Inhibition of DNA replication in preimplantation mouse embryos by aphidicolin

The regulation of trophectoderm differentiation in mouse embryos was studied by inhibiting DNA synthesis with aphidicolin, a specific inhibitor of DNA polymerase alpha. Embryos were exposed to aphidicolin (0.5 micrograms/ml) for 16 h at various preimplantation stages and scored for their ability to form a blastocyst and develop beyond the blastocyst stage. Embryos were most sensitive to aphidicolin at the late 4-cell stage and became progressively less sensitive as they developed. Aphidicolin inhibited blastocyst formation by 70%, 100%, 77%, and 24% after treatment at the 2-cell, 4-cell, noncompacted 8-cell, and compacted 8-cell stages, respectively. Although the inhibitory effect of aphidicolin on blastocyst formation decreased markedly as 8-cell embryos underwent compaction, developmental capacity beyond the blastocyst stage was poor after treatment of either noncompacted or compacted 8-cell embryos. Treatment at the morula and early blastocyst stages was less harmful to embryos than treatment at earlier stages but reduced the number of trophoblast outgrowths by interfering with hatching. Autoradiographic analysis showed that during aphidicolin treatment, incorporation of 3H-thymidine was inhibited over 90% at all stages examined, indicating an inhibition of DNA synthesis. Because inhibition of blastocyst formation by aphidicolin decreased at the compacted 8-cell stage, we suggest that approximately the first half of the fourth DNA replication cycle is critical for subsequent blastocyst formation. Furthermore, the poor further development of blastocysts formed after aphidicolin treatment of compacted 8-cell embryos suggests that the DNA replication requirements for initial trophectoderm differentiation are distinct from requirements for further development of blastocysts in vitro.     

The Effect of α-Amanitin on Nucleic Acid Synthesis in Preimplantation Mouse Embryos

It has been hypothesized that multiple forms of RNA polymerase may play a role in the control of development and differentiation in eukaryotic organisms. For this to be true, three criteria must be met. First, multiple forms of RNA polymerase must be demonstrated. Second, the relative proportion of the enzyme forms must be shown to change with development or differentiation. And third, the types of RNA synthesized must correlate with the types of RNA polymerase present at each developmental stage. We have previously reported data satisfying the first two criteria for preimplantation mouse embryos. The present paper probes the third criterion in this differentiating system.
It was found that although the proportion of the RNA polymerase enzyme forms changes from the 8-cell to the blastocyst stage of development, the types of newly synthesized nucleic acids at each of these stages were similar. Furthermore, inhibition of rRNA, mRNA, and tRNA, by α-amanitin, was identical for 8-cell and blastocyst embryos. The only difference between these two stages was that DNA synthesis in blastocysts was more sensitive to inhibition by α-amanitin than DNA synthesis in 8-cell embryos. We conclude that the synthesis of different classes of RNA by preimplantation mouse embryos is not simply controlled by changes in the levels of the multiple forms of RNA polymerase.     

Variations in the amounts of RNA polymerase forms I, II and III during preimplantation development in the mouse.

DNA-dependent RNA polymerase has been measured at various stages of preimplantation development in mouse embryos. The total RNA polymerase activity per embryo increases rapidly from the 8-cell stage to the blastocyst stage. Studies with low α-amanitin concentrations, which inhibit form II RNA polymerase, and high α-amanitin concentrations, which inhibit both form II and III RNA polymerases indicate that the relative proportions of the three forms change significantly during preimplantation development. The changes which occur in the types and levels of RNA polymerase appear to parallel corresponding changes in the synthesis of the major classes of RNA.     

Early DNA synthesis during the germination of wheat embryos

Both [³H]thymidine and [³H]deoxyadenosine were found to be incorporated into the nuclear DNA of wheat embryos immediately after dry embryos were allowed to imbibe aqueous solutions of the radioactive precursors. The early labelled DNA sedimented in a manner suggesting that replicative intermediates were already formed within the first 90 min of germination. However, aphidicolin remained without any effect on this early DNA synthesis. Likewise, a cell-free system derived from early embryos incorporated [³H]dCTP into DNA independently of the presence of aphidicolin. On the contrary, dideoxyTTP inhibited the DNA synthesis considerably. It is concluded that a proportion of the resting wheat embryo cells is able to initiate a replicative DNA synthesis immediately upon imbibition. The synthesis seems, however, to proceed with the participation of a γ-like, rather than an α-like, DNA polymerase.     

