Modulation of synapse formation by cyclic adenosine monophosphate

Mouse Neuroblastoma CB1 Cannabinoid Receptor-Stimulated [35S]GTPɣS Binding: Total and Antibody-Targeted Gα Protein-Specific Scintillation Proximity Assays

G protein-coupled receptors (GPCRs) are important regulators of cellular signaling functions and therefore are a major target for drug discovery. The CB1 cannabinoid receptor is among the most highly expressed GPCRs in neurons, where it regulates many differentiated neuronal functions. One model system for studying the biochemistry of neuronal responses is the use of neuroblastoma cells originating from the C1300 tumor in the A/J mouse, including cloned cell lines NS20, N2A, N18TG2, N4TG1, and N1E-115, and various immortalized hybrids of neurons with N18TG2 cells. GPCR signal transduction is mediated through interaction with multiple types and subtypes of G proteins that transduce the receptor stimulus to effectors. The [35S]GTPɣS assay provides a valuable pharmacological method to evaluate efficacy and potency in the first step in GPCR signaling. Here, we present detailed protocols for the [35S]GTPɣS-binding assay to measure the total G protein binding and the antibody-targeted scintillation proximity assay to measure specific Gα proteins in neuroblastoma cell membrane preparations. This chapter presents step-by-step methods from cell culture, membrane preparation, assay procedures, and data analysis.

Neurotransmitter release: vacuolar ATPase V0 sector c-subunits in possible gene or cell therapies for Parkinson's, Alzheimer's, and psychiatric diseases

We overview the 16-kDa proteolipid mediatophore, the transmembrane c-subunit of the V0 sector of the vacuolar proton ATPase (ATP6V0C) that was shown to mediate the secretion of acetylcholine. Acetylcholine, serotonin, and dopamine (DA) are released from cell soma and/or dendrites if ATP6V0C is expressed in cultured cells. Adeno-associated viral vector-mediated gene transfer of ATP6V0C into the caudate putamen enhanced the depolarization-induced overflow of endogenous DA in Parkinson-model mice. Motor impairment was ameliorated in hemiparkinsonian model mice when ATP6V0C was expressed with DA-synthesizing enzymes. The review discusses application in the future as a potential tool for gene therapy, cell transplantation therapy, and inducible pluripotent stem cell therapy in neurological diseases, from the view point of recent findings regarding vacuolar ATPase.

Somato-axodendritic release of oxytocin into the brain due to calcium amplification is essential for social memory

Oxytocin (OT) is released into the brain from the cell soma, axons, and dendrites of neurosecretory cells in the hypothalamus. Locally released OT can activate OT receptors, form inositol-1,4,5-trisphosphate and elevate intracellular free calcium (Ca²⁺) concentrations [(Ca²⁺)_i] in self and neighboring neurons in the hypothalamus, resulting in further OT release: i.e., autocrine or paracrine systems of OT-induced OT release. CD38-dependent cyclic ADP-ribose (cADPR) is also involved in this autoregulation by elevating [Ca²⁺]_i via Ca²⁺ mobilization through ryanodine receptors on intracellular Ca²⁺ pools that are sensitive to both Ca²⁺ and cADPR. In addition, it has recently been reported that heat stimulation and hyperthermia enhance [Ca²⁺]_i increases by Ca²⁺ influx, probably through TRPM2 cation channels, suggesting that cADPR and TRPM2 molecules act as Ca²⁺ signal amplifiers. Thus, OT release is not simply due to depolarization–secretion coupling. Both of these molecules play critical roles not only during labor and milk ejection in reproductive females, but also during social behavior in daily life in both genders. This was clearly demonstrated in CD38 knockout mice in that social behavior was impaired by reduction of [Ca²⁺]_i elevation and subsequent OT secretion. Evidence for the associations of CD38 with social behavior and psychiatric disorder is discussed, especially in subjects with autism spectrum disorder.

SIRT1 knockdown promotes neural differentiation and attenuates the heat shock response

Neurons have a limited capacity for heat shock protein (HSP) induction and are vulnerable to the pathogenic consequence of protein misfolding and aggregation as seen in age-related neurodegenerative diseases. Sirtuin 1 (SIRT1), an NAD(+) -dependent lysine deacetylase with important biological functions, has been shown to sustain the DNA-binding state of HSF1 for HSP induction. Here we show that differentiation and maturation of embryonic cortical neurons and N2a neuroprogenitor cells is associated with decreases in SIRT1 expression and heat shock-dependent induction of HSP70 protein. Tests of a pharmacological activator and an inhibitor of SIRT1 affirm the regulatory role of SIRT1 in HSP70 induction. Protein cross-linking studies show that nuclear SIRT1 and HSF1 form a co-migrating high molecular weight complex upon stress. The use of retroviral vectors to manipulate SIRT1 expression in N2a cells show that shRNA-mediated knock down of SIRT1 causes spontaneous neurite outgrowth coincident with reduced growth rate and decreased induction of hsp70-reporter gene, whereas SIRT1 over-expression blocks the induced neural differentiation of N2a cells. Our results suggest that decreased SIRT1 expression is conducive to neuronal differentiation and this decrease contributes to the attenuated induction of HSPs in neurons.

Neurochemical and morphological studies on differentiation of NG108-15 cells by phorbol ester and forskolin

12-O-tetradecanoylphorbol 13-acetate (TPA), forskolin or dibutyryl cAMP induced neurite outgrowth and inhibition of cell growth in NG108-15 cells. TPA, forskolin and dibutyryl cAMP significantly increased specific activity of choline acetyltransferase. Forskolin markedly stimulated cAMP accumulation, but not TPA, suggesting that forskolin could induce differentiation by increasing the cAMP content via adenylate cyclase activation, but TPA-induced differentiation seems not to be due to the raise of the cAMP level. Incubation of the cells with TPA, forskolin or dibutyryl cAMP for 24 h resulted in enhancement of 50 mM K(+)-evoked Ca(2+) influx and neurite elongation, although incubation with these agents for 1 h didn't affect these events. From these results, it is suggested that TPA and forskolin induce differentiation of NG108-15 cells to acetylcholine neurons via different mechanisms: protein kinase C activation by TPA and cAMP-dependent protein kinase activation by forskolin. In addition, it is likely that Ca(2+) channels in cells differentiated by TPA, forskolin or dibutyryl cAMP become sensitive to depolarization.

