alpha A-crystallin is expressed in non-ocular tissues

αA-crystallin and αB-crystallin reside in separate subcellular compartments in the developing ocular lens

αA-Crystallin (αA) and αB-crystallin (αB), the two prominent members of the small heat shock family of proteins are considered to be two subunits of one multimeric protein, α-crystallin, within the ocular lens. Outside of the ocular lens, however, αA and αB are known to be two independent proteins, with mutually exclusive expression in many tissues. This dichotomous view is buoyed by the high expression of αA and αB in the lens and their co-fractionation from lens extracts as one multimeric entity, α-crystallin. To understand the biological function(s) of each of these two proteins, it is important to investigate the biological basis of this perceived dichotomy; in this report, we address the question whether αA and αB exist as independent proteins in the ocular lens. Discontinuous sucrose density gradient fractionation and immunoconfocal localization reveal that in early developing rat lens αA is a membrane-associated small heat shock protein similar to αB but with remarkable differences. Employing an established protocol, we demonstrate that αB predominantly sediments with rough endoplasmic reticulum, whereas αA fractionates with smooth membranes. These biochemical observations were corroborated with immunogold labeling and transmission electron microscopy. Importantly, in the rat heart also, which does not contain αA, αB fractionates with rough endoplasmic reticulum, suggesting that αA has no influence on the distribution of αB. These data demonstrate presence of αA and αB in two separate subcellular membrane compartments, pointing to their independent existence in the developing ocular lens.

Pathology-dependent effects linked to small heat shock proteins expression: an update

Small heat shock proteins (small Hsps) are stress-induced molecular chaperones that act as holdases towards polypeptides that have lost their folding in stress conditions or consequently of mutations in their coding sequence. A cellular protection against the deleterious effects mediated by damaged proteins is thus provided to cells. These chaperones are also highly expressed in response to protein conformational and inflammatory diseases and cancer pathologies. Through specific and reversible modifications in their phospho-oligomeric organization, small Hsps can chaperone appropriate client proteins in order to provide cells with resistance to different types of injuries or pathological conditions. By helping cells to better cope with their pathological status, their expression can be either beneficial, such as in diseases characterized by pathological cell degeneration, or deleterious when they are required for tumor cell survival. Moreover, small Hsps are actively released by cells and can act as immunogenic molecules that have dual effects depending on the pathology. The cellular consequences linked to their expression levels and relationships with other Hsps as well as therapeutic strategies are discussed in view of their dynamic structural organization required to interact with specific client polypeptides.

The combined effect of acetylation and glycation on the chaperone and anti-apoptotic functions of human α-crystallin

N(ε)-acetylation occurs on select lysine residues in α-crystallin of the human lens and alters its chaperone function. In this study, we investigated the effect of N(ε)-acetylation on advanced glycation end product (AGE) formation and consequences of the combined N(ε)-acetylation and AGE formation on the function of α-crystallin. Immunoprecipitation experiments revealed that N(ε)-acetylation of lysine residues and AGE formation co-occurs in both αA- and αB-crystallin of the human lens. Prior acetylation of αA- and αB-crystallin with acetic anhydride (Ac(2)O) before glycation with methylglyoxal (MGO) resulted in significant inhibition of the synthesis of two AGEs, hydroimidazolone (HI) and argpyrimidine. Similarly, synthesis of ascorbate-derived AGEs, pentosidine and N(ε)-carboxymethyl lysine (CML), was inhibited in both proteins by prior acetylation. In all cases, inhibition of AGE synthesis was positively related to the degree of acetylation. While prior acetylation further increased the chaperone activity of MGO-glycated αA-crystallin, it inhibited the loss of chaperone activity by ascorbate-glycation in both proteins. BioPORTER-mediated transfer of αA- and αB-crystallin into CHO cells resulted in significant protection against hyperthermia-induced apoptosis. This effect was enhanced in acetylated and MGO-modified αA- and αB-crystallin. Caspase-3 activity was reduced in α-crystallin transferred cells. Glycation of acetylated proteins with either MGO or ascorbate produced no significant change in the anti-apoptotic function. Collectively, these data demonstrate that lysine acetylation and AGE formation can occur concurrently in α-crystallin of human lens, and that lysine acetylation improves anti-apoptotic function of α-crystallin and prevents ascorbate-mediated loss of chaperone function.

Heat shock proteins and heat shock factor 1 in carcinogenesis and tumor development: an update

Heat shock proteins (HSP) are a subset of the molecular chaperones, best known for their rapid and abundant induction by stress. HSP genes are activated at the transcriptional level by heat shock transcription factor 1 (HSF1). During the progression of many types of cancer, this heat shock transcriptional regulon becomes co-opted by mechanisms that are currently unclear, although evidently triggered in the emerging tumor cell. Concerted activation of HSF1 and the accumulation of HSPs then participate in many of the traits that permit the malignant phenotype. Thus, cancers of many histologies exhibit activated HSF1 and increased HSP levels that may help to deter tumor suppression and evade therapy in the clinic. We review here the extensive work that has been carried out and is still in progress aimed at (1) understanding the oncogenic mechanisms by which HSP genes are switched on, (2) determining the roles of HSF1/HSP in malignant transformation and (3) discovering approaches to therapy based on disrupting the influence of the HSF1-controlled transcriptome in cancer.

Biologic activities of molecular chaperones and pharmacologic chaperone imidazole-containing dipeptide-based compounds: natural skin care help and the ultimate challenge: implication for adaptive responses in the skin

Accumulation of molecular damage and increased molecular heterogeneity are hallmarks of photoaged skin and pathogenesis of human cutaneous disease. Growing evidence demonstrates the ability of molecular chaperone proteins and of pharmacologic chaperones to decrease the environmental stress and ameliorate the oxidation stress-related and glycation disease phenotypes, suggesting that the field of chaperone therapy might hold novel treatments for skin diseases and aging. In this review, we examine the evidence suggesting a role for molecular chaperone proteins in the skin and their inducer and protecting agents: pharmacologic chaperone imidazole dipeptide-based agents (carcinine and related compounds) in cosmetics and dermatology. Furthermore, we discuss the use of chaperone therapy for the treatment of skin photoaging diseases and other skin pathologies that have a component of increased glycation and/or free radical-induced oxidation in their genesis. We examine biologic activities of molecular and pharmacologic chaperones, including strategies for identifying potential chaperone compounds and for experimentally demonstrating chaperone activity in in vitro and in vivo models of human skin disease. This allows the protein to function and traffic to the appropriate location in the skin, thereby increasing protein activity and cellular function and reducing stress on skin cells. The benefits of imidazole dipeptide antioxidants with transglycating activity (such as carcinine) in skin care are that they help protect and repair cell membrane damage and help retain youthful, younger-looking skin. All skin types will benefit from daily, topical application of pharmacologic chaperone antioxidants, anti-irritants, in combination with water-binding protein agents that work to mimic the structure and function of healthy skin. General strategies are presented addressing ground techniques to improve absorption of usually active chaperone proteins and dipeptide compounds, include encapsulation into hydrophobic carriers, a combination with penetration enhancers, active electrical transport, or chemical modification to increase hydrophobicity.

