Visualization of IP(3) dynamics reveals a novel AMPA receptor-triggered IP(3) production pathway mediated by voltage-dependent Ca(2+) influx in Purkinje cells

Investigation of Brain Functions with Fluorescence Imaging Techniques

An intricate interplay of complex spatio-temporal events underlies brain functions. Therefore, clarifying these dynamic processes is indispensable for revealing the mechanisms of brain functions. Fluorescence imaging is a powerful technique for visualizing cellular and molecular dynamics in the brain. Recent developments in fluorescent indicators and specialized optics have advanced research in the field of neuroscience. In this review, I will exemplify the power and beauty of fluorescence imaging by discussing my work focusing on the molecular dynamics of metabotropic glutamate receptor (mGluR) signaling at the synapse. By developing novel fluorescent indicators for glutamate, inositol 1,4,5-trisphosphate and Ca2+ within the endoplasmic reticulum, I succeeded in imaging the spatio-temporal dynamics of synaptic mGluR signaling, which led to the identification of novel mechanisms of mGluR-mediated glutamatergic neurotransmission. These discoveries highlight the importance of the development and application of novel fluorescence imaging techniques for the investigation of brain functions.

Efficient non-degenerate two-photon excitation for fluorescence microscopy

Non-degenerate two-photon excitation (ND-TPE) has been explored in two-photon excitation microscopy. However, a systematic study of the efficiency of ND-TPE to guide the selection of fluorophore excitation wavelengths is missing. We measured the relative non-degenerate two-photon absorption cross-section (ND-TPACS) of several commonly used fluorophores (two fluorescent proteins and three small-molecule dyes) and generated 2-dimensional ND-TPACS spectra. We observed that the shape of a ND-TPACS spectrum follows that of the corresponding degenerate two-photon absorption cross-section (D-TPACS) spectrum, but is higher in magnitude. We found that the observed enhancements are higher than theoretical predictions.

Metabotropic Acetylcholine and Glutamate Receptors Mediate PI(4,5)P2 Depletion and Oscillations in Hippocampal CA1 Pyramidal Neurons in situ

The sensitivity of many ion channels to phosphatidylinositol-4,5-bisphosphate (PIP2) levels in the cell membrane suggests that PIP2 fluctuations are important and general signals modulating neuronal excitability. Yet the PIP2 dynamics of central neurons in their native environment remained largely unexplored. Here, we examined the behavior of PIP2 concentrations in response to activation of Gq-coupled neurotransmitter receptors in rat CA1 hippocampal neurons in situ in acute brain slices. Confocal microscopy of the PIP2-selective molecular sensors tubbyCT-GFP and PLCδ1-PH-GFP showed that pharmacological activation of muscarinic acetylcholine (mAChR) or group I metabotropic glutamate (mGluRI) receptors induces transient depletion of PIP2 in the soma as well as in the dendritic tree. The observed PIP2 dynamics were receptor-specific, with mAChR activation inducing stronger PIP2 depletion than mGluRI, whereas agonists of other Gαq-coupled receptors expressed in CA1 neurons did not induce measureable PIP2 depletion. Furthermore, the data show for the first time neuronal receptor-induced oscillations of membrane PIP2 concentrations. Oscillatory behavior indicated that neurons can rapidly restore PIP2 levels during persistent activation of Gq and PLC. Electrophysiological responses to receptor activation resembled PIP2 dynamics in terms of time course and receptor specificity. Our findings support a physiological function of PIP2 in regulating electrical activity.

Dandelion Root Extract Induces Intracellular Ca2+ Increases in HEK293 Cells

Dandelion (Taraxacum officinale Weber ex F.H.Wigg.) has been used for centuries as an ethnomedical remedy. Nonetheless, the extensive use of different kinds of dandelion extracts and preparations is based on empirical findings. Some of the tissue-specific effects reported for diverse dandelion extracts may result from their action on intracellular signaling cascades. Therefore, the aim of this study was to evaluate the effects of an ethanolic dandelion root extract (DRE) on Ca²⁺ signaling in human embryonic kidney (HEK) 293 cells. The cytotoxicity of increasing doses of crude DRE was determined by the Calcein viability assay. Fura-2 and the fluorescence resonance energy transfer (FRET)-based probe ERD1 were used to measure cytoplasmic and intraluminal endoplasmic reticulum (ER) Ca²⁺ levels, respectively. Furthermore, a green fluorescent protein (GFP)-based probe was used to monitor phospholipase C (PLC) activation (pleckstrin homology [PH]–PLCδ–GFP). DRE (10–400 µg/mL) exposure, in the presence of external Ca²⁺, dose-dependently increased intracellular Ca²⁺ levels. The DRE-induced Ca²⁺ increase was significantly reduced in the absence of extracellular Ca²⁺. In addition, DRE caused a significant Ca²⁺ release from the ER of intact cells and a concomitant translocation of PH–PLCδ–GFP. In conclusion, DRE directly activates both the release of Ca²⁺ from internal stores and a significant Ca²⁺ influx at the plasma membrane. The resulting high Ca²⁺ levels within the cell seem to directly stimulate PLC activity.

Knockdown of Myo-Inositol Transporter SMIT1 Normalizes Cholinergic and Glutamatergic Function in an Immortalized Cell Line Established from the Cerebral Cortex of a Trisomy 16 Fetal Mouse, an Animal Model of Human Trisomy 21 (Down Syndrome)

The Na⁺/myo-inositol cotransporter (SMIT1) is overexpressed in human Down syndrome (DS) and in trisomy 16 fetal mice (Ts16), an animal model of the human condition. SMIT1 overexpression determines increased levels of intracellular myo-inositol, a precursor of phophoinositide synthesis. SMIT1 is overexpressed in CTb cells, an immortalized cell line established from the cerebral cortex of a Ts16 mouse fetus. CTb cells exhibit impaired cytosolic Ca²⁺ signals in response to glutamatergic and cholinergic stimuli (increased amplitude and delayed time-dependent kinetics in the decay post-stimulation), compared to our CNh cell line, derived from the cerebral cortex of a euploid animal. Considering the role of myo-inositol in intracellular signaling, we normalized SMIT1 expression in CTb cells using specific mRNA antisenses. Forty-eight hours post-transfection, SMIT1 levels in CTb cells reached values comparable to those of CNh cells. At this time, decay kinetics of Ca²⁺ signals induced by either glutamate, nicotine, or muscarine were accelerated in transfected CTb cells, to values similar to those of CNh cells. The amplitude of glutamate-induced cytosolic Ca²⁺ signals in CTb cells was also normalized. The results suggest that SMIT1 overexpression contributes to abnormal cholinergic and glutamatergic Ca²⁺ signals in the trisomic condition, and knockdown of DS-related genes in our Ts16-derived cell line could constitute a relevant tool to study DS-related neuronal dysfunction.

