The role of chondrocyte senescence in the pathogenesis of canine osteoarthritis

The aims of this study were to (1) evaluate cellular senescence in chondrocytes from osteoarthritic articular cartilage, (2) investigate the hypothesis that oxidative stress is a feature of canine OA chondrocytes and that oxidative stress contributes to cellular senescence in canine chondrocytes, (3) investigate the hypothesis that osteoarthritic chondrocytes alter the gene expression of adjacent normal chondrocytes in OA joints leading to modulation of genes known to play a role in the pathogenesis of OA and (4) evaluate the presentation of dogs undergoing femoral head excision in veterinary referral practice in the UK as a treatment for osteoarthritis of the coxofemoral joint, and to categorise the distribution and severity of associated pathological lesions. Chondrocytes from osteoarthritic and normal cartilage were examined for levels of senescence. Initially chondrocytes were cultured using an alginate bead culture system, thought to mimic the extracellular matrix of articular cartilage. However, these chondrocytes showed almost no growth as compared to monolayer culture where they grew rapidly. OA chondrocytes entered the senescent state after 1.5 to 4.9 population doublings in monolayer culture, while normal chondrocytes underwent 4.8 to 14.6 population doublings before entering the senescent state. Osteoarthritic chondrocytes had increased levels of markers of cellular senescence (senescence associated beta-galactosidase accumulation and p16 protein accumulation) as compared to normal chondrocytes, suggesting that chondrocyte senescence is a feature of canine osteoarthritis. An experimental model for the induction of oxidative stress in chondrocyte cell culture was developed using tert-Butyl hydroperoxide and total cellular glutathione was measured as an indicator of cellular oxidative stress levels. Experimental induction of oxidative stress in both normal and osteoarthritic chondrocytes in cell culture resulted in increased amounts of cellular senescence, shown by an increase in levels of senescence associated beta-galactosidase accumulation and decreased replicative capacity. Experimental induction of oxidative stress also resulted in altered gene expression of three genes important to the degradation of the extracellular matrix; MMP-13, MMP-3 and Col-3A1, measured by RT-PCR, in normal canine chondrocytes in monolayer cell culture. MMP-3 showed the greatest relative expression change, with a fold-change of between 1.43 and 4.78. MMP-13 had a fold change of 1.16 to 1.38. Col-3A1 was down regulated, with a fold-change of between 0.21 and 0.31. These data demonstrate that experimentally induced oxidative stress in chondrocytes in monolayer culture increases levels of cellular senescence and alters the expression of genes relevant to the pathogenesis of canine OA. Coculture of osteoarthritic chondrocytes with normal canine chondrocytes resulted in gene modulation in the normal chondrocytes. Altered gene expression of ten genes known to play a role in the pathogenesis of osteoarthritis was detected in the normal chondrocytes (fold change shown in brackets); TNF-alpha (11.95), MMP-13 (5.93), MMP-3 (5.48), IL-4 (7.03), IL-6 (5.3), IL-8 (4.92), IL-F3 (4.22), COL-3A1 (4.12), ADAMTS-4 (3.78) and ADAMTS-5 (4.27). In total, 594 genes were significantly modulated suggesting that osteoarthritic chondrocytes contribute to the disease propagation by altering the gene expression of adjacent normal chondrocytes, thus recruiting them into the disease process. Gene expression changes were measured by microarray analysis and validated by RT-PCR and Western blot analysis. An epidemiological study of femoral heads collected from dogs undergoing total hip replacement surgery as a treatment for osteoarthritis of the coxofemoral joint secondary to canine hip dysplasia revealed that there was no characteristic pattern of cartilage lesion for canine hip dysplasia. Severe pathology of the femoral head with cartilage erosion occurred in 63.9% of cases and exposure of subchondral bone in 31.3% of cases. The work presented in this thesis has demonstrated that cellular senescence is a feature of chondrocytes from canine osteoarthritic cartilage and suggests that cellular senescence and oxidative stress play an important role in the pathogenesis of osteoarthritis in dogs.