Penaeidins, a family of antimicrobial peptides from penaeid shrimp (Crustacea, Decapoda)

The production of antimicrobial peptides represents a first-line host defense mechanism of innate immunity that is widespread in nature. Only recently such effectors were isolated in crustacean species, whereas numerous antimicrobial peptides have been characterized from other arthropods, both insects and chelicerates. This review presents findings on a family of antimicrobial peptides, named penaeidins, isolated from the shrimp Penaeus vannamei. Their structure and antimicrobial properties as well as their immune function will be discussed through analyses of penaeidin gene expression and peptide distribution upon microbial challenge. Received 21 January 2000; received after revision 10 March 2000; accepted 10 March 2000     

The role of mammalian antimicrobial peptides and proteins in awakening of innate host defenses and adaptive immunity

Since we live in a dirty environment, we have developed many host defenses to contend with microorganisms. The epithelial lining of our skin, gastrointestinal tract and bronchial tree produces a number of antibacterial peptides, and our phagocytic neutrophils rapidly ingest and enzymatically degrade invading organisms, as well as produce peptides and enzymes with antimicrobial activities. Some of these antimicrobial moieties also appear to alert host cells involved in both innate host defense and adaptive immune responses. The epithelial cells are a source of constitutively produced beta defensin (HBD1) and proinflammatory cytokine-inducible beta defensins (HBD2 and -3) and cathelicidin (LL37). The neutrophils-derived antimicrobial peptides are released on demand from their cytoplasmic granules. They include the enzymes cathepsin G and chymase, azurocidin, a defensins and cathelicidin. In contrast, C5a and C3b are produced by activation of the serum complement cascade. The antimicrobial moieties direct the migration and activate target cells by interacting with selected G-protein-coupled seven-transmembrane receptors (GPCRs) on cell surfaces. The beta defensins interact with the CCR6 chemokine GPCRs, whereas cathelicidins interact with the low-affinity FPRL-1 receptors. The neutrophil-derived cathepsin G acts on the high-affinity FMLP receptor (GPCR) known as FPR, while the receptors for chymase and azurocidin have not been identified as yet. The serum-derived C5a uses a GPCR known as C5aR to mediate its chemotactic and cell-activating effects. Consequently, all these ligand-receptor interactions in addition to mediating chemotaxis also activate receptor-expressing cells to produce other mediators of inflammation.     

Antimicrobial and cytolytic peptides of venomous arthropods

As a response to invading microorganisms, the innate immune system of arthropods has evolved a complex arrangement of constitutive and inducible antimicrobial peptides that immediately destroy a large variety of pathogens. At the same time, venomous arthropods have developed an additional offensive system in their venom glands to subdue their prey items. In this complex venom system, several enzymes, low-molecular-mass compounds, neurotoxins, antimicrobial and cytolytic peptides interact together, resulting in extremely rapid immobilization and/or killing of prey or aggressors. This review provides an overview of antimicrobial peptides identified in the hemolymph of venomous arthropods, and especially of cytolytic peptides in their venom. For these peptides a dual role is proposed: acting as antimicrobials as well as increasing the potency of the venom by influencing excitable cells.Received 17 March 2003; received after revision 11 June 2003; accepted 17 June 2003     

Properties and mechanisms of action of naturally occurring antifungal peptides

Antimicrobial peptides are a vital component of the innate immune system of all eukaryotic organisms and many of these peptides have potent antifungal activity. They have potential application in the control of fungal pathogens that are a serious threat to both human health and food security. Development of antifungal peptides as therapeutics requires an understanding of their mechanism of action on fungal cells. To date, most research on antimicrobial peptides has focused on their activity against bacteria. Several antimicrobial peptides specifically target fungal cells and are not active against bacteria. Others with broader specificity often have different mechanisms of action against bacteria and fungi. This review focuses on the mechanism of action of naturally occurring antifungal peptides from a diverse range of sources including plants, mammals, amphibians, insects, crabs, spiders, and fungi. While antimicrobial peptides were originally proposed to act via membrane permeabilization, the mechanism of antifungal activity for these peptides is generally more complex and often involves entry of the peptide into the cell.     