Schwann cell mitochondria as key regulators in the development and maintenance of peripheral nerve axons

Formation of myelin sheaths by Schwann cells (SCs) enables rapid and efficient transmission of action potentials in peripheral axons, and disruption of myelination results in disorders that involve decreased sensory and motor functions. Given that construction of SC myelin requires high levels of lipid and protein synthesis, mitochondria, which are pivotal in cellular metabolism, may be potential regulators of the formation and maintenance of SC myelin. Supporting this notion, abnormal mitochondria are found in SCs of neuropathic peripheral nerves in both human patients and the relevant animal models. However, evidence for the importance of SC mitochondria in myelination has been limited, until recently. Several studies have recently used genetic approaches that allow SC-specific ablation of mitochondrial metabolic activity in living animals to show the critical roles of SC mitochondria in the development and maintenance of peripheral nerve axons. Here, we review current knowledge about the involvement of SC mitochondria in the formation and dysfunction of myelinated axons in the peripheral nervous system.

The endoplasmic reticulum-mitochondria interface is perturbed in PARK2 knockout mice and patients with PARK2 mutations

Mutations in PARK2, encoding the E3 ubiquitin protein ligase Parkin, are a common cause of autosomal recessive Parkinson's disease (PD). Loss of PARK2 function compromises mitochondrial quality by affecting mitochondrial biogenesis, bioenergetics, dynamics, transport and turnover. We investigated the impact of PARK2 dysfunction on the endoplasmic reticulum (ER)-mitochondria interface, which mediates calcium (Ca²⁺) exchange between the two compartments and is essential for Parkin-dependent mitophagy. Confocal and electron microscopy analyses showed the ER and mitochondria to be in closer proximity in primary fibroblasts from PARK2 knockout (KO) mice and PD patients with PARK2 mutations than in controls. Ca²⁺ flux to the cytosol was also modified, due to enhanced ER-to-mitochondria Ca²⁺ transfers, a change that was also observed in neurons derived from induced pluripotent stem cells of a patient with PARK2 mutations. Subcellular fractionation showed the abundance of the Parkin substrate mitofusin 2 (Mfn2), which is known to modulate the ER-mitochondria interface, to be specifically higher in the mitochondrion-associated ER membrane compartment in PARK2 KO tissue. Mfn2 downregulation or the exogenous expression of normal Parkin restored cytosolic Ca²⁺ transients in fibroblasts from patients with PARK2 mutations. In contrast, a catalytically inactive PD-related Parkin variant had no effect. Overall, our data suggest that Parkin is directly involved in regulating ER-mitochondria contacts and provide new insight into the role of the loss of Parkin function in PD development.

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.

Phosphoinositide-specific phospholipase C in health and disease

Phospholipases are widely occurring and can be found in several different organisms, including bacteria, yeast, plants, animals, and viruses. Phospholipase C (PLC) is a class of phospholipases that cleaves phospholipids on the diacylglycerol (DAG) side of the phosphodiester bond producing DAGs and phosphomonoesters. Among PLCs, phosphoinositide-specific PLC (PI-PLC) constitutes an important step in the inositide signaling pathways. The structures of PI-PLC isozymes show conserved domains as well as regulatory specific domains. This is important, as most PI-PLCs share a common mechanism, but each of them has a peculiar role and can have a specific cell distribution that is linked to a specific function. More importantly, the regulation of PLC isozymes is fundamental in health and disease, as there are several PLC-dependent molecular mechanisms that are associated with the activation or inhibition of important physiopathological processes. Moreover, PI-PLC alternative splicing variants can play important roles in complex signaling networks, not only in cancer but also in other diseases. That is why PI-PLC isozymes are now considered as important molecules that are essential for better understanding the molecular mechanisms underlying both physiology and pathogenesis, and are also potential molecular targets useful for the development of innovative therapeutic strategies.

Dual transgene expression in murine cerebellar Purkinje neurons by viral transduction in vivo

Viral-vector mediated gene transfer to cerebellar Purkinje neurons in vivo is a promising avenue for gene therapy of cerebellar ataxias and for genetic manipulation in functional studies of animal models of cerebellar disease. Here, we report the results of experiments designed to identify efficient methods for viral transduction of adult murine Purkinje neurons in vivo. For these analyses, several lentiviral and an adeno-associated virus (AAV), serotype 1, vector with various promoter combinations were generated and compared for in situ transduction efficiency, assayed by fluorescent reporter protein expression in Purkinje neurons. Additional experiments were also conducted to identify the optimal experimental strategy for co-expression of two proteins in individual Purkinje neurons. Of the viruses tested, AAV1 with a CAG promoter exhibited the highest specificity for Purkinje neurons. To deliver two proteins to the same Purkinje neuron, several methods were tested, including: an internal ribosome entry site (IRES), a 2A sequence, a dual promoter vector, and co-injection of two viruses. Efficient expression of both proteins in the same Purkinje neuron was only achieved by co-injecting two AAV1-CAG viruses. We found that use of an AAV1-CAG virus outperformed similar lentivirus vectors and that co-injection of two AAV1-CAG viruses could be used to efficiently deliver two proteins to the same Purkinje neuron in adult mice. AAV1 with a CAG promoter is highly efficient and selective at transducing adult cerebellar Purkinje neurons and two AAV-CAG viruses can be used to efficiently express two proteins in the same neuron in vivo.

Mechanisms of [Ca2+]i elevation following P2X receptor activation in the guinea-pig small mesenteric artery myocytes

BACKGROUND

There is growing evidence suggesting involvement of L-type voltage-gated Ca2+ channels (VGCCs) in purinergic signaling mechanisms. However, detailed interplay between VGCCs and P2X receptors in intracellular Ca2+ mobilization is not well understood. This study examined relative contribution of the Ca2+ entry mechanisms and induced by this entry Ca2+ release from the intracellular stores engaged by activation of P2X receptors in smooth muscle cells (SMCs) from the guinea-pig small mesenteric arteries.

METHODS

P2X receptors were stimulated by the brief local application of αβ-meATP and changes in [Ca2+]i were monitored in fluo-3 loaded SMCs using fast x-y confocal Ca2+ imaging. The effects of the block of L-type VGCCs and/or depletion of the intracellular Ca2+ stores on αβ-meATP-induced [Ca2+]i transients were analyzed.

RESULTS

Our analysis revealed that Ca2+ entry via L-type VGCCs is augmented by the Ca2+-induced Ca2+ release significantly more than Ca2+ entry via P2X receptors, even though net Ca2+ influxes provided by the two mechanisms are not significantly different.

CONCLUSIONS

Thus, arterial SMCs upon P2X receptor activation employ an effective mechanism of the Ca2+ signal amplification, the major component of which is the Ca2+ release from the SR activated by Ca2+ influx via L-type VGCCs. This signaling pathway is engaged by depolarization of the myocyte membrane resulting from activation of P2X receptors, which, being Ca2+ permeable, per se form less effective Ca2+ signaling pathway. This study, therefore, rescales potential targets for therapeutic intervention in purinergic control of vascular tone.