Transcriptional regulation of small HSP-HSF1 and beyond

The members of the small heat shock protein (sHSP) family are molecular chaperones that play major roles in development, stress responses, and diseases, and have been envisioned as targets for therapy, particularly in cancer. The molecular mechanisms that regulate their transcription, in normal, stress, or pathological conditions, are characterized by extreme complexity and subtlety. Although historically linked to the heat shock transcription factors (HSFs), the stress-induced or developmental expression of the diverse members, including HSPB1/Hsp27/Hsp25, αA-crystallin/HSPB4, and αB-crystallin/HSPB5, relies on the combinatory effects of many transcription factors. Coupled with remarkably different cis-element architectures in the sHsp regulatory regions, they confer to each member its developmental expression or stress-inducibility. For example, multiple regulatory pathways coordinate the spatio-temporal expression of mouse αA-, αB-crystallin, and Hsp25 genes during lens development, through the action of master genes, like the large Maf family proteins and Pax6, but also HSF4. The inducibility of Hsp27 and αB-crystallin transcription by various stresses is exerted by HSF-dependent mechanisms, by which concomitant induction of Hsp27 and αB-crystallin expression is observed. In contrast, HSF-independent pathways can lead to αB-crystallin expression, but not to Hsp27 induction. Not surprisingly, deregulation of the expression of sHSP is associated with various pathologies, including cancer, neurodegenerative, or cardiac diseases. However, many questions remain to be addressed, and further elucidation of the developmental mechanisms of sHsp gene transcription might help to unravel the tissue- and stage-specific functions of this fascinating class of proteins, which might prove to be crucial for future therapeutic strategies. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.

Small heat shock protein αA-crystallin prevents photoreceptor degeneration in experimental autoimmune uveitis

The small heat shock protein, αA-crystallin null (αA−/−) mice are known to be more prone to retinal degeneration than the wild type mice in Experimental Autoimmune Uveoretinitis (EAU). In this report we demonstrate that intravenous administration of αA preserves retinal architecture and prevents photoreceptor damage in EAU. Interestingly, only αA and not αB-crystallin (αB), a closely related small heat shock protein works, pointing to molecular specificity in the observed retinal protection. The possible involvement of αA in retinal protection through immune modulation is corroborated by adaptive transfer experiments, (employing αA−/− and wild type mice with EAU as donors and Rag2−/− as the recipient mice), which indicate that αA protects against the autoimmune challenge by modulating the systemic B and T cell immunity. We show that αA administration causes marked reduction in Th1 cytokines (TNF-α, IL-12 and IFN-γ), both in the retina and in the spleen; notably, IL-17 was only reduced in the retina suggesting local intervention. Importantly, expression of Toll-like receptors and their associated adaptors is also inhibited suggesting that αA protection, against photoreceptor loss in EAU, is associated with systemic suppression of both the adaptive and innate immune responses.

Specific phosphorylation of αA-crystallin is required for the αA-crystallin-induced protection of astrocytes against staurosporine and C2-ceramide toxicity

We previously reported that αA-crystallin and protease-activated receptor are involved in protection of astrocytes against C2-ceramide- and staurosporine-induced cell death (Li et al., 2009). Here, we investigated the molecular mechanism of αA-crystallin-mediated cytoprotection. We found that the expression of mutants mimicking specific phosphorylation of αA-crystallin increases the protection of astrocytes. However, the expression of mutants mimicking unphosphorylation of αA-crystallin results in loss of protection. These data revealed that the phosphorylation of αA-crystallin at Ser122 and Ser148 is required for protection. Furthermore, we explored the mechanism of cytoprotection of astrocytes by αA-crystallin. Application of specific inhibitors of p38 and ERK abrogates the protection of astrocytes by over-expression of αA-crystallin. Thus, p38 and ERK contribute to protective processes by αA-crystallin. This is comparable to our previous results which demonstrated that p38 and ERK regulated protease-activated receptor-2 (PAR-2)/αB-crystallin-mediated cytoprotection. Furthermore, we found that PAR-2 activation increases the expression of αA-crystallin. Thus, endogenous αA-crystallin protects astrocytes via mechanisms, which regulate the expression and/or phosphorylation status of αA-crystallin.

Identification of the HSPB4/TLR2/NF-κB axis in macrophage as a therapeutic target for sterile inflammation of the cornea

Sterile inflammation underlies many diseases of the cornea including serious chemical burns and the common dry eye syndrome. In search for therapeutic targets for corneal inflammation, we defined the kinetics of neutrophil infiltration in a model of sterile injury to the cornea and identified molecular and cellular mechanisms triggering inflammatory responses. Neutrophil infiltration occurred in two phases: a small initial phase (Phase I) that began within 15 min after injury, and a larger second phase (Phase II) that peaked at 24–48 h. Temporal analysis suggested that the neuropeptide secretoneurin initiated Phase I without involvement of resident macrophages. Phase II was initiated by the small heat shock protein HSPB4 that was released from injured keratocytes and that activated resident macrophages via the TLR2/NF-κB pathway. The Phase II inflammation was responsible for vision-threatening opacity and was markedly suppressed by different means of inhibition of the HSPB4/TLR2/NF-κB axis: in mice lacking HSPB4 or TLR2, by antibodies to HSPB4 or by TNF-α stimulated gene/protein 6 that CD44-dependently inhibits the TLR2/NF-κB pathway. Therefore, our data identified the HSPB4/TLR2/NF-κB axis in macrophages as an effective target for therapy of corneal inflammation.

Hydroimidazolone modification of the conserved Arg12 in small heat shock proteins: studies on the structure and chaperone function using mutant mimics

Methylglyoxal (MGO) is an α-dicarbonyl compound present ubiquitously in the human body. MGO reacts with arginine residues in proteins and forms adducts such as hydroimidazolone and argpyrimidine in vivo. Previously, we showed that MGO-mediated modification of αA-crystallin increased its chaperone function. We identified MGO-modified arginine residues in αA-crystallin and found that replacing such arginine residues with alanine residues mimicked the effects of MGO on the chaperone function. Arginine 12 (R12) is a conserved amino acid residue in Hsp27 as well as αA- and αB-crystallin. When treated with MGO at or near physiological concentrations (2–10 µM), R12 was modified to hydroimidazolone in all three small heat shock proteins. In this study, we determined the effect of arginine substitution with alanine at position 12 (R12A to mimic MGO modification) on the structure and chaperone function of these proteins. Among the three proteins, the R12A mutation improved the chaperone function of only αA-crystallin. This enhancement in the chaperone function was accompanied by subtle changes in the tertiary structure, which increased the thermodynamic stability of αA-crystallin. This mutation induced the exposure of additional client protein binding sites on αA-crystallin. Altogether, our data suggest that MGO-modification of the conserved R12 in αA-crystallin to hydroimidazolone may play an important role in reducing protein aggregation in the lens during aging and cataract formation.