Differential expression of the 5-HT(3A) and 5-HT(3B) receptor in differentiated NG108-15 cells

Previous work from this laboratory has shown that the serotonin (5-HT) induced response is significantly augmented in differentiated NG108-15 (NG) cells treated with dibutyryl cAMP (Bt(2)cAMP) due to qualitative and quantitative changes in the expression of the 5-HT(3) receptor as demonstrated by specific [(3)H] LY-278584 (a selective 5HT(3) receptor antagonist) binding. In this study, we investigated whether there is any change in the relative expression of the 5-HT(3A) and 5-HT(3B) subunits in NG cells differentiated following Bt(2)cAMP treatment cells. The major findings of this study were that the relative amount of 5-HT(3B) subunit mRNA in Bt(2)cAMP-treated NG cells 5 days following Bt(2)cAMP-treatment was greater than that in the untreated cells. In contrast, the relative expression of the 5-HT(3B) subunit protein in the Bt(2)cAMP-treated NG cells was much less than in the untreated cells, but the relative expression of the 5-HT(3A) subunit in the Bt(2)cAMP-treated NG cells was similar to the untreated cells. Therefore, no relationship between mRNA and protein expression for 5-HT(3A) and 5-HT(3B) subunits in Bt(2)cAMP treated and untreated NG cells were observed. It was also found that fluorescent intensity for the 5-HT(3B) subunit in the cell body of the Bt(2)cAMP treated and untreated NG cells gradually decreased from the day 1-5 after Bt(2)cAMP treatment. However, in specific areas such as the varicosity and nerve endings of the Bt(2)cAMP treated cells, staining intensity for the 5-HT(3B) subunits was stronger than in the untreated cells at the all time points, peaking at day 5 post-treatment. These results suggest that the augmented response induced by 5-HT acting via 5-HT(3) receptors in differentiated NG cells may be due to changes in the relative amount of the 5-HT(3B) subunit, particularly the ratio and distribution of the 5-HT(3A) to (3B) subunits.

A personal view from a long-lasting collaborator on the research strategies of Marshall Nirenberg

In this review, I summarized transition in Dr. Marshall Nirenberg's research interests during 1970s, from a view of a long-lasting collaborator. Nirenberg switched his research filed to neurobiology after his success in deciphering genetic code and being honored with the Nobel Prize in Physiology or Medicine in 1968. His targets were to obtain genetically pure population of neurons, i.e. neuroblastoma clones, to make somatic hydrid cells, to culture neuronal and muscle cells, and to produce monoclonal antibodies against whole retinal or neuroblastoma cells. He studied neurotransmitters, receptors, cyclic nucleotides, cell differentiation, secretion, synapse formation, and chemical recognition. Especially he liked his hypothesis for opiate tolerance and dependency as a model of cellular memory. Through these studies, he seemed to devote all his time of about 50 years from 1960s to decoding brain memory processes.

Ect2, an ortholog of Drosophila Pebble, regulates formation of growth cones in primary cortical neurons

In collaboration with Marshall Nirenberg, we performed in vivo RNA interference (RNAi) genome-wide screening in Drosophila embryos. Pebble has been shown to be involved in Drosophila neuronal development. We have also reported that depletion of Ect2, a mammalian ortholog of Pebble, induces differentiation in NG108-15 neuronal cells. However, the precise role of Ect2 in neuronal development has yet to be studied. Here, we confirmed in PC12 pheochromocytoma cells that inhibition of Ect2 expression by RNAi stimulated neurite outgrowth, and in the mouse embryonic cortex that Ect2 was accumulated throughout the ventricular and subventricular zones with neuronal progenitor cells. Next, the effects of Ect2 depletion were studied in primary cultures of mouse embryonic cortical neurons: Loss of Ect2 did not affect the differentiation stages of neuritogenesis, the number of neurites, or axon length, while the numbers of growth cones and growth cone-like structures were increased. Taken together, our results suggest that Ect2 contributes to neuronal morphological differentiation through regulation of growth cone dynamics.

CD38 and its role in oxytocin secretion and social behavior

Here, we review the functional roles of cyclic ADP-ribose and CD38, a transmembrane protein with ADP-ribosyl cyclase activity, in mouse social behavior via the regulation of oxytocin (OXT) release, an essential component of social cognition. Herein we describe data detailing the molecular mechanism of CD38-dependent OXT secretion in CD38 knockout mice. We also review studies that used OXT, OXT receptor (OXTR), or CD38 knockout mice. Additionally, we compare the behavioral impairments that occur in these knockout mice in relation to the OXT system and CD38. This review also examines autism spectrum disorder (ASD), which is characterized by social and communication impairments, in relation to defects in the OXT system. Two single nucleotide polymorphisms (SNPs) in the human CD38 gene are possible risk factors for ASD via inhibition of OXT function. Further analysis of CD38 in relation to the OXT system may provide a better understanding of the neuroendocrinological roles of OXT and CD38 in the hypothalamus and of the pathophysiology of ASD. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Social memory, amnesia, and autism: brain oxytocin secretion is regulated by NAD+ metabolites and single nucleotide polymorphisms of CD38

Previously, we demonstrated that CD38, a transmembrane protein with ADP-ribosyl cyclase activity, plays a critical role in mouse social behavior by regulating the release of oxytocin (OXT), which is essential for mutual recognition. When CD38 was disrupted, social amnesia was observed in Cd38 knockout mice. The autism spectrum disorders (ASDs), characterized by defects in reciprocal social interaction and communication, occur either sporadically or in a familial pattern. However, the etiology of ASDs remains largely unknown. Therefore, the theoretical basis for pharmacological treatments has not been established. Hence, there is a rationale for investigating single nucleotide polymorphisms (SNPs) in the human CD38 gene in ASD subjects. We found several SNPs in this gene. The SNP rs3796863 (C>A) was associated with high-functioning autism (HFA) in American samples from the Autism Gene Resource Exchange. Although this finding was partially confirmed in low-functioning autism subjects in Israel, it has not been replicated in Japanese HFA subjects. The second SNP of interest, rs1800561 (4693C>T), leads to the substitution of an arginine (R) at codon 140 by tryptophan (W; R140W) in CD38. This mutation was found in four probands of ASD and in family members of three pedigrees with variable levels of ASD or ASD traits. The plasma levels of OXT in ASD subjects with the R140W allele were lower than those in ASD subjects lacking this allele. The OXT levels were unchanged in healthy subjects with or without this mutation. One proband with the R140W allele receiving intranasal OXT for approximately 3years showed improvement in areas of social approach, eye contact and communication behaviors, emotion, irritability, and aggression. Five other ASD subjects with mental deficits received nasal OXT for various periods; three subjects showed improved symptoms, while two showed little or no effect. These results suggest that SNPs in CD38 may be possible risk factors for ASD by abrogating OXT function and that some ASD subjects can be treated with OXT in preliminary clinical trials.

Target-based drug discovery, genetic diseases, and biologics

The last fifteen years have witnessed a major strategic shift in drug discovery away from an empiric approach based on incremental improvements of proven therapies, to a more theoretical, target-based approach. This arose as a consequence of three technical advances: (1) generation and interpretation of genome sequences, which facilitated identification and characterization of potential drug targets; (2) efficient production of candidate ligands for these putative targets through combinatorial chemistry or generation of monoclonal antibodies; and (3) high-throughput screening for rapid evaluation of interactions of these putative ligands with the selected targets. The basic idea underlying all three of these technologies is in keeping with Marshall Nirenberg's dictum that science progresses best when there are simple assays capable of generating large data sets rapidly. Furthermore, practical implementation of target-based drug discovery was enabled directly by technologies that either were originated or nurtured by Marshall, his post-docs and fellows. Chief among these was the genetic code. Also important was adoption of clonal cell lines for pharmacological investigations, as well as the use of hybridomas to generate molecular probes that allowed physical purchase on signaling elements that had previously been only hypothetical constructs. Always the pure scientist, Marshall's contributions nevertheless enabled fruitful applications in the pharmaceutical industry, several of them by his trainees. Both the successes and the shortcomings of target-based drug discovery are worthy of consideration, as are its implications for the choices of therapeutic goals and modalities by the pharmaceutical industry.

