A transgenic mouse model for human autosomal dominant cataract

Measurement of absolute abundance of crystallins in human and αA N101D transgenic mouse lenses using 15N-labeled crystallin standards

Stable isotope labeled standards of all major human lens crystallins were created to measure the abundance of lens endogenous crystallins from birth to adulthood. All major human crystallins (αA, αB, βA2, βA3/A1, βA4, βB1, βB2, βB3, γA, γB, γC, γD, γS) were cloned with N-terminal 6 x His tagged SUMO for ease of purification and the ability to generate natural N-termini by SUMO protease cleavage when producing crystallins for structure/function studies. They were then expressed in 15N-enriched media, quantified by mass spectrometry, and mixed in proportions found in young human lens to act as an artificial lens standard. The absolute quantification method was tested using soluble protein from 5-day, 23-day, 18-month, and 18-year-old human lenses spiked with the 15N artificial lens standard. Proteins were trypsinized, relative ratios of light and heavy labeled peptides determined using high-resolution precursor and data independent MS2 scans, and data analysis performed using Skyline software. Crystallin abundances were measured in both human donor lenses and in transgenic mouse αA N101D cataract lenses. Technical replicates of human crystallin abundance measurements were performed with average coefficients of variation of approximately 2% across all 13 crystallins. αA crystallin comprised 27% of the soluble protein of 5-day-old lens and decreased to 16% by 18-years of age. Over this time period αB increased from 6% to 9% and the αA/αB ratio decreased from 4.5/1 to 2/1. γS-crystallin also increased nearly 2-fold from 7% to 12%, becoming the 3rd most abundant protein in adult lens, while βB1 increased from 14% to 20%, becoming the most abundant crystallin of adult lens. Minor crystallins βA2, βB3, and γA comprised only about 1% each of the newborn lens soluble protein, and their abundance dropped precipitously by adulthood. While 9 of the SUMO tagged crystallins were useful for purification of crystallins for structural studies, γA, γB, γC, and γD were resistant to cleavage by SUMO protease. The abundance of WT and N101D human αA in transgenic mouse lenses was approximately 40-fold lower than endogenous mouse αA, but the deamidation mimic human αA N101D was less soluble than human WT αA. The high content of αA and the transient abundance of βA2, βB3, and γA in young lens suggest these crystallins play a role in early lens development and growth. βB1 becoming the most abundant crystallin may result from its role in promoting higher order β-crystallin oligomerization in mature lens. The full set of human crystallin expression vectors in the Addgene repository should be a useful resource for future crystallin studies. 15N labeling of these crystallins will be useful to accurately quantify crystallins in lens anatomic regions, as well as measure the composition of insoluble light scattering crystallin aggregates. The standards will also be useful to measure the abundance of crystallins expressed in transgenic animal models.

Role of ATP-Small Heat Shock Protein Interaction in Human Diseases

Adenosine triphosphate (ATP) is an important fuel of life for humans and Mycobacterium species. Its potential role in modulating cellular functions and implications in systemic, pulmonary, and ocular diseases is well studied. Plasma ATP has been used as a diagnostic and prognostic biomarker owing to its close association with disease’s progression. Several stresses induce altered ATP generation, causing disorders and illnesses. Small heat shock proteins (sHSPs) are dynamic oligomers that are dominantly β-sheet in nature. Some important functions that they exhibit include preventing protein aggregation, enabling protein refolding, conferring thermotolerance to cells, and exhibiting anti-apoptotic functions. Expression and functions of sHSPs in humans are closely associated with several diseases like cataracts, cardiovascular diseases, renal diseases, cancer, etc. Additionally, there are some mycobacterial sHSPs like Mycobacterium leprae HSP18 and Mycobacterium tuberculosis HSP16.3, whose molecular chaperone functions are implicated in the growth and survival of pathogens in host species. As both ATP and sHSPs, remain closely associated with several human diseases and survival of bacterial pathogens in the host, therefore substantial research has been conducted to elucidate ATP-sHSP interaction. In this mini review, the impact of ATP on the structure and function of human and mycobacterial sHSPs is discussed. Additionally, how such interactions can influence the onset of several human diseases is also discussed.

Modeling congenital cataract in vitro using patient-specific induced pluripotent stem cells

Congenital cataracts are the leading cause of childhood blindness. To date, surgical removal of cataracts is the only established treatment, but surgery is associated with multiple complications, which often lead to visual impairment. Therefore, mechanistic studies and drug-candidate screening have been intrigued by the aims of developing novel therapeutic strategies. However, these studies have been hampered by a lack of an appropriate human-disease model of congenital cataracts. Herein, we report the establishment of a human congenital cataract in vitro model through differentiation of patient-specific induced pluripotent stem cells (iPSCs) into regenerated lenses. The regenerated lenses derived from patient-specific iPSCs with known causative mutations of congenital cataracts (CRYBB2 [p. P24T] and CRYGD [p. Q155X]) showed obvious opacification that closely resembled that seen in patients’ cataracts in terms of opacification severity and disease course accordingly, as compared with lentoid bodies (LBs) derived from healthy individuals. Increased protein aggregation and decreased protein solubility corresponding to the patients’ cataract severity were observed in the patient-specific LBs and were attenuated by lanosterol treatment. Taken together, the in vitro model described herein, which recapitulates patient-specific clinical manifestations of congenital cataracts and protein aggregation in patient-specific LBs, provides a robust system for research on the pathological mechanisms of cataracts and screening of drug candidates for cataract treatment.

Transgenic zebrafish models reveal distinct molecular mechanisms for cataract-linked αA-crystallin mutants

Mutations in the small heat shock proteins α-crystallins have been linked to autosomal dominant cataracts in humans. Extensive studies in vitro have revealed a spectrum of alterations to the structure and function of these proteins including shifts in the size of the oligomer, modulation of subunit exchange and modification of their affinity to client proteins. Although mouse models of these mutants were instrumental in identifying changes in cellular proliferation and lens development, a direct comparative analysis of their effects on lens proteostasis has not been performed. Here, we have transgenically expressed cataract-linked mutants of αA- and αB-crystallin in the zebrafish lens to dissect the underlying molecular changes that contribute to the loss of lens optical properties. Zebrafish lines expressing these mutants displayed a range of morphological lens defects. Phenotype penetrance and severity were dependent on the mutation even in fish lines lacking endogenous α-crystallin. The mechanistic origins of these differences were investigated by the transgenic co-expression of a destabilized human γD-crystallin mutant. We found that the R49C but not the R116C mutant of αA-crystallin drove aggregation of γD-crystallin, although both mutants have similar affinity to client proteins in vitro. Our working model attributes these differences to the propensity of R49C, located in the buried N-terminal domain of αA-crystallin, to disulfide crosslinking as previously demonstrated in vitro. Our findings complement and extend previous work in mouse models and emphasize the need of investigating chaperone/client protein interactions in appropriate cellular context.

