NADK-mediated proline synthesis enhances high-salinity tolerance in the razor clam

Euryhaline organisms can accumulate organic osmolytes to maintain osmotic balance between their internal and external environments. Proline is a pivotal organic small molecule and plays an important role in osmoregulation that enables marine shellfish to tolerate high-salinity conditions. During high-salinity challenge, NAD kinase (NADK) is involved in de novo synthesis of NADP(H) in living organisms, which serves as a reducing agent for the biosynthetic reactions. However, the role of shellfish NADK in proline biosynthesis remains elusive. In this study, we show the modulation of NADK on proline synthesis in the razor clam (Sinonovacula constricta) in response to osmotic stress. Under acute hypersaline conditions, gill tissues exhibited a significant increase in the expression of ScNADK. To elucidate the role of ScNADK in proline biosynthesis, we performed dsRNA interference in the expression of ScNADK in gill tissues to assess proline content and the expression levels of key enzyme genes involved in proline biosynthesis. The results indicate that the knock-down of ScNADK led to a significant decrease in proline content (P<0.01), as well as the expression levels of two proline synthetase genes P5CS and P5CR involved in the glutamate pathway. Razor clams preferred to use ornithine as substrate for proline synthesis when the glutamate pathway is blocked. Exogenous administration of proline greatly improved cell viability and mitigated cell apoptosis in gills. In conclusion, our results demonstrate the important role of ScNADK in augmenting proline production under high-salinity stress, by which the razor clam is able to accommodate salinity variations in the ecological niche.

Identification and functional analysis of Toll-like receptor 2 from razor clam Sinonovacula constricta

Toll-like receptor 2 (TLR2) is a member of TLR family that plays important roles in the innate immune system, such as pathogen recognition and inflammation regulation. In this study, the TLR2 homologue was cloned from razor clam Sinonovacula constricta (denoted as ScTLR2) and its immune function was explored. The full-length cDNA of ScTLR2 comprised 2890 nucleotides with a 5'-UTR of 218 bp, an open reading frame of 2169 bp encoding 722 amino acids and a 3'-UTR of 503 bp. The deduced amino acid of ScTLR2 showed similar structure to TLR2 homologue with a conserved signal peptide, four LRR domains, one LRR-TYP domain, one LRR-CT domain, one transmembrane domain and a conserved TIR domain. ScTLR2 mRNA was detected in all examined tissues with the highest expression in the gill. After Vibrio parahaemolyticus challenge, the mRNA expression of ScTLR2 was significantly induced both in gill and haemocytes. The recombinant ScTLR2-LRR protein could bind all tested PAMPs including LPS, PGN and MAN. Bacterial agglutination assay showed that rScTLR2 could agglutinate the six tested bacteria with a calcium dependent manner. More importantly, ScTLR2 silencing by siRNA transfection could significantly depress the mRNA expression of Myd88, NF-κB, Tollip, IRF1, and IRF8. The survival rate of S. constricta was markedly decreased after V. parahaemolyticus challenge under this condition. Our current study demonstrated that ScTLR2 served as a pattern recognition receptor to induce immune response against invasive pathogen.

Effects of acute hypoxia and reoxygenation on histological structure, antioxidant response, and apoptosis in razor clam Sinonovacula constricta

Hypoxia is one of the major environmental problems limiting the healthy development of intensive aquaculture. Marine benthic shellfish are encountering heightened problems related to hypoxic stress as a result of ongoing human activities and aquaculture operations. Razor clam Sinonovacula constricta, a commercially valuable shellfish, has not yet been reported in studies on physiological changes caused by hypoxia and reoxygenation. To understand the negative effects of hypoxia and reoxygenation on the clams, we set up two low-oxygen concentration groups (DO 2.0 mg/L and DO 0.5 mg/L) and assessed multiple aspects of oxidative damage to their hepatopancreas and gills. After the hypoxic stress, the two tissues of the razor clam suffered varying degrees of damage, including cell degeneration and disruption of mitochondrial cristae. After reoxygenation, the 2.0 mg/L group recovered substantially, but the clams in the 0.5 mg/L group still unrecovered. The activities of antioxidant enzymes (MDA, T-AOC, SOD, GPX, and CAT) in clams were considerably altered by acute hypoxia and reoxygenation. Briefly, there was a growing and then declining trend in MDA, T-AOC, and SOD activities in the hepatopancreas, whereas GPX and CAT activities showed the converse trend. In the hepatopancreas and gills, the level of anti-apoptotic gene Bcl-2 transcripts gradually decreased with the duration of hypoxia and increased following reoxygenation. However, changes in the transcript level of the pro-apoptotic gene Bax were in contrast to that of Bcl-2. The TUNEL assay revealed that hypoxia caused apoptosis. Furthermore, at DO 0.5 mg/L, the degree of apoptosis was more significant than at DO 2.0 mg/L, and hepatopancreatic apoptosis was more severe than gill apoptosis. Collectively, our findings imply that hypoxia induces oxidative stress, histological damage, and apoptosis in razor clams in a concentration-dependent and tissue-specific manner. These consequences serve as a reminder that prolonged recovery periods may be required for razor clams to fully recover from oxidative damage resulting from hypoxia-reoxygenation episodes.

Investigating the Impact of Humic Acid on Copper Accumulation in Sinonovacula constricta Using a Toxicokinetic-Toxicodynamic Model

In aquatic ecosystems, the interaction between heavy metals and dissolved organic carbon (DOC) plays a pivotal role in modifying the bioavailability of these metals. This study, employing a toxicokinetic-toxicodynamic model, delves into the interactive effects of humic acid (HA), a significant component of DOC, on the bioaccumulation and toxicity of copper (Cu) in the estuarine economic bivalve Sinonovacula constricta. Utilizing the stable isotope 65Cu as a tracer, we evaluated Cu uptake in S. constricta under varied DOC concentrations in a controlled laboratory setting. Our findings reveal that at DOC concentrations below 3.05 mg L-1, the bioavailability of Cu is reduced due to shifts in the speciation distribution of Cu, resulting in decreased bioaccumulation within S. constricta. Conversely, at DOC levels exceeding 3.05 mg L-1, the formation of colloidal Cu-HA complexes allows its entry into the bivalves' digestive system. Moreover, toxicity assays demonstrate an increase in S. constricta survival rates with higher DOC concentrations, suggesting a protective effect of DOC against Cu toxicity. The integration of accumulation and toxicity data infers that Cu-HA complexes, when ingested via the digestive tract, exhibit lower toxicity compared to Cu directly assimilated from the water phase. These findings emphasize the need to consider environmental DOC levels in assessing Cu pollution risks and provide insights for managing heavy metal toxicity in estuarine aquaculture.

Function of taurine and its synthesis-related genes in hypertonic regulation of Sinonovacula constricta

Salinity changes affect the osmotic gradient across the gill epithelium of marine species. Taurine is an important osmoregulator with a crucial role in osmoregulation in marine bivalves. This study determined the osmolality, taurine content, key enzymes involved in taurine synthesis (cysteine dioxygenase (CDO), cysteine sulfinic acid decarboxylase (CSAD), and taurine transporter (TauT)) and related gene expression in razor clam Sinonovacula constricta in response to high salt stress [high salt seawater (S30) versus high salt seawater with taurine supplementation (S30T) versus natural salinity control]. The data were recorded at 0, 6, 12, 24, 48, 72 h. Serum osmolality significantly increased under high salt conditions compared with the control group (P < 0.05). When serum osmolality had stabilized (after 48 h), there was no significant difference in serum osmolality between the S30T and control groups (P > 0.05), but serum osmolality was significantly lower in the S30 versus control group (P < 0.05). Taurine content significantly increased under high salt stress and remained high (P < 0.05). CSAD and CDO content was higher in S30 than in S30T, whereas TauT was significantly lower in S30 than in the control group eventually (P < 0.05). Expression of CDO and CSAD in the S30 and S30T groups was significantly higher than in control animals (P < 0.05), with that in S30 being higher than in S30T. By contrast, TauT expression peaked 6 h after stress in S30 and S30T, but was lower in S30 than in the control group (P < 0.05). These results demonstrate that S. constricta is an osmoconformer, with exogenous taurine relieving the stress of osmoregulation caused by insufficient endogenous taurine in cells. These findings further enhance our understanding of the regulatory mechanisms underlying the response of S. constricta to high salinity stress.

