Protein kinase C activation inhibits Cav1.3 calcium channel at NH2-terminal serine 81 phosphorylation site

CaM-dependent modulation of human CaV1.3 whole-cell and single-channel currents by C-terminal CaMKII phosphorylation site S1475

Phosphorylation enables rapid modulation of voltage-gated calcium channels (VGCC) in physiological and pathophysiological conditions. How phosphorylation modulates human CaV1.3 VGCC, however, is largely unexplored. We characterized modulation of CaV1.3 gating via S1475, the human equivalent of a phosphorylation site identified in the rat. S1475 is highly conserved in CaV1.3 but absent from all other high-voltage activating calcium channel types co-expressed with CaV1.3 in similar tissues. Further, it is located in the C-terminal EF-hand motif, which binds calmodulin (CaM). This is involved in calcium-dependent channel inactivation (CDI). We used amino acid exchanges that mimic either sustained phosphorylation (S1475D) or phosphorylation resistance (S1475A). Whole-cell and single-channel recordings of phosphorylation state imitating CaV1.3 variants in transiently transfected HEK-293 cells revealed functional relevance of S1475 in human CaV1.3. We obtained three main findings: (1) CaV1.3_S1475D, imitating sustained phosphorylation, displayed decreased current density, reduced CDI and (in-) activation kinetics shifted to more depolarized voltages compared with both wildtype CaV1.3 and the phosphorylation-resistant CaV1.3_S1475A variant. Corresponding to the decreased current density, we find a reduced open probability of CaV1.3_S1475D at the single-channel level. (2) Using CaM overexpression or depletion, we find that CaM is necessary for modulating CaV1.3 through S1475. (3) CaMKII activation led to CaV1.3_WT-current properties similar to those of CaV1.3_S1475D, but did not affect CaV1.3_S1475A, confirming that CaMKII modulates human CaV1.3 via S1475. Given the physiological and pathophysiological importance of CaV1.3, our findings on the S1475-mediated interplay of phosphorylation, CaM interaction and CDI provide hints for approaches on specific CaV1.3 modulation under physiological and pathophysiological conditions. KEY POINTS: Phosphorylation modulates activity of voltage-gated L-type calcium channels for specific cellular needs but is largely unexplored for human CaV1.3 channels. Here we report that S1475, a CaMKII phosphorylation site identified in rats, is functionally relevant in human CaV1.3. Imitating phosphorylation states at S1475 alters current density and inactivation in a calmodulin-dependent manner. In wildtype CaV1.3 but not in the phosphorylation-resistant variant S1475A, CaMKII activation elicits effects similar to constitutively mimicking phosphorylation at S1475. Our findings provide novel insights on the interplay of modulatory mechanisms of human CaV1.3 channels, and present a possible target for CaV1.3-specific gating modulation in physiological and pathophysiological conditions.

Pathophysiology of Cav1.3 L-type calcium channels in the heart

Ca²⁺ plays a crucial role in excitation-contraction coupling in cardiac myocytes. Dysfunctional Ca²⁺ regulation alters the force of contraction and causes cardiac arrhythmias. Ca²⁺ entry into cardiomyocytes is mediated mainly through L-type Ca²⁺ channels, leading to the subsequent Ca²⁺ release from the sarcoplasmic reticulum. L-type Ca²⁺ channels are composed of the conventional Ca_v1.2, ubiquitously expressed in all heart chambers, and the developmentally regulated Ca_v1.3, exclusively expressed in the atria, sinoatrial node, and atrioventricular node in the adult heart. As such, Ca_v1.3 is implicated in the pathogenesis of sinoatrial and atrioventricular node dysfunction as well as atrial fibrillation. More recently, Ca_v1.3 de novo expression was suggested in heart failure. Here, we review the functional role, expression levels, and regulation of Ca_v1.3 in the heart, including in the context of cardiac diseases. We believe that the elucidation of the functional and molecular pathways regulating Ca_v1.3 in the heart will assist in developing novel targeted therapeutic interventions for the aforementioned arrhythmias.

