The Heat Shock Response
The eukaryotic heat shock response (HSR) is universal and regulated by the stress-induced activation of heat shock transcription factors. In yeast, C. elegans and Drosophila this involves HSF-1, whereas vertebrates encode a family of HSFs (HSF1-4). The regulation of HSFs involves both positive and negative regulatory factors that sense the stress signal and the molecular events associated with activation of HSF-1 from an inert non-DNA binding state to a trimeric DNA binding transcription factor, extensive post-translational modification by acetylation, sumoylation, ubiqutination and phosphorylation and attenuation of HSF-1 activity to its inert state. Our interests on the transcription of the human Hsp70 gene has been to understand the occupancy of the human Hsp70 promoter with basal and inducible transcription factors at the level of chromatin structure and promoter organization in response to stress, cell growth, oncogenes, and development, and the interplay between the family of HSFs and other trans-factors that regulate Hsp70, Hsp90 and other HSF-1 regulated genes. This is also discussed in the section on the PROTEOSTASIS NETWORK and in recent reviews (Åkerfelt et al., 2010; Li et al., 2017).
At the cellular level of the heat shock response, Jose Cotto and Caroline Jolly showed that HSF-1 exhibits a complex movement from the inert monomer state in the cytoplasm or nucleus to form distinctive nuclear localized stress granules with properties of membraneless condensates that contain the activated HSF1 trimers, a marker of heat shock activation in human cells (Jolly et al., 1997; 1999; Cotto et al., 1997). In the nuclei of human cells, Caroline Jolly showed that HSF-1 stress granules are in proximity to satellite III repeats on chromosome 9 (Jolly et al., 2002) which suggests an organizational role for HSF-1 stress granules to ensure that heat shock gene transcription is rapidly induced within seconds and that heat shock gene transcription is coordinately regulated.
The regulatory complexity of the heat shock response in C. elegans was studied by Eric Guisbert who 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 (Guisbert et al., 2013). These modifiers function in specific steps of gene expression, protein synthesis, protein folding, trafficking, and protein clearance that are essential for the metazoan heat shock regulatory network (HSN). Whereas the positive regulators function in all tissues of C. elegans, nearly all of the negative regulators were tissue-selective. For example, knockdown of the subunits of the proteasome strongly induced heat shock reporter activity only in the intestine and spermatheca but not in muscle cells, while knockdown of subunits of the TRiC/CCT chaperonin induced heat shock reporter activity only in muscle cells. 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.
A more direct understanding of HSF-1 in C. elegans was pursued by Jian Li who employed genome-wide “omic” approaches to show why HSF-1 is essential during development. Jian identified the targets for HSF-1 binding during development and demonstrated that the in vivo binding of HSF-1 requires the simultaneous binding of the cell cycle factor E2F/DP to a GC-rich motif adjacent to degenerate heat-shock elements (HSE) in genes regulated by HSF-1 during development (Li et al., 2016). The HSF-1 transcriptional program in larval development is distinct from the HSR; in early development, both E2F is essential for HSF-1-dependent developmental expression, whereas in response to heat shock, only tandem canonical HSEs are necessary. The HSF-1 developmental network regulates the expression of a specific subset of chaperone genes for protein biogenesis and anabolic metabolism (Li et al., 2017).
While it has long been suggested that the HSR declines in aging, a molecular basis had not been identified until 2015 when John Labbadia showed that multiple cell stress responses, highlighted by a dramatic collapse of the HSR, are severely compromised at an early point in adulthood associated with fecundity (Labbadia and Morimoto, 2015). The timing of the HSR collapse is controlled by reduced levels of the H3K27 demethylase jmjd-3.1 in response to signals from germline stem cells. The programmed repression of the HSR results from placement of H3K27me3 marks at heat shock genes, condensing the chromatin and preventing the binding of HSF-1 and RNA Pol II at heat shock gene promoters. Repression of the HSR at the onset of reproductive maturity suggests that the lack of inducible chaperones may be among the earliest event of molecular aging, which over time contributes to a decline in all cellular processes. We suggest that this logic may be important for free-living organisms to ensure that adult animals do not compete for limiting resources, in line with the disposable soma theory of aging (Labbadia and Morimoto, 2015).
Ahn, S.-G.., P. Liu, K. Klyachko, R.I. Morimoto, and D. Thiele. The Loop Domain of Heat Shock Transcription Factor 1 Dictates DNA Binding Specificity and Responses to Heat Stress. Genes and Development 15: 2134-2145. (2001).
