A role for the actin cytoskeleton in the initiation and maintenance of store-mediated calcium entry in human platelets. Evidence for conformational coupling

Regulation of store-operated Ca2+ entry by IP3 receptors independent of their ability to release Ca2

Loss of endoplasmic reticular (ER) Ca2+ activates store-operated Ca2+ entry (SOCE) by causing the ER localized Ca2+ sensor STIM to unfurl domains that activate Orai channels in the plasma membrane at membrane contact sites (MCS). Here, we demonstrate a novel mechanism by which the inositol 1,4,5 trisphosphate receptor (IP3R), an ER-localized IP3-gated Ca2+ channel, regulates neuronal SOCE. In human neurons, SOCE evoked by pharmacological depletion of ER-Ca2+ is attenuated by loss of IP3Rs, and restored by expression of IP3Rs even when they cannot release Ca2+, but only if the IP3Rs can bind IP3. Imaging studies demonstrate that IP3Rs enhance association of STIM1 with Orai1 in neuronal cells with empty stores; this requires an IP3-binding site, but not a pore. Convergent regulation by IP3Rs, may tune neuronal SOCE to respond selectively to receptors that generate IP3.

Calcium sequestration by human platelet acidic organelles is regulated by the actin cytoskeleton and autocrine 5-hydroxytryptamine

Human platelets regulate agonist-evoked Ca²⁺ signalling through Ca²⁺ release from and sequestration into acidic organelles. Previous studies have pharmacologically characterised the presence of a Ca²⁺-H⁺ exchanger in these organelles. This exchanger appears to regulate a secondary plateau phase in agonist-evoked cytosolic Ca²⁺ signals in fura-2-loaded human platelets. Here we demonstrate that cytochalasin D treatment removes the secondary plateau in ADP-evoked Ca²⁺ signals elicited in the absence of external Ca²⁺. This effect was reversed by pretreatment with nigericin, a K⁺/H⁺ exchanger that short-circuits the Ca²⁺-H⁺ exchanger. Using Fluo-5N- or Lysosensor Green-loaded cells, cytochalasin D was found to enhance Ca²⁺ sequestration into acidic organelles by preventing their alkalinisation. Additional experiments demonstrated that ADP-evoked alkalinisation of acidic organelles and subsequent slowing of acidic organellar Ca²⁺ sequestration was mediated by autocrine 5-HT signalling. Enhancing this 5-HT signalling using fluoxetine overcame the inhibitory effect of cytochalasin D on ADP-evoked Ca²⁺ signals, indicating that cytochalasin D interferes with 5-HT autocrine signalling. The ability of Cytochalasin D to interfere with autocrine 5-HT signalling was downstream of the 5-HT_2A receptor as secretion of [³H]-5-HT from ADP-stimulated human platelets was not reduced. These data provide the first evidence that the pH gradient across acidic organelles is dynamically regulated upon human platelet activation, and that this can play a significant role in controlling human platelet function by modulating Ca²⁺-H⁺ exchange and so [Ca²⁺]_i.

Noncanonical function of long myosin light chain kinase in increasing ER-PM junctions and augmentation of SOCE

Increased endothelial permeability leads to excessive exudation of plasma proteins and leukocytes in the interstitium, which characterizes several vascular diseases including acute lung injury. The myosin light chain kinase long (MYLK-L) isoform is canonically known to regulate the endothelial permeability by phosphorylating myosin light chain (MLC-P). Compared to the short MYLK isoform, MYLK-L contains an additional stretch of ~919 amino acid at the N-terminus of unknown function. We show that thapsigargin and thrombin-induced SOCE was markedly reduced in Mylk-L-/- endothelial cells (EC) or MYLK-L-depleted human EC. These agonists also failed to increase endothelial permeability in MYLK-L-depleted EC and Mylk-L-/- lungs, thus demonstrating the novel role of MYLK-L-induced SOCE in increasing vascular permeability. MYLK-L augmented SOCE by increasing endoplasmic reticulum (ER)-plasma membrane (PM) junctions and STIM1 translocation to these junctions. Transduction of N-MYLK domain (amino acids 1-919 devoid of catalytic activity) into Mylk-L-/- EC rescued SOCE to the level seen in control EC in a STIM1-dependent manner. N-MYLK-induced SOCE augmented endothelial permeability without MLC-P via an actin-binding motif, DVRGLL. Liposomal-mediated delivery of N-MYLK mutant but not ∆DVRGLL-N-MYLK mutant in Mylk-L-/- mice rescued vascular permeability increase in response to endotoxin, indicating that targeting of DVRGLL motif within MYLK-L may limit SOCE-induced vascular hyperpermeability.

Intracellular G-actin targeting of peripheral sensory neurons by the multifunctional engineered protein C2C confers relief from inflammatory pain

The engineered multifunctional protein C2C was tested for control of sensory neuron activity by targeted G-actin modification. C2C consists of the heptameric oligomer, C2II-CI, and the monomeric ribosylase, C2I. C2C treatment of sensory neurons and SH-SY5Y cells in vitro remodeled actin and reduced calcium influx in a reversible manner. C2C prepared using fluorescently labeled C2I showed selective in vitro C2I delivery to primary sensory neurons but not motor neurons. Delivery was dependent on presence of both C2C subunits and blocked by receptor competition. Immunohistochemistry of mice treated subcutaneously with C2C showed colocalization of subunit C2I with CGRP-positive sensory neurons and fibers but not with ChAT-positive motor neurons and fibers. The significance of sensory neuron targeting was pursued subsequently by testing C2C activity in the formalin inflammatory mouse pain model. Subcutaneous C2C administration reduced pain-like behaviors by 90% relative to untreated controls 6 h post treatment and similarly to the opioid buprenorphene. C2C effects were dose dependent, equally potent in female and male animals and did not change gross motor function. One dose was effective in 2 h and lasted 1 week. Administration of C2I without C2II-CI did not reduce pain-like behavior indicating its intracellular delivery was required for behavioral effect.

Calcium Signaling in Glioma Cells: The Role of Nucleotide Receptors

Calcium signaling is probably one of the evolutionary oldest and the most common way by which the signal can be transmitted from the cell environment to the cytoplasmic calcium binding effectors. Calcium signal is fast and due to diversity of calcium binding proteins it may have a very broad effect on cell behavior. Being a crucial player in neuronal transmission it is also very important for glia physiology. It is responsible for the cross-talk between neurons and astrocytes, for microglia activation and motility. Changes in calcium signaling are also crucial for the behavior of transformed glioma cells. The present chapter summarizes molecular mechanisms of calcium signal formation present in glial cells with a strong emphasis on extracellular nucleotide-evoked signaling pathways. Some aspects of glioma C6 signaling such as the cross-talk between P2Y₁ and P2Y₁₂ nucleotide receptors in calcium signal generation will be discussed in-depth, to show complexity of machinery engaged in formation of this signal. Moreover, possible mechanisms of modulation of the calcium signal in diverse environments there will be presented herein. Finally, the possible role of calcium signal in glioma motility is also discussed. This is a very important issue, since glioma cells, contrary to the vast majority of neoplastic cells, cannot spread in the body with the bloodstream and, at least in early stages of tumor development, may expand only by means of sheer motility.

