Molecular characterization and induction of heat shock protein 90 in the Antarctic bivalve Laternula elliptica

Anatomy and behavior of Laternula elliptica, a keystone species of the Antarctic benthos (Bivalvia: Anomalodesmata: Laternulidae)

Laternula elliptica (P. P. King, 1832) is the sole representative of the anomalodesmatan family Laternulidae and the largest bivalve in the Antarctic and Subantarctic. A keystone species of the regional benthic communities, it has reached model status, having been studied in hundreds of scientific works across many biological disciplines. In contrast, its anatomy has remained poorly known, with prior published data limited to partial descriptions based on chemically preserved specimens. Based on observations of aquarium-maintained living animals at the Brazilian Comandante Ferraz Antarctic Station, gross-morphological dissections, and histological sectioning, the comparative anatomy, functional morphology, and aspects of behavior of L. elliptica are described and discussed. Special focus is placed on the pallial organs (including elucidation of cleansing and feeding sorting mechanisms in the mantle cavity) and the musculature. Among the noteworthy findings are the presence of well-developed siphons furnished with sensory tentacles at its tips, some of which bearing eyes; large, folded gills and labial palps capable of sorting the material entering the mantle cavity; an inter-chamber communication in the posterior region of the mantle cavity; an ample ventral mantle fusion with an anterior pedal gape; the absence of a 4^th pallial opening; and the absence of a ligamental lithodesma in adult specimens. This study reevaluates the available anatomical data in the literature, both supplementing and correcting previously published accounts.

Identification and Characterization of HSP90 Gene Family Reveals Involvement of HSP90, GRP94 and Not TRAP1 in Heat Stress Response in Chlamys farreri

Heat shock proteins 90 (HSP90s) are a class of ubiquitous, highly conserved, and multi-functional molecular chaperones present in all living organisms. They assist protein folding processes to form functional proteins. In the present study, three HSP90 genes, CfHSP90, CfGRP94 and CfTRAP1, were successfully identified in the genome of Chlamys farreri. The length of CfHSP90, CfGRP94 and CfTRAP1 were 7211 bp, 26,457 bp, and 28,699 bp, each containing an open reading frame (ORF) of 2181 bp, 2397 bp, and 2181 bp, and encoding proteins of 726, 798, and 726 amino acids, respectively. A transcriptomic database demonstrated that CfHSP90 and CfGRP94 were the primary functional executors with high expression during larval development and in adult tissues, while CfTRAP1 expression was low. Furthermore, all of the three CfHSP90s showed higher expression in gonads and ganglia as compared with other tissues, which indicated their probable involvement in gametogenesis and nerve signal transmission in C. farreri. In addition, under heat stress, the expressions of CfHSP90 and CfGRP94 were significantly up-regulated in the mantle, gill, and blood, but not in the heart. Nevertheless, the expression of CfTRAP1 did not change significantly in the four tested tissues. Taken together, in coping with heat stress, CfHSP90 and CfGRP94 could help correct protein folding or salvage damaged proteins for cell homeostasis in C. farreri. Collectively, a comprehensive analysis of CfHSP90s in C. farreri was conducted. The study indicates the functional diversity of CfHSP90s in growth, development, and environmental response, and our findings may have implications for the subsequent in-depth exploration of HSP90s in invertebrates.

Transcriptomic analyses provide insights into the adaptive responses to heat stress in the ark shells, Scapharca subcrenata

