The RhoA regulators Myo9b and GEF-H1 are targets of cyclic nucleotide-dependent kinases in platelets

Platelet inhibition by hypochlorous acid involves cAMP signalling

Hypochlorous acid (HOCl), made by neutrophil-derived myeloperoxidase, has been suggested to inhibit platelets, however, the mechanisms involved have not been described. Here we confirm that HOCl exposure changes platelet morphology and inhibits platelet spreading and aggregation. HOCl effects could be reversed by glutathione suggesting a role for cysteine oxidation. Mass spectrometry-based proteomics of HOCl-exposed platelets revealed oxidised cysteine residues in 37 proteins including adenylate cyclase 6 and Rap1B. Adenylate cyclase is involved in the inhibitory cAMP pathway triggered by endothelium-derived prostacyclin and Rap1 is a small G protein required for integrin αIIbβ3 activation and platelet aggregation. We show that HOCl exposure stimulates cAMP production and phosphorylation of the cAMP-dependent protein kinase substrate VASP in platelets and transfected HEK293T cells. In addition, HOCl inhibited Rap1-GTP formation. These data suggest that HOCl inhibits platelets at least in part through the cAMP pathway and by regulating Rap1. Thus, this study provides new insights into HOCl mediated crosstalk between neutrophils and platelets.

Roles of G proteins and their GTPase-activating proteins in platelets

Platelets are small anucleate blood cells supporting vascular function. They circulate in a quiescent state monitoring the vasculature for injuries. Platelets adhere to injury sites and can be rapidly activated to secrete granules and to form platelet/platelet aggregates. These responses are controlled by signalling networks that include G proteins and their regulatory guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Recent proteomics studies have revealed the complete spectrum of G proteins, GEFs, and GAPs present in platelets. Some of these proteins are specific for platelets and very few have been characterised in detail. GEFs and GAPs play a major role in setting local levels of active GTP-bound G proteins in response to activating and inhibitory signals encountered by platelets. Thus, GEFs and GAPs are highly regulated themselves and appear to integrate G protein regulation with other cellular processes. This review focuses on GAPs of small G proteins of the Arf, Rab, Ras, and Rho families, as well as of heterotrimeric G proteins found in platelets.

RhoGAP6 interacts with COPI to regulate protein transport

RhoGAP6 is the most highly expressed GTPase-activating protein (GAP) in platelets specific for RhoA. Structurally RhoGAP6 contains a central catalytic GAP domain surrounded by large, disordered N- and C-termini of unknown function. Sequence analysis revealed three conserved consecutive overlapping di-tryptophan motifs close to the RhoGAP6 C-terminus which were predicted to bind to the mu homology domain (MHD) of δ-COP, a component of the COPI vesicle complex. We confirmed an endogenous interaction between RhoGAP6 and δ-COP in human platelets using GST-CD2AP which binds an N-terminal RhoGAP6 SH3 binding motif. Next, we confirmed that the MHD of δ-COP and the di-tryptophan motifs of RhoGAP6 mediate the interaction between both proteins. Each of the three di-tryptophan motifs appeared necessary for stable δ-COP binding. Proteomic analysis of other potential RhoGAP6 di-tryptophan motif binding partners indicated that the RhoGAP6/δ-COP interaction connects RhoGAP6 to the whole COPI complex. 14-3-3 was also established as a RhoGAP6 binding partner and its binding site was mapped to serine 37. We provide evidence of potential cross-regulation between 14-3-3 and δ-COP binding, however, neither δ-COP nor 14-3-3 binding to RhoGAP6 impacted RhoA activity. Instead, analysis of protein transport through the secretory pathway demonstrated that RhoGAP6/δ-COP binding increased protein transport to the plasma membrane, as did a catalytically inactive mutant of RhoGAP6. Overall, we have identified a novel interaction between RhoGAP6 and δ-COP which is mediated by conserved C-terminal di-tryptophan motifs, and which might control protein transport in platelets.

The Role of NO/sGC/cGMP/PKG Signaling Pathway in Regulation of Platelet Function

Circulating blood platelets are controlled by stimulatory and inhibitory factors, and a tightly regulated equilibrium between these two opposing processes is essential for normal platelet and vascular function. NO/cGMP/ Protein Kinase G (PKG) pathways play a highly significant role in platelet inhibition, which is supported by a large body of studies and data. This review focused on inconsistent and controversial data of NO/sGC/cGMP/PKG signaling in platelets including sources of NO that activate sGC in platelets, the role of sGC/PKG in platelet inhibition/activation, and the complexity of the regulation of platelet inhibitory mechanisms by cGMP/PKG pathways. In conclusion, we suggest that the recently developed quantitative phosphoproteomic method will be a powerful tool for the analysis of PKG-mediated effects. Analysis of phosphoproteins in PKG-activated platelets will reveal many new PKG substrates. A future detailed analysis of these substrates and their involvement in different platelet inhibitory pathways could be a basis for the development of new antiplatelet drugs that may target only specific aspects of platelet functions.

