Molecular characterization of kappa class glutathione S-transferase from the disk abalone (Haliotis discus discus) and changes in expression following immune and stress challenges

Antioxidant Defence in Labeo rohita to Biotic and Abiotic Stress: Insight from mRNA Expression, Molecular Characterization and Recombinant Protein-Based ELISA of Catalase, Glutathione Peroxidase, CuZn Superoxide Dismutase, and Glutathione S-Transferase

Fish possess numerous enzymatic antioxidant systems as part of their innate immunity. These systems have been poorly studied in Labeo rohita (rohu). The present study characterized and investigated the role of antioxidant genes in the defence mechanisms against two types of stressors, including infection and ammonia stress. Four key genes associated with antioxidant activity-catalase, glutathione peroxidase, glutathione S-transferase, and CuZn superoxide dismutase were successfully cloned and sequenced. These genes were found to be expressed in different tissues and developmental stages of rohu. The expression levels of these antioxidant genes in the liver and anterior kidney tissues of rohu juveniles were modulated in response to bacterial infection (Aeromonas hydrophila), parasite infection (Argulus siamensis), poly I:C stimulation and ammonia stress. Additionally, the recombinant proteins derived from these genes exhibited significant antioxidant and antibacterial activities. These proteins also demonstrated a protective effect against A. hydrophila infection in rohu and had an immunomodulatory role. Furthermore, indirect ELISA assay systems were developed to measure these protein levels in healthy as well as A. hydrophila and ammonia-induced rohu serum. Overall, this study characterized and emphasised the importance of the antioxidant mechanism in rohu's defence against oxidative damage and microbial diseases.

GST-Mu of Cristaria plicata is regulated by Nrf2/Keap1 pathway in detoxification microcystin and has antioxidant function

Glutathione S-transferase is a crucial phase II metabolic enzyme involved in detoxification and metabolism in aquatic organisms. This study aimed to investigate the regulation of Nrf2/Keap1 pathway on microcystin-induced CpGST-Mu expression and CpGST-Mu resistance to hydrogen peroxide. A mu class GST from Cristaria plicata (CpGST-Mu) was identified. The full-length cDNA was 1026 bp, with an open reading frame of 558 bp. Subcellular localization revealed that CpGST-Mu was localized in cytoplasm. The optimum pH and temperature for the catalytic activity of CpGST-Mu protein was pH 6 and 40 °C, respectively. The results of Real-time quantitative PCR showed that CpGST-Mu mRNA was constitutively expressed in tissues, with the highest expression level in hepatopancreas and the lowest expression level in gill. The mRNA level of CpGST-Mu was significantly increased under the stress of microcystins and hydrogen peroxide. CpGST-Mu had an antagonistic effect on hydrogen peroxide. In the knockdown experiments, the mRNA levels of CpGST-Mu exhibited corresponding changes while Nrf2 and Keap1 genes were individually knocked down. These findings indicated that GST-Mu exhibited antioxidant properties and its expression was regulated by Nrf2/Keap1 signaling pathway. The study provided new information on the function of GST-Mu and could contribute to future studies on how to excrete microcystins in molluscs.

Molecular Cloning and Functional Characterization of Catalase in Stress Physiology, Innate Immunity, Testicular Development, Metamorphosis, and Cryopreserved Sperm of Pacific Abalone

Catalase is a crucial enzyme of the antioxidant defense system responsible for the maintenance of cellular redox homeostasis. The aim of the present study was to evaluate the molecular regulation of catalase (Hdh-CAT) in stress physiology, innate immunity, testicular development, metamorphosis, and cryopreserved sperm of Pacific abalone. Hdh-CAT gene was cloned from the digestive gland (DG) of Pacific abalone. The 2894 bp sequence of Hdh-CAT had an open reading frame of 1506 bp encoding 501 deduced amino acids. Fluorescence in situ hybridization confirmed Hdh-CAT localization in the digestive tubules of the DG. Hdh-CAT was induced by different types of stress including thermal stress, H₂O₂ induction, and starvation. Immune challenges with Vibrio, lipopolysaccharides, and polyinosinic-polycytidylic acid sodium salt also upregulated Hdh-CAT mRNA expression and catalase activity. Hdh-CAT responded to cadmium induced-toxicity by increasing mRNA expression and catalase activity. Elevated seasonal temperature also altered Hdh-CAT mRNA expression. Hdh-CAT mRNA expression was relatively higher at the trochophore larvae stage of metamorphosis. Cryopreserved sperm showed significantly lower Hdh-CAT mRNA expression levels compared with fresh sperm. Hdh-CAT mRNA expression showed a relationship with the production of ROS. These results suggest that Hdh-CAT might play a role in stress physiology, innate immunity, testicular development, metamorphosis, and sperm cryo-tolerance of Pacific abalone.

