Myopathy-associated αB-crystallin Mutants

Desmin and its molecular chaperone, the αB-crystallin: How post-translational modifications modulate their functions in heart and skeletal muscles?

Maintenance of the highly organized striated muscle tissue requires a cell-wide dynamic network through protein-protein interactions providing an effective mechanochemical integrator of morphology and function. Through a continuous and complex trans-cytoplasmic network, desmin intermediate filaments ensure this essential role in heart and in skeletal muscle. Besides their role in the maintenance of cell shape and architecture (permitting contractile activity efficiency and conferring resistance towards mechanical stress), desmin intermediate filaments are also key actors of cell and tissue homeostasis. Desmin participates to several cellular processes such as differentiation, apoptosis, intracellular signalisation, mechanotransduction, vesicle trafficking, organelle biogenesis and/or positioning, calcium homeostasis, protein homeostasis, cell adhesion, metabolism and gene expression. Desmin intermediate filaments assembly requires αB-crystallin, a small heat shock protein. Over its chaperone activity, αB-crystallin is involved in several cellular functions such as cell integrity, cytoskeleton stabilization, apoptosis, autophagy, differentiation, mitochondria function or aggresome formation. Importantly, both proteins are known to be strongly associated to the aetiology of several cardiac and skeletal muscles pathologies related to desmin filaments disorganization and a strong disturbance of desmin interactome. Note that these key proteins of cytoskeleton architecture are extensively modified by post-translational modifications that could affect their functional properties. Therefore, we reviewed in the herein paper the impact of post-translational modifications on the modulation of cellular functions of desmin and its molecular chaperone, the αB-crystallin.

HspB5 Chaperone Structure and Activity Are Modulated by Chemical-Scale Interactions in the ACD Dimer Interface

Small heat shock proteins (sHsps) are a family of ATP-independent molecular chaperones that function as "holdases" and prevent protein aggregation due to changes in temperature, pH, or oxidation state. sHsps have a conserved α-crystallin domain (ACD), which forms the dimer building block, flanked by variable N- and C-terminal regions. sHsps populate various oligomeric states as a function of their sequestrase activity, and these dynamic structural features allow the proteins to interact with a plethora of cellular substrates. However, the molecular mechanisms of their dynamic conformational assembly and the interactions with various substrates remains unclear. Therefore, it is important to gain insight into the underlying physicochemical properties that influence sHsp structure in an effort to understand their mechanism(s) of action. We evaluated several disease-relevant mutations, D109A, F113Y, R116C, R120G, and R120C, in the ACD of HspB5 for changes to in vitro chaperone activity relative to that of wildtype. Structural characteristics were also evaluated by ANS fluorescence and CD spectroscopy. Our results indicated that mutation Y113F is an efficient holdase, while D109A and R120G, which are found in patients with myofibrillar myopathy and cataracts, respectively, exhibit a large reduction in holdase activity in a chaperone-like light-scattering assay, which indicated alterations in substrate-sHsp interactions. The extent of the reductions in chaperone activities are different among the mutants and specific to the substrate protein, suggesting that while sHsps are able to interact with many substrates, specific interactions provide selectivity for some substrates compared to others. This work is consistent with a model for chaperone activity where key electrostatic interactions in the sHsp dimer provide structural stability and influence both higher-order sHsp interactions and facilitate interactions with substrate proteins that define chaperone holdase activity.

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.

Crystallins as Important Pathogenic Targets for Accumulation of Structural Damages Resulting in Protein Aggregation and Cataract Development: Introduction to This Special Issue of Biochemistry (Moscow)

This issue of Biochemistry (Moscow) is dedicated to the role of protein misfolding and aggregation in cataract development. In fact, many genetic mutations or chemical and physical deleterious factors can initiate alterations in the macrostructural order and proper folding of eye lens proteins, which in some cases result in the formation of large light-scattering aggregates, affecting the quality of vision and making lens more prone to cataract development. Diabetes mellitus, which is associated with oxidative stress and mass production of highly reactive compounds, can accelerate unfolding and aggregation of eye lens proteins. This journal issue contains reviews and research articles that describe the destructive effects of mutations and highly reactive metabolites on the structure and function of lens crystallin proteins, as well important molecules in the lens's natural defense system involved in protection against deleterious effects of the physical and chemical factors.

Phosphorylation of the Chaperone-Like HspB5 Rescues Trafficking and Function of F508del-CFTR

Cystic Fibrosis is a lethal monogenic autosomal recessive disease linked to mutations in Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) protein. The most frequent mutation is the deletion of phenylalanine at position 508 of the protein. This F508del-CFTR mutation leads to misfolded protein that is detected by the quality control machinery within the endoplasmic reticulum and targeted for destruction by the proteasome. Modulating quality control proteins as molecular chaperones is a promising strategy for attenuating the degradation and stabilizing the mutant CFTR at the plasma membrane. Among the molecular chaperones, the small heat shock protein HspB1 and HspB4 were shown to promote degradation of F508del-CFTR. Here, we investigated the impact of HspB5 expression and phosphorylation on transport to the plasma membrane, function and stability of F508del-CFTR. We show that a phosphomimetic form of HspB5 increases the transport to the plasma membrane, function and stability of F508del-CFTR. These activities are further enhanced in presence of therapeutic drugs currently used for the treatment of cystic fibrosis (VX-770/Ivacaftor, VX-770+VX-809/Orkambi). Overall, this study highlights the beneficial effects of a phosphorylated form of HspB5 on F508del-CFTR rescue and its therapeutic potential in cystic fibrosis.

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.