Regulation of L-type CaV1.3 channel activity and insulin secretion by the cGMP-PKG signaling pathway

cGMP is a second messenger widely used in the nervous system and other tissues. One of the major effectors for cGMP is the serine/threonine protein kinase, cGMP-dependent protein kinase (PKG), which catalyzes the phosphorylation of a variety of proteins including ion channels. Previously, it has been shown that the cGMP-PKG signaling pathway inhibits Ca²⁺ currents in rat vestibular hair cells and chromaffin cells. This current allegedly flow through voltage-gated Ca_V1.3L-type Ca²⁺ channels, and is important for controlling vestibular hair cell sensory function and catecholamine secretion, respectively. Here, we show that native L-type channels in the insulin-secreting RIN-m5F cell line, and recombinant Ca_V1.3 channels heterologously expressed in HEK-293 cells, are regulatory targets of the cGMP-PKG signaling cascade. Our results indicate that the Ca_Vα₁ ion-conducting subunit of the Ca_V1.3 channels is highly expressed in RIN-m5F cells and that the application of 8-Br-cGMP, a membrane-permeable analogue of cGMP, significantly inhibits Ca²⁺ macroscopic currents and impair insulin release stimulated with high K⁺. In addition, KT-5823, a specific inhibitor of PKG, prevents the current inhibition generated by 8-Br-cGMP in the heterologous expression system. Interestingly, mutating the putative phosphorylation sites to residues resistant to phosphorylation showed that the relevant PKG sites for Ca_V1.3 L-type channel regulation centers on two amino acid residues, Ser793 and Ser860, located in the intracellular loop connecting the II and III repeats of the Ca_Vα₁ pore-forming subunit of the channel. These findings unveil a novel mechanism for how the cGMP-PKG signaling pathway may regulate Ca_V1.3 channels and contribute to regulate insulin secretion.

Potential Role of Caffeine in the Treatment of Parkinson's Disease

Parkinson’s disease [PD] is the second most common neurodegenerative disorder after Alzheimer’s disease, affecting 1% of the population over the age of 55. The underlying neuropathology seen in PD is characterised by progressive loss of dopaminergic neurons in the substantia nigra pars compacta with the presence of Lewy bodies. The Lewy bodies are composed of aggregates of α-synuclein. The motor manifestations of PD include a resting tremor, bradykinesia, and muscle rigidity. Currently there is no cure for PD and motor symptoms are treated with a number of drugs including levodopa [L-dopa]. These drugs do not delay progression of the disease and often provide only temporary relief. Their use is often accompanied by severe adverse effects. Emerging evidence from both in vivo and in vitro studies suggests that caffeine may reduce parkinsonian motor symptoms by antagonising the adenosine A_2A receptor, which is predominately expressed in the basal ganglia. It is hypothesised that caffeine may increase the excitatory activity in local areas by inhibiting the astrocytic inflammatory processes but evidence remains inconclusive. In addition, the co-administration of caffeine with currently available PD drugs helps to reduce drug tolerance, suggesting that caffeine may be used as an adjuvant in treating PD. In conclusion, caffeine may have a wide range of therapeutic effects which are yet to be explored, and therefore warrants further investigation in randomized clinical trials.

GABA(B) receptors couple to Gαq to mediate increases in voltage-dependent calcium current during development