Type 1 inositol trisphosphate receptor regulates cerebellar circuits by maintaining the spine morphology of purkinje cells in adult mice

The structural maintenance of neural circuits is critical for higher brain functions in adulthood. Although several molecules have been identified as regulators for spine maintenance in hippocampal and cortical neurons, it is poorly understood how Purkinje cell (PC) spines are maintained in the mature cerebellum. Here we show that the calcium channel type 1 inositol trisphosphate receptor (IP3R1) in PCs plays a crucial role in controlling the maintenance of parallel fiber (PF)-PC synaptic circuits in the mature cerebellum in vivo. Significantly, adult mice lacking IP3R1 specifically in PCs (L7-Cre;Itpr1(flox/flox)) showed dramatic increase in spine density and spine length of PCs, despite having normal spines during development. In addition, the abnormally rearranged PF-PC synaptic circuits in mature cerebellum caused unexpectedly severe ataxia in adult L7-Cre;Itpr1(flox/flox) mice. Our findings reveal a specific role for IP3R1 in PCs not only as an intracellular mediator of cerebellar synaptic plasticity induction, but also as a critical regulator of PF-PC synaptic circuit maintenance in the mature cerebellum in vivo; this mechanism may underlie motor coordination and learning in adults.

The physiological roles of primary phospholipase C

The roles of phosphoinositide-specific phospholipase C (PLC) have been extensively investigated in diverse cell lines and pathological conditions. Among the PLC isozmes, primary PLCs, PLC-β and PLC-γ, are directly activated by receptor activation, unlike other secondary PLCs (PLC-ɛ, PLC-δ1, and PLC-η1). PLC-β isozymes are activated by G protein couple receptor and PLC-γ isozymes are activated by receptor tyrosine kinase (RTK). Primary PLCs are differentially expressed in different tissues, suggesting their specific roles in diverse tissues and regulate a variety of physiological and pathophysiological functions. Thus, dysregulation of phospholipases contributes to a number of human diseases and primary PLCs have been identified as therapeutic targets for prevention and treatment of diseases. Here we review the roles of primary PLCs in physiology and their impact in pathology.

Acute inhibition of diacylglycerol lipase blocks endocannabinoid-mediated retrograde signalling: evidence for on-demand biosynthesis of 2-arachidonoylglycerol

The endocannabinoid (eCB) 2-arachidonoylglycerol (2-AG) produced by diacylglycerol lipase α (DGLα) is one of the best-characterized retrograde messengers at central synapses. It has been thought that 2-AG is produced 'on demand' upon activation of postsynaptic neurons. However, recent studies propose that 2-AG is pre-synthesized by DGLα and stored in neurons, and that 2-AG is released from such 'pre-formed pools' without the participation of DGLα. To address whether the 2-AG source for retrograde signalling is the on-demand biosynthesis by DGLα or the mobilization from pre-formed pools, we examined the effects of acute pharmacological inhibition of DGL by a novel potent DGL inhibitor, OMDM-188, on retrograde eCB signalling triggered by Ca(2+) elevation, Gq/11 protein-coupled receptor activation or synergy of these two stimuli in postsynaptic neurons. We found that pretreatment for 1 h with OMDM-188 effectively blocked depolarization-induced suppression of inhibition (DSI), a purely Ca(2+)-dependent form of eCB signalling, in slices from the hippocampus, striatum and cerebellum. We also found that at parallel fibre-Purkinje cell synapses in the cerebellum OMDM-188 abolished synaptically induced retrograde eCB signalling, which is known to be caused by the synergy of postsynaptic Ca(2+) elevation and group I metabotropic glutamate receptor (I-mGluR) activation. Moreover, brief OMDM-188 treatments for several minutes were sufficient to suppress both DSI and the I-mGluR-induced retrograde eCB signalling in cultured hippocampal neurons. These results are consistent with the hypothesis that 2-AG for synaptic retrograde signalling is supplied as a result of on-demand biosynthesis by DGLα rather than mobilization from presumptive pre-formed pools.

Single-cell imaging techniques for the real-time detection of IP₃ in live cells

Inositol 1,4,5-trisphosphate (IP(3)) is a ubiquitous second messenger, derived from the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP(2)) by enzymes of the phospholipase C (PLC) family. Binding of IP(3) to its cognate receptor in the endoplasmic reticulum membrane leads to release of Ca(2+) into the cytoplasm, which is involved in the regulation of an array of cellular functions. Traditional techniques for the detection of IP(3) have required the extraction of a large number of cells, with limitations in the time resolution of changes in IP(3) and an inability to obtain detailed information on the dynamics of this second messenger in single cells. Recent progress in this field has led to the development of a number of genetically encoded fluorescent biosensors, which upon recombinant expression are able selectively to detect real-time changes in IP(3) in single live cells. In this chapter, I detail protocols for the expression, visualization (by confocol or fluorescence microscopy), and interpretation of data obtained with such biosensors expressed in mammalian cells.

Dendritic calcium signaling in cerebellar Purkinje cell

The Purkinje cells in the cerebellum are unique neurons that generate local and global Ca(2+) signals in response to two types of excitatory inputs, parallel fiber and climbing fiber, respectively. The spatiotemporal distribution and interaction of these synaptic inputs produce complex patterns of Ca(2+) dynamics in the Purkinje cell dendrites. The Ca(2+) signals originate from Ca(2+) influx through voltage-gated Ca(2+) channels and Ca(2+) release from intracellular stores that are mediated by the metabotropic glutamate receptor signaling pathway. These Ca(2+) signals are essential for the induction of various forms of synaptic plasticity and for controlling the input-output relationship of Purkinje cells. In this article we review Ca(2+) signaling in Purkinje cell dendrites.

Molecular mechanisms of muscarinic acetylcholine receptor-stimulated increase in cytosolic free Ca(2+) concentration and ERK1/2 activation in the MIN6 pancreatic β-cell line

Muscarinic acetylcholine receptor (mAChR) activation of pancreatic β-cells elevates intracellular Ca(2+) and potentiates glucose-stimulated insulin secretion. In addition, it activates a number of signaling molecules, including ERK1/2, whose activation has been shown to play an important role in regulating pancreatic β-cell function and mass. The aim of this work was to determine how mAChR activation elevates intracellular Ca(2+) concentration ([Ca(2+)]( i )) and activates ERK1/2 in the pancreatic β-cell line MIN6. We demonstrate that agonist-stimulated ERK1/2 activation is dependent on the activation of phospholipase C and an elevation in [Ca(2+)]( i ), but is independent of the activation of diacylglycerol-dependent protein kinase C isoenzymes. Using a pharmacological approach, we provide evidence that agonist-induced increases in [Ca(2+)]( i ) and ERK activity require (1) IP(3) receptor-mediated mobilization of Ca(2+) from the endoplasmic reticulum, (2) influx of extracellular Ca(2+) through store-operated channels, (3) closure of K(ATP) channels, and (4) Ca(2+) entry via L-type voltage-operated Ca(2+) channels. Moreover, this Ca(2+)-dependent activation of ERK is mediated via both Ras-dependent and Ras-independent mechanisms. In summary, this study provides important insights into the multifactorial signaling mechanisms linking mAChR activation to increases in [Ca(2+)]( i ) and ERK activity.

