Different localizations and functions of L-type and N-type calcium channels during development of hippocampal neurons

Extracellular transglutaminase-2, nude or associated with astrocytic extracellular vesicles, modulates neuronal calcium homeostasis

We have uncovered a novel role for astrocytes-derived extracellular vesicles (EVs) in controlling intraneuronal Ca²⁺ concentration ([Ca²⁺]_i) and identified transglutaminase-2 (TG2) as a surface-cargo of astrocytes-derived EVs. Incubation of hippocampal neurons with primed astrocyte-derived EVs have led to an increase in [Ca²⁺]_i, unlike EVs from TG2-knockout astrocytes. Exposure of neurons or brain slices to extracellular TG2 promoted a [Ca²⁺]_i rise, which was reversible upon TG2 removal and was dependent on Ca²⁺ influx through the plasma membrane. Patch-clamp and calcium imaging recordings revealed TG2-dependent neuronal membrane depolarization and activation of inward currents, due to the Na⁺/Ca²⁺-exchanger (NCX) operating in the reverse mode and indirect activation of L-type VOCCs, as indicated by VOCCs/NCX pharmacological inhibitors. A subunit of Na⁺/K⁺-ATPase was selected by comparative proteomics and identified as being functionally inhibited by extracellular TG2, implicating Na⁺/K⁺-ATPase inhibition in NCX reverse mode-switching leading to Ca²⁺ influx and higher basal [Ca²⁺]_i. These data suggest that reactive astrocytes control intraneuronal [Ca²⁺]_i through release of EVs with TG2 as responsible cargo, which could have a significant impact on synaptic activity in brain inflammation.

Spontaneous Ca2+ events are linked to the development of neuronal firing during maturation in mice primary hippocampal culture cells

Calcium is one of the most vital intracellular secondary messengers that tightly regulates a variety of cell physiology processes, especially in the brain. Using a fluorescent Ca²⁺-sensitive Oregon Green probe, we revealed three different amplitude distributions of spontaneous Ca²⁺ events (SCEs) in neurons between 15 and 26 days in vitro (DIV) culture maturation. We detected a series of amplitude events: micro amplitude SCE (microSCE) 25% increase from the baseline, intermediate amplitude SCE (interSCE) as 25-75%, and macro amplitude SCE (macroSCE) - over 75%. The SCEs were fully dependent on extracellular Ca²⁺ and neuronal network activity and vanished in the Ca²⁺-free solution, 10 mM Mg²⁺-block, or in the presence of voltage-gated Na⁺-channel blocker, tetrodotoxin. Combined patch-clamp and Ca²⁺-imaging techniques revealed that microSCE match single action potential (AP), interSCE - burst of 3-12 APs, and macroSCE - 'superburst' of 10+ APs. MicroSCEs were blocked by a common α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainic acid (KA) receptor antagonist, CNQX. The γ-aminobutyric acid (GABA) A-type receptor (GABAAR) picrotoxin blockade and L-type voltage-dependent Ca²⁺-channel inhibitor diltiazem significantly reduced microSCE frequency. InterSCEs were inhibited by CNQX, but picrotoxin treatment significantly increased its amplitude. The N-methyl-d-aspartate (NMDA) receptor antagonist, D-APV, voltage-gated K⁺-channel blocker, tetraethylammonium, noticeably suppressed interSCE amplitude. We also demonstrate that macroSCEs were AMPA/KA receptor-independent.

Design of an Imaging Probe to Monitor Real-Time Redistribution of L-type Voltage-Gated Calcium Channels in Astrocytic Glutamate Signaling

PURPOSE

In the brain, astrocytes are non-excitable cells that undergo rapid morphological changes when stimulated by the excitatory neurotransmitter glutamate. We developed a chemical probe to monitor how glutamate affects the density and distribution of astrocytic L-type voltage-gated calcium channels (LTCC).

PROCEDURES

The imaging probe FluoBar1 was created from a barbiturate ligand modified with a fluorescent coumarin moiety. The probe selectivity was examined with colocalization analyses of confocal fluorescence imaging in U118-MG and transfected COS-7 cells. Living cells treated with 50 nM FluoBar1 were imaged in real time to reveal changes in density and distribution of astrocytic LTCCs upon exposure to glutamate.

RESULTS

FluoBar1 was synthesized in ten steps. The selectivity of the probe was demonstrated with immunoblotting and confocal imaging of immunostained cells expressing the Ca_V1.2 isoform of LTCCs proteins. Applying FluoBar1 to astrocyte model cells U118-MG allowed us to measure a fivefold increase in fluorescence density of LTCCs upon glutamate exposure.

CONCLUSIONS

Imaging probe FluoBar1 allows the real-time monitoring of LTCCs in living cells, revealing for first time that glutamate causes a rapid increase of LTCC membranar density in astrocyte model cells. FluoBar1 may help tackle previously intractable questions about LTCC dynamics in cellular events.

Delta-opioid receptor-mediated modulation of excitability of individual hippocampal neurons: mechanisms involved

BACKGROUND

Delta-opioid receptor (DOR)-mediated modulation of hippocampal neural networks is involved in emotions, cognition, and in pathophysiology and treatment of mood disorders. In this study, we examined the effects of DOR agonist (SNC80) and antagonist (naltrindole) on the excitability of individual hippocampal neurons.

METHODS

Primary neuronal cultures were prepared from hippocampi of newborn rats and cultivated in vitro for 8-14 days (DIV8-14). The effects of SNC80 naltrindole on evoked and spontaneous action potentials (APs) were measured at DIV8-9 and DIV13-14, respectively.

RESULTS

SNC80 (100 µM) potentiated spontaneous AP firing and stimulated sodium current; naltrindole had opposite effects. The stimulatory effect of 100 µM of SNC80 was revoked by pre-administration of 1 µM of naltrindole. SNC80 and naltrindole induced similar inhibitory effects on the evoked AP firing and on the calcium current. Further, SNC80 inhibited both peak and sustained potassium currents. Naltrindole had no effect on potassium currents.

CONCLUSION

We suggest that the effects of naltrindole and high concentration of SNC80 on the sodium currents are mediated via DORs and underlying the changes in spontaneous activity. The inhibitory effects of SNC80 on calcium and potassium currents might also be DOR-dependent; these currents might mediate SNC80 effect on the evoked AP firing. The inhibitory effects of naltrindole on calcium and of low doses of SNC80 on sodium currents might be however DOR independent. The behavioral effects of SNC80 and naltrindole, observed in previous studies, might be mediated, at least in part, via the modulatory effect of these ligands on the excitability of hippocampal neurons.

Different effects of metformin and phenformin on hypoxia-induced Ca2+ fluxes in cultured primary neurons

Recent evidence suggests that metformin and phenformin may exert beneficial effects against neuronal injury in the ischemic brain, however, the difference of action between these two drugs and the molecular mechanism of such protection is not clear. In this study, we investigated whether mild hypoxia-affected neurons exhibit changes in cytosolic calcium handling and whether metformin and phenformin exert any effect on calcium homeostasis in hypoxia-affected neurons. Cultured primary rat cortical cells were stained with calcium sensitive dye Oregon Green 488 BAPTA-1 AM and spontaneous calcium dependent changes of fluorescence were recorded. Using obtained fluorescence traces we estimated changes in relative amplitude of recorded spontaneous signals, changes in frequency of spontaneous activity, and changes in decay of fluorescence traces. We found that hypoxia caused reduction of the relative signal amplitude, increased the spontaneous activity, and slowed the decay of calcium concentration. After pre-treatment of cells with 0.1-0.5 mM metformin, the relative signal amplitude increased and the frequency of spontaneous signals decreased in hypoxia-affected neurons. However, pre-treatment with 1-25 µM phenformin neither increased the relative signal amplitude nor reduced the frequency of spontaneous signals. The decay of fluorescence traces became faster after application of metformin or phenformin comparing to neurons under hypoxic conditions. These results suggest different action of metformin and phenformin in improvement of Ca²⁺ homeostasis in hypoxia-affected neurons, which may have different effects on neuronal survival and functions after hypoxia/ischemia.