Neuromuscular Diseases Due to Chaperone Mutations: A Review and Some New Results

Skeletal muscle and the nervous system depend on efficient protein quality control, and they express chaperones and cochaperones at high levels to maintain protein homeostasis. Mutations in many of these proteins cause neuromuscular diseases, myopathies, and hereditary motor and sensorimotor neuropathies. In this review, we cover mutations in DNAJB6, DNAJB2, αB-crystallin (CRYAB, HSPB5), HSPB1, HSPB3, HSPB8, and BAG3, and discuss the molecular mechanisms by which they cause neuromuscular disease. In addition, previously unpublished results are presented, showing downstream effects of BAG3 p.P209L on DNAJB6 turnover and localization.

Exploring the multifaceted roles of heat shock protein B8 (HSPB8) in diseases

HSPB8 is a member of ubiquitous small heat shock protein (sHSP) family, whose expression is induced in response to a wide variety of unfavorable physiological and environmental conditions. Investigation of HSPB8 structure indicated that HSPB8 belongs to the group of so-called intrinsically disordered proteins and possesses a highly flexible structure. Unlike most other sHSPs, HSPB8 tends to form small-molecular-mass oligomers and exhibits substrate-dependent chaperone activity. In cooperation with BAG3, the chaperone activity of HSPB8 was reported to be involved in the delivery of misfolded proteins to the autophagy machinery. Through this way, HSPB8 interferes with pathological processes leading to neurodegenerative diseases. Accordingly, published studies have identified genetic links between mutations of HSPB8 and some kind of neuromuscular diseases, further supporting its important role in neurodegenerative disorders. In addition to their anti-aggregation properties, HSPB8 is indicated to interact with a wide range of client proteins, modulating their maturations and activities, and therefore, regulates a large repertoire of cellular functions, including apoptosis, proliferation, inflammation and etc. As a result, HSPB8 has key roles in cancer biology, autoimmune diseases, cardiac diseases and cerebral vascular diseases.

Loss of αB-crystallin function in zebrafish reveals critical roles in the development of the lens and stress resistance of the heart

Genetic mutations in the human small heat shock protein αB-crystallin have been implicated in autosomal cataracts and skeletal myopathies, including heart muscle diseases (cardiomyopathy). Although these mutations lead to modulation of their chaperone activity in vitro, the in vivo functions of αB-crystallin in the maintenance of both lens transparency and muscle integrity remain unclear. This lack of information has hindered a mechanistic understanding of these diseases. To better define the functional roles of αB-crystallin, we generated loss-of-function zebrafish mutant lines by utilizing the CRISPR/Cas9 system to specifically disrupt the two αB-crystallin genes, αBa and αBb We observed lens abnormalities in the mutant lines of both genes, and the penetrance of the lens phenotype was higher in αBa than αBb mutants. This finding is in contrast with the lack of a phenotype previously reported in αB-crystallin knock-out mice and suggests that the elevated chaperone activity of the two zebrafish orthologs is critical for lens development. Besides its key role in the lens, we uncovered another critical role for αB-crystallin in providing stress tolerance to the heart. The αB-crystallin mutants exhibited hypersusceptibility to develop pericardial edema when challenged by crowding stress or exposed to elevated cortisol stress, both of which activate glucocorticoid receptor signaling. Our work illuminates the involvement of αB-crystallin in stress tolerance of the heart presumably through the proteostasis network and reinforces the critical role of the chaperone activity of αB-crystallin in the maintenance of lens transparency.

The growing world of small heat shock proteins: from structure to functions

Small heat shock proteins (sHSPs) are present in all kingdoms of life and play fundamental roles in cell biology. sHSPs are key components of the cellular protein quality control system, acting as the first line of defense against conditions that affect protein homeostasis and proteome stability, from bacteria to plants to humans. sHSPs have the ability to bind to a large subset of substrates and to maintain them in a state competent for refolding or clearance with the assistance of the HSP70 machinery. sHSPs participate in a number of biological processes, from the cell cycle, to cell differentiation, from adaptation to stressful conditions, to apoptosis, and, even, to the transformation of a cell into a malignant state. As a consequence, sHSP malfunction has been implicated in abnormal placental development and preterm deliveries, in the prognosis of several types of cancer, and in the development of neurological diseases. Moreover, mutations in the genes encoding several mammalian sHSPs result in neurological, muscular, or cardiac age-related diseases in humans. Loss of protein homeostasis due to protein aggregation is typical of many age-related neurodegenerative and neuromuscular diseases. In light of the role of sHSPs in the clearance of un/misfolded aggregation-prone substrates, pharmacological modulation of sHSP expression or function and rescue of defective sHSPs represent possible routes to alleviate or cure protein conformation diseases. Here, we report the latest news and views on sHSPs discussed by many of the world's experts in the sHSP field during a dedicated workshop organized in Italy (Bertinoro, CEUB, October 12-15, 2016).

Cardiac arrhythmia and late-onset muscle weakness caused by a myofibrillar myopathy with unusual histopathological features due to a novel missense mutation in FLNC

Myofibrillar myopathies (MFM) are mostly adult-onset diseases characterized by progressive morphological alterations of the muscle fibers beginning in the Z-disk and the presence of protein aggregates in the sarcoplasm. They are mostly caused by mutations in different genes that encode Z-disk proteins, including DES, CRYAB, LDB3, MYOT, FLNC and BAG3. A large family of French origin, presenting an autosomal dominant pattern, characterized by cardiac arrhythmia associated to late-onset muscle weakness, was evaluated to clarify clinical, morphological and genetic diagnosis. Muscle weakness began during adult life (over 30 years of age), and had a proximal distribution. Histology showed clear signs of a myofibrillar myopathy, but with unusual, large inclusions. Subsequently, genetic testing was performed in MFM genes available for screening at the time of clinical/histological diagnosis, and desmin (DES), αB-crystallin (CRYAB), myotilin (MYOT) and ZASP (LDB3), were excluded. LMNA gene screening found the p.R296C variant which did not co-segregate with the disease. Genome wide scan revealed linkage to 7q.32, containing the FLNC gene. FLNC direct sequencing revealed a heterozygous c.3646T>A p.Tyr1216Asn change, co-segregating with the disease, in a highly conserved amino acid of the protein. Normal filamin C levels were detected by Western-blot analysis in patient muscle biopsies and expression of the mutant protein in NIH3T3 showed filamin C aggregates. This is an original FLNC mutation in a MFM family with an atypical clinical and histopathological presentation, given the presence of significantly focal lesions and prominent sarcoplasmic masses in muscle biopsies and the constant heart involvement preceding significantly the onset of the myopathy. Though a rare etiology, FLNC gene should not be excluded in early-onset arrhythmia, even in the absence of myopathy, which occurs later in the disease course.

