Osteoarthritis (OA) is the most common form of arthritis and a major cause of pain and disability in older adults (1) Often OA is referred to as degenerative joint disease “(DJD)” This is a misnomer because OA is not simply a process of wear and tear but rather abnormal remodeling of joint tissues driven by a host of inflammatory mediators within the affected joint The most common risk factors for OA include age, gender, prior joint injury, obesity, genetic predisposition, and mechanical factors, including malalignment and abnormal joint shape (2, 3) Despite the multifactorial nature of OA, the pathological changes seen in osteoarthritic joints have common features that affect the entire joint structure resulting in pain, deformity and loss of function

The pathologic changes seen in OA joints (Figures 1 and ​and2)2) include degradation of the articular cartilage, thickening of the subchondral bone, osteophyte formation, variable degrees of synovial inflammation, degeneration of ligaments and, in the knee, the menisci, and hypertrophy of the joint capsule There can also be changes in periarticular muscles, nerves, bursa, and local fat pads that may contribute to OA or the symptoms of OA The findings of pathological changes in all of the joint tissues are the impetus for considering OA as a disease of the joint as an organ resulting in “joint failure” In this review, we will summarize the key features of OA in the various joint tissues affected and provide an overview of the basic mechanisms currently thought to contribute to the pathological changes seen in these tissues






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Figure 1


Sagittal inversion recovery (A–C) and coronal fast spin echo (D–F) images illustrating the magnetic resonance imaging findings of osteoarthritis (A) reactive synovitis (thick white arrow), (B) subchondral cyst formation (white arrow), (C) bone marrow edema (thin white arrows), (D) partial thickness cartilage wear (thick black arrow), (E–F) full thickness cartilage wear (thin black arrows), subchondral sclerosis (arrowhead) and marginal osteophyte formation (double arrow) Image courtesy of Drs Hollis Potter and Catherine Hayter, Hospital for Special Surgery, New York, NY

https://onlinelibrary.wiley.com/doi/pdf/10.1002/art.34453

Osteoarthritis: A Disease of the Joint as an Organ

Purpose of review
This review focuses on the novel stress-induced and proinflammatory mechanisms underlying the pathogenesis of osteoarthritis, with particular attention to the role of synovitis and the contributions of other joint tissues to cellular events that lead to the onset and progression of the disease and irreversible cartilage damage.

Inflammation in osteoarthritis

Background The glucagon-like peptide-1 (GLP-1) is a multifaceted hormone with broad pharmacological potential. Among the numerous metabolic effects of GLP-1 are the glucose-dependent stimulation of insulin secretion, decrease of gastric emptying, inhibition of food intake, increase of natriuresis and diuresis, and modulation of rodent β-cell proliferation. GLP-1 also has cardio- and neuroprotective effects, decreases inflammation and apoptosis, and has implications for learning and memory, reward behavior, and palatability. Biochemically modified for enhanced potency and sustained action, GLP-1 receptor agonists are successfully in clinical use for the treatment of type-2 diabetes, and several GLP-1-based pharmacotherapies are in clinical evaluation for the treatment of obesity. Scope of review In this review, we provide a detailed overview on the multifaceted nature of GLP-1 and its pharmacology and discuss its therapeutic implications on various diseases. Major conclusions Since its discovery, GLP-1 has emerged as a pleiotropic hormone with a myriad of metabolic functions that go well beyond its classical identification as an incretin hormone. The numerous beneficial effects of GLP-1 render this hormone an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, and neurodegenerative disorders

https://spiral.imperial.ac.uk/bitstream/10044/1/74273/2/Glucagon-like%20peptide%201%20%28GLP-1%29%20Molecular%20Metabolism.pdf

Glucagon-like peptide 1 (GLP-1)

The mammalian proprotein convertases constitute a family of nine secretory serine proteases that are related to bacterial subtilisin and yeast kexin. Seven of these (proprotein convertase 1 (PC1), PC2, furin, PC4, PC5, paired basic amino acid cleaving enzyme 4 (PACE4) and PC7) activate cellular and pathogenic precursor proteins by cleavage at single or paired basic residues, whereas subtilisin kexin isozyme 1 (SKI-1) and proprotein convertase subtilisin kexin 9 (PCSK9) regulate cholesterol and/or lipid homeostasis via cleavage at non-basic residues or through induced degradation of receptors. Proprotein convertases are now considered to be attractive targets for the development of powerful novel therapeutics. In this Review, we summarize the physiological functions and pathological implications of the proprotein convertases, and discuss proposed strategies to control some of their activities, including their therapeutic application and validation in selected disease states.

