Acquisition of the heat-shock response and thermotolerance during early development of Xenopus laevis

What's Genetic Variation Got to Do with It? Starvation-Induced Self-Fertilization Enhances Survival in Paramecium

The pervasiveness of sex despite its well-known costs is a long-standing puzzle in evolutionary biology. Current explanations for the success of sex in nature largely rely on the adaptive significance of the new or rare genotypes that sex may generate. Less explored is the possibility that sex-underlying molecular mechanisms can enhance fitness and convey benefits to the individuals that bear the immediate costs of sex. Here, we show that the molecular environment associated with self-fertilization can increase stress resistance in the ciliate Paramecium tetraurelia. This advantage is independent of new genetic variation, coupled with a reduced nutritional input, and offers fresh insights into the mechanistic origin of sex. In addition to providing evidence that the molecular underpinnings of sexual reproduction and the stress response are linked in P. tetraurelia, these findings supply an integrative explanation for the persistence of self-fertilization in this ciliate.

Fish embryo vulnerability to combined acidification and warming coincides with a low capacity for homeostatic regulation

The vulnerability of fish embryos and larvae to environmental factors is often attributed to a lack of adult-like organ systems (gills) and thus insufficient homeostatic capacity. However, experimental data supporting this hypothesis are scarce. Here, by using Atlantic cod (Gadus morhua) as a model, the relationship between embryo vulnerability (to projected ocean acidification and warming) and homeostatic capacity was explored through parallel analyses of stage-specific mortality and in vitro activity and expression of major ion pumps (ATP-synthase, Na+/K+-ATPase, H+-ATPase) and co-transporters (NBC1, NKCC1). Immunolocalization of these transporters was used to study ionocyte morphology in newly hatched larvae. Treatment-related embryo mortality until hatching (+20% due to acidification and warming) occurred primarily during an early period (gastrulation) characterized by extremely low ion transport capacity. Thereafter, embryo mortality decreased in parallel with an exponential increase in activity and expression of all investigated ion transporters. Significant changes in transporter activity and expression in response to acidification (+15% activity) and warming (-30% expression) indicate some potential for short-term acclimatization, although this is probably associated with energetic trade-offs. Interestingly, whole-larvae enzyme activity (supported by abundant epidermal ionocytes) reached levels similar to those previously measured in gill tissue of adult cod, suggesting that early-life stages without functional gills are better equipped in terms of ion homeostasis than previously thought. This study implies that the gastrulation period represents a critical transition from inherited (maternal) defenses to active homeostatic regulation, which facilitates enhanced resilience of later stages to environmental factors.

Development of the embryonic heat shock response and the impact of repeated thermal stress in early stage lake whitefish (Coregonus clupeaformis) embryos

Lake whitefish (Coregonus clupeaformis) embryos were exposed to thermal stress (TS) at different developmental stages to determine when the heat shock response (HSR) can be initiated and if it is altered by exposure to repeated TS. First, embryos were subject to one of three different TS temperatures (6, 9, or 12°C above control) at 4 points in development (21, 38, 60 and 70 days post-fertilisation (dpf)) for 2h followed by a 2h recovery to understand the ontogeny of the HSR. A second experiment explored the effects of repeated TS on the HSR in embryos from 15 to 75 dpf. Embryos were subjected to one of two TS regimes; +6°C TS for 1h every 6 days or +9°C TS for 1h every 6 days. Following a 2h recovery, a subset of embryos was sampled. Our results show that embryos could initiate a HSR via upregulation of heat shock protein 70 (hsp70) mRNA at all developmental ages studied, but that this response varied with age and was only observed with a TS of +9 or +12°C. In comparison, when embryos received multiple TS treatments, hsp70 was not induced in response to the 1h TS and 2h recovery, and a downregulation was observed at 39 dpf. Downregulation of hsp47 and hsp90α mRNA was also observed in early age embryos. Collectively, these data suggest that embryos are capable of initiating a HSR at early age and throughout embryogenesis, but that repeated TS can alter the HSR, and may result in either reduced responsiveness or a downregulation of inducible hsps. Our findings warrant further investigation into both the short- and long-term effects of repeated TS on lake whitefish development.

Thermotolerance and HSP70 expression in the Mediterranean fruit fly Ceratitis capitata

The relationship between Hsp70 expression and thermotolerance has been well documented in Drosophila melanogaster. However, there is limited information on this relationship in other insect species. In this report we describe the Hsp70-thermotolerance relationship in one of the major fruit fly pests, Ceratitis capitata (medfly). Hsp70 expression and thermotolerance were assayed at a range of temperatures in several stages of medfly development. The most thermotolerant stage was found to be the late larval stage (100% survival at 41 degrees C) followed by adult flies and late embryos (100% survival at 39 degrees C). These three stages showed a positive relationship between Hsp70 expression and thermotolerance. Mid-larval and mid-embryonic stages were found less thermotolerant and the Hsp70-thermotolerance relationship was not evident. Early embryos did not express Hsp70 at any temperature and exhibited the lowest thermotolerance. The relationship between Hsp70 and inducible thermotolerance was also studied in late larvae. A pretreatment at 37-39 degrees C increased thermotolerance at higher temperatures by approximately 1 degrees C. In parallel, the pretreatment increased Hsp70 expression suggesting a close link between Hsp70 expression and inducible thermotolerance. The increased Hsp70 levels after pretreatment were found to be due to the increased levels of the hsp70 RNA.

Expression of heat shock protein70 in pig oocytes: Heat shock response during oocyte growth

The heat shock response of growing and fully-grown pig oocytes was analyzed in vitro by determining heat shock protein70 (HSP70) synthesis under both normal conditions (39 degrees C; 0 and 6h) and after heat shock (43 degrees C; 1, 4 and 6h). The expression of HSP70 in oocytes was detected by immunoblotting analysis. Growing oocytes measuring 80-99 microm synthesized a high number of HSP70 without heat shock effect, and these were capable of increasing the synthesis of HSP70 after heat shock to a maximum after 1h. Growing oocytes measuring 100-115 microm also synthesized HSP70 without heat shock and after it, but the HSP70 synthesis was not statistically changed by increasing duration of heat shock. In fully-grown oocytes, great amounts of HSP70 were found without heat shock treatment, and the contents of HSP70 significantly decreased after heat shock. These results indicate that growing oocytes are able to synthesize HSP70 after heat shock. This ability declines at the end of the growth period, and fully-grown oocytes are unable to induce HSP70 synthesis after heat shock. HSP70 is synthesized and stored during oocyte growth. The high HSP70 synthesis in non-heat-treated growing oocytes and a great amount of HSP70 in fully-grown oocytes support the hypothesis that HSP70 is important for oocyte growth and maturation.

