Synaptogenesis of cultured striatal neurons in serum-free medium: a morphological and biochemical study

Enteric Neural Network Assembly Was Promoted by Basic Fibroblast Growth Factor and Vitamin A but Inhibited by Epidermal Growth Factor

Extending well beyond the original use of propagating neural precursors from the central nervous system and dorsal root ganglia, neurosphere medium (NSM) and self-renewal medium (SRM) are two distinct formulas with widespread popularity in enteric neural stem cell (ENSC) applications. However, it remains unknown what growth factors or nutrients are crucial to ENSC development, let alone whether the discrepancy in their components may affect the outcomes of ENSC culture. Dispersed enterocytes from murine fetal gut were nurtured in NSM, SRM or their modifications by selective component elimination or addition to assess their effects on ENSC development. NSM generated neuriteless neurospheres, whereas SRM, even deprived of chicken embryo extract, might wire ganglia together to assemble neural networks. The distinct outcomes came from epidermal growth factor, which inhibited enteric neuronal wiring in NSM. In contrast, basic fibroblast growth factor promoted enteric neurogenesis, gangliogenesis, and neuronal wiring. Moreover, vitamin A derivatives might facilitate neuronal maturation evidenced by p75 downregulation during ENSC differentiation toward enteric neurons to promote gangliogenesis and network assembly. Our results might help to better manipulate ENSC propagation and differentiation in vitro, and open a new avenue for the study of enteric neuronal neuritogenesis and synaptogenesis.

Brain-derived neurotrophic factor predominantly regulates the expression of synapse-related genes in the striatum: Insights from in vitro transcriptomics

Aim

The striatum, a main component of the basal ganglia, is a critical part of the motor and reward systems of the brain. It consists of GABAergic and cholinergic neurons and receives projections of dopaminergic, glutamatergic, and serotonergic neurons from other brain regions. Brain‐derived neurotrophic factor (BDNF) plays multiple roles in the central nervous system, and striatal BDNF has been suggested to be involved in psychiatric and neurodegenerative disorders. However, the transcriptomic impact of BDNF on the striatum remains largely unknown. In the present study, we performed transcriptomic profiling of striatal cells stimulated with BDNF to identify enriched gene sets (GSs) and their novel target genes in vitro.

Methods

We carried out RNA sequencing (RNA‐Seq) of messenger RNA extracted from primary dissociated cultures of rat striatum stimulated with BDNF and conducted Generally Applicable Gene‐set Enrichment (GAGE) analysis on 10599 genes. Significant differentially expressed genes (DEGs) were determined by differential expression analysis for sequence count data 2 (DESeq2).

Results

GAGE analysis identified significantly enriched GSs that included GSs related to regulation and dysregulation of synaptic functions, such as synaptic vesicle cycle and addiction to nicotine and morphine, respectively. It also detected GSs related to various types of synapses, including not only GABAergic and cholinergic synapses but also dopaminergic and glutamatergic synapses. DESeq2 revealed 72 significant DEGs, among which the highest significance was observed in the apolipoprotein L domain containing 1 (Apold1).

Conclusions

The present study indicates that BDNF predominantly regulates the expression of synaptic‐function‐related genes and that BDNF promotes synaptogenesis in various subtypes of neurons in the developing striatum. Apold1 may represent a unique target gene of BDNF in the striatum.

In the present study, we performed transcriptomic profiling of striatal cells stimulated with BDNF to identify enriched gene sets (GSs) in vitro. Generally Applicable Gene‐set Enrichment (GAGE) analysis followed by differential expression analysis for sequence count data 2 (DESeq2) suggested that BDNF predominantly regulates the expression of synaptic‐function‐related genes and that BDNF promotes synaptogenesis in various subtypes of neurons in the developing striatum.

The necessity for standardization of glioma stem cell culture: a systematic review

BACKGROUND

The cancer stem cell hypothesis is an old idea which has been revived in recent years for many cancers, including gliomas. However, this concept has become controversial due to a series of studies with conflicting results.

METHODS

A systematic literature search was conducted in PubMed and the Web of Science database to analyze studies using serum-free medium and its components in glioma stem cells, glioma stem-like cells, glioma-initiating cells, or glioma neurosphere cultures. All the studies reviewed were published between 1970 and 2019. We found that no standardized culture method was used, and the data were incomparable due to differing culture conditions and the use of media with different components.

CONCLUSIONS

Here, we review the most commonly used serum-free media and added components for glioma stem cell culture while highlighting the function of each component used in the media. We emphasize the necessity for standardization of glioma stem cell culture and propose a standard culture medium to prevent bias in glioma stem cell research.

Developmental changes in trak-mediated mitochondrial transport in neurons

Previous studies established that the kinesin adaptor proteins, TRAK1 and TRAK2, play an important role in mitochondrial transport in neurons. They link mitochondria to kinesin motor proteins via a TRAK acceptor protein in the mitochondrial outer membrane, the Rho GTPase, Miro. TRAKs also associate with enzyme, O-linked N-acetylglucosamine transferase (OGT), to form a quaternary, mitochondrial trafficking complex. A recent report suggested that TRAK1 preferentially controls mitochondrial transport in axons of hippocampal neurons whereas TRAK2 controls mitochondrial transport in dendrites. However, it is not clear whether the function of any of these proteins is exclusive to axons or dendrites and if their mechanisms of action are conserved between different neuronal populations and also, during maturation. Here, a comparative study was carried out into TRAK-mediated mitochondrial mobility in axons and dendrites of hippocampal and cortical neurons during maturation in vitro using a shRNA gene knockdown approach. It was found that in mature hippocampal and cortical neurons, TRAK1 predominantly mediates axonal mitochondrial transport whereas dendritic transport is mediated via TRAK2. In young, maturing neurons, TRAK1 and TRAK2 contribute similarly in mitochondrial transport in both axons and dendrites in both neuronal types. These findings demonstrate maturation regulation of mitochondrial transport which is conserved between at least two distinct neuronal subtypes.

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Mitochondrial transport and velocity changes during neuronal maturation.

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TRAK1 and TRAK2 contribute to transport in axons and dendrites of immature neurons.

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In mature neurons TRAK1 controls axonal mitochondrial transport.

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In mature neurons TRAK2 controls dendritic mitochondrial transport.

PGC-1α/ERRα-Sirt3 Pathway Regulates DAergic Neuronal Death by Directly Deacetylating SOD2 and ATP Synthase β

AIMS

Parkinson's disease (PD) heavily affects humans and little is known about its cause and pathogenesis. Sirtuin 3 (Sirt3) plays a key role in regulating mitochondrial dysfunction, which is the main cause of DAergic neuronal loss in PD. We investigated the mechanisms of neuroprotective role of Sirt3 in DAergic neuronal survival.

RESULTS

Sirt3 was reduced in 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP)-treated neurons with its overexpression being neuroprotective. We identified that Sirt3 interacted with manganese superoxide dismutase (SOD2) and adenosine triphosphate (ATP) synthase β and modulated their activities by deacetylating SOD2 (K130) and ATP synthase β (K485) to prevent reactive oxygen species accumulation and ATP depletion, and to alleviate DAergic neuronal death upon MPTP treatment. Peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α) interacted with estrogen-related receptor alpha (ERRα) that bound to the Sirt3 promoter as its transcription factor to regulate Sirt3 expression and DAergic neuronal death. In the mouse midbrain, MPTP administration led to the loss of PGC-1α and Sirt3, high acetylation level of SOD2 and ATP synthase β, and the specific loss of DAergic neurons, while Sirt3 overexpression could protect against DAergic neuronal loss. Sirt3 knockout mice exhibited more sensitive and more DAergic neuronal loss to MPTP treatment.

INNOVATION

The study provides new insights into a critical PGC-1α/ERRα-Sirt3 pathway, linking regulation of mitochondrial protein acetylation and DAergic neuronal death in PD pathogenesis, which provide a potential therapeutic strategy and target in PD treatment.

CONCLUSION

These results provide a vital PGC-1α/ERRα-Sirt3 pathway that protects against DAergic neuronal death by directly deacetylating SOD2 (K130) and ATP synthase β (K485) in PD.

α-synuclein assemblies sequester neuronal α3-Na+/K+-ATPase and impair Na+ gradient

Extracellular α-synuclein (α-syn) assemblies can be up-taken by neurons; however, their interaction with the plasma membrane and proteins has not been studied specifically. Here we demonstrate that α-syn assemblies form clusters within the plasma membrane of neurons. Using a proteomic-based approach, we identify the α3-subunit of Na+/K+-ATPase (NKA) as a cell surface partner of α-syn assemblies. The interaction strength depended on the state of α-syn, fibrils being the strongest, oligomers weak, and monomers none. Mutations within the neuron-specific α3-subunit are linked to rapid-onset dystonia Parkinsonism (RDP) and alternating hemiplegia of childhood (AHC). We show that freely diffusing α3-NKA are trapped within α-syn clusters resulting in α3-NKA redistribution and formation of larger nanoclusters. This creates regions within the plasma membrane with reduced local densities of α3-NKA, thereby decreasing the efficiency of Na+ extrusion following stimulus. Thus, interactions of α3-NKA with extracellular α-syn assemblies reduce its pumping activity as its mutations in RDP/AHC.

