Delayed activation of the plasma membrane calcium pump by a sudden increase in Ca2+: fast pumps reside in fast cells

Ca2+ puffs underlie adhesion-triggered Ca2+ microdomains in T cells

Ca2+ signalling is pivotal in T cell activation, an essential process in adaptive immune responses. Key to this activation are Ca2+ microdomains, which are transient increases in cytosolic Ca2+ concentration occurring within narrow regions between the endoplasmic reticulum (ER) and the plasma membrane (PM), lasting a few tens of milliseconds. Adhesion Dependent Ca2+ Microdomains (ADCM) rely on store-operated Ca2+ entry (SOCE) via the ORAI/STIM system. The nanometric scale at which these microdomains form poses challenges for direct experimental observation. Following the previous work of Gil et al. [1], which introduced a three-dimensional model of the ER-PM junction, this study combines a detailed description of the Ca2+ fluxes at the junction with stochastic dynamics of a cluster of D-myo-inositol 1,4,5 trisphosphate receptors (IP3R) located in the ER surrounding the junction. Because the consideration of Ca2+ release through the IP3R calls for the simulation of a portion of the cytoplasm considerably larger than the junction, our study also investigates the spatial distribution of PMCAs, revealing their likely localization outside the ER-PM junction. Simulations indicate that Ca2+ puffs implying the opening of 2-6 IP3Rs create ADCMs by provoking local depletions of ER Ca2+ stimulating Ca2+ entry through the ORAI1 channels. Such conditions allow the reproduction of the amplitude, duration and spatial extent of the observed ADCMs. By integrating advanced computational techniques with insights from experimental studies, our approach provides valuable information on the mechanisms governing early Ca2+ signalling in T cell activation, paving the way for a deeper understanding of immune responses.

Analysis of cholesterol-recognition motifs of the plasma membrane Ca2+-ATPase

The plasma membrane Ca2+-ATPase (PMCA) is crucial for the fine tuning of intracellular calcium levels in eukaryotic cells. In this study, we show the presence of CARC sequences in all human and rat PMCA isoforms and we performed further analysis by molecular dynamics simulations. This analysis focuses on PMCA1, containing three CARC motifs, and PMCA4, with four CARC domains. In PMCA1, two CARC motifs reside within transmembrane domains, while the third is situated at the intracellular interface. The simulations depict more stable RMSD values and lower RMSF fluctuations in the presence of cholesterol, emphasizing its potential stabilizing effect. In PMCA4, a distinct dynamic was found. Notably, the total energy differences between simulations with cholesterol and phospholipids are pronounced in PMCA4 compared to PMCA1. RMSD values for PMCA4 indicate a more energetically favorable conformation in the presence of cholesterol, suggesting a robust interaction between CARCs and this lipid in the membranes. Furthermore, RMSF analysis for CARCs in both PMCA isoforms exhibit lower values in the presence of cholesterol compared to POPC alone. The analysis of H-bond occupancy and total energy values strongly suggests the potential interaction of CARCs with cholesterol. Given the crucial role of PMCAs in physiological calcium regulation and their involvement in diverse pathological processes, this study underscores the significance of CARC motifs and their interaction with cholesterol in elucidating PMCA function. These insights into the energetic preferences associated with CARC-cholesterol interactions offer valuable implications for understanding PMCA function in maintaining calcium homeostasis and addressing potential associated pathologies.

Plasma Membrane Calcium ATPase Regulates Stoichiometry of CD4+ T-Cell Compartments

Immune responses involve mobilization of T cells within naïve and memory compartments. Tightly regulated Ca²⁺ levels are essential for balanced immune outcomes. How Ca²⁺ contributes to regulating compartment stoichiometry is unknown. Here, we show that plasma membrane Ca²⁺ ATPase 4 (PMCA4) is differentially expressed in human CD4⁺ T compartments yielding distinct store operated Ca²⁺ entry (SOCE) profiles. Modulation of PMCA4 yielded a more prominent increase of SOCE in memory than in naïve CD4⁺ T cell. Interestingly, downregulation of PMCA4 reduced the effector compartment fraction and led to accumulation of cells in the naïve compartment. In silico analysis and chromatin immunoprecipitation point towards Ying Yang 1 (YY1) as a transcription factor regulating PMCA4 expression. Analyses of PMCA and YY1 expression patterns following activation and of PMCA promoter activity following downregulation of YY1 highlight repressive role of YY1 on PMCA expression. Our findings show that PMCA4 adapts Ca²⁺ levels to cellular requirements during effector and quiescent phases and thereby represent a potential target to intervene with the outcome of the immune response.

Near-Infrared Light Triggered-Release in Deep Brain Regions Using Ultra-photosensitive Nanovesicles

Remote and minimally-invasive modulation of biological systems with light has transformed modern biology and neuroscience. However, light absorption and scattering significantly prevents penetration to deep brain regions. Herein, we describe the use of gold-coated mechanoresponsive nanovesicles, which consist of liposomes made from the artificial phospholipid Rad-PC-Rad as a tool for the delivery of bioactive molecules into brain tissue. Near-infrared picosecond laser pulses activated the gold-coating on the surface of nanovesicles, creating nanomechanical stress and leading to near-complete vesicle cargo release in sub-seconds. Compared to natural phospholipid liposomes, the photo-release was possible at 40 times lower laser energy. This high photosensitivity enables photorelease of molecules down to a depth of 4 mm in mouse brain. This promising tool provides a versatile platform to optically release functional molecules to modulate brain circuits.

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.

Evidence for a role of plasma membrane calcium pumps in neurodegenerative disease: Recent developments

Plasma membrane Ca2+ ATPases (PMCAs) are a major system for calcium extrusion from all cells. Different PMCA isoforms and splice variants are involved in the precise temporal and spatial handling of Ca2+ signals and the re-establishment of resting Ca2+ levels in the nervous system. Lack or inappropriate expression of specific PMCAs leads to characteristic neuronal phenotypes, which may be reciprocally exacerbated by genetic predisposition through alleles in other genes that modify PMCA interactions, regulation, and function. PMCA dysfunction is often poorly compensated in neurons and may lead to changes in synaptic transmission, altered excitability and, with long-term calcium overload, eventual cell death. Decrease and functional decline of PMCAs are hallmarks of neurodegeneration during aging, and mutations in specific PMCAs are responsible for neuronal dysfunction and accelerated neurodegeneration in many sensory and cognitive diseases.