Evaluation of the activities of the medfly and Drosophila hsp70 promoters in vivo in germ-line transformed medflies

The promoter of the hsp70 gene of Drosophila melanogaster has been widely used for the expression of foreign genes in other insects. It has been generally assumed that because this gene is highly conserved, its promoter will function efficiently in other species. We report the results of a quantitative comparison of the activities of the medfly and D. melanogaster hsp70 promoters in vivo in transformed medflies. We constructed transformed lines containing the lacZ reporter gene under the control of the two promoters by using Minos-mediated germ-line transformation. The activity of each promoter was evaluated in 15 transformed lines by beta-galactosidase quantitative assays. The heat-inducible activity of the medfly promoter was found several times higher than the respective activity of the heterologous D. melanogaster promoter. These results were confirmed by northern blot analysis and indicate that the D. melanogaster promoter does not work efficiently in medfly. The -263/+105 medfly promoter region that was used in this study was found able to drive heat shock expression of the lacZ reporter gene in all stages of medfly, except early embryonic stages, in a similar fashion to the endogenous hsp70 genes. However the heat inducible RNA levels driven from this promoter region were significantly lower than the endogenous hsp70 RNA levels, suggesting that additional upstream and/or downstream sequences to the -263/+105 region may be necessary for optimum function of the medfly hsp70 promoter in vivo.

Development-dependent differences in intracellular localization of stress proteins (hsps) in rainbow trout, Oncorhynchus mykiss, following heat shock

Using antibodies specific for salmonid fish, we have determined the intracellular localization of hsp70, hsc70 and hsp90 before and after an acute heat shock in juvenile and mature rainbow trout. We found that both hsp70 and hsp90 were primarily located outside the nucleus in both the liver and the heart of juvenile and mature fish and heat shock resulted in an increase in these proteins in all cellular fractions examined. In mature fish, liver hsp70 was predominantly found in the membranes and organelles after heat shock, while in juvenile fish, hsp70 was mostly cytoplasmic. Hsc70 was found in all cellular compartments examined both before and after heat shock in the livers and hearts of juvenile and mature fish. Heat shock resulted in a significant induction of hsp90 in the liver tissue of both juvenile and mature fish; however, in juvenile fish, this increase was seen in the membranes and organelles whereas in mature fish, hsp90 increased in the nucleus and cytoplasm. Hsp90 was only induced in the hearts of mature fish with heat shock, where it increased in both the nucleus and the cytoplasm. These results indicate that the cells of juvenile and mature fish respond differently to acute temperature stress. While the nucleus appears to be an important target for hsp protection following heat shock, the presence of hsps in all subcellular fractions examined suggests multifunctional roles for these proteins in the cellular response to temperature stress in fish.

Examination of the stress-induced expression of the collagen binding heat shock protein, hsp47, in Xenopus laevis cultured cells and embryos

HSP47 is an endoplasmic reticulum (ER)-resident molecular chaperone involved in collagen production. This study examined the stress-induced pattern of hsp47 gene expression in Xenopus cultured cells and embryos. Sequence analysis revealed that protein encoded by the hsp47 cDNA exhibited 70-77% identity with fish, avian and mammalian HSP47. In A6 kidney epithelial cells hsp47 mRNA and HSP47 were present constitutively and inducible by heat shock but not ER stressors including tunicamycin and A23187, both of which enhanced BiP mRNA. Furthermore A23187 treatment inhibited constitutive accumulation of hsp47 mRNA and retarded heat-induced accumulation of hsp47 and hsp70 mRNA. Interestingly, hsp47 gene expression but not hsp70 or BiP mRNA accumulation was enhanced by treatment with a procollagen-specific stressor, beta-aminopropionitrile. In Xenopus embryos hsp47 mRNA was present constitutively throughout development. In tailbud embryos hsp47 mRNA was enriched in tissues associated with collagen production including notochord, somites and head region. Heat shock-induced accumulation of hsp47 mRNA was enhanced primarily in embryonic tissues already exhibiting hsp47 mRNA accumulation. These studies suggest that the pattern of Xenopus hsp47 gene expression is similar to hsp70 in response to heat shock but also displays unique features including a response to a procollagen-specific stressor and preferential expression in collagen-containing tissues.

Medfly promoters relevant to the sterile insect technique

This review summarizes structural and functional studies on medfly promoters and regulatory elements that can be used for driving sex-specific, conditional and constitutive gene expression in this species. Sex-specific and conditional promoters are important for generating transgenic sexing strains that could increase the performance of the Sterile Insect Technique while strong constitutive promoters are necessary for developing sensitive transgenic marker systems. The review focuses on the functional analysis of the promoters of two male-specific and heat shock medfly genes. A special emphasis is put on the potential utility of these promoters for developing transgenic sexing strains.

Expression and function of small heat shock protein genes during Xenopus development

The hsp30 small heat shock protein family is a stress-inducible group of molecular chaperones in the frog, Xenopus laevis. Hsp30 genes are intronless and present in clusters. Expression of these genes are developmentally regulated likely at the level of chromatin structure. Also heat-induced hsp30 transcripts and protein are enriched in selected embryonic tissues. In vitro studies revealed that multimeric hsp30 binds to heat denatured target protein, inhibits their aggregation and maintains them in a folding-competent state until reactivated by other cellular chaperones. Finally optimal chaperone activity and secondary structure of hsp30 can be inhibited by phosphorylation or mutagenesis of the C-terminal end.

