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PubMed2004

14-3-3 gamma associates with muscle specific kinase and regulates synaptic gene transcription at vertebrate neuromuscular synapse.

The muscle-specific receptor tyrosine kinase (MuSK) is part of a receptor complex, activated by neural agrin, that orchestrates the differentiation of the neuromuscular junction (NMJ). To gain insight into the function of the MuSK complex, we have developed a proteomic approach to identify new MuSK partners. MS analysis of MuSK crosslink products from postsynaptic membranes of the Torpedo electrocytes identified the adaptor protein 14-3-3 gamma. The 14-3-3 gamma protein was found localized at the adult rat NMJ. Cotransfection experiments in COS-7 cells showed that MuSK codistributed with the 14-3-3 gamma protein at the plasma membrane. Furthermore, 14-3-3 gamma was copurified by affinity chromatography with MuSK from transfected COS-7 cells and myotubes. The 14-3-3 gamma protein did not colocalize with agrin-elicited acetylcholine receptor (AChR) aggregates in cultured myotubes, suggesting that it is not involved in AChR clustering. Expression of 14-3-3 gamma specifically repressed the transcription of several synaptic reporter genes in cultured myotubes. This repression was potentiated by MuSK expression. Moreover, the expression of 14-3-3 gamma in muscle fibers in vivo caused both the repression of synaptic genes transcription and morphological perturbations of the NMJ. Our data extend the notion that, apart from its well documented role in AChR clustering, the MuSK complex might also be involved in the regulation of synaptic gene expression at the NMJ.

14-3-3 Proteins↗
PubMed1989

Monitoring of the activation of receptor-operated calcium channels in human platelets.

Activation of receptor-operated calcium channels has been monitored by measurements of the quenching of the fluorescence of intracellularly trapped fura-2 by Mn entering from the extracellular medium. Release of calcium from intracellular stores was followed simultaneously by measurements of the ratio of the fluorescence excited at 340 and 380 nm. Thrombin, ADP, platelet-activating-factor (PAF) and collagen, all produced both release of calcium from the intracellular stores and uptake of Mn from the extracellular medium. The uptake of Mn, but not the increase of (Ca2+)i, was blocked by nickel. These results suggest the existence of plasma membrane calcium channels which can be activated by the different agonists tested here. The activation of calcium channels was very fast and transient with ADP and PAF, fast and maintained with thrombin, and delayed with collagen.

Adenosine Diphosphate↗
PubMed1992

Early stages of myogenesis in a large mammal: formation of successive generations of myotubes in sheep tibialis cranialis muscle.

The generation of myotubes was studied in the tibialis cranialis muscle in the sheep hindlimb from the earliest stage of primary myotube formation until a stage shortly before muscle fascicles began to segregate. Primary myotubes were first seen on embryonic day 32 (E32) and reached their maximum number by E38. Small numbers of secondary myotubes were first identified at E38, and secondary myotube numbers continued to increase during the period of study. The ratio of adult muscle fibre to primary myotube numbers was approximately 70:1, making it seem unlikely that every later generation myotube used a primary myotube as scaffold for its formation, as described in small mammals. By E62, some secondary myotubes were supporting the formation of a third generation of myotubes. Experiments with diffusible dye markers showed that primary myotubes extended from tendon to tendon of the muscle, whereas most adult fibres ran for only part of the muscle length, terminating with myo-myonal attachments to other muscle fibres in a series arrangement. Acetylcholinesterase (AChE) and acetylcholine receptor (AChR) aggregations appeared in multiple bands across the muscle shortly after formation of the primary generation of myotubes was complete. The number of bands and their pattern of distribution across the muscle as they were first formed was the same as in the adult. Primary myotubes teased from early muscles had multiple focal AChE and AChR deposits regularly spaced along their lengths. We suggest that the secondary generation of myotubes forms at endplate sites in a series arrangement along the length of single primary myotubes, and that tertiary and possibly later generations of myotubes in their turn use the earlier generation myofibres as a scaffold. Although the fundamental cellular mechanisms appear to be similar, the process of muscle fibre generation in large mammalian muscles is more complex than that described from previous studies in small laboratory rodents.

Acetylcholinesterase↗
PubMed1985

Characterization of dog platelet alpha-adrenergic receptor: lack of in vivo down regulation by adrenergic agonist treatments.

