379 TREATMENT OF 293T CELLS WITH XENOPUS EGG EXTRACT INDUCES NOT-STRESS-RELATED EPIGENETIC REMODELING OF RIBOSOMAL GENES

Undefined mechanisms involved in reprogramming of somatic cells by Xenopus leavis egg extract have, to date, prevented clinical applications for cell replacement therapy. The aim of this study was to evaluate the immediate response of somatic cells to exposure to Xenopus leavis egg extract using ribosomal genes (rDNA) as a sensitive marker for stress and/or chromatin remodeling. Human epithelial 293T cells in standard culture (control), starved in glucose-free medium (stress-control), or treated for 1 h with egg extract were fixed at 6 and 24 h after treatment and analyzed for pre-rRNA synthetic activity by quantitative RT-PCR; level of H3K9me3 by immunofluorescence; and for occupancy of rDNA promoter by markers of normal activity (UBF), stress silencing (SIRT1, SUV39H1), and remodeling (SNF2H) using chromatin immunoprecipitation. Relative levels of pre-rRNA decreased in all treated groups to 20 to 35% of control levels. Chromatin immunoprecipitation did not reveal any significant effect of starvation or extract treatment on UBF and SIRT1 binding to the rDNA promoter, presumably because of the pleiotropic property of these proteins. However, starvation specifically led to enhanced SUV39H1 binding 6 h after treatment, indicating early cell response to stress. No differences were found in SUV39H1 binding between extract-treated cells and control. In contrast, binding of SNF2H at 6 h increased in extract-treated cells but decreased in starved cells. The relative level of H3K9me3 increased first after 24 h of starvation equally in all cells, suggesting later involvement of this histone modification in rDNA silencing. Extract treatment, however, led to a decrease in H3K9me3 level at 6 h, and after 24 h, 2 main cell populations were observed: one (85.4% of cells) that retained decreased H3K9me3 and one (12.4% of cells) with significantly enhanced levels. Moreover, H3K9me3 foci in the last group were associated with the periphery of presumptive nucleoli. Thus, extract treatment apparently does not initiate a stress-induced silencing of the rDNA genes; rather, it activates SNF2H-dependent chromatin remodeling, resulting in a long-term decrease in pre-rRNA synthetic activity. The research was supported by Carlsberg Foundation 2008-01-0105.

Toward reprogramming cells to pluripotency

The possibility of turning one somatic cell type into another may in the long run have beneficial applications in regenerative medicine. Somatic cell nuclear transfer (therapeutic cloning) may offer this possibility; however, ethical guidelines prevent application of this technology in many in countries. As a result, alternative approaches are being developed for altering cell fate. This communication discusses recent non-nuclear transfer-based in vitro approaches for reprogramming cells and enhancing their potential for differentiation toward various lineages.

220 CpG METHYLATION PROFILE OF ADIPOGENIC AND ENDOTHELIAL GENE PROMOTERS IN HUMAN ADIPOSE STEM CELLS SUGGESTS A RESTRICTIVE ENDOTHELIAL DIFFERENTIATION POTENTIAL

Mesenchymal stem cells (MSCs) have received intense research interest due to their perceived potential application in regenerative medicine; nevertheless, MSCs are primarily restricted to form mesodermal cell types. Adipose stem cells (ASCs) with a CD34+ CD105+ CD45– CD31– immunophenotype can be obtained in an uncultured state with high purity from the stromal vascular fraction of human liposuction material (Boquest et al. 2005 Mol. Biol. Cell 16, 1131–1141). While ASCs differentiate readily into adipocytes, their endothelial lineage commitment has been scarcely reported, and controversy remains regarding ASC contribution to vascularization. To address the epigenetic commitment of ASCs to adipogenic and endothelial lineages, we carried out a bisulfite sequencing analysis of CpG methylation in the promoters of adipogenic (LEP, PPARG2, FABP4, LPL), endothelial (CD31, CD144), and myogenic (MYOG) genes in freshly isolated and in clonal ASC cultures in relation to gene expression and differentiation potential. Uncultured ASCs display mosaic hypomethylation of adipogenic promoters, in contrast to MYOG, CD31, or CD144 which are methylated (Noer et al. 2006 Mol. Biol. Cell 17, in press). Nevertheless, CpG methylation does not reflect transcriptional status of these genes in undifferentiated cells. Culture and adipogenic differentiation of ASCs maintains the hypomethylated profile of adipogenic promoters and the hypermethylation of non-adipogenic promoters. Endothelial stimulation of ASCs in methylcellulose elicits tubule-like networks, up-regulation of CD31 and CD144, and restrictive induction of a CD31+ CD144+ immunophenotype. Discrete and lineage-specific changes in CpG methylation in the CD31 and CD144 promoters take place but no global demethylation that marks endothelial cells occurs. Promoters not involved in endothelial differentiation retain a methylation profile characteristic of undifferentiated cells. Hypermethylation of CD31 and CD144 suggests a restricted commitment of ASCs to the endothelial lineage. This contrasts with hypomethylation of adipogenic promoters which reflects a propensity toward adipogenic differentiation. Despite the up-regulation of lineage-specific transcripts, overall maintenance of promoter methylation after adipogenic, osteogenic, and endothelial differentiation suggests the maintenance of an epigenetic signature characteristic of undifferentiated cells. Analysis of CpG methylation at lineage-specific promoters should provide a robust assessment of epigenetic commitment of stem cells to a specific lineage.

Dedifferentiation of cells: new approaches

Reprogramming of a differentiated cell into a cell capable of giving rise to many different cell types, a pluripotent cell, which in turn could repopulate or repair sick or damaged tissue, would present beneficial applications in regenerative medicine. Somatic cell nuclear transfer may offer this possibility, but technical hurdles and ethical frameworks currently prevent application of this technology in several countries. As a result, alternative strategies to reprogramming cell fate are being developed. This review briefly addresses somatic cell nuclear transfer and focuses on recent non-nuclear transfer-based approaches for reprogramming somatic cells and enhancing their differentiation potential. These include the fusion of somatic cells with embryonic stem cells, the treatment of somatic cells with extract of pluripotent cells and the retroviral transduction of somatic cells to overexpress pluripotency genes.

221 EPIGENETIC DYNAMICS OF PLURIPOTENCY GENES IN THE CONTEXT OF DIFFERENTIATION AND NUCLEAR REPROGRAMMING DETERMINED BY Q2ChIP, A QUICK AND QUANTITATIVE CHROMATIN IMMUNOPRECIPITATION TECHNIQUE APPLICABLE TO SMALL CELL SAMPLES

Interactions between proteins and DNA are essential for cellular functions such as genomic stability, DNA replication and repair, chromosome segregation, transcription, and epigenetic silencing of gene expression. Chromatin immunoprecipitation (ChIP) is a key technique for mapping histone modifications and transcription factor binding on DNA and thereby unraveling the role of epigenetics in the regulation of gene expression. Current ChIP protocols require extensive sample handling and large numbers of cells (5-10 million). primarily owing to ample loss of material during the procedure. We altered critical steps of conventional ChIP to develop a quick and quantitative (Q2) ChIP assay suitable for cell numbers 100- to 1000-fold lower than those required for conventional ChIP. Key modifications of the ChIP procedure include (i) formaldehyde DNA–protein cross-linking in suspended cells, (ii) cross-linking in the presence of 20 mM sodium butyrate to enhance specificity of precipitation of acetylated histones, (iii) transfer of washed precipitated immune complexes to a clean tube ('tube shift') to increase ChIP specificity by virtually eliminating nonspecifically bound chromatin, and (iv) combination of cross-link reversal, protein digestion, and DNA elution into a single 2-h step. We used Q2ChIP to monitor changes in 6 histone H3 modifications on the human developmentally regulated genes OCT4 (POU5F1), NANOG, and LMNA (lamin A) in the context of retinoic acid (RA)-mediated differentiation of embryonal carcinoma cells and upon reprogramming of kidney epithelial 293T cells to pluripotency in carcinoma cell extract (Taranger et al. 2005 Mol. Biol. Cell 16, 5719–5735). Real-time PCR analysis of precipitated DNA unravels an unexpected two-step heterochromatin assembly elicited by RA on the OCT4 proximal promoter, proximal enhancer, and distal enhancer, and on the NANOG promoter, whereby methylation of H3K9 and H3K27 is followed by H3K9 deacetylation. H3K4 di- and trimethylation remain relatively unaffected by RA treatment. In contrast, reprogramming of 293T cells in carcinoma extract promotes assembly of histone marks characteristic of transcriptional induction of OCT4 and NANOG, such as acetylation and demethylation of H3K9. The results argue toward ordered chromatin repackaging at developmentally regulated promoters upon differentiation or, conversely, nuclear reprogramming to pluripotency.