Effect of histone deacetylase inhibitors on heat shock protein gene expression during Xenopus development

We examined the effect of histone deacetylase inhibitors (HDIs), trichostatin A (TSA), valproic acid (VPA), and sodium butyrate (NaB) on heat shock protein (hsp) gene expression during early Xenopus laevis development. HDIs enhance histone acetylation and result in the relief of repressed chromatin domains and ultimately increase the accessibility of transcription factors to target cis-acting regulatory sites. Treatment of embryos with HDIs enhanced the heat shock-induced accumulation of hsp70 mRNA in post-midblastula stage embryos. No effect was observed with actin mRNA or other hsp70 family members including heat shock cognate 70 and immunoglobulin binding protein. Normally, hsp30 genes are not heat-inducible until the late neurula or early tailbud stage of development. Treatment with HDIs resulted in heat-induced expression of hsp30 genes at the gastrula stage with enhanced heat-induced accumulation in neurula and tailbud stages. HDI treatment alone did not induce the accumulation of hsp70 or hsp30 mRNA. Whole-mount in situ hybridization verified the RNA blot analyses and additionally revealed that TSA treatment did not result in any major alterations in the spatial pattern of stress-induced hsp70 or hsp30 mRNA accumulation in early embryos. This study suggests that the states of Xenopus hsp70 and 30 chromatin are subject to repression beyond the midblastula transition.

Developmental regulation of the heat shock response by nuclear transport factor karyopherin-α3

During early stages of Drosophila development the heat-shock response cannot be induced. It is reasoned that the adverse effects on cell cycle and cell growth brought about by Hsp70 induction must outweigh the beneficial aspects of Hsp70 induction in the early embryo. Although the Drosophila heat shock transcription factor (dHSF) is abundant in the early embryo it does not enter the nucleus in response to heat shock. In older embryos and in cultured cells the factor is localized within the nucleus in an apparent trimeric structure that binds DNA with high affinity. The domain responsible for nuclear localization upon stress resides between residues 390 and 420 of the dHSF. Using that domain as bait in a yeast two-hybrid system we now report the identification and cloning of a Drosophila nuclear transport protein karyopherin-α3 (dKap-α3). Biochemical methods demonstrate that the dKap-α3 protein binds specifically to the dHSF’s nuclear localization sequence (NLS). Furthermore, the dKap-α3 protein does not associate with NLSs that contain point mutations, which are not transported in vivo. Nuclear docking studies also demonstrate specific nuclear targeting of the NLS substrate by dKap-α3. Consistant with previous studies demonstrating that early Drosophila embryos are refractory to heat shock as a result of dHSF nuclear exclusion, we demonstrate that the early embryo is deficient in dKap-α3 protein through cycle 12. From cycle 13 onward the transport factor is present and the dHSF is localized within the nucleus thus allowing the embryo to respond to heat shock.

Constitutive and stress-inducible expression of the endoplasmic reticulum heat shock protein 70 gene family member, immunoglobulin-binding protein (BiP), during Xenopus laevis early development

We have characterized the constitutive and stress-inducible pattern of immunoglobulin-binding protein (BiP) gene expression during Xenopus early development. Whole mount in situ hybridization analysis revealed that BiP mRNA was detected in unfertilized eggs, cleavage and blastula stage embryos. In gastrulae, BiP mRNA was present across the surface of the embryo, while in neurulae BiP mRNA was enriched in the neural plate, neural fold, and around the blastopore. In early and late tailbud embryos, BiP mRNA was found primarily in the dorsal region. Tunicamycin and A23187, the calcium ionophore, enhanced BiP mRNA accumulation first at the neurula stage, while heat shock induced BiP mRNA accumulation first at the gastrula stage. Compared to control, A23187- and heat shock-treated neurulae displayed relatively high levels of BiP mRNA in selected tissues, including the neural plate, neural folds, around the blastopore, and ectoderm. At the early tailbud stage, A23187 and heat shock enhanced BiP mRNA accumulation primarily in the head, somites, tail, and along the spinal cord. A similar situation was found with A23187- and heat shock-treated late tailbud embryos, except that heat-shocked embryos also displayed enhanced BiP mRNA accumulation in the epidermis. These studies demonstrate a preferential accumulation of BiP mRNA in selected tissues during development and in response to stress.

Effects of cyclin A2 noncoding regions on reporter gene translation during early development of Xenopus laevis

The repression of translation of Xenopus cyclin A2 transcripts during early development was examined by analyzing the effects of cyclin A2 noncoding regions using a CAT reporter system. On their own, the 5' and 3' UTRs (untranslated regions) were unable to inhibit reporter translation until approximately the time of the midblastula transition. Transcripts containing the 3' UTR were polyadenylated after fertilization and the midblastula transition. When both noncoding regions flanked a CAT reporter gene, translation was repressed at all stages of development examined in spite of their polyadenylation after fertilization. From these data, we conclude that the 5' and 3' UTRs interact synergically to prevent translation during early development and that the poly(A) tail is insufficient to promote their translation.

Stress-induced, tissue-specific enrichment of hsp70 mRNA accumulation in Xenopus laevis embryos

In this study, we have employed whole-mount, in situ hybridization to study the spatial pattern of hsc70 and hsp70 mRNA accumulation in normal and heat shocked embryos during Xenopus laevis development. Our findings revealed that hsc70 mRNA was constitutively present in a global fashion throughout the embryo and was not heat inducible. Accumulation of hsp70 mRNA, however, was detected only in heat shocked embryos. Furthermore, hsp70 mRNA accumulation was enriched in a tissue-specific manner in X. laevis tailbud embryos within 15 minutes of a 33 degrees C heat shock. Abundant levels of heat shock-induced hsp70 mRNA were detected in the head region, including the lens placode, the cement gland, and in the somitic region and proctodeum. Preferential heat-induced accumulation of hsp70 mRNA was first detected at a heat shock temperature of 30 degrees C. Placement of embryos at 22 degrees C after a 1-hour, 33 degrees C heat shock resulted in decreased hsp70 mRNA with time, but the message persisted in selected tissues, including the lens placode and somites. Treatment of tailbud embryos with either sodium arsenite or zinc chloride induced a tissue-specific enrichment of hsp70 mRNA in the lens placode and somitic region. These studies reveal the complex nature of the heat shock response in different embryonic tissues and suggest the presence of regulatory mechanisms that lead to a stressor-induced, tissue-specific enrichment of hsp70 mRNA.