The role of the serotonin receptor subtypes 5-HT1A and 5-HT7 and its interaction in emotional learning and memory

Serotonin [5-hydroxytryptamine (5-HT)] is a multifunctional neurotransmitter innervating cortical and limbic areas involved in cognition and emotional regulation. Dysregulation of serotonergic transmission is associated with emotional and cognitive deficits in psychiatric patients and animal models. Drugs targeting the 5-HT system are widely used to treat mood disorders and anxiety-like behaviors. Among the fourteen 5-HT receptor (5-HTR) subtypes, the 5-HT_1AR and 5-HT₇R are associated with the development of anxiety, depression and cognitive function linked to mechanisms of emotional learning and memory. In rodents fear conditioning and passive avoidance (PA) are associative learning paradigms to study emotional memory. This review assesses the role of 5-HT_1AR and 5-HT₇R as well as their interplay at the molecular, neurochemical and behavioral level. Activation of postsynaptic 5-HT_1ARs impairs emotional memory through attenuation of neuronal activity, whereas presynaptic 5-HT_1AR activation reduces 5-HT release and exerts pro-cognitive effects on PA retention. Antagonism of the 5-HT_1AR facilitates memory retention possibly via 5-HT₇R activation and evidence is provided that 5HT₇R can facilitate emotional memory upon reduced 5-HT_1AR transmission. These findings highlight the differential role of these 5-HTRs in cognitive/emotional domains of behavior. Moreover, the results indicate that tonic and phasic 5-HT release can exert different and potentially opposing effects on emotional memory, depending on the states of 5-HT_1ARs and 5-HT₇Rs and their interaction. Consequently, individual differences due to genetic and/or epigenetic mechanisms play an essential role for the responsiveness to drug treatment, e.g., by SSRIs which increase intrasynaptic 5-HT levels thereby activating multiple pre- and postsynaptic 5-HTR subtypes.

GABAA receptor activity shapes the formation of inhibitory synapses between developing medium spiny neurons

Basal ganglia play an essential role in motor coordination and cognitive functions. The GABAergic medium spiny neurons (MSNs) account for ~95% of all the neurons in this brain region. Central to the normal functioning of MSNs is integration of synaptic activity arriving from the glutamatergic corticostriatal and thalamostriatal afferents, with synaptic inhibition mediated by local interneurons and MSN axon collaterals. In this study we have investigated how the specific types of GABAergic synapses between the MSNs develop over time, and how the activity of GABA_A receptors (GABA_ARs) influences this development. Isolated embryonic (E17) MSNs form a homogenous population in vitro and display spontaneous synaptic activity and functional properties similar to their in vivo counterparts. In dual whole-cell recordings of synaptically connected pairs of MSNs, action potential (AP)-activated synaptic events were detected between 7 and 14 days in vitro (DIV), which coincided with the shift in GABA_AR operation from depolarization to hyperpolarization, as detected indirectly by intracellular calcium imaging. In parallel, the predominant subtypes of inhibitory synapses, which innervate dendrites of MSNs and contain GABA_AR α1 or α2 subunits, underwent distinct changes in the size of postsynaptic clusters, with α1 becoming smaller and α2 larger over time, while both the percentage and the size of mixed α1/α2-postsynaptic clusters were increased. When activity of GABA_ARs was under chronic blockade between 4–7 DIV, the structural properties of these synapses remained unchanged. In contrast, chronic inhibition of GABA_ARs between 7–14 DIV led to reduction in size of α1- and α1/α2-postsynaptic clusters and a concomitant increase in number and size of α2-postsynaptic clusters. Thus, the main subtypes of GABAergic synapses formed by MSNs are regulated by GABA_AR activity, but in opposite directions, and thus appear to be driven by different molecular mechanisms.

Quantitative phosphoproteomics unravels biased phosphorylation of serotonin 2A receptor at Ser280 by hallucinogenic versus nonhallucinogenic agonists

The serotonin 5-HT_2A receptor is a primary target of psychedelic hallucinogens such as lysergic acid diethylamine, mescaline, and psilocybin, which reproduce some of the core symptoms of schizophrenia. An incompletely resolved paradox is that only some 5-HT_2A receptor agonists exhibit hallucinogenic activity, whereas structurally related agonists with comparable affinity and activity lack such a psychoactive activity. Using a strategy combining stable isotope labeling by amino acids in cell culture with enrichment in phosphorylated peptides by means of hydrophilic interaction liquid chromatography followed by immobilized metal affinity chromatography, we compared the phosphoproteome in HEK-293 cells transiently expressing the 5-HT_2A receptor and exposed to either vehicle or the synthetic hallucinogen 1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI) or the nonhallucinogenic 5-HT_2A agonist lisuride. Among the 5995 identified phosphorylated peptides, 16 sites were differentially phosphorylated upon exposure of cells to DOI versus lisuride. These include a serine (Ser²⁸⁰) located in the third intracellular loop of the 5-HT_2A receptor, a region important for its desensitization. The specific phosphorylation of Ser²⁸⁰ by hallucinogens was further validated by quantitative mass spectrometry analysis of immunopurified receptor digests and by Western blotting using a phosphosite specific antibody. The administration of DOI, but not of lisuride, to mice, enhanced the phosphorylation of 5-HT_2A receptors at Ser²⁸⁰ in the prefrontal cortex. Moreover, hallucinogens induced a less pronounced desensitization of receptor-operated signaling in HEK-293 cells and neurons than did nonhallucinogenic agonists. The mutation of Ser²⁸⁰ to aspartic acid (to mimic phosphorylation) reduced receptor desensitization by nonhallucinogenic agonists, whereas its mutation to alanine increased the ability of hallucinogens to desensitize the receptor. This study reveals a biased phosphorylation of the 5-HT_2A receptor in response to hallucinogenic versus nonhallucinogenic agonists, which underlies their distinct capacity to desensitize the receptor.

Isolation and characterization of cancer stem cells in vitro

The cancer stem cell hypothesis is an appealing concept to account for intratumoral heterogeneity and the observation that systemic metastasis and treatment failure are often associated with the survival of a small number of cancer cells. Whilst in vivo evidence forms the foundation of this concept, in vitro methods and reagents are attractive as they offer opportunities to perform experiments that are not possible in an animal model. While there is abundant evidence that existing cancer cell lines are not reliable models of tumor heterogeneity, recent advances based on well validated novel cancer cell lines established de novo in defined serum-free media are encouraging, particularly in the study of glioblastoma multiforme. In this chapter we wish to broadly outline the process of establishing, characterizing, and managing novel cancer cell lines in defined serum-free media, and discuss the limitations and potential opportunities that may arise from these model systems.

BDNF may play a differential role in the protective effect of the mGluR2/3 agonist LY379268 on striatal projection neurons in R6/2 Huntington's disease mice

We have found that daily subcutaneous injection with a maximum tolerated dose (MTD) of the mGluR2/3 agonist LY379268 (20mg/kg) beginning at 4 weeks dramatically improves the phenotype in R6/2 mice. For example, we observed normalization of motor function in distance traveled, speed, the infrequency of pauses, and the ability to locomote in a straight line, and a rescue of a 15-20% striatal neuron loss at 10 weeks. As acute LY379268 treatment is known to increase cortical BDNF production, and BDNF is known to be beneficial for striatal neurons, we investigated if the benefit of daily LY379268 in R6/2 mice for striatal projection neurons was associated with increases in corticostriatal BDNF, with assessments done at 10 weeks of age after daily MTD treatment since the fourth week of life. We found that LY379268 increased BDNF expression in layer 5 neurons in motor cortex, which project to striatum, partly rescued a preferential loss of enkephalinergic striatal neurons, and enhanced substance P (SP) expression by SP striatal projection neurons. The enhanced survival of enkephalinergic striatal neurons was correlated with the cortical BDNF increase, but the enhanced SP expression by SP striatal neurons was not. Thus, LY379268 may protect the two main striatal projection neuron types by different mechanisms, enkephalinergic neurons by the trophic benefit of BDNF, and SP neurons by a mechanism not involving BDNF. The SP neuron benefit may perhaps instead involve the anti-excitotoxic action of mGluR2/3 receptor agonists.

Ca(2+) channel currents of cortical neurons from pure and mixed cultures

Voltage-gated Ca(2+) channels (VGCCs) are key regulators of many neuronal functions, and involved in multiple central nervous system diseases. In the last 30 years, a large number of injury and disease models have been established based on cultured neurons. Culture with serum develops a mixture of neurons and glial cells, while culture without serum develops pure neurons. Both of these neuronal-culture methods are widely used. However, the properties of Ca(2+) currents in neurons from these two cultures have not been compared. In this study, we cultured rat cortical neurons in serum-containing or -free medium and then recorded the Ca(2+) channel currents using patch-clamp technique. Our results showed that there were significant differences in the amplitude and activation properties of whole-cell Ca(2+) channel currents, and of non-L-type Ca(2+) channel currents between the neurons from these two culture systems. Our data suggested that the difference of whole-cell Ca(2+) currents may result from the differences in non-L-type currents. Understanding of these properties will considerably advance studies of VGCCs in neurons from pure or mixed culture.

