Germinal vesicle breakdown in the Xenopus laevis oocyte: description of a transient microtubular structure

Actin-driven chromosome clustering facilitates fast and complete chromosome capture in mammalian oocytes

Accurate chromosome segregation during meiosis is crucial for reproduction. Human and porcine oocytes transiently cluster their chromosomes before the onset of spindle assembly and subsequent chromosome segregation. The mechanism and function of chromosome clustering are unknown. Here we show that chromosome clustering is required to prevent chromosome losses in the long gap phase between nuclear envelope breakdown and the onset of spindle assembly, and to promote the rapid capture of all chromosomes by the acentrosomal spindle. The initial phase of chromosome clustering is driven by a dynamic network of Formin-2- and Spire-nucleated actin cables. The actin cables form in the disassembling nucleus and migrate towards the nuclear centre, moving the chromosomes centripetally by interacting with their arms and kinetochores as they migrate. A cage of stable microtubule loops drives the late stages of chromosome clustering. Together, our data establish a crucial role for chromosome clustering in accurate progression through meiosis.

X-linked α-thalassemia with mental retardation is downstream of protein kinase A in the meiotic cell cycle signaling cascade in Xenopus oocytes and is dynamically regulated in response to DNA damage†

X-linked α-thalassemia with mental retardation (ATRX) is a chromatin remodeling protein that belongs to the SWItch/sucrose non-fermentable (SWI2/SNF2) family of helicase/ATPases. During meiosis, ATRX is necessary for heterochromatin formation and maintenance of chromosome stability in order to ensure proper assembly of the metaphase II spindle. Previously, we established ATRX as a novel progesterone regulated protein during bovine meiotic maturation, in addition to being dynamically regulated in response to DNA damage in oocytes. In the present study, we utilize the Xenopus laevis model system to further elucidate the signaling pathways regulating ATRX expression within the oocyte. Here, we present an analysis of endogenous ATRX protein expression during oogenesis, oocyte meiotic maturation, and early embryonic development. ATRX expression is dynamically regulated as evidenced by loss of the protein in metaphase II of meiosis. The downstream activation of meiosis via protein kinase A inhibition resulted in a similar decrease in ATRX protein expression. We demonstrate that the ATRX protein is detected in ubiquitin immuno-precipitates from germinal vesicle oocyte extracts and experimentally demonstrate that proteosomal degradation is responsible for the decreased expression of ATRX during meiosis. ATRX expression is significantly increased in response to gamma-irradiation induced DNA damage in oocytes and embryos. This increased expression is independent of p53 protein expression in apoptotic embryos, as determined by the expression of active caspase-3. Thus, regulation of ATRX protein expression impacts on G2-M progression and ultimately has consequences for cell survival.

Inhibition of ectopic microtubule assembly by the kinesin-13 KLP-7 prevents chromosome segregation and cytokinesis defects in oocytes

In most species, oocytes lack centrosomes. Accurate meiotic spindle assembly and chromosome segregation - essential to prevent miscarriage or developmental defects - thus occur through atypical mechanisms that are not well characterized. Using quantitative in vitro and in vivo functional assays in the C. elegans oocyte, we provide novel evidence that the kinesin-13 KLP-7 promotes destabilization of the whole cellular microtubule network. By counteracting ectopic microtubule assembly and disorganization of the microtubule network, this function is strictly required for spindle organization, chromosome segregation and cytokinesis in meiotic cells. Strikingly, when centrosome activity was experimentally reduced, the absence of KLP-7 or the mammalian kinesin-13 protein MCAK (KIF2C) also resulted in ectopic microtubule asters during mitosis in C. elegans zygotes or HeLa cells, respectively. Our results highlight the general function of kinesin-13 microtubule depolymerases in preventing ectopic, spontaneous microtubule assembly when centrosome activity is defective or absent, which would otherwise lead to spindle microtubule disorganization and aneuploidy.

Actin-based spindle positioning: new insights from female gametes

Asymmetric divisions are essential in metazoan development, where they promote the emergence of cell lineages. The mitotic spindle has astral microtubules that contact the cortex, which act as a sensor of cell geometry and as an integrator to orient cell division. Recent advances in live imaging revealed novel pools and roles of F-actin in somatic cells and in oocytes. In somatic cells, cytoplasmic F-actin is involved in spindle architecture and positioning. In starfish and mouse oocytes, newly discovered meshes of F-actin control chromosome gathering and spindle positioning. Because oocytes lack centrosomes and astral microtubules, F-actin networks are key players in the positioning of spindles by transmitting forces over long distances. Oocytes also achieve highly asymmetric divisions, and thus are excellent models to study the roles of these newly discovered F-actin networks in spindle positioning. Moreover, recent studies in mammalian oocytes provide a further understanding of the organisation of F-actin networks and their biophysical properties. In this Commentary, we present examples of the role of F-actin in spindle positioning and asymmetric divisions, with an emphasis on the most up-to-date studies from mammalian oocytes. We also address specific technical issues in the field, namely live imaging of F-actin networks and stress the need for interdisciplinary approaches.

Recent insights into spindle function in mammalian oocytes and early embryos

Errors in chromosome segregation in oocytes and early embryos lead to embryo aneuploidy, which contributes to early pregnancy loss. At the heart of chromosome segregation is the spindle, a dynamic biomechanical machine fashioned from microtubules, which is tasked with gathering and sorting chromosomes and dispatching them to the daughter cells at the time of cell division. Understanding the causes of segregation error in the oocyte and early embryo will undoubtedly hinge on a thorough understanding of the mechanism of spindle assembly and function in these highly specialized cellular environments. The recent advent of live imaging approaches to observe chromosome segregation in real-time in oocytes and embryos, paired with gene-silencing techniques and specific inhibition for assessing the function of a protein of interest, has led to a substantial advance in our understanding of chromosome segregation in early mammalian development. These studies have uncovered numerous mechanistic differences between oocytes, embryos, and traditional model systems. In addition, a flurry of recent studies using naturally aged mice as the model for human aging have begun to shed light on the increased levels of aneuploidy seen in embryos from older mothers. Here we review these recent developments and consider what has been learned about the causes of chromosome missegregation in early development.

