Voltage-Gated Calcium Channel Α Δ Subunits
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F1000Research 2018, 7(F1000 Faculty Rev):1830 Last updated: 21 NOV 2018 REVIEW Voltage-gated calcium channel α2δ subunits: an assessment of proposed novel roles [version 1; referees: 2 approved] Annette C. Dolphin Department of Neuroscience, Physiology and Pharmacology, University College London, Gower Street, London, WC1E 6BT, UK First published: 21 Nov 2018, 7(F1000 Faculty Rev):1830 ( Open Peer Review v1 https://doi.org/10.12688/f1000research.16104.1) Latest published: 21 Nov 2018, 7(F1000 Faculty Rev):1830 ( https://doi.org/10.12688/f1000research.16104.1) Referee Status: Abstract Invited Referees Voltage-gated calcium (CaV) channels are associated with β and α2δ auxiliary 1 2 subunits. This review will concentrate on the function of the α2δ protein family, which has four members. The canonical role for α δ subunits is to convey a version 1 2 published variety of properties on the CaV1 and CaV2 channels, increasing the density of 21 Nov 2018 these channels in the plasma membrane and also enhancing their function. More recently, a diverse spectrum of non-canonical interactions for α2δ proteins has been proposed, some of which involve competition with calcium F1000 Faculty Reviews are commissioned channels for α δ or increase α δ trafficking and others which mediate roles 2 2 from members of the prestigious F1000 completely unrelated to their calcium channel function. The novel roles for α δ 2 Faculty. In order to make these reviews as proteins which will be discussed here include association with low-density comprehensive and accessible as possible, lipoprotein receptor-related protein 1 (LRP1), thrombospondins, α-neurexins, prion proteins, large conductance (big) potassium (BK) channels, and N peer review takes place before publication; the -methyl-d-aspartate (NMDA) receptors. referees are listed below, but their reports are not formally published. Keywords calcium channel, alpha2delta, interaction 1 Emilio Carbone, Lab of Cellular Physiology and Molecular Neuroscience, Italy 2 Jutta Engel, Saarland University, School of Medicine, Germany Discuss this article Comments (0) Page 1 of 14 F1000Research 2018, 7(F1000 Faculty Rev):1830 Last updated: 21 NOV 2018 Corresponding author: Annette C. Dolphin ([email protected]) Author roles: Dolphin AC: Conceptualization, Writing – Original Draft Preparation, Writing – Review & Editing Competing interests: No competing interests were disclosed. Grant information: My own work referenced in this study has been funded by Wellcome Trust and MRC. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Copyright: © 2018 Dolphin AC. This is an open access article distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. How to cite this article: Dolphin AC. Voltage-gated calcium channel α 2δ subunits: an assessment of proposed novel roles [version 1; referees: 2 approved] F1000Research 2018, 7(F1000 Faculty Rev):1830 (https://doi.org/10.12688/f1000research.16104.1) First published: 21 Nov 2018, 7(F1000 Faculty Rev):1830 (https://doi.org/10.12688/f1000research.16104.1) Page 2 of 14 F1000Research 2018, 7(F1000 Faculty Rev):1830 Last updated: 21 NOV 2018 Introduction channels13, a process which it has been suggested influences cal- cium channel function by sequestering the subunits. For (b), Voltage-gated calcium (CaV) channels are ubiquitously present α2δ in excitable cells and are essential for their function. They I will discuss novel roles associated with the association of α2δ with thrombospondins (TSPs), an interaction which has been can be divided into three classes (CaV1–3). All except the found to influence synaptogenesis in some systems14. I will also CaV3 (T type) channels are associated with several auxiliary discuss the proposed association of with N-methyl-D-aspartate subunits—termed α2δ and β—together with an additional γ α2δ subunit in skeletal muscle1,2 (Figure 1). One of these subu- (NMDA) receptors15 (Figure 2). It is possible that the gabapentinoid drugs also act by influencing these various novel targets. nits, α2δ, conveys a variety of properties on the channels but recently has also been reported to have distinct effects on both other ion channels and other biological processes. These novel Topology, domain structure, and biochemical aspects of function are the subject of this review. This topic α2δ properties of α2δ proteins is important, as α2δ-1 is the therapeutic target of the α2δ ligand The α2δ subunit was first identified as two proteins—α2 and (gabapentinoid) class of drugs3,4, which are widely prescribed δ—co-purifying as integral constituents of the calcium channel for several indications, including many types of neuropathic pain. complex present in skeletal muscle T-tubules16–18. It was found that α2δ is encoded by a single gene and is subsequently proc- essed into and 17,18. Four mammalian