Carboxyl terminal sequence of human phospholipase Cgamma2

Phospholipase Cgamma2 (PLCgamma2), the predominant isoform of phospholipase C expressed in platelets, plays a major role in activation of platelets by collagen. Although PLCgamma2 has been shown to be tyrosine phosphorylated upon collagen-induced activation, the phosphorylation sites are yet to be determined. We have sequenced the 3' terminal cDNA of human phospholipase C-gamma-2 and found it different from the human PLCgamma2 cDNA sequence previously reported by Ohta et al. (Ohta S, Matsui A, Nazawa Y, Kagawa Y. FEBS Lett 1988; 242: 31-5). There is an extra guanosine at position 3723 which causes a shift in the reading frame. The new carboxyl terminal amino acid (aa) sequence beyond the frame shift is 88% identical to that of rat (21 out of 24 aa residues) which is considerably higher than the identity with published sequence (26% identity). The new deduced aa sequence contains two tyrosine residues at positions 1245 and 1264 which might be phosphorylated upon stimulation and hence might be important for the activation of the PLCgamma2.

Isolation and characterization of EMS16, a C-lectin type protein from Echis multisquamatus venom, a potent and selective inhibitor of the alpha2beta1 integrin

We have isolated and characterized EMS16, a potent and selective inhibitor of the alpha2beta1 integrin, from Echis multisquamatus venom. It belongs to the family of C-lectin type of proteins (CLPs), and its amino acid sequence is homologous with other members of this protein family occurring in snake venoms. EMS16 (M(r) approximately 33K) is a heterodimer composed of two distinct subunits linked by S-S bonds. K562 cells transfected with alpha2 integrin selectively adhere to immobilized EMS16, but not to two other snake venom-derived CLPs, echicetin and alboaggregin B. EMS16 inhibits adhesion of alpha2beta1-expressing cells to immobilized collagen I at picomolar concentrations, and the platelet/collagen I interaction in solution at nanomolar concentrations. EMS16 inhibits binding of isolated, recombinant I domain of alpha2 integrin to collagen in an ELISA assay, but not the interaction of isolated I domain of alpha1 integrin with collagen IV. Studies with monoclonal antibodies suggested that EMS16 binds to the alpha2 subunit of the integrin. EMS16 inhibits collagen-induced platelet aggregation, but has no effect on aggregation induced by other agonists such as ADP, thromboxane analogue (U46619), TRAP, or convulxin. EMS16 also inhibits collagen-induced, but not convulxin-induced, platelet cytosolic Ca(2+) mobilization. In addition, EMS16 inhibits HUVEC migration in collagen I gel. In conclusion, we report a new, potent viper venom-derived inhibitor of alpha2beta1 integrin, which does not belong to the disintegrin family.

The P2Y1 receptor mediates ADP-induced p38 kinase-activating factor generation in human platelets

U46619, a thromboxane A2 mimetic, but not ADP, caused activation of p38 mitogen activated protein (MAP) kinase in aspirin-treated platelets. In nonaspirinated human platelets ADP activated p38 MAP kinase in both a time-and concentration-dependent manner, suggesting that ADP-induced p38 MAP kinase activation requires generation of thromboxane A2. However, neither a thromboxane A2/prostaglandin H2 receptor antagonist SQ29548 and a thromboxane synthase inhibitor, furegrelate, either alone or together, nor indomethacin blocked ADP-induced p38 kinase activation in nonaspirinated platelets. Other cycloxygenase products, PGE2, PGD2, and PGF2alpha, failed to activate p38 kinase in aspirin-treated platelets. Hence, ADP must be generating an agonist, other than thromboxane A2, via an aspirin-sensitive pathway, which is capable of activating p38 kinase. AR-C66096, a P2TAC (platelet ADP receptor coupled to inhibition of adenylate cyclase) antagonist, did not inhibit ADP-induced p38 MAP kinase activation. The P2X receptor selective agonist, alpha, beta-methylene ATP, failed to activate p38 MAP kinase. On the other hand, the P2Y1 receptor selective antagonist, adenosine-2'-phosphate-5'-phosphate inhibited ADP-induced p38 kinase activation in a concentration-dependent manner, indicating that the P2Y1 receptor alone mediates ADP-induced generation of the p38 kinase-activating factor. These results demonstrate that ADP causes the generation of a factor in human platelets, which can activate p38 kinase, and that this response is mediated by the P2Y1 receptor. Neither the P2TAC receptor nor the P2X1 receptor has any significant role in this response.

