Structural aspects and biological properties of the cathelicidin PMAP-36

Antimicrobial Activities of α-Helix and β-Sheet Peptides against the Major Bovine Respiratory Disease Agent, Mannheimia haemolytica

Bovine respiratory disease (BRD) is the leading cause of morbidity and mortality in cattle raised in North America. At the feedlot, cattle are subject to metaphylactic treatment with macrolides to prevent BRD, a practice that may promote antimicrobial resistance and has resulted in an urgent need for novel strategies. Mannheimia haemolytica is one of the major bacterial agents of BRD. The inhibitory effects of two amphipathic, α-helical (PRW4, WRL3) and one β-sheet (WK2) antimicrobial peptides were evaluated against multidrug-resistant (MDR) M. haemolytica isolated from Alberta feedlots. WK2 was not cytotoxic against bovine turbinate (BT) cells by the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. All three peptides inhibited M. haemolytica, with WK2 being the most efficacious against multiple isolates. At 8-16 µg/mL, WK2 was bactericidal against Mh 330 in broth, and at 32 µg/mL in the presence of BT cells, it reduced the population by 3 logs CFU/mL without causing cytotoxic effects. The membrane integrity of Mh 330 was examined using NPN (1-N-phenylnaphthylamine) and ONPG (o-Nitrophenyl β-D-galactopyranoside), with both the inner and outer membranes being compromised. Thus, WK2 may be a viable alternative to the use of macrolides as part of BRD prevention and treatment strategies.

Antibacterial activity of the antimicrobial peptide PMAP-36 in combination with tetracycline against porcine extraintestinal pathogenic Escherichia coli in vitro and in vivo

The increase in the emergence of antimicrobial resistance has led to great challenges in controlling porcine extraintestinal pathogenic Escherichia coli (ExPEC) infections. Combinations of antimicrobial peptides (AMPs) and antibiotics can synergistically improve antimicrobial efficacy and reduce bacterial resistance. In this study, we investigated the antibacterial activity of porcine myeloid antimicrobial peptide 36 (PMAP-36) in combination with tetracycline against porcine ExPEC PCN033 both in vitro and in vivo. The minimum bactericidal concentrations (MBCs) of AMPs (PMAP-36 and PR-39) against the ExPEC strains PCN033 and RS218 were 10 μM and 5 μM, respectively. Results of the checkerboard assay and the time-kill assay showed that PMAP-36 and antibiotics (tetracycline and gentamicin) had synergistic bactericidal effects against PCN033. PMAP-36 and tetracycline in combination led to PCN033 cell wall shrinkage, as was shown by scanning electron microscopy. Furthermore, PMAP-36 delayed the emergence of PCN033 resistance to tetracycline by inhibiting the expression of the tetracycline resistance gene tetB. In a mouse model of systemic infection of PCN033, treatment with PMAP-36 combined with tetracycline significantly increased the survival rate, reduced the bacterial load and dampened the inflammatory response in mice. In addition, detection of immune cells in the peritoneal lavage fluid using flow cytometry revealed that the combination of PMAP-36 and tetracycline promoted the migration of monocytes/macrophages to the infection site. Our results suggest that AMPs in combination with antibiotics may provide more therapeutic options against multidrug-resistant porcine ExPEC.

Structural and biological characterization of shortened derivatives of the cathelicidin PMAP-36

Cathelicidins, a family of host defence peptides in vertebrates, play an important role in the innate immune response, exhibiting antimicrobial activity against many bacteria, as well as viruses and fungi. This work describes the design and synthesis of shortened analogues of porcine cathelicidin PMAP-36, which contain structural changes to improve the pharmacokinetic properties. In particular, 20-mers based on PMAP-36 (residues 12-31) and 13-mers (residues 12-24) with modification of amino acid residues at critical positions and introduction of lipid moieties of different lengths were studied to identify the physical parameters, including hydrophobicity, charge, and helical structure, required to optimise their antibacterial activity. Extensive conformational analysis, performed by CD and NMR, revealed that the substitution of Pro25-Pro26 with Ala25-Lys26 increased the α-helix content of the 20-mer peptides, resulting in broad-spectrum antibacterial activity against Escherichia coli, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Staphylococcus epidermidis strains. Interestingly, shortening to just 13 residues resulted in only a slight decrease in antibacterial activity. Furthermore, two sequences, a 13-mer and a 20-mer, did not show cytotoxicity against HaCat cells up to 64 µM, indicating that both derivatives are not only effective but also selective antimicrobial peptides. In the short peptide, the introduction of the helicogenic α-aminoisobutyric acid forced the helix toward a prevailing 310 structure, allowing the antimicrobial activity to be maintained. Preliminary tests of resistance to Ser protease chymotrypsin indicated that this modification resulted in a peptide with an increased in vivo lifespan. Thus, some of the PMAP-36 derivatives studied in this work show a good balance between chain length, antibacterial activity, and selectivity, so they represent a good starting point for the development of even more effective and proteolysis-resistant active peptides.

