Formation of the sea urchin male pronucleus in cell-free extracts

In vitro assembly of nuclear envelope in tobacco cultured cells

The coordinated regulation of the nucelar envelope (NE) reassembly during cell division is an essential event. However, there is little information on the molecular components involved in NE assembly in plant cells. Here we developed an in vitro assay of NE assembly using tobacco BY-2 cultured cells. To start the NE assembly reaction, the demembranated nuclei and the S12 fraction (cytosol and microsomes) were mixed in the presence of GTP and ATP nucleotides. Time-course analysis indicated that tubule structures were extended from the microsomal vesicles that accumulated on the demembranated nuclei, and finally sealed the NE. Immunofluorescence confirmed that the assembled membrane contains a component of nuclear pore complex. The efficiency of the NE assembly is significantly inhibited by GTPγS that suppresses membrane fusion. This in-vitro assay system may elucidate the role of specific proteins and provide important insights into the molecular machinery of NE assembly in plant cells.

Building a nuclear envelope at the end of mitosis: coordinating membrane reorganization, nuclear pore complex assembly, and chromatin de-condensation

The metazoan nucleus is disassembled and re-built at every mitotic cell division. The nuclear envelope, including nuclear pore complexes, breaks down at the beginning of mitosis to accommodate the capture of massively condensed chromosomes by the spindle apparatus. At the end of mitosis, a nuclear envelope is newly formed around each set of segregating and de-condensing chromatin. We review the current understanding of the membrane restructuring events involved in the formation of the nuclear membrane sheets of the envelope, the mechanisms governing nuclear pore complex assembly and integration in the nascent nuclear membranes, and the regulated coordination of these events with chromatin de-condensation.

MAP kinase dependent cyclinE/cdk2 activity promotes DNA replication in early sea urchin embryos

Sea urchins provide an excellent model for studying cell cycle control mechanisms governing DNA replication in vivo. Fertilization and cell cycle progression are tightly coordinated by Ca(2+) signals, but the mechanisms underlying the onset of DNA replication after fertilization remain less clear. In this study we demonstrate that calcium-dependent activation of ERK1 promotes accumulation of cyclinE/cdk2 into the male and female pronucleus and entry into first S-phase. We show that cdk2 activity rises quickly after fertilization to a maximum at 4 min, corresponding in timing to the early ERK1 activity peak. Abolishing MAP kinase activity after fertilization with MEK inhibitor, U0126, substantially reduces the early peak of cdk2 activity and prevents cyclinE and cdk2 accumulation in both sperm pronucleus and zygote nucleus in vivo. Both p27(kip1) and roscovitine, cdk2 inhibitors, prevented DNA replication suggesting cdk2 involvement in this process in sea urchin. Inhibition of cdk2 activity using p27(kip1) had no effect on the phosphorylation of MBP by ERK, but completely abolished phosphorylation of retinoblastoma protein, a cdk2 substrate, indicating that cdk2 activity is downstream of ERK1 activation. This pattern of regulation of DNA synthesis conforms to the pattern observed in mammalian somatic cells.

In vitro reassembly of nuclear envelopes and organelles in Xenopus egg extracts

We reconstituted bilayer nuclear membranes, multilayer membranes, and organelles from mixtures of Xenopus laevis egg extracts and demembranated Xenopus sperm nuclei. Varying proportions of the cytosolic and vesicular fractions from the eggs were used in the reconstitution mixtures. A cytosol:vesicle ratio of 10:1 promoted reassembly of the normal bilayer nuclear membrane with inserted nuclear pore complexes around the decondensed Xenopus sperm chromatin. A cytosol:vesicle ratio of 5:1 caused decondensed and dispersed sperm chromatin to be either surrounded by or divided by unusual multilayer membrane structures with inlaid pore complexes. A cytosol:vesicle ratio of 2.5:1 promoted reconstitution of mitochondria, endoplasmic reticulum networks, and Golgi apparatus. During reassembly of the endoplasmic reticulum and Golgi apparatus, vesicular fragments of the corresponding organelles fused together and changed their shape to form flattened cisternae, which were then stacked one on top of another.

Expression and transient nuclear translocation of proprotein convertase 1 (PC1) during mouse preimplantation embryonic development

