A Causal Role Of Dicer During Activation-Induced Cell Death

During B cells development in the bone marrow, autoreactive cells encountering receptor-self-antigen interactions are eliminated by an active form of apoptosis called activation-induced cell death (AICD). Recent studies in our lab have been focused on the role of microRNAs (miRNAs) in regulating AICD. A global approach to study miRNA-regulated mechanisms is to ablate or suppress expression of Dicer ribonuclease, which is a central enzyme in the processing of miRNAs, double strand RNA (dsRNA) and other small RNAs. To do so we used shRNA to knockdown Dicer expression in B cells and found that loss of Dicer suppresses biogenesis of miRNAs and enhances sensitivity to AICD imposed by anti-B cell receptor stimulation. Moreover, in a conditional mouse model enabling inducible ablation of Dicer (Dicer^flox/flox Mx-Cre) we show that resistance to AICD of mature splenic B cells is lost upon ablation of Dicer. Further, using wt and Dicer-deficient HEK cells we show that apoptosis induced by TNFa is enhanced in the absence of Dicer. These studies suggest that through controlling biogenesis of miRNAs Dicer plays a central regulator in determination of cell fate.

Yet, in recent years cleavage of Dicer was suggested to play an active direct role in the process of apoptosis. In agreement with this, we find that Dicer is specifically cleaved in B lymphocytes undergoing AICD and that this cleavage is correlated with elevation in active caspase 3. Our plans are to test whether cleaved Dicer functions as DNAs and to study whether cleavage-resistant form of Dicer will enhance resistance to AICD. We think this study may uncover novel mechanism for activation-induced apoptosis.