Targeted deletion of RIC8A in mouse neural precursor cells interferes with the development of the brain, eyes, and muscles

Autosomal recessive disorders such as Fukuyama congenital muscular dystrophy, Walker-Warburg syndrome, and the muscle-eye-brain disease are characterized by defects in the development of patient's brain, eyes, and skeletal muscles. These syndromes are accompanied by brain malformations like type II lissencephaly in the cerebral cortex with characteristic overmigrations of neurons through the breaches of the pial basement membrane. The signaling pathways activated by laminin receptors, dystroglycan and integrins, control the integrity of the basement membrane, and their malfunctioning may underlie the pathologies found in the rise of defects reminiscent of these syndromes. Similar defects in corticogenesis and neuromuscular disorders were found in mice when RIC8A was specifically removed from neural precursor cells. RIC8A regulates a subset of G-protein α subunits and in several model organisms, it has been reported to participate in the control of cell division, signaling, and migration. Here, we studied the role of RIC8A in the development of the brain, muscles, and eyes of the neural precursor-specific conditional Ric8a knockout mice. The absence of RIC8A severely affected the attachment and positioning of radial glial processes, Cajal-Retzius' cells, and the arachnoid trabeculae, and these mice displayed additional defects in the lens, skeletal muscles, and heart development. All the discovered defects might be linked to aberrancies in cell adhesion and migration, suggesting that RIC8A has a crucial role in the regulation of cell-extracellular matrix interactions and that its removal leads to the phenotype characteristic to type II lissencephaly-associated diseases. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 78: 374-390, 2018.

Effect of Geranylgeranylacetone on Ultraviolet Radiation Type B-Induced Cataract in Heat-Shock Transcription Factor 1 Heterozygous Mouse

PURPOSE

We investigated whether heat-shock transcription factor 1 (HSF1) was involved in ultraviolet radiation type B (UVR-B)-induced lens opacity (cataract) using HSF1 heterozygous mice. We also examined the effects of geranylgeranylacetone (GGA), an inducer of heat-shock proteins via activation of HSF, on the UVR-B-induced cataract.

MATERIAL AND METHODS

Male HSF1^+/- and WT mice were unilaterally exposed to UVR-B (total: 1200mJ) at 16 weeks of age. At 48 h after the last UVR-B irradiation, the lens was isolated and the induction of the cataract was quantified as the cataract area ratio (opacity area/anterior capsule). GGA was orally administered at a dosage of 500 mg/kg once a day for two days before the first UVR-B exposure until the end of the experiment (21days in total).

RESULTS

The HSF1 expression was more greatly decreased in the lens from HSF1^+/- mice than in that from WT mice (p < 0.01). UVR-B exposure could mainly induce cataracts in the anterior capsule in both HSF1^+/- and WT mice, while the opacity of the lens was markedly enhanced in HSF^1+/- mice compared to that in WT mice(p (0.01). GGA treatment could prevent the induction of lens opacity by UVR-B exposure in both WT and HSF1^+/- mice as compared with the non-administration group (p < 0.01). No obvious alteration by the UVR-B radiation was seen in lens protein levels of αA-crystallin, αB-crystallin, or γ-crystallin with or without GGA administration among all groups of mice. In contrast to the crystallins, the lens protein level of HSP25 was decreased by UVR-B exposure in both HSF1^+/- and WT mice, and was significantly recovered in WT mice by the GGA treatment (p < 0.01). The induction of HSP25 was suppressed in HSF1^+/- mice compared with that in WT mice.

CONCLUSIONS

These data suggest that HSF1 plays an important role in the occurrence of UVR-B-induced cataracts, possibly via regulation of HSPs such as HSP25.

Differential role of arginine mutations on the structure and functions of α-crystallin

BACKGROUND

α-Crystallin is a major protein of the eye lens in vertebrates. It is composed of two subunits, αA- and αB-crystallin. α-Crystallin is an oligomeric protein having these two subunits in 3:1 ratio. It belongs to small heat shock protein family and exhibits molecular chaperone function, which plays an important role in maintaining the lens transparency. Apart from chaperone function, both subunits also exhibit anti-apoptotic property. Comparison of their primary sequences reveals that αA- and αB-crystallin posses 13 and 14 arginine residues, respectively. Several of them undergo mutations which eventually lead to various eye diseases such as congenital cataract, juvenile cataract, and retinal degeneration. Interestingly, many arginine residues of these subunits are modified during glycation and even some are truncated during aging. All these facts indicate the importance of arginine residues in α-crystallin.

SCOPE OF REVIEW

In this review, we will emphasize the recent in vitro and in vivo findings related to congenital cataract causing arginine mutations in α-crystallin.

MAJOR CONCLUSIONS

Congenital cataract causing arginine mutations alters the structure and decreases the chaperone function of α-crystallin. These mutations also affect the lens morphology and phenotypes. Interestingly, non-natural arginine mutations (generated for mimicking the glycation and truncation environment) improve the chaperone function of α-crystallin which may play an important role in maintaining the eye lens transparency during aging.

GENERAL SIGNIFICANCE

The neutralization of positive charge on the guanidino group of arginine residues is not always detrimental to the functionality of α-crystallin. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Mutation analysis of two families with inherited congenital cataracts

The present study aimed to identify the genetic mutations in two families affected with congenital cataracts. Detailed family histories and clinical data of the family members were recorded. The family members with affected phenotypes were recruited, and candidate gene sequencing was performed to determine the disease‑causing mutation. Bioinformatics analysis was performed to predict the function of the mutant gene. Green fluorescent protein‑tagged human wild‑type CRYAA and GJA8 were sub‑cloned, and the mutants were generated by site‑directed mutagenesis. A novel mutation, c.416T>C (p.L139P), in CRYAA and a known mutation, c.139G>A (p.D47N), in GJA8 were identified. These mutations co‑segregated with all affected individuals in each family and were not observed in the unaffected family members or in unrelated controls. The results of the bioinformatics analysis indicated that the amino acid at position 139 was highly conserved and that the p.L139P mutation was predicted to be damaging, as with p.D47N. Finally, overexpression of the two mutants revealed marked alterations, compared with the wild‑type proteins. These results extend the mutation spectrum of CRYAA and provides further evidence that the p.D47N mutation in GJA8 is a hot-spot mutation.