Alpha-tocopheryl succinate induces rapid and reversible phosphatidylserine externalization in histiocytic lymphoma through the caspase-independent pathway

Phosphatidylserine (PS) externalization is a key feature of apoptotic cell death and plays an important role in clearance of apoptotic cells by phagocytes. PS externalization during apoptosis is generally an irreversible event mediated by caspase activation and is accompanied by other apoptotic events. We report here that an apoptosis inducer alpha-tocopheryl succinate (TOS) can induce PS externalization that is independent of apoptosis and reversible in the absence of fetal bovine serum (FBS) in histiocytic lymphoma U937 cells. In the presence of FBS, TOS induced PS externalization via a caspase-dependent mechanism accompanied by mitochondrial depolarization, cell shrinkage, increase of caspase-3 activity, and chromatin condensation. In contrast, in the absence of FBS, TOS induced the rapid PS externalization which was not accompanied by other apoptotic events. The PS externalization was reversible by removing TOS and was not involved in Ca(2+)-dependent scramblase activation and thiol oxidation of aminophospholipid translocase. A similar PS externalization was also induced by cholesteryl hemisuccinate (CS), the other succinate ester. These results suggested that the mechanism of TOS- and CS-induced PS externalization in the absence of FBS was different from it occurring during typical apoptosis.

Physical interaction of calmodulin with the 5-hydroxytryptamine2C receptor C-terminus is essential for G protein-independent, arrestin-dependent receptor signaling

The serotonin (5-hydroxytryptamine; 5-HT)(2C) receptor is a G protein-coupled receptor (GPCR) exclusively expressed in CNS that has been implicated in numerous brain disorders, including anxio-depressive states. Like many GPCRs, 5-HT(2C) receptors physically interact with a variety of intracellular proteins in addition to G proteins. Here, we show that calmodulin (CaM) binds to a prototypic Ca(2+)-dependent "1-10" CaM-binding motif located in the proximal region of the 5-HT(2C) receptor C-terminus upon receptor activation by 5-HT. Mutation of this motif inhibited both beta-arrestin recruitment by 5-HT(2C) receptor and receptor-operated extracellular signal-regulated kinase (ERK) 1,2 signaling in human embryonic kidney-293 cells, which was independent of G proteins and dependent on beta-arrestins. A similar inhibition was observed in cells expressing a dominant-negative CaM or depleted of CaM by RNA interference. Expression of the CaM mutant also prevented receptor-mediated ERK1,2 phosphorylation in cultured cortical neurons and choroid plexus epithelial cells that endogenously express 5-HT(2C) receptors. Collectively, these findings demonstrate that physical interaction of CaM with recombinant and native 5-HT(2C) receptors is critical for G protein-independent, arrestin-dependent receptor signaling. This signaling pathway might be involved in neurogenesis induced by chronic treatment with 5-HT(2C) receptor agonists and their antidepressant-like activity.

Inverse agonist and neutral antagonist actions of antidepressants at recombinant and native 5-hydroxytryptamine2C receptors: differential modulation of cell surface expression and signal transduction

Despite the importance of 5-hydroxytryptamine (5-HT)(2C) (serotonin) receptors in the control of depressive states, actions of antidepressants at these receptors remain poorly characterized. This issue was addressed both in human embryonic kidney (HEK)-293 cells coexpressing unedited human 5-HT(2CINI) receptors and Galpha(q) protein and in cultured mouse cortical neurons. Indicative of constitutive activity, the inverse agonist SB206,553 decreased basal inositol phosphate (IP) production in HEK-293 cells. The tetracyclic antidepressants mirtazapine and mianserin likewise suppressed basal IP formation. Conversely, the tricyclics amitriptyline and clomipramine, the m-chlorophenylpiperazine derivatives trazodone and nefazodone, and the 5-HT reuptake inhibitors fluoxetine and citalopram were inactive alone, although they blocked 5-HT-induced IP production. Inverse agonist actions of 5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2,3-f]indole (SB206,553) and mirtazapine were abolished by the neutral antagonist 6-chloro-5-methyl-1-[6-(2-methylpyridin-3-yloxy)pyridin-3-ylcarbamoyl]indoline (SB242,084), which was inactive alone. As assessed by confocal microscopy and enzyme-linked immunosorbent assay, prolonged treatment of HEK-293 cells with SB206,553, mirtazapine, or mianserin, but not the other antidepressants, enhanced cell surface expression of 5-HT(2C) receptors: 5-HT-induced IP production was also increased, and both these actions were blocked by SB242,084. Cortical neurons were shown by reverse transcription-polymerase chain reaction to predominantly express constitutively active 5-HT(2C) receptor isoforms. Prolonged pretreatment with SB206,553 or mirtazapine triggered an otherwise absent 5-HT-induced elevation in cytosolic Ca(2+) concentrations. SB242,084, which was inactive alone, abolished these effects of SB206,553 and mirtazapine. In conclusion, the tetracyclic antidepressants mirtazapine and mianserin, but not other clinically established antidepressants, suppress constitutive activity at recombinant and native 5-HT(2C) receptors. The clinical significance of inverse agonist versus neutral antagonist properties both during and after drug administration will be of interest to elucidate.

Neuronal activity enhances aryl hydrocarbon receptor-mediated gene expression and dioxin neurotoxicity in cortical neurons

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor activated by dioxin and polyaromatic hydrocarbons. Recent studies have revealed that AhR activity in central neurons depends on the NMDA receptor. In this study, we investigated how the neuronal activity influence AhR-mediated dioxin-responsive gene expression and neurotoxicity. Our results show that activation of AhR by the selective agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin induced dioxin-responsive gene expression and calcium entry, which were attenuated by AhR small interfering RNA, the NMDA receptor channel blocker MK801, and the action potential blocker tetrodotoxin (TTX). In addition, AhR-mediated gene expression was enhanced in neurons during synaptogenesis (10 days in vitro) compared with younger neurons (4 days in vitro), as was sensitivity to TTX and MK801. Furthermore, TTX and MK801 differentially affected the association of AhR and its transcriptional co-activator cAMP-responsive-element binding protein with the cytochrome P450 1A1 (cyp1A1) gene enhancer. Calcium/calmodulin-dependent protein kinase IV, the cAMP-responsive-element binding protein activating enzyme, was also activated by 2,3,7,8-tetrachlorodibenzo-p-dioxin in an activity-dependent manner. Finally, we found that neuronal susceptibility to dioxin insult was also maturation and activity-dependent. Together, the results suggest that neuronal activity may facilitate AhR-mediated calcium signaling, which in turn enhances AhR-mediated gene regulation and mediated maturation-dependent dioxin neurotoxicity.

Nicotine-induced phosphorylation of ERK in mouse primary cortical neurons: evidence for involvement of glutamatergic signaling and CaMKII

Extracellular signal-regulated kinase (ERK) is activated in vivo in a number of brain areas by nicotine and other drugs of abuse. Here we show that nicotine stimulation of cultured mouse cortical neurons leads to a robust induction of ERK phosphorylation that is dependent on nicotine concentration and duration of exposure. Calcium/calmodulin-dependent protein kinase II activity is necessary for nicotine-induced ERK phosphorylation and neither cAMP-dependent protein kinase or protein kinase C appear to be involved. Activity of glutamate receptors, L-type voltage-gated calcium channels, and voltage-gated sodium channels are also required for nicotine-induced ERK phosphorylation. Nicotine-induced ERK phosphorylation was inhibited by high concentrations of mecamylamine, however it was not blocked by other broad nicotinic acetylcholine receptor (nAChR) inhibitors (including hexamethonium and chlorisondamine) or nAChR subtype selective inhibitors (such as methyllycaconitine, alpha-bungarotoxin, dihydro-beta-erythroidine, and alpha-conotoxin Au1B). In accord with these pharmacological results, nicotine-induced ERK phosphorylation was normal in primary cultures made from beta2 or alpha7 nAChR subunit knockout mice. The alpha3/beta4 nAChR agonist cytisine did not induce ERK phosphorylation suggesting that alpha3/beta4 nAChRs were not involved in this process. Taken together, these data define a necessary role for glutamatergic signaling and calcium/calmodulin-dependent protein kinase II in nicotine-induced ERK phosphorylation in cortical neurons and do not provide evidence for the involvement of classical nAChRs.

d-myo-Inositol 1-phosphate as a surrogate of d-myo-inositol 1,4,5-tris phosphate to monitor G protein-coupled receptor activation

Phospholipase C beta (PLC-beta)-coupled G protein-coupled receptor (GPCR) activities traditionally are assessed by measuring Ca2+ triggered by D-myo-inositol 1,4,5-trisphosphate (IP3), a PLC-beta hydrolysis product, or by measuring the production of inositol phosphate using cumbersome radioactive assays. A specific detection of IP3 production was also established using IP3 binding proteins. The short lifetime of IP3 makes this detection very challenging in measuring GPCR responses. Indeed, this IP3 rapidly enters the metabolic inositol phosphate cascade. It has been known for decades that lithium chloride (LiCl) leads to D-myo-inositol 1-phosphate accumulation on GPCR activation by inhibiting inositol monophosphatase, the final enzyme of the IP3 metabolic cascade. We show here that IP1 can be used as a surrogate of IP3 to monitor GPCR activation. We developed a novel homogeneous time-resolved fluorescence (HTRF) assay that correlates perfectly with existing methods and is easily amenable to high-throughput screening. The IP-One assay was validated on various GPCR models. It has the advantage over the traditional Ca2+ assay of allowing the measurement of inverse agonist activity as well as the analysis of PLC-beta activity in any nontransfected primary cultures. Finally, the high assay specificity for D-myo-inositol 1 monophosphate (IP1(1)) opens new possibilities in developing selective assays to study the functional roles of the various isoforms of inositol phosphates.

