An Insect Defensin-Derived β‑Hairpin Peptide with Enhanced Antibacterial Activity

Insect defensins are a class of small, cysteine-rich antimicrobial peptides primarily active on Gram-positive bacteria. Their roles in maggot therapy for treating chronic wound infection have been reported recently. However, a relatively narrow antibacterial spectrum together with the lack of a cost-effective means of commercial-scale production has limited their application. To further exploit the therapeutic potential of these molecules, we engineered the carboxyl-terminal β-sheet of navidefensin2-2, an insect defensin from Nasonia vitripennis, based on its structural similarity to naturally occurring microbicidal β-hairpin peptides. The designed peptide of 14 residues, referred to as NvBH, spans the β-sheet region of the defensin with two amino acids substituted for assembly of a disulfide-bonded amphipathic β-hairpin structure. Liquid chromatography–tandem mass spectrometry (LC–MS/MS) combined with circular dichroism (CD) analysis shows that the oxidized NvBH (oNvBH), produced from the synthetic peptide by air oxidization in an alkaline environment, folds into a typical β-hairpin structure linked by two disulfide bridges (Cys1–Cys4; Cys2–Cys3). However, such a structure appears not to be functionally necessary as synthetic NvBH with a spontaneously oxidized disulfide bridge (Cys2–Cys3) (termed poNvBH) displayed similar antibacterial potency to oNvBH. In comparison with oNvBH, poNvBH exhibited higher serum stability and more resistance on tryptic digestion. These two forms of peptides are capable of killing an array of Gram-positive (including antibiotic-resistant strains of Staphylococcus) and Gram-negative bacterial pathogens at low micromolar concentrations through a membrane disruptive mode of action. Our work indicates that the β-sheet region of insect defensins is a promising subdomain of proteins in anti-infective drug discovery

Image2.PDF

Animal venoms are a mixture of peptides and proteins that serve two basic biological functions: predation and defense against both predators and microbes. Antimicrobial peptides (AMPs) are a common component extensively present in various scorpion venoms (herein abbreviated as svAMPs). However, their roles in predation and defense against predators and potential as drugs are poorly understood. Here, we report five new venom peptides with antimicrobial activity from two Mesobuthus scorpion species. These α-helical linear peptides displayed highly bactericidal activity toward all the Gram-positive bacteria used here but differential activity against Gram-negative bacteria and fungi. In addition to the antibiotic activity, these AMPs displayed lethality to houseflies and hemotoxin-like toxicity on mice by causing hemolysis, tissue damage and inducing inflammatory pain. Unlike AMPs from other origins, these venom-derived AMPs seem to be unsuitable as anti-infective drugs due to their high hemolysis and low serum stability. However, MeuTXKβ1, a known two-domain Mesobuthus AMP, is an exception since it exhibits high activity toward antibiotic resistant Staphylococci clinical isolates with low hemolysis and high serum stability. The findings that the classical AMPs play predatory and defensive roles indicate that the multifunctionality of scorpion venom components is an intrinsic feature likely evolved by natural selection from microbes, prey and predators of scorpions. This definitely provides an excellent system in which one can study how a protein adaptively evolves novel functions in a new environment. Meantimes, new strategies are needed to remove the toxicity of svAMPs on eukaryotic cells when they are used as leads for anti-infective drugs.</p

Comparison of channel blockade and antinociceptive effects of DrTx(1-42) and its mutants.

<p>Note: IC₅₀ means the concentration of 50% inhibition on TTX-R Na⁺ currents in rat DRG neurons Antinociceptive effects of DrTx(1-42) and its mutants were obtained by using 1 mg/kg of dose (i.v.) compared to vehicle-treated group (saline). Data are shown as mean ± S.E.</p

Data_Sheet_1_Enhancement of SARS-CoV-2 receptor-binding domain activity by two microbial defensins.PDF

Peptide binders are of great interest to both basic and biomedical research due to their unique properties in manipulating protein functions in a precise spatial and temporal manner. The receptor-binding domain (RBD) of the SARS-CoV-2 Spike protein is a ligand that captures human angiotensin-converting enzyme 2 (ACE2) to initiate infection. The development of binders of RBDs has value either as antiviral leads or as versatile tools to study the functional properties of RBDs dependent on their binding positions on the RBDs. In this study, we report two microbe-derived antibacterial defensins with RBD-binding activity. These two naturally occurring binders bind wild-type RBD (WT RBD) and RBDs from various variants with moderate-to-high affinity (7.6–1,450 nM) and act as activators that enhance the ACE2-binding activity of RBDs. Using a computational approach, we mapped an allosteric pathway in WT RBD that connects its ACE2-binding sites to other distal regions. The latter is targeted by the defensins, in which a cation-π interaction could trigger the peptide-elicited allostery in RBDs. The discovery of the two positive allosteric peptides of SARS-CoV-2 RBD will promote the development of new molecular tools for investigating the biochemical mechanisms of RBD allostery.</p

