Caged inositol 1,4,5-trisphosphate for studying release of Ca2+ from intracellular stores

Custom-made modification of a commercial confocal microscope to photolyze caged compounds using the conventional illumination module and its application to the observation of Inositol 1,4,5-trisphosphate-mediated calcium signals

The flash photolysis of "caged" compounds is a powerful experimental technique for producing rapid changes in concentrations of bioactive signaling molecules. These caged compounds are inactive and become active when illuminated with ultraviolet light. This paper describes an inexpensive adaptation of an Olympus confocal microscope that uses as source of ultraviolet light the mercury lamp that comes with the microscope for conventional fluorescence microscopy. The ultraviolet illumination from the lamp (350 - 400 nm) enters through an optical fiber that is coupled to a nonconventional port of the microscope. The modification allows to perform the photolysis of caged compounds over wide areas (∼ 200 μm) and obtain confocal fluorescence images simultaneously. By controlling the ultraviolet illumination exposure time and intensity it is possible to regulate the amount of photolyzed compounds. In the paper we characterize the properties of the system and show its capabilities with experiments done in aqueous solution and in Xenopus Laevis oocytes. The latter demonstrate its applicability for the study of Inositol 1,4,5-trisphosphate-mediated intracellular calcium signals.

Quantifying calcium fluxes underlying calcium puffs in Xenopus laevis oocytes

We determine the calcium fluxes through inositol 1,4,5-trisphosphate receptor/channels underlying calcium puffs of Xenopus laevis oocytes using a simplified version of the algorithm of Ventura et al. An analysis of 130 puffs obtained with Fluo-4 indicates that Ca2+ release comes from a region of width approximately 450 nm, that the release duration is peaked around 18 s and that the underlying Ca2+ currents range between 0.12 and 0.95 pA. All these parameters are independent of IP(3) concentration. We explore what distributions of channels that open during a puff, N(p), and what relations between current and number of open channels, I(N(p)), are compatible with our findings and with the distribution of puff-to-trigger amplitude ratio reported in Rose et al. To this end, we use simple "mean field" models in which all channels open and close simultaneously. We find that the variability among clusters plays an important role in shaping the observed puff amplitude distribution and that a model for which I(N(p)) approximately N(p) for small N(p) and I(N(p)) approximately N(p)(1/alpha) (alpha > 1) for large N(p), provides the best agreement. Simulations of more detailed models in which channels open and close stochastically show that this nonlinear behavior can be attributed to the limited time resolution of the observations and to the averaging procedure that is implicit in the mean-field models. These conclusions are also compatible with observations of approximately 400 puffs obtained using the dye Oregon green.

Inositol-1,4,5-trisphosphate receptor-mediated Ca2+ waves in pyramidal neuron dendrites propagate through hot spots and cold spots

We studied inositol-1,4,5-trisphosphate (IP(3)) receptor-dependent intracellular Ca(2+) waves in CA1 hippocampal and layer V medial prefrontal cortical pyramidal neurons using whole-cell patch-clamp recordings and Ca(2+) fluorescence imaging. We observed that Ca(2+) waves propagate in a saltatory manner through dendritic regions where increases in the intracellular concentration of Ca(2+) ([Ca(2+)](i)) were large and fast ('hot spots') separated by regions where increases in [Ca(2+)](i) were comparatively small and slow ('cold spots'). We also observed that Ca(2+) waves typically initiate in hot spots and terminate in cold spots, and that most hot spots, but few cold spots, are located at dendritic branch points. Using immunohistochemistry, we found that IP(3) receptors (IP(3)Rs) are distributed in clusters along pyramidal neuron dendrites and that the distribution of inter-cluster distances is nearly identical to the distribution of inter-hot spot distances. These findings support the hypothesis that the dendritic locations of Ca(2+) wave hot spots in general, and branch points in particular, are specially equipped for regenerative IP(3)R-dependent internal Ca(2+) release. Functionally, the observation that IP(3)R-dependent [Ca(2+)](i) rises are greater at branch points raises the possibility that this novel Ca(2+) signal may be important for the regulation of Ca(2+)-dependent processes in these locations. Futhermore, the observation that Ca(2+) waves tend to fail between hot spots raises the possibility that influences on Ca(2+) wave propagation may determine the degree of functional association between distinct Ca(2+)-sensitive dendritic domains.

