Phosphorylation-induced change of the oligomerization state of alpha B-crystallin

Lens Cytoskeleton: An Update on the Etiopathogenesis of Human Cataracts

A cataract is a loss of the transparency of a normal crystalline lens. Multiple factors, including age as the major risk factor for cataracts, can disturb the transparency of the crystalline lens due to cumulative damage from environmental insults to proteins, particularly crystallins. Lens proteins do not turnover, and crystallins undergo extensive post-translational modifications (PTMs) with age in order to interact with each other and maintain their soluble basis for lens transparency. These PTMs include truncation, oxidation, deamidation, acetylation, phosphorylation, and glycosylation. Cataract formation, apart from protein PTMs, involves protein crosslinking, protein insolubilization, and aggregation. Oxidation is a key feature in age-related cataract formation. Due to the role of genetic and environmental factors, as well as its variable clinical presentation, we consider cataracts to be a multifactorial disease. The preliminary results of our study indicate that proteins implicated in the pathway of a structural constituent of the eye lens (BFSP1, BFSP2, CRYAA, CRYAB, CRYBA, CRYBB, CRYGC, CRYGD, CRYGS, KRTs, and VIM), together with AQP1 and AQP5, may also be involved in lens aging.

Identification of CryAB as a target of NUAK kinase activity in Drosophila muscle tissue

Phosphorylation reactions performed by protein kinases are one of the most studied post-translational modifications within cells. Much is understood about conserved residues within protein kinase domains that perform catalysis of the phosphotransfer reaction, yet the identity of the target substrates and downstream biological effects vary widely among cells, tissues, and organisms. Here, we characterize key residues essential for NUAK kinase activity in Drosophila melanogaster myogenesis and homeostasis. Creation of a NUAK kinase-dead mutation using Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 results in lethality at the embryo to larval transition, while loss of NUAK catalytic function later in development produces aggregation of the chaperone protein αB-crystallin/CryAB in muscle tissue. Yeast 2-hybrid assays demonstrate a physical interaction between NUAK and CryAB. We further show that a phospho-mimetic version of NUAK promotes the phosphorylation of CryAB and this post-translational modification occurs at 2 previously unidentified phosphosites that are conserved in the primary sequence of human CryAB. Mutation of these serine residues in D. melanogaster NUAK abolishes CryAB phosphorylation, thus, proving their necessity at the biochemical level. These studies together highlight the importance of kinase activity regulation and provide a platform to further explore muscle tissue proteostasis.

Small Heat Shock Proteins in Retinal Diseases

This review summarizes the latest findings on small heat shock proteins (sHsps) in three major retinal diseases: glaucoma, diabetic retinopathy, and age-related macular degeneration. A general description of the structure and major cellular functions of sHsps is provided in the introductory remarks. Their role in specific retinal diseases, highlighting their regulation, role in pathogenesis, and possible use as therapeutics, is discussed.

Phosphorylation activates the yeast small heat shock protein Hsp26 by weakening domain contacts in the oligomer ensemble

Hsp26 is a small heat shock protein (sHsp) from S. cerevisiae. Its chaperone activity is activated by oligomer dissociation at heat shock temperatures. Hsp26 contains 9 phosphorylation sites in different structural elements. Our analysis of phospho-mimetic mutations shows that phosphorylation activates Hsp26 at permissive temperatures. The cryo-EM structure of the Hsp26 40mer revealed contacts between the conserved core domain of Hsp26 and the so-called thermosensor domain in the N-terminal part of the protein, which are targeted by phosphorylation. Furthermore, several phosphorylation sites in the C-terminal extension, which link subunits within the oligomer, are sensitive to the introduction of negative charges. In all cases, the intrinsic inhibition of chaperone activity is relieved and the N-terminal domain becomes accessible for substrate protein binding. The weakening of domain interactions within and between subunits by phosphorylation to activate the chaperone activity in response to proteotoxic stresses independent of heat stress could be a general regulation principle of sHsps.

H101G Mutation in Rat Lens αB-Crystallin Alters Chaperone Activity and Divalent Metal Ion Binding

BACKGROUND

The molecular chaperone function of αB-crystallins is heavily involved in maintaining lens transparency and the development of cataracts.

OBJECTIVE

To study whether divalent metal ion binding improves the stability and αB-crystallin chaperone activity.

METHOD

In this study, we have developed an H101G αB-crystallin mutant and compared the surface hydrophobicity, chaperone activity, and secondary and tertiary structure with the wild type in the presence and absence of metal ions.

RESULTS

Substitution of His101 with glycine resulted in structural and functional changes. Spectral analysis and chaperone-like activity assays showed that substitution of glycine resulted in a higher percentage of random coils, increased hydrophobicity, and 22±2% higher chaperone-like activity. Whereas in the presence of the Cu²⁺ ion, H101G exhibited 32±1% less chaperone-like activity compared to the wild type.

CONCLUSION

Cu²⁺ has been reported to enhance the chaperone-like activity of lens α-crystallin. Our results indicate that H101 is the predominant Cu²⁺binding site, and the mutation resulted in a partial unfolding that impaired the binding of Cu²⁺ to H101 residue. In conclusion, this study further helps to understand the important binding site for Cu²⁺ to αB-crystallin.

HspB5/αB-crystallin phosphorylation at S45 and S59 is essential for protection of the dendritic tree of rat hippocampal neurons

Rarefaction of the dendritic tree leading to neuronal dysfunction is a hallmark of many neurodegenerative diseases and we have shown previously that heat shock protein B5 (HspB5)/αB-crystallin is able to increase dendritic complexity in vitro. The aim of this study was to investigate if this effect is also present in vivo, if HspB5 can counteract dendritic rarefaction under pathophysiological conditions and the impact of phosphorylation of HspB5 in this process. HspB5 and eight mutants inhibiting or mimicking phosphorylation at the three phosphorylation sites serine (S)19, S45, and S59 were over-expressed in cultured rat hippocampal neurons with subsequent investigation of the complexity of the dendritic tree. Sholl analysis revealed significant higher complexity of the dendritic tree after over-expression of wild-type HspB5 and the mutant HspB5-AEE. All other mutants showed no or minor effects. For in vivo investigation in utero electroporation of mouse embryos was applied. At embryonal day E15.5 the respective plasmids were injected, cornu ammonis 1 (CA1) pyramidal cells transfected by electroporation and their basal dendritic trees were analyzed at post-natal day P15. In vivo, HspB5 and HspB5-AEE led to an increase of total dendritic length as well as a higher complexity. Finally, the dendritic effect of HspB5 was investigated under a pathophysiological condition, that is, iron deficiency which reportedly results in dendritic rarefaction. HspB5 and HspB5-AEE but not the non-phosphorylatable mutant HspB5-AAA significantly counteracted the dendritic rarefaction. Thus, our data suggest that up-regulation and selective phosphorylation of HspB5 in neurodegenerative diseases may preserve dendritic morphology and counteract neuronal dysfunction.

