Defect of Mitotic Vimentin Phosphorylation Causes Microophthalmia and Cataract via Aneuploidy and Senescence in Lens Epithelial Cells

Abstract

Vimentin, a type III intermediate filament (IF) protein, is phosphorylated predominantly in mitosis. The expression of a phosphorylation-compromised vimentin mutant in T24 cultured cells leads to cytokinetic failure, resulting in binucleation (multinucleation). The physiological significance of intermediate filament phosphorylation during mitosis for organogenesis and tissue homeostasis was uncertain. Here, we generated knock-in mice expressing vimentin that have had the serine sites phosphorylated during mitosis substituted by alanine residues. Homozygotic mice (VIMSA/SA) presented with microophthalmia and cataracts in the lens, whereas heterozygotic mice (VIMWT/SA) were indistinguishable from WT (VIMWT/WT) mice. In VIMSA/SA mice, lens epithelial cell number was not only reduced but the cells also exhibited chromosomal instability, including binucleation and aneuploidy. Electron microscopy revealed fiber membranes that were disorganized in the lenses of VIMSA/SA, reminiscent of similar characteristic changes seen in age-related cataracts. Because the mRNA level of the senescence (aging)-related gene was significantly elevated in samples from VIMSA/SA, the lens phenotype suggests a possible causal relationship between chromosomal instability and premature aging. Vimentin, a type III intermediate filament (IF) protein, is phosphorylated predominantly in mitosis. The expression of a phosphorylation-compromised vimentin mutant in T24 cultured cells leads to cytokinetic failure, resulting in binucleation (multinucleation). The physiological significance of intermediate filament phosphorylation during mitosis for organogenesis and tissue homeostasis was uncertain. Here, we generated knock-in mice expressing vimentin that have had the serine sites phosphorylated during mitosis substituted by alanine residues. Homozygotic mice (VIMSA/SA) presented with microophthalmia and cataracts in the lens, whereas heterozygotic mice (VIMWT/SA) were indistinguishable from WT (VIMWT/WT) mice. In VIMSA/SA mice, lens epithelial cell number was not only reduced but the cells also exhibited chromosomal instability, including binucleation and aneuploidy. Electron microscopy revealed fiber membranes that were disorganized in the lenses of VIMSA/SA, reminiscent of similar characteristic changes seen in age-related cataracts. Because the mRNA level of the senescence (aging)-related gene was significantly elevated in samples from VIMSA/SA, the lens phenotype suggests a possible causal relationship between chromosomal instability and premature aging.