SERCA2b and 3 play a regulatory role in store-operated calcium entry in human platelets

Carboxyl PEGylation of magnetic nanoparticles as antithrombotic and thrombolytic agents by calcium binding

Known to be biocompatible and hemocompatible, polyethylene glycol (PEG) has been widely used as anti-fouling coating of biomaterials. Nanoparticles coated with functionalized PEG were also investigated for their nano-cell interactions, but seldomly on the coagulation system, especially with platelets. Both experiments and molecular dynamic simulations indicate that terminal carboxylation of PEG promotes its binding with calcium, especially in the ionized form, which makes it potential anticoagulants. Further, the carboxyl PEGylated magnetic nanoparticle (HOOC-PEG₂₀₀₀-MNP) exhibits significantly increased anticoagulant and antiplatelet properties, by entering the open canalicular system (OCS) of human platelets and binding with the cytoplasmic calcium ions. HOOC-PEG₂₀₀₀-MNP also acts as effective thrombolytic agents in dissolving mature blood clots under oscillating magnetic field both in vitro and in vivo. Therefore, the carboxyl PEGylated magnetic nanoparticles are prototype agents for antithrombotic and thrombolytic therapies and provide a versatile platform for targeted and effective treatments of acute cardiovascular diseases.

Natural Polyphenols as SERCA Activators: Role in the Endoplasmic Reticulum Stress-Related Diseases

Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) is a key protein responsible for transporting Ca²⁺ ions from the cytosol into the lumen of the sarco/endoplasmic reticulum (SR/ER), thus maintaining Ca²⁺ homeostasis within cells. Accumulating evidence suggests that impaired SERCA function is associated with disruption of intracellular Ca²⁺ homeostasis and induction of ER stress, leading to different chronic pathological conditions. Therefore, appropriate strategies to control Ca²⁺ homeostasis via modulation of either SERCA pump activity/expression or relevant signaling pathways may represent a useful approach to combat pathological states associated with ER stress. Natural dietary polyphenolic compounds, such as resveratrol, gingerol, ellagic acid, luteolin, or green tea polyphenols, with a number of health-promoting properties, have been described either to increase SERCA activity/expression directly or to affect Ca²⁺ signaling pathways. In this review, potential Ca²⁺-mediated effects of the most studied polyphenols on SERCA pumps or related Ca²⁺ signaling pathways are summarized, and relevant mechanisms of their action on Ca²⁺ regulation with respect to various ER stress-related states are depicted. All data were collected using scientific search tools (i.e., Science Direct, PubMed, Scopus, and Google Scholar).

Mechanisms Underlying Dichotomous Procoagulant COAT Platelet Generation-A Conceptual Review Summarizing Current Knowledge

Procoagulant platelets are a subtype of activated platelets that sustains thrombin generation in order to consolidate the clot and stop bleeding. This aspect of platelet activation is gaining more and more recognition and interest. In fact, next to aggregating platelets, procoagulant platelets are key regulators of thrombus formation. Imbalance of both subpopulations can lead to undesired thrombotic or bleeding events. COAT platelets derive from a common pro-aggregatory phenotype in cells capable of accumulating enough cytosolic calcium to trigger specific pathways that mediate the loss of their aggregating properties and the development of new adhesive and procoagulant characteristics. Complex cascades of signaling events are involved and this may explain why an inter-individual variability exists in procoagulant potential. Nowadays, we know the key agonists and mediators underlying the generation of a procoagulant platelet response. However, we still lack insight into the actual mechanisms controlling this dichotomous pattern (i.e., procoagulant versus aggregating phenotype). In this review, we describe the phenotypic characteristics of procoagulant COAT platelets, we detail the current knowledge on the mechanisms of the procoagulant response, and discuss possible drivers of this dichotomous diversification, in particular addressing the impact of the platelet environment during in vivo thrombus formation.