Ultrafast Near-Infrared Light-triggered Intracellular Uncaging to Probe Cell Signaling

The possibility of regulating cell signaling with high spatial and temporal resolution within individual cells and complex cellular networks has important implications in biomedicine. In this report, we demonstrate a general strategy that uses near-infrared tissue-penetrating laser pulses to uncage biomolecules from plasmonic gold-coated liposomes, i.e. plasmonic liposomes, to activate cell signaling in a non-thermal, ultrafast and highly controllable fashion. Near-infrared picosecond laser pulse induces transient nanobubbles around plasmonic liposomes. The mechanical force generated from the collapse of nanobubbles rapidly ejects encapsulated compound within 0.1 ms. We showed that single pulse irradiation triggers the rapid intracellular uncaging of calcein from plasmonic liposomes inside endo-lysosomes. The uncaged calcein then evenly distributes over the entire cytosol and nucleus. Furthermore, we demonstrated the ability to trigger calcium signaling in both an immortalized cell line and primary dorsal root ganglion (DRG) neurons by intracellular uncaging of inositol triphosphate (IP3), an endogenous cell calcium signaling second messenger. Compared with other uncaging techniques, this ultrafast near-infrared light-driven molecular uncaging method is easily adaptable to deliver a wide range of bioactive molecules with an ultrafast optical switch, enabling new possibilities to investigate signaling pathways within individual cells and cellular networks.

The ataxia related G1107D mutation of the plasma membrane Ca2+ ATPase isoform 3 affects its interplay with calmodulin and the autoinhibition process

The plasma membrane Ca²⁺ ATPases (PMCA pumps) have a long, cytosolic C-terminal regulatory region where a calmodulin-binding domain (CaM-BD) is located. Under basal conditions (low Ca²⁺), the C-terminal tail of the pump interacts with autoinhibitory sites proximal to the active center of the enzyme. In activating conditions (i.e., high Ca²⁺), Ca²⁺-bound CaM displaces the C-terminal tail from the autoinhibitory sites, restoring activity. We have recently identified a G1107D replacement within the CaM-BD of isoform 3 of the PMCA pump in a family affected by X-linked congenital cerebellar ataxia. Here, we investigate the effects of the G1107D replacement on the interplay of the mutated CaM-BD with both CaM and the pump core, by combining computational, biochemical and functional approaches. We provide evidence that the affinity of the isolated mutated CaM-BD for CaM is significantly reduced with respect to the wild type (wt) counterpart, and that the ability of CaM to activate the pump in vitro is thus decreased. Multiscale simulations support the conclusions on the detrimental effect of the mutation, indicating reduced stability of the CaM binding. We further show that the G1107D replacement impairs the autoinhibition mechanism of the PMCA3 pump as well, as the introduction of a negative charge perturbs the contacts between the CaM-BD and the pump core. Thus, the mutation affects both the ability of the pump to optimally transport Ca²⁺ in the activated state, and the autoinhibition mechanism in its resting state.

A quantitative description of tubular system Ca(2+) handling in fast- and slow-twitch muscle fibres

KEY POINTS

Current methods do not allow a quantitative description of Ca(2+) movements across the tubular (t-) system membrane without isolating the membranes from their native skeletal muscle fibre. Here we present a fluorescence-based method that allows determination of the t-system [Ca(2+) ] transients and derivation of t-system Ca(2+) fluxes in mechanically skinned skeletal muscle fibres. Differences in t-system Ca(2+) -handling properties between fast- and slow-twitch fibres from rat muscle are resolved for the first time using this new technique. The method can be used to study Ca(2+) handling of the t-system and allows direct comparisons of t-system Ca(2+) transients and Ca(2+) fluxes between groups of fibres and fibres from different strains of animals.

ABSTRACT

The tubular (t-) system of skeletal muscle is an internalization of the plasma membrane that maintains a large Ca(2+) gradient and exchanges Ca(2+) between the extracellular and intracellular environments. Little is known of the Ca(2+) -handling properties of the t-system as the small Ca(2+) fluxes conducted are difficult to resolve with conventional methods. To advance knowledge in this area we calibrated t-system-trapped rhod-5N inside skinned fibres from rat and [Ca(2+) ]t-sys , allowing confocal measurements of Ca(2+) -dependent changes in rhod-5N fluorescence during rapid changes in the intracellular ionic environment to be converted to [Ca(2+) ] transients in the t-system ([Ca(2+) ]t-sys (t)). Furthermore, t-system Ca(2+) -buffering power was determined so that t-system Ca(2+) fluxes could be derived from [Ca(2+) ]t-sys (t). With this new approach, we show that rapid depletion of sarcoplasmic reticulum (SR) Ca(2+) induced a robust store-operated Ca(2+) entry (SOCE) in fast- and slow-twitch fibres, reducing [Ca(2+) ]t-sys to < 0.1 mm. The rapid activation of SOCE upon Ca(2+) release was consistent with the presence of STIM1L in both fibre types. Abruptly introducing internal solutions with 1 mm Mg(2+) and [Ca(2+) ]cyto (28 nm-1.3 μm) to Ca(2+) -depleted fibres generated t-system Ca(2+) uptake rates dependent on [Ca(2+) ]cyto with [Ca(2+) ]t-sys reaching final plateaus in the millimolar range. For the same [Ca(2+) ]cyto , t-system Ca(2+) fluxes of fast-twitch fibres were greater than that in slow-twitch fibres. In addition, simultaneous imaging of t-system and SR Ca(2+) signals indicated that both membrane compartments accumulated Ca(2+) at similar rates and that SOCE was activated early during SR Ca(2+) depletion.

Multifaceted plasma membrane Ca(2+) pumps: From structure to intracellular Ca(2+) handling and cancer

Plasma membrane Ca(2+) ATPases (PMCAs) are intimately involved in the control of intracellular Ca(2+) concentration. They reduce Ca(2+) in the cytosol not only by direct ejection, but also by controlling the formation of inositol-1,4,5-trisphosphate and decreasing Ca(2+) release from the endoplasmic reticulum Ca(2+) pool. In mammals four genes (PMCA1-4) are expressed, and alternative RNA splicing generates more than twenty variants. The variants differ in their regulatory characteristics. They localize into highly specialized membrane compartments and respond to the incoming Ca(2+) with distinct temporal resolution. The expression pattern of variants depends on cell type; a change in this pattern can result in perturbed Ca(2+) homeostasis and thus altered cell function. Indeed, PMCAs undergo remarkable changes in their expression pattern during tumorigenesis that might significantly contribute to the unbalanced Ca(2+) homeostasis of cancer cells. This article is part of a Special Issue entitled: Calcium and Cell Fate. Guest Editors: Jacques Haiech, Claus Heizmann, Joachim Krebs, Thierry Capiod and Olivier Mignen.

