Synovial fibrosis is an important pathological change in osteoarthritis (OA) and is closely associated with disease progression. During the progression of synovial fibrosis in OA, fibroblasts differentiate into myofibroblasts, leading to abnormal deposition of extracellular matrix (ECM) and resulting in clinical symptoms such as joint pain and limited mobility. This study aims to investigate the role and cellular and molecular mechanisms of the fibroblast growth factor 7 (FGF7)-fibroblast growth factor receptor 1 (FGFR1) signaling axis in OA synovial fibrosis.

① By mining publicly available synovial single-cell sequencing data, combined with differential analysis, enrichment analysis, fibrosis scoring, correlation analysis, and virtual knockout techniques, the relationship between FGF7/FGFR1 and OA synovial fibrosis was explored. ② An OA mouse model was established via destabilization of the medial meniscus (DMM), and the mice receiving sham surgery served as the control group. Recombinant FGF7 protein (2 μg/10 μL) or PBS was injected into the joint cavity once weekly for 4 weeks. Functional assessments were performed at 4 (n=6) and 8 weeks (n=5) after modeling via behavioral testing, including gait analysis, hindlimb balance test, and knee joint range-of-motion evaluation. Histopathological analyses, including HE, Masson's trichrome, and Sirius Red staining, as well as immunostaining for collagen type Ⅰ alpha 1 chain (COL1A1) and alpha-smooth muscle actin (α-SMA), were conducted to evaluate synovial fibrosis and inflammatory changes. ③ EdU, CCK-8 assay, wound-healing assay, qPCR, and Western blotting were applied to investigate the role of FGF7 in fibroblast-to-myofibroblast differentiation.

① FGF7 expression was positively correlated with synovial fibrosis scores. ② Exogenous FGF7 markedly aggravated OA-related joint dysfunction in DMM mice. At 4 weeks post-DMM, compared with the PBS group, the FGF7 group showed decreased distribution of right hindlimb ground contact time (P<0.01), right hindlimb weight-bearing (P<0.01), and knee joint range-of-motion (P<0.001), with a similar trend observed at 8 weeks post-DMM. ③ FGF7 promoted ECM deposition and upregulated fibrosis markers in the synovial tissues. Histological analysis revealed that FGF7 treatment significantly aggravated synovial inflammation at 4 weeks (P<0.0001) and 8 weeks (P<0.01), and promoted ECM deposition compared with the PBS group. COL1A1 immunohistochemical staining indicated increased type Ⅰ collagen deposition in the FGF7 group at 4 (P<0.05) and 8 weeks (P<0.0001), and immunofluorescence assay showed increased α-SMA expression in the FGF7 group at 4 (P<0.001) and 8 weeks (P<0.01). ④ In in vitro study, EdU and CCK-8 assays indicated that FGF7 promoted fibroblast proliferation (P<0.05); the scratch assay showed that FGF7 enhanced fibroblast migration (P<0.01); FGF7 upregulated α-SMA (P<0.001) and COL1A1 (P<0.001) expression in fibroblasts. ⑤Mechanistically, FGF7 exerted pro-fibrotic effects through the FGFR1 pathway. FGF7 activated p-FGFR1 (P<0.05) and p-ERK (P<0.01) expression in fibroblasts; BGJ398 inhibited the FGF7-induced upregulation of p-FGFR1 (P<0.05), p-ERK (P<0.05), COL1A1 (P<0.05), and α-SMA (P<0.01) in fibroblasts.

Exogenous FGF7 exacerbates synovial fibrosis in OA mice. Our findings revealed that FGF7 promotes fibroblast proliferation, migration, and differentiation into myofibroblasts via activation of the FGFR1 pathway, thereby providing a potential novel target for the treatment of OA synovial fibrosis.

To investigate the impact of a high-altitude and cold environment on skeletal muscle function and the pathological processes of osteoarthritis (OA), to elucidate the interaction mechanisms between skeletal muscle atrophy, fibrosis, and joint degeneration under cold condition, aiming to found a theoretical basis for the health protection of the musculoskeletal system in military personnel stationed in high-altitude cold regions.

Destabilization of the medial meniscus (DMM) was established on C57BL/6 mice (11 weeks old) by cutting the attaching point at the anterior intercondylar region where anterior cruciate ligament blends with the medial meniscus. Forty male C57BL/6 mice (11 weeks old) were stratified by 2 postoperative time points (8 and 12 weeks). At each time point, the mice were divided randomly divided into 4 groups (n=5): 22 ℃ sham operation (22 ℃ Sham), 4 ℃ sham operation (4 ℃ Sham), 22 ℃ DMM, and 4 ℃ DMM. From 2 weeks after DMM modelling, the 4 ℃ Sham and 4 ℃ DMM groups were placed in a cold chamber [4 ℃, humidity (50±5)%] for 12 h per day until the respective endpoint, meanwhile, the 22 ℃ Sham and 22 ℃ DMM groups were kept at room temperature. At 8 and 12 weeks after intervention, the following assessments were carried out: ① Behavioral tests: Gait parameters were quantified using the CatWalk system; Limb load-bearing capacity was evaluated with bipedal balance test; Spontaneous activity distance and time were recorded in the open field test; Twitch force and tetanic force in the quadriceps were measured, and the force-frequency curve was plotted. ② Histopathological analysis: Quadriceps and knee joint tissues were harvested, embedded in paraffin, sectioned and stained with HE and safranin O-fast green to assess synovial inflammatory cell infiltration, cartilage surface integrity, and OARSI score from Osteoarthritis Research Society International System. Additionally, Masson's trichrome staining was applied to observe collagen deposition in muscle fibers. ③ Molecular detection: Immunofluorescence assay for dystrophin antibody was used to label muscle fiber boundaries and calculate cross-sectional area; Immunohistochemistry assay was employed to determine the expression levels of IL-1β and TNF-α in synovial tissue.

