Characterization of calumenin-SERCA2 interaction in mouse cardiac sarcoplasmic reticulum

Characterize direct protein interactions with enrichable, cleavable and latent bioreactive unnatural amino acids

Latent bioreactive unnatural amino acids (Uaas) have been widely used in the development of covalent drugs and identification of protein interactors, such as proteins, DNA, RNA and carbohydrates. However, it is challenging to perform high-throughput identification of Uaa cross-linking products due to the complexities of protein samples and the data analysis processes. Enrichable Uaas can effectively reduce the complexities of protein samples and simplify data analysis, but few cross-linked peptides were identified from mammalian cell samples with these Uaas. Here we develop an enrichable and multiple amino acids reactive Uaa, eFSY, and demonstrate that eFSY is MS cleavable when eFSY-Lys and eFSY-His are the cross-linking products. An identification software, AixUaa is developed to decipher eFSY mass cleavable data. We systematically identify direct interactomes of Thioredoxin 1 (Trx1) and Selenoprotein M (SELM) with eFSY and AixUaa.

Structural Plasticity Allows Calumenin-1 to Moonlight as a Ca2+-Independent Chaperone: Pb2+ Enables Probing Alternate Inhibitory Conformation

Human calumenin-1 (HsCalu-1) is an endoplasmic reticulum (ER) and Golgi-resident Ca2+-binding protein of the hepta-EF-hand superfamily that plays a vital role in maintaining the cytoplasmic Ca2+ concentration below toxic levels by interacting with Sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) and ryanodine receptors (RyR), indicating its role in Ca2+ homeostasis in the ER. HsCalu-1 seems to be able to exhibit structural plasticity to achieve its plethora of functions. In this study, we demonstrate that HsCalu-1 acts as a chaperone in both its intrinsically disordered state (apo form) and the structured state (Ca2+-bound form). HsCalu-1 chaperone activity is independent of Ca2+ and Pb2+ binding attenuating its chaperone-like activity. Incidentally, Pb2+ binds to HsCalu-1 with lower affinity (KD = 38.46 μM) (compared to Ca2+-binding), leading to the formation of a less-stable conformation as observed by a sharp drop in its melting temperature Tm from 67 °C in the Ca2+-bound form to 43 °C in the presence of Pb2+. The binding site for Pb2+ was mapped as being in the EF-Hand-234 domain of HsCalu-1, a region that overlaps with the Ca2+-dependent initiator of its functional fold. A change in the secondary and tertiary structure, leading to a less-stable but compact conformation upon Pb2+ binding, is the mechanism by which the chaperone-like activity of HsCalu-1 is diminished. Our results not only demonstrate the chaperone activity by a protein in its disordered state but also explain, using Pb2+ as a probe, that the multiple functions of calumenin are due to its ability to adopt a quasi-stable conformation.

Physiology of intracellular calcium buffering

Calcium signaling underlies much of physiology. Almost all the Ca2+ in the cytoplasm is bound to buffers, with typically only ∼1% being freely ionized at resting levels in most cells. Physiological Ca2+ buffers include small molecules and proteins, and experimentally Ca2+ indicators will also buffer calcium. The chemistry of interactions between Ca2+ and buffers determines the extent and speed of Ca2+ binding. The physiological effects of Ca2+ buffers are determined by the kinetics with which they bind Ca2+ and their mobility within the cell. The degree of buffering depends on factors such as the affinity for Ca2+, the Ca2+ concentration, and whether Ca2+ ions bind cooperatively. Buffering affects both the amplitude and time course of cytoplasmic Ca2+ signals as well as changes of Ca2+ concentration in organelles. It can also facilitate Ca2+ diffusion inside the cell. Ca2+ buffering affects synaptic transmission, muscle contraction, Ca2+ transport across epithelia, and the killing of bacteria. Saturation of buffers leads to synaptic facilitation and tetanic contraction in skeletal muscle and may play a role in inotropy in the heart. This review focuses on the link between buffer chemistry and function and how Ca2+ buffering affects normal physiology and the consequences of changes in disease. As well as summarizing what is known, we point out the many areas where further work is required.

Upregulation of miR-335-5p Contributes to Right Ventricular Remodeling via Calumenin in Pulmonary Arterial Hypertension

Right ventricular (RV) failure determines the prognosis in pulmonary arterial hypertension (PAH), but the underlying mechanism is still unclear. Growing evidence has shown that microRNAs participate in RV remodeling. This study is undertaken to explore the role of miR-335-5p in regulating RV remodeling induced by PAH. Two PAH models were used in the study, including the monocrotaline rat model and hypoxia/su5416 mouse model. miRNA sequencing and RT-qPCR validation identified that miR-335-5p was elevated in the RV of PAH rats. In vitro, miR-335-5p expression was increased after angiotensin II treatment, and miR-335-5p inhibition relieved angiotensin II-induced cardiomyocyte hypertrophy. The luciferase reporter assay showed that calumenin was a target gene for miR-335-5p. Pretreatment with miR-335-5p inhibitors could rescue calumenin downregulation induced by angiotensin II in H9C2 cells. Moreover, intracellular Ca²⁺ concentration and apoptosis were increased after angiotensin II treatment, and miR-335-5p inhibition decreased intracellular Ca²⁺ accumulation and apoptosis. Finally, in vivo miR-335-5p downregulation (antagomir miR-335-5p) attenuated RV remodeling and rescued calumenin downregulation under conditions of hypoxia/su5416 exposure. Our work highlights the role of miR-335-5p and calumenin in RV remodeling and may lead to the development of novel therapeutic strategies for right heart failure.

