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At least 1,423 records · Page 79Linked to original sources
PubMed2002

Receptor-facilitated antigen presentation requires the recruitment of B cell linker protein to Igalpha.

Ags that cross-link the B cell Ag receptor are preferentially and rapidly delivered to the MHC class II-enriched compartment for processing into peptides and subsequent loading onto MHC class II. Proper sorting of Ag/receptor complexes requires the recruitment of Syk to the phosphorylated immunoreceptor tyrosine-based activation motif tyrosines of the B cell Ag receptor constituent Igalpha. We postulated that the Igalpha nonimmunoreceptor tyrosine-based activation motif tyrosines, Y(176) and Y(204), contributed to receptor trafficking. Igalpha(YDeltaF(176,204))/Igbeta receptors were targeted to late endosomes, but were excluded from the vesicle lumen and could not facilitate the presentation of Ag to T cells. Subsequent analysis demonstrated that phosphorylation of Y(176)/Y(204) recruited the B cell linker protein, Vav, and Grb2. Reconstitution of Igalpha(YDeltaF(176,204))/Igbeta with the B cell linker protein rescued both receptor-facilitated Ag presentation and entry into the MHC class II-enriched compartment. Thus, aggregation accelerates receptor trafficking by recruiting two separate signaling modules required for transit through sequential checkpoints.

Adaptor Proteins, Signal Transducing↗
PubMed1976

In vivo and in vitro activation of T-antigen receptors on leukocytes and platelets.

Serological studies on a patient whose red cells are polyagglutinable due to T activation have demonstrated concomitant T activity of the separated leukocytes and platelets. Normal leukocytes and platelets are not T active, but activation can be induced in vitro by treatment with neuraminidase or with pneumococcus type III filtrate. Such T-active cells absorb anti-T from Arachis hypogea lectin. Tests on idfferent types of separated leukocytes show that both neutrophils and lymphocytes have latent T antigen receptors. Neuraminidase treatment of platelets does not change their ability to promote clot retraction, to aggregate with ADP, or to take up serotonin.

Absorption↗
PubMed1998

Analysis of membrane protein cluster densities and sizes in situ by image correlation spectroscopy.

Communication between cells invariably involves interactions of a signalling molecule with a receptor at the surface of the cell. Typically, the receptor is imbedded in the membrane and it is hypothesized that the binding of the signalling molecule causes a change in the state of aggregation of the receptor which, in turn, initiates a biochemical signal within the cell. Subsequently, many of the occupied receptors bind to membrane-associated structures, called coated pits, which invaginate and pinch off to form coated vesicles, thereby removing the receptors from the cell surface. The state of aggregation of membrane receptors is obviously in constant flux. Any useful approach to measuring the state of aggregation must, therefore, allow for dynamic measurements in living cells. It is possible to use fluorescently labelled signalling molecules or antibodies directed at the receptor of interest to visualize the receptor on the cell surface with a fluorescence microscope. By employing a laser confocal microscope, high resolution images can be produced in which the fluorescence intensity is quantitatively imaged as a function of position across the surface of the cell. Calculations of autocorrelation functions of these images provide direct and accurate measures of the density of fluorescent particles on the surface. Combined with the average intensity in the image, which reflects the total average number of molecules, it is possible to estimate the degree of aggregation of the receptor molecules. We refer to this analysis as image correlation spectroscopy (ICS). We show how ICS can be used to measure the density of several receptors on a variety of cells and how it can be used to measure the density of coated pits and the number of molecules per coated pit. We also show how the technique can be used to monitor fusion of virus particles to cell membranes. Further, we illustrate that, by calculating cross-correlation functions between pairs of images, we can extend the analysis to measurements of the distributions as a function of time, on the second timescale, as well as to measurements of the movement of the receptor aggregates on the surface. Finally, we illustrate that, by this approach, we can measure the extent of interaction between two different receptors as a function of time. This represents the most quantitative measurement of the extent of co-localization of receptors available and is independent of the spatial resolution of the confocal microscope. The theory of ICS and image cross-correlation spectroscopy (ICCS), focussing on the interpretation of the data in terms of the biological phenomenon being probed, is discussed.

