Alternative splicing of Bim and Erk-mediated BimEL phosphorylation are dispensable for hematopoietic homeostasis in vivo

Abstract

The pro-apoptotic BH3-only protein Bim has a major role in hematopoietic homeostasis, particularly in the lymphocyte compartment, where it strongly affects immune function. The three major Bim isoforms (Bim_EL, Bim_L and Bim_S) are generated by alternative splicing. Bim_EL, the most abundant isoform, contains a unique sequence that has been reported to be the target of phosphorylation by several MAP kinases. In particular, Erk1/2 has been shown to interact with Bim_EL through the DEF2 domain of Bim_EL and specifically phosphorylate this isoform, thereby targeting it for ubiquitination and proteasomal degradation. To examine the physiological importance of this mechanism of regulation and of the alternative splicing of Bim, we have generated several Bim knock-in mouse strains and analyzed their hematopoietic system. Although mutation in the DEF2 domain reduces Bim_EL degradation in some circumstances, this mutation did not significantly increase Bim's pro-apoptotic activity in vivo nor impact on the homeostasis of the hematopoietic system. We also show that Bim_EL and Bim_L are interchangeable, and that Bim_S is dispensable for the function of Bim. Hence, we conclude that physiological regulation of Bim relies on mechanisms independent of its alternative splicing or the Erk-dependent phosphorylation of Bim_EL.