Background/objective: The development of regenerative therapies for osteoarthritis and intervertebral disc degeneration requires non-destructive methods to spatiotemporally monitor extracellular matrix (ECM) synthesis. This study aimed to establish a method for the use of bioorthogonal click chemistry for longitudinal and spatially resolved tracking of newly synthesized proteoglycans in cartilaginous tissues, including tools to assess specificity, enable quantification and for future application for noninvasive real-time tracking of ECM production by imaging. Methods: Bovine articular cartilage explants, human nucleus pulposus tissue, and human chondrocyte cultures were metabolically labeled using azide-modified N-acetylgalactosamine (Ac4 GalNAz) incorporation with copper-catalyzed and copper-free click chemistry. GAG-specific incorporation was verified by enzymatic digestion with chondroitinase ABC(ChABC) and hyaluronidase (HAse). Sequential labeling with spectrally distinct fluorophores enabled temporal tracking of matrix deposition. Volumetric quantification was performed using ImageJ on confocal microscopy Z-stacks. Near-infrared (NIR) dyes were employed for in vitro and subcutaneous imaging, with photoacoustic imaging used to enhance spatial resolution. Results: Ac4GalNAz demonstrated negligible cytotoxicity at 50 μM in cartilage explants. In tissue explants, metabolic labeling showed high specificity for chondroitin sulfate (CS)-containing proteoglycans, confirmed by ChABC digestion. In contrast, 2D chondrocyte cultures showed ChABC-resistant labeling, suggesting incorporation into membrane-associated glycoproteins. Sequential copper-free labeling distinguished temporally distinct matrix synthesis within the same sample. Using NIR-labels after incorporation, explants were detectable both in vitro and following subcutaneous implantation, with only a moderate loss of signal intensity. Photoacoustic imaging appeared to provide higher spatial resolution, as shown by visualization of implant contours. Conclusions: Bioorthogonal metabolic labeling enables non-destructive, spatially resolved, and quantitative monitoring of proteoglycan synthesis in cartilaginous tissues. Labeling specificity is tissue-context dependent, requiring enzymatic validation for each model system. Integration with NIR fluorescence and photoacoustic imaging provides a foundation for future in vivo applications. Statement of significance Monitoring newly synthesized extracellular matrix (ECM) in cartilaginous tissues is a critical challenge in regenerative medicine for osteoarthritis and intervertebral disc degeneration. Building on prior work demonstrating metabolic labeling feasibility in cartilage, this study substantially advances the field by establishing quantitative, spatiotemporally resolved proteoglycan tracking across multiple clinically relevant model systems, including nucleus pulposus tissue. We introduce volumetric 3D quantification of newly synthesized ECM from confocal z-stacks, sequential temporal labeling to distinguish matrix deposition at different timepoints, and near-infrared and photoacoustic imaging enabling subcutaneous detection. Critically, we identify model-dependent labeling specificity, highlighting the need for validation in each model. Together, these advances provide a comprehensive framework for non-destructive monitoring of ECM regeneration with direct translational potential.

Osteoarthritis (OA) is a common degenerative inflammatory joint disease with progressive loss of articular cartilage that undermines patients’ quality of life. There are no regulatory-approved, disease-modifying OA medications, despite a great deal of studies done to elucidate OA pathogenesis. Until now, OA pharmacological treatment focused mainly on generalized inhibition of inflammation and pain. Currently, monoclonal antibodies and nucleic-acid aptamers emerge as targeted therapies offering potential alternatives by addressing the complex challenges posed by OA, such as specifically reducing inflammation and pain in the joint targeting specific molecular key players, instead of a systemic and generalized approach like with non-steroidal anti-inflammatory drugs. Aptamers’ properties, including structure versatility, reduced immunogenicity, and flexible administration methods, position them as high-potential candidates for OA treatment. This review summarizes results from clinical trials applying monoclonal antibodies to treat OA, preclinical research, and the development of aptamers as a new generation of targeting agents. Meanwhile, it provides a comprehensive comparison of the characteristics, advantages, and limitations of aptamers versus monoclonal antibodies. Notably, the promising applications of aptamers, demonstrated in other inflammatory and degenerative conditions, underscore their potential for OA therapy. We anticipate that the application of aptamer could offer a new way of OA pharmacological intervention.

