The current study is the first Mendelian randomization study to explore the potential causal relationship between multiple metabolites and multiple joint diseases. In previous studies, confounding factors could not be controlled for because researchers were unable to conduct randomized controlled trials. The conclusions drawn from existing observational studies may be due to confounding bias or causality reversal, so no consistent conclusions have been drawn in this regard, so here a GWAS-based MR method was applied to effectively avoid confounding factors and causality reversal and to analyze the causal associations between metabolites and joint disorders starting from the genetic level.The results of the present study are consistent with our initial hypothesis. We found that Gonarthrosis was significantly and causally associated with Glycine levels, N-acetylglycine levels, Glycine to serine ratio, and Propionylcarnitine (c3) levels.
There was a clear causal relationship between Glycine levels The development of metabolomics technology has helped to detect early biological changes in osteoarthritis through changes in metabolites [36]. Blood, synovial fluid, cartilage, and subchondral bone are typical sample sources for metabolomics identification. Global and targeted metabolomics on synovial fluid identified six differential osteoarthritis metabolites [37]. Metabolomic analyses of human plasma showed that an overactive arginine to ornithine pathway contributes to arginine depletion in patients with osteoarthritis of the knee, ultimately leading to an imbalance between cartilage repair and degeneration [38]. Comparison of urine from osteoarthritis patients to normal patients similarly demonstrated an altered metabolic profile in both [39]. Results The results of the present study and previous studies suggest metabolite disorders as a potential pathogenesis of Gonarthrosis, with the most commonly reported metabolites being amino acids, taurine, and phospholipids.
Our study found that five metabolites in the amino acid metabolic pathway were causally associated with different outcome factors. Amino acids have irreplaceable physiological functions in the organism and are precursors for the composition of many small molecules (e.g., nitric oxide, dopamine, 5-hydroxytryptamine, polyamines, and glutathione) [40–41]. A study showed that IRF1-STAT1-driven chemokines are essential for leukocyte recruitment to arthritic joints. Mice on a low-protein diet can reduce the severity of RA by inhibiting the expression of IRF1-STAT1-driven chemokines [42]. Tryptophan is an essential amino acid. The liver is the main organ involved in the oxidative catabolism of tryptophan. Tryptophan and its catabolic metabolites, kynurenine, and 3-hydroxy-o-tryptophan (3-HAA), have anti-inflammatory effects [43–44]. Targeted quantitative tryptophan metabolomics analyses of sera from patients with rheumatoid arthritis and healthy patients revealed reduced levels of kynurenine (KYNA) and xanthurenine (XANA), as well as indole derivatives, and increased levels of quinolinic acid (QUIN) in sera from patients with RA. They were positively correlated with disease severity and negatively correlated with quality-of-life scores. Further animal studies revealed that QUIN induces mitochondrial respiration and glycolysis, favors the proliferation of human fibroblast-like synoviocytes, and affects their cellular metabolism [45]. The tryptophan pathway and its catabolic metabolite, the kynurenine pathway, are likewise major pathways in osteoarthritis pathology. Observational clinical studies have confirmed the important role of the tryptophan pathway in osteoarthritis [46], and similarly, it has been demonstrated in a rat osteoarthritic model that activation of the tryptophan metabolic pathway may promote the expression of the hydrocarbon receptor (AhR) on chondrocytes, accelerating osteoarthritis progression [47].
Glycine is the simplest naturally occurring amino acid in terms of chemical structure, and glycine is synthesized mainly through the serine pathway [48]. It has been shown in animal studies to be an effective therapeutic agent in a variety of diseases and has broad-spectrum anti-inflammatory, cytoprotective, and immunomodulatory effects [49]. Metabolomics analysis showed that serum levels of glycine were reduced in patients with severe osteoarthritis, and glycine levels were negatively correlated with the severity of OA [50]. Glycine, a member of the functional amino acids, is a potential therapeutic target for osteoarthritis due to its anti-inflammatory and antioxidant properties [51]. The present study similarly demonstrated a causal relationship between glycine and knee osteoarthritis. The expression levels of glycine also differ in RA and juvenile idiopathic arthritis [52–53]. Currently, with the development of metabolomics studies, more and more amino acids are strongly associated with joint diseases, with arginine, alanine, and histidine all serving as potential biomarkers for different joint diseases [54–56], and thus amino acid metabolic pathways may be a source of new diagnostic, therapeutic, or preventive options.