Platelet shape change is mediated by both calcium-dependent and -independent signaling pathways. Role of p160 Rho-associated coiled-coil-containing protein kinase in platelet shape change

Platelets undergo shape change upon activation with agonists. During shape change, disc-shaped platelets turn into spiculated spheres with protruding filopodia. When agonist-induced cytosolic Ca(2+) increases were prevented using the cytosolic Ca(2+) chelator, 5, 5'-dimethyl-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (5, 5'-dimethyl-BAPTA), platelets still underwent shape change, although the onset was delayed and the initial rate was dramatically decreased. In the absence of cytosolic Ca(2+), agonist-stimulated myosin light chain phosphorylation was significantly inhibited. The myosin light chain was maximally phosphorylated at 2 s in control platelets compared with 30 s in 5,5'-dimethyl-BAPTA-treated platelets. ADP, thrombin, or U46619-induced Ca(2+)-independent platelet shape change was significantly reduced by staurosporine, a nonselective kinase inhibitor, by the selective p160 Rho-associated coiled-coil-containing protein kinase inhibitor Y-27632, or by HA 1077. Both Y-27632 and HA 1077 reduced peak levels of ADP-induced platelet shape change and myosin light chain phosphorylation in control platelets. In 5,5'-dimethyl-BAPTA-treated platelets, Y-27632 and HA 1077 completely abolished both ADP-induced platelet shape change and myosin light chain phosphorylation. Our results indicate that Ca(2+)/calmodulin-stimulated myosin light chain kinase and p160 Rho-associated coiled-coil-containing protein kinase independently contribute to myosin light chain phosphorylation and platelet shape change, through Ca(2+)-sensitive and Ca(2+)-insensitive pathways, respectively.

Role of intracellular signaling events in ADP-induced platelet aggregation

Human platelets express two distinct G protein-coupled ADP receptors, one coupled to phospholipase C through Gq, P2Y1, and the other to inhibition of adenylyl cyclase through Gi, P2TAC. We have recently shown that concomitant intracellular signaling from both the P2TAC and P2Y1 receptors is essential for ADP-induced platelet aggregation. Previous studies have tested whether ADP causes a decrease in the basal cAMP level and this reduction promotes platelet aggregation, but did not study the effect of decreased cAMP levels when the Gq pathway is selectively activated. Since we are now aware that platelet aggregation requires activation of two receptors, we investigated whether the function of P2TAC receptor activation, leading to inhibition of platelet adenylyl cyclase, could be replaced by direct inhibition of adenylyl cyclase, when Gq pathway is also activated, a possibility that has not been addressed to date. In the present study, we supplemented the P2Y1 mediated Gq signaling pathway with inhibition of the platelet adenylyl cyclase by using SQ22536 or dideoxyadenosine, or by selective activation of the alpha2A adrenoceptors with epinephrine. Although SQ22536, dideoxyadenosine, and epinephrine reduced the cAMP levels, only epinephrine could mimic the P2TAC receptor mediated signaling events, suggesting that reduction in basal cAMP levels does not directly contribute to ADP-induced platelet activation. Adenosine-5'-phosphate-3'-phosphosulfate, a P2Y1 receptor antagonist, completely blocked ADP-induced inositol 1,4,5-trisphosphate and inositol 1,3.4-trisphosphate formation suggesting that P2TAC-mediated activation of Gi (or other G proteins) does not activate phospholipase C. These results suggest that a signaling event downstream from Gi, independent of the inhibition of platelet adenylyl cyclase, contributes to alphaIIb beta3 activation.

