Is heat shock protein re-induction during tolerance related to the stressor-specific induction of heat shock proteins?

The existence of stressor-specific induction programs of heat shock proteins (hsps) leads us to analyze the possible occurrence of a stressor-specific tolerance induced by either heat shock, arsenite, or cadmium. As a measure of this tolerance re-induction of hsps was studied. In this paper, we tested whether the refractory state is either valid for each specific hsp (implying independent regulation of every member of the heat shock protein family) or extends from small subsets of the hsp-family to even larger groups of proteins (indicating a more common denominator in their regulation). (Re-)induction of hsps does not seem to be regulated at the level of each individual hsp since differences in induced synthesis of hsps between two stressor conditions are not supplemented systematically upon the sequential application of the two stressors. The most notable example in this respect is hsp60. A pretreatment with cadmium, which hardly induces synthesis of this hsp, does induce a tolerance to (re)-induction by heat shock, which normally induces hsp60. This suggests the existence of a more common denominator regulating the coordinate expression of at least some hsps. From our data we conclude that the degree, but not the pattern, of hsp re-induction is influenced by the type of stressor used in the pretreatment. The pattern of hsps induced by a secondary applied stressor still shows most of its stressor-specificity and seems to be independent of any pretreatment. The possible implications of stressor-specificity are discussed. © 1996 Wiley-Liss, Inc.     

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

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

Small heat shock proteins and the cytoskeleton: An essential interplay for cell integrity?

The cytoskeleton is a highly complex network of three major intracellular filaments, microfilaments (MFs), microtubules (MTs) and intermediate filaments (IFs). This network plays a key role in the control of cell shape, division, functions and interactions in animal organs and tissues. Dysregulation of the network can contribute to numerous human diseases. Although small HSPs (sHSPs) and in particular HSP27 (HSPB1) or αB-crystallin (HSPB5) display a wide range of cellular properties, they are mostly known for their ability to protect cells under stress conditions. Mutations in some sHSPs have been found to affect their ability to interact with cytoskeleton proteins, leading to IF aggregation phenotypes that mimick diseases related to disorders in IF proteins (i.e. desmin, vimentin and neuro-filaments). The aim of this review is to discuss new findings that point towards the possible involvement of IFs in the cytoprotective functions of sHSPs, both in physiological and pathological settings, including the likelihood that sHSPs such as HSPB1 may play a role during epithelial-to-mesenchymal transition (EMT) during fibrosis or cancer progression. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.     

Walter’s two-layer hypothesis revisited: back to the roots!

Walter (Jahrb Wiss Bot 87:750–860, 1939) proposed a two-layer hypothesis, an equilibrium explanation for coexistence of savanna trees and grasses. This hypothesis relies on vertical niche partitioning and assumed that grasses are more water-use efficient than trees and use subsurface water while trees also have access to deeper water sources. Thus, in open savannas, grasses were predicted to predominate because of their water use efficiency and access to subsurface water. This hypothesis has been a prominent part of the savanna literature since first proposed. We review the literature on Walter’s hypothesis and reconsider his original intentions. Walter intended this hypothesis to be restricted to dry savannas. In his opinion, mesic and humid savannas were controlled by biotic factors and disturbances. We surveyed the global savanna literature for records of vertical niche partitioning by grasses and trees. We find that, within the scope of Walter’s original intentions, this hypothesis works remarkably well, and in some cases is appropriate for deserts as well as for dry temperate systems and even some mesic savannas.     

Actin cytoskeleton and small heat shock proteins: how do they interact?

Actin and small heat shock proteins (sHsps) are ubiquitous and multifaceted proteins that exist in 2 reversible forms, monomers and multimers, ie, the microfilament of the cytoskeleton and oligomers of the sHsps, generally, supposed to be in a spherical and hollow form. Two situations are described in the literature, where the properties of actin are modulated by sHsps; the actin polymerization is inhibited in vitro by some sHsps acting as capping proteins, and the actin cytoskeleton is protected by some sHsps against the disruption induced by various stressful conditions. We propose that a direct actin-sHsp interaction occurs to inhibit actin polymerization and to participate in the in vivo regulation of actin filament dynamics. Protection of the actin cytoskeleton would result from an F-actin-sHsp interaction in which microfilaments would be coated by small oligomers of phosphorylated sHsps. Both proteins share common structural motives suggesting direct binding sites, but they remain to be demonstrated. Some sHsps would behave with the actin cytoskeleton as actin-binding proteins capable of either capping a microfilament when present as a nonphosphorylated monomer or stabilizing and protecting the microfilament when organized in small, phosphorylated oligomers.     

