Alpha A- and alpha B-crystallin in the retina. Association with the post-Golgi compartment of frog retinal photoreceptors

Proteomic analysis of diabetic retinas

Introduction

As a metabolic disease, diabetes often leads to health complications such as heart failure, nephropathy, neurological disorders, and vision loss. Diabetic retinopathy (DR) affects as many as 100 million people worldwide. The mechanism of DR is complex and known to impact both neural and vascular components in the retina. While recent advances in the field have identified major cellular signaling contributing to DR pathogenesis, little has been reported on the protein post-translational modifications (PTM) - known to define protein localization, function, and activity - in the diabetic retina overall. Protein glycosylation is the enzymatic addition of carbohydrates to proteins, which can influence many protein attributes including folding, stability, function, and subcellular localization. O-linked glycosylation is the addition of sugars to an oxygen atom in amino acids with a free oxygen atom in their side chain (i.e., threonine, serine). To date, more than 100 congenital disorders of glycosylation have been described. However, no studies have identified the retinal O-linked glycoproteome in health or disease. With a critical need to expedite the discovery of PTMomics in diabetic retinas, we identified both global changes in protein levels and the retinal O-glycoproteome of control and diabetic mice.

Methods

We used liquid chromatography/mass spectrometry-based proteomics and high throughput screening to identify proteins differentially expressed and proteins differentially O-glycosylated in the retinas of wildtype and diabetic mice.

Results

Changes in both global expression levels of proteins and proteins differentially glycosylated in the retinas of wild-type and diabetic mice have been identified. We provide evidence that diabetes shifts both global expression levels and O-glycosylation of metabolic and synaptic proteins in the retina.

Discussion

Here we report changes in the retinal proteome of diabetic mice. We highlight alterations in global proteins involved in metabolic processes, maintaining cellular structure, trafficking, and neuronal processes. We then showed changes in O-linked glycosylation of individual proteins in the diabetic retina.

Therapeutic Potential of α-Crystallins in Retinal Neurodegenerative Diseases

The chaperone and anti-apoptotic activity of α-crystallins (αA- and αB-) and their derivatives has received increasing attention due to their tremendous potential in preventing cell death. While originally known and described for their role in the lens, the upregulation of these proteins in cells and animal models of neurodegenerative diseases highlighted their involvement in adaptive protective responses to neurodegeneration associated stress. However, several studies also suggest that chronic neurodegenerative conditions are associated with progressive loss of function of these proteins. Thus, while external supplementation of α-crystallin shows promise, their potential as a protein-based therapeutic for the treatment of chronic neurodegenerative diseases remains ambiguous. The current review aims at assessing the current literature supporting the anti-apoptotic potential of αA- and αB-crystallins and its potential involvement in retinal neurodegenerative diseases. The review further extends into potentially modulating the chaperone and the anti-apoptotic function of α-crystallins and the use of such functionally enhanced proteins for promoting neuronal viability in retinal neurodegenerative disease.

Tissue Transparency In Vivo

In vivo tissue transparency in the visible light spectrum is beneficial for many research applications that use optical methods, whether it involves in vivo optical imaging of cells or their activity, or optical intervention to affect cells or their activity deep inside tissues, such as brain tissue. The classical view is that a tissue is transparent if it neither absorbs nor scatters light, and thus absorption and scattering are the key elements to be controlled to reach the necessary transparency. This review focuses on the latest genetic and chemical approaches for the decoloration of tissue pigments to reduce visible light absorption and the methods to reduce scattering in live tissues. We also discuss the possible molecules involved in transparency.

Localization of αA-Crystallin in Rat Retinal Müller Glial Cells and Photoreceptors

Transparent cells in the vertebrate optical tract, such as lens fiber cells and corneal epithelium cells, have specialized proteins that somehow permit only a low level of light scattering in their cytoplasm. It has been shown that both cell types contain (1) beaded intermediate filaments as well as (2) α-crystallin globulins. It is known that genetic and chemical alterations to these specialized proteins induce cytoplasmic opaqueness and visual complications. Crystallins were described previously in the retinal Müller cells of frogs. In the present work, using immunocytochemistry, fluorescence confocal imaging, and immuno-electron microscopy, we found that αA-crystallins are present in the cytoplasm of retinal Müller cells and in the photoreceptors of rats. Given that Müller glial cells were recently described as "living light guides" as were photoreceptors previously, we suggest that αA-crystallins, as in other highly transparent cells, allow Müller cells and photoreceptors to minimize intraretinal scattering during retinal light transmission.

Alzheimer's and Danish dementia peptides induce cataract and perturb retinal architecture in rats

Familial Danish dementias (FDDs) are autosomal dominant neurodegenerative disorders that are associated with visual defects. In some aspects, FDD is similar to Alzheimer's disease (AD)- the amyloid deposits in FDD and AD are made of short peptides: amyloid β (Aβ) in AD and ADan in FDD. Previously, we demonstrated an interaction between the dementia peptides and α-crystallin leading to lens opacification in organ culture due to impaired chaperone activity of α-crystallin. Herein, we report the in vivo effects of ADan and Aβ on the eye. ADan [reduced (ADan-red) and oxidized (ADan-oxi)] and Aβ (Aβ1-40 and Aβ1-42) were injected intravitreally in rats. The onset of cataract was seen after injection of all the peptides, but the cataract matured by 2 weeks in the case of ADan-red, 5 weeks for ADan-oxi and 6 weeks for Aβ1-40, while Aβ1-42 had minimal effect on cataract progression. The severity of cataract is associated with insolubilization and alterations in crystallins and loss of chaperone activity of α-crystallin. Further, disruption of the architecture of the retina was evident from a loss of rhodopsin, increased gliosis, and the thinning of the retina. These results provide a basis for the dominant heredo-otoophthalmo-encephalopathy (HOOE)/FDD syndrome and indicate that ADan peptides are more potent than Aβpeptides in inflicting visual impairment.

Heat shock proteins in the retina: Focus on HSP70 and alpha crystallins in ganglion cell survival

Heat shock proteins (HSPs) belong to a superfamily of stress proteins that are critical constituents of a complex defense mechanism that enhances cell survival under adverse environmental conditions. Cell protective roles of HSPs are related to their chaperone functions, antiapoptotic and antinecrotic effects. HSPs' anti-apoptotic and cytoprotective characteristics, their ability to protect cells from a variety of stressful stimuli, and the possibility of their pharmacological induction in cells under pathological stress make these proteins an attractive therapeutic target for various neurodegenerative diseases; these include Alzheimer's, Parkinson's, Huntington's, prion disease, and others. This review discusses the possible roles of HSPs, particularly HSP70 and small HSPs (alpha A and alpha B crystallins) in enhancing the survival of retinal ganglion cells (RGCs) in optic neuropathies such as glaucoma, which is characterized by progressive loss of vision caused by degeneration of RGCs and their axons in the optic nerve. Studies in animal models of RGC degeneration induced by ocular hypertension, optic nerve crush and axotomy show that upregulation of HSP70 expression by hyperthermia, zinc, geranyl-geranyl acetone, 17-AAG (a HSP90 inhibitor), or through transfection of retinal cells with AAV2-HSP70 effectively supports the survival of injured RGCs. RGCs survival was also stimulated by overexpression of alpha A and alpha B crystallins. These findings provide support for translating the HSP70- and alpha crystallin-based cell survival strategy into therapy to protect and rescue injured RGCs from degeneration associated with glaucomatous and other optic neuropathies.