Role of exosomes in malignant glioma: microRNAs and proteins in pathogenesis and diagnosis

Malignant gliomas are the most common and deadly type of central nervous system tumors. Despite some advances in treatment, the mean survival time remains only about 1.25 years. Even after surgery, radiotherapy and chemotherapy, gliomas still have a poor prognosis. Exosomes are the most common type of extracellular vesicles with a size range of 30 to 100 nm, and can act as carriers of proteins, RNAs, and other bioactive molecules. Exosomes play a key role in tumorigenesis and resistance to chemotherapy or radiation. Recent evidence has shown that exosomal microRNAs (miRNAs) can be detected in the extracellular microenvironment, and can also be transferred from cell to cell via exosome secretion and uptake. Therefore, many recent studies have focused on exosomal miRNAs as important cellular regulators in various physiological and pathological conditions. A variety of exosomal miRNAs have been implicated in the initiation and progression of gliomas, by activating and/or inhibiting different signaling pathways. Exosomal miRNAs could be used as therapeutic agents to modulate different biological processes in gliomas. Exosomal miRNAs derived from mesenchymal stem cells could also be used for glioma treatment. The present review summarizes the exosomal miRNAs that have been implicated in the pathogenesis, diagnosis and treatment of gliomas. Moreover, exosomal proteins could also be involved in glioma pathogenesis. Exosomal miRNAs and proteins could also serve as non-invasive biomarkers for prognosis and disease monitoring. Video Abstract.

Proteinaceous Transformers: Structural and Functional Variability of Human sHsps

The proteostasis network allows organisms to support and regulate the life cycle of proteins. Especially regarding stress, molecular chaperones represent the main players within this network. Small heat shock proteins (sHsps) are a diverse family of ATP-independent molecular chaperones acting as the first line of defense in many stress situations. Thereby, the promiscuous interaction of sHsps with substrate proteins results in complexes from which the substrates can be refolded by ATP-dependent chaperones. Particularly in vertebrates, sHsps are linked to a broad variety of diseases and are needed to maintain the refractive index of the eye lens. A striking key characteristic of sHsps is their existence in ensembles of oligomers with varying numbers of subunits. The respective dynamics of these molecules allow the exchange of subunits and the formation of hetero-oligomers. Additionally, these dynamics are closely linked to the chaperone activity of sHsps. In current models a shift in the equilibrium of the sHsp ensemble allows regulation of the chaperone activity, whereby smaller oligomers are commonly the more active species. Different triggers reversibly change the oligomer equilibrium and regulate the activity of sHsps. However, a finite availability of high-resolution structures of sHsps still limits a detailed mechanistic understanding of their dynamics and the correlating recognition of substrate proteins. Here we summarize recent advances in understanding the structural and functional relationships of human sHsps with a focus on the eye-lens αA- and αB-crystallins.

Come Together: Protein Assemblies, Aggregates and the Sarcostat at the Heart of Cardiac Myocyte Homeostasis

Homeostasis in vertebrate systems is contingent on normal cardiac function. This, in turn, depends on intricate protein-based cellular machinery, both for contractile function, as well as, durability of cardiac myocytes. The cardiac small heat shock protein (csHsp) chaperone system, highlighted by αB-crystallin (CRYAB), a small heat shock protein (sHsp) that forms ∼3–5% of total cardiac mass, plays critical roles in maintaining proteostatic function via formation of self-assembled multimeric chaperones. In this work, we review these ancient proteins, from the evolutionarily preserved role of homologs in protists, fungi and invertebrate systems, as well as, the role of sHsps and chaperones in maintaining cardiac myocyte structure and function. We propose the concept of the “sarcostat” as a protein quality control mechanism in the sarcomere. The roles of the proteasomal and lysosomal proteostatic network, as well as, the roles of the aggresome, self-assembling protein complexes and protein aggregation are discussed in the context of cardiac myocyte homeostasis. Finally, we will review the potential for targeting the csHsp system as a novel therapeutic approach to prevent and treat cardiomyopathy and heart failure.

Fam20C regulates protein secretion by Cab45 phosphorylation

The TGN is a key compartment for the sorting and secretion of newly synthesized proteins. At the TGN, soluble proteins are sorted based on the instructions carried in their oligosaccharide backbones or by a Ca2+-mediated process that involves the cargo-sorting protein Cab45. Here, we show that Cab45 is phosphorylated by the Golgi-specific protein kinase Fam20C. Mimicking of phosphorylation translocates Cab45 into TGN-derived vesicles, which goes along with an increased export of LyzC, a Cab45 client. Our findings demonstrate that Fam20C plays a key role in the export of Cab45 clients by fine-tuning Cab45 oligomerization and thus impacts Cab45 retention in the TGN.

Regulation of small heat-shock proteins by hetero-oligomer formation

Small heat-shock proteins (sHsps) compose the most widespread family of molecular chaperones. The human genome encodes 10 different sHsps (HspB1-10). It has been shown that HspB1 (Hsp27), HspB5 (αB-crystallin), and HspB6 (Hsp20) can form hetero-oligomers in vivo However, the impact of hetero-oligomerization on their structure and chaperone mechanism remains enigmatic. Here, we analyzed hetero-oligomer formation in human cells and in vitro using purified proteins. Our results show that the effect of hetero-oligomer formation on the composition of the sHsp ensembles and their chaperone activities depends strongly on the respective sHsps involved. We observed that hetero-oligomer formation between HspB1 and HspB5 leads to an ensemble that is dominated by species larger than the individual homo-oligomers. In contrast, the interaction of dimeric HspB6 with either HspB1 or HspB5 oligomers shifted the ensemble toward smaller oligomers. We noted that the larger HspB1-HspB5 hetero-oligomers are less active and that HspB6 activates HspB5 by dissociation to smaller oligomer complexes. The chaperone activity of HspB1-HspB6 hetero-oligomers, however, was modulated in a substrate-specific manner, presumably due to the specific enrichment of an HspB1-HspB6 heterodimer. These heterodimeric species may allow the tuning of the chaperone properties toward specific substrates. We conclude that sHsp hetero-oligomerization exerts distinct regulatory effects depending on the sHsps involved.

