Determinants of G protein inhibition of presynaptic calcium channels

The life cycle of voltage-gated Ca2+ channels in neurons: an update on the trafficking of neuronal calcium channels

Neuronal voltage-gated Ca²⁺ (Ca_V) channels play a critical role in cellular excitability, synaptic transmission, excitation-transcription coupling and activation of intracellular signaling pathways. Ca_V channels are multiprotein complexes and their functional expression in the plasma membrane involves finely tuned mechanisms, including forward trafficking from the endoplasmic reticulum (ER) to the plasma membrane, endocytosis and recycling. Whether genetic or acquired, alterations and defects in the trafficking of neuronal Ca_V channels can have severe physiological consequences. In this review, we address the current evidence concerning the regulatory mechanisms which underlie precise control of neuronal Ca_V channel trafficking and we discuss their potential as therapeutic targets.

Fast Inactivation of CaV2.2 Channels Is Prevented by the Gβ1 Subunit in Rat Sympathetic Neurons

Voltage-dependent regulation of Ca_V2.2 channels by G-proteins is performed by the β (Gβ) subunit. Most studies of regulation by G-proteins have focused on channel activation; however, little is known regarding channel inactivation. This study investigated inactivation of Ca_V2.2 channels in superior cervical ganglion neurons that overexpressed Gβ subunits. Ca_V2.2 currents were recorded by whole-cell patch clamping configuration. We found that the Gβ₁ subunit reduced inactivation, while Gβ₅ subunit did not alter at all inactivation kinetics compared to control recordings. Ca_V2.2 current decay in control neurons consisted of both fast and slow inactivation; however, Gβ₁-overexpressing neurons displayed only the slow inactivation. Fast inactivation was restored by a strong depolarization of Gβ₁-overexpressing neurons, therefore, through a voltage-dependent mechanism. The Gβ₁ subunit shifted the voltage dependence of inactivation to more positive voltages and reduced the fraction of Ca_V2.2 channels resting in the inactivated state. These results support that the Gβ₁ subunit inhibits the fast inactivation of Ca_V2.2 channels in SCG neurons. They explain the long-observed sustained Ca²⁺ current under G-protein modulation.

Regulator of G protein signaling 1 suppresses CXCL12-mediated migration and AKT activation in RPMI 8226 human plasmacytoma cells and plasmablasts

Migration of plasma cells to the bone marrow is critical factor to humoral immunity and controlled by chemokines. Regulator of G protein signaling 1 (RGS1) is a GTPase-activating protein that controls various crucial functions such as migration. Here, we show that RGS1 controls the chemotactic migration of RPMI 8226 human plasmacytoma cells and human plasmablasts. LPS strongly increased RGS1 expression and retarded the migration of RPMI 8226 cells by suppressing CXCL12-mediated AKT activation. RGS1 knockdown by siRNA abolished the retardation of migration and AKT suppression by LPS. RGS1-dependent regulation of migration via AKT is also observed in cultured plasmablasts. We propose novel functions of RGS1 that suppress AKT activation and the migration of RPMI 8226 cells and plasmablasts in CXCL12-mediated chemotaxis.

G protein regulation of neuronal calcium channels: back to the future

Neuronal voltage-gated calcium channels have evolved as one of the most important players for calcium entry into presynaptic endings responsible for the release of neurotransmitters. In turn, and to fine-tune synaptic activity and neuronal communication, numerous neurotransmitters exert a potent negative feedback over the calcium signal provided by G protein-coupled receptors. This regulation pathway of physiologic importance is also extensively exploited for therapeutic purposes, for instance in the treatment of neuropathic pain by morphine and other μ-opioid receptor agonists. However, despite more than three decades of intensive research, important questions remain unsolved regarding the molecular and cellular mechanisms of direct G protein inhibition of voltage-gated calcium channels. In this study, we revisit this particular regulation and explore new considerations.

The many faces of T-type calcium channels

Since the discovery of low-voltage-activated T-type calcium channels in sensory neurons and the initial characterization of their physiological function mainly in inferior olive and thalamic neurons, studies on neuronal T-type currents have predominantly focused on the generation of low-threshold spike (and associated action potential burst firing) which is strictly conditioned by a preceding hyperpolarization. This T-type current mediated activity has become an archetype of the function of these channels, constraining our view of the potential physiological and pathological roles that they may play in controlling the excitability of single cells and neural networks. However, greatly helped by the recent availability of the first potent and selective antagonists for this class of calcium channels, novel T-type current functions are rapidly being uncovered, including their surprising involvement in neuronal excitability at depolarized membrane potentials and their complex control of dendritic integration and neurotransmitter release. These and other data summarized in this short review clearly indicate a much wider physiological involvement of T-type channels in neuronal activity than previously expected.

