Phosphorylations of alpha A- and alpha B-crystallin

microRNA 146a ameliorates retinal damage in experimental autoimmune uveitis

Introduction

Uveitis and related intraocular inflammations are a major cause of blindness due to retinal damage caused by degeneration and loss of the photoreceptor cells. In mouse experimental autoimmune uveitis (EAU) previously we have shown mitochondrial oxidative stress with marked upregulation of αA crystallin in the inner segments of the photoreceptors. Furthermore, αA crystallin treatment prevented photoreceptor mitochondrial oxidative stress by suppressing innate and adaptive immunity in EAU.

Methods

Since these immune processes are modulated by microRNAs, in this study we investigated (a) modulation of microRNAs during development of EAU by αA crystallin administration and (b) microRNA therapeutic intervention.

Results

Few microRNAs were significantly upregulated in EAU mice with intravenous injection of αA crystallin and among these, computational bioinformatic analysis revealed that the upregulated microRNA 146a targets the innate and adaptive immune responses. In EAU, intravenous as well as intravitreal administration of this microRNA prevented inflammatory cell infiltration in uvea and retina and preserved photoreceptor cells.

Discussion

This protective function suggests that microRNA146a can be a novel therapeutic agent in preventing retinal damage in uveitis.

Proteomic characterization of the human lens and Cataractogenesis

INTRODUCTION

The goal of this review is to highlight the triumphs and frontiers in measurement of the lens proteome as it relates to onset of age-related nuclear cataract. As global life expectancy increases, so too does the frequency of age-related nuclear cataracts. Molecular therapeutics do not exist for delay or relief of cataract onset in humans. Since lens fiber cells are incapable of protein synthesis after initial maturation, age-related changes in proteome composition and post-translational modification accumulation can be measured with various techniques. Several of these modifications have been associated with cataract onset.

AREAS COVERED

We discuss the impact of long-lived proteins on the lens proteome and lens homeostasis as well as proteomic techniques that may be used to measure proteomes at various levels of proteomic specificity and spatial resolution.

EXPERT OPINION

There is clear evidence that several proteome modifications are correlated with cataract formation. Past studies should be enhanced with cutting-edge, spatially resolved mass spectrometry techniques to enhance the specificity and sensitivity of modification detection as it relates to cataract formation.

Regulation of αA- and αB-crystallins via phosphorylation in cellular homeostasis

αA-Crystallin (αA) and αB-crystallin (αB) are small heat shock proteins responsible for the maintenance of transparency in the lens. In non-lenticular tissues, αB is involved in both maintenance of the cytoskeleton and suppression of neurodegeneration amongst other roles. Despite their importance in maintaining cellular health, modifications and mutations to αA and αB appear to play a role in disease states such as cataract and myopathies. The list of modifications that have been reported is extensive and include oxidation, disulphide bond formation, C- and N-terminal truncation, acetylation, carboxymethylation, carboxyethylation, carbamylation, deamidation, phosphorylation and methylation. Such modifications, notably phosphorylation, are alleged to cause changes to chaperone activity by inducing substructural changes and altering subunit exchange dynamics. Although the effect modification has on the activities of αA and αB is contentious, it has been proposed that these changes are responsible for the induction of hyperactivity and are thereby indirectly responsible for protein deposition characteristic of many diseases associated with αA and αB. This review compiles all reported sites of αA and αB modifications, and investigates the role phosphorylation, in particular, plays in cellular processes.

Phosphorylation of αB-crystallin: Role in stress, aging and patho-physiological conditions

BACKGROUND

αB-crystallin, once thought to be a lenticular protein, is ubiquitous and has critical roles in several cellular processes that are modulated by phosphorylation. Serine residues 19, 45 and 59 of αB-crystallin undergo phosphorylation. Phosphorylation of S45 is mediated by p44/42 MAP kinase, whereas S59 phosphorylation is mediated by MAPKAP kinase-2. Pathway involved in S19 phosphorylation is not known.

SCOPE OF REVIEW

The review highlights the role of phosphorylation in (i) oligomeric structure, stability and chaperone activity, (ii) cellular processes such as apoptosis, myogenic differentiation, cell cycle regulation and angiogenesis, and (iii) aging, stress, cardiomyopathy-causing αB-crystallin mutants, and in other diseases.

