Reduced IP3 sensitivity of IP3 receptor in Purkinje neurons

Purkinje cell models: past, present and future

The investigation of the dynamics of Purkinje cell (PC) activity is crucial to unravel the role of the cerebellum in motor control, learning and cognitive processes. Within the cerebellar cortex (CC), these neurons receive all the incoming sensory and motor information, transform it and generate the entire cerebellar output. The relatively homogenous and repetitive structure of the CC, common to all vertebrate species, suggests a single computation mechanism shared across all PCs. While PC models have been developed since the 70's, a comprehensive review of contemporary models is currently lacking. Here, we provide an overview of PC models, ranging from the ones focused on single cell intracellular PC dynamics, through complex models which include synaptic and extrasynaptic inputs. We review how PC models can reproduce physiological activity of the neuron, including firing patterns, current and multistable dynamics, plateau potentials, calcium signaling, intrinsic and synaptic plasticity and input/output computations. We consider models focusing both on somatic and on dendritic computations. Our review provides a critical performance analysis of PC models with respect to known physiological data. We expect our synthesis to be useful in guiding future development of computational models that capture real-life PC dynamics in the context of cerebellar computations.

mGluR1 signaling in cerebellar Purkinje cells: Subcellular organization and involvement in cerebellar function and disease

The cerebellum is essential for the control, coordination, and learning of movements, and for certain aspects of cognitive function. Purkinje cells are the sole output neurons in the cerebellar cortex and therefore play crucial roles in the diverse functions of the cerebellum. The type 1 metabotropic glutamate receptor (mGluR1) is prominently enriched in Purkinje cells and triggers downstream signaling pathways that are required for functional and structural plasticity, and for synaptic responses. To understand how mGluR1 contributes to cerebellar functions, it is important to consider not only the operational properties of this receptor, but also its spatial organization and the molecular interactions that enable its proper functioning. In this review, we highlight how mGluR1 and its related signaling molecules are organized into tightly coupled microdomains to fulfill physiological functions. We also describe emerging evidence that altered mGluR1 signaling in Purkinje cells underlies cerebellar dysfunction in ataxias of human patients and mouse models.

Visualization of Ca2+ Filling Mechanisms upon Synaptic Inputs in the Endoplasmic Reticulum of Cerebellar Purkinje Cells

The endoplasmic reticulum (ER) plays crucial roles in intracellular Ca(2+) signaling, serving as both a source and sink of Ca(2+), and regulating a variety of physiological and pathophysiological events in neurons in the brain. However, spatiotemporal Ca(2+) dynamics within the ER in central neurons remain to be characterized. In this study, we visualized synaptic activity-dependent ER Ca(2+) dynamics in mouse cerebellar Purkinje cells (PCs) using an ER-targeted genetically encoded Ca(2+) indicator, G-CEPIA1er. We used brief parallel fiber stimulation to induce a local decrease in the ER luminal Ca(2+) concentration ([Ca(2+)]ER) in dendrites and spines. In this experimental system, the recovery of [Ca(2+)]ER takes several seconds, and recovery half-time depends on the extent of ER Ca(2+) depletion. By combining imaging analysis and numerical simulation, we show that the intraluminal diffusion of Ca(2+), rather than Ca(2+) reuptake, is the dominant mechanism for the replenishment of the local [Ca(2+)]ER depletion immediately following the stimulation. In spines, the ER filled almost simultaneously with parent dendrites, suggesting that the ER within the spine neck does not represent a significant barrier to Ca(2+) diffusion. Furthermore, we found that repetitive climbing fiber stimulation, which induces cytosolic Ca(2+) spikes in PCs, cumulatively increased [Ca(2+)]ER. These results indicate that the neuronal ER functions both as an intracellular tunnel to redistribute stored Ca(2+) within the neurons, and as a leaky integrator of Ca(2+) spike-inducing synaptic inputs.