Alpha crystallins in the retinal pigment epithelium and implications for the pathogenesis and treatment of age-related macular degeneration

BACKGROUND

αA- and αB crystallins are principal members of the small heat shock protein family and elicit both a cell protective function and a chaperone function. α-Crystallins have been found to be prominent proteins in normal and pathological retina emphasizing the importance for in-depth understanding of their function and significance.

SCOPE OF REVIEW

Retinal pigment epithelial cells (RPE) play a vital role in the pathogenesis of age-related macular degeneration (AMD). This review addresses a number of cellular functions mediated by α-crystallins in the retina. Prominent expression of αB crystallin in mitochondria may serve to protect cells from oxidative injury. αB crystallin as secretory protein via exosomes can offer neuroprotection to adjacent RPE cells and photoreceptors. The availability of chaperone-containing minipeptides of αB crystallin could prove to be a valuable new tool for therapeutic treatment of retinal disorders.

MAJOR CONCLUSIONS

α-Crystallins are expressed in cytosol and mitochondria of RPE cells and are regulated during oxygen-induced retinopathy and during development. α-Crystallins protect RPE from oxidative-and ER stress-induced injury and autophagy. αB-Crystallin is a modulator of angiogenesis and vascular endothelial growth factor. αB Crystallin is secreted via exosomal pathway. Minichaperone peptides derived from αB Crystallin prevent oxidant induced cell death and have therapeutic potential.

GENERAL SIGNIFICANCE

Overall, this review summarizes several novel properties of α-crystallins and their relevance to maintaining normal retinal function. In particular, the use of α-crystallin derived peptides is a promising therapeutic strategy to combat retinal diseases such as AMD. This article is part of a Special Issue entitled Crystallin biochemistry in health and disease.

Therapeutic potential of α-crystallin

BACKGROUND

The findings that α-crystallins are multi-functional proteins with diverse biological functions have generated considerable interest in understanding their role in health and disease. Recent studies have shown that chaperone peptides of α-crystallin could be delivered into cultured cells and in experimental animals with beneficial effects against protein aggregation, oxidation, inflammation and apoptosis.

SCOPE OF REVIEW

In this review, we will summarize the latest developments on the therapeutic potential of α-crystallins and their functional peptides.

MAJOR CONCLUSIONS

α-Crystallins and their functional peptides have shown significant favorable effects against several diseases. Their targeted delivery to tissues would be of great therapeutic benefit. However, α-crystallins can also function as disease-causing proteins. These seemingly contradictory functions must be carefully considered prior to their therapeutic use.

GENERAL SIGNIFICANCE

αA and αB-Crystallin are members of the small heat shock protein family. These proteins exhibit molecular chaperone and anti-apoptotic activities. The core crystallin domain within these proteins is largely responsible for these prosperities. Recent studies have identified peptides within the crystallin domain of both α- and αB-crystallins with remarkable chaperone and anti-apoptotic activities. Administration of α-crystallin or their functional peptides has shown substantial inhibition of pathologies in several diseases. However, α-crystallins have been shown to promote disease-causing pathways. These two sides of the proteins are discussed in this review. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Alpha-crystallin-mediated protection of lens cells against heat and oxidative stress-induced cell death

In addition to their key role as structural lens proteins, α-crystallins also appear to confer protection against many eye diseases, including cataract, retinitis pigmentosa, and macular degeneration. Exogenous recombinant α-crystallin proteins were examined for their ability to prevent cell death induced by heat or oxidative stress in a human lens epithelial cell line (HLE-B3). Wild type αA- or αB-crystallin (WT-αA and WT-αB) and αA- or αB-crystallins, modified by the addition of a cell penetration peptide (CPP) designed to enhance the uptake of proteins into cells (gC-αB, TAT-αB, gC-αA), were produced by recombinant methods. In vitro chaperone-like assays were used to assay the ability of α-crystallins to protect client proteins from chemical or heat induced aggregation. In vivo viability assays were performed in HLE-B3 to determine whether pre-treatment with α-crystallins reduced death after exposure to oxidative or heat stress. Most of the five recombinant α-crystallin proteins tested conferred some in vitro protection from protein aggregation, with the greatest effect seen with WT-αB and gC-αB. All α-crystallins displayed significant protection to oxidative stress induced cell death, while only the αB-crystallins reduced cell death induced by thermal stress. Our findings indicate that the addition of the gC tag enhanced the protective effect of αB-crystallin against oxidative but not thermally-induced cell death. In conclusion, modifications that increase the uptake of α-crystallin proteins into cells, without destroying their chaperone-like activity and anti-apoptotic functions, create the potential to use these proteins therapeutically.

A degenerative retinal process in HIV-associated non-infectious retinopathy

HIV retinopathy is the most common non-infectious complication in the eyes of HIV-positive individuals. Oncotic lesions in the retinal nerve fiber layer, referred to as cotton wool spots (CWS), and intraretinal (IR) hemorrhages are frequently observed but are not unique to this pathology. HIV-positive patients have impaired color vision and contrast sensitivity, which worsens with age. Evidence of inner-retinal lesions and damage have been documented ophthalmoscopically, however their long term structural effect has not been investigated. It has been hypothesized that they may be partially responsible for loss of visual function and visual field. In this study we utilized clinical data, retinal imaging and transcriptomics approaches to comprehensively interrogate non-infectious HIV retinopathy. The methods employed encompassed clinical examinations, fundus photography, indirect ophthalmoscopy, Farmsworth-Munsell 100 hue discrimination testing and Illumina BeadChip analyses. Here we show that changes in the outer retina, specifically in the retinal pigment epithelium (RPE) and photoreceptor outer segments (POS) contribute to vision changes in non-infectious HIV retinopathy. We find that in HIV-positive retinae there is an induction of rhodopsin and other transcripts (including PDE6A, PDE6B, PDE6G, CNGA1, CNGB1, CRX, NRL) involved in visual transduction, as well as structural components of the rod photoreceptors (ABCA4 and ROM1). This is consistent with an increased rate of renewal of rod outer segments induced via increased phagocytosis by HIV-infected RPE previously reported in culture. Cone-specific transcripts (OPN1SW, OPN1LW, PDE6C, PDE6H and GRK7) are uniformly downregulated in HIV positive retina, likely due to a partial loss of cone photoreceptors. Active cotton wool spots and intraretinal hemorrhages (IRH) may not affect photoreceptors directly and the interaction of photoreceptors with the aging RPE may be the key to the progressive vision changes in HIV-positive patients.

New insights into the mechanism of lens development using zebra fish

On the basis of recent advances in molecular biology, genetics, and live-embryo imaging, direct comparisons between zebra fish and human lens development are being made. The zebra fish has numerous experimental advantages for investigation of fundamental biomedical problems that are often best studied in the lens. The physical characteristics of visible light can account for the highly coordinated cell differentiation during formation of a beautifully transparent, refractile, symmetric optical element, the biological lens. The accessibility of the zebra fish lens for direct investigation during rapid development will result in new knowledge about basic functional mechanisms of epithelia-mesenchymal transitions, cell fate, cell-matrix interactions, cytoskeletal interactions, cytoplasmic crowding, membrane transport, cell adhesion, cell signaling, and metabolic specialization. The lens is well known as a model for characterization of cell and molecular aging. We review the recent advances in understanding vertebrate lens development conducted with zebra fish.

