Calumenin, a multiple EF-hands Ca2+-binding protein, interacts with ryanodine receptor-1 in rabbit skeletal sarcoplasmic reticulum

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.

Calumenin, a Ca2+ Binding Protein, Is Required for Dauer Formation in Caenorhabditis elegans

Caenorhabditis elegans can adapt and survive in dynamically changing environments by the smart and delicate switching of molecular plasticity. C. elegans dauer diapause is a form of phenotypic and developmental plasticity that induces reversible developmental arrest upon environmental cues. An ER (endoplasmic reticulum)-resident Ca²⁺ binding protein, calumenin has been reported to function in a variety of malignant diseases in vertebrates and in the process of muscle contraction-relaxation. In C. elegans, CALU-1 is known to function in Ca²⁺-regulated behaviors (pharyngeal pumping and defecation) and cuticle formation. The cuticles of dauer larvae are morphologically distinct from those of larvae that develop in favorable conditions. The structure of the dauer cuticle is thicker and more highly reinforced than that of other larval stages to protect dauer larvae from various environmental insults. Since the calu-1(tm1783) mutant exhibited abnormal cuticle structures such as highly deformed annuli and alae, we investigated whether CALU-1 is involved in dauer formation or not. Ascaroside pheromone (ascr#2) and crude daumone were used under starvation conditions to analyze the rate of dauer formation in the calu-1(tm1783) mutant. Surprisingly, the dauer ratio of the calu-1(tm1783) mutant was extremely low compared to that of the wild type. In fact, the calu-1(tm1783) mutants were mostly unable to enter diapause. We also found that calu-1 is expressed in body-wall muscle and AIA interneurons at the dauer stage. Taken together, our results suggest that CALU-1 is required for normal entry into diapause in C. elegans.

CaV1.1 Calcium Channel Signaling Complexes in Excitation-Contraction Coupling: Insights from Channelopathies

In skeletal muscle, excitation-contraction (EC) coupling relies on the mechanical coupling between two ion channels: the L-type voltage-gated calcium channel (CaV1.1), located in the sarcolemma and functioning as the voltage sensor of EC coupling, and the ryanodine receptor 1 (RyR1), located on the sarcoplasmic reticulum serving as the calcium release channel. To this day, the molecular mechanism by which these two ion channels are linked remains elusive. However, recently, skeletal muscle EC coupling could be reconstituted in heterologous cells, revealing that only four proteins are essential for this process: CaV1.1, RyR1, and the cytosolic proteins CaVβ1a and STAC3. Due to the crucial role of these proteins in skeletal muscle EC coupling, any mutation that affects any one of these proteins can have devastating consequences, resulting in congenital myopathies and other pathologies.Here, we summarize the current knowledge concerning these four essential proteins and discuss the pathophysiology of the CaV1.1, RyR1, and STAC3-related skeletal muscle diseases with an emphasis on the molecular mechanisms. Being part of the same signalosome, mutations in different proteins often result in congenital myopathies with similar symptoms or even in the same disease.

Excitation-contraction coupling in mammalian skeletal muscle: Blending old and last-decade research

