Cloning, expression, and chaperone-like activity of human alphaA-crystallin

Nano-assemblies enhance chaperone activity, stability, and delivery of alpha B-crystallin-D3 (αB-D3)

Crystallins, small heat shock chaperone proteins that prevent protein aggregation, are of potential value in treating protein aggregation disorders. However, their therapeutic use is limited by their low potency and poor intracellular delivery. One approach to facilitate the development of crystallins is to improve their activity, stability, and delivery. In this study, zinc addition to αB-crystallin-D3 (αB-D3) formed supramolecular nano- and micro- assemblies, induced dose-dependent changes in structure (beta-sheet to alpha-helix) and increased surface hydrophobicity and chemical stability. Further, crystallin assemblies exhibited a size-dependent chaperone activity, with the nano-assemblies being superior to micro-assemblies and 4.3-fold more effective than the native protein in preventing β-mercaptoethanol induced aggregation of insulin. Insulin rescued by crystallin assemblies retained the activity as evidenced by glucose uptake in 3T3-L1 cells. The most active nano-assemblies enhanced protein stability, in the presence of urea, by 1.6-fold, whereas intracellular delivery was enhanced by 3.0-fold. The αB-D3 crystallin nano-assemblies exhibit uniquely enhanced stability, activity, and delivery compared to the native protein.

Plant-Based Diets Induce Transcriptomic Changes in Muscle of Zebrafish and Atlantic Salmon

With the expansion of the aquaculture industry in the last two decades, there has been a large increase in the use of plant ingredients in aquafeeds, which has created new challenges in fish growth, health and welfare. Fish muscle growth is an important trait that is strongly affected by diet, but our knowledge on the effect of plant protein-based diets on global gene expression in muscle is still scant. The present study evaluated nutrigenomic effects of the inclusion of proteins from pea, soy and wheat into aquafeeds, compared to a control diet with fishmeal as the main protein source using the zebrafish model by RNA-seq; these results were extended to an important aquaculture species by analyzing selected differentially expressed genes identified in the zebrafish model on on-growing Atlantic salmon fed with equivalent plant protein-based diets. Expression of selected Atlantic salmon paralogues of the zebrafish homologs was analyzed using paralogue-specific qPCR assays. Global gene expression changes in muscle of zebrafish fed with plant-based diets were moderate, with the highest changes observed in the soy diet-fed fish, and no change for the pea diet-fed fish compared to the control diet. Among the differentially expressed genes were mylpfb, hsp90aa1.1, col2a1a, and odc1, which are important in regulating muscle growth, maintaining muscle structure and function, and muscle tissue homeostasis. Furthermore, those genes and their paralogues were differentially expressed in Atlantic salmon fed with the equivalent percentage of soy or wheat protein containing diets. Some of these genes were similarly regulated in both species while others showed species-specific regulation. The present study expands our understanding on the molecular effects of plant ingredients in fish muscle. Ultimately, the knowledge gained would be of importance for the improved formulation of sustainable plant-based diets for the aquaculture industry.

Replacing the eleven native tryptophans by directed evolution produces an active P-glycoprotein with site-specific, non-conservative substitutions

P-glycoprotein (Pgp) pumps an array of hydrophobic compounds out of cells, and has major roles in drug pharmacokinetics and cancer multidrug resistance. Yet, polyspecific drug binding and ATP hydrolysis-driven drug export in Pgp are poorly understood. Fluorescence spectroscopy using tryptophans (Trp) inserted at strategic positions is an important tool to study ligand binding. In Pgp, this method will require removal of 11 endogenous Trps, including highly conserved Trps that may be important for function, protein-lipid interactions, and/or protein stability. Here, we developed a directed evolutionary approach to first replace all eight transmembrane Trps and select for transport-active mutants in Saccharomyces cerevisiae. Surprisingly, many Trp positions contained non-conservative substitutions that supported in vivo activity, and were preferred over aromatic amino acids. The most active construct, W(3Cyto), served for directed evolution of the three cytoplasmic Trps, where two positions revealed strong functional bias towards tyrosine. W(3Cyto) and Trp-less Pgp retained wild-type-like protein expression, localization and transport function, and purified proteins retained drug stimulation of ATP hydrolysis and drug binding affinities. The data indicate preferred Trp substitutions specific to the local context, often dictated by protein structural requirements and/or membrane lipid interactions, and these new insights will offer guidance for membrane protein engineering.

The structure and oxidation of the eye lens chaperone αA-crystallin

The small heat shock protein αA-crystallin is a molecular chaperone important for the optical properties of the vertebrate eye lens. It forms heterogeneous oligomeric ensembles. We determined the structures of human αA-crystallin oligomers by combining cryo-electron microscopy, cross-linking/mass spectrometry, NMR spectroscopy and molecular modeling. The different oligomers can be interconverted by the addition or subtraction of tetramers, leading to mainly 12-, 16- and 20-meric assemblies in which interactions between N-terminal regions are important. Cross-dimer domain-swapping of the C-terminal region is a determinant of αA-crystallin heterogeneity. Human αA-crystallin contains two cysteines, which can form an intramolecular disulfide in vivo. Oxidation in vitro requires conformational changes and oligomer dissociation. The oxidized oligomers, which are larger than reduced αA-crystallin and destabilized against unfolding, are active chaperones and can transfer the disulfide to destabilized substrate proteins. The insight into the structure and function of αA-crystallin provides a basis for understanding its role in the eye lens.

Sequence specific 1H, 13C and 15N resonance assignments of the C-terminal domain of human γS-crystallin

The high solubility and stability of crystallins present in the human eye lens maintains its transparency and refractive index with negligible protein turnover. Monomeric γ-crystallins and oligomeric β-crystallins are made up of highly homologous double Greek key domains. These domains are symmetric and possess higher stability as a result of the complex topology of individual Greek key motifs. γS-crystallin is one of the most abundant structural βγ-crystallins present in the human eye lens. In order to understand the structural stability of individual domains of human γS-crystallin in isolation vis-à-vis full length protein, we set out to structurally characterize its C-terminal domain (abbreviated hereafter as γS-CTD) by solution NMR. In this direction, we have cloned, over-expressed, isolated and purified the γS-CTD. The 2D [¹⁵N-¹H] HSQC recorded with uniformly ¹³C/¹⁵N labeled γS-CTD showed a highly dispersed spectrum indicating the protein to adopt an ordered conformation. In this paper, we report almost complete sequence-specific ¹H, ¹³C and ¹⁵N resonance assignments of γS-CTD using a suite of heteronuclear 3D NMR experiments.

