Embryonic beginnings of definitive hematopoietic stem cells

The ability of the many cell types within the adult blood system to be constantly replenished and renewed from hematopoietic stem cells is an interesting problem in development and differentiation and has led to questions concerning how, when and where these stem cells for the adult hematopoietic system are generated within the embryo. During embryonic development many mature hematopoietic cells appear before adult-type hematopoietic stem cells thus the notion of a conventional hematopoietic hierarchy is challenged. Experiments probing the development of hematopoietic stem cells in the mouse embryo strongly suggest that at least two independent hematopoietic sites generate blood cells during development; the yolk sac, which produces the transient embryonic hematopoietic system, and the AGM (aorta-gonad-mesonephros) region, which initiates the long-lived adult hematopoietic system.

Gene interactions in gonadal development

The acquisition of a sexually dimorphic phenotype is a critical event in mammalian development. Although the maturation of sexual function and reproduction occurs after birth, essentially all of the critical developmental steps take place during embryogenesis. Temporally, these steps can be divided into two different phases: sex determination, the initial event that determines whether the gonads will develop as testes or ovaries; and sexual differentiation, the subsequent events that ultimately produce either the male or the female sexual phenotype. A basic tenet of sexual development in mammals is that genetic sex--determined by the presence or absence of the Y chromosome--directs the embryonic gonads to differentiate into either testes or ovaries. Thereafter, hormones produced by the testes direct the developmental program leading to male sexual differentiation. In the absence of testicular hormones, the pathway of sexual differentiation is female. This chapter reviews the anatomic and cellular changes that constitute sexual differentiation and discusses SRY and other genes, including SF-1, WT1, DAX-1, and SOX9, that play key developmental roles in this process. Dose-dependent interactions among these genes are critical for sex determination and differentiation.

The Wilms' tumor suppressor gene (wt1) product regulates Dax-1 gene expression during gonadal differentiation

Gonadal differentiation is dependent upon a molecular cascade responsible for ovarian or testicular development from the bipotential gonadal ridge. Genetic analysis has implicated a number of gene products essential for this process, which include Sry, WT1, SF-1, and DAX-1. We have sought to better define the role of WT1 in this process by identifying downstream targets of WT1 during normal gonadal development. We have noticed that in the developing murine gonadal ridge, wt1 expression precedes expression of Dax-1, a nuclear receptor gene. We document here that the spatial distribution profiles of both proteins in the developing gonad overlap. We also demonstrate that WT1 can activate the Dax-1 promoter. Footprinting analysis, transient transfections, promoter mutagenesis, and mobility shift assays suggest that WT1 regulates Dax-1 via GC-rich binding sites found upstream of the Dax-1 TATA box. We show that two WT1-interacting proteins, the product of a Denys-Drash syndrome allele of wt1 and prostate apoptosis response-4 protein, inhibit WT1-mediated transactivation of Dax-1. In addition, we demonstrate that WT1 can activate the endogenous Dax-1 promoter. Our results indicate that the WT1-DAX-1 pathway is an early event in the process of mammalian sex determination.

Origin of the gonad in the juvenile of a solitary ascidian, Ciona intestinalis

In the just-metamorphosed juveniles of Ciona intestinalis, a round mass of tissue debris derived from the resorbed tadpole tail is situated in the broad space enclosed by the peritoneal membrane and the epidermis around the ventral side of the esophagus. In living juveni es, the origin of the gonad rudiment was traced back to the mass of tissue debris. Electron microscopically, the round mass was a clump of irregular-shaped phagocytotic cells engulfing degenerated cell fragments. On the surface of the cell clump, a small number of singly occurring round cells were found and identified as primordial germ cells on the basis of morphological continuity to obvious germ cells in later stages. Presence of nuage around the nucleus characterized the germ cells. In a few days the germ cells assembled to form a solid slender body (gonad rudiment) together with smaller somatic cells. The gonad rudiment left the space around the esophagus, moving into the narrow mesenteric space connecting the stomach and intestine on the fourth day after metamorphosis. It gradually increased in size by proliferation of the germ cells and somatic cells. The solid gonad rudiment changed into an oval vesicle with an eccentrically located cavity on about the seventh day after metamorphosis. The vesicle comprised a thinner wall made of a simple epithelium without germ cells and a thicker wall containing germ cells and somatic cells.

