Red blood cell lipids and the plasma membrane

Computational study of the conformational ensemble of CX3C chemokine receptor 1 (CX3CR1) and its interactions with antagonist and agonist ligands

The CX3C chemokine receptor 1 (CX3CR1), a member of the class A of G Protein-Coupled Receptors (GPCR) superfamily, and its ligand fractalkine constitute an important biochemical axis that influence many cellular pathways involving homeostatic and inflammatory processes. They participate in the activation, chemotaxis and recruitment of multiple immunological cells such as microglia, macrophages and monocytes, and play a critical role in neuroinflammatory conditions such as Alzheimer's disease and multiple sclerosis, in the recovery from central nervous system injuries, in several chronic, peripheral inflammatory entities and in some infective processes including HIV-AIDS. In this work we present the study of the CX3CR1 receptor employing extensive atomistic Molecular Dynamics (MD) simulations with the aim to characterize the conformational ensemble of the receptor in the presence of its antagonist and agonist ligands. We analyzed the receptor conformational changes and described interactions within its key regions and the bounded ligands to identify their notable differences. Finally, we classify the features that would allow the identification of patterns that characterize a functional state to contribute to the understanding of the complexity of the GPCR superfamily.

The P2X7 receptor forms a dye-permeable pore independent of its intracellular domain but dependent on membrane lipid composition

The P2X7 receptor mediates extracellular ATP signaling implicated in the development of devastating diseases such as chronic pain and cancer. Activation of the P2X7 receptor leads to opening of the characteristic dye-permeable membrane pore for molecules up to ~900 Da. However, it remains controversial what constitutes this peculiar pore and how it opens. Here we show that the panda receptor, when purified and reconstituted into liposomes, forms an intrinsic dye-permeable pore in the absence of other cellular components. Unexpectedly, we found that this pore opens independent of its unique C-terminal domain. We also found that P2X7 channel activity is facilitated by phosphatidylglycerol and sphingomyelin, but dominantly inhibited by cholesterol through direct interactions with the transmembrane domain. In combination with cell-based functional studies, our data suggest that the P2X7 receptor itself constitutes a lipid-composition dependent dye-permeable pore, whose opening is facilitated by palmitoylated cysteines near the pore-lining helix.

Cholesterol segregates into submicrometric domains at the living erythrocyte membrane: evidence and regulation

Although cholesterol is essential for membrane fluidity and deformability, the level of its lateral heterogeneity at the plasma membrane of living cells is poorly understood due to lack of appropriate probe. We here report on the usefulness of the D4 fragment of Clostridium perfringens toxin fused to mCherry (theta*), as specific, non-toxic, sensitive and quantitative cholesterol-labeling tool, using erythrocyte flat membrane. By confocal microscopy, theta* labels cholesterol-enriched submicrometric domains in coverslip-spread but also gel-suspended (non-stretched) fresh erythrocytes, suggesting in vivo relevance. Cholesterol domains on spread erythrocytes are stable in time and space, restricted by membrane:spectrin anchorage via 4.1R complexes, and depend on temperature and sphingomyelin, indicating combined regulation by extrinsic membrane:cytoskeleton interaction and by intrinsic lipid packing. Cholesterol domains partially co-localize with BODIPY-sphingomyelin-enriched domains. In conclusion, we show that theta* is a useful vital probe to study cholesterol organization and demonstrate that cholesterol forms submicrometric domains in living cells.

Endogenous sphingomyelin segregates into submicrometric domains in the living erythrocyte membrane

We recently reported that trace insertion of exogenous fluorescent (green BODIPY) analogs of sphingomyelin (SM) into living red blood cells (RBCs), partially spread onto coverslips, labels submicrometric domains, visible by confocal microscopy. We here extend this feature to endogenous SM, upon binding of a SM-specific nontoxic (NT) fragment of the earthworm toxin, lysenin, fused to the red monomeric fluorescent protein, mCherry [construct named His-mCherry-NT-lysenin (lysenin*)]. Specificity of lysenin* binding was verified with composition-defined liposomes and by loss of (125)I-lysenin* binding to erythrocytes upon SM depletion by SMase. The (125)I-lysenin* binding isotherm indicated saturation at 3.5 × 10(6) molecules/RBC, i.e., ∼3% of SM coverage. Nonsaturating lysenin* concentration also labeled sub-micrometric domains on the plasma membrane of partially spread erythrocytes, colocalizing with inserted green BODIPY-SM, and abrogated by SMase. Lysenin*-labeled domains were stable in time and space and were regulated by temperature and cholesterol. The abundance, size, positioning, and segregation of lysenin*-labeled domains from other lipids (BODIPY-phosphatidylcholine or -glycosphingolipids) depended on membrane tension. Similar lysenin*-labeled domains were evidenced in RBCs gently suspended in 3D-gel. Taken together, these data demonstrate submicrometric compartmentation of endogenous SM at the membrane of a living cell in vitro, and suggest it may be a genuine feature of erythrocytes in vivo.

