Figure 7.

Non-canonical antagonism of phosphoinositide 3-kinase (PI3K) by Itpkb delays thymocyte β-selection and renders it Notch dependent. (A) T cells develop in the thymus from HSC and CLP-derived early thymocyte progenitors (ETPs) through several successive CD4⁻CD8⁻ “double-negative” stages (DN2-DN4) and a CD8⁺ immature single-positive (ISP) stage into CD4⁺CD8⁺ double-positive (DP) thymocytes (93, 94). DP cells then undergo positive and negative selection to mature into CD4⁺ or CD8⁺ T cells. At the DN3a stage, expression of a pre-T cell receptor (TCR) composed of an invariant pre-TCRα (pTα) chain and a fully rearranged TCRβ chain triggers metabolic activation, proliferation, survival, β chain allelic exclusion, the initiation of TCRα chain somatic gene rearrangements, acquisition of the α/β T cell fate, and developmental progression to the DP stage. DP thymocytes express a mature TCR composed of fully rearranged α and β chains. The DN3-to-DP transition requires pre-TCR and costimulatory Notch signals. This process is termed β-selection, because it allows only DN3 cells expressing a functional TCRβ chain to survive and mature. (B) Based on studies in Itpkb^−/− mice, we recently proposed a model in which pre-TCR and Notch signaling both activate PI3K to produce PIP₃ in DN3 cells. PIP₃ then recruits and activates Akt to increase glucose metabolism via the Akt/mammalian or mechanistic target of rapamycin (mTOR) pathway. This is required for β-selection. However, pre-TCR signaling also activates Itpkb to produce IP₄, which competes with PIP₃ for Akt pleckstrin homology domain binding and limits Akt recruitment and signaling in pre-TCR expressing DN3 cells. By limiting downstream glucose metabolism, this “IP₄ brake” delays the kinetics of β-selection and renders this process dependent on Notch costimulation (27). (C) Without Itpkb, IP₄ no more dampens Akt activation. In the presence of Notch signals, Akt is now hyperactivated and causes an accelerated DN3-to-DP cell differentiation (indicated by bold arrows). (D) In absence of Itpkb, pre-TCR signaling alone sufficiently activates Akt/mTOR to trigger DP cell development without Notch engagement (27).