Identification and expressional analysis of two cathepsins from half-smooth tongue sole (Cynoglossus semilaevis)

Characterization, expression profiling, and immunological role of Cathepsin D in Sebastes schlegelii during bacterial infection

Cathepsin D (CTSD), a ubiquitous aspartate hydrolase in eukaryotes, is predominantly localized in lysosomes and involved in the process of substance hydrolysis. While extensive studies have highlighted the importance of CTSD in various physiological and pathological conditions in mammals, its functional roles and mechanisms in fish in responses to bacterial infections remain poorly understood. In this study, two CTSD genes, SsCTSDa and SsCTSDb, were identified in Sebastes schlegelii, and their characteristics were systematically investigated through phylogenetic analysis, syntenic analysis, and tissue-specific expression profiling under both healthy and bacterial infection conditions. Additionally, their immune-related properties, including subcellular localization, microbial ligand-binding capacity, and agglutination activity, were explored. Firstly, SsCTSDa encodes a 396-amino acid protein with a molecular mass of 43.01 kDa, while SsCTSDb encodes a 339-amino acid protein with a molecular mass of 43.36 kDa. Furthermore, both genes were ubiquitously expressed in all examined tissues, with the highest expression levels observed in the spleen. Moreover, SsCTSDa and SsCTSDb exhibited distinct expression patterns following bacterial infection, showing significant upregulation in the kidney and gill. Functional assays demonstrated that recombinant SsCTSDa (rSsCTSDa) and SsCTSDb (rSsCTSDb) exhibited strong binding affinity to microbial ligands, including LPS, PGN, LTA, and Poly (I:C). Notably, rSsCTSDb displayed broad-spectrum agglutination activity against both Gram-positive and Gram-negative bacteria, whereas rSsCTSDa specifically agglutinated Gram-negative bacteria. This study suggests that CTSD plays a crucial role in the immune responses of teleosts, highlighting its potential as a key mediator in host-pathogen interactions.

CTSL-2 upon specifically recognizing Vibrio splendidus directly cleaves complement C3 to promote the bacterial phagocytosis and degradation in oyster

Cathepsin L (CTSL) as a cysteine cathepsin protease mediates complement C3 cleavage and pathogen degradation. In the present study, a CTSL homolog was identified from Crassostrea gigas (designated as CgCTSL-2). Its mRNA expression increased significantly in hemocytes after Vibrio splendidus stimulation. The activity of rCgCTSL-2 was induced after incubation with LPS or V. splendidus in Ca2+-dependent manner. rCgCTSL-2 could specifically bound V. splendidus in Ca2+-dependent manner. The co-localization of rCgCTSL-2 and V. splendidus was observed in cell-free hemolymph. Upon binding V. splendidus, CgCTSL-2 interacted with CgC3 in cell-free hemolymph and hemocytes. CgC3 fragments in CgCTSL-2-RNAi oysters and full length CgC3 in rCgCTSL-2-treated oysters were both reduced in cell-free hemolymph, respectively. CgC3 fragments were accumulated in CgCTSL-2-RNAi or rCgCTSL-2-treated oysters. The co-localizations of V. splendidus, CgC3, CgCD18, CgCTSL-2 and lysosomes were observed in hemocytes. These results suggested that CgCTSL-2 upon binding V. splendidus directly interacted with CgC3 to lead to CgC3 cleavage and then CgC3 fragments coated on V. splendidus were mediated by CgCD18 into CTSL-2-lysosome pathway.

The essential function of cathepsin X of the orange-spotted grouper, Epinephelus coioides during SGIV infection

Cathepsin X, a class of cysteine proteases in the lysosome, involved in intracellular protein degradation processes. Numerous reports revealed that many kinds of cysteine proteases played a crucial role in pathogen invasion. To investigate the relationship between cathepsin X of teleost fish and virus infection, EcCX was cloned and characterized in the orange-spotted grouper, Epinephelus coioides. The open reading frame (ORF) of EcCX included 909 nucleotides and encoded a protein consisting of 302 amino acids, which shared 75% and 56% identity with zebrafish and humans, respectively. The protein EcCX mainly consisted of a signal peptide (1-19 aa), a pro-pre-peptide region (20-55 aa), and a mature cysteine protease region (56-302 aa). Subcellular localization analysis showed that EcCX was mainly distributed in the cytoplasm, but EcCX ectoped to the vicinity of apoptotic vesicles in FHM cells during SGIV infection. Following stimulation with SGIV or Poly (dA:dT), there was a notable rise in the expression levels of EcCX. EcCX overexpression facilitated virus infection, upregulated the production of inflammatory factors, and induced the activation of the NF-κB promoter. Furthermore, the overexpression of EcCX also accelerated the process of SGIV-induced apoptosis, potentially by enhancing the promoter activity of P53 and AP-1. Overall, our findings demonstrated a correlation between the function of EcCX and SGIV infection, providing a new understanding of the mechanisms involved in fish virus infection.

