Neuronal calcium sparks and intracellular calcium "noise"

On the genesis and unique functions of zinc neuromodulation

In addition to the essential structural and catalytic functions of zinc, evolution has adopted synaptic zinc as a neuromodulator. In the brain, synaptic zinc is released primarily from glutamatergic neurons, notably in the neocortex, hippocampus, amygdala, and auditory brainstem. In these brain areas, synaptic zinc is essential for neuronal and sensory processing fine-tuning. But what niche does zinc fill in neural signaling that other neuromodulators do not? Here, we discuss the evolutionary history of zinc as a signaling agent and its eventual adoption as an essential neuromodulator in the mammalian brain. We then attempt to describe the unique roles that zinc has carved out of the vast and diverse landscape of neuromodulators.

Spontaneous and action potential-evoked Ca2+ release from endoplasmic reticulum in neocortical synaptic boutons

Although the endoplasmic reticulum (ER) is present throughout axons, and IP3 and ryanodine receptors are widely expressed in nerve terminals, whether Ca2+ release from presynaptic stores contributes to action potential (AP)-evoked Ca2+ transients remains controversial. We investigated the release of Ca2+ from ER stores in boutons en passant of neocortical layer 5 pyramidal neurons. A hallmark of these stores is that they spontaneously release Ca2+ at a low frequency. Using a high-affinity Ca2+ indicator, we documented and characterised such spontaneous Ca2+ transients (sCaTs), which occurred at a rate of ~0.2 per min and raised the intracellular Ca2+ concentration ([Ca2+]i) by ~2 µM in the absence of exogenous buffers. Caffeine increased the average frequency of sCaTs by 90%, without affecting their amplitude and decay kinetics. Therefore, presynaptic ryanodine receptors were likely involved. To determine if presynaptic ER stores contribute to intracellular Ca2+ accumulation during repetitive stimulation, we measured [Ca2+]i during 2 s long trains of APs evoked at 10-50 Hz. We found that for frequencies <20 Hz, [Ca2+]i reached a steady state within ~500 ms after stimulation onset. However, for higher frequencies, [Ca2+]i continued to increase with AP number, suggesting that the rate of Ca2+ entry exceeded the rate of clearance. Comparison between measured and predicted values indicates supralinear summation of Ca2+. Block of the sarco/endoplasmic reticulum Ca2+-ATPase reduced the supralinearity of summation, without reducing the amplitude of a single AP-evoked Ca2+ transient. Together, our results implicate presynaptic ER stores as a source of Ca2+ during repetitive stimulation.

Ryanodine Receptor 1 Polymorphism Is Not Associated with Aneurysmal Subarachnoid Hemorrhage or its Clinical Sequelae

OBJECTIVE

The pathophysiologic mechanisms underlying cerebral vasospasm after aneurysmal subarachnoid hemorrhage (aSAH) remain poorly understand. Ryanodine receptors (RYR) are intracellular calcium channels involved in the regulation of vascular smooth muscle cells and cerebrovascular tone and diameter. Previous work reported an association between an RYR polymorphism and cerebral vasospasm. Here, we sought to assess the impact of that RYR polymorphism on aSAH and its clinical sequelae.

METHODS

Blood samples from all patients enrolled in the CARAS (Cerebral Aneurysm Renin Angiotensin System) study were used for genetic evaluation. The RYR1 single nucleotide polymorphism (SNP) rs35364374 was detected using 5'exonuclease (Taqman) genotyping assays. Associations between the RYR1 polymorphism and aSAH and its clinical sequelae were analyzed.

RESULTS

Samples from 149 patients with aSAH and 50 controls were available for analysis. Multivariable regression analysis did not show an association of RYR1 SNP rs35364374 with aSAH. Moreover, there was no association of RYR1 SNP rs35364374 with clinical vasospasm, delayed cerebral ischemia, functional outcome at discharge, or functional outcome at last follow-up.

CONCLUSIONS

Contrary to a previous report, the RYR1 SNP rs35364374 was not associated with aSAH or its clinical sequelae.

