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

The relative importance of platelet integrins in hemostasis, thrombosis and beyond

Integrins are heterodimeric transmembrane receptors composed of α and β chains, with an N-terminal extracellular domain forming a globular head corresponding to the ligand binding site. Integrins regulate various cellular functions including adhesion, migration, proliferation, spreading and apoptosis. On platelets, integrins play a central role in adhesion and aggregation on subendothelial matrix proteins of the vascular wall, thereby ensuring hemostasis. Platelet integrins belong either to the β1 family (α2β1, α5β1 and α6β1) or to the β3 family (αIIbβ3 and αvβ3). On resting platelets, integrins can engage their ligands when the latter are immobilized but not in their soluble form. The effects of various agonists promote an inside-out signal in platelets, increasing the affinity of integrins for their ligands and conveying a modest signal reinforcing platelet activation, called outside-in signaling. This outside-in signal ensures platelet adhesion, shape change, granule secretion and aggregation. In this review, we examine the role of each platelet integrin in hemostatic plug formation, hemostasis and arterial thrombosis and also beyond these classical functions, notably in tumor metastasis and sepsis.

Structure, signal transduction, activation, and inhibition of integrin αIIbβ3

Integrins are heterodimeric receptors comprising α and β subunits. They are expressed on the cell surface and play key roles in cell adhesion, migration, and growth. Several types of integrins are expressed on the platelets, including αvβ3, αIIbβ3, α2β1, α5β1, and α6β1. Among these, physically αIIbβ3 is exclusively expressed on the platelet surface and their precursor cells, megakaryocytes. αIIbβ3 adopts at least three conformations: i) bent-closed, ii) extended-closed, and iii) extended-open. The transition from conformation i) to iii) occurs when αIIbβ3 is activated by stimulants. Conformation iii) possesses a high ligand affinity, which triggers integrin clustering and platelet aggregation. Platelets are indispensable for maintaining vascular system integrity and preventing bleeding. However, excessive platelet activation can result in myocardial infarction (MI) and stroke. Therefore, finding a novel strategy to stop bleeding without accelerating the risk of thrombosis is important. Regulation of αIIbβ3 activation is vital for this strategy. There are a large number of molecules that facilitate or inhibit αIIbβ3 activation. The interference of these molecules can accurately control the balance between hemostasis and thrombosis. This review describes the structure and signal transduction of αIIbβ3, summarizes the molecules that directly or indirectly affect integrin αIIbβ3 activation, and discusses some novel antiαIIbβ3 drugs. This will advance our understanding of the activation of αIIbβ3 and its essential role in platelet function and tumor development.

Fully automated platelet differential interference contrast image analysis via deep learning

Platelets mediate arterial thrombosis, a leading cause of myocardial infarction and stroke. During injury, platelets adhere and spread over exposed subendothelial matrix substrates of the damaged blood vessel wall. The mechanisms which govern platelet activation and their interaction with a range of substrates are therefore regularly investigated using platelet spreading assays. These assays often use differential interference contrast (DIC) microscopy to assess platelet morphology and analysis performed using manual annotation. Here, a convolutional neural network (CNN) allowed fully automated analysis of platelet spreading assays captured by DIC microscopy. The CNN was trained using 120 generalised training images. Increasing the number of training images increases the mean average precision of the CNN. The CNN performance was compared to six manual annotators. Significant variation was observed between annotators, highlighting bias when manual analysis is performed. The CNN effectively analysed platelet morphology when platelets spread over a range of substrates (CRP-XL, vWF and fibrinogen), in the presence and absence of inhibitors (dasatinib, ibrutinib and PRT-060318) and agonist (thrombin), with results consistent in quantifying spread platelet area which is comparable to published literature. The application of a CNN enables, for the first time, automated analysis of platelet spreading assays captured by DIC microscopy.

Introduction to DOK2 and its potential role in cancer

Cancer is a complex, multifactorial disease that modern medicine ultimately aims to overcome. Downstream of tyrosine kinase 2 (DOK2) is a well-known tumor suppressor gene, and a member of the downstream protein DOK family of tyrosine kinases. Through a search of original literature indexed in PubMed and other databases, the present review aims to extricate the mechanisms by which DOK2 acts on cancer, thereby identifying more reliable and effective therapeutic targets to promote enhanced methods of cancer prevention and treatment. The review focuses on the role of DOK2 in multiple tumor types in the lungs, intestines, liver, and breast. Additionally, we discuss the potential mechanisms of action of DOK2 and the downstream consequences via the Ras/MPAK/ERK or PI3K/AKT/mTOR signaling pathways.

