Plasma membrane Ca(2+)-ATPase associates with the cytoskeleton in activated platelets through a PDZ-binding domain

Tubulin Regulates Plasma Membrane Ca2+-ATPase Activity in a Lipid Environment-dependent Manner

Ca2+ plays a crucial role in cell signaling, cytosolic Ca2+ can change up to 10,000-fold in concentration due to the action of Ca2+-ATPases, including PMCA, SERCA and SCR. The regulation and balance of these enzymes are essential to maintain cytosolic Ca2+ homeostasis. Our laboratory has discovered a novel PMCA regulatory system, involving acetylated tubulin alone or in combination with membrane lipids. This regulation controls cytosolic Ca2+ levels and influences cellular properties such as erythrocyte rheology. This review summarizes the findings on the regulatory mechanism of PMCA activity by acetylated tubulin in combination with lipids. The combination of tubulin cytoskeleton and membrane lipids suggests a novel regulatory system for PMCA, which consequently affects cytosolic Ca2+ content, depending on cytoskeletal and plasma membrane dynamics. Understanding the interaction between acetylated tubulin, lipids and PMCA activity provides new insights into Ca2+ signaling and cell function. Further research may shed light on potential therapeutic targets for diseases related to Ca2+ dysregulation. This discovery contributes to a broader understanding of cellular processes and offers opportunities to develop innovative approaches to treat Ca2+-related disorders. By elucidating the complex regulatory mechanisms of Ca2+ homeostasis, we advance our understanding of cell biology and its implications for human health.

The Plasma Membrane Ca2+ Pump PMCA4b Regulates Melanoma Cell Migration through Remodeling of the Actin Cytoskeleton

Simple Summary

Earlier we demonstrated that the plasma membrane Ca²⁺ pump PMCA4b inhibits migration and metastatic activity of BRAF mutant melanoma cells, however, the exact mechanism has not been fully understood. Here we demonstrate that PMCA4b acted through actin cytoskeleton remodeling in generating a low migratory melanoma cell phenotype resulting in increased cell–cell connections, lamellipodia and stress fiber formation. Both proper trafficking and calcium transporting activity of the pump were essential to complete these tasks indicating that controlling Ca²⁺ concentration levels at specific plasma membrane locations such as the cell front played a role. Our findings suggest that PMCA4b downregulation is likely one of the mechanisms that leads to the perturbed cancer cell cytoskeleton organization resulting in enhanced melanoma cell migration and metastasis.

Abstract

We demonstrated that the plasma membrane Ca²⁺ ATPase PMCA4b inhibits migration and metastatic activity of BRAF mutant melanoma cells. Actin dynamics are essential for cells to move, invade and metastasize, therefore, we hypothesized that PMCA4b affected cell migration through remodeling of the actin cytoskeleton. We found that expression of PMCA4b in A375 BRAF mutant melanoma cells induced a profound change in cell shape, cell culture morphology, and displayed a polarized migratory character. Along with these changes the cells became more rounded with increased cell–cell connections, lamellipodia and stress fiber formation. Silencing PMCA4b in MCF-7 breast cancer cells had a similar effect, resulting in a dramatic loss of stress fibers. In addition, the PMCA4b expressing A375 cells maintained front-to-rear Ca²⁺ concentration gradient with the actin severing protein cofilin localizing to the lamellipodia, and preserved the integrity of the actin cytoskeleton from a destructive Ca²⁺ overload. We showed that both PMCA4b activity and trafficking were essential for the observed morphology and motility changes. In conclusion, our data suggest that PMCA4b plays a critical role in adopting front-to-rear polarity in a normally spindle-shaped cell type through F-actin rearrangement resulting in a less aggressive melanoma cell phenotype.

The role of Plasma Membrane Calcium ATPases (PMCAs) in neurodegenerative disorders

Selective degeneration of differentiated neurons in the brain is the unifying feature of neurodegenerative disorders such as Parkinson's disease (PD) or Alzheimer's disease (AD). A broad spectrum of evidence indicates that initially subtle, but temporally early calcium dysregulation may be central to the selective neuronal vulnerability observed in these slowly progressing, chronic disorders. Moreover, it has long been evident that excitotoxicity and its major toxic effector mechanism, neuronal calcium overload, play a decisive role in the propagation of secondary neuronal death after acute brain injury from trauma or ischemia. Under physiological conditions, neuronal calcium homeostasis is maintained by a fine-tuned interplay between calcium influx and releasing mechanisms (Ca²⁺-channels), and calcium efflux mechanisms (Ca²⁺-pumps and -exchangers). Central functional components of the calcium efflux machinery are the Plasma Membrane Calcium ATPases (PMCAs), which represent high-affinity calcium pumps responsible for the ATP-dependent removal of calcium out of the cytosol. Beyond a growing body of experimental evidence, it is their high expression level, their independence of secondary ions or membrane potential, their profound redox regulation and autoregulation, their postsynaptic localization in close proximity to the primary mediators of pathological calcium influx, i.e. NMDA receptors, as well as evolutionary considerations which all suggest a pivotal role of the PMCAs in the etiology of neurodegeneration and make them equally challenging and alluring candidates for drug development. This review aims to summarize the recent literature on the role of PMCAs in the pathogenesis of neurodegenerative disorders.

