Nonlinear propagation of agonist-induced cytoplasmic calcium waves in single astrocytes

Exploring poisonous mechanism of honeybee, Apis mellifera ligustica Spinola, caused by pyrethroids

As the important intracellular secondary messengers, calcium channel is the target of many neurotoxic pesticides as calcium homeostasis in the neuroplasm play important role in neuronal functions and behavior in insects. This study investigated the effect of deltamethrin (DM) on calcium channel in the brain nerve cells of adult workers of Apis mellifera ligustica Spinola that were cultured in vitro. The results showed that the intracellular calcium concentration was significantly elevated even with a very low concentration of the DM (3.125×10^-2mg/L). Further testing revealed that T-type voltage-gated calcium channels (VGCCs), except for sodium channels, was one of the target of DM on toxicity of Apis mellifera, while DM has no significant effect on the L-type VGCCs, N-methyl-d-aspartate receptor-gated calcium channels and calcium store. These results suggesting that the DM may act on T-type VGCCs in brain cells of honeybees and result in behavioral abnormalities including swarming, feeding, learning, and acquisition.

Adrenergic stimulation of single rat astrocytes results in distinct temporal changes in intracellular Ca(2+) and cAMP-dependent PKA responses

During the arousal and startle response, locus coeruleus neurons, innervating practically all brain regions, release catecholamine noradrenaline, which reaches neural brain cells, including astrocytes. These glial cells respond to noradrenergic stimulation by simultaneous activation of the α- and β-adrenergic receptors (ARs) in the plasma membrane with increasing cytosolic levels of Ca(2+) and cAMP, respectively. AR-activation controls a myriad of processes in astrocytes including glucose metabolism, gliosignal vesicle homeostasis, gene transcription, cell morphology and antigen-presenting functions, all of which have distinct temporal characteristics. It is known from biochemical studies that Ca(2+) and cAMP signals in astrocytes can interact, however it is presently unclear whether the temporal properties of the two second messengers are time associated upon AR-activation. We used confocal microscopy to study AR agonist-induced intracellular changes in Ca(2+) and cAMP in single cultured cortical rat astrocytes by real-time monitoring of the Ca(2+) indicator Fluo4-AM and the fluorescence resonance energy transfer-based nanosensor A-kinase activity reporter 2 (AKAR2), which reports the activity of cAMP via its downstream effector protein kinase A (PKA). The results revealed that the activation of α1-ARs by phenylephrine triggers periodic (phasic) Ca(2+) oscillations within 10s, while the activation of β-ARs by isoprenaline leads to a ∼10-fold slower tonic rise to a plateau in cAMP/PKA activity devoid of oscillations. Thus the concomitant activation of α- and β-ARs triggers the Ca(2+) and cAMP second messenger systems in astrocytes with distinct temporal properties, which appears to be tailored to regulate downstream effectors in different time domains.

Perisynaptic astroglial processes: dynamic processors of neuronal information

Neuroglial interactions are now recognized as essential to brain functions. Extensive research has sought to understand the modalities of such dialog by focusing on astrocytes, the most abundant glial cell type of the central nervous system. Neuron-astrocyte exchanges occur at multiple levels, at different cellular locations. With regard to information processing, regulations occurring around synapses are of particular interest as synaptic networks are thought to underlie higher brain functions. Astrocytes morphology is tremendously complex in that their processes exceedingly branch out to eventually form multitudinous fine leaflets. The latter extremities have been shown to surround many synapses, forming perisynaptic astrocytic processes, which although recognized as essential to synaptic functioning, are poorly defined elements due to their tiny size. The current review sums up the current knowledge on their molecular and structural properties as well as the functional characteristics making them good candidates for information processing units.

Computational models of neuron-astrocyte interaction in epilepsy

Astrocytes actively shape the dynamics of neurons and neuronal ensembles by affecting several aspects critical to neuronal function, such as regulating synaptic plasticity, modulating neuronal excitability, and maintaining extracellular ion balance. These pathways for astrocyte-neuron interaction can also enhance the information-processing capabilities of brains, but in other circumstances may lead the brain on the road to pathological ruin. In this article, we review the existing computational models of astrocytic involvement in epileptogenesis, focusing on their relevance to existing physiological data.

Calcium Signaling and Gliotransmission in Normal vs. Reactive Astrocytes

A prominent area of neuroscience research over the past 20 years has been the acute modulation of neuronal synaptic activity by Ca²⁺-dependent release of the transmitters ATP, D-serine, and glutamate (called gliotransmitters) by astrocytes. Although the physiological relevance of this mechanism is under debate, emerging evidence suggests that there are critical factors in addition to Ca²⁺ that are required for gliotransmitters to be released from astrocytes. Interestingly, these factors include activated microglia and the proinflammatory cytokine Tumor Necrosis Factor α (TNFα), chemotactic cytokine Stromal cell-Derived Factor-1α (SDF-1α), and inflammatory mediator prostaglandin E2 (PGE₂). Of note, microglial activation and release of inflammatory molecules from activated microglia and reactive astrocytes can occur within minutes of a triggering stimulus. Therefore, activation of astrocytes by inflammatory molecules combined with Ca²⁺ elevations may lead to gliotransmitter release, and be an important step in the early sequence of events contributing to hyperexcitability, excitotoxicity, and neurodegeneration in the damaged or diseased brain. In this review, we will first examine evidence questioning Ca²⁺-dependent gliotransmitter release from astrocytes in healthy brain tissue, followed by a close examination of recent work suggesting that Ca²⁺-dependent gliotransmitter release occurs as an early event in the development of neurological disorders and neuroinflammatory and neurodegenerative diseases.

Calcium signaling in cerebral vasoregulation

The tight coupling of regional neurometabolic activity with synaptic activity and regional cerebral blood perfusion constitutes a single functional unit, described generally as a neurovascular unit. This is central to any discussion of haemodynamic response linked to any neuronal activation. In normal as well as in pathologic conditions, neurons, astrocytes and endothelial cells of the vasculature interact to generate the complex activity-induced cerebral haemodynamic responses, with astrocytes not only partaking in the signaling but actually controlling it in many cases. Neurons and astrocytes have highly integrated signaling mechanisms, yet they form two separate networks. Bidirectional neuron-astrocyte interactions are crucial for the function and survival of the central nervous system. The primary purpose of such regulation is the homeostasis of the brain's microenvironment. In the maintenance of such homeostasis, astrocytic calcium response is a crucial variable in determining neurovascular control. Future work will be directed towards resolving the nature and extent of astrocytic calcium-mediated mechanisms for gene transcription, in modelling neurovascular control, and in determining calcium sensitive imaging assays that can capture disease variables.

