Structure-function relationships in the Ca(2+)-ATPase of sarcoplasmic reticulum: facts, speculations and questions for the future

Structure-Function Relationships and Modifications of Cardiac Sarcoplasmic Reticulum Ca2+-Transport

Sarcoplasmic reticulum (SR) is a specialized tubular network, which not only maintains the intracellular concentration of Ca2+ at a low level but is also known to release and accumulate Ca2+ for the occurrence of cardiac contraction and relaxation, respectively. This subcellular organelle is composed of several phospholipids and different Ca2+-cycling, Ca2+-binding and regulatory proteins, which work in a coordinated manner to determine its function in cardiomyocytes. Some of the major proteins in the cardiac SR membrane include Ca2+-pump ATPase (SERCA2), Ca2+-release protein (ryanodine receptor), calsequestrin (Ca2+-binding protein) and phospholamban (regulatory protein). The phosphorylation of SR Ca2+-cycling proteins by protein kinase A or Ca2+-calmodulin kinase (directly or indirectly) has been demonstrated to augment SR Ca2+-release and Ca2+-uptake activities and promote cardiac contraction and relaxation functions. The activation of phospholipases and proteases as well as changes in different gene expressions under different pathological conditions have been shown to alter the SR composition and produce Ca2+-handling abnormalities in cardiomyocytes for the development of cardiac dysfunction. The post-translational modifications of SR Ca2+ cycling proteins by processes such as oxidation, nitrosylation, glycosylation, lipidation, acetylation, sumoylation, and O GlcNacylation have also been reported to affect the SR Ca2+ release and uptake activities as well as cardiac contractile activity. The SR function in the heart is also influenced in association with changes in cardiac performance by several hormones including thyroid hormones and adiponectin as well as by exercise-training. On the basis of such observations, it is suggested that both Ca2+-cycling and regulatory proteins in the SR membranes are intimately involved in determining the status of cardiac function and are thus excellent targets for drug development for the treatment of heart disease.

Tracking the sarcoplasmic reticulum membrane voltage in muscle with a FRET biosensor

The sarcoplasmic reticulum membrane contains ion channels, but it is unknown whether it experiences voltage changes during cellular activity. By expressing voltage-sensitive fluorescence biosensors in this membrane, Sanchez et al. suggest that it remains electrically silent during muscle activation.

Ion channel activity in the plasma membrane of living cells generates voltage changes that are critical for numerous biological functions. The membrane of the endoplasmic/sarcoplasmic reticulum (ER/SR) is also endowed with ion channels, but whether changes in its voltage occur during cellular activity has remained ambiguous. This issue is critical for cell functions that depend on a Ca²⁺ flux across the reticulum membrane. This is the case for contraction of striated muscle, which is triggered by opening of ryanodine receptor Ca²⁺ release channels in the SR membrane in response to depolarization of the transverse invaginations of the plasma membrane (the t-tubules). Here, we use targeted expression of voltage-sensitive fluorescence resonance energy transfer (FRET) probes of the Mermaid family in differentiated muscle fibers to determine whether changes in SR membrane voltage occur during depolarization–contraction coupling. In the absence of an SR targeting sequence, FRET signals from probes present in the t-tubule membrane allow calibration of the voltage sensitivity and amplitude of the response to voltage-clamp pulses. Successful SR targeting of the probes was achieved using an N-terminal domain of triadin, which completely eliminates voltage-clamp–activated FRET signals from the t-tubule membrane of transfected fibers. In fibers expressing SR-targeted Mermaid probes, activation of SR Ca²⁺ release in the presence of intracellular ethyleneglycol-bis(β-amino-ethyl ether)-N,N,N′,N′-tetra acetic acid (EGTA) results in an accompanying FRET signal. We find that this signal results from pH sensitivity of the probe, which detects cytosolic acidification because of the release of protons upon Ca²⁺ binding to EGTA. When EGTA is substituted with either 1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid or the contraction blocker N-benzyl-p-toluene sulfonamide, we find no indication of a substantial change in the FRET response caused by a voltage change. These results suggest that the ryanodine receptor–mediated SR Ca²⁺ efflux is well balanced by concomitant counterion currents across the SR membrane.

Effects of prior exercise and a low-carbohydrate diet on muscle sarcoplasmic reticulum function during cycling in women

The effects of exercise and diet on sarcoplasmic reticulum Ca(2+)-cycling properties in female vastus lateralis muscle were investigated in two groups of women following four different conditions. The conditions were 4 days of a low-carbohydrate (Lo CHO) and glycogen-depleting exercise plus a Lo CHO diet (Ex + Lo CHO) (experiment 2) and 4 days of normal CHO (Norm CHO) and glycogen-depleting exercise plus Norm CHO (Ex + Norm CHO) (experiment 1). Peak aerobic power (Vo2peak)) was 38.1 +/- 1.4 (SE); n = 9 and 35.6 +/- 1.4 ml.kg(-1).min(-1); n = 9, respectively. Sarcoplasmic reticulum properties measured in vitro in homogenates (micromol.g protein(-1).min(-1)) indicated exercise-induced reductions (P < 0.05) in maximal Ca(2+)-ATPase activity (0 > 30, 60 min > fatigue), Ca(2+) uptake (0 > 30 > 60 min, fatigue), and Ca(2+) release, both phase 1 (0, 30 > 60 min, fatigue) and phase 2 (0 > 30, 60 min, fatigue; 30 min > fatigue) in Norm CHO. Exercise was without effect in altering the Hill slope (n(H)), defined as the slope of relationship between Ca(2+)-ATPase activity and Ca(2+) concentration. No differences were observed between Norm CHO and Ex+Norm CHO. Compared with Norm CHO, Lo CHO resulted in a lower (P < 0.05) Ca(2+) uptake, phase 1 Ca(2+) release (30 min), and n(H). Ex + Lo CHO resulted in a greater (P < 0.05) Ca(2+) uptake and n(H) compared with Lo CHO. The results demonstrate that Lo CHO alone can disrupt SR Ca(2+) cycling and that, with the exception of Ca(2+) release, a glycogen-depleting session of exercise before Lo CHO can reverse the effects.

