A continuous spectrophotometric assay for simultaneous measurement of calcium uptake and ATP hydrolysis in sarcoplasmic reticulum

Sarcolipin Promotes Uncoupling of the SERCA Ca2+ Pump by Inducing a Structural Rearrangement in the Energy-Transduction Domain

We have performed microsecond (μs) molecular dynamics simulation (MDS) to identify structural mechanisms for sarcolipin (SLN) uncoupling of Ca2+ transport from ATP hydrolysis for the sarcoplasmic reticulum Ca2+-ATPase (SERCA). SLN regulates muscle metabolism and energy expenditure to provide resistance against diet-induced obesity and extreme cold. MDS demonstrated that the cytosolic domain of SLN induces a salt bridge-mediated structural rearrangement in the energy-transduction domain of SERCA. We propose that this structural change uncouples SERCA by perturbing Ca2+ occlusion at residue E309 in transport site II, thus facilitating Ca2+ backflux to the cytosol. Our results have important implications for designing muscle-based therapies for human obesity.

Direct detection of SERCA calcium transport and small-molecule inhibition in giant unilamellar vesicles

We have developed a charge-mediated fusion method to reconstitute the sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) in giant unilamellar vesicles (GUV). Intracellular Ca²⁺ transport by SERCA controls key processes in human cells such as proliferation, signaling, and contraction. Small-molecule effectors of SERCA are urgently needed as therapeutics for Ca²⁺ dysregulation in human diseases including cancer, diabetes, and heart failure. Here we report the development of a method for efficiently reconstituting SERCA in GUV, and we describe a streamlined protocol based on optimized parameters (e.g., lipid components, SERCA preparation, and activity assay requirements). ATP-dependent Ca²⁺ transport by SERCA in single GUV was detected directly using confocal fluorescence microscopy with the Ca²⁺ indicator Fluo-5F. The GUV reconstitution system was validated for functional screening of Ca²⁺ transport using thapsigargin (TG), a small-molecule inhibitor of SERCA currently in clinical trials as a prostate cancer prodrug. The GUV system overcomes the problem of inhibitory Ca²⁺ accumulation for SERCA in native and reconstituted small unilamellar vesicles (SUV). We propose that charge-mediated fusion provides a widely-applicable method for GUV reconstitution of clinically-important membrane transport proteins. We conclude that GUV reconstitution is a technological advancement for evaluating small-molecule effectors of SERCA.

A microplate technique to simultaneously assay calcium accumulation in endoplasmic reticulum and SERCA release of inorganic phosphate

Traditional analyses of calcium homeostasis have separately quantified either calcium accumulation or release mechanisms. To define the system as a whole, however, requires multiple experimental techniques to examine both accumulation and release. Here we describe a technique that couples the simultaneous quantification of radio-labeled calcium accumulation in endoplasmic reticulum (ER) microsomes with the release of inorganic phosphate (Pi) by the hydrolytic activity of sarco-endoplasmic reticulum calcium ATPase (SERCA) all in the convenience of a 96-well format.

Inhibition of a cardiac sarcoplasmic reticulum chloride channel by tamoxifen

Anion and cation channels present in the sarcoplasmic reticulum (SR) are believed to be necessary to maintain the electroneutrality of SR membrane during Ca(2+) uptake by the SR Ca(2+) pump (SERCA). Here we incorporated canine cardiac SR ion channels into lipid bilayers and studied the effects of tamoxifen and other antiestrogens on these channels. A Cl(-) channel was identified exhibiting multiple subconductance levels which could be divided into two primary conductance bands. Tamoxifen decreases the time the channel spends in its higher, voltage-sensitive band and the mean channel current. The lower, voltage-insensitive, conductance band is not affected by tamoxifen, nor is a K(+) channel present in the cardiac SR preparation. By examining SR Ca(2+) uptake, SERCA ATPase activity, and SR ion channels in the same preparation, we also estimated SERCA transport current, SR Cl(-) and K(+) currents, and the density of SERCA, Cl(-), and K(+) channels in cardiac SR membranes.

