Voltage-independent inhibition of P/Q-type Ca2+ channels in adrenal chromaffin cells via a neuronal Ca2+ sensor-1-dependent pathway involves Src family tyrosine kinase

Dopamine D2 autoreceptor interactome: Targeting the receptor complex as a strategy for treatment of substance use disorder

Dopamine D2 autoreceptors (D2ARs), located in somatodendritic and axon terminal compartments of dopamine (DA) neurons, function to provide a negative feedback regulatory control on DA neuron firing, DA synthesis, reuptake and release. Dysregulation of D2AR-mediated DA signaling is implicated in vulnerability to substance use disorder (SUD). Due to the extreme low abundance of D2ARs compared to postsynaptic D2 receptors (D2PRs) and the lack of experimental tools to differentiate the signaling of D2ARs from D2PRs, the regulation of D2ARs by drugs of abuse is poorly understood. The recent availability of conditional D2AR knockout mice and newly developed virus-mediated gene delivery approaches have provided means to specifically study the function of D2ARs at the molecular, cellular and behavioral levels. There is a growing revelation of novel mechanisms and new proteins that mediate D2AR activity, suggesting that D2ARs act cooperatively with an array of membrane and intracellular proteins to tightly control DA transmission. This review highlights D2AR-interacting partners including transporters, G-protein-coupled receptors, ion channels, intracellular signaling modulators, and protein kinases. The complexity of the D2AR interaction network illustrates the functional divergence of D2ARs. Pharmacological targeting of multiple D2AR-interacting partners may be more effective to restore disrupted DA homeostasis by drugs of abuse.

Chromaffin Cells of the Adrenal Medulla: Physiology, Pharmacology, and Disease

Chromaffin cells (CCs) of the adrenal gland and the sympathetic nervous system produce the catecholamines (epinephrine and norepinephrine; EPI and NE) needed to coordinate the bodily "fight-or-flight" response to fear, stress, exercise, or conflict. EPI and NE release from CCs is regulated both neurogenically by splanchnic nerve fibers and nonneurogenically by hormones (histamine, corticosteroids, angiotensin, and others) and paracrine messengers [EPI, NE, adenosine triphosphate, opioids, γ-aminobutyric acid (GABA), etc.]. The "stimulus-secretion" coupling of CCs is a Ca2+ -dependent process regulated by Ca2+ entry through voltage-gated Ca2+ channels, Ca2+ pumps, and exchangers and intracellular organelles (RE and mitochondria) and diffusible buffers that provide both Ca2+ -homeostasis and Ca2+ -signaling that ultimately trigger exocytosis. CCs also express Na+ and K+ channels and ionotropic (nAChR and GABAA ) and metabotropic receptors (mACh, PACAP, β-AR, 5-HT, histamine, angiotensin, and others) that make CCs excitable and responsive to autocrine and paracrine stimuli. To maintain high rates of E/NE secretion during stressful conditions, CCs possess a large number of secretory chromaffin granules (CGs) and members of the soluble NSF-attachment receptor complex protein family that allow docking, fusion, and exocytosis of CGs at the cell membrane, and their recycling. This article attempts to provide an updated account of well-established features of the molecular processes regulating CC function, and a survey of the as-yet-unsolved but important questions relating to CC function and dysfunction that have been the subject of intense research over the past 15 years. Examples of CCs as a model system to understand the molecular mechanisms associated with neurodegenerative diseases are also provided. Published 2019. Compr Physiol 9:1443-1502, 2019.

Calcium Sensors in Neuronal Function and Dysfunction

Calcium signaling in neurons as in other cell types can lead to varied changes in cellular function. Neuronal Ca²⁺ signaling processes have also become adapted to modulate the function of specific pathways over a wide variety of time domains and these can have effects on, for example, axon outgrowth, neuronal survival, and changes in synaptic strength. Ca²⁺ also plays a key role in synapses as the trigger for fast neurotransmitter release. Given its physiological importance, abnormalities in neuronal Ca²⁺ signaling potentially underlie many different neurological and neurodegenerative diseases. The mechanisms by which changes in intracellular Ca²⁺ concentration in neurons can bring about diverse responses is underpinned by the roles of ubiquitous or specialized neuronal Ca²⁺ sensors. It has been established that synaptotagmins have key functions in neurotransmitter release, and, in addition to calmodulin, other families of EF-hand-containing neuronal Ca²⁺ sensors, including the neuronal calcium sensor (NCS) and the calcium-binding protein (CaBP) families, play important physiological roles in neuronal Ca²⁺ signaling. It has become increasingly apparent that these various Ca²⁺ sensors may also be crucial for aspects of neuronal dysfunction and disease either indirectly or directly as a direct consequence of genetic variation or mutations. An understanding of the molecular basis for the regulation of the targets of the Ca²⁺ sensors and the physiological roles of each protein in identified neurons may contribute to future approaches to the development of treatments for a variety of human neuronal disorders.

Emerging Roles of Neuronal Ca2+ Sensor-1 in Cardiac and Neuronal Tissues: A Mini Review

The EF-hand calcium (Ca²⁺)-binding protein, neuronal Ca²⁺ sensor-1 (NCS-1/frequenin), is predominantly expressed in neuronal tissues and plays a crucial role in neuronal functions, including synaptic transmission and plasticity. NCS-1 has diverse functional roles, as elucidated in the past 15 years, which include the regulation of phosphatidylinositol 4-kinase IIIβ (PI-4K-β) and several ion channels such as voltage-gated K⁺ and Ca²⁺ channels, the D2 dopamine receptors, and inositol 1,4,5-trisphosphate receptors (InsP₃Rs). Functional analyses demonstrated that NCS-1 enhances exocytosis and neuronal survival after injury, as well as promotes learning and memory in mice. NCS-1 is also expressed in the heart including the Purkinje fibers (PFs) of the conduction system. NCS-1 interacts with K_V4 K⁺ channels together with dipeptidyl peptidase-like protein-6 (DPP-6), and this macromolecule then composes the transient outward current in PFs and contributes to the repolarization of PF action potential, thus being responsible for idiopathic arrhythmia. Moreover, NCS-1 expression was reported to be significantly high at the immature stage and at hypertrophy in adults. That report demonstrated that NCS-1 positively regulates cardiac contraction in immature hearts by increasing intracellular Ca²⁺ signals through interaction with InsP₃Rs. With the related signals, NCS-1 activates nuclear Ca²⁺ signals, which would be a mechanism underlying hormone-induced cardiac hypertrophy. Furthermore, NCS-1 contributes to stress tolerance in cardiomyocytes by activating mitochondrial detoxification pathways, with a key role in Ca²⁺-dependent pathways. In this review, we will discuss recent findings supporting the functional significance of NCS-1 in the brain and heart and will address possible underlying molecular mechanisms.

Interaction between neuronal calcium sensor protein 1 and lithium in pedunculopontine neurons

Bipolar disorder is characterized by sleep dysregulation, suggesting a role for the reticular activating system (RAS). Postmortem studies showed increased expression of neuronal calcium sensor protein 1 (NCS-1) in the brains of some bipolar disorder patients, and reduced or aberrant gamma band activity is present in the same disorder. Lithium (Li+) has been shown to effectively treat the mood disturbances in bipolar disorder patients. We previously showed that NCS-1 at low levels increased, and at high levels decreased, gamma oscillations in RAS pedunculopontine neurons (PPN), and that Li+ decreased these oscillations. We previously described the effects of each agent on oscillations, G-protein mechanisms, and Ca2+ currents. However, we designed the present experiments to determine the nature of the interaction of NCS-1 and Li+ at physiological concentrations that would have an effect within minutes of application. As expected, Li+ decreased gamma oscillation amplitude, while NCS-1 increased the amplitude of gamma oscillations. We identified NCS-1 at 2 μmol/L as a concentration that increased gamma oscillations within 5-10 min, and Li+ at 10 μmol/L as a concentration that decreased gamma oscillations within 5 min. The combined application of NCS-1 and Li+ at these concentrations showed that Li+ reduced the effects of NCS-1 on oscillation amplitude within 5-10 min. These results demonstrate that at physiological levels, Li+ acts to reduce the effects of NCS-1 so that, given over expression of NCS-1, Li+ would have salutary effects.