Acute Salinity Stress Disrupts Gut Microbiota Homeostasis and Reduces Network Connectivity and Cooperation in Razor Clam Sinonovacula constricta

Accumulating evidence demonstrates that it is of great importance to maintain a stable and functional gut microbial community for host's growth and health. However, gut microenvironment is constantly affected by diverse environmental factors. Salinity can cause stress, including hypersaline or hyposaline stress to aquatic species, thereby affecting their growth conditions. Razor clam (Sinonovacula constricta), an economically important bivalve species, inhabits in intertidal and estuarine zones and constantly experiences salinity stress. Yet little is known about how and to what extent clam gut microbiota is affected by salinity stress, while this knowledge is fundamental for clam aquaculture health management. To address this concern, this study compared the temporal differences of gut bacterial signatures and community assembly of S. constricta under normal salinity (NS), low salinity (LS), and high salinity (HS) conditions. Acute salinity stress affected the compositions, structures, and functional potentials of clam gut microbial community, of which salinity stress, hours post stress, and their interaction respectively constrained 7.6%, 16.4%, and 7.9% of community variation. Phylogenetic bin-based null model result revealed that the gut bacterial assembly of three salinity groups seemed to be largely driven by stochastic processes. Network analysis indicated that gut bacterial interspecies interaction exhibited less connected and lower cooperative activity under the conditions of LS and HS compared with NS. Notably, some pathogenic bacteria, including Vibrio and Pseudoalteromonas, were identified as keystone taxa of gut microbial networks in LS and HS groups. Above findings suggest that the clams under LS and HS conditions might be at a higher risk of developing disease. Our findings enhance the mechanism understanding of gut microbial assembly in S. constricta under abiotic factor challenge, which has important implications for clam health control from a microbial ecological perspective.

Genome-Wide Identification of 5-HT Receptor Gene Family in Razor Clam Sinonovacula constricta and Their Circadian Rhythm Expression Analysis

Serotonin (5-HT) is primarily distributed in the gastrointestinal and central nervous systems, where it plays a crucial role in regulating various physiological functions such as digestion, reproduction and establishing animal emotions. 5-HT is an effective oxytocin widely used in molluscan aquaculture, and its physiological functions are performed by binding to corresponding 5-HT receptors (5-HTRs). In this study, seven 5-HTR genes of Sinonovacula constricta (Sc5-HTRs) were identified and analyzed, and they were designated as Sc5-HT1A, Sc5-HT1D, Sc5-HT2-1, Sc5-HT2-2, Sc5-HT2-3, Sc5-HT4 and Sc5-HT6. Phylogenetic analysis showed that the seven Sc5-HTRs were conserved among mollusks, and the Sc5-HTRs were all transmembrane proteins. The seven Sc5-HTR genes were distributed on chromosome 1, 2, 13 and 14. After injecting 5-HT, there was a significant increase in mRNA expression levels of Sc5-HT1A (p < 0.05) and Sc5-HT2-3 (p < 0.01), while Sc5-HT4 decreased significantly (p < 0.01) compared to control groups which might be effective 5-HT receptors. Furthermore, two of the receptors (Sc5-HT2-3 and Sc5-HT4) were expressed in the circadian rhythm patterns, indicating their potential influence on the nocturnal spawning of S. constricta. Overall, these findings provide a theoretical basis for understanding the structures and functions of 5-HTR gene family members, and may facilitate the artificial propagation of mollusks.

Expression and Functional Analysis of AMT1 Gene Responding to High Ammonia Stress in Razor Clam (Sinonovacula constricta)

Ammonium transporter 1 (AMT1), a member of ammonia (NH₃/NH₄⁺) transport proteins, has been found to have ammonia transport activity in plants and microorganisms. However, the functional characteristics and molecular mechanisms of AMT1 in mollusks remain unclear. The razor clam (Sinonovacula constricta) is a suitable model species to explore the molecular mechanism of ammonia excretion because of the high concentration of ambient ammonia it is exposed to in the clam-fish-shrimp polyculture system. Here, the expression of AMT1 in S. constricta (Sc-AMT1) in response to high ammonia (12.85 mmol/L NH₄Cl) stress was identified by real-time quantitative PCR (qRT-PCR), Western blotting, RNA interference, and immunofluorescence analysis. Additionally, the association between the SNP_g.15211125A > T linked with Sc-AMT1 and ammonia tolerance was validated by kompetitive allele-specific PCR (KASP). A significant upregulated expression of Sc-AMT1 was observed during ammonia exposure, and Sc-AMT1 was found to be localized in the flat cells of gill. Moreover, the interference with Sc-AMT1 significantly upregulated the hemolymph ammonia levels, accompanied by the increased mRNA expression of Rhesus glycoprotein (Rh). Taken together, our findings imply that AMT1 may be a primary contributor to ammonia excretion in S. constricta, which is the basis of their ability to inhabit benthic water with high ammonia levels.

Genome-wide characterization of the cytosolic sulfotransferase 1B member 1 (SULT1B1) family and its expression responses to sulfide stress in the razor clam Sinonovacula constricta

The razor clam (Sinonovacula constricta), a typical burrowing organism in the intertidal zones, is often exposed to sulfide environment and shows strong sulfide tolerance. Located downstream of the sulfur metabolism pathway, cytosolic sulfotransferase family 1B member 1 (SULT1B1) is a key enzyme catalysing the sulfonation reaction, and plays an important role in the biotransformation of endogenous substances such as thyroid hormones (THs). To investigate their roles in sulfide resistance, a systematic analysis of S. constricta SULT1B1s (ScSULT1B1s), including genomic distribution, phylogenetic relationships, gene structure, conserved motifs, and expression profiles under sulfide stress, was performed. A total of 10 ScSULT1B1 genes were found in the S. constricta genome. Sequence analysis showed that ScSULT1B1 gene family encoded 155-425 amino acids, containing four catalytic active sites (K, N, H, and S), one PAPS binding domain at the N-terminus, and one PAPS binding and dimerization domain at the C-terminus. The spatial-temporal expression patterns of ScSULT1B1s were further estimated by quantitative real-time PCR (qRT-PCR). Among them, partial ScSULT1B1s showed significantly high expression in the gill, hepatopancreas, and siphon. Furthermore, the response expression of certain ScSULT1B1s significantly fluctuated under sulfide stress. Together, our results suggest that ScSULT1B1s, by mediating the sulfonation reaction, may regulate THs levels to maintain basic metabolic and immune functions, making S. constricta highly sulfide tolerant.

The Inhibitory Effects of RNA-Interference-Mediated Guanylate Cyclase Knockdown on Larval Metamorphosis and Early Progeny Growth of Razor Clam

Guanylate cyclase (GC, cGMPase) is a key enzyme in organisms, catalyzing the synthesis of cGMP from GTP, thus making cGMP work. cGMP plays a vital role in the regulation of cell and biological growth as a second messenger in signaling pathways. In this study, we screened and identified cGMPase from the razor clam Sinonovacula constricta, which encoded 1257 amino acids and was widely expressed in different tissues, especially the gill and liver. We also screened one double-stranded RNA (dsRNA), cGMPase, which was used to knockdown cGMPase at three larval metamorphosis development stages: trochophores-veliger larve, veliger larve-umbo larve, and umbo larve-creeping larvae. We showed that interference at these stages significantly inhibited larval metamorphosis and survival rates. cGMPase knockdown resulted in an average metamorphosis rate of 60% and an average mortality rate of 50% when compared with control clams. After 50 days, shell length and body weight were inhibited to 53% and 66%, respectively. Thus, cGMPase appeared to regulate metamorphosis development and growth in S. constricta. By examining the role of the key gene in the metamorphosis development of S. constricta larvae and the growth and development period, we can provide some data reference for studying the growth and development mechanism of shellfish, and the results provided basic information for the breeding of S. constricta.

Comprehensive Analysis of Whole-Transcriptome Profiles in Response to Acute Hypersaline Challenge in Chinese Razor Clam Sinonovacula constricta

The Chinese razor clam (Sinonovacula constricta) is an important for Chinese aquaculture marine bivalve that naturally occurs across intertidal and estuarine areas subjected to significant changes in salinity level. However, the information on the molecular mechanisms related to high salinity stress in the species remain limited. In this study, nine gill samples of S. constricta treated with 20, 30, and 40 ppt salinity for 24 h were used for whole-transcriptome RNA sequencing, and a regulatory network of competing endogenous RNAs (ceRNAs) was constructed to better understand the mechanisms responsible for adaptation of the species to high salinity. A total of 83,262 lncRNAs, 52,422 mRNAs, 2890 circRNAs, and 498 miRNAs were identified, and 4175 of them displayed differential expression pattern among the three groups examined. The KEGG analyses of differentially expressed RNAs evidenced that amino acid synthesis and membrane transport were the dominant factors involved in the adaptation of the Chinese razor clam to acute salinity increase, while lipid metabolism and signaling played only a supporting role. In addition, lncRNA/circRNA-miRNA-mRNA regulatory networks (ceRNA network) showed clearly regulatory relationships among different RNAs. Moreover, the expression of four candidate genes, including tyrosine aminotransferase (TAT), hyaluronidase 4 (HYAL4), cysteine sulfinic acid decarboxylase (CSAD), and ∆¹-pyrroline-5-carboxylate synthase (P5CS) at different challenge time were detected by qRT-PCR. The expression trend of TAT and HYAL4 was consistent with that of the ceRNA network, supporting the reliability of established network. The expression of TAT, CSAD, and P5CS were upregulated in response to increased salinity. This might be associated with increased amino acid synthesis rate, which seems to play an essential role in adaptation of the species to high salinity stress. In contrast, the expression level of HYAL4 gene decreased in response to elevated salinity level, which is associated with reduction Hyaluronan hydrolysis to help maintain water in the cell. Our findings provide a very rich reference for understanding the important role of ncRNAs in the salinity adaptation of shellfish. Moreover, the acquired information may be useful for optimization of the artificial breeding of the Chinese razor clam under aquaculture conditions.