Inhibition of dopamine transporter activity by G protein βγ subunits

Uptake through the Dopamine Transporter (DAT) is the primary mechanism of terminating dopamine signaling within the brain, thus playing an essential role in neuronal homeostasis. Deregulation of DAT function has been linked to several neurological and psychiatric disorders including ADHD, schizophrenia, Parkinson’s disease, and drug addiction. Over the last 15 years, several studies have revealed a plethora of mechanisms influencing the activity and cellular distribution of DAT; suggesting that fine-tuning of dopamine homeostasis occurs via an elaborate interplay of multiple pathways. Here, we show for the first time that the βγ subunits of G proteins regulate DAT activity. In heterologous cells and brain tissue, a physical association between Gβγ subunits and DAT was demonstrated by co-immunoprecipitation. Furthermore, in vitro pull-down assays using purified proteins established that this association occurs via a direct interaction between the intracellular carboxy-terminus of DAT and Gβγ. Functional assays performed in the presence of the non-hydrolyzable GTP analog GTP-γ-S, Gβγ subunit overexpression, or the Gβγ activator mSIRK all resulted in rapid inhibition of DAT activity in heterologous systems. Gβγ activation by mSIRK also inhibited dopamine uptake in brain synaptosomes and dopamine clearance from mouse striatum as measured by high-speed chronoamperometry in vivo. Gβγ subunits are intracellular signaling molecules that regulate a multitude of physiological processes through interactions with enzymes and ion channels. Our findings add neurotransmitter transporters to the growing list of molecules regulated by G-proteins and suggest a novel role for Gβγ signaling in the control of dopamine homeostasis.

Modulation of distinct isoforms of L-type calcium channels by G(q)-coupled receptors in Xenopus oocytes: antagonistic effects of Gβγ and protein kinase C

L-type voltage dependent Ca(2+) channels (L-VDCCs; Ca(v)1.2) are crucial in cardiovascular physiology. In heart and smooth muscle, hormones and transmitters operating via G(q) enhance L-VDCC currents via essential protein kinase C (PKC) involvement. Heterologous reconstitution studies in Xenopus oocytes suggested that PKC and G(q)-coupled receptors increased L-VDCC currents only in cardiac long N-terminus (NT) isoforms of α(1C), whereas known smooth muscle short-NT isoforms were inhibited by PKC and G(q) activators. We report a novel regulation of the long-NT α(1C) isoform by Gβγ. Gβγ inhibited whereas a Gβγ scavenger protein augmented the G(q)--but not phorbol ester-mediated enhancement of channel activity, suggesting that Gβγ acts upstream from PKC. In vitro binding experiments reveal binding of both Gβγ and PKC to α(1C)-NT. However, PKC modulation was not altered by mutations of multiple potential phosphorylation sites in the NT, and was attenuated by a mutation of C-terminally located serine S1928. The insertion of exon 9a in intracellular loop 1 rendered the short-NT α(1C) sensitive to PKC stimulation and to Gβγ scavenging. Our results suggest a complex antagonistic interplay between G(q)-activated PKC and Gβγ in regulation of L-VDCC, in which multiple cytosolic segments of α(1C) are involved.

Gβ2 and Gβ4 participate in the opioid and adrenergic receptor-mediated Ca2+ channel modulation in rat sympathetic neurons

Cardiac function is regulated in part by the sympathetic branch of the autonomic nervous system via the stellate ganglion (SG) neurons. Neurotransmitters, such as noradrenaline (NA), and neuropeptides, including nociceptin (Noc), influence the excit ability of SG neurons by modulating Ca(2+) channel function following activation of the adrenergic and nociceptin/orphanin FQ peptide (NOP) opioid receptors, respectively. The regulation of Ca(2+) channels is mediated by Gβγ, but the specific Gβ subunit that modulates the channels is not known. In the present study, small interference RNA (siRNA) was employed to silence the natively expressed Gβ proteins in rat SG tissue and to examine the coupling specificity of adrenergic and NOP opioid receptors to Ca(2+) channels employing the whole-cell variant of the patch-clamp technique.Western blotting analysis showed that Gβ1, Gβ2 and Gβ4 are natively expressed. The knockdown of Gβ2 or Gβ4 led to a significant decrease of the NA- and Noc-mediated Ca(2+)current inhibition, while Gβ1 silencing was without effect. However, sustaining low levels of Gβ2 resulted in an increased expression of Gβ4 and a concomitant compensation of both adrenergic and opioid signalling pathways modulating Ca(2+) channels. Conversely, Gβ4-directed siRNA was not accompanied with a compensation of the signalling pathway. Finally, the combined silencing of Gβ2 and Gβ4 prevented any additional compensatory mechanisms.Overall, our studies suggest that in SG neurons, Gβ2 and Gβ4 normally maintain the coupling of Ca(2+) channels with the receptors, with the latter subtype responsible for maintaining the integrity of both pathways.

In vivo expression of G-protein beta1gamma2 dimer in adult mouse skeletal muscle alters L-type calcium current and excitation-contraction coupling

A number of G-protein-coupled receptors are expressed in skeletal muscle but their roles in muscle physiology and downstream effector systems remain poorly investigated. Here we explored the functional importance of the G-protein betagamma (Gbetagamma) signalling pathway on voltage-controlled Ca(2+) homeostasis in single isolated adult skeletal muscle fibres. A GFP-tagged Gbeta(1)gamma(2) dimer was expressed in vivo in mice muscle fibres. The GFP fluorescence pattern was consistent with a Gbeta(1)gamma(2) dimer localization in the transverse-tubule membrane. Membrane current and indo-1 fluorescence measurements performed under voltage-clamp conditions reveal a drastic reduction of both L-type Ca(2+) current density and of peak amplitude of the voltage-activated Ca(2+) transient in Gbeta(1)gamma(2)-expressing fibres. These effects were not observed upon expression of Gbeta(2)gamma(2), Gbeta(3)gamma(2) or Gbeta(4)gamma(2). Our data suggest that the G-protein beta(1)gamma(2) dimer may play an important regulatory role in skeletal muscle excitation-contraction coupling.

