Analysis of the expression and function of the small heat shock protein gene, hsp27, in Xenopus laevis embryos

Insights from a vertebrate model organism on the molecular mechanisms of whole-body dehydration tolerance

Studies on the molecular mechanisms of dehydration tolerance have been largely limited to plants and invertebrates. Currently, research in whole body dehydration of complex animals is limited to cognitive and behavioral effects in humans, leaving the molecular mechanisms of vertebrate dehydration relatively unexplored. The present review summarizes studies to date on the African clawed frog (Xenopus laevis) and examines whole-body dehydration on physiological, cellular and molecular levels. This aquatic frog is exposed to seasonal droughts in its native habitat and can endure a loss of over 30% of its total body water. When coping with dehydration, osmoregulatory processes prioritize water retention in skeletal tissues and vital organs over plasma volume. Although systemic blood circulation is maintained in the vital organs and even elevated in the brain during dehydration, it is done so at the expense of reduced circulation to the skeletal muscles. Increased hemoglobin affinity for oxygen helps to counteract impaired blood circulation and metabolic enzymes show altered kinetic and regulatory parameters that support the use of anaerobic glycolysis. Recent studies with X. laevis also show that pro-survival pathways such as antioxidant defenses and heat shock proteins are activated in an organ-specific manner during dehydration. These pathways are tightly coordinated at the post-transcriptional level by non-coding RNAs, and at the post-translational level by reversible protein phosphorylation. Paired with ongoing research on the X. laevis genome, the African clawed frog is poised to be an ideal animal model with which to investigate the molecular adaptations for dehydration tolerance much more deeply.

Thermal and salinity effects on locomotor performance of Thoropa taophora tadpoles (Anura, Cycloramphidae)

It is well known that environmental temperature influences several biological functions of ectotherms, notably in amphibians. The high permeability of anuran skin, associated with the effect of elevated environmental temperature, potentiates the dehydration process and this combination may restrict locomotor performance. Thoropa taophora is an endemic species from the Atlantic Rainforest whose tadpoles are semiterrestrial and predominantly diurnal, and are found in rocky seashores where they are exposed to sea spray and high temperatures. In this study we investigated how temperature and salinity conditions affect the locomotor performance in Thoropa taophora tadpoles. We also assessed how different osmotic concentrations affect the activity of the metabolic pathways that support muscle function. We measured the sprint speed of tadpoles of various sizes at different temperatures and salinities in the field. We also measured the activity of the enzymes pyruvate kinase (PK), lactate dehydrogenase (LDH) and citrate synthase (CS) in different temperatures and osmotic concentrations, and calculated the thermal sensitivity and the activity constants for each osmolality. Our results showed that, in general, sprint speed decreased with increasing temperature and salinity. However, whereas the effect of increased salinity was similar in smaller and larger tadpoles, increased temperature had a higher negative impact on sprint speed of larger tadpoles, thus indicating low thermal sensitivity of small tadpoles. PK and LDH thermal sensitivities and LDH constant of activity decreased as the osmolality increased. In conclusion, the locomotor capacity of tadpoles was decreased by temperature and salinity, which may be related to a decrease in anaerobic metabolism both in terms of sensitivity and total energy turnover through enzymatic activity. We discuss the ecological consequences, including the potential impacts on predator escape behavior promoted by changes in metabolism and locomotor performance in an early stage of development of this species.

The regulation of heat shock proteins in response to dehydration in Xenopus laevis

African clawed frogs (Xenopus laevis) endure bouts of severe drought in their natural habitats and survive the loss of approximately 30% of total body water due to dehydration. To investigate molecular mechanisms employed by X. laevis during periods of dehydration, the heat shock protein response, a vital component of the cytoprotective stress response, was characterized. Using western immunoblotting and multiplex technology, the protein levels of HSP27, HSP40, HSP60, HSP70, HSC70, and HSP90 were quantified in the liver, skeletal muscle, kidney, lung, and testes from control frogs and those that underwent medium or high dehydration (~16 or ~30% loss of total body water). Dehydration increased HSP27 (1.45-1.65-fold) in the kidneys and lungs, and HSP40 (1.39-2.50-fold) in the liver, testes, and skeletal muscle. HSP60 decreased in response to dehydration (0.43-0.64 of control) in the kidneys and lungs. HSP70 increased in the liver, lungs, and testes (1.39-1.70-fold) during dehydration, but had a dynamic response in the kidneys (levels increased 1.57-fold with medium dehydration, but decreased to 0.56 of control during high dehydration). HSC70 increased in the liver and kidneys (1.20-1.36-fold), but decreased in skeletal muscle (0.27-0.55 of control) during dehydration. Lastly, HSP90 was reduced in the kidney, lung, and skeletal muscle (0.39-0.69 of control) in response to dehydration, but rose in the testes (1.30-fold). Overall, the results suggest a dynamic tissue-specific heat shock protein response to whole body dehydration in X. laevis.

