Defective platelet activation in G alpha(q)-deficient mice

Pathogenicity and virulence of Yersinia

The genus Yersinia includes human, animal, insect, and plant pathogens as well as many symbionts and harmless bacteria. Within this genus are Yersinia enterocolitica and the Yersinia pseudotuberculosis complex, with four human pathogenic species that are highly related at the genomic level including the causative agent of plague, Yersinia pestis. Extensive laboratory, field work, and clinical research have been conducted to understand the underlying pathogenesis and zoonotic transmission of these pathogens. There are presently more than 500 whole genome sequences from which an evolutionary footprint can be developed that details shared and unique virulence properties. Whereas the virulence of Y. pestis now seems in apparent homoeostasis within its flea transmission cycle, substantial evolutionary changes that affect transmission and disease severity continue to ndergo apparent selective pressure within the other Yersiniae that cause intestinal diseases. In this review, we will summarize the present understanding of the virulence and pathogenesis of Yersinia, highlighting shared mechanisms of virulence and the differences that determine the infection niche and disease severity.

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.

Capillary malformations

Capillary malformation (CM), or port wine birthmark, is a cutaneous congenital vascular anomaly that occurs in 0.1%-2% of newborns. Patients with a CM localized on the forehead have an increased risk of developing a neurocutaneous disorder called encephalotrigeminal angiomatosis or Sturge-Weber syndrome (SWS), with complications including seizure, developmental delay, glaucoma, and vision loss. In 2013, a groundbreaking study revealed causative activating somatic mutations in the gene (GNAQ) encoding guanine nucleotide-binding protein Q subunit α (Gαq) in CM and SWS patient tissues. In this Review, we discuss the disease phenotype, the causative GNAQ mutations, and their cellular origin. We also present the endothelial Gαq-related signaling pathways, the current animal models to study CM and its complications, and future options for therapeutic treatment. Further work remains to fully elucidate the cellular and molecular mechanisms underlying the formation and maintenance of the abnormal vessels.

Sustainable inflammatory activation following spinal cord injury is driven by thrombin-mediated dynamic expression of astrocytic chemokines

Acute spinal cord injury (SCI) always results in sustainable recruitment of inflammatory cells driven by sequentially generated chemokines, thereby eliciting excessive neuroinflammation. However, the underlying mechanism of temporally produced chemokines remains elusive. Reactive astrocytes are known to be the main sources of chemokines at the lesion site, which can be immediately activated by thrombin following SCI. In the present study, SCI was shown to induce a sequential production of chemokines CCL2 and CCL5 from astrocytes, which were associated with a persistent infiltration of macrophages/microglia. The rapidly induced CCL2 and later induced CCL5 from astrocytes were regulated by thrombin at the damaged tissues. Investigation of the regulatory mechanism revealed that thrombin facilitated astrocytic CCL2 production through activation of ERK/JNK/NFκB pathway, whereas promoted CCL5 production through PLCβ3/NFκB and ERK/JNK/NFκB signal pathway. Inhibition of thrombin activity significantly decreased production of astrocytic CCL2 and CCL5, and reduced the accumulation of macrophages/microglia at the lesion site. Accordingly, the locomotor function of rats was remarkably improved. The present study has provided a new regulatory mechanism on thrombin-mediated sequential production of astrocytic chemokines, which might be beneficial for clinical therapy of CNS neuroinflammation.

Spatiotemporal Optical Control of Gαq-PLCβ Interactions

Cells experience time-varying and spatially heterogeneous chemokine signals in vivo, activating cell surface proteins including G protein-coupled receptors (GPCRs). The Gαq pathway activation by GPCRs is a major signaling axis with broad physiological and pathological significance. Compared with other Gα members, GαqGTP activates many crucial effectors, including PLCβ (Phospholipase Cβ) and Rho GEFs (Rho guanine nucleotide exchange factors). PLCβ regulates many key processes, such as hematopoiesis, synaptogenesis, and cell cycle, and is therefore implicated in terminal-debilitating diseases, including cancer, epilepsy, Huntington's Disease, and Alzheimer's Disease. However, due to a lack of genetic and pharmacological tools, examining how the dynamic regulation of PLCβ signaling controls cellular physiology has been difficult. Since activated PLCβ induces several abrupt cellular changes, including cell morphology, examining how the other pathways downstream of Gq-GPCRs contribute to the overall signaling has also been difficult. Here we show the engineering, validation, and application of a highly selective and efficient optogenetic inhibitor (Opto-dHTH) to completely disrupt GαqGTP-PLCβ interactions reversibly in user-defined cellular-subcellular regions on optical command. Using this newly gained PLCβ signaling control, our data indicate that the molecular competition between RhoGEFs and PLCβ for GαqGTP determines the potency of Gq-GPCR-governed directional cell migration.

A regulatory node involving Gαq, PLCβ, and RGS proteins modulates platelet reactivity to critical agonists

BACKGROUND

Most platelet agonists work through G protein-coupled receptors, activating pathways that involve members of the Gq, Gi, and G12/G13 families of heterotrimeric G proteins. Gq signaling has been shown to be critical for efficient platelet activation. Growing evidence suggests that regulatory mechanisms converge on G protein-coupled receptors and Gq to prevent overly robust platelet reactivity.

OBJECTIVES

To identify and characterize mechanisms by which Gq signaling is regulated in platelets.

METHODS

Based on our prior experience with a Gαi2 variant that escapes regulation by regulator of G protein signaling (RGS) proteins, a Gαq variant was designed with glycine 188 replaced with serine (G188S) and then incorporated into a mouse line so that its effects on platelet activation and thrombus formation could be studied in vitro and in vivo.

RESULTS AND CONCLUSIONS

As predicted, the G188S substitution in Gαq disrupted its interaction with RGS18. Unexpectedly, it also uncoupled PLCβ-3 from activation by platelet agonists as evidenced by a loss rather than a gain of platelet function in vitro and in vivo. Binding studies showed that in addition to preventing the binding of RGS18 to Gαq, the G188S substitution also prevented the binding of PLCβ-3 to Gαq. Structural analysis revealed that G188 resides in the region that is also important for Gαq binding to PLCβ-3 in platelets. We conclude that the Gαq signaling node is more complex than that has been previously understood, suggesting that there is cross-talk between RGS proteins and PLCβ-3 in the context of Gαq signaling.

Spatiotemporal optical control of Gαq-PLCβ interactions

Cells experience time-varying and spatially heterogeneous chemokine signals in vivo, activating cell surface proteins, including G protein-coupled receptors (GPCRs). The Gαq pathway activation by GPCRs is a major signaling axis with a broad physiological and pathological significance. Compared to other Gα members, GαqGTP activates many crucial effectors, including PLCβ (Phospholipase Cβ) and Rho GEFs (Rho guanine nucleotide exchange factors). PLCβ regulates many key processes, such as hematopoiesis, synaptogenesis, and cell cycle, and is therefore implicated in terminal - debilitating diseases, including cancer, epilepsy, Huntington's Disease, and Alzheimer's Disease. However, due to a lack of genetic and pharmacological tools, examining how the dynamic regulation of PLCβ signaling controls cellular physiology has been difficult. Since activated PLCβ induces several abrupt cellular changes, including cell morphology, examining how the other pathways downstream of Gq-GPCRs contribute to the overall signaling has also been difficult. Here we show the engineering, validation, and application of a highly selective and efficient optogenetic inhibitor (Opto-dHTH) to completely disrupt GαqGTP-PLCβ interactions reversibly in user-defined cellular-subcellular regions on optical command. Using this newly gained PLCβ signaling control, our data indicate that the molecular competition between RhoGEFs and PLCβ for GαqGTP determines the potency of Gq-GPCR-governed directional cell migration.

