Alyteserin-1Ma

Alyteserin-2a (ILGKLLSTAAGLLSNL.NH(2)) is a cationic, amphipathic α-helical cell-penetrating peptide, first isolated from skin secretions of the midwife toad Alytes obstetricans. Structure-activity relationships were investigated by synthesizing analogs of alyteserin-2a in which amino acids on the hydrophobic face of the helix were replaced by L-tryptophan and amino acids on the hydrophilic face were replaced by one or more L-lysine or D-lysine residues. The Trp-containing peptides display increased cytotoxic activity against non-small cell lung adenocarcinoma A549 cells (up to 11-fold), but hemolytic activity against human erythrocytes increases in parallel. The potency of the N15K analog against A549 cells (LC(50) = 13 μM) increases sixfold relative to alyteserin-2a and the therapeutic index (ratio of LC(50) for erythrocytes and tumor cells) increases twofold. Incorporation of a D-Lys(11) residue into the N15K analog generates a peptide that retains potency against A549 cells (LC(50) = 15 μM) but whose therapeutic index is 13-fold elevated relative to the native peptide. [G11k, N15K] alyteserin-2a is also active against human hepatocarcinoma HepG2 cells (LC(50) = 26 μM), breast adenocarcinoma MDA-MB-231 cells (LC(50) = 20 μM), and colorectal adenocarcinoma HT-29 cells (LC(50) = 28 μM). [G11k, N15K] alyteserin-2a, in concentrations as low as 1 μg/mL, significantly (P < 0.05) inhibits the release of the immune-suppressive cytokines IL-10 and TGF-β from unstimulated and concanavalin A-stimulated peripheral blood mononuclear cells. The data suggest a strategy of increasing the cationicity while reducing the helicity of naturally occurring amphipathic α-helical peptides to generate analogs with improved cytotoxicity against tumor cells but decreased activity against non-neoplastic cells.

Both vertebrates and invertebrates employ alpha-helical antimicrobial peptides (AMPs) as an essential component of their innate immune system. However, evolutionary relation of these immune molecules remains unresolved. Venoms, as key weapons of venomous arthropods for prey and defense, receive increasing recognition as an emerging source of such peptides. From a cDNA library prepared from the venom gland of the scorpion Mesobuthus eupeus, clones encoding precursors of two new AMPs, named meucin-13 (IFGAIAGLLKNIF-NH(2)) and meucin-18 (FFGHLFKLATKIIPSLFQ), have been isolated. The precursor of meucins consists of a signal peptide, a mature peptide, and an acidic propeptide, in which dibasic residues as the typical processing signal are located between the mature and propeptide. Meucin-13 is an ortholog of several previously described AMPs from scorpion venom and has also detectable sequence similarity to temporins, a large family of AMPs from frog skin, whereas meucin-18 displays some similarity to AMPs from diverse origin including arthropod venoms, fish mast cells, and frog skins. These two meucin peptides form alpha-helical structure in the presence of 50% trifluoroethanol (TFE), a membrane-mimicking environment, as identified by circular dichroism (CD) spectroscopy. This finding is further verified by their NMR structures that show a typical alpha-helical amphipathic design, a structural prerequisite for cytolytic activity. Meucins exhibit extensive cytolytic effects on both prokaryotic and eukaryotic cells (gram(+) and gram(-) bacteria, fungi, yeasts, rabbit erythrocytes, and rat dorsal root ganglion cells) at micromolar concentrations. It is remarkable that muecin-18 was 2- to >14-fold more potent than meucin-13 against nearly all the cells tested. Structural differences in hydrophilic/hydrophobic balance and cationic amino acid location between two meucins could account for their differential potency. Despite these differences, commonalities at precursor organization, three-dimensional structure, and biological function suggests that meucins are two evolutionarily related AMPs and likely originated from a common ancestor by gene duplication. Our work presented here also provides new insights into an evolutionary link among AMPs from invertebrates and vertebrates and clues for evolutionary convergence between AMPs and virus fusion domains.

Two families of structurally related C-terminally alpha-amidated antimicrobial peptides have been identified in norepinephrine-stimulated skin secretions of the midwife toad Alytes obstetricans (Alytidae). The alyteserin-1 peptides (Gly-Leu-Lys-(Asp/Glu)-Ile-Phe-Lys-Ala-Gly-Leu-Gly-Ser-Leu-Val-Lys-(Gly/Asn)-Ile-Ala-Ala-His-Val-Ala-(Asn/Ser).NH(2)) show limited structural similarity to the ascaphins from the skins of frogs of the family Leiopelmatidae. Alyteserin-2a (Ile-Leu-Gly-Lys-Leu-Leu-Ser-Thr-Ala-Ala-Gly-Leu-Leu-Ser-Asn-Leu.NH(2)) and alyteserin-2b and -2c (Ile-Leu-Gly-Ala-Ile-Leu-Pro-Leu-Val-Ser-Gly-Leu-Leu-Ser-(Asn/Ser)-Lys-Leu x NH(2)) show limited sequence identity with bombinin H6, present in the skins of frogs of the family Bombinatoridae. The alyteserin-1 peptides show selective growth inhibitory activity against the Gram-negative bacteria Escherichia coli (MIC=25 microM) whereas alyteserin-2a is more potent against the Gram-positive bacteria Staphylococcus aureus (MIC=50 microM). The hemolytic activity against human erythrocytes of all peptides tested is relatively weak (LC(50)>100 microM). The data demonstrate that the frogs belonging to the family Alytidae are among those producing dermal antimicrobial peptides that may represent a component of the animal's system of innate immunity.