Osteoarthritis (OA) is one of the most frequent musculoskeletal diseases characterized by degeneration of articular cartilage, subchondral bone remodeling, and synovial membrane inflammation, which is a leading cause of global disability, morbidity, and decreased quality of life. Interpreting the potential mechanisms of OA pathogenesis is essential for developing novel prevention and disease-modifying therapeutic interventions. Gut microbiota is responsible for a series of metabolic, immunological, and structural and neurological functions, potentially elucidating the heterogeneity of OA phenotypes and individual features. In this narrative review, we summarized research evidence supporting the hypothesis of a “gut-joint axis” and the interaction between gut microbiota and the OA-relevant factors, including age, gender, genetics, metabolism, central nervous system, and joint injury, elucidating the underlying mechanisms of this intricate interaction. In the context, we also speculated the promising manipulation of gut microbiota in OA management, such as exercise and fecal microbiota transplantation (FMT), highlighting the clinical values of gut microbiota. Additionally, future research directions, such as more convincing studies by the interventions of gut microbiota, the gene regulation of host contributing to or attributed to the specific phenotypes of gut microbiota related to OA, and the relevance of distinct cell subgroups to gut microbiota, are expected. Moreover, gut microbiota is also the potential biomarker related to inflammation and gut dysbiosis that is able to predict OA progression and monitor the efficacy of therapeutic intervention.

Aims: Post-traumatic osteoarthritis (PTOA) is a subset of osteoarthritis (OA). The gut microbiome is shown to be involved in OA. However, the effect of exercise on gut microbiome in PTOA remains elusive. Methods: A total of 18 eight-week Sprague-Dawley rats were assigned into three groups: Sham/sedentary (Sham/Sed), PTOA/sedentary (PTOA/Sed), and PTOA/treadmill-walking (PTOA/TW). PTOA model was induced by transection of the anterior cruciate ligament (ACLT) and the destabilization of the medial meniscus (DMM). Treadmill-walking (15 m/min, 30 min/d, five days/week for eight weeks) was employed in the PTOA/TW group. The response of cartilage, subchondral bone, serology, and gut microbiome and their correlations were assessed. Results: Eight-week treadmill-walking was effective at maintaining the integrity of cartilage-subchondral bone unit and reducing the elevated systematic inflammation factors and microbiome-derived metabolites. Furthermore, 16S ribosomal ribonucleic acid (rRNA) sequencing showed disease-relevant microbial shifts in PTOA animals, characterized by the decreased abundance of phylum TM7 and the increase of phylum Fusobacteria. At the genus level, the abundance of Lactobacillus, Turicibacter, Adlercreutzia, and Cetobacterium were increased in the PTOA animals, while the increase of Adlercreutzia and Cetobacterium was weakened as a response to exercise. The correlation analysis showed that genus Lactobacillus and Adlercreutzia were correlated to the structural OA phenotypes, while phylum Fusobacteria and genus Cetobacterium may contribute to the effects of exercise on the diminishment of serological inflammatory factors. Conclusion: Exercise is effective at maintaining the integrity of cartilage-subchondral bone unit, and the exercise-induced modification of disease-relevant microbial shifts is potentially involved in the mechanisms of exercise-induced amelioration of PTOA. Cite this article: Bone Joint Res 2022;11(4):214–225.

Endoplasmic reticulum (ER) stress is a cellular phenomenon that arises in response to the accumulation of misfolded proteins within the ER. This process triggers the activation of a signalling pathway known as the unfolded protein response (UPR), which aims to restore ER homeostasis by reducing protein synthesis, increasing protein degradation, and promoting proper protein folding. However, excessive ER stress can perturb regular cellular function and contribute to the development of diverse pathological conditions. As is well known, ferroptosis is a kind of programmed cell death characterized by the accumulation of lipid peroxides and iron-dependent reactive oxygen species (ROS), resulting in oxidative harm to cellular structures. In recent years, there has been increasing evidence indicating that ferroptosis occurs in musculoskeletal disorders (MSDs), with emerging recognition of the complex relationship between ER stress and ferroptosis. This review presents a summary of ER stress and the ferroptosis pathway. Most importantly, it delves into the significance of ER stress in the ferroptosis process within diverse skeletal or muscle cell types. Furthermore, we highlight the potential benefits of targeting the correlation between ER stress and ferroptosis in treating degenerative MSDs.

This study aimed to elucidate the role of SESN2 in the pathological process of osteoarthritis (OA), with a particular focus on its potential mechanism in regulating chondrocyte mitochondrial function. In vitro and in vivo inflammatory models were established using SpragueDawley rats via IL-1β stimulation and surgical destabilization of the medial meniscus (DMM). siRNA, overexpression plasmids, and adeno-associated viruses were used to investigate the role of SESN2 in OA progression, and mechanistic studies were performed to clarify the underlying pathways and downstream targets. SESN2 expression was significantly upregulated in the chondrocytes and cartilage tissues of OA rats. SESN2 overexpression exacerbated IL-1β-induced chondrocyte inflammation and DMM-induced cartilage damage (OARSI score, 2.000 [95% CI: 0.000-5.000]); conversely, SESN2 knockdown alleviated metabolic disorders and mitochondrial dysfunction in the matrix (Relative ATP content, 0.2639 [95% CI: 0.1068-0.4209]) and ameliorated cartilage degeneration in vivo (OARSI score, -2.000 [95% CI: -5.000 to -1.000]). Mechanistic studies revealed that SESN2 induced mitochondrial hyperfusion in chondrocytes (Mean branch length, 0.4821 [95% CI: 0.2707-0.6935]) and exacerbated mitochondrial dysfunction and inflammation via excessive mitochondrial fusion mediated by MFN2. Further investigation revealed that SESN2 upregulated MFN2 protein expression by promoting its dephosphorylation (Relative protein expression, 0.7693 [95% CI: 0.5392-0.9994]), thereby inducing mitochondrial hyperfusion. SESN2 promotes chondrocyte mitochondrial dysfunction and the pathological progression of OA by regulating MFN2 dephosphorylation to induce mitochondrial hyperfusion, thereby providing a new potential target for intervention in OA.

Objective: This study aims to explore the effects of exercise on gut microbiota and short-chain fatty acid (SCFA) metabolism, as well as its relationships with joint degeneration. Methods: To explore the impact of exercise on obesity-associated osteoarthritis (OA), rats were fed a high-fat/high-sucrose (HFS) diet with or without exercise. Histological staining and micro-computed tomography (micro-CT) were used to assess the effects of exercise on articular cartilage and subchondral bone. 16S rRNA sequencing and Gas Chromatography–Mass Spectrometry (GC-MS) were performed to analyze alteration in fecal gut microbiota and serum SCFAs. Results: Exercise prevented articular cartilage degeneration and subchondral bone loss in the HFS diet with exercise (HE) group compared to the HFS-diet control sedentary (HS) group. Gut microbiota analysis revealed that exercise reduced the relative abundances of families Lachnospiraceae, Ruminococcaceae, genera Ruminococcus, Colidextribacter, Caproiciproducens, and the unidentified genus of Lachnospiraceae, while increased the relative abundance of genus Akkermansia. Metabolomic analysis indicated exercise prevented AA reduction in the HE group. In addition, the level of AA was negatively correlated with OA severity and with the abundances of families Lachnospiraceae and Ruminococcaceae, genus Colidextribacter and the unidentified genus of Lachnospiraceae. Conclusions: Exercise effectively preserves the integrity of the cartilage–subchondral bone unit. The observed modifications in gut microbiota and AA levels following exercise intervention may be associated with the protective mechanisms against obesity-associated OA.