It is well established that pacemaker activity of the sino-atrial node (SAN) initiates the heartbeat. However, the atrioventricular node (AVN) can generate viable pacemaker activity in case of SAN failure, but we have limited knowledge of the ionic bases of AVN automaticity. We characterized pacemaker activity and ionic currents in automatic myocytes of the mouse AVN. Pacemaking of AVN cells (AVNCs) was lower than that of SAN pacemaker cells (SANCs), both in control conditions and upon perfusion of isoproterenol (ISO). Block of INa by tetrodotoxin (TTX) or of ICa,L by isradipine abolished AVNCs pacemaker activity. TTX-resistant (INar) and TTX-sensitive (INas) Na+ currents were recorded in mouse AVNCs, as well as T-(ICa,T) and L-type (ICa,L) Ca2+ currents. ICa,L density was lower than in SANCs (51%). The density of the hyperpolarization-activated current, (If) and that of the fast component of the delayed rectifier current (IKr) were, respectively, lower (52%) and higher (53%) in AVNCs than in SANCs. Pharmacological inhibition of If by 3 µM ZD-7228 reduced pacemaker activity by 16%, suggesting a relevant role for If in AVNCs automaticity. Some AVNCs expressed also moderate densities of the transient outward K+ current (Ito). In contrast, no detectable slow component of the delayed rectifier current (IKs) could be recorded in AVNCs. The lower densities of If and ICa,L, as well as higher expression of IKr in AVNCs than in SANCs may contribute to the intrinsically slower AVNCs pacemaking than that of SANCs.
atrioventricular node; sino-atrial node; pacemaker activity; ion channels; electrophysiology; conduction; heart rate; Ca2+ channels; Na+ channels; f-channels; K+ channels
The atrioventricular node controls cardiac impulse conduction and generates pacemaker activity in case of failure of the sino-atrial node. Understanding the mechanisms of atrioventricular automaticity is important for managing human pathologies of heart rate and conduction. However, the physiology of atrioventricular automaticity is still poorly understood. We have investigated the role of three key ion channel-mediated pacemaker mechanisms namely, Cav1.3, Cav3.1 and HCN channels in automaticity of atrioventricular node cells (AVNCs). We studied atrioventricular conduction and pacemaking of AVNCs in wild-type mice and mice lacking Cav3.1 (Cav3.1−/−), Cav1.3 (Cav1.3−/−), channels or both (Cav1.3−/−/Cav3.1−/−). The role of HCN channels in the modulation of atrioventricular cells pacemaking was studied by conditional expression of dominant-negative HCN4 channels lacking cAMP sensitivity. Inactivation of Cav3.1 channels impaired AVNCs pacemaker activity by favoring sporadic block of automaticity leading to cellular arrhythmia. Furthermore, Cav3.1 channels were critical for AVNCs to reach high pacemaking rates under isoproterenol. Unexpectedly, Cav1.3 channels were required for spontaneous automaticity, because Cav1.3−/− and Cav1.3−/−/Cav3.1−/− AVNCs were completely silent under physiological conditions. Abolition of the cAMP sensitivity of HCN channels reduced automaticity under basal conditions, but maximal rates of AVNCs could be restored to that of control mice by isoproterenol. In conclusion, while Cav1.3 channels are required for automaticity, Cav3.1 channels are important for maximal pacing rates of mouse AVNCs. HCN channels are important for basal AVNCs automaticity but do not appear to be determinant for β-adrenergic regulation.
genetically-engineered mice; pacemaker activity; atrioventricular node; congenital heart block; sino-atrial node dysfunction; ion channels; Cav1.3 channels; Cav3.1 channels; HCN channels; electrophysiology; conduction; heart rate
A C-terminal modulatory domain (CTM) tightly regulates the biophysical properties of Cav1.3 L-type Ca2+ channels, in particular the voltage dependence of activation (V0.5) and Ca2+ dependent inactivation (CDI). A functional CTM is present in the long C-terminus of human and mouse Cav1.3 (Cav1.3L), but not in a rat long cDNA clone isolated from superior cervical ganglia neurons (rCav1.3scg). We therefore addressed the question if this represents a species-difference and compared the biophysical properties of rCav1.3scg with a rat cDNA isolated from rat pancreas (rCav1.3L).
