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Review
. 2019 Sep;27S(Suppl):S81-S91.
doi: 10.1016/j.molmet.2019.06.017.

Role of the active zone protein, ELKS, in insulin secretion from pancreatic β-cells

Mica Ohara-Imaizumi  1 , Kyota Aoyagi  2 , Toshihisa Ohtsuka  3
Affiliations
Review

Role of the active zone protein, ELKS, in insulin secretion from pancreatic β-cells

Mica Ohara-Imaizumi et al. Mol Metab. 2019 Sep.

Abstract

Background: Insulin is stored within large dense-core granules in pancreatic beta (β)-cells and is released by Ca2+-triggered exocytosis with increasing blood glucose levels. Polarized and targeted secretion of insulin from β-cells in pancreatic islets into the vasculature has been proposed; however, the mechanisms related to cellular and molecular localization remain largely unknown. Within nerve terminals, the Ca2+-dependent release of a polarized transmitter is limited to the active zone, a highly specialized area of the presynaptic membrane. Several active zone-specific proteins have been characterized; among them, the CAST/ELKS protein family members have the ability to form large protein complexes with other active zone proteins to control the structure and function of the active zone for tight regulation of neurotransmitter release. Notably, ELKS but not CAST is also expressed in β-cells, implying that ELKS may be involved in polarized insulin secretion from β-cells.

Scope of review: This review provides an overview of the current findings regarding the role(s) of ELKS and other active zone proteins in β-cells and focuses on the molecular mechanism underlying ELKS regulation within polarized insulin secretion from islets.

Major conclusions: ELKS localizes at the vascular-facing plasma membrane of β-cells in mouse pancreatic islets. ELKS forms a potent insulin secretion complex with L-type voltage-dependent Ca2+ channels on the vascular-facing plasma membrane of β-cells, enabling polarized Ca2+ influx and first-phase insulin secretion from islets. This model provides novel insights into the functional polarity observed during insulin secretion from β-cells within islets at the molecular level. This active zone-like region formed by ELKS at the vascular side of the plasma membrane is essential for coordinating physiological insulin secretion and may be disrupted in diabetes.

Keywords: Active zone protein; Ca(2+) influx; ELKS; Insulin exocytosis; Pancreatic β-cells; Voltage-dependent Ca(2+) channel.

