Technological Potential of Antimicrobial Peptides: a Systematic Review

*Corresponding Author:

Jane Gelinski
University of the West of Santa Catarina, SC, Brazil
E-mail: jgelinski@yahoo.com.br

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Abstract

Resistance to antibiotics has being considered one of the greatest public health problems worldwide. The objective of this systematic review was to compile important bibliographical references that support the studies related to the biotechnological potential of antimicrobial peptides. Antimicrobial peptides are considered the major components of the innate immune system and work in defence against infections caused by different microorganisms. Many researchers argue that the studies associated with the discoveries of antimicrobial peptides that are more effective in the treatment of infections caused by microorganisms be also directed to the substances produced by insects. The major aspects associated to the technological potential of antimicrobial peptides are, general characteristics, classification, mode of action and prospects for applying, mainly in the pharmaceutical industry. It can be concluded that research for new substances with action against pathogenic microorganisms aim mainly to seek alternatives to the use of antibiotics. In addition, anticancer peptides are small cationic molecules with promising pharmacological use. However, the products for application in the protection of food, treatment of skin infections and its use in the cosmetics industry are those with the greatest potential.

Keywords

Antibiotics, AMPs, immune system, insect, pharmaceutical industry

The discovery of antibiotics is considered one of the great advances in medicine; its application allows controlling more efficiently infections caused by microorganisms, facilitating the achievement of complex surgical procedures. The use of these substances contributes to the control of infectious diseases by significantly reducing the mortality of humans and animals. The efficiency in the treatment placed the antibiotics among the drugs most prescribed in the world; its indiscriminate use has generated a strong selective pressure favoring the evolution of the bacterial resistance[1-4].

The evolution of resistance of pathogens to antibiotics puts at risk the lives of many patients by reducing the success of modern medicine. The first strains resistant to penicillin was first identified in 1947[5] and since then, the number of resistant microorganisms has increased considerably[2-4,6]. Davies and Davies[7] stated that many of us have lived in a period that can be referred to as “post-antibiotic age” since some species of pathogens are resistant to more than 50 % of the antibiotic agents used to fight them[8]. In 2004, about 70 % of the pathogenic bacteria showed resistance against at least one antibiotic[9]. Currently one of the major concerns is the emergence of the so-called super bacteria, which show resistance against multiple antibiotics; in The United States these bacteria cause the death of 99 % of infected people[10,11].

Resistance to antibiotics has determined an increase in mortality rates and costs of treatments, being considered one of the greatest public health problems worldwide[3,12-14]. According to the Centers for Disease Control and Prevention[15] this phenomenon causes around 23 000 deaths each year in the United States. An additional complication is the reduction of investments in the pharmaceutical industry in developing new antibiotics observed in recent decades[1,16]. During this period, the search for new substances with action against pathogenic microorganisms has become a constant between researchers in the pharmaceutical area. Among the substances studied are the antimicrobial peptides (AMPs), also known as peptides of host defense.

The AMPs are known since the beginning of the last century, but their potential for application in the pharmaceutical industry, as a replacement for antibiotics, began to be investigated more consistently with the characterization of cecropin by Hultmark et al.[17]. This substance with antimicrobial action was isolated from pupae of Hyalophora cecropia Linnaeus, 1758 (Saturniidae) after this, many other peptides with this characteristic were discovered[18,19].

The objective of this systematic review was to compile important bibliographical references that support the studies related to the biotechnological potential of AMPs. The main topics covered here are: description of the general characteristics of the AMPs, classification, mode of action and technological potential of use. The authors have prioritized literature review articles from the last 10 y, but some older ones were considered because they included specific and relevant aspects about peptides.

General characteristics of the AMPs:

The AMPs are substances evolutionarily ancient found in bacteria, fungi, plants and animals. These substances differ in molecular size, spectrum of action; they are considered the major components of the innate immune system and work in defense against infections caused by viruses, bacteria and fungi[20-33]. Furthermore, recent studies indicate that the AMP may exercise activities such as immunological modulators[34,35] in the treatment of cancer[36], in the recovery of skin lesions and as an alternative in the treatment of biofilms formed by pathogenic microorganisms[37]. The AMPs, in general, are substances that have fewer than 100 amino acids with a molecular weight below 5000 Daltons, although there have already been found peptides containing between 130 and 150 amino acids. In general, the number of amino acids is less than 60, ranging between 12 and 50 AA. They are amphipathic molecules (with hydrophobic and hydrophilic regions), with a positive charge (varying between +2 and +9) due to the presence of multiple basic amino acids such as lysine, arginine and histidine[38-40].

