The cardiomyopathy and lens cataract mutation in alphaB-crystallin alters its protein structure, chaperone activity, and interaction with intermediate filaments in vitro

Oligomers of mutant glial fibrillary acidic protein (GFAP) Inhibit the proteasome system in alexander disease astrocytes, and the small heat shock protein alphaB-crystallin reverses the inhibition

The accumulation of the intermediate filament protein, glial fibrillary acidic protein (GFAP), in astrocytes of Alexander disease (AxD) impairs proteasome function in astrocytes. We have explored the molecular mechanism that underlies the proteasome inhibition. We find that both assembled and unassembled wild type (wt) and R239C mutant GFAP protein interacts with the 20 S proteasome complex and that the R239C AxD mutation does not interfere with this interaction. However, the R239C GFAP accumulates to higher levels and forms more protein aggregates than wt protein. These aggregates bind components of the ubiquitin-proteasome system and, thus, may deplete the cytosolic stores of these proteins. We also find that the R239C GFAP has a greater inhibitory effect on proteasome system than wt GFAP. Using a ubiquitin-independent degradation assay in vitro, we observed that the proteasome cannot efficiently degrade unassembled R239C GFAP, and the interaction of R239C GFAP with proteasomes actually inhibits proteasomal protease activity. The small heat shock protein, alphaB-crystallin, which accumulates massively in AxD astrocytes, reverses the inhibitory effects of R239C GFAP on proteasome activity and promotes degradation of the mutant GFAP, apparently by shifting the size of the mutant protein from larger oligomers to smaller oligomers and monomers. These observations suggest that oligomeric forms of GFAP are particularly effective at inhibiting proteasome activity.

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.

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.

Shadows of complexity: what biological networks reveal about epistasis and pleiotropy

Pleiotropy, in which one mutation causes multiple phenotypes, has traditionally been seen as a deviation from the conventional observation in which one gene affects one phenotype. Epistasis, or gene-gene interaction, has also been treated as an exception to the Mendelian one gene-one phenotype paradigm. This simplified perspective belies the pervasive complexity of biology and hinders progress toward a deeper understanding of biological systems. We assert that epistasis and pleiotropy are not isolated occurrences, but ubiquitous and inherent properties of biomolecular networks. These phenomena should not be treated as exceptions, but rather as fundamental components of genetic analyses. A systems level understanding of epistasis and pleiotropy is, therefore, critical to furthering our understanding of human genetics and its contribution to common human disease. Finally, graph theory offers an intuitive and powerful set of tools with which to study the network bases of these important genetic phenomena.

The altered expression of inflammation-related molecules and secretion of IL-6 and IL-8 by HUVEC from newborns with maternal inactive systemic lupus erythematosus is modified by estrogens

Systemic lupus erythematosus (SLE) predominantly affects women, especially those in reproductive age. Genetic contributions to disease susceptibility as well as immune dysregulation, particularly persistent inflammatory responses, are considered essential features. Our aim was to determine whether human umbilical vein endothelial cells (HUVEC) isolated from healthy newborns to women with inactive SLE show inflammation-related abnormalities that might lead to an early development of SLE in the offsprings. HUVEC isolated from six women with inactive SLE were stimulated with 2.5 ng/mL of TNF-alpha and/or physiological and pharmacological doses of 17-I(2) estradiol (E2). Then the expression of VCAM-1, ICAM-1, E-selectin, toll-like receptor-9 (TLR-9), heat shock protein 70 (HSP70) and HSP90 were measured. The concentrations of IL-6, IL-8, and IL-10 were also determined in maternal serum and in TNF-alpha stimulated and non-stimulated HUVEC culture supernatant. HUVEC from children with no family history of autoimmune disease served as controls. Our results showed that in HUVEC from SLE+ mothers, a constitutively low expression of adhesion molecules was enhanced by TNF-alpha treatment. The E2 (1 ng/mL) increased the expression of adhesion molecules but had no effect upon TNF-alpha-treated cells. IL-6 was constitutively higher in SLE+ HUVEC, whereas IL-8 was lower; E2 treatment diminished the latter. The E2 had no effect upon IL-6 and IL-8 secretions in TNF-alpha-treated cells. SLE+ HUVEC showed a disordered cytoskeleton and overexpressed HSP70, HSP90, and TLR-9. Our results indicate that endothelial cells of newborns to SLE+ mothers are in a proinflammatory condition which can be upregulated by estrogens.

Protective effect of post-ischaemic viral delivery of heat shock proteins in vivo

Heat shock proteins (HSPs) function as molecular chaperones involved in protein folding, transport and degradation and, in addition, they can promote cell survival both in vitro and in vivo after a range of stresses. Although some in vivo studies have suggested that HSP27 and HSP70 can be neuroprotective, current evidence is limited, particularly when HSPs have been delivered after an insult. The effect of overexpressing HSPs after transient occlusion of the middle cerebral artery in rats was investigated by delivering an attenuated herpes simplex viral vector (HSV-1) engineered to express HSP27 or HSP70 30 mins after tissue reperfusion. Magnetic resonance imaging scans were used to determine lesion size and cerebral blood flow at six different time points up to 1 month after stroke. Animals underwent two sensorimotor tests at the same time points to assess the relationship between lesion size and function. Results indicate that post-ischaemic viral delivery of HSP27, but not of HSP70, caused a statistically significant reduction in lesion size and induced a significant behavioural improvement compared with controls. This is the first evidence of effective post-ischaemic gene therapy with a viral vector expressing HSP27 in an experimental model of stroke.

Lens intermediate filaments

The ocular lens assembles two separate intermediate filament systems sequentially with differentiation. Canonical 8-11 nm IFs composed of Vimentin are assembled in lens epithelial cells and younger fiber cells, while the fiber cell-specific beaded filaments are switched on as fiber cell elongation initiates. Some of the key features of both filament systems are reviewed.

