Cyclotides from Brazilian Palicourea sessilis and Their Effects on Human Lymphocytes

Abstract

Cyclotides are plant-derived peptides found within five families of flowering plants (Violaceae, Rubiaceae, Fabaceae, Solanaceae, and Poaceae) that have a cyclic backbone and six conserved cysteine residues linked by disulfide bonds. Their presence within the Violaceae species seems ubiquitous, yet not all members of other families produce these macrocyclic peptides. The genus Palicourea Aubl. (Rubiaceae) contains hundreds of neotropical species of shrubs and small trees; however, only a few cyclotides have been discovered hitherto. Herein, five previously uncharacterized Möbius cyclotides within Palicourea sessilis and their pharmacological activities are described. Cyclotides were isolated from leaves and stems of this plant and identified as pase A–E, as well as the known peptide kalata S. Cyclotides were de novo sequenced by MALDI-TOF/TOF mass spectrometry, and their structures were solved by NMR spectroscopy. Because some cyclotides have been reported to modulate immune cells, pase A–D were assayed for cell proliferation of human primary activated T lymphocytes, and the results showed a dose-dependent antiproliferative function. The toxicity on other nonimmune cells was also assessed. This study reveals that pase cyclotides have potential for applications as immunosuppressants and in immune-related disorders.

Importance of the Cyclic Cystine Knot Structural Motif for Immunosuppressive Effects of Cyclotides

“…It has been developed from the prototypic anti-proliferative cyclotide kalata B1 isolated from Oldenlandia affinis. Other cyclotide family members (e.g., from the genus Palicourea or Viola ) are also known to halt the proliferation of T-cells. ,, These immunosuppressive cyclotides are characterized by a considerable variation in intercysteine loop sequences but all share the CCK as a highly conserved structural element (Figure S10). Consequently, it was of interest to explore whether, and how, the CCK contributes to the immunosuppressive activity of this peptide family …”

“…A sequence logo was prepared using the webtool Weblogo 2.82 and published sequences of immunosuppressive cyclotides: kB1 and amino acid mutants of it (T20K, G18K, N29K, T20K, N29K), pase peptides A–E, cycloviolacin O1, -O3, -O28, -O32, -T, kalata B2, -B5, -S, vaby B, vibi 6, -E, -K and varv peptide C. ,, The sequence plot shows frequencies of residues at position 1-33 of the core region from cyclotide precursor genes.…”

Notification of dried flowers of Clitoria ternatea L. as a traditional food from a third country pursuant to Article 14 of Regulation (EU) 2015/2283

“…Suppression of proliferation of human lymphocytes after in vitro exposure to cyclotides has been reported (Pinto et al, 2021) while [T20K]kalata B1 was also described to decrease symptoms in experimental autoimmune encephalomyelitis, a mouse model for multiple sclerosis (Thell et al, 2016), which is in line with the antiproliferative effect of lymphocytes in vitro. EFSA notes that even though the cyclotide Kalata B1 has not been detected in the TF, the studies indicate potential effects on the immune system for one cyclotide among a group of substances for which little information on their biological effects and potential toxicology is currently available.…”

Plant derived cyclic peptides

“…Despite the lack of the cyclic backbone, several of these peptides have been shown to have cytotoxic activities similar to their cyclic counterparts [ 18 ]. New cyclotide sequences have also recently been found in Palicourea sessilis , that have potential as immunosuppressants [ 19 ].…”