The Fragile X Mental Retardation Protein Regulates Striatal Medium Spiny Neuron Synapse Density and Dendritic Spine Morphology

The fragile X mental retardation protein (FMRP), an RNA-binding protein that mediates the transport, stability, and translation of hundreds of brain RNAs, is critically involved in regulating synaptic function. Loss of FMRP, as in fragile X syndrome (FXS), is a leading monogenic cause of autism and results in altered structural and functional synaptic plasticity, widely described in the hippocampus and cortex. Though FXS is associated with hyperactivity, impaired social interaction, and the development of repetitive or stereotyped behaviors, all of which are influenced by striatal activity, few studies have investigated the function of FMRP here. Utilizing a cortical-striatal co-culture model, we find that striatal medium spiny neurons (MSNs) lacking FMRP fail to make normal increases in PSD95 expression over a short time period and have significant deficits in dendritic spine density and colocalized synaptic puncta at the later measured time point compared to wildtype (WT) MSNs. Acute expression of wtFMRP plasmid in Fmr1 KO co-cultures results in contrasting outcomes for these measures on MSNs at the more mature time point, reducing spine density across multiple spine types but making no significant changes in colocalized puncta. FMRP’s KH2 and RGG RNA-binding domains are required for normal elimination of PSD95, and interruption of these domains slightly favors elimination of immature spine types. Further, KH2 is required for normal levels of colocalized puncta. Our data are largely consistent with a basal role for FMRP and its RNA-binding domains in striatal synapse stabilization on developing MSNs, and in light of previous findings, suggest distinct regional and/or cell type-specific roles for FMRP in regulating synapse structure.

Effect of Laminin Derived Peptides IKVAV and LRE Tethered to Hyaluronic Acid on hiPSC Derived Neural Stem Cell Morphology, Attachment and Neurite Extension

Low neural tissue extracellular matrix (ECM) content has led to the understudy of its effects on neural cells and tissue. Hyaluronic acid (HA) and laminin are major neural ECM components, but direct comparisons of their cellular effects could not be located in the literature. The current study uses human-induced pluripotent stem-cell-derived neural stem cells to assess the effects of HA, laminin, and HA with laminin-derived peptides IKVAV and LRE on cellular morphology, attachment, neurite extension and ECM remodeling. Increased attachment was observed on HA with and without IKVAV and LRE compared to laminin. Cellular morphology and neurite extension were similar on all surfaces. Using a direct binding inhibitor of Cav2.2 voltage gated calcium channel activity, a known binding partner of LRE, reduced attachment on HA with and without IKVAV and LRE and altered cellular morphology on surfaces with laminin or IKVAV and LRE. HA with IKVAV and LRE reduced the fluorescent intensity of fibronectin staining, but did not alter the localization of ECM remodeling enzymes matrix metalloprotease 2 and 9 staining compared to HA. Overall, the data indicate HA, IKVAV and LRE have complementary effects on human-induced pluripotent stem-cell-derived neural stem cell behavior.

AMPA Induces NO-Dependent cGMP Signals in Hippocampal and Cortical Neurons via L-Type Voltage-Gated Calcium Channels

The nitric oxide (NO)/cGMP signaling cascade has an established role in synaptic plasticity. However, with conventional methods, the underlying cGMP signals were barely detectable. Here, we set out to confirm the well-known NMDA-induced cGMP increases, to test the impact of AMPA on those signals, and to identify the relevant phosphodiesterases (PDEs) using a more sensitive fluorescence resonance energy transfer (FRET)-based method. Therefore, a “knock-in” mouse was generated that expresses a FRET-based cGMP indicator (cGi-500) allowing detection of cGMP concentrations between 100 nM and 3 μM. Measurements were performed in cultured hippocampal and cortical neurons as well as acute hippocampal slices. In hippocampal and cortical neurons, NMDA elicited cGMP signals half as high as the ones elicited by exogenous NO. Interestingly, AMPA increased cGMP independently of NMDA receptors and dependent on NO synthase (NOS) activation. NMDA- and AMPA-induced cGMP signals were not additive indicating that both pathways converge on the level of NOS. Accordingly, the same PDEs, PDE1 and PDE2, were responsible for degradation of NMDA- as well as AMPA-induced cGMP signals. Mechanistically, AMPAR induced calcium influx through L-type voltage-gated calcium channels leading to NOS and finally NO-sensitive guanylyl cyclase activation. Our results demonstrate that in addition to NMDA also AMPA triggers endogenous NO formation and hence cGMP production.

Growth hormone secretagogue receptor constitutive activity impairs voltage-gated calcium channel-dependent inhibitory neurotransmission in hippocampal neurons

KEY POINTS

Presynaptic Ca_V 2 voltage-gated calcium channels link action potentials arriving at the presynaptic terminal to neurotransmitter release. Hence, their regulation is essential to fine tune brain circuitry. Ca_V 2 channels are highly sensitive to G protein-coupled receptor (GPCR) modulation. Our previous data indicated that growth hormone secretagogue receptor (GHSR) constitutive activity impairs Ca_V 2 channels by decreasing their surface density. We present compelling support for the impact of Ca_V 2.2 channel inhibition by agonist-independent GHSR activity exclusively on GABA release in hippocampal cultures. We found that this selectivity arises from a high reliance of GABA release on Ca_V 2.2 rather than on Ca_V 2.1 channels. Our data provide new information on the effects of the ghrelin-GHSR system on synaptic transmission, suggesting a putative physiological role of the constitutive signalling of a GPCR that is expressed at high levels in brain areas with restricted access to its natural agonist.

ABSTRACT

Growth hormone secretagogue receptor (GHSR) displays high constitutive activity, independent of its endogenous ligand, ghrelin. Unlike ghrelin-induced GHSR activity, the physiological role of GHSR constitutive activity and the mechanisms that underlie GHSR neuronal modulation remain elusive. We previously demonstrated that GHSR constitutive activity modulates presynaptic Ca_V 2 voltage-gated calcium channels. Here we postulate that GHSR constitutive activity-mediated modulation of Ca_V 2 channels could be relevant in the hippocampus since this brain area has high GHSR expression but restricted access to ghrelin. We performed whole-cell patch-clamp in hippocampal primary cultures from E16- to E18-day-old C57BL6 wild-type and GHSR-deficient mice after manipulating GHSR expression with lentiviral transduction. We found that GHSR constitutive activity impairs Ca_V 2.1 and Ca_V 2.2 native calcium currents and that Ca_V 2.2 basal impairment leads to a decrease in GABA but not glutamate release. We postulated that this selective effect is related to a higher Ca_V 2.2 over Ca_V 2.1 contribution to GABA release (∼40% for Ca_V 2.2 in wild-type vs. ∼20% in wild-type GHSR-overexpressing cultures). This effect of GHSR constitutive activity is conserved in hippocampal brain slices, where GHSR constitutive activity reduces local GABAergic transmission of the granule cell layer (intra-granule cell inhibitory postsynaptic current (IPSC) size ∼-67 pA in wild-type vs. ∼-100 pA in GHSR-deficient mice), whereas the glutamatergic output from the dentate gyrus to CA3 remains unchanged. In summary, we found that GHSR constitutive activity impairs IPSCs both in hippocampal primary cultures and in brain slices through a Ca_V 2-dependent mechanism without affecting glutamatergic transmission.

Calcium Channels in Postnatal Development of Rat Pancreatic Beta Cells and Their Role in Insulin Secretion

Pancreatic beta cells during the first month of development acquire functional maturity, allowing them to respond to variations in extracellular glucose concentration by secreting insulin. Changes in ionic channel activity are important for this maturation. Within the voltage-gated calcium channels (VGCC), the most studied channels are high-voltage-activated (HVA), principally L-type; while low-voltage-activated (LVA) channels have been poorly studied in native beta cells. We analyzed the changes in the expression and activity of VGCC during the postnatal development in rat beta cells. We observed that the percentage of detection of T-type current increased with the stage of development. T-type calcium current density in adult cells was higher than in neonatal and P20 beta cells. Mean HVA current density also increased with age. Calcium current behavior in P20 beta cells was heterogeneous; almost half of the cells had HVA current densities higher than the adult cells, and this was independent of the presence of T-type current. We detected the presence of α1G, α1H, and α1I subunits of LVA channels at all ages. The Cav 3.1 subunit (α1G) was the most expressed. T-type channel blockers mibefradil and TTA-A₂ significantly inhibited insulin secretion at 5.6 mM glucose, which suggests a physiological role for T-type channels at basal glucose conditions. Both, nifedipine and TTA-A₂, drastically decreased the beta-cell subpopulation that secretes more insulin, in both basal and stimulating glucose conditions. We conclude that changes in expression and activity of VGCC during the development play an important role in physiological maturation of beta cells.

Sigma-1 receptor agonist increases axon outgrowth of hippocampal neurons via voltage-gated calcium ions channels

INTRODUCTION

Sigma-1 receptors (Sig-1Rs) are unique endoplasmic reticulum proteins that have been implicated in both neurodegenerative and ischemic diseases, such as Alzheimer's disease and stroke. Accumulating evidence has suggested that Sig-1R plays a role in neuroprotection and axon outgrowth. The underlying mechanisms of Sig-1R-mediated neuroprotection have been well elucidated. However, the mechanisms underlying the effects of Sig-1R on axon outgrowth are not fully understood.

