Drosophila Small Heat Shock Proteins: An Update on Their Features and Functions

Protective effects of small heat shock proteins in Daphnia magna against heavy metal exposure

Daphnia magna is one of the most commonly used model organisms to assess toxicity of heavy metal and other xenobiotics. However, the lack of knowledge about important stress-resistant molecules limits our understanding of the alteration of phenotypic and physiological traits of D. magna upon stress exposures. In this study, we focused on a chaperone family of small heat shock protein (sHSP) that has been found in archaea, bacteria and eukaryotes and plays an important role in stress tolerance. A total of eleven sHSP genes (termed DmsHSP1 - DmsHSP11) were identified from the D. magna genome, whose expression profiles during exposure to heavy metal (Cd²⁺, Cu²⁺ and Zn²⁺) and a few other potential pollutants were evaluated via qRT-PCR and RNA-Seq analysis. The results highlighted the predominant role of DmsHSP1 with the highest basal expression level in adults and robust upregulation upon exposure to heavy metals (Cu²⁺ > Cd²⁺ > Zn²⁺). In vivo, recombinant protein rDmsHSP1-21 and rDmsHSP11-12.8 could not only prevent model substrates agglutination induced by heavy metals or reducer dithiotreitol (DTT), but also protect tissue proteins and enzymes from denaturation and inactivation caused by heavy metals or high temperature. Ectopically expression of DmsHSP1-21 or DmsHSP11-12.8 in E. coli conferred host enhanced resistance against various abiotic stresses including Cd²⁺, Cu²⁺ and phenazine methosulfate (PMS). Knockdown of DmsHSP1-21 by RNAi, but not for DmsHSP11-12.8, significantly increased the vulnerability of D. magna to heavy metal exposure. Our work provides systematic information on the evolution and function of sHSPs in D. magna and leads to important insights into the mechanisms by which D. magna survive in adverse environments.

Understanding Diversity, Evolution, and Structure of Small Heat Shock Proteins in Annelida Through in Silico Analyses

Small heat shock proteins (sHsps) are oligomeric stress proteins characterized by an α-crystallin domain (ACD). These proteins are localized in different subcellular compartments and play critical roles in the stress physiology of tissues, organs, and whole multicellular eukaryotes. They are ubiquitous proteins found in all living organisms, from bacteria to mammals, but they have never been studied in annelids. Here, a data set of 23 species spanning the annelid tree of life, including mostly transcriptomes but also two genomes, was interrogated and 228 novel putative sHsps were identified and manually curated. The analysis revealed very high protein diversity and showed that a significant number of sHsps have a particular dimeric architecture consisting of two tandemly repeated ACDs. The phylogenetic analysis distinguished three main clusters, two of them containing both monomeric sHsps, and ACDs located downstream in the dimeric sHsps, and the other one comprising the upstream ACDs from those dimeric forms. Our results support an evolutionary history of these proteins based on duplication events prior to the Spiralia split. Monomeric sHsps 76) were further divided into five subclusters. Physicochemical properties, subcellular location predictions, and sequence conservation analyses provided insights into the differentiating elements of these putative functional groups. Strikingly, three of those subclusters included sHsps with features typical of metazoans, while the other two presented characteristics resembling non-metazoan proteins. This study provides a solid background for further research on the diversity, evolution, and function in the family of the sHsps. The characterized annelid sHsps are disclosed as essential for improving our understanding of this important family of proteins and their pleotropic functions. The features and the great diversity of annelid sHsps position them as potential powerful molecular biomarkers of environmental stress for acting as prognostic tool in a diverse range of environments.

