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

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.

Structural and mechanistic implications of metal binding in the small heat-shock protein αB-crystallin

The human small heat-shock protein αB-crystallin (αB) rescues misfolded proteins from irreversible aggregation during cellular stress. Binding of Cu(II) was shown to modulate the oligomeric architecture and the chaperone activity of αB. However, the mechanistic basis of this stimulation is so far not understood. We provide here first structural insights into this Cu(II)-mediated modulation of chaperone function using NMR spectroscopy and other biophysical approaches. We show that the α-crystallin domain is the elementary Cu(II)-binding unit specifically coordinating one Cu(II) ion with picomolar binding affinity. Putative Cu(II) ligands are His(83), His(104), His(111), and Asp(109) at the dimer interface. These loop residues are conserved among different metazoans, but also for human αA-crystallin, HSP20, and HSP27. The involvement of Asp(109) has direct implications for dimer stability, because this residue forms a salt bridge with the disease-related Arg(120) of the neighboring monomer. Furthermore, we observe structural reorganization of strands β2-β3 triggered by Cu(II) binding. This N-terminal region is known to mediate both the intermolecular arrangement in αB oligomers and the binding of client proteins. In the presence of Cu(II), the size and the heterogeneity of αB multimers are increased. At the same time, Cu(II) increases the chaperone activity of αB toward the lens-specific protein β(L)-crystallin. We therefore suggest that Cu(II) binding unblocks potential client binding sites and alters quaternary dynamics of both the dimeric building block as well as the higher order assemblies of αB.

Multiple molecular architectures of the eye lens chaperone αB-crystallin elucidated by a triple hybrid approach

The molecular chaperone αB-crystallin, the major player in maintaining the transparency of the eye lens, prevents stress-damaged and aging lens proteins from aggregation. In nonlenticular cells, it is involved in various neurological diseases, diabetes, and cancer. Given its structural plasticity and dynamics, structure analysis of αB-crystallin presented hitherto a formidable challenge. Here we present a pseudoatomic model of a 24-meric αB-crystallin assembly obtained by a triple hybrid approach combining data from cryoelectron microscopy, NMR spectroscopy, and structural modeling. The model, confirmed by cross-linking and mass spectrometry, shows that the subunits interact within the oligomer in different, defined conformations. We further present the molecular architectures of additional well-defined αB-crystallin assemblies with larger or smaller numbers of subunits, provide the mechanism how "heterogeneity" is achieved by a small set of defined structural variations, and analyze the factors modulating the oligomer equilibrium of αB-crystallin and thus its chaperone activity.

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

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

Protection against protein aggregation by alpha-crystallin as a mechanism of preconditioning

Anesthetic preconditioning occurs when cells previously exposed to inhaled anesthetics are protected against subsequent injury. We hypothesize that inhaled anesthetics may cause slight protein misfolding that involves site-specific dehydration, stimulating cytoprotective mechanisms. Human neuroblastoma cells were exposed to ethanol (as the dehydration agent) followed by quantitative analysis of the expression of five heat shock genes: DNAJC5G, CRYAA, HSPB2, HSF4 and HSF2. There was an ethanol-induced upregulation of all genes except HSF4, similar to previous observations using isoflurane. CRYAA (the gene for alphaA-crystallin) exhibited a 23.19 and 17.15-fold increase at 24 and 48 h post ethanol exposure, respectively. Additionally, we exposed glyceraldehyde 3-phosphate dehydrogenase to ethanol, which altered oligomeric subspecies and caused protein aggregation in a concentration-dependent manner. Ethanol-mediated dehydration-induced protein aggregation was prevented by incubation with alpha-crystallin. These data indicate that ethanol mimics the effects of isoflurane presumably through a cellular preconditioning mechanism that involves dehydration-induced protein aggregation.

Structural and functional roles of deamidation of N146 and/or truncation of NH2- or COOH-termini in human αB-crystallin

PURPOSE

The purpose of the study was to determine the relative effects of deamidation and/or truncation on the structural and functional properties of αB-crystallin.

METHODS

Using wild-type (WT) αB-crystallin and the αB deamidated mutant (i.e., αB N146D), we generated NH(2)-terminal domain deleted (residues no. 1-66; αB-NT), deamidated plus NH(2)-terminal domain deleted (αB N146D-NT), COOH-terminal extension deleted (residues no. 151-175; αB-CT), and deamidated plus COOH-terminal extension deleted (αB N146D-CT) mutants. All of the proteins were purified and their structural and functional (chaperone activity with insulin as target protein) properties were determined and compared to WT αB-crystallin.

RESULTS

The desired deletions in the αB-crystallin mutants were confirmed by DNA sequencing and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometric analysis. The homomers of αB-CT and its deamidated form (αB N146D-CT) became water insoluble, whereas the αB N146D, αB-NT, and αB N146D-NT species remained water-soluble. CD spectroscopic studies revealed that the mutants with deletion of NH(2)- or COOH-termini or deamidation showed increased β-sheet and decreased α-helical contents with the exception of αB N146D-CT, which showed a substantial increase in α-helix and decrease in β-sheet content. Results of intrinsic Trp fluorescence suggested little change in Trp microenvironment of αB N146D relative to WT αB, but substantial alterations on deletion of COOH-terminal extension or a combination of this deletion plus deamidation. Hydrophobic binding studies using the hydrophobic probe 8-anilino-1-naphthalene sulfonate (ANS) showed that, relative to WT αB structure, the N146 deamidation, COOH-terminal extension deletion or a combination of this deamidation and deletion resulted in a relatively compact structure whereas the NH(2)-terminal domain deletion and a combination of this deletion plus deamidation resulted in a relaxed structure. All the αB mutants showed higher molecular mass ranging between 1.2×10(6) to 5.4×10(6) Da, relative to WT αB which had a molecular mass of 5.8×10(5) Da. Chaperone activity across all αB species decreased in the following order: WTαB > αB N146D-CT > αB N146D-NT > αB-NT > αB-CT > αB N146D. Specifically, substantial losses in chaperone activity (only 10% to 20% protection) were seen in αB N146D, αB-NT, and αB-CT. However, in the species with the combination of deamidation plus NH(2)- or COOH-terminal deletion, the percent protection was about 24% in αB N146D-NT and about 40% in αB N146D-CT.

