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V.M. Fowler, R. Nowak, J. Moyer, R.S. Fischer; Mice With Tropomodulin1–Deficient Lenses Develop Y–Shaped Anterior Sutural Cataracts . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4106.
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© ARVO (1962-2015); The Authors (2016-present)
Tropomodulin1 (Tmod1) caps the pointed ends of actin filaments in the lens fiber cell plasma membrane skeleton and is proposed to regulate lens fiber cell elongation and morphogenesis. To understand the role of Tmod1 and actin filament capping in the lens, we have generated a Tmod1 knockout mouse.
Targeted deletion of the Tmod1 gene in mice leads to embryonic lethality between E9.5–10 due to abnormal cardiac development, precluding analysis of Tmod1 function in the lens. Therefore, we rescued the embryonic lethality of the Tmod1 null mouse by crossing with a TOT mouse that expresses a Tmod1 transgene in the heart under the control of the alpha–myosin heavy chain promoter. Genotyping of litters from crosses of Tmod1–/+;TOT+ mice with Tmod1+/– mice demonstrates that viable animals with no endogenous Tmod1 are produced, providing the opportunity to investigate the lens phenotype in the absence of Tmod1.
Western blotting and immunofluorescence staining demonstrates that Tmod1 protein is completely absent from lenses of the rescued Tmod1–/–;TOT+ mice. Instead, the Tmod2 isoform that is normally present in the lens epithelial cells is now increased in the fiber cells of Tmod1–deficient lenses. Histological analysis reveals that embryonic and postnatal development of the lens is grossly normal in the absence of Tmod1, including normal timing and location of organelle loss. Visual inspection of intact eyes from 1–2 month old mice under a dissecting microscope reveals an abnormal dark shadow in the shape of a jagged Y, corresponding to an abnormal anterior suture. In mice aged over 1 year, dissection and light microscopic observation of lenses reveals prominent Y–shaped as well as fleck–like anterior opacities that are present only in the Tmod1–deficient lenses. Western blotting of isolated membranes from these Tmod1–deficient lenses demonstrates that levels of actin and a 27kDa tropomyosin (TM) isoform are decreased, while levels of a 34kDa TM isoform are unchanged.
The absence of Tmod1 in mouse lens fiber cells is not essential for embryonic or postnatal lens development, but leads to formation of anterior sutural cataracts during lens aging. The absence of Tmod1 in lens fiber cells is incompletely compensated by up–regulation of Tmod2, leading to reduced levels of actin and reduced TM on the fiber cell membranes. Alterations in Tmod and TM isoforms may lead to decreased actin filament stability, resulting in defects in fiber cell migration, adhesion and membrane organization, finally resulting in abnormal formation or maintenance of cell–cell contacts at anterior sutures.
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