Heat-shock proteins Hsp104 and Hsp70 reactivate mRNA splicing after heat inactivation

Discovery of N-Benzyl-4-(1-bromonaphthalen-2-yl)oxybutan-1-amine as a Potential Antifungal Agent against Sporidia Growth and Teliospore Germination of Sporisorium scitamineum

The cultivation of sugar cane using perennial roots is the primary planting method, which is one of the reasons for the serious occurrence of sugar cane smut disease caused by the basidiomycetous fungus Sporisorium scitamineum in the sugar cane perennial root planting area. Consequently, it is crucial to eliminate pathogens from perennial sugar cane buds. In this study, we found that MAP kinase Hog1 is necessary for heat stress resistance. Subsequent investigations revealed a significant reduction in the expression of the heat shock protein 104-encoding gene, SsHSP104, in the ss1hog1Δ mutant. Additionally, the overexpression of SsHSP104 partially restored colony growth in the ss1hog1Δ strain following heat stress treatment, demonstrating the crucial role of SsHsp104 in SsHog1-mediated heat stress tolerance. Hence, we constructed the ss1hsp104:eGFP fusion strain in the wild type of S. scitamineum to identify small-molecule compounds that could inhibit the heat stress response, leading to the discovery of N-benzyl-4-(1-bromonaphthalen-2-yl)oxybutan-1-amine as a potential compound that targets the SsHog1 mediation SsHsp104 pathway during heat treatment. Furthermore, the combination of N-benzyl-4-(1-bromonaphthalen-2-yl)oxybutan-1-amine and warm water treatment (45 °C for 15 min) inhibits the growth of S. scitamineum and teliospore germination, thereby reducing the occurrence of sugar cane smut diseases and indicating its potential for eliminating pathogens from perennial sugar cane buds. In conclusion, these findings suggest that N-benzyl-4-(1-bromonaphthalen-2-yl)oxybutan-1-amine is promising as a targeted compound for the SsHog1-mediated SsHsp104 pathway and may enable the reduction of hot water treatment duration and/or temperature, thereby limiting the occurrence of sugar cane smut diseases caused by S. scitamineum.

HSP70 binds to specific non-coding RNA and regulates human RNA polymerase III

Molecular chaperones are critical for protein homeostasis and are implicated in several human pathologies such as neurodegeneration and cancer. While the binding of chaperones to nascent and misfolded proteins has been studied in great detail, the direct interaction between chaperones and RNA has not been systematically investigated. Here, we provide the evidence for widespread interaction between chaperones and RNA in human cells. We show that the major chaperone heat shock protein 70 (HSP70) binds to non-coding RNA transcribed by RNA polymerase III (RNA Pol III) such as tRNA and 5S rRNA. Global chromatin profiling revealed that HSP70 binds genomic sites of transcription by RNA Pol III. Detailed biochemical analyses showed that HSP70 alleviates the inhibitory effect of cognate tRNA transcript on tRNA gene transcription. Thus, our study uncovers an unexpected role of HSP70-RNA interaction in the biogenesis of a specific class of non-coding RNA with wider implications in cancer therapeutics.

Altered splicing factor and alternative splicing events in a mouse model of diet- and polychlorinated biphenyl-induced liver disease

Non-alcoholic fatty liver disease (NAFLD) is associated with human environmental exposure to polychlorinated biphenyls (PCBs). Alternative splicing (AS) is dysregulated in steatotic liver disease and is regulated by splicing factors (SFs) and N-6 methyladenosine (m6A) modification. Here integrated analysis of hepatic mRNA-sequencing data was used to identify differentially expressed SFs and differential AS events (ASEs) in the livers of high fat diet-fed C57BL/6 J male mice exposed to Aroclor1260, PCB126, Aroclor1260 + PCB126, or vehicle control. Aroclor1260 + PCB126 co-exposure altered 100 SFs and replicate multivariate analysis of transcript splicing (rMATS) identified 449 ASEs in 366 genes associated with NAFLD pathways. These ASEs were similar to those resulting from experimental perturbations in m6A writers, readers, and erasers. These results demonstrate specific hepatic SF and AS regulatory mechanisms are disrupted by HFD and PCB exposures, contributing to the expression of altered isoforms that may play a role in NAFLD progression to NASH.

Genetic inactivation of essential HSF1 reveals an isolated transcriptional stress response selectively induced by protein misfolding

Heat Shock Factor 1 (Hsf1) in yeast drives the basal transcription of key proteostasis factors and its activity is induced as part of the core heat shock response. Exploring Hsf1 specific functions has been challenging due to the essential nature of the HSF1 gene and the extensive overlap of target promoters with environmental stress response (ESR) transcription factors Msn2 and Msn4 (Msn2/4). In this study, we constructed a viable hsf1∆ strain by replacing the HSF1 open reading frame with genes that constitutively express Hsp40, Hsp70, and Hsp90 from Hsf1-independent promoters. Phenotypic analysis showed that the hsf1∆ strain grows slowly, is sensitive to heat as well as protein misfolding and accumulates protein aggregates. Transcriptome analysis revealed that the transcriptional response to protein misfolding induced by azetidine-2-carboxylic acid is fully dependent on Hsf1. In contrast, the hsf1∆ strain responded to heat shock through the ESR. Following HS, Hsf1 and Msn2/4 showed functional compensatory induction with stronger activation of the remaining stress pathway when the other branch was inactivated. Thus, we provide a long-overdue genetic test of the function of Hsf1 in yeast using the novel hsf1∆ construct. Our data highlight that the accumulation of misfolded proteins is uniquely sensed by Hsf1-Hsp70 chaperone titration inducing a highly selective transcriptional stress response.

Chaperones directly and efficiently disperse stress-triggered biomolecular condensates

Stresses such as heat shock trigger the formation of protein aggregates and the induction of a disaggregation system composed of molecular chaperones. Recent work reveals that several cases of apparent heat-induced aggregation, long thought to be the result of toxic misfolding, instead reflect evolved, adaptive biomolecular condensation, with chaperone activity contributing to condensate regulation. Here we show that the yeast disaggregation system directly disperses heat-induced biomolecular condensates of endogenous poly(A)-binding protein (Pab1) orders of magnitude more rapidly than aggregates of the most commonly used misfolded model substrate, firefly luciferase. Beyond its efficiency, heat-induced condensate dispersal differs from heat-induced aggregate dispersal in its molecular requirements and mechanistic behavior. Our work establishes a bona fide endogenous heat-induced substrate for long-studied heat shock proteins, isolates a specific example of chaperone regulation of condensates, and underscores needed expansion of the proteotoxic interpretation of the heat shock response to encompass adaptive, chaperone-mediated regulation.

Daily rhythms in gene expression of the human parasite Schistosoma mansoni

Background

The consequences of the earth’s daily rotation have led to 24-h biological rhythms in most organisms. Even some parasites are known to have daily rhythms, which, when in synchrony with host rhythms, can optimise their fitness. Understanding these rhythms may enable the development of control strategies that take advantage of rhythmic vulnerabilities. Recent work on protozoan parasites has revealed 24-h rhythms in gene expression, drug sensitivity and the presence of an intrinsic circadian clock; however, similar studies on metazoan parasites are lacking. To address this, we investigated if a metazoan parasite has daily molecular oscillations, whether they reveal how these longer-lived organisms can survive host daily cycles over a lifespan of many years and if animal circadian clock genes are present and rhythmic. We addressed these questions using the human blood fluke Schistosoma mansoni that lives in the vasculature for decades and causes the tropical disease schistosomiasis.

