Ebook: Osteoarthritis, Inflammation and Degradation: A Continuum

Osteoarthritis is a major public health issue due to its impact in term of handicap. Moreover, Ageing of the world population and outbreak of obesity in industrialized and non-industrialized countries will dramatically increase its incidence in the next years. Regarded as a multi-factorial disease, today mechanistic and inflammatory theories are no more opposed but, on the contrary, are framed within the same continuum: osteoarthritis, inflammation and degeneration. In order to collect major information in a benchmark book on the fundamental aspects of this disease, internationally well-known authors, from multiple specialties, gathered to analyse, dissect and finally try to understand the secrets of a disease which should no more be regarded as the common and relentless result of ageing or of passive wear but much more as an active disease able to benefit from the best targeted pharmacological (anti-cytokines, inhibitors of signalling pathways, inhibitors of proteases, etc.) and non-pharmacological (cellular therapy, gene therapy, cartilage engineering etc.) therapies, current and future.

Update of these therapies goes through a sharp knowledge of the different pathophysiological mechanisms of osteoarthritis. Major new paradigms have emerged in this field in the very last years. For example, it is noteworthy that cartilage used to be the unique tissue involved in the OA process. But in this book, many chapters refer to novel findings on the role of other tissues like bone or synovial tissue which should be of critical importance for the degradative process of the OA joint. Another challenge refers to the possibility in the future to evaluate the potential severity of the disease in a single patient from the very early stage. The recent new advances in imaging and biomarkers detailed in this volume suggest that we are not so far from this capacity. These recent advances have been compiled in this monograph, that should captivate a large audience, scientists and clinicians.

F. Berenbaum, Elected President of OARSI

Osteoarthritis (OA) is a disease that predominantly, but not solely, affects the diarthrodial joints and results from an interaction between a number of complex mechanical and biological processes. Knowledge of the etiopathogenesis of OA has progressed significantly in the past few decades. A major characteristic of OA is articular cartilage destruction, yet it has become obvious that synovial inflammation, although not a primary cause of the disease, is among the significant structural changes that take place during its development. There is compelling evidence suggesting that secreted inflammatory mediators impact on the matrix homeostasis of articular tissue cells by altering their metabolism. Among these mediators that are responsible for the progression of the disease, evidence points to the proinflammatory cytokine interleukin-1 beta (IL-1ß) as the most important factor responsible for the catabolic process in OA. New members of the IL-1 super-family have recently been identified (ILF5-ILF10), some of which are suggested to be of interest for the arthritic diseases. Other proinflammatory cytokines, such as tumor necrosis factor (TNF)-α, IL-6, leukemia inhibitory factor (LIF), oncostatin M (OSM), IL-17, IL-18, and IL-8, are also considered potential contributing factors in the pathogenesis of OA. However, the exact role and importance of each in the OA process is not yet clearly established. In addition to cytokines, other inflammatory mediators also play a major role in the OA pathological process. These include nitric oxide (NO), eicosanoids (prostaglandins and leukotriene), and a newly identified cell membrane receptor family, the protease-activated receptors (PARs), in which an important role for PAR-2 in chronic arthritis has been suggested. All these topics will be discussed in this review and should help the reader to better understand the most recent advances concerning the inflammatory factors involved in the pathophysiology of OA.

This chapter examines the effects of mechanical loading on cartilage metabolism in explant culture and as isolated chondrocytes in high density monolayer and agarose/scaffold cultures. The mechanical loading effects are defined in terms of the two stress states that arise within cartilage, shear stress and hydrostatic pressure. The paper examines possible signaling pathways that could contribute to transduction of changes in the physical environment of articular chondrocyte to modulation of chondrocyte gene and protein expression.

