Multipotential differentiation ability of GATA-1-null erythroid-committed cells

Generation of Red Blood Cells from Human Pluripotent Stem Cells-An Update

Red blood cell (RBC) transfusion is a lifesaving medical procedure that can treat patients with anemia and hemoglobin disorders. However, the shortage of blood supply and risks of transfusion-transmitted infection and immune incompatibility present a challenge for transfusion. The in vitro generation of RBCs or erythrocytes holds great promise for transfusion medicine and novel cell-based therapies. While hematopoietic stem cells and progenitors derived from peripheral blood, cord blood, and bone marrow can give rise to erythrocytes, the use of human pluripotent stem cells (hPSCs) has also provided an important opportunity to obtain erythrocytes. These hPSCs include both human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs). As hESCs carry ethical and political controversies, hiPSCs can be a more universal source for RBC generation. In this review, we first discuss the key concepts and mechanisms of erythropoiesis. Thereafter, we summarize different methodologies to differentiate hPSCs into erythrocytes with an emphasis on the key features of human definitive erythroid lineage cells. Finally, we address the current limitations and future directions of clinical applications using hiPSC-derived erythrocytes.

Data-driven modeling predicts gene regulatory network dynamics during the differentiation of multipotential hematopoietic progenitors

Cellular differentiation during hematopoiesis is guided by gene regulatory networks (GRNs) comprising transcription factors (TFs) and the effectors of cytokine signaling. Based largely on analyses conducted at steady state, these GRNs are thought to be organized as a hierarchy of bistable switches, with antagonism between Gata1 and PU.1 driving red- and white-blood cell differentiation. Here, we utilize transient gene expression patterns to infer the genetic architecture—the type and strength of regulatory interconnections—and dynamics of a twelve-gene GRN including key TFs and cytokine receptors. We trained gene circuits, dynamical models that learn genetic architecture, on high temporal-resolution gene-expression data from the differentiation of an inducible cell line into erythrocytes and neutrophils. The model is able to predict the consequences of gene knockout, knockdown, and overexpression experiments and the inferred interconnections are largely consistent with prior empirical evidence. The inferred genetic architecture is densely interconnected rather than hierarchical, featuring extensive cross-antagonism between genes from alternative lineages and positive feedback from cytokine receptors. The analysis of the dynamics of gene regulation in the model reveals that PU.1 is one of the last genes to be upregulated in neutrophil conditions and that the upregulation of PU.1 and other neutrophil genes is driven by Cebpa and Gfi1 instead. This model inference is confirmed in an independent single-cell RNA-Seq dataset from mouse bone marrow in which Cebpa and Gfi1 expression precedes the neutrophil-specific upregulation of PU.1 during differentiation. These results demonstrate that full PU.1 upregulation during neutrophil development involves regulatory influences extrinsic to the Gata1-PU.1 bistable switch. Furthermore, although there is extensive cross-antagonism between erythroid and neutrophil genes, it does not have a hierarchical structure. More generally, we show that the combination of high-resolution time series data and data-driven dynamical modeling can uncover the dynamics and causality of developmental events that might otherwise be obscured.

The supply of blood cells is replenished by the maturation of hematopoietic progenitor cells into different cell types. Which cell type a progenitor cell develops into is determined by a complex network of genes whose protein products directly or indirectly regulate each others’ expression and that of downstream genes characteristic of the cell type. We inferred the nature and causality of the regulatory connections in a 12-gene network known to affect the decision between erythrocyte and neutrophil cell fates using a predictive machine-learning approach. Our analysis showed that the overall architecture of the network is densely interconnected and not hierarchical. Furthermore, the model inferred that PU.1, considered a master regulator of all white-blood cell lineages, is upregulated during neutrophil development by two other proteins, Cebpa and Gfi1. We validated this prediction by showing that Cebpa and Gfi1 expression precedes that of PU.1 in single-cell gene expression data from mouse bone marrow. These results revise the architecture of the gene network and the causality of regulatory events guiding hematopoiesis. The results also show that combining machine learning approaches with time course data can help resolve causality during development.

