Adenosine A2A receptor null chondrocyte transcriptome resembles that of human osteoarthritic chondrocytes

Biased agonism of G protein-coupled receptors as a novel strategy for osteoarthritis therapy

Osteoarthritis (OA) is a prevalent degenerative joint disorder marked by chronic pain, inflammation, and cartilage loss, with current treatments limited to symptom relief. G protein-coupled receptors (GPCRs) play a pivotal role in OA progression by regulating inflammation, chondrocyte survival, and matrix homeostasis. However, their multifaceted signaling, via G proteins or β-arrestins, poses challenges for precise therapeutic targeting. Biased agonism, where ligands selectively activate specific GPCR pathways, emerges as a promising approach to optimize efficacy and reduce side effects. This review examines biased signaling in OA-associated GPCRs, including cannabinoid receptors (CB1, CB2), chemokine receptors (CCR2, CXCR4), protease-activated receptors (PAR-2), adenosine receptors (A1R, A2AR, A2BR, A3R), melanocortin receptors (MC1R, MC3R), bradykinin receptors (B2R), prostaglandin E2 receptors (EP-2, EP-4), and calcium-sensing receptors (CaSR). We analyze ligands in clinical trials and explore natural products from Traditional Chinese Medicine as potential biased agonists. These compounds, with diverse structures and bioactivities, offer novel therapeutic avenues. By harnessing biased agonism, this review underscores the potential for developing targeted, safer OA therapies that address its complex pathology, bridging molecular insights with clinical translation.

Adenosine metabolism and receptors in aging of the skin, musculoskeletal, immune and cardiovascular systems

Aging populations worldwide face an increasing burden of age-related chronic conditions, necessitating a deeper understanding of the underlying mechanisms. Purine metabolism has emerged as a crucial player in the pathophysiology of aging, affecting various tissues and organs. Dysregulation of purine metabolism, particularly alterations in extracellular adenosine levels and adenosine receptor signaling, contributes to age-related musculoskeletal problems, cardiovascular diseases, inflammation, and impaired immune responses. Changes in purine metabolism are associated with diminished tissue repair and regeneration, altered bone density, and impaired muscle regeneration. Mechanistically, age-related alterations in purine metabolism involve reductions in extracellular adenosine production, impaired autocrine signaling, and dysregulated expression of CD73 and CD39. Targeting adenosine receptors, such as A2A and A2B receptors, emerges as a promising therapeutic approach to mitigate age-related conditions, including sarcopenia, obesity, osteoarthritis, and impaired wound healing. Since we cannot reverse time, understanding the intricate molecular interplay between purine metabolism and aging-related pathologies holds significant potential for developing novel therapeutic strategies to improve the health and quality of life of aging populations. In this review, we compile the findings related to purine metabolism during aging in several tissues and organs and provide insights into how these signals can be manipulated to circumvent the deleterious effects of the passage of time on our body.

Purines and Adenosine Receptors in Osteoarthritis

OA is a common and debilitating condition that restricts mobility and diminishes the quality of life. Recent work indicates that the generation of adenosine at the cell surface is an important mediator of chondrocyte homeostasis, and topical application of adenosine in a slow-release form (liposomes) can halt the progression of OA and diminish the pain associated with OA. Here, we review the evidence indicating that adenosine, acting at A2A receptors, plays a critical role in endogenous and exogenous treatment and reversal of OA.

Adenosine A2A receptor activation reduces chondrocyte senescence

Osteoarthritis (OA) pathogenesis is associated with reduced chondrocyte homeostasis and increased levels of cartilage cellular senescence. Chondrosenescence is the development of cartilage senescence that increases with aging joints and disrupts chondrocyte homeostasis and is associated with OA. Adenosine A2A receptor (A2AR) activation in cartilage via intra-articular injection of liposomal A2AR agonist, liposomal-CGS21680, leads to cartilage regeneration in vivo and chondrocyte homeostasis. A2AR knockout mice develop early OA isolated chondrocytes demonstrate upregulated expression of cellular senescence and aging-associated genes. Based on these observations, we hypothesized that A2AR activation would ameliorate cartilage senescence. We found that A2AR stimulation of chondrocytes reduced beta-galactosidase staining and regulated levels and cell localization of common senescence mediators p21 and p16 in vitro in the human TC28a2 chondrocyte cell line. In vivo analysis similarly showed A2AR activation reduced nuclear p21 and p16 in obesity-induced OA mice injected with liposomal-CGS21680 and increased nuclear p21 and p16 in A2AR knockout mouse chondrocytes compared to wild-type mice. A2AR agonism also increased activity of the chondrocyte Sirt1/AMPK energy-sensing pathway by enhancing nuclear Sirt1 localization and upregulating T172-phosphorylated (active) AMPK protein levels. Lastly, A2AR activation in TC28a2 and primary human chondrocytes reduced wild-type p53 and concomitantly increased p53 alternative splicing leading to increase in an anti-senescent p53 variant, Δ133p53α. The results reported here indicate that A2AR signaling promotes chondrocyte homeostasis in vitro and reduces OA cartilage development in vivo by reducing chondrocyte senescence.

Stage related metabolic profile of the synovial fluid in patients with acute flares of knee osteoarthritis

Background and aim

Osteoarthritis (OA) is the most common joint condition and the leading cause of pain and disability in elderly patients. Currently, there is no biomarker available for the early diagnosis of OA, and limited data is available regarding the molecular basis of progression for OA. For this reason, this study aimed to identify the metabolomic profile of early and late OA using high-performance liquid chromatography coupled with untargeted mass spectrometry (LC-MS).

Methods

31 patients with knee OA and joint effusion were enrolled. Based on Kellgren/Laurence scale, 12 patients were classified as early OA (eOA) and 19 as late OA (lOA). The synovial fluid (SF) was collected and characterized by untargeted LC-MS. Only the metabolites identified in more than 25% of each group were kept for further analysis. Principal component analysis (PCA) enabled the unsupervised clustering of the eOA and lOA groups. Further, for classification, the best three principal components (PCs) were used as input for two machine learning algorithms (random forest and naïve Bayes), which were trained to discriminate between the eOA and lOA groups.

Results

43 metabolites were identified in both eOA and lOA, but after selecting the metabolites present in at least 25% of the patients in each group, the metabolomics analysis yielded a panel of only nine metabolites: four metabolites related to phospholipids (phosphatidylcholine 20:0/18:2 and 18:0/20:2, sphingomyelin, and ceramide), three metabolites belonging to purine metabolites (inosine 5'-phosphate, adenosine thiamine diphosphate, and diadenosine 5',5'-diphosphate), one metabolite was a gonadal steroid hormone (estrone 3-sulfate), and one metabolite represented by heme, with all but ceramide (d18:1/20:0) being enriched in the lOA group. By using as features the best three PCs (PC2, PC8 and PC9), random forest and naïve Bayes machine learning algorithms yielded a classification accuracy of 0.81 and 0.78, respectively.

Conclusion

Our LC-MS analysis of SF from patients with eOA and lOA indicates stage-dependent differences, lOA being associated with a perturbed metabolome of phospholipids, purine metabolites, gonadal steroid hormones (estrone 3-sulfate) and a heme molecule. Specific questions need to be answered regarding the biosynthesis and function of these metabolites in osteoarthritic joints, with the aim of developing new relevant biomarkers and therapeutic strategies.