Novel roles for α-crystallins in retinal function and disease

α-Crystallins are key members of the superfamily of small heat shock proteins that have been studied in detail in the ocular lens. Recently, novel functions for α-crystallins have been identified in the retina and in the retinal pigmented epithelium (RPE). αB-Crystallin has been localized to multiple compartments and organelles including mitochondria, golgi apparatus, endoplasmic reticulum and nucleus. α-Crystallins are regulated by oxidative and endoplasmic reticulum stress, and inhibit apoptosis-induced cell death. α-Crystallins interact with a large number of proteins that include other crystallins, and apoptotic, cytoskeletal, inflammatory, signaling, angiogenic, and growth factor molecules. Studies with RPE from αB-crystallin deficient mice have shown that αB-crystallin supports retinal and choroidal angiogenesis through its interaction with vascular endothelial growth factor. αB-Crystallin has also been shown to have novel functions in the extracellular space. In RPE, αB-crystallin is released from the apical surface in exosomes where it accumulates in the interphotoreceptor matrix and may function to protect neighboring cells. In other systems administration of exogenous recombinant αB-crystallin has been shown to be anti-inflammatory. Another newly described function of αB-crystallin is its ability to inhibit β-amyloid fibril formation. α-Crystallin minichaperone peptides have been identified that elicit anti-apoptotic function in addition to being efficient chaperones. Generation of liposomal particles and other modes of nanoencapsulation of these minipeptides could offer great therapeutic advantage in ocular delivery for a wide variety of retinal degenerative, inflammatory and vascular diseases including age-related macular degeneration and diabetic retinopathy.

αB-crystallin/sHSP protects cytochrome c and mitochondrial function against oxidative stress in lens and retinal cells

BACKGROUND

αB-crystallin/sHSP protects cells against oxidative stress damage. Here, we mechanistically examined its ability to preserve mitochondrial function in lens and retinal cells and protect cytochrome c under oxidative stress conditions.

METHODS

αB-crystallin/sHSP was localized in human lens (HLE-B3) and retinal (ARPE-19) cells. αB-crystallin/sHSP was stably over-expressed and its ability to preserve mitochondrial membrane potential under oxidative stress conditions was monitored. Interactions between αB-crystallin/sHSP and cytochrome c were examined by fluorescent resonance energy transfer (FRET) and by co-immune precipitation. The ability of αB-crystallin/sHSP to protect cytochrome c against methionine-80 oxidation was monitored.

RESULTS

αB-crystallin/sHSP is present in the mitochondria of lens and retinal cells and is translocated to the mitochondria under oxidative conditions. αB-crystallin/sHSP specifically interacts with cytochrome c in vitro and in vivo and its overexpression preserves mitochondrial membrane potential under oxidative stress conditions. αB-crystallin/sHSP directly protects cytochrome c against oxidation.

GENERAL SIGNIFICANCE

These data demonstrate that αB-crystallin/sHSP maintains lens and retinal cells under oxidative stress conditions at least in part by preserving mitochondrial function and by protecting cytochrome c against oxidation. Since oxidative stress and loss of mitochondrial function are associated with eye lens cataract and age-related macular degeneration, loss of these αB-crystallin/sHSP functions likely plays a key role in the development of these diseases. αB-crystallin/sHSP is expressed throughout the body and its ability to maintain mitochondrial function is likely important for the prevention of multiple degenerative diseases.

Small heat shock protein expression and functions during development

The expression of small heat shock proteins is tightly regulated during development in multiple organisms. As housekeeping proteins, small heat shock proteins help protect cells from apoptosis, stabilize the cytoskeleton and contribute to proteostasis. Consistently, depletion of one small heat shock protein is usually not detrimental due to a certain level of redundancy between the functions of each small heat shock protein. However, while their stress-induced expression is regulated by heat shock factors, their constitutive expression is under the control of other specific transcription factors, suggesting the existence of very specialized functions. This review focuses on the expression patterns and functions of small heat shock proteins in various organisms during development. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.

Impact of diabetes on alpha-crystallins and other heat shock proteins in the eye

Diabetes and its related complications represent a major growing health concern and economic burden worldwide. Ocular manifestations of diabetes include cataractogenesis and retinopathy, the latter being the leading cause of blindness in the working-age population. Despite numerous studies and recent progress, the exact pathophysiology of the disease remains to be fully elucidated and development of new and improved therapeutic strategies for this chronic condition are greatly needed. Heat shock proteins (Hsps) are highly conserved families of proteins, which are generally regarded as protective molecules that play a wide variety of roles and can be expressed in response to different types of cellular stresses. In recent years, numerous studies have reported their implication in various ocular diseases including diabetic retinopathy. The present review focuses on the potential implication of Hsps in ocular diabetic complications and discusses their specific mechanisms of regulation with respect to their expression, functions and alteration during diabetes. The review will conclude by examining the potential of Hsps as therapeutic agents or targets for the treatment of diabetic retinopathy.

Regulation of mouse small heat shock protein αb-crystallin gene by aryl hydrocarbon receptor

The stress-inducible small heat shock protein (shsp)/αB-crystallin gene is expressed highly in the lens and moderately in other tissues. Here we provide evidence that it is a target gene of the aryl hydrocarbon receptor (AhR) transcription factor. A sequence (−329/−323, CATGCGA) similar to the consensus xenobiotic responsive element (XRE), called here XRE-like, is present in the αBE2 region of αB-crystallin enhancer and can bind AhR in vitro and in vivo. αB-crystallin protein levels were reduced in retina, lens, cornea, heart, skeletal muscle and cultured muscle fibroblasts of AhR^−/− mice; αB-crystallin mRNA levels were reduced in the eye, heart and skeletal muscle of AhR^−/− mice. Increased AhR stimulated αB-crystallin expression in transfection experiments conducted in conjunction with the aryl hydrocarbon receptor nuclear translocator (ARNT) and decreased AhR reduced αB-crystallin expression. AhR effect on aB-crystallin promoter activity was cell-dependent in transfection experiments. AhR up-regulated αB-crystallin promoter activity in transfected HeLa, NIH3T3 and COS-7 cells in the absence of exogenously added ligand (TCDD), but had no effect on the αB-crystallin promoter in C₂C₁₂, CV-1 or Hepa-1 cells with or without TCDD. TCDD enhanced AhR-stimulated αB-crystallin promoter activity in transfected αTN4 cells. AhR could bind to an XRE-like site in the αB-crystallin enhancer in vitro and in vivo. Finally, site-specific mutagenesis experiments showed that the XRE-like motif was necessary for both basal and maximal AhR-induction of αB-crystallin promoter activity. Our data strongly suggest that AhR is a regulator of αB-crystallin gene expression and provide new avenues of research for the mechanism of tissue-specific αB-crystallin gene regulation under normal and physiologically stressed conditions.