Identification of a novel conserved signaling motif in CD200 receptor required for its inhibitory function

The inhibitory signaling of CD200 receptor 1 (CD200R) has been attributed to its NPxY signaling motif. However, NPxY-motifs are present in multiple protein families and are mostly known to mediate protein trafficking between subcellular locations rather than signaling. Therefore, we investigated whether additional motifs specify the inhibitory function of CD200R. We performed phylogenetic analysis of the intracellular domain of CD200R in mammals, birds, bony fish, amphibians and reptiles. Indeed, the tyrosine of the NPxY-motif is fully conserved across species, in line with its central role in CD200R signaling. In contrast, P295 of the NPxY-motif is not conserved. Instead, a conserved stretch of negatively charged amino acids, EEDE279, and two conserved residues P285 and K292 in the flanking region prior to the NPxY-motif are required for CD200R mediated inhibition of p-Erk, p-Akt308, p-Akt473, p-rpS6 and LPS-induced IL-8 secretion. Altogether, we show that instead of the more common NPxY-motif, CD200R signaling can be assigned to a unique signaling motif in mammals defined by: EEDExxPYxxYxxKxNxxY.

Dok-1 negatively regulates platelet integrin αIIbβ3 outside-in signalling and inhibits thrombosis in mice

Adaptor proteins play a critical role in the assembly of signalling complexes after engagement of platelet receptors by agonists such as collagen, ADP and thrombin. Recently, using proteomics, the Dok (downstream of tyrosine kinase) adapter proteins were identified in human and mouse platelets. In vitro studies suggest that Dok-1 binds to platelet integrin β3, but the underlying effects of Dok-1 on αIIbβ3 signalling, platelet activation and thrombosis remain to be elucidated. In the present study, using Dok-1-deficient (Dok-1-/-) mice, we determined the phenotypic role of Dok-1 in αIIbβ3 signalling. We found that platelets from Dok-1-/- mice displayed normal aggregation, activation of αIIbβ3 (assessed by binding of JON/A), P-selectin surface expression (assessed by anti-CD62P), and soluble fibrinogen binding. These findings indicate that Dok-1 does not affect "inside-out" platelet signalling. Compared with platelets from wild-type (WT) mice, platelets from Dok-1-/- mice exhibited increased clot retraction (p < 0.05 vs WT), increased PLCγ2 phosphorylation, and enhanced spreading on fibrinogen after thrombin stimulation (p < 0.01 vs WT), demonstrating that Dok-1 negatively regulates αIIbβ3 "outside-in" signalling. Finally, we found that Dok-1-/- mice exhibited significantly shortened bleeding times and accelerated carotid artery thrombosis in response to photochemical injury (p < 0.05 vs WT mice). We conclude that Dok-1 modulates thrombosis and haemostasis by negatively regulating αIIbβ3 outside-in signalling.

Dok-2 adaptor protein regulates the shear-dependent adhesive function of platelet integrin αIIbβ3 in mice

The Dok proteins are a family of adaptor molecules that have a well defined role in regulating cellular migration, immune responses, and tumor progression. Previous studies have demonstrated that Doks-1 to 3 are expressed in platelets and that Dok-2 is tyrosine-phosphorylated downstream of integrin αIIbβ3, raising the possibility that it participates in integrin αIIbβ3 outside-in signaling. We demonstrate that Dok-2 in platelets is primarily phosphorylated by Lyn kinase. Moreover, deficiency of Dok-2 leads to dysregulated integrin αIIbβ3-dependent cytosolic calcium flux and phosphatidylinositol(3,4)P2 accumulation. Although agonist-induced integrin αIIbβ3 affinity regulation was unaltered in Dok-2(-/-) platelets, Dok-2 deficiency was associated with a shear-dependent increase in integrin αIIbβ3 adhesive function, resulting in enhanced platelet-fibrinogen and platelet-platelet adhesive interactions under flow. This increase in adhesion was restricted to discoid platelets and involved the shear-dependent regulation of membrane tethers. Dok-2 deficiency was associated with an increased rate of platelet aggregate formation on thrombogenic surfaces, leading to accelerated thrombus growth in vivo. Overall, this study defines an important role for Dok-2 in regulating biomechanical adhesive function of discoid platelets. Moreover, they define a previously unrecognized prothrombotic mechanism that is not detected by conventional platelet function assays.