Methionine sulfoxide reductase A: Structure, function and role in ocular pathology

Methionine is a highly susceptible amino acid that can be oxidized to S and R diastereomeric forms of methionine sulfoxide by many of the reactive oxygen species generated in biological systems. Methionine sulfoxide reductases (Msrs) are thioredoxin-linked enzymes involved in the enzymatic conversion of methionine sulfoxide to methionine. Although MsrA and MsrB have the same function of methionine reduction, they differ in substrate specificity, active site composition, subcellular localization, and evolution. MsrA has been localized in different ocular regions and is abundantly expressed in the retina and in retinal pigment epithelial (RPE) cells. MsrA protects cells from oxidative stress. Overexpression of MsrA increases resistance to cell death, while silencing or knocking down MsrA decreases cell survival; events that are mediated by mitochondria. MsrA participates in protein-protein interaction with several other cellular proteins. The interaction of MsrA with α-crystallins is of utmost importance given the known functions of the latter in protein folding, neuroprotection, and cell survival. Oxidation of methionine residues in α-crystallins results in loss of chaperone function and possibly its antiapoptotic properties. Recent work from our laboratory has shown that MsrA is co-localized with αA and αB crystallins in the retinal samples of patients with age-related macular degeneration. We have also found that chemically induced hypoxia regulates the expression of MsrA and MsrB2 in human RPE cells. Thus, MsrA is a critical enzyme that participates in cell and tissue protection, and its interaction with other proteins/growth factors may provide a target for therapeutic strategies to prevent degenerative diseases.

The anti-apoptotic function of human αA-crystallin is directly related to its chaperone activity

αA-crystallin is a molecular chaperone and an antiapoptotic protein. This study investigated the mechanism of inhibition of apoptosis by human αA-crystallin and determined if the chaperone activity of αA-crystallin is required for the antiapoptotic function. αA-crystallin inhibited chemical-induced apoptosis in Chinese hamster ovary (CHO) cells and HeLa cells by inhibiting activation of caspase-3 and -9. In CHO cells, it inhibited apoptosis induced by the overexpression of human proapoptotic proteins, Bim and Bax. αA-crystallin inhibited doxorubicin-mediated activation of human procaspase-3 in CHO cells and it activated the PI3K/Akt cell survival pathway by promoting the phosphorylation of PDK1, Akt and phosphatase tensin homologue in HeLa cells. The phosphoinositide 3 kinase (PI3K) activity was increased by αA-crystallin overexpression but the protein content was unaltered. Downregulation of PI3K by the expression of a dominant-negative mutant or inhibition by LY294002 abrogated the ability of αA-crystallin to phosphorylate Akt. These antiapoptotic functions of αA-crystallin were enhanced in a mutant protein (R21A) that shows increased chaperone activity than the wild-type (Wt) protein. Interestingly, a mutant protein (R49A) that shows decreased chaperone activity was far weaker than the Wt protein in its antiapoptotic functions. Together, our study results show that αA-crystallin inhibits apoptosis by enhancing PI3K activity and inactivating phosphatase tensin homologue and that the antiapoptotic function is directly related to its chaperone activity.

A quantitative NMR spectroscopic examination of the flexibility of the C-terminal extensions of the molecular chaperones, αA- and αB-crystallin

The principal lens proteins αA- and αB-crystallin are members of the small heat-shock protein (sHsp) family of molecular chaperone proteins. Via their chaperone action, αA- and αB-crystallin play an important role in maintaining lens transparency by preventing crystallin protein aggregation and precipitation. αB-crystallin is found extensively extralenticularly where it is stress inducible and acts as a chaperone to facilitate general protein stabilization. The structure of either αA- or αB-crystallin is not known nor is the mechanism of their chaperone action. Our earlier (1)H NMR spectroscopic studies determined that mammalian sHsps have a highly dynamic, polar and unstructured region at their extreme C-terminus (summarized in Carver (1999) Prog. Ret. Eye Res. 18, 431). This C-terminal extension acts as a solubilizing agent for the relatively hydrophobic protein and the complex it makes with its target proteins during chaperone action. In this study, αA- and αB-crystallin were (15)N-labelled and their (1)H-(15)N through-bond correlation, heteronuclear single-quantum coherence (HSQC) NMR spectra were assigned via standard methods. (1)H-(15)N spin-lattice (T(1)) and spin-spin (T(2)) relaxation times were measured for αA- and αB-crystallin in the absence and presence of a bound target protein, reduced α-lactalbumin. (1)H-(15)N Nuclear Overhauser Effect (NOE) values provide an accurate measure, on a residue-by-residue basis, of the backbone flexibility of polypeptides. From measurement of these NOE values, it was determined that the flexibility of the extension in αA- and αB-crystallin increased markedly at the extreme C-terminus. By contrast, upon chaperone interaction of αA-crystallin with reduced α-lactalbumin, flexibility was maintained in the extension but was distributed evenly across all residues in the extension. Two mutants of αB-crystallin in its C-terminal region: (i) I159A and I161A and (ii) K175L, have altered chaperone ability (Treweek et al. (2007) PLoS One 2, e1046). Comparison of (1)H-(15)N NOE values for these mutants with wild type αB-crystallin revealed alteration in flexibility of the extension, particularly at the extremity of K175L αB-crystallin, which may affect chaperone ability.

The small heat shock protein alphaA-crystallin is expressed in pancreas and acts as a negative regulator of carcinogenesis

The small heat shock protein alphaA-crystallin is a structural protein in the ocular lens. In addition, recent studies have also revealed that it is a molecular chaperone, an autokinase and a strong anti-apoptotic regulator. Besides its lenticular distribution, a previous study demonstrates that a detectable level of alphaA-crystallin is found in other tissues including thymus and spleen. In the present study, we have re-examined the distribution of alphaA-crystallin in various normal human and mouse tissues and found that the normal pancreas expresses a moderate level of alphaA-crystallin. Moreover, alphaA-crystallin is found significantly downregulated in 60 cases of pancreatic carcinoma of different types than it is in 11 normal human pancreas samples. In addition, we demonstrate that alphaA-crystallin can enhance the activity of the activating protein-1 (AP-1) through modulating the function of the MAP kinase, and also upregulates components of TGFbeta pathway. Finally, expression of alphaA-crystallin in a pancreatic cancer cell line, MiaPaCa, results in retarded cell migration. Together, these results suggest that alphaA-crystallin seems to negatively regulate pancreatic carcinogenesis.

The eye lens chaperone alpha-crystallin forms defined globular assemblies

Alpha-crystallins are molecular chaperones that protect vertebrate eye lens proteins from detrimental protein aggregation. alphaB-Crystallin, 1 of the 2 alpha-crystallin isoforms, is also associated with myopathies and neuropathological diseases. Despite the importance of alpha-crystallins in protein homeostasis, only little is known about their quaternary structures because of their seemingly polydisperse nature. Here, we analyzed the structures of recombinant alpha-crystallins using biophysical methods. In contrast to previous reports, we show that alphaB-crystallin assembles into defined oligomers consisting of 24 subunits. The 3-dimensional (3D) reconstruction of alphaB-crystallin by electron microscopy reveals a sphere-like structure with large openings to the interior of the protein. alphaA-Crystallin forms, in addition to complexes of 24 subunits, also smaller oligomers and large clusters consisting of individual oligomers. This propensity might explain the previously reported polydisperse nature of alpha-crystallin.