Modeling of stochastic behavior of pacemaker potential in interstitial cells of Cajal

It is widely accepted that interstitial cells of Cajal (ICCs) generate pacemaker potentials to propagate slow waves along the whole gastrointestinal tract. Previously, we constructed a biophysically based model of ICCs in mouse small intestine to explain the pacemaker mechanism. Our previous model, however, could not explain non-uniformity of pacemaker potentials and random occurrence of unitary potentials, thus we updated our model. The inositol 1,4,5-trisphosphate (IP3)-mediated Ca(2+) mobilization is a key event to drive the cycle of pacemaker activity and was updated to reproduce its stochastic behavior. The stochasticity was embodied by simulating random opening and closing of individual IP3-mediated Ca(2+) channel. The updated model reproduces the stochastic features of pacemaker potentials in ICCs. Reproduced pacemaker potentials are not uniform in duration and interval. The resting and peak potentials are -75.5 ± 1.1 mV and -0.8 ± 0.5 mV, respectively (n = 55). Frequency of pacemaker potential is 14.3 ± 0.4 min(-1) (n = 10). Width at half-maximal amplitude of pacemaker potential is 902 ± 6 ms (n = 55). There are random events of unitary potential-like depolarization. Finally, we compared our updated model with a recently published model to speculate which ion channel is the best candidate to drive pacemaker depolarization. In conclusion, our updated mathematical model could now reproduce stochastic features of pacemaker activity in ICCs.

Observations of calcium dynamics in cortical secretory vesicles

Calcium (Ca(2+)) dynamics were evaluated in fluorescently labeled sea urchin secretory vesicles using confocal microscopy. 71% of the vesicles examined exhibited one or more transient increases in the fluorescence signal that was damped in time. The detection of transient increases in signal was dependent upon the affinity of the fluorescence indicator; the free Ca(2+) concentration in the secretory vesicles was estimated to be in the range of ∼10 to 100 μM. Non-linear stochastic analysis revealed the presence of extra variance in the Ca(2+) dependent fluorescence signal. This noise process increased linearly with the amplitude of the Ca(2+) signal. Both the magnitude and spatial properties of this noise process were dependent upon the activity of vesicle p-type (Ca(v)2.1) Ca(2+) channels. Blocking the p-type Ca(2+) channels with ω-agatoxin decreased signal variance, and altered the spatial noise pattern within the vesicle. These fluorescence signal properties are consistent with vesicle Ca(2+) dynamics and not simply due to obvious physical properties such as gross movement artifacts or pH driven changes in Ca(2+) indicator fluorescence. The results suggest that the free Ca(2+) content of cortical secretory vesicles is dynamic; this property may modulate the exocytotic fusion process.

Theory and applications of geometric scaling of localized calcium release events

Geometric measures of localized calcium release (LCR) events have been used to understand their biophysical basis. We found power law scaling between three such metrics-maximum amplitude (MA), mass above half-maximum amplitude (MHM), and area at half-maximum amplitude (AHM). In an effort to understand this scaling a minimal analytic model was employed to simulate LCR events recorded by confocal line scan. The distribution of logMHM as a function of logAHM, pMHM(pAHM), was dependent on model parameters such as channel open time, current size, line scan offset, and apparent diffusion coefficient. The distribution of log[MHM/AHM] as a function of logMA, p[MHM/AHM](pMA), was invariant, reflecting the gross geometry of the LCR event. The findings of the model were applied to real LCR line scan data from rabbit portal vein myocytes, rat cerebral artery myocytes, and guinea pig fundus knurled cells. pMHM(pAHM) could be used to distinguish two populations of LCR events in portal vein, even at the scale of "calcium noise," and to calculate the relative current of the two. The relative current was 2. pMHM(pAHM) could also be used to study pharmacological effects. The pMHM(pAHM) distribution of knurled cell LCR events was markedly contracted by ryanodine, suggesting a reduction in channel open time. The p[MHM/AHM](pMA) distributions were invariant across all cell types and were consistent with the model, underlying the common physical basis of their geometry. The geometric scaling of LCR events demonstrated here may help with their mechanistic characterization.