Dopamine release via the vacuolar ATPase V0 sector c-subunit, confirmed in N18 neuroblastoma cells, results in behavioral recovery in hemiparkinsonian mice

A 16-kDa proteolipid, mediatophore, in Torpedo electric organs mediates Ca(2+)-dependent acetylcholine release. Mediatophore is identical to the pore-forming stalk c-subunit of the V0 sector of vacuolar proton ATPase (ATP6V0C). The function of ATP6V0C in the mammalian central nervous system is not clear. Here, we report transfection of adeno-associated viral vectors harboring rat ATP6V0C into the mouse substantia nigra, in which high potassium stimulation increased overflow of endogenous dopamine (DA) measured in the striatum by in vivo microdialysis. Next, in the striatum of 6-hydroxydopamine-lesioned mice, a model of Parkinson's disease (PD), human tyrosine hydroxylase, aromatic l-amino-acid decarboxylase and guanosine triphosphate cyclohydrolase 1, together with or without ATP6V0C, were expressed in the caudoputamen for rescue. Motor performance on the accelerating rotarod test and amphetamine-induced ipsilateral rotation were improved in the rescued mice coexpressing ATP6V0C. [(3)H]DA, taken up into cultured N18 neuronal tumor cells transformed to express ATP6V0C, was released by potassium stimulation. These results indicated that ATP6V0C mediates DA release from nerve terminals in the striatum of DA neurons of normal mice and from gene-transferred striatal cells of parkinsonian mice. The results suggested that ATP6V0C may be useful as a rescue molecule in addition to DA-synthetic enzymes in the gene therapy of PD.

Neuronal functions associated with endo- and exocytotic events-cum-molecular trafficking may be cell maturation-dependent: lessons learned from studies on botulism

The passion in the scientific endeavors of Marshall Warren Nirenberg had been his quest for knowledge regarding the storage, retrieval, and processing of information in the cell. After deciphering the genetic code for which he shared the Nobel Prize in Physiology and Medicine in 1968, Nirenberg devoted his attention to unraveling the mysteries in the most complex cellular organization in the body, i.e., the nervous system, especially those governing neuronal development, plasticity, and synaptogenesis. During the tenure of the primary author (RR) as a postdoctoral Staff Fellow in the Nirenberg laboratory in the late seventies to early eighties, he had the opportunity of working on projects related to what Nirenberg used to broadly define as the "synaptic code." The major aspects of these projects dealt with the functional macromolecules relevant to neuronal growth, organization, lineage, selectivity, stabilization, synaptogenesis, and functions such as neuroexocytosis. This author's emphasis was particularly on voltage-gated calcium channels that regulate stimulus-induced neurotransmitter release. One central as well as crucial theme in these studies was the fact that the neurons had to be mature and differentiated in order to study these entities (Science 222: 794-799, 1983; Cold Spring Harb Symp Quant Biol 48: 707-715, 1983). In this communication, we illustrate how did this basic knowledge, i.e., cell maturation-dependent properties being essential for neuronal functions, led to a successful experimental design and demonstration of the validity of the targeted neurologic therapeutic delivery approach based on recombinant botulinum toxin serotype A (BoNT/A) heavy chain (rHC) serving as a neuron-specific targeting molecule (BMC Pharmacol 9: 12, 2009).

Neuroprotective drug riluzole amplifies the heat shock factor 1 (HSF1)- and glutamate transporter 1 (GLT1)-dependent cytoprotective mechanisms for neuronal survival

Heat shock factor 1 (HSF1) mediates the cellular response to stress to increase the production of heat shock protein (HSP) chaperones for proper protein folding, trafficking, and degradation; failure of this homeostatic mechanism likely contributes to neurodegeneration. We show that the neuroprotective drug riluzole increased the amount of HSF1 in NG108-15 neuroprogenitor cells by slowing the specific turnover of HSF1 and supporting a more robust and sustained activation of HSF1. Using Hsp70-luciferase as a functional readout of the activity of HSF1, we show that riluzole amplified the heat shock induction of the reporter gene with an optimal increase at 1 μM. Immunocytochemical staining and Western blot quantitation of HSP70 in NG108-15 neuroprogenitor cells and embryonic spinal cord neurons provided corroborative evidence that riluzole amplified the HSF1-dependent regulation of HSP70 expression. Parallel studies on the GLT1 glutamate transporter showed that riluzole increased GLT1-reporter and GLT1 protein expression and that the increase was enhanced by heat shock and coincident with the increased expression of HSP70 and HSP90. This result is consistent with the anti-glutamatergic profile of riluzole and the presence of multiple heat shock elements on the GLT1 gene promoter, suggesting that riluzole may modulate GLT1 expression through HSF1. The increased HSP chaperones and GLT1 transporter blunted glutamate-induced and N-methyl D-aspartate receptor-mediated excitotoxic death. In summary, we show that riluzole increased the amount and activity of HSF1 to boost the expression of HSPs and GLT1 for neuroprotection under stress.

Changes in characteristics of the specific binding of [3H]LY-278584, a 5-HT3-receptor antagonist, on differentiated NG108-15 cells

We have reported previously that the concentration of intracellular Ca2+ evoked by serotonin (5-HT) was significantly augmented in differentiated NG108-15 (NG) cells treated with dibutyryl cAMP and the enhanced response occurred via 5-HT3 receptors. We investigated changes in the characteristics for specific binding of [(3)H]LY-278584 (a specific antagonist of the 5-HT3 receptor) on membranes from differentiated NG cells. The results indicated that the K(d) and B(max) values for the specific binding to differentiated NG cells were significantly smaller and larger, respectively, than those for undifferentiated NG cells. The binding was significantly inhibited by 10 nM tropisetron, a specific 5-HT3-receptor antagonist, but not by any other types of 5-HT-receptor antagonists. These results suggested that the enhanced response by 5-HT in differentiated NG cells was due to both qualitative and quantitative changes in the 5-HT3 receptor.

