Sizing up the nucleus: nuclear shape, size and nuclear-envelope assembly

Induction of intranuclear membranes by overproduction of Opi1p and Scs2p, regulators for yeast phospholipid biosynthesis, suggests a mechanism for Opi1p nuclear translocation

In the yeast Saccharomyces cerevisiae, the expression of phospholipid biosynthetic genes is suppressed by the Opi1p negative regulator. Opi1p enters into the nucleoplasm from the nuclear membrane to suppress the gene expression under repressing conditions. The binding of Opi1p to the nuclear membrane requires an integral membrane protein, Scs2p and phosphatidic acid (PA). Although it is demonstrated that the association of Opi1p with membranes is affected by PA levels, how Opi1p dissociates from Scs2p is unknown. Here, we found that fluorescently labelled Opi1p accumulated on a perinuclear region in an Scs2p-dependent manner. Electron microscopic analyses indicated that the perinuclear region consists of intranuclear membranes, which may be formed by the invagination of the nuclear membrane due to the accumulation of Opi1p and Scs2p in a restricted area. As expected, localization of Opi1p and Scs2p in the intranuclear membranes was detected by immunoelectron microscopy. Biochemical analysis showed that Opi1p recovered in the membrane fraction was detergent insoluble while Scs2p was soluble, implying that Opi1p behaves differently from Scs2p in the fraction. We hypothesize that Opi1p dissociates from Scs2p after targeting to the nuclear membrane, making it possible to be released from the membrane quickly when PA levels decrease.

Computational prediction of strain-dependent diffusion of transcription factors through the cell nucleus

Nuclear spreading plays a crucial role in stem cell fate determination. In previous works, we reported evidence of multipotency maintenance for mesenchymal stromal cells cultured on three-dimensional engineered niche substrates, fabricated via two-photon laser polymerization. We correlated maintenance of multipotency to a more roundish morphology of these cells with respect to those cultured on conventional flat substrates. To interpret these findings, here we present a multiphysics model coupling nuclear strains induced by cell adhesion to passive diffusion across the cell nucleus. Fully three-dimensional reconstructions of cultured cells were developed on the basis of confocal images: in particular, the level of nuclear spreading resulted significantly dependent on the cell localization within the niche architecture. We assumed that the cell diffusivity varies as a function of the local volumetric strain. The model predictions indicate that the higher the level of spreading of the cell, the higher the flux across the nucleus of small solutes such as transcription factors. Our results point toward nuclear spreading as a primary mechanism by which the stem cell translates its shape into a fate decision, i.e., by amplifying the diffusive flow of transcriptional activators into the nucleus.

Depletion of UBC9 Causes Nuclear Defects during the Vegetative and Sexual Life Cycles in Tetrahymena thermophila

Ubc9p is the sole E2-conjugating enzyme for SUMOylation, and its proper function is required for regulating key nuclear events such as transcription, DNA repair, and mitosis. In Tetrahymena thermophila, the genome is separated into a diploid germ line micronucleus (MIC) that divides by mitosis and a polyploid somatic macronucleus (MAC) that divides amitotically. This unusual nuclear organization provides novel opportunities for the study of SUMOylation and Ubc9p function. We identified the UBC9 gene and demonstrated that its complete deletion from both MIC and MAC genomes is lethal. Rescue of the lethal phenotype with a GFP-UBC9 fusion gene driven by a metallothionein promoter generated a cell line with CdCl2-dependent expression of green fluorescent protein (GFP)-Ubc9p. Depletion of Ubc9p in vegetative cells resulted in the loss of MICs, but MACs continued to divide. In contrast, expression of catalytically inactive Ubc9p resulted in the accumulation of multiple MICs. Critical roles for Ubc9p were also identified during the sexual life cycle of Tetrahymena. Cell lines that were depleted for Ubc9p did not form mating pairs and therefore could not complete any of the subsequent stages of conjugation, including meiosis and macronuclear development. Mating between cells expressing catalytically inactive Ubc9p resulted in arrest during macronuclear development, consistent with our observation that Ubc9p accumulates in the developing macronucleus.

Documentation of Ultrastructural Changes in Nucleus and Microvilli by Fish Oil in Experimental Colon Carcinogenesis

Fish oil (FO) exerts a chemopreventive effect by regulating apoptosis in colon carcinogenesis. The present study reports the ultrastructural changes in various organelles on supplementation of FO in experimental colon carcinogenesis. The carcinogen treatment led to abnormal nuclear shape and alteration in microvilli number indicating cancer establishment. On the other hand, different ratios of FO and corn oil increased chromatin condensation along with an extensive loss of microvilli in a dose- and time-dependent manner which depicts an increase in apoptosis. The associated ultrastuctural alterations support the facilitation of apoptosis by FO as a mechanism for its beneficial effect in colon carcinogenesis.