Caprine humoral response to Burkholderia pseudomallei antigens during acute melioidosis from aerosol exposure

Burkholderia pseudomallei causes melioidosis, a common source of pneumonia and sepsis in Southeast Asia and Northern Australia that results in high mortality rates. A caprine melioidosis model of aerosol infection that leads to a systemic infection has the potential to characterize the humoral immune response. This could help identify immunogenic proteins for new diagnostics and vaccine candidates. Outbred goats may more accurately mimic human infection, in contrast to the inbred mouse models used to date. B. pseudomallei infection was delivered as an intratracheal aerosol. Antigenic protein profiling was generated from the infecting strain MSHR511. Humoral immune responses were analyzed by ELISA and western blot, and the antigenic proteins were identified by mass spectrometry. Throughout the course of the infection the assay results demonstrated a much greater humoral response with IgG antibodies, in both breadth and quantity, compared to IgM antibodies. Pre-infection sera showed multiple immunogenic proteins already reactive for IgG (7-20) and IgM (0-12) in most of the goats despite no previous exposure to B. pseudomallei. After infection, the number of IgG reactive proteins showed a marked increase as the disease progressed. Early stage infection (day 7) showed immune reaction to chaperone proteins (GroEL, EF-Tu, and DnaK). These three proteins were detected in all serum samples after infection, with GroEL immunogenically dominant. Seven common reactive antigens were selected for further analysis using ELISA. The heat shock protein GroEL1 elicited the strongest goat antibody immune response compared to the other six antigens. Most of the six antigens showed the peak IgM reactivity at day 14, whereas the IgG reactivity increased further as the disease progressed. An overall MSHR511 proteomic comparison between the goat model and human sera showed that many immune reactive proteins are common between humans and goats with melioidosis.

Changes in function but not oligomeric size are associated with αB-crystallin lysine substitution

αB-Crystallin, ubiquitously expressed in many tissues including the ocular lens, is a small heat shock protein that can prevent protein aggregation. A number of post-translation modifications are reported to modify αB-crystallin function. Recent studies have identified αB-crystallin lysine residues are modified by acetylation and ubiquitination. Therefore, we sought to determine the effects of lysine to alanine substitution on αB-crystallin functions including chaperone activity and modulation of actin polymerization. Analysis of the ten substitution mutants as recombinant proteins indicated all the proteins were soluble and formed oligomeric complexes similar to wildtype protein. Lysozyme aggregation induced by chemical treatment indicated that K82, K90, K121, K166 and K174/K175 were required for efficient chaperone activity. Thermal induction of γ-crystallin aggregation could be prevented by all αB-crystallin substitution mutants. These αB-crystallin mutants also were able to mediate wildtype levels of actin polymerization. Further analysis of two clones with either enhanced or reduced chaperone activity on individual client substrates or actin polymerization indicated both retained broad chaperone activity and anti-apoptotic activity. Collectively, these studies show the requirements for lysine residues in αB-crystallin function.

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αB-crystallin Lysine-to-alanine mutation yields soluble recombinant protein.

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αB-crystallin mutants form oligomeric complexes similar to wildtype.

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αB-crystallin mutants prevent thermal aggregation of γ-crystallin.

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αB-crystallin mutants have disperse activity in chemical aggregation assays.

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αB-crystallin mutants retain ability to modulate actin polymerization.

Sequence specific 1H, 13C and 15N resonance assignments of a cataract-related variant G57W of human γS-crystallin

γS-crystallin is a major structural component of the human eye lens, which maintains its stability over the lifetime of an organism with negligible turnover. The G57W mutant of human γS-crystallin (abbreviated hereafter as γS-G57W) is associated with dominant congenital cataracts. In order to provide a structural basis for the ability of γS-G57W causing cataract, we have cloned, overexpressed, isolated and purified the protein. The 2D [¹⁵N-¹H]-HSQC spectrum recorded with uniformly ¹³C/¹⁵N-labelled γS-G57W was highly dispersed indicating the protein to adopt an ordered conformation. In this paper, we report almost complete sequence-specific ¹H, ¹³C and ¹⁵N resonance assignments of γS-G57W using a suite of heteronuclear 3D NMR experiments.

Impact of Subunit Composition on the Uptake of α-Crystallin by Lens and Retina

Misfolded protein aggregation, including cataract, cause a significant amount of blindness worldwide. α-Crystallin is reported to bind misfolded proteins and prevent their aggregation. We hypothesize that supplementing retina and lens with α-crystallin may help to delay disease onset. The purpose of this study was to determine if αB-crystallin subunits containing a cell penetration peptide (gC-tagged αB-crystallin) facilitate the uptake of wild type αA-crystallin (WT-αA) in lens and retina. Recombinant human αB-crystallin was modified by the addition of a novel cell penetration peptide derived from the gC gene product of herpes simplex virus (gC-αB). Recombinant gC-αB and wild-type αA-crystallin (WT-αA) were purified from E. coli over-expression cultures. After Alexa-labeling of WT-αA, these proteins were mixed at ratios of 1:2, 1:5 and 1:10, respectively, and incubated at 37°C for 4 hours to allow for subunit exchange. Mixed oligomers were subsequently incubated with tissue culture cells or mouse organ cultures. Similarly, crystallin mixtures were injected into the vitreous of rat eyes. At various times after exposure, tissues were harvested and analyzed for protein uptake by confocal microscopy or flow cytometry. Chaperone-like activity assays were performed on α-crystallins ratios showing optimal uptake using chemically-induced or heat induced substrate aggregation assays. As determined by flow cytometry, a ratio of 1:5 for gC-αB to WT-αA was found to be optimal for uptake into retinal pigmented epithelial cells (ARPE-19). Chaperone-like activity assays demonstrated that hetero-oligomeric complex of gC-αB to WT-αA (in 1:5 ratio) retained protein aggregation protection. We observed a significant increase in protein uptake when optimized (gC-αB to WT-αA (1:5 ratio)) hetero-oligomers were used in mouse lens and retinal organ cultures. Increased levels of α-crystallin were found in lens and retina following intravitreal injection of homo- and hetero-oligomers in rats.