Metabotropic GABA(B) receptors are known to modulate the activity of voltage-dependent calcium channels. Previously, we have shown that GABA(B) receptors couple to a non-Gi/o G-protein to enhance calcium influx through L-type calcium channels by activating protein kinase C in neonatal rat hippocampal neurons. In this study, the components of this signaling pathway were investigated further. Gαq was knocked down using morpholino oligonucleotides prior to examining GABA(B) -mediated enhancement of calcium influx. When Gαq G-proteins were eliminated using morpholino-mediated knockdown, the enhancing effects of the GABA(B) receptor agonist baclofen (10 μM) on calcium current or entry were eliminated. These data suggest that GABA(B) receptors couple to Gαq to regulate calcium influx. Confocal imaging analysis illustrating colocalization of GABA(B) receptors with Gαq supports this hypothesis. Furthermore, baclofen treatment caused translocation of PKCα (protein kinase C α) but not PKCβ or PKCε, suggesting that it is the α isoform of PKC that mediates calcium current enhancement. Inhibition of calcium/calmodulin-dependent kinase II did not affect the baclofen-mediated enhancement of calcium levels. In summary, activation of GABA(B) receptors during development leads to increased calcium in a subset of neurons through Gαq signaling and PKCα activation without the involvement of calcium/calmodulin-dependent kinase II. Activation of GABA(B) receptors in the neonatal rat hippocampus enhances voltage-dependent calcium currents independently of Gi/o . In this study, knockdown of Gαq with morpholino oligonucleotides abolished enhancement of calcium influx and protein kinase Cα was activated by GABA(B) receptors. Therefore, we hypothesize that GABA(B) receptors couple to Gq to activate PKCα leading to enhancement of L-type calcium current.

Roles of PKC Isoforms in PMA-Induced Modulation of the hERG Channel (Kv11.1)

Protein kinases C (PKC) modulate the activity of the Kv11.1 ion channel current (hERG). However, the differential effects of specific PKC subtypes on the biophysics of the channel are unknown. The pharmaceutical tools to selectively modulate PKC subtypes are not membrane permeable and must be added directly to the intracellular solution in electrophysiology studies. Here, the PatchXpress electrophysiology robot was used to voltage clamp up to 16 cells simultaneously yet asynchronously across individual Sealchip chambers. The precision afforded by repeats of automation procedures minimized the experimental errors typical of these assays. Eight well-known PKC selective peptidomimmetics and general synthetic modulators were used to modulate the protein-protein interactions between hERG and the major PKC subtypes. We identified a specific role for the PKCε inhibitory peptidomimmetics in decreasing PKC-induced hERG τ activation (80%) and half-maximum activation voltage (90%) at steady state; a specific PKCε activator exhibited the opposite effect. Disruption of PKCβ, PKCα, and PKCη interactions also showed significant effects albeit of lower magnitudes. The effect of PKCδ inhibitor was only marginal. A significant correlation was observed between the shifts in τ activation and half-maximum voltage at steady state (R(2)= 0.85). Peak current amplitudes and time constant of deactivation remained unaffected in all conditions.

Calreticulin negatively regulates the surface expression of Cav1.3 L-type calcium channel

BACKGROUND

The neuroendocrine Cav1.3 L-type Ca channels have been recently found in the Human fetal heart and shown to play a vital role in Ca entry from the sarcolemma into the cell and in Ca homeostasis. Calreticulin, a Ca binding endoplasmic reticulum (ER) resident protein, has been recently shown to translocate to the cell surface where its role and function are just emerging. Here, we demonstrated a novel mechanism of Cav1.3 and calreticulin interaction resulting in downregulation of Cav1.3 channel densities in native Human fetal cardiac cells and Human Embryonic Kidney cell lines (tsA201).

METHODS AND RESULTS

Cell surface and cytoplasmic staining of calreticulin was demonstrated first in cultured human fetal cardiomyocytes (HFC), gestational age 18-24 weeks, using confocal microscopy thereby establishing that calreticulin is present at the cell surface in HFC. Co-immunoprecipitation from HFC using anti-Cav1.3 Ca channel antibody, and probing with anti-calreticulin antibody revealed a 46 kDa band corresponding to calreticulin suggesting that Cav1.3 Ca channel and calreticulin co-assemble in a macromolecular complex. Co-expression of Cav1.3 and calreticulin in tsA201 cells resulted in a decrease in surface expression of Cav1.3 Ca channels. These findings were consistent with the electrophysiological studies showing that co-transfection of Cav1.3 Ca channel and calreticulin resulted in 55% reduction of Cav1.3 Ca current densities recorded from tsA201 cells.