Intracellular calcium elevation induced by extracellular application of cyclic-ADP-ribose or oxytocin is temperature-sensitive in rodent NG108-15 neuronal cells with or without exogenous expression of human oxytocin receptors

ADP-ribosyl cyclase and/or CD38 are activated after oxytocin receptor stimulation in the hypothalamus and pituitary in adult mice, leading to facilitation of oxytocin secretion. Although cyclic adenosine 5'-diphosphoribose (cADPR) primarily acts as an intracellular second messenger, it has been suggested that extracellular cADPR stimulates intracellular ryanodine receptors after internalisation via the nucleotide-transporting capacity of CD38 in fibroblasts and astrocytes. However, little is known about whether extracellular cADPR activates neurones. To address this question, we used a model neuronal cell line, NG108-15 mouse neuroblastoma x rat glioma hybrid cells possessing CD38 but not oxytocin receptors, and measured cytosolic free calcium concentrations ([Ca(2+)](i)). Extracellular application of cADPR to NG108-15 cells elevated [Ca(2+)](i) at 35 degrees C. The elevation was significantly enhanced when measured at 40 degrees C. The cADPR and heat-induced [Ca(2+)](i) increase were blocked under extracellular Ca(2+)-free conditions and by 2-aminoethoxydiphenyl borate, an antagonist of melastatin-related transient receptor potential channel 2 (TRPM2) cation channels. Reverse transcriptation-polymerase chain reaction analyses indicated that TRPM2 channels were expressed in NG108-15 cells. Application of oxytocin elevated [Ca(2+)](i) in NG108-15 cells transformed to transiently express cloned human oxytocin receptors. The oxytocin-induced [Ca(2+)](i) response was also enhanced by heat. These results indicate that the extracellular application of cADPR, together with heat, activates cation influx downstream of oxytocin receptor signalling in NG108-15 neuronal cells, and suggest the possible involvement of TRPM2 channels in oxytocin release in the mammalian brain.

Influence of cyclic AMP on facial nerve regeneration in rats

Promoting facial nerve regeneration is a significant challenge.

Aim

To evaluate the possible neurotrophic influence of cyclic AMP on facial nerve regeneration of Wistar rats.

Method

The right facial nerve of thirty-two animals were completely transected and immediately sutured, followed by exposure or not to topical cyclic AMP. Behavioral and histometric analyses were done at 14 and 28 days.

Results

Statistical differences (p<0.05) were found in the behavioral and histometric analyses on the 14th day, suggesting an early regenerative response of the facial nerve to cAMP exposure.

Conclusion

This study demonstrates a possible neurotrophic effect of cAMP on facial nerve regeneration in rats.

Characteristics for enhanced response of serotonin-evoked ion dynamics in differentiated NG108-15 cells

Characteristics for the up-regulated response in the concentration of intracellular calcium ion ([Ca(2+)]( i )) and in the sodium ion (Na(+)) current by serotonin (5-HT) were investigated in differentiated neuroblastoma x glioma hybrid NG108-15 (NG) cells. The results for the changes in [Ca(2+)]( i ) by 5-HT were as follows, (1) The 5-HT-induced Ca(2+) response was inhibited by 3 x 10(-9) M tropisetron (a 5-HT(3) receptor blocker), but not by other types of 5-HT receptor blockers; (2) The 5-HT-induced Ca(2+) response was mainly inhibited by calciseptine (a L-type Ca(2+) blocker), but not by other types of Ca(2+) channel blockers or 10(-7) M TTX (a voltage-sensitive Na(+) channel blocker); (3) When the extracellular Na(+) was removed by exchange with choline chloride or N-methyl-D-glucamine, the 5-HT-induced Ca(2+) response was extremely inhibited. The results for the 5-HT-induced Na(+) current by the whole cell patch-clamp technique were as follows, (1) The 5-HT-induced Na(+) current in differentiated cells was significantly larger than that in undifferentiated cells; (2) The ED(50) value for 5-HT-induced Na(+) current in undifferentiated and differentiated cells was almost the same, about 4 x 10(-6) M each other; (3) The 5-HT-induced Na(+) current was completely blocked by 3 x 10(-9) M tropisetron, but not by other 5-HT receptor antagonists and 10(-7) M TTX. These results suggested that 5-HT-induced Ca(2+) response in differentiated NG cells was mainly due to L-type voltage-gated Ca(2+) channels allowing extracellular Na(+) to enter via 5-HT(3) receptors, but not through voltage-gated Na(+) channels.

Hydrogen sulfide evokes neurite outgrowth and expression of high-voltage-activated Ca2+ currents in NG108-15 cells: involvement of T-type Ca2+ channels

We investigated if stimulation of T-type Ca(2+) channels with sodium hydrosulfide (NaHS), a donor of hydrogen sulfide (H(2)S), could cause neuronal differentiation of NG108-15 cells. Like dibutyryl cyclic AMP (db-cAMP), treatment with NaHS at 1.5-13.5 mM for 16 h enhanced neurite outgrowth in a concentration-dependent manner. Synergistic neuritogenic effect was obtained in the cells stimulated with NaHS in combination with db-cAMP at subeffective concentrations. Exposure to NaHS or db-cAMP for 2 days resulted in enhancement of expression of high-voltage-activated currents consisting of N-, P/Q-, L- and also other types, but not of T-type currents. Mibefradil, a pan-T-type channel blocker, abolished the neuritogenesis induced by NaHS, but not by db-cAMP. The NaHS-evoked neuritogenesis was also completely blocked by pretreatment with BAPTA/AM, a chelator of intracellular Ca(2+), and by zinc chloride at a concentration known to selectively inhibit Ca(v)3.2 isoform of T-type Ca(2+) channels, but not Ca(v)3.1 or Ca(v)3.3. Further, L-ascorbate, recently proven to selectively inhibit Ca(v)3.2, abolished the neuritogenic effect of NaHS, but not db-cAMP. Our data thus demonstrate that NaHS/H(2)S is a novel inducer of neuronal differentiation in NG108-15 cells, as characterized by neuritogenesis and expression of high-voltage-activated currents, and suggest the involvement of T-type Ca(2+) channels, especially Ca(v)3.2.

The synergistic inhibitory actions of oxcarbazepine on voltage-gated sodium and potassium currents in differentiated NG108-15 neuronal cells and model neurons

Oxcarbazepine (OXC), one of the newer anti-epileptic drugs, has been demonstrating its efficacy on wide-spectrum neuropsychiatric disorders. However, the ionic mechanism of OXC actions in neurons remains incompletely understood. With the aid of patch-clamp technology, we first investigated the effects of OXC on ion currents in NG108-15 neuronal cells differentiated with cyclic AMP. We found OXC (0.3-30 microm) caused a reversible reduction in the amplitude of voltage-gated Na+ current (INa). The IC50 value required for the inhibition of INa by OXC was 3.1 microm. OXC (3 microm) could shift the steady-state inactivation of INa to a more negative membrane potential by approximately -9 mV with no effect on the slope of the inactivation curve, and produce a significant prolongation in the recovery of INa inactivation. Additionally, OXC was effective in suppressing persistent INa (INa(P)) elicited by long ramp pulses. The blockade of INa by OXC does not simply reduce current magnitude, but alters current kinetics. Moreover, OXC could suppress the amplitude of delayed rectifier K+ current (IK(DR)), with no effect on M-type K+ current (IK(M)). In current-clamp configuration, OXC could reduce the amplitude of action potentials and prolong action-potential duration. Furthermore, the simulations, based on hippocampal pyramidal neurons (Pinsky-Rinzel model) and a network of the Hodgkin-Huxley model, were analysed to investigate the effect of OXC on action potentials. Taken together, our results suggest that the synergistic blocking effects on INa and IK(DR) may contribute to the underlying mechanisms through which OXC affects neuronal function in vivo.

