Single-channel monitoring of reversible L-type Ca(2+) channel Ca(V)α(1)-Ca(V)β subunit interaction

Inhibitory effects on L- and N-type calcium channels by a novel CaVβ1 variant identified in a patient with autism spectrum disorder

Voltage-gated calcium channel (VGCC) subunits have been genetically associated with autism spectrum disorders (ASD). The properties of the pore-forming VGCC subunit are modulated by auxiliary β-subunits, which exist in four isoforms (Ca_Vβ_1-4). Our previous findings suggested that activation of L-type VGCCs is a common feature of Ca_Vβ₂ subunit mutations found in ASD patients. In the current study, we functionally characterized a novel Ca_Vβ_1b variant (p.R296C) identified in an ASD patient. We used whole-cell and single-channel patch clamp to study the effect of Ca_Vβ_{1b_R296C} on the function of L- and N-type VGCCs. Furthermore, we used co-immunoprecipitation followed by Western blot to evaluate the interaction of the Ca_Vβ_1b-subunits with the RGK-protein Gem. Our data obtained at both, whole-cell and single-channel levels, show that compared to a wild-type Ca_Vβ_1b, the Ca_Vβ_{1b_R296C} variant inhibits L- and N-type VGCCs. Interaction with and modulation by the RGK-protein Gem seems to be intact. Our findings indicate functional effects of the Ca_Vβ_{1b_R296C} variant differing from that attributed to Ca_Vβ₂ variants found in ASD patients. Further studies have to detail the effects on different VGCC subtypes and on VGCC expression.

Role of High Voltage-Gated Ca2+ Channel Subunits in Pancreatic β-Cell Insulin Release. From Structure to Function

The pancreatic islets of Langerhans secrete several hormones critical for glucose homeostasis. The β-cells, the major cellular component of the pancreatic islets, secrete insulin, the only hormone capable of lowering the plasma glucose concentration. The counter-regulatory hormone glucagon is secreted by the α-cells while δ-cells secrete somatostatin that via paracrine mechanisms regulates the α- and β-cell activity. These three peptide hormones are packed into secretory granules that are released through exocytosis following a local increase in intracellular Ca2+ concentration. The high voltage-gated Ca2+ channels (HVCCs) occupy a central role in pancreatic hormone release both as a source of Ca2+ required for excitation-secretion coupling as well as a scaffold for the release machinery. HVCCs are multi-protein complexes composed of the main pore-forming transmembrane α1 and the auxiliary intracellular β, extracellular α2δ, and transmembrane γ subunits. Here, we review the current understanding regarding the role of all HVCC subunits expressed in pancreatic β-cell on electrical activity, excitation-secretion coupling, and β-cell mass. The evidence we review was obtained from many seminal studies employing pharmacological approaches as well as genetically modified mouse models. The significance for diabetes in humans is discussed in the context of genetic variations in the genes encoding for the HVCC subunits.

CaV β controls the endocytic turnover of CaV 1.2 L-type calcium channel

Membrane depolarization activates the multisubunit Ca_V 1.2 L-type calcium channel initiating various excitation coupling responses. Intracellular trafficking into and out of the plasma membrane regulates the channel's surface expression and stability, and thus, the strength of Ca_V 1.2-mediated Ca²⁺ signals. The mechanisms regulating the residency time of the channel at the cell membrane are unclear. Here, we coexpressed the channel core complex Ca_V 1.2α₁ pore-forming and auxiliary Ca_V β subunits and analyzed their trafficking dynamics from single-particle-tracking trajectories. Speed histograms obtained for each subunit were best fitted to a sum of diffusive and directed motion terms. The same mean speed for the highest-mobility state underlying directed motion was found for all subunits. The frequency of this component increased by covalent linkage of Ca_V β to Ca_V 1.2α₁ suggesting that high-speed transport occurs in association with Ca_V β. Selective tracking of Ca_V 1.2α₁ along the postendocytic pathway failed to show the highly mobile state, implying Ca_V β-independent retrograde transport. Retrograde speeds of Ca_V 1.2α₁ are compatible with myosin VI-mediated backward transport. Moreover, residency time at the cell surface was significantly prolonged when Ca_V 1.2α₁ was covalently linked to Ca_V β. Thus, Ca_V β promotes fast transport speed along anterograde trafficking and acts as a molecular switch controlling the endocytic turnover of L-type calcium channels.