A wild boar cathelicidin peptide derivative inhibits severe acute respiratory syndrome coronavirus-2 and its drifted variants

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) poses a clear threat to humanity. It has infected over 200 million and killed 4 million people worldwide, and infections continue with no end in sight. To control the pandemic, multiple effective vaccines have been developed, and global vaccinations are in progress. However, the virus continues to mutate. Even when full vaccine coverage is achieved, vaccine-resistant mutants will likely emerge, thus requiring new annual vaccines against drifted variants analogous to influenza. A complimentary solution to this problem could be developing antiviral drugs that inhibit SARS CoV-2 and its drifted variants. Host defense peptides represent a potential source for such an antiviral as they possess broad antimicrobial activity and significant diversity across species. We screened the cathelicidin family of peptides from 16 different species for antiviral activity and identified a wild boar peptide derivative that inhibits SARS CoV-2. This peptide, which we named Yongshi and means warrior in Mandarin, acts as a viral entry inhibitor. Following the binding of SARS-CoV-2 to its receptor, the spike protein is cleaved, and heptad repeats 1 and 2 multimerize to form the fusion complex that enables the virion to enter the cell. A deep learning-based protein sequence comparison algorithm and molecular modeling suggest that Yongshi acts as a mimetic to the heptad repeats of the virus, thereby disrupting the fusion process. Experimental data confirm the binding of Yongshi to the heptad repeat 1 with a fourfold higher affinity than heptad repeat 2 of SARS-CoV-2. Yongshi also binds to the heptad repeat 1 of SARS-CoV-1 and MERS-CoV. Interestingly, it inhibits all drifted variants of SARS CoV-2 that we tested, including the alpha, beta, gamma, delta, kappa and omicron variants.

Gut bacterial extracellular vesicles: important players in regulating intestinal microenvironment

Intestinal microenvironment dysbiosis is one of the major causes of diseases, such as obesity, diabetes, inflammatory bowel disease, and colon cancer. Microbiota-based strategies have excellent clinical potential in the treatment of repetitive and refractory diseases; however, the underlying regulatory mechanisms remain elusive. Identification of the internal regulatory mechanism of the gut microbiome and the interaction mechanisms involving bacteria-host is essential to achieve precise control of the gut microbiome and obtain effective clinical data. Gut bacteria-derived extracellular vesicles (GBEVs) are lipid bilayer nanoparticles secreted by the gut microbiota and are considered key players in bacteria-bacteria and bacteria-host communication. This review focusses on the role of GBEVs in gut microbiota interactions and bacteria-host communication, and the potential clinical applications of GBEVs.

A sustainable and efficient recycling strategy of feather waste into keratin peptides with antimicrobial activity

The study aimed to propose an efficient and eco-friendly strategy to improve the utilization of feather waste and converting it into high-valued antimicrobial products. Under the synergistic effect of instant catapult steam explosion (ICSE) (1.5 MPa-120 s), over 90% of chicken feather powder (CFP) was degraded into soluble peptides via keratinolysis within 3 h, about 90% of which were smaller than 3 kDa, indicating an overwhelming advantage than general proteolysis. Importantly, the keratinolysis hydrolysate of CFP was able to inhibit E. coli growth, among which the fraction < 3 kDa exhibited highest antimicrobial activity with a minimal inhibitory concentration of 30 mg/mL. Compared to other fractions, the fraction < 3 kDa contained higher content of hydrophobic amino acids (364.11 mg/g), in which about 79% of peptides had more than 60% hydrophobic ratio, potentially contributing to its antimicrobial activity. ICSE-keratinolysis process holds potential in reducing both protein resource waste and environmental pollution by valorizing feathers into antimicrobial product.

Multiple roles of ribosomal antimicrobial peptides in tackling global antimicrobial resistance

In the last century, conventional antibiotics have played a significant role in global healthcare. Antibiotics support the body in controlling bacterial infection and simultaneously increase the tendency of drug resistance. Consequently, there is a severe concern regarding the regression of the antibiotic era. Despite the use of antibiotics, host defence systems are vital in fighting infectious diseases. In fact, the expression of ribosomal antimicrobial peptides (AMPs) has been crucial in the evolution of innate host defences and has been irreplaceable to date. Therefore, this valuable source is considered to have great potential in tackling the antimicrobial resistance (AMR) crisis. Furthermore, the possibility of bacterial resistance to AMPs has been intensively investigated. Here, we summarize all aspects related to the multiple applications of ribosomal AMPs and their derivatives in combating AMR.

Outer Membrane Vesicles Protect Gram-Negative Bacteria against Host Defense Peptides

Host defense peptides (HDPs) are part of the innate immune system and constitute a first line of defense against invading pathogens. They possess antimicrobial activity against a broad spectrum of pathogens. However, pathogens have been known to adapt to hostile environments. Therefore, the bacterial response to treatment with HDPs was investigated. Previous observations suggested that sublethal concentrations of HDPs increase the release of outer membrane vesicles (OMVs) in Escherichia coli. First, the effects of sublethal treatment with HDPs CATH-2, PMAP-36, and LL-37 on OMV release of several Gram-negative bacteria were analyzed. Treatment with PMAP-36 and CATH-2 induced release of OMVs, but treatment with LL-37 did not. The OMVs were further characterized with respect to morphological properties. The HDP-induced OMVs often had disc-like shapes. The beneficial effect of bacterial OMV release was studied by determining the susceptibility of E. coli toward HDPs in the presence of OMVs. The minimal bactericidal concentration was increased in the presence of OMVs. It is concluded that OMV release is a means of bacteria to dispose of HDP-affected membrane. Furthermore, OMVs act as a decoy for HDPs and thereby protect the bacterium. IMPORTANCE Antibiotic resistance is a pressing problem and estimated to be a leading cause of mortality by 2050. Antimicrobial peptides, also known as host defense peptides (HDPs), and HDP-derived antimicrobials have potent antimicrobial activity and high potential as alternatives to antibiotics due to low resistance development. Some resistance mechanisms have developed in bacteria, and complete understanding of bacterial defense against HDPs will aid their use in the clinic. This study provides insight into outer membrane vesicles (OMVs) as potential defense mechanisms against HDPs, which will allow anticipation of unforeseen resistance to HDPs in clinical use and possibly prevention of bacterial resistance by the means of OMVs.

