Heat Shock Response Activation Exacerbates Inclusion Body Formation in a Cellular Model of Huntington Disease

The cellular heat shock response (HSR) protects cells from toxicity associated with defective protein folding, and this pathway is widely viewed as a potential pharmacological target to treat neurodegenerative diseases linked to protein aggregation. Here we show that the HSR is not activated by mutant huntingtin (HTT) even in cells selected for the highest expression levels and for the presence of inclusion bodies containing aggregated protein. Surprisingly, HSR activation by HSF1 overexpression or by administration of a small molecule activator lowers the concentration threshold at which HTT forms inclusion bodies in cells expressing aggregation-prone, polyglutamine-expanded fragments of HTT. These data suggest that the HSR does not mitigate inclusion body formation.     

SirT1 regulates energy metabolism and response to caloric restriction in mice

The yeast sir2 gene and its orthologues in Drosophila and C. elegans have well-established roles in lifespan determination and response to caloric restriction. We have studied mice carrying two null alleles for SirT1, the mammalian orthologue of sir2, and found that these animals inefficiently utilize ingested food. These mice are hypermetabolic, contain inefficient liver mitochondria, and have elevated rates of lipid oxidation. When challenged with a 40% reduction in caloric intake, normal mice maintained their metabolic rate and increased their physical activity while the metabolic rate of SirT1-null mice dropped and their activity did not increase. Moreover, CR did not extend lifespan of SirT1-null mice. Thus, SirT1 is an important regulator of energy metabolism and, like its orthologues from simpler eukaryotes, the SirT1 protein appears to be required for a normal response to caloric restriction.     

Convergence of Molecular,Modeling, and Systems Approaches for an Understanding of the Escherichia coli Heat Shock Response

Summary: The heat shock response (HSR) is a homeostatic response that maintains the proper protein-folding environment in the cell. This response is universal, and many of its components are well conserved from bacteria to humans. In this review, we focus on the regulation of one of the most well-characterized HSRs, that of Escherichia coli. We show that even for this simple model organism, we still do not fully understand the central component of heat shock regulation, a chaperone-mediated negative feedback loop. In addition, we review other components that contribute to the regulation of the HSR in E. coli and discuss how these additional components contribute to regulation. Finally, we discuss recent genomic experiments that reveal additional functional aspects of the HSR.     

The Membrane-Associated Transient Receptor Potential Vanilloid Channel Is the Central Heat Shock Receptor Controlling the Cellular Heat Shock Response in Epithelial Cells

The heat shock response (HSR) is a highly conserved molecular response to various types of stresses, including heat shock, during which heat-shock proteins (Hsps) are produced to prevent and repair damages in labile proteins and membranes. In cells, protein unfolding in the cytoplasm is thought to directly enable the activation of the heat shock factor 1 (HSF-1), however, recent work supports the activation of the HSR via an increase in the fluidity of specific membrane domains, leading to activation of heat-shock genes. Our findings support the existence of a plasma membrane-dependent mechanism of HSF-1 activation in animal cells, which is initiated by a membrane-associated transient receptor potential vanilloid receptor (TRPV). We found in various non-cancerous and cancerous mammalian epithelial cells that the TRPV1 agonists, capsaicin and resiniferatoxin (RTX), upregulated the accumulation of Hsp70, Hsp90 and Hsp27 and Hsp70 and Hsp90 respectively, while the TRPV1 antagonists, capsazepine and AMG-9810, attenuated the accumulation of Hsp70, Hsp90 and Hsp27 and Hsp70, Hsp90, respectively. Capsaicin was also shown to activate HSF-1. These findings suggest that heat-sensing and signaling in mammalian cells is dependent on TRPV channels in the plasma membrane. Thus, TRPV channels may be important drug targets to inhibit or restore the cellular stress response in diseases with defective cellular proteins, such as cancer, inflammation and aging.     

Identification of a Tissue-Selective Heat Shock Response Regulatory Network

The heat shock response (HSR) is essential to survive acute proteotoxic stress and has been studied extensively in unicellular organisms and tissue culture cells, but to a lesser extent in intact metazoan animals. To identify the regulatory pathways that control the HSR in Caenorhabditis elegans, we performed a genome-wide RNAi screen and identified 59 genes corresponding to 7 positive activators required for the HSR and 52 negative regulators whose knockdown leads to constitutive activation of the HSR. These modifiers function in specific steps of gene expression, protein synthesis, protein folding, trafficking, and protein clearance, and comprise the metazoan heat shock regulatory network (HSN). Whereas the positive regulators function in all tissues of C. elegans, nearly all of the negative regulators exhibited tissue-selective effects. Knockdown of the subunits of the proteasome strongly induces HS reporter expression only in the intestine and spermatheca but not in muscle cells, while knockdown of subunits of the TRiC/CCT chaperonin induces HS reporter expression only in muscle cells. Yet, both the proteasome and TRiC/CCT chaperonin are ubiquitously expressed and are required for clearance and folding in all tissues. We propose that the HSN identifies a key subset of the proteostasis machinery that regulates the HSR according to the unique functional requirements of each tissue.     

