Antimicrobial function of SHβAP, a novel hemoglobin β chain-related antimicrobial peptide, isolated from the liver of skipjack tuna, Katsuwonus pelamis

Marine Antimicrobial Peptides: An Emerging Nightmare to the Life-Threatening Pathogens

The emergence of multidrug-resistant pathogens due to improper usage of conventional antibiotics has created a global health crisis. Alternatives to antibiotics being an urgent need, the scientific community is forced to search for new antimicrobials. This exploration has led to the discovery of antimicrobial peptides, a group of small peptides occurring in different phyla such as Porifera, Cnidaria, Annelida, Arthropoda, Mollusca, Echinodermata, and Chordata, as a component of their innate immune system. The marine environment, possessing immense diversity of organisms, is undoubtedly one of the richest sources of unique potential antimicrobial peptides. The distinctiveness of marine antimicrobial peptides lies in their broad-spectrum activity, mechanism of action, less cytotoxicity, and high stability, which form the benchmark for developing a potential therapeutic. This review aims to (1) synthesise the available information on the distinctive antimicrobial peptides discovered from marine organisms, particularly over the last decade, and (2) discuss the distinctiveness of marine antimicrobial peptides and their prospects.

Genome-Wide Identification of Trachinotus ovatus Antimicrobial Peptides and Their Immune Response against Two Pathogen Challenges

Golden pompano, Trachinotus ovatus, as a highly nutritious commercially valuable marine fish, has become one of the preferred species for many fish farmers due to its rapid growth, wide adaptability, and ease of feeding and management. However, with the expansion of aquaculture scale, bacterial and parasitic diseases have also become major threats to the golden pompano industry. This study, based on comparative genomics, shows the possibility of preferential evolution of freshwater fish over marine fish by analyzing the phylogenetic relationships and divergence times of 14 marine fish and freshwater fish. Furthermore, we identified antimicrobial peptide genes from 14 species at the genomic level and found that the number of putative antimicrobial peptides may be related to species evolution. Subsequently, we classified the 341 identified AMPs from golden pompano into 38 categories based on the classification provided by the APD3. Among them, TCP represented the highest proportion, accounting for 23.2% of the total, followed by scolopendin, lectin, chemokine, BPTI, and histone-derived peptides. At the same time, the distribution of AMPs in chromosomes varied with type, and covariance analysis showed the frequency of its repeat events. Enrichment analysis and PPI indicated that AMP was mainly concentrated in pathways associated with disease immunity. In addition, our transcriptomic data measured the expression of putative AMPs of golden pompano in 12 normal tissues, as well as in the liver, spleen, and kidney infected with Streptococcus agalactiae and skin infected with Cryptocaryon irritans. As the infection with S. agalactiae and C. irritans progressed, we observed tissue specificity in the number and types of responsive AMPs. Positive selection of AMP genes may participate in the immune response through the MAPK signaling pathway. The genome-wide identification of antimicrobial peptides in the golden pompano provided a complete database of potential AMPs that can contribute to further understanding the immune mechanisms in pathogens. AMPs were expected to replace traditional antibiotics and be developed into targeted drugs against specific bacterial and parasitic pathogens for more precise and effective treatment to improve aquaculture production.

Exploring Oceans for Curative Compounds: Potential New Antimicrobial and Anti-Virulence Molecules against Pseudomonas aeruginosa

Although several antibiotics are already widely used against a large number of pathogens, the discovery of new antimicrobial compounds with new mechanisms of action is critical today in order to overcome the spreading of antimicrobial resistance among pathogen bacteria. In this regard, marine organisms represent a potential source of a wide diversity of unique secondary metabolites produced as an adaptation strategy to survive in competitive and hostile environments. Among the multidrug-resistant Gram-negative bacteria, Pseudomonas aeruginosa is undoubtedly one of the most important species due to its high intrinsic resistance to different classes of antibiotics on the market and its ability to cause serious therapeutic problems. In the present review, we first discuss the general mechanisms involved in the antibiotic resistance of P. aeruginosa. Subsequently, we list the marine molecules identified up until now showing activity against P. aeruginosa, dividing them according to whether they act as antimicrobial or anti-virulence compounds.

