Germ layer formation during Xenopus embryogenesis: the balance between pluripotency and differentiation

The African clawed frog, Xenopus laevis, has long been a model animal for the studies in the fields of animal cloning, developmental biology, biochemistry, cell biology, and physiology. With the aid of Xenopus, major molecular mechanisms that are involved in embryonic development have been understood. Germ layer formation is the first event of embryonic cellular differentiation, which is induced by a few key maternal factors and subsequently by zygotic signals. Meanwhile, another type of signals, the pluripotency factors in ES cells, which maintain the undifferentiated state, are also present during early embryonic cells. In this review, the functions of the pluripotency factors during Xenopus germ layer formation and the regulatory relationship between the signals that promote differentiation and pluripotency factors are discussed.     

BAX-mediated cell death affects early germ cell loss and incidence of testicular teratomas in Dnd1 mice

A homozygous nonsense mutation (Ter) in murine Dnd1 (Dnd1^Ter/Ter) results in a significant early loss of primordial germ cells (PGCs) prior to colonization of the gonad in both sexes and all genetic backgrounds tested. The same mutation also leads to testicular teratomas only on the 129Sv/J background. Male mutants on other genetic backgrounds ultimately lose all PGCs with no incidence of teratoma formation. It is not clear how these PGCs are lost or what factors directly control the strain-specific phenotype variation. To determine the mechanism underlying early PGC loss we crossed Dnd1^Ter/Ter embryos to a Bax-null background and found that germ cells were partially rescued. Surprisingly, on a mixed genetic background, rescued male germ cells also generated fully developed teratomas at a high rate. Double-mutant females on a mixed background did not develop teratomas, but were fertile and produced viable off-spring. However, when Dnd1^Ter/Ter XX germ cells developed in a testicular environment they gave rise to the same neoplastic clusters as mutant XY germ cells in a testis. We conclude that BAX-mediated apoptosis plays a role in early germ cell loss and protects from testicular teratoma formation on a mixed genetic background.     

Germ cell differentiation and synaptonemal complex formation are disrupted in CPEB knockout mice.

CPEB is a sequence-specific RNA binding protein that regulates translation during vertebrate oocyte maturation. Adult female CPEB knockout mice contained vestigial ovaries that were devoid of oocytes; ovaries from mid-gestation embryos contained oocytes that were arrested at the pachytene stage. Male CPEB null mice also contained germ cells arrested at pachytene. The germ cells from the knockout mice harbored fragmented chromatin, suggesting a possible defect in homologous chromosome adhesion or synapsis. Two CPE-containing synaptonemal complex protein mRNAs, which interact with CPEB in vitro and in vivo, contained shortened poly(A) tails and mostly failed to sediment with polysomes in the null mice. Synaptonemal complexes were not detected in these animals. CPEB therefore controls germ cell differentiation by regulating the formation of the synaptonemal complex.     

RNA-binding proteins in pluripotency,differentiation, and reprogramming

Embryonic stem cell maintenance, differentiation, and somatic cell reprogramming require the interplay of multiple pluripotency factors, epigenetic remodelers, and extracellular signaling pathways. RNA-binding proteins （RBPs） are involved in a wide range of regulatory pathways, from RNA metabolism to epigenetic modifications. In recent years we have witnessed more and more studies on the discovery of new RBPs and the assessment of their functions in a variety of biological systems, including stem cells. We review the current studies on RBPs and focus on those that have functional implications in pluripotency, differentiation, and/or reprogramming in both the human and mouse systems.     

Innervation and maturation of muscular tissue in testicular teratomas in strain 129/Sv-ter mice

In strain 129/Sv-ter mice, teratomas develop spontaneously during the 13th day of gestation. These testicular germ cell tumors exhibit characteristics of different germ layers closely resembling normal embryonic tissue. We investigated the interrelationship between nervous and muscular tissues (often found side by side) in teratomas of 4-week-old 129/Sv-ter mice. In well-differentiated mouse teratomas, histochemically and immunohistochemically distinct muscle fiber types could be distinguished, but not with all reactions. According to its aerobic oxidative capacity, teratoma muscle tissue was comparable with normal muscles. However, with respect to myosin-related properties, fiber type differentiation was incomplete. The muscle fibers - generally arranged in bundles - contained one centrally located endplate which was contacted mostly by a single nerve terminal. From this, proper endplate zones within the fiber bundles were formed. Occasionally "type grouping" was encountered, suggesting collateral axonal branching paralleled by synapse elimination. Together with the earlier in vivo observation of muscular contractions, we assume that teratoma muscle fibers are innervated by nerve cells (within the nervous tissue compartments) corresponding to spinal motoneurons. Thus, myogenesis, maturation and innervation of skeletal muscular tissue in mouse teratomas are largely comparable to normal development.     

Critical roles of hMAGEA2 in induced pluripotent stem cell pluripotency,proliferation, and differentiation

Induced pluripotent stem (iPS) cells are important for clinical application and stem cell research. Although human melanoma‐associated antigen A2 (hMAGEA2) expression is known to affect differentiation in embryonic stem cells, its specific role in iPS cells remains unclear. To evaluate the function of hMAGEA2 and its characteristics in iPS cells, we produced hMAGEA2‐overexpressing iPS cells from hMAGEA2‐overexpressing transgenic mice. Although the iPS cells with overexpressed hMAGEA2 did not differ in morphology, their pluripotency, and self‐renewal related genes (Nanog, Oct3/4, Sox2, and Stat3), expression level was significantly upregulated. Moreover, hMAGEA2 contributed to the promotion of cell cycle progression, thereby accelerating cell proliferation. Through embryoid body formation in vitro and teratoma formation in vivo, we demonstrated that hMAGEA2 critically decreases the differentiation ability of iPS cells. These data indicate that hMAGEA2 intensifies the self‐renewal, pluripotency, and degree of proliferation of iPS cells, while significantly repressing their differentiation efficiency. Therefore, our findings prove that hMAGEA2 plays key roles in iPS cells.     