Comparative bioinformatic analysis and biochemical characterization of digestive proteinases from Pacific whiting (Merluccius productus)

The hake fishery plays a crucial role due to its significant economic impact. The genus Merluccius includes 12 extant species found along the coasts of the Americas, Europe, and Africa. However, research on their digestive physiology and the enzymes involved in digestion, including proteases, remains limited. Proteases play a key role in protein digestion, a vital process for all living organisms. This study focused on screening the genomes of eight Merluccius spp. for eight specific proteases previously identified in Merluccius polli. Additionally, the study conducted biochemical analyses of proteases found in the stomach and intestine of Pacific whiting fish (Merluccius productus), comparing the results with the genomic findings. The analysis revealed that proteases across Merluccius spp. are conserved, although with slight variations, particularly in chymotrypsin and aspartic proteases. Biochemical characterization of M. productus identified at least three main proteases in the stomach, active at acidic pH, and at least seven proteases in the intestine, active at alkaline pH, as determined by electrophoresis. Further investigation, including specific inhibition studies, determination of molecular mass, and assessment of pH and temperature preferences for catalysis, revealed that one of the stomach proteases functioning at acidic pH likely belongs to the acid peptidase class, likely pepsin. Similarly, analysis of proteases active at alkaline pH indicated the presence of a chymotrypsin and a trypsin, consistent with genomic findings in M. productus. These results are important as they provide insights into the digestive physiology of Merluccius spp., contributing to a better understanding of their nutritional needs.

Characteristics of cathepsin members and expression responses to poly I:C challenge in Pacific cod (Gadus macrocephalus)

Cathepsins are major lysosomal enzymes that participate in necessary physiological processes, including protein degradation, tissue differentiation, and innate or adaptive immune responses. According to their proteolytic activity, vertebrate cathepsins are classified as cysteine proteases (cathepsins B, C, F, H, K, L, O, S, V, W, and X or Z), aspartic proteases (cathepsin D and E), and serine proteases (cathepsin A and G). Several cathepsins were reported in teleosts, however, no cathepsin gene has been identified from Pacific cod so far. In the present study, a total of 13 cathepsin genes were identified for Pacific cod. The evolutionary path of each cathepsin gene was demonstrated via analysis of phylogenetic trees, multiple alignments, conserved domains, motif compositions, and tertiary structures. Tissue distribution analysis showed that all cathepsin genes were ubiquitously expressed in eight healthy tissues but they exhibited diverse levels of expression. Several cathepsin genes were found to be highly expressed in the kidney, spleen, head kidney and liver, whereas low or modest levels were detected in the gills, skin, intestines, and heart. Temporal-specific expression of cathepsins in early developmental stages of Pacific cod were also conducted. CTSK, S, F, and Z were highly expressed at 1 dph and 5 dph and decreased later, while CTSL, L1, and L.1 transcript levels gradually increased in a time-dependent manner. Additionally, the expression profiles of cathepsin genes in Pacific cod were evaluated in the spleen and liver after poly I:C challenge. The results indicated that all cathepsin genes were significantly upregulated upon poly I:C stimulation, suggesting that they play key roles in antiviral immune responses in Pacific cod. Our findings establish a foundation for future exploration of the molecular mechanisms of cathepsins in modulating antiviral immunity in Pacific cod.

Functional Analysis of the Cathepsin D Gene Response to SGIV Infection in the Orange-Spotted Grouper, Epinephelus coioides