Study on the toxic effects of sodium pentachlorophenol (PCP-Na) on razor clam (Sinonovacula constricta)

Currently, research on toxic effects of PCP Na is greatly insufficient. The aim of this study is to explore the toxic effects of PCP-Na for better conducting future work on PCP-Na toxicology. For this purpose, S. constricta adults were exposed to PCP-Na for toxicity testing. The results showed that PCP-Na could easily bioaccumulate in S. constricta and significantly affected both phrase I and II metabolism enzymes. Meanwhile, PCP-Na strongly activated antioxidant system and caused PC, LPO and DNA damage. In addition, neurotoxicity and immunotoxicity of PCP-Na was demonstrated in this study. Interestingly, we observed that PCP-Na significantly affected the expression of genes of electron transport chain and induced key enzymes of glycolysis, indicating that PCP-Na may act as an uncoupler of oxidative phosphorylation, interfering with energy supply and causing energy compensation. This study is the first to fully analyze and provide a new perspective on the toxicity of PCP-Na.

PDIA6 involves the thermal stress response of razor clam, Sinonovacula constricta

Protein disulfide isomerases A6 (PDIA6), an oxidoreductase and isomerase, catalyzes the oxidation reduction and isomerization of disulfide bonds, and serves as molecular chaperone to prevent the buildup of misfolded proteins under various environmental insults. However, the role of PDIA6 in mollusks remains largely obscure, although its multifunctional protein has been reported in other species under adverse conditions. To fill this gap, we identified PDIA6 from the razor clam Sinonovacula constricta (ScPDIA6) and investigated its expression patterns in response to thermal stress. Tissue distribution showed that the mRNA transcript of ScPDIA6 was ubiquitously expressed in nine tested tissues. Temporal expression profiles by qPCR revealed that ScPDIA6 in the gill and mantle was significantly increased by hyper-thermic treatment. Further, Western blot and immunofluorescence indicated that ScPDIA6 was significantly upregulated by thermal treatment at the protein level. Additionally, the survival test demonstrated that the viability of E. coli cells expressing recombinant ScPDIA6 protein increased at 42 °C compared with empty vector. Overall, these findings suggested that ScPDIA6 may play a pivotal role in counteracting thermal stress. This study will provide valuable reference data resource for understanding the potential role of PDIA6 in mollusks.

Massive expansion of P-selectin genes in two Venerida species, Sinonovacula constricta and Mercenaria mercenaria: evidence from comparative genomics of Bivalvia

Background

P-selectin is a molecule participating in the inflammatory response through mediating cellular adhesion and essential for wound repair. However, studies regarding P-selectin in Bivalvia are rare. This study identified 90 P-selectin genes among nine bivalve genomes and classified them into 4 subfamilies according to phylogenetic analysis.

Results

Notable P-selectin gene expansion was observed in two Venerida species, Sinonovacula constricta and Mercenaria mercenaria. The synteny analysis revealed that P-selectin gene expansion was mostly caused by tandem duplication. In addition, the expression profiles of P-selectin genes in S. constricta showed that many P-selectins were specifically highly expressed in the gills, and the P-selectin expression patterns changed dramatically under low salt stress and ammonia nitrogen stress.

Conclusions

The massive expansion of P-selectins may facilitate the tolerance to environmental stresses. This study sheds light on the characterizations and expression profiles of P-selectin genes in Bivalvia and provides an integrated framework for further investigation of the role of P-selectins in the environmental tolerance of bivalves.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08861-6.

Characteristics of glutathione peroxidase gene and its responses to ammonia-N stress in razor clam Sinonovacula constricta

Glutathione peroxidase (GPX) is a crucial enzyme in the antioxidant defense system. However, the previous studies on the structure and functions of mollusk GPX genes are still very limited. Here, we investigated the GPX gene from Sinonovacula constricta (Sc-GPX), and its expression profiles, protein localization, gene function and association with ammonia tolerance. The full length of sequence of Sc-GPX was 1781 bp, containing an open reading frame (ORF) of 588 bp encoding 195 amino acids. Quantitative expression of seven adult tissues showed that Sc-GPX was most abundant in hepatopancreas, followed by gills. Furthermore, the enzyme activity of Sc-GPX in hepatopancreas increased significantly under different ammonia concentrations (100, 140, and 180 mg/L) (P < 0.01). Further, we explored the mRNA expression level, histological structure and histo-cellular localization in gills and hepatopancreas of Sc-GPX under 140 mg/L ammonia stress. The mRNA expression level in gills and hepatopancreas of Sc-GPX increased significantly (P < 0.05) and immunohistochemistry results suggested that the columnar cells of gills filaments and the endothelial cells of hepatopancreas were the major sites for the action of Sc-GPX protein. In addition, we performed western blotting (WB), RNA interference (RNAi) and single nucleotide polymorphisms (SNPs) in the hepatopancreas of Sc-GPX under ammonia stress (140 mg/L). WB results indicated that the protein expression of Sc-GPX increased significantly (P < 0.01) after ammonia challenge. In addition, expression of Sc-GPX mRNA were significantly downregulated at 24 and 48 h after RNAi (P < 0.01). The association analysis between ammonia-tolerance group and control group identified six SNPs in coding sequence (CDS) of Sc-GPX from 449 individuals. Among them, c.162A > C was missense mutation, which lead to the amino acid change from Lys to Asn. These findings revealed that Sc-GPX may play a critical role in clam ammonia detoxification.

Bacterial community in Sinonovacula constricta intestine and its relationship with culture environment

Although the importance of intestinal microbes to aquaculture animals has been recognized, the intestinal bacteria of Sinonovacula constricta and its culture environment are rarely studied. In this study, high-throughput sequencing was used to explore the intestinal bacterial communities of pond water, sediment, and S. constricta intestine. Significance analysis and principal coordinates analysis (PCoA) showed that there were significant differences in bacterial communities among animals' intestine, pond water, and sediment (p < 0.05). Venn analysis showed that intestinal bacteria shared a considerable number of OTUs (operational taxonomic units) with the sediment and water. SourceTracker analysis suggested that the contribution of sediment to the intestinal bacteria of S. constricta was much larger than that of rearing water. The Kruskal-Wallis test showed that the dominant bacterial taxa differed significantly between animals' intestines and the pond environment, and each of them has a unique bacterial composition. A network diagram indicated the complex positive and negative interactions between intestinal bacteria at the OTU level. Furthermore, BugBase analysis indicated that the bacterial contribution to potential pathogens in the animals' intestines is similar to that in sediments, suggesting that sediment was the main source of potential pathogens in S. constricta intestine. This study provided a theoretical basis for environmental regulation and disease prevention of S. constricta in aquaculture. KEY POINTS: • Culture environment had a significant effect on the intestinal bacterial community in S. constricta. • Sediment was a major source of intestinal bacteria and potentially pathogenic bacteria. • Complex positive and negative interactions existed between intestinal bacteria.

Immunomodulatory effects of the polysaccharide from Sinonovacula constricta on RAW264.7 macrophage cells

This study aimed to evaluate the immunomodulatory effect of the polysaccharide from Sinonovacula constricta (SCP‐1‐1) in RAW264.7 cells. SCP‐1‐1 with a molecular weight of 440.0 kDa consisted of glucose and mannose. The immunomodulatory assay results showed that SCP‐1‐1 could significantly enhance phagocytic ability, NO production, and some cytokines (TNF‐α, IL‐6, and IL‐1β) secretion of RAW264.7 cell in a dose‐dependent manner. Western blot analysis results demonstrated that SCP‐1‐1 could regulate the expression levels of the key proteins in the signaling pathways of RAW264.7 cell and might associated with NF‐κβ and PI3K signaling pathway. These findings could contribute to elucidate the immunomodulatory activities of the polysaccharide from Sinonovacula constricta.

Expression and functional characterization of peptidoglycan recognition protein-S6 involved in antibacterial responses in the razor clam Sinonovacula constricta

It has been recognized that peptidoglycan recognition proteins (PGRPs), structurally conserved molecules, play crucial roles in the innate immunity of invertebrate. However, few studies have been taken to explore their potential functions. In this study, a novel PGRP from the razor clam Sinonovacula constrict designated as ScPGRP-S6 was identified and characterized. The open reading frame (ORF) of ScPGRP-S6 was 666 bp in length, encoding a protein of 221 amino acid with a signal peptide (1-30) and a typical PGRP domain (39-187). The sequence alignment combined with phylogenetic analysis collectively confirmed that ScPGRP-S6 was a novel member belonging to PGRP-S family. The mRNA transcript of ScPGRP-S6 in the hepatopancreases was significantly up-regulated after peptidoglycan (PGN) stimulation, while it was moderately up-regulated after lipopolysaccharide (LPS) stimulation. The result of immunofluorescence detection demonstrated that the positive signal enhanced obviously after Vibrio parahaemolyticus challenge. Notably, the recombinant protein of ScPGRP-S6 (designed as rScPGRP-S6) exhibited high agglutination activity towards V. parahaemolyticus but weak to Staphylococcus aureus. Furthermore, rScPGRP-S6 showed strong amidase and antibacterial activity in the presence of Zn²⁺. Collectively, our results manifested that ScPGRP-S6 could act as a scavenger in the innate immune response of S. constricta.

