alpha B-crystallin and hsp25 in neonatal cardiac cells--differences in cellular localization under stress conditions

Targeting Ser78 phosphorylation of Hsp27 achieves potent antiviral effects against enterovirus A71 infection

A positive-sense (+) single-stranded RNA (ssRNA) virus (e.g. enterovirus A71, EV-A71) depends on viral polypeptide translation for initiation of virus replication after entry. We reported that EV-A71 hijacks Hsp27 to induce hnRNP A1 cytosol redistribution to initiate viral protein translation, but the underlying mechanism is still elusive. Here, we show that phosphorylation-deficient Hsp27-3A (Hsp27S15/78/82A) and Hsp27S78A fail to translocate into the nucleus and induce hnRNP A1 cytosol redistribution, while Hsp27S15A and Hsp27S82A display similar effects to the wild type Hsp27. Furthermore, we demonstrate that the viral 2A protease (2Apro) activity is a key factor in regulating Hsp27/hnRNP A1 relocalization. Hsp27S78A dramatically decreases the IRES activity and viral replication, which are partially reduced by Hsp27S82A. However, Hsp27S15A displays the same activity as the wild-type Hsp27. Peptide S78 potently suppresses EV-A71 protein translation and reproduction through blockage of EV-A71-induced Hsp27 phosphorylation and Hsp27/hnRNP A1 relocalization. A point mutation (S78A) on S78 impairs its inhibitory functions on Hsp27/hnRNP A1 relocalization and viral replication. Taken together, we demonstrate the importance of Ser78 phosphorylation of Hsp27 regulated by virus infection in nuclear translocation, hnRNP A1 cytosol relocation, and viral replication, suggesting a new path (such as peptide S78) for target-based antiviral strategy.

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.

Small Heat Shock Proteins Collaborate with FAIM to Prevent Accumulation of Misfolded Protein Aggregates

Cells and tissues are continuously subject to environmental insults, such as heat shock and oxidative stress, which cause the accumulation of cytotoxic, aggregated proteins. We previously found that Fas Apoptosis Inhibitory Molecule (FAIM) protects cells from stress-induced cell death by preventing abnormal generation of protein aggregates similar to the effect of small heat shock proteins (HSPs). Protein aggregates are often associated with neurodegenerative diseases, including Alzheimer's disease (AD). In this study, we sought to determine how FAIM protein dynamics change during cellular stress and how FAIM prevents the formation of amyloid-β aggregates/fibrils, one of the pathological hallmarks of AD. Here, we found that the majority of FAIM protein shifts to the detergent-insoluble fraction in response to cellular stress. A similar shift to the insoluble fraction was also observed in small heat shock protein (sHSP) family molecules, such as HSP27, after stress. We further demonstrate that FAIM is recruited to sHSP-containing complexes after cellular stress induction. These data suggest that FAIM might prevent protein aggregation in concert with sHSPs. In fact, we observed the additional effect of FAIM and HSP27 on the prevention of protein aggregates using an in vitro amyloid-β aggregation model system. Our work provides new insights into the interrelationships among FAIM, sHSPs, and amyloid-β aggregation.

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

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

Dynamic localization of αB-crystallin at the microtubule cytoskeleton network in beating heart cells

αB-crystallin is highly expressed in the heart and slow skeletal muscle; however, the roles of αB-crystallin in the muscle are obscure. Previously, we showed that αB-crystallin localizes at the sarcomere Z-bands, corresponding to the focal adhesions of cultured cells. In myoblast cells, αB-crystallin completely colocalizes with microtubules and maintains cell shape and adhesion. In this study, we show that in beating cardiomyocytes α-tubulin and αB-crystallin colocalize at the I- and Z-bands of the myocardium, where it may function as a molecular chaperone for tubulin/microtubules. Fluorescence recovery after photobleaching (FRAP) analysis revealed that the striated patterns of GFP-αB-crystallin fluorescence recovered quickly at 37°C. FRAP mobility assay also showed αB-crystallin to be associated with nocodazole-treated free tubulin dimers but not with taxol-treated microtubules. The interaction of αB-crystallin and free tubulin was further confirmed by immunoprecipitation and microtubule sedimentation assay in the presence of 1-100 μM calcium, which destabilizes microtubules. Förster resonance energy transfer analysis showed that αB-crystallin and tubulin were at 1-10 nm apart from each other in the presence of colchicine. These results suggested that αB-crystallin may play an essential role in microtubule dynamics by maintaining free tubulin in striated muscles, such as the soleus or cardiac muscles.

αB-crystallin response to a pro-oxidant non-cytotoxic environment in murine cardiac cells: An "in vitro" and "in vivo" study

The αB-crystallin (HSPB5) protein is modulated in response to a wide variety of stressors generated by multiple physio-pathological conditions, sustained by reactive oxygen species (ROS) production. In cardiac muscle tissue, this protein regulates various cellular processes, such as protein degradation, apoptosis and the stabilization of cytoskeletal elements. In this work, we studied the role of HSPB5 expression, activation and localization in HL-1 murine cardiomyocytes exposed to pro-oxidant and non-cytotoxic H₂O₂ concentration, as well as in cardiac tissue isolated from mice following an acute, non-damaging endurance exercise. Our results demonstrated that HSPB5 is the most abundant HSP in both cardiac muscle tissue and HL-1 cells when compared to HSPB1 or HSPA1A (≈3-8 fold higher protein concentrations, p < 0.01). The acute exposure of cardiac muscle cells to sustainable level of H₂O₂ "in vitro" or to aerobic non-damaging exercise "in vivo" determined a fast and specific increase of HSPB5 phosphorylation (from 3 up to 25 fold increase, p < 0.01) correlated to an increase in lipid peroxidation (p < 0.05). In both experimental models, p-HSPB5 likely facilitated both the interaction with β-actin, desmin, and α-Filamin 1, the last one identified as new HSPB5 substrate in cardiac cells, as well as the sub-localization of HSPB5 within the same cellular compartment or the re-localization between compartments (i.e., nucleus and cytosol). Taken together, these data point out the role of "oxidative eustress" induced by physiological conditions in activating the molecular machinery devoted to cardiomyocytes' protection and candidate HSPB5 as a putative molecular mediator for the health benefits induced in cardiac tissue by exercise training.

The protective role of small heat shock proteins in cardiac diseases: key role in atrial fibrillation

Atrial fibrillation (AF) is the most common tachyarrhythmia which is associated with increased morbidity and mortality. AF usually progresses from a self-terminating paroxysmal to persistent disease. It has been recognized that AF progression is driven by structural remodeling of cardiomyocytes, which results in electrical and contractile dysfunction of the atria. We recently showed that structural remodeling is rooted in derailment of proteostasis, i.e., homeostasis of protein production, function, and degradation. Since heat shock proteins (HSPs) play an important role in maintaining a healthy proteostasis, the role of HSPs was investigated in AF. It was found that especially small heat shock protein (HSPB) levels get exhausted in atrial tissue of patients with persistent AF and that genetic or pharmacological induction of HSPB protects against cardiomyocyte remodeling in experimental models for AF. In this review, we provide an overview of HSPBs as a potential therapeutic target for normalizing proteostasis and suppressing the substrates for AF progression in experimental and clinical AF and discuss HSP activators as a promising therapy to prevent AF onset and progression.

