Nuclear structure: determination of the fate of the nuclear envelope in Drosophila during mitosis using monoclonal antibodies

Confocal Analysis of Nuclear Lamina Behavior during Male Meiosis and Spermatogenesis in Drosophila melanogaster

Lamin family proteins are structural components of a filamentous framework, the nuclear lamina (NL), underlying the inner membrane of nuclear envelope. The NL not only plays a role in nucleus mechanical support and nuclear shaping, but is also involved in many cellular processes including DNA replication, gene expression and chromatin positioning. Spermatogenesis is a very complex differentiation process in which each stage is characterized by nuclear architecture dramatic changes, from the early mitotic stage to the sperm differentiation final stage. Nevertheless, very few data are present in the literature on the NL behavior during this process. Here we show the first and complete description of NL behavior during meiosis and spermatogenesis in Drosophila melanogaster. By confocal imaging, we characterized the NL modifications from mitotic stages, through meiotic divisions to sperm differentiation with an anti-laminDm0 antibody against the major component of the Drosophila NL. We observed that continuous changes in the NL structure occurred in parallel with chromatin reorganization throughout the whole process and that meiotic divisions occurred in a closed context. Finally, we analyzed NL in solofuso meiotic mutant, where chromatin segregation is severely affected, and found the strict correlation between the presence of chromatin and that of NL.

The Ran Pathway in Drosophila melanogaster Mitosis

Over the last two decades, the small GTPase Ran has emerged as a central regulator of both mitosis and meiosis, particularly in the generation, maintenance, and regulation of the microtubule (MT)-based bipolar spindle. Ran-regulated pathways in mitosis bear many similarities to the well-characterized functions of Ran in nuclear transport and, as with transport, the majority of these mitotic effects are mediated through affecting the physical interaction between karyopherins and Spindle Assembly Factors (SAFs)—a loose term describing proteins or protein complexes involved in spindle assembly through promoting nucleation, stabilization, and/or depolymerization of MTs, through anchoring MTs to specific structures such as centrosomes, chromatin or kinetochores, or through sliding MTs along each other to generate the force required to achieve bipolarity. As such, the Ran-mediated pathway represents a crucial functional module within the wider spindle assembly landscape. Research into mitosis using the model organism Drosophila melanogaster has contributed substantially to our understanding of centrosome and spindle function. However, in comparison to mammalian systems, very little is known about the contribution of Ran-mediated pathways in Drosophila mitosis. This article sets out to summarize our understanding of the roles of the Ran pathway components in Drosophila mitosis, focusing on the syncytial blastoderm embryo, arguing that it can provide important insights into the conserved functions on Ran during spindle formation.

Nucleocytoplasmic shuttling mediates the dynamic maintenance of nuclear Dorsal levels during Drosophila embryogenesis

In Drosophila, the NF-kappaB/REL family transcription factor, Dorsal, redistributes from the cytoplasm to nuclei, forming a concentration gradient across the dorsoventral axis of the embryo. Using live imaging techniques in conjunction with embryos expressing a chimeric Dorsal-GFP, we demonstrate that the redistribution of Dorsal from cytoplasm to nucleus is an extremely dynamic process. Nuclear Dorsal concentration changes continuously over time in all nuclei during interphase. While Dorsal appears to be nuclearly localized primarily in ventral nuclei, it is actively shuttling into and out of all nuclei, including nuclei on the dorsal side. Nuclear export is blocked by leptomycin B, a potent inhibitor of Exportin 1 (CRM1)-mediated nuclear export. We have developed a novel in vivo assay revealing the presence of a functional leucine-rich nuclear export signal within the carboxyterminal 44 amino acids of Dorsal. We also find that diffusion of Dorsal is partially constrained to cytoplasmic islands surrounding individual syncitial nuclei. A model is proposed in which the generation and maintenance of the Dorsal gradient is a consequence of an active process involving both restricted long-range diffusion and the balancing of nuclear import with nuclear export.