Alpha B-crystallin suppresses pressure overload cardiac hypertrophy

AlphaB-crystallin (CryAB) is the most abundant small heat shock protein (HSP) constitutively expressed in cardiomyocytes. Gain- and loss-of-function studies demonstrated that CryAB can protect against myocardial ischemia/reperfusion injury. However, the role of CryAB or any HSPs in cardiac responses to mechanical overload is unknown. This study addresses this issue. Nontransgenic mice and mice with cardiomyocyte-restricted transgenic overexpression of CryAB or with germ-line ablation of the CryAB/HSPB2 genes were subjected to transverse aortic constriction or sham surgery. Two weeks later, cardiac responses were analyzed by fetal gene expression profiling, cardiac function analyses, and morphometry. Comparison among the 3 sham surgery groups reveals that CryAB overexpression is benign, whereas the knockout is detrimental to the heart as reflected by cardiac hypertrophy and malfunction at 10 weeks of age. Compared to nontransgenic mice, transgenic mouse hearts showed significantly reduced NFAT transactivation and attenuated cardiac hypertrophic responses to transverse aortic constriction but unchanged cardiac function, whereas NFAT transactivation was significantly increased in cardiac and skeletal muscle of the knockout mice at baseline, and they developed cardiac insufficiency at 2 weeks after transverse aortic constriction. CryAB overexpression in cultured neonatal rat cardiomyocytes significantly attenuated adrenergic stimulation-induced NFAT transactivation and hypertrophic growth. We conclude that CryAB suppresses cardiac hypertrophic responses likely through attenuating NFAT signaling and that CryAB and/or HSPB2 are essential for normal cardiac function.

Protein-protein interactions and lens transparency

Past studies have identified posttranslational modifications of human lens proteins occurring during cataract formation, and have also demonstrated that protein-protein interactions exist between different lens crystallins. Based upon current theories of lens transparency, these posttranslational modifications and their possible effects upon crystallin interactions may be the key to understanding why the lens is able to transmit light, and why transmission is decreased during cataractogenesis. This review will summarize current knowledge of posttranslational modifications during human cataractogenesis, and will propose their possible role in protein-protein interactions that are thought to be necessary for lens transparency. Based upon this premise, model systems will be described that will test the validity of the theory.

Glial fibrillary acidic protein filaments can tolerate the incorporation of assembly-compromised GFAP-delta, but with consequences for filament organization and alphaB-crystallin association

The glial fibrillary acidic protein (GFAP) gene is alternatively spliced to give GFAP-alpha, the most abundant isoform, and seven other differentially expressed transcripts including GFAP-delta. GFAP-delta has an altered C-terminal domain that renders it incapable of self-assembly in vitro. When titrated with GFAP-alpha, assembly was restored providing GFAP-delta levels were kept low (approximately 10%). In a range of immortalized and transformed astrocyte derived cell lines and human spinal cord, we show that GFAP-delta is naturally part of the endogenous intermediate filaments, although levels were low (approximately 10%). This suggests that GFAP filaments can naturally accommodate a small proportion of assembly-compromised partners. Indeed, two other assembly-compromised GFAP constructs, namely enhanced green fluorescent protein (eGFP)-tagged GFAP and the Alexander disease-causing GFAP mutant, R416W GFAP both showed similar in vitro assembly characteristics to GFAP-delta and could also be incorporated into endogenous filament networks in transfected cells, providing expression levels were kept low. Another common feature was the increased association of alphaB-crystallin with the intermediate filament fraction of transfected cells. These studies suggest that the major physiological role of the assembly-compromised GFAP-delta splice variant is as a modulator of the GFAP filament surface, effecting changes in both protein- and filament-filament associations as well as Jnk phosphorylation.

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.

Intermediate filament diseases: desminopathy

Desminopathy is one of the most common intermediate filament human disorders associated with mutations in closely interacting proteins, desmin and alphaB-crystallin. The inheritance pattern in familial desminopathy is characterized as autosomal dominant or autosomal recessive, but many cases have no family history. At least some and likely most sporadic desminopathy cases are associated with de novo DES mutations. The age of disease onset and rate of progression may vary depending on the type of inheritance and location of the causative mutation. Typically, the illness presents with lower and later upper limb muscle weakness slowly spreading to involve truncal, neck-flexor, facial and bulbar muscles. Skeletal myopathy is often combined with cardiomyopathy manifested by conduction blocks, arrhythmias and chronic heart failure resulting in premature sudden death. Respiratory muscle weakness is a major complication in some patients. Sections of the affected skeletal and cardiac muscles show abnormal fibre areas containing chimeric aggregates consisting of desmin and other cytoskeletal proteins. Various DES gene mutations: point mutations, an insertion, small in-frame deletions and a larger exon-skipping deletion, have been identified in desminopathy patients. The majority of these mutations are located in conserved alpha-helical segments, but additional mutations have recently been identified in the tail domain. Filament and network assembly studies indicate that most but not all disease-causing mutations make desmin assembly-incompetent and able to disrupt a pre-existing filamentous network in dominant-negative fashion. AlphaB-crystallin serves as a chaperone for desmin preventing its aggregation under various forms of stress; mutant CRYAB causes cardiac and skeletal myopathies identical to those resulting from DES mutations.

Myopathy-associated αB-crystallin Mutants

Three mutations (R120G, Q151X, and 464delCT) in the small heat shock protein alphaB-crystallin cause inherited myofibrillar myopathy. In an effort to elucidate the molecular basis for the associated myopathy, we have determined the following for these mutant alphaB-crystallin proteins: (i) the formation of aggregates in transfected cells; (ii) the partition into different subcellular fractions; (iii) the phosphorylation status; and (iv) the ability to interact with themselves, with wild-typealphaB-crystallin, and with other small heat shock proteins that are abundant in muscles. We found that all three alphaB-crystallin mutants have an increased tendency to form cytoplasmic aggregates in transfected cells and significantly increased levels of phosphorylation when compared with the wild-type protein. Although wild-type alphaB-crystallin partitioned essentially into the cytosol and membranes/organelles fractions, mutant alphaB-crystallin proteins partitioned additionally into the nuclear and cytoskeletal fractions. By using various protein interaction assays, including quantitative fluorescence resonance energy transfer measurements in live cells, we found abnormal interactions of the various alphaB-crystallin mutants with wild-type alphaB-crystallin, with themselves, and with the other small heat shock proteins Hsp20, Hsp22, and possibly with Hsp27. The collected data suggest that eachalphaB-crystallin mutant has a unique pattern of abnormal interaction properties. These distinct properties of the alphaB-crystallin mutants identified are likely to contribute to a better understanding of the gradual manifestation and clinical heterogeneity of the associated myopathy in patients.