Results

Using round-the-clock transcriptomics of male and female adult worms collected from experimentally infected mice, we discovered that ~ 2% of its genes followed a daily pattern of expression. Rhythmic processes included a stress response during the host’s active phase and a ‘peak in metabolic activity’ during the host’s resting phase. Transcriptional profiles in the female reproductive system were mirrored by daily patterns in egg laying (eggs are the main drivers of the host pathology). Genes cycling with the highest amplitudes include predicted drug targets and a vaccine candidate. These 24-h rhythms may be driven by host rhythms and/or generated by a circadian clock; however, orthologs of core clock genes are missing and secondary clock genes show no 24-h rhythmicity.

Conclusions

There are daily rhythms in the transcriptomes of adult S. mansoni, but they appear less pronounced than in other organisms. The rhythms reveal temporally compartmentalised internal processes and host interactions relevant to within-host survival and between-host transmission. Our findings suggest that if these daily rhythms are generated by an intrinsic circadian clock then the oscillatory mechanism must be distinct from that in other animals. We have shown which transcripts oscillate at this temporal scale and this will benefit the development and delivery of treatments against schistosomiasis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01189-9.

Deletion of the Stress Response Gene DDR48 from Histoplasma capsulatum Increases Sensitivity to Oxidative Stress, Increases Susceptibility to Antifungals, and Decreases Fitness in Macrophages

The stress response gene DDR48 has been characterized in Saccharomyces cerevisiae and Candida albicans to be involved in combating various cellular stressors, from oxidative agents to antifungal compounds. Surprisingly, the biological function of DDR48 has yet to be identified, though it is likely an important part of the stress response. To gain insight into its function, we characterized DDR48 in the dimorphic fungal pathogen Histoplasma capsulatum. Transcriptional analyses showed preferential expression of DDR48 in the mycelial phase. Induction of DDR48 in Histoplasma yeasts developed after treatment with various cellular stress compounds. We generated a ddr48∆ deletion mutant to further characterize DDR48 function. Loss of DDR48 alters the transcriptional profile of the oxidative stress response and membrane synthesis pathways. Treatment with ROS or antifungal compounds reduced survival of ddr48∆ yeasts compared to controls, consistent with an aberrant cellular stress response. In addition, we infected RAW 264.7 macrophages with DDR48-expressing and ddr48∆ yeasts and observed a 50% decrease in recovery of ddr48∆ yeasts compared to wild-type yeasts. Loss of DDR48 function results in numerous negative effects in Histoplasma yeasts, highlighting its role as a key player in the global sensing and response to cellular stress by fungi.

Heat Stress Tolerance Gene FpHsp104 Affects Conidiation and Pathogenicity of Fusarium pseudograminearum

Heat shock protein Hsp104, a homolog of the bacterial chaperone ClpB and plant Hsp100, plays an essential part in the response to heat and various chemical agents in Saccharomyces cerevisiae. However, their functions remain largely unknown in plant fungal pathogens. Here, we report the identification and functional characterization of a plausible ortholog of yeast Hsp104 in Fusarium pseudograminearum, which we termed FpHsp104. Deletion mutant of FpHsp104 displayed severe defects in the resistance of heat shock during F. pseudograminearum mycelia and conidia when exposed to extreme heat. We also found that the protein showed dynamic localization to small particles under high temperature. However, no significant differences were detected in osmotic, oxidative, or cell wall stress responses between the wild-type and Δfphsp104 strains. Quantitative real-time PCR analysis showed that FpHsp104 was upregulated in the conidia, and disruption of FpHsp104 gene resulted in defects in conidia production, morphology, and germination. The transcript levels of conidiation-related genes of FpFluG, FpVosA, FpWetA, and FpAbaA were reduced in the Δfphsp104 mutant vs. the wild-type strain, but heat-shocked mRNA splicing repair was not affected in Δfphsp104. Moreover, Δfphsp104 mutant also showed attenuated virulence, but its DON synthesis was normal. These data from the first study of Hsp104 in F. pseudograminearum strongly suggest that FpHsp104 gene is an important element in the heat tolerance, development, and pathogenicity processes of F. pseudograminearum.

Heat Shock Proteins as the Druggable Targets in Leishmaniasis: Promises and Perils

Leishmania, the causative agent of leishmaniasis, is an intracellular pathogen that thrives in the insect gut and mammalian macrophages to complete its life cycle. Apart from temperature difference (26 to 37°C), it encounters several harsh conditions, including oxidative stress, inflammatory reactions, and low pH. Heat shock proteins (HSPs) play essential roles in cell survival by strategically reprogramming cellular processes and signaling pathways. HSPs assist cells in multiple functions, including differentiation, adaptation, virulence, and persistence in the host cell. Due to cyclical epidemiological patterns, limited chemotherapeutic options, drug resistance, and the absence of a vaccine, control of leishmaniasis remains a far-fetched dream. The essential roles of HSPs in parasitic differentiation and virulence and increased expression in drug-resistant strains highlight their importance in combating the disease. In this review, we highlighted the diverse physiological importance of HSPs present in Leishmania, emphasizing their significance in disease pathogenesis. Subsequently, we assessed the potential of HSPs as a chemotherapeutic target and underlined the challenges associated with it. Furthermore, we have summarized a few ongoing drug discovery initiatives that need to be explored further to develop clinically successful chemotherapeutic agents in the future.

Multi-Omics Analysis Provides Novel Insight into Immuno-Physiological Pathways and Development of Thermal Resistance in Rainbow Trout Exposed to Acute Thermal Stress

In recent years, poikilothermic animals such as fish have increasingly been exposed to stressful high-temperature environments due to global warming. However, systemic changes in fish under thermal stress are not fully understood yet at both the transcriptome and proteome level. Therefore, the objective of this study was to investigate the immuno-physiological responses of fish under extreme thermal stress through integrated multi-omics analysis. Trout were exposed to acute thermal stress by raising water temperature from 15 to 25 °C within 30 min. Head-kidney and plasma samples were collected and used for RNA sequencing and two-dimensional gel electrophoresis. Gene enrichment analysis was performed: differentially expressed genes (DEGs) and differentially expressed proteins (DEPs) were identified to interpret the multi-omics results and identify the relevant biological processes through pathway analysis. Thousands of DEGs and 49 DEPs were identified in fish exposed to thermal stress. Most of these genes and proteins were highly linked to DNA replication, protein processing in the endoplasmic reticulum, cell signaling and structure, glycolysis activation, complement-associated hemolysis, processing of released free hemoglobin, and thrombosis and hypertension/vasoconstriction. Notably, we found that immune disorders mediated by the complement system may trigger hemolysis in thermally stressed fish, which could have serious consequences such as ferroptosis and thrombosis. However, antagonistic activities that decrease cell-free hemoglobin, heme, and iron might be involved in alleviating the side effects of thermally induced immuno-physiological disorders. These factors may represent the major thermal resistance traits that allow fish to overcome extreme thermal stress. Our findings, based on integration of multi-omics data from transcriptomics and proteomics analyses, provide novel insight into the pathogenesis of acute thermal stress and temperature-linked epizootics.

