Megakaryocytes and erythroid cells are thought to derive from a common progenitor during hematopoietic differentiation. Although a number of transcriptional regulators are important for this process, they do not explain the bipotential result. We now show by gain-and loss-offunction studies that erythroid Krü ppellike factor (EKLF), a transcription factor whose role in erythroid gene regulation is well established, plays an unexpected directive role in the megakaryocyte lineage. EKLF inhibits the formation of megakaryocytes while at the same time stimulating erythroid differentiation. Quantitative examination of expression during hematopoiesis shows that, unlike genes whose presence is required for establishment of both lineages, EKLF is uniquely down-regulated in megakaryocytes after formation of the megakaryocyte-erythroid progenitor. Expression profiling and molecular analyses support these observations and suggest that megakaryocytic inhibition is achieved, at least in part, by EKLF repression of Fli-1 message levels.
IntroductionHematopoiesis is the process by which a self-renewing population of stem cells provide a continuous replenishment of differentiated blood cells by generating progeny with sequentially altered gene expression patterns. [1][2][3] Identification of these cells has relied on selective enrichment by cell-surface markers combined with culture and in vivo cellular assays that enable detection of cells at specific stages of differentiation. Although stem cells are multipotent, individual steps of subsequent differentiative decisions are performed by a series of simpler, even bipotential, decisions whereby one cell type gives rise to 2 or 3 descendants of differing character. 4 This has led to a commonly accepted pattern of parent and progeny relations, 2 although variations of it have recently been suggested 5 (but see
Forsberg et al 6 ).A large number of genetic, cellular, and gene expression studies point to the critical importance of cytokine pathways 7 and expression patterns of transcription factors 1,[8][9][10] for establishing and maintaining steady state numbers of lymphoid, myeloid, and erythroid cells that, at the same time, can respond quickly to changes in the organismal environment and increase or decrease the cellularity of specific blood cell types. The megakaryocyte and erythrocyte lineages are proposed to derive from a common precursor, the megakaryocyte-erythroid progenitor (MEP) 4,11,12 (reviewed in Pang et al 13 ). Strikingly, these 2 lineages share a number of commonalities with respect to transcription factors that are absolutely required (eg, GATA1,14,15 FOG1, 16 SCL,17, ). At the same time, the protein partners that form with these factors as differentiation proceeds can be significantly different between lineages. 20 However, because these factors are all positively required for both lineages, we are still left with an incomplete picture of how these lineages are differentially established during hematopoiesis. 13 Erythroid Krüppel-like factor (EKLF; KLF1 21 ) is ...