Steamed Panax notoginseng Attenuates Anemia in Mice With Blood Deficiency Syndrome via Regulating Hematopoietic Factors and JAK-STAT Pathway

Panax notoginseng (Burk.) F. H. Chen is a medicinal herb used to treat blood disorders since ancient times, of which the steamed form exhibits the anti-anemia effect and acts with a “blood-tonifying” function according to traditional use. The present study aimed to investigate the anti-anemia effect and underlying mechanism of steamed P. notoginseng (SPN) on mice with blood deficiency syndrome induced by chemotherapy. Blood deficiency syndrome was induced in mice by cyclophosphamide and acetylphenylhydrazine. A number of peripheral blood cells and organs (liver, kidney, and spleen) coefficients were measured. The mRNA expression of hematopoietic function-related cytokines in the bone marrow of mice was detected by RT-qPCR. The janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathway was screened based on our previous analysis by network pharmacology. The expression of related proteins and cell cycle factors predicted in the pathway was determined by Western blot and RT-qPCR. SPN could significantly increase the numbers of peripheral blood cells and reverse the enlargement of spleen in a dose-dependent manner. The quantities of related hematopoietic factors in bone marrow were also increased significantly after SPN administration. SPN was involved in the cell cycle reaction and activation of immune cells through the JAK-STAT pathway, which could promote the hematopoiesis.

Modeling dynamic functional relationship networks and application to ex vivo human erythroid differentiation

MOTIVATION

Functional relationship networks, which summarize the probability of co-functionality between any two genes in the genome, could complement the reductionist focus of modern biology for understanding diverse biological processes in an organism. One major limitation of the current networks is that they are static, while one might expect functional relationships to consistently reprogram during the differentiation of a cell lineage. To address this potential limitation, we developed a novel algorithm that leverages both differentiation stage-specific expression data and large-scale heterogeneous functional genomic data to model such dynamic changes. We then applied this algorithm to the time-course RNA-Seq data we collected for ex vivo human erythroid cell differentiation.

RESULTS

Through computational cross-validation and literature validation, we show that the resulting networks correctly predict the (de)-activated functional connections between genes during erythropoiesis. We identified known critical genes, such as HBD and GATA1, and functional connections during erythropoiesis using these dynamic networks, while the traditional static network was not able to provide such information. Furthermore, by comparing the static and the dynamic networks, we identified novel genes (such as OSBP2 and PDZK1IP1) that are potential drivers of erythroid cell differentiation. This novel method of modeling dynamic networks is applicable to other differentiation processes where time-course genome-scale expression data are available, and should assist in generating greater understanding of the functional dynamics at play across the genome during development.

AVAILABILITY AND IMPLEMENTATION

The network described in this article is available at http://guanlab.ccmb.med.umich.edu/stageSpecificNetwork.

Transcription factor GATA1 is dispensable for mast cell differentiation in adult mice

Although previous studies have shown that GATA1 is required for mast cell differentiation, the effects of the complete ablation of GATA1 in mast cells have not been examined. Using conditional Gata1 knockout mice (Gata1(-/y)), we demonstrate here that the complete ablation of GATA1 has a minimal effect on the number and distribution of peripheral tissue mast cells in adult mice. The Gata1(-/y) bone marrow cells were capable of differentiating into mast cells ex vivo. Microarray analyses showed that the repression of GATA1 in bone marrow mast cells (BMMCs) has a small impact on the mast cell-specific gene expression in most cases. Interestingly, however, the expression levels of mast cell tryptases in the mouse chromosome 17A3.3 were uniformly reduced in the GATA1 knockdown cells, and GATA1 was found to bind to a 500-bp region at the 5' end of this locus. Revealing a sharp contrast to that observed in the Gata1-null BMMCs, GATA2 deficiency resulted in a significant loss of the c-Kit(+) FcεRIα(+) mast cell fraction and a reduced expression of several mast cell-specific genes. Collectively, GATA2 plays a more important role than GATA1 in the regulation of most mast cell-specific genes, while GATA1 might play specific roles in mast cell functions.

