The clam embryo protein cyclin A induces entry into M phase and the resumption of meiosis in Xenopus oocytes

Intracellular pH change does not appear as a prerequisite for triggering activation of Barnea candida (Mollusca, Pelecypoda) oocytes

Barnea candida oocytes, exposed to excess KCl, ammonia, or digitonin, exhibit germinal vesicle breakdown (GVBD) and reinitiate meiosis, at least up to first polar body extrusion. While we confirm that KCl- but not ammonia-induced activation required external calcium, our findings that digitonin is effective at any pH from 6 to 8, in the presence of calcium, while the phorbol ester TPA and diacylglycerol fail to reinitiate meiosis, strongly suggests calcium as the main trigger for this process. Preliminary experiments using the fluorescent probes fluorescein diacetate and Quin 2/AM show, moreover, that KCl and ammonia produce both an intracellular calcium surge (30 nM) and a slight alkalinization of the intracellular cytoplasm from 7.84 to 8.05.

Regulation of MPF activity in vitro

We have developed a soluble, cell-free system from premeiotic Xenopus oocytes that executes the post-translational activation of a precursor form of maturation promoting factor (MPF). We have distinguished at least two components of this ATP-dependent reaction: pre-MPF, a precursor to MPF that activates independently of added MPF and whose apparent molecular weight changes from 400 kd to 260 kd upon activation; and INH, an inhibitor of pre-MPF activation that confers MPF dependence on the reaction. We present evidence suggesting that INH is a phosphatase and that the activation of pre-MPF occurs via phosphorylation. INH activity itself seems to be regulated by another phosphatase, protein phosphatase-1. We have directly examined the pattern of protein phosphorylation during the activation reaction and have found 92 and 140 kd proteins whose phosphorylation increases when MPF activity appears. This system makes possible a direct examination of the regulation of MPF activity during the cell cycle.

The relationships between early ionic events, the pattern of protein synthesis, and oocyte activation in the surf clam, Spisula solidissima

The ionic events linked to activation of surf clam (Spisula solidissima) oocytes include a transient increased Ca2+ influx and an acid release. The aim of the present work was to further elucidate the respective roles of these two ionic events and to clarify the possible role of protein kinase C in the sequence of events leading to oocyte activation. K+-enriched seawater, ammonium chloride, and the phorbol ester 12-O-tetradecanoyl-13-phorbol acetate (TPA), a protein kinase C activator, were tested for their ability to promote germinal vesicle breakdown (GVBD), an acid release, increased 45Ca2+ uptake, and a shift in the pattern of protein synthesis. Oocytes activated by addition of K+ ions release an amount of H+ similar to that induced by fertilization, with the same time course, show an increased, verapamil-sensitive, 45Ca2+ uptake that is proportional to the amount of added K+, and undergo a shift in their pattern of protein synthesis, which requires the presence of external Ca2+. Ammonium chloride, at concentrations causing a higher production of acid than that induced by K+ ions or fertilization, does not trigger GVBD nor any increased 45Ca2+ uptake or any detectable shift in the pattern of protein synthesis. Combined additions of ammonium chloride with subthreshold concentrations of K+ ions allow GVBD to occur, thus revealing a synergistic effect of ammonia and K+ ions. TPA slowly induces GVBD, an Na+-dependent acid release, and a shift in the pattern of protein synthesis, in the absence of increased 45Ca2+ uptake. Our results lead us to propose the following sequence of events for the activation of Spisula oocytes: an increased Ca2+ influx contributes to activate protein kinase C which causes a Na+-dependent acid release leading to a rise of pHi. This rise of pHi, although insufficient by itself, may set the pHi in a permissive range for activation to occur through the action of other protein kinase C-sensitive events leading to the production of meiosis-inducing proteins.

Translational control of InsP3-induced chromatin condensation during the early cell cycles of sea urchin embryos

The cycles of DNA synthesis and chromatin condensation in dividing cells are controlled by signals from the cytoplasm. Changes in the concentration of free calcium (Cai) in the cytoplasm control a variety of cellular functions and it has thus been suggested that observed variations in Cai during the cell cycle may be the cytoplasmic signal that co-ordinates nuclear and cytoplasmic division. We show here that increases in Cai induced by the calcium-releasing second messenger inositol 1,4,5-triphosphate (Ins(1,4,5)P3), or by calcium buffers, cause premature chromatin condensation and breakdown of the nuclear envelope in sea urchin (Lytechinus pictus) early embryos. Both natural and induced chromatin condensation are prevented by calcium chelators. The nucleus becomes sensitive to the Cai signal 45 min after fertilization, but remains insensitive if protein synthesis is prevented. Our experiments demonstrate that Cai regulates the behaviour of the nucleus during the cell cycle, suggest that Ins(1,4,5)P3 is a cell cycle messenger and indicate that there is an interaction between the protein and ionic signals that control the state of chromatin during the cell cycle.