Engineering functionalized multi-phased silicon/silicon oxide nano-biomaterials to passivate the aggressive proliferation of cancer

Currently, the use of nano silicon in cancer therapy is limited as drug delivery vehicles and markers in imaging, not as manipulative/controlling agents. This is due to limited properties that native states of nano silicon and silicon oxides offers. We introduce nano-functionalized multi-phased silicon/silicon oxide biomaterials synthesized via ultrashort pulsed laser synthesis, with tunable properties that possess inherent cancer controlling properties that can passivate the progression of cancer. This nanostructured biomaterial is composed of individual functionalized nanoparticles made of a homogenous hybrid of multiple phases of silicon and silicon oxide in increasing concentration outwards from the core. The chemical properties of the proposed nanostructure such as number of phases, composition of phases and crystal orientation of each functionalized nanoparticle in the three dimensional nanostructure is defined based on precisely tuned ultrashort pulsed laser-material interaction mechanisms. The amorphous rich phased biomaterial shows a 30 fold (95%) reduction in number of cancer cells compared to bulk silicon in 48 hours. Further, the size of the cancer cells reduces by 76% from 24 to 48 hours. This method exposes untapped properties of combination of multiple phases of silicon oxides and its applications in cancer therapy.

Cell shape dependent regulation of nuclear morphology

Recent studies suggest that actin filaments are essential in how a cell controls its nuclear shape. However, little is known about the relative importance of membrane tension in determining nuclear morphology. In this study, we used adhesive micropatterned substrates to alter the cellular geometry (aspect ratio, size, and shape) that allowed direct membrane tension or without membrane lateral contact with the nucleus and investigate nuclear shape remodeling and orientation on a series of rectangular shapes. Here we showed that at low cell aspect ratios the orientation of the nucleus was regulated by actin filaments while cells with high aspect ratios can maintain nuclear shape and orientation even when actin polymerization was blocked. A model adenocarcinoma cell showed similar behavior in the regulation of nuclear shape in response to changes in cell shape but actin filaments were essential in maintaining cell shape. Our results highlight the two distinct mechanisms to regulate nuclear shape through cell shape control and the difference between fibroblasts and a model cancerous cell in cell adhesion and cell shape control.

The Inner Nuclear Membrane Protein Src1 Is Required for Stable Post-Mitotic Progression into G1 in Aspergillus nidulans

How membranes and associated proteins of the nuclear envelope (NE) are assembled specifically and inclusively around segregated genomes during exit from mitosis is incompletely understood. Inner nuclear membrane (INM) proteins play key roles by providing links between DNA and the NE. In this study we have investigated the highly conserved INM protein Src1 in Aspergillus nidulans and have uncovered a novel cell cycle response during post mitotic formation of G1 nuclei. Live cell imaging indicates Src1 could have roles during mitotic exit as it preferentially locates to the NE abscission points during nucleokinesis and to the NE surrounding forming daughter G1 nuclei. Deletion analysis further supported this idea revealing that although Src1 is not required for interphase progression or mitosis it is required for stable post-mitotic G1 nuclear formation. This conclusion is based upon the observation that in the absence of Src1 newly formed G1 nuclei are structurally unstable and immediately undergo architectural modifications typical of mitosis. These changes include NPC modifications that stop nuclear transport as well as disassembly of nucleoli. More intriguingly, the newly generated G1 nuclei then cycle between mitotic- and interphase-like states. The findings indicate that defects in post-mitotic G1 nuclear formation caused by lack of Src1 promote repeated failed attempts to generate stable G1 nuclei. To explain this unexpected phenotype we suggest a type of regulation that promotes repetition of defective cell cycle transitions rather than preventing progression past the defective cell cycle transition. We suggest the term “reboot regulation” to define this mode of cell cycle regulation. The findings are discussed in relationship to recent studies showing the Cdk1 master oscillator can entrain subservient oscillators that when uncoupled cause cell cycle transitions to be repeated.