Invertebrate lamins

Lamins are the main component of the nuclear lamina and considered to be the ancestors of all intermediate filament proteins. They are localized mainly at the nuclear periphery where they form protein complexes with integral proteins of the nuclear inner membrane, transcriptional regulators, histones and chromatin modifiers. Studying lamins in invertebrate species has unique advantages including the smaller number of lamin genes in the invertebrate genomes and powerful genetic analyses in Caenorhabditis elegans and Drosophila melanogaster. These simpler nuclear lamina systems allow direct analyses of their structure and functions. Here we give an overview of recent advances in the field of invertebrate nuclear lamins with special emphasis on their evolution, assembly and functions.

The nuclear lamina and its functions in the nucleus

The nuclear lamina is a structure near the inner nuclear membrane and the peripheral chromatin. It is composed of lamins, which are also present in the nuclear interior, and lamin-associated proteins. The increasing number of proteins that interact with lamins and the compound interactions between these proteins and chromatin-associated proteins make the nuclear lamina a highly complex but also a very exciting structure. The nuclear lamina is an essential component of metazoan cells. It is involved in most nuclear activities including DNA replication, RNA transcription, nuclear and chromatin organization, cell cycle regulation, cell development and differentiation, nuclear migration, and apoptosis. Specific mutations in nuclear lamina genes cause a wide range of heritable human diseases. These diseases include Emery-Dreifuss muscular dystrophy, limb girdle muscular dystrophy, dilated cardiomyopathy (DCM) with conduction system disease, familial partial lipodystrophy (FPLD), autosomal recessive axonal neuropathy (Charcot-Marie-Tooth disorder type 2, CMT2), mandibuloacral dysplasia (MAD), Hutchison Gilford Progeria syndrome (HGS), Greenberg Skeletal Dysplasia, and Pelger-Huet anomaly (PHA). Genetic analyses in Caenorhabditis elegans, Drosophila, and mice show new insights into the functions of the nuclear lamina, and recent structural analyses have begun to unravel the molecular structure and assembly of lamins and their associated proteins.

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.

Remodeling of nuclear architecture during the cell cycle in Drosophila embryos

Little is known about what determines the nuclear matrix or how its reorganization is regulated during mitosis. In this study we report on a monoclonal antibody, mAb2A, which identifies a novel nuclear structure in Drosophila embryos which forms a diffuse meshwork at interphase but which undergoes a striking reorganization into a spindle-like structure during pro- and metaphase. Double labelings with alpha-tubulin and mAb2A antibodies demonstrate that the microtubules of the mitotic apparatus co-localize with this mAb2A labeled structure during metaphase, suggesting it may serve a role in microtubule spindle assembly and/or function during nuclear division. That the mAb2A-labeled nuclear structure is essential for cell division and/or maintenance of nuclear integrity was directly demonstrated by microinjection of mAb2A into early syncytial embryos which resulted in a disintegration of nuclear morphology and perturbation of mitosis.

Perturbation of nuclear architecture by long-distance chromosome interactions

SUMMARY

Position-effect variegation (PEV) describes the stochastic transcriptional silencing of a gene positioned adjacent to heterochromatin. Using FISH, we have tested whether variegated expression of the eye-color gene brown in Drosophila is influenced by its nuclear localization. In embryonic nuclei, a heterochromatic insertion at the brown locus is always spatially isolated from other heterochromatin. However, during larval development this insertion physically associates with other heterochromatic regions on the same chromosome in a stochastic manner. These observations indicate that the brown gene is silenced by specific contact with centromeric heterochromatin. Moreover, they provide direct evidence for long-range chromosome interactions and their impact on three-dimensional nuclear architecture, while providing a cohesive explanation for the phenomenon of PEV.