Fast calcium waves

Calcium waves are propagated in five main speed ranges which cover a billion-fold range of speeds. We define the fast speed range as 3-30μm/s after correction to a standard temperature of 20°C. Only waves which are not fertilization waves are considered here. 181 such cases are listed here. These are through organisms in all major taxa from cyanobacteria through mammals including human beings except for those through other bacteria, higher plants and fungi. Nearly two-thirds of these speeds lie between 12 and 24μm/s. We argue that their common mechanism in eukaryotes is a reaction-diffusion one involving calcium-induced calcium release, in which calcium waves are propagated along the endoplasmic reticulum. We propose that the gliding movements of some cyanobacteria are driven by fast calcium waves which are propagated along their plasma membranes. Fast calcium waves may drive materials to one end of developing embryos by cellular peristalsis, help coordinate complex cell movements during development and underlie brain injury waves. Moreover, we continue to argue that such waves greatly increase the likelihood that chronic injuries will initiate tumors and cancers before genetic damage occurs. Finally we propose numerous further studies.

Ca2+ spiral waves in a spatially discrete and random medium

It is well known that the spatial distribution of the calcium ion channels in the endoplasmic reticulum is discrete. We study the Ca(2+) spiral pattern formation based on a model in which ion channels are discretely and randomly distributed. Numerical simulations are performed on different types of media with the Ca(2+) release sites uniformly distributed, discretely and uniformly arranged, or discretely and randomly arranged. The comparisons among the different media show that random distribution is necessary for spontaneous initiation of Ca(2+) spiral waves, and the discrete and random distribution is of significance for spiral waves under physiologically reasonable conditions. The period and velocity of spiral waves are also calculated, and they are not prominently changed by varying the type of medium.

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.

What is the role of astrocyte calcium in neurophysiology?

Astrocytes comprise approximately half of the volume of the adult mammalian brain and are the primary neuronal structural and trophic supportive elements. Astrocytes are organized into distinct nonoverlapping domains and extend elaborate and dense fine processes that interact intimately with synapses and cerebrovasculature. The recognition in the mid 1990s that astrocytes undergo elevations in intracellular calcium concentration following activation of G protein-coupled receptors by synaptically released neurotransmitters demonstrated not only that astrocytes display a form of excitability but also that astrocytes may be active participants in brain information processing. The roles that astrocytic calcium elevations play in neurophysiology and especially in modulation of neuronal activity have been intensely researched in recent years. This review will summarize the current understanding of the function of astrocytic calcium signaling in neurophysiological processes and discuss areas where the role of astrocytes remains controversial and will therefore benefit from further study.

Calcium puffs in Xenopus oocytes

The second messenger inositol 1,4,5-trisphosphate (InsP3) functions in large part by liberating calcium ions from intracellular stores. This release process is highly non-linear and shows a regenerative characteristic that allows production of all-or-none calcium spikes which propagate as waves. However, at low concentrations of InsP3 an additional mode of calcium liberation is seen in Xenopus oocytes, transient 'puffs' of cytosolic calcium that last for a few hundred milliseconds and are restricted to within a few micrometres. Puffs are generally of similar size and the amount of calcium released (about 3 x 10(-18) mol) suggests that they arise through the concerted opening of several InsP3-gated calcium release channels. Puff sites are present at a density of about one per 30 microns 2 in the animal hemisphere of the oocyte. Each site functions autonomously, producing puffs at largely random intervals. We conclude that calcium puffs represent 'quantal' units of InsP3-evoked calcium liberation, which may result from local regenerative feedback by cytosolic calcium ions at functionally discrete release sites.

Astrocyte signaling in the presence of spatial inhomogeneities

Astrocytes, a special type of glial cells, were considered to have just a supporting role in information processing in the brain. However, several recent studies have shown that they can be chemically stimulated by various neurotransmitters, such as ATP, and can generate Ca2+ and ATP waves, which can propagate over many cell lengths before being blocked. Although pathological conditions, such as spreading depression and epilepsy, have been linked to abnormal wave propagation in astrocytic cellular networks, a quantitative understanding of the underlying characteristics is still lacking. Astrocytic cellular networks are inhomogeneous, in the sense that the domain they occupy contains passive regions or gaps, which are unable to support wave propagation. Thus, this work focuses on understanding the complex interplay between single-cell signal transduction, domain inhomogeneity, and the characteristics of wave propagation and blocking in astrocytic cellular networks. The single-cell signal transduction model that was employed accounts for ATP-mediated IP3 production, the subsequent Ca2+ release from the ER, and ATP release into the extracellular space. The model is excitable and thus an infinite range of wave propagation is observed if the domain of propagation is homogeneous. This is not always the case for inhomogeneous domains. To model wave propagation in inhomogeneous astrocytic networks, a reaction-diffusion framework was developed and one-gap as well as multiple-gap cases were simulated using an efficient finite-element algorithm. The minimum gap length that blocks the wave was computed as a function of excitability levels and geometric characteristics of the inhomogeneous network, such as the length of the active regions (cells). Complex transient patterns, such as wave reflection, wave trapping, and generation of echo waves, were also predicted by the model, and their relationship to the geometric characteristics of the network was evaluated. Therefore, the proposed model can help in the formulation of testable hypotheses to explain the observed abnormal wave propagation in pathological situations.

Signaling proteins in raft-like microdomains are essential for Ca2+ wave propagation in glial cells

The hypothesis that calcium signaling proteins segregate into lipid raft-like microdomains was tested in isolated membranes of rat oligodendrocyte progenitor (OP) cells and astrocytes using Triton X-100 solubilization and density gradient centrifugation. Western blot analysis of gradient fractions showed co-localization of caveolin-1 with proteins involved in the Ca2+ signaling cascade. These included agonist receptors, P2Y1, and M1, TRPC1, IP3R2, ryanodine receptor, as well as the G protein Galphaq and Homer. Membranes isolated from agonist-stimulated astrocytes showed an enhanced recruitment of phospholipase C (PLCbeta1), IP3R2 and protein kinase C (PKC-alpha) into lipid raft fractions. IP3R2, TRPC1 and Homer co-immunoprecipitated, suggesting protein-protein interactions. Disruption of rafts by cholesterol depletion using methyl-beta-cyclodextrin (beta-MCD) altered the distribution of caveolin-1 and GM1 to non-raft fractions with higher densities. beta-MCD-induced disruption of rafts inhibited agonist-evoked Ca2+ wave propagation in astrocytes and attenuated wave speeds. These results indicate that in glial cells, Ca2+ signaling proteins might exist in organized membrane microdomains, and these complexes may include proteins from different cellular membrane systems. Such an organization is essential for Ca2+ wave propagation.