MAJOR CONCLUSIONS

Depending on the context and extent of phosphorylation, αB-crystallin seems to confer beneficial or deleterious effects. Phosphorylation alters structure, stability, size distribution and dynamics of the oligomeric assembly, thus modulating chaperone activity and various cellular processes. Phosphorylated αB-crystallin has a tendency to partition to the cytoskeleton and hence to the insoluble fraction. Low levels of phosphorylation appear to be protective, while hyperphosphorylation has negative implications. Mutations in αB-crystallin, such as R120G, Q151X and 464delCT, associated with inherited myofibrillar myopathy lead to hyperphosphorylation and intracellular inclusions. An ongoing study in our laboratory with phosphorylation-mimicking mutants indicates that phosphorylation of R120GαB-crystallin increases its propensity to aggregate.

GENERAL SIGNIFICANCE

Phosphorylation of αB-crystallin has dual role that manifests either beneficial or deleterious consequences depending on the extent of phosphorylation and interaction with cytoskeleton. Considering that disease-causing mutants of αB-crystallin are hyperphosphorylated, moderation of phosphorylation may be a useful strategy in disease management. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Site specific oxidation of amino acid residues in rat lens γ-crystallin induced by low-dose γ-irradiation

Although cataracts are a well-known age-related disease, the mechanism of their formation is not well understood. It is currently thought that eye lens proteins become abnormally aggregated, initially causing clumping that scatters the light and interferes with focusing on the retina, and ultimately resulting in a cataract. The abnormal aggregation of lens proteins is considered to be triggered by various post-translational modifications, such as oxidation, deamidation, truncation and isomerization, that occur during the aging process. Such modifications, which are also generated by free radical and reactive oxygen species derived from γ-irradiation, decrease crystallin solubility and lens transparency, and ultimately lead to the development of a cataract. In this study, we irradiated young rat lenses with low-dose γ-rays and extracted the water-soluble and insoluble protein fractions. The water-soluble and water-insoluble lens proteins were digested with trypsin, and the resulting peptides were analyzed by LC-MS. Specific oxidation sites of methionine, cysteine and tryptophan in rat water-soluble and -insoluble γE and γF-crystallin were determined by one-shot analysis. The oxidation sites in rat γE and γF-crystallin resemble those previously identified in γC and γD-crystallin from human age-related cataracts. Our study on modifications of crystallins induced by ionizing irradiation may provide useful information relevant to human senile cataract formation.

Medical implications of understanding the functions of human small heat shock proteins

Small heat shock proteins (sHsps) are ubiquitous molecular chaperones that are implicated in a variety of diseases. Upon stress, they stabilize unfolding proteins and prevent them from aggregating. However, under physiological conditions without severe stress, some sHsps interact with other proteins. In a perspective view, their ability to bind specific client proteins might allow them to fine-tune the availability of the client for other, client-dependent cellular processes. Additionally, some sHsps seem to interact with specific co-chaperones. These co-chaperones are usually part of large protein machineries that are functionally modulated upon sHsps interaction. Finally, secreted human sHsps seem to interact with receptor proteins, potentially as signal molecules transmitting the stress status from one cell to another. This review focuses on the mechanistic description of these different binding modes for human sHsps and how this might help to understand and modulate the function of sHsps in the context of disease.

A rapid, comprehensive liquid chromatography-mass spectrometry (LC-MS)-based survey of the Asp isomers in crystallins from human cataract lenses

Cataracts are caused by clouding of the eye lens and may lead to partial or total loss of vision. The mechanism of cataract development, however, is not well understood. It is thought that abnormal aggregates of lens proteins form with age, causing loss of lens clarity and development of the cataract. Lens proteins are composed of soluble α-, β-, and γ-crystallins, and as long lived proteins, they undergo post-translational modifications including isomerization, deamidation, and oxidation, which induce insolubilization, aggregation, and loss of function that may lead to cataracts. Therefore, analysis of post-translational modifications of individual amino acid residues in proteins is important. However, detection of the optical isomers of amino acids formed in these proteins is difficult because optical resolution is only achieved using complex methodology. In this study, we describe a new method for the analysis of isomerization of individual Asp residues in proteins using LC-MS and the corresponding synthetic peptides containing the Asp isomers. This makes it possible to analyze isomers of Asp residues in proteins precisely and quickly. We demonstrate that Asp-58, -76, -84, and -151 of αA-crystallin and Asp-62 and -96 of αB-crystallin are highly converted to lβ-, dβ-, and dα-isomers. The amount of isomerization of Asp is greater in the insoluble fraction at all Asp sites in lens proteins, therefore indicating that isomerization of these Asp residues affects the higher order structure of the proteins and contributes to the increase in aggregation, insolubilization, and disruption of function of proteins in the lens, leading to the cataract.

