Development of erythroid and myeloid progenitors in the yolk sac and embryo proper of the mouse.

In this study, we have mapped the onset of hematopoietic development in the mouse embryo using colony-forming progenitor assays and PCR-based gene expression analysis. With this approach, we demonstrate that commitment of embryonic cells to hematopoietic fates begins in proximal regions of the egg cylinder at the mid-primitive streak stage (E7.0) with the simultaneous appearance of primitive erythroid and macrophage progenitors. Development of these progenitors was associated with the expression of SCL/tal-1 and GATA-1, genes known to be involved in the development and maturation of the hematopoietic system. Kinetic analysis revealed the transient nature of the primitive erythroid lineage, as progenitors increased in number in the developing yolk sac until early somite-pair stages of development (E8.25) and then declined sharply to undetectable levels by 20 somite pairs (E9.0). Primitive erythroid progenitors were not detected in any other tissue at any stage of embryonic development. The early wave of primitive erythropoiesis was followed by the appearance of definitive erythroid progenitors (BFU-E) that were first detectable at 1-7 somite pairs (E8.25) exclusively within the yolk sac. The appearance of BFU-E was followed by the development of later stage definitive erythroid (CFU-E), mast cell and bipotential granulocyte/macrophage progenitors in the yolk sac. C-myb, a gene essential for definitive hematopoiesis, was expressed at low levels in the yolk sac just prior to and during the early development of these definitive erythroid progenitors. All hematopoietic activity was localized to the yolk sac until circulation was established (E8.5) at which time progenitors from all lineages were detected in the bloodstream and subsequently in the fetal liver following its development. This pattern of development suggests that definitive hematopoietic progenitors arise in the yolk sac, migrate through the bloodstream and seed the fetal liver to rapidly initiate the first phase of intraembryonic hematopoiesis. Together, these findings demonstrate that commitment to hematopoietic fates begins in early gastrulation, that the yolk sac is the only site of primitive erythropoiesis and that the yolk sac serves as the first source of definitive hematopoietic progenitors during embryonic development.     

SCL/tal-1-dependent process determines a competence to select the definitive hematopoietic lineage prior to endothelial differentiation

Hematopoiesis in most vertebrate species occurs in two distinct phases, primitive and definitive, which diverge from FLK1(+)VE-cadherin(-) mesoderm and FLK1(+)VE-cadherin(+) endothelial cells (EC), respectively. This study aimed at determining the stage at which hematopoietic lineage fate is determined by manipulating the SCL/tal-1 expression that is known to be essential for the early development of the primitive and definitive hematopoietic systems. We established SCL-null ES cell lines in which SCL expression is rescued by tamoxifen-inducible Cre recombinase-loxP site-mediated recombination. While no hematopoietic cells (HPC) were detected in SCL-null ES cell differentiation cultures, SCL gene reactivation from day 2 to day 4 after initiation of differentiation could rescue both primitive and definitive hematopoiesis. SCL reactivation at later phases was ineffective. Moreover, generation of VE-cadherin(+) EC that can give rise to definitive HPC required SCL reactivation prior to VE-cadherin expression. These results indicated that the competence to become HPC is acquired at the mesodermal stage by a SCL-dependent process that takes place independently of determination of endothelial fate.     

Serglycin proteoglycan expression and synthesis in embryonic stem cells

The serglycin proteoglycan is expressed in most hematopoietic cells and is packaged into secretory vesicles for constitutive or regulated secretion. We have now shown serglycin mRNA expression in undifferentiated murine embryonic stem (ES) cells and in embryoid bodies, and synthesis and secretion in undifferentiated ES cells. Serglycin was localized to ES cell cytoplasm by immunostaining. Serglycin mRNA is expressed in tal-1((-/-)) ES cells and embryoid bodies; tal-1((-/-)) mice cannot produce hematopoietic cells. Thus, ES serglycin expression is probably not associated with hematopoiesis. Serglycin expression was increased by treatment of ES cells with retinoic acid (RA) and dibutyryl cAMP (dbcAMP). The serglycin core protein obtained from control ES culture medium after chondroitinase digestion appears as a doublet. Only the lower Mr band is present in serglycin secreted from RA-treated and the higher Mr band in RA+dbcAMP-treated cells, suggesting that core protein structure is affected by differentiation.     