167 ISOLATION AND COMPARATIVE PROFILING OF HUMAN ADIPOSE-DERIVED ADULT STEM CELLS

The stromal compartment of mesenchymal tissues is thought to harbor stem cells that display extensive proliferative capacity and multilineage potential. However, despite their potential impact in the field of regenerative medicine, little is known about the biology of stromal stem cells prior to culture. After removing adipocytes and erythrocytes from collagenase digested human adipose tissue, we identified two cell populations using flow cytometry which shared expression of stem cell markers SH2 and CD34, but lacked the phenotypic characteristics of leukocytes (CD45−). However, they were found to be discernible based on CD31 expression, a marker for endothelial cells. Using CD31 conjugated magnetic beads, we separated these cells (CD45-CD31− and CD45-CD31+) from three patients and compared global gene expression profiles using an Affymetrix platform. The prominant feature of CD45-CD31+ cells was the up-regulation of genes associated with endothelial cells. By contrast, CD45-CD31− cells were found to overexpress transcripts involved in cell cycle quiescence and cell signaling elements including those of the WNT pathway thought to be important for maintaining the stem cell state. Upon culture in DMEM/F12 with 20% FCS, only CD45-CD31− cells were capable of adhering to plastic and forming colonies. These cells with fibroblastic morphology met the key criterion of stem cells, the ability to proliferate while retaining the capacity to differentiate into mature tissues. Under appropriate inductive conditions, they were found to exclusively form bone, cartilage, adipose and neuronal-like tissues in vitro. Clonal cell lines generated from individually cultured CD45-CD31− cells displayed multilineage and proliferative capacity, validating our conclusion that they are true stem cells and not simply committed progenitors. We then undertook extensive comparative profiling of CD45-CD31− cells with their cultured counterparts to examine changes that stromal stem cells undergo during culture. Except for the disappearance of CD34, flow cytometry analysis using 52 antibodies revealed little change in cell surface phenotype as a result of culture. However, comparative global gene profiling revealed extensive down-regulation of many genes during culture. These included cell cycle arresting genes, as expected, and genes encoding elements involved in cell signaling including those belonging to the tumor necrosis factor, interleukin, transforming growth factor and chemokine families. The consequences of these changes remain unknown, but ultimately may affect the potential use of adipose tissue stem cells in regenerative medicine.

Dynamics of the nuclear envelope at mitosis and during apoptosis

The nuclear envelope is a highly dynamic structure that reversibly disassembles and reforms at mitosis. The nuclear envelope also breaks down--irreversibly--during apoptosis, a process essential for development and tissue homeostasis. Analyses of fixed cells, time-lapse, imaging studies of live cells and the development of powerful cell-free extracts derived from gametes or mammalian somatic cells have provided insights on the fate of nuclear envelope proteins during mitosis and apoptosis, and on the mechanisms behind nuclear envelope modifications in these processes. In this review, we discuss evidence leading to our understanding of the dynamics of the nuclear envelope alterations at mitosis and during apoptosis. We also present novel imaging and genetic approaches to the study of nuclear envelope dynamics and function.

Regulation of anchoring of the RIIα regulatory subunit of PKA to AKAP95 by threonine phosphorylation of RIIα: implications for chromosome dynamics at mitosis

CDK1 phosphorylates the A-kinase regulatory subunit RIIα on threonine 54 (T54) at mitosis, an event proposed to alter the subcellular localization of RIIα. Using an RIIα-deficient leukemic cell line (Reh) and stably transfected Reh cell clones expressing wild-type RIIα or an RIIα(T54E) mutant, we show that RIIα associates with chromatin-bound A-kinase anchoring protein AKAP95 at mitosis and that this interaction involves phosphorylation of RIIα on T54. During interphase, both RIIα and RIIα(T54E) exhibit a centrosome-Golgi localization, whereas AKAP95 is intranuclear. At mitosis and in a mitotic extract, most RIIα, but not RIIα(T54E), co-fractionates with chromatin, onto which it associates with AKAP95. This correlates with T54 phosphorylation of RIIα. Disrupting AKAP95-RIIα anchoring or depleting RIIα from the mitotic extract promotes premature chromatin decondensation. In a nuclear reconstitution assay that mimics mitotic nuclear reformation, RIIα is threonine dephosphorylated and dissociates from AKAP95 prior to assembly of nuclear membranes. Lastly, the Reh cell line exhibits premature chromatin decondensation in vitro, which can be rescued by addition of wild-type RIIα or an RIIα(T54D) mutant, but not RIIα(T54E, A, L or V) mutants. Our results suggest that CDK1-mediated T54 phosphorylation of RIIα constitutes a molecular switch controlling anchoring of RIIα to chromatin-bound AKAP95, where the PKA-AKAP95 complex participates in remodeling chromatin during mitosis.

CDK1-mediated phosphorylation of the RIIα regulatory subunit of PKA works as a molecular switch that promotes dissociation of RIIα from centrosomes at mitosis

Protein kinase A regulatory subunit RIIα is tightly bound to centrosomal structures during interphase through interaction with the A-kinase anchoring protein AKAP450, but dissociates and redistributes from centrosomes at mitosis. The cyclin B-p34cdc2 kinase (CDK1) has been shown to phosphorylate RIIα on T54 and this has been proposed to alter the subcellular localization of RIIα. We have made stable transfectants from an RIIα-deficient leukemia cell line (Reh) that expresses either wild-type or mutant RIIα (RIIα(T54E)). When expressed, RIIα detaches from centrosomes at mitosis and dissociates from its centrosomal location in purified nucleus-centrosome complexes by incubation with CDK1 in vitro. By contrast, centrosomal RIIα(T54E) is not redistributed at mitosis, remains mostly associated with centrosomes during all phases of the cell cycle and cannot be solubilized by CDK1 in vitro. Furthermore, RIIα is solubilized from particular cell fractions and changes affinity for AKAP450 in the presence of CDK1. D and V mutations of T54 also reduce affinity for the N-terminal RII-binding domain of AKAP450, whereas small neutral residues do not change affinity detected by surface plasmon resonance. In addition, only RIIα(T54E) interacts with AKAP450 in a RIPA-soluble extract from mitotic cells. Finally, microtubule repolymerization from mitotic centrosomes of the RIIα(T54E) transfectant is poorer and occurs at a lower frequency than that of RIIα transfectants. Our results suggest that T54 phosphorylation of RIIα by CDK1 might serve to regulate the centrosomal association of PKA during the cell cycle.

Phosphodiesterase 4D and protein kinase a type II constitute a signaling unit in the centrosomal area

The mediation of cAMP effects by specific pools of protein kinase A (PKA) targeted to distinct subcellular domains raises the question of how inactivation of the cAMP signal is achieved locally and whether similar targeting of phosphodiesterases (PDEs) to sites of cAMP/PKA action could be observed. Here, we demonstrate that Sertoli cells of the testis contain an insoluble PDE4D3 isoform, which is shown by immunofluorescence to target to centrosomes. Staining of PDE4D and PKA shows co-localization of PDE4D with PKA-RIIalpha and RIIbeta in the centrosomal region. Co-precipitation of RII subunits and PDE4D3 from cytoskeletal extracts indicates a physical association of the two proteins. Distribution of PDE4D overlaps with that of the centrosomal PKA-anchoring protein, AKAP450, and AKAP450, PDE4D3, and PKA-RIIalpha co-immunoprecipitate. Finally, both PDE4D3 and PKA co-precipitate with a soluble fragment of AKAP450 encompassing amino acids 1710 to 2872 when co-expressed in 293T cells. Thus, a centrosomal complex that includes PDE4D and PKA constitutes a novel signaling unit that may provide accurate spatio-temporal modulation of cAMP signals.

