The human genome encodes 10 alpha-crystallin-related small heat shock proteins: HspB1-10

The crucial role of small heat shock proteins in prostate cancer: mechanisms and new therapeutic perspectives

As resistance to new anti-androgen drugs occurs more frequently, increasing numbers of researchers are exploring alternative key molecular targets for prostate cancer treatment. The small heat shock protein (sHSP) family is a subclass of heat shock proteins (HSPs). Due to the smaller molecular size of their monomers, they often function as large oligomeric complexes with diverse biological roles, thus garnering increasing attention from urologists. Different members of the sHSP family exhibit distinct biological roles in prostate cancer, offering a new perspective for precision therapy. In this review, we summarize the specific roles of sHSP family members in prostate cancer and analyze their similarities and differences. Additionally, we discuss and review the drugs targeting various sHSPs in prostate cancer, providing new insights into the exploration and further application of sHSP-targeted therapies.

Structural characterization and functional analysis of human αB-crystallin with the p.R11G mutation: Insights into cataractogenesis and cardiomyopathy

αB-crystallin, a member of the small heat-shock protein family, functions as a molecular chaperone and plays a critical role in maintaining cellular homeostasis by preventing the aggregation of misfolded proteins in various tissues. This research investigates the structural and functional consequences of the p.R11G mutation in human αB-crystallin, which is associated with serious health issues, including cataracts, myofibrillar myopathy, and dilated cardiomyopathy. Following the introduction of this mutation through site-directed mutagenesis, the mutant protein was expressed in a prokaryotic host system and purified using ion-exchange chromatography. The structure and stability of the mutant protein were assessed using various spectroscopic techniques. Moreover, the oligomeric structure of the mutant protein was examined using dynamic light scattering and atomic force microscopy. To evaluate the chaperone activity and cytoprotective effects of the protein, UV-Vis spectroscopy and the 2,5-diphenyl-2H-tetrazolium bromide (MTT) assay were utilized. The results demonstrated that the p.R11G mutation significantly alters the protein's structure, leading to enhanced thermal and chemical stability, and formation of the larger oligomers compared to the wild-type protein. Additionally, the mutation was found to increase the protein's chaperone activity and its capacity to inhibit cancer cell death under oxidative stress conditions. Based on the results of our study, the significant changes observed in the structure and activity of human αB-crystallin due to this mutation elucidate the potential role of the mutated chaperone in cataract formation and myopathy. Further research is necessary to fully elucidate the underlying mechanisms and translate these findings into effective therapeutic interventions.

Filamin C dimerisation is regulated by HSPB7

The biomechanical properties and responses of tissues underpin a variety important of physiological functions and pathologies. In striated muscle, the actin-binding protein filamin C (FLNC) is a key protein whose variants causative for a wide range of cardiomyopathies and musculoskeletal pathologies. FLNC is a multi-functional protein that interacts with a variety of partners, however, how it is regulated at the molecular level is not well understood. Here we investigate its interaction with HSPB7, a cardiac-specific molecular chaperone whose absence is embryonically lethal. We find that FLNC and HSPB7 interact in cardiac tissue under biomechanical stress, forming a strong hetero-dimer whose structure we solve by X-ray crystallography. Our quantitative analyses show that the hetero-dimer out-competes the FLNC homo-dimer interface, potentially acting to abrogate the ability of the protein to cross-link the actin cytoskeleton, and to enhance its diffusive mobility. We show that phosphorylation of FLNC at threonine 2677, located at the dimer interface and associated with cardiac stress, acts to favour the homo-dimer. Conversely, phosphorylation at tyrosine 2683, also at the dimer interface, has the opposite effect and shifts the equilibrium towards the hetero-dimer. Evolutionary analysis and ancestral sequence reconstruction reveals this interaction and its mechanisms of regulation to date around the time primitive hearts evolved in chordates. Our work therefore shows, structurally, how HSPB7 acts as a specific molecular chaperone that regulates FLNC dimerisation.

Heat shock protein 22: A new direction for cardiovascular disease (Review)

Small heat shock proteins (sHSPs) are common molecular chaperone proteins that function in various biological processes, and serve indispensable roles in maintaining cellular protein homeostasis and regulating the hydrolysis of unfolded proteins. HSP22 is a member of the sHSP family that is primarily expressed in the heart and skeletal muscle, as well as in various types of cancer. There have been important findings concerning the role of HSP22 in cardiovascular diseases. The aim of the present study was to provide insights into the various molecular mechanisms by which HSP22 functions in the heart, including oxidative stress, autophagy, apoptosis, the subcellular distribution of proteins and the promoting effect of proteasomes. In addition, drugs and cytokines, including geranylgeranylacetone, can exert protective effects on the heart by regulating the expression of HSP22. Based on increasingly abundant research, HSP22 may be considered a potential therapeutic target in cardiovascular diseases.

Key Role of Phosphorylation in Small Heat Shock Protein Regulation via Oligomeric Disaggregation and Functional Activation

Heat shock proteins (HSPs) are essential molecular chaperones that protect cells by aiding in protein folding and preventing aggregation under stress conditions. Small heat shock proteins (sHSPs), which include members from HSPB1 to HSPB10, are particularly important for cellular stress responses. These proteins share a conserved α-crystallin domain (ACD) critical for their chaperone function, with flexible N- and C-terminal extensions that facilitate oligomer formation. Phosphorylation, a key post-translational modification (PTM), plays a dynamic role in regulating sHSP structure, oligomeric state, stability, and chaperone function. Unlike other PTMs such as deamidation, oxidation, and glycation-which are often linked to protein destabilization-phosphorylation generally induces structural transitions that enhance sHSP activity. Specifically, phosphorylation promotes the disaggregation of sHSP oligomers into smaller, more active complexes, thereby increasing their efficiency. This disaggregation mechanism is crucial for protecting cells from stress-induced damage, including apoptosis, inflammation, and other forms of cellular dysfunction. This review explores the role of phosphorylation in modulating the function of sHSPs, particularly HSPB1, HSPB4, and HSPB5, and discusses how these modifications influence their protective functions in cellular stress responses.

Novel mTORC2/HSPB4 Interaction: Role and Regulation of HSPB4 T148 Phosphorylation

HSPB4 and HSPB5 (α-crystallins) have shown increasing promise as neuroprotective agents, demonstrating several anti-apoptotic and protective roles in disorders such as multiple sclerosis and diabetic retinopathy. HSPs are highly regulated by post-translational modification, including deamidation, glycosylation, and phosphorylation. Among them, T148 phosphorylation has been shown to regulate the structural and functional characteristics of HSPB4 and underlie, in part, its neuroprotective capacity. We recently demonstrated that this phosphorylation is reduced in retinal tissues from patients with diabetic retinopathy, raising the question of its regulation during diseases. The kinase(s) responsible for regulating this phosphorylation, however, have yet to be identified. To this end, we employed a multi-tier strategy utilizing in vitro kinome profiling, bioinformatics, and chemoproteomics to predict and discover the kinases capable of phosphorylating T148. Several kinases were identified as being capable of specifically phosphorylating T148 in vitro, and further analysis highlighted mTORC2 as a particularly strong candidate. Altogether, our data demonstrate that the HSPB4-mTORC2 interaction is multi-faceted. Our data support the role of mTORC2 as a specific kinase phosphorylating HSPB4 at T148, but also provide evidence that the HSPB4 chaperone function further strengthens the interaction. This study addresses a critical gap in our understanding of the regulatory underpinnings of T148 phosphorylation-mediated neuroprotection.

ZenoSWATH DIA proteomics and clustering analysis of the effect of cysteamine at the cellular level in cystinotic fibroblasts

Cysteamine, an aminothiol, is the only available treatment for cystinosis, an incurable metabolic recessive disease characterized by detrimental symptoms at the renal, ocular, and muscular levels. Cystinosis is due to mutations in the CTNS gene encoding for the lysosomal symporter cystinosine. Cysteamine treatment only delays the symptoms, presents undesirable side effects and the patients depend on it for life. Thus, it is of paramount importance to find new complementary therapeutic targets for the disease, as well as to understand, at the molecular level, both the beneficial and detrimental effects of cysteamine. Here, we have used ZenoSWATH DIA proteomics and clustering analysis to unravel the differences between cystinotic and non-cystinotic skin fibroblasts, and to study the effect of increasing concentrations of cysteamine. Cystinotic cells present significant differences in proteins related to extracellular matrix structure and detoxification. Only a subset of those proteins is reversed by cysteamine in a dose-dependent manner, partially providing an explanation for its therapeutic benefits. Finally, cysteamine per se alters the levels of a group of lysosomal proteins that are not modulated in basal conditions. Our results will be helpful to understand the benefits, deficiencies, and detrimental effects of the cysteamine treatment.