Growth performance and ecological services evaluation of razor clams based on dynamic energy budget model

Bivalve shellfish aquaculture has been proposed to abate eutrophication and increase carbon sink in integrated multi-trophic aquaculture ecosystems. An individual growth model for razor clams Sinonovacula constricta in an integrated aquaculture pond of Portunus trituberculatus- Penaeus japonicus- S. constricta was constructed based on dynamic energy budget (DEB) theory after parameter measurement and model validation. Goodness-of-fit indices (R-squared, mean difference, and absolute and relative root mean square error) showed that the DEB model accurately reproduced razor clam growth. The growth performance evaluation of razor clams under different environmental conditions showed that warming climate and food shortages inhibited the razor clam growth. The quantification results of ecological service showed that individual razor clam have the potential for nutrient (nitrogen and phosphorus) removal and CO₂ fixation, but exhibit a source of CO₂ in individual month. The possible applications and contribution of this aquaculture model in China are also discussed, and the assessment results can provide important support for the low carbon bivalve integrated aquaculture.

Expression of p38MAPK and its regulation of apoptosis under high temperature stress in the razor clam Sinonovacula constricta

p38MAPK is a key branch of the MAPK (mitogen-activated protein kinase) pathway that plays an important role in physiological processes such as apoptosis, cell proliferation and growth. In this experiment, we screened and identified one p38MAPK gene in the razor clam Sinonovacula constricta, which encoded 359 amino acids and was widely expressed in various adult tissues. After 24 h of high temperature stress at 34 °C, the transcript expression of p38MAPK showed significant changes in all tested tissues. In particular in the gill and hepatopancreas tissues, where the expression increased 1.81 and 7.83 times compared with the control group, respectively (P < 0.01). Furthermore, we examined the expression of the apoptosis suppressor gene Bcl-2 and pro-apoptosis gene Bax by knock-down of p38MAPK with dsRNA interference in the gill and hepatopancreas tissues. The obvious up-regulation expression of Bcl-2 and significant suppression of Bax were observed, respectively (P < 0.01). Moreover, the TUNEL staining technique was used to detect apoptosis before and after interference. The degree of apoptosis in the gill and hepatopancreas tissues was reduced after interference with p38MAPK, and the ROS content was significantly reduced (P < 0.01). The results suggested that p38MAPK had a regulatory role in the heat tolerance of razor clams.

Two fibrinogen-related proteins (FREPs) in the razor clam (Sinonovacula constricta) with a broad recognition spectrum and bacteria agglutination activity

Fibrinogen-related proteins (FREPs) that contain only the fibrinogen-related domain are likely involved in pathogen recognition. In this study, we identified two FREPs from the razor clam (Sinonovacula constricta), called ScFREP-1 and ScFREP-2, and investigated their roles in the immune response. Both ScFREP-1 and ScFREP-2 contained a fibrinogen-related domain at the C-terminal. ScFREP-1 and ScFREP-2 mRNAs were detected in all adult clam tissues tested, with the highest expression levels in the gill and mantle, respectively. Their expression levels were significantly upregulated after microbe infection. Recombinant ScFREPs could bind Gram-positive and Gram-negative bacteria as well as some pathogen-associated molecular patterns (PAMPs), and they could agglutinate those bacteria. These results showed that ScFREPs functioned as potential pattern recognition receptors to mediate immune response by recognizing PAMPs and agglutinating invasive microbes.

Vital Role of Glutamate Dehydrogenase Gene in Ammonia Detoxification and the Association Between its SNPs and Ammonia Tolerance in Sinonovacula constricta

Increasing evidence has revealed accumulated ammonia will cause adverse effects on the growth, reproduction, and survival of aquatic animals. As a marine benthic mollusk, the razor clam Sinonovacula constricta shows better growth and survival under high ammonia nitrogen environment. However, little is known about its adaptation mechanisms to high ammonia stress in an integrated mariculture system. In this study, we analyzed the association between the polymorphism of glutamate dehydrogenase gene (GDH), a key gene involved in ammonia nitrogen detoxification, and ammonia tolerance. The results showed that 26 and 22 single-nucleotide polymorphisms (SNPs) of GDH in S. constricta (denoted as Sc-GDH) were identified from two geographical populations, respectively. Among them, two SNPs (c.323T > C and c.620C > T) exhibited a significant and strong association with ammonia tolerance, suggesting that Sc-GDH gene could serve as a potential genetic marker for molecular marker–assisted selection to increase survival rate and production of S. constricta. To observe the histological morphology and explore the histocellular localization of Sc-GDH, by paraffin section and hematoxylin–eosin staining, the gills were divided into gill filament (contains columnar and flattened cells) and gill cilia, whereas hepatopancreas was made up of individual hepatocytes. The results of immunohistochemistry indicated that the columnar cells of gill filaments and the endothelial cells of hepatocytes were the major sites for Sc-GDH secretion. Under ammonia stress (180 mg/L), the expression levels of Sc-GDH were extremely significantly downregulated at 24, 48, 72, and 96 h (P < 0.01) after RNA interference. Thus, we can speculate that Sc-GDH gene may play an important role in the defense process against ammonia stress. Overall, these findings laid a foundation for further research on the adaptive mechanisms to ammonia–nitrogen tolerance for S. constricta.

Defense responses of sulfur dioxygenase to sulfide stress in the razor clam Sinonovacula constricta

BACKGROUND

Sulfide is a well-known toxicant widely distributed in the culture environment. As a representative burrowing benthic bivalve, the razor clam Sinonovacula constricta is highly sulfide tolerant. Mitochondrial sulfide oxidation is an important way for sulfide detoxification, where sulfur dioxygenase (SDO) is the second key enzyme.

OBJECTIVE

To investigate the mechanism of sulfide tolerance in S. constricta, the molecular characterization of its SDO (designated as ScSDO) was studied.

METHODS

The cDNA sequence of ScSDO was cloned by RACE technique. The response of ScSDO in gills and livers of S. constricta was investigated during sulfide exposure (50, 150, and 300 μM sulfide) for 0, 3, 6, 12, 24, 48, 72, and 96 h by qRT-PCR. Moreover, the temporal expression of ScSDO protein in S. constricta gills after exposure to 150 μM sulfide was detected by Western blot. The subcellular location of ScSDO was identified by TargetP 1.1 prediction and Western Blot analysis.

RESULTS

The full-length cDNA of ScSDO was 2914 bp, encoding a protein of 304 amino acids. The deduced ScSDO protein was highly conserved, containing the signature HXHXDH motif of the metallo-β-lactamase superfamily and two metal-binding sites, of which metal-binding site I is known to be the catalytically active center. Subcellular localization confirmed that ScSDO was located only in the mitochondria. Responding to the sulfide exposure, distinct time-dependent increases in ScSDO expression were detected at both mRNA and protein levels. Moreover, the gills exhibited a higher ScSDO expression level than the livers.

CONCLUSIONS

All of our results suggest that ScSDO plays an important role in mitochondrial sulfide oxidation during sulfide stress, making S. constricta highly sulfide tolerant. In addition, as a respiratory tissue, the gills play a more critical role in sulfide detoxification.

Integrated analysis of transcriptomic and metabolomic data to evaluate responses to hypersalinity stress in the gill of the razor clam (Sinonovacula constricta)

Salinity is an important ecological factor that affects physiological metabolism, survival, and distribution of marine organisms. Despite changes in the osmolarity and composition of the cytosol during salinity shifts, marine mollusks are able to maintain their metabolic function. The razor clam (Sinonovacula constricta) survives the wide range of salinity in the intertidal zone via changes in behavior and physiology. To explore the stress responses and mechanisms of salinity tolerance in razor clams, we collected transcriptomic and metabolomic data from a control group (salinity 20‰, S20) and a salinity-stress group (salinity 35‰, S35). The transcriptome data showed that genes related to the immune system, cytoskeleton remodeling, and signal transduction pathways dominated in the S35 group to counteract hypersalinity stress in the gill. The metabolomic analysis showed that 142 metabolites were significantly different between the S35 and S20 groups and that amino acid and carbohydrate metabolism were affected by hypersalinity stress. Levels of amino acids and energy substances, such as l-proline, isoleucine, and fructose, were higher in the gill of the S35 group. The combination of transcriptomic and metabolomic data indicated that metabolism of amino acids, carbohydrates, and lipids was enhanced in the gill during adaptation to high salinity. These results clarified the complex physiological processes involved in the response to hyperosmotic stress and maintenance of metabolism in the gill of razor clams. These findings provide a reference for further study of the biological responses of euryhaline shellfish to hyperosmotic stress and a molecular basis for the search for populations with high salinity tolerance.