cADPR stimulates SERCA activity in Xenopus oocytes

The intracellular second messenger cyclic ADP-ribose (cADPR) induces Ca(2+) release through the activation of ryanodine receptors (RyRs). Moreover, it has been suggested that cADPR may serve an additional role to modulate sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) pump activity, but studies have been complicated by concurrent actions on RyR. Here, we explore the actions of cADPR in Xenopus oocytes, which lack RyRs. We examined the effects of cADPR on the sequestration of cytosolic Ca(2+) following Ca(2+) transients evoked by photoreleased inositol 1,4,5-trisphosphate (InsP(3)), and by Ca(2+) influx through expressed nicotinic acetylcholine receptors (nAChR) in the oocytes membrane. In both cases the decay of the Ca(2+) transients was accelerated by intracellular injection of a non-metabolizable analogue of cADPR, 3-Deaza-cADPR, and photorelease of cADPR from a caged precursor demonstrated that this action is rapid (a few s). The acceleration was abolished by pre-treatment with thapsigargin to block SERCA activity, and was inhibited by two specific antagonists of cADPR, 8-NH(2)-cADPR and 8-br-cADPR. We conclude that cADPR serves to modulate Ca(2+) sequestration by enhancing SERCA pump activity, in addition to its well-established action on RyRs to liberate Ca(2+).

Localization of puff sites adjacent to the plasma membrane: functional and spatial characterization of Ca2+ signaling in SH-SY5Y cells utilizing membrane-permeant caged IP3

The Xenopus oocyte has been a favored model system in which to study spatio-temporal mechanisms of intracellular Ca2+ dynamics, in large part because this giant cell facilitates intracellular injections of Ca2+ indicator dyes, buffers and caged compounds. However, the recent commercial availability of membrane-permeant ester forms of caged IP3 (ci-IP3) and EGTA, now allows for facile loading of these compounds into smaller mammalian cells, permitting control of [IP3]i and cytosolic Ca2+ buffering. Here, we establish the human neuroblastoma SH-SY5Y cell line as an advantageous experimental system for imaging Ca2+ signaling, and characterize IP3-mediated Ca2+ signaling mechanisms in these cells. Flash photo-release of increasing amounts of i-IP3 evokes Ca2+ puffs that transition to waves, but intracellular loading of EGTA decouples release sites, allowing discrete puffs to be studied over a wide range of [IP3]. Puff activity persists for minutes following a single photo-release, pointing to a slow rate of i-IP3 turnover in these cells and suggesting that repetitive Ca2+ spikes with periods of 20-30s are not driven by oscillations in [IP3]. Puff amplitudes are independent of [IP3], whereas their frequencies increase with increasing photo-release. Puff sites in SH-SY5Y cells are not preferentially localized near the nucleus, but instead are concentrated close to the plasma membrane where they can be visualized by total internal reflection microscopy, offering the potential for unprecedented spatio-temporal resolution of Ca2+ puff kinetics.

Single- and Two-Photon Properties of a Dye-Derivatized Roussin's Red Salt Ester (Fe2(μ-RS)2(NO)4) with a Large TPA Cross Section

The synthesis, characterization, photochemistry, and two-photon photophysical properties of a new dye-derivatized iron sulfur nitrosyl cluster Fe2(mu-RS)2(NO)4 (AFX-RSE, RS = 2-thioethyl ester of N-phenyl-N-(3-(2-ethoxy)phenyl)-7-(benzothiazol-2-yl)-9,9-diethyl-fluoren-2-yl-amine) were investigated. Under continuous photolysis, AFX-RSE decomposes with modest quantum yields (Phi(diss) = (4.9 +/- 0.9) x 10(-3) at lambda(irr) = 436 nm) as measured from the loss of the nitrosyl bands in the IR absorbance spectrum. Nitric oxide (NO) was qualitatively demonstrated to be photochemically produced via single-photon excitation through the use of an NO-specific electrode. Steady-state luminescence measurements have shown that AFX-RSE fluorescence is about 88% quenched relative to the model compound AF-tosyl. This is attributed to a relatively efficient energy transfer from the excited states of the antenna chromophores to the dinuclear metal center, with the subsequent production of NO. In addition, the two-photon absorption (TPA) cross sections (delta) were measured for the AF-chromophores via the two-photon excitation (TPE) photoluminescence technique using a femtosecond excitation source. The TPA cross section of AFX-RSE was found with this technique to be delta = 246 +/- 8 GM (1 GM = 10(-50) cm4 s photon(-1) molecule(-1)).