P2 receptor subtypes in the cardiovascular system

Extracellular nucleotides have been implicated in a number of physiological functions. Nucleotides act on cell-surface receptors known as P2 receptors, of which several subtypes have been cloned. Both ATP and ADP are stored in platelets and are released upon platelet activation. Furthermore, nucleotides are also released from damaged or broken cells. Thus during vascular injury nucleotides play an important role in haemostasis through activation of platelets, modulation of vascular tone, recruitment of neutrophils and monocytes to the site of injury, and facilitation of adhesion of leucocytes to the endothelium. Nucleotides also moderate these functions by generating nitric oxide and prostaglandin I2 through activation of endothelial cells, and by activating different receptor subtypes on vascular smooth muscle cells. In the heart, P2 receptors regulate contractility through modulation of L-type Ca2+ channels, although the molecular mechanisms involved are still under investigation. Classical pharmacological studies have identified several P2 receptor subtypes in the cardiovascular system. Molecular pharmacological studies have clarified the nature of some of these receptors, but have complicated the picture with others. In platelets, the classical P2T receptor has now been resolved into three P2 receptor subtypes: the P2Y1, P2X1 and P2TAC receptors (the last of these, which is coupled to the inhibition of adenylate cyclase, is yet to be cloned). In peripheral blood leucocytes, endothelial cells, vascular smooth muscle cells and cardiomyocytes, the effects of classical P2X, P2Y and P2U receptors have been found to be mediated by more than one P2 receptor subtype. However, the exact functions of these multiple receptor subtypes remain to be understood, as P2-receptor-selective agonists and antagonists are still under development.

Nickel enhances collagen-induced platelet activation acting by increasing the organization of the cytoskeleton

In keratinocytes, osteoclasts and enterocytes, Ni2+ acts as an agonist working through selective activation of the polyvalent cation-sensing receptor. We report here that while Ni2+ alone had no direct ability to induce platelet aggregation or secretion, Ni2+ pretreatment produced these responses when platelets were stimulated with subthreshold concentrations of collagen. In addition, pretreatment with Ni2+ significantly enhanced collagen-induced phospholipase C activation and calcium mobilization. Platelet adhesion to collagen was increased and the inhibition of collagen-induced adhesion normally seen after cytochalasin D treatment was significantly diminished. When Ni2+ was added to platelets alone, tyrosine phosphorylation of p60src was increased. Moreover, Ni2+ enhanced the amount of protein, especially actin, found in the low-speed Triton X-100 insoluble cytoskeleton. Our results indicate that nickel, possibly acting via a platelet cation sensing receptor analogous to that which has been described in other cell types, may cause a rapid tyrosine kinase-dependent cytoskeleton reorganization leading to enhanced adhesion of platelets to collagen and increasing collagen-dependent responses.

Molecular basis for ADP-induced platelet activation. I. Evidence for three distinct ADP receptors on human platelets

Acting through cell surface receptors, ADP activates platelets resulting in shape change, aggregation, thromboxane A2 production, and release of granule contents. ADP also causes a number of intracellular events including inhibition of adenylyl cyclase, mobilization of calcium from intracellular stores, and rapid calcium influx in platelets. However, the receptors that transduce these events remain unidentified and their molecular mechanisms of action have not been elucidated. The receptor responsible for the actions of ADP on platelets has been designated the P2T receptor. In this study we have used ARL 66096, a potent antagonist of ADP-induced platelet aggregation, and a P2X ionotropic receptor agonist, alpha,beta-methylene adenosine 5'-triphosphate, to distinguish the ADP-induced intracellular events. ARL 66096 blocked ADP-induced inhibition of adenylyl cyclase, but did not affect ADP-mediated intracellular calcium increases or shape change. Both ADP and 2-methylthio-ADP caused a 3-fold increase in the level of inositol 1,4,5-trisphosphate over control levels which peaked in a similar fashion to the Ca2+ transient. The increase in inositol 1,3,4-trisphosphate was of similar magnitude to that of inositol 1,4,5-trisphosphate. alpha,beta-Methylene adenosine 5'-triphosphate did not cause an increase in either of the inositol trisphosphates. These results clearly demonstrate the presence of two distinct platelet ADP receptors in addition to the P2X receptor: one coupled to adenylyl cyclase and the other coupled to mobilization of calcium from intracellular stores through inositol trisphosphates.

