The genes encoding mammalian chaperonin 60 and chaperonin 10 are linked head-to-head and share a bidirectional promoter

Sensing, signaling and surviving mitochondrial stress

Mitochondrial fidelity is a key determinant of longevity and was found to be perturbed in a multitude of disease contexts ranging from neurodegeneration to heart failure. Tight homeostatic control of the mitochondrial proteome is a crucial aspect of mitochondrial function, which is severely complicated by the evolutionary origin and resulting peculiarities of the organelle. This is, on one hand, reflected by a range of basal quality control factors such as mitochondria-resident chaperones and proteases, that assist in import and folding of precursors as well as removal of aggregated proteins. On the other hand, stress causes the activation of several additional mechanisms that counteract any damage that may threaten mitochondrial function. Countermeasures depend on the location and intensity of the stress and on a range of factors that are equipped to sense and signal the nature of the encountered perturbation. Defective mitochondrial import activates mechanisms that combat the accumulation of precursors in the cytosol and the import pore. To resolve proteotoxic stress in the organelle interior, mitochondria depend on nuclear transcriptional programs, such as the mitochondrial unfolded protein response and the integrated stress response. If organelle damage is too severe, mitochondria signal for their own destruction in a process termed mitophagy, thereby preventing further harm to the mitochondrial network and allowing the cell to salvage their biological building blocks. Here, we provide an overview of how different types and intensities of stress activate distinct pathways aimed at preserving mitochondrial fidelity.

HSF1 is required for induction of mitochondrial chaperones during the mitochondrial unfolded protein response

The mitochondrial unfolded protein response (UPRmt ) is characterized by the transcriptional induction of mitochondrial chaperone and protease genes in response to impaired mitochondrial proteostasis and is regulated by ATF5 and CHOP in mammalian cells. However, the detailed mechanisms underlying the UPRmt are currently unclear. Here, we show that HSF1 is required for activation of mitochondrial chaperone genes, including HSP60, HSP10, and mtHSP70, in mouse embryonic fibroblasts during inhibition of matrix chaperone TRAP1, protease Lon, or electron transfer complex 1 activity. HSF1 bound constitutively to mitochondrial chaperone gene promoters, and we observed that its occupancy was remarkably enhanced at different levels during the UPRmt . Furthermore, HSF1 supported the maintenance of mitochondrial function under the same conditions. These results demonstrate that HSF1 is required for induction of mitochondrial chaperones during the UPRmt , and thus, it may be one of the guardians of mitochondrial function under conditions of impaired mitochondrial proteostasis.

Mitonuclear conflict and cooperation govern the integration of genotypes, phenotypes and environments

The mitonuclear genome is the most successful co-evolved mutualism in the history of life on Earth. The cross-talk between the mitochondrial and nuclear genomes has been shaped by conflict and cooperation for more than 1.5 billion years, yet this system has adapted to countless genomic reorganizations by each partner, and done so under changing environments that have placed dramatic biochemical and physiological pressures on evolving lineages. From putative anaerobic origins, mitochondria emerged as the defining aerobic organelle. During this transition, the two genomes resolved rules for sex determination and transmission that made uniparental inheritance the dominant, but not a universal pattern. Mitochondria are much more than energy-producing organelles and play crucial roles in nutrient and stress signalling that can alter how nuclear genes are expressed as phenotypes. All of these interactions are examples of genotype-by-environment (GxE) interactions, gene-by-gene (GxG) interactions (epistasis) or more generally context-dependent effects on the link between genotype and phenotype. We provide evidence from our own studies in Drosophila, and from those of other systems, that mitonuclear interactions—either conflicting or cooperative—are common features of GxE and GxG. We argue that mitonuclear interactions are an important model for how to better understand the pervasive context-dependent effects underlying the architecture of complex phenotypes. Future research in this area should focus on the quantitative genetic concept of effect size to place mitochondrial links to phenotype in a proper context.

This article is part of the theme issue ‘Linking the mitochondrial genotype to phenotype: a complex endeavour’.

