Hydramacin-1 in action: scrutinizing the barnacle model

Molecular characterization, expression and antibacterial function of a macin, HdMac, from Haliotis discus hannai

Macins are a family of antimicrobial peptides, which play multiple roles in the elimination of invading pathogens. In the present study, a macin was cloned and characterized from Pacific abalone Haliotis discus hannai (Designated as HdMac). Analysis of the conserved domain suggested that HdMac was a new member of the macin family. In non-stimulated abalones, HdMac transcripts were constitutively expressed in all five tested tissues, especially in hemocytes. After Vibrio harveyi stimulation, the expression of HdMac mRNA in hemocytes was significantly up-regulated at 12 hr (P < 0.01). RNAi-mediated knockdown of HdMac transcripts affected the survival rates of abalone against V. harveyi. Moreover, recombinant protein of HdMac (rHdMac) exhibited high antibacterial activities against invading bacteria, especially for Vibrio anguillarum. In addition, rHdMac possessed binding activities towards glucan, lipopolysaccharides (LPS), and peptidoglycan (PGN), but not chitin in vitro. Membrane integrity analysis revealed that rHdMac could increase the membrane permeability of bacteria. Meanwhile, both the phagocytosis and chemotaxis ability of hemocytes could be significantly enhanced by rHdMac. Overall, the results showed that HdMac could function as a versatile molecule involved in immune responses of H. discus hannai.

Antimicrobial peptides from freshwater invertebrate species: potential for future applications

Invertebrates are a significant source of antimicrobial peptides because they lack an adaptive immune system and must rely on their innate immunity to survive in a pathogen-infested environment. Various antimicrobial peptides that represent major components of invertebrate innate immunity have been described in a number of investigations over the last few decades. In freshwater invertebrates, antimicrobial peptides have been identified in arthropods, annelids, molluscs, crustaceans, and cnidarians. Freshwater invertebrate species contain antimicrobial peptides from the families astacidin, macin, defensin, and crustin, as well as other antimicrobial peptides that do not belong to these families. They show broad spectrum activities greatly directed against bacteria and to a less extent against fungi and viruses. This review focuses on antimicrobial peptides found in freshwater invertebrates, highlighting their features, structure-activity connections, antimicrobial processes, and possible applications in the food industry, animal husbandry, aquaculture, and medicine. The methods for their synthesis, purification, and characterization, as well as the obstacles and strategies for their development and application, are also discussed.

Identification, antibacterial activities and action mode of two macins from manila clam Venerupis philippinarum

In this study, two macins were identified from clam Venerupis philippinarum (designated as VpMacin-1 and VpMacin-2). They showed 64.71% similarity with each other. The highest mRNA expression of VpMacin-1 and VpMacin-2 was detected in gills and hepatopancreas, respectively, in non-stimulated clams, and their expression could be induced significantly in hemocytes after Vibrio anguillarum infection. Silencing of VpMacin-1 and VpMacin-2 led to 22% and 49% mortality 6 days post infection. Escherichia coli cells were killed by recombinant protein rVpMacin-1 and rVpMacin-2 within 1000 and 400 min, respectively, at a concentration of 1.0 × MIC. Compared with rVpMacin-1, rVpMacin-2 not only showed higher broad-spectrum antimicrobial activities towards Vibrio strains, but possessed stronger abilities to inhibit the formation of bacterial biofilm. Both membrane integrity and electrochemical assay indicated that rVpMacins were capable of causing bacterial membrane permeabilization, especially for rVpMacin-2. Besides, rVpMacin-1 significantly induced both phagocytic (0.1 and 1.0 × MIC, p < 0.05) and chemotactic effects (0.1 × MIC, p < 0.01) of hemocytes, while there was no significant increase for rVpMacin-2. Overall, our results suggested that VpMacin-1 and VpMacin-2 play important roles in host defense against invasive pathogens.

A macin identified from Venerupis philippinarum: Investigation on antibacterial activities and action mode

In the present study, a macin was cloned and characterized from clam Venerupis philippinarum (designed as VpMacin). The full-length cDNA of VpMacin was of 579 bp, encoding a peptide of 87 amino acids with the predicted molecular weight of 9.7 kDa. Analysis of the conserved domain suggested that VpMacin was a new member of the macin family. In non-stimulated clams, VpMacin transcripts exhibited different tissue expression pattern, and highly expressed in the tissues of gills and hepatopancreas. Generally, the temporal expression of VpMacin transcripts was significantly induced in hemocytes of clams post Vibrio anguillarum challenge. Moreover, the recombinant VpMacin protein (rVpMacin) showed obvious antimicrobial activities against Gram-positive and Gram-negative bacteria. After incubated with 40 μM rVpMacin, all detected Escherichia coli could be killed within 60 min. Membrane integrity analysis revealed that rVpMacin could increase the membrane permeability of bacteria and then resulted in cell death. Overall, our results suggested that VpMacin had an important function in host defense against invasive pathogens.