TRP Channels Interactome as a Novel Therapeutic Target in Breast Cancer

Breast cancer is one of the most frequent cancer types worldwide and the first cause of cancer-related deaths in women. Although significant therapeutic advances have been achieved with drugs such as tamoxifen and trastuzumab, breast cancer still caused 627,000 deaths in 2018. Since cancer is a multifactorial disease, it has become necessary to develop new molecular therapies that can target several relevant cellular processes at once. Ion channels are versatile regulators of several physiological- and pathophysiological-related mechanisms, including cancer-relevant processes such as tumor progression, apoptosis inhibition, proliferation, migration, invasion, and chemoresistance. Ion channels are the main regulators of cellular functions, conducting ions selectively through a pore-forming structure located in the plasma membrane, protein–protein interactions one of their main regulatory mechanisms. Among the different ion channel families, the Transient Receptor Potential (TRP) family stands out in the context of breast cancer since several members have been proposed as prognostic markers in this pathology. However, only a few approaches exist to block their specific activity during tumoral progress. In this article, we describe several TRP channels that have been involved in breast cancer progress with a particular focus on their binding partners that have also been described as drivers of breast cancer progression. Here, we propose disrupting these interactions as attractive and potential new therapeutic targets for treating this neoplastic disease.

New Fundamentals in Hemostasis

Hemostasis encompasses the tightly regulated processes of blood clotting, platelet activation, and vascular repair. After wounding, the hemostatic system engages a plethora of vascular and extravascular receptors that act in concert with blood components to seal off the damage inflicted to the vasculature and the surrounding tissue. The first important component that contributes to hemostasis is the coagulation system, while the second important component starts with platelet activation, which not only contributes to the hemostatic plug, but also accelerates the coagulation system. Eventually, coagulation and platelet activation are switched off by blood-borne inhibitors and proteolytic feedback loops. This review summarizes new concepts of activation of proteases that regulate coagulation and anticoagulation, to give rise to transient thrombin generation and fibrin clot formation. It further speculates on the (patho)physiological roles of intra- and extravascular receptors that operate in response to these proteases. Furthermore, this review provides a new framework for understanding how signaling and adhesive interactions between endothelial cells, leukocytes, and platelets can regulate thrombus formation and modulate the coagulation process. Now that the key molecular players of coagulation and platelet activation have become clear, and their complex interactions with the vessel wall have been mapped out, we can also better speculate on the causes of thrombosis-related angiopathies.

Thrombin-stimulated discharge of calcium stores in human platelets: analysis of experimental data

The purpose of this research was to analyze experimental data concerning thrombin-stimulated discharge of calcium stores in human platelets contained in calcium-free medium in view of better understanding the mechanisms involved in calcium fluxes. The model curves are reasonably close to experimental data; the parameters of the models are related to the properties of the entities responsible for control or maintenance of cytosolic calcium concentration. It has been shown that: (a) time-course of calcium concentration in cytosol of human platelets can be acceptably modeled on the basis of reasonable assumptions concerning agonist stimulated calcium redistribution in cellular compartments; (b) those assumptions are of fundamental importance for the model (c) some parameters of the model (taken arbitrarily) cannot be estimated independently of others from fitting the model to experimental data available; (d) special experiments are necessary to determine the unknown parameters; (e) agonist-stimulated change of the permeability of endomembrane of calcium stores can be regarded as a pulse of the permeability; it can be modeled as a sequence of transitions of the system from inactive to active and to inactive state again.

STIM1 restores coronary endothelial function in type 1 diabetic mice

RATIONALE

The endoplasmic reticulum (ER) is a major intracellular Ca(2+) store in endothelial cells (ECs). The Ca(2+) concentration in the ER greatly contributes to the generation of Ca(2+) signals that regulate endothelial functions. Many proteins, including stromal interaction molecule 1/2 (STIM1/2), Orai1/2/3, and sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase 3 (SERCA3), are involved in the ER Ca(2+) refilling after store depletion in ECs.