(1) Background: Lysosomal aspartic protease Cathepsin D (CD) is a key regulator and signaling molecule in various biological processes including activation and degradation of intracellular proteins, the antigen process and programmed cell death. However, the function of fish CD in virus infection remains largely unknown. (2) Methods: The functions of the CD gene response to SGIV infection was determined with light microscopy, reverse transcription quantitative PCR, Western blot and flow cytometry. (3) Results: In this study, Ec-Cathepsin D (Ec-CD) was cloned and identified from the orange-spotted grouper, Epinephelus coioides. The open reading frame (ORF) of Ec-CD consisted of 1191 nucleotides encoding a 396 amino acid protein with a predicted molecular mass of 43.17 kDa. Ec-CD possessed typical CD structural features including an N-terminal signal peptide, a propeptide region and a mature domain including two glycosylation sites and two active sites, which were conserved in other CD sequences. Ec-CD was predominantly expressed in the spleen and kidneys of healthy groupers. A subcellular localization assay indicated that Ec-CD was mainly distributed in the cytoplasm. Ec-CD expression was suppressed by SGIV stimulation and Ec-CD-overexpressing inhibited SGIV replication, SGIV-induced apoptosis, caspase 3/8/9 activity and the activation of reporter gene p53 and activating protein-1 (AP-1) in vitro. Simultaneously, Ec-CD overexpression obviously restrained the activated mitogen-activated protein kinase (MAPK) pathways, including extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK). In addition, Ec-CD overexpression negatively regulated the transcription level of pro-inflammatory cytokines and activation of the NF-κB promotor. (4) Conclusions: Our findings revealed that the Ec-CD possibly served a function during SGIV infection.

Antigen Presentation and Autophagy in Teleost Adaptive Immunity

Infectious diseases are a burden for aquaculture. Antigen processing and presentation (APP) to the immune effector cells that fight pathogens is key in the adaptive immune response. At the core of the adaptive immunity that appeared in lower vertebrates during evolution are the variable genes encoding the major histocompatibility complex (MHC). MHC class I molecules mainly present peptides processed in the cytosol by the proteasome and transported to the cell surface of all cells through secretory compartments. Professional antigen-presenting cells (pAPC) also express MHC class II molecules, which normally present peptides processed from exogenous antigens through lysosomal pathways. Autophagy is an intracellular self-degradation process that is conserved in all eukaryotes and is induced by starvation to contribute to cellular homeostasis. Self-digestion during autophagy mainly occurs by the fusion of autophagosomes, which engulf portions of cytosol and fuse with lysosomes (macroautophagy) or assisted by chaperones (chaperone-mediated autophagy, CMA) that deliver proteins to lysosomes. Thus, during self-degradation, antigens can be processed to be presented by the MHC to immune effector cells, thus, linking autophagy to APP. This review is focused on the essential components of the APP that are conserved in teleost fish and the increasing evidence related to the modulation of APP and autophagy during pathogen infection.

Molecular characterization and functional analysis of the lysosomal cathepsin D-like gene in red swamp crayfish, Procambarus clarkii

Aspartic proteinases are one of the four families of proteinase enzymes that are widely present in living organisms. They are involved in various physiological events, such as protein degradation, development, and host defense. However, the characterization and functional roles of aspartic proteinases remain to be elucidated in crustaceans. Here, we characterized a fragment of cathepsin D-like cDNA from red swamp crayfish, Procambarus clarkii (Pc-cathepsin D-like). The open reading frame of the Pc-cathepsin D-like gene contained 1152 bp, encoding a protein of 383 amino acid residues. We also evaluated the immunological role of the Pc-cathepsin D-like gene in vivo. Spatial distribution analysis revealed that the Pc-cathepsin D-like mRNA was high in the hepatopancreas, followed by the gut, gills, and hemocytes of P. clarkii. The expression levels of the Pc-cathepsin D-like gene increased following challenge with viral (polyinosinic: polycytidylic acid) and bacterial (lipopolysaccharides, peptidoglycan) PAMPs compared with PBS injection. The suppression of the Pc-cathepsin D-like gene by RNA interference significantly increased the expression of immune-associated genes. These results showed that the Pc-cathepsin D-like gene has an essential biological role in innate immune responses because it regulates the expression of immune-associated genes.

Pathogenic infection and immune-related gene expression of Chinese sturgeon (Acipenser sinensis) challenged by Citrobacter freundii

Citrobacter freundii is one of the important bacterial diseases responsible for disease outbreaks to wild and cultured fishes globally. However, no known empirical research has focused on exploring relationships between immune response after C. freundii infection in sturgeons. In this study, C. freundii was isolated and identified from artificially breeding Chinese sturgeon, and global measurement of transcriptome response to C. freundii infection in head-kidney and spleen of A. sinensis were conducted to the acknowledgement of the potential mechanisms of pathogen-host interaction triggered by the bacterial infection. In total, differentially expressed genes which significantly associated with immune responses were found to be participated in antigen processing and presentation (MHC I, MHC II, HspA1, Hsp90A, Hsp70, CTSL, and CTSE), and acute phase response (serotransferrin and CP), as well as changing of other immune-related cytokine, such as chemokine and interferon, which proving their reacting and regulatory role during the response of thehost against C. freundii infection in fish. C. freundii can cause serious disease in sturgeon species was first reported in this study, and innate immune responses to C. freundii infection in this study will be conducive to understand the defense mechanisms and making appropriate prevention strategies in A. sinensis aquaculture operations.