METHODS

To clarify this issue, we utilized immunofluorescence to compare the axon lengths of cultured naïve hippocampal neurons before and after the application of the Sig-1R agonist, SA4503. Then, electrophysiology and immunofluorescence were used to examine voltage-gated calcium ion channel (VGCCs) currents in the cell membranes and growth cones.

RESULTS

We found that Sig-1R activation dramatically enhanced the axonal length of the naïve hippocampal neurons. Application of the Sig-1R antagonist NE100 and gene knockdown techniques both demonstrated the effects of Sig-1R. The growth-promoting effect of SA4503 was accompanied by the inhibition of voltage-gated Ca2+ influx and was recapitulated by incubating the neurons with the L-type, N-type, and P/Q-type VGCC blockers, nimodipine, MVIIA and ω-agatoxin IVA, respectively. This effect was unrelated to glial cells. The application of SA4503 transformed the growth cone morphologies from complicated to simple, which favored axon outgrowth.

CONCLUSION

Sig-1R activation can enhance axon outgrowth and may have a substantial influence on neurogenesis and neurodegenerative diseases.

MAP6 interacts with Tctex1 and Cav 2.2/N-type calcium channels to regulate calcium signalling in neurons

MAP6 proteins were first described as microtubule‐stabilizing agents, whose properties were thought to be essential for neuronal development and maintenance of complex neuronal networks. However, deletion of all MAP6 isoforms in MAP6 KO mice does not lead to dramatic morphological aberrations of the brain but rather to alterations in multiple neurotransmissions and severe behavioural impairments. A search for protein partners of MAP6 proteins identified Tctex1 – a dynein light chain with multiple non‐microtubule‐related functions. The involvement of Tctex1 in calcium signalling led to investigate it in MAP6 KO neurons. In this study, we show that functional Ca_v2.2/N‐type calcium channels are deficient in MAP6 KO neurons, due to improper location. We also show that MAP6 proteins interact directly with both Tctex1 and the C‐terminus of Ca_v2.2/N‐type calcium channels. A balance of these two interactions seems to be crucial for MAP6 to modulate calcium signalling in neurons.

Induction of Calcium Influx in Cortical Neural Networks by Nanomagnetic Forces

Nanomagnetic force stimulation with ferromagnetic nanoparticles was found to trigger calcium influx in cortical neural networks without observable cytotoxicity. Stimulated neural networks showed an average of 20% increment in calcium fluorescence signals and a heightened frequency in calcium spiking. These effects were also confined spatially to areas with engineered high magnetic field gradients. Furthermore, blockage of N-type calcium channels inhibited the stimulatory effects of the nanomagnetic forces, suggesting the role of mechano-sensitive ion channels in mediating calcium influx.

In vitro dopaminergic neurotoxicity of pesticides: a link with neurodegeneration?

Around the globe, chemical compounds are used to treat or repel pests and plagues that pose a threat to food and feed production. From epidemiological studies, it is known that there is a link between exposure to certain chemical classes of these so-called pesticides and the prevalence of neurodegenerative disorders such as Parkinson's disease in humans. However, which particular compound(s) account for this link or what underlying mechanisms are involved is still largely unresolved. The degenerative process in Parkinson's disease is largely limited to the dopaminergic neurons in the basal ganglia. Cellular mechanisms that are implicated in parkinsonian neurodegeneration include mitochondrial dysfunction, oxidative stress, disturbance of intracellular calcium homeostasis and endoplasmic reticulum (ER) stress. A major characteristic that distinguishes the dopaminergic neurons in the basal ganglia from other dopaminergic neurons is a particular reliance on intracellular calcium for spontaneous activity. Considering the energy consuming nature of maintenance of the intracellular calcium homeostasis and its involvement in life and death of a neuron, this may explain the specific vulnerability of this neuronal population. Despite a large variation in primary mechanism of action it has been demonstrated that pesticides from different classes disturb intracellular calcium homeostasis, thus interfering with intracellular calcium signalling. This relates to altered dopaminergic signalling, disturbed protein homeostasis and increased oxidative stress. Therefore, effects of (mixtures of) pesticides on the intracellular calcium homeostasis may play a role in the development of Parkinson's disease in humans. Although human exposure to pesticides via e.g. food often occurs in complex mixtures, (human) risk assessment is largely based on the assessment of single compounds. The discovery of common modes of action across different classes of pesticides therefore underpins the urgency of development of new models and approaches in risk assessment.

Defects During Mecp2 Null Embryonic Cortex Development Precede the Onset of Overt Neurological Symptoms

MeCP2 is associated with several neurological disorders; of which, Rett syndrome undoubtedly represents the most frequent. Its molecular roles, however, are still unclear, and data from animal models often describe adult, symptomatic stages, while MeCP2 functions during embryonic development remain elusive. We describe the pattern and timing of Mecp2 expression in the embryonic neocortex highlighting its low but consistent expression in virtually all cells and show the unexpected occurrence of transcriptional defects in the Mecp2 null samples at a stage largely preceding the onset of overt symptoms. Through the deregulated expression of ionic channels and glutamatergic receptors, the lack of Mecp2 during early neuronal maturation leads to the reduction in the neuronal responsiveness to stimuli. We suggest that such features concur to morphological alterations that begin affecting Mecp2 null neurons around the perinatal age and become evident later in adulthood. We indicate MeCP2 as a key modulator of the transcriptional mechanisms regulating cerebral cortex development. Neurological phenotypes of MECP2 patients could thus be the cumulative result of different adverse events that are already present at stages when no obvious signs of the pathology are evident and are worsened by later impairments affecting the central nervous system during maturation and maintenance of its functionality.

(S)-Lacosamide Binding to Collapsin Response Mediator Protein 2 (CRMP2) Regulates CaV2.2 Activity by Subverting Its Phosphorylation by Cdk5

The neuronal circuit remodels during development as well as in human neuropathologies such as epilepsy. Neurite outgrowth is an obligatory step in these events. We recently reported that alterations in the phosphorylation state of an axon specification/guidance protein, the collapsin response mediator protein 2 (CRMP2), play a major role in the activity-dependent regulation of neurite outgrowth. We also identified (S)-LCM, an inactive stereoisomer of the clinically used antiepileptic drug (R)-LCM (Vimpat®), as a novel tool for preferentially targeting CRMP2-mediated neurite outgrowth. Here, we investigated the mechanism by which (S)-LCM affects CRMP2 phosphorylation by two key kinases, cyclin-dependent kinase 5 (Cdk5) and glycogen synthase kinase 3β (GSK-3β). (S)-LCM application to embryonic cortical neurons resulted in reduced levels of Cdk5- and GSK-3β-phosphorylated CRMP2. Mechanistically, (S)-LCM increased CRMP2 binding to both Cdk5- and GSK-3β without affecting binding of CRMP2 to its canonical partner tubulin. Saturation transfer difference nuclear magnetic resonance (STD NMR) and differential scanning fluorimetry (DSF) experiments demonstrated direct binding of (S)-LCM to CRMP2. Using an in vitro luminescent kinase assay, we observed that (S)-LCM specifically inhibited Cdk5-mediated phosphorylation of CRMP2. Cross-linking experiments and analytical ultracentrifugation showed no effect of (S)-LCM on the oligomerization state of CRMP2. The increased association between Cdk5-phosphorylated CRMP2 and CaV2.2 was reduced by (S)-LCM in vitro and in vivo. This reduction translated into a decrease of calcium influx via CaV2.2 in (S)-LCM-treated neurons compared to controls. (S)-LCM, to our knowledge, is the first molecule described to directly inhibit CRMP2 phosphorylation and may be useful for delineating CRMP2-facilitated functions.