GATA factor switching from GATA2 to GATA1 contributes to erythroid differentiation

Transcription factor GATA2 is highly expressed in hematopoietic stem cells and progenitors, whereas its expression declines after erythroid commitment of progenitors. In contrast, the start of GATA1 expression coincides with the erythroid commitment and increases along with the erythroid differentiation. We refer this dynamic transition of GATA factor expression to as the 'GATA factor switching'. Here, we examined contribution of the GATA factor switching to the erythroid differentiation. In Gata1-knockdown embryos that concomitantly express Gata2-GFP reporter, high-level expression of GFP reporter was detected in accumulated immature hematopoietic cells with impaired differentiation, demonstrating that GATA1 represses Gata2 gene expression in hematopoietic progenitors in vivo. We have conducted chromatin immunoprecipitation (ChIP) on microarray analyses of GATA2 and GATA1, and results indicate that the GATA1-binding sites widely overlap with the sites pre-occupied by GATA2 before the GATA1 expression. Importantly, erythroid genes harboring GATA boxes bound by both GATA1 and GATA2 tend to be expressed in immature erythroid cells, whereas those harboring GATA boxes to which GATA1 binds highly but GATA2 binds only weakly are important for the mature erythroid cell function. Our results thus support the contention that preceding binding of GATA2 helps the following binding of GATA1 and thereby secures smooth expression of the transient-phase genes.

Establishment of erythroleukemic GAK14 cells and characterization of GATA1 N-terminal domain

GATA1 is a transcription factor essential for erythropoiesis and megakaryopoiesis. It has been found that Gata1 gene knockdown heterozygous female (Gata1(G1.05/+)) mice spontaneously develop erythroblastic leukemias. In this study, we have generated a novel Gata1 knockdown erythroblastic cell line, designated GAK14, from the leukemia cells in the Gata1(G1.05/+) mice. Although GAK14 cells maintain immature phenotype on OP9 stromal cells in the presence of erythropoietin and stem cell factor, the cells produce Gr-1-, Mac1-, B220-, CD3e- or CD49b-positive hematopoietic cells when co-cultured with DAS104-8 feeder cells. However, GAK14 cells did not produce erythroid and megakaryocytic lineages, perhaps due to the absence of GATA1. Indeed, GAK14 cells became capable of differentiating into mature erythroid cells when complemented with full-length GATA1 and co-cultured with fetal liver-derived FLS5 stromal cells. This differentiation potential was impaired when GATA1 lacking the N-terminal domain was complemented. The N-terminal domain is known to contribute to the pathogenesis of transient abnormal myelopoiesis and acute megakaryoblastic leukemia related to Down syndrome. These results thus showed that GAK14 cells will serve as a powerful tool for dissecting domain function of GATA1 and that the GATA1 N-terminal domain is essential for the erythroid differentiation of GAK14 cells.

Combinatorial motif analysis of regulatory gene expression in Mafb deficient macrophages

Background

Deficiency of the transcription factor MafB, which is normally expressed in macrophages, can underlie cellular dysfunction associated with a range of autoimmune diseases and arteriosclerosis. MafB has important roles in cell differentiation and regulation of target gene expression; however, the mechanisms of this regulation and the identities of other transcription factors with which MafB interacts remain uncertain. Bioinformatics methods provide a valuable approach for elucidating the nature of these interactions with transcriptional regulatory elements from a large number of DNA sequences. In particular, identification of patterns of co-occurrence of regulatory cis-elements (motifs) offers a robust approach.