Role of nuclear material in the early cell cycle of Xenopus embryos

Activated Xenopus eggs show periodic surface contraction waves and oscillations in endogenous protein phosphorylation, MPF, and kinase activities timed with the cleavage cycle of control fertilized eggs. In this paper, we show that in activated eggs lacking the material that originates from the oocyte nucleus, MPF and kinase oscillations occur in the absence of surface contraction waves. Two mitotic phosphoproteins (M116 and M46), previously described by 32P labeling in nucleated eggs, are no longer detected in the enucleated eggs. We conclude that a cytoplasmic temporal control of MPF and kinase activities is likely to be the essential cell cycle oscillator. The oocyte nuclear components normally stored in the cytoplasm of the embryos are not involved in the clock although they appear to be required for the generation of surface contraction waves.

Thiol protease-specific inhibitor E-64 arrests human epidermoid carcinoma A431 cells at mitotic metaphase

E-64-d (ethyl (2S, 3S)-3-[(S)-3-methyl-1-(3-methylbutylcarbamoyl) butylcarbamoyl]oxirane-2-carboxylate), a membrane-permeant derivative of the thiol protease-specific inhibitor E-64, was found to arrest human epidermoid carcinoma A431 cells at mitotic metaphase. This effect was dose-dependent with a threshold of 20 micrograms/ml in chemically defined culture medium. Cell cycle analysis by flow cytometry showed that the relative proportion of the G2/M population increased 2.5-fold after treatment of the cells with E-64 (100 micrograms/ml) for 5 hr. In addition, time-lapse video analysis showed that E-64-treated cells remained at metaphase for an extended period after rounding-up, whereas untreated cells completed mitosis within 42.0 +/- 5.7 min. Some treated cells were able to complete mitosis, while others did not do so within the limits of our observation. As an approach to the molecular basis of this phenomenon, we have shown that several cellular proteins can be labeled by incubation of cells with radioactive E-64-d.

Cyclic activation of histone H1 kinase during sea urchin egg mitotic divisions

Fertilized sea urchin eggs undergo a series of rapid and synchronized mitotic divisions. Extracts were made at various times throughout the first three mitotic divisions and assayed for phosphorylating activity toward histone H1. Histone H1 kinase (HH1K) undergoes a transient activation (8- to 10-fold increase) 20 min before each cleavage. The amplitude of the HH1K peak strongly depends on the synchrony of the egg population. Concomitant cytological observations show that the time-course of HH1K correlates with the time-course of nuclear envelope breakdown and of metaphase. This correlation is observed at each cell division cycle. HH1K from each of the three first mitoses show identical time- and concentration-dependence curves as well as identical dose-inhibition curves with 6-dimethylaminopurine and quercetin, suggesting that the same (group of) kinase(s) is (are) activated before each cleavage. Ionophore A23187 does not trigger, but inhibits, HH1K activation; however, partial activation of the eggs with ammonia at pH 9.0 (but not at pH 8.0) triggers the transient HH1K activation. Appearance of the HH1K cycle requires protein synthesis since it is completely abolished in emetine-treated eggs. Although cytochalasin B blocks egg cleavage, it does not inhibit HH1K activation nor nuclear divisions. A prolonged HH1K activation cycle is observed in eggs arrested in metaphase with colchicine or nocodazole. Despite the existence of a cycle in cAMP concentration during mitosis, forskolin, an activator of adenylate cyclase, does not modify the time-course of HH1K activation and of cell division. The cycling HH1K is independent of calcium-calmodulin, calcium-phospholipids, or cyclic AMP. It clearly resembles the mammalian "growth-associated histone kinase." The relationship between the transient activation of HH1K and the intracellular mitotic factors driving the cell cycle is discussed.

Periodic mitotic events induced in the absence of DNA replication

We have discovered and report here a means of separating a mitotic "subcycle" from the G1- and S-phase events of the mammalian cell cycle. Time-lapse videomicroscopy of Syrian hamster fibroblast (BHK) cells revealed that caffeine could induce multiple entries into mitosis while cells were blocked in DNA synthesis. As with normal mitoses, the abundance of mitosis-specific phosphoproteins was coupled with the condensation of chromatin. The BHK temperature-sensitive mutant tsBN2 also completed multiple entries into mitosis while arrested during DNA replication and raised to the restrictive temperature. Periodic mitotic events occurred even when BHK cells were exposed to low concentrations of serum or cycloheximide, conditions that prevent the cycling of BHK cells by blocking their entry into S phase. These results suggest that an oscillation governing the activation and inactivation of mitotic factors can be generated in mammalian cells and uncoupled from the G1 and DNA replication events of the normal cell cycle. This system will be useful for examining the molecular nature of mitotic factors.