The nuclear F-actin interactome of Xenopus oocytes reveals an actin-bundling kinesin that is essential for meiotic cytokinesis

Nuclei of Xenopus laevis oocytes grow 100 000-fold larger in volume than a typical somatic nucleus and require an unusual intranuclear F-actin scaffold for mechanical stability. We now developed a method for mapping F-actin interactomes and identified a comprehensive set of F-actin binders from the oocyte nuclei. Unexpectedly, the most prominent interactor was a novel kinesin termed NabKin (Nuclear and meiotic actin-bundling Kinesin). NabKin not only binds microtubules but also F-actin structures, such as the intranuclear actin bundles in prophase and the contractile actomyosin ring during cytokinesis. The interaction between NabKin and F-actin is negatively regulated by Importin-β and is responsive to spatial information provided by RanGTP. Disconnecting NabKin from F-actin during meiosis caused cytokinesis failure and egg polyploidy. We also found actin-bundling activity in Nabkin's somatic paralogue KIF14, which was previously shown to be essential for somatic cell division. Our data are consistent with the notion that NabKin/KIF14 directly link microtubules with F-actin and that such link is essential for cytokinesis.

The presence and role of actin filaments in cell nuclei remains incompletely understood. A proteomics approach now reveals a highly distinct set of F-actin-binding proteins in the nucleus, including a novel kinesin family member.

Polar body cytokinesis

Polar body cytokinesis is the physical separation of a small polar body from a larger oocyte or ovum. This maternal meiotic division shares many similarities with mitotic and spermatogenic cytokinesis, but there are several distinctions, which will be discussed in this review. We synthesize results from many different model species, including those popular for their genetics and several that are more obscure in modern cell biology. The site of polar body division is determined before anaphase, by the eccentric, cortically associated meiotic spindle. Depending on the species, either the actin or microtubule cytoskeleton is required for spindle anchoring. Chromatin is necessary and sufficient to elicit differentiation of the associated cortex, via Ran-based signaling. The midzone of the anaphase spindle serves as a hub for regulatory complexes that elicit Rho activation, and ultimately actomyosin contractile ring assembly and contraction. Polar body cytokinesis uniquely requires another Rho family GTPase, Cdc42, for dynamic reorganization of the polar cortex. This is perhaps due to the considerable asymmetry of this division, wherein the polar body and the oocyte/ovum have distinct fates and very different sizes. Thus, maternal meiotic cytokinesis appears to occur via simultaneous polar relaxation and equatorial contraction, since the polar body is extruded from the spherical oocyte through the nascent contractile ring. As such, polar body cytokinesis is an interesting and important variation on the theme of cell division.

Polar body emission

Generation of a haploid female germ cell, the egg, consists of two rounds of asymmetric cell division (meiosis I and meiosis II), yielding two diminutive and nonviable polar bodies and a large haploid egg. Animal eggs are also unique in the lack of centrioles and therefore form meiotic spindles without the pre-existence of the two dominant microtubule organizing centers (centrosomes) found in mitosis. Meiotic spindle assembly is further complicated by the unique requirement of sister chromatid mono-oriented in meiosis I. Nonetheless, the eggs appear to adopt many of the same proteins and mechanisms described in mitosis, with necessary modifications to accommodate their special needs. Unraveling these special modifications will not only help understanding animal reproduction, but should also enhance our understanding of cell division in general.

Acentrosomal spindle assembly and chromosome segregation during oocyte meiosis

The ability to reproduce relies in most eukaryotes on specialized cells called gametes. Gametes are formed by the process of meiosis in which, after a single round of replication, two successive cell divisions reduce the ploidy of the genome. Fusion of gametes at fertilization reconstitutes diploidy. In most animal species, chromosome segregation during female meiosis occurs on spindles assembled in the absence of the major microtubule-organizing center, the centrosome. In mammals, oocyte meiosis is error prone and underlies most birth aneuploidies. Here, we review recent work on acentrosomal spindle formation and chromosome alignment/separation during oocyte meiosis in different animal models.

Actin filaments: key players in the control of asymmetric divisions in mouse oocytes

Meiotic maturation is characterized by the succession of two asymmetric divisions each giving rise to a small polar body and a large oocyte. These highly asymmetric divisions are characteristic of meiosis in higher organisms. They allow most of the maternal stores to be retained in the oocyte, a vital property for further embryo development. In mouse oocytes, the asymmetry is ensured by the migration and the anchoring of the division spindle to the cortex in meiosis I and by its anchoring to the cortex in meiosis II. In addition, and subsequent to this off-centre positioning of the spindle, a differentiation of the cortex overhanging the chromosomes takes place and is necessary for the extrusion of small polar bodies. In the present review, we will emphasize the role of the actin cytoskeleton in the control of spindle positioning, spindle anchoring to the cortex and cortical differentiation.

Interactions between chromosomes, microfilaments and microtubules revealed by the study of small GTPases in a big cell, the vertebrate oocyte

Meiotic divisions during oogenesis in higher eukaryotes are extremely asymmetric giving rise to one gamete, the oocyte, and two polar bodies. In most species, this asymmetric partitioning relies on the eccentric positioning of meiotic spindles. Recent work performed in mouse and frog oocytes has suggested the involvement of small GTPases, such as Cdc42, Rac and Ran both in the control of spindle organization and positioning. The present review summarizes these findings that shed light on the molecular mechanisms by which small GTPases control asymmetric cell divisions in vertebrate oocytes.

A centriole- and RanGTP-independent spindle assembly pathway in meiosis I of vertebrate oocytes

Spindle formation is essential for stable inheritance of genetic material. Experiments in various systems indicate that Ran GTPase is crucial for meiotic and mitotic spindle assembly. Such an important role for Ran in chromatin-induced spindle assembly was initially demonstrated in Xenopus laevis egg extracts. However, the requirement of RanGTP in living meiotic cells has not been shown. In this study, we used a fluorescence resonance energy transfer probe to measure RanGTP-regulated release of importin β. A RanGTP-regulated gradient was established during meiosis I and was centered on chromosomes throughout mouse meiotic maturation. Manipulating levels of RanGTP in mice and X. laevis oocytes did not inhibit assembly of functional meiosis I spindles. However, meiosis II spindle assembly did not tolerate changes in the level of RanGTP in both species. These findings suggest that a mechanism common to vertebrates promotes meiosis I spindle formation in the absence of chromatin-induced microtubule production and centriole-based microtubule organizing centers.

XMAP215, XKCM1, NuMA, and cytoplasmic dynein are required for the assembly and organization of the transient microtubule array during the maturation of Xenopus oocytes

During the maturation of Xenopus oocytes, a transient microtubule array (TMA) is nucleated from a novel MTOC near the base of the germinal vesicle. The MTOC-TMA transports the meiotic chromosomes to the animal cortex, where it serves as the precursor to the first meiotic spindle. To understand more fully the assembly of the MTOC-TMA, we used confocal immunofluorescence microscopy to examine the localization and function of XMAP215, XKCM1, NuMA, and cytoplasmic dynein during oocyte maturation. XMAP215, XKCM1, and NuMA were all localized to the base of the MTOC-TMA and the meiotic spindle. Microinjection of anti-XMAP215 inhibited microtubule (MT) assembly during oocyte maturation, disrupting assembly of the MTOC-TMA and subsequent assembly of the first meiotic spindle. In contrast, microinjection of anti-XKCM1 promoted MT assembly throughout the cytoplasm, disrupting organization of the MTOC-TMA and meiotic spindle. Finally, microinjection of anti-dynein or anti-NuMA disrupted the organization of the MTOC-TMA and subsequent assembly of the meiotic spindles. These results suggest that XMAP215 and XKCM1 act antagonistically to regulate MT assembly and organization during maturation of Xenopus oocytes, and that dynein and NuMA are required for organization of the MTOC-TMA.

