αB-Crystallin Maintains Skeletal Muscle Myosin Enzymatic Activity and Prevents its Aggregation under Heat-shock Stress

Impact of MG132 induced-proteotoxic stress on αB-crystallin and desmin phosphorylation and O-GlcNAcylation and their partition towards cytoskeleton

Small Heat Shock Proteins are considered as the first line of defense when proteostasis fails. Among them, αB-crystallin is expressed in striated muscles in which it interacts with desmin intermediate filaments to stabilize them, maintaining cytoskeleton's integrity and muscular functionalities. Desmin is a key actor for muscle health; its targeting by αB-crystallin is thus crucial, especially in stress conditions. αB-crystallin is phosphorylated and O-GlcNAcylated. Its phosphorylation increases consecutively to various stresses, correlated with its recruitment for cytoskeleton's safeguarding. However, phosphorylation as unique signal for cytoskeleton translocation remains controversial; indeed, O-GlcNAcylation was also proposed to be involved. Thus, there are still some gaps for a deeper comprehension of how αB-crystallin functions are finely regulated by post-translational modifications. Furthermore, desmin also bears both post-translational modifications; while desmin phosphorylation is closely linked to desmin intermediates filaments turnover, it is unclear whereas its O-GlcNAcylation could impact its proper function. In the herein paper, we aim at identifying whether phosphorylation and/or O-GlcNAcylation are involved in αB-crystallin targeting towards cytoskeleton in proteotoxic stress induced by proteasome inhibition in C2C12 myotubes. We demonstrated that proteotoxicity led to αB-crystallin's phosphorylation and O-GlcNAcylation patterns changes, both presenting a dynamic interplay depending on protein subfraction. Importantly, both post-translational modifications showed a spatio-temporal variation correlated with αB-crystallin translocation towards cytoskeleton. In contrast, we did not detect any change of desmin phosphorylation and O-GlcNAcylation. All together, these data strongly support that αB-crystallin phosphorylation/O-GlcNAcylation interplay rather than changes on desmin is a key regulator for its cytoskeleton translocation, preserving it towards stress.

Transcriptional Rearrangements Associated with Thermal Stress and Preadaptation in Baikal Whitefish (Coregonus baicalensis)

In this work, we describe the transcriptional profiles of preadapted and non-adapted one-month-old juvenile Baikal whitefish after heat shock exposure. Preadapted fish were exposed to a repeated thermal rise of 6 °C above the control temperature every three days throughout their embryonic development. One month after hatching, preadapted and non-adapted larvae were either kept at control temperatures (12 °C) or exposed to an acute thermal stress (TS) of 12 °C above the control temperature. In response to this acute stress, an increase in HSP gene expression (HSP-30, HSP-40, HSP-47, HSP-70, and HSP-90) and TRIM16 was detected, independent of preadaptation. The expression levels of genes responsible for the response to oxygen levels, growth factors and the immune response, HBA, HBB, Myosin VI, Myosin VII, MHC, Plumieribetin, TnI, CYP450, and LDB3 were higher in individuals that had previously undergone adaptation. Genes responsible for the regulation of metabolism, MtCK, aFGF, ARF, CRYGB, and D-DT, however, increased their activity in non-adapted individuals. This information on transcriptional profiles will contribute to further understanding of the mechanisms of adaptation of whitefish to their environment.

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.

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.

Function and Fiber-Type Specific Distribution of Hsp60 and αB-Crystallin in Skeletal Muscles: Role of Physical Exercise

Simple Summary

Skeletal muscle represents about 40% of the body mass in humans and it is a copious and plastic tissue, rich in proteins that are subject to continuous rearrangements. Physical exercise is considered a physiological stressor for different organs, in particular for skeletal muscle, and it is a factor able to stimulate the cellular remodeling processes related to the phenomenon of adaptation. All cells respond to various stress conditions by up-regulating the expression and/or activation of a group of proteins called heat shock proteins (HSPs). Although their expression is induced by several stimuli, they are commonly recognized as HSPs due to the first experiments showing their increased transcription after application of heat shock. These proteins are molecular chaperones mainly involved in assisting protein transport and folding, assembling multimolecular complexes, and triggering protein degradation by proteasome. Among the HSPs, a special attention needs to be devoted to Hsp60 and αB-crystallin, proteins constitutively expressed in the skeletal muscle, where they are known to be important in muscle physiopathology. Therefore, here we provide a critical update on their role in skeletal muscle fibers after physical exercise, highlighting the control of their expression, their biological function, and their specific distribution within skeletal muscle fiber-types.

Abstract

Skeletal muscle is a plastic and complex tissue, rich in proteins that are subject to continuous rearrangements. Skeletal muscle homeostasis can be affected by different types of stresses, including physical activity, a physiological stressor able to stimulate a robust increase in different heat shock proteins (HSPs). The modulation of these proteins appears to be fundamental in facilitating the cellular remodeling processes related to the phenomenon of training adaptations such as hypertrophy, increased oxidative capacity, and mitochondrial activity. Among the HSPs, a special attention needs to be devoted to Hsp60 and αB-crystallin (CRYAB), proteins constitutively expressed in the skeletal muscle, where their specific features could be highly relevant in understanding the impact of different volumes of training regimes on myofiber types and in explaining the complex picture of exercise-induced mechanical strain and damaging conditions on fiber population. This knowledge could lead to a better personalization of training protocols with an optimal non-harmful workload in populations of individuals with different needs and healthy status. Here, we introduce for the first time to the reader these peculiar HSPs from the perspective of exercise response, highlighting the control of their expression, biological function, and specific distribution within skeletal muscle fiber-types.

Eccentric Resistance Training Ameliorates Muscle Weakness in a Mouse Model of Idiopathic Inflammatory Myopathies

OBJECTIVE

High-force eccentric contractions (ECCs) have traditionally been excluded from rehabilitation programs that include patients with idiopathic inflammatory myopathies (IIMs) due to unverified fear of causing muscle damage and inflammation. In an IIM animal model that used mice with experimental autoimmune myositis (EAM), we undertook this study to investigate whether ECC training can safely and effectively be used to counteract muscle weakness in IIM.