Calicum microdomains form within neutrophils at the neutrophil-tumor cell synapse: role in antibody-dependent target cell apoptosis

Ca(2+) messages are broadly important in cellular signal transduction. In immune cells, Ca(2+) signaling is an essential step in many forms of activation. Neutrophil-mediated antibody-dependent cell-mediated cytotoxicity (ADCC) is one form of leukocyte activation that plays an important role in tumor cell killing in vitro and in patient care. Using fluorescence methodologies, we found that neutrophils exhibit Ca(2+) signals during ADCC directed against breast fibrosarcoma cells. Importantly, these signals were localized to Ca(2+) microdomains at the neutrophil-to-tumor cell interface where they display dynamic features such as movement, fusion, and fission. These signals were blocked by the intracellular Ca(2+) buffer BAPTA. At the neutrophil-tumor cell synapse, the neutrophil's cytoplasm was enriched in STIM1, a crucial mediator of Ca(2+) signaling, whereas the Ca(2+)-binding proteins calbindin and parvalbumin were not affected. Our findings suggest that Ca(2+) microdomains are due to an active signaling process. As Ca(2+) signals within neutrophils were necessary for specific tumor cell apoptosis, a central role of microdomains in leukocyte-mediated tumor cell destruction is indicated.

Calcium sparks

The calcium ion (Ca(2+)) is the simplest and most versatile intracellular messenger known. The discovery of Ca(2+) sparks and a related family of elementary Ca(2+) signaling events has revealed fundamental principles of the Ca(2+) signaling system. A newly appreciated "digital" subsystem consisting of brief, high Ca(2+) concentration over short distances (nanometers to microns) comingles with an "analog" global Ca(2+) signaling subsystem. Over the past 15 years, much has been learned about the theoretical and practical aspects of spark formation and detection. The quest for the spark mechanisms [the activation, coordination, and termination of Ca(2+) release units (CRUs)] has met unexpected challenges, however, and raised vexing questions about CRU operation in situ. Ample evidence shows that Ca(2+) sparks catalyze many high-threshold Ca(2+) processes involved in cardiac and skeletal muscle excitation-contraction coupling, vascular tone regulation, membrane excitability, and neuronal secretion. Investigation of Ca(2+) sparks in diseases has also begun to provide novel insights into hypertension, cardiac arrhythmias, heart failure, and muscular dystrophy. An emerging view is that spatially and temporally patterned activation of the digital subsystem confers on intracellular Ca(2+) signaling an exquisite architecture in space, time, and intensity, which underpins signaling efficiency, stability, specificity, and diversity. These recent advances in "sparkology" thus promise to unify the simplicity and complexity of Ca(2+) signaling in biology.

Fast XYT imaging of elementary calcium release events in muscle with multifocal multiphoton microscopy and wavelet denoising and detection

We used multifocal multiphoton microscopy to image fast, localized elevations of the cytosolic Ca2+ concentration in two spatial dimensions plus time (XYT). This technique extends the common spatially 1-D XT imaging and allows the acquisition of more than ten times longer time series (>500 images) and ten times larger areas of interest than for previously used confocal XYT imaging techniques due to lower phototoxicity and fast multifocal scanning. We recorded spontaneously occurring elementary Ca2+ release events in chemically permeabilized adult mammalian skeletal muscle fibers using two-photon excitation of the fluorescent dye Fluo-4. The resulting time series were analyzed with an automated denoising and detection algorithm based on the à trous implementation of the discrete wavelet transform. Wavelet coefficient hard-thresholding is used for denoising and event detection is performed across several wavelet scales. The spatiotemporal characteristics of the detected Ca2+ release events are followed throughout the XYT stack and are parametrized using a biophysically valid anisotropic Gaussian event model. The proposed method allows a detailed spatiotemporal analysis of elementary Ca2+ release events underlying the excitation-contraction coupling process in muscle.

