Ca2+ Channels As Integrators of G Protein-Mediated Signaling in Neurons

Cortisol Interaction with Aquaporin-2 Modulates Its Water Permeability: Perspectives for Non-Genomic Effects of Corticosteroids

Aquaporins (AQPs) are water channels widely distributed in living organisms and involved in many pathophysiologies as well as in cell volume regulations (CVR). In the present study, based on the structural homology existing between mineralocorticoid receptors (MRs), glucocorticoid receptors (GRs), cholesterol consensus motif (CCM) and the extra-cellular vestibules of AQPs, we investigated the binding of corticosteroids on the AQP family through in silico molecular dynamics simulations of AQP2 interactions with cortisol. We propose, for the first time, a putative AQPs corticosteroid binding site (ACBS) and discussed its conservation through structural alignment. Corticosteroids can mediate non-genomic effects; nonetheless, the transduction pathways involved are still misunderstood. Moreover, a growing body of evidence is pointing toward the existence of a novel membrane receptor mediating part of these rapid corticosteroids' effects. Our results suggest that the naturally produced glucocorticoid cortisol inhibits channel water permeability. Based on these results, we propose a detailed description of a putative underlying molecular mechanism. In this process, we also bring new insights on the regulatory function of AQPs extra-cellular loops and on the role of ions in tuning the water permeability. Altogether, this work brings new insights into the non-genomic effects of corticosteroids through the proposition of AQPs as the membrane receptor of this family of regulatory molecules. This original result is the starting point for future investigations to define more in-depth and in vivo the validity of this functional model.

Taste Cells of the Type III Employ CASR to Maintain Steady Serotonin Exocytosis at Variable Ca2+ in the Extracellular Medium

Type III taste cells are the only taste bud cells which express voltage-gated (VG) Ca²⁺ channels and employ Ca²⁺-dependent exocytosis to release neurotransmitters, particularly serotonin. The taste bud is a tightly packed cell population, wherein extracellular Ca²⁺ is expected to fluctuate markedly due to the electrical activity of taste cells. It is currently unclear whether the Ca²⁺ entry-driven synapse in type III cells could be reliable enough at unsteady extracellular Ca². Here we assayed depolarization-induced Ca²⁺ signals and associated serotonin release in isolated type III cells at varied extracellular Ca²⁺. It turned out that the same depolarizing stimulus elicited invariant Ca²⁺ signals in type III cells irrespective of bath Ca²⁺ varied within 0.5–5 mM. The serotonin release from type III cells was assayed with the biosensor approach by using HEK-293 cells co-expressing the recombinant 5-HT4 receptor and genetically encoded cAMP sensor Pink Flamindo. Consistently with the weak Ca²⁺ dependence of intracellular Ca²⁺ transients produced by VG Ca²⁺ entry, depolarization-triggered serotonin secretion varied negligibly with bath Ca²⁺. The evidence implicated the extracellular Ca²⁺-sensing receptor in mediating the negative feedback mechanism that regulates VG Ca²⁺ entry and levels off serotonin release in type III cells at deviating Ca²⁺ in the extracellular medium.

cAMP signaling through protein kinase A and Epac2 induces substance P release in the rat spinal cord

Using neurokinin 1 receptor (NK1R) internalization to measure of substance P release in rat spinal cord slices, we found that it was induced by the adenylyl cyclase (AC) activator forskolin, by the protein kinase A (PKA) activators 6-Bnz-cAMP and 8-Br-cAMP, and by the activator of exchange protein activated by cAMP (Epac) 8-pCPT-2-O-Me-cAMP (CPTOMe-cAMP). Conversely, AC and PKA inhibitors decreased substance P release induced by electrical stimulation of the dorsal root. Therefore, the cAMP signaling pathway mediates substance P release in the dorsal horn. The effects of forskolin and 6-Bnz-cAMP were not additive with NMDA-induced substance P release and were decreased by the NMDA receptor blocker MK-801. In cultured dorsal horn neurons, forskolin increased NMDA-induced Ca²⁺ entry and the phosphorylation of the NR1 and NR2B subunits of the NMDA receptor. Therefore, cAMP-induced substance P release is mediated by the activating phosphorylation by PKA of NMDA receptors. Voltage-gated Ca²⁺ channels, but not by TRPV1 or TRPA1, also contributed to cAMP-induced substance P release. Activation of PKA was required for the effects of forskolin and the three cAMP analogs. Epac2 contributed to the effects of forskolin and CPTOMe-cAMP, signaling through a Raf - mitogen-activated protein kinase pathway to activate Ca²⁺ channels. Epac1 inhibitors induced NK1R internalization independently of substance P release. In rats with latent sensitization to pain, the effect of 6-Bnz-cAMP was unchanged, whereas the effect of forskolin was decreased due to the loss of the stimulatory effect of Epac2. Hence, substance P release induced by cAMP decreases during pain hypersensitivity.

Impact of Perfluorooctane Sulfonate on Reproductive Ability of Female Mice through Suppression of Estrogen Receptor α-Activated Kisspeptin Neurons

Perfluorooctane sulfonate (PFOS) is used extensively in industrial and household applications. High exposure to PFOS has been associated with increased odds of irregular and long menstrual cycles in women. However, the underlying mechanisms remain to be elucidated. Herein, we show that adult female mice appeared prolongation of diestrus and reduction of corpora luteum within a week of oral administration of PFOS (10 mg/kg), which are associated with decreases in the levels of serum progesterone, LH and hypothalamic GnRH. The number of AVPV-kisspeptin neurons and the AVPV-kisspeptin expression were increased in proestrus mice or OVX-mice treated with high-dose estradiol benzoate (0.05 mg/kg), which were suppressed by the administration of PFOS. The administration of PFOS or GPR54 antagonist P234 prevented the generation of LH-surge in OVX-mice treated with high-dose E2. In hypothalamic slices incubated in 100 nM E2 for 4 h, the AVPV-kisspeptin expression was significantly enhanced, which was inhibited by PFOS in a dose-dependent manner or estrogen receptor α (ERα) antagonist MPP, but not ERβ antagonist PHTPP. The incubation of ERα agonist PPT rather than ERβ agonist DPN could increase the level of AVPV-kisspeptin expression, which was sensitive to the treatment with PFOS. The administration of GPR54 agonist kisspeptin-10 in PFOS-mice could correct the prolongation of diestrus and reduction of corpora luteum, and recover the LH-surge and the levels of LH and GnRH. The results indicate that exposure to PFOS suppressed ERα-induced activation of AVPV-kisspeptin neurons leads to diestrus prolongation and ovulation reduction.