Small heat-shock proteins: important players in regulating cellular proteostasis

Small heat-shock proteins (sHsps) are a diverse family of intra-cellular molecular chaperone proteins that play a critical role in mitigating and preventing protein aggregation under stress conditions such as elevated temperature, oxidation and infection. In doing so, they assist in the maintenance of protein homeostasis (proteostasis) thereby avoiding the deleterious effects that result from loss of protein function and/or protein aggregation. The chaperone properties of sHsps are therefore employed extensively in many tissues to prevent the development of diseases associated with protein aggregation. Significant progress has been made of late in understanding the structure and chaperone mechanism of sHsps. In this review, we discuss some of these advances, with a focus on mammalian sHsp hetero-oligomerisation, the mechanism by which sHsps act as molecular chaperones to prevent both amorphous and fibrillar protein aggregation, and the role of post-translational modifications in sHsp chaperone function, particularly in the context of disease.

AP-2α is required after lens vesicle formation to maintain lens integrity

BACKGROUND

Transcription factors are critical in regulating lens development. The AP-2 family of transcription factors functions in differentiation, cell growth and apoptosis, and in lens and eye development. AP-2α, in particular, is important in early lens development, and when conditionally deleted at the placode stage defective separation of the lens vesicle from the surface ectoderm results. AP-2α's role during later stages of lens development is unknown. To address this, the MLR10-Cre transgene was used to delete AP-2α from the lens epithelium beginning at embryonic day (E) 10.5.

RESULTS

The loss of AP-2α after lens vesicle separation resulted in morphological defects beginning at E18.5. By P4, a small highly vacuolated lens with a multilayered epithelium was evident in the MLR10-AP-2α mutants. Epithelial cells appeared elongated and expressed fiber cell specific βB1 and γ-crystallins. Epithelial cell polarity and lens cell adhesion was disrupted and accompanied by the misexpression of ZO-1, N-Cadherin, and β-catenin. Cell death was observed in the mutant lens epithelium between postnatal day (P) 14 and P30, and correlated with altered arrangements of cells within the epithelium.

CONCLUSIONS

Our findings demonstrate that AP-2α continues to be required after lens vesicle separation to maintain a normal lens epithelial cell phenotype and overall lens integrity and to ensure correct fiber cell differentiation.

Molecular mechanism of formation of cortical opacity in CRYAAN101D transgenic mice

PURPOSE

The CRYAAN101D transgenic mouse model expressing deamidated αA-crystallin (deamidation at N101 position to D) develops cortical cataract at the age of 7 to 9 months. The present study was carried out to explore the molecular mechanism that leads to the development of cortical opacity in CRYAAN101D lenses.

METHODS

RNA sequence analysis was carried out on 2- and 4-month-old αA-N101D and wild type (WT) lenses. To understand the biologic relevance and function of significantly altered genes, Ingenuity Pathway Analysis (IPA) was done. To elucidate terminal differentiation defects, immunohistochemical, and Western blot analyses were carried out.

RESULTS

RNA sequence and IPA data suggested that the genes belonging to gene expression, cellular assembly and organization, and cell cycle and apoptosis networks were altered in N101D lenses. In addition, the tight junction signaling and Rho A signaling were among the top three canonical pathways that were affected in N101D mutant. Immunohistochemical analysis identified a series of terminal differentiation defects in N101D lenses, specifically, increased proliferation and decreased differentiation of lens epithelial cells (LEC) and decreased denucleation of lens fiber cells (LFC). The expression of Rho A was reduced in different-aged N101D lenses, and, conversely, Cdc42 and Rac1 expressions were increased in the N101D mutants. Moreover, earlier in development, the expression of major membrane-bound molecular transporter Na,K-ATPase was drastically reduced in N101D lenses.

CONCLUSIONS

The results suggest that the terminal differentiation defects, specifically, increased proliferation and decreased denucleation are responsible for the development of lens opacity in N101D lenses.

In vivo substrates of the lens molecular chaperones αA-crystallin and αB-crystallin

αA-crystallin and αB-crystallin are members of the small heat shock protein family and function as molecular chaperones and major lens structural proteins. Although numerous studies have examined their chaperone-like activities in vitro, little is known about the proteins they protect in vivo. To elucidate the relationships between chaperone function, substrate binding, and human cataract formation, we used proteomic and mass spectrometric methods to analyze the effect of mutations associated with hereditary human cataract formation on protein abundance in αA-R49C and αB-R120G knock-in mutant lenses. Compared with age-matched wild type lenses, 2-day-old αA-R49C heterozygous lenses demonstrated the following: increased crosslinking (15-fold) and degradation (2.6-fold) of αA-crystallin; increased association between αA-crystallin and filensin, actin, or creatine kinase B; increased acidification of βB1-crystallin; increased levels of grifin; and an association between βA3/A1-crystallin and αA-crystallin. Homozygous αA-R49C mutant lenses exhibited increased associations between αA-crystallin and βB3-, βA4-, βA2-crystallins, and grifin, whereas levels of βB1-crystallin, gelsolin, and calpain 3 decreased. The amount of degraded glutamate dehydrogenase, α-enolase, and cytochrome c increased more than 50-fold in homozygous αA-R49C mutant lenses. In αB-R120G mouse lenses, our analyses identified decreased abundance of phosphoglycerate mutase, several β- and γ-crystallins, and degradation of αA- and αB-crystallin early in cataract development. Changes in the abundance of hemoglobin and histones with the loss of normal α-crystallin chaperone function suggest that these proteins also play important roles in the biochemical mechanisms of hereditary cataracts. Together, these studies offer a novel insight into the putative in vivo substrates of αA- and αB-crystallin.