Ryanodine receptor interaction with the SNARE-associated protein snapin

The ryanodine receptor (RyR) is a widely expressed intracellular calcium (Ca(2+))-release channel regulating processes such as muscle contraction and neurotransmission. Snapin, a ubiquitously expressed SNARE-associated protein, has been implicated in neurotransmission. Here, we report the identification of snapin as a novel RyR2-interacting protein. Snapin binds to a 170-residue predicted ryanodine receptor cytosolic loop (RyR2 residues 4596-4765), containing a hydrophobic segment required for snapin interaction. Ryanodine receptor binding of snapin is not isoform specific and is conserved in homologous RyR1 and RyR3 fragments. Consistent with peptide fragment studies, snapin interacts with the native ryanodine receptor from skeletal muscle, heart and brain. The snapin-RyR1 association appears to sensitise the channel to Ca(2+) activation in [(3)H]ryanodine-binding studies. Deletion analysis indicates that the ryanodine receptor interacts with the snapin C-terminus, the same region as the SNAP25-binding site. Competition experiments with native ryanodine receptor and SNAP25 suggest that these two proteins share an overlapping binding site on snapin. Thus, regulation of the association between ryanodine receptor and snapin might constitute part of the elusive molecular mechanism by which ryanodine-sensitive Ca(2+) stores modulate neurosecretion.

Calcium microdomains in regulated exocytosis

Katz and co-workers showed that Ca(2+) triggers exocytosis. The existence of sub-micrometer domains of greater than 100 microM [Ca(2+)](i) was postulated on theoretical grounds. Using a modified, low-affinity aequorin, Llinas et al. were the first to demonstrate the existence of Ca(2+) 'microdomains' in squid presynaptic terminals. Over the past several years, it has become clear that individual Ca(2+) nano- and microdomains forming around the mouth of voltage-gated Ca(2+) channels ascertain the tight coupling of fast synaptic vesicle release to membrane depolarization by action potentials. Recent work has established different geometric arrangements of vesicles and Ca(2+) channels at different central synapses and pointed out the role of Ca(2+) syntillas - localized, store operated Ca(2+) signals - in facilitation and spontaneous release. The coupling between Ca(2+) increase and evoked exocytosis is more sluggish in peripheral terminals and neuroendocrine cells, where channels are less clustered and Ca(2+) comes from different sources, including Ca(2+) influx via the plasma membrane and the mobilization of Ca(2+) from intracellular stores. Finally, also non- (electrically) excitable cells display highly localized Ca(2+) signaling domains. We discuss in particular the organization of structural microdomains of Bergmann glia, specialized astrocytes of the cerebellum that have only recently been considered as secretory cells. Glial microdomains are the spatial substrate for functionally segregated Ca(2+) signals upon metabotropic activation. Our review emphasizes the large diversity of different geometric arrangements of vesicles and Ca(2+) sources, leading to a wide spectrum of Ca(2+) signals triggering release.

Two system-size-resonance behaviors for calcium signaling: for optimal cell size and for optimal network size

We have studied the collective calcium signaling behavior of an array of coupled N cells, taking into account the internal noises resulting from the small cell size V. The system's performance was characterized by the reciprocal coefficient of variance (RCV) of the calcium spike train. Two system-size resonances were observed, namely, the RCV value shows a clear peak when both N and V are optimal. Therefore, an optimal number of cells of optimal size work the best as a whole.

Dihydropyridine receptors and type 1 ryanodine receptors constitute the molecular machinery for voltage-induced Ca2+ release in nerve terminals

Ca2+ stores were studied in a preparation of freshly dissociated terminals from hypothalamic magnocellular neurons. Depolarization from a holding level of -80 mV in the absence of extracellular Ca2+ elicited Ca2+ release from intraterminal stores, a ryanodine-sensitive process designated as voltage-induced Ca2+ release (VICaR). The release took one of two forms: an increase in the frequency but not the quantal size of Ca2+ syntillas, which are brief, focal Ca2+ transients, or an increase in global [Ca2+]. The present study provides evidence that the sensors of membrane potential for VICaR are dihydropyridine receptors (DHPRs). First, over the range of -80 to -60 mV, in which there was no detectable voltage-gated inward Ca2+ current, syntilla frequency was increased e-fold per 8.4 mV of depolarization, a value consistent with the voltage sensitivity of DHPR-mediated VICaR in skeletal muscle. Second, VICaR was blocked by the dihydropyridine antagonist nifedipine, which immobilizes the gating charge of DHPRs but not by Cd2+ or FPL 64176 (methyl 2,5 dimethyl-4[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylate), a non-dihydropyridine agonist specific for L-type Ca2+ channels, having no effect on gating charge movement. At 0 mV, the IC50 for nifedipine blockade of VICaR in the form of syntillas was 214 nM in the absence of extracellular Ca2+. Third, type 1 ryanodine receptors, the type to which DHPRs are coupled in skeletal muscle, were detected immunohistochemically at the plasma membrane of the terminals. VICaR may constitute a new link between neuronal activity, as signaled by depolarization, and a rise in intraterminal Ca2+.