Binding of PKA-RIIalpha to the Adenovirus E1A12S oncoprotein correlates with its nuclear translocation and an increase in PKA-dependent promoter activity

The adenovirus type 12 (Ad12) E1A12S oncoprotein utilizes the cAMP/protein kinase A (PKA) signal transduction pathway to activate expression of the viral E2 gene, the products of which are essential for viral replication. A central unsolved question is, however, whether E1A12S interacts directly with PKA in the process of promoter activation. We show here that E1A12S binds to the regulatory subunits (R) of PKA in vitro and in vivo. Interaction depends on the N-terminus and the conserved region 1 (CR1) of E1A12S. Both domains are also essential for the activation of viral E2 gene expression. Infection of cells with Ad12 leads to the cellular redistribution of RIIalpha from the cytoplasm into the nucleus. Furthermore, RIIalpha is also located in the nucleus of cells transformed by E1 of Ad12 and transient expression of E1A12S leads to the redistribution of RIIalpha into the nucleus in a N-terminus- and CR1-dependent manner. Cotransfection of E1A12S with RIIalpha results in strong activation of the E2 promoter. Based on these results we conclude that E1A12S functions as a viral A-kinase anchoring protein redistributing RIIalpha from the cytoplasm into the nucleus where it is involved in E1A12S-mediated activation of the E2 promoter.

HA95 is a protein of the chromatin and nuclear matrix regulating nuclear envelope dynamics

We report a role for HA95, a nuclear protein with high homology to the nuclear A-kinase anchoring protein AKAP95, in the regulation of nuclear envelope-chromatin interactions. Biochemical and photobleaching data indicate that HA95 is tightly associated with chromatin and the nuclear matrix/lamina network in interphase, and bound to chromatin at mitosis. HA95 resides in a complex together with lamin B receptor (LBR), lamina-associated polypeptide (LAP)2 and emerin, integral proteins of the inner nuclear membrane. Cross-linking experiments, however, illustrate a tight association of HA95 with LBR and LAP2 only. Intra-nuclear blocking of HA95 with anti-HA95 antibodies abolishes nuclear breakdown in a mitotic HeLa cell extract. The antibodies inhibit nuclear membrane breakdown and chromatin condensation - the latter independently of nuclear membranes. However, lamina disassembly is not affected, as judged by immunological analyses of A/C- and B-type lamins. In contrast, immunoblocking of HA95 bound to condensed chromosomes does not impair chromatin decondensation, nuclear membrane reassembly or lamina reformation. Our results argue for a role for HA95 in anchoring nuclear membranes and lamins to chromatin in interphase, and in releasing membranes from chromatin at mitosis. The data also suggest that HA95 is not involved in initial binding of membranes to chromatin upon nuclear reassembly. We propose that HA95 is a central platform at the chromatin/nuclear matrix interface implicated in regulating nuclear envelope-chromatin interactions during the cell cycle.

Recruitment of protein phosphatase 1 to the nuclear envelope by A-kinase anchoring protein AKAP149 is a prerequisite for nuclear lamina assembly

Subcellular targeting of cAMP-dependent protein kinase (protein kinase A [PKA]) and of type 1 protein phosphatase (PP1) is believed to enhance the specificity of these enzymes. We report that in addition to anchoring PKA, A-kinase anchoring protein AKAP149 recruits PP1 at the nuclear envelope (NE) upon somatic nuclear reformation in vitro, and that PP1 targeting to the NE is a prerequisite for assembly of B-type lamins. AKAP149 is an integral membrane protein of the endoplasmic reticulum/NE network. The PP1-binding domain of AKAP149 was identified as K(153)GVLF(157). PP1 binds immobilized AKAP149 in vitro and coprecipitates with AKAP149 from purified NE extracts. Affinity isolation of PP1 from solubilized NEs copurifies AKAP149. Upon reassembly of somatic nuclei in interphase extract, PP1 is targeted to the NE. Targeting is inhibited by a peptide containing the PP1-binding domain of AKAP149, abolished in nuclei assembled with membranes immunodepleted of AKAP149, and restored after reincorporation of AKAP149 into nuclear membranes. B-type lamins do not assemble into a lamina when NE targeting of PP1 is abolished, and is rescued upon recruitment of PP1 to the NE. We propose that kinase and phosphatase anchoring at the NE by AKAP149 plays in a role in modulating nuclear reassembly at the end of mitosis.

Localization of a novel human A-kinase-anchoring protein, hAKAP220, during spermatogenesis

Using a combination of protein kinase A type II overlay screening, rapid amplification of cDNA ends, and database searches, a contig of 9923 bp was assembled and characterized in which the open reading frame encoded a 1901-amino-acid A-kinase-anchoring protein (AKAP) with an apparent SDS-PAGE mobility of 220 kDa, named human AKAP220 (hAKAP220). The hAKAP220 amino acid sequence revealed high similarity to rat AKAP220 in the 1167 C-terminal residues, but contained 727 residues in the N-terminus not present in the reported rat AKAP220 sequence. The hAKAP220 mRNA was expressed at high levels in human testis and in isolated human pachytene spermatocytes and round spermatids. The hAKAP220 protein was present in human male germ cells and mature sperm. Immunofluorescent labeling with specific antibodies indicated that hAKAP220 was localized in the cytoplasm of premeiotic pachytene spermatocytes and in the centrosome of developing postmeiotic germ cells, while a midpiece/centrosome localization was found in elongating spermatocytes and mature sperm. The hAKAP220 protein together with a fraction of PKA types I and II and protein phosphatase I was resistant to detergent extraction of sperm tails, suggesting an association with cytoskeletal structures. In contrast, S-AKAP84/D-AKAP1, which is also present in the midpiece, was extracted under the same conditions. Anti-hAKAP220 antisera coimmunoprecipitated both type I and type II regulatory subunits of PKA in human testis lysates, indicating that hAKAP220 interacts with both classes of R subunits, either through separate or through a common binding motif(s).

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

At fertilization, the dormant sperm nucleus undergoes morphological and biochemical transformations leading to the development of a functional male pronucleus. We have investigated the formation of the male pronucleus in a cell-free system consisting of permeabilized sea urchin sperm nuclei incubated in fertilized sea urchin egg extract containing membrane vesicles. The first sperm nuclear transformation observed in vitro is the disassembly of the sperm nuclear lamina as a result of lamin B phosphorylation mediated by egg protein kinase C. The conical sperm nucleus then decondenses into a spherical pronucleus in an ATP-dependent manner. The new nuclear envelope (NE) forms by ATP-dependent binding of vesicles to chromatin and GTP-dependent fusion of vesicles with one another. Three cytoplasmic vesicle fractions with distinct properties are required for the formation of the male pronuclear envelope. Binding of each fraction to chromatin requires two detergent-resistant lipophilic structures at each pole of the sperm nucleus, which are incorporated into the NE by membrane fusion. Targeting of the bulk of NE vesicles to chromatin is mediated by a lamin B receptor (LBR)-like integral membrane protein. The last step of male pronuclear formation involves nuclear swelling. Nuclear swelling is associated with import of soluble lamin B into the nucleus and growth of the NE. In the nucleus, lamin B associates with LBR, which apparently tethers the NE to the lamina. Thus, formation of the male pronuclear envelope involves a highly ordered series of reactions.

Gene-Gun-Mediated Transfer of Reporter Genes to Somatic Zebrafish (Danio rerio) Tissues

We describe the use of gene-gun-mediated transfer of luciferase and green fluorescent protein (GFP) reporter genes in zebrafish (Danio rerio). Optimization of DNA transfer parameters indicated highest overall luciferase expression in epidermis and dermis using 1-μm microcarriers and 1 μg of pCMVL plasmid DNA at a delivery pressure of 200 psi. Time course studies revealed luciferase activity peaking at 18 hours and decreasing to 30% of the maximum at day 8 after DNA transfer. Onset of reporter gene (GFP) expression was detected at 13 minutes after DNA delivery, and by 65 minutes approximately 100% of the cells in the target area exhibited GFP expression. No germline association or integration events were detected in a screen of approximately 250,000 zebrafish sperm cells by fluorescence in situ hybridization at 15 or 30 days after delivery of 1 μg of pCMVL DNA, suggesting incidental male germline integration should not be considered as a risk factor when using the biolistic DNA delivery parameters and target tissues described.