The expression and function of hsp30-like small heat shock protein genes in amphibians, birds, fish, and reptiles

Small heat shock proteins (sHSPs) are a superfamily of molecular chaperones with important roles in protein homeostasis and other cellular functions. Amphibians, reptiles, fish and birds have a shsp gene called hsp30, which was also referred to as hspb11 or hsp25 in some fish and bird species. Hsp30 genes, which are not found in mammals, are transcribed in response to heat shock or other stresses by means of the heat shock factor that is activated in response to an accumulation of unfolded protein. Amino acid sequence analysis revealed that representative HSP30s from different classes of non-mammalian vertebrates were distinct from other sHSPs including HSPB1/HSP27. Studies with amphibian and fish recombinant HSP30 determined that they were molecular chaperones since they inhibited heat- or chemically-induced aggregation of unfolded protein. During non-mammalian vertebrate development, hsp30 genes were differentially expressed in selected tissues. Also, heat shock-induced stage-specific expression of hsp30 genes in frog embryos was regulated at the level of chromatin structure. In adults and/or tissue culture cells, hsp30 gene expression was induced by heat shock, arsenite, cadmium or proteasomal inhibitors, all of which enhanced the production of unfolded/damaged protein. Finally, immunocytochemical analysis of frog and chicken tissue culture cells revealed that proteotoxic stress-induced HSP30 accumulation co-localized with aggresome-like inclusion bodies. The congregation of damaged protein in aggresomes minimizes the toxic effect of aggregated protein dispersed throughout the cell. The current availability of probes to detect the presence of hsp30 mRNA or encoded protein has resulted in the increased use of hsp30 gene expression as a marker of proteotoxic stress in non-mammalian vertebrates.

Expression and localization of the Xenopus laevis small heat shock protein, HSPB6 (HSP20), in A6 kidney epithelial cells

Small heat shock proteins (sHSPs) are molecular chaperones that bind to unfolded protein, inhibit the formation of toxic aggregates and facilitate their refolding and/or degradation. Previously, the only sHSPs that have been studied in detail in the model frog system, Xenopus laevis, were members of the HSP30 family and HSPB1 (HSP27). We now report the analysis of X. laevis HSPB6, an ortholog of mammalian HSPB6. X. laevis HSPB6 cDNA encodes a 168 aa protein that contains an α-crystallin domain, a polar C-terminal extension and some possible phosphorylation sites. X. laevis HSPB6 shares 94% identity with a X. tropicalis HSPB6, 65% with turtle, 59% with humans, 49% with zebrafish and only 50% and 43% with X. laevis HSPB1 and HSP30C, respectively. Phylogenetic analysis revealed that X. laevis HSPB6 grouped more closely with mammalian and reptilian HSPB6s than with fish HSPB6. X. laevis recombinant HSPB6 displayed molecular chaperone properties since it had the ability to inhibit heat-induced aggregation of citrate synthase. Immunoblot analysis determined that HSPB6 was present constitutively in kidney epithelial cells and that heat shock treatment did not upregulate HSPB6 levels. While treatment with the proteasomal inhibitor, MG132, resulted in a 2-fold increase in HSPB6 levels, exposure to cadmium chloride produced a slight increase in HSPB6. These findings were in contrast to HSP70, which was enhanced in response to all three stressors. Finally, immunocytochemical analysis revealed that HSPB6 was present in the cytoplasm in the perinuclear region with some in the nucleus.

