Selective rescue of early haematopoietic progenitors in Scl–/– mice by expressing Scl under the control of a stem cell enhancer

Identification of a TAL1 target gene reveals a positive role for the LIM domain-binding protein Ldb1 in erythroid gene expression and differentiation

The TAL1 (or SCL) gene, originally identified from its involvement by a recurrent chromosomal translocation, encodes a basic helix-loop-helix transcription factor essential for erythropoiesis. Although presumed to regulate transcription, its target genes are largely unknown. We show here that a nuclear complex containing TAL1, its DNA-binding partner E47, zinc finger transcription factor GATA-1, LIM domain protein LMO2, and LIM domain-binding protein Ldb1 transactivates the protein 4.2 (P4.2) gene through two E box GATA elements in its proximal promoter. Binding of this complex to DNA was dependent on the integrity of both E box and GATA sites and was demonstrated to occur on the P4.2 promoter in cells. Maximal transcription in transiently transfected cells required both E box GATA elements and expression of all five components of the complex. This complex was shown, in addition, to be capable of linking in solution double-stranded oligonucleotides corresponding to the two P4.2 E box GATA elements. This DNA-linking activity required Ldb1 and increased with dimethyl sulfoxide-induced differentiation of murine erythroleukemia (MEL) cells. In contrast, enforced expression in MEL cells of dimerization-defective mutant Ldb1, as well as wild-type Ldb1, significantly decreased E box GATA DNA-binding activities, P4.2 promoter activity, and accumulation of P4.2 and beta-globin mRNAs. These studies define a physiologic target for a TAL1- and GATA-1-containing ternary complex and reveal a positive role for Ldb1 in erythroid gene expression and differentiation.

Manipulation of mouse hematopoietic progenitors by specific retroviral infection

Previous studies have identified an enhancer 3' of the scl gene that can direct transgene expression to hematopoietic progenitors and stem cells. Here we use this enhancer to restrict expression of the avian leukosis virus receptor, TVA, to hematopoietic stem cells and progenitors in bone marrow and fetal liver and demonstrate that retroviral infection can be used to specifically introduce exogenous sequences. We show that a majority of CFU-S12 multipotential progenitor cells can be transduced in vitro. Uniquely, transduction of TVA+ progenitors with a retrovirus encoding a puromycin resistance gene allows selection and expansion of a multipotential hematopoietic progenitor population that can be superinfected with high efficiency. Using this system we show for the first time that v-Myb oncoproteins expressed from avian viruses can induce a leukemic transformation in the mouse. The phenotype of the transformed cells is similar to that which is seen in the chicken and is likewise dependent on the particular structure of v-Myb. This implies that the basic mechanisms of action of mutated transcription factors in the etiology of leukemia are conserved between birds and mammals.

Control of erythroid cell production via caspase-mediated cleavage of transcription factor SCL/Tal-1

SCL/Tal-1 is a helix-loop-helix (HLH) transcription factor required for blood cell development, whose abnormal expression is responsible for induction of T-cell acute lymphoblastic leukemia. We show here that SCL/Tal-1 is a key target of caspases in developing erythroblasts. SCL/Tal-1 degradation occurred rapidly after caspase activation and preceded the cleavage of the major erythroid transcription factor GATA-1. Expression of a caspase-resistant SCL/Tal-1 in erythroid progenitors was able to prevent amplification of caspase activation, GATA-1 degradation and impaired erythropoiesis induced by growth factor deprivation or death receptor triggering. The potent proerythropoietic activity of uncleavable SCL/Tal-1 was clearly evident in the absence of erythropoietin, a condition that did not allow survival of normal erythroid cells or expansion of erythroblasts expressing caspase-resistant GATA-1. In the absence of erythropoietin, cells expressing caspase-resistant SCL/Tal-1 maintain high levels of Bcl-X(L), which inhibits amplification of the caspase cascade and mediates protection from apoptosis. Thus, SCL/TAL-1 is a survival factor for erythroid cells, whereas caspase-mediated cleavage of SCL/Tal-1 results in amplification of caspase activation, GATA-1 degradation and impaired erythropoiesis.

The critical regulator of embryonic hematopoiesis, SCL, is vital in the adult for megakaryopoiesis, erythropoiesis, and lineage choice in CFU-S12

Gene targeting studies have shown that the transcription factor SCL is critically important for embryonic hematopoiesis, but the early lethality of SCL null mice has precluded the genetic analysis of its function in the adult. We have now generated a conditional knockout of SCL by using CreLox technology and an IFN-inducible Cre transgenic mouse. Deletion of SCL in adult mice perturbed megakaryopoiesis and erythropoiesis with the loss of early progenitor cells in both lineages. This led to a blunted response to the hematopoietic stress induced by polyinosinic-polycytidylic acid, with a persistently low platelet count and hematocrit compared with controls. In contrast, progenitors of granulocyte and macrophage lineages were not affected, even in the setting of stress. Immature progenitor cells (day 12 colony-forming unit spleen) with multilineage capacity were still present in the SCL null bone marrow, but these progenitors had lost the capacity to generate erythroid and megakaryocyte cells, and colonies were composed of only myeloid cells. These results suggest that SCL is critical for megakaryopoiesis and erythropoiesis, but is dispensable for production of myeloid cells during adult hematopoiesis.

