[Show abstract][Hide abstract] ABSTRACT: Defensins constitute a major class of cationic antimicrobial peptides in mammals and vertebrates, acting as effectors of innate immunity against infectious microorganisms. It is generally accepted that defensins are bactericidal by disrupting the anionic microbial membrane. Here, we provide evidence that membrane activity of human alpha-defensins does not correlate with antibacterial killing. We further show that the alpha-defensin human neutrophil peptide-1 (HNP1) binds to the cell wall precursor lipid II and that reduction of lipid II levels in the bacterial membrane significantly reduces bacterial killing. The interaction between defensins and lipid II suggests the inhibition of cell wall synthesis as a novel antibacterial mechanism of this important class of host defense peptides.
[Show abstract][Hide abstract] ABSTRACT: Human defensins are a family of small antimicrobial proteins found predominantly in leukocytes and epithelial cells that play important roles in the innate and adaptive immune defense against microbial infection. The most distinct molecular feature of defensins is cationicity, manifested by abundant Arg and/or Lys residues in their sequences. Sequence analysis indicates that Arg is strongly selected over Lys in alpha-defensins but not in beta-defensins. To understand this Arg/Lys disparity in defensins, we chemically synthesized human alpha-defensin 1 (HNP1) and several HNP1 analogs where three Arg residues were replaced by each of the following six alpha-amino acids: Lys, ornithine (Orn), diaminobutyric acid (Dab), diaminopropionic acid (Dap), N,N-dimethyl-Lys ((diMe)Lys), and homo-Arg ((homo)Arg). In addition, we prepared human beta-defensin 1 (hBD1) and (Lys-->Arg)hBD1 in which all four Lys residues were substituted for Arg. Bactericidal activity assays revealed the following. 1) Arg-containing HNP1 and (Lys-->Arg)hBD1 are functionally better than Lys-HNP1 and hBD1, respectively; the difference between Arg and Lys is more evident in the alpha-defensin than in the beta-defensin and is more evident at low salt concentrations than at high salt concentrations. 2) For HNP1, the Arg/Lys disparity is much more pronounced with Staphylococcus aureus than with Escherichia coli, and the Arg-rich HNP1 kills bacteria faster than its Lys-rich analog. 3) Arg and Lys appear to have optimal chain lengths for bacterial killing as shortening Lys or lengthening Arg in HNP1 invariably becomes functionally deleterious. Our findings provide insights into the Arg/Lys disparity in defensins, and shed light on the cationicity of defensins with respect to their antimicrobial activity and specificity.
[Show abstract][Hide abstract] ABSTRACT: Human neutrophil alpha-defensins (HNPs) are synthesized in vivo as inactive precursor proteins, i.e. preproHNPs. A series of sequential proteolytic events excise the N-terminal inhibitory pro peptide, leading to defensin maturation and storage in azurophilic granules. The anionic pro peptide, required for correct sub-cellular trafficking and sorting of proHNPs, inhibits the antimicrobial activity of cationic defensins, either inter or intra-molecularly, presumably through charge neutralization. To better understand the role of the pro peptide in the folding and functioning of alpha-defensins and/or pro alpha-defensins, we chemically attached the proHNP1 pro peptide or (wt)pro peptide and the following artificial pro segments to the N terminus of HNP1: polyethylene glycol (PEG), Arg(10) (polyR), Ser(10) (polyS), and (cr)pro peptide, a charge-reversing mutant of the pro peptide where Arg/Lys residues were changed to Asp, and Asp/Glu residues to Lys. Comparative in vitro folding suggested that while all artificial pro segments chaperoned defensin folding, with PEG being the most efficient, the pro peptide catalyzed the folding of proHNPs likely through two independent mechanisms: solubilization of and interaction with the C-terminal defensin domain. Further, the N-terminal artificial pro segments dramatically altered the bactericidal activity of HNP1 against both Escherichia coli and Staphylococcus aureus. Surprisingly, (cr)pro peptide and (wt)pro peptide showed similar properties with respect to intra-molecular and inter-molecular catalysis of defensin folding as well as alpha-defensin binding, although their binding modes appeared different. Our findings identify a dual chaperone activity of the pro peptide and may shed light on the molecular mechanisms by which pro alpha-defensins fold in vivo.