Preimplantation embryos express a number of hormones, neuropeptides, and membrane receptors known to derive from proteolytic activation of their precursors by the seven-member family of subtilisin-like, calcium-dependent serine proteinases known as proprotein convertases (PCs). The goal of this study was to determine the pattern of PC expression in mouse preimplantation embryos. Transcripts for all PCs, except PC2, were detected by reverse transcription-polymerase chain reaction (RT-PCR) in unfertilized and fertilized eggs. Furin, PACE4, PC1, and PC7 transcripts remained present at subsequent stages of preimplantation embryonic development, whereas the levels of transcripts for PC4 and PC5 gradually disappeared after the 2-cell stage. Proprotein convertase 1 (PC1) expression was further examined at the protein level. Immunoblotting revealed the presence of the zymogen and mature forms of this enzyme in eggs and embryos. Immunofluorescence laser confocal microscopy showed PC1-specific staining throughout the cytoplasm of unfertilized eggs. After fertilization, surprisingly, the staining was concentrated in pronuclei. It relocated to the cytoplasm at postzygotic stages and was particularly strong at junctions between blastomeres. The nuclear translocation of PC1 in fertilized eggs is probably mediated by its prodomain. Indeed, when transduced in human colon carcinoma LoVo cells, a mutant proPC1 incapable of cleaving off its prodomain was shown to accumulate in the nucleus. Furthermore, when N-terminally fused to green fluorescent protein, this domain was able to direct the reporter protein to the nucleus of these cells. Collectively, these data establish that eggs and preimplantation embryos express various PCs necessary for proteolytic activation of precursors of hormones and growth factors. They also raise the possibility of a nuclear function for PC1 during zygote formation.

Comparative studies on in vitro sperm decondensation and pronucleus formation in egg extracts between gynogenetic and bisexual fish

A cell-free system based upon the egg extracts from gynogenetic gibel carp (Carassius auratus gibelio) or bisexual red common carp (Cyprinus carpio red variety) was developed to investigate developmental behaviors of the demembranated sperm nuclei. Both red common carp and gibel carp sperm nuclei could decondense fully and form pronuclei in the red common carp egg extracts. Gibel carp sperm nuclei could also decondense fully and form pronuclei in the gibel carp egg extracts, but red common carp sperm nuclei could not decondense sufficiently in the same extracts. The significant differences of morphological changes were further confirmed by ultrastructural observation of transmission electron microscopy. The data further offer cytological evidence for gonochoristic reproduction in the gynogenetically reproducing gibel carp. In addition, the sperm nuclei in vitro decondensation is dependent on the pH in the extracts, and the decondensed efficiency is optimal at pH 7. However, no DNA replication was observed in the two kinds of egg extracts during the incubation period of the sperm nuclei. It is suggested that the egg extracts prepared from the gynogenetic gibel carp should be a valid in vitro system for studying molecular mechanism on gynogenesis and reproduction mode diversity in fish.

Remodeling of sperm chromatin after fertilization involves nucleosomes formed by sperm histones H2A and H2B and two CS histone variants

The composition of nucleosomes at an intermediate stage of male pronucleus formation was determined in sea urchins. Nucleosomes were isolated from zygotes harvested 10 min post-insemination, whole nucleoprotein particles were obtained from nucleus by nuclease digestion, and nucleosomes were subsequently purified by a sucrose gradient fractionation. The nucleosomes derived from male pronucleus were separated from those derived from female pronucleus by immunoadsorption to antibodies against sperm specific histones (anti-SpH) covalently bound to Sepharose 4B (anti-SpH-Sepharose). The immunoadsorbed nucleosomes were eluted, and the histones were analyzed by Western blots. Sperm histones (SpH) or alternatively, the histones from unfertilized eggs (CS histone variants), were identified with antibodies directed against each set of histones. It was found that these nucleosomes are organized by a core formed by sperm histones H2A and H2B combined with two major CS histone variants. Such a hybrid histone core interacts with DNA fragments of approximately 100 bp. It was also found that these atypical nucleosome cores are subsequently organized in a chromatin fiber that exhibits periodic nuclease hypersensitive sites determined by DNA fragments of 500 bp of DNA. It was found that these nucleoprotein particles were organized primarily by the hybrid nucleosomes described above. We postulate that this unique chromatin organization defines an intermediate stage of male chromatin remodeling after fertilization.

The Ran GTPase as a marker of chromosome position in spindle formation and nuclear envelope assembly

The small GTPase Ran is a key regulator of nucleocytoplasmic transport during interphase. The asymmetric distribution of the GTP-bound form of Ran across the nuclear envelope--that is, large quantities in the nucleus compared with small quantities in the cytoplasm--determines the directionality of many nuclear transport processes. Recent findings that Ran also functions in spindle formation and nuclear envelope assembly during mitosis suggest that Ran has a general role in chromatin-centred processes. Ran functions in these events as a signal for chromosome position.

Nuclear assembly of demembranated Xenopus sperm in plant cell-free extracts from Nicotiana ovules

The cell-free preparation derived from Nicotiana tabaccum ovules induced chromatin decondensation and pronuclear formation from demembranated Xenopus laevis sperm nuclei. Fluorescent microscope and phase-contrast microscope visualization of assembly intermediates indicated that 95.6% of X. leavis sperm changed their tadpole-like shape to circular shape or elliptical shape after over 1.5 h of incubation. Transmission electron microscope visualization showed that nuclear membrane was assembled around the periphery of the dispersed chromatin. Nuclear envelope of most reassembled nuclei was composed of a double membrane inlaid with a little single membrane. Nucleosome assembly was verified by means of micrococcal nuclease digestion. After 2 to 5 h of incubation, digestion of the product of nuclear assembly with micrococcal nuclease produced at least six nucleosome fragments of about 250 bp each.