The razor clam Sinonovacula constricta uses the strategy of conversion of toxic ammonia to glutamine in response to high environmental ammonia exposure

High ammonia can inhibit the survival and growth, and even cause mortality of razor clam (S. constricta). The accumulation of ammonia to lethal concentrations in some invertebrates may be partially prevented by converting some of the ammonia into glutamine (Gln). Glutamine dehydrogenase (GDH) and glutamine synthetase (GS) have been widely implicated a central role in response to ammonia stress. However, the molecular and physiological response of GDH and GS to ammonia alterations has not yet been determined in clams. To investigate the possible participatory role of GDH and GS genes in altered ammonia conditions, we have cloned their gene sequences and examined the mRNA expression and western blotting under ammonia exposure in S. constricta (ScGDH and ScGS), and detected the levels of GS and GDH, and the content of glutamate (Glu) and Gln. The full-length cDNA of ScGDH was 3924 bp, with a 1629 bp open reading frame (ORF) encoding a 542 amino-acid polypeptide. The complete cDNA sequence for ScGS had 2739 bp with an ORF of 1110 bp encoding 369 amino acids. To investigate ammonia detoxification strategies, the clams were exposed to ammonia for 96 h at four different concentrations (0, 100, 140, and 180 mg/L). Exposure to ammonia resulted in a significant increase of glutamate concentration and Gln in the haemocytes. GDH activity, GDH relative mRNA and protein expression, GS activity, GS relative mRNA and protein expression increased significantly and showed a pronounced time and dosage interaction in the liver. The results suggested that the protective strategies of Gln formation existed in S. constricta, which could convert ammonia to non- or less toxic nitrogenous compounds on the exposure of ammonia. Glutamate content in the haemocytes increased significantly, which is to ensure sufficient Glu to meet the needs for GS to catalyze the conversion of ammonia to Gln. We proposed that the induction of Glu synthesis-related genes and the subsequent formation of the active protein occurred in preparation for the increased capacity of the body to convert ammonia, into Gln. The results of this study suggested that GDH and GS play an important role in the synthesis of Gln, emphasizing, the protective strategies of Gln formation in S. constricta convert ammonia to nontoxic or less toxic nitrogenous compounds upon exposure to ammonia.

Transcriptional regulation mechanism of sterol regulatory element binding proteins on Δ6 fatty acyl desaturase in razor clam Sinonovacula constricta

The razor clam, Sinonovacula constricta, contains high levels of long-chain PUFA (LC-PUFA), which are critical for human health. In addition, S. constricta is the first marine mollusc demonstrated to possess Δ6 fatty acyl desaturase (Fad) and complete LC-PUFA biosynthetic ability, providing a good representative to investigate the molecular mechanism of sterol regulatory element binding proteins (SREBP) in regulating Δ6 Fad for LC-PUFA biosynthesis in marine molluscs. Herein, S. constricta SREBP and Δ6 Fad promoter were cloned and characterised. Subsequently, dual luciferase and electrophoretic mobility shift assays were conducted to explore the SREBP binding elements in the core regulatory region of S. constricta Δ6 Fad promoter. Results showed that S. constricta SREBP had a very conservative basic helix-loop-helix-leucine zipper motif, while S. constricta Δ6 Fad promoter exhibited very poor identity with teleost Fads2 promoters, indicating their differentiation during evolution. A 454 bp region harbouring a core sequence in S. constricta Δ6 Fad promoter was predicted to be essential for the transcriptional activation by SREBP. This was the first report on the regulatory mechanism of LC-PUFA biosynthesis in marine molluscs, which would facilitate optimising the LC-PUFA biosynthetic pathway of bivalves in further studies.

RNAi-mediated knock-down of the dopamine beta-hydroxylase gene changes growth of razor clams

Dopamine beta-hydroxylase (DβH) plays an essential role in the synthesis of catecholamines (CA) in neuroendocrine networks. In the razor clam, Sinonovacula constricta a novel gene for DβH (ScDβH-α) was identified that belongs to the copper type II ascorbate-dependent monooxygenase family. Expression analysis revealed ScDβH-α gene transcripts were abundant in the liver and expressed throughout development. Knock-down of ScDβH-α in adult clams using siRNA caused a reduction in the growth rate compared to control clams. Reduced growth was associated with strong down-regulation of gene transcripts for the growth-related factors, platelet derived growth factors A (PDGF-A) (P < 0.001) 24 h after ScDβH-α knock-down, vascular endothelial growth factor (VEGF1) (P < 0.001) and platelet derived growth factor B (PDGF-B-2) (P < 0.001) 24 h and 48 h after ScDβH-α knock-down and transforming growth factor beta (TGF-β1) (P < 0.001) 48 h and 72 h after ScDβH-α knock-down. Taken together the results suggest that the novel ScDβH-α gene through its role in CA synthesis is involved in growth regulation in the razor clam and possibly other bivalves.

Integrated mRNA and Small RNA Sequencing Reveals Regulatory Expression of Larval Metamorphosis of the Razor Clam

The razor clam, Sinonovacula constricta, is an important economic marine shellfish, and its larval development involves obvious morphological and physiological changes. MicroRNA plays a key role in the physiological changes of the organism through regulating targeted mRNA. This study performed miRNA-mRNA sequencing for eight different developmental stages of S. constricta using Illumina sequencing. A total of 2156 miRNAs were obtained, including 2069 known miRNAs and 87 novel miRNAs. In addition, target genes were predicted for key miRNAs differentially expressed between adjacent development samples by integrating the mRNA transcriptome. Further analysis revealed that the differentially expressed genes were enriched in complement activation, alternative pathways, translation, and negative regulation of monocyte molecular protein-1 production. KEGG pathway annotation showed significant enrichment in the regulation of the ribosome, phagosome, tuberculosis and fluid shear stress, and atherosclerosis. Ten mRNAs and ten miRNAs that are related to larval metamorphosis were identified using real-time PCR. Furthermore, the double luciferase experiment validated the negative regulatory relationship between miR-133 and peroxisome proliferator-activated receptor-γ (PPAR-γ). These results indicated that the target genes regulated by these differentially expressed miRNAs may play an important regulatory role in the metamorphosis development of S. constricta.

Involvement of a novel Ca2+-independent C-type lectin from Sinonovacula constricta in food recognition and innate immunity

Bivalve lectins perform a crucial function in recognition of foreign particles, such as microalgae and pathogenic bacteria. In this study, a novel C-type lectin form Sinonovacula constricta (ScCL) was characterized. The full-length cDNA of ScCL was 1645 bp, encoding a predicted polypeptide of 273 amino acids with one typical carbohydrate-recognition domain. ScCL has the highest similarity and closest phylogenetic relationship with the C-type lectin from Solen grandis. Real-time PCR analysis showed that ScCL was expressed in all tested tissues, with the highest expression in the foot and the lowest expression in hemocytes. Agglutination activity of ScCL was Ca²⁺-independent. ScCL showed the strongest agglutination on Chlorella vulgaris, the modest agglutination on Platymonas subcordiformis, Nannochloropsis sp., and Thalassiosira pseudonana, the weakest agglutination on Chaetoceros sp., and no agglutination on Isochrysis zhanjiangensis. Meanwhile, agglutination tests and western blot analysis revealed that the recombinant ScCL protein could agglutinate Staphylococcus aureus and Vibrio harveyi, but could not agglutinate Vibrio anguillarum, Bacillus cereus, or Vibrio parahaemolyticus. Furthermore, ScCL had a high binding activity with LPS and mannose, a low binding activity with LTA, and no binding activity with PGN. The expression of ScCL in the gill of S. constricta fed with C. vulgaris and T. pseudonana was significantly increased at 1 and/or 3 h. After injection with S. aureus, the expression of ScCL in the gill was significantly increased at 3, 6, and 24 h. These results indicated that ScCL was involved in food particle recognition and immunity of S. constricta.

A Potential Role of Bone Morphogenetic Protein 7 in Shell Formation and Growth in the Razor Clam Sinonovacula constricta

Bone morphogenetic proteins (BMPs) not only play essential roles in bone development but also are involved in embryonic growth, organogenesis cell proliferation and differentiation. However, the previous studies on the functions of shellfish BMPs genes are still very limited. To better understand its molecular structure and biological function, BMP7 of the razor clam Sinonovacula constricta (Sc-BMP7) was cloned and characterized in this study. The full length of Sc-BMP7 is 2252 bp, including an open reading frame (ORF) of 1257 bp encoding 418 amino acids. The protein sequence included a signal peptide (1–32 aa), a prodomain (38–270 aa) and a TGF-β domain (317–418 aa). The quantitative expression of eleven adult tissues showed that Sc-BMP7 was significantly higher expressed in the gill, foot, and mantle (P < 0.05), but lower in hemocytes and hepatopancreas. In the early development stages, low expression was detected in the stages of unfertilized mature eggs, fertilized eggs, 4-cell embryos, blastula, gastrulae, whereas it increased after the stage of trochophore and demonstrated the highest expression in umbo larvae (P < 0.01). In shell repair experiment, Sc-BMP7 showed increasing expression level after 12 h. The higher expression of Sc-BMP7 was detected while Ca²⁺ concentration was reduced in seawater. After inhibiting Sc-BMP7 expression using RNA interference (RNAi) technology, expression of Sc-BMP7 mRNA and protein were significantly down-regulated (P < 0.05) in the central zone of mantle (nacre formation related tissue) and the pallial zone of mantle (prismatic layer formation related tissue). Association analysis identified two shared SNPs in exon of Sc-BMP7 gene from 246 individuals of two groups. These results indicated that BMP7 might be involved in shell formation and growth. These results would contribute to clarify the role of Sc-BMP7 in the regulation of growth and shell formation, and provide growth-related markers for molecular marker assisted breeding of this species.