Calcium ions and integration in neural circuits

Integration in the nervous system is achieved by signal processing within dynamic functional ensembles formed by highly complex neuronal-glial cellular circuits. The interactions between electrically excitable neuronal networks and electrically non-excitable glial syncytium occur through either chemical transmission, which involves the release of transmitters from presynaptic terminals or from astroglial cells, or via direct intercellular contacts, gap junctions. Calcium ions act as a universal intracellular signalling system, which controls many aspects of neuronal-glial communications. In neurones, calcium signalling events regulate the exocytosis of neurotransmitters and establish the link between excitation of postsynaptic cells and integrative intracellular events, which control synaptic strength, expression of genes and memory function. In glial cells metabotropic receptor mediated release of calcium ions from the intracellular endoplasmic reticulum calcium store provide specific form of glial excitability. Glial calcium signals ultimately result in vesicular secretion of "glio" transmitters, which affect neuronal networks thus closing the glial-neuronal circuits. Cellular signalling through calcium ions therefore can be regarded as a molecular mechanism of integration in the nervous system.

Modeling of ATP-mediated signal transduction and wave propagation in astrocytic cellular networks

Astrocytes, a special type of glial cells, were considered to have supporting role in information processing in the brain. However, several recent studies have shown that they can be chemically stimulated by neurotransmitters and use a form of signaling, in which ATP acts as an extracellular messenger. Pathological conditions, such as spreading depression, have been linked to abnormal range of wave propagation in astrocytic cellular networks. Nevertheless, the underlying intra- and inter-cellular signaling mechanisms remain unclear. Motivated by the above, we constructed a model to understand the relationship between single-cell signal transduction mechanisms and wave propagation and blocking in astrocytic networks. The model incorporates ATP-mediated IP3 production, the subsequent Ca2+ release from the ER through IP3R channels and ATP release into the extracellular space. For the latter, two hypotheses were tested: Ca2+- or IP3-dependent ATP release. In the first case, single astrocytes can exhibit excitable behavior and frequency-encoded oscillations. Homogeneous, one-dimensional astrocytic networks can propagate waves with infinite range, while in two dimensions, spiral waves can be generated. However, in the IP3-dependent ATP release case, the specific coupling of the driver ATP-IP3 system with the driven Ca2+ subsystem leads to one- and two-dimensional wave patterns with finite range of propagation.

Bax affects intracellular Ca2+ stores and induces Ca2+ wave propagation

In the present study, we evaluated proapoptotic protein Bax on mitochondria and Ca2+ homeostasis in primary cultured astrocytes. We found that recombinant Bax (rBax, 10 and 100 ng/ml) induces a loss in mitochondrial membrane potential (Delta Psi m). This effect might be related to the inhibition of respiratory rates and a partial release of cytochrome c, which may change mitochondrial morphology. The loss of Delta Psi m and a selective permeabilization of mitochondrial membranes contribute to the release of Ca2+ from the mitochondria. This was inhibited by cyclosporin A (5 microM) and Ruthenium Red (1 microg/ml), indicating the involvement of mitochondrial Ca2+ transport mechanisms. Bax-induced mitochondrial Ca2+ release evokes Ca2+ waves and wave propagation between cells. Our results show that Bax induces mitochondrial alteration that affects Ca2+ homeostasis and signaling. These changes show that Ca2+ signals might be correlated with the proapoptotic activities of Bax.

Selection of intracellular calcium patterns in a model with clustered Ca2+ release channels

A two-dimensional model is proposed for intracellular Ca2+ waves, which incorporates both the discrete nature of Ca2+ release sites in the endoplasmic reticulum membrane and the stochastic dynamics of the clustered inositol 1,4,5-triphosphate (IP3) receptors. Depending on the Ca2+ diffusion coefficient and concentration of IP3, various spontaneous Ca2+ patterns, such as calcium puffs, local waves, abortive waves, global oscillation, and tide waves, can be observed. We further investigate the speed of the global waves as a function of the IP3 concentration and the Ca2+ diffusion coefficient and under what conditions the spatially averaged Ca2+ response can be described by a simple set of ordinary differential equations.

On the conservation of fast calcium wave speeds

Calcium waves were first seen about 25 years ago as the giant, 10 micro m/s wave or tsunami which crosses the cytoplasm of an activating medaka fish egg [J Cell Biol 76 (1978) 448]. By 1991, reports of such waves with approximately 10 micro m/s velocities through diverse, activating eggs and with approximately 30 micro m/s velocities through diverse, fully active systems had been compiled to form a class of what are now called fast calcium waves [Proc Natl Acad Sci USA 88 (1991) 9883; Bioessays 21 (1999) 657]. This compilation is now updated to include organisms from algae and sponges up to blowflies, squid and men and organizational levels from mammalian brains and hearts as well as chick embryos down to muscle, nerve, epithelial, blood and cancer cells and even cell-free extracts. Plots of these data confirm the narrow, 2-3-fold ranges of fast wave speeds through activating eggs and 3-4-fold ones through fully active systems at a given temperature. This also indicate Q(10)'s of 2.7-fold per 10 degrees C for both activating eggs and for fully activated cells.Speeds through some ultraflat preparations which are a few-fold above the conserved range are attributed to stretch propagated calcium entry (SPCE) rather than calcium-induced calcium release (CICR).

Astrocytes in adult rat brain express type 2 inositol 1,4,5-trisphosphate receptors

Astrocytes respond to neuronal activity by propagating Ca(2+) waves elicited through the inositol 1,4,5-trisphosphate pathway. We have previously shown that wave propagation is supported by specialized Ca(2+) release sites, where a number of proteins, including inositol 1,4,5-trisphosphate receptors (IP(3)R), occur together in patches. The specific IP(3)R isoform expressed by astrocytes in situ in rat brain is unknown. In the present report, we use isoform-specific antibodies to localize immunohistochemically the IP(3)R subtype expressed in astrocytes in rat brain sections. Astrocytes were identified using antibodies against the astrocyte-specific markers, S-100 beta, or GFAP. Dual indirect immunohistochemistry showed that astrocytes in all regions of adult rat brain express only IP(3)R2. High-resolution analysis showed that hippocampal astrocytes are endowed with a highly branched network of processes that bear fine hair-like extensions containing punctate patches of IP(3)R2 staining in intimate contact with synapses. Such an organization is reminiscent of signaling microdomains found in cultured glial cells. Similarly, Bergmann glial cell processes in the cerebellum also contained fine hair-like processes containing IP(3)R2 staining. The IP(3)R2-containing fine terminal branches of astrocyte processes in both brain regions were found juxtaposed to presynaptic terminals containing synaptophysin as well as PSD 95-containing postsynaptic densities. Corpus callosum astrocytes had an elongated morphology with IP(3)R2 studded processes extending along fiber tracts. Our data suggest that PLC-mediated Ca(2+) signaling in astrocytes in rat brain occurs predominantly through IP(3)R2 ion channels. Furthermore, the anatomical arrangement of the terminal astrocytic branches containing IP(3)R2 ensheathing synapses is ideal for supporting glial monitoring of neuronal activity.