NFκB is a central regulator of protein quality control in response to protein aggregation stresses via autophagy modulation

NFκB is a master regulator of protein quality control. It helps the cells to survive proteotoxicity by modulating autophagy via up-regulation of BAG3 and HspB8 expression, a molecular mechanism relevant to protein conformational diseases.

During cell life, proteins often misfold, depending on particular mutations or environmental changes, which may lead to protein aggregates that are toxic for the cell. Such protein aggregates are the root cause of numerous diseases called “protein conformational diseases,” such as myofibrillar myopathy and familial amyotrophic lateral sclerosis. To fight against aggregates, cells are equipped with protein quality control mechanisms. Here we report that NFκB transcription factor is activated by misincorporation of amino acid analogues into proteins, inhibition of proteasomal activity, expression of the R120G mutated form of HspB5 (associated with myofibrillar myopathy), or expression of the G985R and G93A mutated forms of superoxide dismutase 1 (linked to familial amyotrophic lateral sclerosis). This noncanonical stimulation of NFκB triggers the up-regulation of BAG3 and HspB8 expression, two activators of selective autophagy, which relocalize to protein aggregates. Then NFκB-dependent autophagy allows the clearance of protein aggregates. Thus NFκB appears as a central and major regulator of protein aggregate clearance by modulating autophagic activity. In this context, the pharmacological stimulation of this quality control pathway might represent a valuable strategy for therapies against protein conformational diseases.

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.

Drosophila small heat shock protein CryAB ensures structural integrity of developing muscles, and proper muscle and heart performance

Molecular chaperones, such as the small heat shock proteins (sHsps), maintain normal cellular function by controlling protein homeostasis in stress conditions. However, sHsps are not only activated in response to environmental insults, but also exert developmental and tissue-specific functions that are much less known. Here, we show that during normal development the Drosophila sHsp CryAB [L(2)efl] is specifically expressed in larval body wall muscles and accumulates at the level of Z-bands and around myonuclei. CryAB features a conserved actin-binding domain and, when attenuated, leads to clustering of myonuclei and an altered pattern of sarcomeric actin and the Z-band-associated actin crosslinker Cheerio (filamin). Our data suggest that CryAB and Cheerio form a complex essential for muscle integrity: CryAB colocalizes with Cheerio and, as revealed by mass spectrometry and co-immunoprecipitation experiments, binds to Cheerio, and the muscle-specific attenuation of cheerio leads to CryAB-like sarcomeric phenotypes. Furthermore, muscle-targeted expression of CryAB(R120G), which carries a mutation associated with desmin-related myopathy (DRM), results in an altered sarcomeric actin pattern, in affected myofibrillar integrity and in Z-band breaks, leading to reduced muscle performance and to marked cardiac arrhythmia. Taken together, we demonstrate that CryAB ensures myofibrillar integrity in Drosophila muscles during development and propose that it does so by interacting with the actin crosslinker Cheerio. The evidence that a DRM-causing mutation affects CryAB muscle function and leads to DRM-like phenotypes in the fly reveals a conserved stress-independent role of CryAB in maintaining muscle cell cytoarchitecture.

HSPB8 and the Cochaperone BAG3 Are Highly Expressed During the Synthetic Phase of Rat Myometrium Programming During Pregnancy

The small heat shock protein (HSP) B family of proteins are a group of molecular chaperones that enable tissues to adapt to changes in their local environments during differentiation, stress, or disease conditions. The objective of this research was to characterize the expression of HSPB8 and its cochaperone Bcl2-associated athanogene 3 (BAG3) in nonpregnant (NP) and pregnant rat myometrium during myometrial programming. Rat myometrium was collected from NP and pregnant rats as well as 1 day postpartum (PP) and samples prepared for immunoblot and immunofluorescence analysis. Immunoblot analysis determined that HSPB8 protein expression was significantly elevated at Day (D) 15, D17, and D19 compared to expression at NP and D6, while BAG3 expression was significantly elevated at D15 compared to NP, and D17 compared to NP, D6, D23, and PP time points (P < 0.05). In situ, HSPB8 and BAG3 were predominantly localized to myometrial cells throughout pregnancy, with intense cytoplasmic HSPB8 and BAG3 detection on D15 and D17 in both longitudinal and circular muscle layers. Immunoblot analysis of HSPB8 and BAG3 protein expression in myometrium from unilateral pregnancies also revealed that expression of both proteins was significantly increased at D15 in gravid compared to nongravid horns. Thus, HSPB8 and BAG3 are highly expressed during the synthetic phase of myometrial differentiation marked by initiation of uterine distension and myometrial hypertrophy. HSPB8 and BAG3 could be regulators of the protein quality control required for this process.

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.

