Protein kinase C-mediated inhibition of recombinant T-type Cav3.2 channels by neurokinin 1 receptors

Adiponectin receptor 1-mediated stimulation of Cav3.2 channels in trigeminal ganglion neurons induces nociceptive behaviors in mice

BACKGROUND

Adipokines, including adiponectin, are implicated in nociceptive pain; however, the underlying cellular and molecular mechanisms remain unknown.

METHODS

Using electrophysiological recording, immunostaining, molecular biological approaches and animal behaviour tests, we elucidated a pivotal role of adiponectin in regulating membrane excitability and pain sensitivity by manipulating Cav3.2 channels in trigeminal ganglion (TG) neurons.

RESULTS

Adiponectin enhanced T-type Ca2+ channel currents (IT) in TG neurons through the activation of adiponectin receptor 1 (adipoR1) but independently of heterotrimeric G protein-mediated signaling. Coimmunoprecipitation revealed a physical association between AdipoR1 and casein kinase II alpha-subunits (CK2α) in the TG, and inhibiting CK2 activity by chemical inhibitor or siRNA targeting CK2α prevented the adiponectin-induced IT response. Adiponectin significantly activated protein kinase C (PKC), and this effect was abrogated by CK2α knockdown. Adiponectin increased the membrane abundance of PKC beta1 (PKCβ1). Blocking PKCβ1 pharmacologically or genetically abrogated the adiponectin-induced IT increase. In heterologous expression systems, activation of adipoR1 induced a selective enhancement of Cav3.2 channel currents, dependent on PKCβ1 signaling. Functionally, adiponectin increased TG neuronal excitability and induced mechanical pain hypersensitivity, both attenuated by T-type channel blockade. In a trigeminal neuralgia model induced by chronic constriction injury of infraorbital nerve, blockade of adipoR1 signaling suppressed mechanical allodynia, which was prevented by silencing Cav3.2.

CONCLUSION

Our study elucidates a novel signaling cascade wherein adiponectin stimulates TG Cav3.2 channels via adipoR1 coupled to a novel CK2α-dependent PKCβ1. This process induces neuronal hyperexcitability and pain hypersensitivity. Insight into adipoR-Cav3.2 signaling in sensory neurons provides attractive targets for pain treatment.

Voltage-Gated T-Type Calcium Channel Modulation by Kinases and Phosphatases: The Old Ones, the New Ones, and the Missing Ones

Calcium (Ca2+) can regulate a wide variety of cellular fates, such as proliferation, apoptosis, and autophagy. More importantly, changes in the intracellular Ca2+ level can modulate signaling pathways that control a broad range of physiological as well as pathological cellular events, including those important to cellular excitability, cell cycle, gene-transcription, contraction, cancer progression, etc. Not only intracellular Ca2+ level but the distribution of Ca2+ in the intracellular compartments is also a highly regulated process. For this Ca2+ homeostasis, numerous Ca2+ chelating, storage, and transport mechanisms are required. There are also specialized proteins that are responsible for buffering and transport of Ca2+. T-type Ca2+ channels (TTCCs) are one of those specialized proteins which play a key role in the signal transduction of many excitable and non-excitable cell types. TTCCs are low-voltage activated channels that belong to the family of voltage-gated Ca2+ channels. Over decades, multiple kinases and phosphatases have been shown to modulate the activity of TTCCs, thus playing an indirect role in maintaining cellular physiology. In this review, we provide information on the kinase and phosphatase modulation of TTCC isoforms Cav3.1, Cav3.2, and Cav3.3, which are mostly described for roles unrelated to cellular excitability. We also describe possible potential modulations that are yet to be explored. For example, both mitogen-activated protein kinase and citron kinase show affinity for different TTCC isoforms; however, the effect of such interaction on TTCC current/kinetics has not been studied yet.

