Calcium Signalling through Ligand-Gated Ion Channels such as P2X1 Receptors in the Platelet and other Non-Excitable Cells

High-throughput assessment identifying major platelet Ca2+ entry pathways via tyrosine kinase-linked and G protein-coupled receptors

In platelets, elevated cytosolic Ca²⁺ is a crucial second messenger, involved in most functional responses, including shape change, secretion, aggregation and procoagulant activity. The platelet Ca²⁺ response consists of Ca²⁺ mobilization from endoplasmic reticulum stores, complemented with store-operated or receptor-operated Ca²⁺ entry pathways. Several channels can contribute to the Ca²⁺ entry, but their relative contribution is unclear upon stimulation of ITAM-linked receptors such as glycoprotein VI (GPVI) and G-protein coupled receptors such as the protease-activated receptors (PAR) for thrombin. We employed a 96-well plate high-throughput assay with Fura-2-loaded human platelets to perform parallel [Ca²⁺]_i measurements in the presence of EGTA or CaCl₂. Per agonist condition, this resulted in sets of EGTA, CaCl₂ and Ca²⁺ entry ratio curves, defined by six parameters, reflecting different Ca²⁺ ion fluxes. We report that threshold stimulation of GPVI or PAR, with a variable contribution of secondary mediators, induces a maximal Ca²⁺ entry ratio of 3-7. Strikingly, in combination with Ca²⁺-ATPase inhibition by thapsigargin, the maximal Ca²⁺ entry ratio increased to 400 (GPVI) or 40 (PAR), pointing to a strong receptor-dependent enhancement of store-operated Ca²⁺ entry. By pharmacological blockage of specific Ca²⁺ channels in platelets, we found that, regardless of GPVI or PAR stimulation, the Ca²⁺ entry ratio was strongest affected by inhibition of ORAI1 (2-APB, Synta66) > Na⁺/Ca²⁺ exchange (NCE) > P2×₁ (only initial). In contrast, inhibition of TRPC6, Piezo1/2 or STIM1 was without effect. Together, these data reveal ORAI1 and NCE as dominating Ca²⁺ carriers regulating GPVI- and PAR-induced Ca²⁺ entry in human platelets.

Inhibition of platelet aggregation by activation of platelet intermediate conductance Ca2+ -activated potassium channels

BACKGROUND

Within the vasculature platelets and endothelial cells play crucial roles in hemostasis and thrombosis. Platelets, like endothelial cells, possess intermediate conductance Ca2+ -activated K+ (IKCa ) channels and generate nitric oxide (NO). Although NO limits platelet aggregation, the role of IKCa channels in platelet function and NO generation has not yet been explored.

OBJECTIVES

We investigated whether IKCa channel activation inhibits platelet aggregation, and per endothelial cells, enhances platelet NO production.

METHODS

Platelets were isolated from human volunteers. Aggregometry, confocal microscopy, and a novel flow chamber model, the Quartz Crystal Microbalance (QCM) were used to assess platelet function. Flow cytometry was used to measure platelet NO production, calcium signaling, membrane potential, integrin αIIb /β3 activation, granule release, and procoagulant platelet formation.

RESULTS

Platelet IKCa channel activation with SKA-31 inhibited aggregation in a concentration-dependent manner, an effect reversed by the selective IKCa channel blocker TRAM-34. The QCM model along with confocal microscopy demonstrated that SKA-31 inhibited platelet aggregation under flow conditions. Surprisingly, IKCa activation by SKA-31 inhibited platelet NO generation, but this could be explained by a concomitant reduction in platelet calcium signaling. IKCa activation by SKA-31 also inhibited dense and alpha-granule secretion and integrin αIIb /β3 activation, but maintained platelet phosphatidylserine surface exposure as a measure of procoagulant response.

CONCLUSIONS

Platelet IKCa channel activation inhibits aggregation by reducing calcium-signaling and granule secretion, but not by enhancing platelet NO generation. IKCa channels may be novel targets for the development of antiplatelet drugs that limit atherothrombosis, but not coagulation.