Neuropathy- and myopathy-associated mutations in human small heat shock proteins: Characteristics and evolutionary history of the mutation sites

Mutations in four of the ten human small heat shock proteins (sHSP) are associated with various forms of motor neuropathies and myopathies. In HspB1, HspB3, and HspB8 all known mutations cause motor neuropathies, whereas in HspB5 they cause myopathies. Several features are common to the majority of these mutations: (i) they are missense mutations, (ii) most associated disease phenotypes exhibit a dominant inheritance pattern and late disease onset, (iii) in the primary protein sequences, the sites of most mutations are located in the conserved α-crystallin domain and the variable C-terminal extensions, and (iv) most human mutation sites are highly conserved among the vertebrate orthologs and have been historically exposed to significant purifying selection. In contrast, a minor fraction of these mutations deviate from these rules: they are (i) frame shifting, nonsense, or elongation mutations, (ii) associated with recessive or early onset disease phenotypes, (iii) positioned in the N-terminal domain of the proteins, and (iv) less conserved among the vertebrates and were historically not subject to a strong selective pressure. In several vertebrate sHSPs (including primate sHSPs), homologous sites differ from the human sequence and occasionally even encode the same amino acid residues that cause the disease in humans. Apparently, a number of these mutations sites are not crucial for the protein function in single species or entire taxa, and single species even seem to have adopted mechanisms that compensate for potentially adverse effects of 'mutant-like' sHSPs. The disease-associated dominant sHSP missense mutations have a number of cellular consequences that are consistent with gain-of-function mechanisms of genetic dominance: dominant-negative effects, the formation of cytotoxic amyloid protein oligomers and precipitates, disruption of cytoskeletal networks, and increased downstream enzymatic activities. Future therapeutic concepts should aim for reducing these adverse effects of mutant sHSPs in patients. Indeed, initial experimental results are encouraging.

Analysis of the dominant effects mediated by wild type or R120G mutant of αB-crystallin (HspB5) towards Hsp27 (HspB1)

Several human small heat shock proteins (sHsps) are phosphorylated oligomeric chaperones that enhance stress resistance. They are characterized by their ability to interact and form polydispersed hetero-oligomeric complexes. We have analyzed the cellular consequences of the stable expression of either wild type HspB5 or its cataracts and myopathies inducing R120G mutant in growing and oxidative stress treated HeLa cells that originally express only HspB1. Here, we describe that wild type and mutant HspB5 induce drastic and opposite effects on cell morphology and oxidative stress resistance. The cellular distribution and phosphorylation of these polypeptides as well as the oligomerization profile of the resulting hetero-oligomeric complexes formed by HspB1 with the two types of exogenous polypeptides revealed the dominant effects induced by HspB5 polypeptides towards HspB1. The R120G mutation enhanced the native size and salt resistance of HspB1-HspB5 complex. However, in oxidative conditions the interaction between HspB1 and mutant HspB5 was drastically modified resulting in the aggregation of both partners. The mutation also induced the redistribution of HspB1 phosphorylated at serine 15, originally observed at the level of the small oligomers that do not interact with wild type HspB5, to the large oligomeric complex formed with mutant HspB5. This phosphorylation stabilized the interaction of HspB1 with mutant HspB5. A dominant negative effect towards HspB1 appears therefore as an important event in the cellular sensitivity to oxidative stress mediated by mutated HspB5 expression. These observations provide novel data that describe how a mutated sHsp can alter the protective activity of another member of this family of chaperones.

O-GlcNAcylation of αB-crystallin regulates its stress-induced translocation and cytoprotection

Under normal conditions, the ubiquitously expressed αB-crystallin functions as a chaperone. αB-crystallin has been implicated in a variety of pathologies, consistent with a build-up of protein aggregates, such as neuromuscular disorders, myofibrillar myopathies, and cardiomyopathies. αB-crystallins' cardioprotection is partially attributed to its translocation and binding to cytoskeletal elements in response to stress. The triggers for this translocation are not clearly understood. In the heart, αB-crystallin undergoes at least three significant post-translational modifications: phosphorylation at ser-45 and 59 and O-GlcNAcylation (O-linked attachment of the monosaccharide β-N-acetyl-glucosamine) at thr-170. Whether phosphorylation status drives translocation remains controversial. Therefore, we evaluated the role of αB-crystallins' O-GlcNAcylation in its stress-induced translocation and cytoprotection in cardiomyocytes under stress. Immunoblotting and precipitation experiments with anti-O-GlcNAc antibody (CTD110.6) and glycoprotein staining (Pro-Q Emerald) both demonstrate robust stress-induced O-GlcNAcylation of αB-crystallin. A non-O-GlcNAcylatable αB-crystallin mutant (αB-T170A) showed diminished translocation in response to heat shock and robust phosphorylation at both ser-45 and ser-59. Cell survival assays show a loss of overexpression-associated cytoprotection with the non-glycosylatable mutant to multiple stresses. While ectopic expression of wild-type αB-crystallin strongly stabilized ZsProSensor, a fusion protein rapidly degraded by the proteasome, the non-O-GlcNAcylatable version did not. Therefore, we believe the O-GlcNAcylation of αB-crystallin is a dynamic and important regulator of both its localization and function.

Peptide aptamers: tools to negatively or positively modulate HSPB1(27) function

Human HSP27 (HSPB1) is a molecular chaperone sensor which, through dynamic changes in its phosphorylation and oligomerization, allows cells to adapt to changes in their physiology and/or mount a protective response to injuries. In pathological conditions, the high level of HSPB1 expression can either be beneficial, such as in diseases characterized by cellular degenerations, or be malignant in cancer cells where it promotes tumourigenesis, metastasis and anti-cancer drug resistance. Structural changes allow HSPB1 to interact with specific client protein partners in order to modulate their folding/activity and/or half-life. Therefore, the search is open for therapeutic compounds aimed at either down- or upregulating HSPB1 activity. In this respect, we have previously described two peptide aptamers (PA11 and PA50) that specifically interact with HSPB1 small oligomers and decrease its anti-apoptotic and tumourigenic activities. A novel analysis of the different HSPB1-interacting aptamers that were isolated earlier revealed that one aptamer (PA23) has the intriguing ability to stimulate the protective activity of HSPB1. We show here that this aptamer abolishes the dominant negative effect induced by the R120G mutant of αB-crystallin (HSPB5) by disrupting its interaction with HSPB1. Hence, developing structure-based interfering strategies could lead to the discovery of HSPB1-based therapeutic drugs.