The mitochondrial unfolded protein response and mitohormesis: a perspective on metabolic diseases

Mitochondria perform essential roles as crucial organelles for cellular and systemic energy homeostasis, and as signaling hubs, which coordinate nuclear transcriptional responses to the intra- and extra-cellular environment. Complex human diseases, including diabetes, obesity, fatty liver disease and aging-related degenerative diseases are associated with alterations in mitochondrial oxidative phosphorylation (OxPhos) function. However, a recent series of studies in animal models have revealed that an integrated response to tolerable mitochondrial stress appears to render cells less susceptible to subsequent aging processes and metabolic stresses, which is a key feature of mitohormesis. The mitochondrial unfolded protein response (UPRmt) is a central part of the mitohormetic response and is a retrograde signaling pathway, which utilizes the mitochondria-to-nucleus communication network. Our understanding of the UPRmt has contributed to elucidating the role of mitochondria in metabolic adaptation and lifespan regulation. In this review, we discuss and integrate recent data from the literature on the present status of mitochondrial OxPhos function in the development of metabolic diseases, relying on evidence from human and other animal studies, which points to alterations in mitochondrial function as a key factor in the regulation of metabolic diseases and conclude with a discussion on the specific roles of UPRmt and mitohormesis as a novel therapeutic strategy for the treatment of obesity and insulin resistance.

The different axes of the mammalian mitochondrial unfolded protein response

Mitochondria are sensitive to numerous environmental stresses, which can lead to activation of mitochondrial stress responses (MSRs). Of particular recent interest has been the mitochondrial unfolded protein response (UPR^mt), activated to restore protein homeostasis (proteostasis) upon mitochondrial protein misfolding. Several axes of the UPR^mt have been described, creating some confusion as to the nature of the different responses. While distinct molecularly, these different axes are likely mutually beneficial and activated in parallel. This review aims at describing and distinguishing the different mammalian MSR/UPR^mt axes to define key processes and members and to examine the involvement of protein misfolding.

Effects of a Mutation in the HSPE1 Gene Encoding the Mitochondrial Co-chaperonin HSP10 and Its Potential Association with a Neurological and Developmental Disorder

We here report molecular investigations of a missense mutation in the HSPE1 gene encoding the HSP10 subunit of the HSP60/ HSP10 chaperonin complex that assists protein folding in the mitochondrial matrix. The mutation was identified in an infant who came to clinical attention due to infantile spasms at 3 months of age. Clinical exome sequencing revealed heterozygosity for a HSPE1 NM_002157.2:c.217C>T de novo mutation causing replacement of leucine with phenylalanine at position 73 of the HSP10 protein. This variation has never been observed in public exome sequencing databases or the literature. To evaluate whether the mutation may be disease-associated we investigated its effects by in vitro and ex vivo studies. Our in vitro studies indicated that the purified mutant protein was functional, yet its thermal stability, spontaneous refolding propensity, and resistance to proteolytic treatment were profoundly impaired. Mass spectrometric analysis of patient fibroblasts revealed barely detectable levels of HSP10-p.Leu73Phe protein resulting in an almost 2-fold decrease of the ratio of HSP10 to HSP60 subunits. Amounts of the mitochondrial superoxide dismutase SOD2, a protein whose folding is known to strongly depend on the HSP60/HSP10 complex, were decreased to approximately 20% in patient fibroblasts in spite of unchanged SOD2 transcript levels. As a likely consequence, mitochondrial superoxide levels were increased about 2-fold. Although, we cannot exclude other causative or contributing factors, our experimental data support the notion that the HSP10-p.Leu73Phe mutation could be the cause or a strong contributing factor for the disorder in the described patient.

Bovine TWINKLE and mitochondrial ribosomal protein L43 genes are regulated by an evolutionary conserved bidirectional promoter

TWINKLE is a mitochondrial DNA helicase playing an important role in mitochondrial DNA replication. In human, mutations in this gene cause progressive external ophtalmoplegia and mitochondrial DNA depletion syndrome-7. TWINKLE is well conserved among multicellular eukaryotes and is believed to be a key regulator of mitochondrial DNA copy number in mammals. Despite its involvement in several diseases and its important function in mitochondrial DNA metabolism, nothing is known about the regulation of the expression of TWINKLE. We have analysed the 5'-flanking genomic region of the bovine TWINKLE gene and found it was localised adjacent to the MRPL43 gene in a head-to-head orientation, suggesting that both genes are regulated by a shared bidirectional promoter. The bovine 75-bp long intergenic region shows substantial homology across different species and contains several conserved putative transcription factor binding sites. A TATA box, however, was lacking. Using a dual fluorescent reporter system and transient transfection assays, we have analysed the bovine intergenic region between TWINKLE and MRPL43. This small genomic fragment showed a bidirectional promoter activity. As the TWINKLE/MRPL43 bidirectional promoter tested was highly conserved, it is likely that the results we obtained here in cattle may be extended to the other species.