A two-photon antenna for photochemical delivery of nitric oxide from a water-soluble, dye-derivatized iron nitrosyl complex using NIR light

The experiments described here demonstrate the use of two-photon excitation (TPE) to sensitize nitric oxide (NO) release from a dye-derivatized iron/sulfur/nitrosyl cluster Fe2(mu-RS)2(NO)4 (Fluor-RSE, RS = 2-thioethyl ester of fluorescein) with near-infrared (NIR) light in the form of femtosecond pulses from a Ti:sapphire laser. TPE at 800 nm leads both to weak fluorescence from the organic chromophore at lambda(max) = 532 nm and to NO labilization from the cluster. Since the emission from the reference compound Fluor-Et (the ethyl ester of fluorescein) under identical conditions (50/50 CH3CN/phosphate buffer (1 mM) at pH 7.4) is considerably more intense, the weaker emission from Fluor-RSE and the NO generation indicate that the fluorescein excited states initially formed by TPE are largely quenched by energy transfer to the cluster core. The two-photon absorption (TPA) cross section of Fluor-RSE at 800 nm was determined to be delta = 63 +/- 7 GM via the TPA photoluminescence technique. This can be compared to the TPA cross section of 36 GM reported for fluorescein dye in pH 11 aqueous solution and of 32 +/- 3 GM for Fluor-Et measured under conditions comparable to those used for Fluor-RSE. Pulse intensity dependence studies showed that the quantity of NO released from the latter as the result of NIR photoexcitation follows a quadratic relationship to excitation intensity, consistent with the expectation for a TPE process. These studies demonstrate the potential utility of a two-photon antenna for sensitization of the photochemical release of an active agent (in this case, NO) from a photoactive pro-drug.

Graded recruitment and inactivation of single InsP3 receptor Ca2+-release channels: implications for quantal [corrected] Ca2+release

Modulation of cytoplasmic free Ca2+ concentration ([Ca2+]i) by receptor-mediated generation of inositol 1,4,5-trisphosphate (InsP3) and activation of its receptor (InsP3R), a Ca2+-release channel in the endoplasmic reticulum, is a ubiquitous signalling mechanism. A fundamental aspect of InsP3-mediated signalling is the graded release of Ca2+ in response to incremental levels of stimuli. Ca2+ release has a transient fast phase, whose rate is proportional to [InsP3], followed by a much slower one even in constant [InsP3]. Many schemes have been proposed to account for quantal Ca2+ release, including the presence of heterogeneous channels and Ca2+ stores with various mechanisms of release termination. Here, we demonstrate that mechanisms intrinsic to the single InsP3R channel can account for quantal Ca2+ release. Patch-clamp electrophysiology of isolated insect Sf9 cell nuclei revealed a consistent and high probability of detecting functional endogenous InsP3R channels, enabling InsP3-induced channel inactivation to be identified as an inevitable consequence of activation, and allowing the average number of activated channels in the membrane patch (N(A)) to be accurately quantified. InsP3-activated channels invariably inactivated, with average duration of channel activity reduced by high [Ca2+]i and suboptimal [InsP3]. Unexpectedly, N(A) was found to be a graded function of both [Ca2+]i and [InsP3]. A qualitative model involving Ca2+-induced InsP3R sequestration and inactivation can account for these observations. These results suggest that apparent heterogeneous ligand sensitivity can be generated in a homogeneous population of InsP3R channels, providing a mechanism for graded Ca2+ release that is intrinsic to the InsP3R Ca2+ release channel itself.

Imaging Ca2+ signals in Xenopus oocytes

Xenopus oocytes have become a favored preparation in which to study the spatiotemporal dynamics of intracellular Ca2+ signaling. Advantages of the oocyte as a model cell system include its large size, lack of intracellular Ca2+ release channels other than the type 1 inositol trisphosphate receptor, and ease of expression of foreign receptors and channels. We describe the use of high-resolution fluorescence imaging techniques to visualize Ca2+ signals in Xenopus oocytes at levels ranging from global Ca2+ waves to single-channel Ca2+ microdomains.