Spatial and Functional Crosstalk between the Mitochondrial Na+-Ca2+ Exchanger NCLX and the Sarcoplasmic Reticulum Ca2+ Pump SERCA in Cardiomyocytes

The mitochondrial Na⁺-Ca²⁺ exchanger, NCLX, was reported to supply Ca²⁺ to sarcoplasmic reticulum (SR)/endoplasmic reticulum, thereby modulating various cellular functions such as the rhythmicity of cardiomyocytes, and cellular Ca²⁺ signaling upon antigen receptor stimulation and chemotaxis in B lymphocytes; however, there is little information on the spatial relationships of NCLX with SR Ca²⁺ handling proteins, and their physiological impact. Here we examined the issue, focusing on the interaction of NCLX with an SR Ca²⁺ pump SERCA in cardiomyocytes. A bimolecular fluorescence complementation assay using HEK293 cells revealed that the exogenously expressed NCLX was localized in close proximity to four exogenously expressed SERCA isoforms. Immunofluorescence analyses of isolated ventricular myocytes showed that the NCLX was localized to the edges of the mitochondria, forming a striped pattern. The co-localization coefficients in the super-resolution images were higher for NCLX–SERCA2, than for NCLX–ryanodine receptor and NCLX–Na⁺/K⁺ ATPase α-1 subunit, confirming the close localization of endogenous NCLX and SERCA2 in cardiomyocytes. The mathematical model implemented with the spatial and functional coupling of NCLX and SERCA well reproduced the NCLX inhibition-mediated modulations of SR Ca²⁺ reuptake in HL-1 cardiomyocytes. Taken together, these results indicated that NCLX and SERCA are spatially and functionally coupled in cardiomyocytes.

Golgi Metal Ion Homeostasis in Human Health and Diseases

The Golgi apparatus is a membrane organelle located in the center of the protein processing and trafficking pathway. It consists of sub-compartments with distinct biochemical compositions and functions. Main functions of the Golgi, including membrane trafficking, protein glycosylation, and sorting, require a well-maintained stable microenvironment in the sub-compartments of the Golgi, along with metal ion homeostasis. Metal ions, such as Ca²⁺, Mn²⁺, Zn²⁺, and Cu²⁺, are important cofactors of many Golgi resident glycosylation enzymes. The homeostasis of metal ions in the secretory pathway, which is required for proper function and stress response of the Golgi, is tightly regulated and maintained by transporters. Mutations in the transporters cause human diseases. Here we provide a review specifically focusing on the transporters that maintain Golgi metal ion homeostasis under physiological conditions and their alterations in diseases.

Update on Calcium Signaling in Cystic Fibrosis Lung Disease

Cystic fibrosis (CF) is an autosomal recessive disorder characterized by mutations in the cystic fibrosis transmembrane conductance regulator gene, which causes multifunctional defects that preferentially affect the airways. Abnormal viscosity of mucus secretions, persistent pathogen infections, hyperinflammation, and lung tissue damage compose the classical pathological manifestation referred to as CF lung disease. Among the multifunctional defects associated with defective CFTR, increasing evidence supports the relevant role of perturbed calcium (Ca²⁺) signaling in the pathophysiology of CF lung disease. The Ca²⁺ ion is a critical player in cell functioning and survival. Its intracellular homeostasis is maintained by a fine balance between channels, transporters, and exchangers, mediating the influx and efflux of the ion across the plasma membrane and the intracellular organelles. An abnormal Ca²⁺ profile has been observed in CF cells, including airway epithelial and immune cells, with heavy repercussions on cell function, viability, and susceptibility to pathogens, contributing to proinflammatory overstimulation, organelle dysfunction, oxidative stress, and excessive cytokines release in CF lung. This review discusses the role of Ca²⁺ signaling in CF and how its dysregulation in airway epithelial and immune cells contributes to hyperinflammation in the CF lung. Finally, we provide an outlook on the therapeutic options that target the Ca²⁺ signaling to treat the CF lung disease.

Structure and Mechanism of P-Type ATPase Ion Pumps

P-type ATPases are found in all kingdoms of life and constitute a wide range of cation transporters, primarily for H⁺, Na⁺, K⁺, Ca²⁺, and transition metal ions such as Cu(I), Zn(II), and Cd(II). They have been studied through a wide range of techniques, and research has gained very significant insight on their transport mechanism and regulation. Here, we review the structure, function, and dynamics of P2-ATPases including Ca²⁺-ATPases and Na,K-ATPase. We highlight mechanisms of functional transitions that are associated with ion exchange on either side of the membrane and how the functional cycle is regulated by interaction partners, autoregulatory domains, and off-cycle states. Finally, we discuss future perspectives based on emerging techniques and insights.

Calumenin-1 Interacts with Climp63 to Cooperatively Determine the Luminal Width and Distribution of Endoplasmic Reticulum Sheets

The ER is composed of distinct structures like tubules, matrices, and sheets, all of which are important for its various functions. However, how these distinct ER structures, especially the perinuclear ER sheets, are formed remains unclear. We report here that the ER membrane protein Climp63 and the ER luminal protein calumenin-1 (Calu1) collaboratively maintain ER sheet morphology. We show that the luminal length of Climp63 is positively correlated with the luminal width of ER sheets. Moreover, the lumen-only mutant of Climp63 dominant-negatively narrows the lumen of ER sheets, demonstrating that Climp63 acts as an ER luminal bridge. We also reveal that Calu1 specifically interacts with Climp63 and antagonizes Climp63 in terms of both ER sheet distribution and luminal width. Together, our data provide insight into how the structure of ER sheets is maintained and regulated.

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Climp63 determines the luminal width of ER sheets

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ER luminal protein Calumenin-1 (Calu1) interacts with Climp63

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Knockout of Calu1 triggers ER sheet accumulation and wider sheet lumen

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Calu1 regulates ER sheet morphology in a Climp63-dependent manner

Sarco/endoplasmic reticulum Ca2+-ATPase is a more effective calcium remover than sodium-calcium exchanger in human embryonic stem cell-derived cardiomyocytes

Human pluripotent stem cell (hPSCs)-derived ventricular (V) cardiomyocytes (CMs) display immature Ca2+–handing properties with smaller transient amplitudes and slower kinetics due to such differences in crucial Ca2+-handling proteins as the poor sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) pump but robust Na+-Ca2+ exchanger (NCX) activities in human embryonic stem cell (ESC)-derived VCMs compared with adult. Despite their fundamental importance in excitation-contraction coupling, the relative contribution of SERCA and NCX to Ca2+-handling of hPSC-VCMs remains unexplored. We systematically altered the activities of SERCA and NCX in human embryonic stem cell-derived ventricular cardiomyocytes (hESC-VCMs) and their engineered microtissues, followed by examining the resultant phenotypic consequences. SERCA overexpression in hESC-VCMs shortened the decay of Ca2+ transient at low frequencies (0.5 Hz) without affecting the amplitude, SR Ca2+ content and Ca2+ baseline. Interestingly, short hairpin RNA-based NCX suppression did not prolong the transient decay, indicating a compensatory response for Ca2+ removal. Although hESC-VCMs and their derived microtissues exhibited negative frequency-transient/force responses, SERCA overexpression rendered them less negative at high frequencies (>2 Hz) by accelerating Ca2+ sequestration. We conclude that for hESC-VCMs and their microtissues, SERCA, rather than NCX, is the main Ca2+ remover during diastole; poor SERCA expression is the leading cause for immature negative-frequency/force responses, which can be partially reverted by forced expression. Combinatorial approach to mature calcium handling in hESC-VCMs may help shed further mechanistic insights.

