Kinetics of calcium uptake by isolated sarcoplasmic reticulum vesicles using flash photolysis of caged adenosine 5'-triphosphate

Optogenetic toolkit for precise control of calcium signaling

Calcium acts as a second messenger to regulate a myriad of cell functions, ranging from short-term muscle contraction and cell motility to long-term changes in gene expression and metabolism. To study the impact of Ca2+-modulated 'ON' and 'OFF' reactions in mammalian cells, pharmacological tools and 'caged' compounds are commonly used under various experimental conditions. The use of these reagents for precise control of Ca2+ signals, nonetheless, is impeded by lack of reversibility and specificity. The recently developed optogenetic tools, particularly those built upon engineered Ca2+ release-activated Ca2+ (CRAC) channels, provide exciting opportunities to remotely and non-invasively modulate Ca2+ signaling due to their superior spatiotemporal resolution and rapid reversibility. In this review, we briefly summarize the latest advances in the development of optogenetic tools (collectively termed as 'genetically encoded Ca2+ actuators', or GECAs) that are tailored for the interrogation of Ca2+ signaling, as well as their applications in remote neuromodulation and optogenetic immunomodulation. Our goal is to provide a general guide to choosing appropriate GECAs for optical control of Ca2+ signaling in cellulo, and in parallel, to stimulate further thoughts on evolving non-opsin-based optogenetics into a fully fledged technology for the study of Ca2+-dependent activities in vivo.

Binding and Transport of Metal Ions at the Dimer Interface of the Escherichia coli Metal Transporter YiiP

YiiP is a representative member of the cation diffusion facilitator (CDF) family, a class of ubiquitous metal transporters that play an essential role in metal homeostasis. Recently, a pair of Zn2+/Cd2+-selective binding sites has been localized to two highly conserved aspartyl residues (Asp157), each in a 2-fold-symmetry-related transmembrane segment 5 (TM5) of a YiiP homodimer. Here we report the functional and structural interactions between Asp157 and yet another highly conserved Asp49 in the TM2. Calorimetric binding analysis indicated that Asp49 and Asp157 contribute to a common Cd2+ binding site in each subunit. Copper phenanthroline oxidation of YiiP(D49C), YiiP(D157C), and YiiP(D49C/D157C) yielded inter- and intra-subunit cross-links among Cys49 and Cys157, consistent with the spatial proximity of two (Asp49-Asp157) sites at the dimer interface. Hg2+ binding to YiiP(D49C) or YiiP(D49C/D157C) also yielded a Cys49-Hg2+-Cys49 biscysteinate complex across the dimer interface, further establishing the interfacial location of a (Asp49-Asp157)2 bimetal binding center. Two bound Cd2+ ions were found transported cooperatively with a sigmoidal dependence on the Cd2+ concentration (n = 1.4). The binding affinity, transport cooperativity, and rate were modestly reduced by either a D49C or D157C mutation, but greatly diminished when all the bidentate aspartate O-ligands in (Asp49-Asp157)2 were replaced by the monodentate cysteine S-ligands. The functional significance of these findings is discussed based on the unique coordination chemistry of aspartyl residues and a model for the translocation pathway of metal ions at the YiiP dimer interface.

Photoreaction of caged ATP studied by the time-resolved transient grating method

The photoreaction of caged ATP, P3-[1-(2-nitrophenyl)ethyl]adenosine-5'-triphosphate, has been investigated using the time-resolved transient grating (TG) method. We found that a feature of the TG signal time profile depends sensitively on the grating wavenumber (q) after the photoexcitation of caged ATP. This q-dependent feature of the TG signal was interpreted based on a model where the ATP release rate is comparable to the molecular diffusion process. We found that the TG signals at various q can be consistently analyzed based on this model and the ATP release rate determined. The enthalpy and volume changes of the reaction have been determined by quantitative measurement of the grating and photoacoustic signals.