Polycystin 2 interacts with type I inositol 1,4,5-trisphosphate receptor to modulate intracellular Ca2+ signaling

Autosomal dominant polycystic kidney disease, a common cause of renal failure, arises from mutations in either the PKD1 or the PKD2 gene. The precise function of both PKD gene products polycystins (PCs) 1 and 2 remain controversial. PC2 has been localized to numerous cellular compartments, including the endoplasmic reticulum, plasma membrane, and cilia. It is unclear what pools are the most relevant to its physiological function as a putative Ca2+ channel. We employed a Xenopus oocyte Ca2+ imaging system to directly investigate the role of PC2 in inositol 1,4,5-trisphosphate (IP3)-dependent Ca2+ signaling. Cytosolic Ca2+ signals were recorded following UV photolysis of caged IP3 in the absence of extracellular Ca2+. We demonstrated that overexpression of PC2, as well as type I IP3 receptor (IP3R), significantly prolonged the half-decay time (t1/2) of IP3-induced Ca2+ transients. However, overexpressing the disease-associated PC2 mutants, the point mutation D511V, and the C-terminally truncated mutation R742X did not alter the t1/2. In addition, we found that D511V overexpression significantly reduced the amplitude of IP3-induced Ca2+ transients. Interestingly, overexpression of the C terminus of PC2 not only significantly reduced the amplitude but also prolonged the t1/2. Co-immunoprecipitation assays indicated that PC2 physically interacts with IP3R through its C terminus. Taken together, our data suggest that PC2 and IP3R functionally interact and modulate intracellular Ca2+ signaling. Therefore, mutations in either PC1 or PC2 could result in the misregulation of intracellular Ca2+ signaling, which in turn could contribute to the pathology of autosomal dominant polycystic kidney disease.

Spatiotemporal patterning of IP3-mediated Ca2+ signals in Xenopus oocytes by Ca2+-binding proteins

Ca(2+)-binding proteins (CaBPs) are expressed in a highly specific manner across many different cell types, yet the physiological basis underlying their selective distribution patterns remains unclear. We used confocal line-scan microscopy together with photo-release of IP(3) in Xenopus oocytes to investigate the actions of mobile cytosolic CaBPs on the spatiotemporal properties of IP(3)-evoked Ca(2+) signals. Parvalbumin (PV), a CaBP with slow Ca(2+)-binding kinetics, shortened the duration of IP(3)-evoked Ca(2+) signals and 'balkanized' global responses into discrete localized events (puffs). In contrast, calretinin (CR), a presumed fast buffer, prolonged Ca(2+) responses and promoted 'globalization' of spatially uniform Ca(2+) signals at high [IP(3)]. Oocytes loaded with CR or PV showed Ca(2+) puffs following photolysis flashes that were subthreshold in controls, and the spatiotemporal properties of these localized events were differentially modulated by PV and CR. In comparison to results we previously obtained with exogenous Ca(2+) buffers, PV closely mimicked the actions of the slow buffer EGTA, whereas CR showed important differences from the fast buffer BAPTA. Most notably, puffs were never observed after loading BAPTA, and this exogenous buffer did not show the marked sensitization of IP(3) action evident with CR. The ability of Ca(2+) buffers and CaBPs with differing kinetics to fine-tune both global and local intracellular Ca(2+) signals is likely to have significant physiological implications.