Developmental expression of the Sturge-Weber syndrome-associated genetic mutation in Gnaq: a formal test of Happle's paradominant inheritance hypothesis

Sturge-Weber Syndrome (SWS) is a sporadic (non-inherited) syndrome characterized by capillary vascular malformations in the facial skin, leptomeninges, or the choroid. A hallmark feature is the mosaic nature of the phenotype. SWS is caused by a somatic mosaic mutation in the GNAQ gene (p.R183Q), leading to activation of the G protein, Gαq. Decades ago, Rudolf Happle hypothesized SWS as an example of "paradominant inheritance", that is, a "lethal gene (mutation) surviving by mosaicism". He predicted that the "presence of the mutation in the zygote will lead to death of the embryo at an early stage of development". We have created a mouse model for SWS using gene targeting to conditionally express the GNAQ p.R183Q mutation. We have employed two different Cre-drivers to examine the phenotypic effects of expression of this mutation at different levels and stages of development. As predicted by Happle, global, ubiquitous expression of this mutation in the blastocyst stage results in 100% embryonic death. The majority of these developing embryos show vascular defects consistent with the human vascular phenotype. By contrast, global but mosaic expression of the mutation enables a fraction of the embryos to survive, but those that survive to birth and beyond do not exhibit obvious vascular defects. These data validate Happle's paradominant inheritance hypothesis for SWS and suggest the requirement of a tight temporal and developmental window of mutation expression for the generation of the vascular phenotype. Furthermore, these engineered murine alleles provide the template for the development of a mouse model of SWS that acquires the somatic mutation during embryonic development, but permits the embryo to progress to live birth and beyond, so that postnatal phenotypes can also be investigated. These mice could then also be employed in pre-clinical studies of novel therapies.

Clinical pharmacology of antiplatelet drugs

INTRODUCTION

Platelets play a key role in arterial thrombosis and antiplatelet therapy is pivotal in the treatment of cardiovascular disease. Current antiplatelet drugs target different pathways of platelet activation and show specific pharmacodynamic and pharmacokinetic characteristics, implicating clinically relevant drug-drug interactions.

AREAS COVERED

This article reviews the role of platelets in hemostasis and cardiovascular thrombosis, and discusses the key pharmacodynamics, drug-drug interactions and reversal strategies of clinically used antiplatelet drugs.

EXPERT OPINION

Antiplatelet therapies target distinct pathways of platelet activation: thromboxane A₂ synthesis, adenosine diphosphate-mediated signaling, integrin α_IIbβ₃ (GPIIb/IIIa), thrombin-mediated platelet activation via the PAR1 receptor and phosphodiesterases. Key clinical drug-drug interactions of antiplatelet agents involve acetylsalicylic acid - ibuprofen, clopidogrel - omeprazole, and morphine - oral P2Y₁₂ inhibitors, all of which lead to an attenuated antiplatelet effect. Platelet function and genetic testing and the use of scores (ARC-HBR, PRECISE-DAPT, ESC ischemic risk definition) may contribute to a more tailored antiplatelet therapy. High on-treatment platelet reactivity presents a key problem in the acute management of ST-elevation myocardial infarction (STEMI). A treatment strategy involving early initiation of an intravenous antiplatelet agent may be able to bridge the gap of insufficient platelet inhibition in high ischemic risk patients with STEMI.

G protein-coupled receptor signaling: transducers and effectors

G protein-coupled receptors (GPCRs) are of considerable interest due to their importance in a wide range of physiological functions and in a large number of Food and Drug Administration (FDA)-approved drugs as therapeutic entities. With continued study of their function and mechanism of action, there is a greater understanding of how effector molecules interact with a receptor to initiate downstream effector signaling. This review aims to explore the signaling pathways, dynamic structures, and physiological relevance in the cardiovascular system of the three most important GPCR signaling effectors: heterotrimeric G proteins, GPCR kinases (GRKs), and β-arrestins. We will first summarize their prominent roles in GPCR pharmacology before transitioning into less well-explored areas. As new technologies are developed and applied to studying GPCR structure and their downstream effectors, there is increasing appreciation for the elegance of the regulatory mechanisms that mediate intracellular signaling and function.

The Emerging Key Role of the mGluR1-PKCγ Signaling Pathway in the Pathogenesis of Spinocerebellar Ataxias: A Neurodevelopmental Viewpoint

Spinocerebellar ataxias (SCAs) are a heterogeneous group of autosomal dominantly inherited progressive disorders with degeneration and dysfunction of the cerebellum. Although different subtypes of SCAs are classified according to the disease-associated causative genes, the clinical syndrome of the ataxia is shared, pointing towards a possible convergent pathogenic pathway among SCAs. In this review, we summarize the role of SCA-associated gene function during cerebellar Purkinje cell development and discuss the relationship between SCA pathogenesis and neurodevelopment. We will summarize recent studies on molecules involved in SCA pathogenesis and will focus on the mGluR1-PKCγ signaling pathway evaluating the possibility that this might be a common pathway which contributes to these diseases.

Unfolded Protein Response Differentially Modulates the Platelet Phenotype

BACKGROUND

Unfolded protein response (UPR) is a multifaceted signaling cascade that alleviates protein misfolding. Although well studied in nucleated cells, UPR in absence of transcriptional regulation has not been described. Intricately associated with cardiovascular diseases, platelets, despite being anucleate, respond rapidly to stressors in blood. We investigate the UPR in anucleate platelets and explore its role, if any, on platelet physiology and function.

METHODS

Human and mouse platelets were studied using a combination of ex vivo and in vivo experiments. Platelet lineage-specific knockout mice were generated independently for each of the 3 UPR pathways, PERK (protein kinase RNA [PKR]-like endoplasmic reticulum kinase), XBP1 (X-binding protein), and ATF6 (activating transcription factor 6). Diabetes patients were prospectively recruited, and platelets were evaluated for activation of UPR under chronic pathophysiological disease conditions.

RESULTS

Tunicamycin induced the IRE1α (inositol-requiring enzyme-1alpha)-XBP1 pathway in human and mouse platelets, while oxidative stress predominantly activated the PERK pathway. PERK deletion significantly increased platelet aggregation and apoptosis and phosphorylation of PLCγ2, PLCβ3, and p38 MAPK. Deficiency of XBP1 increased platelet aggregation, with higher PLCβ3 and PKCδ activation. ATF6 deletion mediated a relatively modest effect on platelet phenotype with increased PKA (protein kinase A). Platelets from diabetes patients exhibited a positive correlation between disease severity, platelet activation, and protein aggregation, with only IRE1α-XBP1 activation. Moreover, IRE1α inhibition increased platelet aggregation, while clinically approved chemical chaperone, sodium 4-phenylbutyrate reduced the platelet hyperactivation.

CONCLUSIONS

We show for the first time, that UPR activation occurs in platelets and can be independent of genomic regulation, with selective induction being specific to the source and severity of stress. Each UPR pathway plays a key role and can differentially modulate the platelet activation pathways and phenotype. Targeting the specific arms of UPR may provide a new antiplatelet strategy to mitigate thrombotic risk in diabetes and other cardiovascular diseases.

