Manipulating heat shock factor-1 in Xenopus tadpoles: neuronal tissues are refractory to exogenous expression

HSB-1 Inhibition and HSF-1 Overexpression Trigger Overlapping Transcriptional Changes To Promote Longevity in Caenorhabditis elegans

Heat shock factor 1 (HSF-1) is a component of the heat shock response pathway that is induced by cytoplasmic proteotoxic stress. In addition to its role in stress response, HSF-1 also acts as a key regulator of the rate of organismal aging. Overexpression of HSF-1 promotes longevity in C. elegans via mechanisms that remain less understood. Moreover, genetic ablation of a negative regulator of HSF-1, termed as heat shock factor binding protein 1 (HSB-1), results in hsf-1-dependent life span extension in animals. Here we show that in the absence of HSB-1, HSF-1 acquires increased DNA binding activity to its genomic target sequence. Using RNA-Seq to compare the gene expression profiles of the hsb-1 mutant and hsf-1 overexpression strains, we found that while more than 1,500 transcripts show ≥1.5-fold upregulation due to HSF-1 overexpression, HSB-1 inhibition alters the expression of less than 500 genes in C. elegans. Roughly half of the differentially regulated transcripts in the hsb-1 mutant have altered expression also in hsf-1 overexpressing animals, with a strongly correlated fold-expression pattern between the two strains. In addition, genes that are upregulated via both HSB-1 inhibition and HSF-1 overexpression include numerous DAF-16 targets that have known functions in longevity regulation. This study identifies how HSB-1 acts as a specific regulator of the transactivation potential of HSF-1 in non-stressed conditions, thus providing a detailed understanding of the role of HSB-1/HSF-1 signaling pathway in transcriptional regulation and longevity in C. elegans.

Eye-specific gene expression following embryonic ethanol exposure in zebrafish: roles for heat shock factor 1

The mechanisms through which ethanol exposure results in developmental defects remain unclear. We used the zebrafish model to elucidate eye-specific mechanisms that underlie ethanol-mediated microphthalmia (reduced eye size), through time-series microarray analysis of gene expression within eyes of embryos exposed to 1.5% ethanol. 62 genes were differentially expressed (DE) in ethanol-treated as compared to control eyes sampled during retinal neurogenesis (24-48 h post-fertilization). The EDGE (extraction of differential gene expression) algorithm identified >3000 genes DE over developmental time in ethanol-exposed eyes as compared to controls. The DE lists included several genes indicating a mis-regulated cellular stress response due to ethanol exposure. Combined treatment with sub-threshold levels of ethanol and a morpholino targeting heat shock factor 1 mRNA resulted in microphthalmia, suggesting convergent molecular pathways. Thermal preconditioning partially prevented ethanol-mediated microphthalmia while maintaining Hsf-1 expression. These data suggest roles for reduced Hsf-1 in mediating microphthalmic effects of embryonic ethanol exposure.

Neuronal responses to physiological stress

Physiological stress can be defined as any external or internal condition that challenges the homeostasis of a cell or an organism. It can be divided into three different aspects: environmental stress, intrinsic developmental stress, and aging. Throughout life all living organisms are challenged by changes in the environment. Fluctuations in oxygen levels, temperature, and redox state for example, trigger molecular events that enable an organism to adapt, survive, and reproduce. In addition to external stressors, organisms experience stress associated with morphogenesis and changes in inner chemistry during normal development. For example, conditions such as intrinsic hypoxia and oxidative stress, due to an increase in tissue mass, have to be confronted by developing embryos in order to complete their development. Finally, organisms face the challenge of stochastic accumulation of molecular damage during aging that results in decline and eventual death. Studies have shown that the nervous system plays a pivotal role in responding to stress. Neurons not only receive and process information from the environment but also actively respond to various stresses to promote survival. These responses include changes in the expression of molecules such as transcription factors and microRNAs that regulate stress resistance and adaptation. Moreover, both intrinsic and extrinsic stresses have a tremendous impact on neuronal development and maintenance with implications in many diseases. Here, we review the responses of neurons to various physiological stressors at the molecular and cellular level.

Discovery of variants unmasked by hemizygous deletions

Array-based genome-wide segmental aneuploidy screening detects both de novo and inherited copy number variations (CNVs). In sporadic patients de novo CNVs are interpreted as potentially pathogenic. However, a deletion, transmitted from a healthy parent, may be pathogenic if it overlaps with a mutated second allele inherited from the other healthy parent. To detect such events, we performed multiplex enrichment and next-generation sequencing of the entire coding sequence of all genes within unique hemizygous deletion regions in 20 patients (1.53 Mb capture footprint). Out of the detected 703 non-synonymous single-nucleotide variants (SNVs), 8 represented variants being unmasked by a hemizygous deletion. Although evaluation of inheritance patterns, Grantham matrix scores, evolutionary conservation and bioinformatic predictions did not consistently indicate pathogenicity of these variants, no definitive conclusions can be drawn without functional validation. However, in one patient with severe mental retardation, lack of speech, microcephaly, cheilognathopalatoschisis and bilateral hearing loss, we discovered a second smaller deletion, inherited from the other healthy parent, resulting in loss of both alleles of the highly conserved heat shock factor binding protein 1 (HSBP1) gene. Conceivably, inherited deletions may unmask rare pathogenic variants that may exert a phenotypic impact through a recessive mode of gene action.

