Unfolded protein response leading to cataractogenesis in a microphthalmia cataract mouse mutant

Abstract

Congenital cataract is a leading cause of visual disability among children worldwide, it has a heterogeneous genetic basis but the cellular and molecular bases for cataractogenesis remain elusive. In our laboratory we have a spontaneously occurring, autosomal dominant mouse mutant named Secc, which displays combined features of small eye, cataract and closed eyelid. By mapping and sequencing we identified a single point deletion at nucleotide 273 of the Cryga gene, leading to a frame-shift from the 3rd Greek Key motif of the gamma-A-crystallin. Cataract features were initiated in E14.5 Secc mutant, the nuclei of the primary lens fibres were scattered and failed to align in the equatorial region. By E16.5, the secondary lens fibre cells were abnormally arranged with poor lens suture formation. Apoptotic cells were found in the centre of the lens as shown by TUNEL assay, the cytoskeleton and cell adhesion in the lens centre were disturbed as shown in immunohistochemistry analysis. By western blotting we found that mutant gamma-crystallins were reduced in amount and enriched in the insoluble fraction, suggesting that mutant gamma-a-crystallins were misfolded and protein aggregates were formed. We found that the expression of genes involved in the unfolded protein response (UPR) pathways including BiP, CHOP and spliced variant of XBP-1 were all up-regulated significantly in E14.5 and 16.5 mutant lenses. Therefore, the mutant gamma-A-crystallin appeared to trigger UPR and cell death in the fibre cells. The mutant cells lost their normal cell adhesion, failed to maintain the proper lens architecture, leading to cataract formation.