Modulation the alternative splicing of GLA (IVS4+919G>A) in Fabry disease

While a base substitution in intron 4 of GLA (IVS4+919G>A) that causes aberrant alternative splicing resulting in Fabry disease has been reported, its molecular mechanism remains unclear. Here we reported that upon IVS4+919G>A transversion, H3K36me3 was enriched across the alternatively spliced region. PSIP1, an adapter of H3K36me3, together with Hsp70 and NONO were recruited and formed a complex with SF2/ASF and SRp20, which further promoted GLA splicing. Amiloride, a splicing regulator in cancer cells, could reverse aberrant histone modification patterns and disrupt the association of splicing complex with GLA. It could also reverse aberrant GLA splicing in a PP1-dependant manner. Our findings revealed the alternative splicing mechanism of GLA (IVS4+919G>A), and a potential treatment for this specific genetic type of Fabry disease by amiloride in the future.

Heat shock increases lifetime of a small RNA and induces its accumulation in cells

4.5SH and 4.5SI RNA are two abundant small non-coding RNAs specific for several related rodent families including Muridae. These RNAs have a number of common characteristics such as the short length (about 100nt), transcription by RNA polymerase III, and origin from Short Interspersed Elements (SINEs). However, their stabilities in cells substantially differ: the half-life of 4.5SH RNA is about 20min, while that of 4.5SI RNA is 22h. Here we studied the influence of cell stress such as heat shock or viral infection on these two RNAs. We found that the level of 4.5SI RNA did not change in stressed cells; whereas heat shock increased the abundance of 4.5SH RNA 3.2-10.5 times in different cell lines; and viral infection, 5 times. Due to the significant difference in the turnover rates of these two RNAs, a similar activation of their transcription by heat shock increases the level of the short-lived 4.5SH RNA and has minor effect on the level of the long-lived 4.5SI RNA. In addition, the accumulation of 4.5SH RNA results not only from the induction of its transcription but also from a substantial retardation of its decay. To our knowledge, it is the first example of a short-lived non-coding RNA whose elongated lifetime contributes significantly to its accumulation in stressed cells.

Cytosolic splice isoform of Hsp70 nucleotide exchange factor Fes1 is required for the degradation of misfolded proteins in yeast

Yeast Hsp70 nucleotide exchange factor Fes1 is expressed by rare alternative splicing as two isoforms. Fes1L is targeted to the nucleus, and Fes1S localizes to the cytosol and is required for the efficient proteasomal degradation of cytosolic misfolded proteins, as well as of species that are imported into the nucleus for degradation.

Cells maintain proteostasis by selectively recognizing and targeting misfolded proteins for degradation. In Saccharomyces cerevisiae, the Hsp70 nucleotide exchange factor Fes1 is essential for the degradation of chaperone-associated misfolded proteins by the ubiquitin-proteasome system. Here we show that the FES1 transcript undergoes unique 3′ alternative splicing that results in two equally active isoforms with alternative C-termini, Fes1L and Fes1S. Fes1L is actively targeted to the nucleus and represents the first identified nuclear Hsp70 nucleotide exchange factor. In contrast, Fes1S localizes to the cytosol and is essential to maintain proteostasis. In the absence of Fes1S, the heat-shock response is constitutively induced at normally nonstressful conditions. Moreover, cells display severe growth defects when elevated temperatures, amino acid analogues, or the ectopic expression of misfolded proteins, induce protein misfolding. Importantly, misfolded proteins are not targeted for degradation by the ubiquitin-proteasome system. These observations support the notion that cytosolic Fes1S maintains proteostasis by supporting the removal of toxic misfolded proteins by proteasomal degradation. This study provides key findings for the understanding of the organization of protein quality control mechanisms in the cytosol and nucleus.

Widespread use of non-productive alternative splice sites in Saccharomyces cerevisiae

Saccharomyces cerevisiae has been used as a model system to investigate the mechanisms of pre-mRNA splicing but only a few examples of alternative splice site usage have been described in this organism. Using RNA-Seq analysis of nonsense-mediated mRNA decay (NMD) mutant strains, we show that many S. cerevisiae intron-containing genes exhibit usage of alternative splice sites, but many transcripts generated by splicing at these sites are non-functional because they introduce premature termination codons, leading to degradation by NMD. Analysis of splicing mutants combined with NMD inactivation revealed the role of specific splicing factors in governing the use of these alternative splice sites and identified novel functions for Prp17p in enhancing the use of branchpoint-proximal upstream 3′ splice sites and for Prp18p in suppressing the usage of a non-canonical AUG 3′-splice site in GCR1. The use of non-productive alternative splice sites can be increased in stress conditions in a promoter-dependent manner, contributing to the down-regulation of genes during stress. These results show that alternative splicing is frequent in S. cerevisiae but masked by RNA degradation and that the use of alternative splice sites in this organism is mostly aimed at controlling transcript levels rather than increasing proteome diversity.

Accurate gene expression requires the transfer of gene information from DNA to RNA. When DNA is transcribed into RNA, part of the RNA needs to be removed (spliced) to generate a proper copy of the genetic information. This process needs to be very accurate to preserve the genetic information that will be transferred into proteins. Our study shows that in baker's yeast, the splicing process does not always produce the correctly spliced products, as RNA splicing events frequently utilize incorrect splice sites. However, these deficient RNA molecules are eliminated from cells by a quality control mechanism to preserve the integrity of the genetic information. However, incorrect splicing is not useless, as it can be used to regulate the quantity of RNA that is generated.

The metazoan protein disaggregase and amyloid depolymerase system: Hsp110, Hsp70, Hsp40, and small heat shock proteins

A baffling aspect of metazoan proteostasis is the lack of an Hsp104 ortholog that rapidly disaggregates and reactivates misfolded polypeptides trapped in stress induced disordered aggregates, preamyloid oligomers, or amyloid fibrils. By contrast, in bacteria, protozoa, chromista, fungi, and plants, Hsp104 orthologs are highly conserved and confer huge selective advantages in stress tolerance. Moreover, in fungi, the amyloid remodeling activity of Hsp104 has enabled deployment of prions for various beneficial modalities. Thus, a longstanding conundrum has remained unanswered: how do metazoan cells renature aggregated proteins or resolve amyloid fibrils without Hsp104? Here, we highlight recent advances that unveil the metazoan protein-disaggregase machinery, comprising Hsp110, Hsp70, and Hsp40, which synergize to dissolve disordered aggregates, but are unable to rapidly solubilize stable amyloid fibrils. However, Hsp110, Hsp70, and Hsp40 exploit the slow monomer exchange dynamics of amyloid, and can slowly depolymerize amyloid fibrils from their ends in a manner that is stimulated by small heat shock proteins. Upregulation of this system could have key therapeutic applications in various protein-misfolding disorders. Intriguingly, yeast Hsp104 can interface with metazoan Hsp110, Hsp70, and Hsp40 to rapidly eliminate disease associated amyloid. Thus, metazoan proteostasis is receptive to augmentation with exogenous disaggregases, which opens a number of therapeutic opportunities.

Mechanisms of heat shock response in mammals

Heat shock (HS) is one of the best-studied exogenous cellular stresses. The cellular response to HS utilizes ancient molecular networks that are based primarily on the action of stress-induced heat shock proteins and HS factors. However, in one way or another, all cellular compartments and metabolic processes are involved in such a response. In this review, we aimed to summarize the experimental data concerning all aspects of the HS response in mammalian cells, such as HS-induced structural and functional alterations of cell membranes, the cytoskeleton and cellular organelles; the associated pathways that result in different modes of cell death and cell cycle arrest; and the effects of HS on transcription, splicing, translation, DNA repair, and replication.