Dynamic fibrillar assembly of αB-crystallin induced by perturbation of the conserved NT-IXI motif resolved by cryo-EM

αB-crystallin is an archetypical member of the small heat shock proteins (sHSPs) vital for cellular proteostasis and mitigating protein misfolding diseases. Gaining insights into the principles defining their molecular organization and chaperone function have been hindered by intrinsic dynamic properties and limited high-resolution structural analysis. To disentangle the mechanistic underpinnings of these dynamical properties, we ablate a conserved IXI-motif located within the N-terminal (NT) domain of human αB-crystallin implicated in subunit exchange dynamics and client sequestration. This results in a profound structural transformation, from highly polydispersed caged-like native assemblies into an elongated fibril state amenable to high-resolution cryo-EM analysis. The reversible nature of this variant facilitates interrogation of functional effects due to perturbation of the NT-IXI motif in both the native-like oligomer and fibril states. Together, our investigations unveil several features thought to be key mechanistic attributes to sHSPs and point to a critical significance of the NT-IXI motif in αB-crystallin assembly, polydispersity, and chaperone activity.

O-GlcNAc modification of HSP27 alters its protein interactions and promotes refolding of proteins through the BAG3/HSP70 co-chaperone

Almost all types of cellular stress induce post-translational O-GlcNAc modifications of proteins, and this increase promotes cell survival. We previously demonstrated that O-GlcNAc on certain small heat shock proteins (sHSPs), including HSP27, directly increases their chaperone activity as one potential protective mechanism. Here, we furthered our use of synthetic proteins to prepare biotinylated sHSPs and show that O-GlcNAc modification of HSP27 also changes how it interacts within the sHSP system and the broader HSP network. Specifically, we show that O-GlcNAc modified HSP27 binds more strongly to the co-chaperone protein BAG3, which then promotes refolding of a model substrate by HSP70. We use proteomics to identify other potential HSP27 interactions that are changed by O-GlcNAc, including one that we confirm with another sHSP, αB-crystallin. These findings add additional evidence for O-GlcNAc as a switch for regulating protein-protein interactions and for modifications of chaperones as one mechanism by which O-GlcNAc protects against protein aggregation.

HSPB6: A lipid-dependent molecular chaperone inhibits α-synuclein aggregation

The process of protein misfolding and aggregation is associated with various cytotoxic effects. Understanding how this phenomenon is regulated by the protein homeostasis system, however, is difficult, since it takes place through a complex non-linear network of coupled microscopic steps, including primary nucleation, fibril elongation, and secondary nucleation, which depend on environmental factors. To address this problem, we studied how the aggregation of α-synuclein, a protein associated with Parkinson's disease, is modulated by molecular chaperones and lipid membranes. We focused on small heat shock proteins (sHSPs/HSPBs), which interact with proteins and lipids and are upregulated during aging, a major risk factor for protein misfolding diseases. HSPBs act on different microscopic steps to prevent α-synuclein aggregation, with HSPB6 showing a lipid-dependent chaperone activity. Our findings provide an example of how HSPBs diversified their mechanisms of action to reach an efficient regulation of protein misfolding and aggregation within the complex cellular environment.

Universal Adapter Protein Bag3 and Small Heat Shock Proteins

Bag3 (Bcl-2-associated athanogene 3) protein contains a number of functional domains and interacts with a wide range of different partner proteins, including small heat shock proteins (sHsps) and heat shock protein Hsp70. The ternary Bag3-sHsp-and Hsp70 complex binds denatured proteins and transports them to phagosomes, thus playing a key role in the chaperone-assisted selective autophagy (CASA). This complex also participates in the control of formation and disassembly of stress granules (granulostasis) and cytoskeleton regulation. As Bag3 and sHsps participate in multiple cellular processes, mutations in these proteins are often associated with neurodegenerative diseases and cardiomyopathy. The review discusses the role of sHsps in different processes regulated by Bag3.

The α-crystallin Chaperones Undergo a Quasi-ordered Co-aggregation Process in Response to Saturating Client Interaction

Small heat shock proteins (sHSPs) are ATP-independent chaperones vital to cellular proteostasis, preventing protein aggregation events linked to various human diseases including cataract. The α-crystallins, αA-crystallin (αAc) and αB-crystallin (αBc), represent archetypal sHSPs that exhibit complex polydispersed oligomeric assemblies and rapid subunit exchange dynamics. Yet, our understanding of how this plasticity contributes to chaperone function remains poorly understood. Using biochemical and biophysical analyses combined with single-particle electron microscopy (EM), we examined structural changes in αAc, αBc and native heteromeric lens α-crystallins (αLc) in their apo-states and at varying degree of chaperone saturation leading to co-aggregation, using lysozyme and insulin as model clients. Quantitative single-particle analysis unveiled a continuous spectrum of oligomeric states formed during the co-aggregation process, marked by significant client-triggered expansion and quasi-ordered elongation of the sHSP oligomeric scaffold, whereby the native cage-like sHSP assembly displays a directional growth to accommodate saturating conditions of client sequestration. These structural modifications culminated in an apparent amorphous collapse of chaperone-client complexes, resulting in the creation of co-aggregates capable of scattering visible light. Intriguingly, these co-aggregates maintain internal morphological features of highly elongated sHSP oligomers with striking resemblance to polymeric α-crystallin species isolated from aged lens tissue. This mechanism appears consistent across αAc, αBc and αLc, albeit with varying degrees of susceptibility to client-induced co-aggregation. Importantly, our findings suggest that client-induced co-aggregation follows a distinctive mechanistic and quasi-ordered trajectory, distinct from a purely amorphous process. These insights reshape our understanding of the physiological and pathophysiological co-aggregation processes of α-crystallins, carrying potential implications for a pathway toward cataract formation.

Heat Shock Response and Heat Shock Proteins: Current Understanding and Future Opportunities in Human Diseases

The heat shock response is an evolutionarily conserved mechanism that protects cells or organisms from the harmful effects of various stressors such as heat, chemicals toxins, UV radiation, and oxidizing agents. The heat shock response triggers the expression of a specific set of genes and proteins known as heat shock genes/proteins or molecular chaperones, including HSP100, HSP90, HSP70, HSP60, and small HSPs. Heat shock proteins (HSPs) play a crucial role in thermotolerance and aiding in protecting cells from harmful insults of stressors. HSPs are involved in essential cellular functions such as protein folding, eliminating misfolded proteins, apoptosis, and modulating cell signaling. The stress response to various environmental insults has been extensively studied in organisms from prokaryotes to higher organisms. The responses of organisms to various environmental stressors rely on the intensity and threshold of the stress stimuli, which vary among organisms and cellular contexts. Studies on heat shock proteins have primarily focused on HSP70, HSP90, HSP60, small HSPs, and ubiquitin, along with their applications in human biology. The current review highlighted a comprehensive mechanism of heat shock response and explores the function of heat shock proteins in stress management, as well as their potential as therapeutic agents and diagnostic markers for various diseases.

Dynamic fibrillar assembly of αB-crystallin induced by perturbation of the conserved NT-IXI motif resolved by cryo-EM

αB-crystallin is an archetypical member of the small heat-shock proteins (sHSPs) vital for cellular proteostasis and mitigating protein misfolding diseases. Gaining insights into the principles defining their molecular organization and chaperone function have been hindered by intrinsic dynamic properties and limited high-resolution structural analysis. To disentangle the mechanistic underpinnings of these dynamical properties, we mutated a conserved IXI-motif located within the N-terminal (NT) domain of human αB-crystallin. This resulted in a profound structural transformation, from highly polydispersed caged-like native assemblies into a comparatively well-ordered helical fibril state amenable to high-resolution cryo-EM analysis. The reversible nature of the induced fibrils facilitated interrogation of functional effects due to perturbation of the NT-IXI motif in both the native-like oligomer and fibril states. Together, our investigations unveiled several features thought to be key mechanistic attributes to sHSPs and point to a critical significance of the NT-IXI motif in αB-crystallin assembly, dynamics and chaperone activity.