SNARE-dependent membrane fusion initiates α-granule matrix decondensation in mouse platelets

Platelet α-granule cargo release is fundamental to both hemostasis and thrombosis. Granule matrix hydration is a key regulated step in this process, yet its mechanism is poorly understood. In endothelial cells, there is evidence for 2 modes of cargo release: a jack-in-the-box mechanism of hydration-dependent protein phase transitions and an actin-driven granule constriction/extrusion mechanism. The third alternative considered is a prefusion, channel-mediated granule swelling, analogous to the membrane "ballooning" seen in procoagulant platelets. Using thrombin-stimulated platelets from a set of secretion-deficient, soluble N-ethylmaleimide factor attachment protein receptor (SNARE) mutant mice and various ultrastructural approaches, we tested predictions of these mechanisms to distinguish which best explains the α-granule release process. We found that the granule decondensation/hydration required for cargo expulsion was (1) blocked in fusion-protein-deficient platelets; (2) characterized by a fusion-dependent transition in granule size in contrast to a preswollen intermediate; (3) determined spatially with α-granules located close to the plasma membrane (PM) decondensing more readily; (4) propagated from the site of granule fusion; and (5) traced, in 3-dimensional space, to individual granule fusion events at the PM or less commonly at the canalicular system. In sum, the properties of α-granule decondensation/matrix hydration strongly indicate that α-granule cargo expulsion is likely by a jack-in-the-box mechanism rather than by gradual channel-regulated water influx or by a granule-constriction mechanism. These experiments, in providing a structural and mechanistic basis for cargo expulsion, should be informative in understanding the α-granule release reaction in the context of hemostasis and thrombosis.

Transition from Actin-Driven to Water-Driven Cell Migration Depends on External Hydraulic Resistance

Cells in vivo can reside in diverse physical and biochemical environments. For example, epithelial cells typically live in a two-dimensional (2D) environment, whereas metastatic cancer cells can move through dense three-dimensional matrices. These distinct environments impose different kinds of mechanical forces on cells and thus potentially can influence the mechanism of cell migration. For example, cell movement on 2D flat surfaces is mostly driven by forces from focal adhesion and actin polymerization, whereas in confined geometries, it can be driven by water permeation. In this work, we utilize a two-phase model of the cellular cytoplasm in which the mechanics of the cytosol and the F-actin network are treated on an equal footing. Using conservation laws and simple force balance considerations, we are able to describe the contributions of water flux, actin polymerization and flow, and focal adhesions to cell migration both on 2D surfaces and in confined spaces. The theory shows how cell migration can seamlessly transition from a focal adhesion- and actin-based mechanism on 2D surfaces to a water-based mechanism in confined geometries.

Flavonoids and Platelet-Derived Thrombotic Disorders

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Thrombotic disorders are characterized by an increase in the probability of the formation of unnecessary thrombi that might be due to the activation of the coagulation cascade or the circulating platelets. Platelets or thrombocytes play an essential role in hemostasis but abnormal platelet function leads to the development of a number of cardiovascular complications, including thrombotic disorders. Under pathological conditions, platelets are associated with the development of different thrombotic disorders, including atherosclerosis, arterial thrombosis and stroke, deep venous thrombosis and pulmonary embolism; therefore, platelets are the target of a number of anti-thrombotic strategies. Flavonoids, a large group of polyphenols ubiquitously expressed in fruits and vegetables that have attracted considerable attention because of their benefits in human health, including the reduction of the risk of cardiovascular disease. Flavonoids have been reported to reduce platelet activity by attenuating agonist-induced GPIIb/IIIa receptor activation, mobilization of intracellular free Ca2+, granule exocytosis, as well as activation of different signaling molecules such as mitogen- activated protein kinases or phospholipases. This review summarizes the current studies concerning the modulation of platelet activation by flavonoids, giving especial attention to those events associated to thrombotic disorders.

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.

Drebrin Regulation of Calcium Signaling in Immune Cells

Store-operated Ca²⁺ channels are plasma membrane channels that are activated by depletion of intracellular Ca²⁺ stores, resulting in an increase in intracellular Ca²⁺; however, little is known about their regulation. Our work has shown that the immunosuppressant compound BTP2, which blocks Ca²⁺ influx into cells, interacts with the actin-reorganizing protein, drebrin. Here we review the role of drebrin in the regulation of calcium signaling, with a focus on immune cells.

Platelet WDR1 suppresses platelet activity and is associated with cardiovascular disease

Platelet activity plays a major role in hemostasis with increased platelet activity likely contributing to the pathogenesis of atherothrombosis. We sought to identify associations between platelet activity variability and platelet-related genes in healthy controls. Transcriptional profiling of platelets revealed that WD-40 repeat domain 1 (WDR1), an enhancer of actin-depolymerizing factor activity, is downregulated in platelet messenger RNA (mRNA) from subjects with a hyperreactive platelet phenotype. We used the human megakaryoblastic cell line MEG-01 as an in vitro model for human megakaryocytes and platelets. Stimulation of MEG-01 with thrombin reduced levels of WDR1 transcripts and protein. WDR1 knockdown (KD) in MEG-01 cells increased adhesion and spreading in both the basal and activated states, increased F-actin content, and increased the basal intracellular calcium concentration. Platelet-like particles (PLPs) produced by WDR1 KD cells were fewer in number but larger than PLPs produced from unmodified MEG-01 cells, and had significantly increased adhesion in the basal state and upon thrombin activation. In contrast, WDR1 overexpression reversed the WDR1 KD phenotype of megakaryocytes and PLPs. To translate the clinical significance of these findings, WDR1 expression was measured in platelet RNA from subjects with established cardiovascular disease (n = 27) and age- and sex-matched controls (n = 10). The WDR1 mRNA and protein level was significantly lower in subjects with cardiovascular disease. These data suggest that WDR1 plays an important role in suppressing platelet activity, where it alters the actin cytoskeleton dynamics, and downregulation of WDR1 may contribute to the platelet-mediated pathogenesis of cardiovascular disease.

