The plasma membrane Ca2+ pump isoform 4a differs from isoform 4b in the mechanism of calmodulin binding and activation kinetics: implications for Ca2+ signaling

17β-estradiol induces hyperresponsiveness in guinea pig airway smooth muscle by inhibiting the plasma membrane Ca2+-ATPase

High serum estrogen concentrations are associated with asthma development and severity, suggesting a link between estradiol and airway hyperresponsiveness (AHR). 17β-estradiol (E2) has non-genomic effects via Ca2+ regulatory mechanisms; however, its effect on the plasma membrane Ca2+-ATPases (PMCA1 and 4) and sarcoplasmic reticulum Ca2+-ATPase (SERCA) is unknown. Hence, in the present study, we aim to demonstrate if E2 favors AHR by increasing intracellular Ca2+ concentrations in guinea pig airway smooth muscle (ASM) through a mechanism involving Ca2+-ATPases. In guinea pig ASM, Ca2+ microfluorometry, muscle contraction, and Western blot were evaluated. Then, we performed molecular docking analysis between the estrogens and Ca2+ ATPases. In tracheal rings, E2 produced AHR to carbachol. In guinea pig myocytes, acute exposure to physiological levels of E2 modified the transient Ca2+ peak induced by caffeine to a Ca2+ plateau. The incubation with PMCA inhibitors (lanthanum and carboxyeosin, CE) partially reversed the E2-induced sustained plateau in the caffeine response. In contrast, cyclopiazonic acid (SERCA inhibitor), U-0126 (an inhibitor of ERK 1/2), and choline chloride did not modify the Ca2+ plateau produced by E2. The mitochondrial uniporter activity and the capacitative Ca2+ entry were unaffected by E2. In guinea pig ASM, Western blot analysis demonstrated PMCA1 and PMCA4 expression. The results from the docking modeling demonstrate that E2 binds to both plasma membrane ATPases. In guinea pig tracheal smooth muscle, inhibiting the PMCA with CE, induced hyperresponsiveness to carbachol. 17β-estradiol produces hyperresponsiveness by inhibiting the PMCA in the ASM and could be one of the mechanisms responsible for the increase in asthmatic crisis in women.

Monomeric α-synuclein activates the plasma membrane calcium pump

Alpha-synuclein (aSN) is a membrane-associated and intrinsically disordered protein, well known for pathological aggregation in neurodegeneration. However, the physiological function of aSN is disputed. Pull-down experiments have pointed to plasma membrane Ca2+ -ATPase (PMCA) as a potential interaction partner. From proximity ligation assays, we find that aSN and PMCA colocalize at neuronal synapses, and we show that calcium expulsion is activated by aSN and PMCA. We further show that soluble, monomeric aSN activates PMCA at par with calmodulin, but independent of the autoinhibitory domain of PMCA, and highly dependent on acidic phospholipids and membrane-anchoring properties of aSN. On PMCA, the key site is mapped to the acidic lipid-binding site, located within a disordered PMCA-specific loop connecting the cytosolic A domain and transmembrane segment 3. Our studies point toward a novel physiological role of monomeric aSN as a stimulator of calcium clearance in neurons through activation of PMCA.

Identification of ATP2B4 Regulatory Element Containing Functional Genetic Variants Associated with Severe Malaria

Genome-wide association studies for severe malaria (SM) have identified 30 genetic variants mostly located in non-coding regions. Here, we aimed to identify potential causal genetic variants located in these loci and demonstrate their functional activity. We systematically investigated the regulatory effect of the SNPs in linkage disequilibrium (LD) with the malaria-associated genetic variants. Annotating and prioritizing genetic variants led to the identification of a regulatory region containing five ATP2B4 SNPs in LD with rs10900585. We found significant associations between SM and rs10900585 and our candidate SNPs (rs11240734, rs1541252, rs1541253, rs1541254, and rs1541255) in a Senegalese population. Then, we demonstrated that both individual SNPs and the combination of SNPs had regulatory effects. Moreover, CRISPR/Cas9-mediated deletion of this region decreased ATP2B4 transcript and protein levels and increased Ca²⁺ intracellular concentration in the K562 cell line. Our data demonstrate that severe malaria-associated genetic variants alter the expression of ATP2B4 encoding a plasma membrane calcium-transporting ATPase 4 (PMCA4) expressed on red blood cells. Altering the activity of this regulatory element affects the risk of SM, likely through calcium concentration effect on parasitaemia.