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

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

Analysis of the alphaB-crystallin domain responsible for inhibiting tubulin aggregation

The cytoskeleton has a unique property such that changes of conformation result in polymerization into a filamentous form. alphaB-Crystallin, a small heat shock protein (sHsp), has chaperone activities for various substrates, including proteins constituting the cytoskeleton, such as actin; intermediate filament; and tubulin. However, it is not clear whether the "alpha-crystallin domain" common to sHsps also has chaperone activity for the protein cytoskeleton. To investigate the possibility that the C-terminal alpha-crystallin domain of alpha-crystallin has the aggregation-preventing ability for tubulin, we constructed an N-terminal domain deletion mutant of alphaB-crystallin. We characterized its structural properties and chaperone activities. Far-ultraviolet (UV) circular dichroism measurements showed that secondary structure in the alpha-crystallin domain of the deletion mutant is maintained. Ultracentrifuge analysis of molecular masses indicated that the deletion mutant formed smaller oligomers than did the full-length protein. Chaperone activity assays demonstrated that the N-terminal domain deletion mutant suppressed heat-induced aggregation of tubulin well. Comparison of chaperone activities for 2 other substrates (citrate synthase and alcohol dehydrogenase) showed that it was less effective in the suppression of their aggregation. These results show that alphaB-crystallin recognizes a variety of substrates and especially that alpha-crystallin domain binds free cytoskeletal proteins. We suggest that this feature would be advantageous in its functional role of holding or folding multiple proteins denatured simultaneously under stress conditions.

Human alpha B-crystallin mutation causes oxido-reductive stress and protein aggregation cardiomyopathy in mice

The autosomal dominant mutation in the human alphaB-crystallin gene inducing a R120G amino acid exchange causes a multisystem, protein aggregation disease including cardiomyopathy. The pathogenesis of cardiomyopathy in this mutant (hR120GCryAB) is poorly understood. Here, we show that transgenic mice overexpressing cardiac-specific hR120GCryAB recapitulate the cardiomyopathy in humans and find that the mice are under reductive stress. The myopathic hearts show an increased recycling of oxidized glutathione (GSSG) to reduced glutathione (GSH), which is due to the augmented expression and enzymatic activities of glucose-6-phosphate dehydrogenase (G6PD), glutathione reductase, and glutathione peroxidase. The intercross of hR120GCryAB cardiomyopathic animals with mice with reduced G6PD levels rescues the progeny from cardiac hypertrophy and protein aggregation. These findings demonstrate that dysregulation of G6PD activity is necessary and sufficient for maladaptive reductive stress and suggest a novel therapeutic target for abrogating R120GCryAB cardiomyopathy and heart failure in humans.

Residue R120 is essential for the quaternary structure and functional integrity of human alphaB-crystallin

The missense mutation Arg-120 to Gly (R120G) in the human alphaBeta-crystallin sequence has been reported to be associated with autosomal dominant myopathy, cardiomyopathy, and cataract. Previous studies of the mutant showed a significant ability to aggregate in cultured cells and an increased oligomeric size coupled to an important loss of the chaperone-like activity in vitro. The aim of this study was to further analyze the role of the R120 residue in the structural and functional properties of alphaBeta-crystallin. The following mutants were generated, Arg-120 to Gly (R120G), Cys (R120C), Lys (R120K), and Asp (R120D). In cellulo, after expression in two cultured cell lines, NIH-3T3 and Cos-7, the capacity of the wild-type and mutant crystallins to aggregate was evaluated and the protein location was determined by immunofluorescence. In vitro, the wild-type and mutant crystallins were expressed in Escherichia coli cells, purified by size exclusion chromatography, and characterized using dynamic light scattering, electron microscopy, and chaperone-like activity assays. Aggregate sizes in cellulo and in vitro were analyzed. The whole of the data showed that the preservation of an Arg residue at position 120 of alphaBeta-crystallin is critical for the structural and functional integrity of the protein and that each mutation results in specific changes in both structural and functional characteristics.

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.

Interactive domains in the molecular chaperone human alphaB crystallin modulate microtubule assembly and disassembly

Background

Small heat shock proteins regulate microtubule assembly during cell proliferation and in response to stress through interactions that are poorly understood.

Methodology

Novel functions for five interactive sequences in the small heat shock protein and molecular chaperone, human αB crystallin, were investigated in the assembly/disassembly of microtubules and aggregation of tubulin using synthetic peptides and mutants of human αB crystallin.

Principal Findings

The interactive sequence ₁₁₃FISREFHR₁₂₀ exposed on the surface of αB crystallin decreased microtubule assembly by ∼45%. In contrast, the interactive sequences, ₁₃₁LTITSSLSSDGV₁₄₂ and ₁₅₆ERTIPITRE₁₆₄, corresponding to the β8 strand and the C-terminal extension respectively, which are involved in complex formation, increased microtubule assembly by ∼34–45%. The αB crystallin peptides, ₁₁₃FISREFHR₁₂₀ and ₁₅₆ERTIPITRE₁₆₄, inhibited microtubule disassembly by ∼26–36%, and the peptides ₁₁₃FISREFHR₁₂₀ and ₁₃₁LTITSSLSSDGV₁₄₂ decreased the thermal aggregation of tubulin by ∼42–44%. The ₁₃₁LTITSSLSSDGV₁₄₂ and ₁₅₆ERTIPITRE₁₆₄ peptides were more effective than the widely used anti-cancer drug, Paclitaxel, in modulating tubulin↔microtubule dynamics. Mutagenesis of these interactive sequences in wt human αB crystallin confirmed the effects of the αB crystallin peptides on microtubule assembly/disassembly and tubulin aggregation. The regulation of microtubule assembly by αB crystallin varied over a narrow range of concentrations. The assembly of microtubules was maximal at αB crystallin to tubulin molar ratios between 1∶4 and 2∶1, while molar ratios >2∶1 inhibited microtubule assembly.

Conclusions and Significance

Interactive sequences on the surface of human αB crystallin collectively modulate microtubule assembly through a dynamic subunit exchange mechanism that depends on the concentration and ratio of αB crystallin to tubulin. These are the first experimental results in support of the functional importance of the dynamic subunit model of small heat shock proteins.