Alteration of hemoglobin ß gene expression in mucosal tissues of Japanese flounder, Paralichthys olivaceus, in response to heat stress, Edwardsiella piscicida infection, and immunostimulants administration

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Hbβ gene expression in the gills is decreased upon heat stress.

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Epidermal Hbß gene expression was increased upon AsA and LF feeding or infection.

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Mucosal Hbβ expression may be a useful indicator for monitoring fish health status.

Hemoglobin beta (Hbß) is a heme-binding protein capable of oxygen delivery. The oligopeptides derived from Hbβ in fish mucus are active against a variety of gram-negative bacteria and protozoa. To gain information on the physiological and immunological roles of Hbβ in the mucosal tissues of fish, we analyzed changes in Hbß gene expression levels in the epidermis, gills, and intestine of Japanese flounder, Paralichthys olivaceus, in response to heat stress, Edwardsiella piscicida infection, and trial feeding of immunostimulants, high-concentration ascorbic acid (AsA) or lactoferrin (LF). The results of quantitative real-time PCR showed that expression of the Hbß gene in the gills decreased markedly when exposed to heat stress, whereas that in the epidermis exhibited an increase 3h after infection with E. piscicida. Seven days after starting to feed either immunostimulant, epidermal Hbß gene expression in all AsA or LF dose groups was significantly higher than in the control group. The results of in situ hybridization showed that the abundance and intensity of the stained cells in the epidermis and in the gills were consistent with the expression levels of Hbß gene obtained from the infection and immunosuppressant experiments and the heat stress experiment, respectively. Our results suggest that mucosal Hbβ gene expression is closely related to physiological and immunological status and could be a useful indicator for monitoring condition of fish health.

Discovery of Marine Natural Products as Promising Antibiotics against Pseudomonas aeruginosa

Pseudomonas aeruginosa, one of the most intractable Gram-negative bacteria, has become a public health threat due to its outer polysaccharide layer, efflux transporter system, and high level of biofilm formation, all of which contribute to multi-drug resistance. Even though it is a pathogen of the highest concern, the status of the antibiotic development pipeline is unsatisfactory. In this review, we summarize marine natural products (MNPs) isolated from marine plants, animals, and microorganisms which possess unique structures and promising antibiotic activities against P. aeruginosa. In the last decade, nearly 80 such MNPs, ranging from polyketides to alkaloids, peptides, and terpenoids, have been discovered. Representative compounds exhibited impressive in vitro anti-P. aeruginosa activities with MIC values in the single-digit nanomolar range and in vivo efficacy in infectious mouse models. For some of the compounds, the preliminary structure-activity-relationship (SAR) and anti-bacterial mechanisms of selected compounds were introduced. Compounds that can disrupt biofilm formation or membrane integrity displayed potent inhibition of multi-resistant clinical P. aeruginosa isolates and could be considered as lead compounds for future development. Challenges on how to translate hits into useful candidates for clinical development are also proposed and discussed.

Enhancement of immune proteins expression in skin mucus of Japanese flounder Paralicthys olivaceus upon feeding a diet supplemented with high concentration of ascorbic acid

To search immune defense proteins in skin mucus of Japanese flounder fed with a diet containing high concentration of ascorbic acid, we carried out 2D-PAGE and compared the resolved pattern of proteins between control group that fed commercial diet and ascorbic acid supplemented group (AsA group) fed a diet supplemented with high concentration of ascorbic acid (2,000 mg/kg) for 7 days. The results revealed that there were many proteins exhibited distinct increase in AsA group. Among them, 6 regions that showed a dramatic elevation were chosen for protein identification using LC-MS/MS analysis and Mascot database search. Six proteins were identified, i.e. serotransferrin (Sero), transferrin (Trans), warm temperature acclimation-related 65 kDa protein (Wap65), complement component c3 (C3), hemoglobin beta-A chain (Hbß) and apolipoprotein A-1 (Apo). Quantitative RT-PCR analysis showed that the mRNA level of Hbß in epidermis of AsA group gave much higher increase (11.6 folds) than control group; the levels of Sero/Trans, Wap65, C3 and Apo showed no apparent difference between the two groups. The mRNA levels of wap65 and c3 in the liver and Apo in the kidney of AsA group exhibited significant increase in comparison to control group. In the case of secreted immunoglobulin M (IgM) and lysozyme (lyz), no difference of the mRNA levels of IgM in epidermis, gill, kidney, spleen and intestine, and lyz in epidermis, gill, spleen and intestine, was observed. The results of in situ hybridization confirmed the elevation of Hbß mRNA level in the epidermis tissue of AsA group. Our present study provided additional evidence showing the effectiveness of AsA in activating innate immune defense system in skin mucosal tissue of fish.

