Rapamycin conditionally inhibits Hsp90 but not Hsp70 mRNA translation in Drosophila: implications for the mechanisms of Hsp mRNA translation

RNA structure mediated thermoregulation: What can we learn from plants?

RNA molecules have the capacity to form a multitude of distinct secondary and tertiary structures, but only the most energetically favorable conformations are adopted at any given time. Formation of such structures strongly depends on the environment and consequently, these structures are highly dynamic and may refold as their surroundings change. Temperature is one of the most direct physical parameters that influence RNA structure dynamics, and in turn, thermosensitive RNA structures can be harnessed by a cell to perceive and respond to its temperature environment. Indeed, many thermosensitive RNA structures with biological function have been identified in prokaryotic organisms, but for a long time such structures remained elusive in eukaryotes. Recent discoveries, however, reveal that thermosensitive RNA structures are also found in plants, where they affect RNA stability, pre-mRNA splicing and translation efficiency in a temperature-dependent manner. In this minireview, we provide a short overview of thermosensitive RNA structures in prokaryotes and eukaryotes, highlight recent advances made in identifying such structures in plants and discuss their similarities and differences to established prokaryotic RNA thermosensors.

Genotype and Trait Specific Responses to Rapamycin Intake in Drosophila melanogaster

Simple Summary

Rapamycin is commonly used as an immunosuppressant, but also as an anti-aging medicine. Despite its widespread use, results suggest that there is large variability in drug efficiency among patients, and limited knowledge exists about potential side-effects. In the present study, we investigated the effects of rapamycin using the common fruit fly as model organism. Six genetically distinct lines were exposed to rapamycin, and the phenotypic consequence on fecundity, longevity and heat stress tolerance was quantified. Flies exposed to rapamycin had increased longevity and heat stress tolerance, however a side effect in the form of decreased fecundity was also observed. Our data clearly show that the costs and benefits of rapamycin treatment is strongly genotype dependent. These observations are important as they imply that a ‘one size fits all’ approach when it comes to rapamycin treatment is not advisable. Future studies should address the underlying genetic component that drive the drug response variability.

Abstract

Rapamycin is a powerful inhibitor of the TOR (Target of Rapamycin) pathway, which is an evolutionarily conserved protein kinase, that plays a central role in plants and animals. Rapamycin is used globally as an immunosuppressant and as an anti-aging medicine. Despite widespread use, treatment efficiency varies considerably across patients, and little is known about potential side effects. Here we seek to investigate the effects of rapamycin by using Drosophila melanogaster as model system. Six isogenic D. melanogaster lines were assessed for their fecundity, male longevity and male heat stress tolerance with or without rapamycin treatment. The results showed increased longevity and heat stress tolerance for male flies treated with rapamycin. Conversely, the fecundity of rapamycin-exposed individuals was lower than for flies from the non-treated group, suggesting unwanted side effects of the drug in D. melanogaster. We found strong evidence for genotype-by-treatment interactions suggesting that a ‘one size fits all’ approach when it comes to treatment with rapamycin is not recommendable. The beneficial responses to rapamycin exposure for stress tolerance and longevity are in agreement with previous findings, however, the unexpected effects on reproduction are worrying and need further investigation and question common believes that rapamycin constitutes a harmless drug.

Combined Transcriptomic and Proteomic Analysis of Perk Toxicity Pathways

In Drosophila, endoplasmic reticulum (ER) stress activates the protein kinase R-like endoplasmic reticulum kinase (dPerk). dPerk can also be activated by defective mitochondria in fly models of Parkinson's disease caused by mutations in pink1 or parkin. The Perk branch of the unfolded protein response (UPR) has emerged as a major toxic process in neurodegenerative disorders causing a chronic reduction in vital proteins and neuronal death. In this study, we combined microarray analysis and quantitative proteomics analysis in adult flies overexpressing dPerk to investigate the relationship between the transcriptional and translational response to dPerk activation. We identified tribbles and Heat shock protein 22 as two novel Drosophila activating transcription factor 4 (dAtf4) regulated transcripts. Using a combined bioinformatics tool kit, we demonstrated that the activation of dPerk leads to translational repression of mitochondrial proteins associated with glutathione and nucleotide metabolism, calcium signalling and iron-sulphur cluster biosynthesis. Further efforts to enhance these translationally repressed dPerk targets might offer protection against Perk toxicity.

