Human Hsp60 with its mitochondrial import signal occurs in solution as heptamers and tetradecamers remarkably stable over a wide range of concentrations

Crystal structures of dimeric and heptameric mtHsp60 reveal the mechanism of chaperonin inactivation

Mitochondrial Hsp60 (mtHsp60) plays a crucial role in maintaining the proper folding of proteins in the mitochondria. mtHsp60 self-assembles into a ring-shaped heptamer, which can further form a double-ring tetradecamer in the presence of ATP and mtHsp10. However, mtHsp60 tends to dissociate in vitro, unlike its prokaryotic homologue, GroEL. The molecular structure of dissociated mtHsp60 and the mechanism behind its dissociation remain unclear. In this study, we demonstrated that Epinephelus coioides mtHsp60 (EcHsp60) can form a dimeric structure with inactive ATPase activity. The crystal structure of this dimer reveals symmetrical subunit interactions and a rearranged equatorial domain. The α4 helix of each subunit extends and interacts with its adjacent subunit, leading to the disruption of the ATP-binding pocket. Furthermore, an RLK motif in the apical domain contributes to stabilizing the dimeric complex. These structural and biochemical findings provide new insights into the conformational transitions and functional regulation of this ancient chaperonin.

Oligomeric State and Holding Activity of Hsp60

Similar to its bacterial homolog GroEL, Hsp60 in oligomeric conformation is known to work as a folding machine, with the assistance of co-chaperonin Hsp10 and ATP. However, recent results have evidenced that Hsp60 can stabilize aggregation-prone molecules in the absence of Hsp10 and ATP by a different, "holding-like" mechanism. Here, we investigated the relationship between the oligomeric conformation of Hsp60 and its ability to inhibit fibrillization of the Ab40 peptide. The monomeric or tetradecameric form of the protein was isolated, and its effect on beta-amyloid aggregation was separately tested. The structural stability of the two forms of Hsp60 was also investigated using differential scanning calorimetry (DSC), light scattering, and circular dichroism. The results showed that the protein in monomeric form is less stable, but more effective against amyloid fibrillization. This greater functionality is attributed to the disordered nature of the domains involved in subunit contacts.

Hereditary spastic paraplegia SPG13 mutation increases structural stability and ATPase activity of human mitochondrial chaperonin

Human mitochondrial chaperonin mHsp60 is broadly associated with various human health conditions and the V72I mutation in mHsp60 causes a form of hereditary spastic paraplegia, a neurodegenerative disease. The main function of mHsp60 is to assist folding of mitochondrial proteins in an ATP-dependent manner. In this study, we unexpectedly found that mutant mHsp60^V72I was more stable structurally and more active in the ATPase activity than the wildtype. Analysis of our recently solved cryo-EM structure of mHsp60 revealed allosteric roles of V72I in structural stability and ATPase activity, which were supported by studies including those using the V72A mutation. Despite with the increases in structural stability and ATPase activity, mHsp60^V72I was less efficient in folding malate dehydrogenase, a putative mHsp60 substrate protein in mitochondria and also commonly used in chaperonin studies. In addition, although mHsp60^V72I along with its cochaperonin mHsp10 was able to substitute the E. coli chaperonin system in supporting cell growth under normal temperature of 37 °C, it was unable under heat shock temperature of 42 °C. Our results support the importance of structural dynamics and an optimal ATP turnover that mHsp60 has evolved for its function and physiology. We propose that unproductive energy utilization, or hyperactive ATPase activity and compromised folding function, not mutually exclusive, are responsible for the V72I pathology in neurodegenerative disease.

Heat shock proteins in adaptation to physical activity

The review article presents the author’s model of one of the blocks of the integrated adaptation mechanism to physical activity and the accompanying moderate heat effects. The participation of heat shock proteins in the stabilization of the tertiary structure and in the restoration of the function of proteins damaged by temperature and physical stressors but performing catalytic, transport, reception or protective role and being involved in the processes of contraction- relaxation and muscle and bone tissue remodeling is discussed.

The Possible Role of the Type I Chaperonins in Human Insulin Self-Association

Insulin is a hormone that attends to energy metabolism by regulating glucose levels in the bloodstream. It is synthesised within pancreas beta-cells where, before being released into the serum, it is stored in granules as hexamers coordinated by Zn²⁺ and further packaged in microcrystalline structures. The group I chaperonin cpn60, known for its assembly-assisting function, is present, together with its cochaperonin cpn10, at each step of the insulin secretory pathway. However, the exact function of the heat shock protein in insulin biosynthesis and processing is still far from being understood. Here we explore the possibility that the molecular machine cpn60/cpn10 could have a role in insulin hexameric assembly and its further crystallization. Moreover, we also evaluate their potential protective effect in pathological insulin aggregation. The experiments performed with the cpn60 bacterial homologue, GroEL, in complex with its cochaperonin GroES, by using spectroscopic methods, microscopy and hydrodynamic techniques, reveal that the chaperonins in vitro favour insulin hexameric organisation and inhibit its aberrant aggregation. These results provide new details in the field of insulin assembly and its related disorders.