HSPB6 Deficiency Promotes the Development of Aortic Dissection and Rupture

To better understand the pathogenesis of acute type A aortic dissection, high-sensitivity liquid chromatography-tandem mass spectrometry/mass spectrometry (LC-MS/MS)-based proteomics and phosphoproteomics approaches were used to identify differential proteins. Heat shock protein family B (small) member 6 (HSPB6) in aortic dissection was significantly reduced in human and mouse aortic dissection samples by real-time PCR, western blotting, and immunohistochemical staining techniques. Using an HSPB6-knockout mouse, we investigated the potential role of HSPB6 in β-aminopropionitrile monofumarate-induced aortic dissection. We found increased mortality and increased probability of ascending aortic dissection after HSPB6 knockout compared with wild-type mice. Mechanistically, our data suggest that HSPB6 deletion promoted vascular smooth muscle cell apoptosis. More importantly, HSPB6 deletion attenuated cofilin activity, leading to excessive smooth muscle cell stiffness and eventually resulting in the development of aortic dissection and rupture. Our data suggest that excessive stiffness of vascular smooth muscle cells caused by HSPB6 deficiency is a new pathogenetic mechanism leading to aortic dissection.

Molecular Insights into the Inhibitory Role of α-Crystallin against γD-Crystallin Aggregation

Cataracts, a major cause of global blindness, contribute significantly to the overall prevalence of blindness. The opacification of the lens, resulting in cataract formation, primarily occurs due to the aggregation of crystallin proteins within the eye lens. Despite the high concentration of these crystallins, they remarkably maintain the lens transparency and refractive index. α-Crystallins (α-crys), acting as chaperones, play a crucial role in preventing crystallin aggregation, although the exact molecular mechanism remains uncertain. In this study, we employed a combination of molecular docking, all-atom molecular dynamics simulations, and advanced free energy calculations to investigate the interaction between γD-crystallin (γD-crys), a major structural protein of the eye lens, and α-crystallin proteins. Our findings demonstrate that α-crys exhibits an enhanced affinity for the NTD2 and CTD4 regions of γD-crys. The NTD2 and CTD4 regions form the interface between the N-terminal domain (NTD) and the C-terminal domain (CTD) of the γD-crys protein. By binding to the interface region between the NTD and CTD of the protein, α-crys effectively inhibits the formation of domain-swapped aggregates and mitigates protein aggregation. Analysis of the Markov state models using molecular dynamics trajectories confirms that minimum free energy conformations correspond to the binding of the α-crystallin domain (ACD) of α-crys to NTD2 and CTD4 that form the interdomain interface.

The major inducible small heat shock protein HSP20-3 in the tardigrade Ramazzottius varieornatus forms filament-like structures and is an active chaperone

The tardigrade Ramazzottius varieornatus has remarkable resilience to a range of environmental stresses. In this study, we have characterised two members of the small heat shock protein (sHSP) family in R. varieornatus, HSP20-3 and HSP20-6. These are the most highly upregulated sHSPs in response to a 24 h heat shock at 35 0C of adult tardigrades with HSP20-3 being one of the most highly upregulated gene in the whole transcriptome. Both R. varieornatus sHSPs and the human sHSP, CRYAB (HSPB5), were produced recombinantly for comparative structure-function studies. HSP20-3 exhibited a superior chaperone activity than human CRYAB in a heat-induced protein aggregation assay. Both tardigrade sHSPs also formed larger oligomers than CRYAB as assessed by size exclusion chromatography and transmission electron microscopy of negatively stained samples. Whilst both HSP20-3 and HSP20-6 formed particles that were variable in size and larger than the particles formed by CRYAB, only HSP20-3 formed filament-like structures. The particles and filament-like structures formed by HSP20-3 appear inter-related as the filament-like structures often had particles located at their ends. Sequence analyses identified two unique features; an insertion in the middle region of the N-terminal domain (NTD) and preceding the critical-sequence identified in CRYAB, as well as a repeated QNTN-motif located in the C-terminal domain of HSP20-3. The NTD insertion is expected to affect protein-protein interactions and subunit oligomerisation. Removal of the repeated QNTN-motif abolished HSP20-3 chaperone activity and also affected the assembly of the filament-like structures. We discuss the potential contribution of HSP20-3 to protein condensate formation.

API5-Hsp20 axis regulate apoptosis and viral infection in mud crab (Scylla paramamosain)

Apoptosis Inhibitor 5 (API5) is a widely concerned nuclear protein with diverse functions in organisms, so far, study of API5 is still quite limited in lower animals, and its role in viral immune response has not been addressed. Here, we explored the function of API5 in mud crab (Scylla paramamosain) during White Spot Syndrome Virus (WSSV) infection. The interacting protein Hsp20 of API5 was screened by pull-down assay, and API5 and hsp20 were knocked down by RNAi interference. The results showed that API5 was upregulated along with virus infection, silencing of API5 led to increased WSSV copy numbers and apoptotic rate of hemocytes, highlighting its significance in the immune response. Moreover, we discovered a novel interaction between API5 and Heat Shock Protein 20 (Hsp20), and then revealed that Hsp20 could promote cell apoptosis of hemocytes and reduce viral copy numbers by suppressing API5. The current study therefore improves the knowledge of API5-Hsp20 axis and provides novel insights into intricate mechanisms governing the antiviral response in marine crustaceans.

The heat shock protein 20 gene family in large yellow croaker (Larimichthys crocea): Identification, phylogenetic relationships, expression analyses

Large yellow croaker (Larimichthys crocea) is an economically important fish in China, but its aquaculture industry has been threatened by both biotic and abiotic stressors such as hypoxia and pathogens. In the current study, hsp20 genes were identified and analyzed systematically for the first time from the genome of large yellow croaker, and their roles in hypoxia response and Aeromonas hydrophila, Pseudomonas plecoglossicida infection were investigated. Herein, 11 hsp20 genes were identified and annotated, phylogenetic analysis and selection pressure analysis showed that the hsp20 genes were evolutionarily-constrained and their function was conserved among fishes. Besides, we observed the expression patterns of the hsp20 genes under hypoxia and two pathogens' stress. In brief, seven, four, seven genes responded to hypoxia stress, A. hydrophila infection and P. plecoglossicida challenge, respectively, which indicated that they were involved in hypoxia and disease responses. Furthermore, pathogen- and time-specific pattern was observed after A. hydrophila and P. plecoglossicida infection whereas tissue-specific pattern was observed after hypoxia exposure, revealing that hsp20 genes showed differential functions in response to hypoxia and immune stress. Taken together, these results provided preliminary information for future analysis of the roles of hsp20 genes in both biotic and abiotic stress response in fish.

Genes related to heat tolerance in cattle-a review

Heat stress is described as the cumulative detrimental effect caused by an imbalance between heat production within the body and heat dissipation. When cattle are exposed to heat stress with skin surface temperatures exceeding 35 °C, gene networks within and across cells respond to environmental heat loads with both intra and extracellular signals that coordinate cellular and whole-animal metabolism changes to store heat and rapidly increase evaporative heat loss. In this study, we examined evidence from genes known to be associated with heat tolerance (Hsp70, HSF1, HspB8, SOD1, PRLH, ATP1A1, MTOR, and EIF2AK4). This information could serve as valuable resource material for breeding programs aimed at increasing the thermotolerance of cattle.

Insight into the biochemical and cell biological function of an intrinsically unstructured heat shock protein, Hsp12 of Ustilago maydis

Small heat shock proteins (sHsps) play diverse roles in the stress response and maintenance of cellular functions. The Ustilago maydis genome codes for few sHsps. Among these, Hsp12 has previously been demonstrated to be involved in the pathogenesis of the fungus by our group. In the present study we further investigated the biological function of the protein in the pathogenic development of U. maydis. Analysis of the primary amino acid sequence of Hsp12 in combination with spectroscopic methods to analyse secondary protein structures revealed an intrinsically disordered nature of the protein. We also carried out detailed analysis on the protein aggregation prevention activity associated with Hsp12. Our data suggest Hsp12 has trehalose-dependent protein aggregation prevention activity. Through assaying the interaction of Hsp12 with lipid membranes in vitro we also showed the ability of U. maydis Hsp12 to induce stability in lipid vesicles. U. maydis hsp12 deletion mutants exhibited defects in the endocytosis process and delayed completion of the pathogenic life cycle. Therefore, U. maydis Hsp12 contributes to the pathogenic development of the fungus through its ability to relieve proteotoxic stress during infection as well as its membrane-stabilizing function.

