Heat shock-induced accumulation of ubiquitin mRNA in Xenopus laevis embryos is developmentally regulated

RNA-seq analyses of cellular responses to elevated body temperature in the high Antarctic cryopelagic nototheniid fish Pagothenia borchgrevinki

Through evolution in the isolated, freezing (-1.9°C) Southern Ocean, Antarctic notothenioid fish have become cold-adapted as well as cold-specialized. Notothenioid cold specialization is most evident in their limited tolerance to heat challenge, and an apparent loss of the near universal inducible heat shock (HSP70) response. Beyond these it remains unclear how broadly cold specialization pervades the underlying tissue-wide cellular responses. We report the first analysis of massively parallel RNA sequencing (RNA-seq) to identify gene expression changes in the liver in response to elevated body temperature of a high-latitude Antarctic nototheniid, the highly cold-adapted and cold-specialized cryopelagic bald notothen, Pagothenia borchgrevinki. From a large (14,873) mapped set of qualified, annotated liver transcripts, we identified hundreds of significantly differentially expressed genes following two and four days of 4°C exposure, suggesting substantial transcriptional reorganization in the liver when body temperature was raised 5°C above native water temperature. Most notably, and in sharp contrast to heat stressed non-polar fish species, was a widespread down-regulation of nearly all classes of molecular chaperones including HSP70, as well as polyubiquitins that are associated with proteosomal degradation of damaged proteins. In parallel, genes involved in the cell cycle were down-regulated by day two of 4°C exposure, signifying slowing cellular proliferation; by day four, genes associated with transcriptional and translational machineries were down-regulated, signifying general slowing of protein biosynthesis. The log2 fold differential transcriptional changes are generally of small magnitudes but significant, and in total portray a broad down turn of cellular activities in response to four days of elevated body temperature in the cold-specialized bald notothen.

Expression, activity, and subcellular localization of the Yin Yang 1 transcription factor in Xenopus oocytes and embryos

Yin Yang 1 (YY1) is a multifunctional transcription factor that acts as an activator, repressor, or initiator of transcription of numerous cellular and viral genes. Previous studies in tissue culture model systems suggest YY1 plays a role in development and differentiation in multiple cell types, but the biological role of YY1 in vertebrate oocytes and embryos is not well understood. Here we analyzed expression, activity, and subcellular localization profiles of YY1 during Xenopus laevis development. Abundant levels of YY1 mRNA and protein were detected in early stage oocytes and in all subsequent stages of oocyte and embryonic development through to swimming larval stages. The DNA binding activity of YY1 was detected only in early oocytes (stages I and II) and in embryos after the midblastula transition (MBT), which suggested that its potential to modulate gene expression may be specifically repressed in the intervening period of development. Experiments to determine transcriptional activity showed that addition of YY1 recognition sites upstream of the thymidine kinase promoter had no stimulatory or repressive effect on basal transcription in oocytes and post-MBT embryos. Although the apparent transcriptional inactivity of YY1 in oocytes could be explained by the absence of DNA binding activity at this stage of development, the lack of transcriptional activity in post-MBT embryos was not expected given the ability of YY1 to bind its recognition elements. Subsequent Western blot and immunocytochemical analyses showed that YY1 is localized in the cytoplasm in oocytes and in cells of developing embryos well past the MBT. These findings suggest a novel mode of YY1 regulation during early development in which the potential transcriptional function of the maternally expressed factor is repressed by cytoplasmic localization.

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.

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.

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.

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.

Polyubiquitin RNA characteristics and conditional induction in sea urchin embryos

A cDNA of the sea urchin Strongylocentrotus purpuratus was identified as encoding polyubiquitin and used to detect a single gene with transcripts containing multiple ubiquitin coding units. Polyubiquitin transcripts exist as a 3.2-kb RNA in polyribosomes and as three higher molecular weight RNAs in purified nuclei. The amount of polyubiquitin RNA is essentially constant at 10(4) -10(5) transcripts per embryo during the egg-to-blastula period and then declines during further development. Heat shock elicits a transient increase in the level of polyubiquitin RNA, while Zn(II) ions induce a sustained accumulation, that is influenced by developmental parameters: One round of Zn(II) induction elicits the accumulation of the nuclear 7.6- and 5.6-kb RNAs, as well as the 3.2-kb polysomal RNA; however, a second round of induction yields only the 5.6- and 3.2-kb RNAs, suggestive of a change in pre-mRNA size or processing. Polyubiquitin RNA is expressed equally in ectodermal and mesoendodermal tissues and is induced in both tissue fractions by treatment of pluteus larvae with Zn(II). However, in isolated and cultured tissue fractions, polyubiquitin RNA is not inducible by Zn(II), in contrast to the full inducibility of metallothionein mRNAs. Polyubiquitin RNA induction thus appears to be conditioned by the integrity of the embryo, as well as by previous exposure to inducer.

Functions of maternal mRNA in early development

In this review, the types of mRNAs found in oocytes and eggs of several animal species, particularly Drosophila, marine invertebrates, frogs, and mice, are described. The roles that proteins derived from these mRNAs play in early development are discussed, and connections between maternally inherited information and embryonic pattern are sought. Comparisons between genetically identified maternally expressed genes in Drosophila and maternal mRNAs biochemically characterized in other species are made when possible. Regulation of the meiotic and early embryonic cell cycles is reviewed, and translational control of maternal mRNA following maturation and/or fertilization is discussed with regard to specific mRNAs.

Induction of glucose-regulated proteins in Xenopus laevis A6 cells

We have characterized the induction of glucose-regulated proteins (GRPs) in Xenopus laevis A6 cells, a kidney epithelial cell line. Exposure of A6 cells to medium in which 2-deoxyglucose replaced galactose resulted in enhanced synthesis of two proteins at 78 and 98 kd. The 78 kd protein was determined by two-dimensional PAGE to consist of two isoelectric variants with pls of 5.3 and 5.2 whereas the 98 kd protein resolved into a single spot with a pl of 5.1. The 78 kd protein cross-reacted with antiserum against chicken GRP78 (glucose-regulated protein), suggesting that the Xenopus protein shares homology with a previously characterized GRP. This was supported by the finding that a rat GRP78 probe hybridized with a 2-deoxyglucose-inducible mRNA. Synthesis of the two proteins was also induced by tunicamycin, 2-deoxygalactose, and dithiothreitol. However, the GRPs were not induced by glucosamine or calcium ionophore A23187 at concentrations and exposure periods that have previously been shown to elicit a GRP response in mammalian and avian cells. Enhanced synthesis of the two GRPs by 2-deoxyglucose was transient, reaching maximal levels by 12-24 h and decreasing to near control levels by 48 h. Removal of the stress at the point of peak synthesis resulted in decreased synthesis of both proteins within 6 h and a return to control levels within 24 h of recovery. These data suggest that Xenopus cells have a GRP response that is similar, but not identical, to that found in mammalian cells.