The endocytic adaptor protein Disabled-2 is required for cellular uptake of fibrinogen

Endocytosis is pivotal for uptake of fibrinogen from plasma into megakaryocytes and platelet α-granules. Due to the complex adaptor and cargo contents in endocytic vehicles, the underlying mechanism of fibrinogen uptake is not yet completely elucidated. In this study, we investigated whether the endocytic adaptor protein Disabled-2 (DAB2) mediates fibrinogen uptake in an adaptor-specific manner. By employing primary megakaryocytes and megakaryocytic differentiating human leukemic K562 cells as the study models, we found that fibrinogen uptake is associated with the expression of integrin αIIbβ3 and DAB2 and is mediated through clathrin-dependent manner. Accordingly, constitutive and inducible knockdown of DAB2 by small interfering RNA reduced fibrinogen uptake for 53.2 ± 9.8% and 59.0 ± 10.7%, respectively. Culturing the cells in hypertonic solution or in the presence of clathrin inhibitor chlorpromazine abrogated clathrin-dependent endocytosis and diminished the uptake of fibrinogen. Consistent with these findings, 72.2 ± 0.2% of cellular DAB2 was colocalized with clathrin, whereas 56.4±4.1% and 54.6 ± 2.0% of the internalized fibrinogen were colocalized with clathrin and DAB2, respectively. To delineate whether DAB2 mediates fibrinogen uptake in an adaptor-specific manner, K562 stable cell lines with knockdown of the adaptor protein-2 (AP-2) or double knockdown of AP-2/DAB2 were established. The AP-2 knockdown cells elicited normal fibrinogen uptake activity but the uptake of collagen was diminished. In addition, collagen uptake was further reduced in DAB2/AP-2 knockdown cells. These findings thereby define an adaptor-specific mechanism in the control of fibrinogen uptake and implicate that DAB2 is the key adaptor in the clathrin-associated endocytic complexes to mediate fibrinogen internalization.

PlateletWeb: a systems biologic analysis of signaling networks in human platelets

Understanding the cellular mechanisms of platelet activation and their pharmacologic modulation is of major interest for basic and clinical research. Here we introduce a comprehensive human platelet repository (PlateletWeb) for systems biologic analysis of platelets in the functional context of integrated networks. Functional, drug, and pathway associations provide a first systemic insight into various aspects of platelet functionality and pharmacologic regulation. Detailed manual curation of recent platelet proteome and transcriptome studies yielded more than 5000 platelet proteins. Integration of protein-protein interactions with kinase-substrate relationships unraveled the platelet signaling network involving more than 70% of all platelet proteins. Analysis of the platelet kinome in the context of the kinase phylogenetic background revealed an over-representation of tyrosine kinase substrates. The extraction and graphical visualization of specific subnetworks allow identification of all major signaling modules involved in activation and inhibition. An in-depth analysis of DOK1 signaling identifies putative signal modulators of the integrin network. Through integration of various information sources and high curation standards, the PlateletWeb knowledge base offers the systems biologic background for the investigation of signal transduction in human platelets (http://plateletweb.bioapps.biozentrum.uni-wuerzburg.de).

The proximal signaling network of the BCR-ABL1 oncogene shows a modular organization