HSP27 Inhibitory Activity against Caspase-3 Cleavage and Activation by Caspase-9 Is Enhanced by Chaperone O-GlcNAc Modification in Vitro

One of the O-GlcNAc modifications is the protection of cells against a variety of stressors that result in cell death. Previous experiments have focused on the overall ability of O-GlcNAc to prevent protein aggregation under stress as well as its ability to affect stress-response signaling pathways. Less attention has been paid to the potential role for O-GlcNAc in the direct inhibition of a major cell-death pathway, apoptosis. Apoptosis involves the sequential activation of caspase proteases, including the transfer of cell-stress information from initiator caspase-9 to effector caspase-3. Cells have multiple mechanisms to slow the apoptotic cascade, including heat shock protein HSP27, which can directly inhibit the activation of caspase-3 by caspase-9. We have previously shown that O-GlcNAc modification increases the chaperone activity of HSP27 against amyloid aggregation, raising the question as to whether this modification may play important roles in other facets of HSP27 biology. Here, we use protein chemistry to generate different versions of O-GlcNAc modified HSP27 and demonstrate that the modification enhances this antiapoptotic function of the chaperone, at least in an in vitro context. These results provide additional molecular insight into how O-GlcNAc functions as a mediator of cellular stress with important implications for human diseases like cancer and neurodegeneration.

The α-crystallin chaperones undergo a quasi-ordered co-aggregation process in response to saturating client interaction

Small heat shock proteins (sHSPs) are ATP-independent chaperones vital to cellular proteostasis, preventing protein aggregation events linked to various human diseases including cataract. The α-crystallins, αA-crystallin (αAc) and αB-crystallin (αBc), represent archetypal sHSPs that exhibit complex polydispersed oligomeric assemblies and rapid subunit exchange dynamics. Yet, our understanding of how this plasticity contributes to chaperone function remains poorly understood. This study investigates structural changes in αAc and αBc during client sequestration under varying degree of chaperone saturation. Using biochemical and biophysical analyses combined with single-particle electron microscopy (EM), we examined αAc and αBc in their apo-states and at various stages of client-induced co-aggregation, using lysozyme as a model client. Quantitative single-particle analysis unveiled a continuous spectrum of oligomeric states formed during the co-aggregation process, marked by significant client-triggered expansion and quasi-ordered elongation of the sHSP scaffold. These structural modifications culminated in an apparent amorphous collapse of chaperone-client complexes, resulting in the creation of co-aggregates capable of scattering visible light. Intriguingly, these co-aggregates maintain internal morphological features of highly elongated sHSP scaffolding with striking resemblance to polymeric α-crystallin species isolated from aged lens tissue. This mechanism appears consistent across both αAc and αBc, albeit with varying degrees of susceptibility to client-induced co-aggregation. Importantly, our findings suggest that client-induced co-aggregation follows a distinctive mechanistic and quasi-ordered trajectory, distinct from a purely amorphous process. These insights reshape our understanding of the physiological and pathophysiological co-aggregation processes of sHSPs, carrying potential implications for a pathway toward cataract formation.

Identification and functional analysis of αB-crystallins in Pteromalus puparum

Heat shock proteins, including αB-crystallins (CRYAB), are pivotal in cellular defense mechanisms and stress response. This study presents a comprehensive investigation of heat shock proteins (HSPs), with a specific focus on the CRYAB family, within the genome of Pteromalus puparum. The analysis encompasses the identification of these proteins, exploration of their phylogenetic relationships, examination of conserved domains, and evaluation of their response to high temperature conditions. A total of 46 HSPs were identified in the P. puparum genome, and the differential expression of mRNA at 35°C and 25°C drew attention to five genes belonging to the CRYAB family, namely, PpCRYAB-1 to PpCRYAB-5. The conservation level of CRYAB family genes across different species was observed to be relatively modest. Through genome-wide screening of 22 species representing six insect orders, a total of 235 CRYAB proteins were identified, with P. puparum harboring eight CRYAB proteins, indicative of a moderate abundance compared to other species. Intriguingly, evolutionary analysis highlighted PpCRYAB-4 with potentially intricate differentiation in comparison to other members of the CRYAB family. Furthermore, RNA interference (RNAi) results demonstrated significant regulatory effects on adult lifespan under heat stress at 35°C for PpCRYAB-4 and PpCRYAB-5. These findings lay a groundwork for future investigations into stress resistance mechanisms in parasitic wasps, providing fresh insights for the study of insect resilience amidst the backdrop of global climate change.

Whole Exome Sequencing of 20 Spanish Families: Candidate Genes for Non-Syndromic Pediatric Cataracts

Non-syndromic pediatric cataracts are defined as opacification of the crystalline lens that occurs during the first years of life without affecting other organs. Given that this disease is one of the most frequent causes of reversible blindness in childhood, the main objective of this study was to propose new responsible gene candidates that would allow a more targeted genetic approach and expand our genetic knowledge about the disease. We present a whole exome sequencing (WES) study of 20 Spanish families with non-syndromic pediatric cataracts and a previous negative result on an ophthalmology next-generation sequencing panel. After ophthalmological evaluation and collection of peripheral blood samples from these families, WES was performed. We were able to reach a genetic diagnosis in 10% of the families analyzed and found genes that could cause pediatric cataracts in 35% of the cohort. Of the variants found, 18.2% were classified as pathogenic, 9% as likely pathogenic, and 72.8% as variants of uncertain significance. However, we did not find conclusive results in 55% of the families studied, which suggests further studies are needed. The results of this WES study allow us to propose LONP1, ACACA, TRPM1, CLIC5, HSPE1, ODF1, PIKFYVE, and CHMP4A as potential candidates to further investigate for their role in pediatric cataracts, and AQP5 and locus 2q37 as causal genes.

Evaluation of the Small Heat Shock Protein Family Members HSPB2 and HSPB3 in Bladder Cancer Prognosis and Progression

Bladder cancer (BlCa) represents the sixth most commonly diagnosed type of male malignancy. Due to the clinical heterogeneity of BlCa, novel markers would optimize treatment efficacy and improve prognosis. The small heat shock proteins (sHSP) family is one of the major groups of molecular chaperones responsible for the maintenance of proteome functionality and stability. However, the role of sHSPs in BlCa remains largely unknown. The present study aimed to examine the association between HSPB2 and HSPB3 expression and BlCa progression in patients, and to investigate their role in BlCa cells. For this purpose, a series of experiments including reverse transcription-quantitative PCR, Western blotting, MTT assay and flow cytometry were performed. Initial analyses revealed increased vs. human transitional carcinoma cells, expression levels of the HSPB2 and HSPB3 genes and proteins in high grade BlCa cell lines. Therefore, we then evaluated the clinical significance of the HSPB2 and HSPB3 genes expression levels in bladder tumor samples and matched adjusted normal bladder specimens. Total RNA from 100 bladder tumor samples and 49 paired non-cancerous bladder specimens were isolated, and an accurate SYBR-Green based real-time quantitative polymerase chain reaction (qPCR) protocol was developed to quantify HSPB2 and HSPB3 mRNA levels in the two cohorts of specimens. A significant downregulation of the HSPB2 and HSPB3 genes expression was observed in bladder tumors as compared to matched normal urothelium; yet, increased HSPB2 and HSPB3 levels were noted in muscle-invasive (T2-T4) vs. superficial tumors (TaT1), as well as in high-grade vs. low-grade tumors. Survival analyses highlighted the significantly higher risk for post-treatment disease relapse in TaT1 patients poorly expressing HSPB2 and HSPB3 genes; this effect tended to be inverted in advanced disease stages (muscle-invasive tumors) indicating the biphasic impact of HSPB2, HSPB3 genes in BlCa progression. The pro-survival role of HSPB2 and HSPB3 in advanced tumor cells was also evident by our finding that HSPB2, HSPB3 genes expression silencing in high grade BlCa cells enhanced doxorubicin toxicity. These findings indicate that the HSPB2, HSPB3 chaperone genes have a likely pro-survival role in advanced BlCa; thus, they can be targeted as novel molecular markers to optimize treatment efficacy in BlCa and to limit unnecessary interventions.