Mechanotransduction Mechanisms for Intraventricular Diastolic Vortex Forces and Myocardial Deformations: Part 2

Epigenetic mechanisms are fundamental in cardiac adaptations, remodeling, reverse remodeling, and disease. A primary goal of translational cardiovascular research is recognizing whether disease-related changes in phenotype can be averted by eliminating or reducing the effects of environmental epigenetic risks. There may be significant medical benefits in using gene-by-environment interaction knowledge to prevent or reverse organ abnormalities and disease. This survey proposes that "environmental" forces associated with diastolic RV/LV rotatory flows exert important, albeit still unappreciated, epigenetic actions influencing functional and morphological cardiac adaptations. Mechanisms analogous to Murray's law of hydrodynamic shear-induced endothelial cell modulation of vascular geometry are likely to link diastolic vortex-associated shear, torque and "squeeze" forces to RV/LV adaptations. The time has come to explore a new paradigm in which such forces play a fundamental epigenetic role, and to work out how heart cells react to them. Findings from various imaging modalities, computational fluid dynamics, molecular cell biology and cytomechanics are considered. The following are examined, among others: structural dynamics of myocardial cells (endocardium, cardiomyocytes, and fibroblasts), cytoskeleton, nucleoskeleton, and extracellular matrix; mechanotransduction and signaling; and mechanical epigenetic influences on genetic expression. To help integrate and focus relevant pluridisciplinary research, rotatory RV/LV filling flow is placed within a working context that has a cytomechanics perspective. This new frontier in cardiac research should uncover versatile mechanistic insights linking filling vortex patterns and attendant forces to variable expressions of gene regulation in RV/LV myocardium. In due course, it should reveal intrinsic homeostatic arrangements that support ventricular myocardial function and adaptability.

A novel multiplexed, image-based approach to detect phenotypes that underlie chromosome instability in human cells

Chromosome instability (CIN) is characterized by a progressive change in chromosome numbers. It is a characteristic common to virtually all tumor types, and is commonly observed in highly aggressive and drug resistant tumors. Despite this information, the majority of human CIN genes have yet to be elucidated. In this study, we developed and validated a multiplexed, image-based screen capable of detecting three different phenotypes associated with CIN. Large-scale chromosome content changes were detected by quantifying changes in nuclear volumes following RNAi-based gene silencing. Using a DsRED-LacI reporter system to fluorescently label chromosome 11 within a human fibrosarcoma cell line, we were able to detect deviations from the expected number of two foci per nucleus (one focus/labelled chromosome) that occurred following CIN gene silencing. Finally, micronucleus enumeration was performed, as an increase in micronucleus formation is a classic hallmark of CIN. To validate the ability of each assay to detect phenotypes that underlie CIN, we silenced the established CIN gene, SMC1A. Following SMC1A silencing we detected an increase in nuclear volumes, a decrease in the number of nuclei harboring two DsRED-LacI foci, and an increase in micronucleus formation relative to controls (untreated and siGAPDH). Similar results were obtained in an unrelated human fibroblast cell line. The results of this study indicate that each assay is capable of detecting CIN-associated phenotypes, and can be utilized in future experiments to uncover novel human CIN genes, which will provide novel insight into the pathogenesis of cancer.

Regulation of nucleocytoplasmic transport by ADP-ribosylation: the emerging role of karyopherin-β1 mono-ADP-ribosylation by ARTD15

Post-translational modifications of a cellular protein by mono- and poly-ADP-ribosylation involve the cleavage of NAD (+) , with the release of its nicotinamide moiety. This is accompanied by the transfer of a single (mono-) or several (poly-) ADP-ribose molecules from NAD (+) to a specific amino-acid residue of the protein. Recent reports have shed new light on the correlation between NAD (+) -dependent ADP-ribosylation reactions and the endoplasmic reticulum, in addition to the well-documented roles of these reactions in the nucleus and mitochondria. We have demonstrated that ARTD15/PARP16 is a novel mono-ADP-ribosyltransferase with a new intracellular location, as it is associated with the endoplasmic reticulum. The endoplasmic reticulum, which is a membranous network of interconnected tubules and cisternae, is responsible for specialised cellular functions, including protein folding and protein transport. Maintenance of specialised cellular functions requires the correct flow of information between separate organelles that is made possible through the nucleocytoplasmic trafficking of proteins. ARTD15 appears to have a role in nucleocytoplasmic shuttling, through karyopherin-β1 mono-ADP-ribosylation. This is in line with the emerging role of ADP-ribosylation in the regulation of intracellular trafficking of cellular proteins. Indeed, other, ADP-ribosyltransferases like ARTD1/PARP1, have been reported to regulate nucleocytoplasmic trafficking of crucial proteins, including p53 and NF-κB, and as a consequence, to modulate the subcellular localisation of these proteins under both physiological and pathological conditions.