Time-resolved, in vivo studies of mitotic spindle formation and nuclear lamina breakdown in Drosophila early embryos

Time-resolved, two-component, three-dimensional fluorescence light microscopy imaging in living Drosophila early embryos is used to demonstrate that a large fraction of the nuclear envelope lamins remain localized to a rim in the nuclear periphery until well into metaphase. The process of lamin delocalization and dispersal, typical of ‘open’ forms of mitosis, does not begin until about the time the final, metaphase geometry of the mitotic spindle is attained. Lamin dispersal is completed about the time that the chromosomal movements of anaphase begin. This pattern of nuclear lamina breakdown appears to be intermediate between traditional designations of ‘open’ and ‘closed’ mitoses. These results thus clarify earlier observations of lamins in mitosis in fixed Drosophila early embryos, clearly showing that the observed lamin localization does not result from a structurally defined ‘spindle envelope’ that persists throughout mitosis. During this extended time interval of lamin localization in the nuclear periphery, the lamina undergoes an extensive series of structural rearrangements that are closely coupled to, and likely driven by, the movements of the centrosomes and microtubules that produce the mitotic spindle. Furthermore, throughout this time the nuclear envelope structure is permeable to large macromolecules, which are excluded in interphase. While the functional significance of these structural dynamics is not yet clear, it is consistent with a functional role for the lamina in mitotic spindle formation.

Monoclonal antibody raised against murine IL-1 alpha peptide cross-reacts with a 60-kDa antigen in early Drosophila melanogaster embryo

Whole-mounts of Drosophila embryos were stained with the monoclonal antibody Vmp 18, raised against the peptide 199-208 of murine interleukin 1/ alpha. Immunofluorescence observations showed that the antibody cross-reacted with an antigenic determinant that changed in localization during Drosophila development. In syncytial Drosophila embryos, the antibody recognized an epitope localized on the nuclear envelope throughout mitotic division. As cellularization occurred, the fluorescence was mainly concentrated in the apical region of the blastoderm cells. Western blot analysis of whole Drosophila embryo extracts showed that the antibody recognized a 60-kDa protein in syncytial embryos and during germ band elongation. This suggests that the 60-kDa antigen undergoes dynamic redistribution during embryogenesis.

Autonomous replication in Drosophila melanogaster tissue culture cells

This study addresses the ability of DNA fragments from various sources to mediate autonomous DNA replication in cultured Drosophila melanogaster cells. We created a series of plasmids containing genomic DNA fragments from the Ultrabithorax gene of Drosophila and test ed them for autonomous replication after transfection into Schneider line 2 cells. We found that all plasmids containing Drosophila DNA were able to replicate autonomously, as were random human and Escherichia coli genomic DNA fragments. Most of the plasmids were detectable 18 days after transfection in the absence of selection, suggesting that transfected DNA is maintained in Drosophila cells without rapid loss or degradation. The finding that all plasmids containing Drosophila, human or bacterial DNA replicate autonomously in Drosophila cells suggests that the signals that direct autonomous replication in Drosophila contain a low degree of sequence specificity. A two-dimensional gel analysis of initiation on one of the plasmids was consistent with many dispersed initiation sites. Low sequence specificity and dispersed initiation sites also characterize autonomous replication in human cells and Xenopus eggs and may be general properties of autonomous replication in animal cells.

An extended view of nuclear lamin structure, function, and dynamics

Molecularly-based studies of nuclear lamins have progressed at a rapid rate in the last decade. However, we still have no answer to the most important question: what are the functions of lamins? In this review we describe recent experiments which challenge traditional views of lamin function and structure. These surprising results indicate that much lamin functionality remains to be discovered, and that more global approaches to lamin structure and function are especially appropriate at this time.