Contribution of small heat shock proteins to muscle development and function

Investigations undertaken over the past years have led scientists to introduce the concept of protein quality control (PQC) systems, which are responsible for polypeptide processing. The PQC system monitors proteostasis and involves activity of different chaperones such as small heat shock proteins (sHSPs). These proteins act during normal conditions as housekeeping proteins regulating cellular processes, and during stress conditions. They also mediate the removal of toxic misfolded polypeptides and thereby prevent development of pathogenic states. It is postulated that sHSPs are involved in muscle development. They could act via modulation of myogenesis or by maintenance of the structural integrity of signaling complexes. Moreover, mutations in genes coding for sHSPs lead to pathological states affecting muscular tissue functioning. This review focuses on the question how sHSPs, still relatively poorly understood proteins, contribute to the development and function of three types of muscle tissue: skeletal, cardiac and smooth.

Distinct patterns of HSP30 and HSP70 degradation in Xenopus laevis A6 cells recovering from thermal stress

Heat shock proteins (HSPs) are molecular chaperones that assist in protein synthesis, folding and degradation and prevent stress-induced protein aggregation. In this study, we examined the pattern of accumulation of HSP30 and HSP70 in Xenopus laevis A6 kidney epithelial cells recovering from heat shock. Immunoblot analysis revealed the presence of elevated levels of HSP30 after 72h of recovery. However, the relative levels of HSP70 declined to near control levels after 24h. The relative levels of both hsp30 and hsp70 mRNA were reduced to low levels after 24h of recovery from heat shock. Pretreatment of cells with cycloheximide, a translational inhibitor, produced a rapid decline in HSP70 but not HSP30. The cycloheximide-associated decline of HSP70 was blocked by the proteasomal inhibitor, MG132, but had little effect on the relative level of HSP30. Also, treatment of cells with the phosphorylation inhibitor, SB203580, in addition to cycloheximide treatment enhanced the stability of HSP30 compared to cycloheximide alone. Immunocytochemical studies detected the presence of HSP30 accumulation in a granular pattern in the cytoplasm of recovering cells and its association with aggresome-like structures, which was enhanced in the presence of SB203580. This study has shown that the relative levels of heat shock-induced HSP30 persist during recovery in contrast to HSP70. While HSP70 is degraded by the ubiquitin-proteasome system, it is likely that the presence of HSP30 multimeric complexes that are known to associate with unfolded protein as well as its association with aggresome-like structures may delay its degradation.

Small heat shock protein HSPB1 regulates growth of embryonic zebrafish craniofacial muscles

The small heat shock protein HspB1 (Hsp27) is abundantly expressed in embryonic muscle tissues of a wide variety of vertebrate species. However, the functional significance of this expression pattern is not well established. In the present study, we observed specific, high level expression of HspB1 protein and an HspB1 gene reporter in developing craniofacial muscles of the zebrafish, Danio rerio, and examined the consequences of reducing HspB1 expression to the development and growth of these muscles. Quantitative morphometric analyses revealed a reduction in the cross-sectional area of myofibers in embryos expressing reduced HspB1 levels by as much as 47% compared to controls. In contrast, we detected no differences in the number of myofibrils or associated nuclei, nor the number, size or development of chondrocytes in surrounding tissues. We also did not detect changes to the overall organization of sarcomeres or myofibrils in embryos expressing reduced levels of HspB1. Together our results reveal a critical role for HspB1 in the growth of myofibrils and provide new insight into the mechanism underlying its developmental function.

Small heat shock protein expression and functions during development

The expression of small heat shock proteins is tightly regulated during development in multiple organisms. As housekeeping proteins, small heat shock proteins help protect cells from apoptosis, stabilize the cytoskeleton and contribute to proteostasis. Consistently, depletion of one small heat shock protein is usually not detrimental due to a certain level of redundancy between the functions of each small heat shock protein. However, while their stress-induced expression is regulated by heat shock factors, their constitutive expression is under the control of other specific transcription factors, suggesting the existence of very specialized functions. This review focuses on the expression patterns and functions of small heat shock proteins in various organisms during development. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.

Silkworm egg proteins at the germ-band formation stage and a functional analysis of BmEP80 protein

The patterning of embryos in early stages is a critical process for embryo development. In order to understand the molecular mechanism of early embryogenesis in silkworm, 2-DE combined with MALDI-TOF-MS technologies were used to analyze the proteins from diapause-destined eggs at the germ-band formation stage. From over 1000 spots, 93 were selected for analysis and data were obtained from 59 revealing 42 proteins. Gene Ontology annotation showed these proteins were involved in several biological processes at the germ-band formation stage, including cell stress response and protein folding, cell growth and migration, termination of diapause, and nutrition storage. Prominent among them was a new 80 kDa protein, named Bombyx mori egg protein 80 (BmEP80). BmEP80 was a component of the eggshell which was secreted by follicle cells during the late vitellogenesis stage to early choriogenesis stage (FCs -5 to +10). It disappears during early embryogenesis and RNAi against it resulted in the collapse of eggs, thus it is likely that BmEP80 is a new component of the silkworm vitelline membrane.

