Transcriptional and Translational Analysis of the Murine 84- and 86-kDa Heat Shock Proteins

Single-animal, single-tube RNA extraction for quantitative analysis of transcripts in the tardigrade Hypsibius exemplaris

The tardigrade Hypsibius exemplaris is an emerging model organism renowned for its ability to survive environmental extremes.1-3 To explore the molecular mechanisms and genetic basis of such extremotolerance, many studies rely on transcriptional profiling4, 5 and RNA interference (RNAi)6 to define molecular targets. Such studies require efficient, accurate, and robust RNA extraction methods; however, obtaining high-quality quantitative levels of RNA from H. exemplaris has been challenging6, 7. Possessing a layer of firm chitinous cuticle, tardigrade tissues are difficult to disrupt by chemical or mechanical means8. Here we present an efficient single-tardigrade, single-tube RNA extraction method (STST) that not only reliably isolates RNA from individual tardigrades but dramatically reduces the time required for extraction. We show that this RNA extraction method yields robust quantities of cDNA and can be used to amplify multiple transcripts by qRT-PCR. To validate the method, we use it to compare dynamic changes in expression of genes encoding two heat-shock-regulated proteins, Heat-Shock Protein 70 β2 (HSP70β2) and Heat-Shock Protein 90α (HSP90α) by quantifying their expression levels in heat-exposed and cold-exposed individuals using qRT-PCR across long-term and short-term heat stressors. Our method effectively complements existing bulk RNA extraction methods7, permitting rapid examination of individual tardigrade transcriptional data and quantification of phenotypic variations in expression profiles amongst individuals.

Metamorphism in TDP-43 prion-like domain determines chaperone recognition

The RNA binding protein TDP-43 forms cytoplasmic inclusions via its C-terminal prion-like domain in several neurodegenerative diseases. Aberrant TDP-43 aggregation arises upon phase de-mixing and transitions from liquid to solid states, following still unknown structural conversions which are primed by oxidative stress and chaperone inhibition. Despite the well-established protective roles for molecular chaperones against protein aggregation pathologies, knowledge on the determinants of chaperone recognition in disease-related prions is scarce. Here we show that chaperones and co-chaperones primarily recognize the structured elements in TDP-43´s prion-like domain. Significantly, while HSP70 and HSP90 chaperones promote TDP-43 phase separation, co-chaperones from the three classes of the large human HSP40 family (namely DNAJA2, DNAJB1, DNAJB4 and DNAJC7) show strikingly different effects on TDP-43 de-mixing. Dismantling of the second helical element in TDP-43 prion-like domain by methionine sulfoxidation impacts phase separation and amyloid formation, abrogates chaperone recognition and alters phosphorylation by casein kinase-1δ. Our results show that metamorphism in the post-translationally modified TDP-43 prion-like domain encodes determinants that command mechanisms with major relevance in disease.

Cytosolic Hsp90 Isoform-Specific Functions and Clinical Significance

The heat shock protein 90 (Hsp90) is a molecular chaperone and a key regulator of proteostasis under both physiological and stress conditions. In mammals, there are two cytosolic Hsp90 isoforms: Hsp90α and Hsp90β. These two isoforms are 85% identical and encoded by two different genes. Hsp90β is constitutively expressed and essential for early mouse development, while Hsp90α is stress-inducible and not necessary for survivability. These two isoforms are known to have largely overlapping functions and to interact with a large fraction of the proteome. To what extent there are isoform-specific functions at the protein level has only relatively recently begun to emerge. There are studies indicating that one isoform is more involved in the functionality of a specific tissue or cell type. Moreover, in many diseases, functionally altered cells appear to be more dependent on one particular isoform. This leaves space for designing therapeutic strategies in an isoform-specific way, which may overcome the unfavorable outcome of pan-Hsp90 inhibition encountered in previous clinical trials. For this to succeed, isoform-specific functions must be understood in more detail. In this review, we summarize the available information on isoform-specific functions of mammalian Hsp90 and connect it to possible clinical applications.