Regulation of the Plasma Membrane Calcium ATPases by the actin cytoskeleton

Associations between the cortical cytoskeleton and the components of the plasma membrane are no longer considered to be merely of structural and mechanical nature but are nowadays recognized as dynamic interactions that modulate a plethora of cellular responses. Reorganization of actin filaments upon diverse stimuli - among which is the rise in cytosolic Ca²⁺ - is involved in cell motility and adhesion, phagocytosis, cytokinesis, and secretion. Actin dynamics also participates in the regulation of ion transport across the membranes where it not only plays a key role in the delivery and stabilization of channels and transporters in the plasma membrane but also in the regulation of their activity. The recently described functional interaction between actin and the Plasma Membrane Ca²⁺-ATPase (PMCA) represents a novel regulatory mechanism of the pump at the time that unveils a new pathway by which the cortical actin cytoskeleton participates in the regulation of cytosolic Ca²⁺ homeostasis. In this review, we summarize the current knowledge on the interaction between the cortical actin cytoskeleton and the PMCA and discuss the possible mechanisms that may explain the pump's modulation.

Metabolic regulation of the PMCA: Role in cell death and survival

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The PMCA is an ATP-driven Ca²⁺ pump critical for the maintenance of low cytosolic calcium.

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The PMCA has an important but paradoxical role in cell death and survival.

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The PMCA can be differentially regulated by caspase/calpain cleavage.

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Glycolytic ATP supply may be sufficient to fuel the PMCA during metabolic stress.

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The ATP sensitivity of the PMCA can be regulated by acidic phospholipids.

The plasma membrane Ca²⁺-ATPase (PMCA) is a ubiquitously expressed, ATP-driven Ca²⁺ pump that is critical for maintaining low resting cytosolic Ca²⁺ ([Ca²⁺]_i) in all eukaryotic cells. Since cytotoxic Ca²⁺ overload has such a central role in cell death, the PMCA represents an essential “linchpin” for the delicate balance between cell survival and cell death. In general, impaired PMCA activity and reduced PMCA expression leads to cytotoxic Ca²⁺ overload and Ca²⁺ dependent cell death, both apoptosis and necrosis, whereas maintenance of PMCA activity or PMCA overexpression is generally accepted as being cytoprotective. However, the PMCA has a paradoxical role in cell death depending on the cell type and cellular context. The PMCA can be differentially regulated by Ca²⁺-dependent proteolysis, can be maintained by a localised glycolytic ATP supply, even in the face of global ATP depletion, and can be profoundly affected by the specific phospholipid environment that it sits within the membrane. The major focus of this review is to highlight some of the controversies surrounding the paradoxical role of the PMCA in cell death and survival, challenging the conventional view of ATP-dependent regulation of the PMCA and how this might influence cell fate.

Plasma membrane calcium ATPases: From generic Ca(2+) sump pumps to versatile systems for fine-tuning cellular Ca(2.)

The plasma membrane calcium ATPases (PMCAs) are ATP-driven primary ion pumps found in all eukaryotic cells. They are the major high-affinity calcium extrusion system for expulsion of Ca(2+) ions from the cytosol and help restore the low resting levels of intracellular [Ca(2+)] following the temporary elevation of Ca(2+) generated during Ca(2+) signaling. Due to their essential role in the maintenance of cellular Ca(2+) homeostasis they were initially thought to be "sump pumps" for Ca(2+) removal needed by all cells to avoid eventual calcium overload. The discovery of multiple PMCA isoforms and alternatively spliced variants cast doubt on this simplistic assumption, and revealed instead that PMCAs are integral components of highly regulated multi-protein complexes fulfilling specific roles in calcium-dependent signaling originating at the plasma membrane. Biochemical, genetic, and physiological studies in gene-manipulated and mutant animals demonstrate the important role played by specific PMCAs in distinct diseases including those affecting the peripheral and central nervous system, cardiovascular disease, and osteoporosis. Human PMCA gene mutations and allelic variants associated with specific disorders continue to be discovered and underline the crucial role of different PMCAs in particular cells, tissues and organs.

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.