Honey bee sHSP are responsive to diverse proteostatic stresses and potentially promising biomarkers of honey bee stress

The pollination services provided by the honey bee are critical in both natural and agricultural ecosystems. Honey bee colonies in the United States have suffered from an increased rate of die-off in recent years, stemming from a complex set of interacting stresses that remain poorly described. Defining specific common cellular processes and cellular stress responses impacted by multiple stressors represent a key step in understanding these synergies. Proteotoxic stresses negatively impact protein synthesis, folding, and degradation. Diverse proteotoxic stresses induce expression of genes encoding small heat shock proteins (sHSP) of the expanded lethal (2) essential for life (l(2)efl) gene family. In addition to upregulation by the Integrated Stress Response (ISR), the Heat Shock Response (HSR), and the Oxidative Stress Response (OSR), our data provide first evidence that sHSP genes are upregulated by the Unfolded Protein Response (UPR). As these genes appear to be part of a core stress response that could serve as a useful biomarker for cellular stress in honey bees, we designed and tested an RT-LAMP assay to detect increased l(2)efl gene expression in response to heat-stress. While this assay provides a powerful proof of principle, further work will be necessary to link changes in sHSP gene expression to colony-level outcomes, to adapt our preliminary assay into a Point of Care Testing (POCT) assay appropriate for use as a diagnostic tool for use in the field, and to couple assay results to management recommendations.

Loss of Hsp67Bc leads to autolysosome enlargement in the Drosophila brain

Hsp67Bc is a small heat shock protein found in Drosophila melanogaster. Apart from performing a function (common for all small heat shock proteins) of preventing aggregation of misfolded proteins, it is involved in macroautophagy regulation alongside the Starvin protein. Overexpression of the D. melanogaster Hsp67Bc gene has been shown to stimulate macroautophagy in S2 cell culture. Nonetheless, it has been unknown how the absence of the Hsp67Bc gene may affect it. Here, we studied the effect of Hsp67Bc gene deletion on the macroautophagy induced by the pathogenic Wolbachia wMelPop strain in D. melanogaster. We detected Wolbachia inside autophagic vacuoles in fly neurons, thereby proving that these endosymbionts were being eliminated via macroautophagy. Nevertheless, we did not register any difference in brain bacterial load between Hsp67Bc-null and control flies at all tested stages of ontogenesis. Moreover, the abundance of autophagic vacuoles was similar between neurons of the mutant and control flies, yet the cross-sectional area of autolysosomes on ultrathin sections was more than 1.5-fold larger in Hsp67Bc-null fly brains than in the control line. Our findings suggest that the product of the Hsp67Bc gene does not participate in the initiation of endosymbiont-induced macroautophagy but may mediate autophagosome maturation: the deletion of the Hsp67Bc gene leads to the increase in autolysosome size.

Evolution and genomic organization of the insect sHSP gene cluster and coordinate regulation in phenotypic plasticity

Background

Conserved syntenic gene complexes are rare in Arthropods and likely only retained due to functional constraint. Numerous sHSPs have been identified in the genomes of insects, some of which are located clustered in close proximity. Previous phylogenetic analyses of these clustered sHSP have been limited to a small number of holometabolous insect species and have not determined the pattern of evolution of the clustered sHSP genes (sHSP-C) in insect or Arthropod lineages.

Results

Using eight genomes from representative insect orders and three non-insect arthropod genomes we have identified that a syntenic cluster of sHSPs (sHSP-C) is a hallmark of most Arthropod genomes. Using 11 genomes from Hymenopteran species our phylogenetic analyses have refined the evolution of the sHSP-C in Hymenoptera and found that the sHSP-C is order-specific with evidence of birth-and-death evolution in the hymenopteran lineage. Finally we have shown that the honeybee sHSP-C is co-ordinately expressed and is marked by genomic features, including H3K27me3 histone marks consistent with coordinate regulation, during honeybee ovary activation.

Conclusions

The syntenic sHSP-C is present in most insect genomes, and its conserved coordinate expression and regulation implies that it is an integral genomic component of environmental response in arthropods.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01885-8.

Dynamics of heat shock protein responses to thermal stress changes after metamorphosis in a lepidopteran insect

In the last decade, unexpected high temperatures have been frequent in spring and early summer. Numerous studies have shown that such thermal stress has substantial effects on life-history traits that influence fitness of insects, but few have examined expression dynamics of heat shock proteins (Hsps) across developmental stages, especially as regards potential carry-over effects at the transcriptional level across metamorphosis. We exposed pupae of the oriental fruit moth ("OFM," Grapholita molesta Busck) to mild heat stress (38°C, 6 h) and then quantified expression patterns of six Hsps (Hsp90, 70, 60, 40, 21, and 11) from pupal through adult stages. Almost all Hsps showed a higher expression immediately after pupae were heat-stressed, but later dropped to normal levels after metamorphosis. Although upregulation of Hsps is transient and the effects carry over longer to early adult stage, upregulation will nonetheless have positive effects on adult fitness. The fitness of some insects may benefit from higher expression of chaperon genes after mild stress, in the form of higher fecundity and longer lifespan, as a carry-over effect. These results suggest that mild thermal stress can change genetic expression that later boosts adult fitness through a cascade effect.