Spatial pattern of constitutive and heat shock-induced expression of the small heat shock protein gene family, Hsp30, in Xenopus laevis tailbud embryos

We employed whole-mount in situ hybridization and immunohistochemistry to study the spatial pattern of hsp30 gene expression in normal and heatshocked embryos during Xenopus laevis development. Our findings revealed that hsp30 mRNA accumulation was present constitutively only in the cement gland of early and midtailbud embryos, while hsp30 protein was detected until at least the early tadpole stage. Heat shock-induced accumulation of hsp30 mRNA and protein was first observed in early and midtailbud embryos with preferential enrichment in the cement gland, somitic region, lens placode, and proctodeum. In contrast, cytoskeletal actin mRNA displayed a more generalized pattern of accumulation which did not change following heat shock. In heat shocked midtailbud embryos the enrichment of hsp30 mRNA in lens placode and somitic region was first detectable after 15 min of a 33 degrees C heatshock. The lowest temperature capable of inducing this pattern was 30 degrees C. Placement of embryos at 22 degrees C following a 1-h 33 degrees C heat shock resulted in decreased hsp30 mRNA in all regions with time, although enhanced hsp30 mRNA accumulation still persisted in the cement gland after 11 h compared to control. In late tailbud embryos the basic midtailbud pattern of hsp30 mRNA accumulation was enhanced with additional localization to the spinal cord as well as enrichment across the embryo surface. These studies demonstrate that hsp30 gene expression can be detected constitutively in the cement gland of tailbud embryos and that heat shock results in a preferential accumulation of hsp30 mRNA and protein in certain tissues.

Heat shock-induced acquisition of thermotolerance at the levels of cell survival and translation in Xenopus A6 kidney epithelial cells

In this study we have investigated the acquisition of thermotolerance in a Xenopus laevis kidney A6 epithelial cell line at both the level of cell survival and translation. In cell survival studies, A6 cells were incubated at temperatures ranging from 22 to 35°C for 2 h followed by a thermal challenge at 39°C for 2 h and a recovery period at 22°C for 24 h. Optimal acquisition of thermotolerance occurred at 33°C. For example, exposure of A6 cells to 39°C for 2 h resulted in only 3.4% survival of the cells whereas prior exposure to 33°C for 2 h enhanced the survival rate to 69%. This state of thermotolerance in A6 cells was detectable after 1 h at 33°C and was maintained even after 18 h of incubation. Cycloheximide inhibited the acquisition of thermotolerance at 33°C suggesting the requirement for ongoing protein synthesis. The optimal temperature for the acquisition of translational thermotolerance also occurred at 33°C. Treatment of A6 cells at 39°C for 2 h resulted in an inhibition of labeled amino acid incorporation into protein which recovered to approximately 14% of control after 19 h at 22°C whereas cells treated at 33°C for 2 h prior to the thermal challenge recovered to 58% of control levels. These translationally thermotolerant cells displayed relatively high levels of the heat shock proteins hsp30, hsp70, and hsp90 compared to pretreatment at 22, 28, 30, or 35°C. These studies demonstrate that Xenopus A6 cells can acquire a state of thermotolerance and that it is correlated with the synthesis of heat shock proteins.Key words: Xenopus laevis, heat shock protein, hsps, A6 cells, chaperone, thermotolerance.

Heat shock protein expression during gametogenesis and embryogenesis

When cells are subjected to various stress factors, they increase the production of a group of proteins called heat shock proteins (hsp). Heat shock proteins are highly conserved proteins present in organisms ranging from bacteria to man. Heat shock proteins enable cells to survive adverse environmental conditions by preventing protein denaturation. Thus the physiological and pathological potential of hsps is enormous and has been studied widely over the past two decades. The presence or absence of hsps influences almost every aspect of reproduction. They are among the first proteins produced during mammalian embryo development. In this report, the production of hsps in gametogenesis and early embryo development is described. It has been suggested that prolonged and asymptomatic infections trigger immunity to microbial hsp epitopes that are also expressed in man. This may be relevant for human reproduction, since many couples with fertility problems have had a previous genital tract infection. Antibodies to bacterial and human hsps are present at high titers in sera of many patients undergoing in vitro fertilization. In a mouse embryo culture model, these antibodies impaired the mouse embryo development at unique developmental stages. The gross morphology of these embryos resembled cells undergoing apoptosis. The TUNEL (terminal deoxynucleotidyl transferase-mediated X-dUTP nick end labeling) staining pattern, which is a common marker of apoptosis, revealed that embryos cultured in the presence of hsp antibodies stained TUNEL-positive more often than unexposed embryos. These data extend preexisting findings showing the detrimental effect of immune sensitization to hsps on embryo development.

Developmentally regulated nuclear transport of transcription factors in Drosophila embryos enable the heat shock response

Hsp70 is a broadly conserved thermotolerance factor, but inhibits growth at normal temperatures and cannot be induced in early embryos. We report that in Drosophila embryos the temporal and spatial patterns of Hsp70 inducibility were unexpectedly complex, with striking differences between the soma and the germline. In both, regulation occurred at the level of transcription. During the refractory period for Hsp70 induction, HSF (heat-shock transcription factor) exhibited specific DNA-binding activity characteristic of activation in extracts of heated embryos. Remarkably, however, HSF was restricted to the cytoplasm in intact embryos even after heat shock. HSF moved from the cytoplasm to the nucleus in the absence of heat precisely when the capacity to induce Hsp70 was acquired (cycle 12 of the germline, cycle 13 in the soma). During oogenesis, Hsp70 inducibility was lost in nurse cells around stage 10, in a posterior-to-anterior gradient and HSF redistributed from nucleus to cytoplasm in the same spatiotemporal pattern. In a highly inbred derivative of the Samarkind strain, HSF moved into embryonic nuclei earlier than in our standard wild-type strain. Correspondingly, Hsp70 was inducible earlier, confirming that nuclear transport of HSF controls the inducibility of Hsp70 in early embryos. We also report for the first time the nuclear import patterns of two general transcription factors, RNA polymerase subunit Ilc and TATA binding protein (TBP). Both enter nuclei in a highly synchronous manner, independently of each other and of HSF. The import of TBP coincides with the first reported appearance of transcripts in the embryo. We suggest that the potentiation of general and heat shock-specific transcription in Drosophila embryos is controlled by the developmentally programmed relocalization of general and heat shock-specific transcription factors. Restricted nuclear entry of HSF represents a newly described mechanism for regulating the heat-shock response.

