Drug discovery from natural products is still one of the biggest sources of novel lead compounds. In particular, plant cyclotides, disulfide-rich peptides comprising three conserved disulfide bonds in a knotted arrangement, known as cyclic cystine knot motif, and a head-to-tail cyclization, have been extensively investigated over the last four decades for their use as scaffolds in drug development. However, their distribution among flowering plants still remains limited to few species of the families of Rubiaceae (coffee), Violaceae (violet), Cucurbitaceae (cucurbit), Fabaceae (bean) and recently Solanaceae (potato family), but it is very likely that cyclotides are more widely distributed since their predicted number in Rubiaceae alone is ~50.000. Additionally, the pharmacological validation of plants used in traditional medicines may trigger the discovery of novel uterotonic compounds .
Methods and results
Based on the use of plants in traditional Nigerian medicine during pregnancy and childbirth, we analyzed several plants from different families to evaluate their uterotonic properties at cellular level and to identify cyclotides as active molecules. Using a MALDI-TOF/TOF-based screening protocol we were able to identify many novel cyclotide-containing species which was confirmed by manual de novo sequencing and automated database identification. The aqueous extracts and semipure peptide fractions have been tested further in a collagen gel contractility assay model and showed varying ability to induce contractions in human myometrial smooth muscle cells.
In conclusion, our results underpin earlier suggestions that cyclotides are one of the largest peptide classes within plants, covering a large chemical space based on their high sequence diversity. The evaluation of contractile properties of plants used in traditional medicines offers new starting points for the discovery and development of peptide-based uterotonic drug leads.
This work is funded by the Austrian Science Fund (FWF, grant P22889) and an “Ernst-Mach” Scholarship from the Austrian Agency for International Mobility and Cooperation in Education, Science and Research (OeAD) (A.A.).