Critical role for calcium mobilization in activation of the NLRP3 inflammasome

The NLRP3 (nucleotide-binding domain, leucine-rich-repeat-containing family, pyrin domain-containing 3) inflammasome mediates production of inflammatory mediators, such as IL-1β and IL-18, and as such is implicated in a variety of inflammatory processes, including infection, sepsis, autoinflammatory diseases, and metabolic diseases. The proximal steps in NLRP3 inflammasome activation are not well understood. Here we elucidate a critical role for Ca(2+) mobilization in activation of the NLRP3 inflammasome by multiple stimuli. We demonstrate that blocking Ca(2+) mobilization inhibits assembly and activation of the NLRP3 inflammasome complex, and that during ATP stimulation Ca(2+) signaling is pivotal in promoting mitochondrial damage. C/EPB homologous protein, a transcription factor that can modulate Ca(2+) release from the endoplasmic reticulum, amplifies NLRP3 inflammasome activation, thus linking endoplasmic reticulum stress to activation of the NLRP3 inflammasome. Our findings support a model for NLRP3 inflammasome activation by Ca(2+)-mediated mitochondrial damage.

Involvement of inositol 1,4,5-trisphosphate formation in the voltage-dependent regulation of the Ca(2+) concentration in porcine coronary arterial smooth muscle cells

The involvement of inositol 1,4,5-trisphosphate (IP(3)) formation in the voltage-dependent regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) was examined in smooth muscle cells of the porcine coronary artery. Slow ramp depolarization from -90 to 0 mV induced progressive [Ca(2+)](i) increase. The slope was reduced or increased in the presence of Cd(2+) or (±)-1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-[trifluoromethyl]-phenyl)pyridine-3-carboxlic acid methyl ester (Bay K 8644), respectively. The decrease in [Ca(2+)](i) via the membrane hyperpolarization induced by K(+) channel openers (levcromakalim and Evans blue) under current clamp was identical to that under voltage clamp. The step hyperpolarization from -40 to -80 mV reduced [Ca(2+)](i) uniformly over the whole-cell area with a time constant of ∼10 s. The [Ca(2+)](i) at either potential was unaffected by heparin, an inhibitor of IP(3) receptors. Alternatively, [Ca(2+)](i) rapidly increased in the peripheral regions by depolarization from -80 to 0 mV and stayed at that level (∼400 nM) during a 60-s pulse. When the pipette solution contained IP(3) pathway blockers [heparin, 2-aminoethoxydiphenylborate, xestospongin C, or 1-[6-[((17β)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione (U73122)], the peak [Ca(2+)](i) was unchanged, but the sustained [Ca(2+)](i) was gradually reduced by ∼250 nM within 30 s. In the presence of Cd(2+), a long depolarization period slightly increased the [Ca(2+)](i), which was lower than that in the presence of heparin alone. In coronary arterial myocytes, the sustained increase in the [Ca(2+)](i) during depolarization was partly caused by the Ca(2+) release mediated by the enhanced formation of IP(3). The initial [Ca(2+)](i) elevation triggered by the Ca(2+) influx though voltage-dependent Ca(2+) channels may be predominantly responsible for the activation of phospholipase C for IP(3) formation.

Optogenetic reporters: Fluorescent protein-based genetically encoded indicators of signaling and metabolism in the brain

Fluorescent protein technology has evolved to include genetically encoded biosensors that can monitor levels of ions, metabolites, and enzyme activities as well as protein conformation and even membrane voltage. They are well suited to live-cell microscopy and quantitative analysis, and they can be used in multiple imaging modes, including one- or two-photon fluorescence intensity or lifetime microscopy. Although not nearly complete, there now exists a substantial set of genetically encoded reporters that can be used to monitor many aspects of neuronal and glial biology, and these biosensors can be used to visualize synaptic transmission and activity-dependent signaling in vitro and in vivo. In this review, we present an overview of design strategies for engineering biosensors, including sensor designs using circularly permuted fluorescent proteins and using fluorescence resonance energy transfer between fluorescent proteins. We also provide examples of indicators that sense small ions (e.g., pH, chloride, zinc), metabolites (e.g., glutamate, glucose, ATP, cAMP, lipid metabolites), signaling pathways (e.g., G protein-coupled receptors, Rho GTPases), enzyme activities (e.g., protein kinase A, caspases), and reactive species. We focus on examples where these genetically encoded indicators have been applied to brain-related studies and used with live-cell fluorescence microscopy.

An efficient method for the long-term and specific expression of exogenous cDNAs in cultured Purkinje neurons

We present a simple and efficient method for expressing cDNAs in Purkinje neurons (PNs) present in heterogeneous mouse cerebellar cultures. The method combines the transfection of freshly dissociated cerebellar cells via nucleofection with the use of novel expression plasmids containing a fragment of the L7 (Pcp2) gene that, within the cerebellum, drives PN-specific expression. The efficiency of PN transfection (determined 13 days post nucleofection) is approximately 70%. Double and triple transfections are routinely achieved at slightly lower efficiencies. Expression in PNs is obvious after one week in culture and still strong after three weeks, by which time these neurons are well-developed. Moreover, high-level expression is restricted almost exclusively to the PNs present in these mixed cultures, which greatly facilitates the characterization of PN-specific functions. As proof of principle, we used this method to visualize (1) the morphology of living PNs expressing mGFP, (2) the localization and dynamics of the dendritic spine proteins PSD-93 and Homer-3a tagged with mGFP and (3) the interaction of live PNs expressing mGFP with other cerebellar neurons expressing mCherry from a β-Actin promoter plasmid. Finally, we created a series of L7-plasmids containing different fluorescent protein cDNAs that are suited for the expression of cDNAs of interest as N- and C-terminally tagged fluorescent fusion proteins. In summary, this procedure allows for the highly efficient, long-term, and specific expression of multiple cDNAs in differentiated PNs, and provides a favorable alternative to two procedures (viral transduction, ballistic gene delivery) used previously to express genes in cultured PNs.

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.

Phospholipase C-η1 is activated by intracellular Ca(2+) mobilization and enhances GPCRs/PLC/Ca(2+) signaling

Phospholipase C-η1 (PLC-η1) is the most recently identified PLC isotype and is primarily expressed in nerve tissue. However, its functional role is unclear. In the present study, we report for the first time that PLC-η1 acts as a signal amplifier in G protein-coupled receptor (GPCR)-mediated PLC and Ca(2+) signaling. Short-hairpin RNA (shRNA)-mediated knockdown of endogenous PLC-η1 reduced lysophosphatidic acid (LPA)-, bradykinin (BK)-, and PACAP-induced PLC activity in mouse neuroblastoma Neuro2A (N2A) cells, indicating that PLC-η1 participates in GPCR-mediated PLC activation. Interestingly, ionomycin-induced PLC activity was significantly decreased by PLC-η1, but not PLC-η2, knockdown. In addition, we found that intracellular Ca(2+) source is enough for PLC-η1 activation. Furthermore, the IP(3) receptor inhibitor, 2-APB, inhibited LPA-induced PLC activity in control N2A cells, whereas this effect was not observed in PLC-η1 knockdown N2A cells, suggesting a pivotal role of intracellular Ca(2+) mobilization in PLC-η1 activation. Finally, we found that LPA-induced ERK1/2 phosphorylation and expression of the downstream target gene, krox-24, were significantly decreased by PLC-η1 knockdown, and these knockdown effects were abolished by 2-APB. Taken together, our results strongly suggest that PLC-η1 is activated via intracellular Ca(2+) mobilization from the ER, and therefore amplifies GPCR-mediated signaling.

