Effects of phosphoinositides on calcium movements in human platelet membrane vesicles

Activation of store-operated I(CRAC) by hydrogen peroxide

Reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) play a role in both innate immunity as well as cellular injury. H2O2 induces changes in intracellular calcium ([Ca(2+)]i) in many cell types and this seems to be at least partially mediated by transient receptor potential melastatin 2 (TRPM2) in cells that express this channel. Here we show that low concentrations of H2O2 induce the activation of the Ca(2+)-release activated Ca(2+) current I(CRAC). This effect is not mediated by direct CRAC channel activation, since H2O2 does not activate heterologously expressed CRAC channels independently of stromal interaction molecule (STIM). Instead, I(CRAC) activation is partially mediated by store depletion through activation of inositol 1,4,5 trisphosphate receptors (IP3R), since pharmacological inhibition of IP3 receptors by heparin or molecular knock-out of all IP3 receptors in DT40 B cells strongly reduce H2O2-induced I(CRAC). The remainder of H2O2-induced I(CRAC) activation is likely mediated by IP3R-independent store-depletion. Our data suggest that H2O2 can activate Ca(2+) entry through TRPM2 as well as store-operated CRAC channels, thereby adding a new facet to ROS-induced Ca(2+) signaling.

Cellular calcium mobilization in response to phosphoinositide delivery

Intracellular calcium [Ca(2+)](i) is mobilized in many cell types in response to activation of phosphoinositide (PIP(n)) signaling pathways involving PtdIns(4,5)P(2) or PtdIns(3,4,5)P(3). To further explore the relationship between increases in intracellular PIP(n) concentrations and mobilization of [Ca(2+)](i), each of the seven phosphorylated phosphoinositides (PIP(n)s) were delivered into cells and the metabolism and physiological effects of the exogenously administered PIP(n)s were determined. The efficient cellular delivery of fluorophore-tagged and native PIP(n)s was accomplished using histone protein, neomycin, and dendrimeric polyamines. PtdIns(4,5)P(2) fluorophore-tagged analogs with short- and long-acyl chains were substrates for cellular enzymes in vitro and for phospholipases in stimulated fibroblasts. PtdIns(4)P, PtdIns(3,4)P(2) and PtdIns(4,5)P(2), each induced calcium mobilization rapidly after exogenous addition to fibroblasts. PtdIns(3,4,5)P(3) induced a significant, but smaller increase in intracellular calcium. These observations suggest that PIP(n)s other than PtdIns(4,5)P(2) or PtdIns(3,4,5)P(3) may have direct roles in signaling involving [Ca(2+)](i).

Frequency decoding of fast calcium oscillations by calpain

We report the development of a novel procedure for generating fast, high-frequency Ca2+ oscillations in vitro and the frequency-dependent activation of m-calpain, the Ca2+-activated intracellular cysteine protease. The procedure is based upon liberating Ca2+ from a cage, DM-Nitrophen, by repetitive UV laser pulses and its concomitant binding by a 'slow' chelator, 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetate (DOTA). It is shown that a full control over the pattern of oscillations can be readily achieved because the half-life of individual spikes is determined by DOTA concentration and pH, whereas peak amplitude can be adjusted by light intensity. Frequency is only limited by the physical parameters of the light source. The sensitivity of calpain activation to the frequency of Ca2+ oscillations was monitored by the cleavage of microtubule-associated protein 2, a very sensitive physiological substrate of the enzyme. One hundred transients at a peak Ca2+ concentration of 10 microM were presented at various pH values and frequencies ranging from 1 to 50 Hz. At pH 6.0 and 7.0 significant activation occurred at high frequencies (20 and 50 Hz), but here Ca2+ accumulated due to the overlap of transients; at low frequencies (1 and 3 Hz) where Ca2+ accumulation was negligible, there was no calpain activation. At pH 8.0, where individual transients do not overlap even at 50 Hz, frequency-dependence of activation is seen when calpain is sensitized to Ca2+ by autolysis and by the addition of a phospholipid, phosphatidylinositol-4,5-bisphosphate. Our results show that calpain is sensitive to the frequency of fast Ca2+ oscillations in vitro, which is of potential physiological significance.