A potential role of crystallin in the vitreous bodies of rats after ischemia-reperfusion injury

PURPOSE

Ischemia-reperfusion injury (I/R injury) is known not only to induce hypoxic and oxidative stress, but also to cause retinal degeneration in rats. Crystallins, known to inhibit the formation of reactive oxygen species, reduce apoptotic cell death. Our goal was to clarify not only the role of I/R injury-mediated crystallins, but also to evaluate the correlation of these compounds to anti-inflammation in the vitreous body.

METHODS

Twenty-four Sprague-Dawley rats were used in this study. We induced I/R injury by clamping the optic nerve for 30 minutes and then releasing it. The vitreous bodies were obtained from the experimental and control subjects 24, 48, and 72 hours after I/R injury. Two-dimensional electrophoresis was performed, and the targeted spots were further investigated using matrix-assisted laser desorption-ionization time-of-flight mass spectrometry, spectrophotometry, Western blotting, and histological examination.

RESULTS

After I/R injury, 23 spots were identified as crystallins. The βB2 crystallins were transcriptionally and post-translationally regulated, whereas the αB crystallins were controlled by post-translational modifications in the vitreous bodies of the rats. The total amounts of αA and β crystallins (including isotypes of β crystalline) had increased 48 hours after injury. The phosphorylation of αB crystallin (at serine residues 19, 45, and 59) was significantly increased 48 hours later, whereas phosphorylation of ERK1/2 showed the greatest decrease.

CONCLUSIONS

During hypoxic and oxidation stress, our results suggest that phosphorylated αB crystalline inhibits RAS, resulting in the inactivation of ERK1/2. The phosphorylation of αB crystallin may be associated with the inflammatory suppression in the vitreous body via the I/R injury model system.

Alphab-crystallin expression in epiretinal membrane of human proliferative diabetic retinopathy

PURPOSE

To examine the expression of alphaB-crystallin and its colocalization with vascular endothelial growth factor in the epiretinal membrane of human proliferative diabetic retinopathy.

METHODS

Ten epiretinal membranes of proliferative diabetic retinopathy and three normal retinas surgically excised were included in this study. Paraformaldehyde-fixed, paraffin-embedded tissue sections were processed for immunohistochemistry with alphaB-crystallin, vascular endothelial growth factor, and CD31 antibodies.

RESULTS

AlphaB-crystallin was expressed in all epiretinal membranes examined. The immunolocalization of alphaB-crystallin was detected in the cytoplasm of CD31-positive endothelial cells, but not in normal retinal blood vessels. Furthermore, alphaB-crystallin immunoreactivity was colocalized in vascular endothelial growth factor-positive endothelial cells in proliferative diabetic retinopathy membranes.

CONCLUSION

AlphaB-crystallin was expressed in proliferative diabetic retinopathy membranes, and colocalized with vascular endothelial growth factor-positive neovessels. AlphaB-crystallin may play a potential role in the pathogenesis of epiretinal membranes in proliferative diabetic retinopathy, together with vascular endothelial growth factor.

Flicker downregulates the content of crystallin proteins in form-deprived C57BL/6 mouse retina