Cell Cycle Regulation by Heat Shock Transcription Factors

Cell division and cell cycle mechanism has been studied for 70 years. This research has revealed that the cell cycle is regulated by many factors, including cyclins and cyclin-dependent kinases (CDKs). Heat shock transcription factors (HSFs) have been noted as critical proteins for cell survival against various stresses; however, recent studies suggest that HSFs also have important roles in cell cycle regulation-independent cell-protective functions. During cell cycle progression, HSF1, and HSF2 bind to condensed chromatin to provide immediate precise gene expression after cell division. This review focuses on the function of these HSFs in cell cycle progression, cell cycle arrest, gene bookmarking, mitosis and meiosis.

Extracellular HSP90 Machineries Build Tumor Microenvironment and Boost Cancer Progression

HSP90 is released by cancer cells in the tumor microenvironment where it associates with different co-chaperones generating complexes with specific functions, ranging from folding and activation of extracellular clients to the stimulation of cell surface receptors. Emerging data indicate that these functions are essential for tumor growth and progression. The understanding of the exact composition of extracellular HSP90 complexes and the molecular mechanisms at the basis of their functions in the tumor microenvironment may represent the first step to design innovative diagnostic tools and new effective therapies. Here we review the impact of extracellular HSP90 complexes on cancer cell signaling and behavior.

RNA-binding proteins and heat-shock protein 90 are constituents of the cytoplasmic capping enzyme interactome

The N7-methylguanosine cap is added in the nucleus early in gene transcription and is a defining feature of eukaryotic mRNAs. Mammalian cells also possess cytoplasmic machinery for restoring the cap at uncapped or partially degraded RNA 5' ends. Central to both pathways is capping enzyme (CE) (RNA guanylyltransferase and 5'-phosphatase (RNGTT)), a bifunctional, nuclear and cytoplasmic enzyme. CE is recruited to the cytoplasmic capping complex by binding of a C-terminal proline-rich sequence to the third Src homology 3 (SH3) domain of NCK adapter protein 1 (NCK1). To gain broader insight into the cellular context of cytoplasmic recapping, here we identified the protein interactome of cytoplasmic CE in human U2OS cells through two complementary approaches: chemical cross-linking and recovery with cytoplasmic CE and protein screening with proximity-dependent biotin identification (BioID). This strategy unexpectedly identified 66 proteins, 52 of which are RNA-binding proteins. We found that CE interacts with several of these proteins independently of RNA, mediated by sequences within its N-terminal triphosphatase domain, and we present a model describing how CE-binding proteins may function in defining recapping targets. This analysis also revealed that CE is a client protein of heat shock protein 90 (HSP90). Nuclear and cytoplasmic CEs were exquisitely sensitive to inhibition of HSP90, with both forms declining significantly following treatment with each of several HSP90 inhibitors. Importantly, steady-state levels of capped mRNAs decreased in cells treated with the HSP90 inhibitor geldanamycin, raising the possibility that the cytotoxic effect of these drugs may partially be due to a general reduction in translatable mRNAs.

Client Proteins and Small Molecule Inhibitors Display Distinct Binding Preferences for Constitutive and Stress-Induced HSP90 Isoforms and Their Conformationally Restricted Mutants

The two cytosolic/nuclear isoforms of the molecular chaperone HSP90, stress-inducible HSP90α and constitutively expressed HSP90β, fold, assemble and maintain the three-dimensional structure of numerous client proteins. Because many HSP90 clients are important in cancer, several HSP90 inhibitors have been evaluated in the clinic. However, little is known concerning possible unique isoform or conformational preferences of either individual HSP90 clients or inhibitors. In this report, we compare the relative interaction strength of both HSP90α and HSP90β with the transcription factors HSF1 and HIF1α, the kinases ERBB2 and MET, the E3-ubiquitin ligases KEAP1 and RHOBTB2, and the HSP90 inhibitors geldanamycin and ganetespib. We observed unexpected differences in relative client and drug preferences for the two HSP90 isoforms, with HSP90α binding each client protein with greater apparent affinity compared to HSP90β, while HSP90β bound each inhibitor with greater relative interaction strength compared to HSP90α. Stable HSP90 interaction was associated with reduced client activity. Using a defined set of HSP90 conformational mutants, we found that some clients interact strongly with a single, ATP-stabilized HSP90 conformation, only transiently populated during the dynamic HSP90 chaperone cycle, while other clients interact equally with multiple HSP90 conformations. These data suggest different functional requirements among HSP90 clientele that, for some clients, are likely to be ATP-independent. Lastly, the two inhibitors examined, although sharing the same binding site, were differentially able to access distinct HSP90 conformational states.