The excitation–contraction coupling (ECC) in skeletal muscle refers to the Ca²⁺-mediated link between the membrane excitation and the mechanical contraction. The initiation and propagation of an action potential through the membranous system of the sarcolemma and the tubular network lead to the activation of the Ca²⁺-release units (CRU): tightly coupled dihydropyridine and ryanodine (RyR) receptors. The RyR gating allows a rapid, massive, and highly regulated release of Ca²⁺ from the sarcoplasmic reticulum (SR). The release from triadic places generates a sarcomeric gradient of Ca²⁺ concentrations ([Ca²⁺]) depending on the distance of a subcellular region from the CRU. Upon release, the diffusing Ca²⁺ has multiple fates: binds to troponin C thus activating the contractile machinery, binds to classical sarcoplasmic Ca²⁺ buffers such as parvalbumin, adenosine triphosphate and, experimentally, fluorescent dyes, enters the mitochondria and the SR, or is recycled through the Na⁺/Ca²⁺ exchanger and store-operated Ca²⁺ entry (SOCE) mechanisms. To commemorate the 7^th decade after being coined, we comprehensively and critically reviewed “old”, historical landmarks and well-established concepts, and blended them with recent advances to have a complete, quantitative-focused landscape of the ECC. We discuss the: 1) elucidation of the CRU structures at near-atomic resolution and its implications for functional coupling; 2) reliable quantification of peak sarcoplasmic [Ca²⁺] using fast, low affinity Ca²⁺ dyes and the relative contributions of the Ca²⁺-binding mechanisms to the whole concert of Ca²⁺ fluxes inside the fibre; 3) articulation of this novel quantitative information with the unveiled structural details of the molecular machinery involved in mitochondrial Ca²⁺ handing to understand how and how much Ca²⁺ enters the mitochondria; 4) presence of the SOCE machinery and its different modes of activation, which awaits understanding of its magnitude and relevance in situ; 5) pharmacology of the ECC, and 6) emerging topics such as the use and potential applications of super-resolution and induced pluripotent stem cells (iPSC) in ECC. Blending the old with the new works better!

Calumenin knockdown, by intronic artificial microRNA, to improve expression efficiency of the recombinant human coagulation factor IX

OBJECTIVES

To improve the expression efficiency of recombinant hFIX, by enhancing its γ-carboxylation, which is inhibited by Calumenin (CALU), we used intronic artificial microRNAs (amiRNAs) for the CALU downregulation.

METHODS

Two human CALU (hCALU)-specific amiRNAs were designed, validated and inserted within a truncated form of the hFIX intron 1, in either 3'- or 5'-untranslated regions of the hFIX cDNA, in an expression vector. After transfections of a human cell line with the recombinant constructs, processing of the miRNAs confirmed by RT-PCR, using stem-loop primers. The hFIX and hCALU expression assessments were done based on RT-PCR results. The Gamma(γ)-carboxylation of the expressed hFIX was examined by a barium citrate precipitation method, followed by Enzyme-Linked Immunosorbent Assay.

RESULTS

Efficient CALU down regulations, with more than 30-fold decrease, occurred in the cells carrying either of the two examined the 3'-located amiRNAs. The CALU downregulation in the same cells doubled the FIX γ-carboxylation, although the transcription of the FIX decreased significantly. On the other hand, while the expression of the amiRNAs from the 5'-located intron had no decreasing effect on the expression level of CALU, the level of hFIX transcription in these cells increased almost twofold compared to the construct without amiRNA.

CONCLUSION

The CALU downregulation, consistent with efficient hFIX γ-carboxylation, occurred in the cells carrying either of the two amiRNAs containing constructs, although it was affected by the locations of the amiRNA carrying introns, suggesting a possible need to optimize the conditions for the amiRNAs expression.

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.

Calumenin contributes to epithelial-mesenchymal transition and predicts poor survival in glioma

Background

Calumenin (CALU) has been reported to be associated with invasiveness and metastasis in some malignancies. However, in glioma, the role of CALU remains unclear.

Methods

Clinical and transcriptome data of 998 glioma patients, including 301 from CGGA and 697 from TCGA dataset, were included. R language was used to perform statistical analyses.

Results

CALU expression was significantly upregulated in more malignant gliomas, including higher grade, IDH wildtype, mesenchymal, and classical subtype. Gene Ontology analysis revealed that CALU-correlated genes were mainly enriched in cell/biological adhesion, response to wounding, and extracellular matrix/structure organization, all of which were strongly correlated with the epithelial-mesenchymal transition (EMT) phenotype. GSEA further validated the profound involvement of CALU in EMT. Subsequent GSVA suggested that CALU was particularly correlated with three EMT signaling pathways, including TGFβ, PI3K/AKT, and hypoxia pathway. Furthermore, CALU played synergistically with EMT key markers, including N-cadherin, vimentin, snail, slug, and TWIST1. Survival and Cox regression analysis showed that higher CALU predicted worse survival, and the prognostic value was independent of WHO grade and age.

Conclusions

CALU was correlated with more malignant phenotypes in glioma. Moreover, CALU seemed to serve as a pro-EMT molecular target and could contribute to predict prognosis independently in glioma.