Image degradation by loss of higher spatial frequencies causes form-deprivation myopia (FDM) in humans and animals, and cyclical illumination (flicker) at certain frequencies may prevent FDM. The molecular mechanisms underlying FDM and its prevention by flicker are poorly known. To understand them better, we have identified proteins that differ in amount in form-deprived (FD) mouse retinas, under steady versus flickering light. Male C57BL/6 mice (age 27-29 days) were randomly divided into three groups: Experimental - monocularly form-deprived, and kept under either normal room light ("FD-Only") or 20 Hz flickering light ("FD-Flicker"), throughout the 12-hour light phase; and Control ("Open-Control") - kept under normal illumination, without form deprivation. After two weeks of treatment, retinal proteins were extracted and separated by two-dimensional gel electrophoresis (2D-GE); proteins that differ in content in FD-only versus FD-flicker retinas were identified by mass spectroscopy ("MS"), and their identities were verified by western blotting. The contents of three identified proteins differed statistically in FD-only compared to FD-flicker retinas. These proteins were identified by MS as α-A-crystallin, crystallin β A2 and crystallin β A1. Quantitative western blotting showed that the relative amount of α-A-crystallin in FD-only retinas was significantly higher than that in FD-Flicker and control retinas. In conclusion, form deprivation induced significant increases in the amounts of crystallins in mouse retinas. These increases were significantly reduced by exposure to 20 Hz flicker. Since form deprivation is known to induce myopia development, and flicker to prevent it, our data suggest that FD- and flicker-responsive changes in the content of crystallin proteins may be involved causally or protectively in myopia development.

Expression of alpha-crystallin in retinoblastoma

OBJECTIVE

To examine the expression of alpha-crystallin, a small heat-shock protein family, and apoptosis in retinal neoplastic cells.

METHODS

Thirteen enucleated globes were included in this study, 1 with retinocytoma and 12 with retinoblastoma. Formalin-fixed paraffin-embedded tissue sections were processed for immunohistochemistry with alpha-crystallin antibodies. Apoptotic cells were detected using the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) method.

RESULTS

In the retinocytoma, alphaA-crystallin was expressed in the cytoplasm of all tumor cells, whereas alphaB-crystallin immunoreactivity was only weakly positive. Apoptotic cells were rarely noted in retinocytoma cells; the apoptotic index was 0.29. Examination of the retinoblastoma globes revealed 6 cases (50%) that were strongly positive for alphaA-crystallin. The mean (SD) apoptotic indices in the strongly and weakly positive cases were 3.55 (2.61) and 7.50 (2.61), respectively. The apoptotic index was significantly higher in those cases that were weakly positive for alphaA-crystallin than in those that were strongly positive (P < .05). No correlation was observed between apoptotic index and alphaB-crystallin immunoreactivity, although 50% of retinoblastomas were strongly positive for alphaB-crystallin.

CONCLUSIONS

The alphaA- and alphaB-crystallins are expressed in retinoblastomas, and alphaA-crystallin expression may prevent apoptosis of neoplastic cells. Clinical Relevance Suppression of alphaA-crystallin may be useful in controlling tumor growth.

Enhanced degradation and decreased stability of eye lens alpha-crystallin upon methylglyoxal modification

Methylglyoxal (MGO), a potent glycating agent, forms advanced glycation end products (AGEs) with proteins. Several diabetic complications including cataract are thought to be the result of accumulation of these protein-AGEs. alpha-Crystallin, molecular chaperone of the eye lens, plays an important role in maintaining the transparency of the lens by preventing the aggregation/inactivation of several proteins/enzymes in addition to its structural role. Binding of adenosine-5-triphosphate (ATP) to alpha-crystallin has been shown to enhance its chaperone-like function and protection against proteolytic degradation. In the earlier study, we have shown that modification of alpha-crystallin by MGO caused altered chaperone-like activity along with structural changes, cross-linking, coloration and subsequent insolubilization leading to scattering of light [Biochem. J. 379 (2004) 273]. In the present study, we have investigated ATP binding, stability and degradation of MGO-modified alpha-crystallin. Proteolytic digestion with trypsin and chymotrypsin showed that MGO-modified alpha-crystallin is more susceptible to degradation compared to native alpha-crystallin. Furthermore, ATP was able to protect native alpha-crystallin against proteolytic cleavage but not MGO-modified alpha-crystallin. Interestingly, binding studies indicate decreased ATP binding to MGO-modified alpha-crystallin and support the decreased protection by ATP against proteolysis. In addition, differential scanning calorimetric and denaturant-induced unfolding studies indicate that modification of alpha-crystallin by MGO leads to decreased stability. These results indicate that MGO-modification of alpha-crystallin causes partial unfolding and decreased stability leading to enhanced proteolysis. Cross-linking of these degraded products could result in aggregation and subsequent insolubilization as observed in senile and diabetic cataract lenses.

