Some properties of human small heat shock protein Hsp22 (H11 or HspB8)

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.

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

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

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

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

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

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

Small heat shock protein 22 kDa can modulate the aggregation and liquid-liquid phase separation behavior of tau

Alzheimer's disease is a progressive fatal neurodegenerative disease with no cure or effective treatments. The hallmarks of disease include extracellular plaques and intracellular tangles of aggregated protein. The intracellular tangles consist of the microtubule associated protein tau. Preventing the pathological aggregation of tau may be an important therapeutic approach to treat disease. In this study we show that small heat shock protein 22 kDa (Hsp22) can prevent the aggregation of tau in vitro. Additionally, tau can undergo liquid-liquid phase separation (LLPS) in the presence of crowding reagents which causes it to have an increased aggregation rate. We show that Hsp22 can modulate both the aggregation and LLPS behavior of tau in vitro.

Small Heat Shock Proteins and Human Neurodegenerative Diseases

The review discusses the role of small heat shock proteins (sHsps) in human neurodegenerative disorders, such as Charcot-Marie-Tooth disease (CMT), Parkinson's and Alzheimer's diseases, and different forms of tauopathies. The effects of CMT-associated mutations in two small heat shock proteins (HspB1 and HspB8) on the protein stability, oligomeric structure, and chaperone-like activity are described. Mutations in HspB1 shift the equilibrium between different protein oligomeric forms, leading to the alterations in its chaperone-like activity and interaction with protein partners, which can induce damage of the cytoskeleton and neuronal death. Mutations in HspB8 affect its interaction with the adapter protein Bag3, as well as the process of autophagy, also resulting in neuronal death. The impact of sHsps on different forms of amyloidosis is discussed. Experimental studies have shown that sHsps interact with monomers or small oligomers of amyloidogenic proteins, stabilize their structure, prevent their aggregation, and/or promote their specific proteolytic degradation. This effect might be due to the interaction between the β-strands of sHsps and β-strands of target proteins, which prevents aggregation of the latter. In cooperation with the other heat shock proteins, sHsps can promote disassembly of oligomers formed by amyloidogenic proteins. Despite significant achievements, further investigations are required for understanding the role of sHsps in protection against various neurodegenerative diseases.

Small heat shock proteins: Simplicity meets complexity

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

The Role of the Arginine in the Conserved N-Terminal Domain RLFDQxFG Motif of Human Small Heat Shock Proteins HspB1, HspB4, HspB5, HspB6, and HspB8

Although the N-terminal domain of vertebrate small heat shock proteins (sHsp) is poorly conserved, it contains a core motif preserved in many members of the sHsp family. The role of this RLFDQxFG motif remains elusive. We analyzed the specific role of the first arginine residue of this conserved octet sequence in five human sHsps (HspB1, HspB4, HspB5, HspB6, and HspB8). Substitution of this arginine with an alanine induced changes in thermal stability and/or intrinsic fluorescence of the related HspB1 and HspB8, but yielded only modest changes in the same biophysical properties of HspB4, HspB5, and HspB6 which together belong to another clade of vertebrate sHsps. Removal of the positively charged Arg side chain resulted in destabilization of the large oligomers of HspB1 and formation of smaller size oligomers of HspB5. The mutation induced only minor changes in the structure of HspB4 and HspB6. In contrast, the mutation in HspB8 was accompanied by shifting the equilibrium from dimers towards the formation of larger oligomers. We conclude that the RLFDQxFG motif plays distinct roles in the structure of several sHsp orthologs. This role correlates with the evolutionary relationship of the respective sHsps, but ultimately, it reflects the sequence context of this motif.

