B cell antigen receptor signalling : regulation and targets of the P13K/AKT pathway

The B cell receptor (BCR) is a major regulator of B cell development, activation, and cell death. The misregulation of these processes results in autoimmunity and B cell lymphoma. Engagement of the BCR activates multiple signalling pathways that are essential for normal B cell responses. However, the roles of individual signalling pathways in mediating these responses is not completely understood. Activation of the phosphatidylinositol-3-kinase (PI3K) pathway is important for the proper regulation of B cell survival and development. In this thesis I have investigated the regulation and targets of the PI3K pathway in B cells. A putative target of the PI3K pathway is β-catenin, a transcriptional activator with important roles in early development. I found that the BCR regulates β-catenin levels via the activation of the phospholipase-C/protein kinase C/glycogen synthase kinase-3 pathway and is partially dependent on PI3K. Signalling by the BCR also activates the Rap GTPases, putative antagonists of Ras-mediated signalling. Ras can activate both the Raf-1/ERK 1/21/2 pathway and the PI3K/Akt pathway, pathways that can promote cell survival. I investigated whether Rap activation limits the activation of either of these pathways. I found that Rap activation had no effect on the BCR-induced activation of the Raf-1/ERK1/21/2 pathway. However, endogenous Rap limited the BCRinduced activation of the POK/Akt pathway, opposed the Akt-mediated inhibition of the FKHR/p27[sup Kip1] pro-apoptotic module, and enhanced cell death. Therefore, Rap can oppose the pro-survival role of the PI3K pathway. BCR-induced activation of the PI3K effector Akt leads to changes in gene transcription through the Akt-mediated activation or inhibition of transcription factors which can promote cell survival. I used cDNA microarray technology to identify novel gene targets of Akt in B cells. I found that Akt activation resulted in changes in expression of genes involved in the regulation of cell cycle progression, cell adhesion and apoptosis. In summary, I have identified novel targets and novel mechanisms of regulating the PI3K/Akt pathway in B cells. This work will hopefully contribute to the overall understanding of how the PI3K pathway affects the regulation of B cell development, activation, and survival.Science, Faculty ofMicrobiology and Immunology, Department ofGraduat