Expression of cell death regulatory genes and limited apoptosis induction in avian blastodermal cells

Apoptosis is a well-established cellular mechanism for selective cell deletion during development. However, little is known about the expression of an apoptotic pathway and its role in determining the relative sensitivity of the early, pre-gastrula, avian embryo to stress-induced cell death. We examined the sensitivity of avian blastodermal cells to engage in apoptosis upon exposure to etoposide, a topoisomerase II-inhibitor that rapidly and efficiently induces apoptosis in many cell types. We found that while the blastodermal cells are capable of engaging in apoptosis, they are highly resistant to such induction with respect to both concentration of drug required and length of exposure, even when compared to a tumor cell line with a known multi-drug resistant phenotype. Additionally, we assessed the expression of several candidate regulatory genes in blastodiscs from infertile eggs (i.e., maternal RNA transcripts), blastodermal cells immediately following oviposition, and various stages of early development up to gastrulation. This analysis revealed that several genes whose products have anti-apoptotic activity, including bcl-2, bcl-xL, hsp70, grp78 and the glutathione S-transferases, are expressed as early as the stage 1 embryo in the newly oviposited egg. These transcripts are also found in the infertile blastodisc, suggesting a role for maternally derived transcripts in the protection of the oocyte and zygote. Significantly, constitutive levels of hsp70 mRNA exceeded those of the other anti-apoptotic genes in the blastodermal cells. These results contribute to an emerging picture of stress resistance at the earliest stages of avian embryo development which involves multiple anti-apoptotic genes that act at different regulatory points in the apoptotic cascade.

Stimulation of translation and cytoplasmic polyadenylation by the Xenopus c-mycI 3'-untranslated region

Expression of messenger RNA (mRNA) in both embryonic and adult cells may be profoundly influenced by untranslated sequences in the 3'-end. Elements in the 3'-untranslated regions (UTRs) of messengers are known to influence messenger stability, polyadenylation, and translation. We have examined the effects of the 3'-UTR of Xenopus laevis c-mycI (either alone or in combination with the 5'-first exon) on the expression of a chloramphenicol acetyltransferase (CAT) reporter in Xenopus embryos. The Xenopus c-mycI 3'-UTR enhanced messenger translation independent of the 5'-UTR. RNase H analysis indicated that the Xenopus c-mycI 3'-UTR can promote the cytoplasmic polyadenylation of CAT mRNA in embryos. The result suggests that the post-fertilization enhancement of translation caused by the c-mycI 3'-UTR may be a consequence of cytoplasmic polyadenylation. A uridine (U)-rich sequence in the Xenopus c-mycI 3'-UTR that may be responsible for polyadenylation is similar to an element that destabilizes mammalian c-myc transcripts. We discuss the possibility that U-rich sequences may play a dual role by destabilizing growth-related transcripts in adult cells and stimulating their polyadenylation during development, and we propose that a switch in the role of such sequences in adult cells could lead to stabilization of these messengers, increased translation, and abnormal growth control.

Isolation and characterization of a cDNA encoding a Xenopus immunoglobulin binding protein, BiP (grp78)

We have isolated a full-length cDNA clone encoding a Xenopus laevis immunoglobulin binding protein (BiP; also called glucose-regulated protein or grp78). The Bip cDNA sequence includes an open reading frame of 1,965 bp encoding a 655 amino acid protein with an N-terminal hydrophobic leader sequence and a C-terminal KDEL tetrapeptide which has been found in other lumenal proteins of the endoplasmic reticulum. The 3' untranslated region contains a polyadenylation and an adenylation control element (ACE) as well as a putative mRNA instability sequence. The Xenopus BiP amino acid sequence displayed high identity with BiP from other vertebrates including chicken (91.3%), rat (90.7%), and human (89.9%). Northern hybridization analysis demonstrated that BiP mRNA was present constitutively in the Xenopus A6 kidney epithelial cell line and that BiP mRNA levels could be enhanced by treatment of the cells with galactose-free media, 2-deoxyglucose, 2-deoxygalactose, glucosamine, tunicamycin, heat shock, dithiothreitol, and the calcium ionophore, A23187. Finally, while BiP mRNA was detected in all of the adult tissues examined, the relative level of BiP mRNA differed dramatically between organs. For example, relatively high levels of BiP mRNA were detected in liver with moderate levels in testis, ovary and heart and reduced levels in eye and muscle tissue.

Isolation and characterization of a cDNA encoding a Xenopus 70-kDa heat shock cognate protein, Hsc70.I

We isolated a full-length cDNA clone encoding a Xenopus laevis 70-kDa heat shock cognate protein, hsc70.I. The protein coding region exhibits a high degree of identity with a number of mammalian hsc70 proteins, such as rat hsc71 (92%), whereas the identity to Xenopus hsp70 is only 80%. These data suggest that the inducible and constitutive forms of hsp70 diverged long before the emergence of amphibians. The Xenopus hsc70.I contains a number of conserved elements, including the ATP-binding domain, a nuclear localization signal and the carboxy-terminal EEVD motif, which has been implicated in several activities associated with chaperonin function. Northern blot analyses revealed that maternal hsc70.I mRNA is present in cleavage and early blastula stages of Xenopus development. After the onset of zygotic transcription at the midblastula stage, the levels of hsc70.I message increase through to the tadpole stages. Furthermore, in contrast to hsp70 mRNA, the relative levels of hsc70.I mRNA are not enhanced after heat shock in embryos and in the kidney epithelial cell line, A6. The levels of hsc70.I mRNA are high in adult spleen and testis, with moderate levels in eye, heart, liver and brain and comparatively low levels in hindlimb muscle.