Identification of five small heat shock protein genes in Spodoptera frugiperda and expression analysis in response to different environmental stressors

Spodoptera frugiperda (J. E. Smith) is a highly adaptable polyphagous migratory pest in tropical and subtropical regions. Small heat shock proteins (sHsps) are molecular chaperones that play important roles in the adaptation to various environment stressors. The present study aimed to clarify the response mechanisms of S. frugiperda to various environmental stressors. We obtained five S. furcifera sHsp genes (SfsHsp21.3, SfsHsp20, SfsHsp20.1, SfsHsp19.3, and SfsHsp29) via cloning. The putative proteins encoded by these genes contained a typical α-crystallin domain. The expression patterns of these genes during different developmental stages, in various tissues of male and female adults, as well as in response to extreme temperatures and UV-A stress were studied via real-time quantitative polymerase chain reaction. The results showed that the expression levels of all five SfsHsp genes differed among the developmental stages as well as among the different tissues of male and female adults. The expression levels of most SfsHsp genes under extreme temperatures and UV-A-induced stress were significantly upregulated in both male and female adults. In contrast, those of SfsHsp20.1 and SfsHsp19.3 were significantly downregulated under cold stress in male adults. Therefore, the different SfsHsp genes of S. frugiperda play unique regulatory roles during development as well as in response to various environmental stressors.

Heat stress-induced HSP90 expression is dependent on ERK and HSF1 activation in turbot (Scophthalmus maximus) kidney cells

Mitogen-activated protein kinases (MAPKs) and heat shock proteins (HSPs) are ubiquitous proteins that are functional mediators in both normal and stressed states of the cell. In this study, we performed heat stress (37 °C) experiments on turbot kidney (TK) cells. Heat stress expression patterns of HSP90, as well as the expression and phosphorylation levels of extracellular-regulated signal kinases (ERKs) and the transcription factor HSF1 and c-Fos, were examined. The results show that heat stress activates ERK1/2 and HSF1, and induces HSP90 gene expression in TK cells. Inhibition of ERK activation attenuates heat stress-induced HSP90 gene expression. The double luciferase reporter gene experiment showed that HSF1 is an important transcription factor for heat-induced HSP90 gene expression. Likewise, c-Fos does not directly regulate the heat-induced expression of HSP90 in turbot kidney cells. To our knowledge, this is the first study to report a signaling pathway that regulates the heat shock response in turbot cells. Our results may facilitate an understanding of the underlying molecular mechanisms of the cellular stress response in marine fish.

Analysis of insect nuclear small heat shock proteins and interacting proteins

The small heat shock proteins (sHsps) are a ubiquitous family of ATP-independent stress proteins found in all domains of life. Drosophila melanogaster Hsp27 (DmHsp27) is the only known nuclear sHsp in insect. Here analyzing sequences from HMMER, we identified 56 additional insect sHsps with conserved arginine-rich nuclear localization signal (NLS) in the N-terminal region. At this time, the exact role of nuclear sHsps remains unknown. DmHsp27 protein-protein interaction analysis from iRefIndex database suggests that this protein, in addition to a putative role of molecular chaperone, is likely involved in other nuclear processes (i.e., chromatin remodeling and transcription). Identification of DmHsp27 interactors should provide key insights on the cellular and molecular functions of this nuclear chaperone.