α-Crystallin Domains of Five Human Small Heat Shock Proteins (sHsps) Differ in Dimer Stabilities and Ability to Incorporate Themselves into Oligomers of Full-Length sHsps

The α-crystallin domain (ACD) is the hallmark of a diverse family of small heat shock proteins (sHsps). We investigated some of the ACD properties of five human sHsps as well as their interactions with different full-length sHsps. According to size-exclusion chromatography, at high concentrations, the ACDs of HspB1 (B1ACD), HspB5 (B5ACD) and HspB6 (B6ACD) formed dimers of different stabilities, which, upon dilution, dissociated to monomers to different degrees. Upon dilution, the B1ACD dimers possessed the highest stabilities, and those of B6ACD had the lowest. In striking contrast, the ACDs of HspB7 (B7ACD) and HspB8 (B8ACD) formed monomers in the same concentration range, which indicated the compromised stabilities of their dimer interfaces. B1ACD, B5ACD and B6ACD transiently interacted with full-length HspB1 and HspB5, which are known to form large oligomers, and modulated their oligomerization behavior. The small oligomers formed by the 3D mutant of HspB1 (mimicking phosphorylation at Ser15, Ser78 and Ser82) effectively interacted with B1ACD, B5ACD and B6ACD, incorporating these α-crystallin domains into their structures. The inherently dimeric full-length HspB6 readily formed heterooligomeric complexes with B1ACD and B5ACD. In sharp contrast to the abovementioned ACDs, B7ACD and B8ACD were unable to interact with full-length HspB1, the 3D mutant of HspB1, HspB5 or HspB6. Thus, their high sequence homology notwithstanding, B7ACD and B8ACD differ from the other three ACDs in their inability to form dimers and interact with the full-length small heat shock proteins. Having conservative primary structures and being apparently similar, the ACDs of the different sHsps differ in terms of their dimer stabilities, which can influence the heterooligomerization preferences of sHsps.

The chaperone-assisted selective autophagy complex dynamics and dysfunctions

Each protein must be synthesized with the correct amino acid sequence, folded into its native structure, and transported to a relevant subcellular location and protein complex. If any of these steps fail, the cell has the capacity to break down aberrant proteins to maintain protein homeostasis (also called proteostasis). All cells possess a set of well-characterized protein quality control systems to minimize protein misfolding and the damage it might cause. Autophagy, a conserved pathway for the degradation of long-lived proteins, aggregates, and damaged organelles, was initially characterized as a bulk degradation pathway. However, it is now clear that autophagy also contributes to intracellular homeostasis by selectively degrading cargo material. One of the pathways involved in the selective removal of damaged and misfolded proteins is chaperone-assisted selective autophagy (CASA). The CASA complex is composed of three main proteins (HSPA, HSPB8 and BAG3), essential to maintain protein homeostasis in muscle and neuronal cells. A failure in the CASA complex, caused by mutations in the respective coding genes, can lead to (cardio)myopathies and neurodegenerative diseases. Here, we summarize our current understanding of the CASA complex and its dynamics. We also briefly discuss how CASA complex proteins are involved in disease and may represent an interesting therapeutic target.Abbreviation ALP: autophagy lysosomal pathway; ALS: amyotrophic lateral sclerosis; AMOTL1: angiomotin like 1; ARP2/3: actin related protein 2/3; BAG: BAG cochaperone; BAG3: BAG cochaperone 3; CASA: chaperone-assisted selective autophagy; CMA: chaperone-mediated autophagy; DNAJ/HSP40: DnaJ heat shock protein family (Hsp40); DRiPs: defective ribosomal products; EIF2A/eIF2α: eukaryotic translation initiation factor 2A; EIF2AK1/HRI: eukaryotic translation initiation factor 2 alpha kinase 1; GABARAP: GABA type A receptor-associated protein; HDAC6: histone deacetylase 6; HSP: heat shock protein; HSPA/HSP70: heat shock protein family A (Hsp70); HSP90: heat shock protein 90; HSPB8: heat shock protein family B (small) member 8; IPV: isoleucine-proline-valine; ISR: integrated stress response; KEAP1: kelch like ECH associated protein 1; LAMP2A: lysosomal associated membrane protein 2A; LATS1: large tumor suppressor kinase 1; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MTOC: microtubule organizing center; MTOR: mechanistic target of rapamycin kinase; NFKB/NF-κB: nuclear factor kappa B; NFE2L2: NFE2 like bZIP transcription factor 2; PLCG/PLCγ: phospholipase C gamma; polyQ: polyglutamine; PQC: protein quality control; PxxP: proline-rich; RAN translation: repeat-associated non-AUG translation; SG: stress granule; SOD1: superoxide dismutase 1; SQSTM1/p62: sequestosome 1; STUB1/CHIP: STIP1 homology and U-box containing protein 1; STK: serine/threonine kinase; SYNPO: synaptopodin; TBP: TATA-box binding protein; TARDBP/TDP-43: TAR DNA binding protein; TFEB: transcription factor EB; TPR: tetratricopeptide repeats; TSC1: TSC complex subunit 1; UBA: ubiquitin associated; UPS: ubiquitin-proteasome system; WW: tryptophan-tryptophan; WWTR1: WW domain containing transcription regulator 1; YAP1: Yes1 associated transcriptional regulator.

Emerging therapeutic roles of small heat shock protein-derived mini-chaperones and their delivery strategies

The small heat shock protein (sHsp) family is a group of proteins in which some are induced in response to external stimuli, such as environmental and pathological stresses, while others are constitutively expressed. They show chaperone-like activity, protect cells from apoptosis, and maintain cytoskeletal architecture. Short sequences or fragments ranging from approximately 19-20 residues in sHsps were shown to display chaperone activity in vitro. These sequences are termed sHsp-derived mini-peptides/mini-chaperones. These peptides offer an advantage in providing protective and therapeutic effects over full-length proteins owing to their small molecular weight and easy uptake into the cells. Research on sHsp mini-chaperone therapy has recently received attention and advanced tremendously. sHsp mini-chaperones have shown a wide range of therapeutic effects, such as anti-aggregation of proteins, anti-apoptotic, anti-inflammatory, anti-oxidant, senolytic, and anti-platelet activity. The administration of mini-chaperones into the several disease animal models, including experimental autoimmune encephalomyelitis, cataract, age-related macular degeneration, glaucoma, and thrombosis through various routes reduced symptoms or prevented the progression of the disease. However, it was found that the therapeutic potential of sHsp mini-chaperones is limited by their short turnover and enzymatic degradation in circulation. Nonetheless, carrier molecules approach such as nanoparticles, cell penetration peptides, and extracellular vesicles increased their efficacy by enhancing the uptake, retention time, protection from enzymatic degradation, and site-specific delivery without altering their biological activity. In this context, this review highlights the recent advances in the therapeutic potential of sHsp-derived mini-chaperones, their effect in experimental animal models, and approaches for increasing their efficacy.

The permanently chaperone-active small heat shock protein Hsp17 from Caenorhabditis elegans exhibits topological separation of its N-terminal regions

Small Heat shock proteins (sHsps) are a family of molecular chaperones that bind nonnative proteins in an ATP-independent manner. Caenorhabditis elegans encodes 16 different sHsps, among them Hsp17, which is evolutionarily distinct from other sHsps in the nematode. The structure and mechanism of Hsp17 and how these may differ from other sHsps remain unclear. Here, we find that Hsp17 has a distinct expression pattern, structural organization, and chaperone function. Consistent with its presence under nonstress conditions, and in contrast to many other sHsps, we determined that Hsp17 is a mono-disperse, permanently active chaperone in vitro, which interacts with hundreds of different C. elegans proteins under physiological conditions. Additionally, our cryo-EM structure of Hsp17 reveals that in the 24-mer complex, 12 N-terminal regions are involved in its chaperone function. These flexible regions are located on the outside of the spherical oligomer, whereas the other 12 N-terminal regions are engaged in stabilizing interactions in its interior. This allows the same region in Hsp17 to perform different functions depending on the topological context. Taken together, our results reveal structural and functional features that further define the structural basis of permanently active sHsps.