Phosphorylation of αB-Crystallin Alters Chaperone Function through Loss of Dimeric Substructure

Phosphorylation is the most common posttranslational modification of the alpha-crystallins in the human lens. These phosphorylated forms are not only important because of their abundance in aging lenses and the implications for cataract but also because they have been identified in patients with degenerative brain disease. By using mimics corresponding to the reported in vivo phosphorylation sites in the human lens, we have examined the effects of phosphorylation upon the chaperone-like properties and structure of alphaB-crystallin. Here we show that phosphorylation of alphaB-crystallin at Ser-45 results in uncontrolled aggregation. By using an innovative tandem mass spectrometry approach, we demonstrate how this alteration in behavior stems from disruption of dimeric substructure within the polydisperse alphaB-crystallin assembly. This structural perturbation appears to disturb the housekeeping role of alphaB-crystallin and consequently has important implications for the disease states caused by protein aggregation in the lens and deposition in non-lenticular tissue.

Induction of cortical cataracts in cultured mouse lenses with H-89, an inhibitor of protein kinase A

PURPOSE

To compare the effects of two serine-threonine protein kinase inhibitors in a mouse lens culture system previously designed to investigate cortical cataracts caused by L-buthionine sulfoximine (BSO), inhibitor of GSH biosynthesis.

METHODS

Cataract development in HL-1 medium was evaluated visually or by measurement of lens Na+/K+ ratio through atomic absorption. Protein changes were evaluated by 32P-labeling, 2D-gel electrophoresis, phosphorimaging and mass spectrometry. Results. H-7 (50 microM), inhibitor of protein kinase A (PKA) and protein kinase C (PKC), did not cause cataracts, but inhibited BSO cataract development. By contrast, 25 microM H-89, selective inhibitor of PKA, caused large annular cortical cataracts and 100-fold elevation of Na+/K+ within 30 hr in day 10 lenses, in either the presence or absence of BSO. H-89 cataracts were also seen in day 12 and day 21 lenses. 32P-labeling of day 12 lenses pretreated with H-89 displayed more than 80% decrease in phosphorylation of alphaA crystallin, a known substrate of PKA, in the insoluble protein fraction. 2D-gel electrophoresis of day 12 H-89 cataract lens fractions revealed limited degradation of alpha and beta crystallins, degradation of cytoskeletal proteins, and elevated lens Ca2+ (>4 nmol/mg wet wt.), suggesting Ca2+-activated proteolysis. Conclusions. High Na+/K+ cataracts are induced by H-89, selective inhibitor of PKA, but not by H-7, an inhibitor of both PKA and PKC that impeded BSO-induced Na+/K+ elevation and cataract. These results suggest contrasting effects of PKA and PKC on lens cation transport and cortical cataract development.

Prediction of possible sites for posttranslational modifications in human gamma crystallins: Effect of glycation on the structure of human gamma-b-crystallin as analyzed by molecular modeling

Crystallins are recognized as one of the long-lived proteins of lens tissue that might serve as the target for several posttranslational modifications leading to cataract development. We have studied several such sites present in the human gamma-crystallins based either on PROSITE pattern search results or earlier experimental evidences. Their probabilities were examined on the basis of the database analysis of the gamma-crystallin sequences and on their specific locations in the constructed homology models. An N-glycosylation site in human gammaD-crystallin and several phosphorylation sites in all four human gamma-crystallins were predicted by the PROSITE search. Some of these sites were found to be strongly conserved in the gamma-crystallin sequences from different sources. An extensive analysis of these sites was performed to predict their probabilities as potential sites for protein modifications. Glycation studies were performed separately by attaching sugars to the human gammaB-crystallin model, and the effect of binding was analyzed. The studies showed that the major effect of alphaD-glucose (alphaD-G) and alphaD-glucose-6-phosphate (alphaD-G6P) binding was the disruption of charges not only at the surface but also within the molecule. Only a minor alteration in the distances of sulfhydryl groups of cysteines and on their positions in the three-dimensional models were observed, leading us to assume that glycation alone is not responsible for intra- and intermolecular disulfide bond formation.