Titin: central player of hypertrophic signaling and sarcomeric protein quality control

The giant sarcomeric protein titin has multiple important functions in striated muscle cells. Due to its gigantic size, its central position in the sarcomere and its elastic I-band domains, titin is a scaffold protein that is important for sarcomere assembly, and serves as a molecular spring that defines myofilament distensibility. This review focuses on the emerging role of titin in mechanosensing and hypertrophic signaling, and further highlights recent evidence that links titin to sarcomeric protein turnover.

Effect of hypoxia on the expression of αB-crystallin in head and neck squamous cell carcinoma

Background

The presence of hypoxia in head and neck squamous cell carcinoma (HNSCC) is associated with therapeutic resistance and increased risk of metastasis formation. αB-crystallin (HspB5) is a small heat shock protein, which is also associated with metastasis formation in HNSCC. In this study, we investigated whether αB-crystallin protein expression is increased in hypoxic areas of HNSCC biopsies and analyzed whether hypoxia induces αB-crystallin expression in vitro and in this way may confer hypoxic cell survival.

Methods

In 38 HNSCC biopsies, the overlap between immunohistochemically stained αB-crystallin and pimonidazole-adducts (hypoxiamarker) was determined. Moreover, expression levels of αB-crystallin were analyzed in HNSCC cell lines under hypoxia and reoxygenation conditions and after exposure to reactive oxygen species (ROS) and the ROS scavenger N-acetylcysteine (NAC). siRNA-mediated knockdown was used to determine the influence of αB-crystallin on cell survival under hypoxic conditions.

Results

In all biopsies αB-crystallin was more abundantly present in hypoxic areas than in normoxic areas. Remarkably, hypoxia decreased αB-crystallin mRNA expression in the HNSCC cell lines. Only after reoxygenation, a condition that stimulates ROS formation, αB-crystallin expression was increased. αB-crystallin mRNA levels were also increased by extracellular ROS, and NAC abolished the reoxygenation-induced αB-crystallin upregulation. Moreover, it was found that decreased αB-crystallin levels reduced cell survival under hypoxic conditions.

Conclusions

We provide the first evidence that hypoxia stimulates upregulation of αB-crystallin in HNSCC. This upregulation was not caused by the low oxygen pressure, but more likely by ROS formation. The higher expression of αB-crystallin may lead to prolonged survival of these cells under hypoxic conditions.

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.

αB-crystallin stimulates VEGF secretion and tumor cell migration and correlates with enhanced distant metastasis in head and neck squamous cell carcinoma

Background

αB-crystallin is able to modulate vascular endothelial growth factor (VEGF) secretion. In many solid tumors VEGF is associated with angiogenesis, metastasis formation and poor prognosis. We set out to assess whether αB-crystallin expression is correlated with worse prognosis and whether this is related to VEGF secretion and cell motility in head and neck squamous cell carcinoma (HNSCC).

Methods

αB-crystallin expression was determined immunohistochemically in tumor biopsies of 38 HNSCC patients. Locoregional control (LRC) and metastasis-free survival (MFS) of the patients were analyzed in relation to αB-crystallin expression. Additionally, the effects of αB-crystallin knockdown on VEGF secretion and cell motility were studied in vitro.

Results

Patients with higher staining fractions of αB-crystallin exhibited a significantly shorter MFS (Log-Rank test, p < 0.005). Under normoxic conditions αB-crystallin knockdown with two different siRNAs in a HNSCC cell line reduced VEGF secretion 1.9-fold and 2.1-fold, respectively. Under hypoxic conditions, a similar reduction of VEGF secretion was observed, 1.9-fold and 2.2-fold, respectively. The effect on cell motility was assessed by a gap closure assay, which showed that αB-crystallin knockdown decreased the rate by which HNSCC cells were able to close a gap by 1.5- to 2.0-fold.

Conclusions

Our data suggest that αB-crystallin expression is associated with distant metastases formation in HNSCC patients. This association might relate to the chaperone function of αB-crystallin in mediating folding and secretion of VEGF and stimulating cell migration.

Study of αB-crystallin expression in Gerbil BCAO model of transient global cerebral ischemia

αB-crystallin (α-BC), the fifth member of mammalian small heat shock protein family (HspB5), is known to be expressed in many tissues and has a distinctive interaction with cytoskeleton components. In this study, we investigated that α-BC and microtubule-associated protein-2 (MAP-2), a neuron-specific cytoskeleton protein, were coexpressed in neurons of Gerbil cortex, while in subcortex Gerbil brains, we found that several MAP-2-negative glia cells also express α-BC. When subjected to 10-minute bilateral carotid artery occlusion (BCAO), an increment was observed in α-BC-positive cells after 6-hour reperfusion and peaked at around 7 days after. In the same circumstances, the number and the staining concentration of MAP-2 positive neurons significantly decreased immediately after 6-hour reperfusion, followed by a slow recovery, which is consistent with the increase of α-BC. Our results suggested that α-BC plays an important role in brain ischemia, providing the early protection of neurons by giving intracellular supports through the maintenance of cytoskeleton and extracellular supports through the protection of glia cells.

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.

Myristoylated methionine sulfoxide reductase A protects the heart from ischemia-reperfusion injury

Methionine sulfoxide reductase A (MsrA) catalytically scavenges reactive oxygen species and also repairs oxidized methionines in proteins. Increasing MsrA protects cells and organs from a variety of oxidative stresses while decreasing MsrA enhances damage, but the mechanisms of action have not been elucidated. A single gene encodes MsrA of which ∼25% is targeted to the mitochondria, a major site of reactive oxygen species production. The other ∼75% is targeted to the cytosol and is posttranslationally modified by myristoylation. To determine the relative importance of MsrA in each compartment in protecting against ischemia-reperfusion damage, we created a series of transgenic mice overexpressing MsrA targeted to the mitochondria or the cytosol. We used a Langendorff model of ischemia-reperfusion and assayed both the rate pressure product and infarct size following ischemia and reperfusion as measures of injury. While the mitochondrially targeted MsrA was expected to be protective, it was not. Notably, the cytosolic form was protective but only if myristoylated. The nonmyristoylated, cytosolic form offered no protection against injury. We conclude that cytosolic MsrA protects the heart from ischemia-reperfusion damage. The requirement for myristoylation suggests that MsrA must interact with a hydrophobic domain to provide protection.