Protein Kinase A in cellular migration-Niche signaling of a ubiquitous kinase

Cell migration requires establishment and maintenance of directional polarity, which in turn requires spatial heterogeneity in the regulation of protrusion, retraction, and adhesion. Thus, the signaling proteins that regulate these various structural processes must also be distinctly regulated in subcellular space. Protein Kinase A (PKA) is a ubiquitous serine/threonine kinase involved in innumerable cellular processes. In the context of cell migration, it has a paradoxical role in that global inhibition or activation of PKA inhibits migration. It follows, then, that the subcellular regulation of PKA is key to bringing its proper permissive and restrictive functions to the correct parts of the cell. Proper subcellular regulation of PKA controls not only when and where it is active but also specifies the targets for that activity, allowing the cell to use a single, promiscuous kinase to exert distinct functions within different subcellular niches to facilitate cell movement. In this way, understanding PKA signaling in migration is a study in context and in the elegant coordination of distinct functions of a single protein in a complex cellular process.

Recent developments in Phos-tag electrophoresis for the analysis of phosphoproteins in proteomics

INTRODUCTION

Phosphate-binding tag (Phos-tag) sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) is an important development capable of analyzing the phosphorylation state of proteins. Conventionally, proteins were separated via SDS-PAGE and Phos-tag SDS-PAGE that use different gels to identify phosphorylated proteins. However, it was often difficult to compare the electrophoretic mobility of the proteins in the different gels used. The recently developed Phos-tag diagonal electrophoresis has been able to solve this problem. It can indicate the SDS-PAGE and Phos-tag SDS-PAGE patterns on a single gel; therefore, phosphorylated proteins can be distinguished easily from non-phosphorylated proteins.

AREAS COVERED

This review assesses the importance of Phos-tag electrophoresis, which enables the analysis of protein phosphorylation states, in the field of proteomics. Additionally, this review describes the significance and actual experimental technique of Phos-tag diagonal electrophoresis, which was recently developed to overcome the drawbacks of Phos-tag SDS-PAGE.

EXPERT OPINION

Although shotgun analysis of proteins allows detecting many phosphorylation sites, it is challenging to clarify the differences in the phosphorylation states of protein molecules using this technique. Therefore, Phos-tag SDS-PAGE is frequently used to determine the phosphorylation state of proteins. This technique has become more powerful with the recent development of Phos-tag diagonal electrophoresis.

Turning Platelets Off and On: Role of RhoGAPs and RhoGEFs in Platelet Activity

Platelet cytoskeletal reorganisation is a critical component of platelet activation and thrombus formation in haemostasis. The Rho GTPases RhoA, Rac1 and Cdc42 are the primary drivers in the dynamic reorganisation process, leading to the development of filopodia and lamellipodia which dramatically increase platelet surface area upon activation. Rho GTPases cycle between their active (GTP-bound) and inactive (GDP-bound) states through tightly regulated processes, central to which are the guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). GEFs catalyse the dissociation of GDP by inducing changes in the nucleotide binding site, facilitating GTP binding and activating Rho GTPases. By contrast, while all GTPases possess intrinsic hydrolysing activity, this reaction is extremely slow. Therefore, GAPs catalyse the hydrolysis of GTP to GDP, reverting Rho GTPases to their inactive state. Our current knowledge of these proteins is constantly being updated but there is considerably less known about the functionality of Rho GTPase specific GAPs and GEFs in platelets. In the present review, we discuss GAP and GEF proteins for Rho GTPases identified in platelets, their regulation, biological function and present a case for their further study in platelets.

Fine-Tuning of Platelet Responses by Serine/Threonine Protein Kinases and Phosphatases-Just the Beginning

Comprehensive proteomic analyses of human and murine platelets established an extraordinary intracellular repertoire of signaling components, which control crucial functions. The spectrum of platelet serine/threonine protein kinases (more than 100) includes the AGC family (protein kinase A, G, C [PKA, PKG, PKC]), the mitogen-activated protein kinases (MAPKs), and others. PKA and PKG have multiple significantly overlapping substrates in human platelets, which possibly affect functions with clear "signaling nodes" of regulation by multiple protein kinases/phosphatases. Signaling nodes are intracellular Ca²⁺ stores, the contractile system (myosin light chains), and other signaling components such as G-proteins, protein kinases, and protein phosphatases. An example for this fine-tuning is the tyrosine kinase Syk, a crucial component of platelet activation, which is controlled by several serine/threonine and tyrosine protein kinases as well as phosphatases. Other protein kinases including PKA/PKG modulate protein phosphatase 2A, which may be a master regulator of MAPK signaling in human platelets. Protein kinases and in particular MAPKs are targeted by an increasing number of clinically used inhibitors. However, the precise regulation and fine-tuning of these protein kinases and their effects on other signaling components in platelets are only superficially understood-just the beginning. However, promising future approaches are in sight.