Latrunculin B modulates electrophysiological characteristics and arrhythmogenesis in pulmonary vein cardiomyocytes

AF (atrial fibrillation) is the most common sustained arrhythmia, and the PVs (pulmonary veins) play a critical role in triggering AF. Stretch causes structural remodelling, including cytoskeleton rearrangement, which may play a role in the genesis of AF. Lat-B (latrunculin B), an inhibitor of actin polymerization, is involved in Ca(2+) regulation. However, it is unclear whether Lat-B directly modulates the electrophysiological characteristics and Ca(2+) homoeostasis of the PVs. Conventional microelectrodes, whole-cell patch-clamp, and the fluo-3 fluorimetric ratio technique were used to record ionic currents and intracellular Ca(2+) within isolated rabbit PV preparations, or within isolated single PV cardiomyocytes, before and after administration of Lat-B (100 nM). Langendorff-perfused rabbit hearts were exposed to acute and continuous atrial stretch, and we studied PV electrical activity. Lat-B (100 nM) decreased the spontaneous electrical activity by 16±4% in PV preparations. Lat-B (100 nM) decreased the late Na(+) current, L-type Ca(2+) current, Na(+)/Ca(2+) exchanger current, and stretch-activated BKCa current, but did not affect the Na(+) current in PV cardiomyocytes. Lat-B reduced the transient outward K(+) current and ultra-rapid delayed rectifier K(+) current, but increased the delayed rectifier K(+) current in isolated PV cardiomyocytes. In addition, Lat-B (100 nM) decreased intracellular Ca(2+) transient and sarcoplasmic reticulum Ca(2+) content in PV cardiomyocytes. Moreover, Lat-B attenuated stretch-induced increased spontaneous electrical activity and trigger activity. The effects of Lat-B on the PV spontaneous electrical activity were attenuated in the presence of Y-27632 [10 μM, a ROCK (Rho-associated kinase) inhibitor] and cytochalasin D (10 μM, an actin polymerization inhibitor). In conclusion, Lat-B regulates PV electrophysiological characteristics and attenuates stretch-induced arrhythmogenesis.

Store-operated Ca2+ Entry-associated Regulatory factor (SARAF) Plays an Important Role in the Regulation of Arachidonate-regulated Ca2+ (ARC) Channels

The store-operated Ca(2+)entry-associated regulatory factor (SARAF) has recently been identified as a STIM1 regulatory protein that facilitates slow Ca(2+)-dependent inactivation of store-operated Ca(2+)entry (SOCE). Both the store-operated channels and the store-independent arachidonate-regulated Ca(2+)(ARC) channels are regulated by STIM1. In the present study, we show that, in addition to its location in the endoplasmic reticulum, SARAF is constitutively expressed in the plasma membrane, where it can interact with plasma membrane (PM)-resident ARC forming subunits in the neuroblastoma cell line SH-SY5Y. Using siRNA-based and overexpression approaches we report that SARAF negatively regulates store-independent Ca(2+)entry via the ARC channels. Arachidonic acid (AA) increases the association of PM-resident SARAF with Orai1. Finally, our results indicate that SARAF modulates the ability of AA to promote cell survival in neuroblastoma cells. In addition to revealing new insight into the biology of ARC channels in neuroblastoma cells, these findings provide evidence for an unprecedented location of SARAF in the plasma membrane.

Historical Overview of Store-Operated Ca(2+) Entry

Calcium influx is an essential mechanism for the activation of cellular functions both in excitable and non-excitable cells. In non-excitable cells, activation of phospholipase C by occupation of G protein-coupled receptors leads to the generation of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG), which, in turn, initiate two Ca(2+) entry pathways: Ca(2+) release from intracellular Ca(2+) stores, signaled by IP3, leads to the activation of store-operated Ca(2+) entry (SOCE); on the other hand, DAG activates a distinct second messenger-operated pathway. SOCE is regulated by the filling state of the intracellular calcium stores. The search for the molecular components of SOCE has identified the stromal interaction molecule 1 (STIM1) as the Ca(2+) sensor in the endoplasmic reticulum and Orai1 as a store-operated channel (SOC) subunit. Furthermore, a number of reports have revealed that several members of the TRPC family of channels also take part of the SOC macromolecular complex. This introductory chapter summarizes the early pieces of evidence that led to the concept of SOCE and the components of the store-operated signaling pathway.

Regulation of Platelet Function by Orai, STIM and TRP

Agonist-induced changes in cytosolic Ca(2+) concentration ([Ca(2+)]c) are central events in platelet physiology. A major mechanism supporting agonist-induced Ca(2+) signals is store-operated Ca(2+) entry (SOCE), where the Ca(2+) sensor STIM1 and the channels of the Orai family, as well as TRPC members are the key elements. STIM1-dependent SOCE plays a major role in collagen-stimulated Ca(2+) signaling, phosphatidylserine exposure and thrombin generation. Furthermore, studies involving Orai1 gain-of-function mutants and platelets from Orai1-deficient mice have revealed the importance of this channel in thrombosis and hemostasis to those found in STIM1-deficient mice indicating that SOCE might play a prominent role in thrombus formation. Moreover, increase in TRPC6 expression might lead to thrombosis in humans. The role of STIM1, Orai1 and TRPCs, and thus SOCE, in thrombus formation, suggests that therapies directed against SOCE and targeting these molecules during cardiovascular and cerebrovascular events could significantly improve traditional anti-thrombotic treatments.

Modest Interference with Actin Dynamics in Primary T Cell Activation by Antigen Presenting Cells Preferentially Affects Lamellal Signaling

Dynamic subcellular distributions of signaling system components are critical regulators of cellular signal transduction through their control of molecular interactions. Understanding how signaling activity depends on such distributions and the cellular structures driving them is required for comprehensive insight into signal transduction. In the activation of primary murine T cells by antigen presenting cells (APC) signaling intermediates associate with various subcellular structures, prominently a transient, wide, and actin-associated lamellum extending from an interdigitated T cell:APC interface several micrometers into the T cell. While actin dynamics are well established as general regulators of cellular organization, their role in controlling signaling organization in primary T cell:APC couples and the specific cellular structures driving it is unresolved. Using modest interference with actin dynamics with a low concentration of Jasplakinolide as corroborated by costimulation blockade we show that T cell actin preferentially controls lamellal signaling localization and activity leading downstream to calcium signaling. Lamellal localization repeatedly related to efficient T cell function. This suggests that the transient lamellal actin matrix regulates T cell signaling associations that facilitate T cell activation.