Extracellular matrix calcification with calcium pyrophosphate dihydrate (CPPD) and/or hydroxyapatite crystals commonly develops in osteoarthritic (OA) cartilage. Primary forms of articular cartilage CPPD crystal deposition and less commonly hydroxyapatite deposition can present as degenerative joint disease. Moreover, CPPD and hydroxyapatite crystals can traffic from cartilage to synovium and induce cytotoxic, catabolic, and inflammatory responses of chondrocytes and synovial lining cells, and promote synovial proliferation. Such changes have the potential to not only contribute to low-grade synovitis and inflammatory symptoms in OA but also to accelerate the progression of OA. In addition, CPPD and hydroxyapatite crystals are commonly found in joints with advanced OA. However, it is not clear that CPPD and hydroxyapatite crystal deposition actually worsen the course of primary OA. Instead, it appears that CPPD and hydroxyapatite crystal deposition in OA articular cartilage reflect aging, inflammation, altered IGF-I and TGFβ responsiveness, chondrocyte hypertrophic differentiation, changes in the closely linked metabolism of ATP, PP_i, and P_i, and possibly local changes in PTHrP expression and systemic changes in PTH. As such, OA pathogenesis richly informs us on mechanisms that drive articular cartilage calcification. Conversely, the presence of cartilage calcification informs us about pathogenesis and progression factors in subsets of affected subjects with OA.

The prevalence of obesity and obesity-related diseases focussed an increasing interest, over the last 10 years, on white adipose tissue and its derived bioactive peptides, being the discovery of leptin in 1994 the trigger of the renaissance of the studies about adipose tissue.

Leptin was initially depicted as the most important anorexigenic factor with neuroendocrine actions, but it has been later shown to significantly modulate immune and inflammatory processes. Leptin is a dual molecule: apart from its previously envisaged metabolic activities, increasing evidence frames leptin as a novel pro-inflammatory adipokine and, at present it might be easily considered one of the relevant links among immune system, inflammatory response and neuroendocrine system. Leptin regulates and participates both in immune homeostasis and inflammatory processes by acting as a modulator of cell activity and playing an active role in articular degenerative inflammatory diseases such as osteoarthritis and rheumatoid arthritis, but also in a host of autoimmune inflammatory conditions such as encephalomyelitis, type-1 diabetes, and bowel inflammation. This review will be focussed more on the adipokine facet of leptin, even though its role as metabolic hormone will be also addressed. In addition, the role od other relevant adipokines in inflammation will be covered.

The ability of degradation products of the extracellular matrix (ECM) to regulate cartilage homeostasis has now been well documented. There are now numerous observations that different types of products derived from the damaged matrix can provide additional signals that can amplify catabolic processes that serve either to clear tissue components for repair or to initiate reparative signals. These fragments include fibronectin fragments (Fn-f), collagen fragments (Col-f) and hyaluronan fragments (HA-f) and likely link protein fragments (LP-f). Active fragments of other ECM components may be found in the future. ECM fragments can arise during cartilage degeneration with enhanced levels of proteinases and normal rates of matrix synthesis. Ironically and theoretically, fragments might also arise from enhanced synthesis of their native precursors but only with basal levels of proteinases and this might lead to enhanced proteinases. Further, certain types of fragments might arise from synovial tissue. The linkage between catabolic and anabolic pathways in cartilage is amply illustrated by the properties of Fn-fs in that the damage pathways initiated by Fn-fs also initiate anabolic pathways of attempted repair. Observations with Fn-fs show that lower concentrations that initiate the lowest levels of matrix metalloproteinases (MMPs) can initiate anabolic processes while higher concentrations also enhance catabolic protease driven pathways that swamp out the anabolic pathway. Anabolism might be enhanced through post-translational events such as proteolytic activation of ECM bound growth factors although other explanations are possible. Thus, fragment systems may be operative not only during damage, but also during normal metabolism and in either case, may shift metabolism in either direction, depending on the concentration of the fragments. Regulation of the fragment pathways may be through native ligands, since the ECM fragments are likely inhibitors of the native ligands and vice versa. These ECM fragment pathways may define a global pathway in which: (1) one type of fragment, such as a Fn-f, can bind either Fn or type II collage and affect not only Fn integrins but also collagen integrins and (2) one type of fragment may bind one type of integrin proximal to another type and affect integrin complexes or clusters. The signaling pathway of Fn-fs suggest that they bind to receptors and disrupt receptor clusters and this may allow internalization of receptors and initiate new pathways involving MAP kinases, Nf-kB activation and ultimately cytokine and MMP upregulation. It will be important to continue to compare the Fn-f, Col-f and HA-f pathways to determine if there is a single global mechanism that might be subject to therapeutic intervention. There are still some basic questions that need to be addressed such as whether these fragments initiate cartilage degeneration or simply amplify ongoing processes or where they are positioned in the early stages of i.e. osteoarthritis and whether or not partial vs complete inhibition of these pathways would be beneficial in a degradative state. More information of the mechanisms is needed especially far upstream at the level of membrane receptors, where re-distribution of integrins may be the key initiating event in the pathway.