Deflagellation of Chlamydomonas reinhardtii follows a rapid transitory accumulation of inositol 1,4,5-trisphosphate and requires Ca2+ entry

C. reinhardtii sheds its flagella in response to acidification. Previously, we showed correlations between pH shock, deflagellation, and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] production, but 100% of cells deflagellated by 5 s, which was the earliest that Ins(1,4,5)P3 accumulation could be accurately measured by techniques available to us at that time (Quarmby, L. M., Y. G. Yueh, J. L. Cheshire, L. R. Keller, W. J. Snell, and R. C. Crain. J. Cell Biol. 1992. 116:737-744). To learn about the causal relationship between Ins(1,4,5)P3 accumulation and deflagellation, we extended these studies to early times using a continuous-flow rapid-quench device. Within 1 s of acidification to pH 4.3-4.5, 100% of cells deflagellated. A transient peak of Ins(1,4,5)P3 was observed 250-350 ms after pH shock, preceding deflagellation. Preincubation with 10 microM neomycin, which prevents hydrolysis of phosphatidylinositol 4,5-bisphosphate, inhibited both the transient production of Ins(1,4,5)P3 and the subsequent deflagellation. The nonspecific Ca2+ channel blockers La3+ and Cd2+ prevented flagellar excision induced by mastoparan without inhibiting rapid Ins(1,4,5)P3 production. Likewise, the Ins(1,4,5)P3-gated channel inhibitors ruthenium red and heparin blocked deflagellation in response to mastoparan. These studies were extended to mutants defective in flagellar excision. Fa-1, a mutant defective in flagellar structure, produced Ins(1,4,5)P3 but failed to deflagellate. These results support a model in which acid pH activates a putative cellular receptor leading to G-protein dependent activation of phospholipase C and accumulation of Ins(1,4,5)P3. These events are upstream of Ins(1,4,5)P3-dependent Ca2+ entry from the medium, and of deflagellation.

BW755C or staurosporine inhibits collagen-stimulated phosphoinositide phosphorylation in platelets

Stimulation of platelets by collagen results in increased formation of the polyphosphoinositides, phosphatidylinositol phosphate (PtdInsP) and phosphatidylinositol bisphosphate (PtdInsP2) through stimulation of phosphoinositide kinase activities. We investigated a possible regulatory role of endogenous thromboxane formation and protein kinase C (PKC) activation in the induction of phosphoinositide phosphorylation following collagen stimulation, as well as following stimulation by the thromboxane mimetic, U-46619. Human platelets were prelabeled with [3H]inositol and stimulated with collagen (2 micrograms/mL) or U-46619 (1 microM), in the absence or presence of either the cyclo-oxygenase/lipoxygenase inhibitor, BW755C, or staurosporine, a putative inhibitor or PKC. Collagen stimulation resulted in a time-dependent increase in [3H]inositol-labeled PtdInsP and PtdInsP2 which was completely inhibited in the presence of BW755C. Addition of U-46619 to BW755C-treated, collagen-stimulated platelets restored the increased polyphosphoinositide formation. Stimulation of platelets with U-46619 alone also resulted in increased formation of [3H]PtdInsP and [3H]PtdInsP2, but this was not affected by the presence of BW755C. These results suggest that the collagen-induced activation of phosphoinositide kinases was dependent upon thromboxane formation, but that U-46619-induced phosphoinositide formation was rather independent of further thromboxane production. Pretreatment of platelets with staurosporine, prior to agonist addition, completely blocked the collagen-stimulated rise in radiolabeled PtdInsP and the U-46619-induced PtdInsP and PtdInsP2 generations, suggesting that protein kinase, possibly PKC, may play a role in the activation of phosphoinositide kinases by these agonists.

Purification and characterization of phosphatidylinositol 4-kinase from human erythrocyte membranes

Two species of PtdIns 4-kinase with molecular masses of 50 kDa and 45 kDa were detected in human erythrocyte membranes using SDS/PAGE. These enzymes were purified to near homogeneity and found to display very similar enzymatic characteristics. The purification scheme consisted of solubilization from erythrocyte membranes in the presence of Triton X-100, followed by Cibacron-blue-Sephadex, phosphocellulose and Mono Q anion-exchange chromatography. The final step in the purification protocol was preparative SDS/PAGE, followed by electroelution and renaturation of the enzyme. This procedure afforded an about 4000-fold purification of the enzyme from erythrocyte membranes. Characterization of the [32P]PtdInsP products formed by the purified PtdIns kinases indicated that these enzymes specifically phosphorylated the D-4 position of the inositol ring. The Km values of both PtdIns 4-kinase species for PtdIns and ATP were found to be 0.2 mM and 0.1 mM, respectively. The enzymes are both activated by Mg2+, and inhibited by Ca2+ and by adenosine. The potential importance of these effectors for the regulation of PtdIns phosphorylation in cells is discussed.