The Heat Shock Response Is Modulated by and Interferes with Toxic Effects of Scrapie Prion Protein and Amyloid β

The heat shock response (HSR) is an evolutionarily conserved pathway designed to maintain proteostasis and to ameliorate toxic effects of aberrant protein folding. We have studied the modulation of the HSR by the scrapie prion protein (PrP^Sc) and amyloid β peptide (Aβ) and investigated whether an activated HSR or the ectopic expression of individual chaperones can interfere with PrP^Sc- or Aβ-induced toxicity. First, we observed different effects on the HSR under acute or chronic exposure of cells to PrP^Sc or Aβ. In chronically exposed cells the threshold to mount a stress response was significantly increased, evidenced by a decreased expression of Hsp72 after stress, whereas an acute exposure lowered the threshold for stress-induced expression of Hsp72. Next, we employed models of PrP^Sc- and Aβ-induced toxicity to demonstrate that the induction of the HSR ameliorates the toxic effects of both PrP^Sc and Aβ. Similarly, the ectopic expression of cytosolic Hsp72 or the extracellular chaperone clusterin protected against PrP^Sc- or Aβ-induced toxicity. However, toxic signaling induced by a pathogenic PrP mutant located at the plasma membrane was prevented by an activated HSR or Hsp72 but not by clusterin, indicating a distinct mode of action of this extracellular chaperone. Our study supports the notion that different pathological protein conformers mediate toxic effects via similar cellular pathways and emphasizes the possibility to exploit the heat shock response therapeutically.     

SIRT3, a mitochondrial sirtuin deacetylase, regulates mitochondrial function and thermogenesis in brown adipocytes

SIRT3 is one of the seven mammalian sirtuin homologs of the yeast Sir2 gene, which mediates the effect of caloric restriction on life span extension in yeast and Caenorhabditis elegans. Because adipose tissue is essential in energy homeostasis and also plays a role in life span determination, we decided to investigate the function of sirtuin members in fat. We report here that murine SIRT3 is expressed in brown adipose tissue and is localized on the mitochondria inner membrane. Caloric restriction activates SIRT3 expression in both white and brown adipose. Additionally, cold exposure up-regulates SIRT3 expression in brown fat, whereas elevated climate temperature reduces the expression. Enforced expression of SIRT3 in the HIB1B brown adipocytes enhances the expression of the uncoupling protein PGC-1alpha, UCP1, and a series of mitochondria-related genes. Both ADP-ribosyltransferase and deacetylase activities of SIRT3 are required for this action. Furthermore, the SIRT3 deacetylase mutant exhibits a dominant negative effect by inhibiting UCP1 expression. This inhibitive effect can be abolished by the coexpression of PGC-1alpha, indicating a major role of PGC-1alpha in the SIRT3 action. In addition, SIRT3 stimulates CREB phosphorylation, which reportedly activates PGC-1alpha promoter directly. Functionally, sustained expression of SIRT3 decreases membrane potential and reactive oxygen species production while increasing cellular respiration. Finally, SIRT3, along with genes related to mitochondrial function, is down-regulated in the brown adipose tissue of several genetically obese mice. In summary, our results demonstrate that SIRT3 activates mitochondria functions and plays an important role in adaptive thermogenesis in brown adipose.     

Heat Shock Protein 90 Homeostasis Controls Stage Differentiation in Leishmania donovani

The differentiation of Leishmania parasites from the insect stage, the promastigote, toward the pathogenic mammalian stage, the amastigote, is triggered primarily by the rise in ambient temperature encountered during the insect-to-mammal transmission. We show here that inactivation of heat shock protein (Hsp) 90, with the use of the drugs geldanamycin or radicicol, mimics transmission and induces the differentiation from the promastigote to the amastigote stage. Geldanamycin also induces a growth arrest of cultured promastigotes that can be forestalled by overexpression of the cytoplasmic Hsp90. Moreover, we demonstrate that Hsp90 serves as a feedback inhibitor of the cellular heat shock response in Leishmania. Our results are consistent with Hsp90 homeostasis serving as cellular thermometer for these primitive eukaryotes, controlling both the heat shock response and morphological differentiation.     

Some like it hot, some like it cold: the heat shock response is found in New Zealand but not Antarctic notothenioid fishes

Previous research on Antarctic notothenioids has demonstrated that cells of cold-adapted Antarctic notothenioids lack a common cellular defense mechanism called the heat shock response (HSR), the induction of a family of heat shock proteins (Hsps) in response to elevated temperatures. The goal of this study was to address how widespread the loss of the HSR is within the Notothenioidei suborder and, specifically, to ask whether cold temperate non-Antarctic notothenioids possess the HSR. In general, Antarctic fish have provided an important opportunity for physiologists to examine responses to selection in the environment and to ask whether traits of the notothenioids represent cold adaptation, or whether the traits are related to history and are characteristics of the notothenioid lineage. Using in vivo metabolic labeling, results indicate that one of the two New Zealand notothenioids possess an HSR. The thornfish, Bovichtus variegatus Richardson, 1846, expressed heat shock proteins (Hsp) in response to heat stress, whereas the black cod, Notothenia angustata Hutton, 1875, did not display robust stress-inducible Hsp synthesis at the protein-level. However, further analysis using Northern blotting clearly demonstrated that mRNA for a common Hsp gene, hsp70, was present in cells of both New Zealand species following exposure to elevated temperatures. Overall, combined evidence on the HSR in notothenioid fishes from temperate New Zealand waters indicate that the loss of the HSR in Antarctic notothenioid fishes occurred after the separation of Bovichtidae from the other Antarctic notothenioid families, and that the HSR was most likely lost during evolution at cold and constant environmental temperatures.