Analysis of puff dynamics in oocytes: interdependence of puff amplitude and interpuff interval

Puffs are localized Ca(2+) signals that arise in oocytes in response to inositol 1,4,5-trisphosphate (IP(3)). They are analogous to the sparks of myocytes and are believed to be the result of the liberation of Ca(2+) from the endoplasmic reticulum through the coordinated opening of IP(3) receptor/channels clustered at a functional release site. In this article, we analyze sequences of puffs that occur at the same site to help elucidate the mechanisms underlying puff dynamics. In particular, we show a dependence of the interpuff time on the amplitude of the preceding puff, and of the amplitude of the following puff on the preceding interval. These relationships can be accounted for by an inhibitory role of the Ca(2+) that is liberated during puffs. We construct a stochastic model for a cluster of IP(3) receptor/channels that quantitatively replicates the observed behavior, and we determine that the characteristic time for a channel to escape from the inhibitory state is of the order of seconds.

Ca2+ sparks and secretion in dorsal root ganglion neurons

Ca(2+) sparks as the elementary intracellular Ca(2+) release events are instrumental to local control of Ca(2+) signaling in many types of cells. Here, we visualized neural Ca(2+) sparks in dorsal root ganglion (DRG) sensory neurons and investigated possible role of DRG sparks in the regulation of secretion from the somata of the cell. DRG sparks arose mainly from type 3 ryanodine receptor Ca(2+) release channels on subsurface cisternae of the endoplasmic reticulum, rendering a striking subsurface localization. Caffeine- or 3,7-dimethyl-1-(2-propynyl)xanthine-induced store Ca(2+) release, in the form of Ca(2+) sparks, triggered exocytosis, independently of membrane depolarization and external Ca(2+). The spark-secretion coupling probability was estimated to be between 1 vesicle per 6.6 sparks and 1 vesicle per 11.4 sparks. During excitation, subsurface sparks were evoked by physiological Ca(2+) entry via the Ca(2+)-induced Ca(2+) release mechanism, and their synergistic interaction with Ca(2+) influx accounted for approximately 60% of the Ca(2+)-dependent exocytosis. Furthermore, inhibition of Ca(2+)-induced Ca(2+) release abolished endotoxin-induced secretion of pain-related neuropeptides. These findings underscore an important role for Ca(2+) sparks in the amplification of surface Ca(2+) influx and regulation of neural secretion.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

Imaging microdomain Ca2+ in muscle cells

Ca2+ ions passing through a single or a cluster of Ca2+-permeable channels create microscopic, short-lived Ca2+ gradients that constitute the building blocks of cellular Ca2+ signaling. Over the last decade, imaging microdomain Ca2+ in muscle cells has unveiled the exquisite spatial and temporal architecture of intracellular Ca2+ dynamics and has reshaped our understanding of Ca2+ signaling mechanisms. Major advances include the visualization of "Ca2+ sparks" as the elementary events of Ca2+ release from the sarcoplasmic reticulum (SR), "Ca2+ sparklets" produced by openings of single Ca2+-permeable channels, miniature Ca2+ transients in single mitochondria ("marks"), and SR luminal Ca2+ depletion transients ("scraps"). As a model system, a cardiac myocyte contains a 3-dimensional grid of 104 spark ignition sites, stochastic activation of which summates into global Ca2+ transients. Tracking intermolecular coupling between single L-type Ca2+ channels and Ca2+ sparks has provided direct evidence validating the local control theory of Ca2+-induced Ca2+ release in the heart. In vascular smooth muscle myocytes, Ca2+ can paradoxically signal both vessel constriction (by global Ca2+ transients) and relaxation (by subsurface Ca2+ sparks). These findings shed new light on the origin of Ca2+ signaling efficiency, specificity, and versatility. In addition, microdomain Ca2+ imaging offers a novel modality that complements electrophysiological approaches in characterizing Ca2+ channels in intact cells.