GqPCR-stimulated dephosphorylation of AKT is induced by an IGBP1-mediated PP2A switch

BACKGROUND

G protein-coupled receptors (GPCRs) usually regulate cellular processes via activation of intracellular signaling pathways. However, we have previously shown that in several cell lines, GqPCRs induce immediate inactivation of the AKT pathway, which leads to JNK-dependent apoptosis. This apoptosis-inducing AKT inactivation is essential for physiological functions of several GqPCRs, including those for PGF2α and GnRH.

METHODS

Here we used kinase activity assays of PI3K and followed phosphorylation state of proteins using specific antibodies. In addition, we used coimmunoprecipitation and proximity ligation assays to follow protein-protein interactions. Apoptosis was detected by TUNEL assay and PARP1 cleavage.

RESULTS

We identified the mechanism that allows the unique stimulated inactivation of AKT and show that the main regulator of this process is the phosphatase PP2A, operating with the non-canonical regulatory subunit IGBP1. In resting cells, an IGBP1-PP2Ac dimer binds to PI3K, dephosphorylates the inhibitory pSer608-p85 of PI3K and thus maintains its high basal activity. Upon GqPCR activation, the PP2Ac-IGBP1 dimer detaches from PI3K and thus allows the inhibitory dephosphorylation. At this stage, the free PP2Ac together with IGBP1 and PP2Aa binds to AKT, causing its dephosphorylation and inactivation.

CONCLUSION

Our results show a stimulated shift of PP2Ac from PI3K to AKT termed "PP2A switch" that represses the PI3K/AKT pathway, providing a unique mechanism of GPCR-stimulated dephosphorylation. Video Abstract.

Gαq activation modulates autophagy by promoting mTORC1 signaling

The mTORC1 node plays a major role in autophagy modulation. We report a role of the ubiquitous Gαq subunit, a known transducer of plasma membrane G protein-coupled receptors signaling, as a core modulator of mTORC1 and autophagy. Cells lacking Gαq/11 display higher basal autophagy, enhanced autophagy induction upon different types of nutrient stress along with a decreased mTORC1 activation status. They are also unable to reactivate mTORC1 and thus inactivate ongoing autophagy upon nutrient recovery. Conversely, stimulation of Gαq/11 promotes sustained mTORC1 pathway activation and reversion of autophagy promoted by serum or amino acids removal. Gαq is present in autophagic compartments and lysosomes and is part of the mTORC1 multi-molecular complex, contributing to its assembly and activation via its nutrient status-sensitive interaction with p62, which displays features of a Gαq effector. Gαq emerges as a central regulator of the autophagy machinery required to maintain cellular homeostasis upon nutrient fluctuations.

Sex and brain region-specific regulation of serotonin transporter activity in synaptosomes in guanine nucleotide-binding protein G(q) alpha knockout mice

The regulation of the serotonin transporter (SERT) by guanine nucleotide-binding protein alpha (Gα) q was investigated using Gαq knockout mice. In the absence of Gαq, SERT-mediated uptake of 5-hydroxytryptamine (5HT) was enhanced in midbrain and frontal cortex synaptosomes, but only in female mice. The mechanisms underlying this sexual dimorphism were investigated using quantitative western blot analysis revealing brain region-specific differences. In the frontal cortex, SERT protein expression was decreased in male knockout mice, seemingly explaining the sex-dependent variation in SERT activity. The differential expression of Gαi1 in female mice contributes to the sex differences in the midbrain. In fact, Gαi1 levels inversely correlate with 5HT uptake rates across both sexes and genotypes. Likely due to differential SERT regulation as well as sex differences in the expression of tryptophan hydroxylase 2, Gαq knockout mice also displayed sex- and genotype-dependent alterations in total 5HT tissue levels as determined by high-performance liquid chromatography. Gαq inhibitors, YM-254890 and BIM-46187, differentially affected SERT activity in both, synaptosomes and cultured cells. YM-254890 treatment mimicked the effect of Gαq knockout in the frontal cortex. BIM-46187, which promotes the nucleotide-free form of Gα proteins, substantially inhibited 5HT uptake, prompting us to hypothesise that Gαq interacts with SERT similarly as with G-protein-coupled receptors and inhibits SERT activity by modulating transport-associated conformational changes. Taken together, our findings reveal a novel mechanism of SERT regulation and impact our understanding of sex differences in diseases associated with dysregulation of serotonin transmission, such as depression and anxiety.

Is Antiplatelet Therapy Contraindicated After Platelet-Rich Plasma Treatment? A Narrative Review

Background

Antiplatelet therapies are often withheld before and after platelet-rich plasma product (PRPP) administration due to theoretical concerns that therapies that inhibit the function of platelets would inhibit the effects of PRPP.

Purpose/Hypothesis

The purpose of this study was to evaluate the effect that antiplatelet therapies have on the ability of PRPP to stimulate wound healing and tissue regeneration. Our hypothesis was that antiplatelet therapies would have highly heterogeneous effects on the biological activity of PRPP.

Study Design

Narrative review.

Methods

The Medline database was searched via PubMed to identify all studies related to PRPP and antiplatelet therapies, yielding 1417 publications. After the search was confined to articles published after 1995, there were 901 articles remaining. All abstracts were then screened to identify animal or human clinical studies that focused on growth factor or inflammatory cytokine production or treatment outcomes. We limited our analysis to studies reporting on orthopaedic pathologies and in vitro studies of antiplatelet therapies. Ultimately, 12 articles fit the search criteria.

Results

The majority of studies reported on the use of nonsteroidal anti-inflammatory drugs as antiplatelet therapy. The majority of studies were in vitro analyses of growth factors, inflammatory cytokines, or cell viability, whereas 1 study examined clinical outcomes in an animal model. None of the studies investigated clinical outcomes in humans. All of the studies showed no effect or mixed effects of antiplatelet therapies on PRPP efficacy. One study showed PRPP recovery to baseline function after a 1-week washout period.

Conclusion

The literature did not provide support for the common clinical practice of withholding antiplatelet therapies in patients being treated with PRPP.

The effect of platelet G proteins on platelet extravasation and tumor growth in the murine model of ovarian cancer

We and other investigators have shown that platelets promote metastasis and the growth of tumors. Our rationale for conducting this study is that platelets' prometastatic and progrowth effects depend on a close encounter between platelets and cancer cells. This interaction occurs inside blood vessels with circulating tumor cells and outside blood vessels with cancer cells residing in the tumor parenchyma. Our hypothesis was that platelet extravasation is required for the effect of platelets on tumor growth. Platelets respond to environmental stimuli by activation of G protein-coupled receptors on their surface. We investigated the impact of various platelet G proteins on the growth of ovarian cancer tumors and platelet extravasation. We used mice with platelet-specific deficiency of Gαi2 (Gi), Gα13 (G13), or Gαq (Gq) in a syngeneic ovarian cancer model. We measured the total weight of tumor nodules resected from tumor-bearing mice. We developed methods for automated whole-slide image acquisition and unbiased computerized image analysis to quantify extravasated platelets. We compared the number of platelets inside tumor nodules of platelet G protein-deficient tumor-bearing mice. We found that deficiency of Gi and G13, but not Gq, in platelets resulted in smaller tumors compared with those in corresponding littermates. Deficiency of Gi and G13 in platelets reduced the number of extravasated platelets by >90%, but deficiency of Gq did not reduce the number of extravasated platelets significantly. The lack of Gi or G13 in platelets reduced platelet extravasation into the tumor and tumor growth.