HMG-CoA reductase guides migrating primordial germ cells

The enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase is best known for catalysing a rate-limiting step in cholesterol biosynthesis, but it also participates in the production of a wide variety of other compounds. Some clinical benefits attributed to inhibitors of HMG-CoA reductase are now thought to be independent of any serum cholesterol-lowering effect. Here we describe a new cholesterol-independent role for HMG-CoA reductase, in regulating a developmental process: primordial germ cell migration. We show that in Drosophila this enzyme is highly expressed in the somatic gonad and that it is necessary for primordial germ cells to migrate to this tissue. Misexpression of HMG-CoA reductase is sufficient to attract primordial germ cells to tissues other than the gonadal mesoderm. We conclude that the regulated expression of HMG-CoA reductase has a critical developmental function in providing spatial information to guide migrating primordial germ cells.

Role of Ahch in gonadal development and gametogenesis

Ahch (also known as Dax1) encodes a transcription factor that has been implicated in sex determination and gonadal differentiation. Mutations in human AHC cause X-linked, adrenal hypoplasia congenita (AHC) and hypogonadotropic hypogonadism (HH). Duplication of the Xp21 dosage-sensitive sex reversal (DSS) region, which contains the Ahch locus, and transgenic overexpression of Ahch cause male-to-female sex reversal. Using Cre-mediated disruption of Ahch, we have generated a mouse model of AHC-HH that allows the function of Ahch to be examined in both males and females. Although Ahch has been postulated to function as an ovarian determination gene, the loss of Ahch function in females does not affect ovarian development or fertility. Ahch is instead essential for the maintenance of spermatogenesis. Lack of Ahch causes progressive degeneration of the testicular germinal epithelium independent of abnormalities in gonadotropin and testosterone production and results in male sterility. Ahch is thus not an ovarian determining gene, but rather has a critical role in spermatogenesis.

Sertoli cells of the mouse testis originate from the coelomic epithelium

During mouse development, the gonad begins to form shortly before 10. 5 days postcoitum (dpc) on the ventromedial side of the mesonephros. The XY gonad consists of germ cells and somatic cells. The origin of the germ cells is clearly established; however, the origin of the somatic cells, especially the epithelial supporting cell lineages, called Sertoli cells, is still unclear. Sertoli cells are the first somatic cell type to differentiate in the testis and are thought to express Sry, the male sex-determining gene, and to play a crucial role in directing testis development. Previous data have suggested that the somatic cells of the gonad may arise from the mesonephric tubules, the mesonephric mesenchyme, or the coelomic epithelium. Immunohistochemical staining of the gonad at 11.5 dpc showed that the basement membrane barrier under the coelomic epithelium is discontinuous, suggesting that cells in the coelomic epithelium at this stage might move inward. To test this possibility directly, cells of the coelomic epithelium were labeled using the fluorescent lipophilic dye, DiI. We show that when labeled at tail somite 15-17 stages, corresponding to 11.2-11.4 dpc, the coelomic epithelial cells of both sexes migrated into the gonad. In XY gonads, the migrating coelomic epithelial cells became Sertoli cells, as well as interstitial cells. This ability of the coelomic epithelium to give rise to Sertoli cells was developmentally regulated. When labeled at tail somite 18-20 stages, corresponding to 11.5-11.7 dpc, the coelomic epithelial cells no longer became Sertoli cells. Instead, cells that migrated into the gonad stayed outside testis cords, in the interstitium. Migration gradually decreased and ceased by tail somite 30 stage, corresponding to 12.5 dpc, after testis cords had formed and the basement membrane layer underlying the coelomic epithelium had thickened to form the tunica albuginea. In XX gonads, coelomic epithelial cells also migrated into the gonad, but there was no obvious fate restriction during the same developmental period. Taken together, our data show that the coelomic epithelium is a source of Sertoli cells as well as other somatic cells of the gonad in the developing mouse testis.