Regulation and function of the human HSP90AA1 gene

Heat shock protein 90α (Hsp90α), encoded by the HSP90AA1 gene, is the stress inducible isoform of the molecular chaperone Hsp90. Hsp90α is regulated differently and has different functions when compared to the constitutively expressed Hsp90β isoform, despite high amino acid sequence identity between the two proteins. These differences are likely due to variations in nucleotide sequence within non-coding regions, which allows for specific regulation through interaction with particular transcription factors, and to subtle changes in amino acid sequence that allow for unique post-translational modifications. This article will specifically focus on the expression, function and regulation of Hsp90α.

Effects of heat shock on the production of human erythropoietin from recombinant CHO cells

The production of recombinant proteins by mammalian cells demands a highly controlled environment for cell cultivation. Temperature stress represents a commonly encountered disturbance in both research and process environments. In this study, we examined the effects of heat shock on the expression of recombinant human erythropoietin (EPO) in a Chinese hamster ovary (CHO) cell line. Biosynthetic radiolabeling experiments indicated that the cells exposed to a 42 degrees C/1-hour heat shock exhibit a transient reprogramming of transcription and translation characterized by the inhibition of protein synthesis and induction of heat shock proteins. The rate of protein synthesis decreased by 50% after the heat shock, while the rate of RNA synthesis increased by 50% initially and then quickly reduced to 80% of the control level. The protein and RNA synthesis rates were fully recovered in approximately 48 hours after the heat shock. However, we found that the expression of EPO was not arrested by the heat shock. The glycosylation patterns, as examined by isoelectric focusing, of either the culture supernatant or the purified EPO were not affected by the heat shock. In contrast, a 45 degrees C/1-hour heat shock terminated RNA and protein synthesis immediately and caused culture death in 12 hours. Cellular responses to temperature stress were affected by the metabolic state of the cells; cells maintained in serum-free medium were more sensitive than cells growing exponentially in the presence of serum. We have also examined the kinetics of metabolic responses of the cells to heat shock with respect to nutrient uptake and metabolite accumulation.

Regulation of uterine hsp90alpha, hsp72 and HSF-1 transcription in B6C3F1 mice by beta-estradiol and bisphenol A: involvement of the estrogen receptor and protein kinase C

We have previously demonstrated that bisphenol A (BPA)- and beta-estradiol (E2)-induced increases in uterine weight and heat shock protein (hsp) 90alpha and hsp72 levels are mediated through the estrogen receptor (ER). It is not, however, clear if BPA and E2 regulation of hsps is at the transcriptional or post-transcriptional level. Therefore, in this study we examined the ability of BPA and E2 to increase uterine weight and regulate transcription of these hsps and of heat shock factor (HSF)-1 in ovariectomized B6C3F1 mice at 6 or 24 h after a single subcutaneous injection of E2 (1 microg/kg) or BPA (100 mg/kg). The role of the ER and protein kinase C (PKC) in these E2 and BPA effects was evaluated by co-administration of the antiestrogen ICI 182,780 (5 mg/kg) or the PKC inhibitor GF 109203X (0.5 mg/kg), respectively. The results demonstrated ER involvement in uterine weight increases. Uterine hsp mRNA levels are increased by E2 and BPA through a direct effect on their transcription and/or, in the case of E2, through an increase in HSF-1 mRNA. PKC is involved in the BPA-induced increases in hsp90alpha mRNA levels. We conclude that E2 and BPA regulate hsp90alpha and hsp72alpha transcription via similar and distinct pathways.

Cloning and characterization of the promoter for murine 84-kDa heat-shock protein

The 90-kDa heat-shock (HS) proteins (HSP90) are members of the HSP family. Their synthesis is inducible by HS and a variety of stress signals. HSP90 is also abundant under normal physiological conditions and its synthesis can be regulated during growth and differentiation. Therefore, HSP90 is speculated to have important biological functions, in addition to its role in mediating stress responses. However, the mechanism(s) regulating hsp90 gene expression in nonstressed cells is poorly understood. As a prerequisite towards understanding the basis for hsp90 regulation, we have cloned and characterized the 5' flanking region of murine hsp84, one of two genes which code for HSP90 proteins. Full basal promoter activity of hsp84 was found to be associated with a 627-bp region immediately upstream from the transcription start point (tsp). Sequence analysis revealed several putative regulatory elements, including a HS element (HSE), an AP1-binding site (AP1), a cyclic AMP response element (CRE), and four stimulatory protein-1-binding sites (SP1). HS inducibility required the HSE which was bound by HS transcription factor-1(HSF-1) present in extracts prepared from cells exposed to HS. The HSE was not required for basal (non-HS) expression, but, interestingly, two protein-HSE complexes, devoid of HSF-1 and HSF-2, were formed under these conditions. The potential significance of these findings to the expression of hsp84 under normal physiological conditions is discussed.