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

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

The Chaperone Activity and Substrate Spectrum of Human Small Heat Shock Proteins

Small heat shock proteins (sHsps) are a ubiquitous family of molecular chaperones that suppress the unspecific aggregation of miscellaneous proteins. Multicellular organisms contain a large number of different sHsps, raising questions as to whether they function redundantly or are specialized in terms of substrates and mechanism. To gain insight into this issue, we undertook a comparative analysis of the eight major human sHsps on the aggregation of both model proteins and cytosolic lysates under standardized conditions. We discovered that sHsps, which form large oligomers (HspB1/Hsp27, HspB3, HspB4/αA-crystallin, and HspB5/αB-crystallin) are promiscuous chaperones, whereas the chaperone activity of the other sHsps is more substrate-dependent. However, all human sHsps analyzed except HspB7 suppressed the aggregation of cytosolic proteins of HEK293 cells. We identified ∼1100 heat-sensitive HEK293 proteins, 12% of which could be isolated in complexes with sHsps. Analysis of their biochemical properties revealed that most of the sHsp substrates have a molecular mass from 50 to 100 kDa and a slightly acidic pI (5.4-6.8). The potency of the sHsps to suppress aggregation of model substrates is correlated with their ability to form stable substrate complexes; especially HspB1 and HspB5, but also B3, bind tightly to a variety of proteins, whereas fewer substrates were detected in complex with the other sHsps, although these were also efficient in preventing the aggregation of cytosolic proteins.

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

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

Small heat shock proteins: Role in cellular functions and pathology

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

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.

Charcot-Marie-Tooth disease and intracellular traffic

Mutations of genes whose primary function is the regulation of membrane traffic are increasingly being identified as the underlying causes of various important human disorders. Intriguingly, mutations in ubiquitously expressed membrane traffic genes often lead to cell type- or organ-specific disorders. This is particularly true for neuronal diseases, identifying the nervous system as the most sensitive tissue to alterations of membrane traffic. Charcot-Marie-Tooth (CMT) disease is one of the most common inherited peripheral neuropathies. It is also known as hereditary motor and sensory neuropathy (HMSN), which comprises a group of disorders specifically affecting peripheral nerves. This peripheral neuropathy, highly heterogeneous both clinically and genetically, is characterized by a slowly progressive degeneration of the muscle of the foot, lower leg, hand and forearm, accompanied by sensory loss in the toes, fingers and limbs. More than 30 genes have been identified as targets of mutations that cause CMT neuropathy. A number of these genes encode proteins directly or indirectly involved in the regulation of intracellular traffic. Indeed, the list of genes linked to CMT disease includes genes important for vesicle formation, phosphoinositide metabolism, lysosomal degradation, mitochondrial fission and fusion, and also genes encoding endosomal and cytoskeletal proteins. This review focuses on the link between intracellular transport and CMT disease, highlighting the molecular mechanisms that underlie the different forms of this peripheral neuropathy and discussing the pathophysiological impact of membrane transport genetic defects as well as possible future ways to counteract these defects.

HspB2/myotonic dystrophy protein kinase binding protein (MKBP) as a novel molecular chaperone: structural and functional aspects

The small heat shock protein, human HspB2, also known as Myotonic Dystrophy Kinase Binding Protein (MKBP), specifically associates with and activates Myotonic Dystrophy Protein Kinase (DMPK), a serine/threonine protein kinase that plays an important role in maintaining muscle structure and function. The structure and function of HspB2 are not well understood. We have cloned and expressed the protein in E.coli and purified it to homogeneity. Far-UV circular dichroic spectrum of the recombinant HspB2 shows a β-sheet structure. Fluorescence spectroscopic studies show that the sole tryptophan residue at the 130^th position is almost completely solvent-exposed. Bis-ANS binding shows that though HspB2 exhibits accessible hydrophobic surfaces, it is significantly less than that exhibited by another well characterized small HSP, αB-crystallin. Sedimentation velocity measurements show that the protein exhibits concentration-dependent oligomerization. Fluorescence resonance energy transfer study shows that HspB2 oligomers exchange subunits. Interestingly, HspB2 exhibits target protein-dependent chaperone-like activity: it exhibits significant chaperone-like activity towards dithiothreitol (DTT)-induced aggregation of insulin and heat-induced aggregation of alcohol dehydrogenase, but only partially prevents the heat-induced aggregation of citrate synthase, co-precipitating with the target protein. It also significantly prevents the ordered amyloid fibril formation of α-synuclein. Thus, our study, for the first time, provides biophysical characterization on the structural aspects of HspB2, and shows that it exhibits target protein-dependent chaperone-like activity.

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.