Light induced and circadian effects on retinal photoreceptor cell crystallins

Crystallins in the retina may serve a chaperone-like protective function. In this study we measured mRNA levels for alpha-, beta- and gamma-crystallins in rat retinas following treatment with potentially damaging levels of light. We also determined crystallin protein patterns in photoreceptor cell rod outer segments (ROSs) isolated from rats exposed to intense light. Weanling albino rats were maintained in a dim cyclic light environment or in darkness for 40days. At P60 animals were treated with intense visible light, for as long as 8h, beginning at various times of the day or night. Retinas were excised immediately after light treatment and used for quantitative RT-PCR, or to prepare ROSs for western analysis. Some eyes were frozen in OCT for crystallin immunohistochemistry. Intense light exposure led to increases in mRNA expression for all retinal crystallins and to changes in ROS crystallin immunoreactivity. These light-induced changes were found to depend on the time of day that exposure started, duration of light treatment and previous light rearing history. We suggest that crystallin synthesis in retina exhibits a dependence on both light stress and circadian rhythm and that within photoreceptor cells crystallins appear to migrate in a light-independent, circadian fashion.

Mutation screen of beta-crystallin genes in 274 patients with age-related macular degeneration

PURPOSE

The crystallin family of proteins comprise the main structural proteins of the vertebrate lens and have been classified into alpha-, beta-, and gamma- families. Several of the beta-crystallin proteins have been detected in the retina where they are each localized to different compartments of rod and cone photoreceptors. Functionally, beta-crystallins have been implicated in the protection of the retina from intense light exposure. Two members of the beta-crystallins, CRYBB1 and CRYBB2, have been identified in drusen preparations isolated from the retina of donor eyes of patients with age-related macular degeneration (AMD), the leading cause of blindness in the elderly population of developed countries. We therefore investigated CRYBB1 and CRYBB2 as candidate genes for AMD in 274 unrelated patients.

RESULTS

A mutation screen of the entire coding region of the CRYBB1gene uncovered eight sequence variations, including three missense changes, two intronic changes and three isocoding changes. A mutation screen of the entire coding region of the CRYBB2 gene uncovered three sequence variations, one isocoding change and two intronic changes.

CONCLUSIONS

Although variant alleles of the CRYBB1 and CRYBB2 genes were found, none are considered pathogenic.

Up-regulation of crystallins is involved in the neuroprotective effect of wolfberry on survival of retinal ganglion cells in rat ocular hypertension model

Wolfberry (fruit of Lycium barbarum Linn) has been known for balancing 'Yin' and 'Yang' in the body, nourishing the liver and kidney, improving visual acuity for more than 2,500 years in oriental countries. The active components in wolfberry include L. barbarum polysaccharide (LBP), zeaxanthine, betaine, cerebroside and trace amounts of zinc, iron, and copper. Each of them confers distinct beneficial effects and together they help to explain widespread use of wolfberry in the eastern world. Earlier study reported the neuroprotective effects of LBP on retinal ganglion cell (RGC) in an experimental model of glaucoma and the underlying in vivo cellular mechanisms of LBP neuroprotection deserve further exploration. In this study, we adopted proteomics, functional genomics, to evaluate pharmacological effects of LBP on the neuronal survival pathways. Among the significantly changed proteins induced by LBP feeding on ocular hypertension (OH) retinas, only proteins in crystallin family were focused in this study. The proteomic results were further confirmed using the Western blotting of the retinas and immunohistochemical staining of the retinal sections. We demonstrated that neuroprotective effect of-wolfberry extract-LBP on the survival of RGCs may be mediated via direct up-regulation of neuronal survival signal betaB2-crystallin.

Molecular chaperones and photoreceptor function

Molecular chaperones facilitate and regulate protein conformational change within cells. This encompasses many fundamental cellular processes: including the correct folding of nascent chains; protein transport and translocation; signal transduction and protein quality control. Chaperones are, therefore, important in several forms of human disease, including neurodegeneration. Within the retina, the highly specialized photoreceptor cell presents a fascinating paradigm to investigate the specialization of molecular chaperone function and reveals unique chaperone requirements essential to photoreceptor function. Mutations in several photoreceptor proteins lead to protein misfolding mediated neurodegeneration. The best characterized of these are mutations in the molecular light sensor, rhodopsin, which cause autosomal dominant retinitis pigmentosa. Rhodopsin biogenesis is likely to require chaperones, while rhodopsin misfolding involves molecular chaperones in quality control and the cellular response to protein aggregation. Furthermore, the specialization of components of the chaperone machinery to photoreceptor specific roles has been revealed by the identification of mutations in molecular chaperones that cause inherited retinal dysfunction and degeneration. These chaperones are involved in several important cellular pathways and further illuminate the essential and diverse roles of molecular chaperones.

Application of proteomic technology in eye research: a mini review

Proteomics is a rapidly growing research area for the study of the protein cognate of genomic data. This review gives a brief overview of the modern proteomic technology. In addition to general applications of proteomics, we highlight its contribution to studying the physiology of different ocular tissues. We also summarise the published proteomic literature in the broad context of ophthalmic diseases, such as cataract, age-related maculopathy, diabetic retinopathy, glaucoma and myopia. The proteomic technology is a useful research tool and it will continue to advance our understanding of a variety of molecular processes in ocular tissues and diseases.

Interactions between Important Regulatory Proteins and Human αB Crystallin

Protein pin arrays assessed interactions between alphaB crystallin and 12 regulatory proteins, including EGF, FGF-2, IGF-1, NGF-beta, TGF-beta, VEGF, insulin, beta-catenin, caspase-3, caspase-8, Bcl-2, and Bcl-xL, which are important in cellular differentiation, proliferation, signaling, cytoskeletal assembly, and apoptosis. FGF-2, NGF-beta, VEGF, insulin, and beta-catenin had strong interactions with human alphaB crystallin peptides, and the alphaB crystallin interactive sequences for these proteins were identified. The seven remaining proteins (EGF, IGF-1, TGF-beta, caspase-3, caspase-8, BCl-2, and Bcl-xL) did not interact with alphaB crystallin. The alphaB crystallin sequences that interacted with FGF-2, NGF-beta, VEGF, insulin, and beta-catenin overlapped with sequences that selectively interact with partially unfolded proteins, suggesting a common function for alphaB crystallin in chaperone activity and the regulation of cell growth and differentiation. Chaperone assays conducted with full-length alphaB crystallin and synthetic alphaB crystallin peptides confirmed the ability of alphaB crystallin to protect against the aggregation of FGF-2 and VEGF, suggesting that alphaB crystallin protects these proteins against unfolding and aggregation under conditions of stress. This is the first report in which sequences involved in interactions with regulatory proteins, including FGF-2, NGF-beta, VEGF, insulin, and beta-catenin, were identified in a small heat shock protein.

Paradoxical effects of substitution and deletion mutation of Arg56 on the structure and chaperone function of human alphaB-crystallin

Human alphaB-crystallin is a small heat-shock protein that functions as a molecular chaperone. Recent studies indicate that deletion of a peptide (54FLRAPSWF61) from its N-terminus makes it a better chaperone, and this particular sequence is thought to participate in substrate interaction and subunit exchange with alphaA-crystallin. To determine whether the positive charge on arginine 56 (R56) influences these functions, we prepared human alphaB-crystallin mutants in which R56 was deleted (DeltaR56) or replaced by alanine (R56A). To determine if the effects are specific to R56, we generated two additional mutant proteins in which the two neighboring amino acids were deleted (DeltaL55 and DeltaA57). Dynamic light scattering studies suggested that none of the mutations affected the oligomeric mass of the protein. Far-ultraviolet circular dichroism (UV CD) spectra revealed greater helicity in the secondary structures of R56A and DeltaR56 compared to that of the wild-type (Wt) protein. Near-UV CD spectra showed that the tertiary structure is perturbed in all mutants. Insulin and citrate synthase aggregation assays showed 38 and 30% improvement of chaperone function in DeltaR56 compared to that of the Wt. In contrast, the R56A mutant lost most of its chaperone function. Deletion mutants, DeltaL55 and DeltaA57, showed no significant changes in the chaperone function compared to that of the Wt. The DeltaR56 mutant had a higher surface hydrophobicity than the Wt, but the R56A mutant had a lower hydrophobicity. Our data show paradoxical effects of the deletion and substitution of R56 and imply that the chaperone function of human alphaB-crystallin is dictated not only by the positive charge on R56 but also by the conformational change that it bestows on the protein.