Structure and function of α-crystallins: Traversing from in vitro to in vivo

BACKGROUND

The two α-crystallins (αA- and αB-crystallin) are major components of our eye lenses. Their key function there is to preserve lens transparency which is a challenging task as the protein turnover in the lens is low necessitating the stability and longevity of the constituent proteins. α-Crystallins are members of the small heat shock protein family. αB-crystallin is also expressed in other cell types.

SCOPE OF THE REVIEW

The review summarizes the current concepts on the polydisperse structure of the α-crystallin oligomer and its chaperone function with a focus on the inherent complexity and highlighting gaps between in vitro and in vivo studies.

MAJOR CONCLUSIONS

Both α-crystallins protect proteins from irreversible aggregation in a promiscuous manner. In maintaining eye lens transparency, they reduce the formation of light scattering particles and balance the interactions between lens crystallins. Important for these functions is their structural dynamics and heterogeneity as well as the regulation of these processes which we are beginning to understand. However, currently, it still remains elusive to which extent the in vitro observed properties of α-crystallins reflect the highly crowded situation in the lens.

GENERAL SIGNIFICANCE

Since α-crystallins play an important role in preventing cataract in the eye lens and in the development of diverse diseases, understanding their mechanism and substrate spectra is of importance. To bridge the gap between the concepts established in vitro and the in vivo function of α-crystallins, the joining of forces between different scientific disciplines and the combination of diverse techniques in hybrid approaches are necessary. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Electrostatic origin of in vitro aggregation of human γ-crystallin

The proteins α-, β-, and γ-crystallins are the major components of the lens in the human eye. Using dynamic light scattering method, we have performed in vitro investigations of protein-protein interactions in dilute solutions of human γ-crystallin and α-crystallin. We find that γ-crystallin spontaneously aggregates into finite-sized clusters in phosphate buffer solutions. There are two distinct populations of unaggregated and aggregated γ-crystallins in these solutions. On the other hand, α-crystallin molecules are not aggregated into large clusters in solutions of α-crystallin alone. When α-crystallin and γ-crystallin are mixed in phosphate buffer solutions, we demonstrate that the clusters of γ-crystallin are prevented. By further investigating the roles of temperature, protein concentration, pH, salt concentration, and a reducing agent, we show that the aggregation of γ-crystallin under our in vitro conditions arises from non-covalent electrostatic interactions. In addition, we show that aggregation of γ-crystallin occurs under the dilute in vitro conditions even in the absence of oxidizing agents that can induce disulfide cross-links, long considered to be responsible for human cataracts. Aggregation of γ-crystallin when maintained under reducing conditions suggests that oxidation does not contribute to the aggregation in dilute solutions.

Addition of αA-crystallin sequence 164-173 to a mini-chaperone DFVIFLDVKHFSPEDLT alters the conformation but not the chaperone-like activity

It has been shown that αA-mini-chaperone, a peptide representing the chaperone binding site in αA-crystallin, prevents destabilized protein aggregation. αA-Mini-chaperone has been shown to form amyloid fibrils. This study was undertaken to improve the stability of αA-mini-chaperone while preserving its anti-aggregation activity by fusing the flexible and solvent-exposed C-terminal 164-173 region of αA-crystallin to the mini-chaperone sequence DFVIFLDVKHFSPEDLT. The resulting chimeric chaperone peptide, DFVIFLDVKHFSPEDLTEEKPTSAPSS (designated CP1), was characterized. Circular dichroism studies showed that unlike αA-mini-chaperone with its β-sheet structure, the CP1 peptide exhibited a random structure. Transmission electron microscopy (TEM) examination of the CP1 peptide incubated in a shaker at 37 °C for 72 h did not reveal amyloid fibrils, whereas αA-mini-chaperone showed distinct fibrils. Consistent with TEM observation, the thioflavin T binding assay showed an increased level of dye binding in the mini-chaperone incubated at 37 °C and subjected to shaking but not of the CP1 peptide incubated under similar conditions. The chaperone activity of the CP1 peptide was comparable to that of αA-mini-chaperone against denaturing alcohol dehydrogenase, citrate synthase, and α-lactalbumin. Transduction of both peptide chaperones to COS-7 cells showed no cytotoxic effects. The antioxidation assay involving the H2O2 treatment of COS-7 cells revealed that αA-mini-chaperone and the CP1 peptide have comparable cytoprotective properties against H2O2-induced oxidative damage in COS-7 cells. This study therefore shows that the addition of C-terminal sequence 164-173 of αA-crystallin to αA-mini-chaperone influences the conformation of αA-mini-chaperone without affecting its chaperone function or cytoprotective activity.

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

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

Crystallins are regulated biomarkers for monitoring topical therapy of glaucomatous optic neuropathy

Optic nerve atrophy caused by abnormal intraocular pressure (IOP) remains the most common cause of irreversible loss of vision worldwide. The aim of this study was to determine whether topically applied IOP-lowering eye drugs affect retinal ganglion cells (RGCs) and retinal metabolism in a rat model of optic neuropathy. IOP was elevated through cauterization of episcleral veins, and then lowered either by the daily topical application of timolol, timolol/travoprost, timolol/dorzolamide, or timolol/brimonidine, or surgically with sectorial iridectomy. RGCs were retrogradely labeled 4 days prior to enucleation, and counted. Two-dimensional polyacrylamide gel electrophoresis (2D-PAGE), matrix-assisted laser desorption ionization mass spectrometry, Western blotting, and immunohistochemistry allowed the identification of IOP-dependent proteomic changes. Genomic changes were scrutinized using microarrays and qRT-PCR. The significant increase in IOP induced by episcleral vein cauterization that persisted until 8 weeks of follow-up in control animals (p<0.05) was effectively lowered by the eye drops (p<0.05). As anticipated, the number of RGCs decreased significantly following 8 weeks of elevated IOP (p<0.05), while treatment with combination compounds markedly improved RGC survival (p<0.05). 2D-PAGE and Western blot analyses revealed an IOP-dependent expression of crystallin cry-βb2. Microarray and qRT-PCR analyses verified the results at the mRNA level. IHC demonstrated that crystallins were expressed mainly in the ganglion cell layer. The data suggest that IOP and either topically applied antiglaucomatous drugs influence crystallin expression within the retina. Neuronal crystallins are thus suitable biomarkers for monitoring the progression of neuropathy and evaluating any neuroprotective effects.