METHODS

EAM was induced in BALB/c mice by immunization with 3 injections of myosin emulsified in Freund's complete adjuvant. Controls (n = 12) and mice with EAM (n = 12) were exposed to either an acute bout of 100 ECCs or 4 weeks of ECC training (20 ECCs every other day). To induce ECCs, plantar flexor muscles were electrically stimulated while the ankle was forcibly dorsiflexed.

RESULTS

Less cell damage, as assessed by Evans blue dye uptake, was observed in the muscles of mice with EAM, compared to controls, after an acute bout of 100 ECCs (P < 0.05). Maximum Ca²⁺ -activated force was decreased in skinned gastrocnemius muscle fibers from mice with EAM, and this was accompanied by increased expression of endoplasmic reticulum (ER) stress proteins, including Gsp78 and Gsp94 (P < 0.05). ECC training prevented the decrease in force and the increase in ER stress proteins and also enhanced the expression and myofibrillar binding of small heat-shock proteins (HSPs) (P < 0.05), which can stabilize myofibrillar structure and function.

CONCLUSION

ECC training protected against the reduction in myofibrillar force-generating capacity in an IIM mouse model, and this occurred via inhibition of ER stress responses and small HSP-mediated myofibrillar stabilization.

Under construction: The dynamic assembly, maintenance, and degradation of the cardiac sarcomere

The sarcomere is the basic contractile unit of striated muscle and is a highly ordered protein complex with the actin and myosin filaments at its core. Assembling the sarcomere constituents into this organized structure in development, and with muscle growth as new sarcomeres are built, is a complex process coordinated by numerous factors. Once assembled, the sarcomere requires constant maintenance as its continuous contraction is accompanied by elevated mechanical, thermal, and oxidative stress, which predispose proteins to misfolding and toxic aggregation. To prevent protein misfolding and maintain sarcomere integrity, the sarcomere is monitored by an assortment of protein quality control (PQC) mechanisms. The need for effective PQC is heightened in cardiomyocytes which are terminally differentiated and must survive for many years while preserving optimal mechanical output. To prevent toxic protein aggregation, molecular chaperones stabilize denatured sarcomere proteins and promote their refolding. However, when old and misfolded proteins cannot be salvaged by chaperones, they must be recycled via degradation pathways: the calpain and ubiquitin-proteasome systems, which operate under basal conditions, and the stress-responsive autophagy-lysosome pathway. Mutations to and deficiency of the molecular chaperones and associated factors charged with sarcomere maintenance commonly lead to sarcomere structural disarray and the progression of heart disease, highlighting the necessity of effective sarcomere PQC for maintaining cardiac function. This review focuses on the dynamic regulation of assembly and turnover at the sarcomere with an emphasis on the chaperones involved in these processes and describes the alterations to chaperones - through mutations and deficient expression - implicated in disease progression to heart failure.

Eccentric training enhances the αB-crystallin binding to the myofibrils and prevents skeletal muscle weakness in adjuvant-induced arthritis rat

Patients with rheumatoid arthritis (RA) frequently suffer from muscle weakness. We examined whether eccentric training prevents skeletal muscle weakness in adjuvant-induced arthritis (AIA) rat, a widely used animal model for RA. AIA was induced in the knees of Wistar rats by injection of complete Freund’s adjuvant. To induce eccentric contractions (ECCs), neuromuscular electrical stimulation (45 V) was applied to the plantar flexor muscles simultaneously with forced dorsiflexion of the ankle joint (0–40°) and was given every 6 s. ECC exercise was applied every other day for a total of 11 sessions and consisted of 4 sets of 5 contractions. There was a significant reduction in in vitro maximum Ca2+-activated force in skinned fibers in gastrocnemius muscle from AIA rats. These changes were associated with reduced expression levels of contractile proteins (i.e., myosin and actin), increased levels of inflammation redox stress-related biomarkers (i.e., TNF-α, malondialdehyde-protein adducts, NADPH oxidase 2, and neuronal nitric oxide synthase), and autolyzed active calpain-1 in AIA muscles. ECC training markedly enhanced the steady-state levels of αB-crystallin, a small heat shock protein, and its binding to the myofibrils and prevented the AIA-induced myofibrillar dysfunction, reduction in contractile proteins, and inflammation-oxidative stress insults. Our findings demonstrate that ECC training preserves myofibrillar function without muscle damage in AIA rats, which is at least partially attributable to the protective effect of αB-crystallin on the myofibrils against oxidative stress-mediated protein degeneration. Thus ECC training can be a safe and effective intervention, counteracting the loss of muscle strength in RA patients.

NEW & NOTEWORTHY Eccentric contractions (ECCs) are regarded as an effective way to increase muscle strength. No studies, however, assess safety and effectiveness of ECC training on muscle weakness associated with rheumatoid arthritis. Here, we used adjuvant-induced arthritis (AIA) rats to demonstrate that ECC training prevents intrinsic contractile dysfunction without muscle damage in AIA rats, which may be attributed to the protective effect of αB-crystallin on the myofibrils against inflammation-oxidative stress insults.

The associations between proteomic biomarkers and beef tenderness depend on the end-point cooking temperature, the country origin of the panelists and breed

Steaks of 74 animals from 3 young bull breeds (Aberdeen Angus, Limousin and Blond d'Aquitaine) were cooked at two end-point cooking temperatures (55 and 74 °C) and evaluated for tenderness by trained panelists from France (FR) and the United Kingdom (UK). Using principal component regressions, the tenderness scores of each breed, country origin of the panelists and cooking temperature were linked with the abundances of 21 protein biomarkers belonging to five biological pathways. Twelve regression equations were built and explained 68 to 95% of tenderness variability. A high dissimilarity in the retained biomarkers was observed among the equations and differences exist among breeds, cooking temperatures and country origin of the panelists. Among the 21 biomarkers, 6 proteins including structural (MyHC-I, MyHC-IIa, MyHC-IIx), oxidative stress (DJ-1, PRDX6) and proteolysis (CAPN1) were retained robustly in positive or negative directions in the tenderization process of Longissimus thoracis, regardless the breed, the end-point cooking temperature or the country origin of the panelist.