Cutting Edge: Drebrin-Regulated Actin Dynamics Regulate IgE-Dependent Mast Cell Activation and Allergic Responses

Mast cells play critical roles in allergic responses. Calcium signaling controls the function of these cells, and a role for actin in regulating calcium influx into cells has been suggested. We have previously identified the actin reorganizing protein Drebrin as a target of the immunosuppressant 3,5-bistrifluoromethyl pyrazole, which inhibits calcium influx into cells. In this study, we show that Drebrin(-/-) mice exhibit reduced IgE-mediated histamine release and passive systemic anaphylaxis, and Drebrin(-/-) mast cells also exhibit defects in FcεRI-mediated degranulation. Drebrin(-/-) mast cells exhibit defects in actin cytoskeleton organization and calcium responses downstream of the FcεRI, and agents that relieve actin reorganization rescue mast cell FcεRI-induced degranulation. Our results indicate that Drebrin regulates the actin cytoskeleton and calcium responses in mast cells, thus regulating mast cell function in vivo.

Store-operated calcium entry: unveiling the calcium handling signalplex

Store-operated Ca(2+) entry (SOCE) is an important mechanism for Ca(2+) influx in non-excitable cells, also present in excitable cells. The activation of store-operated channels (SOCs) is finely regulated by the filling state of the intracellular agonist-sensitive Ca(2+) compartments, and both, the mechanism of sensing the Ca(2+) stores and the nature and functional properties of the SOCs, have been a matter of intense investigation and debate. The identification of STIM1 as the endoplasmic reticulum Ca(2+) sensor and both Orai1, as the pore-forming subunit of the channels mediating the Ca(2+)-selective store-operated current, and the members of the TRPC subfamily of proteins, as the channels mediating the cation-permeable SOCs, has shed new light on the underlying events. This review summarizes the initial hypothesis and the current advances on the mechanism of activation of SOCE.

A role for actin polymerization in persistent pulmonary hypertension of the newborn

Persistent pulmonary hypertension of the newborn (PPHN) is defined as the failure of normal pulmonary vascular relaxation at birth. Hypoxia is known to impede postnatal disassembly of the actin cytoskeleton in pulmonary arterial myocytes, resulting in elevation of smooth muscle α-actin and γ-actin content in elastic and resistance pulmonary arteries in PPHN compared with age-matched controls. This review examines the original histological characterization of PPHN with attention to cytoskeletal structural remodeling and actin isoform abundance, reviews the existing evidence for understanding the biophysical and biochemical forces at play during neonatal circulatory transition, and specifically addresses the role of the cortical actin architecture, primarily identified as γ-actin, in the transduction of mechanical force in the hypoxic PPHN pulmonary circuit.

Systems modeling of Ca(2+) homeostasis and mobilization in platelets mediated by IP3 and store-operated Ca(2+) entry

Resting platelets maintain a stable level of low cytoplasmic calcium ([Ca(2+)]cyt) and high dense tubular system calcium ([Ca(2+)]dts). During thrombosis, activators cause a transient rise in inositol trisphosphate (IP3) to trigger calcium mobilization from stores and elevation of [Ca(2+)]cyt. Another major source of [Ca(2+)]cyt elevation is store-operated calcium entry (SOCE) through plasmalemmal calcium channels that open in response to store depletion as [Ca(2+)]dts drops. A 34-species systems model employed kinetics describing IP3-receptor, DTS-plasmalemma puncta formation, SOCE via assembly of STIM1 and Orai1, and the plasmalemma and sarco/endoplasmic reticulum Ca(2+)-ATPases. Four constraints were imposed: calcium homeostasis before activation; stable in zero extracellular calcium; IP3-activatable; and functional SOCE. Using a Monte Carlo method to sample three unknown parameters and nine initial concentrations in a 12-dimensional space near measured or expected values, we found that model configurations that were responsive to stimuli and demonstrated significant SOCE required high inner membrane electric potential (>-70 mV) and low resting IP3 concentrations. The absence of puncta in resting cells was required to prevent spontaneous store depletion in calcium-free media. Ten-fold increases in IP3 caused saturated calcium mobilization. This systems model represents a critical step in being able to predict platelets' phenotypes during hemostasis or thrombosis.

Myeloperoxidase modulates human platelet aggregation via actin cytoskeleton reorganization and store-operated calcium entry

Myeloperoxidase (MPO) is a heme-containing enzyme released from activated leukocytes into the extracellular space during inflammation. Its main function is the production of hypohalous acids that are potent oxidants. MPO can also modulate cell signaling and inflammatory responses independently of its enzymatic activity. Because MPO is regarded as an important risk factor for cardiovascular diseases associated with increased platelet activity, we studied the effects of MPO on human platelet functional properties. Laser scanning confocal microscopy was used to reveal carbohydrate-independent MPO binding to human platelet membrane. Adding MPO to platelets did not activate their aggregation under basal conditions (without agonist). In contrast, MPO augmented agonist-induced platelet aggregation, which was not prevented by MPO enzymatic activity inhibitors. It was found that exposure of platelets to MPO leads to actin cytoskeleton reorganization and an increase in their elasticity. Furthermore, MPO evoked a rise in cytosolic Ca²⁺ through enhancement of store-operated Ca²⁺ entry (SOCE). Together, these findings indicate that MPO is not a direct agonist but rather a mediator that binds to human platelets, induces actin cytoskeleton reorganization and affects the mechanical stiffness of human platelets, resulting in potentiating SOCE and agonist-induced human platelet aggregation. Therefore, an increased activity of platelets in vascular disease can, at least partly, be provided by MPO elevated concentrations.

Cytoskeletal and scaffolding proteins as structural and functional determinants of TRP channels

Transient receptor potential (TRP) channels are six transmembrane-spanning proteins, with variable selectivity for cations, that play a relevant role in intracellular Ca(2+) homeostasis. There is a large body of evidence that shows association of TRP channels with the actin cytoskeleton or even the microtubules and demonstrating the functional importance of this interaction for TRP channel function. Conversely, cation currents through TRP channels have also been found to modulate cytoskeleton rearrangements. The interplay between TRP channels and the cytoskeleton has been demonstrated to be essential for full activation of a variety of cellular functions. Furthermore, TRP channels have been reported to take part of macromolecular complexes including different signal transduction proteins. Scaffolding proteins play a relevant role in the association of TRP proteins with other signaling molecules into specific microdomains. Especially relevant are the roles of the Homer family members for the regulation of TRPC channel gating in mammals and INAD in the modulation of Drosophila TRP channels. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Calcium signaling in glioma cells--the role of nucleotide receptors

Calcium signaling is probably one of the evolutionary oldest and the most common way by which the signal can be transmitted from the cell environment to the cytoplasmic calcium binding effectors. Calcium signal is fast and due to diversity of calcium binding proteins it may have a very broad effect on cell behavior. Being a crucial player in neuronal transmission it is also very important for glia physiology. It is responsible for the cross-talk between neurons and astrocytes, for microglia activation and motility. Changes in calcium signaling are also crucial for the behavior of transformed glioma cells. The present Chapter summarizes molecular mechanisms of calcium signal formation present in glial cells with a strong emphasis on extracellular nucleotide-evoked signaling pathways. Some aspects of glioma C6 signaling such as the cross-talk between P2Y(1) and P2Y(12) nucleotide receptors in calcium signal generation will be discussed in-depth, to show complexity of machinery engaged in formation of this signal. Moreover, possible mechanisms of modulation of the calcium signal in diverse environments there will be presented herein. Finally, the possible role of calcium signal in glioma motility is also discussed. This is a very important issue, since glioma cells, contrary to the vast majority of neoplastic cells, cannot spread in the body with the bloodstream and, at least in early stages of tumor development, may expand only by means of sheer motility.