Recombinant active Peptides and their Therapeutic functions

Recombinant active peptides are utilized as diagnostic and biotherapeutics in various maladies and as bacterial growth inhibitors in the food industry. This consequently stimulated the need for recombinant peptides' production, which resulted in about 19 approved biotech peptides of 1-100 amino acids commercially available. While most peptides have been produced by chemical synthesis, the production of lengthy and complicated peptides comprising natural amino acids has been problematic with low quantity. Recombinant peptide production has become very vital, cost-effective, simple, environmentally friendly with satisfactory yields. Several reviews have focused on discussing expression systems, advantages, disadvantages, and alternatives strategies. Additionally, the information on the antimicrobial activities and other functions of multiple recombinant peptides is challenging to access and is scattered in literature apart from the food and drug administration (FDA) approved ones. From the reports that come to our knowledge, there is no existing review that offers substantial information on recombinant active peptides developed by researchers and their functions. This review provides an overview of some successfully produced recombinant active peptides of ≤100 amino acids by focusing on their antibacterial, antifungal, antiviral, anticancer, antioxidant, antimalarial, and immune-modulatory functions. It also elucidates their modes of expression that could be adopted and applied in future investigations. We expect that the knowledge available in this review would help researchers involved in recombinant active peptide development for therapeutic uses and other applications.

Porcine Myeloid Antimicrobial Peptides: A Review of the Activity and Latest Advances

Traditional antibiotics have made great contributions to human health and animal husbandry since the discovery of penicillin in 1928, but bacterial resistance and drug residues are growing threats to global public health due to the long-term uncontrolled application of antibiotics. There is a critical need to develop new antimicrobial drugs to replace antibiotics. Antimicrobial peptides (AMPs) are distributed in all kingdoms of life, presenting activity against pathogens as well as anticancer, anti-inflammatory, and immunomodulatory activities; consequently, they have prospects as new potential alternatives to antibiotics. Porcine myeloid antimicrobial peptides (PMAPs), the porcine cathelicidin family of AMPs, have been reported in the literature in recent years. PMAPs have become an important research topic due to their strong antibacterial activity. This review focuses on the universal trends in the biochemical parameters, structural characteristics and biological activities of PMAPs.

Opossum Cathelicidins Exhibit Antimicrobial Activity Against a Broad Spectrum of Pathogens Including West Nile Virus

This study aimed to characterize cathelicidins from the gray short-tailed opossum in silico and experimentally validate their antimicrobial effects against various pathogenic bacteria and West Nile virus (WNV). Genome-wide in silico analysis against the current genome assembly of the gray short-tailed opossum yielded 56 classical antimicrobial peptides (AMPs) from eight different families, among which 19 cathelicidins, namely ModoCath1 – 19, were analyzed in silico to predict their antimicrobial domains and three of which, ModoCath1, -5, and -6, were further experimentally evaluated for their antimicrobial activity, and were found to exhibit a wide spectrum of antimicroial effects against a panel of gram-positive and gram-negative bacterial strains. In addition, these peptides displayed low-to-moderate cytotoxicity in mammalian cells as well as stability in serum and various salt and pH conditions. Circular dichroism analysis of the spectra resulting from interactions between ModoCaths and lipopolysaccharides (LPS) showed formation of a helical structure, while a dual-dye membrane disruption assay and scanning electron microscopy analysis revealed that ModoCaths exerted bactericidal effects by causing membrane damage. Furthermore, ModoCath5 displayed potent antiviral activity against WNV by inhibiting viral replication, suggesting that opossum cathelicidins may serve as potentially novel antimicrobial endogenous substances of mammalian origin, considering their large number. Moreover, analysis of publicly available RNA-seq data revealed the expression of eight ModoCaths from five different tissues, suggesting that gray short-tailed opossums may be an interesting source of cathelicidins with diverse characteristics.

Enhanced Expression and Functional Characterization of the Recombinant Putative Lysozyme-PMAP36 Fusion Protein

The porcine myeloid antimicrobial peptide (PMAP), one of the cathelicidin family members, contains small cationic peptides with amphipathic properties. We used a putative lysozyme originated from the bacteriophage P22 (P22 lysozyme) as a fusion partner, which was connected to the N-terminus of the PMAP36 peptide, to markedly increase the expression levels of recombinant PMAP36. The PMAP36-P22 lysozyme fusion protein with high solubility was produced in Escherichia coli. The final purified yield was approximately 1.8 mg/L. The purified PMAP36-P22 lysozyme fusion protein exhibited antimicrobial activity against both Gram-negative and Gram-positive bacteria (Staphylococcus aureus, Salmonella enterica serovar Typhimurium, Pseudomonas aeruginosa, and Bacillus subtilis). Furthermore, we estimated its hemolytic activity against pig erythrocytes as 6% at the high concentration (128 μM) of the PMAP36-P22 lysozyme fusion protein. Compared with the PMAP36 peptide (12%), our fusion protein exhibited half of the hemolytic activity. Overall, our recombinant PMAP36-P22 lysozyme fusion protein sustained the antimicrobial activity with the lower hemolytic activity associated with the synthetic PMAP36 peptide. This study suggests that the PMAP36-P22 lysozyme fusion system could be a crucial addition to the plethora of novel antimicrobials.