Oxytocin pretreatment of pregnant rat uterus inhibits Ca2+ uptake in plasma membrane and sarcoplasmic reticulum

Myometrium from rats in varying stages of pregnancy (from 17 to 22 days) was treated with oxytocin (0.1-10 microM) and plasma membranes and sarcoplasmic reticulum were isolated using a Percoll gradient. When the myometrium had been treated with oxytocin, Ca2+ uptake was reduced by 29.4% in the plasma membrane and by 32.6% in the sarcoplasmic reticulum. The inhibitory action of oxytocin was highly dependent upon the stage of gestation: Only membranes from rats close to term (21-22 days) exhibited reduced Ca2+ transport activity after hormone treatment. This effect correlated highly with a significant decrease in the serum progesterone level of these animals. In plasma membrane vesicles, oxytocin reduced the maximal velocity of the Ca2+ pump without significantly affecting the affinity for Ca2+. Oxytocin did not affect the passive permeability of the plasma membranes, nor their proportion of sealed inside-out vesicles nor the amount of Ca2(+)-pump protein in these membranes. In addition, oxytocin caused no change in the passive permeability of the membrane nor in the rate of inositol trisphosphate-induced Ca2+ release from the sarcoplasmic reticulum. These results suggest that there is a specific action of oxytocin on the activity of the myometrial plasma membrane and sarcoplasmic reticulum Ca2+ pumps which may contribute to the maintenance of an elevated intracellular calcium level during parturition.

Effect of genistein, a tyrosine kinase inhibitor, on U46619-induced phosphoinositide phosphorylation in human platelets

Recent evidence suggests that the agonist-induced formation of phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2) via PI and PIP kinases may play an important role in transmembrane signalling. In the present work, the effect of genistein, a specific inhibitor of protein-tyrosine kinase, on phosphoinositide phosphorylation was studied in human platelets stimulated with the endoperoxide analogue, U46619. At 100 microM concentration, genistein, but not the related compounds, flavone and biochanin A, which possess only weak anti-protein-tyrosine kinase activity, significantly inhibited the U46619-induced accumulation of [3H]PIP (by 71%) and [3H]PIP2. These data suggest that phosphoinositide phosphorylation may be regulated, in part, by tyrosine phosphorylation in U46619-stimulated platelets.

Inhibition of mural thrombus formation by novel nipecotoylpiperazine antiplatelet agents

The effectiveness of two closely related nipecotoylpiperazine derivatives, BPAT-143 and BPAT-117, as antiplatelet agents was measured by their ability to inhibit the accumulation of human blood platelets on collagen-coated (type 1) glass in a parallel plate flow chamber. Whole human blood, with fluorescently labeled platelets, was perfused through the flow chamber, and epi-fluorescent video microscopy was used to visualize the dynamics of individual platelet adhesion and thrombus formation on the collagen-coated surface. Digital image processing was used to analyze the dynamics of thrombus growth on the surfaces. The collagen-coated surface serves as a model for the damaged blood vessel wall, as collagen is a primary component of the matrix beneath endothelial cells. At a concentration of 50 microM, BPAT-117 (the considerably more hydrophobic molecule) inhibited platelet accumulation by striking 90 +/- 2% (+/- S.E.), while it took 2- to 4-fold higher concentrations of BPAT-143 to register meaningful to comparable effects (52 +/- 6% and 80 +/- 4%, respectively). This further corroborates the substantial impact of hydrophobic features within the matrix of appropriately structured molecules on their ability to alter platelet function.

Effect of albumin-bound DHA on phosphoinositide phosphorylation in collagen stimulated human platelets