Results

Here, the directional relationships among several functional motifs were evaluated using the Log-linear Graphical Model (LGM) after extraction and search for evolutionarily conserved motifs. This analysis highlighted GATA-1 motifs and 5’AT-rich half Maf recognition elements (MAREs) in promoter regions of 18 genes that were down-regulated in Mafb deficient macrophages. GATA-1 motifs and MafB motifs could regulate expression of these genes in both a negative and positive manner, respectively. The validity of this conclusion was tested with data from a luciferase assay that used a C1qa promoter construct carrying both the GATA-1 motifs and MAREs. GATA-1 was found to inhibit the activity of the C1qa promoter with the GATA-1 motifs and MafB motifs.

Conclusions

These observations suggest that both the GATA-1 motifs and MafB motifs are important for lineage specific expression of C1qa. In addition, these findings show that analysis of combinations of evolutionarily conserved motifs can be successfully used to identify patterns of gene regulation.

Concise Review: Alchemy of Biology: Generating Desired Cell Types from Abundant and Accessible Cells

A major goal of regenerative medicine is to produce cells to participate in the generation, maintenance, and repair of tissues that are damaged by disease, aging, or trauma, such that function is restored. The establishment of induced pluripotent stem cells, followed by directed differentiation, offers a powerful strategy for producing patient-specific therapies. Given how laborious and lengthy this process can be, the conversion of somatic cells into lineage-specific stem/progenitor cells in one step, without going back to, or through, a pluripotent stage, has opened up tremendous opportunities for regenerative medicine. However, there are a number of obstacles to overcome before these cells can be widely considered for clinical applications. Here, we focus on induced transdifferentiation strategies to convert mature somatic cells to other mature cell types or progenitors, and we summarize the challenges that need to be met if the potential applications of transdifferentiation technology are to be achieved.

Role of transcription factors in differentiation and reprogramming of hematopoietic cells

Differentiation of hematopoietic cells is a sequential process of cell fate decision originating from hematopoietic stem cells (HSCs), allowing multi- or oligopotent progenitors to commit to certain lineages. HSCs are cells that are able to self-renew and repopulate the marrow for the long term. They first differentiate into multipotent progenitors (MPPs), which give rise to common lymphoid progenitors (CLPs) and common myeloid progenitors (CMPs). CMPs then differentiate into granulocyte monocyte progenitors (GMPs) and megakaryocyte erythroid progenitors (MEPs), which are the precursors of granulocytes/monocytes and erythrocytes/megakaryocytes, respectively. Lineage specification at differentiation branch points is dictated by the activation of lineage-specific transcription factors such as C/EBPα, PU.1, and GATA-1. The role of these transcription factors is generally instructive, and the expression of a single factor can often determine cell fate. Differentiation was long regarded as an irreversible process, and it was believed that somatic cells would not change their fate once they were differentiated. This paradigm was first challenged by the finding that ectopic cytokine signals could change the fate of differentiation, probably through modulating internal transcription networks. Subsequently, we and others showed that virtually all progenitors, including CLPs, CMPs, GMPs, and MEPs, still retain differentiation plasticity, and they can be converted into lineages other than their own by ectopic activation of only a single lineage-specific transcription factor. These findings established a novel paradigm for cellular differentiation and opened up an avenue for artificially manipulating cell fate for clinical use.

A map for lineage restriction of progenitors during hematopoiesis: the essence of the myeloid-based model

Most hematology and immunology textbooks describe that the first branch point from the hematopoietic stem cells (HSCs) produces two progenitors, one for myelo-erythroid cells and the other for lymphoid cells including T and B cells. This model is based on the concept that the blood cell family can be subdivided into two major lineages, a myelo-erythroid lineage and a lymphoid lineage. Several alternative models have been proposed during the last three decades. We proposed the myeloid-based model in 2001, in which myeloid potential is retained in an early stage of branches toward erythroid, T-, and B-cell lineages. In this review, we focus on the point that cell differentiation models have two different facets: as a map of developmental potential and a cell fate map. These two are expressed in other words as a map for lineage restriction and a map for physiological production routes. We argue that a map of potential is first and foremost essential for the study of molecular mechanisms of lineage commitment, which is the least clarified aspect of cell differentiation. The validity of the myeloid-based model of hematopoiesis will be discussed in reference to these two issues, developmental potential and cell fate.

