Mitochondrial dysfunction in osteoarthritis

Abstract

In osteoarthritis (OA) a time or age dependent process leads to aberrant cartilage structure which is characterized by reduced number of chondrocytes, loss of existing cartilage extracellular matrix, the production of matrix with abnormal composition and pathologic matrix calcification. Because chondrocyte matrix synthesis and mineralization are modulated by the balance between ATP generation and consumption, the mechanism by which chondrocytes generate energy have been a topic of interest. The analysis of mitochondrial respiratory chain (MRC) activity in OA chondrocytes shows a significant decrease in complexes II and III compared to normal chondrocytes. On the other hand, mitochondrial mass is increased in OA, as demonstrated by a significant rise in CS activity. Furthermore, OA cells show a reduction in the mitochondrial membrane potential (Δψm) as demonstrated by using the fluorescent probe JC-1. OA cartilage contains high number of apoptotic chondrocytes, and mitochondria play a key role in apoptosis. Interestingly, OA cartilages show markedly elevated Bcl-2 and caspasa-3 expression. This expression is also correlated with chondrocyte apoptosis and OA lesions. The pathogenesis of OA includes elaboration of increased amounts of NO as a consequence of up-regulation of chondrocyte-inducible NO synthase induced by IL-1, TNF-α and other factors. NO reduces chondrocyte survival and induces cell death with morphologic changes characteristic of chondrocyte apoptosis. NO reduces the activity of complex IV and decreases the Δψm as measured as the ratio of red/green fluorescence. Furthermore, NO induces the mRNA expression of caspase-3 and -7, and it reduces the expression of mRNA bcl-2 and the bcl-2 protein synthesis.

Some studies suggest that the chondrocyte mitochondria are specialized for calcium transport and are important in the calcification of the extracellular matrix. Mineral formation has been demonstrated in matrix vesicles (MV) and within mitochondria. Direct suppression of mitochondrial respiration promoted MV-mediated mineralization in chondrocytes. Regulation of MRC may be one of the signaling pathways by which NO modulates articular cartilage matrix biosynthesis and pathologic mineralization. After age 40, the incidence of OA in humans increases progressively with increasing age. Studies show a trend to statistic significance between the age and the reduction of complex I activity of human normal chondrocytes. However, the study of relation between age and Δψm in normal chondrocytes do not demonstrate any significant correlation. It has been reported that as the number of population doublings increased, mitochondrial DNA was degraded and the number of mitochondria per chondrocyte decline. One approach for determining the role of mitochondria in OA is to determine the effects of the MRC inhibition and to compare them with the findings in OA. Inhibition of MRC with antimycin prevents the normal ability of TGFβ to increase excretion of Pi, thereby worsening deposition of pathologic HA crystals. In chondrocytes, the inhibition of complex IV with NaN₃ modified both the Δψm and the survival of cells inducing apoptosis. Inhibition of complex I with rotenone increases the expression and synthesis of Bcl-2 and Cox-2, both effects are similar effects to produced by IL-1 in human chondrocytes.

Introduction

Principally two forms of joint pathologies can be distinguished, inflammatory arthropathies, such as rheumatoid arthritis (RA) and degenerative arthropathies such as osteoarthritis (OA). Joint pathologies affect approximately 15% of the total US population, approximately 40 million adults, and of these, 16 million (43%) are affected by OA (Scott et al., 1999). In both pathologies the articular cartilage is degradated. However while in RA, the cartilage destruction is the result of an aberrant immune and inflammatory response in the synovium which leads to the degradation of the articular cartilage, in OA a time or age dependent process leads to aberrant cartilage structure which is characterized by reduced number of chondrocytes, loss of existing cartilage extracellular matrix, the production of matrix with abnormal composition and pathologic matrix calcification (Blanco et al., 1998, Bullough, 1984).

Articular human normal cartilage is composed of a hydrated extensive extracellular matrix (ECM) in which a small number of cells—chondrocytes—(2–3% of the total tissue volume) are embedded. Chondrocytes are the only cell type in normal mature articular cartilage and within the tissue are metabolically very active and they normally do not divide after adolescence. They live in an anoxic environment and are believed to carry out their metabolism mainly through anaerobic pathways. Each chondrocyte can be thought of as a metabolically functional unit of cartilage, isolated from neighboring cells but ultimately responsible for the elaboration and maintenance of the ECM. The major component of the ECM of human adult articular cartilage is water (65–70% of the total weight). This water is tightly bound within the ECM due to the physical properties of the macromolecular components of the tissue which are composed of collagens (collagen type II), proteoglycans (aggrecan) and non-collagenous glycoproteins. The concentration and metabolic balance among the various ECM macromolecules and their structural relationships and interactions determine the biochemical properties, and hence the function, of articular cartilage within different joints.

Articular cartilage has no blood vessels or lymphatics, the delivery of nutrients to, and the removal of waste products from, the cells occurs via diffusion through the ECM. There are also no nerve fibers; neural signals are thus not directly transmitted from the tissue. Articular cartilage can be divided into several layers or zones. From the surface inward, these zones are: superficial or tangential zone, the transitional or middle zone, the radial or deep zone and the calcified zone. These zones are composed by different proportions and disposition of cells and ECM proteins (Fig. 1).

Chondrocytes are differentiated mesenchymal cells which play an important role in skeletal development during endochondral bone formation. Endochondral ossification is an ordered process of cartilage formation and resorption. Chondrocytes progress through different phases of differentiation from a proliferative to a hypertrophic phase. Endochondral bone formation is dependent on the formation of matrix vesicles, subcellular membrane-enclosed particles that are critical in the initial deposition of bone mineral. Terminal or hypertrophic differentiation of chondrocytes is physiologically associated with apoptosis. Death of hypertrophic chondrocytes is an important event in matrix calcification and associated with vascular invasion of the cartilage. Chondrocytes in articular cartilage and in growth plate are maintained at distinct stages of differentiation. Chondrocytes in articular cartilage normally do not undergo terminal differentiation as in growth plate, but remain at the pre-hypertrophic stage where they produce the cartilage specific extracellular matrix which is not calcified.