Cathelicidins PMAP-36, LL-37 and CATH-2 are similar peptides with different modes of action

Host defense peptides (HDPs) play a pivotal role in innate immunity and have, in addition to antimicrobial activity, also important immunomodulatory functions. Bacteria are less likely to develop resistance against HDPs because these peptides target and kill bacteria in multiple ways, as well as modulate the immune system. Therefore, HDPs, and derivatives thereof, are promising alternatives to traditional antibiotics. Hardly anything is known about the immunomodulatory functions of porcine cathelicidin PMAP-36. In this study, we aimed to determine both antibacterial and immunomodulatory activities of PMAP-36 comparing the properties of PMAP-36 analogs with two well-studied peptides, human LL-37 and chicken CATH-2. Transmission electron microscopy revealed different killing mechanisms of E. coli for PMAP-36, CATH-2 and LL-37. LL-37 binds LPS very weakly in contrast to PMAP-36, but it inhibits LPS activation of macrophages the strongest. The first 11 amino acids of the N-terminal side of PMAP-36 are dispensable for E. coli killing, LPS-neutralization and binding. Deletion of four additional amino acids resulted in a strong decrease in activity. The activity of full length PMAP-36 was not affected by monomerization, whereas the shorter analogs require dimerization for proper immunomodulatory activity but not for their antibacterial activity.

Antimicrobial peptides: Promising alternatives in the post feeding antibiotic era

Antimicrobial peptides (AMPs), critical components of the innate immune system, are widely distributed throughout the animal and plant kingdoms. They can protect against a broad array of infection-causing agents, such as bacteria, fungi, parasites, viruses, and tumor cells, and also exhibit immunomodulatory activity. AMPs exert antimicrobial activities primarily through mechanisms involving membrane disruption, so they have a lower likelihood of inducing drug resistance. Extensive studies on the structure-activity relationship have revealed that net charge, hydrophobicity, and amphipathicity are the most important physicochemical and structural determinants endowing AMPs with antimicrobial potency and cell selectivity. This review summarizes the recent advances in AMPs development with respect to characteristics, structure-activity relationships, functions, antimicrobial mechanisms, expression regulation, and applications in food, medicine, and animals.

Human apolipoprotein E as a reservoir of cryptic bioactive peptides: The case of ApoE 133-167

Bioactive peptides derived from the receptor-binding region of human apolipoprotein E have previously been reported. All these peptides, encompassing fragments of this region or designed on the basis of short repeated cationic sequences identified in the same region, show toxic activities against a broad spectrum of bacteria and interesting immunomodulatory effects. However, the ability of these molecules to exert antibiofilm properties has not been described so far. In the present work, we report the characterization of a novel peptide, corresponding to residues 133 to 167 of human apolipoprotein E, here named ApoE (133-167). This peptide, besides presenting interesting properties comparable with those reported for other ApoE-derived peptides, such as a direct killing activity against a broad spectrum of bacteria or the ability to downregulate lipopolysaccharide-induced cytokine release, is also endowed with significant antibiofilm properties. Indeed, the peptide is able to strongly affect the formation of the extracellular matrix and also the viability of encapsulated bacteria. Noteworthy, ApoE (133-167) is not toxic toward human and murine cell lines and is able to assume ordered conformations in the presence of membrane mimicking agents. Taken together, collected evidences about biological and structural properties of ApoE (133-167) open new perspectives in the design of therapeutic agents based on human-derived bioactive peptides.

Genomewide Analysis of the Antimicrobial Peptides in Python bivittatus and Characterization of Cathelicidins with Potent Antimicrobial Activity and Low Cytotoxicity

In this study, we sought to identify novel antimicrobial peptides (AMPs) in Python bivittatus through bioinformatic analyses of publicly available genome information and experimental validation. In our analysis of the python genome, we identified 29 AMP-related candidate sequences. Of these, we selected five cathelicidin-like sequences and subjected them to further in silico analyses. The results showed that these sequences likely have antimicrobial activity. The sequences were named Pb-CATH1 to Pb-CATH5 according to their sequence similarity to previously reported snake cathelicidins. We predicted their molecular structure and then chemically synthesized the mature peptide for three putative cathelicidins and subjected them to biological activity tests. Interestingly, all three peptides showed potent antimicrobial effects against Gram-negative bacteria but very weak activity against Gram-positive bacteria. Remarkably, ΔPb-CATH4 showed potent activity against antibiotic-resistant clinical isolates and also was observed to possess very low hemolytic activity and cytotoxicity. ΔPb-CATH4 also showed considerable serum stability. Electron microscopic analysis indicated that ΔPb-CATH4 exerts its effects via toroidal pore preformation. Structural comparison of the cathelicidins identified in this study to previously reported ones revealed that these Pb-CATHs are representatives of a new group of reptilian cathelicidins lacking the acidic connecting domain. Furthermore, Pb-CATH4 possesses a completely different mature peptide sequence from those of previously described reptilian cathelicidins. These new AMPs may be candidates for the development of alternatives to or complements of antibiotics to control multidrug-resistant pathogens.

Cathelicidins Inhibit Escherichia coli-Induced TLR2 and TLR4 Activation in a Viability-Dependent Manner

Activation of the immune system needs to be tightly regulated to provide protection against infections and, at the same time, to prevent excessive inflammation to limit collateral damage to the host. This tight regulation includes regulating the activation of TLRs, which are key players in the recognition of invading microbes. A group of short cationic antimicrobial peptides, called cathelicidins, have previously been shown to modulate TLR activation by synthetic or purified TLR ligands and may play an important role in the regulation of inflammation during infections. However, little is known about how these cathelicidins affect TLR activation in the context of complete and viable bacteria. In this article, we show that chicken cathelicidin-2 kills Escherichia coli in an immunogenically silent fashion. Our results show that chicken cathelicidin-2 kills E. coli by permeabilizing the bacterial inner membrane and subsequently binds the outer membrane-derived lipoproteins and LPS to inhibit TLR2 and TLR4 activation, respectively. In addition, other cathelicidins, including human, mouse, pig, and dog cathelicidins, which lack antimicrobial activity under cell culture conditions, only inhibit macrophage activation by nonviable E. coli In total, this study shows that cathelicidins do not affect immune activation by viable bacteria and only inhibit inflammation when bacterial viability is lost. Therefore, cathelicidins provide a novel mechanism by which the immune system can discriminate between viable and nonviable Gram-negative bacteria to tune the immune response, thereby limiting collateral damage to the host and the risk for sepsis.