Control of protein synthesis during blastocyst formation in the mouse.

In order to evaluate the dependence of the embryo on new mRNA synthesis during the period leading to blastulation, quantitative and qualitative aspects of protein synthesis in developing mouse morulae were investigated using α-amanitin, an inhibitor of RNA polymerase II. Only 1 of 423 early morulae cultured for 27 hr in the presence of 11 μg/ml α-amanitin cavitated, although most progressed as far as fully compacted morulae. About two-thirds of the untreated embryos cavitated during the same period. Incorporation of [³⁵S]methionine into protein was measured at 3- or 4-hr intervals over a 24-hr period and showed a two- to fivefold increase in control embryos. This increase was blocked in the α-amanitin-treated group although initial levels of incorporation were maintained. Total uptake of the amino acid appeared to be unaffected by the inhibitor. RNA synthesis, as measured by [³H]uridine incorporation over the same period, was reduced by between 5 and 52%, and the preblastulation surge in RNA synthesis was also blocked by α-amanitin. Two-dimensional polyacrylamide gel electrophoresis of labeled polypeptides synthesized by the embryos after 24-hr incubation in the presence or absence of the inhibitor revealed three distinct classes of polypeptide. The majority of polypeptides continued to be synthesized in the presence of α-amanitin whereas a small number of polypeptides, the synthesis of which would normally have increased during the development of the morula to the blastocyst, were prevented from doing so. A few polypeptides which normally cease to be synthesized over this period continued to be synthesized in the presence of α-amanitin. It is concluded that, while most of the proteins detectable at the morula stage are synthesized on mRNA templates of relatively long translational life, the general surge in protein synthesis, including the increased synthesis of a few species of polypeptide, are dependent on continuous translational activity.     

Effects of amino acids and α-amanitin on bovine embryo development in a simple protein-Free medium

Five experiments, utilizing 3741 embryos produced in vitro, were designed to test the effects of Eagle's nonessential amino acids, and combinations of Eagle's essential amino acids and the RNA polymerase inhibitor α-amanitin on the development of preimplantation bovine embryos in a modified protein-free KSOM medium. Embryos were cultured in 5% O₂:5% CO₂:90% N₂ at 39°C for the first 40–44 hr in modified KSOM, and embryos with ≥4 cells were cultured in modified KSOM-PVA with different amino acids in experiments 1–4, and with the addition of α-amanitin in experiment 5. In experiment 1, addition of 0.5× of the essential amino acids, with different concentrations of nonessential amino acids significantly increased hatching of blastocysts and decreased blastocyst degeneration, but increasing the nonessential amino acids from 1× to 5×, did not stimulate embryo development. In experiments 2–4, increasing only the glycine concentration, or adding each of the 12 essential amino acids singly or several in combination to the medium containing nonessential amino acids, did not significantly improve embryo development. Taurine (0.4 mM) in the modified KSOM medium reduced blastocyst degeneration. In experiment 5, α-amanitin (20 μM) completely inhibited further embryo development when it was added at several stages from 4-cell embryos to morulae. The study with protein-free KSOM plus amino acids provided a completely defined simple medium for culturing bovine embryos, with evidence that continuous mRNA activity and presumed protein synthesis was obligatory to meet the complex and continuous requirements for proteins by the developing blastocyst. Mol. Reprod. Dev. 46:278–285, 1997. © 1997 Wiley-Liss, Inc.     

Phorbol ester-mediated modulation of cell proliferation and primary differentiation of mouse preimplantation embryos

The tumour promoter, phorbol myristate acetate (PMA) at concentrations of 5–50 ng/ml substantially affected 2-, 4-, 8-cell and morula mouse embryos cultured in vitro. PMA evoked a delay of cell growth and caused premature cell differentiation. In the former there was a formation of binuclear blastomeres, in the latter of giant cell formation in trophectoderm of blastocyst and premature cavitation. PMA-mediated delay of growth rate was completely reversible in 8-cell embryos, partially reversible in 4-cell embryos and poorly reversible, if at all, in 2-cell embryos. In the presence of PMA, nuclear DNA synthesis proceeded although the rate of nuclear labelling with [³H]thymidine was lower than in the control. Blastomeres of some 2-cell embryos treated with PMA fused, resulting in the formation of 1-cell embryos.     