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

BACKGROUND

The aging related decline of heat shock factor-1 (HSF1) signaling may be causally related to protein aggregation diseases. To model such disease, we tried to cripple HSF1 signaling in the Xenopus tadpole.

RESULTS

Over-expression of heat shock factor binding protein-1 did not inhibit the heat shock response in Xenopus. RNAi against HSF1 mRNA inhibited the heat shock response by 70% in Xenopus A6 cells, but failed in transgenic tadpoles. Expression of XHSF380, a dominant-negative HSF1 mutant, was embryonic lethal, which could be circumvented by delaying expression via a tetracycline inducible promoter. HSF1 signaling is thus essential for embryonic Xenopus development. Surprisingly, transgenic expression of the XHSF380 or of full length HSF1, whether driven by a ubiquitous or a neural specific promoter, was not detectable in the larval brain.

CONCLUSIONS

Our finding that the majority of neurons, which have little endogenous HSF1, refused to accept transgene-driven expression of HSF1 or its mutant suggests that HSF1 levels are strictly controlled in neuronal tissue.

Hsp27 is persistently expressed in zebrafish skeletal and cardiac muscle tissues but dispensable for their morphogenesis

Constitutive expression of Hsp27 has been demonstrated in vertebrate embryos, especially in developing skeletal and cardiac muscle. Results of several previous studies have indicated that Hsp27 could play a role in the development of these tissues. For example, inhibition of Hsp27 expression has been reported to cause defective development of mammalian myoblasts in vitro and frog embryos in vivo. In contrast, transgenic mice lacking Hsp27 develop normally. Here, we examined the distribution of Hsp27 protein in developing and adult zebrafish and effects of suppressing Hsp27 expression using phosphorodiamidate morpholino oligonucleotides (PMO) on zebrafish development. Consistent with our previous analysis of hsp27 messenger RNA expression, we detected the protein Hsp27 in cardiac, smooth, and skeletal muscle of both embryonic and adult zebrafish. However, embryos lacking detectable Hsp27 after injection of antisense hsp27 PMO exhibited comparable heart beat rates to that of control embryos and cardiac morphology was indistinguishable in the presence or absence of Hsp27. Loss of Hsp27 also had no effect on the structure of the skeletal muscle myotomes in the developing embryo. Finally, embryos injected with antisense hsp27 and scrambled control PMO displayed equal motility. We conclude that Hsp27 is dispensable for zebrafish morphogenesis but could play a role in long-term maintenance of heart and muscle tissues.

The small heat shock protein (sHSP) genes in the silkworm, Bombyx mori, and comparative analysis with other insect sHSP genes

Background

Small heat shock proteins (sHSPs) are products of heat shock response and of other stress responses, and ubiquitous in all three domains of life, archaea, bacteria, and eukarya. They mainly function as molecular chaperones to protect proteins from being denatured in extreme conditions. Study on insect sHSPs could provide some insights into evolution of insects that have adapted to diverse niches in the world.

Results

Taking advantage of the newly assembled genome sequence, we performed a genome-wide analysis of the candidate sHSP genes in the silkworm, Bombyx mori. Based on known silkworm sHSP sequences, we identified 16 silkworm sHSP genes. Most of them are distributed on two silkworm chromosomes 5 and 27, respectively. 15 of 16 silkworm sHSPs have expression evidence. The comparative analysis of insect sHSPs from B. mori, Drosophila melanogaster, Apis mellifera, Tribolium castaneum, and Anopheles gambiae revealed that there is only one orthologous cluster whereas remaining clusters are species-specific on the phylogenetic tree. This suggested that most of sHSPs might have diverged in function across insects investigated. In addition, the data presented in this study also revealed that sHSPs in the insect orthologous cluster are highly conserved in both sequence and expression pattern. In sum, insect sHSPs show a completely different evolutionary pattern from that found in vertebrate sHSPs.

Conclusion

B. mori has the largest number of insect sHSP genes characterized to date, including 16 genes. The inference that most species-specific sHSPs might have diverged in function across insects investigated will help us understand the adaptability of these insects to diverse environments.