Heightened activation of embryonic megakaryocytes causes aneurysms in the developing brain of mice lacking podoplanin

During early embryonic development in mammals, including humans and mice, megakaryocytes (Mks) first originate from primitive hematopoiesis in the yolk sac. These embryonic Mks (eMks) circulate in the vasculature with unclear function. Herein, we report that podoplanin (PDPN), the ligand of C-type lectin-like receptor (CLEC-2) on Mks/platelets, is temporarily expressed in neural tissue during midgestation in mice. Loss of PDPN or CLEC-2 resulted in aneurysms and spontaneous hemorrhage, specifically in the lower diencephalon during midgestation. Surprisingly, more eMks/platelets had enhanced granule release and localized to the lower diencephalon in mutant mouse embryos than in wild-type littermates before hemorrhage. We found that PDPN counteracted the collagen-1-induced secretion of angiopoietin-1 from fetal Mks, which coincided with enhanced TIE-2 activation in aneurysm-like sprouts of PDPN-deficient embryos. Blocking platelet activation prevented the PDPN-deficient embryo from developing vascular defects. Our data reveal a new role for PDPN in regulating eMk function during midgestation.

NK Cell Interaction With Platelets and Myeloid Cells in the Tumor Milieu

Natural killer (NK) cells recognize and kill tumor cells via germ-line encoded receptors and polarized degranulation of cytotoxic molecules, respectively. As such, NK cells help to inhibit the development of cancers. The activating receptor NKG2D induces NK cell-mediated killing of metastasizing tumor cells by recognition of the stress-induced ligands MICA, MICB, and ULBP1-6. However, platelets enable escape from this immune surveillance mechanism by obstructing the interactions between NK cells and tumor cells or by cleaving the stress-induced ligands. It is also being increasingly appreciated that NK cells play additional roles in cancer immunity, including chemokine-mediated recruitment of antigen presenting cells in the tumor microenvironment that is followed by generation of adaptive immunity. However, the NK cell interplays with dendritic cells, and macrophages are extremely complex and involve molecular interactions via NKG2D and cytokine receptors. Specifically, NKG2D-mediated chronic interaction between NK cells and tumor-infiltrating macrophages causes immune suppression by differentiating NK cells toward a dysfunctional state. Here we discuss the underlying mechanisms of NK cell control by platelets and myeloid cells with focus on NKG2D and its ligands, and provide a timely perspective on how to harness these pathways with novel immunotherapeutic approaches.

The role of phospho-tyrosine signaling in platelet biology and hemostasis

Platelets are small enucleated cell fragments specialized in the control of hemostasis, but also playing a role in angiogenesis, inflammation and immunity. This plasticity demands a broad range of physiological processes. Platelet functions are mediated through a variety of receptors, the concerted action of which must be tightly regulated, in order to allow specific and timely responses to different stimuli. Protein phosphorylation is one of the main key regulatory mechanisms by which extracellular signals are conveyed. Despite the importance of platelets in health and disease, the molecular pathways underlying the activation of these cells are still under investigation. Here, we review current literature on signaling platelet biology and in particular emphasize the newly emerging role of phosphatases in these processes.

Heterotrimeric G Protein Subunit Gαq Is a Master Switch for Gβγ-Mediated Calcium Mobilization by Gi-Coupled GPCRs

Mechanisms that control mobilization of cytosolic calcium [Ca²⁺]_i are key for regulation of numerous eukaryotic cell functions. One such paradigmatic mechanism involves activation of phospholipase Cβ (PLCβ) enzymes by G protein βγ subunits from activated Gα_i-Gβγ heterotrimers. Here, we report identification of a master switch to enable this control for PLCβ enzymes in living cells. We find that the Gα_i-Gβγ-PLCβ-Ca²⁺ signaling module is entirely dependent on the presence of active Gα_q. If Gα_q is pharmacologically inhibited or genetically ablated, Gβγ can bind to PLCβ but does not elicit Ca²⁺ signals. Removal of an auto-inhibitory linker that occludes the active site of the enzyme is required and sufficient to empower "stand-alone control" of PLCβ by Gβγ. This dependence of Gi-Gβγ-Ca²⁺ on Gα_q places an entire signaling branch of G-protein-coupled receptors (GPCRs) under hierarchical control of Gq and changes our understanding of how Gi-GPCRs trigger [Ca²⁺]_i via PLCβ enzymes.

Tetrahydroimidazo[1,2-a]pyrazine Derivatives: Synthesis and Evaluation as Gαq -Protein Ligands

The 5,6,7,8-tetrahydroimidazo[1,2-a]pyrazine derivative BIM-46174 and its dimeric form BIM-46187 (1) are heterocyclized dipeptides that belong to the very few cell-permeable compounds known to preferentially silence Gαq proteins. To explore the chemical space of Gαq inhibitors of the BIM chemotype, a combinatorial approach was conducted towards a library of BIM molecules. This library was evaluated in a second messenger-based fluorescence assay to analyze the activity of Gαq proteins through the determination of intracellular myo-inositol 1-phosphate. Structure-activity relationships were deduced and structural requirements for biological activity obtained, which were (i) a redox reactive thiol/disulfane substructure, (ii) an N-terminal basic amino group, (iii) a cyclohexylalanine moiety, and (iv) a bicyclic skeleton. Active compounds exhibited cellular toxicity, which was investigated in detail for the prototypical inhibitor 1. This compound affects the structural cytoskeletal dynamics in a Gαq/11 -independent manner.

Direct Gαq Gating Is the Sole Mechanism for TRPM8 Inhibition Caused by Bradykinin Receptor Activation

Activation of Gα_q-coupled receptors by inflammatory mediators inhibits cold-sensing TRPM8 channels, aggravating pain and inflammation. Both Gα_q and the downstream hydrolysis of phosphatidylinositol 4, 5-bisphosphate (PIP₂) inhibit TRPM8. Here, I demonstrate that direct Gα_q gating is essential for both the basal cold sensitivity of TRPM8 and TRPM8 inhibition elicited by bradykinin in sensory neurons. The action of Gα_q depends on binding to three arginine residues in the N terminus of TRPM8. Neutralization of these residues markedly increased sensitivity of the channel to agonist and membrane voltage and completely abolished TRPM8 inhibition by both Gα_q and bradykinin while sparing the channel sensitivity to PIP₂. Interestingly, the bradykinin receptor B2R also binds to TRPM8, rendering TRPM8 insensitive to PIP₂ depletion. Furthermore, TRPM8-Gα_q binding impaired Gα_q coupling and signaling to PLCβ-PIP₂. The crosstalk in the TRPM8-Gα_q-B2R complex thus determines Gα_q gating rather than PIP₂ as a sole means of TRPM8 inhibition by bradykinin.

Proteinase-Activated Receptor 4 Activation Triggers Cell Membrane Blebbing through RhoA and β-Arrestin

Proteinase-activated receptors (PARs) are a four-member family of G-protein-coupled receptors that are activated via proteolysis. PAR4 is a member of this family that is cleaved and activated by serine proteinases such as thrombin, trypsin, and cathepsin-G. PAR4 is expressed in a variety of tissues and cell types, including platelets, vascular smooth muscle cells, and neuronal cells. In studying PAR4 signaling and trafficking, we observed dynamic changes in the cell membrane, with spherical membrane protrusions that resemble plasma membrane blebbing. Since nonapoptotic membrane blebbing is now recognized as an important regulator of cell migration, cancer cell invasion, and vesicular content release, we sought to elucidate the signaling pathway downstream of PAR4 activation that leads to such events. Using a combination of pharmacological inhibition and CRISPR/CRISPR-associated protein 9 (Cas9)-mediated gene editing approaches, we establish that PAR4-dependent membrane blebbing occurs independently of the Gα _q/11- and Gα _i-signaling pathways and is dependent on signaling via the β-arrestin-1/2 and Ras homolog family member A (RhoA) signaling pathways. Together these studies provide further mechanistic insight into PAR4 regulation of cellular function. SIGNIFICANCE STATEMENT: We find that the thrombin receptor PAR4 triggers cell membrane blebbing in a RhoA-and β-arrestin-dependent manner. In addition to identifying novel cellular responses mediated by PAR4, these data provide further evidence for biased signaling in PAR4 since membrane blebbing was dependent on some, but not all, signaling pathways activated by PAR4.

