Nuclear acceptor sites for androgen-receptor complexes in seminal-vesicle epithelium.

An assay method in vitro was developed and applied to quantify acceptor binding of steroid-receptor complexes in nuclei from isolated epithelium of guinea-pig seminal vesicle. Steroid-receptor complex prepared from 1-day-castrated animals was incubated with purified nuclei from 1-28 day-castrated animals in a medium containing 0.15 M-KCl. Free and bound steroid-receptor complexes were measured and the data were submitted to Scatchard analysis. With nuclei from 1-day-castrated animals the Kd for binding of cytosolic [3H]dihydrotestosterone-receptor complexes was found to be 0.83 X 10(-10) M and the capacity for binding was 0.35 pmol/mg of nuclear DNA. Scatchard analysis consistently disclosed only a single line of constant slope and gave the same kinetic constants for nuclei obtained from animals castrated up to 28 days before assay. Administration of 2 mg of dihydrotestosterone, 3 alpha-androstanediol or androsterone or 100 microgram of oestradiol-17 beta 1 h before killing of the 1-day-castrated animals that provided the nuclei resulted in a significant decrease in nuclear acceptor binding of the steroid-receptor complex compared with untreated animals. Thus our assay method disclosed nuclear acceptor sites that may be involved in responses to androgens (and oestrogens) in vivo. We conclude that there is a class of nuclear accept or sites of high affinity and limited capacity that may be occupied by steroid-receptor complexes in vivo.     

Nuclear envelope structure.

The past 18 months have seen significant advances in our knowledge of the constituents of the nuclear envelope, their interactions during interphase and the mechanisms involved in their mitotic dynamics. Although most of the new data are in general agreement with, and contribute detail to, our traditional image of the nuclear envelope, a few observations appear to mark the beginning of new and important directions in research.     

Nuclear envelope dynamics.

The nuclear envelope (NE) provides a semi permeable barrier between the nucleus and cytoplasm and plays a central role in the regulation of macromolecular trafficking between these two compartments. In addition to this transport function, the NE is a key determinant of interphase nuclear architecture. Defects in NE proteins such as A-type lamins and the inner nuclear membrane protein, emerin, result in several human diseases that include cardiac and skeletal myopathies as well as lipodystrophy. Certain disease-linked A-type lamin defects cause profound changes in nuclear organization such as loss of peripheral heterochromatin and redistribution of other nuclear envelope components. While clearly essential in maintenance of nuclear integrity, the NE is a highly dynamic organelle. In interphase it is constantly remodeled to accommodate nuclear growth. During mitosis it must be completely dispersed so that the condensed chromosomes may gain access to the mitotic spindle. Upon completion of mitosis, dispersed NE components are reutilized in the assembly of nuclei within each daughter cell. These complex NE rearrangements are under precise temporal and spatial control and involve interactions with microtubules, chromatin, and a variety of cell-cycle regulatory molecules.     

Nuclear envelope inclusions demonstrated by freeze-fracture.

Inclusions in the perinuclear space of the nuclear envelope of human diploid (MRC-5) fibroblasts, limpet (Patella vulgata) haemocytes, and yeast (Saccharomyces cerevisiae) as observed with the freeze-fracture technique are described. The significance of these inclusions is discussed and it is tentatively concluded that they represent vesicles engaged in transporting macromolecules between nucleus and cytoplasm. Although the inclusions were infrequently observed, their demonstration in mammalian, invertebrate and lower eukaryotic cell types raises the possibility that this form of nucleocytoplasmic exchange may potentially be adopted under appropriate circumstances by the eukaryotic cell in general.     

Nuclear envelope breakdown proceeds by microtubule-induced tearing of the lamina.

The mechanism of nuclear envelope breakdown (NEBD) was investigated in live cells. Early spindle microtubules caused folds and invaginations in the NE up to one hour prior to NEBD, creating mechanical tension in the nuclear lamina. The first gap in the NE appeared before lamin B depolymerization, at the site of maximal tension, by a tearing mechanism. Gap formation relaxed this tension and dramatically accelerated the rate of chromosome condensation. The hole produced in the NE then rapidly expanded over the nuclear surface. NE fragments remaining on chromosomes were removed toward the centrosomes in a microtubule-dependent manner, suggesting a mechanism mediated by a minus-end-directed motor.     