The Drosophila maternal-effect gene fs(1)Ya encodes a cell cycle-dependent nuclear envelope component required for embryonic mitosis

The maternal-effect gene fs(1)Ya is specifically required for embryonic mitosis in Drosophila. fs(1)Ya is involved in the initiation of the first embryonic mitosis and may also be necessary for subsequent embryonic mitotic divisions. fs(1)Ya encodes a 91.3 kd hydrophilic protein containing two putative MPF phosphorylation target sites and two potential nuclear localization signals. This protein is synthesized during postoogenic maturation from its maternal RNA and persists throughout embryogenesis. In early embryos, the fs(1)Ya protein is localized to the nuclear envelope from interphase to metaphase. During anaphase and telophase, it is dispersed in the nucleoplasm and cytoplasm, a behavior that is different from that of both the nuclear envelope and lamins. These results suggest that the fs(1)Ya protein is a cell cycle-dependent component of the nuclear envelope that specifically functions in embryonic mitosis.

A monoclonal antibody recognizing a common antigen on Drosophila embryos and human fibroblasts

We used a monoclonal antibody specific for vimentin from human fibroblasts to stain whole mounts of Drosophila embryos. In immunofluorescence observations this antibody cross-reacts with an antigenic determinant localized throughout mitosis at the nuclear boundary. Double fluorescence observations with the Rb188 antibody that specifically recognizes a centrosomal protein of the Drosophila embryo [Whitfield et al., 1988] showed that the anti-vimentin antibody cross-reacts with an antigen localized in the centrosomal region.

Optical sectioning and three-dimensional reconstruction of diploid and polytene nuclei

To build a coherent picture of mitosis and cell fates during blastoderm and through the complex movements of gastrulation, it will be important to localize and follow several markers simultaneously in live specimens, ideally in 3D, using high-resolution, specific, noninjurious staining and observation procedures. The study of early Drosophila development has already profited from the use of fluorescent labeling and low-light-level imaging of live embryos using a CCD camera. Chromosomes in fixed samples have been labeled using DNA-specific dyes, making the pattern of mitotic patches visible. In vivo, 3D microscopy of fluorescently tagged chromosomes, in conjunction with computerized image processing, has permitted the first direct cell lineage analysis in the early Drosophila embryo. Moreover, the techniques adapted to study Drosophila development have been used for analysis of Drosophila chromosome structure, mitosis, and cell cycle, and are general enough to be applied to a myriad of problems in cell biology. "Optical sectioning" has always been used to scrutinize everything from onion roots to frog eggs, focusing up and down through the specimen, with the observer's brain responsible for the image processing. However, the volume of raw data generated by the high-resolution approach detailed above requires the use of sophisticated and adaptable computer systems to analyze and organize the results. Software designed to extract information from these complex images, either automatically or through an interactive approach, will become essential tools for cell and developmental biology. The brain of the experimenter remains the most important component in any image-processing system, but the support of technology will be essential.

Interphase nuclear envelope lamins form a discontinuous network that interacts with only a fraction of the chromatin in the nuclear periphery

Antibodies directed against nuclear envelope lamin proteins have been used in conjunction with three-dimensional light and electron microscope methodologies to determine the spatial organization of lamins in diploid interphase nuclei and to relate this organization to the positions of chromatin in the nuclear periphery. Using Drosophila early embryos, Drosophila Kc cells, and human HeLa cells, it is qualitatively and quantitatively observed that lamins are organized as a highly discontinuous, apparently fibrillar network that leaves large voids in the nuclear periphery containing little or no lamin. Using fluorescence microscopy to compare and quantitate the relationship between chromatin and the lamin network, it is found that although there is a strong tendency for the most peripheral chromatin to be positioned directly underneath a lamin fiber, only a small fraction of the chromatin in the nuclear periphery is sufficiently close to a lamin fiber to possibly be in direct contact.