CONCLUSIONS

Although all mutants formed oligomers even after deamidation, on deletion of either NH(2)-terminal domain or COOH-terminal extension or a combination of these deletions and deamidation, their structural properties were substantially altered. The results suggested that the NH(2)-terminal domain is relatively more important than the COOH-terminal extension for the chaperone function of αB. The non-deamidated N146 residue, NH(2)-terminal domain and COOH-terminal extension are also of critical importance to the maintenance of αB-crystallin chaperone activity.

Structural and functional properties of NH(2)-terminal domain, core domain, and COOH-terminal extension of αA- and αB-crystallins

PURPOSE

The purpose of the present study was to determine the biophysical and chaperone properties of the NH(2)-terminal domain, core domain and COOH-terminal extension of human αA- and αB-crystallins and correlate these properties to those of wild type (WT) αA- and αB-crystallins.

METHODS

WT αA- and αB-crystallins cloned into pET 100D TOPO vector, were used as templates to generate different constructs encoding specific regions (NH(2)-terminal domain [NTD], core domain [CD], and COOH-terminal extension, [CTE]). The specific regions amplified by PCR using plasmid DNA from WT αA and WT αB were: αA NTD (residues 1-63), αA CD (residues 64-142), αA CTE (residues 143-173), αB NTD (residues 1-66), αB CD (residues 67-146), and αB CTE (residues 147-175). Resultant blunt-end PCR products were ligated to a pET 100 Directional TOPO vector. DNA sequencing results confirmed the desired constructs. Positive clones were transformed into the BL21 Star (DE3) expression cell line. Protein expression and solubility were confirmed by SDS-PAGE and western blot analysis using a monoclonal antibody against a 6× His-tag epitope. Proteins were purified using Ni(2+)-affinity column chromatography, under native or denaturing conditions, and used for biophysical and chaperone function analyses.

RESULTS

A total of five constructs were successfully generated: αA NTD, αA CD, αB NTD, αB CD, and αB CTE. SDS-PAGE and western blot analyses showed that αA CD and αB CD were present in both the soluble and insoluble fractions, whereas mutant preparations with NTD alone became insoluble and the mutant with CTE alone became soluble. All purified constructs showed alterations in biophysical properties and chaperone function compared to WT α-crystallins. αA NTD and αB CTE exhibited the most notable changes in secondary structural content. Also, αA NTD and all αB-crystallin constructs showed altered surface hydrophobicity compared to their respective WT α-crystallins.

CONCLUSIONS

Although the individual α-crystallin regions (i.e., NH(2)-terminal domain, core domain, and COOH-terminal extension) exhibited varied biophysical properties, each region alone retained some level of chaperone function. The NH(2)-terminal domains of αA and αB each showed the maximum chaperone activity of the three regions with respect to their WT crystallins.

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.

Enhanced molecular chaperone activity of the small heat-shock protein alphaB-cystallin following covalent immobilization onto a solid-phase support

The well-characterized small heat-shock protein, alphaB-crystallin, acts as a molecular chaperone by interacting with unfolding proteins to prevent their aggregation and precipitation. Structural perturbation (e.g., partial unfolding) enhances the in vitro chaperone activity of alphaB-crystallin. Proteins often undergo structural perturbations at the surface of a synthetic material, which may alter their biological activity. This study investigated the activity of alphaB-crystallin when covalently bound to a support surface; alphaB-crystallin was immobilized onto a range of solid material surfaces, and its characteristics and chaperone activity were assessed. Immobilization was achieved via a plasma-deposited thin polymeric interlayer containing aldehyde surface groups and reductive amination, leading to the covalent binding of alphaB-crystallin lysine residues to the surface aldehyde groups via Schiff-base linkages. Immobilized alphaB-crystallin was characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and quartz crystal microgravimetry, which showed that 300 ng cm(-2) (dry mass) of oligomeric alphaB-crystallin was bound to the surface. Immobilized alphaB-crystallin exhibited a significant enhancement (up to 5000-fold, when compared with the equivalent activity of alphaB-crystallin in solution) of its chaperone activity against various proteins undergoing both amorphous and amyloid fibril forms of aggregation. The enhanced molecular chaperone activity of immobilized alphaB-crystallin has potential applications in preventing protein misfolding, including against amyloid disease processes, such as dialysis-related amyloidosis, and for biodiagnostic detection of misfolded proteins.

Enhancement of proteasomal function protects against cardiac proteinopathy and ischemia/reperfusion injury in mice

The ubiquitin-proteasome system degrades most intracellular proteins, including misfolded proteins. Proteasome functional insufficiency (PFI) has been observed in proteinopathies, such as desmin-related cardiomyopathy, and implicated in many common diseases, including dilated cardiomyopathy and ischemic heart disease. However, the pathogenic role of PFI has not been established. Here we created inducible Tg mice with cardiomyocyte-restricted overexpression of proteasome 28 subunit α (CR-PA28αOE) to investigate whether upregulation of the 11S proteasome enhances the proteolytic function of the proteasome in mice and, if so, whether the enhancement can rescue a bona fide proteinopathy and protect against ischemia/reperfusion (I/R) injury. We found that CR-PA28αOE did not alter the homeostasis of normal proteins and cardiac function, but did facilitate the degradation of a surrogate misfolded protein in the heart. By breeding mice with CR-PA28αOE with mice representing a well-established model of desmin-related cardiomyopathy, we demonstrated that CR-PA28αOE markedly reduced aberrant protein aggregation. Cardiac hypertrophy was decreased, and the lifespan of the animals was increased. Furthermore, PA28α knockdown promoted, whereas PA28α overexpression attenuated, accumulation of the mutant protein associated with desmin-related cardiomyopathy in cultured cardiomyocytes. Moreover, CR-PA28αOE limited infarct size and prevented postreperfusion cardiac dysfunction in mice with myocardial I/R injury. We therefore conclude that benign enhancement of cardiac proteasome proteolytic function can be achieved by CR-PA28αOE and that PFI plays a major pathogenic role in cardiac proteinopathy and myocardial I/R injury.

