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      IP 3Rs puff along: A SNAPpy dance with IP 3 and Ca 2+

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          Abstract

          Concerted openings of clustered inositol 1,4,5-trisphosphate receptors (IP 3Rs) result in short, localized Ca 2+ bursts, also called puffs, which are crucial regulators of Ca 2+-dependent signaling processes. However, the processes regulating Ca 2+ puff amplitude (average ∼0.5 ΔF/F 0) and duration (at half-maximal; average ∼25-30 ms) have yet to be elucidated. A recent study in JBC by Smith and Taylor determined that Ca 2+ puff amplitude is independent of IP 3R cluster density and that the termination of IP 3R Ca 2+ puff is regulated by IP 3 dissociation, illuminating the steps of this regulatory dance.

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          Ca 2+ signals initiate at immobile IP 3 receptors adjacent to ER-plasma membrane junctions

          Nagendra Babu Thillaiappan, Alap P. Chavda, Stephen Tovey … (2017)
          IP3 receptors (IP3Rs) release Ca2+ from the ER when they bind IP3 and Ca2+. The spatial organization of IP3Rs determines both the propagation of Ca2+ signals between IP3Rs and the selective regulation of cellular responses. Here we use gene editing to fluorescently tag endogenous IP3Rs, and super-resolution microscopy to determine the geography of IP3Rs and Ca2+ signals within living cells. We show that native IP3Rs cluster within ER membranes. Most IP3R clusters are mobile, moved by diffusion and microtubule motors. Ca2+ signals are generated by a small population of immobile IP3Rs. These IP3Rs are licensed to respond, but they do not readily mix with mobile IP3Rs. The licensed IP3Rs reside alongside ER-plasma membrane junctions where STIM1, which regulates store-operated Ca2+ entry, accumulates after depletion of Ca2+ stores. IP3Rs tethered close to ER-plasma membrane junctions are licensed to respond and optimally placed to be activated by endogenous IP3 and to regulate Ca2+ entry.
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            Structural basis for the regulation of inositol trisphosphate receptors by Ca 2+ and IP 3

            Navid Paknejad, Richard Hite (2018)
            Inositol trisphosphate receptors (IP3R) are ubiquitous Ca2+-permeable channels that mediate release of Ca2+ from the endoplasmic reticulum to regulate numerous processes including cell division, cell death, differentiation and fertilization. IP3R is activated by both IP3 and its permeant ion Ca2+. At high concentrations, however, Ca2+ inhibits activity ensuring precise spatiotemporal control over intracellular Ca2+. Despite extensive characterization of IP3R, the mechanisms by which these molecules control channel gating have remained elusive. Here, we present structures of full-length human type 3 IP3R in ligand-bound and ligand-free states. Multiple IP3-bound structures demonstrate that the large cytoplasmic domain provides a platform for propagation of long-range conformational changes to the ion conduction gate. Structures in the presence of Ca2+ reveal two Ca2+ binding sites that induce the disruption of numerous interactions between subunits, thereby inhibiting IP3R. These structures thus begin to provide a mechanistic basis for understanding the regulation of IP3R.
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              Size Matters: Ryanodine Receptor Cluster Size Affects Arrhythmogenic Sarcoplasmic Reticulum Calcium Release

              Samuel Galice, Yuanfang Xie, Yi Yang … (2018)
              Background Ryanodine receptors (RyR) mediate sarcoplasmic reticulum calcium (Ca2+) release and influence myocyte Ca2+ homeostasis and arrhythmias. In cardiac myocytes, RyRs are found in clusters of various sizes and shapes, and RyR cluster size may critically influence normal and arrhythmogenic Ca2+ spark and wave formation. However, the actual RyR cluster sizes at specific Ca2+ spark sites have never been measured in the physiological setting. Methods and Results Here we measured RyR cluster size and Ca2+ sparks simultaneously to assess how RyR cluster size influences Ca2+ sparks and sarcoplasmic reticulum Ca2+ leak. For small RyR cluster sizes (<50), Ca2+ spark frequency is very low but then increases dramatically at larger cluster sizes. In contrast, Ca2+ spark amplitude is nearly maximal even at relatively small RyR cluster size (≈10) and changes little at larger cluster size. These properties agreed with computational simulations of RyR gating within clusters. Conclusions Our study explains how this combination of properties may limit arrhythmogenic Ca2+ sparks and wave propagation (at many junctions) while preserving the efficacy and spatial synchronization of Ca2+‐induced Ca2+‐release during normal excitation‐contraction coupling. However, variations in RyR cluster size among individual junctions and RyR sensitivity could exacerbate heterogeneity of local sarcoplasmic reticulum Ca2+ release and arrhythmogenesis under pathological conditions.
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                Author and article information

                Contributors
                David A. Jacobson
                Journal
                Journal ID (nlm-ta): J Biol Chem
                Journal ID (iso-abbrev): J Biol Chem
                Title: The Journal of Biological Chemistry
                Publisher: American Society for Biochemistry and Molecular Biology
                ISSN (Print): 0021-9258
                ISSN (Electronic): 1083-351X
                Publication date PMC-release: 10 February 2023
                Publication date Collection: March 2023
                Publication date (Electronic): 10 February 2023
                Volume: 299
                Issue: 3
                Electronic Location Identifier: 103010
                Affiliations
                [1]Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee, USA
                Author notes
                [∗ ]For correspondence: David A. Jacobson david.a.jacobson@ 123456vanderbilt.edu
                Article
                Publisher Item ID: S0021-9258(23)00142-4 Publisher ID: 103010
                DOI: 10.1016/j.jbc.2023.103010
                PMC ID: 9996433
                PubMed ID: 36773801
                SO-VID: c0d64b0d-8bf3-4e78-9ab1-7dc7cd9fd138
                Copyright © © 2023 The Authors
                License:

                This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

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                ScienceOpen disciplines: Biochemistry
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                ScienceOpen disciplines: Biochemistry

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