The impact of different mutations at arginine141 on the structure, subunit exchange dynamics and chaperone activity of Hsp16.3

Hsp16.3, a molecular chaperone, plays a vital role in the growth and survival of Mycobacterium tuberculosis inside the host. We previously reported that deletion of three amino acid residues (¹⁴² STN¹⁴⁴ ) from C-terminal extension (CTE) of Hsp16.3 triggers its structural perturbation and increases its chaperone activity, which reaches its apex upon the deletion of its entire CTE (¹⁴¹ RSTN¹⁴⁴ ). Thus, we hypothesized that Arg141 (R141) and Ser142 (S142) in the CTE of Hsp16.3 possibly hold the key in maintaining its native-like structure and chaperone activity. To test this hypothesis, we generated two deletion mutants in which R141 and S142 were deleted individually (Hsp16.3ΔR141 and Hsp16.3ΔS142) and three substitution mutants in which R141 was replaced by lysine (Hsp16.3R141K), alanine (Hsp16.3R141A), and glutamic acid (Hsp16.3R141E), respectively. Hsp16.3ΔS142 or Hsp16.3R141K mutant has native-like structure and chaperone activity. Deletion of R141 from the CTE (Hsp16.3ΔR141) perturbs the secondary and tertiary structure, lowers the subunit exchange dynamics and decreases the chaperone activity of Hsp16.3. But, the substitution of R141 with alanine (Hsp16.3R141A) or glutamic acid (Hsp16.3R141E) perturbs its secondary and tertiary structure. Surprisingly, such charge tampering of R141 enhances the subunit exchange dynamics and chaperone activity of Hsp16.3. Interestingly, neither the deletion of R141/S142 nor the substitution of R141 with lysine, alanine and glutamic acid affects the oligomeric mass/size of Hsp16.3. Overall, our study suggests that R141 (especially the positive charge on R141) plays a crucial role in maintaining the native-like structure as well as in regulating subunit exchange dynamics and chaperone activity of Hsp16.3.

Unconventional secretion of α-Crystallin B requires the Autophagic pathway and is controlled by phosphorylation of its serine 59 residue

α-Crystallin B (CRYAB or HspB5) is a chaperone member of the small heat-shock protein family that prevents aggregation of many cytosolic client proteins by means of its ATP-independent holdase activity. Surprisingly, several reports show that CRYAB exerts a protective role also extracellularly, and it has been recently demonstrated that CRYAB is secreted from human retinal pigment epithelial cells by an unconventional secretion pathway that involves multi-vesicular bodies. Here we show that autophagy is crucial for this unconventional secretion pathway and that phosphorylation at serine 59 residue regulates CRYAB secretion by inhibiting its recruitment to the autophagosomes. In addition, we found that autophagosomes containing CRYAB are not able to fuse with lysosomes. Therefore, CRYAB is capable to highjack and divert autophagosomes toward the exocytic pathway, inhibiting their canonical route leading to the lysosomal compartment. Potential implications of these findings in the context of disease-associated mutant proteins turn-over are discussed.

Spatiotemporal changes in the human lens proteome: Critical insights into long-lived proteins

The ocular lens is a unique tissue that contains an age gradient of cells and proteins ranging from newly differentiated cells containing newly synthesized proteins to cells and proteins that are as old as the organism. Thus, the ocular lens is an excellent model for studying long-lived proteins (LLPs) and the effects of aging and post-translational modifications on protein structure and function. Given the architecture of the lens, with young fiber cells in the outer cortex and the oldest cells in the lens nucleus, spatially-resolved studies provide information on age-specific protein changes. In this review, experimental strategies and proteomic methods that have been used to examine age-related and cataract-specific changes to the human lens proteome are described. Measured spatio-temporal changes in the human lens proteome are summarized and reveal a highly consistent, time-dependent set of modifications observed in transparent human lenses. Such measurements have led to the discovery of cataract-specific modifications and the realization that many animal systems are unsuitable to study many of these modifications. Mechanisms of protein modifications such as deamidation, racemization, truncation, and protein-protein crosslinking are presented and the implications of such mechanisms for other long-lived proteins in other tissues are discussed in the context of age-related neurological diseases. A comprehensive understanding of LLP modifications will enhance our ability to develop new therapies for the delay, prevention or reversal of age-related diseases.

Interplay of disordered and ordered regions of a human small heat shock protein yields an ensemble of 'quasi-ordered' states

Small heat shock proteins (sHSPs) are nature's 'first responders' to cellular stress, interacting with affected proteins to prevent their aggregation. Little is known about sHSP structure beyond its structured α-crystallin domain (ACD), which is flanked by disordered regions. In the human sHSP HSPB1, the disordered N-terminal region (NTR) represents nearly 50% of the sequence. Here, we present a hybrid approach involving NMR, hydrogen-deuterium exchange mass spectrometry, and modeling to provide the first residue-level characterization of the NTR. The results support a model in which multiple grooves on the ACD interact with specific NTR regions, creating an ensemble of 'quasi-ordered' NTR states that can give rise to the known heterogeneity and plasticity of HSPB1. Phosphorylation-dependent interactions inform a mechanism by which HSPB1 is activated under stress conditions. Additionally, we examine the effects of disease-associated NTR mutations on HSPB1 structure and dynamics, leveraging our emerging structural insights.

Mechanisms of Small Heat Shock Proteins

Small heat shock proteins (sHSPs) are ATP-independent chaperones that delay formation of harmful protein aggregates. sHSPs' role in protein homeostasis has been appreciated for decades, but their mechanisms of action remain poorly understood. This gap in understanding is largely a consequence of sHSP properties that make them recalcitrant to detailed study. Multiple stress-associated conditions including pH acidosis, oxidation, and unusual availability of metal ions, as well as reversible stress-induced phosphorylation can modulate sHSP chaperone activity. Investigations of sHSPs reveal that sHSPs can engage in transient or long-lived interactions with client proteins depending on solution conditions and sHSP or client identity. Recent advances in the field highlight both the diversity of function within the sHSP family and the exquisite sensitivity of individual sHSPs to cellular and experimental conditions. Here, we will present and highlight current understanding, recent progress, and future challenges.

Small Heat Shock Proteins, Big Impact on Protein Aggregation in Neurodegenerative Disease

Misfolding, aggregation, and aberrant accumulation of proteins are central components in the progression of neurodegenerative disease. Cellular molecular chaperone systems modulate proteostasis, and, therefore, are primed to influence aberrant protein-induced neurotoxicity and disease progression. Molecular chaperones have a wide range of functions from facilitating proper nascent folding and refolding to degradation or sequestration of misfolded substrates. In disease states, molecular chaperones can display protective or aberrant effects, including the promotion and stabilization of toxic protein aggregates. This seems to be dependent on the aggregating protein and discrete chaperone interaction. Small heat shock proteins (sHsps) are a class of molecular chaperones that typically associate early with misfolded proteins. These interactions hold proteins in a reversible state that helps facilitate refolding or degradation by other chaperones and co-factors. These sHsp interactions require dynamic oligomerization state changes in response to diverse cellular triggers and, unlike later steps in the chaperone cascade of events, are ATP-independent. Here, we review evidence for modulation of neurodegenerative disease-relevant protein aggregation by sHsps. This includes data supporting direct physical interactions and potential roles of sHsps in the stewardship of pathological protein aggregates in brain. A greater understanding of the mechanisms of sHsp chaperone activity may help in the development of novel therapeutic strategies to modulate the aggregation of pathological, amyloidogenic proteins. sHsps-targeting strategies including modulators of expression or post-translational modification of endogenous sHsps, small molecules targeted to sHsp domains, and delivery of engineered molecular chaperones, are also discussed.