Glowing zebrafish: integration, transmission, and expression of a single luciferase transgene promoted by noncovalent DNA-nuclear transport peptide complexes

The development of vehicles driving foreign DNA into the cell nucleus is essential for effective cellular gene transfer applications. We report that noncovalent binding of nuclear localization signal (NLS) peptides to plasmid DNA enhances nuclear uptake of the DNA and promotes germline integration, inheritance, and expression of a single copy of a luciferase reporter gene in zebrafish. As few as 10 DNA-NLS complexes (0.06 fg plasmid DNA) cytoplasmically injected are sufficient to produce germline-transgenic zebrafish bearing a single copy of the transgene. This corresponds to a 10(5)-fold reduction in DNA concentration compared to commonly used procedures. Use of 10(3) or 10(4) DNA-NLS complexes augments the number of transgene integrations, which occur mostly within 1-4 distinct insertion sites in the genome. In situ hybridization analyses and transmission studies show that transgene integration into the germline and somatic tissues is mosaic, and that the extent of mosaicism is negatively correlated with the amount of DNA-NLS injected. In addition, a larger proportion of zebrafish harboring a single copy of the transgene expresses luciferase, albeit at a 10-fold lower level than those containing numerous transgene insertions. The data demonstrate the potential use of nuclear targeting peptides noncovalently bound to vector DNA to enhance the efficiency of biotechnological nonviral gene transfer applications.

Rearrangements of sea urchin egg cytoplasmic membrane domains at fertilization

Fertilization in the sea urchin is accompanied by rapid reorganization of the egg endoplasmic reticulum (ER). ER-derived vesicles contribute to one of three classes of membranes used in assembling the male pronuclear envelope in vitro. We provide here biochemical evidence for the rearrangement of sea urchin egg cytoplasmic membrane domains at fertilization up to the first mitosis, with respect to two nuclear envelope markers, lamin B and lamin B receptor (LBR), using purified vesicles prepared from homogenates fractionated by floatation on sucrose gradients. In unfertilized eggs, immunoprecipitation data indicate that most of lamin B and LBR are localized in the same vesicles but do not interact. By 3 min post-fertilization, both proteins are more widely distributed across the gradients and by 12 min most of lamin B and LBR are localized in vesicles of different densities. This partitioning is maintained throughout S phase. At mitosis, most lamin B and LBR remain in distinct vesicles, while a small proportion of lamin B and LBR, likely derived from the disassembled nuclear envelope, associate in a minor subset of vesicles. The results illustrate a dynamic reorganization of egg cytoplasmic membranes at fertilization, and the establishment of distinct membrane domains enriched in specific nuclear envelope markers during the first cell cycle of sea urchin development. Additionally, we demonstrate that male pro-nuclear membrane assembly occurs only when both cytosol and membranes originate from fertilized but not unfertilized eggs, suggesting that fertilization-induced membrane rearrangements contribute to the ability of the egg to assemble the male pronuclear envelope.

Sorting nuclear membrane proteins at mitosis

The nuclear envelope (NE) breaks down reversibly and reassembles at mitosis. Two models of mitotic nuclear membrane disassembly and reformation have emerged from studies of NE dynamics in somatic cells and egg extracts. One model suggests that nuclear membranes fragment reversibly by vesiculation, producing NE-derived vesicles separate from the endoplasmic reticulum. The second model proposes that nuclear membranes vanish by diffusion of their integral proteins through a continuous endoplasmic reticulum. Here, we discuss critically the grounds for the elaboration of these apparently mutually exclusive views. Our conclusions favour a model in which nuclear membranes do not vesiculate during mitosis.

Identification, cloning and characterization of a novel nuclear protein, HA95, homologous to A-kinase anchoring protein 95

Previously, we have identified and characterized nuclear AKAP95 from man which targets cyclic AMP (cAMP)-dependent protein kinase (PKA)-type II to the condensed chromatin/spindle region at mitosis. Here we report the cloning of a novel nuclear protein with an apparent molecular mass of 95 kDa that is similar to AKAP95 and is designated HA95 (homologous to AKAP95). HA95 cDNA sequence encodes a protein of 646 amino acids that shows 61% homology to the deduced amino acid sequence of AKAP95. The HA95 gene is located on chromosome 19p13.1 immediately upstream of the AKAP95 gene. Both HA95 and AKAP95 genes contain 14 exons encoding similar regions of the respective proteins, indicating a previous gene duplication event as the origin of the two tandem genes. Despite their apparent similarity, HA95 does not bind RII in vitro. HA95 contains a putative nuclear localization signal in its N-terminal domain. It is localized exclusively into the nucleus as demonstrated in cells transfected with HA95 fused to either green fluorescence protein or the c-myc epitope. In the nucleus, the HA95 protein is found as complexes directly associated with each other or indirectly associated via other nuclear proteins. In interphase, HA95 is co-localized with AKAP95, but the two proteins are not biochemically associated. At metaphase, both proteins co-localize with condensed chromosomes. The similarity in sequence and localization of HA95 and AKAP95 suggests that the two molecules constitute a novel family of nuclear proteins that may exhibit related functions.

Cytoplasmic control of nuclear assembly

The reconstitution of a replication-competent, transcriptionally active nucleus following mitosis, fertilization or nuclear transplantation involves a stepwise series of reactions, most (if not all) of which are controlled by the cytoplasmic environment. This review discusses the nature of cytoplasmic contributions to the development of the male pronucleus at fertilization, and the effect of altering the cytoplasmic environment on nuclear assembly. The system used to investigate these regulations consists of permeabilized sea urchin sperm nuclei incubated under controlled conditions in a cell-free extract of fertilized sea urchin eggs. (1) In egg cytoplasmic extract, male pronuclear formation is initiated by the disassembly of the sperm nuclear lamina as a result of lamin phosphorylation by a cytosolic protein kinase C. (2) Sperm histones are phosphorylated by an as yet unidentified soluble kinase. (3) The conical sperm nucleus decondenses into a spherical pronucleus in an ATP-and cytosolic pH-dependent manner. (4) Chromatin decondensation is associated with the replacement of sperm histones by maternal histones. (5) Nuclear membranes form by ATP-dependent binding of vesicles to chromatin and GTP-dependent fusion of these vesicles to one another. (6) Three cytoplasmic vesicle populations with distinct biochemical, chromatin-binding and fusion properties are required for nuclear envelope assembly. (7) Targeting of the bulk of nuclear membrane vesicles to chromatin is mediated by an integral membrane protein similar to human lamin B receptor. (8) The last step of male pronuclear formation, nuclear swelling, is promoted by the assembly of nuclear pores, nuclear import of soluble lamins and growth of the nuclear membranes. (9) Once inside the nucleus, lamin B associates with lamin B receptors, presumably to tether the inner nuclear membrane with the lamina. Overall, these processes are similar to those characterizing nuclear reconstitution after mitosis in somatic cells or nuclear remodeling following transplantation into oocytes or eggs. The influence of the egg cytoplasmic environment on some aspects of nuclear remodeling after nuclear transplantation is also discussed.

The A-kinase-anchoring protein AKAP95 is a multivalent protein with a key role in chromatin condensation at mitosis

Protein kinase A (PKA) and the nuclear A-kinase-anchoring protein AKAP95 have previously been shown to localize in separate compartments in interphase but associate at mitosis. We demonstrate here a role for the mitotic AKAP95-PKA complex. In HeLa cells, AKAP95 is associated with the nuclear matrix in interphase and redistributes mostly into a chromatin fraction at mitosis. In a cytosolic extract derived from mitotic cells, AKAP95 recruits the RIIalpha regulatory subunit of PKA onto chromatin. Intranuclear immunoblocking of AKAP95 inhibits chromosome condensation at mitosis and in mitotic extract in a PKA-independent manner. Immunodepletion of AKAP95 from the extract or immunoblocking of AKAP95 at metaphase induces premature chromatin decondensation. Condensation is restored in vitro by a recombinant AKAP95 fragment comprising the 306-carboxy-terminal amino acids of the protein. Maintenance of condensed chromatin requires PKA binding to chromatin-associated AKAP95 and cAMP signaling through PKA. Chromatin-associated AKAP95 interacts with Eg7, the human homologue of Xenopus pEg7, a component of the 13S condensin complex. Moreover, immunoblocking nuclear AKAP95 inhibits the recruitment of Eg7 to chromatin in vitro. We propose that AKAP95 is a multivalent molecule that in addition to anchoring a cAMP/PKA-signaling complex onto chromosomes, plays a role in regulating chromosome structure at mitosis.