Cloning and characterization of a cDNA encoding the collagen-binding stress protein hsp47 in zebrafish

Hsp47 is a major stress-inducible protein that is localized to the endoplasmic reticulum of avian and mammalian cells and is thought to act as a molecular chaperone specific for the processing of procollagen. Although hsp47 is coordinately expressed together with several collagen types, and vertebrate embryos are known to express collagen genes in complex spatial and temporal patterns, limited information is available regarding the function or regulation of hsp47 during early embryonic development. We have initiated an examination of hsp47 in the zebrafish, Danio rerio, which offers a number of features that make it attractive as a model developmental system with which to examine the expression and function of hsp47. A polymerase chain reaction (PCR)-based cloning strategy was used to isolate a hsp47 cDNA from an embryonic zebrafish cDNA library. The deduced translation product of the cDNA is a 404-amino-acid polypeptide that is 72% identical to chicken, 64% identical to mouse and rat, and 69% identical to human hsp47. The protein contains a typical hydrophobic signal sequence, an RDEL endoplasmic reticulum retention signal, and a serine protease inhibitor signature sequence, all of which are characteristic of hsp47 in higher vertebrates. Thus, it is likely that hsp47 in zebrafish is also localized to the endoplasmic reticulum and may play a similar role to its counterpart in higher vertebrates. Northern blot analysis revealed that the hsp47 gene is expressed at relatively low levels in embryos during normal development but is strongly induced following exposure to heat shock at the gastrula, midsomitogenesis, 2-day, and 3-day larval stages. The level of induction was much higher than has previously been reported in chicken and mouse cells.

Role of chromatin and Xenopus laevis heat shock transcription factor in regulation of transcription from the X. laevis hsp70 promoter in vivo

Xenopus laevis oocytes activate transcription from the Xenopus hsp70 promoter within a chromatin template in response to heat shock. Expression of exogenous Xenopus heat shock transcription factor 1 (XHSF1) causes the activation of the wild-type hsp70 promoter within chromatin. XHSF1 activates transcription at normal growth temperatures (18 degrees C), but heat shock (34 degrees C) facilitates transcriptional activation. Titration of chromatin in vivo leads to constitutive transcription from the wild-type hsp70 promoter. The Y box elements within the hsp70 promoter facilitate transcription in the presence or absence of chromatin. The presence of the Y box elements prevents the assembly of canonical nucleosomal arrays over the promoter and facilitates transcription. In a mutant hsp70 promoter lacking Y boxes, exogenous XHSF1 activates transcription from a chromatin template much more efficiently under heat shock conditions. Activation of transcription from the mutant promoter by exogenous XHSF1 correlates with the disappearance of a canonical nucleosomal array over the promoter. Chromatin structure on a mutant hsp70 promoter lacking Y boxes can restrict XHSF1 access; however, on both mutant and wild-type promoters, chromatin assembly can also restrict the function of the basal transcriptional machinery. We suggest that chromatin assembly has a physiological role in establishing a transcriptionally repressed state on the Xenopus hsp70 promoter in vivo.

The cDNA encoding Xenopus laevis heat-shock factor 1 (XHSF1): nucleotide and deduced amino-acid sequences, and properties of the encoded protein

We have isolated a cDNA encoding Xenopus laevis (Xl) heat-shock factor 1 (XHSF1). XHSF1, translated from the mRNA synthesized in vitro, will bind specifically to the Xl hsp70 promoter (hsp70). Microinjection of XHSF1 mRNA into Xl oocytes leads to synthesis of XHSF1 which accumulates in the nucleus and selectively activates Xl phsp70p activity at 18 degrees C.

Identification of members of the HSP30 small heat shock protein family and characterization of their developmental regulation in heat-shocked Xenopus laevis embryos

In the present study we have characterized the synthesis of members of the HSP30 family during Xenopus laevis development using a polyclonal antipeptide antibody derived from the carboxyl end of HSP30C. Two-dimensional PAGE/immunoblot analysis was unable to detect any heat-inducible small HSPs in cleavage, blastula, gastrula, or neurula stage embryos. However, heat-inducible accumulation of a single protein was first detectable in early tailbud embryos with an additional 5 HSPs at the late tailbud stage and a total of 13 small HSPs at the early tadpole stage. In the Xenopus A6 kidney epithelial cell line, a total of eight heat-inducible small HSPs were detected by this antibody. Comparison of the pattern of protein synthesis in embryos and somatic cells revealed a number of common and unique heat inducible proteins in Xenopus embryos and cultured kidney epithelial cells. To specifically identify the protein product of the HSP30C gene, we made a chimeric gene construct with the Xenopus HSP30C coding sequence under the control of a constitutive promoter. This construct was microinjected into fertilized eggs and resulted in the premature and constitutive synthesis of the HSP30C protein in gastrula stage embryos. Through a series of mixing experiments, we were able to specifically identify the protein encoded by the HSP30C gene in embryos and somatic cells and to conclude that HSP30C synthesis was first head-inducible at the early tailbud stage of development. The differential pattern of heat-inducible accumulation of members of the HSP30 family during Xenopus development suggests that these proteins may have distinct functions at specific embryonic stages during a stress response.

Involvement of differential gene expression and mRNA stability in the developmental regulation of the hsp 30 gene family in heat-shocked Xenopus laevis embryos

Four complete hsp 30 genes have been isolated from Xenopus laevis: hsp 30A, hsp 30B (a pseudogene), hsp 30C, and hsp 30D. The hsp 30A and hsp 30C genes are first heat inducible at the early tailbud stage, as determined by RNase protection and RT-PCR assays. In this study, we determined by RT-PCR that the hsp 30D gene was first heat inducible (33 degrees C for 1 h) at the mid-tailbud stage, approximately 1 day later in development than hsp 30A and hsp 30C. Furthermore, using Northern blot analysis, we detected the presence of very low levels of hsp 30 mRNA at the heat-shocked late blastula stage. The relative levels of these pre-tailbud (PTB) hsp 30 mRNAs increased at the gastrula and neurula stage followed by a dramatic enhancement in heat shocked tailbud and tadpole stage embryos (50- to 100- fold relative to late blastula). Interestingly, treatment of blastula or gastrula embryos at high temperatures (37 degrees C for 1 h) or with the protein synthesis inhibitor, cycloheximide, followed by heat shock, led to enhanced accumulation of the pre-tailbud (PTB) hsp 30 mRNAs. hsp 70, hsp 87, and actin messages were not stabilized at high temperatures or by cycloheximide treatment. Finally, hsp 30D mRNA was not detected by RT-PCR analysis of cycloheximide-treated, heat-shocked blastula stage embryos, confirming that it is not a member of the PTB hsp 30 mRNAs. This study indicates that differential gene expression and mRNA stability are involved in the regulation of hsp 30 gene expression during early Xenopus laevis development.