Identification and characterization of the novel genes encoding glutathione S-transferases in Mytilus galloprovincialis

The superfamily of glutathione S-transferases (GST) plays an essential role in the xenobiotic metabolism, binding compounds to the glutathione, and is like a cell protector during the influence of various negative external factors. Nevertheless, there are very few works devoted to the investigation of these genes in marine invertebrates. Up to this time, only three classes of cytosolic GSTs for one of the leading commercial molluscs Mytilus galloprovincialis were described. We sequenced the whole transcriptome from the gill tissues and, using bioinformatic analysis, detected ten classes of glutathione S-transferases, which are expressed in the mussel M. galloprovincialis. For the first time, two subfamilies were described: mitochondrial GST (kappa class) and microsomal (MAPEG), as well as five classes of the family of cytosolic GSTs (mu, omega, rho, tau, theta). Omega and sigma GST classes might be rapidly regulated genes due to the lack of introns and this assumption was confirmed by the investigation of short-term hypoxia on M. galloprovincialis. Seven new classes of GST revealed a greater gene variety of this detoxifying enzyme in mussels than expected. The obtained nucleotide sequences are necessary for future investigations of GSTs expression in response to various external factors (pollution, oxygen starvation, infection, etc.).

Cloning and expression of the ChGstα and ChGstκ genes in the gills of Crassostrea hongkongensis under nanoparticulate and ionic Zn stress

Nanoparticulate and ionic Zn have potential impacts on the detoxification systems of organisms, and Gst genes play key roles in the detoxification of xenobiotics. In this study, we cloned the ChGstα and ChGstκ genes of C. hongkongensis, and studied their expression in gills under nanoparticulate and ionic Zn stress. The results showed that the coding sequences of the ChGstα and ChGstκ genes were 684 and 675 bp, respectively, and had no signal peptide; ChGstα was cytoplasmic, while ChGstκ was mitochondrial. The two genes were expressed in all 8 tested samples, with the most abundant expression observed in hemocytes for ChGstα and digestive glands for ChGstκ. After ZnCl₂ or ZnoNP challenge, the expression of ChGstα decreased significantly in the ZnCl₂ groups, and its expression was higher in the ZnoNP groups than in the ZnCl₂ groups. The expression of ChGstκ was significantly decreased in the ZnCl₂ and ZnoNP groups, and its expression was higher in the ZnoNP groups than in the ZnCl₂ groups except at 3 h post metal Zn stress, which suggested that ChGstα and ChGstκ were more sensitive to ZnoNP than ZnCl₂.

Recent progress in natural product-based inhibitor screening with enzymatic fluorescent probes

Drug discovery is a complex process in which many challenges need to be overcome, from the discovery of a drug candidate to ensuring the efficacy and safety of the candidate in humans. Modern analytical methods allow tens of thousands of drug candidates to be screened for their inhibition of specific enzymes or receptors. In recent years, fluorescent probes have been used for the detection and diagnosis of human pathogens as well as high-throughput screening. This review focuses on recent progress in organic small-molecule based enzyme-activated fluorescent probes for screening of inhibitors from natural products. The contents include the construction of fluorescent probes, working mechanism and the process of inhibitor screening. The progress suggests that fluorescent probes are a vital and rapidly growing technology for inhibitor screening of enzymes, in particular, inhibitor screening in situ.

