Selective Activation of STAT5 Unveils Its Role in the Maintenance of Hematopoietic Stem Cells and the Development of Myeloproliferative Disorder.

Recent studies have implicated the janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway in the maintenance of stem cells, such as mouse embryonic stem cells and Drosophila germ cells. We have previously reported that thrombopoietin (TPO) can support in vitro self-renewal division of murine hematopoietic stem cells (HSCs) (CD34^−/lowc-Kit⁺Sca-1⁺lineage marker-negative; CD34⁻KSL cells). Signal transducers and activators of transcription 5 (STAT5) is one of the major signaling molecules that mediate TPO signals. All these findings suggest that STAT5 could be an attractive candidate for therapeutic manipulation of HSCs. Cytokines activate JAK/STAT5 pathway along with other signaling pathways, causing difficulty to dissect STAT5-specific functions in hematopoietic stem cells (HSCs). Here we took advantage of constitutively active STAT5 mutants to selectively activate STAT5 signaling pathway in HSCs. The mutants used are STAT5A 1*6 that harbors two amino acid mutations S710F and H298R in the effecter domain, and STAT5A #2 that harbors a point mutation N642H in the SH2 domain. Retroviral transduction of either STAT5 1*6 or STAT5#2 mutant into purified CD34⁻KSL HSCs caused a drastic expansion of multipotential progenitors in vitro and promoted multi-lineage differentiation in vitro. During 7 days of culture supplemented with SCF and TPO, the number of high proliferative potential colonies (HPPC) increased ten-fold compared with the GFP control and half of them were derived from multipotential progenitor cells. Notably, even in the culture supplemented with SCF only, expression of STAT5 mutants in HSCs supported a similar mode of expansion of progenitors cells and multi-lineage differentiation, indicating that activation of STAT5 can substitute major biological effects of TPO in HSCs. In all in vitro experiments, STAT5 1*6 showed stronger effects than STAT5#2. To evaluate the effect of STAT5A mutants in the maintenance of long-term bone marrow repopulating HSC ex vivo, cultured transduced cells corresponding to 30 initial CD34⁻KSL HSCs were transplanted into lethally irradiated mice 7 to 10 days after transduction. Although rapid hamatopoietic repopulation was observed with HSCs expressing STAT5A 1*6, mice developed myeloproliferative disease (MPD) and succumbed to death within two months. In contrast, HSCs expressing STAT5A #2 presented significantly higher long-term repopulating capacity than the GFP control. These data indicate that selective activation of STAT5 maintains long-term repopulating ability of HSCs ex vivo. Oncostatin M, a well known STAT5 target gene, has been postulated to be involved in the development of MPD and was actually induced STAT5A 1*6-expressing cells. However, transplantation of OSM−/− HSCs expressing STAT5A 1*6 similarly caused a lethal MPD in wild-type mice, indicating that Oncostatin is not the main target for STAT5 in MPD development. Taken together, our findings establish a role for STAT5 in the self-renewal of HSCs and provide STAT5 as novel target for therapeutic manipulation of HSCs ex vivo.