Presynaptic Calcium Channels

Presynaptic Ca²⁺ entry occurs through voltage-gated Ca²⁺ (Ca_V) channels which are activated by membrane depolarization. Depolarization accompanies neuronal firing and elevation of Ca²⁺ triggers neurotransmitter release from synaptic vesicles. For synchronization of efficient neurotransmitter release, synaptic vesicles are targeted by presynaptic Ca²⁺ channels forming a large signaling complex in the active zone. The presynaptic Ca_V2 channel gene family (comprising Ca_V2.1, Ca_V2.2, and Ca_V2.3 isoforms) encode the pore-forming α1 subunit. The cytoplasmic regions are responsible for channel modulation by interacting with regulatory proteins. This article overviews modulation of the activity of Ca_V2.1 and Ca_V2.2 channels in the control of synaptic strength and presynaptic plasticity.

Mechanisms of μ-opioid receptor inhibition of NMDA receptor-induced substance P release in the rat spinal cord

The interaction between NMDA receptors and μ-opioid receptors in primary afferent terminals was studied by using NMDA to induce substance P release, measured as neurokinin 1 receptor internalization. In rat spinal cord slices, the μ-opioid receptor agonists morphine, DAMGO and endomorphin-2 inhibited NMDA-induced substance P release, whereas the antagonist CTAP right-shifted the concentration response of DAMGO. In vivo, substance P release induced by intrathecal NMDA after priming with BDNF was inhibited by DAMGO. ω-Conotoxins MVIIC and GVIA inhibited about half of the NMDA-induced substance P release, showing that it was partially mediated by the opening of voltage-gated calcium (Cav) channels. In contrast, DAMGO or ω-conotoxins did not inhibit capsaicin-induced substance P release. In cultured DRG neurons, DAMGO but not ω-conotoxin inhibited NMDA-induced increases in intracellular calcium, indicating that μ-opioid receptors can inhibit NMDA receptor function by mechanisms other than inactivation of Cav channels. Moreover, DAMGO decreased the ω-conotoxin-insensitive component of the substance P release. Potent inhibition by ifenprodil showed that these NMDA receptors have the NR2B subunit. Activators of adenylyl cyclase and protein kinase A (PKA) induced substance P release and this was decreased by the NMDA receptor blocker MK-801 and by DAMGO. Conversely, inhibitors of adenylyl cyclase and PKA, but not of protein kinase C, decreased NMDA-induced substance P release. Hence, these NMDA receptors are positively modulated by the adenylyl cyclase-PKA pathway, which is inhibited by μ-opioid receptors. In conclusion, μ-opioid receptors inhibit NMDA receptor-induced substance P release through Cav channel inactivation and adenylyl cyclase inhibition.

Native metabotropic glutamate receptor 4 depresses synaptic transmission through an unusual Gαq transduction pathway

In cerebellar cortex, mGlu₄ receptors located on parallel fibers play an essential role in normal motor function, but the molecular mechanisms involved are not yet completely understood. Using a strategy combining biochemical and electrophysiological approaches in the rodent cerebellum, we demonstrate that presynaptic mGlu₄ receptors control synaptic transmission through an atypical activation of Gα_q proteins. First, the Gα_q subunit, PLC and PKC signaling proteins present in cerebellar extracts are retained on affinity chromatography columns grafted with different sequences of the cytoplasmic domain of mGlu₄ receptor. The i2 loop and the C terminal domain were used as baits, two domains that are known to play a pivotal role in coupling selectivity and efficacy. Second, in situ proximity ligation assays show that native mGlu₄ receptors and Gα_q subunits are in close physical proximity in cerebellar cortical slices. Finally, electrophysiological experiments demonstrate that the molecular mechanisms underlying mGlu₄ receptor-mediated inhibition of transmitter release at cerebellar Parallel Fiber (PF) - Molecular Layer Interneuron (MLI) synapses involves the Gα_q-PLC signaling pathway. Taken together, our results provide compelling evidence that, in the rodent cerebellar cortex, mGlu₄ receptors act by coupling to the Gα_q protein and PLC effector system to reduce glutamate synaptic transmission.

Pedunculopontine Gamma Band Activity and Development

This review highlights the most important discovery in the reticular activating system in the last 10 years, the manifestation of gamma band activity in cells of the reticular activating system (RAS), especially in the pedunculopontine nucleus, which is in charge of waking and rapid eye movement (REM) sleep. The identification of different cell groups manifesting P/Q-type Ca(2+) channels that control waking vs. those that manifest N-type channels that control REM sleep provides novel avenues for the differential control of waking vs. REM sleep. Recent discoveries on the development of this system can help explain the developmental decrease in REM sleep and the basic rest-activity cycle.

μ-Opioid receptor inhibition of substance P release from primary afferents disappears in neuropathic pain but not inflammatory pain

Opiate analgesia in the spinal cord is impaired during neuropathic pain. We hypothesized that this is caused by a decrease in μ-opioid receptor inhibition of neurotransmitter release from primary afferents. To investigate this possibility, we measured substance P release in the spinal dorsal horn as neurokinin 1 receptor (NK1R) internalization in rats with chronic constriction injury (CCI) of the sciatic nerve. Noxious stimulation of the paw with CCI produced inconsistent NK1R internalization, suggesting that transmission of nociceptive signals by the injured nerve was variably impaired after CCI. This idea was supported by the fact that CCI produced only small changes in the ability of exogenous substance P to induce NK1R internalization or in the release of substance P evoked centrally from site of nerve injury. In subsequent experiments, NK1R internalization was induced in spinal cord slices by stimulating the dorsal root ipsilateral to CCI. We observed a complete loss of the inhibition of substance P release by the μ-opioid receptor agonist [D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO) in CCI rats but not in sham-operated rats. In contrast, DAMGO still inhibited substance P release after inflammation of the hind paw with complete Freund's adjuvant and in naïve rats. This loss of inhibition was not due to μ-opioid receptor downregulation in primary afferents, because their colocalization with substance P was unchanged, both in dorsal root ganglion neurons and primary afferent fibers in the dorsal horn. In conclusion, nerve injury eliminates the inhibition of substance P release by μ-opioid receptors, probably by hindering their signaling mechanisms.

