Identification, eukaryotic expression and structure & function characterizations of β-defensin like homologues from Pelodiscus sinensis

Crocodile defensin (CpoBD13) antifungal activity via pH-dependent phospholipid targeting and membrane disruption

Crocodilians are an order of ancient reptiles that thrive in pathogen-rich environments. The ability to inhabit these harsh environments is indicative of a resilient innate immune system. Defensins, a family of cysteine-rich cationic host defence peptides, are a major component of the innate immune systems of all plant and animal species, however crocodilian defensins are poorly characterised. We now show that the saltwater crocodile defensin CpoBD13 harbors potent antifungal activity that is mediated by a pH-dependent membrane-targeting action. CpoBD13 binds the phospholipid phosphatidic acid (PA) to form a large helical oligomeric complex, with specific histidine residues mediating PA binding. The utilisation of histidine residues for PA engagement allows CpoBD13 to exhibit differential activity at a range of environmental pH values, where CpoBD13 is optimally active in an acidic environment.

Efficacy of natural antimicrobial peptides versus peptidomimetic analogues: a systematic review

Aims: This systematic review was carried out to determine whether synthetic peptidomimetics exhibit significant advantages over antimicrobial peptides in terms of in vitro potency. Structural features - molecular weight, charge and length - were examined for correlations with activity. Methods: Original research articles reporting minimum inhibitory concentration  values against Escherichia coli, indexed until 31 December 2020, were searched in PubMed/ScienceDirect/Google Scholar and evaluated using mixed-effects models. Results: In vitro antimicrobial activity of peptidomimetics resembled that of antimicrobial peptides. Net charge significantly affected minimum inhibitory concentration values (p < 0.001) with a trend of 4.6% decrease for increments in charge by +1. Conclusion: AMPs and antibacterial peptidomimetics exhibit similar potencies, providing an opportunity to exploit the advantageous stability and bioavailability typically associated with peptidomimetics.

The Two Domains of the Avian Double-β-Defensin AvBD11 Have Different Ancestors, Common with Potential Monodomain Crocodile and Turtle Defensins

Simple Summary

Vertebrate defensins are a multigene family of antimicrobial peptides that evolved following a series of gene duplication and divergence events during the expansion of vertebrates. In birds, the repertoire of avian defensins contains an atypical defensin, namely AvBD11 (avian beta-defensin 11), which consists of two repeated but divergent defensin units (or domains) while most vertebrate defensins only possess one unit. In this study, we investigated the evolutionary scenario leading to the formation of this double defensin in birds by comparing each defensin unit of AvBD11 with other defensins from birds and closely related reptiles (crocodile, turtles) predicted to have a single defensin unit. Our most outstanding results suggest that the double defensin AvBD11 probably appeared following a fusion of two ancestral genes or from an ancestral double defensin, but not from a recent internal duplication as it can be observed in other types of proteins with domain repeats.

Abstract

Beta-defensins are an essential group of cysteine-rich host-defence peptides involved in vertebrate innate immunity and are generally monodomain. Among bird defensins, the avian β-defensin 11 (AvBD11) is unique because of its peculiar structure composed of two β-defensin domains. The reasons for the appearance of such ‘polydefensins’ during the evolution of several, but not all branches of vertebrates, still remain an open question. In this study, we aimed at exploring the origin and evolution of the bird AvBD11 using a phylogenetic approach. Although they are homologous, the N- and C-terminal domains of AvBD11 share low protein sequence similarity and possess different cysteine spacing patterns. Interestingly, strong variations in charge properties can be observed on the C-terminal domain depending on bird species but, despite this feature, no positive selection was detected on the AvBD11 gene (neither on site nor on branches). The comparison of AvBD11 protein sequences in different bird species, however, suggests that some amino acid residues may have undergone convergent evolution. The phylogenetic tree of avian defensins revealed that each domain of AvBD11 is distant from ovodefensins (OvoDs) and may have arisen from different ancestral defensins. Strikingly, our phylogenetic analysis demonstrated that each domain of AvBD11 has common ancestors with different putative monodomain β-defensins from crocodiles and turtles and are even more closely related with these reptilian defensins than with their avian paralogs. Our findings support that AvBD11′s domains, which differ in their cysteine spacing and charge distribution, do not result from a recent internal duplication but most likely originate from a fusion of two different ancestral genes or from an ancestral double-defensin arisen before the Testudines-Archosauria split.

