[Energy-dependent translocation of amino-phospholipids in the erythrocyte membrane]

The proteome of sickle cell disease: insights from exploratory proteomic profiling

The expanding realm of exploratory proteomics has added a unique dimension to the study of the complex pathophysiology involved in sickle cell disease. A review of proteomic studies published on sickle cell erythrocytes and plasma shows trends of upregulation of antioxidant proteins, an increase in cytoskeletal defects, an increase in protein repair and turnover components, a decrease in lipid raft proteins and apolipoprotein dysregulation. Many of these findings are consistent with the pathophysiology of sickle cell disease, including high oxidant burden, resulting in damage to cytoskeletal and other proteins, and erythrocyte rigidity. More unexpected findings, such as a decrease in lipid raft components and apolipoprotein dysregulation, offer previously unexplored targets for future investigation and potential therapeutic intervention. Exploratory proteomic profiling is a valuable source of hypothesis generation for the cellular and molecular pathophysiology of sickle cell disease.

Red blood cell (RBC) membrane proteomics--Part I: Proteomics and RBC physiology

Membrane proteomics is concerned with accurately and sensitively identifying molecules involved in cell compartmentalisation, including those controlling the interface between the cell and the outside world. The high lipid content of the environment in which these proteins are found often causes a particular set of problems that must be overcome when isolating the required material before effective HPLC-MS approaches can be performed. The membrane is an unusually dynamic cellular structure since it interacts with an ever changing environment. A full understanding of this critical cell component will ultimately require, in addition to proteomics, lipidomics, glycomics, interactomics and study of post-translational modifications. Devoid of nucleus and organelles in mammalian species other than camelids, and constantly in motion in the blood stream, red blood cells (RBCs) are the sole mammalian oxygen transporter. The fact that mature mammalian RBCs have no internal membrane-bound organelles, somewhat simplifies proteomics analysis of the plasma membrane and the fact that it has no nucleus disqualifies microarray based methods. Proteomics has the potential to provide a better understanding of this critical interface, and thereby assist in identifying new approaches to diseases.

Multi drug resistance-dependent “vacuum cleaner” functionality potentially driven by the interactions between endocytosis, drug size and Pgp-like transporters surface density

In cells, multi drug resistance (MDR) is associated with Pgp-like transporters expression extruding drugs from cellular membranes. MDR is efficiently generated with a relatively small fraction of membrane transporters. As the insertion of drugs into cellular membranes is widespread, there are no reasons why a drug should incorporate the membrane in the vicinity of a transporter. As a result a further elusive hypothesis is usually invoked: these transporters act like "vacuum cleaners" of drugs embedded in the membrane. Nonetheless, how these transporters attract drugs remains obscure. To clarify the "vacuum cleaner" notion, we suggest that during its residency time in cellular membranes, the lateral movement of drugs from their point of insertion to transporters is governed by Brownian's diffusion, which allows the drugs/transporters interaction. Taking into account the functionality of Pgp-like transporters, namely the extrusion of drugs from the plasma membrane inner leaflet, we characterize how the state of drug resistance is triggered involving: membrane endocytosis, drug physico-chemical properties and the surface density of Pgp-like transporters. In addition, the theory developed provides for the first time a theoretical proof of Lipinski's second rule with regard to drugs' size (or MW) selectivity on their permeation across cellular membranes.

New effects in polynucleotide release from cationic lipid carriers revealed by confocal imaging, fluorescence cross-correlation spectroscopy and single particle tracking

We report on new insights into the mechanisms of short single and double stranded oligonucleotide release from cationic lipid complexes (lipoplexes), used in gene therapy. Specifically, we modeled endosomal membranes using giant unilamellar vesicles and investigated the roles of various individual cellular phospholipids in interaction with lipoplexes. Our approach uses a combination of confocal imaging, fluorescence cross-correlation spectroscopy and single particle tracking, revealing several new aspects of the release: (a) phosphatidylserine and phosphatidylethanolamine are equally active in disassembling lipoplexes, while phosphatidylcholine and sphingomyelin are inert; (b) in contrast to earlier findings, phosphatidylethanolamine alone, in the absence of anionic phosphatidylserine triggers extensive release; (c) a double-stranded DNA structure remains well preserved after release; (d) lipoplexes exhibited preferential binding to transient lipid domains, which appear at the onset of lipoplex attachment to originally uniform membranes and vanish after initiation of polynucleotide release. The latter effect is likely related to phosphatidyleserine redistribution in membranes due to lipoplex binding. Real time tracking of single DOTAP/DOPE and DOTAP/DOPC lipoplexes showed that both particles remained compact and associated with membranes up to 1-2 min before fusion, indicating that a more complex mechanism, different from suggested earlier rapid fusion, promotes more efficient transfection by DOTAP/DOPE complexes.

