Cell specificity, anti-inflammatory activity, and plausible bactericidal mechanism of designed Trp-rich model antimicrobial peptides

Abstract

To develop novel short Trp-rich antimicrobial peptides (AMPs) with potent cell specificity (targeting bacteria but not eukaryotic cells) and anti-inflammatory activity, a series of 11-meric Trp-rich model peptides with different ratios of Leu and Lys/Arg residues, XXWXXWXXWXX-NH 2 (X indicates Leu or Lys/Arg), was synthesized. K 6L 2W 3 displayed an approximately 40-fold increase in cell specificity, compared with the natural Trp-rich AMP indolicidin (IN). Lys-containing peptides (K 8W 3, K 7LW 3 and K 6L 2W 3) showed approximately 2- to 4-fold higher cell specificities than did their counterparts, the Arg-containing peptides (R 8W 3, R 7LW 3 and R 6L 2W 3), indicating that multiple Lys residues are more important than multiple Arg residues in the design of AMPs with good cell specificity. The excellent resistance of d-enantiomers (K 6L 2W 3-D and R 6L 2W 3-D) and Orn/Nle-containing peptides (O 6L 2W 3 and O 6L 2W 3) to trypsin digestion compared with the rapid breakdown of the l-enantiomers (K 6L 2W 3 and R 6L 2W 3), highlights the clinical potential of such peptides. K 6L 2W 3, R 6L 2W 3, K 6L 2W 3-D and R 6L 2W 3-D caused weak dye leakage from bacterial membrane-mimicking negatively charged EYPG/EYPE (7:3, v/v) liposomes. Confocal microscopy showed that these peptides penetrated the cell membrane of Escherichia coli and accumulated in the cytoplasm, as observed for buforin-2. Gel retardation studies revealed that the peptides bound more strongly to DNA than did IN. These results suggested that one possible peptide bactericidal mechanism may relate to the inhibition of intracellular functions via interference with DNA/RNA synthesis. Furthermore, some model peptides, containing K 6L 2W 3, K 5L 3W 3, R 6L 2W 3, O 6L 2W 3, O 6L 2W 3, and K 6L 2W 3-D inhibited LPS-induced inducible nitric oxide synthase (iNOS) mRNA expression, the release of nitric oxide (NO) following LPS stimulation in RAW264.7 cells and had powerful LPS binding activities at bactericidal concentrations. Collectively, our results indicated that these peptides have potential for future development as novel antimicrobial and anti-inflammatory agents.