Cell penetration peptides for enhanced entry of αB-crystallin into lens cells

PURPOSE

The prevalence of cataract increases with age. Conversely, the abundance of native α-crystallin diminishes with age and cataract development. We hypothesize replenishing lens α-crystallin may delay or prevent cataract. Herein we investigated the ability of cell penetration peptides (CPP) to enhance entry of α-crystallins into lens-derived cells.

METHODS

Recombinant αB-crystallins were modified by the addition of CPPs. Candidate CPP were designed with reference to the HSV-1 glycoprotein C gene (gC) or the HIV-1 TAT peptide. αB-crystallins produced by fusing gC or TAT were over-expressed in E. coli. Purified proteins were subjected to size exclusion chromatography (SEC) to characterize oligomeric complexes (OC). Chaperone-like activity (CLA) was evaluated by measuring the ability of α-crystallins to suppress chemically-induced protein aggregation. To evaluate protein uptake, labeled α-crystallins were incubated with HLE B3 cells and monitored by fluorescence microscopy for 48 hours.

RESULTS

We examined the effects of the addition of CPP on the structure, CLA, and cell transduction properties of αB-crystallins. C-terminal CPP fused crystallins had poor solubility. In contrast, N-terminal tagged αB-crystallins were soluble. These modified αB-crystallins formed OC that were larger than wild-type based on SEC. Wild-type and gC tagged αB-crystallin displayed robust CLA. Subunit exchange was observed when gC-fused αB-crystallin was mixed with αA. In contrast to wild-type, modified α-crystallins accumulated in HLE B3 cells.

CONCLUSIONS

Addition of CPP improves the uptake of αB-crystallins into HLE B3 cells. No undesirable changes to the chaperone-like abilities of α-crystallins were observed in αB-crystallin modified by the addition of the gC-derived CPP.

Mutations in human αA-crystallin/sHSP affect subunit exchange interaction with αB-crystallin

Background

Mutation in αA-crystallin contributes to the development of congenital cataract in humans. Heterooligomerization of αA-crystallin and αB-crystallin is essential for maintaining transparency in the eye lens. The effect of congenital cataract causing mutants of αA-crystallin on subunit exchange and interaction with αB-crystallin is unknown. In the present study, interaction of the mutants of αA-crystallin with αB-crystallin was studied both in vitro and in situ by the fluorescence resonance energy transfer (FRET) technique.

Methodology/Principal Findings

In vitro FRET technique was used to demonstrate the rates of subunit exchange of αB-wt with the following αA-crystallin mutants: R12C, R21L, R21W, R49C, R54C, and R116C. The subunit exchange rates (k values) of R21W and R116C with αB-wt decreased drastically as compared to αA-wt interacting with αB-wt. Moderately decreased k values were seen with R12C, R49C and R54C while R21L showed nearly normal k value. The interaction of αA- mutants with αB-wt was also assessed by in situ FRET. YFP-tagged αA mutants were co-expressed with CFP-tagged αB-wt in HeLa cells and the spectral signals were captured with a confocal microscope before and after acceptor laser photobleaching. The interaction of R21W and R116C with αB-wt was decreased nearly 50% as compared to αA-wt while the rest of the mutants showed slightly decreased interaction. Thus, there is good agreement between the in vitro and in situ FRET data.

Conclusions/Significance

Structural changes occurring in these mutants, as reported earlier, could be the underlying cause for the decreased interaction with αB may contribute to development of congenital cataract.

Identification and characterization of a copper-binding site in αA-crystallin

Previous studies have shown that both αA- and αB-crystallins bind Cu2+, suppress the formation of Cu2+-mediated active oxygen species, and protect ascorbic acid from oxidation by Cu2+. αA- and αB-crystallins are small heat shock proteins with molecular chaperone activity. In this study we show that the mini-αA-crystallin, a peptide consisting of residues 71-88 of αA-crystallin, prevents copper-induced oxidation of ascorbic acid. Evaluation of binding of copper to mini-αA-crystallin showed that each molecule of mini-αA-crystallin binds one copper molecule. Isothermal titration calorimetry and nanospray mass spectrometry revealed dissociation constants of 10.72 and 9.9 μM, respectively. 1,1'-Bis(4-anilino)naphthalene-5,5'-disulfonic acid interaction with mini-αA-crystallin was reduced after binding of Cu2+, suggesting that the same amino acids interact with these two ligands. Circular dichroism spectrometry showed that copper binding to mini-αA-crystallin peptide affects its secondary structure. Substitution of the His residue in mini-αA-crystallin with Ala abolished the redox-suppression activity of the peptide. During the Cu2+-induced ascorbic acid oxidation assay, a deletion mutant, αAΔ70-77, showed about 75% loss of ascorbic acid protection compared to the wild-type αA-crystallin. This difference indicates that the 70-77 region is the primary Cu2+-binding site(s) in human native full-size αA-crystallin. The role of the chaperone site in Cu2+ binding in native αA-crystallin was confirmed by the significant loss of chaperone activity by the peptide after Cu2+ binding.