Temporins, anti-infective peptides with expanding properties

Antimicrobial peptides are effector molecules of the innate immune response of all pluricellular organisms, providing them with first-line defence against pathogens. Amphibian skin secretions represent one of the richest natural sources for such peptide antibiotics, and temporins, a large family of antimicrobial peptides from frog skin, are among the smallest ones found in nature to date. Their functional role and modes of action have been described, along with their interesting and unique properties. These properties make temporins good molecules for an in-depth understanding of host defence peptides in general. Furthermore, they are attractive templates for the future design of new therapeutics against infectious diseases with new modes of action, urgently needed due to the increasing resistance of microorganisms to the available drugs. Received 8 November 2005; received after revision 19 December 2005; accepted 18 January 2006     

Intestinal epithelial barrier and mucosal immunity

The innate immune system plays a crucial role in maintaining the integrity of the intestine and protecting the host against a vast number of potential microbial pathogens from resident and transient gut microflora. Mucosal epithelial cells and Paneth cells produce a variety of antimicrobial peptides (defensins, cathelicidins, crytdinrelated sequence peptides, bactericidal/permeabilityincreasing protein, chemokine CCL20) and bacteriolytic enzymes (lysozyme, group IIA phospholipase A2) that protect mucosal surfaces and crypts containing intestinal stem cells against invading microbes. Many of the intestinal antimicrobial molecules have additional roles of attracting leukocytes, alarming the adaptive immune system or neutralizing proinflammatory bacterial molecules. Dysfunction of components of the innate immune system has recently been implicated in chronic inflammatory bowel diseases such as Crohn's disease and ulcerative colitis, illustrating the pivotal role of innate immunity in maintaining the delicate balance between immune tolerance and immune response in the gut.     

Host-defense peptides: from biology to therapeutic strategies

Primitive innate defense mechanisms in the form of gene-encoded antimicrobial peptides are now considered as potential candidates for the development of new therapeutics. They are well known for their function as the first protective barrier of all organisms against microbial infections. In addition, emerging studies reveal that they assist in modulating the host immune system. The biological properties of these host-defense peptides, their role in human health, their cell selectivity and related molecular mechanisms are discussed in this multi-author review along with the strategies to transform them or their peptidomimetics into clinically usable drugs.     

Multifunctional cationic host defence peptides and their clinical applications

With the rapid rise in the emergence of bacterial strains resistant to multiple classes of antimicrobial agents, there is an urgent need to develop novel antimicrobial therapies to combat these pathogens. Cationic host defence peptides (HDPs) and synthetic derivatives termed innate defence regulators (IDRs) represent a promising alternative approach in the treatment of microbial-related diseases. Cationic HDPs (also termed antimicrobial peptides) have emerged from their origins as nature’s antibiotics and are widely distributed in organisms from insects to plants to mammals and non-mammalian vertebrates. Although their original and primary function was proposed to be direct antimicrobial activity against bacteria, fungi, parasites and/or viruses, cationic HDPs are becoming increasingly recognized as multifunctional mediators, with both antimicrobial activity and diverse immunomodulatory properties. Here we provide an overview of the antimicrobial and immunomodulatory activities of cationic HDPs, and discuss their potential application as beneficial therapeutics in overcoming infectious diseases.     