Centrosome maturation requires YB-1 to regulate dynamic instability of microtubules for nucleus reassembly

Microtubule formation from the centrosome increases dramatically at the onset of mitosis. This process is termed centrosome maturation. However, regulatory mechanisms of microtubule assembly from the centrosome in response to the centrosome maturation are largely unknown. Here we found that YB-1, a cellular cancer susceptibility protein, is required for the centrosome maturation. Phosphorylated YB-1 accumulated in the centrosome at mitotic phase. By YB-1 knockdown, microtubules were found detached from the centrosome at telophase and an abnormal nuclear shape called nuclear lobulation was found due to defective reassembly of nuclear envelope by mis-localization of non-centrosomal microtubules. In conclusion, we propose that YB-1 is important for the assembly of centrosomal microtubule array for temporal and spatial regulation of microtubules.

Rac1 nucleocytoplasmic shuttling drives nuclear shape changes and tumor invasion

Nuclear membrane microdomains are proposed to act as platforms for regulation of nuclear function, but little is known about the mechanisms controlling their formation. Organization of the plasma membrane is regulated by actin polymerization, and the existence of an actin pool in the nucleus suggests that a similar mechanism might operate here. We show that nuclear membrane organization and morphology are regulated by the nuclear level of active Rac1 through actin polymerization-dependent mechanisms. Rac1 nuclear export is mediated by two internal nuclear export signals and through its interaction with nucleophosmin-1 (B23), which acts as a Rac1 chaperone inside the nucleus. Rac1 nuclear accumulation alters the balance between cytosolic Rac1 and Rho, increasing RhoA signaling in the cytoplasm and promoting a highly invasive phenotype. Nuclear Rac1 shuttling is a finely tuned mechanism for controlling nuclear shape and organization and cell invasiveness.

Quantitative analysis of mouse pancreatic islet architecture by serial block-face SEM

We have applied serial block-face scanning electron microscopy (SBF-SEM) to measure parameters that describe the architecture of pancreatic islets of Langerhans, microscopic endocrine organs that secrete insulin and glucagon for control of blood glucose. By analyzing entire mouse islets, we show that it is possible to determine (1) the distributions of alpha and beta cells, (2) the organization of blood vessels and pericapillary spaces, and (3) the ultrastructure of the individual secretory cells. Our results show that the average volume of a beta cell is nearly twice that of an alpha cell, and the total mitochondrial volume is about four times larger. In contrast, nuclear volumes in the two cell types are found to be approximately equal. Although the cores of alpha and beta secretory granules have similar diameters, the beta granules have prominent halos resulting in overall diameters that are twice those of alpha granules. Visualization of the blood vessels revealed that every secretory cell in the islet is in contact with the pericapillary space, with an average contact area of 9±5% of the cell surface area. Our data show that consistent results can be obtained by analyzing small numbers of islets. Due to the complicated architecture of pancreatic islets, such precision cannot easily be achieved by using TEM of thin sections.

Steatosis-induced proteins adducts with lipid peroxidation products and nuclear electrophilic stress in hepatocytes

Accumulating evidence suggests that fatty livers are particularly more susceptible to several pathological conditions, including hepatic inflammation, cirrhosis and liver cancer. However the exact mechanism of such susceptibility is still largely obscure. The current study aimed to elucidate the effect of hepatocytes lipid accumulation on the nuclear electrophilic stress. Accumulation of intracellular lipids was significantly increased in HepG2 cells incubated with fatty acid (FA) complex (1 mM, 2:1 oleic and palmitic acids). In FA-treated cells, lipid droplets were localized around the nucleus and seemed to induce mechanical force, leading to the disruption of the nucleus morphology. Level of reactive oxygen species (ROS) was significantly increased in FA-loaded cells and was further augmented by treatment with moderate stressor (CoCl₂). Increased ROS resulted in formation of reactive carbonyls (aldehydes and ketones, derived from lipid peroxidation) with a strong perinuclear accumulation. Mass-spectroscopy analysis indicated that lipid accumulation per-se can results in modification of nuclear protein by reactive lipid peroxidation products (oxoLPP). 235 Modified proteins involved in transcription regulation, splicing, protein synthesis and degradation, DNA repair and lipid metabolism were identified uniquely in FA-treated cells. These findings suggest that steatosis can affect nuclear redox state, and induce modifications of nuclear proteins by reactive oxoLPP accumulated in the perinuclear space upon FA-treatment.

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Effects of fatty acids on modification of nuclear proteins in hepatocytes was studied.

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Lipid accumulation was associated with abnormal nuclear morphology.

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Lipid accumulation promoted mitochondrial activity and enhanced ROS generation.

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Nuclear proteins were modified by lipid peroxidation products.

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Data suggest nuclear stress as a mechanism for fatty liver disease progression.

cPKC regulates interphase nuclear size during Xenopus development

During Xenopus development, increased nuclear cPKC activity and decreased nuclear association of lamins mediate nuclear scaling.