Genetic susceptibility to testicular autoimmunity: comparison between postthymectomy and postvasectomy models in mice

The genetic susceptibility was compared in six mouse strains to both postthymectomy (post-Tx) autoimmune orchitis, induced by thymectomy at Day 3, and postvasectomy (post-Vx) autoimmune testicular lesions, induced by bilateral vasectomy with ligation at 2 months of age. Results from mice examined at 4 months of age showed that the strain distribution of the post-Vx testicular lesion was similar to that of post-Tx orchitis. The SWR/J strain was susceptible, the DBA/1J, C57L/J, BALB/cJ and probably C57BL/10J strains were resistant, and the A/J strain was intermediate. Anti-sperm antibodies (IgG) were produced to varying degrees in all strains of vasectomized mice irrespective of the testicular lesion. Neither the development of the testicular lesion nor antibody formation was associated with the particular H-2 haplotypes of mice in either system. Neonatal thymectomy significantly enhanced anti-sperm antibody formation in the post-Vx system and resulted in a mild testicular atrophy in a resistant strain BALB/cBy. All these data suggest that a gene(s) outside the major histocompatibility complex may determine the genetic susceptibility to testicular autoimmunity, possibly at the testicular level.     

Effect of the W mutation, for white belly spot, on testicular germ cell differentiation in mice.

The effect of the mutation for white belly spot controlled by the dominant gene W on spermatogenesis in mice was examined by experimental cryptorchidism and its surgical reversal. The course of spermatogenesis from spermatogonia to spermatid was normal in intact testes of W/+ mice. In cryptorchid testes, there was no difference in the number and activity of Type A spermatogonia between the testes of W/+ and +/+ mice, in mitotic and labelling indices. Although surgical reversal of the cryptorchid testis resulted in regenerative differentiation of germ cells in both genotypes, the recovery of cell differentiation in the W/+ testis was slower than in the +/+ testis. There were fewer germ cells, such as intermediate-Type B spermatogonia or more advanced ones, in W/+ testes. On Day 17 after surgical reversal, cell associations in W/+ testes were abnormal and the numbers of intermediate-Type B spermatogonia, spermatocytes and spermatids were approximately 70, 50 and 15%, respectively, of those in +/+ testes. These results indicate that the W gene affects spermatogenic cell differentiation in adult mice.     

Enhancers and suppressors of testicular cancer susceptibility in single- and double-mutant mice

Susceptibility to spontaneous testicular germ cell tumors (TGCTs), a common cancer affecting young men, shows unusual genetic complexity. Despite remarkable progress in the genetics analysis of susceptibility to many cancers, TGCT susceptibility genes have not yet been identified. Various mutations that are inherited as Mendelian traits in laboratory mice affect susceptibility to spontaneous TGCTs on the 129/Sv inbred genetic background. We compared the frequency of spontaneous TGCTs in single- and double-mutant mice to identify combinations that show evidence of enhancer or suppressor effects. The lower-than-expected TGCT frequencies in mice with partial deficiencies of TRP53 and MGF-SLJ and in 129.MOLF-Chr19 (M19) consomic mice that were heterozygous for the A(y) mutation suggest that either these genes complement each other to restore normal functionality in TGCT stem cells or together these genes activate mechanisms that suppress incipient TGCTs. By contrast, the higher-than-expected TGCT frequencies in Mgf(Sl-J)-M19 compound heterozygous mice suggest that these mutations exacerbate each other's effects. Together, these results provide clues to the genetic and molecular basis for susceptibility to TGCTs in mice and perhaps in humans.     

Extracellular matrix regulates Sertoli cell differentiation, testicular cord formation, and germ cell development in vitro

Sertoli cell preparations isolated from 10-day-old rats were cultured on three different substrates: plastic, a matrix deposited by co-culture of Sertoli and peritubular myoid cells, and a reconstituted basement membrane gel from the EHS tumor. When grown on plastic, Sertoli cells formed a squamous monolayer that did not retain contaminating germ cells. Grown on the matrix deposited by Sertoli-myoid cell co-cultures, Sertoli cells were more cuboidal and supported some germ cells but did not allow them to differentiate. After 3 wk however, the Sertoli cells flattened to resemble those grown on plastic. In contrast, the Sertoli cells grown on top of the reconstituted basement membrane formed polarized monolayers virtually identical to Sertoli cells in vivo. They were columnar with an elaborate cytoskeleton. In addition, they had characteristic basally located tight junctions and maintained germ cells for at least 5 wk in the basal aspect of the monolayer. However, germ cells did not differentiate. Total protein, androgen binding protein, transferrin, and type I collagen secretion were markedly greater when Sertoli cells were grown on the extracellular matrices than when they were grown on plastic. When Sertoli cells were cultured within rather than on top of reconstituted basement membrane gels they reorganized into cords. After one week, tight junctional complexes formed between adjacent Sertoli cells, functionally compartmentalizing the cords into central (adluminal) and peripheral (basal) compartments. Germ cells within the cords continued to differentiate. Thus, Sertoli cells cultured on top of extracellular matrix components assume a phenotype and morphology more characteristic of the in vivo, differentiated cells. Growing Sertoli cells within reconstituted basement membrane gels induces a morphogenesis of the cells into cords, which closely resemble the organ from which the cells were dissociated and which provide an environment permissive for germ cell differentiation.