Project summary: The vast majority of mammalian genes produce alternatively processed mRNAs through alternative splicing and alternative polyadenylation (APA). Different mRNA isoforms produced from the same gene can encode distinct proteins and/or they may be differentially regulated. Recent studies have revealed essential roles of mRNA alternative processing in many biological processes and mis-regulation of alternative splicing and APA has been causally linked to a wide range of diseases, including cancer and neurodegenerative diseases. However, the mechanism and functions of alternative mRNA processing remain poorly understood.Antibody secretion by B cells is a major component of our immune response and mis-regulated antibody response underlies many auto-immune diseases. B cell activation and differentiation require a sophisticated gene regulation cascade. Previous works, including ours, have provided insights into the transcriptional regulation mechanisms governing this process. However, it is clear that post- transcriptional gene regulation, such as alternative splicing and APA, also play an important role. In 1980, several landmark studies reported the first example of alternative RNA processing: the Immunoglobulin M (IgM) heavy chain gene (IghM) produces two APA isoforms, which encode a membrane-bound and a secreted IgM respectively. Additionally the IghM APA is developmentally regulated. Subsequent studies, however, have failed to provide a consistent mechanistic model for this APA switch. Furthermore, it remains unknown how widespread the APA regulation network is and what the functional impact of APA regulation is during B cell activation and differentiation. In our preliminary studies, we provided evidence that transcription factors, core mRNA 3’ processing factors, and RNA-binding proteins regulate IghM APA. In addition, we discovered that B cell activation leads to a significantly change in the APA patterns of ~900 genes, including those encoding key cell fate regulators and signaling proteins. Based on these preliminary results, we hypothesize that the APA of IghM and a large gene network are regulated at multiple levels and that APA regulation plays an important role in B cell functions. To test these hypotheses, we have designed the following specific aims: 1) Identify regulators of B cell activation-induced IghM APA switch using a biochemical and genetic approach; 2) Systematically characterize the mechanisms of B cell activation-induced IghM APA switch; 3) Determine the role of APA regulation in B cell activation and differentiation. Successful completion of the proposed studies will provide fundamental insights into APA regulation and function. More importantly, our results will reveal the role of post-transcriptional gene regulation in B cell development and B cell- mediated immune response, which will pave the way for better strategies for developing vaccines and treatment for autoimmune diseases.