HSPB8 is methylated in hematopoietic malignancies and overexpression of HSPB8 exhibits antileukemia effect

HSPB8 has been shown to be involved in regulation of cell proliferation and apoptosis, and it has also been found to have divergent properties in solid tumors. The purpose of this study was to investigate the expression and function of HSPB8 in hematopoietic malignancies. Expression and induced expression of HSPB8 was evaluated in hematopoietic tumor cell lines and bone marrow samples from patients with leukemia. Methylation status was investigated by methylation-specific polymerase chain reaction. The role of HSPB8 in hematopoietic malignancies was addressed by reintroducing HSPB8 expression into the K562 (leukemia) and Namalwa (lymphoma) cell lines. Expression of HSPB8 was absent in hematopoietic tumor cell lines and primary patient and normal volunteer samples. Promoter DNA methylation of HSPB8 was observed in these cells. HSPB8 expression could be restored after demethylation treatment with 5-aza-2'-deoxycytidine. Overexpression of HSPB8 reduced colony formation of both K562 and Namalwa cell lines, inhibited the cell growth of Namalwa in vitro, and suppressed tumor formation of K562 cells in vivo. The present study demonstrates that HSPB8 is silenced by DNA methylation in hematopoietic malignant and normal cells and its expression can be induced by treatment with 5-aza-2'-deoxycytidine. Overexpression of HSPB8 may have an antitumor activity in chronic myelogenous leukemia and lymphoma.

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

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

The pivotal role of the beta 7 strand in the intersubunit contacts of different human small heat shock proteins

Human alpha B-crystallin and small heat shock proteins HspB6 and HspB8 were mutated so that all endogenous Cys residues were replaced by Ser and the single Cys residue was inserted in a position homologous to that of Cys137 of human HspB1, i.e. in a position presumably located in the central part of beta 7 strand of the alpha-crystallin domain. The secondary, tertiary, and quaternary structures of thus obtained Cys-mutants as well as their chaperone-like activity were similar to those of their wild-type counterparts. Mild oxidation of Cys-mutants leads to formation of disulfide bond crosslinking neighboring monomers thus indicating participation of the beta 7 strand in intersubunit interaction. Oxidation weakly affects the secondary and tertiary structure, does not affect the quaternary structure of alpha B-crystallin and HspB6, and shifts equilibrium between monomer and dimer of HspB8 towards dimer formation. It is concluded that the beta 7 strand participates in the intersubunit interaction of four human small heat shock proteins (alpha B-crystallin, HspB1, HspB6, HspB8) having different structure of beta2 strand of alpha-crystallin domain and different length and composition of variable N- and C-terminal tails.

Characterization of orchardgrass p23, a flowering plant Hsp90 cohort protein

p23 is a heat shock protein 90 (Hsp90) co-chaperone and stabilizes the Hsp90 heterocomplex in mammals and yeast. In this study, we isolated a complementary DNA (cDNA) encoding p23 from orchardgrass (Dgp23) and characterized its functional roles under conditions of thermal stress. Dgp23 is a 911 bp cDNA with an open reading frame predicted to encode a 180 amino acid protein. Northern analysis showed that expression of Dgp23 transcripts was heat inducible. Dgp23 has a well-conserved p23 domain and interacted with an orchardgrass Hsp90 homolog in vivo, like mammalian and yeast p23 homologs. Recombinant Dgp23 is a small acidic protein with a molecular mass of approximately 27 kDa and pI 4.3. Dgp23 was also shown to function as a chaperone protein by suppression of malate dehydrogenase thermal aggregation. Differential scanning calorimetry thermograms indicated that Dgp23 is a heat-stable protein, capable of increasing the T (m) of lysozyme. Moreover, overexpression of Dgp23 in a yeast p23 homolog deletion strain, Deltasba1, increased cell viability. These results suggest that Dgp23 plays a role in thermal stress-tolerance and functions as a co-chaperone of Hsp90 and as a chaperone.