Differential regulation of Cav 3.2 and Cav 2.2 calcium channels by CB1 receptors and cannabidiol

BACKGROUND AND PURPOSE

Cannabinoids are a promising therapeutic avenue for chronic pain. However, clinical trials often fail to report analgesic efficacy of cannabinoids. Inhibition of voltage gate calcium (Ca_v ) channels is one mechanism through which cannabinoids may produce analgesia. We hypothesized that cannabinoids and cannabinoid receptor agonists target different types of Ca_v channels through distinct mechanisms.

EXPERIMENTAL APPROACH

Electrophysiological recordings from tsA-201 cells expressing either Ca_v 3.2 or Ca_v 2.2 were used to assess inhibition by HU-210 or cannabidiol (CBD) in the absence and presence of the CB₁ receptor. Homology modelling assessed potential interaction sites for CBD in both Ca_v 2.2 and Ca_v 3.2. Analgesic effects of CBD were assessed in mouse models of inflammatory and neuropathic pain.

KEY RESULTS

HU-210 (1 μM) inhibited Ca_v 2.2 function in the presence of CB₁ receptor but had no effect on Ca_v 3.2 regardless of co-expression of CB₁ receptor. By contrast, CBD (3 μM) produced no inhibition of Ca_v 2.2 and instead inhibited Ca_v 3.2 independently of CB₁ receptors. Homology modelling supported these findings, indicating that CBD binds to and occludes the pore of Ca_v 3.2, but not Ca_v 2.2. Intrathecal CBD alleviated thermal and mechanical hypersensitivity in both male and female mice, and this effect was absent in Ca_v 3.2 null mice.

CONCLUSION AND IMPLICATIONS

Our findings reveal differential modulation of Ca_v 2.2 and Ca_v 3.2 channels by CB₁ receptors and CBD. This advances our understanding of how different cannabinoids produce analgesia through action at different voltage-gated calcium channels and could influence the development of novel cannabinoid-based therapeutics for treatment of chronic pain.

Constitutive activity of the dopamine (D5 ) receptor, highly expressed in CA1 hippocampal neurons, selectively reduces CaV 3.2 and CaV 3.3 currents

BACKGROUND AND PURPOSE

Ca_V 3.1-3 currents differentially contribute to neuronal firing patterns. Ca_V 3 are regulated by G protein-coupled receptors (GPCRs) activity, but information about Ca_V 3 as targets of the constitutive activity of GPCRs is scarce. We investigate the impact of D₅ recpetor constitutive activity, a GPCR with high levels of basal activity, on Ca_V 3 functionality. D₅ recpetor and Ca_V 3 are expressed in the hippocampus and have been independently linked to pathophysiological states associated with epilepsy.

EXPERIMENTAL APPROACH

Our study models were HEK293T cells heterologously expressing D₁ or D₅ receptor and Ca_V 3.1-3, and mouse brain slices containing the hippocampus. We used chlorpromazine (D₁ /D₅ inverse agonist) and a D₅ receptor mutant lacking constitutive activity as experimental tools. We measured Ca_V 3 currents and excitability parameters using the patch-clamp technique. We completed our study with computational modelling and imaging technique.

KEY RESULTS

We found a higher sensitivity to TTA-P2 (Ca_V 3 blocker) in CA1 pyramidal neurons obtained from chlorpromazine-treated animals compared with vehicle-treated animals. We found that Ca_V 3.2 and Ca_V 3.3-but not Ca_V 3.1-are targets of D₅ receptor constitutive activity in HEK293T cells. Finally, we found an increased firing rate in CA1 pyramidal neurons from chlorpromazine-treated animals in comparison with vehicle-treated animals. Similar changes in firing rate were observed on a neuronal model with controlled Ca_V 3 currents levels.

CONCLUSIONS AND IMPLICATIONS

Native hippocampal Ca_V 3 and recombinant Ca_V 3.2-3 are sensitive to D₅ receptor constitutive activity. Manipulation of D₅ receptor constitutive activity could be a valuable strategy to control neuronal excitability, especially in exacerbated conditions such as epilepsy.