Cyclin synthesis, modification and destruction during meiotic maturation of the starfish oocyte

The pattern of protein synthesis in oocytes of starfish Marthasterias glacialis changes during 1-methyladenine-induced meiotic maturation. One of the newly synthesized proteins, a major 54-kDa polypeptide, was synthesized continuously after activation but was destroyed abruptly just before appearance of the polar bodies at each meiotic division. This protein thus resembles the cyclin proteins identified in cleaving sea urchin and clam embryos. RNA extracted from oocytes before and after maturation encoded virtually identical polypeptides when translated in the reticulocyte lysate. However, there was poor correspondence between the in vitro translation products and the labelling pattern of intact cells. There was no exact in vitro counterpart to the in vivo-labelled cyclin. Instead, a major polypeptide of 52 kDa was seen which appears to be a precursor of the 54-kDa form of cyclin. The 52-kDa polypeptide was identified as cyclin by hybrid arrest of translation. Cyclin mRNA is ot translated to a significant extent before oocyte activation and is present in oocytes as nonadenylated form. It becomes polyadenylated when the oocytes mature. This behavior is also seen in the case of the mRNA for the small subunit of ribonucleotide reductase, another abundant maternal mRNA whose translation is activated at maturation.

Molecular cloning and characterization of the mRNA for cyclin from sea urchin eggs

We have isolated a cDNA clone encoding sea urchin cyclin and determined its sequence. It contains a single open reading frame of 409 amino acids which shows homology with clam cyclins. RNA transcribed in vitro from this sequence was efficiently translated in reticulocyte lysates, yielding full-length cyclin. Injection of nanogram amounts of this synthetic mRNA into Xenopus oocytes caused them to mature more rapidly than with progesterone treatment. The sea urchin cyclin underwent two posttranslational modifications in the Xenopus oocytes during maturation. The first occurred at about the time that maturation became cycloheximide-resistant, when a small apparent increase in the molecular weight of cyclin was observed. The second modification involved destruction of the cyclin at about the time of white spot appearance, just as would have occurred at the metaphase/anaphase transition in the natural environment of a cleaving sea urchin embryo.

Cellular factors couple recombination with growth phase: characterization of a new component in the lambda site-specific recombination pathway

Here we characterize FIS (factor for inversion stimulation), a new cellular component of the lambda site-specific recombination pathway. This host protein binds to a specific region in the lambda attP overlapping the Xis binding sites and can bind cooperatively with Xis to these sites. FIS stimulates lambda excision up to 20-fold in vitro in the presence of suboptimal Xis concentrations, but has no effect in the presence of saturating Xis; FIS has no effect on integrative recombination. FIS can replace one Xis molecule in a series of cooperative and competitive interactions but cannot carry out excision in the absence of Xis. FIS's role in the regulation of recombination has been inferred from in vivo modification of DNA. In exponentially growing cells the lambda FIS site is fully occupied, whereas in stationary-phase cells this binding site is vacant.

Three translationally regulated mRNAs are stored in the cytoplasm of clam oocytes

In situ hybridization was used to examine the spatial distributions of three translationally controlled maternal RNAs in oocytes and two-cell embryos of the clam Spisula. 3H-labeled single-stranded RNA probes were generated from SP6 recombinant clones containing DNA inserts encoding portions of histone H3 (the DNA sequence which is presented here), cyclin A, and the small subunit of ribonucleotide reductase. Hybridization of these probes to oocytes, in which the mRNAs are translationally inactive, shows that these mRNAs are stored in the cytoplasm. There is no evidence for sequestration of any of the RNAs within the nucleus or any other discrete structure. Instead they appear to be evenly distributed throughout the cytoplasm.

Reactivation of spindle elongation in vitro is correlated with the phosphorylation of a 205 kd spindle-associated protein

Mitotic spindles isolated from the diatom Stephanopyxis turris consist of two half-spindles of closely interdigitating microtubules that slide relative to one another in the presence of ATP, reinitiating spindle elongation (anaphase B) in vitro. Purified spindles that have been exposed to ATP-gamma-S undergo ATP-dependent reactivation more readily than do control spindles. Thiophosphorylated proteins in such spindles are located in the spindle midzone, kinetochores, and a portion of the pole complex. One major thiophosphorylated peptide of 205 kd is detected in extracts prepared from spindles labeled with [35S]ATP-gamma-S, and is also localized in the spindle midzone by using an antibody that recognizes thiophosphorylated proteins. It is likely that this 205 kd peptide is either a positive regulator or mechanochemical transducer of microtubule sliding when it is in a phosphorylated state.