Nitric oxide-mediated modulation of calcium/calmodulin-dependent protein kinase II

The mechanisms of NO inhibition of CaMK [Ca2+/CaM (calmodulin)-dependent protein kinase] II activity were studied. In rat pituitary tumour GH3 cells, TRH [thyrotrophin (TSH)-releasing hormone]-stimulated phosphorylation of nNOS [neuronal NOS (NO synthase)] at Ser847 was sensitive to an inhibitor of CaMKs, KN-93, and was enhanced by inhibition of nNOS with 7NI (7-nitroindazole). Enzyme activity of CaMKII following in situ treatment with 7NI was also increased. The in vitro activity of CaMKII was inhibited by co-incubation either with nNOS and L-arginine or with NO donors SNAP (S-nitroso-N-acetyl-DL-penicillamine) and DEA-NONOate [diethylamine-NONOate (diazeniumdiolate)]. Once inhibited by these treatments, CaMKII was observed to undergo full reactivation on the addition of a reducing reagent, DTT (dithiothreitol). In transfected cells expressing CaMKII and nNOS, treatment with the calcium ionophore A23187 further revealed nNOS phosphorylation at Ser847, which was enhanced by 7NI and CaMKII S-nitrosylation. Mutated CaMKII (C6A), in which Cys6 was substituted with an alanine residue, was refractory to 7NI-induced enhancement of nNOS phosphorylation or to CaMKII S-nitrosylation. Furthermore, we could identify Cys6 as a direct target for S-nitrosylation of CaMKII using MS. In addition, treatment with glutamate caused an increase in CaMKII S-nitrosylation in rat hippocampal slices. This glutamate-induced S-nitrosylation was blocked by 7NI. These results suggest that inactivation of CaMKII mediated by S-nitrosylation at Cys6 may contribute to NO-induced neurotoxicity in the brain.

Neural differentiation and the attenuated heat shock response

Differentiation of neural progenitor cells of neuroblastoma, pheochromocytoma, and surrogate stem cell lineages from a state resembling stem cells to a state resembling neurons is accompanied by a marked attenuation in induction of the heat shock protein 70 promoter driven-luciferase reporter gene, and induction of the reporter gene in primary embryonic neurons from hippocampus, cortex, and spinal cord is lower still when compared to the differentiated cells. Neural specificity of this phenotype is demonstrated by a negative correlation of hsp70-reporter gene expression and neurite extension under various experimental conditions. Analysis of biochemical events involved in induction of the heat shock response (HSR) reveal a blunted activation of HSF1 DNA-binding activity, and decreased induction of the mRNA(hsp70) and the 72 kDa HSP70 protein. Immunocytochemical staining for HSP70 demonstrates a cytoplasmic staining pattern; heat shock greatly increased the HSP70 staining intensity in the undifferentiated cells and less so in the differentiated cells. Vulnerability of the differentiated cells towards the oxidizer, arsenite, and the excitotoxic glutamate/glycine is demonstrated by the dose-dependent cytotoxic effects of these agents on cell viability and activation of caspase 3/7. Importantly, conditioning heat shock as well as increased expression of HSP70 by gene transfer conferred protection against such cytotoxicity. Together, our results show that neural differentiation is associated with a decreased induction of the heat shock response and an increased vulnerability to stress induced pathologies and death.

Changes in the regulation of heat shock gene expression in neuronal cell differentiation

Neuronal differentiation of the NG108-15 neuroblastoma-glioma hybrid cells is accompanied by a marked attenuation in the heat shock induction of the Hsp70-firefly luciferase reporter gene activity. Analysis of the amount and activation of heat shock factor 1, induction of mRNA(hsp), and the synthesis and accumulation of heat shock proteins (HSPs) in the undifferentiated and differentiated cells suggest a transcriptional mechanism for this attenuation. Concomitant with a decreased induction of the 72-kDa Hsp70 protein in the differentiated cells, there is an increased abundance of the constitutive 73-kDa Hsc70, a protein known to function in vesicle trafficking. Assessment of sensitivity of the undifferentiated and differentiated cells against stress-induced cell death reveals a significantly greater vulnerability of the differentiated cells toward the cytotoxic effects of arsenite and glutamate/glycine. This study shows that changes in regulation of the HSP and HSC proteins are components of the neuronal cell differentiation program and that the attenuated induction of HSPs likely contributes to neuronal vulnerability whereas the increased expression of Hsc70 likely has a role in neural-specific functions.

Enhancement of sodium current in NG108-15 cells during neural differentiation is mainly due to an increase in NaV1.7 expression

It is well known that morphological and functional changes during neural differentiation sometimes accompany the expression of various voltage-gated ion channels. In this work, we investigated whether the enhancement of sodium current in differentiated neuroblastoma x glioma NG108-15 cells treated with dibutyryl cAMP is related to the expression of voltage-gated sodium channels. The results were as follows. (1) Sodium current density on peak voltage in differentiated cells was significantly enhanced compared with that in undifferentiated cells, as detected by the whole-cell patch clamp method. The steady-state inactivation curve in differentiated cells was similar to that for undifferentiated cells, but a hyperpolarized shift in the activation curve for differentiated cells was observed. The sodium currents of differentiated and undifferentiated cells were completely inhibited by 10(-7) M tetrodotoxin (TTX). (2) The only Na(V) mRNA with an increased expression level during neuronal differentiation was that for NaV1.7, as observed by real-time PCR analysis. (3) The increase in the level of NaV1.7 alpha subunit expression during neuronal differentiation was also observed by immunocytochemistry; in particular, the localization of NaV1.7 alpha subunits on the soma, varicosities and growth cone was significant. These results suggest that the enhancement of TTX-sensitive sodium current density in differentiated NG108-15 cells is mainly due to the increase in the expression of the TTX-sensitive voltage-gated Na+ channel, NaV1.7.

Enhancement of serotonin- and bradykinin-evoked calcium ion dynamics in differentiated NG108-15 cells

Dynamic changes in the concentration of intracellular free-calcium ion ([Ca(2+)](i)) by carbachol (CCh) and neurotransmitter candidates was investigated in undifferentiated and differentiated neuroblastomaxglioma hybrid NG108-15 (NG) cells. [Ca(2+)](i) was increased in a dose-dependent manner by bradykinin (BK) and serotonin (5-HT) in differentiated NG cells, and the response to BK and 5-HT was significantly greater than that in undifferentiated NG cells. The EC(50) value of BK was approximately 1.5 x 10(-8)M in both undifferentiated and differentiated NG cells. The EC(50) value of 5-HT in differentiated NG cells was about 5 x 10(-6)M. The response to BK and 5-HT was almost completely inhibited by 10 nM Hoe140 (a BK B2 receptor antagonist) and 3 nM tropisetron (a 5-HT(3) receptor antagonist), respectively. These results suggest that there are some mechanisms by which the response evoked by BK and 5-HT is up-regulated in differentiated NG cells.