In vitro generation of HSC-like cells from murine ESCs/iPSCs by enforced expression of LIM-homeobox transcription factor Lhx2

Identification of genes involved in in vitro differentiation induction of embryonic stem cells (ESCs) into hematopoietic stem cells (HSCs) has been challenged during last decade. To date, a homeobox transcription factor Hoxb4 has been only demonstrated to possess such an effect in mice. Here, we show that HSC-like cells were efficiently induced from mouse ESCs by enforced expression of Lhx2, a LIM-homeobox transcription factor. Transduction of Lhx2 into ESC-derived mesodermal cells resulted in robust differentiation of c-Kit(+)/Sca-1(+)/Lineage(-) (KSL) cells in vitro. The KSL cell induction frequency was superior to the case of Hoxb4. Furthermore, transplantation of Lhx2-transduced hematopoietic cells into lethally irradiated mice resulted in multilineage repopulation of hematopoietic cells over 4 months. Transduction of Lhx2 into induced pluripotent stem cells (iPSCs) was also effective in generating KSL cells in vitro, as well as HSC-like activities in vivo. These results demonstrate that ectopic expression of Lhx2 confers an in vivo engrafting capacity to ESC/iPSC-derived hematopoietic cells and in vivo behavior of iPSC-derived hematopoietic cells is almost identical to that of ESC-derived cells.

Transcriptional regulation by GATA1 and GATA2 during erythropoiesis

The transcription factor GATA1 regulates multiple genes in erythroid lineage cells. However, the means by which GATA1 regulates the expression of target genes during erythropoiesis remains to be elucidated. Three mechanisms have been postulated for the regulation of genes by GATA1. First, individual target genes may have multiple discrete thresholds for cellular GATA1. GATA1 has a dynamic expression profile during erythropoiesis, thus the expression of a set of GATA1 target genes may be triggered at a given stage of differentiation by cellular GATA1. Second, the expression of GATA1 target genes may be modified, at least in part, by GATA2 occupying the GATA-binding motifs. GATA2 is expressed earlier in erythropoiesis than GATA1, and prior GATA2 binding may afford GATA1 access to GATA motifs through epigenetic remodeling and thus facilitate target gene expression. Third, other regulatory molecules specific to each target gene may function cooperatively with GATA1. If GATA1 is required for the expression of such cofactors, a regulatory network will be formed and relevant gene expression will be delayed. We propose that the stage-specific regulation of erythroid genes by GATA1 is tightly controlled through a combination of these mechanisms in vivo.

SENP1-mediated GATA1 deSUMOylation is critical for definitive erythropoiesis

Small ubiquitin-like modifier (SUMO) modification of proteins (SUMOylation) and deSUMOylation have emerged as important regulatory mechanisms for protein function. SENP1 (SUMO-specific protease) deconjugates SUMOs from modified proteins. We have created SENP1 knockout (KO) mice based on a Cre-loxP system. Global deletion of SENP1 (SENP1 KO) causes anemia and embryonic lethality between embryonic day 13.5 and postnatal day 1, correlating with erythropoiesis defects in the fetal liver. Bone marrow transplantation of SENP1 KO fetal liver cells to irradiated adult recipients confers erythropoiesis defects. Protein analyses show that the GATA1 and GATA1-dependent genes are down-regulated in fetal liver of SENP1 KO mice. This down-regulation correlates with accumulation of a SUMOylated form of GATA1. We further show that SENP1 can directly deSUMOylate GATA1, regulating GATA1-dependent gene expression and erythropoiesis by in vitro assays. Moreover, we demonstrate that GATA1 SUMOylation alters its DNA binding, reducing its recruitment to the GATA1-responsive gene promoter. Collectively, we conclude that SENP1 promotes GATA1 activation and subsequent erythropoiesis by deSUMOylating GATA1.

