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R. Maddala, P. V. Rao; Enrichment of Rap 1 GTPase at Cell-Cell Junctions of the Lens Fibers. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4766.
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Cell-cell junctions are recognized as being critical for symmetric packing of lens fiber cells and lens transparency, however, little is known about the regulation of cell-cell junctions and cell adhesive interactions of lens fibers. In this study, to explore our hypothesis that Rap1 GTPase may play a critical role in formation of the lens fiber cell adherens junctions, we determined the spatial distribution profile of Rap1 in lens fibers.
Mouse lens saggital and equatorial cryosections, and lens epithelial explants were immunostained for Rap1 GTPase, N-cadherins, beta-catenin and F-actin, in conjunction with confocal fluorescence imaging analysis to determine the co-distribution of Rap1 with cell junctional proteins. Expression of Rap1 activating proteins including cAMP activated nucleotide exchange factors (Epac1 and Epac2) was determined in the mouse lens by RT-PCR analysis. The effects of Rap1 activation by Epac on cell-cell junctions was evaluated using cultured lens epithelial cells.
Rap1 GTPase immunostaining exhibited a distinct localization to the lens fiber cell lateral membrane and cell-cell junctions in mouse lens cryosections and epithelial explants. Rap1 colocalized perfectly with N-cadherin, beta-catenin and F-actin at vertices and along the long and short sides of the hexagonal lens fiber cells. Epac1 and Epac2, the activators of Rap1 were confirmed to be expressed in the mouse lens. Importantly, activation of Epac in lens primary epithelial cells by the cAMP analog 8-pCPT-2'-O-methyl-cAMP (O-Me-cAMP) induced Rap1 relocalization to the cell-cell junctions and increased the formation of adherens junctions.
Unlike the Rho and Rac GTPases, Rap1 GTPase was found to distinctly localize at the cell-cell junctions of the lens fibers. Moreover, Rap1 co-distributed with N-cadherin, beta-catenin and F-actin at the lens fiber cell-cell junctions indicating a potential role for Rap1 in regulation of lens fiber cell adherens junction formation and organization. This study also uncovers the potential significance of the cAMP/Epac/Rap1 signaling pathway in regulation of cell-cell junctions in the lens tissue.
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