Small heat shock protein Hsp67Bc plays a significant role in Drosophila melanogaster cold stress tolerance

Hsp67Bc in Drosophila melanogaster is a member of the small heat shock protein family, the main function of which is to prevent the aggregation of misfolded or damaged proteins. Hsp67Bc interacts with Starvin and Hsp23, which are known to be a part of the cold stress response in the fly during the recovery phase. In this study, we investigated the role of the Hsp67Bc gene in the cold stress response. We showed that in adult Drosophila, Hsp67Bc expression increases after cold stress and decreases after 1.5 h of recovery, indicating the involvement of Hsp67Bc in short-term stress recovery. We also implemented a deletion in the D. melanogaster Hsp67Bc gene using imprecise excision of a P-element, and analysed the cold tolerance of Hsp67Bc-null mutants at different developmental stages. We found that Hsp67Bc-null homozygous flies are viable and fertile but display varying cold stress tolerance throughout the stages of ontogenesis: the survival after cold stress is slightly impaired in late third instar larvae, unaffected in pupae, and notably affected in adult females. Moreover, the recovery from chill coma is delayed in Hsp67Bc-null adults of both sexes. In addition, the deletion in the Hsp67Bc gene caused more prominent up-regulation of Hsp70 following cold stress, suggesting the involvement of Hsp70 in compensation of the lack of the Hsp67Bc protein. Taken together, our results suggest that Hsp67Bc is involved in the recovery of flies from a comatose state and contributes to the protection of the fruit fly from cold stress.

Induced expression of small heat shock proteins is associated with thermotolerance in female Laodelphax striatellus planthoppers

Insects are often exposed to high temperature stress in natural environments, but the mechanisms involved in thermotolerance in many insect groups like Hemiptera are not well known. To explore possible mechanisms of thermotolerance in the hemipteran pest Laodelphax striatellus, which damages rice through direct feeding and viral transmission, small heat shock proteins (sHsps) implicated in thermotolerance in other insect groups were identified. The seven sHsps identified have a conserved alpha crystallin domain, a variable N-terminal region, and shared relative low identities to each other. Three of the sHsp genes (LsHsp20.5, LsHsp21.5, and LsHsp21.6) exhibited higher basal expression than the other four genes but showed weak or no heat-induced expression. The other four genes (LsHsp20.1, LsHsp21.2, LsHsp21.4, and LsHsp22.0) were induced up to 3306-fold by heat. Injection of dsRNA indicated that expression of these sHsps was associated with thermotolerance, and Escherichia coli transformed with LsHsp21.2 and LsHsp20.1 showed relatively higher thermotolerance. These results point to an important functional role of these sHsps for thermotolerance in L. striatellus.

Maternal loading of a small heat shock protein increases embryo thermal tolerance in Drosophila melanogaster

Maternal investment is likely to have direct effects on offspring survival. In oviparous animals whose embryos are exposed to the external environment, maternal provisioning of molecular factors like mRNAs and proteins may help embryos cope with sudden changes in the environment. Here, we sought to modify the maternal mRNA contribution to offspring embryos and test for maternal effects on acute thermal tolerance in early embryos of Drosophila melanogaster We drove in vivo overexpression of a small heat shock protein gene (Hsp23) in female ovaries and measured the effects of acute thermal stress on offspring embryonic survival and larval development. We report that overexpression of the Hsp23 gene in female ovaries produced offspring embryos with increased thermal tolerance. We also found that brief heat stress in the early embryonic stage (0-1 h old) caused decreased larval performance later in life (5-10 days old), as indexed by pupation height. Maternal overexpression of Hsp23 protected embryos against this heat-induced defect in larval performance. Our data demonstrate that transient products of single genes have large and lasting effects on whole-organism environmental tolerance. Further, our results suggest that maternal effects have a profound impact on offspring survival in the context of thermal variability.

Oligomeric structure and chaperone-like activity of Drosophila melanogaster mitochondrial small heat shock protein Hsp22 and arginine mutants in the alpha-crystallin domain

The structure and chaperone function of DmHsp22WT, a small Hsp of Drosophila melanogaster localized within mitochondria were examined. Mutations of conserved arginine mutants within the alpha-crystallin domain (ACD) domain (R105G, R109G, and R110G) were introduced, and their effects on oligomerization and chaperone function were assessed. Arginine to glycine mutations do not induce significant changes in tryptophan fluorescence, and the mutated proteins form oligomers that are of equal or smaller size than the wild-type protein. They all form oligomer with one single peak as determined by size exclusion chromatography. While all mutants demonstrate the same efficiency as the DmHsp22WT in a DTT-induced insulin aggregation assay, all are more efficient chaperones to prevent aggregation of malate dehydrogenase. Arginine mutants of DmHsp22 are efficient chaperones to retard aggregation of CS and Luc. In summary, this study shows that mutations of arginine to glycine in DmHsp22 ACD induce a number of structural changes, some of which differ from those described in mammalian sHsps. Interestingly, only the R110G-DmHsp22 mutant, and not the expected R109G equivalent to human R140-HspB1, R116-HspB4, and R120-HspB5, showed different structural properties compared with the DmHsp22WT.