Insights into the Natural Defenses of a Coral Reef Fish Against Gill Ectoparasites: Integrated Metabolome and Microbiome Approach

Understanding natural defense mechanisms against parasites can be a valuable tool for the development of innovative therapies. We have previously identified a butterflyfish species (Chaetodon lunulatus) that avoids gill monogenean parasites while living amongst closely related parasitized species. The metabolome and microbiome of several sympatric butterflyfish species from the island of Moorea (French Polynesia) were previously described. In this study, we used the previously generated datasets in an attempt to identify metabolites and bacteria potentially involved in parasite defense mechanisms. We investigated the interplay between the gill mucus metabolome and microbiome of the non-susceptible C. lunulatus versus sympatric butterflyfish species that were always found parasitized in the Central and Eastern Indo-Pacific. After observing significant differences between the metabolome and bacteria of susceptible versus non-susceptible fish, we obtained the discriminant metabolites and operational taxonomic units (OTUs) using a supervised analysis. Some of the most important discriminant metabolites were identified as peptides, and three new peptides derived from β-subunit hemoglobin from C. lunulatus (CLHbβ-1, CLHbβ-2, and CLHbβ-3) were purified, characterized and synthesized to confirm their structures. We also identified specific bacterial families and OTUs typical from low-oxygen habitats in C. lunulatus gill mucus. By using a correlation network between the two datasets, we found a Fusobacteriaceae strain exclusively present in C. lunulatus and highly correlated to the peptides. Finally, we discuss the possible involvement of these peptides and Fusobacteriaceae in monogenean avoidance by this fish species.

Proteomic profiling of yellow catfish (Pelteobagrus fulvidraco) skin mucus identifies differentially-expressed proteins in response to Edwardsiella ictaluri infection

Fish mucus acts as a physiological and immunological barrier for maintaining normal fish physiology and conferring defense against pathogens infection. Here we report proteomic profiling of skin mucus of yellow catfish before and after E. ictaluri infection by Label-free LC-MS/MS approach. A total of 918 non-redundant proteins were identified from 54443 spectra referring to yellow catfish genome database. Further annotation via GO and KEGG database revealed complex protein composition of yellow catfish mucus. Besides structural proteins in mucus, a lot of immune-related proteins were retrieved, such as lectins, complement components, antibacterial peptides and immunoglobins. 133 differentially-expressed proteins (DEPs), including 76 up-regulated and 57 down-regulated proteins, were identified, most of which were enriched into 17 pathways centering on "immune system" category with 33 proteins involved. Consistently, significant proliferation of mucus-secreting goblet cells and CYPA-expressing cells were observed along outside of yellow catfish skin after E. ictaluri infection, indicating an enhanced immune response to E. ictaluri infection in yellow catfish skin mucus. The proteomic data provide systematic protein information to comprehensively understand the biological function of yellow catfish skin mucus in response to bacterial infection.

Biochemical and molecular identification of a novel hepcidin type 2-like antimicrobial peptide in the skin mucus of the pufferfish Takifugu pardalis

Fish skin mucus is considered to act as the first line of defense against waterborne pathogens and to be potential source of novel antimicrobial components. Here we report the purification and characterization of a novel hepcidin type 2-like antimicrobial peptide (TpHAMP2) from the skin mucus of the pufferfish Takifugu pardalis. The purified TpHAMP2 comprised of 23 amino acids (AAs) with eight Cys residues that form four intramolecular disulfide bonds. The TpHAMP2 gene shared overall structural characteristics with all known hepcidins, which have a tripartite exon-intron gene organization and three structural signatures in the precursor protein. Phylogenetically, TpHAMP2 was classified as HAMP2 class in acanthopterygian fish. Interestingly, the AA sequence of TpHAMP2 did not contain a proprotein cleavage site (RXXR motif) that conserved in most hepcidins and showed a highly positive charged (RKR-) short N-terminus and Val¹⁸ and Gly²² residues, which are distinctive structures compared to other known active hepcidins. Recombinant TpHAMP2 identical to the native form exhibited a broad spectrum and potent antimicrobial activity against tested gram-positive and -negative bacteria. Expression of TpHAMP2 mRNA was predominant in the liver and was upregulated in the liver, the spleen, the intestine, and the skin of T. pardalis post immune challenge. Thus, our findings suggests that TpHAMP2 might be of importance in the framework of discovering the fish hepcidins, especially type 2s, and provide noteworthy insight into its gene structure and expression and in the innate immunity as well as the mucosal immunity in regard to hepcidins' evolutionary history in fish species.

