The role of integrin mediated adhesion and signaling during Xenopus retinal development
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Integrins are the major family of cell adhesion receptors for extracellular matrix (ECM) components and are involved in cell-cell and cell-ECM adhesion. Integrins also participate in cytoskeletal rearrangements, activation of signal transductions, and co-regulation of growth factor activities.;In this dissertation, I have investigated the role of beta1 integrin-mediated adhesion and signaling during early Xenopus retinal development. First, I characterized the expression patterns of focal adhesion associated proteins, beta1 integrin, talin, vinculin, paxillin and phosphotyrosine in retinal glial cell line XR1 and in the developing Xenopus retina. These proteins are colocalized at focal adhesions located at the termini of F-actin filaments in XR1 cells. These proteins display spatial and temporal expression patterns, which are related with specific morphologies during retinal development. The similar and differential expression patterns suggest that focal adhesion proteins are involved in integrin-mediated adhesion and signaling, and are likely to be essential in regulating retinal morphogenesis. Second, I tested the hypothesis that protein tyrosine kinase (PTK) activity is essential for Xenopus retinal development. Inhibition of tyrosine kinase activity blocked focal adhesion assembly in cultures of dissociated retinal neuroepithelial cells and XR1 glial cells. Application of PTK inhibitors to embryonic retina disrupted retinal lamination and photoreceptor morphogenesis. The expression patterns of phosphotyrosine and beta1 integrins were also disrupted. Finally, I directly blocked integrin-ECM interactions with a disintegrin echistatin and observed the effects of the blockage on the retinal development. Echistatin inhibited XR1 glial cell attachment to fibronectin substrates, and also blocked cell spreading and focal adhesion assembly in XR1 cells. In addition, echistatin reduced the tyrosine phosphorylation levels of paxillin. Furthermore, application of echistatin to embryonic retina disrupted the retinal lamination and induced rosette structures with ectopic photoreceptors in the outer retina.;Taken together, these results provide evidence that integrins are likely to play an important role in selective cell-ECM interactions, cell migration, differentiation and neurite outgrowth during retinal development. Focal adhesion associated proteins and tyrosine kinase activities are involved in the modulation of integrin-mediated adhesion and signal transduction pathways. This research contributes to our understanding of the functions and mechanisms of integrin-mediated adhesion during neural development.