Role of major histocompatibility complex II antigen-presentation pathway genes in orange-spotted grouper infected with Cryptocaryon irritans

Cryptocaryon irritans, a pathogen model for fish mucosal immunity, causes skin mucosal and systematic humoral immune response. Where and how MHC II antigen presentation occurs in fish infected with C. irritans remain unknown. In this study, the full-length cDNA of the grouper cysteine protease CTSS was cloned. The expression distributions of six genes (CTSB, CTSL, CTSS, GILT, MHC IIA and MHC IIB) involved in MHC II antigen presentation pathway were tested. These genes were highly expressed in systematic immune tissues and skin and gill mucosal-associated immune tissues. All six genes were upregulated in skin at most time points. Five genes expected CTSS was upregulated in spleen at most time points. CTSB, CTSL and MHC IIA were upregulated in the gill and head kidney at some time points. These results indicate that the presentation of MHC II antigen intensively occurred in local infected skin and gill. Spleen, not head kidney, had the most extensive systematic antigen presentation. In skin, six genes most likely peaked at day 2, earlier than in spleen (5-7 days), marking an earlier skin antibody peak than any recorded in serum previously. This significant and earlier mucosal antigen presentation indicates that specific immune response occurs in local mucosal tissues.

Characterization of the cathepsin D in Procambarus clarkii and its biological role in innate immune responses

Cathepsin D belongs to aspartic protease family, produced in the rough endoplasmic reticulum, and then transported to lysosomes, where it participates in various physiological processes. Despite its importance, only a few reports available on the functional role of cathepsin D in crustaceans. Herein, we cloned a cDNA fragment of cathepsin D from the hepatopancreas of the red swamp crayfish, Procambarus clarkii (Pc-cathepsin D) for the first time. It included 1158 base pairs open reading frame, encoding a protein of 385 amino acids. Multiple alignment analysis confirmed the presence of aspartic proteinase active sites and N glycosylation sites. Pc-cathepsin D mRNA expression was high in the gills followed by gut, heart, hepatopancreas of P. clarkii. At different time points post-infection with lipopolysaccharides, peptidoglycan, or polyinosinic polycytidylic acid, Pc-cathepsin D mRNA expression significantly enhanced compared with the control group. Knockdown of the Pc-cathepsin D by double-stranded RNA, strikingly, changed the expression of all the tested P. clarkii immune-associated genes, including Pc-Toll, Pc-lectin, Pc-cactus, Pc-anti-lipopolysaccharide factor, Pc-phospholipase, and Pc-sptzale. Altogether, these results suggest that Pc-cathepsin D is needed to confer innate immunity against microbial pathogens by modulating the expression of crucial transcripts that encode immune-associated genes.

Characterization of the immune roles of cathepsin L in turbot (Scophthalmus maximus L.) mucosal immunity

Cathepsin L (CTSL) is one of the crucial enzymes in cathepsin family, which has been widely known for its involvement in the innate immunity. However, it still remains poorly understood how CTSL modulates the immune system of teleosts. In this study, we captured three cathepsin L genes (SmCTSL, SmCTSL.1 and SmCTSL1) from turbot (Scophthalmus maximus). The coding sequences of SmCTSL, SmCTSL.1 and SmCTSL1 are 1,026 bp, 1,005 bp and 1,017 bp in length and encode 341, 334 and 338 amino acids, respectively. In details, transcripts of CTSL genes share same domains as other CTSL genes, one signal peptide, one propeptide and one papain family cysteine protease domain. Protein interaction network analysis indicated that turbot CTSL genes may play important roles in apoptotic signaling and involve in innate immune response. Evidence from subcellular localization demonstrated that the three Cathepsin L proteins were ubiquitous in nucleus and cytoplasm. The cathepsin L genes were widely expressed in all the tested tissues with the highest expression level of SmCTSL in spleen, and SmCTSL.1 and SmCTSL1 in intestine. Following Vibrio anguillarum, Edwardsiella tarda and Streptococcus iniae challenge, these cathepsin L genes were significantly regulated in mucosal tissues in all the challenges, especially significant down-regulation occurred rapidly in intestine in all the three challenges. In addition, the three cathepsin L genes showed strong binding ability to all the examined microbial ligands (LPS, PGN and LTA). Further studies should be used to analyze the specific function of these three cathepsin L genes. By then, we can use their function to maintain the integrity of the mucosal barrier, thereby promoting the disease resistance line and family selection in turbot.