Constitutive expression of a microinjected glucose-regulated protein (grp78) fusion gene during early Xenopus laevis development

In this study we have found that a rat glucose-regulated protein (grp) 78 chloramphenicol acetyltransferase (CAT) fusion gene deleted to -456 bp at the 5' end and injected into fertilized Xenopus eggs was first expressed in a constitutive manner in late blastula stage embryos and displayed increased expression as the embryos developed to the gastrula and neurula stages. Using a series of internal deletion mutants and linker-scanner mutants of the rat grp78 promoter, we have found that a CCAAT box and CCAAT-like element within the region -129 to -90 were essential for constitutive expression of the chimeric genes in neurula stage embryos. These results suggest conservation of the regulatory sequences within the grp78 promoter between rat and Xenopus. Interestingly, deletion or alteration of sequences between -130 and -149 had a dramatic stimulatory effect on basal promoter activity. This effect, which was not observed previously in rat cells, may be the result of upstream elements that are transcriptionally active in Xenopus and that can compensate for the mutated or deleted sequences. It is also possible that these results indicate the presence of a negative regulatory element that is recognized by the Xenopus transcriptional apparatus.

Activity of a microinjected inducible murine hsp68 gene promoter depends on plasmid configuration and the presence of heat shock elements in mouse dictyate oocytes but not in two-cell embryos

After fertilization in the mouse, the zygotic genome is activated in two-cell embryos by the spontaneous expression, among other genes, of the major inducible heat shock gene, hsp68, in the absence of heat-inducibility of heat shock genes. To obtain information on this phenomenon, we have probed one- and two-cell embryo's ability to express microinjected reporter DNA constructs, containing the Escherichia coli lacZ gene driven by promoters from early SV40 genes, the human beta-actin gene, and the normal or HSE-deleted mouse hsp68 gene. Activity of these promoters was also tested in mouse granulosa cells and dictyate oocytes, as a function of circular/linear construct configuration and occurrence of heat shock. The hsp68 promoter was heat-inducible in both granulosa cells and oocytes. Its heat activation required the presence of HSEs and, in the oocytes, of construct linear configuration. In the embryos however, this promoter was expressed independently of the presence of HSEs and of construct configuration, and its activity was not affected by heat shock. When constructs with early SV40 and beta-actin promoters were injected into one-cell embryos, they appeared to be inactivated with the first embryonic cleavage, in agreement with previous observations [Wiekowski et al., 1992]. By contrast, both normal and HSE-deleted hsp68 promoters maintained their activity through the first cleavage, providing the first evidence of a gene escaping such transcriptional repression. Present results confirm previous findings on hsp68 expression during early mouse development, and suggest that this activation is mediated by a factor(s) other than HSF.

Expression of endogenous and microinjected hsp 30 genes in early Xenopus laevis embryos

In the present study, we have examined the regulation of expression of a newly isolated member of the hsp 30 gene family, hsp 30C. Using RT-PCR, we found that this gene was first heat-inducible at the tailbud stage of development. We also examined the expression of two microinjected modified hsp 30C gene constructs in Xenopus embryos. One of the constructs had 404 bp of hsp 30C 5'-flanking region, whereas the other had 3.6 kb. Both gene constructs had 1 kb of 3'-flanking region. RT-PCR assays were employed to detect the expression of these microinjected genes. The presence of extensive 5'- and 3'-flanking regions of the hsp 30C gene did not confer proper developmental regulation, since heat-inducible expression of both of the microinjected constructs was detectable at the midblastula stage. The premature expression of the microinjected hsp 30 gene was not a result of high plasmid copy number of the presence of plasmid DNA sequences. These results suggest that the microinjected genes contain all the cis-acting DNA sequences required for correct heat-inducible regulation but do not contain the elements required for the proper regulation of hsp 30 gene expression during development. It is possible that regulatory elements controlling the developmental expression of the hsp30 genes may reside upstream or downstream of the entire cluster.

The induction of pyruvate kinase synthesis by heat shock in Xenopus laevis embryos

Heat-shocked Xenopus embryos have an unusually complex heat shock response. The dominant heat shock protein (Hsp) has a relative molecular mass (M(r)) of 62,000 D (Hsp62). Affinity-purified IgGs against the glycolytic enzyme pyruvate kinase (PK; EC 2.7.1.40) specifically immunoprecipitated Hsp62 from extracts of embryos that had been heat-shocked at 37 degrees C for 30 min. Thus, Hsp62 and pyruvate kinase are immunologically cross-reacting. Electrophoretic separation of PK isoforms suggests that heat-shocked Xenopus embryos increase synthesis of an isoform of PK. Thermal denaturation studies suggest that this isoform has enhanced thermal stability. The identification of PK as an Hsp is discussed within the context of a physiological requirement for elevated levels of anaerobic glycolysis in heat-stressed cells as a vital component of the acquisition of thermotolerance.

Characterization of a new member of the sea urchin Paracentrotus lividus hsp70 gene family and its expression

We have sequenced a second gene of the hsp70 family derived from a genomic clone of the sea urchin, Paracentrotus lividus. The structure of this gene, named hsp70IV gene, is interrupted by one intron and differs from the previously analyzed sea urchin hsp70II gene, which contains several introns. Two open reading frames of hsp70IV gene encode a predicted protein of 639 amino acids with an M(r) of 69,672. The 5' flanking region of the gene contains a putative TATA element, three heat-shock elements made up of some arrays of the 5-bp units, NGAAN and NTTCN (N = A,C,G or T), a canonic consensus sequence for binding of the regulatory activating transcription factor (ATF), and a purine box. The 3' flanking region contains four putative polyadenylation sites located at different sites downstream from the stop codon. Using Northern blot hybridization analysis, carried out using a probe corresponding to a 3' noncoding fragment (UTR) peculiar to hsp70IV gene, we found that this gene is transcribed only under heat shock (Hs) and that the transcript can be recovered from the polysomal pellet. The hsp70IV gene may be classified as a Hs gene 70 although it contains one intron.