Increase in surface hydrophobicity of the cataract-associated P23T mutant of human gammaD-crystallin is responsible for its dramatically lower, retrograde solubility

The cataract-associated Pro23 to Thr (P23T) mutation in human gammaD-crystallin (HGD) has a variety of phenotypes and is geographically widespread. Therefore, there is considerable interest in understanding the molecular basis of cataract formation due to this mutation. We showed earlier [Pande, A., et al. (2005) Biochemistry 44, 2491-2500] that the probable basis of opacity in this case is the severely compromised, retrograde solubility and aggregation of P23T relative to HGD. The dramatic solubility change occurs even as the structure of the mutant protein remains essentially unchanged in vitro. We proposed that the retrograde solubility and aggregation of P23T were mediated by net hydrophobic, protein-protein interactions. On the basis of these initial findings for P23T and related mutants, and the subsequent finding that they show atypical phase behavior [McManus, J. J., et al. (2007) Proc. Natl. Acad. Sci. U.S.A. 104, 16856-16861], we concluded that the protein clusters formed in solutions of the mutant proteins were held together by net hydrophobic, anisotropic interactions. Here we show, using chemical probes, that the surface hydrophobicities of these mutants are inversely related to their solubility. Furthermore, by probing the isolated N-terminal domains of HGD and P23T directly, we find that the increase in the surface hydrophobicity of P23T is localized in the N-terminal domain. Modeling studies suggest the presence of sticky patches on the surface of the N-terminal domain that could be engaged in the formation of protein clusters via hydrophobic protein-protein interactions. This work thus provides direct evidence of the dominant role played by net hydrophobic and anisotropic protein-protein interactions in the aggregation of P23T.

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.

αA-Crystallin associates with α6 integrin receptor complexes and regulates cellular signaling

α-Crystallins are small heat-shock proteins important to lens transparency that provide the lens with its refractive properties. In their role as molecular chaperones, these crystallins also prevent protein aggregation, affect cytoskeletal remodeling, enhance resistance to cell stress, and provide lens cells with protection against apoptosis. While many of the functions assigned to αA-crystallin are attributable to its presence in the cytoplasm of lens cells, αA-crystallin also has been detected at the lens plasma membrane. However, how αA-crystallin becomes linked to the plasma membrane or what its functions are at this site has remained unknown. In this study, we examined the mechanisms by which αA-crystallin becomes associated with the lens membrane, focusing specifically on its interaction with membrane receptors, and the differentiation-specificity of these interactions. We also determined how the long-term absence of αA-crystallin alters receptor-linked signaling pathways. αA-crystallin association with membrane receptors was determined by co-immunoprecipitation analysis; its membrane localization was examined by confocal imaging; and the effect of αA-crystallin loss-of-function on the activation state of signaling molecules in pathways linked to membrane receptors was determined by immunoblot analysis. The results show that, in lens epithelial cells, plasma membrane αA-crystallin was primarily localized to apicolateral borders, reflecting the association of αA-crystallin with E-cadherin complexes. These studies also provide the first evidence that αA-crystallin maintained its association with the plasma membrane in lens cortical fiber cells, where it was localized to lateral interfaces, and further show that this association was mediated, in part, by αA-crystallin interaction with α6 integrin receptor complexes. We report that the absence of αA-crystallin led to constitutive activation of the stress kinases p38 and JNK, classical inducers of apoptotic cell death, and the loss of the phospho-Bad pro-survival signal, effects that were greatest in differentiating lens fiber cells. Concurrent with this, activation of FAK and ERK kinases was increased, demonstrating that these receptor-linked pathways also were dysregulated in the absence of αA-crystallin. These data link αA-crystallin plasma membrane association to its differentiation-state-specific interaction with E-cadherin and α6 integrin receptor complexes. The changes in cell signaling in αA-crystallin-null lenses suggest that dysregulation of receptor-linked cell-signaling pathways that accompany the failure of αA-crystallin to associate with membrane receptors may be responsible for the induction of apoptosis. The observed changes in lens cell signaling likely reflect long-term functional adaptations to the absence of the αA-crystallin chaperone/small heat-shock protein.

Partially folded aggregation intermediates of human gammaD-, gammaC-, and gammaS-crystallin are recognized and bound by human alphaB-crystallin chaperone

Human gamma-crystallins are long-lived, unusually stable proteins of the eye lens exhibiting duplicated, double Greek key domains. The lens also contains high concentrations of the small heat shock chaperone alpha-crystallin, which suppresses aggregation of model substrates in vitro. Mature-onset cataract is believed to represent an aggregated state of partially unfolded and covalently damaged crystallins. Nonetheless, the lack of cell or tissue culture for anucleate lens fibers and the insoluble state of cataract proteins have made it difficult to identify the conformation of the human gamma-crystallin substrate species recognized by human alpha-crystallin. The three major human lens monomeric gamma-crystallins, gammaD, gammaC, and gammaS, all refold in vitro in the absence of chaperones, on dilution from denaturant into buffer. However, off-pathway aggregation of the partially folded intermediates competes with productive refolding. Incubation with human alphaB-crystallin chaperone during refolding suppressed the aggregation pathways of the three human gamma-crystallin proteins. The chaperone did not dissociate or refold the aggregated chains under these conditions. The alphaB-crystallin oligomers formed long-lived stable complexes with their gammaD-crystallin substrates. Using alpha-crystallin chaperone variants lacking tryptophans, we obtained fluorescence spectra of the chaperone-substrate complex. Binding of substrate gamma-crystallins with two or three of the four buried tryptophans replaced by phenylalanines showed that the bound substrate remained in a partially folded state with neither domain native-like. These in vitro results provide support for protein unfolding/protein aggregation models for cataract, with alpha-crystallin suppressing aggregation of damaged or unfolded proteins through early adulthood but becoming saturated with advancing age.

Expression and purification of his-tagged recombinant mouse zeta-crystallin

Zeta-crystallin is an NADPH-binding protein consisting of four identical 35kD subunits. The protein possesses quinone oxidoreductase activity, and is present in large amounts in the lenses of camelids, certain hystricomorphic rodents, and the Japanese tree frog, and in lower catalytic amounts in certain tissues of various species. In this study, recombinant methods were used to produce substantial quantities of his-tagged recombinant mouse zeta-crystallin, which was then purified to homogeneity. The yield of pure recombinant mouse zeta-crystallin was five times that obtained previously for purification of recombinant guinea pig zeta-crystallin. The quinone oxidoreductase activity of purified his-tagged recombinant mouse zeta-crystallin was comparable to that of purified native guinea pig lens zeta-crystallin, and to that previously reported for recombinant guinea pig zeta-crystallin. The method permits production of substantial amounts of recombinant zeta-crystallin for conducting studies on the biological role of this interesting protein, which exists in such high concentration in the lenses of certain species.