Desmin-related cardiomyopathy: an unfolding story

The intermediate filament protein desmin is an integral component of the cardiomyocyte and serves to maintain the overall structure and cytoskeletal organization within striated muscle cells. Desmin-related myopathy can be caused by mutations in desmin or associated proteins, which leads to intracellular accumulation of misfolded protein and production of soluble pre-amyloid oligomers, which leads to weakened skeletal and cardiac muscle. In this review, we examine the cellular phenotypes in relevant animal models of desmin-related cardiomyopathy. These models display characteristic sarcoplasmic protein aggregates. Aberrant protein aggregation leads to mitochondrial dysfunction, abnormal metabolism, and altered cardiomyocyte structure. These deficits to cardiomyocyte function may stem from impaired cellular proteolytic mechanisms. The data obtained from these models allow a more complete picture of the pathology in desmin-related cardiomyopathy to be described. Moreover, these studies highlight the importance of desmin in maintaining cardiomyocyte structure and illustrate how disrupting this network can be deleterious to the heart. We emphasize the similarities observed between desmin-related cardiomyopathy and other protein conformational disorders and speculate that therapies to treat this disease may be broadly applicable to diverse protein aggregation-based disorders.

NMR spectroscopy of 14-3-3ζ reveals a flexible C-terminal extension: differentiation of the chaperone and phosphoserine-binding activities of 14-3-3ζ

Intracellular 14-3-3 proteins bind to many proteins, via a specific phosphoserine motif, regulating diverse cellular tasks including cell signalling and disease progression. The 14-3-3ζ isoform is a molecular chaperone, preventing the stress-induced aggregation of target proteins in a manner comparable with that of the unrelated sHsps (small heat-shock proteins). 1H-NMR spectroscopy revealed the presence of a flexible and unstructured C-terminal extension, 12 amino acids in length, which protrudes from the domain core of 14-3-3ζ and is similar in structure and length to the C-terminal extension of mammalian sHsps. The extension stabilizes 14-3-3ζ, but has no direct role in chaperone action. Lys49 is an important functional residue within the ligand-binding groove of 14-3-3ζ with K49E 14-3-3ζ exhibiting markedly reduced binding to phosphorylated and non-phosphorylated ligands. The R18 peptide binds to the binding groove of 14-3-3ζ with high affinity and also reduces the interaction of 14-3-3ζ ligands. However, neither the K49E mutation nor the presence of the R18 peptide affected the chaperone activity of 14-3-3ζ, implying that the C-terminal extension and binding groove of 14-3-3ζ do not mediate interaction with target proteins during chaperone action. Other region(s) in 14-3-3ζ are most likely to be involved, i.e. the protein's chaperone and phosphoserine-binding activities are functionally and structurally separated.

αB-crystallin, a small heat shock protein, modulates NF-κB activity in a phosphorylation-dependent manner and protects muscle myoblasts from TNF-α induced cytotoxicity

αB-crystallin, a member of the small heat shock protein family, has been implicated in various biological functions including response to heat shock, differentiation and apoptosis, the mechanisms of which have not been well understood. Myoblasts, the precursor cells in muscle regeneration, when subjected to growth factor deprivation differentiate to form myotubes or undergo apoptosis. During differentiation, myoblasts express elevated levels of αB-crystallin as well as TNF-α but the connecting link between these proteins in cell signaling is not clearly understood. We have therefore investigated the role of αB-crystallin in TNF-α induced regulation of NF-κB. We demonstrate that in response to TNF-α treatment, αB-crystallin associates with IKKβ and activate its kinase activity, facilitating the degradation of phosphorylated I-kBα, a prime step in NF-κB activation. Reducing the level of αB-crystallin using the RNAi approach reduces the translocation of p65, further confirming the role of αB-crystallin in NF-κB activation. Our study shows that the ability of αB-crystallin to activate NF-κB depends on its phosphorylation status. The present study shows that αB-crystallin-dependent NF-κB activation protects myoblasts from TNF-α induced cytoxicity by enhancing the expression of the anti-apoptotic protein, Bcl 2. Thus, our study identifies yet another mechanism by which αB-crystallin exerts its anti-apoptotic activity.

A knock-in mouse model for the R120G mutation of αB-crystallin recapitulates human hereditary myopathy and cataracts

An autosomal dominant missense mutation in αB-crystallin (αB-R120G) causes cataracts and desmin-related myopathy, but the underlying mechanisms are unknown. Here, we report the development of an αB-R120G crystallin knock-in mouse model of these disorders. Knock-in αB-R120G mice were generated and analyzed with slit lamp imaging, gel permeation chromatography, immunofluorescence, immunoprecipitation, histology, and muscle strength assays. Wild-type, age-matched mice were used as controls for all studies. Both heterozygous and homozygous mutant mice developed myopathy. Moreover, homozygous mutant mice were significantly weaker than wild-type control littermates at 6 months of age. Cataract severity increased with age and mutant gene dosage. The total mass, precipitation, and interaction with the intermediate filament protein vimentin, as well as light scattering of αB-crystallin, also increased in mutant lenses. In skeletal muscle, αB-R120G co-aggregated with desmin, became detergent insoluble, and was ubiquitinated in heterozygous and homozygous mutant mice. These data suggest that the cataract and myopathy pathologies in αB-R120G knock-in mice share common mechanisms, including increased insolubility of αB-crystallin and co-aggregation of αB-crystallin with intermediate filament proteins. These knock-in αB-R120G mice are a valuable model of the developmental and molecular biological mechanisms that underlie the pathophysiology of human hereditary cataracts and myopathy.