Advances in the Development of Anticancer HSP-based Vaccines

Current advances in cancer treatment are based on the recent discoveries of molecular mechanisms of tumour maintenance. It was shown that heat shock proteins (HSPs) play a crucial role in the development of immune response against tumours. Thus, HSPs represent multifunctional agents not only with chaperone functions, but also possessing immunomodulatory properties. These properties are exploited for the development of HSP-based anticancer vaccines aimed to induce cytotoxic responses against tumours. To date, a number of strategies have been suggested to facilitate HSP-based vaccine production and to increase its effectiveness. The present review focuses on the current trend for the development of HSPbased vaccines aimed at inducing strong immunological tumour-specific responses against cancer cells of distinct etiology and localization.

The early response of αB-crystallin to a single bout of aerobic exercise in mouse skeletal muscles depends upon fiber oxidative features

Besides its substantial role in eye lens, αB-crystallin (HSPB5) retains fundamental function in striated muscle during physiological or pathological modifications. In this study, we aimed to analyse the cellular and molecular factors driving the functional response of HSPB5 protein in different muscles from mice subjected to an acute bout of non-damaging endurance exercise or in C2C12 myocytes upon exposure to pro-oxidant environment, chosen as “in vivo” and “in vitro” models of a physiological stressing conditions, respectively.

To this end, red (GR) and white gastrocnemius (GW), as sources of slow-oxidative and fast-glycolytic/oxidative fibers, as well as the soleus (SOL), mainly composed of slow-oxidative type fibers, were obtained from BALB/c mice, before (CTRL) and at different times (0′, 15′, 30′ 120′) following 1-h of running. Although the total level of HSPB5 protein was not affected by exercise, we found a significantly increase of phosphorylated HSPB5 (p-HSPB5) only in GR and SOL skeletal muscle with a higher amount of type I and IIA/X myofibers. The fiber-specific activation of HSPB5 was correlated to its interaction with the actin filaments, as well as to an increased level of lipid peroxidation and carbonylated proteins. The role of the pro-oxidant environment in HSPB5 response was investigated in terminally differentiated C2C12 myotubes, where most of HSPB5/pHSPB5 pool was present in the cytosolic compartment in standard culture conditions. As a result of exposure to pro-oxidizing, but not cytotoxic, H₂O₂ concentration, the p-38MAPK-mediated phosphorylation of HSPB5 resulted functional to promote its interaction with the myofibrillar components, such as β-actin, desmin and filamin 1.

This study provides novel information on the molecular pathway underlying the HSPB5 physiological function in skeletal muscle, confirming the contribution of the pro-oxidant environment in HSPB5 activation and interaction with substrate/client myofibrillar proteins, offering new insights for the study of myofibrillar myopathies and cardiomyopathies.

Preconditioning contractions prevent the delayed onset of myofibrillar dysfunction after damaging eccentric contractions

KEY POINTS

We examined the mechanisms underlying the positive effect of preconditioning contractions (PCs) on the recovery of muscle force after damaging eccentric contractions (ECCs). The mechanisms underlying the immediate force decrease after damaging ECCs differ from those causing depressed force with a few days' delay, where reactive oxygen species (ROS) produced by invading immune cells play an important causative role. PCs counteracted the delayed onset force depression and this could be explained by prevention of immune cell invasion, which resulted in decreased myeloperoxidase-mediated ROS production, hence avoiding cell membrane disruption, calpain activation and degenerative changes in myosin and actin molecules.

ABSTRACT

Preconditioning contractions (PCs) have been shown to result in markedly improved contractile function during the recovery periods after muscle damage from eccentric contractions (ECCs). Here, we examined the mechanisms underlying the beneficial effect of PCs with a special focus on the myofibrillar function. Rat medial gastrocnemius muscles were exposed to 100 repeated damaging ECCs in situ and excised immediately (recovery 0, REC0) or after 4 days (REC4). PCs with 10 repeated non-damaging ECCs were applied 2 days before the damaging ECCs. PCs improved in situ maximal isometric torque at REC4. Skinned muscle fibres were used to directly assess changes in myofibrillar function. PCs prevented the damaging ECC-induced depression in maximum Ca²⁺ -activated force at REC4. PCs also prevented the following damaging ECC-induced effects at REC4: (i) the reduction in myosin heavy chain and actin content; (ii) calpain activation; (iii) changes in redox homeostasis manifested as increased expression levels of malondialdehyde-protein adducts, NADPH oxidase 2, superoxide dismutase 2 and catalase, and activation of myeloperoxidase (MPO); (iv) infiltration of immune cells and loss of cell membrane integrity. Additionally, at REC0, PCs enhanced the expression levels of heat shock protein (HSP) 70, HSP25, and αB-crystallin in the myofibrils and prevented the increased mRNA levels of granulocyte-macrophage colony-stimulating factor and interleukin-6. In conclusion, PCs prevent the delayed force depression after damaging ECCs by an HSP-dependent inhibition of degenerative changes in myosin and actin molecules caused by myeloperoxidase-induced membrane lysis and subsequent calpain activation, which were triggered by an inflammatory reaction with immune cells invading damaged muscles.

The role of αB-crystallin in skeletal and cardiac muscle tissues

All organisms and cells respond to various stress conditions such as environmental, metabolic, or pathophysiological stress by generally upregulating, among others, the expression and/or activation of a group of proteins called heat shock proteins (HSPs). Among the HSPs, special attention has been devoted to the mutations affecting the function of the αB-crystallin (HSPB5), a small heat shock protein (sHsp) playing a critical role in the modulation of several cellular processes related to survival and stress recovery, such as protein degradation, cytoskeletal stabilization, and apoptosis. Because of the emerging role in general health and disease conditions, the main objective of this mini-review is to provide a brief account on the role of HSPB5 in mammalian muscle physiopathology. Here, we report the current known state of the regulation and localization of HSPB5 in skeletal and cardiac tissue, making also a critical summary of all human HSPB5 mutations known to be strictly associated to specific skeletal and cardiac diseases, such as desmin-related myopathies (DRM), dilated (DCM) and restrictive (RCM) cardiomyopathy. Finally, pointing to putative strategies for HSPB5-based therapy to prevent or counteract these forms of human muscular disorders.