Cdc2-cyclin B triggers H3 kinase activation of Aurora-A in Xenopus oocytes

Xenopus oocytes are arrested in meiotic prophase I and resume meiotic divisions in response to progesterone. Progesterone triggers activation of M-phase promoting factor (MPF) or Cdc2-cyclin B complex and neosynthesis of Mos kinase, responsible for MAPK activation. Both Cdc2 and MAPK activities are required for the success of meiotic maturation. However, the signaling pathway induced by progesterone and leading to MPF activation is poorly understood, and most of the targets of both Cdc2 and MAPK in the oocyte remain to be determined. Aurora-A is a Ser/Thr kinase involved in separation of centrosomes and in spindle assembly during mitosis. It has been proposed that in Xenopus oocytes Aurora-A could be an early component of the progesterone-transduction pathway, acting through the regulation of Mos synthesis upstream Cdc2 activation. We addressed here the question of Aurora-A regulation during meiotic maturation by using new in vitro and in vivo experimental approaches. We demonstrate that Cdc2 kinase activity is necessary and sufficient to trigger both Aurora-A phosphorylation and kinase activation in Xenopus oocyte. In contrast, these events are independent of the Mos/MAPK pathway. Aurora-A is phosphorylated in vivo at least on three residues that regulate differentially its kinase activity. Therefore, Aurora-A is under the control of Cdc2 in the Xenopus oocyte and could be involved in meiotic spindle establishment.

G2 arrest in Xenopus oocytes depends on phosphorylation of cdc25 by protein kinase A

Xenopus oocytes, which are arrested in G(2) of meiosis I, contain complexes of cyclin B-cdc2 (M phase-promoting factor) that are kept repressed by inhibitory phosphorylations on cdc2 at Thr-14 and Tyr-15. Progesterone induces a cytoplasmic signaling pathway that leads to activation of cdc25, the phosphatase that removes these phosphorylations, catalyzing entry into M phase. It has been known for 25 years that high levels of cAMP and protein kinase A (PKA) are required to maintain the G(2) arrest and that a drop in PKA activity is required for M phase-promoting factor activation, but no physiological targets of PKA have been identified. We present evidence that cdc25 is a critical target of PKA. (i) In vitro, cdc25 Ser-287 serves as a major site of phosphorylation by PKA, resulting in sequestration by 14-3-3. (ii) Endogenous cdc25 is phosphorylated on Ser-287 in oocytes and dephosphorylated in response to progesterone just before cdc2 dephosphorylation and M-phase entry. (iii) High PKA activity maintains phosphorylation of Ser-287 in vivo, whereas inhibition of PKA by its heat-stable inhibitor (PKI) induces dephosphorylation of Ser-287. (iv) Overexpression of mutant cdc25 (S287A) bypasses the ability of PKA to maintain oocytes in G(2) arrest. These findings argue that cdc25 is a physiologically relevant target of PKA in oocytes. In the early embryonic cell cycles, Ser-287 is phosphorylated during interphase and dephosphorylated just before cdc2 activation and mitotic entry. Thus, in addition to its role in checkpoint arrest, cdc25 Ser-287 serves as a site for regulation during normal, unperturbed cell cycles.

Multiple isoforms of the high molecular weight microtubule associated protein XMAP215 are expressed during development in Xenopus

We have cloned and sequenced cDNAs encoding two isoforms of XMAP215, a high molecular weight microtubule-associated protein identified in Xenopus eggs. XMAP215 is approximately 80% identical in amino acid sequence to the product of ch-TOG, a cDNA that is over expressed in certain human tumors [Charrasse et al., 1995: Eur J Biochem 234:406-413]. Northern and Western blots demonstrated that XMAP215 is expressed throughout development, from oogenesis to tadpole. We identified two XMAP215 transcripts differing only in the presence of a 108-bp sequence encoding a 36 amino acid insert. RT-PCR revealed that the transcripts encoding these two isoforms are expressed at distinct times during development: a transcript containing the insert (encoding XMAP215(M)) is expressed during oogenesis and is present through gastrulation. The second transcript (encoding XMAP215(Z)) lacks the 108-bp insert and is expressed from gastrulation onward. In situ hybridization demonstrated that XMAP215 transcripts are localized to the ectoderm of early embryos and in the developing nervous system during later development. These results suggest that XMAP215 plays important roles in at least two phases of development: (1) regulating the assembly of MTs during the rapid cell divisions after fertilization, and (2) regulating MT assembly during the development of the nervous system.

Progesterone regulates the accumulation and the activation of Eg2 kinase in Xenopus oocytes

Xenopus prophase oocytes reenter meiotic division in response to progesterone. The signaling pathway leading to Cdc2 activation depends on neosynthesized proteins and a decrease in PKA activity. We demonstrate that Eg2 protein, a Xenopus member of the Aurora/Ipl1 family of protein kinases, accumulates in response to progesterone and is degraded after parthenogenetic activation. The polyadenylation and cap ribose methylation of Eg2 mRNA are not needed for the protein accumulation. Eg2 protein accumulation is induced by progesterone through a decrease in PKA activity, upstream of Cdc2 activation. Eg2 kinase activity is undetectable in prophase and is raised in parallel with Cdc2 activation. In contrast to Eg2 protein accumulation, Eg2 kinase activation is under Cdc2 control. Furthermore, by using an anti-sense strategy, we show that Eg2 accumulation is not required in the transduction pathway leading to Cdc2 activation. Altogether, our results strongly suggest that Eg2 is not necessary for Cdc2 activation, though it could participate in the organization of the meiotic spindles, in agreement with the well-conserved roles of the members of the Aurora family, from yeast to man.