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 pump: new ways to look at an old enzyme

The three-dimensional structure of the PMCA pump has not been solved, but its basic mechanistic properties are known to repeat those of the other Ca(2+) pumps. However, the pump also has unique properties. They concern essentially its numerous regulatory mechanisms, the most important of which is the autoinhibition by its C-terminal tail. Other regulatory mechanisms involve protein kinases and the phospholipids of the membrane in which the pump is embedded. Permanent activation of the pump, e.g. by calmodulin, is physiologically as harmful to cells as its absence. The concept is now emerging that the global control of cell Ca(2+) may not be the main function of the pump; in some cell types, it could even be irrelevant. The main pump role would be the regulation of Ca(2+) in cell microdomains in which the pump co-segregates with partners that modulate the Ca(2+) message and transduce it to important cell functions.

Apart from its known function, the plasma membrane Ca²⁺ATPase can regulate Ca²⁺ signaling by controlling phosphatidylinositol 4,5-bisphosphate levels

Plasma membrane Ca(2+) ATPases (PMCAs, also known as ATP2B1-ATP2B4) are known targets of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂], but if and how they control the PtdIns(4,5)P₂ pool has not been considered. We demonstrate here that PMCAs protect PtdIns(4,5)P₂ in the plasma membrane from hydrolysis by phospholipase C (PLC). Comparison of active and inactive PMCAs indicates that the protection operates by two mechanisms; one requiring active PMCAs, the other not. It appears that the mechanism requiring activity is the removal of the Ca(2+) required for sustained PLC activity, whereas the mechanism not requiring activity is PtdIns(4,5)P₂ binding. We show that in PMCA overexpressing cells, PtdIns(4,5)P₂ binding can lead to less inositol 1,4,5-triphosphate (InsP₃) and diminished Ca(2+) release from intracellular Ca(2+) pools. Inspection of a homology model of PMCA suggests that PMCAs have a conserved cluster of basic residues forming a 'blue collar' at the interface between the membrane core and the cytoplasmic domains. By molecular dynamics simulation, we found that the blue collar forms four binding pockets for the phosphorylated inositol head group of PtdIns(4,5)P₂; these pockets bind PtdIns(4,5)P₂ strongly and frequently. Our studies suggest that by having the ability to bind PtdIns(4,5)P₂, PMCAs can control the accessibility of PtdIns(4,5)P₂ for PLC and other PtdIns(4,5)P₂-mediated processes.

A mathematical model of T lymphocyte calcium dynamics derived from single transmembrane protein properties

Fate decision processes of T lymphocytes are crucial for health and disease. Whether a T lymphocyte is activated, divides, gets anergic, or initiates apoptosis depends on extracellular triggers and intracellular signaling. Free cytosolic calcium dynamics plays an important role in this context. The relative contributions of store-derived calcium entry and calcium entry from extracellular space to T lymphocyte activation are still a matter of debate. Here we develop a quantitative mathematical model of T lymphocyte calcium dynamics in order to establish a tool which allows to disentangle cause-effect relationships between ion fluxes and observed calcium time courses. The model is based on single transmembrane protein characteristics which have been determined in independent experiments. This reduces the number of unknown parameters in the model to a minimum and ensures the predictive power of the model. Simulation results are subsequently used for an analysis of whole cell calcium dynamics measured under various experimental conditions. The model accounts for a variety of these conditions, which supports the suitability of the modeling approach. The simulation results suggest a model in which calcium dynamics dominantly relies on the opening of channels in calcium stores while calcium entry through calcium-release activated channels (CRAC) is more associated with the maintenance of the T lymphocyte calcium levels and prevents the cell from calcium depletion. Our findings indicate that CRAC guarantees a long-term stable calcium level which is required for cell survival and sustained calcium enhancement.

The critical role of STIM1-dependent Ca2+ signalling during T-cell development and activation

T lymphocytes are key cellular effectors of adaptive immunity able to recognize a virtually limitless number of antigenic peptides and mount an immune response. Ca(2+) signals are crucial to the development and activation of T cells and Stromal Interaction Molecule 1 (STIM1) has been identified as a critical modulator of intracellular Ca(2+) levels in T cells. Although the role of STIM1 in T cell activation has been extensively investigated, the role of STIM1 in T cell development has been somewhat controversial. Indeed, deficiencies in STIM1 expression and function lead to both developmental defects associated with the development of autoimmunity yet also interfere with T cell activation leading to severe combined immunodeficiency signifying a multifaceted role of STIM1 in T cell physiology and pathophysiology.

A C-terminal di-leucine motif controls plasma membrane expression of PMCA4b

Recent evidences show that the localization of different plasma membrane Ca(2+) ATPases (PMCAs) is regulated in various complex, cell type-specific ways. Here we show that in low-density epithelial and endothelial cells PMCA4b localized mostly in intracellular compartments and its plasma membrane localization was enhanced upon increasing density of cells. In good correlation with the enhanced plasma membrane localization a significantly more efficient Ca(2+) clearance was observed in confluent versus non-confluent HeLa cell cultures expressing mCherry-PMCA4b. We analyzed the subcellular localization and function of various C-terminally truncated PMCA4b variants and found that a truncated mutant PMCA4b-ct24 was mostly intracellular while another mutant, PMCA4b-ct48, localized more to the plasma membrane, indicating that a protein sequence corresponding to amino acid residues 1158-1181 contained a signal responsible for the intracellular retention of PMCA4b in non-confluent cultures. Alteration of three leucines to alanines at positions 1167-1169 resulted in enhanced cell surface expression and an appropriate Ca(2+) transport activity of both wild type and truncated pumps, suggesting that the di-leucine-like motif (1167)LLL was crucial in targeting PMCA4b. Furthermore, upon loss of cell-cell contact by extracellular Ca(2+) removal, the wild-type pump was translocated to the early endosomal compartment. Targeting PMCA4b to early endosomes was diminished by the L(1167-69)A mutation, and the mutant pump accumulated in long tubular cytosolic structures. In summary, we report a di-leucine-like internalization signal at the C-tail of PMCA4b and suggest an internalization-mediated loss of function of the pump upon low degree of cell-cell contact.