High Specific Selectivity and Membrane-Active Mechanism of Synthetic Cationic Hybrid Antimicrobial Peptides Based on the Peptide FV7

Hybrid peptides integrating different functional domains of peptides have many advantages, such as remarkable antimicrobial activity, lower hemolysis and ideal cell selectivity, compared with natural antimicrobial peptides. FV7 (FRIRVRV-NH₂), a consensus amphiphilic sequence was identified as being analogous to host defense peptides. In this study, we designed a series of hybrid peptides FV7-LL-37 (17-29) (FV-LL), FV7-magainin 2 (9-21) (FV-MA) and FV7-cecropin A (1-8) (FV-CE) by combining the FV7 sequence with the small functional sequences LL-37 (17-29) (LL), magainin 2 (9-21) (MA) and cecropin A (1-8) (CE) which all come from well-described natural peptides. The results demonstrated that the synthetic hybrid peptides, in particular FV-LL, had potent antibacterial activities over a wide range of Gram-negative and Gram-positive bacteria with lower hemolytic activity than other peptides. Furthermore, fluorescent spectroscopy indicated that the hybrid peptide FV-LL exhibited marked membrane destruction by inducing outer and inner bacterial membrane permeabilization, while scanning electron microscopy (SEM) and transmission electron microscopy (TEM) demonstrated that FV-LL damaged membrane integrity by disrupting the bacterial membrane. Inhibiting biofilm formation assays also showed that FV-LL had similar anti-biofilm activity compared with the functional peptide sequence FV7. Synthetic cationic hybrid peptides based on FV7 could provide new models for combining different functional domains and demonstrate effective avenues to screen for novel antimicrobial agents.

Interspecies cathelicidin comparison reveals divergence in antimicrobial activity, TLR modulation, chemokine induction and regulation of phagocytosis

Cathelicidins are short cationic peptides initially described as antimicrobial peptides, which can also modulate the immune system. Because most findings have been described in the context of human LL-37 or murine CRAMP, or have been investigated under varying conditions, it is unclear which functions are cathelicidin specific and which functions are general cathelicidin properties. This study compares 12 cathelicidins from 6 species under standardized conditions to better understand the conservation of cathelicidin functions. Most tested cathelicidins had strong antimicrobial activity against E. coli and/or MRSA. Interestingly, while more physiological culture conditions limit the antimicrobial activity of almost all cathelicidins against E. coli, activity against MRSA is enhanced. Seven out of 12 cathelicidins were able to neutralize LPS and another 7 cathelicidins were able to neutralize LTA; however, there was no correlation found with LPS neutralization. In contrast, only 4 cathelicidins enhanced DNA-induced TLR9 activation. In conclusion, these results provide new insight in the functional differences of cathelicidins both within and between species. In addition, these results underline the importance not to generalize cathelicidin functions and indicates that caution should be taken in extrapolating results from LL-37- or CRAMP-related studies to other animal settings.

Do antimicrobial peptides PR-39, PMAP-36 and PMAP-37 have any effect on bacterial growth and quality of liquid-stored boar semen?

The use of antimicrobial peptides (AMP) has become one of the most promising alternatives to the use of antibiotics (Abs) in semen extender's formulation to overcome the increasing bacterial resistance to antibiotics. However, AMP may impair boar sperm quality, so that their deleterious effects might be higher than their effectiveness against bacteria. Thus, the aim of this study was to determine whether three different AMP, the proline-arginine-rich antimicrobial peptide PR-39 (PR-39), and the porcine myeloid antimicrobial peptides 36 (PMAP-36) and 37 (PMAP-37) had any effect upon boar sperm quality and bacterial growth. For this purpose, three different concentrations of each peptide (1 μM, 10 μM and 20 μM for PR-39 and 0.5 μM, 1 μM and 3 μM for PMAP-36 and PMAP-37) were added to 2 mL of a pool of extended semen with BTS without Abs; two controls, one without AMPs and Abs, and the other with Abs only were used for each peptide (n = 3). Total (TMOT) and progressive (PMOT) sperm motility, sperm viability and bacterial concentration were assessed before the addition of each AMP or Abs and at 1, 3, 6, 8 and 10 days post-addition. For each AMP, results revealed a drop in the TMOT and PMOT in all treatments and controls. In regard to sperm viability, while PR-39 at 10 μM maintained it in values similar to those of the control with Abs and PMAP-36 kept also the sperm viability in a similar fashion to the treatment with Abs, PMAP-37 was more effective in keeping sperm viability than controls (P < 0.05). Whereas PR-39 at 20 μM and PMAP-37 at 3 μM were quite effective in controlling bacterial load, PMAP-36 did not avoid bacterial growth at any concentration tested. In conclusion, taking all results together, PMAP-37 seems to be a suitable candidate to replace Abs in extended semen, as it hardly impairs sperm viability and controls the bacterial load. Nevertheless, further studies are still required to improve its effectiveness.