Molecular characterization and promoter analysis of crustacean heat shock protein 10 in Scylla paramemosain

Heat shock proteins (Hsps) are an evolutionarily conserved group of molecules present in all eukaryotic and prokaryotic organisms. Hsp10 and Hsp60 were originally described as the essential mitochondrial proteins involved in protein folding. Recent studies demonstrate that Hsp10 has additional roles including immune modulation. In our study, an homologous Hsp10 (Sp-Hsp10) was identified in the mud crab Scylla paramemosain, and its genomic DNA organization was determined. The cDNA sequence of Sp-Hsp10 contains an open reading frame of 309 bp, encoding a putative protein of 102 amino acid residues with approximately 10 kDa. The Sp-Hsp10 gene is located next to the Sp-Hsp60 gene and shares a 1916-bp intergenic region. The promoter activity of the Sp-Hsp10 flanking gene was analyzed using luciferase reporter assays in transfected endothelial progenitor cells. The upregulation of Sp-Hsp10 expression was detected after exposure of hemocytes to a heat shock of 1 h at 37 °C compared with unstressed hemocytes raised at 20 °C. To our knowledge, this is the first report characterizing the genomic organization of a new Hsp10 in a crustacean.

Mitochondrial stress: a bridge between mitochondrial dysfunction and metabolic diseases?

Under pathophysiological conditions such as obesity, excessive oxidation of nutrients may induce mitochondrial stress, leading to mitochondrial unfolded protein response (UPR(mt)) and initiation of a retrograde stress signaling pathway. Defects in the UPR(mt) and the retrograde signaling pathways may disrupt the integrity and homeostasis of the mitochondria, resulting in endoplasmic reticulum stress and insulin resistance. Improving the capacity of mitochondria to reduce stress may be an effective approach to improve mitochondria function and to suppress obesity-induced metabolic disorders such as insulin resistance and type 2 diabetes.

The mechanism underlying the synergetic hypocholesterolemic effect of sesamin and α-tocopherol in rats fed a high-cholesterol diet

Sesamin is a major lignan in sesame seed. We confirmed that ingestion of sesamin and α-tocopherol synergistically reduced the concentration of blood cholesterol in rats given a high-cholesterol diet. To elucidate the molecular mechanism behind this effect, we analyzed the gene-expression profiles in rat liver after co-ingestion of sesamin and α-tocopherol. Six-week-old male Sprague-Dawley rats were fed a 1% cholesterol diet (HC) or HC containing 0.2% sesamin, 1% α-tocopherol or sesamin + α-tocopherol for 10 days. Blood samples were collected on days 1, 3, 7, and 10 and livers were excised on day 10. The gene expressions of ATP-binding cassette, sub-family G (WHITE), members 5 (ABCG5) and 8 (ABCG8) were significantly increased, while the gene expression of apolipoprotein (Apo) A4 was significantly decreased. ABCG5 and ABCG8 form a functional heterodimer that acts as a cholesterol efflux transporter, which contributes to the excretion of cholesterol from the liver. ApoA4 controls the secretion of ApoB, which is a component of low-density-lipoprotein cholesterol. These studies indicate that the cholesterol-lowering mechanism underlying the effects of co-ingestion of sesamin and α-tocopherol might be attributable to increased biliary excretion of cholesterol and reduced ApoB secretion into the bloodstream.

Heat shock proteins: cellular and molecular mechanisms in the central nervous system

Emerging evidence indicates that heat shock proteins (HSPs) are critical regulators in normal neural physiological function as well as in cell stress responses. The functions of HSPs represent an enormous and diverse range of cellular activities, far beyond the originally identified roles in protein folding and chaperoning. HSPs are now understood to be involved in processes such as synaptic transmission, autophagy, ER stress response, protein kinase and cell death signaling. In addition, manipulation of HSPs has robust effects on the fate of cells in neurological injury and disease states. The ongoing exploration of multiple HSP superfamilies has underscored the pluripotent nature of HSPs in the cellular context, and has demanded the recent revamping of the nomenclature referring to these families to reflect a re-organization based on structure and function. In keeping with this re-organization, we first discuss the HSP superfamilies in terms of protein structure, regulation, expression and distribution in the brain. We then explore major cellular functions of HSPs that are relevant to neural physiological states, and from there we discuss known and proposed HSP impacts on major neurological disease states. This review article presents a three-part discussion on the array of HSP families relevant to neuronal tissue, their cellular functions, and the exploration of therapeutic targets of these proteins in the context of neurological diseases.

