Chondrocytes in cartilage are responsible for laying down the extensive extracellular matrix that functions as a shock absorber for the underlying bone. The molecular composition of cartilage matrix is predominantly the fibrillar collagen, type II, and the proteoglycan, aggrecan. Until just a few years ago, osteoarthritis (OA) was considered an unavoidable consequence of cartilage aging, where the tissue wears out and degenerates over time.
Traditionally, it was thought that cartilage was unable to repair the extracellular matrix, and consequently, degeneration of the matrix occurred passively. However, when it became possible to tag newly synthesized molecules with radiolabeled proline, Floman and colleagues (1) demonstrated high levels of collagen synthesis in an animal model of OA, and Lippiello and colleagues (2) discovered that human OA cartilage synthesized high levels of collagen. Another leap in our understanding occurred when Aigner and colleagues (3) used the technique of in situ hybridization to messenger RNA (mRNA) to demonstrate that in humans, even at the time of total joint replacement, the cells in OA cartilage were actively trying to rebuild their extracellular matrix (3). At about the same time, enzymes were discovered that could degrade collagen and aggrecan. The first enzymes discovered were matrix metalloproteinases (MMPs). MMP-13 is the primary enzyme that cleaves type II collagen (4). Aggrecans can be cleaved by MMPs (5), but it is currently thought that the enzymes ADAMTS-4 and ADAMTS-5 are responsible for the initial cleavage of aggrecan (6, 7). In fact, mice lacking ADAMTS-5 are not susceptible to injury-induced cartilage degeneration (8).
These discoveries highlight the active nature of the chondrocyte in the degenerative process, and it has been proposed that OA is characterized by a process by which the chondrocytes are stimulated by mechanical events (use or trauma) or genetic assault (mutation of a matrix molecule), and they respond by trying to remove and repair the damaged matrix (9). Often, fragments of molecules that are produced during cartilage degeneration, such as fibronectin (10) or link protein fragments (11), stimulate the cells even more. Unfortunately, these events result in a “vicious circle” that ends when the balance of catabolic events outstrips the anabolic events, leading to rapid degeneration of the cartilage.
The “key events” in the pathology of OA have not yet been completely established. However, we may have reached a point in the study of OA where the events that are now known to occur can begin to be staged. In this issue of Arthritis & Rheumatism, Xu and colleagues (12) present the hypothesis that stimulation of a specific collagen-degrading enzyme, MMP-13, by collagen itself via the discoidin domain receptor 2 (DDR-2) may be a key step in establishing the threshold for irreversible degeneration in both human and mouse OA. In previous studies, this group of investigators has shown that MMP-13 and DDR-2 are specifically and consistently increased in 2 genetic mouse models of early-onset OA, Col9a1–/– mice, which lack the α1-chain of type IX collagen (13), and Col11a1cho/+ mice, which lack the α1-chain of type XI collagen and are heterozygous for chondrodysplasia (cho/+) (14). In addition, they have shown that activation of the DDR-2 collagen receptor via intact collagen results in increased expression of MMP-13 in chondrocytes in vitro (14). In the current study, they extend these findings to human OA cartilage and an additional mouse model of surgically induced OA (12). Thus, they propose that the stimulation of collagen degeneration via DDR-2 is a key event in OA, in that it tips the balance of chondrocyte metabolism to the catabolic side, from which it cannot recover.
Why does collagen stimulate DDR in OA and not in normal cartilage? The answer to this question potentially lies in the structure of the normal chondrocyte extracellular matrix. In normal cartilage, DDR does not “see” collagen fibrils; this is likely due to protection of the receptor by proteoglycans surrounding the cell. In OA, according to the hypothesis of Xu and colleagues, removal of proteoglycans unmasks collagen fibrils that can in turn bind to the DDR-2. The binding to the DDR-2 leads to an increase in the expression of the receptor itself as well as of MMP-13. The genetic mouse models used in previous studies by Xu and colleagues (13, 14) demonstrated that proteoglycans are depleted from the matrix prior to increases in MMP-13 and DDR-2. In the current study, they used a surgically induced mouse model and found that immunodetectable DDR-2 and MMP-13 were increased when the proteoglycan degradation became evident, at 4 weeks after surgery. This hypothesis is supported by clinical studies that have suggested that the proteoglycan degradation occurs prior to the breakdown of type II collagen in articular cartilage (15, 16). The study with human OA tissue by Xu and colleagues (12) supports the clinical findings and the mouse models. Other studies have suggested that aggrecan actually protects collagen from degradation by blocking proteolytic access to the collagen fibril or access of cytokines and proteases from the synovial fluid (17). Thus, there is a consensus that loss of aggrecan is an early event in OA, and its destruction could be considered a key event in the pathogenesis of OA. However, this early step is still potentially reversible.