Cell adhesion and intracellular calcium signaling in neurons

Cell adhesion molecules (CAMs) play indispensable roles in the developing and mature brain by regulating neuronal migration and differentiation, neurite outgrowth, axonal fasciculation, synapse formation and synaptic plasticity. CAM-mediated changes in neuronal behavior depend on a number of intracellular signaling cascades including changes in various second messengers, among which CAM-dependent changes in intracellular Ca²⁺ levels play a prominent role. Ca²⁺ is an essential secondary intracellular signaling molecule that regulates fundamental cellular functions in various cell types, including neurons. We present a systematic review of the studies reporting changes in intracellular Ca²⁺ levels in response to activation of the immunoglobulin superfamily CAMs, cadherins and integrins in neurons. We also analyze current experimental evidence on the Ca²⁺ sources and channels involved in intracellular Ca²⁺ increases mediated by CAMs of these families, and systematically review the role of the voltage-dependent Ca²⁺ channels (VDCCs) in neurite outgrowth induced by activation of these CAMs. Molecular mechanisms linking CAMs to VDCCs and intracellular Ca²⁺ stores in neurons are discussed.

Sex-specific signaling in the blood-brain barrier is required for male courtship in Drosophila

Soluble circulating proteins play an important role in the regulation of mating behavior in Drosophila melanogaster. However, how these factors signal through the blood–brain barrier (bbb) to interact with the sex-specific brain circuits that control courtship is unknown. Here we show that male identity of the blood–brain barrier is necessary and that male-specific factors in the bbb are physiologically required for normal male courtship behavior. Feminization of the bbb of adult males significantly reduces male courtship. We show that the bbb–specific G-protein coupled receptor moody and bbb–specific Go signaling in adult males are necessary for normal courtship. These data identify sex-specific factors and signaling processes in the bbb as important regulators of male mating behavior.

Complex behaviors such as mating behavior are controlled by the brain. Ensembles of brain cells work in networks to ensure proper behavior at the right time. While the state of these cells plays an important role in whether and how the behavior is displayed, information from outside the brain is also required. Often, this information is provided by hormones that are present in the circulating fluid (such as the blood). However, the brain is protected by a layer of very tight cells, the so-called blood–brain barrier, that keeps unwanted molecules out. So how then do hormones and other regulatory factors “talk” to the brain? We are studying this question by examining the mating behavior of males of a model organism, the fruit fly Drosophila melanogaster. We have found that the blood–brain barrier cells themselves contain male-specific molecules that play an important role. When they are absent, courtship behavior is compromised. We have also identified how outside factors talk to the brain: by using a cellular signaling protein and a particular signaling pathway. Together they are well suited to pass on outside information to the brain network that regulates mating behavior.

Support for calcium channel gene defects in autism spectrum disorders

BACKGROUND

Alternation of synaptic homeostasis is a biological process whose disruption might predispose children to autism spectrum disorders (ASD). Calcium channel genes (CCG) contribute to modulating neuronal function and evidence implicating CCG in ASD has been accumulating. We conducted a targeted association analysis of CCG using existing genome-wide association study (GWAS) data and imputation methods in a combined sample of parent/affected child trios from two ASD family collections to explore this hypothesis.

METHODS

A total of 2,176 single-nucleotide polymorphisms (SNP) (703 genotyped and 1,473 imputed) covering the genes that encode the α1 subunit proteins of 10 calcium channels were tested for association with ASD in a combined sample of 2,781 parent/affected child trios from 543 multiplex Caucasian ASD families from the Autism Genetics Resource Exchange (AGRE) and 1,651 multiplex and simplex Caucasian ASD families from the Autism Genome Project (AGP). SNP imputation using IMPUTE2 and a combined reference panel from the HapMap3 and the 1,000 Genomes Project increased coverage density of the CCG. Family-based association was tested using the FBAT software which controls for population stratification and accounts for the non-independence of siblings within multiplex families. The level of significance for association was set at 2.3E-05, providing a Bonferroni correction for this targeted 10-gene panel.

RESULTS

Four SNPs in three CCGs were associated with ASD. One, rs10848653, is located in CACNA1C, a gene in which rare de novo mutations are responsible for Timothy syndrome, a Mendelian disorder that features ASD. Two others, rs198538 and rs198545, located in CACN1G, and a fourth, rs5750860, located in CACNA1I, are in CCGs that encode T-type calcium channels, genes with previous ASD associations.

CONCLUSIONS

These associations support a role for common CCG SNPs in ASD.

Norepinephrine: a neuromodulator that boosts the function of multiple cell types to optimize CNS performance

Norepinephrine (NE) is a neuromodulator that in multiple ways regulates the activity of neuronal and non-neuronal cells. NE participates in the rapid modulation of cortical circuits and cellular energy metabolism, and on a slower time scale in neuroplasticity and inflammation. Of the multiple sources of NE in the brain, the locus coeruleus (LC) plays a major role in noradrenergic signaling. Processes from the LC primarily release NE over widespread brain regions via non-junctional varicosities. We here review the actions of NE in astrocytes, microglial cells, and neurons based on the idea that the overarching effect of signaling from the LC is to maximize brain power, which is accomplished via an orchestrated cellular response involving most, if not all cell types in CNS.

G(o) activation is required for both appetitive and aversive memory acquisition in Drosophila

Heterotrimeric G(o) is an abundant brain protein required for negatively reinforced short-term associative olfactory memory in Drosophila. G(o) is the only known substrate of the S1 subunit of pertussis toxin (PTX) in fly, and acute expression of PTX within the mushroom body neurons (MB) induces a reversible deficit in associative olfactory memory. We demonstrate here that the induction of PTX within the α/β and γ lobe MB neurons leads to impaired memory acquisition without affecting memory stability. The induction of PTX within these MB neurons also leads to a significant defect in an optimized positively reinforced short-term memory paradigm; however, this PTX-induced learning deficit is noticeably less severe than found with the negatively reinforced paradigm. Both negatively and positively reinforced memory phenotypes are rescued by the constitutive expression of G(o)α transgenes bearing the Cys(351)Ile mutation. Since this mutation renders the G(o) molecule insensitive to PTX, the results isolate the effect of PTX on both forms of olfactory associative learning to the inhibition of the G(o) activation.

L-histidine sensing by calcium sensing receptor inhibits voltage-dependent calcium channel activity and insulin secretion in β-cells

AIMS

Our goal was to test the hypothesis that the histidine-induced activation of calcium sensing receptor (CaR) can regulate calcium channel activity of L-type voltage dependent calcium channel (VDCC) due to increased spatial interaction between CaR and VDCC in β-cells and thus modulate glucose-induced insulin secretion.

MAIN METHODS

Rat insulinoma (RINr1046-38) insulin-producing β-cells were cultured in RPMI-1640 medium on 25 mm diameter glass coverslips in six-well culture plates in a 5% CO(2) incubator at 37°C. The intracellular calcium concentration, [Ca(2+)](i), was determined by ratio fluorescence microscopy using Fura-2AM. The spatial interactions between CaR and L-type VDCC in β-cells were measured by immunofluorescence confocal microscopy using a Nikon C1 laser scanning confocal microscope. The insulin release was determined by enzyme-linked immunosorbent assay (ELISA).

