Fibroblasts and other eukaryotic non-muscle cells exert substantial forces on their extracellular matrix protein environment in the course of normal physiological processes such as wound healing and embryogenesis. Such forces are thought to mediate the orderly organization, contraction, and condensation of fibrillar collagens in tissue and, as such, are likely to be essential to normal development and homeostasis. However, dysregulated force generation by fibroblasts can lead debilitating contractures of scar tissue; for example, after burn injury or as a consequence of silicone breast implant leakage. Similar dysregulafion of cell mediated force may play a role in fibrocontractive diseases such as hepatic cirrhosis, pulmonary fibrosis, and even scleroderma. In vitro models that approximate some elements of cell mediated force generation have been developed. If appropriately stimulated, fibroblasts are able to generate substantial forces when suspended in artificial gels containing type I collagen. These cells organize and contract such gels, a process that has been extensively studied as an in vitro correlate of the contraction phase of normal wound healing as well as the exaggerated tissue contraction in fibrocontractive diseases. Although fibroblasts have multiple integrin and non-integrin receptors for type I collagen, we have demonstrated that the process of gel contraction is dependant upon the expression and function of only one: alpha2beta1 integrin. The central role of alpha2beta1 integrin in the generation of force has now been extended to complex normal tissues containing multiple matrix proteins: we have shown that contraction of vitreous explants (a biological gel containing type II collagen) is also mediated by this integrin, a finding that has relevance to a class of sight threatening human diseases characterized by cell mediated vitreous contraction and tractional retinal detachment. To better understand the molecular basis of the important and general biological process of cell mediated force generation, we propose to: i) measure quantitatively forces generated in collagen gels to define more precisely the relative role of alpha2beta1 in isometric force generation, and attempt to interfere with force generation using specific blocking antibodies and peptides; 2) identify the requisite elements of the short cytoplasmic domain of the alpha2 chain that we have shown is critical for collagen gel contraction; and 3) study the role of alpha1beta1, a structurally similar but functionally distinct integrin on fibroblasts that may competitively inhibit force generation mediated by alpha2beta1. Understanding how alpha2beta1 is involved in the generation of force and how this process may be antagonized by exogenous and endogenous factors will help define new approaches to the study and treatment of fibrocontractive disease and wound healing.
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