Animals↗
PubMed1994

Species variability in platelet and other cellular responsiveness to thrombin receptor-derived peptides.

The aggregation of platelets from a variety of animal species in response to thrombin receptor-derived activating peptides was evaluated. A series of 14-(SFLLRNPNDKYEPF), 7-(SFLLRNP-NH2), 6-(SFLLRN-HN2) or 5-(SFLLR-NH2) residue peptides, the structures of which were based on the deduced amino acid sequence of the human thrombin receptor, promoted full aggregation of platelets in plasma from humans, African Green and Rhesus monkeys, baboons and guinea pigs at 4-50 microM depending on the peptide used. Platelets in plasma from rabbit, dog, pig, and hamster underwent a shape change but failed to aggregate in response to these peptides over 3 log units of peptide up to 800 microM, despite being fully responsive to human thrombin. However, because the receptor peptides induced shape change in the platelets from these non-aggregating species, they apparently can activate some of the intracellular signaling system(s) usually initiated by thrombin in these platelets. In contrast, platelets from rats did not undergo shape change or aggregate in response to the peptides. A 7-residue receptor-derived peptide based on the deduced amino acid sequence of the clone of the hamster thrombin receptor (SFFLRNP-N2) was nearly as efficacious as the corresponding human receptor-derived 7-residue peptide to promote aggregation of human platelets. However, the hamster peptide could not promote aggregation of hamster platelets in plasma at up to 800 microM peptide, while a shape change response was elicited. Platelets from rats, rabbits and pigs also did not aggregate in response to this peptide derived from the hamster thrombin receptor, but all species except the rat underwent a shape change.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1998

Depressed platelet status in an elderly patient with hemorrhagic stroke after thrombolysis for acute myocardial infarction. GUSTO-III Investigators.

BACKGROUND: Impaired platelet function has been reported in acute myocardial infarction (AMI) and stroke. However, prospective data on the changes of platelet status in patients before the occurrence of hemorrhagic stroke after thrombolytic therapy are unavailable. CASE DESCRIPTION: An 86-year-old male patient was among the 23 AMI patients enrolled in the platelet study for the GUSTO-III trial. He received 325 mg of aspirin daily for at least 6 years, suffered an AMI, and was successfully reperfused with alteplase, but after 44 hours developed a large hemorrhagic stroke resulting in paraplegia. Platelet aggregation and receptor expression were measured by flow cytometry and ELISA before thrombolysis and at 3, 6, 12, and 24 hours thereafter. The percentage of platelet aggregation was lower in the stroke patient at every time point when induced by 5 micromol/L of ADP, by 10 micromol/L of ADP, and by thrombin than in the rest of the AMI group. Ristocetin and collagen-induced aggregability were within the group range. Decreased platelet glycoprotein Ib, IIb, IIIa, and IIb/IIIa and vitronectin receptor expression were observed in the stroke patient. No other differences in p24 (CD9), very late antigen-2, P-selectin, and platelet/endothelial cell adhesion molecule-1 expression were determined. CONCLUSIONS: Profound depression of platelet status preceded the occurrence of hemorrhagic stroke in an elderly long-term aspirin user treated with thrombolytic therapy. Initial "exhausted" platelets may be responsible for the increased risk for hemorrhagic stroke after coronary thrombolysis.

Adenosine Diphosphate↗
PubMed2003

Steroid-porphyrin conjugate for saccharide sensing in protic media.