Mitochondrial DNA variability modulates mRNA and intra-mitochondrial protein levels of HSP60 and HSP75: experimental evidence from cybrid lines

To explore possible relationships between mitochondrial DNA (mtDNA) polymorphism and the expression levels of stress-responder nuclear genes we assembled five cybrid cell lines by repopulating 143B.TK(-) cells, depleted of their own mtDNA (Rho(0) cells), with foreign mitochondria with different mtDNA sequences (lines H, J, T, U, X). We evaluated, at both basal and under heat stress conditions, gene expression (mRNA) and intra-mitochondrial protein levels of HSP60 and HSP75, two key components in cellular stress response. At basal conditions, the levels of HSP60 and HSP75 mRNA were lower in one cybrid (H) than in the others (p = 0.005 and p = 0.001, respectively). Under stress conditions, the H line over-expressed both genes, so that the inter-cybrid difference was abolished. Moreover, the HSP60 intra-mitochondrial protein levels differed among the cybrid lines (p = 0.001), with levels higher in H than in the other cybrid lines. On the whole, our results provide further experimental evidence that mtDNA variability influences the cell response to stressful conditions by modulating components involved in this response. Sentence summary of the article: the results reported in the present study provide important experimental evidence that in human cells mtDNA variability is able to influence the cellular response to heat stress by modulating both the transcription of genes involved in this response and their intra-mitochondrial protein levels.

LEDGF binding to stress response element increases alphaB-crystallin expression in astrocytes with oxidative stress

AlphaB-crystallin, known as a vertebrate lens protein, is a member of the small heat shock proteins (sHSP). AlphaB-crystallin is abundantly expressed in the vertebrate lens and striated muscles and it is also expressed constitutively in other tissues including the central nervous system (CNS). In our previous report, we showed alphaB-crystallin induction in activated astrocytes, which are enriched in the penumbra after transient focal cerebral ischemia. We also reported that alphaB-crystallin is significantly induced in astrocytes in the CA3 region of the hippocampus following KA-induced seizure. Here, we report that the expression of alphaB-crystallin is upregulated in H2O2-treated primary astrocyte cultures, which was prepared from newborn male Sprague-Dawley rats and that the proximal 408 bp of the alphaB-crystallin promoter harboring stress response element (STRE) is responsible for this induction. This effect of H2O2 was found to be virtually abolished by introducing mutations into STRE, and these mutations also impaired increased lens epithelial derived growth factor (LEDGF) binding to STRE after H2O2 treatment. Moreover, LEDGF was induced in primary astrocyte cultures after H2O2 treatment and alphaB-crystallin induction was significantly suppressed by transfecting small interfering RNA (siRNA) targeting LEDGF. Together these results indicate that the H2O2-induced upregulations of alphaB-crystallin in astrocytes are mediated by the LEDGF-STRE interaction on alphaB-crystallin promoter.

Interferon-gamma-induced expressions of heat shock protein 60 and heat shock protein 10 in C6 astroglioma cells: identification of the signal transducers and activators of transcription 3-binding site in bidirectional promoter

Heat shock protein 60 and heat shock protein 10 are mitochondrial chaperonin proteins. Here, we report that the expressions of heat shock protein 60 and heat shock protein 10 were upregulated in interferon-gamma-treated C6 astroglioma cells, and the 582 bp in the bidirectional promoter of the heat shock protein 60 and heat shock protein 10 genes is responsible for interferon-gamma-induced induction. The induction of heat shock protein 60 and heat shock protein 10 by interferon-gamma was virtually abolished by introducing mutations into the putative signal transducers and activators of transcription 3-response element in the promoter, and the same mutation impaired increment of the signal transducers and activators of transcription 3-binding after interferon-gamma treatment. Moreover, Rac1 GTPase was required for maximal heat shock protein 10 and heat shock protein 60 inductions by interferon-gamma. These results suggest that interferon-gamma-induced upregulations of heat shock protein 60 and heat shock protein 10 in C6 astroglioma cells are mediated by the signal transducers and activators of transcription 3-binding site, localized in the bidirectional promoter.

Heat Shock Protein 60 or 70 Activates Nitric-oxide Synthase (NOS) I- and Inhibits NOS II-associated Signaling and Depresses the Mitochondrial Apoptotic Cascade during Brain Stem Death

