Mechanismen der Inostitol (1,4,5) trisphosphate 3-kinase B (ltpkB) Upstream-Reguation in Thymozyten
Zusammenfassung der Projektergebnisse
lnositol(1,4,5)trisphosphate (IP3) 3-kinase B (ItpkB) phosphorylates the Ca2+-release mediating soluble second messenger molecule IP3 into soluble inositol(1,3,4,5)tetrakisphosphate (IP4). In its best characterized in vivo function, IP4 acts as a soluble analog of the phosphatidylinostiol 3-kinase (P13K) lipid messenger product phosphatidylinositol(3,4,5)trisphosphate (PIP3), a membrane-recruiting ligand for PH-domain containing effectors such as Akt. IP4 can promote or competitively inhibit PIP3-mediated protein membrane recruitment. Dr. Sauer's group recently identified IP4 as an essential signaling mediator for the maturation of CD4+ CD8+ DP thymocytes. The goal of the present study was to investigate the ItpkB upstream regulation in DP thymocytes. My studies show that additionally to its role in developing thymocytes, ItpkB has a hitherto unknown essential role for correct lymphoid vs. myeloid lineage development and hematopoietic stem cell (HSC) homeostasis - a very provocative finding that profoundly expands the significance of ItpkB in hematopoiesis. These findings complicate interpretations of the DP cell phenotype, because any effects at this late stage could indirectly result from the perturbed earlier development. Therefore, the originally proposed studies of ItpkB upstream regulation in DP thymocytes were postponed until suitable conditional ItpkB knockout mice, that delete the ItpkB gene only in DP thymocytes, are available. Because of the innovative nature of my findings that ItpkB is a novel effector of HSC homeostasis, and because of the translational importance and applicability of these findings, the role of ItpkB in HSC homeostasis was investigated. Tight regulation of HSC homeostasis is essential to sustain hematopoiesis life-long, prevent blood cancers and avert BM failure syndromes. Here, I identify production of the soluble messenger IP4 by ItpkB as a novel mechanism that limits HSC cell cycle activity and function by restricting cytokineinduced phosphoinositide 3-kinase (PI3K) signaling. ItpkB-/- mice contained increased numbers of phenotypic long- and short-term reconstituting HSC with increased viability, proliferation and cell size. HSC mobilization and extramedullary hematopoiesis in ItpkB-/- mice associated with reduced CXCR4 chemokine-receptor expression, VCAM-1 integrin expression, tissue homing and long-term repopulating potential of ItpkB-/- HSC. In vitro, ItpkB-/- HSC showed increased stem-cell-factor (SCF) activation of the PI3K effector Akt reversed by treatment with exogenous IP4. Thus, IP4 acts at least in part by limiting PI3K signaling within HSC. Elevated numbers of phenotypic long- and short-tenn repopulating HSC upon conditional ItpkB deletion in hematopoietic cells but not HBC niche cells and reduced in vitro longevity ItpkB-/- HSC suggest that ItpkB maintains HSC homeostasis by HSC-intrinsic mechanisms. The presented data have important clinical implications: HSC transplantation is the standard approach for restoring hematopoiesis in immune disease or myeloablated cancer patients. However, new methods for the safe expansion of few graft-derived HSC are required to improve reconstitution efficacy. Cytostatic drug-resistant quiescent leukemic stem cells (LSC) are a main cause of disease relapse, and new LSC activating agents are urgently needed. ItpkB-loss expands HBC without causing cancer or myeloproliferative disease. Thus, my data identify ItpkB as potential target to safely mobilize or expand HBC, or to overcome LSC drug resistance by activating them and rendering them cytostatic drug-sensitive. Translational studies exploring transient ItpkB small-molecule inhibition for expanding human HSC therapeutically or for rendering human LSC cytostatic-sensitive are currently in progress. Additionally, I contributed to studies identifying an important role of ItpkB in developing NK cells. NK cells have gained clinical interest in treating certain cancers and infectious diseases, and the findings could lead to the development of drugs that improve NK cell functions. The NK cell study was reported to the public on the homepage of The Scripps Research Institute (http://wvw.scripps.edu/news/press/2012/20121127sauer.html).
Projektbezogene Publikationen (Auswahl)
- "Balancing pro-and anti-inflammatory TLR4 signaling', Nat. Immunol. 2012 Nov;13(11):1031-3
Siegemund, S., Sauer, K.
- „Inositol Tetrakisphosphate Limits NK Cell Effector Functions by Controlling Phosphoinositide 3-Kinase Signaling", Blood 2013 Jan 10;121(2):286-97
Sauer, K., Park, E., Siegemund, S., French, A.R., Wahle, J. A., Sternberg, L., Jonsson, A. H., Yokoyama, W. M. and Huang, Y. H.