Antimicrobial and immunomodulatory activity of beta-defensin from the Chinese spiny frog (Quasipaa spinosa)

The β-defensins are important components of the vertebrate innate immune system. While mammalian β-defensins have wide-ranging antibacterial and immunomodulatory activities, those of amphibians remain largely uncharacterised. In this study, β-defensin cDNA was identified from the skin transcriptome of the Chinese spiny frog Quasipaa spinosa. This β-defensin (QS-BD) consists of a signal and a mature peptide. Sequence alignments with other amphibian β-defensins showed conservation of the functional mature peptide and that its closest relative is β-defensin from Zhangixalus puerensis. Synthetic QS-BD showed antibacterial activity against Vibrio vulnificus, Vibrio harveyi, Streptococcus iniae, and Aeromonas hydrophila. QS-BD showed bactericidal activity by destroying the cell membrane integrity, but did not hydrolyse genomic DNA. QS-BD treatment promoted respiratory bursts and upregulated the expression of interleukin-1β and tumour necrosis factor-α in the murine leukemic monocyte/macrophage cell line RAW264.7. This is the first demonstration of immunomodulatory activity by an amphibian β-defensin.

Structural and functional characterizations and heterogenous expression of the antimicrobial peptides, Hidefensins, from black soldier fly, Hermetia illucens (L.)

Insect defensins are effector components of the innate defense system. Defensins, which are widely distributed among insects, are a type of small cysteine-rich plant antimicrobial peptides with broad-spectrum antimicrobial activity. Here, the cDNAs of the black soldier fly, Hermetia illucens (L.), encoding six defensins, designated herein as Hidefensin1-1, 2, 3, 4, 5, 6. Moreover, Hidefensin1-1, 2, and 5 were identified for the first time by genome-targeted analysis. These Hidefensins were found to mainly adopt α-helix and β-sheet conformation homology as modeled by PRABI, Swiss-Model and ProFunc server. Six conserved cysteine residues that contribute to three disulfide bonds formed the spacing pattern "C-X₁₂-C-X₃-C-X₉-C-X₅-C-X-C", which play a vital role in the molecular stability of Hidefensins. Phylogenetic analysis revealed that the homology of five Hidefensins (except Hidefensin4) was about 59%-92% compared with other insect defensins, indicating that they are novel antimicrobial peptides genes in black soldier fly. Furthermore, the Hidefensin1-1 was expressed in the Escherichia coli strain BL21(DE3) as a fusion protein with thioredoxin. Results showed that the purified TRX-Hidefensin1-1 exerted strong inhibitory effects against the Gram-positive bacteria Staphylococcus aureus and the Gram-negative bacteria Escherichia coli. The inhibitory efficacy of TRX-Hidefensin1-1 against Gram-positive bacteria was better than that against Gram-negative bacteria. These results indicated that Hidefensin1-1 has potent antimicrobial activities against test pathogens.

Identification of a crocodylian β-defensin variant from Alligator mississippiensis with antimicrobial and antibiofilm activity

β-defensin host defense peptides are important components of the innate immune system of vertebrates. Although evidence of their broad antimicrobial, antibiofilm and immunomodulatory activities in mammals have been presented, β-defensins from other vertebrate species, like crocodylians, remain largely unexplored. In this study, five new crocodylian β-defensin variants from Alligator mississippiensis and Crocodylus porosus were selected for synthesis and characterization based on their charge and hydrophobicity values. Linear peptides were synthesized, folded, purified and then evaluated for their antimicrobial and antibiofilm activities against the bacterial pathogens, Salmonella enterica serovar Typhimurium, Staphylococcus aureus, Enterobacter cloacae and Acinetobacter baumannii. The Am23SK variant (SCRFSGGYCIWNWERCRSGHFLVALCPFRKRCCK) from A. mississippiensis displayed promising activity against both planktonic cells and bacterial biofilms, outperforming the human β-defensin 3 under the experimental conditions. Moreover, Am23SK exhibited no cytotoxicity towards mammalian cells and exerted immunomodulatory effects in vitro, moderately suppressing the production of proinflammatory mediators from stimulated human bronchial epithelial cells. Overall, our results have expanded the activity landscape of crocodylian and reptilian β-defensin in general.

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.

Five subfamilies of β-defensin genes are present in salmonids: Evolutionary insights and expression analysis in Atlantic salmon Salmo salar

β-defensins (BD) are the largest family of vertebrate defensins with potent antimicrobial, chemotactic and immune-regulatory activities. Four BD genes (BD1-4) have been cloned previously in rainbow trout but none have been reported in other salmonids. In this study seven BD genes (BD1a-b, 2-4, 5a-b) are characterised in Atlantic salmon and additional BD genes (BD1b and BD5) in rainbow trout. Bioinformatic analysis revealed up to seven BD genes in the genomes of other salmonids that belong to five subfamilies (BD1-5) due to whole genome duplications. BD1-2 and BD4-5 are also present in basal teleosts but only BD1 and/or BD5 are present in advanced teleosts due to loss of one chromosomal locus. BD3 is salmonid specific. Fish BD have a unique three-coding exon structure. Fish BD are highly divergent between subfamilies but conserved within each subfamily. Atlantic salmon BD genes are differentially expressed in tissues, often with low level expression in systemic immune organs (head kidney and spleen) yet with at least one BD gene highly expressed in mucosal tissues, heart, blood and liver. This suggests an important role of these BD genes in innate immunity in mucosa, liver and blood in Atlantic salmon.