The dog platelet alpha-adrenergic receptor was characterized using [3H]clonidine and [3H]yohimbine. The binding of both radioligands was rapid and reversible at 25 degrees C; saturation and kinetic experiments revealed a single population of binding sites. The number of [3H]yohimbine sites was 2-3-fold higher than the number of [3H]clonidine sites as reported in other tissues containing alpha2-adrenoceptors. The various alpha-agonists and antagonists displaced [3H]clonidine and [3H]yohimbine with an order of potency indicating alpha2-adrenoceptor specificity. Neither (-)adrenaline nor clonidine infusions (0.5 micrograms/min/kg during 3 hr) modified the number of [3H]yohimbine and [3H]clonidine sites or the affinity of the ligands for the alpha2-sites of the dog platelet. Oral administration of clonidine (3 X 150 micrograms/day) did not alter the binding parameters of either ligand.

Adenosine Diphosphate↗
PubMed1983

Redistribution of fodrin (a component of the cortical cytoplasm) accompanying capping of cell surface molecules.

Fodrin, a protein composed of two polypeptides with molecular weights of 250,000 and 240,000, is concentrated in the cortical cytoplasm of neurons, and moves down the axons by the process of axonal transport. We have used immunofluorescence techniques to determine whether fodrin antigens also move in non-neuronal cells when cell surface ligands are induced to redistribute by crosslinking them. A redistribution of fodrin antigens occurred in the following instances: (i) when 3T3 cells were incubated with concanavalin A and anti-concanavalin A, surface concanavalin A receptors formed aggregates and fodrin antigens formed corresponding intracellular aggregates; (ii) when B lymphocytes were incubated with anti-Ig, the surface Ig formed caps and fodrin antigens formed intracellular subcaps; (iii) when T lymphocytes were treated with anti-H-2 followed by a secondary antibody, the H-2 antigen formed caps and fodrin formed corresponding subcaps. These observations show that fodrin antigens can move within non-neuronal cells, as well as in axons, and that their organization can be regulated by interaction between surface proteins and environmental stimuli. They also raise the possibility that fodrin, together with other proteins that form subcaps in lymphocytes (e.g., actin, myosin, and alpha-actinin) is a component of the cellular machinery responsible for the capping process. We consider whether the similarities between the movements of fodrin in lymphocyte capping and axonal transport may indicate that certain aspects of these two processes are related.

Animals↗
PubMed2001

Proteinase-activated receptors.

Proteinase-activated receptors are a recently described, novel family of seven-transmembrane G-protein-coupled receptors. Rather then being stimulated through ligand receptor occupancy, activation is initiated by cleavage of the N terminus of the receptor by a serine protease resulting in the generation of a new tethered ligand that interacts with the receptor within extracellular loop-2. To date, four proteinase-activated receptors (PARs) have been identified, with distinct N-terminal cleavage sites and tethered ligand pharmacology. In addition to the progress in the generation of PAR-1 antagonists, we describe the role of thrombin in such processes as wound healing and the evidence implicating PAR-1 in vascular disorders and cancer. We also identify advances in the understanding of PAR-1-mediated intracellular signaling and receptor desensitization. The cellular functions, signaling events, and desensitization processes involved in PAR-2 activation are also assessed. However, other major aspects of PAR-2 are highlighted, in particular the ability of several serine protease enzymes, in addition to trypsin, to function as activators of PAR-2. The likely physiological and pathophysiological roles for PAR-2 in skin, intestine, blood vessels, and the peripheral nervous system are considered in the context of PAR-2 activation by multiple serine proteases. The recent discovery of PAR-3 and PAR-4 as additional thrombin-sensitive PARs further highlights the complexity in assessing the effects of thrombin in several different systems, an issue that remains to be fully addressed. These discoveries have also highlighted possible PAR-PAR interactions at both functional and molecular levels. The future identification of other PARs and their modes of activation are an important future direction for this expanding field of study.

Animals↗
PubMed1986

Current issues in thrombosis prevention with antiplatelet drugs.

