Role of the Alarmin S100A9 protein in inducing Achilles tendinopathy in rats

Abstract

BACKGROUND This study aimed to investigate the correlation between the Alarmin S100A9 protein and Achilles tendinopathy (AT), and to reveal the role of this protein in inducing AT. METHODS In this study, 40 male Sprague-Dawley rats were randomly divided into four groups: Control group (received no treatment), Injury group (Achilles tendon tissues were cut intraoperatively), S100A9 group (received a subcutaneous injection of rhS100A9 solution), and S100A9 + Paquinimod group [received a subcutaneous injection of rhS100A9 and Paquinimod (1:1 ratio) into the Achilles tendon]. At 1 week postoperatively, the four groups of rats were euthanized, and the Achilles tendon tissues were isolated for histological staining, immunohistochemistry (IHC), immunofluorescence, Sirius Red (SR) staining, and terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling assay. RESULTS Compared with both the Control and S100A9 + Paquinimod groups, the Injury and S100A9 groups exhibited higher expression levels of S100A9 protein, matrix metalloproteinase-3 (MMP-3), and inflammatory factors. Regarding histomorphology [hematoxylin-eosin (HE) staining and Safranin O/fast green (SOFG; fast green and Safranin) training], the Achilles tendon tissues in the Injury and S100A9 groups showed AT-like changes, and the fibers were extremely disorderly, non-bundled, and ruptured, and some nuclei were spindles. As for collagen (Col) remodeling, a large number of fresh collagen fibers had formed, the amounts of Col-I and Col-II were lower, and a large quantity of Col-III was present. Additionally, a large number of tendon cells had died in both the Injury and S100A9 groups. CONCLUSIONS This study showed that Alarmin S100A9 can induce AT-like morphological changes and local inflammatory reactions, trigger collagen fiber remodeling and tendon cell apoptosis, and ultimately induce AT.