Understanding the Diversity and Distribution of Cyclotides from Plants of Varied Genetic Origin

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.

Screening for Cyclotides in Sri Lankan Medicinal Plants: Discovery, Characterization, and Bioactivity Screening of Cyclotides from Geophila repens

Cyclotides are an intriguing class of structurally stable circular miniproteins of plant origin with numerous potential pharmaceutical and agricultural applications. To investigate the occurrence of cyclotides in Sri Lankan flora, 50 medicinal plants were screened, leading to the identification of a suite of new cyclotides from Geophila repens of the family Rubiaceae. Cycloviolacin O2-like (cyO2-like) gere 1 and the known cyclotide kalata B7 (kB7) were among the cyclotides characterized at the peptide and/or transcript level together with several putative enzymes, likely involved in cyclotide biosynthesis. Five of the most abundant cyclotides were isolated, sequenced, structurally characterized, and screened in antimicrobial and cytotoxicity assays. All gere cyclotides showed cytotoxicity (IC50 of 2.0-10.2 μM), but only gere 1 inhibited standard microbial strains at a minimum inhibitory concentration of 4-16 μM. As shown by immunohistochemistry, large quantities of the cyclotides were localized in the epidermis of the leaves and petioles of G. repens. Taken together with the cytotoxicity and membrane permeabilizing activities, this implicates gere cyclotides as potential plant defense molecules. The presence of cyO2-like gere 1 in a plant in the Rubiaceae supports the notion that phylogenetically distant plants may have coevolved to express similar cytotoxic cyclotides for a specific functional role, most likely involving host defense.

Using the Cyclotide Scaffold for Targeting Biomolecular Interactions in Drug Development

This review provides an overview of the properties of cyclotides and their potential for developing novel peptide-based therapeutics. The selective disruption of protein–protein interactions remains challenging, as the interacting surfaces are relatively large and flat. However, highly constrained polypeptide-based molecular frameworks with cell-permeability properties, such as the cyclotide scaffold, have shown great promise for targeting those biomolecular interactions. The use of molecular techniques, such as epitope grafting and molecular evolution employing the cyclotide scaffold, has shown to be highly effective for selecting bioactive cyclotides.

Tropical vibes from Sri Lanka - cyclotides from Viola betonicifolia by transcriptome and mass spectrometry analysis

Cyclotides are an extremely stable class of peptides, ubiquitously distributed in Violaceae. The aim of the present study was to investigate the presence of cyclotides in Sri Lankan Violaceae plants, using combined tools of transcriptomics and mass spectrometry. New cyclotides were discovered for the first time in the wild flora of Sri Lanka, within Viola betonicifolia, a plant used in traditional medicine as an antimicrobial. Plant extracts prepared in small scale from Viola betonicifolia were first subjected to LC-MS analysis. Subsequent transcriptome de novo sequencing of Viola betonicifolia uncovered 25 new (vibe 1-25) and three known (varv A/kalata S, viba 17, viba 11) peptide sequences from Möbius and bracelet cyclotide subfamilies as well as hybrid cyclotides. Among the transcripts, putative linear acyclotide sequences (vibe 4, vibe 10, vibe 11 and vibe 22) that lack a conserved asparagine or aspartic acid vital for cyclisation were also present. Four asparagine endopeptidases (AEPs), VbAEP1-4 were found within the Viola betonicifolia transcriptome, including a peptide asparaginyl ligase (PAL), potentially involved in cyclotide backbone cyclisation, showing >93% sequence homology to Viola yedoensis peptide asparaginyl ligases, VyPALs. In addition, we identified two protein disulfide isomerases (PDIs), VbPDI1-2, likely involved in cyclotide oxidative folding, having high sequence homology (>74%) with previously reported Rubiaceae and Violaceae PDIs. The current study highlights the ubiquity of cyclotides in Violaceae as well as the utility of transcriptomic analysis for cyclotides and their putative processing enzyme discovery. The high variability of cyclotide sequences in terms of loop sizes and residues in V. betonicifolia showcase the cyclotide structure as an adaptable scaffold as well as their importance as a combinatorial library, implicated in plant defense.