Prion-like nuclear aggregation of TDP-43 during heat shock is regulated by HSP40/70 chaperones

TDP-43 aggregation in the cytoplasm or nucleus is a key feature of the pathology of amyotrophic lateral sclerosis and frontotemporal dementia and is observed in numerous other neurodegenerative diseases, including Alzheimer's disease. Despite this fact, the inciting events leading to TDP-43 aggregation remain unclear. We observed that endogenous TDP-43 undergoes reversible aggregation in the nucleus after the heat shock and that this behavior is mediated by the C-terminal prion domain. Substitution of the prion domain from TIA-1 or an authentic yeast prion domain from RNQ1 into TDP-43 can completely recapitulate heat shock-induced aggregation. TDP-43 is constitutively bound to members of the Hsp40/Hsp70 family, and we found that heat shock-induced TDP-43 aggregation is mediated by the availability of these chaperones interacting with the inherently disordered C-terminal prion domain. Finally, we observed that the aggregation of TDP-43 during heat shock led to decreased binding to hnRNPA1, and a change in TDP-43 RNA-binding partners suggesting that TDP-43 aggregation alters its function in response to misfolded protein stress. These findings indicate that TDP-43 shares properties with physiologic prions from yeast, in that self-aggregation is mediated by a Q/N-rich disordered domain, is modulated by chaperone proteins and leads to altered function of the protein. Furthermore, they indicate that TDP-43 aggregation is regulated by chaperone availability, explaining the recurrent observation of TDP-43 aggregates in degenerative diseases of both the brain and muscle where protein homeostasis is disrupted.

Quercetin suppresses drug-resistant spheres via the p38 MAPK-Hsp27 apoptotic pathway in oral cancer cells

Background

Treatment failure in oral squamous cell carcinoma (OSCC) leading to local recurrence(s) and metastases is mainly due to drug resistance. Cancer stem cells (CSCs) are thought be responsible for the development of drug resistance. However, the correlations between CSCs, drug resistance, and new strategy against drug resistance in OSCC remain elusive.

Methods

A drug-resistant sphere (DRSP) model was generated by using a nonadhesive culture system to induce drug-resistant cells from SCC25 oral cancer cells. A comparative analysis was performed between the parent control cells and DRSPs with a related treatment strategy focusing on the expression of epithelial–mesenchymal transition (EMT)-associated markers, drug-resistance-related genes, and CSC properties in vitro, as well as tumorigenicity and the regimen for tumor regression in vivo.

Results

Our data show the presence of a phenomenon of EMT with gradual cellular transition from an epithelioid to mesenchymal-like spheroid morphology during induction of drug resistance. The characterization of DRSPs revealed the upregulation of the drug-resistance-related genes ABCG2 and MDR-1 and of CSC-representative markers, suggesting that DRSPs have greater resistance to cisplatin (Cis) and stronger CSC properties compared with the control. Moreover, overexpression of phosphorylated heat-shock protein 27 (p-Hsp27) via the activation of p38 MAPK signaling was observed in DRSPs. Knockdown of Hsp27 decreased Cis resistance and induced apoptosis in DRSPs. Furthermore, an inhibitor of Hsp27, quercetin (Qu), suppressed p-Hsp27 expression, with alterations of the EMT signature, leading to the promotion of apoptosis in DRSPs. A xenographic study also confirmed the increase of tumorigenicity in DRSPs. The combination of Qu and Cis can reduce tumor growth and decrease drug resistance in OSCC.

Conclusions

The p38 MAPK–Hsp27 axis plays an important role in CSCs-mediated drug resistance in OSCC. Targeting this axis using Qu combined with Cis may be a treatment strategy to improve prognosis in patients with OSCC.

Dual effect of heat shock on DNA replication and genome integrity

The data presented here suggest that in an asynchronous cell culture, heat shock might affect DNA integrity both directly and via arrest of replication fork progression and that the phosphorylation of histone H2AX has a protective effect on the arrested replication forks in addition to its known DNA damage signaling function.

Heat shock (HS) is one of the better-studied exogenous stress factors. However, little is known about its effects on DNA integrity and the DNA replication process. In this study, we show that in G1 and G2 cells, HS induces a countable number of double-stranded breaks (DSBs) in the DNA that are marked by γH2AX. In contrast, in S-phase cells, HS does not induce DSBs but instead causes an arrest or deceleration of the progression of the replication forks in a temperature-dependent manner. This response also provoked phosphorylation of H2AX, which appeared at the sites of replication. Moreover, the phosphorylation of H2AX at or close to the replication fork rescued the fork from total collapse. Collectively our data suggest that in an asynchronous cell culture, HS might affect DNA integrity both directly and via arrest of replication fork progression and that the phosphorylation of H2AX has a protective effect on the arrested replication forks in addition to its known DNA damage signaling function.

Diverse environmental stresses elicit distinct responses at the level of pre-mRNA processing in yeast

Gene expression in eukaryotic cells is profoundly influenced by the post-transcriptional processing of mRNAs, including the splicing of introns in the nucleus and both nuclear and cytoplasmic degradation pathways. These processes have the potential to affect both the steady-state levels and the kinetics of changes to levels of intron-containing transcripts. Here we report the use of a splicing isoform-specific microarray platform to investigate the effects of diverse stress conditions on pre-mRNA processing. Interestingly, we find that diverse stresses cause distinct patterns of changes at this level. The responses we observed are most dramatic for the RPGs and can be categorized into three major classes. The first is characterized by accumulation of RPG pre-mRNA and is seen in multiple types of amino acid starvation regimes; the magnitude of splicing inhibition correlates with the severity of the stress. The second class is characterized by a rapid decrease in both pre- and mature RPG mRNA and is seen in many stresses that inactivate the TORC1 kinase complex. These decreases depend on nuclear turnover of the intron-containing pre-RNAs. The third class is characterized by a decrease in RPG pre-mRNA, with only a modest reduction in the mature species; this response is observed in hyperosmotic and cation-toxic stresses. We show that casein kinase 2 (CK2) makes important contributions to the changes in pre-mRNA processing, particularly for the first two classes of stress responses. In total, our data suggest that complex post-transcriptional programs cooperate to fine-tune expression of intron-containing transcripts in budding yeast.

The elusive middle domain of Hsp104 and ClpB: location and function

Hsp104 in yeast and ClpB in bacteria are homologous, hexameric AAA+ proteins and Hsp100 chaperones, which function in the stress response as ring-translocases that drive protein disaggregation and reactivation. Both Hsp104 and ClpB contain a distinctive coiled-coil middle domain (MD) inserted in the first AAA+ domain, which distinguishes them from other AAA+ proteins and Hsp100 family members. Here, we focus on recent developments concerning the location and function of the MD in these hexameric molecular machines, which remains an outstanding question. While the atomic structure of the hexameric assembly of Hsp104 and ClpB remains uncertain, recent advances have illuminated that the MD is critical for the intrinsic disaggregase activity of the hexamer and mediates key functional interactions with the Hsp70 chaperone system (Hsp70 and Hsp40) that empower protein disaggregation.