NEW & NOTEWORTHY In this study of human pluripotent stem cell-derived cardiomyocytes, we studied the role of sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) and Na+-Ca2+ exchanger (NCX) in Ca2+ handling. Our data support the notion that SERCA is more effective in cytosolic calcium removal than the NCX.

Structures of the heart specific SERCA2a Ca2+-ATPase

The sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase 2a (SERCA2a) performs active reuptake of cytoplasmic Ca²⁺ and is a major regulator of cardiac muscle contractility. Dysfunction or dysregulation of SERCA2a is associated with heart failure, while restoring its function is considered as a therapeutic strategy to restore cardiac performance. However, its structure has not yet been determined. Based on native, active protein purified from pig ventricular muscle, we present the first crystal structures of SERCA2a, determined in the CPA-stabilized E2-AlF4- form (3.3 Å) and the Ca²⁺-occluded [Ca₂]E1-AMPPCP form (4.0 Å). The structures are similar to the skeletal muscle isoform SERCA1a pointing to a conserved mechanism. We seek to explain the kinetic differences between SERCA1a and SERCA2a. We find that several isoform-specific residues are acceptor sites for post-translational modifications. In addition, molecular dynamics simulations predict that isoform-specific residues support distinct intramolecular interactions in SERCA2a and SERCA1a. Our experimental observations further indicate that isoform-specific intramolecular interactions are functionally relevant, and may explain the kinetic differences between SERCA2a and SERCA1a.

The Differential Response to Ca2+ from Vertebrate and Invertebrate Calumenin Is Governed by a Single Amino Acid Residue

Calumenin (Calu) is a well-conserved multi-EF-hand-containing Ca²⁺-binding protein. In this work, we focused on the alterations that calumenin has undergone during evolution. We demonstrate that vertebrate calumenin is significantly different from its invertebrate homologues with respect to its response to Ca²⁺ binding. Human calumenin (HsCalu1) is intrinsically unstructured in the Ca²⁺ free form and responds to Ca²⁺ with a dramatic gain in structure. Calumenin from Caenorhabditis elegans (CeCalu) is structured even in the apo form, with no conformational change upon binding of Ca²⁺. We decode this structural and functional distinction by identifying a single "Leu" residue-based switch located in the fourth EF-hand of HsCalu1, occupied by "Gly" in the invertebrate homologues. We demonstrate that replacing Leu with Gly (L150G) in HsCalu1 enables the protein to adopt a structural fold even in the Ca²⁺ free form, similar to CeCalu, leading to ligand compensation (adoption of structure in the absence of Ca²⁺). The fourth (of seven) EF-hand of HsCalu1 nucleates the structural fold of the protein depending on the switch residue (Gly or Leu). Our analyses reveal that the Leu that replaced Gly from fishes onward is absolutely conserved in higher vertebrates, while lower organisms have Gly, not only enlarging the scope of Ca²⁺-dependent structural transitions but also drawing a boundary between the invertebrate and vertebrate calumenin. The evolutionary selection of the switch residue strongly corroborates the change in the structure of the protein and its pleiotropic functions and seems like it can be extended to the presence or absence of a heart in that organism.

Comparative proteomic analysis of different developmental stages of swamp buffalo testicular seminiferous tubules

With ageing, many protein components change markedly during mammalian spermatogenesis. Most of these proteins have yet to be characterized and verified. Here, we have employed two-dimensional electrophoresis coupled to tandem mass spectrometry to explore the different proteins from pre-pubertal, pubertal and post-pubertal swamp buffalo testicular seminiferous tubules. The results showed that 25 protein spots were differentially expressed among developmental stages, and 13 of them were successfully identified by mass spectrometry. Of which four proteins were up-regulated and three proteins were down-regulated with age, and the remaining six proteins were fluctuated among developmental stages. Bioinformatics analysis indicates that these proteins were probably related to cellular developmental process (53.8%), cell differentiation (53.8%), spermatogenesis (15.4%), apoptotic process and cell death (30.8%). Expression profiles of calumenin (CALU) and galectin-1 (LGALS1) were further verified via Western blotting. In summary, the results help to develop an understanding of molecular mechanisms associated with buffalo spermatogenesis.

Calumenin relieves cardiac injury by inhibiting ERS-initiated apoptosis during viral myocarditis

Viral myocarditis (VMC) is a common disease causing heart failure (HF) for which no specific treatments are available. As apoptosis of cardiomyoctes is a hallmark of VMC and HF, strategies targeting apoptosis are an effective way of prevention and treatment of HF. Recent studies found endoplasmic reticulum stress (ERS) reaction is a new signal transduction pathway mediating apoptosis. Calumenin protein (CP) is located within the endoplasmic reticulum Ca²⁺ binding proteins, and is important in ER-initiated apoptosis. The aim of this study was to investigate whether the function of CP was influenced in cardiomyocytes infected by coxsackievirus B3. The expression of CP was down-regulated in cardiomyocytes infected by coxsackievirus B3. TUNEL studies showed that apoptosis was increased in CP-deficient and ΔCP-mutant cardiomyocytes infected by coxsackievirus B3. Additionally, ERS-associated proteins (GRP78, p-PERK, p-eIF2α, ATF4 and CHOP) were up-regulated in coxsackievirus B3-infected CP-deficient and ΔCP-mutant cardiomyocytes compared to wild type control cells. These results suggested ER-initiated apoptosis was induced by coxsackievirus B3-infected cardiomyocytes and caused apoptosis through ER stress. CP can relieve ERS-initiated apoptosis in viral myocarditis.