Role of calcium channels in bipolar disorder

Bipolar disorder is characterized by a host of sleep-wake abnormalities that suggests that the reticular activating system (RAS) is involved in these symptoms. One of the signs of the disease is a decrease in high frequency gamma band activity, which accounts for a number of additional deficits. Bipolar disorder has also been found to overexpress neuronal calcium sensor protein 1 (NCS-1). Recent studies showed that elements in the RAS generate gamma band activity that is mediated by high threshold calcium (Ca2+) channels. This mini-review provides a description of recent findings on the role of Ca2+ and Ca2+ channels in bipolar disorder, emphasizing the involvement of arousal-related systems in the manifestation of many of the disease symptoms. This will hopefully bring attention to a much-needed area of research and provide novel avenues for therapeutic development.

Src-family tyrosine kinases and the Ca2+ signal

In this review, we shall describe the rich crosstalk between non-receptor Src-family kinases (SFKs) and the Ca²⁺ transient generated in activated cells by a variety of extracellular and intracellular stimuli, resulting in diverse signaling events. The exchange of information between SFKs and Ca²⁺ is reciprocal, as it flows in both directions. These kinases are main actors in pathways leading to the generation of the Ca²⁺ signal, and reciprocally, the Ca²⁺ signal modulates SFKs activity and functions. We will cover how SFKs participate in the generation of the cytosolic Ca²⁺ rise upon activation of a series of receptors and the mechanism of clearance of this Ca²⁺ signal. The role of SFKs modulating Ca²⁺-translocating channels participating in these events will be amply discussed. Finally, the role of the Ca²⁺ sensor protein calmodulin on the activity of c-Src, and potentially on other SFKs, will be outlined as well. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Lithium decreases the effects of neuronal calcium sensor protein 1 in pedunculopontine neurons

Human postmortem studies reported increased expression of neuronal calcium sensor protein 1 (NCS-1) in the brains of some bipolar disorder patients, and reduced or aberrant gamma band activity is present in the same disorder. Bipolar disorder is characterized by sleep dysregulation, suggesting a role for the reticular activating system (RAS). Lithium (Li(+)) has been shown to effectively treat the mood disturbances in bipolar disorder patients and was proposed to act by inhibiting the interaction betweenNCS-1 and inositol 1,4,5-triphosphate receptor protein (InsP3R).NCS-1 is known to enhance the activity of InsP3R, and of Ca(2+)-mediated gamma oscillatory activity in the pedunculopontine nucleus (PPN), part of theRAS This study aimed to determine the nature of some of the intracellular mechanisms of Li(+)on ratPPNcells and to identify the interaction between Li(+)andNCS-1. Since Li(+)has been shown to act by inhibiting the enhancing effects ofNCS-1, we tested the hypothesis that Li(+)would reduced the effects of overexpression ofNCS-1 and prevent the downregulation of gamma band activity. Li(+)decreased gamma oscillation frequency and amplitude by downregulating Ca(2+)channel activity, whereasNCS-1 reduced the effect of Li(+)on Ca(2+)currents. These effects were mediated by a G-protein overinhibition of Ca(2+)currents. These results suggest that Li(+)affected intracellular pathways involving the activation of voltage-gated Ca(2+)channels mediated by an intracellular mechanism involving voltage-dependent activation of G proteins, thereby normalizing gamma band oscillations mediated by P/Q-type calcium channels modulated byNCS-1.

Comparative phosphoproteomic analysis of intestinal phosphorylated proteins in active versus aestivating sea cucumbers

The sea cucumber Apostichopus japonicus is becoming an excellent model marine invertebrate for studies of environmentally-induced aestivation. Reversible protein phosphorylation as a regulatory mechanism in aestivation is known for some terrestrial aestivators but has never before been documented in sea cucumbers. The present study provides a global quantitative analysis of the role of reversible phosphorylation in sea cucumber aestivation by using tandem mass tag (TMT) labeling followed by an IMAC enrichment strategy to map aestivation-responsive changes in the phosphoproteome of sea cucumber intestine. We identified 2295 unique phosphosites derived from 1283 phosphoproteins and, of these, 211 hyperphosphorylated and 65 hypophosphorylated phosphoproteins were identified in intestine during deep aestivation compared with the active state based on the following criterion: quantitative ratios over 1.5 or less than 0.67 with corrected p-value <0.05. Six major functional classes of proteins exhibited changes in their phosphorylation status during aestivation: (1) protein synthesis, (2) transcriptional regulators, (3) kinases, (4) signaling, (5) transporter, (6) DNA binding. These data on the global involvement of phosphorylation in sea cucumber aestivation significantly improve our understanding of the regulatory mechanisms involved in metabolic arrest when marine invertebrates face environmental stress and provide substantial candidate phosphorylated proteins that could be important for identifying functionally adaptive variation in marine invertebrates.

SIGNIFICANCE

Sea cucumber Apostichopus japonicus is an excellent model organism for studies of environmentally-induced aestivation by a marine invertebrate. The present study provides the first quantitative phosphoproteomic analysis of sea cucumber aestivation using isobaric tag based TMT labeling followed by an IMAC enrichment strategy. These data on the global involvement of phosphorylation in sea cucumber aestivation significantly improve our understanding of the regulatory mechanism involved in metabolic arrest when marine invertebrates face environmental stress and provide substantial candidate phosphorylated proteins that could be important for identifying functionally adaptive variation in marine invertebrates. This study also demonstrates the usefulness of the TMT-based quantitative phosphoproteomics approach to explore the survival responses of a non-model marine invertebrate species to seasonal changes in its environment.

Modulation of CaV2.1 channels by neuronal calcium sensor-1 induces short-term synaptic facilitation

Facilitation and inactivation of P/Q-type Ca2+ currents mediated by Ca2+/calmodulin binding to Ca(V)2.1 channels contribute to facilitation and rapid depression of synaptic transmission, respectively. Other calcium sensor proteins displace calmodulin from its binding site and differentially modulate P/Q-type Ca2 + currents, resulting in diverse patterns of short-term synaptic plasticity. Neuronal calcium sensor-1 (NCS-1, frequenin) has been shown to enhance synaptic facilitation, but the underlying mechanism is unclear. We report here that NCS-1 directly interacts with IQ-like motif and calmodulin-binding domain in the C-terminal domain of Ca(V)2.1 channel. NCS-1 reduces Ca2 +-dependent inactivation of P/Q-type Ca2+ current through interaction with the IQ-like motif and calmodulin-binding domain without affecting peak current or activation kinetics. Expression of NCS-1 in presynaptic superior cervical ganglion neurons has no effect on synaptic transmission, eliminating effects of this calcium sensor protein on endogenous N-type Ca2+ currents and the endogenous neurotransmitter release machinery. However, in superior cervical ganglion neurons expressing wild-type Ca(V)2.1 channels, co-expression of NCS-1 induces facilitation of synaptic transmission in response to paired pulses and trains of depolarizing stimuli, and this effect is lost in Ca(V)2.1 channels with mutations in the IQ-like motif and calmodulin-binding domain. These results reveal that NCS-1 directly modulates Ca(V)2.1 channels to induce short-term synaptic facilitation and further demonstrate that CaS proteins are crucial in fine-tuning short-term synaptic plasticity.

Sense and specificity in neuronal calcium signalling

Changes in the intracellular free calcium concentration ([Ca²⁺]i) in neurons regulate many and varied aspects of neuronal function over time scales from microseconds to days. The mystery is how a single signalling ion can lead to such diverse and specific changes in cell function. This is partly due to aspects of the Ca²⁺ signal itself, including its magnitude, duration, localisation and persistent or oscillatory nature. The transduction of the Ca²⁺ signal requires Ca²⁺binding to various Ca²⁺ sensor proteins. The different properties of these sensors are important for differential signal processing and determine the physiological specificity of Ca(2+) signalling pathways. A major factor underlying the specific roles of particular Ca²⁺ sensor proteins is the nature of their interaction with target proteins and how this mediates unique patterns of regulation. We review here recent progress from structural analyses and from functional analyses in model organisms that have begun to reveal the rules that underlie Ca²⁺ sensor protein specificity for target interaction. We discuss three case studies exemplifying different aspects of Ca²⁺ sensor/target interaction. This article is part of a special issue titled the 13th European Symposium on Calcium.