Lens crystallin modifications and cataract in transgenic mice overexpressing acylpeptide hydrolase

The accumulation of crystallin fragments in vivo and their subsequent interaction with crystallins are responsible, in part, for protein aggregation in cataracts. Transgenic mice overexpressing acylpeptide hydrolase (APH) specifically in the lens were prepared to test the role of protease in the generation and accumulation of peptides. Cataract development was seen at various postnatal days in the majority of mice expressing active APH (wt-APH). Cataract onset and severity of the cataracts correlated with the APH protein levels. Lens opacity occurred when APH protein levels were >2.6% of the total lens protein and the specific activity, assayed using Ac-Ala-p-nitroanilide substrate, was >1 unit. Transgenic mice carrying inactive APH (mt-APH) did not develop cataract. Cataract development also correlated with N-terminal cleavage of the APH to generate a 57-kDa protein, along with an increased accumulation of low molecular weight (LMW) peptides, similar to those found in aging human and cataract lenses. Nontransgenic mouse lens proteins incubated with purified wt-APH in vitro resulted in a >20% increase in LMW peptides. Crystallin modifications and cleavage were quite dramatic in transgenic mouse lenses with mature cataract. Affected lenses showed capsule rupture at the posterior pole, with expulsion of the lens nucleus and degenerating fiber cells. Our study suggests that the cleaved APH fragment might exert catalytic activity against crystallins, resulting in the accumulation of distinct LMW peptides that promote protein aggregation in lenses expressing wt-APH. The APH transgenic model we developed will enable in vivo testing of the roles of crystallin fragments in protein aggregation.

Acetylation of lysine 92 improves the chaperone and anti-apoptotic activities of human αB-crystallin

αB-Crystallin is a chaperone and an anti-apoptotic protein that is strongly expressed in many tissues, including the lens, retina, heart, and kidney. In the human lens, several lysine residues in αB-crystallin are acetylated. We have previously shown that such acetylation is predominant at lysine 92 (K92) and lysine 166 (K166). We have investigated the effect of lysine acetylation on the structure and functions of αB-crystallin by the specific introduction of an N(ε)-acetyllysine (AcK) mimic at K92. The introduction of AcK slightly altered the secondary and tertiary structures of the protein. The introduction of AcK also resulted in an increase in the molar mass and hydrodynamic radius of the protein, and the protein became structurally more open and more stable than the native protein. The acetyl protein acquired higher surface hydrophobicity and exhibited 25-55% higher chaperone activity than the native protein. The acetyl protein had more client protein binding per subunit of the protein and higher binding affinity relative to that of the native protein. The acetyl protein was at least 20% more effective in inhibiting chemically induced apoptosis than the native protein. Molecular modeling suggests that acetylation of K92 makes the "α-crystallin domain" more hydrophobic. Together, our results reveal that the acetylation of a single lysine residue in αB-crystallin makes the protein structurally more stable and improves its chaperone and anti-apoptotic activities. Our findings suggest that lysine acetylation of αB-crystallin is an important chemical modification for enhancing αB-crystallin's protective functions in the eye.

Phenotype of cardiomyopathy in cardiac-specific heat shock protein B8 K141N transgenic mouse

A K141N missense mutation in heat shock protein (HSP) B8, which belongs to the small HSP family, causes distal hereditary motor neuropathy, which is characterized by the formation of inclusion bodies in cells. Although the HSPB8 gene causes hereditary motor neuropathy, obvious expression of HSPB8 is also observed in other tissues, such as the heart. The effects of a single mutation in HSPB8 upon the heart were analyzed using rat neonatal cardiomyocytes. Expression of HSPB8 K141N by adenoviral infection resulted in increased HSPB8-positive aggregates around nuclei, whereas no aggregates were observed in myocytes expressing wild-type HSPB8. HSPB8-positive aggresomes contained amyloid oligomer intermediates that were detected by a specific anti-oligomer antibody (A11). Expression of HSPB8 K141N induced slight cellular toxicity. Recombinant HSPB8 K141N protein showed reactivity against the anti-oligomer antibody, and reactivity of the mutant HSPB8 protein was much higher than that of wild-type HSPB8 protein. To extend our in vitro study, cardiac-specific HSPB8 K141N transgenic (TG) mice were generated. Echocardiography revealed that the HSPB8 K141N TG mice exhibited mild hypertrophy and apical fibrosis as well as slightly reduced cardiac function, although no phenotype was detected in wild-type HSPB8 TG mice. A single point mutation of HSPB8, such as K141N, can cause cardiac disease.

Crystal structure of an activated variant of small heat shock protein Hsp16.5

How does the sequence of a single small heat shock protein (sHSP) assemble into oligomers of different sizes? To gain insight into the underlying structural mechanism, we determined the crystal structure of an engineered variant of Methanocaldococcus jannaschii Hsp16.5 wherein a 14 amino acid peptide from human heat shock protein 27 (Hsp27) was inserted at the junction of the N-terminal region and the α-crystallin domain. In response to this insertion, the oligomer shell expands from 24 to 48 subunits while maintaining octahedral symmetry. Oligomer rearrangement does not alter the fold of the conserved α-crystallin domain nor does it disturb the interface holding the dimeric building block together. Rather, the flexible C-terminal tail of Hsp16.5 changes its orientation relative to the α-crystallin domain which enables alternative packing of dimers. This change in orientation preserves a peptide-in-groove interaction of the C-terminal tail with an adjacent β-sandwich, thereby holding the assembly together. The interior of the expanded oligomer, where substrates presumably bind, retains its predominantly nonpolar character relative to the outside surface. New large windows in the outer shell provide increased access to these substrate-binding regions, thus accounting for the higher affinity of this variant to substrates. Oligomer polydispersity regulates sHSPs chaperone activity in vitro and has been implicated in their physiological roles. The structural mechanism of Hsp16.5 oligomer flexibility revealed here, which is likely to be highly conserved across the sHSP superfamily, explains the relationship between oligomer expansion observed in disease-linked mutants and changes in chaperone activity.

Temperature-dependent structural and functional properties of a mutant (F71L) αA-crystallin: molecular basis for early onset of age-related cataract

Previously we identified a novel mutation (F71L) in the αA-crystallin gene associated with early onset of age-related cataract. However, it is not known how the missense substitution translates into reduced chaperone-like activity (CLA), and how the structural and functional changes lead to early onset of the disease. Herein, we show that under native conditions the F71L-mutant is not significantly different from wild-type with regard to secondary and tertiary structural organization, hydrophobicity and the apparent molecular mass of oligomer but has substantial differences in structural and functional properties following a heat treatment. Wild-type αA-crystallin demonstrated increased CLA, whereas the F71L-mutant substantially lost its CLA upon heat treatment. Further, unlike the wild-type αA-subunit, F71L-subunit did not protect the αB-subunit in hetero-oligomeric complex from heat-induced aggregation. Moreover, hetero-oligomer containing F71L and αB in 3:1 ratio had significantly lower CLA upon thermal treatment compared to its unheated control. These results indicate that α-crystallin complexes containing F71L-αA subunits are less stable and have reduced CLA. Therefore, F71L may lead to earlier onset of cataract due to interaction with several environmental factors (e.g., temperature in this case) along with the aging process.

