cADP-ribose formation by blood platelets is not responsible for intracellular calcium mobilization

NAADP/SERCA3-Dependent Ca2+ Stores Pathway Specifically Controls Early Autocrine ADP Secretion Potentiating Platelet Activation

RATIONALE

Ca²⁺ signaling is a key and ubiquitous actor of cell organization and its modulation controls many cellular responses. SERCAs (sarco-endoplasmic reticulum Ca²⁺-ATPases) pump Ca²⁺ into internal stores that play a major role in the cytosolic Ca²⁺ concentration rise upon cell activation. Platelets exhibit 2 types of SERCAs, SERCA2b and SERCA3 (SERCA3 deficient mice), which may exert specific roles, yet ill-defined. We have recently shown that Ca²⁺ mobilization from SERCA3-dependent stores was required for full platelet activation in weak stimulation conditions.

OBJECTIVE

To uncover the signaling mechanisms associated with Ca²⁺ mobilization from SERCA3-dependent stores leading to ADP secretion.

METHODS AND RESULTS

Using platelets from wild-type or Serca3-deficient mice, we demonstrated that an early (within 5-10 s following stimulation) secretion of ADP specifically dependent on SERCA3 stored Ca²⁺ is exclusively mobilized by nicotinic acid adenosine dinucleotide-phosphate (NAADP): both Ca²⁺ mobilization from SERCA3-dependent stores and primary ADP secretion are blocked by the NAADP receptor antagonist Ned-19, and reciprocally both are stimulated by permeant NAADP. In contrast, Ca²⁺ mobilization from SERCA3-dependent stores and primary ADP secretion were unaffected by inhibition of the production of IP3 (inositol-1,4,5-trisphosphate) by phospholipase-C and accordingly were not stimulated by permeant IP3.

CONCLUSIONS

Upon activation, an NAADP/SERCA3 Ca²⁺ mobilization pathway initiates an early ADP secretion, potentiating platelet activation, and a secondary wave of ADP secretion driven by both an IP3/SERCA2b-dependent Ca²⁺ stores pathway and the NAADP/SERCA3 pathway. This does not exclude that Ca²⁺ mobilized from SERCA3 stores may also enhance platelet global reactivity to agonists. Because of its modulating effect on platelet activation, this NAADP-SERCA3 pathway may be a relevant target for anti-thrombotic therapy. Graphic Abstract: A graphic abstract is available for this article.

Relationship between calcium mobilization and platelet α- and δ-granule secretion. A role for TRPC6 in thrombin-evoked δ-granule exocytosis

Changes in cytosolic Ca(2+) concentration ([Ca(2+)]c) regulate granule secretion in different cell types. Thrombin activates PAR1 and PAR4 receptors and promotes release of Ca(2+) from distinct intracellular stores, which, in turn, activates store-operated Ca(2+) entry (SOCE). A crucial step during platelet function is the release of physiological agonists stored in secretory granules to the extracellular compartment during activation. We aim to study the role of Ca(2+) mobilization from the extracellular compartment or from different intracellular stores in platelet granule secretion. By using flow cytometry, we have found that α- and δ-granules are secreted in thrombin-stimulated platelets in the absence of extracellular Ca(2+), and in a concentration-dependent manner. Our findings show that thrombin-stimulated granule secretion depends on Ca(2+) mobilization from intracellular stores. Analysis of the kinetics of granule secretion reveals that platelet stimulation with thrombin results in rapid release of α-granules which precedes the secretion of δ-granules. Incubation of platelets with a specific antibody, which recognizes the extracellular amino acid sequence 573-586 of TRPC6, inhibited thrombin-evoked δ-granule exocytosis. Our results indicate that the mechanisms underlying thrombin-induced α- and δ-granule secretion show differences in dependency on Ca(2+) mobilization.

Critical role for CD38-mediated Ca2+ signaling in thrombin-induced procoagulant activity of mouse platelets and hemostasis

CD38, a multifunctional enzyme that catalyzes the synthesis of intracellular Ca(2+) messengers, cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP), is known to be expressed on platelets. However, the role of CD38 in platelets remains unclear. Our present results show that treatment of platelets with thrombin results in a rapid and sustained Ca(2+) signal, resulting from a coordinated interplay of Ca(2+)-mobilizing messengers, inositol 1,4,5-trisphosphate, cADPR, and NAADP. By dissecting the signaling pathway using various agents, we delineated that cADPR and NAADP are sequentially produced through CD38 internalization by protein kinase C via myosin heavy chain IIA following phospholipase C activation in thrombin-induced platelets. An inositol 1,4,5-trisphosphate receptor antagonist blocked the thrombin-induced formation of cADPR and NAADP as well as Ca(2+) signals. An indispensable response of platelets relying on cytosolic calcium is the surface exposure of phosphatidylserine (PS), which implicates platelet procoagulant activity. Scrutinizing this parameter reveals that CD38(+/+) platelets fully express PS on the surface when stimulated with thrombin, whereas this response was decreased on CD38(-/-) platelets. Similarly, PS exposure and Ca(2+) signals were attenuated when platelets were incubated with 8-bromo-cADPR, bafilomycin A1, and a PKC inhibitor. Furthermore, in vivo, CD38-deficient mice exhibited longer bleeding times and unstable formation of thrombus than wild type mice. These results demonstrate that CD38 plays an essential role in thrombin-induced procoagulant activity of platelets and hemostasis via Ca(2+) signaling mediated by its products, cADPR and NAADP.