Bax translocation to mitochondria subsequent to a rapid loss of mitochondrial membrane potential

Bax, a pro-apoptotic member of the Bcl-2 family, is a cytosolic protein that inserts into mitochondrial membranes upon induction of cell death. Using the green fluorescent protein fused to Bax (GFP-Bax) to quantitate mitochondrial binding in living cells we have investigated the cause of Bax association with mitochondria and the time course relative to endogenous and induced changes in mitochondrial membrane potential (DeltaPsi(m)). We have found that staurosporine (STS) induces a loss in DeltaPsi(m) before GFP-Bax translocation can be measured. The onset of the DeltaPsi(m) loss is followed by a rapid and complete collapse of DeltaPsi(m) which is followed by Bax association with mitochondria. The mitochondria uncoupler FCCP, in the presence of the F(1)-F(0) ATPase inhibitor oligomycin, can trigger Bax translocation to mitochondria suggesting that when ATP levels are maintained a collapse of DeltaPsi(m) induces Bax translocation. Neither FCCP nor oligomycin alone alters Bax location. Bax association with mitochondria is also triggered by inhibitors of the electron transport chain, antimycin and rotenone, compounds that collapse DeltaPsi(m) without inducing rapid ATP hydrolysis that typically occurs with uncouplers such as FCCP. Taken together, our results suggest that alterations in mitochondrial energization associated with apoptosis can initiate Bax docking to mitochondria.

Control of astrocyte Ca(2+) oscillations and waves by oscillating translocation and activation of protein kinase C

BACKGROUND

Glutamate-induced Ca2+ oscillations and waves coordinate astrocyte signaling responses, which in turn regulate neuronal excitability. Recent studies have suggested that the generation of these Ca2+ oscillations requires a negative feedback that involves the activation of conventional protein kinase C (cPKC). Here, we use total internal reflection fluorescence (TIRF) microscopy to investigate if and how periodic plasma membrane translocation of cPKC is used to generate Ca2+ oscillations and waves.

RESULTS

Glutamate stimulation of astrocytes triggered highly localized GFP-PKCgamma plasma membrane translocation events, induced rapid oscillations in GFP-PKCgamma translocation, and generated GFP-PKCgamma translocation waves that propagated across and between cells. These translocation responses were primarily mediated by the Ca2+-sensitive C2 domains of PKCgamma and were driven by localized Ca2+ spikes, by oscillations in Ca2+ concentration, and by propagating Ca(2+) waves, respectively. Interestingly, GFP-conjugated C1 domains from PKCgamma or PKCdelta that have been shown to bind diacylglycerol (DAG) also oscillated between the cytosol and the plasma membrane after glutamate stimulation, suggesting that PKC is repetitively activated by combined oscillating increases in Ca(2+) and DAG concentrations. The expression of C1 domains, which increases the DAG buffering capacity and thereby delays changes in DAG concentrations, led to a marked prolongation of Ca(2+) spikes, suggesting that PKC activation is involved in terminating individual Ca(2+) spikes and waves and in defining the time period between Ca(2+) spikes.

CONCLUSIONS

Our study suggests that cPKCs have a negative feedback role on Ca(2+) oscillations and waves that is mediated by their repetitive activation by oscillating DAG and Ca(2+) concentrations. Periodic translocation and activation of cPKC can be a rapid and markedly localized signaling event that can limit the duration of individual Ca(2+) spikes and waves and can define the Ca(2+) spike and wave frequencies.

Sparks and puffs in oligodendrocyte progenitors: cross talk between ryanodine receptors and inositol trisphosphate receptors

Investigating how calcium release from the endoplasmic reticulum (ER) is triggered and coordinated is crucial to our understanding of how oligodendrocyte progenitor cells (OPs) develop into myelinating cells. Sparks and puffs represent highly localized Ca(2+) release from the ER through ryanodine receptors (RyRs) and inositol trisphosphate receptors (IP(3)Rs), respectively. To study whether sparks or puffs trigger Ca(2+) waves in OPs, we performed rapid high-resolution line scan recordings in fluo-4-loaded OP processes. We found spontaneous and evoked sparks and puffs, and we have identified functional cross talk between IP(3)Rs and RyRs. Local events evoked using the IP(3)-linked agonist methacholine (MeCh) showed significantly different morphology compared with events evoked using the caffeine analog 3,7-dimethyl-1-propargylxanthine (DMPX). Pretreatment with MeCh potentiated DMPX-evoked events, whereas inhibition of RyRs potentiated events evoked by low concentrations of MeCh. Furthermore, activation of IP(3)Rs but not RyRs was critical for Ca(2+) wave initiation. Using immunocytochemistry, we show OPs express the specific Ca(2+) release channel subtypes RyR3 and IP(3)R2 in patches along OP processes. RyRs are coexpressed with IP(3)Rs in some patches, but IP(3)Rs are also found alone. This differential distribution pattern may underlie the differences in local and global Ca(2+) signals mediated by these two receptors. Thus, in OPs, interactions between IP(3)Rs and RyRs determine the spatial and temporal characteristics of calcium signaling, from microdomains to intracellular waves.

Müller cell Ca2+ waves evoked by purinergic receptor agonists in slices of rat retina

We have measured agonist evoked Ca2+ waves in Müller cells in situ within freshly isolated retinal slices. Using an eye cup dye loading procedure we were able to preferentially fill Müller glial cells in retinal slices with calcium green. Fluorescence microscopy revealed that bath perfusion of slices with purinergic agonists elicits Ca2+ waves in Müller cells, which propagate along their processes. These Ca2+ signals were insensitive to tetrodotoxin (TTX, 1.0 microM) pretreatment. Cells were readily identified as Müller cells by their unique morphology and by subsequent immunocytochemical labeling with glial fibrillary acidic protein antibodies. While cells never exhibited spontaneous Ca2+ oscillations, purinoreceptor agonists, ATP, 2 MeSATP, ADP, 2 MeSADP, and adenosine readily elicited Ca2+ waves. These waves persisted in the absence of [Ca2+]o but were abolished by thapsigargin pretreatment, suggesting that the purinergic agonists tested act by releasing Ca2+ from intracellular Ca2+ stores. The rank order of potency of different purines and pyrimidines for inducing Ca2+ signals was 2 MeSATP = 2MeSADP > ADP > ATP >> alphabetameATP = uridine triphosphate (UTP) > uridine diphosphate (UDP). The Ca2+ signals evoked by ATP, ADP, and 2 MeSATP were inhibited by reactive blue (100 microM) and suramin (200 microM), and the adenosine induced signals were abolished only by 3,7-dimethyl-1-propargylxanthine (200 microM) and not by 1,3-dipropyl-8-(2-amino-4-chlorophenyl)-xanthine) or 8-cyclopentyl-1,3-dipropylxanthine at the same concentration. Based on these pharmacological characteristics and the dose-response relationships for ATP, 2 MeSATP, 2 MeSADP, ADP, and adenosine, we concluded that Müller cells express the P1A2 and P2Y1 subtypes of purinoceptors. Analysis of Ca2+ responses showed that, similar to glial cells in culture, wave propagation occurred by regenerative amplification at specialized Ca2+ release sites (wave amplification sites), where the rate of Ca2+ release was significantly enhanced. These data suggest that Müller cells in the retina may participate in signaling, and this may serve as an extra-neuronal signaling pathway.