The cataract-associated R14C mutant of human gamma D-crystallin shows a variety of intermolecular disulfide cross-links: a Raman spectroscopic study

The Arg14 to Cys (R14C) mutation in the human gammaD-crystallin (HGD) gene has been associated with a juvenile-onset hereditary cataract. We showed previously [Pande, A., et al. (2000) Proc. Natl. Acad. Sci. U.S.A. 97, 1993-1998] that rapid oxidation of Cys14 in the mutant leads to the formation of intermolecular, disulfide-cross-linked aggregates at physiological pH. Here we present a Raman spectroscopic analysis of R14C and HGD and show that R14C forms such aggregates even at pH 4.5. The lower pH enabled us to monitor the evolution of a variety of disulfide cross-links with distinct conformations around the CC-SS-CC dihedral angles. At least three cysteine residues are involved, forming protein-protein cross-links through disulfide-exchange reactions. From the pattern of the S-S and Trp Raman bands, we infer that Cys32 is likely to be involved in the cross-linking. The data suggest that protein precipitation in the mutant may not be the direct result of disulfide cross-linking, although such cross-linking is the initiating event. Thus, our Raman data not only enhance the understanding of the reactivity of Cys14 in the R14C mutant and the mechanism of opacity, but also shed light on the mechanism of oxidative degradation during long-term storage of thiol-containing pharmaceuticals.

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.

A novel mutation (F71L) in alphaA-crystallin with defective chaperone-like function associated with age-related cataract

Age-related cataract (ARC) is a multifactorial disease and the leading cause of blindness worldwide. Genetic predisposition in association with other etiological factors may contribute to ARC. However, gene mutation studies on ARC are scanty. In the present work, we identified a genetic variation (F71L) in the exon-2 of CRYAA (alphaA-crystallin) gene in three unrelated female sporadic cases among 711 ARC patients but not in 265 normal non-cataractous controls by SSCP and RFLP analysis. By comparing human recombinant wild-type and F71L-alphaA-crystallin, we characterized the functional significance of this missense mutation. Chromatography, fluorescence and far- and near-UV CD studies indicated that F71L missense mutation did not significantly affect the apparent molecular mass, secondary and tertiary structures and hydrophobicity of alphaA-crystallin. While the mutant alphaA-crystallin displayed significant (35-90%) loss of chaperone-like activity (CLA) in thermal aggregation of carbonic anhydrase, betaL- and gamma-crystallins, it showed moderate (10-50%) loss in CLA in DTT-induced aggregation of insulin and lysozyme. This is the first report of an alphaA-F71L mutation being associated with ARC and suggests that ARC in individuals carrying this mutation (F71L) might be due to the overall loss of in vivo chaperone activity due to interaction with other environmental factors.

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

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

Lens aging: effects of crystallins

The primary function of the eye lens is to focus light on the retina. The major proteins in the lens--alpha, beta, and gamma-crystallins--are constantly subjected to age-related changes such as oxidation, deamidation, truncation, glycation, and methylation. Such age-related modifications are cumulative and affect crystallin structure and function. With time, the modified crystallins aggregate, causing the lens to increasingly scatter light on the retina instead of focusing light on it and causing the lens to lose its transparency gradually and become opaque. Age-related lens opacity, or cataract, is the major cause of blindness worldwide. We review deamidation, and glycation that occur in the lenses during aging keeping in mind the structural and functional changes that these modifications bring about in the proteins. In addition, we review proteolysis and discuss recent observations on how crystallin fragments generated in vivo, through their anti-chaperone activity may cause crystallin aggregation in aging lenses. We also review hyperbaric oxygen treatment induced guinea pig and 'humanized' ascorbate transporting mouse models as suitable options for studies on age-related changes in lens proteins.

Age-related changes in the spatial distribution of human lens alpha-crystallin products by MALDI imaging mass spectrometry

PURPOSE

To develop a protocol for MALDI (matrix-assisted laser desorption ionization) imaging mass spectrometry for mapping the distributions of alpha-crystallin and its modified forms in human lens tissue as a function of lens age and cataract.

METHODS

Frozen human lenses were cryosectioned equatorially and axially into 20-mum-thick sections, and the sections were mounted onto conductive glass slides by methanol soft-landing. An ethanol washing procedure facilitated uniform matrix crystal formation by a two-step matrix deposition procedure to produce high-quality mass spectral data. Molecular images of modified and unmodified alpha-crystallin subunits were obtained from mass spectral data acquired in 100-mum steps across normal and cataractous lens sections. Proteins extracted from the lens sections were digested with endoproteinase Glu-C and subjected to mass spectrometric analysis for identification of modifications.

RESULTS

Intact alpha-crystallin signals were detected primarily in the outer cortical fiber cells in lenses up to 29 years of age. Multiple truncation products were observed for alpha-crystallin that increased in abundance, both with distance into the lens and with lens age. Phosphorylated alphaB-crystallin forms were most abundant in the cortical region of older lenses. In axial sections, no significant anterior-posterior pole variation was observed. A previously unreported alphaA-crystallin mutation was detected in an age-matched cataractous human lens.

CONCLUSIONS

A method has been developed to spatially map the age-related changes of human lens alpha-crystallin by MALDI imaging mass spectrometry including a novel L52F alphaA-crystallin mutation in a cataractous lens. Application of this spatially resolved proteomic technique to lens biology enhances the understanding of alpha-crystallin protein processing in aging and diseased human lenses.

