Activation of the Unfolded Protein Response in the Lens Results in Cataracts

Abstract

Purpose:: Human diseases caused by mutations in extracellular matrix (ECM) genes are often associated with an increased risk of cataract, lens capsular rupture, and lens dislocation. In other systems, activation of the Unfolded Protein Response (UPR) cascade triggers alterations in gene and protein expression and/or apoptosis. Here we test the hypothesis that the presence of abnormal ECM in the lens results in cataract due to activation of the UPR.

Methods:: Transgenic mice were created harboring constructs consisting of the hybrid promoter ΔEN/αA-crystallin, which is active in all lens cells starting at the late lens pit stage, driving expression of either the mouse collagen α3(IV) or α4(IV) cDNA. Alterations in lens structure were measured by darkfield microscopy and conventional histology. The expression of the transgenes and evaluation of the molecular markers of UPR was tested by immunohistochemistry, rt-PCR and Western blotting.

Results:: Mice overexpressing either collagen α3(IV) or α4(IV) had small lenses with obvious opacities. They showed a 90% reduction in total protein although the overall crystallin profile was not altered. Immunostaining detected a significant retention of ectopic collagen IV in transgenic lens cells and reduced collagen IV staining in the lens capsule. However laminin staining was seen exclusively in the lens capsule of transgenics which indicates it is processed and secreted normally. Both types of transgenic lenses exhibited a large upregulation of BiP, an ER resident chaperone whose expression is usually upregulated when the protein folding capacity of the ER is challenged. In other systems, the ER transmembrane kinase/endoribonuclease ER stress sensor IRE1 initiates the processing of XBP1 mRNA to produce a transcript encoding a potent basic leucine zipper transcription factor that activates BiP expression. This pathway is activated in collagen IV transgenic lenses since we detected a dramatic elevation of spliced XBP1 mRNA by rt-PCR.

Conclusions:: Overexpressed single Collagen IV chains can not form regular protomers, thus misfold and are retained in the ER. This retention is specific to ectopic Collagen IV since laminin is secreted normally. The retention of misfolded collagen IV in these transgenic mice upregulates BiP expression, attenuates global protein synthesis and activates the IRE1 branch of the unfolded protein response resulting in cataracts. These data support the hypothesis that activation of UPR pathways in the lens is an important cataractogenic mechanism.

Keywords: cataract • extracellular matrix • genetics