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Understanding the transcriptional mechanisms that control hematopoiesis and the interaction between hematopoietic stem cells and the bone marrow (BM) microenvironment in vivo is of considerable interest. The calcineurin-dependent transcription factor NFAT (Nuclear Factor of Activated T cells) is known as master regulator of cytokine production in T lymphocytes and therefore central for T cell-dependent immune reactions, but has also been shown to regulate a process of differentiation and tissue adaptation in various cell types. The activation of NFAT is dependent on the calcium level within the cell. In resting cells, calcium levels are low and NFAT is cytoplasmic and inactive. A sustained increase in the internal calcium concentration within an external stimuli leads to activation of the calcium-dependent calcineurin, followed by dephosphorylation and nuclear translocation of NFAT. We have previously shown that NFATc2, a member of the NFAT family, is expressed in CD34+ hematopoietic stem cells (HSC). A mouse model harboring NFATc2 deficiency provides the opportunity for in vivo investigation of the role of NFATc2 in hematopoiesis. Our recent observations showed that aged mice lacking the transcription factor NFATc2 develop peripheral blood anemia and thrombocytopenia, BM hypoplasia and extramedullary hematopoiesis in spleen and liver. The proliferation and differentiation of NFATc2-deficient hematopoietic stem cells ex vivo, however, was found to be intact. It remained therefore unclear whether the disturbed hematopoiesis in NFATc2-deficient mice was caused by the hematopoietic or the stroma component of the BM hematopoietic niche. In the current study we dissected the relative contribution of hematopoietic and stroma cells to the phenotype of the NFATc2-deficent mice by transplanting immuno-magnetically purified NFATc2-deficient (KO) HSCs to lethally irradiated wild type (WT) mice, and vice versa. After a post-transplantation period of 6-8 months, peripheral blood, BM as well as spleen and liver of the transplanted animals were analyzed and compared to WT and KO mice transplanted with control cells. Transplantation of NFATc2-deficient HSCs into WT recipients (KO WT) induced similar hematological abnormalities as those occurring in non-transplanted KO mice or in KO mice transplanted with KO cells (KO KO). Compared to WT mice transplanted with WT cells (WT WT), KO WT mice showed evidence of anemia, thrombocytopenia and a significantly reduced number of hematopoietic cells in their BM. Likewise, KO WT mice developed clear signs of extramedullary hematopoiesis in spleen and liver, which was not the case in WT WT control animals. In addition to the hematopoietic abnormalities, transplantation of NFATc2-deficient HSC also induced osteogenic abnormalities such as BM sclerosis and fibrosis in WT mice. This phenomenon was rather subtle and of incomplete penetrance, but never seen in mice transplanted with WT cells. These data demonstrate for the first time, that the NFATc2 transcription factor directly regulates the intrinsic function of hematopoietic stem cells in vivo. However, the transcriptional targets for NFAT in these cells are yet unknown. In addition to hematopoietic stem cells, NFATc2 has been shown to be expressed in a lineage-specific manner during myeloid differentiation and, notably, is maintained during megakaryopoiesis while it is suppressed during the differentiation of neutrophils. Bone marrow megakaryocytes are the precursors of peripheral blood platelets and therefore constitute an integral part of primary hemostasis, thrombosis and wound healing. The biological role of NFAT in megakaryocytes is unknown. We have recently shown that NFATc2 is not necessary for megakaryocytic differentiation. On the other hand, recent evidence suggests that NFATc2 is required for the transcription of specific megakaryocytic genes. In this study, we showed that activation of the calcineurin/NFAT pathway in either primary megakaryocytes or CMK megakaryocytic cells forces the cells to go into apoptosis. Cell death in megakaryocytes is induced by treating the cells with the calcium ionophore ionomycin and suppressed by either the pan-caspase inhibitor zVAD or the calcineurin inhibitor cyclosporin A (CsA). Ionomycin stimulation of megakaryocytes leads to the expression of Fas Ligand (FASLG), a pro-apoptotic member of the tumor necrosis factor superfamily. Expression of FASLG was detectable as early as four hours after stimulation on the membrane of ionomycin-treated megakaryocytes, was augmented in cells stably overexpressing NFATc2, and was suppressed in cells either pretreated with CsA or expressing the specific peptide inhibitor of NFAT, VIVIT. To investigate the physiological relevance of FASLG expression on megakaryocytes, we performed co-cultures of megakaryocytes with Fas-expressing T-lymphocytes, in which CMK cells were left either unstimulated or pre-stimulated with ionomycin and then added to Jurkat cells. The presence of ionomycin-stimulated CMK cells, but not of unstimulated cells or cells stimulated in the presence of CsA, significantly induced apoptosis in Jurkat cells. Overexpression of NFATc2 in CMK cells enhanced their potency to induce apoptosis in Jurkat cells, while cells expressing VIVIT were less effective. Apoptosis induction of Jurkat cells by stimulated CMK cells was partially blocked by the presence of either a neutralizing antibody against FASLG or an antagonistic antibody to Fas during the co-culture period, indicating involvement of the FASLG/Fas apoptosis pathway. These results represent the first clear evidence for a biological function of the calcineurin/NFAT pathway in megakaryocytes, namely the regulation of Fas/FASLG-dependent apoptosis. Second, they underline that the biological role of megakaryocytes is not restricted to the production of proteins and other cellular structures for platelet assembly, but that this population of cells fulfills an independent regulatory function in the context of the surrounding tissue. Finally, we have identified by RNA sequencing analysis of NFATc2-expressing and -deficient cells, the entire set of genes which is induced by NFATc2 in stimulated megakaryocytes. Functional pathway analysis suggests an involvement of NFATc2 in pro-inflammatory pathways in these cells. The significance of these findings has to be addressed in further studies.