Synchronization of cell division in eight-cell bovine embryos produced in vitro: effects of aphidicolin

To date, methods for synchronizing the cell division of ungulate embryos without reducing their developmental potential have not been reliable or simple. The overall objective of this study was to determine the reliability of aphidicolin, a powerful inhibitor of eukaryotic DNA synthesis, to arrest and synchronize blastomere division in cleavage-stage bovine embryos and to assess its reversibility and toxicity in vitro. Eight-cell stage embryos obtained at 58 h post insemination were treated with several concentrations of aphidicolin for 12 h. Treated embryos were assessed for cleavage arrest, chromatin morphology and DNA synthesis; scored for blastocyst formation and hatching rate; and fixed for determination of the number of nuclei. Complete arrest of cell division was observed at aphidicolin concentrations of 1.4 microM and above. At these concentrations, no morphological alteration to interphase chromatin was observed in treated embryos compared with the controls. Removal of aphidicolin led to at least a 4-h delay before resumption of DNA synthesis and cleavage. The ability of treated embryos to reach the blastocyst stage in vitro, the hatching rate and the number of cells per blastocyst were significantly reduced compared with the control group. Since the ability of treated embryos to develop to the blastocyst stage was significantly reduced even at the minimal effective dosage, it is concluded that aphidicolin is unlikely to provide suitable cell cycle synchronization without damage to the embryos.     

Preimplantation embryo development in the mouse: Role of histidine decarboxylase

The present study determines the effect of a specific and an irreversible inhibitor of histidine decarboxylase (HDC), α-fluoromethylhistidine (α-FMH) on the mouse preimplantation embryo development in vitro. The embryo culture technique was used to assess the effect of α-FMH. Embryos recovered at 0800–0900 hr (AM) on day 3 of pregnancy were 4–8 cells, whereas those recovered at 1600–1630 hr were mostly 8-cell compacted embryos. Of the day 3-AM embryos, 81.3 ± 4.3% developed to blastocysts within 48 hr when cultured in the medium alone, but addition of α-FMH (0.19 or 0.38 mM) drastically reduced the blastocyst formation to 26.6 ± 7 or 16.8 ± 4.3%. Most of them were arrested before the compaction stage. Addition of L-histidine, the substrate for HDC, did not alter the inhibition of blastocyst formation in the presence of α-FMH (37.2 ± 10.9%). Of the day 3-PM embryos, 99.3 ± 0.7% developed to blastocyst stage when cultured in the medium alone and addition of α-FMH (0.19 or 0.38 mM) did not affect the embryo development (92.1 ± 4.3 or 81.9 ± 9.9% developed to blastocysts). The birth of healthy young following transfer of these blastocysts into pseudopregnant mice indicates normal development of the embryos under this condition. The results suggest that histamine synthesis may be required for the process of compaction and thus the formation of blastocyst.     

A comparative study of mouse liver and mouse blastocyst DNA-dependent RNA polymerases.

DNA-dependent RNA polymerase has been studied in adult mouse liver and mouse blastocysts. The enzyme from mouse liver was resolved into three enzyme forms by DEAE-Sephadex chromatography. Two of the forms, IA and IB, are insensitive to α-amanitin, have low $\text{Mn}{}^{\mathrm{2+}}\text{Mg}{}^{\mathrm{2+}}$ activity ratios, and are optimally active at low ionic strength. Form II is inhibited by α-amanitin, has a higher $\text{Mn}{}^{\mathrm{2+}}\text{Mg}{}^{\mathrm{2+}}$ activity ratio, and is most active at high ionic strength. An optimal reaction temperature of 37 ° C was found for all enzyme forms. All of the isolated enzyme forms are inhibited by the exotoxin from Bacillus thuringiensis and the inhibition can be partially reversed by increased ATP levels. Forms IA and IB are most active with native template while form II prefers denatured DNA.The blastocyst RNA polymerase activity exhibits similar requirements for divalent metal ions and ionic strength to the purified liver enzymes. The maximum inhibition of blastocyst RNA polymerase obtained with α-amanitin and exotoxin differs from that observed for purified liver enzymes but is similar to the inhibition of liver homogenate. However, the concentrations of inhibitor required for maximum inhibition by α-amanitin and exotoxin is different for the blastocyst and liver homogenate enzymes.     