FOG1 requires NuRD to promote hematopoiesis and maintain lineage fidelity within the megakaryocytic-erythroid compartment

Nuclear factors regulate the development of complex tissues by promoting the formation of one cell lineage over another. The cofactor FOG1 interacts with transcription factors GATA1 and GATA2 to control erythroid and megakaryocyte (MK) differentiation. In contrast, FOG1 antagonizes the ability of GATA factors to promote mast cell (MC) development. Normal FOG1 function in late-stage erythroid cells and MK requires interaction with the chromatin remodeling complex NuRD. Here, we report that mice in which the FOG1/NuRD interaction is disrupted (Fog(ki/ki)) produce MK-erythroid progenitors that give rise to significantly fewer and less mature MK and erythroid colonies in vitro while retaining multilineage capacity, capable of generating MCs and other myeloid lineage cells. Gene expression profiling of Fog(ki/ki) MK-erythroid progenitors revealed inappropriate expression of several MC-specific genes. Strikingly, aberrant MC gene expression persisted in mature Fog(ki/ki) MK and erythroid progeny. Using a GATA1-dependent committed erythroid cell line, select MC genes were found to be occupied by NuRD, suggesting a direct mechanism of repression. Together, these observations suggest that a simple heritable silencing mechanism is insufficient to permanently repress MC genes. Instead, the continuous presence of GATA1, FOG1, and NuRD is required to maintain lineage fidelity throughout MK-erythroid ontogeny.

Blasts in transient leukaemia in neonates with Down syndrome differentiate into basophil/mast-cell and megakaryocyte lineages in vitro in association with down-regulation of truncated form of GATA1

Mutations of GATA1, leading to aberrant expression of a truncated form of GATA1 (called GATA1s), are present in transient leukaemia (TL) in neonates with Down syndrome. Using these molecular markers of TL, we investigated the growth and differentiation potential of TL blasts in the presence of hematopoietic growth factors (HGFs). Interleukin-3, stem cell factor and granulocyte-macrophage colony-stimulating factor potently stimulated the growth of TL blast progenitors and induced differentiation towards basophil/mast cell lineages, whereas thrombopoietin induced differentiation towards megakaryocytes. GATA1s was expressed in TL blasts in all five patients examined but was down-regulated during differentiation induced by these HGFs, while full-length GATA1 was not expressed throughout the culture. GATA1 mutations were detected in TL blasts in four patients, including one patient with two distinct mutations. The cells of this patient exhibited identical and only mutated sequences both before and after culture with HGFs, confirming the leukemic cell origin of these differentiated cells. Erythroid differentiation of TL blasts was not evident with any HGFs. These data indicate that TL blasts have the potential to grow and differentiate towards particular hematopoietic lineages in the presence of specific HGFs and that the down-regulation of GATA1s might be involved in blast cell differentiation.

Forcing cells to change lineages

The ability to produce stem cells by induced pluripotency (iPS reprogramming) has rekindled an interest in earlier studies showing that transcription factors can directly convert specialized cells from one lineage to another. Lineage reprogramming has become a powerful tool to study cell fate choice during differentiation, akin to inducing mutations for the discovery of gene functions. The lessons learnt provide a rubric for how cells may be manipulated for therapeutic purposes.

New light on the biology and developmental potential of haematopoietic stem cells and progenitor cells

Even though stem cells have been identified in several tissues, one of the best understood somatic stem cells is the bone marrow residing haematopoietic stem cell (HSC). These cells are able to generate all types of blood cells found in the periphery over the lifetime of an animal, making them one of the most profound examples of tissue-restricted stem cells. HSC therapy also represents one of the absolutely most successful cell-based therapies applied both in the treatment of haematological disorders and cancer. However, to fully explore the clinical potential of HSCs we need to understand the molecular regulation of cell maturation and lineage commitment. The extensive research effort invested in this area has resulted in a rapid development of the understanding of the relationship between different blood cell lineages and increased understanding for how a balanced composition of blood cells can be generated. In this review, several of the basic features of HSCs, as well as their multipotent and lineage-restricted offspring, are addressed, providing a current view of the haematopoietic development tree. Some of the basic mechanisms believed to be involved in lineage restriction events including activities of permissive and instructive external signals are also discussed, besides transcription factor networks and epigenetic alterations to provide an up-to-date view of early haematopoiesis.