Comparison of regulatory and structural regions of the Xenopus laevis small heat-shock protein-encoding gene family

We have isolated several unique Xenopus laevis hsp30 (encoding heat-shock protein 30) genomic clones, one of which contains two complete hsp30 genes (hsp30C and hsp30D), as well as the promoter and N-terminal coding region of a third gene (hsp30E). Nucleotide sequence and restriction enzyme analysis revealed that this gene cluster is different from a cluster isolated previously. The hsp30C and hsp30D genes encode proteins of approx. 24 kDa. In all, the hsp30 gene family contains a minimum of seven genes. The strand exchange and breakage of the duplication events which generated this gene family appear to have occurred within tracts of DNA which potentially can assume a Z-DNA conformation. Comparing the amino acid (aa) sequences of each known Hsp30 protein with bovine alpha-crystallin revealed a high degree of shared conservation of aa that constitute the major structural feature(s) of alpha-crystallin.

Developmental regulation of transcription factor AP-2 during Xenopus laevis embryogenesis

We have isolated a cDNA clone encoding the Xenopus homologue of the transcription factor AP-2 (XAP-2). The predicted amino acid sequence derived from the Xenopus cDNA shows very strong conservation with the amino acid sequence of human AP-2, suggesting that this protein is evolutionarily conserved, at least among vertebrates. This is further substantiated by the demonstration that an in vitro translation product of XAP-2 cDNA bound specifically to an AP-2 binding site from the human MT-IIA gene. Northern blot analysis of Xenopus embryo RNA revealed the existence of three major XAP-2 mRNA species that were only detectable after the midblastula transition (when embryonic transcription is activated), with peak accumulation of the transcripts occurring during gastrulation. Therefore, in contrast to other Xenopus transcription factors, XAP-2 is not maternally derived but arises exclusively from zygotic transcription. Unlike the situation in cultured human teratocarcinoma (NT2) cells, retinoic acid treatment did not induce XAP-2 mRNA in Xenopus embryos, even though the treatment had a pronounced morphogenetic effect on the embryos. Our results suggest that XAP-2 may play a distinctive role during Xenopus embryogenesis.

Developmental regulation of heat shock protein synthesis and HSP 70 RNA accumulation during postimplantation rat embryogenesis

Exposure of postimplantation rat embryos on days 9, 10, 11, and 12 of gestation to an in vitro heat shock of 43 degrees C for 30 min results in the induction of heat shock proteins (HSPs) in day 9 and 10 embryos, a severely attenuated response in day 11 embryos, and no detectable response in day 12 embryos. The heat shock response in day 9 embryos (presomite stage) is characterized by the synthesis of HSPs with molecular weights of 28-78 kDa. In heat shocked day 10 embryos, two additional HSPs are induced (34 and 82 kDa). In addition, two HSPs present on day 9 are absent on day 10. In day 11 heat shocked embryos, only three HSPs (31, 39, and 69 kDa) are induced, while in day 12 embryos no detectable HSPs are induced. Northern blot analysis of HSP 70 RNA levels indicates that the accumulation of this RNA, but not actin RNA, varies depending on developmental stage at the time of exposure to heat as well as the duration of the heat shock. Day 9 embryos exhibit the most pronounced accumulation of HSP 70 RNA while embryos on days 10-12 exhibit an increasingly attenuated accumulation of HSP 70 RNA, particularly after the more acute exposures (43 degrees C for 30 or 60 min). Thus, the ability to synthesize HSP 70 and to accumulate HSP 70 RNA changes dramatically as rat embryos develop from day 9 to day 12 (presomite to 31-35 somite stages).

Analysis of CCAAT box transcription factor binding activity during early Xenopus laevis embryogenesis

Unfertilized eggs and pre-midblastula (MBT) stage Xenopus embryos were found to contain a large pool of maternally derived CCAAT box-binding transcription factor (CBTF). DNA mobility shift experiments using embryonic extracts prepared with either low or high salt buffers suggest that Xenopus CBTF may not interact with embryonic DNA until the late blastula stage, a time point coincident with the increase in zygotic transcription. Additionally, photoaffinity-labeling experiments revealed that both pre- and post-MBT CBTF-binding activities were composed of at least three proteins having relative molecular masses of 68, 52, and 42 kDa.

Developmental regulation of heat-shock response in mouse oogenesis: identification of differentially responsive oocyte classes during Graafian follicle development

The response to heat (hs response) of dictyate mouse oocytes at various differentiation stages was analyzed in vitro, by determining patterns of oocyte heat-shock (hs) gene expression and heat-shock protein (HSP) synthesis, under both normal conditions and after an hs. Growing oocytes constitutively synthesized HSP89 and HSC70, and, in contrast to preovulatory oocytes which do not display an hs response, displayed a heat-elicited, transcription-dependent synthesis of two HSP68 isoforms, but not of other inducible HSPs. To determine the developmental schedule of hs response disappearance during oogenesis, fully grown oocytes from Graafian follicles were morphologically sorted into three discrete classes related to the follicle development, namely, loosely associated with granulosa cells (LA oocytes, from small Graafian follicles), intermediately associated with granulosa cells (IA oocytes, from medium-sized Graafian follicles), and cumulus-associated (CA oocytes, from mature follicles). LA oocytes displayed an hs response qualitatively similar to, but smaller in extent than, that of growing oocytes, and were able to resume and complete spontaneous meiotic maturation in vitro at a high rate after hs. We conclude that hs response of mouse dictyate oocytes is maximal during growth period, significantly declines with acquisition of full oocyte size and antrum formation within the follicle, and is finally shut off with oocyte/follicle terminal differentiation.

Tunicamycin-inducible polypeptide synthesis during Xenopus laevis embryogenesis

Tunicamycin treatment of Xenopus laevis embryos enhanced the synthesis of a specific set of polypeptides with molecular masses of 98, 78, 59 and 58 kDa. The 78-kDa polypeptide was tentatively identified as glucose-regulated protein (GRP) 78 on the basis of molecular mass, pl (5.2), and tunicamycin inducibility, which took place upon treating embryos after the midblastula transition (MBT). The synthesis of a polypeptide with this electrophoretic mobility was detected but was not tunicamycin-inducible at stages prior to the MBT. GRP78 mRNA was detectable before the MBT but was not inducible by tunicamycin until the tailbud stage. A comparison of tunicamycin-induced polypeptide synthesis in Xenopus embryos, A6 cell line, and white blood cells by 2D-PAGE and fluorography revealed three spots in the GRP78 region of the gel. One was observed in both embryos and adult cells; another was adult-specific; and the third one was possibly an embryo-specific form. These results suggest that GRP78 synthesis might undergo a switch from an embryonic to an adult pattern during Xenopus development.