Opposite effects of PSD-95 and MPP3 PDZ proteins on serotonin 5-hydroxytryptamine2C receptor desensitization and membrane stability

PSD-95/Disc large/Zonula occludens 1 (PDZ) domain-containing proteins (PDZ proteins) play an important role in the targeting and the trafficking of transmembrane proteins. Our previous studies identified a set of PDZ proteins that interact with the C terminus of the serotonin 5-hydroxytryptamine (5-HT)(2C) receptor. Here, we show that the prototypic scaffolding protein postsynaptic density-95 (PSD-95) and another membrane-associated guanylate kinase, MAGUK p55 subfamily member 3 (MPP3), oppositely regulate desensitization of the receptor response in both heterologous cells and mice cortical neurons in primary culture. PSD-95 increased desensitization of the 5-HT(2C) receptor-mediated Ca(2+) response, whereas MPP3 prevented desensitization of the Ca(2+) response. The effects of the PDZ proteins on the desensitization of the Ca(2+) response were correlated with a differential regulation of cell surface expression of the receptor. Additional experiments were performed to assess how PDZ proteins globally modulate desensitization of the 5-HT(2C) receptor response in neurons, by using a peptidyl mimetic of the 5-HT(2C) receptor C terminus fused to the human immunodeficiency virus type-1 Tat protein transduction domain, which disrupts interaction between the 5-HT(2C) receptor and PDZ proteins. Transduction of this peptide inhibitor into cultured cortical neurons increased the desensitization of the 5-HT(2C) receptor-mediated Ca(2+) response. This indicates that, overall, interaction of 5-HT(2C) receptors with PDZ proteins inhibits receptor desensitization in cortical neurons.

Neuronal 5-HT metabotropic receptors: fine-tuning of their structure, signaling, and roles in synaptic modulation

Serotonin (5-hydroxytryptamine, 5-HT) is, without doubt, the neurotransmitter for which the number of receptors is the highest. Fifteen genes encoding functional 5-HT receptors have been cloned in mammalian brain. 5-HT(3) receptors are ionotropic receptors, whereas all the others are metabotropic G-protein-coupled receptors (GPCRs). 5-HT receptor diversity is further increased by post-genomic modifications, such as alternative splicing (up to 10 splice variants for the 5-HT(4) receptor) or by mRNA editing in the case of 5-HT(2C) receptors. The cellular and behavioral implications of 5-HT(2C) receptor editing are of great physiological importance. Signaling of 5-HT receptors involves a great variety of pathways, but only some of these have been demonstrated in neurons. The classical view of neurotransmitter receptors localized within the synaptic cleft cannot be applied to 5-HT receptors, which are mostly (but not exclusively) localized at extra-synaptic locations either pre- or post-synaptically. 5-HT receptors are engaged in pre- or post-synaptic complexes composed of many GPCR-interacting proteins. The functions of these proteins are starting to be revealed. These proteins have been implicated in targeting, trafficking to or from the membrane, desensitization, and fine-tuning of signaling.

Differences in mitochondrial movement and morphology in young and mature primary cortical neurons in culture

Mitochondria have many roles critical to the function of neurons including the generation of ATP and regulation of intracellular Ca2+. Mitochondrial movement is highly dynamic in neurons and is thought to direct mitochondria to specific cellular regions of increased need and to transport damaged or old mitochondria to autophagosomes. Morphology also varies between individual mitochondria and is modulated by fusion and fission proteins such as mitofusin-1 and dynamin-related protein-1, respectively. Although mitochondrial movement and morphology are thought to be modulated to best meet cellular demands, few regulatory signals have been identified. In this study, we examined how the different cellular environments of synaptically immature and mature rat cortical neurons affect mitochondrial movement, morphology, distribution and function. In younger cells, mitochondria were more mobile, were shorter, occupied a smaller percentage of neuronal processes, and expressed greater mitofusin-1 and lower dynamin-related protein-1 protein levels compared with older cells. However, the number of mitochondria per mum of neuronal process, mitochondrial membrane potential and the amount of basally sequestered mitochondrial Ca2+ were similar. Our results suggest that while mitochondria in young neurons are functionally similar to mature neurons, their enhanced motility may permit faster energy dispersal for cellular demands, such as synaptogenesis. As cells mature, mitochondria in the processes may then elongate and reduce their motility for long-term support of synaptic structures.

In vitro assay of primary astrocyte migration as a tool to study Rho GTPase function in cell polarization

Rho GTPases are key players in cell migration. The contribution of Rho, Rac, and Cdc42 to the regulation of the actin and microtubule cytoskeletons is essential for membrane protrusion and cell retraction (Etienne-Manneville and Hall, 2002). The polarization of these protrusive and retracting activities in a migrating cell is also under the control of Rho GTPases, in particular Cdc42 (Nobes and Hall, 1999). In vitro study of cell migration has shown that Cdc42 activity is required for polarized cell migration in several cell types, including fibroblasts, neutrophils, macrophages, and astrocytes (Allen et al., 1998; Etienne-Manneville, 2004; Etienne-Manneville and Hall, 2001; Palazzo et al., 2001; Srinivasan et al., 2003). Using scratch-induced migration assay, we have previously used primary astrocytes as a tool to study the molecular mechanisms controlling cell polarization at the onset of migration (Etienne-Manneville and Hall, 2001, 2003). On scratching of the monolayer, astrocytes polarize perpendicularly to the scratch to migrate and close the wound. Astrocyte polarization is characterized by the formation of a protrusion in the direction of migration, the elongation of the microtubules that fill the protrusion, and the reorientation of the centrosome, which serves as a microtubule-organizing center toward the direction of migration. This in vitro migration assay allows us to simultaneously investigate the mechanisms controlling cell migration, cell protrusion, and cell polarization. Primary astrocytes, although more constraining, provide a more physiological model than immortalized cell lines. Moreover, astrocyte culture can be obtained in a large number and, therefore, also allows biochemical analysis. Here I describe the procedure by which we can obtain and purify primary rat astrocytes and the different assays we have previously used to analyze the role of Rho GTPases and their downstream targets in cell migration and polarization.

New sorting nexin (SNX27) and NHERF specifically interact with the 5-HT4a receptor splice variant: roles in receptor targeting

The 5-hydroxytryptamine type 4 receptor (5-HT4R) is involved in learning, feeding, respiratory control and gastrointestinal transit. This receptor is one of the G-protein-coupled receptors for which alternative mRNA splicing generates the most variants that differ in their C-terminal extremities. Some 5-HT4R variants (a, e and f) express canonical PDZ ligands at their C-termini. Here, we have examined whether some mouse 5-HT4R variants associate with specific sets of proteins, using a proteomic approach based on peptide-affinity chromatography, two-dimensional electrophoresis and mass spectrometry. We have identified ten proteins that interact specifically with the 5-HT4(a)R and three that only associate with the 5-HT4(e)R. Most of them are PDZ proteins. Among the proteins that associated specifically with the 5-HT4(a)R variant, NHERF greatly modified its subcellular localization. Moreover, NHERF recruited the 5-HT4(a)R to microvilli, where it localized with activated ezrin, consistent with the role of 5-HT4(a)R in cytoskeleton remodelling. The 5-HT4(a)R also interacted with both the constitutive and inducible (upon methamphetamine treatment) forms of the recently cloned sorting nexin 27 (SNX27a and b, respectively). We found that SNX27a redirected part of 5-HT4(a)R to early endosomes. The interaction of the 5-HT4R splice variants with distinct sets of PDZ proteins might specify their cellular localization as well as their signal transduction properties.

TREK-1, a K+ channel involved in neuroprotection and general anesthesia

TREK-1 is a two-pore-domain background potassium channel expressed throughout the central nervous system. It is opened by polyunsaturated fatty acids and lysophospholipids. It is inhibited by neurotransmitters that produce an increase in intracellular cAMP and by those that activate the Gq protein pathway. TREK-1 is also activated by volatile anesthetics and has been suggested to be an important target in the action of these drugs. Using mice with a disrupted TREK-1 gene, we now show that TREK-1 has an important role in neuroprotection against epilepsy and brain and spinal chord ischemia. Trek1-/- mice display an increased sensitivity to ischemia and epilepsy. Neuroprotection by polyunsaturated fatty acids, which is impressive in Trek1+/+ mice, disappears in Trek1-/- mice indicating a central role of TREK-1 in this process. Trek1-/- mice are also resistant to anesthesia by volatile anesthetics. TREK-1 emerges as a potential innovative target for developing new therapeutic agents for neurology and anesthesiology.