Inositol 1,4,5-trisphosphate signaling maintains the activity of glutamate uptake in Bergmann glia

The maintenance of synaptic functions is essential for neuronal information processing in the adult brain. Astrocytes express glutamate transporters that rapidly remove glutamate from the extracellular space and they play a critical role in the precise operation of glutamatergic transmission. However, how the glutamate clearance function of astrocytes is maintained remains elusive. Here, we describe a maintenance mechanism for the glutamate uptake capacity of Bergmann glial cells (BGs) in the cerebellum. When inositol 1,4,5-trisphosphate (IP(3) ) signaling was chronically and selectively inhibited in BGs in vivo, the retention time of glutamate around parallel fiber-Purkinje cell synapses was increased. Under these conditions, a decrease in the level of the glutamate/aspartate transporter (GLAST) in BGs was observed. The same effects were observed after chronic in vivo inhibition of purinergic P2 receptors in the cerebellar cortex. These results suggest that the IP(3) signaling cascade is involved in regulating GLAST levels in BGs to maintain glutamate clearance in the mature cerebellum.

Spatiotemporal dynamics of Ca2+ signaling and its physiological roles

Changes in the intracellular Ca(2+) concentration regulate numerous cell functions and display diverse spatiotemporal dynamics, which underlie the versatility of Ca(2+) in cell signaling. In many cell types, an increase in the intracellular Ca(2+) concentration starts locally, propagates within the cell (Ca(2+) wave) and makes oscillatory changes (Ca(2+) oscillation). Studies of the intracellular Ca(2+) release mechanism from the endoplasmic reticulum (ER) showed that the Ca(2+) release mechanism has inherent regenerative properties, which is essential for the generation of Ca(2+) waves and oscillations. Ca(2+) may shuttle between the ER and mitochondria, and this appears to be important for pacemaking of Ca(2+) oscillations. Importantly, Ca(2+) oscillations are an efficient mechanism in regulating cell functions, having effects supra-proportional to the sum of duration of Ca(2+) increase. Furthermore, Ca(2+) signaling mechanism studies have led to the development of a method for specific inhibition of Ca(2+) signaling, which has been used to identify hitherto unrecognized functions of Ca(2+) signals.

N-methyl-D-aspartate receptors mediate the phosphorylation and desensitization of muscarinic receptors in cerebellar granule neurons

Changes in synaptic strength mediated by ionotropic glutamate N-methyl-d-asparate (NMDA) receptors is generally considered to be the molecular mechanism underlying memory and learning. NMDA receptors themselves are subject to regulation through signaling pathways that are activated by G-protein-coupled receptors (GPCRs). In this study we investigate the ability of NMDA receptors to regulate the signaling of GPCRs by focusing on the G_q/11-coupled M₃-muscarinic receptor expressed endogenously in mouse cerebellar granule neurons. We show that NMDA receptor activation results in the phosphorylation and desensitization of M₃-muscarinic receptors through a mechanism dependent on NMDA-mediated calcium influx and the activity of calcium-calmodulin-dependent protein kinase II. Our study reveals a complex pattern of regulation where GPCRs (M₃-muscarinic) and NMDA receptors can feedback on each other in a process that is likely to influence the threshold value of signaling networks involved in synaptic plasticity.

Cytosolic PLA2(alpha) activation in Purkinje neurons and its role in AMPA-receptor trafficking

Cytosolic phospholipase A(2)alpha (cPLA(2)alpha) selectively releases arachidonic acid from membrane phospholipids and has been proposed to be involved in the induction of long-term depression (LTD), a form of synaptic plasticity in the cerebellum. This enzyme requires two events for its full activation: Ca(2+)-dependent translocation from the cytosol to organelle membranes in order to access phospholipids as substrates, and phosphorylation by several kinases. However, the subcellular distribution and activation of cPLA(2)alpha in Purkinje cells and the role of arachidonic acid in cerebellar LTD have not been fully elucidated. In cultured Purkinje cells, stimulation of AMPA receptors, but not metabotropic glutamate receptors, triggered translocation of cPLA(2)alpha to the somatic and dendritic Golgi compartments. This translocation required Ca(2+) influx through P-type Ca(2+) channels. AMPA plus PMA, a chemical method for inducing LTD, released arachidonic acid via phosphorylation of cPLA(2)alpha. AMPA plus PMA induced a decrease in surface GluR2 for more than 2 hours. Interestingly, this reduction was occluded by a cPLA(2)alpha-specific inhibitor. Furthermore, PMA plus arachidonic acid caused the prolonged internalization of GluR2 without activating AMPA receptors. These results suggest that cPLA(2)alpha regulates the persistent decrease in the expression of AMPA receptors, underscoring the role of cPLA(2)alpha in cerebellar LTD.

Temporal profiling of changes in phosphatidylinositol 4,5-bisphosphate, inositol 1,4,5-trisphosphate and diacylglycerol allows comprehensive analysis of phospholipase C-initiated signalling in single neurons

Phosphatidylinositol 4,5-bisphosphate (PIP₂) fulfils vital signalling roles in an array of cellular processes, yet until recently it has not been possible selectively to visualize real-time changes in PIP₂ levels within living cells. Green fluorescent protein (GFP)-labelled Tubby protein (GFP-Tubby) enriches to the plasma membrane at rest and translocates to the cytosol following activation of endogenous Gα_q/11-coupled muscarinic acetylcholine receptors in both SH-SY5Y human neuroblastoma cells and primary rat hippocampal neurons. GFP-Tubby translocation is independent of changes in cytosolic inositol 1,4,5-trisphosphate and instead reports dynamic changes in levels of plasma membrane PIP₂. In contrast, enhanced GFP (eGFP)-tagged pleckstrin homology domain of phospholipase C (PLCδ1) (eGFP-PH) translocation reports increases in cytosolic inositol 1,4,5-trisphosphate. Comparison of GFP-Tubby, eGFP-PH and the eGFP-tagged C1₂ domain of protein kinase C-γ [eGFP-C1(2); to detect diacylglycerol] allowed a selective and comprehensive analysis of PLC-initiated signalling in living cells. Manipulating intracellular Ca²⁺ concentrations in the nanomolar range established that GFP-Tubby responses to a muscarinic agonist were sensitive to intracellular Ca²⁺ up to 100–200 nM in SH-SY5Y cells, demonstrating the exquisite sensitivity of agonist-mediated PLC activity within the range of physiological resting Ca²⁺ concentrations. We have also exploited GFP-Tubby selectively to visualize, for the first time, real-time changes in PIP₂ in hippocampal neurons.