Phospholipid requirement and pH optimum for the in vitro enzymatic activity of the E. coli P-type ATPase ZntA

Detergent solubilization and purification of the E. coli heavy metal P-type ATPase ZntA yields an enzyme with reduced hydrolytic activity in vitro. Here, it is shown that the in vitro hydrolytic activity of detergent solubilized ZntA is increased in the presence of negatively charged phospholipids and at slightly acidic pH. The protein-lipid interaction of ZntA was characterized by enzyme-coupled ATPase assays and fluorescence spectroscopy. Among the most abundant naturally occurring phospholipids, only phosphatidyl-glycerol lipids (PG) enhance the in vitro enzymatic ATPase activity of ZntA. Re-lipidation of detergent purified ZntA with 1,2-dioleoylphosphatidyl-glycerol (DOPG) increases the ATPase activity four-fold compared to the purified state. All other E. coli phospholipids fail to activate the ATPase. Among the phosphatidyl-glycerol family, highest activity was observed for 1,2-dioleoyl-PG followed by 1,2-dimyristoyl-PG, 1,2-dipalmitoyl-PG and 1,2-distearoyl-PG. Increasing intrinsic Trp fluorescence quantum yield upon relipidation of ZntA was used to determine a pH maximum for lipid binding at pH 6.7. The pH dependence of the lipid binding was confirmed by pH-dependent ATPase assays showing maximum activity at pH 6.7. The biophysical characterization of detergent solubilized membrane proteins crucially relies on the conformational stability and functional integrity of the protein under investigation. The present study describes how the E. coli ZntA P-type ATPase can be stabilized and functionally activated in a detergent solubilized system.

Oral Pretreatment with Ebselen Enhances Heat Shock Protein 72 Expression and Reduces Myocardial Infarct Size

Reactive oxygen species (ROS) enhance myocardial ischemia-reperfusion (I/R) injury. Ebselen, a seleno-organic glutathione peroxidase (GPx) mimetic, has a protective effect against tissue injury induced by ROS. However, the cardio-protective effect of orally administered ebselen has never been investigated in cardiac I/R injury. We investigated the effects and mechanisms of orally administered ebselen on experimental myocardial infarction. Isolated perfused rabbit hearts underwent 30 min of global ischemia and 60 min of reperfusion, with or without oral administration of ebselen 24 h before I/R, with or without enhanced oxidative stress by H202 infusion for the first 1 min of reperfusion. The recovery of left ventricular developed pressure (LVDP) was significantly improved, and the myocardial infarct size was significantly reduced by ebselen. The recovery of LVDP and the myocardial infarct size were markedly aggravated by H202 infusion. These enhancements by H202 were dose-dependently suppressed by ebselen, along with a reduction in myocardial 8-hydroxydeoxyguanosine levels, a marker for oxidative DNA damage. The myocardial reduced glutathione (GSH) level was preserved by ebselen. Ebselen markedly enhanced myocardial heat shock protein (HSP) 72 expression. The cardioprotective effect of ebselen-induced HSP72 was confirmed by MTT assay in isolated cardiomyocytes using KNK437, a novel HSP inhibitor. In conclusion, an oral administration of ebselen 24 h before I/R provided excellent cardioprotective effects, at least in part through HSP72 induction and GSH preservation.

Differential effects of repetitive activity on sarcoplasmic reticulum responses in rat muscles of different oxidative potential

We investigated the hypothesis that muscles of different oxidative potential would display differences in sarcoplasmic reticulum (SR) Ca2+ handling responses to repetitive contractile activity and recovery. Repetitive activity was induced in two muscles of high oxidative potential, namely, soleus (SOL) and red gastrocnemius (RG), and in white gastrocnemius (WG), a muscle of low oxidative potential, by stimulation in adult male rats. Measurements of SR properties, performed in crude homogenates, were made on control and stimulated muscles at the start of recovery (R0) and at 25 min of recovery (R25). Maximal Ca2+-ATPase activity (Vmax, micromol x g protein(-1) x min(-1)) at R0 was lower in stimulated SOL (105 +/- 9 vs. 135 +/- 7) and RG (269 +/- 22 vs. 317 +/- 26) and higher (P < 0.05) in WG (795 +/- 32 vs. 708 +/- 34). At R25, Vmax remained lower (P < 0.05) in SOL and RG but recovered in WG. Ca2+ uptake, measured at 2,000 nM, was depressed (P < 0.05) in SOL and RG by 34 and 13%, respectively, in stimulated muscles at R0 and remained depressed (P < 0.05) at R25. In contrast, Ca2+ uptake was elevated (P < 0.05) in stimulated WG at R0 by 9% and remained elevated (P < 0.05) at R25. Ca2+ release, unaltered in SOL and RG at both R0 and R25, was increased (P < 0.05) in stimulated WG at both R0 and R25. We conclude that SR Ca2+-handling responses to repetitive contractile activity and recovery are related to the oxidative potential of muscle.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Metabolic and sarcoplasmic reticulum Ca2+cycling responses in human muscle 4 days following prolonged exercise