Peroxisome proliferators activated receptors (PPAR) are ligandinducible nuclear transacting factors comprising 3 subtypes, PPARα, PPARβ/δ and PPARγ, which play a key role in lipids and glucose homeostasis. All PPAR subtypes have been identified in joint cells and their activation resulted in a transcriptional repression of pro-inflammatory cytokines (IL-1, TNFα), early inflammatory genes (NOS₂, COX-2, mPGES-1) or matrix metalloproteases (MMP-1, MMP-13), at least for the γ subtype. These anti-inflammatory and anti-catabolic properties were confirmed in animal models of joint diseases although much less data are available for experimental osteoarthritis (OA) than for polyarthritis. PPAR agonists were also shown to stimulate IL-1 receptor antagonist (IL-1Ra) production by cytokines-stimulated cells in a subtype-dependent manner. So, PPAR agonists are able to reduce joint inflammation and to prevent cartilage destruction, although many effects were obtained at a higher concentration than required to restore insulin sensitivity or to lower circulating lipids levels. Besides, PPAR agonists were able to modulate the differentiation and/or activity of bone cells, but data are lacking for their effect on OA-associated sclerosis of subchondral bone. Although promising, the therapeutic insight of PPAR agonists in OA warrants additional proofs that could be obtained indirectly from the follow-up of diabetic and/or hyperlipidemic patients with OA treated daily with glitazones or fibrates.

Mitogen-activated protein (MAP) kinase activation by cytokines and other soluble mediators in articular chondrocyte cultures was shown to reproduce critical components relevant to cartilage development, synovial joint inflammation as well as human and animal osteoarthritic pathology. MAP kinase activation has been shown to be critical in cartilage formation Cytokines, such as interleukin-1β and tumor necrosis factor-α, soluble mediators, such as nitric oxide, and growth factors, namely fibroblast growth factor, connective tissue growth factor, vascular endothelial growth factor and hepatocyte growth factor activate specific MAP kinases resulting in nuclear factor-κB activation. NF-κB is a transcription factor which regulates matrix metalloproteinase gene expression, induces chondrocyte programmed cell death, up-regulates chondrocyte cytokine gene transcription as well as suppressing extracellular matrix protein biosynthesis, events that are consistent with synovial inflammation and the resultant destruction of articular cartilage in osteoarthritis.