An Endoplasmic Reticulum CREC Family Protein Regulates the Egress Proteolytic Cascade in Malaria Parasites

The endoplasmic reticulum (ER) is thought to play an essential role during egress of malaria parasites because the ER is assumed to be required for biogenesis and secretion of egress-related organelles. However, no proteins localized to the parasite ER have been shown to play a role in egress of malaria parasites. In this study, we generated conditional mutants of the Plasmodium falciparum endoplasmic reticulum-resident calcium-binding protein (PfERC), a member of the CREC family. Knockdown of the PfERC gene showed that this gene is essential for asexual growth of P. falciparum Analysis of the intraerythrocytic life cycle revealed that PfERC is essential for parasite egress but is not required for protein trafficking or calcium storage. We found that PfERC knockdown prevents the rupture of the parasitophorous vacuole membrane. This is because PfERC knockdown inhibited the proteolytic maturation of the subtilisin-like serine protease SUB1. Using double mutant parasites, we showed that PfERC is required for the proteolytic maturation of the essential aspartic protease plasmepsin X, which is required for SUB1 cleavage. Further, we showed that processing of substrates downstream of the proteolytic cascade is inhibited by PfERC knockdown. Thus, these data establish that the ER-resident CREC family protein PfERC is a key early regulator of the egress proteolytic cascade of malaria parasites.IMPORTANCE The divergent eukaryotic parasites that cause malaria grow and divide within a vacuole inside a host cell, which they have to break open once they finish cell division. The egress of daughter parasites requires the activation of a proteolytic cascade, and a subtilisin-like protease initiates a proteolytic cascade to break down the membranes blocking egress. It is assumed that the parasite endoplasmic reticulum plays a role in this process, but the proteins in this organelle required for egress remain unknown. We have identified an early ER-resident regulator essential for the maturation of the recently discovered aspartic protease in the egress proteolytic cascade, plasmepsin X, which is required for maturation of the subtilisin-like protease. Conditional loss of PfERC results in the formation of immature and inactive egress proteases that are unable to breakdown the vacuolar membrane barring release of daughter parasites.

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.

Cab45-Unraveling key features of a novel secretory cargo sorter at the trans-Golgi network

The accurate and efficient delivery of proteins to specific domains of the plasma membrane or to the extracellular space is critical for the ordered function of surface receptors and proteins such as insulin, collagens, antibodies, extracellular proteases. The trans-Golgi network is responsible for sorting proteins onto specific carriers for transport to their final destination. The role of the mannose-6-phosphate receptor in the sorting of hydrolases destined for lysosomes has been studied extensively, but the sorting mechanisms for secreted proteins remains poorly understood. We recently described a novel process that links the cytoplasmic actin cytoskeleton to the membrane-anchored Ca²⁺ ATPase SPCA1 and the lumenal Ca²⁺-binding protein Cab45, which mediates sorting of a subset of secretory proteins at the TGN. In response to Ca²⁺ influx, Cab45 forms oligomers, enabling it to bind a variety of specific cargo molecules. Thus, we suggest that this represents a novel way to export cargo molecules without the need for a bona fide transmembrane cargo receptor. This review focuses on Cab45's molecular function and highlights its possible role in disease.

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.

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.

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.

Possible Biomarkers for the Early Detection of HIV-associated Heart Diseases: A Proteomics and Bioinformatics Prediction