Activation of the thyroid hormone receptor alpha gene promoter by the orphan nuclear receptor ERR alpha

The superfamily of nuclear receptors comprises transcription factors that depend on a ligand for their activity. In addition, the superfamily includes a number of orphan receptors, for which no ligand is known. We report here that the orphan receptor estrogen receptor related alpha receptor (ERR alpha) stimulates the expression of the thyroid hormone receptor alpha (TR alpha) gene promoter. We characterized a responsive site that is both necessary and sufficient for ERR alpha-induced transactivation. In addition, we show that both TR alpha and ERR alpha are coexpressed in embryonic intestine, brown fat and heart as well as in the adult gonads. In the testis, expression of both receptors can be found in the seminiferous tubes where it is totally restricted to spermatocytes I. Altogether this suggests that TR alpha is an in vivo target of ERR alpha.

Wilms' tumor suppressor gene (WT1) as a target gene of SRY function in a mouse ES cell line transfected with SRY

With the aim of identifying the gene(s) located downstream from SRY, we transfected an ES cell line with XX karyotype, TMA-18, with a Sry DNA construct and established cell lines, TS18-1 and TS18-2, where the transfected Sry was expressed in the functional linear mRNA form. Among the five potential SRY-target genes examined, i.e., MIS, SF1, P450arom, Sox9 and WT1, only the expression of WT1 was induced de novo by the unscheduled expression of Sry in the transfected cell lines. No clear indication of Sry-induced enhancement of Sox9 expression was obtained in the present series of experiments. Function of a yet unidentified gene(s) located on the Y chromosome might be needed for the up-regulation of Sox 9 expression which takes place during the development of male gonads. Quantitative RT-PCR analysis of the patterns of WT1 expression in developing fetal gonads revealed that although both male and female fetal gonads express WT1, male gonads invariably expressed WT1 mRNA at higher levels than female ones after the Sry expression. Immunohistochemical analysis of the male fetal gonads between 10.5 and 13.5 dpc demonstrated the presence of strong WT1 immunoreactivity in Sertoli cells of the primordial testes. Suggestions were made in the past indicating that both SF1 and WT1 proteins might be active in a common pathway upstream from Sry. Our results showed that WT1 is located downstream, rather than upstream from Sry and behaves independently from SF1. Analysis using an appropriate in vitro system will be essential to understand the molecular mechanisms of SRY action within cells.

Cytoskeletal actin genes function downstream of HNF-3beta in ascidian notochord development

We have examined the expression and regulation of cytoskeletal actin genes in ascidians with tailed (Molgula oculata) and tailless larvae (Molgula occulta). Four cDNA clones were isolated representing two pairs of orthologous cytoskeletal actin genes (CA1 and CA2), which encode proteins differing by five amino acids in the tailed and tailless species. The CA1 and CA2 genes are present in one or two copies, although several related genes may also be present in both species. Maternal CA1 and CA2 mRNA is present in small oocytes but transcript levels later decline, suggesting a role in early oogenesis. In the tailed species, embryonic CA1 and CA2 mRNAs first appear in the presumptive mesenchyme and muscle cells during gastrulation, subsequently accumulate in the presumptive notochord cells, and can be detected in these tissues through the tadpole stage. CA1 mRNAs accumulate initially in the same tissues in the tailless species but subsequently disappear, in concert with the arrest of notochord and tail development. In contrast, CA2 mRNAs were not detected in embryos of the tailless species. Fertilization of eggs of the tailless species with sperm of the tailed species, which restores the notochord and the tail, also results in the upregulation of CA1 and CA2 gene expression in hybrid embryos. Antisense oligodeoxynucleotide experiments suggest that CA1 and CA2 expression in the notochord, but not in the muscle cells, is dependent on prior expression of Mocc FHI, an ascidian HNF-3beta-like gene. The expression of the CA1 and CA2 genes in the notochord in the tailed species, downregulation in the tailless species, upregulation in interspecific hybrids, and dependence on HNF-3beta activity is consistent with a role of these genes in development of the ascidian notochord.