The headache of a hyperactive calcium channel

Migraine is thought to be triggered by excessive neocortical neuronal excitability that leads to cortical spreading depression. In this issue of Neuron, Tottene et al. study a mouse model of familial hemiplegic migraine type 1, and provide evidence for the hyperactivity of P/Q-type calcium channel-mediated cortical glutamatergic synaptic transmission as an underlying mechanism for the susceptibility of cortical spreading depression initiation in migraine disorders.

Lipid raft segregation modulates TRPM8 channel activity

Transient receptor potential channels are a family of cation channels involved in diverse cellular functions. Most of these channels are expressed in the nervous system and play a key role in sensory physiology. TRPM8 (transient receptor potential melastatine 8), a member of this family, is activated by cold, cooling substances such menthol and icilin and voltage. Although TRPM8 is a thermosensitive channel highly expressed in cold sensory neurons, the mechanisms underlying its temperature sensitivity are still poorly understood. Here we show that, in sensory neurons, TRPM8 channel is localized in cholesterol-rich specialized membrane domains known as lipid rafts. We also show that, in heterologous expression systems, lipid raft segregation of TRPM8 is favored by glycosylation at the Asn(934) residue of the polypeptide. In electrophysiological and imaging experiments, using cold and menthol as agonists, we also demonstrate that lipid raft association modulates TRPM8 channel activity. We found that menthol- and cold-mediated responses of TRPM8 are potentiated when the lipid raft association of the channel is prevented. In addition, lipid raft disruption shifts the threshold for TRPM8 activation to a warmer temperature. In view of these data, we suggest a role for lipid rafts in the activity and temperature sensitivity of TRPM8. We propose a model wherein different lipid membrane environments affect the cold sensing properties of TRPM8, modulating the response of cold thermoreceptors.

Membrane signalling complexes: implications for development of functionally selective ligands modulating heptahelical receptor signalling

Technological development has considerably changed the way in which we evaluate drug efficacy and has led to a conceptual revolution in pharmacological theory. In particular, molecular resolution assays have revealed that heptahelical receptors may adopt multiple active conformations with unique signalling properties. It is therefore becoming widely accepted that ligand ability to stabilize receptor conformations with distinct signalling profiles may allow to direct the stimulus generated by an activated receptor towards a specific signalling pathway. This capacity to induce only a subset of the ensemble of responses regulated by a given receptor has been termed "functional selectivity" (or "stimulus trafficking"), and provides the bases for a highly specific regulation of receptor signalling. Concomitant with these observations, heptahelical receptors have been shown to associate with G proteins and effectors to form multimeric arrays. These complexes are constitutively formed during protein synthesis and are targeted to the cell surface as integral signalling units. Herein we summarize evidence supporting the existence of such constitutive signalling arrays and analyze the possibility that they may constitute viable targets for developing ligands with "functional selectivity".

Importance of voltage-dependent inactivation in N-type calcium channel regulation by G-proteins

Direct regulation of N-type calcium channels by G-proteins is essential to control neuronal excitability and neurotransmitter release. Binding of the G(betagamma) dimer directly onto the channel is characterized by a marked current inhibition ("ON" effect), whereas the pore opening- and time-dependent dissociation of this complex from the channel produce a characteristic set of biophysical modifications ("OFF" effects). Although G-protein dissociation is linked to channel opening, the contribution of channel inactivation to G-protein regulation has been poorly studied. Here, the role of channel inactivation was assessed by examining time-dependent G-protein de-inhibition of Ca(v)2.2 channels in the presence of various inactivation-altering beta subunit constructs. G-protein activation was produced via mu-opioid receptor activation using the DAMGO agonist. Whereas the "ON" effect of G-protein regulation is independent of the type of beta subunit, the "OFF" effects were critically affected by channel inactivation. Channel inactivation acts as a synergistic factor to channel activation for the speed of G-protein dissociation. However, fast inactivating channels also reduce the temporal window of opportunity for G-protein dissociation, resulting in a reduced extent of current recovery, whereas slow inactivating channels undergo a far more complete recovery from inhibition. Taken together, these results provide novel insights on the role of channel inactivation in N-type channel regulation by G-proteins and contribute to the understanding of the physiological consequence of channel inactivation in the modulation of synaptic activity by G-protein coupled receptors.