HspB8 participates in protein quality control by a non-chaperone-like mechanism that requires eIF2{alpha} phosphorylation

Aggregation of mutated proteins is a hallmark of many neurodegenerative disorders, including Huntington disease. We previously reported that overexpression of the HspB8.Bag3 chaperone complex suppresses mutated huntingtin aggregation via autophagy. Classically, HspB proteins are thought to act as ATP-independent molecular chaperones that can bind unfolded proteins and facilitate their processing via the help of ATP-dependent chaperones such as the Hsp70 machine, in which Bag3 may act as a molecular link between HspB, Hsp70, and the ubiquitin ligases. However, here we show that HspB8 and Bag3 act in a non-canonical manner unrelated to the classical chaperone model. Rather, HspB8 and Bag3 induce the phosphorylation of the alpha-subunit of the translation initiator factor eIF2, which in turn causes a translational shut-down and stimulates autophagy. This function of HspB8.Bag3 does not require Hsp70 and also targets fully folded substrates. HspB8.Bag3 activity was independent of the endoplasmic reticulum (ER) stress kinase PERK, demonstrating that its action is unrelated to ER stress and suggesting that it activates stress-mediated translational arrest and autophagy through a novel pathway.

Human mutation in the anti-apoptotic heat shock protein 20 abrogates its cardioprotective effects

The small heat shock protein Hsp20 protects cardiomyocytes against apoptosis, and phosphorylation at its Ser16 site enhances its cardioprotection. To determine whether genetic variants exist in human Hsp20, which may modify these beneficial effects, we sequenced the coding region of the Hsp20 gene in 1347 patients suffering from dilated cardiomyopathy and 744 subjects with no heart disease. We identified a C59T substitution in the human Hsp20 gene in one patient and three individuals without heart disease. All subjects were heterozygous for this mutation, which changes a fully conserved proline residue into leucine at position 20 (P20L), resulting in secondary structural alterations. To examine the potential functional significance of the P20L-Hsp20 human variant, adult rat cardiomyocytes were infected with Ad.GFP (where Ad is adenovirus and GFP is green fluorescent protein), Ad.WT-Hsp20 (where WT is wild-type), and Ad.P20L-Hsp20 and subjected to simulated ischemia/reperfusion injury. Expression of WT-Hsp20 resulted in significant attenuation of apoptosis compared with the GFP control. However, the P20L-Hsp20 mutant showed no protection against apoptosis, assessed by Hoechst staining and DNA fragmentation. The loss of cardioprotection by the mutant Hsp20 was associated with its diminished phosphorylation at Ser16 compared with WT-Hsp20. Furthermore, maximal stimulation of cardiomyocytes with isoproterenol or protein kinase A-mediated phosphorylation in vitro confirmed the impaired ability of the mutant Hsp20 to become phosphorylated at Ser16. In conclusion, we have identified a P20L substitution in human Hsp20, which is associated with diminished phosphorylation at Ser16 and complete abrogation of the Hsp20 cardioprotective effects which may adversely affect the ability of human carriers to cope with cellular stress.

Induction of Hsp22 (HspB8) by estrogen and the metalloestrogen cadmium in estrogen receptor-positive breast cancer cells

Estrogen (E2) plays a critical role in the etiology and progression of human breast cancer. The estrogenic response is complex and not completely understood, including in terms of the involved responsive genes. Here we show that Hsp22 (synonyms: HspB8, E2lG1, H11), a member of the small heat shock protein (sHSP) superfamily, was induced by E2 in estrogen receptor-positive MCF-7 breast cancer cells, resulting in an elevated Hsp22 protein level, whereas it was not induced in estrogen receptor-negative MDA-MB-231 cells. This induction was prevented by the pure anti-estrogen ICI182780 (faslodex, fulvestrant), whereas tamoxifen, a substance with mixed estrogenic and antiestrogenic properties, had no major inhibitory effect on this induction, nor did it induce Hsp22 on its own. Cadmium (Cd) is an environmental pollutant with estrogenic properties (metalloestrogen) that has been implicated in breast cancer. Treatment of MCF-7 cells with Cd also resulted in induction of Hsp22, and this induction was also inhibited by ICI182780. In live MCF-7 cells, Hsp22 interacted at the level of dimers with Hsp27, a related sHSP, as was shown by quantitative fluorescence resonance energy transfer measurements. In cytosolic extracts of MCF-7 cells, most of the E2- and Cd-induced Hsp22 was incorporated into high-molecular mass complexes. In part, Hsp22 and Hsp27 were components of distinct populations of these complexes. Finally, candidate elements in the Hsp22 promoter were identified by sequence analysis that could account for the induction of Hsp22 by E2 and Cd. Taken together, Hsp22 induction represents a new aspect of the estrogenic response with potential significance for the biology of estrogen receptor-positive breast cancer cells.