Graded repression of PU.1/Sfpi1 gene transcription by GATA factors regulates hematopoietic cell fate

GATA-1 and PU.1 are essential hematopoietic transcription factors that control erythromegakaryocytic and myelolymphoid differentiation, respectively. These proteins antagonize each other through direct physical interaction to repress alternate lineage programs. We used immortalized Gata1(-) erythromegakaryocytic progenitor cells to study how PU.1/Sfpi1 expression is regulated by GATA-1 and GATA-2, a related factor that is normally expressed at earlier stages of hematopoiesis. Both GATA factors bind the PU.1/Sfpi1 gene at 2 highly conserved regions. In the absence of GATA-1, GATA-2 binding is associated with an undifferentiated state, intermediate level PU.1/Sfpi1 expression, and low-level expression of its downstream myeloid target genes. Restoration of GATA-1 function induces erythromegakaryocytic differentiation. Concomitantly, GATA-1 replaces GATA-2 at the PU.1/Sfpi1 locus and PU.1/Sfpi1 expression is extinguished. In contrast, when GATA-1 is not present, shRNA knockdown of GATA-2 increases PU.1/Sfpi1 expression by 3-fold and reprograms the cells to become macrophages. Our findings indicate that GATA factors act sequentially to regulate lineage determination during hematopoiesis, in part by exerting variable repressive effects at the PU.1/Sfpi1 locus.

Ablation of Gata1 in adult mice results in aplastic crisis, revealing its essential role in steady-state and stress erythropoiesis

The transcription factor Gata1 is expressed in several hematopoietic lineages and plays essential roles in normal hematopoietic development during embryonic stages. The lethality of Gata1-null embryos has precluded determination of its role in adult erythropoiesis. Here we have examined the effects of Gata1 loss in adult erythropoiesis using conditional Gata1 knockout mice expressing either interferon- or tamoxifen-inducible Cre recombinase (Mx-Cre and Tx-Cre, respectively). Mx-Cre-mediated Gata1 recombination, although incomplete, resulted in maturation arrest of Gata1-null erythroid cells at the proerythroblast stage, thrombocytopenia, and excessive proliferation of megakaryocytes in the spleen. Tx-Cre-mediated Gata1 recombination resulted in depletion of the erythroid compartment in bone marrow and spleen. Formation of the early and late erythroid progenitors in bone marrow was significantly reduced in the absence of Gata1. Furthermore, on treatment with a hemolytic agent, these mice failed to activate a stress erythropoietic response, despite the rising erythropoietin levels. These results indicate that, in addition to the requirement of Gata1 in adult megakaryopoiesis, Gata1 is necessary for steady-state erythropoiesis and for erythroid expansion in response to anemia. Thus, ablation of Gata1 in adult mice results in a condition resembling aplastic crisis in human.

Differential context-dependent effects of friend of GATA-1 (FOG-1) on mast-cell development and differentiation

Friend of GATA-1 (FOG-1) is a binding partner of GATA-1, a zinc finger transcription factor with crucial roles in erythroid, megakaryocytic, and mast-cell differentiation. FOG-1 is indispensable for the function of GATA-1 during erythro/megakaryopoiesis, but FOG-1 is not expressed in mast cells. Here, we analyzed the role of FOG-1 in mast-cell differentiation using a combined experimental system with conditional gene expression and in vitro hematopoietic induction of mouse embryonic stem cells. Expression of FOG-1 during the progenitor period inhibited the differentiation of mast cells and enhanced the differentiation of neutrophils. Analysis using a mutant of PU.1, a transcription factor that positively or negatively cooperates with GATA-1, revealed that this lineage skewing was caused by disrupted binding between GATA-1 and PU.1, which is a prerequisite for mast-cell differentiation. However, FOG-1 expression in mature mast cells brought approximately a reversible loss of the mast-cell phenotype. In contrast to the lineage skewing, the loss of the mast-cell phenotype was caused by down-regulation of MITF, a basic helix-loop-helix transcription factor required for mast-cell differentiation and maturation. These results indicate that FOG-1 inhibits mast-cell differentiation in a differentiation stage-dependent manner, and its effects are produced via different molecular mechanisms.