Structure, Function and Regulation of the Plasma Membrane Calcium Pump in Health and Disease

In this review, I summarize the present knowledge of the structural and functional properties of the mammalian plasma membrane calcium pump (PMCA). It is outlined how the cellular expression of the different spliced isoforms of the four genes are regulated under normal and pathological conditions.

The plasma membrane calcium pumps-The old and the new

The plasma membrane Ca²⁺-ATPase (PMCA) pumps play a critical role in the maintenance of calcium (Ca²⁺) homeostasis, crucial for optimal neuronal function and cell survival. Loss of Ca²⁺ homeostasis is a key precursor in neuronal dysfunction associated with brain aging and in the pathogenesis of neurodegenerative disorders. In this article, we review evidence showing age-related changes in the PMCAs in synaptic plasma membranes (SPMs) and lipid raft microdomains isolated from rat brain. Both PMCA activity and protein levels decline progressively with increasing age. However, the loss of activity is disproportionate to the reduction of protein levels suggesting the presence of dysfunctional PMCA molecules in aged brain. PMCA activity is also diminished in post-mortem human brain samples from Alzheimer's disease and Parkinson's disease patients and in cell models of these neurodegenerative disorders. Experimental reduction of the PMCAs not only alter Ca²⁺ homeostasis but also have diverse effects on neurons such as reduced neuritic network, impaired release of neurotransmitter and increased susceptibility to stressful stimuli, particularly to agents that elevate intracellular Ca²⁺ [Ca²⁺]_i. Loss of PMCA is likely to contribute to neuronal dysfunction observed in the aging brain and in the development of age-dependent neurodegenerative disorders. Therapeutic (pharmacological and/or non-pharmacological) approaches that can enhance PMCA activity and stabilize [Ca²⁺]_i homeostasis may be capable of preventing, slowing, and/or reversing neuronal degeneration.

Implications of the thyroid hormone on neuronal development with special emphasis on the calmodulin-kinase IV pathway

Thyroid hormones influence brain development through regulation of gene expression. This is especially true for Ca²⁺-dependent regulation since a major pathway is controlled by the Ca²⁺/calmodulin-dependent protein kinase IV (CaMKIV) which in turn is induced by the thyroid hormone T₃. In addition, CaMKIV is involved in regulation of alternative splicing of a number of protein isoforms, among them PMCA1a, the neuronal specific isoform of the plasma membrane calcium pump. On the other hand, hypothyroidism or CaMKIV deficiency can have a severe influence on brain development. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Epididymosomes: transfer of fertility-modulating proteins to the sperm surface

A variety of glycosylphosphatidylinositol (GPI)-linked proteins are acquired on spermatozoa from epididymal luminal fluids (ELF) during sperm maturation. These proteins serve roles in immunoprotection and in key steps of fertilization such as capacitation, acrosomal exocytosis and sperm-egg interactions. Their acquisition on sperm cells is mediated both by membrane vesicles (epididymosomes, EP) which were first reported to dock on the sperm surface, and by lipid carriers which facilitate the transfer of proteins associated with the membrane-free fraction of ELF. While the nonvesicular fraction is more efficient, both pathways are dependent on hydrophobic interactions between the GPI-anchor and the external lipid layer of the sperm surface. More recently proteomic and hypothesis-driven studies have shown that EP from several mammals carry transmembrane (TM) proteins, including plasma membrane Ca 2 + -ATPase 4 (PMCA4). Synthesized in the testis, PMCA4 is an essential protein and the major Ca 2 + efflux pump in murine spermatozoa. Delivery of PMCA4 to spermatozoa from bovine and mouse EP during epididymal maturation and in vitro suggests that the docking of EP on the sperm surface precedes fusion, and experimental evidence supports a fusogenic mechanism for TM proteins. Fusion is facilitated by CD9, which generates fusion-competent sites on membranes. On the basis of knowledge of PMCA4's interacting partners a number of TM and membrane-associated proteins have been identified or are predicted to be present, in the epididymosomal cargo deliverable to spermatozoa. These Ca 2 + -dependent proteins, undetected in proteomic studies, play essential roles in sperm motility and fertility, and their detection highlights the usefulness of the hypothesis-driven approach.