Single-Channel Resolution of the Interaction between C-Terminal CaV1.3 Isoforms and Calmodulin

Voltage-dependent calcium (Ca_V) 1.3 channels are involved in the control of cellular excitability and pacemaking in neuronal, cardiac, and sensory cells. Various proteins interact with the alternatively spliced channel C-terminus regulating gating of Ca_V1.3 channels. Binding of a regulatory calcium-binding protein calmodulin (CaM) to the proximal C-terminus leads to the boosting of channel activity and promotes calcium-dependent inactivation (CDI). The C-terminal modulator domain (CTM) of Ca_V1.3 channels can interfere with the CaM binding, thereby inhibiting channel activity and CDI. Here, we compared single-channel gating behavior of two natural Ca_V1.3 splice isoforms: the long Ca_V1.3₄₂ with the full-length CTM and the short Ca_V1.3_42A with the C-terminus truncated before the CTM. We found that CaM regulation of Ca_V1.3 channels is dynamic on a minute timescale. We observed that at equilibrium, single Ca_V1.3₄₂ channels occasionally switched from low to high open probability, which perhaps reflects occasional binding of CaM despite the presence of CTM. Similarly, when the amount of the available CaM in the cell was reduced, the short Ca_V1.3_42A isoform showed patterns of the low channel activity. CDI also underwent periodic changes with corresponding kinetics in both isoforms. Our results suggest that the competition between CTM and CaM is influenced by calcium, allowing further fine-tuning of Ca_V1.3 channel activity for particular cellular needs.

Stapled Voltage-Gated Calcium Channel (CaV) α-Interaction Domain (AID) Peptides Act As Selective Protein-Protein Interaction Inhibitors of CaV Function

For many voltage-gated ion channels (VGICs), creation of a properly functioning ion channel requires the formation of specific protein–protein interactions between the transmembrane pore-forming subunits and cystoplasmic accessory subunits. Despite the importance of such protein–protein interactions in VGIC function and assembly, their potential as sites for VGIC modulator development has been largely overlooked. Here, we develop meta-xylyl (m-xylyl) stapled peptides that target a prototypic VGIC high affinity protein–protein interaction, the interaction between the voltage-gated calcium channel (Ca_V) pore-forming subunit α-interaction domain (AID) and cytoplasmic β-subunit (Ca_Vβ). We show using circular dichroism spectroscopy, X-ray crystallography, and isothermal titration calorimetry that the m-xylyl staples enhance AID helix formation are structurally compatible with native-like AID:Ca_Vβ interactions and reduce the entropic penalty associated with AID binding to Ca_Vβ. Importantly, electrophysiological studies reveal that stapled AID peptides act as effective inhibitors of the Ca_Vα₁:Ca_Vβ interaction that modulate Ca_V function in an Ca_Vβ isoform-selective manner. Together, our studies provide a proof-of-concept demonstration of the use of protein–protein interaction inhibitors to control VGIC function and point to strategies for improved AID-based Ca_V modulator design.

Protein partners of the calcium channel β subunit highlight new cellular functions

Calcium plays a key role in cell signalling by its intervention in a wide range of physiological processes. Its entry into cells occurs mainly via voltage-gated calcium channels (VGCC), which are found not only in the plasma membrane of excitable cells but also in cells insensitive to electrical signals. VGCC are composed of different subunits, α1, β, α2δ and γ, among which the cytosolic β subunit (Cavβ) controls the trafficking of the channel to the plasma membrane, its regulation and its gating properties. For many years, these were the main functions associated with Cavβ. However, a growing number of proteins have been found to interact with Cavβ, emphasizing the multifunctional role of this versatile protein. Interestingly, some of the newly assigned functions of Cavβ are independent of its role in the regulation of VGCC, and thus further increase its functional roles. Based on the identity of Cavβ protein partners, this review emphasizes the diverse cellular functions of Cavβ and summarizes both past findings as well as recent progress in the understanding of VGCC.

The role of auxiliary subunits for the functional diversity of voltage-gated calcium channels

Voltage-gated calcium channels (VGCCs) represent the sole mechanism to convert membrane depolarization into cellular functions like secretion, contraction, or gene regulation. VGCCs consist of a pore-forming α₁ subunit and several auxiliary channel subunits. These subunits come in multiple isoforms and splice-variants giving rise to a stunning molecular diversity of possible subunit combinations. It is generally believed that specific auxiliary subunits differentially regulate the channels and thereby contribute to the great functional diversity of VGCCs. If auxiliary subunits can associate and dissociate from pre-existing channel complexes, this would allow dynamic regulation of channel properties. However, most auxiliary subunits modulate current properties very similarly, and proof that any cellular calcium channel function is indeed modulated by the physiological exchange of auxiliary subunits is still lacking. In this review we summarize available information supporting a differential modulation of calcium channel functions by exchange of auxiliary subunits, as well as experimental evidence in support of alternative functions of the auxiliary subunits. At the heart of the discussion is the concept that, in their native environment, VGCCs function in the context of macromolecular signaling complexes and that the auxiliary subunits help to orchestrate the diverse protein–protein interactions found in these calcium channel signalosomes. Thus, in addition to a putative differential modulation of current properties, differential subcellular targeting properties and differential protein–protein interactions of the auxiliary subunits may explain the need for their vast molecular diversity. J. Cell. Physiol. 999: 00–00, 2015. © 2015 The Authors. Journal of Cellular Physiology Published by Wiley Periodicals, Inc. J. Cell. Physiol. 230: 2019–2031, 2015. © 2015 Wiley Periodicals, Inc.