Neural Burst Firing and Its Roles in Mental and Neurological Disorders

Neural firing patterns are critical for specific information coding and transmission, and abnormal firing is implicated in a series of neural pathologies. Recent studies have indicated that enhanced burst firing mediated by T-type voltage-gated calcium channels (T-VGCCs) in specific neuronal subtypes is involved in several mental or neurological disorders such as depression and epilepsy, while suppression of T-VGCCs relieve related symptoms. Burst firing consists of groups of relatively high-frequency spikes separated by quiescence. Neurons in a variety of brain areas, including the thalamus, hypothalamus, cortex, and hippocampus, display burst firing, but the ionic mechanisms that generating burst firing and the related physiological functions vary among regions. In this review, we summarize recent findings on the mechanisms underlying burst firing in various brain areas, as well as the roles of burst firing in several mental and neurological disorders. We also discuss the ion channels and receptors that may regulate burst firing directly or indirectly, with these molecules highlighted as potential intervention targets for the treatment of mental and neurological disorders.

Regulation of CaV3.2 channels by the receptor for activated C kinase 1 (Rack-1)

This study describes the interaction between Ca_V3.2 calcium channels and the receptor for activated C kinase 1 (Rack-1), a scaffold protein which has recently been implicated in neuropathic pain. The coexpression of Ca_V3.2 and Rack-1 in tsA-201 cells led to a reduction in the magnitude of whole-cell Ca_V3.2 currents and Ca_V3.2 channel expression at the plasma membrane. Co-immunoprecipitations from transfected cells show the formation of a molecular protein complex between Cav3.2 channels and Rack-1. We determined that the interaction of Rack-1 occurs at the intracellular II-III loop and the C-terminus of the channel. Finally, the coexpression of PKCβII abolished the effect of Rack-1 on current densities. Altogether, our findings show that Rack-1 regulates Ca_V3.2-mediated calcium entry in a PKC-dependent manner.

Two naturally occurring mutations of human GPR103 define distinct G protein selection bias

The neuropeptide 26RFa plays important roles in the regulation of many physiological functions. 26RFa has been recognized as an endogenous ligand for receptor GPR103. In the present study, we demonstrate that GPR103 dually couples to Gαq and Gαi/o proteins. However, two naturally occurring missense mutations were identified from a young male patient. In the first, Y68H, induction of Ca2+ mobilization was noted without detection of ERK1/2 activation. In the second, R371W, the potential to activate ERK1/2 signaling was retained but with failure to evoke Ca2+ mobilization. Further analysis provides evidence that Gαq, L-type Ca2+ channel and PKCβI and βII are involved in the Y68H-mediated signaling pathway, whereas Gαi/o, Gβγ, and PKCζ are implicated in the R371W-induced signaling. Our results demonstrate that two point mutations, Y68H and R371W, affect the equilibrium between the different receptor conformations, leading to alteration of G protein-coupling preferences. Importantly, these findings provide a foundation for future elucidation of GPCR-mediated biased signaling and the physiological implications of their bias.

Serotonin Regulates the Firing of Principal Cells of the Subiculum by Inhibiting a T-type Ca2+ Current

The subiculum is the main output of the hippocampal formation. A high proportion of its principal neurons fire action potentials in bursts triggered by the activation of low threshold calcium currents. This firing pattern promotes synaptic release and regulates spike-timing-dependent plasticity. The subiculum receives a high density of fibers originating from the raphe nuclei, suggesting that serotonin (5-HT) modulates subicular neurons. Here we investigated if and how 5-HT modulates the firing pattern of bursting neurons. By combining electrophysiological analysis with pharmacology, optogenetics and calcium imaging, we demonstrate that 5-HT_2C receptors reduce bursting activity by inhibiting a low-threshold calcium current mediated by T-type Ca²⁺ channels in principal cells of the subiculum. In addition, we show that the activation of this novel pathway decreases bursting activity and the occurrence of epileptiform discharges induced in in vitro models for temporal lobe epilepsy (TLE).