Calumenin contributes to ER-Ca2+ homeostasis in bronchial epithelial cells expressing WT and F508del mutated CFTR and to F508del-CFTR retention

Cystic Fibrosis (CF) is the most frequent fatal genetic disease in Caucasian populations. Mutations in the chloride channel CF Transmembrane Conductance Regulator (CFTR) gene are responsible for functional defects of the protein and multiple associated dysregulations. The most common mutation in patients with CF, F508del-CFTR, causes defective CFTR protein folding. Thus minimal levels of the receptor are expressed at the cell surface as the mutated CFTR is retained in the endoplasmic reticulum (ER) where it correlates with defective calcium (Ca²⁺) homeostasis. In this study, we discovered that the Ca²⁺ binding protein Calumenin (CALU) is a key regulator in the maintenance of ER-Ca²⁺ calcium homeostasis in both wild type and F508del-CFTR expressing cells. Calumenin modulates SERCA pump activity without drastically affecting ER-Ca²⁺ concentration. In addition, reducing Calumenin expression in CF cells results in a partial restoration of CFTR activity, highlighting a potential function of Calumenin in CFTR maturation. These findings demonstrate a pivotal role for Calumenin in CF cells, providing insights into how modulation of Calumenin expression or activity may be used as a potential therapeutic tool to correct defects in F508del-CFTR.

MicroRNAs for Restenosis and Thrombosis After Vascular Injury

Percutaneous revascularization revolutionized the therapy of patients with coronary artery disease. Despite continuous technical advances that substantially improved patients' outcome after percutaneous revascularization, some issues are still open. In particular, restenosis still represents a challenge, even though it was dramatically reduced with the advent of drug-eluting stents. At the same time, drug-eluting stent thrombosis emerged as a major concern because of incomplete or delayed re-endothelialization after vascular injury. The discovery of microRNAs revealed a previously unknown layer of regulation for several biological processes, increasing our knowledge on the biological mechanisms underlying restenosis and stent thrombosis, revealing novel promising targets for more efficient and selective therapies. The present review summarizes recent experimental and clinical evidence on the role of microRNAs after arterial injury, focusing on practical aspects of their potential therapeutic application for selective inhibition of smooth muscle cell proliferation, enhancement of endothelial regeneration, and inhibition of platelet activation after coronary interventions. Application of circulating microRNAs as potential biomarkers is also discussed.

Total Flavonoids of Astragalus Plays a Cardioprotective Role in Viral Myocarditis

BACKGROUND

Viral myocarditis is initiated by viral infection of myocardial tissue leading to dilated cardiomyopathy and congestive heart failure. Recent studies have linked viral myocarditis with dysfunctions in endoplasmic reticulum (ER) mediated Ca(2+) homeostasis and the unfolded protein response (UPR). Currently there are no effective treatments for this viral infection.

METHODS

We employed the use of a well-characterized pathogen coxsackievirus B3 (CVB3) to induce mouse viral myocarditis. After intraperitoneal administration of total flavonoids of Astragalus (TFA), we examined the protective effect of TFA on CVB3-induced heart function impairment and decreased calumenin mRNA levels. Furthermore, calumenin protein level was studied in vivo and in vitro with CVB3 infection in the presence or absence of TFA. The interaction between calumenin and the sarco/endoplasmic reticulum Ca(2+)-ATPase 2 (SERCA2) was also tested in HL-1 cells.

RESULTS

Whereas customarily we would expect that CVB3 infection would decrease mRNA and protein levels of the Ca(2+) binding ER chaperone calumenin, here TFA treatment prevented this decline in both CVB3 infected mice and in an in vitro system of infected HL-1 cardiomyocytes. CVB3 infection in HL-1 cells prevented the association of calumenin with the calcium mobilizing protein SERCA2, and TFA treatment rescued this interaction.

CONCLUSIONS

This study identified that CVB3 infection promotes cardiomyocyte dysfunction by effecting expression levels and activity of the cardio protective ER chaperone calumenin. For the first time, TFA was shown to prevent loss of mRNA and protein levels of calumenin and also rescued the association of this protein with SERCA2.

KEY WORDS

Calumenin; Sarco/endoplasmic reticulum Ca(2+)-ATPase; Total flavonoids of Astragalus; Viral myocarditis.

Ca-Dependent Folding of Human Calumenin

Human calumenin (hCALU) is a six EF-hand protein belonging to the CREC family. As other members of the family, it is localized in the secretory pathway and regulates the activity of SERCA2a and of the ryanodine receptor in the endoplasmic reticulum (ER). We have studied the effects of Ca²⁺ binding to the protein and found it to attain a more compact structure upon ion binding. Circular Dichroism (CD) measurements suggest a major rearrangement of the protein secondary structure, which reversibly switches from disordered at low Ca²⁺ concentrations to predominantly alpha-helical when Ca²⁺ is added. SAXS experiments confirm the transition from an unfolded to a compact structure, which matches the structural prediction of a trilobal fold. Overall our experiments suggest that calumenin is a Ca²⁺ sensor, which folds into a compact structure, capable of interacting with its molecular partners, when Ca²⁺ concentration within the ER reaches the millimolar range.

Analysis of the effect of a novel therapeutic for type 2 diabetes on the proteome of a muscle cell line

Elevated serum retinol-binding protein (RBP) concentration has been implicated in the development of insulin resistance and type 2 diabetes. Two series of small molecules have been designed to lower serum levels by reducing secretion of the transthyretin-RBP complex from the liver and enhancing RBP clearance through the kidney. These small molecules were seen to improve glucose and insulin tolerance tests and to reduce body weight gain in mice rendered diabetic through a high fat diet. A proteomics study was conducted to better understand the effects of these compounds in muscle cells, muscle being the primary site for energy expenditure. One lead compound, RTC-15, is seen to have a significant effect on proteins involved in fat and glucose metabolism. This could indicate that the compound is having a direct effect on muscle tissue to improve energy homeostasis as well as a whole body effect on circulating RBP levels. This newly characterized group of antidiabetic compounds may prove useful in the treatment and prevention of insulin resistance and obesity.