Effect of Ca2+ binding on the profile structure of the sarcoplasmic reticulum membrane using time-resolved x-ray diffraction

A number of studies have indicated that Ca(2+)-ATPase, the integral membrane protein of the sarcoplasmic reticulum (SR) membrane, undergoes some structural change upon Ca2+ binding to its high affinity binding sites (i.e., upon conversion of the E1 to the CaxE1 form of the enzyme). We have used x-ray diffraction to study the changes in the electron density profile of the SR membrane upon high-affinity Ca2+ binding to the enzyme in the absence of enzyme phosphorylation. The photolabile Ca2+ chelator DM-nitrophen was used to rapidly release Ca2+ into the extravesicular spaces throughout an oriented SR membrane multilayer and thereby synchronously in the vicinity of the high affinity binding sites of each enzyme molecule in the multilayer. A critical control was developed to exclude possible artifacts arising from heating and non-Ca2+ photolysis products in the membrane multilayer specimens upon photolysis of the DM-nitrophen. Upon photolysis, changes in the membrane electron density profile arising from high-affinity Ca2+ binding to the enzyme are found to be localized to three different regions within the profile. These changes can be attributed to the added electron density of the Ca2+ bound at three discrete sites centered at 5, approximately 30, and approximately 67 A in the membrane profile, but they also require decreased electron density within the cylindrically averaged profile structure of the Ca(2+)-ATPase immediately adjacent (< 15 A) to these sites. The locations of these three Ca2+ binding sites in the SR membrane profile span most of the membrane profile in the absence of enzyme phosphorylation,in agreement with the locations of lanthanide (Tb3+ and La3+) binding sites in the membrane profile determined independently by using resonance x-ray diffraction.

Simultaneous internalization and binding of calcium during the initial phase of calcium uptake by the sarcoplasmic reticulum Ca pump

The kinetics of Ca2+ transport mediated by the sarcoplasmic reticulum (SR) Ca-ATPase were investigated by rapid kinetic techniques that either measure the disappearance of Ca2+ from the medium [stopped-flow photometry of Ca2+ indicators or rapid filtration (method 1)] or directly detect the changes in the accessibility of Ca2+ to the exterior of the membrane, i.e., occlusion of Ca2+ within the Ca pump and Ca2+ transport into the lumen of SR vesicles [EGTA quench (method 2)]. SR vesicles were preincubated in micromolar Ca2+ to form the E.2Cacyt intermediate of the Ca-ATPase, and then Ca2+ transport was initiated by addition of ATP. It was found that Ca2+ uptake measured by method 1 began with no lag phase, in spite of the prediction of kinetic models of the Ca-ATPase. Instead, the time course of Ca2+ uptake was found to have two components: a fast and a slow phase, similar to that obtained using method 2, although the rate constant of the fast phase determined by method 1 was considerably lower than that measured by method 2. The fast phase of Ca2+ uptake measured by method 1 was not influenced by either Ca2+ ionophore or detergent treatment, whereas the slow phase was diminished.(ABSTRACT TRUNCATED AT 250 WORDS)

A new method for monitoring the kinetics of calcium binding to the sarcoplasmic reticulum Ca(2+)-ATPase employing the flash-photolysis of caged-calcium

The kinetics of Ca2+ binding to the high-affinity sites of the sarcoplasmic reticulum (SR) Ca2(+)-ATPase were directly investigated by continuously monitoring the extravesicular calcium concentration via the metallochromic indicator Arsenazo III following the release of Ca2+ from a photolabile caged-calcium molecule, 1-(2-nitro-4,5-dimethoxyphenyl)-N,N,N',N'-tetrakis [(oxycarbony)methyl]-1,2-ethanediamine (DM-nitrophen), utilizing a pulsed Nd:YAG laser for photolysis. The nature of the binding kinetics is at least biphasic over the first 400 ms for vesicular dispersions of SR. The stoichiometry for calcium binding expressed as Ca:E1 approximately P has been calculated to be approximately 1.4:1 for the pure SR preparation under the reaction conditions employed.

