Abstract
BackgroundCellular prion-related protein (PrPc) is a cell-surface protein that is ubiquitously expressed in the human body. The multifunctionality of PrPc, and presence of an exposed cationic and heparin-binding N-terminus, a feature characterizing many antimicrobial peptides, made us hypotesize that PrPc could exert antimicrobial activity.Methodology and Principal FindingsIntact recombinant PrP exerted antibacterial and antifungal effects at normal and low pH. Studies employing recombinant PrP and N- and C-terminally truncated variants, as well as overlapping peptide 20mers, demonstrated that the antimicrobial activity is mediated by the unstructured N-terminal part of the protein. Synthetic peptides of the N-terminus of PrP killed the Gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa, and the Gram-positive Bacillus subtilis and Staphylococcus aureus, as well as the fungus Candida parapsilosis. Fluorescence studies of peptide-treated bacteria, paired with analysis of peptide effects on liposomes, showed that the peptides exerted membrane-breaking effects similar to those seen after treatment with the “classical” human antimicrobial peptide LL-37. In contrast to LL-37, however, no marked helix induction was detected for the PrP-derived peptides in presence of negatively charged (bacteria-mimicking) liposomes. PrP furthermore showed an inducible expression during wounding of human skin ex vivo and in vivo, as well as stimulation of keratinocytes with TGF-α in vitro.ConclusionsThe demonstration of an antimicrobial activity of PrP, localisation of its activity to the N-terminal and heparin-binding region, combined with results showing an increased expression of PrP during wounding, indicate that PrPs could have a previously undisclosed role in host defense.
Highlights
The innate immune system, based on antimicrobial peptides (AMP), provides a rapid and non-specific response against potentially invasive pathogenic microorganisms
The high quality peptides LVL20; LVLFVATWSDLGLCKKRPKP, KKR20; KKRPKPGGWNTGGSRYPGQG, MAN28; MANLGCWMLVLFVATWSDLGLCKKRPKP, GHH20; GHHPHGHHPHGHHPHGHHPH and AHH24; AHHAHAAHH AHAAHHAHAAHHAHA were synthesized by Biopeptide Co., San Diego, USA, with the exception of LL-37, which was obtained from Innovagen AB, Lund, Sweden
Since the latter form, lacking the first 90 amino acids of PrP significantly lost its antimicrobial potential, the results indicated that the major antimicrobial activity was dependant on an intact N-terminal region of PrP
Summary
The innate immune system, based on antimicrobial peptides (AMP), provides a rapid and non-specific response against potentially invasive pathogenic microorganisms. The majority of AMPs are characterized by an amphipathic structure, composed of hydrophobic and cationic amino acids spatially organized in sectors of the molecules. AMPs comprise linear peptides, many of which may adopt ahelical and amphipathic conformation upon bacterial binding, peptides forming cysteine-linked antiparallel b-sheets, as well as cysteineconstrained loop structures. AMPs may be found among peptides not displaying such ordered structures as long as these are characterized by an over-representation of certain amino acids[1,4,5,6]. The multifunctionality of PrPc, and presence of an exposed cationic and heparin-binding N-terminus, a feature characterizing many antimicrobial peptides, made us hypotesize that PrPc could exert antimicrobial activity
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