Ca(++)-dependent vesicle release from erythrocytes involves stomatin-specific lipid rafts, synexin (annexin VII), and sorcin

Imaging morphological details and pathological differences of red blood cells using tapping-mode AFM

The surface topography of red blood cells (RBCs) was investigated under near-physiological conditions using atomic force microscopy (AFM). An immobilization protocol was established where RBCs are coupled via molecular bonds of the membrane glycoproteins to wheat germ agglutinin (WGA), which is covalently and flexibly tethered to the support. This results in a tight but non-invasive attachment of the cells. Using tapping-mode AFM, which is known as gentle imaging mode and therefore most appropriate for soft biological samples like erythrocytes, it was possible to resolve membrane skeleton structures without major distortions or deformations of the cell surface. Significant differences in the morphology of RBCs from healthy humans and patients with systemic lupus erythematosus (SLE) were observed on topographical images. The surface of RBCs from SLE patients showed characteristic circular-shaped holes with approx. 200 nm in diameter under physiological conditions, a possible morphological correlate to previously published changes in the SLE erythrocyte membrane.

Lipid raft-associated protein sorting in exosomes

Exosomes are small membrane vesicles secreted by cells upon fusion of multivesicular endosomes with the cell surface. The mechanisms underlying the specific sorting of proteins in exosomal membranes are far from being unraveled. We demonstrate here, using different cells, that some molecules are released in the extracellular medium via their association with lipid raft domains of the exosomal membrane. Various typical raft-associated molecules could be detected by immunoblot in exosomes and Triton X-100-insoluble fractions isolated from exosomes of different origins. Partial localization of major histocompatibility complex (MHC) class II molecules with detergent-resistant fractions isolated from Daudi-secreted exosomes was demonstrated by immunoblot and confirmed by electron microscopy colocalization of MHC class II molecules and ganglioside GM1. Moreover, we found that exosome-associated Lyn (1) had a lower molecular weight compared with Lyn detected in cell-isolated detergent-resistant domains, (2) was absent from the Triton X-100-insoluble fraction isolated from exosomes, and (3) had lost its partitioning capacity in Triton X-114. Exosomal Lyn is probably cleaved by a caspase-3-like activity contained in secreted vesicles. All together, the data highlight the presence of lipid microdomains in exosomal membranes and suggest their participation in vesicle formation and structure, as well as the direct implication of exosomes in regulatory mechanisms.

Lipid raft-associated protein sorting in exosomes

Exosomes are small membrane vesicles secreted by cells upon fusion of multivesicular endosomes with the cell surface. The mechanisms underlying the specific sorting of proteins in exosomal membranes are far from being unraveled. We demonstrate here, using different cells, that some molecules are released in the extracellular medium via their association with lipid raft domains of the exosomal membrane. Various typical raft-associated molecules could be detected by immunoblot in exosomes and Triton X-100-insoluble fractions isolated from exosomes of different origins. Partial localization of major histocompatibility complex (MHC) class II molecules with detergent-resistant fractions isolated from Daudi-secreted exosomes was demonstrated by immunoblot and confirmed by electron microscopy colocalization of MHC class II molecules and ganglioside GM1. Moreover, we found that exosome-associated Lyn (1) had a lower molecular weight compared with Lyn detected in cell-isolated detergent-resistant domains, (2) was absent from the Triton X-100-insoluble fraction isolated from exosomes, and (3) had lost its partitioning capacity in Triton X-114. Exosomal Lyn is probably cleaved by a caspase-3-like activity contained in secreted vesicles. All together, the data highlight the presence of lipid microdomains in exosomal membranes and suggest their participation in vesicle formation and structure, as well as the direct implication of exosomes in regulatory mechanisms. (Blood. 2003;102:4336-4344)

Antigenic characterization of endothelial cell-derived microparticles and their detection ex vivo

BACKGROUND

Endothelial activation and dysfunction are associated with several diseases. However, hardly any specific markers are available. Microparticles (MP) from endothelial cells (EC; EMP) were reported in patient groups and healthy individuals. The antibodies used to detect EMP, however, were mainly directed against antigens without EC specificity.

OBJECTIVES

We evaluated the antigens on EC and EMP to establish proper markers for EMP detection.

METHODS

EMP were isolated from supernatants of resting and interleukin (IL)-1alpha activated human umbilical vein EC (HUVEC; n=3; 0-72 h), stained with annexin V and monoclonal antibodies, and analyzed by flow cytometry. Human platelet-MP (PMP), the main MP population in plasma, were prepared in vitro. EMP and PMP were studied in plasma from systemic lupus erythematosus (SLE) patients (n=11) and healthy individuals (n=10).