Phosphoproteome study reveals Hsp27 as a novel signaling molecule involved in GDNF-induced neurite outgrowth

Glial-cell-line-derived neurotrophic factor (GDNF) is a most potent survival factor for dopaminergic neurons. In addition, GDNF was also found to promote neurite outgrowth in dopaminergic neurons. However, despite the potential clinical and physiological importance of GDNF, its mechanism of action is unclear. Therefore, we employed a state-of-the-art proteomic technique, DIGE (Difference in two-dimensional gel electrophoresis), to quantitatively compare profiles of phosphoproteins of PC12-GFRalpha1-RET cells (that stably overexpress GDNF receptor alpha1 and RET) 0.5 and 10 h after GDNF challenge with control. A total of 92 differentially expressed proteins were successfully identified by mass spectrometry. Among them, the relative levels of phosphorylated Hsp27 increased significantly both in 0.5 and 10 h GDNF-treated PC12-GFRalpha1-RET cells. Confocal microscopy and Western blot results showed that the phosphorylation of Hsp27 after GDNF treatment was accompanied by its nuclear translocation. After the mRNA of Hsp27 was interfered, neurite outgrowth of PC12-GFRalpha1-RET cells induced by GDNF was significantly blocked. Furthermore, the percentage of neurite outgrowth induced by GDNF was also reduced by the expression of dominant-negative mutants of Hsp27, in which specific serine phosphorylation residues (Ser15, Ser78 and Ser82) were substituted with alanine. Our data also revealed that p38 MAPK and ERK are the upstream regulators of Hsp27 phosphorylation. Hence, in addition to the numerous novel proteins that are potentially important in GDNF mediated differentiation of dopaminergic cells revealed by our study, our data has indicated that Hsp27 is a novel signaling molecule involved in GDNF-induced neurite outgrowth of dopaminergic neurons.

Hsp27 is persistently expressed in zebrafish skeletal and cardiac muscle tissues but dispensable for their morphogenesis

Constitutive expression of Hsp27 has been demonstrated in vertebrate embryos, especially in developing skeletal and cardiac muscle. Results of several previous studies have indicated that Hsp27 could play a role in the development of these tissues. For example, inhibition of Hsp27 expression has been reported to cause defective development of mammalian myoblasts in vitro and frog embryos in vivo. In contrast, transgenic mice lacking Hsp27 develop normally. Here, we examined the distribution of Hsp27 protein in developing and adult zebrafish and effects of suppressing Hsp27 expression using phosphorodiamidate morpholino oligonucleotides (PMO) on zebrafish development. Consistent with our previous analysis of hsp27 messenger RNA expression, we detected the protein Hsp27 in cardiac, smooth, and skeletal muscle of both embryonic and adult zebrafish. However, embryos lacking detectable Hsp27 after injection of antisense hsp27 PMO exhibited comparable heart beat rates to that of control embryos and cardiac morphology was indistinguishable in the presence or absence of Hsp27. Loss of Hsp27 also had no effect on the structure of the skeletal muscle myotomes in the developing embryo. Finally, embryos injected with antisense hsp27 and scrambled control PMO displayed equal motility. We conclude that Hsp27 is dispensable for zebrafish morphogenesis but could play a role in long-term maintenance of heart and muscle tissues.

Heat shock proteins and exercise: a primer

Heat shock proteins (HSPs) are, in general, prosurvival molecules within the cellular environment, and the overexpression of even just 1 family of HSPs can lead to protection against and improvements after a variety of stressors. Not surprisingly, a fertile area of study has grown out of efforts to exploit the innate biologic behaviour of HSPs. Exercise, because of the inherent physiologic stresses associated with it, is but 1 stimulus that can result in a robust increase in various HSPs in several tissues, not the least of which happen to be the heart and skeletal muscle. The purpose of this review is to introduce the reader to the major HSP families, the control of their expression, and some of their biologic functions, specifically with respect to the influence of exercise. Moreover, as the first in a series of reviews from a common symposium, we will briefly introduce the concepts presented by the other authors, which include the effects of different exercise paradigms on skeletal muscle HSPs in the adult and aged systems, HSPs as regulators of inflammation, and the ion channel stabilizing effects of HSPs.

Small heat shock proteins in smooth muscle

The small heat shock proteins (HSPs) HSP20, HSP27 and alphaB-crystallin are chaperone proteins that are abundantly expressed in smooth muscles are important modulators of muscle contraction, cell migration and cell survival. This review focuses on factors regulating expression of small HSPs in smooth muscle, signaling pathways that regulate macromolecular structure and the biochemical and cellular functions of small HSPs. Cellular processes regulated by small HSPs include chaperoning denatured proteins, maintaining cellular redox state and modifying filamentous actin polymerization. These processes influence smooth muscle proliferation, cell migration, cell survival, muscle contraction and synthesis of signaling proteins. Understanding functions of small heat shock proteins is relevant to mechanisms of disease in which dysfunctional smooth muscle causes symptoms, or is a target of drug therapy. One example is that secreted HSP27 may be a useful marker of inflammation during atherogenesis. Another is that phosphorylated HSP20 which relaxes smooth muscle may prove to be highly relevant to treatment of hypertension, vasospasm, asthma, premature labor and overactive bladder. Because small HSPs also modulate smooth muscle proliferation and cell migration they may prove to be targets for developing effective, novel treatments of clinical problems arising from remodeling of smooth muscle in vascular, respiratory and urogenital systems.

Nucleocytoplasmic trafficking of the molecular chaperone Hsp104 in unstressed and heat-shocked cells

Hsp104 is a molecular chaperone in yeast that restores solubility and activity to inactivated proteins after severe heat shock. We investigated the mechanisms that influence Hsp104 subcellular distribution in both unstressed and heat-shocked cells. In unstressed cells, Hsp104 and a green fluorescent protein-Hsp104 fusion protein were detected in both the nucleus and the cytoplasm. We demonstrate that a 17-amino-acid sequence of Hsp104 nuclear localization sequence 17 (NLS17) is sufficient to target a reporter molecule to the nucleus and is also necessary for normal Hsp104 subcellular distribution. The nuclear targeting function of NLS17 is genetically dependent on KAP95 and KAP121. In addition, wild-type Hsp104, but not an NLS17-mutated Hsp104 variant, accumulated in the nucleus of cells depleted for the general export factor Xpo1. Interestingly, severe, nonlethal heat shock enhances the nuclear levels of Hsp104 in an NLS17-independent manner. Under these conditions, we demonstrate that karyopherin-mediated nuclear transport is impaired, while the integrity of the nuclear-cytoplasmic barrier remains intact. Based on these observations, we propose that Hsp104 continues to access the nucleus during severe heat shock using a karyopherin-independent mechanism.