Molecular basis for ADP-induced platelet activation. II. The P2Y1 receptor mediates ADP-induced intracellular calcium mobilization and shape change in platelets

ADP is an important platelet agonist causing shape change from smooth discoid shape to spiculated spheres and platelet aggregation. However, the molecular mechanisms involved in ADP-induced platelet activation have not been elucidated. We demonstrated earlier the existence of two distinct ADP receptors on platelets, one coupled to phospholipase C, P2TPLC, and the other to inhibition of adenylyl cyclase, P2TAC (Daniel, J. L., Dangelmaier, C., Jin, J., Ashby, B., Smith, J. B., and Kunapuli, S. P. (1998) J. Biol. Chem. 273, 2024-2029), in addition to the previously described P2X1 receptor. Here we report the cloning of a cDNA clone encoding the P2Y1 receptor from a human platelet cDNA library by homology screening with radiolabeled P2Y1-P2Y6 receptor cDNAs. ADP or 2-methyl(thio)-ADP-induced intracellular calcium increases were inhibited by the P2Y1 receptor-specific antagonists, adenosine 3'-phosphate 5'-phosphosulfate (A3P5PS), adenosine 3'-phosphate 5'-phosphate (A3P5P), and adenosine 2'-phosphate 5'-phosphate (A2P5P), in a concentration-dependent manner, but not by ARL 66096 or alpha, beta-MeATP. A3P5PS, A3P5P, and A2P5P also inhibited the shape change of aspirinated platelets induced by 10 microM ADP or 3 microM 2-methyl-(thio)-ADP in a concentration-dependent manner, with complete inhibition occurring at 300 microM. On the other hand ARL 66096 (100 nM), a potent P2TAC antagonist and alpha, beta-methylene-ATP (40 microM), a P2X1 receptor agonist, had no effect on ADP-induced platelet shape change. On the contrary, ADP-induced inhibition of adenylyl cyclase was blocked by ARL 66096, but not by alpha, beta-MeATP or the P2Y1 receptor-specific antagonists, A3P5PS, A3P5P, or A2P5P. These results demonstrate the role of the P2Y1 receptor in ADP-induced platelet shape change and calcium mobilization and support the idea that several P2 receptors are involved in the regulation of different aspects of platelet stimulus-response coupling.

Molecular cloning of a novel P2 purinoceptor from human erythroleukemia cells

Screening of a human erythroleukemia cell cDNA library with radiolabeled chicken P2Y3 cDNA at low stringency revealed a cDNA clone encoding a novel G protein-coupled receptor with homology to P2 purinoceptors. This receptor, designated P2Y7, has 352 amino acids and shares 23-30% amino acid identity with the P2Y1-P2Y6 purinoceptors. The P2Y7 cDNA was transiently expressed in COS-7 cells: binding studies thereon showed a very high affinity for ATP (37 +/- 6 nM), much less for UTP and ADP (approximately 1300 nM), and a novel rank order of affinities in the binding series studied of 8 nucleotides and suramin. The P2Y7 receptor sequence appears to denote a different subfamily from that of all the other known P2Y purinoceptors, with only a few of their characteristic sequence motifs shared. The P2Y7 receptor mRNA is abundantly present in the human heart and the skeletal muscle, moderately in the brain and liver, but not in the other tissues tested. The P2Y7 receptor mRNA was also abundantly present in the rat heart and cultured neonatal rat cardiomyocytes. The P2Y7 receptor is functionally coupled to phospholipase C in COS-7 cells transiently expressing this receptor. The P2Y7 gene was shown to be localized to human chromosome 14. We have thus cloned a unique member of the P2Y purinoceptor family which probably plays a role in the regulation of cardiac muscle contraction.