Activation of unfolded protein response in transgenic mouse lenses

PURPOSE

Overloading of unfolded or misfolded proteins in the endoplasmic reticulum (ER) can cause ER stress and activate the unfolded protein response (UPR) in the cell. The authors tested whether transgene overexpression in the mouse lens would activate the UPR.

METHODS

Transgenic mice expressing proteins that either enter the ER secretory pathway or are synthesized in cytosol were selected. Activation of the UPR was assessed by determining the expression levels of the ER chaperone protein BiP, the spliced form of X-box binding protein-1 (Xbp-1) mRNA, and the transcription factor CHOP. Changes in the ubiquitin-proteasome system in the mouse lens were detected by ubiquitin immunofluorescence.

RESULTS

BiP expression was upregulated in the fiber cells of transgenic mouse lenses expressing platelet-derived growth factor-A (PDGF-A), dominant-negative fibroblast growth factor receptor (DN-FGFR), or DN-Sprouty2 (DN-Spy2). BiP upregulation occurred around embryonic day 16.5, primarily in the fiber cells adjacent to the organelle free zone. Fiber cell differentiation was disrupted in the PDGF-A and DN-Spry2 lenses, whereas the fiber cells were degenerating in the DN-FGFR lens. High levels of UPR activation and ubiquitin-labeled protein aggregates were found in the DN-FGFR lens, indicating inefficient disposal of unfolded/misfolded proteins in the fiber cells.

CONCLUSIONS

This study implies that overexpression of some transgenes in the lens can induce ER or overall cell stress in fiber cells, resulting in the activation of UPR signaling pathways. Therefore, investigators should assess the levels of UPR activation when they analyze the downstream effects of transgene expression in the lens.

The common modification in alphaA-crystallin in the lens, N101D, is associated with increased opacity in a mouse model

To elucidate the morphological and cellular changes due to introduction of a charge during development and the possible mechanism that underlies cataract development in humans as a consequence of an additional charge, we generated a transgenic mouse model mimicking deamidation of Asn at position 101. The mouse model expresses a human αA-crystallin gene in which Asn-101 was replaced with Asp, which is referred to as αAN101D-transgene and is considered to be "deamidated" in this study. Mice expressing αAN101D-transgene are referred to here CRYAA(N101D) mice. All of the lines showed the expression of αAN101D-transgene. Compared with the lenses of mice expressing wild-type (WT) αA-transgene (referred to as CRYAA(WT) mice), the lenses of CRYAA(N101D) mice showed (a) altered αA-crystallin membrane protein (aquaporin-0 (AQP0), a specific lens membrane protein) interaction, (b) extracellular spaces between outer cortical fiber cells, (c) attenuated denucleation during confocal microscopic examination, (d) disrupted normal fiber cell organization and structure during scanning electron microscopic examination, (e) distorted posterior suture lines by bright field microscopy, and (f) development of a mild anterior lens opacity in the superior cortical region during the optical coherence tomography scan analysis. Relative to lenses with WT αA-crystallin, the lenses containing the deamidated αA-crystallin also showed an aggregation of αA-crystallin and a higher level of water-insoluble proteins, suggesting that the morphological and cellular changes in these lenses are due to the N101D mutation. This study provides evidence for the first time that expression of deamidated αA-crystallin caused disruption of fiber cell structural integrity, protein aggregation, insolubilization, and mild cortical lens opacity.

Cat-Map: putting cataract on the map

Lens opacities, or cataract(s), may be inherited as a classic Mendelian disorder usually with early-onset or, more commonly, acquired with age as a multi-factorial or complex trait. Many genetic forms of cataract have been described in mice and other animal models. Considerable progress has been made in mapping and identifying the genes and mutations responsible for inherited forms of cataract, and genetic determinants of age-related cataract are beginning to be discovered. To provide a convenient and accurate summary of current information focused on the increasing genetic complexity of Mendelian and age-related cataract we have created an online chromosome map and reference database for cataract in humans and mice (Cat-Map).

Aldo-keto reductases in the eye

Aldose reductase (AKR1B1) is an NADPH-dependent aldo-keto reductase best known as the rate-limiting enzyme of the polyol pathway. Accelerated glucose metabolism through this pathway has been implicated in diabetic cataract and retinopathy. Some human tissues contain AKR1B1 as well as AKR1B10, a closely related member of the aldo-keto reductase gene superfamily. This opens the possibility that AKR1B10 may also contribute to diabetic complications. The goal of the current study was to characterize the expression profiles of AKR1B1 and AKR1B10 in the human eye. Using quantitative reverse transcriptase-PCR and immunohistochemical staining, we observed expression of both AKR genes in cornea, iris, ciliary body, lens, and retina. Expression of AKR1B1 was the highest in lens and retina, whereas AKR1B10 was the highest in cornea. Lenses from transgenic mice designed for overexpression of AKR1B10 were not significantly different from nontransgenic controls, although a significant number developed a focal defect in the anterior lens epithelium following 6 months of experimentally induced diabetes. However, lenses from AKR1B10 mice remained largely transparent following longterm diabetes. These results indicate that AKR1B1 and AKR1B10 may have different functional properties in the lens and suggest that AKR1B10 does not contribute to the pathogenesis of diabetic cataract in humans.

Hydroimidazolone modification of human alphaA-crystallin: Effect on the chaperone function and protein refolding ability

AlphaA-crystallin is a molecular chaperone; it prevents aggregation of denaturing proteins. We have previously demonstrated that upon modification by a metabolic alpha-dicarbonyl compound, methylglyoxal (MGO), alphaA-crystallin becomes a better chaperone. AlphaA-crystallin also assists in refolding of denatured proteins. Here, we have investigated the effect of mild modification of alphaA-crystallin by MGO (with 20-500 microM) on the chaperone function and its ability to refold denatured proteins. Under the conditions used, mildly modified protein contained mostly hydroimidazolone modifications. The modified protein exhibited an increase in chaperone function against thermal aggregation of beta(L)- and gamma-crystallins, citrate synthase (CS), malate dehydrogenase (MDH) and lactate dehydrogenase (LDH) and chemical aggregation of insulin. The ability of the protein to assist in refolding of chemically denatured beta(L)- and gamma-crystallins, MDH and LDH, and to prevent thermal inactivation of CS were unchanged after mild modification by MGO. Prior binding of catalytically inactive, thermally denatured MDH or the hydrophobic probe, 2-p-toluidonaphthalene-6-sulfonate (TNS) abolished the ability of alphaA-crystallin to assist in the refolding of denatured MDH. However, MGO modification of chaperone-null TNS-bound alphaA-crystallin resulted in partial regain of the chaperone function. Taken together, these results demonstrate that: 1) hydroimidazolone modifications are sufficient to enhance the chaperone function of alphaA-crystallin but such modifications do not change its ability to assist in refolding of denatured proteins, 2) the sites on the alphaA-crystallin responsible for the chaperone function and refolding are the same in the native alphaA-crystallin and 3) additional hydrophobic sites exposed upon MGO modification, which are responsible for the enhanced chaperone function, do not enhance alphaA-crystallin's ability to refold denatured proteins.

