Physical and Biological Regulation of Proteoglycan Turnover around Chondrocytes in Cartilage Explants: Implications for Tissue Degradation and Repair

Abstract

The development of clinical strategies for cartilage repair and inhibition of matrix degradation may be facilitated by a better understanding of (1) the chondrocyte phenotype in the context of a damaged extracellular matrix, and (2) the roles of biochemical and biomechanical pathways by which matrix metabolism is mediated. Using methods of quantitative autoradiography, we examined the cell-length scale patterns of proteoglycan deposition and turnover in the cell-associated matrices of chondrocytes in adult bovine and calf cartilage explants. Results highlight a rapid turnover in the pericellular matrix, which may indicate spatial organization of PG metabolic pools, and specific biomechanical roles for different matrix regions. Subsequent to injurious compression of calf explants, which resulted in grossly visible tissue cracks and caused a decrease in the number of viable chondrocytes within explants, cell-mediated matrix catabolic processes appeared to increase, resulting in apparently increased rates of proteoglycan turnover around active cells. Furthermore, the influences of cell-stimulatory factors such as IL-1 beta appeared to be delayed in their effects subsequent to injurious compression, suggesting interactions between biomechanical and biochemical pathways of PG degradation. These results may provide a useful reference point in the development of in vitro models for cartilage injury and disease, and hint at possible new approaches in the development of cartilage repair strategies.

Matrix and cell injury due to sub‐impact loading of adult bovine articular cartilage explants: effects of strain rate and peak stress

“…Sustained elevated rates of GAG release in the most aggressively loaded explants (Fig. 3) may have indicated a cellular response to matrix injury involving increased rates of matrix turnover and remodelling, as previously observed [21,22]. It may also have reflected increased passive loss of proteoglycan due to mechanical disruption of the matrix.…”

“…The apparatus consisted of a motor controller (PM50O-C, Newport Instruments, Schlieren, Switzerland) and displacement actuator (PM500-1A, Newport) mounted opposite a load cell (Sensotec Model 31, Transmetra, Schafhausen, Switzerland) in a custom built aluminum and stainless steel frame. These peak stresses were chosen because they are close to the high end of the physiological range [I21 and they are similar to those previously found to be associated with cartilage matrix and cell injury in impact [1,10,24] and sub-impact loading studies [21,22]. Preliminary control studies indicated that this range of compression parameters would provide a wide range of severity of injury to osteochondral explants, thereby allowing for the identification of thresholds at which injury occurs.…”

ADAMTS4 Cleaves at the Aggrecanase Site (Glu373-Ala374) and Secondarily at the Matrix Metalloproteinase Site (Asn341-Phe342) in the Aggrecan Interglobular Domain

“…The major silverstained protein isolated by this protocol migrated at ϳ70 kDa and corresponded to the major anti-ADAMTS4 antibody-reactive species present (Fig. 1 [1][2][3][4][5]. This showed, as expected, the gradual appearance of a 65-kDa product with the electrophoretic properties of G1-NITEGE 373 .…”

“…Aggrecan is the major cartilage hyalectan (1), which, together with the collagen network, provides this tissue with its unique mechanical properties of compressibility and stiffness (2)(3)(4). Extraction of aggrecan in its native form (5) and subsequent structural analysis (6) have revealed that the molecular organization of aggrecan is perfectly suited to its central functional role in articular cartilage.…”

Mechanical injury potentiates proteoglycan catabolism induced by interleukin‐6 with soluble interleukin‐6 receptor and tumor necrosis factor α in immature bovine and adult human articular cartilage

“…While the mechanism responsible for the synergistic action between injury and proinflammatory cytokines is unclear, previous studies have shown that the modest loss of GAG observed during the initial 24-hour period after mechanical injury alone was due to mechanical microdamage of the matrix instead of cellmediated proteolysis (45), since inhibitors of aggre- canases, MMPs, and cell biosynthesis could not reduce this immediate GAG loss (9). However, injury dramatically up-regulated expression of the ADAMTS-5 gene (10), and the observed release of GAG during the 2-week period following injury alone was associated in part with aggrecanase activity (23).…”