Rapalog resistance is associated with mesenchymal-type changes in Tsc2-null cells

Tuberous Sclerosis Complex (TSC) and Lymphangioleiomyomatosis (LAM) are caused by inactivating mutations in TSC1 or TSC2, leading to mTORC1 hyperactivation. The mTORC1 inhibitors rapamycin and analogs (rapalogs) are approved for treating of TSC and LAM. Due to their cytostatic and not cytocidal action, discontinuation of treatment leads to tumor regrowth and decline in pulmonary function. Therefore, life-long rapalog treatment is proposed for the control of TSC and LAM lesions, which increases the chances for the development of acquired drug resistance. Understanding the signaling perturbations leading to rapalog resistance is critical for the development of better therapeutic strategies. We developed the first Tsc2-null rapamycin-resistant cell line, ELT3-245, which is highly tumorigenic in mice, and refractory to rapamycin treatment. In vitro ELT3-245 cells exhibit enhanced anchorage-independent cell survival, resistance to anoikis, and loss of epithelial markers. A key alteration in ELT3-245 is increased β-catenin signaling. We propose that a subset of cells in TSC and LAM lesions have additional signaling aberrations, thus possess the potential to become resistant to rapalogs. Alternatively, when challenged with rapalogs TSC-null cells are reprogrammed to express mesenchymal-like markers. These signaling changes could be further exploited to induce clinically-relevant long-term remissions.

The effect of aging on the autophagic and heat shock response in human peripheral blood mononuclear cells

Autophagy is a lysosome degradation pathway through which damaged organelles and macromolecules are degraded within the cell. A decrease in activity of the autophagic process has been linked to several age-associated pathologies, including triglyceride accumulation, mitochondrial dysfunction, muscle degeneration, and cardiac malfunction. Here, we examined the differences in the autophagic response using autophagy-inducer rapamycin (Rapa) in peripheral blood mononuclear cells (PBMCs) from young (21.8 ± 1.9 years) and old (64.0 ± 3.7 years) individuals. Furthermore, we tested the interplay between the heat shock response and autophagy systems. Our results showed a significant increase in LC3-II protein expression in response to Rapa treatment in young but not in old individuals. This was associated with a decreased response in MAP1LC3B mRNA levels, but not SQSTM1/p62. Furthermore, HSPA1A mRNA was upregulated only in young individuals, despite no differences in HSP70 protein expression. The combined findings suggest a suppressed autophagic response following Rapa treatment in older individuals.

Chronic exposure to rapamycin and episodic serum starvation modulate ageing of human fibroblasts in vitro

Mild stress-induced activation of stress response (SR) pathways, such as autophagy, heat shock response, oxidative SR, DNA damage response, and inflammatory response, can be potentially health beneficial. Using the model system of cellular ageing and replicative senescence in vitro, we have studied the ageing modulatory effects of the two conditions, rapamycin and serum starvation. Chronic exposure to 0.1, 1 and 10 nM rapamycin positively modulated the survival, growth, morphology, telomere length, DNA methylation levels, 8-oxo-dG level in DNA, N⁶-methyl-adenosine level in RNA, and ethanol stress tolerance of serially passaged normal human skin fibroblasts. Furthermore, episodic (once a week) serum starvation of human skin fibroblasts extended their replicative lifespan by about 22%, along with the maintenance of early passage youthful morphology even in late passage cultures. Although the results of this study may be considered preliminary, it can be inferred that intermittent and episodic induction of SR, rather than chronic up-regulation of SR, is more effective and applicable in the practice of hormesis for healthy ageing and longevity.