This study investigated the effects of prolonged exercise on muscle sarcoplasmic reticulum (SR) Ca2+cycling properties and the metabolic responses with and without a session of exercise designed to reduce muscle glycogen reserves while on a normal carbohydrate (CHO) diet. Eight untrained males (VO2peak = 3.81 ± 0.12 L/min, mean ± SE) performed a standardized cycle-to-fatigue at 55% VO2peakwhile on a normal CHO diet (Norm CHO) and 4 days following prolonged exercise while on a normal CHO diet (Ex+Norm CHO). Compared to rest, exercise in Norm CHO to fatigue resulted in significant reductions (p < 0.05) in Ca2+uptake (3.17 ± 0.21 vs. 2.47 ± 0.12 µmol·(g protein)–1·min–1), maximal Ca2+ATPase activity (Vmax, 152 ± 12 vs. 119 ± 9 µmol·(g protein)–1·min–1) and both phase 1 (15.1 ± 0.98 vs. 13.1 ± 0.28 µmol·(g protein)–1·min–1) and phase 2 (6.56 ± 0.33 vs. 4.91 ± 0.28 µmol·(g protein)–1·min–1) Ca2+release in vastus lateralis muscle. No differences were observed between Norm CHO and Ex-Norm CHO in the response of these properties to exercise. Compared with Norm CHO, Ex+Norm CHO resulted in higher (p < 0.05) resting Ca2+uptake (3.17 ± 0.21 vs. 3.49 ± 0.24 µmol·(g protein)·min–1and higher ionophore ratio, defined as the ratio of Vmaxmeasured with and without the Ca2+-ionophore A23187, (2.3 ± 0.3 vs. 4.4 ± 0.3 µmol·(g protein)·min–1) at fatigue. No differences were observed between conditions in the concentration of muscle glycogen, the high-energy phosphates (ATP and PCr), or metabolites (Pi, Cr, and lactate). Ex+Norm CHO also failed to modify the exercise-induced changes in CHO and fat oxidation. We conclude that prolonged exercise to fatigue performed 4 days following glycogen-depleting exercise while on a normal CHO diet elevates resting Ca2+uptake and prevents increases in SR membrane permeability to Ca2+as measured by the ionophore ratio. Key words: Ca2+cycling, glycogen depletion, contractile activity, recovery.

Muscle sarcoplasmic reticulum Ca2+ cycling adaptations during 16 h of heavy intermittent cycle exercise

The repetition-dependent effects of a repetitive heavy exercise protocol previously shown to alter muscle mechanic behavior (Green HJ, Duhamel TA, Ferth S, Holloway GP, Thomas MM, Tupling AR, Rich SM, and Yau JE. J Appl Physiol 97: 2166-2175, 2004) on muscle sarcoplasmic reticulum (SR) Ca2+-transport properties, measured in vitro, were examined in 12 untrained volunteers [peak aerobic power (VO2(peak)) = 44.3 +/- 0.66 ml x kg(-1) x min(-1)]. The protocol involved 6 min of cycle exercise performed at approximately 91% VO2(peak) once per hour for 16 h. Tissue samples were obtained from the vastus lateralis before (B) and after (A) exercise at repetitions 1 (R1), 2 (R2), 9 (R9), and 16 (R16). Reductions (P < 0.05) in maximal Ca2+-ATPase activity (Vmax) of 26 and 12% with exercise were only observed at R1 and R16, respectively. Vmax remained depressed (P < 0.05) at R2 (B) but not at R9 (B) and R16 (B). No changes were observed in two other kinetic properties of the enzyme, namely the Hill coefficient (defined as the slope of the relationship between Ca2+-ATPase activity and free Ca2+ concentration) and the Ca50 (defined as the free Ca2+ concentration needed to elicit 50% Vmax). Changes in Ca2+ uptake (measured at 2,000 nM) with exercise and recovery generally paralleled Vmax. The apparent coupling ratio, defined as the ratio between Ca2+ uptake and Vmax, was unaffected by the intermittent protocol. Reductions (P < 0.05) in phase 1 Ca2+ release (32%) were only observed at R1. No differences were observed between B and A for R2, R9, and R16 or between B and B for R1, R2, R9, and R16. The changes in phase 2 Ca2+ release were as observed for phase 1 Ca2+ release. It is concluded that the SR Ca2+-handling properties, in general, display rapid adaptations to repetitive exercise.

Effects of progressive exercise and hypoxia on human muscle sarcoplasmic reticulum function

This study examined the effects of progressive exercise to fatigue in normoxia (N) on muscle sarcoplasmic reticulum (SR) Ca2+cycling and whether alterations in SR Ca2+cycling are related to the blunted peak mechanical power output (POpeak) and peak oxygen consumption (V̇o2 peak) observed during progressive exercise in hypoxia (H). Nine untrained men (20.7 ± 0.42 yr) performed progressive cycle exercise to fatigue on two occasions, namely during N (inspired oxygen fraction = 0.21) and during H (inspired oxygen fraction = 0.14). Tissue extracted from the vastus lateralis before exercise and at power output corresponding to 50 and 70% of V̇o2 peak(as determined during N) and at fatigue was used to investigate changes in homogenate SR Ca2+-cycling properties. Exercise in H compared with N resulted in a 19 and 21% lower ( P < 0.05) POpeakand V̇o2 peak, respectively. During progressive exercise in N, Ca2+-ATPase kinetics, as determined by maximal activity, the Hill coefficient, and the Ca2+concentration at one-half maximal activity were not altered. However, reductions with exercise in N were noted in Ca2+uptake (before exercise = 357 ± 29 μmol·min−1·g protein−1; at fatigue = 306 ± 26 μmol·min−1·g protein−1; P < 0.05) when measured at free Ca2+concentration of 2 μM and in phase 2 Ca2+release (before exercise = 716 ± 33 μmol·min−1·g protein−1; at fatigue = 500 ± 53 μmol·min−1·g protein−1; P < 0.05) when measured in vitro in whole muscle homogenates. No differences were noted between N and H conditions at comparable power output or at fatigue. It is concluded that, although structural changes in SR Ca2+-cycling proteins may explain fatigue during progressive exercise in N, they cannot explain the lower POpeakand V̇o2 peakobserved during H.