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

Background

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

Methods

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

Results

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

Conclusion

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

Mechanism of suppression of protein aggregation by α-crystallin

This review summarizes experimental data illuminating the mechanism of suppression of heat-induced protein aggregation by alpha-crystallin, one of the small heat shock proteins. The dynamic light scattering data show that the initial stage of thermal aggregation of proteins is the formation of the initial aggregates involving hundreds of molecules of the denatured protein. Further sticking of the starting aggregates proceeds in a regime of diffusion-limited cluster-cluster aggregation. The protective effect of alpha-crystallin is due to transition of the aggregation process to the regime of reaction-limited cluster-cluster aggregation, wherein the sticking probability for the colliding particles becomes lower than unity.

Transcriptional regulation of mouse alpha A-crystallin gene in a 148kb Cryaa BAC and its derivates

Background

αA-crystallin is highly expressed in the embryonic, neonatal and adult mouse lens. Previously, we identified two novel distal control regions, DCR1 and DCR3. DCR1 was required for transgenic expression of enhanced green fluorescent protein, EGFP, in lens epithelium, whereas DCR3 was active during "late" stages of lens primary fiber cell differentiation. However, the onset of transgenic EGFP expression was delayed by 12–24 hours, compared to the expression of the endogenous Cryaa gene.

Results

Here, we used bacterial artificial chromosome (BAC) and standard transgenic approaches to examine temporal and spatial regulation of the mouse Cryaa gene. Two BAC transgenes, with EGFP insertions into the third coding exon of Cryaa gene, were created: the intact αA-crystallin 148 kb BAC (αA-BAC) and αA-BAC(ΔDCR3), which lacks approximately 1.0 kb of genomic DNA including DCR3. Expression of EGFP in the majority of both BAC transgenics nearly recapitulated the endogenous expression pattern of the Cryaa gene in lens, but not outside of the lens. The number of cells expressing αA-crystallin in the lens pit was higher compared to the number of cells expressing EGFP. Next, we generated additional lines using a 15 kb fragment of αA-crystallin locus derived from αA-BAC(ΔDCR3), 15 kb Cryaa/EGFP. A 15 kb region of Cryaa/EGFP supported the expression pattern of EGFP also in the lens pit. However, co-localization studies of αA-crystallin and EGFP indicated that the number of cells that showed transgenic expression was higher compared to cells expressing αA-crystallin in the lens pit.

Conclusion

We conclude that a 148 kb αA-BAC likely contains all of the regulatory regions required for αA-crystallin expression in the lens, but not in retina, spleen and thymus. In addition, while the 15 kb Cryaa/EGFP region also supported the expression of EGFP in the lens pit, expression in regions such as the hindbrain, indicate that additional genomic regions may play modulatory functions in regulating extralenticular αA-crystallin expression. Finally, deletion of DCR3 in either αA-BAC(ΔDCR3) or Cryaa (15 kb) transgenic mice result in EGFP expression patterns that are consistent with DCR's previously established role as a distal enhancer active in "late" primary lens fiber cells.

Alpha-crystallin protected axons from optic nerve degeneration after crushing in rats

In mature mammals, optic nerve injury results in apoptosis of retinal ganglion cells. The literature confirms that lens injury enhances retinal ganglion cells survival, but the mechanism is not very clear. Using silver staining method and computer image analysis techniques, the effect of alpha-crystallin, a major component of the lens in the survival of retinal ganglion cell axons, was investigated in vivo after intravitreal injections. The results showed that enhanced survival of axotomized axons was observed beyond the crush site after a single intravitreal administration of alpha-crystallin at the time of axotomy. Axonal density of the retinal ganglion cell was significantly greater than in the untreated controls until 2 weeks after injection. This effect declined by 4 weeks after injection but survival of axons remained greater than controls. These findings indicate that alpha-crystallin plays a key role in protecting axons after optic nerve injury.

Differential rate constants of racemization of aspartyl and asparaginyl residues in human alpha A-crystallin mutants

Asp58 and Asp151 in alpha A-crystallin of human eye lenses become highly inverted and isomerized to d-beta-Asp residues with age. Racemization was previously shown to proceed rapidly when the residue on the carboxyl side of the Asp residue is small. Asn was also demonstrated to be more susceptible to racemization than Asp in protein. In this study, the changes of rate constants for racemization at Asp58 and Asp151 and at Asn58 and Asn151 were investigated using D58N, S59T, D151N and A152V mutants obtained through site-directed mutagenesis. The rate constant of racemization at Asn151 in D151N was found to be 1.5 times more rapid than Asp151 in the wild-type. For A152V, the rate constant at Asp151 was 1/4 that of the wild-type. There were no significant differences in the rate constants of racemization for both Asp58 and Asn58 residues. The aggregate size of D58N, S59T and D151N mutants increased or increased in polydispersity and their chaperone activities decreased. The size and chaperone activity of A152V was unchanged. These results suggest that structures close to Asp58 and Asp151 residues in the protein affect the rate constant of Asp racemization and the size and chaperone function of alpha A-crystallin.

Effect of methylglyoxal modification on stress-induced aggregation of client proteins and their chaperoning by human αA-crystallin

α-Crystallin prevents protein aggregation under various stress conditions through its chaperone-like properties. Previously, we demonstrated that MGO (methylglyoxal) modification of αA-crystallin enhances its chaperone function and thus may affect transparency of the lens. During aging of the lens, not only αA-crystallin, but its client proteins are also likely to be modified by MGO. We have investigated the role of MGO modification of four model client proteins (insulin, α-lactalbumin, alcohol dehydrogenase and γ-crystallin) in their aggregation and structure and the ability of human αA-crystallin to chaperone them. We found that MGO modification (10–1000 μM) decreased the chemical aggregation of insulin and α-lactalbumin and thermal aggregation of alcohol dehydrogenase and γ-crystallin. Surface hydrophobicity in MGO-modified proteins decreased slightly relative to unmodified proteins. HPLC and MS analyses revealed argpyrimidine and hydroimidazolone in MGO-modified client proteins. The degree of chaperoning by αA-crystallin towards MGO-modified and unmodified client proteins was similar. Co-modification of client proteins and αA-crystallin by MGO completely inhibited stress-induced aggregation of client proteins. Our results indicate that minor modifications of client proteins and αA-crystallin by MGO might prevent protein aggregation and thus help maintain transparency of the aging lens.