Elevated levels of ribosomal proteins eL36 and eL42 control expression of Hsp90 in rhabdomyosarcoma

Mammalian 90 kDa heat shock protein (Hsp90) is a ubiquitous molecular chaperone whose expression is selectively upregulated during stress, although the precise control mechanism of this increase is yet to be fully elucidated. We used polysome profiling to show that Hsp90α mRNA is selectively translated, while global translation is inhibited during heat stress. Furthermore, we have identified 2 ribosomal proteins, eL36 and eL42 that modulate Hsp90α expression under both normal and heat shock conditions. Importantly, we noted that expression of eL36 and eL42 is elevated in a panel of human rhabdomyosarcomas where it drives high expression of Hsp90 and modulates sensitivity of these cells to an Hsp90 inhibitor 17-AAG.

Stress Beyond Translation: Poxviruses and More

Poxviruses are large double-stranded DNA viruses that form viral factories in the cytoplasm of host cells. These viruses encode their own transcription machinery, but rely on host translation for protein synthesis. Thus, poxviruses have to cope with and, in most cases, reprogram host translation regulation. Granule structures, called antiviral granules (AVGs), have been observed surrounding poxvirus viral factories. AVG formation is associated with abortive poxvirus infection, and AVGs contain proteins that are typically found in stress granules (SGs). With certain mutant poxviruses lack of immunoregulatory factor(s), we can specifically examine the mechanisms that drive the formation of these structures. In fact, cytoplasmic macromolecular complexes form during many viral infections and contain sensing molecules that can help reprogram transcription. More importantly, the similarity between AVGs and cytoplasmic structures formed during RNA and DNA sensing events prompts us to reconsider the cause and consequence of these AVGs. In this review, we first summarize recent findings regarding how poxvirus manipulates host translation. Next, we compare and contrast SGs and AVGs. Finally, we review recent findings regarding RNA- and especially DNA-sensing bodies observed during viral infection.

ncRNAs and thermoregulation: a view in prokaryotes and eukaryotes

During cellular stress response, a widespread inhibition of transcription and blockade of splicing and other post-transcriptional processing is detected, while certain specific genes are induced. In particular, free-living cells constantly monitor temperature. When the thermal condition changes, they activate a set of genes coding for proteins that participate in the response. Non-coding RNAs, ncRNAs, and conformational changes in specific regions of mRNAs seem also to be crucial regulators that enable the cell to adjust its physiology to environmental changes. They exert their effects following the same principles in all organisms and may affect all steps of gene expression. These ncRNAs and structural elements as related to thermal stress response in bacteria are reviewed. The resemblances to eukaryotic ncRNAs are highlighted.

Preferential translation of Hsp83 in Leishmania requires a thermosensitive polypyrimidine-rich element in the 3' UTR and involves scanning of the 5' UTR

Heat shock proteins (HSPs) provide a useful system for studying developmental patterns in the digenetic Leishmania parasites, since their expression is induced in the mammalian life form. Translation regulation plays a key role in control of protein coding genes in trypanosomatids, and is directed exclusively by elements in the 3' untranslated region (UTR). Using sequential deletions of the Leishmania Hsp83 3' UTR (888 nucleotides [nt]), we mapped a region of 150 nt that was required, but not sufficient for preferential translation of a reporter gene at mammalian-like temperatures, suggesting that changes in RNA structure could be involved. An advanced bioinformatics package for prediction of RNA folding (UNAfold) marked the regulatory region on a highly probable structural arm that includes a polypyrimidine tract (PPT). Mutagenesis of this PPT abrogated completely preferential translation of the fused reporter gene. Furthermore, temperature elevation caused the regulatory region to melt more extensively than the same region that lacked the PPT. We propose that at elevated temperatures the regulatory element in the 3' UTR is more accessible to mediators that promote its interaction with the basal translation components at the 5' end during mRNA circularization. Translation initiation of Hsp83 at all temperatures appears to proceed via scanning of the 5' UTR, since a hairpin structure abolishes expression of a fused reporter gene.