Effects of prolonged exercise and recovery on sarcoplasmic reticulum Ca2+ cycling properties in rat muscle homogenates

AIM

To examine the effects of exercise and exercise plus active and passive recovery on sarcoplasmic reticulum (SR) Ca2+-handling properties.

METHODS

Crude muscle homogenates were prepared from adult rat gastrocnemius muscle from two experiments. In one experiment, the muscle was extracted immediately after prolonged treadmill running (RUN), after a 45 min period of reduced exercise intensity (RUN+) following RUN and compared with controls (CON). In the second experiment, muscle was extracted during passive recovery following the same run protocol at 10 min (REC10), 25 min (REC25) and 45 min (REC45) and compared with CON.

RESULTS

Sarcoplasmic reticulum Ca2+-uptake was 31% higher (P < 0.05) in RUN+ compared with CON and RUN. Higher values (P < 0.05) were also found in REC25 (48%) and REC45 (50%) compared with CON. Maximal Ca2+-ATPase was increased by 23% (P < 0.05) in RUN+ compared with CON and RUN and by 65-68% (P < 0.05) in REC25 and REC45 compared with CON. A higher (P < 0.05) Hill coefficient for Ca2+-ATPase activity was observed in RUN+ (2.3 +/- 0.2) compared with CON (1.7 +/- 0.2) or RUN (1.6 +/- 0.2), but not for any REC conditions. In addition, the coupling ratio (Ca2+-uptake/Ca2+-ATPase activity) was higher (P < 0.05) in RUN+ (2.2 +/- 0.10) compared with CON (1.9 +/- 0.05) and RUN (1.9 +/- 0.08).

CONCLUSIONS

It is concluded that in crude homogenates, SR Ca2+-uptake and Ca2+-ATPase activity are elevated in recovery following prolonged running and that the elevation in these properties is more pronounced during passive compared with active recovery.

Adaptations in human muscle sarcoplasmic reticulum to prolonged submaximal training

In this study, we employed single-leg submaximal cycle training, conducted over a 10-wk period, to investigate adaptations in sarcoplasmic reticulum (SR) Ca(2+)-regulatory proteins and processes of the vastus lateralis. During the final weeks, the untrained volunteers (age 21.4 +/- 0.3 yr; means +/- SE, n = 10) were exercising 5 times/wk and for 60 min/session. Analyses were performed on tissue extracted by needle biopsy approximately 4 days after the last training session. Compared with the control leg, the trained leg displayed a 19% reduction (P < 0.05) in homogenate maximal Ca(2+)-ATPase activity (192 +/- 11 vs. 156 +/- 18 micromol. g protein(-1). min(-1)), a 4.3% increase (P < 0.05) in pCa(50), defined as the Ca(2+) concentration at half-maximal activity (6.01 +/- 0.05 vs. 6.26 +/- 0.07), and no change in the Hill coefficient (1.75 +/- 0.15 vs. 1.76 +/- 0.21). Western blot analysis using monoclonal antibodies (7E6 and A52) revealed a 13% lower (P < 0.05) sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) 1 in trained vs. control in the absence of differences in SERCA2a. Training also resulted in an 18% lower (P < 0.05) SR Ca(2+) uptake and a 26% lower (P < 0.05) Ca(2+) release. It is concluded that a downregulation in SR Ca(2+) cycling in vastus lateralis occurs with aerobic-based training, which at least in the case of Ca(2+) uptake can be explained by reduction in Ca(2+)-ATPase activity and SERCA1 protein levels.

Mechanisms underlying increases in SR Ca2+-ATPase activity after exercise in rat skeletal muscle

Prolonged exercise followed by a brief period of reduced activity has been shown to result in an overshoot in maximal sarcoplasmic reticulum (SR) Ca(2+)-ATPase activity [maximal velocity (V(max))] in rat locomoter muscles (Ferrington DA, Reijneveld JC, Bär PR, and Bigelow DJ. Biochim Biophys Acta 1279: 203-213, 1996). To investigate the functional significance and underlying mechanisms for the increase in V(max), we analyzed Ca(2+)-ATPase activity and Ca(2+) uptake in SR vesicles from the fast rat gastrocnemius muscles after prolonged running (RUN) and after prolonged running plus 45 min of low-intensity activity (RUN+) or no activity (REC45) and compared them with controls (Con). Although no differences were observed between RUN and Con, both V(max) and Ca(2+) uptake were higher (P < 0.05) by 43 and 63%, respectively, in RUN+ and by 35 and 34%, respectively, in REC45. The increase in V(max) was accompanied by increases (P < 0.05) in the phosphorylated enzyme intermediate measured by [gamma-(32)P]ATP. No differences between groups for each condition were found for the fluorescent probes FITC and (N-cyclohexyl-N(1)-dimethylamino-alpha-naphthyl)carbodiimide, competitive inhibitors of the nucleotide-binding and Ca(2+)-binding sites on the enzyme, respectively. Similarly, no differences for the Ca(2+)-ATPase were observed between groups in nitrotyrosine and phosphoserine residues, a measure of nitrosylation and phosphorylation states, respectively. Western blots indicated no changes in relative isoform content of sarcoendoplasmic reticulum (SERCA)1 and SERCA2a. It is concluded that the increase in V(max) of the Ca(2+)-ATPase observed in recovery is not the result of changes in enzyme nitroslyation or phosphorylation, changes in ATP and Ca(2+)-binding affinity, or changes in protein content of the Ca(2+)-ATPase.