BAG3 directly interacts with mutated alphaB-crystallin to suppress its aggregation and toxicity

A homozygous disruption or genetic mutation of the bag3 gene causes progressive myofibrillar myopathy in mouse and human skeletal and cardiac muscle disorder while mutations in the small heat shock protein αB-crystallin gene (CRYAB) are reported to be responsible for myofibrillar myopathy. Here, we demonstrate that BAG3 directly binds to wild-type αB-crystallin and the αB-crystallin mutant R120G, via the intermediate domain of BAG3. Peptides that inhibit this interaction in an in vitro binding assay indicate that two conserved Ile-Pro-Val regions of BAG3 are involved in the interaction with αB-crystallin, which is similar to results showing BAG3 binding to HspB8 and HspB6. BAG3 overexpression increased αB-crystallin R120G solubility and inhibited its intracellular aggregation in HEK293 cells. BAG3 suppressed cell death induced by αB-crystallin R120G overexpression in differentiating C2C12 mouse myoblast cells. Our findings indicate a novel function for BAG3 in inhibiting protein aggregation caused by the genetic mutation of CRYAB responsible for human myofibrillar myopathy.

Chaperone activity of α B-crystallin is responsible for its incorrect assignment as an autoantigen in multiple sclerosis

For 15 y, α B-crystallin (heat shock protein [Hsp] B5) has been labeled an autoantigen in multiple sclerosis (MS) based on humoral and cellular responses found in humans and animal models. However, there have been several scientific inconsistencies with this assignment, ranging from studies demonstrating small differences in anticrystallin responses between patients and healthy individuals to the inability of crystallin-specific T cells to induce symptoms of experimental allergic encephalomyelitis in animal models. Experiments in this article demonstrate that the putative anti-HspB5 Abs from 23 MS patients cross-react with 7 other members of the human small Hsp family and were equally present in normal plasma. Biolayer interferometry demonstrates that the binding was temperature dependent, and that the calculated K(a) increased as the concentration of the sHsp decreased. These two patterns are characteristic of multiple binding sites with varying affinities, the composition of which changes with temperature, supporting the hypothesis that HspB5 bound the Ab and not the reverse. HspB5 also precipitated Ig heavy and L chains from sera from patients with MS. These results establish that small Hsps bind Igs with high affinity and refute much of the serological data used to assign α B-crystallin as an autoantigen.

Behavioral defects in chaperone-deficient Alzheimer's disease model mice

Molecular chaperones protect cells from the deleterious effects of protein misfolding and aggregation. Neurotoxicity of amyloid-beta (Aβ) aggregates and their deposition in senile plaques are hallmarks of Alzheimer's disease (AD). We observed that the overall content of αB-crystallin, a small heat shock protein molecular chaperone, decreased in AD model mice in an age-dependent manner. We hypothesized that αB-crystallin protects cells against Aβ toxicity. To test this, we crossed αB-crystallin/HspB2 deficient (CRYAB⁻/⁻HSPB2⁻/⁻) mice with AD model transgenic mice expressing mutant human amyloid precursor protein. Transgenic and non-transgenic mice in chaperone-sufficient or deficient backgrounds were examined for representative behavioral paradigms for locomotion and memory network functions: (i) spatial orientation and locomotion was monitored by open field test; (ii) sequential organization and associative learning was monitored by fear conditioning; and (iii) evoked behavioral response was tested by hot plate method. Interestingly, αB-crystallin/HspB2 deficient transgenic mice were severely impaired in locomotion compared to each genetic model separately. Our results highlight a synergistic effect of combining chaperone deficiency in a transgenic mouse model for AD underscoring an important role for chaperones in protein misfolding diseases.

Crystal structures of Xanthomonas small heat shock protein provide a structural basis for an active molecular chaperone oligomer

Small heat shock proteins (sHsps) are ubiquitous low-molecular-weight chaperones that prevent protein aggregation under cellular stresses. sHsps contain a structurally conserved α-crystallin domain (ACD) of about 100 amino acid residues flanked by varied N- and C-terminal extensions and usually exist as oligomers. Oligomerization is important for the biological functions of most sHsps. However, the active oligomeric states of sHsps are not defined yet. We present here crystal structures (up to 1.65 Å resolution) of the sHspA from the plant pathogen Xanthomonas (XaHspA). XaHspA forms closed or open trimers of dimers (hexamers) in crystals but exists predominantly as 36mers in solution as estimated by size-exclusion chromatography. The XaHspA monomer structures mainly consist of α-crystallin domain with disordered N- and C-terminal extensions, indicating that the extensions are flexible and not essential for the formation of dimers and 36mers. Under reducing conditions where α-lactalbumin (LA) unfolds and aggregates, XaHspA 36mers formed complexes with one LA per XaHspA dimer. Based on XaHspA dimer-dimer interactions observed in crystals, we propose that XaHspA 36mers have four possible conformations, but only XaHspA 36merB, which is formed by open hexamers in 12mer-6mer-6mer-12mer with protruding dimers accessible for substrate (unfolding protein) binding, can bind to 18 reduced LA molecules. Together, our results unravel the structural basis of an active sHsp oligomer.

