Networks in signal transduction: the role of adaptor proteins in platelet activation

The adaptor protein Disabled-2: new insights into platelet biology and integrin signaling

Multiple functions of platelets in various physiological and pathological conditions have prompted considerable attention on understanding how platelets are generated and activated. Of the adaptor proteins that are expressed in megakaryocytes and platelets, Disabled-2 (Dab2) has been demonstrated in the past decades as a key regulator of platelet signaling. Dab2 has two alternative splicing isoforms p82 and p59. However, the mode of Dab2’s action remains to be clearly defined. In this review, we highlight the current understanding of Dab2 expression and function in megakaryocytic differentiation, platelet activation and integrin signaling. Accordingly, Dab2 is upregulated when the human K562 cells, human CD34⁺ hematopoietic stem cells, and murine embryonic stem cells were undergone megakaryocytic differentiation. Appropriate level of Dab2 expression is essential for fate determination of mesodermal and megakaryocytic differentiation. Dab2 is also shown to regulate cell-cell and cell-fibrinogen adhesion, integrin αIIbβ3 activation, fibrinogen uptake, and intracellular signaling of the megakaryocytic cells. In human platelets, p82 is the sole Dab2 isoform present in the cytoplasm and α-granules. Dab2 is released from the α-granules and forms two pools of Dab2 on the outer surface of the platelet plasma membrane, one at the sulfatide-bound and the other at integrin αIIbβ3-bound forms. The balance between these two pools of Dab2 controls the extent of clotting reaction, platelet-fibrinogen interactions and outside-in signaling. In murine platelets, p59 is the only Dab2 isoform and is required for platelet aggregation, fibrinogen uptake, RhoA-ROCK activation, adenosine diphosphate release and integrin αIIbβ3 activation stimulated by low concentration of thrombin. As a result, the bleeding time is prolonged and thrombus formation is impaired for the megakaryocyte lineage-restricted Dab2 deficient mouse. Although discrepancies of Dab2 function and isoform expression are noted between human and murine platelets, the studies up-to-date define Dab2 playing a pivotal role in integrin signaling and platelet activation. With the new tools such as CRISPR and TALEN in the generation of genetically modified animals, the progress in gaining new insights into the functions of Dab2 in megakaryocyte and platelet biology is expected to accelerate.

Kaempferol suppresses collagen-induced platelet activation by inhibiting NADPH oxidase and protecting SHP-2 from oxidative inactivation

Reactive oxygen species (ROS) generated upon collagen stimulation act as second messengers to propagate various platelet-activating events. Among the ROS-generating enzymes, NADPH oxidase (NOX) plays a prominent role in platelet activation. Thus, NOX has been suggested as a novel target for anti-platelet drug development. Although kaempferol has been identified as a NOX inhibitor, the influence of kaempferol on the activation of platelets and the underlying mechanism have never been investigated. Here, we studied the effects of kaempferol on NOX activation, ROS-dependent signaling pathways, and functional responses in collagen-stimulated platelets. Superoxide anion generation stimulated by collagen was significantly inhibited by kaempferol in a concentration-dependent manner. More importantly, kaempferol directly bound p47(phox), a major regulatory subunit of NOX, and significantly inhibited collagen-induced phosphorylation of p47(phox) and NOX activation. In accordance with the inhibition of NOX, ROS-dependent inactivation of SH2 domain-containing protein tyrosine phosphatase-2 (SHP-2) was potently protected by kaempferol. Subsequently, the specific tyrosine phosphorylation of key components (Syk, Vav1, Btk, and PLCγ2) of collagen receptor signaling pathways was suppressed by kaempferol. Kaempferol also attenuated downstream responses, including cytosolic calcium elevation, P-selectin surface exposure, and integrin-αIIbβ3 activation. Ultimately, kaempferol inhibited platelet aggregation and adhesion in response to collagen in vitro and prolonged in vivo thrombotic response in carotid arteries of mice. This study shows that kaempferol impairs collagen-induced platelet activation through inhibition of NOX-derived ROS production and subsequent oxidative inactivation of SHP-2. This effect suggests that kaempferol has therapeutic potential for the prevention and treatment of thrombovascular diseases.

Future innovations in anti-platelet therapies

Platelets have long been recognized to be of central importance in haemostasis, but their participation in pathological conditions such as thrombosis, atherosclerosis and inflammation is now also well established. The platelet has therefore become a key target in therapies to combat cardiovascular disease. Anti-platelet therapies are used widely, but current approaches lack efficacy in a proportion of patients, and are associated with side effects including problem bleeding. In the last decade, substantial progress has been made in understanding the regulation of platelet function, including the characterization of new ligands, platelet-specific receptors and cell signalling pathways. It is anticipated this progress will impact positively on the future innovations towards more effective and safer anti-platelet agents. In this review, the mechanisms of platelet regulation and current anti-platelet therapies are introduced, and strong, and some more speculative, potential candidate target molecules for future anti-platelet drug development are discussed.

Differential regulation of adapter proteins Dok2 and Dok1 in platelets, leading to an association of Dok2 with integrin alphaIIbbeta3

BACKGROUND

We previously demonstrated that Dok2 is rapidly phosphorylated on tyrosine residues in platelets in response to thrombin, the immunoreceptor tyrosine-based activation motif-coupled collagen receptor glycoprotein (GP) VI, and by integrin alphaIIbbeta3.