FKBP52 is involved in the regulation of SOCE channels in the human platelets and MEG 01 cells

Immunophilins are FK506-binding proteins that have been involved in the regulation of calcium homeostasis, either by modulating Ca(2+) channels located in the plasma membrane or in the rough endoplasmic reticulum (RE). We have investigated whether immunophilins would participate in the regulation of stored-operated Ca(2+) entry (SOCE) in human platelets and MEG 01. Both cell types were loaded with fura-2 for determining cytosolic calcium concentration changes ([Ca(2+)](c)), or stimulated and fixed to evaluate the protein interaction profile by performing immunoprecipitation and western blotting. We have found that incubation of platelets with FK506 increases Ca(2+) mobilization. Thapsigargin (TG)-evoked, Thr-evoked SOCE and TG-evoked Mn(2+) entry resulted in significant reduction by treatment of platelets with immunophilin antagonists. We confirmed by immunoprecipitation that immunophilins interact with transient receptor potential channel 1 (TRPC1) and Orai1 in human platelets. FK506 and rapamycin reduced the association between TRPC1 and Orai1 with FK506 binding protein (52) (FKBP52) in human platelets, and between TRPC1 and the type II IP(3)R, which association is known to be crucial for the maintenance of SOCE in human platelets. FKBP52 role in SOCE activation was confirmed by silencing FKBP52 using SiRNA FKBP52 in MEG 01 as demonstrated by single cell configuration imaging technique. TRPC1 silencing and depletion of cell of TRPC1 and FKBP52 simultaneously, impair activation of SOCE evoked by TG in MEG 01. Finally, in MEG 01 incubated with FK506 we observed a reduction in TRPC1/FKBP52 coupling, and similarly, FKBP52 silencing reduced the association between IP3R type II and TRPC1 during SOCE. All together, these results demonstrate that immunophilins participate in the regulation of SOCE in human platelets.

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 tyrosine-phosphorylation is required for STIM1-Orai1 association in human platelets

Stromal interaction molecule 1 (STIM1) is a key element of the store-operated Ca(2+) entry mechanism (SOCE). Recently, regulation of STIM1 by glycosylation and phosphorylation on serine/threonine or proline residues has been described; however other modes of phosphorylation that are important for activating SOCE in platelets, such as tyrosine phosphorylation, have been poorly investigated. Here we investigate the latency of STIM1 phosphorylation on tyrosine residues during the first steps of SOCE activation. Human platelets were stimulated and fixed at desired times using rapid kinetic assays instruments, and immunoprecipitation and western blotting techniques were then used to investigate the pattern of STIM1 tyrosine phosphorylation during the first steps of SOCE activation. We have found that maximal STIM1 tyrosine phosphorylation occurred 2.5s after stimulation of human platelets with thapsigargin (Tg). STIM1 localized in the plasma membrane were also phosphorylated in platelets stimulated with Tg. By using chemical inhibitors that target different members of the Src family of tyrosine kinases (SKFs), two independent signaling pathways involved in STIM1 tyrosine phosphorylation during the first steps of SOCE activation were identified. We finally conclude that STIM1 tyrosine phosphorylation is a key event for the association of STIM1 with plasma membrane Ca(2+) channels such as Orai1, hence it is required for conducting SOCE activation.

The cytoskeleton plays a modulatory role in the association between STIM1 and the Ca2+ channel subunits Orai1 and TRPC1

Store-operated Ca(2+) entry (SOCE) is a major pathway for Ca(2+) influx in non-excitable cells. Recent studies favour a conformational coupling mechanism between the endoplasmic reticulum (ER) Ca(2+) sensor STIM1 and Ca(2+) permeable channels in the plasma membrane to explain SOCE. Previous studies have reported a role for the cytoskeleton modulating the activation of SOCE; therefore, here we have investigated whether the interaction between STIM1 and the Ca(2+) permeable channels is modulated by the actin or microtubular network. In HEK-293 cells, treatment with the microtubular disrupter colchicine enhanced both the activation of SOCE and the association between STIM1 and Orai1 or TRPC1 induced by thapsigargin (TG). Conversely, stabilization of the microtubules by paclitaxel attenuated TG-evoked activation of SOCE and the interaction between STIM1 and the Ca(2+) channels Orai1 and TRPC1, altogether suggesting that the microtubules act as a negative regulator of SOCE. Stabilization of the cortical actin filament layer results in inhibition of TG-evoked both association between STIM1, Orai1 and TRPC1 and SOCE. Interestingly, disruption of the actin filament network by cytochalasin D did not significantly modify TG-evoked association between STIM1 and Orai1 or TRPC1 but enhanced TG-stimulated SOCE. Finally, inhibition of calmodulin by calmidazolium enhances TG-evoked SOCE and disruption of the actin cytoskeleton results in inhibition of TG-evoked association of calmodulin with Orai1 and TRPC1. Thus, we demonstrate that the cytoskeleton plays an essential role in the regulation of SOCE through the modulation of the interaction between their main molecular components.

Constitutive recycling of the store-operated Ca2+ channel Orai1 and its internalization during meiosis

The egg's competency to activate at fertilization and transition to embryogenesis is dependent on its ability to generate a fertilization-specific Ca(2+) transient. To endow the egg with this capacity, Ca(2+) signals remodel during oocyte maturation, including inactivation of the primary Ca(2+) influx pathway store-operated Ca(2+) entry (SOCE). SOCE inactivation is coupled to internalization of the SOCE channel, Orai1. In this study, we show that Orai1 internalizes during meiosis through a caveolin (Cav)- and dynamin-dependent endocytic pathway. Cav binds to Orai1, and we map a Cav consensus-binding site in the Orai1 N terminus, which is required for Orai1 internalization. Furthermore, at rest, Orai1 actively recycles between an endosomal compartment and the cell membrane through a Rho-dependent endocytic pathway. A significant percentage of total Orai1 is intracellular at steady state. Store depletion completely shifts endosomal Orai1 to the cell membrane. These results define vesicular trafficking mechanisms in the oocyte that control Orai1 subcellular localization at steady state, during meiosis, and after store depletion.