KEY FINDINGS

The addition of increasing concentrations of L-histidine along with 10 mM glucose resulted in 57% decrease in [Ca(2+)](i). The confocal fluorescence imaging data showed 5.59 to 8.62-fold increase in colocalization correlation coefficient between CaR and VDCC in β-cells exposed to L-histidine thereby indicating increased membrane delimited spatial interactions between these two membrane proteins. The insulin ELISA data showed 54% decrease in the 1st phase of glucose-induced insulin secretion in β-cells exposed to increasing concentrations of L-histidine.

SIGNIFICANCE

L-histidine-induced increased spatial interaction of CaR with VDCC can inhibit calcium channel activity of VDCC and consequently regulate glucose-induced insulin secretion by β-cells. The L-type VDCC could therefore be a potential therapeutic target in diabetes.

Glucose-mediated spatial interactions of voltage dependent calcium channels and calcium sensing receptor in insulin producing β-cells

AIMS

The voltage dependent calcium channel (VDCC) e.g., L-type VDCC plays critical roles in the spatio-temporal regulation of intracellular calcium concentration ([Ca(2+)](i)) and insulin secretion by β-cell. This study describes the involvement of 2.5 to 15mM glucose-induced spatial interactions between a calcium sensing receptor (CaR) and L-type VDCC in controlling Ca(2+) channel activity and insulin secretion in β-cells in association with the nuclear translocation of a transcription factor nuclear factor kappa B (NF-κB).

MAIN METHODS

The insulin producing β-cells were exposed to 2.5, 5, 7.5, 10, and 15 mM glucose for 24 h at 37 °C. The confocal fluorescence imaging data was obtained by using antibodies against CaR and L-type VDCC. The nuclear translocation of NF-κB was measured by confocal fluorescence imaging using antibody against NF-κB. The insulin release was determined by enzyme-linked immunosorbent assay (ELISA).

KEY FINDINGS

The confocal imaging data showed 6 to 12-fold enhancement in the colocalization correlation coefficient between CaR and VDCC in β-cells exposed to glucose thereby indicating increased membrane delimited spatial interactions between these two membrane proteins. The confocal fluorescence imaging data showed that addition of glucose to β-cells led to 1.8 to 2.7-fold increase in the nuclear translocation of NF-κB. The insulin ELISA data showed a significant increase in the 1st phase of glucose-induced insulin secretion in β-cells exposed to increasing concentrations of glucose.

SIGNIFICANCE

The results described in the present study further strengthen that VDCC and CaR can interact spatially to allow control over calcium channel activity and therefore glucose-induced insulin secretion by β-cells.

The ß subunit of voltage-gated Ca2+ channels

Calcium regulates a wide spectrum of physiological processes such as heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca(2+) in excitable cells are high-voltage activated (HVA) Ca(2+) channels. These are plasma membrane proteins composed of several subunits, including α(1), α(2)δ, β, and γ. Although the principal α(1) subunit (Ca(v)α(1)) contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit (Ca(v)β) plays an essential role in regulating the surface expression and gating properties of HVA Ca(2+) channels. Ca(v)β is also crucial for the modulation of HVA Ca(2+) channels by G proteins, kinases, and the Ras-related RGK GTPases. New proteins have emerged in recent years that modulate HVA Ca(2+) channels by binding to Ca(v)β. There are also indications that Ca(v)β may carry out Ca(2+) channel-independent functions, including directly regulating gene transcription. All four subtypes of Ca(v)β, encoded by different genes, have a modular organization, consisting of three variable regions, a conserved guanylate kinase (GK) domain, and a conserved Src-homology 3 (SH3) domain, placing them into the membrane-associated guanylate kinase (MAGUK) protein family. Crystal structures of Ca(v)βs reveal how they interact with Ca(v)α(1), open new research avenues, and prompt new inquiries. In this article, we review the structure and various biological functions of Ca(v)β, with both a historical perspective as well as an emphasis on recent advances.

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

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

The central nervous system as target for antihypertensive actions of a proline-rich peptide from Bothrops jararaca venom

Pyroglutamyl proline-rich oligopeptides, present in the venom of the pit viper Bothrops jararaca (Bj-PROs), are the first described naturally occurring inhibitors of the angiotensin I-converting enzyme (ACE). The inhibition of ACE by the decapeptide Bj-PRO-10c (<ENWPHPQIPP) and other Bj-PROs was classically used to explain the pharmacological effects of these venom peptides in mammals resulting in a decrease of blood pressure. Recent studies, however, suggest that ACE inhibition alone is not sufficient for explaining the antihypertensive actions exerted by these peptides. In this study, we show that intracerebroventricular injection of Bj-PRO-10c induced a significant reduction of mean arterial pressure (MAP) together with a decrease of heart rate (HR) in spontaneously hypertensive rats, indicating that Bj-PRO-10c may act on the central nervous system. In agreement with its supposed neuronal action, this peptide dose-dependently evoked elevations of intracellular calcium concentration ([Ca(2+)](i)) in primary culture from postnatal rat brain. The N-terminal sequence of the peptide was not essential for induction of calcium fluxes, while any changes of C-terminal Pro or Ile residues affected Bj-PRO-10c's activity. Using calcium imaging by confocal microscopy and fluorescence imaging plate reader analysis, we have characterized Bj-PRO-10c-induced [Ca(2+)](i) transients in rat brain cells as being independent from bradykinin-mediated effects and ACE inhibition. Bj-PRO-10c induced pertussis toxin-sensitive G(i/o)-protein activity mediated through a yet unknown receptor, influx and liberation ofcalcium from intracellular stores, as well as reduction of intracellular cAMP levels. Bj-PRO-10c promoted glutamate and GABA release that may be responsible for its antihypertensive activity and its effect on HR.