In this review, the major current problems related to the pharmacology and clinical use of antiplatelet drugs are discussed in relation to the physiopathology of the platelet-vessel wall interaction and arterial thrombus formation. Although platelet adhesion to injured vessels is a crucial step in thrombogenesis, none of the currently used antiaggregating drugs prevents this phenomenon. Why the normal endothelium does not react with platelets is not known. Thus we are unable to pharmacologically restore endothelial 'non-thrombogenicity' when lost by single or repeated injury. In contrast, more information is available on the mechanisms controlling and amplifying platelet activation by physiological stimuli (such as collagen and thrombin), and on their pharmacological modulation. The 3 main amplification loops involve arachidonic acid metabolism, ADP release and possibly the availability of a phospholipid platelet activating factor. These pathways are in turn activated by the phosphatidylinositol cycle. The most widely used antiaggregating drug is aspirin. It prevents the formation of arachidonic acid metabolites both in platelets and in vascular cells. The use of low-dose aspirin, thromboxane-synthase inhibitors, thromboxane receptor antagonists, epoprostenol (prostacyclin) and its stable analogues, and ticlopidine all appear to be promising pharmacological approaches, but none has so far been tested in clinical trials for thrombosis prevention. On the other hand, aspirin (in relatively large doses of 300 to 1500 mg daily), sulphinpyrazone and dipyridamole have been tested alone or in combination in the secondary prevention of thromboembolic complications. Aspirin has significantly reduced both the occurrence of myocardial infarction and mortality rate in patients with unstable angina and/or previous myocardial infarction; it has also proved beneficial in cerebrovascular disease. The beneficial effect of aspirin was dose-independent. In some of these trials aspirin was combined with either dipyridamole or sulphinpyrazone. When used alone, the latter compound has reduced sudden death or thromboembolic complications in patients with myocardial infarction. It remains to be established whether antiplatelet therapy may prevent or stop the progression of atherosclerosis.

Aspirin↗
PubMed2003

Organization of multiple cytochrome P450s with NADPH-cytochrome P450 reductase in membranes.

Microsomal P450-mediated monooxygenase activity supported by NADPH requires an interaction between flavoprotein NADPH-cytochrome P450 reductase and cytochrome P450. These proteins have been identified as the simplest system (with the inclusion of a phospholipid (PL) component) that possesses monooxygenase function; however, little is known about the organization of these proteins in the microsomal membrane. Although reductase and P450 are known to form a 1:1 functional complex, there exists a 10- to 20-fold excess of P450 over the reductase. This raises several questions including "How are the enzymes of the P450 system organized in the microsomal membrane?" and "Can one P450 enzyme affect the functional characteristics of another P450?" This review summarizes evidence supporting the potential for enzymes involved in the P450 system to interact, focusing on the interactions between reductase and P450 and interactions between multiple P450 enzymes. Studies on the aggregation characteristics of P450 as well as on rotational diffusion are detailed, with a special emphasis on the potential for P450 enzymes to produce oligomeric complexes and to suggest the environment in which P450 exists in the endoplasmic reticulum. Finally, more recent studies describing the potential for multiple P450s to exist as complexes and their effect on P450 function are presented, including studies using reconstituted systems as well as systems where two P450s are coexpressed in the presence of reductase. An understanding of the interactions among reductase and multiple P450s is important for predicting conditions where the drug disposition may be altered by the direct effects of P450-P450 complex formation. Furthermore, the potential for one P450 enzyme to affect the behavior of another P450 may be extremely important for drug screening and development, requiring metabolic screening of a drug with reconstituted systems containing multiple P450s rather than simpler systems containing only a single form.

Cell Membrane↗
PubMed1987

Role of alpha 2-adrenergic receptor in platelet activation during insulin-induced hypoglycemia in normal subjects.

The effects of alpha 2-adrenergic-receptor blocker mianserin on the responses of blood glucose, plasma beta-thromboglobulin (beta-TG), and various counterregulatory hormones to insulin-induced hypoglycemia were studied in nine healthy male subjects. The alpha 2-adrenoceptor-blocking action of mianserin was confirmed by its inhibitory effect on platelet activation in vitro. Mianserin was given orally 90 min before insulin injection; the same study without mianserin was performed on another day as the control study. The time courses of blood glucose and serum C-peptide (0, 20, 45, and 180 min after the insulin injection) were identical in both studies, indicating that mianserin has no effect on these parameters. However, a significant increase of beta-TG at 45 min after insulin injection was completely suppressed by the administration of mianserin (mean +/- SE, 68.5 +/- 6.0 vs. 28.8 +/- 7.6 ng/ml, n = 6, P less than .05). No significant differences were obtained between the two studies in the responses of plasma or serum catecholamines, cortisol, glucagon, growth hormone, thromboxane B2, and 6-ketoprostaglandin F1 alpha. These results suggest that epinephrine is responsible for some, if not all, of the beta-TG release from the platelets during insulin-induced hypoglycemia.