Cyclotide host-defense tailored for species and environments in violets from the Canary Islands

Cyclotides are cyclic peptides produced by plants. Due to their insecticidal properties, they are thought to be involved in host defense. Violets produce complex mixtures of cyclotides, that are characteristic for each species and variable in different environments. Herein, we utilized mass spectrometry (LC–MS, MALDI-MS), transcriptomics and biological assays to investigate the diversity, differences in cyclotide expression based on species and different environment, and antimicrobial activity of cyclotides found in violets from the Canary Islands. A wide range of different habitats can be found on these islands, from subtropical forests to dry volcano peaks at high altitudes. The islands are inhabited by the endemic Viola palmensis, V. cheiranthifolia, V. anagae and the common V. odorata. The number of cyclotides produced by a given species varied in plants from different environments. The highest diversity was noted in V. anagae which resides in subtropical forest and the lowest in V. cheiranthifolia from the Teide volcano. Transcriptome sequencing and LC–MS were used to identify 23 cyclotide sequences from V. anagae. Cyclotide extracts exhibited antifungal activities with the lowest minimal inhibitory concentrations noted for V. anagae (15.62 μg/ml against Fusarium culmorum). The analysis of the relative abundance of 30 selected cyclotides revealed patterns characteristic to both species and populations, which can be the result of genetic variability or environmental conditions in different habitats. The current study exemplifies how plants tailor their host defense peptides for various habitats, and the usefulness of cyclotides as markers for chemosystematics.

Isolation and characterization of cyclotides from the leaves of Viola odorata L. using peptidomic and bioinformatic approach

Cyclotides are true gene products characterized by the presence of six conserved cysteine residues and knotted arrangement of three disulfide bonds. These macrocyclic peptides show exceptional resistance to thermal, chemical and enzymatic degradation which is defined due to their three-dimensional folding. The current study describes an efficient strategy involving reduction, enzymatic digestion and mass spectroscopy sequencing for the identification of the precursor sequences and the cyclotide domains present in the leaf tissue of Viola odorata. We observed 122 partial peptide sequences containing 31 cyclotide domains along with 19 unique sequences consisting of putative novel cyclotides and acyclotides. Four precursor sequences consisting of putative new and already reported domains were further characterized for cyclotide domains, their structure and subfamilies. The sequences revealed the presence of classic knotted cyclotide folds with similar six characteristic loops but different amino acid residues. Molecular modeling indicated that the secondary structures present in the cyclotides are mainly α-helix and random coils. Variation in the sequences and conservation in cysteine residues in the cyclotides was revealed by protein diversity wheel. The significant information observed in the current study expands our knowledge about the structure and type of cyclic peptides in V. odorata leaves.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02763-2.

Exploring the Sequence Diversity of Cyclotides from Vietnamese Viola Species

Viola is the largest genus in the Violaceae plant family and is known for its ubiquitous natural production of cyclotides. Many Viola species are used as medicinal herbs across Asia and are often consumed by humans in teas for the treatment of diseases, including ulcers and asthma. Previous studies reported the isolation of cyclotides from Viola species in many countries in the hope of discovering novel compounds with anti-cancer activities; however, Viola species from Vietnam have not been investigated to date. Here, the discovery of cyclotides from three Viola species (V. arcuata, V. tonkinensis, and V. austrosinensis) collected in the northern mountainous region of Vietnam is reported. Ten cyclotides were isolated from these three Viola species: four are novel and six were previously reported to be expressed in other plants. The structures of three of the new bracelet cyclotides are similar to that of cycloviolacin O2. Because cycloviolacin O2 has previously been shown to have potent activity against a wide range of cancer cell lines including HeLa (human cervical cancer cells) and PC-3 (human prostate cancer cells), the cancer cytotoxicity of the cyclotides isolated from V. arcuata was assessed. All tested cyclotides were cytotoxic against cancer cells, albeit to varying degrees. The sequences discovered in this study significantly expand the understanding of cyclotide diversity, especially in comparison with other cyclotides found in plants from the Asian region.

