Targeted disruption of the voltage-dependent calcium channel alpha2/delta-1-subunit

Trafficking and stability of voltage-gated calcium channels

Voltage-gated calcium channels are important mediators of calcium influx into electrically excitable cells. The amount of calcium entering through this family of channel proteins is not only determined by the functional properties of channels embedded in the plasma membrane but also by the numbers of channels that are expressed at the cell surface. The trafficking of channels is controlled by numerous processes, including co-assembly with ancillary calcium channel subunits, ubiquitin ligases, and interactions with other membrane proteins such as G protein coupled receptors. Here we provide an overview about the current state of knowledge of calcium channel trafficking to the cell membrane, and of the mechanisms regulating the stability and internalization of this important ion channel family.

Comprehensive description of the N-glycoproteome of mouse pancreatic β-cells and human islets

Cell surface N-glycoproteins provide a key interface of cells to their environment and therapeutic entry points for drug and biomarker discovery. Their comprehensive description denotes therefore a formidable challenge. The β-cells of the pancreas play a crucial role in blood glucose homeostasis, and disruption of their function contributes to diabetes. By combining cell surface and whole cell capturing technologies with high-throughput quantitative proteomic analysis, we report on the identification of a total of 956 unique N-glycoproteins from mouse MIN6 β-cells and human islets. Three-hundred-forty-nine of these proteins encompass potential surface N-glycoproteins and include orphan G-protein-coupled receptors, novel proteases, receptor protein kinases, and phosphatases. Interestingly, stimulation of MIN6 β-cells with glucose and the hormone GLP1, known stimulators of insulin secretion, causes significant changes in surface N-glycoproteome expression. Taken together, this β-cell N-glycoproteome resource provides a comprehensive view on the composition of β-cell surface proteins and expands the scope of signaling systems potentially involved in mediating responses of β-cells to various forms of (patho)physiologic stress and the extent of dynamic remodeling of surface N-glycoprotein expression associated with metabolic and hormonal stimulation. Moreover, it provides a foundation for the development of diabetes medicines that target or are derived from the β-cell surface N-glycoproteome.

Divergent biophysical properties, gating mechanisms, and possible functions of the two skeletal muscle Ca(V)1.1 calcium channel splice variants

Voltage-gated calcium channels are multi-subunit protein complexes that specifically allow calcium ions to enter the cell in response to membrane depolarization. But, for many years it seemed that the skeletal muscle calcium channel Ca(V)1.1 is the exception. The classical splice variant Ca(V)1.1a activates slowly, has a very small current amplitude and poor voltage sensitivity. In fact adult muscle fibers work perfectly well even in the absence of calcium influx. Recently a new splice variant of the skeletal muscle calcium channel Ca(V)1.1e has been characterized. The lack of the 19 amino acid exon 29 in this splice variant results in a rapidly activating calcium channel with high current amplitude and good voltage sensitivity. Ca(V)1.1e is the dominant channel in embryonic muscle, where the expression of this high calcium-conducting Ca(V)1.1 isoform readily explains developmental processes depending on L-type calcium currents. Moreover, the availability of these two structurally similar but functionally distinct channel variants facilitates the analysis of the molecular mechanisms underlying the unique current properties of the classical Ca(V)1.1a channel.

The alpha1 subunit EGL-19, the alpha2/delta subunit UNC-36, and the beta subunit CCB-1 underlie voltage-dependent calcium currents in Caenorhabditis elegans striated muscle

Voltage-gated calcium channels, which play key roles in many physiological processes, are composed of a pore-forming α1 subunit associated with up to three auxiliary subunits. In vertebrates, the role of auxiliary subunits has mostly been studied in heterologous systems, mainly because of the severe phenotypes of knock-out animals. The genetic model Caenorhabditis elegans has all main types of voltage-gated calcium channels and strong loss-of-function mutations in all pore-forming and auxiliary subunits; it is therefore a useful model to investigate the roles of auxiliary subunits in their native context. By recording calcium currents from channel and auxiliary subunit mutants, we molecularly dissected the voltage-dependent calcium currents in striated muscle of C. elegans. We show that EGL-19 is the only α1 subunit that carries calcium currents in muscle cells. We then demonstrate that the α2/δ subunit UNC-36 modulates the voltage dependence, the activation kinetics, and the conductance of calcium currents, whereas another α2/δ subunit TAG-180 has no effect. Finally, we characterize mutants of the two β subunits, CCB-1 and CCB-2. CCB-1 is necessary for viability, and voltage-dependent calcium currents are abolished in the absence of CCB-1 whereas CCB-2 does not affect currents. Altogether these results show that EGL-19, UNC-36, and CCB-1 underlie voltage-dependent calcium currents in C. elegans striated muscle.