Aberrant mast-cell differentiation in mice lacking the stem-cell leukemia gene

The stem cell leukemia (SCL) gene encodes a basic helix-loop-helix transcription factor expressed in erythroid, megakaryocyte, and mast-cell lineages. SCL is essential for growth of megakaryocyte and erythroid progenitors. We have used a conditional knockout of SCL (SCL(-/Delta)) to examine its function in mast cells, critical effectors of the immune system. SCL(-/Delta) mice had markedly increased numbers of mast-cell progenitors (MCPs) within the peritoneal fluid, bone marrow, and spleen. Fractionation of bone marrow myeloid progenitors demonstrated that these MCPs were present in the megakaryocyte-erythroid-restricted cell fraction. In contrast, unilineage MCPs from control mice were present in the cell fraction with granulocyte-macrophage potential. The aberrant mast-cell differentiation of SCL(-/Delta) megakaryocyte-erythroid progenitors was associated with increased expression of GATA-2. Despite increased numbers of MCPs in SCL(-/Delta) mice, numbers of mature tissue mast cells were not increased unless SCL(-/Delta) mice were treated with IL-3 and stem-cell factor. In part, this may be due to a requirement for SCL in normal mast-cell maturation: SCL(-/Delta) mast cells had reduced expression of the high-affinity IgE receptor and mast cell proteases, MCP-5 and MCP-6. Together, these studies suggest that loss of SCL leads to aberrant mast-cell differentiation of megakaryocyte-erythroid progenitors.

The hypomorphic Gata1low mutation alters the proliferation/differentiation potential of the common megakaryocytic-erythroid progenitor

Recent evidence suggests that mutations in the Gata1 gene may alter the proliferation/differentiation potential of hemopoietic progenitors. By single-cell cloning and sequential replating experiments of prospectively isolated progenitor cells, we demonstrate here that the hypomorphic Gata1low mutation increases the proliferation potential of a unique class of progenitor cells, similar in phenotype to adult common erythroid/megakaryocytic progenitors (MEPs), but with the "unique" capacity to generate erythroblasts, megakaryocytes, and mast cells in vitro. Conversely, progenitor cells phenotypically similar to mast cell progenitors (MCPs) are not detectable in the marrow from these mutants. At the single-cell level, about 11% of Gata1low progenitor cells, including MEPs, generate cells that will continue to proliferate in cultures for up to 4 months. In agreement with these results, trilineage (erythroid, megakaryocytic, and mastocytic) cell lines are consistently isolated from bone marrow and spleen cells of Gata1low mice. These results confirm the crucial role played by Gata1 in hematopoietic commitment and identify, as a new target for the Gata1 action, the restriction point at which common myeloid progenitors become either MEPs or MCPs.

Hematopoietic stem cells: the paradigmatic tissue-specific stem cell

The recent prospective isolation of a wide variety of somatically derived stem cells has affirmed the notion that homeostatic maintenance of most tissues and organs is mediated by tissue-specific stem and progenitor cells and fueled enthusiasm for the use of such cells in strategies aimed at repairing or replacing damaged, diseased, or genetically deficient tissues and organs. Hematopoietic stem cells (HSCs) are arguably the most well-characterized tissue-specific stem cell, with decades of basic research and clinical application providing not only a profound understanding of the principles of stem cell biology, but also of its potential pitfalls. It is our belief that emerging stem cell fields can benefit greatly from an understanding of the lessons learned from the study of HSCs. In this review we discuss some general concepts regarding stem cell biology learned from the study of HSCs with a highlight on recent work pertaining to emerging topics of interest for stem cell biology.