Insect Antimicrobial Peptides: History
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Insects are the organisms from which the greatest amount of peptides are isolated. A single insect produces a mixture of 15–20 peptides, the concentration of which in the hemolymph increases rapidly during infection. Their presence in the hemolymph enables the body’s systemic response to infection, while peptides synthesized in epithelial cells participate in local reactions involving the gates of infection. With over a million described species, insects make up the largest class of organisms in the world. Insects show adaptability to repeated changes and resistance to a wide range of pathogens. The mechanism of resistance developed by insects is associated with an immune system based solely on the innate immune response, which allows for a quick and broad response to attacking organisms. Insect antimicrobial peptides (AMPs) have been increasingly used in pharmacy as well as in agriculture. With a growing number of identified peptides that can inhibit human pathogens, insect AMPs are of great interest for biomedical applications. Insect AMPs represent a highly promising alternative to overcome medical problems associated with antibiotic resistance.

  • antimicrobial peptides (AMPs)
  • insects
  • glycine-rich peptides
  • defensins

1. Defensins (Cysteine-Rich Peptides)

The largest group of insect antimicrobial peptides are defensins. Defensins are also found in a variety of invertebrates, vertebrates, and plants. In insects, defensins constitute a very large group of antimicrobial peptides. They were described, inter alia, with representatives of Diptera, Hemipter, Coleoptera, Lepidoptera, Odonata and Hymenoptera. Currently, almost 170 defensins are known to be present in invertebrates. In insects, defensin genes are mainly expressed in the fat body of insects, but in hematophages (blood-sucking insects), e.g., Anopheles gambiae or Stomoxys calcitrans, also in the middle intestine. The first insect defensins: sapecin and defensins A and B were obtained from the Sarcophaga peregrina (Diptera) cell line and from the hemolymph of immunized Phormia terranovae (Diptera) larvae, respectively [1]. Insect defensins contain 32–52 amino acid residues (3–6 kDa) [2]. The number of amino acids forming the α-helix and the two β-sheets is highly conserved in all known insect defensins, while the length of the N-terminal loop is variable. In evolutionarily older insects such as dragonflies (Aeshna), the defensin loop is shorter compared to the Coleoptera or Hemiptera defensins [3][4]. The defensins A (3.8 kDa) and B (3.84 kDa) Chironomus plumosus (Diptera) have a spatial structure similar to Protophormia terranovae defensins, but their N-terminal loops are four amino acid residues shorter, which is interesting because such short loops characterize defensins of species evolutionarily distant from Diptera (dragonflies, scorpions, clams). In representatives of the same order of insects, the sequences of defensins show a high degree of homology, while between different orders of insects, this phenomenon is not observed. The mechanism of action of defensins is to change the permeability of the cytoplasmic membrane of Gram-positive bacteria, creating channels in it. Against Gram-negative bacteria, defensins usually lack antibacterial activity [5][6]. The mechanism of action of defensin A isolated from consists in the formation of oligomers in the bacterial membrane, which leads to its damage, partial depolarization, loss of K+ ions from the cell, inhibition of respiratory processes, and reduction in the cytoplasmic level of adenosine triphosphate (ATP) [7]. Insect defensins are especially active against Gram-positive bacteria such as Bacillus subtilis, Staphylococcus aureus, Bacillus megaterium, and Micrococcus luteus. However, some of the insect defensins also show antimicrobial activity against Gram-negative bacteria such as E. coli [8]. Insects, in addition to the production of antimicrobial defensins, are able to synthesize cysteine-rich peptides that have antifungal activity. To date, three antifungal defensins have been characterized: [8] heliomicin from the tobacco budworm Heliothis virescens; drosomycin from Drosophila melanogaster; and termicin from the termite Pseudacanthotermes spiniger [9].