Effects of inositol 1,4,5-trisphosphate receptor-mediated intracellular stochastic calcium oscillations on activation of glycogen phosphorylase

In various cell types cytosolic calcium (Ca(2+)) is an important regulator. The possible role of Ca(2+) release from the inositol 1,4,5-trisphosphate (IP(3)) receptor channel in the regulation of the phosphorylation-dephosphorylation cycle process involved in glycogen degradation by glycogen phosphorylase have theoretically investigated by using the Li-Rinzel model for cytosolic Ca(2+) oscillations. For the case of deterministic cytosolic Ca(2+) oscillations, there exists an optimal frequency of cytosolic Ca(2+) oscillations at which the average fraction of active glycogen phosphorylase reaches a maximum value, and a mutation for the average fraction of active glycogen phosphorylase occurs at the higher bifurcation point of Ca(2+) oscillations. For the case of stochastic cytosolic Ca(2+) oscillations, the fraction of active phosphorylase is strongly affected by the number of IP(3) receptor channels and the level of IP(3) concentration. Small number of IP(3) receptor channels can potentiate the sensitivity of the activity of glycogen phosphorylase. The average frequency and amplitude of active phosphorylase stochastic oscillations are increased with the level of increasing IP(3) stimuli. The various distributions for the amplitude of active glycogen phosphorylase oscillations in parameters plane are discussed.

Ca2+ syntillas, miniature Ca2+ release events in terminals of hypothalamic neurons, are increased in frequency by depolarization in the absence of Ca2+ influx

Localized, brief Ca2+ transients (Ca2+ syntillas) caused by release from intracellular stores were found in isolated nerve terminals from magnocellular hypothalamic neurons and examined quantitatively using a signal mass approach to Ca2+ imaging. Ca2+ syntillas (scintilla, L., spark, from a synaptic structure, a nerve terminal) are caused by release of approximately 250,000 Ca ions on average by a Ca2+ flux lasting on the order of tens of milliseconds and occur spontaneously at a membrane potential of -80 mV. Syntillas are unaffected by removal of extracellular Ca2+, are mediated by ryanodine receptors (RyRs) and are increased in frequency, in the absence of extracellular Ca2+, by physiological levels of depolarization. This represents the first direct demonstration of mobilization of Ca2+ from intracellular stores in neurons by depolarization without Ca2+ influx. The regulation of syntillas by depolarization provides a new link between neuronal activity and cytosolic [Ca2+] in nerve terminals.

Calcium regulates exocytosis at the level of single vesicles

Ca2+ microdomains that form during the opening of voltage-gated Ca2+ channels have been implicated in regulating the kinetics of hormone and transmitter release. Direct assessment of the interaction between a single Ca2+ microdomain and a single secretory vesicle has been impossible because of technical limitations. Using evanescent field imaging of near-membrane micromolar Ca2+ concentration ([Ca2+]) and fluorescently labeled vesicles, we have observed exocytosis of individual chromaffin dense-core vesicles that was triggered by Ca2+ microdomains. Ca2+ microdomains selectively triggered the release of vesicles that were docked within 300 nm. Not all vesicles exposed to a Ca2+ microdomain were released, indicating that some vesicles are docked but are not ready for release. In addition to its established role as a trigger for release, elevated near-membrane [Ca2+] reduced the distance between docked vesicles and Ca2+ entry sites. Our results suggest a new mechanism for stimulation-dependent facilitation of exocytosis, whereby vesicles are moved closer to Ca2+ entry sites, thereby increasing a Ca2+ microdomain's efficacy to trigger vesicle fusion.