The Dami cell possesses the platelet collagen receptor VLA-2, but does not mobilize calcium

Flow cytometric and Western blot analysis showed that Dami cells possess the major platelet collagen adhesion receptor, the integrin VLA-2, and that VLA-2 was expressed in higher levels in a time-dependent manner in DMSO-induced Dami cells. Both control and DMSO-induced Dami cells were able to adhere to collagen as measured in a microtiter-based adhesion assay. It appeared that collagen adhesion was solely mediated by VLA-2, since inclusion of a monoclonal antibody directed against the alpha-2 subunit of VLA-2 in the adhesion assay was able to totally inhibit adhesion. Although Dami cells possess a variety of platelet markers, and are able to mobilize intracellular calcium in response to ADP, U-46619, and thrombin, they were unable to respond to collagen challenge. We concluded that Dami cells may lack some key transducing element present in platelets that prevents them from being activated by collagen.

Pharmacological profiles of two bombesin analogues in cells transfected with human neuromedin B receptors

We examined the effect of two des-Met-bombesin analogues, [(CH3)2CHCO-His-Trp-Ala-Val-D-Ala-His-Leu-NHCH3] (ICI 216140) and [D-Phe6,des-Met14]bombesin(6-14) ethylamide (DPDM-bombesin ethylamide), on neuromedin B-induced Ca2+ and [3H]arachidonate release in BALB 3T3 cells transfected with human neuromedin B receptors. ICI 216140 and DPDM-bombesin ethylamide both stimulated Ca2+ mobilisation in a concentration-dependent manner but were less potent and efficacious than neuromedin B. BIM 23042 [D-Nal-Cys-Tyr-D-Trp-Lys-Val-Cys-Nal-NH2], a selective neuromedin B antagonist and [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P, a broad-spectrum peptide receptor antagonist inhibited neuromedin B-, ICI 216140 and DPDM-bombesin ethylamide-induced Ca2+ release. Pretreatment of cells with either des-Met-bombesin analogue attenuated neuromedin B-induced Ca2+ elevations, suggesting similar agonist-sensitive Ca2+ pools. The pharmacological profiles revealed from the [3H]arachidonate assay were similar, although ICI 216140 was less potent and efficacious than DPDM-bombesin ethylamide. The data suggest that ICI 216140 and DPDM-bombesin ethylamide behave as agonists at the neuromedin B receptor, perhaps as a consequence of neuromedin B receptor overexpression.

Protein kinase C activation is not a key step in ADP-mediated exposure of fibrinogen receptors on human platelets

A selective inhibitor of protein kinase C (PKC), Ro 31-8220, blocks pleckstrin (P47) phosphorylation in platelets activated with either ADP, ADP plus synthetic thromboxane agonist U46619 and ADP plus U46619 plus epinephrine, while inducing a weak inhibition of platelet aggregation, and no significant effect on the fibrinogen binding. In platelets activated by U46619 alone, P47 phosphorylation, platelet aggregation, fibrinogen binding and serotonin release are all inhibited by Ro 31-8220. In the presence of an ADP scavenger system, U46619 induces pleckstrin phosphorylation, serotonin release and calcium mobilization but not platelet aggregation and fibrinogen binding, unless epinephrine is added.

IN CONCLUSION

(1) PKC activation is required for ADP secretion; (2) ADP or epinephrine are essential for fibrinogen receptor exposure induced by U46619; (3) fibrinogen receptor exposure induced by ADP is independent of activation of PKC.

Fibrinogen binding is independent of an increase in intracellular calcium concentration in thrombin degranulated platelets

In a suspension of thrombin degranulated platelets (TDP), ADP and epinephrine can induce platelet aggregation, whereas the synthetic agonist of the thromboxane/endoperoxide receptor U46619 causes only shape change. However, U46619 can enhance platelet aggregation induced by ADP and epinephrine. In this paper, we have measured fibrinogen binding in relation to phospholipase C (PLC) activation and calcium mobilization in TDP activates by ADP, epinephrine and U46619. ADP caused fibrinogen binding in TDP but neither activated PLC nor caused a calcium mobilization. The requirement for ADP in inducing exposure of fibrinogen binding sites was not absolute since the combination of epinephrine and U46619 produced an increase in fibrinogen binding. U46619 caused significant PLC activation and cytosolic calcium release but not fibrinogen binding. These results suggest that in TDP the exposure of fibrinogen binding sites, after agonist activation, is independent of both PLC activation and calcium mobilization.