Interactive sequences in the stress protein and molecular chaperone human alphaB crystallin recognize and modulate the assembly of filaments

Molecular chaperones including the small heat shock proteins, alphaB crystallin and sHSP27 participate in the assembly, disassembly, and reorganization of the cytoskeleton during cell development and differentiation. While alphaB crystallin and sHSP27 stabilize and modulate filament assembly and re-organization, the sequences and structural domains mediating interactions between these proteins and filaments are unknown. It is important to define these interactive domains in order to understand differential interactions between chaperones and stable or unfolding filaments and their function in the cellular stress response. Protein pin arrays identified sequences in human alphaB crystallin that selectively interacted with native or partially unfolded filament proteins desmin, glial-fibrillary acidic protein, and actin. Circular dichroism spectroscopy determined differences in the structure of these filaments at 23 and 45 degrees C. Seven alphaB crystallin sequences had stronger interactions with desmin and six sequences had stronger interactions with glial-fibrillary acidic protein at 23 degrees C than at 45 degrees C. The alphaB crystallin sequences (33)LESDLFPTSTSLSPFYLRPPSFLR(56) and (129)DPLTITSSLSSDGV(145) had the strongest interactions with actin at 23 degrees C, while (57)APSWFDTG(64), (111)HGFISREF(118), (145)VNGPRKQVSG(154), and (155)PERTIPITREEK(165) had the strongest interactions with actin at 45 degrees C. The actin interactive sequences of alphaB crystallin overlapped with previously identified alphaB crystallin chaperone sequences and were synthesized to evaluate their effect on the assembly and aggregation of actin. Full-length alphaB crystallin and the core domain chaperone sequence (131)LTITSSLSSDGV(143) promoted actin polymerization at 37 degrees C and inhibited depolymerization and aggregation at 50 degrees C. The results support the hypothesis that interactive domains in alphaB crystallin have multiple functions in stabilizing the cytoskeleton and protecting cytosolic proteins from unfolding.

Role of heat shock protein 70 in induction of stress fiber formation in rat arterial endothelial cells in response to stretch stress

We investigated the mechanism by which endothelial cells (ECs) resist various forms of physical stress using an experimental system consisting of rat arterial EC sheets. Formation of actin stress fibers (SFs) and expression of endothelial heat-shock stress proteins (HSPs) in response to mechanical stretch stress were assessed by immunofluorescence microscopy. Stretch stimulation increased expression of HSPs 25 and 70, but not that of HSP 90. Treatment with SB203580, a p38 MAP kinase inhibitor that acts upstream of the HSP 25 activation cascade, or with geldanamycin, an inhibitor of HSP 90, had no effect on the SF formation response to mechanical stretch stress. In contrast, treatment with quercetin, an HSP 70 inhibitor, inhibited both upregulation of endothelial HSP 70 and formation of SFs in response to tensile stress. In addition, treatment of stretched ECs with cytochalasin D, which disrupts SF formation, did not adversely affect stretch-induced upregulation of endothelial HSP 70. Our data suggest that endothelial HSP 70 plays an important role in inducing SF formation in response to tensile stress.

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.

A transgenic mouse model for human autosomal dominant cataract

PURPOSE

To characterize lenses from transgenic mice designed to express mutant and wild-type alphaA-crystallin subunits.

METHODS

A series of transgenic mouse strains was created to express mutant (R116C) and wild-type human alphaA-crystallin in fiber cells of the lens. Dissected lenses were phenotypically scored for the presence and extent of opacities, fiber cell morphology, and posterior suture morphology. Gene transcripts derived from integrated transgenes were detected by reverse transcriptase-PCR. Distribution of expressed transgenic protein was determined by immunohistochemical staining of lens tissue sections. The abundance of endogenous and transgenic lens proteins was estimated by quantitative Western blot analysis.

RESULTS

Expression of R116C mutant alphaA-crystallin subunits resulted in posterior cortical cataracts and abnormalities associated with the posterior suture. The severity of lens abnormalities did not increase between the ages of 9 and 30 weeks. With respect to opacities and morphologic abnormalities, lenses from transgenic mice that express wild-type human alphaA-crystallin subunits were indistinguishable from age-matched nontransgenic control mice. Similar phenotypes were observed in different independent lines of R116C transgenic mice that differed by at least two orders of magnitude in the expression level of the mutant transgenic protein.

CONCLUSIONS

The results show that lens opacities and posterior sutural defects occur when mutant R116C alphaA-crystallin subunits are expressed on the background of wild-type endogenous mouse alpha-crystallins. Low levels of R116C alphaA-crystallin subunits are sufficient to induce lens opacities and sutural defects.

Fluorescence resonance energy transfer study of subunit exchange in human lens crystallins and congenital cataract crystallin mutants

Lens alpha-crystallin is an oligomeric protein with a molecular mass of 500-1000 kDa and a polydispersed assembly. It consists of two types of subunits, alphaA and alphaB, each with a molecular mass of 20 kDa. The subunits also form homo-oligomers in some other tissues and in vitro. Their quaternary structures, which are dynamic and characterized by subunit exchange, have been studied by many techniques, including fluorescence resonance energy transfer (FRET) and mass spectrometry analysis. The proposed mechanism of subunit exchange has been either by dissociation/association of monomeric subunits or by rapid equilibrium between oligomers and suboligomers. To explore the nature of subunit exchange further, we performed additional FRET measurements and analyses using a fluorescent dye-labeled W9F alphaA-crystallin as the acceptor probe and Trp in other crystallins (wild-type and R116C alphaA, wild-type and R120G alphaB, wild-type and Q155* betaB2) as the donor probe and calculated the transfer efficiency, Förster distance, and average distance between two probes. The results indicate only slight decreased efficiency and increased distance between two probes for the R116C alphaA and R120G alphaB mutations despite conformational changes.

Conformational changes resulting from pseudophosphorylation of mammalian small heat shock proteins--a two-hybrid study

The human genome codes for 10 so-called mammalian small heat shock or stress proteins (sHsp) with the various tissues expressing characteristic sets of sHsps. Most sHsps interact with each other and form homo- and heterooligomeric complexes. Some of the sHsps are phosphoproteins in vivo, and phosphorylation has been implicated in the regulation of complex size and composition. In this study, we analyze, by the 2-hybrid method, the reporter gene activation pattern of several sHsp pairs that previously have been demonstrated to interact. We show that pseudophosphorylation (mimicry of phosphorylation) of the homologous phosphorylation sites Ser15 and Ser16 in Hsp27 and Hsp20, respectively, modulates characteristics of these sHsps that can be detected by their ability to activate reporter genes in suitable 2-hybrid assays. Pseudophosphorylation of the separated N-terminus of Hsp27 alone is not sufficient for the activation of the reporter genes, whereas the separated C-terminus is sufficient. We conclude that pseudophosphorylation of Hsp27 and Hsp20 at their N-termini results in conformational changes that can be detected by their interaction with other sHsps. Pseudophosphorylation of alphaB-crystallin at Ser19, in contrast, had no detectable consequences.