Dynamics of Platelet Behaviors as Defenders and Guardians: Accumulations in Liver, Lung, and Spleen in Mice

Systemic platelet behaviors in experimental animals are often assessed by infusion of isotope-labeled platelets and measuring them under anesthesia. However, such procedures alter, therefore may not reveal, real-life platelet behaviors. 5-Hydroxytryptamine (5HT or serotonin) is present within limited cell-types, including platelets. In our studies, by measuring 5HT as a platelet-marker in non-anesthetized mice, we identified stimulation- and time-dependent accumulations in liver, lung, and/or spleen as important systemic platelet behaviors. For example, intravenous, intraperitoneal, or intragingival injection of lipopolysaccharide (LPS, a cell-wall component of Gram-negative bacteria), interleukin (IL)-1, or tumor necrosis factor (TNF)-α induced hepatic platelet accumulation (HPA) and platelet translocation into the sinusoidal and perisinusoidal spaces or hepatocytes themselves. These events occurred "within a few hours" of the injection, caused hypoglycemia, and exhibited protective or causal effects on hepatitis. Intravenous injection of larger doses of LPS into normal mice, or intravenous antigen-challenge to sensitized mice, induced pulmonary platelet accumulation (PPA), as well as HPA. These reactions occurred "within a few min" of the LPS injection or antigen challenge and resulted in shock. Intravenous injection of 5HT or a catecholamine induced a rapid PPA "within 6 s." Intravenous LPS injection, within a minute, increased the pulmonary catecholamines that mediate the LPS-induced PPA. Macrophage-depletion from liver and spleen induced "day-scale" splenic platelet accumulation, suggesting the spleen is involved in clearing senescent platelets. These findings indicate the usefulness of 5HT as a marker of platelet behaviors, and provide a basis for a discussion of the roles of platelets as both "defenders" and "guardians."

Comparative Analysis of Microfluidics Thrombus Formation in Multiple Genetically Modified Mice: Link to Thrombosis and Hemostasis

Genetically modified mice are indispensable for establishing the roles of platelets in arterial thrombosis and hemostasis. Microfluidics assays using anticoagulated whole blood are commonly used as integrative proxy tests for platelet function in mice. In the present study, we quantified the changes in collagen-dependent thrombus formation for 38 different strains of (genetically) modified mice, all measured with the same microfluidics chamber. The mice included were deficient in platelet receptors, protein kinases or phosphatases, small GTPases or other signaling or scaffold proteins. By standardized re-analysis of high-resolution microscopic images, detailed information was obtained on altered platelet adhesion, aggregation and/or activation. For a subset of 11 mouse strains, these platelet functions were further evaluated in rhodocytin- and laminin-dependent thrombus formation, thus allowing a comparison of glycoprotein VI (GPVI), C-type lectin-like receptor 2 (CLEC2) and integrin α₆β₁ pathways. High homogeneity was found between wild-type mice datasets concerning adhesion and aggregation parameters. Quantitative comparison for the 38 modified mouse strains resulted in a matrix visualizing the impact of the respective (genetic) deficiency on thrombus formation with detailed insight into the type and extent of altered thrombus signatures. Network analysis revealed strong clusters of genes involved in GPVI signaling and Ca²⁺ homeostasis. The majority of mice demonstrating an antithrombotic phenotype in vivo displayed with a larger or smaller reduction in multi-parameter analysis of collagen-dependent thrombus formation in vitro. Remarkably, in only approximately half of the mouse strains that displayed reduced arterial thrombosis in vivo, this was accompanied by impaired hemostasis. This was also reflected by comparing in vitro thrombus formation (by microfluidics) with alterations in in vivo bleeding time. In conclusion, the presently developed multi-parameter analysis of thrombus formation using microfluidics can be used to: (i) determine the severity of platelet abnormalities; (ii) distinguish between altered platelet adhesion, aggregation and activation; and (iii) elucidate both collagen and non-collagen dependent alterations of thrombus formation. This approach may thereby aid in the better understanding and better assessment of genetic variation that affect in vivo arterial thrombosis and hemostasis.

Arhgef1 Plays a Vital Role in Platelet Function and Thrombogenesis

Background Platelets are the cellular mediators of hemostasis and thrombosis, and their function is regulated by a number of molecular mediators, such as small GTP ases. These small GTP ases are themselves regulated by guanine nucleotide exchange factors such as Arhgefs, several of which are found in platelets, including the highly expressed Arhgef1. However, the role of Arhgef1 in platelets has not yet been investigated. Methods and Results We employed mice with genetic deletion of Arhgef1 (ie, Arhgef1-/-) and investigated their platelet phenotype by employing a host of in vivo and in vitro platelet assays. Our results indicate that Arhgef1-/- mice had prolonged carotid artery occlusion and tail bleeding times. Moreover, platelets from these mice exhibited defective aggregation, dense and α granule secretion, α II bβ3 integrin activation, clot retraction and spreading, in comparison to their wild-type littermates. Finally, we also found that the mechanism by which Arhgef1 regulates platelets is mediated in part by a defect in the activation of the RhoA-Rho-associated kinase axis, but not Rap1b. Conclusions Our data demonstrate, for the first time, that Arhgef1 plays a critical role in platelet function, in vitro and in vivo.

Structural design principles that underlie the multi-specific interactions of Gαq with dissimilar partners

Gα_q is a ubiquitous molecular switch that activates the effectors phospholipase-C-β3 (PLC-β3) and Rho guanine-nucleotide exchange factors. Gα_q is inactivated by regulators of G protein signaling proteins, as well as by PLC-β3. Gα_q further interacts with G protein-coupled receptor kinase 2 (GRK2), although the functional role of this interaction is debated. While X-ray structures of Gα_q bound to representatives of these partners have revealed details of their interactions, the mechanistic basis for differential Gα_q interactions with multiple partners (i.e., Gα_q multi-specificity) has not been elucidated at the individual residue resolution. Here, we map the structural determinants of Gα_q multi-specificity using structure-based energy calculations. We delineate regions that specifically interact with GTPase Activating Proteins (GAPs) and residues that exclusively contribute to effector interactions, showing that only the Gα_q “Switch II” region interacts with all partners. Our analysis further suggests that Gα_q-GRK2 interactions are consistent with GRK2 functioning as an effector, rather than a GAP. Our multi-specificity analysis pinpoints Gα_q residues that uniquely contribute to interactions with particular partners, enabling precise manipulation of these cascades. As such, we dissect the molecular basis of Gα_q function as a central signaling hub, which can be used to target Gα_q-mediated signaling in therapeutic interventions.