Glutamic acid residues in the C-terminal extension of small heat shock protein 25 are critical for structural and functional integrity

Small heat shock proteins (sHsps) are intracellular molecular chaperones that prevent the aggregation and precipitation of partially folded and destabilized proteins. sHsps comprise an evolutionarily conserved region of 80-100 amino acids, denoted the alpha-crystallin domain, which is flanked by regions of variable sequence and length: the N-terminal domain and the C-terminal extension. Although the two domains are known to be involved in the organization of the quaternary structure of sHsps and interaction with their target proteins, the role of the C-terminal extension is enigmatic. Despite the lack of sequence similarity, the C-terminal extension of mammalian sHsps is typically a short, polar segment which is unstructured and highly flexible and protrudes from the oligomeric structure. Both the polarity and flexibility of the C-terminal extension are important for the maintenance of sHsp solubility and for complexation with its target protein. In this study, mutants of murine Hsp25 were prepared in which the glutamic acid residues in the C-terminal extension at positions 190, 199 and 204 were each replaced with alanine. The mutants were found to be structurally altered and functionally impaired. Although there were no significant differences in the environment of tryptophan residues in the N-terminal domain or in the overall secondary structure, an increase in exposed hydrophobicity was observed for the mutants compared with wild-type Hsp25. The average molecular masses of the E199A and E204A mutants were comparable with that of the wild-type protein, whereas the E190A mutant was marginally smaller. All mutants displayed markedly reduced thermostability and chaperone activity compared with the wild-type. It is concluded that each of the glutamic acid residues in the C-terminal extension is important for Hsp25 to act as an effective molecular chaperone.

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

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

Mechanism of insolubilization by a single-point mutation in alphaA-crystallin linked with hereditary human cataracts

AlphaA-crystallin is a small heat shock protein that functions as a molecular chaperone and a lens structural protein. The R49C single-point mutation in alphaA-crystallin causes hereditary human cataracts. We have previously investigated the in vivo properties of this mutant in a gene knock-in mouse model. Remarkably, homozygous mice carrying the alphaA-R49C mutant exhibit nearly complete lens opacity concurrent with small lenses and small eyes. Here we have investigated the 90 degrees light scattering, viscosity, refractive index, and bis-ANS fluorescence of lens proteins isolated from the alphaA-R49C mouse lenses and found that the concentration of total water-soluble proteins showed a pronounced decrease in alphaA-R49C homozygous lenses. Light scattering measurements on proteins separated by gel permeation chromatography showed a small amount of high-molecular mass aggregated material in the void volume which still remains soluble in alphaA-R49C homozygous lens homogenates. An increased level of binding of beta- and gamma-crystallin to the alpha-crystallin fraction was observed in alphaA-R49C heterozygous and homozygous lenses but not in wild-type lenses. Quantitative analysis with the hydrophobic fluorescence probe bis-ANS showed a pronounced increase in fluorescence yield upon binding to alpha-crystallin from mutant as compared with the wild-type lenses. These results suggest that the decrease in the solubility of the alphaA-R49C mutant protein was due to an increase in its hydrophobicity and supra-aggregation of alphaA-crystallin that leads to cataract formation. Our study further shows that analysis of mutant proteins from the mouse model is an effective way to understand the mechanism of protein insolubilization in hereditary cataracts.

Truncation of alphaB-crystallin by the myopathy-causing Q151X mutation significantly destabilizes the protein leading to aggregate formation in transfected cells

Here we investigate the effects of a myopathy-causing mutation in alphaB-crystallin, Q151X, upon its structure and function. This mutation removes the C-terminal domain of alphaB-crystallin, which is expected to compromise both its oligomerization and chaperone activity. We compared this to two other alphaB-crystallin mutants (450delA, 464delCT) and also to a series of C-terminal truncations (E164X, E165X, K174X, and A171X). We find that the effects of the Q151X mutation were not always as predicted. Specifically, we have found that although the Q151X mutation decreased oligomerization of alphaB-crystallin and even increased some chaperone activities, it also significantly destabilized alphaB-crystallin causing it to self-aggregate. This conclusion was supported by our analyses of both the other disease-causing mutants and the series of C-terminal truncation constructs of alphaB-crystallin. The 450delA and 464delCT mutants could only be refolded and assayed as a complex with wild type alphaB-crystallin, which was not the case for Q151X alphaB-crystallin. From these studies, we conclude that all three disease-causing mutations (450delA, 464delCT, and Q151X) in the C-terminal extension destabilize alphaB-crystallin and increase its tendency to self-aggregate. We propose that it is this, rather than a catastrophic loss of chaperone activity, which is a major factor in the development of the reported diseases for the three disease-causing mutations studied here. In support of this hypothesis, we show that Q151X alphaB-crystallin is found mainly in the insoluble fraction of cell extracts from transient transfected cells, due to the formation of cytoplasmic aggregates.

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

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

Site-directed mutations in the C-terminal extension of human alphaB-crystallin affect chaperone function and block amyloid fibril formation

Background

Alzheimer's, Parkinson's and Creutzfeldt-Jakob disease are associated with inappropriate protein deposition and ordered amyloid fibril assembly. Molecular chaperones, including αB-crystallin, play a role in the prevention of protein deposition.

Methodology/Principal Findings

A series of site-directed mutants of the human molecular chaperone, αB-crystallin, were constructed which focused on the flexible C-terminal extension of the protein. We investigated the structural role of this region as well as its role in the chaperone function of αB-crystallin under different types of protein aggregation, i.e. disordered amorphous aggregation and ordered amyloid fibril assembly. It was found that mutation of lysine and glutamic acid residues in the C-terminal extension of αB-crystallin resulted in proteins that had improved chaperone activity against amyloid fibril forming target proteins compared to the wild-type protein.

Conclusions/Significance

Together, our results highlight the important role of the C-terminal region of αB-crystallin in regulating its secondary, tertiary and quaternary structure and conferring thermostability to the protein. The capacity to genetically modify αB-crystallin for improved ability to block amyloid fibril formation provides a platform for the future use of such engineered molecules in treatment of diseases caused by amyloid fibril formation.