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.

Computational reconstitution of spine calcium transients from individual proteins

We have built a stochastic model in the program MCell that simulates Ca(2+) transients in spines from the principal molecular components believed to control Ca(2+) entry and exit. Proteins, with their kinetic models, are located within two segments of dendrites containing 88 intact spines, centered in a fully reconstructed 6 × 6 × 5 μm(3) cube of hippocampal neuropil. Protein components include AMPA- and NMDA-type glutamate receptors, L- and R-type voltage-dependent Ca(2+) channels, Na(+)/Ca(2+) exchangers, plasma membrane Ca(2+) ATPases, smooth endoplasmic reticulum Ca(2+) ATPases, immobile Ca(2+) buffers, and calbindin. Kinetic models for each protein were taken from published studies of the isolated proteins in vitro. For simulation of electrical stimuli, the time course of voltage changes in the dendritic spine was generated with the desired stimulus in the program NEURON. Voltage-dependent parameters were then continuously re-adjusted during simulations in MCell to reproduce the effects of the stimulus. Nine parameters of the model were optimized within realistic experimental limits by a process that compared results of simulations to published data. We find that simulations in the optimized model reproduce the timing and amplitude of Ca(2+) transients measured experimentally in intact neurons. Thus, we demonstrate that the characteristics of individual isolated proteins determined in vitro can accurately reproduce the dynamics of experimentally measured Ca(2+) transients in spines. The model will provide a test bed for exploring the roles of additional proteins that regulate Ca(2+) influx into spines and for studying the behavior of protein targets in the spine that are regulated by Ca(2+) influx.

Plasma membrane Ca2+-ATPase 4: interaction with constitutive nitric oxide synthases in human sperm and prostasomes which carry Ca2+/CaM-dependent serine kinase

Deletion of the gene encoding the widely conserved plasma membrane calcium ATPase 4 (PMCA4), a major Ca(2+) efflux pump, leads to loss of sperm motility and male infertility in mice. PMCA4's partners in sperm and how its absence exerts its effect on fertility are unknown. We hypothesize that in sperm PMCA4 interacts with endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS) which are rapidly activated by Ca(2+), and that these fertility-modulating proteins are present in prostasomes, which deliver them to sperm. We show that in human sperm PMCA4 is present on the acrosome, inner acrosomal membrane, posterior head, neck, midpiece and the proximal principal piece. PMCA4 localization showed inter- and intra-individual variation and was most abundant at the posterior head/neck junction, co-localizing with NOSs. Co-immunoprecipitations (Co-IP) revealed a close association of PMCA4 and the NOSs in Ca(2+) ionophore-treated sperm but much less so in uncapacitated untreated sperm. Fluorescence resonance energy transfer (FRET) showed a similar Ca(2+)-related association: PMCA4 and the NOSs are within 10 nm apart, and preferentially so in capacitated, compared with uncapacitated, sperm. FRET efficiencies varied, being significantly (P < 0.001) higher at high cytosolic Ca(2+) concentration ([Ca(2+)]c) in capacitated sperm than at low [Ca(2+)]c in uncapacitated sperm for the PMCA4-eNOS complex. These dynamic interactions were not seen for PMCA4-nNOS complexes, which had the highest FRET efficiencies. Further, along with Ca(2+)/CaM-dependent serine kinase (CASK), PMCA4 and the NOSs are present in the seminal plasma, specifically in prostasomes where Co-IP showed complexes similar to those in sperm. Finally, flow cytometry demonstrated that following co-incubation of sperm and seminal plasma, PMCA4 and the NOSs can be delivered in vitro to sperm via prostasomes. Our findings indicate that PMCA4 interacts simultaneously with the NOSs preferentially at high [Ca(2+)]c in sperm to down-regulate them, and thus prevent elevated levels of NO, known to induce asthenozoospermia via oxidative stress. Our studies point to the potential underlying cause of infertility in PMCA4's absence, and suggest that inactivating mutations of PMCA4 could lead to asthenozoospermia and human infertility. Screening for these mutations may serve both diagnostic and therapeutic purposes.