Active transgenes in zebrafish are enriched in acetylated histone H4 and dynamically associate with RNA Pol II and splicing complexes

We have investigated the functional organization of active and silent integrated luciferase transgenes in zebrafish, with the aim of accounting for the variegation of transgene expression in this species. We demonstrate the enrichment of transcriptionally active transgenes in acetylated histone H4 and the dynamic association of the transgenes with splicing factor SC35 and RNA Pol II. Analysis of interphase nuclei and extended chromatin fibers by immunofluorescence and in situ hybridization reveals a co-localization of transgenes with acetylated H4 in luciferase-expressing animals only. Enrichment of expressed transgenes in acetylated H4 is further demonstrated by their co-precipitation from chromatin using anti-acetylated H4 antibodies. Little correlation exists, however, between the level of histone acetylation and the degree of transgene expression. In transgene-expressing zebrafish, most transgenes co-localize with Pol II and SC35, whereas no such association occurs in non-expressing individuals. Inhibition of Pol II abolishes transgene expression and disrupts association of transgenes with SC35, although inactivated transgenes remains enriched in acetylated histones. Exposure of embryos to the histone deacetylation inhibitor TSA induces expression of most silent transgenes. Chromatin containing activated transgenes becomes enriched in acetylated histones and the transgenes recruit SC35 and Pol II. The results demonstrate a correlation between H4 acetylation and transgene activity, and argue that active transgenes dynamically recruit splicing factors and Pol II. The data also suggest that dissociation of splicing factors from transgenes upon Pol II inhibition is not a consequence of changes in H4 acetylation.

Sequential PKC- and Cdc2-mediated phosphorylation events elicit zebrafish nuclear envelope disassembly

Molecular markers of the zebrafish inner nuclear membrane (NEP55) and nuclear lamina (L68) were identified, partially characterized and used to demonstrate that disassembly of the zebrafish nuclear envelope requires sequential phosphorylation events by first PKC, then Cdc2 kinase. NEP55 and L68 are immunologically and functionally related to human LAP2beta and lamin B, respectively. Exposure of zebrafish nuclei to meiotic cytosol elicits rapid phosphorylation of NEP55 and L68, and disassembly of both proteins. L68 phosphorylation is completely inhibited by simultaneous inhibition of Cdc2 and PKC and only partially blocked by inhibition of either kinase. NEP55 phosphorylation is completely prevented by inhibition or immunodepletion of cytosolic Cdc2. Inhibition of cAMP-dependent kinase, MEK or CaM kinase II does not affect NEP55 or L68 phosphorylation. In vitro, nuclear envelope disassembly requires phosphorylation of NEP55 and L68 by both mammalian PKC and Cdc2. Inhibition of either kinase is sufficient to abolish NE disassembly. Furthermore, novel two-step phosphorylation assays in cytosol and in vitro indicate that PKC-mediated phosphorylation of L68 prior to Cdc2-mediated phosphorylation of L68 and NEP55 is essential to elicit nuclear envelope breakdown. Phosphorylation elicited by Cdc2 prior to PKC prevents nuclear envelope disassembly even though NEP55 is phosphorylated. The results indicate that sequential phosphorylation events elicited by PKC, followed by Cdc2, are required for zebrafish nuclear disassembly. They also argue that phosphorylation of inner nuclear membrane integral proteins is not sufficient to promote nuclear envelope breakdown, and suggest a multiple-level regulation of disassembly of nuclear envelope components during meiosis and at mitosis.

Modulation of plasmid DNA methylation and expression in zebrafish embryos

Gene expression is under the influence of DNA methylation and assembly of chromatin structure. This paper reports the modulation of transgene expression in zebrafish embryos by altering DNA methylation with 5-azacytidine and heterochromatin formation with sodium butyrate, an inhibitor of histone deacetylation. A CMV promoter-luciferase fusion gene construct (pCMVL) microinjected into zebrafish eggs becomes gradually methylated during development, starting at approximately 12 h post-injection. When methylated in vitro by Hpa II methylase prior to injection, the construct is rapidly demethylated in vivo before being de novo methylated. Demethylation is independent of DNA replication, indicating that it is an active DNA repair process. Demethylating activity has been characterized in zebrafish embryo nuclear extracts, in which this activity is heat-labile, sensitive to protease and RNase and requires ATP hydrolysis. Demethylating activity in vitro is dependent on the developmental stage of the embryo from which extracts are prepared. In vivo , luciferase transcripts are detected prior to de novo plasmid methylation. Furthermore, incubation of pCMVL-injected embryos with 5-azacytidine or butyrate immediately after injection inhibits plasmid methylation and extends the period of luciferase expression. When applied after de novo methylation has occurred, both inhibitors prevent methylation of newly replicated DNA and promote transgene expression. These data suggest that methylation of the injected construct during early development induces repression of the transgene, perhaps by converting the construct to a repressive chromatin structure.

Nuclear localization signals enhance germline transmission of a transgene in zebrafish

We report that cytoplasmic injection into zebrafish eggs of 10(4) copies of plasmid DNA complexed to nuclear localization signal (NLS) peptides, as compared to 10(6) copies of naked DNA, increased nuclear uptake of transgene DNA early during embryo development and enhanced transgene integration frequency into the germline of founders. Monitoring the dynamics of nuclear uptake of DNA-NLS complexes by fluorescence in situ hybridization (FISH) of interphase nuclei indicates that NLS enhances both the proportion of nuclei importing DNA during early embryo development, and the amount of DNA imported by individual nuclei. The use of NLS increases the proportion of germline transgenic founders from 14 to 43% (P < 0.01) as assessed by polymerase chain reaction analysis of F1s. From germline transgenic DNA-NLS-injected founders, 47% transgenic F1s are obtained in wild-type crosses, as opposed to 6% from naked DNA-injected founders (P < 0.01). In both cases, the transgene is transmitted to the F2 generation. In addition, high-resolution FISH analysis of transgenic F1s reveals that the use of NLS increases the number of distinct transgene integration sites along chromatin fibres.

Nuclear envelope disassembly in mitotic extract requires functional nuclear pores and a nuclear lamina

Using sea urchin embryonic and in-vitro-assembled nuclei incubated in sea urchin mitotic extract, I provide evidence for a requirement for functional nuclear pores and a nuclear lamina for nuclear envelope disassembly in vitro. In interphase gastrula nuclei, lamin B interacts with p56, an integral protein of inner nuclear membrane cross-reacting with antibodies to human lamin B receptor. Incubation of gastrula nuclei in mitotic cytosol containing an ATP-generating system rapidly induces hyperphosphorylation of p56 and lamin B. Subsequently, p56-lamin B interactions are weakened and the two proteins segregate into distinct nuclear envelope-derived vesicles upon disassembly of nuclear membranes and of the lamina. Nuclear disassembly is accompanied by chromatin condensation. Blocking nuclear pore function with wheat germ agglutinin or antibodies to nucleoporins prevents p56 and lamin B hyperphosphorylation, nuclear membrane breakdown and lamina solubilization. These events are not rescued by permeabilization of nuclear membranes to molecules of 150, 000 Mr with lysolecithin. In-vitro-assembled nuclei containing nuclear membranes with functional pores but no lamina do not disassemble in mitotic cytosol in spite of p56 hyperphosphorylation. Nuclear import of soluble lamin B and reformation of a lamina in interphase extract restores nuclear disassembly in mitotic cytosol. The data indicate a role for functional nuclear pores in nuclear disassembly in vitro. They show that p56 hyperphosphorylation is not sufficient for nuclear membrane disassembly in mitotic cytosol and argue that the nuclear lamina plays a critical role in nuclear disassembly at mitosis.

Nuclear localization signals: a driving force for nuclear transport of plasmid DNA in zebrafish

Nuclear localization signals (NLSs) are short peptides required for nuclear transport of karyophilic proteins. We review in this paper how the nuclear targeting property of NLS peptides has been taken advantage of to enhance the efficiency of nuclear uptake of transgene DNA in zebrafish and how it may improve the efficiency of transgenesis in this species. Synthetic NLS peptides can bind to plasmid DNA by ionic interactions. Cytoplasmic injection of DNA-NLS complexes in zebrafish eggs enhances the rate and the amount of plasmid DNA taken up by embryonic nuclei. Nuclear import of DNA-NLS complexes has been duplicated in vitro and exhibits energetic and cytosolic requirements similar to those for nuclear protein import. Furthermore, binding NLSs to DNA increases expression frequency of the transgene. We suggest that NLS peptides may constitute a valuable tool to improve the efficiency of transgenesis in zebrafish and other species.

