Antimicrobial Activity of Snake β-Defensins and Derived Peptides

β-defensins are antimicrobial peptides presenting in vertebrate animals. They participate in innate immunity, but little is known about them in reptiles, including snakes. Although several β-defensin genes were described in Brazilian snakes, their function is still unknown. The peptide sequence from these genes was deduced, and synthetic peptides (with approximately 40 amino acids and derived peptides) were tested against pathogenic bacteria and fungi using microbroth dilution assays. The linear peptides, derived from β-defensins, were designed applying the bioisosterism strategy. The linear β-defensins were more active against Escherichia coli, Micrococcus luteus, Citrobacter freundii, and Staphylococcus aureus. The derived peptides (7–14 mer) showed antibacterial activity against those bacteria and on Klebsiella pneumoniae. Nonetheless, they did not present activity against Candida albicans, Cryptococcus neoformans, Trychophyton rubrum, and Aspergillus fumigatus showing that the cysteine substitution to serine is deleterious to antifungal properties. Tryptophan residue showed to be necessary to improve antibacterial activity. Even though the studied snake β-defensins do not have high antimicrobial activity, they proved to be attractive as template molecules for the development of antibiotics.

Self-Control of Inflammation during Tail Regeneration of Lizards

Lizards can spontaneously regenerate their lost tail without evoking excessive inflammation at the damaged site. In contrast, tissue/organ injury of its mammalian counterparts results in wound healing with a formation of a fibrotic scar due to uncontrolled activation of inflammatory responses. Unveiling the mechanism of self-limited inflammation occurring in the regeneration of a lizard tail will provide clues for a therapeutic alternative to tissue injury. The present review provides an overview of aspects of rapid wound healing and roles of antibacterial peptides, effects of leukocytes on the tail regeneration, self-blocking of the inflammatory activation in leukocytes, as well as inflammatory resistance of blastemal cells or immature somatic cells during lizard tail regeneration. These mechanistic insights of self-control of inflammation during lizard tail regeneration may lead in the future to the development of therapeutic strategies to fight injury-induced inflammation.

Reptilian β-defensins: Expanding the repertoire of known crocodylian peptides

One of the major families of host defense peptides (HDPs) in vertebrates are β-defensins. They constitute important components of innate immunity and have remained an interesting topic of research for more than two decades. While many β-defensin sequences in mammals and birds have been identified and their properties and functions characterized, β-defensin peptides from other groups of vertebrates, particularly reptiles, are still largely unexplored. In this review, we focus on reptilian β-defensins and summarize different aspects of their biology, such as their genomic organization, evolution, structure, and biological activities. Reptilian β-defensin genes exhibit similar genomic organization to birds and their number and gene structure are variable among different species. During the evolution of reptiles, several gene duplication and deletion events have occurred and the functional diversification of β-defensins has been mainly driven by positive selection. These peptides display broad antimicrobial activity in vitro, but a deeper understanding of their mechanisms of action in vivo, including their role as immunomodulators, is still lacking. Reptilian β-defensins constitute unique polypeptide sequences to expand our current understanding of innate immunity in these animals and elucidate core biological functions of this family of HDPs across amniotes.

The Komodo dragon (Varanus komodoensis) genome and identification of innate immunity genes and clusters

Background

We report the sequencing, assembly and analysis of the genome of the Komodo dragon (Varanus komodoensis), the largest extant lizard, with a focus on antimicrobial host-defense peptides. The Komodo dragon diet includes carrion, and a complex milieu of bacteria, including potentially pathogenic strains, has been detected in the saliva of wild dragons. They appear to be unaffected, suggesting that dragons have robust defenses against infection. While little information is available regarding the molecular biology of reptile immunity, it is believed that innate immunity, which employs antimicrobial host-defense peptides including defensins and cathelicidins, plays a more prominent role in reptile immunity than it does in mammals. .

Results

High molecular weight genomic DNA was extracted from Komodo dragon blood cells. Subsequent sequencing and assembly of the genome from the collected DNA yielded a genome size of 1.6 Gb with 45x coverage, and the identification of 17,213 predicted genes. Through further analyses of the genome, we identified genes and gene-clusters corresponding to antimicrobial host-defense peptide genes. Multiple β-defensin-related gene clusters were identified, as well as a cluster of potential Komodo dragon ovodefensin genes located in close proximity to a cluster of Komodo dragon β-defensin genes. In addition to these defensins, multiple cathelicidin-like genes were also identified in the genome. Overall, 66 β-defensin genes, six ovodefensin genes and three cathelicidin genes were identified in the Komodo dragon genome.

Conclusions

Genes with important roles in host-defense and innate immunity were identified in this newly sequenced Komodo dragon genome, suggesting that these organisms have a robust innate immune system. Specifically, multiple Komodo antimicrobial peptide genes were identified. Importantly, many of the antimicrobial peptide genes were found in gene clusters. We found that these innate immunity genes are conserved among reptiles, and the organization is similar to that seen in other avian and reptilian species. Having the genome of this important squamate will allow researchers to learn more about reptilian gene families and will be a valuable resource for researchers studying the evolution and biology of the endangered Komodo dragon.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-6029-y) contains supplementary material, which is available to authorized users.