In vertebrates the synthesis of some AMPs occur in a constitutive way and the production of others can be induced during inflammatory processes; in the aquatic invertebrates these peptides are synthesized rapidly in response to infections caused by microorganisms[38,39]. In vertebrates they are produced by several types of cells, such as phagocytes, lymphocytes, epithelial cells of the gastrointestinal tract and urogenital system[40]. In insects such substances are produced mainly in grease and promptly released to the hemolymph. Each species, in general, produces a single repertoire of these peptides. The number of AMPs can vary a lot between different species, in insects: for example, more than 50 types of AMPs were identified in Harmonia axiids[41] and only 6 in Apis melifera[42].

Studies on the evolution of genes that encode AMPs indicate the occurrence of cases of duplication followed by evolutionary divergence. This analysis indicated the presence of positive selection favoring changes in load of amino acids, promoting diversification. It was also identified the occurrence of negative selection in areas important for the functional activities of these substances[40,43]. The genes that encode antimicrobial proteins are rapidly evolving, which indicated their involvement in the arms race against microorganisms[21,40].

Classes and families:

The classification of AMPs can be based on several different criteria, i.e., despite of their importance there is a definitive method for their categorization. The database for AMPs (AMP database)[44] presents seven different ways for the classification of these substances, the mechanism of synthesis, the origin, the biological function, the properties of the peptides, in threedimensional structure, in the standards of covalent bounds and molecular target. The classification used more frequently is based on structural characteristics, number of amino acids and size, which defines four classes or major groups that could represent several families (fig. 1).

Figure 1: Classes of antimicrobial peptides
Class I- α-helix, class II- β-sheet, class III- extended helix and class IV- β-hairpin or loops. Modified from Peters et al.[45]

The description below is based on the works of various authors[45-49]. Class I (α-helix)- linear peptides that assume the setting called α-helix, examples include cecropin, magainin, pexiganan, dermaseptin and dipteran. Class II (β-sheet)- peptides that have two or more β chains that are stabilized by the presence of disulfide bonds, such as defensin, protegrin and heliomicin. Class III (extended)- linear peptides that have high amounts of amino acids like proline, histidine, arginine or glycine, examples include drosocin, lebocin and moricin, pyrrhocoricin, indolicidine and histatins. Class IV (β-hairpin or loops)- peptides that have structures similar to staples connected by bridges of disulfides and possess high quantities of residues of proline, examples, tachyplesins, bactenecin and dodecapeptide. The classes I and II (α-helix and β-sheet) are the AMPs more common and more studied; as an example of important families are the cecropin and defensin[48,49], respectively (fig. 2).

Figure 2: 3D structures of class I and II AMPs
A: Cecropin-like peptide; B: defensin NSD7. Images from RCSB Protein Data Bank (PDB) A. PDB ID 2MMM[48] and B. PDB ID 5KK4[49]

Cecropin was characterized by Hultmark et al.[17] and, after that, it was identified in different organisms. The study of this peptide has enabled the division of family in five subfamilies or subtypes in consequence of the difference in the composition of amino acids. The precursors of cecropin family cecropin have between 58 and 64 amino acids; the mature peptides are released by cleavage of the signal peptide and have between 35 and 39 amino acids. The mature peptides have no residues of cysteine and form two α helices (an amphiphilic N-terminal and a C-terminal hydrophobic). The family offers a broad spectrum of activity against bacteria (Gram-positive or negative) and fungi[18,21,50].

The members of the family defensin were described in several species and their main feature is the presence of 6 to 8 residues of cysteine, which are involved in the formation of bridges of molybdenum disulfide that maintain the structure of the peptide (β-sheet). They are alkaline peptides rich in arginine containing between 16 and 50 amino acids; these are synthesized as a prepeptide that would go through several modifications before being released in the active form. The members of this family differ in size, being known, currently, the subfamilies α, β and θ defensin. In addition, the family defensin have members with action against bacteria, fungi and protozoa[18,21,51].

The diversity and variation in the composition of amino acids observed between the AMP is one of the difficulties for the definition of procedures for classification and identification of these substances, especially the definition of families. As a result, in recent years several works based in bioinformatics have been developed, aiming to establish methodologies and most appropriate criteria for the classification of AMPs[43,50-53].