Purification and characterization of the chaperone-like Hsp26 from Saccharomyces cerevisiae

sHsps are ubiquitous ATP-independent molecular chaperones, which efficiently prevent the unspecific aggregation of non-native proteins. Here, we described the purification of the small heat shock protein Hsp26 from a Saccharomyces cerevisiae strain harboring a multicopy plasmid carrying HSP26 gene under the control of its native promoter. A 26 kDa protein was purified to apparent homogeneity with a recovery of 74% by a very reproducible three steps procedure consisting of ethanol precipitation, sucrose gradient ultracentrifugation, and heat inactivation of residual contaminants. The purified polypeptide was unequivocally identified as Hsp26 using a specific Hsp26 polyclonal antibody as a probe. The analysis of the purified protein by electron microscopy revealed near spherical particles with a diameter of 12.0 nm (n=57, standard deviation +/-1.6 nm), displaying a dispersion in size ranging from 9.2 to 16.1 nm, identical to Methanococcus jannaschii Hsp16.5 and in the range of the size estimated for yeast Hsp26, in a previous report. Purified yeast Hsp26 was able to suppress 72% of the heat-induced aggregation of citrate synthase at a ratio of 1:1 (Hsp26 24-mer complex to citrate synthase dimer), and 86% of the heat-induced aggregation of lysozyme at a molar ratio of 1:16 (Hsp26 24-mer complex to lysozyme monomer). In conclusion, the Hsp26 protein purified as described here has structure and activity similar to the previously described preparations. As advantages, this new protocol is very reproducible and requires simple apparatuses which are found in all standard biochemistry laboratories.

Structural and functional roles for beta-strand 7 in the alpha-crystallin domain of p26, a polydisperse small heat shock protein from Artemia franciscana

Oviparous development in the extremophile crustacean, Artemia franciscana, generates encysted embryos which enter a profound state of dormancy, termed diapause. Encystment is marked by the synthesis of p26, a polydisperse small heat shock protein thought to protect embryos from stress. In order to elucidate structural/functional relationships within p26 and other polydisperse small heat shock proteins, and to better define the protein's role during diapause, amino acid substitutions R110G, F112R, R114A and Y116D were generated within the p26 alpha-crystallin domain by site-directed mutagenesis. These residues were chosen because they are highly conserved across species boundaries, and molecular modelling indicates that they are part of a key structural interface between dimers. The F112R mutation, which had the greatest impact on oligomerization, placed two charged residues at the p26 dimer-dimer interface, demonstrating the importance of beta-strand 7 in tetramer formation. All mutated versions of p26 were less able than wild-type p26 to confer thermotolerance on transformed bacteria and they exhibited diminished chaperone action in three in vitro assays; however, all variants retained protective activity. This apparent stability of p26 may, by prolonging effective chaperone life in vivo, enhance embryo stress resistance. All substitutions modified p26 intrinsic fluorescence, surface hydrophobicity and secondary structure, and the pronounced changes in variant R114A, as indicated by these physical measurements, correlated with the greatest loss of function. Although mutation R114A had the greatest effect on p26 chaperoning, it had the least on oligomerization. These results demonstrate that in contrast to many other small heat shock proteins, p26 effectiveness as a chaperone is independent of oligomerization. The results also reinforce the idea, occasioned by modelling, that R114 is removed slightly from dimer-dimer interfaces. Moreover, beta-strand 7 is shown to have an important role in oligomerization of p26, a function first proposed for this structural element upon crystallization of wheat Hsp16.9, a small heat shock protein with different quaternary structure.

Studies of alphaB crystallin subunit dynamics by surface plasmon resonance

The molecular chaperone activity of alphaB crystallin, an important stress protein in humans, is regulated by physiological factors, including temperature, pH, Ca2+, and ATP. In this study, the role of these factors in regulating the subunit dynamics of human alphaB crystallin was investigated using surface plasmon resonance (SPR). SPR experiments indicate that at temperatures above 37 degrees C, where alphaB crystallin has been reported to have higher chaperone activity, the subunit dynamics of alphaB crystallin were increased with faster association and dissociation rates. SPR experiments also indicate that interactions between alphaB crystallin subunits were enhanced with much faster association and slower dissociation rates at pH values below 7.0, where alphaB crystallin has been reported to have lower chaperone activity. The results suggest that the dynamic and rapid subunit exchange rate may regulate the chaperone activity of alphaB crystallin. The effect of Ca2+ and ATP on the subunit dynamics of alphaB crystallin was minimal, suggesting that Ca2+ and ATP modulate the chaperone activity of alphaB crystallin without altering the subunit dynamics. Based on the SPR results and previously reported biochemical data for the chaperone activity of alphaB crystallin under different conditions of temperature and pH, a model for the relationship between the subunit dynamics and chaperone activity of alphaB crystallin is established. The model is consistent with previous biochemical data for the chaperone activity and subunit dynamics of small heat shock proteins (sHSPs) and establishes a working hypothesis for the relationship between complex assembly and chaperone activity for sHSPs.

Interactions of HSP22 (HSPB8) with HSP20, αB-crystallin, and HSPB3

Seven of the 10 mammalian small heat shock proteins (sHSP) are expressed in muscle where they constitute 3% or more of total protein. sHSPs interact with one another, and these interactions are believed to be important for their functions. In cell types expressing multiple sHSPs, it is of interest to know which sHSPs interact with one another. We have previously shown that HSP22 interacts with itself as well as with HSP27, MKBP, and cvHSP. Using yeast two-hybrid assays and Förster resonance energy transfer microscopy, we now show that HSP22 also can interact with two additional members of the sHSP family, alphaB-crystallin and HSP20. We also show that HSP22 is found in HPLC fractions of primate cardiac muscle containing high molecular weight complexes that include alphaB-crystallin and HSP20. Our results suggest that a variety of oligomers composed of different proportions of different sHSPs may form in cell types expressing multiple sHSPs.