The Golgi apparatus is a functionally distinct Ca2+ store regulated by the PKA and Epac branches of the β1-adrenergic signaling pathway

Ca(2+) release from the Golgi apparatus regulates key functions of the organelle, including vesicle trafficking. We found that the Golgi apparatus was the source of prolonged Ca(2+) release events that originated near the nuclei of primary cardiomyocytes. Golgi Ca(2+) release was unaffected by depletion of sarcoplasmic reticulum Ca(2+), and disruption of the Golgi apparatus abolished Golgi Ca(2+) release without affecting sarcoplasmic reticulum function, suggesting functional and spatial independence of Golgi and sarcoplasmic reticulum Ca(2+) stores. β1-Adrenoceptor stimulation triggers the production of the second messenger cAMP, which activates the Epac family of Rap guanine nucleotide exchange factors and the kinase PKA (protein kinase A). Phosphodiesterases (PDEs), including those in the PDE3 and PDE4 families, degrade cAMP. Activation of β1-adrenoceptors stimulated Golgi Ca(2+) release, an effect that required activation of Epac, PKA, and the kinase CaMKII. Inhibition of PDE3s or PDE4s potentiated β1-adrenergic-induced Golgi Ca(2+) release, which is consistent with compartmentalization of cAMP signaling near the Golgi apparatus. Interventions that stimulated Golgi Ca(2+) release appeared to increase the trafficking of vascular endothelial growth factor receptor-1 (VEGFR-1) from the Golgi apparatus to the surface membrane of cardiomyocytes. In cardiomyocytes from rats with heart failure, decreases in the abundance of PDE3s and PDE4s were associated with increased Golgi Ca(2+) release events. These data suggest that the Golgi apparatus is a focal point for β1-adrenergic-stimulated Ca(2+) signaling and that the Golgi Ca(2+) store functions independently from the sarcoplasmic reticulum and the global Ca(2+) transients that trigger contraction in cardiomyocytes.

Fibulin-1C, C1 Esterase Inhibitor and Glucose Regulated Protein 75 Interact with the CREC Proteins, Calumenin and Reticulocalbin

Affinity purification, immunoprecipitation, gel electrophoresis and mass spectrometry were used to identify fibulin-1C, C1 esterase inhibitor and glucose regulated protein 75, grp75, as binding partners of the CREC proteins, calumenin and reticulocalbin. Surface plasmon resonance was used to verify the interaction of all three proteins with each of the CREC proteins. Fibulin-1C interacts with calumenin and reticulocalbin with an estimated dissociation constant around 50-60 nM. The interaction, at least for reticulocalbin, was not dependent upon the presence of Ca²⁺. C1 esterase inhibitor interacted with both proteins with an estimated dissociation constant at 1 μM for reticulocalbin and 150 nM for calumenin. The interaction, at least for calumenin, was dependent upon the presence of Ca²⁺ with strong interaction at 3.5 mM while no detectable interaction could be found at 0.1 mM. Grp75 binds with an affinity of approximately 3-7 nM with reticulocalbin as well as with calumenin. These interactions suggest functional participation of the CREC proteins in chaperone activity, cell proliferation and transformation, cellular aging, haemostasis and thrombosis as well as modulation of the complement system in fighting bacterial infection.

Zebrafish vitamin K epoxide reductases: expression in vivo, along extracellular matrix mineralization and under phylloquinone and warfarin in vitro exposure

Vitamin K (VK) acts as a cofactor driving the biological activation of VK-dependent proteins and conferring calcium-binding properties to them. As a result, VK is converted into VK epoxide, which must be recycled by VK epoxide reductases (Vkors) before it can be reused. Although VK has been shown to play a central role in fish development, particularly during skeletogenesis, pathways underlying VK actions are poorly understood, while good and reliable molecular markers for VK cycle/homeostasis are still lacking in fish. In the present work, expression of 2 zebrafish vkor genes was characterized along larval development and in adult tissues through qPCR analysis. Zebrafish cell line ZFB1 was used to evaluate in vitro regulation of vkors and other VK cycle-related genes during mineralization and upon 24 h exposure to 0.16 and 0.8 µM phylloquinone (VK1), 0.032 µM warfarin, or a combination of both molecules. Results showed that zebrafish vkors are differentially expressed during larval development, in adult tissues, and during cell differentiation/mineralization processes. Further, several VK cycle intermediates were differentially expressed in ZFB1 cells exposed to VK1 and/or warfarin. Present work provides data identifying different developmental stages and adult tissues where VK recycling is probably highly required, and shows how genes involved in VK cycle respond to VK nutritional status in skeletal cells. Expression of vkor genes can represent a reliable indicator to infer VK nutritional status in fish, while ZFB1 cells could represent a suitable in vitro tool to get insights into the mechanisms underlying VK action on fish bone.

Calumenin and fibulin-1 on tumor metastasis: Implications for pharmacology

Tumor metastasis is a key cause of cancer mortality, and inhibiting migration of cancer cells is one of the major directions of anti-metastatic drug development. Calumenin and fibulin-1 are two extracellular proteins that synergistically inhibit cell migration and tumor metastasis, and could potentially be served as targets for pharmacological research of anti-metastatic drugs. This review briefly introduces the multi-function of these two proteins, and discusses the mechanism of how they regulate cell migration and tumor metastasis.

Circulating Pneumolysin Is a Potent Inducer of Cardiac Injury during Pneumococcal Infection

Streptococcus pneumoniae accounts for more deaths worldwide than any other single pathogen through diverse disease manifestations including pneumonia, sepsis and meningitis. Life-threatening acute cardiac complications are more common in pneumococcal infection compared to other bacterial infections. Distinctively, these arise despite effective antibiotic therapy. Here, we describe a novel mechanism of myocardial injury, which is triggered and sustained by circulating pneumolysin (PLY). Using a mouse model of invasive pneumococcal disease (IPD), we demonstrate that wild type PLY-expressing pneumococci but not PLY-deficient mutants induced elevation of circulating cardiac troponins (cTns), well-recognized biomarkers of cardiac injury. Furthermore, elevated cTn levels linearly correlated with pneumococcal blood counts (r=0.688, p=0.001) and levels were significantly higher in non-surviving than in surviving mice. These cTn levels were significantly reduced by administration of PLY-sequestering liposomes. Intravenous injection of purified PLY, but not a non-pore forming mutant (PdB), induced substantial increase in cardiac troponins to suggest that the pore-forming activity of circulating PLY is essential for myocardial injury in vivo. Purified PLY and PLY-expressing pneumococci also caused myocardial inflammatory changes but apoptosis was not detected. Exposure of cultured cardiomyocytes to PLY-expressing pneumococci caused dose-dependent cardiomyocyte contractile dysfunction and death, which was exacerbated by further PLY release following antibiotic treatment. We found that high PLY doses induced extensive cardiomyocyte lysis, but more interestingly, sub-lytic PLY concentrations triggered profound calcium influx and overload with subsequent membrane depolarization and progressive reduction in intracellular calcium transient amplitude, a key determinant of contractile force. This was coupled to activation of signalling pathways commonly associated with cardiac dysfunction in clinical and experimental sepsis and ultimately resulted in depressed cardiomyocyte contractile performance along with rhythm disturbance. Our study proposes a detailed molecular mechanism of pneumococcal toxin-induced cardiac injury and highlights the major translational potential of targeting circulating PLY to protect against cardiac complications during pneumococcal infections.