The ark shell, Scapharca subcrenata, is susceptible to high temperature which may lead to mass mortality in hot summers. Herein, we conducted the transcriptomic analyses of haemocytes in ark shells under thermal stress, to reveal the underlying molecular mechanisms of heat resistance in these animals. The results showed that a total of 7773, 11,500 and 13,046 unigenes were expressed differentially at 12, 24 and 48 h post thermal stress, respectively. The expression levels of key DEGs as revealed by RNA-seq were confirmed by quantitative real-time PCR. GO and KEGG enrichment analyses showed that the DEGs were mainly associated with apoptosis, NF-kappa B signaling pathway, TNF signaling pathway and RIG-I-like receptor signaling pathway. Among the DEGs, 40 were candidate heat stress response-related genes and 169 were identified to be involved in antioxidant defense, cell detoxification, protein metabolism and endoplasmic reticulum stress responses. It seemed that ark shells may adapt to short term thermal stress through regulation of protein metabolism, DNA replication and anti-apoptotic system. However, if the stress sustains, it may cause irreparable injury gradually in the animals due to oxygen limitation and metabolic dysregulation. Noteworthily, the expression of DEGs involved in protein biosynthesis and proteolysis was significantly elevated in ark shells under heat stress. These findings may provide preliminary insights into the molecular response of ark shells to acute thermal stress and lay the groundwork for marker-assisted selection of heat-resistant strains in S. subcrenata.

Effects of thermal stress on mortality and HSP90 expression levels in the noble scallops Chlamys nobilis with different total carotenoid content

The noble scallop Chlamys nobilis is an economically important marine bivalve cultivated in the southern sea of China since the 1980s. Unfortunately, mass mortality of this scallop species often occurs in summer. The present study was conducted to investigate whether the expression of heat shock protein 90 (HSP90) and level of carotenoids could enhance high-temperature stress resistance in scallop. First, the HSP90 homolog of C. nobilis (designated CnHSP90) was identified and cloned. The complete cDNA sequence of CnHSP90 was 2631 bp, including a 2181-bp open reading frame (ORF) encoding a 726 amino acid polypeptide with five HSP90 family signatures, and sharing high homology with members of the HSP90 family. CnHSP90 was ubiquitously expressed in all examined tissues including the intestine, kidney, adductor, mantle, gill, and gonad, with the highest in the gonad. Golden and brown scallops, which contain significantly different total carotenoid content (TCC), were subjected to acute thermal challenge, and the LTE50 (semi-lethal temperature at 36 h heat shock) and LTI50 (semi-lethal time after heat shock) as well as the correlation between CnHSP90 gene expression and TCC were determined. The LTE50 of golden scallop (32.14 °C) was higher than that of brown scallops (31.19 °C), with longer LTI50 at all tested temperatures, indicating that golden scallops were more resistant to thermal stress than brown scallops. Similarly, the mRNA expression levels of CnHSP90 in gill of golden scallops were significantly higher (P < 0.05) than that of brown scallops at 6, 12, 24, and 36 h, with a strong positive correlation between CnHSP90 expression level and TCC. This suggests that both carotenoids and HSP90 levels could improve thermal resistance in the noble scallops.

Thermal stress induces a distinct transcriptome profile in the Pacific oyster Crassostrea gigas

Oysters are frequently subjected to heat stress during tidal emersion/immersion cycles in their habitats due to attachment on the rocky shore. To understand the effect of temperature elevation on the whole transcriptome over time, the Pacific oyster Crassostrea gigas was exposed to seawater temperature 32°C for 72h from the control 20°C. RNA-seq identified differentially expressed stress responsive transcripts upon thermal stress in the gill tissues of C. gigas. The primary effect of heat stress appears to be significantly induced transcription of molecular chaperones, including members of the heat shock protein (hsp) families, while genes typically associated with protein metabolism, such as those involved in protein degradation (e.g. ATP-dependent proteolysis pathway) and biosynthesis (e.g. ribosomal protein genes), were repressed. In particular, several hsp70 isoforms and a small hsp20 maintained prolonged mRNA expressions for 72h. This study provides preliminary insights into the molecular response of C. gigas to heat stress and suggests a basis for future studies examining molecular adaptation or thermotolerance metabolism in the Pacific oyster.