Contribution of the kinetics of G protein dissociation to the characteristic modifications of N-type calcium channel activity

Direct G protein inhibition of N-type calcium channels is recognized by characteristic biophysical modifications. In this study, we quantify and simulate the importance of G protein dissociation on the phenotype of G protein-regulated whole-cell currents. Based on the observation that the voltage-dependence of the time constant of recovery from G protein inhibition is correlated with the voltage-dependence of channel opening, we depict all G protein effects by a simple kinetic scheme. All landmark modifications in calcium currents, except inhibition, can be successfully described using three simple biophysical parameters (extent of block, extent of recovery, and time constant of recovery). Modifications of these parameters by auxiliary beta subunits are at the origin of differences in N-type channel regulation by G proteins. The simulation data illustrate that channel reluctance can occur as the result of an experimental bias linked to the variable extent of G protein dissociation when peak currents are measured at various membrane potentials. To produce alterations in channel kinetics, the two most important parameters are the extents of initial block and recovery. These data emphasize the contribution of the degree and kinetics of G protein dissociation in the modification of N-type currents.

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.

Arrestin is required for agonist-induced trafficking of voltage-dependent calcium channels

Many metabotropic receptors in the nervous system act through signaling pathways that result in the inhibition of voltage-dependent calcium channels. Our previous findings showed that activation of seven-transmembrane receptors results in the internalization of calcium channels. This internalization takes place within a few seconds, raising the question of whether the endocytic machinery is in close proximity to the calcium channel to cause such rapid internalization. Here we show that voltage-dependent calcium channels are pre-associated with arrestin, a protein known to play a role in receptor trafficking. Upon GABAB receptor activation, receptors are recruited to the arrestin-channel complex and internalized. beta-Arrestin 1 selectively binds to the SNARE-binding region of the calcium channel. Peptides containing the arrestin-binding site of the channel disrupt agonist-induced channel internalization. Taken together these data suggest a novel neuronal role for arrestin.

Scanning Mutagenesis Reveals a Role for Serine 189 of the Heterotrimeric G-Protein Beta 1 Subunit in the Inhibition of N-Type Calcium Channels

Direct interactions between the presynaptic N-type calcium channel and the β subunit of the heterotrimeric G-protein complex cause voltage-dependent inhibition of N-type channel activity, crucially influencing neurotransmitter release and contributing to analgesia caused by opioid drugs. Previous work using chimeras of the G-protein β subtypes Gβ1 and Gβ5 identified two 20–amino acid stretches of structurally contiguous residues on the Gβ1 subunit as critical for inhibition of the N-type channel. To identify key modulation determinants within these two structural regions, we performed scanning mutagenesis in which individual residues of the Gβ1 subunit were replaced by corresponding Gβ5 residues. Our results show that Gβ1 residue Ser189 is critical for N-type calcium channel modulation, whereas none of the other Gβ1 mutations caused statistically significant effects on the ability of Gβ1 to inhibit N-type channels. Structural modeling shows residue 189 is surface exposed, consistent with the idea that it may form a direct contact with the N-type calcium channel α1 subunit during binding interactions.

Voltage-Gated Calcium Channels and Idiopathic Generalized Epilepsies

The idiopathic generalized epilepsies encompass a class of epileptic seizure types that exhibit a polygenic and heritable etiology. Advances in molecular biology and genetics have implicated defects in certain types of voltage-gated calcium channels and their ancillary subunits as important players in this form of epilepsy. Both T-type and P/Q-type channels appear to mediate important contributions to seizure genesis, modulation of network activity, and genetic seizure susceptibility. Here, we provide a comprehensive overview of the roles of these channels and associated subunits in normal and pathological brain activity within the context of idiopathic generalized epilepsy.

Extracellular domains of alpha-neurexins participate in regulating synaptic transmission by selectively affecting N- and P/Q-type Ca2+ channels

Neurexins constitute a large family of highly variable cell-surface molecules that may function in synaptic transmission and/or synapse formation. Each of the three known neurexin genes encodes two major neurexin variants, alpha- and beta-neurexins, that are composed of distinct extracellular domains linked to identical intracellular sequences. Deletions of one, two, or all three alpha-neurexins in mice recently demonstrated their essential role at synapses. In multiple alpha-neurexin knock-outs, neurotransmitter release from excitatory and inhibitory synapses was severely reduced, primarily probably because voltage-dependent Ca2+ channels were impaired. It remained unclear, however, which neurexin variants actually influence exocytosis and Ca2+ channels, which domain of neurexins is required for this function, and which Ca2+-channel subtypes are regulated. Here, we show by electrophysiological recordings that transgenic neurexin 1alpha rescues the release and Ca2+-current phenotypes, whereas transgenic neurexin 1beta has no effect, indicating the importance of the extracellular sequences for the function of neurexins. Because neurexin 1alpha rescued the knock-out phenotype independent of the alpha-neurexin gene deleted, these data are consistent with a redundant function among different alpha-neurexins. In both knock-out and transgenically rescued mice, alpha-neurexins selectively affected the component of neurotransmitter release that depended on activation of N- and P/Q-type Ca2+ channels, but left L-type Ca2+ channels unscathed. Our findings indicate that alpha-neurexins represent organizer molecules in neurotransmission that regulate N- and P/Q-type Ca2+ channels, constituting an essential role at synapses that critically involves the extracellular domains of neurexins.