Role of platelet plasma membrane Ca2+-ATPase in health and disease

Platelets have essential roles in both health and disease. Normal platelet function is required for hemostasis. Inhibition of platelet function in disease or by pharmacological treatment results in bleeding disorders. On the other hand, hyperactive platelets lead to heart attack and stroke. Calcium is a major second messenger in platelet activation, and elevated intracellular calcium leads to hyperactive platelets. Elevated platelet calcium has been documented in hypertension and diabetes; both conditions increase the likelihood of heart attack and stroke. Thus, proper regulation of calcium metabolism in the platelet is extremely important. Plasma membrane Ca²⁺-ATPase (PMCA) is a major player in platelet calcium metabolism since it provides the only significant route for calcium efflux. In keeping with the important role of calcium in platelet function, PMCA is a highly regulated transporter. In human platelets, PMCA is activated by Ca²⁺/calmodulin, by cAMP-dependent phosphorylation and by calpain-dependent removal of the inhibitory peptide. It is inhibited by tyrosine phosphorylation and calpain-dependent proteolysis. In addition, the cellular location of PMCA is regulated by a PDZ-domain-dependent interaction with the cytoskeleton during platelet activation. Rapid regulation by phosphorylation results in changes in the rate of platelet activation, whereas calpain-dependent proteolysis and interaction with the cytoskeleton appears to regulate later events such as clot retraction. In hypertension and diabetes, PMCA expression is upregulated while activity is decreased, presumably due to tyrosine phosphorylation. Clearly, a more complete understanding of PMCA function in human platelets could result in the identification of new ways to control platelet function in disease states.

Disruption of actin filaments induces mitochondrial Ca2+ release to the cytoplasm and [Ca2+]c changes in Arabidopsis root hairs

BACKGROUND

Mitochondria are dynamic organelles that move along actin filaments, and serve as calcium stores in plant cells. The positioning and dynamics of mitochondria depend on membrane-cytoskeleton interactions, but it is not clear whether microfilament cytoskeleton has a direct effect on mitochondrial function and Ca2+ storage. Therefore, we designed a series of experiments to clarify the effects of actin filaments on mitochondrial Ca2+ storage, cytoplasmic Ca2+ concentration ([Ca2+]c), and the interaction between mitochondrial Ca2+ and cytoplasmic Ca2+ in Arabidopsis root hairs.

RESULTS

In this study, we found that treatments with latrunculin B (Lat-B) and jasplakinolide (Jas), which depolymerize and polymerize actin filaments respectively, decreased membrane potential and Ca2+ stores in the mitochondria of Arabidopsis root hairs. Simultaneously, these treatments induced an instantaneous increase of cytoplasmic Ca2+, followed by a continuous decrease. All of these effects were inhibited by pretreatment with cyclosporin A (Cs A), a representative blocker of the mitochondrial permeability transition pore (mPTP). Moreover, we found there was a Ca2+ concentration gradient in mitochondria from the tip to the base of the root hair, and this gradient could be disrupted by actin-acting drugs.

CONCLUSIONS

Based on these results, we concluded that the disruption of actin filaments caused by Lat-B or Jas promoted irreversible opening of the mPTP, resulting in mitochondrial Ca2+ release into the cytoplasm, and consequent changes in [Ca2+]c. We suggest that normal polymerization and depolymerization of actin filaments are essential for mitochondrial Ca2+ storage in root hairs.

SERCA2b activity is regulated by cyclophilins in human platelets

OBJECTIVE

The role of cyclophilins (chaperones that are widely expressed in different cell types, including human platelets) was explored in sarcoendoplasmic calcium (Ca(2+)) adenosine triphosphatase (SERCA) activity.

METHODS AND RESULTS

Cyclophilin inhibition by cyclosporin A (CsA) evoked a time- and concentration-dependent reduction of Ca(2+) uptake by SERCA2b. However, other Ca(2+)-adenosine triphosphatases expressed in platelets, such as SERCA3 and plasma membrane Ca(2+) adenosine triphophatase, remained unaltered after CsA treatment. Cypermethrin, a non-CsA-related calcineurin inhibitor, did not alter SERCA2b activity. Furthermore, SERCA2b was affected by other CsA analogues, which do not interfere with calcineurin, such as PKF-211-811-NX5 (NIM811) and sanglifehrin A. Inhibition of the immunophilin family members using FK506 (tacrolimus) did not alter SERCA2b ability to sequester Ca(2+) into the dense tubular system. Coimmunoprecipitation experiments confirmed that cyclophilin A associates with SERCA2b and stromal interaction molecule-1 in resting platelets. This interaction is attenuated by the physiological agonist thrombin but enhanced by treatment with CsA or sanglifehrin A.

CONCLUSIONS

Cyclophilin A is a regulator of SERCA2b in human platelets.