Crystal structure of R120G disease mutant of human αB-crystallin domain dimer shows closure of a groove

Small heat shock proteins form large cytosolic assemblies from an “α-crystallin domain” (ACD) flanked by sequence extensions. Mutation of a conserved arginine in the ACD of several human small heat shock protein family members causes many common inherited diseases of the lens and neuromuscular system. The mutation R120G in αB-crystallin causes myopathy, cardiomyopathy and cataract. We have solved the X-ray structure of the excised ACD dimer of human αB R120G close to physiological pH and compared it with several recently determined wild-type vertebrate ACD dimer structures. Wild-type excised ACD dimers have a deep groove at the interface floored by a flat extended “bottom sheet.” Solid-state NMR studies of large assemblies of full-length αB-crystallin have shown that the groove is blocked in the ACD dimer by curvature of the bottom sheet. The crystal structure of R120G ACD dimer also reveals a closed groove, but here the bottom sheet is flat. Loss of Arg120 results in rearrangement of an extensive array of charged interactions across this interface. His83 and Asp80 on movable arches on either side of the interface close the groove by forming two new salt bridges. The residues involved in this extended set of ionic interactions are conserved in Hsp27, Hsp20, αA- and αB-crystallin sequences. They are not conserved in Hsp22, where mutation of the equivalent of Arg120 causes neuropathy. We speculate that the αB R120G mutation disturbs oligomer dynamics, causing the growth of large soluble oligomers that are toxic to cells by blocking essential processes.

Myofibrillar myopathies

Myofibrillar myopathies represent a group of muscular dystrophies with a similar morphologic phenotype. They are characterized by a distinct pathologic pattern of myofibrillar dissolution associated with disintegration of the Z-disk, accumulation of myofibrillar degradation products, and ectopic expression of multiple proteins and sometimes congophilic material. The clinical features of myofibrillar myopathies are more variable. These include progressive muscle weakness, that often involves or begins in distal muscles but limb-girdle or scapuloperoneal distributions can also occur. Cardiomyopathy and peripheral neuropathy are frequent associated features. EMG of the affected muscles reveals myopathic motor unit potentials and abnormal irritability often with myotonic discharges. Rarely, neurogenic motor unit potentials or slow nerve conductions are present. The generic diagnosis of myofibrillar myopathies is based on muscle biopsy findings in frozen sections. To date, all myofibrillar myopathy mutations have been traced to Z-disk-associated proteins, namely, desmin, αB-crystallin, myotilin, ZASP, filamin C and Bag3. However, in the majority of the myofibrillar myopathy patients the disease gene awaits discovery.

Myofibrillar myopathies

Myofibrillar myopathies (MFMs) represent a group of muscular dystrophies with a similar morphological phenotype. The diagnosis is established by muscle biopsy. The MFMs are characterized by a distinct pathological pattern of myofibrillar dissolution associated with disintegration of the Z-disk, accumulation of myofibrillar degradation products, and ectopic expression of multiple proteins that include desmin, αB-crystallin, dystrophin, and sometimes congophilic material. The clinical features of MFMs are more variable. These include progressive muscle weakness that often involves or begins in distal muscles, but limb-girdle or scapuloperoneal distributions can also occur. Cardiomyopathy and peripheral neuropathy are frequent associated features. Electromyography of the affected muscles reveals myopathic motor unit potentials and abnormal irritability, often with myotonic discharges. Rarely, neurogenic motor unit potentials or slowing of nerve conduction velocities are present. To date, all MFM mutations have appeared in Z-disk-associated proteins: namely, desmin, αB-crystallin, myotilin, ZASP, filamin C, and Bag3. However, in the majority of patients with MFM, the disease gene awaits discovery.

Characterization of a mutant R11H αB-crystallin associated with human inherited cataract

αB-Crystallin plays an important part in cataract development. A novel mutation (R11H) was previously detected by our group. In the present study, we set out to investigate the possible molecular mechanism by which the R11H mutation causes cataract. We found that the mutant αB-crystallin exhibits folding defects, decreased surface hydrophobicity and enhanced chaperone-like activity compared with the wild-type αB-crystallin. The mutant protein shows nearly the same molecular mass and thermal stability as the wild-type form. Transfection studies revealed that the R11H mutant was remarkably similar to the wild-type protein in its subcellular distribution, but has an abnormal ability to induce cell apoptosis. These results suggest that the changes in hydrophobic exposure and the abnormal ability to induce programmed cell death of the mutant protein are likely to be responsible for the onset of cataract.

Selective degradation of aggregate-prone CryAB mutants by HSPB1 is mediated by ubiquitin-proteasome pathways

Disease-causing mutations of genes encoding small MW heat shock proteins (sHSPs) constitute a growing family of inherited myofibrillar disorders. In the present work, we found that three structurally-distinct CryAB mutants R120G, 450delA and 464delCT are mostly present in the detergent-insoluble fractions when overexpressed in H9c2 rat heart cells. We found that either over-expression or knockdown of HSPB1, a related sHSP, affects the solubility, stability, and degradation of aggregation-prone CryAB mutants. HSPB1 overexpression has negligible effects on the solubility and protein aggregates of either R120G and/or 450delA but increased the solubility and prevented formation of 464delCT aggregates. HSPB1 knockdown decreased solubility and increased protein aggregates of all CryAB mutants, indicating a key role for HSPB1 in clearance of CryAB mutants under basal conditions. We provide four lines of evidence that such selective clearance of R120G, 450delA and 464delCT mutants by HSPB1 is mediated by the ubiquitin-proteasome system (UPS). First, we found that treatment with the proteasome inhibitors increased the levels of all CryAB mutants. Second, R120G and 450delA overexpression corresponded to the accumulation of their specific ubiquitin conjugates in H9c2 cells. Third, HSPB1 knockdown directly increased the levels of all polyubiquitin conjugates. And fourth, the selective attenuation of 464delCT expression by HSPB1 over-expression was abrogated by the proteasome inhibition. We conclude that such selective interactions between CryAB mutants and HSPB1 overexpression might have important implications for the clinical manifestations and potential treatment.