Demonstration of binding of neuronal calcium sensor-1 to the cav2.1 p/q-type calcium channel

In neurons, entry of extracellular calcium (Ca(2+)) into synaptic terminals through Cav2.1 (P/Q-type) Ca(2+) channels is the driving force for exocytosis of neurotransmitter-containing synaptic vesicles. This class of Ca(2+) channel is, therefore, pivotal during normal neurotransmission in higher organisms. In response to channel opening and Ca(2+) influx, specific Ca(2+)-binding proteins associate with cytoplasmic regulatory domains of the P/Q channel to modulate subsequent channel opening. Channel modulation in this way influences synaptic plasticity with consequences for higher-level processes such as learning and memory acquisition. The ubiquitous Ca(2+)-sensing protein calmodulin (CaM) regulates the activity of all types of mammalian voltage-gated Ca(2+) channels, including the P/Q class, by direct binding to specific regulatory motifs. More recently, experimental evidence has highlighted a role for additional Ca(2+)-binding proteins, particularly of the CaBP and NCS families in the regulation of P/Q channels. NCS-1 is a protein found from yeast to humans and that regulates a diverse number of cellular functions. Physiological and genetic evidence indicates that NCS-1 regulates P/Q channel activity, including calcium-dependent facilitation, although a direct physical association between the proteins has yet to be demonstrated. In this study, we aimed to determine if there is a direct interaction between NCS-1 and the C-terminal cytoplasmic tail of the Cav2.1 α-subunit. Using distinct but complementary approaches, including in vitro binding of bacterially expressed recombinant proteins, fluorescence spectrophotometry, isothermal titration calorimetry, nuclear magnetic resonance, and expression of fluorescently tagged proteins in mammalian cells, we show direct binding and demonstrate that CaM can compete for it. We speculate about how NCS-1/Cav2.1 association might add to the complexity of calcium channel regulation mediated by other known calcium-sensing proteins and how this might help to fine-tune neurotransmission in the mammalian central nervous system.

Role of neuronal calcium sensor-1 in the cardiovascular system

Calcium (Ca(2+)) is an important intracellular messenger, regulating myocyte contraction via excitation-contraction (EC) coupling and gene transcription underlying hypertrophy in the heart. Although the mechanisms of EC coupling in the immature heart are believed to be different from those in the adult heart because of the structural immaturity of the sarcoplasmic reticulum in the young heart, the details of these mechanisms are not completely understood. Neuronal Ca(2+) sensor-1 (NCS-1) is an EF-hand Ca(2+)-binding protein that is highly expressed in young hearts; however, little is known about its cardiac functions. In this review, we summarize our recent findings indicating that NCS-1 acts as a novel regulator enhancing Ca(2+) signals in the heart and hence promoting contraction in the immature heart and hypertrophy in the adult heart. Possible signal transduction pathways are also discussed.

Constitutive activity of the A2A adenosine receptor and compartmentalised cyclic AMP signalling fine-tune noradrenaline release

Neuroblastoma SH-SY5Y (SH) cells endogenously express A(2A) adenosine receptors and can be differentiated into a sympathetic neuronal phenotype, capable of depolarisation-dependent noradrenaline release. Using differentiated SH culture, we here explored the link between A(2A)-receptor signalling and neurotransmitter release. In response to the receptor agonist CGS21680, the cells produced cyclic AMP (cAMP), and when depolarised, they released increased amounts of noradrenaline. An A(2A)-receptor antagonist, XAC, as well as an inhibitor of cAMP-dependent protein kinase A (PKA), H89, depressed agonist-dependent release. In the presence of XAC or H89, noradrenaline release was found to be below basal values. This suggested that release facilitation also owes to constitutive receptor activity. We demonstrate that even in the absence of an agonist, the native A(2A)-receptor stimulated cAMP production, leading to the activation of PKA and enhanced noradrenaline release. Ancillary, non-cAMP-dependent effects of the receptor (i.e. phosphorylation of CREB, of Rabphilin3A) were refractory to constitutive activation. PKA-dependent facilitation of noradrenaline release was recapitulated with membrane-permeable 8-Br-cAMP; in addition to facilitation, 8-Br-cAMP caused marked inhibition of release, an effect not observed upon receptor activation. Inhibition by receptor-independent cAMP was likely due to suppression of voltage-dependent calcium current (VDCC) and increased activity of Src-family kinases. Receptor-mediated release facilitation was reproduced in the presence of tetrodotoxin (blocking action potentials); hence, the signalling occurred at the active zone comprising release sites. Our findings thus support (1) presynaptic localisation of the A(2A)-receptor and (2) suggest that compartmentalised pathways transmit cAMP signalling in order to facilitate depolarisation-dependent neurotransmitter release.

Multiple roles for frequenin/NCS-1 in synaptic function and development

The calcium-binding protein frequenin (Frq), discovered in the fruit fly Drosophila, and its mammalian homologue neuronal calcium sensor 1 (NCS-1) have been reported to affect several aspects of synaptic transmission, including basal levels of neurotransmission and short- and long-term synaptic plasticities. However, discrepant reports leave doubts about the functional roles of these conserved proteins. In this review, we attempt to resolve some of these seemingly contradictory reports. We discuss how stimulation protocols, sources of calcium (voltage-gated channels versus internal stores), and expression patterns (presynaptic versus postsynaptic) of Frq may result in the activation of various protein targets, leading to different synaptic effects. In addition, the potential interactions of Frq's C-terminal and N-terminal domains with other proteins are discussed. Frq also has a role in regulating neurite outgrowth, axonal regeneration, and synaptic development. We examine whether the effects of Frq on neurotransmitter release and neurite outgrowth are distinct or interrelated through homeostatic mechanisms. Learning and memory are affected by manipulations of Frq probably through changes in synaptic transmission and neurite outgrowth, raising the possibility that Frq may be implicated in human pathological conditions, including schizophrenia, bipolar disorder, and X-linked mental retardation.

Ca(2+) signaling mechanisms in bovine adrenal chromaffin cells

Calcium (Ca(2+)) is a crucial intracellular messenger in physiological aspects of cell signaling. Adrenal chromaffin cells are the secretory cells from the adrenal gland medulla that secrete catecholamines, which include epinephrine and norepinephrine important in the 'fight or flight' response. Bovine adrenal chromaffin cells have long been used as an important model for secretion -(exocytosis) not only due to their importance in the short-term stress response, but also as a neuroendocrine model of neurotransmtter release, as they have all the same exocytotic proteins as neurons but are easier to prepare, culture and use in functional assays. The components of the Ca(2+) signal transduction cascade and it role in secretion has been extensively characterized in bovine adrenal chromaffin cells. The Ca(2+) sources, signaling molecules and how this relates to the short-term stress response are reviewed in this book chapter in an endeavor to generally -overview these mechanisms in a concise and uncomplicated manner.

Inhibition of Ca2+ channels and adrenal catecholamine release by G protein coupled receptors

Catecholamines and other transmitters released from adrenal chromaffin cells play central roles in the "fight-or-flight" response and exert profound effects on cardiovascular, endocrine, immune, and nervous system function. As such, precise regulation of chromaffin cell exocytosis is key to maintaining normal physiological function and appropriate responsiveness to acute stress. Chromaffin cells express a number of different G protein coupled receptors (GPCRs) that sense the local environment and orchestrate this precise control of transmitter release. The primary trigger for catecholamine release is Ca2+ entry through voltage-gated Ca2+ channels, so it makes sense that these channels are subject to complex regulation by GPCRs. In particular G protein βγ heterodimers (Gbc) bind to and inhibit Ca2+ channels. Here I review the mechanisms by which GPCRs inhibit Ca2+ channels in chromaffin cells and how this might be altered by cellular context. This is related to the potent autocrine inhibition of Ca2+ entry and transmitter release seen in chromaffin cells. Recent data that implicate an additional inhibitory target of Gβγ on the exocytotic machinery and how this might fine tune neuroendocrine secretion are also discussed.