ORL1 receptor–mediated internalization of N-type calcium channels

The inhibition of N-type calcium channels by opioid receptor like receptor 1 (ORL1) is a key mechanism for controlling the transmission of nociceptive signals. We recently reported that signaling complexes consisting of ORL1 receptors and N-type channels mediate a tonic inhibition of calcium entry. Here we show that prolonged ( approximately 30 min) exposure of ORL1 receptors to their agonist nociceptin triggers an internalization of these signaling complexes into vesicular compartments. This effect is dependent on protein kinase C activation, occurs selectively for N-type channels and cannot be observed with mu-opioid or angiotensin receptors. In expression systems and in rat dorsal root ganglion neurons, the nociceptin-mediated internalization of the channels is accompanied by a significant downregulation of calcium entry, which parallels the selective removal of N-type calcium channels from the plasma membrane. This may provide a new means for long-term regulation of calcium entry in the pain pathway.

Location and function of vesicle clusters, active zones and Ca2+ channels in the lamprey presynaptic terminal

Synaptic transmission requires spatial and temporal coordination of a specific sequence of events. The trigger for synaptic vesicle exocytosis is Ca(2)(+) entry into presynaptic terminals, leading to neurotransmitter release at highly specialized sites known as active zones. Ca(2)(+) channel proximity to exocytotic proteins and vesicle clusters at active zones have been inferred from biochemical, histological and ultrastructural data, but direct evidence about functional relationships between these elements in central synapses is absent. We have utilized the lamprey giant reticulospinal synapse to characterize functional colocalization of known synaptic markers in the presynaptic terminal, as well as their reliability during repeated activation. Recycling vesicle clusters, surrounding actin filaments, and physiologically relevant Ca(2)(+) influx all show identical morphological distribution. Ca(2)(+) influx is mediated by clusters of Ca(2)(+) channels that colocalize with the vesicle clusters, defined by imaged sites of vesicle recycling and actin localization. Synaptic transmission is inhibited by block of actin depolymerization, but Ca(2)(+) signalling is unaffected. Functional Ca(2)(+) channels are localized to presynaptic clusters, and Ca(2)(+) transients at these sites account for neurotransmitter release based on their spatial and temporal profiles. Ca(2)(+) transients evoked by single axonal action potentials are mediated solely by voltage-operated Ca(2)(+) channel activation, and slower Ca(2)(+) rises observed throughout the axon result from Ca(2)(+) diffusion from the synaptic regions. We conclude that at lamprey giant reticulospinal synapses, Ca(2)(+) channels and release sites colocalize, creating a close spatial relationship between active zones and Ca(2)(+) entry sites, which is necessary for site-specific, Ca(2)(+)-dependent secretion.

G protein-gated inhibitory module of N-type (ca(v)2.2) ca2+ channels

Voltage-dependent G protein (Gbetagamma) inhibition of N-type (CaV2.2) channels supports presynaptic inhibition and represents a central paradigm of channel modulation. Still controversial are the proposed determinants for such modulation, which reside on the principal alpha1B channel subunit. These include the interdomain I-II loop (I-II), the carboxy tail (CT), and the amino terminus (NT). Here, we probed these determinants and related mechanisms, utilizing compound-state analysis with yeast two-hybrid and mammalian cell FRET assays of binding among channel segments and G proteins. Chimeric channels confirmed the unique importance of NT. Binding assays revealed selective interaction between NT and I-II elements. Coexpressing NT peptide with Gbetagamma induced constitutive channel inhibition, suggesting that the NT domain constitutes a G protein-gated inhibitory module. Such inhibition was limited to NT regions interacting with I-II, and G-protein inhibition was abolished within alpha1B channels lacking these NT regions. Thus, an NT module, acting via interactions with the I-II loop, appears fundamental to such modulation.

G protein beta2 subunit-derived peptides for inhibition and induction of G protein pathways. Examination of voltage-gated Ca2+ and G protein inwardly rectifying K+ channels

Voltage-gated Ca2+ channels of the N-, P/Q-, and R-type and G protein inwardly rectifying K+ channels (GIRK) are modulated via direct binding of G proteins. The modulation is mediated by G protein betagamma subunits. By using electrophysiological recordings and fluorescence resonance energy transfer, we characterized the modulatory domains of the G protein beta subunit on the recombinant P/Q-type channel and GIRK channel expressed in HEK293 cells and on native non-L-type Ca2+ currents of cultured hippocampal neurons. We found that Gbeta2 subunit-derived deletion constructs and synthesized peptides can either induce or inhibit G protein modulation of the examined ion channels. In particular, the 25-amino acid peptide derived from the Gbeta2 N terminus inhibits G protein modulation, whereas a 35-amino acid peptide derived from the Gbeta2 C terminus induced modulation of voltage-gated Ca2+ channels and GIRK channels. Fluorescence resonance energy transfer (FRET) analysis of the live action of these peptides revealed that the 25-amino acid peptide diminished the FRET signal between G protein beta2gamma3 subunits, indicating a reorientation between G protein beta2gamma3 subunits in the presence of the peptide. In contrast, the 35-amino acid peptide increased the FRET signal between GIRK1,2 channel subunits, similarly to the Gbetagamma-mediated FRET increase observed for this GIRK subunit combination. Circular dichroism spectra of the synthesized peptides suggest that the 25-amino acid peptide is structured. These results indicate that individual G protein beta subunit domains can act as independent, separate modulatory domains to either induce or inhibit G protein modulation for several effector proteins.

