Differential tissue specific, temporal and spatial expression patterns of the Aggrecan gene is modulated by independent enhancer elements

The transcriptional mechanism through which chondrocytes control the spatial and temporal composition of the cartilage tissue has remained largely elusive. The central aim of this study was to identify whether transcriptional enhancers played a role in the organisation of the chondrocytes in cartilaginous tissue. We focused on the Aggrecan gene (Acan) as it is essential for the normal structure and function of cartilage and it is expressed developmentally in different stages of chondrocyte maturation. Using transgenic reporter studies in mice we identified four elements, two of which showed individual chondrocyte developmental stage specificity. In particular, one enhancer (-80) distinguishes itself from the others by being predominantly active in adult cartilage. Furthermore, the -62 element uniquely drove reporter activity in early chondrocytes. The remaining chondrocyte specific enhancers, +28 and -30, showed no preference to chondrocyte type. The transcription factor SOX9 interacted with all the enhancers in vitro and mutation of SOX9 binding sites in one of the enhancers (-30) resulted in a loss of its chondrocyte specificity and ectopic enhancer reporter activity. Thus, the Acan enhancers orchestrate the precise spatiotemporal expression of this gene in cartilage types at different stages of development and adulthood.