The present study similarly found the relevance of metabolites of lipid metabolic pathways such as Docosatrienoate, Propionylcarnitine, and Glycohyocholate with joint diseases. Lipids are the most abundant cellular metabolites and have important roles in energy supply and storage, cell membrane construction, and signal transduction [57]. Synovial fluid provides nutrition and lubrication to articular cartilage. In terms of lipid content, synovial fluid from RA patients had increased levels of palmitic acid, total saturated fatty acids, and long-chain unsaturated fatty acids [58], and lower levels of myristic acid and palmitoleic acid than non-rheumatoid arthritis patients [59]. An analysis of synovial fluid from different joints found that patients with RA had an elevated proportion of arachidonic acid (20:4n-6) and docosahexaenoic acid (22:6n-3) and a reduced proportion of the monounsaturated fatty acid oleic acid (18:1n-9) in the shoulder joint [60]. A study examining the association of lipidomic profiles with osteoarthritis (OA) severity showed that OA severity outcomes were correlated with lipidomics of bound and free arachidonic acid, bound palmitoleic acid, oleic acid, linoleic acid and docosapentaenoic acid [61]. Another study also found differences in lipid expression levels in patients with psoriatic arthritis, with higher levels of 12-hydroxyheptadecenoic acid (12-HHTrE) [62].
The current study also identified some other metabolites such as N-formylanthranilic acid and propionylcarnitine (c3) have a risk causal relationship with arthropathy, but there are no studies to prove the mechanism by which these metabolites affect arthropathy, and further studies are needed. The current study also identified some pathways that we have analyzed in fatty acid metabolism (Beta Oxidation of Very Chain Fatty Acids, Oxidation of Branched Chain Fatty Acids, Alpha Linolenic Acid, and Linoleic Acid Metabolism) related pathways. We also identified Caffeine metabolism and Carnitine Synthesis as potential metabolic pathways. Previous studies have similarly demonstrated that caffeine is a risk factor for osteoarthritis and RA [63–64]. Adenosine receptor activation has anti-inflammatory and anabolic effects on chondrocytes, and caffeine inhibits adenosine receptor expression in articular cartilage, thereby exacerbating articular cartilage destruction [65–66]. Carnitine metabolism is likewise an important pathway in arthropathy. Studies have shown that patients with juvenile arthritis and RA have reduced blood levels of free carnitine and acylcarnitines [67–68]. L-carnitine significantly transfers long-chain fatty acids from the inner mitochondrial membrane to peripheral tissues, it reduces plasma-free fatty acids, utilizes oxygen for ATP synthesis, and reduces oxidation and inflammation [69]. Thus, animal studies have found that ingestion of L-carnitine KOA symptoms and serum matrix metallopeptidase 13 [70].
The present study explored the causal relationship between metabolites and joint disease using a two-sample Mendelian randomization study approach, which achieves causality analysis of exposure and outcome by using public GWAS data and is more resource-efficient compared to the randomized controlled trial approach. Although the results of this study provide strong evidence for an association between metabolites and joint disease, it is important to recognize that there are some limitations to this study. Firstly, only the causal relationship between metabolites and joint disease was considered, while the influence of other potential factors was not explored. Arthrogryposis is a complex pathological process that may be influenced by a variety of factors, which could be considered in further studies. Second, the GWAS data used in this study were mainly from European populations, which may limit the applicability of the findings to other population groups. Third, the majority of patients with rheumatoid arthritis are female, and the composition of metabolites varies by gender. However, our work could not analyze the two genders separately. Fourth, the estimates of the lifetime effects of metabolites on arthropathy provided by MR do not provide much clinical relevance for age-specific interventions.
In conclusion, this study highlights the complex pattern of action between metabolites and multiple joint diseases. The underlying mechanisms of the causal relationships identified still need to be further explored. The practical implications of this finding, which expands the horizons of joint disease treatment, may provide useful clues for prevention and intervention in joint diseases.