AlphaA-crystallin R49Cneo mutation influences the architecture of lens fiber cell membranes and causes posterior and nuclear cataracts in mice

Background

αA-crystallin (CRYAA/HSPB4), a major component of all vertebrate eye lenses, is a small heat shock protein responsible for maintaining lens transparency. The R49C mutation in the αA-crystallin protein is linked with non-syndromic, hereditary human cataracts in a four-generation Caucasian family.

Methods

This study describes a mouse cataract model generated by insertion of a neomycin-resistant (neo^r) gene into an intron of the gene encoding mutant R49C αA-crystallin. Mice carrying the neo^r gene and wild-type Cryaa were also generated as controls. Heterozygous knock-in mice containing one wild type gene and one mutated gene for αA-crystallin (WT/R49C^neo) and homozygous knock-in mice containing two mutated genes (R49C^neo/R49C^neo) were compared.

Results

By 3 weeks, WT/R49C^neo mice exhibited large vacuoles in the cortical region 100 μm from the lens surface, and by 3 months posterior and nuclear cataracts had developed. WT/R49C^neo mice demonstrated severe posterior cataracts at 9 months of age, with considerable posterior nuclear migration evident in histological sections. R49C^neo/R49C^neo mice demonstrated nearly complete lens opacities by 5 months of age. In contrast, R49C mice in which the neo^r gene was deleted by breeding with CreEIIa mice developed lens abnormalities at birth, suggesting that the neo^r gene may suppress expression of mutant R49C αA-crystallin protein.

Conclusion

It is apparent that modification of membrane and cell-cell interactions occurs in the presence of the αA-crystallin mutation and rapidly leads to lens cell pathology in vivo.

Interactions between small heat shock protein alpha-crystallin and galectin-related interfiber protein (GRIFIN) in the ocular lens

As a member of the small heat shock protein superfamily, alpha-crystallin has a chaperone-like ability to recognize and bind denatured or unfolded proteins and prevent their aggregation. Recent studies suggest that alpha-crystallin may also interact with a variety of proteins under native conditions in vitro. To identify potential binding partners for alpha-crystallin in the intact ocular lens, we conducted cross-linking studies in transgenic mouse lenses designed for overexpression of His-tagged human alphaA-crystallin. Interacting proteins were copurified with the epitope-tagged crystallin complexes and were identified by tandem mass spectrometry. This approach identified GRIFIN (galectin-related interfiber protein) as a novel binding partner. Consistent with results from cross-linking, GRIFIN subunits copurified with alpha-crystallin complexes during size exclusion chromatography of nontransgenic mouse lens extracts prepared without chemical cross-linking. Equilibrium binding to GRIFIN was studied using native alpha-crystallin isolated from calf lenses as well as oligomeric complexes reconstituted from recombinant alphaA- and alphaB-crystallin subunits. Calf lens alpha-crystallin binds GRIFIN with relatively high affinity (K(d) = 6.5 +/- 0.8 microM) at a stoichiometry of 0.25 +/- 0.01 GRIFIN monomer/alpha-crystallin subunit. The binding interaction between alpha-crystallin and GRIFIN is enhanced up to 5-fold in the presence of 3 mM ATP. These binding data support the hypothesis that GRIFIN is a novel binding partner of alpha-crystallin in the lens.

Structure and mechanism of protein stability sensors: chaperone activity of small heat shock proteins

Small heat shock proteins (sHSP) make up a remarkably diverse group of molecular chaperones possessing a degree of structural plasticity unparalleled in other protein superfamilies. In the absence of chemical energy input, these stability sensors can sensitively recognize and bind destabilized proteins, even in the absence of gross misfolding. Cellular conditions regulate affinity toward client proteins, allowing tightly controlled switching and tuning of sHSP chaperone capacity. Perturbations of this regulation, through chemical modification or mutation, directly lead to a variety of disease states. This review explores the structural basis of sHSP oligomeric flexibility and the corresponding functional consequences in the context of a model describing sHSP activity with a set of three coupled thermodynamic equilibria. As current research illuminates many novel physiological roles for sHSP outside of their traditional duties as molecular chaperones, such a conceptual framework provides a sound foundation for describing these emerging functions in physiological and pathological processes.

Mechanism of cataract formation in alphaA-crystallin Y118D mutation

PURPOSE

The aim of this study was to elucidate the molecular mechanisms that lead to a dominant nuclear cataract in a mouse harboring the Y118D mutation in the alphaA-crystallin gene.

METHODS

The physicochemical properties of alpha-crystallin obtained from mouse lenses with the Y118D mutation as well as a recombinant Y118D alphaA-crystallin were studied using gel filtration, two-dimensional (2D) gel electrophoresis, multi-angle light scattering, circular dichroism, fluorescence, and chaperone activities.

RESULTS

Both native alpha-crystallin from mutant lens and recombinant alphaA-Y118D displayed higher molecular mass distribution than the wild-type. Circular dichroism spectra indicated changes in the secondary structures of alphaA-Y118D. The alphaA-Y118D protein prevented nonspecific protein aggregation more effectively than wild-type alphaA-crystallin. The gel filtration and 2D gel electrophoresis analysis showed a significant reduction of Y118D mutant protein in comparison with wild-type alphaA protein of heterozygous mutant lenses. Quantitative RT-PCR results confirmed a decrease in alphaA and alphaB transcripts in the homozygous mutant alpha A(Y118D/Y118D) lenses.

CONCLUSIONS

The alphaA-Y118D mutant protein itself displays an increased chaperone-like activity. However, the dominant nuclear cataract is associated with a significant decrease in the amount of alphaA-crystallin, leading to a reduction in total chaperone capacity needed for maintaining lens transparency.

