The role of the scaffold protein RACK1 in mast cell activation

Abstract

Mast cells are immunoregulatory cells that are implicated in innate and adaptive immunity. Mast cells participate in different biological events such as host protection, angiogenesis, inflammatory and allergic reactions. The action of mast cells is related directly to their activation and subsequent mediator release. The best characterized mast cell activation pathway is that via the high-affinity receptor for immunoglobulin E (FcµRI). Binding of a multivalent antigen to immunoglobulin E (IgE) that is already bound to FcµRI results in the crosslinking of FcµRI and the initiation of intracellular signaling events. Although there have been many previous studies related to the molecular mechanisms of mast cell activation, the function of some regulatory proteins involved in mast cell signal transduction remain unknown. RACK1 (Receptor for Activated C Kinase 1) is a highly conserved intracellular adaptor protein that interacts with signaling proteins (kinases and phosphatases), transcriptional factors and the cytoplasmic domains of transmembrane proteins. As a scaffold protein, RACK1 integrates inputs from distinct signaling pathways and actively participates in signal transduction. Earlier studies have shown that RACK1 participates in the activation of immune cells and regulates the inflammatory response. Protein-protein interaction networks associated with asthma identified the gene GNB2L1, which encodes RACK1 protein, as a target gene related to cellular signaling in asthma. Acute asthma involves atopic and anaphylactic reactions triggered by mast cells. RACK1 is required as a scaffold protein for several signaling proteins that are engaged during mast cell activation. Recent data obtained in our laboratory identified, for the first time, the presence of RACK1 in RBL-2H3 mast cells. RACK1 is located in lipid rafts and is up regulated when GD1b-derived gangliosides are removed from these microdomains. Preliminary results showed that after mast cell stimulation via FcµRI, the majority of RACK1 was translocated from the cytoplasm to the nucleus. However, some RACK1 remained adjacent to the plasma membrane. These findings suggest that RACK1 may be involved in the organization and function of signaling complexes in lipid rafts, as well as, in signal transduction pathways that regulates the release of inflammatory mediators in mast cells. The present study aims to characterize the functional role of RACK1 in mast cell activation and migration, which is required during allergy and inflammation. The role of RACK1 in mast cells will be investigated using interference RNAs in RBL-2H3 mast cells. The role of RACK1 in anaphylaxis will also be investigated using mice injected with iRNA. An understanding of the role of RACK1 in mast cell activation may lead to new therapeutic targets for allergic and inflammatory process.