Different effects of lysophosphatidic acid on L-type calcium current in neonatal rat ventricular myocytes with and without H2O2 treatment

L-type calcium current (I(Ca-L)) alterations are implicated in various cardiac diseases, and the lysophosphatidic acid (LPA) level increases in several ischemic heart diseases. We investigated the effects of LPA on I(Ca-L) in normal and H2O2-treated neonatal rat ventricular myocytes. LPA treatment (24h) increased the action potential duration (APD) and I(Ca-L) in normal ventricular myocytes, but it decreased these parameters in H2O2-treated myocytes. LPA increased the single-channel open probability of L-type calcium channels in both normal and H2O2-treated myocytes. LPA activated calcineurin (CaN) and induced the cytoplasm-to-nucleus translocation of nuclear factor of activated T-cells (NFAT) in H2O2-treated cardiomyocytes. In H2O2-treated cardiomyocytes, LPA decreased Ca(v)1.2 mRNA and protein expression levels at 4 and 8h, respectively. A CaN inhibitor (FK-506) prevented LPA-induced APD, I(Ca-L), and Ca(v)1.2 mRNA and protein down-regulation. The LPA-induced I(Ca-L) increase in normal cardiomyocytes was CaN-NFAT signaling-independent, and LPA did not affect Ca(v)1.2 mRNA or protein expression. In conclusion, LPA increases the I(Ca-L) in normal ventricular myocytes by increasing the single-channel open probability of L-type calcium channels, and LPA decreases I(Ca-L) in H2O2-treated cardiomyocytes via the CaN-NFAT pathway.

Structure-function of proteins interacting with the α1 pore-forming subunit of high-voltage-activated calcium channels

Openings of high-voltage-activated (HVA) calcium channels lead to a transient increase in calcium concentration that in turn activate a plethora of cellular functions, including muscle contraction, secretion and gene transcription. To coordinate all these responses calcium channels form supramolecular assemblies containing effectors and regulatory proteins that couple calcium influx to the downstream signal cascades and to feedback elements. According to the original biochemical characterization of skeletal muscle Dihydropyridine receptors, HVA calcium channels are multi-subunit protein complexes consisting of a pore-forming subunit (α1) associated with four additional polypeptide chains β, α2, δ, and γ, often referred to as accessory subunits. Twenty-five years after the first purification of a high-voltage calcium channel, the concept of a flexible stoichiometry to expand the repertoire of mechanisms that regulate calcium channel influx has emerged. Several other proteins have been identified that associate directly with the α1-subunit, including calmodulin and multiple members of the small and large GTPase family. Some of these proteins only interact with a subset of α1-subunits and during specific stages of biogenesis. More strikingly, most of the α1-subunit interacting proteins, such as the β-subunit and small GTPases, regulate both gating and trafficking through a variety of mechanisms. Modulation of channel activity covers almost all biophysical properties of the channel. Likewise, regulation of the number of channels in the plasma membrane is performed by altering the release of the α1-subunit from the endoplasmic reticulum, by reducing its degradation or enhancing its recycling back to the cell surface. In this review, we discuss the structural basis, interplay and functional role of selected proteins that interact with the central pore-forming subunit of HVA calcium channels.

Stable incorporation versus dynamic exchange of β subunits in a native Ca2+ channel complex

Voltage-gated Ca(2+) channels are multi-subunit membrane proteins that transduce depolarization into cellular functions such as excitation-contraction coupling in muscle or neurotransmitter release in neurons. The auxiliary β subunits function in membrane targeting of the channel and modulation of its gating properties. However, whether β subunits can reversibly interact with, and thus differentially modulate, channels in the membrane is still unresolved. In the present study we applied fluorescence recovery after photobleaching (FRAP) of GFP-tagged α1 and β subunits expressed in dysgenic myotubes to study the relative dynamics of these Ca(2+) channel subunits for the first time in a native functional signaling complex. Identical fluorescence recovery rates of both subunits indicate stable interactions, distinct recovery rates indicate dynamic interactions. Whereas the skeletal muscle β1a isoform formed stable complexes with CaV1.1 and CaV1.2, the non-skeletal muscle β2a and β4b isoforms dynamically interacted with both α1 subunits. Neither replacing the I-II loop of CaV1.1 with that of CaV2.1, nor deletions in the proximal I-II loop, known to change the orientation of β relative to the α1 subunit, altered the specific dynamic properties of the β subunits. In contrast, a single residue substitution in the α interaction pocket of β1aM293A increased the FRAP rate threefold. Taken together, these findings indicate that in skeletal muscle triads the homologous β1a subunit forms a stable complex, whereas the heterologous β2a and β4b subunits form dynamic complexes with the Ca(2+) channel. The distinct binding properties are not determined by differences in the I-II loop sequences of the α1 subunits, but are intrinsic properties of the β subunit isoforms.