Calcium current homeostasis and synaptic deficits in hippocampal neurons from Kelch-like 1 knockout mice

Kelch-like 1 (KLHL1) is a neuronal actin-binding protein that modulates voltage-gated Ca_V2.1 (P/Q-type) and Ca_V3.2 (α_1H T-type) calcium channels; KLHL1 knockdown experiments (KD) cause down-regulation of both channel types and altered synaptic properties in cultured rat hippocampal neurons (Perissinotti et al., 2014). Here, we studied the effect of ablation of KLHL1 on calcium channel function and synaptic properties in cultured hippocampal neurons from KLHL1 knockout (KO) mice. Western blot data showed the P/Q-type channel α_1A subunit was less abundant in KO hippocampus compared to wildtype (WT); and P/Q-type calcium currents were smaller in KO neurons than WT during early days in vitro, although this decrease was compensated for at late stages by increases in L-type calcium current. In contrast, T-type currents did not change in culture. However, biophysical properties and western blot analysis revealed a differential contribution of T-type channel isoforms in the KO, with Ca_V3.2 α_1H subunit being down-regulated and Ca_V3.1 α_1G up-regulated. Synapsin I levels were also reduced in the KO hippocampus and cultured neurons displayed a concomitant reduction in synapsin I puncta and decreased miniature excitatory postsynaptic current (mEPSC) frequency. In summary, genetic ablation of the calcium channel modulator resulted in compensatory mechanisms to maintain calcium current homeostasis in hippocampal KO neurons; however, synaptic alterations resulted in a reduction of excitatory synapse number, causing an imbalance of the excitatory-inhibitory synaptic input ratio favoring inhibition.

Assessment of the neurotoxic potential of exposure to 50Hz extremely low frequency electromagnetic fields (ELF-EMF) in naïve and chemically stressed PC12 cells

Increasing exposure to extremely low frequency electromagnetic fields (ELF-EMF), generated by power lines and electric appliances, raises concern about potential adverse health effects of ELF-EMF. The central nervous system is expected to be particularly vulnerable to ELF-EMF as its function strongly depends on electrical excitability. We therefore investigated effects of acute (30min) and sub-chronic (48h) exposure to 50Hz ELF-EMF on naïve and chemically stressed pheochromocytoma (PC12) cells. The latter have higher levels of iron and/or reactive oxygen species (ROS) and display increased vulnerability to environmental insults. Effects of ELF-EMF on Ca(2+)-homeostasis, ROS production and membrane integrity were assessed using Fura-2 single cell fluorescence microscopy, H2-DCFDA and CFDA assays, respectively. Our data demonstrate that acute exposure of naïve PC12 cells to 50Hz ELF-EMF up to 1000μT fails to affect basal or depolarization-evoked [Ca(2+)]i. Moreover, sub-chronic ELF-EMF exposure up to 1000μT has no consistent effects on Ca(2+)-homeostasis in naïve PC12 cells and does not affect ROS production and membrane integrity. Notably, in chemically stressed PC12 cells both acute and sub-chronic ELF-EMF exposure also failed to exert consistent effects on Ca(2+)-homeostasis, ROS production and membrane integrity. Our combined findings thus indicate that exposure to 50Hz ELF-EMF up to 1000μT, i.e. 10,000 times above background exposure, does not induce neurotoxic effects in vitro, neither in naïve nor in chemically stressed PC12 cells. Though our data require confirmation, e.g. in developing neuronal cells in vitro or (developing) animals, it appears that the neurotoxic risk of ELF-EMF exposure is limited.

Mapping CRMP3 domains involved in dendrite morphogenesis and voltage-gated calcium channel regulation

Although hippocampal neurons are well-distinguished by the morphological characteristics of their dendrites and their structural plasticity, the mechanisms involved in regulating their neurite initiation, dendrite growth, network formation and remodeling are still largely unknown, in part because the key molecules involved remain elusive. Identifying new dendrite-active cues could uncover unknown molecular mechanisms that would add significant understanding to the field and possibly lead to the development of novel neuroprotective therapy because these neurons are impaired in many neuropsychiatric disorders. In our previous studies, we deleted the gene encoding CRMP3 in mice and identified the protein as a new endogenous signaling molecule that shapes diverse features of the hippocampal pyramidal dendrites without affecting axon morphology. We also found that CRMP3 protects dendrites against dystrophy induced by prion peptide PrP(106-126). Here, we report that CRMP3 has a profound influence on neurite initiation and dendrite growth of hippocampal neurons in vitro. Our deletional mapping revealed that the C-terminus of CRMP3 probably harbors its dendritogenic capacity and supports an active transport mechanism. By contrast, overexpression of the C-terminal truncated CRMP3 phenocopied the effect of CRMP3 gene deletion with inhibition of neurite initiation or decrease in dendrite complexity, depending on the stage of cell development. In addition, this mutant inhibited the activity of CRMP3, in a similar manner to siRNA. Voltage-gated calcium channel inhibitors prevented CRMP3-induced dendritic growth and somatic Ca(2+) influx in CRMP3-overexpressing neurons was augmented largely via L-type channels. These results support a link between CRMP3-mediated Ca(2+) influx and CRMP3-mediated dendritic growth in hippocampal neurons.

Calcineurin signaling mediates activity-dependent relocation of the axon initial segment

The axon initial segment (AIS) is a specialized neuronal subcompartment located at the beginning of the axon that is crucially involved in both the generation of action potentials and the regulation of neuronal polarity. We recently showed that prolonged neuronal depolarization produces a distal shift of the entire AIS structure away from the cell body, a change associated with a decrease in neuronal excitability. Here, we used dissociated rat hippocampal cultures, with a major focus on the dentate granule cell (DGC) population, to explore the signaling pathways underlying activity-dependent relocation of the AIS. First, a pharmacological screen of voltage-gated calcium channels (VGCCs) showed that AIS relocation is triggered by activation of L-type Cav1 VGCCs with negligible contribution from any other VGCC subtypes. Additional pharmacological analysis revealed that downstream signaling events are mediated by the calcium-sensitive phosphatase calcineurin; inhibition of calcineurin with either FK506 or cyclosporin A totally abolished both depolarization- and optogenetically-induced activity-dependent AIS relocation. Furthermore, calcineurin activation is sufficient for AIS plasticity, because expression of a constitutively active form of the phosphatase resulted in relocation of the AIS of DGCs without a depolarizing stimulus. Finally, we assessed the role of calcineurin in other forms of depolarization-induced plasticity. Neither membrane resistance changes nor spine density changes were affected by FK506 treatment, suggesting that calcineurin acts via a separate pathway to modulate AIS plasticity. Together, these results emphasize calcineurin as a vital player in the regulation of intrinsic plasticity as governed by the AIS.

Azole fungicides disturb intracellular Ca2+ in an additive manner in dopaminergic PC12 cells

Humans are exposed to complex mixtures of pesticides and other compounds, mainly via food. Azole fungicides are broad spectrum antifungal compounds used in agriculture and in human and veterinary medicine. The mechanism of antifungal action relies on inhibition of CYP51, resulting in inhibition of fungal cell growth. Known adverse health effects of azole fungicides are mainly linked to CYP inhibition. Additionally, azole fungicide-induced neurotoxicity has been reported, though the underlying mechanism(s) are largely unknown. We therefore investigated the effects of a group of six azole fungicides (imazalil, flusilazole, fluconazole, tebuconazole, triadimefon, and cyproconazole) on cell viability using a combined alamar Blue/CFDA-AM assay and on oxidative stress using a H2-DCFDA fluorescent assay. As calcium plays a pivotal role in neuronal survival and functioning, effects of these six azole fungicides and binary and quaternary mixtures of azole fungicides on the intracellular calcium concentration ([Ca(2+)]i) were investigated using single-cell fluorescence microscopy in dopaminergic PC12 cells loaded with the calcium-sensitive fluorescent dye Fura-2. Only modest changes in cell viability and ROS production were observed. However, five out of six azole fungicides induced a nonspecific inhibition of voltage-gated calcium channels (VGCCs), though with varying potency. Experiments using binary IC20 and quaternary IC10 mixtures indicated that the inhibitory effects on VGCCs are additive. The combined findings demonstrate modulation of intracellular Ca(2+) via inhibition of VGCCs as a novel mode of action of azole fungicides. Furthermore, mixtures of azole fungicides display additivity, illustrating the need to take mixture effects into account in human risk assessment.

A simple method to generate adipose stem cell-derived neurons for screening purposes

Strategies involved in mesenchymal stem cell (MSC) differentiation toward neuronal cells for screening purposes are characterized by quality and quantity issues. Differentiated cells are often scarce with respect to starting undifferentiated population, and the differentiation process is usually quite long, with high risk of contamination and low yield efficiency. Here, we describe a novel simple method to induce direct differentiation of MSCs into neuronal cells, without neurosphere formation. Differentiated cells are characterized by clear morphological changes, expression of neuronal specific markers, showing functional response to depolarizing stimuli and electrophysiological properties similar to those of developing neurons. The method described here represents a valuable tool for future strategies aimed at personalized screening of therapeutic agents in vitro.

