New Aspects of the Contribution of ER to SOCE Regulation
The junctions between the endoplasmic reticulum and the plasma membrane are essential platforms for the activation of store operated Ca2+ influx. These junctions have specific dimensions and are non-uniformly distributed in polarized cells. The mechanisms
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The Role of the ER and ER-Plasma Membrane Junctions in the Regulation of SOCE Hayley Dingsdale, Lee Haynes, Gyorgy Lur, and Alexei Tepikin
10.1
Introduction
The endoplasmic reticulum (ER) is an intracellular organelle present in all eukaryotic cells, which is critical for both the survival and correct functioning of all cell types. Its functions include the synthesis, modification and transport of proteins, and, centrally to this chapter, it also acts as the main intracellular calcium (Ca2+) store (Berridge et al. 2000). These diverse functions all require an ER Ca2+ concentration ([Ca2+]ER) of approximately 100–800 mM at rest; a sustained depletion of ER Ca2+ stores is detrimental to cells, and can lead to cell stress, inhibition of protein synthesis and apoptosis (Burdakov et al. 2005; Groenendyk and Michalak 2005). Cells therefore employ a number of mechanisms to prevent prolonged store depletion, using Ca2+binding proteins in the ER to buffer Ca2+ levels (Michalak et al. 2002), and sarcoplasmic/endoplasmic reticulum Ca2+-ATPases (SERCAs) to actively re-load the store (Carafoli 2002). Crucially, a reduction of the [Ca2+]ER will also activate store-operated Ca2+ entry (SOCE), which occurs at junctions between the ER and the plasma membrane (PM) (Putney 2007). This chapter will describe the basic structural and functional characteristics of the ER, and will look in-depth at ER-PM junctions and the role of the ER structure in the regulation of SOCE.
H. Dingsdale (*) • L. Haynes • A. Tepikin The Physiological Laboratory, Department of Cellular and Molecular Physiology The University of Liverpool, Liverpool, UK e-mail: [email protected] G. Lur Department of Neurobiology, Yale School of Medicine, New Haven, USA K. Groschner et al. (eds.), Store-operated Ca2+ entry (SOCE) pathways, DOI 10.1007/978-3-7091-0962-5_10, # Springer-Verlag/Wien 2012
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10.2
H. Dingsdale et al.
The Endoplasmic Reticulum
10.2.1 ER Structure The ER is an intracellular organelle with a bilayer membrane and a multitude of different functions. Some of these functions (e.g. protein synthesis) have specific structural requirements; cells cater for this by utilizing three distinct types of ER – rough ER, smooth ER and the nuclear envelope (Voeltz et al. 2002). Additionally, some ER classifiers mention transitional ER, a specialized ER domain involved in communication with the Golgi apparatus (Jamieson and Palade 1967; Voeltz et al. 2002). The surface of rough ER is characteristically studded with ribosomes, nucleoprotein complexes responsible for protein synthesis. These large structures (approximately 26 nm in diameter (Lur et al. 2009)) give the ER a “rough” appearance when seen on moderate magnification electron micrographs, and thus give the rough ER its name (Palade 1955). Ribosomes are absent from smooth ER, which is primarily involved in the synthesis of lipids and sterols. A specialized form of smooth ER – the sarcoplasmic reticulum – plays a key role in Ca2+ signaling and excitation-contraction coupling in myocytes (Rossi a
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