Identification of a mitochondrial-targeting secretory protein from Nocardia seriolae which induces apoptosis in fathead minnow cells

Construction of Heme-Binding Protein Deleted Strain and Using It as an Attenuated Vaccine Against Nocardia seriolae in Hybrid Snakehead (Channa maculata ♀ × Channa argus ♂)

Nocardia seriolae is the main pathogen causing fish nocardiosis, which can infect various aquatic freshwater and marine fish species. We have previously confirmed that the heme-binding protein of N. seriolae (NsHBP) is a secreted protein relating to bacterial virulence. In the present study, a ΔNsHBP mutant of N. seriolae was successfully generated by deleting NsHBP gene, and it was further used for attenuated vaccine development and evaluation in hybrid snakehead (Channa maculata ♀ × Channa argus ♂). The LD50 of strain ΔNsHBP by intraperitoneal injection in hybrid snakehead was presented at 5.47 × 107 CFU/mL, significantly (p < 0.01) higher than that of the wild-type strain at 5.28 × 105 CFU/mL, indicating the virulent reduction of strain ΔNsHBP in comparison to wild-type strain. The live attenuated vaccine used strain ΔNsHBP at sub-clinical dosages (LD15, 1.26 × 107 CFU/mL) to immunise the fish by intraperitoneal injection for 56 days could provide a relative percentage survival (RPS) at 82.40% against artificial challenge with wild-type strain ZJ0503. Additionally, serum enzymes activities of lysozyme (LZM), peroxidase (POD), acid phosphatase (ACP), alkaline phosphatase (AKP), superoxide dismutase (SOD) as well as specific antibody (IgM) titers were induced in fish following ΔNsHBP vaccination. Furthermore, the expression of immune-related genes (TNFα, IL-1β, MHCIα, MHCIIα, CD4 and CD8α) was significantly increased in comparison to the control group during immunisation period with ΔNsHBP in fish. Taken together, it is revealed that the deleted strain ΔNsHBP could be a potential candidate for live attenuated vaccine development to control fish nocardiosis in aquaculture.

Plasma-derived exosomes of Edwardsiella piscicida challenged olive flounder (Paralichthys olivaceus): Characterization and miRNA profiling for potential biomarkers screening

Exosomes are released from multiple cell types as part of their normal physiology as well as during acquired abnormalities. In this study, we investigated the effect of pathogenic Edwardsiella piscicida infection on olive flounder (Paralichthys olivaceus) exosomes at morphometric, physicochemical, and molecular levels. Unique cup-shaped exosomes were isolated from the plasma of non-infected (PBS-Exo) and E. piscicida experimentally challenged (Ep-Exo) olive flounder using ultracentrifugation. The average particle size, concentration, and zeta potential were 150.9 ± 6.9 nm, 5.67 × 1010 particles/mL, and -25.6 ± 1.36 mV for PBS-Exo while 138.7 ± 1.9 nm, 1.22 × 1011 particles/mL, and -35 ± 1.82 mV for Ep-Exo, respectively. Expression of tetraspanin markers (CD81, CD9, and CD63) confirmed the presence of olive flounder exosomes. Differentially expressed (DE) known (9) and novel (29) miRNAs (log2 fold change ≥1; p < 0.05) were identified in the Ep-Exo that could be potential as diagnostic biomarkers for the infection. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses revealed that the predicted target genes of the DE miRNAs were highly enriched in metabolic and immune roles. Both PBS-Exo and Ep-Exo were non-toxic in vitro (up to 100 μg/mL) and in vivo (up to 400 μg/mL). Compared to the vehicle, PBS-Exo at 50 μg/mL induced Nf-kB (>1.50-fold) while at 100 μg/mL, Il8, Il10, Nf-kB, P53, and Inf were induced (>1.50-fold) in fathead minnow cells (FHMs). This suggests that the PBS-Exo contains molecules that moderately stimulate gene expression. In the future, validating the exact olive flounder immune response target genes that interact with DE miRNAs in Ep-Exo will be crucial for investigating the host-pathogen interactions, immune defense mechanisms, and therapeutic targets for olive flounder against E. piscicida infection.