Regulation of stress-induced intracellular sorting and chaperone function of Hsp27 (HspB1) in mammalian cells

In vitro, small Hsps (heat-shock proteins) have been shown to have chaperone function capable of keeping unfolded proteins in a form competent for Hsp70-dependent refolding. However, this has never been confirmed in living mammalian cells. In the present study, we show that Hsp27 (HspB1) translocates into the nucleus upon heat shock, where it forms granules that co-localize with IGCs (interchromatin granule clusters). Although heat-induced changes in the oligomerization status of Hsp27 correlate with its phosphorylation and nuclear translocation, Hsp27 phosphorylation alone is not sufficient for effective nuclear translocation of HspB1. Using firefly luciferase as a heat-sensitive reporter protein, we demonstrate that HspB1 expression in HspB1-deficient fibroblasts enhances protein refolding after heat shock. The positive effect of HspB1 on refolding is completely diminished by overexpression of Bag-1 (Bcl-2-associated athanogene), the negative regulator of Hsp70, consistent with the idea of HspB1 being the substrate holder for Hsp70. Although HspB1 and luciferase both accumulate in nuclear granules after heat shock, our results suggest that this is not related to the refolding activity of HspB1. Rather, granular accumulation may reflect a situation of failed refolding where the substrate is stored for subsequent degradation. Consistently, we found 20S proteasomes concentrated in nuclear granules of HspB1 after heat shock. We conclude that HspB1 contributes to an increased chaperone capacity of cells by binding unfolded proteins that are hereby kept competent for refolding by Hsp70 or that are sorted to nuclear granules if such refolding fails.

Expression of the small heat shock protein gene, hsp30, in Rana catesbeiana fibroblasts

In the present study, we examined the expression of the Rana catesbeiana small heat shock protein gene, hsp30, in an FT fibroblast cell line. Northern and western blot analyses revealed that hsp30 mRNA or HSP30 protein was not present constitutively but was strongly induced at a heat shock temperature of 35 degrees C. However, treatment of FT cells with sodium arsenite at concentrations that induced hsp gene expression in other amphibian systems caused cell death. Non-lethal concentrations of sodium arsenite (10 microM) induced only minimal accumulation of hsp30 mRNA or protein after 12 h. Immunocytochemical analyses employing laser scanning confocal microscopy detected the presence of heat-inducible HSP30, in a granular or punctate pattern. HSP30 was enriched in the nucleus with more diffuse localization in the cytoplasm. The nuclear localization of HSP30 was more prominent with continuous heat shock. These heat treatments did not alter FT cell shape or disrupt actin cytoskeletal organization. Also, HSP30 did not co-localize with the actin cytoskeleton.

Cryoelectron Microscopy and EPR Analysis of Engineered Symmetric and Polydisperse Hsp16.5 Assemblies Reveals Determinants of Polydispersity and Substrate Binding

We have identified sequence and structural determinants of oligomer size, symmetry, and polydispersity in the small heat shock protein super family. Using an insertion mutagenesis strategy that mimics evolutionary sequence divergence, we induced the ordered oligomer of Methanococcus jannaschii Hsp16.5 to transition to either expanded symmetric or polydisperse assemblies. A hybrid approach combining spin labeling EPR and cryoelectron microscopy imaging at 10A resolution reveals that the underlying plasticity is mediated by a packing interface with minimal contacts and a flexible C-terminal tether between dimers. Twenty-four dimeric building blocks related by octahedral symmetry assemble into the expanded symmetric oligomer. In contrast, the polydisperse variant has an ordered dimeric building block that heterogeneously packs to yield oligomers of various sizes. Increased exposure of the N-terminal region in the Hsp16.5 variants correlates with enhanced binding to destabilized mutants of T4 lysozyme, whereas deletion of this region reduces binding. Transition to larger intermediates with enhanced substrate binding capacity has been observed in other small heat shock proteins including lens alpha-crystallin mutants linked to congenital cataract. Together, these results provide a mechanistic perspective on substrate recognition and binding by the small heat shock protein superfamily.

Recruitment of phosphorylated small heat shock protein Hsp27 to nuclear speckles without stress

During stress, the mammalian small heat shock protein Hsp27 enters cell nuclei. The present study examines the requirements for entry of Hsp27 into nuclei of normal rat kidney (NRK) renal epithelial cells, and for its interactions with specific nuclear structures. We find that phosphorylation of Hsp27 is necessary for the efficient entry into nuclei during heat shock but not sufficient for efficient nuclear entry under control conditions. We further report that Hsp27 is recruited to an RNAse sensitive fraction of SC35 positive nuclear speckles, but not other intranuclear structures, in response to heat shock. Intriguingly, Hsp27 phosphorylation, in the absence of stress, is sufficient for recruitment to speckles found in post-anaphase stage mitotic cells. Additionally, pseudophosphorylated Hsp27 fused to a nuclear localization peptide (NLS) is recruited to nuclear speckles in unstressed interphase cells, but wildtype and nonphosphorylatable Hsp27 NLS fusion proteins are not. The expression of NLS-Hsp27 mutants does not enhance colony forming abilities of cells subjected to severe heat shock, but does regulate nuclear speckle morphology. These data demonstrate that phosphorylation, but not stress, mediates Hsp27 recruitment to an RNAse soluble fraction of nuclear speckles and support a site-specific role for Hsp27 within the nucleus.

Heat shock protein upregulation protects against pacing-induced myolysis in HL-1 atrial myocytes and in human atrial fibrillation

Atrial fibrillation (AF) causes myocyte stress by inducing structural changes, predominantly myolysis, which is related to the progression of AF. As heat shock proteins (Hsp) protect against cellular stress, their efficacy in preventing myolysis was investigated in a tachy-paced cell model for AF and in patients with AF. HL-1 atrial myocytes were subjected to tachy-pacing, which induced myolysis. Hsp overexpression was accomplished by a mild heat shock or by the drug geranylgeranylacetone (GGA). Hsp-gene-transfection studies were carried out to investigate roles of individual Hsp. In left and/or right atrial appendages from patients with paroxysmal (n=14), persistent (n=17) AF and controls (n=13) in sinus rhythm (SR), Hsp levels (Westerns) and localization (confocal microscopy) were determined. Heat shock and GGA administered prior to tachy-pacing resulted in almost complete protection against tachy-pacing-induced myolysis. Overexpression of Hsp27, but not of Hsp70, also provided complete protection against pacing-induced myolysis. In patients with paroxysmal AF, Hsp27 expression was significantly increased compared to SR and persistent AF. No changes in Hsp40, Hsc70, Hsp70 and Hsp90 expression levels were observed. Hsp27 levels correlated inversely with the duration of paroxysmal and persistent AF and the extent of myolysis. Furthermore, Hsp27 was localized on myofibrils in tachy-paced HL-1 myocytes and in human cardiomyocytes. These data demonstrate that upregulation of Hsp, especially Hsp27, protects tachy-paced atrial myocytes from myolysis. Therefore, the observed elevated Hsp27 expression in patients with paroxysmal AF might serve to protect myocytes from myolysis and limit the progression to persistent AF. Pharmacological induction of Hsp, with drugs such as GGA, may represent a novel therapeutic approach in AF.

