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Tuning the biological properties of amphipathic alpha-helical antimicrobial peptides: rational use of minimal amino acid substitutions.
No full textIn nature, alpha-helical antimicrobial peptides present the small and flexible residue glycine at positions 7 or 14 with a significant frequency. Based on the sequence of the non-proteinogenic alpha-helical model peptide P1(Aib7), with a potent, broad spectrum antimicrobial activity, six peptides were designed by effecting a single amino acid substitution to investigate how tuning the structural characteristics at position 7 could lead to optimization of selectivity without affecting antimicrobial activity against a broad panel of multidrug resistant bacterial and yeast indicator strains. The relationship between structural features (size/hydrophobicity of the side chain as well as conformation and flexibility) and biological activity, in terms of minimum inhibitory concentration, membrane permeabilization kinetics and lysis of red blood cells are discussed. On conversion of the peptide to proteinogenic residues, these principles allowed development of a potent antimicrobial peptide with a reduced cytotoxicity. However, while results suggest that both hydrophobicity of residue 7 and chain flexibility at this position can be modulated to improve selectivity, position 14 is less tolerant of substitutions
Controlled alteration of the shape and conformational stability of alpha-helical cell-lytic peptides: effect on mode of action and cell specificity
No full textA novel method, based on the rational and systematic modulation
of macroscopic structural characteristics on a template originating
from a large number of natural, cell-lytic, amphipathic α-helical
peptides, was used to probe how the depths and shapes of hydrophobic
and polar faces and the conformational stability affect
antimicrobial activity and selectivity with respect to eukaryotic
cells. A plausible mode of action explaining the peptides’ behaviour
in model membranes, bacteria and host cells is proposed.
Cytotoxic activity, in general, correlated strongly with the hydrophobic
sector depth, and required a majority of aliphatic residue
side chains having more than two carbon atoms. It also correlated
significantly with the size of polar sector residues, which determines
the penetration depth of the peptide via the so-called
snorkel effect. Both an oblique gradient of long to short aliphatic
residues along the hydrophobic face and a stabilized helical
structure increased activity against host cells but not against bacteria,
as revealed by haemolysis, flow cytofluorimetric studies on
lymphocytes and surface plasmon resonance studies with model
phosphatidylcholine/cholesterol membranes. The mode of interaction
changes radically for a peptide with a stable, preformed
helical conformation compared with others that form a structure
only on membrane binding. The close correlation between effects
observed in biological andmodel systems suggests that the ‘carpet
model’ correctly represents the type of peptides that are bacteriaselective,
whereas the behaviour of those that lyse host cells is
more complex
Mammalian Defensins: Structures and Mechanism of Antibiotic Activity
No full textAntibiotic peptides are important effector molecules in host-parasite interactions throughout the living world. In vertebrates, they function in first-line host defense by antagonizing a wide range of microbes including bacteria, fungi, and enveloped viruses. The antibiotic activity is thought to be based on their cationic, amphipathic nature, which enables the peptides to impair vital membrane functions. Molecular details for such activities have been elaborated with model membranes; however, there is increasing evidence that these models may not reflect the complex processes involved in the killing of microbes. For example, the overall killing activity of the bacterial peptide antibiotic nisin is composed of independent activities such as the formation of target-mediated pores, inhibition of cell-wall biosynthesis, formation of nontargeted pores, and induction of autolysis. We studied the molecular modes of action of human defense peptides and tried to determine whether they impair membrane functions primarily and whether additional antibiotic activities may be found. We compared killing kinetics, solute efflux kinetics, membrane-depolarization assays, and macromolecular biosynthesis assays and used several strains of Gram-positive cocci as test strains. We found that membrane depolarization contributes to rapid killing of a significant fraction of target cells within a bacterial culture. However, substantial subpopulations appear to survive the primary effects on the membrane. Depending on individual strains and species and peptide concentrations, such subpopulations may resume growth or be killed through additional activities of the peptides. Such activities can include the activation of cell-wall lytic enzymes, which appears of particular importance for killing of staphylococcal strains
Mode of action of human β-defensin 3 (hBD3) against Staphylococcus aureus and transcriptional analysis of responses to defensin challenge
No full textHuman β-defensin 3 (hBD3) is a highly charged (+11) cationic defence peptide, which retains activity against staphylococci even at elevated salt concentrations. We studied the antibiotic mode of action of hBD3 against Staphylococcus aureus SG511, using whole-cell assays and analysing the transcriptional response to hBD3 treatment. hBD3 caused rapid killing and simultaneously blocked all biosynthetic pathways, however, significant depolarisation was not observed and permeabilisation of the membrane was incomplete. The transcriptional response pattern was in part similar to those of strongly cationic amphiphiles, e.g. in that anaerobic energy production was downregulated. Significantly, part of the staphylococcal cell wall stress stimulon were upregulated. The most prominent microbial counterstrategy appears to be based on the upregulation of ABC transporters possibly functioning in detoxification of the membrane environment; the transporter genes were highly upregulated in both, a short-term response and a long-term adaptation experiment. Knockout of the most highly induced transporter VraDE significantly enhanced hBD3 susceptibility. We propose that the antibiotic activity of hBD3 is based on interference with the organisation over space and time of membrane-bound multienzyme machineries such as the electron transport chain and, in particular, the cell wall biosynthesis complex rather than on formation of defined transmembrane pores. © 2008 Elsevier GmbH. All rights reserved
Identification and optimization of an antimicrobial peptide from the ant venom toxin pilosulin
No full textA template based on positional residue frequencies in the N-terminal stretch of natural α-helical antimicrobial peptides was used to prepare sequence patterns and to scan the Swiss-Prot Database, using the ScanProsite tool. This search identified a segment in pilosulin 1, a cytotoxic peptide from the venom of the jumper ant Myrmecia pilosula, as a potential novel antimicrobial peptide sequence. This segment, corresponding to the 20 N-terminal residues, was synthesized and its structural properties and biological activities were investigated. It showed a potent and broad spectrum antimicrobial activity including standard and multi-drug resistant gram-positive and gram-negative bacteria and Candida albicans, confirming the validity of the search method. A rational redesign approach resulting in four amino acid substitutions yielded a variant with improved antibacterial and significantly reduced hemolytic activity
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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