Release of arachidonic acid by 2-arachidonoyl glycerol and HU210 in PC12 cells; roles of Src, phospholipase C and cytosolic phospholipase A(2)alpha

The phospholipase A(2) (PLA(2))-prostanoid cascade is involved in cannabinoid receptor-mediated neuronal functions. We investigated the signaling mechanism for the release of arachidonic acid by cannabinoids, 2-arachidonoyl glycerol (2-AG) and HU210, in rat PC12 cells and in primary cultured cells from the mouse cerebellum. The effect of selective inhibitors for signaling pathways and/or enzymes (alpha type cytosolic PLA(2) (cPLA(2)alpha), G protein, Src kinases, phospholipase C, protein kinase C) was assessed. Methods included translocation of the chimeric protein GFP-cPLA(2)alpha, the activities of Src family kinases, Ca(2+)-dependent fluorescence and cyclic AMP accumulation. Treatment with 2-AG and HU210 at greater concentrations than 3 muM caused the release of arachidonic acid, and the response was inhibited by AM251 (an antagonist of cannabinoid CB(1) receptor) and by pyrrophenone (a selective inhibitor of cPLA(2)alpha) in PC12 cells. The cannabinoid treatment caused the intracellular translocation of cPLA(2)alpha and an increase in the intracellular Ca(2+) level. Treatment with HU210 caused tyrosine phosphorylation of Src and Fyn, and increased their kinase activities. Pretreatment with inhibitors of tyrosine kinases or phospholipase C abolished the cannabinoids-induced release of arachidonic acid and Ca(2+) response, and protein kinase C inhibitor reduced the release of arachidonic acid. 2-AG caused the release of arachidonic acid from cultured cells of the mouse cerebellum via similar mechanisms. These data reveal that cannabinoids activated cPLA(2)alpha in a Src-phospholipase C-protein kinase C-dependent manner probably via cannabinoid CB(1) receptor and/or CB(1)-like receptor in neuronal cells.

Regulation of neurite growth by spontaneous Ca2+ oscillations in astrocytes

Astrocytes play a pivotal role in the regulation of neurite growth, but the intracellular signaling mechanism in astrocytes that mediates this regulation remains unclarified. We studied the relationship between spontaneous Ca(2+) oscillations in astrocytes and the astrocyte-mediated neurite growth. We generated Ca(2+) signal-deficient astrocytes in which spontaneous Ca(2+) oscillations were abolished by a chronic inhibition of IP(3) signaling. When hippocampal neurons were cultured on a monolayer of Ca(2+) signal-deficient astrocytes, the growth of dendrites and axons was inhibited. Time-lapse imaging of the advancement of axonal growth cones indicated the involvement of membrane-bound molecules for this inhibition. Among six candidate membrane-bound molecules that may modulate neuronal growth, N-cadherin was downregulated in Ca(2+) signal-deficient astrocytes. Although a blocking antibody to N-cadherin suppressed the axonal growth on control astrocytes, extrinsic N-cadherin expression rescued the suppressed axonal growth on Ca(2+) signal-deficient astrocytes. These findings suggest that spontaneous Ca(2+) oscillations regulate the astrocytic function to promote neurite growth by maintaining the expression of specific growth-enhancing proteins on their surface, and that N-cadherin is one of such molecules.

Receptor- and calcium-dependent induced inositol 1,4,5-trisphosphate increases in PC12h cells as shown by fluorescence resonance energy transfer imaging

The production and further metabolism of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] require several calcium-dependent enzymes, but little is known about subsequent calcium-dependent changes in cellular Ins(1,4,5)P3. To study the calcium dependence of muscarinic acetylcholine receptor-induced Ins(1,4,5)P3 increases in PC12h cells, we utilized an Ins(1,4,5)P3 imaging system based on fluorescence resonance energy transfer and using green fluorescent protein variants fused with the pleckstrin homology domain of phospholipase C-delta1. The intracellular calcium concentration, monitored by calcium imaging, was adjusted by thapsigargin pretreatment or alterations in extracellular calcium concentration, enabling rapid receptor-independent changes in calcium concentration via store-operated calcium influx. We found that Ins(1,4,5)P3 production was increased by a combination of receptor- and calcium-dependent components, rather than by calcium alone. The level of Ins(1,4,5)P3 induced by the receptor was found to be half that induced by the combined receptor and calcium components. Increases in calcium levels prior to receptor activation did not affect the subsequent receptor-induced Ins(1,4,5)P3 increase, indicating that calcium does not influence Ins(1,4,5)P3 production without receptor activation. Removal of both the receptor agonists and calcium rapidly restored calcium and Ins(1,4,5)P3 levels, whereas removal of calcium alone restored calcium to its basal concentration. Similar calcium-dependent increases in Ins(1,4,5)P3 were also observed in Chinese hamster ovary cells expressing m1 muscarinic acetylcholine receptor, indicating that the observed calcium dependence is common to Ins(1,4,5)P3 production. To our knowledge, our results are the first showing receptor- and calcium-dependent components within cellular Ins(1,4,5)P3.

Carbonic anhydrase related protein 8 mutation results in aberrant synaptic morphology and excitatory synaptic function in the cerebellum

Carbonic anhydrase related protein 8 (Car8) is known to be abundantly expressed in Purkinje cells (PCs), and its genetic mutation causes a motor coordination defect. To determine the underlying mechanism, we analyzed the mouse cerebellum carrying a Car8 mutation. Electrophysiological analysis showed that spontaneous excitatory transmission was largely diminished while paired pulse ratio at parallel fiber-PC synapses was comparable to wild-type, suggesting functional synapses have normal release probability but the number of functional synapses may be lower in mutants. Light microscopic study revealed an abnormal extension of climbing fibers to the distal PC dendrites. At the ultrastructural level, we found numerous PC spines not forming synapses primarily in distal dendrites and occasionally multiple spines contacting a single varicosity. These abnormalities of parallel fiber-PC synapses may underlie the functional defect in excitatory transmission. Thus, Car8 plays a critical role in synaptogenesis and/or maintenance of proper synaptic morphology and function in the cerebellum.

Persistent activation of protein kinase Calpha is not necessary for expression of cerebellar long-term depression

Protein kinase Calpha (PKCalpha) plays a major role in the induction of long-term depression (LTD) in a cerebellar Purkinje cell (PC). The sequential activation model for classical PKC states that PKCalpha translocates to the plasma membrane by binding Ca(++) and then becomes fully activated by binding diacylglycerol (DAG), which enables estimation of the activity by monitoring its localization. Here, we performed simultaneous electrophysiological recording and fluorescence imaging in a cultured PC expressing GFP-tagged PKCalpha. When a PC was depolarized, PKCalpha transiently translocated to the plasma membrane in a Ca(++)-dependent manner. Application of membrane permeable DAG or the blocker of DAG lipase prolonged the translocation. These results suggest that the sequential activation model is applicable to PCs. Conjunctive applications of glutamate and depolarization pulse induced LTD, but did not prolong the translocation. Thus, our results imply that persistent activation of PKCalpha is not necessary for the expression of LTD.

