The regulation of collagen (I) homotrimer synthesis and its role in musculoskeletal dysfunction

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Musculoskeletal diseases such as osteoarthritis, osteoporosis and soft-tissue injuries together with tissue and organ fibrosis impose a huge healthcare burden, particularly in the ageing population. Therefore understanding the regulation of abnormal collagen (I) synthesis and its role in bone and joint function is of critical importance in order to develop strategies to target these debilitating diseases.Collagen (type I) is the most abundant structural protein in the body and is the major component of bone and joint tissues. Type I collagen forms fibres that surround cells and make tissues resilient to mechanical loading. The natural form of type I collagen can be degraded and reformed by cells using biological enzymes and this process allows skeletal tissues to adapt to changes in mechanical loading. When the tissue structure is inadequate to resist external loads, tissue injury including fractures and ruptures can occur. In the general population such problems are manifested as diseases including osteoporosis (weak bone), osteoarthritis (cartilage loss and bone overgrowth) and soft tissue injuries. Over-production of type I collagen (termed fibrosis) furthermore restricts tissue function leading to disability and increased morbidity and mortality. Genetic and biochemical studies have found that an abnormal form of type I collagen, termed collagen (I) homotrimer, is present in both degenerative and fibrotic diseases. This abnormal collagen alters the biophysical properties of collagen fibrils and is resistant to enzymatic breakdown. Collagen (I) homotrimer may therefore affect the ability of tissues to respond to changing mechanical loads and to counteract fibrosis. The aim of this project is to determine whether relative collagen (I) mRNA (COL1A1 and COL1A2) levels direct collagen (I) homotrimer synthesis and if collagen (I) homotrimer produces an inadequate but persistent fibrillar matrix that leads to age-related musculoskeletal disease and fibrosis, or whether pathology could be accounted for by cellular stress caused by over-production of the collagen alpha-1(I) chain.A complex series of cellular interactions normally results in collagen (I) heterotrimer but this project will test the hypothesis that increased levels of the COL1A1 mRNA overwhelms the ability of the cells to control heterotrimer synthesis and results in the concurrent production of abnormal collagen (I) homotrimer. Cells contain an elaborate system of controls that regulate gene activity and protein production and small RNA molecules termed microRNAs appear to be particularly important. MicroRNAs bind to the mRNA intermediates (between gene activity and protein production) often decreasing their effectiveness. miR-133 is known to target COL1A1 rather than COL1A2 and is less abundant in several fibroses. This project will test how increasing or decreasing miR-133 activity affects collagen (I) homotrimer synthesis and will determine whether it could be a novel target for musculoskeletal and fibrotic diseases.To reveal how collagen fibrils containing collagen (I) homotrimer affect musculoskeletal tissues a comprehensive analysis of the structural and biomechanical alterations in hard and soft collagenous tissues will be performed in mice lacking the COL1A2 gene. Preliminary phenotyping data (IMPC) indicates that these mice have abnormal bone morphology and defects in soft collagenous tissues. A well-characterised osteogenesis imperfecta mouse model ('oim') that produces collagen (I) homotrimer along with truncated alpha-2(I) chains will be used as a control. The apparently more severe oim phenotype may result from cellular stress, therefore cellular stress will be evaluated in genetically manipulated cell cultures and mouse tissues. Cellular stress can be targeted by several pharmaceuticals so could potentially be reduced to help treat these diseases.