Regulation and Role of Store-Operated Ca Entry in Cellular Proliferation

Ca is a ubiquitous intracellular messenger that transduces a variety of cellular responses downstream of the activation of G-protein-coupled or tyrosine kinase receptors. Depending on the agonist and cellular context, Ca can mediate different responses in the same cell [1]. The specific cellular response transduced downstream of the particular Ca transient is encoded in the spatial and temporal dynamics of the Ca signal, leading to the activation of a subset of Ca-dependent effectors and the ensuing cellular response. As such, the duration, amplitude, frequency, and spatial localization of Ca signals encode targeted signals that activate Ca-sensitive effectors to define a particular cellular response. To generate and fine-tune those Ca signals, cells use two main Ca sources: entry of extracellular Ca and Ca release from intracellular stores. The primary intracellular Ca store is the endoplasmic reticulum (ER), which can concentrate Ca in the hundreds of μM range [2]. In contrast, cytoplasmic Ca concentration is kept at rest at extremely low levels (∼100 nM or lower), thus providing a low-noise background for detection of complex Ca dynamics [3]. The Ca-signaling machinery includes Ca entry and extrusion pathways in the plasma membrane (PM), ER membrane Ca release channels, and Ca reuptake ATPases within the ER membrane [4]. These Ca transport pathways, in addition to intracellular Ca buffers and Ca uptake and release through other intracellular organelles, primarily the mitochondria, combine to shape highly tuned and dynamic Ca transients that regulate cellular functions [5]. Under physiological conditions in non-excitable cells, Ca transients are typically initiated downstream of agonist stimulation through the activation of the PLC-IP signal transduction cascade, which leads to the opening of intracellular Ca channel inositol 1,4,5-trisphosphate receptors (IPRs) to release Ca from intracellular stores [6]. Ca release depletes the stores and activates a Ca influx pathway in the PM termed store-operated Ca entry (SOCE). SOCE is mediated by two key players: ER transmembrane Ca sensors represented by the STIM family of proteins and PM Ca channels of the Orai family that link directly to STIMs (see Chapters 1 through 3). The N-terminus of STIM1 faces the ER lumen and consists of two EF-hand domains that detect luminal Ca concentration. The loss of STIM1 Ca binding upon store depletion leads to conformational changes in the protein and its aggregation into clusters that translocate and stabilize into ER-PM junctions with very close apposition (∼20 nm) [7]. STIM1 within these ER-PM junctions binds to and recruits Orai1 through a diffusional trap mechanism, resulting in opening Orai1 channels and Ca entry [8]. As such, the STIM-Orai clusters at ER-PM junctions define a specific microdomain at ER-PM junctions that also include the ER Ca-ATPase (SERCA) [9,10]. The tightly regulated remodeling of the Ca-signaling machinery upon store depletion allows for specific Ca signaling in the midrange between Ca microdomains and global Ca waves [10] (see Chapter 5). Spatially, Ca signaling can occur in localized spatially restricted elementary Ca release events that activate effectors located in the immediate proximity of the Ca channel. Alternatively, Ca signals/waves occur/spread through the entire cell resulting in a global spatially unrestricted signal. We have recently described a SOCE-dependent Ca-signaling modularity that signals in the midrange between these two spatial extremes [10]. Store depletion downstream of receptor activation and IP generation results in a localized Ca entry point source at the SOCE clusters that induces Ca entry into the cytoplasm, which is readily taken up into the ER lumen through SERCA activity only to be released again through open IPRs distally to the SOCE entry site and gate Ca-activated Cl channels as downstream Ca effectors. This mechanism, referred to as “Ca teleporting,” allows for specific activation of Ca effectors that are distant from the point source Ca channel without inducing a global Ca wave, thus providing a novel module in the Ca-signaling repertoire. A cartoon summary of Ca teleporting is found in Figure 12.1. The relationship between Ca signaling and cellular proliferation is complex with Ca transients detected at various stages of the cell cycle [2]. These transients are thought to activate a multitude of Ca effectors downstream of the initial Ca signal, which were shown to be important for cellular proliferation, including, for example, calmodulin (CaM) and Ca-CaM-dependent protein kinase II (CaMKII). However, there are a few cases where Ca signals have been shown directly to be critical for cell cycle progression, including in mitosis for nuclear envelope breakdown and for chromosome disjunction [11]. In contrast, nuclear envelope breakdown during meiosis in vertebrate oocytes occurs independently of Ca, but Ca is required for the completion of meiosis I in vertebrate oocytes [12–14]. Interestingly, multiple Ca signaling pathways are modified during M-phase of the cell cycle, with the best defined example being oocyte maturation [15]. Several Ca influx pathways have been implicated in cell proliferation and cell cycle progression, including TRP channels, voltage-gated Ca channels (Ca), purinergic P2X receptors, ionotropic glutamate receptors, and SOCE [16]. Blockers of voltage-gated Ca channels were shown to slow down cell growth, arguing for a role for these channels in cell cycle progression [16–18]. Experimental manipulation of the expression levels of members of the TRPC, TRPV, and TRPM families of cation channels, which are Ca permeable, was linked to cell proliferation with differential effects depending on the particular channel studied [16]. However, some of these studies are difficult to interpret because channel knockdown or overexpression could have significantly broader effects on Ca signaling than affecting Ca influx through the specific channel in question, as it may lead to changes in expression of other Ca-signaling pathways as a compensatory mechanism. Furthermore, the majority of TRP channels conduct cations with some Ca permeability and are not Ca selective like Orai1 or Ca channels, with the exception of TRPV5 and TRPV6 (see Chapter 13). Hence, changes in their expression is likely to affect the ionic balance across the cell membrane with effects on resting membrane potential, which may in turn affect cell proliferation. The relationship between SOCE and cell proliferation is an intimate one that goes beyond the well-recognized roles of Ca signaling in cellular growth and proliferation. SOCE is dramatically downregulated during the division phase of the cell cycle through mechanisms that have not been fully elucidated. This is in line with the significant remodeling of the Ca-signaling machinery during M-phase, which has been well characterized during oocyte meiosis. Furthermore, there is mounting evidence from multiple neoplasms for an important role for SOCE in metastasis. This chapter presents a brief overview of our current knowledge as to the mechanisms regulating SOCE during cell cycle from cellular proliferation to metastasis with an emphasis on SOCE regulation during cell division (mitosis and meiosis).