Separate Ca2+ sources are buffered by distinct Ca2+ handling systems in aplysia neuroendocrine cells

Although the contribution of Ca(2+) buffering systems can vary between neuronal types and cellular compartments, it is unknown whether distinct Ca(2+) sources within a neuron have different buffers. As individual Ca(2+) sources can have separate functions, we propose that each is handled by unique systems. Using Aplysia californica bag cell neurons, which initiate reproduction through an afterdischarge involving multiple Ca(2+)-dependent processes, we investigated the role of endoplasmic reticulum (ER) and mitochondrial sequestration, as well as extrusion via the plasma membrane Ca(2+)-ATPase (PMCA) and Na(+)/Ca(2+) exchanger, to the clearance of voltage-gated Ca(2+) influx, Ca(2+)-induced Ca(2+)-release (CICR), and store-operated Ca(2+) influx. Cultured bag cell neurons were filled with the Ca(2+) indicator, fura-PE3, to image Ca(2+) under whole-cell voltage clamp. A 5 Hz, 1 min train of depolarizing voltage steps elicited voltage-gated Ca(2+) influx followed by EGTA-sensitive CICR from the mitochondria. A compartment model of Ca(2+) indicated the effect of EGTA on CICR was due to buffering of released mitochondrial Ca(2+) rather than uptake competition. Removal of voltage-gated Ca(2+) influx was dominated by the mitochondria and PMCA, with no contribution from the Na(+)/Ca(2+) exchanger or sarcoplasmic/endoplasmic Ca(2+)-ATPase (SERCA). In contrast, CICR recovery was slowed by eliminating the Na(+)/Ca(2+) exchanger and PMCA. Last, store-operated influx, evoked by ER depletion, was removed by the SERCA and depended on the mitochondrial membrane potential. Our results demonstrate that distinct buffering systems are dedicated to particular Ca(2+) sources. In general, this may represent a means to differentially regulate Ca(2+)-dependent processes, and for Aplysia, influence how reproductive behavior is triggered.

Significant differences in genotoxicity induced by retrovirus integration in human T cells and induced pluripotent stem cells

Retrovirus is frequently used in the genetic modification of mammalian cells and the establishment of induced pluripotent stem cells (iPSCs) via cell reprogramming. Vector-induced genotoxicity could induce profound effect on the physiology and function of these stem cells and their differentiated progeny. We analyzed retrovirus-induced genotoxicity in somatic cell Jurkat and two iPSC lines. In Jurkat cells, retrovirus frequently activated host gene expression and gene activation was not dependent on the distance between the integration site and the transcription start site of the host gene. In contrast, retrovirus frequently down-regulated host gene expression in iPSCs, possibly due to the action of chromatin silencing that spreads from the provirus to the nearby host gene promoter. Our data raises the issue that some of the phenotypic variability observed among iPSC clones derived from the same parental cell line may be caused by retrovirus-induced gene expression changes rather than by the reprogramming process itself. It also underscores the importance of characterizing retrovirus integration and carrying out risk assessment of iPSCs before they can be applied in basic research and clinics.

A theory of plasma membrane calcium pump stimulation and activity

The ATP-driven Plasma Membrane Calcium pump or Ca(2+)-ATPase (PMCA) is characterized by a high affinity for calcium and a low transport rate compared to other transmembrane calcium transport proteins. It plays a crucial role for calcium extrusion from cells. Calmodulin is an intracellular calcium buffering protein which is capable in its Ca(2+) liganded form of stimulating the PMCA by increasing both the affinity to calcium and the maximum calcium transport rate. We introduce a new model of this stimulation process and derive analytical expressions for experimental observables in order to determine the model parameters on the basis of specific experiments. We furthermore develop a model for the pumping activity. The pumping description resolves the seeming contradiction of the Ca(2+):ATP stoichiometry of 1:1 during a translocation step and the observation that the pump binds two calcium ions at the intracellular site. The combination of the calcium pumping and the stimulation model correctly describes PMCA function. We find that the processes of calmodulin-calcium complex attachment to the pump and of stimulation have to be separated. Other PMCA properties are discussed in the framework of the model. The presented model can serve as a tool for calcium dynamics simulations and provides the possibility to characterize different pump isoforms by different type-specific parameter sets.

Plasma membrane calcium ATPases as novel candidates for therapeutic agent development

Plasma membrane Ca2+ ATPases (PMCAs) are highly regulated transporters responsible for Ca2+ extrusion from all eukaryotic cells. Different PMCA isoforms are implicated in various tasks of Ca2+ regulation including bulk Ca2+ transport and localized Ca2+ signaling in specific membrane microdomains. Accumulating evidence shows that loss, mutation or inappropriate expression of different PMCAs is associated with pathologies ranging from hypertension, low bone density and male infertility to hearing loss and cerebellar ataxia. Compared to Ca2+ influx channels, PMCAs have lagged far behind as targets for drug development, mainly due to the lack of detailed understanding of their structure and specific function. This is rapidly changing thanks to integrated efforts combining biochemical, structural, cellular and physiological studies suggesting that selective modulation of PMCA isoforms may be of therapeutic value in the management of different and complex diseases. Both structurally informed rational design and high-throughput small molecule library screenings are promising strategies that are expected to lead to specific and isoform-selective modulators of PMCA function. This short review will provide an overview of the diverse roles played by PMCA isoforms in different cells and tissues and their emerging involvement in pathophysiological processes, summarize recent progress in obtaining structural information on the PMCAs, and discuss current and future strategies to develop specific PMCA inhibitors and activators for potential therapeutic applications.

Functional integration of calcium regulatory mechanisms at Purkinje neuron synapses

Cerebellar Purkinje neurons receive synaptic inputs from three different sources: the excitatory parallel fibre and climbing fibre synapses as well as the inhibitory synapses from molecular layer stellate and basket cells. These three synaptic systems use distinct mechanisms in order to generate Ca(2+) signals that are specialized for specific modes of neurotransmitter release and post-synaptic signal integration. In this review, we first describe the repertoire of Ca(2+) regulatory mechanisms that generate and regulate the amplitude and timing of Ca(2+) fluxes during synaptic transmission and then explore how these mechanisms interact to generate the unique functional properties of each of the Purkinje neuron synapses.

Light-driven calcium signals in mouse cone photoreceptors

Calcium mediates various neuronal functions. The complexity of neuronal Ca²⁺ signaling is well exemplified by retinal cone photoreceptors, which, with their distinct compartmentalization, offer unique possibilities for studying the diversity of Ca²⁺ functions in a single cell. Measuring subcellular Ca²⁺ signals in cones under physiological conditions is not only fundamental for understanding cone function, it also bears important insights into pathophysiological processes governing retinal neurodegeneration. However, due to the proximity of light-sensitive outer segments to other cellular compartments, optical measurements of light-evoked Ca²⁺ responses in cones are challenging. We addressed this problem by generating a transgenic mouse (HR2.1:TN-XL) in which both short- and middle-wavelength-sensitive cones selectively express the genetically encoded ratiometric Ca²⁺ biosensor TN-XL. We show that HR2.1:TN-XL allows recording of light-evoked Ca²⁺ responses using two-photon imaging in individual cone photoreceptor terminals and to probe phototransduction and its diverse regulatory mechanisms with pharmacology at subcellular resolution. To further test this system, we asked whether the classical, nitric oxide (NO)-soluble guanylyl-cyclase (sGC)-cGMP pathway could modulate Ca²⁺ in cone terminals. Surprisingly, NO reduced Ca²⁺ resting levels in mouse cones, without evidence for direct sGC involvement. In conclusion, HR2.1:TN-XL mice offer unprecedented opportunities to elucidate light-driven Ca²⁺ dynamics and their (dys)regulation in cone photoreceptors.