Identification of the molecular chaperone alpha B-crystallin in demineralized bone powder and osteoblast-like cells

Bone is subjected to a variety of physiological, as well as cell-deforming biomechanical stresses, including hydrostatic compression and fluid flow. However, little is known about the molecular mechanisms that protect bone cells from mechanical, ischemic, or oxidative damage. Crystallins are 20 kD heat shock proteins that function as molecular chaperones. We tested the hypothesis that alpha B-crystallin (alphaB-crystallin), the most widely expressed vertebrate crystallin, is present in bone and osteoblast-like cells. Noncollagenous proteins (NCPs) were extracted from human demineralized bone matrix with 4 M guanidine HCI containing 0.5 M CaCl2 and protease inhibitors, defatted, dialyzed against 0.2% (v/v) Triton X-100 in 100 mM Tris-HCI (pH 7.2) and water, centrifuged, and lyophilized. The NCPs were separated by 2D IEF/SDS-PAGE. The two most abundant 20 kD spots, with apparent pIs of 7.85 and 7.42 in urea gels, were excised, subjected to matrix-assisted laser desorption ionization/time-of-flight mass spectrometry, and identified as alphaB-crystallins. Indirect immunofluorescence localized alphaB-crystallin to the interphase nucleus, cytoskeleton and cytoplasm of proliferating MC3T3-E1 mouse osteoblast-like cells, as well as the cytoskeleton and cytoplasm of confluent cells. In conclusion, alphaB-crystallin is present in bone and osteoblast-like cells. We hypothesize that alphaB-crystallin may play a role in protecting the osteoblast cytoskeleton from mechanical stress and may be important in modulating nuclear or cellular functions, such as transcription or apoptosis, as observed in other tissues.

Functional similarities between the small heat shock proteins Mycobacterium tuberculosis HSP 16.3 and human alphaB-crystallin

Mycobacterium tuberculosis heat shock protein 16.3 (MTB HSP 16.3) accumulates as the dominant protein in the latent stationary phase of tuberculosis infection. MTB HSP 16.3 displays several characteristics of small heat shock proteins (sHsps): its expression is increased in response to stress, it protects against protein aggregation in vitro, and it contains the core 'alpha-crystallin' domain found in all sHsps. In this study we characterized the chaperone activity of recombinant MTB HSP 16.3 in several different assays and compared the results to those obtained with recombinant human alphaB-crystallin, a well characterized member of the sHsp family. Recombinant MTB HSP 16.3 was expressed in Escherichia coli and purified to apparent homogeneity. Similar to alphaB-crystallin, MTB HSP16.3 suppressed citrate synthase aggregation and in the presence of 3.5 mm ATP the chaperone activity was enhanced by twofold. ATP stabilized MTB HSP 16.3 against proteolysis by chymotrypsin, and no effect was observed with ATPgammaS, a nonhydrolyzable analog of ATP. Increased expression of MTB HSP 16.3 resulted in protection against thermal killing in E. coli at 48 degrees C. While the sequence similarity between human alphaB-crystallin and MTB HSP 16.3 is only 18%, these results suggest that the functional similarities between these proteins containing the core 'alpha-crystallin' domain are much closer.

The lens protein alpha-B-crystallin of the blind subterranean mole-rat: high homology with sighted mammals

Blind subterranean mole rats, Spalax ehrenbergi, retain a subcutaneous, degenerated eye, which is visually non-functional but which does function in circadian entrainment. Crystallins, members of the small heat shock protein family, constitute approximately 90% of the water-soluble proteins of the transparent eye lens and are crucial for its optical properties, but they are also expressed in other tissues. In our attempt to understand the role of the eye in the blind mole-rat, we now describe the cloning, sequencing, and expression of the cDNA of alpha-B-Crystallin from two species of Spalax (S. galili and S. Judaei, with diploid chromosome numbers 2n=52 and 60, respectively). Spalax alpha- B-Crystallin is highly conserved. It is expressed in many tissues of Spalax, among them Spalax eye. The sequence of the cDNA of alpha-B-Crystallin in the eye and in the heart of Spalax is identical. Further studies are essential to clarify the role of this gene in the lens of an atrophied eye of a visually blind mammal.