Effects of different light spectra on embryo development and the performance of newly hatched turbot (Scophthalmus maximus) larvae

Light is a key environmental factor that synchronizes various life stages from embryo development to sexual maturation in fish. For turbot, light spectra have the most influence at the larval and juvenile stages. In the current study, differences in the development of embryos and the performance of newly hatched turbot larvae exposed to five different spectra: full spectrum (LDF), blue (LDB, peak at 450 nm), green (LDG, peak at 533 nm), orange (LDO, peak at 595 nm) and red (LDR, peak at 629 nm), were examined. At 62.8 h post fertilization, a higher number of embryos exposed to short-wavelengths (LDG and LDB) had developed a heartbeat in comparison with embryos exposed to other wavelengths. Larvae exposed to the green spectrum had higher malformation rates than larvae exposed to the other spectra, indicating that larvae exposed to green light may have significantly reduced survival rates. The results of non-specific immunity parameters showed that the mRNA expression levels of cathepsin D (CTSD), cathepsin F (CTSF), catalase (CAT) and metallothionein (MT) in larvae exposed to LDB were significantly higher than those exposed to other spectra, but CAT activity in larvae exposed to LDB was significantly lower than larvae exposed to the other spectra. There was no significant difference in MT activity in larvae exposed to the five different spectra. The mRNA expression level of lysozyme (LZM) in larvae exposed to LDR was significantly higher than other spectra, while there was no significant difference in LZM activity observed in larvae exposed to LDR, LDG, LDB and LDF. The difference of the enzyme activity of total superoxide dismutase (T-SOD) was not significant among larvae exposed to the five spectra. mRNA expression of the heat shock protein 70 (HSP70) was significantly higher in newly hatched larvae exposed to LDB, LDR and LDG, indicating that larvae exposed to LDB, LDG and LDR exhibited a stress response. The mRNA expression level of the insulin-like growth factor-1 (IGF-1) and growth parameters in the newly hatched larvae exposed to the different spectra were not significantly different. The results of the present study indicate that LDO and LDF should be used for embryo incubation and newly hatched larvae when rearing turbot. This study provides a theoretical basis for optimizing the incubation light environment for fertilized turbot eggs, promoting immunity and reducing stress responses in newly hatched larvae.

Characterization of cathepsin C from orange-spotted grouper, Epinephelus coioides involved in SGIV infection

The lysosomal cysteine protease cathepsin C plays a pivotal role in regulation of inflammatory and immune responses. However, the function of fish cathepsin C in virus replication remains largely unknown. In this study, cathepsin C gene (Ec-CC) was cloned and characterized from orange-spotted grouper, Epinephelus coioides. The full-length Ec-CC cDNA was composed of 2077 bp. It contained an open reading frame (ORF) of 1374 bp and encoded a 458-amino acid protein which shared 89% identity to cathepsin C from bicolor damselfish (Stegastes partitus). Amino acid alignment analysis showed that Ec-CC contained an N-terminal signal peptide, the propeptide region and the mature peptide. RT-PCR analysis showed that Ec-CC transcript was expressed in all the examined tissues which abundant in spleen and head kidney. After challenged with Singapore grouper iridovirus (SGIV) stimulation, the relative expression of EC-CC was significantly increased at 24 h post-infection. Subcellular localization analysis revealed that Ec-CC was distributed mainly in the cytoplasm. Further studies showed that overexpression of Ec-CC in vitro significantly delayed the cytopathic effect (CPE) progression evoked by SGIV and inhibited the viral genes transcription. Moreover, overexpression of Ec-CC significantly increased the expression of proinflammatory cytokines during SGIV infection. Taken together, our results demonstrated that Ec-CC might play a functional role in SGIV infection by regulating the inflammation response.

Molecular cloning and functional characterization of cathepsin B from the sea cucumber Apostichopus japonicus

