Different agonists recruit different stromal interaction molecule proteins to support cytoplasmic Ca2+ oscillations and gene expression.

Stimulation of cells with physiological concentrations of calcium-mobilizing agonists often results in the generation of repetitive cytoplasmic Ca(2+) oscillations. Although oscillations arise from regenerative Ca(2+) release, they are sustained by store-operated Ca(2+) entry through Ca(2+) release-activated Ca(2+) (CRAC) channels. Here, we show that following stimulation of cysteinyl leukotriene type I receptors in rat basophilic leukemia (RBL)-1 cells, large amplitude Ca(2+) oscillations, CRAC channel activity, and downstream Ca(2+)-dependent nuclear factor of activated T cells (NFAT)-driven gene expression are all exclusively maintained by the endoplasmic reticulum Ca(2+) sensor stromal interaction molecule (STIM) 1.

Mitofusin 2 regulates STIM1 migration from the Ca2+ store to the plasma membrane in cells with depolarized mitochondria.

Store-operated Ca2+ channels in the plasma membrane (PM) are activated by the depletion of Ca2+ from the endoplasmic reticulum (ER) and constitute a widespread and highly conserved Ca2+ influx pathway. After store emptying, the ER Ca2+ sensor STIM1 forms multimers, which then migrate to ER-PM junctions where they activate the Ca2+ release-activated Ca2+ channel Orai1. Movement of an intracellular protein to such specialized sites where it gates an ion channel is without precedence, but the fundamental question of how STIM1 migrates remains unresolved.

Mast cell CRAC channel as a novel therapeutic target in allergy.

Purpose Of Review: This review describes recent advances in our understanding of a major Ca-entry pathway, the Ca release-activated Ca (CRAC) channel, that is central to mast cell activation.

Recent Findings: Animals in which the genes encoding the CRAC channel have been deleted have severely compromised mast cell function and reduced allergic responses. These functional consequences reflect the ability of CRAC channels to activate a range of spatially and temporally distinct responses in mast cells, which contribute to both rapid and slow phases of an allergic response.

Targeting Ca2+ release-activated Ca2+ channel channels and leukotriene receptors provides a novel combination strategy for treating nasal polyposis.

Background: Nasal polyposis is a chronic inflammatory disease of the upper respiratory tract that affects around 2% of the population and almost 67% of patients with aspirin-intolerant asthma. Polyps are rich in mast cells and eosinophils, resulting in high levels of the proinflammatory cysteinyl leukotrienes.

Objectives: To better understand the role of the proinflammatory leukotrienes in nasal polyposis, we asked the following questions: (1) How do nasal polyps produce leukotriene C(4) (LTC(4))? (2) Can LTC(4) feed back in a paracrine way to maintain mast cell activation? (3) Could a combination therapy targeting the elements of this feed-forward loop provide a novel therapy for allergic disease?

Methods: We have used immunohistochemistry, enzyme immunoassay, and cytoplasmic calcium ion (Ca(2+)) imaging to address these questions on cultured and acutely isolated human mast cells from patients with polyposis.

CRAC channels and Ca2+ signaling in mast cells.

Mast cells are integral members of the immune system. Upon activation by a rise in cytoplasmic Ca2+, they release a battery of paracrine signals, chemokines, and cytokines, which help sculpt the subsequent immune response. Ca2+ entry through store-operated Ca2+ release-activated Ca2+ (CRAC) channels in the plasma membrane is central for driving most of these responses.

Decoding of cytoplasmic Ca(2+) oscillations through the spatial signature drives gene expression.

Cytoplasmic Ca(2+) oscillations are a universal signaling mode that activates numerous cellular responses [1, 2]. Oscillations are considered the physiological mechanism of Ca(2+) signaling because they occur at low levels of stimulus intensity [3]. Ca(2+) oscillations are proposed to convey information in their amplitude and frequency, leading to activation of specific downstream targets [4-6].

Intercellular Ca2+ wave propagation involving positive feedback between CRAC channels and cysteinyl leukotrienes.

Mast cells are key components of the immune system, where they help orchestrate the inflammatory response. Aberrant mast cell activation is linked to a variety of allergic diseases, including asthma, eczema, rhinitis, and nasal polyposis, which in combination affect up to 20% of the population in industrialized countries. On activation, mast cells release a variety of signals that target the bronchi and vasculature and recruit other immune cells to the inflammatory site.

Sustained activation of the tyrosine kinase Syk by antigen in mast cells requires local Ca2+ influx through Ca2+ release-activated Ca2+ channels.

Mast cell activation involves cross-linking of IgE receptors followed by phosphorylation of the non-receptor tyrosine kinase Syk. This results in activation of the plasma membrane-bound enzyme phospholipase Cgamma1, which hydrolyzes the minor membrane phospholipid phosphatidylinositol 4,5-bisphosphate to generate diacylglycerol and inositol trisphosphate. Inositol trisphosphate raises cytoplasmic Ca2+ concentration by releasing Ca2+ from intracellular stores.

Local Ca2+ influx through Ca2+ release-activated Ca2+ (CRAC) channels stimulates production of an intracellular messenger and an intercellular pro-inflammatory signal.

Ca2+ entry through store-operated Ca2+ channels drives the production of the pro-inflammatory molecule leukotriene C4 (LTC4) from mast cells through a pathway involving Ca2+-dependent protein kinase C, mitogen-activated protein kinases ERK1/2, phospholipase A2, and 5-lipoxygenase. Here we examine whether local Ca2+ influx through store-operated Ca2+ release-activated Ca2+ (CRAC) channels in the plasma membrane stimulates this signaling pathway. Manipulating the amplitude and spatial extent of Ca2+ entry by altering chemical and electrical gradients for Ca2+ influx or changing the Ca2+ buffering of the cytoplasm all impacted on protein kinase C and ERK activation, generation of arachidonic acid and LTC4 secretion, with little change in the bulk cytoplasmic Ca2+ rise.

All-or-none activation of CRAC channels by agonist elicits graded responses in populations of mast cells.

In nonexcitable cells, receptor stimulation evokes Ca(2+) release from the endoplasmic reticulum stores followed by Ca(2+) influx through store-operated Ca(2+) channels in the plasma membrane. In mast cells, store-operated entry is mediated via Ca(2+) release-activated Ca(2+) (CRAC) channels. In this study, we find that stimulation of muscarinic receptors in cultured mast cells results in Ca(2+)-dependent activation of protein kinase Calpha and the mitogen activated protein kinases ERK1/2 and this is required for the subsequent stimulation of the enzymes Ca(2+)-dependent phospholipase A(2) and 5-lipoxygenase, generating the intracellular messenger arachidonic acid and the proinflammatory intercellular messenger leukotriene C(4).