Activation of Unfolded Protein Response in Transgenic Mouse Lenses

In eukaryotic cells, secretory and transmembrane proteins enter the endoplasmic reticulum (ER) as unfolded polypeptide chains, which fold and mature in the lumen of the ER in a highly assisted process. The ER lumen contains molecular chaperones, folding enzymes, and quality control factors that assist in folding and trafficking of newly synthesized polypeptides.1,2 If the influx exceeds the folding or processing capacity of the ER, the normal physiological balance of the ER is perturbed, a condition called ER stress. Physiologically, ER stress can be caused by the synthesis of mutated proteins, the absence of cofactors, or a drastic increase in the amount of otherwise normal cargo proteins during cell differentiation. A range of environmental stresses and pathogenic conditions (e.g., nutrient deprivation and viral infection) can also trigger ER stress and result in the accumulation of unfolded proteins in the ER. In response to ER stress, cells activate adaptive intracellular signal transduction pathways, known as the unfolded protein response (UPR), so that the cells can recover from the stress.3–5 Three ER-localized transmembrane protein sensors can initiate the UPR. These are inositol requiring kinase 1α (IRE1α), protein kinase-like ER kinase (PERK), and activating transcription factor 6α (ATF6α). The combined activation of IRE1α, PERK, and ATF6α initially produces cytoprotective outputs, such as enhanced ER protein folding capacity, reduced protein translation, and clearance of misfolded ER proteins.6 For instance, the activated IRE1α endonuclease splices out a 26-nucleotide intron of Xbp-1 mRNA, leading to a frameshift in codon reading and consequently creating an additional potent transactivation domain in the C terminus of the Xbp-1 protein.7 Xbp-1 controls a large group of UPR-response genes, including the ER chaperone BiP, to cope with the unfolded protein load in the ER.8,9 However, when cells undergo prolonged and severe ER stress and ER homeostasis cannot be reestablished, proapoptotic signals are triggered, presumably to protect the organism from these irreversibly damaged cells.10,11 The molecular mechanisms of ER stress–induced cell death are not completely understood. The UPR signaling pathway induced by PERK can contribute to cell death. PERK phosphorylates eukaryotic initiation factor 2α (eIF2α), which results in the enhanced translation of ATF4, a b-ZIP transcription factor. One of the important UPR-specific target genes of ATF4 is CHOP/GADD153, a b-ZIP transcription factor, which plays a role in apoptosis during ER stress.12–14 Recently, ER stress has been found to play a fundamental role in the pathogenesis of many human diseases, including neurodegenerative diseases and diabetes.15 In the eye, common diseases such as retinal degeneration and glaucoma have been implicated as protein misfolding diseases arising from persistent ER stress and activation of UPR signaling.16–18 Recently, Firtina et al.19 reported that the accumulation of collagen chains in the lens can activate the UPR and cause cataract formation in lens fiber cells of transgenic and mutant mice.19 Previous studies20,21 led by Shinohara's group showed that many cataractogenic stressors can activate the UPR in lens cell culture systems. For instance, both glucose deprivation and hyperglycemia can activate the UPR, which results in apoptosis in cultured lens epithelial cells.21 The mechanism of UPR activation under these conditions is unclear but is likely due to overall cellular stress. Despite extensive investigations on the molecular pathways of the UPR, many questions remain to be explored regarding the in vivo impact of the UPR in tissue development, physiology, and homeostasis. The ocular lens serves as a good model system for such analyses. Studies from past years have demonstrated that lens cells are highly susceptible to ER stress by environmental factors and mutations.22–24 During the past few years, transgenic mice have been used as an in vivo model to study gene function in lens development and cataractogenesis.25–28 We took advantage of the availability of various types of transgenic mice to ask the following questions: Could overexpression of a transgene in the mouse lens induce ER stress and activate the UPR? What are the potential biological effects of UPR activation on lens development?