Antimicrobial activity, improved cell selectivity and mode of action of short PMAP-36-derived peptides against bacteria and Candida

Antimicrobial peptides (AMPs) have recently attracted a great deal of attention as promising antibiotic candidates, but some obstacles such as toxicity and high synthesis cost must be addressed before developing them further. For developing short peptides with improved cell selectivity, we designed a series of modified PMAP-36 analogues. Antimicrobial assays showed that decreasing chain length in a certain range retained the high antimicrobial activity of the parental peptide and reduced hemolysis. The 18-mer peptide RI18 exhibited excellent antimicrobial activity against both bacteria and fungi, and its hemolytic activity was observably lower than PMAP-36 and melittin. The selectivity indexes of RI18 against bacteria and fungi were improved approximately 19-fold and 108-fold, respectively, compared to PMAP-36. In addition, serum did not affect the antibacterial activity of RI18 against E. coli but inhibited the antifungal efficiency against C. albicans. Flow cytometry and electron microscopy observation revealed that RI18 killed microbial cells primarily by damaging membrane integrity, leading to whole cell lysis. Taken together, these results suggest that RI18 has potential for further therapeutic research against frequently-encountered bacteria and fungi. Meanwhile, modification of AMPs is a promising strategy for developing novel antimicrobials to overcome drug-resistance.

Antimicrobial peptides in echinoderm host defense

Antimicrobial peptides (AMPs) are important effector molecules in innate immunity. Here we briefly summarize characteristic traits of AMPs and their mechanisms of antimicrobial activity. Echinoderms live in a microbe-rich marine environment and are known to express a wide range of AMPs. We address two novel AMP families from coelomocytes of sea urchins: cysteine-rich AMPs (strongylocins) and heterodimeric AMPs (centrocins). These peptide families have conserved preprosequences, are present in both adults and pluteus stage larvae, have potent antimicrobial properties, and therefore appear to be important innate immune effectors. Strongylocins have a unique cysteine pattern compared to other cysteine-rich peptides, which suggests a novel AMP folding pattern. Centrocins and SdStrongylocin 2 contain brominated tryptophan residues in their native form. This review also includes AMPs isolated from other echinoderms, such as holothuroidins, fragments of beta-thymosin, and fragments of lectin (CEL-III). Echinoderm AMPs are crucial molecules for the understanding of echinoderm immunity, and their potent antimicrobial activity makes them potential precursors of novel drug leads.

Importance of Tryptophan in Transforming an Amphipathic Peptide into a Pseudomonas aeruginosa-Targeted Antimicrobial Peptide

Here, we found that simple substitution of amino acids in the middle position of the hydrophobic face of an amphipathic peptide RI16 with tryptophan (T9W) considerably transformed into an antimicrobial peptide specifically targeting Pseudomonas aeruginosa. Minimal inhibitory concentration (MIC) results demonstrated that T9W had a strong and specifically antimicrobial activity against P. aeruginosa, including antibiotic-resistant strains, but was not active against Escherichia coli, Salmonella typhimurium, Staphylococcus aureus and Staphyfococcus epidermidis. Fluorescent spectroscopic assays indicated that T9W interacted with the membrane of P. aeruginosa, depolarizing the outer and the inner membrane of bacterial cells. Salt susceptibility assay showed that T9W still maintained its strong anti-pseudomonas activity in the presence of salts at physiological concentrations, and in hemolytic and MTT assays T9W also showed no toxicity against human blood cells and macrophages. In vivo assay demonstrated that T9W also displayed no toxicity to Chinese Kun Ming (KM) mice. Furthermore, the strong antibiofilm activity was also observed with the peptide T9W, which decreased the percentage of biomass formation in a dose-dependent manner. Overall, these findings indicated that design of single-pathogen antimicrobial agents can be achieved by simple amino acid mutation in naturally occurring peptide sequences and this study suggested a model of optimization/design of anti-pseudomonas drugs in which the tryptophan residue was a conserved element.

In vitro and in vivo activity of the dimer of PMAP-36 expressed in Pichia pastoris

The antimicrobial peptide PMAP-36 exists as a homodimer stabilized by an intermolecular disulfide bridge. The dimer of PMAP-36 exhibits a potent and rapid microbicidal activity against a wide spectrum of microorganisms. The gene encoding the antiparallel dimer (PMAP-36)2 was designed and codon-optimized according to bias of Pichia pastoris. The gene was then expressed in the P. pastoris strain GS115. The concentration of the recombinant product reached 106 mg/l. In vitro activity assays indicated that the recombinant peptide showed antimicrobial activities against Gram-positive and Gram-negative bacteria but did not cause hemolysis of chicken erythrocytes. Subsequently, 120 7-day-old male Arbor Acres broilers were used to evaluate the in vivo activities of the peptide. A prophylactic dose of ciprofloxacin lactate was supplemented as the control. The results showed that recombinant (PMAP-36)2 significantly increased the serum IgM content of the birds (p < 0.05). The recombinant peptide significantly increased the amounts of Bifidobacterium and decreased the amount of Escherichia coli cells in the ceca of the experimental birds (p < 0.05). The results obtained in the present study indicate that the recombinant (PMAP-36)2 has a potent in vitro and in vivo activity and can be used as an alternative to antibiotic treatment.