Altered phase diagram due to a single point mutation in human gammaD-crystallin

The P23T mutant of human gammaD-crystallin (HGD) is associated with cataract. We have previously investigated the solution properties of this mutant, as well as those of the closely related P23V and P23S mutants, and shown that although mutations at site 23 of HGD do not produce a significant structural change in the protein, they nevertheless profoundly alter the solubility of the protein. Remarkably, the solubility of the mutants decreases with increasing temperature, in sharp contrast to the behavior of the native protein. This inverted solubility corresponds to a strong increase in the binding energy with temperature. Here we have investigated the liquid-liquid coexistence curve and the diffusivity of the P23V mutant and find that these solution properties are unaffected by the mutation. This means that the chemical potentials in the solution phase are essentially unaltered. The apparent discrepancy between the interaction energies in the solution phase, as compared with the solid phase, is explicable in terms of highly anisotropic interprotein interactions, which are averaged out in the solution phase but are fully engaged in the solid phase.

C-Terminal truncation affects subunit exchange of human alphaA-crystallin with alphaB-crystallin

In human lenses, C-terminal cleavage of alphaA-crystallin at residues 172,168, and 162 have been reported. The effect of C-terminal truncation of alphaA-crystallin on subunit exchange and heterooligomer formation with alphaB-crystallin and homooligomer formation with native alphaA-crystallin is not known. We have conducted fluorescence resonance energy transfer studies which have shown that the rates of subunit exchange of alphaA(1-172 )and alphaA(1-168 )with alphaB-wt were two-fold lower than for alphaA-wt interacting with alphaB-wt. The subunit exchange rate between alphaA(1-162) and alphaB-wt was six-fold lower. These data suggest that cleavage of the C-terminal residues could significantly affect heterooligomerization. On the other hand, the subunit exchange rates between alphaA-wt and the truncated alphaA-crystallins were either unchanged or only slightly decreased, which suggest that homooligomerization may not be significantly influenced by C-terminal truncation. The main conclusion from this study is that cleavage of C-terminal residues of alphaA-crystallin including the nine residues of the flexible tail is expected to significantly affect the formation of heteroaggregates. Reconstitution experiments showed that the presence of an intact C-terminus is essential for the formation of fully integrated heteroaggregates with equal proportion of alphaA and alphaB subunits.

Characterisation of amyloid fibril formation by small heat-shock chaperone proteins human alphaA-, alphaB- and R120G alphaB-crystallins

AlphaB-Crystallin is a ubiquitous small heat-shock protein (sHsp) renowned for its chaperone ability to prevent target protein aggregation. It is stress-inducible and its up-regulation is associated with a number of disorders, including those linked to the deposition of misfolded proteins, such as Alzheimer's and Parkinson's diseases. We have characterised the formation of amyloid fibrils by human alphaB-crystallin in detail, and also that of alphaA-crystallin and the disease-related mutant R120G alphaB-crystallin. We find that the last 12 amino acid residues of the C-terminal region of alphaB-crystallin are predicted from their physico-chemical properties to have a very low propensity to aggregate. (1)H NMR spectroscopy reveals that this hydrophilic C-terminal region is flexible both in its solution state and in amyloid fibrils, where it protrudes from the fibrillar core. We demonstrate, in addition, that the equilibrium between different protofilament assemblies can be manipulated and controlled in vitro to select for particular alphaB-crystallin amyloid morphologies. Overall, this study suggests that there could be a fine balance in vivo between the native functional sHsp state and the formation of amyloid fibrils.

Crystallins in the eye: Function and pathology

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

Cleavage of the C-Terminal Serine of Human αA-Crystallin Produces αA1-172 with Increased Chaperone Activity and Oligomeric Size

This study aimed to study the oligomeric size, structure, hydrodynamic properties, and chaperone function of the C-terminally truncated human alphaA-crystallin mutants with special emphasis on alphaA1-172 which is the cleavage product of the Ser172-Ser173 bond, unique to human lenses and constituting a major part of alphaA-crystallin. Various truncated forms of human alphaA-crystallins were prepared by site-directed mutagenesis. The proteins were expressed in Escherichia coli BL21(DE3) pLysS cells and purified by size exclusion column chromatography. Molecular masses and the other hydrodynamic properties were determined by dynamic light scattering measurements. The secondary and tertiary structural changes were assessed by far- and near-UV CD spectra measurements, respectively. Chaperone activity was determined by using ADH, insulin, and betaL-crystallin as the target proteins. alphaAlpha1-172 exhibited a significant increase in oligomeric size, i.e., 866 kDa by light scattering measurements as compared to 702 kDa in alphaA-wt. alphaAlpha1-172 and alphaA-wt had similar secondary structure, but the former exhibited slightly altered tertiary structure. The most interesting observation was that alphaAlpha1-172 behaved as a 28-46% better chaperone than alphaA-wt. The oligomeric size and structure of alphaAlpha1-168 were similar to those of alphaA-wt, while the chaperone activity was decreased by 12-23%. alphaAlpha1-162, on the other hand, had an oligomeric size of 400 kDa, a decrease in chaperone activity of 80-100%, and significantly altered secondary and tertiary structures. The data show that the overall chaperone function of alphaA-crystallin will be significantly improved by the presence of the major truncated product alphaAlpha1-172. This will be beneficial to the lens undergoing oxidative stress. Since alphaAlpha1-168 and alphaAlpha1-162 are present only in small amounts, their effect would be minimal.

Elongation of axons during regeneration involves retinal crystallin beta b2 (crybb2)

Adult retinal ganglion cells (RGCs) can regenerate their axons in vitro. Using proteomics, we discovered that the supernatants of cultured retinas contain isoforms of crystallins with crystallin beta b2 (crybb2) being clearly up-regulated in the regenerating retina. Immunohistochemistry revealed the expression of crybb within the retina, including in filopodial protrusions and axons of RGCs. Cloning and overexpression of crybb2 in RGCs and hippocampal neurons increased axonogenesis, which in turn could be blocked with antibodies against beta-crystallin. Conditioned medium from crybb2-transfected cell cultures also supported the growth of axons. Finally real time imaging of the uptake of green fluorescent protein-tagged crybb2 fusion protein showed that this protein becomes internalized. These data are the first to show that axonal regeneration is related to crybb2 movement. The results suggest that neuronal crystallins constitute a novel class of neurite-promoting factors that likely operate through an autocrine mechanism and that they could be used in neurodegenerative diseases.