Structure, properties, and functions of the human small heat-shock protein HSP22 (HspB8, H11, E2IG1): A critical review

The recently described human HSP22 belongs to the superfamily of small heat-shock proteins containing a conservative alpha-crystallin domain. HSP22 seems to be involved in regulation of cell proliferation, cardiac hypertrophy, apoptosis, and carcinogenesis, and expression of point mutants of HSP22 correlates with development of different neuromuscular diseases. Therefore, an investigation of the structure and properties of HSP22 is desirable for understanding its multiple functions. HSP22 seems to belong to the group of so-called intrinsically disordered proteins and possesses a highly flexible structure. HSP22 tends to form small-molecular-mass oligomers and interacts with biological membranes and many different proteins, among them glycolytic enzymes and different protein kinases. HSP22 possesses chaperonelike activity and prevents aggregation of denatured proteins both in vitro and in vivo. Depending on the cell type and its expression, HSP22 might have either pro- or anti-apoptotic effects. Chaperonelike activity seems to be important for antiapoptotic effects, whereas interaction with and regulation of certain protein kinases might be important for the proapoptotic effects of HSP22. Expression of K141N or K141E mutants of HSP22 correlates with development of distal hereditary motor neuropathy and/or Charcot-Marie-Tooth disease. These mutations destabilize the structure of HSP22, affect its interaction with other small heat-shock proteins, and decrease its chaperonelike activity. HSP22 decreases or prevents aggregation of Huntingtin fragments and amyloid-beta peptide 1-40 of the Dutch type. Thus, HSP22 seems to play an important role in the nervous system, and further investigations are needed to understand the molecular mechanisms of its functioning.

Structure, function, property, and role in neurologic diseases and other diseases of the sHsp22

Small heat shock proteins are members of the heat shock proteins family. They share important identical features: 1) they form the conserved structure 'alpha-crystallin domain' with about 80-100 residues in the C-terminal part of the proteins; 2) they have monomeric molecular masses ranging in 12-43 kDa; 3) they associate into large oligomers consisting in many cases of subunits; 4) they increase expression under stress conditions; 5) they exhibit a highly dynamic structure; and 6) they play a chaperone-like role. Hsp22 (also known as HspB8, H11, and E2IG1) retains the structural motif of the 'alpha-crystallin' family of Hsps and is a member of the superfamily of sHsps. Hsp22 displays chaperone activity, autokinase activity, and trigger or block apoptosis activity. It differs from canonical family members existing as a monomer. A decrease in the HspB8 activity may contribute to the development of some neurologic diseases and others.

Interaction of mammalian Hsp22 with lipid membranes

Hsp22/HspB8 is a member of the small heat-shock protein family, whose function is not yet completely understood. Our immunolocalization studies in a human neuroblastoma cell line, SK-N-SH, using confocal microscopy show that a significant fraction of Hsp22 is localized to the plasma membrane. We therefore investigated its interactions with lipid vesicles in vitro. Intrinsic tryptophan fluorescence is quenched in the presence of lipid vesicles derived from either bovine brain lipid extract or purified lipids. Time-resolved fluorescence studies show a decrease in the lifetimes of the tryptophan residues. Both of these results indicate burial of some tryptophan residues of Hsp22 upon interaction with lipid vesicles. Membrane interactions also lead to increase in fluorescence polarization of Hsp22. Gel-filtration chromatography shows that Hsp22 binds stably with lipid vesicles; the extent of binding depends on the nature of the lipid. Hsp22 binds more strongly to vesicles made of lipids containing a phosphatidic acid, phosphatidylinositol or phosphatidylserine headgroup (known to be present in the inner leaflet of plasma membrane) compared with lipid vesicles made of a phosphatidylcholine head-group alone. Far-UV CD spectra reveal conformational changes upon binding to the lipid vesicles or in membrane-mimetic solvent, trifluoroethanol. Thus our fluorescence, CD and gel-filtration studies show that Hsp22 interacts with membrane and this interaction leads to stable binding and conformational changes. The present study therefore clearly demonstrates that Hsp22 exhibits potential membrane interaction that may play an important role in its cellular functions.