An atypical unfolded protein response in heat shocked cells

Background

The heat shock response (HSR) and the unfolded protein response (UPR) are both activated by proteotoxic stress, although in different compartments, and share cellular resources. How these resources are allocated when both responses are active is not known. Insight in possible crosstalk will help understanding the consequences of failure of these systems in (age-related) disease.

Results

In heat stressed HEK293 cells synthesis of the canonical UPR transcription factors XBP1s and ATF4 was detected as well as HSF1 independent activation of the promoters of the ER resident chaperones HSPA5 (BiP) and DNAJB9 (ERdj4). However, the heat stress activation of the DNAJB9 promoter, a XBP1s target, was not blocked in cells expressing a dominant negative IRE1α mutant, and thus did not require XBP1s. Furthermore, the DNA element required for heat stress activation of the DNAJB9 promoter is distinct from the ATF4 and ATF6 target elements; even though inhibition of eIF2α phosphorylation resulted in a decreased activation of the DNAJB9 promoter upon heat stress, suggesting a role for an eIF2α phosphorylation dependent product.

Conclusions

The initial step in the UPR, synthesis of transcription factors, is activated by heat stress but the second step, transcriptional transactivation by these factors, is blocked and these pathways of the UPR are thus not productive. Expression of canonical ER chaperones is part of the response of heat stressed cells but another set of transcription factors has been recruited to regulate expression of these ER chaperones.

Heat shock factor 2 is required for maintaining proteostasis against febrile-range thermal stress and polyglutamine aggregation

HSF2 regulates proteostasis capacity against febrile-range thermal stress, which provides temperature-dependent mechanisms of cellular adaptation to thermal stress. Furthermore, HSF2 has a strong impact on disease progression of Huntington's disease R6/2 mice, suggesting that it could be a promising therapeutic target for protein misfolding diseases.

Heat shock response is characterized by the induction of heat shock proteins (HSPs), which facilitate protein folding, and non-HSP proteins with diverse functions, including protein degradation, and is regulated by heat shock factors (HSFs). HSF1 is a master regulator of HSP expression during heat shock in mammals, as is HSF3 in avians. HSF2 plays roles in development of the brain and reproductive organs. However, the fundamental roles of HSF2 in vertebrate cells have not been identified. Here we find that vertebrate HSF2 is activated during heat shock in the physiological range. HSF2 deficiency reduces threshold for chicken HSF3 or mouse HSF1 activation, resulting in increased HSP expression during mild heat shock. HSF2-null cells are more sensitive to sustained mild heat shock than wild-type cells, associated with the accumulation of ubiquitylated misfolded proteins. Furthermore, loss of HSF2 function increases the accumulation of aggregated polyglutamine protein and shortens the lifespan of R6/2 Huntington's disease mice, partly through αB-crystallin expression. These results identify HSF2 as a major regulator of proteostasis capacity against febrile-range thermal stress and suggest that HSF2 could be a promising therapeutic target for protein-misfolding diseases.

HSF1 is essential for the resistance of zebrafish eye and brain tissues to hypoxia/reperfusion injury

Ischemia and subsequent reperfusion (IR) produces injury to brain, eye and other tissues, contributing to the progression of important clinical pathologies. The response of cells to IR involves activation of several signaling pathways including those activating hypoxia and heat shock responsive transcription factors. However, specific roles of these responses in limiting cell damage and preventing cell death after IR have not been fully elucidated. Here, we have examined the role of heat shock factor 1 (HSF1) in the response of zebrafish embryos to hypoxia and subsequent return to normoxic conditions (HR) as a model for IR. Heat shock preconditioning elevated heat shock protein expression and protected zebrafish embryo eye and brain tissues against HR-induced apoptosis. These effects were inhibited by translational suppression of HSF1 expression. Reduced expression of HSF1 also increased cell death in brain and eye tissues of embryos subjected to hypoxia and reperfusion without prior heat shock. Surprisingly, reduced expression of HSF1 had only a modest effect on hypoxia-induced expression of Hsp70 and no effect on hypoxia-induced expression of Hsp27. These results establish the zebrafish embryo as a model for the study of ischemic injury in the brain and eye and reveal a critical role for HSF1 in the response of these tissues to HR. Our results also uncouple the role of HSF1 expression from that of Hsp27, a well characterized heat shock protein considered essential for cell survival after hypoxia. Alternative roles for HSF1 are considered.

Heat shock factor 1 ameliorates proteotoxicity in cooperation with the transcription factor NFAT

Heat shock transcription factor 1 (HSF1) is an important regulator of protein homeostasis (proteostasis) by controlling the expression of major heat shock proteins (Hsps) that facilitate protein folding. However, it is unclear whether other proteostasis pathways are mediated by HSF1. Here, we identified novel targets of HSF1 in mammalian cells, which suppress the aggregation of polyglutamine (polyQ) protein. Among them, we show that one of the nuclear factor of activated T cells (NFAT) proteins, NFATc2, significantly inhibits polyQ aggregation in cells and is required for HSF1-mediated suppression of polyQ aggregation. NFAT deficiency accelerated disease progression including aggregation of a mutant polyQ-huntingtin protein and shortening of lifespan in R6/2 Huntington's disease mice. Furthermore, we found that HSF1 and NFAT cooperatively induce the expression of the scaffold protein PDZK3 and αB-crystallin, which facilitate the degradation of polyQ protein. These results show the first mechanistic basis for the observation that HSF1 has a much more profound effect on proteostasis than individual Hsp or combination of different Hsps, and suggest a new pathway for ameliorating protein-misfolding diseases.