Induction of embryonic hematopoietic and endothelial stem/progenitor cells by hedgehog-mediated signals

Blood and vascular endothelial cells form in all vertebrates during gastrulation, a process in which the mesoderm of the embryo is induced and then patterned by molecules whose identity is still largely unknown. Blood islands' of primitive hematopoietic cell clusters surrounded by a layer of endothelial cells form in the yolk sac, external to the developing embryo proper. These lineages arise from a layer of extraembryonic mesoderm that is closely apposed with a layer of primitive (visceral) endoderm. Despite the identification of genes such as Flk1, SCL/tal-1, Cbfa2/Runx1/AML1 and CD34 that are expressed during the induction of primitive hematopoiesis and vasculogenesis, the early molecular and cellular events involved in these processes are not well understood. Recent work has demonstrated that extracellular signals secreted by visceral endoderm surrounding the embryo are essential for the initiation of these events. A member of the Hedgehog family of signaling molecules (Indian hedgehog) is produced by visceral endoderm, can induce formation of blood and endothelial cells in explant cultures and can reprogram prospective neurectoderm along hematopoietic and endothelial cell lineages. Hedgehog proteins also stimulate proliferation of definitive hematopoietic stem/progenitor cells. These findings may have important implications for regulating hematopoiesis and vascular development for therapeutic purposes in humans and for the development of new sources of stem cells for transplantation and gene therapy.     

Gene targeting and expression analysis of mouse Tem1/endosialin using a lacZ reporter

TEM1 (endosialin) expression is increased in the stroma and tumor vasculature of several common human cancers. The exact physiological role of TEM1 is still unknown since Tem1-deficient mice are viable and show only a lower rate of abdominal site-specific tumor invasion in tumor transplantation experiments. Previous studies have reported Tem1 expression in mouse embryos and adults, but did not determine the timing or location of the earliest expression, and did not examine all organ systems. Using the highly sensitive Bluo-Gal staining method for detecting temporal and spatial Tem1-lacZ activity in lacZ knock-in (+/lacZ) mice, we found that Tem1 gene expression was initially detectable in the dorsal aortic wall, the heart, the umbilical vessels, the first branchial arch, and the cephalic mesenchyme at E9.5. From E10.5 to E14.5, Tem1 gene expression was additionally seen mainly in the genital tubercle, the mesonephros, the whisker follicles, the mesenchymal tissues around the eye, and the lung. Remarkably, the kidney expressed abundant Tem1-lacZ starting from E16.5. Postnatally, Tem1 expression decreased in most organs but elevated expression persisted in the renal glomerulus and the uterus, where the expression pattern varied at different estrous cycle stages. Co-localization studies indicated that most vimentin-positive cells co-expressed Tem1-lacZ, while a large portion of CD31- or desmin-positive cells were also positive for Tem1-lacZ. Taken together, our observations suggest that Tem1 is expressed throughout embryonic and adult development in several types of mesenchymal cells closely related to blood vessels.     

Gene transfer to pre-hematopoietic and committed hematopoietic precursors in the early mouse Yolk Sac: a comparative study between <Emphasis Type="Italic">in situ</Emphasis> electroporation and retroviral transduction

Background  

Hematopoietic development in vertebrate embryos results from the sequential contribution of two pools of precursors independently generated. While intra-embryonic precursors harbour the features of hematopoietic stem cells (HSC), precursors formed earlier in the yolk sac (YS) display limited differentiation and self-renewal potentials. The mechanisms leading to the generation of the precursors in both sites are still largely unknown, as are the molecular basis underlying their different potential. A possible approach to assess the role of candidate genes is to transfer or modulate their expression/activity in both sites. We thus designed and compared transduction protocols to target either native extra-embryonic precursors, or hematopoietic precursors.     

Retina restored and brain abnormalities ameliorated by single-copy knock-in of human NR2E1 in null mice

Nr2e1 encodes a stem cell fate determinant of the mouse forebrain and retina. Abnormal regulation of this gene results in retinal, brain, and behavioral abnormalities in mice. However, little is known about the functionality of human NR2E1. We investigated this functionality using a novel knock-in humanized-mouse strain carrying a single-copy bacterial artificial chromosome (BAC). We also documented, for the first time, the expression pattern of the human BAC, using an NR2E1-lacZ reporter strain. Unexpectedly, cerebrum and olfactory bulb hypoplasia, hallmarks of the Nr2e1-null phenotype, were not fully corrected in animals harboring one functional copy of human NR2E1. These results correlated with an absence of NR2E1-lacZ reporter expression in the dorsal pallium of embryos and proliferative cells of adult brains. Surprisingly, retinal histology and electroretinograms demonstrated complete correction of the retina-null phenotype. These results correlated with appropriate expression of the NR2E1-lacZ reporter in developing and adult retina. We conclude that the human BAC contained all the elements allowing correction of the mouse-null phenotype in the retina, while missing key regulatory regions important for proper spatiotemporal brain expression. This is the first time a separation of regulatory mechanisms governing NR2E1 has been demonstrated. Furthermore, candidate genomic regions controlling expression in proliferating cells during neurogenesis were identified.