Small-conductance Ca2+-activated K+ channels modulate action potential-induced Ca2+ transients in hippocampal neurons

In hippocampal pyramidal neurons, voltage-gated Ca(2+) channels open in response to action potentials. This results in elevations in the intracellular concentration of Ca(2+) that are maximal in the proximal apical dendrites and decrease rapidly with distance from the soma. The control of these action potential-evoked Ca(2+) elevations is critical for the regulation of hippocampal neuronal activity. As part of Ca(2+) signaling microdomains, small-conductance Ca(2+)-activated K(+) (SK) channels have been shown to modulate the amplitude and duration of intracellular Ca(2+) signals by feedback regulation of synaptically activated Ca(2+) sources in small distal dendrites and dendritic spines, thus affecting synaptic plasticity in the hippocampus. In this study, we investigated the effect of the activation of SK channels on Ca(2+) transients specifically induced by action potentials in the proximal processes of hippocampal pyramidal neurons. Our results, obtained by using selective SK channel blockers and enhancers, show that SK channels act in a feedback loop, in which their activation by Ca(2+) entering mainly through L-type voltage-gated Ca(2+) channels leads to a reduction in the subsequent dendritic influx of Ca(2+). This underscores a new role of SK channels in the proximal apical dendrite of hippocampal pyramidal neurons.

Characterization of L-type voltage-gated Ca(2+) channel expression and function in developing CA3 pyramidal neurons

Voltage-gated calcium channels (VGCCs) play a major role during the development of the central nervous system (CNS). Ca(2+) influx via VGCCs regulates axonal growth and neuronal migration as well as synaptic plasticity. Specifically, L-type VGCCs have been well characterized to be involved in the formation and refinement of the connections within the CA3 region of the hippocampus. The majority of the growth, formation, and refinement in the CNS occurs during the third trimester of human pregnancy. An equivalent developmental time period in rodents occurs during the first 2weeks of post-natal life, and the expression pattern of L-type VGCCs during this time period has not been well characterized. In this study, we show that Cav1.2 channels are more highly expressed during this developmental period compared to adolescence (post-natal day 30) and that L-type VGCCs significantly contribute to the overall Ca(2+) currents. These findings suggest that L-type VGCCs are functionally expressed during the crucial developmental period.

Timothy syndrome is associated with activity-dependent dendritic retraction in rodent and human neurons

L-type voltage gated calcium channels have an important role in neuronal development by promoting dendritic growth and arborization. A point mutation in the gene encoding Ca(V)1.2 causes Timothy syndrome, a neurodevelopmental disorder associated with autism spectrum disorders (ASDs). We report that channels with the Timothy syndrome alteration cause activity-dependent dendrite retraction in rat and mouse neurons and in induced pluripotent stem cell (iPSC)-derived neurons from individuals with Timothy syndrome. Dendrite retraction was independent of calcium permeation through the mutant channel, was associated with ectopic activation of RhoA and was inhibited by overexpression of the channel-associated GTPase Gem. These results suggest that Ca(V)1.2 can activate RhoA signaling independently of Ca(2+) and provide insights into the cellular basis of Timothy syndrome and other ASDs.

Differential effects of crambescins and crambescidin 816 in voltage-gated sodium, potassium and calcium channels in neurons

Crambescins and crambescidins are two families of guanidine alkaloids from the marine sponge Crambe crambe. Although very little information about their biological effect has been reported, it is known that crambescidin 816 (Cramb816) blocks calcium channels in a neuroblastoma X glioma cell line. Taking this into account, and the fact that ion channels are frequent targets for natural toxins, we examined the effect of Cramb816 and three compounds from the crambescin family, norcrambescin A2 (NcrambA2), crambescin A2 (CrambA2), and crambescin C1 (CrambC1), in the main voltage-dependent ion channels in neurons: sodium, potassium, and calcium channels. Electrophysiological recordings of voltage gated sodium, potassium, and calcium currents, in the presence of these guanidine alkaloids, were performed in cortical neurons from embryonic mice. Different effects were discovered: crambescins inhibited K(+) currents with the following potency: NcrambA2 > CrambC1 > CrambA2, while Cramb816 lacked an effect. Only CrambC1 and Cramb816 partially blocked Na(+) total current. However, Cramb816 partially blocked Ca(2+) , while NcrambA2 did not. Since the blocking effect of Cramb816 on calcium currents has not been previously reported in detail, we further pharmacologically isolated the two main fractions of HVA Ca(2+) channels in neurons and investigated the Cramb816 effect on them. Here, we revealed that Cav1 or L-type calcium channels are the main target for Cramb816. These two families of guanidine alkaloids clearly showed a structure-activity relationship with the crambescins acting on voltage-gated potassium channels, while Cramb816 blocks the voltage-gated calcium channel Cav1 with higher potency than nifedipine. The novel evidence that Cramb816 partially blocked CaV and NaV channels in neurons suggests that this compound might be involved in decreasing the neurotransmitter release and synaptic transmission in the central nervous system. The findings presented here provide the first detailed approach on the different effects of crambescin and crambescidin compounds in voltage-gated sodium, potassium, and calcium channels in neurons and thus provide a basis for future studies.

Sex and steroid hormones in early brain injury

Brain injury during development can have severe, long-term consequences. Using an array of animal models, we have an understanding of the etiology of perinatal brain injury. However, we have only recently begun to address the consequences of endogenous factors such as genetic sex and developmental steroid hormone milieu. Our limited understanding has sometimes led researchers to make over-generalizing and potentially dangerous statements regarding treatment for brain injury. Therefore this review acts as a cautionary tale, speaking to our need to understand the effects of sex and steroid hormone environment on the response to brain trauma in the neonate.

Regional expression and subcellular localization of the voltage-gated calcium channel β subunits in the developing mouse brain

Ca(2+) channel β subunits determine the maturation, biophysical properties and cell surface expression of high voltage-activated channels. Thus, we have analysed the expression, regional distribution and subcellular localization of the Ca(v) β subunit family in mice from birth to adulthood. In the hippocampus and cerebellum, Ca(v) β(1), Ca(v) β(3) and Ca(v) β(4) protein levels increased with age, although there were marked region- and developmental stage-specific differences in their expression. Ca(v) β(1) was predominantly expressed in the strata oriens and radiatum of the hippocampus, and only weakly in the cerebellum. The Ca(v) β(3) subunit was mainly expressed in the strata radiatum and lucidum of the hippocampus and in the molecular layer of the cerebellum. During development, Ca(v) β(3) protein expression in the cerebellum peaked at postnatal days (P) 15 and 21, and had diminished drastically by P60, and in the hippocampus increased with age throughout all subfields. Ca(v) β(4) protein was detected throughout the cerebellum, particularly in the molecular layer, and in contrast to the other subunits, Ca(v) β(4) was mainly detected in the molecular layer and the hilus of the hippocampus. At the subcellular level, Ca(v) β(1) and Ca(v) β(3) were predominantly located post-synaptically in hippocampal pyramidal cells and cerebellar Purkinje cells. Ca(v) β(4) subunits were detected in the pre-synaptic and post-synaptic compartments of both regions, albeit more strongly at post-synaptic sites. These results shed new light on the developmental regulation and subcellular localization of Ca(v) β subunits, and their possible role in pre- and post-synaptic transmission.

Ethanol alters calcium signaling in axonal growth cones

Calcium (Ca2+) channels are sensitive to ethanol and Ca2+ signaling is a critical regulator of axonal growth and guidance. Effects of acute and chronic exposure to ethanol (22, 43, or 87 mM) on voltage-gated Ca2+ channels (VGCCs) in whole cells, and KCl-induced Ca2+ transients in axonal growth cones, were examined using dissociated hippocampal cultures. Whole-cell patch-clamp analysis in neurons with newly-formed axons (Stage 3) revealed that rapidly inactivating, low-voltage activated (LVA) and non-inactivating, high-voltage activated (HVA) currents were both inhibited in a dose-dependent manner by acute ethanol, with relatively greater inhibition of HVA currents. When assessed by Fluo-4-AM imaging, baseline fluorescence and Ca2+ response to ethanol in Stage 3 neurons was similar compared to neurons without axons, but peak Ca2+ transient amplitudes in response to bath-applied KCl were greater in Stage 3 neurons and were decreased by acute ethanol. The amplitude of Ca2+ transients elicited specifically in axonal growth cones by focal application of KCl was also inhibited by acute exposure to moderate-to-high concentrations of ethanol (43 or 87 mM), whereas a lower concentration (22 mM) had no effect. When 43 or 87 mM ethanol was present continuously in the medium, KCl-evoked Ca2+ transient amplitudes were also reduced in growth cones. In contrast, Ca2+ transients were increased by continuous exposure to 22 mM ethanol. Visualization using a fluorescent dihydropyridine analog revealed that neurons continuously exposed to ethanol expressed increased amounts of L-type Ca2+ channels, with greater increases in axonal growth cones than cell bodies. Thus, acute ethanol reduces Ca2+ current and KCl-induced Ca2+ responses in whole cells and axonal growth cones, respectively, and chronic exposure is also generally inhibitory despite apparent up-regulation of L-type channel expression. These results are consistent with a role for altered growth cone Ca2+ signaling in abnormal neuromorphogenesis associated with fetal alcohol spectrum disorders.