The cellular and molecular basis of brain stem death remains an enigma. As the origin of a "life-and-death" signal that reflects the progression toward brain stem death, the rostral ventrolateral medulla (RVLM) is a suitable neural substrate for mechanistic delineation of this phenomenon. Here, we evaluated the hypothesis that heat shock proteins (HSPs) play a neuroprotective role in the RVLM during brain stem death and delineated the underlying mechanisms, using a clinically relevant animal model that employed the organophosphate pesticide mevinphos (Mev) as the experimental insult. In Sprague-Dawley rats, proteomic, Western blot, and real-time PCR analyses demonstrated that Mev induced de novo synthesis of HSP60 or HSP70 in the RVLM without affecting HSP90 level. Loss-of-function manipulations of HSP60 or HSP70 in the RVLM using anti-serum or antisense oligonucleotide potentiated Mev-elicited cardiovascular depression alongside reduced nitric-oxide synthase (NOS) I/protein kinase G signaling, enhanced NOS II/peroxynitrite cascade, intensified nucleosomal DNA fragmentation, elevated cytoplasmic histone-associated DNA fragments or activated caspase-3, and augmented the cytochrome c/caspase-3 cascade of apoptotic signaling in the RVLM. Co-immunoprecipitation experiments further revealed a progressive increase in the complex formed between HSP60 and mitochondrial or cytosolic Bax or mitochondrial Bcl-2 during Mev intoxication, alongside a dissociation of the cytosolic HSP60-Bcl-2 complex. We conclude that HSP60 and HSP70 confer neuroprotection against Mev intoxication by ameliorating cardiovascular depression via an anti-apoptotic action in the RVLM. The possible underlying intracellular processes include enhancing NOS I/protein kinase G signaling and inhibiting the NOS II/peroxynitrite cascade. In addition, HSP60 exerts its effects against apoptosis by blunting Mev-induced activation of the Bax/cytochrome c/caspase-3 cascade.

The promoter of the rat 5alpha-reductase type 1 gene is bidirectional and Sp1-dependent

In many androgen target tissues, testosterone is reduced to the more potent androgen, dihydrotestosterone, by steroid 5alpha-reductase. Two isoforms of 5alpha-reductase, type 1 and type 2, have been cloned. They are differentially expressed and regulated. To determine the mechanisms of regulation of 5alpha-reductase type 1 expression, we have cloned its 5'upstream region and defined its promoter. The proximal 5'upstream region of 5alpha-reductase type 1 displays all the features of a CpG island and has numerous Sp1 binding sites. By transient transfection assays, we have identified a bidirectional promoter activity in this region; this activity was highest in the negative orientation, in the direction of the methyltransferase Nsun2 (predicted) gene. Promoter activity, in either orientation, was lost in Sp1 deficient cells but was rescued following co-transfection with a Sp1 expression vector. Thus, the 5'upstream region of rat 5alpha-reductase type 1 contains a bidirectional promoter with an activity that is Sp1-dependent.

NO-induced downregulation of HSP10 and HSP60 expression in the postischemic brain

Heat shock protein 60 (HSP60) and HSP10 are mitochondrial chaperonin proteins and are responsible for the folding of newly imported proteins and 13 polypeptides encoded by mitochondrial DNA. The expressions of HSP60 and HSP10 are regulated simultaneously because these genes are localized head to head on chromosome, separated by a bidirectional promoter, which harbors heat shock element (HSE), CHOP, STAT3, and SP1 binding sites. In the present study, we show that the expressions of HSP60 and HSP10 in the brain after middle cerebral artery occlusion (MCAO) are induced significantly. Interestingly, the expressions of HSP60 and HSP10 were further upregulated by the administration of aminoguanidine (AG), an inhibitor of the inducible nitric oxide synthase (iNOS), which is known to reduce ischemic damage in an animal model after MCAO. The results obtained in the present study suggest that HSP10 and HSP60 are induced in the postischemic brain, yet are downregulated by NO generated from 12 hr after MCAO/reperfusion. The downregulation of HSP60 and HSP10 by NO were confirmed in vitro, wherein HSP10 and SHP60 expressions were increased by treatment of LPS and IFN gamma (LPS/INF gamma) in C6 astroglioma cells and further upregulated by NMMA, another iNOS inhibitor. Reporter gene analysis combined with deletion and mutation studies showed that STAT3 binding site in the bidirectional promoter is responsible for LPS/INF gamma-induced upregulation and for downregulation by NO. Our results indicated that NO suppresses HSP60 and HSP10 inductions in the postischemic brain by suppressing STAT3 binding to its recognition site.

Heat shock protein 60 in rostral ventrolateral medulla reduces cardiovascular fatality during endotoxaemia in the rat