Chaperone-independent mitochondrial translocation and protection by αB-crystallin in RPE cells

αB-crystallin is a small heat shock protein that exhibits chaperone activity and can protect multiple cell types against oxidative stress damage. Altered levels and specific mutations of αB-crystallin are associated with multiple degenerative diseases. We previously found that αB-crystallin translocates to lens and retinal cell mitochondria upon oxidative stress exposure where it provides protection against oxidative stress damage. To date, the role of the chaperone function of αB-crystallin in mitochondrial translocation and protection has not been established. Here, we sought to determine the relationship between the chaperone activity of αB-crystallin and its ability to translocate to and protect retinal cell mitochondria against oxidative stress damage. Our data provide evidence that three forms of αB-crystallin exhibiting different chaperone activity levels including wild-type, R120G (decreased chaperone activity) and M68A (increased chaperone activity) provide comparable levels of mitochondrial translocation and protection to retinal cells exposed to oxidative stress. The results provide evidence that mitochondrial translocation and protection by αB-crystallin is independent of its chaperone activity and that other functions of αB-crystallin may also be independent of its chaperone activity.

A combined laser microdissection and mass spectrometry approach reveals new disease relevant proteins accumulating in aggregates of filaminopathy patients

Filaminopathy is a subtype of myofibrillar myopathy caused by mutations in FLNC, the gene encoding filamin C, and histologically characterized by pathologic accumulation of several proteins within skeletal muscle fibers. With the aim to get new insights in aggregate composition, we collected aggregates and control tissue from skeletal muscle biopsies of six myofibrillar myopathy patients harboring three different FLNC mutations by laser microdissection and analyzed the samples by a label-free mass spectrometry approach. A total of 390 proteins were identified, and 31 of those showed significantly higher spectral indices in aggregates compared with patient controls with a ratio >1.8. These proteins included filamin C, other known myofibrillar myopathy associated proteins, and a striking number of filamin C binding partners. Across the patients the patterns were extremely homogeneous. Xin actin-binding repeat containing protein 2, heat shock protein 27, nebulin-related-anchoring protein, and Rab35 could be verified as new filaminopathy biomarker candidates. In addition, further experiments identified heat shock protein 27 and Xin actin-binding repeat containing protein 2 as novel filamin C interaction partners and we could show that Xin actin-binding repeat containing protein 2 and the known interaction partner Xin actin-binding repeat containing protein 1 simultaneously associate with filamin C. Ten proteins showed significant lower spectral indices in aggregate samples compared with patient controls (ratio <0.56) including M-band proteins myomesin-1 and myomesin-2. Proteomic findings were consistent with previous and novel immunolocalization data. Our findings suggest that aggregates in filaminopathy have a largely organized structure of proteins also interacting under physiological conditions. Different filamin C mutations seem to lead to almost identical aggregate compositions. The finding that filamin C was detected as highly abundant protein in aggregates in filaminopathy indicates that our proteomic approach may be suitable to identify new candidate genes among the many MFM patients with so far unknown mutation.

Pathology-dependent effects linked to small heat shock proteins expression: an update

Small heat shock proteins (small Hsps) are stress-induced molecular chaperones that act as holdases towards polypeptides that have lost their folding in stress conditions or consequently of mutations in their coding sequence. A cellular protection against the deleterious effects mediated by damaged proteins is thus provided to cells. These chaperones are also highly expressed in response to protein conformational and inflammatory diseases and cancer pathologies. Through specific and reversible modifications in their phospho-oligomeric organization, small Hsps can chaperone appropriate client proteins in order to provide cells with resistance to different types of injuries or pathological conditions. By helping cells to better cope with their pathological status, their expression can be either beneficial, such as in diseases characterized by pathological cell degeneration, or deleterious when they are required for tumor cell survival. Moreover, small Hsps are actively released by cells and can act as immunogenic molecules that have dual effects depending on the pathology. The cellular consequences linked to their expression levels and relationships with other Hsps as well as therapeutic strategies are discussed in view of their dynamic structural organization required to interact with specific client polypeptides.

Expression of the molecular chaperone αB-crystallin in infiltrating ductal breast carcinomas and the significance thereof: an immunohistochemical and proteomics-based strategy

This study aims to evaluate αB-crystallin expression in infiltrating ductal breast carcinomas (IDCAs), as well as, its prognostic significance. Using a two-dimensional electrophoresis matrix-assisted laser desorption/ionisation-time of flight mass spectrometry investigation coupled to an immunohistochemical approach, we have assessed the expression of αB-crystallin in IDCAs, as well as, in other types of breast tumors (invasive lobular carcinomas, medullary carcinomas, and in situ ductal carcinomas). Correlation between αB-crystallin expression and clinicopathological parameters of breast cancer has also been investigated. Proteomic analyses revealed an increased expression of αB-crystallin in IDCA tumors compared to adjacent nontumor tissues. Overexpression of this molecular chaperone was further confirmed in 51 tumor specimens. Statistical analyses revealed, however, no significant correlations between αB-crystallin expression and clinicopathological parameters of the disease (tumor stage, patient age, hormone receptors, SBR grade, and lymph node metastases). This study demonstrates the upregulation of αB-crystallin in IDCA tissues which may highlight its possible involvement in breast cancer development. Our findings do not, however, support the involvement of this molecular chaperone in the progression of this disease.