Titin isoforms are increasingly protected against oxidative modifications in developing rat cardiomyocytes

During the perinatal adaptation process N2BA titin isoforms are switched for N2B titin isoforms leading to an increase in cardiomyocyte passive tension (F_passive). Here we attempted to reveal how titin isoform composition and oxidative insults (i.e. sulfhydryl (SH)-group oxidation or carbonylation) influence F_passive of left ventricular (LV) cardiomyocytes during rat heart development. Moreover, we also examined a hypothetical protective role for titin associated small heat shock proteins (sHSPs), Hsp27 and αB-crystallin in the above processes. Single, permeabilized LV cardiomyocytes of the rat (at various ages following birth) were exposed either to 2,2'-dithiodipyridine (DTDP) to provoke SH-oxidation or Fenton reaction reagents (iron(II), hydrogen peroxide (H₂O₂), ascorbic acid) to induce protein carbonylation of cardiomyocytes in vitro. Thereafter, cardiomyocyte force measurements for F_passive determinations and Western immunoblot assays were carried out for the semiquantitative determination of oxidized SH-groups or carbonyl-groups of titin isoforms and to monitor sHSPs' expressions. DTDP or Fenton reagents increased F_passive in 0- and 7-day-old rats to relatively higher extents than in 21-day-old and adult animals. The degrees of SH-group oxidation or carbonylation declined with cardiomyocyte age to similar extents for both titin isoforms. Moreover, the above characteristics were mirrored by increasing levels of HSP27 and αB-crystallin expressions during cardiomyocyte development. Our data implicate a gradual build-up of a protective mechanism against titin oxidation through the upregulation of HSP27 and αB-crystallin expressions during postnatal cardiomyocyte development.

Muscle plasticity related to changes in tubulin and αB-crystallin levels induced by eccentric contraction in rat skeletal muscles

We used the model of eccentric contraction of the hindlimb muscle by Ochi et al. to examine the role of eccentric contraction in muscle plasticity. This model aims to focus on stimulated skeletal muscle responses by measuring tissue weights and tracing the quantities of αB-crystallin and tubulin. The medial gastrocnemius muscle (GCM) responded to electrically induced eccentric contraction (EIEC) with significant increases in tissue weight (p < 0.01) and the ratio of tissue weight to body weight (p < 0.05); however, there was a decrease in soleus muscle weight after EIEC. EIEC in the GCM caused contractile-induced sustenance of the traced proteins, but the soleus muscle exhibited a remarkable decrease in α-tubulin and a 19% decrease in αB-crystallin. EIEC caused fast-to-slow myosin heavy chain (MHC) isoform type-oriented shift within both the GCM and soleus muscle. These results have shown that different MHC isoform type-expressing slow and fast muscles commonly undergo fast-to-slow type MHC isoform transformation. This suggests that different levels of EIEC affected each of the slow and fast muscles to induce different quantitative changes in the expression of αB-crystallin and α-tubulin.

Controlled Heat Stress Promotes Myofibrillogenesis during Myogenesis

Hyperthermia therapy has recently emerged as a clinical modality used to finely tune heat stress inside the human body for various biomedical applications. Nevertheless, little is known regarding the optimal timing or temperature of heat stress that is needed to achieve favorable results following hyperthermia therapy for muscle regeneration purposes. The regeneration of skeletal muscle after injury is a highly complex and coordinated process that involves a multitude of cellular mechanisms. The main objective of this study was to characterize the effects of hyperthermal therapy on the overall behavior of myoblasts during myogenic differentiation. Various cellular processes, including myogenesis, myofibrillogenesis, hypertrophy/atrophy, and mitochondrial biogenesis, were studied using systematic cellular, morphological, and pathway-focused high-throughput gene expression profiling analyses. We found that C2C12 myoblasts exhibited distinctive time and temperature-dependence in biosynthesis and regulatory events during myogenic differentiation. Specifically, we for the first time observed that moderate hyperthermia at 39°C favored the growth of sarcomere in myofibrils at the late stage of myogenesis, showing universal up-regulation of characteristic myofibril proteins. Characteristic myofibrillogenesis genes, including heavy polypeptide 1 myosin, heavy polypeptide 2 myosin, alpha 1 actin, nebulin and titin, were all significantly upregulated (p<0.01) after C2C12 cells differentiated at 39°C over 5 days compared with the control cells cultured at 37°C. Furthermore, moderate hyperthermia enhanced myogenic differentiation, with nucleus densities per myotube showing 2.2-fold, 1.9-fold and 1.6-fold increases when C2C12 cells underwent myogenic differentiation at 39°C over 24 hours, 48 hours and 72 hours, respectively, as compared to the myotubes that were not exposed to heat stress. Yet, atrophy genes were sensitive even to moderate hyperthermia, indicating that strictly controlled heat stress is required to minimize the development of atrophy in myotubes. In addition, mitochondrial biogenesis was enhanced following thermal induction of myoblasts, suggesting a subsequent shift toward anabolic demand requirements for energy production. This study offers a new perspective to understand and utilize the time and temperature-sensitive effects of hyperthermal therapy on muscle regeneration.