The interaction of unfolding α-lactalbumin and malate dehydrogenase with the molecular chaperone αB-crystallin: a light and X-ray scattering investigation

PURPOSE

The molecular chaperone αB-crystallin is found in high concentrations in the lens and is present in all major body tissues. Its structure and the mechanism by which it protects its target protein from aggregating and precipitating are not known.

METHODS

Dynamic light scattering and X-ray solution scattering techniques were used to investigate structural features of the αB-crystallin oligomer when complexed with target proteins under mild stress conditions, i.e., reduction of α-lactalbumin at 37 °C and malate dehydrogenase when heated at 42 °C. In this investigation, the size, shape and particle distribution of the complexes were determined in real-time following the induction of stress.

RESULTS

Overall, it is observed that the mass distribution, hydrodynamic radius, and spherical shape of the αB-crystallin oligomer do not alter significantly when it complexes with its target protein.

CONCLUSIONS

The data are consistent with the target protein being located in the outer protein shell of the αB-crystallin oligomer where it is readily accessible for possible refolding via the action of other molecular chaperones.

Constitutively active calcineurin induces cardiac endoplasmic reticulum stress and protects against apoptosis that is mediated by alpha-crystallin-B

Cardiac-specific overexpression of a constitutively active form of calcineurin A (CNA) leads directly to cardiac hypertrophy in the CNA mouse model. Because cardiac hypertrophy is a prominent characteristic of many cardiomyopathies, we deduced that delineating the proteomic profile of ventricular tissue from this model might identify novel, widely applicable therapeutic targets. Proteomic analysis was carried out by subjecting fractionated cardiac samples from CNA mice and their WT littermates to gel-free liquid chromatography linked to shotgun tandem mass spectrometry. We identified 1,918 proteins with high confidence, of which 290 were differentially expressed. Microarray analysis of the same tissue provided us with alterations in the ventricular transcriptome. Because bioinformatic analyses of both the proteome and transcriptome demonstrated the up-regulation of endoplasmic reticulum stress, we validated its occurrence in adult CNA hearts through a series of immunoblots and RT-PCR analyses. Endoplasmic reticulum stress often leads to increased apoptosis, but apoptosis was minimal in CNA hearts, suggesting that activated calcineurin might protect against apoptosis. Indeed, the viability of cultured neonatal mouse cardiomyocytes (NCMs) from CNA mice was higher than WT after serum starvation, an apoptotic trigger. Proteomic data identified α-crystallin B (Cryab) as a potential mediator of this protective effect and we showed that silencing of Cryab via lentivector-mediated transduction of shRNAs in NCMs led to a significant reduction in NCM viability and loss of protection against apoptosis. The identification of Cryab as a downstream effector of calcineurin-induced protection against apoptosis will permit elucidation of its role in cardiac apoptosis and its potential as a therapeutic target.

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 pivotal role of the beta 7 strand in the intersubunit contacts of different human small heat shock proteins

Human alpha B-crystallin and small heat shock proteins HspB6 and HspB8 were mutated so that all endogenous Cys residues were replaced by Ser and the single Cys residue was inserted in a position homologous to that of Cys137 of human HspB1, i.e. in a position presumably located in the central part of beta 7 strand of the alpha-crystallin domain. The secondary, tertiary, and quaternary structures of thus obtained Cys-mutants as well as their chaperone-like activity were similar to those of their wild-type counterparts. Mild oxidation of Cys-mutants leads to formation of disulfide bond crosslinking neighboring monomers thus indicating participation of the beta 7 strand in intersubunit interaction. Oxidation weakly affects the secondary and tertiary structure, does not affect the quaternary structure of alpha B-crystallin and HspB6, and shifts equilibrium between monomer and dimer of HspB8 towards dimer formation. It is concluded that the beta 7 strand participates in the intersubunit interaction of four human small heat shock proteins (alpha B-crystallin, HspB1, HspB6, HspB8) having different structure of beta2 strand of alpha-crystallin domain and different length and composition of variable N- and C-terminal tails.

Proteasome inhibition induces hsp30 and hsp70 gene expression as well as the acquisition of thermotolerance in Xenopus laevis A6 cells

Previous studies have shown that inhibiting the activity of the proteasome leads to the accumulation of damaged or unfolded proteins within the cell. In this study, we report that proteasome inhibitors, lactacystin and carbobenzoxy-L-leucyl-L-leucyl-L-leucinal (MG132), induced the accumulation of ubiquitinated proteins as well as a dose- and time-dependent increase in the relative levels of heat shock protein (HSP)30 and HSP70 and their respective mRNAs in Xenopus laevis A6 kidney epithelial cells. In A6 cells recovering from MG132 exposure, HSP30 and HSP70 levels were still elevated after 24 h but decreased substantially after 48 h. The activation of heat shock factor 1 (HSF1) may be involved in MG132-induced hsp gene expression in A6 cells since KNK437, a HSF1 inhibitor, repressed the accumulation of HSP30 and HSP70. Exposing A6 cells to simultaneous MG132 and mild heat shock enhanced the accumulation of HSP30 and HSP70 to a much greater extent than with each stressor alone. Immunocytochemical studies determined that HSP30 was localized primarily in the cytoplasm of lactacystin- or MG132-treated cells. In some cells treated with higher concentrations of MG132 or lactacystin, we observed in the cortical cytoplasm (1) relatively large HSP30 staining structures, (2) colocalization of actin and HSP30, and (3) cytoplasmic areas that were devoid of HSP30. Lastly, MG132 treatment of A6 cells conferred a state of thermotolerance such that they were able to survive a subsequent thermal challenge.

Protein aggregates and novel presenilin gene variants in idiopathic dilated cardiomyopathy

BACKGROUND

Heart failure is a debilitating condition resulting in severe disability and death. In a subset of cases, clustered as idiopathic dilated cardiomyopathy (iDCM), the origin of heart failure is unknown. In the brain of patients with dementia, proteinaceous aggregates and abnormal oligomeric assemblies of beta-amyloid impair cell function and lead to cell death.