[Show abstract][Hide abstract] ABSTRACT: Human neutrophil alpha-defensin 2 (HNP2) was N-terminally acetylated and/or C-terminally amidated, resulting in three terminally modified analogs, Ac-HNP2, HNP2-NH2 and Ac-HNP2-NH2. We examined their bactericidal activity against E. coli and S. aureus and their ability to induce leakage from large unilamellar vesicles. Loss of the N-terminal positive charge was functionally deleterious, whereas removal of the C-terminal negative charge enhanced microbial killing and membrane permeabilization. Our findings validate the importance of electrostatic forces in defensin-microbe interactions and point to the bacterial cytoplasmic membrane as a target of HNP2 activity.
[Show abstract][Hide abstract] ABSTRACT: Defensins are cationic antimicrobial mini-proteins that play important roles in the innate immune defense against microbial infection. Six invariant Cys residues in each defensin form three structurally indispensable intramolecular disulfide bridges. The only other residue invariant in all known mammalian defensins is a Gly. Structural studies indicate that the invariant Gly residue is located in an atypical, classic-type beta-bulge with the backbone torsion angles (Phi, Psi) disallowed for L-amino acids but permissible for D-enantiomers. We replaced the invariant Gly17 residue in human neutrophil alpha-defensin 2 (HNP2) by L-Ala or one of the D-amino acids Ala, Glu, Phe, Arg, Thr, Val, or Tyr. Although L-Ala17-HNP2 could not be folded, resulting in massive aggregation, all of the D-amino acid-substituted analogs folded with high efficiency. The high resolution x-ray crystal structures of dimeric D-Ala17-HNP2 were determined in three different crystal forms, showing a well preserved beta-bulge identical to those found in other defensins. The seven D-analogs of HNP2 exhibited highly variable bactericidal activity against Gram-positive and Gram-negative test strains, consistent with the premise that interplay between charge and hydrophobicity dictates how amphiphilic defensins kill. Further, the bactericidal activity of these d-amino acid analogs of HNP2 correlated well with their ability to induce leakage from large unilamellar vesicles, supporting membrane permeabilization as the lethal event in microbial killing by HNP2. Our findings identify a conformational prerequisite in the beta-bulge of defensins essential for correct folding and native structure, thereby explaining the molecular basis of the Gly-Xaa-Cys motif conserved in all mammalian defensins.
[Show abstract][Hide abstract] ABSTRACT: Amoxicillin mucoadhesive microspheres (Amo-ad-ms) were prepared using ethylcellulose (Ec) as matrix and carbopol 934P as mucoadhesive polymer for the potential use of treating gastric and duodenal ulcers, which were associated with Helicobacter pylori. The morphological characteristics of the mucoadhesive microspheres were studied under scanning electron microscope. In vitro release test showed that amoxicillin released faster in pH 1.0 hydrochloric acid (HCl) than in pH 7.8 phosphate buffer. Yet, it would be degraded to some extent in a pH 1.0 HCl medium at 37 degrees C, which indicated that amoxicillin was not stable in an acidic surrounding. It was also found that amoxicillin entrapped within the microspheres could keep stable. In vitro and in vivo mucoadhesive tests showed that Amo-ad-ms adhered more strongly to gastric mucous layer than nonadhesive amoxicillin microspheres (Amo-Ec-ms) did and could retain in gastrointestinal tract for an extended period of time. Amo-ad-ms and amoxicillin powder were orally administered to rats. The amoxicillin concentration in gastric tissue was higher in the Amo-ad-ms group. In vivo H. pylori clearance tests were also carried out by administering, respectively, Amo-ad-ms or amoxicillin powder, to H. pylori infectious BALB/c mice under fed conditions at single or multiple dose(s) in oral administration. The results showed that Amo-ad-ms had a better clearance effect than amoxicillin powder did. In conclusion, the prolonged gastrointestinal residence time and enhanced amoxicillin stability resulting from the mucoadhesive microspheres of amoxicillin might make contribution to H. pylori clearance.