Cytotoxic Cyclotides from Anchietea pyrifolia, a South American Plant Species

Structure and Activity of Reconstructed Pseudo-Ancestral Cyclotides

Cyclotides are cyclic peptides that are promising scaffolds for the design of drug candidates and chemical tools. However, despite there being hundreds of reported cyclotides, drug design studies have commonly focussed on a select few prototypic examples. Here, we explored whether ancestral sequence reconstruction could be used to generate new cyclotides for further optimization. We show that the reconstructed 'pseudo-ancestral' sequences, named Ancy-m (for the ancestral cyclotide of the Möbius sub-family) and Ancy-b (for the bracelet sub-family), have well-defined structures like their extant members, comprising the core structural feature of a cyclic cystine knot. This motif underpins efforts to re-engineer cyclotides for agrochemical and therapeutic applications. We further show that the reconstructed sequences are resistant to temperatures approaching boiling, bind to phosphatidyl-ethanolamine lipid bilayers at micromolar affinity, and inhibit the growth of insect cells at inhibitory concentrations in the micromolar range. Interestingly, the Ancy-b cyclotide had a higher oxidative folding yield than its comparator cyclotide cyO2, which belongs to the bracelet cyclotide subfamily known to be notoriously difficult to fold. Overall, this study provides new cyclotide sequences not yet found naturally that could be valuable starting points for the understanding of cyclotide evolution and for further optimization as drug leads.

Characterization and evaluation of cytotoxic and antimicrobial activities of cyclotides from Viola japonica

Cyclotides are a type of defense peptide most commonly found in the Violaceae family of plants, exhibiting various biological activities. In this study, we focused on the Viola japonica as our research subject and conducted transcriptome sequencing and analysis using high-throughput transcriptomics techniques. During this process, we identified 61 cyclotides, among which 25 were previously documented, while the remaining 36 were designated as vija 1 to vija 36. Mass spectrometry detection showed that 21 putative cyclotides were found in the extract of V. japonica. Through isolation, purification and tandem mass spectrometry, we characterized and investigated the activities of five cyclotides. Our results demonstrated inhibitory effects of these cyclotides on the growth of Acinetobacter baumannii and Bacillus subtilis, with minimum inhibitory concentrations (MICs) of 4.2 μM and 2.1 μM, respectively. Furthermore, time killing kinetic assays revealed that cyclotides at concentration of 4 MICs achieved completely bactericidal effects within 2 h. Additionally, fluorescence staining experiments confirmed that cyclotides disrupt microbial membranes. Moreover, cytotoxicity studies showed that cyclotides possess cytotoxic effects, with IC50 values ranging from 0.1 to 3.5 μM. In summary, the discovery of new cyclotide sequences enhances our understanding of peptide diversity and the exploration of their activity lays the foundation for a deeper investigation into the mechanisms of action of cyclotides.