Elevation of resting mitochondrial membrane potential of neural cells by cyclosporin A, BAPTA-AM, and bcl-2

This study tested the hypothesis that the activity of the mitochondrial membrane permeability transition pore (PTP) affects the resting mitochondrial membrane potential (DeltaPsi) of normal, healthy cells and that the anti-apoptotic gene product Bcl-2 inhibits the basal activity of the PTP. DeltaPsi was measured by both fluorometric and nonfluorometric methods with SY5Y human neuroblastoma cells and with GT1-7 hypothalamic cells and PC12 pheochromocytoma cells in the absence and presence of Bcl-2 gene overexpression. The resting DeltaPsi of Bcl-2 nonexpressing PC12 and wild-type SY5Y cells was increased significantly by the presence of the PTP inhibitor cyclosporin A (CsA) or by intracellular Ca(2+) chelation through exposure to the acetoxymethyl ester of 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM). The DeltaPsi of Bcl-2-overexpressing PC12 cells was larger than that of Bcl-2-negative cells and not significantly increased by CsA or by Ca(2+) chelation. CsA did not present a significant effect on the DeltaPsi monitored in unstressed GT1-7 cells but did inhibit the decrease in DeltaPsi elicited by the addition of t-butyl hydroperoxide, an oxidative inducer of the mitochondrial permeability transition. These results support the hypothesis that an endogenous PTP activity can contribute to lowering the basal DeltaPsi of some cells and that Bcl-2 can regulate the endogenous activity of the mitochondrial PTP.

Mapping of IP(3)-mediated Ca(2+) signals in single human neuroblastoma SH-SY5Y cells: cell volume shaping the Ca(2+) signal

Fast confocal laser-scanning microscopy was used to study spatiotemporal properties of IP(3)-mediated Ca(2+) release signals in human SH-SY5Y neuroblastoma cells. [Ca(2+)](i) increases were not affected by ryanodine (30 microgM) or caffeine (10 mM) and largely insensitive to removal of external Ca(2+), indicating predominance of IP(3)-induced Ca(2+) release. Ca(2+) signals evoked by high concentration (10 microM) of the muscarinic agonist carbachol appeared as self-propagating waves initiating in cell processes. At low carbachol concentrations (500 nM) Ca(2+) changes in most cells displayed striking spatiotemporal heterogeneity. The Ca(2+) response in the cell body was delayed and had a smaller amplitude and a slower rise time than that in processes. Ca(2+) changes in processes either occurred in a homogeneous manner throughout the whole process or were sometimes confined to hot spots. Regional differences in surface-to-volume ratio appear to be critical clues that determine the spatiotemporal pattern of intracellular Ca(2+) release signals.

The local control of cytosolic Ca2+ as a propagator of CNS communication--integration of mitochondrial transport mechanisms and cellular responses

Ca2+ signals propagate in wave form along individual cells of the central nervous system (CNS) and through networks of connected cells of neuronal and multiple glial cell types. In order for wave fronts to convey information, signaling mechanisms are required that allow waves to propagate reproducibly and without decrement in signal strength over long distances. CNS Ca2+ waves are under specific integrated local control, made possible by interactions at local subcellular microdomains between endoplasmic reticulum and mitochondria. Active mitochondria located near the mouth of inositol trisphosphate receptor (InsP3R) channel clusters in glia take up Ca2+, which may prevent a buildup of Ca2+ around the InsP3R channel, thereby decreasing the rate of Ca2+-induced receptor inactivation, and prolonging channel open time. Mitochondria may amplify InsP3-dependent Ca2+ signals by a transient permeability transition in response to Ca2+ uptake into the mitochondrion. Other evidence suggests privileged access into mitochondria for Ca2+ entering neurons by glutamatergic receptor channels. This enables specific signal modulation as the Ca2+ wave is propagated into neurons, such that mitochondria located close to glutamate channels can prolong the neuronal cytosolic response time by successive uptake and release of Ca2+. Disruption of mitochondrial function deregulates the ability of CNS-derived cells to undergo normal Ca2+ signaling and wave propagation.

Microscopic properties of elementary Ca2+ release sites in non-excitable cells

BACKGROUND

Elementary Ca2+ signals, such as 'Ca2+ puffs', that arise from the activation of clusters of inositol 1 ,4,5,-trisphosphate (InsP3) receptors are the building blocks for local and global Ca2+ signalling. We previously found that one, or a few, Ca2+ puff sites within agonist-stimulated cells act as 'pacemakers' to initiate global Ca2+ waves. The factors that distinguish these pacemaker Ca2+ puff sites from the other Ca2+ release sites that simply participate in Ca2+ wave propagation are unknown.

RESULTS

The spatiotemporal properties of Ca2+ puffs were investigated using confocal microscopy of fluo3-loaded HeLa cells. The same pacemaker Ca2+ puff sites were activated during stimulation of cells with different agonists. The majority of agonist-stimulated pacemaker Ca2+ puffs originated in a perinuclear location. The positions of such Ca2+ puff sites were stable for up to 2 hours, and were not affected by disruption of the actin cytoskeleton. A similar perinuclear distribution of Ca2+ puff sites was also observed when InsP3 receptors were directly stimulated with thimerosal or membrane-permeant InsP3 esters. Immunostaining indicated that the perinuclear position of pacemaker Ca2+ puffs was not due to the localised expression of InsP3 receptors.

CONCLUSIONS

The pacemaker Ca2+ puff sites that initiate Ca2+ responses are temporally and spatially stable within cells. These Ca2+ release sites are distinguished from their neighbours by an intrinsically higher InsP3 sensitivity.

Intercellular Ca(2+) waves induce temporally and spatially distinct intracellular Ca(2+) oscillations in glia

Mechanically induced intercellular Ca(2+) waves propagated for approximately 300 microm in primary glial cultures. Following the wave propagation, 34% of the cells displayed Ca(2+) oscillations in a zone 60-120 microm from the stimulated cell. The initiation, frequency, and duration of these Ca(2+) oscillations were dependent on the cells' distance from the wave origin but were not dependent on the cell type nor on the magnitude of the Ca(2+) wave. When an individual cell propagated two sequential intercellular Ca(2+) waves originating from different sites, the characteristics of the Ca(2+) oscillations initiated by each wave were determined by the distance of the cell from the origin of each wave. Each Ca(2+) oscillation commonly occurred as an intracellular Ca(2+) wave that was initiated from a specific site within the cell. The position of the initiation site and the direction of the intracellular Ca(2+) wave were independent of the orientation of the initial intercellular Ca(2+) wave. Because initiation and frequency of Ca(2+) oscillations are dependent on the intracellular inositol trisphosphate concentration ([IP(3)](i)), we propose that the zone of cells displaying Ca(2+) oscillations is determined by an intercellular gradient of [IP(3)](i), established by the diffusion of IP(3) through gap junctions during the propagation of the intercellular Ca(2+) wave. Exposure to acetylcholine, a muscarinic agonist that initiates IP(3) production, shifted the zone of oscillating cells about 45 microm farther away from the origin of the mechanically induced wave. These findings indicate that a glial syncytium can resolve information provided by a local Ca(2+) wave into a distinct spatial and temporal pattern of Ca(2+) oscillations.