The rostral ventrolateral medulla (RVLM) is the origin of a 'life-and-death' signal that reflects central cardiovascular regulatory failure during brain stem death. Using an experimental endotoxaemia model, we evaluated the hypothesis that the 60 kDa heat shock protein 60 (HSP60) reduces cardiovascular fatality during brain stem death via an anti-apoptotic action in the RVLM. In Sprague-Dawley rats maintained under propofol anaesthesia, proteomic or Western blot analysis revealed a progressive augmentation of HSP60 expression in the RVLM after intravenous administration of Escherichia coli lipopolysaccharide (30 mg kg(-1)). Pretreatment with a microinjection of actinomycin D or cycloheximide into bilateral RVLM significantly blunted this HSP60 increase, whereas real-time PCR showed progressive augmentation of hsp60 mRNA. Intriguingly, superimposed on the augmented expression was a progressive decline in mitochondrial, or elevation in cytosolic, HSP60 in ventrolateral medulla. Loss-of-function manipulations in the RVLM using anti-HSP60 antiserum or antisense hsp60 oligonucleotide exacerbated mortality by potentiating the cardiovascular depression during experimental endotoxaemia, alongside intensified nucleosomal DNA fragmentation, elevated cytoplasmic histone-associated DNA fragments or augmented cytochromec-caspase-3 cascade of apoptotic signalling in the RVLM. Immunoprecipitation coupled with immunoblot analysis further revealed a progressive increase in the complex formed between HSP60 and mitochondrial or cytosolic Bax or mitochondrial Bcl-2 during endotoxaemia, alongside a dissociation of the cytosolic HSP60-Bcl-2 complex. We conclude that HSP60 redistributed from mitochondrion to cytosol in the RVLM confers neuroprotection against fatal cardiovascular depression during endotoxaemia via reduced activation of the cytochrome c-caspase-3 cascade of apoptotic signalling through enhanced interactions with mitochondrial or cytosolic Bax or Bcl-2.

Regulation of the ferritin heavy-chain homologue gene in the yellow fever mosquito, Aedes aegypti

In the yellow fever mosquito Aedes aegypti, the ferritin heavy-chain homologue (HCH) gene is induced by blood feeding. This suggests that ferritin may serve as a cytotoxic protector against the oxidative challenge of the blood meal and may be essential for the survival of the insect. In this study, various cis-acting elements for the gene were identified and mapped. Transfection assays showed that the strength and activity of a subset of these elements are orientation-dependent. The shift observed for the ferritin HCH cis-acting elements is unique among known ferritin genes. DNase I footprinting data together with Transfac analyses identified a number of putative sites known for their involvement in developmental and cell proliferation processes.

The DNA element controlling expression of the varicella-zoster virus open reading frame 28 and 29 genes consists of two divergent unidirectional promoters which have a common USF site

The mechanism of the divergent expression of the varicella-zoster virus (VZV) ORF 28 and ORF 29 genes from a common intergenic DNA element, the ORF 28/29 promoter, is of interest based on the observation that both genes are expressed during VZV lytic infection but only the ORF 29 gene is expressed in latently infected neurons. In the work presented here, expression driven by the ORF 28/29 intergenic region was examined. We found that the promoter activity towards the ORF 29 direction is more responsive to activation by the major viral transactivator IE62 than that towards the ORF 28 direction in the context of our experimental system. Analysis of the functional DNA elements involved in IE62 activation of the bidirectional ORF 28/29 regulatory element revealed that in both transfected and VZV-superinfected cells it is a fusion of two unidirectional promoters overlapping an essential USF binding site but with distinct TATA elements. A single TATA element directs expression in the ORF 28 direction, whereas the two TATA elements directing ORF 29 gene expression are alternatively and differentially utilized for transcription initiation. We also identified an Sp1 site localized proximal to the ORF 28 gene which functions as an activator element for expression in both directions. These results indicate that the ORF 28 and ORF 29 genes can be expressed either coordinately or independently and that the observed expression of only the ORF 29 gene during VZV latency may involve neuron-specific cellular factors and/or structural aspects of the latent viral genome.

Characterization of the bidirectional promoter region between the human genes encoding VLCAD and PSD-95

Bidirectional promoters are widely known among lower organisms but rare in mammals. A shared promoter between the two human genes encoding very long chain acyl-CoA dehydrogenase (VLCAD) and postsynaptic density protein 95 (PSD-95) is an ideal model to investigate bidirectional transcription in mammals. VLCAD associates with the inner mitochondrial membrane and catalyzes the initial step in mitochondrial long-chain fatty acid beta-oxidation. PSD-95, a component protein of the PSD, plays an essential role in clustering the transmembrane proteins in synaptic membranes. Interestingly, the human genes encoding VLCAD (ACADVL) and PSD-95 (DLG4) are adjacently located in the head-to-head orientation on chromosome 17p. The transcribed regions of the two genes overlap, while the two transcription start sites stand approximately 220 bp apart. To analyze the common transcriptional control region shared by the two genes, we generated serial promoter partial deletion constructs using firefly luciferase as the reporter gene. Our results showed that the essential promoter activity of PSD-95 is carried within an approximately 400-bp region, which covers the entire approximately 270-bp minimal promoter of VLCAD. The results from di-(2-ethylhexyl) phthalate (DEHP)-treated HepG2 cells revealed that the minimal VLCAD promoter is able to up-regulate VLCAD expression in response to DEHP treatment. Site-directed mutagenesis experiments showed that a mutated activator protein 2-binding site markedly reduced the transcriptional activity of both promoters and abolished the minimal VLCAD promoter's response to DEHP treatment.