Cellular Functions and Mechanisms of Action of Small Heat Shock Proteins

Small heat shock proteins (sHsps) constitute a diverse chaperone family that shares the α-crystallin domain, which is flanked by variable, disordered N- and C-terminal extensions. sHsps act as the first line of cellular defense against protein unfolding stress. They form dynamic, large oligomers that represent inactive storage forms. Stress conditions cause a rapid increase in cellular sHsp levels and trigger conformational rearrangements, resulting in exposure of substrate-binding sites and sHsp activation. sHsps bind to early-unfolding intermediates of misfolding proteins in an ATP-independent manner and sequester them in sHsp/substrate complexes. Sequestration protects substrates from further uncontrolled aggregation and facilitates their refolding by ATP-dependent Hsp70-Hsp100 disaggregases. Some sHsps with particularly strong sequestrase activity, such as yeast Hsp42, are critical factors for forming large, microscopically visible deposition sites of misfolded proteins in vivo. These sites are organizing centers for triaging substrates to distinct quality control pathways, preferentially Hsp70-dependent refolding and selective autophagy.

CRYAB protects cardiomyocytes against heat stress by preventing caspase-mediated apoptosis and reducing F-actin aggregation

CRYAB is a small heat shock protein (sHSP) that has previously been shown to protect the heart against various cellular stresses; however, its precise function in myocardial cell injury caused by heat stress remains unclear. This study aimed to investigate the molecular mechanism by which CRYAB protects cardiomyocytes against heat stress. We constructed two H9C2 cell lines that stably express CRYAB protein to differing degrees: CRYAB-5 and CRYAB-7. Both CRYAB-5 and CRYAB-7 showed significantly reduced granular degeneration and vacuolar degeneration following heat stress compared to control cells. In addition, CRYAB overexpression in H9C2 cells relieved cell cycle proportion at the G0/G1 phase following heat stress compared to control cells. These protective effects were associated with the level of CRYAB protein expression. Our immunofluorescence analysis showed CRYAB could translocate from the cytoplasm to the nucleus under heat stress conditions, but that CRYAB co-localized with F-actin (which accumulates under stress conditions). Indeed, overexpression of CRYAB significantly reduced the aggregation of F-actin in H9C2 cells caused by heat stress. Furthermore, overexpressing CRYAB protein significantly reduced the apoptosis of cardiomyocytes induced by heat stress, likely by reducing the expression of cleaved-caspase 3. Collectively, our results show overexpression of CRYAB significantly increases the heat resistance of H9C2 cardiomyocytes, likely by reducing F-actin aggregation (thus stabilizing the cytoskeleton), regulating the cell cycle, and preventing caspase-mediated apoptosis.

HSPB5 engages multiple states of a destabilized client to enhance chaperone activity in a stress-dependent manner

Small heat shock proteins (sHSPs) delay protein aggregation in an ATP-independent manner by interacting with client proteins that are in states susceptible to aggregation, including destabilized states related to cellular stress. Up-regulation of sHSPs under stress conditions supports their critical role in cellular viability. Widespread distribution of sHSPs in most organisms implies conservation of function, but it remains unclear whether sHSPs implement common or distinct mechanisms to delay protein aggregation. Comparisons among various studies are confounded by the use of different model client proteins, different assays for both aggregation and sHSP/client interactions, and variable experimental conditions used to mimic cellular stress. To further define sHSP/client interactions and their relevance to sHSP chaperone function, we implemented multiple strategies to characterize sHSP interactions with α-lactalbumin, a model client whose aggregation pathway is well defined. We compared the chaperone activity of human αB-crystallin (HSPB5) with HSPB5 variants that mimic states that arise under conditions of cellular stress or disease. The results show that these closely related sHSPs vary not only in their activity under identical conditions but also in their interactions with clients. Importantly, under nonstress conditions, WT HSPB5 delays client aggregation solely through transient interactions early in the aggregation pathway, whereas HSPB5 mutants that mimic stress-activated conditions can also intervene at later stages of the aggregation pathway to further delay client protein aggregation.

Small heat shock proteins: Simplicity meets complexity

Small heat shock proteins (sHsps) are a ubiquitous and ancient family of ATP-independent molecular chaperones. A key characteristic of sHsps is that they exist in ensembles of iso-energetic oligomeric species differing in size. This property arises from a unique mode of assembly involving several parts of the subunits in a flexible manner. Current evidence suggests that smaller oligomers are more active chaperones. Thus, a shift in the equilibrium of the sHsp ensemble allows regulating the chaperone activity. Different mechanisms have been identified that reversibly change the oligomer equilibrium. The promiscuous interaction with non-native proteins generates complexes that can form aggregate-like structures from which native proteins are restored by ATP-dependent chaperones such as Hsp70 family members. In recent years, this basic paradigm has been expanded, and new roles and new cofactors, as well as variations in structure and regulation of sHsps, have emerged.

Small heat shock protein speciation: novel non-canonical 44 kDa HspB5-related protein species in rat and human tissues

When analyzing small stress proteins of rat and human tissues by electrophoretic methods followed by western blotting, and using the anti-HspB1/anti-HspB5 antibody clone 8A7, we unexpectedly found a protein with a molecular mass of ~44 kDa. On two-dimensional gels, this protein resolved into four distinct species. Electrophoretic and immunological evidence suggests that this 44 kDa protein is a derivative of HspB5, most likely a covalently linked HspB5 dimer. This HspB5-like 44 kDa protein (HspB5L-P44) is particularly abundant in rat heart, brain, and renal cortex and glomeruli. HspB5L-P44 was also found in human brains, including those from patients with Alexander disease, a condition distinguished by cerebral accumulation of HspB5. Gray matter of such a patient contained an elevated amount of HspB5L-P44. A spatial model of structurally ordered dimeric HspB5 α-crystallin domains reveals the exposed and adjacent position of the two peptide segments homologous to the HspB1-derived 8A7 antigen determinant peptide (epitope). This explains the observed extraordinary high avidity of the 8A7 antibody towards HspB5L-P44, as opposed to commonly used HspB5-specific antibodies which recognize other epitopes. This scenario also explains the remarkable fact that no previous study reported the existence of HspB5L-P44 species. Exposure of rat endothelial cells to UV light, an oxidative stress condition, temporarily increased HspB5L-P44, suggesting physiological regulation of the dimerization. The existence of HspB5L-P44 supports the protein speciation discourse and fits to the concept of the protein code, according to which the expression of a given gene is reflected only by the complete set of the derived protein species.

