Recent insights into mammalian ER–PM junctions

doi:10.1016/j.ceb.2018.12.011

Current Opinion in Cell Biology

Volume 57, April 2019, Pages 99-105

https://doi.org/10.1016/j.ceb.2018.12.011 Get rights and content

ER–PM junctions are subcellular sites where the endoplasmic reticulum (ER) and the plasma membrane (PM) are kept in close appositions, providing a platform for inter-organelle contact. These membrane contact sites are important for many physiological functions in mammalian cells, including excitation–contraction coupling, store-operated Ca²⁺ entry, and non-vesicular transfer of lipids between the ER and the PM. Here we review recent insights into the 3D structure and spatial organization of ER–PM junctions in mammalian cells as well as molecular mechanisms underlying the formation and functions of mammalian ER–PM junctions.

Introduction

The endoplasmic reticulum (ER) is an extensive membrane organelle important for Ca²⁺ storage, biosynthesis of proteins and lipids, and protein folding. Communication between the ER and the plasma membrane (PM) is important for cells to properly respond to extracellular stimulation and maintain cellular homeostasis. Close appositions between the ER and the PM at ER–PM junctions, also referred to as ER–PM contacts or ER–PM contact sites, enable direct contacts between these two membrane compartments for signaling and molecular trafficking [1].

ER–PM junctions were first observed using electron microscopy (EM) in muscle cells and referred to as dyads and triads [2]. Dyads and triads are found to be essential for excitation–contraction (E–C) coupling underlying muscle contraction [3]. At dyads and triads, depolarization of voltage-gated Ca²⁺ channels at the PM triggers the juxtaposed ryanodine receptors at the ER to release stored Ca²⁺ stimulating contraction of muscle cells. Junctophilins (JPHs) enriched at dyads and triads are important for the formation of these ER–PM junctions and E–C coupling [4]. Deficiency in JPHs leads to cardiac arrests and lethality in mice.

Nearly five decades after, ER–PM junctions were identified as the spatial platform of the ubiquitous store-operated Ca²⁺ entry (SOCE) pathway for Ca²⁺ signaling and homeostasis [5,6]. Following a decrease in ER Ca²⁺, SOCE is activated by translocation of the ER membrane protein STIM1 to ER–PM junctions, enabling its interaction with the PM Ca²⁺ channel Orai1 [7]. SOCE is essential for numerous physiological processes such as the immune response, secretion, cell migration, and tissue development. Humans deficient in SOCE suffer severe combined immunodeficiency, skeletal myopathy, and ectodermal dysplasia [8].

More recently, it was found that lipid exchange at ER–PM junctions is pivotal to the phosphatidylinositol (PI) cycle, a fundamental process first observed in 1953 in receptor-stimulated pancreas slices [9]. The PI cycle involves transfer of phosphatidic acid (PA), which is converted from diacylglycerol (DAG) generated by stimulation-induced breakdown of PI 4,5-bisphosphate (PI(4,5)P₂), from the PM to the ER where it can be converted to PI and transferred back to the PM for PI(4,5)P₂ re-synthesis to sustain cell signaling [10,11]. The production of PA following receptor stimulation induces translocation of the cytosolic PI/PA transfer protein Nir2 to ER–PM junctions to mediate the PI cycle [12, 13, 14]. The PI cycle is not only important for sustaining receptor-induced signaling, but also required to maintain numerous PI(4,5)P₂-dependent cellular functions such as ion transport, membrane trafficking, and cytoskeleton dynamics.

The known functions of mammalian ER–PM junctions continue to expand. Additional proteins that constitutively or dynamically enriched at ER–PM junctions have been identified, including extended synaptotagmins (E-Syts), TMEM24, Kv2 voltage-gated K⁺ channels, oxysterol-binding protein (OSBP)-related proteins (ORP) 5 and ORP8, and GRAM domain-containing proteins (GRAMDs). Moreover, recent developments in imaging technologies and tools enable the visualization of the 3D structure of ER–PM junctions and their spatial organization in mammalian cells, advancing our understanding of these membrane junctions. The aim of this review is to discuss recent insights into this fast growing field of research on the structure, functions, and regulation of mammalian ER–PM junctions, complementing several comprehensive reviews on this topic [1,10,15,16].

Section snippets

3D structure and spatial organization of mammalian ER–PM junctions

Early EM studies on the structure of mammalian ER–PM junctions were mostly performed using muscle and neuronal cells. ER–PM junctions in muscle cells were observed in specific regions of sarcomere, the contraction unit of the muscle [3]. The gap distance between the ER and the PM at these junctions is kept at 9–12 nm. Correlations between the localization, number, and gap distance of ER–PM junctions with the contraction speed of different types of muscles were observed, consistent with an

Molecular mechanisms underlying formation and functions of mammalian ER–PM junctions

The ER and the PM are kept in close apposition at ER–PM junctions by protein–protein and protein–lipid interactions between the two membrane compartments. Many proteins have been reported to support the formation and functions of mammalian ER–PM junctions. Here we discuss seven groups of proteins that harbor well-characterized dual targeting mechanisms for the ER and the PM, enabling their constitutive or dynamic enrichment at ER–PM junctions (Figure 2).

