Molecular characterization, antiviral activity, and UV-B damage responses of Caspase-9 from Amphiprion clarkii

Apoptosis plays a vital role in maintaining cellular homeostasis in multicellular organisms. Caspase-9 (casp-9) is one of the major initiator caspases that induces apoptosis by activating downstream intrinsic apoptosis pathway genes. Here, we isolated the cDNA sequence (1992 bp) of caspase-9 from Amphiprion clarkii (Accasp-9) that consists of a 1305 bp coding region and encodes a 434 aa protein. In silico analysis showed that Accasp-9 has a theoretical isoelectric point of 5.81 and a molecular weight of 48.45 kDa. Multiple sequence alignment revealed that the CARD domain is located at the N-terminus, whereas the large P-20 and small P-10 domains are located at the C-terminus. Moreover, a highly conserved pentapeptide active site (²⁹⁶QACGG³⁰¹), as well as histidine and cysteine active sites, are also retained at the C-terminus. In phylogenetic analysis, Accasp-9 formed a clade with casp-9 from different species, distinct from other caspases. Accasp-9 was highly expressed in the gill and intestine compared with other tissues analyzed in healthy A. clarkii. Accasp-9 expression was significantly elevated in the blood after stimulation with Vibrio harveyi and polyinosinic:polycytidylic acid (poly I:C; 12-48 h), but not with lipopolysaccharide. The nucleoprotein expression of the viral hemorrhagic septicemia virus was significantly reduced in Accasp-9 overexpressed fathead minnow (FHM) cells compared with that in the control. In addition, other in vitro assays revealed that cell apoptosis was significantly elevated in poly I:C and UV-B-treated Accasp-9 transfected FHM cells. However, H^248P or C^298S mutated Accasp-9 significantly reduced apoptosis in UV-B irradiated cells. Collectively, our results show that Accasp-9 might play a defensive role against invading pathogens and UV-B radiation and H²⁴⁸ and C²⁹⁸ active residues are significantly involved in apoptosis in teleosts.

Molecular characterization and immune regulatory, antioxidant, and antiapoptotic activities of thioredoxin domain-containing protein 17 (TXNDC17) in yellowtail clownfish (Amphiprion clarkii)

Thioredoxin domain-containing protein 17 (TXNDC17) is an important, highly conserved oxidoreductase protein, ubiquitously expressed in all living organisms. It is a small (~14 kDa) protein mostly co-expressed with thioredoxin 1 (TRx1). In the present study, we obtained the TXNDC17 gene sequence from a previously constructed yellowtail clownfish (Amphiprion clarkii) (AcTXNDC17) database and studied its phylogeny as well as the protein's molecular characteristics, antioxidant, and antiapoptotic effects. The full length of the AcTXNDC17 cDNA sequence was 862 bp with a 372 bp region encoding a 123 amino acid (aa) protein. The predicted molecular mass and isoelectric point of AcTXNDC17 were 14.2 kDa and 5.75, respectively. AcTXNDC17 contained a TRX-related protein 14 domain and a highly conserved N-terminal Cys⁴³-Pro⁴⁴-Asp⁴⁵-Cys⁴⁶ motif. qPCR analysis revealed that AcTXNDC17 transcripts were ubiquitously and differently expressed in all the examined tissues. AcTXNDC17 expression in the spleen tissue was significantly upregulated in a time-dependent manner upon stimulation with lipopolysaccharide (LPS), polyinosinic-polycytidylic (poly I:C), and Vibrio harveyi. Besides, LPS-induced intrinsic apoptotic pathway (TNF-α, caspase-8, Bid, cytochrome C, caspase-9, and caspase-3) gene expression was significantly lower in AcTXNDC17-overexpressing RAW264.7 cells, as were NF-κB activation and nitric oxide (NO) production. Furthermore, the viability of H₂O₂-stimulated macrophages was significantly improved under AcTXNDC17 overexpression. Collectively, our findings indicate that AcTXNDC17 is involved in the innate immune response of the yellowtail clownfish.

A newly discovered teleost disulfide isomerase, thioredoxin domain containing 5 (TXNDC5), from big-belly seahorse (Hippocampus abdominalis): Insights into its molecular and functional properties and immune regulatory functions

The thioredoxin domain containing 5 (TXNDC5) is a recently discovered member of the protein disulfide isomerase family (PDI), which is mainly involved in the proper folding of and the correct formation of disulfide bonds in newly synthesized proteins via its disulfide isomerase and chaperone activities. Although the structural and functional features of mammalian TXNDC5 have been explored in previous studies, no studies have reported the functional characteristics of TXNDC5 in teleost fish. In this study, we report the identification and characterization of TXNDC5 from big-belly seahorse (Hippocampus abdominalis) (ShTXNDC5) accompanied by functional studies. The in-silico analysis revealed that the gene encodes a 433 amino acid (aa) long polypeptide chain with a predicted molecular weight of 49.3 kDa. According to homology analysis, ShTXNDC5 shares more than 55% sequence similarity with other teleost TXNDC5 proteins, and the alignment of the gene sequence convincingly reflects the accepted phylogeny of teleost. Analysis of the spatial distribution of ShTXNDC5 expression showed that its highest expression was observed in the ovary, gill, and pouch of seahorses. Moreover, significant upregulation of ShTXNDC5 transcription was noted in seahorse blood and kidney tissues in a time-dependent manner upon viral and bacterial immune challenges. Furthermore, considerable NADPH turnover, insulin reduction ability and significant cell survival effects of ShTXNDC5 were determined by the functional assay, revealing its capability to overcome cellular oxidative stress. Altogether, these findings expand our understanding of TXNDC5 at the molecular and functional levels, and its putative role in seahorse immunity.

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.