Multipotentiality of neuronal cells after spontaneous fusion with embryonic stem cells and nuclear reprogramming in vitro

Primary mouse brain cells were cultured with HPRT (hypoxanthine phosphoribosyl transferase)-deficient ES (embryonic stem) cells to see if the ES cells could provide cues sufficient to reprogram a pluripotential state. After 5 days of coculture, HPRT-deficient ES cells were killed by selection in HAT (hypoxanthine, aminopterin, thymidine) medium. We observed islands of HAT-resistant ES-like cells surrounded by differentiated cells. Cell lines generated from three such "islands" proved to be spontaneous, pluripotential ES-neural hybrids, and gave rise to a chimera following blastocyst injection. Re-expression of the ES-specific gene Foxd3 from somatic-derived chromosomes suggested that the somatic nucleus had been reprogrammed. Our results raise the intriguing possibility that ASCs shown to contribute to multiple tissues in blastocyst-injection studies may not contribute as a result of pluripotency. Instead contributions may arise from spontaneous fusion events in which phenotype is determined by either cytoplasmic dominance, nuclear reprogramming, or both.

N-Methyl-D-aspartate receptor activation inhibits protein synthesis in cortical neurons independently of its ionic permeability properties

Transient cerebral ischemia, which is accompanied by a sustained release of glutamate, strongly depresses protein synthesis. We have previously demonstrated in cortical neurons that a glutamate-induced increase in intracellular Ca(2+) is likely responsible for the blockade of the elongation step of protein synthesis. In this study, we provide evidence indicating that NMDA mobilizes a thapsigargin-sensitive pool of intracellular Ca(2+). Exposure of cortical neurons to NMDA, in the absence of external Ca(2+), produced a transient rise in intracellular Ca(2+) that was suppressed by pretreatment with thapsigargin. This rise in intracellular Ca(2+) did not result from an influx of Na(+) via reversal of the mitochondrial Na(+)/Ca(2+) exchanger since it persisted in a Na(+)-free medium or in the presence of CGP 37157, an inhibitor of the exchanger. Moreover, the NMDA-induced increase in intracellular Ca(2+) required the presence of D-serine, was blocked by D(-)-2-amino-5-phosphonopentanoic acid, but was not reduced in the presence of external Mg(2+). This unexpected non-ionotropic effect of NMDA was associated with an inhibition of protein synthesis that was also insensitive to the absence of external Ca(2+) or Na(+), or presence of Mg(2+). NMDA treatment resulted in an increase in the phosphorylation of eEF-2 in the absence or presence of external Ca(2+). The initiation step of protein synthesis was not blocked by NMDA since the phosphorylation of initiation factor eIF-2alpha subunit was not altered by NMDA treatment. In conclusion, we provide evidence indicating that NMDA can inhibit protein synthesis in cortical neurons through a process that involves the mobilization of intracellular Ca(2+) stores via a mechanism that is not linked to the ionic properties of NMDA receptors.

Zinc-induced inhibition of protein synthesis and reduction of connexin-43 expression and intercellular communication in mouse cortical astrocytes

Zinc released from a subpopulation of glutamatergic synapses, mainly localized in the cerebral cortex and the hippocampus, facilitates or reduces glutamatergic transmission by acting on neuronal AMPA and NMDA receptors, respectively. However, neurons are not the only targets of zinc. In the present study, we provide evidence that zinc inhibits protein synthesis in cultured astrocytes from the cerebral cortex of embryonic mice. This inhibition, which reached 85% in the presence of 100 micro m zinc, was partially and slowly reversible and resulted from the successive inhibition of the elongation and the initiation steps of the protein translation process. This was assessed by measuring the phosphorylation level of the elongation factor eEF-2 and of the alpha subunit of the initiation factor eIF-2. Due to the rapid turnover of connexin-43 that forms junction channels in cultured astrocytes, the zinc-induced decrease of protein synthesis led to a partial disappearance of connexin-43, which was associated with an inhibition of the cellular coupling in the astrocytic syncitium. In conclusion, zinc not only inhibits protein synthesis in neurons, as previously demonstrated, but also in astrocytes. The resulting decrease in the intercellular communication between astrocytes should alter the function of surrounding neurons as well as their survival.

GABA is toxic for mouse striatal neurones through a transporter-mediated process

In the present study, GABA was shown to induce a necrotic neuronal death in cultured striatal neurones from mouse embryos. This effect did not depend on the activation of GABA(A), GABA(B) or GABA(C) receptors as it was neither antagonized by bicuculline, saclofen or picrotoxin, respectively, nor reproduced by the GABA receptor agonists, muscimol and baclofen. Excluding the participation of glutamate, GABA neurotoxicity persisted in the presence of either the antagonists of ionotropic and metabotropic glutamate receptors or glutamate pyruvate transaminase, which induces an immediate catabolism of glutamate. A GABA transport-associated process is involved in GABA neurotoxicity as nipecotic acid and NO 711, two inhibitors of the high-affinity neuronal GABA transporters (GAT-1, in particular), completely prevented the neurotoxic effect of GABA. The activation of a subset of G proteins is also implicated in the GABA transport-mediated neuronal death as GABA neurotoxicity was completely suppressed when striatal neurones were pre-treated with pertussis toxin. Further demonstrating the specificity of this neurotoxic process, GABA-induced neurotoxicity was not observed in cortical neurones which, in contrast to striatal neurones, are largely represented by glutamatergic neurones. In conclusion, our study suggests that glutamate is not the sole neurotransmitter that can be responsible for brain damage and that GABA neurotoxicity involves both GABA transport and G protein transduction pathways.

Effect of Glutamate and Ionomycin on the Release of Arachidonic Acid, Prostaglandins and HETEs from Cultured Neurons and Astrocytes

The release of arachidonic acid (ArA) metabolites from mouse neurons and astrocytes in primary culture has been studied in response to ionomycin or glutamate stimulation. Cells were preincubated with [3H]ArA for 24 h and the radioactivity released was examined by HPLC. In striatal, cortical and hippocampal neurons, glutamate and ionomycin strongly stimulated the release of ArA, but neither prostaglandins (PGs) nor hydroxyeicosatetraenoic acids (HETEs) could be detected. If they were released, these latter compounds represented < 0.02% of the amount of ArA. In contrast, in astrocyte cultures, ionomycin (but not glutamate) strongly stimulated the release of PGs and HETEs as well as ArA. Reversed- and straight-phase HPLC analysis revealed the presence of PGD2, PGE2, PGF2alpha, 12-hydroxyheptadeca-5,8,10-trienoic acid (HHT) and HETEs (15-HETE, 11-HETE and 5-HETE). Indomethacin inhibited the release of PGs and HHT, but also that of 11- and 15-HETE, indicating that these two HETEs may be produced through the cyclooxygenase pathway. Metabolism of [3H]ArA was also examined in cellular homogenates. Although > 50% of the [3H]ArA was metabolized to PGF2alpha, PGE2, PGD2, HHT, 15- and 11-HETE in cultured astrocyte homogenates, no [3H]ArA metabolism could be detected in cultured striatal neuron homogenates. Moreover, neuronal homogenates did not inhibit the metabolism of [3H]ArA observed in either astrocyte or platelet homogenates. These results indicate that central neurons in primary culture possess very low lipoxygenase and cyclooxygenase activities. They emphasize the need to identify the cellular source of ArA metabolites in the brain, particularly when considering the multiple new messenger roles proposed for these molecules, such as that of retrograde messengers involved in synaptic plasticity phenomena.

A Nitric Oxide Synthase Activity Selectively Stimulated by NMDA Receptors Depends on Protein Kinase C Activation in Mouse Striatal Neurons

In mouse striatal neurons in primary culture, the maximal increase in intracellular cyclic guanosine monophosphate level evoked by N-methyl-d-aspartic acid (NMDA) receptor activation was twice that induced by kainate, KCl and ionomycin. Quisqualate was almost inactive. All responses were mediated by nitric oxide (NO) production since they were blocked by haemoglobin (a NO scavenger) and by l-NG-monomethylarginine and l-NG-nitroarginine, the effects of both arginine analogues being reversed by an excess of l-arginine. Several results indicate that NMDA receptors stimulate a specific NO synthase activity. This specifically NMDA-activated NO synthase was blocked by nanomolar concentrations of l-NG-nitroarginine, whereas the responses evoked by other agents, including kainate, KCl and ionomycin, were only blocked by micromolar concentrations of this NO synthase inhibitor. The NMDA response could not be totally reproduced by an increase in cytosolic calcium (Ca2+) alone. In contrast, in the presence of staurosporine, an inhibitor of protein kinases C (PKC), as well as after desensitization of PKC induced by long-term treatment with the phorbol ester, phorbol-12, 13-dibutyrate, NMDA-stimulated NO production was selectively reduced, reaching the level evoked by kainate or Ca2+ increase. In conclusion, our results suggest that in striatal neurons, NMDA selectively stimulates a NO synthase activity which is inhibited by low concentrations of l-NG-nitro-arginine, through a mechanism involving PKC.