Dramatic changes in cell and nuclear size occur during development and differentiation, and aberrant nuclear size is associated with many disease states. However, the mechanisms that regulate nuclear size are largely unknown. A robust system for investigating nuclear size is early Xenopus laevis development, during which reductions in nuclear size occur without changes in DNA content. To identify cellular factors that regulate nuclear size during development, we developed a novel nuclear resizing assay wherein nuclei assembled in Xenopus egg extract become smaller in the presence of cytoplasmic interphase extract isolated from post-gastrula Xenopus embryos. We show that nuclear shrinkage depends on conventional protein kinase C (cPKC). Increased nuclear cPKC localization and activity and decreased nuclear association of lamins mediate nuclear size reductions during development, and manipulating cPKC activity in vivo during interphase alters nuclear size in the embryo. We propose a model of steady-state nuclear size regulation whereby nuclear expansion is balanced by an active cPKC-dependent mechanism that reduces nuclear size.

Exfoliative cytology of buccal squames: A quantitative cytomorphometric analysis of patients with diabetes

BACKGROUND

Diabetes is a third leading cause of mortality and morbidity in the world. Diabetes is one of the most common endocrine metabolic disorders and its prevalence has been increasing worldwide. Oral exfoliative cytology may be a more appropriate adjunctive diagnostic tool in conditions like diabetes mellitus, where the invasive techniques lose viability.

AIMS

The purpose of this study is to analyze the cytomorphometric changes in the exfoliated cells of the oral mucosa, as an adjunct to the diagnosis of diabetes.

MATERIALS AND METHODS

Smears were taken from the buccal mucosa of 30 diabetes patients (study group) and 30 healthy individuals (control group). All the smears were stained with rapid Papanicolaou stain (PAP). In the PAP smears, the nuclear area (NA), cytoplasmic area (CA), and cytoplasmic-to-nuclear ratio (CNR) were evaluated for 50 cells in each smear, using the Image Analysis Software (Magnus Pro™) and research microscope (Lawrence and Mayo™).

RESULTS

The results showed that the mean NA was significantly higher (P < 0.001) in the study group, whereas, the mean CA did not exhibit a statistically significant difference (P > 0.001). The mean CNR was significantly lower in the study group (P < 0.001).

INTERPRETATION AND CONCLUSION

The results associated with the clinical observations suggest that diabetes can produce morphological and functional alterations in the oral epithelial cells, detectable by microscopic and cytomorphometric analysis using exfoliative cytology, which can be used in the diagnosis of the disease.

The diagnostic pathology of the nuclear envelope in human cancers

Cancer is still diagnosed on the basis of altered tissue and cellular morphology. The criteria that pathologists use for diagnosis include many morphologically distinctive alterations in the nuclear envelope (NE). With the expectation that diagnostic NE changes will have biological relevance to cancer, a classification of the various types of NE structural changes into three groups is proposed. The first group predicts chromosomal instability. The changes in this group include pleomorphism of lamina size and shape, as if constraints to maintain a spherical shape were lost. Also characteristic of chromosomal instability are the presence of micronuclei, a specific structural feature likely related to the newly described physiology of chromothripsis. The second group is predicted to be functionally important during clonal evolution, because the NE changes in this group are conserved during the clonal evolution of genetically unstable tumors. Two examples of this group include increased ratio of nuclear volume to cytoplasmic volume and the relatively fragile nuclei of small-cell carcinomas. The third and most interesting group develops in a near-diploid, genetically stable background. Many of these (perhaps ultimately all) are directly related to the activation of particular oncogenes. The changes in this group so far include long inward folds of the NE and spherical invaginations of cytoplasm projecting partially into the nucleus ("intranuclear cytoplasmic inclusions"). This group is exemplified by papillary thyroid carcinoma in which RET and TRK tyrosine kinases, and probably B-Raf mutations, directly lead to diagnostic longitudinal folds of the lamina ("nuclear grooves") and intranuclear cytoplasmic inclusions. B-Raf activation may also be linked to intranuclear cytoplasmic inclusions in melanoma and to nuclear grooves in Langerhans cell histiocytosis. Nuclear grooves in granulosa cell tumor may be related to mutations in the FOXL2 oncogene. Uncovering the precise mechanistic basis for any of these lamina alterations would provide a valuable objective means for improving diagnosis, and will likely reflect new types of functional changes, relevant to particular forms of cancer.