The Role of Small Heat Shock Proteins in Protein Misfolding Associated Motoneuron Diseases

Motoneuron diseases (MNDs) are neurodegenerative conditions associated with death of upper and/or lower motoneurons (MNs). Proteostasis alteration is a pathogenic mechanism involved in many MNDs and is due to the excessive presence of misfolded and aggregated proteins. Protein misfolding may be the product of gene mutations, or due to defects in the translation process, or to stress agents; all these conditions may alter the native conformation of proteins making them prone to aggregate. Alternatively, mutations in members of the protein quality control (PQC) system may determine a loss of function of the proteostasis network. This causes an impairment in the capability to handle and remove aberrant or damaged proteins. The PQC system consists of the degradative pathways, which are the autophagy and the proteasome, and a network of chaperones and co-chaperones. Among these components, Heat Shock Protein 70 represents the main factor in substrate triage to folding, refolding, or degradation, and it is assisted in this task by a subclass of the chaperone network, the small heat shock protein (sHSPs/HSPBs) family. HSPBs take part in proteostasis by bridging misfolded and aggregated proteins to the HSP70 machinery and to the degradative pathways, facilitating refolding or clearance of the potentially toxic proteins. Because of its activity against proteostasis alteration, the chaperone system plays a relevant role in the protection against proteotoxicity in MNDs. Here, we discuss the role of HSPBs in MNDs and which HSPBs may represent a valid target for therapeutic purposes.

Is the small heat shock protein HSPB7 (cvHsp) a genuine actin-binding protein?

It is postulated that the small heat shock proteins directly interact with actin, affect formation and stabilize actin filaments. To verify this suggestion, we have analyzed interaction of recombinant human small heat shock protein HspB7 with skeletal muscle actin. In blot overlay HspB7 binds both G- and F-actin. The sites of interaction are located in the C-terminal large core domain of actin. In the course of ultracentrifugation F-actin and F-actin/tropomyosin complexes were pelleted and trapped HspB7. However, HspB7 pelleting was nonspecific and saturation was not achieved even at very high HspB7 concentration. HspB7 was unable to retard or prevent heat-induced F-actin aggregation. Native gel electrophoresis and chemical crosslinking failed to detect interaction of G-actin with HspB7, although both these methods clearly demonstrated formation of complexes formed by G-actin with DNAse I and cofilin-2. It is concluded that HspB7 is not a genuine actin-binding protein and its effect on actin filaments seems to be determined by interaction of HspB7 with minor regulatory proteins of actin filaments.

Role of Small Heat Shock Proteins in the Remodeling of Actin Microfilaments

Small heat shock proteins (sHsps) play an important role in the maintenance of proteome stability and, particularly, in stabilization of the cytoskeleton and cell contractile apparatus. Cell exposure to different types of stress is accompanied by the translocation of sHsps onto actin filaments; therefore, it is commonly believed that the sHsps are true actin-binding proteins. Investigations of last years have shown that this assumption is incorrect. Stress-induced translocation of sHsp to actin filaments is not the result of direct interaction of these proteins with intact actin, but results from the chaperone-like activity of sHsps and their interaction with various actin-binding proteins. HspB1 and HspB5 interact with giant elastic proteins titin and filamin thus providing an integrity of the contractile apparatus and its proper localization in the cell. HspB6 binds to the universal adapter protein 14-3-3 and only indirectly affects the structure of actin filament. HspB7 interacts with filamin C and controls actin filament assembly. HspB8 forms tight complex with the universal regulatory and adapter protein Bag3 and participates in the chaperone-assisted selective autophagy (CASA) of actin-binding proteins (e.g., filamin), as well as in the actin-depending processes taking place in mitoses. Hence, the mechanisms of sHsp participation in the maintenance of the contractile apparatus and cytoskeleton are much more complicated and diverse than it has been postulated earlier and are not limited to direct interactions of sHsps with actin. The old hypothesis on the direct binding of sHsps to intact actin should be revised and further detailed investigation on the sHsp interaction with minor proteins participating in the formation and remodeling of actin filaments is required.

Human HspB1, HspB3, HspB5 and HspB8: Shaping these disease factors during vertebrate evolution

Small heat shock proteins (sHSPs) emerged early in evolution and occur in all domains of life and nearly in all species, including humans. Mutations in four sHSPs (HspB1, HspB3, HspB5, HspB8) are associated with neuromuscular disorders. The aim of this study is to investigate the evolutionary forces shaping these sHSPs during vertebrate evolution. We performed comparative evolutionary analyses on a set of orthologous sHSP sequences, based on the ratio of non-synonymous: synonymous substitution rates for each codon. We found that these sHSPs had been historically exposed to different degrees of purifying selection, decreasing in this order: HspB8 > HspB1, HspB5 > HspB3. Within each sHSP, regions with different degrees of purifying selection can be discerned, resulting in characteristic selective pressure profiles. The conserved α-crystallin domains were exposed to the most stringent purifying selection compared to the flanking regions, supporting a 'dimorphic pattern' of evolution. Thus, during vertebrate evolution the different sequence partitions were exposed to different and measurable degrees of selective pressures. Among the disease-associated mutations, most are missense mutations primarily in HspB1 and to a lesser extent in the other sHSPs. Our data provide an explanation for this disparate incidence. Contrary to the expectation, most missense mutations cause dominant disease phenotypes. Theoretical considerations support a connection between the historic exposure of these sHSP genes to a high degree of purifying selection and the unusual prevalence of genetic dominance of the associated disease phenotypes. Our study puts the genetics of inheritable sHSP-borne diseases into the context of vertebrate evolution.

Understanding Diversity, Evolution, and Structure of Small Heat Shock Proteins in Annelida Through in Silico Analyses

Small heat shock proteins (sHsps) are oligomeric stress proteins characterized by an α-crystallin domain (ACD). These proteins are localized in different subcellular compartments and play critical roles in the stress physiology of tissues, organs, and whole multicellular eukaryotes. They are ubiquitous proteins found in all living organisms, from bacteria to mammals, but they have never been studied in annelids. Here, a data set of 23 species spanning the annelid tree of life, including mostly transcriptomes but also two genomes, was interrogated and 228 novel putative sHsps were identified and manually curated. The analysis revealed very high protein diversity and showed that a significant number of sHsps have a particular dimeric architecture consisting of two tandemly repeated ACDs. The phylogenetic analysis distinguished three main clusters, two of them containing both monomeric sHsps, and ACDs located downstream in the dimeric sHsps, and the other one comprising the upstream ACDs from those dimeric forms. Our results support an evolutionary history of these proteins based on duplication events prior to the Spiralia split. Monomeric sHsps 76) were further divided into five subclusters. Physicochemical properties, subcellular location predictions, and sequence conservation analyses provided insights into the differentiating elements of these putative functional groups. Strikingly, three of those subclusters included sHsps with features typical of metazoans, while the other two presented characteristics resembling non-metazoan proteins. This study provides a solid background for further research on the diversity, evolution, and function in the family of the sHsps. The characterized annelid sHsps are disclosed as essential for improving our understanding of this important family of proteins and their pleotropic functions. The features and the great diversity of annelid sHsps position them as potential powerful molecular biomarkers of environmental stress for acting as prognostic tool in a diverse range of environments.

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.

Insights Into the Role of Heat Shock Protein 27 in the Development of Neurodegeneration

Small heat shock protein 27 is a critically important chaperone, that plays a key role in several essential and varied physiological processes. These include thermotolerance, apoptosis, cytoskeletal dynamics, cell differentiation, protein folding, among others. Despite its relatively small size and intrinsically disordered termini, it forms large and polydisperse oligomers that are in equilibrium with dimers. This equilibrium is driven by transient interactions between the N-terminal region, the α-crystallin domain, and the C-terminal region. The continuous redistribution of binding partners results in a conformationally dynamic protein that allows it to adapt to different functions where substrate capture is required. However, the intrinsic disorder of the amino and carboxy terminal regions and subsequent conformational variability has made structural investigations challenging. Because heat shock protein 27 is critical for so many key cellular functions, it is not surprising that it also has been linked to human disease. Charcot-Marie-Tooth and distal hereditary motor neuropathy are examples of neurodegenerative disorders that arise from single point mutations in heat shock protein 27. The development of possible treatments, however, depends on our understanding of its normal function at the molecular level so we might be able to understand how mutations manifest as disease. This review will summarize recent reports describing investigations into the structurally elusive regions of Hsp27. Recent insights begin to provide the required context to explain the relationship between a mutation and the resulting loss or gain of function that leads to Charcot-Marie Tooth disease and distal hereditary motor neuropathy.