Of mice and models: improved animal models for biomedical research

The ability to engineer the mouse genome has profoundly transformed biomedical research. During the last decade, conventional transgenic and gene knockout technologies have become invaluable experimental tools for modeling genetic disorders, assigning functions to genes, evaluating drugs and toxins, and by and large helping to answer fundamental questions in basic and applied research. In addition, the growing demand for more sophisticated murine models has also become increasingly evident. Good state-of-principle knowledge about the enormous potential of second-generation conditional mouse technology will be beneficial for any researcher interested in using these experimental tools. In this review we will focus on practice, pivotal principles, and progress in the rapidly expanding area of conditional mouse technology. The review will also present an internet compilation of available tetracycline-inducible mouse models as tools for biomedical research (http://www.zmg.uni-mainz.de/tetmouse/).

Identification of endoglin as a functional marker that defines long-term repopulating hematopoietic stem cells

We describe a strategy to obtain highly enriched long-term repopulating (LTR) hematopoietic stem cells (HSCs) from bone marrow side-population (SP) cells by using a transgenic reporter gene driven by a stem cell enhancer. To analyze the gene-expression profile of the rare HSC population, we developed an amplification protocol termed "constant-ratio PCR," in which sample and control cDNAs are amplified in the same PCR. This protocol allowed us to identify genes differentially expressed in the enriched LTR-HSC population by oligonucleotide microarray analysis using as little as 1 ng of total RNA. Endoglin, an ancillary transforming growth factor beta receptor, was differentially expressed by the enriched HSCs. Importantly, endoglin-positive cells, which account for 20% of total SP cells, contain all the LTR-HSC activity within bone marrow SP. Our results demonstrate that endoglin, which plays important roles in angiogenesis and hematopoiesis, is a functional marker that defines LTR HSCs. Our overall strategy may be applicable for the identification of markers for other tissue-specific stem cells.

Rescue of the lethal scl(-/-) phenotype by the human SCL locus

The stem cell leukemia (SCL) gene encodes a basic helix-loop-helix transcription factor with a critical role in the development of both blood and endothelium. Loss-of-function studies have shown that SCL is essential for the formation of hematopoietic stem cells, for subsequent erythroid development and for yolk sac angiogenesis. SCL exhibits a highly conserved pattern of expression from mammals to teleost fish. Several murine SCL enhancers have been identified, each of which directs reporter gene expression in vivo to a subdomain of the normal SCL expression pattern. However, regulatory elements necessary for SCL expression in erythroid cells remain to be identified and the size of the chromosomal domain needed to support appropriate SCL transcription is unknown. Here we demonstrate that a 130-kilobase (kb) yeast artificial chromosome (YAC) containing the human SCL locus completely rescued the embryonic lethal phenotype of scl(-/-) mice. Rescued YAC(+) scl(-/-) mice were born in appropriate Mendelian ratios, were healthy and fertile, and exhibited no detectable abnormality of yolk sac, fetal liver, or adult hematopoiesis. The human SCL protein can therefore substitute for its murine homologue. In addition, our results demonstrate that the human SCL YAC contains the chromosomal domain necessary to direct expression to the erythroid lineage and to all other tissues in which SCL performs a nonredundant essential function.

The role of the stem cell leukemia (SCL) gene in hematopoietic and endothelial lineage specification

Anatomical observations made at the beginning of the twentieth century revealed an intimate association between the ontogeny of blood and endothelium and led to the hypothesis of a common cell of origin termed the hemangioblast. However, the precise nature of the cellular intermediates involved in the development of both lineages from uncommitted precursors to mature cell types is still the subject of ongoing studies, as are the molecular mechanisms driving this process. There is clear evidence that lineage-restricted transcription factors play a central role in the genesis of mature lineage committed cells from multipotent progenitors. Amongst these, the basic helix-loop-helix (bHLH) family is of key importance for cell fate determination in the development of the hematopoietic system and beyond. This article will review the current evidence for the common origin of blood and endothelium, focusing on the function of the bHLH protein encoded by the stem cell leukemia (SCL) gene, and its role as a pivotal regulator of hematopoiesis and vasculogenesis.