Glucosylceramide and glucosylsphingosine modulate calcium mobilization from brain microsomes via different mechanisms

We recently demonstrated that elevation of intracellular glucosylceramide (GlcCer) levels results in increased functional Ca2+ stores in cultured neurons, and suggested that this may be due to modulation of ryanodine receptors (RyaRs) by GlcCer (Korkotian, E., Schwarz, A., Pelled, D., Schwarzmann, G., Segal, M. and Futerman, A. H. (1999) J. Biol. Chem. 274, 21673-21678). We now systematically examine the effects of exogenously added GlcCer, other glycosphingolipids (GSLs) and their lyso-derivatives on Ca2+ release from rat brain microsomes. GlcCer had no direct effect on Ca2+ release, but rather augmented agonist-stimulated Ca2+ release via RyaRs, through a mechanism that may involve the redox sensor of the RyaR, but had no effect on Ca2+ release via inositol 1,4,5-trisphosphate receptors. Other GSLs and sphingolipids, including galactosylceramide, lactosylceramide, ceramide, sphingomyelin, sphingosine 1-phosphate, sphinganine 1-phosphate, and sphingosylphosphorylcholine had no effect on Ca2+ mobilization from rat brain microsomes, but both galactosylsphingosine (psychosine) and glucosylsphingosine stimulated Ca2+ release, although only galactosylsphingosine mediated Ca2+ release via the RyaR. Finally, we demonstrated that GlcCer levels were approximately 10-fold higher in microsomes prepared from the temporal lobe of a type 2 Gaucher disease patient compared with a control, and Ca2+ release via the RyaR was significantly elevated, which may be of relevance for explaining the pathophysiology of neuronopathic forms of Gaucher disease.

Presence and functional significance of presynaptic ryanodine receptors

Ca(2+)-induced Ca(2+) release (CICR) mediated by sarcoplasmic reticulum resident ryanodine receptors (RyRs) has been well described in cardiac, skeletal and smooth muscle. In brain, RyRs are localised primarily to endoplasmic reticulum (ER) and have been demonstrated in postsynaptic entities, astrocytes and oligodendrocytes where they regulate intracellular Ca(2+) concentration ([Ca(2+)](i)), membrane potential and the activity of a variety of second messenger systems. Recently, the contribution of presynaptic RyRs and CICR to functions of central and peripheral presynaptic terminals, including neurotransmitter release, has received increased attention. However, there is no general agreement that RyRs are localised to presynaptic terminals, nor is it clear that RyRs regulate a large enough pool of intracellular Ca(2+) to be physiologically significant. Here, we review direct and indirect evidence that on balance favours the notion that ER and RyRs are found in presynaptic terminals and are physiologically significant. In so doing, it became obvious that some of the controversy originates from issues related to (i) the ability to demonstrate conclusively the physical presence of ER and RyRs, (ii) whether the biophysical properties of RyRs are such that they can contribute physiologically to regulation of presynaptic [Ca(2+)](i), (iii) how ER Ca(2+) load and feedback gain of CICR contributes to the ability to detect functionally relevant RyRs, (iv) the distance that Ca(2+) diffuses from plasma membranes to RyRs to trigger CICR and from RyRs to the Active Zone to enhance vesicle release, and (v) the experimental conditions used. The recognition that ER Ca(2+) stores are able to modulate local Ca(2+) levels and neurotransmitter release in presynaptic terminals will aid in the understanding of the cellular mechanisms controlling neuronal function.

Stochastic properties of Ca(2+) release of inositol 1,4,5-trisphosphate receptor clusters

Intracellular Ca(2+) release is controlled by inositol 1,4,5-trisphosphate (IP(3)) receptors or ryanodine receptors. These receptors are typically distributed in clusters with several or tens of channels. The random opening and closing of these channels introduces stochasticity into the elementary calcium release mechanism. Stochastic release events have been experimentally observed in a variety of cell types and have been termed sparks and puffs. We put forward a stochastic version of the Li-Rinzel model (the deactivation binding process is described by a Markovian scheme) and a computationally more efficient Langevin approach to model the stochastic Ca(2+) oscillation of single clusters. Statistical properties such as Ca(2+) puff amplitudes, lifetimes, and interpuff intervals are studied with both models and compared with experimental observations. For clusters with tens of channels, a simply decaying amplitude distribution is typically observed at low IP(3) concentration, while a single peak distribution appears at high IP(3) concentration.