Comparison of models for IP3 receptor kinetics using stochastic simulations

Inositol 1,4,5-trisphosphate receptor (IP₃R) is a ubiquitous intracellular calcium (Ca²⁺) channel which has a major role in controlling Ca²⁺ levels in neurons. A variety of computational models have been developed to describe the kinetic function of IP₃R under different conditions. In the field of computational neuroscience, it is of great interest to apply the existing models of IP₃R when modeling local Ca²⁺ transients in dendrites or overall Ca²⁺ dynamics in large neuronal models. The goal of this study was to evaluate existing IP₃R models, based on electrophysiological data. This was done in order to be able to suggest suitable models for neuronal modeling. Altogether four models (Othmer and Tang, 1993; Dawson etal., 2003; Fraiman and Dawson, 2004; Doi etal., 2005) were selected for a more detailed comparison. The selection was based on the computational efficiency of the models and the type of experimental data that was used in developing the model. The kinetics of all four models were simulated by stochastic means, using the simulation software STEPS, which implements the Gillespie stochastic simulation algorithm. The results show major differences in the statistical properties of model functionality. Of the four compared models, the one by Fraiman and Dawson (2004) proved most satisfactory in producing the specific features of experimental findings reported in literature. To our knowledge, the present study is the first detailed evaluation of IP₃R models using stochastic simulation methods, thus providing an important setting for constructing a new, realistic model of IP₃R channel kinetics for compartmental modeling of neuronal functions. We conclude that the kinetics of IP₃R with different concentrations of Ca²⁺ and IP₃ should be more carefully addressed when new models for IP₃R are developed.

Computational analysis of calcium signaling and membrane electrophysiology in cerebellar Purkinje neurons associated with ataxia

Background

Mutations in the smooth endoplasmic reticulum (sER) calcium channel Inositol Trisphosphate Receptor type 1 (IP3R1) in humans with the motor function coordination disorders Spinocerebellar Ataxia Types 15 and 16 (SCA15/16) and in a corresponding mouse model, the IP3R1^{delta18/delta18} mice, lead to reduced IP3R1 levels. We posit that increasing IP3R1 sensitivity to IP3 in ataxias with reduced IP3R1 could restore normal calcium response. On the other hand, in mouse models of the human polyglutamine (polyQ) ataxias, SCA2, and SCA3, the primary finding appears to be hyperactive IP3R1-mediated calcium release. It has been suggested that the polyQ SCA1 mice may also show hyperactive IP3R1. Yet, SCA1 mice show downregulated gene expression of IP3R1, Homer, metabotropic glutamate receptor (mGluR), smooth endoplasmic reticulum Ca-ATP-ase (SERCA), calbindin, parvalbumin, and other calcium signaling proteins.

Results

We create a computational model of pathological alterations in calcium signaling in cerebellar Purkinje neurons to investigate several forms of spinocerebellar ataxia associated with changes in the abundance, sensitivity, or activity of the calcium channel IP3R1. We find that increasing IP3R1 sensitivity to IP3 in computational models of SCA15/16 can restore normal calcium response if IP3R1 abundance is not too low. The studied range in IP3R1 levels reflects variability found in human and mouse ataxic models. Further, the required fold increases in sensitivity are within experimental ranges from experiments that use IP3R1 phosphorylation status to adjust its sensitivity to IP3. Results from our simulations of polyglutamine SCAs suggest that downregulation of some calcium signaling proteins may be partially compensatory. However, the downregulation of calcium buffer proteins observed in the SCA1 mice may contribute to pathology. Finally, our model suggests that the calcium-activated voltage-gated potassium channels may provide an important link between calcium metabolism and membrane potential in Purkinje cell function.

Conclusion

Thus, we have established an initial platform for computational evaluation and prediction of ataxia pathophysiology. Specifically, the model has been used to investigate SCA15/16, SCA1, SCA2, and SCA3. Results suggest that experimental studies treating mouse models of any of these ataxias with appropriately chosen peptides resembling the C-terminal of IP3R1 could adjust receptor sensitivity, and thereby modulate calcium release and normalize IP3 response. In addition, the model supports the hypothesis of IP3R1 supersensitivity in SCA1.

Simulation of postsynaptic glutamate receptors reveals critical features of glutamatergic transmission