Distribution of voltage gated calcium channel β subunits in the mouse retina

Voltage gated calcium channels (VGCCs) are essential to neuronal excitation and signal transduction. They are multimeric in structure and comprised of an alpha subunit that functions as a calcium pore and two additional subunits: an alpha2delta subunit and a cytoplasmic beta subunit. To better understand the role of VGCCs in the retina we used immunohistochemical methods to determine the distribution of VGCC β subunits in normal and mutant mice. To verify the specificity of each antibody and to examine the potential for subunit redistribution when beta subunit expression is perturbed, we used 4 mutant mouse lines that each lack a specific β subunit isoform (β(1)-β(4)). We found the β(1) subunit distributed on cell bodies in the inner nuclear layer (INL) and on processes within both the inner and outer limiting membrane; the β(2) subunit localized to the outer plexiform layer (OPL) and inner plexiform layer (IPL); the β(3) subunit was localized to three narrow and distinct bands within the IPL; the β(4) subunit was localized to three diffuse bands within the IPL. Loss of one β subunit affected labeling intensity but not general distribution patterns of other β subunits. It is likely that VGCCs critical for retinal signal transmission are comprised of the β(2) subunit in the OPL and any of the 4 β subunits in the IPL. Our results suggest that within the OPL the α(1F) subunit pairs predominantly with the β(2) subunit while within the IPL it may pair with either any β subunit.

The mechanisms underlying ICa heterogeneity across murine left ventricle

L-type calcium current (I(Ca)) plays a critical role in excitation-contraction coupling (ECC). Unlike transient outward K(+) current (I(to)), it is controversial whether I(Ca) transmural gradient exists in left ventricle. Although previous studies have shown some evidences for I(Ca) heterogeneity, the mechanism is still unknown. In this study, the authors recorded I(Ca) from epicardial (EPI) and endocardial (ENDO) myocytes isolated from murine left ventricle using patch-clamp technique. It was found that I(Ca) density was obviously larger in EPI than in ENDO (7.3 ± 0.3 pA/pF vs. 6.2 ± 0.2 pA/pF, at test potential of +10 mV, P < 0.05). The characteristics of I(Ca) showed no difference between these two regions except for the fast inactivation time constants (9.9 ± 0.9 ms in EPI vs. 13.5 ± 0.9 ms in ENDO, at test potential of +10 mV, P < 0.05). In addition, it was explored the molecular mechanism underlying I(Ca) transmural gradient by Western blot. The authors demonstrated that a higher activity of CaMKII in ENDO cells induced more nuclear translocation of p65, a component of nuclear factor-kappa B (NF-kB). Consequently, p65 in ENDO inhibited more transcription of Cav1.2, the main encoding gene for L-type calcium channels (LTCCs). These results reveal a difference in CaMKII/p65 signal pathway between EPI and ENDO that underlies this mechanism of I(Ca) heterogeneity in murine left ventricle.

Cardiac L-type calcium channel (Cav1.2) associates with gamma subunits

The cardiac voltage-gated Ca(2+) channel, Ca(v)1.2, mediates excitation-contraction coupling in the heart. The molecular composition of the channel includes the pore-forming α1 subunit and auxiliary α2/δ-1 and β subunits. Ca(2+) channel γ subunits, of which there are 8 isoforms, consist of 4 transmembrane domains with intracellular N- and C-terminal ends. The γ1 subunit was initially detected in the skeletal muscle Ca(v)1.1 channel complex, modulating current amplitude and activation and inactivation properties. The γ1 subunit also shifts the steady-state inactivation to more negative membrane potentials, accelerates current inactivation, and increases peak currents, when coexpressed with the cardiac α1c subunit in Xenopus oocytes and human embryonic kidney (HEK) 293 cells. The γ1 subunit is not expressed, however, in cardiac muscle. We sought to determine whether γ subunits that are expressed in cardiac tissue physically associate with and modulate Ca(v)1.2 function. We now demonstrate that γ4, γ6, γ7, and γ8 subunits physically interact with the Ca(v)1.2 complex. The γ subunits differentially modulate Ca(2+) channel function when coexpressed with the β1b and α2/δ-1 subunits in HEK cells, altering both activation and inactivation properties. The effects of γ on Ca(v)1.2 function are dependent on the subtype of β subunit. Our results identify new members of the cardiac Ca(v)1.2 macromolecular complex and identify a mechanism by which to increase the functional diversity of Ca(v)1.2 channels.

The ß subunit of voltage-gated Ca2+ channels

Calcium regulates a wide spectrum of physiological processes such as heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca(2+) in excitable cells are high-voltage activated (HVA) Ca(2+) channels. These are plasma membrane proteins composed of several subunits, including α(1), α(2)δ, β, and γ. Although the principal α(1) subunit (Ca(v)α(1)) contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit (Ca(v)β) plays an essential role in regulating the surface expression and gating properties of HVA Ca(2+) channels. Ca(v)β is also crucial for the modulation of HVA Ca(2+) channels by G proteins, kinases, and the Ras-related RGK GTPases. New proteins have emerged in recent years that modulate HVA Ca(2+) channels by binding to Ca(v)β. There are also indications that Ca(v)β may carry out Ca(2+) channel-independent functions, including directly regulating gene transcription. All four subtypes of Ca(v)β, encoded by different genes, have a modular organization, consisting of three variable regions, a conserved guanylate kinase (GK) domain, and a conserved Src-homology 3 (SH3) domain, placing them into the membrane-associated guanylate kinase (MAGUK) protein family. Crystal structures of Ca(v)βs reveal how they interact with Ca(v)α(1), open new research avenues, and prompt new inquiries. In this article, we review the structure and various biological functions of Ca(v)β, with both a historical perspective as well as an emphasis on recent advances.

