Characterization of B Cell Development and Activation in the Absence of Akt or Presenilin

The biochemical pathways critical to B cell development remain poorly defined. Here I characterize a critical role for two separate families of proteins, Akt and Presenilin in the development and activation of B cells. The absence of Akt1 and Akt2 leads to a block in marginal zone (MZ) and B1B cell development, as well as decreased cellularity of splenic follicular B cells. In addition, I find the combined loss of Akt1 and Akt2 causes altered B cell receptor repertoire and poor competitive ability when matched against wild-type B cells. Similar to deficiencies in the Akt pathway the combined loss of Presenilin1 and Presenilin2 results in defective MZ, B1B cell development, and altered BCR repertoire selection. Furthermore, I find that these defects are independent of the Notch pathway and that Presenilins are required for optimal responses to cross-linking of the BCR. Collectively, these findings identify and phenotypically characterize two novel pathways important to B cell development and function

WIP is a chaperone for Wiskott–Aldrich syndrome protein (WASP)

Wiskott–Aldrich syndrome protein (WASP) is in a complex with WASP-interacting protein (WIP). WASP levels, but not mRNA levels, were severely diminished in T cells from WIP(−/−) mice and were increased by introduction of WIP in these cells. The WASP binding domain of WIP was shown to protect WASP from degradation by calpain in vitro. Treatment with the proteasome inhibitors MG132 and bortezomib increased WASP levels in T cells from WIP(−/−) mice and in T and B lymphocytes from two WAS patients with missense mutations (R86H and T45M) that disrupt WIP binding. The calpain inhibitor calpeptin increased WASP levels in activated T and B cells from the WASP patients, but not in primary T cells from the patients or from WIP(−/−) mice. Despite its ability to increase WASP levels proteasome inhibition did not correct the impaired IL-2 gene expression and low F-actin content in T cells from the R86H WAS patient. These results demonstrate that WIP stabilizes WASP and suggest that it may also be important for its function

Akt1 and Akt2 promote peripheral B-cell maturation and survival

Although the 3 isoforms of Akt regulate cell growth, proliferation, and survival in a wide variety of cell types, their role in B-cell development is unknown. We assessed B-cell maturation in the bone marrow (BM) and periphery in chimeras established with fetal liver progenitors lacking Akt1 and/or Akt2. We found that the generation of marginal zone (MZ) and B1 B cells, 2 key sources of antibacterial antibodies, was highly dependent on the combined expression of Akt1 and Akt2. In contrast, Akt1/2 deficiency did not negatively affect the generation of transitional or mature follicular B cells in the periphery or their precursors in the BM. However, Akt1/2-deficient follicular B cells exhibited a profound survival defect when forced to compete against wild-type B cells in vivo. Altogether, these studies show that Akt signaling plays a key role in peripheral B-cell maturation and survival

AKT1 and AKT2 maintain hematopoietic stem cell function by regulating reactive oxygen species

Although AKT is essential for multiple cellular functions, the role of this kinase family in hematopoietic stem cells (HSCs) is unknown. Thus, we analyzed HSC function in mice deficient in the 2 isoforms most highly expressed in the hematopoietic compartment, AKT1 and AKT2. Although loss of either isoform had only a minimal effect on HSC function, AKT1/2 double-deficient HSCs competed poorly against wild-type cells in the development of myeloid and lymphoid cells in in vivo reconstitution assays. Serial transplantations revealed an essential role for AKT1 and AKT2 in the maintenance of long-term HSCs (LT-HSCs). AKT1/2 double-deficient LT-HSCs were found to persist in the G0 phase of the cell cycle, suggesting that the long-term functional defects are caused by increased quiescence. Furthermore, we found that the intracellular content of reactive oxygen species (ROS) is dependent on AKT because double-deficient HSCs demonstrate decreased ROS. The importance of maintaining ROS for HSC differentiation was shown by a rescue of the differentiation defect after pharmacologically increasing ROS levels in double-deficient HSCs. These data implicate AKT1 and AKT2 as critical regulators of LT-HSC function and suggest that defective ROS homeostasis may contribute to failed hematopoiesis