Plasticins: membrane-damaging peptides with ‘chameleon-like’ properties

Plasticins belong to the dermaseptin superfamily of gene-encoded, membrane-active host defense peptides produced by the skin of hylid frogs. The plasticins, which are rich in Gly and Leu residues arranged in regular 5-mer motifs GXXXG (where X is any amino acid residue), have very similar amino acid sequences, hydrophobicities, and amphipathicities but differ markedly in their net charge, conformational plasticity, and activity spectra. The intrinsic flexibility and structural malleability of plasticins modulate their ability to bind to and disrupt the membranes of prokaryotic and eukaryotic cells, and/or to reach intracellular targets, therefore triggering functional versatility. This family of closely related but functionally divergent peptides constitutes a good model to address the relationships between structural polymorphism, membrane-interacting properties, and the biological activity of antimicrobial, cell-penetrating, and viral fusion peptides. Plasticins could thus serve as templates to design potent multifunctional drugs that could act simultaneously against bacterial pathogens and viruses. Received 26 September 2007; received after revision 22 October 2007; accepted 29 October 2007     

The trefoil factor family – small peptides with multiple functionalities

The trefoil factor family (TFF) comprises a group of small peptides which are highly expressed in tissues containing mucus-producing cells – especially in the mucosa lining the gastrointestinal tract. The peptides seem crucial for epithelial restitution and may work via other pathways than the conventional factors involved in restitution. In vitro studies have shown that the TFFs promote restitution using multiple mechanisms. The peptides also have other functionalities including interactions with the immune system. Moreover, therapeutic effects of the TFFs have been shown in several animal models of gastrointestinal damage. Still it is not clear which of their in vitro properties are involved in the in vivo mode of action. This review describes the TFF family with emphasis on their biological properties and involvement in mucosal protection and repair. Received 10 October 2008; received after revision 07 November 2008; accepted 10 November 2008     

Epithelial antimicrobial peptides: innate local host response elements

Multicellular organisms have to survive in an environment laden with numerous microorganisms, which represent a potential hazard to life. Different strategies have been developed to ward off infections by preventing microorganisms from entering surfaces and by preventing the attack of microorganisms that have already entered the epithelia. Therefore, it is not surprising that epithelia are equipped with various antimicrobial substances that act rapidly to kill a broad range of microorganisms. This review summarizes our present knowledge about epithelial peptide antibiotics produced in plants, invertebrates, and vertebrates including humans. There is now strong evidence that in addition to constitutively secreted peptide antibiotics, others are induced upon contact with microorganisms or by proinflammatory cytokines. beta-Defensins represent one family of vertebrate antimicrobial peptides, members of which are inducible and have recently been identified in humans. The defensin-characteristic local expression pattern may indicate that specialized surfaces express a characteristic surface antimicrobial peptide pattern that might define the characteristic microflora as well as the density of microorganisms present on the surface.     

Structural diversity and species distribution of host-defense peptides in frog skin secretions

Cationic peptides that adopt an amphipathic α-helical conformation in a membrane-mimetic environment are synthesized in the skins of many frog species. These peptides often display cytolytic activities against bacteria and fungi consistent with the idea that they play a role in the host’s system of defense against pathogenic microorganisms, but their importance in the survival strategy of the animal is not clearly understood. Despite the common misconception that antimicrobial peptides are synthesized in the skins of all anurans, the species distribution is sporadic, suggesting that their production may confer some evolutionary advantage to the organism but is not necessary for survival. The low potency of many frog skin antimicrobial peptides is consistent with the hypothesis that cutaneous symbiotic bacteria may provide the major system of defense against pathogenic microorganisms in the environment with antimicrobial peptides assuming a supplementary role in some species.     

Role of host-defence peptides in eye diseases

The eye and its associated tissues including the lacrimal system and lids have evolved several defence mechanisms to prevent microbial invasion. Included among this armory are several host-defence peptides. These multifunctional molecules are being studied not only for their endogenous antimicrobial properties but also for their potential therapeutic effects. Here the current knowledge of host-defence peptide expression in the eye will be summarised. The role of these peptides in eye disease will be discussed with the primary focus being on infectious keratitis, inflammatory conditions including dry eye and wound healing. Finally the potential of using host-defence peptides and their mimetics/derivatives for the treatment and prevention of eye diseases is addressed.     

