News and Publications

Cytoskeletal Regulation of IgE-Mediated Degranulation in Mast Cells

J.R. Apgar, L.G. Frigeri, T. Kawakami*

* La Jolla Institute of Allergy and Immunology, La Jolla, CA

Mast cells are a central component in many allergic and inflammatory conditions such as asthma, urticaria, atopic dermatitis, and allergic rhinitis. These cells express a high-affinity IgE receptor that binds to IgE in a 1:1 ratio. Under these conditions, the cells are sensitized, but they are inactive. Cross-linking of the receptor by multivalent antigen activates signaling pathways, leading to the release of a variety of inflammatory mediators such as histamine, eicosanoids, and cytokines. The major goal of our research is to determine how actin microfilaments are involved in the regulation of receptor-mediated signaling leading to degranulation. This relationship is complex because microfilaments can be involved in upregulation as well as downregulation of the response.

We use rat basophilic leukemia cells, which are a major model for studying the activation of mast cells through the high-affinity IgE receptor. The rat cells adhere and spread on extracellular matrix proteins such as fibronectin. These processes depend on the polymerization and reorganization of the microfilaments. Antigen-activated cells, which are adherent and spread, degranulate to a greater extent than do cells that are in suspension. Studies have indicated that spreading of the cells, not simply adhesion, is important in the increased activation of the cells. Furthermore, all of the signaling pathways are also upregulated. Currently, the tyrosine kinases, Lyn, Syk, and Btk, are being investigated to determine whether they are also upregulated. Of particular interest is Btk, which contains a pleckstrin homology domain that allows the kinase to bind to actin filaments.

Adhesion and spreading of the cells are an example of how microfilaments may be involved in the upregulation of the signaling pathways. However, activation of the cells with antigen also leads to the polymerization of actin, and these newly formed microfilaments are thought to be involved in downregulation. Inhibi-tion of antigen-induced actin polymerization leads to increased degranulation as well as increased signaling. Evidence indicates that tyrosine phosphorylation of the receptor by Lyn, which is the first step in the signaling cascade, is increased. This finding suggests that these newly formed microfilaments may be involved in downregulating the responses by uncoupling the cross-linked receptors from Lyn. Thus, in a negative feedback loop, these microfilaments downregulate the same pathways that lead to actin polymerization.

PUBLICATIONS

Frigeri, L.G., Apgar, J.R. The role of actin microfilaments in the downregulation of the degranulation response in RBL-2H3 mast cells. J. Immunol. 162:2243, 1999.

Yao, L., Janmey, P., Frigeri, L.G., Han, W., Fujita, J., Kawakami, Y., Apgar, J.R., Kawakami, T. Pleckstrin homology domains interact with filamentous actin. J. Biol. Chem. 274:19752, 1999.