Buffer kinetics shape the spatiotemporal patterns of IP3-evoked Ca2+ signals

Ca2+ liberation through inositol 1,4,5-trisphosphate receptors (IP3Rs) plays a universal role in cell regulation, and specificity of cell signalling is achieved through the spatiotemporal patterning of Ca2+ signals. IP3Rs display Ca2+-induced Ca2+ release (CICR), but are grouped in clusters so that regenerative Ca2+ signals may remain localized to individual clusters, or propagate globally between clusters by successive cycles of Ca2+ diffusion and CICR. We used confocal microscopy and photoreleased IP3 in Xenopus oocytes to study how these properties are modulated by mobile cytosolic Ca2+ buffers. EGTA (a buffer with slow 'on-rate') speeded Ca2+ signals and 'balkanized' Ca2+ waves by dissociating them into local signals. In contrast, BAPTA (a fast buffer with similar affinity) slowed Ca2+ responses and promoted 'globalization' of spatially uniform Ca2+ signals. These actions are likely to arise through differential effects on Ca2+ feedback within and between IP3R clusters, because Ca2+ signals evoked by influx through voltage-gated channels were little affected. We propose that cell-specific expression of Ca2+-binding proteins with distinct kinetics may shape the time course and spatial distribution of IP3-evoked Ca2+ signals for specific physiological roles.

Photoremovable protecting groups: reaction mechanisms and applications

Photolabile protecting groups enable biochemists to control the release of bioactive compounds in living tissue. 'Caged compounds' (photoactivatable bioagents) have become an important tool to study the events that follow chemical signalling in, e.g., cell biology and the neurosciences. The possibilities are by no means exhausted. Progress will depend on the development of photoremovable protecting groups that satisfy the diverse requirements of new applications--a challenging task for photochemists.

Synthesis and application of caged peptides and proteins

Caged compounds have covalently attached groups that are rapidly cleaved upon exposure to UV light. Attachment of photolabile groups makes the molecule inert until photolysis releases it in its bioactive form. When caged compounds are applied to the experimental system in advance, the concentration jump of biologically active substances can be brought about immediately in a limited area upon irradiation with pulsed and focused UV light. Therefore, caged compounds of low molecular weight, which are commercially available, have been used effectively to study the mechanisms of temporal biological phenomena, such as muscle contraction, intracellular signaling, and neurotransmission. Because many proteins and peptides play important roles in these phenomena, their caged derivatives should serve as powerful tools to clarify complex biological systems. To prepare caged proteins and peptides, several groups have improved upon a chemical modification method, as well as developed two new methods: (1) nonsense codon suppression and (2) solid-phase peptide synthesis. In this review, we summarize recent advances made in the design, preparation, and application of caged peptides and proteins.

Subcellular mechanisms of presenilin-mediated enhancement of calcium signaling

Mutations in presenilin-1 (PS1), the leading cause of early-onset, autosomal-dominant familial Alzheimer's disease (FAD), enhance calcium signaling mediated by inositol 1,4,5-trisphosphate (IP3). To elucidate the subcellular mechanisms underlying this enhancement, we used high resolution line-scanning confocal microscopy to image elementary calcium release events ("puffs") in Xenopus oocytes expressing wild-type or mutant PS1. Here we report that mutant PS1-rendered puffs more sensitive to IP3 and increased both the magnitude and the rate of calcium release during each event. These effects were not attributable to quantitative changes in the levels of IP3 receptors or their distribution on the ER, but were instead associated with an abnormal elevation of ER calcium stores. Together, our results suggest that the effects of mutant PS1 on calcium signaling are manifested predominantly at the level of the regulation of calcium stores rather than via perturbations in the numbers or activity of IP3-activated calcium release channels.

Sperm-induced Ca(2+) oscillations in mouse oocytes and eggs can be mimicked by photolysis of caged inositol 1,4,5-trisphosphate: evidence to support a continuous low level production of inositol 1, 4,5-trisphosphate during mammalian fertilization

During mouse fertilization the spermatozoon induces a series of low-frequency long-lasting Ca(2+) oscillations. It is generally accepted that these oscillations are due to Ca(2+) release through the inositol 1,4,5-trisphosphate (InsP(3)) receptor. However, InsP(3) microinjection does not mimic sperm-induced Ca(2+) oscillations, leading to the suggestion that the spermatozoon causes Ca(2+) release by sensitizing the InsP(3) receptor to basal levels of InsP(3). This contradicts recent evidence that the spermatozoon triggers Ca(2+) oscillations by introducing a phospholipase C or else an activator of phospholipase C. Here we show for the first time that sperm-induced Ca(2+) oscillations may be mimicked by the photolysis of caged InsP(3) in both mouse metaphase II eggs and germinal vesicle stage oocytes. Eggs, and also oocytes that had displayed spontaneous Ca(2+) oscillations, gave long-lasting Ca(2+) oscillations when fertilized or when caged InsP(3) was photolyzed. In contrast, oocytes that had shown no spontaneous Ca(2+) oscillations did not generate many oscillations when fertilized or following photolysis of caged InsP(3). Fertilization in eggs was most closely mimicked when InsP(3) was uncaged at relatively low amounts for extended periods. Here we observed an initial Ca(2+) transient with superimposed spikes, followed by a series of single transients with a low frequency; all characteristics of the Ca(2+) changes at fertilization. We therefore show that InsP(3) can mimic the distinctive pattern of Ca(2+) release in mammalian eggs at fertilization. It is proposed that a sperm Ca(2+)-releasing factor operates by generating a continuous small amount of InsP(3) over an extended period of time, consistent with the evidence for the involvement of a phospholipase C.