The heat shock response: life on the verge of death

Organisms must survive a variety of stressful conditions, including sudden temperature increases that damage important cellular structures and interfere with essential functions. In response to heat stress, cells activate an ancient signaling pathway leading to the transient expression of heat shock or heat stress proteins (Hsps). Hsps exhibit sophisticated protection mechanisms, and the most conserved Hsps are molecular chaperones that prevent the formation of nonspecific protein aggregates and assist proteins in the acquisition of their native structures. In this Review, we summarize the concepts of the protective Hsp network.

Heat shock-induced SRSF10 dephosphorylation displays thermotolerance mediated by Hsp27

Gene regulation in response to environmental stress is critical for the survival of all organisms. From Saccharomyces cerevisiae to humans, it has been observed that splicing of mRNA precursors is repressed upon heat shock. However, a mild heat pretreatment often prevents splicing inhibition in response to a subsequent and more severe heat shock, a phenomenon called splicing thermotolerance. We have shown previously that the splicing regulator SRSF10 (formerly SRp38) is specifically dephosphorylated by the phosphatase PP1 in response to heat shock and that dephosphorylated SRSF10 is responsible for splicing repression caused by heat shock. Here we report that a mild heat shock protects SRSF10 from dephosphorylation during a second and more severe heat shock. Furthermore, this "thermotolerance" of SRSF10 phosphorylation, like that of splicing, requires de novo protein synthesis, specifically the synthesis of heat shock proteins. Indeed, overexpression of one of these proteins, Hsp27, inhibits SRSF10 dephosphorylation in response to heat shock and does so by interaction with SRSF10. Our data thus provide evidence that splicing thermotolerance is acquired through maintenance of SRSF10 phosphorylation and that this is mediated at least in part by Hsp27.

Plant Hsp100/ClpB-like proteins: poorly-analyzed cousins of yeast ClpB machine

ClpB/Hsp100 proteins act as chaperones, mediating disaggregation of denatured proteins. Recent work shows that apart from cytoplasm, these proteins are localized to nuclei, chloroplasts, mitochondria and plasma membrane. While ClpB/Hsp100 genes are essentially stress-induced (mainly heat stress) in vegetative organs of the plant body, expression of ClpB/Hsp100 proteins is noted to be constitutive in plant reproductive structures like pollen grains, developing embryos, seeds etc. With global warming looming large on the horizon, ways to genetically engineer plants against high temperature stress are urgently needed. Yeast mutants unable to synthesize active ClpB/Hsp100 protein show a clear thermosensitive phenotype. ClpB/Hsp100 proteins are implicated in high temperature stress tolerance in plants. We herein highlight the selected important facets of this protein family in plants.

[NSI (+)]: a novel non-Mendelian nonsense suppressor determinant in Saccharomyces cerevisiae

Non-Mendelian determinants that control heritable traits in yeast are subdivided into two major groups-one that includes DNA- or RNA-based elements and another that comprises protein-based factors that are analogous to mammalian prion. All yeast non-Mendelian determinants show dominant inheritance, and some of them demonstrate cytoplasmic infectivity. Only prions, however, harbor-specific features, such as high frequency of induction following overproduction of prion-encoding protein, loss of the protein's normal function, and reversible curability. Here, we describe a novel nonchromosomal determinant that, in addition to [PSI (+)] and [ISP (+)], is involved in epigenetic control of nonsense suppression. This determinant, which we have designated [NSI (+)], causes nonsense suppression in the strains bearing the N-terminal-deleted or -modified SUP35 gene, but has no manifestation in the strains with the intact copy of SUP35. [NSI (+)] shows dominant non-Mendelian inheritance, reversible curability and may be transmitted by cytoduction, albeit with low frequency. Similar to yeast prions, this determinant can be cured by deletion or mutational inactivation of Hsp104. We have shown that [NSI (+)] does not correspond to the already identified yeast prions. Based on the data obtained, we hypothesize that [NSI (+)] is a novel prion factor involved in epigenetic control of nonsense suppression.

Role of the heat shock transcription factor, Hsf1, in a major fungal pathogen that is obligately associated with warm-blooded animals

All organisms have evolved mechanisms that protect them against environmental stress. The major fungal pathogen of humans, Candida albicans, has evolved robust stress responses that protect it against human immune defences and promote its pathogenicity. However, C. albicans is unlikely to be exposed to heat shock as it is obligatorily associated with warm-blooded animals. Therefore, we examined the role of the heat shock transcription factor (Hsf1) in this pathogen. We show that C. albicans expresses an evolutionarily conserved Hsf1 (orf19.4775) that is phosphorylated in response to heat shock, induces transcription via the heat shock element (HSE), contributes to the global transcriptional response to heat shock, and is essential for viability. Why has Hsf1 been conserved in this obligate animal saprophyte? We reasoned that Hsf1 might contribute to medically relevant stress responses. However, this is not the case, as an Hsf1-specific HSE-lacZ reporter is not activated by oxidative, osmotic, weak acid or pH stress. Rather, Hsf1 is required for the expression of essential chaperones in the absence of heat shock (e.g. Hsp104, Hsp90, Hsp70). Furthermore, Hsf1 regulates the expression of HSE-containing genes in response to growth temperature in C. albicans. Therefore, the main role of Hsf1 in this pathogen might be the homeostatic modulation of chaperone levels in response to growth temperature, rather than the activation of acute responses to sudden thermal transitions.

Spp382p interacts with multiple yeast splicing factors, including possible regulators of Prp43 DExD/H-Box protein function

Prp43p catalyzes essential steps in pre-mRNA splicing and rRNA biogenesis. In splicing, Spp382p stimulates the Prp43p helicase to dissociate the postcatalytic spliceosome and, in some way, to maintain the integrity of the spliceosome assembly. Here we present a dosage interference assay to identify Spp382p-interacting factors by screening for genes that when overexpressed specifically inhibit the growth of a conditional lethal prp38-1 spliceosome assembly mutant in the spp382-1 suppressor background. Identified, among others, are genes encoding the established splicing factors Prp8p, Prp9p, Prp11p, Prp39p, and Yhc1p and two poorly characterized proteins with possible links to splicing, Sqs1p and Cwc23p. Sqs1p copurifies with Prp43p and is shown to bind Prp43p and Spp382p in the two-hybrid assay. Overexpression of Sqs1p blocks pre-mRNA splicing and inhibits Prp43p-dependent steps in rRNA processing. Increased Prp43p levels buffer Sqs1p cytotoxicity, providing strong evidence that the Prp43p DExD/H-box protein is a target of Sqs1p. Cwc23p is the only known yeast splicing factor with a DnaJ motif characteristic of Hsp40-like chaperones. We show that similar to SPP382, CWC23 activity is critical for efficient pre-mRNA splicing and intron metabolism yet, surprisingly, this activity does not require the canonical DnaJ/Hsp40 motif. These and related data establish the value of this dosage interference assay for finding genes that alter cellular splicing and define Sqs1p and Cwc23p as prospective modulators of Spp382p-stimuated Prp43p function.