Adult↗
PubMed1988

Basal lamina components are concentrated in premuscle masses and at early acetylcholine receptor clusters in chick embryo hindlimb muscles.

As an initial step in characterizing the function of basal lamina components during muscle cell differentiation and innervation in vivo, we have determined immunohistochemically the pattern of expression of three components--laminin, proteins related to agrin (an acetylcholine receptor (AChR)-aggregating protein), and a heparan sulfate proteoglycan--during the development of chick embryo hindlimb muscles. Monoclonal antibodies against agrin were used to purify the protein from the Torpedo ray and to characterize agrin-like proteins from embryonic and adult chicken. In early hindlimb buds (stage 19), antibodies against laminin and agrin stained the ectodermal basement membrane and bound to limb mesenchyme with a generalized, punctate distribution. However, as dorsal and ventral premuscle masses condensed (stage 22-23), mesenchymal immunoreactivity for laminin and agrin-like proteins, but not the proteoglycan, became concentrated in these myogenic regions. Significantly, the preferential accumulation of these molecules in myogenic regions of the limb preceded by 1-2 days the appearance of muscle-specific proteins, myoblast fusion, and muscle innervation. All three basal lamina components were preferentially associated with all AChR clusters from the time we first observed them on newly formed myotubes at stage 26. Localization of these antigens in three-dimensional collagen gel cultures of limb mesenchyme, explanted prior to innervation of the limb, paralleled the staining patterns seen during limb development in the embryo. These results indicate that basal lamina molecules intrinsic to limb mesenchyme are early markers for myogenic and synaptic differentiation, and suggest that these components play important roles during the initial phases of myogenesis and synaptogenesis.

Agrin↗
PubMed1975

B lymphocytes lacking surface IG in patients with immune deficiency: initiation of IG synthesis in culture in cells of a patient with thymoma.

A 64-year-old woman with a syndrome of thymoma, severe hypogammaglobulinemia, seemingly normal cell-mediated immunity and aplastic anemia, was found to have virtually no immunoglobulin- (Ig) bearing peripheral blood lymphocytes (PBL). However, 7.8 +/- 3.4% of the PBL were positive for another B-cell marker, the receptor for aggregated IgG, while the remaining cells bound sheep erythrocytes. Those cells which were aggregate-reactive appeared to be immature or incomplete B cells. Cultures of peripheral blood leukocytes from the patient in various serum-containing media were studied by 3 independent technics for the development of lymphocyte surface Ig and for Ig in the culture supernatants. In vitro the patient's cells were able to develop surface Ig in media supplemented with fetal calf serum (FCS) or normal serum; in media supplemented with autologous serum, the cells developed no surface Ig. During the cultures in FCS, human Ig determinants became detectable in the medium, and both medium and cell-surface Ig underwent a shift from mu determinants early in the culture period to gamma and alpha determinants later. The development of Ig on the cells was not inhibited by the presence of autologous serum if FCS was included in the medium. These data support the concept that a factor, missing from this patient's serum, is required at an early stage in the maturation of the B cell. A patient with X-linked agammaglobulinemia had a population of circulating lymphocytes with surface characteristics similar to the B cells of the thymoma case. In contrast, no Ig synthesis by this patient's cultured cells could be demonstrated, indicating a different level of block in the 2 cases despite their similarity at the level of the cell surface.

Agammaglobulinemia↗
PubMed2001

The dual role of thrombin's anion-binding exosite-I in the recognition and cleavage of the protease-activated receptor 1.