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Figures

Figure 1
Figure 1
Molecular structure of CAST and ELKS in presynaptic active zones. (A) Molecular structure of CAST and ELKS. CAST and ELKS are predicted to contain four coiled-coil structures, illustrated as zig-zag objects. The red triangle is the PDZ-binding IWA motif. Arrows indicate binding sites of interacting molecules. ELKSε is the C-terminal spliced isoform. (B) Schema of the interactions of presynaptic active zone proteins and ELKS/CAST-binding proteins.
Figure 2
Figure 2
ELKS is expressed in pancreatic islet β-cells. (A) Immunoblot analysis of pancreatic islet lysates using anti-ELKS and anti-CAST antibodies. Rat brain homogenate was used as a positive control. (B) Pancreatic sections were stained for ELKS and insulin. (C) Localization of ELKS, VE-cadherin (an endothelial cell marker), and Syntaxin 1 in islets. Islets were double-stained using anti-ELKS pAb and anti-VE-cadherin mAb or anti-Syntaxin 1 mAb. (D) Ultrastructural localization of ELKS in β-cells. Note that immunoreactivity of ELKS (small gold particles) was frequently detected close to insulin-containing granules (large gold particles) with docking at the plasma membrane facing a blood capillary (arrows). B: β-cell, ECS: extracellular space, E: endothelial cell, C: blood capillary. Bar, 0.2 μm. (E) ELKS clusters are sites for insulin granule docking and fusion, with docking and fusion of insulin granules occurring at these clusters. TIRF image of GFP-tagged insulin granules and Cy3-labeled ELKS clusters in MIN6 cells and dual-image analysis of GFP-tagged insulin granule motion at ELKS clusters following 50 mM KCl stimulation. The box (1 × 1 μm) indicates the granule to be fused. Timestamp (min:sec:msec) was overlaid. Time 0 indicates the addition of KCl. (F) Sequential images (1 × 1 μm, 300-ms intervals) of a single insulin granule (green) at an ELKS cluster (red) upon stimulation with 50 mM KCl. Adapted from Ohara-Imaizumi et al. (2005) .
Figure 3
Figure 3
ELKS controls Ca2+influx via L-type Ca2+channels and first-phase insulin secretion. (A) Immunohistochemical staining for insulin and ELKS in pancreatic islets from control and ELKS βKO mice. Scale bars = 50 μm. (B) Insulin secretion in perfused pancreas in response to glucose (16.7 mM). (C, D) Fura-2 Ca2+ imaging in β-cells from control and ELKS βKO mice stimulated with glucose (22 mM) (C) and high K+ (40 mM) (D). (E) ELKS deficiency reduces the L-type voltage-dependent Ca2+ current. The current (pA/pF)–voltage (mV) relationships recorded in control and ELKS-KO β-cells with or without of nifedipine are shown (nif: 1 μM). (F, G) G-CaMP8b Ca2+ imaging of the β-cell plasma membrane in control (F) and ELKS-KO (G) islets stimulated with glucose (22 mM). The asterisk indicates the vasculature labeled with tomato lectin. (H, I) Typical time course of the fluorescence intensity of G-CaMP8b in the three regions of the β-cell plasma membrane (the vascular-facing plasma membrane, lateral region, and side opposite the vasculature) during glucose stimulation in control (H) and ELKS βKO (I) islets. (J, K) Typical time course of the fluorescence intensity of G-CaMP8b in the three regions of the β-cell plasma membrane (the vascular-facing plasma membrane, lateral region, and side opposite the vasculature) during glucose stimulation in islets from db/m+ mice (J) and db/db mice (K). Scale bars = 10 μm. Time stamps (min:sec:msec) are shown for each image. Results are presented as the means ± SEM. *P < 0.05; **P < 0.01. Adapted from Ohara-Imaizumi et al. (2019) .
Figure 4
Figure 4
Schema for regulation of ELKS during insulin secretion from islet β-cells. (A) ELKS has a role in regulating the opening of L-type VDCCs through direct binding to VDCC-β2/3. (B) In pancreatic islets, ELKS localizes at the vascular-facing plasma membrane of β-cells where it forms a complex with L-type VDCCs for insulin exocytosis. This complex controls the polarity of the initial increase in Ca2+ and first-phase insulin secretion into the central venous vasculature. Adapted from Ohara-Imaizumi et al. (2019) .
Figure 5
Figure 5
Interaction model of ELKS and other active zone proteins in pancreatic β-cells. ELKS acts as a platform for a dynamic multicomplex of active zone proteins in order to control insulin secretion.
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References

    1. Ashcroft F.M., Rorsman P. Diabetes mellitus and the β cell: the last ten years. Cell. 2012;148(6):1160–1171. - PMC - PubMed
    1. Bonner-Weir S. Morphological evidence for pancreatic polarity of β-cell within islets of langerhans. Diabetes. 1988;37(5):616–621. - PubMed
    1. Weir G.C., Bonner-Weir S. Islets of Langerhans: the puzzle of intraislet interactions and their relevance to diabetes. Journal of Clinical Investigation. 1990;85(4):983–987. - PMC - PubMed
    1. Granot Z., Swisa A., Magenheim J., Stolovich-Rain M., Fujimoto W., Manduchi E. LKB1 regulates pancreatic beta cell size, polarity, and function. Cell Metabolism. 2009;10(4):296–308. - PMC - PubMed
    1. Roscioni S.S., Migliorini A., Gegg M., Lickert H. Impact of islet architecture on β-cell heterogeneity, plasticity and function. Nature Reviews Endocrinology. 2016;12(12):695–709. - PubMed

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