The Proteome of Cataract Markers: Focus on Crystallins

Cataract is a major cause of blindness worldwide. It is characterized by lens opacification and is accompanied by extensive posttranslational modifications (PTMs) in various proteins. PTMs play an essential role in lens opacification. Several PTMs have been described in proteins isolated from relatively old human lenses, including phosphorylation, deamidation, racemization, truncation, acetylation, and methylation. An overwhelming majority of previous cataract proteomic studies have exclusively focused on crystallin proteins, which are the most abundant proteome components of the lens. To investigate the proteome of cataract markers, this chapter focuses on the proteomic research on the functional relevance of the major PTMs in crystallins of human cataractous lenses. Elucidating the role of these modifications in cataract formation has been a challenging task because they are among the most difficult PTMs to study analytically. The proteomic status of some amides presents similar properties in normal aged and cataractous lenses, whereas some may undergo greater PTMs in cataract. Therefore, it is of great importance to review the current proteomic research on crystallins, the major protein markers in different types of cataract, to elucidate the pathogenesis of this major human-blinding condition.

Dephosphorylation enhances postmortem degradation of myofibrillar proteins

Protein degradation is primarily responsible for postmortem meat tenderization, which might be affected by phosphorylation. The objective of this study was to investigate the effect of phosphorylation on myofibrillar proteins degradation in muscle during postmortem. Here we modulated the phosphorylation status of protein by protein kinase inhibitor and phosphatase inhibitor, and the effect of these inhibitors on myofibrillar protein degradation was evaluated. Generally, myofibril fragmentation index of samples with lower phosphorylation level was higher. Troponin T and heat shock protein 27 were degraded faster in protein kinase inhibited (low phosphorylation level) muscle, compared with the other two groups, while the degradation of desmin was not affected by inhibitors. Meanwhile, myosin heavy chain, actin and tropomyosin showed limited degradation in postmortem muscle. This study showed that dephosphorylation enhances the degradation of some myofibrillar proteins, indicating that protein phosphorylation may play an important role in postmortem meat tenderization.

Using bicistronic constructs to evaluate the chaperone activities of heat shock proteins in cells

Heat shock proteins (Hsps) are molecular chaperones that prevent the aggregation of client proteins by facilitating their refolding, or trafficking them for degradation. The chaperone activities of Hsps are dependent on dynamic protein-protein interactions, including their oligomerisation into large multi-subunit complexes. Thus, tagging Hsps with fluorescent proteins can interfere with their chaperone activity. To overcome this limitation, we have exploited bicistronic constructs for the concurrent expression of a non-tagged Hsp and fluorescent reporter from a single mRNA in cells. We used the Hsp-encoding bicistronic constructs in a cell-based model of protein aggregation, using a destabilised (mutant) form of firefly luciferase (mFluc) that forms inclusion bodies in cells. Expression of Hsp40, Hsp70, or Hsp40 and Hsp70 in cells expressing mFluc decreased the formation of inclusion bodies by 25–46% compared to controls. Moreover, there was a concentration-dependent decrease in the proportion of cells with inclusions when Hsp70, or Hsp40 and Hsp70 were co-expressed with mFluc in cells. The Hsp-encoding bicistronic constructs enable transfection efficiencies and concentration-dependent effects of Hsp expression to be determined using fluorescence based techniques, without the need to tag the Hsp with a fluorescent protein.

Effect of N-terminal region of nuclear Drosophila melanogaster small heat shock protein DmHsp27 on function and quaternary structure

The importance of the N-terminal region (NTR) in the oligomerization and chaperone-like activity of the Drosophila melanogaster small nuclear heat shock protein DmHsp27 was investigated by mutagenesis using size exclusion chromatography and native gel electrophoresis. Mutation of two sites of phosphorylation in the N-terminal region, S58 and S75, did not affect the oligomerization equilibrium or the intracellular localization of DmHsp27 when transfected into mammalian cells. Deletion or mutation of specific residues within the NTR region delineated a motif (FGFG) important for the oligomeric structure and chaperone-like activity of this sHsp. While deletion of the full N-terminal region, resulted in total loss of chaperone-like activity, removal of the (FGFG) at position 29 to 32 or single mutation of F29A/Y, G30R and G32R enhanced oligomerization and chaperoning capacity under non-heat shock conditions in the insulin assay suggesting the importance of this site for chaperone activity. Unlike mammalian sHsps DmHsp27 heat activation leads to enhanced association of oligomers to form large structures of approximately 1100 kDa. A new mechanism of thermal activation for DmHsp27 is presented.

Phosphorylation of αB-crystallin in the myocardium: Analysis of relations with aging and cardiomyopathy

Phosphorylation is a major post-translational modification of αB-crystallin (CryaB) and determines this protein's chaperone activity, intracellular distribution, translocation, and cytoprotective functions. Phosphorylation of CryaB manifests itself as either beneficial or deleterious consequences depending on the extent of phosphorylation and interaction with the cytoskeleton. Herein, for the first time, we compared the age-related alterations of the expression and phosphorylation (on Ser59: pS59) of CryaB in the myocardium of Wistar and senescence-accelerated OXYS rats. The latters, as we demonstrated here, develop cardiomyopathy by the age of 12 months against the background of hypertension. Rats at the age of 20 days, 3, 12, and 24 months were used. The expression of CryaB mRNA (studied by RT-PCR) and of the CryaB protein (analyzed by western blotting) increased with age in the myocardium of both Wistar and OXYS rats, but only at the age of 24 months did their levels become lower in OXYS rats. Phosphorylation of CryaB increased with age in all rats. There was no association of cardiomyopathy with the pS59-CryaB amount in the detergent-soluble fraction either. Moreover, immunostaining of the myocardium revealed that the amount of pS59-CryaB was greater in OXYS rats than in the control animals. This phenomenon was the result of translocation of pS59-CryaB from the detergent-soluble protein fraction to the detergent-insoluble one. The amount of pS59-CryaB in striated sarcomeres (detergent-insoluble) of the myocardium increased with age in both strains but faster in the myocardium of OXYS rats, and its accumulation preceded the development of cardiomyopathy. Translocation of phosphorylated CryaB to sarcomeres affects functional and structural properties (of cardiomyocytes) that are crucial for contractile function and myofibrillar organization and may be an important component of an endogenous mechanism of aging of the myocardium.