CONCLUSIONS

The results show the first evidence that calreticulin: (1) is localized outside the ER on the cell surface of HFC; (2) coimmunoprecipitates with Cav1.3 L-type Ca channel; (3) negatively regulates Cav1.3 surface expression thus resulting in decreased Cav1.3 Ca current densities. The data demonstrate a novel mechanism of modulation of Cav1.3 Ca channel by calreticulin, which may be involved in pathological settings such as autoimmune associated congenital heart block where Cav1.3 Ca channels are downregulated.

Drosophila MagT1 is upregulated by PKC activation

Magnesium transporter subtype 1 (MagT1) is a newly discovered and evolutionarily conservative magnesium membrane transporter with channel like properties. Previous reports have demonstrated that MagT1 is important to cellular magnesium homeostasis. In this study, we investigated whether drosophila MagT1 (dMagT1) was functionally regulated by PKC activation in vitro. With patch clamping, we have observed that whole cell currents of wild type dMagT1 were magnesium selective and non-voltage dependent when expressed in a human neuroblastoma SH-SY5Y cell line. Furthermore, dMagT1 currents were significantly increased in cells treated with a non specific PKC activator PMA, but not in cells treated with the inactive form of PMA, 4α-PMA. Lastly, we have demonstrated that upregulation of dMagT1 currents by PKC activation involves specific PKC phorsphorylation sites in dMagT1. Of all three dMagT1 mutants created for testing the putative PKC phorsphorylation sites, dMagT1-S35A displayed a significant increase of whole cell currents while dMagT1-S100A and -S108A were not affected by PKC activation. Thus, we have demonstrated that dMagT1 is a magnesium selective transporter with basic biophysical characters similar to its mammalian homolog and can be functionally upregulated by PKC activation. Both dMagT1 Ser100 and Ser106 are equally important to this PKC-dependent modulation, therefore the most likely molecular sites for PKC phorsphorylation. The data presented here may establish a general regulatory mechanism for MagT1 by PKC activation.

Voltage-dependent amplification of synaptic inputs in respiratory motoneurones

The role of persistent inward currents (PICs) in cat respiratory motoneurones (phrenic inspiratory and thoracic expiratory) was investigated by studying the voltage-dependent amplification of central respiratory drive potentials (CRDPs), recorded intracellularly, with action potentials blocked with the local anaesthetic derivative, QX-314. Decerebrate unanaesthetized or barbiturate-anaesthetized preparations were used. In expiratory motoneurones, plateau potentials were observed in the decerebrates, but not under anaesthesia. For phrenic motoneurones, no plateau potentials were observed in either state (except in one motoneurone after the abolition of the respiratory drive by means of a medullary lesion), but all motoneurones showed voltage-dependent amplification of the CRDPs, over a wide range of membrane potentials, too wide to result mainly from PIC activation. The measurements of the amplification were restricted to the phase of excitation, thus excluding the inhibitory phase. Amplification was found to be greatest for the smallest CRDPs in the lowest resistance motoneurones and was reduced or abolished following intracellular injection of the NMDA channel blocker, MK-801. Plateau potentials were readily evoked in non-phrenic cervical motoneurones in the same (decerebrate) preparations. We conclude that the voltage-dependent amplification of synaptic excitation in phrenic motoneurones is mainly the result of NMDA channel modulation rather than the activation of Ca2+ channel mediated PICs, despite phrenic motoneurones being strongly immunohistochemically labelled for CaV1.3 channels. The differential PIC activation in different motoneurones, all of which are CaV1.3 positive, leads us to postulate that the descending modulation of PICs is more selective than has hitherto been believed.