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

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

Stressed out! Effects of environmental stress on mRNA metabolism

Exposure of yeast cells to environmental stresses can disrupt essential intracellular processes, especially those carried out by large macromolecular complexes. The production of mature, translatable mRNAs is most sensitive to stress owing to the inhibition of messenger RNA splicing and alterations in the export of mRNA from the nucleus. Changes in the cytoplasmic pools of mRNAs also occur following exposure to stress conditions. Messenger RNAs accumulate in discrete cytoplasmic foci such as processing bodies and stress granules. These dynamic changes in RNA metabolism, following exposure to stress, ensure the preferential production and export of heat-shock mRNAs and the sequestering of general cellular mRNAs in the nucleus or in cytoplasmic foci, thus allowing for a redirection of the translational machinery to encode stress proteins, which aid in cellular recovery following stress. Stress proteins, such as Hsp70p and Hsp104p, have been shown to play a direct role in the repair of macromolecular complexes involved in RNA metabolism in yeast cells, thus ensuring that the cell returns to homeostasis.

Intracellular localization of constitutive and inducible heat shock protein 70 in rat liver after in vivo heat stress

The level and intracellular redistribution of the two nucleo-cytoplasmic members of 70 kDa heat shock protein family (constitutive, Hsc70 or Hsp73, and inducible, Hsp72) were studied in rat liver during a 24-h period after exposure of the animals to 41 degrees C whole body hyperthermic stress. The examined proteins were detected in the liver cytosol and nuclei by Western blotting and immunohistochemical staining of paraffin sections, as well as by immnocytochemical staining of isolated nuclear smears. All three techniques applied were based on the use of monoclonal antibodies recognizing both constitutive and inducible Hsp70 isoforms or only the inducible isoform, and gave consistent results. The exposure of the animals to in vivo heat stress was shown to induce the synthesis of otherwise non-existing Hsp72, rendering Hsc70 level unchanged in comparison to unstressed controls. However, immediately after the stress the intracellular redistribution of Hsc70, i.e. its nuclear accumulation, was observed. The maximal level of Hsp70 both in the cytoplasm and in the nuclei was registered 5 h after the stress, which coincided with the maximal level of Hsp72 induction. The alterations in the level and intracellular distribution of examined proteins were still noticeable 24 h after the stress. The results of this study could shed some more light on, as yet uncertain, differences between cellular functions of these two proteins, as well as on the role of the constitutive form under normal and stress conditions.

The yeast splicing factor Prp40p contains functional leucine-rich nuclear export signals that are essential for splicing

To investigate the function of the essential U1 snRNP protein Prp40p, we performed a synthetic lethal screen in Saccharomyces cerevisiae. Using an allele of PRP40 that deletes 47 internal residues and causes only a slight growth defect, we identified aphenotypic mutations in three distinct complementation groups that conferred synthetic lethality. The synthetic phenotypes caused by these mutations were suppressed by wild-type copies of CRM1 (XPO1), YNL187w, and SME1, respectively. The strains whose synthetic phenotypes were suppressed by CRM1 contained no mutations in the CRM1 coding sequence or promoter. This indicates that overexpression of CRM1 confers dosage suppression of the synthetic lethality. Interestingly, PRP40 and YNL187w encode proteins with putative leucine-rich nuclear export signal (NES) sequences that fit the consensus sequence recognized by Crm1p. One of Prp40p's two NESs lies within the internal deletion. We demonstrate here that the NES sequences of Prp40p are functional for nuclear export in a leptomycin B-sensitive manner. Furthermore, mutation of these NES sequences confers temperature-sensitive growth and a pre-mRNA splicing defect. Although we do not expect that yeast snRNPs undergo compartmentalized biogenesis like their metazoan counterparts, our results suggest that Prp40p and Ynl187wp contain redundant NESs that aid in an important, Crm1p-mediated nuclear export event.

Stress response in yeast mRNA export factor: reversible changes in Rat8p localization are caused by ethanol stress but not heat shock

Ethanol stress (10% v/v) causes selective mRNA export in Saccharomyces cerevisiae in a similar manner to heat shock (42 degrees C). Bulk poly(A)(+) mRNA accumulates in the nucleus, whereas heat shock protein mRNA is exported under such conditions. Here we investigated the effects of stress on mRNA export factors. In cells treated with ethanol stress, the DEAD box protein Rat8p showed a rapid and reversible change in its localization, accumulating in the nucleus. This change correlated closely with the blocking of bulk poly(A)(+) mRNA export caused by ethanol stress. We also found that the nuclear accumulation of Rat8p is caused by a defect in the Xpo1p/Crm1p exportin. Intriguingly, the localization of Rat8p did not change in heat shocked cells, suggesting that the mechanisms blocking bulk poly(A)(+) mRNA export differ for heat shock and ethanol stress. These results suggest that changes in the localization of Rat8p contribute to the selective export of mRNA in ethanol stressed cells, and also indicate differences in mRNA export between the heat shock response and ethanol stress response.

Gle2p is essential to induce adaptation of the export of bulk poly(A)+ mRNA to heat shock in Saccharomyces cerevisiae

The export of bulk poly(A)(+) mRNA is blocked under heat-shocked (42 degrees C) conditions in Saccharomyces cerevisiae. We found that an mRNA export factor Gle2p rapidly dissociated from the nuclear envelope and diffused into the cytoplasm at 42 degrees C. However, in exponential phase cells pretreated with mild heat stress (37 degrees C for 1 h), Gle2p did not dissociate at 42 degrees C, and the export of bulk poly(A)(+) mRNA continued. Cells in stationary phase also continued with the export of bulk poly(A)(+) mRNA at 42 degrees C without the dissociation of Gle2p from the nuclear envelope. The dissociation of Gle2p was caused by increased membrane fluidity and correlated closely with blocking of the export of bulk poly(A)(+) mRNA. Furthermore, the mutants gle2Delta and rip1Delta could not induce such an adaptation of the export of bulk poly(A)(+) mRNA to heat shock. Our findings indicate that Gle2p plays a crucial role in mRNA export especially under heat-shocked conditions. Our findings also indicate that the nuclear pore complexes that Gle2p constitutes need to be stabilized for the adaptation and that the increased membrane integrity caused by treatment with mild heat stress or by survival in stationary phase is likely to contribute to the stabilization of the association between Gle2p and the nuclear pore complexes.

Dephosphorylated SRp38 acts as a splicing repressor in response to heat shock

The cellular response to stresses such as heat shock involves changes in gene expression. It is well known that the splicing of messenger RNA precursors is generally repressed on heat shock, but the factors responsible have not been identified. SRp38 is an SR protein splicing factor that functions as a general repressor of splicing. It is activated by dephosphorylation and required for splicing repression in M-phase cells. Here we show that SRp38 is also dephosphorylated on heat shock and that this dephosphorylation correlates with splicing inhibition. Notably, depletion of SRp38 from heat-shocked cell extracts derepresses splicing, and adding back dephosphorylated SRp38 specifically restores inhibition. We further show that dephosphorylated SRp38 interacts with a U1 small nuclear ribonucleoprotein particle (snRNP) protein, and that this interaction interferes with 5'-splice-site recognition by the U1 snRNP. Finally, SRp38-deficient DT40 cells show an altered cell-cycle profile consistent with a mitotic defect; they are also temperature sensitive and defective in recovery after heat shock. SRp38 thus plays a crucial role in cell survival under stress conditions by inhibiting the splicing machinery.