Neuronal calcium sensor-1 regulation of calcium channels, secretion, and neuronal outgrowth

Calcium (Ca(2+)) is an important intracellular messenger underlying cell physiology. Ca(2+) channels are the main entry route for Ca(2+) into excitable cells, and regulate processes such as neurotransmitter release and neuronal outgrowth. Neuronal Calcium Sensor-1 (NCS-1) is a member of the Calmodulin superfamily of EF-hand Ca(2+) sensing proteins residing in the subfamily of NCS proteins. NCS-1 was originally discovered in Drosophila as an overexpression mutant (Frequenin), having an increased frequency of Ca(2+)-evoked neurotransmission. NCS-1 is N-terminally myristoylated, can bind intracellular membranes, and has a Ca(2+) affinity of 0.3 μM. Over 10 years ago it was discovered that NCS-1 overexpression enhances Ca(2+)-evoked secretion in bovine adrenal chromaffin cells. The mechanism was unclear, but there was no apparent direct effect on the exocytotic machinery. It was revealed, again in chromaffin cells, that NCS-1 regulates voltage-gated Ca(2+) channels (Cavs) in G-Protein Coupled Receptor (GPCR) signaling pathways. This work in chromaffin cells highlighted NCS-1 as an important modulator of neurotransmission. NCS-1 has since been shown to regulate and/or directly interact with many proteins including Cavs (P/Q, N, and L), TRPC1/5 channels, GPCRs, IP3R, and PI4 kinase type IIIβ. NCS-1 also affects neuronal outgrowth having roles in learning and memory affecting both short- and long-term synaptic plasticity. It is not known if NCS-1 affects neurotransmission and synaptic plasticity via its effect on PIP2 levels, and/or via a direct interaction with Ca(2+) channels or their signaling complexes. This review gives a historical account of NCS-1 function, examining contributions from chromaffin cells, PC12 cells and other models, to describe how NCS-1's regulation of Ca(2+) channels allows it to exert its physiological effects.

Src family kinases mediate the inhibition of substance P release in the rat spinal cord by μ-opioid receptors and GABA(B) receptors, but not α2 adrenergic receptors

GABA(B) , μ-opioid and adrenergic α(2) receptors inhibit substance P release from primary afferent terminals in the dorsal horn. Studies in cell expression systems suggest that μ-opioid and GABA(B) receptors inhibit transmitter release from primary afferents by activating Src family kinases (SFKs), which then phosphorylate and inhibit voltage-gated calcium channels. This study investigated whether SFKs mediate the inhibition of substance P release by these three receptors. Substance P release was measured as neurokinin 1 receptor (NK1R) internalization in spinal cord slices and in vivo. In slices, NK1R internalization induced by high-frequency dorsal root stimulation was inhibited by the μ-opioid agonist DAMGO and the GABA(B) agonist baclofen. This inhibition was reversed by the SFK inhibitor PP1. NK1R internalization induced by low-frequency stimulation was also inhibited by DAMGO, but PP1 did not reverse this effect. In vivo, NK1R internalization induced by noxious mechanical stimulation of the hind paw was inhibited by intrathecal DAMGO and baclofen. This inhibition was reversed by intrathecal PP1, but not by the inactive PP1 analog PP3. PP1 produced no effect by itself. The α(2) adrenergic agonists medetomidine and guanfacine produced a small but statistically significant inhibition of NK1R internalization induced by low-frequency dorsal root stimulation. PP1 did not reverse the inhibition by guanfacine. These results show that SFKs mediate the inhibition of substance P release by μ-opioid and GABA(B) receptors, but not by α(2) receptors, which is probably mediated by the binding of G protein βγ subunits to calcium channels.

Neuronal calcium sensor-1 modulation of optimal calcium level for neurite outgrowth

Neurite extension and branching are affected by activity-dependent modulation of intracellular Ca2+, such that an optimal window of [Ca2+] is required for outgrowth. Our understanding of the molecular mechanisms regulating this optimal [Ca2+]i remains unclear. Taking advantage of the large growth cone size of cultured primary neurons from pond snail Lymnaea stagnalis combined with dsRNA knockdown, we show that neuronal calcium sensor-1 (NCS-1) regulates neurite extension and branching, and activity-dependent Ca2+ signals in growth cones. An NCS-1 C-terminal peptide enhances only neurite branching and moderately reduces the Ca2+ signal in growth cones compared with dsRNA knockdown. Our findings suggest that at least two separate structural domains in NCS-1 independently regulate Ca2+ influx and neurite outgrowth, with the C-terminus specifically affecting branching. We describe a model in which NCS-1 regulates cytosolic Ca2+ around the optimal window level to differentially control neurite extension and branching.

NCS-1 differentially regulates growth cone and somata calcium channels in Lymnaea neurons

Local voltage-gated calcium channels, which regulate intracellular Ca2+ levels by allowing Ca2+ influx, play an important role in guiding and shaping growth cones, and in regulating the outgrowth and branching of neurites. Therefore, elucidating the mechanisms that regulate the biophysical properties of whole-cell calcium currents in the growth cones and somata of growing neurons is important to improving our understanding of neuronal development and regeneration. In this study, taking advantage of the large size of the pedal A (PeA) neurons in Lymnaea stagnalis, we compared the biophysical properties of somata and growth cone whole-cell calcium channel currents using Ba2+ and Ca2+ as current carriers. We found that somata and growth cone currents exhibit similar high-voltage activation properties. However, Ba2+ and Ca2+ currents in growth cones and somata are differentially affected by a dominant-negative peptide containing the C-terminal amino acid sequence of neuronal calcium sensor-1 (NCS-1). The peptide selectively reduces the peak and sustained components of current densities and the slope conductance in growth cones, and shifts the reversal potential of the growth cone currents to more hyperpolarized voltages. In contrast, the peptide had no significant effect on the somata calcium channels. Thus, we conclude that NCS-1 differentially modulates Ca2+ currents in the somata and growth cones of regenerating neurons, and may serve as a key regulator to facilitate the growth cone calcium channel activity.

Differential regulation of endogenous N- and P/Q-type Ca2+ channel inactivation by Ca2+/calmodulin impacts on their ability to support exocytosis in chromaffin cells

P/Q-type (Ca(V)2.1) and N-type (Ca(V)2.2) Ca2+ channels are critical to stimulus-secretion coupling in the nervous system; feedback regulation of these channels by Ca2+ is therefore predicted to profoundly influence neurotransmission. Here we report divergent regulation of Ca2+-dependent inactivation (CDI) of native N- and P/Q-type Ca2+ channels by calmodulin (CaM) in adult chromaffin cells. Robust CDI of N-type channels was observed in response to prolonged step depolarizations, as well as repetitive stimulation with either brief step depolarizations or action potential-like voltage stimuli. Adenoviral expression of Ca2+-insensitive calmodulin mutants eliminated CDI of N-type channels. This is the first demonstration of CaM-dependent CDI of a native N-type channel. CDI of P/Q-type channels was by comparison modest and insensitive to expression of CaM mutants. Cloning of the C terminus of the Ca(V)2.1 alpha1 subunit from chromaffin cells revealed multiple splice variants lacking structural motifs required for CaM-dependent CDI. The physiological relevance of CDI on stimulus-coupled exocytosis was revealed by combining perforated-patch voltage-clamp recordings of pharmacologically isolated Ca2+ currents with membrane capacitance measurements of exocytosis. Increasing stimulus intensity to invoke CDI resulted in a significant decrease in the exocytotic efficiency of N-type channels compared with P/Q-type channels. Our results reveal unexpected diversity in CaM regulation of native Ca(V)2 channels and suggest that the ability of individual Ca2+ channel subtypes to undergo CDI may be tailored by alternative splicing to meet the specific requirements of a particular cellular function.

Localization of synaptic proteins involved in neurosecretion in different membrane microdomains

A number of proteins and signalling molecules modulate voltage-gated calcium channel activity and neurosecretion. As recent findings have indicated the presence of Ca(v)2.1 (P/Q-type) channels and soluble N-ethyl-maleimide-sensitive fusion protein attachment protein receptors (SNAREs) in the cholesterol-enriched microdomains of neuroendocrine and neuronal cells, we investigated whether molecules known to modulate neurosecretion, such as the heterotrimeric G proteins and neuronal calcium sensor-1 (NCS-1), are also localized in these microdomains. After immuno-isolation, flotation gradients from Triton X-100-treated synaptosomal membranes revealed the presence of different detergent-resistant membranes (DRMs) containing proteins of the exocytic machinery (Ca(v)2.1 channels and SNAREs) or NCS-1; both DRM subtypes contained aliquots of heterotrimeric G protein subunits and phosphatidylinositol-4,5-bisphosphate. In line with the biochemical data, confocal imaging of immunolabelled membrane sheets revealed the localization of SNARE proteins and NCS-1 in different dot-like structures. This distribution was largely impaired by treatment with methyl-beta-cyclodextrin, thus suggesting the localization of all three proteins in cholesterol-dependent domains. Finally, bradykinin (which is known to activate the NCS-1 pathway) caused a significant increase in NCS-1 in the DRMs. These findings suggest that different membrane microdomains are involved in the spatial organization of the complex molecular network that converges on calcium channels and the secretory machinery.