Thermal instability of rat muscle sarcoplasmic reticulum Ca(2+)-ATPase function

To examine the thermal instability and the role of sulfhydryl (SH) oxidation on sarcoplasmic reticulum (SR) Ca(2+)-ATPase function, crude homogenates were prepared from the white portion of the gastrocnemius (WG) adult rat muscles (n = 9) and incubated in vitro for < or =60 min either at a normal resting body temperature (37 degrees C) or at a temperature indicative of exercise-induced hyperthermia (41 degrees C) with DTT and without DTT (CON). In general, treatment with DTT resulted in higher Ca(2+)-ATPase and Ca(2+) uptake values (nmol. mg protein(-1). min(-1), P < 0.05), an effect that was not specific to time of incubation. Incubations at 41 degrees C resulted in lower (P < 0.05) Ca(2+) uptake rates (156 +/- 18 and 35.9 +/- 3.3) compared with 37 degrees C (570 +/- 54 and 364 +/- 26) at 30 and 60 min, respectively. At 37 degrees C, ryanodine (300 microM), which was used to block Ca(2+) release from the calcium release channel, prevented the time-dependent decrease in Ca(2+) uptake. A general inactivation (P < 0.05) of maximal Ca(2+)-ATPase activity (V(max)) in CON was observed with incubation time (0 > 30 > 60 min), with the effect being more pronounced (P < 0.05) at 41 degrees C compared with 37 degrees C. The Hill slope, a measure of co-operativity, and the pCa(50), the cytosolic Ca(2+) concentration required for half-maximal activation of Ca(2+)-ATPase activity, decreased (P < 0.05) at 41 degrees C only. Treatment with DTT attenuated the alterations in enzyme kinetics. The increase in V(max) with the Ca(2+) ionophore A-23187 was less pronounced at 41 degrees C compared with 37 degrees C. It is concluded that exposure of homogenates to a temperature typically experienced in exercise results in a reduction in the coupling ratio, which is mediated primarily by lower Ca(2+) uptake and occurs as a result of increases in membrane permeability to Ca(2+). Moreover, the decreases in Ca(2+)-ATPase kinetics in WG with sustained heat stress result from SH oxidation.

Role of the self-association of beta subunits in the oligomeric structure of Na+/K+-ATPase

The two subunits of Na(+)/K(+)-ATPase that are essential for function are alpha and beta. Previous cross-linking studies on the oligomeric structure of the membrane-bound enzyme identified alpha,beta and alpha,alpha associations, but only the former and not the latter could be detected after solubilization. To study the possibility of direct beta,beta association, the purified membrane enzyme and a trypsin-digested enzyme that occludes cations and contains an essentially intact beta and fragments of alpha were subjected to oxidative cross-linking in the presence of Cu(2+)-phenanthroline. Resolution of products on polyacrylamide gels, N-terminal analysis and reactivity with anti-beta antibody showed that, in addition to previously identified products (e.g. alpha,alpha and alpha,beta dimers), a beta,beta dimer, most likely linked through intramembrane Cys(44) residues of two chains, is also formed. This dimer was also noted when digitonin-solubilized intact enzyme, and the trypsin-digested enzyme solubilized with digitonin or polyoxyethylene 10-laurylether were subjected to cross-linking, indicating that the detected beta,beta association was not due to random collisions. In the digested enzyme, K(+) but not Na(+) enhanced beta,beta dimer formation. The alternative cross-linking of beta-Cys(44) to a Cys residue of a transmembrane alpha-helix was antagonized specifically by K(+) or Na(+). The findings (i) indicate the role of beta,beta association in maintaining the minimum oligomeric structure of (alpha,beta)(2), (ii) provide further support for conformation-dependent flexibilities of the spatial relations of the transmembrane helices of alpha and beta and (iii) suggest the possibility of significant differences between the quaternary structures of the P-type ATPases that do and do not contain a beta subunit.

Silver ions induce Ca2+ release from the SR in vitro by acting on the Ca2+ release channel and the Ca2+ pump

Silver nitrate (AgNO3) is a sulfhydryl oxidizing agent that induces a biphasic Ca2+ release from isolated sarcoplasmic reticulum (SR) vesicles by presumably oxidizing critical sulfhydryl groups in the Ca2+ release channel (CRC), causing the channel to open. To further examine the effects of AgNO3 on the CRC and the Ca2+-ATPase, Ca2+ release was measured in muscle homogenates prepared from rat hindlimb muscle using indo 1. Cyclopiazonic acid (CPA) and ruthenium red (RR) were used to inhibit the Ca2+-ATPase and block the CRC, respectively, before inducing Ca2+ release with both AgNO3 and 4-chloro-m-cresol (4-CMC), a releasing agent specific for the CRC. With AgNO3 and CPA, the early rapid rate of release (phase 1) was increased (P < 0.05) by 42% (314 +/- 5 vs. 446 +/- 39 micromol x g protein(-1) x min(-1)), whereas the slower, more prolonged rate of release (phase 2) was decreased (P < 0.05) by 72% (267 +/- 39 vs. 74 +/- 7.7 micromol x g protein(-1) x min(-1)). RR, in combination with AgNO3, had no effect on phase 1 (P > 0.05) (314 +/- 51 vs. 334 +/- 43 micromol x g protein(-1) x min(-1)) and decreased phase 2 (P < 0.05) by 65% (245 +/- 34 vs. 105 +/- 8.2 micromol x g protein(-1) x min(-1)). With 4-CMC, CPA had no effect (P > 0.05) on either phase 1 or 2. With addition of RR, phase 1 was reduced (P < 0.05) by 59% (2,468 +/- 279 vs. 1,004 +/- 87 micromol x g protein(-1) x min(-1)), and RR completely blocked phase 2. Both AgNO3 and 4-CMC fully inhibited Ca2+-ATPase activity measured in homogenates. These findings indicate that AgNO3, but not 4-CMC, induces Ca2+ release by acting on both the CRC and the Ca2+-ATPase.