Dynamics of fast synaptic excitation during trains of stimulation in myenteric neurons of guinea-pig ileum

Fast excitatory postsynaptic potentials (fEPSPs) occur in bursts in the myenteric plexus during evoked motor reflexes in the guinea-pig ileum in vitro. This study used electrophysiological methods to study fEPSPs during stimulus trains to mimic bursts of synaptic activity in vitro. The amplitude of fEPSPs or fast excitatory postsynaptic currents (EPSCs) declined (rundown) during stimulus trains at frequencies of 0.5, 5, 10 and 20 Hz. At 0.5 Hz, fEPSP or fEPSC amplitude declined by 50% after the first stimulus but remained constant for the remainder of the train. At 5, 10 and 20 Hz, synaptic responses ran down completely with time constants of 0.35, 0.21 and 0.11 s, respectively. Recovery from rundown occurred with a time constant of 7 s. Mecamylamine, a nicotinic cholinergic receptor antagonist, or PPADS, a P2X receptor antagonist, reduced fEPSP amplitude, but they had no effect on rundown. Responses caused by trains of ionophoretically applied ATP or ACh (to mimic fEPSPs) did not rundown. Blockade of presynaptic inhibitory muscarinic, adenosine A1, opioid, alpha2-adrenergic and 5-HT1A receptors or pertussis toxin (PTX) treatment did not alter rundown. Antidromic action potentials followed a 10-Hz stimulus train. Iberiotoxin (100 nM), a blocker of large conductance calcium activated K+ (BK) channels, did not alter rundown. These data suggest that synaptic rundown is not due to: (a) action potential failure; (b) nicotinic or P2X receptor desensitization; (c) presynaptic inhibition mediated by pertussis-toxin sensitive G-proteins, or (d) BK channel activation. Synaptic rundown is likely due to depletion of a readily releasable pool (RRP) of neurotransmitter.

Antinociception depends on the presence of G protein gamma2-subunits in brain

We have shown previously [Hosohata, K., Logan, J.K., Varga, E., Burkey, T.H., Vanderah, T.W., Porreca, F., Hruby, V.J., Roeske, W.R., Yamamura, H.I., 2000. The role of the G protein gamma2 subunit in opioid antinociception in mice. Eur. J. Pharmacol. 392, R9-R11] that intracerebroventricular (i.c.v.) treatment of mice with a phosphorothioate oligodeoxynucleotide antisense to the gamma2 subunit (Ggamma2) of the heterotrimeric G proteins (antisense ODN) significantly attenuates antinociception by a delta-opioid receptor agonist. In the present study, we examined the involvement of Ggamma2 in antinociception mediated by other (mu- or kappa-opioid, cannabinoid, alpha2-adrenoreceptor) analgesic agents in a warm (55 degrees C) water tail-flick test in mice. Interestingly, i.c.v. treatment with the antisense ODN attenuated antinociception by each analgesic agent. Missense phosphorothioate oligodeoxynucleotide treatment, on the other hand, had no effect on antinociception mediated by these agonists. The antinociceptive response recovered in 6 days after the last antisense ODN injection, indicating a lack of nonspecific tissue damage in the animals. These results suggest a pervasive role for the G protein gamma2 subunits in supraspinal antinociception.

A Single Gβ Subunit Locus Controls Cross-talk between Protein Kinase C and G Protein Regulation of N-type Calcium Channels

The modulation of N-type calcium channels is a key factor in the control of neurotransmitter release. Whereas N-type channels are inhibited by Gbetagamma subunits in a G protein beta-isoform-dependent manner, channel activity is typically stimulated by activation of protein kinase C (PKC). In addition, there is cross-talk among these pathways, such that PKC-dependent phosphorylation of the Gbetagamma target site on the N-type channel antagonizes subsequent G protein inhibition, albeit only for Gbeta(1)-mediated responses. The molecular mechanisms that control this G protein beta subunit subtype-specific regulation have not been described. Here, we show that G protein inhibition of N-type calcium channels is critically dependent on two separate but adjacent approximately 20-amino acid regions of the Gbeta subunit, plus a highly conserved Asn-Tyr-Val motif. These regions are distinct from those implicated previously in Gbetagamma signaling to other effectors such as G protein-coupled inward rectifier potassium channels, phospholipase beta(2), and adenylyl cyclase, thus raising the possibility that the specificity for G protein signaling to calcium channels might rely on unique G protein structural determinants. In addition, we identify a highly specific locus on the Gbeta(1) subunit that serves as a molecular detector of PKC-dependent phosphorylation of the G protein target site on the N-type channel alpha(1) subunit, thus providing for a molecular basis for G protein-PKC cross-talk. Overall, our results significantly advance our understanding of the molecular details underlying the integration of G protein and PKC signaling pathways at the level of the N-type calcium channel alpha(1) subunit.