Microtubules in Xenopus oocytes are oriented with their minus-ends towards the cortex

Despite lacking centrosomes, stage VI Xenopus oocytes contain extensive networks of cytoplasmic microtubules (MTs). To gain additional insight into the factors regulating MT organization during oogenesis, we have used electron microscopy and "hook decoration" to examine the distribution and orientation of MTs in Xenopus oocytes. A limited survey of two "undecorated" stage VI oocytes revealed 218 MTs in images covering approximately 2,500 microm(2), and indicated that the MT number density of the animal cytoplasm was greater than that of the vegetal cytoplasm. Examination of five "decorated" stage VI oocytes (three animal and five vegetal hemispheres) revealed 653 MTs. Of these, 76% could be scored as having exclusively counterclockwise (CCW) or clockwise (CW) hooks. In the animal hemispheres, 93% of the scored MTs exhibited CCW hooks when viewed from the direction of the cortex, indicating that most MTs were oriented with their minus-ends out. MT orientation appeared relatively uniform throughout the animal cytoplasm: more than 90% of the scored MTs in the cortical (90%), subcortical (96%), or perinuclear (98%) cytoplasm were oriented with their minus-ends out. In the vegetal hemispheres, approximately 80% of the scored MTs exhibited CCW hooks, and thus were oriented with their minus-ends out; 96% of the scored MTs in stage III oocytes were oriented minus-end out. These observations support a model in which the cortex plays a significant role in MT nucleation and organization in Xenopus oocytes, and have significant implications for the MT-dependent transport and localization of cytoplasmic organelles and RNAs during oogenesis.

Exclusively juvenile centrioles in Xenopus laevis oocytes injected with preparations of mature centrioles

Activated oocytes of Xenopus laevis were injected with centriole preparations isolated either from spermatozoa of loach fish Misgurnus fossilis or from rat liver. These injections induced the development of cytasters in the ooplasm and egg cleavage. Electron microscopic study of cytasters was made at the stage that corresponded to interphase between first and second cleavage divisions. This study revealed in cytasters singleton centrioles surrounded by pericentriolar material and numerous microtubules. Surprisingly, the ultrastructure of centrioles in cytasters corresponded to that of juvenile, newly formed vertebrate centrioles, whereas the injected preparations contained only adult mature centrioles. We suggested that xenogenic centrioles injected to Xenopus laevis oocytes could dissolve after formation of centrioles made from molecules of oocyte origin. A special mechanism that eliminates male centrioles after egg fertilization is speculated.

Confocal microscopy and 3-D reconstruction of the cytoskeleton of Xenopus oocytes

Xenopus oocytes contain a complex cytoskeleton composed of three filament systems: (1) microtubules, composed of tubulin and at least three different microtubule-associated proteins (XMAPs); (2) microfilaments composed of actin and associated proteins; and (3) intermediate filaments, composed of keratins. For the past several years, we have used confocal immunofluorescence microscopy to characterize the organization of the oocyte cytoskeleton throughout the course of oogenesis. Together with computer-assisted reconstruction of the oocyte in three dimensions, confocal microscopy gives an unprecedented view of the assembly and reorganization of the cytoskeleton during oocyte growth and differentiation. Results of these studies, combined with the effects of cytoskeletal inhibitors, suggest that organization of the cytoskeleton in Xenopus oocytes is dependent upon a hierarchy of interactions between microtubules, microfilaments, and keratin filaments. This article presents a gallery of confocal images and 3-D reconstructions depicting the assembly and organization of the oocyte cytoskeleton during stages 0-VI of oogenesis, a discussion of the mechanisms that might regulate cytoskeletal organization during oogenesis, and speculates on the potential roles of the oocyte cytoskeleton during oogenesis and axis formation.

XMAP230 is required for the assembly and organization of acetylated microtubules and spindles in Xenopus oocytes and eggs

We used affinity-purified polyclonal antibodies to characterize the distribution and function of XMAP230, a heat-stable microtubule-associated protein isolated from Xenopus eggs, during oogenesis. Immunoblots revealed that XMAP230 was present throughout oogenesis and early development, but was most abundant in late stage oocytes, eggs, and early embryos. Immunofluorescence microscopy revealed that XMAP230 was associated with microtubules in oogonia, post-mitotic stage 0 oocytes, early stage I oocytes, and during stage IV-VI of oogenesis. However, staining of microtubules by anti-XMAP230 was not detectable during late stage I through stage III. In stage VI oocytes, anti-XMAP230 stained a large subset of microtubules that were also stained with monoclonal antibodies specific for acetylated (α)-tubulin. During oocyte maturation, XMAP230 was associated with the transient microtubule array that serves as the precursor of the first meiotic spindle, as well as both first and second meiotic spindles. The extensive array of cytoplasmic microtubules present throughout maturation was not detectably stained by anti-XMAP230. Microinjection of anti-XMAP230 locally disrupted the organization and acetylation of microtubules in stage VI oocytes, and reduced the re-acetylation of microtubules during recovery from cold-induced microtubule disassembly. Subsequent maturation of oocytes injected with anti-XMAP230 resulted in defects in the assembly of the transient microtubules array and first meiotic spindle. These observations suggest that XMAP230 is required for the stabilization and organization of cytoplasmic and spindle microtubules in Xenopus oocytes and eggs.

Function and regulation of cdc25 protein phosphate through mitosis and meiosis

Activation of the cyclin B-cdc2 kinase mitotic inducer involves dephosphorylation of two inhibitory residues, tyrosine 15 and threonine 14, cdc25 is the specific phosphatase that directly dephosphorylates and activates the cdc2 kinase, cdc25 activity is regulated by phosphorylation. Both phosphatases 1 and 2A could act as cdc25-specific inhibitory phosphatases. Although the cyclin B-cdc2 complex plays a role in activating cdc25, it is highly probable that a distinct protein kinase is involved as a trigger in cdc25 activation. The implication of raf kinase as a cdc25-specific activating kinase in human cells and Xenopus oocytes is discussed.

Immunolocalization of alpha-tubulin, gamma-tubulin, and CENP-E in male rat and male mouse meiotic divisions: pathway of meiosis I spindle formation in mammalian spermatocytes

Recent findings on cell division suggest that differences exist in spindle organization not only between mitotic and meiotic systems, but also between female and male meiosis. In mammals, this has been difficult to demonstrate due to lack of appropriate methods. By taking advantage of the strict organization and ordered kinetics of mammalian spermatogenesis, we harvested highly enriched populations of dividing mouse and rat spermatocytes using transillumination-assisted micro-dissection of seminiferous tubules. In the spermatocytes, we examined the localization and distribution of microtubules, centrosomes, and kinetochores at different phases of the first meiotic division using immunohistochemistry with antibodies against alpha-tubulin, gamma-tubulin, and CENP-E, respectively. Fluorescence and confocal microscope analysis of dividing spermatocytes provides evidence that the formation of the male mammalian meiosis I spindle differs from that of female meiosis and mitosis. A short (1-2 microns) bipolar aggregate of microtubules is nucleated by two adjacent centrosomes located next to the nucleus. After nuclear envelope breakdown, adjacent centrosomes and the short spindle become surrounded by the mass of paired meiotic chromosomes. At prometaphase the distance between the centrosomes increases resulting in elongation of the microtubule arrays and eventually formation of a full-length metaphase spindle (12-14 microns). Based on these results we suggest a model for spindle morphogenesis in mammalian spermatocytes.