Autophagic dysfunction in Alzheimer’s disease: Cellular and molecular mechanistic approaches to halt Alzheimer’s pathogenesis

Autophagy is a preserved cytoplasmic self-degradation process and endorses recycling of intracellular constituents into bioenergetics for the controlling of cellular homeostasis. Functional autophagy process is essential in eliminating cytoplasmic waste components and helps in the recycling of some of its constituents. Studies have revealed that neurodegenerative disorders may be caused by mutations in autophagy-related genes and alterations of autophagic flux. Alzheimer’s disease (AD) is an irrevocable deleterious neurodegenerative disorder characterized by the formation of senile plaques and neurofibrillary tangles (NFTs) in the hippocampus and cortex. In the central nervous system of healthy people, there is no accretion of amyloid β (Aβ) peptides due to the balance between generation and degradation of Aβ. However, for AD patients, the generation of Aβ peptides is higher than lysis that causes accretion of Aβ. Likewise, the maturation of autophagolysosomes and inhibition of their retrograde transport creates favorable conditions for Aβ accumulation. Furthermore, increasing mammalian target of rapamycin (mTOR) signaling raises tau levels as well as phosphorylation. Alteration of mTOR activity occurs in the early stage of AD. In addition, copious evidence links autophagic/lysosomal dysfunction in AD. Compromised mitophagy is also accountable for dysfunctional mitochondria that raises Alzheimer’s pathology. Therefore, autophagic dysfunction might lead to the deposit of atypical proteins in the AD brain and manipulation of autophagy could be considered as an emerging therapeutic target. This review highlights the critical linkage of autophagy in the pathogenesis of AD, and avows a new insight to search for therapeutic target for blocking Alzheimer’s pathogenesis.     

Effect of thymopentin on the production of cytokines, heat shock proteins, and NF-κB signaling proteins

In vivo effects of thymopentin, an active fragment of the naturally occurring thymic hormone thymopoietin, on the production of cytokines, nitric oxide, heat shock proteins, and signaling proteins NF-κB, phNF-κB, and IκB-α in lymphoid cells of male NMRI mice was studied. Activation of production of several cytokines (IL-1α, IL-2, IL-6, IL-10, and IFN-γ), nitric oxide, and heat shock proteins (HSP70 and HSP90) was observed in peritoneal macrophages and spleen lymphocytes of mice that received intraperitoneal injections of thymopentin (15μg per 100 g body weight). Thymopentin apparently produces stress-like rather than damaging effects. A probable action mechanism of this hormone is activation of the NF-κB signaling pathway, which is most pronounced at the NF-κB phosphorylation stage.     

Effect of single and repeated heat stress on chemical signals of heat shock response cascade in the rat’s heart

Exposure to sublethal heat stress activates a complex cascade of signaling events, such as activators (NO), signal molecules (PKCε), and mediators (HSP70 and COX-2), leading to implementation of heat preconditioning, an adaptive mechanism which makes the organism more tolerant to additional stress. We investigated the time frame in which these chemical signals are triggered after heat stress (41?±?0.5°С/45 min), single or repeated (24 or 72 h after the first one) in heart tissue of male Wistar rats. The animals were allowed to recover 24, 48 or 72 h at room temperature. Single heat stress caused a significant increase of the concentration of HSP70, NO, and PKC level and decrease of COX-2 level 24 h after the heat stress, which in the next course of recovery gradually normalized. The second heat stress, 24 h after the first one, caused a significant reduction of the HSP70 levels, concentration of NO and PKC?, and significant increase of COX-2 concentration. The second exposure, 72 h after the first heat stress, caused more expressive changes of HSP70 and NO in the 24 h-recovery groups. The level of PKC? was not significantly changed, but there was significantly increased COX-2 concentration during recovery. Serum activity of AST, ALT, and CK was reduced after single exposure and increased after repeated exposure to heat stress, in both time intervals. In conclusion, a longer period of recovery (72 h) between two consecutive sessions of heat stress is necessary to achieve more expressive changes in mediators (HSP70) and triggers (NO) of heat preconditioning.     