RESULTS

Platelet-endothelial cell adhesion molecule-1 (PECAM-1), alphanu and beta3 were constitutively exposed on HUVEC, but (almost) absent on EMP (<15% positive for alphanu and beta3), or only exposed on a subpopulation (PECAM-1; 30-60%). Activated HUVEC (>80%) and (subpopulations of) EMP exposed E-selectin and tissue factor. PMP strongly exposed PECAM-1, beta3, and glycoprotein (GP)Ib (CD42b), but not alphanu or E-selectin. GPIb and P-selectin (CD62P) were absent on EMP. Plasma samples contained 0.5% MP staining for E-selectin and/or alphanu. Plasma from one SLE patient contained E-selectin exposing MP (21%), but little alphanu-positive MP.

CONCLUSIONS

EC release EMP in vitro. The antigenic phenotype of EMP released from resting and IL-1alpha-stimulated EC differs among each other as well as from resting and stimulated EC, respectively. E-selectin exposed on IL-1alpha-stimulated EC is a valid marker for EMP detection ex vivo to establish endothelial cell activation.

Erythrocyte detergent-resistant membrane proteins: their characterization and selective uptake during malarial infection

Infection of human erythrocytes by the apicomplexan malaria parasite Plasmodium falciparum results in endovacuolar uptake of 4 host proteins that reside in erythrocyte detergent-resistant membranes (DRMs). Whether this vacuolar transport reflects selective uptake of host DRM proteins remains unknown. A further complication is that DRMs of vastly different protein and cholesterol contents have been isolated from erythrocytes. Here we show that isolated DRMs containing the highest cholesterol-to-protein ratio have low protein mass. Liquid chromatography, mass spectrometry, and antibody-based studies reveal that the major DRM proteins are band 3, flotillin-1 and -2, peroxiredoxin-2, and stomatin. Band 3 and stomatin, which reflect the bulk mass of erythrocyte DRM proteins, and all tested non-DRM proteins are excluded from the vacuolar parasite. In contrast, flotillin-1 and -2 and 8 minor DRM proteins are recruited to the vacuole. These data suggest that DRM association is necessary but not sufficient for vacuolar recruitment and there is active, vacuolar uptake of a subset of host DRM proteins. Finally, the 10 internalized DRM proteins show varied lipid and peptidic anchors indicating that, contrary to the prevailing model of apicomplexan vacuole formation, DRM association, rather than lipid anchors, provides the preferred criteria for protein recruitment to the malarial vacuole.

The "stomatin" gene and protein in overhydrated hereditary stomatocytosis

In overhydrated hereditary stomatocytosis (OHSt), Coomassie- and silver-stained polyacrylamide gels show an apparently complete deficit of the 32-kDa membrane protein, stomatin. We have used an antistomatin antibody to examine peripheral blood films, bone marrow, splenic tissue, and hepatic tissue from these patients by immunocytochemistry. This technique revealed that, in fact, some red cells did show positive stomatin immunoreactivity; and consistent with this result, Western blot analysis of the red cell membranes confirmed that about one twentieth to one fiftieth of the normal amount of stomatin was in fact present. Flow cytometry, combining immunoreactive quantitation of stomatin expression with thiazole orange staining for reticulocytes, showed that in OHSt, it was the young cells that had more stomatin. Magnetic-activated cell separation studies, using beads to which an anti-transferrin receptor antibody was conjugated, confirmed that in OHSt there was a correspondence between expression of stomatin and the transferrin receptor. Immunocytochemistry and Western blotting revealed that in OHSt patients, the protein was present in spleen, liver, neutrophils, platelets, monocytes, and about 50% of the peripheral lymphocytes, with the same distribution as in healthy controls. Neither Southern blots, nor direct sequencing of multiple subclones of the cDNA, nor sequencing of amplicons from genomic DNA revealed any significant abnormality in stomatin gene sequence in these patients. The deficiency of stomatin from red cells appears to be due to a loss of stomatin from these red cells on maturation in the bone marrow and in the circulation.

Function, expression and localization of annexin A7 in platelets and red blood cells: insights derived from an annexin A7 mutant mouse

BACKGROUND

Annexin A7 is a Ca2+- and phospholipid-binding protein expressed as a 47 and 51 kDa isoform, which is thought to be involved in membrane fusion processes. Recently the 47 kDa isoform has been identified in erythrocytes where it was proposed to be a key component in the process of the Ca2+-dependent vesicle release, a process with which red blood cells might protect themselves against an attack by for example complement components.

RESULTS

The role of annexin A7 in red blood cells was addressed in erythrocytes from anxA7-/- mice. Interestingly, the Ca2+-mediated vesiculation process was not impaired. Also, the membrane organization appeared not to be disturbed as assessed using gradient fractionation studies. Instead, lack of annexin A7 led to an altered cell shape and increased osmotic resistance of red blood cells. Annexin A7 was also identified in platelets. In these cells its loss led to a slightly slower aggregation velocity which seems to be compensated by an increased number of platelets. The results appear to rule out an important role of annexin A7 in membrane fusion processes occurring in red blood cells. Instead the protein might be involved in the organization of the membrane cytoskeleton. Red blood cells may represent an appropriate model to study the role of annexin A7 in cellular processes.