Extracellular heat shock proteins in cancer: From early diagnosis to new therapeutic approach

In cancer, human cells lose the ability to properly control the series of events that occur constantly during cell growth and division, including protein expression, stability, and dynamics. Heat shock proteins (Hsps) are key molecules in these events, constitutively expressed at high levels and could furthermore be induced by the response to cancer-induced stress. In tumor cells, Hsps have been shown to be implicated in the regulation of apoptosis, immune responses, angiogenesis and metastasis; in some cases, they can be overexpressed and dysregulated, representing important cancer hallmarks. In the past few years, it has been demonstrated that Hsps can be released by tumor cells through several secreting pathways, including the extracellular vesicles (EVs), thus modulating the tumor microenvironment as well as long-distance intercellular communication and metastatization. In this review, we discuss the role of extracellular Hsps in cancer, with a particular interest in Hsps in EVs. We would also like to highlight the importance of fully understanding of the role of extracellular Hsps released by EVs and encourage further research in this field the use of Hsps as early cancer biomarkers and therapeutic targets.

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

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

JNK pathway and heat shock response mediate the survival of C26 colon carcinoma bearing mice fed with the mushroom Pleurotus eryngii var. eryngii without affecting tumor growth or cachexia

In the last few years, there has been emerging interest in developing treatments against human diseases using natural bioactive content. Here, the powder of the edible mushroom Pleurotus eryngii var. eryngii was mixed with the normal diet of mice bearing C26 colon carcinoma. Interestingly, it was evidenced by a significant increase in the survival rate of C26 tumor-bearing mice accompanied by a significant increase in Hsp90 and Hsp27 protein levels in the tumors. These data were paralleled by a decrease in Hsp60 levels. The mushroom introduced in the diet induced the inhibition of the transcription of the pro-inflammatory cytokines IL-6 and IL-1 exerting an anti-inflammatory action. The effects of the mushroom were mediated by the activation of c-Jun NH₂-terminal kinases as a result of metabolic stress induced by the micronutrients introduced in the diet. In the tumors of C26 bearing mice fed with Pleurotus eryngii there was also a decreased expression of the mitotic regulator survivin and the anti-apoptotic factor Bcl-xL as well as an increase in the expression levels of Atg7, a protein that drives autophagy. In our hypothesis the interplay of these molecules favored the survival of the mice fed with the mushroom. These data are promising for the introduction of Pleurotus eryngii as a dietary supplement or as an adjuvant in anti-cancer therapy.

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

Simple Summary

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

Abstract

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

Extracellular Chaperones as Novel Biomarkers of Overall Cancer Progression and Efficacy of Anticancer Therapy

Exosomal heat shock proteins (Hsps) are involved in intercellular communication both in physiological and pathological conditions. They play a role in key processes of carcinogenesis including immune system regulation, cell differentiation, vascular homeostasis and metastasis formation. Thus, exosomal Hsps are emerging biomarkers of malignancies and possible therapeutic targets. Adolescents and young adults (AYAs) are patients aged 15–39 years. This age group, placed between pediatric and adult oncology, pose a particular challenge for cancer management. New biomarkers of cancer growth and progression as well as prognostic factors are desperately needed in AYAs. In this review, we attempted to summarize the current knowledge on the role of exosomal Hsps in selected solid tumors characteristic for the AYA population and/or associated with poor prognosis in this age group. These included malignant melanoma, brain tumors, and breast, colorectal, thyroid, hepatocellular, lung and gynecological tract carcinomas. The studies on exosomal Hsps in these tumors are limited; however; some have provided promising results. Although further research is needed, there is potential for future clinical applications of exosomal Hsps in AYA cancers, both as novel biomarkers of disease presence, progression or relapse, or as therapeutic targets or tools for drug delivery.

Heat shock protein 60 and cardiovascular diseases: An intricate love-hate story

Cardiovascular diseases (CVDs) are the result of complex pathophysiological processes in the tissues comprising the heart and blood vessels. Inflammation is the main culprit for the development of cardiovascular dysfunction, and it may be traced to cellular stress events including apoptosis, oxidative and shear stress, and cellular and humoral immune responses, all of which impair the system's structure and function. An intracellular chaperone, heat shock protein 60 (HSP60) is an intriguing example of a protein that may both be an ally and a foe for cardiovascular homeostasis; on one hand providing protection against cellular injury, and on the other triggering damaging responses through innate and adaptive immunity. In this review we will discuss the functions of HSP60 and its effects on cells and the immune system regulation, only to later address its implications in the development and progression of CVD. Lastly, we summarize the outcome of various studies targeting HSP60 as a potential therapeutic strategy for cardiovascular and other diseases.