Chaperones in preventing protein denaturation in living cells and protecting against cellular stress

A variety of cellular internal and external stress conditions can be classified as proteotoxic stresses. Proteotoxic stresses can be defined as stresses that increase the fraction of proteins that are in an unfolded state, thereby enhancing the probability of the formation of intracellular aggregates. These aggregates, if not disposed, can lead to cell death. In response to the appearance of damaged proteins, cells induce the expression of heat shock proteins. These can function as molecular chaperones to prevent protein aggregation and to keep proteins in a state competent for either refolding or degradation. Most knowledge of the function and regulation (by co-factors) of individual heat shock proteins comes from cell free studies on refolding of heat- or chemically denatured, purified proteins. Unlike the experimental situation in a test tube, cells contain multiple chaperones and co-factors often moving in and out different subcompartments that contain a variety of protein substrates at different folding states. Also, within cells folding competes with the degradative machinery. In this chapter, an overview will be provided on how the main cytosolic/nuclear chaperone Hsp70 is regulated, what is known about its interaction with other main cytosolic/nuclear chaperone families (Hsp27, Hsp90, and Hsp110), and how it may function as a molecular chaperone in living mammalian cells to protect against proteotoxic stresses.

Small heat-shock proteins function in the insoluble protein complex

Small heat-shock proteins (sHSPs) represent an abundant and ubiquitous family of molecular chaperones. The current model proposes that sHSPs function to prevent irreversible aggregation of non-native proteins by forming soluble complex. The chaperone activity of sHSPs is usually determined by the capacity to suppress thermally or chemically induced protein aggregation. However, sHSPs were frequently found in the insoluble complex particularly in vivo. In this report, it is clearly revealed that the insoluble sHSP/substrate complex is formed when sHSP is overloaded with non-native substrates, which is the very case under in vivo conditions. The proposal that sHSPs function to prevent the protein aggregation seems misleading. sHSPs appear to promote the elimination of protein aggregates by incorporating into the insoluble protein complex.

p38 MAPK/HSP25 signaling mediates cadmium-induced contraction of mesangial cells and renal glomeruli

The environmental pollutant cadmium affects human health, with the kidney being a primary target. In addition to proximal tubules, glomeruli and their contractile mesangial cells have also been identified as targets of cadmium nephrotoxicity. Glomerular contraction is thought to contribute to reduced glomerular filtration, a characteristic of cadmium nephrotoxicity. Because p38 MAPK/HSP25 signaling has been implicated in smooth muscle contraction, we examined its role in cadmium-induced contraction of mesangial cells. We report that exposure of mesangial cells to cadmium resulted in 1) cell contraction, 2) activation of MAP kinases, 3) increased HSP25 phosphorylation coincident with p38 MAP kinase activation, 4) sequential phosphorylation of the two phosphorylation sites of mouse HSP25 with Ser15 being phosphorylated before Ser86, 5) reduction of oligomeric size of HSP25, and 6) association of HSP25 with microfilaments. Exposure of isolated rat glomeruli to cadmium also resulted in contraction and increased HSP25 phosphorylation. The cadmium-induced responses were inhibited by the specific p38 MAP kinase inhibitor SB-203580, and cadmium-induced phosphorylation of HSP25 was inhibited by expression of a dominant-negative p38 MAP kinase mutant. These findings tentatively suggest that cadmium-induced nephrotoxicity results, in part, from glomerular contraction due to p38 MAP kinase/HSP25 signaling-dependent contraction of mesangial cells. With regard to the cellular action of HSP25, these data support a change in paradigm: in addition to its well-established cytoprotective function, HSP25 may also be involved in processes that ultimately lead to adverse effects, as is observed in the response of mesangial cells to cadmium.

Cloning, characterization, and heat stress-induced redistribution of a protein homologous to human hsp27 in the zebrafish Danio rerio

Hsp27 is a small heat shock protein (shsp) regulating stress tolerance and increasingly thought to play roles in tissue homeostasis and differentiation. The zebrafish Danio rerio is an important model for the study of developmental processes, but little is known regarding shsps in this animal. Here, we report the sequence, expression, regulation, and function of a zebrafish protein (zfHsp27) homologous to human Hsp27. zfHsp27 contains three conserved phosphorylatable serines and a cysteine important for regulation of apoptosis, but it lacks much of a C-terminal tail domain and shows low homology in two putative actin interacting domains that are features of mammalian Hsp27. zfHsp27 mRNA is most abundant in adult skeletal muscle and heart and is upregulated during early embryogenesis. zfHsp27 expressed in mammalian fibroblasts was phosphorylated in response to heat stress and anisomycin, and this phosphorylation was prevented by treatment with SB202190, an inhibitor of p38 MAPK. Expression of zfHsp27 and human Hsp27 in mammalian fibroblasts promoted a similar degree of tolerance to heat stress. zfHsp27 fusion proteins entered the nucleus and associated with the cytoskeleton of heat stressed cells in vitro and in zebrafish embryos. These results reveal conservation in regulation and function of mammalian and teleost Hsp27 proteins and define zebrafish as a new model for the study of Hsp27 function.

alphaB-crystallin is phosphorylated during myocardial infarction: involvement of platelet-derived growth factor-BB

alphaB-crystallin is the most abundant low-molecular-weight heat shock protein in heart and recent studies have demonstrated that it plays a cardioprotective role during myocardial infarction both in vivo and in vitro. On the other hand, platelet-derived growth factor (PDGF), a potent serum mitogen, has been reported to improve cardiac function after myocardial infarction. In the present study, using a mouse myocardial infarction model, we investigated whether alphaB-crystallin is phosphorylated during myocardial infarction and the implication of PDGF-BB. Phosphorylation of alphaB-crystallin at Ser-59 was time dependently induced and plasma PDGF-BB levels were concomitantly increased. Moreover, PDGF-BB-stimulated phosphorylation of alphaB-crystallin was suppressed by SB203580, a specific inhibitor of p38 mitogen-activated protein (MAP) kinase, in primary cultured cardiac myocytes. Our results indicate that PDGF-BB induces phosphorylation of alphaB-crystallin via p38 MAP kinase during myocardial infarction.

