Characterizing the role of CARMA1 in lymphocyte and mast cell signaling
Pappu, Bhanu Prakash
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The transcription factor NF-κB plays an important role in the development, activation and survival of lymphocytes. The precise mechanism by which T cell receptor (TCR)-induced signaling activates the NF-κB pathway is still unclear. CARMA1 (CARD domain and MAGUK-domain containing protein-1) has been shown to play a critical role in antigen receptor-induced NF-κB activation in T and B-lymphocytes. However, the requirement for CARMA1 in T cell activation and proliferation remains unclear. In this study, we show that naïve T cell activation and proliferation are independent of CARMA1. In contrast, cytokine production and differentiation of T cells require CARMA1-mediated activation of NF-κB pathway. Based on our results, we propose a revised model for T cell activation. According to this model, signals that induce T cell proliferation are mediated through PKC-[straight theta] but independent of CARMA1. Additionally, the PKC-[straight theta]-mediated CARMA1---NF-κB pathway plays a specific role in the cytokine production and differentiation of activated T cells. Stimulation through the CD40 receptor on B cells promotes proliferation, somatic hypermutation, class-switch recombination and survival. CARMA1 KO B cells are defective in CD40-induced proliferation but the molecular mechanisms responsible for this defect are yet to be identified. In this study we find that NF-κB and MAPKinase signaling cascades induced by CD40 stimulation in CARMA1 KO B cells remain intact and may not contribute to defective proliferation. However, our data indicate that partially defective cell cycle progression in these B cells may result in impaired proliferation. Further analysis indicated that MZ B cells in the spleen are the primary responders to CD40-induced proliferation and that CARMA1 KO mice have greatly reduced numbers of MZ B cells. Therefore, we conclude that the CD40-induced proliferation defect in CARMA1 KO mice is primarily due to reduced numbers of MZ B population. Although MZ B cell numbers are greatly diminished in CARMA1 KO mice, CD40-induced proliferation in these cells is similar to WT MZ B cells. The observation further strengthens the argument that defective proliferation of CARMA1 KO B cells is a direct consequence of reduced numbers of MZ B cells. Therefore, we propose that BCR-induced NF-κB pathway is crucial for the development of MZ B cells in the spleen. Activated mast cells play a major role in allergic and inflammatory diseases by secreting histamines, prostaglandins and other proinflammatory cytokines. Cross-linking of Fc[varepsilon]RI by a multivalent antigen or allergen leads to mast cell activation and the NF-κB pathway plays a critical role in this activation process. PKC-β has been shown to mediate Fc[varepsilon]RI-induced NF-κB activation, but the downstream signaling molecules that link PKC-β to NF-κB activation are not yet characterized. As described in chapter 5, we find that CARMA1 and Bcl10-deficient mast cells are defective in Fc[varepsilon]RI-induced NF-κB activation. Our data suggest that CARMA1 and Bcl10 form the molecular bridge that connects PKC-β to NF-κB activation in mast cells. The results from our studies provide novel insights into the role of NF-κB activation pathway in lymphocyte and mast cell function. This knowledge will be helpful in designing novel strategies to modulate the activation of lymphocytes and mast cells to treat allergic and autoimmune diseases and certain lymphoid malignancies.