Perspectives

Recent findings indicate that mammalian ER–PM junctions are membrane contact sites supported by cortical actin and multiple families of proteins at a given time. Since the enrichment of these proteins at ER–PM junctions can be regulated by various factors such as ER/cytosolic Ca²⁺ levels, PM PA/PS/PI4P/PI(4,5)P₂/cholesterol levels, or phosphorylation, it is likely that the composition of single ER–PM junctions is heterogeneous and controlled by the local environment and stimulated state of a

Conflict of interest statement

Nothing declared.

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

• of special interest
•• of outstanding interest

Acknowledgements

We thank the Liou Laboratory members for valuable discussions. This work was supported by National Institutes of Health grant GM113079, Welch Foundation Grant I-1789, and National Institutes of Health Cell and Molecular Biology Training ProgramT32 GM008203. J. Liou is a Sowell Family Scholar in Medical Research.

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Endoplasmic reticulum-plasma membrane contact sites (ER-PM MCS) are a specialised domain involved in the control of Ca²⁺ dynamics and various Ca²⁺-dependent cellular processes. Intracellular Ca²⁺ signals are broadly supported by Ca²⁺ release from intracellular Ca²⁺ channels such as inositol 1,4,5-trisphosphate receptors (IP₃Rs) and subsequent store-operated Ca²⁺ entry (SOCE) across the PM to replenish store content. IP₃Rs sit in close proximity to the PM where they can easily access newly synthesised IP₃, interact with binding partners such as actin, and localise adjacent to ER-PM MCS populated by the SOCE machinery, STIM1–2 and Orai1–3, to possibly form a locally regulated unit of Ca²⁺ influx. PtdIns(4,5)P₂ is a multiplex regulator of Ca²⁺ signalling at the ER-PM MCS interacting with multiple proteins at these junctions such as actin and STIM1, whilst also being consumed as a substrate for phospholipase C to produce IP₃ in response to extracellular stimuli. In this review, we consider the mechanisms regulating the synthesis and turnover of PtdIns(4,5)P₂ via the phosphoinositide cycle and its significance for sustained signalling at the ER-PM MCS. Furthermore, we highlight recent insights into the role of PtdIns(4,5)P₂ in the spatiotemporal organization of signalling at ER-PM junctions and raise outstanding questions on how this multi-faceted regulation occurs.
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Clusters of STIM1 at ER-PM MCS recruit Orai1 by diffusional trapping thus stabilizing the STIM1-Orai1 macromolecular complex at the junction and providing the Ca2+ entry source. ER-PM MCSs are not induced by store depletion but are present at rest and are stabilized through tethering proteins such as the e-synaptotagmin family, ORPs, and GRAMDs [11], some of which like the ORPs play a role in non-vesicular lipid transfer at the junction. Therefore, store depletion leads to a major restructuring of the junctions resulting in a point source Ca2+ influx that not only refills the ER stores, but also serves important signaling functions by activating multiple cellular pathways, including secretion, transcription, crosstalk with other signal transduction pathway, and neuronal signaling.
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View full text

Recent insights into mammalian ER–PM junctions

Introduction

Section snippets

3D structure and spatial organization of mammalian ER–PM junctions

Molecular mechanisms underlying formation and functions of mammalian ER–PM junctions

Perspectives

Conflict of interest statement

References and recommended reading

Acknowledgements

Biochim Biophys Acta Mol Cell Res

Cell Calcium

Curr Biol

J Biol Chem

Biochim Biophys Acta

Biochim Biophys Acta

Cell Rep

J Biol Chem

Dev Cell

Neuron

J Cell Sci

Mol Biol Cell

Curr Biol

PLoS One

Neuroscience

Cell

Studies on the endoplasmic reticulum. III. Its form and distribution in striated muscle cells

J Biophys Biochem Cytol

The relationship between form and function throughout the history of excitation-contraction coupling

J Gen Physiol

Ca2+ store depletion causes STIM1 to accumulate in ER regions closely associated with the plasma membrane

J Cell Biol

Store-operated calcium channels

Physiol Rev

Diseases caused by mutations in ORAI1 and STIM1

Ann N Y Acad Sci

STIM proteins and the endoplasmic reticulum-plasma membrane junctions

Annu Rev Biochem

Endoplasmic reticulum-plasma membrane contact sites

Annu Rev Biochem

Subsurface cisterns and their relationship to the neuronal plasma membrane

J Cell Biol

From the cover: STIM1-induced precortical and cortical subdomains of the endoplasmic reticulum

Proc Natl Acad Sci U S A

Three-dimensional architecture of extended synaptotagmin-mediated endoplasmic reticulum-plasma membrane contact sites

Proc Natl Acad Sci U S A

Cortical actin contributes to spatial organization of ER-PM junctions

Mol Biol Cell

Ca²⁺ store depletion causes STIM1 to accumulate in ER regions closely associated with the plasma membrane