Exercise-induced ROS in heat shock proteins response

Cells have evolved multiple and sophisticated stress response mechanisms aiming to prevent macromolecular (including proteins, lipids, and nucleic acids) damage and to maintain or re-establish cellular homeostasis. Heat shock proteins (HSPs) are among the most highly conserved, ubiquitous, and abundant proteins in all organisms. Originally discovered more than 50 years ago through heat shock stress, they display multiple, remarkable roles inside and outside cells under a variety of stresses, including also oxidative stress and radiation, recognizing unfolded or misfolded proteins and facilitating their restructuring. Exercise consists in a combination of physiological stresses, such as metabolic disturbances, changes in circulating levels of hormones, increased temperature, induction of mild to severe inflammatory state, increased production of reactive oxygen and nitrogen species (ROS and RNS). As a consequence, exercise is one of the main stimuli associated with a robust increase in different HSPs in several tissues, which appears to be also fundamental in facilitating the cellular remodeling processes related to the training regime. Among all factors involved in the exercise-related modulation of HSPs level, the ROS production in the contracting muscle or in other tissues represents one of the most attracting, but still under discussion, mechanism. Following exhaustive or damaging muscle exercise, major oxidative damage to proteins and lipids is likely involved in HSP expression, together with mechanically induced damage to muscle proteins and the inflammatory response occurring several days into the recovery period. Instead, the transient and reversible oxidation of proteins by physiological concentrations of ROS seems to be involved in the activation of stress response following non-damaging muscle exercise. This review aims to provide a critical update on the role of HSPs response in exercise-induced adaptation or damage in humans, focusing on experimental results where the link between redox homeostasis and HSPs expression by exercise has been addressed. Further, with the support of in vivo and in vitro studies, we discuss the putative molecular mechanisms underlying the ROS-mediated modulation of HSP expression and/or activity during exercise.

Size dependent classification of heat shock proteins: a mini-review

Molecular chaperones are ubiquitous and abundant within cellular environments, functioning as a defense mechanism against outer environment. The range of molecular chaperones varies from 10 to over 100 kDa. Depending on the size, the specific locations and physiological roles of molecular chaperones vary within the cell. Multifunctionality of heat shock proteins (HSPs) expressed via various cyto-stress including heat shock have been spotlighted as a reliable prognostic target biomarker for therapeutic purpose in neuromuscular disease or cancer related studies. HSP also plays a critical role in the maintenance of proteins and cellular homeostasis in exercise-induced adaptation. Such various functions of HSPs give scientists insights into intracellular protective mechanisms in the living body thus HSPs can be target molecules to know the defense mechanism in cellular environment. Based on experimental results regarding small to large scaled HSPs, this review aims to provide updated important information regarding the modality of responses of intracellular HSPs towards extracellular stimulations. Further, the expressive mechanisms of HSPs data from tremendous in vivo and in vitro studies underlying the enhancement of the functionality of living body will be discussed.

Getting folded: chaperone proteins in muscle development, maintenance and disease

Chaperone proteins are critical for protein folding and stability, and hence are necessary for normal cellular organization and function. Recent studies have begun to interrogate the role of this specialized class of proteins in muscle biology. During development, chaperone-mediated folding of client proteins enables their integration into nascent functional sarcomeres. In addition to assisting with muscle differentiation, chaperones play a key role in the maintenance of muscle tissues. Furthermore, disruption of the chaperone network can result in neuromuscular disease. In this review, we discuss how chaperones are involved in myofibrillogenesis, sarcomere maintenance, and muscle disorders. We also consider the possibilities of therapeutically targeting chaperones to treat muscle disease.

Changes in αB-crystallin, tubulin, and MHC isoforms by hindlimb unloading show different expression patterns in various hindlimb muscles

[Purpose]

αB-crystallin is a small heat shock protein that acts as a molecular chaperone under various stress conditions. Microtubules, which consist of tubulin, are related to maintain the intracellular organelles and cellular morphology. These two proteins have been shown to be related to the properties of different types of myofibers based on their contractile properties. The response of these proteins during muscular atrophy, which induces a myofibril component change, is not clearly understood.

[Methods]

We performed 15 days of hindlimb unloading on rats to investigate the transitions of these proteins by analyzing their absolute quantities. Protein contents were analyzed in the soleus, plantaris, and gastrocnemius muscles of the unloading and control groups (N = 6).

[Results]

All three muscles were significantly atrophied by hindlimb unloading (P < 0.01): soleus (47.5%), plantaris (16.3%), and gastrocnemius (21.3%) compared to each control group. αB-crystallin was significantly reduced in all three examined unloaded hindlimb muscles compared to controls (P < 0.01) during the transition of the myosin heavy chain to fast twitch muscles. α-Tubulin responded only in the unloaded soleus muscle. Muscle atrophy induced the reduction of αB-crystallin and α-tubulin expressions in plantar flexor muscles with a shift to the fast muscle fiber compared to the control.

[Conclusion]

The novel finding of this study is that both proteins, αB-crystallin and α-tubulin, were downregulated in slow muscles (P < 0.01); However, α-tubulin was not significantly reduced compared to the control in fast muscles (P < 0.01).

Contribution of small heat shock proteins to muscle development and function

Investigations undertaken over the past years have led scientists to introduce the concept of protein quality control (PQC) systems, which are responsible for polypeptide processing. The PQC system monitors proteostasis and involves activity of different chaperones such as small heat shock proteins (sHSPs). These proteins act during normal conditions as housekeeping proteins regulating cellular processes, and during stress conditions. They also mediate the removal of toxic misfolded polypeptides and thereby prevent development of pathogenic states. It is postulated that sHSPs are involved in muscle development. They could act via modulation of myogenesis or by maintenance of the structural integrity of signaling complexes. Moreover, mutations in genes coding for sHSPs lead to pathological states affecting muscular tissue functioning. This review focuses on the question how sHSPs, still relatively poorly understood proteins, contribute to the development and function of three types of muscle tissue: skeletal, cardiac and smooth.

Human myocytes are protected from titin aggregation-induced stiffening by small heat shock proteins

Small heat shock proteins translocate to unfolded titin Ig domains under stress conditions to prevent titin aggregation and myocyte stiffening.

In myocytes, small heat shock proteins (sHSPs) are preferentially translocated under stress to the sarcomeres. The functional implications of this translocation are poorly understood. We show here that HSP27 and αB-crystallin associated with immunoglobulin-like (Ig) domain-containing regions, but not the disordered PEVK domain (titin region rich in proline, glutamate, valine, and lysine), of the titin springs. In sarcomeres, sHSP binding to titin was actin filament independent and promoted by factors that increased titin Ig unfolding, including sarcomere stretch and the expression of stiff titin isoforms. Titin spring elements behaved predominantly as monomers in vitro. However, unfolded Ig segments aggregated, preferentially under acidic conditions, and αB-crystallin prevented this aggregation. Disordered regions did not aggregate. Promoting titin Ig unfolding in cardiomyocytes caused elevated stiffness under acidic stress, but HSP27 or αB-crystallin suppressed this stiffening. In diseased human muscle and heart, both sHSPs associated with the titin springs, in contrast to the cytosolic/Z-disk localization seen in healthy muscle/heart. We conclude that aggregation of unfolded titin Ig domains stiffens myocytes and that sHSPs translocate to these domains to prevent this aggregation.

