Direct Vesicle-Vesicle Exchange of Phospholipids Mediated by Polymyxin B

doi:10.1006/bbrc.1995.1722

Biochemical and Biophysical Research Communications

Volume 210, Issue 3, 25 May 1995, Pages 746-752

https://doi.org/10.1006/bbrc.1995.1722 Get rights and content

Abstract

Direct and rapid intermembrane exchange of phospholipids without fusion is shown to occur across stable vesicle-vesicle contacts formed by stoichiometric amounts of polymyxin B. The exchange is selective for monoanionic, but not dianionic, glycerophospholipids irrespective of their chain length or phase properties. Selective transfer mediated by protein contacts between membranes could serve as a model for stable fusion intermediates and provide a structural and organizational basis for direct phospholipid transfer as implicated in entry of enveloped viruses, secretion, endocytosis, and intracellular transport and targetting of lipids. It is also proposed that the loss of the specificity of phospholipid composition in the inner and outer membranes of gram negative bacteria mediated by polymyxin B could be the basis for its bactericidal action.

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With the rising incidences of antimicrobial resistance (AMR) and the diminishing options of novel antimicrobial agents, it is paramount to decipher the molecular mechanisms of action and the emergence of resistance to the existing drugs. Polymyxin, a cationic antimicrobial lipopeptide, is used to treat infections by Gram-negative bacterial pathogens as a last option. Though polymyxins were identified almost seventy years back, their use has been restricted owing to toxicity issues in humans. However, their clinical use has been increasing in recent times resulting in the rise of polymyxin resistance. Moreover, the detection of “mobile colistin resistance (mcr)” genes in the environment and their spread across the globe have complicated the scenario. The mechanism of polymyxin action and the development of resistance is not thoroughly understood. Specifically, the polymyxin-bacterial lipopolysaccharide (LPS) interaction is a challenging area of investigation. The use of advanced biophysical techniques and improvement in molecular dynamics simulation approaches have furthered our understanding of this interaction, which will help develop polymyxin analogs with better bactericidal effects and lesser toxicity in the future. In this review, we have delved deeper into the mechanisms of polymyxin-LPS interactions, highlighting several models proposed, and the mechanisms of polymyxin resistance development in some of the most critical Gram-negative pathogens.
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Antimicrobial resistance is a major global health challenge. As few new efficacious antibiotics will become available in the near future, peptide antibiotics continue to be major therapeutic options for treating infections caused by multidrug-resistant pathogens. Rational use of antibiotics requires optimisation of the pharmacokinetics and pharmacodynamics for the treatment of different types of infections. Toxicodynamics must also be considered to improve the safety of antibiotic use and, where appropriate, to guide therapeutic drug monitoring. This review focuses on the pharmacokinetics/pharmacodynamics/toxicodynamics of peptide antibiotics against multidrug-resistant Gram-negative and Gram-positive pathogens. Optimising antibiotic exposure at the infection site is essential for improving their efficacy and minimising emergence of resistance.
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Multidrug-resistant Gram-negative bacteria have increased the prevalence of a variety of serious diseases in modern times. Polymyxins are used as the last-line therapeutic options for the treatment of infections. However, the mechanism of action of polymyxins remains in dispute. In this work, we used a coarse-grained molecular dynamics simulation to investigate the mechanism of the cationic antimicrobial peptide polymyxin B (PmB) interacting with both the inner and outer membrane models of bacteria. Our results show that the binding of PmB disturbs the outer membrane by displacing the counterions, decreasing the orientation order of the lipopolysaccharide tail, and creating more lipopolysaccharide packing defects. Upon binding onto the inner membrane, in contrast to the traditional killing mechanism that antimicrobial peptides usually use to induce holes in the membrane, PmBs do not permeabilize the inner membrane but stiffen it by filling up the lipid packing defect, increasing the lipid tail order and the membrane bending rigidity as well as restricting the lipid diffusion. PmBs also mediate intermembrane contact and adhesion. These joint effects suggest that PmBs deprive the biological activity of Gram-negative bacteria by sterilizing the cell.
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Role of 57-72 loop in the allosteric action of bile salts on pancreatic IB phospholipase A<inf>2</inf>: Regulation of fat and cholesterol homeostasis
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Citation Excerpt :
With suitable precautions estimated uncertainty in the reported rates is 10%. The pH-stat protocol was also used for monitoring the rate of hydrolysis of DMPM (0.4 mM) vesicles in the presence of polymyxin B [35,38,39], or of mixed-micelles of 1 mM POPC with bile salts in 1:2 ratio [20,40], or of dioctanoylphosphatidylcholine micelles under the low salt conditions [41]. If suitably adopted and controlled, these kinetic assays provide significant mechanistic insights [16].
Mono- and biphasic kinetic effects of bile salts on the pancreatic IB phospholipase A2 (PLA2) catalyzed interfacial hydrolysis are characterized. This novel phenomenon is modeled as allosteric action of bile salts with PLA2 at the interface. The results and controls also show that these kinetic effects are not due to surface dilution or solubilization or disruption of the bilayer interface where in the mixed-micelles substrate replenishment becomes the rate-limiting step. The PLA2-catalyzed rate of hydrolysis of zwitterionic dimyristoylphosphatidylcholine (DMPC) vesicles depends on the concentration and structure of the bile salt. The sigmoidal rate increase with cholate saturates at 0.06 mole fraction and changes little at the higher mole fractions. Also, with the rate-lowering bile salts (B), such as taurochenodeoxycholate (TCDOC), the initial sigmoidal rate increase at lower mole fraction is followed by nearly complete reversal to the rate at the pre-activation level at higher mole fractions. The rate-lowering effect of TCDOC is not observed with the (62–66)-loop deleted ΔPLA2, or with the Naja venom PLA2 that is evolutionarily devoid of the loop. The rate increase is modeled with the assumption that the binding of PLA2 to DMPC interface is cooperatively promoted by bile salt followed by allosteric k_cat^⁎-activation of the bound enzyme by the anionic interface. The rate-lowering effect of bile salts is attributed to the formation of a specific catalytically inert E^⁎B complex in the interface, which is noticeably different than the 1:1 EB complex in the aqueous phase. The cholate-activated rate of hydrolysis is lowered by hypolidemic ezetimibe and guggul extract which are not interfacial competitive inhibitors of PLA2. We propose that the biphasic modulation of the pancreatic PLA2 activity by bile salts regulates gastrointestinal fat metabolism and cholesterol homeostasis.
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Regular ArticleDirect Vesicle-Vesicle Exchange of Phospholipids Mediated by Polymyxin B

Abstract

Regular Article
Direct Vesicle-Vesicle Exchange of Phospholipids Mediated by Polymyxin B