HspB5/αB-crystallin increases dendritic complexity and protects the dendritic arbor during heat shock in cultured rat hippocampal neurons

The small heat shock protein ΗspΒ5 (αB-crystallin) exhibits generally cytoprotective functions and possesses powerful neuroprotective capacity in the brain. However, little is known about the mode of action of ΗspΒ5 or other members of the HspB family particularly in neurons. To get clues of the neuronal function of HspBs, we overexpressed several HspBs in cultured rat hippocampal neurons and investigated their effect on neuronal morphology and stress resistance. Whereas axon length and synapse density were not affected by any HspB, dendritic complexity was enhanced by HspB5 and, to a lesser extent, by HspB6. Furthermore, we could show that this process was dependent on phosphorylation, since a non-phosphorylatable mutant of HspB5 did not show this effect. Rarefaction of the dendritic arbor is one hallmark of several neurodegenerative diseases. To investigate if HspB5, which is upregulated at pathophysiological conditions, might be able to protect dendrites during such situations, we exposed HspB5 overexpressing neuronal cultures to heat shock. HspB5 prevented heat shock-induced rarefaction of dendrites. In conclusion, we identified regulation of dendritic complexity as a new function of HspB5 in hippocampal neurons.

Differential phosphorylation-based regulation of αB-crystallin chaperone activity for multipass transmembrane proteins

We have previously shown that αB-crystallin (CRYAB), a small heat shock protein (sHsp) that prevents irreversible aggregation of unfolded protein by an ATP-independent chaperone activity, plays a pivotal role in the biogenesis of multipass transmembrane proteins (TMPs) assisting their folding from the cytosolic side of the endoplasmic reticulum (ER) (D'Agostino et al., 2013). Here we present evidence, based on phosphomimetic substitutions, that the three phosphorytable serine residues at position 19, 45 and 59 of CRYAB play a different regulatory role in this novel chaperone activity: S19 and S45 have a strong inhibitory effect, either alone or in combination, while S59 has not and counteracts the inhibition caused by single phosphomimetic substitutions at S19 and S45. Interestingly, all phosphomimetic substitutions determine the formation of smaller oligomeric complexes containing CRYAB, indicating that the inhibitory effect seen for S19 and S45 cannot be ascribed to the reduction of oligomerization frequently associated to a decreased chaperone activity. These results indicate that phosphorylation finely regulates the chaperone activity of CRYAB with multipass TMPs and suggest a pivotal role for S59 in this process.

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CRYAB chaperone activity toward ATP7B-H1069Q and Fz4-FEVR.

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Phosphomimetic S19D and S45D inhibit CRYAB chaperone activity.

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Phosphomimetic S59D protects CRYAB chaperone activity.

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Pseudo-phosphorylation decreases CRYAB oligomerization.

Enhancement of the Chaperone Activity of Alkyl Hydroperoxide Reductase C from Pseudomonas aeruginosa PAO1 Resulting from a Point-Specific Mutation Confers Heat Tolerance in Escherichia coli

Alkyl hydroperoxide reductase subunit C from Pseudomonas aeruginosa PAO1 (PaAhpC) is a member of the 2-Cys peroxiredoxin family. Here, we examined the peroxidase and molecular chaperone functions of PaAhpC using a site-directed mutagenesis approach by substitution of Ser and Thr residues with Cys at positions 78 and 105 located between two catalytic cysteines. Substitution of Ser with Cys at position 78 enhanced the chaperone activity of the mutant (S78C-PaAhpC) by approximately 9-fold compared with that of the wild-type protein (WT-PaAhpC). This increased activity may have been associated with the proportionate increase in the high-molecular-weight (HMW) fraction and enhanced hydrophobicity of S78C-PaAhpC. Homology modeling revealed that mutation of Ser(78) to Cys(78) resulted in a more compact decameric structure than that observed in WT-PaAhpC and decreased the atomic distance between the two neighboring sulfur atoms of Cys(78) in the dimer-dimer interface of S78C-PaAhpC, which could be responsible for the enhanced hydrophobic interaction at the dimer-dimer interface. Furthermore, complementation assays showed that S78C-PaAhpC exhibited greatly improved the heat tolerance, resulting in enhanced survival under thermal stress. Thus, addition of Cys at position 78 in PaAhpC modulated the functional shifting of this protein from a peroxidase to a chaperone.

Induction of expression and phosphorylation of heat shock protein B5 (CRYAB) in rat myometrium during pregnancy and labour

During pregnancy the myometrium undergoes a programme of differentiation induced by endocrine, cellular, and biophysical inputs. Small heat shock proteins (HSPs) are a family of ten (B1–B10) small-molecular-weight proteins that not only act as chaperones, but also assist in processes such as cytoskeleton rearrangements and immune system activation. Thus, it was hypothesized that HSPB5 (CRYAB) would be highly expressed in the rat myometrium during the contractile and labour phases of myometrial differentiation when such processes are prominent. Immunoblot analysis revealed that myometrial CRYAB protein expression significantly increased from day (D) 15 to D23 (labour;P<0.05). In correlation with these findings, serine 59-phosphorylated (pSer59) CRYAB protein expression significantly increased from D15 to D23, and was also elevated 1-day post-partum (P<0.05). pSer59-CRYAB was detected in the cytoplasm of myocytes within both uterine muscle layers mid- to late-pregnancy. In unilaterally pregnant rats, pSer59-CRYAB protein expression was significantly elevated in the gravid uterine horns at both D19 and D23 of gestation compared with non-gravid horns. Co-immunolocalization experiments using the hTERT-human myometrial cell line and confocal microscopy demonstrated that pSer59-CRYAB co-localized with the focal adhesion protein FERMT2 at the ends of actin filaments as well as with the exosomal marker CD63. Overall, pSer59-CRYAB is highly expressed in myometrium during late pregnancy and labour and its expression appears to be regulated by uterine distension. CRYAB may be involved in the regulation of actin filament dynamics at focal adhesions and could be secreted by exosomes as a prelude to involvement in immune activation in the myometrium.

αB-crystallin: Portrait of a malignant chaperone as a cancer therapeutic target

αB-crystallin is a widely expressed member of the small heat shock protein family that protects cells from stress by its dual function as a molecular chaperone to preserve proteostasis and as a cell death antagonist that negatively regulates components of the conserved apoptotic cell death machinery. Deregulated expression of αB-crystallin occurs in a broad array of solid tumors and has been linked to tumor progression and poor clinical outcomes. This review will focus on new insights into the molecular mechanisms by which oncogenes, oxidative stress, matrix detachment and other tumor microenvironmental stressors deregulate αB-crystallin expression. We will also review accumulating evidence pointing to an essential role for αB-crystallin in the multi-step metastatic cascade whereby tumor cells colonize distant organs by circumventing a multitude of barriers to cell migration and survival. Finally, we will evaluate emerging strategies to therapeutically target αB-crystallin and/or interacting proteins to selectively activate apoptosis and/or derail the metastatic cascade in an effort to improve outcomes for patients with metastatic disease.