Antibodies against heat shock proteins in environmental stresses and diseases: friend or foe?

Heat shock proteins (Hsps) can be found in two forms, intracellular and extracellular. The intracellular Hsps are induced as a result of stress and have been found to be cytoprotective in many instances due to their chaperone functions in protein folding and in protein degradation. The origin and role of extracellular Hsps is less clear. Although they were suspected originally to be released from damaged cells (necrosis), their presence in most normal individuals rather suggests that they have regulatory functions in circulation. As immunodominant molecules, Hsps can stimulate the immune system, leading to the production of autoantibodies recognizing epitopes shared by microbial and human Hsps. Thus, extracellular Hsps can influence the inflammatory response as evidenced by the production of inflammatory cytokines. Antibodies to Hsps have been found under normal conditions but seem to be increased in certain stresses and diseases. Such antibodies could regulate the inflammatory response positively or negatively. Here, we review the literature on the findings of antibodies to Hsps in situations of environmental or occupational stress and in a number of diseases and discuss their possible significance for the diagnosis, prognosis, or pathogenesis of these diseases.     

Tackling neurodegenerative diseases: animal models of Alzheimer’s disease and Parkinson’s disease

Our ageing society is confronted with a dramatic increase in incidence of age-related neurodegenerative diseases; biomedical research leading to novel therapeutic strategies is crucial to address this problem. Animal models of neurodegenerative conditions are invaluable in improving our understanding of the molecular basis of pathology, potentially revealing novel targets for intervention. Here, we review transgenic animal models of Alzheimer’s and Parkinson’s disease reported in mice, zebrafish, Caenorhabditis elegans and Drosophila melanogaster. This information will enable researchers to compare different animal models targeting disease-associated molecules by genomic engineering and to facilitate the development of novel animal models for any particular study, depending on the ultimate research goals.     

Strasburger’s legacy to mitosis and cytokinesis and its relevance for the Cell Theory

Eduard Strasburger was one of the most prominent biologists contributing to the development of the Cell Theory during the nineteenth century. His major contribution related to the characterization of mitosis and cytokinesis and especially to the discovery of the discrete stages of mitosis, which he termed prophase, metaphase and anaphase. Besides his observations on uninucleate plant and animal cells, he also investigated division processes in multinucleate cells. Here, he emphasised the independent nature of mitosis and cytokinesis. We discuss these issues from the perspective of new discoveries in the field of cell division and conclude that Strasburger's legacy will in the future lead to a reformulation of the Cell Theory and that this will accommodate the independent and primary nature of the nucleus, together with its complement of perinuclear microtubules, for the organisation of the eukaryotic cell.     

Heat shock proteins hsp32 and hsp70 as biomarkers of an early response?: In vitro induction of heat shock proteins after exposure of cell culture to carcinogenic compounds and their real mixtures

Heat shock proteins (hsp) are a highly conserved group of proteins that are synthesized as a response to different forms of stress (heat, toxic chemicals, diseases, non-physiological pH changes). Because of their high sensitivity to changes in the environment, these proteins were suggested as possible early biomarkers of exposure in ecotoxicological studies. The purpose of the present study was to check the suitability of hsp32 and hsp70 as biomarkers of in vitro exposure to environmentally relevant carcinogens: polycyclic aromatic hydrocarbons (PAHs), their nitro-derivates, aromatic amines, acrylonitrile (ACN) and the mixture of organic compounds adsorbed onto ambient airborne particles (extractable organic matter, EOM).The expression of hsp32 and hsp70 was studied in human diploid lung fibroblasts (HEL cells) and human monocytic leukaemia cells (THP-1 cells) incubated in vitro with different concentrations of dibenzo[a,l]pyrene (DB[a,l]P), 1-nitropyrene, (NP), 4-aminobiphenyl (ABP), ACN and EOM for different periods of time. The incubation of cells with DB[a,l]P, NP, ABP and EOM did not result in increased levels of hsp32 or hsp70, either in dose- or time-dependent manner. ACN induced the expression of hsp32 as well as hsp70 in HEL and THP-1 cells, which probably reflects its ability to induce oxidative stress. We conclude that hsp32 and hsp70 are not suitable biomarkers of an early exposure to PAHs, their nitro-derivates, aromatic amines or EOM under the conditions used.     