How many Ca2+ATPase isoforms are expressed in a cell type? A growing family of membrane proteins illustrated by studies in platelets

Ca(2+) signaling plays a key role in normal and abnormal platelet functions. Understanding platelet Ca(2+) signaling requires the knowledge of proteins involved in this process. Among these proteins are Ca(2+)ATPases or Ca(2+) pumps that deplete the cytosol of Ca(2+) ions. Here, we will particularly focus on two Ca(2+) pump families: the plasma membrane Ca(2+)ATPases (PMCAs) that extrude cytosolic Ca(2+) towards the extracellular medium and the sarco/endoplasmic reticulum Ca(2+)ATPases (SERCAs) that pump Ca(2+) into the endoplasmic reticulum (ER). In the present review, we will summarize data on platelet Ca(2+)ATPases including their identification and biogenesis. First of all, we will present the Ca(2+)ATPase genes and their isoforms expressed in platelets. We will especially focus on a member of the SERCA family, SERCA3, recently found to give rise to a number of species-specific isoforms. Next, we will describe the differences in Ca(2+)ATPase patterns observed in human and rat platelets. Last, we will analyze how the expression of Ca(2+)ATPase isoforms changes during megakaryocytic maturation and show that megakaryocytopoiesis is associated with a profound reorganization of the expression and/or activity of Ca(2+)ATPases. Taken together, these data provide new aspects of investigations to better understand normal and abnormal platelet Ca(2+) signaling.

Muscarinic-induced recruitment of plasma membrane Ca2+-ATPase involves PSD-95/Dlg/Zo-1-mediated interactions

Efflux of cytosolic Ca2+ mediated by plasma membrane Ca2+-ATPases (PMCA) plays a key role in fine tuning the magnitude and duration of Ca2+ signaling following activation of G-protein-coupled receptors. However, the molecular mechanisms that underpin the trafficking of PMCA to the membrane during Ca2+ signaling remain largely unexplored in native cell models. One potential mechanism for the recruitment of proteins to the plasma membrane involves PDZ interactions. In this context, we investigated the role of PMCA interactions with the Na+/H+ exchanger regulatory factor 2 (NHERF-2) during muscarinic-induced Ca2+ mobilization in the HT-29 epithelial cell line. GST pull-downs in HT-29 cell lysates showed that the PDZ2 module of NHERF-2 bound to the PDZ binding motif on the C terminus of PMCA. Co-immunoprecipitations confirmed that PMCA1b and NHERF-2 associated under normal conditions in HT-29 cells. Cell surface biotinylations revealed significant increases in membrane-associated NHERF-2 and PMCA within 60 s following muscarinic activation, accompanied by increased association of the two proteins as seen by confocal microscopy. The recruitment of NHERF-2 to the membrane preceded that of PMCA, suggesting that NHERF-2 was involved in nucleating an efflux complex at the membrane. The muscarinic-mediated translocation of PMCA was abolished when NHERF-2 was silenced, and the rate of relative Ca2+ efflux was also reduced. These experiments also uncovered a NHERF-2-independent PMCA retrieval mechanism. Our findings describe rapid agonist-induced translocation of PMCA in a native cell model and suggest that NHERF-2 plays a key role in scaffolding and maintaining PMCA at the cell membrane.

Mechanism of modulation of the plasma membrane Ca(2+)-ATPase by arachidonic acid

The intracellular level of long chain fatty acids controls the Ca(2+) concentration in the cytoplasm. The molecular mechanisms underlying this Ca(2+) mobilization are not fully understood. We show here that the addition of low micromolar concentrations of fatty acids directly to the purified plasma membrane Ca(2+)-ATPase enhance ATP hydrolysis, while higher concentration decrease activity, exerting a dual effect on the enzyme. The effect of arachidonic acid is similar in the presence or absence of calmodulin, acidic phospholipids or ATP at the regulatory site, thereby precluding these sites as probable acid binding sites. At low arachidonic acid concentrations, neither the affinity for calcium nor the phosphoenzyme levels are significantly modified, while at higher concentrations both are decreased. The action of arachidonic acid is isoenzyme specific. The increase on ATP hydrolysis, however, is uncoupled from calcium transport, because arachidonic acid increases the permeability of erythrocyte membranes to calcium. Oleic acid has no effect on membrane permeability while linoleic acid shows an effect similar to that of arachidonic acid. Such effects might contribute to the entry of extracellular Ca(2+) following to fatty acid release.

Exporting calcium from cells

All eukaryotic cells import Ca2+ through a number of variously gated plasma membrane channels. Once inside cells, Ca2+ transmits information to a large number of (enzyme) targets. Eventually, it must be exported again, to prevent the overloading of the cytosol with Ca2+. Two systems export Ca2+ from cells: a high affinity, low capacity Ca2+-ATPase, and a lower affinity, but much larger capacity, Na+/Ca2+ exchanger. The ATPase (commonly called the Ca2+ pump) is the fine-tuner of cell Ca2+, as it functions well even if the concentration of the ion drops below the microM level. It is a large enzyme, with 10 transmembrane domains and a C-terminal cytosolic tail that contains regulatory sites, including a calmodulin-binding domain. Four distinct gene products plus a large number of splice variants have been described. Some are tissue specific, the isoform 2 being specifically expressed in the sensorial cells of the Corti organ in the inner-ear. Its genetic absence causes deafness in mice. Two different families of the Na+/Ca2+ exchanger exist, one of which, originally described in photoreceptors, transports K+ and Ca2+ in exchange for Na+. The exchanger is particularly active in excitable cells, e.g., heart, where the necessity cyclically arises to rapidly eject large amounts of Ca2+. In addition to heart, the exchanger is particularly important to neurons: the cleavage of the most important neuronal isoform (NCX3) by calpains activated by excitotoxic treatments generates Ca2+ overload and eventually cell death.