Cannabinoid CB1 receptor facilitation of substance P release in the rat spinal cord, measured as neurokinin 1 receptor internalization

The contribution of CB1 receptors in the spinal cord to cannabinoid analgesia is still unclear. The objective of this study was to investigate the effect of CB1 receptors on substance P release from primary afferent terminals in the spinal cord. Substance P release was measured as neurokinin 1 (NK1) receptor internalization in lamina I neurons. It was induced in spinal cord slices by dorsal root stimulation and in live rats by a noxious stimulus. In spinal cord slices, the CB1 receptor antagonists AM251, AM281 and rimonabant partially but potently inhibited NK1 receptor internalization induced by electrical stimulation of the dorsal root. This was due to an inhibition of substance P release and not of NK1 receptor internalization itself, because AM251 and AM281 did not inhibit NK1 receptor internalization induced by exogenous substance P. The CB1 receptor agonist ACEA increased NK1 receptor internalization evoked by dorsal root stimulation. The effects of AM251 and ACEA cancelled each other. In vivo, AM251 injected intrathecally decreased NK1 receptor internalization in spinal segments L5 and L6 induced by noxious hind paw clamp. Intrathecal AM251 also produced analgesia to radiant heat stimulation of the paw. The inhibition by AM251 of NK1 receptor internalization was reversed by antagonists of mu-opioid and GABA(B) receptors. This indicates that CB1 receptors facilitate substance P release by inhibiting the release of GABA and opioids next to primary afferent terminals, producing disinhibition. This results in a pronociceptive effect of CB1 receptors in the spinal cord.

In search of analgesia: emerging roles of GPCRs in pain

Of all clinically marketed drugs, greater than thirty percent are modulators of G protein-coupled receptors (GPCRs). Nearly 400 GPCRs (i.e., excluding odorant and light receptors) are encoded within the human genome, but only a small fraction of these seven-transmembrane proteins have been identified as drug targets. Chronic pain affects more than one-third of the population, representing a substantial societal burden in use of health care resources and lost productivity. Furthermore, currently available treatments are often inadequate, underscoring the significant need for better therapeutic strategies. The expansion of the identified human GPCR repertoire, coupled with recent insights into the function and structure of GPCRs, offers new opportunities for the development of novel analgesic therapeutics.

Analgesic alpha-conotoxins Vc1.1 and Rg1A inhibit N-type calcium channels in rat sensory neurons via GABAB receptor activation

alpha-Conotoxins Vc1.1 and Rg1A are peptides from the venom of marine Conus snails that are currently in development as a treatment for neuropathic pain. Here we report that the alpha9alpha10 nicotinic acetylcholine receptor-selective conotoxins Vc1.1 and Rg1A potently and selectively inhibit high-voltage-activated (HVA) calcium channel currents in dissociated DRG neurons in a concentration-dependent manner. The post-translationally modified peptides vc1a and [P6O]Vc1.1 were inactive, as were all other alpha-conotoxins tested. Vc1.1 inhibited the omega-conotoxin-sensitive HVA currents in DRG neurons but not those recorded from Xenopus oocytes expressing Ca(V)2.2, Ca(V)2.1, Ca(V)2.3, or Ca(V)1.2 channels. Inhibition of HVA currents by Vc1.1 was not reversed by depolarizing prepulses but was abolished by pertussis toxin (PTX), intracellular GDPbetaS, or a selective inhibitor of pp60c-src tyrosine kinase. These data indicate that Vc1.1 does not interact with N-type calcium channels directly but inhibits them via a voltage-independent mechanism involving a PTX-sensitive, G-protein-coupled receptor. Preincubation with a variety of selective receptor antagonists demonstrated that only the GABA(B) receptor antagonists, [S-(R*,R*)][-3-[[1-(3,4-dichlorophenyl)ethyl]amino]-2-hydroxy propyl]([3,4]-cyclohexylmethyl) phosphinic acid hydrochloride (2S)-3[[(1S)-1-(3,4-dichlorophenyl)-ethyl]amino-2-hydroxypropyl](phenylmethyl) phosphinic acid and phaclofen, blocked the effect of Vc1.1 and Rg1A on Ca2+ channel currents. Together, the results identify Ca(V)2.2 as a target of Vc1.1 and Rg1A, potentially mediating their analgesic actions. We propose a novel mechanism by which alpha-conotoxins Vc1.1 and Rg1A modulate native N-type (Ca(V)2.2) Ca2+ channel currents, namely acting as agonists via G-protein-coupled GABA(B) receptors.

Caveolin-1 alters Ca(2+) signal duration through specific interaction with the G alpha q family of G proteins

Caveolae are membrane domains having caveolin-1 (Cav1) as their main structural component. Here, we determined whether Cav1 affects Ca(2+) signaling through the Galpha(q)-phospholipase-Cbeta (PLCbeta) pathway using Fischer rat thyroid cells that lack Cav1 (FRTcav(-)) and a sister line that forms caveolae-like domains due to stable transfection with Cav1 (FRTcav(+)). In the resting state, we found that eCFP-Gbetagamma and Galpha(q)-eYFP are similarly associated in both cell lines by Forster resonance energy transfer (FRET). Upon stimulation, the amount of FRET between Galpha(q)-eYFP and eCFP-Gbetagamma remains high in FRTcav(-) cells, but decreases almost completely in FRTcav(+) cells, suggesting that Cav1 is increasing the separation between Galpha(q)-Gbetagamma subunits. In FRTcav(-) cells overexpressing PLCbeta, a rapid recovery of Ca(2+) is observed after stimulation. However, FRTcav(+) cells show a sustained level of elevated Ca(2+). FRET and colocalization show specific interactions between Galpha(q) and Cav1 that increase upon stimulation. Fluorescence correlation spectroscopy studies show that the mobility of Galpha(q)-eGFP is unaffected by activation in either cell type. The mobility of eGFP-Gbetagamma remains slow in FRTcav(-) cells but increases in FRTcav(+) cells. Together, our data suggest that, upon stimulation, Galpha(q)(GTP) switches from having strong interactions with Gbetagamma to Cav1, thereby releasing Gbetagamma. This prolongs the recombination time for the heterotrimer, thus causing a sustained Ca(2+) signal.

Presynaptic signaling by heterotrimeric G-proteins

G-proteins (guanine nucleotide-binding proteins) are membrane-attached proteins composed of three subunits, alpha, beta, and gamma. They transduce signals from G-protein coupled receptors (GPCRs) to target effector proteins. The agonistactivated receptor induces a conformational change in the G-protein trimer so that the alpha-subunit binds GTP in exchange for GDP and alpha-GTP, and betagamma-subunits separate to interact with the target effector. Effector-interaction is terminated by the alpha-subunit GTPase activity, whereby bound GTP is hydrolyzed to GDP. This is accelerated in situ by RGS proteins, acting as GTPase-activating proteins (GAPs). Galpha-GDP and Gbetagamma then reassociate to form the Galphabetagamma trimer. G-proteins primarily involved in the modulation of neurotransmitter release are G(o), G(q) and G(s). G(o) mediates the widespread presynaptic auto-inhibitory effect of many neurotransmitters (e.g., via M2/M4 muscarinic receptors, alpha(2) adrenoreceptors, micro/delta opioid receptors, GABAB receptors). The G(o) betagamma-subunit acts in two ways: first, and most ubiquitously, by direct binding to CaV2 Ca(2+) channels, resulting in a reduced sensitivity to membrane depolarization and reduced Ca(2+) influx during the terminal action potential; and second, through a direct inhibitory effect on the transmitter release machinery, by binding to proteins of the SNARE complex. G(s) and G(q) are mainly responsible for receptor-mediated facilitatory effects, through activation of target enzymes (adenylate cyclase, AC and phospholipase-C, PLC respectively) by the GTP-bound alpha-subunits.