The plethora of PMCA isoforms: Alternative splicing and differential expression

In this review the four different genes of the mammalian plasma membrane calcium ATPase (PMCA) and their spliced isoforms are discussed with respect to their tissue distribution, their differences during development and their importance for regulating Ca²⁺ homeostasis under different conditions. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

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

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

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.

Expression and secretion of plasma membrane Ca2+-ATPase 4a (PMCA4a) during murine estrus: association with oviductal exosomes and uptake in sperm

PMCA4, a membrane protein, is the major Ca²⁺ efflux pump in murine sperm where its deletion leads to a severe loss of hyperactivated motility and to male infertility. We have previously shown that the PMCA4b splice variant interacts with CASK (Ca^2+/CaM-dependent serine kinase) in regulating sperm Ca²⁺. More recently we detected that PMCA4a isoform, in addition to its presence in testis, is secreted in the epididymal luminal fluid and transferred to sperm. Here we show that Pmca4 mRNA is expressed in both the 4a and 4b variants in the vagina, uterus, and oviduct. Immunofluorescence reveals that PMCA4a is similarly expressed and is elevated during estrus, appearing in the glandular and luminal epithelia. Western analysis detected PMCA4a in all tissues and in the luminal fluids (LF) of the vagina (VLF), uterus (ULF), and the oviduct (OLF) collected during estrus. It was ~9- and 4-fold higher in OLF than in VLF and ULF, and only marginally present in LF collected at metestrus/diestrus. Fractionation of the LF collected at estrus, via ultracentrifugation, revealed that 100% of the PMCA4a resides in the vesicular fraction of the ULF and OLF. Transmission electron microscopy (TEM) revealed that OLF vesicles have an exosomal orientation (with the cytoplasmic-side inward), a size range of 25-100 nm, with the characteristic CD9 biomarker. Thus, we dubbed these vesicles “oviductosomes”, to which PMCA4a was immunolocalized. Incubation of caudal sperm in the combined LF or exosomes resulted in up to a ~3-fold increase of sperm PMCA4a, as detected by flow cytometry, indicating in vitro uptake. Our results are consistent with the increased requirement of Ca²⁺ efflux in the oviduct. They show for the first time the presence of oviductal exosomes and highlight their role, along with uterosomes and vaginal exosomes, in post-testicular sperm acquisition of PMCA4a which is essential for hyperactivated motility and fertility.

Plasma membrane Ca2+-ATPase 4 in murine epididymis: secretion of splice variants in the luminal fluid and a role in sperm maturation