Heat shock proteins and heat shock factor 1 in carcinogenesis and tumor development: an update

Heat shock proteins (HSP) are a subset of the molecular chaperones, best known for their rapid and abundant induction by stress. HSP genes are activated at the transcriptional level by heat shock transcription factor 1 (HSF1). During the progression of many types of cancer, this heat shock transcriptional regulon becomes co-opted by mechanisms that are currently unclear, although evidently triggered in the emerging tumor cell. Concerted activation of HSF1 and the accumulation of HSPs then participate in many of the traits that permit the malignant phenotype. Thus, cancers of many histologies exhibit activated HSF1 and increased HSP levels that may help to deter tumor suppression and evade therapy in the clinic. We review here the extensive work that has been carried out and is still in progress aimed at (1) understanding the oncogenic mechanisms by which HSP genes are switched on, (2) determining the roles of HSF1/HSP in malignant transformation and (3) discovering approaches to therapy based on disrupting the influence of the HSF1-controlled transcriptome in cancer.

The protective and therapeutic function of small heat shock proteins in neurological diseases

Historically, small heat shock proteins (sHSPs) have been extensively studied in the context of being intracellular molecular chaperones. However, recent studies looking at the role of sHSPs in neurological diseases have demonstrated a near universal upregulation of certain sHSPs in damaged and diseased brains. Initially, it was thought that sHSPs are pathological in these disease states because they are found in the areas of damage. However, transgenic overexpression and exogenous administration of sHSPs in various experimental disease paradigms have shown just the contrary – that sHSPs are protective, not pathological. This review examines sHSPs in neurological diseases and highlights the potential for using these neuroprotective sHSPs as novel therapeutics. It first addresses the endogenous expression of sHSPs in a variety of neurological disorders. Although many studies have examined the expression of sHSPs in neurological diseases, there are no review articles summarizing these data. Furthermore, it focuses on recent studies that have investigated the therapeutic potential of sHSPs for neurological diseases. Finally, it will explain what we think is the function of endogenous sHSPs in neurological diseases.

Small heat shock proteins HSP27 (HspB1), αB-crystallin (HspB5) and HSP22 (HspB8) as regulators of cell death

Hsp27, αB-crystallin and HSP22 are ubiquitous small heat shock proteins (sHsp) whose expression is induced in response to a wide variety of unfavorable physiological and environmental conditions. These sHsp protect cells from otherwise lethal conditions mainly by their involvement in cell death pathways such as necrosis, apoptosis or autophagy. At a molecular level, the mechanisms accounting for sHsp functions in cell death are (1) prevention of denatured proteins aggregation, (2) regulation of caspase activity, (3) regulation of the intracellular redox state, (4) function in actin polymerization and cytoskeleton integrity and (5) proteasome-mediated degradation of selected proteins. In cancer cells, these sHsp are often overexpressed and associated with increased tumorigenicity, cancer cells metastatic potential and resistance to chemotherapy. Altogether, these properties suggest that Hsp27, αB-crystallin and Hsp22 are appropriate targets for modulating cell death pathways. In the present, we briefly review recent reports showing molecular evidence of cell death regulation by these sHsp and co-chaperones. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.

Small heat shock proteins and α-crystallins: dynamic proteins with flexible functions

The small heat shock proteins (sHSPs) and the related α-crystallins (αCs) are virtually ubiquitous proteins that are strongly induced by a variety of stresses, but that also function constitutively in multiple cell types in many organisms. Extensive research has demonstrated that a majority of sHSPs and αCs can act as ATP-independent molecular chaperones by binding denaturing proteins and thereby protecting cells from damage due to irreversible protein aggregation. As a result of their diverse evolutionary history, their connection to inherited human diseases, and their novel protein dynamics, sHSPs and αCs are of significant interest to many areas of biology and biochemistry. However, it is increasingly clear that no single model is sufficient to describe the structure, function or mechanism of action of sHSPs and αCs. In this review, we discuss recent data that provide insight into the variety of structures of these proteins, their dynamic behavior, how they recognize substrates, and their many possible cellular roles.

Large potentials of small heat shock proteins

Modern classification of the family of human small heat shock proteins (the so-called HSPB) is presented, and the structure and properties of three members of this family are analyzed in detail. Ubiquitously expressed HSPB1 (HSP27) is involved in the control of protein folding and, when mutated, plays a significant role in the development of certain neurodegenerative disorders. HSPB1 directly or indirectly participates in the regulation of apoptosis, protects the cell against oxidative stress, and is involved in the regulation of the cytoskeleton. HSPB6 (HSP20) also possesses chaperone-like activity, is involved in regulation of smooth muscle contraction, has pronounced cardioprotective activity, and seems to participate in insulin-dependent regulation of muscle metabolism. HSPB8 (HSP22) prevents accumulation of aggregated proteins in the cell and participates in the regulation of proteolysis of unfolded proteins. HSPB8 also seems to be directly or indirectly involved in regulation of apoptosis and carcinogenesis, contributes to cardiac cell hypertrophy and survival and, when mutated, might be involved in development of neurodegenerative diseases. All small heat shock proteins play important "housekeeping" roles and regulate many vital processes; therefore, they are considered as attractive therapeutic targets.

Tumor suppressor Alpha B-crystallin (CRYAB) associates with the cadherin/catenin adherens junction and impairs NPC progression-associated properties

Alpha B-crystallin (CRYAB) maps within the nasopharyngeal carcinoma (NPC) tumor-suppressive critical region 11q22-23 and its downregulation is significantly associated with the progression of NPC. However, little is known about the functional impact of CRYAB on NPC progression. In this study we evaluated the NPC tumor-suppressive and progression-associated functions of CRYAB. Activation of CRYAB suppressed NPC tumor formation in nude mice. Overexpression of CRYAB affected NPC progression-associated phenotypes such as loss of cell adhesion, invasion, interaction with the tumor microenvironment, invasive protrusion formation in three dimensional Matrigel culture, as well as expression of epithelial-mesenchymal transition-associated markers. CRYAB mediates this ability to suppress cancer progression by inhibition of E-cadherin cytoplasmic internalization and maintenance of β-catenin in the membrane that subsequently reduces the levels of expression of critical downstream targets such as cyclin-D1 and c-myc. Both ectopically expressed and recombinant CRYAB proteins were associated with endogenous E-cadherin and β-catenin, and, thus, the cadherin/catenin adherens junction. The CRYAB α-crystallin core domain is responsible for the interaction of CRYAB with both E-cadherin and β-catenin. Taken together, these results indicate that CRYAB functions to suppress NPC progression by associating with the cadherin/catenin adherens junction and modulating the β-catenin function.