Calsenilin reverses presenilin-mediated enhancement of calcium signaling

Most cases of autosomal-dominant familial Alzheimer's disease are linked to mutations in the presenilin genes (PS1 and PS2). In addition to modulating beta-amyloid production, presenilin mutations also produce highly specific and selective alterations in intracellular calcium signaling. Although the molecular mechanisms underlying these changes are not known, one candidate molecular mediator is calsenilin, a recently identified calcium-binding protein that associates with the C terminus of both PS1 and PS2. In this study, we investigated the effects of calsenilin on calcium signaling in Xenopus oocytes expressing either wild-type or mutant PS1. In this system, mutant PS1 potentiated the amplitude of calcium signals evoked by inositol 1,4,5-trisphosphate and also accelerated their rates of decay. We report that calsenilin coexpression reverses both of these potentially pathogenic effects. Notably, expression of calsenilin alone had no discernable effects on calcium signaling, suggesting that calsenilin modulates these signals by a mechanism independent of simple calcium buffering. Our findings further suggest that the effects of presenilin mutations on calcium signaling are likely mediated through the C-terminal domain, a region that has also been implicated in the modulation of beta-amyloid production and cell death.

Phasic characteristic of elementary Ca(2+) release sites underlies quantal responses to IP(3)

Ca(2+) liberation by inositol 1,4,5-trisphosphate (IP(3)) is 'quantal', in that low [IP(3)] causes only partial Ca(2+) release, but further increasing [IP(3)] evokes more release. This characteristic allows cells to generate graded Ca(2+) signals, but is unexpected, given the regenerative nature of Ca(2+)-induced Ca(2+) release through IP(3) receptors. Two models have been proposed to resolve this paradox: (i) all-or-none Ca(2+) release from heterogeneous stores that empty at varying [IP(3)]; and (ii) phasic liberation from homogeneously sensitive stores. To discriminate between these hypotheses, we imaged subcellular Ca(2+) puffs evoked by IP(3) in Xenopus oocytes where release sites were functionally uncoupled using EGTA. Puffs were little changed by 300 microM intracellular EGTA, but sites operated autonomously and did not propagate waves. Photoreleased IP(3) generated flurries of puffs-different to the prolonged Ca(2+) elevation following waves in control cells-and individual sites responded repeatedly to successive increments of [IP(3)]. These data support the second hypothesis while refuting the first, and suggest that local Ca(2+) signals exhibit rapid adaptation, different to the slower inhibition following global Ca(2+) waves.

Construction of a confocal microscope for real-time x-y and x-z imaging

We describe the construction of a simple 'real-time' laser-scanning confocal microscope, and illustrate its use for rapid imaging of elementary intracellular calcium signaling events. A resonant scanning galvanometer (8 kHz) allows x-y frame acquisition rates of 15 or 30 Hz, and the use of mirrors to scan the laser beam permits use of true, pin-hole confocal detection to provide diffraction-limited spatial resolution. Furthermore, use of a piezoelectric device to rapidly focus the objective lens allows axial (x-z) images to be obtained from thick specimens at similar frame rates. A computer with image acquisition and graphics cards converts the output from the microscope to a standard video signal, which can then be recorded on videotape and analyzed by regular image processing systems. The system is largely made from commercially available components and requires little custom construction of mechanical parts or electronic circuitry. It costs only a small fraction of that of comparable commercial instruments, yet offers greater versatility and similar or better performance.