Involvement of protein kinase C and IP3-mediated Ca2+ release in activity modulation by paraoxon in snail neurons

We have previously reported that paraoxon, an organophosphate compound, at submicromolar concentrations effectively suppresses Ca2+ action potentials and modulates the activity of snail neurons. This effect was unrelated to acetylcholinesterase inhibition but was found to involve the direct or indirect modulation of ion channels [Vatanparast, J., Janahmadi, M., Asgari, A.R., Sepehri, H., Haeri-Rohani, A., 2006a. Paraoxon suppresses Ca2+ action potential and afterhyperpolarization in snail neurons: Relevance to the hyperexcitability induction. Brain Res. 1083 (1), 110-117]. In the present work, the interaction of paraoxon with protein kinase C (PKC) and inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+ release, on the modulation of Ca2+ action potentials and neuronal activity was investigated. Phorbol 12, 13 dibutyrate (PdBu), the activator of PKC, suppressed afterhyperpolarization and increased the activity of snail neurons without any significant effect on the Ca2+ action potential duration. Pretreatment with PKC activator attenuated the suppressing effect of paraoxon on the duration of Ca2+ action potentials. Staurosporine, a selective blocker of PKC, did not block the effect of paraoxon on Ca2+ action potential suppression and hyperexcitability induction. Our findings did not support the involvement PKC in the paraoxon induced Ca2+ action potential suppression and neuronal activity modulation, although activation of this protein kinase could attenuate some effects of paraoxon. Pretreatment with 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-8), an antagonist of IP3-mediated Ca2+ release, abolished the secondary silencing effect of paraoxon, which is observed after primary paraoxon-induced hyperexcitability. It was concluded that slow activation of intracellular cascades by paraoxon could induce an IP3 mediated Ca2+ release from intracellular stores and participate to its secondary silencing effect by mechanisms dependent on intracellular calcium homeostasis.

Different kinases regulate activation of voltage-dependent calcium channels by depolarization in GH3 cells

The L-type Ca2+ channel is the primary voltage-dependent Ca2+-influx pathway in many excitable and secretory cells, and direct phosphorylation by different kinases is one of the mechanisms involved in the regulation of its activity. The aim of this study was to evaluate the participation of Ser/Thr kinases and tyrosine kinases (TKs) in depolarization-induced Ca2+ influx in the endocrine somatomammotrope cell line GH3. Intracellular Ca2+ concentration ([Ca2+]i) was measured using a spectrofluorometric method with fura 2-AM, and 12.5 mM KCl (K+) was used as a depolarization stimulus. K+ induced an abrupt spike (peak) in [Ca2+]i that was abolished in the presence of nifedipine, showing that K+ enhances [Ca2+]i, preferably activating L-type Ca2+ channels. H89, a selective PKA inhibitor, significantly reduced depolarization-induced Ca2+ mobilization in a concentration-related manner when it was applied before or after K+, and okadaic acid, an inhibitor of Ser/Thr phosphatases, which has been shown to regulate PKA-stimulated L-type Ca2+ channels, increased K+-induced Ca2+ entry. When PKC was activated by PMA, the K+-evoked peak in [Ca2+]i, as well as the plateau phase, was significantly reduced, and chelerythrine (a PKC inhibitor) potentiated the K+-induced increase in [Ca2+]i, indicating an inhibitory role of PKC in voltage-dependent Ca2+ channel (VDCC) activity. Genistein, a TK inhibitor, reduced the K+-evoked increase in [Ca2+]i, but, unexpectedly, the tyrosine phosphatase inhibitor orthovanadate reduced not only basal Ca2+ levels but, also, Ca2+ influx during the plateau phase. Both results suggest that different TKs may act differentially on VDCC activation. Activation of receptor TKs with epidermal growth factor (EGF) or vascular endothelial growth factor potentiated K+-induced Ca2+ influx, and AG-1478 (an EGF receptor inhibitor) decreased it. However, inhibition of the non-receptor TK pp60 c-Src enhanced K+-induced Ca2+ influx. The present study strongly demonstrates that a complex equilibrium among different kinases and phosphatases regulates VDCC activity in the pituitary cell line GH3: PKA and receptor TKs, such as vascular endothelial growth factor receptor and EGF receptor, enhance depolarization-induced Ca2+ influx, whereas PKC and c-Src have an inhibitory effect. These kinases modulate membrane depolarization and may therefore participate in the regulation of a plethora of intracellular processes, such as hormone secretion, gene expression, protein synthesis, and cell proliferation, in pituitary cells.

Neuronal calcium channels: splicing for optimal performance

Calcium ion channels coordinate an astounding number of cellular functions. Surprisingly, only 10 Ca(V)alpha(1) subunit genes encode the structural cores of all voltage-gated calcium channels. What mechanisms exist to modify the structure of calcium channels and optimize their coupling to the rich spectrum of cellular functions? Growing evidence points to the contribution of post-translational alternative processing of calcium channel RNA as the main mechanism for expanding the functional potential of this important gene family. Alternative splicing of RNA is essential during neuronal development where fine adjustments in protein signaling promote and inhibit cell-cell interactions and underlie axonal guidance. However, attributing a specific functional role to an individual splice isoform or splice site has been difficult. In this regard, studies of ion channels are advantageous because their function can be monitored with precision, allowing even subtle changes in channel activity to be detected. Such studies are especially insightful when coupled with information about isoform expression patterns and cellular localization. In this paper, we focus on two sites of alternative splicing in the N-type calcium channel Ca(V)2.2 gene. We first describe cassette exon 18a that encodes a 21 amino acid segment in the II-III intracellular loop region of Ca(V)2.2. Here, we show that e18a is upregulated in the nervous system during development. We discuss these new data in light of our previous reports showing that e18a protects the N-type channel from cumulative inactivation. Second, we discuss our published data on exons e37a and e37b, which encode 32 amino acids in the intracellular C-terminus of Ca(V)2.2. These exons are expressed in a mutually exclusive manner. Exon e37a-containing Ca(V)2.2 mRNAs and their resultant channels express at higher density in dorsal root ganglia and, as we showed recently, e37a increases N-type channel sensitivity to G-protein-mediated inhibition, as compared to generic e37b-containing N-type channels.