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.

Genetic, age and light mediated effects on crystallin protein expression in the retina

To probe for possible relationships between retinal crystallins and retinal degenerations, protein expression was compared in normal Sprague-Dawley rats, treated or not with intense light, Royal College of Surgeons (RCS) rats and transgenic rats expressing rhodopsin mutations. Rats were reared in dim cyclic light for 21-75 days. Photoreceptor cell DNA levels were determined at various ages to assess the rates of visual cell loss. 1D- and 2D-gel electrophoresis was used to profile retinal protein expression. Crystallins were identified by western analysis and by tandem mass spectrometry. In normal rat retinas, alpha, beta and gamma crystallins were present, although alphaA- and gamma-crystallins exhibited some increase with age. As measured by DNA levels, the rate of genetically induced photoreceptor cell loss was greater in rats with faster degenerating retinas (RCS, S334-ter Line 4, P23H Line 3) than in rats with slower degenerating retinas (S334-ter Line 9, P23H Line 2). In genetic models of retinal degeneration increased levels of immunoreactivity for all crystallins, especially alphaA-insert, correlated with the different rates of photoreceptor loss. In the light induced degeneration model alphaA-insert was unchanged, truncated alphaB-crystallin levels were increased and gamma-crystallins were greatly reduced. In the RCS rat retina 16 different crystallins were identified. Our data suggests that an increase in crystallin expression occurs during various retinal degenerations and that the increases may be related to the severity, type and onset of retinal degeneration.

Study of posttranslational modifications in lenticular αA-Crystallin of mice using proteomic analysis techniques

In the present work the complexity in the 2D-gel protein pattern of murin lenticular alphaA-Crystallin was analyzed. An in depth study of the different protein isoforms was done combining different proteomic tools. Lens proteins of four different ages, from embryo to 100-week-old mice, were separated by large 2D-PAGE, revealing an increase in the number and intensity of the spots of alphaA-Crystallin during the process of aging. For further analyses the oldest mice were chosen. Comparison and evaluation of two different staining methods proved Imidazole-Zinc to be a good alternative to the generally used Coomassie stain. The characterization of the different alphaA-Crystallin protein species was done using nanoLC-ESI-MS/MS (liquid chromatography electrospray ionisation tandem mass spectrometry). Data interpretation was done by database searching, manual validation and a new MS/MS-interpretation tool for posttranslational modifications--the PTM-Explorer. Using this way, eight different phosphorylation sites were identified and localized; the identification of four of them was not published so far. Furthermore, quantitative N-terminal acetylation of alphaA-Crystallin and variable C-terminal truncation was observed, also not published in this extent yet. The results of the mass spectrometric analysis were validated by immunoblotting experiments using two different alphaA-Crystallin specific antibodies. In addition, a fluorescent phospho-specific stain was used to detect the protein spots including phosphorylation groups. Re-separation 2D-PAGE was done to round off the present study and explain the appearance of some of the protein spots in the gel as artifacts of the 2D-PAGE separation.

Biological characterization of gene response in Rpe65-/- mouse model of Leber's congenital amaurosis during progression of the disease

RPE65 is the retinal isomerase essential for conversion of all-trans-retinyl ester to 11-cis-retinol in the visual cycle. Leber's congenital amaurosis (LCA), an autosomal recessive form of RP resulting in blindness, is commonly caused by mutations in the Rpe65 gene. Whereas the molecular mechanisms by which these mutations contribute to retinal disease remain largely unresolved, affected patients show marked RPE damage and photoreceptor degeneration. We evaluated gene expression in Rpe65-/- mouse model of LCA before and at the onset of photoreceptor cell death in 2, 4, and 6 month old animals. Microarray analysis demonstrates altered expression of genes involved in phototransduction, apoptosis regulation, cytoskeleton organization, and extracellular matrix (ECM) constituents. Cone-specific phototransduction genes are strongly decreased, reflecting early loss of cones. In addition, remaining rods show modified expression of genes encoding components of the cytoskeleton and ECM. This may affect rod physiology and interaction with the adjacent RPE and lead to loss of survival signals, as reflected by the alteration of apoptosis-related genes Together, these results suggest that RPE65 defect triggers an overall remodeling of the neurosensitive retina that may, in turn, disrupt photoreceptor homeostasis and induce apoptosis signaling cascade toward retinal cell death.

Regulation and function of small heat shock protein genes during amphibian development

Small heat shock proteins (shsps) are molecular chaperones that are inducible by environmental stress such as elevated temperature or exposure to heavy metals or arsenate. Recent interest in shsps has been propelled by the finding that shsp synthesis or mutations are associated with various human diseases. While much is known about shsps in cultured cells, less is known about their expression and function during early animal development. In amphibian model systems, shsp genes are developmentally regulated under both normal and environmental stress conditions. For example, in Xenopus, the shsp gene family, hsp30, is repressed and not heat-inducible until the late neurula/early tailbud stage whereas other hsps are inducible at the onset of zygotic genome activation at the midblastula stage. Furthermore, these shsp genes are preferentially induced in selected tissues. Recent studies suggest that the developmental regulation of these shsp genes is controlled, in part, at the level of chromatin structure. Some shsps including Xenopus and Rana hsp30 are synthesized constitutively in selected tissues where they may function in the prevention of apoptosis. During environmental stress, amphibian multimeric shsps bind to denatured target protein, inhibittheir aggregation and maintain them in a folding-competent state until reactivated by other cellular chaperones. Phosphorylation of shsps appears to play a major role in the regulation of their function.

Esterified and unesterified cholesterol in drusen and basal deposits of eyes with age-related maculopathy

To address the potential for an outer segment (OS) contribution to the sub-retinal pigment epithelium (RPE) lesions of age-related maculopathy (ARM), we quantified esterified and unesterified cholesterol (EC, UC) with the sterol-specific fluorescent probe filipin in cryosections of ARM eyes. Twenty six eyes from 20 donors were preserved <5 hr after death in 4% paraformaldehyde (n = 16) or 2.5% glutaraldehyde/1% paraformaldehyde (n = 10). Eyes had exudative late ARM (n = 6), geographic atrophy (n = 15), and drusen > or =125 microm (n = 11). Sections were stained with filipin for UC or were extracted and hydrolysed with cholesterol esterase before filipin staining for EC. Drusen varied in cholesterol content, with a rough correlation between EC and UC. Dome-shaped drusen contained distinctive, loosely packed UC-rich loops. In basal deposits, EC and UC were more prominent near Bruch's membrane than near the RPE. A UC-rich material was localized within the subretinal space (n = 4). Maximum filipin fluorescence due to UC was quantified in 47 lesions (19 drusen, 24 basal deposits, and 4 sub-retinal) from 12 ARM eyes and compared to OS and inner plexiform layer (IPL) of uninvolved retina in the same sections. Relative to IPL, UC fluorescence was higher in lesions (mean+/-s.d: 1.63+/-0.69) and lower in OS (0.64+/-0.18). If only the packing of membranes explained fluorescence intensity, then one would expect much higher intensities in membrane-rich OS than in lesions. Because the converse is true, the membranous material in lesions must be more highly enriched in cholesterol on a per unit area basis. UC in sub-RPE deposits cannot be derived directly from OS without considerable intracellular processing within RPE, additional cholesterol sources, or both.