Complete regression and systemic protective immune responses obtained in B16 melanomas after treatment with LTX-315

Malignant melanoma is the most aggressive and deadliest form of skin cancer due to its highly metastatic potential, which calls for new and improved therapies. Cationic antimicrobial peptides (CAPs) are naturally occurring molecules found in most species, in which they play a significant role in the first line of defense against pathogens, and several CAPs have shown promising potential as novel anticancer agents. Structure-activity relationship studies on the CAP bovine lactoferricin allowed us to de novo design short chemically modified lytic anticancer peptides. In the present study, we investigated the in vivo antitumor effects of LTX-315 against intradermally established B16 melanomas in syngeneic mice. Intratumoral administration of LTX-315 resulted in tumor necrosis and the infiltration of immune cells into the tumor parenchyma followed by complete regression of the tumor in the majority of the animals. LTX-315 induced the release of danger-associated molecular pattern molecules such as the high mobility group box-1 protein in vitro and the subsequent upregulation of proinflammatory cytokines such as interleukin (IL) 1β, IL6 and IL18 in vivo. Animals cured by LTX-315 treatment were protected against a re-challenge with live B16 tumor cells both intradermally and intravenously. Together, our data indicate that intratumoral treatment with LTX-315 can provide local tumor control followed by protective immune responses and has potential as a new immunotherapeutic agent.

The effect of dietary zinc deficiency on the lipid composition of the rat erythrocyte membrane

The effect of dietary zinc deficiency in the rat on the lipid composition of the erythrocyte membrane was determined. Weanling male Wistar rats were fed an egg white-based diet containing less than 1.0 mg Zn/kg diet ad libitum. Control rats were either pair-fed or ad libitum-fed the basal diet supplemented with 100 mg Zn/kg diet. A zinc refed group was fed the -Zn diet until day 18 and then pair-fed the +Zn diet until day 21. The voluntary feed restriction associated with dietary zinc deficiency resulted in erythrocyte membranes that had depressed phospholipid/protein and elevated cholesterol/phospholipid ratios. Similarly, all feed restricted groups had elevated 22-carbon n-3 polyunsaturated fatty acids (PUFA) and depressed 22-carbon n-6 PUFA concentrations in alkenyl-acyl and diacyl glycerophosphoethanolamine, phosphatidylserine and phosphatidylcholine; they also had depressed 24:2n-6 levels in sphingomyelin. The relative concentrations of phospholipids in the membrane was similar between -Zn and +Zn (ad libitum) groups; however, the -Zn group had significantly less phosphatidyl-serine relative to +Zn (pair-fed) controls.

The effect on photohaemolysis of variation in the structure of the porphyrin photosensitizer

A comparison of the photosensitizing ability of a variety of porphyrins for photohaemolysis gives the following order of activity: protoporphyrin greater than deuteroporphyrin, mesoporphyrin, haematoporphyrin dimethyl ester much greater than haematoporphyrin diacetate, haematoporphyrin greater than haematoporphyrin monoacetate, coproporphyrin III, haematoporphyrin derivative, coproporphyrin III tetramethyl ester greater than uroporphyrin I, meso-tetra-(N-methyl-4-pyridinium)porphyrin tetratoluene-p-sulphonate, meso-tetra-(p-carboxyphenyl)porphyrin, protoporphyrin dimethyl ester, meso-tetra-(p-hydroxy-sulphonylphenyl)porphyrin tetrasodium salt, uroporphyrin III, deuteroporphyrin-3,8-disulphonic acid and protohaemin. The results for the metal-free porphyrins are rationalized in terms of solubility and partition properties, and a model is proposed for the incorporation of amphipathic porphyrins into the membrane lipid bilayer. Experiments with erythrocytes from patients with erythropoeitic protoporphyria and with normal erythrocytes to which porphyrin was added in a deuterium oxide medium do not lead to an increase in the rate of photohaemolysis. A possible explanation for this somewhat surprising observation is outlined.

Isolation and characterization of the erythrocyte surface membrane of the smooth dogfish, Mustelus canis

1. The plasma membrane of the dogfish erythrocyte is characterized. 2. Surface membranes were isolated from dogfish red cells. 3. Cells suspended in a hypotonic zinc chloride buffer (0.5 mM ZnCl2, 5 mM Tris-HCl, pH 8.0) tended to enucleate spontaneously. 4. By gentle homogenization relatively high yields of red cell "ghosts" were formed; these were purified using discontinuous gradients of sucrose and of glycerol solutions. Analyses of protein, carbohydrates and lipids were carried out. 5. There did not appear to be marked overall differences in the composition of the surface membrane of the nucleated dogfish erythrocyte compared to those of other species.