Large-scale structural changes in the sarcoplasmic reticulum ATPase appear essential for calcium transport

Model refinement calculations utilizing the results from time-resolved x-ray diffraction studies indicate that specific, large-scale changes (i.e., structural changes over a large length scale or long range) occur throughout the cylindrically averaged profile structure of the sarcoplasmic reticulum ATPase upon its phosphorylation during calcium active transport. Several physical-chemical factors, all of which slow the kinetics of phosphoenzyme formation, induce specific, large-scale changes throughout the profile structure of the unphosphorylated enzyme that in general are opposite to those observed upon phosphorylation. These results suggest that such large-scale structural changes in the ATPase occurring upon its phosphorylation are required for its calcium transport function.

Effect of Mg2+ concentration on Ca2+ uptake kinetics and structure of the sarcoplasmic reticulum membrane

Direct measurements of phosphorylation of the Ca2+ ATPase of the sarcoplasmic reticulum (SR) have shown that the lifetime of the first phosphorylated intermediate in the Ca2+ transport cycle, E1 approximately P, increases with decreasing [Mg2+] (Dupont, Y. 1980. Eur. J. Biochem. 109:231-238). Previous x-ray diffraction work (Pascolini, D., and J.K. Blasie. 1988. Biophys. J. 54:669-678) under high [Mg2+] conditions (25 mM) indicated that changes in the profile structure of the SR membrane could be responsible for the low-temperature transient trapping of E1 approximately P that occurs at temperatures below 2-3 degrees C, the upper characteristic temperature th for lipid lateral phase separation in the membrane. We now present results of our study of the Ca2+ uptake kinetics and of the structure of the SR membrane at low [Mg2+] (less than or equal to 100 microM). Our results show a slowing in the kinetics of both phases of the Ca2+ uptake process and an increase in the duration of the plateau of the fast phase before the onset of the slow phase, indicating an increase in the lifetime (transient trapping) of E1 approximately P. Calcium uptake kinetics at low [Mg2+] and moderately low temperature (approximately 0 degree C) are similar to those observed at much lower temperatures (approximately -10 degrees C) at high [Mg2+]. The temperature-induced structural changes that we observed at low [Mg2+] are much more pronounced than those found to occur at higher [Mg2+]. Also, at the lower [Mg2+] the upper characteristic temperature th for lipid lateral phase separation was found to be higher, at approximately 8-10 degrees C. Our studies indicate that both temperature and [Mg2+] affect the structure and the functionality (as measured by changes in the kinetics of Ca2+ uptake) of the SR membrane. Membrane lipid phase behavior and changes in the Ca2+ ATPase profile structure seem to be related, and we have found that structural changes are responsible for the slowing of the kinetics of the fast phase of Ca2+ uptake, and could also mediate the effect that [Mg2+] has on E1 approximately P lifetime.

Flash photolysis of caged compounds: new tools for cellular physiology

Caged compounds are molecules or ions of physiological interest, e.g. ATP, IP3, cAMP, cGMP, GTP and Ca2+ rendered inactive by chemical modification. The modification introduces a photochemically labile bond, which on exposure to ultraviolet light cleaves rapidly, releasing the active compound. This article reviews some of the major advances and applications of the photorelease approach, and illustrates its potential in several areas of interest to cellular neuroscientists.