Induction of inclusion formation and disruption of lamin A/C structure by premutation CGG-repeat RNA in human cultured neural cells

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disorder that affects some adult carriers of pre-mutation alleles (55-200 CGG repeats) of the fragile X mental retardation 1 (FMR1) gene. FXTAS is thought to be caused by a toxic 'gain-of-function' of the expanded CGG-repeat FMR1 mRNA, which is found in the neuronal and astrocytic intranuclear inclusions associated with the disorder. Using a reporter construct with a FMR1 5' untranslated region harboring an expanded (premutation) CGG repeat, we have demonstrated that intranuclear inclusions can be formed in both primary neural progenitor cells and established neural cell lines. As with the inclusions found in post-mortem tissue, the inclusions induced by the expanded CGG repeat are alphaB-crystallin-positive; however, inclusions in culture are not associated with ubiquitin, indicating that incorporation of ubiquitinated proteins is a later event in the disease process. The absence of ubiquitinated proteins also argues against a model in which inclusion formation is due to a failure of the proteasomal degradative machinery. The presence of the expanded CGG repeat, as RNA, results in reduced cell viability as well as the disruption of the normal architecture of lamin A/C within the nucleus. This last observation, and the findings that lamin A/C is present in both the inclusions of FXTAS patients and the inclusions in cell culture, suggests that lamin A/C dysregulation may be a component of the pathogenesis of FXTAS; in particular, the Charcot-Marie-Tooth-type neuropathy associated with FXTAS may represent a functional laminopathy.

Intrasarcoplasmic amyloidosis impairs proteolytic function of proteasomes in cardiomyocytes by compromising substrate uptake

The presence of increased ubiquitinated proteins and amyloid oligomers in failing human hearts strikingly resembles the characteristic pathology in the brain of many neurodegenerative diseases. The ubiquitin-proteasome system (UPS) is responsible for degradation of most cellular proteins and plays essential roles in virtually all cellular processes. UPS impairment by aberrant protein aggregation was previously shown in cell culture but remains to be demonstrated in intact animals. Mechanisms underlying the impairment are poorly understood. We report here that UPS proteolytic function is severely impaired in the heart of a mouse model of intrasarcoplasmic amyloidosis caused by cardiac-restricted expression of a human desmin-related myopathy-linked missense mutation of alphaB-crystallin (CryAB(R120G)). The UPS impairment was detected before cardiac hypertrophy, and failure became discernible, suggesting that defective protein turnover likely contributes to cardiac remodeling and failure in this model. Further analyses reveal that the impairment is likely attributable to insufficient delivery of substrate proteins into the 20S proteasomes, and depletion of key components of the 19S subcomplex may be responsible. The derangement is likely caused by aberrant protein aggregation rather than loss of function of the CryAB gene because UPS malfunction was not evident in CryAB-null hearts and inhibition of aberrant protein aggregation by Congo red or a heat shock protein significantly attenuated CryAB(R120G)-induced UPS malfunction in cultured cardiomyocytes. Because of the central role of the UPS in cell regulation and the high intrasarcoplasmic amyloidosis prevalence in failing human hearts, our data suggest a novel pathogenic process in cardiac disorders with abnormal protein aggregation.

Nuclear import of {alpha}B-crystallin is phosphorylation-dependent and hampered by hyperphosphorylation of the myopathy-related mutant R120G

Phosphorylation modulates the functioning of alphaB-crystallin as a molecular chaperone. We here explore the role of phosphorylation in the nuclear import and cellular localization of alphaB-crystallin in HeLa cells. Inhibition of nuclear export demonstrated that phosphorylation of alphaB-crystallin is required for import into the nucleus. As revealed by mutant analysis, phosphorylation at Ser-59 is crucial for nuclear import, and phosphorylation at Ser-45 is required for speckle localization. Co-immunoprecipitation experiments suggested that the import of alphaB-crystallin is possibly regulated by its phosphorylation-dependent interaction with the survival motor neuron (SMN) protein, an important factor in small nuclear ribonucleoprotein nuclear import and assembly. This interaction was supported by co-localization of endogenous phosphorylated alphaB-crystallin with SMN in nuclear structures. The cardiomyopathy-causing alphaB-crystallin mutant R120G was found to be excessively phosphorylated, which disturbed SMN interaction and nuclear import, and resulted in the formation of cytoplasmic inclusions. Like for other protein aggregation disorders, hyperphosphorylation appears as an important aspect of the pathogenicity of alphaB-crystallin R120G.

The small heat shock proteins and their role in human disease

Small heat shock proteins (sHSPs) function as molecular chaperones, preventing stress induced aggregation of partially denatured proteins and promoting their return to native conformations when favorable conditions pertain. Sequence similarity between sHSPs resides predominately in an internal stretch of residues termed the alpha-crystallin domain, a region usually flanked by two extensions. The poorly conserved N-terminal extension influences oligomer construction and chaperone activity, whereas the flexible C-terminal extension stabilizes quaternary structure and enhances protein/substrate complex solubility. sHSP polypeptides assemble into dynamic oligomers which undergo subunit exchange and they bind a wide range of cellular substrates. As molecular chaperones, the sHSPs protect protein structure and activity, thereby preventing disease, but they may contribute to cell malfunction when perturbed. For example, sHSPs prevent cataract in the mammalian lens and guard against ischemic and reperfusion injury due to heart attack and stroke. On the other hand, mutated sHSPs are implicated in diseases such as desmin-related myopathy and they have an uncertain relationship to neurological disorders including Parkinson's and Alzheimer's disease. This review explores the involvement of sHSPs in disease and their potential for therapeutic intervention.

Intermediate filaments interact with dormant ezrin in intestinal epithelial cells

Ezrin connects the apical F-actin scaffold to membrane proteins in the apical brush border of intestinal epithelial cells. Yet, the mechanisms that recruit ezrin to the apical domain remain obscure. Using stable CACO-2 transfectants expressing keratin 8 (K8) antisense RNA under a tetracycline-responsive element, we showed that the actin-ezrin scaffold cannot assemble in the absence of intermediate filaments (IFs). Overexpression of ezrin partially rescued this phenotype. Overexpression of K8 in mice also disrupted the assembly of the brush border, but ezrin distributed away from the apical membrane in spots along supernumerary IFs. In cytochalasin D-treated cells ezrin localized to a subapical compartment and coimmunoprecipitated with IFs. Overexpression of ezrin in undifferentiated cells showed a Triton-insoluble ezrin compartment negative for phospho-T567 (dormant) ezrin visualized as spots along IFs. Pulse-chase analysis showed that Triton-insoluble, newly synthesized ezrin transiently coimmunoprecipitates with IFs during the first 30 min of the chase. Dormant, but not active (p-T567), ezrin bound in vitro to isolated denatured keratins in Far-Western analysis and to native IFs in pull-down assays. We conclude that a transient association to IFs is an early step in the polarized assembly of apical ezrin in intestinal epithelial cells.

The biology of desmin filaments: how do mutations affect their structure, assembly, and organisation?

