Essential Role Of Sin1-mTORC2 In B Cell Growth,Development,Metabolism And Function

mTOR is a highly conserved serine-threonine kinase expressed in all eukaryotes.mTOR is crucial for coordinating growth factors,amino acid and energy availability with cell growth,proliferation,metabolism and survival.mTOR associates with two functionally distinct multiprotein complexes called mTOR complex 1(mTORC1)and mTORC2.Nutrients,growth factors,hormones,and energy signals activate mTORC1 which in turn phosphorylates the translational regulators S6 K and 4E-BP1 leading to increased cellular protein synthesis and ribosome biogenesis.Meanwhile,mTORC2 regulates activity of the AGC kinases family phosphorylation,including Akt,SGK1 and PKC,to promote cell growth,metabolism and survival.Stress-activated protein kinase interacting protein 1(Sin1)is an essential component for mTORC2,while the role of Sin1 in B cells development,growth and metabolism is largely unknown.In this study,bone marrow(BM)cells from Sin1 deficient mice shown elevated percentage of precursor B(pro/pre B)cells,while significantly reduced in mature B cells.In peripheral,Sin1 deficient mice shown increased percentage of transitional 1(T1),while decreased percentage of T2/T3 and mature B cells,suggesting Sin1 deficient mice show B cell development defect at pro/pre and T1 B cell stages.Meanwhile,cell size of immature B to T1 and T2 B cells from Sin1 deficient mice were smaller than those of WT mice,and cell size dynamic change was positively correlated with Akt Ser473 phosphorylation.Sin1 deficient B cells shown less survival and proliferation under anti-IgM treatment,and the basal B cell metabolism and metabolic change after anti-IgM treatment were imparied in Sin1 KO mice compared to WT group.Further signaling studies shown spleen B cells treated with anti-IgM resulted in upregulated Akt Ser473 phosphorylation and mTORC1 activity,inactivated GSK3α/β by phosphorylation at Ser21/9,leading to increased expression of c-Myc,which is required in promoting cell growth,metabolism,proliferation and differentiation.In addition,Sin1-deficient mice produced less antigen-specific antibody after model antigen immunization.Sin1 deficient mice show more severe flu symptoms and produce significantly lower levels of protective HA-IgG compared to WT mice.Collectively,our study revealed that Sin1 is a critical regulator of B cell development,growth,differentiation and maintaining the homeostasis of humoral immunity against infection through multiple mechanisms,which provides a clue for treating B lymphocytes defects and dysfunction by targeting Sin1-mTORC2.