Solid-state NMR and functional measurements indicate that the conserved tyrosine residues of sarcolipin are involved directly in the inhibition of SERCA1

The transmembrane protein sarcolipin regulates calcium storage in the sarcoplasmic reticulum of skeletal and cardiac muscle cells by modulating the activity of sarco(endo)plasmic reticulum Ca(2+)-ATPases (SERCAs). The highly conserved C-terminal region ((27)RSYQY-COOH) of sarcolipin helps to target the protein to the sarcoplasmic reticulum membrane and may also participate in the regulatory interaction between sarcolipin and SERCA. Here we used solid-state NMR measurements of local protein dynamics to illuminate the direct interaction between the Tyr(29) and Tyr(31) side groups of sarcolipin and skeletal muscle Ca(2+)-ATPase (SERCA1a) embedded in dioleoylphosphatidylcholine bilayers. Further solid-state NMR experiments together with functional measurements on SERCA1a in the presence of NAc-RSYQY, a peptide representing the conserved region of sarcolipin, suggest that the peptide binds to the same site as the parent protein at the luminal face of SERCA1a, where it reduces V(max) for calcium transport and inhibits ATP hydrolysis with an IC(50) of approximately 200 microM. The inhibitory effect of NAc-RSYQY is remarkably sequence-specific, with the native aromatic residues being essential for optimal inhibitory activity. This combination of physical and functional measurements highlights the importance of aromatic and polar residues in the C-terminal region of sarcolipin for regulating calcium cycling and muscle contractility.

Solubilization, purification and reconstitution of Ca2+-ATPase from bovine pulmonary artery smooth muscle microsomes by different detergents: Preservation of native structure and function of the enzyme by DHPC

The properties of Ca(2+)-ATPase purified and reconstituted from bovine pulmonary artery smooth muscle microsomes {enriched with endoplasmic reticulum (ER)} were studied using the detergents 1,2-diheptanoyl-sn-phosphatidylcholine (DHPC), poly(oxy-ethylene)8-lauryl ether (C(12)E(8)) and Triton X-100 as the solubilizing agents. Solubilization with DHPC consistently gave higher yields of purified Ca(2+)-ATPase with a greater specific activity than solubilization with C(12)E(8) or Triton X-100. DHPC was determined to be superior to C(12)E(8); while that the C(12)E(8) was determined to be better than Triton X-100 in active enzyme yields and specific activity. DHPC solubilized and purified Ca(2+)-ATPase retained the E1Ca-E1*Ca conformational transition as that observed for native microsomes; whereas the C(12)E(8) and Triton X-100 solubilized preparations did not fully retain this transition. The coupling of Ca(2+) transported to ATP hydrolyzed in the DHPC purified enzyme reconstituted in liposomes was similar to that of the native micosomes, whereas that the coupling was much lower for the C(12)E(8) and Triton X-100 purified enzyme reconstituted in liposomes. The specific activity of Ca(2+)-ATPase reconstituted into dioleoyl-phosphatidylcholine (DOPC) vesicles with DHPC was 2.5-fold and 3-fold greater than that achieved with C(12)E(8) and Triton X-100, respectively. Addition of the protonophore, FCCP caused a marked increase in Ca(2+) uptake in the reconstituted proteoliposomes compared with the untreated liposomes. Circular dichroism analysis of the three detergents solubilized and purified enzyme preparations showed that the increased negative ellipticity at 223 nm is well correlated with decreased specific activity. It, therefore, appears that the DHPC purified Ca(2+)-ATPase retained more organized and native secondary conformation compared to C(12)E(8) and Triton X-100 solubilized and purified preparations. The size distribution of the reconstituted liposomes measured by quasi-elastic light scattering indicated that DHPC preparation has nearly similar size to that of the native microsomal vesicles whereas C(12)E(8) and Triton X-100 preparations have to some extent smaller size. These studies suggest that the Ca(2+)-ATPase solubilized, purified and reconstituted with DHPC is superior to that obtained with C(12)E(8) and Triton X-100 in many ways, which is suitable for detailed studies on the mechanism of ion transport and the role of protein-lipid interactions in the function of the membrane-bound enzyme.

