Knee osteoarthritis (KOA) is a complex and multifactorial chronic degenerative disease of the bones and joints, characterized by synovitis, thickening of the joint capsule, progressive degeneration of articular cartilage, narrowing of the joint space, formation of osteophytes and subchondral sclerosis[1]. This leads to chronic joint pain and loss of function. The pathogenesis factors involved are intricate and interactive, encompassing age, sex, biomechanical changes, infiltration of inflammatory factors, obesity, hormone levels, genetic factors, immunological factors as well as abnormal metabolism in articular cartilage[2].The incidence of KOA has been steadily increasing as the global aging population grows. Epidemiological surveys have shown that KOA is particularly common in individuals over 50 years old, primarily affecting those aged 56 to 75 years old. The estimated number of KOA patients worldwide exceeds 300 million, accounting for a total prevalence rate of approximately 16%[3], significantly impacting the quality of life among older individuals.
Studies have demonstrated that tumor necrosis factor-alpha (TNF-α), interleukin-2 (IL-2), interleukin-6 (IL-6), and other inflammatory factors accelerate cartilage matrix degradation, disrupt cartilage tissue, and induce chondrocyte apoptosis[4, 5].In recent years, there has been a growing focus on the gut microorganisms in joint inflammatory diseases. The concept of the "Gut-joint" axis was proposed by Professor Bradtzaeg from Oslo University in 1997 [6]. It is suggested that this axis involves immune cells migrating from the intestinal mucosa to the inflamed synovial site through vascular homing [7]. Although there is no anatomical connection between the intestines and joints, epidemiological investigations have indicated a potential correlation between intestinal inflammation and joint diseases[8], compromised gut integrity can lead to endotoxemia and a pro-inflammatory state, contributing to OA. The "Gut-joint" axis operates through the circulatory system, linking the overall structure of intestinal flora, intestinal mucosal tissue, bones, and joints closely together. Intestinal diseases may induce joint lesions[9, 10], while conversely, joint diseases can also affect the intestines.
The occurrence of KOA is characterized not only by joint degeneration but also as a chronic systemic autoimmune inflammatory disease, in which the immune response may be regulated by the "Gut-joint" axis. Disturbances in intestinal flora can impact the body's immunity, leading to systemic inflammatory responses. The intestinal microbiota plays a role in the pathogenesis of various diseases through its involvement in immune-inflammatory processes. The persistent knee pain experienced by KOA patients is associated with long-term inflammatory responses within the body. The pathogenesis of gut microbiota and knee osteoarthritis remains incompletely understood; however, several studies have suggested that the influence of gut microbes on the immune system, metabolic processes, and other factors may contribute to the development of knee osteoarthritis. On one hand, gut microbes can impact immune cell activity and function, thereby modulating the immune response. On the other hand, metabolites produced by gut microbes can affect joint tissue metabolism, leading to articular cartilage damage and inflammatory responses. There are intricate interactions between gut microbes and the immune system as they promote the development and functional maturation of immune cells through stimulation of the intestinal mucosa's immune system. Additionally, metabolites from gut microbes can serve as signaling molecules to regulate immune cell activity and function. This interaction is crucial for maintaining intestinal homeostasis. During knee osteoarthritis pathogenesis, an imbalance in gut microbes may result in aberrant activation of the immune system which subsequently triggers an inflammatory response and damages joint tissues.
Small molecules secreted by intestinal flora such as peptidoglycan lipopolysaccharide and bacterial DNA fragments can enter the circulatory system through gaps in intestinal endothelial cells' barrier function and distribute throughout the body, triggering local inflammatory reactions[11, 12]. Relevant studies have found that lipopolysaccharide(lipopolysaccharide,LPS) is a major component of gram-negative bacteria cell walls within the intestines[13]. An appropriate amount of LPS can activate normal immune function and maintain homeostasis within the body microenvironment. However, when disturbances occur within the gut microbiota resulting in an increase of opportunistic pathogens producing excessive amounts of LPS that enters into circulation causing endotoxemia; this overactivation leads to severe immune-inflammatory responses including systemic inflammation syndrome[14, 15]. Under various influencing factors' interference, dysbiosis of gut microorganisms may recruit immune cytokines circulating systemically or lymphatically inducing low-grade inflammation specifically localized to areas within intestines[16].Local inflammatory factors infiltrate the intestine, potentially contributing to systemic low-grade inflammation. Cytokines derived from intestinal inflammation are transported systemically or via lymphatic circulation, facilitated by chemokines and adhesion molecules. These cytokines bind to receptors on immune cells' surfaces in joint synovium, triggering immune reactions that potentially promote KOA onset and progression[17].