Complex Destabilization in the Mitochondrial Chaperonin Hsp60 Leads to Disease

Several neurological disorders have been linked to mutations in chaperonin genes and more specifically to the HSPD1 gene. In humans, HSPD1 encodes the mitochondrial Heat Shock Protein 60 (mtHsp60) chaperonin, which carries out essential protein folding reactions that help maintain mitochondrial and cellular homeostasis. It functions as a macromolecular complex that provides client proteins an environment that favors proper folding in an ATP-dependent manner. It has been established that mtHsp60 plays a crucial role in the proper folding of mitochondrial proteins involved in ATP producing pathways. Recently, various single-point mutations in the mtHsp60 encoding gene have been directly linked to neuropathies and paraplegias. Individuals who harbor mtHsp60 mutations that negatively impact its folding ability display phenotypes with highly compromised muscle and neuron cells. Carriers of these mutations usually develop neuropathies and paraplegias at different stages of their lives mainly characterized by leg stiffness and weakness as well as degeneration of spinal cord nerves. These phenotypes are likely due to hindered energy producing pathways involved in cellular respiration resulting in ATP deprived cells. Although the complete protein folding mechanism of mtHsp60 is not well understood, recent work suggests that several of these mutations act by destabilizing the oligomeric stability of mtHsp60. Here, we discuss recent studies that highlight key aspects of the mtHsp60 mechanism with a focus on some of the known disease-causing point mutations, D29G and V98I, and their effect on the protein folding reaction cycle.

Hsp60 Post-translational Modifications: Functional and Pathological Consequences

Hsp60 is a chaperone belonging to the Chaperonins of Group I and typically functions inside mitochondria in which, together with the co-chaperonin Hsp10, maintains protein homeostasis. In addition to this canonical role, Hsp60 plays many others beyond the mitochondria, for instance in the cytosol, plasma-cell membrane, extracellular space, and body fluids. These non-canonical functions include participation in inflammation, autoimmunity, carcinogenesis, cell replication, and other cellular events in health and disease. Thus, Hsp60 is a multifaceted molecule with a wide range of cellular and tissue locations and functions, which is noteworthy because there is only one hsp60 gene. The question is by what mechanism this protein can become multifaceted. Likely, one factor contributing to this diversity is post-translational modification (PTM). The amino acid sequence of Hsp60 contains many potential phosphorylation sites, and other PTMs are possible such as O-GlcNAcylation, nitration, acetylation, S-nitrosylation, citrullination, oxidation, and ubiquitination. The effect of some of these PTMs on Hsp60 functions have been examined, for instance phosphorylation has been implicated in sperm capacitation, docking of H2B and microtubule-associated proteins, mitochondrial dysfunction, tumor invasiveness, and delay or facilitation of apoptosis. Nitration was found to affect the stability of the mitochondrial permeability transition pore, to inhibit folding ability, and to perturb insulin secretion. Hyperacetylation was associated with mitochondrial failure; S-nitrosylation has an impact on mitochondrial stability and endothelial integrity; citrullination can be pro-apoptotic; oxidation has a role in the response to cellular injury and in cell migration; and ubiquitination regulates interaction with the ubiquitin-proteasome system. Future research ought to determine which PTM causes which variations in the Hsp60 molecular properties and functions, and which of them are pathogenic, causing chaperonopathies. This is an important topic considering the number of acquired Hsp60 chaperonopathies already cataloged, many of which are serious diseases without efficacious treatment.

Formation of tRNA Wobble Inosine in Humans Is Disrupted by a Millennia-Old Mutation Causing Intellectual Disability

The formation of inosine at the wobble position of eukaryotic tRNAs is an essential modification catalyzed by the ADAT2/ADAT3 complex. In humans, a valine-to-methionine mutation (V144M) in ADAT3 that originated ∼1,600 years ago is the most common cause of autosomal recessive intellectual disability (ID) in Arabia. While the mutation is predicted to affect protein structure, the molecular and cellular effects of the V144M mutation are unknown.