GABA(B) receptor-mediated ERK1/2 phosphorylation via a direct interaction with Ca(V)1.3 channels

Neuronal L-type Ca(2+) channels play pivotal roles in regulating gene expression, cell survival, and synaptic plasticity. The Ca(V)1.2 and Ca(V)1.3 channels are 2 main subtypes of neuronal L-type Ca(2+) channels. However, the specific roles of Ca(V)1.2 and Ca(V)1.3 in L-type Ca(2+) channel-mediated neuronal responses and their cellular mechanisms are poorly elucidated. On the basis of our previous study demonstrating a physical interaction between the Ca(V)1.3 channel and GABA(B) receptor (GABA(B)R), we further examined the involvement of Ca(V)1.2 and Ca(V)1.3 in the GABA(B)R-mediated activation of ERK(1/2), a kinase involved in both CREB activation and synaptic plasticity. After confirming the involvement of L-type Ca(2+) channels in baclofen-induced ERK(1/2) phosphorylation, we examined a specific role of Ca(V)1.2 and Ca(V)1.3 channels in the baclofen effect. Using siRNA-mediated silencing of Ca(V)1.2 or Ca(V)1.3 messenger, we determined the relevance of each channel subtype to baclofen-induced ERK(1/2) phosphorylation in a mouse hippocampal cell line (HT-22) and primary cultured rat neurons. In the detailed characterization of each subtype using HEK293 cells transfected with Ca(V)1.2 or Ca(V)1.3, we found that GABA(B)R can increase ERK(1/2) phosphorylation and Ca(V)1.3 channel activity through direct interaction with Ca(V)1.3 channels. These results suggest a functional interaction between Ca(V)1.3 and GABA(B)R and important implications of Ca(V)1.3/GABA(B)R clusters for translating synaptic activity into gene expression alterations.

Regulation of cardiac excitability by protein kinase C isozymes

Cardiac excitability and electrical activity are determined by the sum of individual ion channels, gap junctions and exchanger activities. Electrophysiological remodeling during heart disease involves changes in membrane properties of cardiomyocytes and is related to higher prevalence of arrhythmia-associated morbidity and mortality. Pharmacological and genetic manipulation of cardiac cells as well as animal models of cardiovascular diseases are used to identity changes in electrophysiological properties and the molecular mechanisms associated with the disease. Protein kinase C (PKC) and several other kinases play a pivotal role in cardiac electrophysiological remodeling. Therefore, identifying specific therapies that regulate these kinases is the main focus of current research. PKC, a family of serine/threonine kinases, has been implicated as potential signaling nodes associated with biochemical and biophysical stress in cardiovascular diseases. In this review, we describe the role of PKC isozymes that are involved in cardiac excitability and discuss both genetic and pharmacological tools that were used, their attributes and limitations. Selective and effective pharmacological interventions to normalize cardiac electrical activities and correct cardiac arrhythmias will be of great clinical benefit.

Alternative perspective on intestinal calcium absorption: proposed complementary actions of Ca(v)1.3 and TRPV6

Transcellular models of dietary Ca(2+) absorption by the intestine assign essential roles to TRPV6 and calbindin-D(9K) . However, studies with gene-knockout mice challenge this view. Something fundamental is missing. The L-type channel Ca(v) 1.3 is located in the apical membrane from the duodenum to the ileum. In perfused rat jejunum in vivo and in Caco-2 cells, Ca(v) 1.3 mediates sodium glucose transporter 1 (SGLT1)-dependent and prolactin-induced active, transcellular Ca(2+) absorption, respectively. TRPV6 is activated by hyperpolarization and is vitamin D dependent; in contrast, Ca(v) 1.3 is activated by depolarization and is independent of calbindin-D(9K) and vitamin D. This review considers evidence supporting the idea that Ca(v) 1.3 and TRPV6 have complementary roles in the regulation of intestinal Ca(2+) absorption as depolarization and repolarization of the apical membrane occur during and between digestive periods, respectively, and as chyme moves from one intestinal segment to another and food transit times increase. Reassessment of current arguments for paracellular flow reveals that key phenomena have alternative explanations within the integrated Ca(v) 1.3/TRPV6 view of transcellular Ca(2+) absorption.