Purine signaling has emerged as a novel focus for investigation in the field of traditional Chinese medicine.Purine signaling refers to the release of adenosine triphosphate (ATP) from the cell, which is then decomposed into adenosine diphosphate (ADP), adenosine monophosphate (AMP), and adenosine (ADO)[18]. These substances interact with corresponding purine signaling receptors, playing a role in various physiological and pathological effects. Among the 19 receptors involved in purine signaling, the P2X7 receptor has received significant attention, particularly in studies related to inflammation, tumors, and neurological diseases. The studies have revealed that extracellular ATP and ADO play a crucial role in regulating the tumor microenvironment (TME), which is closely associated with immune responses. ATP acts as a proinflammatory factor, actively released during inflammatory states. Accumulation of extracellular ATP can effectively trigger an inflammatory response by binding to its specific receptor, thereby exerting potent chemotactic and immune stimulatory effects.The P2X7 receptor may serve as a key target for treating various diseases[19]. In recent years, there have been notable advancements in understanding the role of the P2Y14 receptor in inflammation, pain management, tumors, and other related diseases[20].
Currently, the role of the P2X7 receptor in inflammation and pain has been widely acknowledged and substantiated. The activation of the P2X7 receptor can lead to the development of pain by mediating the release of inflammatory mediators. ATP activates the P2X7 receptor, opening a non-selective cation channel, activating various cellular signals, releasing diverse inflammatory cytokines, and inducing pain. In the animal model of KOA, the use of P2X7 receptor inhibitors can significantly reduce the concentration of intra-articular inflammatory factors such as IL-1β, IL-6, and TNF-α to alleviate KOA pain[21]. Therefore, targeting the P2X7 receptor may hold potential for arthritis treatment.
The Gancao Fuzi decoction(GCFZD) was initially documented in the book "Synopsis of Golden Chamber" by Zhang Zhongjing, a renowned physician during the Eastern Han Dynasty. This medicinal preparation comprises the radix and rhizome of Glycyrrhiza uralensis Fisch. (Leguminosae), the processed lateralis radix of Aconitum carmichaelii Debeaux (Ranunculaceae), the rhizome of Atractylodes macrocephala Koidz (Asteraceae) and twigs of Cinnamomum cassia (L.). J. Presl (Lauraceae). It possesses meridian-clearing properties, alleviates cold and dampness, relieves rheumatism, and is extensively employed in clinical practice due to its remarkable therapeutic efficacy. Throughout history, the licorice aconite soup has been commonly prescribed for treating joint diseases. Although research on this herbal remedy primarily originates from Chinese scholars who predominantly focus on its therapeutic effects, there are limited studies conducted at the mechanistic level.Zhao, X et al [22]demonstrated the therapeutic effect of GCFZD on mice with CIA, and the mechanism is related to an improvement in the Th17/Treg cell imbalance by targeting Foxp3 via miR-34a.Sun Bo et al [23] confirmed that the toxicity of lateral root cause rats was attenuated by licorice in the context of metabonomics research, as discussed based on nuclear magnetic resonance (NMR),Co-administration of Gancao with Fuzi mitigates toxicity at the metabolic level.Liu, K et al [24] was carried out based on the network pharmacology, and molecular docking monkshood - licorice medicine for treating nonalcoholic fatty liver disease (NAFLD) curative effect and mechanism of prediction research. Zhao,J et al [25] confirmed the key active components and potential molecular mechanism of Gancao Fuzi decoction (GFD) in the treatment of cold-dampness obstruction-type knee osteoarthritis (KOA) remain unclear. We investigated the mechanism by which GCFZD treats KOA in rats through the "intestinal joint" axis, and elucidated the potential involvement of purine receptors P2X7 and P2Y14. We hope that these research findings will contribute to expanding the clinical application of GCFZD.