Rictor positively regulates B cell receptor signaling by modulating actin reorganization via ezrin

As the central hub of the metabolism machinery, the mammalian target of rapamycin complex 2 (mTORC2) has been well studied in lymphocytes. As an obligatory component of mTORC2, the role of Rictor in T cells is well established. However, the role of Rictor in B cells still remains elusive. Rictor is involved in B cell development, especially the peripheral development. However, the role of Rictor on B cell receptor (BCR) signaling as well as the underlying cellular and molecular mechanism is still unknown. This study used B cell–specfic Rictor knockout (KO) mice to investigate how Rictor regulates BCR signaling. We found that the key positive and negative BCR signaling molecules, phosphorylated Brutons tyrosine kinase (pBtk) and phosphorylated SH2-containing inositol phosphatase (pSHIP), are reduced and enhanced, respectively, in Rictor KO B cells. This suggests that Rictor positively regulates the early events of BCR signaling. We found that the cellular filamentous actin (F-actin) is drastically increased in Rictor KO B cells after BCR stimulation through dysregulating the dephosphorylation of ezrin. The high actin-ezrin intensity area restricts the lateral movement of BCRs upon stimulation, consequently reducing BCR clustering and BCR signaling. The reduction in the initiation of BCR signaling caused by actin alteration is associated with a decreased humoral immune response in Rictor KO mice. The inhibition of actin polymerization with latrunculin in Rictor KO B cells rescues the defects of BCR signaling and B cell differentiation. Overall, our study provides a new pathway linking cell metablism to BCR activation, in which Rictor regulates BCR signaling via actin reorganization.
Author summary As the central hub of cell metabolism, the mammalian target of rapamycin complex (mTORC) integrates immune signals and metabolic cues for the maintenance and activation of these systems. Rictor is the core component of the mammalian target of rapamycin complex 2 (mTORC2), and loss of this protein leads to an immunodeficiency that involves (among other things) impaired antibody production. B cell receptor (BCR) signaling is critical for antibody generation and although it has been shown that loss of Rictor in B cells negatively impacts this function, the underlying molecular mechanisms are unknown. Here, we show that both early and distal BCR signaling is reduced in Rictor knockout (KO) B cells. We find that the reduction in BCR signaling stems from defective clustering of BCRs during early B cell activation. This seems to be caused by the uncontrolled activation of the actin-connecting protein ezrin, which leads to a rigid actin fence that restricts the lateral movement of BCRs in the membrane. Interestingly, treatment of Rictor KO mice with an actin inhibitor rescues the BCR signaling. Our findings suggest that Rictor helps to allow effective BCR signaling in B cells by triggering reorganization of the actin network, thereby enabling an appropriate antibody response during infection.