① Progressive deterioration of behavioral function: Compared with the 22 ℃ DMM group, the 4 ℃ DMM group exhibited significantly exacerbated functional impairments at both 8 and 12 weeks after intervention, in a time-dependent worsening trend. a. Loss of gait coordination: At 8 weeks, the relative stance phase time of the right hindlimb was shorten by 0.113 (P < 0.001), print area was reduced by 0.102(P < 0.05), swing phase time was extended by 0.115 (P < 0.05), and single-limb stand time was prolonged by 0.146 (P < 0.01). These abnormalities still existed at 12 weeks, with the difference of print area expanding to 0.154(P < 0.05) and that of swing phase time maintaining an elevated magnitude of 0.113 (P < 0.05). b. Decline in weight-bearing capacity: Percentage of right paw weight-bearing was decreased by 2.777% at 8 weeks (P < 0.001), with the deficit further expanding to 5.156% at 12 weeks (P < 0.0001). c. Suppression of spontaneous activity: Total locomotor distance was decreased by 1069 cm at 8 weeks (P < 0.0001) and remained reduced by 429.3 cm at 12 weeks (P < 0.05). ② Exacerbation of joint histopathological damage: Cold exposure combined with DMM surgery significantly aggravated synovial inflammation and cartilage destruction. a. Synovial pathology: The 4 ℃ DMM group displayed increased synovial pathological score of 2.133 at 8 weeks (P < 0.01) and then 2.833 at 12 weeks (P < 0.001) relative to the 22 ℃ DMM group, indicating a sustained inflammatory response.b. Cartilage degeneration: The 4 ℃ DMM group showed the maximum score of OARSI elevated by 1.000 (P < 0.05) and total score by 2.000 (P < 0.01) at 8 weeks, while, at 12 weeks, the maximum score remained elevated by 0.750, and the total score maintained a differential trend of 2.000. c. Upregulation of pro-inflammatory factors: The synovial expression area of IL-1β in the 4 ℃ DMM group was increased by 1.047% (P < 0.05) and 1.892% (P < 0.05) at 8 and 12 weeks, respectively, and that of TNF-α expression was increased by 3.850%(P < 0.0001) and 3.806% (P < 0.0001), separately at the same time points. ③ Composite impairment of skeletal muscle structure and contractile function: Cold intervention induced triple pathological changes in quadriceps encompassing atrophy, mechanical performance decline, and fibrotic remodeling. a. Depletion of muscular force reserve: Twitch force was decreased by 34.77 mN·m/kg (P < 0.01) and 52.87 mN·m/kg (P < 0.05) at 8 and 12 weeks, respectively; Tetanus force was reduced by 115.0 mN·m/kg (P < 0.05) and 138.3 mN·m/kg (P < 0.0001) at the same time points, with a downward shift of the force-frequency curve. b. Muscle atrophy: The mass ratio of quadriceps was decreased by 0.076%(P < 0.05) and 0.102% (P < 0.0001), and the muscle fiber cross-sectional area (CSA) was reduced by 261.3 μm~2 (P < 0.05) and 233.5 μm~2 (P < 0.01) at 8 and 12 weeks, respectively, accompanied with an increased proportion of small-diameter fibers. c. Fibrotic deposition: Relative collagen fiber area in muscle tissue was increased by 4.072% (P < 0.01) and 2.238% (P < 0.05) at 8 and 12 weeks, respectively, suggesting abnormal extracellular matrix remodeling.

High-altitude cold environment not only directly aggravates synovial inflammation and cartilage degeneration in OA but also further compromises joint structure and function by inducing quadriceps atrophy, reduced muscle strength, and fibrosis.

To explore the changes in biochemical indicators among soldiers with different endurance qualities after high altitude exposure and their adaptation to plateau environment.

A status survey was carried out on 253 young male soldiers who were planned to station in Malone, Yunnan Province（2 100 m）for 4 weeks from July to August, 2018. The sampling method is cluster sampling. The endurance quality and biochemical indicators were assessed and measured before exposure and after 4 weeks' training. Their endurance quality was evaluated with grading 3-km armed cross-country. The included biochemical indicators were levels of creatine kinase, hemoglobin, average hemoglobin and cardiac troponin, and counts of blood red cells, lymphocytes and basophils.

Based on the results of the 3-km armed cross-country, the participants were divided into excellent（12%）, good（21%）, passing（40%）and failing（27%）groups. Except for basophil counts, levels of creatine kinase, cardiac troponin, hemoglobin and average hemoglobin, and counts of red blood cells and lymphocyte changed significantly in all the 4 groups before and after high altitude exposure（P<0.01）. While, obvious changes were observed in cardiac troponin level and lymphocyte count among the 4 groups after they were stationed at high altitude（P<0.01）, and these changes were correlated with endurance quality（cardiac troponin: r_s=-0.409, P<0.01; lymphocytes: r_s=0.205, P<0.01）.

There are some differences in blood biochemical indicators in soldiers with different endurance qualities at high altitude. Our results suggest that soldiers with higher endurance quality have a relatively lower risk of myocardial injury and stronger immune function after entering high altitude.