Remodeling the sperm nucleus into a male pronucleus at fertilization

After fertilization, the dormant sperm nucleus undergoes morphological and biochemical transformations leading to the development of a functional nucleus, the male pronucleus. We have investigated the formation of the male pronucleus in a cell-free system consisting of permeabilized sea urchin sperm nuclei incubated in fertilized sea urchin egg extract containing membrane vesicles. The first sperm nuclear alteration in vitro is the disassembly of the sperm nuclear lamina as a result of lamin phosphorylation mediated by egg protein kinase C. The conical sperm nucleus decondenses into a spherical pronucleus in an ATP-dependent manner. The new nuclear envelope (NE) forms by ATP-dependent binding of vesicles to chromatin and GTP-dependent fusion of vesicles to each other. Three cytoplasmic membrane vesicle fractions with distinct biochemical, chromatin-binding and fusion properties, are required for pronuclear envelope assembly. Binding of each fraction to chromatin requires two detergent-resistant lipophilic structures at each pole of the sperm nucleus, which are incorporated into the NE by membrane fusion. Targeting of the bulk of NE vesicles to chromatin is mediated by a lamin B receptor (LBR)-like integral membrane protein. The last step of male pronuclear formation involves nuclear swelling. Nuclear swelling is associated with import of soluble lamin B into the nucleus and growth of the nuclear envelope by fusion of additional vesicles. In the nucleus, lamin B associates with LBR, which apparently tethers the NE to the lamina. Thus male pronuclear envelope assembly in vitro involves a highly ordered series of events. These events are similar to those characterizing the remodeling of somatic and embryonic nuclei transplanted into oocytes. The relationship between sperm nuclear remodeling at fertilization and nuclear remodeling after nuclear transplantation is discussed.

A functional study of the salmon GnRH promoter

The Pa and Pb promoters of the Atlantic salmon (Salmo salar) GnRH gene were fused together or individually to the Escherichia coli lacZ gene, and their transcriptional activities were measured in transient expression assays in zebrafish (Danio rerio). In 48-hour embryos, both promoters were preferentially expressed in the brain, whereas a cytomegalovirus (CMV) promoter-lacZ fusion gene displayed high levels of activity in nonbrain tissues. Pa and Pb exhibited different cell specificity in the forebrain. Pb was active in large neuron-like cells exclusive in the olfactory placode region, whereas Pa appeared active in nonneuron-like cells in the forebrain. In Atlantic salmon forebrain tissue, both Pa and Pb exhibited endogenous activity, as assessed by reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. However, only the Pb transcript contained the prepro-GnRH exon II-IV sequences, suggesting that Pa activity may not be related to GnRH production in this species.

Nuclear envelope dynamics during male pronuclear development

Upon fertilization, the sperm nucleus undergoes reactivation. The poreless sperm nuclear envelope is replaced by a functional male pronuclear envelope and the highly compact male chromatin decondenses. Here some recent evidence is examined: that disassembly of the sperm lamina is required for chromatin decondensation, that remnant portions of the sperm nuclear envelope target the binding of egg membrane vesicles that form the male pronuclear envelope, that functional male pronuclear envelopes containing lamin B receptor assemble prior to lamin import and lamina formation, and that lamina assembly drives male pronuclear swelling. Several unresolved issues are discussed.

Protein kinase C-mediated interphase lamin B phosphorylation and solubilization

Disassembly of the sperm nuclear envelope at fertilization is one of the earliest events in the development of the male pronucleus. We report that nuclear lamina disassembly in interphase sea urchin egg cytosol is a result of lamin B phosphorylation mediated by protein kinase C (PKC). Lamin B of permeabilized sea urchin sperm nuclei incubated in fertilized egg G1 phase cytosolic extract is phosphorylated within 1 min of incubation and solubilized prior to sperm chromatin decondensation. Phosphorylation is Ca2+-dependent. It is reversibly inhibited by the PKC-specific inhibitor chelerythrine, a PKC pseudosubstrate inhibitor peptide, and a PKC substrate peptide, but not by inhibitors of PKA, p34(cdc2) or calmodulin kinase II. Phosphorylation is inhibited by immunodepletion of cytosolic PKC and restored by addition of purified rat brain PKC. Sperm lamin B is a substrate for rat brain PKC in vitro, resulting in lamin B solubilization. Two-dimensional phosphopeptide maps of lamin B phosphorylated by the cytosolic kinase and by purified rat PKC are virtually identical. These data suggest that PKC is the major kinase required for interphase disassembly of the sperm lamina.

Rapid targeting of plasmid DNA to zebrafish embryo nuclei by the nuclear localization signal of SV40 T antigen

Binding SV40 T antigen nuclear localization signals (NLSs) to plasmid DNA promotes transgene expression following injection of DNA-NLS complexes into the cytoplasm of zebrafish eggs. We now demonstrate that NLS peptides mediate import of DNA from the cytoplasm into embryo nuclei, under conditions in which naked DNA is not imported. Plasmid DNA was localized by polymerase chain reaction (PCR) in isolated nuclei, and relative amounts were quantified by densitometry. Binding DNA to NLSs, but not to nuclear-import-deficient peptides, promoted rapid targeting of DNA-NLS complexes to nuclei, and transport across the nuclear envelope. Import of DNA-NLS complexes was competed by co-injected albumin-NLS conjugates. NLS, but not reverse NLS, was detected on blots of nuclei probed with 32P-labeled DNA. The results suggest that NLS-mediated DNA transfer into nuclei may constitute a valuable tool for several gene transfer applications.

Nuclear localization signal of SV40 T antigen directs import of plasmid DNA into sea urchin male pronuclei in vitro

Nuclear import of plasmid DNA mediated by a nuclear localization signal (NLS) derived from SV40 T antigen was investigated in a cell-free extract. In vitro assembled sea urchin male pronuclei were incubated in a 100,000g supernatant of a zebrafish fertilized egg lysate, together with fluorescently labeled plasmid DNA bound to NLS or nuclear import deficient reverse NLS (revNLS) peptides. After 3 hr, DNA-NLS, but not DNA-revNLS, complexes were bound around the nuclear periphery. We demonstrate that nuclear import of DNA-NLS complexes is a two-step process involving binding to, and translocation across, the nuclear envelope. Binding is ATP-independent, occurs at 0 degree C and is Ca(2+)-independent. By contrast, translocation requires ATP hydrolysis, Ca2+, is temperature dependent and is blocked by the lectin wheat germ agglutinin. Both binding and translocation are competitively inhibited by albumin-NLS conjugates, require heat-labile cytosolic factors, and are inhibited by N-ethylmaleimide treatment of the cytosol. Binding and translocation are differentially affected by cytosol dilutions, suggesting that at least two distinct soluble fractions are required for nuclear import. The requirements for NLS-mediated nuclear import of plasmid DNA are similar to those for nuclear import of protein-NLS conjugates in permeabilized cells.

Targeting of membranes to sea urchin sperm chromatin is mediated by a lamin B receptor-like integral membrane protein

We have identified an integral membrane protein of sea urchin gametes with an apparent molecular mass of 56 kD that cross-reacts with an antibody against the nucleoplasmic NH2-terminal domain of human lamin B receptor (LBR). In mature sperm, p56 is located at the tip and base of the nucleus from where it is removed by egg cytosol in vitro. In the egg, p56 is present in a subset of cytoplasmic membranes (MV2 beta) which contributes the bulk of the nuclear envelope during male pronuclear formation. p56-containing vesicles are required for nuclear envelope assembly and have a chromatin-binding capacity that is mediated by p56. Lamin B is not present in these vesicles and is imported into the nucleus from a soluble pool at a later stage of pronuclear formation. Lamin B incorporation and addition of new membranes are necessary for pronuclear swelling and nuclear envelope growth. We suggest that p56 is a sea urchin LBR homologue that targets membranes to chromatin and later anchors the membrane to the lamina.