SERCA activity is required for timely progression through G1/S

Changes in intracellular Ca2+ correlate with specific events in the cell cycle. Here we investigated the role of Ca2+ in the G1 phase. HEK 293 cells were arrested in mitosis and subjected to short-term treatments that alter Ca2+ homeostasis prior to their release into G1. Treatment with thapsigargin (TG), an irreversible inhibitor of the sarco-endoplasmic reticulum Ca2+ ATPase (SERCA) lengthened the G1 phase. Moreover, TG treatment also resulted in a dramatic alteration in cellular morphology and attachment and in the reduction of MAPK activity and lower levels of cyclin D1 and cyclin E proteins. Treatments with reagents that transiently increase or decrease cytosolic Ca2+ or that temporarily inactivate SERCA did not alter any of the above parameters. Cells expressing a TG-resistant form of SERCA progressed normally through the G1/S transition after TG treatment. These results suggest that long-term SERCA inactivation affects cell cycle-dependent events and compromises progression through G1/S.

SERCA1 truncated proteins unable to pump calcium reduce the endoplasmic reticulum calcium concentration and induce apoptosis

By pumping calcium from the cytosol to the ER, sarco/endoplasmic reticulum calcium ATPases (SERCAs) play a major role in the control of calcium signaling. We describe two SERCA1 splice variants (S1Ts) characterized by exon 4 and/or exon 11 splicing, encoding COOH terminally truncated proteins, having only one of the seven calcium-binding residues, and thus unable to pump calcium. As shown by semiquantitative RT-PCR, S1T transcripts are differentially expressed in several adult and fetal human tissues, but not in skeletal muscle and heart. S1T proteins expression was detected by Western blot in nontransfected cell lines. In transiently transfected cells, S1T homodimers were revealed by Western blot using mildly denaturing conditions. S1T proteins were shown, by confocal scanning microscopy, to colocalize with endogenous SERCA2b into the ER membrane. Using ER-targeted aequorin (erAEQ), we have found that S1T proteins reduce ER calcium and reverse elevation of ER calcium loading induced by SERCA1 and SERCA2b. Our results also show that SERCA1 variants increase ER calcium leakage and are consistent with the hypothesis of a cation channel formed by S1T homodimers. Finally, when overexpressed in liver-derived cells, S1T proteins significantly induce apoptosis. These data reveal a further mechanism modulating Ca(2+) accumulation into the ER of nonmuscle cells and highlight the relevance of S1T proteins to the control of apoptosis.

Effects of a 21-day expedition to 6,194 m on human skeletal muscle SR Ca2+-ATPase

We investigated the effects of a 21-day expedition to the summit of Mount Denali, Alaska (6,194 m) on selected Ca2+ sequestration properties of sarcoplasmic reticulum (SR) calcium pump in vastus lateralis muscle. Muscle samples were obtained by biopsy from 5 male climbers (peak oxygen consumption, VO2peak = 52.3 +/- 2.1 mL.kg(-1).min(-1)) approximately 7 days prior to (PRE) and 4 days following (POST) the expedition. A comparison of PRE versus POST measures of maximal Ca2+-ATPase activities (117 +/- 8.5 vs. 97.6 +/- 5.6 nmol.mg protein(-1).min(-1)) and Ca2+-uptake (204 +/- 15 vs. 161 +/- 11 nmol.mg protein(-1).min(-1)) measured in crude homogenates obtained from pre-exercised muscle, indicated only an effect (p < 0.05) of the expedition on Ca2+-uptake. The reduction in Ca2+-ATPase activity, representing 16.6%, was not significant (p = 0.089). The sarco endoplasmic reticulum calcium (SERCA)-ATPase isoforms, measured using Western blotting techniques, revealed a small reduction (p < 0.05) in SERCA 1 (-4.6 +/- 1.9%), but not in SERCA 2a (+2.0 +/- 1.4%). Prior to the expedition, both Ca2+-ATPase activity and Ca2+-uptake were reduced (p < 0.05) by approximately 34 and 18%, respectively, following 40 min of a two-step continuous cycling task (20 min at 59% VO2peak and 20 min at 74% VO2peak). The exercise-induced reduction in Ca2+-ATPase activity was independent of fiber type. Only in the case of Ca2+-uptake was a lower exercise response (p < 0.05) observed following the expedition, an effect that was due to the lower resting value. It is concluded that acclimatization as experienced during a mountaineering expedition induces changes in the properties of the SR Ca2+-pump, and particularly to Ca2+-sequestering function.