Developmental expression patterns of the zebrafish small heat shock proteins

Small heat shock proteins (sHSPs), or alpha-crystallins, are low-molecular weight proteins found in every kingdom and nearly every species examined to date. Many, if not all, sHSPs act as molecular chaperones. Several also have functions independent of their chaperone activity, and at least a few are expressed in specific spatiotemporal patterns during embryonic and/or juvenile stages, suggesting specific roles during development. To date, however, no one has systematically characterized the expression patterns of all of the sHSPs during development in any organism. We have characterized the normal heat shock-induced expression patterns of all 13 zebrafish sHSPs during development. Seven of the sHSPs are expressed in a tissue-specific manner during development, and five are upregulated by heat shock. The results of these studies provide a foundation for analysis of sHSP function during normal development and their roles in protecting cells from the effects environmental stressors.

Small heat shock protein Hsp27 is required for proper heart tube formation

The small heat shock protein Hsp27 has been shown to be involved in a diverse array of cellular processes, including cellular stress response, protein chaperone activity, regulation of cellular glutathione levels, apoptotic signaling, and regulation of actin polymerization and stability. Furthermore, mutation within Hsp27 has been associated with the human congenital neuropathy Charcot-Marie Tooth (CMT) disease. Hsp27 is known to be expressed in developing embryonic tissues; however, little has been done to determine the endogenous requirement for Hsp27 in developing embryos. In this study, we show that depletion of XHSP27 protein results in a failure of cardiac progenitor fusion resulting in cardia bifida. Furthermore, we demonstrate a concomitant disorganization of actin filament organization and defects in myofibril assembly. Moreover, these defects are not associated with alterations in specification or differentiation. We have thus demonstrated a critical requirement for XHSP27 in developing cardiac and skeletal muscle tissues.

ArHsp21, a developmentally regulated small heat-shock protein synthesized in diapausing embryos of Artemia franciscana

Embryos of the crustacean, Artemia franciscana, undergo alternative developmental pathways, producing either larvae or encysted embryos (cysts). The cysts enter diapause, characterized by exceptionally high resistance to environmental stress, a condition thought to involve the sHSP (small heat-shock protein), p26. Subtractive hybridization has revealed another sHSP, termed ArHsp21, in diapause-destined Artemia embryos. ArHsp21 shares sequence similarity with p26 and sHSPs from other organisms, especially in the α-crystallin domain. ArHsp21 is the product of a single gene and its synthesis occurred exclusively in diapause-destined embryos. Specifically, ArHsp21 mRNA appeared 2 days post-fertilization, followed 1 day later by the protein, and then increased until embryo release at day 5. No ArHsp21 protein was detected in embryos developing directly into larvae, although there was a small amount of mRNA at 3 days post-fertilization. The protein was degraded during post-diapause development and had disappeared completely from second instar larvae. ArHsp21 formed large oligomers in encysted embryos and transformed bacteria. When purified from bacteria, ArHsp21 functioned as a molecular chaperone in vitro, preventing heat-induced aggregation of citrate synthase and reduction-driven denaturation of insulin. Sequence characteristics, synthesis patterns and functional properties demonstrate clearly that ArHsp21 is an sHSP able to chaperone other proteins and contribute to stress tolerance during diapause. As such, ArHsp21 would augment p26 chaperone activity and it may also possess novel activities that benefit Artemia embryos exposed to stress.

Genome-wide analysis and expression profiling of the small heat shock proteins in zebrafish

Small Heat Shock Proteins (sHSPs) have important roles in preventing disease and promoting resistance to environmental stressors. Mutations in any one of a number of sHSPs, including HSP27 (HSPB1), HSP22 (HSPB8), alphaA-crystallin (HSPB4), or alphaB-crystallin (HSPB5) can result in neuronal degeneration, myopathy, and/or cataract in humans. Ten sHSPs are known in humans, and thirteen have been identified in teleost fish. Here we report the identification of thirteen zebrafish sHSPs. Using a combination of phylogenetic analysis and analysis of synteny, we have determined that ten are likely orthologs of human sHSPs. We have used quantitative RT-PCR to determine the relative expression levels of all thirteen sHSPs during development and in response to heat shock. Our findings indicate that most of the zebrafish sHSPs are expressed during development, and five of these genes are transcriptionally upregulated by heat shock at one or more stages of development.