A potential role for β- and γ-crystallins in the vascular remodeling of the eye

We demonstrate that expression of beta- and gamma-crystallins is associated with intraocular vessels during normal vascular development of the eye and also in the Nuc1 rat, a mutant in which the hyaloid vascular system fails to regress normally. Real-Time RT PCR, Western blot and metabolic labeling studies indicate an increased expression of beta- and gamma-crystallins in Nuc1 retina. The increased expression of crystallins was localized to the astrocytes surrounding the intraocular vessels. A similar pattern of crystallin expression was also observed in the retinal vessels during normal development. Cultured human astrocytes exposed to 3-nitropropionic acid, an established model of neuronal hypoxia, increased VEGF expression, as expected, but also increased expression of crystallins. Our data suggest that crystallins may function together with VEGF during vascular remodeling. Interestingly, in human PFV (persistent fetal vasculature) disease, where the hyaloid vasculature abnormally persists after birth, we show that astrocytes express both VEGF and crystallins.

Small Heat Shock Protein αB-Crystallin Is Part of Cell Cycle-dependent Golgi Reorganization

AlphaB-crystallin is a developmentally regulated small heat shock protein known for its binding to a variety of denatured polypeptides and suppression of protein aggregation in vitro. Elevated levels of alphaB-crystallin are known to be associated with a number of neurodegenerative pathologies such as Alzheimer disease and multiple sclerosis. Mutations in alphaB-crystallin gene have been linked to desmin related cardiomyopathy and cataractogenesis. The physiological function of this protein, however, is unknown. Using discontinuous sucrose density gradient fractionation of post-nuclear supernatants, prepared from rat tissues and human glioblastoma cell line U373MG, we have identified discrete membrane-bound fractions of alphaB-crystallin, which co-sediment with the Golgi matrix protein, GM130. Confocal microscopy reveals co-localization of alphaB-crystallin with BODIPY TR ceramide and the Golgi matrix protein, GM130, in the perinuclear Golgi in human glioblastoma U373MG cells. Examination of synchronized cultures indicated that alphaB-crystallin follows disassembly of the Golgi at prometaphase and its reassembly at the completion of cytokinesis, suggesting that this small heat shock protein, with its chaperone-like activity, may have an important role in the Golgi reorganization during cell division.

Modified alpha A crystallin in the retina: altered expression and truncation with aging

Crystallins are small heat shock proteins with chaperone function that prevent heat- and oxidative stress-induced aggregation of proteins. This is the first report describing modifications of alphaA crystallin in the sensory retina, including altered content and truncation with aging. Proteins from adult, middle age, and old Fischer 344 Brown Norway rats were compared. Western immunoblotting was used to evaluate alphaA crystallin content and identify protein spots on two-dimensional gels containing alphaA crystallin. The type and site of multiple post-translational modifications were identified by mass spectrometry. We found the content of alphaA crystallin was significantly decreased in the oldest rats. On two-dimensional gels, retinal crystallins resolved into multiple spots with altered migration, indicative of changes in intrinsic charge and/or truncation. Post-translational modifications that were identified included oxidation, phosphorylation, deamidation, acetylation, and truncation. In samples from rats of all ages, a highly modified N-terminus containing these modifications was found. We also observed an age-dependent difference in the extent of N- and C-terminal truncation. These results suggest that protection against stress-induced protein aggregation is compromised in the aged retina.

Protein profile of aging and its retardation by caloric restriction in neural retina

Aging is a slow, gradual deterioration process of an organism. The only experimental intervention, which can reliably retard aging and age-related degenerative diseases, is dietary caloric restriction (CR). To gain insight into the mechanism of CR intervention, we have investigated the protein profile of aging and its retardation by CR in the neural retina of Brown Norway (BN) rats using the comprehensive proteomic approach. We found that the intensities of 18 proteins decreased significantly with age. CR intervention can completely prevent seven of them, and partially protect eight of them, from such age-related declines. The major protein targets protected by CR intervention appear to be glycolytic enzymes and molecular chaperones. These data are the first to suggest that CR may retard the age-related degeneration of retina by maintaining sufficient glucose metabolism, by ensuring proper protein folding, and/or by preventing protein denaturation in the neural retina.

Morphological characterization of the Alpha A- and Alpha B-crystallin double knockout mouse lens

BACKGROUND

One approach to resolving some of the in vivo functions of alpha-crystallin is to generate animal models where one or both of the alpha-crystallin gene products have been eliminated. In the single alpha-crystallin knockout mice, the remaining alpha-crystallin may fully or partially compensate for some of the functions of the missing protein, especially in the lens, where both alpha A and alpha B are normally expressed at high levels. The purpose of this study was to characterize gross lenticular morphology in normal mice and mice with the targeted disruption of alpha A- and alpha B-crystallin genes (alpha A/BKO).

METHODS

Lenses from 129SvEvTac mice and alpha A/BKO mice were examined by standard scanning electron microscopy and confocal microscopy methodologies.

RESULTS

Equatorial and axial (sagittal) dimensions of lenses for alpha A/BKO mice were significantly smaller than age-matched wild type lenses. No posterior sutures or fiber cells extending to the posterior capsule of the lens were found in alpha A/BKO lenses. Ectopical nucleic acid staining was observed in the posterior subcapsular region of 5 wk and anterior subcapsular cortex of 54 wk alpha A/BKO lenses. Gross morphological differences were also observed in the equatorial/bow, posterior and anterior regions of lenses from alpha A/BKO mice as compared to wild mice.

CONCLUSION

These results indicated that both alpha A- and alpha B-crystallin are necessary for proper fiber cell formation, and that the absence of alpha-crystallin can lead to cataract formation.

Intense light exposure changes the crystallin content in retina

Toward a better understanding of light-induced photoreceptor damage, the crystallin content of rat retina was examined following intense light exposure. Nine crystallin species were identified by mass spectrometric analysis of rat retina fractionated by 2D gel electrophoresis. The Coomassie blue staining intensity of all crystallin 2D gel components was 2- to 3-fold greater in light exposed than in control retinas. Following light exposure, anti-alphaB-crystallin immunoreactivity was increased in rod outer segments and retinal pigment epithelium. These findings support a possible role for crystallins in protecting photoreceptors from light damage.