The major soluble tubulins are found in mega dalton (MDa) fractions in fully-grown oocytes and eggs but not in brain of the frog, Rana pipiens

During progesterone-induced oocyte maturation, the cell is converted from prophase I to metaphase II, which is known to involve microtubule changes. We hypothesize that progesterone affects the polymeric state of tubulins, isoform dynamics, as well as tubulin synthesis and degradation. To test the former, leopard frog (Rana pipiens) oocytes were treated with progesterone, homogenized and centrifuged at 16,000 x g. The resulting pellet and supernatant fractions were analyzed using immunoblots probed with anti-alpha-tubulin antibody DM1A and anti-beta-tubulin antibody DM1B. The results indicate that the majority of tubulins are in supernatant fractions (soluble cellular component) and further indicate changes in size of pelletable tubulins with time post-progesterone treatment. To characterize the supernatant tubulins, Superose-6b columns were used to fractionate 16,000 x g supernatant materials. The results indicate tubulins in fractions of up to 5 MDa (equivalent in size to c. 50 tubulin dimers or 5,000 kDa) predominate, while tubulin dimers and monomers of 110 kDa and 55 kDa, respectively, are relatively minor components. The tubulin in MDa fraction pools in the oocyte soluble component seem to be tissue specific, since the reverse patterns were found in either frog brain or rat (Rattus norvegicus) brain tissue under the same conditions. Microtubule poisons, taxol and nocodazole, were used to test the dynamics of tubulin in MDa fractions and the results were opposite to that expected, i.e., taxol, a microtubule stabilizer, decreased tubulin in large MDa fractions while nocodazole, a microtubule destabilizer, increased it. Preliminary data also indicated that progesterone treatment alters the tubulin size classes (size-shift) in comparison with that in immature oocytes. These results, in combination with other research reports, suggest that tubulin in MDa fractions may either be associated with other cellular components or act as intermediates in microtubule dynamics, perhaps via oligomerization, during oocyte maturation and early development.

In vivo regulation of cytostatic activity in Xenopus metaphase II-arrested oocytes

Metaphase II arrest of Xenopus oocyte is characterized by the presence of M-phase-promoting factor (MPF) and of a microtubular spindle, both of which are stable in the presence of protein synthesis inhibitors. We studied in vivo this equilibrium state that is settled during meiotic maturation. At time of germinal vesicle breakdown (GVBD), cdc2 kinase and MAP kinase activities are stimulated. A component of the cyclin ubiquitin ligase, CDC27, is phosphorylated at the same time and remains phosphorylated until fertilization, indicating that an important component of the ligase complex is modified as early as GVBD. During a first period extending from GVBD until the cortical anchorage of the metaphase II spindle, homogeneous pools of cdc2 kinase and mitogen-activated protein (MAP) kinase activities are present in oocyte and are strictly dependent on protein turnover, since protein synthesis inhibition induces their total inactivation and drives oocytes into interphase. The metaphase II spindle, once anchored into the cortex, is no more sensitive to protein synthesis inhibition, likewise MAP kinase activity. During this cellular arrest, cdc2 kinase is divided into two distinctly regulated pools. The first one contains cyclin B that actively turns over and is subjected to a microtubular checkpoint. The second one is stable. Alteration of intracellular compartmentation of metaphase II oocytes either by gentle centrifugation or by cold shock inactivates MAP kinase and targets all cyclin B molecules for full destruction. We therefore suggest that MAP kinase participates to the cytostatic activity by preventing part of cyclin B molecules from entering the ubiquitination/degradation machinery which is still turned on in metaphase II oocytes.

Microinjection of anti-alpha-tubulin antibody (DM1A) inhibits progesterone-induced meiotic maturation and deranges the microtubule array in follicle-enclosed oocytes of the frog, Rana pipiens

Microinjection of anti-alpha-tubulin (Dm1A) inhibited progesterone-induced meiotic maturation in large follicle-enclosed oocytes of the frog, Rana pipiens. DM1A (46 nl; 10 mg/ml) injection significantly increased the ED50 value for progesterone as determined by germinal vesicle dissolution (GVD) bioassay. By contrast, low doses of microinjected DM1A (46 nl; 2.5 mg/ml), anti-actin (clone KJ43A), anti-cytokeratin (C-11), anti-intermediate filament antibody (IFA), generic IgG (46 nl; 20 mg/ml) or sodium azide (46 nl; 1 mg/ml), an antibody preservative, were without inhibitory effect in this bioassay. Microinjected, affinity-purified DM1A (46 nl; 7.5 mg/ml) was also inhibitory, but preabsorption with pure tubulin prior to injection significantly reduced the inhibitory effect. DM1A injection had no effect on centrifugation-induced germinal vesicle migration (GVM). Previous work indicated that drugs (e.g. demecolcine and nocodazole), which destabilise microtubules, enhance both centrifugation-induced GVM and progesterone-induced GVD in Rana oocytes. Taking these results together, it is suggested that DM1A injection may have differential effects on microtubules in this cell. Thus, while the majority of microtubules were apparently depolymerised by DM1A (46 nl; 10 mg/ml) injection, a small subpopulation appeared to be stabilised as bundles. Confocal immunofluorescence microscopy of follicle-enclosed oocytes after DM1A injection revealed a major loss of microtubules throughout the cell; however, apparent sparse bundles of microtubules arranged in an approximately 600 microns shell were associated with the injectate region 24 h post-injection. By contrast, control follicle-enclosed oocytes topically labelled with DM1A post-fixation had extensive microtubule arrays similar to those previously reported in Xenopus oocytes. Intracellular recording after DM1A injection and progesterone treatment yielded an intermediate membrane potential (Vm = -31.8 mV) compared with control (immature) DM1A-injected cells (Vm = -44.7 mV) or potassium balanced salt solution (KBS)-injected cells matured with progesterone (Vm = -13.9 mV). These results suggest that DM1A injection does not completely inhibit electrophysiological changes initiated by progesterone. Working hypotheses are proposed that suggest a role for microtubules in the action of progesterone which normally lifts the prophase I block in the Rana follicle-enclosed oocyte.