The formation of inosine at the wobble position of eukaryotic tRNAs is an essential modification catalyzed by the ADAT2/ADAT3 complex. In humans, a valine-to-methionine mutation (V144M) in ADAT3 that originated ∼1,600 years ago is the most common cause of autosomal recessive intellectual disability (ID) in Arabia. While the mutation is predicted to affect protein structure, the molecular and cellular effects of the V144M mutation are unknown. Here, we show that cell lines derived from ID-affected individuals expressing only ADAT3-V144M exhibit decreased wobble inosine in certain tRNAs. Moreover, extracts from the same cell lines of ID-affected individuals display a severe reduction in tRNA deaminase activity. While ADAT3-V144M maintains interactions with ADAT2, the purified ADAT2/3-V144M complexes exhibit defects in activity. Notably, ADAT3-V144M exhibits an increased propensity to form aggregates associated with cytoplasmic chaperonins that can be suppressed by ADAT2 overexpression. These results identify a key role for ADAT2-dependent folding of ADAT3 in wobble inosine modification and indicate that proper formation of an active ADAT2/3 complex is crucial for proper neurodevelopment.

Inhibition of Aβ1-42 Fibrillation by Chaperonins: Human Hsp60 Is a Stronger Inhibitor than Its Bacterial Homologue GroEL

Alzheimer's disease is a chronic neurodegenerative disease characterized by the accumulation of pathological aggregates of amyloid beta peptide. Many efforts have been focused on understanding peptide aggregation pathways and on identification of molecules able to inhibit aggregation in order to find an effective therapy. As a result, interest in neuroprotective proteins, such as molecular chaperones, has increased as their normal function is to assist in protein folding or to facilitate the disaggregation and/or clearance of abnormal aggregate proteins. Using biophysical techniques, we evaluated the effects of two chaperones, human Hsp60 and bacterial GroEL, on the fibrillogenesis of Aβ_1-42. Both chaperonins interfere with Aβ_1-42 aggregation, but the effect of Hsp60 is more significant and correlates with its more pronounced flexibility and stronger interaction with ANS, an indicator of hydrophobic regions. Dose-dependent ThT fluorescence kinetics and SAXS experiments reveal that Hsp60 does not change the nature of the molecular processes stochastically leading to the formation of seeds, but strongly delays them by recognition of hydrophobic sites of some peptide species crucial for triggering amyloid formation. Hsp60 reduces the initial chaotic heterogeneity of Aβ_1-42 sample at high concentration regimes. The understanding of chaperone action in counteracting pathological aggregation could be a starting point for potential new therapeutic strategies against neurodegenerative diseases.

GroEL and the GroEL-GroES Complex

Chaperonin is categorized as a molecular chaperone and mediates the formation of the native conformation of proteins by first preventing folding during synthesis or membrane translocation and subsequently by mediating the step-wise ATP-dependent release that result in proper folding. In the GroEL-GroES complex, a single heptameric GroEL ring binds one GroES ring in the presence of ATP/ADP, in this vein, the double ring GroEL tetradecamer is present in two distinct types of GroEL-GroES complexes: asymmetric 1:1 "bullet"-shaped GroEL:GroES and symmetric 1:2 "football" (American football)-shaped GroEL:GroES₂. There have been debates as to which complex is critical to the productive protein folding mediated by the GroEL-GroES complex, and how GroES coordinates with GroEL in the chaperonin reaction cycle in association with regulation by adenine nucleotides and through the interplay of substrate proteins. A lot of knowledge on chaperonins has been accumulating as if expanding as ripples spread around the GroEL-GroES from Escherichia coli. In this article, an overview is presented on GroEL and the GroEL-GroES complex, with emphasis on their morphological variations, and some potential applications to the fabrication of nanocomposites using GroEL as a nano-block. In parallel, a guideline is presented that supports the recognition that the E. coli and its GroEL-GroES complex do not always receive in standard literature because the biochemical features of chaperonins derived from others special, such as mammals, are not always the same as those confirmed using GroEL-GroES derived from E. coli.

Strangers in strange lands: mitochondrial proteins found at extra-mitochondrial locations

The mitochondrial proteome is estimated to contain ∼1100 proteins, the vast majority of which are nuclear-encoded, with only 13 proteins encoded by the mitochondrial genome. The import of these nuclear-encoded proteins into mitochondria was widely believed to be unidirectional, but recent discoveries have revealed that many these ‘mitochondrial’ proteins are exported, and have extra-mitochondrial activities divergent from their mitochondrial function. Surprisingly, three of the exported proteins discovered thus far are mitochondrially encoded and have significantly different extra-mitochondrial roles than those performed within the mitochondrion. In this review, we will detail the wide variety of proteins once thought to only reside within mitochondria, but now known to ‘emigrate’ from mitochondria in order to attain ‘dual citizenship’, present both within mitochondria and elsewhere.