The roles of steroidogenic factor 1 in endocrine development and function

The orphan nuclear receptor steroidogenic factor 1 (SF-1) was initially isolated as a key regulator of the cytochrome P450 steroid hydroxylases in adrenocortical and gonadal cells. Subsequent analyses of SF-1 knockout mice have expanded considerably our understanding of the roles that SF-1 plays in endocrine development. These SF-1 knockout mice lacked adrenal glands and gonads, with consequent male-to-female sex reversal of their internal and external genitalia. Thus, SF-1 is essential for the embryonic survival of the primary steroidogenic organs. They further exhibited impaired gonadotrope function and agenesis of the ventromedial hypothalamic nucleus, establishing that SF-1 contributes to reproductive function at all three levels of the hypothalamic-pituitary-gonadal axis. This report reviews experiments that have defined these critical roles of SF-1 in endocrine development, and highlights areas of ongoing investigation.

Function of steroidogenic factor 1 (SF1) ligand-binding domain in gene activation and interaction with AP1

The nuclear receptor SF1 is an essential mediator in ventromedial hypothalamus-pituitary-gonadal development. As with other nuclear receptors, SF1 possesses a DNA-binding domain composed of two zinc fingers and a ligand-binding domain containing a ligand-dependent activation sequence termed AF2. To dissect the domain function of SF1, we examined various SF1 mutants in mouse adrenocortical Y1 cells and human placental JEG3 cells. Destruction of the AF2 structure removed 73-90% transactivation activity, suggesting that AF2 is indispensable for transactivation. Mutants carrying the DNA-binding domain but lacking the AF2 or the ligand-binding domain blocked the activity of normal SF1. Disrupting the zinc finger diminished the dominant negative effect of mutant. Cotransfection of SF1 with AP1 showed that the two transcription factors cooperated to activate gene expression. Some mutants lost the synergistic action with AP1, while some retained partial activity. These experiments delineate the functional domains of SF1.

Stimulation of mouse and human primitive hematopoiesis by murine embryonic aorta-gonad-mesonephros-derived stromal cell lines

We report here on a novel stromal cell line, AGM-S3, derived from the aorta-gonad-mesonephros (AGM) region of a 10.5 days postcoitum (dpc) mouse embryo. The AGM-S3 cells promoted production of hematopoietic progenitors and day-12 spleen colony-forming cells from Lin-c-Kit+Sca-1(+) murine primitive hematopoietic cells. They also supported for 6 weeks generation of human multipotential progenitors from cord blood CD34(+)CD38(-) primitive hematopoietic cells. Human long-term repopulating hematopoietic stem cells (LTR-HSC) with the potential to reconstitute hematopoiesis in NOD/SCID mice were maintained on AGM-S3 cells for at least 4 weeks. Flow cytometric analysis showed that CD13, vascular cellular adhesion molecule-1, and Sca-1 were expressed on AGM-S3 cells. Because stem cell factor, interleukin-6 (IL-6), and oncostatin M, but not IL-3, IL-11, leukemia- inhibitory factor, granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, thrombopoietin, and Flk2 ligand were detected in reverse transcription-polymerase chain reaction analysis of AGM-S3 cells, the cells seem to express species-cross reactive molecule(s) other than the cytokines examined and which act on primitive hematopoietic progenitor/stem cells. This cell line is expected to elucidate molecular mechanisms regulating early hematopoiesis and pave the way for developing strategies for expansion of human transplantable HSC.