The role of thrombin anion-binding exosite-I in the recognition and cleavage of the extracellular domain of the seven transmembrane domain thrombin receptor (PAR1) was determined using site-directed mutagenesis. Basic residues in anion-binding exosite-I (Arg35, Arg36, Arg67, Arg73, Arg75, Arg77A, Lys81, Lys109, Lys110 and Lys149E) were substituted with glutamines and the resultant recombinant mutant thrombins were used to determine kinetic parameters for the cleavage of a peptide (PAR38-60) based on the PAR1 extracellular domain. Compared with wild-type thrombin, replacement of Arg67 and Arg73 had a dramatic effect on the cleavage of PAR38-60 (k(cat)/K(m) = 1.8 x 10(6) and 4.6 x 10(6) vs 9.2 x 10(7) M(-1).s(-1)), whereas the remaining mutations of the anion-binding exosite-I of thrombin had a less pronounced effect, with k(cat)/K(m) values ranging from 3.3 x 10(7) M(-1). s(-1) (R77(a)Q) to 5.8 x 10(7) M(-1).s(-1) (K109Q). The ability of thrombin mutants to activate platelets paralleled that of PAR38-60 cleavage, whereas their ability to clot fibrinogen differed profoundly, as did their susceptibility to hirudin inhibition. Results are interpreted with respect to known interactions of thrombin with thrombomodulin, hirudin, rhodniin and heparin cofactor II. We conclude that the basic residues of anion-binding exosite-I contribute significantly to enhancing the rate of complex formation in two ways; the first (general) ensures electrostatic steering of ligands with complementary electrostatic fields, the second (specific) involves a combination of molecular contacts within the complex that is unique for each ligand.

Amino Acid Substitution↗
PubMed1996

Human neutrophil interactions of a bispecific monoclonal antibody targeting tumor and human Fc gamma RIII.

2B1 is a bispecific murine monoclonal antibody (bsmAb) targeting the c-erbB-2 and CD16 (Fc gamma RIII) antigens. c-erbB-2 is over-expressed by a variety of adenocarcinomas, and CD16, the low-affinity Fc gamma receptor for aggregated immunoglobulins, is expressed by polymorphonuclear leukocytes (PMN), natural killer (NK) cells and differentiated mononuclear phagocytes. 2B1 potentiates the in vitro lysis of c-erb-2 over-expressing tumors by NK cells and macrophages. In this report, the interactions between 2B1 and PMN were investigated to assess the impact of these associations on in vitro 2B1-promoted tumor cytotoxicity by human NK cells. The peak binding of 2B1 to PMN was observed at a concentration of 10 microgram/ml 2B1. However, 2B1 rapidly dissociated from PMN in vitro at 37 degrees C in non-equilibrium conditions. This dissociation was not caused by CD16 shedding. When PMN were labeled witn 125I-2B1 and incubated at 37 degrees C and the supernatants examined by HPLC analysis, the Fab regions of dissociated 2B1 were not complexed with shed CD16 extracellular domain. While most of the binding of 2B1 PMN was solely attributable to Fab-directed binding to Fc gamma RIII, PMN-associated 2B1 also bound through Fc gamma-domain/Fc gamma RII interactions. 2B1 did not promote in vitro PMN cytotoxicity against c-erbB-2-expressing SK-OV-3 tumor cells. When PMN were coincubated with peripheral blood lymphocytes, SK-OV-3 tumor and 2B1, the concentration of 2B1 required for maximal tumor lysis was lowered. Although PMN may serve as a significant competitive binding pool of systemically administered 2B1 in vivo, the therapeutic potential of the targeted cytotoxicity properties of this bsmAb should not be compromised.

Antibodies, Bispecific↗
PubMed1998

Identification in collagen type I of an integrin alpha2 beta1-binding site containing an essential GER sequence.

The collagen type I-derived fragment alpha1(I)CB3 is known to recognize the platelet collagen receptor integrin alpha2beta1 as effectively as the parent collagen, although it lacks platelet-aggregatory activity. We have synthesized the fragment as seven overlapping peptides that spontaneously assemble into triple helices. On the basis of their capacity to bind purified alpha2 beta1 and the recombinant alpha2 A-domain, and their ability to support alpha2 beta1-mediated cell adhesion, we identified two peptides, CB3(I)-5 and -6, which contain an alpha2 beta1 recognition site. Synthesis of the peptide CB3(I)-5/6, containing the overlap sequence between peptides 5 and 6, allowed us to locate the binding site within the 15-residue sequence, GFP*GERGVEGPP*GPA (where P* represents hydroxyproline), corresponding to residues 502-516 of the collagen type I alpha1 chain. The Glu and Arg residues in the GER triplet were found to be essential for recognition since substitution of either residue with Ala caused a loss of alpha2 A-domain binding. By contrast, substitution of the Glu in GVE did not reduce binding, but rather enhanced it slightly. We were unable to detect significant recognition of alpha2 beta1 by the peptide CB3(I)-2 containing the putative alpha2 beta1 recognition sequence DGEA. Peptides CB3(I)-1 to -6, together with peptide CB3(I)-5/6, exhibited good platelet-aggregatory activity, in some cases better than collagen. However, peptide CB3(I)-7 was inactive, suggesting the presence of an inhibitory element that might account for the lack of aggregatory activity of the parent alpha1(I)CB3 fragment.