Human uterine lower segment myometrial cell and nuclear volume at term: influence of maternal age

Little is known about the cytoarchitecture of human myometrial cells in pregnancy, and whether or not this may be influenced by maternal characteristics such as age, parity and body mass index (BMI). The aim of this study was primarily to evaluate human myometrial smooth muscle cell (SMC) and nuclear volume in the third trimester of human pregnancy, and secondarily to investigate if these parameters are altered in relation to the maternal characteristics outlined above. Myometrial biopsies were obtained from 30 women undergoing elective caesarean delivery at term. One-micrometer sections were prepared for light microscopy and 100-nm sections for electron microscopy. The nucleator technique was used to assess nuclear volume from the light microscopy images. Point-counting methodology was used on transmission electron micrographs to assess the percentage of the cell volume occupied by the nucleus. Cell volume was calculated from these measurements. The euchromatin to heterochromatin (Eu/Het) ratio was determined to ascertain whether differences in nuclear volume were due to an increased range of genes being transcribed. The mean (± SEM) nuclear volume was 175 ± 10 μm(3) , the nucleus occupied 1.5 ± 0.1% of the SMC and the mean cell size was 14 047 ± 1352 μm(3) . The Eu/Het ratio was 7.54 ± 0.4. The mean volume of heterochromatin and euchromatin in the nucleus was 21.5 ± 1.7 and 149 ± 9 μm(3) , respectively. A multivariate regression analysis revealed that advanced maternal age was associated with an increase in the percentage of the cell occupied by nucleus (R(2)  = 0.32, P = 0.004). There were no other significant effects of maternal age, BMI or parity on the measured parameters. These findings provide reliable volumes for human myometrial cells and their nuclei at term gestation, and show that nuclear volume fraction may be influenced by maternal age.

The contractile strength of vascular smooth muscle myocytes is shape dependent

Vascular smooth muscle cells in muscular arteries are more elongated than those in elastic arteries. Previously, we reported changes in the contractility of engineered vascular smooth muscle tissue that appeared to be correlated with the shape of the constituent cells, supporting the commonly held belief that elongated muscle geometry may allow for the better contractile tone modulation required in response to changes in blood flow and pressure. To test this hypothesis more rigorously, we developed an in vitro model by engineering human vascular smooth muscle cells to take on the same shapes as those seen in elastic and muscular arteries and measured their contraction during stimulation with endothelin-1. We found that in the engineered cells, actin alignment and nuclear eccentricity increased as the shape of the cell elongated. Smooth muscle cells with elongated shapes exhibited lower contractile strength but greater percentage increase in contraction after endothelin-1 stimulation. We analysed the relationship between smooth muscle contractility and subcellular architecture and found that changes in contractility were correlated with actin alignment and nuclear shape. These results suggest that elongated smooth muscle cells facilitate muscular artery tone modulation by increasing its dynamic contractile range.

A light-sheet microscope compatible with mobile devices for label-free intracellular imaging and biosensing

The inner structure, especially the nuclear structure, of cells carries valuable information about disease and health conditions of a person. Here we demonstrate a label-free technique to enable direct observations and measurements of the size, shape and morphology of the cell nucleus. With a microfabricated lens and a commercial CMOS imager, we form a scanning light-sheet microscope to produce a dark-field optical scattering image of the cell nucleus that overlays with the bright-field image produced in a separate regime of the same CMOS sensor. We have used the device to detect nuclear features that characterize the life cycle of cells and have used the nucleus volume as a new parameter for cell classification. The device can be developed into a portable, low-cost, point-of-care device leveraging the capabilities of the CMOS imagers to be pervasive in mobile electronics.

Nucleosome assembly is required for nuclear pore complex assembly in mouse zygotes

Packaging of DNA into nucleosomes not only helps to store genetic information but also creates diverse means for regulating DNA-templated processes. Attempts to reveal additional functions of the nucleosome have been unsuccessful, owing to cell lethality caused by nucleosome deletion. Taking advantage of the mammalian fertilization process, in which sperm DNA assembles into nucleosomes de novo, we generated nucleosome-depleted (ND) paternal pronuclei by depleting maternal histone H3.3 or its chaperone HIRA in mouse zygotes. We found that the ND pronucleus forms a nuclear envelope devoid of nuclear pore complexes (NPCs). Loss of NPCs is accompanied by defective localization of ELYS, a nucleoporin essential for NPC assembly, to the nuclear rim. Interestingly, tethering ELYS to the nuclear rim of the ND nucleus rescues NPC assembly. Our study thus demonstrates that nucleosome assembly is a prerequisite for NPC assembly during paternal pronuclear formation.

