B cell antigen receptor (BCR) signaling is not only critical for antibody-mediated immunity to pathogens but also maintains tolerance to self. Hyperactive BCR signaling is a hallmark of autoimmune diseases such as systemic lupus erythematosus (SLE), but the molecular mechanisms controlling B cell hyper responsiveness to antigen are poorly understood. Therefore, identification of proteins that drive B cell hyperactivation and autoantibody production is crucial for development of effective therapeutic strategies in SLE. Lyn-/- mice display B cell hyperactivation as well as pathogenic symptoms associated with human SLE. Human GWAS studies have shown a genetic association between the Src family kinase Lyn and SLE, making Lyn-/- mice a clinically relevant model for this disease. We have previously reported that the membrane-cytoskeleton linker protein ezrin regulates membrane dynamics and BCR signaling in antigen-stimulated B cells through its ability to undergo dynamic changes in phosphorylation. Interestingly, conditional deletion of ezrin in B cells of Lyn-/- mice leads to decreased B cell activation and differentiation, and reduced autoantibody production. These data suggest that ezrin-mediated regulation is a critical mechanism governing the hyper activation of B cells in the absence of Lyn. Ezrin is hyperphosphorylated in Lyn-/- B cells, and dephosphorylation of ezrin leads to reduced BCR microclustering, calcium signaling and cytokine secretion. Our data suggest a positive feedback model whereby stronger calcium signaling in Lyn-/- B cells leads to increased calcium-dependent protein kinase C activation, which results in hyperphosphorylation of ezrin, which in turn increases BCR microclustering and signaling. Collectively, our data reveal a novel collaborative interface between ezrin and Lyn that regulates the strength of BCR signaling in SLE, and may be a potential target for therapeutic intervention. We hypothesize that ezrin regulates B cell hyper activation and autoimmunity in Lyn-/- mice via its ability to regulate BCR spatial organization and signaling.
Our specific aims are, (1) To determine the impact of ezrin deletion on development of B cell autoimmunity in Lyn-/- mice, (2) To investigate the effect of ezrin deletion on BCR organization and signaling in Lyn-/- B cells, and (3) To elucidate the mechanism by which hyperphosphorylated ezrin mediates hyperactivity of Lyn-/- B cells. We will employ genetic, biochemical, molecular biological, immunological and cutting-edge high resolution imaging techniques to accomplish our aims. Our results will provide insights into spatial regulation of BCR response to antigen in lupus B cells, and elucidate the mechanism by which ezrin mediates the hyperactivity of Lyn-/- B cells. Ultimately, our findings will lead to the development of novel strategies for treatment of lupus and other B cell disorders that exhibit aberrant BCR signaling.
B cells respond to environmental pathogens such as bacteria and viruses by secreting specific antibodies that clear infections. This also provides the basis fo memory that is generated during early childhood vaccinations. Malfunctioning B cells are associated with autoimmune diseases such as systemic lupus erythematosus. We hypothesize that ezrin is a protein that regulates both protective and pathogenic functions of B cells via its ability to dynamically link the cell membrane to the cellular skeleton. In this application, we propose to test the role of ezrin in B cell activation in the context of autoimmunity. Mechanistic insights from understanding the role of ezrin in these processes will lead to the design of novel strategies for treatment of lupus.