Ultra-low exposure to α-7 nicotinic acetylcholine receptor partial agonists elicits an improvement in cognition that corresponds with an increase in α-7 receptor expression in rodents: implications for low dose clinical efficacy

Αlpha-7 neuronal nicotinic receptors (NNRs) are considered targets for cognitive enhancement in schizophrenia and Alzheimer's disease. AZD0328 is an alpha-7 NNR partial agonist that enhances cognition in rodents and nonhuman primates at sub-microgram to microgram doses. We hypothesized that increased expression of the alpha-7 receptor contributes to this beneficial activity at low doses and tested this by examining the effect of AZD0328 using in vivo and ex vivo binding, RT-PCR and cognitive function in rodents. AZD0328 (0.00178 mg/kg) was subcutaneously administered to mice 4, 24, 48 and 72 hours prior to testing in novel object recognition and produced a significant increase in cognition at 4, 24 and 48 h post-dosing. In vivo binding was examined in rat brain using [(3)H]AZ11637326 and there was a dose-dependent reduction in receptor binding at higher doses of AZD0328 (0.001-3 mg/kg), and a second alpha-7 partial agonist, SSR180711 (0.01-30 mg/kg). Lower doses of both compounds (0.0001 mg/kg) produced a significant increase in binding of [(3)H]AZ11637326. Ex vivo binding using [(125)I]-α-bungarotoxin, showed a significant increase in receptor number (B(max.)) in the frontal cortex or hippocampus with no significant effect on receptor affinity (K(d)) 2 h post administration of AZD0328. [(3)H]AZ11637326 administered 1.5 h following AZD0328 produced a significant increase in specific binding in rat brain regions. We found that the effect on receptor number was long-lasting, with [(125)I]-α-bungarotoxin binding increased in rats given AZD0328 for 2-48 h, but this was not accompanied by increased mRNA synthesis. SSR180711 produced a similar increase in B(max.) and specific binding with no effect on K(d). Therefore, trace dose of alpha-7 partial agonists has rapid onset and produces a profound, sustained effect on novel object recognition in mice that corresponds by dose to an increase in receptor number in rat brain. These findings provide an explanation for the acute and sustained benefit of alpha-7 receptor activation in working memory in nonhuman primates and guidance for drug development initiatives and treatment regimens for nicotinic partial agonists.

Endogenous SNAP-25 regulates native voltage-gated calcium channels in glutamatergic neurons

In addition to its primary role as a fundamental component of the SNARE complex, SNAP-25 also modulates voltage-gated calcium channels (VGCCs) in various overexpression systems. Although these studies suggest a potential negative regulatory role of SNAP-25 on VGCC activity, the effects of endogenous SNAP-25 on native VGCC function in neurons are unclear. In the present study, we investigated the VGCC properties of cultured glutamatergic and GABAergic rat hippocampal neurons. Glutamatergic currents were dominated by P/Q-type channels, whereas GABAergic cells had a dominant L-type component. Also, glutamatergic VGCC current densities were significantly lower with enhanced inactivation rates and shifts in the voltage dependence of activation and inactivation curves compared with GABAergic cells. Silencing endogenous SNAP-25 in glutamatergic neurons did not alter P/Q-type channel expression or localization but led to increased VGCC current density without changes in the VGCC subtype proportions. Isolation of the P/Q-type component indicated that increased current in the absence of SNAP-25 was correlated with a large depolarizing shift in the voltage dependence of inactivation. Overexpressing SNAP-25 in GABAergic neurons reduced current density without affecting the VGCC subtype proportion. Accordingly, VGCC current densities in glutamatergic neurons from Snap-25(+/-) mice were significantly elevated compared with wild type glutamatergic neurons. Overall, this study demonstrates that endogenous SNAP-25 negatively regulates native VGCCs in glutamatergic neurons which could have important implications for neurological diseases associated with altered SNAP-25 expression.

Voltage-activated calcium channel expression profiles in mouse brain and cultured hippocampal neurons

The importance and diversity of calcium signaling in the brain is mirrored by the expression of a multitude of voltage-activated calcium channel (Ca(V)) isoforms. Whereas the overall distributions of alpha(1) subunits are well established, the expression patterns of distinct channel isoforms in specific brain regions and neurons, as well as those of the auxiliary beta and alpha(2)delta subunits are still incompletely characterized. Further it is unknown whether neuronal differentiation and activity induce changes of Ca(V) subunit composition. Here we combined absolute and relative quantitative TaqMan reverse transcription PCR (RT-PCR) to analyze mRNA expression of all high voltage-activated Ca(V) alpha(1) subunits and all beta and alpha(2)delta subunits. This allowed for the first time the direct comparison of complete Ca(V) expression profiles of mouse cortex, hippocampus, cerebellum, and cultured hippocampal neurons. All brain regions expressed characteristic profiles of the full set of isoforms, except Ca(V)1.1 and Ca(V)1.4. In cortex development was accompanied by a general down regulation of alpha(1) and alpha(2)delta subunits and a shift from beta(1)/beta(3) to beta(2)/beta(4). The most abundant Ca(V) isoforms in cerebellum were Ca(V)2.1, beta(4), and alpha(2)delta-2, and in hippocampus Ca(V)2.3, beta(2), and alpha(2)delta-1. Interestingly, cultured hippocampal neurons also expressed the same Ca(V) complement as adult hippocampus. During differentiation specific Ca(V) isoforms experienced up- or down-regulation; however blocking electrical activity did not affect Ca(V) expression patterns. Correlation analysis of alpha(1), beta and alpha(2)delta subunit expression throughout all examined preparations revealed a strong preference of Ca(V)2.1 for beta(4) and alpha(2)delta-2 and vice versa, whereas the other alpha(1) isoforms were non-selectively expressed together with each of the other beta and alpha(2)delta isoforms. Together our results revealed a remarkably stable overall Ca(2+) channel complement as well as tissue specific differences in expression levels. Developmental changes are likely determined by an intrinsic program and not regulated by changes in neuronal activity.

GLP-2 potentiates L-type Ca2+ channel activity associated with stimulated glucose uptake in hippocampal neurons

Glucagon-like peptide-2 (GLP-2) is a neuropeptide secreted from endocrine cells in the gut and neurons in the brain. GLP-2 stimulates intestinal crypt cell proliferation and mucosal blood flow while decreasing gastric emptying and gut motility. However, a GLP-2-mediated signaling network has not been fully established in primary cells. Since the GLP-2 receptor mRNA and protein were highly expressed in the mouse hippocampus, we further characterized that human (125)I-labeled GLP-2(1-33) specifically bound to cultured hippocampal neurons with K(d) = 0.48 nM, and GLP-2 acutely induced subcellular translocalization of the early gene c-Fos. Using the whole cell patch clamp, we recorded barium currents (I(Ba)) flowing through voltage-gated Ca(2+) channels (VGCC) in those neurons in the presence of GLP-2 with and without inhibitors. We showed that GLP-2 (20 nM) enhanced the whole cell I(Ba) mediated by L-type VGCC that was defined using an L-type Ca(2+) channel blocker (nifedipine, 10 microM). Moreover, GLP-2-potentiation of L-type VGCC was abolished in neurons pretreated with a PKA inhibitor (PKI(14-22), 1 microM). Finally, using a fluorescent nonmetabolized glucose analog (6-NBDG) tracing imaging, we showed that glucose was taken up directly by cultured neurons. GLP-2 increased 2-deoxy-d-[(3)H]glucose uptake that was dependent upon dosage, activation of PKA, and potentiation of L-type VGCC. We conclude that GLP-2 potentiates L-type VGCC activity through activating PKA signaling, partially stimulating glucose uptake by primary cultured hippocampal neurons. The potentiation of L-type VGCC may be physiologically relevant to GLP-2-induced neuroendocrine modulation of neurotransmitter release and hormone secretion.

