The alpha5beta1 integrin as mechanotransducer in chondrocytes

Mechanical loading has been proposed as a mechanism for improving the functionality of engineered cartilage tissue. However, the precise mechanotransduction pathways are unknown. Recent studies have shown that integrins can act as mechanoreceptors in articular cartilage. In this study, we examined the role of integrin a5ß1 in the anoregulation of both ECM gene expression and ECM protein synthesis in tissue-engineered hondrocyte/agarose constructs. Chondrocytes, isolated and seeded in 3% agarose constructs, were compressed at 0.33 and 1 Hz, in the presence or absence of GRGDSP (a blocker for integrin a5ß1). mRNA levels for aggrecan, collagen II and MMP-3 were determined at several time points up to 24 hours post-stimulation. Cell viability, DNA and sGAG content were determined at several time points up to 28 days post-stimulation. mRNA levels for all genes were upregulated upon loading, except for collagen II, loaded at 0.33 Hz. Incubation with GRGDSP counteracted upregulation. sGAG levels were significantly lower in constructs loaded at 0.33 Hz compared to the unstrained control at day 28. In contrast, loading at 1 Hz caused a significant increase in sGAG deposition at this timepoint, which was counteracted by blocking the a5ß1 integrin. We conclude that the a5ß1 integrin acts as a mechanotransducer in the regulation of both ECM gene expression and matrix biosynthesis for chondrocytes seeded in agarose under the loading regimes applied. However, this regulation appears frequency dependent.