Changes in oxidative stress biomarkers in Sinonovacula constricta in response to toxic metal accumulation during growth in an aquaculture farm

Clam farming comprises an important part of China's economy. However, increasing pollution in the ocean caused by toxic metals has led to the bioaccumulation of toxic metals in marine animals, especially the bivalves such as clams, and the consequence of heavy metal-associated toxicity in these animals. Such toxicity can enhance the production of reactive oxygen species (ROS) within the tissues of the animals. In aquatic species, oxidative stress mechanisms have been studied by measuring the antioxidant and oxidative damage index in the tissues. The objectives of this study were to investigate the levels of different toxic metals and the extent of oxidative stress responses in the clam Sinonovacula constricta at different growth periods (from May to October) in an aquaculture farm in Wengyang, an important economic shellfish culture zone in Zhejiang Province, China. Water and sediment samples taken from the farm were subjected to Pb, Hg, Cd, Cr assays. Overall, the levels of these metals in the water and sediment could be considered as light pollution, though the levels of Hg in the water (0.266) and Cd in the sediment (0.813) could be considered as reaching moderate pollution. In addition, the levels of these metals, H₂O₂, MDA and GSH content, antioxidant enzyme (CAT, SOD, GPx) activities as well as the level of metallothioneins (MT) mRNA in the tissues of S. constricta were also analyzed. The levels of Pb, Hg, Cd, Cr increased with increasing culturing time, and a higher level of these metals was accumulated in the visceral mass than in the foot. The levels of MDA and GSH, as well as the level of SOD activity in the viscera and foot of S. constricta increased with increasing metal accumulation. However, CAT and GPX activities, H₂O₂ level and the expression of MT initially increased and then decreased. This suggested that S. constricta might have the ability to control oxidative damage by triggering antioxidant defense in coordination with the metal sequestering response. The results also implied that toxic metal pollution should be taken into account when selecting the site to be used as an aquaculture farm. In addition, the visceral mass should be considered to be a good tissue for measuring the level of metal pollutants.

Dopamine beta-hydroxylase and its role in regulating the growth and larval metamorphosis in Sinonovacula constricta

Dopamine beta-hydroxylase (DβH) plays a key role in the synthesis of catecholamines (CAs) in the neuroendocrine regulatory network. The DβH gene was identified from the razor clam Sinonovacula constricta and referred to as ScDβH. The ScDβH gene is a copper type II ascorbate-dependent monooxygenase with a DOMON domain and two Cu2_monooxygen domains. ScDβH transcript expression was abundant in liver and hemolymph. During early development, ScDβH expression significantly increased at the umbo larval stage. Furthermore, the inhibitors and siRNA of DβH were screened. After challenge with DβH inhibitor, the larval metamorphosis and survival rates, and juvenile growth were obviously decreased. Under the siRNA stress, the larval metamorphosis and survival rates were also significantly decreased. Therefore, ScDβH may play an important regulating role in larval metamorphosis and juvenile growth.

Chromosome-level genome assembly of the razor clam Sinonovacula constricta (Lamarck, 1818)

Bivalves, a highly diverse and the most evolutionarily successful class of invertebrates native to aquatic habitats, provide valuable molecular resources for understanding the evolutionary adaptation and aquatic ecology. Here, we reported a high-quality chromosome-level genome assembly of the razor clam Sinonovacula constricta using Pacific Bioscience single-molecule real-time sequencing, Illumina paired-end sequencing, 10X Genomics linked-reads and Hi-C reads. The genome size was 1,220.85 Mb, containing scaffold N50 of 65.93 Mb and contig N50 of 976.94 Kb. A total of 899 complete (91.92%) and seven partial (0.72%) matches of the 978 metazoa Benchmarking Universal Single-Copy Orthologs were determined in this genome assembly. And Hi-C scaffolding of the genome resulted in 19 pseudochromosomes. A total of 28,594 protein-coding genes were predicted in the S. constricta genome, of which 25,413 genes (88.88%) were functionally annotated. In addition, 39.79% of the assembled genome was composed of repetitive sequences, and 4,372 noncoding RNAs were identified. The enrichment analyses of the significantly expanded and contracted genes suggested an evolutionary adaptation of S. constricta to highly stressful living environments. In summary, the genomic resources generated in this work not only provide a valuable reference genome for investigating the molecular mechanisms of S. constricta biological functions and evolutionary adaptation, but also facilitate its genetic improvement and disease treatment. Meanwhile, the obtained genome greatly improves our understanding of the genetics of molluscs and their comparative evolution.

Identification of a dopamine receptor in Sinonovacula constricta and its antioxidant responses

The dopamine (DA) D2 receptor is a member of the G protein-coupled receptors of organisms and plays an important role in immune system regulation. The presence of DA receptors has been widely reported in vertebrates, but few studies have been conducted in shellfish. Here, we identified a novel DA-D2 receptor gene, ScDopR2-1, in the razor clam Sinonovacula constricta. ScDopR2-1 belongs to the family of G protein-coupled receptors, containing seven hydrophobic transmembrane domains, along with 16 predicted N-glycosylation sites and 69 phosphorylation sites. A longer third intracellular loop and a shorter C-terminus in ScDopR2-1 are characteristic features of D2 receptors. ScDopR2-1 is widely expressed in tissues from adult clams, showing high expression in siphon and foot tissues. Furthermore, in response to Vibrio anguillarum challenge, ScDopR2-1 expression levels are significantly increased in liver tissue. Moreover, changes in the activities of catalase (CAT) and superoxide dismutase (SOD) also indicate that the organism causes an immune response. In summary, the results indicate that ScDopR2-1 plays a pivotal role in antioxidant responses in S. constricta.

Crystallographic characterization of ferritin from Sinonovacula constricta

Ferritins are ubiquitous iron-binding proteins that are mainly related to iron storage, detoxification and innate immunity. Here, we present the crystal structure of a marine invertebrate ferritin from Sinonovacula constricta at a resolution of 1.98 Å. The S. constricta ferritin (ScFer) possessed some structural similarities with vertebrate ferritins, and they shared a well-conserved architecture composed of five α-helical bundles that assembled into a cage-like structure with 24-subunits. The structure of ScFer also showed iron binding sites in the 3-fold channel, ferroxidase center, and putative nucleation sites. Further, electrostatic potential calculations suggested that the electrostatic gradient of the 3-fold channel could provide a guidance mechanism for iron entering the ferritin cavity.

Polymorphisms of LAP3 gene and their association with the growth traits in the razor clam Sinonovacula constricta

Leucine aminopeptidase 3 (LAP3) is an important proteolytic enzyme that catalyzes the hydrolysis of leucine residues from the amino termini of protein or peptide substrates and plays a critical role in protein metabolism and growth. In the present study, we investigated the full-length cDNA sequence of the LAP3 gene in Sinonovacula constricta (ScLAP3) using expressed sequence tags and rapid amplification of cDNA ends. The full-length ScLAP3 cDNA was 2885 bp, with a 1560 bp open reading frame encoding 519 amino acids. Sequence analysis revealed that ScLAP3 shared 70.9% identity with LAP3 from the blood clam Tegillarca granosa and 62.0-68.0% with other species. ScLAP3 was expressed in all six tested tissues, with significantly higher expression levels in the foot compared with mantle, adductor muscle, liver, gills, and siphon tissues in adults (P < 0.01). In the eight developmental stages, ScLAP3 expression gradually increased, with significantly higher levels in D-shaped larvae compared with other developmental stages (P < 0.01), suggesting that it may be involved in the formation of certain organs during early development. Association analysis identified three shared single nucleotide polymorphisms (SNPs), c.1073A > G, c.1139C > T and c.1154A > G in exons of ScLAP3 gene from 177 individuals of two groups, one selective strain and one wild population, which had significant effects on growth traits of S. constricta. The results provided candidate genetic markers to assist selective breeding of razor clams toward improved growth.