During cartilage formation and maintenance, the expression of chondrocyte-specific genes, such as those encoding type II collagen (COL2A1), aggrecan, and cartilage-derived retinoic acid sensitive protein (CD-RAP), is regulated by both activators and repressors that interact with the promoter or enhancer regions of these genes. Cascades of both positive and negative transcription factors have been found to determine developmental events in the embryonic growth plate. The high mobility group protein Sox9, which is required for COL2A1 transcription along with l-Sox5 and Sox6, plays a key role in cartilage formation and maintenance, while Sp1 and the coactivator, CBP/p300, are required for constitutive activity. The bHLH, HOX, SMAD, ETS, and STAT families consist of both positive and negative regulators that directly or indirectly influence COL2A1 and CD-RAP during chondrogenesis and chondrocyte hypertrophy. In osteoarthritis, activation of mature articular chondrocytes may result in recapitulation of these developmental events and phenotypic modulation by the associated transcription factors. Cytokine-induced transcription factors, including NF-κB, C/EBP, ETS, and AP-1 family members that activate catabolic and proinflammatory genes, may then suppress chondrocyte phenotype and cartilage repair mechanisms by inhibiting expression of cartilage-specific genes. This review will focus on the transcriptional regulation of COL2A1 and CD-RAP genes by factors involved in cartilage formation and homeostasis, as well as in inflammatory and catabolic events that adversely affect cartilage integrity.

The recent development of high throughput genomic profiling technologies such as cDNA microarrays combined with advanced computational approaches have provided basic and clinical investigators with the ability to identify and characterize high-resolution expression profiles of numerous disease states and to dissect molecular networks that underlie specific disease phenotypes. In the field of osteoarthritis (OA) and cartilage research, the application of microarray technology holds the promise that it may allow the identification of molecular signatures specific for OA in articular cartilage chondrocytes which could provide clues to the elucidation of the pathogenetic mechanisms involved or responsible for the disease. Some of these molecular signatures may also be of great value in patient management and clinical care by providing potential biomarkers of utility as diagnostic or prognostic tools and as markers of the effectiveness of disease modifying therapies for OA. The aim of this chapter is to provide an overview of the relatively few investigations that applied microarrays to the study of human articular cartilage and OA.

Mitochondria are critical regulators of cell function and cellular survival. Many lines of evidence suggest that mitochondria have a central role in ageing-related diseases. Mutations in mitochondrial DNA and oxidative stress both contribute to ageing. Osteoarthritis (OA) is a rheumatic disease associated to aging and it is characterized by articular cartilage degradation and increases of chondrocyte death. Articular cartilage chondrocytes must survive and maintain tissue integrity in an avascular environment. Then chondrocytes from deep and superficial zones may require adaptively increased anaerobic glucolysis and aerobic respiration respectively to support ATP synthesis. Recent ex vivo studie reported dysfunction of mitochondrial human OA chondrocytes. The analysis of mitochondrial electron transport chain activity in OA chondrocytes shows a significant decrease in Complexes I, II and III compared to normal chondrocytes. This mitochondrial dysfunction can mediate several pathways implicated in cartilage degradation such as, oxidative stress, inadequacy of chondrocyte biosynthetic and growth responses, up-regulated chondrocyte cytokine-induced inflammation and matrix catabolism, pathologic cartilage matrix calcification and increased chondrocyte death (necrosis or apoptosis). Mitochondrial dysfunction in OA chondrocytes may be originated by somatic mutations in mtDNA or by direct effect of pro-inflammatory mediators (cytokines, prostaglandin, ROS and NO) on mitochondrial activity.

Osteoarthritis (OA) is considered a complex illness in which cross-talk between the different tissues of the joint plays a significant role in the evolution of the disease. Although we may not yet completely know all the initiating factors involved in the degeneration of the articular tissues, significant progress has been made with respect to the proposal of new concepts regarding the etiopathogenesis of this disease. For decades, the prevailing concept centered around the destruction of articular cartilage as the focal pathological feature of OA. Consequently, it is not surprising that investigators concentrated their efforts at identifying mechanisms involved in the destruction of this tissue. There is now substantial evidence, from preclinical and clinical studies, that changes in the subchondral bone metabolism comprise an integral component of the disease process, and its key role in the initiation and/or progression of cartilage degeneration may have been largely underestimated. This concept as well as the complex pathophysiological mechanisms taking place in this tissue during OA, are the focus of this chapter.