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

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

HSPB1, HSPB6, HSPB7 and HSPB8 protect against RhoA GTPase-induced remodeling in tachypaced atrial myocytes

Background

We previously demonstrated the small heat shock protein, HSPB1, to prevent tachycardia remodeling in in vitro and in vivo models for Atrial Fibrillation (AF). To gain insight into its mechanism of action, we examined the protective effect of all 10 members of the HSPB family on tachycardia remodeling. Furthermore, modulating effects of HSPB on RhoA GTPase activity and F-actin stress fiber formation were examined, as this pathway was found of prime importance in tachycardia remodeling events and the initiation of AF.

Methods and Results

Tachypacing (4 Hz) of HL-1 atrial myocytes significantly and progressively reduced the amplitude of Ca²⁺ transients (CaT). In addition to HSPB1, also overexpression of HSPB6, HSPB7 and HSPB8 protected against tachypacing-induced CaT reduction. The protective effect was independent of HSPB1. Moreover, tachypacing induced RhoA GTPase activity and caused F-actin stress fiber formation. The ROCK inhibitor Y27632 significantly prevented tachypacing-induced F-actin formation and CaT reductions, showing that RhoA activation is required for remodeling. Although all protective HSPB members prevented the formation of F-actin stress fibers, their mode of action differs. Whilst HSPB1, HSPB6 and HSPB7 acted via direct prevention of F-actin formation, HSPB8-protection was mediated via inhibition of RhoA GTPase activity.

Conclusion

Overexpression of HSPB1, as well as HSPB6, HSPB7 and HSPB8 independently protect against tachycardia remodeling by attenuation of the RhoA GTPase pathway at different levels. The cardioprotective role for multiple HSPB members indicate a possible therapeutic benefit of compounds able to boost the expression of single or multiple members of the HSPB family.

Myofibrillar myopathies

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

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

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

αB-crystallin is elevated in highly infiltrative apoptosis-resistant glioblastoma cells

We have previously established two distinct glioma phenotypes by serial xenotransplantation of human glioblastoma (GBM) biopsies in nude rats. These tumors undergo a gradual transition from a highly invasive nonangiogenic to a less-invasive angiogenic phenotype. In a protein screen to identify molecular markers associated with the infiltrative phenotype, we identified α-basic-crystallin (αBc), a small heat-shock protein with cytoprotective properties. Its increased expression in the infiltrative phenotype was validated by immunohistochemistry and Western blots, confirming its identity to be tumor-derived and not from the host. Stereotactic human GBM biopsies taken from MRI-defined areas verified stronger αBc expression in the infiltrative edge compared to the tumor core. Cell migration assays and immunofluorescence staining showed αBc to be expressed by migrating cells in vitro. To determine αBc function, we altered its expression levels. αBc siRNA depletion caused a loss of migrating tumor cells from biopsy spheroids and delayed monolayer wound closure. In contrast, glioma cell migration in a Boyden chamber assay was unaffected by either αBc knockdown or overexpression, indicating that αBc is not functionally linked to the cell migration machinery. However, after siRNA αBc depletion, a significant sensitization of cells to various apoptotic inducers was observed (actinomycin, tumor necrosis factor α, and TNF-related apoptosis-inducing ligand [TRAIL]). In conclusion, αBc is overexpressed by highly migratory glioma cells where it plays a functional role in apoptosis resistance.

Abnormal interaction of motor neuropathy-associated mutant HspB8 (Hsp22) forms with the RNA helicase Ddx20 (gemin3)

A number of missense mutations in the two related small heat shock proteins HspB8 (Hsp22) and HspB1 (Hsp27) have been associated with the inherited motor neuron diseases (MND) distal hereditary motor neuropathy and Charcot-Marie-Tooth disease. HspB8 and HspB1 interact with each other, suggesting that these two etiologic factors may act through a common biochemical mechanism. However, their role in neuron biology and in MND is not understood. In a yeast two-hybrid screen, we identified the DEAD box protein Ddx20 (gemin3, DP103) as interacting partner of HspB8. Using co-immunoprecipitation, chemical cross-linking, and in vivo quantitative fluorescence resonance energy transfer, we confirmed this interaction. We also show that the two disease-associated mutant HspB8 forms have abnormally increased binding to Ddx20. Ddx20 itself binds to the survival-of-motor-neurons protein (SMN protein), and mutations in the SMN1 gene cause spinal muscular atrophy, another MND and one of the most prevalent genetic causes of infant mortality. Thus, these protein interaction data have linked the three etiologic factors HspB8, HspB1, and SMN protein, and mutations in any of their genes cause the various forms of MND. Ddx20 and SMN protein are involved in spliceosome assembly and pre-mRNA processing. RNase treatment affected the interaction of the mutant HspB8 with Ddx20 suggesting RNA involvement in this interaction and a potential role of HspB8 in ribonucleoprotein processing.