Permeability transition pore regulates both mitochondrial membrane potential and agonist-evoked Ca2+ signals in oligodendrocyte progenitors

In this study, we investigated the importance of mitochondrial permeability transition pore (PTP) in agonist-evoked cytosolic Ca2+ ([Ca2+]c) signals in oligodendrocyte progenitor cells (OP cells). We measured transmembrane potential across the mitochondrial inner membrane (delta psi m) and [Ca2+]c in the immediate vicinity simultaneously using tetramethylrhodamine ethyl ester (TMRE) and calcium green respectively. Stimulation of OP cells with methacholine evoked robust [Ca2+]c signals in approximately 80% of cells which were either oscillatory or showed a peak followed by a plateau. Elevations in [Ca2+]c induced by supramaximal concentrations of the agonist (> 200 microM) were accompanied by changes in delta psi m in 33-42% of the total mitochondria investigated. The mitochondria that responded either depolarized (26-29%), hyperpolarized (7-13%) or showed no change (58-67%). Thus, of the responsive mitochondria, most (70%) depolarized during agonist-evoked [Ca2+]c signals. Blockade of PTP with cyclosporin A (CSA) reduced the number of mitochondria that depolarized with a corresponding increase in the number that hyperpolarized. In addition, CSA or its analogue methyl valine-4- CSA (MeVal-CSA), reduced the frequency of agonist-evoked global [Ca2+]c oscillations. In resting cells, CSA (63%) and MeVal-CSA (77%) hyperpolarized a majority of the mitochondria suggesting that PTP is constitutively active and may show flickering openings. Such hyperpolarizations were not mimicked by either cyclosporine H or verapamil and were inhibited by Ru360, which blocks the mitochondrial uniporter. This observation suggested that in resting cells, Ca2+ ions might redistribute between cytosol and mitochondrial matrix through the uniporter and the PTP. Taken together, these data suggest that PTP may play an important role in regulating delta psi m and local [Ca2+]c signals during agonist stimulation in OP cells.

Calcium signalling in glial cells

Calcium signals are the universal way of glial responses to the various types of stimulation. Glial cells express numerous receptors and ion channels linked to the generation of complex cytoplasmic calcium responses. The glial calcium signals are able to propagate within glial cells and to create a spreading intercellular Ca2+ wave which allow information exchange within the glial networks. These propagating Ca2+ waves are primarily mediated by intracellular excitable media formed by intracellular calcium storage organelles. The glial calcium signals could be evoked by neuronal activity and vice versa they may initiate electrical and Ca2+ responses in adjacent neurones. Thus glial calcium signals could integrate glial and neuronal compartments being therefore involved in the information processing in the brain.

Images of Ca2+ flux in astrocytes: evidence for spatially distinct sites of Ca2+ release and uptake

In this study, we have developed a mathematical method to derive the Ca2+ fluxes underlying agonist-evoked Ca2+ waves in cultured rat cortical astrocytes. Astrocytes were stimulated with norepinephrine (100 nM) to evoke Ca2+ waves, which were recorded by measuring Fluo-3 fluorescence changes with high spatial and temporal resolution. Normalized fluorescence (delta F/F) was analyzed in discrete cellular spaces in a series of successive slices along the length of the cell. From these data, Ca2+ flux was then calculated using a one dimensional reaction-diffusion equation which utilizes the temporal and spatial derivatives of the fluorescence data and the diffusion coefficient of Ca2+ in the cytosol. This method identified distinct sites of positive flux (Ca2+ release into the cytosol) and of negative flux (Ca2+ removal from cytosol) and showed that in astrocytes, sites of Ca2+ release from stores regularly alternate with sites of Ca2+ removal from the cytosol. Cross correlation analysis of the two distribution patterns gave positive correlation at 2 microns out of phase and a negative correlation in phase. Thapsigargin-induced Ca2+ waves were analyzed to determine if the negative flux was due to Ca2+ uptake via thapsigargin-sensitive Ca2+ pumps. Negative flux sites were still found under these conditions, suggesting that multiple mechanisms of Ca2+ removal from the cytosol may contribute to negative flux sites. This method of calculation of flux may serve as a means to describe the distribution of functional ion channels and pumps participating in cellular Ca2+ signalling.

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.

Characterization of ryanodine receptors in oligodendrocytes, type 2 astrocytes, and O-2A progenitors

In this study we have investigated the expression of ryanodine receptors (RyRs), and the ability of caffeine to evoke RyR-mediated elevation of intracellular Ca2+ levels ([Ca2+]i) in glial cells of the oligodendrocyte/type 2 astrocyte lineage. Immunocytochemistry with specific antibodies identified ryanodine receptors in cultured oligodendrocytes, type 2 astrocytes, and O-2A progenitor cells, at high levels in the perinuclear region and in a variegated pattern along processes. Glia acutely isolated from rat brain and in aldehydefixed sections of cortex were similarly found to express RyRs. Caffeine (5-50 mM) caused an increase in [Ca2+]i in most cultured type 2 astrocytes and in 50% of oligodendrocytes. Responses elicited by caffeine were inhibited by pretreatment with ryanodine (10 microM) or thapsigargin (1 microM), and the peak response was unaffected by removal of [Ca2+]o. O-2A progenitor cells, in contrast, were largely unresponsive to caffeine treatment. Pretreatment with kainate (200 microM) to activate Ca2+ entry increased the magnitude of caffeine-evoked [Ca2+]i elevations in type 2 astrocytes and oligodendrocytes, and caused caffeine to activate responses in a significant proportion of previously non-responding O-2A progenitors. In both type 2 astrocytes and oligodendrocytes, caffeine evoked Ca2+ changes which propagated as wavefronts from several initiation sites. These wave amplification sites were characterized by significantly higher local Ca2+ release kinetics. Our results indicate that several glial cell types express RyRs, and that their functionality differs within different cell types of the oligodendrocyte lineage. In addition, ionotropic glutamate receptor activation fills the caffeine-sensitive Ca2+ stores in these cells.