Dendritic and axonal targeting of type 5 metabotropic glutamate receptor is regulated by homer1 proteins and neuronal excitation

The physiological actions of neurotransmitter receptors are intimately linked to their proper neuronal compartment localization. Here we studied the effect of the metabotropic glutamate receptor (mGluR)-interacting proteins, Homer1a, b, and c, in the targeting of mGluR5 in neurons. We found that mGluR5 was exclusively localized in cell bodies when transfected alone in cultured cerebellar granule cells. In contrast, mGluR5 was found also in dendrites when coexpressed with Homer1b or Homer1c, and in both dendrites and axons when cotransfected with Homer1a. In dendrites, cotransfected mGluR5 and Homer1b/c formed clusters that colocalized with the synaptic marker synaptophysin. Interestingly when transfected alone, the Homer proteins were also translocated to neurites but did not form such clusters. Depolarization of the neurons with a mixture of ionotropic glutamate receptor agonists, NMDA and kainate, or potassium channel blockers, tetraethylammonium and 4-aminopyridine, induced transient expression of endogenous Homer1a and persistent neuritic localization of transfected mGluR5 even long after degradation of Homer1a. These results suggest that Homer1a/b/c proteins are involved in the targeting of mGluR5 to dendritic synaptic sites and/or axons and that this effect can be regulated by neuronal activity. Because the activity-dependent effect of endogenous Homer1a was also long-lasting, the axonal targeting of mGluR5 by this protein is likely to play an important role in synaptic plasticity.

Inhibition of protein synthesis in cortical neurons during exposure to hydrogen peroxide

Transient cerebral ischemia, which is accompanied by a sustained release of glutamate and zinc, as well as H(2)O(2) formation during the reperfusion period, strongly depresses protein synthesis. We have previously demonstrated that the glutamate-induced increase in cytosolic Ca(2+) is likely responsible for blockade of the elongation step of protein synthesis, whereas Zn(2+) preferentially inhibits the initiation step. In this study, we provide evidence indicating that H(2)O(2) and thapsigargin mobilized a common intracellular Ca(2+) pool. H(2)O(2) treatment stimulated a slow increase in intracellular Ca(2+), and precluded the effect of thapsigargin on Ca(2+) mobilization. H(2)O(2) stimulated the phosphorylation of both eIF-2alpha and eEF-2, in a time- and dose-dependent manner, suggesting that both the blockade of the elongation and of the initiation step are responsible for the H(2)O(2)-induced inhibition of protein synthesis. However, kinetic data indicated that, at least during the first 15 min of H(2)O(2) treatment, the inhibition of protein synthesis resulted mainly from the phosphorylation of eEF-2. In conclusion, H(2)O(2) inhibits protein translation in cortical neurons by a process that involves the phosphorylation of both eIF-2alpha and eEF-2 and the relative contribution of these two events depends on the duration of H(2)O(2) treatment.

Dendritic and axonal targeting of type 5 metabotropic glutamate receptor is regulated by homer1 proteins and neuronal excitation

The physiological actions of neurotransmitter receptors are intimately linked to their proper neuronal compartment localization. Here we studied the effect of the metabotropic glutamate receptor (mGluR)-interacting proteins, Homer1a, b, and c, in the targeting of mGluR5 in neurons. We found that mGluR5 was exclusively localized in cell bodies when transfected alone in cultured cerebellar granule cells. In contrast, mGluR5 was found also in dendrites when coexpressed with Homer1b or Homer1c, and in both dendrites and axons when cotransfected with Homer1a. In dendrites, cotransfected mGluR5 and Homer1b/c formed clusters that colocalized with the synaptic marker synaptophysin. Interestingly when transfected alone, the Homer proteins were also translocated to neurites but did not form such clusters. Depolarization of the neurons with a mixture of ionotropic glutamate receptor agonists, NMDA and kainate, or potassium channel blockers, tetraethylammonium and 4-aminopyridine, induced transient expression of endogenous Homer1a and persistent neuritic localization of transfected mGluR5 even long after degradation of Homer1a. These results suggest that Homer1a/b/c proteins are involved in the targeting of mGluR5 to dendritic synaptic sites and/or axons and that this effect can be regulated by neuronal activity. Because the activity-dependent effect of endogenous Homer1a was also long-lasting, the axonal targeting of mGluR5 by this protein is likely to play an important role in synaptic plasticity.

Agonist stimulation provokes dendritic and axonal dopamine D(1) receptor redistribution in primary cultures of striatal neurons

To investigate the influence of neurotransmitter on G-protein-coupled receptor trafficking and compartimentalization in neurons, we have developed a model of primary neuronal cultures from fetal rat striatum on which we have studied the cellular and subcellular distribution and trafficking of the D(1) dopaminergic receptor. This receptor is known to be somatodendritic and axonal targeted in vivo, mostly to extrasynaptic locations. Immunohistochemical studies at the light and electron microscopic levels showed that, in cultures, the D(1) dopaminergic receptor is expressed in the absence of dopamine stimulation. The pattern of D(1) dopaminergic receptor immunostaining after stimulation by the D(1) dopaminergic receptor agonist SKF 82958 (1 microM) is dramatically modified with a decrease of the number of labeled D(1) dopaminergic receptor puncta (-40%) and an increase of their size in both dendrites (+120%) and axons (+240%). Seven hours after removal of the agonist, return to normal pattern was observed. The D(1) dopaminergic receptor antagonist SCH 23390 (2 microM) abolishes the effect of SKF 82958. Electron microscopy demonstrated, in dendrites, a translocation of the labeling from the plasma membrane to endosomes. Axonal D(1) dopaminergic receptor redistribution after acute stimulation indicates that the D(1) dopaminergic receptor is membrane targeted and responsive to stimulation. These results validate primary culture of striatal neurons to study subcellular localization and intraneuronal trafficking of G-protein-coupled receptors. This preparation will be useful to address various questions concerning the behavior and the trafficking of these receptors in neurons in relation to the neurotransmitter environment.

Imidazoline-induced neuroprotective effects result from blockade of NMDA receptor channels in neuronal cultures

Imidazolines have been shown to be neuroprotective in focal and global ischemia in the rat. However, their mechanism of action is still unclear. We have studied the neuroprotective effects of imidazolines against NMDA-induced neuronal death and hypoxic insult in cerebellar and striatal neuronal cultures. All of the imidazolines tested decreased the NMDA-mediated neurotoxicity in a non-competitive manner. Antazoline was the most effective (IC(50) of 5 microM, maximal neuroprotection reaching 90% at 100 microM). The neuroprotective effects were still present when the imidazolines were applied during the post-insult period. Antazoline, idazoxan and guanabenz also showed neuroprotective effects against hypoxia-induced neuronal death (neuroprotection reaching 95% for antazoline at 100 microM). Antazoline was still active if applied during the reoxygenation period (15% neuroprotection). To determine the mechanism of the neuroprotective effects, the possible interaction of imidazolines with NMDA receptors was studied. Imidazolines dose-dependently and non-competitively inhibited NMDA currents. As found for the neuroprotective effects, antazoline was the most effective imidazoline, with an IC(50) of 4 microM and a maximal inhibition of 90% at 100 microM. This blockade was rapid, reversible and voltage-dependent. We compared these effects to those of the classical non-competitive antagonist of NMDA channels, MK-801. In contrast to imidazolines, blockade of the NMDA current by MK-801 was voltage-independent and reversible only at positive potentials. When co-applied with MK-801, antazoline prevented the long lasting blockade of the NMDA current by MK-801. These results are consistent with the existence of overlapping binding sites for these drugs on the NMDA receptor channel. They indicate that imidazolines exert a strong neuroprotective effect against excitotoxicity and hypoxia in cerebellar and striatal primary neuronal cultures by inhibiting NMDA receptors. Since these effects were non-competitive, imidazolines appear to be interesting new drugs with therapeutic potential.

Influence of the target on distribution and functioning of the varicosities of Helix pomatia metacerebral cell C1 in dissociated cell culture

The serotonergic metacerebral giant cell (C1) of Helix pomatia was isolated with its bifurcate axon and plated in culture under five conditions: (i) with no target; (ii) with the appropriate target B2 near the stump of the bigger branch (CBC); (iii) with B2 near the stump of the smaller branch (CC); (iv) with a wrong target (C3) near the stump of the CBC branch and (v) with B2 and C3 positioned near the CBC and CC stump, respectively. The counting of anti-serotonin antibody-labelled varicosities of the C1 neuron showed that the presence of the appropriate target in either axonal domain both down-regulated the number of varicosities of the contralateral neuritic field, and increased their average size, whereas the wrong target induced an overall reduction of the number of C1 neuron varicosities, and inhibited the evoked transmitter release. The action potential-evoked calcium concentration increase in the neuritic terminals of the C1 neuron cultured alone, or in presence of the appropriate target, reached a value significantly higher than that reached in presence of the wrong target. These results provide evidence that the postsynaptic neuron regulates both morphological and functional development of presynaptic terminals.

Routes of zinc entry in mouse cortical neurons: role in zinc-induced neurotoxicity

Exposure of central neurons to Zn2+ triggers neuronal death. The routes of Zn2+ entry were investigated in living cortical neurons from the mouse using the specific Zn2+ fluorescent dye N-(6-methoxy-8-quinolyl)-p-toluene sulphonamide (TSQ), which preferentially detects membrane-bound Zn2+. Exposure of cortical neurons to increasing concentrations of Zn2+ (1-100 microM) induced a progressive increase in the fluorescence of TSQ. This fluorescence signal was not attenuated by the permeation of plasma membrane with digitonin. Accordingly, the major part of TSQ fluorescence (two-thirds) was associated to the particulate fraction of cortical neurons exposed to Zn2+. These results suggest that Zn2+ detected with TSQ in neurons is mainly bound to membranes. TSQ fluorescence measured in neurons exposed to 3 microM Zn2+ was enhanced by Na+-pyrithione, a Zn2+ ionophore, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), N-methyl-D-aspartate (NMDA) or KCl-induced depolarization. However, in the absence of any treatment, TSQ labelling of neurons exposed to 3 microM Zn2+ was only decreased by NMDA receptor antagonists, whereas it remained unaltered in the presence of antagonists of AMPA receptors or L-type voltage-gated Ca2+ channels. Zn2+ entry through NMDA receptors did not contribute to Zn2+-induced neuronal death, as it was prevented by antagonists of NMDA receptors only when they were added after the Zn2+ exposure. Finally, Zn2+ induced a delayed accumulation of extracellular glutamate which might be responsible for the delayed NMDA receptor activation that leads to neuronal death.