The mode of antifungal action of plant, insect and human defensins

Defensins are small (~5 kDa), basic, cysteine-rich antimicrobial peptides that fulfill an important role in the innate immunity of their host by combating pathogenic invading micro-organisms. Defensins can inhibit the growth or virulence of microorganisms directly or can do so indirectly by enhancing the host's immune system. Because of their wide distribution in nature, defensins are believed to be ancient molecules with a common ancestor that arose more than a billion years ago. This review summarizes current knowledge concerning the mode of antifungal action of plant, insect and human defensins.     

Host defense peptides as new weapons in cancer treatment

In the last decade intensive research has been conducted to determine the role of innate immunity host defense peptides (also termed antimicrobial peptides) in the killing of prokaryotic and eukaryotic cells. Many antimicrobial peptides damage the cellular membrane as part of their killing mechanism. However, it is not clear what makes cancer cells more susceptible to some of these peptides, and what the molecular mechanisms underlying these activities are. Two general mechanisms were suggested: (i) plasma membrane disruption via micellization or pore formation, and (ii) induction of apoptosis via mitochondrial membrane disruption. To be clinically used, these peptides need to combine high and specific anticancer activity with stability in serum. Although so far very limited, new studies have paved the way for promising anticancer host defense peptides with a new mode of action and with a broad spectrum of anticancer activity.     

Vernix caseosa as a multi-component defence system based on polypeptides, lipids and their interactions

Vernix caseosa is a white cream-like substance that covers the skin of the foetus and the newborn baby. Recently, we discovered antimicrobial peptides/proteins such as LL-37 in vernix, suggesting host defence functions of vernix. In a proteomic approach, we have continued to characterize proteins in vernix and have identified 20 proteins, plus additional variant forms. The novel proteins identified, considered to be involved in host defence, are cystatin A, UGRP-1, and calgranulin A, B and C. These proteins add protective functions to vernix such as antifungal activity, opsonizing capacity, protease inhibition and parasite inactivation. The composition of the lipids in vernix has also been characterized and among these compounds the free fatty acids were found to exhibit antimicrobial activity. Interestingly, the vernix lipids enhance the antimicrobial activity of LL-37 in vitro, indicating interactions between lipids and antimicrobial peptides in vernix. In conclusion, vernix is a balanced cream of compounds involved in host defence, protecting the foetus and newborn against infection.     

Proline-rich antimicrobial peptides: converging to a non-lytic mechanism of action

Proline-rich antimicrobial peptides are a group of cationic host defense peptides of vertebrates and invertebrates characterized by a high content of proline residues, often associated with arginine residues in repeated motifs. Those isolated from some mammalian and insect species, although not evolutionarily related, use a similar mechanism to selectively kill Gram-negative bacteria, with a low toxicity to animals. Unlike other types of antimicrobial peptides, their mode of action does not involve the lysis of bacterial membranes but entails penetration into susceptible cells, where they then act intracellularly. Some aspects of the transport system and cytoplasmic targets have been elucidated. These features make them attractive both as anti-infective lead compounds and as a new class of potential cell-penetrating peptides capable of internalising membrane-impermeant drugs into both bacterial and eukaryotic cells     

Fluorescence spectroscopy and molecular dynamics simulations in studies on the mechanism of membrane destabilization by antimicrobial peptides

Since their initial discovery, 30 years ago, antimicrobial peptides (AMPs) have been intensely investigated as a possible solution to the increasing problem of drug-resistant bacteria. The interaction of antimicrobial peptides with the cellular membrane of bacteria is the key step of their mechanism of action. Fluorescence spectroscopy can provide several structural details on peptide–membrane systems, such as partition free energy, aggregation state, peptide position and orientation in the bilayer, and the effects of the peptides on the membrane order. However, these “low-resolution” structural data are hardly sufficient to define the structural requirements for the pore formation process. Molecular dynamics simulations, on the other hand, provide atomic-level information on the structure and dynamics of the peptide–membrane system, but they need to be validated experimentally. In this review we summarize the information that can be obtained by both approaches, highlighting their versatility and complementarity, suggesting that their synergistic application could lead to a new level of insight into the mechanism of membrane destabilization by AMPs.     

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.