The Orphan Nuclear Receptor Nr4a1 Specifies Long-Term, Myeloid-Biased HSCs and Responds To PGE2

The Nr4a family of transcription factors regulates the fate of cells from multiple tissues. The absence of Nr4a1 and Nr4a3 in the hematopoietic system results in acute myeloid leukemia, suggesting that this protein family is critically involved in the control of blood cell proliferation and development. In order to better understand the role played by Nr4a1 in particular, we have taken advantage of our Nr4a1(Nur77)^GFP reporter mouse and investigated Nr4a1 expression during hematopoiesis. We find that Nr4a1 is expressed within the hematopoietic stem cell (HSC) population and most highly expressed among quiescent, long term (LT)-HSC. The association of Nr4a1 expression with LT-HSCs was directly confirmed by engraftment experiments. Sorted Lin-Sca+c-Kit+ (LSK) Nr4a1^GFPhigh cell populations from CD45.2+ donors reconstituted the blood of CD45.1+ irradiated recipients for at least 20 weeks after transplant; an average of 20.3% of blood cells were CD45.2+. In contrast, Nr4a1^GFPlow populations of LSK cells only transiently reconstituted the blood; well below 1% of blood cells were CD45.2+. Furthermore, blood cells arising from Nr4a1^GFPhigh LSKs were dominated by myeloid cells: the lymphoid (L) to myeloid (M) ratio (ρ) was 13.7. The few Nr4a1^GFPlow cells that remained after 20 weeks were almost exclusively lymphoid (L/M ratio = 99.3). Examination of the bone marrow from engrafted mice at 20 weeks showed that HSC populations derived from Nr4a1^GFPhigh cells included a higher percentage of CD150+CD48- LT-HSCs than cells arising from whole bone marrow competitors. Given the association of Nr4a1 expression with long-term engraftment potential, and given that Nr4a1 has been shown to be highly responsive to environmental stimuli in a variety of tissues, we tested whether HSCs express Nr4a1 in response to signals generated by the bone marrow stroma. We evaluated the influence of a variety of factors, including GCSF, SCF, IL-6, TNFa, and PGE2. We found that PGE2 and, to a lesser extent, TNFa specifically upregulate Nr4a1 in HSCs. In summary, we show that Nr4a1 is expressed by a population of long-term, myeloid-biased HSCs, which closely resemble the potent long-term, myeloid biased HSC subset recently identified by Oguro et al. We further show that Nr4a1 expression is responsive to PGE2, which is known to enhance HSC engraftment. We hypothesize that Nr4a1 expression by HSCs reflects and integrates bone marrow environmental inductive cues critical for long-term stem cell homeostasis, including quiescence.