HSP-72 synthesis is promoted by increase in [Ca2+]i or activation of G proteins but not pHi or cAMP

The family of 70-kDa heat-shock proteins (HSP-70) is evolutionarily highly conserved and has been shown to enhance cell survival from thermal injury. This study characterized HSP-72 induction in human epidermoid A-431 cells exposed to 45 degrees C for 10 min and determined the relationship between HSP-72, intracellular pH (pHi), adenosine 3',5'-cyclic monophosphate (cAMP), G proteins, and intracellular cytosolic free Ca2+ concentration ([Ca2+]i). Heat shock induced HSP-72 production, which was dependent on both temperature and the duration of heating. This HSP-72 induction was confirmed by Western blot analysis. HSP-72 levels in cells that had been heated then returned to 37 degrees C were elevated at 2 h (1.5 +/- 0.1 x control), reached a maximum at 8 h (2.7 +/- 0.1 x control), and remained above baseline for up to 4 days. Levels of HSP-72 mRNA, determined by dot-blot analysis, reached a maximum at 2 h and returned to baseline within 8 h. Both actinomycin D and cycloheximide blocked HSP-72 induction. Because heating causes intracellular acidification and increases in cAMP and [Ca2+]i, we studied the effect of pHi, cellular cAMP, and [Ca2+]i on HSP-72 induction. The reduction of pHi to 6.9 by acid loading did not affect the basal level of HSP-72 in unheated cells. Treatment with pertussis toxin, cholera toxin, or forskolin, but not 8-bromo-cAMP, 3-isobutyl-1-methylxanthine, or N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide potentiated heat-induced HSP-72 production. Inhibition of the heat-induced increase in [Ca2+]i attenuated, but failed to completely block, heat-induced HSP-72 production, mRNA synthesis, and the heat-shock transcriptional factor-heat-shock element binding complex formation, which suggests there are Ca(2+)-dependent and -independent processes involved in HSP-72 synthesis. Our results show that an increase in [Ca2+]i or activation of G proteins, but not pHi and cAMP, enhances HSP-72 induction.

Tissue-specific expression of heat shock proteins 70 and 90: potential implication for differential sensitivity of tissues to glucocorticoids

Heat shock proteins (hsps) and glucocorticoids are key elements of the organism's adaptive response to adverse physiological conditions. Glucocorticoids are pleiotropic hormones acting through receptor-mediated processes and eliciting tissue-specific biologic effects. The inactive form of the glucocorticoid receptor (GR) in the cytoplasm appears to be bound to hsps of the 90K family (hsp90 alpha and hsp90 beta). This interaction facilitates binding of glucocorticoid to its receptor, and depends on the relative amounts of the interacting components, GR and hsp90. To gain insight into the mechanisms of glucocorticoid regulation in a physiological context, the level of the hsp70/90 system in a panel of tissues, including testis, spleen, liver, thymus, pituitary, hypothalamus, hippocampus, brain cortex, pineal and adrenal, was examined by Western blotting. The hsp90 component showed greater variation (up to about forty-fold) relative to the less variable (up to about three-fold) hsp70 component of the system. The relative distribution of the hsp90 alpha and beta forms in the various tissues was also examined by a combination of Western and Northern blotting techniques. It was found that the alpha form predominated in the brain and the testis and the beta form predominated in the other peripheral organs. There was no relation between tissue hsp90 content and differential expression of either form. These findings suggest that tissue hsp90 content, an important physiological parameter of cellular homeostasis, may confer tissue specificity and sensitivity to glucocorticoids.