Åkerfelt, M., R.I. Morimoto, and L. Sistonen. Heat Shock Factors: Integrators of Cell Stress, Development, and Lifespan. Nature Reviews Molecular Cell Biology 11: 545-555. (2010).
Cotto J. Fox S. Morimoto R . HSF1 granules: a novel stress-induced nuclear compartment of human cells. Journal of Cell Science. 110 ( Pt 23):2925-34. (1997).
Cotto JJ. Kline M. Morimoto RI . Activation of heat shock factor 1 DNA binding precedes stress-induced serine phosphorylation. Evidence for a multistep pathway of regulation. Journal of Biological Chemistry 271(7): 3355-8. (1996).
Guisbert, E., D. M. Czyz, K. Richter, P. D. McMullen, and R. I. Morimoto. Identification of a Tissue-Selective Heat Shock Response Regulatory Network. PLoS Genetics 9(4): DOI: 10.1371, PMID: 23637632 (2013).
Guisbert, E. and R.I. Morimoto. Regulation of the Heat Shock Response. In: Protein Quality Control in Neurodegenerative Diseases, Research and Perspectives in Alzheimer’s Disease. p. 1-18, Springer Press. (2012).
Holmberg CI, Hietakangas V, Mikhailov A, Rantanen JO, Kallio M, Meinander A, Hellman J, Morrice N, MacKintosh C, Morimoto RI, Eriksson JE, Sistonen L. Phosphorylation of serine 230 promotes inducible transcriptional activity of heat shock factor 1. EMBO J 20: 3800-10 (2001).
Jolly, C., L. Konecny, D.L. Grady, Y.A. Kutskova, J.J. Cotto, R.I. Morimoto, and C. Vourch. In vivo Binding of Active HSF1 to Human Chromosome 9 Heterochromatin During Stress. J. Cell Biology 156: 775-781. (2002).
Jolly C. Morimoto R. Robert-Nicoud M. Vourc’h C . HSF1 transcription factor concentrates in nuclear foci during heat shock: relationship with transcription sites. Journal of Cell Science 110 ( Pt 23): 2935-41. (1997).
Jolly C. Usson Y. Morimoto RI. Rapid and reversible relocalization of heat shock factor 1 within seconds to nuclear stress granules. Proc National Academy of Science USA 8; 96(12):6769-74. (1999).
Kanei-Ishii C. Tanikawa J. Nakai A. Morimoto RI. Ishii S . Activation of heat shock transcription factor 3 by c-Myb in the absence of cellular stress. Science 277(5323): 246-8. (1997).
Kline MP. Morimoto RI . Repression of the heat shock factor 1 transcriptional activation domain is modulated by constitutive phosphorylation. Molecular & Cellular Biology 17(4): 2107-15. (1997).
Labbadia, J., R. Brielmann, M. Neto, Y.-F. Lin, C.M. Haynes, and R.I. Morimoto. Mitochondrial Stress Restores the Heat Shock Response and Prevents Proteostasis Collapse During Aging. Cell Reports 21: 1481-1494. DOI.org/10.1016/ j.celrep.2017.10.038 (2017).
Labbadia, J. and R.I. Morimoto. Repression of the Heat Shock Response is a Programmed Event at the Onset of Reproduction. Molecular Cell 59: 639-650. DOI 10.1016/j. molcel.2015.06.027 PMID: 266212459 (2015).
Lee BS. Chen J. Angelidis C. Jurivich DA. Morimoto RI. Pharmacological modulation of heat shock factor 1 by antiinflammatory drugs results in protection against stress-induced cellular damage. Proceedings of the National Academy of Sciences of the United States of America 92(16): 7207-11. (1995).
Li, J., L. Chauve, G. Phelps, R. Brielmann, and R.I. Morimoto. E2F Co-regulates an Essential HSF Developmental Program Distinct from the Heat Shock Response. Genes and Development 30: 2062-2075. PMID 27688402 (2016).
Li, J., J. Labbadia, and R.I. Morimoto. Rethinking the Roles of HSF-1 in Cell Stress, Development and Organismal Health. Trends in Cell Biology 12: DOI: org/10.1016/ j.tcb.2017.08.002 (2017).
Mathew A, Mathur, SK, Jolly, C, Fox, SG, Kim, S and Morimoto RI. Stress-specific activation and repression of heat shock factors 1 and 2. Mol Cell Biol 21: 7163-71. (2001).