Identification of A Novel Antibacterial Peptide from Atlantic Mackerel belonging to the GAPDH-Related Antimicrobial Family and Its In Vitro Digestibility

The Atlantic mackerel, Scomber scombrus, is one of the most fished species in the world, but it is still largely used for low-value products, such as bait; mainly for crustacean fishery. This resource could be transformed into products of high value and may offer new opportunities for the discovery of bioactive molecules. Mackerel hydrolysate was investigated to discover antibacterial peptides with biotechnological potential. The proteolytic process generated a hydrolysate composed of 96% proteinaceous compounds with molecular weight lower than 7 kDa. From the whole hydrolysate, antibacterial activity was detected against both Gram-negative and Gram-positive bacteria. After solid phase extraction, purification of the active fraction led to the identification of 4 peptide sequences by mass spectrometry. The peptide sequence N-KVEIVAINDPFIDL-C, called Atlantic Mackerel GAPDH-related peptide (AMGAP), was selected for chemical synthesis to confirm the antibacterial activity and to evaluate its stability through in vitro digestibility. Minimal inhibitory concentrations of AMGAP revealed that Listeria strains were the most sensitive, suggesting potential as food-preservative to prevent bacterial growth. In addition, in vitro digestibility experiments found rapid (after 20 min) and early digestibility (stomach). This study highlights the biotechnological potential of mackerel hydrolysate due to the presence of the antibacterial AMGAP peptide.

Whole Genome Sequencing of the Blue Tilapia (Oreochromis aureus) Provides a Valuable Genetic Resource for Biomedical Research on Tilapias

Blue tilapia (Oreochromis aureus) has been an economically important fish in Asian countries. It can grow and reproduce in both freshwater and brackish water conditions, whereas it is also considered as a significant invasive species around the world. This species has been widely used as the hybridization parent(s) for tilapia breeding with a major aim to produce novel strains. However, available genomic resources are still limited for this important tilapia species. Here, we for the first time sequenced and assembled a draft genome for a seawater cultured blue tilapia (0.92 Gb), with 97.8% completeness and a scaffold N50 of 1.1 Mb, which suggests a relatively high quality of this genome assembly. We also predicted 23,117 protein-coding genes in the blue tilapia genome. Comparisons of predicted antimicrobial peptides between the blue tilapia and its close relative Nile tilapia proved that these immunological genes are highly similar with a genome-wide scattering distribution. As a valuable genetic resource, our blue tilapia genome assembly will benefit for biomedical researches and practical molecular breeding for high resistance to various diseases, which have been a critical problem in the aquaculture of tilapias.

Development of novel antimicrobial peptides derived from anti-lipopolysaccharide factor of the swimming crab, Portunus trituberculatus