Hsp60 in Skeletal Muscle Fiber Biogenesis and Homeostasis: From Physical Exercise to Skeletal Muscle Pathology

Hsp60 is a molecular chaperone classically described as a mitochondrial protein with multiple roles in health and disease, participating to the maintenance of protein homeostasis. It is well known that skeletal muscle is a complex tissue, rich in proteins, that is, subjected to continuous rearrangements, and this homeostasis is affected by many different types of stimuli and stresses. The regular exercise induces specific histological and biochemical adaptations in skeletal muscle fibers, such as hypertrophy and an increase of mitochondria activity and oxidative capacity. The current literature is lacking in information regarding Hsp60 involvement in skeletal muscle fiber biogenesis and regeneration during exercise, and in disease conditions. Here, we briefly discuss the functions of Hsp60 in skeletal muscle fibers during exercise, inflammation, and ageing. Moreover, the potential usage of Hsp60 as a marker for disease and the evaluation of novel treatment options is also discussed. However, some questions remain open, and further studies are needed to better understand Hsp60 involvement in skeletal muscle homeostasis during exercise and in pathological condition.

Heat Shock Proteins as Immunomodulants

Heat shock proteins (Hsps) are conserved molecules whose main role is to facilitate folding of other proteins. Most Hsps are generally stress-inducible as they play a particularly important cytoprotective role in cells exposed to stressful conditions. Initially, Hsps were generally thought to occur intracellulary. However, recent work has shown that some Hsps are secreted to the cell exterior particularly in response to stress. For this reason, they are generally regarded as danger signaling biomarkers. In this way, they prompt the immune system to react to prevailing adverse cellular conditions. For example, their enhanced secretion by cancer cells facilitate targeting of these cells by natural killer cells. Notably, Hsps are implicated in both pro-inflammatory and anti-inflammatory responses. Their effects on immune cells depends on a number of aspects such as concentration of the respective Hsp species. In addition, various Hsp species exert unique effects on immune cells. Because of their conservation, Hsps are implicated in auto-immune diseases. Here we discuss the various metabolic pathways in which various Hsps manifest immune modulation. In addition, we discuss possible experimental variations that may account for contradictory reports on the immunomodulatory function of some Hsps.

Heat Shock Proteins in Alzheimer's Disease: Role and Targeting

Among diseases whose cure is still far from being discovered, Alzheimer’s disease (AD) has been recognized as a crucial medical and social problem. A major issue in AD research is represented by the complexity of involved biochemical pathways, including the nature of protein misfolding, which results in the production of toxic species. Considering the involvement of (mis)folding processes in AD aetiology, targeting molecular chaperones represents a promising therapeutic perspective. This review analyses the connection between AD and molecular chaperones, with particular attention toward the most important heat shock proteins (HSPs) as representative components of the human chaperome: Hsp60, Hsp70 and Hsp90. The role of these proteins in AD is highlighted from a biological point of view. Pharmacological targeting of such HSPs with inhibitors or regulators is also discussed.

Folding and assembly defects of pyruvate dehydrogenase deficiency-related variants in the E1α subunit of the pyruvate dehydrogenase complex

The pyruvate dehydrogenase complex (PDC) bridges glycolysis and the citric acid cycle. In human, PDC deficiency leads to severe neurodevelopmental delay and progressive neurodegeneration. The majority of cases are caused by variants in the gene encoding the PDC subunit E1α. The molecular effects of the variants, however, remain poorly understood. Using yeast as a eukaryotic model system, we have studied the substitutions A189V, M230V, and R322C in yeast E1α (corresponding to the pathogenic variants A169V, M210V, and R302C in human E1α) and evaluated how substitutions of single amino acid residues within different functional E1α regions affect PDC structure and activity. The E1α A189V substitution located in the heterodimer interface showed a more compact conformation with significant underrepresentation of E1 in PDC and impaired overall PDC activity. The E1α M230V substitution located in the tetramer and heterodimer interface showed a relatively more open conformation and was particularly affected by low thiamin pyrophosphate concentrations. The E1α R322C substitution located in the phosphorylation loop of E1α resulted in PDC lacking E3 subunits and abolished overall functional activity. Furthermore, we show for the E1α variant A189V that variant E1α accumulates in the Hsp60 chaperonin, but can be released upon ATP supplementation. Our studies suggest that pathogenic E1α variants may be associated with structural changes of PDC and impaired folding of E1α.