Alternative pathways for association and dissociation of the calmodulin-binding domain of plasma membrane Ca(2+)-ATPase isoform 4b (PMCA4b)

The calmodulin (CaM)-binding domain of isoform 4b of the plasma membrane Ca(2+) -ATPase (PMCA) pump is represented by peptide C28. CaM binds to either PMCA or C28 by a mechanism in which the primary anchor residue Trp-1093 binds to the C-terminal lobe of the extended CaM molecule, followed by collapse of CaM with the N-terminal lobe binding to the secondary anchor Phe-1110 (Juranic, N., Atanasova, E., Filoteo, A. G., Macura, S., Prendergast, F. G., Penniston, J. T., and Strehler, E. E. (2010) J. Biol. Chem. 285, 4015-4024). This is a relatively rapid reaction, with an apparent half-time of ~1 s. The dissociation of CaM from PMCA4b or C28 is much slower, with an overall half-time of ~10 min. Using targeted molecular dynamics, we now show that dissociation of Ca(2+)-CaM from C28 may occur by a pathway in which Trp-1093, although deeply embedded in a pocket in the C-terminal lobe of CaM, leaves first. The dissociation begins by relatively rapid release of Trp-1093, followed by very slow release of Phe-1110, removal of C28, and return of CaM to its conformation in the free state. Fluorescence measurements and molecular dynamics calculations concur in showing that this alternative path of release of the PMCA4b CaM-binding domain is quite different from that of binding. The intermediate of dissociation with exposed Trp-1093 has a long lifetime (minutes) and may keep the PMCA primed for activation.

The development, distribution and density of the plasma membrane calcium ATPase 2 calcium pump in rat cochlear hair cells

Calcium is tightly regulated in cochlear outer hair cells (OHCs). It enters mainly via mechanotransducer (MT) channels and is extruded by the plasma membrane calcium ATPase (PMCA)2 isoform of the PMCA, mutations in which cause hearing loss. To assess how pump expression matches the demands of Ca(2+) homeostasis, the distribution of PMCA2 at different cochlear locations during development was quantified using immunofluorescence and post-embedding immunogold labeling. The PMCA2 isoform was confined to stereociliary bundles, first appearing at the base of the cochlea around post-natal day (P)0 followed by the middle and then the apex by P3, and was unchanged after P8. The developmental appearance matched the maturation of the MT channels in rat OHCs. High-resolution immunogold labeling in adult rats showed that PMCA2 was distributed along the membranes of all three rows of OHC stereocilia at similar densities and at about a quarter of the density in inner hair cell stereocilia. The difference between OHCs and inner hair cells was similar to the ratio of their MT channel resting open probabilities. Gold particle counts revealed no difference in PMCA2 density between low- and high-frequency OHC bundles despite larger MT currents in high-frequency OHCs. The PMCA2 density in OHC stereocilia was determined in low- and high-frequency regions from calibration of immunogold particle counts as 2200/μm(2) from which an extrusion rate of ∼200 ions/s per pump was inferred. The limited ability of PMCA2 to extrude the Ca(2+) load through MT channels may constitute a major cause of OHC vulnerability and high-frequency hearing loss.

Plasma membrane calcium pump (PMCA) isoform 4 is targeted to the apical membrane by the w-splice insert from PMCA2

Local Ca(2+) signaling requires proper targeting of the Ca(2+) signaling toolkit to specific cellular locales. Different isoforms of the plasma membrane Ca(2+) pump (PMCA) are responsible for Ca(2+) extrusion at the apical and basolateral membrane of polarized epithelial cells, but the mechanisms and signals for differential targeting of the PMCAs are not well understood. Recent work demonstrated that the alternatively spliced w-insert in PMCA2 directs this pump to the apical membrane. We now show that inserting the w-insert into the corresponding location of the PMCA4 isoform confers apical targeting to this normally basolateral pump. Mutation of a di-leucine motif in the C-tail thought to be important for basolateral targeting did not enhance apical localization of the chimeric PMCA4(2w)/b. In contrast, replacing the C-terminal Val residue by Leu to optimize the PDZ ligand site for interaction with the scaffolding protein NHERF2 enhanced the apical localization of PMCA4(2w)/b, but not of PMCA4x/b. Functional studies showed that both apical PMCA4(2w)/b and basolateral PMCA4x/b handled ATP-induced Ca(2+) signals with similar kinetics, suggesting that isoform-specific functional characteristics are retained irrespective of membrane targeting. Our results demonstrate that the alternatively spliced w-insert provides autonomous apical targeting information in the PMCA without altering its functional characteristics.

STIM1 is required for attenuation of PMCA-mediated Ca2+ clearance during T-cell activation

T-cell activation involves a complex signalling cascade uniquely dependent on elevated cytosolic Ca(2+) levels. Further, the spatiotemporal characteristics of this Ca(2+) signal play a critical role in this process via selective activation of transcription factors. In T cells, store-operated Ca(2+) entry (SOCe) is the primary Ca(2+) influx pathway; however, cytosolic Ca(2+) concentration depends upon the balance between Ca(2+) influx and extrusion. The plasma membrane Ca(2+) ATPase (PMCA) has previously been identified as a critical player in Ca(2+) clearance in T cells. Here, we provide data revealing both functional and physical links between the activation of stromal interacting molecule 1 (STIM1) and PMCA-mediated Ca(2+) clearance. Due to the ubiquitous expression of both STIM1 and PMCA, these findings have wide-ranging implications for Ca(2+) signalling in multiple cell types.

Converging evidence for an association of ATP2B2 allelic variants with autism in male subjects

BACKGROUND

Autism is a severe developmental disorder, with strong genetic underpinnings. Previous genome-wide scans unveiled a linkage region spanning 3.5 Mb, located on human chromosome 3p25. This region encompasses the ATP2B2 gene, encoding the plasma membrane calcium-transporting ATPase 2 (PMCA2), which extrudes calcium (Ca2+) from the cytosol into the extracellular space. Multiple lines of evidence support excessive intracellular Ca2+ signaling in autism spectrum disorder (ASD), making ATP2B2 an attractive candidate gene.