Design of embedded-hybrid antimicrobial peptides with enhanced cell selectivity and anti-biofilm activity

Antimicrobial peptides have attracted considerable attention because of their broad-spectrum antimicrobial activity and their low prognostic to induce antibiotic resistance which is the most common source of failure in bacterial infection treatment along with biofilms. The method to design hybrid peptide integrating different functional domains of peptides has many advantages. In this study, we designed an embedded-hybrid peptide R-FV-I16 by replacing a functional defective sequence RR7 with the anti-biofilm sequence FV7 embedded in the middle position of peptide RI16. The results demonstrated that the synthetic hybrid the peptide R-FV-I16 had potent antimicrobial activity over a wide range of Gram-negative and Gram-positive bacteria, as well as anti-biofilm activity. More importantly, R-FV-I16 showed lower hemolytic activity and cytotoxicity. Fluorescent assays demonstrated that R-FV-I16 depolarized the outer and the inner bacterial membranes, while scanning electron microscopy and transmission electron microscopy further indicated that this peptide killed bacterial cells by disrupting the cell membrane, thereby damaging membrane integrity. Results from SEM also provided evidence that R-FV-I16 inherited anti-biofilm activity from the functional peptide sequence FV7. Embedded-hybrid peptides could provide a new pattern for combining different functional domains and showing an effective avenue to screen for novel antimicrobial agents.

Antimicrobial properties and membrane-active mechanism of a potential α-helical antimicrobial derived from cathelicidin PMAP-36

Antimicrobial peptides (AMPs), which present in the non-specific immune system of organism, are amongst the most promising candidates for the development of novel antimicrobials. The modification of naturally occurring AMPs based on their residue composition and distribution is a simple and effective strategy for optimization of known AMPs. In this study, a series of truncated and residue-substituted derivatives of antimicrobial peptide PMAP-36 were designed and synthesized. The 24-residue truncated peptide, GI24, displayed antimicrobial activity comparable to the mother peptide PMAP-36 with MICs ranging from 1 to 4 µM, which is lower than the MICs of bee venom melittin. Although GI24 displayed high antimicrobial activity, its hemolytic activity was much lower than melittin, suggesting that GI24 have optimal cell selectivity. In addition, the crucial site of GI24 was identified through single site-mutation. An amino acid with high hydrophobicity at position 23 played an important role in guaranteeing the high antimicrobial activity of GI24. Then, lipid vesicles and whole bacteria were employed to investigate the membrane-active mechanisms. Membrane-simulating experiments showed that GI24 interacted strongly with negatively charged phospholipids and weakly with zwitterionic phospholipids, which corresponded well with the data of its biological activities. Membrane permeabilization and flow cytometry provide the evidence that GI24 killed microbial cells by permeabilizing the cell membrane and damaging membrane integrity. GI24 resulted in greater cell morphological changes and visible pores on cell membrane as determined using scanning electron microscopy (SEM) and transmission electron microscope (TEM). Taken together, the peptide GI24 may provide a promising antimicrobial agent for therapeutic applications against the frequently-encountered bacteria.

Design of imperfectly amphipathic α-helical antimicrobial peptides with enhanced cell selectivity

Antimicrobial peptides (AMPs), which are produced by multicellular organisms as a defense mechanism against competing pathogenic microbes, appear to be excellent candidates for the development of novel antimicrobial agents. Amphipathicity is traditionally believed to be crucial to the de novo design or systematic optimization of AMPs. In this study, we designed a series of short α-helical AMPs with imperfect amphipathicity to augment the arsenal of strategies and to gain further insights into their antimicrobial and hemolytic activity. These imperfectly amphipathic α-helical AMPs were designed by replacing the paired charged amino acid residues on the polar face of an amphipathic peptide with tryptophan residues on the basis of α-helical protein folding principles. PRW4, an imperfectly amphipathic α-helical AMP with hydrogen bonds formed by paired tryptophan residues, was observed to be more selective towards bacterial cells than toward human red blood cells. PRW4 was also effective against Gram-negative and Gram-positive bacteria, and fluorescence spectroscopy, flow cytometry, scanning electron microscopy and transmission electron microscopy indicated that PRW4 killed microbial cells by permeabilizing the cell membrane and damaging their membrane integrity. Therefore, disruptive amphipathicity has excellent potential for the rational design and optimization of AMPs with promising antimicrobial activities.

Antimicrobial activity of a halocidin-derived peptide resistant to attacks by proteases

Cationic antimicrobial peptides (AMPs) have attracted a great deal of interest as a promising candidate for a novel class of antibiotics that might effectively treat recalcitrant infections caused by a variety of microbes that are resistant to currently available drugs. However, the AMPs are inherently limited in that they are inevitably susceptible to attacks by proteases generated by human and pathogenic microbes; this vulnerability severely hinders their pharmaceutical use in human therapeutic protocols. In this study, we report that a halocidin-derived AMP, designated HG1, was found to be resistant to proteolytic degradation. As a result of its unique structural features, HG1 proved capable of preserving its antimicrobial activity after incubation with trypsin, chymotrypsin, and human matrix metalloprotease 7 (MMP-7). Additionally, HG1 was observed to exhibit profound antimicrobial activity in the presence of fluid from human skin wounds or proteins extracted from the culture supernatants of Staphylococcus aureus and Pseudomonas aeruginosa. Greater understanding of the structural motifs of HG1 required for its protease resistance might provide feasible ways to solve the problems intrinsic to the development of an AMP-based antibiotic.

Antimicrobial peptides isolated from the blood of farm animals

The development of antimicrobial resistance by pathogenic bacteria has fuelled the search for alternatives to conventional antibiotics. Endogenous antimicrobial peptides have the potential to be used as new antimicrobial substances because they have low minimum inhibitory concentration in vitro, have broad-spectrum activity, neutralise lipopolysaccharides, promote wound healing and have synergistic effects with conventional antibiotics. Farm animals, in particular the blood that is a by-product of the meat and poultry industries, are an abundant, and currently underutilised, source of such antimicrobial peptides. These antimicrobial peptides could be isolated and developed into high-value products such as biopreservatives, topical neutraceutical products and pharmaceuticals. There have been some clinical trials of antimicrobial peptides as pharmaceutical products, but up to now, the trials have shown disappointing results. Further research and development is still needed before such peptides can be commercialised and full advantage taken of this waste product of the meat and poultry industries.