Exacerbation of retinal degeneration in the absence of alpha crystallins in an in vivo model of chemically induced hypoxia

This study evaluated the role of crystallins in retinal degeneration induced by chemical hypoxia. Wild-type, alphaA-crystallin (-/-), and alphaB-crystallin (-/-) mice received intravitreal injection of 12 nmol (low dose), 33 nmol (intermediate dose) or 60 nmol (high dose) cobalt chloride (CoCl(2)). Hematoxylin and eosin and TdT-mediated dUTP nick-end labeling (TUNEL) stains were performed after 24 h, 96 h, and 1 week post-injection, while immunofluorescent stains for alphaA- and alphaB-crystallin were performed 1 week post-injection. The in vitro effects of CoCl(2) on alphaB-crystallin expression in ARPE-19 cells were determined by real time RT-PCR, Western blot, and confocal microscopy and studies evaluating subcellular distribution of alphaB-crystallin in the mitochondria and cytosol were also performed. Histologic studies revealed progressive retinal degeneration with CoCl(2) injection in wild-type mice. Retinas of CoCl(2) injected mice showed transient increased expression of HIF-1alpha which was maximal 24h after injection. Intermediate-dose CoCl(2) injection was associated with increased retinal immunofluorescence for both alphaA- and alphaB-crystallin; however, after high-dose injection, increased retinal degeneration was associated with decreased levels of crystallin expression. Injection of CoCl(2) at either intermediate or high dose in alphaA-crystallin (-/-) and alphaB-crystallin (-/-) mice resulted in much more severe retinal degeneration compared to wild-type eyes. A decrease in ARPE-19 total and cytosolic alphaB-crystallin expression with increasing CoCl(2) treatment and an increase in mitochondrial alphaB-crystallin were found. We conclude that lack of alpha-crystallins accentuates retinal degeneration in chemically induced hypoxia in vivo.

Mixed Oligomer Formation between Human αA-Crystallin and its Cataract-causing G98R Mutant: Structural, Stability and Functional Differences

Mutation of the glycine 98 residue to arginine in alphaA-crystallin has been shown to cause presenile cataract in an Indian family. Our earlier study showed that the mutant protein exhibits folding defects that lead to aggregation and inclusion body formation in Escherichia coli. Despite the presence of a normal copy, the pathology is seen in the heterozygous individuals. Formation of mixed oligomers between wild-type and the mutant subunits might be crucial for manifestation of such dominant negative character. We have investigated the role of G98R mutation in alphaA-crystallin in its structural stability and subunit exchange. G98R alphaA-crystallin unfolds at lower concentrations of urea compared to wild-type alphaA-crystallin. The mutant protein is more susceptible to proteolysis than the wild-type protein and transiently populates fragments that are prone to aggregation. Subunit exchange studies using fluorescence resonance energy transfer show that the mutant protein forms mixed oligomers with the wild-type protein. The mutant protein is more susceptible to thermal aggregation, whereas mixed oligomer formation leads to a decreased propensity to aggregate. Co-expression of wild-type alphaA-crystallin with G98R alphaA-crystallin in E. coli rescues the mutant alphaA-crystallin from formation of inclusion bodies. These observations may underlie the molecular basis for the presenile onset, not congenital cataract, in spite of severe folding defect and aggregation of the mutant. Our study shows that the mixed oligomers of wild-type and G98R alphaA-crystallin exhibit properties dominated by those of the mutant protein in structural aspects, oligomeric size, urea-induced unfolding and, more importantly, the chaperone activity, which may provide the molecular basis for presenile cataract formation in affected individuals.

Small heat shock protein alphaA-crystallin regulates epithelial sodium channel expression

Integral membrane proteins are synthesized on the cytoplasmic face of the endoplasmic reticulum (ER). After being translocated or inserted into the ER, they fold and undergo post-translational modifications. Within the ER, proteins are also subjected to quality control checkpoints, during which misfolded proteins may be degraded by proteasomes via a process known as ER-associated degradation. Molecular chaperones, including the small heat shock protein alphaA-crystallin, have recently been shown to play a role in this process. We have now found that alphaA-crystallin is expressed in cultured mouse collecting duct cells, where apical Na(+) transport is mediated by epithelial Na(+) channels (ENaC). ENaC-mediated Na(+) currents in Xenopus oocytes were reduced by co-expression of alphaA-crystallin. This reduction in ENaC activity reflected a decrease in the number of channels expressed at the cell surface. Furthermore, we observed that the rate of ENaC delivery to the cell surface of Xenopus oocytes was significantly reduced by co-expression of alphaA-crystallin, whereas the rate of channel retrieval remained unchanged. We also observed that alphaA-crystallin and ENaC co-immunoprecipitate. These data are consistent with the hypothesis that small heat shock proteins recognize ENaC subunits at ER quality control checkpoints and can target ENaC subunits for ER-associated degradation.

Unfolding and Refolding of Bovine α-Crystallin in Urea and Its Chaperone Activity

We undertook an unfolding and refolding study of alpha(L)-crystallin in presence of urea to explore the breakdown and formation of various levels of structure and to find out whether the breakdown of various levels of structure occurs simultaneously or in a hierarchal manner. We used various techniques such as circular dichroism, fluorescence spectroscopy, light scattering, polarization to determine the changes in secondary, tertiary, and quaternary structure. Unfolding and refolding occurred through a number of intermediates. The results showed that all levels of structure in alpha(L)-crystallin collapsed or reformed simultaneously. The intermediates that occurred in the 2-4 M urea concentration range during unfolding and refolding differed from each other in terms of the polarity of the tryptophan environment. The ANS binding experiments revealed that refolded alpha(L)-crystallin had higher number of hydrophobic pockets compared to native one. On the other hand, polarity of these pockets remained same as that of the native protein. Both light scattering and polarization measurements showed smaller oligomeric size of refolded alpha(L)-crystallin. Thus, although the secondary structural changes were almost reversible, the tertiary and quaternary structural changes were not. The refolded alpha(L)-crystallin had more exposed hydrophobic sites with increased binding affinity. The refolded form also showed higher chaperone activity than native one. Since the refolded form was smaller in oligomeric size, some buried hydrophobic sites were available. The higher chaperone activity of lower sized oligomer of alpha(L)-crystallin again revealed that chaperone activity was dependent on hydrophobicity and not on oligomeric size.

Modulation of alpha and beta crystallin expression in rat retinas with ocular hypertension-induced ganglion cell degeneration

The expression of alpha (alphaA and alphaB) and beta (betaA1/A3, betaA2, betaA4, and betaB2) crystallin genes were analyzed at the mRNA and protein levels in rat retinas with ocular hypertension-induced ganglion cell death. An animal model with progressive loss of retinal ganglion cells (RGC) was generated by elevation of intraocular pressure (IOP). The estimated RGC loss was approximately 8% and 20% at 2 and 5 weeks post IOP elevation, respectively. mRNA and protein quantification showed that alpha and beta crystallin genes were downregulated at both transcriptional (alphaA, alphaB, betaA1/A3, betaA4, and betaB2 approximately 50% and betaA2~40%) and protein (alphaA~50%, alphaB~63%, betaA1/A3~70%, and betaB2~38%) levels 2 weeks after IOP elevation. In experimental retinas 5 weeks after IOP elevation, the levels of crystallin mRNAs were higher than at 2 weeks and were comparable to that of control retinas. However, the levels of the corresponding proteins were still lower (alphaA, alphaB, and betaB2 approximately 37% and betaA1/A3~70%) than in control retinas. Furthermore, we found that the expression of these genes in the retina is predominantly localized to the cells in the GCL and to a lesser degree in the INL and ONL. Colocalization of the crystallin-positive and Fluorogold retrogradely labeled cells indicated that the cells expressing alpha and beta crystallins in the GCL are RGCs. In summary, we showed that alpha and beta crystallins are expressed in the retina predominantly by RGCs and that their expression is affected by ocular hypertension.

