Relevant Mechanisms

Once the FC-epsilon-RI/IgE complex is activated by the binding of a specific antigen (in this case, peanut protein Ara h 2), a signaling cascade is initiated that culminates in the degranulation of the mast cell.

The process begins with the auto-phosphorylation of LYN, a tyrosine kinase in the SRC family (shown in pink in Figure 15, below), which is attached to the beta-subunit of the FC-epsilon-RI receptor (shown in purple), at tyrosine residue 396.

Figure 15 – Overview of how authophosphorylation of Lyn Src tyrosine-kinase leads to phosphorylation of ITAM and subsequent mast cell degranulation.

The LYN kinase proceeds to phosphorylate the proximal tyrosine residue on the immunoreceptor tyrosine-based activation motif (ITAM, shown in green in Figure 15, above) located on the beta-subunit of FC-epsilon-RI. The arrow-pushing mechanism by which the ITAM is phosphorylated is shown in Figure 16, below.

A generic base on the previously-phosphorylated LYN kinase removes a protein from the ITAM’s Tyr residue, leaving a deprotonated oxygen to attack the phosphate group at the phosphorus atom. The Y396 residue on the LYN kinase then receives the proton from the generic base. This mechanism is reminiscent of many other phosphorylation reactions throughout biochemistry.

Figure 16 – Arrow pushing mechanism for the phosphorylation of the immunoreceptor tyrosine-based activation motif ^{18, 19}.

After the phosphorylation of the ITAM, a series of similar phosphorylation reactions among enzymes and adapter molecules occurs. This signalling pathway is depicted in Figure 17. After its first phosphorylation of the beta-chain ITAM on Fc-epsilon-RI, LYN kinase proceeds to phosphorylate a second ITAM on the gamma-chain of the receptor. Both ITAMs serve as docking sites for two spleen tyrosine-kinase (SYK) molecules, which are also phosphorylated. The LYN and SYK kinases then phosphorylate the transmembrane adaptor molecule, LAT (linker for activation of T-cells). From here, LAT coordinates a collection of enzymes, resulting not only in degranulation but the production of various other promoters of the inflammatory immune response.

First, PLC-gamma-1 (the main signalling enzyme for mast cell degranulation, phospholipase C-gamma-1) interacts with the four terminal tyrosine residues on LAT and is activated.

Figure 17 – Overview of the signalling pathway initiated by the autophosphorylation of Lyn kinase bound to the FC-epsilon-RI receptor ²⁰

Activated PLC-gamma then catalyzes the hydrolysis of phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2, found in the plasma membrane), to inositol-1,4,5-triphosphate (InsP3) and diacylglycerol (DAG). The hydrolysis reaction and its general arrow-pushing mechansim are shown in Figure 18, below. InsP3 and DAG act as secondary messengers to increase cytosolic calcium and activate protein kinase C (PKC).

Figure 18 – Hydrolysis of phosphatidylinositol-4,5-bisphosphate into inositol-1,4,5-triphosphate and diacylglycerol ²¹

The induced calcium signal and the activation of PKC are what elicit mast cell degranulation. The implications of this degranulation are shown in Figure 19, below.

Figure 19 – Consequences of mast cell degranulation producing a body-wide allergic response