Increased response to high KCl-induced elevation in the intracellular-Ca(2+) concentration in differentiated NG108-15 cell and the inhibitory effect of the L-type Ca(2+) channel blocker, calciseptine

Characteristics of the increasing effect for the concentration of intracellular calcium ions ([Ca(2+)](i)) by high-KCl application were investigated in the neuroblastomaxglioma hybrid NG108-15 cell line (NG108-15 cells). The present study confirmed that the increasing effect of [Ca(2+)](i) by high-KCl application in single NG108-15 cells, differentiated with dibutyryl cAMP (Bt(2)cAMP), was significantly enhanced, compared to undifferentiated cells. The following observations were made at first: (1) The response to high-KCl application, in both undifferentiated and differentiated cells, was significantly inhibited by calciseptine (CaS), an L-type Ca(2+) channel blocker, but not by N-, P- and R-type Ca(2+) channel blockers. The IC(50) values for CaS in both undifferentiated and differentiated cell was almost identical. (2) The inhibitory effect of CaS was irreversible. (3) The increasing effect for [Ca(2+)](i) by high-KCl application was completely dependent on the presence of extracellular calcium ions. (4) The increased [Ca(2+)](i) by high-KCl application under a plateau concentration was quickly decreased to basal levels when the high-KCl solution was exchanged for a high-KCl solution containing EGTA (without CaCl(2)). Together, these results suggest that the enhancement of the response effect of [Ca(2+)](i) by high-KCl application in differentiated single NG108-15 cells was mainly due to the quantitative increase of L-type voltage-sensitive calcium channels (VSCCs), which were irreversibly inhibited by CaS.

Enhancement of veratridine-induced sodium dynamics in NG108-15 cells during differentiation

Developmental changes in dynamics of Na+ were studied in neuroblastomaxglioma hybrid NG108-15 cells during differentiation which was induced by dibutyryl cAMP (Bt2cAMP). Ratiometric Na+ imaging with a Na+-sensitive fluorescent dye SBFI (sodium-binding benzofuran isophthalate) revealed that the intracellular Na+ concentration ([Na+]i) was not affected by the application of high K+ (60 mM) solution to either control or differentiated cells. When cells were exposed to 50 microM veratridine (Vtd), an agonist of voltage-sensitive sodium channels (VSSCs), a significant increase in [Na+]i was observed in differentiated but not in undifferentiated cells. Calculated mean [Na+]i value increased from the basal 10.4 to 44.1 mM in response to 50 microM Vtd. This Vtd response was reversibly inhibited by tetrodotoxin (TTX), a specific blocker for VSSCs, in a dose-dependent manner (IC50 = 1 nM). It is suggested that VSSCs in NG108-15 cells are sensitive to TTX and Vtd and that the number of VSSCs increases during differentiation.

Merkel Cell-Nerve Cell Interaction Undergoes Formation of a Synapse-like Structure in a Primary Culture

Merkel cells have been assumed to guide nerve fibers to the skin. However, there has been little in vitro evidence that supports this hypothesis, because there is no suitable established culture system of Merkel cells. Here we show that Merkel cells isolated from rat footpad skin were successfully cultured in a monolayer with keratinocytes. Keratinocytes did not affect any structural changes in Merkel cells. When nerve cells (NG108-15 or PC12) were added to the culture system, both nerve fibers and cytoplasmic processes of Merkel cells outgrew and cooperatively organized synapse-like structures at their contact points. Nerve cells promoted Merkel cell survival, compared with keratinocytes only. Merkel cell proliferation was not detected in all conditions, even with nerve growth factor, neurotrophin-3, interleukin-6 and tumor necrosis factor-alpha. The data suggest, firstly, that Merkel cells may guide nerve fibers to the skin by interacting with nerve cells; and, secondly, that nerve cells, but not keratinocytes, may produce some survival factors other than the cytokines above for Merkel cells, although Merkel cells may be a terminally differentiated cell type. Our method could open a way to study Merkel cell biology.

Cyclin-dependent kinase 5 activators p35 and p39 facilitate formation of functional synapses

Cyclin-dependent kinase 5 (Cdk5) has emerged as a key coordinator of cell signaling in neurite outgrowth. Cdk5 needs to associate with one of the regulatory proteins p35 or p39 to be an active enzyme. To investigate if Cdk5 plays a role in the establishment of functional synapses, we have characterized the expression of Cdk5, p35, and p39 in the neuroblastoma-glioma cell line NG108-15, and recorded postsynaptic activity in myotubes in response to presynaptic overexpression of Cdk5, p35, and p39. Endogenous Cdk5 and p35 protein levels increased with cellular differentiation and preferentially distributed to soluble pools, whereas the level of p39 protein remained low and primarily was present in membrane and cytoskeletal fractions. Transient transfection of a dominant-negative mutant of Cdk5 in NG108-15 cells and subsequent culturing on differentiating muscle cells resulted in a significant reduction in synaptic activity, as measured by postsynaptic miniature endplate potentials (mEPPs). Overexpression of either Cdk5/p35 or Cdk5/p39 resulted in a substantial increase in synaptic structures that displayed postsynaptic activities, as well as mEPP frequency. These findings demonstrate that Cdk5, p35, and p39 are endogenously expressed in NG108-15 cells, exhibit distinct subcellular localizations, and that both Cdk5/p35 and Cdk5/p39 are central in formation of functional synapses.

Inhibition of neuronal nitric-oxide synthase by phosphorylation at Threonine1296 in NG108-15 neuronal cells

We demonstrate that neuronal nitric-oxide synthase (nNOS) is directly inhibited through the phosphorylation of Thr(1296) in NG108-15 neuronal cells. Treatment of NG108-15 cells expressing nNOS with calyculin A, an inhibitor of protein phosphatase 1 and 2A, revealed a dose-dependent inhibition of nNOS enzyme activity with concomitant phosphorylation of Thr(1296) residue. Cells expressing a phosphorylation-deficient mutant in which Thr(1296) was changed to Ala proved resistant to phosphorylation and suppression of NOS activity. Mimicking phosphorylation mutant of nNOS in which Thr(1296) is changed to Asp showed a significant decrease in nNOS enzyme activity, being competitive with NADPH, relative to the wild-type enzyme. These data suggest that phosphorylation of nNOS at Thr(1296) may involve the attenuation of nitric oxide production in neuronal cells through the decrease of NADPH-binding to the enzyme.

Persistent sodium current is a target for cAMP-induced neuronal plasticity in a state-setting modulatory interneuron

We have identified a TTX-resistant low-threshold persistent inward sodium current in the cerebral giant cells (CGCs) of Lymnaea, an important state-setting modulatory cell type of molluscan feeding networks. This current has slow voltage-dependent activation and de-activation kinetics, ultra-slow inactivation kinetics and fast de-inactivation kinetics. It activates at approximately -90 mV, peaks at approximately -30 mV, reverses at approximately +35 mV and does not show full voltage-dependent inactivation even at positive voltage steps. Lithium-sodium replacement experiments indicate that the persistent sodium current makes a significant contribution to the CGC membrane potential. Injection of cyclic adenosine monophosphate (cAMP) into the CGC cell body produces a large increase in the persistent sodium current that lasts for several hours. cAMP injection also leads to increased bursting, a significant decrease in the resistance and a significant depolarization of the soma membrane, indicating that cAMP-dependent mechanisms induce prolonged neuronal plasticity in the CGCs. Our observations provide the first link between cAMP-mediated modulation of a TTX-resistant persistent sodium current and prolonged neuronal plasticity in an identified modulatory cell type that plays an important role in behavioral state setting.