Immunocytochemical detection of beta-defensins and cathelicidins in the secretory granules of the tongue in the lizard Anolis carolinensis

Previous molecular studies indicated that antimicrobial peptides in lizard are expressed in the skin and tongue among other epithelial organs. The present ultrastructural immunogold study aimed to detect the specific location of three broadly expressed antimicrobial peptides in the tongue of the lizard Anolis carolinensis. The immunocytochemical study indicated that beta-defensin-15, the likely main defensin of granulocytes and skin, is poorly expressed in some dense and medium-dense granules of glandular cells of the papillated tongue. Conversely beta-defensin-27 appears highly expressed in numerous pale and cribriform dense granules of glandular cells and is also secreted on the tongue surface. The immunostaining for cathelicidin-1 indicated a variable but however positive immunolabeling in numerous granules in the tongue glands, suggesting that this antimicrobial peptide previously found on the epidermal surface is also present in the tongue secretions and participates to the formation of the anti-microbial oral barrier. The study suggests that among the numerous beta-defensins and cathelicidins identified in the genome of this lizard is present a specific distribution of different peptide subtypes in various body regions, including the tongue, and that these peptides contribute to the formation of local antimicrobial barriers.

Antimicrobial peptides in reptiles

Reptiles are among the oldest known amniotes and are highly diverse in their morphology and ecological niches. These animals have an evolutionarily ancient innate-immune system that is of great interest to scientists trying to identify new and useful antimicrobial peptides. Significant work in the last decade in the fields of biochemistry, proteomics and genomics has begun to reveal the complexity of reptilian antimicrobial peptides. Here, the current knowledge about antimicrobial peptides in reptiles is reviewed, with specific examples in each of the four orders: Testudines (turtles and tortosises), Sphenodontia (tuataras), Squamata (snakes and lizards), and Crocodilia (crocodilans). Examples are presented of the major classes of antimicrobial peptides expressed by reptiles including defensins, cathelicidins, liver-expressed peptides (hepcidin and LEAP-2), lysozyme, crotamine, and others. Some of these peptides have been identified and tested for their antibacterial or antiviral activity; others are only predicted as possible genes from genomic sequencing. Bioinformatic analysis of the reptile genomes is presented, revealing many predicted candidate antimicrobial peptides genes across this diverse class. The study of how these ancient creatures use antimicrobial peptides within their innate immune systems may reveal new understandings of our mammalian innate immune system and may also provide new and powerful antimicrobial peptides as scaffolds for potential therapeutic development.

Ultrastructural immunocytochemistry suggests that periderm granules in embryonic chick epidermis contain beta-defensins

The capability to express an innate antimicrobial action through the production of beta-defensins in the skin of chick embryos has been studied by immunocytochemistry. The immunolabeling of periderm granules present in the initial embryonic epidermis of the chick is obtained with a lizard anti-defensin antibody (Ac-BD-15) that recognizes a homologous peptide in the chick beta-defensin 9. Similar granules, termed honeycomb granules, are also stored in some basophilic granulocytes of adult and hatchling chicks, but did not show immunoreactivity to the antibody. It is unclear whether this morphology indicates different organelles, content or lack of available antigens due to organelle packaging. The remarkable ultrastructural similarity between periderm granules and the honeycomb granules present in basophil granulocytes further indicates that embryonic keratinocytes can synthesize antimicrobial peptides together with corneous proteins. These peptides may participate in the formation of an epidermal microbial barrier during ontogenesis or protect the embryo in case of damage to the amniotic-periderm barrier. These transient organelles, apart from the early keratinization of the epidermis that forms the initial barrier before hatching may also contribute to the formation of an efficient anti-microbial barrier against the penetration of microbes during embryonic life until hatching when the adaptive immunity system is still immature.

Ultrastructural immunolocalization of chatelicidin-like peptides in granulocytes of normal and regenerating lizard tissues

The presence and localization of cathelicidin anti-microbial peptides in the lizard, Anolis carolinensis, were investigated by immunocytochemistry. The study showed that immunoreactivity for cathelicidins 1 and 2 was only present in large granules of heterophilic-basophilic granulocytes, rarely found in the dermis and sub-dermal muscle in normal and more frequently in wound and regenerating skin tissues or in the blood. Some immunopositive granulocytes were also observed among the keratinocytes of the wound epithelium covering the tail stump and occasionally in the regenerating epidermis of the tail. Immunolabeling for cathelicidins was also seen in low electrondense amorphous material present on the surface of the wound epidermis and on the plasma membrane of bacteria present on the surface of corneocytes of the epidermis. Immunolabeling for cathelicidins was absent in the other cell types and in control sections. The study suggests that cathelicidins are normally stored in granulocytes in the blood or in connective tissues, while keratinocytes can be stimulated to produce and possibly release these molecules only after injury or microbial invasion.

Ultrastructural immunolocalization of beta-defensin-27 in granulocytes of the dermis and wound epidermis of lizard suggests they contribute to the anti-microbial skin barrier

The high resistance to infections in lizard wounds suggests that these reptiles possess effective antimicrobial peptides in their tissues. The present immunocytochemical study shows the cellular localization of beta-defensin 27 in tail tissues and in the blood, a defensin previously identified in the lizard Anolis carolinensis through biomolecular methods. Beta-defensin-27 immunoreactivity is only observed in some large granules mainly contained in heterophilic granulocytes that are sparse within the dermis of the skin or in the isolated blood. This peptide is absent in other cell types of the skin, in keratinocytes and in subdermal muscle tissue of the tail in normal conditions. Pre-corneous keratinocytes of the regenerating tail epidermis are unlabeled or show a weak labeling for the peptide only in sparse cytoplasmic areas or in the extracellular spaces among corneocytes of the wound and regenerating epidermis. The study suggests that beta-defensin 27 is normally stored in granulocytes present in the blood or in connective tissues while in the epidermis keratinocytes do not show the presence of this peptide unless these cells are stimulated from injury to produce and likely release beta-defensins.