Mimicking phosphorylation of the small heat-shock protein alphaB-crystallin recruits the F-box protein FBX4 to nuclear SC35 speckles

The mammalian small heat shock protein alphaB-crystallin can be phosphorylated at three different sites, Ser19, Ser45 and Ser59. We compared the intracellular distribution of wild-type, nonphosphorylatable and all possible pseudophosphorylation mutants of alphaB-crystallin by immunoblot and immunocytochemical analyses of stable and transiently transfected cells. We observed that pseudophosphorylation at two (especially S19D/S45D) or all three (S19D/S45D/S59D) sites induced the partial translocation of alphaB-crystallin from the detergent-soluble to the detergent-insoluble fraction. Double immunofluorescence studies showed that the pseudophosphorylation mutants localized in nuclear speckles containing the splicing factor SC35. The alphaB-crystallin mutants in these speckles were resistant to mild detergent treatment, and also to DNase I or RNase A digestion, indicating a stable interaction with one or more speckle proteins, not dependent on intact DNA or RNA. We further found that FBX4, an adaptor protein of the ubiquitin-protein isopeptide ligase SKP1/CUL1/F-box known to interact with pseudophosphorylated alphaB-crystallin, was also recruited to SC35 speckles when cotransfected with the pseudophosphorylation mutants. Because SC35 speckles also react with an antibody against alphaB-crystallin endogenously phosphorylated at Ser45, our findings suggest that alphaB-crystallin has a phosphorylation-dependent role in the ubiquitination of a component of SC35 speckles.

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

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

Association of the Chaperone αB-crystallin with Titin in Heart Muscle

alphaB-crystallin, a major component of the vertebrate lens, is a chaperone belonging to the family of small heat shock proteins. These proteins form oligomers that bind to partially unfolded substrates and prevent denaturation. alphaB-crystallin in cardiac muscle binds to myofibrils under conditions of ischemia, and previous work has shown that the protein binds to titin in the I-band of cardiac fibers (Golenhofen, N., Arbeiter, A., Koob, R., and Drenckhahn, D. (2002) J. Mol. Cell. Cardiol. 34, 309-319). This part of titin extends as muscles are stretched and is made up of immunoglobulin-like modules and two extensible regions (N2B and PEVK) that have no well defined secondary structure. We have followed the position of alphaB-crystallin in stretched cardiac fibers relative to a known part of the titin sequence. alphaB-crystallin bound to a discrete region of the I-band that moved away from the Z-disc as sarcomeres were extended. In the physiological range of sarcomere lengths, alphaB-crystallin bound in the position of the N2B region of titin, but not to PEVK. In overstretched myofibrils, it was also in the Ig region between N2B and the Z-disc. Binding between alphaB-crystallin and N2B was confirmed using recombinant titin fragments. The Ig domains in an eight-domain fragment were stabilized by alphaB-crystallin; atomic force microscopy showed that higher stretching forces were needed to unfold the domains in the presence of the chaperone. Reversible association with alphaB-crystallin would protect I-band titin from stress liable to cause domain unfolding until conditions are favorable for refolding to the native state.

Platelet-derived growth factor-BB phosphorylates heat shock protein 27 in cardiac myocytes

It is recognized that heat shock protein 27 (HSP27) is highly expressed in heart. In the present study, we investigated whether platelet-derived growth factor (PDGF) phosphorylates HSP27 in mouse myocytes, and the mechanism underlying the HSP27 phosphorylation. Administration of PDGF-BB induced the phosphorylation of HSP27 at Ser-15 and -85 in mouse cardiac muscle in vivo. In primary cultured myocytes, PDGF-BB time dependently phosphorylated HSP27 at Ser-15 and -85. PDGF-BB stimulated the phosphorylation of p44/p42 mitogen-activated protein (MAP) kinase, p38 MAP kinase, and stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) among the MAP kinase superfamily. SB203580, a specific inhibitor of p38 MAP kinase, reduced the PDGF-BB-stimulated phosphorylation of HSP27 at both Ser-15 and -85, and phosphorylation of p38 MAP kinase. However, PD98059, a specific inhibitor of MEK, or SP600125, a specific inhibitor of SAPK/JNK, failed to affect the HSP27 phosphorylation. These results strongly suggest that PDGF-BB phosphorylates HSP27 at Ser-15 and -85 via p38 MAP kinase in cardiac myocytes.

Nuclear staining for the small heat shock protein alphaB-crystallin colocalizes with splicing factor SC35

AlphaB-Crystallin has for a long time been considered a specific eye lens protein. Later on it appeared that this protein belongs to the family of the small heat shock proteins and that it occurs also extra-lenticularly in many different cell types. AlphaB-Crystallin is mainly present in the cytoplasm, but there are some indications that it might have a function in the nucleus too. However, till now its presence in the nucleus is uncertain. We therefore compared the localization of alphaB-crystallin in nine cell lines cultured under normal conditions using four different antisera. All four antisera gave a diffuse staining for alphaB-crystallin in the cytoplasm, but one of the antibodies consistently showed nuclear staining in eight of the cell types, in the form of distinct speckles. These speckles are equally pronounced in the different cell types, whether or not cytoplasmic alphaB-crystallin is present. Preabsorption of the antiserum with alphaB-crystallin abolished the staining. Furthermore we demonstrate that if only minor amounts of alphaB-crystallin are present, the protein seems to be located exclusively in the nucleus. However, in case of higher amounts of protein, alphaB-crystallin is distributed between cytoplasm and nucleus. The nuclear alphaB-crystallin exists, like the cytoplasmic alphaB-crystallin, in non-phosphorylated and phosphorylated forms, is Triton-insoluble but can be extracted by 2 M NaCl. These data suggest that alphaB-crystallin might be bound to the nuclear matrix per se or to nuclear matrix proteins via other proteins. In agreement with other nuclear matrix proteins, nuclear alphaB-crystallin staining turns diffuse upon mitosis and leaves the chromosomes unstained. Double staining experiments revealed colocalization of alphaB-crystallin with the splicing factor SC35 in nuclear speckles, suggesting a role for alphaB-crystallin in splicing or protection of the splicing machinery.

Heat shock suppresses the permeability transition in rat liver mitochondria

Heat shock proteins inhibit apoptotic and necrotic cell death in various cell types. However, the specific mechanism underlying protection by heat shock proteins remains unclear. To test the hypothesis that heat shock proteins inhibit cell death by blocking opening of mitochondrial permeability transition (MPT) pores, mitochondria from heat-preconditioned rat livers were isolated by differential centrifugation. Heat shock inhibited MPT pore opening induced by 50 microm CaCl(2) plus 5 microm HgCl(2) or 1 microm mastoparan and by 200 microm CaCl(2) alone. Half-maximal swelling was delayed 15 min or more after heat shock compared with control. Heat shock also increased the threshold of unregulated (Ca(2+)-independent and cyclosporin A-insensitive) MPT pore opening induced by higher doses of HgCl(2) and mastoparan. Heat shock treatment decreased mitochondrial reactive oxygen species formation by 27% but did not change mitochondrial respiration, membrane potential, Ca(2+) uptake, or total glutathione in mitochondrial and cytosolic extracts of liver. Western blot analysis showed that mitochondrial Hsp25 increased, whereas Hsp10, Hsp60, Hsp70, Hsp75, cyclophilin D, and voltage-dependent anion channel did not change after heat shock. These results indicate that heat shock causes resistance to opening of MPT pores, which may contribute to heat shock protection against cellular injury.