Characterization of DNA synthesis during the 2-cell stage and the production of tetraploid chimeric pig embryos

The DNA content of nuclei during the 2-cell stage as well as in presumptive tetraploid embryos was investigated. In vivo produced pig zygotes were cultured to the 2-cell stage and either monitored for cleavage to the 4-cell stage or mounted at various times postcleavage and DNA content determined. The length of the 2-cell stage was 14.8 ± 3.0 hr. There was a significant increase in the length of the 2-cell stage due to the time in vitro as a zygote (P < 0.001: R² = 0.866). The DNA content increased (P < 0.05) each 2 hr postcleavage until 10 hr postcleavage. This suggested that there is a short G₁ and G₂ phase and a relatively long phase of DNA synthesis. Next, 2-cell stage embryos were pulsed with electricity to induce cell-to-cell fusion. Whereas only about half fused within 30 min (55%), most (96%) developed to the blastocyst stage. The DNA content of the nuclei of the embryos was consistent with them being tetraploid. A final experiment was designed to evaluate the ability of the tetraploid embryo to form a chimera with isolated inner cell mass (ICM) cells. Inner cell masses were isolated from d 6 embryos, cut into thirds, labeled with DiO (a membrane die) and injected into the perivitelline space of 4-cell-stage tetraploid embryos. Twelve of 17 formed blastocysts. In most 8/12), the ICM of the resulting blastocyst was labeled, whereas in one the only fluorescence was in the trophectoderm, and in two fluorescence was evenly distributed between the ICM and trophectoderm. These results suggest that it may be possible to create a fetus derived from ICM cells, or potentially stem cells, that has a tetraploid trophoblast. © 1996 Wiley-Liss, Inc.     

Nuclear protein synthesis in regenerating rat liver. Selective histone inhibition by alpha-amanitin

Nuclear protein synthesis has been studied in regenerating rat hepatocytes after partial hepatectomy and α-amanitin treatment. The toxin induced a marked and precocious inhibition of histone synthesis without affecting the acidic nuclear proteins. This inhibition preceded the inhibition of DNA synthesis. The modification of polyribosome profile and of [¹⁴C]lysine incorporation on synthesized polypeptides were consistent with a reduction of specific mRNAs.     

Effect of amino acids and α-amanitin on the development of rabbit embryos in modified protein-free KSOM with HEPES

Four experiments were conducted to test the effects of Eagle's non-essential amino acids (NEAA) and essential amino acids (EAA), glycine, and the RNA polymerase inhibitor α-amanitin, on the development of preimplantation rabbit embryos in modified protein-free KSOM medium. Embryos were distributed randomly into different treatments and cultured in 5% O₂:5% CO₂:90% N₂. In experiment 1, 100% of the embryos became blastocysts in the medium with Eagle's IX NEAA and 0.5X EAA, but 100% stopped development at the morula stage in KSOM without amino acids. These morulae failed to develop further when transferred to amino acid supplemented medium after 72 hr of culture. Glycine alone in modified KSOM (experiment 2) was ineffective in supporting development of 8–16-cell stage embryos past the morula stage. In experiment 3, the addition of IX NEAA and 0.5X EAA at 0, 12, 24, 36, and 48 hr of culture resulted, respectively, in 57, 65, 65, 44, and 14% blastocysts on Day 3 (P<0.05) and 86, 77, 77, 78, and 69% on Day 5 (P<0.05). Omission of Eagle's amino acids until 48 hr clearly delayed embryo development. In experiment 4, when α-amanitin (20 μM) was added to the medium containing Eagle's amino acids after 0, 12, 24, 36, and 48 hr of culture most embryos cleaved only once or twice after adding the α-amanitin. Without the inhibitor, 94% of the zygotes developed into blastocysts. These results indicate that modified KSOM or KSOM plus glycine could not support rabbit embryo development past the morula stage, but this block was overcome by adding Eagle's amino acids. An exogenous source of amino acids was not critical for embryo development during the first 24 hr of culture, but was required after that for development to equal controls. Addition of α-amanitin at multiple pre-blastocyst stages limited further embryo development to one or two cleavage divisions, with no blastocyst development. © 1996 Wiley-Liss, Inc.