BCR-ABL1 is a fusion tyrosine kinase, which causes multiple types of leukemia. We used an integrated proteomic approach that includes label-free quantitative protein complex and phosphorylation profiling by mass spectrometry to systematically characterize the proximal signaling network of this oncogenic kinase. The proximal BCR-ABL1 signaling network shows a modular and layered organization with an inner core of three leukemia transformation-relevant adaptor protein complexes (Grb2/Gab2/Shc1 complex, CrkI complex and Dok1/Dok2 complex). We introduced an 'interaction directionality' analysis, which annotates static protein networks with information on the directionality of phosphorylation-dependent interactions. In this analysis, the observed network structure was consistent with a step-wise phosphorylation-dependent assembly of the Grb2/Gab2/Shc1 and the Dok1/Dok2 complexes on the BCR-ABL1 core. The CrkI complex demonstrated a different directionality, which supports a candidate assembly on the Nedd9 (Hef1, CasL) scaffold. As adaptor protein family members can compensate for each other in leukemic transformation, we compared members of the Dok and Crk protein families and found both overlapping and differential binding patterns. We identified an additional level of regulation for the CrkII protein via binding to 14-3-3 proteins, which was independent from its inhibitory phosphorylation. We also identified novel components of the inner core complexes, including the kinases Pragmin (Sgk223) and Lrrk1 (Lrrk2 paralog). Pragmin was found as a component of the CrkI complex and is a potential link between BCR-ABL1/CrkI and RhoA signaling. Lrrk1 is an unusual kinase with a GTPase domain. We detected Lrrk1 as a component of the Grb2/Gab2/Shc1 complex and found that it functionally interacts with the regulator of small GTPases Arap1 (Centd2) and possibly participates in the mitogen-activated protein kinase response to cellular stresses. This modular and phosphorylation-driven interaction network provides a framework for the integration of pleiotropic signaling effects of BCR-ABL1 toward leukemic transformation.

Proteomic analysis of integrin alphaIIbbeta3 outside-in signaling reveals Src-kinase-independent phosphorylation of Dok-1 and Dok-3 leading to SHIP-1 interactions

BACKGROUND AND OBJECTIVES

Outside-in integrin alphaIIbbeta3 signaling involves a series of tyrosine kinase reactions that culminate in platelet spreading on fibrinogen. The aim of this study was to identify novel tyrosine phosphorylated signaling proteins downstream of alphaIIbbeta3, and explore their role in platelet signaling.

METHODS AND RESULTS

Utilizing proteomics to search for novel platelet proteins that contribute to outside-in signaling by the integrin alphaIIbbeta3, we identified 27 proteins, 17 of which were not previously shown to be part of a tyrosine phosphorylation-based signaling complex downstream of alphaIIbbeta3. The proteins identified include the novel immunoreceptors G6f and G6b-B, and two members of the Dok family of adapters, Dok-1 and Dok-3, which underwent increased tyrosine phosphorylation following platelet spreading on fibrinogen. Dok-3 was also inducibly phosphorylated in response to the GPVI-specific agonist collagen-related peptide (CRP) and the PAR-1 and -4 agonist thrombin, independently of the integrin alphaIIbbeta3. Tyrosine phosphorylation of Dok-1 and Dok-3 was primarily Src kinase-independent downstream of the integrin, whereas it was Src kinase-dependent downstream of GPVI. Moreover, both proteins inducibly interacted with Grb-2 and SHIP-1 in fibrinogen-spread platelets.

CONCLUSIONS

This study provides new insights into the molecular mechanism regulating alphaIIbbeta3-mediated platelet spreading on fibrinogen. The novel platelet adapter Dok-3 and the structurally related Dok-1 are tyrosine phosphorylated in an Src kinase-independent manner downstream of alphaIIbbeta3 in human platelets, leading to an interaction with Grb2 and SHIP-1.

Regulation of adaptor protein GIT1 in platelets, leading to the interaction between GIT1 and integrin alpha(IIb)beta3

GIT1 is an adaptor protein, which links signaling proteins to focal adhesion, thereby regulating cytoskeletal reorganization. Platelets undergo dynamic cytoskeletal reorganization during platelet activation, for which a large number of adaptor proteins are required. However, there has been no report of GIT1 in platelets. We found that GIT1 was abundantly expressed in platelets and underwent tyrosine phosphorylation downstream of integrin alpha(IIb)beta(3), which was inhibited by the Src kinase inhibitor PP2. Furthermore, GIT1 constitutively associated with betaPIX, a guanine nucleotide exchange factor (GEF) for Rac. The GIT1/betaPIX complex associated with alpha(IIb)beta(3), concomitantly with GIT1 tyrosine phosphorylation. Moreover, both GIT1 and alpha(IIb)beta(3) rapidly translocated to the cytoskeletal fraction during platelet aggregation, which was not observed in the absence of aggregation. These results suggest that tyrosine phosphorylation of GIT1 by Src kinases may regulate cytoskeletal reorganization downstream of alpha(IIb)beta(3) by bringing the Rac GEF betaPIX to the vicinity of the integrin.