A new saccharide receptor in protic media has been designed and synthesized. The receptor combines advantages of steroids, which are responsible for saccharide binding, and of the porphyrin moiety acting as a signalling component of the molecule due to changes in UV-vis electronic spectra. The synthesis is based on condensation of steroid aldehyde with pyrrole to form the porphyrin unit with four protected steroid moieties. After deprotection, meso-substituted porphyrin contains 12-hydroxy groups on the steroidal part. The receptor is soluble in aqueous solutions and exhibits high complexation affinity towards saccharides. Because the receptor extensively aggregates in water, most of the experiments were performed in 50% aqueous 2-propanol where aggregation is significantly eliminated. Binding is evidenced by spectral changes in the Soret region of the receptor in UV-vis absorption spectra allowing the evaluation of the binding constants. Additional confirmation of binding is obtained using 1H NMR, Raman and IR spectroscopies and the surface plasmon resonance technique. The receptor exhibits higher selectivity for oligosaccharides over monosaccharide. The results point to the importance of a combination of multiple binding via H-bonding and hydrophobic interactions.

Biosensing Techniques↗
PubMed2001

Apoptosis of haematopoietic cells upon thymidylate synthase inhibition is independent of p53 accumulation and CD95-CD95 ligand interaction.

Treatment of haematopoietic BA/F3 cells with the thymidylate synthase inhibitor 5-fluoro-2'-deoxyuridine (FUdR) activated apoptosis through a mechanism that required continuous protein synthesis and was inhibited by Bcl-2 over-expression. Analysis of p53 levels in cells treated with FUdR indicated a marked accumulation of this protein. Accumulation of p53 was also observed in cells over-expressing Bcl-2. In BA/F3 cells transfected with a cDNA coding for the human papilloma virus protein E6, p53 accumulation after FUdR treatment was inhibited markedly. However, apoptosis was induced in both control and E6 cells to a similar extent. The role of the CD95/CD95 ligand (CD95L) system in FUdR-induced apoptosis was also assessed. As determined by reverse transcriptase PCR, BA/F3 expressed a low constitutive level of CD95L mRNA, which decreased following FUdR treatment. Moreover, blocking CD95-CD95L interactions with antagonistic CD95 monoclonal antibody did not prevent drug-induced apoptosis. Furthermore, analysis of caspase involvement showed important differences in apoptosis induced by CD95-triggering or FUdR treatment. In summary, these results suggest that apoptosis induced by thymineless stress in haematopoietic BA/F3 cells occurs by a mechanism that does not require accumulation of p53 and which is independent of CD95-CD95L interactions.

Animals↗
PubMed1986

Desensitization of epinephrine-initiated platelet aggregation does not alter binding to the alpha 2-adrenergic receptor or receptor coupling to adenylate cyclase.

Several investigators have shown that incubating unstirred platelets with epinephrine blunts subsequent aggregation when the platelets are stirred. Using aspirin-treated platelets, we further characterized this desensitization of alpha 2-adrenergic receptor-initiated aggregation. Desensitization occurred with a t1/2 of 3-6 min and was maximal at 20-30 min, at which time the initial rate of aggregation and its maximal extent were about half that of control platelets. When we preincubated platelets with epinephrine, and then added phentolamine to block the alpha 2-receptors, ADP-initiated aggregation occurred normally. Thus, the desensitization of epinephrine-initiated aggregation was not associated with a generalized impairment of aggregation. At concentrations too low to initiate aggregation, epinephrine is known to potentiate aggregation initiated by other agents. Clonidine also acts at alpha 2-receptors to potentiate aggregation initiated by other agents, but it does not initiate aggregation by itself. Preincubating clonidine with platelets for 30 min abolished its potentiating effect on ADP-initiated aggregation. Thus, the ability of alpha 2-receptors to both potentiate and initiate aggregation desensitizes after a brief preincubation with agonist. We performed several types of experiments to investigate the mechanism of this desensitization. Platelet alpha 2-receptors are coupled to an inhibition of adenylate cyclase. We found, however, that alpha 2-mediated inhibition of prostaglandin E1-stimulated cAMP accumulation occurred normally in desensitized platelets. Similarly, epinephrine inhibited basal adenylate cyclase activity normally in membranes prepared from desensitized platelets. In membranes prepared from desensitized platelets, epinephrine competed normally for [3H]rauwolscine binding, and this competition was modulated normally by guanine nucleotides. Thus, the properties of the alpha 2-receptors, as measured in radioligand binding experiments, were unchanged by densensitization. In conclusion, desensitization of alpha 2-adrenergic receptor-mediated aggregation occurs without change in the alpha 2-adrenergic receptors or in their coupling to an inhibition of adenylate cyclase.