Single-Ring Intermediates Are Essential for Some Chaperonins

Chaperonins are macromolecular complexes found throughout all kingdoms of life that assist unfolded proteins reach a biologically active state. Historically, chaperonins have been classified into two groups based on sequence, subunit structure, and the requirement for a co-chaperonin. Here, we present a brief review of chaperonins that can form double- and single-ring conformational intermediates in their protein-folding catalytic pathway. To date, the bacteriophage encoded chaperonins ϕ-EL and OBP, human mitochondrial chaperonin and most recently, the bacterial groEL/ES systems, have been reported to form single-ring intermediates as part of their normal protein-folding activity. These double-ring chaperonins separate into single-ring intermediates that have the ability to independently fold a protein. We discuss the structural and functional features along with the biological relevance of single-ring intermediates in cellular protein folding. Of special interest are the ϕ-EL and OBP chaperonins which demonstrate features of both group I and II chaperonins in addition to their ability to function via single-ring intermediates.

Chaperonin of Group I: Oligomeric Spectrum and Biochemical and Biological Implications

Chaperonins play various physiological roles and can also be pathogenic. Elucidation of their structure, e.g., oligomeric status and post-translational modifications (PTM), is necessary to understand their functions and mechanisms of action in health and disease. Group I chaperonins form tetradecamers with two stacked heptameric rings. The tetradecamer is considered the typical functional complex for folding of client polypeptides. However, other forms such as the monomer and oligomers with smaller number of subunits than the classical tetradecamer, also occur in cells. The properties and functions of the monomer and oligomers, and their roles in chaperonin-associated diseases are still incompletely understood. Chaperonin I in eukaryotes occurs in various locations, not just the mitochondrion, which is its canonical place of residence and function. Eukaryotic Chaperonin I, namely Hsp60 (designated HSP60 or HSPD1 in humans) has, indeed, been found in the cytosol; the plasma-cell membrane; on the outer surface of cells; in the intercellular space; in biological liquids such as lymph, blood, and cerebrospinal fluid; and in secretions, for instance saliva and urine. Hsp60 has also been found in cell-derived vesicles such as exosomes. The functions of Hsp60 in all these non-canonical locales are still poorly characterized and one of the questions not yet answered is in what form, i.e., monomer or oligomer, is the chaperonin present in these non-canonical locations. In view of the steady increase in interest on chaperonopathies over the last several years, we have studied human HSP60 to determine its role in various diseases, its locations in cells and tissues and migrations in the body, and its post-translational modifications that might have an impact on its location and function. We also carried out experiments to characterize the oligomeric status of extramitochondrial of HSP60 in solution. Here, we provide an overview of our results, focusing on the oligomeric equilibrium and stability of the various forms of HSP60 in comparison with GroEL. We also discuss post-translational modifications associated with anti-cancer drugs to indicate the potential of Hsp60 in Medicine, as a biomarker and etiopathogenic factor.

Exosomal HSP60: a potentially useful biomarker for diagnosis, assessing prognosis, and monitoring response to treatment

INTRODUCTION

Cell-to-cell communication is imperative for life and it is mediated by sending and receiving information via the secretion and subsequent receptor-mediated detection of biological molecules. Exosomes (EXs) secreted from cells to the extracellular environment play an important role in intercellular communication in normal and pathological conditions. Areas covered: New evidence indicates that tumor cells-derived EXs contribute to cancer progression through the modulation of tumor microenvironment. The exosomal heat shock protein 60 (HSP60) is very likely a key player in intercellular cross-talk, particularly during the progress of diseases, such as cancer. Many studies have focused on the extracellular roles played by HSP60 that pertain to cancer development and immune system stimulation. Our experimental data in vitro and in vivo demonstrated that HSP60 occurs on the surface of EXs secreted by tumour cells. Expert commentary: Exosomal HSP60 has great potential for clinical applications, as a 'liquid biopsy', including its use as biomarker for diagnostics, assessing prognosis, and monitoring disease progression and response to treatment, particularly in cancer.

Elevated sodium leads to the increased expression of HSP60 and induces apoptosis in HUVECs

Atherosclerosis is the leading cause of death in the world. We have previously shown that expression of heat shock protein 60 (HSP60) on the surface of endothelial cells is the main cause of initiating the disease as it acts as a T cell auto-antigen and can be triggered by classical atherosclerosis risk factors, such as infection (e.g. Chlamydia pneumoniae), chemical stress (smoking, oxygen radicals, drugs), physical insult (heat, shear blood flow) and inflammation (inflammatory cytokines, lipopolysaccharide, oxidized low density lipoprotein, advanced glycation end products). In the present study, we show that increasing levels of sodium chloride can also induce an increase in intracellular and surface expression of HSP60 protein in human umbilical vein endothelial cells. In addition, we found that elevated sodium induces apoptosis.

