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Abby L. Grabitz, Melinda K. Duncan; Sip1 Function in the Lens is Different than That Reported in Other Cell Types. Invest. Ophthalmol. Vis. Sci. 2011;52(14):2774.
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© ARVO (1962-2015); The Authors (2016-present)
Sip-1, or Smad interacting protein 1, is a member of the Zeb protein family and is a known binding partner of Smad proteins that co-regulate TGFB induced epithelial-mesenchymal transition. Deletion of Sip-1 early in eye development demonstrated that it is critical for the separation of the lens vesicle from the head ectoderm. In non-ocular tissues, Sip1 appears to repress epithelial specific gene expression while activating mesenchymal gene expression. This work tests the hypothesis that Sip-1 continues to function in the lens after lens vesicle formation and seeks to determine the mechanism by which the Zeb proteins function in vivo.
Mice homozygous for the Sip-1 flox allele were bred with mice carrying the MLR10 Cre gene, which is first expressed at the lens vesicle stage. Gene deletion was assessed by immunofluorescent localization of Sip1 and PCR. The structure of lenses lacking Sip1 was assessed by scanning electron microscopy (SEM) and conventional histological methods. Crystallin expression was investigated using gel electrophoresis and protein staining. The expression and evaluation of epithelial and mesenchymal cell markers was tested by immunohistochemistry, qRT-PCR, and Western blotting. C57Bl/6 mice were used as the wild type controls.
Wild type mice express low levels of endogenous Sip-1 protein in the lens placode, but these levels increase during lens vesicle formation. During embryonic development, Sip-1 expression is maintained in the lens epithelial cells, while it is expressed in both the epithelium and newly formed fiber cells in later post-natal stages. Conditional deletion of the Sip1 gene results in profound defects in lens fiber cell shape and structure beginning at approximately the lens vesicle stage of embryonic development while later in development lens epithelial defects are also seen. Crystallin expression, however, is not dramatically altered upon the loss of Sip1. The deletion of Sip1 in the lens decreases E-Cadherin protein and mRNA expression levels, while the levels of Vimentin are increased.
These data indicate that Sip-1 is important for both lens epithelial gene expression and fiber cell structure. However, our data suggest that Sip1 is not essential for crystallin expression in the lens. Notably, the changes in E-Cadherin and Vimentin levels upon Sip-1 deletion in the lens are opposite that expected from prior studies of Sip1 function in epithelial cancers suggesting that Sip1 plays a novel functional role in the lens. Future work will investigate additional targets of Sip1 in the lens and how these targets regulate lens structure.
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