Porcine host defense peptides: expanding repertoire and functions

Host defense peptides (HDPs) are a large group of innate immune effectors that are also termed antimicrobial peptides. Because of the rapid progress that has been made in completing several animal genomes, many HDPs have been systemically defined using bioinformatic analysis and partially characterized using reverse genomic approaches. In pigs, about 30 HDPs have been identified and partially characterized relative to structure and function. Antimicrobial activity of porcine HDPs has been extensively evaluated against a broad spectrum of microorganisms in vitro and evaluated for their protective role in vivo. Increasing evidence indicates that HDPs are functionally differentiated during posttranslational and postsecretory processing, and that the structural units for antimicrobial and immunoregulatory functions are separate. These findings suggest promising new avenues for therapeutic drug design based on HDPs, including porcine HDPs. This review summarizes and discusses advances in porcine HDPs research during the last decade with an emphasis on the rapidly expanding profiles and biological functions.

Structural features of distinctin affecting peptide biological and biochemical properties

The antimicrobial peptide distinctin consists of two peptide chains linked by a disulfide bridge; it presents a peculiar fold in water resulting from noncovalent dimerization of two heterodimeric molecules. To investigate the contribution of each peptide chain and the S-S bond to distinctin biochemical properties, different monomeric and homodimeric peptide analogues were synthesized and comparatively evaluated with respect to the native molecule. Our experiments demonstrate that the simultaneous occurrence of both peptide chains and the disulfide bond is essential for the formation of the quaternary structure of distinctin in aqueous media, able to resist protease action. In contrast, distinctin and monomeric and homodimeric analogues exhibited comparable antimicrobial activities, suggesting only a partial contribution of the S-S bond to peptide killing effectiveness. Relative bactericidal properties paralleled liposome permeabilization results, definitively demonstrating that microbial membranes are the main target of distinctin activity. Various biophysical experiments performed in membrane-mimicking media, before and after peptide addition, provided information about peptide secondary structure, lipid bilayer organization, and lipid-peptide orientation with respect to membrane surface. These data were instrumental in the generation of putative models of peptide-lipid supramolecular pore complexes.

Inherent antibacterial activity of a peptide-based beta-hairpin hydrogel

Among several important considerations for implantation of a biomaterial, a main concern is the introduction of infection. We have designed a hydrogel scaffold from the self-assembling peptide, MAX1, for tissue regeneration applications whose surface exhibits inherent antibacterial activity. In experiments where MAX1 gels are challenged with bacterial solutions ranging in concentrations from 2 x 10(3) colony forming units (CFUs)/dm2 to 2 x 10(9) CFUs/dm2, gel surfaces exhibit broad-spectrum antibacterial activity. Results show that the hydrogel surface is active against Gram-positive (Staphylococcus epidermidis, Staphylococcus aureus, and Streptococcus pyogenes) and Gram-negative (Klebsiella pneumoniae and Escherichia coli) bacteria, all prevalent in hospital settings. Live-dead assays employing laser scanning confocal microscopy show that bacteria are killed when they engage the surface. In addition, the surface of MAX1 hydrogels was shown to cause inner and outer membrane disruption in experiments that monitor the release of beta-galactosidase from the cytoplasm of lactose permease-deficient E. coli ML-35. These data suggest a mechanism of antibacterial action that involves membrane disruption that leads to cell death upon cellular contact with the gel surface. Although the hydrogel surface exhibits bactericidal activity, co-culture experiments indicate hydrogel surfaces show selective toxicity to bacterial versus mammalian cells. Additionally, gel surfaces are nonhemolytic toward human erythrocytes, which maintain healthy morphologies when in contact with the surface. These material attributes make MAX1 gels attractive candidates for use in tissue regeneration, even in nonsterile environments.

Determination of the antibacterial and lipopolysaccharide-neutralizing regions of guinea pig neutrophil cathelicidin peptide CAP11

Previously, we revealed that a cationic antibacterial polypeptide of 11 kDa (CAP11), a member of the cathelicidins isolated from guinea pig neutrophils, exhibits not only potent antibacterial activity but also lipopolysaccharide (LPS)-neutralizing activity. In this study, to determine the biologically active regions of CAP11, we isolated or synthesized the partial peptides of CAP11 and evaluated their antibacterial and LPS-neutralizing activities. Although CAP11 has a unique homodimeric structure with a disulfide bridge, the biological activities of dimeric and monomeric forms of CAP11 were almost the same. Moreover, the G(1)-E(33) peptide of CAP11 showed the same activities as CAP11, whereas the C-terminal region (Y(34) to I(43)) possessed no biological activities. In addition, the three 18-mer peptides (G(1)-R(18), T(9)-K(26), and L(16)-E(33)) with overlapping sequences were synthesized, and their activities were determined. The three 18-mer peptides retained the antibacterial activities, and G(1)-R(18) was the most potent. In contrast, the LPS-neutralizing activities of these peptides were markedly reduced. Together, these observations indicate that the active region with antibacterial activity is localized at G(1) to R(18) of CAP11, whereas longer sequences (such as G(1) to E(33)) would be required for the expression of LPS-neutralizing activity. Furthermore, the C-terminal region (Y(34) to I(43)) and a disulfide bridge are not essential for the antibacterial and LPS-neutralizing activities of CAP11.