Plasma membrane Ca(2+)-ATPase isoform 4 (PMCA4) is the primary Ca(2+) efflux pump in murine sperm, where it regulates motility. In Pmca4 null sperm, motility loss results in infertility. We have shown that murine sperm PMCA4b interacts with Ca(2+)/CaM-dependent serine kinase (CASK) in regulating Ca(2+) homeostasis and motility. However, recent work indicated that the bovine PMCA4a splice variant (missing in testis) is epididymally expressed, along with 4b, and may be transferred to sperm. Here we show, via conventional and in situ RT-PCR, that both the splice variants of Pmca4 mRNA are expressed in murine testis and throughout the epididymis. Immunofluorescence localized PMCA4a to the apical membrane of the epididymal epithelium, and Western analysis not only confirmed its presence but showed for the first time that PMCA4a and PMCA4b are secreted in the epididymal luminal fluid (ELF), from which epididymosomes containing PMCA4a were isolated. Flow cytometry indicated the presence of PMCA4a on mature caudal sperm where it was increased ~5-fold compared to caput sperm (detected by Western blotting) and ~2-fold after incubation in ELF, revealing in vitro uptake and implicating PMCA4a in epididymal sperm maturation. Coimmunoprecipitation using pan-PMCA4 antibodies, revealed that both variants associate with CASK, suggesting their presence in a complex. Because they have different kinetic properties for Ca(2+) transport and different abilities to bind to CASK, our study suggests a mechanism for combining the functional attributes of both PMCA4 variants, leading to heightened efficiency of the pump in the maintenance of Ca(2+) homeostasis, which is crucial for normal motility and male fertility.

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.

Distinct mechanisms of calmodulin binding and regulation of adenylyl cyclases 1 and 8

Calmodulin (CaM), by mediating the stimulation of the activity of two adenylyl cyclases (ACs), plays a key role in integrating the cAMP and Ca(2+) signaling systems. These ACs, AC1 and AC8, by decoding discrete Ca(2+) signals can contribute to fine-tuning intracellular cAMP dynamics, particularly in neurons where they predominate. CaM comprises an α-helical linker separating two globular regions at the N-terminus and the C-terminus that each bind two Ca(2+) ions. These two lobes have differing affinities for Ca(2+), and they can interact with target proteins independently. This study explores previous indications that the two lobes of CaM can regulate AC1 and AC8 differently and thereby yield different responses to cellular transitions in [Ca(2+)](i). We first compared by glutathione S-transferase pull-down assays and offline nanoelectrospray ionization mass spectrometry the interaction of CaM and Ca(2+)-binding deficient mutants of CaM with the internal CaM binding domain (CaMBD) of AC1 and the two terminal CaMBDs of AC8. We then examined the influence of these three CaMBDs on Ca(2+) binding by native and mutated CaM in stopped-flow experiments to quantify their interactions. The three CaMBDs show quite distinct interactions with the two lobes of CaM. These findings establish the critical kinetic differences between the mechanisms of Ca(2+)-CaM activation of AC1 and AC8, which may underpin their different physiological roles.

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.

Linear motifs: lost in (pre)translation

Pretranslational modification by alternative splicing, alternative promoter usage and RNA editing enables the production of multiple protein isoforms from a single gene. A large quantity of data now supports the notion that short linear motifs (SLiMs), which are protein interaction modules enriched within intrinsically disordered regions, are key for the functional diversification of these isoforms. The inclusion or removal of these SLiMs can switch the subcellular localisation of an isoform, promote cooperative associations, refine the affinity of an interaction, coordinate phase transitions within the cell, and even create isoforms of opposing function. This article discusses the novel functionality enabled by the addition or removal of SLiM-containing exons by pretranslational modifications, such as alternative splicing and alternative promoter usage, and how these alterations enable the creation and modulation of complex regulatory and signalling pathways.

Calcium microdomains at the immunological synapse: how ORAI channels, mitochondria and calcium pumps generate local calcium signals for efficient T-cell activation

Cell polarization enables restriction of signalling into microdomains. Polarization of lymphocytes following formation of a mature immunological synapse (IS) is essential for calcium-dependent T-cell activation. Here, we analyse calcium microdomains at the IS with total internal reflection fluorescence microscopy. We find that the subplasmalemmal calcium signal following IS formation is sufficiently low to prevent calcium-dependent inactivation of ORAI channels. This is achieved by localizing mitochondria close to ORAI channels. Furthermore, we find that plasma membrane calcium ATPases (PMCAs) are re-distributed into areas beneath mitochondria, which prevented PMCA up-modulation and decreased calcium export locally. This nano-scale distribution-only induced following IS formation-maximizes the efficiency of calcium influx through ORAI channels while it decreases calcium clearance by PMCA, resulting in a more sustained NFAT activity and subsequent activation of T 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.