[Seasonal variation of microflora in Sinonovacula constricta and its aquacultural pond based on high-throughput sequencing]

In order to explore the seasonal changes of the bacterial community structure and the interaction of environmental factors in Sinonovacula constricta ponds, we used high throughput sequencing technology to examine the bacteria of water, sediment, and viscera. The results showed that microflora structure of water samples in winter was significantly different from that in spring, summer and autumn, while there was no significant difference in bacterial community structure of sediment and viscera in different seasons. There was no significant difference of Shannon diversity index in water across different seasons. The Shannon diversity index of sediment and viscera was the lowest in summer and the highest in winter. At the phylum level, Cyanobacteria, Proteobacteria and Tenericutes were the most predominant bacteria in water, sediment, and viscera, respectively. At the genus level, NS3a_marine_group was predominant in winter water, and Synechococcus in the other three seasons. By contrast, dominant bacteria in sediments were norank_f_Anaerolineacea and Nitrospira, and Mycoplasma and Arcobacter were the most abundant bacterial genera in viscera. Synechococcus had a positive correlation with water temperature, COD, PO₄^--P, NH₄⁺-N, pH, and transparency. The norank_f_Anaerolineacea was positively correlated with water temperature, COD, and TP. Mycoplasma was positively correlated with water temperature, PO₄^--P, NH₄⁺-N, pH, and transparency. Our results suggest that there were significant differences in the composition and diversity of microflora of S. constricta and ponds in different seasons. Bacteria in water was obviously affected by various environmental factors, especially water temperature and the concentrations of nitrogen and phosphorus.

Complement factor B/C2 in molluscs regulates agglutination and illuminates evolution of the Bf/C2 family

Complement factor B/C2 family (Bf/C2F) proteins are core complement system components in vertebrates that are absent in invertebrates and have been lost by numerous species, raising evolutionary questions. At least 3 duplication events have occurred from Cnidaria (ancestor) to mammals. Type II Bf/C2 genes appeared during separation of Proterostomia and Deuterostomes. The second event occurred during separation of vertebrates and invertebrates, yielding type II-2 Bf/C2. The third event occurred when jawed and jawless fish were separated, eventually producing Bf and C2 genes. Herein, we report the second mollusc Sinonovacula constricta Bf/C2-type gene (ScBf). ScBf is similar to Ruditapes decussatus Bf-like because both lack the first complement control protein module at the N terminus present in mammalian Bf/C2 proteins. Uniquely, the Ser protease (SP) module at the C terminus of ScBf is ∼50 aa longer than in other complement factor B/C2-type (Bf/C2T) proteins, and is Glu-rich. Bf/C2T proteins in molluscs lack the catalytic Ser in the SP module. Surprisingly, ScBf regulates rabbit erythrocyte agglutination, during which it is localized on the erythrocyte surface. Thus, ScBf may mediate the agglutination cascade and may be an upstream regulator of this process. Our findings provide new insight into the origin of the Bf/C2F.-Peng, M., Li, Z., Niu, D., Liu, X., Dong, Z., Li, J. Complement factor B/C2 in molluscs regulates agglutination and illuminates evolution of the Bf/C2 family.

Tolerance, Growth, and Physiological Responses of the Juvenile Razor Clam (Sinonovacula constricta) to Environmental Ca2+ and Mg2+ Concentrations

To facilitate transplanting razor clam (Sinonovacula constricta) populations to inland saline-alkaline waters (ISWs), we evaluated the tolerance of juvenile S. constricta (JSC) to Ca²⁺ and Mg²⁺ concentrations, and determined the effects of these ions on JSC growth and physiological parameters. After 30 days stress, the tolerable ranges of JSC to Ca²⁺ and Mg²⁺ were determined to be 0.19 mmol⋅L^-1-19.46 mmol⋅L^-1 and 0 mmol⋅L^-1-29.54 mmol⋅L^-1, respectively. The concentrations of Ca²⁺ (less than 0.65 mmol⋅L^-1 or more than 3.24 mmol⋅L^-1) and Mg²⁺ (less than 0.37 mmol⋅L^-1 or more than 14.17 mmol⋅L^-1) significantly inhibit JSC growth. Physiological enzyme activity no significant response when the concentrations range of Ca²⁺ and Mg²⁺ are 0.93 mmol⋅L^-1-6.49 mmol⋅L^-1 and 0.37 mmol⋅L^-1-14.77 mmol⋅L^-1, respectively. For transplantation practice, these data indicate that only high concentrations of Ca²⁺ (3.24-6.825 mmol⋅L^-1) and Mg²⁺ (14.77-33.69 mmol⋅L^-1) in target inland saline-alkaline water had significantly impact on growth and physiological response. In addition, present study suggests that the increase in Ca²⁺ and Mg²⁺ ion concentrations caused by ocean acidification will not affect the survival, growth and physiology of S. constricta. Current research suggests that S. constricta can adapt to extreme changes in the marine environment (Ca²⁺ and Mg²⁺) and may be an excellent candidate for inland saline-alkaline water transplantation practice.

Gene identification and antimicrobial activity analysis of a novel lysozyme from razor clam Sinonovacula constricta

Lysozymes are important immune effectors present in phylogenetically diverse organisms. They play vital roles in bacterial elimination during early immune responses. In the present study, a second invertebrate-type (i-type) lysozyme gene from razor clam Sinonovacula constricta (denoted as ScLYZ-2) was cloned by RACE and nested PCR methods. The full-length cDNA sequences of ScLYZ-2 were 1558 bp, including a 5' untranslated region (UTR) of 375 bp, an open reading frame of 426 bp, and a 3'-UTR of 757 bp with polyadenylation signal sequence (AATAAA) located upstream of the poly(A) tail. SMART analysis showed that ScLYZ-2 contains a signal peptide in the first 16 amino acid (AA) sequences and a destabilase domain located from 24 to 134 AA sequences. The deduced AA sequences of ScLYZ-2 were highly similar (42%-58%) to other known lysozyme genes of bivalve species. Multiple alignments of AA sequences showed that ScLYZ-2 possesses the classical i-type lysozyme family signature of two motifs ["MDVGSLSCGP(Y/F)QIK" and "CL(E/L/R/H)C(I/M)C"] and two catalytic residues (Glu³⁵ and Asp⁴⁶). Moreover, phylogenetic analysis showed that ScLYZ-2 is a new member of the i-type lysozyme family. In healthy razor clams, ScLYZ-2 was highly expressed in the hepatopancreas, followed by the gills, water pipes, and abdominal foot. Lysozyme activity and ScLYZ-2 expression levels were significantly upregulated in the hepatopancreas and gills after being infected with V. splendidus, V. harveyi, V. parahaemolyticus and S. aureus and M. luteus. Moreover, the recombinant ScLYZ-2 had strong antimicrobial activities against V. splendidus, V. harveyi, and V. parahaemolyticus. Furthermore, the minimal inhibitory concentration of the recombinant ScLYZ-2 against V. parahaemolyticus was 7.2 μmol/mL. Taken together, our results show that ScLYZ-2 plays an important role in the immune defense of razor clam by eliminating pathogenic microorganisms.

Characterization of the ScghC1q-1 gene in Sinonovacula constricta and its role in innate immune responses

C1q is an important immune gene that can mediate a variety of immune regulatory functions, and is involved in complement pathway activation. In the present study, a ghC1q gene from the razor clam Sinonovacula constricta was identified and named ScghC1q-1. The complete ScghC1q-1 gene is 692 bp in length, with an open reading frame (ORF) of 489 bp encoding a protein of 162 amino acids. ScghC1q-1 mRNA was widely expressed in various tissues, and transcript levels in the hemolymph were significantly up-regulated following Staphylococcus aureus or Vibrio anguillarum challenge. Recombinant ScghC1q-1 protein was found to agglutinate both Gram-positive and Gram-negative bacteria. These results indicate that ScghC1q-1 plays an essential role in the immune defense of S. constricta.

Identification of a novel C1q complement component in razor clam Sinonovacula constricta and its role in antibacterial activity

The serum complement component C1q mediates a variety of immune regulatory functions. Herein, we identified a globular head C1q (ghC1q) gene in razor clam Sinonovacula constricta. The complete Sc-ghC1q gene was 872 bp long included an 81 bp 5'-untranslated region (UTR), a 95 bp 3'-UTR with a poly(A) tail, and an open reading frame (ORF) of 696 bp. The mRNA expression of Sc-ghC1q was upregulated in hepatopancreas and hemocytes. After Staphylococcus aureus or Vibrio anguillarum challenge, Sc-ghC1q mRNA transcript abundance was significantly upregulated in hemolymph. Recombinant Sc-ghC1q protein could bind lipopolysaccharide (LPS) and lipoteichoic acid (LTA), and it could agglutinate both Gram-positive and Gram-negative bacteria. Additionally, flow cytometry revealed that Sc-ghC1q strongly promoted phagocytosis in hemocytes. Together, these results demonstrated that Sc-ghC1q played an important role in innate immunity in S. constricta.