genes have been The α2δ subunits have a well-established canonical role to α2 δ α2δ cloned (CACNA2D1–4)16,19–21. influence the trafficking and function of the CaV1 and CaV2 channels, increasing the density of these channels on the plasma 5 22,23 membrane . They also direct trafficking of the channels to All the α2δ proteins have highly related topology , with an specific subcellular sites, including neuronal processes5,6. In N-terminal signal sequence, indicating that the N-terminus is addition, the α2δ subunits increase CaV function by influencing extracellular (Figure 3). The hydrophobic C-terminus of α2δ, the biophysical properties of the calcium currents7–10, over and and its behavior as an integral membrane protein, led to its being above their effect on trafficking6. categorized as a transmembrane protein17,18. However, it was subsequently identified to have a strongly predicted glycosyl- phosphatidylinositol (GPI)-anchor -site24. Indeed, multiple More recently, α2δ-1 proteins have been proposed to have non- ω classic functions of two types: (a) additional functions related pieces of experimental evidence indicate that α2δ-1, α2δ-2, and 24–26 to calcium channels, either to link the calcium channel com- α2δ-3 (and probably α2δ-4 by prediction) are GPI-anchored . plexes to other proteins or to influence calcium channel function, and (b) roles not associated with calcium channel function. The α2δ subunit genes encode a single precursor protein, which is post-translationally proteolytically processed into two For (a), I will discuss several topics, including the associa- polypeptides. The folding of α2δ in the endoplasmic reticu- lum involves the formation of multiple disulfide bonds both tion of α2δ proteins with α-neurexins to influence synaptic 11,12 within and between the and moieties, so that, despite their transmission . The α2δ-1 protein has also been found to α2 δ interact potentially with large conductance (big) potassium (BK) cleavage, the α2 and δ polypeptides remain disulfide-bonded Figure 1. The subunit structure of voltage-gated calcium channels of the CaV1 and CaV2 family. The CaV α1 subunit with 24 transmembrane segments and the intracellular β and the extracellular α2δ subunits are shown. The γ subunit (γ1) is associated with CaV1.1 only and is not depicted. Page 3 of 14 F1000Research 2018, 7(F1000 Faculty Rev):1830 Last updated: 21 NOV 2018 Figure 2. Summary of α2δ interactions with other proteins. The various ion channels and other proteins with which α2δ subunits have been found to interact are shown. BK, large conductance (big) potassium; LRP1, low-density lipoprotein receptor-related protein 1; NMDA, N-methyl-D-aspartate; TSP, thrombospondin. Figure 3. The post-translational processing of α2δ subunits. The hydrophobic N-terminal signal sequence is a signal for the polypeptide to co-translationally pass through the membrane of the endoplasmic reticulum (ER). This signal sequence is cleaved off. The glycosylphosphatidylinositol (GPI) anchor is added in the ER by an endopeptidase transamidase, which cleaves the C-terminal signal peptide at the ω-site and adds a pre-formed GPI lipid anchor. Multiple disulfide bonds are formed as the protein folds in the ER, and N-glycosylation occurs at multiple sites. Mature glycosylation is then completed in the Golgi complex, and it is likely that proteolytic cleavage of α2δ also occurs here 27. The GPI anchor can also be modified during trafficking. Page 4 of 14 F1000Research 2018, 7(F1000 Faculty Rev):1830 Last updated: 21 NOV 2018 17,18 together . The role for the proteolytic cleavage between α2 lack many components of the native environment) are rein- and δ has been shown to be key to the mature function of these forced by parallel experiments in more intact systems, including 6,28 20,38–42 proteins , and CaV2.2 associates to a greater extent with the using tissues from α2δ knockout mice and small interfering 28 43 mature cleaved form of α2δ-1 than with the uncleaved form . RNA (siRNA) knockdown of α2δ-1 in skeletal muscle cells or cardiac myocytes44. A von Willebrand factor A (VWA) domain is present in the α2 29,30 moiety of all α2δ proteins ; these widespread domains are Role for α2δ-1 in calcium channel trafficking generally involved in extracellular protein–protein interactions. The effect of α2δ subunits to increase calcium current density A key motif in VWA domains is the metal ion-dependent can be partially explained by an increase in the trafficking of the adhesion site (MIDAS), which involves coordination of the diva- channels to augment the amount on the cell surface5. The exact 29 lent cation by a ring of up to five polar or charged residues . mechanism whereby α2δ increases the density of CaV chan- 30 α2δ-1 and α2δ-2 have a “perfect” MIDAS site , whereas α2δ-3 nels in the plasma membrane is still unclear. There was no effect 29 5 and α2δ-4 have a missing polar residue .