RNAseq analysis of murine ITPKB deficient versus wild type LT-HSC

Tight regulation of hematopoietic stem cell (HSC) homeostasis is essential for life-long hematopoiesis, for preventing blood cancers and for averting bone marrow failure. The underlying mechanisms are incompletely understood. Here, we identify production of inositol-tetrakisphosphate (IP4) by inositoltrisphosphate 3-kinase B (ItpkB) as essential for HSC quiescence and function. Young ItpkB-/- mice accumulated phenotypic HSC and showed extramedullary hematopoiesis. ItpkB-/- HSC were less quiescent and proliferated more than wildtype controls. They downregulated quiescence and stemness associated mRNAs, but upregulated activation, oxidative metabolism, protein synthesis and lineage associated transcripts. Although they showed no significant homing defects, ItpkB-/- HSC had a severely reduced competitive long-term repopulating potential. Aging ItpkB-/- mice lost hematopoietic stem and progenitor cells and died with severe anemia. Wildtype HSC normally repopulated ItpkB-/- hosts, incidating a HSC-intrinsic ItpkB requirement. ItpkB-/- HSC had reduced cobblestone-area forming cell activity in vitro and showed increased stem-cell-factor activation of the phosphoinositide 3-kinase (PI3K) effector Akt, reversed by exogenous provision of the known PI3K/Akt antagonist IP4. They also showed transcriptome changes consistent with hyperactive Akt/mTOR signaling. Thus, we propose that ItpkB ensures HSC quiescence by limiting cytokine-induced PI3K signaling in HSC.