The Novel Nuclear Envelope Protein KAKU4 Modulates Nuclear Morphology in Arabidopsis

In animals, the nuclear lamina is a fibrillar meshwork on the inner surface of the nuclear envelope, composed of coiled-coil lamin proteins and lamin binding membrane proteins. Plants also have a meshwork on the inner surface of the nuclear envelope, but little is known about its composition other than the presence of members of the CROWDED NUCLEI (CRWN) protein family, possible plant lamin analogs. Here, we describe a candidate lamina component, based on two Arabidopsis thaliana mutants (kaku2 and kaku4) with aberrant nuclear morphology. The responsible gene in kaku2 encodes CRWN1, and the responsible gene in kaku4 encodes a plant-specific protein of unknown function (KAKU4) that physically interacts with CRWN1 and its homolog CRWN4. Immunogold labeling revealed that KAKU4 localizes at the inner nuclear membrane. KAKU4 deforms the nuclear envelope in a dose-dependent manner, in association with nuclear membrane invagination and stack formation. The KAKU4-dependent nuclear envelope deformation was enhanced by overaccumulation of CRWN1, although KAKU4 can deform the nuclear envelope even in the absence of CRWN1 and/or CRWN4. Together, these results suggest that plants have evolved a unique lamina-like structure to modulate nuclear shape and size.

A Crowdsourced nucleus: understanding nuclear organization in terms of dynamically networked protein function

The spatial organization of the nucleus results in a compartmentalized structure that affects all aspects of nuclear function. This compartmentalization involves genome organization as well as the formation of nuclear bodies and plays a role in many functions, including gene regulation, genome stability, replication, and RNA processing. Here we review the recent findings associated with the spatial organization of the nucleus and reveal that a common theme for nuclear proteins is their ability to participate in a variety of functions and pathways. We consider this multiplicity of function in terms of Crowdsourcing, a recent phenomenon in the world of information technology, and suggest that this model provides a novel way to synthesize the many intersections between nuclear organization and function. This article is part of a Special Issue entitled: Chromatin and epigenetic regulation of animal development.

Sizing and shaping the nucleus: mechanisms and significance

The size and shape of the nucleus are tightly regulated, indicating the physiological significance of proper nuclear morphology, yet the mechanisms and functions of nuclear size and shape regulation remain poorly understood. Correlations between altered nuclear morphology and certain disease states have long been observed, most notably many cancers are diagnosed and staged based on graded increases in nuclear size. Here we review recent studies investigating the mechanisms regulating nuclear size and shape, how mitotic events influence nuclear morphology, and the role of nuclear size and shape in subnuclear chromatin organization and cancer progression.

Physiological intestinal oxygen modulates the Caco-2 cell model and increases sensitivity to the phytocannabinoid cannabidiol

The Caco-2 cell model is widely used as a model of colon cancer and small intestinal epithelium but, like most cell models, is cultured in atmospheric oxygen conditions (∼21%). This does not reflect the physiological oxygen range found in the colon. In this study, we investigated the effect of adapting the Caco-2 cell line to routine culturing in a physiological oxygen (5%) environment. Under these conditions, cells maintain a number of key characteristics of the Caco-2 model, such as increased formation of tight junctions and alkaline phosphatase expression over the differentiation period and maintenance of barrier function. However, these cells exhibit differential oxidative metabolism, proliferate less and become larger during differentiation. In addition, these cells were more sensitive to cannabidiol-induced antiproliferative actions through changes in cellular energetics: from a drop of oxygen consumption rate and loss of mitochondrial membrane integrity in cells treated under atmospheric conditions to an increase in reactive oxygen species in intact mitochondria in cells treated under low-oxygen conditions. Inclusion of an additional physiological parameter, sodium butyrate, into the medium revealed a cannabidiol-induced proliferative response at low doses. These effects could impact on its development as an anticancer therapeutic, but overall, the data supports the principle that culturing cells in microenvironments that more closely mimic the in vivo conditions is important for drug screening and mechanism of action studies.

Mechanisms of nuclear size regulation in model systems and cancer

Changes in nuclear size have long been used by cytopathologists as an important parameter to diagnose, stage, and prognose many cancers. Mechanisms underlying these changes and functional links between nuclear size and malignancy are largely unknown. Understanding mechanisms of nuclear size regulation and the physiological significance of proper nuclear size control will inform the interplay between altered nuclear size and oncogenesis. In this chapter we review what is known about molecular mechanisms of nuclear size control based on research in model experimental systems including yeast, Xenopus, Tetrahymena, Drosophila, plants, mice, and mammalian cell culture. We discuss how nuclear size is influenced by DNA ploidy, nuclear structural components, cytoplasmic factors and nucleocytoplasmic transport, the cytoskeleton, and the extracellular matrix. Based on these mechanistic insights, we speculate about how nuclear size might impact cell physiology and whether altered nuclear size could contribute to cancer development and progression. We end with some outstanding questions about mechanisms and functions of nuclear size regulation.