METHODS AND RESULTS

We have similarly characterized fibrillar and oligomeric assemblies in the hearts of iDCM patients, pointing to abnormal protein aggregation as a determinant of iDCM. We also showed that oligomers alter myocyte Ca(2+) homeostasis. Additionally, we have identified 2 new sequence variants in the presenilin-1 (PSEN1) gene promoter leading to reduced gene and protein expression. We also show that presenilin-1 coimmunoprecipitates with SERCA2a.

CONCLUSIONS

On the basis of these findings, we propose that 2 mechanisms may link protein aggregation and cardiac function: oligomer-induced changes on Ca(2+) handling and a direct effect of PSEN1 sequence variants on excitation-contraction coupling protein function.

Increased monomerization of mutant HSPB1 leads to protein hyperactivity in Charcot-Marie-Tooth neuropathy

Small heat shock proteins are molecular chaperones capable of maintaining denatured proteins in a folding-competent state. We have previously shown that missense mutations in the small heat shock protein HSPB1 (HSP27) cause distal hereditary motor neuropathy and axonal Charcot-Marie-Tooth disease. Here we investigated the biochemical consequences of HSPB1 mutations that are known to cause peripheral neuropathy. In contrast to other chaperonopathies, our results revealed that particular HSPB1 mutations presented higher chaperone activity compared with wild type. Hyperactivation of HSPB1 was accompanied by a change from its wild-type dimeric state to a monomer without dissociation of the 24-meric state. Purification of protein complexes from wild-type and HSPB1 mutants showed that the hyperactive isoforms also presented enhanced binding to client proteins. Furthermore, we show that the wild-type HSPB1 protein undergoes monomerization during heat-shock activation, strongly suggesting that the monomer is the active form of the HSPB1 protein.

Muscle giants: molecular scaffolds in sarcomerogenesis

Myofibrillogenesis in striated muscles is a highly complex process that depends on the coordinated assembly and integration of a large number of contractile, cytoskeletal, and signaling proteins into regular arrays, the sarcomeres. It is also associated with the stereotypical assembly of the sarcoplasmic reticulum and the transverse tubules around each sarcomere. Three giant, muscle-specific proteins, titin (3-4 MDa), nebulin (600-800 kDa), and obscurin (approximately 720-900 kDa), have been proposed to play important roles in the assembly and stabilization of sarcomeres. There is a large amount of data showing that each of these molecules interacts with several to many different protein ligands, regulating their activity and localizing them to particular sites within or surrounding sarcomeres. Consistent with this, mutations in each of these proteins have been linked to skeletal and cardiac myopathies or to muscular dystrophies. The evidence that any of them plays a role as a "molecular template," "molecular blueprint," or "molecular ruler" is less definitive, however. Here we review the structure and function of titin, nebulin, and obscurin, with the literature supporting a role for them as scaffolding molecules and the contradictory evidence regarding their roles as molecular guides in sarcomerogenesis.

Methionine sulfoxide reductase A (MsrA) restores alpha-crystallin chaperone activity lost upon methionine oxidation

BACKGROUND

Lens cataract is associated with protein oxidation and aggregation. Two proteins that cause cataract when deleted from the lens are methionine sulfoxide reductase A (MsrA) that repairs protein methionine sulfoxide (PMSO) oxidized proteins and alpha-crystallin which is a two-subunit (alphaA and alphaB) chaperone. Here, we tested whether PMSO formation damages alpha-crystallin chaperone function and whether MsrA could repair PMSO-alpha-crystallin.

METHODS

Total alpha-crystallin was oxidized to PMSO and evaluated by CNBr-cleavage and mass spectrometry. Chaperone activity was measured by light scattering using lysozyme as target. PMSO-alpha-crystallin was treated with MsrA, and repair was assessed by CNBr cleavage, mass spectrometry and recovery of chaperone function. The levels of alpha-crystallin-PMSO in the lenses of MsrA-knockout relative to wild-type mice were determined.

RESULTS

PMSO oxidation of total alpha-crystallin (met 138 of alphaA and met 68 of alphaB) resulted in loss of alpha-crystallin chaperone activity. MsrA treatment of PMSO-alpha-crystallin repaired its chaperone activity through reduction of PMSO. Deletion of MsrA in mice resulted in increased levels of PMSO-alpha-crystallin.

CONCLUSIONS

Methionine oxidation damages alpha-crystallin chaperone function and MsrA can repair PMSO-alpha-crystallin restoring its chaperone function. MsrA is required for maintaining the reduced state of alpha-crystallin methionines in the lens.

SIGNIFICANCE

Methionine oxidation of alpha-crystallin in combination with loss of MsrA repair causes loss of alpha-crystallin chaperone function. Since increased PMSO levels and loss of alpha-crystallin function are hallmarks of cataract, these results provide insight into the mechanisms of cataract development and likely those of other age-related diseases.

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.

Tragedy in a heartbeat: malfunctioning desmin causes skeletal and cardiac muscle disease

Muscle fiber deterioration resulting in progressive skeletal muscle weakness, heart failure, and respiratory distress occurs in more than 20 inherited myopathies. As discussed in this Review, one of the newly identified myopathies is desminopathy, a disease caused by dysfunctional mutations in desmin, a type III intermediate filament protein, or alphaB-crystallin, a chaperone for desmin. The range of clinical manifestations in patients with desminopathy is wide and may overlap with those observed in individuals with other myopathies. Awareness of this disease needs to be heightened, diagnostic criteria reliably outlined, and molecular testing readily available; this would ensure prevention of sudden death from cardiac arrhythmias and other complications.