Anti-lipopolysaccharide factors (ALFs) are a representative host defense protein in crustaceans. In this study, we successfully developed two novel antimicrobial peptides (AMPs), named crab-ALF2A and crab-ALF6A, which contain changes to the amino acid sequences of the lipopolysaccharide binding domain and signal peptide, respectively, of the ALF of the swimming crab Portunus trituberculatus. The crab-ALF2A peptide showed potent antimicrobial activity against the Gram-positive bacteria Bacillus cereus, Staphylococcus aureus, and Streptococcus iniae (minimal effective concentration [MEC] 1.51-1.93 μg/mL) and the Gram-negative bacteria Pseudomonas aeruginosa and Escherichia coli (MEC 1.87-1.98 μg/mL), with maximal bactericidal activity at a peptide concentration of 5 μg/mL. The crab-ALF6A peptide also showed potent antimicrobial activity against B. cereus, S. aureus, and S. iniae (MEC 1.49-2.3 μg/mL) and P. aeruginosa and E. coli (MEC 1.72-1.19 μg/mL) at a peptide concentration of 5 μg/mL. Notably, the crab-ALF2A and crab-ALF6A peptides exhibited strong activity against Candida albicans (MECs of 2.11 and 1.95 μg/mL, respectively). These activities were stable following heat treatment. Moreover, the effect of crab-ALF2A and crab-ALF6A peptide treatment on microbe cell morphology was confirmed by scanning electron microscopy. Membrane disruption and damage, and the leakage of cytoplasmic content were clearly observed. A downsizing peptide approach illustrated that the hexapeptide ALF6A8 (RVLLRL) was the shortest peptide showing significant antimicrobial activity. Our approach allows for the generation of novel antimicrobial peptides in a cost effective manner as potential next-generation antibiotics.

Characterization, Preparation, and Purification of Marine Bioactive Peptides

Marine bioactive peptides, as a source of unique bioactive compounds, are the focus of current research. They exert various biological roles, some of the most crucial of which are antioxidant activity, antimicrobial activity, anticancer activity, antihypertensive activity, anti-inflammatory activity, and so forth, and specific characteristics of the bioactivities are described. This review also describes various manufacturing techniques for marine bioactive peptides using organic synthesis, microwave assisted extraction, chemical hydrolysis, and enzymes hydrolysis. Finally, purification of marine bioactive peptides is described, including gel or size exclusion chromatography, ion-exchange column chromatography, and reversed-phase high-performance liquid chromatography, which are aimed at finding a fast, simple, and effective method to obtain the target peptides.

Antibacterial Activity of AI-Hemocidin 2, a Novel N-Terminal Peptide of Hemoglobin Purified from Arca inflata

The continued emergence of antibiotic resistant bacteria in recent years is of great concern. The search for new classes of antibacterial agents has expanded to non-traditional sources such as shellfish. An antibacterial subunit of hemoglobin (Hb-I) was purified from the mantle of Arca inflata by phosphate extraction and ion exchange chromatography. A novel antibacterial peptide, AI-hemocidin 2, derived from Hb-I, was discovered using bioinformatics analysis. It displayed antibacterial activity across a broad spectrum of microorganisms, including several Gram-positive and Gram-negative bacteria, with minimal inhibitory concentration (MIC) values ranging from 37.5 to 300 μg/mL, and it exhibited minimal hemolytic or cytotoxic activities. The antibacterial activity of AI-hemocidin 2 was thermostable (25–100 °C) and pH resistant (pH 3–10). The cellular integrity was determined by flow cytometry. AI-hemocidin 2 was capable of permeating the cellular membrane. Changes in the cell morphology were observed with a scanning electron microscope. Circular dichroism spectra suggested that AI-hemocidin 2 formed an α-helix structure in the membrane mimetic environment. The results indicated that the anti-bacterial mechanism for AI-hemocidin 2 occurred through disrupting the cell membrane. AI-hemocidin 2 might be a potential candidate for tackling antibiotic resistant bacteria.

Hemerythrin-related antimicrobial peptide, msHemerycin, purified from the body of the Lugworm, Marphysa sanguinea

A ∼1.7 kDa antimicrobial peptide was purified from the acidified body extract of the Lugworm, Marphysa sanguinea, by preparative acid-urea-polyacrylamide gel electrophoresis and C18 reversed-phase high performance liquid chromatography (HPLC). The identified peptide is composed of 14 amino acids with the N-terminal acetylation. Comparison of the identified amino acid sequences and molecular weight of this peptide with those of other known proteins or peptides revealed that this peptide had high identity to the N-terminus of hemerythrin of marine invertebrates and named the msHemerycin. The full-length hemerythrin cDNA of Lugworm was contained 1027-bp, including a 5'-untranslated region (UTR) of 60-bp, a 3'-UTR of 595-bp, and an open reading frame of 372-bp encoding 123 amino acids including the msHemerycin at the N-terminus. Tissue distribution of the msHemerycin mRNA suggests that it is constitutively expressed as a non-tissue-specific manner, however, a relatively higher expression level was observed in muscle (6.8-fold) and brain (6.3-fold), and the lowest level in digestive gland. The secondary structural prediction and homology modeling studies indicate that the msHemerycin might form an unordered structure and might act via unconventional mechanism. Our results suggest that the msHemerycin might be an innate immune component related to the host defenses in the Lugworm. This is the first report on the antimicrobial function of the peptide derived from the N-terminus of hemerythrin in the Lugworm, Marphysa sanguinea.