Intracellular Ca(2+) pools in neuronal signalling

Different intracellular pools participate in generating Ca(2+) signals in neuronal cells and in shaping their spatio-temporal patterns. They include the endoplasmic reticulum (endowed with different classes of Ca(2+) channels, with distinct functional properties and highly defined expression patterns in the brain), the Golgi apparatus, and the mitochondria. The release of Ca(2+) from intracellular pools plays an important role in controlling processes such as neurite outgrowth, synaptic plasticity, secretion and neurodegeneration.

Intracellular calcium dynamics--sparks of insight

Calcium ions are of key importance in a large number of cellular functions. In the past decade a large variety of cells have been found to show localized increases in the intracellular calcium concentration named calcium sparks. In this brief review, the methodology of detecting calcium sparks by confocal microscopy is summarized. Some of the properties of calcium sparks in muscle (cardiac, skeletal and smooth muscles), neurons, nerve terminals and oocytes aredescribed. Speculations are put forward regarding their possible role in microcontrol of cell function.

Cerebellar neurons lacking complex gangliosides degenerate in the presence of depolarizing levels of potassium

Mice engineered to lack GM2/GD2 synthase (GalNAc-T), with resultant deficit of GM2, GD2, and all gangliotetraose gangliosides, were originally described as showing a relatively normal phenotype with only a slight reduction in nerve conduction. However, a subsequent study showed that similar animals suffer axonal degeneration, myelination defects, and impaired motor coordination. We have examined the behavior of cerebellar granule neurons from these neonatal knockouts in culture and have found evidence of impaired capacity for Ca2+ regulation. These cells showed relatively normal behavior when grown in the presence of physiological or moderately elevated K+ but gradually degenerated in the presence of high K+. This degeneration in depolarizing medium was accompanied by progressive elevation of intracellular calcium and onset of apoptosis, phenomena not observed with normal cells. No differences were detected in cells from normal vs. heterozygous mice. These findings suggest that neurons from GalNAc-T knockout mice are lacking a calcium regulatory mechanism that is modulated by one or more of the deleted gangliosides, and they support the hypothesis that maintenance of calcium homeostasis is one function of complex gangliosides during, and perhaps subsequent to, neuronal development.

Cerebellar neurons lacking complex gangliosides degenerate in the presence of depolarizing levels of potassium

Mice engineered to lack GM2/GD2 synthase (GalNAc-T), with resultant deficit of GM2, GD2, and all gangliotetraose gangliosides, were originally described as showing a relatively normal phenotype with only a slight reduction in nerve conduction. However, a subsequent study showed that similar animals suffer axonal degeneration, myelination defects, and impaired motor coordination. We have examined the behavior of cerebellar granule neurons from these neonatal knockouts in culture and have found evidence of impaired capacity for Ca2+ regulation. These cells showed relatively normal behavior when grown in the presence of physiological or moderately elevated K+ but gradually degenerated in the presence of high K+. This degeneration in depolarizing medium was accompanied by progressive elevation of intracellular calcium and onset of apoptosis, phenomena not observed with normal cells. No differences were detected in cells from normal vs. heterozygous mice. These findings suggest that neurons from GalNAc-T knockout mice are lacking a calcium regulatory mechanism that is modulated by one or more of the deleted gangliosides, and they support the hypothesis that maintenance of calcium homeostasis is one function of complex gangliosides during, and perhaps subsequent to, neuronal development.

Calcium stores in hippocampal synaptic boutons mediate short-term plasticity, store-operated Ca2+ entry, and spontaneous transmitter release

Evoked transmitter release depends upon calcium influx into synaptic boutons, but mechanisms regulating bouton calcium levels and spontaneous transmitter release are obscure. To understand these processes better, we monitored calcium transients in axons and presynaptic terminals of pyramidal neurons in hippocampal slice cultures. Action potentials reliably evoke calcium transients in axons and boutons. Calcium-induced calcium release (CICR) from internal stores contributes to the transients in boutons and to paired-pulse facilitation of EPSPs. Store depletion activates store-operated calcium channels, influencing the frequency of spontaneous transmitter release. Boutons display spontaneous Ca2+ transients; blocking CICR reduces the frequency of these transients and of spontaneous miniature synaptic events. Thus, spontaneous transmitter release is largely calcium mediated, driven by Ca2+ release from internal stores. Bouton store release is important for short-term synaptic plasticity and may also contribute to long-term plasticity.