Activation of several subtypes of glutamate receptors contributes to changes in postsynaptic calcium concentration at hippocampal synapses, resulting in various types of changes in synaptic strength. Thus, while activation of NMDA receptors has been shown to be critical for long-term potentiation (LTP) and long term depression (LTD) of synaptic transmission, activation of metabotropic glutamate receptors (mGluRs) has been linked to either LTP or LTD. While it is generally admitted that dynamic changes in postsynaptic calcium concentration represent the critical elements to determine the direction and amplitude of the changes in synaptic strength, it has been difficult to quantitatively estimate the relative contribution of the different types of glutamate receptors to these changes under different experimental conditions. Here we present a detailed model of a postsynaptic glutamatergic synapse that incorporates ionotropic and mGluR type I receptors, and we use this model to determine the role of the different receptors to the dynamics of postsynaptic calcium with different patterns of presynaptic activation. Our modeling framework includes glutamate vesicular release and diffusion in the cleft and a glutamate transporter that modulates extracellular glutamate concentration. Our results indicate that the contribution of mGluRs to changes in postsynaptic calcium concentration is minimal under basal stimulation conditions and becomes apparent only at high frequency of stimulation. Furthermore, the location of mGluRs in the postsynaptic membrane is also a critical factor, as activation of distant receptors contributes significantly less to calcium dynamics than more centrally located ones. These results confirm the important role of glutamate transporters and of the localization of mGluRs in postsynaptic sites in their signaling properties, and further strengthen the notion that mGluR activation significantly contributes to postsynaptic calcium dynamics only following high-frequency stimulation. They also provide a new tool to analyze the interactions between metabotropic and ionotropic glutamate receptors.

mGluR1/TRPC3-mediated Synaptic Transmission and Calcium Signaling in Mammalian Central Neurons

Metabotropic glutamate receptors type 1 (mGluR1s) are required for a normal function of the mammalian brain. They are particularly important for synaptic signaling and plasticity in the cerebellum. Unlike ionotropic glutamate receptors that mediate rapid synaptic transmission, mGluR1s produce in cerebellar Purkinje cells a complex postsynaptic response consisting of two distinct signal components, namely a local dendritic calcium signal and a slow excitatory postsynaptic potential. The basic mechanisms underlying these synaptic responses were clarified in recent years. First, the work of several groups established that the dendritic calcium signal results from IP(3) receptor-mediated calcium release from internal stores. Second, it was recently found that mGluR1-mediated slow excitatory postsynaptic potentials are mediated by the transient receptor potential channel TRPC3. This surprising finding established TRPC3 as a novel postsynaptic channel for glutamatergic synaptic transmission.

Imaging of Ca2+ and related signaling molecules and investigation of their functions in the brain

Intracellular Ca(2+) signaling, and related mechanisms involving inositol 1,4,5-trisphosphate (IP(3)), nitric oxide, and the excitatory neurotransmitter glutamate, play a major role in the regulation of cellular function in the brain. Due to the complex morphology of central neurons, the correct spatiotemporal distribution of signaling molecules is essential. Thus, imaging studies have been particularly useful in elucidating the functions of these signaling molecules. The advancement of imaging methods, together with the development of a new method for the specific inhibition of intracellular IP(3) signaling, have made it possible to identify pathways that are regulated by Ca(2+) signals in the brain, including Ca(2+)-dependent synaptic maintenance and glial cell-dependent neurite growth. Further investigation of Ca(2+)-related signaling is expected to increase our understanding of brain function in the future.

IP(3) mobilization and diffusion determine the timing window of Ca(2+) release by synaptic stimulation and a spike in rat CA1 pyramidal cells

Synaptically activated calcium release from internal stores in CA1 pyramidal neurons is generated via metabotropic glutamate receptors by mobilizing IP(3). Ca(2+) release spreads as a large amplitude wave in a restricted region of the apical dendrites of these cells. These Ca(2+) waves have been shown to induce certain forms of synaptic potentiation and have been hypothesized to affect other forms of plasticity. Pairing a single backpropagating action potential (bAP) with repetitive synaptic stimulation evokes Ca(2+) release when synaptic stimulation alone is subthreshold for generating release. We examined the timing window for this synergistic effect under conditions favoring Ca(2+) release. The window, measured from the end of the train, lasted 250-500 ms depending on the duration of stimulation tetanus. The window appears to correspond to the time when both IP(3) concentration and [Ca(2+)](i) are elevated at the site of the IP(3) receptor. Detailed analysis of the mechanisms determining the duration of the window, including experiments using different forms of caged IP(3) instead of synaptic stimulation, suggest that the most significant processes are the time for IP(3) to diffuse away from the site of generation and the time course of IP(3) production initiated by activation of mGluRs. IP(3) breakdown, desensitization of the IP(3) receptor, and the kinetics of IP(3) unbinding from the receptor may affect the duration of the window but are less significant. The timing window is short but does not appear to be short enough to suggest that this form of coincidence detection contributes to conventional spike timing-dependent synaptic plasticity in these cells.