Modulation of Gq-protein-coupled inositol trisphosphate and Ca2+ signaling by the membrane potential

Gq-protein-coupled receptors (GqPCRs) are widely distributed in the CNS and play fundamental roles in a variety of neuronal processes. Their activation results in phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis and Ca2+ release from intracellular stores via the phospholipase C (PLC)-inositol 1,4,5-trisphosphate (IP3) signaling pathway. Because early GqPCR signaling events occur at the plasma membrane of neurons, they might be influenced by changes in membrane potential. In this study, we use combined patch-clamp and imaging methods to investigate whether membrane potential changes can modulate GqPCR signaling in neurons. Our results demonstrate that GqPCR signaling in the human neuronal cell line SH-SY5Y and in rat cerebellar granule neurons is directly sensitive to changes in membrane potential, even in the absence of extracellular Ca2+. Depolarization has a bidirectional effect on GqPCR signaling, potentiating thapsigargin-sensitive Ca2+ responses to muscarinic receptor activation but attenuating those mediated by bradykinin receptors. The depolarization-evoked potentiation of the muscarinic signaling is graded, bipolar, non-inactivating, and with no apparent upper limit, ruling out traditional voltage-gated ion channels as the primary voltage sensors. Flash photolysis of caged IP3/GPIP2 (glycerophosphoryl-myo-inositol 4,5-bisphosphate) places the voltage sensor before the level of the Ca2+ store, and measurements using the fluorescent bioprobe eGFP-PH(PLCdelta) (enhanced green fluorescent protein-pleckstrin homology domain-PLCdelta) directly demonstrate that voltage affects muscarinic signaling at the level of the IP3 production pathway. The sensitivity of GqPCR IP3 signaling in neurons to voltage itself may represent a fundamental mechanism by which ionotropic signals can shape metabotropic receptor activity in neurons and influence processes such as synaptic plasticity in which the detection of coincident signals is crucial.

A mathematical model for simultaneous spatio-temporal dynamics of calcium and inositol 1,4,5-trisphosphate in Madin-Darby canine kidney epithelial cells

The landmark paper by Hirose et al. (Hirose, K., Kadowaki, S., Tanabe, M., Takeshima, H., Iino, M., Science 284:1527-1530, 1999) presented experimental investigations to show that not only can calcium upregulate IP(3), but that it can also have an inhibitory effect on IP(3). In this paper, we present a preliminary model, which is consistent with these experiments. This model includes positive and negative feedback between calcium and IP(3) and is able to reproduce more precisely the data presented in Hirose et al. (Hirose, K., Kadowaki, S., Tanabe, M., Takeshima, H., Iino, M., Science 284:1527-1530, 1999). In the second part of the paper, the intracellular and intercellular calcium movement in Madin-Darby canine kidney epithelial cells is investigated. With the aid of the model we are able to identify the aspects of IP(3) and calcium signalling, which should be studied further experimentally before refining the model.

Simultaneous measurement of superoxide generation and intracellular Ca2+ concentration reveals the effect of extracellular Ca2+ on rapid and transient contents of superoxide generation in differentiated THP-1 cells

We invented a simultaneous measuring instrument of fluorescence and chemiluminescence, realizing the analysis of chronological correlation between change in intracellular Ca2+ concentration ([Ca2+]i) and superoxide generation. A human monocytic cell line, THP-1, differentiated to be neutrophil-like cells generated superoxide with increase in intracellular Ca2+ concentration when stimulated with formyl-methionyl-leucyl-phenylalanine (fMLP) whereas PMA, phorbol ester-stimulated superoxide response occurred without change in [Ca2+]i. The cells treated with TMB-8, an intracellular Ca2+ antagonist, generated superoxide rapidly as well as transiently with transient [Ca2+]i elevation after stimulation with fMLP, whereas EGTA-treated cells generated superoxide slowly as well as persistently with transient [Ca2+]i elevation after the stimulation. These results suggest that the rapid and transient contents of superoxide generation are specific for Ca2+ influx from the extracellular domain. Verapamil, voltage-dependent Ca2+ channel blocker, dose-dependently inhibited fMLP-stimulated extracellular Ca2+ influx and superoxide generation without affecting PMA-stimulated superoxide generation. Other channel blockers tested, nifedipine and diltiazem, similarly inhibited these fMLP-stimulated responses. Numerical analysis of the values of the response curves elucidated that TMB-8 or the channel blocker reveals or eliminates the same contents of superoxide generation by the antagonism of intracellular Ca2+ release or extracellular Ca2+ influx, respectively. Taking these results together, the characteristic extracellular Ca2+ influx essential for superoxide generation was first revealed by the simultaneous measurement of superoxide generation and change in [Ca2+]i.

Postsynaptic inositol 1,4,5-trisphosphate signaling maintains presynaptic function of parallel fiber-Purkinje cell synapses via BDNF

The maintenance of synaptic functions is essential for neuronal information processing, but cellular mechanisms that maintain synapses in the adult brain are not well understood. Here, we report an activity-dependent maintenance mechanism of parallel fiber (PF)-Purkinje cell (PC) synapses in the cerebellum. When postsynaptic metabotropic glutamate receptor (mGluR) or inositol 1,4,5-trisphosphate (IP(3)) signaling was chronically inhibited in vivo, PF-PC synaptic strength decreased because of a decreased transmitter release probability. The same effects were observed when PF activity was inhibited in vivo by the suppression of NMDA receptor-mediated inputs to granule cells. PF-PC synaptic strength similarly decreased after the in vivo application of an antibody against brain-derived neurotrophic factor (BDNF). Furthermore, the weakening of synaptic connection caused by the blockade of mGluR-IP(3) signaling was reversed by the in vivo application of BDNF. These results indicate that a signaling cascade comprising PF activity, postsynaptic mGluR-IP(3) signaling and subsequent BDNF signaling maintains presynaptic functions in the mature cerebellum.

Synergistic control of protein kinase Cgamma activity by ionotropic and metabotropic glutamate receptor inputs in hippocampal neurons

Conventional protein kinase C (PKC) isoforms are abundant neuronal signaling proteins with important roles in regulating synaptic plasticity and other neuronal processes. Here, we investigate the role of ionotropic and metabotropic glutamate receptor (iGluR and mGluR, respectively) activation on the generation of Ca2+ and diacylglycerol (DAG) signals and the subsequent activation of the neuron-specific PKCgamma isoform in hippocampal neurons. By combining Ca2+ imaging with total internal reflection microscopy analysis of specific biosensors, we show that elevation of both Ca2+ and DAG is necessary for sustained translocation and activation of EGFP (enhanced green fluorescent protein)-PKCgamma. Both DAG production and PKCgamma translocation were localized processes, typically observed within discrete microdomains along the dendritic branches. Markedly, intermediate-strength NMDA receptor (NMDAR) activation or moderate electrical stimulation generated Ca2+ but no DAG signals, whereas mGluR activation generated DAG but no Ca2+ signals. Both receptors were needed for PKCgamma activation. This suggests that a coincidence detection process exists between iGluRs and mGluRs that relies on a molecular coincidence detection process based on the corequirement of Ca2+ and DAG for PKCgamma activation. Nevertheless, the requirement for costimulation with mGluRs could be overcome for maximal NMDAR stimulation through a direct production of DAG via activation of the Ca2+-sensitive PLCdelta (phospholipase Cdelta) isoform. In a second important exception, mGluRs were sufficient for PKCgamma activation in neurons in which Ca2+ stores were loaded by previous electrical activity. Together, the dual activation requirement for PKCgamma provides a plausible molecular interpretation for different synergistic contributions of mGluRs to long-term potentiation and other synaptic plasticity processes.