Protein kinase C alpha enhances sodium-calcium exchange during store-operated calcium entry in mouse platelets

A rise in intracellular calcium concentration ([Ca(2+)](i)) is necessary for platelet activation. A major component of the [Ca(2+)](i) elevation occurs through store-operated Ca(2+) entry (SOCE). The aim of this study was to understand the contribution of the classical PKC isoform, PKCα to platelet SOCE, using platelets from PKCα-deficient mice. SOCE was reduced by approximately 50% in PKCα(-/-) platelets, or following treatment with bisindolylmaleimide I, a PKC inhibitor. However, TG-induced Mn(2+) entry was unaffected, which suggests that divalent cation entry through store-operated channels is not directly regulated. Blocking the autocrine action of secreted ADP or 5-HT on its receptors did not reproduce the effect of PKCα deficiency. In contrast, SN-6, a Na(+)/Ca(2+) exchanger inhibitor, did reduce SOCE to the same extent as loss of PKCα, as did replacing extracellular Na(+) with NMDG(+). These treatments had no further effect in PKCα(-/-) platelets. These data suggest that PKCα enhances the extent of SOCE in mouse platelets by regulating Ca(2+) entry through the Na(+)/Ca(2+) exchanger.

Redox regulation of morphology, cell stiffness, and lectin-induced aggregation of human platelets

Redox regulation and carbohydrate recognition are potent molecular mechanisms which can contribute to platelet aggregation in response to various stimuli. The purpose of this study is to investigate the relationship between these mechanisms and to examine whether cell surface glycocalyx and cell stiffness of human platelets are sensitive to the redox potential formed by glutathione. To this end, human platelets were treated with different concentrations (0.05 μM to 6 mM) and ratios of reduced or oxidized glutathione (GSH or GSSG), and platelet morphological, mechanical, and functional properties were determined using conventional light microscopy, atomic force microscopy, and lectin-induced cell aggregation analysis. It was found that lowering the glutathione redox potential changed platelet morphology and increased platelet stiffness as well as modulated nonuniformly platelet aggregation in response to plant lectins with different carbohydrate-binding specificity including wheat germ agglutinin, Sambucus nigra agglutinin, and Canavalia ensiformis agglutinin. Extracellular redox potential and redox buffering capacity of the GSSG/2GSH couple were shown to control the availability of specific lectin-binding glycoligands on the cell surface, while the intracellular glutathione redox state affected the general functional ability of platelets to be aggregated independently of the type of lectins. Our data provide the first experimental evidence that glutathione as a redox molecule can affect the mechanical stiffness of human platelets and induce changes of the cell surface glycocalyx, which may represent a new mechanism of redox regulation of intercellular contacts.

beta-Dystroglycan binds caveolin-1 in smooth muscle: a functional role in caveolae distribution and Ca2+ release

The dystrophin-glycoprotein complex (DGC) links the extracellular matrix and actin cytoskeleton. Caveolae form membrane arrays on smooth muscle cells; we investigated the mechanism for this organization. Caveolin-1 and beta-dystroglycan, the core transmembrane DGC subunit, colocalize in airway smooth muscle. Immunoprecipitation revealed the association of caveolin-1 with beta-dystroglycan. Disruption of actin filaments disordered caveolae arrays, reduced association of beta-dystroglycan and caveolin-1 to lipid rafts, and suppressed the sensitivity and responsiveness of methacholine-induced intracellular Ca2+ release. We generated novel human airway smooth muscle cell lines expressing shRNA to stably silence beta-dystroglycan expression. In these myocytes, caveolae arrays were disorganized, caveolae structural proteins caveolin-1 and PTRF/cavin were displaced, the signaling proteins PLCbeta1 and G(alphaq), which are required for receptor-mediated Ca2+ release, were absent from caveolae, and the sensitivity and responsiveness of methacholine-induced intracellular Ca2+ release, was diminished. These data reveal an interaction between caveolin-1 and beta-dystroglycan and demonstrate that this association, in concert with anchorage to the actin cytoskeleton, underpins the spatial organization and functional role of caveolae in receptor-mediated Ca2+ release, which is an essential initiator step in smooth muscle contraction.

Lipid rafts modulate the activation but not the maintenance of store-operated Ca(2+) entry

Different studies have reported that proteins involved in Ca(2+) entry are localized in discrete plasma membrane domains known as lipid rafts, which have been suggested to support store-operated Ca(2+) entry by facilitating STIM1 clustering in endoplasmic reticulum-plasma membrane junctions as well as the interaction of STIM1 with TRPC1. Here we report that treatment of HEK293 cells with thapsigargin (TG) results in the activation of Ca(2+) entry with two components, an early, La(3+)-sensitive, component and a late component that shows both La(3+)-sensitive and -insensitive constituents. Preincubation with methyl-beta-cyclodextrin (MbetaCD) prevented TG-induced activation of Ca(2+) entry but, in contrast, enhanced this process after its activation. Addition of MbetaCD after store depletion did not modify the La(3+)-sensitive store-operated divalent cation entry but increased La(3+)-insensitive non-capacitative Ca(2+) entry. Cell stimulation with TG results in a transient increase in Orai1 co-immunoprecipitation with STIM1, TRPC1 and TRPC6. TG-induced association of these proteins was significantly attenuated by preincubation for 30 min with MbetaCD, without altering surface expression of Orai1 or TRPCs. In contrast, the association of Orai1 with STIM1 or TRPC1 was unaffected when MbetaCD was added after store depletion with TG. Addition of MbetaCD to TG-treated cells promoted dissociation between Orai1 and TRPC6, as well as non-capacitative Ca(2+) entry. TRPC6 expression silencing indicates that MbetaCD-enhanced non-capacitative Ca(2+) entry was mediated by TRPC6. In conclusion, lipid raft domains are necessary for the activation but not the maintenance of SOCE probably due to the support of the formation of Ca(2+) signalling complexes involving Orai1, TRPCs and STIM1.

Store-operated calcium entry channels in pulmonary endothelium: the emerging story of TRPCS and Orai1

Cells of diverse origin utilize shifts in cytosolic calcium concentrations as intracellular signals to elicit physiological responses. In endothelium, inflammatory first messengers increase cytosolic calcium as a signal to disrupt cell-cell borders and produce inter-cellular gaps. Calcium influx across the plasma membrane is required to initiate barrier disruption, although the calcium entry mechanism responsible for this effect remains poorly understood. This chapter highlights recent efforts to define the molecular anatomy of the ion channel responsible for triggering endothelial cell gap formation. Resolving the identity and function of this calcium channel will pave the way for new anti-inflammatory therapeutic targets.