Role of cysteine residues in the enhancement of chaperone function in methylglyoxal-modified human alpha A-crystallin

We have previously demonstrated that the reaction of a physiological dicarbonyl, methylglyoxal (MGO) enhances the chaperone function of human alpha A-crystallin. MGO can react with cysteine, arginine, and lysine residues in proteins. Although the role of arginine and lysine residues in the enhancement of chaperone function has been investigated, the role of cysteine residues is yet to be determined. In this study, we have investigated the effect of MGO modification on the structure and chaperone function of alpha A-crystallin mutant proteins in which C131 and C142 were replaced either individually or simultaneously with isoleucine. MGO-modification resulted in improved chaperone function in all three alpha A-crystallin mutants, including the cysteine-free double mutant. The enhanced chaperone function was due to increased surface hydrophobicity and increased binding of client proteins. These results suggest that the two cysteine residues, even though they could be modified, do not take part in the MGO-induced improvement in the chaperone function of human alpha A-crystallin.

Structural and functional roles of deamidation and/or truncation of N- or C-termini in human alpha A-crystallin

The purpose of the study was to compare the effects of deamidation alone, truncation alone, or both truncation and deamidation on structural and functional properties of human lens alphaA-crystallin. Specifically, the study investigated whether deamidation of one or two sites in alphaA-crystallin (i.e., alphaA-N101D, alphaA-N123D, alphaA-N101/123D) and/or truncation of the N-terminal domain (residues 1-63) or C-terminal extension (residues 140-173) affected the structural and functional properties relative to wild-type (WT) alphaA. Human WT-alphaA and human deamidated alphaA (alphaA-N101D, alphaA-N123D, alphaA-N101/123D) were used as templates to generate the following eight N-terminal domain (residues 1-63) deleted or C-terminal extension (residues 140-173) deleted alphaA mutants and deamidated plus N-terminal domain or C-terminal extension deleted mutants: (i) alphaA-NT (NT, N-terminal domain deleted), (ii) alphaA-N101D-NT, (iii) alphaA-N123D-NT, (iv) alphaA-N101/123D-NT, (v) alphaA-CT (CT, C-terminal extension deleted), (vi) alphaA-N101D-CT, (vii) alphaA-N123D-CT, and (viii) alphaA-N101/123D-CT. All of the proteins were purified and their structural and functional (chaperone activity) properties determined. The desired deletions in the alphaA-crystallin mutants were confirmed by matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectrometric analysis. Relative to WT-alphaA homomers, the mutant proteins exhibited major structural and functional changes. The maximum decrease in chaperone activity in homomers occurred on deamidation of N123 residue, but it was substantially restored after N- or C-terminal truncations in this mutant protein. Far-UV circular dichroism (CD) spectral analyses generally showed an increase in the beta-contents in alphaA mutants with deletions of N-terminal domain or C-terminal extension and also with deamidation plus above N- or C-terminal deletions. Intrinsic tryptophan (Trp) and total fluorescence spectral studies suggested altered microenvironments in the alphaA mutant proteins. Similarly, the ANS (8-anilino-1-naphthalenesulfate) binding showed generally increased fluorescence with blue shift on deletion of the N-terminal domain in the deamidated mutant proteins, but opposite effects were observed on deletion of the C-terminal extension. Molecular mass, polydispersity of homomers, and the rate of subunit exchange with WT-alphaB-crystallin increased on deletion of the C-terminal extension in the deamidated alphaA mutants, but on N-terminal domain deletion these values showed variable results based on the deamidation site. In summary, the data suggested that the deamidation alone showed greater effect on chaperone activity than the deletion of N-terminal domain or C-terminal extension of alphaA-crystallin. The N123 residue of alphaA-crystallin plays a crucial role in maintaining its chaperone function. However, both the N-terminal domain and C-terminal extension are also important for the chaperone activity of alphaA-crystallin because the activity was partially or fully recovered following either deletion in the alphaA-N123D mutant. The results of subunit exchange rates among alphaA mutants and WT-alphaB suggested that such exchange is an important determinant in maintenance of chaperone activity following deamidation and/or deletion of the N-terminal domain or C-terminal extension in alphaA-crystallin.

The other side of the Maillard reaction

The Maillard reaction plays an important role in eye lens aging and cataract formation. Methylglyoxal (MGO) is a metabolic dicarbonyl compound present in the lens. It reacts with arginine residues in lens proteins to form advanced glycation end products (AGEs), such as hydroimidazolones and argpyrimidine. alpha-Crystallin, comprising alphaA- and alphaB-crystallin, is a major protein of the lens and it functions as a chaperone protein. We have found that upon reaction with MGO, human alphaA-crystallin becomes a more effective chaperone. Modification of specific arginine residues to AGEs appears to be the reason. Mutation of these arginine residues to alanine mirrors the effect of MGO, suggesting neutralization of the positive charge on arginine residues as a cause for improved chaperone function. Reaction with MGO also blocks the loss of the chaperone function of alphaA-crystallin caused by nonenzymatic glycation by ascorbate and ribose. These findings suggest that low levels of MGO might help the lens remain transparent during aging.

Clinical variability of autosomal dominant cataract, microcornea and corneal opacity and novel mutation in the alpha A crystallin gene (CRYAA)

We studied 28 individuals from a four-generation Chilean family (ADC54) including 13 affected individuals with cataracts, microcornea and/or corneal opacity. All individuals underwent a complete ophthalmologic exam. We screened with a panel of polymorphic DNA markers for known loci that cause autosomal dominant cataracts, if mutated, and refined the locus using the ABI Prism Linkage Mapping Set Version 2.5, and calculated two-point lod scores. Novel PCR primers were designed for the three coding exons, including intron-exon borders, of the candidate gene alpha A crystallin (CRYAA). Clinically, affected individuals had diverse and novel cataracts with variable morphology (anterior polar, cortical, embryonal, fan-shaped, anterior subcapsular). Microcornea and corneal opacity was evident in some. Marker D21S171 gave a lod score of 4.89 (theta(m) = theta(f) = 0). CRYAA had a G414A transition that segregated with the disease and resulted in an amino acid alteration (R116H). The phenotypic variability within this family was significant with novel features of the cataracts and a corneal opacity. With the exception of iris coloboma, the clinical features in all six previously reported families with mutations in the CRYAA gene were found in this family. We identified a novel G414A transition in exon 3 of CRYAA that co-segregated with an autosomal dominant phenotype. The resulting amino acid change R116H is in a highly conserved region and represents a change in charge. The genotype-phenotype correlation of this previously unreported mutation provides evidence that other factors, genetic and/or environmental, may influence the development of cataract as a result of this alteration.