Protein Aggregation and Cataract: Role of Age-Related Modifications and Mutations in α-Crystallins

* The article is published as a part of the Special Issue "Protein Misfolding and Aggregation in Cataract Disorders" (Vol. 87, No. 2). ** To whom correspondence should be addressed. Cataract is a major cause of blindness. Due to the lack of protein turnover, lens proteins accumulate age-related and environmental modifications that alter their native conformation, leading to the formation of aggregation-prone intermediates, as well as insoluble and light-scattering aggregates, thus compromising lens transparency. The lens protein, α-crystallin, is a molecular chaperone that prevents protein aggregation, thereby maintaining lens transparency. However, mutations or post-translational modifications, such as oxidation, deamidation, truncation and crosslinking, can render α-crystallins ineffective and lead to the disease exacerbation. Here, we describe such mutations and alterations, as well as their consequences. Age-related modifications in α-crystallins affect their structure, oligomerization, and chaperone function. Mutations in α-crystallins can lead to the aggregation/intracellular inclusions attributable to the perturbation of structure and oligomeric assembly and resulting in the rearrangement of aggregation-prone regions. Such rearrangements can lead to the exposure of hitherto buried aggregation-prone regions, thereby populating aggregation-prone state(s) and facilitating amorphous/amyloid aggregation and/or inappropriate interactions with cellular components. Investigations of the mutation-induced changes in the structure, oligomer assembly, aggregation mechanisms, and interactomes of α-crystallins will be useful in fighting protein aggregation-related diseases.

Insights on Human Small Heat Shock Proteins and Their Alterations in Diseases

The family of the human small Heat Shock Proteins (HSPBs) consists of ten members of chaperones (HSPB1-HSPB10), characterized by a low molecular weight and capable of dimerization and oligomerization forming large homo- or hetero-complexes. All HSPBs possess a highly conserved centrally located α-crystallin domain and poorly conserved N- and C-terminal domains. The main feature of HSPBs is to exert cytoprotective functions by preserving proteostasis, assuring the structural maintenance of the cytoskeleton and acting in response to cellular stresses and apoptosis. HSPBs take part in cell homeostasis by acting as holdases, which is the ability to interact with a substrate preventing its aggregation. In addition, HSPBs cooperate in substrates refolding driven by other chaperones or, alternatively, promote substrate routing to degradation. Notably, while some HSPBs are ubiquitously expressed, others show peculiar tissue-specific expression. Cardiac muscle, skeletal muscle and neurons show high expression levels for a wide variety of HSPBs. Indeed, most of the mutations identified in HSPBs are associated to cardiomyopathies, myopathies, and motor neuropathies. Instead, mutations in HSPB4 and HSPB5, which are also expressed in lens, have been associated with cataract. Mutations of HSPBs family members encompass base substitutions, insertions, and deletions, resulting in single amino acid substitutions or in the generation of truncated or elongated proteins. This review will provide an updated overview of disease-related mutations in HSPBs focusing on the structural and biochemical effects of mutations and their functional consequences.

Alpha B-Crystallin in Muscle Disease Prevention: The Role of Physical Activity

HSPB5 or alpha B-crystallin (CRYAB), originally identified as lens protein, is one of the most widespread and represented of the human small heat shock proteins (sHSPs). It is greatly expressed in tissue with high rates of oxidative metabolism, such as skeletal and cardiac muscles, where HSPB5 dysfunction is associated with a plethora of human diseases. Since HSPB5 has a major role in protecting muscle tissues from the alterations of protein stability (i.e., microfilaments, microtubules, and intermediate filament components), it is not surprising that this sHSP is specifically modulated by exercise. Considering the robust content and the protective function of HSPB5 in striated muscle tissues, as well as its specific response to muscle contraction, it is then realistic to predict a specific role for exercise-induced modulation of HSPB5 in the prevention of muscle diseases caused by protein misfolding. After offering an overview of the current knowledge on HSPB5 structure and function in muscle, this review aims to introduce the reader to the capacity that different exercise modalities have to induce and/or activate HSPB5 to levels sufficient to confer protection, with the potential to prevent or delay skeletal and cardiac muscle disorders.

Heat Shock Protein 22 in Physiological and Pathological Hearts: Small Molecule, Large Potentials

Small heat shock protein 22 (HSP22) belongs to the superfamily of heat shock proteins and is predominantly expressed in the heart, brain, skeletal muscle, and different types of cancers. It has been found that HSP22 is involved in variant cellular functions in cardiomyocytes and plays a vital role in cardiac protection against cardiomyocyte injury under diverse stress. This review summarizes the multiple functions of HSP22 in the heart and the underlying molecular mechanisms through modulating gene transcription, post-translational modification, subcellular translocation of its interacting proteins, and protein degradation, facilitating mitochondrial function, cardiac metabolism, autophagy, and ROS production and antiapoptotic effect. We also discuss the association of HSP22 in cardiac pathologies, including human dilated cardiomyopathy, pressure overload-induced heart failure, ischemic heart diseases, and aging-related cardiac metabolism disorder. The collected information would provide insights into the understanding of the HSP22 in heart diseases and lead to discovering the therapeutic targets.

Oligomeric Structural Transition of HspB1 from Chinese Hamster

HspB1 is a mammalian sHsp that is ubiquitously expressed in almost all tissues and involved in regulating many vital functions. Although the recent crystal structure of human HspB1 showed that 24 monomers form the oligomeric complex of human HspB1 in a spherical configuration, the molecular architecture of HspB1 is still controversial. In this study, we examined the oligomeric structural change of CgHspB1 by sedimentation velocity analytical ultracentrifugation. At the low temperature of 4 °C, CgHspB1 exists as an 18-mer, probably a trimeric complex of hexamers. It is relatively unstable and partially dissociates into small oligomers, hexamers, and dodecamers. At elevated temperatures, the 24-mer was more stable than the 18-mer. The 24-mer is also in dynamic equilibrium with the dissociated oligomers in the hexameric unit. The hexamer further dissociates to dimers. The disulfide bond between conserved cysteine residues seems to be partly responsible for the stabilization of hexamers. The N-terminal domain is involved in the assembly of dimers and the interaction between hexamers. It is plausible that CgHspB1 expresses a chaperone function in the 24-mer structure.

Quaternary Structure and Hetero-Oligomerization of Recombinant Human Small Heat Shock Protein HspB7 (cvHsp)

In this study, a reliable and simple method of untagged recombinant human HspB7 preparation was developed. Recombinant HspB7 is presented in two oligomeric forms with an apparent molecular weight of 36 kDa (probably dimers) and oligomers with an apparent molecular weight of more than 600 kDa. By using hydrophobic and size-exclusion chromatography, we succeeded in preparation of HspB7 dimers. Mild oxidation promoted the formation of large oligomers, whereas the modification of Cys 126 by iodoacetamide prevented it. The deletion of the first 13 residues or deletion of the polySer motif (residues 17–29) also prevented the formation of large oligomers of HspB7. Cys-mutants of HspB6 and HspB8 containing a single-Cys residue in the central part of the β7 strand in a position homologous to that of Cys137 in HspB1 can be crosslinked to the wild-type HspB7 through a disulfide bond. Immobilized on monoclonal antibodies, the wild-type HspB6 interacted with the wild-type HspB7. We suppose that formation of heterodimers of HspB7 with HspB6 and HspB8 may be important for the functional activity of these small heat shock proteins.

Interplay between Heat Shock Proteins, Inflammation, and Pain: A promising Therapeutic Approach

Heat Shock Proteins (HSPs) are important molecular chaperones that facilitate many functions of the cells. They also play a pivotal role in cell survival, especially in the presence of stressors, including nutritional deprivation, lack of oxygen, fever, alcohol, inflammation, oxidative stress, heavy metals, as well as conditions that cause injury and necrosis. In the face of a painful stimulus encounter, many factors could be associated with pain that may include nitric oxide, excitatory amino acids, reactive oxygen species (ROS) formation, prostaglandins, and inflammatory cytokines. One influential factor affecting pain reduction is the expression of HSPs that act as a ROS scavenger, regulate the inflammatory cytokines, and reduce pain responses subsequently. Hence, we assembled information on the painkilling attributes of HSPs. In this field of research, new painkillers could be developed by targetting HSPs to alleviate pain and widen our grasp of pain in pathological conditions and neurological diseases.