H2S protects hippocampal neurons against hypoxia-reoxygenation injury by promoting RhoA phosphorylation at Ser188

Inhibition of RhoA-ROCK pathway is involved in the H₂S-induced cerebral vasodilatation and H₂S-mediated protection on endothelial cells against oxygen-glucose deprivation/reoxygenation injury. However, the inhibitory mechanism of H₂S on RhoA-ROCK pathway is still unclear. The aim of this study was to investigate the target and mechanism of H₂S in inhibition of RhoA/ROCK. GST-RhoA^wild and GST-RhoA^S188A proteins were constructed and expressed, and were used for phosphorylation assay in vitro. Recombinant RhoA^wild-pEGFP-N1 and RhoA^S188A-pEGFP-N1 plasmids were constructed and transfected into primary hippocampal nerve cells (HNCs) to evaluate the neuroprotective mechanism of endothelial H₂S by using transwell co-culture system with endothelial cells from cystathionine-γ-lyase knockout (CSE^-/-) mice and 3-mercaptopyruvate sulfurtransferase knockout (3-MST^-/-) rats, respectively. We found that NaHS, exogenous H₂S donor, promoted RhoA phosphorylation at Ser188 in the presence of cGMP-dependent protein kinase 1 (PKG1) in vitro. Besides, both exogenous and endothelial H₂S facilitated the RhoA phosphorylation at Ser188 in HNCs, which induced the reduction of RhoA activity and membrane transposition, as well as ROCK₂ activity and expression. To further investigate the role of endothelial H₂S on RhoA phosphorylation, we detected H₂S release from ECs of CSE^+/+ and CSE^-/- mice, and 3-MST^+/+ and 3-MST^-/- rats, respectively, and found that H₂S produced by ECs in the culture medium is mainly catalyzed by CSE synthase. Moreover, we revealed that both endothelial H₂S, mainly catalyzed by CSE, and exogenous H₂S protected the HNCs against hypoxia-reoxygenation injury via phosphorylating RhoA at Ser188.

Proteomics: A Tool to Study Platelet Function

Platelets are components of the blood that are highly reactive, and they quickly respond to multiple physiological and pathophysiological processes. In the last decade, it became clear that platelets are the key components of circulation, linking hemostasis, innate, and acquired immunity. Protein composition, localization, and activity are crucial for platelet function and regulation. The current state of mass spectrometry-based proteomics has tremendous potential to identify and quantify thousands of proteins from a minimal amount of material, unravel multiple post-translational modifications, and monitor platelet activity during drug treatments. This review focuses on the role of proteomics in understanding the molecular basics of the classical and newly emerging functions of platelets. including the recently described role of platelets in immunology and the development of COVID-19.The state-of-the-art proteomic technologies and their application in studying platelet biogenesis, signaling, and storage are described, and the potential of newly appeared trapped ion mobility spectrometry (TIMS) is highlighted. Additionally, implementing proteomic methods in platelet transfusion medicine, and as a diagnostic and prognostic tool, is discussed.

Phosphoproteomics identify arachidonic-acid-regulated signal transduction pathways modulating macrophage functions with implications for ovarian cancer

Arachidonic acid (AA) is a polyunsaturated fatty acid present at high concentrations in the ovarian cancer (OC) microenvironment and associated with a poor clinical outcome. In the present study, we have unraveled a potential link between AA and macrophage functions.

Methods: AA-triggered signal transduction was studied in primary monocyte-derived macrophages (MDMs) by phosphoproteomics, transcriptional profiling, measurement of intracellular Ca²⁺ accumulation and reactive oxygen species production in conjunction with bioinformatic analyses. Functional effects were investigated by actin filament staining, quantification of macropinocytosis and analysis of extracellular vesicle release.

Results: We identified the ASK1 - p38δ/α (MAPK13/14) axis as a central constituent of signal transduction pathways triggered by non-metabolized AA. This pathway was induced by the Ca²⁺-triggered activation of calmodulin kinase II, and to a minor extent by ROS generation in a subset of donors. Activated p38 in turn was linked to a transcriptional stress response associated with a poor relapse-free survival. Consistent with the phosphorylation of the p38 substrate HSP27 and the (de)phosphorylation of multiple regulators of Rho family GTPases, AA impaired actin filament organization and inhibited actin-driven macropinocytosis. AA also affected the phosphorylation of proteins regulating vesicle biogenesis, and consistently, AA enhanced the release of tetraspanin-containing exosome-like vesicles. Finally, we identified phospholipase A₂ group 2A (PLA2G2A) as the clinically most relevant enzyme producing extracellular AA, providing further potentially theranostic options.

Conclusion: Our results suggest that AA contributes to an unfavorable clinical outcome of OC by impacting the phenotype of tumor-associated macrophages. Besides critical AA-regulated signal transduction proteins identified in the present study, PLA2G2A might represent a potential prognostic tool and therapeutic target to interfere with OC progression.