Significance of urinary 11-dehydro-thromboxane B2 in age-related diseases: Focus on atherothrombosis

Platelet activation plays a key role in atherogenesis and atherothrombosis. Biochemical evidence of increased platelet activation in vivo can be reliably obtained through non-invasive measurement of thromboxane metabolite (TXM) excretion. Persistent biosynthesis of TXA₂ has been associated with several ageing-related diseases, including acute and chronic cardio-cerebrovascular diseases and cardiovascular risk factors, such as cigarette smoking, type 1 and type 2 diabetes mellitus, obesity, hypercholesterolemia, hyperhomocysteinemia, hypertension, chronic kidney disease, chronic inflammatory diseases. Given the systemic nature of TX excretion, involving predominantly platelet but also extraplatelet sources, urinary TXM may reflect either platelet cyclooxygenase-1 (COX-1)-dependent TX generation or COX-2-dependent biosynthesis by inflammatory cells and/or platelets, or a combination of the two, especially in clinical settings characterized by low-grade inflammation or enhanced platelet turnover. Although urinary 11-dehydro-TXB₂ levels are largely suppressed with low-dose aspirin, incomplete TXM suppression by aspirin predicts the future risk of vascular events and death in high-risk patients and may identify individuals who might benefit from treatments that more effectively block in vivo TX production or activity. Several disease-modifying agents, including lifestyle intervention, antidiabetic drugs and antiplatelet agents besides aspirin have been shown to reduce TX biosynthesis. Taken together, these aspects may contribute to the development of promising mechanism-based therapeutic strategies to reduce the progression of atherothrombosis. We intended to critically review current knowledge on both the pathophysiological significance of urinary TXM excretion in clinical settings related to ageing and atherothrombosis, as well as its prognostic value as a biomarker of vascular events.

Cyclic-Nucleotide- and HCN-Channel-Mediated Phototransduction in Intrinsically Photosensitive Retinal Ganglion Cells

Non-image-forming vision in mammals is mediated primarily by melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs). In mouse M1-ipRGCs, by far the best-studied subtype, melanopsin activates PLCβ4 (phospholipase C-β4) to open TRPC6,7 channels, mechanistically similar to phototransduction in fly rhabdomeric (microvillous) photoreceptors. We report here that, surprisingly, mouse M4-ipRGCs rely on a different and hitherto undescribed melanopsin-driven, ciliary phototransduction mechanism involving cyclic nucleotide as the second messenger and HCN channels rather than CNG channels as the ion channel for phototransduction. Even more surprisingly, within an individual mouse M2-ipRGC, this HCN-channel-dependent, ciliary phototransduction pathway operates in parallel with the TRPC6,7-dependent rhabdomeric pathway. These findings reveal a complex heterogeneity in phototransduction among ipRGCs and, more importantly, break a general dogma about segregation of the two phototransduction motifs, likely with strong evolutionary implications.

Loss of Gαq impairs regulatory B-cell function

Background

Recent studies have shown a crucial role of Gαq in immune regulation, but how Gαq modulates regulatory B-cell (Breg) function is still unclear. We address this here.

Methods

CD19⁺IL-10⁺ Bregs of wild-type (WT) and Gnaq^−/− mice were analyzed by flow cytometry after stimulation by lipopolysaccharide. The WT and Gnaq^−/− Bregs were isolated and cocultured with WT CD4⁺CD25⁻ T cells in the presence of T-activator, and the proliferation of T cells and differentiation of regulatory T cells (Tregs) were analyzed by flow cytometry. We used inhibitors of PI3 kinase (PI3K), extracellular regulated protein kinases 1/2 (Erk1/2), and p38 mitogen-activated protein kinase (p38 MAPK) to detect the pathways involved in the regulation of Gαq on Breg differentiation, which were confirmed by western blot analysis. Furthermore, the expression level of Gαq was assessed by quantitative real-time PCR in peripheral blood mononuclear cells (PBMCs) from healthy controls and rheumatoid arthritis patients. The frequency of CD19⁺CD24^hiCD38^hi B cells in PBMCs was detected by flow cytometry, and the association of the Gαq mRNA expression level and the frequency of CD19⁺CD24^hiCD38^hi B cells was analyzed by Spearman test.

Results

The differentiation of CD19⁺IL-10⁺ Bregs was inhibited in the Gnaq^−/− mice. In addition, Gαq depletion showed an impaired suppressive function of Bregs on T-cell proliferation, which might be due to the decreased Treg expansion. Mechanically, our data demonstrated that the PI3K, Erk1/2, and p38 MAPK signaling pathways were required for regulation of Gαq on Bregs, and blockage of these signaling pathways impaired Breg differentiation. Consistent with our previous studies, we also found a decreased frequency of CD19⁺CD24^hiCD38^hi Bregs in rheumatoid arthritis patients. As expected, a significantly positive correlation was investigated between CD19⁺CD24^hiCD38^hi Bregs with Gαq mRNA expression.

Conclusions

Our results indicate that Gαq plays a critical role in the differentiation and immunosuppression of Bregs, and it may provide a new therapeutic target for autoimmune diseases.

Electronic supplementary material

The online version of this article (10.1186/s13075-018-1682-0) contains supplementary material, which is available to authorized users.

Platelet Signaling and Disease: Targeted Therapy for Thrombosis and Other Related Diseases

Platelets are essential for clotting in the blood and maintenance of normal hemostasis. Under pathologic conditions such as atherosclerosis, vascular injury often results in hyperactive platelet activation, resulting in occlusive thrombus formation, myocardial infarction, and stroke. Recent work in the field has elucidated a number of platelet functions unique from that of maintaining hemostasis, including regulation of tumor growth and metastasis, inflammation, infection, and immune response. Traditional therapeutic targets for inhibiting platelet activation have primarily been limited to cyclooxygenase-1, integrin αIIbβ3, and the P2Y12 receptor. Recently identified signaling pathways regulating platelet function have made it possible to develop novel approaches for pharmacological intervention in the blood to limit platelet reactivity. In this review, we cover the newly discovered roles for platelets as well as their role in hemostasis and thrombosis. These new roles for platelets lend importance to the development of new therapies targeted to the platelet. Additionally, we highlight the promising receptor and enzymatic targets that may further decrease platelet activation and help to address the myriad of pathologic conditions now known to involve platelets without significant effects on hemostasis.

Protease-Activated Receptor 1 Inhibitors: Novel Antiplatelet Drugs in Prevention of Atherothrombosis

Protease-activated receptor (PAR)-1 inhibitors have recently become popular in the use of atherosclerosis among clinicians. Atherosclerosis can cause cardiovascular and cerebrovascular events leading to one of the major causes of mortality worldwide. Thrombin-mediated platelets can cause atherosclerotic plaques, and these platelets are activated by thrombin through the PAR-1. Vorapaxar and atopaxar are novel antiplatelet drugs that inhibit the thrombin-induced platelet activation by antagonizing the PAR-1. The objective of this article is to review the mechanism of action of vorapaxar and atopaxar and explain the rationale for using them in atherothrombosis patients including myocardial infarction, peripheral arterial disease, and stroke.

Retrospective Proteomic Analysis of a Novel, Cancer Metastasis-Promoting RGD-Containing Peptide

Patients who undergo surgical extirpation of a primary liver carcinoma followed by radiotherapy and chemotherapy leading to complete remission are nevertheless known to develop cancerous metastases 3-10 years later. We retrospectively examined the blood sera collected over 8 years from 30 patients who developed bone metastases after the complete remission of liver cancer to identify serum proteins showing differential expression compared to patients without remission. We detected a novel RGD (Arg-Gly-Asp)-containing peptide derived from the C-terminal portion of fibrinogen in the sera of metastatic patients that appeared to control the EMT (epithelial-mesenchymal transition) of cancer cells, in a process associated with miR-199a-3p. The RGD peptide enhanced new blood vessel growth and increased vascular endothelial growth factor levels when introduced into fertilized chicken eggs. The purpose of this study was to enable early detection of metastatic cancer cells using the novel RGD peptide as a biomarker, and thereby develop new drugs for the treatment of metastatic cancer.