Conserved F84 and P86 residues in alphaB-crystallin are essential to effectively prevent the aggregation of substrate proteins

Previously, we have shown that residues 73-92 (sequence DRFSVNLDVKHFSPEELKVK) in alphaB-crystallin are involved in preventing the formation of light scattering aggregates by substrate proteins. In this study, we made single substitutions of three conserved amino acid residues (H83 --> A, F84 --> G, and P86 --> A) and a nonconserved amino acid residue (K90 --> C) in the functional region of alphaB-crystallin and evaluated their role in anti-aggregation activity. Mutation of conserved residues led to changes in intrinsic tryptophan intensity, bis-ANS binding, and in the secondary and tertiary structures. The H83A mutation led to a twofold increase in molar mass, while the other mutants did not produce significant changes in the molar mass when compared to that of wild-type protein. The chaperone-like activity of the H83A mutant was enhanced by 15%-20%, and the chaperone-like activity of F84G and P86A mutants was reduced by 50%-65% when compared to the chaperone-like activity of wild-type alphaB-crystallin. The substitution of the nonconserved residue (K90 --> C) did not induce an appreciable change in the structure and function of the mutant protein. Fluorescence resonance energy transfer (FRET) assay demonstrated that destabilized ADH interacted near the K90 region in alphaB-crystallin. The data show that F84 and P86 residues are essential for alphaB-crystallin to effectively prevent the aggregation of substrate proteins. This study further supports the involvement of the residues in the 73-92 region of alphaB-crystallin in substrate protein binding and chaperone-like action.

Structure, stability, and chaperone function of αA-crystallin: Role of N-terminal region

Small heat shock protein alphaA-crystallin, the major protein of the eye lens, is a molecular chaperone. It consists of a highly conserved central domain flanked by the N-terminal and C-terminal regions. In this article we studied the role of the N-terminal domain in the structure and chaperone function of alphaA-crystallin. Using site directed truncation we raised several deletion mutants of alphaA-crystallin and their protein products were expressed in Escherichia coli. Size exclusion chromatography of these purified proteins showed that deletion from the N-terminal beyond the first 20 residues drastically reduced the oligomeric association of alphaA-crystallin and its complete removal resulted in a tetramer. Chaperone activity of alphaA-crystallin, determined by thermal and nonthermal aggregation and refolding assay, decreased with increasing length of deletion and little activity was observed for the tetramer. However it was revealed that N-terminal regions were not responsible for specific recognition of natural substrates and that low affinity substrate binding sites existed in other part of the molecule. The number of exposed hydrophobic sites and the affinity of binding hydrophobic probe bis-ANS as well as protein substrates decreased with N-terminal deletion. The stability of the mutant proteins decreased with increase in the length of deletion. The role of thermodynamic stability, oligomeric size, and surface hydrophobicity in chaperone function is discussed. Detailed analysis showed that the most important role of N-terminal region is to control the oligomerization, which is crucial for the stability and in vivo survival of this protein molecule.

Differential recognition of natural and nonnatural substrate by molecular chaperone α-crystallin—A subunit exchange study

alpha-Crystallin is a molecular chaperone that recognizes proteins substrates in stress. It binds to the unstable conformer of a large variety of related or unrelated substrates and thus prevents them aggregating and holds them in a folding competent state. In this article, we have tried to critically analyze, from experimental point of view, whether alpha-crystallin has any preference for its natural substrates compared to the nonnatural one. Our results clearly show that alpha-crystallin is exceptionally active and sensitive in preventing aggregation of its natural substrates and can fully prevent such an aggregation in a substoichiometric ratio, but nonnatural substrates require a considerably higher amount of alpha-crystallin. Using suitable fluorescent-labeled alpha-crystallins and performing fluorescence resonance energy transfer experiments, we were able to determine the subunit exchange kinetics between the alpha-crystallin oligomers. It was found that while alpha-crystallin was bound to its natural substrate, the rate of subunit exchange was slightly decreased. But, when a nonnatural substrate carbonic anhydrase remained bound to the chaperone, further loss in subunit exchange rate was observed. Nonnatural substrate was found to create higher activation energy barrier for the subunit exchange reaction compared to the native substrates. Similarities in major beta-sheet structure of both alpha-crystallin and its natural substrates may be the reason for the preference in molecular recognition in comparison with the nonnatural substrate.

Differential susceptibility of alpha A- and alpha B-crystallin to gamma-ray irradiation

Alpha-crystallin, a major protein of all vertebrate lenses, consists of two different subunits, alpha A and alpha B, which form polymeric aggregates with an average molecular mass of 300-800 kDa. Both the alpha A and alpha B subunit have a molecular mass of about 20 kDa. It is not known why alpha crystallin aggregates comprise two different subunits, given that the physicochemical properties of these proteins are very similar. The present study compares the susceptibility of the alpha A and alpha B subunits to gamma-rays. We prepared a recombinant form of human alpha A- and alpha B-crystallin and then irradiated the proteins with gamma-rays. Based on far-UV CD spectra, alpha A-crystallin retained beta-sheet conformation after gamma irradiation up to 3.0 kGy, whereas alpha B-crystallin lost beta-sheet conformation upon exposure to gamma irradiation at >1.0 kGy. Size exclusion chromatography showed that the aggregation and polydispersity of recombinant alpha A-crystallin increased slightly after >1.0 kGy irradiation. In contrast, irradiation of alpha B-crystallin at 1.0 kGy resulted in the formation of huge aggregates and a marked increase in heterogeneity. We have also compared the chaperone activities of gamma-irradiated alpha A- and alpha B-crystallin aggregates. The activity of irradiated alpha A-crystallin was retained while that of the irradiated alpha B-crystallin was became inactive after irradiation of >0.5 kGy. Our results indicate that alpha A-crystallin is more stable to gamma irradiation than alpha B-crystallin.

Eye lens proteomics

The eye lens is a fascinating organ as it is in essence living transparent matter. Lenticular transparency is achieved through the peculiarities of lens morphology, a semi-apoptotic process where cells elongate and loose their organelles and the precise molecular arrangement of the bulk of soluble lenticular proteins, the crystallins. The 16 crystallins ubiquitous in mammals and their modifications have been extensively characterized by 2-DE, liquid chromatography, mass spectrometry and other protein analysis techniques. The various solubility dependant fractions as well as subproteomes of lenticular morphological sections have also been explored in detail. Extensive post translational modification of the crystallins is encountered throughout the lens as a result of ageing and disease resulting in a vast number of protein species. Proteomics methodology is therefore ideal to further comprehensive understanding of this organ and the factors involved in cataractogenesis.

Temperature dependence of chaperone-like activity and oligomeric state of αB-crystallin

The chaperone-like activity and the oligomeric state of alphaB-crystallin were studied at different temperatures and in the presence of urea and thiocyanate. The activity, assessed measuring the ability of alphaB-crystallin to prevent the aggregation of denatured insulin, strongly depends on temperature. While a significant activity increase was detected at 42 degrees C, the presence of urea and thiocyanate does not affect the protein activity in an irreversible way. In-solution SAXS measurements performed in the same experimental conditions showed that alphaB-crystallin forms near-spherical, hollowed, polydisperse oligomers, whose dimensions change above 42 degrees C. Moreover, in the presence of urea and thiocyanate, a global fit analysis confirms the high stability of alphaB-crystallin assemblies in relationship with their variable quaternary structure. In particular, the changes in the inner radius as well as the thickness and dispersion of the protein shell, account for the preservation of the chaperone-like activity.