1-Methyl-3-isobutylxanthine↗
PubMed1989

Monocyte Fc gamma receptor recognition of cell-bound and aggregated IgG.

Monocyte and macrophage Fc gamma receptors are important components in the recognition of IgG-coated cells and IgG-containing immune complexes. Two proteins have been identified on human peripheral blood monocytes that can function as Fc gamma receptors, Fc gamma RI (70 Kd) and Fc gamma RII (40 Kd). We studied the role of Fc gamma RI and Fc gamma RII on human monocytes by examining their binding of IgG-sensitized cells (human IgG anti-D-coated RBCs and rabbit IgG-sensitized sheep RBCs) and their binding of human trimeric IgG. To examine the function of monocyte Fc gamma RII, we used an anti-Fc gamma RII monoclonal antibody (MoAb) that competes for the Fc gamma RII ligand binding site. Preincubation of monocytes with saturating concentrations of anti-Fc gamma RII MoAb did not alter the recognition of IgG (anti-D)-sensitized human RBCs by monocytes. Furthermore, ligand-binding studies demonstrated that anti-Fc gamma RII antibody altered neither the number nor the affinity of monocyte-binding sites for human IgG trimer. Anti-Fc gamma RII inhibited monocyte binding of rabbit IgG-sensitized sheep RBCs, but only at low ionic strength or temperature when increased numbers of monocyte Fc gamma RII were expressed. At low ionic strength and 4 degrees C, anti-Fc gamma RII also partially inhibited monocyte binding of human trimeric IgG. Thus, monocyte Fc gamma RII does not appear to recognize IgG-sensitized RBCs or trimeric IgG at physiologic temperatures and ionic strength. The data suggest that Fc gamma RI is the primary Fc gamma receptor on monocytes involved in the binding of IgG (anti-D)-sensitized erythrocytes and low mol wt complexes of IgG.

Animals↗
PubMed1984

Inhibition of 5-hydroxytryptamine-potentiated aggregation of human blood platelets by 5-hydroxytryptamine receptor-blocking agents.

Blood platelets possess specific receptors for 5-hydroxytryptamine (5-HT) which differ from the carrier for the amine uptake. Activation of 5-HT receptors causes shape change and aggregation of platelets. We studied the influence of drugs with 5-HT receptor blocking properties on 5-HT-potentiated, ADP-induced aggregation of human blood platelets. The synergistic effect of 5-HT on aggregation was inhibited by the compounds tested in the order of potency: cyproheptadine, pizotifen, spiperone, methysergide, methiothepine , mianserin, ketanserin, dihydroergotamine. The IC50 values lay in the range from 14 to 40 nmol/l. Chlorpromazine and haloperidol were less effective by more than one order of magnitude. Our results indicate a certain similarity between the 5-HT receptors on platelets and the 5-HT receptors on serotonergic neurons and vascular smooth muscle which belong to the 5-HT2 receptor type.

Adenosine Diphosphate↗
PubMed1993

Interaction of the 43 kd postsynaptic protein with all subunits of the muscle nicotinic acetylcholine receptor.

The 43 kd postsynaptic protein (43K) plays a key role in the aggregation of muscle nicotinic acetylcholine receptors (AChRs) in the postsynaptic membrane of the neuromuscular junction. By transiently coexpressing 43K and a single AChR subunit (alpha, beta, gamma, or delta) in the quail fibroblast cell line, QT-6, we show that 43K interacts with each subunit to form cell surface clusters in which 43K and receptor subunit are precisely colocalized. Although the level of cell surface expression of single subunits is much lower than that of fully assembled receptor, the clustering of both single subunits and fully assembled AChR occurs efficiently. In addition, 43K-induced clustering is specific for AChR subunits. From these results, we conclude that each pentameric AChR has five potential sites for interacting with 43K during cluster formation.