Switch of PMCA4 splice variants in bovine epididymis results in altered isoform expression during functional sperm maturation

Ca(2+) and Ca(2+)-dependent signals are essential for sperm maturation and fertilization. In mouse sperm the plasma membrane Ca(2+)-ATPase (PMCA) isoform 4 plays a crucial role in Ca(2+) transport. The two major splice variants of PMCA4 are PMCA4a and PMCA4b. PMCA4a differs from PMCA4b in the mechanism of calmodulin binding and activation. PMCA4a shows a much higher basal activity and is more effective than PMCA4b in returning Ca(2+) to resting levels. Knock-out mice carrying a PMCA4-null mutation are infertile because their sperm cannot achieve a hyperactivated state of motility. As sperm reach functional maturity during their transit through the epididymis, the expression of PMCA4a and 4b was assessed in bull testis and epididymis. Quantitative PCR revealed that PMCA4b is the major splice variant in testis, caput, and corpus epididymidis. In contrast, PMCA4a is the major splice variant in cauda epididymidis, whereas sperm are transcriptionally silent. Immunohistochemical staining using a new antibody against bovine PMCA4a located the PMCA4a to the apical membrane of the epithelium of cauda epididymidis, whereas testis, caput, and corpus epididymidis were negative. Western blotting of testis, epididymis, and sperm isolated from caput and cauda epididymidis showed a much higher level of PMCA4a in cauda epididymidis and sperm from cauda epididymidis compared with testis membranes and sperm from caput epididymidis. These findings suggest that PMCA4a is transferred to bovine sperm membranes in cauda epididymidis. This isoform switch may facilitate a higher calcium turnover in sperm necessary to traverse the female genital tract.

The plasma membrane Ca²+ ATPase and the plasma membrane sodium calcium exchanger cooperate in the regulation of cell calcium

Calcium is an ambivalent signal: it is essential for the correct functioning of cell life, but may also become dangerous to it. The plasma membrane Ca(2+) ATPase (PMCA) and the plasma membrane Na(+)/Ca(2+) exchanger (NCX) are the two mechanisms responsible for Ca(2+) extrusion. The NCX has low Ca(2+) affinity but high capacity for Ca(2+) transport, whereas the PMCA has a high Ca(2+) affinity but low transport capacity for it. Thus, traditionally, the PMCA pump has been attributed a housekeeping role in maintaining cytosolic Ca(2+), and the NCX the dynamic role of counteracting large cytosolic Ca(2+) variations (especially in excitable cells). This view of the roles of the two Ca(2+) extrusion systems has been recently revised, as the specific functional properties of the numerous PMCA isoforms and splicing variants suggests that they may have evolved to cover both the basal Ca(2+) regulation (in the 100 nM range) and the Ca(2+) transients generated by cell stimulation (in the μM range).

Kinetic analysis reveals differences in the binding mechanism of calmodulin and calmodulin-like protein to the IQ motifs of myosin-10

Myo10 is an unconventional myosin with important functions in filopodial motility, cell migration, and cell adhesion. The neck region of Myo10 contains three IQ motifs that bind calmodulin (CaM) or the tissue-restricted calmodulin-like protein (CLP) as light chains. However, little is known about the mechanism of light chain binding to the IQ motifs in Myo10. Binding of CaM and CLP to each IQ motif was assessed by nondenaturing gel electrophoresis and by stopped-flow experiments using fluorescence-labeled CaM and CLP. Although the binding kinetics are different in each case, there are similarities in the mechanism of binding of CaM and CLP to IQ1 and IQ2: for both IQ motifs Ca(2+) increased the binding affinity, mainly by increasing the rate of the forward steps. The general kinetic mechanism comprises a two-step process, which in some cases may involve the binding of a second IQ motif with lower affinity. For IQ3, however, the kinetics of CaM binding is very different from that of CLP. In both cases, binding in the absence of Ca(2+) is poor, and addition of Ca(2+) decreases the K(d) to below 10 nM. However, while the CaM binding kinetics are complex and best fitted by a multistep model, binding of CLP is fitted by a relatively simple two-step model. The results show that, in keeping with growing structural evidence, complexes between CaM or CaM-like myosin light chains and IQ motifs are highly diverse and depend on the specific sequence of the particular IQ motif as well as the light chain.