Mechanism of cataract formation in alphaA-crystallin Y118D mutation

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

alphaB-crystallin: a hybrid solid-state/solution-state NMR investigation reveals structural aspects of the heterogeneous oligomer

Atomic-level structural information on alphaB-Crystallin (alphaB), a prominent member of the small heat-shock protein family, has been a challenge to obtain due its polydisperse oligomeric nature. We show that magic-angle spinning solid-state NMR can be used to obtain high-resolution information on an approximately 580-kDa human alphaB assembled from 175-residue 20-kDa subunits. An approximately 100-residue alpha-crystallin domain is common to all small heat-shock proteins, and solution-state NMR was performed on two different alpha-crystallin domain constructs isolated from alphaB. In vitro, the chaperone-like activities of full-length alphaB and the isolated alpha-crystallin domain are identical. Chemical shifts of the backbone and C(beta) resonances have been obtained for residues 64-162 (alpha-crystallin domain plus part of the C-terminus) in alphaB and the isolated alpha-crystallin domain by solid-state and solution-state NMR, respectively. Both sets of data strongly predict six beta-strands in the alpha-crystallin domain. A majority of residues in the alpha-crystallin domain have similar chemical shifts in both solid-state and solution-state, indicating similar structures for the domain in its isolated and oligomeric forms. Sites of intersubunit interaction are identified from chemical shift differences that cluster to specific regions of the alpha-crystallin domain. Multiple signals are observed for the resonances of M68 in the oligomer, identifying the region containing this residue as existing in heterogeneous environments within alphaB. Evidence for a novel dimerization motif in the human alpha-crystallin domain is obtained by a comparison of (i) solid-state and solution-state chemical shift data and (ii) (1)H-(15)N heteronuclear single quantum coherence spectra as a function of pH. The isolated alpha-crystallin domain undergoes a dimer-monomer transition over the pH range 7.5-6.8. This steep pH-dependent switch may be important for alphaB to function optimally (e.g., to preserve the filament integrity of cardiac muscle proteins such as actin and desmin during cardiac ischemia, which is accompanied by acidosis).

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Build it up-Tear it down: protein quality control in the cardiac sarcomere

The assembly and maintenance of the cardiac sarcomere, which contains the basic contractile components of actin and myosin, are essential for cardiac function. While often described as a static structure, the sarcomere is actually dynamic and undergoes constant turnover, allowing it to adapt to physiological changes while still maintaining function. A host of new factors have been identified that play a role in the regulation of protein quality control in the sarcomere, including chaperones that mediate the assembly of sarcomere components and ubiquitin ligases that control their specific degradation. There is clear evidence of sarcomere disorganization in animal models lacking muscle-specific chaperone proteins, illustrating the importance of these molecules in sarcomere structure and function. Although ubiquitin ligases have been found within the sarcomere structure itself, the role of the ubiquitin proteasome system in cardiac sarcomere regulation, and the factors that control its activity, are only just now being elucidated. The number of ubiquitin ligases identified with specificity for sarcomere proteins, each with distinct target substrates, is growing, allowing for tight regulation of this system. In this review, we highlight the dynamic interplay between sarcomere-specific chaperones and ubiquitin-dependent degradation of sarcomere proteins that is necessary in order to maintain structure and function of the cardiac sarcomere.

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.

Significance of α-crystallin heteropolymer with a 3:1 αA/αB ratio: chaperone-like activity, structure and hydrophobicity

The small heat-shock protein α-crystallin isolated from the eye lens exists as a large (700 kDa) heteropolymer composed of two subunits, αA and αB, of 20 kDa each. Although trace amounts of αA-crystallin are found in other tissues, non-lenticular distribution of α-crystallin is dominated by the αB homopolymer. In most vertebrate lens, the molar ratio of αA to αB is generally 3:1. However, the importance of this ratio in the eye lens is not known. In the present study, we have investigated the physiological significance of the 3:1 ratio by determining the secondary/tertiary structure, hydrophobicity and chaperone-like activity of αA- and αB-homopolymers and heteropolymers with different ratios of αA to αB subunits. Although, under physiologically relevant conditions, the αB-homopolymer (37–40 °C) has shown relatively higher activity, the αA-homopolymer or the heteropolymer with a higher αA proportion (3:1 ratio) has shown greater chaperone-like activity at elevated temperatures (>50 °C) and also upon structural perturbation. Furthermore, higher chaperone activity at elevated temperatures as well as upon structural perturbation is mainly mediated through increased hydrophobicity of αA. Although homopolymers and heteropolymers of α-crystallin did not differ in their secondary structure, changes in tertiary structure due to structural perturbations upon pre-heating are mediated predominantly by αA. Interestingly, the heteropolymer with higher αA proportion (3:1) or the αA-homopolymer seems to be better chaperones in protecting lens β- and γ-crystallins at both normal and elevated temperatures. Thus lens might have favoured a combination of these qualities to achieve optimal protection under both native and stress (perturbed) conditions for which the heteropolymer with αA to αB in the 3:1 ratio appears to be better suited.

Small heat shock protein activity is regulated by variable oligomeric substructure

The alpha-crystallins are members of the small heat shock protein family of molecular chaperones that have evolved to minimize intracellular protein aggregation; however, they are also implicated in a number of protein deposition diseases. In this study, we employed novel mass spectrometry techniques to investigate the changes in quaternary structure associated with this switch from chaperone to adjuvant of aggregation. We replicated the oligomeric rearrangements observed for post-translationally modified alpha-crystallins, without altering the protein sequence, by refolding the alpha-crystallins in vitro. This refolding resulted in a loss of dimeric substructure concomitant with an augmentation of substrate affinity. We show that packaging of small heat shock proteins into dimeric units is used to control the level of chaperone function by regulating the exposure of hydrophobic surfaces. We propose that a bias toward monomeric substructure is responsible for the aberrant chaperone behavior associated with the alpha-crystallins in protein deposition diseases.

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.