Identification of circRNAs in the Liver of Whitespotted Bamboo Shark (Chiloscyllium plagiosum)

Whitespotted bamboo shark (Chiloscyllium plagiosum), a member of the cartilaginous fish family, has an extremely large liver and demonstrates a strong regeneration ability and immune regulation. Circular RNAs (circRNAs) is an important class of non-coding RNAs. Increasing evidences suggest that circRNAs are a kind of potential regulators. Recently, researchers have isolated and identified different circRNAs from various species, while few reports were on the circRNAs of C. plagiosum. In this study, we have identified a total of 4,558 circRNAs in the liver of C. plagiosum. This finding suggests that circRNAs are not evenly distributed in the chromosomes and follow the GT-AG rule during cyclization. Alternative back-splicing might exist in shark circRNAs as shown by the authenticity identification of predicted circRNAs. The binding strength of circRNAs (<2,000 bp) and the detected miRNAs in shark liver were simultaneously analyzed to construct an mRNA–miRNA–circRNA network for the Glutathione S-transferase P1 gene, and the circRNA authenticity was simultaneously verified. Our data provide not only novel insights into the rich existence of circRNAs in marine animals, but also a basis for characterizing functions of identified circRNAs in the liver homeostasis of C. plagiosum.

Glutathione S-transferase (GST) genes from marine copepods Acartia tonsa: cDNA cloning and mRNA expression in response to 1,2-dimethylnaphthalene

The calanoid copepod, Acartia tonsa, is relatively sensitive to marine pollution. Glutathione S-transferase (GST) multifunctional enzyme, as a biomarker, play an important role in detoxification metabolism of exogenous substances. In the present study, GST-theta and GST-mu class homology genes (designated as AtGSTT1 and AtGSTM2) were identified and characterized from A. tonsa. The coding sequence of AtGSTT1 comprised 726 bp and encoded a putative protein of 241 amino acid residues. AtGSTM2 contained an open reading frame of 678 bp that encoded a putative 227 amino acid polypeptide. Both proteins contained a conserved GST-N domain and a GST-C domain. Structural analysis revealed the characteristic N-terminal G-site. Three-dimensional structure analysis showed that AtGSTT1 and AtGSTM2 have two typical domains of GST family: The βαβαββα topology structure at the N- terminus and the superhelical structure at the C- terminus. Subsequently, the expression levels of the two GST genes were detected in A. tonsa using real-time quantitative PCR after exposure to 1,2-dimethylnaphthalene (C2-NAPH) at different concentrations (0.574, 5.736 and 57.358 μg/L) for 24, 48, 72, and 96 h. AtGSTT1 mRNA expression was significantly up-regulated in a time-dependent manner and the highest mRNA expression occurred at 5.736 μg/L C2-NAPH exposure for 96 h. AtGSTM2 mRNA expression peaked at 72 h in 0.574 μg/L and 5.736 μg/L dose groups. The expression level of AtGSTM2 showed an increasing trend in a time-dependent manner at 57.358 μg/L of C2-NAPH. These results suggested that GST genes may play an important role in protecting A. tonsa from C2-NAPH pollution, and provide a theoretical basis for further study on the molecular mechanism of polycyclic aromatic hydrocarbon (PAHs) pollution on zooplankton.

The glutathione S-transferase genes in marine rotifers and copepods: Identification of GSTs and applications for ecotoxicological studies

Various xenobiotics are constantly being released and accumulated into the aquatic environments and consequently, the aquatic organisms are continuously being exposed to exogenous stressors. Among various xenobiotic detoxifying enzymes, Glutathione S-transferase (GST) is one of the major xenobiotic detoxifying enzyme which is widely distributed among living organisms and thus, understanding of the nature of GSTs is crucial. Previous studies have shown GST activity in response to various xenobiotics yet, full identification of GSTs in marine invertebrates is still limited. This review covers information on the importance of GSTs as a biomarker for emerging chemicals and their response to wide ranges of environmental pollutants as well as in-depth phylogenetic analysis of marine invertebrates, including recently identified GSTs belonging to rotifers (Brachionus spp.) and copepods (Tigriopus japonicus and Paracyclopina nana), with unique class-specific features of GSTs, as well as a new suggestion of GST evolutionary pathway.