The frequency of cardiovascular disorders is increasing in HIV-infected individuals despite a significant reduction in the viral load by antiretroviral therapies (ART). Since the CD4 + T-cells are responsible for the viral load as well as immunological responses, we hypothesized that chronic HIV-infection of T-cells produces novel proteins/enzymes that cause cardiac dysfunctions. To identify specific factors that might cause cardiac disorders without the influence of numerous cofactors produced by other pathogenic microorganisms that co-inhabit most HIV-infected individuals, we analyzed genome-wide proteomes of a CD4 + T-cell line at different stages of HIV replication and cell growth over > 6 months. Subtractive analyses of several hundred differentially regulated proteins from HIV-infected and uninfected counterpart cells and comparisons with proteins expressed from the same cells after treating with the antiviral drug Zidovudine/AZT and inhibiting virus replication, identified a well-coordinated network of 12 soluble/diffusible proteins in HIV-infected cells. Functional categorization, bioinformatics and statistical analyses of each protein predicted that the expression of cardiac-specific Ca2 + kinase together with multiple Ca2 + release channels causes a sustained overload of Ca2 + in the heart which induces fetal/cardiac myosin heavy chains (MYH6 and MYH7) and a myosin light-chain kinase. Each of these proteins has been shown to cause cardiac stress, arrhythmia, hypertrophic signaling, cardiomyopathy and heart failure (p = 8 × 10^− 11). Translational studies using the newly discovered proteins produced by HIV infection alone would provide additional biomarkers that could be added to the conventional markers for an early diagnosis and/or development of specific therapeutic interventions for heart diseases in HIV-infected individuals.

The excitation-contraction coupling mechanism in skeletal muscle

First coined by Alexander Sandow in 1952, the term excitation-contraction coupling (ECC) describes the rapid communication between electrical events occurring in the plasma membrane of skeletal muscle fibres and Ca2+ release from the SR, which leads to contraction. The sequence of events in twitch skeletal muscle involves: (1) initiation and propagation of an action potential along the plasma membrane, (2) spread of the potential throughout the transverse tubule system (T-tubule system), (3) dihydropyridine receptors (DHPR)-mediated detection of changes in membrane potential, (4) allosteric interaction between DHPR and sarcoplasmic reticulum (SR) ryanodine receptors (RyR), (5) release of Ca2+ from the SR and transient increase of Ca2+ concentration in the myoplasm, (6) activation of the myoplasmic Ca2+ buffering system and the contractile apparatus, followed by (7) Ca2+ disappearance from the myoplasm mediated mainly by its reuptake by the SR through the SR Ca2+ adenosine triphosphatase (SERCA), and under several conditions movement to the mitochondria and extrusion by the Na+/Ca2+ exchanger (NCX). In this text, we review the basics of ECC in skeletal muscle and the techniques used to study it. Moreover, we highlight some recent advances and point out gaps in knowledge on particular issues related to ECC such as (1) DHPR-RyR molecular interaction, (2) differences regarding fibre types, (3) its alteration during muscle fatigue, (4) the role of mitochondria and store-operated Ca2+ entry in the general ECC sequence, (5) contractile potentiators, and (6) Ca2+ sparks.

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.

The intracellular transport and secretion of calumenin-1/2 in living cells

Calumenin isoforms 1 and 2 (calu-1/2), encoded by the CALU gene, belong to the CREC protein family. Calu-1/2 proteins are secreted into the extracellular space, but the secretory process and regulatory mechanism are largely unknown. Here, using a time-lapse imaging system, we visualized the intracellular transport and secretory process of calu-1/2-EGFP after their translocation into the ER lumen. Interestingly, we observed that an abundance of calu-1/2-EGFP accumulated in cellular processes before being released into the extracellular space, while only part of calu-1/2-EGFP proteins were secreted directly after attaching to the cell periphery. Moreover, we found the secretion of calu-1/2-EGFP required microtubule integrity, and that calu-1/2-EGFP-containing vesicles were transported by the motor proteins Kif5b and cytoplasmic dynein. Finally, we determined the export signal of calu-1/2-EGFP (amino acid positions 20–46) and provided evidence that the asparagine at site 131 was indispensable for calu-1/2-EGFP stabilization. Taken together, we provide a detailed picture of the intracellular transport of calu-1/2-EGFP, which facilitates our understanding of the secretory mechanism of calu-1/2.

