Endoplasmic Reticulum-Plasma Membrane Junctions as Sites of Depolarization-Induced Ca2+ Signaling in Excitable Cells

Abstract

Membrane contact sites between endoplasmic reticulum (ER) and plasma membrane (PM), or ER-PM junctions, are found in all eukaryotic cells. In excitable cells they play unique roles in organizing diverse forms of Ca²⁺ signaling as triggered by membrane depolarization. ER-PM junctions underlie crucial physiological processes such as excitation-contraction coupling, smooth muscle contraction and relaxation, and various forms of activity-dependent signaling and plasticity in neurons. In many cases the structure and molecular composition of ER-PM junctions in excitable cells comprise important regulatory feedback loops linking depolarization-induced Ca²⁺ signaling at these sites to the regulation of membrane potential. Here, we describe recent findings on physiological roles and molecular composition of native ER-PM junctions in excitable cells. We focus on recent studies that provide new insights into canonical forms of depolarization-induced Ca²⁺ signaling occurring at junctional triads and dyads of striated muscle, as well as the diversity of ER-PM junctions in these cells and in smooth muscle and neurons.

Keywords: cardiac muscle; ion channel; membrane contact sites; neuron; second messenger; skeletal muscle; smooth muscle.

Figure 1.. SR-PM junctions as platforms for depolarization-induced Ca²⁺ signaling in muscle.

a) Depiction of a skeletal muscle triad with the typical SR-TT-SR arrangement where t-tubule localized Ca_V1.1 channels (blue) physically interact with jSR-localized RyR1 (teal) to orchestrate Ca²⁺ signaling that leads to contraction during AP-induced depolarizations. Triadic proteins JPH1 and JPH2 (dark blue) tether the junctions while triad morphology is supported by MG29 (dark red) and BIN1 (grey). b) Illustration of a cardiac muscle dyad showing Ca_V1.2 channels (light link) within nanometer proximity of SR-localized RyR2 (teal). This arrangement facilitates CICR and myocardial contraction. Dyadic proteins NEXN (orange), BIN1 (grey) and JPH2 (dark blue) play roles in dyad regulation, membrane folding, and tethering respectively. c) Cartoon of smooth muscle peripheral couplings, focusing on those between the caveolar PM and peripheral SR. (i) A MCS containing PM BK channels (gold) and ER RyR2 (teal) allows for BK channel activation by RyR2-mediated Ca²⁺ release, leading to reduced Ca_V1.2 (pink) activity and favoring smooth muscle relaxation. (ii) ER-PM junctions at which activation of PM TRPM4 channels (purple) by Ca²⁺ release from closely associated ER IP₃Rs (dark green) triggers membrane depolarization that enhances Ca_V1.2 (pink) activity favoring smooth muscle contraction. (iii) A MCS containing PM Ca²⁺ activated Cl⁻ channels (red) that are activated by Ca²⁺ release from nearby ER IP₃R (green) or RyR (teal) leads to membrane depolarization that activates Cav1.2 (pink) L-type Ca²⁺ channels leading to smooth muscle contraction. Figure created with Biorender.com. CICR: Ca²⁺-induced Ca²⁺ release; ER: endoplasmic reticulum; ER-PM: endoplasmic reticulum-plasma membrane: jSR: junctional sarcoplasmic reticulum; MCS: membrane contact site; SR: sarcoplasmic reticulum; STIC: spontaneous transient inward currents mediated by Ca²⁺ activated Cl⁻ channels; STOC: spontaneous transient outward currents mediated by Ca²⁺ activated K⁺ channels; TT: t-tubule.

Figure 2.. Neuronal ER-PM junctions as platforms for depolarization-induced Ca²⁺ signaling.

a) K_V2 containing ER-PM junctions in hippocampal pyramidal neurons comprise PM K_V2.1 (or K_V2.2) channels (light blue) in association with ER VAP proteins (black). K_V2 channels recruit PM Ca_V1.2 L-type Ca²⁺ channels (pink) leading to their increased clustering that enhances their activity and brings them into close spatial and functional association with ER RyRs (teal) facilitating depolarization-induced CICR and downstream Ca²⁺ signaling pathways including excitation-transcription coupling. b) PM Ca_V2.1 voltage-gated Ca²⁺ channels (light green) are localized at ER-PM junctions in cartwheel interneurons of dorsal cochlear nucleus bringing them into close spatial and functional association with ER RyRs (teal) facilitating depolarization-induced CICR that activates Ca²⁺-activated BK channels (light gold) to control AP firing patterns. c) In hippocampal neurons, Ca_V1.3 LTCCs (light pink) are localized at ER-PM junctions stabilized by JPH3 and JPH4 (dark blue) bringing them into close spatial and functional association with ER RyRs (teal) facilitating depolarization-induced CICR that activates Ca²⁺-activated IK channels (orange) to generate the slow AHP. Figure created with Biorender.com. AHP: afterhyperpolarization; AP: action potential; CICR: Ca²⁺-induced Ca²⁺ release; ER: endoplasmic reticulum; ER-PM: endoplasmic reticulum-plasma membrane; PM: plasma membrane.