Cloning, characterization and expression analysis of glutathione S-transferase from the Antarctic yeast Rhodotorula mucilaginosa AN5

The gene for glutathione S-transferase (GST) in Antarctic sea-ice yeast Rhodotorula mucilaginosa AN5 was cloned and expressed in Escherichia coli and named RmGST. Sequence analysis showed that the RmGST gene contained a 843 bp open reading frame, which encoded 280 amino acid residues with a calculated molecular mass of 30.4 kDa and isoelectric point of 5.40. RmGST has the typical C- and N-terminal double domains of glutathione S-transferase. Recombinant RmGST (rRmGST) was expressed in E. coli to produce heterologous protein that had a high specific activity of 60.2 U/mg after purification. The apparent K_m values of rRmGST for glutathione and 1-chloro-2,4-dinitrobenzene were 0.35 mM and 0.40 mM, respectively. Optimum enzyme activity was measured at 35 °C and at pH 7.0 and complete inactivation was observed after incubation at 55 °C for 60 min rRmGST tolerated high salt concentrations (1.0 M NaCl) and was stable at pH 3.0. Additionally, the recombinant protein nearly kept whole activity in Hg²⁺ and Mn²⁺, and could tolerate Ca²⁺, Cu²⁺, Mg²⁺, Cd²⁺, EDTA, thiourea, urea, Tween-80, H₂O₂ and Triton X-100. Real-time quantitative PCR showed that relative expression of the GST gene was significantly increased under Cu²⁺ and low temperature stress. These results indicate that rRmGST is a typical low thermostable enzyme, while its other characteristics, heavy metal and low temperature tolerance, might be related to its Antarctic home environment.

Molecular characterization and functional analysis of glutathione S-transferase kappa 1 (GSTκ1) from the big belly seahorse (Hippocampus abdominalis): Elucidation of its involvement in innate immune responses

Glutathione S-transferases (GSTs) are essential enzymes for the bioactivation of xenobiotics through the conjugation of the thiol group of glutathione (GSH). In this study, a kappa class of GST was identified from the big belly seahorse (Hippocampus abdominalis) (HaGSTκ1) and its biochemical and functional properties were analyzed. HaGSTκ1 has 231 amino acids encoded by a 696 bp open reading frame (ORF). The protein has a predicted molecular mass of 26.04 kDa and theoretical isoelectric point (pI) of 8.28. It comprised a thioredoxin domain, disulfide bond formation protein A (DsbA) general fold, and Ser15 catalytic site as well as GSH-binding and polypeptide-binding sites. Phylogenetic analysis revealed that HaGSTκ1 is closely clustered with the kappa class of GSTs from teleost fishes. The recombinant (rHaGSTκ1) protein exhibited activity toward 1-chloro-2,4-dinitrobenzene (CDNB), 4-nitrobenzyl (4-NBC), and 4-nitrophenethyl bromide (4-NPB) but not 1,2-dichloro-4-nitrobenzene (DCNB). The optimum pH and temperature were 8 and 30 °C, respectively, for the catalysis of CDNB and the universal substrate of GSTs. The rHaGSTκ1 activity was efficiently inhibited in the presence of Cibacron blue (CB) as compared with hematin. Most prominent expression of HaGSTκ1 was observed in the liver and kidney among the fourteen different tissues of normal seahorse. After challenge with lipopolysaccharide (LPS), polyinosinic-polycytidylic (poly I:C), gram-negative Edwardsiella tarda, and gram-positive Streptococcus iniae, HaGSTκ1 expression was significantly modulated in the liver and blood tissues. Altogether, our study proposes the plausible important role of HaGSTκ1 in innate immunity and detoxification of harmful xenobiotics.

Immune and xenobiotic responses of glutathione S-Transferase theta (GST-θ) from marine invertebrate disk abalone (Haliotis discus discus): With molecular characterization and functional analysis