Molecular cloning and sequence analysis of heat shock proteins 70 (HSP70) and 90 (HSP90) and their expression analysis when exposed to benzo(a)pyrene in the clam Ruditapes philippinarum

HSP70 and HSP90 are the most important heat shock proteins (HSPs), which play the key roles in the cell as molecular chaperones and may involve in metabolic detoxification. The present research has obtained full-length cDNAs of genes HSP70 and HSP90 from the clam Ruditapes philippinarum and studied the transcriptional responses of the two genes when exposed to benzo(a)pyrene (BaP). The full-length RpHSP70 cDNA was 2336bp containing a 5' untranslated region (UTR) of 51bp, a 3' UTR of 335bp and an open reading frame (ORF) of 1950bp encoding 650 amino acid residues. The full-length RpHSP90 cDNA was 2839bp containing a 107-bp 5' UTR, a 554-bp 3' UTR and a 2178-bp ORF encoding 726 amino acid residues. The deduced amino acid sequences of RpHSP70 and RpHSP90 shared the highest identity with the sequences of Paphia undulata, and the phylogenetic trees showed that the evolutions of RpHSP70 and RpHSP90 were almost in accord with the evolution of species. The RpHSP70 and RpHSP90 mRNA expressions were detected in all tested tissues in the adult clams (digestive gland, gill, adductor muscle and mantle) and the highest mRNA expression level was observed in the digestive gland compared to other tissues. Quantitative real-time RT-PCR analysis revealed that mRNA expression levels of the clam RpHSP70, RpHSP90 and other xenobiotic metabolizing enzymes (XMEs) (AhR, DD, GST, GPx) in the digestive gland of R. philippinarum were induced by benzo(a)pyrene (BaP) and the absolute expression levels of these genes showed a temporal and dose-dependent response. The results suggested that RpHSP70 and RpHSP90 were involved in the metabolic detoxification of BaP in the clam R. philippinarum.

Molecular characterization and mRNA expression of grp78 and hsp90A in the estuarine copepod Eurytemora affinis

The present study aimed to develop a method of quantification of heat shock protein transcript levels in the estuarine copepod Eurytemora affinis. For that, the full-length cDNA of the 78-kDa glucose-regulated protein (Ea-grp78) and the cytosolic 90-kDa heat shock protein (Ea-hsp90A) from this species have been cloned. These cDNA revealed, respectively, 2,370 and 2,299 bp with 1,971 and 2,124 bp open reading frames encoding 656 and 707 amino acids. Main features, sequence identities and phylogenetic analysis with other species were described. Then, the expression profiles were analysed using reverse transcription/real-time quantitative PCR method from copepods subjected to different thermic and osmotic stresses in laboratory, and from copepods directly sampled into the natural population of the Seine Estuary (France) along a salinity gradient. Thermic shock (7.5°C, 22.5°C and 30°C during 90 min) significantly induced increases of transcript quantities ranged between 1.7- and 19.7-fold the levels observed in control conditions (15°C). Hypo- and hyper-osmotic shocks (salinities of 1 and 30 during 90 min) caused a 2-fold induction of Ea-hsp90A transcript level in comparison to controls (salinity of 15) whereas no significant change was measured for Ea-grp78. On the other hand, similar expression profiles were observed for the two transcripts after 72 h of exposition to salinities of 1 and 25 with a significant 2-fold induction observed for the lower salinity. To finish, strong expression inductions of both Ea-grp78 and Ea-hsp90A genes were observed in field copepods sampled at low salinity during the campaigns of June 2009 and May 2010. These results tend to show that the low salinity and the increase of temperature seem to have a synergic effect on stress condition of copepods.