Complement-activation fragment C4a mediates effector functions by binding as untethered agonist to protease-activated receptors 1 and 4

C4a is a small protein released from complement component C4 upon activation of the complement system's classical and lectin pathways, which are important constituents of innate immune surveillance. Despite the structural similarity between C4a and well-described anaphylatoxins C3a and C5a, the binding partner and biological function of C4a have remained elusive. Using a cell-based reporter assay, we screened C4a against a panel of both known and orphan G protein-coupled receptors and now provide evidence that C4a is a ligand for protease-activated receptor (PAR)1 and PAR4. Whereas C4a showed no activity toward known anaphylatoxin receptors, it acted as an agonist for both PAR1 and PAR4 with nanomolar activity. In human endothelial cells, ERK activation by C4a was mediated through both PAR1 and PAR4 in a Gα_i-independent signaling pathway. Like other PAR1 activators, C4a induced calcium mobilization through the PAR1/Gα_q/PLCβ signaling axis. Moreover, C4a increased stress fiber formation and enhanced endothelial permeability, both of which were reduced by PAR1 antagonists. In sum, our study identifies C4a as an untethered agonist for PAR1 and PAR4 with effects on cellular activation and endothelial permeability, thereby revealing another instance of cross-talk between the complement system and other host defense pathways.

New Concepts and Mechanisms of Platelet Activation Signaling

Upon blood vessel injury, platelets are exposed to adhesive proteins in the vascular wall and soluble agonists, which initiate platelet activation, leading to formation of hemostatic thrombi. Pathological activation of platelets can induce occlusive thrombosis, resulting in ischemic events such as heart attack and stroke, which are leading causes of death globally. Platelet activation requires intracellular signal transduction initiated by platelet receptors for adhesion proteins and soluble agonists. Whereas many platelet activation signaling pathways have been established for many years, significant recent progress reveals much more complex and sophisticated signaling and amplification networks. With the discovery of new receptor signaling pathways and regulatory networks, some of the long-standing concepts of platelet signaling have been challenged. This review provides an overview of the new developments and concepts in platelet activation signaling.

Placental hemostasis and sterile inflammation: New insights into gestational vascular disease

Activation of the coagulation and inflammatory systems are physiologically occurring during pregnancy. However, excess activation of either system is well documented in gestational vascular diseases (GVD). GVD are placenta-mediated pregnancy complications and a major cause of feto-maternal morbidity and mortality. The causal relevance of excess coagulation and inflammatory responses for GVD remains largely unknown. Deciphering the causal relationship of excess coagulation and inflammation in GVD may allow conceptualizing new therapeutic approaches to combat GVD. Platelet activation and procoagulant extracellular vesicles (EVs) provide a link between coagulation and inflammation and their activation or generation in GVD is well established. As recently shown EVs cause sterile placental inflammation by activating maternal platelets that release ATP and activate purinergic receptor signaling and NLRP3 inflammasome in the embryonic trophoblast. This thrombo-inflammatory mechanism suggests a novel link between coagulation activation and sterile inflammation in GVD. These findings highlight a role of anti-platelet therapies in GVD. In addition, targeting the inflammasome alone or in combination with platelet inhibition may provide a new therapeutic strategy in GVD.

Platelet Shape Changes and Cytoskeleton Dynamics as Novel Therapeutic Targets for Anti-Thrombotic Drugs

Platelets play an essential role in hemostasis through aggregation and adhesion to vascular injury sites but their unnecessary activation can often lead to thrombotic diseases. Upon exposure to physical or biochemical stimuli, remarkable platelet shape changes precede aggregation or adhesion. Platelets shape changes facilitate the formation and adhesion of platelet aggregates, but are readily reversible in contrast to the irrevocable characteristics of aggregation and adhesion. In this dynamic phenomenon, complex molecular signaling pathways and a host of diverse cytoskeleton proteins are involved. Platelet shape change is easily primed by diverse pro-thrombotic xenobiotics and stimuli, and its inhibition can modulate thrombosis, which can ultimately contribute to the development or prevention of thrombotic diseases. In this review, we discussed the current knowledge on the mechanisms of platelet shape change and also pathological implications and therapeutic opportunities for regulating the related cytoskeleton dynamics.

p45 NF-E2 regulates syncytiotrophoblast differentiation by post-translational GCM1 modifications in human intrauterine growth restriction

Placental insufficiency jeopardizes prenatal development, potentially leading to intrauterine growth restriction (IUGR) and stillbirth. Surviving fetuses are at an increased risk for chronic diseases later in life. IUGR is closely linked with altered trophoblast and placental differentiation. However, due to a paucity of mechanistic insights, suitable biomarkers and specific therapies for IUGR are lacking. The transcription factor p45 NF-E2 (nuclear factor erythroid derived 2) has been recently found to regulate trophoblast differentiation in mice. The absence of p45 NF-E2 in trophoblast cells causes IUGR and placental insufficiency in mice, but mechanistic insights are incomplete and the relevance of p45 NF-E2 for human syncytiotrophoblast differentiation remains unknown. Here we show that p45 NF-E2 negatively regulates human syncytiotrophoblast differentiation and is associated with IUGR in humans. Expression of p45 NF-E2 is reduced in human placentae complicated with IUGR compared with healthy controls. Reduced p45 NF-E2 expression is associated with increased syncytiotrophoblast differentiation, enhanced glial cells missing-1 (GCM1) acetylation and GCM1 desumoylation in IUGR placentae. Induction of syncytiotrophoblast differentiation in BeWo and primary villous trophoblast cells with 8-bromo-adenosine 3',5'-cyclic monophosphate (8-Br-cAMP) reduces p45 NF-E2 expression. Of note, p45 NF-E2 knockdown is sufficient to increase syncytiotrophoblast differentiation and GCM1 expression. Loss of p45 NF-E2 using either approach resulted in CBP-mediated GCM1 acetylation and SENP-mediated GCM1 desumoylation, demonstrating that p45 NF-E2 regulates post-translational modifications of GCM1. Functionally, reduced p45 NF-E2 expression is associated with increased cell death and caspase-3 activation in vitro and in placental tissues samples. Overexpression of p45 NF-E2 is sufficient to repress GCM1 expression, acetylation and desumoylation, even in 8-Br-cAMP exposed BeWo cells. These results suggest that p45 NF-E2 negatively regulates differentiation and apoptosis activation of human syncytiotrophoblast by modulating GCM1 acetylation and sumoylation. These studies identify a new pathomechanism related to IUGR in humans and thus provide new impetus for future studies aiming to identify new biomarkers and/or therapies of IUGR.