RA-RhoGAP, Rap-activated Rho GTPase-activating protein implicated in neurite outgrowth through Rho

Rap1 and Rho small G proteins have been implicated in the neurite outgrowth, but the functional relationship between Rap1 and Rho in the neurite outgrowth remains to be established. Here we identified a potent Rho GTPase-activating protein (GAP), RA-RhoGAP, as a direct downstream target of Rap1 in the neurite outgrowth. RA-RhoGAP has the RA and GAP domains and showed GAP activity specific for Rho, which was enhanced by the binding of the GTP-bound active form of Rap1 to the RA domain. Overexpression of RA-RhoGAP induced inactivation of Rho for promoting the neurite outgrowth in a Rap1-dependent manner. Knockdown of RA-RhoGAP reduced the Rap1-induced neurite outgrowth. These results indicate that RA-RhoGAP transduces a signal from Rap1 to Rho and regulates the neurite outgrowth.

Regulation of SNAREs by tomosyn and ROCK: implication in extension and retraction of neurites

Extension of neurites requires the SNARE-dependent fusion of plasmalemmal precursor vesicles with the plasma membrane of growth cones. Here, we show that tomosyn localizes at the palm of growth cones and inhibits the fusion of the vesicles there, thus promoting transport of the vesicles to the plasma membrane of the leading edges of growth cones. Tomosyn localizes because ROCK activated by Rho small G protein phosphorylates syntaxin-1, which increases the affinity of syntaxin-1 for tomosyn and forms a stable complex with tomosyn, resulting in inhibition of the formation of the SNARE complex. In retraction of neurites, tomosyn localizes all over the edges of the neurites and inhibits fusion of the vesicles with the plasma membrane. Thus, tomosyn demarcates the plasma membrane by binding to syntaxin-1 phosphorylated by ROCK, and thereby regulates extension and retraction of neurites.

Signal transduction cascades underlying de novo protein synthesis required for neuronal morphogenesis in differentiating neurons

Differentiating neurons must acquire many unique morphological and functional characteristics in creating the precise neural circuits of the mature nervous system. The phenomenon of 'neuronal differentiation' includes a special set of simple, separate processes, that is, neuritogenesis, neurite outgrowth, pathfinding, targeting and synaptogenesis. All of these processes are critically dependent on the reorganization of actin cytoskeleton by many actin-binding proteins that function downstream of Rho-family GTPases. Furthermore, de novo synthesis of key proteins are critically involved in the reorganization of actin cytoskeleton during neuronal differentiation. In this article, we review recent progresses in the general mechanisms that control actin dynamics by various actin-binding proteins in differentiating neurons, including a series of recent studies from our laboratory on de novo synthesis of several key proteins that are essential for actin reorganization induced by second messengers. We demonstrated that dual regulation of cyclic AMP and Ca2+ determines cofilin (an actin-binding protein) phosphorylation states and LIM kinase 1 (a cofilin kinase) expression level during neuritogenesis.

Dual role of cyclic AMP-dependent protein kinase in neuritogenesis and synaptogenesis during neuronal differentiation

To create precise neural circuits in the nervous system, neuritogenesis and synaptogenesis are the critical cellular processes during neuronal differentiation. We examined the cyclic AMP (cAMP)-responsible signaling pathways for regulating neuritogenesis and synaptogenesis in NG108-15 cells. A rise in intracellular cAMP concentration by a membrane-permeable cAMP analog, dibutyryl cAMP (DBcAMP), led to an increase in the number of neurites and varicosities. Inhibition of cAMP-dependent protein kinase (PKA) activity by a PKA inhibitor (H89) accelerated this neuritogenesis and neurite outgrowth rate. Treatment with H89, however, decreased the number of varicosities and the frequency of postsynaptic miniature current recorded in the cultured cells, resulting in suppression of synaptogenesis. Immunoblot analyses revealed that PKA activity mediates phosphorylation of a gene transcription factor, cAMP-response element binding protein (CREB). On the other hand, inhibition of a mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) pathway by a MAPK/ERK kinase (MEK) inhibitor (PD98059) suppressed both neuritogenesis and neurite outgrowth without CREB phosphorylation. These results suggest strongly that PKA simultaneously plays two different roles in neuronal differentiation: inhibition of neuritogenesis and stimulation of synaptogenesis, via CREB-mediated gene expression.

Neuronal T-type alpha 1H calcium channels induce neuritogenesis and expression of high-voltage-activated calcium channels in the NG108-15 cell line

Neuronal differentiation involves both morphological and electrophysiological changes, which depend on calcium influx. Voltage-gated calcium channels (VGCCs) represent a major route for calcium entry into neurons. The recently cloned low-voltage-activated T-type calcium channels (T-channels) are the first class of VGCCs functionally expressed in most developing neurons, as well as in neuroblastoma cell lines, but their roles in neuronal development are yet unknown. Here, we document the part played by T-channels in neuronal differentiation. Using NG108-15, a cell line that recapitulates early steps of neuronal differentiation, we demonstrate that blocking T-currents by nickel, mibefradil, or the endogenous cannabinoid anandamide prevents neuritogenesis without affecting neurite outgrowth. Similar results were obtained using antisense oligodeoxynucleotides directed against the alpha1H T-channel subunit. Furthermore, we describe that inhibition of alpha1H T-channel activity impairs concomitantly, but independently, both high-voltage-activated calcium channel expression and neuritogenesis, providing strong evidence for a dual role of T-channels in both morphological and electrical changes at early stages of neuronal differentiation.

Arachidonate 12-lipoxygenases

Arachidonate 12-lipoxygenase introduces a molecular oxygen at carbon 12 of arachidonic acid to generate a 12-hydroperoxy derivative. The enzymes generate 12-hydroperoxy derivatives with either S- or R-configurations. There are three isoforms of 12S-lipoxygenases named after the cells where they were first identified; platelet, leukocyte and epidermis. The leukocyte-type enzyme is widely distributed among cells, but the tissue distribution varies substantially from species to species. The platelet and epidermal enzymes are present in only a relatively limited number of cell types. Although the structures and enzymatic properties of the three isoforms of 12S-lipoxygenases have been elucidated, the physiological roles of the 12S-lipoxygenases are not yet fully understood. There are important roles for the enzymes and their products in several biological systems including those involved in atherosclerosis and neurotransmission.

