Stim1, PKCδ and RasGRP proteins set a threshold for pro-apoptotic Erk signaling during B cell development (109.6)

Abstract

Abstract Antigen encounter induces proliferation in mature B cells but triggers apoptosis in developing B cells. Thus, developing B cells that strongly recognize auto-antigens undergo apoptosis, one mechanism that contributes to prevent autoimmunity. Developing B cells display increased Ca2+ entry in response to antigen as compared to mature B cells, and Ca2+ is required for antigen-induced apoptosis. However, the Ca2+-driven signals involved are poorly defined. Here we identify and characterize a previously unrecognized Ca2+-driven Erk activation pathway, which is pro-apoptotic and biochemically distinct from diacylglycerol (DAG)-induced Erk activation. This pathway requires PKCδ and RasGRP proteins and depends on Stim1 levels, which control the magnitude of Ca2+ entry. We identify a novel phosphorylation site on RasGRP1 as a putative target of PKCδ that is absolutely required for Ca2+-dependent Erk activation but is dispensable for DAG-mediated Erk activation. Developmental regulation of these proteins is associated with selective activation of the pathway in bone marrow B cells prone to negative selection. This checkpoint is impaired in PKCδ-/- mice, which develop B cell autoimmunity. Conversely, Stim1 overexpression confers a competitive disadvantage to developing B cells, an effect that depends on PKCδ. Our findings demonstrate that DAG and Ca2+ can mediate the activation of functionally distinct Erk pathways to determine whether B cells proliferate or die upon antigen encounter.