OBJECTIVE

This study is designed to examine the role of Ca(2+) in the ER in coronary endothelial dysfunction in diabetes.

METHODS AND RESULTS

Mouse coronary ECs (MCECs) isolated from diabetic mice exhibited (1) a significant decrease in the Ca(2+) mobilization from the ER when the cells were treated by SERCA inhibitor, and (2) significant downregulation of STIM1 and SERCA3 protein expression in comparison to the controls. Overexpression of STIM1 restored (1) the increase in cytosolic Ca(2+) concentration due to Ca(2+) leak from the ER in diabetic MCECs, (2) the Ca(2+) concentration in the ER, and (3) endothelium-dependent relaxation that was attenuated in diabetic coronary arteries.

CONCLUSIONS

Impaired ER Ca(2+) refilling in diabetic MCECs, due to the decrease in STIM1 protein expression, attenuates endothelium-dependent relaxation in diabetic coronary arteries, while STIM1 overexpression has a beneficial and therapeutic effect on coronary endothelial dysfunction in diabetes.

The first comprehensive and quantitative analysis of human platelet protein composition allows the comparative analysis of structural and functional pathways

Antiplatelet treatment is of fundamental importance in combatting functions/dysfunction of platelets in the pathogenesis of cardiovascular and inflammatory diseases. Dysfunction of anucleate platelets is likely to be completely attributable to alterations in posttranslational modifications and protein expression. We therefore examined the proteome of platelets highly purified from fresh blood donations, using elaborate protocols to ensure negligible contamination by leukocytes, erythrocytes, and plasma. Using quantitative mass spectrometry, we created the first comprehensive and quantitative human platelet proteome, comprising almost 4000 unique proteins, estimated copy numbers for ∼ 3700 of those, and assessed intersubject (4 donors) as well as intrasubject (3 different blood samples from 1 donor) variations of the proteome. For the first time, our data allow for a systematic and weighted appraisal of protein networks and pathways in human platelets, and indicate the feasibility of differential and comprehensive proteome analyses from small blood donations. Because 85% of the platelet proteome shows no variation between healthy donors, this study represents the starting point for disease-oriented platelet proteomics. In the near future, comprehensive and quantitative comparisons between normal and well-defined dysfunctional platelets, or between platelets obtained from donors at various stages of chronic cardiovascular and inflammatory diseases will be feasible.

Carbon nanotubes activate store-operated calcium entry in human blood platelets

Carbon nanotubes (CNTs) are known to potentiate arterial thrombosis in animal models, which raises serious safety issues concerning environmental or occupational exposure to CNTs and their use in various biomedical applications. We have shown previously that different CNTs, but not fullerene (nC60), induce the aggregation of human blood platelets. To date, however, a mechanism of potentially thrombogenic CNT-induced platelet activation has not been elucidated. Here we show that pristine multiwalled CNTs (MWCNTs) penetrate platelet plasma membrane without any discernible damage but interact with the dense tubular system (DTS) causing depletion of platelet intracellular Ca(2+) stores. This process is accompanied by the clustering of stromal interaction molecule 1 (STIM1) colocalized with Orai1, indicating the activation of store-operated Ca(2+) entry (SOCE). Our findings reveal the molecular mechanism of CNT-induced platelet activation which is critical in the evaluation of the biocompatibility of carbon nanomaterials with blood.

The Ca2+ pumps of the endoplasmic reticulum and Golgi apparatus

The various splice variants of the three SERCA- and the two SPCA-pump genes in higher vertebrates encode P-type ATPases of the P(2A) group found respectively in the membranes of the endoplasmic reticulum and the secretory pathway. Of these, SERCA2b and SPCA1a represent the housekeeping isoforms. The SERCA2b form is characterized by a luminal carboxy terminus imposing a higher affinity for cytosolic Ca(2+) compared to the other SERCAs. This is mediated by intramembrane and luminal interactions of this extension with the pump. Other known affinity modulators like phospholamban and sarcolipin decrease the affinity for Ca(2+). The number of proteins reported to interact with SERCA is rapidly growing. Here, we limit the discussion to those for which the interaction site with the ATPase is specified: HAX-1, calumenin, histidine-rich Ca(2+)-binding protein, and indirectly calreticulin, calnexin, and ERp57. The role of the phylogenetically older and structurally simpler SPCAs as transporters of Ca(2+), but also of Mn(2+), is also addressed.