Actin polymerisation regulates thrombin-evoked Ca2+signalling after activation of PAR-4 but not PAR-1 in human platelets

The role of actin polymerisation in regulating thrombin-evoked Ca(2+) signalling was investigated in human platelets. We have previously reported that cytochalasin D (Cyt D) inhibits thapsigargin-evoked store-operated Ca(2+) entry (SOCE), which is believed to contribute a major component of thrombin-evoked Ca(2+) entry in platelets. In contrast, Cyt D increased thrombin-evoked Ca(2+) entry to 147.5 +/- 9.2% and Sr(2+) entry to 134.2 +/- 6.4% of control. Similar results were obtained with latrunculin A. This potentiation was not affected if protein kinase C was inhibited using Ro-31-8220, suggesting that it did not involve PKC-dependent non-capacitative Ca(2+) entry. Ca(2+) entry evoked by the PAR-4 agonist, AYPGKF, was increased to 133.7 +/- 12.8% of control by Cyt D, whereas Ca(2+) signalling evoked by the PAR-1 agonist, SFLLRN, was unaffected. The PAR-4 antagonist, tcY-NH(2), abolished the effect of Cyt D on thrombin-evoked Ca(2+) entry. Biotinylation of cell-surface proteins showed that PAR-4 was internalised after stimulation by thrombin. Cyt D reduced this internalisation. These data suggest that Cyt D prevents the internalisation of PAR-4, which may lead to prolonged signalling from this receptor. This may mask a direct effect of Cyt D on the activation of SOCE after the activation of PAR-4.

Calcium signaling in platelets

Agonist-induced elevation in cytosolic Ca2+ concentrations is essential for platelet activation in hemostasis and thrombosis. It occurs through Ca2+ release from intracellular stores and Ca2+ entry through the plasma membrane (PM). Ca2+ store release is a well-established process involving phospholipase (PL)C-mediated production of inositol-1,4,5-trisphosphate (IP3), which in turn releases Ca2+ from the intracellular stores through IP3 receptor channels. In contrast, the mechanisms controlling Ca2+ entry and the significance of this process for platelet activation have been elucidated only very recently. In platelets, as in other non-excitable cells, the major way of Ca2+ entry involves the agonist-induced release of cytosolic sequestered Ca2+ followed by Ca2+ influx through the PM, a process referred to as store-operated calcium entry (SOCE). It is now clear that stromal interaction molecule 1 (STIM1), a Ca2+ sensor molecule in intracellular stores, and the four transmembrane channel protein Orai1 are the key players in platelet SOCE. The other major Ca2+ entry mechanism is mediated by the direct receptor-operated calcium (ROC) channel, P2X1. Besides these, canonical transient receptor potential channel (TRPC) 6 mediates Ca2+ entry through the PM. This review summarizes the current knowledge of platelet Ca2+ homeostasis with a focus on the newly identified Ca2+ entry mechanisms.

Microtubule remodeling mediates the inhibition of store-operated calcium entry (SOCE) during mitosis in COS-7 cells

Regulation of the intracellular calcium ion concentration ([Ca(2+)](i)) is critical, because calcium signaling controls diverse and vital cellular processes such as secretion, proliferation, division, gene transcription, and apoptosis. Store-operated calcium entry (SOCE) is the main mechanism through which non-excitable cells replenish and thus maintain this delicate balance. There is limited evidence which indicates that SOCE may be inhibited during mitosis, and the mechanisms leading to the presumed inhibition has not been elucidated. In the present study, we examined and compared the [Ca(2+)](i) dynamics of COS-7 cells in mitotic and non-mitotic phases with special reference paid to SOCE. Laser scanning confocal microscopy to monitor [Ca(2+)](i) dynamics revealed that SOCE was progressively inhibited in mitosis and became virtually absent during the metaphase. We used various cytoskeletal modifying drugs and immunofluorescence to assess the contribution of microtubule and actin filaments in SOCE signaling. Nocodazole treatment caused microtubule reorganization and retraction from the cell periphery that mimicked the natural mitotic microtubule remodeling that was also accompanied by SOCE inhibition. Short exposure to paclitaxel, a microtubule-stabilizing drug, bolstered SOCE, whereas long exposure resulted in microtubule disruption and SOCE inhibition. Actin-modifying drugs did not affect SOCE. These findings indicate that mitotic microtubule remodeling plays a significant role in the inhibition of SOCE during mitosis.

A key role for dense granule secretion in potentiation of the Ca2+ signal arising from store-operated calcium entry in human platelets

Recent work has demonstrated a role for Na(+)/Ca(2+) exchange in potentiation of the Ca(2+) entry elicited through the human platelet store-operated channel by controlling a Mn(2+)-impermeable Ca(2+) entry pathway. Here we demonstrate that this involves control over the secretion of dense granules by a Na(+)/Ca(2+) exchanger (NCX) and so autocrine signalling between platelets. NCX inhibition reduced dense granule secretion. The reduction in SOCE elicited by NCX inhibition could be reversed by the addition of uninhibited donor cells, their releasate alone, or exogenous ADP and 5-HT. The use of specific receptor antagonists indicated that ATP, ADP and 5-HT all played a role in NCX-dependent autocrine signalling between platelets following thapsigargin stimulation, by activating Mn(2+)-impermeable Ca(2+) entry pathways. These data provide further insight into the mechanisms underlying the known interrelationship between platelet Ca(2+) signalling and dense granule secretion, and suggest an important role for the NCX in potentiation of platelet activation via dense granule secretion and so autocrine signalling. Our results caution the interpretation of platelet Ca(2+) signalling studies involving pharmacological or other manipulations that do not assess possible effects on NCX activity and dense granule secretion.

Reduced levels of intracellular calcium releasing in spermatozoa from asthenozoospermic patients

BACKGROUND

Asthenozoospermia is one of the most common findings present in infertile males characterized by reduced or absent sperm motility, but its aetiology remains unknown in most cases. In addition, calcium is one of the most important ions regulating sperm motility. In this study we have investigated the progesterone-evoked intracellular calcium signal in ejaculated spermatozoa from men with normospermia or asthenozoospermia.

METHODS

Human ejaculates were obtained from healthy volunteers and asthenospermic men by masturbation after 4-5 days of abstinence. For determination of cytosolic free calcium concentration, spermatozoa were loaded with the fluorescent ratiometric calcium indicator Fura-2.