Phosphorylation negatively regulates exosome mediated secretion of cryAB in glioma cells

Exosomes mediate secretion of crystallin alphaB (cryAB), a well characterized molecular chaperone with anti-apoptotic activity. However, the mechanisms governing its packaging and secretion remained unexplored. In glioma cells, notwithstanding extensive phosphorylation of cryAB at Ser59 followed by Ser45 (Ser19 is largely unphosphorylated), we discovered that the majority of secreted exosomal cryAB is nonphosphorylated. Transient ectopic expression of a yellow fluorescent protein (YFP) tagged triple phosphomimic (3-SD) cryAB construct in cryAB absent glioma cells led to the formation of large cytosolic inclusions. Our findings demonstrate that mimicking phosphorylation significantly reduces cryAB secretion via exosomes. Moreover, decreased colocalization of 3-SD YFP-cryAB with multivesicular endosome (MVE) and exosome marker, CD63 or Rab27, a small GTPase regulating exocytosis of MVEs, suggests that phosphorylation deters packaging of cryAB in vesicles bound for secretion as exosomes. Additionally, we found that preventing O-GlcNAcylation on cryAB also curtailed its colocalization with CD63 and Rab27 resulting in reduced exosomal secretion. Thus, our study points to O-GlcNAcylation and lack of phosphorylation as being the selective processes involved in the packaging and secretion of cryAB via exosomes.

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

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

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

BACKGROUND

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

SCOPE OF REVIEW

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

MAJOR CONCLUSIONS

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

GENERAL SIGNIFICANCE

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

A Mechanism of Subunit Recruitment in Human Small Heat Shock Protein Oligomers

Small heat shock proteins (sHSPs) make up a class of molecular chaperones broadly observed across organisms. Many sHSPs form large oligomers that undergo dynamic subunit exchange that is thought to play a role in chaperone function. Though remarkably heterogeneous, sHSP oligomers share three types of intermolecular interactions that involve all three defined regions of a sHSP: the N-terminal region (NTR), the conserved α-crystallin domain (ACD), and a C-terminal region (CTR). Here we define the structural interactions involved in incorporation of a subunit into a sHSP oligomer. We demonstrate that a minimal ACD dimer of the human sHSP, HSPB5, interacts with an HSPB5 oligomer through two types of interactions: (1) interactions with CTRs in the oligomer and (2) via exchange into and out of the dimer interface composed of two ACDs. Unexpectedly, although dimers are thought to be the fundamental building block for sHSP oligomers, our results clearly indicate that subunit exchange into and out of oligomers occurs via monomers. Using structure-based mutants, we show that incorporation of a subunit into an oligomer is predicated on recruitment of the subunit via its interaction with CTRs on an oligomer. Both the rate and extent of subunit incorporation depend on the accessibility of CTRs within an HSPB5 oligomer. We show that this mechanism also applies to formation of heterooligomeric sHSP species composed of HSPB5 and HSPB6 and is likely general among sHSPs. Finally, our observations highlight the importance of NTRs in the thermodynamic stability of sHSP oligomers.

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

BACKGROUND

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

SCOPE OF THE REVIEW

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

MAJOR CONCLUSIONS

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

GENERAL SIGNIFICANCE

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

Alpha crystallins in the retinal pigment epithelium and implications for the pathogenesis and treatment of age-related macular degeneration

BACKGROUND

αA- and αB crystallins are principal members of the small heat shock protein family and elicit both a cell protective function and a chaperone function. α-Crystallins have been found to be prominent proteins in normal and pathological retina emphasizing the importance for in-depth understanding of their function and significance.

SCOPE OF REVIEW

Retinal pigment epithelial cells (RPE) play a vital role in the pathogenesis of age-related macular degeneration (AMD). This review addresses a number of cellular functions mediated by α-crystallins in the retina. Prominent expression of αB crystallin in mitochondria may serve to protect cells from oxidative injury. αB crystallin as secretory protein via exosomes can offer neuroprotection to adjacent RPE cells and photoreceptors. The availability of chaperone-containing minipeptides of αB crystallin could prove to be a valuable new tool for therapeutic treatment of retinal disorders.

MAJOR CONCLUSIONS

α-Crystallins are expressed in cytosol and mitochondria of RPE cells and are regulated during oxygen-induced retinopathy and during development. α-Crystallins protect RPE from oxidative-and ER stress-induced injury and autophagy. αB-Crystallin is a modulator of angiogenesis and vascular endothelial growth factor. αB Crystallin is secreted via exosomal pathway. Minichaperone peptides derived from αB Crystallin prevent oxidant induced cell death and have therapeutic potential.

GENERAL SIGNIFICANCE

Overall, this review summarizes several novel properties of α-crystallins and their relevance to maintaining normal retinal function. In particular, the use of α-crystallin derived peptides is a promising therapeutic strategy to combat retinal diseases such as AMD. This article is part of a Special Issue entitled Crystallin biochemistry in health and disease.

Role of crystallins in diabetic complications

BACKGROUND

Crystallins are the major structural proteins of vertebrate eye lens responsible for maintaining the refractive index of the lens. However, recent studies suggest that they also have a functional significance in non-lenticular tissues. Prolonged uncontrolled diabetes results in the development of macro and microvascular complications that are the leading causes of morbidity and mortality in diabetic patients all over the world.

SCOPE OF REVIEW

Recent studies have shown that crystallins play an instrumental role in diabetes and its complications. Therefore, this review highlights the current data on the impact of chronic hyperglycemia on expression, distribution, glycation, phosphorylation, chaperone-like function and, anti-apoptotic activity of crystallins. Furthermore, we discussed the insights for developing therapeutic strategies for diabetic complications including natural agents, peptides, and pharmacological chaperones that modulate or mimic chaperone activity of α-crystallins.

MAJOR CONCLUSIONS

Upregulation of crystallins appears to be a common feature of chronic diabetes. Further, chronic hyperglycemia induces the glycation and phosphorylation of crystallins, mainly α-crystallins and thereby alters their properties. The disturbed interaction of αB-crystallin with various apoptotic mediators including Bax and caspases is also an important factor for increased cell death in diabetes. Numerous dietary agents, peptides, and chemical chaperones prevent apoptosis and the loss of chaperone activity in diabetes.