Alternative splicing controls G protein-dependent inhibition of N-type calcium channels in nociceptors

Neurotransmitter release from mammalian sensory neurons is controlled by Ca(V)2.2 N-type calcium channels. N-type channels are a major target of neurotransmitters and drugs that inhibit calcium entry, transmitter release and nociception through their specific G protein-coupled receptors. G protein-coupled receptor inhibition of these channels is typically voltage-dependent and mediated by Gbetagamma, whereas N-type channels in sensory neurons are sensitive to a second G protein-coupled receptor pathway that inhibits the channel independent of voltage. Here we show that preferential inclusion in nociceptors of exon 37a in rat Cacna1b (encoding Ca(V)2.2) creates, de novo, a C-terminal module that mediates voltage-independent inhibition. This inhibitory pathway requires tyrosine kinase activation but not Gbetagamma. A tyrosine encoded within exon 37a constitutes a critical part of a molecular switch controlling N-type current density and G protein-mediated voltage-independent inhibition. Our data define the molecular origins of voltage-independent inhibition of N-type channels in the pain pathway.

Physical association between neuropeptide FF and micro-opioid receptors as a possible molecular basis for anti-opioid activity

Neuropeptide FF (NPFF) modulates the opioid system by exerting functional anti-opioid activity on neurons, the mechanism of which is unknown. By using a model of SH-SY5Y cells, we recently postulated that anti-opioid activity likely takes place upstream from the signaling cascade, suggesting that NPFF receptors could block opioid receptors by physical interaction. In the present study, fluorescence techniques were used to monitor the physical association and the dynamic of NPFF2 and micro-opioid (MOP) receptors tagged with variants of the green fluorescent protein. Importantly, cyan fluorescent protein-tagged NPFF2 receptors retained their capacity to antagonize opioid receptors. Fluorescence resonance energy transfer (FRET) and coimmunoprecipitation studies indicate that NPFF and MOP receptors are close enough to generate a basal FRET signal. The opioid agonist Tyr-D-Ala-Gly-NMe-Phe-Gly-ol disrupts by 20-30% this FRET signal, mainly because it concomitantly induces 40% internalization of receptors. In contrast, the NPFF analog 1DMe significantly increases by 10-15% the basal FRET signal, suggesting an association between both receptors. In addition, 1DMe reduces, by half, MOP receptor internalization, indicating that, besides a functional blockade of opioid receptors, the NPFF analog also inhibits their internalization. Finally, as a first report showing the modulation of the mobility of a G-protein-coupled receptor by another one, fluorescence recovery after photobleaching analysis reveals that 1DMe modifies the lateral diffusion of MOP receptors in the cell membrane, changing them from a confined to a freely diffusing state. By promoting NPFF-MOP receptor heteromerization, 1DMe could disrupt the domain organization of MOP receptors in the membrane, resulting in a reduction of opioid response.

Localization of synaptic proteins involved in neurosecretion in different membrane microdomains

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

The role of voltage-gated calcium channels in pancreatic beta-cell physiology and pathophysiology

Voltage-gated calcium (CaV) channels are ubiquitously expressed in various cell types throughout the body. In principle, the molecular identity, biophysical profile, and pharmacological property of CaV channels are independent of the cell type where they reside, whereas these channels execute unique functions in different cell types, such as muscle contraction, neurotransmitter release, and hormone secretion. At least six CaValpha1 subunits, including CaV1.2, CaV1.3, CaV2.1, CaV2.2, CaV2.3, and CaV3.1, have been identified in pancreatic beta-cells. These pore-forming subunits complex with certain auxiliary subunits to conduct L-, P/Q-, N-, R-, and T-type CaV currents, respectively. beta-Cell CaV channels take center stage in insulin secretion and play an important role in beta-cell physiology and pathophysiology. CaV3 channels become expressed in diabetes-prone mouse beta-cells. Point mutation in the human CaV1.2 gene results in excessive insulin secretion. Trinucleotide expansion in the human CaV1.3 and CaV2.1 gene is revealed in a subgroup of patients with type 2 diabetes. beta-Cell CaV channels are regulated by a wide range of mechanisms, either shared by other cell types or specific to beta-cells, to always guarantee a satisfactory concentration of Ca2+. Inappropriate regulation of beta-cell CaV channels causes beta-cell dysfunction and even death manifested in both type 1 and type 2 diabetes. This review summarizes current knowledge of CaV channels in beta-cell physiology and pathophysiology.

Presynaptic Ca2+ channels--integration centers for neuronal signaling pathways

Calcium influx into presynaptic nerve terminals via voltage-gated Ca2+ channels is an essential step in neurotransmitter release. The predominant Ca2+ channel species in synaptic nerve terminals are P/Q-type and N-type channels, with their relative levels of expression varying across the nervous system. The different distributions of these two channel subtypes are reflected in their distinct physiological and pathological roles, yet their activity is regulated by common mechanisms and both function as part of larger signaling complexes that enable their precise regulation and subcellular targeting. Here, we provide a broad overview of molecular and cellular mechanisms that regulate Ca2+ channels, and how these cellular signaling pathways are integrated at the level of the channel protein.

Molecular basis of inherited calcium channelopathies: role of mutations in pore-forming subunits

The pore-forming alpha subunits of voltage-gated calcium channels contain the essential biophysical machinery that underlies calcium influx in response to cell depolarization. In combination with requisite auxiliary subunits, these pore subunits form calcium channel complexes that are pivotal to the physiology and pharmacology of diverse cells ranging from sperm to neurons. Not surprisingly, mutations in the pore subunits generate diverse pathologies, termed channelopathies, that range from failures in excitation-contraction coupling to night blindness. Over the last decade, major insights into the mechanisms of pathogenesis have been derived from animals showing spontaneous or induced mutations. In parallel, there has been considerable growth in our understanding of the workings of voltage-gated ion channels from a structure-function, regulation and cell biology perspective. Here we document our current understanding of the mutations underlying channelopathies involving the voltage-gated calcium channel alpha subunits in humans and other species.