METHODS

We performed a family-based association study in an exploratory sample of 277 autism genetic resource exchange families and in a replication sample including 406 families primarily recruited in Italy.

RESULTS

Several markers were significantly associated with ASD in the exploratory sample, and the same risk alleles at single nucleotide polymorphisms rs3774180, rs2278556, and rs241509 were found associated with ASD in the replication sample after correction for multiple testing. In both samples, the association was present in male subjects only. Markers associated with autism are all comprised within a single block of strong linkage disequilibrium spanning several exons, and the "risk" allele seems to follow a recessive mode of transmission.

CONCLUSIONS

These results provide converging evidence for an association between ATP2B2 gene variants and autism in male subjects, spurring interest into the identification of functional variants, most likely involved in the homeostasis of Ca2+ signaling. Additional support comes from a recent genome-wide association study by the Autism Genome Project, which highlights the same linkage disequilibrium region of the gene.

STIM1L is a new actin-binding splice variant involved in fast repetitive Ca2+ release

A newly identified splice variant of STIM1 called STIM1L forms constitutive clusters that interact with actin and Orai1 and allows fast repetitive Ca²⁺ release.

Cytosolic Ca²⁺ signals encoded by repetitive Ca²⁺ releases rely on two processes to refill Ca²⁺ stores: Ca²⁺ reuptake from the cytosol and activation of a Ca²⁺ influx via store-operated Ca²⁺ entry (SOCE). However, SOCE activation is a slow process. It is delayed by >30 s after store depletion because stromal interaction molecule 1 (STIM1), the Ca²⁺ sensor of the intracellular stores, must form clusters and migrate to the membrane before being able to open Orai1, the plasma membrane Ca²⁺ channel. In this paper, we identify a new protein, STIM1L, that colocalizes with Orai1 Ca²⁺ channels and interacts with actin to form permanent clusters. This property allowed the immediate activation of SOCE, a characteristic required for generating repetitive Ca²⁺ signals with frequencies within seconds such as those frequently observed in excitable cells. STIM1L was expressed in several mammalian tissues, suggesting that many cell types rely on this Ca²⁺ sensor for their Ca²⁺ homeostasis and intracellular signaling.

Calcium occlusion in plasma membrane Ca2+-ATPase

In this work, we set out to identify and characterize the calcium occluded intermediate(s) of the plasma membrane Ca(2+)-ATPase (PMCA) to study the mechanism of calcium transport. To this end, we developed a procedure for measuring the occlusion of Ca(2+) in microsomes containing PMCA. This involves a system for overexpression of the PMCA and the use of a rapid mixing device combined with a filtration chamber, allowing the isolation of the enzyme and quantification of retained calcium. Measurements of retained calcium as a function of the Ca(2+) concentration in steady state showed a hyperbolic dependence with an apparent dissociation constant of 12 ± 2.2 μM, which agrees with the value found through measurements of PMCA activity in the absence of calmodulin. When enzyme phosphorylation and the retained calcium were studied as a function of time in the presence of La(III) (inducing accumulation of phosphoenzyme in the E(1)P state), we obtained apparent rate constants not significantly different from each other. Quantification of EP and retained calcium in steady state yield a stoichiometry of one mole of occluded calcium per mole of phosphoenzyme. These results demonstrate for the first time that one calcium ion becomes occluded in the E(1)P-phosphorylated intermediate of the PMCA.

Auditory and vestibular hair cell stereocilia: relationship between functionality and inner ear disease

The stereocilia of the inner ear are unique cellular structures which correlate anatomically with distinct cochlear functions, including mechanoelectrical transduction, cochlear amplification, adaptation, frequency selectivity and tuning. Their function is impaired by inner ear stressors, by various types of hereditary deafness, syndromic hearing loss and inner ear disease (e.g. Ménière's disease). The anatomical and physiological characteristics of stereocilia are discussed in relation to inner ear malfunctions.

Apical localization of PMCA2w/b is enhanced in terminally polarized MDCK cells

The "w" splice forms of PMCA2 localize to distinct membrane compartments such as the apical membrane of the lactating mammary epithelium, the stereocilia of inner ear hair cells or the post-synaptic density of hippocampal neurons. Previous studies indicated that PMCA2w/b was not fully targeted to the apical domain of MDCK cells but distributed more evenly to the lateral and apical membrane compartments. Overexpression of the apical scaffold protein NHERF2, however, greatly increased the amount of the pump in the apical membrane of these epithelial cells. We generated a stable MDCK cell line expressing non-tagged, full-length PMCA2w/b to further study the localization and function of this protein. Here we demonstrate that PMCA2w/b is highly active and shows enhanced apical localization in terminally polarized MDCK cells grown on semi-permeable filters. Reversible surface biotinylation combined with confocal microscopy of fully polarized cells show that the pump is stabilized in the apical membrane via the apical membrane cytoskeleton with the help of endogenous NHERF2 and ezrin. Disruption of the actin cytoskeleton removed the pump from the apical actin patches without provoking its internalization. Our data suggest that full polarization is a prerequisite for proper positioning of the PMCA2w variants in the apical membrane domain of polarized cells.

Emanuel Strehler’s work on calcium pumps and calcium signaling

Cells are equipped with mechanisms to control tightly the influx, efflux and resting level of free calcium (Ca²⁺). Inappropriate Ca²⁺ signaling and abnormal Ca²⁺ levels are involved in many clinical disorders including heart disease, Alzheimer’s disease and stroke. Ca²⁺ also plays a major role in cell growth, differentiation and motility; disturbances in these processes underlie cell transformation and the progression of cancer. Accordingly, research in the Strehler laboratory is focused on a better understanding of the molecular “toolkit” needed to ensure proper Ca²⁺ homeostasis in the cell, as well as on the mechanisms of localized Ca²⁺ signaling. A long-term focus has been on the plasma membrane calcium pumps (PMCAs), which are linked to multiple disorders including hearing loss, neurodegeneration, and heart disease. Our work over the past 20 years or more has revealed a surprising complexity of PMCA isoforms with different functional characteristics, regulation, and cellular localization. Emerging evidence shows how specific PMCAs contribute not only to setting basal intracellular Ca²⁺ levels, but also to local Ca²⁺ signaling and vectorial Ca²⁺ transport. A second major research area revolves around the calcium sensor protein calmodulin and an enigmatic calmodulin-like protein (CALML3) that is linked to epithelial differentiation. One of the cellular targets of CALML3 is the unconventional motor protein myosin-10, which raises new questions about the role of CALML3 and myosin-10 in cell adhesion and migration in normal cell differentiation and cancer.