Characterization, expression and functional analysis of Hsp40 during LPS challenge in blood parrot Amphilophus citrinellus ×Vieja melanura

Heat shock protein 40 belonging to heat shock protein family plays an important role in the immune responses of organisms. In this study, the full length cDNA of Hsp40 was 2426 bp including a 1368 bp open reading frame (ORF) encoding 455 amino acids with a molecular weight of 49.16 kDa and a theoretical isoelectric point of 9.34 in blood parrot Vieja synspila ♀ × Amphilophus citrinellus ♂, an important ornamental fish in China. It had three conserved domains DnaJ, CRR and DnaJ_C. Phylogenetic analysis showed that the sequence of Hsp40 among species was conserved, and the blood parrot Hsp40 was closely related to Neolamprologus brichardi. Blood parrot Hsp40 mRNA could be detected in all of the tissues examined and mainly distributed in the cytoplasm. The expression of Hsp40 was upregulated during lipopolysaccharide (LPS) challenge. Upregulated Hsp40 inhibited the activity of nuclear factor κB (NF-κB) and activated protein 1 (AP-1) and reduced the ratio of Bax/Bcl-2 mRNA expression. This study provides a theoretical basis for further exploring the role of Hsp40 gene in the anti-bacterial immunity of blood parrot.

Selenomethionine Mitigates Effects of Nocardia cyriacigeorgica-Induced Inflammation, Oxidative Stress, and Apoptosis in Bovine Mammary Epithelial Cells

Nocardia cyriacigeorgica causes bovine mastitis, reduces milk quantity and quality, and is often resistant to antimicrobials. Selenomethionine (SeMet) is a form of selenium, which reduces reactive oxygen species (ROS)-mediated apoptosis and intramammary infections. However, the protective effects of SeMet on N. cyriacigeorgica-infected bovine mammary epithelial cells (bMECs) are unclear. The objective of this study was to evaluate whether SeMet mitigated N. cyriacigeorgica-induced inflammatory injury, oxidative damage and apoptosis in bMECs. Cells were cultured with or without being pretreated with 40 µM of SeMet for 12 h, then challenged with N. cyriacigeorgica (multiplicity of infection = 5:1) for 6 h. Although N. cyriacigeorgica was resistant to lincomycin, erythromycin, enrofloxacin, penicillin, amoxicillin, cephalonium, cephalexin, and ceftriaxone, 40 μM SeMet increased cell viability and inhibited lactate dehydrogenase release in infected bMECs. Furthermore, N. cyriacigeorgica significantly induced mRNA production and protein expression of TNF-α, IL-1β, IL-6, and IL-8 at 6 h. Cell membrane rupture, cristae degeneration and mitochondria swelling were evident with transmission electron microscopy. Superoxide dismutase (SOD) and glutathione peroxidase (GSH-px) activities were down-regulated after 3, 6, or 12 h, whereas malondialdehyde (MDA) and ROS contents were significantly upregulated, with cell damage and apoptosis rapidly evident (the latter increased significantly in a time-dependent manner). In contrast, bMECs pretreated with 40 μM SeMet before infection, SOD, and GSH-px activities were upregulated (p < 0.05); MDA and ROS concentrations were downregulated (p < 0.05), and apoptosis was reduced (p < 0.05). In conclusion, 40 μM SeMet alleviated inflammation, oxidative stress and apoptosis induced by N. cyriacigeorgica in bMECs cultured in vitro.