CONCLUSION

We have demonstrated the presence of both annexin A7 isoforms in red blood cells and the presence of the small isoform in platelets. In both cell types the loss of annexin A7 impairs cellular functions. The defects observed are however not compatible with a crucial role for annexin A7 in membrane fusion processes in these cell types.

Information transfer in the penta-EF-hand protein sorcin does not operate via the canonical structural/functional pairing. A study with site-specific mutants

Sorcin is a typical penta-EF-hand protein that participates in Ca2+-regulated processes by translocating reversibly from cytosol to membranes, where it interacts with different target proteins in different tissues. Binding of two Ca2+/monomer triggers translocation, although EF1, EF2, and EF3 are potentially able to bind calcium at micromolar concentrations. To identify the functional pair, the conserved bidentate -Z glutamate in these EF-hands was mutated to yield E53Q-, E94A-, and E124A-sorcin, respectively. Limited structural perturbations occur only in E124A-sorcin due to involvement of Glu-124 in a network of interactions that comprise the long D helix connecting EF3 to EF2. The overall affinity for Ca2+ and for two sorcin targets, annexin VII and the ryanodine receptor, follows the order wild-type > E53Q- > E94A- > E124A-sorcin, indicating that disruption of EF3 has the largest functional impact and that disruption of EF2 and EF1 has progressively smaller effects. Based on this experimental evidence, EF3 and EF2, which are not paired in the canonical manner, are the functional EF-hands. Sorcin is proposed to be activated upon Ca2+ binding to EF3 and transmission of the conformational change at Glu-124 via the D helix to EF2 and from there to EF1 via the canonical structural/functional pairing. This mechanism may be applicable to all penta-EF-hand proteins.

Identification of teleost Thy-1 and association with the microdomain/lipid raft reggie proteins in regenerating CNS axons

During regeneration, retinal ganglion cell axons in fish upregulate a cell surface protein that is recognized by the monoclonal antibody (mAB) M802. M802 antigen appeared to be linked to the intracellular, membrane-associated lipid raft/microdomain proteins reggie-1 and reggie-2 that were previously shown to be reexpressed in axon-regenerating neurons [Development 124 (1997), 577]. Here, we report the isolation of the M802 antigen and its identification as the teleost homolog of mammalian Thy-1. Fish Thy-1 is detected in the same detergent-insoluble lipid raft fractions from a fibroblast cell line and from axon regenerating retinae as reggie-1 and 2. Importantly, mAB M802 coimmunoprecipitates reggie-1 and 2 from this lipid raft fraction, implying that fish Thy-1 and reggies interact. This correlates with their colocalization in growing cell processes after M802 antigen/Thy-1 activation with mAB M802. These findings suggest a role of clustered M802 antigen/Thy-1 in reggie raft microdomains for cell growth and axon regeneration.

Plasmodium falciparum merozoite surface protein 1

In addition to the major carbohydrate moieties of the glycosylphosphatidylinositol (GPI) anchor, we report that Plasmodium falciparum merozoite surface protein 1 (MSP-1) bears O-GlcNAc modifications predominantly in beta-anomeric configuration, in both the C- and N-terminal portions of the protein. Subcellular fractionation of parasitized erythrocytes in the late trophozoite/schizont stage reveals that GPI-anchored C-terminal fragments of MSP-1 are recovered in Triton X-100 resistant, low-density membrane fractions. Our results suggest that O-GlcNAc-modified MSP-1 N-terminal fragments tend to localize within the parasitophorous vacuolar membrane while GPI-anchored MSP-1 C-terminal fragments associate with low-density, Triton X-100 resistant membrane domains (rafts), redistribute in the parasitized erythrocyte and are eventually shed as membrane vesicles that also contain the endogenous, GPI-linked CD59.

Incidence of hypothyroidism after 131I therapy for hyperthyroidism. Relation to pretherapy serum levels of T3, T4 and thyroid antibodies

A correlation is reported between serum levels of triiodothyronine (S-T3) and thyroxine (S-T4) before treatment, as well as levels of thyroid antibodies before treatment, and the development of hypothyroidism following 131I therapy in 86 patients with hyperthyroidism. Patients with marked elevation of S-T3 or S-T4 had demonstrable antibodies to thyroid cytoplasmic antigen more often than those with normal or moderately elevated levels, and patients with markedly elevated levels of S-T3 also had a higher incidence of hypothyroidism after treatment. Patients with nodular thyroid glands and with markedly elevated levels of S-T3 required a larger number of 131I doses before no signs of hyperthyroidism persisted in comparison to those with moderately elevated levels.