Low temperature molecular adaptation of the skeletal muscle sarco(endo)plasmic reticulum Ca2+-ATPase 1 (SERCA 1) in the wood frog (Rana sylvatica)

We have compared the primary sequence and enzymatic properties of the sarcoplasmic reticulum Ca(2+)-ATPases from a cold-tolerant frog Rana sylvatica with those of a closely related cold-intolerant frog, Rana clamitans. Sarcoplasmic reticulum isolated from leg muscles of both species contains a major protein ( approximately 100 kDa) that reacts with a monoclonal antibody against sarco(endo)plasmic reticulum Ca(2+)-ATPase type 1 (SERCA1). The apparent molecular mass of R. sylvatica SERCA1 is 115 kDa, whereas that of R. clamitans is 105 kDa. However, the deduced amino acid sequences obtained from cDNAs do not indicate a difference in molecular weight, thus suggesting post-translational protein modification of R. sylvatica SERCA1. Comparison of the temperature dependence of both ATP hydrolysis and Ca(2+) transport indicates that R. sylvatica SERCA1 exhibits significantly lower activation energy below 20 degrees C and an approximately 2-fold greater Ca(2+)-ATPase activity near 0 degrees C. Furthermore, R. sylvatica SERCA1 exhibits simple Michaelis-Menten kinetics with ATP and Ca(2+) as opposed to the two-site ATP kinetics and positive cooperativity with Ca(2+) observed for R. clamitans and mammalian SERCA1s. Cooperativity has been linked to protein-protein interaction in SERCA1, and this property may be altered in R. sylvatica SERCA1. Primary sequence comparison shows that R. sylvatica SERCA1 exhibits seven unique amino acid substitutions, three of which are in the ATP binding domain. We also report for the first time the presence of alternative splicing in the frog, resulting in isoforms SERCA1a and SERCA1b. Thus, it appears that the low temperature muscle contractility of R. sylvatica can be explained partially by significant functional and structural differences in SERCA1.

Gene knockout studies of Ca2+-transporting ATPases

The biochemical functions of intracellular and plasma membrane Ca2+-transporting ATPases in the control of cytosolic and organellar Ca2+ levels are well established, but the physiological roles of specific isoforms are less well understood. There appear to be three different types of Ca2+ pumps in mammalian tissues: the sarco(endo)plasmic reticulum Ca2+-ATPases (SERCAs), which sequester Ca2+ within the endoplasmic or sarcoplasmic reticulum, the plasma membrane Ca2+-ATPases (PMCAs), which extrude Ca2+ from the cell, and the putative secretory pathway Ca2+-ATPase (SPCA), the function of which is poorly understood. This review describes the results of recent analyses of mouse models with null mutations in the genes encoding SERCA and PMCA isoforms and genetic studies of SERCA and SPCA dysfunction in both humans and model organisms. These studies are yielding important insights regarding the physiological functions of individual Ca2+-transporting ATPases in vivo.

Hepatitis B virus-related insertional mutagenesis implicates SERCA1 gene in the control of apoptosis

We have used the Hepatitis B Virus DNA genome as a probe to identify genes clonally mutated in vivo, in human liver cancers. In a tumor, HBV-DNA was found to be integrated into the gene encoding Sarco/Endoplasmic Reticulum Calcium ATPase (SERCA), which pumps calcium, an important intracellular messenger for cell viability and growth, from the cytosol to the endoplasmic reticulum. The HBV X gene promoter cis-activates chimeric HBV X/SERCA1 transcripts, with splicing of SERCA1 exon 11, encoding C-terminally truncated SERCA1 proteins. Two chimeric HBV X/SERCA1 proteins accumulate in the tumor and form dimers. In vitro analyses have demonstrated that these proteins localize to the ER, determine its calcium depletion and induce cell death. We have also shown that these biological effects are related to expression of the SERCA, rather than of the viral moiety. This report involves for the first time the expression of mutated SERCA proteins in vivo in a tumor cell proliferation and in vitro in the control of cell viability. Oncogene (2000).

Structure and dynamics of membrane proteins as studied by infrared spectroscopy

Infrared (IR) spectroscopy is a useful technique in the study of protein conformation and dynamics. The possibilities of the technique become apparent specially when applied to large proteins in turbid suspensions, as is often the case with membrane proteins. The present review describes the applications of IR spectroscopy to the study of membrane proteins, with an emphasis on recent work and on spectra recorded in the transmission mode, rather than using reflectance techniques. Data treatment procedures are discussed, including band analysis and difference spectroscopy methods. A technique for the analysis of protein secondary and tertiary structures that combines band analysis by curve-fitting of original spectra with protein thermal denaturation is described in detail. The assignment of IR protein bands in H2O and in D2O, one of the more difficult points in protein IR spectroscopy, is also reviewed, including some cases of unclear assignments such as loops, beta-hairpins, or 3(10)-helices. The review includes monographic studies of some membrane proteins whose structure and function have been analysed in detail by IR spectroscopy. Special emphasis has been made on the role of subunit III in cytochrome c oxidase structure, and the proton pathways across this molecule, on the topology and functional cycle of sarcoplasmic reticulum Ca(2+)-ATPase, and on the role of lipids in determining the structure of the nicotinic acetylcholine receptor. In addition, shorter descriptions of retinal proteins and references to other membrane proteins that have been studied less extensively are also included.

Influence of various nucleotides on the in situ crystallization of Ca2+-ATPase

A reproducible in situ crystallization of the Ca2+-ATPase in isolated sarcoplasmic reticulum (SR) membranes was studied. The addition of various nucleotides to the washing buffer allowed the formation of tubular crystals, which is induced by vanadate. SR membranes washed with nucleotide-free buffer could not form tubular crystals upon subsequent incubation with vanadate.