The human mitochondrial Hsp60 in the APO conformation forms a stable tetradecameric complex

The human mitochondrial chaperonin is a macromolecular machine that catalyzes the proper folding of mitochondrial proteins and is of vital importance to all cells. This chaperonin is composed of 2 distinct proteins, Hsp60 and Hsp10, that assemble into large oligomeric complexes that mediate the folding of non-native polypeptides in an ATP dependent manner. Here, we report the bacterial expression and purification of fully assembled human Hsp60 and Hsp10 recombinant proteins and that Hsp60 forms a stable tetradecameric double-ring conformation in the absence of co-chaperonin and nucleotide. Evidence of the stable double-ring conformation is illustrated by the 15 Å resolution electron microscopy reconstruction presented here. Furthermore, our biochemical analyses reveal that the presence of a non-native substrate initiates ATP-hydrolysis within the Hsp60/10 chaperonin to commence protein folding. Collectively, these data provide insight into the architecture of the intermediates used by the human mitochondrial chaperonin along its protein folding pathway and lay a foundation for subsequent high resolution structural investigations into the conformational changes of the mitochondrial chaperonin.

Dynamic Complexes in the Chaperonin-Mediated Protein Folding Cycle

The GroEL–GroES chaperonin system is probably one of the most studied chaperone systems at the level of the molecular mechanism. Since the first reports of a bacterial gene involved in phage morphogenesis in 1972, these proteins have stimulated intensive research for over 40 years. During this time, detailed structural and functional studies have yielded constantly evolving concepts of the chaperonin mechanism of action. Despite of almost three decades of research on this oligomeric protein, certain aspects of its function remain controversial. In this review, we highlight one central aspect of its function, namely, the active intermediates of its reaction cycle, and present how research to this day continues to change our understanding of chaperonin-mediated protein folding.

Hsp60, amateur chaperone in amyloid-beta fibrillogenesis

BACKGROUND

Molecular chaperones are a very special class of proteins that play essential roles in many cellular processes like folding, targeting and transport of proteins. Moreover, recent evidence indicates that chaperones can act as potentially strong suppressor agents in Alzheimer's disease (AD). Indeed, in vitro experiments demonstrate that several chaperones are able to significantly slow down or suppress aggregation of Aβ peptide and in vivo studies reveal that treatment with specific chaperones or their overexpression can ameliorate some distinct pathological signs characterizing AD.

METHODS

Here we investigate using a biophysical approach (fluorescence, circular dichroism (CD), transmission electron (TEM) and atomic force (AFM) microscopy, size exclusion chromatography (SEC)) the effect of the human chaperonin Hsp60 on Aβ fibrillogenesis.

RESULTS

We found that Hsp60 powerfully inhibits Aβ amyloid aggregation, by closing molecular pathways leading to peptide fibrillogenesis.

CONCLUSIONS

We observe that Hsp60 inhibits Aβ aggregation through a more complex mechanism than a simple folding chaperone action. The action is specifically directed toward the early oligomeric species behaving as aggregation seeds for on-pathway amyloid fibrillogenesis.

GENERAL SIGNIFICANCE

Understanding the specificity of the molecular interactions of Hsp60 with amyloid Aβ peptide allowed us to emphasize the important aspects to be taken into consideration when considering the recent promising therapeutic strategies for neurodegeneration.

Alcoholic Liver Disease: A Mouse Model Reveals Protection by Lactobacillus fermentum

OBJECTIVES

Alcoholism is one of the most devastating diseases with high incidence, but knowledge of its pathology and treatment is still plagued with gaps mostly because of the inherent limitations of research with patients. We developed an animal model for studying liver histopathology, Hsp (heat-shock protein)-chaperones involvement, and response to treatment.

METHODS

The system was standardized using mice to which ethanol was orally administered alone or in combination with Lactobacillus fermentum following a precise schedule over time and applying, at predetermined intervals, a battery of techniques (histology, immunohistochemistry, western blotting, real-time PCR, immunoprecipitation, 3-nitrotyrosine labeling) to assess liver pathology (e.g., steatosis, fibrosis), and Hsp60 and iNOS (inducible form of nitric oxide synthase) gene expression and protein levels, and post-translational modifications.

RESULTS

Typical ethanol-induced liver pathology occurred and the effect of the probiotic could be reliably monitored. Steatosis score, iNOS levels, and nitrated proteins (e.g., Hsp60) decreased after probiotic intake.

CONCLUSIONS

We describe a mouse model useful for studying liver disease induced by chronic ethanol intake and for testing pertinent therapeutic agents, e.g., probiotics. We tested L. fermentum, which reduced considerably ethanol-induced tissue damage and deleterious post-translational modifications of the chaperone Hsp60. The model is available to test other agents and probiotics with therapeutic potential in alcoholic liver disease.