Global gene expression analysis of rodent motor neurons following spinal cord injury associates molecular mechanisms with development of postinjury spasticity

Spinal cord injury leads to severe problems involving impaired motor, sensory, and autonomic functions. After spinal injury there is an initial phase of hyporeflexia followed by hyperreflexia, often referred to as spasticity. Previous studies have suggested a relationship between the reappearance of endogenous plateau potentials in motor neurons and the development of spasticity after spinalization. To unravel the molecular mechanisms underlying the increased excitability of motor neurons and the return of plateau potentials below a spinal cord injury we investigated changes in gene expression in this cell population. We adopted a rat tail-spasticity model with a caudal spinal transection that causes a progressive development of spasticity from its onset after 2 to 3 wk until 2 mo postinjury. Gene expression changes of fluorescently identified tail motor neurons were studied 21 and 60 days postinjury. The motor neurons undergo substantial transcriptional regulation in response to injury. The patterns of differential expression show similarities at both time points, although there are 20% more differentially expressed genes 60 days compared with 21 days postinjury. The study identifies targets of regulation relating to both ion channels and receptors implicated in the endogenous expression of plateaux. The regulation of excitatory and inhibitory signal transduction indicates a shift in the balance toward increased excitability, where the glutamatergic N-methyl-d-aspartate receptor complex together with cholinergic system is up-regulated and the gamma-aminobutyric acid type A receptor system is down-regulated. The genes of the pore-forming proteins Cav1.3 and Nav1.6 were not up-regulated, whereas genes of proteins such as nonpore-forming subunits and intracellular pathways known to modulate receptor and channel trafficking, kinetics, and conductivity showed marked regulation. On the basis of the identified changes in global gene expression in motor neurons, the present investigation opens up for new potential targets for treatment of motor dysfunction following spinal cord injury.

Supramolecular assemblies and localized regulation of voltage-gated ion channels

This review addresses the localized regulation of voltage-gated ion channels by phosphorylation. Comprehensive data on channel regulation by associated protein kinases, phosphatases, and related regulatory proteins are mainly available for voltage-gated Ca2+ channels, which form the main focus of this review. Other voltage-gated ion channels and especially Kv7.1-3 (KCNQ1-3), the large- and small-conductance Ca2+-activated K+ channels BK and SK2, and the inward-rectifying K+ channels Kir3 have also been studied to quite some extent and will be included. Regulation of the L-type Ca2+ channel Cav1.2 by PKA has been studied most thoroughly as it underlies the cardiac fight-or-flight response. A prototypical Cav1.2 signaling complex containing the beta2 adrenergic receptor, the heterotrimeric G protein Gs, adenylyl cyclase, and PKA has been identified that supports highly localized via cAMP. The type 2 ryanodine receptor as well as AMPA- and NMDA-type glutamate receptors are in close proximity to Cav1.2 in cardiomyocytes and neurons, respectively, yet independently anchor PKA, CaMKII, and the serine/threonine phosphatases PP1, PP2A, and PP2B, as is discussed in detail. Descriptions of the structural and functional aspects of the interactions of PKA, PKC, CaMKII, Src, and various phosphatases with Cav1.2 will include comparisons with analogous interactions with other channels such as the ryanodine receptor or ionotropic glutamate receptors. Regulation of Na+ and K+ channel phosphorylation complexes will be discussed in separate papers. This review is thus intended for readers interested in ion channel regulation or in localization of kinases, phosphatases, and their upstream regulators.