Acute effects of sex-specific sex hormones on heat shock proteins in fast muscle of male and female rats

Heat shock protein (HSP) expression and sex hormone levels have been shown to influence several aspects of skeletal muscle physiology (e.g., hypertrophy, resistance to oxidative stress), suggesting that sex hormone levels can effect HSP expression. This study evaluated the effects of differing levels of sex-specific sex hormones (i.e., testosterone in males and estrogen in females) on the expression of 4: HSP70, HSC70, HSP25, and αB-crystallin in the quadriceps muscles of male and female rats. Animals were assigned to 1 of 3 groups (n = 5 M and F/group). The first group (Ctl) consisted of typically cage-housed animals that served as controls. The second group (H) was gonadectomized and received either testosterone (males) or estradiol (females) via injection for 12 consecutive days. The third group (Gx) was gonadectomized and injected as above, but with vehicle only, rather than hormones. Significant sex by condition interactions (P < 0.05 by two-way MANOVA) were found for all 4 proteins studied, except for HSP70, which exhibited a significant effect of condition only. The expression of all HSPs was greater (1.9-2.5-fold) in males vs. females in the Ctl group, except for HSP70, which was no different. Generally, gonadectomy appeared to have greater effects in males than females, but administration of the exogenous sex hormones tended to produce more robust relative changes in females than males. There were no differences in myosin composition in any of the groups, suggesting that changes in fiber type were not a factor in the differential protein expression. These data may have implications for sex-related differences in muscular responses to exercise, disuse, and injury.

Chaperones: needed for both the good times and the bad times

In this issue, we explore the assembly roles of protein chaperones, mainly through the portal of their associated human diseases (e.g. cardiomyopathy, cataract, neurodegeneration, cancer and neuropathy). There is a diversity to chaperone function that goes beyond the current emphasis in the scientific literature on their undoubted roles in protein folding and refolding. The focus on chaperone-mediated protein folding needs to be broadened by the original Laskey discovery that a chaperone assists the assembly of an oligomeric structure, the nucleosome, and the subsequent suggestion by Ellis that other chaperones may function in assembly processes, as well as in folding. There have been a number of recent discoveries that extend this relatively neglected aspect of chaperone biology to include proteostasis, maintenance of the cellular redox potential, genome stability, transcriptional regulation and cytoskeletal dynamics. So central are these processes that we propose that chaperones stand at the crossroads of life and death because they mediate essential functions, not only during the bad times, but also in the good times. We suggest that chaperones facilitate the success of a species, and hence the evolution of individuals within populations, because of their contributions to so many key cellular processes, of which protein folding is only one.

A network-based approach for predicting Hsp27 knock-out targets in mouse skeletal muscles

Thanks to genomics, we have previously identified markers of beef tenderness, and computed a bioinformatic analysis that enabled us to build an interactome in which we found Hsp27 at a crucial node. Here, we have used a network-based approach for understanding the contribution of Hsp27 to tenderness through the prediction of its interactors related to tenderness. We have revealed the direct interactors of Hsp27. The predicted partners of Hsp27 included proteins involved in different functions, e.g. members of Hsp families (Hsp20, Cryab, Hsp70a1a, and Hsp90aa1), regulators of apoptosis (Fas, Chuk, and caspase-3), translation factors (Eif4E, and Eif4G1), cytoskeletal proteins (Desmin) and antioxidants (Sod1). The abundances of 15 proteins were quantified by Western blotting in two muscles of HspB1-null mice and their controls. We observed changes in the amount of most of the Hsp27 predicted targets in mice devoid of Hsp27 mainly in the most oxidative muscle. Our study demonstrates the functional links between Hsp27 and its predicted targets. It suggests that Hsp status, apoptotic processes and protection against oxidative stress are crucial for post-mortem muscle metabolism, subsequent proteolysis, and therefore for beef tenderness.

Impact of exercise and metabolic disorders on heat shock proteins and vascular inflammation

Heat shock proteins (Hsp) play critical roles in the body's self-defense under a variety of stresses, including heat shock, oxidative stress, radiation, and wounds, through the regulation of folding and functions of relevant cellular proteins. Exercise increases the levels of Hsp through elevated temperature, hormones, calcium fluxes, reactive oxygen species (ROS), or mechanical deformation of tissues. Isotonic contractions and endurance- type activities tend to increase Hsp60 and Hsp70. Eccentric muscle contractions lead to phosphorylation and translocation of Hsp25/27. Exercise-induced transient increases of Hsp inhibit the generation of inflammatory mediators and vascular inflammation. Metabolic disorders (hyperglycemia and dyslipidemia) are associated with type 1 diabetes (an autoimmune disease), type 2 diabetes (the common type of diabetes usually associated with obesity), and atherosclerotic cardiovascular disease. Metabolic disorders activate HSF/Hsp pathway, which was associated with oxidative stress, increased generation of inflammatory mediators, vascular inflammation, and cell injury. Knock down of heat shock factor-1 (HSF1) reduced the activation of key inflammatory mediators in vascular cells. Accumulating lines of evidence suggest that the activation of HSF/Hsp induced by exercise or metabolic disorders may play a dual role in inflammation. The benefits of exercise on inflammation and metabolism depend on the type, intensity, and duration of physical activity.