Ca2+-Dependent Inositol 1,4,5-Trisphosphate and Nitric Oxide Signaling in Cerebellar Neurons

Intracellular Ca(2+) signals are important for the regulation of synaptic functions in the central nervous system. In this review, I summarize findings of our recent studies on upstream and downstream Ca(2+) signaling mechanisms in cerebellar synapses using novel molecular imaging methods. Inositol 1,4,5-trisphosphate (IP(3))-induced Ca(2+) release plays a pivotal role in central synapses. The visualization of IP(3) at fine dendrites of Purkinje cells (PCs) using a fluorescent IP(3) indicator showed that intracellular Ca(2+) concentration has a stimulatory effect on phospholipase C activity, which catalyzes IP(3) production. This indicates that metabotropic and ionotropic glutamate receptors collaborate to generate IP(3) signals. Using a novel nitric oxide (NO) indicator, the spatial distribution of NO signals originating from parallel fiber (PF) terminals was visualized. Our results show that the NO signal decays steeply with distance from the site of production in the cerebellum and is dependent on PF stimulation frequency in a biphasic manner. NO released from PF terminals generated a synapse-specific long-term potentiation of PF-PC synapse when PF was stimulated at certain frequencies. These imaging studies clarified new aspects of the regulatory mechanisms of synaptic functions.

Calcium From Internal Stores Triggers GABA Release From Retinal Amacrine Cells

The Ca2+ that promotes transmitter release is generally thought to enter presynaptic terminals through voltage-gated Ca2+channels. Using electrophysiology and Ca2+ imaging, we show that, in amacrine cell dendrites, at least some of the Ca2+ that triggers transmitter release comes from endoplasmic reticulum Ca2+ stores. We show that both inositol 1,4,5-trisphosphate receptors (IP3Rs) and ryanodine receptors (RyRs) are present in these dendrites and both participate in the elevation of cytoplasmic [Ca2+] during the brief depolarization of a dendrite. Only the Ca2+ released through IP3Rs, however, seems to promote the release of transmitter. Antagonists for the IP3R reduced transmitter release, whereas RyR blockers had no effect. Application of an agonist for metabotropic glutamate receptor, known to liberate Ca2+ from internal stores, enhanced both spontaneous and evoked transmitter release.

The latest phospholipase C, PLCeta, is implicated in neuronal function

Members of the phosphoinositide-specific phospholipase C (PLC) family have key roles in cell signalling. In response to many extracellular stimuli, such as hormones, neurotransmitters, antigens and growth factors, PLCs catalyse the hydrolysis of phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P(2)], thereby generating two well-established second messengers, inositol (1,4,5)-trisphosphate and diacylglycerol. Eleven PLC isozymes encoded by different genes have been identified in mammals and, on the basis of their structure and sequence relationships, have been classified into five families designated PLCbeta (1-4), PLCgamma (1 and 2), PLCdelta (1, 3 and 4), PLCepsilon (1) and PLCzeta (1). All PLCs contain the catalytic X and Y domain, in addition to other regulatory domains including the C2 domain and the EF-hand domain. In 2005, four groups independently identified an entirely new family of PLCs--eta1 and eta2--using data mining of mammalian genomes. The properties of the PLCeta enzyme suggest that it might act as a Ca(2+) sensor, in particular, functioning during formation and maintenance of the neuronal network in the postnatal brain.

Inositol (1,4,5)-trisphosphate dynamics and intracellular calcium oscillations in pancreatic beta-cells

Glucose-stimulated insulin secretion is associated with transients of intracellular calcium concentration ([Ca2+]i) in the pancreatic beta-cell. We tested the hypothesis that inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] [Ca2+]i release is incorporated in glucose-induced [Ca2+]i oscillations in mouse islets and MIN6 cells. We found that depletion of intracellular Ca2+ stores with thapsigargin increased the oscillation frequency by twofold and inhibited the slow recovery phase of [Ca2+]i oscillations. We employed a pleckstrin homology domain-containing fluorescent biosensor, phospholipase C partial differential pleckstrin homology domain-enhanced green fluorescent protein, to visualize Ins(1,4,5)P3 dynamics in insulin-secreting MIN6 cells and mouse islets in real time using a video-rate confocal system. In both types of cells, stimulation with carbamoylcholine (CCh) and depolarization with KCl results in an increase in Ins(1,4,5)P3 accumulation in the cytoplasm. When stimulated with glucose, the Ins(1,4,5)P3 concentration in the cytoplasm oscillates in parallel with oscillations of [Ca2+]i. Maximal accumulation of Ins(1,4,5)P3 in these oscillations coincides with the peak of [Ca2+]i and tracks changes in frequencies induced by the voltage-gated K+ channel blockade. We show that Ins(1,4,5)P3 release in insulin-secreting cells can be stimulated by depolarization-induced Ca2+ flux. We conclude that Ins(1,4,5)P3 concentration oscillates in parallel with [Ca2+]i in response to glucose stimulation, but it is not the driving force for [Ca2+]i oscillations.

Identification and characterization of a novel Delphilin variant with an alternative N-terminus

Delphilin is identified as a Glutamate receptor delta2 (GluRdelta2) subunit interacting protein, consisting of a PDZ domain and formin homology (FH) domains 1 and 2, in addition to a C-terminal coiled-coil structure. Delphilin has been shown to be selectively expressed in cerebellar Purkinje cells where it co-localizes with the GluRdelta2 subunit at the Purkinje cell-parallel fiber synapses. Although Delphilin specifically interacts with the GluRdelta2 C-terminus via its PDZ domain, the physiological role of the interaction is not yet understood. Here, we report that the Delphilin protein exhibits diversity at its N-terminus by variable usage of the first several exons. Interestingly, the two Delphilin mRNAs which correspond to the first one initially identified (now designated as Delphilin alpha) and the second that contains a newly identified first exon (designated as Delphilin beta), show different chronological expression profiles. Delphilin beta mRNA was not decreased throughout the cerebellar development in vivo and in vitro, while in vivo Delphilin alpha mRNA gradually decreases following the first postnatal week. Delphilins alpha and beta also revealed different subcellular distribution with some overlap. Specifically, the cerebellar synaptosomal membrane fraction contained the Delphilin beta protein. Both Delphilin alpha and beta localized at the dendritic spines with GluRdelta2; however, dendritic shafts in cultured Purkinje cells also included Delphilin beta. In MDCK cells upon becoming confluent, Delphilin alpha moved to the cell-cell junction area, whereas Delphilin beta maintained a diffuse distribution pattern throughout the cytoplasm. Taken as a whole, these two different Delphilins seemed to play functionally different roles in developing and matured cerebellar Purkinje cells.