OBJECTIVE: We and others previously demonstrated that Sirt1 regulates apoptosis and cartilage-specific gene expression in human chondrocytes and mouse models. To determine if Sirt1 enzymatic activity plays a protective role in cartilage homeostasis in vivo, we investigated mice carrying sirt1 mutations to characterize their cartilage.
METHOD: Articular cartilage was harvested from 5- and 6-month-old mice paws and knees, homozygous for wild type or sirt1(tm2.1Mcby) (sirt1(y/y) ), an allele carrying a point mutation that encodes a Sirt1 protein with no enzymatic activity. Mice aged 2 days and 6/7 days were also examinated. Joint cartilage was processed for histologic examination or biochemical analyses of chondr... More
OBJECTIVE: We and others previously demonstrated that Sirt1 regulates apoptosis and cartilage-specific gene expression in human chondrocytes and mouse models. To determine if Sirt1 enzymatic activity plays a protective role in cartilage homeostasis in vivo, we investigated mice carrying sirt1 mutations to characterize their cartilage.
METHOD: Articular cartilage was harvested from 5- and 6-month-old mice paws and knees, homozygous for wild type or sirt1(tm2.1Mcby) (sirt1(y/y) ), an allele carrying a point mutation that encodes a Sirt1 protein with no enzymatic activity. Mice aged 2 days and 6/7 days were also examinated. Joint cartilage was processed for histologic examination or biochemical analyses of chondrocyte cultures.
RESULTS: We found that articular cartilage tissue sections from sirt1(y/y) mice, up to 6 months of age, contained reduced levels of type 2 collagen, aggrecan, and glycosaminoglycan. In contrast, protein levels of MMP-8, MMP-9, and MMP-13 were elevated in the sirt1(y/y) mice cartilage. Additional results showed elevated chondrocyte apoptosis in Sirt1 mutant mice as compared to wild type littermates. In line with these observations, PTP1b (protein tyrosine phosphatase b) was elevated in the sirt1(y/y) mice.
CONCLUSION: Our in vivo findings from this animal model demonstrated that mice with defective Sirt1 also had defective cartilage with elevated rates of cartilage degradation with age. Hence, normal cartilage homeostasis requires enzymatically active Sirt1 protein. ? 2012 American College of Rheumatology.