Presenilin-2 mutations modulate amplitude and kinetics of inositol 1, 4,5-trisphosphate-mediated calcium signals

Mutations in the two presenilin genes (PS1, PS2) account for the majority of early-onset familial Alzheimer's disease (FAD) cases. Converging evidence from a variety of experimental systems, including fibroblasts from FAD patients and transgenic animals, indicates that PS1 mutations modulate intracellular calcium signaling pathways. Despite the potential relevance of these changes to the pathogenesis of FAD, a comparable effect for PS2 has not yet been demonstrated experimentally. We examined the effects of wild-type PS2, and both of the identified FAD mutations in PS2, on intracellular calcium signaling in Xenopus oocytes. Inositol 1,4, 5-trisphosphate (IP(3))-evoked calcium signals were significantly potentiated in cells expressing either of the PS2 mutations relative to wild-type PS2-expressing cells and controls. Decay rates of calcium signals were also significantly accelerated in mutant PS2-expressing cells in a manner dependent upon IP(3) concentration. The finding that mutations in both PS1 and PS2 modulate intracellular calcium signaling suggests that these disturbances may represent a common pathogenic mechanism of presenilin-associated FAD.

Radial localization of inositol 1,4,5-trisphosphate-sensitive Ca2+ release sites in Xenopus oocytes resolved by axial confocal linescan imaging

The radial localization and properties of elementary calcium release events ("puffs") were studied in Xenopus oocytes using a confocal microscope equipped with a piezoelectric focussing unit to allow rapid (>100 Hz) imaging of calcium signals along a radial line into the cell with a spatial resolution of <0.7 micrometer. Weak photorelease of caged inositol 1,4,5-trisphosphate (InsP3) evoked puffs arising predominantly within a 6-micrometer thick band located within a few micrometers of the cell surface. Approximately 25% of puffs had a restricted radial spread, consistent with calcium release from a single site. Most puffs, however, exhibited a greater radial spread (3.25 micrometer), likely involving recruitment of radially neighboring release sites. Calcium waves evoked by just suprathreshold stimuli exhibited radial calcium distributions consistent with inward diffusion of calcium liberated at puff sites, whereas stronger flashes evoked strong, short-latency signals at depths inward from puff sites, indicating deep InsP3-sensitive stores activated at higher concentrations of InsP3. Immunolocalization of InsP3 receptors showed punctate staining throughout a region corresponding to the localization of puffs and subplasmalemmal endoplasmic reticulum. The radial organization of puff sites a few micrometers inward from the plasma membrane may have important consequences for activation of calcium-dependent ion channels and "capacitative" calcium influx. However, on the macroscopic (hundreds of micrometers) scale of global calcium waves, release can be considered to occur primarily within a thin, essentially two-dimensional subplasmalemmal shell.

Hemispheric asymmetry of macroscopic and elementary calcium signals mediated by InsP3 in Xenopus oocytes

1. The mechanisms underlying hemispheric asymmetry of the inositol 1, 4,5-trisphosphate (InsP3)-calcium signalling pathway in Xenopus oocytes were examined by fluorescence imaging of calcium signals and recording calcium-activated Cl- currents (ICl,Ca) evoked by intracellular calcium injections and photorelease of InsP3. 2. The maximal ICl,Ca evoked by strong photorelease of InsP3 was 8 times greater in the animal than the vegetal hemisphere, but the average threshold amounts of InsP3 required to evoke detectable currents were similar in each hemisphere. 3. Currents evoked by injections of calcium were about 2.5 times greater near the animal pole than near the vegetal pole, whereas fluorescence signals evoked by injections were similar in each hemisphere. 4. Calcium waves were evoked by photolysis flashes of similar strengths in both hemispheres of albino oocytes, but peak calcium levels evoked by supramaximal stimuli were 70 % greater in the animal hemisphere. 5. Elementary calcium release events (puffs) in the animal hemisphere had amplitudes about double that in the vegetal hemisphere, and more often involved coupled release from adjacent sites. Calcium release sites were more closely packed in the animal hemisphere, with a mean spacing of about 1.5 micro m compared with 2.25 micro m in the vegetal hemisphere. 6. The larger amplitude of currents mediated by InsP3 in the animal hemisphere, therefore, involves an increased flux of calcium at individual release units, a more dense packing of release units and a higher density of Cl- channels.