When expressed in tsA-201 cells under identical experimental conditions rCav1.3L exhibited Ca2+ current properties indistinguishable from human and mouse Cav1.3L, compatible with the presence of a functional CTM. In contrast, rCav1.3scg showed gating properties similar to human short splice variants lacking a CTM. rCav1.3scg differs from rCav1.3L at three single amino acid (aa) positions, one alternative spliced exon (exon31), and a N-terminal polymethionine stretch with two additional lysines. Two aa (S244, A2075) in rCav1.3scg explained most of the functional differences to rCav1.3L. Their mutation to the corresponding residues in rCav1.3L (G244, V2075) revealed that both contributed to the more negative V0.5, but caused opposite effects on CDI. A2075 (located within a region forming the CTM) additionally permitted higher channel open probability. The cooperative action in the double-mutant restored gating properties similar to rCav1.3L. We found no evidence for transcripts containing one of the single rCav1.3scg mutations in rat superior cervical ganglion preparations. However, the rCav1.3scg variant provided interesting insight into the structural machinery involved in Cav1.3 gating.
cacna1d; Cav1.3; voltage-gated Ca2+ channels; species differences; gating; mutagenesis
An intramolecular interaction between a distal (DCRD) and a proximal regulatory
domain (PCRD) within the C terminus of long Cav1.3 L-type
Ca2+ channels (Cav1.3L) is a major
determinant of their voltage- and Ca2+-dependent gating
kinetics. Removal of these regulatory domains by alternative splicing generates
Cav1.342A channels that activate at a more negative
voltage range and exhibit more pronounced Ca2+-dependent
inactivation. Here we describe the discovery of a novel short splice variant
(Cav1.343S) that is expressed at high levels in the
brain but not in the heart. It lacks the DCRD but, in contrast to
Cav1.342A, still contains PCRD. When expressed
together with α2δ1 and β3 subunits in tsA-201 cells,
Cav1.343S also activated at more negative voltages
like Cav1.342A but Ca2+-dependent
inactivation was less pronounced. Single channel recordings revealed much higher
channel open probabilities for both short splice variants as compared with
Cav1.3L. The presence of the proximal C terminus in
Cav1.343S channels preserved their modulation by
distal C terminus-containing Cav1.3- and Cav1.2-derived
C-terminal peptides. Removal of the C-terminal modulation by alternative
splicing also induced a faster decay of Ca2+ influx during
electrical activities mimicking trains of neuronal action potentials. Our
findings extend the spectrum of functionally diverse Cav1.3 L-type
channels produced by tissue-specific alternative splicing. This diversity may
help to fine tune Ca2+ channel signaling and, in the case of
short variants lacking a functional C-terminal modulation, prevent excessive
Ca2+ accumulation during burst firing in neurons. This may
be especially important in neurons that are affected by
Ca2+-induced neurodegenerative processes.
Alternative Splicing; Calcium Channels; Calcium Signaling; Cell Signaling; Ion Channels; CaV1.3; L-type Calcium Channels; Alternative Splicing; Cellular Excitability; Ion Channels
Auxiliary β subunits are critical determinants of membrane expression and gating properties of voltage-gated calcium channels. Mutations in the β4 subunit gene cause ataxia and epilepsy. However, the specific function of β4 in neurons and its causal relation to neurological diseases are unknown. Here we report the localization of the β4 subunit in the nuclei of cerebellar granule and Purkinje cells. β4b was the only β isoform showing nuclear targeting when expressed in neurons and skeletal myotubes. Its specific nuclear targeting property was mapped to an N-terminal double-arginine motif, which was necessary and sufficient for targeting β subunits into the nucleus. Spontaneous electrical activity and calcium influx negatively regulated β4b nuclear localization by a CRM-1-dependent nuclear export mechanism. The activity-dependent shuttling of β4b into and out of the nucleus indicates a specific role of this β subunit in neurons, in communicating the activity of calcium channels to the nucleus.
voltage-gated calcium channels; CACNB4; Ca2+; nuclear export; hippocampal neurons; cerebellum
In response to light, the mouse retinal pigment epithelium (RPE) generates a series of slow changes in potential that are referred to as the c-wave, fast oscillation (FO), and light peak (LP) of the electroretinogram (ERG). The LP is generated by a depolarization of the basolateral RPE plasma membrane by the activation of a calcium-sensitive chloride conductance. We have previously shown that the LP is reduced in both mice and rats by nimodipine, which blocks voltage-dependent calcium channels (VDCCs) and is abnormal in lethargic mice, carrying a null mutation in the calcium channel β4 subunit. To define the α1 subunit involved in this process, we examined mice lacking CaV1.3. In comparison with wild-type (WT) control littermates, LPs were reduced in CaV1.3−/− mice. This pattern matched closely with that previously noted in lethargic mice, confirming a role for VDCCs in regulating the signaling pathway that culminates in LP generation. These abnormalities do not reflect a defect in rod photoreceptor activity, which provides the input to the RPE to generate the c-wave, FO, and LP, because ERG a-waves were comparable in WT and CaV1.3−/− littermates. Our results identify CaV1.3 as the principal pore-forming subunit of VDCCs involved in stimulating the ERG LP.