Mathew A, Mathur SK, Morimoto RI Heat shock response and protein degradation: regulation of HSF2 by the ubiquitin- proteasome pathway. Mol Cell Biol: 18(9):5091-8. (1998).
Morimoto, RI. Regulation of the heat shock transcriptional response: Cross talk between a family of heat shock factors, molecular chaperones, and negative regulators. (Review) Genes & Development 12:3788-3796. (1998).
Morimoto, R.I. Dynamic Remodeling of Transcription Complexes by Molecular Chaperones. Cell 110: 281-284. (2002).
Morley, J.F. and Morimoto, R. Regulation of Longevity in Caenorhabditis elegans by Heat Shock Factor and Molecular Chaperones. Mol Biol Cell Feb;15: 657-64. (2004).
Petre, I., C.L. Hyder, A. Mizera, A. Meinander, A. Mikhailov, R.I. Morimoto, L. Sistonen, J.E. Eriksson, and R-J. Back. A Simple Mass-Action Model for the Eukaryotic Heat Shock Response and its Mathematic Validation. Natural Computing 10: 595-612. DOI: 10.1007/s11047-010-9216-y (2011).
Prahlad, V., T. Cornelius and R.I. Morimoto, . Regulation of the Cellular Heat Shock Response in Caenorhabditis elegans by Thermosensory Neurons. Science 320: 811-814. (2008).
Prahlad, V., and R.I. Morimoto. Integrating the Stress Response: Lessons for Neurodegenerative Diseases from C. elegans. Trends in Cell Biology 19: 52-61. (2009).
Rieger, T., R.I. Morimoto, and V. Hatzimanikatis. Mathematical Modeling of the Eukaryotic Heat Shock Response: Dynamics of the Hsp70 Promoter. Biophysical Journal 88: 1646-1658. (2005).
Satyal SH, Chen D, Fox SG, Kramer JM, Morimoto RI Negative regulation of the heat shock transcriptional response by HSBP1. Genes Dev Jul 1;12(13): 1962-7. (1998).
Shi Y, Kroeger PE and Morimoto RI. The carboxyl-terminal transactivation domain of heat shock factor 1 is negatively regulated and stress responsive. Mol. Cell Biol 15: 4309-4318. (1995).
Shi Y, Mosser DD, Morimoto RI Molecular chaperones as HSF1-specific transcriptional repressors. Genes Dev Mar 1;12(5): 654-66. (1998).
Silva, M. C., M. D. Amaral, and R.I. Morimoto. Neuronal Reprogramming of Protein Homeostasis by Calcium-Dependent Regulation of the Heat Shock Response. PLoS Genetics 9(8): DOI: 10.1371, e1003711, PMID: 24009518 (2013).
Tai LJ, McFall SM, Huang K, Demeler B, Fox SG, Brubaker K, Radhakrishnan I, Morimoto RI. Structure-function analysis of the heat shock factor binding protein reveals a protein composed solely of a highly conserved and dynamic coiled-coil trimerization domain. J Biol Chem. (2001).
Tanabe M, Kawazoe Y, Takeda S, Morimoto RI, Nagata K, Nakai A Disruption of the HSF3 gene results in the severe reduction of heat shock gene expression and loss of thermotolerance. EMBO J Mar 16;17(6):1750-8. (1998).
Westerheide, S.D., J. Anckar, S.M. Stevens, Jr., L. Sistonen, and R.I. Morimoto. Stress-Inducible Regulation of Heat Shock Factor 1 by the Deacetylase SIRT1. Science 20: 1063-1066. (2009).
Westerheide, S., J. Bosman, B. Mbadugha, T. Kawahara, G. Matsumoto, W. Gu, S. Kim, J. Devlin, R. Silverman, and R.I. Morimoto. Celastrols as Inducers of the Heat Shock Response and Cytoprotection. J. Biol. Chem 279: 56053-60. (2004).
Westerheide, S., and R.I. Morimoto. Heat Shock Response Modulators as Therapeutic Tools for Diseases of Protein Conformation. J. Biol. Chem 380: 33097-33100. (2005).
Westerheide, S., T. Kawahara, K. Orton, and R.I. Morimoto, Triptolide, an Inhibitor of the Human Heat Shock Response that Enhances Stress-Induced Cell Death. J. Biol. Chem 281: 9616-9622. (2006).