THE IMMEDIACY OF GENOMIC CONTROL DURING EARLY DEVELOPMENT

Bisphenol A directly targets tubulin to disrupt spindle organization in embryonic and somatic cells

There is increasing concern that animal and human reproduction may be adversely affected by exposure to xenoestrogens that activate estrogen receptors. There is evidence that one such compound, Bisphenol A (BPA), also induces meiotic and mitotic aneuploidy, suggesting that these kinds of molecules may also have effects on cell division. In an effort to understand how Bisphenol A might disrupt cell division, a phenotypic analysis was carried out using sea urchin eggs, whose early embryonic divisions are independent of zygotic transcription. Fertilized Lytechinus pictus eggs exposed to BPA formed multipolar spindles resulting in failed cytokinesis in a dose-dependent, transcriptionally independent manner. By use of novel biotinylated BPA affinity probes to fractionate cell-free extracts, tubulin was identified as a candidate binding protein by mass spectrometry, and BPA promoted microtubule polymerization and centrosome-based microtubule nucleation in vitro but did not appear to display microtubule-stabilizing activity. Treatment of mammalian cells demonstrated that BPA as well as a series of Bisphenol A derivatives induced ectopic spindle pole formation in the absence of centrosome overduplication. Together, these results suggest a novel mechanism by which Bisphenol A affects the nucleation of microtubules, disrupting the tight spatial control associated with normal chromosome segregation, resulting in aneuploidy.

The reproduction of centrosomes: nuclear versus cytoplasmic controls

The tight coordination normally found between nuclear events and the doubling of centrosomes at each cell cycle suggests that nuclear activities may be part of the mechanism that controls the reproduction of centrosomes. To determine if this is the case, we used a micropipette to completely remove the nucleus from eggs of the sea urchin Lytechinus variegatus at prophase of the first mitosis, leaving only one centrosome in the cell. The subsequent behavior of this centrosome was then followed in vivo with the polarization microscope. In all cases the centrosome reproduced in a precise 1:2:4:8 fashion with a periodicity that was slightly slower than the centrosome cycle of control eggs. The cell cycle-related changes in centrosome morphology were identical to those of control eggs in that: (a) the astral birefringence varied cyclically to a normal extent, (b) the astral focus enlarged and then flattened during the telophase equivalent, (c) cleavage furrows were initiated as the astral birefringence faded, and (d) daughter centrosomes separated before the increase in astral birefringence at the onset of each mitosis. To determine if centrioles also reproduced normally, enucleate eggs were followed in vivo until they contained eight centrosomes. They were then individually removed from the preparations, fixed, and embedded. Each egg was serially 0.25-micron sectioned for observation with the high voltage electron microscope. We completely reconstructed 23 centrosomes in four eggs; all centrosomes contained two centrioles apiece. These results demonstrate that the subunits for complete centrosome assembly can be stockpiled ahead of time and that the properly controlled use of these subunits for centrosome reproduction does not require nuclear transcription or nuclear DNA synthesis at each cell cycle.

[Autoradiographic Studies on RNA and Protein Synthesis in the Early Embryogenesis ofMusca domestica]

1) ³H-Uridine incorporation into the RNA of eggs ofMusca domestica was measured by means of autoradiography. In addition, protein synthesis was studied by measuring incorporation of³H-amino acids. 2) The onset of embryonic RNA synthesis was found to occur after the 10. division of the cleavage nuclei. 3) Protein synthesis occurs in mature oocytes as well as in freshly laid eggs. The rate of synthesis in fertilized eggs begins to increase when the cleavage nuclei penetrate the periplasma. At the end of blastodermal development, the rate of protein synthesis reaches a value which remains constant until differentiation of the germ layers. Immediately after deposition of the eggs, the rate of incorporation of³H-amino acids is higher in unfertilized eggs than in fertilized ones. 4) At oviposition the radioactivity is uniformly distributed over the whole egg. During further development more radioactivity is accumulated in the periplasm than in the inner part of the egg. Already before penetration of the cleavage nuclei, a higher rate of protein synthesis is observed in the posterior pole-plasm than in any other part of the periplasm. 5) Immediately before the beginning of organogenesis, a region specific incorporation of³H-amino acids into nuclei can be observed. 6) The rÔle of the maternal RNA in protein synthesis in the developing egg as well as its rÔle in the determination of certain egg regions (especially of the posterior pole-plasm) is discussed. The beginning of RNA synthesis and the simultaneous increase in the rate of protein synthesis are interpreted as processes of embryonic differentiation. It is assumed that the maternal RNA stored in the pole-plasm has a determinative function.

Ribosomal RNA synthesis during cleavage of Ascaris lumbricoides eggs

The appearance of well-defined nucleoli in 4-celled Ascaris embryos is associated with the synthesis of ribosomal RNA.

Cilia regeneration in the sea urchin embryo: evidence for a pool of ciliary proteins

Late gastrulae of paracentrotus lividus regenerated cilia after being deciliated in hypertonic sea water. Regeneration was not affected by actinomycin D or puromycin. Actinomycin D also did not affect ciliary protein synthesis during regeneration, although overall embryonic synthesis was depressed. Puromycin inhibited both total embryonic and ciliary protein synthesis. The data indicate that ciliary protein synthesis is controlled by a stable template and that the regenerating cilia are formed from a pool of ciliary proteins. It is suggested that the proteins of the mitotic apparatus and of the ciliamay be related.

A comparative study in vivo and in vitro of the ability of ribosomes from Xenopus liver and ovary to incorporate L-[U-14C]leucine

1. A system for the incorporation in vitro of amino acids into protein is described for the South African clawed toad (Xenopus laevis laevis Daudin). 2. The incorporation of l-[U-(14)C]leucine by Xenopus-liver microsomes is very much greater per mg. of microsomal RNA than the incorporation by ovary microsomes. 3. The incorporation by Xenopus-liver and -ovary polysomes is approximately the same when expressed per mg. of polysomal RNA. 4. It was predicted from the above results that ovary microsomes should contain a ribosomal fraction inactive in protein synthesis. This was shown to be the case by a labelling experiment in vivo with l-[U-(14)C]leucine. 5. The labelling experiment in vivo also showed that the active polysomal fraction in ovary is associated with membranes and is liberated by treatment with deoxycholate; this is also true of liver microsomes in vivo. 6. The results are discussed in relation to previous work on the synthesis of proteins by amphibian ovarian tissue, and on the role of bound and free ribosomes in protein synthesis.

Rate of protein synthesis: regulation during first division cycle of sea urchin eggs

Protein synthesis in fertilized sea urchin eggs, or in 12,000g supernatants derived from them, increased linearly during the period preceding prophase of the first mitotic cycle, dropped during metaphase and anaphase, and increased again after telophase. Similar results were observed for whole cells incubated in the presence of colchicine. These changes in the rate of protein synthesis during the mitotic cycle may be regulated at the translational level.

Continuity of protein synthesis through cleavage metaphase

Protein synthesis continues without a decline in rate throughout the period of chromosome condensation and of cytokinesis in the first two cleavages of sea urchin embryos. The natural synchrony of the egg populations and the conditions of measurement allowed even a partial inhibition of synthesis to be observed. Our results do not explain the mechanism of inhibition of protein synthesis that occurs at metaphase in cultured mammalian cells, but it shows that such a change in rate is neither universal nor obligatory.