The effect of exogenous albumin-bound docosahexaenoic acid (22:6n-3) (DHA), arachidonic acid (20:4n-6) (AA), and eicosapendaenoic acid (20:5n-3) (EPA) on phosphoinositide metabolism following collagen stimulation was studied using [3H]inositol prelabelled platelets. Collagen stimulation (3 min, 1.8 micrograms/ml) increased the labelling of both phosphatidylinositol 4-monophosphate (PIP), and phosphatidylinositol 4,5-biphosphate (PIP2). Of the fatty acids tested, only pre-incubation (2 min) with DHA (20 microM) significantly attenuated the collagen-induced increased PIP and PIP2 labelling; EPA was without effect, while AA enhanced PIP labelling. Forty microM DHA was less effective at attenuating the increased PIP and PIP2 labelling even though this concentration of DHA resulted in greater inhibition of platelet aggregation. Neither concentration of DHA attenuated the increased polyphosphoinositide labelling resulting from stimulation by the endoperoxide analogue U46619, or the phorbol ester, PMA. These data suggest that the effect of DHA on attenuating the increased PIP and PIP2 labelling following collagen stimulation likely occurs before thromboxane receptor occupancy, may not occur at the level of protein kinase C activation, and could be mediated in part via a lessened synthesis of thromboxane A2.

Inhibition of thrombus formation in vivo by novel antiplatelet agent

The antithrombotic and antihemostatic effects of the novel antiplatelet compound, alpha, alpha'-bis[3-(N,N-diethylcarbamoyl)piperidino]-p-xylene dihydrobromide (GT-12), were investigated in a baboon model of platelet-dependent thrombosis under high flow conditions. In this model, segments of collagen-coated tubing and Dacron vascular graft were placed as thrombus-inducing extension devices in exteriorized femoral arteriovenous shunts. The deposition of 111In-labeled platelets was measured for each thrombogenic segment throughout 1 hour by using gamma camera imaging. In addition, the 125I-fibrin retained in the forming thrombus was measured. Intravenous infusion of GT-12 (100 mumol/kg, 63.3 mg/kg) over a 15-minute period before the insertion of the test segments prolonged the bleeding time from a baseline value of 4.4 +/- 0.4 min to 7.6 +/- 1.0 min (p = 0.036) and inhibited platelet aggregation ex vivo induced by adenosine diphosphate (ED50 4.7 +/- 0.9 to 10.3 +/- 2.2 microM; p less than 0.02) and collagen (ED50 2.0 +/- 0.4 to 8.0 +/- 2.4 micrograms/ml; p less than 0.05). Deposition of platelets and fibrin was decreased in concert by 30% (p less than 0.05) for vascular grafts and possibly collagen segments at the end of the 60-minute observation period. We conclude that GT-12 is antithrombotic for Dacron graft-induced thrombus formation in vivo.

Thrombin-induced activation of calcium transport pathways and their role in platelet functions

In human platelets thrombin-induced calcium release from intracellular stores, the consequent influx of extracellular calcium, as well as their role in the aggregation and ATP-secretion reactions were examined. In indo-1-loaded platelets intracellular calcium release was studied in the presence of excess EGTA in the incubation medium, while calcium influx was followed after a rapid repletion of external calcium. After thrombin-stimulation both calcium release and calcium influx produced about the same peak levels of cytoplasmic free calcium but in the first case it was only a transient response, while in the latter one a sustained calcium signal was observed. Increased calcium influx could be evoked for several minutes after the addition of thrombin, it was selectively inhibited by Mg2+ (20 mM) and Ni2+ (1 mM) ions, by neomycin and by PCMB, a non-penetrating SH-group reagent. This calcium influx was practically insensitive to organic calcium channel blockers. Thrombin-induced platelet aggregation was only partial in the absence of external calcium, even if excess magnesium was present in the media, while the aggregation response became complete if external calcium was repleted. A significantly reduced aggregation could be seen in calcium-containing media if calcium influx was selectively inhibited. Platelet ATP-secretion under the same conditions did not depend on external calcium or on calcium influx. These data indicate that in thrombin-stimulated platelets the opening of specific plasma membrane calcium channels can be selectively modulated and these channels play a major role in the development of a full-scale aggregation.

Relationships between chemical structure and inhibition of epinephrine-induced human blood platelet aggregation

The effect of structural features in a series of carbamoylpiperidine and nipecotoylpiperazine congeners on epinephrine-induced aggregation of human blood platelets is examined. Epinephrine-induced primary aggregation is effectively inhibited by the nipecotoylpiperazine derivatives (culminating at an IPA50 of 11.9 microM). While among nipecotoylpiperazine as well as carbamoylpiperidine congeners there are potent inhibitors of ADP-stimulated platelet function (cresting at an IA50 of 12.4 and 11.4 microM, respectively), the carbamoylpiperidine analogs are much less active (e.g., IPA50 of 298.1), or practically inactive, in impeding epinephrine-induced primary aggregation (PA).