The nuclear localization sequence of the SV40 T antigen promotes transgene uptake and expression in zebrafish embryo nuclei

We report luciferase expression in zebrafish embryos after cytoplasmic injection of low copy numbers of plasmid DNA coupled to the SV40 T antigen nuclear localization sequence (NLS). Binding of NLS to plasmid DNA (pCMVL) occurs at room temperature in 0.25 M KCl, as assayed by gel retardation at molar ratios of NLS:pCMVL of at least 100:1. Luciferase expression is induced in 35% of embryos with as low as 10(3) NLS-bound pCMVL copies. With 10(4) copies, the proportion of expression increases from 6% at 0:1 to 70% 100:1 NLS:pCMVL (p < 0.01). The beneficial effect of NLS is abolished at DNA concentrations promoting high frequencies of transgene expression without NLS. Regardless of the DNA concentration, the use of NLS does not affect embryo viability for at least up to 10 days. The specificity of NLS on luciferase expression was tested by using a nuclear import deficient reverse NLS peptide (revNLS). revNLS binds to pCMVL, causing gel retardation similarly to NLS, but does not promote transgene expression. Binding of equimolar amounts of revNLS and NLS to DNA reduces by 50% the beneficial effect of NLS on transgene expression. The results suggest efficient targeting of MLS-bound plasmid DNA to the nucleus, and subsequent enhanced uptake of DNA by the nucleus. The data suggest that the use of NLS may reduce the need for using elevated DNA copy numbers in some gene transfer applications.

Conserved binding recognition elements of sperm chromatin, sperm lipophilic structures and nuclear envelope precursor vesicles

Detergent-resistant, lipophilic structures (LSs) at the apex and base of the conical sea urchin sperm nucleus are targets for cytoplasmic membrane vesicle (MV) binding and fuse with these vesicles to form a nuclear envelope in vitro. We report similar LSs associated with trout, frog and mammalian (mouse and bovine) sperm nuclei. The LSs are located at the implantation fossae of all species examined, as well as in the ventral hook region of mouse sperm nuclei. LSs can be removed from, and reconstituted back to, their original sites on nuclei. LS removal prevents MV binding, and reconstitution restores MV binding and GTP-induced fusion activities. Binding of LSs to chromatin or to MVs is mediated, at least in part, by proteins on each structure. Inter-specific LS-chromatin reconstitutions using sea urchin, fish, frog, and mammalian LSs indicate that site-specific binding is not dependent on species. All LSs also bind to MVs of sea urchin or bovine origin, but sea urchin MVs will only fuse with sea urchin LSs, and mammalian MVs only with mammalian LSs. These results demonstrate the conservation between echinoderms, fish, amphibians, and mammals of recognition elements for LS-chromatin and LS-MV binding. The mechanism for fusion of LSs with cytoplasmic MVs, however, is apparently not conserved, and close apposition of MVs on the chromatin surface mediated by LSs is not sufficient to permit MV-MV fusion in the presence of GTP.

Reducing the amount of cytoplasm available for early embryonic development decreases the quality but not quantity of embryos produced by in vitro fertilization and nuclear transplantation

The effect of reducing the amount of cytoplasm available for early embryonic development was investigated in embryos produced by in vitro fertilization (IVF) and nuclear transplantation. In Experiment 1, approximately 1/2 or 1/20 of the cytoplasm was removed from bovine embryos at the pronuclear-stage of development. The percentage of embryos developing to the compact morula or blastocyst stage was significantly higher in non-manipulated controls (26%) than in embryos with 1/20 of the cytoplasm removed (16%), and those with 1/2 of the cytoplasm removed (10%; P < 0.05). There was also a significant difference in the average number of cells between blastocysts in which 1/20 of their cytoplasm was removed (67), those with 1/2 of their cytoplasm removed (55), and nonmanipulated controls (77; P < 0.05). In Experiment 2, nuclear transfer embryos were produced in which approximately 1/2 or 1/20 of the cytoplasm was removed during oocyte enucleation. The percentage of embryos developing to the blastocyst stage was 17% for both groups of nuclear transfer embryos compared to 44% for control embryos (P < 0.05). The mean number of cells in blastocysts produced by nuclear transfer in which 1/20 of the cytoplasm was removed during oocyte enucleation (61) was no different than that in control embryos (66), but significantly higher than the mean number of cells in blastocysts produced by nuclear transfer in which 1/2 of the cytoplasm was removed (42; P < 0.05). There was no indication that altering the amount of cytoplasm available for early embryonic development of IVF embryos affected the timing of differentiation events, including those of embryo compaction and blastocyst formation.

Distinct egg membrane vesicles differing in binding and fusion properties contribute to sea urchin male pronuclear envelopes formed in vitro

We have identified three distinct membrane vesicle populations from sea urchin egg cytoplasm that cooperate in assembling the male pronuclear envelope in vitro. Membranes from sea urchin egg homogenates were separated by buoyant density into five vesicle fractions, three of which bind to demembranated sperm nuclei. Each requires a membranous element (lipophilic structure) derived from the sperm nuclear envelope at the tip and base (poles) of the nucleus in order to bind. Binding is differentially sensitive to protease, high salt and N-ethyl maleimide treatment of the membrane vesicles. MV1 binds at the poles and is required for fusion of the membrane vesicle fractions to each other and to the lipophilic structures. MV2 beta binds over the entire chromatin surface and is enriched in an endoplasmic reticulum marker enzyme. MV2 alpha binds at the nuclear poles, is enriched in a Golgi enzyme marker and is required for fusion of MV2 beta. All three fractions are required for nuclear envelope formation in vitro. The results suggest a multistep process for nuclear envelope formation involving contributions from both sperm and egg, roles for both endoplasmic reticulum and non-endoplasmic reticulum-derived vesicles, and the localization of a critical element of the fusion machinery in MV1.

Formation of the sea urchin male pronucleus in vitro: membrane-independent chromatin decondensation and nuclear envelope-dependent nuclear swelling

We demonstrate that complete sea urchin male pronuclear development in vitro is a two-step process involving membrane-independent chromatin decondensation and nuclear envelope-dependent pronuclear swelling. In the absence of cytoplasmic membrane vesicles (MVs), permeabilized sperm chromatin decondenses into a spherical nucleus of approximately 4 microns in diameter. Pronuclear swelling to approximately 7 microns requires an intact nuclear envelope, and the degree of swelling is limited by the amount of MVs assembled on the chromatin. Furthermore, after a nuclear envelope is formed, swelling can occur in the absence of additional cytoplasmic MVs. Nuclear swelling also requires ATP hydrolysis, Ca2+ and cytosolic factors, some of which are sensitive to heat and to the sulfhydryl alkylating agent, N-ethylmaleimide. The requirement for a nuclear envelope and the rate of pronuclear swelling are consistent with previous in vivo observations.

Histone phosphorylation during sea urchin development

Studies on histone phosphorylation during transitions in chromatin structure occurring in vivo during spermatogenesis and early embryogenesis in sea urchins are reviewed and evaluated in the light of recent studies on histone phosphorylation occurring during chromatin synthesis in frog egg extracts in vitro and evidence that protein kinases and phosphatases play direct roles in the regulation of cellular structure. Sperm-specific histone variants Sp H1 and Sp H2B are maintained as phosphorylated derivatives N and O/P throughout spermatogenesis and early embryogenesis and egg specific histone variants CS H1 and CS H2A are phosphorylated during early embryogenesis. These developmental correlations provide clues about the roles of histone phosphorylation in control of chromatin structure in vivo and provide a basis for the interpretation of data obtained from in-vitro sperm chromatin remodeling in egg extracts and from biochemical studies on the effects of histone phosphorylation on DNA binding. The potential consequences for chromatin structure of the various histone phosphorylation events observed in sea urchins and frog egg extracts are discussed.

Lamin dynamics during sea urchin male pronuclear formation in vitro

The dynamics of lamin disassembly and reassembly during sea urchin male pronuclear development in vitro was investigated. Using five anti-lamin antibodies, we monitored by immunofluorescence and immunoblotting the changes in lamins during sperm chromatin decondensation, nuclear envelope (NE) formation, and male pronuclear swelling in fertilized sea urchin egg cytoplasmic extracts. We report the existence of five proteins in sperm nuclei and swollen male pronuclei (p49, p54, p65, p72, p84) which react with the antibodies. The sperm antigens resist membrane permeabilization with lysolecithin and 0.1% Triton X (TX)-100, but are removed from the lateral aspects of the nuclei by 1% TX-100. All five are completely removed from nuclei within 10 min of incubation in egg extracts. Initial chromatin decondensation and NE formation occur without reassembly of the putative lamins, but all lamins or lamin epitope-containing peptides assemble coordinately during pronuclear swelling promoted by adenosine 5'-triphosphate. Of the five pronuclear antigens, p49 and p54 appear to originate exclusively from the sperm. p65, p72, and p84 are also present in the egg cytoplasm and may be contributed to pronuclei by either source. Assembly of putative lamins and nuclear swelling, but not chromatin decondensation and nuclear envelope formation, are prevented in lamin-depleted cytoplasmic extracts. Our results indicate that p49 and p54 are not necessary for nuclear swelling, cytoplasmic p65 is required for swelling, and p72 and p84 are by themselves not sufficient but may be involved with p65 in nuclear swelling and full pronuclear development in vitro.