Analysis of plasma membrane Ca2+-ATPase gene expression during epileptogenesis employing single hippocampal CA1 neurons

Disruption of calcium homeostasis in epileptic cells is characterized by both short- and long-term perturbations of Ca(2+) buffering systems. Along with the Na(+)/Ca(2+) exchanger, the plasma membrane Ca(2+)-ATPase (PMCA) plays an important role in excitable cells. The involvement of PMCAs in epileptogenesis has primarily been studied in brief intervals after various stimuli; however, the specific contribution of this molecule to epileptogenesis is not yet fully understood. Our aim has been to investigate whether PMCA expression in the chronic stages of epilepsy is altered. Through an interdisciplinary approach, involving whole-cell recordings and real-time reverse transcriptase-polymerase chain reaction, we have shown that epileptic neurons in our preparation consistently show changes in electrical properties during the period of chronic epilepsy. These changes included increased spike frequency, altered resting membrane potential and changes in passive membrane properties. Following these observations, which indicate an altered excitability in the epileptic cells studied, PMCA mRNA transcripts were studied. It was found that while PMCA1 transcripts are significantly increased one month following the pilocarpine epileptogenic stimulus, PMCA3, an isoform important in excitable tissues, was significantly, decreased. These findings suggest that, in the long-term, a slow PMCA (PMCA1) plays a role in the reestablishment of a new calcium homeostasis attained by epileptic cells. Overall, this phenomenon points out the fact that in seizure disorders, changes that take place in the balance of the different molecules and their isoforms in charge of maintaining neuronal calcium homeostasis, are fundamental in the survival of affected cells.

The expression of plasma membrane Ca(2+)-ATPase isoform 2 and its splice variants at sites A and C in the neonatal rat cochlea

OBJECTIVE

To study the expression of plasma membrane Ca(2+)-ATPase isoform 2 (PMCA2) and its alternative splicing at sites A (the first intracellular loop) and C (the C-terminal region) in the neonatal rat cochlea.

METHODS

The cochleae from rats postnatal day 3 to postnatal day 4 (P3-P4) were dissected, fixed, embedded, and sectioned. Meanwhile, the cochlear coils from neonatal rats were isolated and fixed. Using immunofluorescence staining, the expression of PMCA2 was respectively examined in the cochlear sections and cochlear coils. In addition, the total RNAs of basilar membrane (BM, including the organ of corti, the same below), spiral ganglion (SG), spiral ligament (SL, including SV, the same below), and the whole cochlea from neonatal rats were respectively extracted and reverse transcribed to cDNAs, then subjected to primers flanking site A or C in the PMCA2 gene using reverse transcription polymerase chain reaction (RT-PCR). Western blot was also applied to detect the expression of PMCA2 isoforms in the cochlear tissues.

RESULTS

We found that PMCA2 is strongly expressed in outer hair cell (OHC) bundles, SG, and stria vascularis (SV), weakly expressed in Reissner's membrane (RM), and occasionally expressed in inner hair cell (IHC) bundles. Moreover, w/a is the major splice form of PMCA2 present in hair cell bundles, z/b and z/c are the major splice forms of PMCA2 present in SG, and w/a and w/c are the major splice forms of PMCA2 present in SV. In the whole cochlea, variants w, y, and z were detected at site A, and variants a, b, and c were detected at site C. Using Western blot, variant a or b was also detectable in the same cochlear tissues mentioned above.

CONCLUSIONS

PMCA2 and its splice variants at sites A and C are differentially expressed in cochlear tissues of neonatal rat.

Transcriptional mechanisms regulating Ca(2+) homeostasis

Ca(2+) is a dynamic cellular secondary messenger which mediates a vast array of cellular responses. Control over these processes is achieved via an extensive combination of pumps and channels which regulate the concentration of Ca(2+) within not only the cytosol but also all intracellular compartments. Precisely how these pumps and channels are regulated is only partially understood, however, recent investigations have identified members of the Early Growth Response (EGR) family of zinc finger transcription factors as critical players in this process. The roles of several other transcription factors in control of Ca(2+) homeostasis have also been demonstrated, including Wilms Tumor Suppressor 1 (WT1), Nuclear Factor of Activated T cells (NFAT) and c-myc. In this review, we will discuss not only how these transcription factors regulate the expression of the major proteins involved in control of Ca(2+) homeostasis, but also how this transcriptional remodeling of Ca(2+) homeostasis affects Ca(2+) dynamics and cellular responses.

Contribution of plasma membrane Ca2+ ATPase to cerebellar synapse function

The cerebellum expresses one of the highest levels of the plasma membrane Ca²⁺ ATPase, isoform 2 in the mammalian brain. This highly efficient plasma membrane calcium transporter protein is enriched within the main output neurons of the cerebellar cortex; i.e. the Purkinje neurons (PNs). Here we review recent evidence, including electrophysiological and calcium imaging approaches using the plasma membrane calcium ATPase 2 (PMCA2) knockout mouse, to show that PMCA2 is critical for the physiological control of calcium at cerebellar synapses and cerebellar dependent behaviour. These studies have also revealed that deletion of PMCA2 throughout cerebellar development in the PMCA2 knockout mouse leads to permanent signalling and morphological alterations in the PN dendrites. Whilst these findings highlight the importance of PMCA2 during cerebellar synapse function and development, they also reveal some limitations in the use of the PMCA2 knockout mouse and the need for additional experimental approaches including cell-specific and reversible manipulation of PMCAs.

Reduced expression of the Ca(2+) transporter protein PMCA2 slows Ca(2+) dynamics in mouse cerebellar Purkinje neurones and alters the precision of motor coordination

Cerebellar Purkinje neurones (PNs) express high levels of the plasma membrane calcium ATPase, PMCA2, a transporter protein critical for the clearance of calcium from excitable cells. Genetic deletion of one PMCA2 encoding gene in heterozygous PMCA2 knock-out (PMCA2(+/-) mice enabled us to determine how PMCA2 influences PN calcium regulation without the complication of the severe morphological changes associated with complete PMCA2 knock-out (PMCA2(-/-) in these cells. The PMCA2(+/-) cerebellum expressed half the normal levels of PMCA2 and this nearly doubled the time taken for PN dendritic calcium transients to recover (mean fast and slow recovery times increased from 70 ms to 110 ms and from 600 ms to 1100 ms). The slower calcium recovery had distinct consequences for PMCA2(+/-) PN physiology. The PNs exhibited weaker climbing fibre responses, prolonged outward Ca(2+)-dependent K(+) current (mean fast and slow recovery times increased from 136 ms to 192 ms and from 595 ms to 1423 ms) and a slower mean frequency of action potential firing (7.4 Hz compared with 15.8 Hz). Our findings were consistent with prolonged calcium accumulation in the cytosol of PMCA2(+/-) Purkinje neurones. Although PMCA2(+/-) mice exhibited outwardly normal behaviour and little change in their gait pattern, when challenged to run on a narrow beam they exhibited clear deficits in hindlimb coordination. Training improved the motor performance of both PMCA2(+/-) and wild-type mice, although PMCA2(+/-) mice were always impaired. We conclude that reduced calcium clearance perturbs calcium dynamics in PN dendrites and that this is sufficient to disrupt the accuracy of cerebellar processing and motor coordination.