Background: Cartilage defects result in joint inflammation. The presence of proinflammatory factors has been described to negatively affect cartilage formation. Purpose: To evaluate the effect and timing of administration of triamcinolone acetonide (TAA), an anti-inflammatory drug, on cartilage repair using a mouse model. Study Design: Controlled laboratory study. Methods: A full-thickness cartilage defect was created in the trochlear groove of 10-week-old male DBA/1 mice (N = 80). Mice received an intra-articular injection of TAA or saline on day 1 or 7 after induction of the defect. Mice were euthanized on days 10 and 28 for histological evaluation of cartilage defect repair, synovial inflammation, and synovial membrane thickness. Results: Mice injected with TAA had significantly less synovial inflammation at day 10 than saline-injected mice independent of the time of administration. At day 28, the levels of synovitis dropped toward healthy levels; nevertheless, the synovial membrane was thinner in TAA- than in saline-injected mice, reaching statistical significance in animals injected on day 1 (70.1 ± 31.9 µm vs 111.9 ± 30.9 µm, respectively; P =.01) but not in animals injected on day 7 (68.2 ± 21.86 µm vs 90.2 ± 21.29 µm, respectively; P =.26). A thinner synovial membrane was moderately associated with less filling of the defect after 10 and 28 days (r = 0.42, P =.02; r = 0.47, P =.01, respectively). Whereas 10 days after surgery there was no difference in the area of the defect filled and the cell density in the defect area between saline- and TAA-injected knees, filling of the defect at day 28 was lower in TAA- than in saline-injected knees for both injection time points (day 1 injection, P =.04; day 7 injection, P =.01). Moreover, there was less collagen type 2 staining in the filled defect area in TAA- than in saline-injected knees after 28 days, reaching statistical significance in day 1–injected knees (2.6% vs 18.5%, respectively; P =.01) but not in day 7–injected knees (7.4% vs 15.8%, respectively; P =.27). Conclusion: Intra-articular injection of TAA reduced synovial inflammation but negatively affected cartilage repair. This implies that inhibition of inflammation may inhibit cartilage repair or that TAA has a direct negative effect on cartilage formation. Clinical Relevance: Our findings show that TAA can inhibit cartilage defect repair. Therefore, we suggest not using TAA to reduce inflammation in a cartilage repair setting.

Background and Purpose: Corticosteroids are intra-articularly injected to relieve pain in joints with osteoarthritis (OA) or acute tissue damage such as ligament or tendon tears, despite its unverified contraindication in unstable joints. Biomaterial-based sustained delivery may prolong reduction of inflammatory pain, while avoiding harmful peak drug concentrations. Experimental Approach: The applicability of prolonged corticosteroid exposure was examined in a rat model of anterior cruciate ligament and medial meniscus transection (ACLT + pMMx) with ensuing degenerative changes. Key Results: Intra-articular injection of a bolus of the corticosteroid triamcinolone acetonide (TAA) resulted in enhanced joint instability in 50% of the joints, but neither instability-induced OA cartilage degeneration, synovitis, nor the OA-related bone phenotype was affected. However, biomaterial microsphere-based extended TAA release enhanced instability in 94% of the animals and induced dystrophic calcification and exacerbation of cartilage degeneration. In healthy joints, injection with TAA releasing microspheres had no effect at all. In vitro, TAA inhibited cell migration out of joint tissue explants, suggesting inhibited tissue healing in vivo as mechanisms for enhanced instability and subsequent cartilage degeneration. Conclusions and Implications: We conclude that short-term TAA exposure has minor effects on surgically induced unstable joints, but its extended presence is detrimental by extending instability and associated joint degeneration through compromised healing. This supports a contraindication of prolonged corticosteroid exposure in tissue damage-associated joint instability, but not of brief exposure.

Tissue engineering and regenerative medicine are two therapeutic strategies to treat, and to potentially cure, diseases affecting cartilaginous tissues, such as osteoarthritis and cartilage defects. Insights into the processes occurring during regeneration are essential to steer and inform development of the envisaged regenerative strategy, however tools are needed for longitudinal and quantitative monitoring of cartilage matrix components. In this study, we introduce a contrast-enhanced computed tomography (CECT)-based method using a cationic iodinated contrast agent (CA4+) for longitudinal quantification of glycosaminoglycans (GAG) in cartilage-engineered constructs. CA4+ concentration and scanning protocols were first optimized to ensure no cytotoxicity and a facile procedure with minimal radiation dose. Chondrocyte and mesenchymal stem cell pellets, containing different GAG content were generated and exposed to CA4+. The CA4+ content in the pellets, as determined by micro computed tomography, was plotted against GAG content, as measured by 1,9-dimethylmethylene blue analysis, and showed a high linear correlation. The established equation was used for longitudinal measurements of GAG content over 28 days of pellet culture. Importantly, this method did not adversely affect cell viability or chondrogenesis. Additionally, the CA4+ distribution accurately matched safranin-O staining on histological sections. Hence, we show proof-of-concept for the application of CECT, utilizing a positively charged contrast agent, for longitudinal and quantitative imaging of GAG distribution in cartilage tissue-engineered constructs. Statement of Significance: Tissue engineering and regenerative medicine are promising therapeutic strategies for different joint pathologies such as cartilage defects or osteoarthritis. Currently, in vitro assessment on the quality and composition of the engineered cartilage mainly relies on destructive methods. Therefore, there is a need for the development of techniques that allow for longitudinal and quantitative imaging and monitoring of cartilage-engineered constructs. This work harnesses the electrostatic interactions between the negatively-charged glycosaminoglycans (GAGs) and a positively-charged contrast agent for longitudinal and non-destructive quantification of GAGs, providing valuable insight on GAG development and distribution in cartilage engineered constructs. Such technique can advance the development of regenerative strategies, not only by allowing continuous monitoring but also by serving as a pre-implantation screening tool.