Neurotransmitter-induced novel modulation of a nonselective cation channel by a cAMP-dependent mechanism in rat pineal cells

In the rat, circadian rhythm in melatonin is regulated by noradrenergic and neuropeptide inputs to the pineal via adenosine 3',5'-cyclic monophosphate (cAMP)- and Ca2+-dependent mechanisms. We have identified a large conductance (170 pS), voltage-dependent, nonselective cation channel on rat pineal cells in culture that shows a novel mode of modulation by cAMP. Pituitary adenylate cyclase activating peptide (PACAP), norepinephrine, or 8-Br-cAMP increase channel open probability (Po) with a hyperpolarizing shift in voltage dependence such that the channel becomes active at resting membrane potentials. The increase in Po was accompanied by a change in current rectification properties such that the channel was transformed from being inactive at rest to an inwardly rectifying cation conductance in the presence of agonist, which depolarizes the cell. This channel is calcium insensitive, is blocked by Cs+, and shows a permeability sequence: K+ > Na+ >/= NH+4 > Li+. The data suggest that PACAP and norepinephrine acting through a cAMP-dependent mechanism modulate this nonselective cation channel, resulting in a slow onset depolarization that may be important in regulation of pineal cell excitability.

Mitochondrial Ca2+ uptake and release influence metabotropic and ionotropic cytosolic Ca2+ responses in rat oligodendrocyte progenitors

1. Many physiologically important activities of oligodendrocyte progenitor cells (O-2A cells), including proliferation, migration and differentiation, are regulated by cytosolic Ca2+ signals. However, little is known concerning the mechanisms of Ca2+ signalling in this cell type. We have studied the interactions between Ca2+ entry, Ca2+ release from endoplasmic reticulum and Ca2+ regulation by mitochondria in influencing cytosolic Ca2+ responses in O-2A cells. 2. Methacholine (MCh; 100 microM) activated Ca2+ waves that propagated from several initiation sites along O-2A processes. 3. During a Ca2+ wave evoked by MCh, mitochondrial membrane potential was often either depolarized (21 % of mitochondria) or hyperpolarized (20 % of mitochondria), as measured by changes in the fluorescence of 5,5',6,6'-tetrachloro-1,1',3, 3'-tetraethylbenzimidazole carbocyanine iodide (JC-1). 4. Stimulation with kainate (100 microM) evoked a slowly rising, sustained cytosolic Ca2+ elevation in O-2A cells. This also, in some cases, resulted in either a depolarization (15 % of mitochondria) or hyperpolarization (12 % of mitochondria) of mitochondrial membrane potential. 5. Simultaneous measurement of cytosolic (fluo-3 AM) and mitochondrial (rhod-2 AM) Ca2+ responses revealed that Ca2+ elevations in the cytosol evoked by either MCh or kainate were translated into long-lasting Ca2+ elevations in subpopulations of mitochondria. In some mitochondria, Ca2+ signals appeared to activate Ca2+ release into the cytosol. 6. Inhibition of the mitochondrial Na+-Ca2+ exchanger by CGP-37157 (25 microM) decreased kainate Ca2+ response amplitude and increased the rate of return of the response to basal Ca2+ levels. 7. Thus, both ionotropic and metabotropic stimulation evoke changes in mitochondrial membrane potential and Ca2+ levels in O-2A cells. Ca2+ uptake into some mitochondria is activated by Ca2+ entry into cells or release from stores. Mitochondrial Ca2+ release appears to play a key role in shaping kainate-evoked Ca2+ responses.

Thapsigargin and receptor-mediated activation of Drosophila TRPL channels stably expressed in a Drosophila S2 cell line

The Drosophila melanogaster genes, transient receptor potential (trp) and transient receptor potential-like (trpl) encode putative plasma membrane cation channels TRP and TRPL, respectively. We have stably co-expressed Drosophila TRPL with a Drosophila muscarinic acetylcholine receptor (DM1) in a Drosophila cell line (S2 cells). Basal Ca2+ levels measured using Fura-2/AM in unstimulated S2-DM1-TRPL cells were low and indistinguishable from untransfected cells, indicating that the TRPL channels were not constitutively active in this expression system. Activation of DM1 receptor in S2-DM1-TRPL cells by 100 microM carbamylcholine induced Ca2+ release from an intracellular Ca2+ pool followed by a Gd(3+)-insensitive Ca2+ influx. Pretreatment of S2-DM1-TRPL cells with 10 microM atropine abolished Gd(3+)-insensitive Ca2+ influx triggered by carbamylcholine, but the response was not blocked by prior incubation with pertussis toxin. TRPL channels could also be reliably activated by bath application of 1 microM thapsigargin for 10 min or 100 nM thapsigargin for 60 min in Ca(2+)-free solution. In some cells, TRPL channels activated by thapsigargin could further be activated by carbamylcholine. The findings suggest that, when stably expressed in the S2 cell line, TRPL may be regulated by two distinct mechanisms: (i) store depletion; and (ii) stimulation of DM1 receptor via pertussis-toxin insensitive G-protein (or the subsequent activation of PLC), but without further requirement for Ca2+ release.

Glial calcium: homeostasis and signaling function

Glial cells respond to various electrical, mechanical, and chemical stimuli, including neurotransmitters, neuromodulators, and hormones, with an increase in intracellular Ca2+ concentration ([Ca2+]i). The increases exhibit a variety of temporal and spatial patterns. These [Ca2+]i responses result from the coordinated activity of a number of molecular cascades responsible for Ca2+ movement into or out of the cytoplasm either by way of the extracellular space or intracellular stores. Transplasmalemmal Ca2+ movements may be controlled by several types of voltage- and ligand-gated Ca(2+)-permeable channels as well as Ca2+ pumps and a Na+/Ca2+ exchanger. In addition, glial cells express various metabotropic receptors coupled to intracellular Ca2+ stores through the intracellular messenger inositol 1,4,5-triphosphate. The interplay of different molecular cascades enables the development of agonist-specific patterns of Ca2+ responses. Such agonist specificity may provide a means for intracellular and intercellular information coding. Calcium signals can traverse gap junctions between glial cells without decrement. These waves can serve as a substrate for integration of glial activity. By controlling gap junction conductance, Ca2+ waves may define the limits of functional glial networks. Neuronal activity can trigger [Ca2+]i signals in apposed glial cells, and moreover, there is some evidence that glial [Ca2+]i waves can affect neurons. Glial Ca2+ signaling can be regarded as a form of glial excitability.

Calcium waves in a model with a random spatially discrete distribution of Ca2+ release sites

We study the propagation of intracellular calcium waves in a model that features Ca2+ release from discrete sites in the endoplasmic reticulum membrane and random spatial distribution of these sites. The results of our simulations qualitatively reproduce the experimentally observed behavior of the waves. When the level of the channel activator inositol trisphosphate is low, the wave undergoes fragmentation and eventually vanishes at a finite distance from the region of initiation, a phenomenon we refer to as an abortive wave. With increasing activator concentration, the mean distance of propagation increases. Above a critical level of activator, the wave becomes stable. We show that the heterogeneous distribution of Ca2+ channels is the cause of this phenomenon.