Relationship between 90-kilodalton heat shock protein, estrogen receptor, and progesterone receptor in human mammary tumors

To determine, if, as previously observed with rodent uterus, the 90 kilodalton heat shock protein (HSP-90) might be under estrogenic regulation in human mammary tumors, we analyzed its relationship with estrogen and progesterone receptors. A positive relationship between estrogen receptor and HSP-90 was observed when these were normalized to HSP-70 levels (to normalize for the intrinsic variations in each tumor, independent of any hormonal dependence).

Mapping of the mouse 86-kDa heat-shock protein expressed gene (Hsp86-1) on chromosome 12 and related genes on chromosomes 3, 4, 9, and 11

The HSP86 gene family in BALB/c, AKR/J, C58/J, and NFS/N inbred mice comprises an intron-containing expressed gene and, depending on the strain, two to four other HSP86-related members that are apparently processed pseudogenes. The expressed gene locus, Hsp86-1, was identified by its sequence identity with the mouse HSP86 cDNA coding region together with the presence of an intron at the same position as in the homologous human gene. Hsp86-1 was mapped 11.6 cM from the immunoglobulin heavy chain gene IgH on Chromosome 12 using an intersubspecies backcross. Two of the other loci that were common to all inbred strains tested, designated Hsp86-ps1 and Hsp86-ps2, were mapped to positions on Chromosomes 11 and 3, respectively. An HSP86-related locus specific to NFS/N and C58/J mice, designated Hsp86-ps3, was mapped on Chromosome 9. Also, an HSP86-related locus that was unique to NFS/N mice, designated Hsp86-ps4, was mapped to Chromosome 4.

Molecular cloning and characterization of two distinct hsp85 sequences from the steroid responsive fungus Achlya ambisexualis

In Achlya ambisexualis, hsp85 is one of the characteristic mycelial heat shock proteins induced in response to a rapid elevation in temperature (Silver et al. 1983). This heat shock protein has the same electrophoretic mobility on two-dimensional gels and is antigenically related to an 85 kDa steroid hormone-regulated protein which constitutes a component of the putative Achlya steroid hormone-receptor complex. We report here the isolation of two distinct, yet highly related, hsp85 gene sequences from Achlya genomic libraries. Northern analyses, using these two Achlya genomic sequences as probes, suggest that there are two hsp85 message population in Achlya and that at least one of these is regulated by the steroid hormone antheridiol.

Stage and lineage-regulated expression of two hsp90 transcripts during mouse germ cell differentiation and embryogenesis

The expression of members of the heat shock protein 90 (hsp90) gene family during testicular and embryonic development was investigated. Two different hsp90 transcripts were detected in RNA from mouse testis, approximately 3.2 kb and 2.9 kb in size, and were shown to exhibit cellular and developmental stage specificity of expression. The larger, more abundant transcript was expressed at high levels in the germinal compartment of the testis, particularly in germ cells in meiotic prophase. The smaller hsp90 transcript was expressed predominantly in the somatic compartment of the testis. Expression of the two hsp90 transcripts was observed in testes of other species, suggesting an important role for hsp90 in mammalian testicular function. In addition, expression of both hsp90 transcripts was detected in the embryonic and extra-embryonic compartments of mid-gestation embryos.

Characterization of the mouse 84-kD heat shock protein gene family

The nucleotide sequence of a 6-kb region containing the gene for mouse 84-kD heat shock protein, HSP84, was determined. The hsp84 gene codes for a 5,500-base transcript and consists of 11 exons and 10 introns, ranging in length from 94 to 357 bp and 85 to 1,271 bp, respectively. One of the exons codes for a stretch of highly charged amino acids with two known phosphorylation sites. The presence of numerous introns in the hsp84 gene suggests that synthesis of the HSP84 protein would be precluded during severe heat shock, since such conditions interfere with splicing. The first intron, which is the largest, is located at the exact boundary between the 5'-untranslated region and the coding region and contains a sequence homologous to the heat shock element (HSE), an enhancer that is a characteristic feature of heat-inducible genes. A 71% homology was found between a 569-bp stretch within the first intron of the hsp84 gene, which includes the HSE-like sequence, and a portion of the first intron of the previously reported sequence of the human hsp89 beta gene. The promoter region of the hsp84 gene contained G + C-rich upstream sequences, potential binding sites for transcription factor Sp1, and a canonical TATA box. The hsp84 gene family includes at least six different hsp84-related pseudogenes, which arose about 2-3 million years ago.