The study of OA has been greatly restricted by the inability to investigate the progression of the disease over time. Classically, OA has been studied in 2 ways: in human cartilage obtained at the time of joint replacement surgery and in animal models of cartilage degeneration. The animal models fall into 3 broad categories: spontaneous degeneration of unknown, but genetic, origin; degeneration due to a mutation in a cartilage extracellular matrix gene; and surgically induced instability, which leads to degeneration. No one really knows if any of these animal models mimic human OA, but accumulating evidence indicates that many aspects of human OA can be demonstrated in the mouse models. Suffice it to say that the mouse models do lead to cartilage degeneration, and the process can be studied over time. At the other extreme, human cartilage obtained at the time of joint replacement can be considered to represent end-stage disease, but often it, provides enough material for analyses. In normal adult cartilage, very little cellular activity is necessary to maintain cartilage structure. One of the remarkable features of OA uncovered by studies of end-stage OA has been the increase in metabolic activity of the chondrocytes. This has been well documented, first, by in situ hybridization to mRNA (matrix molecules and enzymes) and second, by gene array analyses (18). This hyperanabolism has become a hallmark of OA cartilage. The overriding assumption made is that the hyperanabolism is an attempt to repair the tissue that eventually fails.
In order to study human cartilage at earlier stages of OA, some investigators have begun to use human cartilage obtained from tissue banks or at the time of autopsy of a donor who had not been diagnosed as having a history of OA (i.e., the presence of osteophytes and joint space narrowing). This “normal” or “asymptomatic” cartilage can be graded for degeneration, and if some degeneration is present (grade 2–3 on the Mankin scale), the cartilage is considered to represent early OA. In other studies, cartilage with different stages of degeneration taken from the same joint has been used to represent “early” (i.e., intact cartilage) and “late” (i.e., fibrillated cartilage) OA.
Cole, Poole, and colleagues have published a number of studies of knee and ankle cartilage obtained from a tissue bank and have identified early stages of OA in the samples. In a recent study, they examined 2 markers of anabolism as well as markers of degradation (19). Using the anabolic markers, the C-propeptide of collagen and epitope 846 of aggrecan, they found that the ankle OA cartilage was more reactive than the knee OA cartilage at the same early stage of disease. These studies indicated that there may be differences between cartilage sources and that cartilage from the ankle may repair more readily than cartilage from the knee. Thus, at these early stages of the disease, it is possible to begin to study the potential repair process separately from the degenerative process. In a separate set of studies, this same group of investigators examined knee lesions with a greater degree of degeneration and found increases in type II collagen synthesis, aggrecan turnover, and markers of degeneration (20). While these studies are not definitive, they do indicate that changes in anabolism occur over the course of OA progression, prior to the point at which joint replacement becomes necessary.
Other efforts have been made to establish the sequence of events in OA. Lorenzo and colleagues (21) compared the rate of biosynthesis of cartilage molecules in tissue obtained during early and late stages of OA. They looked at the synthesis and content of extracellular noncollagenous matrix macromolecules in early and late-stage human OA cartilage obtained at the time of surgery for sarcomas in the lower extremities (designated “normal” and “early OA”) or for total knee replacement (late-stage OA). The early OA samples were further subdivided into those with some fibrillation or those showing no fibrillation, as determined by visual assessment. They found that compared with normal cartilage, there were increases in the synthesis of cartilage oligomeric protein, fibronectin, and cartilage intermediate-layer protein in both early and late-stage OA cartilage samples. Collagen synthesis appeared to be significantly increased only in the late stage of OA. The altered composition and the pattern of biosynthesis indicate that the joint undergoes metabolic alterations early in the disease process, even before there is overt fibrillation of the tissue. Furthermore, the early, nonfibrillated samples showed a generally lower level of matrix synthesis than did the fibrillated samples.
Taken together, the biosynthetic studies strongly suggest that there is a change in the metabolism of chondrocytes that may effect the progression of OA. At this time, there has not been enough work done to sort out when and where the cells may be making the proper matrix for repair or whether some populations of chondrocytes can effect repairs better than others. The sequence of changes in biosynthesis, however, indicates that there may be differences in the synthesis of matrix components in OA cartilage as compared with normal cartilage. These differences could result in the laying down of matrix that is less structurally sound in OA cartilage as compared with normal cartilage, thus contributing to the perpetuation of degeneration.
In addition to matrix synthesis, cell stimulation also leads to an increase in enzymes that are capable of degrading extracellular matrix components. Temporally, aggrecan is one of the first macromolecules to undergo measurable loss in arthritis (22). This observation implies that the synthesis and activation of enzymes necessary to cleave aggrecan precede actual cleavage of the proteoglycan. Thus, the key events in OA can begin to be sorted out, along with establishment of the temporal and molecular stages of OA. Clearly, the cascade of events will generally follow in this order: cell stimulation (increase in matrix synthesis as well as enzyme synthesis), proteoglycan degradation, and collagen degradation. Commitment to any of these events could be considered key to the pathogenesis, but the event that tips the balance from attempted repair to matrix loss may indeed be collagen stimulation of its own degradation, as suggested by Xu and colleagues (12).