The small heat-shock protein alpha B-crystallin promotes FBX4-dependent ubiquitination

AlphaB-crystallin is a small heat-shock protein in which three serine residues (positions 19, 45, and 59) can be phosphorylated under various conditions. We describe here the interaction of alphaB-crystallin with FBX4, an F-box-containing protein that is a component of the ubiquitin-protein isopeptide ligase SCF (SKP1/CUL1/F-box). The interaction with FBX4 was enhanced by mimicking phosphorylation of alphaB-crystallin at both Ser-19 and Ser-45 (S19D/S45D), but not at other combinations. Ser-19 and Ser-45 are preferentially phosphorylated during the mitotic phase of the cell cycle. Also alphaB-crystallin R120G, a mutant found to co-segregate with a desmin-related myopathy, displayed increased interaction with FBX4. Both alphaB-crystallin S19D/S45D and R120G specifically translocated FBX4 to the detergent-insoluble fraction and stimulated the ubiquitination of one or a few yet unknown proteins. These findings implicate the involvement of alphaB-crystallin in the ubiquitin/proteasome pathway in a phosphorylation- and cell cycle-dependent manner and may provide new insights into the alphaB-crystallin-induced aggregation in desmin-related myopathy.

Mutation of COOH-terminal lysines in overexpressed alpha B-crystallin abrogates ischemic protection in cardiomyocytes

High levels of alpha B-crystallin are present in the cardiomyocyte, yet little is understood about the function and importance of this protein. Like many other small heat shock proteins, alpha B-crystallin forms large oligomeric complexes whose size can be regulated by posttranslational modifications. The size of these complexes can modify the function of the protein. A naturally occurring COOH-terminal mutant has many detrimental effects in the lens of the eye and altered oligomerization. Therefore, we mutated the two COOH-terminal lysines of alpha B-crystallin to glycines (K174/175G) and adenovirally mounted them to transduce cardiomyocytes. We analyzed the effect of this mutation on oligomerization, microtubular stabilization, and ischemic outcome. A nearly 45% downward shift in complex size was observed with the mutant by native PAGE followed by immunoblotting. The overexpressed protein no longer protected the tubulin cytoskeleton against ischemic stress by confocal analysis. The mutant caused a 30% increase in cytosolic enzyme release with ischemia compared with control, whereas a 33% decrease was associated with wild-type alpha B-crystallin overexpression. We conclude that the COOH terminus of alpha B-crystallin is crucial to its proper function.

Inhibition of Rho-kinase induces alphaB-crystallin expression in lens epithelial cells

The small heat shock protein, alphaB-crystallin, has been shown to interact with actin and intermediate filament proteins. However, little is known regarding the cellular mechanisms regulating such interactions. In this study, we explored the role of the Rho/Rho-kinase pathway in alphaB-crystallin distribution and expression in porcine lens epithelial cells. alphaB-crystallin was distributed uniformly throughout the cytoplasm and did not exhibit any unique redistribution in response to actin depolymerization induced by Rho/Rho-kinase inhibitors (C3-exoenzyme or Y-27632) or by overexpression of the dominant negative mutant of Rho-kinase (DNRK) in porcine lens epithelial cells. Interestingly, alphaB-crystallin levels markedly increased in lens epithelial cells treated with the inhibitors of Rho/Rho-kinase proteins (lovastatin, Y-27632 or DNRK) while a protein kinase C inhibitor (GF109203x) was found to have no effect. Further, Y-27632 showed a dose (2-50 microM) response effect on alphaB-crystallin induction. Nocodazole, a microtubule-depolymerizing agent, elicited an increase in alphaB-crystallin levels but latrunculin, an actin depolymerizing agent, did not show any significant effect. Pretreatment with cycloheximide or genistein blocked the Rho-kinase inhibitor-induced increase in alphaB-crystallin protein levels. Rho-kinase inhibitor-induced increases in alphaB-crystallin levels were found to be associated with activation of P38 mitogen-activated protein kinase (MAPK). These results suggest that Rho/Rho-kinase negatively regulates alphaB-crystallin expression, and this response appears to be dependent on tyrosine-protein kinase and P38 MAPK function. Finally, alphaB-crystallin induction appears to be better correlated with the direct inhibition of Rho/Rho-kinase than with actin depolymerization per se.

Glucocorticoid treatment induces expression of small heat shock proteins in human satellite cell populations: consequences for a desmin-related myopathy involving the R120G alpha B-crystallin mutation

A missense mutation (R120G) of the molecular chaperone alpha B-crystallin has recently been linked to a familial form of desmin-related myopathy characterized by intrasarcoplasmic aggregates of desmin. It was previously demonstrated that the mutant R120G had a defective chaperone-like function. However, the cellular and physiopathological consequences of R120G mutant expression in human muscle cells are as yet unclear. Thus, we developed a cellular model for the study of this R120G alpha B-crystallin-related desmin-related myopathy. We demonstrate that dexamethasone enhances alpha B-crystallin and HSP27 expression in normal and desmin-related myopathy-derived muscle cells. In the undifferentiated desmin-related myopathy satellite cell population no intracytoplasmic aggregates were observed. However, in differentiated satellite cells derived from a related myopathy patient, we observed an enhanced plasma membrane localization of alpha B-crystallin following glucocorticoid. We also observed that the protective effect against stress of alpha B-crystallin is altered upon glucocorticoid-induced small heat shock protein expression for the desmin-related myopathy-derived cells.