Brain-derived neurotrophic factor-mediated protection of striatal neurons in an excitotoxic rat model of Huntington's disease, as demonstrated by adenoviral gene transfer

Huntington's disease (HD) is a genetic disorder leading to the degeneration of striatal GABA-ergic output neurons. No treatment is currently available for this devastating disorder, although several neurotrophic factors, including brain-derived neurotrophic factor (BDNF), have been shown to be beneficial for striatal neuron survival. We analyzed the effect of adenovirus-mediated transfer of the BDNF gene in a model of HD. Using a stereological procedure, three groups of rats were given an intrastriatal injection of adenovirus encoding BDNF, beta-galactosidase, or sham surgery. Two weeks after treatment, the animals were lesioned with quinolinic acid (QUIN), a toxin that induces striatal neuron death by an excitotoxic process. One month after the lesion, histological study revealed that striatal neurons were protected only in rats treated with the BDNF adenovirus. Volume measurements showed that the QUIN-induced lesions were 55% smaller in the BDNF adenovirus-treated group than in the beta-galactosidase adenovirus-treated group (p < 0.05), and the sham-treated group (p < 0.05). To determine the survival of striatal GABA-ergic output neurons after the QUIN-induced lesion, we immunostained brain sections with DARPP-32, an antibody specific for striatal output neurons. Prior treatment with the BDNF adenovirus resulted in a cell survival of 64%, whereas that after beta-galactosidase treatment was 46% (p < 0.05), showing that the BDNF adenovirus protected the striatal neurons. These results indicate that transfer of the BDNF gene is of therapeutic value for Huntington's disease.

Zinc inhibits protein synthesis in neurons. Potential role of phosphorylation of translation initiation factor-2alpha

In the central nervous system, Zn(2+) is concentrated in the cerebral cortex and hippocampus and has been found to be toxic to neurons. In this study, we show that exposure of cultured cortical neurons from mouse to increasing concentrations of Zn(2+) (10-300 microM) induces a progressive decrease in global protein synthesis. The potency of Zn(2+) was increased by about 2 orders of magnitude in the presence of Na(+)-pyrithione, a Zn(2+) ionophore. The basal rate of protein synthesis was restored 3 h after Zn(2+) removal. Zn(2+) induced a sustained increase in phosphorylation of the alpha subunit of the translation eukaryotic initiation factor-2 (eIF-2alpha), whereas it triggered a transient increase in phosphorylation of eukaryotic elongation factor-2 (eEF-2). Protein synthesis was still depressed 60 min after the onset of Zn(2+) exposure while the state of eEF-2 phosphorylation had already returned to its basal level. Moreover, Zn(2+) was less effective than glutamate to increase eEF-2 phosphorylation, whereas it induced a more profound inhibition of protein synthesis. These results suggest that Zn(2+)-induced inhibition of protein synthesis mainly correlates with the increase in eIF-2alpha phosphorylation. Supporting further that Zn(2+) acts at the initiation step of protein synthesis, it strongly decreased the amount of polyribosomes.

mGluR7-like receptor and GABA(B) receptor activation enhance neurotoxic effects of N-methyl-D-aspartate in cultured mouse striatal GABAergic neurones

Presynaptic metabotropic glutamate receptors (mGluRs) of group III constitute possible targets for putative neuroprotective drugs acting against glutamate excitotoxic insults. Indeed, in glutamatergic cerebellar granule neurones in culture, high concentrations of L-2-amino-4-phosphonobutyrate (L-AP4, above 0.3 mM, thus activating mGluR7) inhibit NMDA-induced cell death. In contrast, in striatal cultures which are enriched in GABAergic neurones, we show that high concentrations of L-AP4 increased neuronal death in control as well as in NMDA-stimulated cultures. Moreover, similar results were obtained with the GABA(B)R agonist. baclofen. Both the neuroprotective effects in cerebellar granule cells and the neurotoxic effects in striatal neurones were mediated via Gi-Go-coupled mGluRs, suggesting that these effects were probably mediated by mGluR7a or b and GABA(B)R expressed in these neurones. In striatal neurones, we found that L-AP4 and baclofen inhibited both basal and NMDA-stimulated GABA release. These inhibitions of GABA release may be responsible for the increase in basal and NMDA-stimulated neuronal death. Indeed, blockade of GABA(A) receptors with bicuculline increased neuronal death of control and NMDA-treated striatal cultures. Taken together, these results suggest that L-AP4 and baclofen, via mGluR7 and GABA(B)R, reduced the neuroprotective effect of GABA present in striatal cultures acting via GABA(A) receptors. Although caution must be taken when extrapolating from in vitro to in vivo situations, the present experiments and the recent observations that mGluR7 and GABA(B)R are expressed in heterologous synapses, should be taken into consideration when evaluating the neuroprotective action of future mGluR7 specific agonists or GABA(B)R specific antagonists.

Pyruvate and lactate protect striatal neurons against N-methyl-D-aspartate-induced neurotoxicity

A sustained release of glutamate contributes to neuronal loss during cerebral ischaemia. Using cultured mouse striatal neurons, we observed that glucose deprivation, which occurs in this pathological process, enhanced the N-Methyl-D-aspartate (NMDA)- or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-induced neurotoxicity. The end products of glycolysis, lactate and pyruvate, strongly protected neurons from these neurotoxic effects. The neuroprotective effect of pyruvate (which is more prominent in the absence of glucose) was not related to its ability to react with H2O2 by a decarboxylation process. Pyruvate and L-lactate strongly counteracted the deep decrease in the neuronal ATP content induced by NMDA, indicating that they might protect striatal neurons by rescuing cellular energy charge. Addition of MK-801 after the NMDA withdrawal completely protected neurons, suggesting that NMDA neurotoxicity resulted from a delayed NMDA receptor activation probably linked to a delayed release of an endogenous agonist in the extracellular medium. The strong accumulation of extracellular glutamate which was found in both sham and NMDA-treated cultures was markedly decreased by pyruvate. Thus, pyruvate might also exert its protecting activity by decreasing the delayed accumulation of glutamate which seemed to be neurotoxic only after a preexposure of neurons to NMDA.

Increase in external glutamate and NMDA receptor activation contribute to H2O2-induced neuronal apoptosis

The present study aims to investigate the role of extracellular glutamate and NMDA receptor stimulation in the neuronal death induced by a transient exposure to H2O2 of cultured neurons originating from mouse cerebral cortex. Most of the neuronal loss following a transient exposure to H2O2 of cortical neurons results from an apoptotic process involving a secondary stimulation of NMDA receptors, which occurs after H2O2 washout. Indeed, (a) the neurotoxic effect of H2O2 was strongly reduced by antagonists of NMDA receptors, (b) the neurotoxic effect of H2O2 was enhanced in the absence of Mg2+, (c) the protective effect of MK-801 progressively decayed when it was applied with increasing delay time after H2O2 exposure, and (d), finally, the extracellular concentration of glutamate was increased after H2O2 exposure. The major part of H2O2-induced neurotoxicity is mediated by the formation of hydroxyl radicals, which might be involved in (a) the delayed accumulation of extracellular glutamate and NMDA receptor activation and (b) the poly(ADP-ribose) polymerase activation and the related NAD content decrease. The combination of these two mechanisms could lead to both an increase in ATP consumption and a decrease of ATP synthesis. The resulting large decrease in ATP content might be finally responsible for the neuronal death.

NMDA and non-NMDA receptor-mediated excitotoxicity are potentiated in cultured striatal neurons by prior chronic depolarization

The excitatory input from cortex and/or thalamus to striatum appears to promote the maturation of glutamate receptors on striatal neurons, but the mechanisms by which it does so have been uncertain. To explore the possibility that the excitatory input to striatum might influence glutamate receptor maturation on striatal neurons, at least in part, by its depolarizing effect on striatal neurons, we examined the influence of chronic KCl depolarization on the development of glutamate receptor-mediated excitotoxic vulnerability and glutamate receptors in cultured striatal neurons. Dissociated striatal neurons from E17 rat embryos were cultured for 2 weeks in Barrett's medium containing either low (3 mM) or high (25 mM) KCl. The vulnerability of these neurons to NMDA receptor agonists (NMDA and quinolinic acid), non-NMDA receptor agonists (AMPA and KA), and a metabotropic glutamate receptor agonist (trans-ACPD) was examined by monitoring cell loss 24 h after a 1-h agonist exposure. We found that high-KCl rearing potentiated the cell loss observed with 500 microM NMDA or 250 microM KA and yielded cell loss with 250 microM AMPA that was not evident under low KCl rearing. In contrast, neither QA up to 5 mM nor trans-ACPD had a significant toxic effect in either KCl group. ELISA revealed that chronic high KCl doubled the abundance of NMDA NR2A/B, AMPA GluR2/3, and KA GluR5-7 receptor subunits on cultured striatal neurons and more than doubled AMPA GluR1 and GluR4 subunits, but had no effect on NMDA NR1 subunit levels. These receptor changes may contribute to the potentiation of NMDA and non-NMDA receptor-mediated excitotoxicity shown by these neurons following chronic high-KCl rearing. Our studies suggest that membrane depolarization produced by corticostriatal and/or thalamostriatal innervation may be required for maturation of glutamate receptors on striatal neurons, and such maturation may be important for expression of NMDA and non-NMDA receptor-mediated excitotoxicity by striatal neurons. Striatal cultures raised under chronically depolarized conditions may, thus, provide a more appropriate culture model to study the role of NMDA or non-NMDA receptor subtypes in excitotoxicity in striatum.