Muscle composition slightly affects in vitro digestion of aged and cooked meat: identification of associated proteomic markers

Meat is an appropriate source of proteins and minerals for human nutrition. Technological treatments modify the physical-chemical properties of proteins, making them liable to decrease the nutritional potential of meat. To counteract this damage, antioxidants and chaperone proteins in muscle cells can prevent oxidation, restore the function of denatured proteins, and thus prevent aggregation. This study aimed to explore the impact of indoor vs outdoor-reared meat protein composition on digestion and to associate protein markers to in vitro digestion parameters. Indoor-reared meat tended to show less oxidation and denaturation than outdoor-reared meat and was characterised by an overexpression of contractile and chaperone proteins. Outdoor-reared meat showed amplification of antioxidant and detoxification metabolism defending against oxidised compounds. Impacts on digestion remained minor. Several protein markers of in vitro digestion parameters were found for aged and cooked meat, linked to the detoxification process and to muscle contraction.

Influences of temperature, oxidative stress, and phosphorylation on binding of heat shock proteins in skeletal muscle fibers

Heat shock proteins (HSPs) help maintain cellular function in stressful situations, but the processes controlling their interactions with target proteins are not well defined. This study examined the binding of HSP72, HSP25, and αB-crystallin in skeletal muscle fibers following various stresses. Rat soleus (SOL) and extensor digitorum longus (EDL) muscles were subjected in vitro to heat stress or strongly fatiguing stimulation. Superficial fibers were “skinned” by microdissection and HSP diffusibility assessed from the extent of washout following 10- to 30 min exposure to a physiological intracellular solution. In fibers from nonstressed (control) SOL muscle, >80% of each HSP is readily diffusible. However, after heating a muscle to 40°C for 30 min ∼95% of HSP25 and αB-crystallin becomes tightly bound at nonmembranous myofibrillar sites, whereas HSP72 bound at membranous sites only after heat treatment to ≥44°C. The ratio of reduced to oxidized cytoplasmic glutathione (GSH:GSSG) decreased approximately two- and fourfold after heating muscles to 40° and 45°C, respectively. The reducing agent dithiothreitol reversed HSP72 binding in heated muscles but had no effect on the other HSPs. Intense in vitro stimulation of SOL muscles, sufficient to elicit substantial oxidation-related loss of maximum force and approximately fourfold decrease in the GSH:GSSG ratio, had no effect on diffusibility of any of the HSPs. When skinned fibers from heat-treated muscles were bathed with additional exogenous HSP72, total binding increased approximately two- and 10-fold, respectively, in SOL and EDL fibers, possibly reflective of the relative sarco(endo)plasmic reticulum Ca2+-ATPase pump densities in the two fiber types. Phosphorylation at Ser59 on αB-crystallin and Ser85 on HSP25 increased with heat treatment but did not appear to determine HSP binding. The findings highlight major differences in the processes controlling binding of HSP72 and the two small HSPs. Binding was not directly related to cytoplasmic oxidative status, but oxidation of cysteine residues influenced HSP72 binding.

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.

Absolute amounts and diffusibility of HSP72, HSP25, and αB-crystallin in fast- and slow-twitch skeletal muscle fibers of rat

Heat shock proteins (HSPs) are essential for normal cellular stress responses. Absolute amounts of HSP72, HSP25, and αB-crystallin in rat extensor digitorum longus (EDL) and soleus (SOL) muscle were ascertained by quantitative Western blotting to better understand their respective capabilities and limitations. HSP72 content of EDL and SOL muscle was only ∼1.1 and 4.6 μmol/kg wet wt, respectively, and HSP25 content approximately twofold greater (∼3.4 and ∼8.9 μmol/kg, respectively). αB-crystallin content of EDL muscle was ∼4.9 μmol/kg but in SOL muscle was ∼30-fold higher (∼140 μmol/kg). To examine fiber heterogeneity, HSP content was also assessed in individual fiber segments; every EDL type II fiber had less of each HSP than any SOL type I fiber, whereas the two SOL type II fibers examined were indistinguishable from the EDL type II fibers. Sarcolemma removal (fiber skinning) demonstrated that 10-20% of HSP25 and αB-crystallin was sarcolemma-associated in SOL fibers. HSP diffusibility was assessed from the extent and rate of diffusion out of skinned fiber segments. In unstressed SOL fibers, 70-90% of each HSP was readily diffusible, whereas ∼95% remained tightly bound in fibers from SOL muscles heated to 45°C. Membrane disruption with Triton X-100 allowed dispersion of HSP72 and sarco(endo)plasmic reticulum Ca(2+)-ATPase pumps but did not alter binding of HSP25 or αB-crystallin. The amount of HSP72 in unstressed EDL muscle is much less than the number of its putative binding sites, whereas SOL type I fibers contain large amounts of αB-crystallin, suggesting its importance in normal cellular function without upregulation.

Infantile muscular dystrophy in Canadian aboriginals is an αB-crystallinopathy

OBJECTIVE

A recessively transmitted fatal hypertonic infantile muscular dystrophy has been described in Canadian aboriginals. The affected infants present with progressive limb and axial muscle stiffness and develop severe respiratory insufficiency, and most die in the first year of life. We sought to determine the genetic basis of this disease.

METHODS

We performed histochemical, immunocytochemical, electron microscopy, and molecular genetic studies in a cohort of 12 patients affected by this disease.

RESULTS

Conventional histochemical and electron microscopy studies suggested myofibrillar myopathy (MFM). Therefore, we searched for ectopic expression of multiple proteins typical of MFM. Alpha B-crystallin (αBC) expression was absent from all fibers using a monoclonal antibody raised against the entire protein. However, a monoclonal antibody directed against the first 10 residues of αBC immunostained portions of abnormal fibers. Pursuing this clue, we searched for mutations in the gene for αBC (CRYAB) in available DNA samples of 8 patients. All harbored a homozygous deletion, c.60C, predicting a Ser to Ala change at codon 21 and a stop codon after 23 missense residues (p.Ser21AlafsX24). Clinically unaffected parents were heterozygous for this mutation.

INTERPRETATION

The homozygous c.60delC in CRYAB pinpoints the genetic basis of the fatal infantile hypertonic muscular dystrophy of Canadian aboriginals. MFMs are typically transmitted by dominant inheritance, but in this disease the parental phenotype is rescued by limited expression of the highly truncated nonfunctional mutant gene product. The severe patient phenotype is due to homozygosity for the markedly hypomorphic allele. Ann Neurol, 2011.

