Host defence peptides (HDPs) are effector molecules of the innate immune system. They show broad activity against bacteria and cancer cells, usually by perturbing the permeability of cell membranes, leading to the death of pathogens by collapse of transmembrane electrochemical gradients and loss of important metabolites and cellular components. Due to this mechanism of action, development of resistance is unlikely. For this reason, HDPs are excellent candidates as a new class of therapeutics to address the problem of drugresistance pathogens. Pore formation in the pathogen membranes requires a significant accumulation of peptide molecules on the cell surface. Therefore, it is conceivable that other effects, in addition to permeabilization, might take place. In this thesis, the effects on membrane dynamics of four peptides (magainin, melittin, LAH4 and killer-FLIP) were analysed. In all cases, a peptide-induced reduction in lipid mobility and lateral diffusion was observed. These effects appeared to be independent of peptide electrostatic charge. A significant hindering of lipid dynamics might lead to inhibition of the function of membrane proteins, contributing to the bactericidal activity of the peptides. Cell selectivity of HDPs is due to the different composition of pathogen and host membranes, and particularly to the negatively charged lipids present in the membranes of bacteria and cancer cells. As a consequence, the cationic charge of most HDPs is an essential determinant of selectivity. However, it is not sufficient: extensive structure-activity relationship studies with a number of HDPs have revealed that a delicate balance between net charge, amphipathicity, hydrophobicity, and structural propensity is critically important to ensure antimicrobial potency and target cell selectivity. An immediate consequence of HDP amphipathicity is the tendency of several of these peptides to aggregate in water. In this thesis, the role of this phenomenon in cell-selectivity has been investigated, focusing on the cationic peptide killer-FLIP, which is strongly selective for cancer cells. We observed that this property is linked to the formation of aggregates. Notwithstanding the cationic charge, the monomeric peptide has a significant affinity towards all membrane compositions, because the water-exposed apolar residues provide a hydrophobic driving force for binding and insertion into neutral membranes of normal cells. By contrast, in the aggregates the hydrophobic sidechains are buried, thus reducing the affinity towards neutral membranes. At the same time, the aggregates still tend to associate to the anionic membranes of cancer cells, driven by electrostatic attraction. Overall, these findings provide a better understanding of the structural determinants of the membrane-perturbing activity and selectivity of HDPs, which is essential for the development of novel HDP-inspired drugs to effectively fight resistant infections and cancer, with minimum toxicity to eukaryotic cells.
(2015). Host defence peptides: mechanisms of membrane perturbation and target cell selectivity.
|Titolo:||Host defence peptides: mechanisms of membrane perturbation and target cell selectivity|
|Data di pubblicazione:||2015|
|Corso di dottorato:||Scienze chimiche|
|Settore Scientifico Disciplinare:||Settore CHIM/03 - Chimica Generale e Inorganica|
|Tipologia:||Tesi di dottorato|
|Citazione:||(2015). Host defence peptides: mechanisms of membrane perturbation and target cell selectivity.|
|Appare nelle tipologie:||07 - Tesi di dottorato|