Small heat shock protein Hsp20 (HspB6) as a partner of 14-3-3gamma

Interaction of human 14-3-3gamma with the small heat shock protein Hsp20 was analyzed by means of size-exclusion chromatography and chemical crosslinking. Unphosphorylated Hsp20 and its mutant S16D mimicking phosphorylation by cAMP-dependent protein kinase did not interact with 14-3-3. Phosphorylated Hsp20 formed a tight complex with 14-3-3 in which dimer of 14-3-3 was bound to dimer of Hsp20. 14-3-3 did not affect the chaperone activity of unphosphorylated Hsp20 but increased the chaperone activity of phosphorylated Hsp20 if insulin was used as a model substrate. Estimation of the effect of 14-3-3 on the chaperone activity of Hsp20 with other model substrates was complicated by the fact that under in vitro conditions isolated 14-3-3 possessed its own high chaperone activity. Taken into account high content of Hsp20 in different muscles it is supposed that upon phosphorylation Hsp20 might effectively compete with multiple protein targets of 14-3-3 and by this means indirectly affect many intracellular processes.

Structure and properties of K141E mutant of small heat shock protein HSP22 (HspB8, H11) that is expressed in human neuromuscular disorders

Some properties of the K141E mutant of human HSP22 that is expressed in distal hereditary motor neuropathy were investigated. This mutation slightly decreased intrinsic fluorescence of HSP22 and induced changes in the far UV CD spectra that correlate with increase of disordered structure. Destabilized K141E mutant was more susceptible to trypsinolysis than the wild type protein. Mutation K141E did not significantly affect the hydrophobic properties measured by bis-ANS binding and did not affect the quaternary structure of HSP22. With insulin as a substrate the chaperone-like activity of K141E mutant and the wild type protein were similar. However with alcohol dehydrogenase and rhodanese the chaperone-like activity of K141E mutant was remarkably lower than the corresponding activity of the wild type protein. It is concluded that K141E mutation induces destabilization of HSP22 structure and probably by this means diminish the chaperone-like activity of HSP22 with certain protein substrates.

Conformational changes resulting from pseudophosphorylation of mammalian small heat shock proteins--a two-hybrid study

The human genome codes for 10 so-called mammalian small heat shock or stress proteins (sHsp) with the various tissues expressing characteristic sets of sHsps. Most sHsps interact with each other and form homo- and heterooligomeric complexes. Some of the sHsps are phosphoproteins in vivo, and phosphorylation has been implicated in the regulation of complex size and composition. In this study, we analyze, by the 2-hybrid method, the reporter gene activation pattern of several sHsp pairs that previously have been demonstrated to interact. We show that pseudophosphorylation (mimicry of phosphorylation) of the homologous phosphorylation sites Ser15 and Ser16 in Hsp27 and Hsp20, respectively, modulates characteristics of these sHsps that can be detected by their ability to activate reporter genes in suitable 2-hybrid assays. Pseudophosphorylation of the separated N-terminus of Hsp27 alone is not sufficient for the activation of the reporter genes, whereas the separated C-terminus is sufficient. We conclude that pseudophosphorylation of Hsp27 and Hsp20 at their N-termini results in conformational changes that can be detected by their interaction with other sHsps. Pseudophosphorylation of alphaB-crystallin at Ser19, in contrast, had no detectable consequences.