Phosphorylation-dependent cellular localization and thermoprotective role of heat shock protein 25 in hippocampal progenitor cells

The present study examined phosphorylation-dependent cellular localization and the thermoprotective role of heat shock protein (HSP) 25 in hippocampal HiB5 cells. HSP25 was induced and phosphorylated by heat shock (at 43 degrees C for 3 h). HSP25, which was located in the cytoplasm in the normal condition, translocated into the nucleus after the heat shock. Transfection experiments with hsp27 mutants in which specific serine phosphorylation residues (Ser(78) and Ser(82)) were substituted with alanines or aspartic acids showed that phosphorylation of HSP27 is accompanied by its nuclear translocation. Phosphorylation of mitogen-activated protein kinases (MAPKs) such as p38 MAPK and ERK was markedly increased by the heat shock, and SB203580 (a p38 MAPK kinase inhibitor) and/or PD098059 (a MEK inhibitor) inhibited the phosphorylation of HSP25, indicating that p38 MAPK and ERK are upstream regulators of HSP25 phosphorylation in the heat shock condition. In the absence of heat shock, actin filament stability was not affected by SB203580 and/or PD098059. Heat shock caused disruption of the actin filament and cell death when phosphorylation of HSP25 was inhibited by SB203580 and/or PD098059. In addition, actin filament was more stable in Asp(78,82)-hsp27 (mimics the phosphorylated form) transfected HiB5 cells than in the normal and Ala(78,82)-hsp27 (nonphosphorylative form) transfected cells. In accordance with actin filament stability, the survival rate against the heat shock increased markedly in Asp(15,78,82)-hsp27 expressing HiB5 cells but decreased in Ala(15,78,82)-hsp27 expressing cells. These results support the idea that phosphorylation of HSP25 is critical for the maintenance of actin filament and enhancement of thermoresistance. Interestingly, HSP25 was dephosphorylated and returned to cytoplasm in a recovery time-dependent manner. This phenomenon was accompanied by an increment of apoptotic cell death as determined by nuclear and DNA fragmentation and fluorescence-activated cell sorter analysis. These results suggest that nuclear-translocated HSP25 might function to protect nuclear structure, thereby preventing apoptotic cell death.

Distribution, phosphorylation, and activities of Hsp25 in heat-stressed H9c2 myoblasts: a functional link to cytoprotection

The behavior of the endogenous heat shock protein 25 (Hsp25) in heat-stressed rat H9c2 myoblasts was studied. After mild or severe heating, this protein became less extractable with Triton X-100 and displayed characteristic immunofluorescence patterns, namely (1) granules in the nucleus, and (2) association with F-actin bundles in the cytoplasm. The intranuclear granulation of Hsp25 and its association with F-actin were sensitive to drugs affecting Hsp25 phosphorylation (cantharidin, sodium orthovanadate, SB203580, SB202190). Isoform analysis of Hsp25 translocated to the nucleus-free cytoskeletal fraction revealed only mono- and biphosphorylated Hsp25 and no unphosphorylated Hsp25. Transfected luciferase with initial localization in the nucleosol became colocalized with the Hsp25-containing granules after a heat shock treatment that denatured the enzyme in the cells. The association of Hsp25 with actin filaments after a mild heat stress conferred protection from subsequent F-actin-damaging treatments with cytochalasins (D and B) or severe heat stress. We hypothesize that (1) the binding of heat-denatured nucleosolic proteins to the Hsp25 contained in specific granular structures may serve for the subsequent chaperoning or degradation of the bound proteins, and (2) the actin cytoskeleton is stabilized by the direct targeting of phosphorylated Hsp25 to microfilament bundles.

Translocation of small heat shock proteins to the actin cytoskeleton upon proteasomal inhibition

The role of small heat shock proteins (sHsps) as molecular chaperones is still poorly understood. We therefore investigated the effect of proteasomal inhibition on sHsps in the rat cardiac myoblast cell line H9c2. Proteasomes are responsible for controlled degradation of intracellular proteins. Inhibition of their activities leads to accumulation of unfolded proteins, which can form insoluble "aggresomes" together with proteasomes and heat shock proteins Hsp70 and Hsp90. We here report that upon proteasome inhibition, alpha B-crystallin and Hsp25 translocate from the detergent-soluble cytosolic fraction to the detergent-insoluble nuclear/cytoskeletal fraction. Although phosphorylation of both alpha B-crystallin and Hsp25 is induced, this does not seem to be essential for the translocation. Immunocytochemistry revealed that alpha B-crystallin and Hsp25, which show a diffuse cytoplasmic staining in unstressed H9c2 cells, colocalize with F-actin upon proteasomal inhibition. After transfection in H9c2 cells, other sHsps (alpha A-crystallin, Hsp20, HspB2 and HspB3) showed similar translocation to the actin cytoskeleton. The redistribution of sHsps upon proteasomal inhibition may reflect a mechanism by which cells are protected from damaged intracellular proteins by sequestering them on the cytoskeleton.

Heat shock proteins and cardiovascular pathophysiology

In the eukaryotic cell an intrinsic mechanism is present providing the ability to defend itself against external stressors from various sources. This defense mechanism probably evolved from the presence of a group of chaperones, playing a crucial role in governing proper protein assembly, folding, and transport. Upregulation of the synthesis of a number of these proteins upon environmental stress establishes a unique defense system to maintain cellular protein homeostasis and to ensure survival of the cell. In the cardiovascular system this enhanced protein synthesis leads to a transient but powerful increase in tolerance to such endangering situations as ischemia, hypoxia, oxidative injury, and endotoxemia. These so-called heat shock proteins interfere with several physiological processes within several cell organelles and, for proper functioning, are translocated to different compartments following stress-induced synthesis. In this review we describe the physiological role of heat shock proteins and discuss their protective potential against various stress agents in the cardiovascular system.

AlphaB crystallin translocation and phosphorylation: signal transduction pathways and preconditioning in the isolated rat heart

In this program of studies we have characterized in detail the translocation (assessed by Triton-insolubility) and phosphorylation (using serine-45 or -59 phosphospecific antibodies) of alphaB crystallin during myocardial ischemia [both with or without ischemic preconditioning (IPC)]. Pharmacological activators and inhibitors allowed us to characterize the signaling pathways involved in alphaB crystallin phosphorylation during ischemia. Ischemic preconditioning alone caused 30% of the heart's alphaB crystallin pool to translocate, providing a significant translocation 'head-start' in protected tissue. This enhanced translocation is coupled with increased (3-fold) alphaB crystallin phosphorylation at both serine residues. The possible role of alphaB crystallin in the protection afforded by ischemic preconditioning is supported by the signal transduction data; which showed preconditioning-induced alphaB crystallin phosphorylation can be blocked by tyrosine kinase inhibition (using genistein) and by p38 MAP kinase or PKC inhibition (using SB203580 or bisindolylmaleimide, respectively). The activation of both p38 MAP kinase and PKC are recognized requirements for the induction of preconditioning and their inhibition is known to block protection. Western immunoblotting analysis after isoelectric focusing electrophoresis, confirmed the observations made with the phosphospecific antibodies; but also showed that 27+/-4% of total cardiac crystallin was phosphorylated after 30 min of ischemia. AlphaB crystallin exists as large polymeric aggregates in cardiac tissue under basal conditions (approximately 1 MDa as determined by gel filtration chromatography). We induced phosphorylation of alphaB crystallin during aerobic perfusion by the administration of phenylephrine. However this treatment did not alter the molecular aggregate size of alphaB crystallin. It appears that alphaB crystallin molecular aggregate size is not simply regulated by phosphorylation. AlphaB crystallin may have a role to play in the myocardial protection induced by ischemic preconditioning, as both translocation and phosphorylation are both accelerated and enhanced by ischemic preconditioning.