Potential role of MG53 in the regulation of transforming-growth-factor-β1-induced atrial fibrosis and vulnerability to atrial fibrillation

Atrial fibrosis plays a critical role in atrial fibrillation (AF) by the transforming growth factor (TGF)-β1/Smad pathway. The disordered differentiation, proliferation, migration and collagen deposition of atrial fibroblasts play significant roles in atrial fibrosis. Mitsugumin (MG)53 is predominantly expressed in myocardium of rodents and has multiple biological functions. However, the role of MG53 in cardiac fibrosis remains unclear. This study provided clinical and experimental evidence for the involvement of MG53 in atrial fibrosis in humans and atrial fibrosis phenotype in cultured rat atrial fibroblasts. In atrial tissue from patients we demonstrated that MG53 was expressed in human atrium. Expression of MG53 increased with the extent of atrial fibrosis, which could induce AF. In cultured atrial fibroblasts, depletion of MG53 by siRNA caused down-regulation of the TGF-β1/Smad pathway, while overexpression of MG53 by adenovirus up-regulated the pathway. MG53 regulated the proliferation and migration of atrial fibroblasts. Besides, exogenous TGF-β1 suppressed expression of MG53. In conclusion, we demonstrated that MG53 was expressed in human atrium, and may be a potential upstream of the TGF-β1/Smad pathway in human atrium and rat atrial fibroblasts. This suggests that MG53 is a potential regulator of atrial fibrosis induced by the TGF-β1/Smad pathway in patients with AF.     

Plasmodium falciparum Hsp70-x: a heat shock protein at the host–parasite interface

Plasmodium falciparum 70?kDa heat shock proteins (PfHsp70s) are expressed at all stages of the pathogenic erythrocytic phase of the malaria parasite life cycle. There are six PfHsp70s, all of which have orthologues in other plasmodial species, except for PfHsp70-x which is unique to P. falciparum. This research highlights a number of original results obtained by a detailed bioinformatics analysis of the protein. Large-scale sequence analysis indicated the presence of an extended transit peptide sequence of PfHsp70-x which potentially directs it to the endoplasmic reticulum (ER). Further analysis showed that PfHsp70-x does not have an ER-retention sequence, suggesting that the protein transits through the ER and is secreted into the parasitophorous vacuole or beyond into the erythrocyte cytosol. These results are consistent with experimental findings. Next, possible interactions between PfHsp70-x and exported P. falciparum Hsp40s or host erythrocyte Hsp40 were interrogated by modelling and docking. Docking results indicated that interaction between PfHsp70-x and each of the Hsp40s, regardless of biological feasibility, seems equally likely. This suggests that J domain might not provide the specificity in the formation of unique Hsp70-Hsp40 complexes, but that the specificity might be provided by other domains of Hsp40s. By studying different structural conformations of PfHsp70-x, it was shown that Hsp40s can only bind when PfHsp70-x is in a certain conformation. Additionally, this work highlighted the possible dependence of the substrate-binding domain residues on the orientation of the α-helical lid for formation of the substrate-binding pocket.     

Calmodulin’s flexibility allows for promiscuity in its interactions with target proteins and peptides

The small bilobal calcium regulatory protein calmodulin (CaM) activates numerous target enzymes in response to transient changes in intracellular calcium concentrations. Binding of calcium to the two helix-loop-helix calcium-binding motifs in each of the globular domains induces conformational changes that expose a methionine-rich hydrophobic patch on the surface of each domain of the protein, which it uses to bind to peptide sequences in its target enzymes. Although these CaM-binding domains typically have little sequence identity, the positions of several bulky hydrophobic residues are often conserved, allowing for classification of CaM-binding domains into recognition motifs, such as the 1–14 and 1–10 motifs. For calcium-independent binding of CaM, a third motif known as the IQ motif is also common. Many CaM-peptide complexes have globular conformations, where CaM’s central linker connecting the two domains unwinds, allowing the protein to wrap around a single predominantly α-helical target peptide sequence. However, novel structures have recently been reported where the conformation of CaM is highly dissimilar to these globular complexes, in some instances with less than a full compliment of bound calcium ions, as well as novel stoichiometries. Furthermore, many divergent CaM isoforms from yeast and plant species have been discovered with unique calcium-binding and enzymatic activation characteristics compared to the single CaM isoform found in mammals.