The influence of dexamethasone on Hsp70 level and association with glucocorticoid receptor in the liver of unstressed and heat-stressed rats

The aim of the present study was to examine the influence of dexamethasone on the levels of heat shock protein Hsp70 and glucocorticoid hormones receptor as well as on the interaction of these two proteins in the liver cytosol and nuclei of unstressed and rats exposed to whole body hyperthermic stress. The results, obtained by quantitative immunoblotting, have shown that dexamethasone provoked a reduction of Hsp70 basal level and an increase in its stress-induced level in the nuclei, supporting the idea that this hormone may be a factor included in the regulation of Hsp70 level both under normal and stress conditions. The cytosolic reduction and nuclear elevation of the glucocorticoid hormones receptor level by dexamethasone were also observed. Co-immunopurification of Hsp70 and glucocorticoid hormones receptor has revealed that the changes of cytosolic and nuclear levels of the two examined proteins resulted in the changes of their interaction within the respective cellular compartments. Thus, 41 ?C heat stress was shown to cause at least two-fold elevation of Hsp70/GR ratio within the glucocorticoid hormones receptor heterocomplexes both in the presence and in the absence of dexamethasone. The results support the view that glucocorticoid hormones signaling pathway and heat shock system are interrelated.

Clp ATPases and their role in protein unfolding and degradation

Although much has been learned about the structure and function of Clp chaperones and their role in proteolysis, the mechanism of protein unfolding catalyzed by Clp ATPases and the mechanism of translocation of the unfolded proteins from Clp ATPases to partner proteases remain unsolved puzzles. However, models in which mechanical force is used to destabilize the structure of the substrate in a processive and directional manner are probable. It also seems likely that when ClpA ATPases are associated with proteases, unfolding is coupled to extrusion of the unfolded protein into the proteolytic cavity. In summary, it is anticipated that the large family of Clp ATPases will accomplish their many important cellular functions by similar mechanisms and what has been learned by studying the prokaryotic members reviewed here will shed a great deal of light on all members of the family.

A bichaperone (Hsp70-Hsp78) system restores mitochondrial DNA synthesis following thermal inactivation of Mip1p polymerase

Mitochondrial DNA synthesis is a thermosensitive process in the yeast Saccharomyces cerevisiae. We found that restoration of mtDNA synthesis following heat treatment of cells is dependent on reactivation of the mtDNA polymerase Mip1p through the action of a mitochondrial bichaperone system consisting of the Hsp70 system and the Hsp78 oligomeric protein. mtDNA synthesis was inefficiently restored after heat shock in yeast lacking either functional component of the bichaperone system. Furthermore, the activity of purified Mip1p was also thermosensitive; however, the purified components of the mitochondrial bichaperone system (Ssc1p, Mdj1p, Mge1p, and Hsp78p) were able to protect its activity under moderate heat shock conditions as well as to reactivate thermally inactivated Mip1p. Interestingly, the reactivation of endogenous Mip1p contributed more significantly to the restoration of mtDNA synthesis than did import of newly synthesized Mip1p from the cytosol. These observations suggest an important link between function of mitochondrial chaperones and the propagation of mitochondrial genomes under ever-changing environmental conditions.

Cytotoxic and genotoxic consequences of heat stress are dependent on the presence of oxygen in Saccharomyces cerevisiae

Lethal heat stress generates oxidative stress in Saccharomyces cerevisiae, and anaerobic cells are several orders of magnitude more resistant than aerobic cells to a 50 degrees C heat shock. Here we characterize the oxidative effects of this heat stress. The thermoprotective effect in anaerobic cells was not due to expression of HSP104 or any other heat shock gene, raising the possibility that the toxicity of lethal heat shock is due mainly to oxidative stress. Aerobic but not anaerobic heat stress caused elevated frequencies of forward mutations and interchromosomal DNA recombination. Oxidative DNA repair glycosylase-deficient strains under aerobic conditions showed a powerful induction of forward mutation frequencies compared to wild-type cells, which was completely abolished under anaerobiosis. We also investigated potential causes for this oxygen-dependent heat shock-induced genetic instability. Levels of sulfhydryl groups, dominated mainly by the high levels of the antioxidant glutathione (reduced form) and levels of vitamin E, decreased after aerobic heat stress but not after anaerobic heat stress. Aerobic heat stress also led to an increase in mitochondrial membrane disruption of several hundredfold, which was 100-fold reduced under anaerobic conditions.

Review: mechanisms of disaggregation and refolding of stable protein aggregates by molecular chaperones

Molecular chaperones are essential for the correct folding of proteins in the cell under physiological and stress conditions. Two activities have been traditionally attributed to molecular chaperones: (1) preventing aggregation of unfolded polypeptides and (2) assisting in the correct refolding of chaperone-bound denatured polypeptides. We discuss here a novel function of molecular chaperones: catalytic solubilization and refolding of stable protein aggregates. In Escherichia coli, disaggregation is carried out by a network of ATPase chaperones consisting of a DnaK core, assisted by the cochaperones DnaJ, GrpE, ClpB, and GroEL-GroES. We suggest a sequential mechanism in which (a) ClpB exposes new DnaK-binding sites on the surface of the stable protein aggregates; (b) DnaK binds the aggregate surfaces and, by doing so, melts the incorrect hydrophobic associations between aggregated polypeptides; (c) ATP hydrolysis and DnaK release allow local intramolecular refolding of native domains, leading to a gradual weakening of improper intermolecular links; (d) DnaK and GroEL complete refolding of solubilized polypeptide chains into native proteins. Thus, active disaggregation by the chaperone network can serve as a central cellular tool for the recovery of native proteins from stress-induced aggregates and actively remove disease-causing toxic aggregates, such as polyglutamine-rich proteins, amyloid plaques, and prions.

The elimination of the yeast [PSI+] prion by guanidine hydrochloride is the result of Hsp104 inactivation

In the yeast Saccharomyces cerevisiae, Sup35p (eRF3), a subunit of the translation termination complex, can take up a prion-like, self-propagating conformation giving rise to the non-Mendelian [PSI+] determinant. The replication of [PSI+] prion seeds can be readily blocked by growth in the presence of low concentrations of guanidine hydrochloride (GdnHCl), leading to the generation of prion-free [psi-] cells. Here, we provide evidence that GdnHCl blocks seed replication in vivo by inactivation of the molecular chaperone Hsp104. Although growth in the presence of GdnHCl causes a modest increase in HSP104 expression (20-90%), this is not sufficient to explain prion curing. Rather, we show that GdnHCl inhibits two different Hsp104-dependent cellular processes, namely the acquisition of thermotolerance and the refolding of thermally denatured luciferase. The inhibitory effects of GdnHCl protein refolding are partially suppressed by elevating the endogenous cellular levels of Hsp104 using a constitutive promoter. The kinetics of GdnHCl-induced [PSI+] curing could be mimicked by co-expression of an ATPase-negative dominant HSP104 mutant in an otherwise wild-type [PSI+] strain. We suggest that GdnHCl inactivates the ATPase activity of Hsp104, leading to a block in the replication of [PSI+] seeds.