Moderate resolution profile structure of the sarcoplasmic reticulum membrane under low temperature conditions for the transient trapping of E1 approximately P

The calcium uptake reaction kinetics of isolated sarcoplasmic reticulum (SR) vesicles have previously been shown to be at least biphasic over a range of temperatures (26 to 10 degrees C) with a fast phase identified with the formation of E1 approximately P and calcium occlusion and a slow phase with Ca2+ translocation across the membrane and turnover of the Ca2+ ATPase ensemble. At "low" temperatures, namely 0 degrees C or lower, E1 approximately P formation is slowed and E1 approximately P is transiently trapped for at least several seconds, as indicated by the absence of the slow phase for 6 s or more. We now report that a reversible, temperature-induced structural transition occurs at about 2-3 degrees C for the isolated SR membrane. We have investigated the nature of this structural transition utilizing meridional and equatorial x-ray diffraction studies of the oriented SR membrane multilayers in the range of temperatures between 7.5 and -2 degrees C. The phase meridional (lamellar) diffraction has provided the profile structure for the SR membrane at the highest vs. lowest temperature at the same moderate resolution of 16-17 A while the equatorial diffraction has provided information on the average lipid chain packing in the SR membrane plane in the two cases. To identify the contribution of each membrane component in producing the differences between the profile structures at 7.5 and -2 degrees C, step-function models have been fitted to the moderate resolution electron density profiles. Lipid lateral phase separation may be responsible for inducing the structural change in the Ca2+ ATPase, thereby resulting in the slowing of E1 approximately P formation and the transient trapping of E1 approximately P at the "lower" temperatures.

Changes in the sarcoplasmic reticulum membrane profile induced by enzyme phosphorylation to E1 approximately P at 16 A resolution via time-resolved x-ray diffraction

Time-resolved x-ray diffraction studies of the isolated sarcoplasmic reticulum (SR) membrane have provided the difference electron density profile for the SR membrane for which the Ca2+ ATPase is transiently trapped exclusively in the first phosphorylated intermediate state, E1 approximately P, in absence of detectable enzyme turnover vs. that before ATP-initiated phosphorylation of the enzyme. These diffraction studies, which utilized the flash-photolysis of caged ATP, were performed at temperatures between 0 and -2 degrees C and with a time-resolution of 2-5 s. Analogous time-resolved x-ray diffraction studies of the SR membrane at 7-8 degrees C with a time resolution of 0.2-0.5 s have previously provided the difference electron density profile for the SR membrane for which the Ca2+ ATPase is only predominately in the first phosphorylated intermediate state under conditions of enzyme turnover vs. that before enzyme phosphorylation. The two difference profiles, compared at the same low resolution (approximately 40 A), are qualitatively similar but nevertheless contain some distinctly different features and have therefore been analyzed via a step-function model analysis. This analysis was based on the refined step-function models for the two different electron density profiles obtained independently from x-ray diffraction studies at higher resolution (16-17 A) of the SR membrane before enzyme phosphorylation at 7.5 and -2 degrees C. The step-function model analysis indicated that the low resolution difference profiles derived from both time-resolved x-ray diffraction experiments arise from a net movement of Ca2+ ATPase protein mass from the outer monolayer to the inner monolayer of the SR membrane lipid bilayer. The conserved redistribution of this protein mass is however somewhat different for the two cases, especially at the extravesicular membrane surface containing the Ca2+ATPase "headpiece." However, the conserved redistribution of protein mass within the SR membrane lipid bilayer common to both cases is clearly due to E1~P formation.

Photolabile chelators for the rapid photorelease of divalent cations

The properties of a recently synthesized photolabile chelator for divalent cations are described, the affinity of which for Ca2+ changes by some 5 orders of magnitude on illumination. The compound 1-(2-nitro-4,5-dimethoxyphenyl)-N,N,N',N'-tetrakis[(oxycarbonyl)me thyl]-1,2-ethanediamine (DM-nitrophen) binds Ca2+ (Kd approximately 5.0 x 10(-9) M) and Mg2+ (Kd approximately 2.5 x 10(-6) M) with relatively high affinities. On exposure of the DM-nitrophen-Ca2+ complex to UV light in the 350-nm range, the chelator is cleaved yielding iminodiacetic products with a much lower affinity for Ca (Kd approximately 3 x 10(-3) M) and the free [Ca2+] increases. The quantum yield for Ca2+ release is 0.18. In experiments with chemically skinned skeletal muscle fibers, a fully relaxed fiber equilibrated with DM-nitrophen-Ca2+ complex produced maximal contraction after a single flash from a frequency-doubled ruby laser (347 nm). Half-maximal tension was achieved in approximately 40 ms, some 5 times faster than that obtained after a rapid solution change from a Ca2+-free to a Ca2+-containing solution. In experiments with resealed human erythrocyte ghosts, irradiation of ghosts containing the DM-nitrophen-Ca2+ complex activates a Ca2+-dependent K+ efflux pathway, which is not observed in the absence of illumination. DM-nitrophen is sufficiently stable and photolabile to be used as a caged Ca (or caged Mg) for the rapid photoinitiation of divalent cation-dependent processes over a wide concentration range with a significant increase in temporal resolution over conventional mixing methods.