Effects of anti-oestrogens and beta-estradiol on calcium uptake by cardiac sarcoplasmic reticulum

1. Tamoxifen and a group of structurally similar non-steroidal, triphenolic compounds inhibit the oestrogen receptor. In addition to this action, these anti-oestrogens are known to inhibit some types of plasma membrane ion channels and other proteins through mechanisms that do not appear to involve their interactions with the estrogen receptor but could be the result of their effect on membrane lipid structure or fluidity. 2. We studied the effects of beta-estradiol and three anti-oestrogens (tamoxifen, 4-hydroxytamoxifen and clomiphene) on Ca(2+) uptake into sarcoplasmic reticulum (SR) vesicles isolated from canine cardiac ventricular tissue. 3. The antiestrogens all inhibit SR Ca(2+) uptake in a concentration-dependent manner (order of potency: tamoxifen > 4-hydroxytamoxifen > or = clomiphene). Although these compounds rapidly inhibit net Ca(2+) uptake they do not have a similar rapid effect on the ATPase activity of the SR Ca pump. beta-estradiol has no effect on Ca(2+) uptake nor does it alter the inhibitory action of tamoxifen on the SR. 4. The differences in the effects of beta-estradiol and the anti-oestrogens on cardiac SR Ca(2+) uptake do not correlate with differences in the ways in which these compounds have been reported to interact with membrane lipids. Our results are consistent, however, with direct effects on a membrane protein (possibly an SR Cl(-) or K(+) channel).

Iodide and bromide inhibit Ca(2+) uptake by cardiac sarcoplasmic reticulum

Recent studies indicate that the Ca(2+) permeability of the sarcoplasmic reticulum (SR) can be affected by its anionic environment. Additionally, anions could directly modulate the SR Ca(2+) pump or the movement of compensatory charge across the SR membrane during Ca(2+) uptake or release. To examine the effect of anion substitution on cardiac SR Ca(2+) uptake, fluorometric Ca(2+) measurements and spectrophotometric ATPase assays were used. Ca(2+) uptake into SR vesicles was inhibited in a concentration-dependent manner when Br(-) or I(-) replaced extravesicular Cl(-) (when Br(-) completely replaced Cl(-), uptake velocity was approximately 70% of control; when I(-) completely replaced Cl(-), uptake velocity was approximately 39% of control). Replacement of Cl(-) with SO(2)(-4) had no effect on SR uptake. Although both I(-) and Br(-) inhibited net Ca(2+) uptake, neither anion directly inhibited the SR Ca(2+) pump nor did they increase the permeability of the SR membrane to Ca(2+). Our results support the hypothesis that an anionic current that occurs during SR Ca(2+) uptake is reduced by the substitution of Br(-) or I(-) for Cl(-).

Characterization of the N-terminal repeat domain of Escherichia coli ClpA-A class I Clp/HSP100 ATPase

The ClpA, ClpB, and ClpC subfamilies of the Clp/HSP100 ATPases contain a conserved N-terminal region of approximately 150 residues that consists of two approximate sequence repeats. This sequence from the Escherichia coli ClpA enzyme is shown to encode an independent structural domain (the R domain) that is monomeric and approximately 40% alpha-helical. A ClpA fragment lacking the R domain showed ATP-dependent oligomerization, protein-stimulated ATPase activity, and the ability to complex with the ClpP peptidase and mediate degradation of peptide and protein substrates, including casein and ssrA-tagged proteins. Compared with the activities of the wild-type ClpA, however, those of the ClpA fragment missing the R domain were reduced. These results indicate that the R domain is not required for the basic recognition, unfolding, and translocation functions that allow ClpA-ClpP to degrade some protein substrates, but they suggest that it may play a role in modulating these activities.

Direct observation of calcium-independent intercellular ATP signaling in astrocytes

Adenosine triphosphate (ATP) is assumed to be involved in the regulation of many extracellular signaling systems including calcium wave propagation. So far all supportive evidence is indirect, such as monitoring changes in intracellular calcium on application of extracellular ATP or off-site measurement of ATP from superfusates. Furthermore, the causal relationships among the various signaling agents are still unclear. A novel chemiluminescence dynamic imaging method was developed to monitor ATP release from living biological cells. The assay has linear response over 3 orders of magnitude for fixed concentrations of enzyme and cofactors, with a correlation coefficient of 0.999. The detectability of ATP is down to 10(-8) M at millisecond exposure times with an intensified charge-coupled device camera. The direct imaging of ATP waves in astrocyte cultures was performed together with Fluo-3-Ca imaging at millisecond temporal resolution and micrometer-scale spatial resolution. We discovered that extracellular ATP mediates intercellular calcium wave propagation, but surprisingly, release and propagation of ATP are not calcium dependent. Therefore, ATP rather than Ca or IP3 is the primary intercellular signaling messenger.