Genetic ablation of tenascin-C expression leads to abnormal hippocampal CA1 structure and electrical activity in vivo

Despite evidence that the extracellular matrix glycoprotein tenascin-C (TNC) is implicated in brain development and plasticity, its roles in the intact adult brain are unknown. Here we report that spontaneous local field potential (LFP) activity in freely moving adult TNC-deficient mice is abnormal. The power of cortical and hippocampal theta and gamma oscillations was enhanced in comparison to wild-type mice. The alteration in hippocampal gamma rhythm was subfield specific, such that CA1 gamma was accentuated while dentate gyrus gamma was normal. Similar to LFP, synaptic transmission and plasticity at perforant path synapses in the dentate gyrus were unaffected by the mutation. Morphological analyses revealed a subfield-specific reduction in the CA1 volume and a reduction in the numbers of somatostatin-positive interneurons in the hippocampus as potential structural substrates of the observed functional aberrations. These findings indicate a role for tenascin-C in structural organization of the CA1 hippocampal subfield and in shaping neural activity.

Transient receptor potential canonical 5 channels activate Ca2+/calmodulin kinase Igamma to promote axon formation in hippocampal neurons

Functionality of neurons is dependent on their compartmentalized polarization of dendrites and an axon. The rapid and selective outgrowth of one neurite, relative to the others, to form the axon is critical in initiating neuronal polarity. Axonogenesis is regulated in part by an optimal intracellular calcium concentration. Our investigation of Ca(2+)-signaling pathways involved in axon formation using cultured hippocampal neurons demonstrates a role for Ca(2+)/calmodulin kinase kinase (CaMKK) and its downstream target Ca(2+)/calmodulin kinase I (CaMKI). Expression of constitutively active CaMKI induced formation of multiple axons, whereas blocking CaMKK or CaMKI activity with pharmacological, dominant-negative, or short hairpin RNA (shRNA) methods significantly inhibited axon formation. CaMKK signals via the gamma-isoform of CaMKI as shRNA to CaMKIgamma, but not the other CaMKI isoforms, inhibited axon formation. Furthermore, overexpression of wild-type CaMKIgamma, but not a mutant incapable of membrane association, accelerated the rate of axon formation. Pharmacological or small interfering RNA inhibition of transient receptor potential canonical 5 (TRPC5) channels, which are present in developing axonal growth cones, suppressed CaMKK-mediated activation of CaMKIgamma as well as axon formation. We demonstrate using biochemical fractionation and immunocytochemistry that CaMKIgamma and TRPC5 colocalize to lipid rafts. These results are consistent with a model in which highly localized calcium influx through the TRPC5 channels activates CaMKK and CaMKIgamma, which subsequently promote axon formation.

Imaging of evoked dense-core-vesicle exocytosis in hippocampal neurons reveals long latencies and kiss-and-run fusion events

Evoked neuropeptide secretion in the central nervous system occurs slowly, but the basis for slow release is not fully understood. Whereas exocytosis of single synaptic vesicles in neurons and of dense-core vesicles (DCVs) in endocrine cells have been directly visualized, single DCV exocytic events in neurons of the central nervous system have not been previously studied. We imaged DCV exocytosis in primary cultured hippocampal neurons using fluorescent propeptide cargo and total internal reflectance fluorescence microscopy. The majority of Ca(2+)-triggered exocytic events occurred from immobile plasma-membrane-proximal DCVs in the cell soma, whereas there were few events in the neurites. Strikingly, DCVs in the cell soma exhibited 50-fold greater release probabilities than those in neurites. Latencies to depolarization-evoked fusion for DCVs were surprisingly long, occurring with an average time constant (tau) of 16 seconds for DCVs in the soma and even longer for DCVs in neurites. All of the single DCV release events exhibited rapid fusion-pore openings and closures, the kinetics of which were highly dependent upon Ca(2+) levels. These ;kiss-and-run' events were associated with limited cargo secretion. Thus, the slow evoked release of neuropeptides could be attributed to very prolonged latencies from stimulation to fusion and transient fusion-pore openings that might limit cargo secretion.

The stargazin-related protein gamma 7 interacts with the mRNA-binding protein heterogeneous nuclear ribonucleoprotein A2 and regulates the stability of specific mRNAs, including CaV2.2

The role(s) of the novel stargazin-like gamma-subunit proteins remain controversial. We have shown previously that the neuron-specific gamma7 suppresses the expression of certain calcium channels, particularly Ca(V)2.2, and is therefore unlikely to operate as a calcium channel subunit. We now show that the effect of gamma7 on Ca(V)2.2 expression is via an increase in the degradation rate of Ca(V)2.2 mRNA and hence a reduction of Ca(V)2.2 protein level. Furthermore, exogenous expression of gamma7 in PC12 cells also decreased the endogenous Ca(V)2.2 mRNA level. Conversely, knockdown of endogenous gamma7 with short-hairpin RNAs produced a reciprocal enhancement of Ca(V)2.2 mRNA stability and an increase in endogenous calcium currents in PC12 cells. Moreover, both endogenous and expressed gamma7 are present on intracellular membranes, rather than the plasma membrane. The cytoplasmic C terminus of gamma7 is essential for all its effects, and we show that gamma7 binds directly via its C terminus to a heterogeneous nuclear ribonucleoprotein (hnRNP A2), which also binds to a motif in Ca(V)2.2 mRNA, and is associated with native Ca(V)2.2 mRNA in PC12 cells. The expression of hnRNP A2 enhances Ca(V)2.2 I(Ba), and this enhancement is prevented by a concentration of gamma7 that alone has no effect on I(Ba). The effect of gamma7 is selective for certain mRNAs because it had no effect on alpha2delta-2 mRNA stability, but it decreased the mRNA stability for the potassium-chloride cotransporter, KCC1, which contains a similar hnRNP A2 binding motif to that in Ca(V)2.2 mRNA. Our results indicate that gamma7 plays a role in stabilizing Ca(V)2.2 mRNA.

Identification of a developmentally regulated pathway of membrane retrieval in neuronal growth cones

The growth-cone plasma membrane constantly reconfigures during axon navigation and upon target recognition. The identity and regulation of the membrane pathway(s) participating in remodeling of the growth-cone surface remain elusive. Here, we identify a constitutive, high-capacity plasma-membrane-recycling activity in the axonal growth cones, which is mediated by a novel bulk endocytic pathway that is mechanistically related to macropinocytosis. This pathway generates large compartments at sites of intense actin-based membrane ruffling through the actions of phosphatidylinositol 3-kinase, the small GTPase Rac1 and the pinocytic chaperone Pincher. At early developmental stages, bulk endocytosis is the primary endocytic pathway for rapid retrieval of the growth-cone plasma membrane. At later stages, during the onset of synaptogenesis, an intrinsic program of maturation leads to downregulation of basal bulk endocytosis and the emergence of depolarization-induced synaptic-vesicle exo-endocytosis. We propose that the control of bulk membrane retrieval contributes to the homeostatic regulation of the axonal plasma membrane and to growth-cone remodeling during axonal outgrowth. In addition, we suggest that the downregulation of bulk endocytosis during synaptogenesis might contribute to the preservation of synaptic-vesicle specificity.

Differential contribution of L-, N-, and P/Q-type calcium channels to [Ca2+]i changes evoked by kainate in hippocampal neurons

We investigated the contribution of L-, N- and P/Q-type Ca(2+) channels to the [Ca(2+)](i) changes, evoked by kainate, in the cell bodies of hippocampal neurons, using a pharmacological approach and Ca(2+) imaging. Selective Ca(2+) channel blockers, namely nitrendipine, omega-Conotoxin GVIA (omega-GVIA) and omega-Agatoxin IVA (omega-AgaIVA) were used. The [Ca(2+)](i) changes evoked by kainate presented a high variability, and were abolished by NBQX, a AMPA/kainate receptor antagonist, but the N-methyl-D-aspartate (NMDA) receptor antagonist, D-AP5, was without effect. Each Ca(2+) channel blocker caused differential inhibitory effects on [Ca(2+)](i) responses evoked by kainate. We grouped the neurons for each blocker in three subpopulations: (1) neurons with responses below 60% of the control; (2) neurons with responses between 60% and 90% of the control, and (3) neurons with responses above 90% of the control. The inhibition caused by nitrendipine was higher than the inhibition caused by omega-GVIA or omega-AgaIVA. Thus, in the presence of nitrendipine, the percentage of cells with responses below 60% of the control was 41%, whereas in the case of omega-GVIA or omega-AgaIVA the values were 9 or 17%, respectively. The results indicate that hippocampal neurons differ in what concerns their L-, N- and P/Q-type Ca(2+) channels activated by stimulation of the AMPA/kainate receptors.