Electrical pump currents generated by the Ca2+-ATPase of sarcoplasmic reticulum vesicles adsorbed on black lipid membranes

Sarcoplasmic reticulum vesicles adsorbed on a black lipid membrane generate an electrical current after a fast increment of the concentration of ATP. This demonstrates directly that the sarcoplasmic Ca2+-ATPase from skeletal muscle acts as an electrogenic ion pump. The increment of the concentration of ATP is achieved by the photolysis of caged ATP (P3-1-(2-nitro)phenylethyl adenosine 5'-triphosphate) a protected analogue of ATP (Kaplan, J.H. et al. (1978) Biochemistry 17, 1929-1935), which is split into ATP and 2-nitroso acetophenone. The release of ATP leads to a transient current flow across the lipid membrane indicating that the vesicles are capacitatively coupled to the underlying lipid membrane. In addition to this transient signal, a stationary current flow is obtained in the presence of ionophores which increase the conductance of the bilayer system and prevent the accumulation of Ca2+ in the lumen of the vesicles. The direction of the transient and the stationary current is in accordance with the concept that Ca2+ is pumped into the lumen of the vesicles. The transient current depends on the concentration of ATP, Ca2+ and Mg2+ as would be the case for a current generated by the sarcoplasmic Ca2+-ATPase. Its amplitude is half-maximal at 10 microM ATP and 1 microM Ca2+. At Ca2+ concentrations above 0.1 mM the amplitude of the current signal declines again. The Mg2+ concentration dependence of the current amplitude at a constant ATP concentration indicates that the MgATP complex is the substrate for the activation of the current. The pump current is inhibited by vanadate and ADP. No current signal is observed if caged ATP is replaced by caged ADP. However, the release of ADP from caged ADP generates a pump current in the presence of an ATP generating system such as creatine phosphate and creatine kinase.

Time-resolved x-ray diffraction studies of the sarcoplasmic reticulum membrane during active transport

X-ray and neutron diffraction studies of oriented multilayers of a highly purified fraction of isolated sarcoplasmic reticulum (SR) have previously provided the separate profile structures of the lipid bilayer and the Ca2+-ATPase molecule within the membrane profile to approximately 10-A resolution. These studies used biosynthetically deuterated SR phospholipids incorporated isomorphously into the isolated SR membranes via phospholipid transfer proteins. Time-resolved x-ray diffraction studies of these oriented SR membrane multilayers have detected significant changes in the membrane profile structure associated with phosphorylation of the Ca2+-ATPase within a single turnover of the Ca2+-transport cycle. These studies used the flash photolysis of caged ATP to effectively synchronize the ensemble of Ca2+-ATPase molecules in the multilayer, synchrotron x-radiation to provide 100-500-ms data collection times, and double-beam spectrophotometry to monitor the Ca2+-transport process directly in the oriented SR membrane multilayer.