Voltage-dependent ebselen and diorganochalcogenides inhibition of 45Ca2+ influx into brain synaptosomes

By mediating the Ca(2+) influx, Ca(2+) channels play a central role in neurotransmission. Chemical agents that potentially interfere with Ca(2+) homeostasis are potential toxic agents. In the present investigation, changes in Ca(2+) influx into synaptosomes by organic forms of selenium and tellurium were examined under nondepolarizing and depolarizing conditions induced by high KCl concentration (135 mM) or by 4-aminopyridine (4-AP). Under nondepolarizing conditions, ebselen (400 micro M) increased Ca(2+) influx; diphenyl ditelluride (40-400 micro M) decreased Ca(2+) in all concentrations tested; and diphenyl diselenide decreased Ca(2+) influx at 40 and 100 micro M, but had no effect at 400 micro M. In the presence of KCl as depolarizing agent, ebselen and diphenyl ditelluride decreased Ca(2+) influx in a linear fashion. In contrast, diphenyl diselenide did not modify Ca(2+) influx into isolated nerve terminals. In the presence of 4-AP (3 mM) as depolarizing agent, ebselen (400 micro M) caused a significant increase, whereas diphenyl diselenide and diphenyl ditelluride inhibited Ca(2+) influx into synaptosomes. The results can be explained by the fact that the mechanism through which 4-AP and high K(+) induced elevation of intracellular Ca(2+) is not exactly coincident. The mechanism by which diphenyl ditelluride and ebselen interact with Ca(2+) channel is unknown, but may be related to reactivity with critical sulfhydryl groups in the protein complex. The results of the present study indicate that the effects of organochalcogenides were rather complex depending on the condition and the depolarizing agent used.

Cavbeta-subunit displacement is a key step to induce the reluctant state of P/Q calcium channels by direct G protein regulation

P/Q Ca(2+) channel activity is inhibited by G protein-coupled receptor activation. Channel inhibition requires a direct Gbetagamma binding onto the pore-forming subunit, Ca(v)2.1. It is characterized by biophysical changes, including current amplitude reduction, activation kinetic slowing, and an I-V curve shift, which leads to a reluctant mode. Here, we have characterized the contribution of the auxiliary beta(3)-subunit to channel regulation by G proteins. The shift in I-V to a P/Q reluctant mode is exclusively observed in the presence of beta(3). Along with the observation that Gbetagamma has no effect on the I-V curve of Ca(v)2.1 alone, we propose that the reluctant mode promoted by Gbetagamma corresponds to a state in which the beta(3)-subunit has been displaced from its channel-binding site. We validate this hypothesis with a beta(3)-I-II(2.1) loop chimera construct. Gbetagamma binding onto the I-II(2.1) loop portion of the chimera releases the beta(3)-binding domain and makes it available for binding onto the I-II loop of Ca(v)1.2, a G protein-insensitive channel. This finding is extended to the full-length Ca(v)2.1 channel by using fluorescence resonance energy transfer. Gbetagamma injection into Xenopus oocytes displaces a Cy3-labeled beta(3)-subunit from a GFP-tagged Ca(v)2.1 channel. We conclude that beta-subunit dissociation from the channel complex constitutes a key step in P/Q calcium channel regulation by G proteins that underlies the reluctant state and is an important process for modulating neurotransmission through G protein-coupled receptors.

Overexpressed Ca(v)beta3 inhibits N-type (Cav2.2) calcium channel currents through a hyperpolarizing shift of ultra-slow and closed-state inactivation

It has been shown that β auxiliary subunits increase current amplitude in voltage-dependent calcium channels. In this study, however, we found a novel inhibitory effect of β3 subunit on macroscopic Ba²⁺ currents through recombinant N- and R-type calcium channels expressed in Xenopus oocytes. Overexpressed β3 (12.5 ng/cell cRNA) significantly suppressed N- and R-type, but not L-type, calcium channel currents at “physiological” holding potentials (HPs) of −60 and −80 mV. At a HP of −80 mV, coinjection of various concentrations (0–12.5 ng) of the β3 with Ca_v2.2α₁ and α₂δ enhanced the maximum conductance of expressed channels at lower β3 concentrations but at higher concentrations (>2.5 ng/cell) caused a marked inhibition. The β3-induced current suppression was reversed at a HP of −120 mV, suggesting that the inhibition was voltage dependent. A high concentration of Ba²⁺ (40 mM) as a charge carrier also largely diminished the effect of β3 at −80 mV. Therefore, experimental conditions (HP, divalent cation concentration, and β3 subunit concentration) approaching normal physiological conditions were critical to elucidate the full extent of this novel β3 effect. Steady-state inactivation curves revealed that N-type channels exhibited “closed-state” inactivation without β3, and that β3 caused an ∼40-mV negative shift of the inactivation, producing a second component with an inactivation midpoint of approximately −85 mV. The inactivation of N-type channels in the presence of a high concentration (12.5 ng/cell) of β3 developed slowly and the time-dependent inactivation curve was best fit by the sum of two exponential functions with time constants of 14 s and 8.8 min at −80 mV. Similar “ultra-slow” inactivation was observed for N-type channels without β3. Thus, β3 can have a profound negative regulatory effect on N-type (and also R-type) calcium channels by causing a hyperpolarizing shift of the inactivation without affecting “ultra-slow” and “closed-state” inactivation properties.

