The Extracellular Matrix: What is it?
The extracellular matrix (ECM) is a complex, three-dimensional network comprised of collagens, glycoproteins, and proteoglycans that provides an adhesive substrate for the organization of cells into tissues. In the adult, dynamic interactions between cells and their surrounding ECM proteins regulate most, if not all, cell and tissue functions. ECM proteins also contribute to the mechanical and permeability properties of the skin, vasculature, lungs, and other organs. Fibronectin is a principal component of the ECM, where it is organized into elongated, branching fibrils. Mechanical forces influence the deposition, organization, and structure of ECM fibronectin fibrils, which in turn, affect cell function, ECM organization and stability, vascular perfusion, blood vessel permeability, and tissue strength. Fibronectin is initially secreted by cells in a soluble, protomeric form. Soluble fibronectins are then polymerized into ECM fibrils by means of a tightly-regulated, cell-dependent mechanism that can be rapidly up- or down-regulated. Fibronectin matrix polymerization is increased in response to tissue injury; altered fibronectin matrix deposition is associated with abnormal tissue repair in a number of chronic inflammatory states, including non-healing wounds, asthma, pulmonary fibrosis, and atherosclerosis. In the body, fibronectin polymerization is a continuous process, with as much as 50% of the fibronectin matrix undergoing turnover every 24 hours. In spite of this remarkable information, the mechanisms of fibronectin matrix polymerization and the function of ECM fibronectin fibrils in vivo is not well understood.
Studies from the Hocking Lab: What have we learned about fibronectin?
Research in the Hocking lab has been directed at determining whether and how this three-dimensional fibronectin matrix regulates cell behaviors that are essential to tissue repair. Our studies have firmly established that cells respond differently to soluble versus ECM fibronectin, and that the ECM form is the primary functional form of fibronectin. We have shown that the conversion of fibronectin into the ECM form specifically stimulates cell spreading, cell migration, and collagen matrix contraction – all cellular events critical to tissue regeneration. We have also shown that the assembly of a fibronectin matrix promotes the co-polymerization of collagen I and enhances the tensile strength of model tissue, thus demonstrating that the organization and functional properties of the ECM are also dependent on fibronectin matrix polymerization. We have localized several of the ECM-specific effects of fibronectin to a single, cryptic heparin-binding site in the first type III module of fibronectin (FNIII-1). This conformation-dependent, or "matricryptic" site is not exposed in soluble fibronectin, but becomes unmasked during fibronectin matrix formation or as cells and tissues exert tension on ECM fibronectin fibrils. Our exciting new studies, conducted in collaboration with the Sarelius Lab, demonstrate that exposure of this matricryptic site in vivo mechanically couples skeletal muscle contraction with local vasodilation. These were the first studies to demonstrate a role for fibronectin fibrils in adult connective tissue in vivo.
Current Research in the Hocking Lab: What questions are we now asking?
Current studies are aimed at understanding how the expression of the matricryptic site in fibronectin fibrils is regulated under both normal and pathological conditions, for example in chronic diabetic wounds, and during abnormal pulmonary matrix remodeling in response to cigarette smoke. In collaboration with the Dalecki Lab, we are also investigating the effects of therapeutic ultrasound on the organization and function of the extracellular fibronectin matrix, as a potential therapeutic approach to wound healing and tissue engineering. Please see below to learn more about ongoing research projects in the Hocking lab.