Epidermal loss of Gαq confers a migratory and differentiation defect in keratinocytes

G-protein coupled receptors (GPCRs), which activate heterotrimeric G proteins, are an essential class of transmembrane receptors that are responsible for a myriad of signaling events in normal and pathologic conditions. Two members of the G protein family, Gα_q and Gα₁₁, activate one of the main GPCR pathways and function as oncogenes by integrating mitogen-stimulated signaling cascades that are active under malignant conditions. Recently, it has been shown that targeted deletion of Gα₁₁ and Gα_q from endothelial cells impairs the Rho-mediated formation of focal adherens junctions, suggesting that Gα_11/q signaling may also play a significant role in cytoskeletal-mediated cellular responses in epithelial cells. Indeed, combined deletion of Gα₁₁ and Gα_q confers a significant migratory defect in keratinocytes that delays cutaneous wound healing in an in vivo setting. This delay can be attributed to a defect during the reepithelialization phase due to significantly attenuated migratory capacity of Gα_q-null keratinocytes under combined Gα₁₁ deficiency. In fact, cells lacking Gα_11/q demonstrate a severely reduced ability to respond to mitogenic and migratory signals in the microenvironment, leading to inappropriate and premature terminal differentiation. These results suggest that Gα_11/q signaling pathways may be critical for integrating mitogenic signals and cytoskeletal function to achieve normal physiological responses. Emergence of a malignant phenotype may therefore arise from both under- and overexpression of Gα_11/q signaling, implicating its upstream regulation as a potential therapeutic target in a host of pathologic conditions.

Receptor-interacting protein kinase 3 promotes platelet activation and thrombosis

Previous studies have shown that receptor-interacting protein kinase 3 (RIP3) is involved in many important biological processes, including necroptosis, apoptosis, and inflammation. Here we show that RIP3 plays a critical role in regulating platelet functions and in vivo thrombosis and hemostasis. Tail bleeding times were significantly longer in RIP3-knockout (RIP3^-/-) mice compared with their wild-type (WT) littermates. In an in vivo model of arteriole thrombosis, mice lacking RIP3 exhibited prolonged occlusion times. WT mice repopulated with RIP3^-/- bone marrow-derived cells had longer occlusion times than RIP3^-/- mice repopulated with WT bone marrow-derived cells, suggesting a role for RIP3-deficient platelets in arterial thrombosis. Consistent with these findings, we observed that RIP3 was expressed in both human and mice platelets. Deletion of RIP3 in mouse platelets caused a marked defect in aggregation and attenuated dense granule secretion in response to low doses of thrombin or a thromboxane A₂ analog, U46619. Phosphorylation of Akt induced by U46619 or thrombin was diminished in RIP3^-/- platelets. Moreover, RIP3 interacted with Gα₁₃ Platelet spreading on fibrinogen and clot retraction were impaired in the absence of RIP3. RIP3 inhibitor dose-dependently inhibited platelet aggregation in vitro and prevented arterial thrombus formation in vivo. These data demonstrate a role for RIP3 in promoting in vivo thrombosis and hemostasis by amplifying platelet activation. RIP3 may represent a novel promising therapeutic target for thrombotic diseases.

Timosaponin AIII induces antiplatelet and antithrombotic activity via Gq-mediated signaling by the thromboxane A2 receptor

The thromboxane (Tx) A₂ pathway is a major contributor to the amplification of initial platelet activation and is therefore a key drug target. To identify potent small-molecule inhibitors of the thromboxane prostaglandin (TP) receptor, we screened a small steroidal saponin library using U46619-induced rat platelet aggregation assays. Timosaponin AIII (TAIII) was identified as a potent inhibitor of U46619-induced rat platelet aggregation and exhibited superior selectivity for the TP receptor versus other G protein-coupled receptors and a PKC activator. TAIII inhibited U46619-induced rat platelet aggregation independent of increases in cAMP and cGMP and the inhibition of TxA2 production. Both PKC and PLC activators restored TAIII-inhibited platelet aggregation, whereas TAIII did not inhibit platelet aggregation induced by co-activation of the G_12/13 and G_z pathways. Furthermore, TAIII did not affect the platelet shape change or ROCK2 phosphorylation evoked by low-dose U46619. In vivo, TAIII prolonged tail bleeding time, reduced the mortality of animals with acute pulmonary thromboembolism and significantly reduced venous thrombus weight. Our study suggests that TAIII, by preferentially targeting Gq-mediated PLC/PKC signaling from the TP receptor, induces stronger in vitro antiplatelet activity and in vivo antithrombotic effects and may be an excellent candidate for the treatment of thrombotic disorders.

Platelets and hemostasis: a new perspective on an old subject

Publisher's Note: This article has a companion Counterpoint by Kapur and Semple. Publisher's Note: Join in the discussion of these articles at Blood Advances Community Conversations.

Loss of epithelial Gq and G11 signaling inhibits TGFβ production but promotes IL-33-mediated macrophage polarization and emphysema

Heterotrimeric guanine nucleotide-binding protein (G protein) signaling links hundreds of G protein-coupled receptors with four G protein signaling pathways. Two of these, one mediated by G_q and G₁₁ (G_q/11) and the other by G₁₂ and G₁₃ (G_12/13), are implicated in the force-dependent activation of transforming growth factor-β (TGFβ) in lung epithelial cells. Reduced TGFβ activation in alveolar cells leads to emphysema, whereas enhanced TGFβ activation promotes acute lung injury and idiopathic pulmonary fibrosis. Therefore, precise control of alveolar TGFβ activation is essential for alveolar homeostasis. We investigated the involvement of the G_q/11 and G_12/13 pathways in epithelial cells in generating active TGFβ and regulating alveolar inflammation. Mice deficient in both Gα_q and Gα₁₁ developed inflammation that was primarily caused by alternatively activated (M2-polarized) macrophages, enhanced matrix metalloproteinase 12 (MMP12) production, and age-related alveolar airspace enlargement consistent with emphysema. Mice with impaired G_q/11 signaling had reduced stretch-mediated generation of TGFβ by epithelial cells and enhanced macrophage MMP12 synthesis but were protected from the effects of ventilator-induced lung injury. Furthermore, synthesis of the cytokine interleukin-33 (IL-33) was increased in these alveolar epithelial cells, resulting in the M2-type polarization of alveolar macrophages independently of the effect on TGFβ. Our results suggest that alveolar G_q/11 signaling maintains alveolar homeostasis and likely independently increases TGFβ activation in response to the mechanical stress of the epithelium and decreases epithelial IL-33 synthesis. Together, these findings suggest that disruption of G_q/11 signaling promotes inflammatory emphysema but protects against mechanically induced lung injury.

Maternal extracellular vesicles and platelets promote preeclampsia via inflammasome activation in trophoblasts

Preeclampsia (PE) is a placenta-induced inflammatory disease associated with maternal and fetal morbidity and mortality. The mechanisms underlying PE remain enigmatic and delivery of the placenta is the only known remedy. PE is associated with coagulation and platelet activation and increased extracellular vesicle (EV) formation. However, thrombotic occlusion of the placental vascular bed is rarely observed and the mechanistic relevance of EV and platelet activation remains unknown. Here we show that EVs induce a thromboinflammatory response specifically in the placenta. Following EV injection, activated platelets accumulate particularly within the placental vascular bed. EVs cause adenosine triphosphate (ATP) release from platelets and inflammasome activation within trophoblast cells through purinergic signaling. Inflammasome activation in trophoblast cells triggers a PE-like phenotype, characterized by pregnancy failure, elevated blood pressure, increased plasma soluble fms-like tyrosine kinase 1, and renal dysfunction. Intriguingly, genetic inhibition of inflammasome activation specifically in the placenta, pharmacological inhibition of inflammasome or purinergic signaling, or genetic inhibition of maternal platelet activation abolishes the PE-like phenotype. Inflammasome activation in trophoblast cells of women with preeclampsia corroborates the translational relevance of these findings. These results strongly suggest that EVs cause placental sterile inflammation and PE through activation of maternal platelets and purinergic inflammasome activation in trophoblast cells, uncovering a novel thromboinflammatory mechanism at the maternal-embryonic interface.