A ditelosomic line of 'Chinese Spring' wheat with augmented acquired thermotolerance

A study of the ditelosomic series of 'Chinese Spring' wheat has yielded a number of lines displaying either an increased or decreased ability to acquire thermotolerance. One such ditelosomic (DT) is termed DT1BS which refers to the missing short arm of chromosome 1 in the B genome. The DT1BS line has the ability to acquire thermotolerance at lower induction temperatures and provide greater protection to the plant against otherwise lethal elevated temperatures. Using a chlorophyll accumulation assay to measure plant health, we show that DT1BS accumulates chlorophyll optimally at the same temperature, and to similar levels as 'Chinese Spring'. We also show that maximum acquired thermotolerance against a 48 degrees C challenge is induced at 40 degrees C, but significant levels of protection can be obtained at temperatures as low as 34 degrees C in DT1BS or 36 degrees C in 'Chinese Spring'. Heat-shock protein accumulation is observed in DT1BS at temperatures 4 degrees C lower than the 'Chinese Spring' and is correlated with the induction of acquired thermotolerance.

Dynamic changes in small nuclear ribonucleoproteins of heat-stressed and thermotolerant HeLa cells

Living organisms when subjected to various forms of environmental stress mount a physiological response to survive the long- and short-term ill-effects of the stress. The stress response may involve selective shut down of non-essential metabolic activities and the repair of macromolecular damage resulting from the stress. Messenger RNA splicing in cultured HeLa cells is one of the processes inhibited by heat stress. Splicing is protected from such inhibition in stress-preconditioned cells that have acquired a tolerant state characterised by increased cell survival and resistance to other environmental stresses. Stress tolerant cells have heat shock proteins (HSPs) that had been induced by the preconditioning process. To examine the biochemical changes induced by stress in the splicing apparatus, we analysed the small nuclear ribonucleoprotein (snRNP) particles associated with spliceosomes in normal, stressed, and stress tolerant cells. We show that (a) the spliceosomal component U4/U5/U6 snRNP particle is disassembled by heat stress into intermediates of splicing assembly, (b) prior induction of stress tolerance protects the structural and functional integrity of snRNPs if cells are subsequently exposed to a severe stress and (c) a novel 65 kDa protein is associated with small nuclear ribonucleoprotein particles in stress tolerant cells.

The stress response is repressed during fermentation in brewery strains of yeast

Yeast cells encounter a variety of environmental stresses during brewing and must respond to ensure cell survival. Cells can respond to stress by inducing a Heat Shock Response in which heat shock proteins (Hsps) are synthesized. In laboratory strains of Saccharomyces cerevisiae, the heat shock protein, Hsp104, plays a major role in the acquisition of tolerance to a variety of stresses such as heat, ethanol and sodium arsenite, and as such acts as an excellent stress indicator. The induction of Hsp104 in bottom-and top-fermenting brewery strains was examined when grown under laboratory and industrial fermentation conditions, and it was found that each brewing strain exhibits its own unique pattern of Hsp104 expression. During industrial fermentations, brewery strains are capable of mounting a stress response at the early stages of fermentation. However, as the fermentation proceeds, the response is repressed. The results suggest that conditions experienced in industrial brewing prevent the activation of the stress response. This study increases our understanding of alterations in gene expression patterns during the brewing process, and yields information that will aid in the definition of best practice in yeast management.

Aggregation of huntingtin in yeast varies with the length of the polyglutamine expansion and the expression of chaperone proteins

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by polyglutamine (polyQ) expansions in the huntingtin (Ht) protein. A hallmark of HD is the proteolytic production of an N-terminal fragment of Ht, containing the polyQ repeat, that forms aggregates in the nucleus and cytoplasm of affected neurons. Proteins with longer polyQ repeats aggregate more rapidly and cause disease at an earlier age, but the mechanism of aggregation and its relationship to disease remain unclear. To provide a new, genetically tractable model system for the study of Ht, we engineered yeast cells to express an N-terminal fragment of Ht with different polyQ repeat lengths of 25, 47, 72, or 103 residues, fused to green fluorescent protein. The extent of aggregation varied with the length of the polyQ repeat: at the two extremes, most HtQ103 protein coalesced into a single large cytoplasmic aggregate, whereas HtQ25 exhibited no sign of aggregation. Mutations that inhibit the ubiquitin/proteasome pathway at three different steps had no effect on the aggregation of Ht fragments in yeast, suggesting that the ubiquitination of Ht previously noted in mammalian cells may not inherently be required for polyQ length-dependent aggregation. Changing the expression levels of a wide variety of chaperone proteins in yeast neither increased nor decreased Ht aggregation. However, Sis1, Hsp70, and Hsp104 overexpression modulated aggregation of HtQ72 and HtQ103 fragments. More dramatically, the deletion of Hsp104 virtually eliminated it. These observations establish yeast as a system for studying the causes and consequences of polyQ-dependent Ht aggregation.

Hsc40, a new member of the hsp40 family, exhibits similar expression profile to that of hsc70 in mammalian cells

Here, we report the identification and characterization of a new hsp40 family member, heat shock cognate 40 (hsc40), which may be a specific functional partner for hsc70. The hsc40 gene consists of five exons and four introns. The previously characterized hsp40 gene contains only three exons and two introns. Despite this difference in the numbers of exons and introns, the structures of these two genes are actually quite similar. If the first two exons and introns of the hsc40 gene are excluded, the rest is essentially the same as the whole hsp40 gene. The intron/exon boundaries of the hsc40 and hsp40 genes are conserved with respect to amino acid coding, indicating that the hsp40 gene might have arisen from a duplicated hsc40 gene, which developed a new transcription-regulatory mechanism in the second intron. Hsc40 is evolutionally conserved. There is over 95% sequence identity between the putative mouse and human hsc40 proteins. However, there is only 60% sequence identity between the hsc40 and hsp40 proteins from either human or mouse cells. Northern blot analysis of various tissues and cells in culture revealed that this gene was expressed under normal conditions, and its expression was further increased after various stress treatments. The expression pattern of the hsc40 gene is very similar to that of the hsc70 gene under both normal and stress conditions, whereas the hsp40 gene exhibited an expression pattern more similar to that of hsp70 genes on the same Northern blots. These results, considered along with the high rate of sequence conservation between the same proteins from different species and high rate of variation between the two different proteins from the same species, strongly suggest that hsc40 and hsp40 may perform different functions and/or have different specificities in the cell.

Concurrent chaperone and protease activities of ClpAP and the requirement for the N-terminal ClpA ATP binding site for chaperone activity

ClpA, a member of the Clp/Hsp100 family of ATPases, is both an ATP-dependent molecular chaperone and the regulatory component of ClpAP protease. We demonstrate that chaperone and protease activities occur concurrently in ClpAP complexes during a single round of RepA binding to ClpAP and ATP-dependent release. This result was substantiated with a ClpA mutant, ClpA(K220V), carrying an amino acid substitution in the N-terminal ATP binding site. ClpA(K220V) is unable to activate RepA, but the presence of ClpP or chemically inactivated ClpP restores its ability to activate RepA. The presence of ClpP simultaneously facilitates degradation of RepA. ClpP must remain bound to ClpA(K220V) for these effects, indicating that both chaperone and proteolytic activities of the mutant complex occur concurrently. ClpA(K220V) itself is able to form stable complexes with RepA in the presence of a poorly hydrolyzed ATP analog, adenosine 5'-O-(thiotriphosphate), and to release RepA upon exchange of adenosine 5'-O-(thiotriphosphate) with ATP. However, the released RepA is inactive in DNA binding, indicating that the N-terminal ATP binding site is essential for the chaperone activity of ClpA. Taken together, these results suggest that substrates bound to the complex of the proteolytic and ATPase components can be partitioned between release/reactivation and translocation/degradation.