Cathepsin B (CTSB), a member of lysosomal cysteine protease, is involved in multiple levels of physiological and biological processes, and also plays crucial roles in host immune defense against pathogen infection in vertebrates. However, the function of CTSB within the innate immune system of invertebrates, particularly in marine echinoderms, has been poorly documented. In this study, the immune function of CTSB in Apostichopus japonicus (designated as AjCTSB), a commercially important and disease vulnerable aquaculture specie, was investigated by integrated molecular and protein approaches. A 2153 bp cDNA representing the full-length of AjCTSB was cloned via overlapping ESTs and RACE fragments. AjCTSB contained an open reading frame of 999 bp encoding a secreted protein of 332 amino acid residues with a predicted molecular mass of 36.8 kDa. The deduced amino acid of AjCTSB shared a typical activity center containing three conserved amino acid residues (Cys¹⁰⁸, His²⁷⁷ and Asn²⁹⁷). Phylogenetic tree analysis also supported that AjCTSB was a new member of CTSB family with clustering firstly with invertebrate CTSBs. Quantitative real time PCR analysis revealed that AjCTSB was ubiquitously expressed in all examined tissues with the highest levels in intestine. The Vibrio splendidus challenged sea cucumber and LPS-exposed coelomocytes could both significantly boost the expression of AjCTSB. Moreover, the purified recombinant AjCTSB exhibited dose-dependent CTSB activities at the concentration ranged from 0 to 0.24 μg μL^-1. Further functional analysis indicated that coelomocytes apoptosis was significantly inhibited by 0.16-fold in vivo and the apoptosis execution Ajcaspase 3 was extremely reduced in Apostichopus japonicus coelomocytes treated with specific AjCTSB siRNA. Collectively, all these results suggested that AjCTSB was an important immune factor and might be served as apoptosis enhancers in pathogen challenged sea cucumber.

Identification, mRNA expression profiling and activity characterization of cathepsin L from red drum (Sciaenops ocellatus)

Cathepsin L is a cysteine protease with a papain-like structure. It is known to be implicated in multiple processes of mammalian immune response to pathogen infection. In teleost fish, the functionality of cathepsin L is less understood. In this work, we characterized a cathepsin L homologue (designated as SoCatL) from red drum Sciaenops ocellatus, an important farmed fish species in China. SoCatL possesses a typical domain arrangement characteristic of cathepsin L, which comprises a proregion and a protease domain with four catalytically essential residues (Gln137, Cys143, His282 and Asn302) conserved in various organisms. SoCatL shares moderate sequence identities with mammalian cathepsin L and relatively high sequence identities with teleost cathepsin L. Phylogenetic analysis revealed that SoCatL is evolutionally close to fish cathepsin L, especially those belonging to the Perciformes order. The homology model of SoCatL was discovered to exhibit a structure resembling human cathepsin L. Transcriptional expression of SoCatL was found ubiquitous in tissues and enhanced after experimental infection with a bacterial pathogen. Recombinant SoCatL purified from Escherichia coli (designated as rSoCatL) displayed apparent proteolytic activity, which was optimal at 50 °C and pH 7.0. The activity of rSoCatL required the catalytic residue Cys143 and was severely reduced by cathepsin inhibitor. These results suggest that SoCatL is a teleost cathepsin L homologue which functions as a cysteine protease and is likely to participate in the host immune response against bacterial infection.

Mucosal expression signatures of two Cathepsin L in channel catfish (Ictalurus punctatus) following bacterial challenge

The mucosal surfaces of fish are the first line of host defense against various pathogens. The mucosal immune responses are the most critical events to prevent pathogen attachment and invasion. Cathepsins are a group of peptidases that involved in different levels of immune responses, but the knowledge of the roles of Cathepsin in mucosal immune responses against bacterial infection are still lacking. Therefore, in the present study we characterized the Cathepsin L gene family in channel catfish, and profiled their expression levels after challenging with two different Gram-negative bacterial pathogens. Here, two Cathepsin L genes were identified from channel catfish and were designated CTSL1a and CTSL.1. Comparing to other fish species, the catfish CTSL genes are highly conserved in their structural features. Phylogenetic analysis was conducted to confirm the identification of CTSL genes. Expression analysis revealed that the CTSL genes were ubiquitously expressed in all tested tissues. Following infection, the CTSL genes were significantly induced at most timepoints in mucosal tissues. But the expression patterns varied depending on both pathogen and tissue types, suggesting that CTSL genes may exert disparate functions or exhibit distinct tissue-selective roles in mucosal immune responses. Our findings here, clearly revealed the key roles of CTSL in catfish mucosal immunity; however, further studies are needed to expand functional characterization and examine whether CTSL may also play additional physiological roles in catfish mucosal tissues.

Genome-wide identification and transcript profile of the whole cathepsin superfamily in the intertidal copepod Tigriopus japonicus