Monitoring the intracellular store Ca2+ concentration in agonist-stimulated, intact human platelets by using Fluo-5N

BACKGROUND

Most Ca(2+) signaling research in platelets has relied solely on monitoring the cytosolic Ca(2+) concentration ([Ca(2+)](cyt)). Changes in [Ca(2+)](cyt) constitute the net effect of Ca(2+) fluxes into the cytosol across the plasma membrane (PM) and from intracellular stores, and Ca(2+) sequestration into the stores and Ca(2+) removal across the PM. This makes interpretation of the effects of pharmacologic or genetic interventions on Ca(2+) signaling difficult and subject to error.

OBJECTIVES

To validate the use of the low-affinity Ca(2+) indicator Fluo-5N to monitor the concentration of Ca(2+) in the intracellular stores ([Ca(2+)](st)) of human platelets as a first step in developing assays for a systems-level analysis of platelet Ca(2+) signaling.

METHODS

Fluo-5N-loaded and Fura-2-loaded human platelets were used to observe the effects of agonist stimulation and other manipulations on [Ca(2+)](cyt) and [Ca(2+)](st).

RESULTS

Fluo-5N fluorescence changed appropriately in response to compounds that induce passive depletion of intracellular Ca(2+) stores and to physiologic agonists. Ca(2+) reuptake inhibitors and blockers of Ca(2+) release channels had the expected effects on Fura-2 and Fluo-5N fluorescence. Agonist-evoked Ca(2+) release was reversed by Ca(2+) addition to the medium, and required intact Ca(2+) reuptake mechanisms. Store refilling was observed in the presence of sarcoplasmic/endoplasmic reticulum Ca(2+) -ATPase (SERCA) inhibitors and ionomycin, suggesting the presence of a non-SERCA Ca(2+) reuptake mechanism. Evidence for a role for Ca(2+) -induced Ca(2+) release in agonist-evoked responses was obtained.

CONCLUSIONS

Our data provide a validation of the use of Fluo-5N as a method for monitoring changes in [Ca(2+)](st) in human platelets.

The plasma membrane calcium ATPase modulates calcium homeostasis, intracellular signaling events and function in platelets

BACKGROUND

The plasma membrane calcium ATPase (PMCA) regulates localized signaling events in a variety of cell types, although its functional role in platelets remains undefined.

OBJECTIVES

To investigate the role of PMCA in determining platelet intracellular calcium concentration ([Ca²(+) ](i) ) at rest and following agonist stimulation, and to define the corresponding effects upon different stages of platelet activation.

METHODS

[Ca²(+) ](i) was continuously measured in Fura-2-loaded platelets and in vitro and in vivo functional analyses performed in the presence of the PMCA inhibitor carboxyeosin (CE).

RESULTS

Concentrations of CE that selectively inhibited Ca²(+) extrusion through PMCA were established in human platelets. [Ca²(+) ](i) was elevated by CE in resting platelets, although collagen-stimulated Ca²(+) release was reduced. Impaired Ca²(+) mobilization upon agonist stimulation was accompanied by reduced dense granule secretion and impaired platelet aggregation. Platelet aggregation responses were also reduced in PMCA4(-/-) mice and in an in vivo mouse model of platelet thromboembolism. Conversely, inhibition of PMCA promoted the early and later stages of platelet activation, observed as enhanced adhesion to fibrinogen, and accelerated clot retraction. Investigations into the signaling mechanisms underlying CE-mediated inhibition of platelet aggregation implicated cGMP-independent vasodilator-stimulated phosphoprotein phosphorylation.