Structure and Interactions of A Host Defense Antimicrobial Peptide Thanatin in Lipopolysaccharide Micelles Reveal Mechanism of Bacterial Cell Agglutination

Host defense cationic Antimicrobial Peptides (AMPs) can kill microorganisms including bacteria, viruses and fungi using various modes of action. The negatively charged bacterial membranes serve as a key target for many AMPs. Bacterial cell death by membrane permeabilization has been well perceived. A number of cationic AMPs kill bacteria by cell agglutination which is a distinctly different mode of action compared to membrane pore formation. However, mechanism of cell agglutinating AMPs is poorly understood. The outer membrane lipopolysaccharide (LPS) or the cell-wall peptidoglycans are targeted by AMPs as a key step in agglutination process. Here, we report the first atomic-resolution structure of thanatin, a cell agglutinating AMP, in complex with LPS micelle by solution NMR. The structure of thanatin in complex with LPS, revealed four stranded antiparallel β-sheet in a ‘head-tail’ dimeric topology. By contrast, thanatin in free solution assumed an antiparallel β-hairpin conformation. Dimeric structure of thanatin displayed higher hydrophobicity and cationicity with sites of LPS interactions. MD simulations and biophysical interactions analyses provided mode of LPS recognition and perturbation of LPS micelle structures. Mechanistic insights of bacterial cell agglutination obtained in this study can be utilized to develop antibiotics of alternative mode of action.

Convergent evolution of defensin sequence, structure and function

Defensins are a well-characterised group of small, disulphide-rich, cationic peptides that are produced by essentially all eukaryotes and are highly diverse in their sequences and structures. Most display broad range antimicrobial activity at low micromolar concentrations, whereas others have other diverse roles, including cell signalling (e.g. immune cell recruitment, self/non-self-recognition), ion channel perturbation, toxic functions, and enzyme inhibition. The defensins consist of two superfamilies, each derived from an independent evolutionary origin, which have subsequently undergone extensive divergent evolution in their sequence, structure and function. Referred to as the cis- and trans-defensin superfamilies, they are classified based on their secondary structure orientation, cysteine motifs and disulphide bond connectivities, tertiary structure similarities and precursor gene sequence. The utility of displaying loops on a stable, compact, disulphide-rich core has been exploited by evolution on multiple occasions. The defensin superfamilies represent a case where the ensuing convergent evolution of sequence, structure and function has been particularly extreme. Here, we discuss the extent, causes and significance of these convergent features, drawing examples from across the eukaryotes.

Honey glycoproteins containing antimicrobial peptides, Jelleins of the Major Royal Jelly Protein 1, are responsible for the cell wall lytic and bactericidal activities of honey

We have recently identified the bacterial cell wall as the cellular target for honey antibacterial compounds; however, the chemical nature of these compounds remained to be elucidated. Using Concavalin A-affinity chromatography, we found that isolated glycoprotein fractions (glps), but not flow-through fractions, exhibited strong growth inhibitory and bactericidal properties. The glps possessed two distinct functionalities: (a) specific binding and agglutination of bacterial cells, but not rat erythrocytes and (b) non-specific membrane permeabilization of both bacterial cells and erythrocytes. The isolated glps induced concentration- and time-dependent changes in the cell shape of both E. coli and B. subtilis as visualized by light and SEM microscopy. The appearance of filaments and spheroplasts correlated with growth inhibition and bactericidal effects, respectively. The time-kill kinetics showed a rapid, >5-log10 reduction of viable cells within 15 min incubation at 1xMBC, indicating that the glps-induced damage of the cell wall was lethal. Unexpectedly, MALDI-TOF and electrospray quadrupole time of flight mass spectrometry, (ESI-Q-TOF-MS/MS) analysis of glps showed sequence identity with the Major Royal Jelly Protein 1 (MRJP1) precursor that harbors three antimicrobial peptides: Jelleins 1, 2, and 4. The presence of high-mannose structures explained the lectin-like activity of MRJP1, while the presence of Jelleins in MRJP1 may explain cell wall disruptions. Thus, the observed damages induced by the MRJP1 to the bacterial cell wall constitute the mechanism by which the antibacterial effects were produced. Antibacterial activity of MRJP1 glps directly correlated with the overall antibacterial activity of honey, suggesting that it is honey's active principle responsible for this activity.

Solution structure and functional studies of the highly potent equine antimicrobial peptide DEFA1

Defensins are small effector molecules of the innate immune system that are present in almost all organisms including plants and animals. These peptides possess antimicrobial activity against a broad range of microbes including bacteria, fungi and viruses and act as endogenous antibiotics. α-Defensins are a subfamily of the defensin family and their expression is limited to specific tissues. Equine DEFA1 is an enteric α-defensin exclusively secreted by Paneth cells and shows an activity against a broad spectrum of microbes, including typical pathogens of the horse such as Rhodococcus equi, various streptococci strains, Salmonella choleraesuis, and Pasteurella multocida. Here, we report the three-dimensional structure of DEFA1 solved by NMR-spectroscopy and demonstrate its specific function of aggregating various phospholipids.

Piloting the membranolytic activities of peptides with a self-organizing map

Antimicrobial peptides (AMPs) show remarkable selectivity toward lipid membranes and possess promising antibiotic potential. Their modes of action are diverse and not fully understood, and innovative peptide design strategies are needed to generate AMPs with improved properties. We present a de novo peptide design approach that resulted in new AMPs possessing low-nanomolar membranolytic activities. Thermal analysis revealed an entropy-driven mechanism of action. The study demonstrates sustained potential of advanced computational methods for designing peptides with the desired activity.