Evidence for an extended duration of GABA-mediated excitation in the developing male versus female hippocampus

Gamma-aminobutyric acid (GABA) is as an excitatory neurotransmitter during brain development. Activation of GABA(A) receptors in neonatal rat hippocampus results in chloride efflux and membrane depolarization sufficient to open voltage sensitive calcium channels. As development progresses, there is a decline in the magnitude of calcium influx subsequent to GABA(A) receptor activation and the number of cells that respond to GABA with excitation. By the second postnatal week in the rat, GABA action in the hippocampus is predominantly inhibitory. The functional consequences and endogenous regulation of developmental GABA-mediated excitation remains under-explored. Hippocampal neurons in the newborn male and female rat respond to GABA(A) receptor activation with increased intracellular calcium and are susceptible to GABA-mediated damage -- both being indicative of the excitatory nature of GABA. In the present study we observed that by postnatal day 7, only males are susceptible to GABA(A) agonist-induced damage and respond to GABA(A) agonist administration with elevated levels of intracellular calcium in cultured hippocampal neurons. By postnatal day 14, GABA(A) agonist administration was without effect on intracellular calcium in both males and females. The age-related sex difference in the impact of GABA(A) receptor activation correlates with a sex difference in chloride co-transporter expression. Males have elevated protein levels of pNKCC1 on PN0 and PN7, with no sex difference by PN14. In contrast, females displayed elevated levels of KCC2 on PN7. This converging evidence infers that sex affects the duration of GABA(A) receptor-mediated excitation during normal hippocampal development, and provides a mechanism by which the effect is mediated.

Effect of O-superfamily conotoxin SO3 on synchronized spontaneous calcium spikes in cultured hippocampal networks

SO3 belongs to the O-superfamily of conotoxins and is known to have analgesic effects in experimental animals. In order to explore the mechanism of its potential pharmacological actions, the effect of SO3 on synchronized spontaneous calcium spikes was examined in cultured hippocampal networks by calcium imaging. Spontaneous oscillations of intracellular concentrations of calcium (Ca(2+)) in the form of waves and spikes are found in cultured hippocampal networks. Exposure to increasing concentrations of SO3 resulted in a progressive decrease in synchronized spontaneous calcium spikes. The higher concentrations (0.1 micromol/L and 1 micromol/L) of SO3 showed the strongest inhibition. The rank order of inhibition was 1 micromol/L > 0.1 micromol/L > 10 micromol/L > 0.01 micromol/L. This action of SO3 in reducing synchronized calcium spikes suggests a possible application for therapeutic treatment of epilepsy.

Physiological development of insulin secretion, calcium channels, and GLUT2 expression of pancreatic rat beta-cells

Insulin secretion in mature beta-cells increases vigorously when extracellular glucose concentration rises. Glucose-stimulated insulin secretion depends on Ca(2+) influx through voltage-gated Ca(2+) channels. During fetal development, this structured response is not well established, and it is after birth that beta-cells acquire glucose sensitivity and a robust secretion. We compared some elements of glucose-induced insulin secretion coupling in beta-cells obtained from neonatal and adult rats and found that neonatal cells are functionally immature compared with adult cells. We observed that neonatal cells secrete less insulin and cannot sense changes in extracellular glucose concentrations. This could be partially explained because in neonates Ca(2+) current density and synthesis of mRNA alpha1 subunit Ca(2+) channel are lower than in adult cells. Interestingly, immunostaining for alpha1B, alpha1C, and alpha1D subunits in neonatal cells is similar in cytoplasm and plasma membrane, whereas it occurs predominantly in the plasma membrane in adult cells. We also observed that GLUT2 expression in adult beta-cells is mostly located in the membrane, whereas in neonatal cells glucose transporters are predominantly in the cytoplasm. This could explain, in part, the insensitivity to extracellular glucose in neonatal beta-cells. Understanding neonatal beta-cell physiology and maturation contributes toward a better comprehension of type 2 diabetes physiopathology, where alterations in beta-cells include diminished L-type Ca(2+) channels and GLUT2 expression that results in an insufficient insulin secretion.

Synapse development: still looking for the forest, still lost in the trees

Synapse development in the vertebrate central nervous system is a highly orchestrated process occurring not only during early stages of brain development, but also (to a lesser extent) in the mature nervous system. During development, the formation of synapses is intimately linked to the differentiation of neuronal cells, the extension of their axons and dendrites, and the course wiring of the nervous system. Subsequently, the stabilization, elimination, and strengthening of synaptic contacts is coupled to the refinement of axonal and dendritic arbors, to the establishment of functionally meaningful connections, and probably also to the day-to-day acquisition, storage, and retrieval of memories, higher order thought processes, and behavioral patterns.

Studies on Ca2+-channel distribution in maturation arrested mouse oocyte

The present study was carried out to identify the existence of voltage-dependent Ca2+-channels (P/Q-, N-, and L-type) and their distributional differences in germinal vesicle (GV) and GV breakdown (GVBD)-arrested mouse oocytes which includes GVBD to telophase I of meiosis I and matured oocytes (MII, metaphase of meiosis II) by using the immunocytochemical method and a confocal laser scanning microscope. (1) Comparison between follicular oocytes (GV) and GV-arrested oocytes after 17 hr of in vitro culture. In follicular oocytes, P/Q-, N-, L (anti-alpha1C anti-alpha1D)-type Ca2+-channels showed both localized and uniform staining. In contrast, GV-arrested oocytes, after in vitro culture for 17 hr, showed no presence of Ca2+-channels in most oocytes. (2) Comparison between GVBD oocytes after culture in vitro for 3 hr and GVBD-arrested oocytes after culture in vitro for 17 hr. In GVBD oocytes, P/Q-, N-, L (anti-1C, anti-alpha1D)-type Ca2+-channels showed both localized and uniform staining. In contrast, in GVBD-arrested oocytes, none of the three types of Ca2+-channels were identified in 72-86% of oocytes. The present study demonstrates that in most GVBD-arrested oocytes that do not mature to MII, there is no Ca2+-channel identified. Therefore, most of the GVBD-arrested oocytes seem to have defects in Ca2+-channel expression/translation. Also, distributional changes of Ca2+-channels take place depending on the maturation progress in GV oocytes and MII stage oocytes (ovulated and 17 hr cultured MII stage oocytes). In addition, we found evidence that a functional voltage-dependent Ca2+-channel (L-type) exists in mouse oocytes (ovulated and cultured MII staged oocytes by a confocal laser scanning microscope).

Adaptation to synaptic inactivity in hippocampal neurons

In response to activity deprivation, CNS neurons undergo slow adaptive modification of unitary synaptic transmission. The changes are comparable in degree to those induced by brief intense stimulation, but their molecular basis is largely unknown. Our data indicate that prolonged AMPAR blockade acts through loss of Ca2+ entry through L-type Ca2+ channels to bring about an increase in both vesicle pool size and turnover rate, as well as a postsynaptic enhancement of the contribution of GluR1 homomers, concentrated at the largest synapses. The changes were consistent with a morphological scaling of overall synapse size, but also featured a dramatic shift toward synaptic drive contributed by the Ca2+-permeable homomeric GluR1 receptors. These results extend beyond "synaptic homeostasis" to involve more profound changes that can be better described as "metaplasticity".

Voltage-gated calcium channels in developing GnRH-1 neuronal system in the mouse

Migration of gonadotropin-releasing hormone-1 (GnRH-1) neurons from the nasal placode into the central nervous system occurs in all vertebrates. This study characterizes the expression of L- and N-type voltage-gated calcium channels (VGCCs) in migrating GnRH-1 neurons in mice. Class C (L-type) and class B (N-type) VGCGs were detected in GnRH-1 cells and cells in the olfactory and vomeronasal epithelium during prenatal development. This expression pattern was mimicked in a nasal explant model known to retain many characteristics of GnRH-1 development in vivo. Using this in vitro system, perturbation studies were performed to elucidate the role of VGCCs in GnRH-1 neuronal development. This report shows that olfactory axon outgrowth and GnRH-1 neuronal migration are attenuated when nasal explants are grown in calcium-free media, and that this effect is temporally restricted to an early developmental period. Blockade of either the L- or the N-type channel did not alter GnRH-1 cell number or overall olfactory axon outgrowth. However, blockade of N-type channels altered the distribution of GnRH-1 neurons in the periphery of the nasal explants. In these explants, more GnRH-1 neurons were located proximal to, and fewer GnRH-1 neurons distal to, the main tissue mass, suggesting a general decrease in the rate of GnRH-1 neuronal migration. These results indicate that extracellular calcium is required for initiating GnRH-1 neuronal migration and that these events are partially dependent on N-type VGCC signals.