Pannexin-1 channel opening is critical for COVID-19 pathogenesis

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) rapidly rampaged worldwide, causing a pandemic of coronavirus disease (COVID -19), but the biology of SARS-CoV-2 remains under investigation. We demonstrate that both SARS-CoV-2 spike protein and human coronavirus 229E (hCoV-229E) or its purified S protein, one of the main viruses responsible for the common cold, induce the transient opening of Pannexin-1 (Panx-1) channels in human lung epithelial cells. However, the Panx-1 channel opening induced by SARS-CoV-2 is greater and more prolonged than hCoV-229E/S protein, resulting in an enhanced ATP, PGE₂, and IL-1β release. Analysis of lung lavages and tissues indicate that Panx-1 mRNA expression is associated with increased ATP, PGE₂, and IL-1β levels. Panx-1 channel opening induced by SARS-CoV-2 spike protein is angiotensin-converting enzyme 2 (ACE-2), endocytosis, and furin dependent. Overall, we demonstrated that Panx-1 channel is a critical contributor to SARS-CoV-2 infection and should be considered as an alternative therapy.

Shiga toxin signals via ATP and its effect is blocked by purinergic receptor antagonism

Shiga toxin (Stx) is the main virulence factor of enterohemorrhagic Escherichia coli (EHEC), that cause gastrointestinal infection leading to hemolytic uremic syndrome. The aim of this study was to investigate if Stx signals via ATP and if blockade of purinergic receptors could be protective. Stx induced ATP release from HeLa cells and in a mouse model. Toxin induced rapid calcium influx into HeLa cells, as well as platelets, and a P2X1 receptor antagonist, NF449, abolished this effect. Likewise, the P2X antagonist suramin blocked calcium influx in Hela cells. NF449 did not affect toxin intracellular retrograde transport, however, cells pre-treated with NF449 exhibited significantly higher viability after exposure to Stx for 24 hours, compared to untreated cells. NF449 protected HeLa cells from protein synthesis inhibition and from Stx-induced apoptosis, assayed by caspase 3/7 activity. The latter effect was confirmed by P2X1 receptor silencing. Stx induced the release of toxin-positive HeLa cell- and platelet-derived microvesicles, detected by flow cytometry, an effect significantly reduced by NF449 or suramin. Suramin decreased microvesicle levels in mice injected with Stx or inoculated with Stx-producing EHEC. Taken together, we describe a novel mechanism of Stx-mediated cellular injury associated with ATP signaling and inhibited by P2X receptor blockade.

Development of a P2X1-eYFP receptor knock-in mouse to track receptors in real time

A P2X1-eYFP knock-in mouse was generated to study receptor expression and mobility in smooth muscle and blood cells. eYFP was added to the C-terminus of the P2X1R and replaced the native P2X1R. Fluorescence corresponding to P2X1-eYFPR was detected in urinary bladder smooth muscle, platelets and megakaryocytes. ATP-evoked currents from wild type and P2X1-eYFP isolated urinary bladder smooth muscle cells had the same peak current amplitude and time-course showing that the eYFP addition had no obvious effect on properties. Fluorescence recovery after photobleaching (FRAP) in bladder smooth muscle cells demonstrated that surface P2X1Rs are mobile and their movement is reduced following cholesterol depletion. Compared to the platelet and megakaryocyte, P2X1-eYFP fluorescence was negligible in red blood cells and the majority of smaller marrow cells. The spatial pattern of P2X1-eYFP fluorescence in the megakaryocyte along with FRAP assessment of mobility suggested that P2X1Rs are expressed extensively throughout the membrane invagination system of this cell type. The current study highlights that the spatiotemporal properties of P2X1R expression can be monitored in real time in smooth muscle cells and megakaryocytes/platelets using the eYFP knock-in mouse model.

