Osteopontin structure and function

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Osteopontin structure and function

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Title: Osteopontin structure and function
Author: Smith, Laura Lee
Abstract: Osteopontin is an adhesive glycoprotein implicated in numerous diseases associated with inflammation and remodeling. There are several structural domains in osteopontin of particular interest. The RGD motif is a cell-attachment sequence shown to be critical for cell adhesion through $\alpha\sb{\rm v}$-containing integrins. In close proximity to the RGD domain is the thrombin cleavage site. Previous observations suggest that thrombin cleavage of osteopontin occurs in vivo and may be physiologically important. To study the functional significance of osteopontin cleavage by thrombin, we made osteopontin fusion proteins that contain either the N- or C-terminal domains expected to be formed following thrombin cleavage. We compared these fragments with native osteopontin in their ability to support adhesion of several cell lines, and identified the receptors mediating these interactions. Our data show that the N-terminal fragment supported adhesion of a melanoma cell line unable to bind native osteopontin, suggesting that osteopontin contains a cryptic binding activity. The receptor was identified as the $\alpha\sb9\beta\sb1$ integrin: a novel osteopontin receptor. In addition to adhesion, we show that $\alpha\sb9\beta\sb1$ can mediate cell migration, a function not previously identified for this integrin. To determine the domain important for $\alpha\sb9\beta\sb1$ interactions, we made mutations in the RGD region of the osteopontin fragment. Mutation of RGD to RAA, or eliminating the RGD completely, failed to support cell adhesion and migration, suggesting that the RGD domain was critical for this interaction. In contrast, $\alpha\sb9\beta\sb1$-mediated adhesion to tenascin was RGD-independent. These data demonstrate that $\alpha\sb9\beta\sb1$ is one of the few integrin receptors that can interact with two distinct RGD-containing ligands through different adhesive domains.CD44, a non-integrin, multifunctional adhesion molecule was recently identified as an osteopontin receptor. To analyze which forms of CD44 bind to osteopontin, we used a variety of CD44-immunoglobulin fusion proteins in enzyme-linked immunosorbant assays. Our data show that although the CD44-hIg fusion proteins could interact with hyaluronic acid as expected, there was no interaction between CD44H, CD44E, CD44v3,v8-v10, or CD44v3 with osteopontin. These studies suggest that CD44-osteopontin interactions may not be common in vivo and may be limited to a specific CD44 isoform(s), and/or a particular modified form of osteopontin.
Description: Thesis (Ph. D.)--University of Washington, 1998
URI: http://hdl.handle.net/1773/6322

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