Licensing of Primordial Germ Cells for Gametogenesis Depends on Genital Ridge Signaling

In mouse embryos at mid-gestation, primordial germ cells (PGCs) undergo licensing to become gametogenesis-competent cells (GCCs), gaining the capacity for meiotic initiation and sexual differentiation. GCCs then initiate either oogenesis or spermatogenesis in response to gonadal cues. Germ cell licensing has been considered to be a cell-autonomous and gonad-independent event, based on observations that some PGCs, having migrated not to the gonad but to the adrenal gland, nonetheless enter meiosis in a time frame parallel to ovarian germ cells -- and do so regardless of the sex of the embryo. Here we test the hypothesis that germ cell licensing is cell-autonomous by examining the fate of PGCs in Gata4 conditional mutant (Gata4 cKO) mouse embryos. Gata4, which is expressed only in somatic cells, is known to be required for genital ridge initiation. PGCs in Gata4 cKO mutants migrated to the area where the genital ridge, the precursor of the gonad, would ordinarily be formed. However, these germ cells did not undergo licensing and instead retained characteristics of PGCs. Our results indicate that licensing is not purely cell-autonomous but is induced by the somatic genital ridge.     

Early Morphogenesis of Chick Gonad in the Absence of Mesonephros

The development of the gonads in male and female chick embryos with induced unilateral mesonephric agenesis was studied using grafting, histoenzymology, and electron microscopy. As in embryos with a mesonephros, proliferation of the coelomic epithelium and its interaction with mesenchymal cells to form the medullary cords take place in the amesonephric gonads. In a similar manner, gonadal sexual differentiation and the differentiation of steroidogenic tissue, detectable by the presence of Δ5-3β-hydroxysteroid dehydrogenase, do not appear to be affected by the absence of an organized mesonephros. However, the initiation of gonadal development, further growth, and the onset of meiosis observable in developing ovaries are retarded. This delay appears to be reversible, as was demonstrated by experiments in which ovaries from chicks with complete mesonephric agenesis were transplanted into the coelomic cavity of male and female 3 1/2-day-old embryos. Meiosis finally occurred in the oocytes of all ovaries, regardless of the sex of the host. Therefore, the presence of a differentiated mesonephros in chick embryos is not required for the establishment of an undifferentiated gonad and sexual differentiation, or for initiation of meiosis.     

Beta-Actin Is Involved in Modulating Erythropoiesis during Development by Fine-Tuning Gata2 Expression Levels

The functions of actin family members during development are poorly understood. To investigate the role of beta-actin in mammalian development, a beta-actin knockout mouse model was used. Homozygous beta-actin knockout mice are lethal at embryonic day (E)10.5. At E10.25 beta-actin knockout embryos are growth retarded and display a pale yolk sac and embryo proper that is suggestive of altered erythropoiesis. Here we report that lack of beta-actin resulted in a block of primitive and definitive hematopoietic development. Reduced levels of Gata2, were associated to this phenotype. Consistently, ChIP analysis revealed multiple binding sites for beta-actin in the Gata2 promoter. Gata2 mRNA levels were almost completely rescued by expression of an erythroid lineage restricted ROSA26-promotor based GATA2 transgene. As a result, erythroid differentiation was restored and the knockout embryos showed significant improvement in yolk sac and embryo vascularization. These results provide new molecular insights for a novel function of beta-actin in erythropoiesis by modulating the expression levels of Gata2 in vivo.     

St?rungen der m?nnlichen Gonadendifferenzierung

XY gonadal dysgenesis is characterized by a failure of testis differentiation and can be caused either by disturbed development of the urogenital ridge to the bipotential gonad or by impaired differentiation of the bipotential gonad to testis. Genes responsible for early gonadal development like WT1 and SF1 can be distinguished from genes involved in testis differentiation such as SRY, SOX9, DMRT, DAX1, WNT4, DHH, CBX2, TSPYL1, ATRX and ARX. In complete XY gonadal dysgenesis, M??llerian but no Wolffian structures are present. In partial XY gonadal dysgenesis, remnants of M??llerian and Wolffian ducts can be present and virilization of the external genitalia can take place. In about a third of cases, XY gonadal dysgenesis occurs in a syndromic form. In these syndromic forms, the extragenital phenotypes can indicate the causative genes, but these genes can also cause non-syndromic forms of XY gonadal dysgenesis.     

Cell aggregation precedes the onset of Sox9-expressing preSertoli cells in the genital ridge of mouse

SOX9 is expressed at the onset of the genital ridge formation in both sexes. It is assumed that SRY, the testis determining gene, turns SOX9 on in male embryos because it is turned off in female embryos. Spatial expression of SRY follows a cranio-caudal pattern. Here, we asked if SOX9 is expressed in the same cell lineage and with a similar pattern as SRY. A correlative study between the structural changes in the genital ridge and the immunocytochemical localization of SOX9-positive cells was undertaken. We used a transgenic strain expressing the green fluorescent protein (GFP) that considerably enhanced the cell context where the first SOX9-positive cells appear. Although SOX9-positive cells are located among loose mesenchymal cells by stages of 8-14 tail somites (ts) in both sexes, they are absent in the thickening coelomic epithelium of females. At 15 ts the first SOX9-positive cells appear within the core of the condensed cells only in male genital ridges. At 17 ts, a gradient of SOX9-positive cells in males is apparent, closely following the cranio-caudal pattern of cell aggregation seen in genital ridges of both sexes. Hence, our results suggest that SOX9 is expressed only in loose mesenchymal cells in both sexes and that expression of SOX9 in males requires the prior aggregation of cells in the genital ridges. The correspondence of SOX9 and SRY pattern of expression supports that both genes are expressed in the preSertoli cell lineage in the core of the genital ridges.     

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.     

GATA4 represses an ileal program of gene expression in the proximal small intestine by inhibiting the acetylation of histone H3, lysine 27

GATA4 is expressed in the proximal 85% of small intestine where it promotes a proximal intestinal (‘jejunal’) identity while repressing a distal intestinal (‘ileal’) identity, but its molecular mechanisms are unclear. Here, we tested the hypothesis that GATA4 promotes a jejunal versus ileal identity in mouse intestine by directly activating and repressing specific subsets of absorptive enterocyte genes by modulating the acetylation of histone H3, lysine 27 (H3K27), a mark of active chromatin, at sites of GATA4 occupancy. Global analysis of mouse jejunal epithelium showed a statistically significant association of GATA4 occupancy with GATA4-regulated genes. Occupancy was equally distributed between down- and up-regulated targets, and occupancy sites showed a dichotomy of unique motif over-representation at down- versus up-regulated genes. H3K27ac enrichment at GATA4-binding loci that mapped to down-regulated genes (activation targets) was elevated, changed little upon conditional Gata4 deletion, and was similar to control ileum, whereas H3K27ac enrichment at GATA4-binding loci that mapped to up-regulated genes (repression targets) was depleted, increased upon conditional Gata4 deletion, and approached H3K27ac enrichment in wild-type control ileum. These data support the hypothesis that GATA4 both activates and represses intestinal genes, and show that GATA4 represses an ileal program of gene expression in the proximal small intestine by inhibiting the acetylation of H3K27.