Modulation of expression of cathepsins in innate immune response has previously been reported in mollusks and large crustaceans including crabs, lobsters, and shrimps in response to immune challenges. However, similar responses in copepods and the related cathepsin members remain under-investigated. To understand molecular and innate immune responses in the intertidal copepod Tigriopus japonicus, we identified the full spectra of cathepsin members (2 aspartyl proteases, 18 cysteine proteases, and 4 serine proteases) and also analyzed transcriptional expression of cathepsin (Tj-cathepsin) genes in developmental stages, lipopolysaccharide (LPS)- and two Vibrio species-exposed T. japonicus. The transcriptional levels of most Tj-cathepsin genes were highly increased during the molting transition from the nauplius to the copepodid stages. LPS treatment induced innate immune response via significant transcriptional increase of serine cathepsin (e.g., cathepsin As) members with induction of several cysteine cathepsin genes. However, Tj-aspartyl cathepsin E-like and a novel cysteine cathepsin were slightly reduced in response to LPS exposure. Interestingly, Vibrio species showed very low transcriptional sensitivity in the expression of entire cathepsins, while LPS induced several cathepsin gene-involved primitive immune responses in T. japonicus. In this paper, we discuss how whole cathepsin expression profiling can be linked to host defense mechanism to better understand and uncover the underlying mechanism of copepods' innate immunity.

Identification and characterization of a cathepsin D homologue from lampreys (Lampetra japonica)

Cathepsin D (EC 3.4.23.5) is a lysosomal aspartic proteinase of the pepsin superfamily which participates in various digestive processes within the cell. In the present study, the full length cDNA of a novel cathepsin D homologue was cloned from the buccal glands of lampreys (Lampetra japonica) for the first time, including a 124-bp 5' terminal untranslated region (5'-UTR), a 1194-bp open reading frame encoding 397 amino acids, and a 472-bp 3'-UTR. Lamprey cathepsin D is composed of a signal peptide (Met 1-Ala 20), a propeptide domain (Leu 21-Ala 48) and a mature domain (Glu 76-Val 397), and has a conserved bilobal structure. Cathepsin D was widely distributed in the buccal glands, immune bodies, hearts, intestines, kidneys, livers, and gills of lampreys. After challenging with Escherichia coli or Staphylococcus aureus, the expression level of lamprey cathepsin D in the buccal gland was 8.5-fold or 6.5-fold higher than that in the PBS group. In addition, lamprey cathepsin D stimulated with Escherichia coli was also up-regulated in the hearts, kidneys, and intestines. As for the Staphylococcus aureus challenged group, the expression level of lamprey cathepsin D was found increased in the intestines. The above results revealed that lamprey cathepsin D may play key roles in immune response to exogenous pathogen and could serve as a potential antibacterial agent in the near future. In addition, lamprey cathepsin D was subcloned into pcDNA 3.1 vector and expressed in the human embryonic kidney 293 cells. The recombinant lamprey cathepsin D could degrade hemoglobin, fibrinogen, and serum albumin which are the major components in the blood, suggested that lamprey cathepsin D may also act as a digestive enzyme during the adaptation to a blood-feeding lifestyle.

Identification, expression, and responses to bacterial challenge of the cathepsin C gene from the razor clam Sinonovacula constricta

Cathepsin C (dipeptidyl-peptidaseI, DPPI) is a lysosomal cysteine proteinase that belongs to the papain superfamily, and it is involved in protein degradation and proenzyme activation. However, very little is known about the function of cathepsin C in bivalves. In the present study, we identified the cathepsin C gene in the razor clam Sinonovacula constricta (Sc-CTSC). The full-length Sc-CTSC cDNA contained a complete open reading frame (ORF) of 1371 nt encoding 456 amino acids, a 98 bp 5' UTR, and a 1043 bp 3' UTR. The ORF of Sc-CTSC consisted of a putative signal peptide of 22 aa, a propeptide of 229 aa, and a mature peptide of 205 aa containing the active site triad of Cys, His, and Asn. The Sc-CTSC transcript was expressed in a wide range of tissues but exhibited the greatest level of expression in the digestive gland. During the early developmental stages, the transcript was detected widely. Upon injection with Vibrio anguillarum, the Sc-CTSC transcript was significantly up-regulated in digestive gland, mantle, and gill tissues. The results provided important information for further exploring the roles of cathepsin C in the innate immune responses.

Cloning, characterisation, and expression analysis of the cathepsin D gene from rock bream (Oplegnathus fasciatus)

Cathepsins are lysosomal cysteine proteases belonging to the papain family, members of which play important roles in normal metabolism for the maintenance of cellular homeostasis. Rock bream (Oplegnathus fasciatus) cathepsin D (RbCTSD) cDNAs were identified by expressed sequence tag analysis of a lipopolysaccharide-stimulated rock bream liver cDNA library. The full-length RbCTSD cDNA (1644 bp) contained an open reading frame of 1191 bp encoding 396 amino acids. Alignment analysis revealed that the active sites and N-glycosylation sites of the deduced protein were well conserved. Phylogenetic analysis revealed that RbCTSD is most closely related to the Mi-iuy croaker (Miichthys miiuy) cathepsin D. RbCTSD was ubiquitously expressed in all the examined tissues, predominantly in muscle and kidneys. RbCTSD mRNA expression was also examined in several tissues under conditions of bacterial and viral challenge. All examined tissues of fish infected with Edwardsiella tarda (E. tarda), Streptococcus iniae (S. iniae), and red sea bream iridovirus (RSIV) showed significant increases in RbCTSD expression compared with the control. In the kidney and spleen, RbCTSD mRNA expression was markedly upregulated following infection with all tested pathogens. These findings indicate that RbCTSD plays an important role in the innate immune response of rock bream. Furthermore, these results provide important information for the identification of other cathepsin D genes in various fish species.

