Osteoarthritis (OA) is a chronic, progressive joint disorder with tremendous social burden. Although OA can affect any synovial joint, knee OA is the most prevalent form, typically presenting with joint pain, stiffness, and reduced mobility. Globally, an estimated 595 million individuals (approximately 7.6% of the population) were affected by OA in 2020, and this number is projected to rise to 1 billion by 2050 [1]. The etiology of idiopathic OA remains incompletely understood, as it involves a complex interplay of factors such as age-related physiological changes, genetic predisposition, ethnicity, and biomechanical stress. Cartilage degradation, the primary pathological feature, is often accompanied by synovial inflammation and subchondral bone remodeling, underscoring OA as a disease that impacts the entire joint organ. This complexity has posed challenges in identifying specific molecular mechanisms that drive disease progression, limiting the development of effective disease-modifying therapies. Currently, OA diagnosis relies on clinical symptoms in conjunction with radiographic imaging, which lacks sensitivity and typically detects disease only at an advanced stage. However, metabolic and molecular alterations occur early in the disease course, preceding observable structural changes. Consequently, there is a critical need to delineate the molecular changes associated with different stages of OA to identify novel therapeutic targets and facilitate timely interventions aimed at delaying or preventing joint replacement.

Previously, we investigated the differential expression of proteins across various stages of OA, providing insight into disease progression at the proteomic level [2]. The current study focuses on another key layer of molecular regulation—microRNAs (miRNAs). Among the biomolecules implicated in OA, circulating miRNAs have gained attention due to their stability in body fluids, tissue specificity, and regulatory potential. These small non-coding RNAs are emerging as promising non-invasive biomarkers and modulators of disease, with growing evidence supporting their roles in maintaining joint homeostasis [3], [4]. While differential expression of miRNAs has been reported in serum, plasma, and cartilage samples from OA patients [5], [6], [7], relatively little is known about synovial fluid (SF)-derived miRNAs and their potential to discriminate between early and late-stage OA [8]. Since SF reflects the intra-articular environment, profiling its miRNA content may offer valuable insight into the local molecular dynamics of OA pathogenesis [9], [10]. In the present study, we comprehensively analyzed miRNA expression profiles in SF supernatants, cell pellets and serum, to investigate extracellular and cellular miRNAs as biomarkers in OA patients.

MicroRNAs are also widely acknowledged as key regulators of gene expression and potential therapeutic targets in OA. For instance, miR-34a-5p is significantly upregulated in the SF and joint tissues of patients with late-stage OA [8], [11], [12]. Its therapeutic relevance was later demonstrated, with intra-articular administration of miR-34a-5p antisense oligonucleotides providing protective effects in murine models of OA [13].

Circulating miRNAs could derive from tissue injury due to cartilage degeneration, apoptosis, and inflammation, or they can be released from the affected tissue cells in the diseased state. Thus, the identification of the source of secretion and downstream pathways is essential in understanding the complexity of the disease. By integrating miRNA profiles from SF, serum, and joint tissues, we aimed to establish a more robust and integrated molecular signature of OA. Thus, the primary objective of this study was to identify key miRNAs capable of differentiating between early and late-stage OA and to elucidate their potential roles in disease development and progression.