Animals↗
PubMed2004

Folate-receptor-targeted delivery of doxorubicin nano-aggregates stabilized by doxorubicin-PEG-folate conjugate.

For folate-receptor-targeted anti-cancer therapy, doxorubicin aggregates in a nano-scale size were produced employing doxorubicin-polyethylene glycol-folate (DOX-PEG-FOL) conjugate. Doxorubicin and folate were respectively conjugated to alpha- and omega-terminal end group of a PEG chain. The conjugates assisted to form doxorubicin nano-aggregates with an average size of 200 nm in diameter when combined with an excess amount of deprotonated doxorubicin in an aqueous phase. Hydrophobically deprotonated doxorubicin molecules were aggregated within the core, while the DOX-PEG-FOL conjugates stabilized the aggregates with exposing folate moieties on the surface. The doxorubicin nano-aggregates showed a greater extent of intracellular uptake against folate-receptor-positive cancer cells than folate-receptor-negative cells, indicating that the cellular uptake occurred via a folate-receptor-mediated endocytosis mechanism. They also exhibited more potent cytotoxic effect on KB cells than free doxorubicin. In a human tumor xenograft nude mouse model, folate-targeted doxorubicin nano-aggregates significantly reduced the tumor volume compared to non-targeted doxorubicin aggregates or free doxorubicin. These results suggested that folate-targeted doxorubicin nano-aggregates could be a potentially useful delivery system for folate-receptor-positive cancer cells.

Animals↗
PubMed2004

Failure of clopidogrel to reduce platelet reactivity and activation following standard dosing in elective stenting: implications for thrombotic events and restenosis.

There is no information on long-term platelet reactivity and activation following elective stenting in patients treated with clopidogrel and aspirin. We measured platelet reactivity and activation at baseline and at 2 h, 24 h, 5 days and 30 days following coronary stenting (n = 94). Patients were treated with the standard aspirin (325 mg) and clopidogrel regimen (300 mg load/75 mg qd). Reactivity was measured by aggregation (5 and 20 microM ADP) and activation was determined by the expression of total and active GP IIb/IIIa. Reactivity and activation were defined as heightened when post-stent aggregation and receptor expression exceeded baseline levels, respectively. Prolonged heightened platelet reactivity was detected by both 5 and 20 microM ADP aggregation. Using 20 microM ADP aggregation, heightened reactivity occurred in 55% of patients at 2 h, 26% at 24 h, 21% at 5 days, and 15% at 30 days post-stenting. A high frequency of heightened platelet activation was detected by both total and active GP IIb/IIIa expression. Using expression of the active GP IIb/IIIa receptor as the marker, activation was greater than baseline in 27% of patients at 2 h, 20% at 24 h, 30% at 5 days, and 22% at 30 days post-stenting. This is the first report demonstrating that a significant percentage of patients receiving standard clopidogrel and aspirin therapy for coronary stenting will have post-drug platelet reactivity and activation above baseline that persists for 30 days after the procedure. These finding suggest insufficient platelet inhibition. The clinical importance of these findings should be further investigated to establish the potential link between insufficient platelet inhibition, stent thrombosis, and restenosis.

Adenosine Diphosphate↗
PubMed1990

The aggregation and dissociation properties of a low molecular weight mannose 6-phosphate receptor from bovine testis.