Purinergic Calcium Signals in Tumor-Derived Endothelium

Tumor microenvironment is particularly enriched with extracellular ATP (eATP), but conflicting evidence has been provided on its functional effects on tumor growth and vascular remodeling. We have previously shown that high eATP concentrations exert a strong anti-migratory, antiangiogenic and normalizing activity on human tumor-derived endothelial cells (TECs). Since both metabotropic and ionotropic purinergic receptors trigger cytosolic calcium increase ([Ca2+]c), the present work investigated the properties of [Ca2+]c events elicited by high eATP in TECs and their role in anti-migratory activity. In particular, the quantitative and kinetic properties of purinergic-induced Ca2+ release from intracellular stores and Ca2+ entry from extracellular medium were investigated. The main conclusions are: (1) stimulation of TECs with high eATP triggers [Ca2+]c signals which include Ca2+ mobilization from intracellular stores (mainly ER) and Ca2+ entry through the plasma membrane; (2) the long-lasting Ca2+ influx phase requires both store-operated Ca2+ entry (SOCE) and non-SOCE components; (3) SOCE is not significantly involved in the antimigratory effect of high ATP stimulation; (4) ER is the main source for intracellular Ca2+ release by eATP: it is required for the constitutive migratory potential of TECs but is not the only determinant for the inhibitory effect of high eATP; (5) a complex interplay occurs among ER, mitochondria and lysosomes upon purinergic stimulation; (6) high eUTP is unable to inhibit TEC migration and evokes [Ca2+]c signals very similar to those described for eATP. The potential role played by store-independent Ca2+ entry and Ca2+-independent events in the regulation of TEC migration by high purinergic stimula deserves future investigation.

Endothelial FAM3A positively regulates post-ischaemic angiogenesis

Background

Angiogenesis improves reperfusion to the ischaemic tissue after vascular obstruction. The underlying molecular mechanisms of post-ischaemic angiogenesis are not clear. FAM3A belongs to the family with sequence similarity 3 (FAM3) genes, but its biological function in endothelial cells in regards to vascular diseases is not well understood.

Methods

Gain- and loss-of-function methods by adenovirus or associated-adenovirus (AAV) in different models were applied to investigate the effects of FAM3A on endothelial angiogenesis. Endothelial angiogenesis was analysed by tube formation, migration and proliferation in vitro, and the blood flow and capillary density in a hind limb ischaemic model in vivo.

Findings

Endothelial FAM3A expression is downregulated under hypoxic conditions. Overexpression of FAM3A promotes, but depletion of FAM3A suppresses, endothelial tube formation, proliferation and migration. Utilizing the mouse hind limb ischaemia model, we also observe that FAM3A overexpression can improve blood perfusion and increase capillary density, whereas FAM3A knockdown has the opposite effects. Mechanistically, mitochondrial FAM3A increases adenosine triphosphate (ATP) production and secretion; ATP binds to P2 receptors and then upregulates cytosolic free Ca²⁺ levels. Increased intracellular Ca²⁺ levels enhance phosphorylation of the transcriptional factor cAMP response element binding protein (CREB) and its recruitment to the VEGFA promoter, thus activating VEGFA transcription and the final endothelial angiogenesis.

Interpretation

In summary, our data demonstrate that FAM3A positively regulates angiogenesis through activation of VEGFA transcription, suggesting that FAM3A may constitute a novel molecular therapeutic target for ischaemic vascular disease.

Store-operated calcium entry in thrombosis and thrombo-inflammation

Cytosolic free calcium (Ca²⁺) is a second messenger regulating a wide variety of functions in blood cells, including adhesion, activation, proliferation and migration. Store-operated Ca²⁺ entry (SOCE), triggered by depletion of Ca²⁺ from the endoplasmic reticulum, provides a main mechanism of regulated Ca²⁺ influx in blood cells. SOCE is mediated and regulated by isoforms of the ion channel proteins ORAI and TRP, and the transmembrane Ca²⁺ sensors stromal interaction molecules (STIMs), respectively. This report provides an overview of the (patho)physiological importance of SOCE in blood cells implicated in thrombosis and thrombo-inflammation, i.e. platelets and immune cells. We also discuss the physiological consequences of dysregulated SOCE in platelets and immune cells and the potential of SOCE inhibition as a therapeutic option to prevent or treat arterial thrombosis as well as thrombo-inflammatory disease states such as ischemic stroke.