Mode of action and technological potential:

The action of AMPs may involve changes in the plasma membrane and intracellular elements, as in DNA, in the processes of synthesis and folding of proteins. The first step of the action of AMPs involves their interaction with the plasma membrane. This interaction depends on the specific characteristics of the membranes of cells and peptides[54].

The AMPs are attracted by electrostatic forces to the negative portions of the phospholipids of the cell membrane which are connected to the lipopolysaccharides in Gram-negative bacteria, to teichoic acid, lipoteichoic and lysylphosphatidylglycerol in Gram-positive bacteria. After that, the AMPs directly interact with the phospholipids of the plasma membrane. The interaction between the AMPs and the double layer of phospholipids stems from the amphiphile nature of both. In this process the positive charges of AMP are important for their link to regions with negative charges of the membrane, while the hydrophobic portion is important for insertion in the double layer[49,55].

The difference in the chemical composition of the plasma membrane of prokaryotes and eukaryotes explains the selectivity of the AMPs for bacteria. Furthermore, the bacterial cells have no cholesterol[56]. The action of the AMPs against tumors is based on differences in the chemical composition of the plasma membrane of the malignant cells[57]. The mechanism of action of AMP does not involve specific receptors, which reduces the speed of evolution of resistance on the part of the pathogens[58]. The recent models known to explain the effects of the AMPs on the plasma membrane are, Barrel-stave model, Toroidal pore model and carpet model. These models differ in how they explain the interaction and/or deterioration caused by AMPs in double layer of phospholipids (fig. 3)[59,60].

Figure 3: Models that explain interaction of AMPs with double layer of phospholipids
Schemes of the 3 models that explain the interaction of AMPs with double layer of phospholipids. A, B and C adapted by permission from Macmillan Publishers Ltd, Nature Reviews Nephology[59] A1, B1 and C1 are modified from López-Meza et al.[60]

The models Barrel-stave and Toroidal involve the insertion of aggregates of AMPs in dual layer and the formation of pores, which can lead to changes in the flow of calcium, membrane depolarization, loss of energy and, in some cases, induce apoptosis. At the model of tappet (carpet) there is the passage of the AMP by double layer of lipids leading to dissolution or destruction of the plasma[54,61].

In the Barrel stave model the process is directed from the hydrophilic interactions of the peptides with the external membrane of the bilayer. From a peptide complex with perpendicular orientation to the membrane, it is inserted through the hydrophobic region of the bilayer forming a channel (fig. 3)[8,60]. Already the Toroidal model (fig. 3) occurs by the transition of the peptide from an inactive state to an active. The peptides are reoriented perpendicularly into the hydrophobic region of the bilayer (active state) and are associated with lipid molecules in a transitional multipore state, known as supramolecular-lipid dynamic complex. The rupture of the membrane becomes irreversible besides increased transmembrane movement of lipids (fig. 3, red arrow)[60].

In the carpet model the positive charges of helical cationic peptides plus negatively charged phospholipid heads interact and are oriented towards the outside of the membrane. Upon reaching a critical concentration, the peptides undergo rotation and the phospholipids present are redirected. Consequently, there is layer collapse and formation of micelles with hydrophobic core and pore formation in the membrane (fig. 3)[8,60].

Recent studies showed the existence of complementary mechanisms which act on the intra cell components. After the interaction with the membrane the AMP bind to intracellular molecules by inhibiting the synthesis of DNA, RNA, proteins and/or components of the plasma membrane[36,62].

Perspectives:

The efficiency of the AMPs has been demonstrated by several studies over time, but nevertheless there are few products available on the market. Among the products marketed are: polymyxin B, colistin, tyrocidin, gramicidin, bactracin and daptomycin, lucinactant, peginesatide, pasireotide, carfilzomib, linaclotide, teduglutide[62]. In recent years, approximately 140 AMPs are in different stages of analysis to the authorization for commercial production[63]. In addition, there are several studies analyzing the efficiency of some AMPs in fighting infections caused by fungi and bacteria in transgenic plants that express the codifying gene of the peptide[38,64].

The studies carried out demonstrated the great potential of AMPs for the pharmaceutical industry, either by their form of action that hinders the development of resistance or by the diversity of types available for tests and assessments. The evolution of resistance to the AMPs would depend on a reconfiguration in the structure of the membrane - a process much more complex and harder to happen[56,65]. On the other hand, the AMPs are produced by all living beings, i.e., are a source almost limitless for research and evaluations[25,32,33,66]. In addition, it is necessary to remember their efficiency and broad spectrum of action.