Voltage-gated Ca2+ channels couple membrane depolarization to Ca2+-dependent intracellular signaling events. This is achieved by mediating Ca2+ ion influx or by direct conformational coupling to intracellular Ca2+ release channels. The family of Cav1 channels, also termed L-type Ca2+ channels (LTCCs), is uniquely sensitive to organic Ca2+ channel blockers and expressed in many electrically excitable tissues. In this review, we summarize the role of LTCCs for human diseases caused by genetic Ca2+ channel defects (channelopathies). LTCC dysfunction can result from structural aberrations within their pore-forming α1 subunits causing hypokalemic periodic paralysis and malignant hyperthermia sensitivity (Cav1.1 α1), incomplete congenital stationary night blindness (CSNB2; Cav1.4 α1), and Timothy syndrome (Cav1.2 α1; reviewed separately in this issue). Cav1.3 α1 mutations have not been reported yet in humans, but channel loss of function would likely affect sinoatrial node function and hearing. Studies in mice revealed that LTCCs indirectly also contribute to neurological symptoms in Ca2+ channelopathies affecting non-LTCCs, such as Cav2.1 α1 in tottering mice. Ca2+ channelopathies provide exciting disease-related molecular detail that led to important novel insight not only into disease pathophysiology but also to mechanisms of channel function.
Channels; Channel gating; Channel activity; Neuronal excitability
Low voltage activation of CaV1.3 L-type Ca2+ channels
controls excitability in sensory cells and central neurons as well as
sinoatrial node pacemaking. CaV1.3-mediated pacemaking determines
neuronal vulnerability of dopaminergic striatal neurons affected in Parkinson
disease. We have previously found that in CaV1.4 L-type
Ca2+ channels, activation, voltage, and calcium-dependent
inactivation are controlled by an intrinsic distal C-terminal modulator.
Because alternative splicing in the CaV1.3 α1 subunit C
terminus gives rise to a long (CaV1.342) and a short
form (CaV1.342A), we investigated if a C-terminal
modulatory mechanism also controls CaV1.3 gating. The biophysical
properties of both splice variants were compared after heterologous expression
together with β3 and α2δ1 subunits in HEK-293 cells.
Activation of calcium current through CaV1.342A channels
was more pronounced at negative voltages, and inactivation was faster because
of enhanced calcium-dependent inactivation. By investigating several
CaV1.3 channel truncations, we restricted the modulator activity to
the last 116 amino acids of the C terminus. The resulting
CaV1.3ΔC116 channels showed gating properties
similar to CaV1.342A that were reverted by co-expression
of the corresponding C-terminal peptide C116. Fluorescence
resonance energy transfer experiments confirmed an intramolecular protein
interaction in the C terminus of CaV1.3 channels that also
modulates calmodulin binding. These experiments revealed a novel mechanism of
channel modulation enabling cells to tightly control CaV1.3 channel
activity by alternative splicing. The absence of the C-terminal modulator in
short splice forms facilitates CaV1.3 channel activation at lower
voltages expected to favor CaV1.3 activity at threshold voltages as
required for modulation of neuronal firing behavior and sinoatrial node
Cav1.2 and Cav1.3 L-type Ca2+ channels (LTCCs) are believed to underlie Ca2+ currents in brain, pancreatic β cells, and the cardiovascular system. In the CNS, neuronal LTCCs control excitation-transcription coupling and neuronal plasticity. However, the pharmacotherapeutic implications of CNS LTCC modulation are difficult to study because LTCC modulators cause card iovascular (activators and blockers) and neurotoxic (activators) effects. We selectively eliminated high dihydropyridine (DHP) sensitivity from Cav1.2 α1 subunits (Cav1.2DHP–/–) without affecting function and expression. This allowed separation of the DHP effects of Cav1.2 from those of Cav1.3 and other LTCCs. DHP effects on pancreatic β cell LTCC currents, insulin secretion, cardiac inotropy, and arterial smooth muscle contractility were lost in Cav1.2DHP–/– mice, which rules out a direct role of Cav1.3 for these physiological processes. Using Cav1.2DHP–/– mice, we established DHPs as mood-modifying agents: LTCC activator–induced neurotoxicity was abolished and disclosed a depression-like behavioral effect without affecting spontaneous locomotor activity. LTCC activator BayK 8644 (BayK) activated only a specific set of brain areas. In the ventral striatum, BayK-induced release of glutamate and 5-HT, but not dopamine and noradrenaline, was abolished. This animal model provides a useful tool to elucidate whether Cav1.3-selective channel modulation represents a novel pharmacological approach to modify CNS function without major peripheral effects.