Interplay between cytosolic dopamine, calcium, and alpha-synuclein causes selective death of substantia nigra neurons

The basis for selective death of specific neuronal populations in neurodegenerative diseases remains unclear. Parkinson's disease (PD) is a synucleinopathy characterized by a preferential loss of dopaminergic neurons in the substantia nigra (SN), whereas neurons of the ventral tegmental area (VTA) are spared. Using intracellular patch electrochemistry to directly measure cytosolic dopamine (DA(cyt)) in cultured midbrain neurons, we confirm that elevated DA(cyt) and its metabolites are neurotoxic and that genetic and pharmacological interventions that decrease DA(cyt) provide neuroprotection. L-DOPA increased DA(cyt) in SN neurons to levels 2- to 3-fold higher than in VTA neurons, a response dependent on dihydropyridine-sensitive Ca2+ channels, resulting in greater susceptibility of SN neurons to L-DOPA-induced neurotoxicity. DA(cyt) was not altered by alpha-synuclein deletion, although dopaminergic neurons lacking alpha-synuclein were resistant to L-DOPA-induced cell death. Thus, an interaction between Ca2+, DA(cyt), and alpha-synuclein may underlie the susceptibility of SN neurons in PD, suggesting multiple therapeutic targets.

PKC and PLA2: probing the complexities of the calcium network

Lipid signaling and phosphorylation cascades are fundamental to calcium signaling networks. In this review, we will discuss the recent laboratory findings for the phospholipase A(2) (PLA(2))/protein kinase C (PKC) pathway within cellular calcium networks. The complexity and connectivity of these ubiquitous cellular signals make interpretation of experimental results extremely challenging. We present here computational methods which have been developed to conquer such complex data, and how they can be used to make models capable of accurately predicting cellular responses within multiple calcium signaling pathways. We propose that information obtained from network analysis and computational techniques provides a rich source of knowledge which can be directly translated to the laboratory benchtop.

Phosphorylation of the consensus sites of protein kinase A on alpha1D L-type calcium channel

The novel alpha(1D) L-type Ca(2+) channel is expressed in supraventricular tissue and has been implicated in the pacemaker activity of the heart and in atrial fibrillation. We recently demonstrated that PKA activation led to increased alpha(1D) Ca(2+) channel activity in tsA201 cells by phosphorylation of the channel protein. Here we sought to identify the phosphorylated PKA consensus sites on the alpha(1) subunit of the alpha(1D) Ca(2+) channel by generating GST fusion proteins of the intracellular loops, N terminus, proximal and distal C termini of the alpha(1) subunit of alpha(1D) Ca(2+) channel. An in vitro PKA kinase assay was performed for the GST fusion proteins, and their phosphorylation was assessed by Western blotting using either anti-PKA substrate or anti-phosphoserine antibodies. Western blotting showed that the N terminus and C terminus were phosphorylated. Serines 1743 and 1816, two PKA consensus sites, were phosphorylated by PKA and identified by mass spectrometry. Site directed mutagenesis and patch clamp studies revealed that serines 1743 and 1816 were major functional PKA consensus sites. Altogether, biochemical and functional data revealed that serines 1743 and 1816 are major functional PKA consensus sites on the alpha(1) subunit of alpha(1D) Ca(2+) channel. These novel findings provide new insights into the autonomic regulation of the alpha(1D) Ca(2+) channel in the heart.

Cholinergic excitation of dopaminergic cells depends on sequential activation of protein kinase C and the L-type calcium channel in ventral tegmental area slices