Is lipid translocation involved during endo- and exocytosis?

Stimulation of the aminophospholipid translocase, responsible for the transport of phosphatidylserine and phosphatidylethanolamine from the outer to the inner leaflet of the plasma membrane, provokes endocytic-like vesicles in erythrocytes and stimulates endocytosis in K562 cells. In this article arguments are given which support the idea that the active transport of lipids could be the driving force involved in membrane folding during the early step of endocytosis. The model is sustained by experiments on shape changes of pure lipid vesicles triggered by a change in the proportion of inner and outer lipids. It is shown that the formation of microvesicles with a diameter of 100-200 nm caused by the translocation of plasma membrane lipids implies a surface tension in the whole membrane. It is likely that cytoskeleton proteins and inner organelles prevent a real cell from undergoing overall shape changes of the type seen with giant unilamellar vesicles. Another hypothesis put forward in this article is the possible implication of the phospholipid 'scramblase' during exocytosis which could favor the unfolding of microvesicles.

Phospholipid domains determine the spatial organization of the Escherichia coli cell cycle: the membrane tectonics model

Escherichia coli normally divides at its equator between segregated nucleoids. Such division is inhibited during perturbations of chromosome replication (even in the absence of inducible division inhibitors); eventually, division resumes at sites which are not at this equator. Escherichia coli will also divide at its poles to generate minicells following overproduction of the FtsZ or MinE proteins. The mechanisms underlying the division inhibition and the positioning of the division sites are unknown. In the membrane tectonics model, I propose that the formation of phospholipid domains within the cytoplasmic membrane positions division sites. The particular phospholipid composition of a domain attracts particular proteins and determines their activity; conversely, particular proteins change the composition of domains. Principally via such proteins, the interaction of the chromosome with the membrane creates a chromosomal domain. The development of chromosomal domains during replication and nucleoid formation contributes to the formation and positioning of a septal domain between them. During septation (cell division), this septal domain matures into a polar domain. Each domain attracts and activates different enzymes. The septal domain attracts and activates enzymes necessary for septation. Preventing the formation of the septal domain by preventing chromosome replication prevents normal division. Altering the composition of the polar domain may allow septation enzymes to function there and generate minicells. A corollary of the model explains how the formation of an origin domain by the attachment of hemi-methylated origin DNA to the membrane may underlie the creation and migration of structures within the envelope, the periseptal annuli.

In vitro synthesis and transbilayer movement of phosphatidylethanolamine molecules labelled with different fatty acids in chick brain microsomes

The transbilayer fatty acid distribution of diacylglycerophosphoethanolamine and the translocation of newly synthesized phosphatidylethanolamine molecules labelled with different fatty acids has been investigated in chick brain microsomes using trinitrobenzensulfonic acid. The determination of the fatty acid composition of diacylglycerophosphoethanolamine in both the outer and the inner leaflet of the microsomal vesicles revealed a similar distribution indicating that both leaflets share the same molecular species. The in vitro incorporation of radioactive fatty acids (16:0, 18:1 and 20:4(n-6] into ethanolamine phospholipids, known to be catalyzed by the lyosphosphatidylethanolamine acyl transferase, showed that the radioactive diacylglycerophosphoethanolamine molecules appeared first in the outer leaflet and were thereafter transferred to the inner leaflet. The apparent rate of translocation of the newly synthesized ethanolamine phospholipid molecules was the highest for those labelled with 16:0 and the lowest for those labelled with 20:4(n-6). The results indicate that the active site of the acyl-CoA:lysophosphatidylethanolamine acyltransferases is located on the outer leaflet of the microsomal vesicles and that the different newly synthesized molecular species of diacylglycerophosphoethanolamine may be translocated from the outer to the inner leaflet at different rates.