Characterization of rat spinal cord neurons cultured in defined media on microelectrode arrays

Previous efforts to utilize mammalian spinal cord neurons as biosensor elements have relied on neuronal: glial co-cultures maintained in serum-containing media. We have examined the feasibility of culturing primary spinal cord neurons in serum-free medium, modified for neuronal longevity, on fabricated microelectrode arrays. Embryonic day 15 rat spinal cord cells were plated on trimethoxysilyl-propyldiethylenetriamine coated microelectrode arrays comprised of gold recording sites passivated with silicon nitride. Immunocytochemistry was performed to verify the presence of neurons and quantitatively assess astrocytes using antibodies against glial fibrillary acidic protein on the silicon nitride substrates. Modifications to culture media enabled viable neuronal culture to extend from approximately 14 days in vitro (DIV) to 40 DIV on the arrays containing only 1.1 +/- 0.5% (mean +/- SEM) astrocytes. Extracellular recording revealed tetrodotoxin-sensitive spontaneous electrical activity from the enriched neuronal culture. Threshold detection of extracellular potentials showed an increase in spike rate as a function of glutamate concentration with neurotoxicity at elevated levels. This approach suggests that functional measures related to biosensor applications, pharmacological screening, or the evaluation of neurological disease models can be implemented in a defined culture system.

Transgenic murine cortical neurons expressing human Bcl-2 exhibit increased resistance to amyloid beta-peptide neurotoxicity

The generation of reactive oxygen species has been implicated in the neurotoxicity of amyloid beta-peptide, the main constituent of the senile plaques that accumulates in the brain of Alzheimer's disease victims. In this study, we have compared the toxicity of amyloid beta-peptide on cultured cortical neurons from control mice and transgenic mice expressing either human copper-zinc superoxide dismutase or human Bcl-2, two proteins that protect cells against oxidative damage. Copper-zinc superoxide dismutase overexpression failed to protect cortical neurons against the toxicity of amyloid beta-peptide(25-35) [the minimal cytotoxic fragment of amyloid beta-peptide(1-42)] as assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction and an enzyme-linked immunoabsorbent assay using an antibody directed against microtubule-associated protein-2 (a specific neuronal protein), ruling out a role for superoxide anion and peroxynitrite in amyloid beta-peptide-evoked neurotoxicity. On the contrary, cortical neurons expressing human copper-zinc superoxide dismutase exhibited increased apoptotic nuclei in both untreated and amyloid beta-peptide(25-35)-exposed neurons. Transgenic neurons expressing human Bcl-2 were partially protected against amyloid beta-peptide-induced neuronal death. This neuroprotection appears to be related to the complete inhibition of apoptosis induced by both amyloid beta-peptide(25-35) and amyloid beta-peptide(1-42). This study may be relevant for developing neuroprotective gene therapy to inhibit neuronal apoptosis in Alzheimer's disease.

A simple method to transfer plasmid DNA into neuronal primary cultures: functional expression of the mGlu5 receptor in cerebellar granule cells

We describe a method to transfer cDNA into neuronal primary cultures with a commercialised cationic lipid, Transfast. Cultures were transfected at a rate of about 5% with green fluorescent protein (GFP) cDNA. Comparing Transfast to other transfection reagents, we found this compound to be the most efficient. GFP-transfected mouse cerebellar granule cells displayed normal whole-cell voltage-sensitive and unitary big K+ channel currents. We also used this transfection method with success to transfer GFP cDNA into primary cultures of striatum and colliculus. Transfast was then used to cotransfect cultured cerebellar cells with GFP cDNA, in conjunction with cDNA coding for the metabotropic glutamate receptor type 5 (mGlu5 receptor). Ninety percent of the cells expressing GFP also expressed mGlu5 receptor. Though neurones were best transfected one day after plating, they still expressed both GFP and mGlu5 receptor proteins 2 weeks after plating, i.e. after full differentiation. A functional test of the expressed mGlu5 receptor was thus performed in GFP-transfected neurones. Stimulation of mGlu5 receptor induced single big K+ channel activity, as it was the case for the native mGlu1 receptor. This indicated that the transfected mGlu5 receptor plasmid was functionally expressed and that both mGlu1 and mGlu5 receptors may share common coupling mechanisms to big K+ channels in neurones.

Members of the JAK/STAT proteins are expressed and regulated during development in the mammalian forebrain

The presence and activation of members of the Janus Kinases/Signal Transducers and Activator of Transcription proteins in response to specific cytokines is currently the focus of intense investigation in the hematopoietic system. Although some evidence suggests that cytokines might play an important role in brain development and brain pathologies, very limited information is available on the presence of the JAK/STAT proteins in the Central Nervous System. Here we provide Western blot and immunohistochemistry data on the presence of Jak2 in vivo in the immature brain, its expression being greater in early stages of the embryonic life and gradually diminishing towards adulthood. Conversely, Jak1 was found expressed at a lower level compared to Jak2 and not modulated during brain maturation. Western blot data also show that specific members of the STAT family, the cytoplasmic substrates of the Janus Kinases, are present in vivo and that the extent of their expression is modulated differently at various stages. In particular, Stat6 protein levels were markedly attenuated at advanced stages of differentiation, as well as in the adult brain, with respect to early embryonic life. On the contrary, Stat3 levels did not vary. Analysis of Statl and Stat5 proteins showed a more complex expression pattern. These data indicate that members of the JAK/STAT proteins are present and modulated in vivo in the embryonic and postnatal brain, therefore supporting their role in the modulation of gene expression during the different stages of brain maturation.

Neurotransmitter regulation of MAP kinase signaling in striatal neurons in primary culture

Glutamate and dopamine are important neurotransmitters in the basal ganglia. Dopamine can act via D1 receptors to activate adenylyl cyclase in striatal neurons, while glutamate stimulation of NMDA receptors leads to an increase in intracellular calcium. Increases in intracellular calcium or cAMP can induce immediate early gene expression in striatal neurons. In the present study, NMDA receptor stimulation or adenylyl cyclase activation resulted in the activation of MAP kinase in striatal neurons in primary culture. The effect of cAMP appeared to involve cAMP-dependent protein kinase, in addition to a tyrosine kinase and MEK. NMDA-induced MAP kinase activation was also dependent on a tyrosine kinase and MEK. The EGF receptor, which has been implicated in calcium- and G protein-induced MAP kinase activation, did not mediate the effects of NMDA or forskolin on MAP kinase. Furthermore, the src kinase inhibitor, herbimycin A, and the phosphoinositol-3-kinase inhibitor, wortmannin, did not prevent MAP kinase activation by these stimuli. However, the ability of both NMDA and forskolin to activate MAP kinase in striatal neurons was blocked by SB 203580, an inhibitor of p38 reactivating kinase. These results indicate that both NMDA receptor activation and elevations in cAMP can result in MEK-induced MAP kinase activation in striatal neurons. However, the signal transduction pathways mediating these responses appear to be distinct from those known to mediate MAP kinase activation by other stimuli.

Hypoxia/reoxygenation induces apoptosis through biphasic induction of protein synthesis in cultured rat brain neurons

To investigate biochemical events accounting for the outcome of central neurons following hypoxia/reoxygenation, cultured neurons from fetal rat forebrain were exposed to hypoxia (95% N2/5% CO2) for 6 h, and then reoxygenated for up to 96 h. Time-dependent changes in macromolecular biosynthesis were analysed by incorporation of [3H]uridine and [3H]leucine and were coupled to cell viability and lactate dehydrogenase leakage. Morphological features of necrosis and apoptosis were scored following nuclear incorporation of the fluorescent dye 4,6-diamidino-2-phenylindole. Hypoxia led to a 36% reduction of cell viability at the end of the reoxygenation period, while 23% of the neurons exhibited apoptosis. A biphasic increase in the rates of protein synthesis was measured 1 h after the onset of hypoxia (77% above controls) and by 48-h postreoxygenation (72%). The presence of cycloheximide during hypoxia inhibited both peaks of synthesis and prevented the development of apoptosis. Protein electrophoresis outlined specific alterations in constitutive proteins, and immunohistochemistry revealed an overexpression of the pro-apoptotic gene products Bax and ICE. Therefore, hypoxia followed by reoxygenation would trigger sequential changes in synthesis of specific proteins, leading to delayed and mainly apoptotic neuronal death.