Cisplatin-induced neuropathic pain is mediated by upregulation of N-type voltage-gated calcium channels in dorsal root ganglion neurons

Cisplatin is important in the treatment of various types of cancer. Although it is highly effective, it also has severe side effects, with neurotoxicity in dorsal root ganglion (DRG) neurons being one of the most common. The key mechanisms of neurotoxicity are still controversially discussed; however, disturbances of the calcium homeostasis in DRG neurons have been suggested to mediate cisplatin neurotoxicity. By using the whole-cell patch-clamp technique, immunostaining and behavioral experiments with Sprague-Dawley rats, we examined the influence of short- and long-term exposure to cisplatin on voltage-gated calcium channel (VGCC) currents (I_Ca(V)) in small DRG neurons. In vitro exposure to cisplatin reduced I_Ca(V) in a concentration-dependent manner (0.01-50μM; 13.8-77.3%; IC₅₀ 5.07μM). Subtype-specific measurements of VGCCs showed differential effects on I_Ca(V). While the I_Ca(V) of P/Q-, L- and T-type VGCCs were reduced, I_Ca(V) of N-type VGCCs were increased by 30.3% during depolarization to 0mV. Exposure of DRG neurons to cisplatin (0.5 or 5μM) for 24-48h in vitro significantly increased a CaMK II-mediated I_Ca(V) current density. Immunostaining and western blot analysis revealed an increase of N-type VGCC protein level in DRG neurons 24h after cisplatin exposure. Cisplatin-mediated activation of caspase-3 was prevented by inhibition of N-type VGCCs using Ɯ-conotoxin MVIIA. Behavioral experiments showed that Ɯ-conotoxin MVIIA treatment prevented neuropathic syndromes in vivo by inhibiting upregulation of the N-type protein level. Here we show evidence for the first time for a crucial role of N-type VGCC in the genesis of cisplatin-induced polyneuropathy.

Protein kinase C activation mediates interferon-β-induced neuronal excitability changes in neocortical pyramidal neurons

BACKGROUND

Cytokines are key players in the interactions of the immune and nervous systems. Recently, we showed that such interplay is mediated by type I interferons (IFNs), which elevate the excitability of neocortical pyramidal neurons. A line of indirect evidence suggested that modulation of multiple ion channels underlies the effect. However, which currents are principally involved and how the IFN signaling cascade is linked to the respective ion channels remains elusive.

METHODS

We tested several single and combined ionic current modulations using an in silico model of a neocortical layer 5 neuron. Subsequently we investigated resulting predictions by whole-cell patch-clamp recordings in layer 5 neurons of ex vivo neocortical rat brain slices pharmacologically reproducing or prohibiting neuronal IFN effects.

RESULTS

The amount and type of modulation necessary to replicate IFN effects in silico suggested protein kinase C (PKC) activation as link between the type I IFN signaling and ion channel modulations. In line with this, PKC activation with 4β-phorbol 12-myristate 13-acetate (4β-PMA) or Bryostatin1 augmented the excitability of neocortical layer 5 neurons comparable to IFN-β in our ex vivo recordings. In detail, both PKC activators attenuated the rheobase and increased the input-output gain as well as the input resistance, thereby augmenting the neuronal excitability. Similar to IFN-β they also left the threshold of action potential generation unaffected. In further support of PKC mediating type I IFN effects, IFN-β, 4β-PMA and Bryostatin1 reduced the amplitude of post-train after-hyperpolarizations in a similar manner. In conjunction with this finding, IFN-β reduced M-currents, which contribute to after-hyperpolarizations and are modulated by PKC. Finally, blocking PKC activation with GF109203X at the catalytic site or calphostin C at the regulatory site prevented the main excitatory effects of IFN-β.