F-actin is required for spindle anchoring and rotation in Xenopus oocytes: a re-examination of the effects of cytochalasin B on oocyte maturation

We used confocal immunofluorescence microscopy to examine spindle migration, morphology and orientation during the maturation of Xenopus oocytes, in the presence or absence of cytochalasin B (CB), an inhibitor of actin assembly. Treatment with CB during maturation (10-50 micrograms/ml beginning 0-3 h prior to addition of progesterone) disrupted the normal organisation of the novel MTOC and transient microtubule array (MTOC-TMA complex) that serves as the immediate precursor of the first meiotic spindle, suggesting that F-actin plays an important role in the assembly or maintenance of this complex. However, CB treatment did not block translocation of the MTOC-TMA complex to the oocyte cortex, suggesting that MTOC-TMA translocation is not dependent on an actin-based mechanism. Bipolar spindles were observed in CB-treated oocytes fixed during both M1 and M2. However, rotation of the M1 and M2 spindles into an orientation orthogonal to the oocyte surface was inhibited by CB. Rhodamine-phalloidin revealed a concentration of F-actin at the site of M1 spindle attachment, further suggesting that cortical actin is required for anchoring and rotation of the meiotic spindles. Finally, the incidence of M1 monasters was significantly increased in CB-treated oocytes, suggesting that interactions between the nascent M1 spindle and cortex are dependent on F-actin.

A Xenopus maternal effect mutant gene affects oocyte meiotic reinitiation and fertilization

No cleavage (nc) is a maternal effect mutant gene, recessive and sex limited. It affects the eggs laid by homozygous mutant females, independently of the male genotype. Contrary to normal oocytes, following germinal vesicle breakdown (GVBD) during maturation, the transient microtubular array (TMA) is not formed, nor are the meiotic spindles. Cytoplasmic asters with condensed chromosomes are present in the majority of oocytes, as well as microtubular bundles and sometimes cytoplasmic spindle-like asters. These mature oocytes exhibit a disturbance in yolk platelet arrangements. The white spot is rather irregular, and the maturation period is longer than normal. Transfers of cytoplasm from nc mature oocytes into normal stage VI oocytes resulted in abnormal maturation of the normal oocytes. Reciprocal transfers (cytoplasm from normal mature oocytes into nc stage VI oocytes) induce the formation of spindles, usually cytoplasmic; this indicates that the deficiency can be partly rescued. Following fertilization, the nc eggs show neither contraction nor rotation; polyspermy is present in the majority of cases. Even in the same egg, simultaneous spindles and nuclei can be observed, revealing a disturbance in the spatial localization of regulators of the cell cycle. Cytokinesis never occurs. Polyspermy results from the absence of cortical reaction following sperm entry. However, when mature nc oocytes are treated with PMA, they show cortical granule exocytosis and the formation of an altered vitelline envelope. The different factors possibly involved in these anomalies are discussed in relation to cytoarchitectural disorganization of the cell and abnormal cell cycle regulation.

Maturation induced internalization of beta 1-integrin by Xenopus oocytes and formation of the maternal integrin pool

A pool of beta 1-integrin, ready to be inserted into the cleavage membranes, is present in the cytoplasm of the Xenopus egg, while its plasma membrane is devoid of this membrane protein (Gawantka et al., 1992). The underlying mechanisms that lead to this specific pattern of beta 1-integrin distribution in the egg have been investigated. beta 1-Integrin is present on the oocyte membrane throughout oogenesis. During maturation the oocyte membrane is cleared of beta 1-integrin via internalization of the protein by the oocyte. Synthesis of beta 1-integrin precursor is stimulated moderately in the maturing oocyte. At the same time processing of the precursor into the mature form of beta 1-integrin and its complexing with a putative alpha-chain is greatly accelerated. This way a maternal integrin pool accumulates in the mature oocyte. It is localized in conspicuous yolk free patches which contain large amounts of endoplasmic reticulum, Golgi complexes and smooth vesicles. We suggest that membrane vesicles harbouring the beta 1-integrin are generated in these cytoplasmic regions and that this store of vesicles provides the material source for the rapid membrane formation during cleavage.

The role of protein kinase C in reorganization of the cortical cytoskeleton during the transition from oocyte to fertilization-competent egg

Fertilization-competent amphibian eggs (metaphase II) are programmed to undergo an actin-myosin based contraction of the cortical cytoplasm (i.e., cortical contraction) in response to an elevation of intracellular-free calcium which accompanies fertilization. This ability to undergo cortical contraction is acquired within a few hours after the meiotically-arrested oocyte is triggered to resume meiosis by exposure to progesterone. This report examines the timing of changes in the contractile potential of the cortical cytoplasm as the oocyte becomes the egg, and in addition, the signal transduction events which induce these changes. We use the bisected oocyte system developed by Christensen et al. ('84; Nature 310: 150-151) to assess the changes in cortical potential during the meiotic resumption. Immediately after progesterone treatment (less than 5% of the way through the meiotic resumption) the cortex acquires the ability to form a contractile ring, an ability which gradually disappears during the meiotic resumption. Eighty percent of the way through the meiotic resumption the cortex of the hemisphere rapidly acquires the ability to undergo cortical contraction. In contrast, when bisected in a medium containing protein kinase C (PKC) agonists, the cortex of the hemisphere undergoes cortical contraction much earlier (i.e., 50% through the meiotic resumption). In addition, treatment of oocytes with PKC agonists alone can mimic the complete spectrum of changes in cortical potential induced by progesterone, suggesting that PKC has a role in reorganization of the cortical cytoskeleton which occurs as a normal response to progesterone. In support of this, antagonists of PKC block the progesterone-induced reorganization of the cortical cytoskeleton.

Role of the cytoskeleton during early development

Oocytes, eggs, and embryos from a diverse array of species have evolved cytoskeletal specializations which allow them to meet the needs of early embryogenesis. While each species studied possesses one or more specializations which are unique, several cytoskeletal features are widely conserved across different animal phyla. These features include highly-developed cortical cytoskeletal domains associated with developmental information, microtubule-mediated pronuclear transport, and rapid intracellular signal-regulated control of cytoskeletal organization.

Microtubule organization during maturation of Xenopus oocytes: assembly and rotation of the meiotic spindles

Assembly of the meiotic spindles during progesterone-induced maturation of Xenopus oocytes was examined by confocal fluorescence microscopy using anti-tubulin antibodies and by time-lapse confocal microscopy of living oocytes microinjected with fluorescent tubulin. Assembly of a transient microtubule array from a disk-shaped MTOC was observed soon after germinal vesicle breakdown. This MTOC-TMA complex rapidly migrated toward the animal pole, in association with the condensing meiotic chromosomes. Four common stages were observed during the assembly of both M1 and M2 spindles: (1) formation of a compact aggregate of microtubules and chromosomes; (2) reorganization of this aggregate resulting in formation of a short bipolar spindle; (3) an anaphase-B-like elongation of the prometaphase spindle, transversely oriented with respect to the oocyte A-V axis; and (4) rotation of the spindle into alignment with the oocyte axis. The rate of spindle elongation observed in M1 (0.7 microns min-1) was slower than that observed in M2 (1.8 microns min-1). Examination of spindles by immunofluorescence with antitubulin revealed numerous interdigitating microtubules, suggesting that prometaphase elongation of meiotic spindles in Xenopus oocytes results from active sliding of antiparallel microtubules. A substantial number of maturing oocytes formed monopolar microtubule asters during M1, nucleated by hollow spherical MTOCs. These monasters were subsequently observed to develop into bipolar M1 spindles and proceed through meiosis. The results presented define a complex pathway for assembly and rotation of the meiotic spindles during maturation of Xenopus oocytes.