Differential impact of environmental stresses on the pea mitochondrial proteome

Exposure to adverse environmental conditions causes oxidative stress in many organisms, leading either to disease and debilitation or to response and tolerance. Mitochondria are a key site of oxidative stress and of cellular response and play important roles in cell survival. We analyzed the response of mitochondria in pea (Pisum sativum) plants to the common stresses associated with drought, cold, and herbicides. These treatments all altered photosynthetic and respiratory rates of pea leaves to various extents, but only herbicides significantly increased lipid peroxidation product accumulation. Mitochondria isolated from the stressed pea plants maintained their electron transport chain activity, but changes were evident in the abundance of uncoupling proteins, non-phosphorylating respiratory pathways, and oxidative modification of lipoic acid moieties on mitochondrial proteins. These data suggest that herbicide treatment placed a severe oxidative stress on mitochondria, whereas chilling and particularly drought were milder stresses. Detailed analysis of the soluble proteome of mitochondria by gel electrophoresis and mass spectrometry revealed differential degradation of key matrix enzymes during treatments with chilling being significantly more damaging than drought. Differential induction of heat shock proteins and specific losses of other proteins illustrated the diversity of response to these stresses at the protein level. Cross-species matching was required for mass spectrometry identification of nine proteins because only a limited number of pea cDNAs have been sequenced, and the full pea genome is not available. Blue-native separation of intact respiratory chain complexes revealed little if any change in response to environmental stresses. Together these data suggest that although many of the molecular events identified by chemical stresses of mitochondria from a range of model eukaryotes are also apparent during environmental stress of plants, their extent and significance can vary substantially.

H11 has dose-dependent and dual hypertrophic and proapoptotic functions in cardiac myocytes

We have shown previously that H11, a serine/threonine kinase, is up-regulated in a heart subjected to ischaemia/reperfusion. In the present study, we have characterized the cellular function of H11, using neonatal rat cardiac myocytes. Although transduction of adenovirus harbouring H11 at low doses increased the cell size, at higher doses it induced apoptosis in cardiac myocytes. Apoptosis was not observed when adenovirus harbouring H11-KI (kinase-inactive mutant of H11) was used, suggesting that the proapoptotic effect of H11 is kinase-dependent. The hypertrophic effect of H11 at high doses was unmasked when apoptosis was inhibited by the caspase inhibitor DEVD-CHO, suggesting that H11 stimulates both hypertrophy and apoptosis in parallel. H11-KI induced hypertrophy even at high doses, indicating that H11 stimulates hypertrophy through kinase-independent mechanisms. H11-KI activated Akt, and cardiac hypertrophy induced by H11-KI was blocked by LY294002, an inhibitor of phosphoinositide 3-kinase. Co-immunoprecipitation analyses indicated that H11 interacts with the alpha subunit of CK2 (casein kinase 2). Overexpression of H11 decreased the kinase activity of CK2. DRB (5,6-dichloro-1-beta-D-ribofuranosyl-benzimidazole), an inhibitor of CK2, mimicked the effect of H11, whereas DRB and H11 failed to exhibit additive effects on apoptosis, suggesting that H11 and DRB utilize a common mechanism to induce apoptosis, namely inhibition of CK2. In summary, H11 is a dual-function kinase in cardiac cells: it induces hypertrophy at low doses through kinase-independent activation of Akt, whereas it causes apoptosis at high doses through protein kinase-dependent mechanisms, in particular by physical interaction with and subsequent inhibition of CK2.

HspB8, a small heat shock protein mutated in human neuromuscular disorders, has in vivo chaperone activity in cultured cells

The family of small heat shock proteins (sHsp) is composed of 10 members in mammals, four of which are found mutated in diseases associated with the accumulation of protein aggregates. Though many sHsp have demonstrated molecular chaperone activity in vitro in cell-free conditions, their activity in vivo in the normal cellular context remains unclear. In the present study, we investigated the capacity of the sHsp, HspB8/Hsp22, to prevent protein aggregation in the cells using the polyglutamine protein Htt43Q as a model. In control conditions, Htt43Q accumulated in perinuclear inclusions composed of SDS-insoluble aggregates. Co-transfected with Htt43Q, HspB8 became occasionally trapped within the inclusions; however, in most cells, HspB8 blocked inclusion formation. Biochemical analyses indicated that HspB8 inhibited the accumulation of SDS-insoluble Htt43Q as efficiently as Hsp40 which was taken as a positive control. Htt43Q then accumulated in the SDS-soluble fraction, provided that protein degradation was blocked by proteasome and autophagy inhibitors. In contrast, the other sHsp Hsp27/HspB1 and alphaB-crystallin/HspB5 had no effect. This suggested that HspB8 functions as a molecular chaperone, maintaining Htt43Q in a soluble state competent for rapid degradation. Analyses of Hsp27-HspB8 chimeric proteins indicated that the C-terminal domain of HspB8 contains the specific sequence necessary for chaperone activity. Missense mutations in this domain at lysine 141, which are found in human motor neuropathies, significantly reduced the chaperone activity of the protein. A decrease in the HspB8 chaperone activity may therefore contribute to the development of these diseases.

