Mechanisms of mast cell signaling in anaphylaxis

Abstract

The recent development of a consensus definition and proposed diagnostic criteria for anaphylaxis offers promise for research efforts and a better understanding of the epidemiology and pathogenesis of this enigmatic and life-threatening disease. This review examines basic principles and recent research advances in the mechanisms of mast cell signaling believed to underlie anaphylaxis. The unfolding complexity of mast cell signaling suggests that the system is sensitive to regulation by any of several individual signaling pathways and intermediates and that complementary pathways regulate mast cell activation by amplified signals. The signaling events underlying anaphylactic reactions have largely been identified through experiments in genetically modified mice and supported by biochemical studies of mast cells derived from these mice. These studies have revealed that signaling pathways exist to both upregulate and downregulate mast cell responses. In this review we will thus describe the key molecular players in these pathways in the context of anaphylaxis.

FIG 1

Signaling pathways that lead from activated KIT and aggregated FcϵRI to mast cell responses. Aggregation of IgE-occupied FcϵRI induces activation of the Src family tyrosine kinase Lyn, whereas stem cell factor–induced dimerization of KIT induces activation of its intrinsic kinase. Phosphorylation (indicated by red circles) of tyrosine residues in the intracellular domains of each of these receptors recruits SH2 domain–containing signaling molecules. In the case of FcϵRI, Syk is recruited through FcϵRI by ITAMs contained in γ chain cytoplasmic domains. Resulting activation of Syk leads to phosphorylation of LAT and LAT2. These proteins then serve as scaffolds for multimolecular signaling complexes for the binding of cytosolic adapter molecules, such as Gads, Grb2, SLP76, and SHC; guanosine triphosphate exchangers, including Sos and Vav1; and the signaling enzymes PLCγ₁ and PLCγ₂. PLCγ catalyzes the hydrolysis of PIP₂ to yield diacylglycerol (DAG) and inositol trisphosphate (IP₃), which, respectively, result in the activation of protein kinase C (PKC) and the liberation of intracellular calcium. These signals lead to mast cell degranulation and eicosanoid generation and contribute to activation of transcription factors required for cytokine and chemokine production. In parallel, PI3K is activated after binding to Gab2 on phosphorylation of this cytosolic adapter molecule by Fyn, Syk, or both; phosphorylation of the p85 adapter subunit of PI3K; and activation of the catalytic subunit by small guanosine triphosphate (GTP)–binding proteins. In the case of KIT, the p85 subunit directly binds to the phosphorylated molecule. The subsequent formation of membrane-associated phosphatidylinositol 3,4,5-trisphosphate (PIP₃) results in the recruitment of pleckstrin homology (PH) domain–containing signaling molecules, such as Btk and PLD. PI3K-regulated pathways serve to enhance/maintain LAT/PLCγ₁-regulated degranulation. KIT- and FcϵRI-mediated activation of the Ras–Raf–mitogen-activated protein kinase (MAPK) pathway after Sos- and Vav-regulated guanosine diphosphate (GDP)–GTP exchange of Ras contributes to these processes. MAPK–extracellular signal-regulated kinase (ERK) 1/2 signaling also regulates phospholipase A₂ (PLA₂) activation, which leads to the generation of eicosanoids. LAT2 can both upregulate and downregulate antigen-mediated responses and appears to be required for KIT to enhance FcϵRI-dependent degranulation. In addition to its role in mast cell mediator release, KIT signaling regulates mast cell proliferation, differentiation, survival, migration, and adhesion. JAK, Janus kinase; STAT, signal transducer and activator of transcription. For other abbreviations, please refer to text. This figure is based on one published in the article by Gilfillan and Rivera.