Calcineurin-NFAT signaling critically regulates early lineage specification in mouse embryonic stem cells and embryos

Self-renewal and pluripotency are hallmarks of embryonic stem cells (ESCs). However, the signaling pathways that trigger their transition from self-renewal to differentiation remain elusive. Here, we report that calcineurin-NFAT signaling is both necessary and sufficient to switch ESCs from an undifferentiated state to lineage-specific cells and that the inhibition of this pathway can maintain long-term ESC self-renewal independent of leukemia inhibitory factor. Mechanistically, this pathway converges with the Erk1/2 pathway to regulate Src expression and promote the epithelial-mesenchymal transition (EMT), a process required for lineage specification in response to differentiation stimuli. Furthermore, calcineurin-NFAT signaling is activated when the earliest differentiation event occurs in mouse embryos, and its inhibition disrupts extraembryonic lineage development. Collectively, our results demonstrate that the NFAT and Erk1/2 cascades form a signaling switch for early lineage segregation in mouse ESCs and provide significant insights into the regulation of the balance between ESC self-renewal and early lineage specification.     

Differential regulation of G protein subunit expression in mouse oocytes, eggs, and early embryos

Pertussis toxin ADP-ribosylation and Western blot analysis using G protein-specific antibodies were used to study G protein expression in mouse oocytes, eggs, and early embryos. A pertussis toxin (PT) substrate of about 40 kDa was observed in all stages, but its level was stage dependent. It decreased dramatically between germinal vesicle stage oocytes and unfertilized eggs, remained relatively constant through the early 2-cell stage, and then declined again with each cell division, reaching the lowest level at the 8- to 16-cell stage. Its level, or perhaps that of a different substrate, then increased at the blastocyst stage. Western blot analysis with antisera to the G protein alpha subunit indicated that the decrease between germinal vesicle stage oocytes and unfertilized eggs was less pronounced for the alpha subunit itself than for the PT substrate. Antisera to G protein beta subunit revealed that the difference in the amount of this subunit in germinal vesicle-stage oocytes versus unfertilized eggs was even greater than that of the PT ADP-ribosylation substrate. These results suggest that during oocyte maturation G protein beta gamma levels decline to a greater extent than alpha levels. Additional evidence supporting this hypothesis was obtained by showing that addition of exogenous beta gamma to unfertilized egg preparations increased the amount of PT substrate. These results indicate that G protein subunit expression is differentially regulated during oocyte maturation.     

Differential expression of mouse Disabled 2 gene in retinoic acid-treated F9 embryonal carcinoma cells and early mouse embryos.

Using a differential display PCR, we identified a differentially expressed cDNA fragment which was detectable in retinoic acid (RA) treated F9 embryonal carcinoma (EC) cells but not in untreated F9 cells. A homology search of the Gene Bank indicated that the cDNA fragment is part of the mouse homolog of the Drosophila Disabled (mDab2) gene. Aggregate cultures of F9 EC cells grown in the presence of the RA differentiated into nonmalignant cells resembling the visceral endoderm of the mouse embryo. Upon induction of endodermal differentiation with 10(-7) M RA, the gene expression of mDab2 was increased gradually during the first 96 h. Neither undifferentiated F9 cells, nor the undifferentiated aggregate cells without RA expressed mDab2. Whole-mount in situ hybridization and quantitative RT-PCR also showed that the temporal expression pattern of the mDab2 gene coincides with the initiation pattern of RA synthesis that occurs during mouse embryogenesis. Also, two alternative splicing messages of mDab2 were detected in a tissue specific manner. All the data indicate that mDab2 may play an important role in RA-induced signal transduction during mouse development.     

Differentiation of X chromosomes in early female mouse embryos

The stalk segment of the heterotrich ciliate, Stentor coeruleus, appears nearly isotropic in the contracted state and develops a characteristic birefringence during extension. The birefringence occurs in stripes and is associated with the myonemes, one of the two longitudinally running subpellicular fiber systems. Electron microscopical investigations reveal changes in the ultrastructure of the myonemes from the extended to the contracted state. The relaxed myonemes consist mainly of 3 nm filaments running in the longitudinal direction, while the contracted myonemes show 10 nm tubular-like filaments, more randomly oriented. It is suggested that during elongation the randomly oriented tubular filaments undergo a conformational change to more regularly arranged thin filaments, thus causing development of birefringence.     