Lysophospholipids elevate [Ca2+]i and trigger exocytosis in bovine chromaffin cells

Sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) are responsible for many physiological functions, including angiogenesis, neuronal survival, and immunity. However, little is known about their effects in modulating the stimulus-secretion coupling in bovine chromaffin cells. The result of PCR showed that at least two receptors (S1P(3) and LPA(1)) were expressed in bovine chromaffin cells. The elevation of [Ca(2+)](i) by S1P was fast and sustaining; but the elevation by LPA was slow and transient. The EC(50) for S1P and LPA in elevating the [Ca(2+)](i) were 0.55+/-0.01 and 0.54+/-0.40microM, respectively. This elevation could be totally blocked by thapsigargin, 2-APB, and U73122. Pertussis toxin pretreatment inhibited about half of the elevation in [Ca(2+)](i) suggesting the involvement of G(i) and other G-proteins. Repetitive [Ca(2+)](i) elevations elicited by S1P, but not LPA, were inhibited by ryanodine. S1P was more effective than LPA in triggering exocytosis as measured by the changes in membrane capacitance. The whole-cell Ca(2+) current was inhibited by both lysophospholipids but Na(+) current was inhibited by S1P only. These results suggest the differential effects of LPA and S1P in releasing Ca(2+) from the intracellular Ca(2+) stores and modulating the stimulus-secretion coupling in bovine chromaffin cells.

Minireview: kisspeptin neurons as central processors in the regulation of gonadotropin-releasing hormone secretion

The Kiss1 gene encodes a family of peptides called kisspeptins, which bind to the G protein-coupled receptor GPR54. Kisspeptin(s) and its receptor are expressed in the forebrain, and the discovery that mice and humans lacking a functional GPR54 fail to undergo puberty and exhibit hypogonadotropic hypogonadism implies that kisspeptin signaling plays an essential role in reproduction. Studies in several mammalian species have shown that kisspeptins stimulate the secretion of gonadotropins from the pituitary by stimulating the release of GnRH from the forebrain after the activation of GPR54, which is expressed by GnRH neurons. Kisspeptin is expressed abundantly in the arcuate nucleus (Arc) and the anteroventral periventricular nucleus (AVPV) of the forebrain. Both estradiol and testosterone regulate the expression of the Kiss1 gene in the Arc and AVPV; however, the response of the Kiss1 gene to these steroids is exactly opposite between these two nuclei. Estradiol and testosterone down-regulate Kiss1 mRNA in the Arc and up-regulate its expression in the AVPV. Thus, kisspeptin neurons in the Arc may participate in the negative feedback regulation of gonadotropin secretion, whereas kisspeptin neurons in the AVPV may contribute to generating the preovulatory gonadotropin surge in the female. Hypothalamic levels of Kiss1 and GPR54 mRNA increase dramatically at puberty, suggesting that kisspeptin signaling could mediate the neuroendocrine events that trigger the onset of puberty. Together, these observations demonstrate that kisspeptin-GPR54 signaling in the brain serves as an important conduit for controlling GnRH secretion in the developing and adult animal.

Protein kinase Calpha modulates depolarizaton-evoked changes of intracellular Ca2+ concentration in a rat pheochromocytoma cell line

Conventional protein kinase C (cPKC) isoforms are activated by a coincident rise in cytosolic Ca(2+) and membrane-bound diacylglycerol. In excitable cells, cPKC may be activated by Ca(2+) influx through voltage-gated Ca(2+) channels (VGCC). cPKCs, in turn, are known to modulate the activity of VGCC. We examined whether PKCalpha, a cPKC, could be activated by depolarization in a neuroendocrine cell line and whether activation occurred on a time scale that modulated the depolarization-evoked intracellular Ca(2+) concentration ([Ca(2+)](i)) signal. Pheochromocytoma cells (PC12 cells) were transfected with wild-type and mutant forms of PKCalpha labeled with yellow fluorescent protein to monitor kinase translocation. Simultaneously, [Ca(2+)](i) changes were monitored with fura-2. Two point mutations that render PKCalpha inactive, D187A in the Ca(2+) binding site and K368R in the ATP binding site, significantly prolonged the time-to-peak of the depolarization-evoked [Ca(2+)](i) signal. A mutation that modulates membrane insertion (W58G) and two mutations of an autophosphorylation site (S657A, S657E) had no effect on the kinetics of the [Ca(2+)](i) signal. We conclude that in PC12 cells, Ca(2+) entry through VGCC rapidly activates PKCalpha, and that PKCalpha can modulate the Ca(2+) signal on a physiologically relevant time scale. Point mutations of PKCalpha can be used as specific and potent modulators of the PKC signaling pathway.

Teaching resources. Regulation of ion channels by G proteins

This Teaching Resource provides lecture notes and slides for a class covering regulation of ion channels by G proteins and is part of the course "Cell Signaling Systems: A Course for Graduate Students." The lecture begins with an overview of calcium channels and then proceeds to describe the interaction of signaling molecules with calcium channels.

A Novel Mechanism for the Suppression of a Voltage-gated Potassium Channel by Glucose-dependent Insulinotropic Polypeptide

The mechanisms involved in glucose regulation of insulin secretion by ATP-sensitive (K(ATP)) and calcium-activated (K(CA)) potassium channels have been extensively studied, but less is known about the role of voltage-gated (K(V)) potassium channels in pancreatic beta-cells. The incretin hormone, glucose-dependent insulinotropic polypeptide (GIP) stimulates insulin secretion by potentiating events underlying membrane depolarization and exerting direct effects on exocytosis. In the present study, we identified a novel role for GIP in regulating K(V)1.4 channel endocytosis. In GIP receptor-expressing HEK293 cells, GIP reduced A-type peak ionic current amplitude of K(V)1.4 via activation of protein kinase A (PKA). Using mutant forms of K(V)1.4 with Ala-Ser/Thr substitutions in a potential PKA phosphorylation site, C-terminal phosphorylation was shown to be linked to GIP-mediated current amplitude decreases. Proteinase K digestion and immunocytochemical studies on mutant K(V)1.4 localization following GIP stimulation demonstrated phosphorylation-dependent rapid endocytosis of K(V)1.4. Expression of K(V)1.4 protein was also demonstrated in human beta-cells; GIP treatment resulting in similar decreases in A-type potassium current peak amplitude to those in HEK293 cells. Transient overexpression in INS-1 beta-cells (clone 832/13) of wild-type (WT) K(V)1.4, or a T601A mutant form resistant to PKA phosphorylation, resulted in reduced glucose-stimulated insulin secretion; WT K(V)1.4 overexpression potentiated GIP-induced insulin secretion, whereas this response was absent in T601A cells. These results strongly support an important novel role for GIP in regulating K(V)1.4 cell surface expression and modulation of A-type potassium currents, which is likely to be critically important for its insulinotropic action.