Reduced expression of the "fast" calcium transporter PMCA2a during homeostatic plasticity

In a model of homeostatic plasticity, hippocampal slice culture CA3 pyramidal neurons responded to excitatory synapse inactivity by enhancing glutamate release through an increased number of miniature excitatory post-synaptic currents, mEPSCs and excitatory pre-synaptic terminals. Also accompanying these changes was a specific reduction in the expression of a "fast" calcium transporter, the plasma membrane calcium ATPase, PMCA2a. This transporter normally influences glutamate release from excitatory terminals where it helps control calcium levels. The reduction in PMCA2a expression occurred within 2 days of synapse inactivity; it was specific and reversible in young and mature hippocampal slice cultures and required removal of NMDA receptor mediated activity. Furthermore, the enhanced mEPSCs in the model were resistant to pharmacological inhibition of PMCA transporter activity. Reduced expression of PMCA2a during homeostatic plasticity therefore provides a mechanism to remodel pre-synaptic Ca2+ dynamics as a flexible way to alter glutamate release.

Role of plasma membrane calcium ATPases in calcium clearance from olfactory sensory neurons

Odorants cause Ca(2+) to rise in olfactory sensory neurons (OSNs) first within the ciliary compartment, then in the dendritic knob, and finally in the cell body. Ca(2+) not only excites but also produces negative feedback on the transduction pathway. To relieve this Ca(2+)-dependent adaptation, Ca(2+) must be cleared from the cilia and dendritic knob by mechanisms that are not well understood. This work focuses on the roles of plasma membrane calcium pumps (PMCAs) through the use of inhibitors and mice missing 1 of the 4 PMCA isoforms (PMCA2). We demonstrate a significant contribution of PMCAs in addition to contributions of the Na(+)/Ca(2+) exchanger and endoplasmic reticulum (ER) calcium pump to the rate of calcium clearance after OSN stimulation. PMCAs in neurons can shape the Ca(2+) signal. We discuss the contributions of the specific PMCA isoforms to the shape of the Ca(2+) transient that controls signaling and adaptation in OSNs.

"Fast" plasma membrane calcium pump PMCA2a concentrates in GABAergic terminals in the adult rat brain

The plasma membrane Ca(2+)-ATPases (PMCA) represent the major high-affinity Ca(2+) extrusion system in the brain. PMCAs comprise four isoforms and over 20 splice variants. Their different functional properties may permit different PMCA splice variants to accommodate different kinds of local [Ca(2+)] transients, but for a specific PMCA to play a unique role in local Ca(2+) handling it must be targeted to the appropriate subcellular compartment. We used immunohistochemistry to study the spatial distribution of PMCA2a-one of the two major carboxyl-terminal splice variants of PMCA2-in the adult rat brain, testing whether this isoform, with especially high basal activity, is targeted to specific subcellular compartments. In striking contrast to the widespread distribution of PMCA2 as a whole, we found that PMCA2a is largely restricted to parvalbumin-positive inhibitory presynaptic terminals throughout the brain. The only major exception to this targeting pattern was in the cerebellar cortex, where PMCA2a also concentrates postsynaptically, in the spines of Purkinje cells. We propose that the fast Ca(2+) activation kinetics and high V(max) of PMCA2a make this pump especially suited for rapid clearance of presynaptic Ca(2+) in fast-spiking inhibitory nerve terminals, which face severe transient calcium loads.

Plasma membrane Ca-ATPase isoform expression in human cataractous lenses compared to age-matched clear lenses

The plasma membrane calcium ATPase (PMCA) pump is the major mechanism by which calcium is removed from the lens. The aim of this study was to determine if mRNA and proteins levels of PMCA isoforms changed with age or lens opacity. mRNA was quantified using a quantitative real-time reverse transcription polymerase chain reaction assay (RT-PCR). PMCA protein levels were quantified using Western blot analysis. No PMCA mRNA or proteins were detected in human lens fiber cells. The mRNA and protein levels of PMCA1, 3 and 4 in the epithelium of cataractous lenses were similar to those of epithelium from age-matched clear lenses and were also the same in younger lenses. PMCA2 mRNA and protein levels were 1.6-2.5 times higher, respectively, in cataractous lenses compared to age-matched clear lenses. Elevated PMCA2 expression in cataractous lenses might be a compensatory mechanism to overcome higher intracellular calcium levels in cataract.

Plasma-membrane Ca(2+) pumps: structural diversity as the basis for functional versatility

Plasma-membrane calcium pumps [PMCAs (plasma-membrane Ca(2+)-ATPases)] expel Ca(2+) from eukaryotic cells to maintain overall Ca(2+) homoeostasis and to provide local control of intracellular Ca(2+) signalling. Recent work indicates functional versatility among PMCA isoforms, with specific pumps being essential for cochlear hair cell function, sperm motility, feedback signalling in the heart and pre- and post-synaptic Ca(2+) regulation in neurons. The functional versatility of PMCAs is due to differences in their regulation by CaM (calmodulin), kinases and other signalling proteins, as well as to their differential targeting and retention in defined plasma membrane domains. The basis for this is the structural diversity of PMCAs. In mammals, four genes encode PMCA isoforms 1-4, and each of these has multiple variants generated by alternative RNA splicing. The alternatively spliced regions are intimately involved in the regulatory interactions and differential membrane localization of the pumps. The alternatively spliced C-terminal tail acts as an autoinhibitory domain by interacting with the catalytic core of the pump. The degree of inhibition and the kinetics of interaction with the major activator CaM differ between PMCA variants. This translates into functional differences in how PMCAs handle Ca(2+) signals of different magnitude and frequency. Accumulating evidence thus demonstrates how structural diversity provides functional versatility in the PMCAs.