Autophagy and UPR in alpha-crystallin mutant knock-in mouse models of hereditary cataracts

Background Knock-in mice provide useful models of congenital and age-related cataracts caused by α-crystallin mutations. R49C αA-crystallin and R120G αB-crystallin mutations are linked with hereditary cataracts. Knock-in αA-R49C +/- heterozygotes develop cataracts by 1-2 months, whereas homozygote mice have cataracts at birth. The R49C mutation drastically reduces lens protein water solubility and causes cell death in knock-in mouse lenses. Mutant crystallin cannot function as a chaperone, which leads to protein aggregation and lens opacity. Protein aggregation disrupts the lens fiber cell structure and normal development and causes cell death in epithelial and fiber cells. We determined what aspects of the wild-type phenotype are age-dependently altered in the mutant lens. Methods Wild-type, heterozygote (αA-R49C +/-), and homozygote (αA-R49C +/+) mouse lenses were assessed pre- and postnatally for lens morphology (electron microscopy, immunohistochemistry), and autophagy or unfolded protein response markers (immunoblotting). Results Morphology was altered by embryonic day 17 in R49C +/+ lenses; R49C +/- lens morphology was unaffected at this stage. Active autophagy in the lens epithelium of mutant lenses was indicated by the presence of autophagosomes using electron microscopy. Protein p62 levels, which are degraded specifically by autophagy, increased in αA-R49C mutant versus wild-type lenses, suggesting autophagy inhibition in the mutant lenses. The unfolded protein response marker XBP-1 was upregulated in adult lenses of αB-R120G +/+ mice, suggesting its role in lens opacification. Conclusions Mutated crystallins alter lens morphology, autophagy, and stress responses. General significance Therapeutic modulation of autophagic pathways may improve protein degradation in cataractous lenses and reduce lens opacity. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.