Equal sensitivity of Cav1.2 and Cav1.3 channels to the opposing modulations of PKA and PKG in mouse chromaffin cells

Mouse chromaffin cells (MCCs) express high densities of L-type Ca2+ channels (LTCCs), which control pacemaking activity and catecholamine secretion proportionally to their density of expression. In vivo phosphorylation of LTCCs by cAMP-PKA and cGMP–PKG, regulate LTCC gating in two opposing ways: the cAMP-PKA pathway potentiates while the cGMP–PKG cascade inhibits LTCCs. Despite this, no attempts have been made to answer three key questions related to the two Cav1 isoforms expressed in MCCs (Cav1.2 and Cav1.3): (i) how much are the two Cav1 channels basally modulated by PKA and PKG?, (ii) to what extent can Cav1.2 and Cav1.3 be further regulated by PKA or PKG activation?, and (iii) are the effects of both kinases cumulative when simultaneously active? Here, by comparing the size of L-type currents of wild-type (WT; Cav1.2+Cav1.3) and Cav1.3−/− KO (Cav1.2) MCCs, we provide new evidence that both PKA and PKG pathways affect Cav1.2 and Cav1.3 to the same extent either under basal conditions or induced stimulation. Inhibition of PKA by H89 (5 μM) reduced the L-type current in WT and KO MCCs by∼60%,while inhibition of PKG by KT 5823 (1 μM) increased by∼40% the same current in both cell types. Given that Cav1.2 and Cav1.3 carry the same quantity of Ca2+ currents, this suggests equal sensitivity of Cav1.2 and Cav1.3 to the two basal modulatory pathways. Maximal stimulation of cAMP–PKA by forskolin (100 μM) and activation of cGMP–PKG by pCPT-cGMP (1mM) uncovered a∼25% increase of L-type currents in the first case and∼65% inhibition in the second case in both WT and KO MCCs, suggesting equal sensitivity of Cav1.2 and Cav1.3 during maximal PKA or PKG stimulation. The effects of PKA and PKG were cumulative and most evident when one pathway was activated and the other was inhibited. The two extreme combinations(PKA activation–PKG inhibition vs. PKG activation-PKA inhibition) varied the size of L-type currents by one order of magnitude (from 180% to 18% of control size). Taken together our data suggest that: (i) Cav1.2 and Cav1.3 are equally sensitive to PKA and PKG action under both basal conditions and maximal stimulation, and (ii) PKA and PKG act independently on both Cav1.2 and Cav1.3, producing cumulative effects when opposingly activated. These extreme Cav1 channel modulations may occur either during high-frequency sympathetic stimulation to sustain prolonged catecholamine release (maximal L-type current) or following activation of the NO–cGMP–PKG signalling pathway (minimal L-type current) to limit the steady release of catecholamines.

Membrane-localized β-subunits alter the PIP2 regulation of high-voltage activated Ca2+ channels

The β-subunits of voltage-gated Ca(2+) (Ca(V)) channels regulate the functional expression and several biophysical properties of high-voltage-activated Ca(V) channels. We find that Ca(V) β-subunits also determine channel regulation by the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP(2)). When Ca(V)1.3, -2.1, or -2.2 channels are cotransfected with the β3-subunit, a cytosolic protein, they can be inhibited by activating a voltage-sensitive lipid phosphatase to deplete PIP(2). When these channels are coexpressed with a β2a-subunit, a palmitoylated peripheral membrane protein, the inhibition is much smaller. PIP(2) sensitivity could be increased by disabling the two palmitoylation sites in the β2a-subunit. To further test effects of membrane targeting of Ca(V) β-subunits on PIP(2) regulation, the N terminus of Lyn was ligated onto the cytosolic β3-subunit to confer lipidation. This chimera, like the Ca(V) β2a-subunit, displayed plasma membrane localization, slowed the inactivation of Ca(V)2.2 channels, and increased the current density. In addition, the Lyn-β3 subunit significantly decreased Ca(V) channel inhibition by PIP(2) depletion. Evidently lipidation and membrane anchoring of Ca(V) β-subunits compete with the PIP(2) regulation of high-voltage-activated Ca(V) channels. Compared with expression with Ca(V) β3-subunits alone, inhibition of Ca(V)2.2 channels by PIP(2) depletion could be significantly attenuated when β2a was coexpressed with β3. Our data suggest that the Ca(V) currents in neurons would be regulated by membrane PIP(2) to a degree that depends on their endogenous β-subunit combinations.