CONCLUSION

Multiple ion channel modulations underlie the neuromodulatory effect of type I IFNs. PKC activation is both sufficient and necessary for mediating the effect, and links the IFN signaling cascade to the intrinsic ion channels. Therefore, we regard PKC activation as unitary mechanism for the neuromodulatory potential of type I IFNs in neocortical neurons.

AKAP-dependent sensitization of Ca(v) 3.2 channels via the EP(4) receptor/cAMP pathway mediates PGE(2) -induced mechanical hyperalgesia

BACKGROUND AND PURPOSE

The Ca(v) 3.2 isoform of T-type Ca(2+) channels (T channels) is sensitized by hydrogen sulfide, a pro-nociceptive gasotransmitter, and also by PKA that mediates PGE(2) -induced hyperalgesia. Here we examined and analysed Ca(v) 3.2 sensitization via the PGE(2) /cAMP pathway in NG108-15 cells that express Ca(v) 3.2 and produce cAMP in response to PGE(2) , and its impact on mechanical nociceptive processing in rats.

EXPERIMENTAL APPROACH

In NG108-15 cells and rat dorsal root ganglion (DRG) neurons, T-channel-dependent currents (T currents) were measured with the whole-cell patch-clamp technique. The molecular interaction of Ca(v) 3.2 with A-kinase anchoring protein 150 (AKAP150) and its phosphorylation were analysed by immunoprecipitation/immunoblotting in NG108-15 cells. Mechanical nociceptive threshold was determined by the paw pressure test in rats.

KEY RESULTS

In NG108-15 cells and/or rat DRG neurons, dibutyryl cAMP (db-cAMP) or PGE(2) increased T currents, an effect blocked by AKAP St-Ht31 inhibitor peptide (AKAPI) or KT5720, a PKA inhibitor. The effect of PGE(2) was abolished by RQ-00015986-00, an EP(4) receptor antagonist. AKAP150 was co-immunoprecipitated with Ca(v) 3.2, regardless of stimulation with db-cAMP, and Ca(v) 3.2 was phosphorylated by db-cAMP or PGE(2) . In rats, intraplantar (i.pl.) administration of db-cAMP or PGE(2) caused mechanical hyperalgesia, an effect suppressed by AKAPI, two distinct T-channel blockers, NNC 55-0396 and ethosuximide, or ZnCl(2) , known to inhibit Ca(v) 3.2 among T channels. Oral administration of RQ-00015986-00 suppressed the PGE(2) -induced mechanical hyperalgesia.

CONCLUSION AND IMPLICATIONS

Our findings suggest that PGE(2) causes AKAP-dependent phosphorylation and sensitization of Ca(v) 3.2 through the EP(4) receptor/cAMP/PKA pathway, leading to mechanical hyperalgesia in rats.

Modulation of low-voltage-activated T-type Ca²⁺ channels

Low-voltage-activated T-type Ca²⁺ channels contribute to a wide variety of physiological functions, most predominantly in the nervous, cardiovascular and endocrine systems. Studies have documented the roles of T-type channels in sleep, neuropathic pain, absence epilepsy, cell proliferation and cardiovascular function. Importantly, novel aspects of the modulation of T-type channels have been identified over the last few years, providing new insights into their physiological and pathophysiological roles. Although there is substantial literature regarding modulation of native T-type channels, the underlying molecular mechanisms have only recently begun to be addressed. This review focuses on recent evidence that the Ca(v)3 subunits of T-type channels, Ca(v)3.1, Ca(v)3.2 and Ca(v)3.3, are differentially modulated by a multitude of endogenous ligands including anandamide, monocyte chemoattractant protein-1, endostatin, and redox and oxidizing agents. The review also provides an overview of recent knowledge gained concerning downstream pathways involving G-protein-coupled receptors. This article is part of a Special Issue entitled: Calcium channels.

