ARC Channels: A Novel Pathway for Receptor-Activated Calcium Entry

Abstract

2+ions ([Ca 2+ ] i ) is a major component of cellular signals generated by the actions of a variety of hormones and neurotransmitters acting on receptors coupled to phospholipase C (PLC). Such [Ca 2+ ] i signals invariably involve both an inositol 1,4,5-trisphosphate (IP 3 )-mediated release of Ca 2+ from intracellular stores and the activation of an enhanced entry of extracellular Ca 2+ . Although the latter is often critical in achieving an effective sustained response from the cells, our understanding of the nature of the relevant Ca 2+ -entry pathway and its regulation has lagged far behind the study of IP 3 -mediated Ca 2+ release, a fact due, as much as anything, to the technical difficulties involved in directly measuring the activities of the generally small conductances involved.For much of the past 18 years, attention has focused on the so-called "capacitative" or storeoperated mechanism of Ca 2+ entry first defined by Putney (24,25). In this, the two processes that make up PLC-dependent [Ca 2+ ] i signaling are intimately and sequentially linked in that the emptying of agonist-sensitive stores alone is both necessary and sufficient to activate Ca 2+ entry. The channels responsible for this entry are generically described as store-operated channels, the archetypal version of which are the Ca 2+ release-activated Ca 2+ channels (CRAC channels) first described in mast cells and Jurkat cells (8,36). Despite extensive study, the molecular identity of these channels and the precise mechanism whereby depletion of the agonist-sensitive Ca 2+ stores induces their activation have yet to be resolved (20,23). Nevertheless, the role of the CRAC channels and other similar store-operated Ca 2+ -selective channels (SOC channels) in cellular Ca 2+ signaling is clear. Entry through these channels determines the level of the sustained elevation in [Ca 2+ ] i observed following stimulation with high agonist concentrations and is responsible for the subsequent refilling of intracellular agonist-sensitive Ca 2+ stores following termination of the signal. However, it is significant that activation of these channels has been shown to require a rather profound and sustained depletion of the intracellular stores (5, 19), although this issue remains controversial (7). In certain cell types (e.g., T lymphocytes), the limited size of the agonist-sensitive store means that such a profound depletion is readily achieved at all levels of stimulation, and in these cells it seems that CRAC channels provide the principal source of Ca 2+ entry under all conditions (23). In many other cells, however, the oscillatory [Ca 2+ ] i signals typically seen following stimulation at lower, more physiologically relevant agonist concentrations reflect only a transient and/or partial release of Ca 2+ from the stores (21). As discussed above, the precise relationship between the extent of store depletion and the magnitude of the CRAC current that results remains controversial. In contrast, it is clear that the activation of CRAC channels is slow,...

Mechanisms of Hypotonicity-Induced Calcium Signaling and Integrin Activation by Arachidonic Acid-Derived Inflammatory Mediators in B Cells

“…6), it is also possible that PLA 2 or AA could be involved in hypotonicity-induced suppression of CRAC channel activation. Reciprocal regulation of store-operated and AA-regulated channels has been observed in a number of cell types (20,21). However, these AA-regulated channels are biophysically and pharmacologically distinct from the AA-sensitive channels we have defined in B cells (22).…”

All-or-None Activation of CRAC Channels by Agonist Elicits Graded Responses in Populations of Mast Cells

“…We also considered the possibility that receptor stimulation might recruit different Ca 2ϩ influx pathways in a concentration-dependent manner. It has been suggested in HEK293 cells for example that low concentrations of agonist recruit a nonstore-operated Ca 2ϩ influx pathway gated by arachidonic acid that is 2-APB insensitive whereas higher agonist concentrations activate store-operated Ca 2ϩ influx (23). However, 2-APB suppressed Ca 2ϩ influx and both Ca 2ϩ -dependent arachidonate release and LTC 4 secretion following stimulation with a low concentration of carbachol (1 M; Fig.…”

A Tridimensional Model of Proangiogenic Calcium Signals in Endothelial Cells

“…Activation process is dependent on the concentration of AA (AA), physiologically released by different enzymes ( [36], see model description). The mechanisms responsible for desensitization of AA-induced calcium current in the presence of the agonist are not known, even if in other cell types a phosphorylation/dephosphorylation balance has been suggested [59,60]: the desensitization factor h is taken as h 2 to fit the time course of the experimental current (Fig. 3).…”