Relationship between chaperone activity and oligomeric size of recombinant human alphaA- and alphaB-crystallin: a tryptic digestion study

Alpha-crystallin, the major eye lens protein, exists as a large oligomer of two subunits, alphaA- and alphaB-crystallin. The individual subunits assemble into the oligomer in vitro. It is generally believed that oligomerization is pre-requisite for chaperone function, although there is no hard data available on this subject. We therefore undertook a study using limited tryptic digestion as a tool for examining the relationship between oligomeric size and chaperone activity of recombinant alphaA- and alphaB-crystallin. We showed that tryptic digested fragments of both alphaA- and alphaB-crystallin much smaller than the original subunits retain considerable chaperone activity. Our results indicate that chaperone activity depends more on the sequence of the reduced peptide than on its oligomeric size. The results also suggest that the presence of the alpha-crystallin domain and hydrophobic clefts on the protein surface, which correlate poorly with oligomeric size, are important for chaperone function.

Alpha-crystallin is a target gene of the farnesoid X-activated receptor in human livers

Alpha-crystallins comprise 35% of soluble proteins in the ocular lens and possess chaperone-like functions. Furthermore, the alphaA subunit (alphaA-crystallin) of alpha crystallin is thought to be "lens-specific" as only very low levels of expression were detected in a few non-lenticular tissues. Here we report that human alphaA-crystallin is expressed in human livers and is regulated by farnesoid X-activated receptor (FXR) in response to FXR agonists. AlphaA-crystallin was identified in a microarray screen as one of the most highly induced genes after treatment of HepG2 cells with the synthetic FXR ligand GW4064. Northern blot and quantitative real-time PCR analyses confirmed that alphaA-crystallin expression was induced in HepG2-derived cell lines and human primary hepatocytes and hepatic stellate cells in response to either natural or synthetic FXR ligands. Transient transfection studies and electrophoretic mobility shift assays revealed a functional FXR response element located in intron 1 of the human alphaA-crystallin gene. Importantly, immunohistochemical staining of human liver sections showed increased alphaA-crystallin expression in cholangiocytes and hepatocytes. As a member of the small heat shock protein family possessing chaperone-like activity, alphaA-crystallin may be involved in protection of hepatocytes from the toxic effects of high concentrations of bile acids, as would occur in disease states such as cholestasis.

α-Crystallin localizes to the leading edges of migrating lens epithelial cells

alpha-crystallin (alphaA and alphaB) is a major lens protein, which belongs to the small heat-shock family of proteins and binds to various cytoskeletal proteins including actin, vimentin and desmin. In this study, we investigated the cellular localization of alphaA and alphaB-crystallins in migrating epithelial cells isolated from porcine lens. Immunofluorescence localization and confocal imaging of alphaB-crystallin in confluent and in migrating subconfluent cell cultures revealed a distinct pattern of subcellular distribution. While alphaB-crystallin localization was predominantly cytoplasmic in confluent cultures, it was strongly localized to the leading edges of cell membrane or the lamellipodia in migrating cells. In accordance with this pattern, we found abundant levels of alphaB-crystallin in membrane fractions compared to cytosolic and nuclear fractions in migrating lens epithelial cells. alphaA-crystallin, which has 60% sequence identity to alphaB-crystallin, also exhibited a distribution profile localizing to the leading edge of the cell membrane in migrating lens epithelial cells. Localization of alphaB-crystallin to the lamellipodia appears to be dependent on phosphorylation of residue serine-59. An inhibitor of p38 MAP kinase (SB202190), but not the ERK kinase inhibitor PD98059, was found to diminish localization of alphaB-crystallin to the lamellipodia, and this effect was found to be associated with reduced levels of Serine-59 phosphorylated alphaB-crystallin in SB202190-treated migrating lens epithelial cells. alphaB-crystallin localization to the lamellipodia was also altered by the treatment with RGD (Arg-Ala-Asp) peptide, dominant negative N17 Rac1 GTPase, cytochalasin D and Src kinase inhibitor (PP2), but not by the Rho kinase inhibitor Y-27632 or the myosin II inhibitor, blebbistatin. Additionally, in migrating lens epithelial cells, alphaB-crystallin exhibited a clear co-localization with the actin meshwork, beta-catenin, WAVE-1, a promoter of actin nucleation, Abi-2, a component of WAVE-1 protein complex and Arp3, a protein of the actin nucleation complex, suggesting potential interactions between alphaB-crystallin and regulatory proteins involved in actin dynamics and cell adhesion. This is the first report demonstrating specific localization of alphaA and alphaB-crystallins to the lamellipodia in migrating lens epithelial cells and our findings indicate a potential role for alpha-crystallin in actin dynamics during cell migration.

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.

Ageing and vision: structure, stability and function of lens crystallins

The alpha-, beta- and gamma-crystallins are the major protein components of the vertebrate eye lens, alpha-crystallin as a molecular chaperone as well as a structural protein, beta- and gamma-crystallins as structural proteins. For the lens to be able to retain life-long transparency in the absence of protein turnover, the crystallins must meet not only the requirement of solubility associated with high cellular concentration but that of longevity as well. For proteins, longevity is commonly assumed to be correlated with long-term retention of native structure, which in turn can be due to inherent thermodynamic stability, efficient capture and refolding of non-native protein by chaperones, or a combination of both. Understanding how the specific interactions that confer intrinsic stability of the protein fold are combined with the stabilizing effect of protein assembly, and how the non-specific interactions and associations of the assemblies enable the generation of highly concentrated solutions, is thus of importance to understand the loss of transparency of the lens with age. Post-translational modification can have a major effect on protein stability but an emerging theme of the few studies of the effect of post-translational modification of the crystallins is one of solubility and assembly. Here we review the structure, assembly, interactions, stability and post-translational modifications of the crystallins, not only in isolation but also as part of a multi-component system. The available data are discussed in the context of the establishment, the maintenance and finally, with age, the loss of transparency of the lens. Understanding the structural basis of protein stability and interactions in the healthy eye lens is the route to solve the enormous medical and economical problem of cataract.

Role of ATP on the interaction of alpha-crystallin with its substrates and its implications for the molecular chaperone function

ATP plays a significant role in the function of molecular chaperones of the large heat shock protein families. However, its role in the functions of chaperones of the small heat shock protein families is not understood very well. We report here a study on the role of ATP on the structure and function of the major eye lens chaperone alpha-crystallin. Our in vitro study shows that at physiological temperature, ATP induces the association of alpha-crystallin with substrate proteins. The association process is reversible and low affinity in nature with unit binding stoichiometry. 4,4'-Dianilino-1,1'-binaphthyl-5,5-disulfonic acid, dipotassium salt, binding studies show that ATP induces the exposure of additional hydrophobic sites on alpha-crystallin, but no appreciable enhancement of the same was observed for the substrate protein gamma-crystallin or carbonic anhydrase. An equilibrium unfolding study reveals that ATP at 3 mgm concentration stabilizes the alpha-crystallin structure by 4.5 kJ/mol. The compactness induced by ATP makes it more resistant to tryptic cleavage. ATP-induced association of chaperone alpha-crystallin with substrate enhanced its aggregation prevention ability and also enhanced the refolding yield of lactate dehydrogenase from the unfolded state. Our results suggest that the binding of ATP to alpha-crystallin and not its hydrolysis is required for all these effects, as replacement of ATP by its nonhydrolyzable analogue adenosine-5'-O-(3-thiotriphosphate), tetralithium salt, reproduced all the results faithfully. The implication of the ATP-induced reversible protein-protein association at physiological temperatures on the functional role of alpha-crystallin in vivo is discussed.