Distinct effects of different calcium-mobilizing agents on cell death in NG108-15 neuroblastoma X glioma cells

The effects of different calcium-mobilizing agents on cell death were characterized in NG108-15 neuroblastoma x glioma hybrid cells. Carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) increased the cytosolic Ca(2+) concentration ([Ca(2+)](i)) and caused cell death. Thapsigargin (TG) not only increased the [Ca(2+)](i) and caused cell death but also induced neurite outgrowth via activation of phospholipase A(2) and cytochrome P450 epoxygenase. In contrast, bradykinin increased the [Ca(2+)](i), but had no effect on cell morphology or cell death. Cell death occurred by two different mechanisms, one of which was caspase-3-dependent and the other caspase-3-independent. Caspase-3 activation was Ca(2+)-dependent, whereas neurite outgrowth was Ca(2+)-independent. TG- or FCCP-induced caspase-3 activation occurred at the same time, but the cell death induced by TG was delayed. TG treatment did not enhance the generation of nitric oxide or cAMP or secretion of glial-derived neurotrophic factor or neurotrophin-3, but activated sphingosine kinase. Furthermore, inhibition of sphingosine kinase accelerated TG-induced cell death, and exogenous sphingosine 1-phosphate (S1P) protected cells from FCCP-induced cell death by about 60%. These results indicate that, in these cells, depletion of intracellular nonmitochondrial or mitochondrial Ca(2+) stores causes cell death, that TG activates phospholipase A(2) and sphingosine kinase, and that arachidonic acid induces neurite outgrowth, whereas S1P delays cell death.

Synergism in cytosolic Ca2+mobilization between bradykinin and agonists for pertussis toxin-sensitive G-protein-coupled receptors in NG 108-15 cells

Bradykinin (BK) induced a transient and pertussis toxin (PT)-insensitive increase in cytosolic Ca2+ ([Ca2+]i) in NG 108-15 neuroblastoma x glioma hybrid cells, whereas leucine-enkephalin (EK), somatostatin, norepinephrine or carbachol showed a weak but PT-sensitive action. When any one of the latter agonists was applied to the cells treated with low doses of BK, however, the level of [Ca2+]i rise caused by the agonist was remarkably increased in a PT-sensitive manner. The decreasing of extracellular Ca2+ only slightly influenced the actions of these agonists. Thus, synergism between a BK receptor and PT-sensitive G-protein-coupled receptors results in marked intracellular Ca2+ mobilization by the latter agonists.

Postsynaptic target regulates functional responses induced by 5-HT3 serotonin receptors on axonal varicosities of NG108-15 hybrid neuroblastoma cells

Rat brain presynaptic 5-HT3 serotonin receptors, members of the ligand-gated ion channel superfamily, induce changes in nerve terminal [Ca2+]i in a manner distinct from that found for somatic 5-HT3 receptors. Here, we assessed the role of postsynaptic target in regulating the nature of presynaptic receptor-induced responses, using the hybrid neuroblastoma cell line NG108-15 as a model neuronal system that expresses 5-HT3 receptors. Using immunocytochemistry, 5-HT3 receptors were found to be present on the presynaptic-like varicosities of differentiated NG108-15 cells, indicating that these receptors possess an inherent ability to localize to potential presynaptic sites. In the absence of postsynaptic target, 5-HT3 receptors localized to the varicosities induce rapid but transient changes in [Ca2+]i that were initiated by voltage-gated Ca2+ channels, as assessed using Ca2+ channel blockers, these properties being typical of those found for somatic 5-HT3 receptors. In co-cultures containing rat myotubes, with which NG108-15 cells form functional cholinergic synapses, the 5-HT3 receptor-induced changes in [Ca2+]i in the axonal varicosities shifted over time (three to 10 days) to that found for brain nerve endings: sustained responses that were insensitive to blockade by antagonists of voltage-gated Ca2+ channels. The effect of co-culturing myotubes with the NG108-15 cells was mimicked by conditioned media from myotube cultures. These results indicate that regulatory molecules from the target postsynaptic cell dictate the functional responses elicited by presynaptic 5-HT3 receptors. Because the target-induced changes required several days before they were evident, we hypothesize that changes in protein expression, perhaps the consequence of altered gene regulation, underlie the changes in the responses to 5-HT3 receptor activation in the axonal varicosities of this neuronal cell line.

Overexpression of rat neuronal calcium sensor-1 in rodent NG108-15 cells enhances synapse formation and transmission

The role of rat neuronal calcium sensor-1 (NCS-1), a Ca2+-binding protein, in synapse formation and transmitter release was examined in mouse neuroblastoma x rat glioma hybrid NG108-15 cells in culture. Wild-type NG108-15 cells expressed rodent NCS-1. Endogenous NCS-1 was partially co-localized with the synaptic protein SNAP-25 at the plasma membrane in both cell bodies and processes, but not with the Golgi marker [beta]-COP, an individual coat subunit of the coatomer complex present on Golgi-derived vesicles. In NG108-15 cells co-cultured with rat myotubes, partial co-localization of SNAP-25 and NCS-1 was observed at the plasma membrane of neurites and growth cones, some of which had synaptic contacts to muscle cells. Transient co-transfection of the rat NCS-1 cDNA and green fluorescent protein (GFP) resulted in NCS-1 overexpression in about 30 % of the cells as determined by fluorescence microscopy. The rate of functional synapse formation with co-cultured rat myotubes increased 2-fold as determined by the presence of miniature endplate potentials (MEPPs) in NCS-1-overexpressing NG108-15 cells compared to non- and mock-transfected cells. The number of neurites per cell, branches per neurite and length of neurites was slightly less in cells that were either transiently transfected (GFP-NCS-1-fluorescence positive) or stably transformed with NCS-1 compared to GFP-NCS-1-negative, non-transfected or mock-transfected NG108-15 cells. The number of action potentials that elicited endplate potentials increased in NG108-15 cells stably transformed with rat NCS-1. The mean number of quanta per impulse (m) increased 5-fold. These results show that NCS-1 functions to facilitate synapse formation, probably because of the increased quantal content of evoked acetylcholine release.

Cyclooxygenase-2 stimulates production of amyloid beta-peptide in neuroblastoma x glioma hybrid NG108-15 cells

Cyclooxygenase (COX) synthesizes bioactive prostaglandins from arachidonic acid, and there are COX-1 and COX-2 isoforms with distinct pathophysiological functions. Recent studies demonstrated that COX-2 expression was up-regulated in the brain of patients with Alzheimer's disease. We established mouse neuroblastoma x rat glioma hybrid NG108-15 cells stably expressing human COX-2. The COX-2-expressing cells showed 3- to 4-fold increases in both COX activity and prostaglandin E(2) production. The mRNA level of amyloid precursor protein (APP) was elevated by approximately 2-fold in the COX-2-expressing cells compared with mock-transfected cells. Amyloid beta-peptide and a secreted form of APP, both derived from APP by proteolysis was also increased. Interestingly, neurite outgrowth was stimulated in the COX-2-expressing cells with concomitant reduction of the cell proliferation rate. A selective COX-2 inhibitor (JTE-522) and a nonselective COX inhibitor (indomethacin) suppressed production of amyloid beta-peptide and a secreted form of APP by inhibition of APP mRNA level, suggesting that COX-2 plays important roles in the neurodegenerative processes of Alzheimer's disease.