A positive charge preservation at position 116 of alpha A-crystallin is critical for its structural and functional integrity

An autosomal dominant congenital cataract associated with a missense mutation, Arg-116 to Cys (R116C), in the coding sequence of human alphaA-crystallin has been reported. Subsequent study of this mutant, generated by site-directed mutagenesis, showed significant changes in secondary and tertiary structures, partial loss of chaperone activity, and substantially increased oligomeric size. The study presented here aims to show whether these changes are due to the loss of a positive charge at this position or due to the presence of an extra Cys. To show this, Arg-116 in alphaA-crystallin was mutated to Lys (R116K), Cys (R116C), Gly (R116G), and Asp (R116D) and expressed in Escherichia coli cells. The wild-type (alphaA-wt) and mutant proteins were purified by size exclusion chromatography and characterized by measurements of circular dichroism, intrinsic tryptophan fluorescence, and TNS fluorescence and by determination of molecular masses and chaperone function which was assessed as the ability to suppress target protein aggregation or enhance target protein refolding. Mutation of Arg-116 to a Cys or Gly showed very similar changes in structure, oligomerization, and chaperone function which suggest that the presence of this Cys per se is not the cause of the changes. The R116K mutant, on the other hand, had nearly the same structure, oligomeric size, and chaperone function as alphaA-wt, whereas the mutant with an acidic amino acid in this position, R116D, showed drastic changes in protein structure. Thus, a positive charge must be preserved at this position for the structural and functional integrity of alphaA-crystallin.

Adaptive loss of ultraviolet-sensitive/violet-sensitive (UVS/VS) cone opsin in the blind mole rat (Spalax ehrenbergi)

In previous studies, fully functional rod and long-wavelength-sensitive (LWS) cone photopigments have been isolated from the eye of the subterranean blind mole rat (Spalax ehrenbergi superspecies). Spalax possesses subcutaneous atrophied eyes and lacks any ability to respond to visual images. By contrast this animal retains the ability to entrain circadian rhythms of locomotor behaviour to environmental light cues. As this is the only known function of the eye, the rod and LWS photopigments are thought to mediate this response. Most mammals are dichromats possessing, in addition to a single rod photopigment, two classes of cone photopigment, LWS and ultraviolet-sensitive/violet-sensitive (UVS/VS) with differing spectral sensitivities which mediate colour vision. In this paper we explore whether Spalax is a dichromat and has the potential to use colour discrimination for photoentrainment. Using immunocytochemistry and molecular approaches we demonstrate that Spalax is a LWS monochromat. Spalax lacks a functional UVS/VS cone photopigment due to the accumulation of several deleterious mutational changes that have rendered the gene nonfunctional. Using phylogenetic analysis we show that the loss of this class of photoreceptor is likely to have arisen from the visual ecology of this species, and is not an artefact of having an ancestor which lacked a functional UVS/VS cone photopigment. We conclude that colour discrimination is not a prerequisite for photoentrainment in this species.

The eye lens protein alphaA-crystallin of the blind mole rat Spalax ehrenbergi: effects of altered functional constraints

The rudimentary eyes of the mole rat Spalax ehrenbergi have lost their visual function, but are still required for the control of circadian rhythms. It has previously been found that alphaA-crystallin, a major eye lens protein in other mammals, evolved much faster in the mole rat than in rodents with normal vision. Yet, although mole rat alphaA-crystallin seems superfluous as a lens protein, its rate of change is still much slower than that of pseudogenes, suggesting some remaining function. The authors therefore studied the structure and function of recombinant mole rat alphaA-crystallin. Circular dichroism (CD), tryptophan fluorescence and gel permeation analyses indicated that the overall structure and stability of mole rat alphaA-crystallin are comparable to that of rat alphaA-crystallin. However, the chaperone-like activity of mole rat alphaA-crystallin is considerably lower than that of its rat orthologue. Two-dimensional NMR spectroscopy of mole rat alphaA-crystallin suggests that this may be in part due to a diminished flexibility of the C-terminal extension, which is thought to be important for the chaperoning capacity. Overall, mole rat alphaA-crystallin appears to still be a viable protein, confirming that it has some as yet elusive role, despite the loss of its primary lens function.

ATP causes small heat shock proteins to release denatured protein

Small heat-shock proteins (sHSPs) are a ubiquitous family of low molecular mass (15-30 kDa) stress proteins that have been found in all organisms. Under stress, sHSPs such as alpha-crystallin can act as chaperones binding partially denatured proteins and preventing further denaturation and aggregation. Recently, it has been proposed that the function of sHSPs is to stabilize stress-denatured protein and then act cooperatively with other HSPs to renature the partially denatured protein in an ATP-dependent manner. However, the process by which this occurs is obscure. As no significant phosphorylation of alpha-crystallin was observed during the renaturation, the role of ATP is not clear. It is now shown that ATP at normal physiological concentrations causes sHSPs to change their confirmation and release denatured protein, allowing other molecular chaperones such as HSP70 to renature the protein and renew its biological activity. In the absence of ATP, sHSPs such as alpha-crystallin are more efficient than HSP70 in preventing stress-induced protein aggregation. This work also indicates that in mammalian systems at normal cellular ATP concentrations, sHSPs are not effective chaperones.

alpha-B- and alpha-A-crystallin prevent irreversible acidification-induced protein denaturation

alpha-Crystallin (alpha), a major structural protein of the mammalian lens, is a large, physically heterogeneous macromolecule with an average molecular weight of approximately 800 kDa and is composed of two 20-kDa polypeptides designated as alphaA and alphaB. A line of evidence strongly suggests that alphaB may have an essential nonlenticular function. Here it is demonstrated that alphaB can bind partially denatured enzymes effectively at acidic pH and prevent their irreversible aggregation, but cannot prevent loss of enzyme activity. However, when the inactive luciferase bound to alphaB was treated with reticulocyte lysate (a rich source of molecular chaperones) and an ATP-generating system, more than 50% of the original luciferase activity could be recovered. Somewhat less activation was observed when alphaA-bound enzyme or the alpha-bound enzyme was renatured similarly. The overall results suggest that alpha acts as a chaperone to stabilize denaturing proteins at acidic pH so that at a later time they can be reactivated by other chaperones.

Unfolding retinal dystrophies: a role for molecular chaperones?

Inherited retinal dystrophy is a major cause of blindness worldwide. Recent molecular studies have suggested that protein folding and molecular chaperones might play a major role in the pathogenesis of these degenerations. Incorrect protein folding could be a common consequence of causative mutations in retinal degeneration disease genes, particularly mutations in the visual pigment rhodopsin. Furthermore, several retinal degeneration disease genes have recently been identified as putative facilitators of correct protein folding, molecular chaperones, on the basis of sequence homology. We also consider whether manipulation of chaperone levels or chaperone function might offer potential novel therapies for retinal degeneration.

alphaA- and alphaB-crystallins protect glucose-6-phosphate dehydrogenase against UVB irradiation-induced inactivation

alpha-Crystallin, a major eye lens protein, has been shown to function like a molecular chaperone by suppressing the aggregation of other proteins induced by various stress conditions. Ultraviolet (UV) radiation is known to cause structural and functional alterations in the lens macromolecules. Earlier we observed that exposure of rat lens to in vitro UV radiation led to inactivation of many lens enzymes including glucose-6-phosphate dehydrogenase (G6PD). In the present paper, we show that alpha-crystallin (alphaA and alphaB) protects G6PD from UVB irradiation induced inactivation. While, at 25 degrees C, there was a time-dependent decrease in G6PD activity upon irradiation at 300 nm, at 40 degrees C there was a complete loss of activity within 30 min even without irradiation. The loss of activity of G6PD was prevented significantly, if alphaA- or alphaB-crystallin was present during irradiation. At 25 degrees C, alphaB-crystallin was slightly a better chaperone in protecting G6PD against UVB inactivation. Interestingly, at 40 degrees C, alphaA- and alphaB-crystallins not only prevent the loss of G6PD activity but also protect against UVB inactivation. However, alphaA- and alphaB-crystallins were equally efficient at 40 degrees C in protecting G6PD.