Evidence for a role for tyrosine phosphorylation of phospholipase C gamma 2 in collagen-induced platelet cytosolic calcium mobilization

(1) The non-specific protein kinase C inhibitor, staurosporine, inhibited collagen-induced increases in cytosolic free Ca2+ while having no effect on Ca2+ mobilization by other platelet agonists. A more specific inhibitor of protein kinase C, Ro 31-8220, did not inhibit collagen-induced Ca2+ mobilization. Neither drug had an effect on platelet adhesion to collagen. (2) Staurosporine inhibited collagen-stimulated tyrosine phosphorylation, while Ro 31-8220 had no effect. (3) It also inhibited collagen-induced phosphatidic acid formation, inositol trisphosphate formation and arachidonic acid liberation. (4) Ro 31-8220 did not inhibit collagen-stimulated arachidonic acid formation, but it enhanced collagen-stimulated phosphatidic acid and inositol trisphosphate formation. (5) Immunoprecipitation of phospholipase C gamma 2 (PLC gamma 2) with a specific antibody demonstrated that PLC gamma 2 was phosphorylated on tyrosine after stimulation by collagen. (6) The phosphorylation of PLC gamma 2 was inhibited by staurosporine but not by Ro 31-8220. These results provide additional evidence that the mechanism of signal transduction for collagen is different from other platelet agonists and indicate that it involves activation of PLC gamma through a tyrosine kinase-dependent mechanism.

The developing cerebral surface. Preliminary report on the patterns of sulcal and gyral maturation--anatomy, ultrasound, and magnetic resonance imaging

This literature review provides data on the temporal sequence in which gyri, sulci, and fissures first become apparent on anatomic specimens of the developing brain surface. Ultrasonic display of these anatomic features lags behind the initial appearance of the features by a variable interval, greater in early gestation and diminishing in late gestation. MR imaging, especially three-dimensional surface rendering, displays the degree of surface maturation to advantage. Different publications have provided variable estimates of the time course of surface maturation, leading to discordance in the date tabulated. None-the-less, the overall sequence is clear and the surface features are useful in assessing the degree of fetal-neonatal maturity by ultrasonography and by MR imaging.

Collagen induces normal signal transduction in platelets deficient in CD36 (platelet glycoprotein IV)

The receptor involved in platelet activation by collagen has not been identified. Platelet glycoprotein IV, now known as CD36, has been implicated in interaction with collagen and also been shown to be associated with intracellular tyrosine kinases. In order to investigate the possible role of collagen-mediated signal transduction via CD36, platelets were obtained from a donor that were deficient in CD36. The collagen-induced intracellular mobilization of Ca2+ in the CD36 deficient cells was of the same magnitude as that seen in platelets from normal donors. In addition, serotonin secretion did not appear to be impaired. Tyrosine phosphorylation was also comparable between the CD36-deficient and normal platelets. Thus, it is unlikely that CD36 plays a major role in collagen-dependent platelet signal transduction.

Two bombesin analogues discriminate between neuromedin B- and bombesin-induced calcium flux in a lung cancer cell line

We examined the profile of two bombesin (BN) antagonists, (CH3)2CHCO-His-Trp-Ala-Val-D-Ala-His-Leu-NHCH3] (ICI 216140) and [D-Phe6,des-Met14]BN(6-14)ethylamide (DPDM-BN EA), against neuromedin B-induced Ca2+ mobilization in the small cell lung cancer (SCLC) line NCI-H345. Neuromedin B (NMB), a BN-like peptide sharing sequence homology with ranatensin, elicited a concentration-dependent Ca2+ release (in part) from intracellular stores. Sequential addition of NMB attenuated Ca2+ mobilization. Desensitization occurred between BN and NMB; depletion of intracellular Ca2+ is a likely mechanism because thapsigargin stimulated Ca2+ release after a maximally desensitizing dose of NMB. ICI 216140 and DPDM-BN EA competitively inhibited BN-induced Ca2+ transients. In contrast, these compounds antagonized NMB-stimulated Ca2+ transients in a noncompetitive manner. The pharmacological profiles obtained support receptor heterogeneity for BN-like peptides on this SCLC line, underscoring the need for thorough examination of dose-response relationships when investigating effects of BN analogues on intact cells.