Molecular chaperone alphaB-crystallin is expressed in the human fetal telencephalon at midgestation by a subset of progenitor cells

Alphab-crystallin (CRYAB) is a small heat shock protein with a chaperoning activity that is present in the postnatal healthy human brain in oligodendrocytes and in a few astrocytes. The involvement of CRYAB in cell differentiation, proliferation, signaling, cytoskeletal assembly, and apoptosis in various model systems has suggested that it might also play a role in the developing human brain. We analyzed the distribution and the levels of this molecular chaperone in healthy and polygenetically compromised (Down syndrome [DS]) human telencephalon at midgestation. We demonstrate that CRYAB is expressed in a temporospatial pattern by numerous radial glial cells and some early oligodendrocyte progenitors, including dividing cells, as well as a few astroglial cells in both healthy and DS fetal brains. We also found abundant phosphorylation of CRYAB at Ser-59, which mediates its antiapoptotic and cytoskeletal functions. There was only marginal phosphorylation at Ser-45.In contrast to our earlier study in young DS subjects, upregulation of phosphorylated CRYAB occurred rarely in DS fetuses. The distribution, the timing of appearance, and the results of colocalization studies suggest that CRYAB assists in the biological processes associated with developmental remodeling/differentiation and proliferation of select subpopulations of progenitor cells in human fetal brain at midgestation.

Proproliferative functions of Drosophila small mitochondrial heat shock protein 22 in human cells

Aging is a complex process accompanied by a decreased capacity of cells to cope with random damages induced by reactive oxygen species, the natural by-products of energy metabolism, leading to protein aggregation in various components of the cell. Chaperones are important players in the aging process as they prevent protein misfolding and aggregation. Small chaperones, such as small heat shock proteins, are involved in the refolding and/or disposal of protein aggregates, a feature of many age-associated diseases. In Drosophila melanogaster, mitochondrial Hsp22 (DmHsp22), is localized in the mitochondrial matrix and is preferentially up-regulated during aging. Its overexpression results in an extension of life span (>30%) (Morrow, G., Samson, M., Michaud, S., and Tanguay, R. M. (2004) FASEB J. 18, 598-599 and Morrow, G., Battistini, S., Zhang, P., and Tanguay, R. M. (2004) J. Biol. Chem. 279, 43382-43385). Long lived flies expressing Hsp22 also have an increased resistance to oxidative stress and maintain locomotor activity longer. In the present study, the cross-species effects of Hsp22 expression were tested. DmHsp22 was found to be functionally active in human cells. It extended the life span of normal fibroblasts, slowing the aging process as evidenced by a lower level of the senescence associated beta-galactosidase. DmHsp22 expression in human cancer cells increased their malignant properties including anchorage-independent growth, tumor formation in nude mice, and resistance to a variety of anticancer drugs. We report that the DmHsp22 interacts and inactivates wild type tumor suppressor protein p53, which may be one possible way of its functioning in human cells.

Mutation in BAG3 causes severe dominant childhood muscular dystrophy

OBJECTIVE

Myofibrillar myopathies (MFMs) are morphologically distinct but genetically heterogeneous muscular dystrophies in which disintegration of Z disks and then of myofibrils is followed by ectopic accumulation of multiple proteins. Cardiomyopathy, neuropathy, and dominant inheritance are frequent associated features. Mutations in alphaB-crystallin, desmin, myotilin, Zasp, or filamin-C can cause MFMs and were detected in 32 of 85 patients of the Mayo MFM cohort. Bag3, another Z-disk-associated protein, has antiapoptotic properties, and its targeted deletion in mice causes fulminant myopathy with early lethality. We therefore searched for mutations in BAG3 in 53 unrelated MFM patients.

METHODS

We searched for mutations in BAG3 by direct sequencing. We analyzed structural changes in muscle by histochemistry, immunocytochemistry, and electron microscopy, examined mobility of the mutant Bag3 by nondenaturing electrophoresis, and searched for abnormal aggregation of the mutant protein in COS-7 (SV-40 transformed monkey kidney fibroblast-7) cells.

RESULTS

We identified a heterozygous p.Pro209Leu mutation in three patients. All presented in childhood, had progressive limb and axial muscle weakness, and experienced development of cardiomyopathy and severe respiratory insufficiency in their teens; two had rigid spines, and one a peripheral neuropathy. Electron microscopy showed disintegration of Z disks, extensive accumulation of granular debris and larger inclusions, and apoptosis of 8% of the nuclei. On nondenaturing electrophoresis of muscle extracts, the Bag3 complex migrated faster in patient than control extracts, and expression of FLAG-labeled mutant and wild-type Bag3 in COS cells showed abnormal aggregation of the mutant protein.

INTERPRETATION

We conclude mutation in Bag3 defines a novel severe autosomal dominant childhood muscular dystrophy.

Myofibrillar myopathies

PURPOSE OF REVIEW

The aim of this communication is to provide an up-to-date overview of myofibrillar myopathies.

RECENT FINDINGS

The most important recent advance in the myofibrillar myopathies has been the discovery that mutations in Z band alternatively spliced PDZ-containing protein and filamin C, as well as in desmin, alphaB-crystallin and myotilin, result in similar pathologic alterations in skeletal muscle that are typical of myofibrillar myopathy. Despite the increasing genetic heterogeneity, the clinical and morphologic phenotypes are remarkably homogeneous. The typical clinical manifestation is slowly progressive proximal, distal or both proximal and distal limb muscle weakness. Cardiomyopathy can be associated and is sometimes the presenting finding. Peripheral neuropathy also occurs in some patients. In every myofibrillar myopathy, there is abnormal accumulation of an array of proteins at ectopic sites as well as accumulation of degraded myofibrillar proteins forming large aggregates. The key issue now is to analyze the molecular mechanisms underlying the cascade of events that destroy the myofibrillar architecture and trigger the aberrant expression of multiple proteins.

SUMMARY

Several disease genes have recently been recognized in myofibrillar myopathies. So far, the disease proteins identified are components of or chaperone for the Z-disk. In each case, the molecular defect leads to a stereotyped cascade of structural events in the muscle fiber.

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

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