Calcium signaling in closely related protozoan groups (Alveolata): non-parasitic ciliates (Paramecium, Tetrahymena) vs. parasitic Apicomplexa (Plasmodium, Toxoplasma)

The importance of Ca2+-signaling for many subcellular processes is well established in higher eukaryotes, whereas information about protozoa is restricted. Recent genome analyses have stimulated such work also with Alveolates, such as ciliates (Paramecium, Tetrahymena) and their pathogenic close relatives, the Apicomplexa (Plasmodium, Toxoplasma). Here we compare Ca2+ signaling in the two closely related groups. Acidic Ca2+ stores have been characterized in detail in Apicomplexa, but hardly in ciliates. Two-pore channels engaged in Ca2+-release from acidic stores in higher eukaryotes have not been stingently characterized in either group. Both groups are endowed with plasma membrane- and endoplasmic reticulum-type Ca2+-ATPases (PMCA, SERCA), respectively. Only recently was it possible to identify in Paramecium a number of homologs of ryanodine and inositol 1,3,4-trisphosphate receptors (RyR, IP3R) and to localize them to widely different organelles participating in vesicle trafficking. For Apicomplexa, physiological experiments suggest the presence of related channels although their identity remains elusive. In Paramecium, IP3Rs are constitutively active in the contractile vacuole complex; RyR-related channels in alveolar sacs are activated during exocytosis stimulation, whereas in the parasites the homologous structure (inner membrane complex) may no longer function as a Ca2+ store. Scrutinized comparison of the two closely related protozoan phyla may stimulate further work and elucidate adaptation to parasitic life. See also "Conclusions" section.

Upregulation of arginase-II contributes to decreased age-related myocardial contractile reserve

Arginase-II (Arg-II) reciprocally regulates nitric oxide synthase (NOS) and offsets basal myocardial contractility. Furthermore, decreased or absent myocardial NOS activity is associated with a depression in myocardial contractile reserve. We therefore hypothesized that upregulation of Arg-II might in part be responsible for depressed myocardial contractility associated with age. We studied arginase activity/expression, NOS expression, NO production in the presence and absence of the arginase inhibitor S-(2-boronoethyl)-L: -cysteine (BEC) in old (22 months) and young (3 months) rat hearts and myocytes. The spatial confinement of Arg-II and NOS was determined with immuno-electron-miocrographic (IEM) and immuno-histochemical studies. We tested the effect of BEC on the force frequency response (FFR) in myocytes, as well as NOS abundance and activity. Arginase activity and Arg-II expression was increased in old hearts (2.27 ± 0.542 vs. 0.439 ± 0.058 nmol urea/mg protein, p = 0.02). This was associated with a decrease in NO production, which was restored with BEC (4.54 ± 0.582 vs. 12.88 ± 0.432 μmol/mg, p < 0.01). IEM illustrates increased mitochondrial density in old myocytes (51.7 ± 1.8 vs. 69 ± 2.2 × 10(6)/cm(2), p < 0.01), potentially contributing to increased Arg-II abundance and activity. Immunohistochemistry revealed an organized pattern of mitochondria and Arg-II that appears disrupted in old myocytes. The FFR was significantly depressed in old myocytes (61.42 ± 16.04 vs. -5.15 ± 5.65%), while inhibition of Arg-II restored the FFR (-5.15 ± 5.65 vs. 70.98 ± 6.10%). NOS-2 is upregulated sixfold in old hearts contributing to increased production of reactive oxygen species which is attenuated with NOS-2 inhibition by 1400 W (4,735 ± 427 vs. 4,014 ± 314 RFU/min/mg protein, p = 0.005). Arg-II upregulation in aging rat hearts contributes to age-related decreased contractile function.

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.

Ryanodine receptors: structure, expression, molecular details, and function in calcium release

Ryanodine receptors (RyRs) are located in the sarcoplasmic/endoplasmic reticulum membrane and are responsible for the release of Ca(2+) from intracellular stores during excitation-contraction coupling in both cardiac and skeletal muscle. RyRs are the largest known ion channels (> 2MDa) and exist as three mammalian isoforms (RyR 1-3), all of which are homotetrameric proteins that interact with and are regulated by phosphorylation, redox modifications, and a variety of small proteins and ions. Most RyR channel modulators interact with the large cytoplasmic domain whereas the carboxy-terminal portion of the protein forms the ion-conducting pore. Mutations in RyR2 are associated with human disorders such as catecholaminergic polymorphic ventricular tachycardia whereas mutations in RyR1 underlie diseases such as central core disease and malignant hyperthermia. This chapter examines the current concepts of the structure, function and regulation of RyRs and assesses the current state of understanding of their roles in associated disorders.