Expression and genomic organization of the zebrafish chaperonin gene complex

Chaperonin 10 and chaperonin 60 monomers exist within the multimeric mitochondrial chaperonin folding complex with a stoichiometry of 2:1. This complex is located in the mitochondrial matrix, where it aids in the folding and acquisition of the tertiary structure of proteins. We have previously isolated the cpn10 cDNA in zebrafish (Danio rerio), and demonstrated that it is ubiquitously expressed during embryonic development and transcriptionally upregulated after exposure to heat shock. In the present study, we have isolated a cDNA encoding chaperonin 60 (cpn60) from zebrafish, and have shown that it is similarly expressed uniformly and ubiquitously throughout early embryonic development of zebrafish. Upregulation of cpn60 expression was also observed after exposure of zebrafish embryos to a heat shock of 1 h at 37 degrees C compared with control embryos raised at 27 degrees C. The induction of the cpn60 heat shock response was greatest after 1 h of heat shock, whereas significant decreases of cpn60 mRNA were observed within 2 h following a return to 27 degrees C. We subsequently isolated genomic DNA sequences for both of these genes, and show that they are also arranged in a head-to-head organization and share a common bidirectional promoter that contains a single heat shock element (HSE). Our database analysis shows that this head-to-head organization is also found in human (Homo sapiens), rat (Rattus norvegicus), pufferfish (Fugu rubripes), and Caenorhabditis elegans, but not in Drosophila or yeast (Saccharomyces cerevisiae). The data suggest that the genomic organization of the cpn gene complex has been conserved across the vertebrates.

A mitochondrial specific stress response in mammalian cells

Cells respond to a wide variety of stresses through the transcriptional activation of genes that harbour stress elements within their promoters. While many of these elements are shared by genes encoding proteins representative of all subcellular compartments, cells can also respond to stresses that are specific to individual organelles, such as the endoplasmic reticulum un folded protein response. Here we report on the discovery and characterization of a mitochondrial stress response in mammalian cells. We find that the accumulation of unfolded protein within the mitochondrial matrix results in the transcriptional upregulation of nuclear genes encoding mitochondrial stress proteins such as chaperonin 60, chaperonin 10, mtDnaJ and ClpP, but not those encoding stress proteins of the endoplasmic reticulum. Analysis of the chaperonin 60/10 bidirectional promoter identified a CHOP element as the mitochondrial stress response element. Dominant-negative mutant forms of CHOP and overexpression of CHOP revealed that this transcription factor, in association with C/EBPbeta, regulates expression of mitochondrial stress genes in response to the accumulation of unfolded proteins.

Import and assembly of proteins into mitochondria of mammalian cells

Most of our knowledge regarding the process of protein import into mitochondria has come from research employing fungal systems. This review outlines recent advances in our understanding of this process in mammalian cells. In particular, we focus on the characterisation of cytosolic molecular chaperones that are involved in binding to mitochondrial-targeted preproteins, as well as the identification of both conserved and novel subunits of the import machineries of the outer and inner mitochondrial membranes. We also discuss diseases associated with defects in import and assembly of mitochondrial proteins and what is currently known about the regulation of import in mammals.

Comparative genome analysis of potential regulatory elements in the ABCG5-ABCG8 gene cluster

The excretion of sterols from the liver and intestine is regulated by the ABCG5 and ABCG8 transporters. To identify potential regulatory elements, 152 kb of the human ABCG5-ABCG8 gene cluster was sequenced and comparative genome analysis was performed. The two genes are oriented in a head-to-head configuration and are separated by a 374-bp intergenic region, which is highly conserved among several species. Using a reporter construct, the intergenic region was found to act as a bidirectional promoter. A conserved GATA site in the intergenic region was shown by site-directed mutagenesis to act as a repressor for the ABCG5 promoter. The intergenic region was also shown to be partially responsive to treatment by LXR agonists. In summary, several potential regulatory elements were found for the ABCG5 and ABCG8 genes, and the intergenic region was found to act as a bidirectional promoter.

Expression of the chaperonin 10 gene during zebrafish development

We have isolated a cDNA encoding chaperonin 10 (cpn10) from the zebrafish. Using northern, western, and in situ hybridization analysis, we observed that the cpn10 gene is expressed uniformly and ubiquitously throughout embryonic development of the zebrafish. Upregulation of cpn10 expression was observed following exposure of zebrafish embryos to a heat shock of 1 hour at 37 degrees C compared to control embryos raised at 27 degrees C. The extracellular form of Cpn10 called early pregnancy factor (EPF), found in the serum of pregnant mammals, was not detected in the serum of either male or female zebrafish. These expression studies suggest that Cpn10 plays a general role in zebrafish development as well as being consistent with the hypothesis that EPF is involved in the embryo implantation process in mammals.