Complement C1q is involved in the activation of membrane attack complexes, regulation of bacterial infectious inflammation, and apoptosis through overexpression in primary cells of silver pomfret (Pampus argenteus) in vitro

The complement system is pivotal in innate immune defense, with Complement 1qb (C1qb) playing a key role in recognizing immune complexes and initiating the classical pathway. In this research, we cloned the full-length cDNA of silver pomfret (Pampus argenteus) c1qb and demonstrated its role in mediating defense responses against Nocardia seriolae (N. seriolae) infection, which notably causes significant economic losses in the aquaculture industry. Our investigation revealed that N. seriolae infection led to tissue damage in fish bodies, as observed in tissue sections. Subsequent analysis of differential genes (DEGs) in the transcriptome highlighted genes linked to apoptosis and inflammation. Through experiments involving overexpression and interference of c1qb in vitro, we confirmed that c1qb could suppress N. seriolae-induced apoptosis and inflammation. Moreover, overexpression of c1qb hindered N. seriolae invasion, and the purified and replicated C1qb protein displayed antimicrobial properties. Additionally, our study unveiled that overexpression of c1qb might stimulate the expression of membrane attack complexes (MAC), potentially enhancing opsonization and antibacterial effects. In conclusion, our findings offer valuable insights into the immune antibacterial mechanisms of c1qb and contribute to the development of strategies for controlling N. seriolae.

Construction of an alanine dehydrogenase gene deletion strain for vaccine development against Nocardia seriolae in hybrid snakehead (Channa maculata ♀ × Channa argus ♂)

Nocardia seriolae is the main pathogen of fish nocardiosis. In our previous study, alanine dehydrogenase was identified as a potential virulence factor of N. seriolae. On the basis of this fact, the alanine dehydrogenase gene of N. seriolae (NsAld) was knocked out to establish the strain ΔNsAld for vaccine development against fish nocardiosis in this study. The LD₅₀ of strain ΔNsAld was 3.90 × 10⁵ CFU/fish, higher than that of wild strain (5.28 × 10⁴ CFU/fish) significantly (p < 0.05). When the strain ΔNsAld was used as a live vaccine to immunize hybrid snakehead (Channa maculata ♀ × Channa argus ♂) at 2.47 × 10⁵ CFU/fish by intraperitoneal injection, the non-specific immune indexes (LZM, CAT, AKP, ACP and SOD activities), specific antibody (IgM) titers and several immune-related genes (CD4, CD8α, IL-1β, MHCIα, MHCIIα and TNFα) were up-regulated in different tissues, indicating that this vaccine could induce humoral and cell-mediated immune responses. Furthermore, the relative percentage survival (RPS) of ΔNsAld vaccine was calculated as 76.48% after wild N. seriolae challenge. All these results suggest that the strain ΔNsAld could be a potential candidate for live vaccine development to control fish nocardiosis in aquaculture.

Construction of an attenuated glutamyl endopeptidase deletion strain of Nocardia seriolae

The glutamyl endopeptidase homolog of Nocardia seriolae (GluNS) has been proved to be a potential virulence factor in our previous study. Present investigation was carried out to construct an attenuated N. seriolae strain by deletion with GluNS gene and evaluate its protective immunity in head snakehead. A deletion strain (NS-ΔGluNS) was established by knockout of gene GluNS from wild strain N. seriolae ZJ0503 via homologous recombination. The LD50 of NS-ΔGluNS in 3.41 × 10⁶ cfu/mL was significantly increased than that of wild strain in 4.75 × 10⁵ cfu/mL, indicating that the virulence of N. seriolae has been attenuated with the knockout of GluNS. Meanwhile, applying NS-ΔGluNS as an attenuated live vaccine to immune hybrid snakehead, the non-specific immunity parameters (serum LYZ, POD, ACP, and AKP activities), specific antibody (IgM) titers production and immune-related genes (MHCIα, CD4, and IL-8) expression were up-regulated in different tissues, which indicated that they were able to trigger humoral and cell-mediated immune responses. Furthermore, the protective efficacy in hybrid snakehead after vaccination with NS-ΔGluNS shown 73.53% relative percentage survival (RPS). Taken together, the attenuated NS-ΔGluNS was obtained successfully and it could elicit strong immune response and supply protective efficacy to hybrid snakehead against N. seriolae wild strain.