Studying isoform-specific inositol 1,4,5-trisphosphate receptor function and regulation

Inositol 1,4,5-trisphosphate receptors (InsP3R) are a family of ubiquitously expressed intracellular Ca2+ channels. Isoform-specific properties of the three family members may play a prominent role in defining the rich diversity of the spatial and temporal characteristics of intracellular Ca2+ signals. Studying the properties of the particular family members is complicated because individual receptor isoforms are typically never expressed in isolation. In this article, we discuss strategies for studying Ca2+ release through individual InsP3R family members with particular reference to methods applicable following expression of recombinant InsP3R and mutant constructs in the DT40-3KO cell line, an unambiguously null InsP3R expression system.

Synapse specificity of calcium release probed by chemical two-photon uncaging of inositol 1,4,5-trisphosphate

Biological messengers can be "caged" by adding a single photosensitive group that can be photolyzed by a light flash to achieve spatially and temporally precise biochemical control. Here we report that photolysis of a double-caged form of the second messenger inositol 1,4,5-trisphosphate (IP3) triggers focal calcium release in Purkinje cell somata, dendrites, and spines as measured by two-photon microscopy. In calbindin knock-out Purkinje cells, peak calcium increased with flash energy with higher cooperativity for double-caged IP3 than for conventional single-caged IP3, consistent with a chemical two-photon effect. Spine photolysis of double-caged IP3 led to local calcium release. Uncaging of glycerophosphoryl-myo-inositol 4,5-bisphosphate (gPIP2), a poorly metabolizable IP3 analog, led to less well localized release. Thus, IP3 breakdown is necessary for spine-specificity. IP3- and gPIP2-evoked signals declined from peak with similar, slow time courses, indicating that release lasts hundreds of milliseconds and is terminated not by IP3 degradation but by intrinsic receptor dynamics. Based on measurements of spine-dendrite coupling, IP3-evoked calcium signals are expected to be at least 2.4-fold larger in their spine of origin than in nearby spines, allowing IP3 to act as a synapse-specific second messenger. Unexpectedly, single-caged IP3 led to less release in somata and was ineffective in dendrites and spines. Calcium release using caged gPIP2 was inhibited by the addition of single-caged IP3, suggesting that single-caged IP3 is an antagonist of calcium release. Caging at multiple sites may be an effective general approach to reducing residual receptor interaction.

Activation of protein kinase C in sensory neurons accelerates Ca2+ uptake into the endoplasmic reticulum

The rate of Ca2+ clearance from the neuronal cytoplasm affects the amplitude, duration, and localization of Ca2+ signals and influences a variety of Ca2+-dependent functions. We reported previously that activation of protein kinase C (PKC) accelerates Ca2+ efflux in rat sensory neurons mediated by the plasma membrane Ca2+-ATPase isoform 4 (PMCA4). Here we show that sarco-endoplasmic reticulum Ca2+-ATPase (SERCA)-mediated Ca2+ uptake into intracellular stores is also accelerated by PKC activation. The rate of intracellular Ca2+ concentration ([Ca2+]i) clearance was studied after small (<350 nM) action potential-induced Ca2+ loads in rat dorsal root ganglion neurons. Under these conditions, mitochondrial Ca2+ uptake and Na+/Ca2+ exchange do not significantly influence [Ca2+]i recovery. Phorbol dibutyrate (PDBu) increased the rate of [Ca2+]i clearance by 71% in a manner sensitive to the selective PKC inhibitors GF109203x (2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)maleimide) and calphostin. PKC-dependent acceleration was still observed (approximately 39%) when the PKC-sensitive PMCA isoform was knocked down by expression of an antisense PMCA4 cDNA (AS4). Direct measurement of Ca2+ in the endoplasmic reticulum (ER) lumen revealed that PKC increased the rate of store refilling more than twofold after depletion by treatment with cyclopiazonic acid. ER refilling was less complete in PDBu-treated cells, although, in AS4-expressing cells, PDBu accelerated the rate without reducing the ER capacity, suggesting that PMCA and SERCA compete for Ca2+. Thus, activation of PKC accelerates the clearance of Ca2+ from the cytoplasm by the concerted stimulation of Ca2+ sequestration and Ca2+ efflux.