Nuclear envelope assembly after mitosis

In higher eukaryotes, the entire nucleus disassembles during prometaphase of the cell cycle and later reassembles around daughter chromosomes. Remarkably, the complex events that occur to create a functional nucleus in vivo can be duplicated in vitro by using cell-free extracts. Current experiments are aimed at understanding the molecular mechanisms of assembly and disassembly of the nuclear pore complexes and nuclear membranes, and the functional roles of four identified inner membrane proteins, two of which bind to both chromatin and the nuclear lamina.     

Nuclear envelope defects in muscular dystrophy

Muscular dystrophies are a heterogeneous group of disorders linked to defects in 20-30 different genes. Mutations in the genes encoding a pair of nuclear envelope proteins, emerin and lamin A/C, have been shown to cause the X-linked and autosomal forms respectively of Emery-Dreifuss muscular dystrophy. A third form of muscular dystrophy, limb girdle muscular dystrophy 1b, has also been linked to mutations in the lamin A/C gene. Given that these two genes are ubiquitously expressed, a major goal is to determine how they can be associated with tissue specific diseases. Recent results suggest that lamin A/C and emerin contribute to the maintenance of nuclear envelope structure and at the same time may modulate the expression patterns of certain mechanosensitive and stress induced genes. Both emerin and lamin A/C may play an important role in the response of cells to mechanical stress and in this way may help to maintain muscle cell integrity.     

Nuclear envelope disease and chromatin organization

The fifth U.K. meeting on nuclear envelope disease and chromatin brought together international experts from across the field of nuclear envelope biology to discuss the advancements in a class of tissue-specific degenerative diseases called the laminopathies. Clinically, these range from relatively mild fat-wasting disorders to the severe premature aging condition known as Hutchinson-Gilford progeria syndrome. Since the first association of the nuclear envelope with human inherited disease in 1994, there has been an exponential increase in an unexpected variety of functions associated with nuclear envelope proteins, ranging from mechanical support and nucleocytoskeletal connections to regulation of chromatin organization and gene expression. This Biochemical Society Focused Meeting reinforced the functional complexity of nuclear-associated diseases, revealed new avenues to be investigated and highlighted the signalling pathways suitable as therapeutic targets.     

Nuclear envelope: nuclear pore complexity

A new study shows that the filamentous fungus, Aspergillus nidulans, which has a closed mitosis, does not maintain a continuous permeability barrier during mitosis. This work challenges current views of the differences between closed and open mitosis and has implications for understanding mitotic specific changes in the nuclear pore complex and Ran GTPase system in lower eukaryotes.     

Nuclear envelope influences on cell-cycle progression

The nuclear envelope is a complex double membrane system that serves as a dynamic interface between the nuclear and cytoplasmic compartments. Among its many roles is to provide an anchor for gene regulatory proteins on its nucleoplasmic surface and for the cytoskeleton on its cytoplasmic surface. Both sets of anchors are proteins called NETs (nuclear envelope transmembrane proteins), embedded respectively in the inner or outer nuclear membranes. Several lines of evidence indicate that the nuclear envelope contributes to cell-cycle regulation. These contributions come from both inner and outer nuclear membrane NETs and appear to operate through several distinct mechanisms ranging from sequestration of gene-regulatory proteins to activating kinase cascades.     

Nuclear lipid metabolism in NEST: Nuclear envelope signal transduction

Summary There is increasing evidence that nuclear lipid metabolism in NEST is an important new component in signal transducing networks and as a result, this metabolism is beginning to attract more attention. While agonistinduced nuclear lipid metabolism adds further complexity to the ever increasing array of signal transduction components, it also provides further avenues by which nuclear activities may be regulated. Identification of the coupling mechanisms, regulation, and physiological roles of nuclear lipid metabolism represents a new and exciting area of research which will have a broad impact in our understanding of signal transduction pathways.     

Nuclear inclusions in the epithelium of the hens oviduct

Summary The nuclear inclusion is described in the epithelial cell lining in the oviduct of the laying hen. The majority of them show a layered structure in respect of variation of electron density. Some are situated in the centre of the nucleus, others in the nuclear membrane and similar bodies were observed in the cytoplasm of other cells. The nature of these inclusions is discussed including the possibility that they represent a form of nuclear secretion.     

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.