Isolation and characterization of plasma-derived exosomes from olive flounder (Paralichthys olivaceus) and their wound healing and regeneration activities

Exosomes have garnered enormous interest for their role in physiological and pathological processes and their potential for therapeutic and diagnostic applications. In this study, exosomes were isolated from plasma of olive flounder (Paralichthys olivaceus) and their physiochemical and morphological characteristics, as well as wound healing and regeneration activities were determined. Isolated exosomes had typical characteristics, including average particle diameter (151.82 ± 9.17 nm), concentration (6.31 × 10¹⁰ particles/mL) with a membrane-bound, cup-shaped morphology. Exosome marker proteins, tetraspanins (CD63, CD9, and CD81), and acetylcholinesterase were detected, indicating the presence of exosomes in olive flounder plasma. Exosomes exhibited no toxicity in in vitro and in vivo studies, even at the highest treatment concentrations (100 and 400 μg/mL, respectively), confirming their suitability for further functional studies. Following exosome treatment (50 and 100 μg/mL), substantial cell migration with rapid closure of the open wound area in in vitro scratch wound healing assay and faster zebrafish larvae fin regeneration rate was observed compared to that of the vehicle. Moreover, exosomes exhibited immunomodulatory properties associated with wound healing, based on mRNA expression patterns in fathead minnow (FHM) cells. In conclusion, exosomes isolated from olive flounder plasma using ultracentrifugation exhibited minimal toxicity and enhanced wound healing and tissue regeneration activities. Identification and in-depth investigation of olive flounder plasma-derived exosome constituents will support the development of exosomes as an efficient therapeutic carrier system for fish medicine in the future.

Identification of a secretory heme-binding protein from Nocardia seriolae involved in cell apoptosis

According to the whole-genome bioinformatics analysis, a heme-binding protein from Nocardia seriolae (HBP) was found. HBP was predicted to be a bacterial secretory protein, located at mitochondrial membrane in eukaryotic cells and have a similar protein structure with the heme-binding protein of Mycobacterium tuberculosis, Rv0203. In this study, HBP was found to be a secretory protein and co-localized with mitochondria in FHM cells. Quantitative analysis of mitochondrial membrane potential value, caspase-3 activity, and transcription level of apoptosis-related genes suggested that overexpression of HBP protein can induce cell apoptosis. In conclusion, HBP was a secretory protein which may target to mitochondria and involve in cell apoptosis in host cells. This research will promote the function study of HBP and deepen the comprehension of the virulence factors and pathogenic mechanisms of N. seriolae.

Molecular Identification of Nocardia seriolae and Comparative Analysis of Spleen Transcriptomes of Hybrid Snakehead (Channa maculata Female × Channa argus Male) With Nocardiosis Disease

Hybrid snakehead (Channa maculata female × Channa argus male) is a new freshwater aquaculture fish species in southern China. During intensive aquaculture, hybrid snakeheads are often infected by Nocardia seriolae. In this study, hybrid snakehead infected suspiciously by N. seriolae in an artificial breeding pond were examined. Diseased hybrid snakeheads swam slowly without food intake, and the clinical symptoms included skin wound, anal swelling and ascites, and white granulomatous in liver, spleen, and kidney of fish. Through bacterial isolation, 16S rDNA sequencing, fluorescence in situ hybridization (FISH) and artificial infection experiment, the pathogen was identified as N. seriolae. Furthermore, the spleen samples from diseased and healthy male hybrid snakeheads in the same pond were used for RNA-Seq analysis. A total of 3,512 unique transcripts (unigenes) were identified as differentially expressed genes (DEGs), and 1,886 of them were up-regulated in diseased fish. The expression patterns of 20 DEGs were verified by quantitative polymerase chain reaction (qPCR). Several immune-related pathways and many immune-related genes were identified. qPCR results showed that the expression patterns of immune-related genes in the liver and kidney of diseased fish were comparable to that in the spleen. This study provides deep-sequencing data of hybrid snakehead spleen and will help understand the immune response of hybrid snakehead to N. seriolae. It is also helpful for the biomarker screening of fish-borne Nocardia spp. and the breeding of nocardiosis-resistant fish species.