Expression of small heat shock protein alphaB-crystallin is induced after hepatic stellate cell activation

Using the differential PCR display method to select cDNA fragments that are differentially expressed after hepatic stellate cell (HSC) activation, we have isolated from activated HSCs a cDNA that corresponds to rat alphaB-crystallin. Northern blots confirmed expression of alphaB-crystallin in culture-activated HSCs but not in quiescent HSCs. Western blot analysis and immunocytochemical staining confirmed expression of alphaB-crystallin protein in activated but not quiescent HSCs. alphaB-crystallin is induced as early as 6 h after plating HSCs on plastic and continues to be expressed for 14 days in culture. Expression of alphaB-crystallin was also induced in vivo in activated HSCs from experimental cholestatic liver fibrosis. Confocal microscopy demonstrated a cytoplasmic distribution of alphaB-crystallin in a cytoskeletal pattern. Heat shock treatment resulted in an immediate perinuclear redistribution that in time returned to a normal cytoskeletal distribution. The expression pattern of alphaB-crystallin was similar to that of HSP25, another small heat shock protein, but differed from the classic heat shock protein HSP70. Therefore, alphaB-crystallin represents an early marker for HSC activation.

alpha-Crystallin facilitates the reactivation of hydrogen peroxide-inactivated rhodanese

It was previously shown that rhodanese, inactivated with hydrogen peroxide, could only be reactivated in the presence of a reductant or the substrate thiosulfate if these reagents were added soon after inactivation and if the oxidant was removed. Here, we report on the facilitated reactivation (75%) of hydrogen peroxide-inactivated rhodanese by the chaperone alpha-crystallin. Reactivation by the chaperone still required a reductant and thiosulfate. Without alpha-crystallin, but in the presence of the reductant and thiosulfate, the inactivated enzyme regained about 39% of its original activity. The alpha-crystallin-assisted reactivation of hydrogen peroxide-inactivated rhodanese was independent of ATP. Further, we found, that alpha-crystallin interacted transiently, but could not form a stable complex with hydrogen peroxide-inactivated rhodanese. Unlike in prior studies that involved denaturation of rhodanese through chemical or thermal means, we have clearly shown that alpha-crystallin can function as a molecular chaperone in the reactivation of an oxidatively inactivated protein.

Small heat-shock proteins and their potential role in human disease

The elevated expression of stress proteins is considered to be a universal response to adverse conditions, representing a potential mechanism of cellular defense against disease and a potential target for novel therapeutics, including gene therapy and chaperone-modulating reagents. Recently, a single mutation in the small heat-shock protein human alphaB-crystallin was linked to desmin-related myopathy, which is characterized by abnormal intracellular aggregates of intermediate filaments in human muscle. New findings demonstrate that the high level of expression of stress proteins can contribute to an autoimmune response and can protect proteins that contribute to disease processes.

ATP and the core "alpha-Crystallin" domain of the small heat-shock protein alphaB-crystallin

Electrospray ionization mass spectrometry (ESI-LC/MS) of tryptic digests of human alphaB-crystallin in the presence and absence of ATP identified four residues located within the core "alpha-crystallin" domain, Lys(82), Lys(103), Arg(116), and Arg(123), that were shielded from the action of trypsin in the presence of ATP. In control experiments, chymotrypsin was used in place of trypsin. The chymotryptic fragments of human alphaB-crystallin produced in the presence and absence of ATP were analyzed using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Seven chymotryptic cleavage sites, Trp(60), Phe(61), Phe(75), Phe(84), Phe(113), Phe(118), and Tyr(122), located near or within the core alpha-crystallin domain, were shielded from the action of chymotrypsin in the presence of ATP. Chemically similar analogs of ATP were less protective than ATP against proteolysis by trypsin or chymotrypsin. ATP had no effect on the enzymatic activity of trypsin and the K(m) for trypsin was 0.031 mM in the presence of ATP and 0.029 mM in the absence of ATP. The results demonstrated an ATP-dependent structural modification in the core alpha-crystallin domain conserved in nearly all identified small heat-shock proteins that act as molecular chaperones.