Oligomerization and chaperone-like activity of Drosophila melanogaster small heat shock protein DmHsp27 and three arginine mutants in the alpha-crystallin domain

The small Hsp DmHsp27 from Drosophila melanogaster is one of the few small heat shock proteins (sHsps) found within the nucleus. We report that its dimerization is independent of disulfide bond formation and seems to rely on salt bridges. Unlike metazoan sHsps, DmHsp27 forms two populations of oligomers not in equilibrium. Mutations at highly conserved arginine residues in mammalian sHsps have been reported to be associated with protein conformational defects and intracellular aggregation. Independent mutation of three highly conserved arginines (R122, R131, and R135) to glycine in DmHsp27 results in only one population of higher molecular weight form. In vitro, the chaperone-like activity of wild-type DmHsp27 was comparable with that of its two isolated populations and to the single population of the R122G, R131G, and R135G using luciferase as substrate. However, using insulin, the chaperone-like activity of wild-type DmHsp27 was lower than that of R122G and R131G mutants. Altogether, the results characterize wild-type DmHsp27 and its alpha-crystallin domain (ACD) arginine mutants and may give insight into protection mechanism of sHsps.

Changes in Drosophila mitochondrial proteins following chaperone-mediated lifespan extension confirm a role of Hsp22 in mitochondrial UPR and reveal a mitochondrial localization for cathepsin D

Hsp22 is a small mitochondrial heat shock protein (sHSP) preferentially up-regulated during aging in Drosophila melanogaster. Its developmental expression is strictly regulated and it is rapidly induced in conditions of stress. Hsp22 is one of the few sHSP to be localized inside mitochondria, and is the first sHSP to be involved in the mitochondrial unfolding protein response (UPR(MT)) together with Hsp60, mitochondrial Hsp70 and TRAP1. The UPR(MT) is a pro-longevity mechanism, and interestingly Hsp22 over-expression by-itself increases lifespan and resistance to stress. To unveil the effect of Hsp22 on the mitochondrial proteome, comparative IEF/SDS polyacrylamide 2D gels were done on mitochondria from Hsp22+ flies and controls. Among the proteins influenced by Hsp22 expression were proteins from the electron transport chain (ETC), the TCA cycle and mitochondrial Hsp70. Hsp22 co-migrates with ETC components and its over-expression is associated with an increase in mitochondrial protease activity. Interestingly, the only protease that showed significant changes upon Hsp22 over-expression in the comparative IEF/SDS-PAGE analysis was cathepsin D, which is localized in mitochondria in addition to lysosome in D. melanogaster as evidenced by cellular fractionation. Together the results are consistent with a role of Hsp22 in the UPR(MT) and in mitochondrial proteostasis.

The impact of green tea polyphenols on development and reproduction in Drosophila melanogaster

Although, green tea has numerous health benefits, adverse effects with excessive consumption have been reported. Using Drosophila melanogaster, a decrease in male fertility with green tea was evidenced. Here, the extent of green tea toxicity on development and reproduction was investigated. Drosophila melanogaster embryos and larvae were exposed to various doses of green tea polyphenols (GTP). Larvae exposed to 10 mg/mL GTP were slower to develop, emerged smaller, and exhibited a dramatic decline in the number of emerged offspring. GTP protected flies against desiccation but sensitized them to starvation and heat stress. Female offspring exhibited a decline in reproductive output and decreased survival while males were unaffected. GTP had a negative impact on reproductive organs in both males and females (e.g., atrophic testes in males, absence of mature eggs in females). Collectively, the data show that high doses of GTP adversely affect development and reproduction of Drosophila melanogaster.