Small heat shock proteins in inherited peripheral neuropathies

Small heat shock proteins (small HSPs) are molecular chaperones that protect cells against stress by assisting in the correct folding of denatured proteins and thus prevent aggregation of misfolded proteins. Small HSPs also modulate apoptotic pathways by interacting with components of programmed cell death. Furthermore, some small HSPs interact with the cytoskeleton to assist in spatial organization and dynamics of its structural elements. The role of small HSPs has been studied in many disorders, including neurodegenerative disease. Recently, mutations in HSPB1 (HSP27) and HSPB8 (HSP22), two members of the small HSP superfamily, have been associated with inherited peripheral neuropathies. In this review, we will summarize the current knowledge of small HSPs, in particular HSPB1 and HSPB8, and discuss their role in health and disease.

The problem of protein kinase activity of small heat shock protein Hsp22 (H11 or HspB8)

The recently described protein denoted H11, Hsp22 or HspB8 seems to participate in regulation of proliferation, apoptosis, and cardiac hypertrophy. Mutation of Hsp22 causes distal motor neuropathy. Multitude action of Hsp22 is supposed to be due to its protein kinase and/or chaperone-like activities. There are many indirect evidences indicating that Hsp22 possesses intrinsic protein kinase activity. However, low homology to protein kinases, low extent of autophosphorylation, lack of significant protein kinase activity with commonly used substrates, and lack of information on stoichiometry, kinetics, and substrate specificity make the existence of intrinsic protein kinase activity of Hsp22 questionable. It is supposed that protein kinase activity ascribed to Hsp22 is due to contaminating protein kinases. Hsp22 is highly homologous to small heat shock proteins and effectively prevents aggregation of denatured protein both in vitro and in vivo. Therefore, it is supposed that chaperone-like activity is of great importance for Hsp22 functioning.

Oligomerization, chaperone activity, and nuclear localization of p26, a small heat shock protein from Artemia franciscana

Artemia franciscana embryos undergo encystment, developmental arrest and diapause, the last characterized by profound metabolic dormancy and extreme stress resistance. Encysted embryos contain an abundant small heat shock protein termed p26, a molecular chaperone that undoubtedly has an important role in development. To understand better the role of p26 in Artemia embryos, the structural and functional characteristics of full-length and truncated p26 expressed in Escherichia coli and COS-1 cells were determined. p26 chaperone activity declined with increasing truncation of the protein, and those deletions with the greatest adverse effect on protection of citrate synthase during thermal stress had the most influence on oligomerization. When produced in either prokaryotic or eukaryotic cells the p26 alpha-crystallin domain consisting of amino acid residues 61-152 existed predominantly as monomers, and p26 variants lacking the amino-terminal domain but with intact carboxyl-terminal extensions were mainly monomers and dimers. The amino terminus was, therefore, required for efficient dimer formation. Assembly of higher order oligomers was enhanced by the carboxyl-terminal extension, although removing the 10 carboxyl-terminal residues had relatively little effect on oligomerization and chaperoning. Full-length and carboxyl-terminal truncated p26 resided in the cytoplasm of transfected COS-1 cells; however, variants missing the complete amino-terminal domain and existing predominantly as monomers/dimers entered the nuclei. A mechanism whereby oligomer disassembly assisted entry of p26 into nuclei was suggested, this of importance because p26 translocates into Artemia embryo nuclei during development and stress. However, when examined in Artemia, the p26 oligomer size was unchanged under conditions that allowed movement into nuclei, suggesting a process more complex than just oligomer dissociation.