The regulatory function of plasma-membrane Ca2+-ATPase (PMCA) in the heart

The PMCA (plasma-membrane Ca2+-ATPase) is a ubiquitously expressed calcium-extruding enzymatic pump important in the control of intracellular calcium concentration. Unlike in non-excitable cells, where PMCA is the only system for calcium extrusion, in excitable cells, such as cardiomyocytes, PMCA has been shown to play only a minor role in calcium homoeostasis compared with the NCX (sodium/calcium exchanger), another system of calcium extrusion. However, increasing evidence points to an important role for PMCA in signal transduction; of particular interest in cardiac physiology is the modulation of nNOS (neuronal nitric oxide synthase) by isoform 4b of PMCA. In the present paper, we will discuss recent advances that support a key role for PMCA4 in modulating the nitric oxide signalling pathway in the heart.

Differential regulation of the apical plasma membrane Ca(2+) -ATPase by protein kinase A in parotid acinar cells

Cross-talk between intracellular calcium ([Ca(2+)](i)) signaling and cAMP defines the specificity of stimulus-response coupling in a variety of cells. Previous studies showed that protein kinase A (PKA) potentiates and phosphorylates the plasma membrane Ca(2+)-ATPase (PMCA) in a Ca(2+)-dependent manner in parotid acinar cells (Bruce, J. I. E., Yule, D. I., and Shuttleworth, T. J. (2002) J. Biol. Chem. 277, 48172-48181). The aim of this study was to further investigate the spatial regulation of [Ca(2+)](i) clearance in parotid acinar cells. Par-C10 cells were used to functionally isolate the apical and basolateral PMCA activity by applying La(3+) to the opposite side to inhibit the PMCA. Activation of PKA (using forskolin) differentially potentiated apical [Ca(2+)](i) clearance in mouse parotid acinar cells and apical PMCA activity in Par-C10 cells. Immunofluorescence of parotid tissue slices revealed that PMCA1 was distributed throughout the plasma membrane, PMCA2 was localized to the basolateral membrane, and PMCA4 was localized to the apical membrane of parotid acinar cells. However, in situ phosphorylation assays demonstrated that PMCA1 was the only isoform phosphorylated by PKA following stimulation. Similarly, immunofluorescence of acutely isolated parotid acinar cells showed that the regulatory subunit of PKA (RIIbeta) translocated to the apical region following stimulation. These data suggest that PKA-mediated phosphorylation of PMCA1 differentially regulates [Ca(2+)](i) clearance in the apical region of parotid acinar cells because of a dynamic translocation of PKA. Such tight spatial regulation of Ca(2+) efflux is likely important for the fine-tuning of Ca(2+)-dependent effectors close to the apical membrane important for the regulation of fluid secretion and exocytosis.

Regulation of plasma membrane Ca2+-ATPase in human platelets by calpain

The plasma membrane Ca(2+)-ATPase (PMCA) plays an essential role in maintaining low cytosolic Ca(2+) in resting human platelets by extruding Ca(2+) from the cytoplasm across the plasma membrane. Since PMCA is the main agent of Ca(2+) efflux in platelets, it is a key point for regulation of platelet Ca(2+) metabolism. PMCA has been shown to be an excellent substrate for the Ca(2+)-activated cysteine protease calpain, a major platelet protein that is turned on during platelet activation. The objectives of the present work were to determine if PMCA is degraded during thrombin- and collagen-mediated platelet activation, and if calpain is responsible. The kinetics of PMCA degradation during platelet activation were analysed using SDS polyacrylamide gel electrophoresis and immunoblotting. The role of calpain was tested using the calpain inhibitors calpeptin and ALLN. Platelet activation mediated by both collagen and thrombin resulted in degradation of 60% of platelet PMCA within 18 minutes. Calpeptin and ALLN significantly inhibited the rate and extent of PMCA degradation. We conclude that calpain-mediated degradation of PMCA during platelet activation likely contributes significantly to Ca(2+) regulation and, therefore, to platelet function.

