97±0.72) mm² and biomechanical property of healed tendon with an improvement of maximum failure load to (71.06±2.48)N and tensile modulus to (9.24±1.56) MPa when compared with those in S and H groups (all <i>P</i><0.01). <b>Conclusion:</b> Injectable Exos-encapsulated hydrogel promotes tendon healing in SD rats with enhancing temporal periphery neuropeptides expression and reducing the nerve sensitization during tendon healing.