Replacement of Arg in the conserved N-terminal RLFDQxFG motif affects physico-chemical properties and chaperone-like activity of human small heat shock protein HspB8 (Hsp22)

The small heat shock protein (sHsp) called HspB8 (formerly, Hsp22) is one of the least typical sHsp members, whose oligomerization status remains debatable. Here we analyze the effect of mutations in a highly conservative sequence located in the N-terminal domain of human HspB8 on its physico-chemical properties and chaperone-like activity. According to size-exclusion chromatography coupled to multi-angle light scattering, the wild type (WT) HspB8 is present as dominating monomeric species (~24 kDa) and a small fraction of oligomers (~60 kDa). The R29A amino acid substitution leads to the predominant formation of 60-kDa oligomers, leaving only a small fraction of monomers. Deletion of the 28–32 pentapeptide (Δ mutant) results in the formation of minor quantities of dimers (~49 kDa) and large quantities of the 24-kDa monomers. Both the WT protein and its Δ mutant efficiently bind a hydrophobic probe bis-ANS and are relatively rapidly hydrolyzed by chymotrypsin, whereas the R29A mutant weakly binds bis-ANS and resists chymotrypsinolysis. In contrast to HspB8 WT and its Δ mutant, which are well phosphorylated by cAMP-dependent and ERK1 protein kinases, the R29A mutant is poorly phosphorylated. R29A mutation affects the chaperone-like activity of HspB8 measured in vitro. It is concluded that the irreplaceable Arg residue located in the only highly conservative motif in the N-terminal domain of all sHsp proteins affects the oligomeric structure and key properties of HspB8.

Small heat-shock protein HSPB3 promotes myogenesis by regulating the lamin B receptor

One of the critical events that regulates muscle cell differentiation is the replacement of the lamin B receptor (LBR)-tether with the lamin A/C (LMNA)-tether to remodel transcription and induce differentiation-specific genes. Here, we report that localization and activity of the LBR-tether are crucially dependent on the muscle-specific chaperone HSPB3 and that depletion of HSPB3 prevents muscle cell differentiation. We further show that HSPB3 binds to LBR in the nucleoplasm and maintains it in a dynamic state, thus promoting the transcription of myogenic genes, including the genes to remodel the extracellular matrix. Remarkably, HSPB3 overexpression alone is sufficient to induce the differentiation of two human muscle cell lines, LHCNM2 cells, and rhabdomyosarcoma cells. We also show that mutant R116P-HSPB3 from a myopathy patient with chromatin alterations and muscle fiber disorganization, forms nuclear aggregates that immobilize LBR. We find that R116P-HSPB3 is unable to induce myoblast differentiation and instead activates the unfolded protein response. We propose that HSPB3 is a specialized chaperone engaged in muscle cell differentiation and that dysfunctional HSPB3 causes neuromuscular disease by deregulating LBR.

Involvement of small heat shock proteins (sHsps) in developmental stages of fall armyworm, Spodoptera frugiperda and its expression pattern under abiotic stress condition

Fall armyworm (FAW), Spodoptera frugiperda a recent invasive pest in India is reported to cause significant damage by feeding voraciously on maize and other economic crops from tropical to temperate provinces. It is becoming an arduous challenge to control the pest as it can survive in a wide range of temperature conditions and is already said to develop resistance towards certain insecticides. The small Heat shock proteins (hereafter, sHsps) are known to play an important role in adaptation of insects under such stress conditions. Our present study involved characterization of the three sHsps genes (sHsp19.74, sHsp20.7 and sHsp19.07) which encoded proteins of about 175, 176 and 165 amino acids with a conserved α-crystalline domain. Phylogenetic analysis of deduced amino acid sequences of the three genes showed strong similarity with the other lepidopteran sHsps. The effect of different growth stages on the expression profile of these stress proteins has also been studied and the Quantitative real time PCR (qRT-PCR) analysis revealed that the transcript level of sHsp19.07 and sHsp20.7 were significantly upregulated under extreme heat (44 °C) and cold (5 °C) stress. However, sHsp19.74 responded only to heat treatment but not to the cold treatment. In addition, the expression profile of all three sHsps was significantly lower in the larval stage (5^th instar). Chlorantraniliprole treatment resulted in maximum expression of sHsp19.07 and sHsp20.7 after 12hr of exposure to the insecticide. Meanwhile, the same expression was observed after 8hr of exposure in case of sHsp19.74. These results proved that the sHsp genes of S. frugiperda were induced and modulated in response to abiotic stress, thus influencing the physiological function leading to survival of FAW in diversified climate in India.

Small Hsps as Therapeutic Targets of Cystic Fibrosis Transmembrane Conductance Regulator Protein

Human small heat shock proteins are molecular chaperones that regulate fundamental cellular processes in normal and pathological cells. Here, we have reviewed the role played by HspB1, HspB4 and HspB5 in the context of Cystic Fibrosis (CF), a severe monogenic autosomal recessive disease linked to mutations in Cystic Fibrosis Transmembrane conductance Regulator protein (CFTR) some of which trigger its misfolding and rapid degradation, particularly the most frequent one, F508del-CFTR. While HspB1 and HspB4 favor the degradation of CFTR mutants, HspB5 and particularly one of its phosphorylated forms positively enhance the transport at the plasma membrane, stability and function of the CFTR mutant. Moreover, HspB5 molecules stimulate the cellular efficiency of currently used CF therapeutic molecules. Different strategies are suggested to modulate the level of expression or the activity of these small heat shock proteins in view of potential in vivo therapeutic approaches. We then conclude with other small heat shock proteins that should be tested or further studied to improve our knowledge of CFTR processing.

Cardio-Vascular Heat Shock Protein (cvHsp, HspB7), an Unusual Representative of Small Heat Shock Protein Family

HspB7 is one of ten human small heat shock proteins. This protein is expressed only in insulin-dependent tissues (heart, skeletal muscle, and fat tissue), and expression of HspB7 is regulated by many different factors. Single nucleotide polymorphism is characteristic for the HspB7 gene and this polymorphism correlates with cardio-vascular diseases and obesity. HspB7 has an unusual N-terminal sequence, a conservative α-crystallin domain, and very short C-terminal domain lacking conservative IPV tripeptide involved in a small heat shock proteins oligomer formation. Nevertheless, in the isolated state HspB7 forms both small oligomers (probably dimers) and very large oligomers (aggregates). HspB7 is ineffective in suppression of amorphous aggregation of model proteins induced by heating or reduction of disulfide bonds, however it is very effective in prevention of aggregation of huntingtin fragments enriched with Gln residues. HspB7 can be an effective sensor of electrophilic agents. This protein interacts with the contractile and cytoskeleton proteins (filamin C, titin, and actin) and participates in protection of the contractile apparatus and cytoskeleton from different adverse conditions. HspB7 possesses tumor suppressive activity. Further investigations are required to understand molecular mechanisms of HspB7 participation in numerous biological processes.

O-GlcNAc modification of small heat shock proteins enhances their anti-amyloid chaperone activity

A major role for the intracellular post-translational modification O-GlcNAc appears to be the inhibition of protein aggregation. Most of the previous studies in this area focused on O-GlcNAc modification of the amyloid-forming proteins themselves. Here we used synthetic protein chemistry to discover that O-GlcNAc also activates the anti-amyloid activity of certain small heat shock proteins (sHSPs), a potentially more important modification event that can act broadly and substoichiometrically. More specifically, we found that O-GlcNAc increases the ability of sHSPs to block the amyloid formation of both α-synuclein and Aβ(1-42). Mechanistically, we show that O-GlcNAc near the sHSP IXI-domain prevents its ability to intramolecularly compete with substrate binding. Finally, we found that, although O-GlcNAc levels are globally reduced in Alzheimer's disease brains, the modification of relevant sHSPs is either maintained or increased, which suggests a mechanism to maintain these potentially protective O-GlcNAc modifications. Our results have important implications for neurodegenerative diseases associated with amyloid formation and potentially other areas of sHSP biology.