Stromal expression of Jagged 1 promotes colony formation by fetal hematopoietic progenitor cells

The Notch signaling system regulates proliferation and differentiation in many tissues. Notch is a transmembrane receptor activated by ligands expressed on adjacent cells. Hematopoietic stem cells and early progenitors express Notch, making the stromal cells which form cell-cell contacts with progenitor cells candidate ligand-presenting cells in the hematopoietic microenvironment. Therefore, we examined primary stromal cell cultures for expression of Notch ligands. Using reverse transcription-polymerase chain reaction, in situ hybridization, immunohistochemistry, and Western blotting, we demonstrate expression of Jagged 1 in primary stromal cultures. To investigate if the stromal expression of Jagged 1 has functional effects on hematopoietic progenitors, we cultured CD34(+), c-kit+ hematopoietic progenitor cells derived from the aorto gonadal mesonephros region of day 11 mouse embryos on the Jagged 1(-) stromal cell line S17 and on S17 cells engineered to express Jagged 1. The presence of Jagged 1 increased the number of colonies formed in subsequent methylcellulose culture fourfold. Larger increases in colony numbers were observed under the same culture conditions with CD34(+), c-kit+ hematopoietic progenitor cells derived from d11 fetal liver. These results obtained in vitro table Jagged 1 as a candidate regulator of stem cell fate in the context of stromal microenvironments in vivo.

Patterns of keratins 8, 18 and 19 during gonadal differentiation in the mouse: sex- and time-dependent expression of keratin 19

The acidic keratins K18 and K19 have been shown to display a sex-specific expression during gonadal differentiation in the rat. To extend these findings, we have undertaken a study of the expression of genes encoding for K18 and K19 and their basic partner K8 in the mouse from 10.5 days of gestation until adulthood, using immunofluorescence, in situ hybridization, and reverse transcriptase polymerase chain reaction (RT-PCR). In the urogenital ridge at 10.5 days of gestation, K18, K19, and K8 are present, in both sexes, in coelomic epithelium in the area of the prospective gonad. At 11 days and 10 h of gestation, they are detected in differentiating gonadal blastema. In male gonads at 11 days and 16 h of gestation the first Sertoli cells differentiate. They are stained for anti-Müllerian hormone by immunofluorescence and appear as dispersed cells throughout the blastema. Progressively, they adhere to each other and form differentiating seminiferous cords. K19 disappears as Sertoli cells differentiate. K18 and K8 continue to be detected in Sertoli cells during fetal life and after birth until 14 days postpartum. In the adult testis, no keratin is observed. In differentiating ovaries, the three keratins are present in somatic cells of the ovigerous cords during fetal life and in primordial follicles differentiating from 1-2 days postpartum. In the course of follicular development, K19 is no longer detected as primordial follicles differentiate into growing follicles. K18 and K18 are present in all stages of follicular development. These results show both differences and similarities with the results previously obtained in the rat. In the mouse, in contrast to the rat, keratins are detected in adult ovaries, and K18 is found in undifferentiated gonads and in ovaries. K18 is, thus, not specific to the testis in the mouse, as it is in the rat. In both species, K19 ceases to be expressed in male gonads as Sertoli cells differentiate and form seminiferous cords. The present observations confirm that downregulation of K19 gene expression in the fetal testis is one of the earliest molecular events attesting the commitment of the undifferentiated gonad to the male differentiative pathway.

epicardin: A novel basic helix-loop-helix transcription factor gene expressed in epicardium, branchial arch myoblasts, and mesenchyme of developing lung, gut, kidney, and gonads

