Matrix Organization and Dimensionality

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Cells interact with biochemically and structurally distinct forms of extracellular matrix in different tissues, at different stages of embryonic development, and during adult wound repair. This project focuses on addressing major questions concerning the mechanisms of these cell-extracellular matrix interactions. For example, what unique mechanisms do different types of mammalian cells use to adhere to and to migrate through different three-dimensional (3D) extracellular matrix environments compared to flat (2D) cell culture substrates, or even 1D fibrillar substrates? We are currently exploring the cell-matrix adhesions formed to 2D compared to 3D fibrillar extracellular matrix under dynamic conditions. An extracellular matrix molecule that has been studied in considerable depth in regular 2D cell culture is fibronectin. We and others previously described two distinct mechanisms for assembly of fibrillar fibronectin, one involving lateral translocation of the alpha5-beta1 integrin along flat tissue culture substrates, and a second mechanism involving the translocation of entire focal adhesions along a basement membrane substrate driven by an actomyosin "contractile winch" system. We have now initiated studies of the mechanisms of fibronectin matrix assembly in a 3D fibrillar collagen matrix. We are also exploring in further depth our previous observation that there is an unusually high level of integrin activation in a 3D fibrillar collagen environment, e.g., what role actomyosin contractility plays in this activation state. The assembly of basement membranes occurs at the basal surfaces of epithelial cells in embryos adhering to their relatively 2D basement membrane substrates. In Drosophila and adult salivary glands, the machinery for synthesis and secretion of basement membrane collagen is at the basal end of the cell close to the site of assembly. In embryonic mouse salivary glands, however, mRNA for collagen IV and laminin is at the opposite, apical end of epithelial cells, along with the endoplasmic reticulum and Golgi apparatus. We are investigating the mechanisms by which these important basement membrane components are transported and secreted appropriately at the basal ends of embryonic mammalian organs. A key mechanism appears to be transport via the microtubule cytoskeleton. These studies are providing insights into the importance of dimensionality in the interactions of cells with the extracellular matrix. They can range from a "1D" interaction as a cell interacts with a single bundle of collagen, to 2D on basement membranes and flat substrates, to 3D in native interstitial extracellular matrices. Understanding the differing nature of these interactions may provide insights useful for rational tissue engineering.