Discovery of Cyclotides from Australasian Plants

This article is part of a special issue celebrating the contributions of Professor Paul Alewood to peptide science. We begin by providing a summary of collaborative projects between the Alewood and Craik groups at The University of Queensland and highlighting the impacts of some of these studies. In particular, studies on the discovery, synthesis, structures, and bioactivities of disulfide-rich toxins from animal venoms have led to a greater understanding of the biology of ion channels and to applications of these bioactive peptides in drug design. The second part of the article focuses on plant-derived disulfide-rich cyclic peptides, known as cyclotides, and includes an analysis of the geographical distribution of Australasian plant species that contain cyclotides as well as an analysis of the diversity of cyclotide sequences found in Australasian plants. This should provide a useful resource for researchers to access native cyclotides and explore their chemistry and biology.

Cyclotides: From Structure to Function

This Review explores the class of plant-derived macrocyclic peptides called cyclotides. We include an account of their discovery, characterization, and distribution in the plant kingdom as well as a detailed analysis of their sequences and structures, biosynthesis and chemical synthesis, biological functions, and applications. These macrocyclic peptides are around 30 amino acids in size and are characterized by their head-to-tail cyclic backbone and cystine knot motif, which render them to be exceptionally stable, with resistance to thermal or enzymatic degradation. Routes to their chemical synthesis have been developed over the past two decades, and this capability has facilitated a wide range of mutagenesis and structure-activity relationship studies. In turn, these studies have both led to an increased understanding of their mechanisms of action as well as facilitated a range of applications in agriculture and medicine, as ecofriendly crop protection agents, and as drug leads or scaffolds for pharmaceutical design. Our overall objective in this Review is to provide readers with a comprehensive overview of cyclotides that we hope will stimulate further work on this fascinating family of peptides.

Reporting a Transcript from Iranian Viola Tricolor, Which May Encode a Novel Cyclotide-Like Precursor: Molecular and in silico Studies

The cyclotides are the largest known family of cyclic proteins, which are found in several plant families including Violaceae. They are circular bioactive peptides consisting of 28-37 amino acids, which possess a cyclic cystine knot (CCK) motif and could be useful in biotechnology and drug design as scaffolds for peptide-based drugs. This study describes our finding of a potentially novel gene transcript from the petals of the Iranian Viola tricolor (V. tricolor) flowers. This study is based on the cDNA screening method employed for isolation of cyclotide precursor genes and in silico analysis. Our study resulted in the finding of a novel cyclotide-like precursor from V. tricolor, which is documented in the NCBI by GenBank accession number: KP065812. The in silico analysis revealed that there are lots of similar sequences in many other plant families and they all exhibit some different features from previously discovered cyclotide precursors. The differences occur particularly in the main cyclotide domain that exists without the usual CCK structure. All of these hypothetical precursors have a conserved ER-signal sequence, a Cysteine (C)-rich sequence forming two zinc finger motifs and a cyclotide-like region containing several conserved elements including two highly conserved C residues. In conclusion, using the cDNA screening method we found a potentially new cyclotide-like precursor gene and in silico studies revealed its significant characteristics that may open up a new research line on the distribution and evolution of cyclotides.