CALU A29809G polymorphism in coronary atherothrombosis: Implications for coronary calcification and prognosis

INTRODUCTION

Arterial calcification is a risk factor for atherosclerosis. Calumenin (CALU), a protein regulating proteins involved in coagulation and arterial calcification also has extracellular functions related to atherosclerosis. We recently described that CALU polymorphism A29809G was related to acenocoumarol requirements, and we wanted to evaluate its role in arterial calcification and prognosis.

PATIENTS AND METHODS

A total of 374 consecutive patients with non-ST-elevation acute coronary syndrome (nSTACS). In 175 of them, who underwent percutaneous coronary intervention, we assessed calcification in each main coronary artery. Follow-up at 1 and 6 months was performed for adverse end-points.

RESULTS

CALU 29809G carriers were more frequent in the low calcium group (P = 0.037). The presence of >or=3 cardiovascular risk factors and CALU polymorphism were associated with arterial calcification (OR 2.34, P = 0.049; and OR 0.34, P = 0.019, respectively). CALU 29809G allele was the only variable associated with events at 1 month (HR 0.42; P = 0.042). Multivariate analysis showed that, at 6 months, age and severe anginal symptoms were associated with worse prognosis (HR 2.13, P = 0.023; and HR 2.01, P = 0.011, respectively), whereas CALU 29809G allele associated with good prognosis (HR 0.59, P = 0.044). Our results suggest that CALU A29809G is associated with arterial calcification and short-term prognosis of the outcome of patients with nSTACS.

Temporal proteome profiling of taxol-induced mitotic arrest and apoptosis

Taxol (Paclitaxel) is a mitotic inhibitor widely used in cancer therapy. Temporal proteome profiling was performed to study changes of proteins during the different cellular states of HeLa cells caused by exposure to taxol. The changes of proteins over time could be associated with various cellular processes such as mitotic arrest, an intermediate between mitotic arrest and apoptosis, apoptosis, and late apoptosis. Calumenin, stress-induced phosphoprotein 1 (STIP1), and translationally controlled tumor protein (TCTP) were assigned to mitotic arrest and selected for further experiments using immunoblotting and subcellular fractionation. Calumenin translocated from membranes to the cytosol during mitotic arrest and late apoptosis, but was significantly reduced in the cytosol during apoptosis. Translocation of STIP1 to the nucleus was observed at apoptosis and to the cytoskeleton at late apoptosis. TCTP increased in the cytosol at mitotic arrest and in membranes at apoptosis. In addition, the quantitative time courses of Bim isoforms revealed differences between BimL and BimS in comparison with BimEL. In summary, temporal proteome profiling of HeLa cells incubated with taxol allowed the assignment of proteins to certain processes and additional experiments with complementary approaches enabled a more comprehensive understanding of spatial changes of selected proteins during mitotic arrest and apoptosis.

Characterization of calumenin-SERCA2 interaction in mouse cardiac sarcoplasmic reticulum

Calumenin is a multiple EF-hand Ca(2+)-binding protein localized in the sarcoplasmic reticulum (SR) with C-terminal SR retention signal HDEF. Recently, we showed evidence that calumenin interacts with SERCA2 in rat cardiac SR (Sahoo, S. K., and Kim, D. H. (2008) Mol. Cells 26, 265-269). The present study was undertaken to further characterize the association of calumenin with SERCA2 in mouse heart by various gene manipulation approaches. Immunocytochemical analysis showed that calumenin and SERCA2 were partially co-localized in HL-1 cells. Knockdown (KD) of calumenin was conducted in HL-1 cells and 80% reduction of calumenin did not induce any expressional changes of other Ca(2+)-cycling proteins. But it enhanced Ca(2+) transient amplitude and showed shortened time to reach peak and decreased time to reach 50% of baseline. Oxalate-supported Ca(2+) uptake showed increased Ca(2+) sensitivity of SERCA2 in calumenin KD HL-1 cells. Calumenin and SERCA2 interaction was significantly lower in the presence of thapsigargin, vanadate, or ATP, as compared with 1.3 mum Ca(2+), suggesting that the interaction is favored in the E1 state of SERCA2. A glutathione S-transferase-pulldown assay of calumenin deletion fragments and SERCA2 luminal domains suggested that regions of 132-222 amino acids of calumenin and 853-892 amino acids of SERCA2-L4 are the major binding partners. On the basis of our in vitro binding data and available information on three-dimensional structure of Ca(2+)-ATPases, a molecular model was proposed for the interaction between calumenin and SERCA2. Taken together, the present results suggest that calumenin is a novel regulator of SERCA2, and its expressional changes are tightly coupled with Ca(2+)-cycling of cardiomyocytes.