Immunological properties of oxygen-transport proteins: hemoglobin, hemocyanin and hemerythrin

It is now well documented that peptides with enhanced or alternative functionality (termed cryptides) can be liberated from larger, and sometimes inactive, proteins. A primary example of this phenomenon is the oxygen-transport protein hemoglobin. Aside from respiration, hemoglobin and hemoglobin-derived peptides have been associated with immune modulation, hematopoiesis, signal transduction and microbicidal activities in metazoans. Likewise, the functional equivalents to hemoglobin in invertebrates, namely hemocyanin and hemerythrin, act as potent immune effectors under certain physiological conditions. The purpose of this review is to evaluate the true extent of oxygen-transport protein dynamics in innate immunity, and to impress upon the reader the multi-functionality of these ancient proteins on the basis of their structures. In this context, erythrocyte-pathogen antibiosis and the immune competences of various erythroid cells are compared across diverse taxa.

A preliminary study on the antibacterial mechanism of Tegillarca granosa hemoglobin by derived peptides and peroxidase activity

The blood clam, Tegillarca granosa, is one of the few bivalve molluscs containing hemoglobin (Hb). In the present study, we purified two types of T. granosa hemoglobin, Tg-HbI and Tg-HbII, using size exclusion chromatography and measured their antibacterial and peroxidase activities. We also tested antibacterial activities of peptides prepared by trypsin digestion of purified Tg-Hb and reversed-phase high-performance liquid chromatography purification. Purified Tg-HbI and Tg-HbII showed antibacterial activity against Escherichia coli, Pseudomonas putida, Bacillus subtilis, and Bacillus firmus, with differences in minimal inhibitory concentrations (MICs), but lacked antibacterial activity against Vibrio alginolyticus, Vibrio parahaemolyticus, Vibrio harveyi and Staphylococcus aureus. In contrast, 7 Tg-Hb derived peptides exhibited varying degrees of antibacterial activity against V. alginolyticus (MICs: 12-200 μg/ml), V. parahaemolyticus (11-100 μg/ml) and V. harveyi (1-200 μg/ml). The antibacterial activity of Hb derived peptides was confirmed by fluorescence microscopy. In addition, peroxidase activity was detected in Tg-HbI and Tg-HbII. The results indicated that in addition to functioning as a respiratory protein T. granosa hemoglobins likely play a role in host antibacterial defense probably via a peroxidase activity of native molecules and some internal peptides released from the proteins.

Tannic acid anchored layer-by-layer covalent deposition of parasin I peptide for antifouling and antimicrobial coatings

Tannic acid and parasin I were deposited alternatively on stainless steel surface by Michael addition/Schiff base reaction-enabled layer-by-layer deposition technique.

Chloroplast-expressed MSI-99 in tobacco improves disease resistance and displays inhibitory effect against rice blast fungus

Rice blast is a major destructive fungal disease that poses a serious threat to rice production and the improvement of blast resistance is critical to rice breeding. The antimicrobial peptide MSI-99 has been suggested as an antimicrobial peptide conferring resistance to bacterial and fungal diseases. Here, a vector harboring the MSI-99 gene was constructed and introduced into the tobacco chloroplast genome via particle bombardment. Transformed plants were obtained and verified to be homoplastomic by PCR and Southern hybridization. In planta assays demonstrated that the transgenic tobacco plants displayed an enhanced resistance to the fungal disease. The evaluation of the antimicrobial activity revealed that the crude protein extracts from the transgenic plants manifested an antimicrobial activity against E. coli, even after incubation at 120 °C for 20 min, indicating significant heat stability of MSI-99. More importantly, the MSI-99-containing protein extracts were firstly proved in vitro and in vivo to display significant suppressive effects on two rice blast isolates. These findings provide a strong basis for the development of new biopesticides to combat rice blast.