Abnormal Expression of Collagen IV in Lens Activates Unfolded Protein Response Resulting in Cataract

Abstract

Human diseases caused by mutations in extracellular matrix genes are often associated with an increased risk of cataract and lens capsular rupture. However, the underlying mechanisms of cataract pathogenesis in these conditions are still unknown. Using two different mouse models, we show that the accumulation of collagen chains in the secretory pathway activates the stress signaling pathway termed unfolded protein response (UPR). Transgenic mice expressing ectopic Col4a3 and Col4a4 genes in the lens exhibited activation of IRE1, ATF6, and PERK associated with expansion of the endoplasmic reticulum and attenuation of general protein translation. The expression of the transgenes had adverse effects on lens fiber cell differentiation and eventually induced cell death in a group of transgenic fiber cells. In Col4a1 ؉/⌬ex40 mutant mice, the accumulation of mutant chains also caused low levels of UPR activation. However, cell death was not induced in mutant lenses, suggesting that low levels of UPR activation are not proapoptotic. Collectively, the results provide in vivo evidence for a role of UPR in cataract formation in response to accumulation of terminally unfolded proteins in the endoplasmic reticulum.The ocular lens is a transparent, cellular structure that refracts light onto the retina, resulting in high resolution vision. Many environmental risk factors and single gene defects are known or hypothesized to result in clouding of the lens, a condition known as cataract. Cataract is the primary cause of blindness worldwide (1, 2), with autosomal dominant congenital cataract being the leading cause of treatable childhood blindness (3, 4). Cataract surgery is the most commonly performed surgical operation in the United States and consumes 60% of the Medicare budget for vision (5, 6).Cataract can be a multifactorial disease and is often associated with systemic or genetic disorders, such as diabetes and Lowe syndrome (7-9). Notably, human diseases caused by mutations in extracellular matrix (ECM) 4 genes are also often associated with an increased risk of cataract. Stickler and Marshall syndromes are two disorders caused by mutations in the COL2A1 gene that are associated with the early onset of distinctive cataracts (10, 11). Alport syndrome, caused by mutations in either the COL4A3, COL4A4, or COL4A5 genes, is also associated with lens capsule abnormalities and cataract formation (12)(13)(14). Humans carrying mutations in the COL4A1 locus often exhibit lens abnormalities and cataracts along with porencephaly and sporadic intracerebral hemorrhage (15-19). To date, approximately 13 independent mutations in the mouse Col4a1 locus and three independent mutations in mouse Col4a2 locus have been found to cause vacuolar cataract and lens abnormalities in mice (19 -21). However, the underlying mechanisms of cataract pathogenesis resulting from these collagen mutations are still unknown.In other tissues, mutations in genes encoding secretory pathway proteins have been found to cause endoplasmic reticulum (ER) stress and...

COL4A2 mutation associated with familial porencephaly and small-vessel disease

“…If collagen chains accumulate in patient cells, then increased storage of HSP47 can be expected. 24,26 Under standard culture conditions, HSP47 staining was comparable in patient and control cells, suggesting no accumulation of unfolded collagen chains (Supplementary Figure S3). Subsequently, we analysed whether KDEL staining under standard and stress conditions, induced by DTT, would detect ER stress in patient cells.…”

“…This accumulation activates the unfolded protein response, a well-known ER stress response preceding apoptosis. 20,24,26 As the c.3206delC COL4A2 mutation predicts a null-allele and no such mutations have been described in human COL4A1 patients, we investigated the pathogenic mechanism of this mutation in cultured skin fibroblasts from proband II-2. We wondered whether ER stress could be induced by abnormal stoichiometry of COL4A1 and COL4A2 chains, because of reduced COL4A2 synthesis.…”

“…Future studies are needed to address this discrepancy, which might be caused for example by the culture conditions compared with experiments performed in mouse tissues. 20,26 Apoptosis is an important mechanism regulating brain development and is involved in the onset of microcephaly. 32 Some of the COL4A2 patients are born microcephalic and the reason is not apparent at the moment, but it could be related to susceptibility of the developing brain to apoptosis.…”

Prox1 and fibroblast growth factor receptors form a novel regulatory loop controlling lens fiber differentiation and gene expression

“…Immunohistochemical staining for β-crystallin was performed as described previously (Firtina et al, 2009). Immunohistochemical staining for pErk1/2 (Cell Signaling, cs4377s; 1:50) and pAKT (Cell Signaling, cs9271s; 1:50) was performed on antigen-retrieved paraffin sections using the CSAII Biotin-Free Tyramide Signal Amplification Kit with the CSA II Rabbit Link (Dako, Agilent).…”

In vivo cellular adaptation to ER stress: survival strategies with double-edged consequences

“…In lens fiber cells, expression of either ectopic collagen IV isoforms or mutant collagen IV -chain induces ER stress and impairs fiber cell differentiation, leading to cataract formation (Firtina et al, 2009); cell death is observed only in a group of ectopic collagen-IV-expressing fiber cells that experience prolonged UPR activation. ER-stress-inducing drugs result in dedifferentiation in a differentiated thyroid cell line through a Srcmediated signaling pathway (Ulianich et al, 2008).…”