Immunohistochemical localization of Ca2+/Calmodulin-dependent protein kinase IV in outer hair cells

A smooth membrane system consisting of subsurface cisternae (SSC) underlies the lateral plasmalemma of auditory outer hair cells (OHCs). The SSC contain Ca-ATPase and are regarded as an intracellular Ca2+ reservoir like the sarcoplasmic reticulum of myocytes. Recently, it has been demonstrated that Ca-ATPase activity in sarcoplasmic reticulum is regulated by Ca2+/calmodulin-dependent protein kinases (CaM kinases). Here we investigated the presence of CaM kinases in OHCs and their possible association with the SSC. Inner ears collected from adult gerbils and from neonates at 2-day intervals between 0 and 20 days after birth were immunostained with antibodies specific for different CaM kinases. A polyclonal antiserum against CaM kinase IV yielded a strong immunostaining reaction along the lateral wall of OHCs. The staining appeared after the tenth postnatal day and continued into adulthood. No other site in the inner ear, including cochlear inner hair cells and vestibular hair cells, was reactive. The kinase's apparent association with the SSC strongly supports its involvement in intracellular Ca2+ homeostasis and suggests a role in regulating the OHCs' slow motile responses.

Amino acid sequence of a Ca(2+)-transporting ATPase from the sarcoplasmic reticulum of the cross-striated part of the adductor muscle of the deep sea scallop: comparison to serca enzymes of other animals

The RT PCR approach was used to obtain the nucleotide sequence of the mRNA of a sarco/endoplasmic reticulum calcium transporting ATPase (SERCA) from the cross-striated (phasic) part of the adductor muscle of the deep sea scallop. Initially, degenerate primers based on consensus sequences among SERCAs and tryptic fragments of the scallop Ca-ATPase were used. The sequence was then extended using homologous primers and the 5' and 3' ends of the transcript determined by 5' and 3' RACE. The mRNA codes for a polypeptide chain 994 amino acid residues long (coded for by 2982 nucleotides) and has a 195 bp 5' untranslated region, with a 697 bp 3' untranslated region. The scallop enzyme shows strongest amino acid similarity to the SERCA enzyme of Loligo, followed by those of Drosophila and Artemia. It resembles the vertebrate SERCA3 in that it does not possess the phospholamban binding motif and so is unlikely to be regulated by protein kinase A mediated signals.

Topology of sarcoplasmic reticulum Ca2+-ATPase: an infrared study of thermal denaturation and limited proteolysis

Sarcoplasmic reticulum Ca2+-ATPase structure and organization in the membrane has been studied by infrared spectroscopy by decomposition of the amide I band. Besides the component bands assignable to secondary structure elements such as alpha-helix, beta-sheet, etc...., two unusual bands, one at 1,645 cm(-1) in H2O buffer and the other at 1,625 cm(-1) in D2O buffer are present. By perturbing the protein using temperature and limited proteolysis, the band at 1,645 cm(-1) is tentatively assigned to alpha-helical segments located in the cytoplasmic domain and coupled to beta-sheet structure, whereas the band at 1,625 cm(-1) arises probably from monomer-monomer contacts in the native oligomeric protein. The secondary structure obtained is 33% alpha-helical segments in the transmembrane plus stalk domain; 20% alpha-helix and 22% beta-sheet in the cytoplasmic domain plus 19% turns and 6% unordered structure. Thermal unfolding of Ca2+-ATPase is a complex process that cannot be described as a two-state denaturation. The results obtained are compatible with the idea that the protein is an oligomer at room temperature. The loss of the 1,625 cm(-1) band upon heating would be consistent with a disruption of the oligomers in a process that later gives rise to aggregates (appearance of the 1,618 cm(-1) band). This picture would also be compatible with early results suggesting that processes governing Ca2+ accumulation and ATPase activity are uncoupled at temperatures above 37 degrees C, so that while ATPase activity proceeds at high rates, Ca2+ accumulation is inhibited.

Ca2+ release from the phosphorylated and the unphosphorylated sarcoplasmic reticulum Ca2+ ATPase results in parallel structural changes. An infrared spectroscopic study

Structural changes of the sarcoplasmic reticulum Ca2+-ATPase occurring in the reaction step involving phosphoenzyme conversion and Ca2+ release (Ca2E1-P --> E2-P) were followed using time-resolved infrared spectroscopy in H2O and 2H2O. The difference spectra measured between 1800 and 1500 cm-1 were almost identical to those of Ca2+ release from the unphosphorylated ATPase (Ca2E1 --> E), implying that parallel structural changes occur in both steps. This suggests that characteristic structural features of the high affinity Ca2+ binding sites of Ca2E1 are still present in the ADP-sensitive phosphoenzyme Ca2E1-P. In both Ca2+ release steps at least two carboxyl groups become protonated, each of them experiencing the same strength of hydrogen bonding irrespective of whether or not the Ca2+ free ATPase is phosphorylated. This suggests that the same amino acid residues are involved and that they are most likely those that participate in high affinity Ca2+ binding and H+ countertransport. We propose that during Ca2+ release from the phosphoenzyme protons from the lumenal side have access to these residues. Our results are consistent with only one pair of Ca2+ binding sites on the ATPase that serves both Ca2+ translocation and H+ countertransport.

Altered Ca2+ responses in muscles with combined mitochondrial and cytosolic creatine kinase deficiencies

We have blocked creatine kinase (CK)-mediated phosphocreatine (PCr) -->/<-- ATP transphosphorylation in skeletal muscle by combining targeted mutations in the genes encoding mitochondrial and cytosolic CK in mice. Contrary to expectation, the PCr level was only marginally affected, but the compound was rendered metabolically inert. Mutant muscles in vivo showed significantly impaired tetanic force output, increased relaxation times, altered mitochondrial volume and location, and conspicuous tubular aggregates of sarcoplasmic reticulum membranes, as seen in myopathies with electrolyte disturbances. In depolarized myotubes cultured in vitro, CK absence influenced both the release and sequestration of Ca2+. Our data point to a direct link between the CK-PCr system and Ca2+-flux regulation during the excitation and relaxation phases of muscle contraction.