A new look at calcium channel α2δ subunits

The classical roles of α(2)δ proteins are as accessory calcium channel subunits, enhancing channel trafficking. They were thought to have type-I transmembrane topology, but we find that they can form GPI-anchored proteins. Moreover α(2)δ-1 and α(2)δ-3 have been shown to have novel functions in synaptogenesis, independent of their effect on calcium channels. In neurons, the α(2)δ-1 subunits are present mainly in presynaptic terminals. Peripheral sensory nerve injury results in the up-regulation of α(2)δ-1 in dorsal root ganglion (DRG) neurons, and there is a consequent increase in trafficking of α(2)δ-1 to their terminals. Furthermore, gabapentinoid drugs, which bind to α(2)δ-1 and α(2)δ-2, not only impair their trafficking, but also affect α(2)δ-1-dependent synaptogenesis. These drugs may interfere with α(2)δ function at several different levels.

The alpha(2)delta subunit augments functional expression and modifies the pharmacology of Ca(V)1.3 L-type channels

The auxiliary Ca(V)alpha(2)delta-1 subunit is an important component of voltage-gated Ca(2+) (Ca(V)) channel complexes in many tissues and of great interest as a drug target. Nevertheless, its exact role in specific cell functions is still unknown. This is particularly important in the case of the neuronal L-type Ca(V) channels where these proteins play a key role in the secretion of neurotransmitters and hormones, gene expression, and the activation of other ion channels. Therefore, using a combined approach of patch-clamp recordings and molecular biology, we studied the role of the Ca(V)alpha(2)delta-1 subunit on the functional expression and the pharmacology of recombinant L-type Ca(V)1.3 channels in HEK-293 cells. Co-expression of Ca(V)alpha(2)delta-1 significantly increased macroscopic currents and conferred the Ca(V)1.3alpha(1)/Ca(V)beta(3) channels sensitivity to the antiepileptic/analgesic drugs gabapentin and AdGABA. In contrast, Ca(V)alpha(2)delta-1 subunits harboring point mutations in N-glycosylation consensus sequences or the proteolytic site as well as in conserved cysteines in the transmembrane delta domain of the protein, reduced functionality in terms of enhancement of Ca(V)1.3alpha(1)/Ca(V)beta(3) currents. In addition, co-expression of the delta domain drastically inhibited macroscopic currents through recombinant Ca(V)1.3 channels possibly by affecting channel synthesis. Together these results provide several lines of evidence that the Ca(V)alpha(2)delta-1 auxiliary subunit may interact with Ca(V)1.3 channels and regulate their functional expression.

L-type Ca(2+) current in ventricular cardiomyocytes

L-type Ca(2+) channels are mediators of Ca(2+) influx and the regulatory events accompanying it and are pivotal in the function and dysfunction of ventricular cardiac myocytes. L-type Ca(2+) channels are located in sarcolemma, including the T-tubules facing the sarcoplasmic reticulum junction, and are activated by membrane depolarization, but intracellular Ca(2+)-dependent inactivation limits Ca(2+) influx during action potential. I(CaL) is important in heart function because it triggers excitation-contraction coupling, modulates action potential shape and is involved in cardiac arrhythmia. L-type Ca(2+) channels are multi-subunit complexes that interact with several molecules involved in their regulations, notably by beta-adrenergic signaling. The present review highlights some of the recent findings on L-type Ca(2+) channel function, regulation, and alteration in acquired pathologies such as cardiac hypertrophy, heart failure and diabetic cardiomyopathy, as well as in inherited arrhythmic cardiac diseases such as Timothy and Brugada syndromes.

Calcium channel diversity: multiple roles of calcium channel subunits

Until recently we held the simple view that voltage-gated calcium channels consisted of an alpha1 subunit, usually associated with auxiliary beta subunits and alpha(2)delta subunits and that skeletal muscle calcium channels were also associated with a gamma subunit. However, as discussed here, there is now evidence that the auxiliary subunits may also perform other roles unrelated to voltage-gated calcium entry. In the past students were taught the simplistic view that second messenger signaling to voltage-gated calcium channels involved mainly phosphorylation of L-type calcium channels, Ca(2+)-dependent inactivation via calmodulin, and direct G-protein-mediated inhibition of the neuronal N and P/Q channels. However, it is now clear that there are many other means of modulating calcium channel activity, including receptor-mediated internalization, proteolytic cleavage, phosphorylation of beta subunits, and interaction of calcium channels with other proteins, including enzymes masquerading as scaffold proteins.