Table_1_Mutation-driven parallel evolution in emergence of ACE2-utilizing sarbecoviruses.XLSX

Mutation and recombination are two major genetic mechanisms that drive the evolution of viruses. They both exert an interplay during virus evolution, in which mutations provide a first ancestral source of genetic diversity for subsequent recombination. Sarbecoviruses are a group of evolutionarily related β-coronaviruses including human severe acute respiratory syndrome coronavirus (SARS-CoV) and SARS-CoV-2 and a trove of related animal viruses called SARS-like CoVs (SL-CoVs). This group of members either use or not use angiotensin-converting enzyme 2 (ACE2) as their entry receptor, which has been linked to the properties of their spike protein receptor binding domains (RBDs). This raises an outstanding question regarding how ACE2 binding originated within sarbecoviruses. Using a combination of analyses of phylogenies, ancestral sequences, structures, functions and molecular dynamics, we provide evidence in favor of an evolutionary scenario, in which three distinct ancestral RBDs independently developed the ACE2 binding trait via parallel amino acid mutations. In this process, evolutionary intermediate RBDs might be firstly formed through loop extensions to offer key functional residues accompanying point mutations to remove energetically unfavorable interactions and to change the dynamics of the functional loops, all required for ACE2 binding. Subsequent optimization in the context of evolutionary intermediates led to the independent emergence of ACE2-binding RBDs in the SARS-CoV and SARS-CoV-2 clades of Asian origin and the clade comprising SL-CoVs of European and African descent. These findings will help enhance our understanding of mutation-driven evolution of sarbecoviruses in their early history.</p

Effects of DrTx(1-42) and its mutants on TTX-R Na⁺ currents in rat DRG neurons (≤25 microm diameter).

<p><i>A</i> Representative current traces of TTX-R Na⁺ currents blocked by DrTx(1-42) and its mutants. TTX-R <i>I</i>_Na was evoked by 50 ms depolarization from a holding potential of −80 mV to −10 mV. <i>B</i> The concentration-dependent inhibition of TTX-R Na⁺ channels by DrTx(1-42) and its mutants. Each point is the mean ± SEM for n = 5–8. These data points (DrTx(1-42), D8K and G9R) were fitted according to the Hill equation, the yielded IC₅₀ values as follows: DrTx(1-42): 4.05±0.35 µM; D8K: 2.25±0.15 µM; G9R: 18.19±1.02 µM. NOTE-Recombinant DrTx(1-42) prepared in this study possesses comparable potency on TTX-R <i>I</i>_Na with that previously reported (Zhu <i>et al.</i>, 2011).</p

The current - voltage (I–V) relationship of D8K on TTX-R currents.

<p><b>I–V</b> relationships of TTX-R currents before (filled circles) and after (open circles) the treatment of 2.5 µM D8K, respectively. Cells were held at −80 mV, and TTX-R currents were elicited by 50 ms depolarizing steps to various potentials ranging from −80 mV to +40 mV in 10 mV increments. Only D8K is presented here for example.</p

Image3.PDF

<p>Animal venoms are a mixture of peptides and proteins that serve two basic biological functions: predation and defense against both predators and microbes. Antimicrobial peptides (AMPs) are a common component extensively present in various scorpion venoms (herein abbreviated as svAMPs). However, their roles in predation and defense against predators and potential as drugs are poorly understood. Here, we report five new venom peptides with antimicrobial activity from two Mesobuthus scorpion species. These α-helical linear peptides displayed highly bactericidal activity toward all the Gram-positive bacteria used here but differential activity against Gram-negative bacteria and fungi. In addition to the antibiotic activity, these AMPs displayed lethality to houseflies and hemotoxin-like toxicity on mice by causing hemolysis, tissue damage and inducing inflammatory pain. Unlike AMPs from other origins, these venom-derived AMPs seem to be unsuitable as anti-infective drugs due to their high hemolysis and low serum stability. However, MeuTXKβ1, a known two-domain Mesobuthus AMP, is an exception since it exhibits high activity toward antibiotic resistant Staphylococci clinical isolates with low hemolysis and high serum stability. The findings that the classical AMPs play predatory and defensive roles indicate that the multifunctionality of scorpion venom components is an intrinsic feature likely evolved by natural selection from microbes, prey and predators of scorpions. This definitely provides an excellent system in which one can study how a protein adaptively evolves novel functions in a new environment. Meantimes, new strategies are needed to remove the toxicity of svAMPs on eukaryotic cells when they are used as leads for anti-infective drugs.</p

Primers used in this study.

<p>Note: Mutated nucleotides are underlined once. All primers listed here were synthesized by SBS Genetech (Beijing, China).</p

Sequence alignment of DrTx(1-42) and its mutants.

<p>The N-turn region derived from BmKITc is shadowed in yellow. Mutated sites are indicated in red.</p