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.

The influence of calcium signaling on the regulation of alternative splicing

In this review the influence of calcium signaling on the regulation of alternative splicing is discussed with respect to its influence on cell- and developmental-specific expression of different isoforms of the plasma membrane calcium pump (PMCA). In a second part the possibility is discussed that due to the interaction of the calcium-binding protein ALG-2 with a spliceosomal regulator of alternative splicing, RBM22, Ca2+-signaling may thus influence its regulatory property.

Functional effects of caloxin 1c2, a novel engineered selective inhibitor of plasma membrane Ca(2+)-pump isoform 4, on coronary artery

Coronary artery smooth muscle expresses the plasma membrane Ca²⁺ pump (PMCA) isoforms PMCA4 and PMCA1. We previously reported the peptide inhibitor caloxin 1b1 that was obtained by using extracellular domain 1 of PMCA4 as the target (Am J Physiol Cell.290 [2006] C1341). To engineer inhibitors with greater affinity and isoform selectivity, we have now created a phage display library of caloxin 1b1-like peptides. We screened this library by affinity chromatography with PMCA from erythrocyte ghosts that contain mainly PMCA4 to obtain caloxin 1c2. Key properties of caloxin 1c2 are (a) Ki = 2.3 ± 0.3 μM which corresponds to a 20× higher affinity for PMCA4 than that of caloxin 1b1 and (b) it is selective for PMCA4 since it has greater than 10-fold affinity for PMCA4 than for PMCA1, 2 or 3. It had the following functional effects on coronary artery smooth muscle: (a) it increased basal tone of the de-endothelialized arteries; the increase being similar at 10, 20 or 50 μM, and (b) it enhanced the increase in the force of contraction at 0.05 but not at 1.6 mM extracellular Ca²⁺ when Ca²⁺ extrusion via the Na⁺–Ca²⁺ exchanger and the sarco/endoplasmic reticulum Ca²⁺ pump were inhibited. We conclude that PMCA4 is pivotal to Ca²⁺ extrusion in coronary artery smooth muscle. We anticipate caloxin 1c2 to aid in understanding the role of PMCA4 in signal transduction and home-ostasis due to its isoform selectivity and ability to act when added extracellularly.

Caloxins: a novel class of selective plasma membrane Ca2+ pump inhibitors obtained using biotechnology

Plasma membrane Ca2+ pumps (PMCA) extrude cellular Ca2+ with a high affinity and hence play a major role in Ca2+ homeostasis and signaling. Caloxins (selective extracellular PMCA inhibitors) would aid in elucidating the physiology of PMCA. PMCA proteins have five extracellular domains (exdoms). Our hypotheses are: 1) peptides that bind selectively to each exdom can be invented by screening a random peptide library, and 2) a peptide can modulate PMCA activity by binding to one of the exdoms. The first caloxin 2a1, selected for binding exdom 2 was selective for PMCA (Ki=529 microM). It has been used to examine the physiological role of PMCA. PMCA isoforms are encoded by four genes. PMCA isoform expression differs in various cell types, with PMCA1 and 4 being the most widely distributed. There are differences between PMCA1-4 exdom 1 sequences, which may be exploited for inventing isoform selective caloxins. Using exdom 1 of PMCA4 as a target, modified screening procedures and mutagenesis led to the high-affinity caloxin 1c2 (Ki=2.3 microM for PMCA4). It is selective for PMCA4 over PMCA1, 2, or 3. We hope that caloxins can be used to discern the roles of individual PMCA isoforms in Ca2+ homeostasis and signaling. Caloxins may also become clinically useful in cardiovascular diseases, neurological disorders, retinopathy, cancer, and contraception.