GENERAL SIGNIFICANCE

Understanding the role of crystallins will aid in developing therapeutic strategies for alleviating pathophysiological conditions such as protein aggregation, inflammation, oxidative stress and apoptosis associated with chronic complications of diabetes including cataract, retinopathy, and cardiomyopathy. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Hyperglycemia induced expression, phosphorylation, and translocation of αB-crystallin in rat skeletal muscle

αB-Crystallin (αBC) is a member of the small heat shock protein family that responds to a variety of stress and prevents the aggregation of partially unfolded proteins. Chronic hyperglycemia created during diabetes results in skeletal muscle atrophy and leads to diabetic myopathy. The aim of this study was to investigate the role of αBC under chronic hyperglycemia in rat skeletal muscle. Diabetes was induced in Wistar rats by a single i.p injection of streptozotocin and maintained for a period of 12 weeks at the end of which the animals were sacrificed and the muscle was collected. The protein levels of αBC and its phosphorylation status in gastrocnemius muscle were analyzed by immunoblotting. The translocation of phosphorylated αBC was analyzed by detergent solubility assay, co-immunoprecipitation (Co-IP), and immunohistochemistry. The cell death was analyzed by TUNEL assay and by apoptotic markers. The interaction of αBC with Bax was analyzed by Co-IP. Chronic hyperglycemia significantly increased the protein levels of αBC and its phosphorylation at S59 by activation of p38 mitogen-activated protein kinase (p38MAPK) and at S45 by activation of the extracellular regulated protein kinase 1/2 (ERK1/2). Further, phosphorylated αBC translocated and interacted with desmin indicating that phosphorylated αBC forms might be involved in protection of sarcomere structures from disruption in chronic hyperglycemia. Further, Co-IP studies showed an impaired interaction of αBC with Bax which could be one of the possible factors for increased cell death as evidenced by TUNEL assay in diabetic muscle. These results suggest that an increased expression, phosphorylation, translocation of αBC, and its involvement in apoptosis might play a significant role in maintenance of cytoskeletal architecture and protection of cells from apoptosis in diabetic skeletal muscle.

Therapeutic potential of α-crystallin

BACKGROUND

The findings that α-crystallins are multi-functional proteins with diverse biological functions have generated considerable interest in understanding their role in health and disease. Recent studies have shown that chaperone peptides of α-crystallin could be delivered into cultured cells and in experimental animals with beneficial effects against protein aggregation, oxidation, inflammation and apoptosis.

SCOPE OF REVIEW

In this review, we will summarize the latest developments on the therapeutic potential of α-crystallins and their functional peptides.

MAJOR CONCLUSIONS

α-Crystallins and their functional peptides have shown significant favorable effects against several diseases. Their targeted delivery to tissues would be of great therapeutic benefit. However, α-crystallins can also function as disease-causing proteins. These seemingly contradictory functions must be carefully considered prior to their therapeutic use.

GENERAL SIGNIFICANCE

αA and αB-Crystallin are members of the small heat shock protein family. These proteins exhibit molecular chaperone and anti-apoptotic activities. The core crystallin domain within these proteins is largely responsible for these prosperities. Recent studies have identified peptides within the crystallin domain of both α- and αB-crystallins with remarkable chaperone and anti-apoptotic activities. Administration of α-crystallin or their functional peptides has shown substantial inhibition of pathologies in several diseases. However, α-crystallins have been shown to promote disease-causing pathways. These two sides of the proteins are discussed in this review. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

A first line of stress defense: small heat shock proteins and their function in protein homeostasis

Small heat shock proteins (sHsps) are virtually ubiquitous molecular chaperones that can prevent the irreversible aggregation of denaturing proteins. sHsps complex with a variety of non-native proteins in an ATP-independent manner and, in the context of the stress response, form a first line of defense against protein aggregation in order to maintain protein homeostasis. In vertebrates, they act to maintain the clarity of the eye lens, and in humans, sHsp mutations are linked to myopathies and neuropathies. Although found in all domains of life, sHsps are quite diverse and have evolved independently in metazoans, plants and fungi. sHsp monomers range in size from approximately 12 to 42kDa and are defined by a conserved β-sandwich α-crystallin domain, flanked by variable N- and C-terminal sequences. Most sHsps form large oligomeric ensembles with a broad distribution of different, sphere- or barrel-like oligomers, with the size and structure of the oligomers dictated by features of the N- and C-termini. The activity of sHsps is regulated by mechanisms that change the equilibrium distribution in tertiary features and/or quaternary structure of the sHsp ensembles. Cooperation and/or co-assembly between different sHsps in the same cellular compartment add an underexplored level of complexity to sHsp structure and function.

Small heat-shock proteins: important players in regulating cellular proteostasis

Small heat-shock proteins (sHsps) are a diverse family of intra-cellular molecular chaperone proteins that play a critical role in mitigating and preventing protein aggregation under stress conditions such as elevated temperature, oxidation and infection. In doing so, they assist in the maintenance of protein homeostasis (proteostasis) thereby avoiding the deleterious effects that result from loss of protein function and/or protein aggregation. The chaperone properties of sHsps are therefore employed extensively in many tissues to prevent the development of diseases associated with protein aggregation. Significant progress has been made of late in understanding the structure and chaperone mechanism of sHsps. In this review, we discuss some of these advances, with a focus on mammalian sHsp hetero-oligomerisation, the mechanism by which sHsps act as molecular chaperones to prevent both amorphous and fibrillar protein aggregation, and the role of post-translational modifications in sHsp chaperone function, particularly in the context of disease.

Small heat shock proteins: Role in cellular functions and pathology

Small heat shock proteins (sHsps) are conserved across species and are important in stress tolerance. Many sHsps exhibit chaperone-like activity in preventing aggregation of target proteins, keeping them in a folding-competent state and refolding them by themselves or in concert with other ATP-dependent chaperones. Mutations in human sHsps result in myopathies, neuropathies and cataract. Their expression is modulated in diseases such as Alzheimer's, Parkinson's and cancer. Their ability to bind Cu2+, and suppress generation of reactive oxygen species (ROS) may have implications in Cu2+-homeostasis and neurodegenerative diseases. Circulating αB-crystallin and Hsp27 in the plasma may exhibit immunomodulatory and anti-inflammatory functions. αB-crystallin and Hsp20 exhitbit anti-platelet aggregation: these beneficial effects indicate their use as potential therapeutic agents. sHsps have roles in differentiation, proteasomal degradation, autophagy and development. sHsps exhibit a robust anti-apoptotic property, involving several stages of mitochondrial-mediated, extrinsic apoptotic as well as pro-survival pathways. Dynamic N- and C-termini and oligomeric assemblies of αB-crystallin and Hsp27 are important factors for their functions. We propose a "dynamic partitioning hypothesis" for the promiscuous interactions and pleotropic functions exhibited by sHsps. Stress tolerance and anti-apoptotic properties of sHsps have both beneficial and deleterious consequences in human health and diseases. Conditional and targeted modulation of their expression and/or activity could be used as strategies in treating several human disorders. The review attempts to provide a critical overview of sHsps and their divergent roles in cellular processes particularly in the context of human health and disease.