Cytochemical evidence of an organized microtubular cytoskeleton in Xenopus laevis oocytes: involvement in the segregation of mitochondrial populations

An organized microtubular cytoskeleton was discovered in the cytoplasm of Xenopus laevis oocytes. The microtubules were observed in 10- to 30-micron cryostat sections by indirect immunoperoxidase labeling using an antibody to tubulin. A gradual extraction of cells with a nonionic detergent was essential for good penetration of the antibody into the cells. In the cytoplasm of all previtellogenic oocytes, a dense network of criss-crossed long microtubules was associated in a basket-like structure surrounding the mitochondrial mass. At the beginning of vitellogenesis, the network meshes enlarged, while clusters of mitochondria migrated, in close association with microtubule bundles. At the beginning of vitellogenesis, the reorganization of the microtubular network, mostly in the vegetal hemisphere, occurred during the segregation of the mitochondrial populations. Reorganization is characterized by (1) a temporary enlargement of the network and close association of mitochondrial clusters with microtubular bundles, and (2) a progressive organization of a ring-shaped microtubular structure in the crown elaboration area. It is hypothesized that these modifications of the microtubular cytoskeleton contribute to the maintenance of cell shape and the polarized organization of the cell.

Intracellular localization of MAP2-related protein (O-map) in prophase I and metaphase II oocytes of Xenopus

An antibody directed against rat brain microtubule-associated protein 2 (MAP2) immunoprecipitated a protein of 240 kDa from a Xenopus oocyte extract. In contrast, in Xenopus brain extract, this antibody recognizes a protein of 280 kDa. The oocyte MAP2-related protein (called O-MAP) is present in both prophase I and metaphase II-blocked oocytes as demonstrated by immunoblotting experiments; it is in vivo phosphorylated. Immunocytochemical studies using the anti-rat brain MAP2 antibody demonstrated that the O-MAP colocalizes within the cortical microtubular array present in both prophase I and metaphase II oocytes. However, O-MAP is not associated with the microtubular structures which are organized during the oocyte prophase-metaphase transition, i.e., a giant cytoplasmic network and both the meiotic spindles. O-MAP therefore appears as a microtubule-associated protein oocyte specific and may play a role in the regulation of microtubule stability and the organization of the oocyte cytoskeleton.

Transformation of the amphibian oocyte into the egg: structural and biochemical events

Amphibian oocytes, arrested in prophase I, are stimulated to progress to metaphase II by progesterone. This process is referred to as meiotic maturation and transforms the oocyte, which cannot support the early events of embryogenesis, into the egg, which can. Meiotic maturation entails global reorganization of cell ultrastructure: In the cell cortex, the plasma membrane flattens and the cortical granules undergo redistribution. In the cell periphery, the annulate lamellae disassemble and the mitochondria become dispersed. In the cell interior, the germinal vesicle becomes disassembled and the meiotic spindles form. Marked changes in the cytoskeleton and mRNA distribution also occur throughout the cell. All of these events are temporally correlated with intracellular signalling events: Fluctuations in cAMP levels, changes in pH, phosphorylation and dephosphorylation, and ion flux changes. Evidence suggests that specific intracellular signals are responsible for specific reorganizations of ultrastructure and mRNA distribution.

In vivo activation of a microtubule-associated protein kinase during meiotic maturation of the Xenopus oocyte

We have characterized a serine/threonine protein kinase from Xenopus metaphase-II-blocked oocytes, which phosphorylates in vitro the microtubule-associated protein 2 (MAP2). The MAP2 kinase activity, undetectable in prophase oocytes, is activated during the progesterone-induced meiotic maturation (G2-M transition of the cell cycle). p-Nitrophenyl phosphate, a phosphatase inhibitor, is required to prevent spontaneous deactivation of the MAP2 kinase in crude preparations; conversely, the partially purified enzyme can be in vitro deactivated by the low-Mr polycation-stimulated (PCSL) phosphatase (also termed protein phosphatase 2A2), working as a phosphoserine/phosphothreonine-specific phosphatase and not as a phosphotyrosyl phosphatase indicating that phosphorylation of serine/threonine is necessary for its activity. S6 kinase, a protein kinase activated during oocyte maturation which phosphorylates in vitro ribosomal protein S6 and lamin C, can be deactivated in vitro by PCSL phosphatase. S6 kinase from prophase oocytes can also be activated in vitro in fractions known to contain all the factors necessary to convert pre-M-phase-promoting factor (pre-MPF) to MPF. Active MAP2 kinase can activate in vitro the inactive S6 kinase present in prophase oocytes or reactivate S6 kinase previously inactivated in vitro by PCSL phosphatase. These data are consistent with the hypothesis that the MAP2 kinase is a link of the meiosis signalling pathway and is activated by a serine/threonine kinase. This will lead to the regulation of further steps in the cell cycle, such as microtubular reorganisation and S6 kinase activation.

Effect of microinjection of a low-Mr human placenta protein tyrosine phosphatase on induction of meiotic cell division in Xenopus oocytes

Homogeneous preparations of a protein phosphatase that is specific for phosphotyrosyl residues (protein tyrosine phosphatase [PTPase] 1B) were isolated from human placenta and microinjected into Xenopus oocytes. This resulted in an increase in activity of up to 10-fold over control levels, as measured in homogenates with use of an artificial substrate (reduced carboxamidomethylated and maleylated lysozyme). Microinjected PTPase was stable for at least 18 h. It is distributed within the oocyte in a manner similar to the endogenous activity and is suggestive of an interaction with cellular structures or molecules located predominantly in the animal hemisphere. The phosphatase markedly retarded (by up to 5 h) maturation induced by insulin. This, in conjunction with the demonstration that PTPase 1B abolished insulin stimulation of an S6 peptide (RRLSSLRA) kinase concomitant with a decrease in the phosphorylation of tyrosyl residues in a protein with the same apparent Mr as the beta subunit of the insulin and insulinlike growth factor 1 receptors (M. F. Cicirelli, N. K. Tonks, C. D. Diltz, E. H. Fischer, and E. G. Krebs, submitted for publication), provides further support for an essential role of protein tyrosine phosphorylation in insulin action. Furthermore, maturation was significantly retarded even when the PTPase was injected 2 to 4 h after exposure of the cells to insulin. PTPase 1B also retarded maturation induced by progesterone and maturation-promoting factor, which presumably do not act through the insulin receptor. These data point to a second site of action of the PTPase in the pathway of meiotic cell division, downstream of the insulin receptor and following the appearance of active maturation-promoting factor.