High density distribution of endoplasmic reticulum proteins and mitochondria at specialized Ca2+ release sites in oligodendrocyte processes

In oligodendrocyte processes, methacholine-evoked Ca2+ waves propagate via regions of specialized Ca2+ release kinetics (wave amplification sites) at which the amplitude and rate of rise of local Ca2+ signals are markedly higher than in surrounding areas (Simpson, P. B., and Russell, J. T. (1996) J. Biol. Chem. 271, 33493-33501). In the present study we have examined the effects of other phosphoinositide-coupled agonists on Ca2+ in these cells, and the structural specializations underlying regenerative wave amplification sites. Both bradykinin and norepinephrine evoke Ca2+ waves, which initiate at the same loci and propagate through the cell body and multiple processes via identical wave amplification sites. Antibodies against type 2 inositol 1,4,5-trisphosphate receptors (InsP3R2) and calreticulin identify expression of these proteins in oligodendrocyte membranes in Western blots. Immunocytochemistry followed by high resolution fluorescence microscopy revealed that both InsP3R2 and calreticulin are expressed in high intensity patches along processes. Cross-correlation analysis of the profiles of local Ca2+ release kinetics during a Ca2+ wave and immunofluorescence for these proteins along cellular processes showed that the domains of high endoplasmic reticulum protein expression correspond closely to wave amplification sites. Staining cells with the mitochondrial dye, MitoTracker(R), showed that mitochondria are only found in intimate association with these sites possessing high density endoplasmic reticulum proteins, and they remain in the same locations over relatively long periods of time. It appears, therefore, that multiple specializations are found at domains of elevated Ca2+ release in oligodendrocyte processes, including high levels of calreticulin, InsP3R2 Ca2+ release channels, and mitochondria.

Role of sarcoplasmic/endoplasmic-reticulum Ca2+-ATPases in mediating Ca2+ waves and local Ca2+-release microdomains in cultured glia

We have characterized the sarcoplasmic-endoplasmic reticulum Ca2+-ATPase (SERCA) pumps in cultured rat cortical type-1 astrocytes, type-2 astrocytes and oligodendrocytes. Perfusion with 10 microM cyclopiazonic acid (CPA) or 1 microM thapsigargin evoked a large and persistent elevation in cytosolic [Ca2+] in normal Ca2+-containing medium and a small and transient increase in nominally Ca2+-free medium. Subtraction of the response in Ca2+-free medium from that in the control revealed a slow-onset Ca2+-entry response to SERCA inhibition, which began after most of the store depletion had occurred. Thapsigargin- and CPA-induced responses propagated as Ca2+ waves, which began in several distinct cellular sites and travelled throughout the cell and through nearby cells, in confluent cultures. Propagation was supported by specialized Ca2+-release sites where the amplitude of the response was significantly higher and the rate of rise steeper. Such higher Ca2+-release kinetics were observed at these sites during Ins(1,4,5)P3-mediated Ca2+ waves in the same cells. Fluorescently tagged thapsigargin labelled SERCA pumps throughout glial cell bodies and processes. In oligodendrocyte processes, multiple domains with elevated SERCA staining were always associated with mitochondria. Our results are consistent with a model in which only a single Ca2+ store, expressing Ins(1,4,5)P3 receptors and SERCAs sensitive to both thapsigargin and CPA, is present in rat cortical glia, and indicate that inhibition of SERCA activates both Ca2+ release as a wavefront and Ca2+ entry via store-operated channels. The spatial relationship between SERCAs and mitochondria is likely to be important for regulating microdomains of elevated Ca2+-release kinetics.

Neurotransmitter- and growth factor-induced cAMP response element binding protein phosphorylation in glial cell progenitors: role of calcium ions, protein kinase C, and mitogen-activated protein kinase/ribosomal S6 kinase pathway

To understand how extracellular signals may produce long-term effects in neural cells, we have analyzed the mechanism by which neurotransmitters and growth factors induce phosphorylation of the transcription factor cAMP response element binding protein (CREB) in cortical oligodendrocyte progenitor (OP) cells. Activation of glutamate receptor channels by kainate, as well as stimulation of G-protein-coupled cholinergic receptors by carbachol and tyrosine kinase receptors by basic fibroblast growth factor (bFGF), rapidly leads to mitogen-activated protein kinase (MAPK) phosphorylation and ribosomal S6 kinase (RSK) activation. Kainate and carbachol activation of the MAPK pathway requires extracellular calcium influx and is accompanied by protein kinase C (PKC) induction, with no significant increase in GTP binding to Ras. Conversely, growth factor-stimulated MAPK phosphorylation is independent of extracellular calcium and is accompanied by Ras activation. Both basal and stimulated MAPK activity in OP cells are influenced by cytoplasmic calcium levels, as shown by their sensitivity to the calcium chelator bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid. The kinetics of CREB phosphorylation in response to the various agonists corresponds to that of MAPK activation. Moreover, CREB phosphorylation and MAPK activation are similarly affected by calcium ions. The MEK inhibitor PD 098059, which selectively prevents activation of the MAPK pathway, strongly reduces induction of CREB phosphorylation by kainate, carbachol, bFGF, and the phorbol ester TPA. We propose that in OPs the MAPK/RSK pathway mediates CREB phosphorylation in response to calcium influx, PKC activation, and growth factor stimulation.

Elementary and global aspects of calcium signalling

Images

Mitochondria support inositol 1,4,5-trisphosphate-mediated Ca2+ waves in cultured oligodendrocytes

We have examined the spatial and temporal nature of Ca2+ signals activated via the phosphoinositide pathway in oligodendrocytes and the cellular specializations underlying oligodendrocyte Ca2+ response characteristics. Cultured cortical oligodendrocytes were incubated with fluo 3 or fura 2, and digital video fluorescence microscopy was used to study the effect of methacholine on [Ca2+]i. Single peaks, oscillations, and steady-state plateau [Ca2+]i elevations were evoked by increasing agonist concentration. The peaks and oscillations were found to be Ca2+ wave fronts, which propagate via distinct amplification regions in the cell where the kinetics of Ca2+ release (amplitude and rate of rise of response) are elevated. Staining with 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolecarbocyanine++ + iodide (JC-1) and 3,3'-dihexyloxacarbocyanine iodide revealed that mitochondria are found in groups of three or more in oligodendrocyte processes and that the groups are distributed with considerable distance separating them. Cross-correlation analysis showed a high degree of correlation between sites where mitochondria are present and peaks in the amplitude and rate of rise of the Ca2+ response. Intramitochondrial Ca2+ concentration, measured using rhod 2, increased upon treatment with methacholine. Methacholine also evoked a rapid change in mitochondrial membrane potential as measured by the J-aggregate fluorescence of JC-1. Pretreatment with the mitochondrial inhibitors carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (1 microM, 2 min) or antimycin (2 microg/ml, 2 min) altered the methacholine-evoked Ca2+ response in most cells studied, responses being either markedly potentiated or inhibited. The results of this study demonstrate that stimulation of phosphoinositide-coupled muscarinic acetylcholinoceptors activates propagating Ca2+ wave fronts in oligodendrocytes and that the characteristics of these waves are dependent on mitochondrial location and function.