Identification and characterisation of a novel small galectin in razor clam (Sinonovacula constricta) with multiple innate immune functions

Galectins are lectins possessing an evolutionarily conserved carbohydrate recognition domain (CRD) with affinity for β-galactoside. The key role played by innate immunity in invertebrates has recently become apparent. Herein, a full-length galectin (ScGal) was identified in razor clam (Sinonovacula constricta). The 528 bp open reading frame encodes a polypeptide of 176 amino acids with a single CRD and no signal peptide. ScGal mRNA transcripts were mainly expressed in hemolymph and gill, and were significantly up-regulated following bacterial challenge. Recombinant rScGal protein binds to and aggregates various bacteria, and has affinity for peptidoglycan, lipoteichoic acid and _d-galactose. The protein also stimulates hemocytes to phagocytose invading bacterial pathogens. ScGal is an important immune factor in innate immunity, and a small protein with multiple important functions.

Novel Ca2+-independent C-type lectin involved in immune defense of the razor clam Sinonovacula constricta

C-type lectins (CTLs) are important pattern recognition molecules that participate in bacterial binding and agglutination by specific recognition of carbohydrates from pathogens. In this study, a full-length cDNA of CTL was cloned from Sinonovacula constricta (designated ScCTL-2). ScCTL-2 has a length of 981 bp, a 5'-untranslated region (UTR) of 47 bp, a short 3'-UTR of 37 bp, and an open reading frame (ORF) of 894 bp, which encodes a polypeptide of 298 amino acid residues. The deduced amino acid of ScCTL-2 possesses a conserved carbohydrate-recognition domain (CRD) similar to that of C³¹-E¹⁷¹. Spatial distribution analysis demonstrated that ScCTL-2 was constitutively expressed in all tested tissues, with dominant expression in foot and siphon and weak expression in hepatopancreas. The mRNA expression level of ScCTL-2 in gills and hepatopancreas was significantly upregulated at 6 and 12 h after challenge with the pathogen Vibrio parahaemolyticus. The recombinant ScCTL-2 showed specific binding and agglutinate capacities to all examined Gram-negative bacterial species, namely, Escherichia coli, Vibro anguillarum, and V. parahaemolyticus in a Ca²⁺-independent manner. However, these binding activities were not detected in Gram-positive Micrococcus luteus. Our results indicated that ScCTL-2 could be a novel pattern recognition receptor that can specifically recognize Gram-negative microorganisms in the innate immunity of S. constricta.

Bioturbation by the razor clam (Sinonovacula constricta) on the microbial community and enzymatic activities in the sediment of an ecological aquaculture wastewater treatment system

Bioturbation by the razor clam, Sinonovacula constricta, influenced the redox conditions and changed the sedimentary environment, providing a suitable micro-environment for microbial growth. However, the mechanism of the integrated razor clam-microbial community in organic matter mineralization remains elusive. To study this mechanism, an in situ experiment was conducted to investigate the bioturbation effects of S. constricta on the microbial community and enzyme activities in the sediment of an ecological aquaculture wastewater treatment system. According to principal coordinate analysis (PCoA) and PERMANOVA, we found that the microbial community was significantly (P < 0.05) influenced by the bioturbation effect of S. constricta. Linear discriminant analysis effect size (LEfSe) showed that species involved in reduced effluent concentrations of TN and TP, such as Prolixibacteraceae, Nitrospira and Actinobacteria, were increased significantly (P < 0.05) by S. constricta. Molecular ecological network (MENs) analysis indicated that the bioturbation effect of S. constricta increased the complexity of interspecies interaction and changed the topological properties of individual OTUs. The results elucidated that S. constricta increased the microbial community network, as substantiated by a higher total number of nodes and a shorter geodesic distance. Zi < 2.5 and Pi <0.62 in MENs showed that the S. constricta treatment significantly increased (P < 0.05) the potential microbial community, with the keystone (OTU747049) Proteobacteria and (OTU74388) Bacteroidetes family Prolixibacteraceae, which connected different co-expressed OTUs. Furthermore, S. constricta significantly increased (P < 0.05) the enzymatic activities (alkaline phosphatase (APA), dehydrogenase and urease) of the substrate at different sampling depths. Overall, this study provides evidence that the bioturbation effect of S. constricta changes the microbial community structure, increases enzymatic activities and accelerates the degradation of organic matter in an aquaculture wastewater environment.

Novel C-type lectin from razor clam Sinonovacula constricta agglutinates bacteria and erythrocytes in a Ca2+-dependent manner

Among its other physiological roles, C-type lectins functioned as pattern recognition receptors (PRR) in innate immunity received much attention. In the present study, a novel C-type lectin was identified and characterized from the invertebrate razor clam Sinonovacula constrict and designated as ScCTL. The complete cDNA sequence of ScCTL was 828 bp in length and coded a secreted polypeptide of 158 amino acids with a typical CRD domain. Multiple sequence alignments combined with phylogenetic analysis both collectively confirmed that ScCTL was a novel member belong to lectin family. Spatial expression distribution analysis revealed that ScCTL was extensively expressed in all of the examined tissues, and the highest expression was detected in the hepatopancreas. After 1 × 10⁷ CFU/mL Vibrio parahaemolyticus challenge by immersion infection, the ScCTL transcript in hepatopancreas and gill were markedly upregulated and arrived the maximum levels at 24 or 12 h after challenge, respectively. Recombinant ScCTL could agglutinate not only all tested bacteria but sheep and mouse erythrocyte in the presence of Ca²⁺. All of our studies suggested that ScCTL performed important roles in protecting cells from pathogenic infection in S. constrict.

Identification of a novel galectin in Sinonovacula constricta and its role in recognition of Gram-negative bacteria

Galectins are soluble lectins that perform a pattern recognition function in invertebrate immunity and specifically recognise β-galactoside residues via conserved carbohydrate recognition domains. However, their function in bivalve molluscs has received little attention. Herein, a galectin (ScGal2) in razor clam (Sinonovacula constricta) consisting of a 507 bp open reading frame encoding a protein of 168 amino acids was identified and characterised. The protein includes a carbohydrate recognition domain (CRD), and several residues involved in dimerisation were found. ScGal2 mRNAs were mainly detected in hemolymph and liver, and expression was upregulated significantly following challenge with Vibrio anguillarum. Recombinant rScGal2 protein displayed strong agglutination activity toward Gram-negative bacteria, and flow cytometry revealed that ScGal2 strongly promoted phagocytosis in hemocytes. These results suggest that ScGal2 plays an indispensable role in innate immunity in razor clam, and likely participates in immune recognition and clearance processes.

Molecular characteristics, expression, and antimicrobial activities of i-type lysozyme from the razor clam Sinonovacula constricta

Lysozyme is a key component of the innate immune system, which plays a pivotal role in early defense against pathogen infection. In this study, an i-type lysozyme homology was identified from the razor clam Sinonovacula constricta (designated as ScLYZ) through RACE approaches. The full-length cDNA of ScLYZ was 768 bp and encoded a polypeptide of 140 amino acid residues. SMART analysis revealed that ScLYZ processed a signal peptide (1-18 aa) and a destabilase domain from 25 to 133 aa. Two catalytic residues (Glu³⁶ and Asp⁴⁷) and two specific motifs ["CL(E/L/R/H)C(I/M)C" and "MDVGSLSCG(P/Y) (F/Y)QIK"] of the i-type lysozyme were highly conserved in the ScLYZ sequence. Multiple sequence alignments and phylogenetic analysis indicated that ScLYZ could be a new member of the i-type lysozyme subfamily. Tissue distribution analysis revealed that ScLYZ was constitutively expressed in all examined tissues, and the highest expression was found in the hepatopancreas. After the razor clams were challenged by Vibrio parahaemolyticus, the mRNA levels of ScLYZ increased in the gill and hepatopancreas. Moreover, the recombinant protein was expressed in Escherichia coli, and the refolded ScLYZ showed highly antimicrobial activities against V. parahaemolyticus and Vibrio splendidus. The minimal inhibitory concentration toward V. parahaemolyticus was 8.2 μmol/mL. All our results supported that ScLYZ was involved in the innate immune defense of razor clam by inhibiting the growth of invasive pathogens.

Hemolytic reactions in the hemolymph of bivalve Sinonovacula constricta show complement-like activity

The complement-like hemolysis method was used to determine the total complement-like activity of the plasma of Sinonovacula constricta. In this study, the effects of both physical and chemical conditions on complement hemolysis of S. constricta were measured. Physical conditions included proportion (S. constricta plasma: 2% rabbit red blood cells), temperature, time, and incubation, while the chemical factors consisted of Lipopolysaccharide (LPS), Flagellin (FLA), Zymosan, Peptidoglycan (PGN), Phenylmethanesulfonyl fluoride (PMSF), Methylamine, and Poly (I: C). The results showed that LPS, flagellin, Zymosan and PGN could activate complement-like activity of S. constricta plasma and cause hemolysis. PMSF and methylamine inhibited complement-like activity, resulting in the disappearance of hemolysis. Poly (I: C) had no effect on plasma complement-like activity. When the reaction temperature was less than 50 °C, hemolytic activity would increase following an increase in temperature. The ratio of plasma to rabbit blood cells had a great impact on the rate of hemolysis. Additionally, incubation with low speed oscillation could improve the hemolysis rate. It is indicated that the hemolytic reactions in the hemolymph of bivalve S. constricta show complement-like activity. The results contribute to further research on immune function of complement in bivalve.