Evolution and function of the ADP ribosyl cyclase/CD38 gene family in physiology and pathology

The membrane proteins CD38 and CD157 belong to an evolutionarily conserved family of enzymes that play crucial roles in human physiology. Expressed in distinct patterns in most tissues, CD38 (and CD157) cleaves NAD(+) and NADP(+), generating cyclic ADP ribose (cADPR), NAADP, and ADPR. These reaction products are essential for the regulation of intracellular Ca(2+), the most ancient and universal cell signaling system. The entire family of enzymes controls complex processes, including egg fertilization, cell activation and proliferation, muscle contraction, hormone secretion, and immune responses. Over the course of evolution, the molecules have developed the ability to interact laterally and frontally with other surface proteins and have acquired receptor-like features. As detailed in this review, the loss of CD38 function is associated with impaired immune responses, metabolic disturbances, and behavioral modifications in mice. CD38 is a powerful disease marker for human leukemias and myelomas, is directly involved in the pathogenesis and outcome of human immunodeficiency virus infection and chronic lymphocytic leukemia, and controls insulin release and the development of diabetes. Here, the data concerning diseases are examined in view of potential clinical applications in diagnosis, prognosis, and therapy. The concluding remarks try to frame all of the currently available information within a unified working model that takes into account both the enzymatic and receptorial functions of the molecules.

Adenylic dinucleotides produced by CD38 are negative endogenous modulators of platelet aggregation

Diadenosine 5',5'''-P1,P2-diphosphate (Ap2A) is one of the adenylic dinucleotides stored in platelet granules. Along with proaggregant ADP, it is released upon platelet activation and is known to stimulate myocyte proliferation. We have previously demonstrated synthesis of Ap2A and of two isomers thereof, called P18 and P24, from their high pressure liquid chromatography retention time, by the ADP-ribosyl cyclase CD38 in mammalian cells. Here we show that Ap2A and its isomers are present in resting human platelets and are released during thrombin-induced platelet activation. The three adenylic dinucleotides were identified by high pressure liquid chromatography through a comparison with the retention times and the absorption spectra of purified standards. Ap2A, P18, and P24 had no direct effect on platelet aggregation, but they inhibited platelet aggregation induced by physiological agonists (thrombin, ADP, and collagen), with mean IC50 values ranging between 5 and 15 microm. Moreover, the three dinucleotides did not modify the intracellular calcium concentration in resting platelets, whereas they significantly reduced the thrombin-induced intracellular calcium increase. Through binding to the purinergic receptor P2Y11, exogenously applied Ap2A, P18, and P24 increased the intracellular cAMP concentration and stimulated platelet production of nitric oxide, the most important endogenous antiaggregant. The presence of Ap2A, P18, and P24 in resting platelets and their release during thrombin-induced platelet activation at concentrations equal to or higher than the respective IC50 value on platelet aggregation suggest a role of these dinucleotides as endogenous negative modulators of aggregation.

Two distinct Ca2+ compartments show differential sensitivity to thrombin, ADP and vasopressin in human platelets

Recent studies propose the existence of two distinct Ca2+ compartments in human platelets based on the expression of different SERCA isoforms with distinct sensitivity to thapsigargin and 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ). Using fura-2-loaded human platelets we have found that depletion of the TBHQ sensitive store reduces thrombin--but not ADP--or vasopressin (AVP)-induced Ca2+ release. Redistribution of cytosolic Ca2+ after thrombin stimulation resulted in overloading of the TBHQ-sensitive store. This phenomenon was not observed with ADP or AVP. We found that NAADP decreases the Ca2+ concentration into the stores in permeabilized platelets, which is prevented by depletion of the TBHQ-sensitive store. Nimodipine, an inhibitor of the NAADP receptor, reduced thrombin-induced Ca2+ release from the TBHQ-sensitive stores, without having any effect on the responses elicited by ADP or AVP. Finally, the phospholipase C inhibitor, U-73122, abolished ADP- and AVP-induced Ca2+ release, suggesting that their responses are entirely dependent on IP3 generation. In contrast, treatment with both U-73122 and nimodipine was required to abolish thrombin-induced Ca2+ release. We suggest that thrombin evokes Ca2+ release from TBHQ-sensitive and insensitive stores, which requires both NAADP and IP3, respectively, while ADP and AVP exert an IP3-dependent release of Ca2+ from the TBHQ-insensitive compartment in human platelets.

Hydrolysis of NADP+ by platelet CD38 in the absence of synthesis and degradation of cyclic ADP-ribose 2'-phosphate

CD38 is a multifunctional cell surface ectoenzyme that catalyzes both the synthesis of cyclic ADP-ribose from NAD+ and its hydrolysis to ADP-ribose. In this work, we investigated the metabolism of NADP+ by CD38 expressed on human platelets. Incubation of either platelet membranes or intact cells with NADP+ resulted in the rapid and time-dependent accumulation of ADP-ribose 2'-phosphate that paralleled the consumption of the substrate. However, under the same conditions, synthesis of cyclic ADP-ribose 2'-phosphate was not observed. By immunoprecipitation experiments, we identified CD38 as the enzyme responsible for the observed NADP+ glycohydrolase activity. The lack of detection of cyclic ADP-ribose 2'-phosphate was not due to its rapid hydrolysis, since direct incubation of platelet membranes with cyclic ADP-ribose 2'-phosphate did not result in the formation of ADP-ribose 2'-phosphate. By contrast, the same membrane samples expressed a significant ability to hydrolyze cyclic ADP-ribose to ADP-ribose. The absence of cyclic ADP-ribose 2'-phosphate hydrolase activity was also confirmed using high concentrations of substrate and by analysing both intact Jurkat T-lymphocytes and immunoprecipitated CD38. These results indicate that CD38, which is a multifunctional enzyme towards NAD+, displays exclusively a NADP+ glycohydrolase activity and is unable to catalyze both the synthesis and the hydrolysis of cyclic ADP-ribose 2'-phosphate.