Amino Acid Sequence↗
PubMed2002

Platelets activated by collagen through immunoreceptor tyrosine-based activation motif play pivotal role in initiation and generation of neointimal hyperplasia after vascular injury.

BACKGROUND: Platelet adhesion on components of the extracellular matrix and platelet activation by those components are crucial for the arrest of posttraumatic bleeding, but they can also harm tissue by occluding diseased vessels. Recent studies have shown that the activation of platelets by collagen is mediated through the same pathway used by immune receptors, with an immunoreceptor tyrosine-based activation motif on the Fc receptor gamma chain (FcRgamma) playing a pivotal role. METHODS AND RESULTS: We examined the role of collagen-stimulated platelets in the development of injury-induced neointimal formation by using mice deficient in FcRgamma. The left femoral arteries of 8- to 12-week-old FcRgamma-deficient mice (n=16) and C57BL/6 (wild-type) mice (n=16) were injured by a straight spring wire (0.35-mm diameter). Segments of the injured and uninjured femoral arteries were excised at 7 days and 28 days after the vascular injury. Arterial segments were examined by immunohistochemistry and electron microscopy. Two hours after injury, electron microscopy showed marked decreases in platelet adhesion and neutrophil attachment to the vascular wall surface in FcRgamma-knockout mice compared with wild-type mice. At 7 days after injury, staining with anti-neutrophil antibody showed fewer neutrophils in FcRgamma-knockout mice than in wild-type mice. Computer-aided morphometry performed to measure the neointimal area, intima/media ratio, and stenotic area at 28 days after injury showed a significantly smaller ratio and area in FcRgamma-knockout mice than in wild-type mice (for neointimal area, 16 635 +/- 1406 versus 31 483 +/- 2309 microm2, respectively; for intima/media ratio, 1.25 +/- 0.40 versus 2.68 +/- 0.04, respectively; and for stenotic area, 26.8 +/- 2.1% versus 49.3 +/- 4.1%, respectively). CONCLUSIONS: These results demonstrate that FcRgamma may play important roles in the initiation and generation of neointimal hyperplasia after balloon injury through the activation of platelets by collagen.

Adaptor Proteins, Signal Transducing↗
PubMed2006

Direct vascular effects of protease-activated receptor type 1 agonism in vivo in humans.

BACKGROUND: Protease-activated receptor type 1 (PAR-1) has been proposed as the principal thrombin receptor in humans, although its actions in vivo have not been defined. The aim of the present study was to determine the direct vascular actions of PAR-1 agonism in humans. METHODS AND RESULTS: Dorsal hand vein diameter was measured by the Aellig technique in 14 healthy volunteers during local intravenous SFLLRN (PAR-1 agonist; 0.05 to 15 nmol/min) and SLIGKV (PAR-2 agonist; 1.6 to 160 nmol/min) infusions. The venous effects of SFLLRN were further assessed in the presence or absence of norepinephrine or the glycoprotein IIb/IIIa antagonist tirofiban. Forearm blood flow was measured by venous occlusion plethysmography in 16 volunteers during infusion of SFLLRN (1 to 50 nmol/min), SLIGKV (160 to 800 nmol/min), and the endothelium-dependent vasodilator bradykinin (100 to 1000 pmol/min). Platelet-monocyte binding (a sensitive measure of platelet activation) and plasma tissue plasminogen activator (tPA), plasminogen-activator inhibitor 1, and von Willebrand factor concentrations were measured at intervals throughout the study. SFLLRN caused dose-dependent venoconstriction (P<0.001) that was unaffected by norepinephrine or tirofiban co-infusion. In forearm resistance vessels, SFLLRN increased forearm blood flow (P<0.001), tPA release (P<0.001), and platelet-monocyte binding (P<0.0001) without affecting plasma plasminogen-activator inhibitor 1 or von Willebrand factor concentrations. SLIGKV caused venous (P<0.001) and arterial (P<0.01) dilatation without tPA release. CONCLUSIONS: We have demonstrated that PAR-1 agonism causes platelet activation, venous constriction, arterial dilatation, and tPA release in vivo in humans. These unique and contrasting effects provide important insights into the physiological and pathophysiological role of thrombin in the human venous and arterial circulations.