Lipophilic organizing structures of sperm nuclei target membrane vesicle binding and are incorporated into the nuclear envelope

We report the existence of lipophilic structures (LS's) associated with demembranated sperm nuclei which together act as a pronuclear envelope organizing center. These structures can be visualized as objects which stain with lipophilic dyes but are resistant to solubilization in 0.1% of the non-ionic detergent Triton X (TX)-100. The structures, located at the acrosomal and centriolar poles of the sea urchin sperm nucleus, initiate ATP-dependent membrane vesicle binding in a fertilized egg extract. The lipophilic material in LS's is incorporated into the nuclear envelope during GTP-induced membrane fusion. Removal of the LS's from sperm nuclei with 1% TX-100 abolishes membrane vesicle binding to the nuclei. LS's recovered from supernatants of extracted nuclei can be reconstituted to their original locations on the stripped nuclei. Rebinding of isolated LS's occurs preferentially at the acrosomal pole. Such reconstituted nuclei direct membrane vesicle binding only to the acrosomal pole region and result in incomplete nuclear envelope assembly following membrane fusion. Binding of LS's and subsequently membrane vesicles to both nuclear poles allows complete nuclear envelope formation.

Inactivation of histone H1 kinase by Ca2+ in rabbit oocytes

The present study investigated the role of intracellular Ca2+ (Ca2+i) elevation on the inactivation of maturation promoting factor (MPF) in rabbit oocytes. The effects of the number of Ca2+ stimulations and of the amplitude of Ca2+i elevation on the profile of histone H1 kinase activity were determined. A Ca2+ stimulation consisted of transferring mature oocytes from culture medium to 0.3 M mannitol containing 0.1-1.0 mM CaCl2, and pulsing them at 1.25 kV/cm for 10 microseconds, or microinjecting 2-8 mM CaCl2 into the oocyte cytoplasm. The number of electrically-induced Ca2+ stimulations was varied, and amplitude of the Ca2+i rise was controlled by altering Ca2+ concentration in the pulsing medium or the injection pipette. Ca2+i concentration was determined with fura-2 dextran; oocytes were snap-frozen at indicated time points and assayed for H1 kinase activity. The activity was quantified by densitometry and expressed as a fraction of activity in nonstimulated oocytes. Electrically-mediated Ca2+i rises inactivated H1 kinase in a manner dependent on the number of Ca2+ stimulations. A single Ca2+ stimulation inactivated H1 kinase to 30-40% of its initial activity. However, H1 kinase inactivation was only transient, regardless of the amplitude of the electrically- or injection-mediated Ca2+i elevation. Increasing the number of Ca2+ stimulations helped to maintain H1 kinase activity at basal (pronuclear) levels. The results show the necessity of a threshold of Ca2+i concentration to trigger MPF inactivation, and suggest a role for the extended period of time over which Ca2+i oscillates at fertilization.

Nuclear transplantation by microinjection of inner cell mass and granulosa cell nuclei

The developmental potential of bovine inner cell mass (ICM) and somatic differentiated (granulosa cell) nuclei was investigated using nuclear transplantation. ICM blastomeres were isolated after immunosurgery of day 7 in vitro produced blastocysts and cumulus granulosa cells recovered from in vitro matured oocytes. Nuclear transplantation was carried out by microinjection of the lysed donor cells into enucleated mature oocytes. Oocytes were activated by three 0.2 kVcm-1/20 microseconds pulses in mannitol containing 100 microM Ca2+, with each pulse 22 min apart. Embryos were cultured in vitro for 7 days and blastocysts were transferred into recipients. ICM and granulosa cell donor nuclei directed 7% (20/304) and 9% (19/213) development to blastocysts, respectively. Fifteen blastocysts from ICM donors resulted in four pregnancies (27%) and two births. No pregnancy was detected with granulosa cell donors. The results illustrate the totipotency of ICM nuclei and indicate that granulosa cell nuclei promote preimplantation development of nuclear transplant embryos.

Influence of recipient oocyte cell cycle stage on DNA synthesis, nuclear envelope breakdown, chromosome constitution, and development in nuclear transplant bovine embryos

Nuclear transplantations into metaphase II (MII) and S phase oocyte cytoplasm were performed to investigate the influence of recipient cell cycle stage on nuclear function and development of bovine nuclear transplant (NT) embryos. Rate of inactivation of histone H1 kinase and duration of DNA synthesis in activated oocytes were determined. The proportion of S phase blastomeres in in vivo produced day 5.5 bovine embryos was measured. DNA synthesis was also assessed in NT embryos after transfer into MII and S phase cytoplasm. MII NT embryos were produced by fusing a blastomere into a MII oocyte; the fusion pulse served to activate the oocyte. S NT embryos were produced by fusing a blastomere into an early S phase oocyte electrically activated 4 h prior to fusion. Nuclear envelope structure, chromosome constitution, and extent of development were examined in MII and S NT embryos. Histone H1 kinase activity dropped to baseline within 2 h of electrical activation. A second electrical pulse did not alter H1 kinase activity when delivered 4 h after the first pulse. The frequency of S phase blastomeres in day 5.5 bovine embryos ranged from 79% to 100%, depending on the duration of culture in 3H-thymidine. Nuclear transplantation into MII cytoplasm resulted in a transient drop in DNA synthesis over 3.5 h. DNA synthesis resumed at 4.5 h post activation (hpa), concomittantly with initiation of DNA replication in activated oocytes. In contrast, DNA synthesis was not interrupted after transfer into S phase cytoplasm. DNA synthesis persisted until 13.5 hpa, as in activated oocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrically induced calcium elevation, activation, and parthenogenetic development of bovine oocytes

The influence of electrical stimulation on the level of intracellular Ca2+ in bovine oocytes, as well as activation and extent of parthenogenetic development, was investigated. Mature oocytes were electrically stimulated at 29 hr of maturation, and intracellular Ca2+ concentration was determined with the Ca2+ indicator fura-2 dextran (fura-2 D). The Ca2+ response of oocytes to a given electrical pulse was variable. Oocytes responded with either no Ca2+ rise from baseline (approximately 12 nM), a short-duration Ca2+ rise (from 12 nM to 300 nM) that returned to baseline within 2 min of the pulse, or a long-duration Ca2+ rise (from 12 nM to 1,000-2,000 nM) that never returned to baseline during the 8 min period over which the oocytes were monitored. In these oocytes, Ca2+ level returned to baseline when oocytes were removed from 0.30 M mannitol and placed in an ionic medium. Increasing field strength or pulse duration tended to increase the proportion of oocytes displaying a Ca2+ rise, and at 1.0 kVcm-1 for 40 microseconds, all oocytes displayed a long-duration Ca2+ elevation. Direct transfer of oocytes from culture medium to mannitol also triggered a Ca2+ rise. Multiple stimulations, either electrical or by transferring to mannitol, produced multiple Ca2+ rises. This mannitol-induced Ca2+ rise could be inhibited by first washing the oocytes in medium containing equal parts of 0.30 M mannitol and phosphate buffered saline (PBS). The level of Ca2+ stimulation affected activation and development of oocytes. Insufficient, or, conversely, excessive Ca2+ stimulation impaired development. Optimum development was obtained with 1) three pulses of 0.2 kVcm-1 for 20 microseconds, each pulse 22 min apart, after direct transfer of oocytes from culture medium to mannitol (22% blastocysts) or 2) three pulses of 1.0 kVcm-1 for 20 microseconds after transfer of oocytes from culture medium to medium containing equal parts mannitol and PBS, then to mannitol (24% blastocysts). This procedure avoided induction of a Ca2+ rise prior to the pulse. The results indicate that the level of Ca2+ stimulation can be regulated by incubation conditions prior to the pulse and, to some extent, by field strength and pulse duration. The level of electrical stimulation influenced oocyte Ca2+ response, activation, and parthenogenetic development.