Desmin, the major intermediate filament (IF) protein of muscle, is evolutionarily highly conserved from shark to man. Recently, an increasing number of mutations of the desmin gene has been described to be associated with human diseases such as certain skeletal and cardiac myopathies. These diseases are histologically characterised by intracellular aggregates containing desmin and various associated proteins. Although there is progress regarding our knowledge on the cellular function of desmin within the cytoskeleton, the impact of each distinct mutation is currently not understood at all. In order to get insight into how such mutations affect filament assembly and their integration into the cytoskeleton we need to establish IF structure at atomic detail. Recent progress in determining the dimer structure of the desmin-related IF-protein vimentin allows us to assess how such mutations may affect desmin filament architecture.

R120G alphaB-crystallin promotes the unfolding of reduced alpha-lactalbumin and is inherently unstable

alpha-Crystallin is the principal lens protein which, in addition to its structural role, also acts as a molecular chaperone, to prevent aggregation and precipitation of other lens proteins. One of its two subunits, alphaB-crystallin, is also expressed in many nonlenticular tissues, and a natural missense mutation, R120G, has been associated with cataract and desmin-related myopathy, a disorder of skeletal muscles [Vicart P, Caron A, Guicheney P, Li Z, Prevost MC, Faure A, Chateau D, Chapon F, Tome F, Dupret JM, Paulin D & Fardeau M (1998) Nat Genet20, 92-95]. In the present study, real-time 1H-NMR spectroscopy showed that the ability of R120G alphaB-crystallin to stabilize the partially folded, molten globule state of alpha-lactalbumin was significantly reduced in comparison with wild-type alphaB-crystallin. The mutant showed enhanced interaction with, and promoted unfolding of, reduced alpha-lactalbumin, but showed limited chaperone activity for other target proteins. Using NMR spectroscopy, gel electrophoresis, and MS, we observed that, unlike the wild-type protein, R120G alphaB-crystallin is intrinsically unstable in solution, with unfolding of the protein over time leading to aggregation and progressive truncation from the C-terminus. Light scattering, MS, and size-exclusion chromatography data indicated that R120G alphaB-crystallin exists as a larger oligomer than wild-type alphaB-crystallin, and its size increases with time. It is likely that removal of the positive charge from R120 of alphaB-crystallin causes partial unfolding, increased exposure of hydrophobic regions, and enhances its susceptibility to proteolysis, thus reducing its solubility and promoting its aggregation and complexation with other proteins. These characteristics may explain the involvement of R120G alphaB-crystallin with human disease states.

Intermediate filament associated proteins

Intermediate filament associated proteins (IFAPs) coordinate interactions between intermediate filaments (IFs) and other cytoskeletal elements and organelles, including membrane-associated junctions such as desmosomes and hemidesmosomes in epithelial cells, costameres in striated muscle, and intercalated discs in cardiac muscle. IFAPs thus serve as critical connecting links in the IF scaffolding that organizes the cytoplasm and confers mechanical stability to cells and tissues. However, in recent years it has become apparent that IFAPs are not limited to structural crosslinkers and bundlers but also include chaperones, enzymes, adapters, and receptors. IF networks can therefore be considered scaffolding upon which associated proteins are organized and regulated to control metabolic activities and maintain cell homeostasis.

Structural instability caused by a mutation at a conserved arginine in the alpha-crystallin domain of Chinese hamster heat shock protein 27

Mutations in the alpha-crystallin domain of 4 of the small heat shock proteins (sHsp) (Hsp27/HspB1, alphaA-crystallin/ HspB4, alphaB-crystallin/HspB5, and HspB8) are responsible for dominant inherited diseases in humans. One such mutation at a highly conserved arginine residue was shown to cause major conformational defects and intracellular aggregation of alphaA- and alphaB-crystallins and HspB8. Here, we studied the effect of this Arg mutation on the structure and function of Hsp27. Chinese hamster Hsp27 with Arg148 replaced by Gly (Hsp27R148G) formed dimers in vitro and in vivo, which contrasted with the 12- or 24-subunit oligomers formed by the wild-type protein (Hsp27WT). Despite these alterations, Hsp27R148G had a chaperone activity almost as high as Hsp27WT. The dimers of Hsp27R148G did not further deoligomerize on phosphorylation and like the dimers formed by phosphorylated Hsp27WT were not affected by the deletion of the N-terminal WD/EPF (single letter amino acid code) motif, suggesting that mutation of Arg148, deletion of the N-terminal WD/EPF motif, and phosphorylation of Ser90 may produce similar structural perturbations. Nevertheless, the structure of Hsp27R148G appeared unstable, and the mutated protein accumulated as aggregates in many cells. Both a lower basal level of phosphorylation of Hsp27R148G and the coexpression of Hsp27WT could reduce the frequency of formation of these aggregates, suggesting possible mechanisms regulating the onset of the sHsp-mediated inherited diseases.

Heat stress-induced localization of small heat shock proteins in mouse myoblasts: intranuclear lamin A/C speckles as target for αB-crystallin and Hsp25

We examined the effect of heat stress on localization of two sHsps, alphaB-crystallin and Hsp25, and of Hsc70, a member of a different class of heat shock proteins (Hsps), in both undifferentiated and differentiated mouse C2C12 cells. Under normal conditions, alphaB-crystallin and Hsp25 are found in the cytoplasm; only alphaB-crystallin is also found in the nucleus, distributed in a speckled pattern. Hsc70 is found to be homogeneously distributed throughout the cell. On heat stress, all these proteins translocate almost entirely into the nucleus and upon recovery relocate to the cytoplasm. Dual staining experiments using C2C12 myoblasts show that alphaB-crystallin and Hsp25, but not Hsc70, colocalize with the intranuclear lamin A/C and the splicing factor SC-35, suggesting interactions of sHsps and intranuclear lamin A/C. Interestingly, none of these proteins are found in the myotube nuclei. Upon heat stress, only Hsc70 translocates into the myotube nuclei. This differential entry of alphaB-crystallin and Hsp25 into the nuclei of myoblasts and myotubes upon heat stress may have functional role in the development and/or in the maintenance of muscle cells. Our study therefore suggests that these sHsps may be a part of the intranuclear lamin A/C network or stabilizing this specific network.

