Systems biology reveals how altered TGFβ signalling with age reduces protection against pro-inflammatory stimuli

Osteoarthritis (OA) is a degenerative condition caused by dysregulation of multiple molecular signalling pathways. Such dysregulation results in damage to cartilage, a smooth and protective tissue that enables low friction articulation of synovial joints. Matrix metalloproteinases (MMPs), especially MMP-13, are key enzymes in the cleavage of type II collagen which is a vital component for cartilage integrity. Transforming growth factor beta (TGFβ) can protect against pro-inflammatory cytokine-mediated MMP expression. With age there is a change in the ratio of two TGFβ type I receptors (Alk1/Alk5), a shift that results in TGFβ losing its protective role in cartilage homeostasis. Instead, TGFβ promotes cartilage degradation which correlates with the spontaneous development of OA in murine models. However, the mechanism by which TGFβ protects against pro-inflammatory responses and how this changes with age has not been extensively studied. As TGFβ signalling is complex, we used systems biology to combine experimental and computational outputs to examine how the system changes with age. Experiments showed that the repressive effect of TGFβ on chondrocytes treated with a pro-inflammatory stimulus required Alk5. Computational modelling revealed two independent mechanisms were needed to explain the crosstalk between TGFβ and pro-inflammatory signalling pathways. A novel meta-analysis of microarray data from OA patient tissue was used to create a Cytoscape network representative of human OA and revealed the importance of inflammation. Combining the modelled genes with the microarray network provided a global overview into the crosstalk between the different signalling pathways involved in OA development. Our results provide further insights into the mechanisms that cause TGFβ signalling to change from a protective to a detrimental pathway in cartilage with ageing. Moreover, such a systems biology approach may enable restoration of the protective role of TGFβ as a potential therapy to prevent age-related loss of cartilage and the development of OA.Author summary: Osteoarthritis (OA) is a debilitating disease that is a consequence of cartilage degeneration, often for a variety of reasons. Age is typically the driving force behind OA, and one reason is that biological pathways change as we get older and can become damaging. Transforming growth factor beta (TGFβ) is one such pathway, which in young tissues has an important role in keeping the joint healthy as well as protecting against damage caused by inflammation. As we age this pathway starts to promote damage. However, how its effect on inflammation changes has not yet been studied in detail. We used a range of experimental and computational techniques to explore how TGFβ helps to protect against inflammation, but also how these interactions change as we age. We found that with ageing TGFβ may lead to a prolonged inflammatory response in older individuals, resulting in more damage than it normally would in a younger individual suggesting a once protective pathway can now lead to prolonged damage. A better understanding of this change may allow us to develop drugs to repair this pathway, and re-establish its once protective effect.