Plasma membrane calcium ATPase and its relationship to nitric oxide signaling in the heart

The plasma membrane calcium/calmodulin-dependent ATPase (PMCA) is a ubiquitously expressed calcium-extruding enzymatic pump. In the majority of cells the main function of PMCA is as the only system to extrude calcium from the cytosol, however, in the excitable cells of the heart it has only a minor role in the bulk removal of calcium compared to the sodium-calcium exchanger. There is increasing evidence to suggest that PMCA has an additional role as a potential modulator of a number of signal transduction pathways. Of key interest in the heart is the functional interaction between the calcium/calmodulin-dependent enzyme neuronal nitric oxide synthase (nNOS) and isoform 4 of PMCA. Nitric oxide production from nNOS is known to be important in the regulation of excitation-contraction (EC) coupling and subsequently contractility. This article will focus on recent evidence suggesting that PMCA4 has a regulatory role in the nitric oxide signaling pathway in the heart.

Effects of plasma membrane Ca(2+) -ATPase tyrosine phosphorylation on human platelet function

BACKGROUND

The plasma membrane Ca(2+)-ATPase (PMCA) plays an essential role in maintaining low intracellular Ca(2+) ([Ca(2+)](i)) in resting platelets. Earlier studies demonstrated that platelet activation by thrombin results in tyrosine phosphorylation of PMCA, which inhibits pump activity.

OBJECTIVES

The objective was to determine the functional consequences of PMCA tyrosine phosphorylation.

METHODS

A decapeptide including the tyrosine phosphorylation site of PMCA and a scrambled version were synthesized and introduced into human platelets using saponin. Fura-2 calcium monitoring and aggregometry were used to characterize the effects of inhibition of tyrosine phosphorylation.

RESULTS

Western blot analysis of immunoprecipitates showed that introduction of the inhibitory peptide decreased tyrosine phosphorylation of PMCA by nearly 60% in saponin-permeabilized, thrombin-treated platelets as compared with the scrambled control peptide. Concomitant with inhibition of PMCA tyrosine phosphorylation was a significant decrease in [Ca(2+)](i) during thrombin-mediated platelet activation. The functional consequence of reduced PMCA tyrosine phosphorylation and decreased [Ca(2+)](i) was a significant delay in the onset of thrombin-mediated platelet aggregation.

CONCLUSIONS

The results demonstrate that PMCA tyrosine phosphorylation regulates [Ca(2+)](i) during platelet activation, which affects downstream events in the activation process. Moreover, PMCA tyrosine phosphorylation and resultant inhibition of PMCA activity produces a positive feedback loop mechanism by enhancing the increase in [Ca(2+)](i) accompanying platelet activation.

Plasma membrane calcium pump activity is affected by the membrane protein concentration: evidence for the involvement of the actin cytoskeleton

Plasma membrane calcium pumps (PMCAs) are integral membrane proteins that actively expel Ca(2+) from the cell. Specific Ca(2+)-ATPase activity of erythrocyte membranes increased steeply up to 1.5-5 times when the membrane protein concentration decreased from 50 microg/ml to 1 microg/ml. The activation by dilution was also observed for ATP-dependent Ca(2+) uptake into vesicles from Sf9 cells over-expressing the PMCA 4b isoform, confirming that it is a property of the PMCA. Dilution of the protein did not modify the activation by ATP, Ca(2+) or Ca(2+)-calmodulin. Treatment with non-ionic detergents did not abolish the dilution effect, suggesting that it was not due to resealing of the membrane vesicles. Pre-incubation of erythrocyte membranes with Cytochalasin D under conditions that promote actin polymerization abolished the dilution effect. Highly-purified, micellar PMCA showed no dilution effect and was not affected by Cytochalasin D. Taken together, these results suggest that the concentration-dependent behavior of the PMCA activity was due to interactions with cytoskeletal proteins. The dilution effect was also observed with different PMCA isoforms, indicating that this is a general phenomenon for all PMCAs.

Perisynaptic organization of plasma membrane calcium pumps in cerebellar cortex

Calcium, a ubiquitous intracellular messenger, regulates numerous intracellular signaling pathways. To permit specificity of signal transduction and prevent unwanted cross-talk between pathways, sites of calcium entry in neurons are localized to specific membrane domains. To test whether Ca(2+) extrusion pumps might exhibit analogous compartmentalization, we used immunohistochemistry to determine the subcellular localization of the two main plasma membrane Ca(2+)-ATPase (PMCA) isoforms in the cortex of the rat cerebellum. We find that both PMCA2 and PMCA3 are targeted to distinct compartments within the plasma membrane. In the molecular layer, both isoforms were at highest levels within synaptic profiles, but PMCA2 was postsynaptic and PMCA3 was presynaptic. Moreover, inside these compartments, both pumps exhibited nonuniform distributions. These data imply that cerebellar neurons possess remarkably effective mechanisms to target and restrict PMCA2 and -3 to specific membrane domains, raising the possibility that calcium pumps contribute to local Ca(2+) signaling.