OA research traditionally focuses on understanding events that occur within the degenerated articular cartilage whereas changes in the synovial membrane are largely neglected. However, inflammatory changes do occur in the synovium that may contribute to the overall effects observed in at least a subsets of OA patients. This implicates that the inflammatory and degradative activities of synoviocytes represents an interesting (therapeutic) target in OA research.

Magnetic resonance imaging (MRI) remains the imaging method of choice for depicting the morphological changes associated with osteoarthritis (OA) and other diseases of cartilage, but the early stages of OA are characterized by tissue level changes which are not evident with standard MRI protocols. Several emerging MR-based techniques show promise for detecting changes in water, collagen, and proteoglycans that are the hallmarks of cartilage degeneration. In this review, the principles and application of several of these techniques, including T2 mapping, T1ρ imaging, delayed gadolinium MRI of cartilage (dGEMRIC), and sodium MRI are outlined and compared.

As a comparatively non-destructive imaging technique into living specimens, fluorescence microscopy has a number of strong advantages over alternative imaging modalities (X-ray, MRI, CT-scan, arthro-scan, etc.). The limited analysis in thick tissue has given rise to the development of other techniques, multiphoton excitation microscopy in particular. A need for increased sensitivity and resolution has been driving the development of new sophisticated fluorescence techniques based on microscopies to study: the tissue microstructure in situ (CLSM, SHG) on deeper thick sections of tissue (Multiphoton), molecular diffusion (FRAP, FCS) with fluorescent protein variants and molecular interaction (spectral, FRET, FLIM). In this paper, we have considered developments based on near infrared (NIR) femtosecond excitation in the imaging of articular tissue and discussed the technical limitations and perspectives.

Molecular markers or biomarkers have recently received growing attention in osteoarthritis (OA), due to their potential usefulness in early diagnosis, assessment of disease activity and severity, and evaluation of drug effects. In this context, biomarkers are ideals, due to their characteristics of non-invasive and nonexpansive measures. Concerning the diagnosis, no marker seems able to satisfy the needs to diagnose OA in pre-radiological stages and to identify different subsets of OA. Instead, biomarkers are useful in the assessment of disease activity and the prevision of its outcome. In the recent years, stimulated by the recent introduction of high sensitive immunoassays, number of studies have suggested a role of C-reactive protein (CRP) as marker of activity or severity of OA. Furthermore, higher CRP levels predict those patients whose disease will progress over 4 years. Since metalloproteases (MMPs) are highly involved in cartilage degradation, their levels or activities have been investigated to obtain information on OA severity or progression. Both in serum and synovial fluid (SF), the most abundant MMP is MMP-3. It has been proposed that pro-matrix MMP-3 may act as a marker for posttraumatic cartilage degradation. The molecular markers most useful in suggesting synthesis or degradation of cartilage originate from different articular sources such as cartilage, bone and synovial tissue. Serum hyaluronan (HA) is the most commonly used marker of synovial proliferation and hyperactivity, which may reflect the OA evolution. Other useful biochemical markers are serum keratin sulphate (KS), cartilage oligomeric matrix protein (COMP), YKL-40, and urinary C-terminal crosslinking telopeptides of collagen types I and II (uCTX-II). COMP concentration in SF from lavage as well as in serum is an early indicator ofradiographic progression at follow up. Furthermore, COMP was the most sensitive test for identifying affected subjectswith the genetic form of premature OA. uCTX-II is well correlated with radiological severity of both knee and hip OA and, in addition, the combined measurements of uCTX-II and serum HA seem the best predictor of the structural progression of hip OA.

The rapid development of tissue engineering today allows us to envisage the clinical use of grafts of chondrocytes, autologous stem cells, biocartilage preparations or gene therapy . However, in spite of the high stakes for orthopedic surgery as well as rhumatology or sports medecine, the answers remain unclear. Clinical research on cell therapy and preparation of biocartilage must continue to be developed in order to better determine the choice of a support matrix (scaffold) the local mechanical forces on the cells (chondrocyte, MSL…).