Cloning and characterization of a thermostable catfish alphaB-crystallin with chaperone-like activity at high temperatures

We have cloned, expressed and characterized catfish alphaB-crystallin (FalphaB). Genomic sequence comparison has revealed conservation of intron splicing sites and coding regions, however, the two intron sequences, 5'- and 3'-untranslated regions of FalphaB gene are shorter than those reported for other vertebrates. In contrast to mammalian homologues with a subunit association ratio (alphaA-crystallin/alphaB-crystallin) of 3:1, alpha-crystallin from catfish lens showed a ratio of 19:1. The biophysical properties and chaperone-like activity of recombinant FalphaB and porcine alphaB-crystallin (PalphaB) were studied and compared by heat denaturation, circular dichroism, intrinsic and dye-binding fluorescence, gel-filtration, and analytical ultracentrifugation. FalphaB shows 50% precipitation occurring at 72 degrees C that is higher than PalphaB at 66 degrees C. Even though FalphaB also possesses more surface hydrophilic regions than PalphaB, FalphaB still possesses higher chaperone activity to prevent aggregation of alcohol dehydrogenase at 60 degrees C. The molecular mass of FalphaB showed a smaller size (450 kDa) than PalphaB (550 kDa), which is also confirmed by analytical ultracentrifugation. In addition, FalphaB possesses better refolding potential after preheating treatment than PalphaB. FalphaB also exhibits higher chaperone-like activity than PalphaB to prevent insulin aggregation induced by dithiothreitol. In contrast to the prevalent notion that fish crystallins generally denature easily, FalphaB with chaperone-like activity appears to be more stable than mammalian homologues towards thermal and chemical denaturation.

Desmin aggregate formation by R120G alphaB-crystallin is caused by altered filament interactions and is dependent upon network status in cells

The R120G mutation in alphaB-crystallin causes desmin-related myopathy. There have been a number of mechanisms proposed to explain the disease process, from altered protein processing to loss of chaperone function. Here, we show that the mutation alters the in vitro binding characteristics of alphaB-crystallin for desmin filaments. The apparent dissociation constant of R120G alphaB-crystallin was decreased while the binding capacity was increased significantly and as a result, desmin filaments aggregated. These data suggest that the characteristic desmin aggregates seen as part of the disease histopathology can be caused by a direct, but altered interaction of R120G alphaB-crystallin with desmin filaments. Transfection studies show that desmin networks in different cell backgrounds are not equally affected. Desmin networks are most vulnerable when they are being made de novo and not when they are already established. Our data also clearly demonstrate the beneficial role of wild-type alphaB-crystallin in the formation of desmin filament networks. Collectively, our data suggest that R120G alphaB-crystallin directly promotes desmin filament aggregation, although this gain of a function can be repressed by some cell situations. Such circumstances in muscle could explain the late onset characteristic of the myopathies caused by mutations in alphaB-crystallin.

Digital image capture and quantification of subtle lens opacities in rodents

A rapid, sensitive, and cost-effective method is reported for the subjective and objective documentation of subtle opacities in lenses of unanesthetized transgenic mice or selenite-injected rat pups as models for cataract formation. Animal eyes were dilated with eye drops and the animal was positioned in front of a Nikon FS2 photo slit lamp. Slit-lamp observations were recorded using a Canon Optura Pi digital video recorder. High-quality images of opacifying lenses were captured from the video and quantified using densitometry at progressive stages of opacification. In mice, targeted genomic deletion of the proteins CP49 (a lens-specific filament) or Six5 (a model for myotonic dystrophy) resulted in subtle cataracts that were easily recorded and quantified using this instrumentation. In rats, the early progressive changes leading to a dense nuclear opacity caused by selenite injection were easily documented using this instrumentation. Low-cost components combined with a conventional slit-lamp ophthalmoscope were used to capture high-quality images of selected stages of cataract formation for quantitative analysis using commercial software.

αB-Crystallin Modulates Protein Aggregation of Abnormal Desmin

αB-crystallin (CryAB) is the most abundant small heat shock protein in the heart. Upregulation of CryAB in desmin-related myopathy and its downregulation in end-stage congestive heart failure have both been reported. We previously demonstrated via cardiac-specific transgenesis that modest increases in normal CryAB are not detrimental to the heart, whereas expression of the R120G mutation of CryAB caused a desminopathy. It is generally believed that CryAB plays an important role in protecting the intermediate filaments, but the underlying mechanism is unclear. We hypothesized that CryAB protects the desmin filaments via preventing abnormal desmin protein from aggregating adversely. To test this hypothesis in vivo, mice expressing a desmin mutation that causes a desmin-related cardiomyopathy (D7) were bred into the R120G-CryAB transgenic (TG) background to examine the accumulation and aberrant aggregation of desmin protein. Despite lower mRNA expression of D7-des than in the D7-des TG hearts, the double-TG myocardium exhibited significantly higher desmin protein levels and dramatically more aberrant desmin aggregates than the D7-des TG hearts. The double-TG mice displayed a significantly stronger cardiac hypertrophic response, with the mice dying of congestive heart failure before 7 weeks. To explore the ability of wild-type (WT) CryAB to protect against mutant desmin, a desmin mutant was expressed in both the conventional and WT-CryAB stably transfected HEK cells. Significantly less aberrant desmin aggregation was observed in the WT-CryAB–overexpressing cells than in the HEK cells. The results suggest that CryAB modulates abnormal desmin aggregation and can serve a cardioprotective role.

Structural and functional defects caused by point mutations in the alpha-crystallin domain of a bacterial alpha-heat shock protein

The diverse family of alpha-crystallin-type small heat shock proteins (alpha-Hsps or sHsps) is characterised by a central, moderately conserved alpha-crystallin domain. Oligomerisation followed by dissociation of subparticles is thought to be a prerequisite for chaperone function. We demonstrate that HspH, a bacterial alpha-Hsp from the soybean-symbiont Bradyrhizobium japonicum, assembles into dynamic complexes freely exchanging subunits with homologous and heterologous complexes. The importance of the alpha-crystallin domain for oligomerisation and chaperone activity was tested by site-directed mutagenesis of 12 different residues. In contrast to mammalian alpha-Hsps, the majority of these mutations elicited severe structural and functional defects in HspH. The individual exchange of five amino acid residues throughout the alpha-crystallin domain was found to compromise oligomerisation to various degrees. Assembly defects resulting in complexes of reduced size correlated with greatly decreased or abolished chaperone activity, reinforcing that complete oligomerisation is required for functionality. Mutation of a highly conserved glycine (G114) at the C-terminal end of the alpha-crystallin domain specifically impaired chaperone activity without interfering with oligomerisation properties, indicating that this residue is critical for substrate interaction. The structural and functional importance of this and other residues is discussed in the context of a modeled three-dimensional structure of HspH.