The biology and therapeutic targeting of the proprotein convertases

https://www.oarsijournal.com/article/S1063-4584(11)00332-3/pdf

Chondrocyte hypertrophy and osteoarthritis: role in initiation and progression of cartilage degeneration?

Objective

Recent published studies have shown that cartilage from ADAMTS-5–knockout mice, but not ADAMTS-4– or ADAMTS-1–knockout mice, is significantly protected from degradation. The present study was undertaken to evaluate the respective roles of these enzymes in human cartilage breakdown, using a small interfering RNA (siRNA) approach to assess the effects of inhibition of each enzyme in normal and osteoarthritic (OA) explants.



Methods

The activities of siRNA specifically targeting ADAMTS-1, -4, and -5 were assessed by transfection into primary human chondrocytes and cultured human cartilage explants. At 24 hours, a cytokine stimulus was applied to normal, but not OA, samples to initiate a catabolic response. At designated times, total RNA was isolated and gene expression was measured by quantitative real-time reverse transcription–polymerase chain reaction. Aggrecan release and aggrecanase-generated neoepitope formation were determined by dye binding analysis and Western blotting, respectively.



Results

Human chondrocytes and explants were efficiently transfected with siRNA that specifically decreased the expression of each targeted gene. Suppression of ADAMTS-4 and ADAMTS-5, individually or in combination, attenuated the degradation of aggrecan in cytokine-stimulated normal cartilage. A reduction in aggrecan degradation was also observed following siRNA-mediated knockdown of either gene in unstimulated OA cartilage. In contrast, knockdown of ADAMTS-1 failed to inhibit aggrecan loss.



Conclusion

Despite the apparent dominant role of ADAMTS-5 in genetically modified mice, our data suggest that both ADAMTS-4 and ADAMTS-5 contribute to the structural damage that characterizes human OA.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/art.22334

Aggrecan degradation in human articular cartilage explants is mediated by both ADAMTS-4 and ADAMTS-5.

Conjugation of therapeutic proteins with high molecular weight polyethylene glycols (HMW PEGs) is used to extend the half-life of biologics. To evaluate the effects of HMW PEGs in animals, we used an immunohistochemical procedure to study the tissue distribution and toxicity of unconjugated HMW PEGs in rats given 100 mg/kg (10K)PEG, (20K)PEG, or (40K)PEG intravenously. Both the PEG cellular distribution and the histology were different between groups. In (10K)PEG and (20K)PEG groups, PEG immunoreactivity was most prominent in the renal tubule epithelium and in alveolar macrophages and hepatic Kupffer cells and cellular vacuolation was absent. In contrast, rats given (40K)PEG had strong PEG immunoreactivity in splenic subcapsular red pulp macrophages, renal interstitial macrophages, and choroid plexus epithelial cells that was frequently associated with cytoplasmic vacuolation. While the vacuolation appeared to be an adaptive response, there was focal renal tubular epithelial degeneration associated with strong PEG immunoreactivity in one rat given (40K)PEG. These data indicate that both the tissue distribution and the vacuolation observed with unconjugated HMW PEGs are markedly influenced by the molecular weight of the PEG and that when vacuolation is observed it is likely an adaptive change that is associated with PEG cytoplasmic immunoreactivity.

https://journals.sagepub.com/doi/pdf/10.1177/0192623312474726

High Molecular Weight Polyethylene Glycol Cellular Distribution and PEG-associated Cytoplasmic Vacuolation Is Molecular Weight Dependent and Does Not Require Conjugation to Proteins

Proprotein convertase activation of aggrecanases in cartilage in situ.