We report the cloning, chromosomal localization, and analysis of the expression pattern of epicardin, a member of the basic helix-loop-helix (bHLH) family of transcription factors. Within its bHLH domain, the human and murine epicardin genes were most similar to paraxis, a bHLH gene important for segmentation of embryonic paraxial mesoderm. In situ hybridization studies revealed strong epicardin expression in murine embryos at 9.5 days postcoitum (dpc) in a region of the septum transversum at the base of the heart known as the proepicardial organ. This mesenchymal structure extends villous projections from which epicardial precursor cells emerge and migrate out over the surface of the myocardium. Strong expression was seen in individual migratory cells and clusters at 9.5 dpc and in a continuous epicardial cell layer in more mature hearts. Also from 9.5 dpc, epicardin transcripts were seen in endocardial cushions of the atrioventricular canal and outflow tract, in skeletal myoblasts within branchial arches and in condensing mesenchyme of gut, kidney, urinary tract, gonads, spleen, and lung. Northern analysis showed that expression persisted in mature visceral organs and heart, but was transient in skeletal muscle. The central role played by bHLH factors in pathways for tissue determination in the embryo suggests a function for epicardin in specification of select mesodermal cell populations associated with heart, cranial skeletal muscle, gut, and urogenital system.

Mammalian sex determination: joining pieces of the genetic puzzle

Mammalian sex determination is controlled by the Y-linked gene SRY. Studies of sex-reversed patients and experimental data obtained with mice have identified other genes, such as DAX1, SOX9, SF1, and WT1, which take part in the process, and have suggested how these genes interact to determine the sex of a mammalian embryo. A recent paper in Nature by Swain et al. provides experimental data that basically confirm the previously proposed hypothesis that SRY acts by inhibiting the action of DSS/DAX1, which is a repressor of genes of the male pathway.

Porcine steroidogenic factor-1 gene (pSF-1) expression and analysis of embryonic pig gonads during sexual differentiation

The porcine steroidogenic factor-1 gene (pSF-1) was cloned using a combination of genomic and RT-PCR based cloning methods. pSF-1 consists of an open reading frame of 1383 nt corresponding to a deduced amino acid sequence of 461 aa, similar to bovine and human SF-1. Sequence homologies between pSF-1 and human, bovine and mouse molecules indicate strong evolutionary conservation at both the nt and aa levels. Northern analysis of pSF-1 expression in adult steroidogenic tissues correlated with porcine steroidogenic acute regulatory protein gene (pStAR) and porcine side chain cleavage (pP450scc) gene expression. Notably, pSF-1 expression was readily detected in neonatal testes, absent at 3 weeks of age, and again readily detected at 3 months and in adult testes. pSF-1 expression was weak but detectable in placental tissues at various times of gestation, and was correlated with pStAR and pP450scc expression, indicating classical steroidogenesis in this organ. In developing gonads from 6-12 weeks of gestation, i.e. during the time of sex differentiation in the pig, Northern analysis demonstrated increasing expression of PSF-1 in fetal testes and no expression in ovaries. This expression pattern was paralleled for pStAR, pP450scc, and porcine Müllerian inhibitory substance (pMIS), consistent with pSF-1 involvement in both steroid and protein hormone secretions of the developing testes during sex differentiation. Porcine SRY HMG-box related gene-9 (pSOX-9) expression also paralleled that of pSF-1 in developing testes. In contrast, DSS-AHC critical region on the X chromosome, gene 1 (pDAX-1) was expressed predominantly in the developing ovaries, indicating a possible reciprocal regulation of pSF-1 and pDAX-1 genes in developing pig testes and ovaries.

Expression of the RNA component of telomerase during human development and differentiation

We used a radioactive in situ method to study expression of the RNA component of human telomerase (hTR) during normal human development and differentiation using archival tissues. In embryonic tissues, the highest and most uniform expression was present in undifferentiated neuroepithelium. Expression was stronger in immature epithelium than in accompanying immature mesenchyme. Differentiation of most tissues was accompanied by decreased or absent expression. Except for testis and adrenal, the adult pattern of expression was present by the 10th postnatal week. In adult tissues, high expression was present in the testis (primary spermatocytes and Sertoli cells), moderate expression was present in lymphoid follicles (germinal centers), and weak expression was present in epithelia (regenerative cells) but was absent in the nervous system and mesenchymal derived tissues. Expression in adult tissues was predominantly limited to dividing cells, although certain differentiated postmitotic cells expressed the hTR. Our studies demonstrate the complex interrelationship of hTR expression with human development, differentiation, and cell division.