Desmin and titin expression in early postimplantation mouse embryos

The expression of the intermediate filament (IF) constituents desmin, vimentin and keratin, as well as the striated-muscle-specific marker titin, was studied in mouse embryos of 8.0 to 9.5 days post coitum (d.p.c.), using the indirect immunofluorescence technique in combination with polyclonal and monoclonal antibodies. During the development of the embryo, desmin was first detected at 8.25 d.p.c. in the ectoderm, where it was transiently coexpressed with keratin and vimentin. At later stages, the ectoderm contained only keratin and to a certain extent also vimentin IF. At 8.5 d.p.c., desmin was found exclusively in the heart rudiment, and remained present with increasing intensity in the myocardial cells during later cardiogenesis. Striation of desmin in the heart muscle cells was observed in 9.5 d.p.c. embryos. At these stages (8.5-9.5 d.p.c.), triple expression of the IF proteins desmin, vimentin and keratin was evident in these cells. From 9.0 d.p.c. onwards, desmin could be detected in the myotomes as well. Immunoblotting studies of 9.5 d.p.c. mouse embryos confirmed the immunohistochemical data. Titin was found in the early heart anlage at stage 8.25 d.p.c., when no desmin expression was observed in this tissue. At this stage the titin appeared in a punctate pattern, similar to that observed in cardiac myofibrils of early chicken embryos (Tokuyasu and Maher, 1987; J. Cell Biol. 105, 2781-2793). In 8.5 d.p.c. mouse embryos, this punctate titin staining pattern was still observed, while, at this stage, a filamentous staining reaction could be seen with the desmin antibodies. During further development, cross-striation was detected within myocardial cells using the polyclonal titin antibody from 9.0 d.p.c. onwards, i.e. before such striation could be detected with the desmin antibodies. From these data, we conclude that titin synthesis may anticipate desmin expression in the developing mouse myocard, although the level of expression of the former protein remains low until 9.0 d.p.c.     

SPARC synthesis in pre-implantation and early post-implantation mouse embryos

Summary SPARC (secreted protein acidic and rich in cysteine), also known as osteonectin and BM-40, is a secreted protein associated with a variety of embryonic and adult tissue and cell types, including placenta, parietal and visceral endoderm, certain epithelia (e.g. gut, skin, glandular epithelia), and regions of active chondrogenesis and osteogenesis. Although much is known concerning the tissue distribution of this protein, neither the time and location of its initial appearance nor its functions during embryogenesis have been clearly established. We identified the location of SPARC on two-dimensional protein gels. By using two-dimensional gel analysis of both pre- and post-implantation stage mouse embryos, we find that SPARC is initially synthesized between 3.5 and 4.5 days of embryogenesis. This is the earliest time during development at which synthesis of SPARC has been demonstrated. Inner cell masses isolated from 4.5 day blastocysts synthesize SPARC indicating that either primitive ectoderm, primitive endoderm, or both produce this protein. SPARC synthesis is also detectable in isolated trophoblast vesicles. Thus, SPARC is synthesized not only in placenta, parietal endoderm, and visceral endoderm, but in the precursors of these tissues as well. Examination of 7.5 day embryos reveals that SPARC is synthesized in isolated parietal yolk sac and in whole extraembryonic and embryonic regions. Relative to other proteins, synthesis of SPARC was most prevalent in the parietal yolk sac. The possible implications of SPARC synthesis as early as 4.5 days are discussed.     

Glucose transporter gene expression in early mouse embryos.

The glucose transporter (GLUT) isoforms responsible for glucose uptake in early mouse embryos have been identified. GLUT 1, the isoform present in nearly every tissue examined including adult brain and erythrocytes, is expressed throughout preimplantation development. GLUT 2, which is normally present in adult liver, kidney, intestine and pancreatic beta cells is expressed from the 8-cell stage onward. GLUT 4, an insulin-recruitable isoform, which is expressed in adult fat and muscle, is not expressed at any stage of preimplantation development or in early postimplantation stage embryos. Genetic mapping studies of glucose transporters in the mouse show that Glut-1 is located on chromosome 4, Glut-2 on chromosome 3, Glut-3 on chromosome 6, and Glut-4 on chromosome 11.