Isolation and Characterization of Plasma-Derived Exosomes from the Marine Fish Rock Bream (Oplegnathus fasciatus) by Two Isolation Techniques

Exosomes are important mediators of intercellular communication and modulate many physiological and pathological processes. Knowledge of secretion, content, and biological functions of fish exosomes during pathological infection is still scarce due to lack of suitable standardized isolation techniques. In this study, we aimed to isolate exosomes from the plasma of marine fish, rock bream (Oplegnathus fasciatus), by two isolation methods: differential ultracentrifugation (UC) and a commercial membrane affinity spin column technique (kit). Morphological and physicochemical characteristics of the isolated exosomes were determined by these two methods, and the efficiencies of the two methods were compared. Exosomes isolated by both methods were in the expected size range (30–200 nm) and had a characteristic cup-shape in transmission electron microscopy observation. Moreover, more intact exosomes were identified using the kit-based method than UC. Nanoparticle tracking analysis demonstrated a heterogeneous population of exosomes with a mean particle diameter of 114.6 ± 4.6 and 111.2 ± 2.2 nm by UC and a kit-based method, respectively. The particle concentration obtained by the kit method (1.05 × 1011 ± 1.23 × 1010 particles/mL) was 10-fold higher than that obtained by UC (4.90 × 1010 ± 2.91 × 109 particles/mL). The kit method had a comparatively higher total protein yield (1.86 mg) and exosome protein recovery (0.55 mg/mL plasma). Immunoblotting analysis showed the presence of exosome marker proteins (CD81, CD63, and HSP90) in the exosomes isolated by both methods and suggests the existence of exosomes. However, the absence of cytotoxicity or adverse immune responses to fish and mammalian cells by the exosomes isolated by the UC procedure indicates its suitability for functional studies in vitro. Overall, our basic characterization results indicate that the kit-based method is more suitable for isolating high-purity exosomes from fish plasma, whereas UC has higher safety in terms of yielding exosomes with low toxicity. This study provides evidence for the existence of typical exosomes in rock beam plasma and facilitates the selection of an efficient exosome isolation procedure for future applications in disease diagnosis and exosome therapy as fish medicine.

Function and Characterization of an Alanine Dehydrogenase Homolog From Nocardia seriolae

Fish nocardiosis is a chronic, systemic, granulomatous disease in aquaculture. Nocardia seriolae has been reported to be one of the main pathogenic bacteria of fish nocardiosis. There are few studies on the associated virulence factors and pathogenesis of N. seriolae. Alanine dehydrogenase (ALD), which may be a secreted protein, was discovered by analysis using bioinformatics methods throughout the whole genomic sequence of N. seriolae. Nevertheless, the roles of ALD and its homologs in the pathogenesis of N. seriolae are not demonstrated. In this study, the function of N. seriolae ALD (NsALD) was preliminarily investigated by gene cloning, host cell subcellular localization, secreted protein identification, and cell apoptosis detection. Identification of the extracellular products of N. seriolae via mass spectrometry (MS) analysis revealed that NsALD is a secreted protein. In addition, subcellular localization of NsALD-GFP recombinant protein in fathead minnow (FHM) cells showed that the strong green fluorescence co-localized with the mitochondria. Moreover, apoptosis assays demonstrated that the overexpression of NsALD induces apoptosis in FHM cells. This study may lay the foundation for further exploration of the function of NsALD and facilitate further understanding of the pathogenic mechanism and the associated virulence factors of N. seriolae.