Activation and co-ordination of InsP3-mediated elementary Ca2+ events during global Ca2+ signals in Xenopus oocytes

1. The activation of elementary calcium release events ('puffs') and their co-ordination to generate calcium waves was studied in Xenopus oocytes by confocal linescan imaging together with photorelease of inositol 1,4,5-trisphosphate (InsP3) from a caged precursor. 2. Weak photolysis flashes evoked no responses or isolated calcium puffs, whereas flashes of increasing strength evoked more frequent puffs, often occurring in flurries as abortive waves, and then a near-simultaneous calcium liberation originating at multiple sites. The numbers of sites activated increased initially as about the fourth power of photoreleased [InsP3]. 3. Following repeated, identical photolysis flashes, puffs arose after stochastically varying latencies of a few hundred milliseconds to several seconds. The cumulative number of events initially increased as about the third power of time. No rise in free [Ca2+] was detected preceding the puffs, suggesting that this co-operativity arises through binding of multiple InsP3 molecules, rather than through calcium feedback. 4. The mean latency to onset of calcium liberation shortened as about the square of the flash strength, and the dispersion in latencies between events reduced correspondingly. 5. Weak stimuli often evoked coupled puffs involving adjacent sites, and stronger flashes evoked saltatory calcium waves, propagating with non-constant velocity. During waves, [Ca2+] rose slowly between puff sites, but more abruptly at active sites following an initial diffusive rise in calcium. 6. Initial rates of rise of local [Ca2+] at release sites were similar during puffs and release induced by much (> 10-fold) greater [InsP3]. In contrast, macroscopic calcium measurements averaged over the scan line showed a graded dependence of rate of calcium liberation upon [InsP3], due to recruitment of additional sites and decreasing dispersion in activation latencies. 7. We conclude that the initiation of calcium liberation depends co-operatively upon [InsP3] whereas the subsequent regenerative increase in calcium flux depends upon local calcium feedback and is largely independent of [InsP3]. Wave propagation is consistent with the diffusive spread of calcium evoking regenerative liberation at heterogeneous discrete sites, the sensitivity of which is primed by InsP3.

A continuum of InsP3-mediated elementary Ca2+ signalling events in Xenopus oocytes

1. The elementary release events underlying inositol 1,4, 5-trisphosphate (InsP3)-mediated calcium signalling were investigated in Xenopus oocytes by means of high-resolution confocal linescan imaging together with flash photolysis of caged InsP3. 2. Weak photolysis flashes evoked localized, transient calcium signals that arose at specific sites following random latencies of up to several seconds. The duration, spatial spread and amplitude of these elementary events varied widely. Event durations (at half-maximal amplitude) were distributed exponentially between about 100 and 600 ms. Fluorescence magnitudes (F/F0 of Oregon Green 488 BAPTA-1) showed a skewed distribution with a peak at about 1.5 and a tail extending as high as 3.5. 3. Individual release sites exhibited both small events (blips) and large events (puffs). The spatiotemporal distribution of calcium signals during puffs was consistent with calcium diffusion from a point source (< a few hundred nanometres), rather than with propagation of a microscopic calcium wave. 4. Estimates of the calcium flux associated with individual events were made by integrating fluorescence profiles along the scan line in three dimensions to derive the 'signal mass' at each time point. The smallest resolved events corresponded to liberation of < 2 x 10-20 mol Ca2+, and large events to about 2 x 10-18 mol Ca2+. The rise of signal mass was more prolonged than that of the fluorescence intensity, suggesting that calcium liberation persists even while the fluorescence begins to decline. Rates of rise of signal mass corresponded to Ca2+ currents of 0.4-2.5 pA. 5. Measurements of signal mass from different events showed a continuous, exponential distribution, arising through variability in magnitude and duration of calcium flux. 6. We conclude that localized calcium transients in the oocyte represent a continuum of events involving widely varying amounts of calcium liberation, rather than falling into separate populations of 'fundamental' and 'elementary' events (blips and puffs) involving, respectively, single and multiple InsP3 receptor channels. This variability probably arises through stochastic variation in both the number of channels recruited and the duration of channel opening.