Chopping off the chondrocyte proteome

Emerging Technologies and Platforms for the Immunodetection of Multiple Biochemical Markers in Osteoarthritis Research and Therapy

Biomarkers, especially biochemical markers, are important in osteoarthritis (OA) research, clinical trials, and drug development and have potential for more extensive use in therapeutic monitoring. However, they have not yet had any significant impact on disease diagnosis and follow-up in a clinical context. Nevertheless, the development of immunoassays for the detection and measurement of biochemical markers in OA research and therapy is an active area of research and development. The evaluation of biochemical markers representing low-grade inflammation or extracellular matrix turnover may permit OA prognosis and expedite the development of personalized treatment tailored to fit particular disease severities. However, currently detection methods have failed to overcome specific hurdles such as low biochemical marker concentrations, patient-specific variation, and limited utility of single biochemical markers for definitive characterization of disease status. These challenges require new and innovative approaches for development of detection and quantification systems that incorporate clinically relevant biochemical marker panels. Emerging platforms and technologies that are already on the way to implementation in routine diagnostics and monitoring of other diseases could potentially serve as good technological and strategic examples for better assessment of OA. State-of-the-art technologies such as advanced multiplex assays, enhanced immunoassays, and biosensors ensure simultaneous screening of a range of biochemical marker targets, the expansion of detection limits, low costs, and rapid analysis. This paper explores the implementation of such technologies in OA research and therapy. Application of novel immunoassay-based technologies may shed light on poorly understood mechanisms in disease pathogenesis and lead to the development of clinically relevant biochemical marker panels. More sensitive and specific biochemical marker immunodetection will complement imaging biomarkers and ensure evidence-based comparisons of intervention efficacy. We discuss the challenges hindering the development, testing, and implementation of new OA biochemical marker assays utilizing emerging multiplexing technologies and biosensors.

Serum NT/CT SIRT1 ratio reflects early osteoarthritis and chondrosenescence

Objective

Previous work has established that the deacetylase sirtuin-1 (SIRT1) is cleaved by cathepsin B in chondrocytes subjected to proinflammatory stress, yielding a stable but inactive N-terminal (NT) polypeptide (75SIRT1) and a C-terminal (CT) fragment. The present work examined if chondrocyte-derived NT-SIRT1 is detected in serum and may serve as an investigative and exploratory biomarker of osteoarthritis (OA).

Methods

We developed a novel ELISA assay to measure the ratio of NT to CT of SIRT1 in the serum of human individuals and mice subjected to post-traumatic OA (PTOA) or age-dependent OA (ADOA). We additionally monitored NT/CT SIRT1 in mice subject to ADOA/PTOA followed by senolytic clearance. Human chondrosenescent and non-senescent chondrocytes were exposed to cytokines and analysed for apoptosis and NT/CT SIRT1 ratio in conditioned medium.

Results

Wild-type mice with PTOA or ADOA of moderate severity exhibited increased serum NT/CT SIRT1 ratio. In contrast, this ratio remained low in cartilage-specific Sirt1 knockout mice despite similar or increased PTOA and ADOA severity. Local clearance of senescent chondrocytes from old mice with post-traumatic injury resulted in a lower NT/CT ratio and reduced OA severity. While primary chondrocytes exhibited NT/CT ratio increased in conditioned media after prolonged cytokine stimulation, this increase was not evident in cytokine-stimulated chondrosenescent cells. Finally, serum NT/CT ratio was elevated in humans with early-stage OA.

Conclusions

Increased levels of serum NT/CT SIRT1 ratio correlated with moderate OA in both mice and humans, stemming at least in part from non-senescent chondrocyte apoptosis, possibly a result of prolonged inflammatory insult.

Detecting early stage osteoarthritis by optical coherence tomography?

Osteoarthritis (OA) is the most common chronic disease of our joints, manifested by a dynamically increasing degeneration of hyaline articular cartilage (AC). While currently no therapy can reverse this process, the few available treatment options are hampered by the inability of early diagnosis. Loss of cartilage surface, or extracellular matrix (ECM), integrity is considered the earliest sign of OA. Despite the increasing number of imaging modalities surprisingly few imaging biomarkers exist. In this narrative review, recent developments in optical coherence tomography are critically evaluated for their potential to assess different aspects of AC quality as biomarkers of OA. Special attention is paid to imaging surface irregularities, ECM organization and the evaluation of posttraumatic injuries by light-based modalities.

Cell Therapy and Tissue Engineering Approaches for Cartilage Repair and/or Regeneration

Articular cartilage injuries caused by traumatic, mechanical and/or by progressive degeneration result in pain, swelling, subsequent loss of joint function and finally osteoarthritis. Due to the peculiar structure of the tissue (no blood supply), chondrocytes, the unique cellular phenotype in cartilage, receive their nutrition through diffusion from the synovial fluid and this limits their intrinsic capacity for healing. The first cellular avenue explored for cartilage repair involved the in situ transplantation of isolated chondrocytes. Latterly, an improved alternative for the above reparative strategy involved the infusion of mesenchymal stem cells (MSC), which in addition to a self-renewal capacity exhibit a differentiation potential to chondrocytes, as well as a capability to produce a vast array of growth factors, cytokines and extracellular matrix compounds involved in cartilage development. In addition to the above and foremost reparative options up till now in use, other therapeutic options have been developed, comprising the design of biomaterial substrates (scaffolds) capable of sustaining MSC attachment, proliferation and differentiation. The implantation of these engineered platforms, closely to the site of cartilage damage, may well facilitate the initiation of an ‘in situ’ cartilage reparation process. In this mini-review, we examined the timely and conceptual development of several cell-based methods, designed to repair/regenerate a damaged cartilage. In addition to the above described cartilage reparative options, other therapeutic alternatives still in progress are portrayed.

Biomarkers of (osteo)arthritis

Arthritic diseases are a major cause of disability and morbidity, and cause an enormous burden for health and social care systems globally. Osteoarthritis (OA) is the most common form of arthritis. The key risk factors for the development of OA are age, obesity, joint trauma or instability. Metabolic and endocrine diseases can also contribute to the pathogenesis of OA. There is accumulating evidence to suggest that OA is a whole-organ disease that is influenced by systemic mediators, inflammaging, innate immunity and the low-grade inflammation induced by metabolic syndrome. Although all joint tissues are implicated in disease progression in OA, articular cartilage has received the most attention in the context of aging, injury and disease. There is increasing emphasis on the early detection of OA as it has the capacity to target and treat the disease more effectively. Indeed it has been suggested that this is the era of "personalized prevention" for OA. However, the development of strategies for the prevention of OA require new and sensitive biomarker tools that can detect the disease in its molecular and pre-radiographic stage, before structural and functional alterations in cartilage integrity have occurred. There is also evidence to support a role for biomarkers in OA drug discovery, specifically the development of disease modifying osteoarthritis drugs. This Special Issue of Biomarkers is dedicated to recent progress in the field of OA biomarkers. The papers in this Special Issue review the current state-of-the-art and discuss the utility of OA biomarkers as diagnostic and prognostic tools.