SERCA and PMCA pumps contribute to the deregulation of Ca2+ homeostasis in human CF epithelial cells

Cystic Fibrosis (CF) disease is caused by mutations in the CFTR gene (CF transmembrane conductance regulator). F508 deletion is the most represented mutation, and F508del-CFTR is absent of plasma membrane and accumulates into the endoplasmic reticulum (ER) compartment. Using specific Ca2+ genetics cameleon probes, we showed in the human bronchial CF epithelial cell line CFBE that ER Ca2+ concentration was strongly increased compared to non-CF (16HBE) cells, and normalized by the F508del-CFTR corrector agent, VX-809. We also showed that ER F508del-CFTR retention increases SERCA (Sarcoplasmic/Reticulum Ca2+ ATPase) pump activity whereas PMCA (Plasma Membrane Ca2+ ATPase) activities were reduced in these CF cells compared to corrected CF cells (VX-809) and non-CF cells. We are showing for the first time CFTR/SERCA and CFTR/PMCA interactions that are modulated in CF cells and could explain part of Ca2+ homeostasis deregulation due to mislocalization of F508del-CFTR. Using ER or mitochondria genetics Ca2+ probes, we are showing that ER Ca2+ content, mitochondrial Ca2+ uptake, SERCA and PMCA pump, activities are strongly affected by the localization of F508del-CFTR protein.

The Sarcoplasmic Reticulum and the Evolution of the Vertebrate Heart

The sarcoplasmic reticulum (SR) is crucial for contraction and relaxation of the mammalian cardiomyocyte, but its role in other vertebrate classes is equivocal. Recent evidence suggests differences in SR function across species may have an underlying structural basis. Here, we discuss how SR recruitment relates to the structural organization of the cardiomyocyte to provide new insight into the evolution of cardiac design and function in vertebrates.

Calumenin-15 facilitates filopodia formation by promoting TGF-β superfamily cytokine GDF-15 transcription

Filopodia, which are actin-rich finger-like membrane protrusions, have an important role in cell migration and tumor metastasis. Here we identify 13 novel calumenin (Calu) isoforms (Calu 3-15) produced by alternative splicing, and find that Calu-15 promotes filopodia formation and cell migration. Calu-15 shuttles between the nucleus and cytoplasm through interacting with importin α, Ran GTPase, and Crm1. The phosphorylation of the threonine at position 73 (Thr-73) by casein kinase 2 (CK2) is essential for the nuclear import of Calu-15, and either Thr-73 mutation or inhibition of CK2 interrupts its nuclear localization. In the nucleus, Calu-15 increases the transcription of growth differentiation factor-15 (GDF-15), a member of the transforming growth factor-β (TGF-β) superfamily, via binding to its promoter region. Furthermore, Calu-15 induces filopodia formation mediated by GDF-15. Together, we identify that Calu-15, a novel isoform of Calu with phosphorylation-dependent nuclear localization, has a critical role in promoting filopodia formation and cell migration by upregulating the GDF-15 transcription.

Proteome modulation in H9c2 cardiac cells by microRNAs miR-378 and miR-378

MicroRNAs are a novel class of powerful endogenous regulators of gene expression. MiR-378 and miR-378* are localized in the first intron of the Ppargc1b gene that codes the transcriptional co-activator PGC-1β. The latter regulates energy expenditure as well as mitochondrial biogenesis. The miR-378:miR-378* hairpin is highly expressed in cardiac cells. To better assess their role in cardiomyocytes, we identified miR-378 and miR-378* targets via a proteomic screen. We established H9c2 cellular models of overexpression of miR-378 and miR-378* and identified a total of 86 down-regulated proteins in the presence of either one of these miRs. Functional annotation clustering showed that miR-378 and miR-378* regulate related pathways in cardiomyocytes, including energy metabolism, notably glycolysis, cytoskeleton, notably actin filaments and muscle contraction. Using bioinformatics algorithms we found that 20 proteins were predicted as direct targets of the miRs. We validated eight of these targets by quantitative RT-PCR and luciferase reporter assay. We found that miR-378 targets lactate dehydrogenase A and impacts on cell proliferation and survival whereas miR-378* targets cytoskeleton proteins actin and vimentin. Proteins involved in endoplasmic reticulum stress response such as chaperone and/or calcium buffering proteins GRP78, PPIA (cyclophilin A), calumenin, and GMMPA involved in glycosylation are repressed by these miRs. Our results show that the miR-378/378* hairpin establishes a connection among energy metabolism, cytoskeleton remodeling, and endoplasmic reticulum function through post-transcriptional regulation of key proteins involved in theses pathways.

BmCREC is an endoplasmic reticulum (ER) resident protein and required for ER/Golgi morphology

Silkworm posterior silkgland is a model for studying intracellular trafficking. Here, using this model, we identify several potential cargo proteins of BmKinesin-1 and focus on one candidate, BmCREC. BmCREC (also known as Bombyx mori DNA supercoiling factor, BmSCF) was previously proposed to supercoil DNA in the nucleus. However, we show here that BmCREC is localized in the ER lumen. Its C-terminal tetrapeptide HDEF is recognized by the KDEL receptor, and subsequently it is retrogradely transported by coat protein I (COPI) vesicles to the ER. Lacking the HDEF tetrapeptide of BmCREC or knocking down COPI subunits results in decreased ER retention and simultaneously increased secretion of BmCREC. Furthermore, we find that BmCREC knockdown markedly disrupts the morphology of the ER and Golgi apparatus and leads to a defect of posterior silkgland tube expansion. Together, our results clarify the ER retention mechanism of BmCREC and reveal that BmCREC is indispensable for maintaining ER/Golgi morphology.

miR-424/322 regulates vascular smooth muscle cell phenotype and neointimal formation in the rat

AIMS

Our aim was to identify new microRNAs (miRNAs) implicated in pathological vascular smooth muscle cells (VSMCs) proliferation and characterize their mechanism of action.