Kinetic, pharmacological and activity-dependent separation of two Ca2+ signalling pathways mediated by type 1 metabotropic glutamate receptors in rat Purkinje neurones

Type 1 metabotropic glutamate receptors (mGluR1) in Purkinje neurones (PNs) are important for motor learning and coordination. Here, two divergent mGluR1 Ca2+-signalling pathways and the associated membrane conductances were distinguished kinetically and pharmacologically after activation by 1-ms photorelease of L-glutamate or by bursts of parallel fibre (PF) stimulation. A new, mGluR1-mediated transient K+ conductance was seen prior to the slow EPSC (sEPSC). It was seen only in PNs previously allowed to fire spontaneously or held at depolarized potentials for several seconds and was slowly inhibited by agatoxin IVA, which blocks P/Q-type Ca2+ channels. It peaked in 148 ms, had well-defined kinetics and, unlike the sEPSC, was abolished by the phospholipase C (PLC) inhibitor U73122. It was blocked by the BK Ca2+-activated K+ channel blocker iberiotoxin and unaffected by apamin, indicating selective activation of BK channels by PLC-dependent store-released Ca2+. The K+ conductance and underlying transient Ca2+ release showed a highly reproducible delay of 99.5 ms following PF burst stimulation, with a precision of 1-2 ms in repeated responses of the same PN, and a subsequent fast rise and fall of Ca2+ concentration. Analysis of Ca2+ signals showed that activation of the K+ conductance by Ca2+ release occurred in small dendrites and subresolution structures, most probably spines. The results show that PF burst stimulation activates two pathways of mGluR1 signalling in PNs. First, transient, PLC-dependent Ca2+ release from stores with precisely reproducible timing and second, slower Ca2+ influx in the cation-permeable sEPSC channel. The priming by prior Ca2+ influx in P/Q-type Ca2+ channels may determine the path of mGluR1 signalling. The precise timing of PLC-mediated store release may be important for interactions of PF mGluR1 signalling with other inputs to the PN.

Endogenous inhibitors of InsP3-induced Ca2+ release in neuroblastoma cells

Cerebellar Purkinje neurons and neuroblastoma N1E-115 cells require 10-50 times more InsP3 to induce Ca2+ release than do a variety of non-neuronal cells (including astrocytes, hepatocytes, endothelial cells, or smooth muscle cells). Given the importance of InsP3-induced Ca2+ release for the development of synaptic plasticity in Purkinje neurons, a low InsP3 sensitivity may facilitate the integration of numerous synaptic inputs before initiating a change in synaptic strength. In the present study, attention is directed at the mechanism underlying this low InsP3 sensitivity of Ca2+ release. We show that permeabilization of neuroblastoma cells with saponin increased InsP3 sensitivity of Ca2+ release, indicating the presence of a diffusible, cytosolic inhibitor(s) of Ca2+ release. Consistent with this hypothesis, gel filtration of the neuroblastoma cytosol yielded three peaks that inhibited InsP3-induced Ca2+ release from permeabilized cells. The prominent inhibitory peak decreased the InsP3 sensitivity of Ca2+ release from permeabilized cells, did not bind 3H-InsP3, and was present in sufficient levels to account for the low InsP3 sensitivity of Ca2+ release in intact neuroblastoma cells. Purification of this prominent inhibitory fraction yielded a protein band that was identified by mass spectrometry as stress-induced phosphoprotein 1 (mSTI1). Furthermore, immunoprecipitation of mSTI1 decreased the inhibitory activity of N1E-115 cytosol, indicating that mSTI1 contributes to the inhibition of InsP3-induced Ca2+ release. Thus, the low InsP3 sensitivity of Ca2+ release in neuroblastoma cells can be explained by the presence of cytosolic inhibitors of Ca2+ release and include stress-induced phosphoprotein 1.

Modeling and analysis of calcium signaling events leading to long-term depression in cerebellar Purkinje cells

Modeling and simulation of the calcium signaling events that precede long-term depression of synaptic activity in cerebellar Purkinje cells are performed using the Virtual Cell biological modeling framework. It is found that the unusually high density and low sensitivity of inositol-1,4,5-trisphosphate receptors (IP3R) are critical to the ability of the cell to generate and localize a calcium spike in a single dendritic spine. The results also demonstrate the model's capability to simulate the supralinear calcium spike observed experimentally during coincident activation of the parallel and climbing fibers. The sensitivity of the calcium spikes to certain biological and geometrical effects is investigated as well as the mechanisms that underlie the cell's ability to generate the supralinear spike. The sensitivity of calcium release rates from the IP3R to calcium concentrations, as well as IP3 concentrations, allows the calcium spike to form. The diffusion barrier caused by the small radius of the spine neck is shown to be important, as a threshold radius is observed above which a spike cannot be formed. Additionally, the calcium buffer capacity and diffusion rates from the spine are found to be important parameters in shaping the calcium spike.