The aggregation state of low molecular weight mannose 6-phosphate receptor from bovine testis was determined in membrane preparations and in purified soluble preparations. The effect of aggregation on binding of the receptor to immobilized pentamannose 6-phosphate was also examined. Nonreducing SDS-PAGE followed by immunoblotting revealed that interchain disulfide bonds exist in detergent-solubilized and purified receptor preparations, but not in membrane-associated receptor. Reduction of the receptor with dithiothreitol abolished its ligand binding activity and drastically altered its ability to bind antibodies. The results of receptor crosslinking and molecular sieving chromatography studies suggest that the receptor exists in membranes as a noncovalently linked dimer and in solution as oligomeric forms, largely as a tetramer. The formation of the tetramer is affected by the concentration of the receptor, but not by its solubilization from membranes with detergent, nor by the presence of mannose 6-phosphate. Mono-, di-, and tetramer forms of 125I-labeled receptor were separated by molecular sieving chromatography and examined for their ability to bind to immobilized ligand, agarose-pentamannose-phosphate. The order of binding observed was tetramer greater than dimer greater than monomer. Binding of the monomer and dimer to immobilized ligand was dependent on the presence of divalent cations while the tetramer had little requirement for divalent cations.

Animals↗
PubMed1986

Plasticity in the phenotypic expression of brain opioid receptors: differential response of forebrain and hindbrain cultures to chemical depolarization.

Potassium chloride at a depolarizing concentration has a differential effect on the expression of opioid receptors in aggregating neuronal cultures prepared from the rat forebrain or hindbrain. The mu-selective enkephalin analogue DAGO was used to indicate that the effect of potassium chloride is not uniform on all subtypes of opioid receptors. These results suggest for the first time that chemical depolarization may modulate the phenotypic expression of opioid receptors.

Animals↗
PubMed2005

Population balance modeling of aggregation kinetics of recombinant human interleukin-1 receptor antagonist.

The kinetics of benzyl alcohol-induced nonnative aggregation of recombinant human interleukin-1 receptor antagonist (rhIL-1ra) were investigated using a population balance model. Steady-state size distributions of rhIL-1ra aggregates formed in a continuous mixed suspension, mixed product removal (MSMPR) reactor were measured and used to extrapolate aggregate nucleation and growth rates parameters. Aggregate growth rate was size-dependent and a linear growth rate model was used to derive a population density function. Addition of 0.9 wt/v% benzyl alcohol increased the nucleation rate by approximately four orders of magnitude. The growth rate for aggregates, however, changed little as a function of benzyl alcohol concentration in the range of 0-0.9%. The addition of sucrose to buffer containing 0.9% benzyl alcohol decreased rhIL1-ra nucleation rate by orders of magnitude and had little impact on growth rate kinetics. The simplicity of the population balance model and the physical relevance of the information obtained from this model render it a useful tool to study protein aggregation kinetics and the effects of excipients on this process.

Benzyl Alcohol↗
PubMed1989

Regulation of thromboxane receptor activation in human platelets.

Thromboxane A2 (TxA2) is a potent platelet agonist that serves as an amplifying signal after exposure of platelets to other stimulants, such as thrombin, in vitro. Exposure of platelets to the TxA2 receptor agonists U46619 and SQ 26,655 (1.4 microM) resulted in a 60-90% decrease in subsequent TxA2 receptor-stimulated aggregation, calcium release, and protein kinase C activation. The desensitization was rapid, with a half-time of 2-3 min. The sequence of events involved in TxA2 receptor desensitization involves initial uncoupling of the receptor from a guanine nucleotide binding (G) protein followed by eventual receptor down-regulation. Consistent with this hypothesis were (i) a 60-70% decrease in SQ 26,655-stimulated platelet GTPase activity, (ii) a shift to the right of the dose-response curve for U46619-stimulated release of calcium [EC50, 275 +/- 51 nM (control)] vs. 475 +/- 71 nM (desensitized); P less than 0.01], and (iii) a delayed loss of receptor sites. In summary, exposure of platelets to TxA2 receptor agonists results in rapid desensitization of the biochemical and functional responses to interaction with its receptor in human platelets. The kinetics of these events are consistent with the hypothesis that this icosanoid functions in the regulation as well as amplification of platelet activation in vivo.

15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5↗