Towards the proteome of the rhodopsin-bearing post-Golgi compartment of retinal photoreceptor cells

Polarized sorting of rhodopsin in retinal rod photoreceptor cells is mediated by post-Golgi carrier membranes that bud from the trans-Golgi network and fuse with the specialized domain of the plasma membrane in the rod inner segment. The identity of the majority of the resident proteins of this organelle still remains elusive, despite multifaceted approaches to study this compartment. In the present study we have taken a proteomic approach to the analysis of the post-Golgi carriers. First, we modified the previously established fractionation protocols in order to achieve greater purity of the isolated membranes. Specifically, the new fractionation scheme depleted the post-Golgi fraction of cytosolic proteins that were the most abundant contaminants complicating analysis of two-dimensional (2-D) gel profiles in our previous preparations. The isolated membranes were subjected to 2-D gel electrophoresis, immunoblotting and microsequencing. This analysis showed that the improved subcellular fractionation yielded a fraction highly enriched in rhodopsin-bearing post-Golgi carrier membranes. Two-dimensional mapping revealed 29 proteins that are preferentially found in this fraction and therefore represent candidates for post-Golgi membrane-specific proteins. This preparation of rhodopsin-bearing post-Golgi carriers is a first step towards the proteomics of this important organelle.

alpha-crystallin prevents irreversible protein denaturation and acts cooperatively with other heat-shock proteins to renature the stabilized partially denatured protein in an ATP-dependent manner

alpha-Crystallin, a major lens protein of approximately 800 kDa with subunits of approximately 20 kDa has previously been shown to act as a chaperone protecting other proteins from stress-induced aggregation. Here it is demonstrated that alpha-crystallin can bind to partially denatured enzymes at 42-43 degrees C and prevent their irreversible aggregation, but cannot prevent loss of enzyme activity. However, the alpha-crystallin-bound enzymes regain activity on interaction with other chaperones. The data indicate that the re-activated enzymes are no longer associated with the alpha-crystallin, and ATP is required for re-activation. When inactive luciferase bound to alpha-crystallin was treated with reticulocyte lysate, a rich source of chaperones, up to 60% of the original luciferase activity could be recovered. Somewhat less re-activation was observed when the alpha-crystallin-bound enzyme was treated with heat-shock protein (HSP)70, HSP40, HSP60 and an ATP-generating system. Similar results were also obtained with citrate synthase. The overall results suggest that alpha-crystallin acts to stabilize denaturing proteins so that they can later be re-activated by other chaperones.

Enhancer-independent promoter activity of the mouse alphaB-crystallin/small heat shock protein gene in the lens and cornea of transgenic mice

The alphaB-crystallin/small heat shock protein gene is expressed very highly in the mouse eye lens and to a lesser extent in many other nonocular tissues, including the heart, skeletal muscle and brain. Previously we showed in transgenic mice that lens-specific alphaB-crystallin promoter activity is directed by a proximal promoter fragment (-164/+44) and that non-lens promoter activity depends on an upstream enhancer (-427/-259) composed of at least 5 cis-control elements. Here we have used truncated alphaB-crystallin promoter-CAT transgenes to test by biphasic CAT assays and/or histochemistry for specific expression in the cornea and lens. Deletion either of 87 bp (-427/-340) from the 5' end of the alphaB-crystallin enhancer or of the whole enhancer (-427/-258) abolished alphaB-crystallin promoter activity in all tissues except the lens and corneal epithelium when examined by the biphasic CAT assay in 4-5-week-old transgenic mice. These truncations also lowered promoter strength in the lens. The -426/+44-CAT, -339/+44-CAT and -164/+44-CAT (previously thought to be lens-specific in transgenic mice) transgenes were all expressed in the 4-6-week-old corneal epithelium when examined histochemically. Immunohistochemical staining confirmed the presence of endogenous alphaB-crystallin in the mature corneal epithelial cells. CAT gene expression driven by the alphaB-crystallin promoter with or without the enhancer was evident in the embryonic and 4-6-week-old lens. By contrast, activity of the alphaB-crystallin promoter/enhancer-CAT transgene was not detectable in the corneal epithelium before birth. Taken together, these results indicate that the intact enhancer of the alphaB-crystallin/small heat shock protein gene is required for promoter activity in all tissues tested except the lens and cornea.

Temperature-dependent chaperone activity and structural properties of human alphaA- and alphaB-crystallins

The chaperone activity and biophysical properties of recombinant human alphaA- and alphaB-crystallins were studied by light scattering and spectroscopic methods. While the chaperone function of alphaA-crystallin markedly improves with an increase in temperature, the activity of alphaB homopolymer appears to change very little upon heating. Compared with alphaB-crystallin, the alphaA-homopolymer is markedly less active at low temperatures, but becomes a more active species at high temperatures. At physiologically relevant temperatures, the alphaB homopolymer appears to be modestly (two times or less) more potent chaperone than alphaA homopolymer. In contrast to very similar thermotropic changes in the secondary structure of both homopolymers, alphaA- and alphaB-crystallins markedly differ with respect to the temperature-dependent surface hydrophobicity profiles. Upon heating, alphaA-crystallin undergoes a conformational transition resulting in the exposure of additional hydrophobic sites, whereas no such transition occurs for alphaB-crystallin. The correlation between temperature-dependent changes in the chaperone activity and hydrophobicity properties of the individual homopolymers supports the view that the chaperone activity of alpha-crystallin is dependent on the presence of surface-exposed hydrophobic patches. However, the present data also show that the surface hydrophobicity is not the sole determinant of the chaperone function of alpha-crystallin.

Mutation of R116C results in highly oligomerized alpha A-crystallin with modified structure and defective chaperone-like function

An autosomal dominant congenital cataract in human is associated with mutation of Arg-116 to Cys (R116C) in alpha A-crystallin. To investigate the molecular basis of cataract formation, rat alpha A-crystallin cDNA was cloned into pET-23d(+), and the site-directed mutants S142C (similar to wild-type human alpha A) and R116C/S142C or R116C (similar to human R116C variant) were generated. These were expressed in E. coli and the recombinant alpha A-crystallins purified by Sephacryl size-exclusion chromatography. The chaperone-like function of mutant R116C determined at 37 degrees C with insulin and alcohol dehydrogenase as target proteins was about 40% lower than those of wild-type and mutant S142C. Based on size-exclusion chromatography data, the oligomeric size of the R116C mutant was about 2000 kDa at 25 degrees C, 1400 kDa at 37 degrees C, and 900 kDa at 45 degrees C. In comparison, alpha A-wild-type and alpha A-S142C ranged from 477 to 581 kDa. Heat stability studies corroborated the effect of temperature on the dynamic quaternary structure of the R116C mutant. Circular dichroism spectra showed secondary and tertiary structural changes, and ANS fluorescence spectra showed loss of surface hydrophobicity in the R116C mutant. These findings suggest that the molecular basis for the congenital cataract with the alpha A-R116C mutation is due to the generation of a highly oligomerized alpha A-crystallin having a modified structure and decreased chaperone-like function.