Viola "inconspicua" No More: An Analysis of Antibacterial Cyclotides

The emergence of rapidly evolving multidrug-resistant pathogens and a deficit of new compounds entering the clinical pipeline necessitate the exploration of alternative sources of antimicrobial therapeutics. Cyclotides revealed in Viola spp. are a class of highly stable, cyclic, and disulfide-bound peptides with diverse intrinsic bioactivities. Herein we have identified a novel complement of 42 putative cyclotide masses in the plant species Viola inconspicua. Cyclotide-containing fractions of a V. inconspicua peptide library revealed potent bioactivities against the Gram-negative bacteria Escherichia coli ATCC 25922 and the highly virulent and multidrug-resistant Klebsiella pneumoniae VK148. As such, six previously uncharacterized cyclotides, cycloviolacins I1-6 (cyI1-cyI6), were prioritized for molecular characterization. Cyclotides cyI3-cyI6 contain a novel "TLNGNPGA" motif in the highly variable loop six region, expanding the already substantial sequence diversity of this peptide class. Library fractions comprised of cyclotides cyI3-cyI6 exhibited MIC values of 18 and 35 μM against E. coli and K. pneumoniae, respectively, whereas isolated cyI3 killed ∼50% of E. coli at 60 μM and isolated cyI4 demonstrated no killing at concentrations >60 μM against both pathogens. This work expands the repertoire of bioactive cyclotides found in Viola spp. and highlights the potential of these antibacterial cyclic peptides.

Pharmaceutical applications of cyclotides

Cyclotides are cyclic peptides, present in several plant families, that show diverse biological properties. Structurally, cyclotides share a distinctive head-to-tail circular knotted topology of three disulfide bonds. This framework provides cyclotides with extraordinary resistance to thermal and chemical denaturation. There is increasing interest in the therapeutic potential of cyclotides, which combine several promising pharmaceutical properties, including binding affinity, target selectivity, and low toxicity towards healthy mammalian cells. Recently, cyclotides have been reported to be orally bioavailable and have proved to be amenable to modifications. Here, we provide an overview of the structure, properties, and pharmaceutical applications of cyclotides.

Transcriptomics Identifies Modules of Differentially Expressed Genes and Novel Cyclotides in Viola pubescens

Viola is a large genus with worldwide distribution and many traits not currently exemplified in model plants including unique breeding systems and the production of cyclotides. Here we report de novo genome assembly and transcriptomic analyses of the non-model species Viola pubescens using short-read DNA sequencing data and RNA-Seq from eight diverse tissues. First, V. pubescens genome size was estimated through flow cytometry, resulting in an approximate haploid genome of 455 Mbp. Next, the draft V. pubescens genome was sequenced and assembled resulting in 264,035,065 read pairs and 161,038 contigs with an N50 length of 3,455 base pairs (bp). RNA-Seq data were then assembled into tissue-specific transcripts. Together, the DNA and transcript data generated 38,081 ab initio gene models which were functionally annotated based on homology to Arabidopsis thaliana genes and Pfam domains. Gene expression was visualized for each tissue via principal component analysis and hierarchical clustering, and gene co-expression analysis identified 20 modules of tissue-specific transcriptional networks. Some of these modules highlight genetic differences between chasmogamous and cleistogamous flowers and may provide insight into V. pubescens' mixed breeding system. Orthologous clustering with the proteomes of A. thaliana and Populus trichocarpa revealed 8,531 sequences unique to V. pubescens, including 81 novel cyclotide precursor sequences. Cyclotides are plant peptides characterized by a stable, cyclic cystine knot motif, making them strong candidates for drug scaffolding and protein engineering. Analysis of the RNA-Seq data for these cyclotide transcripts revealed diverse expression patterns both between transcripts and tissues. The diversity of these cyclotides was also highlighted in a maximum likelihood protein cladogram containing V. pubescens cyclotides and published cyclotide sequences from other Violaceae and Rubiaceae species. Collectively, this work provides the most comprehensive sequence resource for Viola, offers valuable transcriptomic insight into V. pubescens, and will facilitate future functional genomics research in Viola and other diverse plant groups.