CONCLUSIONS

Disruption of PMCA activity perturbs platelet Ca²(+) homeostasis and function in a time-dependent manner, demonstrating that PMCA differentially regulates Ca²(+) -dependent signaling events, and hence function, throughout the platelet activation process.

Dynamic interaction of hTRPC6 with the Orai1-STIM1 complex or hTRPC3 mediates its role in capacitative or non-capacitative Ca(2+) entry pathways

TRPC (canonical transient receptor potential) channel subunits have been shown to assemble into homo- or hetero-meric channel complexes, including different Ca2+-handling proteins, required for the activation of CCE (capacitative Ca2+ entry) or NCCE (non-CCE) pathways. In the present study we found evidence for the dynamic interaction between endogenously expressed hTRPC6 (human TRPC6) with either both Orai1 and STIM1 (stromal interaction molecule 1) or hTRPC3 to participate in CCE or NCCE. Electrotransjection of cells with an anti-hTRPC6 antibody, directed towards the C-terminal region, reduces CCE induced by TPEN [N,N,N',N'-tetrakis-(2-pyridylmethyl)-ethylenediamine], which reduces the intraluminal free Ca2+ concentration. Cell stimulation with thrombin or extensive Ca2+-store depletion by TG (thapsigargin)+ionomycin enhanced the interaction between hTRPC6 and the CCE proteins Orai1 and STIM1. In contrast, stimulation with the diacylglycerol analogue OAG (1-oleoyl-2-acetyl-sn-glycerol) displaces hTRPC6 from Orai1 and STIM1 and enhances the association between hTRPC6 and hTRPC3. The interaction between hTRPC6 and hTRPC3 was abolished by dimethyl-BAPTA [1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid] loading, which indicates that this phenomenon is Ca2+-dependent. These findings support the hypothesis that hTRPC6 participates both in CCE and NCCE through its interaction with the Orai1-STIM1 complex or hTRPC3 respectively.

Intracellular Ca2+ store depletion induces the formation of macromolecular complexes involving hTRPC1, hTRPC6, the type II IP3 receptor and SERCA3 in human platelets

Endogenously expressed human canonical transient receptor potential 1 (hTRPC1) and human canonical transient receptor potential 6 (hTRPC6) have been shown to play a role in store-operated Ca2+ entry (SOCE) in human platelets, where two mechanisms for SOCE, regulated by the dense tubular system (DTS) or the acidic granules, have been identified. In cells preincubated for 1 min with 100 microM flufenamic acid we show that hTRPC6 is involved in SOCE activated by both mechanisms, as demonstrated by selective depletion of the DTS or the acidic stores, using thapsigargin (TG) (10 nM) or 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ) (20 microM), respectively, although it is more relevant after acidic store depletion. Co-immunoprecipitation experiments indicated that depletion of both stores separately results in time-dependent interaction between hTRPC1 and hTRPC6, and also between both hTRPCs and the type II IP3 receptor (IP3RII). The latter was greater after treatment with TG. TBHQ-induced coupling between hTRPC1 and 6 was transient and decreased after 30s of treatment, while that induced by TG increased for at least 3 min. TBHQ induced association between SERCA3, located in the acidic stores, hTRPC1, hTRPC6 and Orai1. TBHQ also evoked coupling between SERCA3 and IP3RII, presumably located in the DTS, thus suggesting interplay between both Ca2+ stores. Similarly, TG induces the interaction of SERCA2b with hTRPC1 and 6 and the IP3RII. The interactions between hTRPC1, hTRPC6, IP3RII and SERCA3 were impaired by disruption of the microtubules, supporting a role for microtubules in Ca2+ homeostasis. In conclusion, the present data demonstrate for the first time that hTRPC1, hTRPC6, IP3RII and SERCA3 are parts of a macromolecular protein complex activated by depletion of the intracellular Ca2+ stores in human platelets.