Control of firing patterns through modulation of axon initial segment T-type calcium channels

Spontaneously active neurons typically fire either in a regular pattern or in bursts. While much is known about the subcellular location and biophysical properties of conductances that underlie regular spontaneous activity, less is known about those that underlie bursts. Here, we show that T-type Ca(2+) channels localized to the site of action potential initiation in the axon initial segment play a pivotal role in spontaneous burst generation. In auditory brainstem interneurons, axon initial segment Ca(2+) influx is selectively downregulated by dopaminergic signalling. This regulation has marked effects on spontaneous activity, converting the predominant mode of spontaneous activity from bursts to regular spiking. Thus, the axon initial segment is a key site, and dopamine a key regulator, of spontaneous bursting activity.

Caveolin-3 regulates protein kinase A modulation of the Ca(V)3.2 (alpha1H) T-type Ca2+ channels

Voltage-gated T-type Ca(2+) channel Ca(v)3.2 (α(1H)) subunit, responsible for T-type Ca(2+) current, is expressed in different tissues and participates in Ca(2+) entry, hormonal secretion, pacemaker activity, and arrhythmia. The precise subcellular localization and regulation of Ca(v)3.2 channels in native cells is unknown. Caveolae containing scaffolding protein caveolin-3 (Cav-3) localize many ion channels, signaling proteins and provide temporal and spatial regulation of intracellular Ca(2+) in different cells. We examined the localization and regulation of the Ca(v)3.2 channels in cardiomyocytes. Immunogold labeling and electron microscopy analysis demonstrated co-localization of the Ca(v)3.2 channel and Cav-3 relative to caveolae in ventricular myocytes. Co-immunoprecipitation from neonatal ventricular myocytes or transiently transfected HEK293 cells demonstrated that Ca(v)3.1 and Ca(v)3.2 channels co-immunoprecipitate with Cav-3. GST pulldown analysis confirmed that the N terminus region of Cav-3 closely interacts with Ca(v)3.2 channels. Whole cell patch clamp analysis demonstrated that co-expression of Cav-3 significantly decreased the peak Ca(v)3.2 current density in HEK293 cells, whereas co-expression of Cav-3 did not alter peak Ca(v)3.1 current density. In neonatal mouse ventricular myocytes, overexpression of Cav-3 inhibited the peak T-type calcium current (I(Ca,T)) and adenovirus (AdCa(v)3.2)-mediated increase in peak Ca(v)3.2 current, but did not affect the L-type current. The protein kinase A-dependent stimulation of I(Ca,T) by 8-Br-cAMP (membrane permeable cAMP analog) was abolished by siRNA directed against Cav-3. Our findings on functional modulation of the Ca(v)3.2 channels by Cav-3 is important for understanding the compartmentalized regulation of Ca(2+) signaling during normal and pathological processes.

G protein-mediated inhibition of Cav3.2 T-type channels revisited

T-type calcium channels are important modulators of both membrane potential and intracellular Ca(2+) concentration, allowing them to play key roles in such diverse processes as aldosterone production from adrenal glomerulosa cells to boosting pain signals in nociceptors. In both these examples, the Ca(v)3.2 isoform mediates Ca(2+) influx. This isoform is also of particular interest because mutations in its gene (CACNA1H) that enhance channel activity have been associated with idiopathic generalized epilepsies, whereas mutations that disrupt its activity have been associated with autism spectrum disorders. Block of T-channel activity has been proposed to contribute to the therapeutic usefulness of a wide variety of drugs, such as antihypertensives, antipsychotics, and antidepressants. Recent evidence strongly supports the hypothesis that block of Ca(v)3.2 channels might be useful in the treatment of neuropathic pain. Therefore, it is of particular interest that Ca(v)3.2 channels are exquisitely regulated by G protein-coupled receptors and various downstream effectors. This Perspective summarizes recent findings (p. 202) on this regulation and the novel pathways specifically activated by either neurokinin I, corticotropin-releasing factor receptor 1, or dopamine D(1) receptors.