OBJECTIVES AND METHODS

In this study we further delineate the regulation of phosphorylation of Dok2 and compare this to the related adapter Dok1.

RESULTS

We demonstrate expression of Dok1 in platelets and the unexpected observation that the adapter protein undergoes tyrosine phosphorylation in response to thrombin but not to GPVI or integrin alphaIIbbeta3. Furthermore, Dok1 phosphorylation is transient, peaking at 30 s and returning to basal by 5 min, whereas Dok2 phosphorylation is delayed but sustained. Dok2 phosphorylation, but not that of Dok1, is inhibited by Src kinase inhibitors and by chelation of intracellular calcium. Further, phosphorylation of Dok2 by thrombin and integrin alphaIIbbeta3 in mouse platelets is independent of Syk and phospholipase Cgamma2. Additionally, Dok2 coimmunoprecipitates with integrin alphaIIbbeta3 downstream of Src kinases.

CONCLUSIONS

These results demonstrate differential modes of regulation of Dok1 and Dok2 in platelets. Further, they raise the interesting possibility that Dok2 plays an important role in integrin outside-in signaling through a physical and functional interaction with integrin alphaIIbbeta3.

Particle simulation approach for subcellular dynamics and interactions of biological molecules

BACKGROUND

Spatio-temporal dynamics within cells can now be visualized at appropriate resolution, due to the advances in molecular imaging technologies. Even single-particle tracking (SPT) and single fluorophore video imaging (SFVI) are now being applied to observation of molecular-level dynamics. However, little is known concerning how molecular-level dynamics affect properties at the cellular level.

RESULTS

We propose an algorithm designed for three-dimensional simulation of the reaction-diffusion dynamics of molecules, based on a particle model. Chemical reactions proceed through the interactions of particles in space, with activation energies determining the rates of these chemical reactions at each interaction. This energy-based model can include the cellular membrane, membranes of other organelles, and cytoskeleton. The simulation algorithm was tested for a reversible enzyme reaction model and its validity was confirmed. Snapshot images taken from simulated molecular interactions on the cell-surface revealed clustering domains (size approximately 0.2 microm) associated with rafts. Sample trajectories of raft constructs exhibited "hop diffusion". These domains corralled the diffusive motion of membrane proteins.

CONCLUSION

These findings demonstrate that our approach is promising for modelling the localization properties of biological phenomena.

Molecular characterization of the ZKT gene encoding a protein with PDZ, K-Box, and TPR motifs in Arabidopsis

The ZKT gene from Arabidopsis encodes a polypeptide of 335 amino acid residues, with a calculated molecular mass of 37.4 kDa. ZKT is a member of a novel protein family present in the plant kingdom, which contains a PDZ, a K-box, and a TPR motif. A BLAST search indicated that the ZKT gene is a single gene in Arabidopsis and that ZKT homologs are present in soybean and rice but not in animals. The level of ZKT mRNA decreased after wounding. Antisera from rabbit immunized with recommbinant ZKT protein recognized a protein of 37 kDa in Arabidopsis. Western analysis with anti-ZKT antibody indicated that the level of ZKT protein does not change after wounding. The ZKT protein has consensus sequence motifs for phosphorylation. Immunoprecipitation with anti-ZKT antibody and western analysis with anti-phosphoamino acid antibody indicated that the ZKT protein is phosophorylated at the threonine and serine residues after wounding. These results suggest that the ZKT protein may act as a molecular adaptor regulated by phosphorylation in wound responses.

The C-terminal peptide of thrombospondin-1 stimulates distinct signaling pathways but induces an activation-independent agglutination of platelets and other cells

A peptide from the C-terminal domain of thrombospondin-1 (4N1-1) has been proposed to stimulate platelet aggregation by a novel mechanism involving both an activation-independent agglutination and an activation-dependent, glycoprotein (GP) IIb/IIIa-mediated aggregation which involves GPVI signaling but does not involve CD47. The present study demonstrates that 4N1-1 stimulated a different pattern of signal transduction pathways than the GPVI agonist convulxin. Furthermore, 4N1-1-induced platelet aggregation was activation-independent and not dependent on GPVI or GPIIb/IIIa. Interestingly, 4N1-1 also stimulated activation-independent agglutination of different megakaryocytic and non-megakaryocytic cells. 4N1-1-induced cell agglutination but not platelet signaling was inhibited by anti-CD47 antibodies.

Characterization of the promoter controlling Mona/Gads expression in the megakaryocytic lineage

Mona/grb2 related adapter downstream of shc is a molecular adapter expressed in platelets, T lymphocytes and myelomonocytic cells. Using human hematopoietic cell lines, we have previously shown that lineage-specific Mona expression is achieved through the production of two transcripts (named 1A and 1B) differing by their 5' untranslated region (5'UTR). Thus, platelets and megakaryocytic cell lines K562 and HEL (Human Erythro-Leukemia) specifically express 1B messenger RNA (mRNA). We report here characterization of the (-2031/+72) genomic region relative to the putative transcription start site of 1B mRNA. We show this region is sufficient to ensure specific reporter gene expression in megakaryocytic cell lines, and that most promoter activity is contained in the (-225/+72) fragment. Electro-mobility shift assay and mutational analyses indicated that GATA-1 and a yet unidentified E-26 family member transcription factor are required for 1B (-2031/+72) promoter activity. Thus, Mona 1B promoter exhibits typical features of megakaryocyte-specific promoters.