Calcium Signaling and Exocytosis in Adrenal Chromaffin Cells

At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca2+concentration ([Ca2+]c) depends on at least four efficient regulatory systems: 1) plasmalemmal calcium channels, 2) endoplasmic reticulum, 3) mitochondria, and 4) chromaffin vesicles. Different mammalian species express different levels of the L, N, P/Q, and R subtypes of high-voltage-activated calcium channels; in bovine and humans, P/Q channels predominate, whereas in felines and murine species, L-type channels predominate. The calcium channels in chromaffin cells are regulated by G proteins coupled to purinergic and opiate receptors, as well as by voltage and the local changes of [Ca2+]c. Chromaffin cells have been particularly useful in studying calcium channel current autoregulation by materials coreleased with catecholamines, such as ATP and opiates. Depending on the preparation (cultured cells, adrenal slices) and the stimulation pattern (action potentials, depolarizing pulses, high K+, acetylcholine), the role of each calcium channel in controlling catecholamine release can change drastically. Targeted aequorin and confocal microscopy shows that Ca2+entry through calcium channels can refill the endoplasmic reticulum (ER) to nearly millimolar concentrations, and causes the release of Ca2+(CICR). Depending on its degree of filling, the ER may act as a sink or source of Ca2+that modulates catecholamine release. Targeted aequorins with different Ca2+affinities show that mitochondria undergo surprisingly rapid millimolar Ca2+transients, upon stimulation of chromaffin cells with ACh, high K+, or caffeine. Physiological stimuli generate [Ca2+]cmicrodomains in which the local subplasmalemmal [Ca2+]crises abruptly from 0.1 to ∼50 μM, triggering CICR, mitochondrial Ca2+uptake, and exocytosis at nearby secretory active sites. The fact that protonophores abolish mitochondrial Ca2+uptake, and increase catecholamine release three- to fivefold, support the earlier observation. This increase is probably due to acceleration of vesicle transport from a reserve pool to a ready-release vesicle pool; this transport might be controlled by Ca2+redistribution to the cytoskeleton, through CICR, and/or mitochondrial Ca2+release. We propose that chromaffin cells have developed functional triads that are formed by calcium channels, the ER, and the mitochondria and locally control the [Ca2+]cthat regulate the early and late steps of exocytosis.

Developmental activation of calmodulin-dependent facilitation of cerebellar P-type Ca2+ current

P-type (CaV2.1) Ca2+ channels are a central conduit of neuronal Ca2+ entry, so their Ca2+ feedback regulation promises widespread neurobiological impact. Heterologous expression of recombinant CaV2.1 channels demonstrates that the Ca2+ sensor calmodulin can trigger Ca2+-dependent facilitation (CDF) of channel opening. This facilitation occurs when local Ca2+ influx through individual channels selectively activates the C-terminal lobe of calmodulin. In neurons, however, such calmodulin-mediated processes have yet to be detected, and CDF of native P-type current has thus far appeared different, arguably triggered by other Ca2+ sensing molecules. Here, in cerebellar Purkinje somata abundant with prototypic P-type channels, we find that the C-terminal lobe of calmodulin does produce CDF, and such facilitation augments Ca2+ entry during stimulation by repetitive action-potential and complex-spike waveforms. Beyond recapitulating key features of recombinant channels, these neurons exhibit an additional modulatory dimension: developmental upregulation of CDF during postnatal week 2. This phenomenon reflects increasing somatic expression of CaV2.1 splice variants that manifest CDF and progressive dendritic targeting of variants lacking CDF. Calmodulin-triggered facilitation is thus fundamental to native CaV2.1 and rapidly enhanced during early development.

Differential regulation of CaV2.1 channels by calcium-binding protein 1 and visinin-like protein-2 requires N-terminal myristoylation

P/Q-type Ca2+ currents through presynaptic CaV2.1 channels initiate neurotransmitter release, and differential modulation of these channels by neuronal calcium-binding proteins (nCaBPs) may contribute to synaptic plasticity. The nCaBPs calcium-binding protein 1 (CaBP1) and visinin-like protein-2 (VILIP-2) differ from calmodulin (CaM) in that they have an N-terminal myristoyl moiety and one EF-hand that is inactive in binding Ca2+. To determine whether myristoylation contributes to their distinctive modulatory properties, we studied the regulation of CaV2.1 channels by the myristoyl-deficient mutants CaBP1/G2A and VILIP-2/G2A. CaBP1 positively shifts the voltage dependence of CaV2.1 activation, accelerates inactivation, and prevents paired-pulse facilitation in a Ca2+-independent manner. Block of myristoylation abolished these effects, leaving regulation that is similar to endogenous CaM. CaBP1/G2A binds to CaV2.1 with reduced stability, but in situ protein cross-linking and immunocytochemical studies revealed that it binds CaV2.1 in situ and is localized to the plasma membrane by coexpression with CaV2.1, indicating that it binds effectively in intact cells. In contrast to CaBP1, coexpression of VILIP-2 slows inactivation in a Ca2+-independent manner, but this effect also requires myristoylation. These results suggest a model in which nonmyristoylated CaBP1 and VILIP-2 bind to CaV2.1 channels and regulate them like CaM, whereas myristoylation allows differential, Ca2+-independent regulation by the inactive EF-hands of CaBP1 and VILIP-2, which differ in their positions in the protein structure. Differential, myristoylation-dependent regulation of presynaptic Ca2+ channels by nCaBPs may provide a flexible mechanism for diverse forms of short-term synaptic plasticity.

Calcium-dependent regulation of exocytosis

A rapid increase in intracellular calcium directly triggers regulated exocytosis. In addition, changes in intracellular calcium concentration can adjust the extent of exocytosis (quantal content) or the magnitude of individual release events (quantal size) in both the short- and long-term. It is generally agreed that calcium achieves this regulation via an interaction with a number of different molecular targets located at or near to the site of membrane fusion. We review here the synaptic proteins with defined calcium-binding domains and protein kinases activated by calcium, summarize what is known about their function in membrane fusion and the experimental evidence in support of their involvement in synaptic plasticity.

Neuronal calcium sensor-1 facilitates neuronal exocytosis through phosphatidylinositol 4-kinase

This work tested the theory that neuronal calcium sensor-1 (NCS-1) has effects on neurotransmitter release beyond its actions on membrane channels. We used nerve-ending preparations where membrane channels are bypassed through membrane permeabilization made by mechanical disruption or streptolysin-O. Nerve ending NCS-1 and phosphatidylinositol 4-kinase (PI4K) are largely or entirely particulate, so their concentrations in nerve endings remain constant after breaching the membrane. Exogenous, myristoylated NCS-1 stimulated nerve ending phosphatidylinositol 4-phosphate [PI(4)P] synthesis, but non-myristoylated-NCS-1 did not. The N-terminal peptide of NCS-1 interfered with PI(4)P synthesis, and with spontaneous and Ca(2+)-evoked release of both [(3)H]-norepinephrine (NA) and [(14)C]-glutamate (glu) in a concentration-dependent manner. An antibody raised against the N-terminal of NCS-1 inhibited perforated nerve ending PI(4)P synthesis, but the C-terminal antibody had no effects. Antibodies against the N- and C-termini of NCS-1 caused significant increases in mini/spontaneous/stimulation-independent release of [(3)H]-NA from perforated nerve endings, but had no effect on [(14)C]-glu release. These results support the idea that NCS-1 facilitates nerve ending neurotransmitter release and phosphoinositide production via PI4K and localizes these effects to the N-terminal of NCS-1. Combined with previous work on the regulation of channels by NCS-1, the data are consistent with the hypothesis that a NCS-1-PI4K (NP, neuropotentiator) complex may serve as an essential linker between lipid and protein metabolism to regulate membrane traffic and co-ordinate it with ion fluxes and plasticity in the nerve ending.