Inositol 1,4,5-trisphosphate-dependent Ca2+ threshold dynamics detect spike timing in cerebellar Purkinje cells

Large Ca2+ signals essential for cerebellar long-term depression (LTD) at parallel fiber (PF)-Purkinje cell synapses are known to be induced when PF activation precedes climbing fiber (CF) activation by 50-200 ms, consistent with cerebellar learning theories. However, large Ca2+ signals and/or LTD can also be induced by massive PF stimulation alone or by photolysis of caged Ca2+ or inositol 1,4,5-trisphosphate (IP3). To understand the spike-timing detection mechanisms in cerebellar LTD, we developed a kinetic model of Ca2+ dynamics within a Purkinje dendritic spine. In our kinetic simulation, IP3 was first produced via the metabotropic pathway of PF inputs, and the Ca2+ influx in response to the CF input triggered regenerative Ca2+-induced Ca2+ release from the internal stores via the IP3 receptors activated by the increased IP3. The delay in IP3 increase caused by the PF metabotropic pathway generated the optimal PF-CF interval. The Ca2+ dynamics revealed a threshold for large Ca2+ release that decreased as IP3 increased, and it coherently explained the different forms of LTD. At 2.5 microM IP3, CF activation after PF activation was essential to reach the threshold for the regenerative Ca2+ release. At 10 microM IP3, the same as achieved experimentally by strong IP3 photolysis, the threshold was lower, and thus large Ca2+ release was generated even without CF stimulation. In contrast, the basal 0.1 microM IP3 level resulted in an extremely high Ca2+ threshold for regenerative Ca2+ release. Thus, the results demonstrated that Ca2+ dynamics can detect spike timing under physiological conditions, which supports cerebellar learning theories.

Fertilization and Inositol 1,4,5-Trisphosphate (IP3)-Induced Calcium Release in Type-1 Inositol 1,4,5-Trisphosphate Receptor Down-Regulated Bovine Eggs1

It is widely believed that stimulation of the phosphoinositide pathway and production of 1,4,5-inositol trisphosphate (IP(3)) underlies the oscillatory changes in the concentration of intracellular free calcium ions ([Ca(2+)](i)) seen during mammalian fertilization. IP(3) promotes Ca(2+) release in eggs by binding to its receptor, the type-1 IP(3) receptor (IP(3)R-1, also known as ITPR1), a ligand-gated Ca(2+) channel located in the membrane of the endoplasmic reticulum, the main Ca(2+) store of the cell. While IP(3)R-1 has been shown to mediate all Ca(2+) release during mouse fertilization, whether or not it plays such an essential role in fertilization-induced Ca(2+) release in large domestic species such as bovine and porcine is presently not known. Accordingly, we have generated metaphase II bovine eggs with a approximately 70%-80% reduction in the number of intact IP(3)R-1 by inducing receptor down-regulation during oocyte maturation. We did so by injecting the nonhydrolyzable IP(3) analogue, adenophostin A. Functional Ca(2+) release analysis revealed that IP(3)R-1 is the predominant Ca(2+) release channel in bovine eggs, requiring as little as 20% of total intact receptor to mount persistent [Ca(2+)](i) oscillations in response to fertilization, expression of PLCzeta (also known as PLCZ1), and adenophostin A. However, lower concentrations of IP(3) and near-physiological concentrations of porcine sperm extract were unable to trigger [Ca(2+)](i) oscillations in this reduced IP(3)R-1 model. Furthermore, we present evidence that the sensitivity of bovine IP(3)R-1 is impaired at the first embryonic interphase. Together, these results demonstrate the essential role of IP(3)R-1-mediated Ca(2+) release during fertilization in bovine eggs, and identify cell cycle regulatory mechanisms of [Ca(2+)](i) oscillations at the level of IP(3)R-1.