Objective

Fibronectin fragments are present at high concentrations in the cartilage of patients with rheumatoid arthritis and patients with osteoarthritis (OA) and have been shown to promote cartilage catabolism in human cartilage cultures, suggesting that fibronectin fragments participate in the initiation and progression of arthritic disease. This study was undertaken to 1) identify the major fibronectin fragments in human OA cartilage and confirm their ability to elicit cartilage catabolism, 2) identify the cleavage sites in fibronectin and generate the corresponding neoepitope antibodies, and 3) explore the utility of fibronectin neoepitopes as biomarkers.



Methods

Fibronectin fragments were purified from human OA cartilage using affinity chromatography; their N-termini were then identified by sequencing. Bovine nasal cartilage was treated with affinity-purified fibronectin fragments and assayed for aggrecan breakdown by monitoring the release of glycosaminoglycans and the aggrecan neoepitope 1771AGEG. Fibronectin neoepitopes were detected by Western blotting in cytokine-treated media of human cartilage explants, and by immunohistochemical analyses of human OA cartilage.



Results

Multiple fibronectin fragments were isolated from human OA cartilage, and all contained the N-terminus 272VYQP. These fragments induced aggrecanase-mediated cartilage catabolism in bovine cartilage explants. Fibronectin fragments with the N-terminus 272VYQP and fragments with the C-terminus VRAA271 were detected following cytokine treatment of human cartilage extracts. These neoepitopes localized with areas of aggrecan loss in OA cartilage.



Conclusion

Human OA cartilage contains fibronectin fragments with catabolic activity and a major cleavage site within fibronectin. This study is the first to characterize fibronectin neoepitopes in OA cartilage, suggesting that they may represent a novel biomarker of arthritis.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/art.22045

Identification of fibronectin neoepitopes present in human osteoarthritic cartilage

Objective

Fibronectin fragments are thought to play a critical role in the initiation and progression of cartilage degradation in arthritis. In a recent study, fibronectin neoepitopes resulting from cleavage of intact fibronectin at the Ala271/Val272 scissile bond, generating an ∼30-kd fragment with the new C-terminus VRAA271 and an ∼50–85-kd fragment with the new N-terminus 272VYQP, were identified in osteoarthritis (OA) cartilage. The present study was undertaken to isolate the enzymes responsible for this cleavage from human OA chondrocytes.



Methods

Fibronectin-degrading activity in human OA chondrocyte–conditioned medium (OACCM) was purified using conventional chromatography. A fluorescent peptide was developed based on the fibronectin scissile bond 269RAAVal272, and this peptide was used to track fibronectinase activity during purification. Western blotting with antibodies that detect the fibronectin neoepitopes VRAA271 and 272VYQP was used to confirm cleavage of intact fibronectin by the enzymatically active fractions. Mass spectrometry was used to identify the proteins found in the fibronectinase-enriched fractions, with further confirmation by Western blotting. In addition, a recombinant enzyme identified by mass spectrometry was tested by Western blotting and dimethylmethylene blue assay for its ability to produce fibronectin neoepitopes in OA cartilage.



Results

Purification of OACCM by chromatography resulted in isolation of a fibronectin-degrading enzyme, and mass spectrometry identified ADAM-8 as the fibronectinase present in these preparations. Furthermore, treatment of OA cartilage with recombinant human ADAM-8 promoted cartilage catabolism.



Conclusion

The results of this study identify ADAM-8 as a fibronectinase in human OA chondrocytes. Because ADAM-8 is capable of producing the fibronectin neoepitopes VRAA271 and 272VYQP in human OA cartilage, this enzyme may be an important mediator of cartilage catabolism.

James T. Alston

Papers

Aggrecan degradation in human articular cartilage explants is mediated by both ADAMTS-4 and ADAMTS-5.

High Molecular Weight Polyethylene Glycol Cellular Distribution and PEG-associated Cytoplasmic Vacuolation Is Molecular Weight Dependent and Does Not Require Conjugation to Proteins

Proprotein convertase activation of aggrecanases in cartilage in situ.

Identification of fibronectin neoepitopes present in human osteoarthritic cartilage

ADAM-8 isolated from human osteoarthritic chondrocytes cleaves fibronectin at Ala271