Hypoxia sensing and pathways of cytosolic Ca2+ increases

Oxygen-sensing and reactivity to changes in the concentration of oxygen is a fundamental property of cellular physiology. This central role is determined, mainly, by, to the fact that oxygen represents the final acceptor of electrons, derived from the normal cellular metabolism, at the end of the mitochondrial respiratory chain. Despite significant advances in molecular characterization of various oxygen-sensitive processes, the nature of the oxygen-sensor molecules and the mechanisms that link sensors to effects remains unclear. One such controversy is about the role and nature of reactive oxygen species (ROS) changes during hypoxia. Irrespective of the mechanisms of oxygen sensing, one of the constant early responses to hypoxia in almost all cell types is an increase in intracellular Ca2+ ([Ca2+]i). In many instances, this increase is mediated by the activation of various plasma membrane Ca2+ conductances. Some of these channels have specific Ca2+ permeability (e.g. voltage-operated Ca2+ channels), whereas others have non-specific cation conductances and are activated by a variety of ligands (ligand-operated channels). In the last decade, a large superfamily of channels with significant Ca2+ permeability has been progressively identified and characterised: the TRP channels. Through their properties, some groups of the TRP channels provide a link to the other hypoxia-activated mechanism of [Ca2+]i increase: the release of Ca2+ from intracellular Ca2+ stores. Since the [Ca2+]i signals, depending on their localization and intensity, are important regulators of the subsequent cellular responses to hypoxia, a deeper understanding of the mechanisms through which hypoxia regulate the activity of these pathways that increase intracellular Ca2+ could point the way towards the development of new therapeutic approaches to reduce or suppress the pathological effects of cellular hypoxia, such as those seen in stroke or myocardial ischemia.

Hippocalcin-deficient mice display a defect in cAMP response element-binding protein activation associated with impaired spatial and associative memory

Hippocalcin is a member of the neuronal calcium sensor (NCS) protein family that is highly expressed in hippocampal pyramidal cells and moderately expressed in the neurons of cerebral cortex, cerebellum and striatum. Here we examined the physiological roles of hippocalcin using targeted gene disruption. Hippocalcin-deficient (-/-) mice displayed no obvious structural abnormalities in the brain including hippocampal formation at the light microscopic level. Deletion of hippocalcin did not result in up-regulation of the hippocalcin-related proteins; neural visinin-like Ca(2+)-binding proteins (NVP) 1, 2, and 3. The synaptic excitability of hippocampal CA1 neurons appeared to be normal, as estimated by the shape of field excitatory postsynaptic potentials elicited by single- and paired-pulse stimuli, and by tetanic stimulation. However, N-methyl-d-aspartate stimulation- and depolarization-induced phosphorylation of cAMP-response element-binding protein (CREB) was significantly attenuated in -/- hippocampal neurons, suggesting an impairment in an activity-dependent gene expression cascade. In the Morris water maze test, the performance of -/- mice was comparable to that of wild-type littermates except in the probe test, where -/- mice crossed the previous location of the platform significantly less often than +/+ mice. Hippocalcin-deficient mice were also impaired on a discrimination learning task in which they needed to respond to a lamp illuminated on the left or right side to obtain food reinforcement. No abnormalities were observed in motor activity, anxiety behavior, or fear learning. These results suggest that hippocalcin plays a crucial role in the Ca(2+)-signaling pathway that underlies long-lasting neural plasticity and that leads to spatial and associative memory.

The neuronal calcium-sensor proteins

Changes in intracellular free Ca(2+) concentration ([Ca(2+)](i)) affect many different aspects of neuronal function ranging from millisecond regulation of ion channels to long term changes in gene expression. These effects of Ca(2+) are transduced by Ca(2+)-binding proteins that act as Ca(2+) sensors by binding Ca(2+), undergoing a conformational change and then modifying the function of additional target proteins. Mammalian species express 14 members of the neuronal calcium sensor (NCS) family of EF hand-containing Ca(2+)-binding proteins which are expressed mainly in photoreceptor cells or neurons. Many of the NCS proteins are membrane targeted through their N-terminal myristoylation either constitutively or following exposure of the myristoyl group after Ca(2+) binding (the Ca(2+)/myristoyl switch). The NCS proteins have been implicated in a wide range of functional roles in neuronal regulation, several of which have been confirmed though molecular genetic analyses.

MGluRs regulate the expression of neuronal calcium sensor proteins NCS-1 and VILIP-1 and the immediate early gene arg3.1/arc in the hippocampus in vivo

The metabotropic glutamate receptor (mGluR) agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) is involved in several forms of hippocampal synaptic plasticity. DHPG application can induce slow-onset potentiation, a form of long-term potentiation (LTP), in the dentate gyrus and in the CA1 region in vivo. The induction of LTP correlates with increased expression levels of neuronal calcium sensor (NCS), considered as key elements for plasticity. In this study we investigated mGluR- and time-dependent changes in the expression of two different NCS proteins. Following DHPG application in vivo NCS-1 and VILIP-1 expression increased, with significant levels reached after 8 and 24h. The effect was attenuated by treatment with the group I mGluR specific antagonist S-4-carboxyphenylglycine. The immediate early gene (IEG) arg3.1/arc showed highest expression levels 2h after DHPG-treatment. Therefore, mGluRs at concentrations which induce synaptic plasticity regulate the expression of IEGs and NCS proteins in different time frames and thus contribute to late phases of synaptic plasticity.

Transient receptor potential vanilloid 4 is essential in chemotherapy-induced neuropathic pain in the rat

The development of treatments for neuropathic pain has been hindered by our limited understanding of the basic mechanisms underlying abnormalities in nociceptor hyperexcitability. We recently showed that the polymodal receptor transient receptor potential vanilloid 4 (TRPV4), a member of the transient receptor potential (TRP) family of ion channels, may play a role in inflammatory pain (Alessandri-Haber et al., 2003). The present study tested whether TRVP4 also contributes to neuropathic pain, using a rat model of Taxol-induced painful peripheral neuropathy. Taxol is the most widely used drug for the treatment of a variety of tumor types, but the dose of Taxol that can be tolerated is limited by the development of a small-fiber painful peripheral neuropathy. We found that Taxol treatment enhanced the nociceptive behavioral responses to both mechanical and hypotonic stimulation of the hind paw. Spinal administration of antisense oligodeoxynucleotides to TRPV4, which reduced the expression of TRPV4 in sensory nerve, abolished Taxol-induced mechanical hyperalgesia and attenuated hypotonic hyperalgesia by 42%. The enhancement of osmotic nociception involves sensitization of osmotransduction in primary afferents because osmotransduction was enhanced in cultured sensory neurons isolated from Taxol-treated rats. Taxol-induced TRPV4-mediated hyperalgesia and the enhanced osmotransduction in cultured nociceptors were dependent on integrin/Src tyrosine kinase signaling. These results suggest that TRPV4 plays a crucial role in a painful peripheral neuropathy, making it a very promising target for the development of a novel class of analgesics.

Ca2+-binding protein-1 facilitates and forms a postsynaptic complex with Cav1.2 (L-type) Ca2+ channels

Ca2+-binding protein-1 (CaBP1) is a Ca2+-binding protein that is closely related to calmodulin (CaM) and localized in somatodendritic regions of principal neurons throughout the brain, but how CaBP1 participates in postsynaptic Ca2+ signaling is not known. Here, we describe a novel role for CaBP1 in the regulation of Ca2+ influx through Ca(v)1.2 (L-type) Ca2+ channels. CaBP1 interacts directly with the alpha1 subunit of Ca(v)1.2 at sites that also bind CaM. CaBP1 binding to one of these sites, the IQ domain, is Ca2+ dependent and competitive with CaM binding. The physiological significance of this interaction is supported by the association of Ca(v)1.2 and CaBP1 in postsynaptic density fractions purified from rat brain. Moreover, in double-label immunofluorescence experiments, CaBP1 and Ca(v)1.2 colocalize in numerous cell bodies and dendrites of neurons, particularly in pyramidal cells in the CA3 region of the hippocampus and in the dorsal cortex. In electrophysiological recordings of cells transfected with Ca(v)1.2, CaBP1 greatly prolonged Ca2+ currents, prevented Ca2+-dependent inactivation, and caused Ca2+-dependent facilitation of currents evoked by step depolarizations and repetitive stimuli. These effects contrast with those of CaM, which promoted strong Ca2+-dependent inactivation of Ca(v)1.2 with these same voltage protocols. Our findings reveal how Ca2+-binding proteins, such as CaM and CaBP1, differentially adjust Ca2+ influx through Ca(v)1.2 channels, which may specify diverse modes of Ca2+ signaling in neurons.

