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Allan F. Wiechmann, Stephen C. Pflugfelder, Eric Howard; Diurnal Expression Of Matrix Metalloproteinases In The Apical Corneal Epithelium Of Xenopus Laevis: A Potential Regulatory Mechanism For Cyclic Desquamation. Invest. Ophthalmol. Vis. Sci. 2011;52(14):303.
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© ARVO (1962-2015); The Authors (2016-present)
It is hypothesized that matrix metalloproteinases (MMPs) are secreted by the corneal epithelium (CE) on a diurnal rhythm for the controlled extracellular cleavage of junctional complexes to enable the surface cells to detach and shed. The purpose of this study was to determine if MMPs are expressed in surface cells of the Xenopus laevis CE, and whether this expression fluctuates on a diurnal basis.
Xenopus corneas were processed for immunocytochemistry using antibodies against MMP-2, a tissue inhibitor of MMP (TIMP-2), and the Mel1a melatonin receptor. In situ zymography was performed to determine the pattern of MMP activity in the surface CE cells. Real time RT-PCR was performed to determine if MMP or TIMP expression is altered in response to time of day or melatonin treatment. Gel zymography was performed to assess the effect of melatonin on MMP activity.
MMP-2 immunoreactivity was localized to the two most superficial layers of the CE and was higher in the dark than in the light period. MMP-2 and TIMP-2 were co-localized to the same cells in the surface layer of the CE and TIMP-2 was elevated early in the light period. Endogenous MMP activity in the surface CE was lower during the light period and was blocked an MMP inhibitor. Dark-adapted corneas showed increased expression of MMP-9, but not MMP-2 RNA, compared to light adapted levels. Mel1a immunoreactivity was localized to the lateral membranes of the surface CE, and was located in the same cells that expressed MMP-2. Melatonin treatment resulted in increased corneal MMP activity.
MMP and TIMP expression and activity are uniquely localized to the apical layers of the Xenopus CE, and may exhibit diurnal changes. Higher MMP expression at night may potentially cause cleavage of surface cell junctional proteins to enable the surface desquamation that may occur early in the light period. Results of these experiments provide support for the hypotheses that circadian signals influence cyclic changes in MMP expression for the controlled extracellular cleavage of junctional complexes to enable daily CE surface cell desquamation.
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