PepSAVI-MS reveals anticancer and antifungal cycloviolacins in Viola odorata

Widespread resistance to antimicrobial and cancer therapeutics is evolving in every country worldwide and has a direct impact on global health, agriculture and the economy. The specificity and selectivity of bioactive peptide natural products present a possible stopgap measure to address the ongoing deficit of new therapeutic compounds. PepSAVI-MS (Statistically-guided bioActive Peptides prioritized VIa Mass Spectrometry) is an adaptable method for the analysis of natural product libraries to rapidly identify bioactive peptides. This pipeline was validated via screening of the cyclotide-rich botanical species Viola odorata and identification of the known antimicrobial and anticancer cyclotide cycloviolacin O2. Herein we present and validate novel bioactivities of the anthelmintic V. odorata cyclotide, cycloviolacin O8 (cyO8), including micromolar anticancer activity against PC-3 prostate, MDA-MB-231 breast, and OVCAR-3 ovarian cancer cell lines and antifungal activity against the agricultural pathogen Fusarium graminearum. A reduction/alkylation strategy in tandem with PepSAVI-MS analysis also revealed several previously uncharacterized putatively bioactive cyclotides. Downstream implementation of ultraviolet photodissociation (UVPD) tandem mass spectrometry is demonstrated for cyO8 as a method to address traditionally difficult-to-sequence cyclotide species. This work emphasizes the therapeutic and agricultural potential of natural product bioactive peptides and the necessity of developing robust analytical tools to deconvolute nature's complexity.

Chemical Synthesis and Functional Analysis of VarvA Cyclotide

Cyclotides are circular peptides found in various plant families. A cyclized backbone, together with multiple disulfide bonds, confers the peptides’ exceptional stability against protease digestion and thermal denaturation. In addition, the features of these antimicrobial molecules make them suitable for use in animal farming, such as aquaculture. Fmoc solid phase peptide synthesis on 2-chlorotrityl chlorine (CTC) resin using the “tea-bag” approach was conducted to generate the VarvA cyclotide identified previously from Viola arvensis. MALDI-TOF mass spectrometry determined the correct peptide amino acid sequence and the cyclization sites-critical in this multicyclic compound. The cyclotide showed antimicrobial activity against various Gram-negative bacteria, including recurrent pathogens present in Chilean aquaculture. The highest antimicrobial activity was found to be against Flavobacterium psychrophilum. In addition, membrane blebbing on the bacterial surface after exposure to the cyclotide was visualized by SEM microscopy and the Sytox Green permeabilization assay showed the ability to disrupt the bacterial membrane. We postulate that this compound can be proposed for the control of fish farming infections.

Investigation of Antimicrobial Peptide Genes Associated with Fungus and Insect Resistance in Maize

Antimicrobial peptides (AMPs) are small defense proteins present in various organisms. Major groups of AMPs include beta-barrelin, hevein, knottin, lipid transfer protein (LTP), thionin, defensin, snakin, and cyclotide. Most plant AMPs involve host plant resistance to pathogens such as fungi, viruses, and bacteria, whereas a few plant AMPs from the cyclotide family carry insecticidal functions. In this research, a genome-wide investigation on antimicrobial peptide genes in maize genome was conducted. AMPs previously identified from various plant species were used as query sequences for maize genome data mining. Thirty-nine new maize AMPs were identified in addition to seven known maize AMPs. Protein sequence analysis revealed 10 distinguishable maize AMP groups. Analysis of mRNA expression of maize AMP genes by quantitative real-time polymerase chain reaction (qRT-PCR) revealed different expression patterns in a panel of 10 maize inbred lines. Five maize AMP genes were found significantly associated with insect or fungus resistance. Identification of maize antimicrobial peptide genes will facilitate the breeding of host plant resistance and improve maize production.

Highlights from the 52nd EUCHEM conference on stereochemistry, Bürgenstock, Switzerland, May 2017

The strong wind that was blowing in Brunnen on the 4th of May 2017 was prophetic of the storm of ideas and creativity that would later fall over the participants of the 52nd edition of the Bürgenstock conference.