METHODS AND RESULTS

MicroRNAs microarray and qRT-PCR results lead us to focus on miR-424 or its rat ortholog miR-322 (miR-424/322). In vitro mir-424/322 level was decreased shortly after the induction of proliferation and increased in a time-dependent manner later on. In vivo its expression increased in the rat carotid artery from Day 4 up to Day 30 after injury. miR-424/322 overexpression in vitro inhibited proliferation and migration without affecting apoptosis and prevented VSMC dedifferentiation. Furthermore, miR-424/322 overexpression resulted in decreased expression of its predicted targets: cyclin D1 and Ca(2+)-regulating proteins calumenin and stromal-interacting molecule 1 (STIM1). Using reporter luciferase assays, we confirmed that cyclin D1 and calumenin mRNAs were direct targets of miR-322, whereas miR-322 effect on STIM1 was indirect. Nevertheless, consistent with the decreased STIM1 level, the store-operated Ca(2+) entry was reduced. We hypothesized that miR-424/322 could be a negative regulator of proliferation overridden in pathological situations. Thus, we overexpressed miR-424/322 in injured rat carotid arteries using an adenovirus, and demonstrated a protective effect against restenosis.

CONCLUSION

Our results demonstrate that miR-424/322 is up-regulated after vascular injury. This is likely an adaptive response to counteract proliferation, although this mechanism is overwhelmed in pathological situations such as injury-induced restenosis.

Evidence for three highly significant QTL for meat quality traits in the Finnish Yorkshire pig breed

Meat quality is important both to consumers and to the meat processing industry. Commonly used measures of porcine meat quality are the pH and color of the meat. The purpose of this study was to identify SNP associated with these meat quality traits in Finnish Yorkshire using the Illumina PorcineSNP60 BeadChip. The association of each SNP with the quality traits was tested with a weighted linear model. The relatedness of samples was accounted for by a random polygenic genetic effect with the accompanying full relationship matrix. The original EBV from single-trait evaluations were deregressed before analysis. The statistical significance of SNP was established using the Bonferroni correction to adjust for multiple testing. Three genomic regions were significant for the meat quality traits. The PRKAG3 region on chromosome 15 was significant for pH measured from loin and ham and for a* (redness) measured from loin. The smallest P-value in the region was obtained for pH measured from loin (ASGA0070634, P-value = 3.8 × 10(-13)). The allele substitution effect (-0.047) of the unfavorable allele A corresponds to 1 SD of the polygenic effect. The second significant region, on chromosome 2 at around 31 megabases (Mb), was associated with pH and L* (lightness) measured from loin. The most significant SNP (ASGA0009814, P-value = 3.89 × 10(-10)) had an allele substitution effect of 0.86, corresponding to 0.7 SD of the polygenic effect of L*. The third region, located on chromosome 6 at around 83 Mb, was significant for a* measured from ham. The P-value of the best SNP (ALGA0035896) was 8.71 × 10(-7) and the allele substitution effect -0.38, corresponding to 0.5 SD of the polygenic effect of a*. The significant association of PRKAG3 with pH was not due to the known AA substitutions. The candidate gene on chromosome 2 associated with color L* is RCN1, which has a high affinity Ca(2+)-binding motif, the EF hand. The significant region on chromosome 6 for color a* contains several genes, so more data are needed to identify the causative gene. Our results indicate that instead of the known AA substitutions of PRKAG3, some yet-unknown AA substitutions are causative for the pH variation in Finnish Yorkshire. Also, a new major QLT for L* was found on chromosome 2. The significant SNP identified in this study can be used in marker-assisted selection.

Tmem64 modulates calcium signaling during RANKL-mediated osteoclast differentiation

Osteoclast maturation and function primarily depend on receptor activator of NF-κB ligand (RANKL)-mediated induction of nuclear factor of activated T cells c1 (NFATc1), which is further activated via increased intracellular calcium ([Ca(2+)](i)) oscillation. However, the coordination mechanism that mediates Ca(2+) oscillation during osteoclastogenesis remains ill defined. Here, we identified transmembrane protein 64 (Tmem64) as a regulator of Ca(2+) oscillation during osteoclastogenesis. We found that Tmem64-deficient mice exhibit increased bone mass due in part to impaired osteoclast formation. Using in vitro osteoclast culture systems, we show here that Tmem64 interacts with sarcoplasmic endoplasmic reticulum Ca(2+) ATPase 2 (SERCA2) and modulates its activity. Consequently, Tmem64 deficiency significantly diminishes RANKL-induced [Ca(2+)](i) oscillation, which results in reduced Ca(2+)/calmodulin-dependent protein kinases (CaMK) IV and mitochondrial ROS, both of which contribute to achieving the CREB activity necessary for osteoclast formation. These data demonstrate that Tmem64 is a positive modulator of osteoclast differentiation via SERCA2-dependent Ca(2+) signaling.

Coupling of ryanodine receptor 2 and voltage-dependent anion channel 2 is essential for Ca2+ transfer from the sarcoplasmic reticulum to the mitochondria in the heart

The structural proximity and functional coupling between the SR (sarcoplasmic reticulum) and mitochondria have been suggested to occur in the heart. However, the molecular architecture involved in the SR–mitochondrial coupling remains unclear. In the present study, we performed various genetic and Ca2+-probing studies to resolve the proteins involved in the coupling process. By using the bacterial 2-hybrid, glutathione transferase pull-down, co-immunoprecipitation and immunocytochemistry assays, we found that RyR2 (ryanodine receptor type 2), which is physically associated with VDAC2 (voltage-dependent anion channel 2), was co-localized in SR–mitochondrial junctions. Furthermore, a fractionation study revealed that VDAC2 was co-localized with RyR2 only in the subsarcolemmal region. VDAC2 knockdown by targeted short hairpin RNA led to an increased diastolic [Ca2+] (calcium concentration) and abolishment of mitochondrial Ca2+ uptake. Collectively, the present study suggests that the coupling of VDAC2 with RyR2 is essential for Ca2+ transfer from the SR to mitochondria in the heart.