Chemical modulation of the chaperone function of human alphaA-crystallin

alphaA-crystallin is abundant in the lens of the eye and acts as a molecular chaperone by preventing aggregation of denaturing proteins. We previously found that chemical modification of the guanidino group of selected arginine residues by a metabolic alpha-dicarbonyl compound, methylglyoxal (MGO), makes human alphaA-crystallin a better chaperone. Here, we examined how the introduction of additional guanidino groups and modification by MGO influence the structure and chaperone function of alphaA-crystallin. alphaA-crystallin lysine residues were converted to homoarginine by guanidination with o-methylisourea (OMIU) and then modified with MGO. LC-ESI-mass spectrometry identified homoargpyrimidine and homohydroimidazolone adducts after OMIU and MGO treatment. Treatment with 0.25 M OMIU abolished most of the chaperone function. However, subsequent treatment with 1.0 mM MGO not only restored the chaperone function but increased it by approximately 40% and approximately 60% beyond that of unmodified alphaA-crystallin, as measured with citrate synthase and insulin aggregation assays, respectively. OMIU treatment reduced the surface hydrophobicity but after MGO treatment, it was approximately 39% higher than control. FRET analysis revealed that alphaA-crystallin subunit exchange rate was markedly retarded by OMIU modification, but was enhanced after MGO modification. These results indicate a pattern of loss and gain of chaperone function within the same protein that is associated with introduction of guanidino groups and their neutralization. These findings support our hypothesis that positively charged guanidino group on arginine residues keeps the chaperone function of alphaA-crystallin in check and that a metabolic alpha-dicarbonyl compound neutralizes this charge to restore and enhance chaperone function.

Mechanism of small heat shock protein function in vivo: a knock-in mouse model demonstrates that the R49C mutation in alpha A-crystallin enhances protein insolubility and cell death

alphaA-crystallin (Cryaa/HSPB4) is a small heat shock protein and molecular chaperone that prevents nonspecific aggregation of denaturing proteins. Several point mutations in the alphaA-crystallin gene cause congenital human cataracts by unknown mechanisms. We took a novel approach to investigate the molecular mechanism of cataract formation in vivo by creating gene knock-in mice expressing the arginine 49 to cysteine mutation (R49C) in alphaA-crystallin (alphaA-R49C). This mutation has been linked with autosomal dominant hereditary cataracts in a four-generation Caucasian family. Homologous recombination in embryonic stem cells was performed using a plasmid containing the C to T transition in exon 1 of the cryaa gene. alphaA-R49C heterozygosity led to early cataracts characterized by nuclear opacities. Unexpectedly, alphaA-R49C homozygosity led to small eye phenotype and severe cataracts at birth. Wild type littermates did not show these abnormalities. Lens fiber cells of alphaA-R49C homozygous mice displayed an increase in cell death by apoptosis mediated by a 5-fold decrease in phosphorylated Bad, an anti-apoptotic protein, but an increase in Bcl-2 expression. However, proliferation measured by in vivo bromodeoxyuridine labeling did not decline. The alphaA-R49C heterozygous and homozygous knock-in lenses demonstrated an increase in insoluble alphaA-crystallin and alphaB-crystallin and a surprising increase in expression of cytoplasmic gamma-crystallin, whereas no changes in beta-crystallin were observed. Co-immunoprecipitation analysis showed increased interaction between alphaA-crystallin and lens substrate proteins in the heterozygous knock-in lenses. To our knowledge this is the first knock-in mouse model for a crystallin mutation causing hereditary human cataract and establishes that alphaA-R49C promotes protein insolubility and cell death in vivo.

Recessive congenital total cataract with microcornea and heterozygote carrier signs caused by a novel missense CRYAA mutation (R54C)

PURPOSE

To determine the genetic basis for congenital total white cataract with microcornea in three affected siblings.

DESIGN

Prospective interventional case series.

METHODS

Clinical ophthalmic examination, venous blood sampling for linkage analyses, and diagnostic testing of identified candidate gene(s).

RESULTS

Three siblings had congenital total white cataract with microcornea; the parents and seven other siblings were asymptomatic. Linkage analysis mapped the phenotype to Hsa 21q22.3, the region of the gene for the alpha-A component of alpha-crystallin (CRYAA), with a logarithm of odds (LOD) score of 2.5. Diagnostic CRYAA sequencing revealed a novel homozygous nonsense mutation (R54C) in the three affected individuals only. One other sibling and the two parents were heterozygotes; these individuals had punctuate lenticular opacities evident by careful slit-lamp biomicroscopy which were not present in the noncarriers, all of whom had unremarkable ophthalmic examinations.

CONCLUSION

R54C is the second reported recessive CRYAA mutation associated with congenital cataract and the first with described morphology: punctuate lenticular opacities in carriers and congenital total white cataract with microcornea in homozygotes. The microcornea may have been caused by an inductive effect on the developing cornea from the abnormal lens and/or reduced CRYAA molecular chaperoning of the cornea.

Crystallins in the eye: Function and pathology

Crystallins are the predominant structural proteins in the lens that are evolutionarily related to stress proteins. They were first discovered outside the vertebrate eye lens by Bhat and colleagues in 1989 who found alphaB-crystallin expression in the retina, heart, skeletal muscles, skin, brain and other tissues. With the advent of microarray and proteome analysis, there is a clearer demonstration that crystallins are prominent proteins both in the normal retina and in retinal pathologies, emphasizing the importance of understanding crystallin functions outside of the lens. There are two main crystallin gene families: alpha-crystallins, and betagamma-crystallins. alpha-crystallins are molecular chaperones that prevent aberrant protein interactions. The chaperone properties of alpha-crystallin are thought to allow the lens to tolerate aging-induced deterioration of the lens proteins without showing signs of cataracts until older age. alpha-crystallins not only possess chaperone-like activity in vitro, but can also remodel and protect the cytoskeleton, inhibit apoptosis, and enhance the resistance of cells to stress. Recent advances in the field of structure-function relationships of alpha-crystallins have provided the first clues to their underlying roles in tissues outside the lens. Proteins of the betagamma-crystallin family have been suggested to affect lens development, and are also expressed in tissues outside the lens. The goal of this paper is to highlight recent work with lens epithelial cells from alphaA- and alphaB-crystallin knockout mice. The use of lens epithelial cells suggests that crystallins have important cellular functions in the lens epithelium and not just the lens fiber cells as previously thought. These studies may be directly relevant to understanding the general cellular functions of crystallins.