Neuronal Ca2+-sensor proteins: multitalented regulators of neuronal function

Many aspects of neuronal activity are regulated by Ca2+ signals. The transduction of temporally and spatially distinct Ca2+ signals requires the action of Ca2+-sensor proteins including various EF-hand-containing Ca2+-binding proteins. The neuronal Ca2+ sensor (NCS) protein family and the related Ca2+-binding proteins (CaBPs) have begun to emerge as key players in neuronal function. Many of these proteins are expressed predominantly or only in neurons, sometimes with cell-specific patterns of expression. Their ability to associate with membranes either constitutively or in response to elevated Ca2+ concentration allows the NCS proteins to discriminate between different spatial and temporal patterns of Ca2+ signals. Recent work has established several physiological roles of these proteins, including diverse actions on gene expression, ion channel function, membrane traffic of ion channels and receptors, and the control of apoptosis.

Role of Lipid Microdomains in P/Q-type Calcium Channel (Cav2.1) Clustering and Function in Presynaptic Membranes

Lipid microdomains can selectively include or exclude proteins and may be important in a variety of functions such as protein sorting, cell signaling, and synaptic transmission. The present study demonstrates that two different voltage-gated calcium channels, which both interact with soluble N-ethyl-maleimide-sensitive fusion protein attachment protein receptor (SNARE) proteins but have distinct subcellular distributions and roles in synaptic transmission, are differently distributed in lipid microdomains; presynaptic P/Q (Cav2.1) but not Lc (Cav1.2) calcium channel subtypes are mainly accumulated in detergent-insoluble complexes. The immunoisolation of multiprotein complexes from detergent-insoluble or detergent-soluble fractions shows that the alpha1A subunits of Cav2.1 colocalize and interact with SNARE complexes in lipid microdomains. The altered organization of these microdomains caused by saponin and methyl-beta-cyclodextrin treatment largely impairs the buoyancy and distribution of Cav2.1 channels and SNAREs in flotation gradients. On the other hand, cholesterol reloading partially reverses the drug effects. Methyl-beta-cyclodextrin treatment alters the colocalization of Cav2.1 with the proteins of the exocytic machinery and also impairs calcium influx in nerve terminals. These results show that lipid microdomains in presynaptic terminals are important in organizing membrane sites specialized for synaptic vesicle exocytosis. The cholesterol-enriched microdomains contribute to optimizing the compartmentalization of exocytic machinery and the calcium influx that triggers synaptic vesicle exocytosis.

Expression and possible role of neuronal calcium sensor-1 in the cerebellum

Neuronal calcium sensor-1 (NCS-1) is a member of EF-hand calcium-binding protein superfamily, which is considered to modulate synaptic transmission and plasticity. In this mini-review, we first summarize distribution of NCS-1 in the cerebellum. NCS-1 is mainly detected in postsynaptic sites, such as somata and dendrites of Purkinje cells, stellate/basket cells and granule cells. In addition, GABAergic inhibitory stellate/basket cell axon terminals also contain NCS-1. Secondly, we describe cerebellar compartmentation defined by NCS-1. The NCS-1 immunostaining displayed characteristic parasagittal-banding pattern in the Purkinje cell layer and molecular layer, whereas there were no apparent bands in the granule cell layer. The alternating positively and negatively NCS-1-labeled Purkinje cell clusters contributed to this cerebellar compartmentation. In contrast, stellate/basket cells were uniformly NCS-1-positive throughout the cerebellum. Interestingly, NCS-1 and zebrin II exhibited a similar parasagittal-banding pattern. But it is noteworthy that NCS-1-negative/zebrin II-positive Purkinje cell clusters were detected selectively in anterior lobule vermis and paraflocculus. These results suggest that NCS-1 defines a novel pattern of cerebellar cortical compartmentation. Lastly, we describe recent data suggesting some relationship between NCS-1 and cerebellar long-term depression-related molecules, and discuss the possible role of NCS-1 in the cerebellum.

Neuronal Calcium Sensor-1 and Phosphatidylinositol 4-Kinase β Regulate IgE Receptor-Triggered Exocytosis in Cultured Mast Cells

We examined the possible occurrence and function of neuronal Ca(2+) sensor 1 (NCS-1/frequenin) in the mast cell line rat basophilic leukemia, RBL-2H3. This protein has been implicated in the control of neurosecretion from dense core granules in neuronal cells as well as in the control of constitutive secretory pathways in both yeast and mammalian cells. We show that RBL-2H3 cells, secretory cells of the immune system, endogenously express the 22-kDa NCS-1 protein as well as an immune-related 50-kDa protein. Both proteins associate in vivo with phosphatidylinositol 4-kinase beta (PI4Kbeta) and colocalize with the enzyme in the Golgi region. We show further that overexpression of NCS-1 in RBL-2H3 cells stimulates the catalytic activity of PI4Kbeta, increases IgE receptor (FcepsilonRI)-triggered hydrolysis of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)), and stimulates FcepsilonRI-triggered, but not Ca(2+) ionophore-triggered, exocytosis. Conversely, expression of a kinase-dead mutant of PI4Kbeta reduces PI4Kbeta activity, decreases FcepsilonRI-stimulated phosphatidylinositol 4,5-bisphosphate hydrolysis, and blocks FcepsilonRI-triggered, but not Ca(2+) ionophore-triggered, exocytosis. Our results indicate that PI(4)P, produced by the Golgi-localized PI4Kbeta, is the rate-limiting factor in the synthesis of the pool of PI(4,5)P(2) that serves as substrate for the generation of lipid-derived second messengers in FcepsilonRI-triggered cells. We conclude that NCS-1 is involved in the control of regulated exocytosis in nonneural cells, where it contributes to stimulus-secretion coupling by interacting with PI4Kbeta and positive regulation of its activity.

Acute changes in short-term plasticity at synapses with elevated levels of neuronal calcium sensor-1

Short-term synaptic plasticity is a defining feature of neuronal activity, but the underlying molecular mechanisms are poorly understood. Depression of synaptic activity might be due to limited vesicle availability, whereas facilitation is thought to result from elevated calcium levels. However, it is unclear whether the strength and direction (facilitation versus depression) of plasticity at a given synapse result from preexisting synaptic strength or whether they are regulated by separate mechanisms. Here we show, in rat hippocampal cell cultures, that increases in the calcium binding protein neuronal calcium sensor-1 (NCS-1) can switch paired-pulse depression to facilitation without altering basal synaptic transmission or initial neurotransmitter release probability. Facilitation persisted during high-frequency trains of stimulation, indicating that NCS-1 can recruit 'dormant' vesicles. Our results suggest that NCS-1 acts as a calcium sensor for short-term plasticity by facilitating neurotransmitter output independent of initial release. We conclude that separate mechanisms are responsible for determining basal synaptic strength and short-term plasticity.

Group I mGlu receptors regulate the expression of the neuronal calcium sensor protein VILIP-1 in vitro and in vivo: implications for mGlu receptor-dependent hippocampal plasticity?

Metabotropic glutamate (mGlu) receptors are involved in several forms of synaptic plasticity in the rat hippocampus. Agonists which activate group I mGlu receptors induce slow-onset potentiation without prior tetanization in the hippocampal area CA1. Activation of group I mGlu receptors induces protein synthesis which may contribute to mGlu receptor-dependent forms of long-term plasticity. Calcium-binding proteins are widely considered to comprise key elements for synaptic plasticity. Therefore, we investigated whether the calcium sensor protein VILIP-1 is associated with group I mGlu receptor-mediated plasticity in the dentate gyrus (DG) in vivo.Application of either the group I and II mGlu agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD) or the selective group I agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) resulted in slow-onset potentiation in the DG of adult rats. In hippocampal cell cultures both agonists elicited an enhanced expression of VILIP-1. In situ hybridization revealed strong hippocampal expression of VILIP-1 and intracerebral application of DHPG to adult rats significantly enhanced hippocampal VILIP-1 expression. The DHPG effects in both, hippocampal cultures and in vivo, were prevented by the group I mGlu receptor antagonist 4-Carboxyphenylglycine (4CPG). Calcium sensor proteins thus appear to be regulated by mGlu receptors in an activity-dependent manner. A specific role for group I mGlu receptors is evident. Furthermore, the sensor proteins may function as molecular switches for the long-term regulation of synaptic plasticity.