Dopaminergic projections from the ventral tegmental area (VTA) constitute the mesolimbocortical system that underlies addiction and psychosis primarily as the result of increased dopaminergic transmission. Dopaminergic neurons in the VTA receive glutamatergic and cholinergic innervations that regulate their firing activities. Both transmitter systems can activate protein kinase C (PKC) by increasing intracellular calcium and lipid second messengers, however, whether PKC mediates increased firing following glutamatergic and cholinergic activation remains unknown. This paper examined the effects of acute PKC inhibition on firing responses to carbachol, NMDA or AMPA using patch clamp recordings from brain slices. The three ligands all induced a reversible increase in firing, however, only carbachol-induced increase in firing was attenuated by the PKC inhibitors chelerythrine or GF 109203X. The L-type calcium channel blocker nifedipine partially blocked carbachol-induced excitation similar to PKC inhibitors. PKC inhibition and L-type channel blockade did not significantly alter NMDA- or AMPA-induced excitation. Concurrent blockade of PKC and L-type channels with chelerythrine and nifedipine did not additively suppress carbachol-induced excitation indicating they were sequential events in the same signaling pathway. Furthermore, preincubation with the PKC inhibitor GF 109203X reduced the carbachol-induced increase in nifedipine-sensitive high-voltage gated calcium currents. These results indicate that cholinergic activation enhances PKC activity, which in turn facilitates L-type channel opening to excite dopaminergic cells, a finding that is in line with reports of increased PKC in the VTA in animals displaying addictive behavior.

Dynamic transcriptomic response to acute hypertension in the nucleus tractus solitarius

Baroreceptor afferents project to the cardiovascular region of the nucleus tractus solitarius (cvNTS), and their cvNTS target neurons may play a role in governing the sensitivity and operating range of the arterial baroreceptor reflex (baroreflexes). Recent studies have shown differential gene and protein expression in the cvNTS in response to changed arterial pressure. However, the extent of these responses is unknown. Therefore, we collected differential global gene expression data in a time series following acute hypertension in awake, freely moving rats. To acquire statistically significant results and place them in functional context, we overcame several quality control requirements and developed novel analytical approaches. The physiologically new findings from the study are that acute hypertension causes very extensive, time-varying gene regulatory changes, many involving neuronal function-specific genes and systems of genes. We use standard genomic analysis methods to manage the large data sets and to develop results such as heat maps to examine patterns and clusters in the gene regulation. We used the Gene Ontology categories to provide functional context. To place our findings in the context of the relevant literature, we developed two graphical representations of the networks implicated, linking receptors and channels to signaling pathways. The results point to the multivariate complexity of the response and implicate a group of receptors as candidates for mediating nucleus tractus solitarius baroreflex function in hypertension by identifying concurrent upregulation of receptor genes. We were able to make transcription factor binding predictions and record dysregulation of heart rate correlated with the transcriptional response.

Protein kinase C activation inhibits alpha1D L-type Ca channel: a single-channel analysis

The recently reported alpha1D Ca channel in the heart is known to be regulated by protein kinase C (PKC) at the whole cell level and has been implicated in atrial fibrillation. The biophysical basis of this regulation at the single-channel level is not known. Therefore, the effect of PKC activation was studied on alpha1D Ca channel expressed in tsA201 cells using cell-attached configuration. Unitary currents were recorded in the presence of 70 mM Ba2+ as the charge carrier at room temperature. Under basal condition, channel activity was rare and infrequent; however, Bay K 8644 (1 microM) induced channel openings with a conductance of 22.3 pS. Single channel analysis of open and closed time distributions were best fitted with a single exponential. PKC activation by 4alpha-phorbol 12-myristate 13-acetate (PMA; 10 nM), a phorbol ester derivative, resulted in a decrease in open probability and increase in closed-time without any significant effect on the conductance of the alpha1D Ca channel. This is consistent with a decreased entry of alpha1D Ca channel into open states in the presence of PMA. PMA effects could not be reproduced by 4-alpha Phorbol, an inactive PMA analogue. These data show, for the first time, (1) the alpha1D Ca channel activity at the single-channel level and (2) the biophysical basis by which PKC activation inhibits the alpha1D Ca channel. The shortening of the open-time and the lengthening of the closed-time constants and the increase in blank sweeps may explain the inhibition of the previously reported whole-cell alpha1D Ca current. Altogether, these data are essential for understanding the complex role of alpha1D Ca channel not only in physiological settings but also in pathological settings such as atrial fibrillation.