Spontaneously active and InsP3-activated ion channels in cell nuclei from rat cerebellar Purkinje and granule neurones

Increases in Ca(2+) concentration in the nucleus of neurones modulate gene transcription and may be involved in activity-dependent long-term plasticity, apoptosis, and neurotoxicity. Little is currently known about the regulation of Ca(2+) in the nuclei of neurones. Investigation of neuronal nuclei is hampered by the cellular heterogeneity of the brain where neurones comprise no more than 10% of the cells. The situation is further complicated by large differences in properties of different neurones. Here we report a method for isolating nuclei from identified central neurones. We employed this technique to study nuclei from rat cerebellar Purkinje and granule neurones. Patch-clamp recording from the nuclear membrane of Purkinje neurones revealed numerous large-conductance channels selective for monovalent cations. The nuclear membrane of Purkinje neurones also contained multiple InsP(3)- activated ion channels localized exclusively in the inner nuclear membrane with their receptor loci facing the nucleoplasm. In contrast, the nuclear membrane of granule neurones contained only a small number of mainly anion channels. Nuclear InsP(3) receptors (InsP(3)Rs) were activated by InsP(3) with EC(50) = 0.67 microm and a Hill coefficient of 2.5. Ca(2+) exhibited a biphasic effect on the receptors elevating its activity at low concentrations and inhibiting it at micromolar concentrations. InsP(3) in saturating concentrations did not prevent the inhibitory effect of Ca(2+), but strongly increased InsP(3)R activity at resting Ca(2+) concentrations. These data are the first evidence for the presence of intranuclear sources of Ca(2+) in neurones. Ca(2+) release from the nuclear envelope may amplify Ca(2+) transients penetrating the nucleus from the cytoplasm or generate Ca(2+) transients in the nucleus independently of the cytoplasm.

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.

Kinesin dependent, rapid, bi-directional transport of ER sub-compartment in dendrites of hippocampal neurons

Although spatially restricted Ca2+ release from the endoplasmic reticulum (ER) through intracellular Ca2+ channels plays important roles in various neuronal activities, the accurate distribution and dynamics of ER in the dendrite of living neurons still remain unknown. To elucidate these, we expressed fluorescent protein-tagged ER proteins in cultured mouse hippocampal neurons, and monitored their movements using time-lapse microscopy. We report here that a sub-compartment of ER forms in relatively large vesicles that are capable, similarly to the reticular ER, of taking up and releasing Ca2+. The vesicular sub-compartment of ER moved rapidly along the dendrites in both anterograde and retrograde directions at a velocity of 0.2-0.3 μm/second. Depletion of microtubules, overexpression of dominant-negative kinesin and kinesin depletion by antisense DNA reduced the number and velocity of the moving vesicles, suggesting that kinesin may drive the transport of the vesicular sub-compartment of ER along microtubules in the dendrite. Rapid transport of the Ca2+-releasable sub-compartment of ER might contribute to rapid supply of fresh ER proteins to the distal part of the dendrite, or to the spatial regulation of intracellular Ca2+ signaling.

Regulation of InsP3 receptor activity by neuronal Ca2+-binding proteins

Inositol 1,4,5-trisphosphate receptors (InsP(3)Rs) were recently demonstrated to be activated independently of InsP(3) by a family of calmodulin (CaM)-like neuronal Ca(2+)-binding proteins (CaBPs). We investigated the interaction of both naturally occurring long and short CaBP1 isoforms with InsP(3)Rs, and their functional effects on InsP(3)R-evoked Ca(2+) signals. Using several experimental paradigms, including transient expression in COS cells, acute injection of recombinant protein into Xenopus oocytes and (45)Ca(2+) flux from permeabilised COS cells, we demonstrated that CaBPs decrease the sensitivity of InsP(3)-induced Ca(2+) release (IICR). In addition, we found a Ca(2+)-independent interaction between CaBP1 and the NH(2)-terminal 159 amino acids of the type 1 InsP(3)R. This interaction resulted in decreased InsP(3) binding to the receptor reminiscent of that observed for CaM. Unlike CaM, however, CaBPs do not inhibit ryanodine receptors, have a higher affinity for InsP(3)Rs and more potently inhibited IICR. We also show that phosphorylation of CaBP1 at a casein kinase 2 consensus site regulates its inhibition of IICR. Our data suggest that CaBPs are endogenous regulators of InsP(3)Rs tuning the sensitivity of cells to InsP(3).