Identification of highly conserved genes: SNZ and SNO in the marine sponge Suberites domuncula: their gene structure and promoter activity in mammalian cells(1)

Recently, we reported that cells from the sponge Suberites domuncula respond to ethylene with an increase in intracellular Ca(2+) level [Ca(2+)](i), and with an upregulation of the expression of (at least) two genes, a Ca(2+)/calmodulin-dependent protein kinase and the potential ethylene-responsive gene, termed SDSNZERR (A. Krasko, H.C. Schröder, S. Perovic, R. Steffen, M. Kruse, W. Reichert, I.M. Müller, W.E.G. Müller, J. Biol. Chem. 274 (1999)). Here, we describe for the first time that also mammalian (3T3) cells respond to ethylene, generated by ethephon, with an immediate and transient, strong increase in [Ca(2+)](i). Next, the promoter for the sponge SDSNZERR gene was isolated from S. domuncula. It was found that the SDSNZERR gene is positioned adjacent to the SNZ-related gene (SNZ-proximal open reading frame) (SDSNO) and linked, as in Saccharomyces cerevisiae, in a head-to-head manner. Until now, neither homologues nor orthologues of these two genes have been identified in higher metazoan phyla. The full-length genes share a bidirectional promoter. 3T3 cells were transfected with this promoter; the activity of the SDSNZERR promoter was strong and twice as high as that of the SV40 promoter, while the SDSNO promoter was less active. Surprisingly, the activity of the SDSNZERR promoter could not be modulated by ethylene or salicylic acid while it is strongly upregulated, by 4-fold, under serum-starved conditions. It is concluded that the modulation of the level of [Ca(2+)](i) by ethylene in mammalian cells is not correlated with an upregulation of the ethylene-responsive gene SDSNZERR. The data indicate that in mammalian cells, the activity of the SDSNZERR promoter is associated with the repression of serum-mediated growth arrest.

Expression of the chaperonin 10 gene during zebrafish development

We have isolated a cDNA encoding chaperonin 10 (cpn10) from the zebrafish. Using northern, western, and in situ hybridization analysis, we observed that the cpn10 gene is expressed uniformly and ubiquitously throughout embryonic development of the zebrafish. Upregulation of cpn10 expression was observed following exposure of zebrafish embryos to a heat shock of 1 hour at 37 degrees C compared to control embryos raised at 27 degrees C. The extracellular form of Cpn10 called early pregnancy factor (EPF), found in the serum of pregnant mammals, was not detected in the serum of either male or female zebrafish. These expression studies suggest that Cpn10 plays a general role in zebrafish development as well as being consistent with the hypothesis that EPF is involved in the embryo implantation process in mammals.

Transcriptional activation of mouse cytosolic chaperonin CCT subunit genes by heat shock factors HSF1 and HSF2

The chaperonin containing TCP-1 (CCT) is a eukaryotic molecular chaperone consisting of eight subunit species and assists in the folding of cytosolic proteins. We show here that all eight mouse CCT subunit genes contain sequences called heat shock elements for binding heat shock transcription factors (HSFs) by electrophoretic mobility shift assays and that these genes are transcriptionally activated by HSFs in reporter gene assays using HeLa cells transiently overexpressing HSFs. These results suggest that HSF1 and/or HSF2 play a role in Cct gene expression.

Sequence analysis and heterologous expression of the groE genes from Thermoanaerobacter sp. Rt8.G4

The groE homologous genes of the anaerobic thermophile Thermoanaerobacter sp. Rt8.G4 (TRt) have been isolated, sequenced and analysed. The TRt groES and groEL encode subunits of chaperonin 10 (Cpn10) and chaperonin 60 (Cpn60) of 94 and 541 amino acids, respectively, and are arranged in that order forming the open reading frames (ORFs) of a bicistronic operon. A controlling inverted repeat of chaperone expression (CIRCE) element lies between the consensus promoter of the operon and TRt groES. At optimum growth temperature (65 degreesC) the chaperonins of TRt are expressed, but production of Cpn60 increases significantly following temperature increases of 3-10 degreesC. Functionally intact recombinant TRt chaperonins were produced in Escherichia coli. However, owing to codon incompatibility, replacement of consecutive AGA codons in the gene encoding TRt Cpn60 was necessary for optimum expression in this heterologous host.