The rapidly expanding CREC protein family: members, localization, function, and role in disease

Although many aspects of the physiological and pathophysiological mechanisms remain unknown, recent advances in our knowledge suggest that the CREC proteins are promising disease biomarkers or targets for therapeutic intervention in a variety of diseases. The CREC family of low affinity, Ca2+-binding, multiple EF-hand proteins are encoded by five genes, RCN1, RCN2, RCN3, SDF4, and CALU, resulting in reticulocalbin, ER Ca2+-binding protein of 55 kDa (ERC-55), reticulocalbin-3, Ca2+-binding protein of 45 kDa (Cab45), and calumenin. Alternative splicing increases the number of gene products. The proteins are localized in the cytosol, in various parts of the secretory pathway, secreted to the extracellular space or localized on the cell surface. The emerging functions appear to be highly diverse. The proteins interact with several different ligands. Rather well-described functions are attached to calumenin with the inhibition of several proteins in the endoplasmic or sarcoplasmic reticulum membrane, the vitamin K(1) 2,3-epoxide reductase, the gamma-carboxylase, the ryanodine receptor, and the Ca2+-transporting ATPase. Other functions concern participation in the secretory process, chaperone activity, signal transduction as well as participation in a large variety of disease processes.

Carbonylation of adipose proteins in obesity and insulin resistance: identification of adipocyte fatty acid-binding protein as a cellular target of 4-hydroxynonenal

Obesity is a state of mild inflammation correlated with increased oxidative stress. In general, pro-oxidative conditions lead to production of reactive aldehydes such as trans-4-hydroxy-2-nonenal (4-HNE) and trans-4-oxo-2-nonenal implicated in the development of a variety of metabolic diseases. To investigate protein modification by 4-HNE as a consequence of obesity and its potential relationship to the development of insulin resistance, proteomics technologies were utilized to identify aldehyde-modified proteins in adipose tissue. Adipose proteins from lean insulin-sensitive and obese insulin-resistant C57Bl/6J mice were incubated with biotin hydrazide and detected using horseradish peroxidase-conjugated streptavidin. High carbohydrate, high fat feeding of mice resulted in a approximately 2-3-fold increase in total adipose protein carbonylation. Consistent with an increase in oxidative stress in obesity, the abundance of glutathione S-transferase A4 (GSTA4), a key enzyme responsible for metabolizing 4-HNE, was decreased approximately 3-4-fold in adipose tissue of obese mice. To identify specific carbonylated proteins, biotin hydrazide-modified adipose proteins from obese mice were captured using avidin-Sepharose affinity chromatography, proteolytically digested, and subjected to LC-ESI MS/MS. Interestingly enzymes involved in cellular stress response, lipotoxicity, and insulin signaling such as glutathione S-transferase M1, peroxiredoxin 1, glutathione peroxidase 1, eukaryotic elongation factor 1alpha-1 (eEF1alpha1), and filamin A were identified. The adipocyte fatty acid-binding protein, a protein implicated in the regulation of insulin resistance, was found to be carbonylated in vivo with 4-HNE. In vitro modification of adipocyte fatty acid-binding protein with 4-HNE was mapped to Cys-117, occurred equivalently using either the R or S enantiomer of 4-HNE, and reduced the affinity of the protein for fatty acids approximately 10-fold. These results indicate that obesity is accompanied by an increase in the carbonylation of a number of adipose-regulatory proteins that may serve as a mechanistic link between increased oxidative stress and the development of insulin resistance.