Immunotolerance toward native alphaA-crystallin in knockout mice deficient in the functional protein

Immune response against self antigens is normally prevented by an elaborate immunotolerance mechanism. A potential problem for recipients of gene therapy is, therefore, an immune response against the newly introduced gene product. To examine this issue we tested the immune response to the native proteins in knockout (KO) mice in which the genes for alphaA- or alphaB-crystallin were disrupted by partial or complete gene deletion, respectively. alphaA- and alphaB-crystallins are two immunologically distinct polypeptides which form the large ( approximately 800 kDa) complex in the lens referred to as alpha-crystallin. When immunized with murine alpha-crystallin, alphaB-crystallin KO mice, in which the corresponding gene was completely deleted, responded well to the absent self antigen. In contrast, alphaA-crystallin KO mice, with the partial gene deletion, resembled wild type (WT) mice in being immunotolerant toward the native crystallin. Although no functional alphaA-crystallin could be detected in the lens of alphaA-crystallin KO mice, mRNA transcript coding for a truncated alphaA-crystallin gene was found in thymi of these mice, suggesting that thymic expression of a residual fragment of the protein is responsible for the tolerance induction. These data suggest that nonfunctional proteins may induce immunotolerance and protect recipients of gene therapy from immunity against the native proteins.

Effects of divalent metal ions on the αB-crystallin chaperone-like activity: spectroscopic evidence for a complex between copper(II) and protein

AlphaB-crystallin is a small heat shock protein, showing chaperone-like activity, that is expressed in the lens and in several other tissues. The role of some metal ions in the alphaB-crystallin biology starts to be well documented. In some neuro-degenerative pathologies, like Parkinson and Alzheimer's diseases, alphaB-crystallin is expressed at high levels. In the same pathologies an accumulation of divalent metal cations is observed. In order to investigate the interactions between human alphaB-crystallin and divalent metal ions, the effect of copper, zinc and calcium on the chaperone-like activity of the protein has been studied. Copper and zinc at concentrations 0.1 and 1 mM significantly increase the chaperone-like activity, whereas calcium 1 mM completely inhibits activity. Electron paramagnetic resonance (EPR) and circular dichroism (CD) spectra indicate the possible complex formation between Cu(II) and protein at physiological pH. Molecular modeling calculations, carried out for the probable Cu(II) binding site, suggest that a complex with three histidine residues is possible.

The Selective Inhibition of Serpin Aggregation by the Molecular Chaperone, α-Crystallin, Indicates a Nucleation-dependent Specificity

Small heat shock proteins (sHsps) are a ubiquitous family of molecular chaperones that prevent the misfolding and aggregation of proteins. However, specific details about their substrate specificity and mechanism of chaperone action are lacking. alpha1-Antichymotrypsin (ACT) and alpha1-antitrypsin (alpha1-AT) are two closely related members of the serpin superfamily that aggregate through nucleation-dependent and nucleation-independent pathways, respectively. The sHsp alpha-crystallin was unable to prevent the nucleation-independent aggregation of alpha1-AT, whereas alpha-crystallin inhibited ACT aggregation in a dose-dependent manner. This selective inhibition of ACT aggregation coincided with the formation of a stable high molecular weight alpha-crystallin-ACT complex with a stoichiometry of 1 on a molar subunit basis. The kinetics of this interaction occur at the same rate as the loss of ACT monomer, suggesting that the monomeric species is bound by the chaperone. 4,4'-Dianilino-1,1'-binaphthyl-5,5'-disulfonic acid (Bis-ANS) binding and far-UV circular dichroism data suggest that alpha-crystallin interacts specifically with a non-native conformation of ACT. The finding that alpha-crystallin does not interact with alpha1-AT under these conditions suggests that alpha-crystallin displays a specificity for proteins that aggregate through a nucleation-dependent pathway, implying that the dynamic nature of both the chaperone and its substrate protein is a crucial factor in the chaperone action of alpha-crystallin and other sHsps.

Roles for alphaB-crystallin and HSPB2 in protecting the myocardium from ischemia-reperfusion-induced damage in a KO mouse model

Overexpression studies have shown that the small heat shock proteins (sHSP) protect the myocardium from ischemia-reperfusion (I/R)-induced damage. However, gene deletion studies are necessary to demonstrate whether sHSPs are required for protection. The genes for alphaB-crystallin (alphaBC) and HSPB2, two sHSPs that are expressed in high levels in the heart, are in close proximity to one another; as a result, both genes were disrupted in a recently generated knockout (KO) mouse line. The alphaBC/HSPB2 KO mouse line is currently the only model that features disruption of sHSPs normally expressed in the heart. Accordingly, we examined the cardiac morphology, function, and response to I/R-induced stress in alphaBC-HSPB2 KO mice. Initial gross, light microscopic and echocardiographic characterization showed that the morphological and functional properties of hearts from adult KO mice were indistinguishable from age-matched wild-type (WT) mice. Electron microscopy showed that, compared with WT mouse hearts, KO mouse heart sarcomeres were relatively normal. Isolated perfused KO mouse hearts displayed normal contractility; however, when compared with WT, after I/R, KO mouse hearts exhibited a twofold reduction in contractile recovery, as well as increased necrosis and apoptosis. Additionally, when compared with WT, KO mouse hearts exhibited 43% less reduced glutathione, which is known to protect from I/R-induced damage. Thus, whereas neither alphaBC nor HSPB2 is essential for myocardial development and function under nonstressful conditions, one or both are required for maximal functional recovery and protection from I/R-induced necrosis and apoptosis.

Crystallins, genes and cataract

Far from being a physical entity, assembled of inanimate structural proteins, the ocular lens epitomizes the biological ingenuity that sustains an essential and near-perfect physical system of immaculate optics. Crystallins (alpha, beta, and gamma) provide transparency by dint of their high concentration, but it is debatable whether proteins that provide transparency are any different, biologically or structurally, from those that are present in non-transparent structures or tissues. It is becoming increasingly clear that crystallins may have a plethora of metabolic and regulatory functions, both within the lens as well as outside of it. Alpha-crystallins are members of a small heat shock family of proteins and beta/gamma-crystallins belong to the family of epidermis-specific differentiation proteins. Crystallin gene expression has been studied from the perspective of the lens specificity of their promoters. Mutations in alpha-, beta-, and gamma-crystallins are linked with the phenotype of the loss of transparency. Understanding catalytic, non-structural properties of crystallins may be critical for understanding the malfunction in molecular cascades that lead to cataractogenesis and its eventual therapeutic amelioration.