The importance of cofilin and Rap GTPases in Fcε Receptor I-mediated actin-dependent mast cell effector functions

Mast cells are central players in inflammatory responses, as they secrete a number of factors that regulate the responses of other immune cells and they participate in pathogen sensing and clearance. However, mast cells are best known for their role in allergic diseases. Mast cells express the cell surface receptor, Fcε Receptor I (FcεRI), which binds the constant region of IgE antibodies. Binding of multivalent antigens to FcεRI-bound IgE initiates intracellular signaling cascades that lead to effector functions such as degranulation and migration. During degranulation, storage granules containing pre-synthesized bioactive molecules such as histamine fuse with the plasma membrane and release their contents. Secretion of granule contents requires dynamic reorganization of the submembrane actin cytoskeleton. Actin dynamics are also required to generate protrusive forces at the leading edge of the cell during antigen-induced mast cell migration. Despite the importance of the actin cytoskeleton for mast cell-mediated immune responses, how actin dynamics are regulated during mast cell activation is poorly understood. Although the actin severing protein, cofilin, is activated by FcεRI signaling, the role of cofilin in actin-dependent mast cell functions has not been investigated. By using siRNA to deplete cofilin in a murine bone marrow-derived mast cell line, I found that cofilin is important for FcεRI-induced actin breakdown as well as mast cell degranulation, cell spreading, and migration. Potential upstream regulators of cofilin activity and actin remodeling are the Rap GTPases but their role in mast cells has not been investigated. The Rap GTPases are members of the Ras family of GTPases that are important for actin reorganization, integrin activation, cell polarity, and proliferation. I showed for the first time that the Rap1 and Rap2 GTPases are activated by FcεRI signaling. Furthermore, I found that Rap1 and Rap2 were important for FcεRI-induced actin breakdown and migration towards antigen but not for degranulation or cell spreading. Interestingly, Rap1 and Rap2 acted independently of cofilin to regulate actin turnover. This work has identified two novel regulatory pathways that facilitate actin-dependent FcεRI-induced mast cell effector functions and broadens our understanding of the signaling pathways that regulate mast cell activation in allergy and inflammation.Science, Faculty ofMicrobiology and Immunology, Department ofGraduat