Site-directed mutations within the core "alpha-crystallin" domain of the small heat-shock protein, human alphaB-crystallin, decrease molecular chaperone functions

Site-directed mutagenesis was used to evaluate the effects on structure and function of selected substitutions within and N-terminal to the core "alpha-crystallin" domain of the small heat-shock protein (sHsp) and molecular chaperone, human alphaB-crystallin. Five alphaB-crystallin mutants containing single amino acid substitutions within the core alpha-crystallin domain displayed a modest decrease in chaperone activity in aggregation assays in vitro and in protecting cell viability of E. coli at 50 degrees C in vivo. In contrast, seven alphaB-crystallin mutants containing substitutions N-terminal to the core alpha-crystallin domain generally resembled wild-type alphaB-crystallin in chaperone activity in vitro and in vivo. Size-exclusion chromatography, ultraviolet circular dichroism spectroscopy and limited proteolysis were used to evaluate potential structural changes in the 12 alphaB-crystallin mutants. The secondary, tertiary and quaternary structures of mutants within and N-terminal to the core alpha-crystallin domain were similar to wild-type alphaB-crystallin. SDS-PAGE patterns of chymotryptic digestion were also similar in the mutant and wild-type proteins, indicating that the mutations did not introduce structural modifications that altered the exposure of proteolytic cleavage sites in alphaB-crystallin. On the basis of the similarities between the sequences of human alphaB-crystallin and the sHsp Mj HSP16.5, the only sHsp for which there exists high resolution structural information, a three-dimensional model for alphaB-crystallin was constructed. The mutations at sites within the core alpha-crystallin domain of alphaB-crystallin identify regions that may be important for the molecular chaperone functions of sHsps.

Mutation R120G in alphaB-crystallin, which is linked to a desmin-related myopathy, results in an irregular structure and defective chaperone-like function

alphaB-crystallin, a member of the small heat shock protein family, possesses chaperone-like function. Recently, it has been shown that a missense mutation in alphaB-crystallin, R120G, is genetically linked to a desmin-related myopathy as well as to cataracts [Vicart, P., Caron, A., Guicheney, P., Li, A., Prevost, M.-C., Faure, A., Chateau, D., Chapon, F., Tome, F., Dupret, J.-M., et al. (1998) Nat. Genet. 20, 92-95]. By using alpha-lactalbumin, alcohol dehydrogenase, and insulin as target proteins, in vitro assays indicated that R120G alphaB-crystallin had reduced or completely lost chaperone-like function. The addition of R120G alphaB-crystallin to unfolding alpha-lactalbumin enhanced the kinetics and extent of its aggregation. R120G alphaB-crystallin became entangled with unfolding alpha-lactalbumin and was a major portion of the resulting insoluble pellet. Similarly, incubation of R120G alphaB-crystallin with alcohol dehydrogenase and insulin also resulted in the presence of R120G alphaB-crystallin in the insoluble pellets. Far and near UV CD indicate that R120G alphaB-crystallin has decreased beta-sheet secondary structure and an altered aromatic residue environment compared with wild-type alphaB-crystallin. The apparent molecular mass of R120G alphaB-crystallin, as determined by gel filtration chromatography, is 1.4 MDa, which is more than twice the molecular mass of wild-type alphaB-crystallin (650 kDa). Images obtained from cryoelectron microscopy indicate that R120G alphaB-crystallin possesses an irregular quaternary structure with an absence of a clear central cavity. The results of this study show, through biochemical analysis, that an altered structure and defective chaperone-like function of alphaB-crystallin are associated with a point mutation that leads to a desmin-related myopathy and cataracts.

Loss of epithelial polarity is accompanied by differential association of proteins with intracellular membranes

Cellular membranes play an important role in the formation and maintenance of epithelial polarity, which is lost early during carcinogenesis. We set out to identify membrane proteins which are altered during loss of cell polarity in mammary epithelium. As a model system we used murine mammary epithelial cells expressing the conditional oncoprotein c-JunER, which induces a reversible loss of polarity upon beta-estradiol-driven activation [1]. When grown either in the absence or presence of hormone, these cells exhibit a polarized or unpolarized phenotype, respectively. Different membrane fractions of polarized or unpolarized cells were analyzed by two-dimensional electrophoresis (2-DE) and differentially expressed membrane proteins were identified. To distinguish between transmembrane orientation and peripheral attachment of these proteins, were performed extractions with carbonate at high pH or with Triton X-114. In addition, cytosolic proteins of both states were analyzed to investigate their differential association with distinct membrane fractions. We found ten protein spots preferentially or exclusively in polarized cells and 17 other proteins as being upregulated during loss of polarity. Some of the peripheral membrane proteins were identified by microsequencing. The resident Golgi protein nucleobindin and fructose-bisphosphate aldolase were preferentially associated with membranes of polarized cells, whereas alphaB crystallin was detected exclusively and in high amounts in unpolarized cells.