Identification of cathepsin B in the razor clam Sinonovacula constricta and its role in innate immune responses

Cathepsin B, a lysosomal cysteine protease, has drawn much attention in vertebrates. However, very little is known about the functions of cathepsin B in bivalves. In this study, we identified the cathepsin B gene in the razor clam Sinonovacula constricta. The protein has a typical cysteine protease structure, comprising a 15-residue putative signal peptide, a 75-residue propeptide and a 249-residue mature domain. In the mature domain, there is an occluding loop, an oxyanion hole (Gln) and a catalytic triad (Cys, His and Asn). The cathepsin B gene is expressed in a wide range of tissues but appears to exhibit greatest level of expression in the liver. During the early developmental stages, the transcript could be detected widely. After the clam was infected with Vibrio anguillarum, the expression of the cathepsin B gene showed the most significant up-regulation in the liver and mantle tissues at 8h after infection. The fact that bacterial infection can induce the expression of the cathepsin B transcript suggests that cathepsin B could play an important role in the innate immunity of clams.

Molecular characterization and expression analysis of four cathepsin L genes in the razor clam, Sinonovacula constricta

Cathepsin L (CTSL) is a lysosomal cysteine protease involved in immune responses in vertebrates. However, few studies exist regarding the role of cathepsin L in bivalves. In this study, we isolated and characterized four cathepsin L genes from the razor clam Sinonovacula constricta, referred to as CTSL1, CTSL2, CTSL3 and CTSL4. These four genes contained typical papain-like cysteine protease structure and enzyme activity sites with ERWNIN-like and GNFD-like motifs in the proregion domain and an oxyanion hole (Gln) and a catalytic triad (Cys, His and Asn) in the mature domain. Expression analysis of the four transcripts revealed a tissue-specific pattern with high expression of CTSL1 and CTSL3 in liver and gonad tissues and high expression of CTSL2 and CTSL4 in liver and gill tissues. During the developmental stages, the four transcripts showed the highest expression in the juvenile stage; however, CTSL3 had a much higher expression level than the other three transcripts during embryogenesis. The four transcripts showed significant changes in expression as early as 4 h or 8 h after infection with Vibrio anguillarum. The fact that bacterial infection can induce expression of the four CTSL transcripts suggests that these transcripts are important components of the innate immunity system of the clam.

Cathepsin L in the orange-spotted grouper, Epinephelus coioides: molecular cloning and gene expression after a Vibrio anguillarum challenge

The orange-spotted grouper, Epinephelus coioides, is an important fish maricultured in many Asian countries. In the present study, the full-length cDNA of cathepsin L, an immunity related gene of fishes, was isolated from E. coioides using rapid amplification of cDNA ends (RACE). It is 1,443 bp in length, including an open reading frame (ORF) of 1,011 bp. The open reading frame encoded a preproprotein of 336 amino acids (aa), which consisted of a signal peptide of 16 aa, a proregion peptide of 98 aa and a mature peptide of 222 aa. The preproprotein contained an oxyanion hole (Gln), a catalytic triad formed by Cys, His and Asn, and the conserved ERWNIN, GNFD and GCNGG motifs, all characteristic of cathepsin L. Homology analysis revealed that the deduced amino acid sequence of E. coioides cathepsin L shared 80.1-94.8 % identity with those of reported fishes. Tissue-dependent mRNA expression analysis showed that the cathepsin L transcript was expressed in all the examined tissues of the healthy E. coioides, being highest in the liver and moderate in the heart, gonad and intestine. After Vibrio anguillarum stimulation, the mRNA expression of cathepsin L in E. coioides was significantly increased in the skin, fin, gills, liver, blood, spleen, head kidney and intestine, with the highest observed in the spleen (10.6-fold) at 12 h post-injection and the next in blood (7.5-fold) at 8 h post-injection. These results provided initial information for further studies on the physiological and immunological roles of the cathepsin L gene in the orange-spotted grouper.