Agonist-independent modulation of N-type calcium channels by ORL1 receptors

We have investigated modulation of voltage-gated calcium channels by nociceptin (ORL1) receptors. In rat DRG neurons and in tsA-201 cells, nociceptin mediated a pronounced inhibition of N-type calcium channels, whereas other calcium channel subtypes were unaffected. In tsA-201 cells, expression of N-type channels with human ORL1 resulted in a voltage-dependent G-protein inhibition of the channel that occurred in the absence of nociceptin, the ORL1 receptor agonist. Consistent with this observation, native N-type channels of small nociceptive dorsal root ganglion (DRG) neurons also had tonic inhibition by G proteins. Biochemical characterization showed the existence of an N-type calcium channel-ORL1 receptor signaling complex, which efficiently exposes N-type channels to constitutive ORL1 receptor activity. Calcium channel activity is thus regulated by changes in ORL1 receptor expression, which provides a possible molecular mechanism for the development of tolerance to opioid receptor agonists.

Multiple serotonergic mechanisms contributing to sensitization in aplysia: evidence of diverse serotonin receptor subtypes

The neurotransmitter serotonin (5-HT) plays an important role in memory encoding in Aplysia. Early evidence showed that during sensitization, 5-HT activates a cyclic AMP-protein kinase A (cAMP-PKA)-dependent pathway within specific sensory neurons (SNs), which increases their excitability and facilitates synaptic transmission onto their follower motor neurons (MNs). However, recent data suggest that serotonergic modulation during sensitization is more complex and diverse. The neuronal circuits mediating defensive reflexes contain a number of interneurons that respond to 5-HT in ways opposite to those of the SNs, showing a decrease in excitability and/or synaptic depression. Moreover, in addition to acting through a cAMP-PKA pathway within SNs, 5-HT is also capable of activating a variety of other protein kinases such as protein kinase C, extracellular signal-regulated kinases, and tyrosine kinases. This diversity of 5-HT responses during sensitization suggests the presence of multiple 5-HT receptor subtypes within the Aplysia central nervous system. Four 5-HT receptors have been cloned and characterized to date. Although several others probably remain to be characterized in molecular terms, especially the Gs-coupled 5-HT receptor capable of activating cAMP-PKA pathways, the multiplicity of serotonergic mechanisms recruited into action during learning in Aplysia can now be addressed from a molecular point of view.

Inhibition of voltage-gated Ca2+ channels by antazoline

We showed recently that imidazolines exert neuroprotection against hypoxia and NMDA toxicity in cerebellar and striatal neuronal cultures, through a voltage-dependent blockade of glutamatergic NMDA receptors. Here, we report that in striatal neuronal cultures from mouse embryos the imidazoline compound, antazoline, inhibits voltage-gated Ca2+ channels by acting at a phencyclidine-like site. This effect was fast, fully reversible, voltage-dependent and predominant on P/Q- and N-type Ca2+ channels. Taken together, these results suggest that imidazolines may elicit neuroprotective effects also by decreasing the release of glutamate through inhibition of presynaptic Ca2+ channels.

Positron emission tomography: measurement of the activity of second messenger systems

Phosphoinositide turnover is closely connected to modulation of synaptic function and is part of an important second messenger-producing system. New radioligands for imaging second messenger systems by positron emission tomography have been developed: carbon-11-labeled 1,2-diacylglycerols. The theoretical background of second messenger imaging is described in detail and the relation between the biologically active compounds and potential tracers for imaging second messenger systems is discussed. We report informative findings on postsynaptic biological responses in the living human brain of healthy normal subjects and with various diseases.

Calcium channel beta subunits differentially regulate the inhibition of N-type channels by individual Gbeta isoforms

The direct inhibition of N- and P/Q-type calcium channels by G protein betagamma subunits is considered a key mechanism for regulating presynaptic calcium levels. We have recently reported that a number of features associated with this G protein inhibition are dependent on the G protein beta subunit isoform (Arnot, M. I., Stotz, S. C., Jarvis, S. E., Zamponi, G. W. (2000) J. Physiol. (Lond.) 527, 203-212; Cooper, C. B., Arnot, M. I., Feng, Z.-P., Jarvis, S. E., Hamid, J., Zamponi, G. W. (2000) J. Biol. Chem. 275, 40777-40781). Here, we have examined the abilities of different types of ancillary calcium channel beta subunits to modulate the inhibition of alpha(1B) N-type calcium channels by the five known different Gbeta subunit subtypes. Our data reveal that the degree of inhibition by a particular Gbeta subunit is strongly dependent on the specific calcium channel beta subunit, with N-type channels containing the beta(4) subunit being less susceptible to Gbetagamma-induced inhibition. The calcium channel beta(2a) subunit uniquely slows the kinetics of recovery from G protein inhibition, in addition to mediating a dramatic enhancement of the G protein-induced kinetic slowing. For Gbeta(3)-mediated inhibition, the latter effect is reduced following site-directed mutagenesis of two palmitoylation sites in the beta(2a) N-terminal region, suggesting that the unique membrane tethering of this subunit serves to modulate G protein inhibition of N-type calcium channels. Taken together, our data suggest that the nature of the calcium channel beta subunit present is an important determinant of G protein inhibition of N-type channels, thereby providing a possible mechanism by which the cellular/subcellular expression pattern of the four calcium channel beta subunits may regulate the G protein sensitivity of N-type channels expressed at different loci throughout the brain and possibly within a neuron.