Characterization and function study of a glutamyl endopeptidase homolog from Nocardia seriolae

Glutamic endopeptidases (Glu), belonging to the class of serine proteases, are a subfamily of chymotrypsin-like proteolytic enzymes, which are regarded as important virulence factors in bacteria. However, the roles of glutamic endopeptidases of Nocardia seriolae in pathogenic process still remain uncertain. Here, a glutamic endopeptidase homolog from N. seriolae (GluNS) was cloned and its function was elucidated. GluNS encoded a 414-aa protein which shared 93% identity to N. concava. In the phylogenetic tree, the glutamic endopeptidases of genus Nocardia clustered together firstly and then clustered with Streptomyces species. Moreover, GluNS was identified to be a secreted protein of N. seriolae and localized in the mitochondria of FHM cells. The transient overexpression of GluNS significantly induced increase in caspase-3 activity and decrease in ΔΨm values in FHM cells. The number of apoptotic bodies was remarkably higher than that in control group. Taken together, GluNS overexpression induced apoptotic characteristics in FHM cells. This study provided new insights into the function of glutamic endopeptidase from N. seriolae.

Molecular characterization and biological function of CXCR1 in Nocardia seriolae-infected largemouth bass (Micropterus salmoides)

Interleukin-8 (IL-8, CXCL8), a pro-inflammatory chemokine secreted by a variety of cell types, plays a critical role in the development of various immune diseases. Interactions between IL-8 and its receptor CXC receptor 1/2 (CXCR1/2) are known to promote chemotaxis and phagocytosis in many immune responses. In this study, we report the molecular characteristics and pharmacological activity of CXCR1 (MsCXCR1) in largemouth bass (Micropterus salmoides) and evaluated the functional involvement of MsCXCR1 in individuals infected with the pathogen Nocardia seriolae. MsCXCR1 was cloned into the pEGFP-N1 plasmid and the subcellular localization of MsCXCR1 on the cell membrane was verified in MsCXCR1-EGFP-expressing HEK293 cells. Following observation of receptor internalization and intracellular signaling detection, we further determined the functional interaction of secreted interleukin-8 (LcIL-8, the ligand for CXCR1 in large yellow croaker) and MsCXCR1 was further determined, and the ERK phosphorylation signal activation mediated by MsCXCR1 was demonstrated. Quantitative real-time PCR assays were conducted to analyze the transcriptional distribution of MsCXCR1 in various tissues of healthy and diseased largemouth bass. These results illustrate the significant elevation of MsCXCR1 expression in the head kidney, spleen and liver of M. salmoides, suggesting that MsCXCR1 was involved in the immune response in N. seriolae-infected largemouth bass and potentially affects the digestive function of this species.

Development of DNA vaccines encoding ribosomal proteins (RplL and RpsA) against Nocardia seriolae infection in fish

Nocardia seriolae, a Gram-positive pathogen, has been identified as the causative agent of fish nocardiosis. DNA vaccination has been proven to be effective in conferring protection against bacterial infection in fish. The 30S ribosomal protein S1 (RpsA) and 50S ribosomal protein L7/L12 (RplL) were identified to be the common immunodominant antigens of three fish pathogenic Nocardia (N. seriolae, N. salmonicida and N. asteroids) by immunoproteomics profiling in our previous study. In current study, the immunogenicity and protective efficacy of two DNA vaccines encoding RplL and RpsA were evaluated and compared in hybrid snakehead. The results showed vaccination of hybrid snakehead with the pcDNA-RplL and pcDNA-RpsA DNA vaccines provided protective efficacy with relative percentage survival (RPS) of 78.31% and 71.08%, respectively. Meanwhile, the immune response of hybrid snakehead induced by these two DNA vaccines were investigated, and it revealed that the non-specific immunity parameters (serum lysozyme (LYZ), peroxidase (POD), acid phosphatase (ACP), alkaline phosphatase (AKP) and superoxide dismutase (SOD) activities), specific antibody (IgM) production and immune-related genes expression (MHCIα, MHCIIα, CD4, CD8α, IL-1β and TNFα) were significantly increased compared with the corresponding control groups after immunization. Taken together, these results indicated that both pcDNA-RplL and pcDNA-RpsA DNA vaccines could boost the innate, humoral and cellular immune responses in hybrid snakehead and show highly protective efficacy against fish nocardiosis, suggesting that ribosomal proteins RplL and RpsA were promising candidates for DNA vaccines and it will promote the vaccine development against fish nocardiosis.