Functional structure and physiological functions of mammalian wild-type HSP60

The Chaperonins comprise a family of molecular chaperones having a double-ring structure and similar sequence homology. These proteins play an essential role in biological reactions that mediate the folding of newly synthesized polypeptides and partially denatured proteins. In the prokaryotic group I chaperonins, structural and reaction cycle analyses of GroEL and its co-chaperone GroES have been performed in detail. While in eukaryotes, there have been limited reports analyzing the group I chaperonin HSP60 and its co-chaperone HSP10. In the present study, we purified the wild type HSP60 from porcine liver and investigated the interaction between HSP60 and HSP10, including conformation and physiological relationships. Based on the results of transmission electron microscopy, native PAGE, and gel filtration column chromatography, the wild type HSP60 displayed a heptameric single-ring structure in the absence of ATP. In contrast, HSP60 formed mainly a "football-type" complex with HSP10 in the presence of ATP and mediated the refolding of denatured substrate protein. The functional conformation cycle of the purified mammalian HSP60 is distinct from the cycle of the prokaryotic GroEL/GroES chaperonin.

Characterization of Precursor PfHsp60 in Plasmodium falciparum Cytosol during Its Asexual Development in Human Erythrocytes

Mitochondrial heat shock protein 60 (Hsp60) is a nuclear encoded gene product that gets post-translationally translocated into the mitochondria. Using multiple approaches such as immunofluorescence experiments, isoelectric point analysis with two-dimensional gel electrophoresis, and mass spectrometric identification of the signal peptide, we show that Hsp60 from Plasmodium falciparum (PfHsp60) accumulates in the parasite cytoplasm during the ring, trophozoite, and schizont stages of parasite development before being imported into the parasite mitochondria. Using co-immunoprecipitation experiments with antibodies specific to cytoplasmic PfHsp90, PfHsp70-1, and PfHsp60, we show association of precursor PfHsp60 with cytoplasmic chaperone machinery. Metabolic labeling involving pulse and chase indicates translocation of the precursor pool into the parasite mitochondrion during chase. Analysis of results obtained with Geldanamycin treatment confirmed precursor PfHsp60 to be one of the clients for PfHsp90. Cytosolic chaperones bind precursor PfHsp60 prior to its import into the mitochondrion of the parasite. Our data suggests an inefficient co-ordination in the synthesis and translocation of mitochondrial PfHsp60 during asexual growth of malaria parasite in human erythrocytes.

Stability and disassembly properties of human naïve Hsp60 and bacterial GroEL chaperonins

Human Hsp60 chaperonin and its bacterial homolog GroEL, in association with the corresponding co-chaperonins Hsp10 and GroES, constitute important chaperone systems promoting the proper folding of several mitochondrial proteins. Hsp60 is also currently described as a ubiquitous molecule with multiple roles both in health conditions and in several diseases. Naïve Hsp60 bearing the mitochondrial import signal has been recently demonstrated to present different oligomeric organizations with respect to GroEL, suggesting new possible physiological functions. Here we present a combined investigation with circular dichroism and small-angle X-ray scattering of structure, self-organization, and stability of naïve Hsp60 in solution in comparison with bacterial GroEL. Experiments have been performed in different concentrations of guanidine hydrochloride, monitoring the dissociation of tetradecamers into heptamers and monomers, until unfolding. GroEL is proved to be more stable with respect to Hsp60, and the unfolding free energy as well as its dependence on denaturant concentration is obtained.

Hsp60 response in experimental and human temporal lobe epilepsy

The mitochondrial chaperonin Hsp60 is a ubiquitous molecule with multiple roles, constitutively expressed and inducible by oxidative stress. In the brain, Hsp60 is widely distributed and has been implicated in neurological disorders, including epilepsy. A role for mitochondria and oxidative stress has been proposed in epileptogenesis of temporal lobe epilepsy (TLE). Here, we investigated the involvement of Hsp60 in TLE using animal and human samples. Hsp60 immunoreactivity in the hippocampus, measured by Western blotting and immunohistochemistry, was increased in a rat model of TLE. Hsp60 was also increased in the hippocampal dentate gyrus neurons somata and neuropil and hippocampus proper (CA3, CA1) of the epileptic rats. We also determined the circulating levels of Hsp60 in epileptic animals and TLE patients using ELISA. The epileptic rats showed circulating levels of Hsp60 higher than controls. Likewise, plasma post-seizure Hsp60 levels in patients were higher than before the seizure and those of controls. These results demonstrate that Hsp60 is increased in both animals and patients with TLE in affected tissues, and in plasma in response to epileptic seizures, and point to it as biomarker of hippocampal stress potentially useful for diagnosis and patient management.

Quaternary structures of GroEL and naïve-Hsp60 chaperonins in solution: a combined SAXS-MD study

Analysis with QUAFIT software combined with SAXS-MD data, allows resolution of GroEL and naïve-Hsp60 oligomeric structures in solution.