Calcium transient alternans in blood-perfused ischemic hearts: observations with fluorescent indicator fura red

Ischemia produces striking electrophysiological abnormalities in blood-perfused hearts that may be caused, in part, by effects of ischemia on intracellular calcium. To test this hypothesis, intracellular Ca2+ concentration ([Ca2+]i) transients were recorded from the epicardial surface of blood- and saline-perfused rabbit hearts using the long-wavelength indicator Fura Red. Calcium transients were much larger than the movement artifact, representing up to 29% of the total signal. Switching the perfusate from saline to blood did not affect the time course of the transients or the apparent level of [Ca2+]i. Compartmentation of Fura Red fluorescence was estimated by exposure to Mn2+. The results were cytosol 60 +/- 3%, organelles 12 +/- 2%, and autofluorescence plus partly deesterified Fura Red 29 +/- 4%. [Ca2+]i transients were calibrated in situ by perfusion of the extracellular space with high-Ca2+ and Ca(2+)-free EGTA solutions. Peak systolic [Ca2+]i was 663 +/- 74 nM, and end-diastolic [Ca2+]i was 279 +/- 59 nm. Ischemia was produced by interruption of aortic perfusion for 2.5 min during pacing (150 beats/min). Ischemia produced broadening of the [Ca2+]i transient, along with beat-to-beat alternations in the peak systolic and end-diastolic level of [Ca2+]i (calcium transient alternans). [Ca2+]i transient alternans occurred in 82% of blood-perfused hearts vs. 43% of saline-perfused hearts. The discrepancy between large and small transients (mean alternans ratio) was larger in the blood-perfused hearts (0.23 +/- 0.04 vs. 0.07 +/- 0.03, P = 0.005). These observations are important because of the apparent relationship of [Ca2+]i transient alternans to electrical alternans and arrhythmias during ischemia.

Preservation of the native structure and function of Ca2+-ATPase from sarcoplasmic reticulum: solubilization and reconstitution by new short-chain phospholipid detergent 1,2-diheptanoyl-sn-phosphatidylcholine

The properties of Ca2+-ATPase purified and reconstituted from rabbit skeletal sarcoplasmic reticulum (SR) has been studied in comparison with the preparations obtained by the commonly used detergent poly(oxyethylene)8-lauryl ether (C12E8) and the bile salt detergents cholate and deoxycholate. 1,2-Diheptanoyl-sn-phosphatidylcholine (DHPC) has been shown to be excellent for solubilizing a wide variety of membrane proteins [Kessi, Poiree, Wehrli, Bachofen, Semenza and Hauser (1994) Biochemistry 33, 10825-10836]. The DHPC method consistently gave higher yields of purified Ca2+-ATPase with a greater specific activity than the methods with C12E8, cholate, or deoxycholate. DHPC and C12E8 were superior to cholate and deoxycholate in active enzyme yields and specific activity. DHPC-solubilized Ca2+-ATPase purified on a density gradient retained the E1Ca-E1(*)Ca conformational transition, whereas the enzyme from the C12E8 purification did not retain this transition. The coupling of Ca2+ transported to ATP hydrolysed in the DHPC-purified enzyme was maximal and matched the values obtained with native SR, whereas the coupling was much lower for the C12E8-purified enzyme. The specific activity of Ca2+-ATPase reconstituted into dioleoylphosphatidylcholine vesicles with DHPC was up to 2-fold greater than that achieved with C12E8, and is comparable to that measured in the native SR. Finally, the dissociation of Ca2+-ATPase into monomers by DHPC preserved the ATPase activity, whereas similar dissociation by C12E8 gave only one-sixth the activity of that obtained with DHPC. These studies show that the Ca2+-ATPase solubilized, purified and reconstituted with DHPC is superior to that obtained with C12E8 in significant ways, making it a preparation suitable for detailed studies on the mechanism of ion transport and the role of protein-lipid interactions in the function of membrane proteins.