Cloning and expression of a heat shock protein (HSP) 90 gene in the haemocytes of Crassostrea hongkongensis under osmotic stress and bacterial challenge

Heat shock protein 90 (HSP90) is a highly conserved and multi-functional molecular chaperone that plays an essential role in both cellular metabolism and stress response. Here, we report the cloning of the HSP90 homologue in Crassostrea hongkongensis (ChHSP90) through SSH in combination with RACE from cDNA of haemocytes. The full-length cDNA of ChHSP90 is 2459 bp in length, consisting of a 3', 5'-untranslated region (UTR) and an open reading frame of 2169 bp encoding 722 amino acids. The identity analysis of the amino acid sequence of HSP90 revealed that ChHSP90 is highly conserved. Distribution of ChHSP90 mRNA in gonad, heart, adductor muscle, mantle, gill, digestive gland, and haemocytes suggested that ChHSP90 is ubiquitously expressed. The mRNA levels of ChHSP90 under salinity and bacterial challenges were analyzed by real-time PCR. Under hypo-osmotic treatment, ChHSP90 mRNA in gonad, heart and haemocytes were significantly up-regulated on day 2 and onwards; while in gill, digestive gland and adductor muscle it was significantly down-regulated; the expression in mantle was decreased significantly on day 2 and 3 (P < 0.01), and then up-regulated on day 4 (P < 0.05). Under hyper-osmotic treatment, the mRNA level in gonad, heart, adductor muscle was increased on day 2 and onwards; in gill, it was firstly increased, and then gradually decreased, reaching a minimum on day 3. On day 4, the expression level in gill recovered to pre-treatment level; in mantle and digestive gland, the expression levels were decreased, reaching to the minimum on day 3. During Vibrio alginolyticus challenge, the mRNA level of ChHSP90 increased 3-fold at 4 h post-infection, returned to its pre-challenge level at 6 h post-infection, then was further up-regulated from 8 to 36 h post-infection. These experiments demonstrate that ChHSP90 mRNA is constitutively expressed in various tissues and apparently inducible in haemocytes under salinity and bacterial challenges, suggesting its important role in response to both osmotic stress and bacterial invasion.

Insights into shell deposition in the Antarctic bivalve Laternula elliptica: gene discovery in the mantle transcriptome using 454 pyrosequencing

Background

The Antarctic clam, Laternula elliptica, is an infaunal stenothermal bivalve mollusc with a circumpolar distribution. It plays a significant role in bentho-pelagic coupling and hence has been proposed as a sentinel species for climate change monitoring. Previous studies have shown that this mollusc displays a high level of plasticity with regard to shell deposition and damage repair against a background of genetic homogeneity. The Southern Ocean has amongst the lowest present-day CaCO₃ saturation rate of any ocean region, and is predicted to be among the first to become undersaturated under current ocean acidification scenarios. Hence, this species presents as an ideal candidate for studies into the processes of calcium regulation and shell deposition in our changing ocean environments.

Results

454 sequencing of L. elliptica mantle tissue generated 18,290 contigs with an average size of 535 bp (ranging between 142 bp-5.591 kb). BLAST sequence similarity searching assigned putative function to 17% of the data set, with a significant proportion of these transcripts being involved in binding and potentially of a secretory nature, as defined by GO molecular function and biological process classifications. These results indicated that the mantle is a transcriptionally active tissue which is actively proliferating. All transcripts were screened against an in-house database of genes shown to be involved in extracellular matrix formation and calcium homeostasis in metazoans. Putative identifications were made for a number of classical shell deposition genes, such as tyrosinase, carbonic anhydrase and metalloprotease 1, along with novel members of the family 2 G-Protein Coupled Receptors (GPCRs). A membrane transport protein (SEC61) was also characterised and this demonstrated the utility of the clam sequence data as a resource for examining cold adapted amino acid substitutions. The sequence data contained 46,235 microsatellites and 13,084 Single Nucleotide Polymorphisms(SNPs/INDELS), providing a resource for population and also gene function studies.

Conclusions

This is the first 454 data from an Antarctic marine invertebrate. Sequencing of mantle tissue from this non-model species has considerably increased resources for the investigation of the processes of shell deposition and repair in molluscs in a changing environment. A number of promising candidate genes were identified for functional analyses, which will be the subject of further investigation in this species and also used in model-hopping experiments in more tractable and economically important model aquaculture species, such as Crassostrea gigas and Mytilus edulis.