Effect of microinjection of a low-Mr human placenta protein tyrosine phosphatase on induction of meiotic cell division in Xenopus oocytes

Homogeneous preparations of a protein phosphatase that is specific for phosphotyrosyl residues (protein tyrosine phosphatase [PTPase] 1B) were isolated from human placenta and microinjected into Xenopus oocytes. This resulted in an increase in activity of up to 10-fold over control levels, as measured in homogenates with use of an artificial substrate (reduced carboxamidomethylated and maleylated lysozyme). Microinjected PTPase was stable for at least 18 h. It is distributed within the oocyte in a manner similar to the endogenous activity and is suggestive of an interaction with cellular structures or molecules located predominantly in the animal hemisphere. The phosphatase markedly retarded (by up to 5 h) maturation induced by insulin. This, in conjunction with the demonstration that PTPase 1B abolished insulin stimulation of an S6 peptide (RRLSSLRA) kinase concomitant with a decrease in the phosphorylation of tyrosyl residues in a protein with the same apparent Mr as the beta subunit of the insulin and insulinlike growth factor 1 receptors (M. F. Cicirelli, N. K. Tonks, C. D. Diltz, E. H. Fischer, and E. G. Krebs, submitted for publication), provides further support for an essential role of protein tyrosine phosphorylation in insulin action. Furthermore, maturation was significantly retarded even when the PTPase was injected 2 to 4 h after exposure of the cells to insulin. PTPase 1B also retarded maturation induced by progesterone and maturation-promoting factor, which presumably do not act through the insulin receptor. These data point to a second site of action of the PTPase in the pathway of meiotic cell division, downstream of the insulin receptor and following the appearance of active maturation-promoting factor.

Characterization of MPF activation by okadaic acid in Xenopus oocyte

Okadaic acid (OA), a specific inhibitor of protein phosphatases, induces a rapid activation (30 min) of MPF when microinjected into the Xenopus oocyte. Neither protein synthesis inhibitors nor cAMP counteract the action of OA. These results indicate that the inhibition of protein phosphatase(s) is sufficient for the in vivo activation of MPF even after the full activation of cAMP-dependent protein kinase. In all experimental conditions (plus or minus inhibitors of protein synthesis; normal or elevated cAMP levels) OA induces a burst of protein phosphorylation together with the activation of MPF. Cytological analysis shows that OA provokes the breakdown of the nuclear envelope, the depolymerization of lamin and the condensation of the chromosomes. However, no metaphase spindles are organized, indicating that inhibition of protein phosphatases strongly affects the function of the microtubule organizing center.

Germinal vesicle migration and dissolution in Rana pipiens oocytes: effect of steroids and microtubule poisons

Germinal vesicle migration (GVM) as evidenced by the appearance of the germinal vesicle at the animal pole surface was induced by nocadazole and demecolcine (colcemid). Nocodazole significantly lowered the progesterone ED50 for germinal vesicle dissolution (GVD). Both demecolcine and nocodazole enhanced centrifugation-induced GVM (i.e., lowered ooplasmic viscoelasticity) after 6-h incubation, and both potentiated the effect of progesterone in this assay. Estradiol, by contrast, inhibited GVM induced by demecolcine in both follicle-enclosed and denuded oocytes. Estradiol was also found to inhibit the normal enhancement of centrifugation-induced GVM by demecolcine or progesterone. Taxol was found to have effects that were generally opposite to those of demecolcine and nocodazole. Taxol inhibited centrifugation-induced GVM either alone or in the presence of progesterone. In addition, taxol significantly increased the progesterone ED50 for GVD induction. Taken together the available data support the hypothesis that microtubules play a role in maintaining the internal position of the germinal vesicle in the prematuration oocyte and that changes occur in the oocyte cytoskeleton during maturation.

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

Fertilized clam embryos synthesize several new cell-cycle-related proteins. The cloned cDNA and derived amino acid sequences of one of these, cyclin A, are presented here. Immunoblots with an anti-cyclin A antibody reveal that cyclin A is undetectable in oocytes, appears within 15 min of fertilization, and is destroyed near the end of each meiosis and mitosis. We directly tested the ability of cyclin A to induce M phase by injecting SP6 cyclin A mRNA into Xenopus oocytes, which are arrested at the G2/M border of first meiosis. The injected mRNA was translated, with the result that the Xenopus oocytes entered meiosis. These findings indicate that the rise in cyclin A plays a direct and natural role in driving cells into M phase.

A possible role for Mg2+ ions in the induction of meiotic maturation of Xenopus oocyte

Progesterone induces in vitro the meiotic cell division of Xenopus full-grown oocytes. Microinjection into oocyte of a solution containing Mg2+ (20 mM) facilitates by one order of magnitude the dose of progesterone which induces 50% of germinal vesicle breakdown. Microinjected in the absence of hormone, Mg2+ and also Mn2+ can induce maturation with efficiencies of, respectively, 24% (SEM = 8; n = 13) and 70% (SEM = 6; n = 23). The dose-response curves of cation-induction of maturation show an optimum of 20 mM for Mg2+ and 15 mM for Mn2+ (pipet concentration); higher doses were less active. Cation-induction of maturation is inhibited when oocytes are preincubated with cholera toxin (500 ng/ml); nevertheless, it cannot be interpreted at the level of cAMP, since both Mg2+ and Mn2+ microinjections provoke an increase in the oocyte cAMP content. Mg2+ induction of maturation is more efficient when oocytes are incubated in trimethylamine at pH 8.2, which is known to increase intracellular pH suggesting an action at the level of alkali pH-sensitive enzymes. Altogether, our results indicate a positive role for Mg2+ ions in the induction of oocyte maturation and raise an attractive hypothesis about the respective roles of cAMP and Mg2+ changes involved in the mechanism of progesterone action. Our results also show that co-injection of 2-glycerophosphate and Mg2+ ions, which are both commonly used in the preparation of the MPF mitotic factors from dividing cells, induces oocyte maturation more efficiently than Mg2+ alone and drastically shortens the kinetics of germinal vesicle breakdown to 1 h 30 min to 2 h 30 min.