Elevation of cAMP in human platelets inhibits thrombin- but not collagen-induced tyrosine phosphorylation

Both thrombin and collagen induced the phosphorylation of tyrosine in numerous proteins in platelets, with collagen causing the phosphorylation of an additional 40 kDa protein. Thrombin-induced phosphorylation was markedly inhibited when cAMP was elevated with iloprost. Iloprost or the combination of iloprost, inhibitors of positive feedback and cytochalasin D also partially inhibited collagen-induced phosphorylation. By contrast, iloprost had no effect on phosphorylation induced by collagen in the presence of inhibitors of positive feedback by released ADP, TxA2 and fibrinogen and in platelets containing BAPTA to prevent increases in cytosolic Ca2+. The results indicate that collagen-induced tyrosine phosphorylation may be a fundamental pathway in hemostasis which can function even when platelet cAMP is elevated.

Cytosolic calcium as a second messenger for collagen-induced platelet responses

We showed previously that direct platelet activation by collagen involves an increase in the platelet cytosolic free Ca2+ concentration ([Ca2+]i) but that this increase is not required for the adhesion of platelets to collagen. We now report that collagen-induced arachidonic acid liberation, myosin phosphorylation and 5-hydroxytryptamine secretion are dependent on increases in [Ca2+]i, as they were markedly inhibited in platelets loaded with the acetoxymethyl ester of the Ca2+ chelator BAPTA but not in cells loaded with the acetoxymethyl ester of the non-chelating diazo-3. BAPTA also partially inhibited the rate of collagen-induced phosphatidic acid (PtdA) formation but had little effect on increases in phosphorylation of pleckstrin (47 kDa protein; P47). From these results we infer that collagen-induced increases in [Ca2+]i are required for dense granule secretion and arachidonic acid liberation, but are not necessary for stimulation of the protein kinase C pathway.

Evidence that adhesion of electrically permeabilized platelets to collagen is mediated by guanine nucleotide regulatory proteins

Adhesion of electrically permeabilized platelets to collagen was found to be essentially independent of free Ca2+ concentration in the medium. Addition of stable GTP analogues increased the proportion of adhering cells about 5-fold. This effect was inhibited by guanosine 5'-[beta-thio]diphosphate, cytochalasin D or monoclonal antibodies to glycoprotein Ia. In contrast, the protein kinase C inhibitor staurosporine had only a small effect on the GTP-analogue-enhanced adhesion of the permeabilized cells to collagen. These results suggest that a guanine nucleotide regulatory (G)-protein is directly linked to the collagen receptor and is involved in the actin-dependent recruitment of additional collagen receptors.

Facile platelet adhesion to collagen requires metabolic energy and actin polymerization and evokes intracellular free calcium mobilization

The attachment of platelets to collagen-coated microtiter plates at 20 degrees C was inhibited strongly by depletion of metabolic energy or by addition of cytochalasins and was slightly inhibited by the intracellular Ca2+ chelator BAPTA. In keeping with their respective potencies as inhibitors of actin polymerization, cytochalasins D and H were the most potent inhibitors of adhesion, while cytochalasin B was the least potent. Energy depletion, cytochalasin D or, to a much lesser extent, BAPTA also inhibited platelet adhesion to collagen in a suspension assay system at 37 degrees C. Collagen-induced platelet cytosolic Ca2+ mobilization was inhibited up to 70% by cytochalasin D and abolished by energy depletion or BAPTA. Elevation of intracellular platelet calcium by treatment with ionomycin had little effect on platelet adhesion to collagen. We propose that rapid platelet spreading along collagen fibers is both energy- and actin-dependent and necessary to produce maximal adhesion needed to elicit Ca2+ mobilization required for subsequent responses.