Trained men display increased basal heat shock protein content of skeletal muscle

PURPOSE

1) To compare the baseline levels of heat shock and antioxidant protein content in the skeletal muscle of trained and untrained humans and 2) to characterize the exercise-induced stress response of aerobically trained human skeletal muscle to an acute exercise challenge.

METHODS

Resting muscle biopsies were obtained from the vastus lateralis muscle of six untrained and six aerobically trained young males. To characterize the stress response of a trained population, the trained subjects also performed a 45-min nondamaging running exercise protocol at an intensity corresponding to 75% of V O2max. Muscle biopsies were obtained from the vastus lateralis muscle at 48 h and 7 d after exercise.

RESULTS

Trained subjects displayed significantly higher (P<0.05) resting levels of heat shock protein 60 (HSP60, 25%), alphaB-crystallin (43%), and manganese superoxide (MnSOD, 45%) protein content compared with untrained subjects. Trained subjects also exhibited no significant change (P > 0.05) in resting levels of HSP70 (16%), HSC70 (13%), and total superoxide dismutase (SOD) activity (46%) compared with untrained subjects. Resting HSP27 levels were unaffected by exercise training (P > 0.05). In the trained subjects, exercise failed to induce significant increases (P>0.05)in muscle content of HSP70, HSC70, HSP60, HSP27, alphaB-crystallin, and MnSOD protein content or in the activity of SOD at any time point after exercise.

CONCLUSION

This study demonstrates for the first time that trained men display a selective up-regulation of basal heat shock and antioxidant protein content and do not exhibit a stress response to customary running exercise. It is suggested that an increase in these protective systems functions to maintain homeostasis during the stress of exercise by protecting against disruptions to the cytoskeleton/contractile machinery, by maintaining redox balance, and by facilitating mitochondrial biogenesis.

Drosophila UNC-45 prevents heat-induced aggregation of skeletal muscle myosin and facilitates refolding of citrate synthase

UNC-45 belongs to the UCS (UNC-45, CRO1, She4p) domain protein family, whose members interact with various classes of myosin. Here we provide structural and biochemical evidence that Escherichia coli-expressed Drosophila UNC-45 (DUNC-45) maintains the integrity of several substrates during heat-induced stress in vitro. DUNC-45 displays chaperone function in suppressing aggregation of the muscle myosin heavy meromyosin fragment, the myosin S-1 motor domain, alpha-lactalbumin and citrate synthase. Biochemical evidence is supported by electron microscopy, which reveals the first structural evidence that DUNC-45 prevents inter- or intra-molecular aggregates of skeletal muscle heavy meromyosin caused by elevated temperatures. We also demonstrate for the first time that UNC-45 is able to refold a denatured substrate, urea-unfolded citrate synthase. Overall, this in vitro study provides insight into the fate of muscle myosin under stress conditions and suggests that UNC-45 protects and maintains the contractile machinery during in vivo stress.

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

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

Protein quality control gets muscle into shape

The synthesis, assembly and organisation of structural and motor proteins during muscle formation requires temporal and spatial control directed by specialized chaperones. For example, alphaB-crystallin, GimC and TRiC facilitate the assembly of sarcomeric proteins such as desmin and actin. Recent studies have demonstrated that the chaperone family of UCS proteins (UNC-45-CRO1-She4p) is required for the proper function of myosin motors. Mutations in the myosin-directed chaperone unc-45, a founding member of this family, lead to disorganisation of striated muscle in Caenorhabditiselegans. In addition to the involvement of client-specific chaperones, myofibrillogenesis also involves ubiquitin-dependent degradation of regulatory muscle proteins. Here, we highlight the interplay between chaperone activity and protein degradation in respect to the formation and maintenance of muscle during physiological and pathological conditions.

Structural perturbation of alphaB-crystallin by zinc and temperature related to its chaperone-like activity

alphaB-crystallin is a small heat shock protein that shows chaperone-like activity, as it protects the aggregation of denatured proteins. In this work, the possible relationships between structural characteristics and the biological activity of alphaB-crystallin were investigated on the native protein and on the protein undergoing the separate effects of metal ligation and temperature. The chaperone-like activity of alphaB-crystallin increased in the presence of zinc and when temperature was increased. By using fluorescent probes to monitor hydrophobic surfaces on alphaB-crystallin, it was found that exposed hydrophobic patches on the protein surface increased significantly both in the presence of zinc and when the temperature was raised from 25 to 37 degrees C. The zinc-induced increased exposure of lipophilic residues is in agreement with theoretical calculations performed on 3D-models of monomeric alphaB-crystallin, and may be significant to its increased biological activity.

Aging skeletal muscle shows a drastic increase in the small heat shock proteins αB-crystallin/HspB5 and cvHsp/HspB7

Most heat shock proteins operate as molecular chaperones and play a central role in the maintenance of normal cellular function. In skeletal muscle, members of the alpha-crystallin domain-containing family of small heat shock proteins are believed to form a cohort of essential stress proteins. Since alphaB-crystallin (alphaBC/HspB5) and the cardiovascular heat shock protein (cvHsp/HspB7) are both implicated in the molecular response to fibre transformation and muscle wasting, it was of interest to investigate the fate of these stress proteins in young adult versus aged muscle. The age-related loss of skeletal muscle mass and strength, now generally referred to as sarcopenia, is one of the most striking features of the senescent organism. In order to better understand the molecular pathogenesis of age-related muscle wasting, we have performed a two-dimensional gel electrophoretic analysis, immunoblotting and confocal microscopy study of aged rat gastrocnemius muscle. Fluorescent labelling of the electrophoretically separated soluble muscle proteome revealed an overall relatively comparable protein expression pattern of young adult versus aged fibres, but clearly an up-regulation of alphaBC and cvHsp. This was confirmed by immunofluorescence microscopy and immunoblot analysis, which showed a dramatic age-induced increase in these small heat shock proteins. Immunodecoration of other major stress proteins showed that they were not affected or less drastically changed in their expression in aged muscle. These findings indicate that the increase in muscle-specific small heat shock proteins constitutes an essential cellular response to fibre aging and might therefore be a novel therapeutic option to treat sarcopenia of old age.

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

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