Inhibition of plasma membrane Ca2+-ATPase by heparin is modulated by potassium

Heparin is related to several protein receptors that control Ca2+ homeostasis. Here, we studied the effects of heparin on the plasma membrane Ca2+-ATPase from erythrocytes. Both ATP hydrolysis and Ca2+ uptake were inhibited by heparin without modification of the steady-state level of phosphoenzyme formed by ATP. Calmodulin did neither modify the inhibition nor the binding of heparin. Inhibition by heparin was counteracted by K+ but not by Li+. This effect was extended to other sulfated polysaccharides with high number of sulfate residues. Hydrolysis of p-nitrophenylphosphate was equally inhibited by heparin. No evidence for enzyme uncoupling was observed: Ca2+ uptake and ATP hydrolysis remained tightly associated at any level of heparin, and heparin did not increase the passive Ca2+ efflux of inside-out vesicles. Vanadate blocked this efflux, indicating that the main point of Ca2+ escape from these vesicles was linked to the Ca2+ pump. It is discussed that sulfated polysaccharides may physiologically increase the steady-state level of Ca2+ in the cytosol by inhibiting the Ca2+ pumps in a K+ (and tissue) regulated way. It is suggested that heparin regulates the plasma membrane Ca2+-ATPase by binding to the E2 conformer.

Collaborative effect of SERCA and PMCA in cytosolic calcium homeostasis in human platelets

Intracellular free Ca2+ concentration ([Ca2+]c) is finely regulated by several mechanisms that either increase or reduce [Ca2+]c. Two different Ca2+ pumps have been described so far as the main mechanisms for Ca2+ removal from the cytosol, either by its sequestration into the stores, mediated by the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) or by Ca2+ extrusion to the extracellular medium, by the plasma membrane Ca2+-ATPase (PMCA). We have used inhibitors of these pumps to analyze their Ca2+ clearance efficacy in human platelets stimulated by the physiological agonist thrombin. Results demonstrate that, after platelet stimulation with thrombin, activation of SERCA precedes that of PMCA, although the ability of PMCA to remove Ca2+ from the cytosol last longer than that of SERCA. The efficacy of SERCA and PMCA removing Ca2+ from the cytosol is reduced when the concentration of thrombin increases. This phenomenon correlates with the greater increase in [Ca2+]c induced by higher concentrations of thrombin, which further confirms that SERCA and PMCA activities are regulated by [Ca2+]c.

Localization of the Cl(-)-ATPase activity on NAP-22 enriched membrane microdomain (raft) of rat brain

Much attention has been paid to the membrane microdomain enriched in cholesterol and sphingolipids called raft. In the central nervous system, however, the physiological role of this domain is not so evident at present, partly because of the complexity of the protein components in the raft fraction. In this study we surveyed ATPase activities in the raft fraction obtained from the synaptic plasma membrane of rat brain and found the enrichment of an ethacrynic acid-sensitive ATPase (Cl(-)-pump) activity. Immunoprecipitation experiments using antibodies to raft-localized proteins showed the co-precipitation of the ATPase activity with NAP-22, a major raft-localized protein. This result suggests the participation of the raft in the regulation of ion transport in addition to the presence of heterogeneity of raft domains in neurons.

Novel functional interaction between the plasma membrane Ca2+ pump 4b and the proapoptotic tumor suppressor Ras-associated factor 1 (RASSF1)

Plasma membrane calmodulin-dependent calcium ATPases (PMCAs) are enzymatic systems implicated in the extrusion of calcium from the cell. We and others have previously identified molecular interactions between the cytoplasmic COOH-terminal end of PMCA and PDZ domain-containing proteins. These interactions suggested a new role for PMCA as a modulator of signal transduction pathways. The existence of other intracellular regions in the PMCA molecule prompted us to investigate the possible participation of other domains in interactions with different partner proteins. A two-hybrid screen of a human fetal heart cDNA library, using the region 652-840 of human PMCA4b (located in the catalytic, second intracellular loop) as bait, revealed a novel interaction between PMCA4b and the tumor suppressor RASSF1, a Ras effector protein involved in H-Ras-mediated apoptosis. Immunofluorescence co-localization, immunoprecipitation, and glutathione S-transferase pull-down experiments performed in mammalian cells provided further confirmation of the physical interaction between the two proteins. The interaction domain has been narrowed down to region 74-123 of RASSF1C (144-193 in RASSF1A) and 652-748 of human PMCA4b. The functionality of this interaction was demonstrated by the inhibition of the epidermal growth factor-dependent activation of the Erk pathway when PMCA4b and RASSF1 were co-expressed. This inhibition was abolished by blocking PMCA/RASSSF1 association with an excess of a green fluorescent protein fusion protein containing the region 50-123 of RASSF1C. This work describes a novel protein-protein interaction involving a domain of PMCA other than the COOH terminus. It suggests a function for PMCA4b as an organizer of macromolecular protein complexes, where PMCA4b could recruit diverse proteins through interaction with different domains. Furthermore, the functional association with RASSF1 indicates a role for PMCA4b in the modulation of Ras-mediated signaling.