Representing a multifunctional complex group of proteins, glutathione S- transferases (GSTs) play a major role in the phase II detoxification process in a wide range of organisms. This study focused on the potential detoxification ability of disk abalone (Haliotis discus discus) GST theta (AbGST-θ) under different stress conditions with special reference to post immune challenges. Characterization of AbGST-θ revealed with 226 amino acids, 26.6 kDa of predicted molecular mass and 8.9 of theoretical isoelectric point. As illustrated in the multiple sequence alignment, eight glutathione binding sites (G-sites) and ten substrate binding sites (H-sites) were identified in well-distinct N-terminal and C-terminal domains of AbGST-θ, respectively. AbGST-θ exhibited its highest sequence identity with Mizuhopecten yessoensis (59.1%) and the phylogenetic tree clearly positioned AbGST-θ with pre-defined GST-θ molluscan homologues. The AbGST-θ was highly expressed in the digestive tract of un-challenged abalones. Upon administering the challenge experiment, AbGST-θ showed significant modulations in their transcriptional levels depending on the time and the tissue type. The optimum temperature was 37 °C and optimum pH was 7.5 for AbGST-θ. The determined enzyme kinetic parameters of AbGST-θ showed low affinity towards 1-Chloro-2,4-dinitrobenzene (CDNB) and glutathione (GSH) as substrates. Nonetheless, with Cibacron blue IC₅₀ (half maximal inhibitory concentration) was calculated to be 0.08 μM while observing 100% inhibition with 100 μM. Furthermore, AbGST-θ resulted in significant protection ability towards H₂O₂, CdCl_2, and ZnCl₂ in the disk diffusion assay. Collectively, this study provides evidences for the detoxification ability and the immunological host defensive capability of AbGST-θ in disk abalone.

Protective Role of Freshwater and Marine Rotifer Glutathione S-Transferase Sigma and Omega Isoforms Transformed into Heavy Metal-Exposed Escherichia coli

Glutathione S-transferases (GSTs) play an important role in phase II of detoxification to protect cells in response to oxidative stress generated by exogenous toxicants. Despite their important role in defense, studies on invertebrate GSTs have mainly focused on identification and characterization. Here, we isolated omega and sigma classes of GSTs from the freshwater rotifer Brachionus calyciflorus and the marine rotifer Brachionus koreanus and explored their antioxidant function in response to metal-induced oxidative stress. The recombinant Bc- and Bk-GSTs were successfully transformed and expressed in Escherichia coli. Their antioxidant potential was characterized by measuring kinetic properties and enzymatic activity in response to pH, temperature, and chemical inhibitor. In addition, a disk diffusion assay, reactive oxygen species assay, and morphological analysis revealed that GST transformed into E. coli significantly protected cells from oxidative stress induced by H₂O₂ and metals (Hg, Cd, Cu, and Zn). Stronger antioxidant activity was exhibited by GST-S compared to GST-O in both rotifers, suggesting that GST-S plays a prominent function as an antioxidant defense mechanism in Brachionus spp. Overall, our study clearly shows the antioxidant role of Bk- and Bc-GSTs in E. coli and provides a greater understanding of GST class-specific and interspecific detoxification in rotifer Brachionus spp.

Cold-Adapted Glutathione S-Transferases from Antarctic Psychrophilic Bacterium Halomonas sp. ANT108: Heterologous Expression, Characterization, and Oxidative Resistance

Glutathione S-transferases are one of the most important antioxidant enzymes to protect against oxidative damage induced by reactive oxygen species. In this study, a novel gst gene, designated as hsgst, was derived from Antarctic sea ice bacterium Halomonas sp. ANT108 and expressed in Escherichia coli (E. coli) BL21. The hsgst gene was 603 bp in length and encoded a protein of 200 amino acids. Compared with the mesophilic EcGST, homology modeling indicated HsGST had some structural characteristics of cold-adapted enzymes, such as higher frequency of glycine residues, lower frequency of proline and arginine residues, and reduced electrostatic interactions, which might be in relation to the high catalytic efficiency at low temperature. The recombinant HsGST (rHsGST) was purified to apparent homogeneity with Ni-affinity chromatography and its biochemical properties were investigated. The specific activity of the purified rHsGST was 254.20 nmol/min/mg. The optimum temperature and pH of enzyme were 25 °C and 7.5, respectively. Most importantly, rHsGST retained 41.67% of its maximal activity at 0 °C. 2.0 M NaCl and 0.2% H₂O₂ had no effect on the enzyme activity. Moreover, rHsGST exhibited its protective effects against oxidative stresses in E. coli cells. Due to its high catalytic efficiency and oxidative resistance at low temperature, rHsGST may be a potential candidate as antioxidant in low temperature health foods.