AAV-mediated knock-down of HRC exacerbates transverse aorta constriction-induced heart failure

Background

Histidine-rich calcium binding protein (HRC) is located in the lumen of sarcoplasmic reticulum (SR) that binds to both triadin (TRN) and SERCA affecting Ca²⁺ cycling in the SR. Chronic overexpression of HRC that may disrupt intracellular Ca²⁺ homeostasis is implicated in pathogenesis of cardiac hypertrophy. Ablation of HRC showed relatively normal phenotypes under basal condition, but exhibited a significantly increased susceptibility to isoproterenol-induced cardiac hypertrophy. In the present study, we characterized the functions of HRC related to Ca²⁺ cycling and pathogenesis of cardiac hypertrophy using the in vitro siRNA- and the in vivo adeno-associated virus (AAV)-mediated HRC knock-down (KD) systems, respectively.

Methodology/Principal Findings

AAV-mediated HRC-KD system was used with or without C57BL/6 mouse model of transverse aortic constriction-induced failing heart (TAC-FH) to examine whether HRC-KD could enhance cardiac function in failing heart (FH). Initially we expected that HRC-KD could elicit cardiac functional recovery in failing heart (FH), since predesigned siRNA-mediated HRC-KD enhanced Ca²⁺ cycling and increased activities of RyR2 and SERCA2 without change in SR Ca²⁺ load in neonatal rat ventricular cells (NRVCs) and HL-1 cells. However, AAV9-mediated HRC-KD in TAC-FH was associated with decreased fractional shortening and increased cardiac fibrosis compared with control. We found that phospho-RyR2, phospho-CaMKII, phospho-p38 MAPK, and phospho-PLB were significantly upregulated by HRC-KD in TAC-FH. A significantly increased level of cleaved caspase-3, a cardiac cell death marker was also found, consistent with the result of TUNEL assay.

Conclusions/Significance

Increased Ca²⁺ leak and cytosolic Ca²⁺ concentration due to a partial KD of HRC could enhance activity of CaMKII and phosphorylation of p38 MAPK, causing the mitochondrial death pathway observed in TAC-FH. Our results present evidence that down-regulation of HRC could deteriorate cardiac function in TAC-FH through perturbed SR-mediated Ca²⁺ cycling.

Focal but reversible diastolic sheet dysfunction reflects regional calcium mishandling in dystrophic mdx mouse hearts

Cardiac dysfunction is a primary cause of patient mortality in Duchenne muscular dystrophy, potentially related to elevated cytosolic calcium. However, the regional versus global functional consequences of cellular calcium mishandling have not been defined in the whole heart. Here we sought for the first time to elucidate potential regional dependencies between calcium mishandling and myocardial fiber/sheet function as a manifestation of dystrophin-deficient (mdx) cardiomyopathy. Isolated-perfused hearts from 16-mo-old mdx (N = 10) and wild-type (WT; N = 10) were arrested sequentially in diastole and systole for diffusion tensor MRI quantification of myocardial sheet architecture and function. When compared with WT hearts, mdx hearts exhibited normal systolic sheet architecture but a lower diastolic sheet angle magnitude (|β|) in the basal region. The regional diastolic sheet dysfunction was normalized by reducing perfusate calcium concentrations. Optical mapping of calcium transients in isolated hearts (3 mdx and 4 WT) revealed a stretch-inducible regional defect of intracellular calcium reuptake, reflected by a 25% increase of decay times (T(50)) and decay constants, at the base of mdx hearts. The basal region of mdx hearts also exhibited greater fibrosis than did the apex, which matched the regional sheet dysfunction. We conclude that myocardial diastolic sheet dysfunction is observed initially in basal segments along with calcium mishandling, ultimately culminating in increased fibrosis. The preservation of relatively normal calcium reuptake and diastolic/systolic sheet mechanics throughout the rest of the heart, together with the rapid reversibility of functional defects by reducing cytosolic calcium, points to the significance of regional mechanical factors in the progression of the disease.

Spatio-temporal expression analysis of the calcium-binding protein calumenin in the rodent brain

Calumenin is a Ca²⁺-binding protein that belongs to the CREC superfamily. It contains six EF-hand domains that exhibit a low affinity for Ca²⁺ as well as an endoplasmic reticulum retention signal. Calumenin exhibits a broad and relatively high expression in various brain regions during development as demonstrated by in situ hybridization. Signal intensity of calumenin is highest during the early development and then declines over time to reach a relatively low expression in adult animals. Immunohistochemistry indicates that at the P0 stage, calumenin expression is most abundant in migrating neurons in the zones around the lateral ventricle. In the brain of adult animals, it is expressed in various glial and neuronal cell types, including immature neurons in subgranular zone of hippocampal dentate gyrus. At the subcellular level, calumenin is identified in punctuate and diffuse distribution mostly in somatic regions where it co-localizes with endoplasmic reticulum (ER) and partially Golgi apparatus. Upon subcellular fractionation, calumenin is enriched in fractions containing membranes and is only weakly present in soluble fractions. This study points to a possible important role of calumenin in migration and differentiation of neurons, and/or in Ca²⁺ signaling between glial cells and neurons.

Comparative proteome analyses of host protein expression in response to Enterovirus 71 and Coxsackievirus A16 infections

Enterovirus 71 (EV71) and Coxsackievirus A16 (CA16) are the main etiological agents of Hand, Foot and Mouth Disease (HFMD), a common disease among children and had caused several outbreaks in the Asia-Pacific region. Although being genetically close to each other, EV71 infection can cause serious and fatal neurological complications like encephalitis, myocarditis, acute flaccid paralysis (AFP) and aseptic meningitis, but not in CA16 infections. In this study, the cellular response of host cells infected with EV71 and CA16 was characterized and compared by 2-dimensional proteome analyses. A total of 16 proteins were identified to be differentially expressed in EV71 and CA16-infected host cells. Desmin and HSP27, both indirectly regulate the contraction of muscle cells, were significantly downregulated as a result of EV71 infection, suggesting a link to acute flaccid paralysis. The ability of EV71 to evade host immune system may be due to the downregulation of MHC-I synthesis proteins like protein disulfide isomerase A3 and calreticulin. Proteins such as nucleophosmin, nuclear ribonucleoprotein C, and eukaryotic translation initiation factor 2 were all downregulated significantly, suggesting the rapid shutting down of host translation machinery by EV71. These findings provide insight into the nature of high virulent EV71 infection as compared to CA16.

The Ca2+ pumps of the endoplasmic reticulum and Golgi apparatus

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