Regulation and Role of Store-Operated Ca2+ Entry in Cellular Proliferation

Excerpt

Ca²⁺ 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, Ca²⁺ can mediate different responses in the same cell [1]. The specific cellular response transduced downstream of the particular Ca²⁺ transient is encoded in the spatial and temporal dynamics of the Ca²⁺ signal, leading to the activation of a subset of Ca²⁺-dependent effectors and the ensuing cellular response. As such, the duration, amplitude, frequency, and spatial localization of Ca²⁺ signals encode targeted signals that activate Ca²⁺-sensitive effectors to define a particular cellular response. To generate and fine-tune those Ca²⁺ signals, cells use two main Ca²⁺ sources: entry of extracellular Ca²⁺ and Ca²⁺ release from intracellular stores. The primary intracellular Ca²⁺ store is the endoplasmic reticulum (ER), which can concentrate Ca²⁺ in the hundreds of μM range [2]. In contrast, cytoplasmic Ca²⁺ concentration is kept at rest at extremely low levels (∼100 nM or lower), thus providing a low-noise background for detection of complex Ca²⁺ dynamics [3].

The Ca²⁺-signaling machinery includes Ca²⁺ entry and extrusion pathways in the plasma membrane (PM), ER membrane Ca²⁺ release channels, and Ca²⁺ reuptake ATPases within the ER membrane [4]. These Ca²⁺ transport pathways, in addition to intracellular Ca²⁺ buffers and Ca²⁺ uptake and release through other intracellular organelles, primarily the mitochondria, combine to shape highly tuned and dynamic Ca²⁺ transients that regulate cellular functions [5].

Under physiological conditions in non-excitable cells, Ca²⁺ transients are typically initiated downstream of agonist stimulation through the activation of the PLC-IP₃ signal transduction cascade, which leads to the opening of intracellular Ca²⁺ channel inositol 1,4,5-trisphosphate receptors (IP₃Rs) to release Ca²⁺ from intracellular stores [6]. Ca²⁺ release depletes the stores and activates a Ca²⁺ influx pathway in the PM termed store-operated Ca²⁺ entry (SOCE). SOCE is mediated by two key players: ER transmembrane Ca²⁺ sensors represented by the STIM family of proteins and PM Ca²⁺ 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 Ca²⁺ concentration. The loss of STIM1 Ca²⁺ 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 Ca²⁺ 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 Ca²⁺-signaling machinery upon store depletion allows for specific Ca²⁺ signaling in the midrange between Ca²⁺ microdomains and global Ca²⁺ waves [10] (see Chapter 5). Spatially, Ca²⁺ signaling can occur in localized spatially restricted elementary Ca²⁺ release events that activate effectors located in the immediate proximity of the Ca²⁺ channel. Alternatively, Ca²⁺ signals/waves occur/spread through the entire cell resulting in a global spatially unrestricted signal.

We have recently described a SOCE-dependent Ca²⁺-signaling modularity that signals in the midrange between these two spatial extremes [10]. Store depletion downstream of receptor activation and IP₃ generation results in a localized Ca²⁺ entry point source at the SOCE clusters that induces Ca²⁺ entry into the cytoplasm, which is readily taken up into the ER lumen through SERCA activity only to be released again through open IP₃Rs distally to the SOCE entry site and gate Ca²⁺-activated Cl^- channels as downstream Ca²⁺ effectors. This mechanism, referred to as “Ca²⁺ teleporting,” allows for specific activation of Ca²⁺ effectors that are distant from the point source Ca²⁺ channel without inducing a global Ca²⁺ wave, thus providing a novel module in the Ca²⁺-signaling repertoire. A cartoon summary of Ca²⁺ teleporting is found in Figure 12.1.

The relationship between Ca²⁺ signaling and cellular proliferation is complex with Ca²⁺ transients detected at various stages of the cell cycle [2]. These transients are thought to activate a multitude of Ca²⁺ effectors downstream of the initial Ca²⁺ signal, which were shown to be important for cellular proliferation, including, for example, calmodulin (CaM) and Ca²⁺-CaM-dependent protein kinase II (CaMKII).

However, there are a few cases where Ca²⁺ 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 Ca²⁺, but Ca²⁺ is required for the completion of meiosis I in vertebrate oocytes [12–14]. Interestingly, multiple Ca²⁺ signaling pathways are modified during M-phase of the cell cycle, with the best defined example being Xenopus oocyte maturation [15].

Several Ca²⁺ influx pathways have been implicated in cell proliferation and cell cycle progression, including TRP channels, voltage-gated Ca²⁺ channels (Ca_V), purinergic P2X receptors, ionotropic glutamate receptors, and SOCE [16]. Blockers of voltage-gated Ca²⁺ 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 Ca²⁺ 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 Ca²⁺ signaling than affecting Ca²⁺ influx through the specific channel in question, as it may lead to changes in expression of other Ca²⁺-signaling pathways as a compensatory mechanism. Furthermore, the majority of TRP channels conduct cations with some Ca²⁺ permeability and are not Ca²⁺ selective like Orai1 or Ca_V 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 Ca²⁺ 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 Ca²⁺-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).