Due to that, a question is evident: what are the major difficulties for the exploitation of this potential by the pharmaceutical industry? Among the main difficulties to use the AMPs as a method of control of microorganisms are the possibility to be toxic to mammalian cells; its proteolytic degradation and the costs for its development for pharmaceutical applications[8,67]. The development of AMP with up to 30 amino acids has a cost within the limit that large companies are willing to invest in the development of new products. The costs for development of larger peptides are considerably high[45,68]. These obstacles are related to up-scaling and licensing of peptides, but despite this, it is estimated that more than 500 derived peptides are under development[68,69].

The AMPs are rapidly degraded by the action of proteases inside the human body; this reduces their availability and makes it difficult to maintain the dose of the medicine at effective concentrations[70]. The problems of stability of the AMP in physiological conditions can be overcome through specific changes in their chemical composition and/or structure, such as the replacement and/or addition of amino acids or other chemical groups. These chemical changes may also contribute to increase the efficiency of AMPs[71]. These changes can be performed using traditional methodologies for the drug´s production.

Among the strategies used to minimize the effects of the AMP in the organism treated and increase its half-life stand out: its association with substances which increase the solubility, association with substances, which increase their aggregation capacity and construction of proteins with a capacity of self-cleavage[71,72]. In recent years, several studies have examined the use of nanotechnology to solve stability problems, application, absorption and movement of peptides inside of the body, facilitating its pharmacological use[73,74].

In addition to the applications in the treatment of infections by microorganism’s products based on AMP may be important in the food industry and cosmetics. The food industry can use the AMP as a substitute of synthetic preservatives for food safer production preventing the growth and development of pathogenic microorganisms and/or avoiding contamination[65,75].

On the other hand, many AMP are active against dermatological pathogens important and relevant to the cosmetics industry. They can be used, therefore, in the making of products for prophylactic application and personal care contributing to maintaining the health of the skin[75,76].

AMPs are, for sure, a great option in the fight of pathogenic microorganisms to humans, animals and plants. A relevant point is the fact that they are substances produced by all living organisms, which puts at the disposal of the researchers an inexhaustible source of studies. The major problems associated with the application of these substances can be overcome by using technologies already applied by the pharmaceutical industry, especially for molecules with fewer than 30 amino acids. The products for application in the protection of food, treatment of skin infections and its use in the cosmetics industry are now those with the greatest potential. Certainly, in the near future, problems for its use in oral and/or intravenous administration will be overcome.

In addition, there are AMPs with selective antitumor mechanisms (cationic peptides) with amphipathic structure that are able to cause cell membrane disruption[54,57]. These anticancer peptides have great in vivo potential but their activity against cancer cells is lower than antimicrobial activity.

Many researchers argue that the studies associated with the discoveries of AMPs more effective in the treatment of infections caused by microorganisms be also directed to the substances produced by insects. The reasons for this suggestion are the evolutive success that allows insects to occupy a variety of habitats; an important part of this success can be attributed to the efficiency of their immune system. In addition, there are more than 30 million species of insects, i.e., a huge source of resources to prospect for new substances with application in medicine, food industry and cosmetics as substitutes or/and for use in conjunction with the antibiotics[38,40,44,45,77].

From evolutionary perspective, even if the researchers find AMP very efficient in control of microorganisms, it must be borne in mind that this success may be temporary, because evolution is an ongoing process[78]. Therefore, it is highly likely that at some time in the future, some strains of bacteria develop resistance or decreased sensitivity to AMP used in the treatment of infections. This is a facet of the mankind’s arms race against pathogenic microorganisms that should not be forgotten. So, part of the resources should be invested continually in the development of new strategies and products for treatment and control of pathogenic agents. An evidence of this need for continuous investment comes from several studies about the possibility of resistance of bacteria to AMPs[68,79-82].

Acknowledgments:

The authors are grateful to the Capes/PNPD-Postdoctoral Fellowship Program, Brazil.

Financial support/funding:

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nivel Superior Brazil (CAPES)-Finance code 001.

Conflict of interest:

The authors declare that they have no conflict of interest. We affirm that funding sponsors had no role in the design of the study; in the collection analyses, or data interpretation in the writing of this manuscript, and the decision to publish this review.

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