Down-regulation of voltage-gated Ca2+ channels by neuronal calcium sensor-1 is beta subunit-specific

Neuronal Ca(2+) sensor protein-1 (NCS-1) is a member of the Ca(2+) binding protein family, with three functional Ca(2+) binding EF-hands and an N-terminal myristoylation site. NCS-1 is expressed in brain and heart during embryonic and postnatal development. In neurons, NCS-1 facilitates neurotransmitter release, but both inhibition and facilitation of the Ca(2+) current amplitude have been reported. In heart, NCS-1 co-immunoprecipitates with K(+) channels and modulates their activity, but the potential effects of NCS-1 on cardiac Ca(2+) channels have not been investigated. To directly assess the effect of NCS-1 on the various types of Ca(2+) channels we have co-expressed NCS-1 in Xenopus oocytes, with Ca(V)1.2, Ca(V)2.1, and Ca(V)2.2 Ca(2+) channels, using various subunit combinations. The major effect of NCS-1 was to decrease Ca(2+) current amplitude, recorded with the three different types of alpha(1) subunit. When expressed with Ca(V)2.1, the depression of Ca(2+) current amplitude induced by NCS-1 was dependent upon the identity of the beta subunit expressed, with no block recorded without beta subunit or with the beta(3) subunit. Current-voltage and inactivation curves were also slightly modified and displayed a different specificity toward the beta subunits. Taken together, these data suggest that NCS-1 is able to modulate cardiac and neuronal voltage-gated Ca(2+) channels in a beta subunit specific manner.

Phosphatidylinositol 4-OH kinase is a downstream target of neuronal calcium sensor-1 in enhancing exocytosis in neuroendocrine cells

Neuronal calcium sensor-1 (NCS-1), the mammalian orthologue of frequenin, belongs to a family of EF-hand-containing Ca(2+) sensors. NCS-1/frequenin has been shown to enhance synaptic transmission in PC12 cells and Drosophila and Xenopus, respectively. However, the precise molecular mechanism for the enhancement of exocytosis is largely unknown. In PC12 cells, NCS-1 potentiated exocytosis evoked by ATP, an agonist to phospholipase C-linked receptors, but had no effect on depolarization-evoked release. NCS-1 also enhanced exocytosis triggered by ionomycin, a Ca(2+) ionophore that bypasses K(+) and Ca(2+) channels. Overexpression of NCS-1 caused a shift in the dose-response curve of inhibition of ATP-evoked secretion using phenylarsine oxide, an inhibitor of phosphatidylinositol 4-OH kinase (PI4K). Plasma membrane phosphatidylinositol 4,5-bisphosphate pools were increased upon NCS-1 transfection as visualized using a phospholipase C-delta pleckstrin homology domain-green fluorescent protein construct. NCS-1-transfected cell extracts displayed increased phosphatidylinositol-4-phosphate biosynthesis, indicating an increase in PI4K activity. Mutations in NCS-1 equivalent to those that abolish the interaction of recoverin, another EF-hand-containing Ca(2+) sensor, with its downstream target rhodopsin kinase, lost their ability to enhance exocytosis. Taken together, the present data indicate that NCS-1 modulates the activity of PI4K, leading to increased levels of phosphoinositides and concomitant enhancement of exocytosis.

Up-regulation of neuronal calcium sensor-1 (NCS-1) in the prefrontal cortex of schizophrenic and bipolar patients

The delineation of dopamine dysfunction in the mentally ill has been a long-standing quest of biological psychiatry. The present study focuses on a recently recognized group of dopamine receptor-interacting proteins as possible novel sites of dysfunction in schizophrenic and bipolar patients. We demonstrate that the dorsolateral prefrontal cortex in schizophrenia and bipolar cases from the Stanley Foundation Neuropathology Consortium display significantly elevated levels of the D2 dopamine receptor desensitization regulatory protein, neuronal calcium sensor-1. These levels of neuronal calcium sensor-1 were not influenced by age, gender, hemisphere, cause of death, postmortem period, alcohol consumption, or antipsychotic and mood stabilizing medications. The present study supports the hypothesis that schizophrenia and bipolar disorder may be associated with abnormalities in dopamine receptor-interacting proteins.

Differential transcriptional expression of Ca2+ BP superfamilies in murine gastrointestinal smooth muscles

Calmodulin (Cal) plays important roles for contractile activity in smooth muscles. Recently, two distinct Ca(2+)-binding protein superfamilies with sequence similarities to Cal have been identified in neuronal cells: neuronal Ca(2+)-binding proteins (NCBPs) and Cal-like Ca(2+)-binding proteins (CaBPs). Some NCBPs and CaBPs play significant roles for Ca(2+)-dependent cellular signaling in the nervous system. In gastrointestinal smooth muscles (GISMs), Cal functions as the regulator of contractile behavior and electrical rhythmicity. However, the molecular identification of NCBPs and CaBPs has not been elucidated in GISMs. Here, we have identified NCBPs and CaBPs expressed in GISMs and determined the expression levels of their transcripts by quantitative RT-PCR. Of 12 NCBPs, the transcripts for neuronal Ca(2+) sensor 1, neural visinin-like proteins 1, 2, and 3, and K(+) channel-interacting proteins 1 and 3 were detected in proximal colon, gastric fundus, gastric antrum, and jejunum. On the other hand, of seven CaBPs including alternatively spliced variants, only CaBP1L transcripts were detected in GISMs.

Identification of Ca2+-dependent binding partners for the neuronal calcium sensor protein neurocalcin delta: interaction with actin, clathrin and tubulin

The neuronal calcium sensors are a family of EF-hand-containing Ca(2+)-binding proteins expressed predominantly in retinal photoreceptors and neurons. One of the family members is neurocalcin delta, the function of which is unknown. As an approach to elucidating the protein interactions made by neurocalcin delta, we have identified brain cytosolic proteins that bind to neurocalcin delta in a Ca(2+)-dependent manner. We used immobilized recombinant myristoylated neurocalcin delta combined with protein identification using MS. We demonstrate a specific interaction with clathrin heavy chain, alpha- and beta-tubulin, and actin. These interactions were dependent upon myristoylation of neurocalcin delta indicating that the N-terminal myristoyl group may be important for protein-protein interactions in addition to membrane association. Direct binding of neurocalcin delta to clathrin, tubulin and actin was confirmed using an overlay assay. These interactions were also demonstrated for endogenous neurocalcin delta by co-immunoprecipitation from rat brain cytosol. When expressed in HeLa cells, neurocalcin delta was cytosolic at resting Ca(2+) levels but translocated to membranes, including a perinuclear compartment (trans-Golgi network) where it co-localized with clathrin, following Ca(2+) elevation. These data suggest the possibility that neurocalcin delta functions in the control of clathrin-coated vesicle traffic.

Differential use of myristoyl groups on neuronal calcium sensor proteins as a determinant of spatio-temporal aspects of Ca2+ signal transduction

The localizations of three members of the neuronal calcium sensor (NCS) family were studied in HeLa cells. Using hippocalcin-EYFP and NCS-1-ECFP, it was found that their localization differed dramatically in resting cells. NCS-1 had a distinct predominantly perinuclear localization (similar to trans-Golgi markers), whereas hippocalcin was present diffusely throughout the cell. Upon the elevation of intracellular Ca(2+), hippocalcin rapidly translocated to the same perinuclear compartment as NCS-1. Another member of the family, neurocalcin delta, also translocated to this region after a rise in Ca(2+) concentration. Permeabilization of transfected cells using digitonin caused loss of hippocalcin and neurocalcin delta in the absence of calcium, but in the presence of 10 microm Ca(2+), both proteins translocated to and were retained in the perinuclear region. NCS-1 localization was unchanged in permeabilized cells regardless of calcium concentration. The localization of NCS-1 was unaffected by mutations in all functional EF hands, indicating that its localization was independent of Ca(2+). A minimal myristoylation motif (hippocalcin-(1-14)) fused to EGFP resulted in similar perinuclear targeting, showing that localization of these proteins is because of the exposure of the myristoyl group. This was confirmed by mutation of the myristoyl motif of NCS-1 and hippocalcin that resulted in both proteins remaining cytosolic, even at elevated Ca(2+) concentration. Dual imaging of hippocalcin-EYFP and cytosolic Ca(2+) concentration in Fura Red-loaded cells demonstrated the kinetics of the Ca(2+)/myristoyl switch in living cells and showed that hippocalcin rapidly translocated with a half-time of approximately 12 s after a short lag period when Ca(2+) was elevated. These results demonstrate that closely related Ca(2+) sensor proteins use their myristoyl groups in distinct ways in vivo in a manner that will determine the time course of Ca(2+) signal transduction.