Transient-state kinetics of the ADP-insensitive phosphoenzyme in sarcoplasmic reticulum: implications for transient-state calcium translocation

The kinetics of formation of the ADP-sensitive (EP) and ADP-insensitive (E*P) phosphoenzyme intermediates of the CaATPase in sarcoplasmic reticulum (SR) were investigated by means of the quenched-flow technique. At 21 degrees C, addition of saturating ADP to SR vesicles phosphorylated for 116 ms with 10 microM ATP gave a triphasic pattern of dephosphorylation in which EP and E*P accounted for 33% and 60% of the total phosphoenzyme, respectively. Inorganic phosphate (Pi) release was less than stoichiometric with respect to E*P decay and was not increased by preincubation with Ca2+ ionophore. The fraction of E*P present after only 6 ms of phosphoenzyme formation was similar to that at 116 ms, indicating that isomerization of EP to E*P occurs very rapidly. Comparison of the time course of E*P formation with intravesicular Ca2+ accumulation measured by quenching with ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid + ADP revealed that Ca2+ release on the inside of the vesicle was delayed with respect to E*P formation. Since Ca2+ should dissociate rapidly dissociation from the low-affinity transport sites, these results suggest that Ca2+ remains "occluded" after phosphoenzyme isomerization and that a subsequent slow transition controls the rate of Ca2+ release at the intravesicular membrane surface. Analysis of the forward and reverse rate constants for the EP to E*P transition gave an expected steady-state distribution of phosphoenzymes strongly favoring the ADP-insensitive form. In contrast, the observed ratio of EP to E*P was about 1:2. To account for this discrepancy, a mechanism is proposed in which stabilization of the ADP-sensitive phosphoenzyme is brought about by a conformational interaction between adjacent subunits in a dimer.

Comparison of the kinetics of calcium transport in vesicular dispersions and oriented multilayers of isolated sarcoplasmic reticulum membranes

Knowledge of the functional properties of the protein in oriented multilayers, in addition to vesicular dispersions, of membranes such as the isolated sarcoplasmic reticulum (SR), extends the variety of techniques that can be effectively used in studies of the membrane protein's structure or structural changes associated with its function. One technique requiring the use of oriented multilayers to provide more direct time-averaged and time-resolved structural investigations of the SR membrane is x-ray diffraction. Therefore, the kinetics of ATP-induced calcium uptake by isolated SR vesicles in dispersions and hydrated, oriented multilayers were compared. Ca2+ uptake was necessarily initiated by the addition of ATP through flash photolysis of caged ATP, P3-1-(2-nitro)phenylethyl adenosine 5'-triphosphate, with either a frequency-doubled ruby laser or a 200 W Hg arc lamp, and measured with two different detector systems that followed the absorbance changes of the metallochromic indicator arsenazo III, which is sensitive to changes in the extravesicular [Ca2+]. The temperature range investigated was -2 degrees to 26 degrees C. The Ca2+ uptake kinetics of SR membranes in both the vesicular dispersions and oriented multilayers consist of at least two phases, an initial fast phase and a subsequent slow phase. The fast phase, generally believed to be associated with the formation of the phosphorylated enzyme, E approximately P, is kinetically comparable in both SR dispersions and multilayers. The slow phase mathematically follows first-order kinetics with specific rate constants of approximately 0.6 s-1 and approximately 1.2 s-1 for the dispersions at 26 degrees C and multilayers at 21 degrees C, respectively, with the given experimental conditions. The slow phase, generally believed to be associated with the translocation of Ca+2, across the membrane profile, appears to be the same process in SR dispersions and multilayers through their virtually identical rate constants and their identical activation energies of 22 +/-1 kcal mol -1. The stoichiometry of ~2 mol Ca2+/mol ATP hydrolyzed was measured in dispersions for the slow phase of Ca2+ uptake. Photolysis of caged ATP with the lamp and the laser provides comparable results for the Ca2+ uptake kinetics in SR dispersions and multilayers. Laser flash photolysis, however, has the advantages of optimal time resolution and effective synchronization of the ensemble of Ca2+-ATPase molecules in the ATP initiated Ca2+ transport process.