15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5↗
PubMed2003

Detection of integrin alpha IIbbeta 3 clustering in living cells.

In platelets, bidirectional signaling across integrin alpha(IIb)beta(3) regulates fibrinogen binding, cytoskeletal reorganization, cell aggregation, and spreading. Because these responses may be influenced by the clustering of alpha(IIb)beta(3) heterodimers into larger oligomers, we established two independent methods to detect integrin clustering and evaluate factors that regulate this process. In the first, weakly complementing beta-galactosidase mutants were fused to the C terminus of individual alpha(IIb) subunits, and the chimeras were stably expressed with beta(3) in Chinese hamster ovary cells. Clustering of alpha(IIb)beta(3) should bring the mutants into proximity and reconstitute beta-galactosidase activity. In the second method, alpha(IIb) was fused to either a green fluorescent protein (GFP) or Renilla luciferase and transiently expressed with beta(3). Here, integrin clustering should stimulate bioluminescence resonance energy transfer between a cell-permeable luciferase substrate and GFP. These methods successfully detected integrin clustering induced by anti-alpha(IIb)beta(3) antibodies. Significantly, they also detected clustering upon soluble fibrinogen binding to alpha(IIb)beta(3). In contrast, no clustering was observed following direct activation of alpha(IIb)beta(3) by MnCl(2) or an anti-alpha(IIb)beta(3)-activating antibody Fab in the absence of fibrinogen. Intracellular events also influenced alpha(IIb)beta(3) clustering. For example, a cell-permeable, bivalent FK506-binding protein (FKBP) ligand stimulated clustering when added to cells expressing an alpha(IIb)(FKBP)(2) chimera complexed with beta(3). Furthermore, alpha(IIb)beta(3) clustering occurred in the presence of latrunculin A or cytochalasin D, inhibitors of actin polymerization. These effects were enhanced by fibrinogen, suggesting that actin-regulated clustering modulates alpha(IIb)beta(3) interaction with ligands. These studies in living cells establish that alpha(IIb)beta(3) clustering is modulated by fibrinogen and actin dynamics. More broadly, they should facilitate investigations of the mechanisms and consequences of integrin clustering.

Actins↗
PubMed2006

PAR4, but not PAR1, signals human platelet aggregation via Ca2+ mobilization and synergistic P2Y12 receptor activation.

Regulation of platelet activation plays a central role in hemostasis and pathophysiological processes such as coronary artery disease. Thrombin is the most potent activator of platelets. Human platelets express two thrombin receptors, PAR1 and PAR4, both of which signal platelet activation. Evidence is lacking on the mechanism by which PAR1 and PAR4 may differentially signal platelet aggregation. Here we show that at the relatively high concentration of agonist most likely found at the site of a local thrombus, dual inhibition of the P2Y12 receptor and calcium mobilization result in a complete inhibition of PAR4-induced aggregation, while having no effect on either thrombin or PAR1-mediated platelet aggregation. Both PAR1- and PAR4mediated aggregation are independent of calcium mobilization. Furthermore, we show that P2Y12 receptor activation is not required for protease-activated receptor-mediated aggregation at higher agonist concentrations and is only partially required for Rap1 as well as GPIIbIIIa activation. P2Y12 receptor inhibitors clinically in use such as clopidogrel are postulated to decrease platelet aggregation through partial inhibition of PAR1 signaling. Our data, however, indicate that at high local concentrations of thrombin, it is the signaling through PAR4 rather than PAR1 that may be regulated through purinergic feedback. Thus, our data identify an intra-platelet mechanism that may function as a future site for therapeutic intervention.

Adenosine Diphosphate↗