Crosstalk between cAMP and Ca2+ signaling in non-excitable cells

An impressive array of cytosolic calcium ([Ca2+](i)) signals exert control over a broad range of physiological processes. The specificity and fidelity of these [Ca2+](i) signals is encoded by the frequency, amplitude, and sub-cellular localization of the response. It is believed that the distinct characteristics of [Ca2+](i) signals underlies the differential activation of effectors and ultimately cellular events. This "shaping" of [Ca2+](i) signals can be achieved by the influence of additional signaling pathways modulating the molecular machinery responsible for generating [Ca2+](i) signals. There is a particularly rich source of potential sites of crosstalk between the cAMP and the [Ca2+](i) signaling pathways. This review will focus on the predominant molecular loci at which these classical signaling systems interact to impact the spatio-temporal pattern of [Ca2+](i) signaling in non-excitable cells.

Plasma membrane Ca2+ATPase isoform 4b is cleaved and activated by caspase-3 during the early phase of apoptosis

The plasma membrane Ca(2+) pump (PMCA) is an essential element in the complex of mechanisms that maintain low intracellular Ca(2+) concentration in the living cell. This pump is tightly regulated by calmodulin through binding to a high affinity calmodulin-binding domain at the C terminus that also serves as an autoinhibitor of the enzyme. Inspection of the C terminus of hPMCA4b, the most widely distributed form of PMCA, revealed a caspase-3 consensus sequence ((1077)DEID(1080)) just a few residues upstream of the calmodulin-binding domain. We demonstrate here that, in the early phase of apoptosis, hPMCA4b is cleaved at aspartic acid Asp(1080) in hPMCA4b-transfected COS-7 cells or in HeLa cells that naturally express this protein. This cleavage of hPMCA4b produces a single 120-kDa fragment that is fully active in the absence of calmodulin, because the whole inhibitory region downstream of the (1077)DEID(1080) sequence is removed. Our experiments show that caspase-3 or a caspase-3-like protease is responsible for the formation of the constitutively active 120-kDa PMCA4b fragment: 1) Pretreatment of the cells with the caspase-3 inhibitor Z-DEVD-FMK (benzyloxycarbonyl-Asp(OMe)-Glu(OMe)-Val-Asp(OMe)-fluoromethyl ketone) was able to block the production of the 120-kDa fragment. 2) In vitro treatment of hPMCA4b with recombinant caspase-3 also generated a 120-kDa cleavage product, consistent with that seen in cells undergoing apoptosis. 3) Mutants in which the caspase-3 consensus sequence was altered ((1077)AEID(1080), (1077)DEIA(1080), and (1077)AEIA(1080) mutants) were resistant to proteolysis. Based on these data, we conclude that hPMCA4b is a newly identified, natural caspase-3 substrate. We suggest that a constitutively active form of this protein, responding much faster to an increase in Ca(2+) concentration than the autoinhibited form, may have an important role in regulating intracellular Ca(2+) concentration in the apoptotic cell.

Plasma membrane Ca2+-atpase isoforms 2b and 4b interact promiscuously and selectively with members of the membrane-associated guanylate kinase family of PDZ (PSD95/Dlg/ZO-1) domain-containing proteins

Spatial and temporal regulation of intracellular Ca(2+) signaling depends on localized Ca(2+) microdomains containing the requisite molecular components for Ca(2+) influx, efflux, and signal transmission. Plasma membrane Ca(2+)-ATPase (PMCA) isoforms of the "b" splice type contain predicted PDZ (PSD95/Dlg/ZO-1) interaction domains. The COOH-terminal tail of PMCA2b isolated the membrane-associated guanylate kinase (MAGUK) protein SAP97/hDlg as a binding partner in a yeast two-hybrid screen. The related MAGUKs SAP90/PSD95, PSD93/chapsyn-110, SAP97, and SAP102 all bound to the COOH-terminal tail of PMCA4b, whereas only the first three bound to the tail of PMCA2b. Coimmunoprecipitations confirmed the interaction selectivity between PMCA4b and SAP102 as opposed to the promiscuity of PMCA2b and 4b in interacting with other SAPs. Confocal immunofluorescence microscopy revealed the exclusive presence and colocalization of PMCA4b and SAP97 in the basolateral membrane of polarized Madin-Darby canine kidney epithelial cells. In hippocampal neurons, PMCA2b was abundant throughout the somatodendritic compartment and often extended into the neck and head of individual spines where it colocalized with SAP90/PSD95. These data show that PMCA "b" splice forms interact promiscuously but also with specificity with different members of the PSD95 family of SAPs. PMCA-SAP interactions may play a role in the recruitment and maintenance of the PMCA at specific membrane domains involved in local Ca(2+) regulation.