Nuclear envelope disassembly and nuclear lamina depolymerization during germinal vesicle breakdown in starfish

During germinal vesicle breakdown (GVBD) in starfish, the nuclear envelope disassembles before the nuclear lamina completely depolymerizes, judging from correlative ultrastructural, immunolabeling, and light microscopic analyses. At 13 degrees C, prophase-arrested oocytes of Pisaster ochraceus begin GVBD and rapidly undergo nuclear envelope disassembly about 50 min after addition of the maturation-inducing hormone 1-methyladenine (1-MA). The nuclear lamina of these oocytes, however, remains present for 10-20 min following the vesiculation of the nuclear envelope. Completion of GVBD, as evidenced by a blending of the nuclear contents with the surrounding cytoplasm, occurs within about 15 min after the nuclear lamina has fully depolymerized. Immunofluorescence studies also indicate that a marked increase in the phosphorylations of nuclear proteins precedes the structural reorganizations of the nuclear envelope and nuclear lamina during GVBD.

A monoclonal antibody recognizes a phosphorylated epitope shared by proteins of the cell nucleus and the erythrocyte membrane skeleton

Monoclonal antibody 3C5 recognizes a family of proteins in the nuclei of cultured cells [(1985) Eur. J. Cell Biol. 38, 344]. This antibody has now been shown to recognize equivalent proteins in liver nuclei and in the Triton-insoluble fraction of tissue extracts. In human erythrocytes the antibody recognized a single protein, present in the membrane skeleton fraction and with the molecular mass and extraction properties of beta-spectrin. The epitope recognized by 3C5 was destroyed by alkaline phosphatase. We conclude that this antibody recognizes a phosphorylation site shared by nuclear proteins and a protein of the erythrocyte membrane skeleton, probably beta-spectrin.

Microtubule arrays present during the syncytial and cellular blastoderm stages of the early Drosophila embryo

The organization of microtubules within the surface caps of Drosophila embryos is described for the mitotic cycles of the syncytial blastoderm stage (particularly cycle 10), and for the subsequent cellularization process. Tubulin was labelled with the well characterized monoclonal antibody YL 1/2 (Kilmartin et al., J cell biol 93 (1982) 576). Each surface cap was found to contain an array of microtubules running around the nucleus. The microtubules originated at prominent centrosomes located close to the apical surface of each cap nucleus. During mitosis the spindle microtubules stained strongly for tubulin. A novel finding was that the spindle microtubules of the interzone region appeared to reduce their connections with the centrosomes at the end of anaphase. The spindle remnant remained in position during telophase but then became smaller in size, disappearing by interphase. At this phase of the cell cycle duplication of the aster centrosomes occurred. The cellular blastoderm stage was marked by a change in the main axis of microtubule orientation. The centrosomes of each cap separated somewhat and formed initiation centres for the development of a well developed basket of microtubules around each nucleus, but now perpendicular to the surface. The microtubule baskets were seen to extend in parallel with nuclear elongation, but not in concert with growth of the cell membranes, which extended some way beneath the bases of the nuclei.

The nuclear lamins. A multigene family of proteins in evolution and differentiation

The nuclear lamina consists of a proteinaceous layer or meshwork situated subjacent to the inner nuclear membrane. It is a karyoskeletal structure formed by a polymer containing one to three major polypeptides collectively termed the lamins. In all cells examined of vertebrates and invertebrates, the lamins exhibit very similar Mr ranging from 60 000 to 80 000. In vertebrates, two groups of lamins can be distinguished by their isoelectric value, one being near-neutral and the other acidic (isoelectric pH values of 5.6 and lower). The lamins represent a family of polypeptides with regions highly conserved during evolution. In certain species, e.g., the amphibian, Xenopus laevis, they exhibit cell type-specific expression during embryonic development, terminal differentiation of certain somatic cells, and gametogenesis. The nuclear lamina of diverse cell types can be composed of one, two or three different lamin polypeptides, without obvious differences in its morphology.

Localization of antigenic determinants in whole Drosophila embryos

Procedures are described for removal of the vitelline membrane from gently fixed Drosophila embryos en masse. The resulting embryos retain excellent structural integrity and are now suitable for a variety of immunocytochemical and biochemical characterizations.