RESULTS

Treatment of spermatozoa from normospermic men with 20 micromolar progesterone plus 1 micromolar thapsigargin in a calcium free medium induced a typical transient increase in cytosolic free calcium concentration due to calcium release from internal stores. Similar results were obtained when spermatozoa were stimulated with progesterone alone. Subsequent addition of calcium to the external medium evoked a sustained elevation in cytosolic free calcium concentration indicative of capacitative calcium entry. However, when progesterone plus thapsigargin were administered to spermatozoa from patients with asthenozoospermia, calcium signal and subsequent calcium entry was much smaller compared to normospermic patients. As expected, pretreatment of normospermic spermatozoa with both the anti-progesterone receptor c262 antibody and with progesterone receptor antagonist RU-38486 decreased the calcium release induced by progesterone. Treatment of spermatozoa with cytochalasin D or jasplakinolide decreased the calcium entry evoked by depletion of internal calcium stores in normospermic patients, whereas these treatments proved to be ineffective at modifying the calcium entry in patients with asthenozoospermia.

CONCLUSION

Our results suggest that spermatozoa from asthenozoospermic patients present a reduced responsiveness to progesterone.

Roles of the actin-binding proteins in intracellular Ca2+ signalling

Starfish oocytes undergo massive intracellular Ca2+ signalling during meiotic maturation and fertilization. Although the igniting stimulus of Ca2+ mobilization may differ in different cell contexts, its final leverage is usually the Ca2+-releasing second messengers such as InsP3, cADPr and NAADP. The general scheme of intracellular Ca2+ release is that the corresponding receptors for these molecules serve as ion channels to release free Ca2+ from its internal stores such as the lumen of the endoplasmic reticulum. However, a growing body of evidence has suggested that intracellular Ca2+ release can be strongly modulated by the actin cytoskeleton. Although it is known that Ca2+ contributes to remodelling of the actin cytoskeleton, whether the actin cytoskeleton modulates Ca2+ signalling in return has not been much explored. An emerging candidate to answer to this reciprocal causality of Ca2+ and the actin cytoskeleton may be actin-binding proteins. In this review, we discuss how the actin cytoskeleton may fit into the known mechanisms of intracellular Ca2+ release, and propose two models to explain the experimental data.

Alteration of the cortical actin cytoskeleton deregulates Ca2+ signaling, monospermic fertilization, and sperm entry

BACKGROUND

When preparing for fertilization, oocytes undergo meiotic maturation during which structural changes occur in the endoplasmic reticulum (ER) that lead to a more efficient calcium response. During meiotic maturation and subsequent fertilization, the actin cytoskeleton also undergoes dramatic restructuring. We have recently observed that rearrangements of the actin cytoskeleton induced by actin-depolymerizing agents, or by actin-binding proteins, strongly modulate intracellular calcium (Ca2+) signals during the maturation process. However, the significance of the dynamic changes in F-actin within the fertilized egg has been largely unclear.

METHODOLOGY/PRINCIPAL FINDINGS

We have measured changes in intracellular Ca2+ signals and F-actin structures during fertilization. We also report the unexpected observation that the conventional antagonist of the InsP(3) receptor, heparin, hyperpolymerizes the cortical actin cytoskeleton in postmeiotic eggs. Using heparin and other pharmacological agents that either hypo- or hyperpolymerize the cortical actin, we demonstrate that nearly all aspects of the fertilization process are profoundly affected by the dynamic restructuring of the egg cortical actin cytoskeleton.

CONCLUSIONS/SIGNIFICANCE

Our findings identify important roles for subplasmalemmal actin fibers in the process of sperm-egg interaction and in the subsequent events related to fertilization: the generation of Ca2+ signals, sperm penetration, cortical granule exocytosis, and the block to polyspermy.

Orai1 mediates the interaction between STIM1 and hTRPC1 and regulates the mode of activation of hTRPC1-forming Ca2+ channels

Orai1 and hTRPC1 have been presented as essential components of store-operated channels mediating highly Ca(2+) selective I(CRAC) and relatively Ca(2+) selective I(SOC), respectively. STIM1 has been proposed to communicate the Ca(2+) content of the intracellular Ca(2+) stores to the plasma membrane store-operated Ca(2+) channels. Here we present evidence for the dynamic interaction between endogenously expressed Orai1 and both STIM1 and hTRPC1 regulated by depletion of the intracellular Ca(2+) stores, using the pharmacological tools thapsigargin plus ionomycin, or by the physiological agonist thrombin, independently of extracellular Ca(2+). In addition we report that Orai1 mediates the communication between STIM1 and hTRPC1, which is essential for the mode of activation of hTRPC1-forming Ca(2+) permeable channels. Electrotransjection of cells with anti-Orai1 antibody, directed toward the C-terminal region that mediates the interaction with STIM1, and stabilization of an actin cortical barrier with jasplakinolide prevented the interaction between STIM1 and hTRPC1. Under these conditions hTRPC1 was no longer involved in store-operated calcium entry but in diacylglycerol-activated non-capacitative Ca(2+) entry. These findings support the functional role of the STIM1-Orai1-hTRPC1 complex in the activation of store-operated Ca(2+) entry.

External calcium facilitates signalling, contractile and secretory mechanisms induced after activation of platelets by collagen

Platelet activation leads to the initiation of intracellular signalling processes, many of which are triggered by Ca2+. We have studied the involvement of exogenous Ca2+ in platelet response to collagen activation. Platelet suspensions were prepared with and without adding external calcium in the suspension buffers. Activation with collagen (Col-I) was carried out, before and after incubation with cytochalasin B (Cyt-B) to block the actin assembly and the cytoskeletal reorganization. We evaluated changes in (i) tyrosine phosphorylation of proteins, in platelet lysates and associated with the cytoskeletal fraction, (ii) the association of contractile proteins to the cytoskeleton, (iii) expression of intraplatelet substances at the surface, and (iv) cytosolic Ca2+ levels ([Ca2+]i). Ultrastructural evaluation of platelets by electron microscopy was also performed. Platelet activation by Col-I in the absence of added Ca2+ was followed by mild association of actin and other contractile proteins, low phosphorylation of proteins at tyrosine residues, lack of expression of intraplatelet substances at the membrane, and absence of aggregation. In the presence of millimolar Ca2+, Col-I induced intense actin filament formation with association of contractile proteins with the cytoskeleton, resulting in profound morphological changes. Under these conditions, Col-I induced signalling through tyrosine phosphorylation, with increases in the [Ca2+]i, release of intragranule content and aggregation. Inhibiting actin polymerization with Cyt-B prevented all these events. Our data indicates that platelet activation by collagen requires external Ca2+. Studies with Cyt-B indicate that assembly of new actin and cytoskeleton-mediated contraction, both dependent on exogenous Ca2+, are key events for platelet activation by collagen. In addition, our results confirm that entrance of exogenous Ca2+ depends on a functional cytoskeleton.