2. Cecropins (α-Helical AMPs)

Cecropins are the best-studied group of peptides. The first described insect antimicrobial peptide was cecropin. It was isolated in 1980 from the hemolymph of immunized Hyalophora cecropia pupae. So far, cecropins have been described in insects belonging to various orders, including Diptera, Coleoptera, and Lepidoptera.
The group of cecropins includes a large number of antibacterial and toxic peptides isolated from various lepidopteran and dipteran species. Cecropins are small proteins with about 35 amino acid residues. In insects, the most common are cecropins A, B, and D, which consist of 35–37 residues without cysteine. Insect cecropins are active against both Gram-positive and Gram-negative bacteria [10]. Cecropins can lyse bacterial cellular membranes, inhibit proline uptake, and cause leaky membranes [11][12]. Insect cecropins also have other names including lepidopteran, bactericidin, sarcotoxin, etc. [13]. To date, several insect cecropins have been studied both structurally and biologically, evaluating their in vitro activity. Cecropin A has been shown to have a stabilized α-helical structure thanks to which it reduces both NADP+ and glutathione levels, causing oxidative stress by creating reactive oxygen species, but the mechanism of action is not known so far [14]. Cecropin A shows activity against the fungus Beauveria bassiana in silkworm larvae [15]. Cecropin B, which is a linear cationic peptide, shows the highest antibacterial activity among the entire cecropin family. The publications contain information that cecropin B reduces the bacterial load of E. coli, and the concentration of endotoxins in plasma shows antifungal activity against Candida albicans [16]. Some cecropins have anti-inflammatory activity [17]. Some cecropins and cecropin derivatives (SB-37 and Shiva) have also been shown to be active against parasites, including Trypanosome and Plasmodium [18][19][20]. In addition, they may inhibit cancer cell proliferation and HIV-1 replication [21]. Cecropins have no hemolytic activity, but ponericins (particles similar to cecropin from ant venom) are lethal to erythrocytes and are potent insecticides. Cecropins, in addition to antibacterial activity, may affect the development of Chagas disease, viruses, and parasites that cause malaria [22].

3. Moricins

Moricins, described only in Lepidoptera, were first isolated from the hemolymph of immunized Bombyx mori caterpillars. Unlike cecropins, the moricin molecule forms a single α-helix structure, the amphipathic amino terminus of which the hydrophobic carboxyl terminus is essential for antimicrobial activity. The structure on which this polypeptide acts is the bacterial cell membrane [23]. Moricins show bactericidal activity, especially against Gram-positive bacteria: B. cereus, S. aureus, and Streptococcus pyogenes, but also against Gram-negative E. coli [7][24]. Gram-positive bacteria are more sensitive to moricin than Gram-negative bacteria. The α-helical structure of the moricin molecule makes it similar to cecropins, except that moricin does not contain an amino nitrogen. In addition, there are no disulfide bridges in the moricin molecule. Moricin, an induced immune protein with a broad spectrum of antibacterial activity, is one of the main humoral factors of the antibacterial defense of the body cavity of the mulberry silkworm.
The antifungal activity of moricin is weak and basically targeted at yeast. Moricin, with a strong effect on Gram-positive bacteria, together with mulberry silkworm cecropins, which act mainly on Gram-negative bacteria, effectively eliminate infections of the body cavity of the Bombyx mori caterpillar [25].

4. Proline-Rich Peptides

Proline-rich AMPs are isolated from insects and mammals and exhibit antibacterial activity mainly against Gram-negative bacteria. Proline-rich insect antimicrobial peptides typically contain 20–35 amino acid residues and function by penetrating and crossing the bacterial cell membrane and entering the periplasmic space [26]. In a cell, peptides inhibit intracellular processes such as the transport system. Due to their special properties, they are among the potential cell-penetrating peptides capable of internalizing impermeable drugs into bacteria and eukaryotic cells [27]. Characteristics of proline-rich AMPs are drosocin and apidicin. Drosocin has been isolated from insects belonging to the order of insects Lepidoptera, Hymenoptera, Hemiptera, and Diptera. The mechanism of action of proline-rich antibacterial peptides is by interfering with DNA and RNA synthesis and by binding to nucleic acids. In addition, proline-rich peptides have been shown to have specific macromolecular targets. Proline-rich peptides exhibit antibacterial activity against Pseudomonas aeruginosa, Escherichia coli, Acinetobacter baumannii, and Klebsiella pneumoniae. Studies conducted for pyrhocoricin, apidaecin, and drosocin have shown that these antibacterial peptides may act in a different, precise way involving the molecular binding of the peptide to bacterial DNaK, thereby inhibiting the action of ATPase and preventing the folding of proteins supported by chaperones [4][28].

5. Glycine-Rich Peptides

The mechanism of action of glycine-rich peptides is the destruction of cell membranes. These peptides are active against Gram-negative bacteria as well as active against fungi and tumor cells. Glycine-rich peptides including sarcotoxin IIA, hymenoptaecin, attacin, diptericin, and coleoptericin have been identified in various insect species, including: Glossina morsitans Westwood (Diptera), Bombyx mori L. (Lepidoptera), Heliothis virescens Fabricius (Lepidoptera), Musca domestica L. (Diptera), Samia ricinidaturidini, (Lepidoptera: Noctuidae), and Trichoplusia ni Hübner (Lepidoptera). The high amount of glycine residues (14–22%) in AMPs affects the function of the tertiary structure of proteins by blocking the synthesis of outer membrane proteins in dividing Gram-negative bacteria such as E. coli, thus disrupting the integrity of the cell wall [29].

This entry is adapted from the peer-reviewed paper 10.3390/pathogens12060854


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