Evaluation of cytological alterations of oral mucosa in smokers and waterpipe users

OBJECTIVE

Oral mucosal epithelia of smokers and waterpipe users are more susceptible to malignant alterations. The aim of this study was morphometric evaluation of the effects of using waterpipe on normal oral mucosa.

MATERIALS AND METHODS

IN A CROSS SECTIONAL STUDY, CYTOLOGIC SMEAR SAMPLES FROM THE FOLLOWING THREE DIFFERENT AREAS: buccal mucosa, lateral surface of the tongue, and floor of the mouth (right) were taken from 40 smokers, 40 waterpipe users, and 40 normal individuals. They were then stained using Papanicolaou staining technique. Quantitative cytologic alterations such as nuclear and cytoplasmic size, nuclear-cytoplasmic (N/C) ratio, Feret ratio (FR), percent of karriorhexis, vacuolization of cytoplasm, two or multilobed nuclei, inflammation, and candida were evaluated. Quantitative evaluation was performed using MoticPlus 2 software, and 50 cells in each slide were studied. Practitioners were matched with age and sex in three groups.

RESULTS

An increase in nuclear size, the N/C ratio, and F.R, while a decrease in cytoplasm size were observed in lateral surface of the tongue, buccal mucosa and floor of the mouth of smokers, waterpipe users and normal individuals, respectively (p≤0.001). No statistically significant differences were observed in percent of karriorhexis, vacuolization of cytoplasm, and two or multilobed nuclei in oral mucosa of smokers, waterpipe users (p=0.8), and normal individuals (p=0.9) in buccal mucosa, tongue, and mouth floor areas. However, the percentage of inflammation and candida in smokers (p<0.001) and waterpipe users (p=0.002) were higher than normal individuals.

CONCLUSION

Smoking and using waterpipe are effective in creating some quantitative cytometric alterations in oral mucosa; however, smoking shows greater effect in the cytometric alterations than using waterpipe. Role of cytology in screening and detection of oral mucosa malignancies in smokers and waterpipe users needs further studies.

Control of nuclear size by NPC proteins

The architecture of the cell nucleus in cancer cells is often altered in a manner associated with the tumor type and aggressiveness. Therefore, it has been the central criterion in the pathological diagnosis and prognosis of cancer. However, the molecular mechanism behind these observed changes in nuclear morphology, including size, remains completely unknown. Based on our current understanding of the physiology of the nuclear pore complex (NPC) and its constituents, which are collectively referred to as nucleoporins (Nups), we discuss how the structural and functional ablation of the NPC and Nups could directly or indirectly contribute to the changes in nuclear size observed in cancer cells.

Estimating podocyte number and density using a single histologic section

The reduction in podocyte density to levels below a threshold value drives glomerulosclerosis and progression to ESRD. However, technical demands prohibit high-throughput application of conventional morphometry for estimating podocyte density. We evaluated a method for estimating podocyte density using single paraffin-embedded formalin-fixed sections. Podocyte nuclei were imaged using indirect immunofluorescence detection of antibodies against Wilms' tumor-1 or transducin-like enhancer of split 4. To account for the large size of podocyte nuclei in relation to section thickness, we derived a correction factor given by the equation CF=1/(D/T+1), where T is the tissue section thickness and D is the mean caliper diameter of podocyte nuclei. Normal values for D were directly measured in thick tissue sections and in 3- to 5-μm sections using calibrated imaging software. D values were larger for human podocyte nuclei than for rat or mouse nuclei (P<0.01). In addition, D did not vary significantly between human kidney biopsies at the time of transplantation, 3-6 months after transplantation, or with podocyte depletion associated with transplant glomerulopathy. In rat models, D values also did not vary with podocyte depletion, but increased approximately 10% with old age and in postnephrectomy kidney hypertrophy. A spreadsheet with embedded formulas was created to facilitate individualized podocyte density estimation upon input of measured values. The correction factor method was validated by comparison with other methods, and provided data comparable with prior data for normal human kidney transplant donors. This method for estimating podocyte density is applicable to high-throughput laboratory and clinical use.

Chromatin-bound Xenopus Dppa2 shapes the nucleus by locally inhibiting microtubule assembly

Nuclear shape and size vary between species, during development, and in many tissue pathologies, but the causes and effects of these differences remain poorly understood. During fertilization, sperm nuclei undergo a dramatic conversion from a heavily compacted form into decondensed, spherical pronuclei, accompanied by rapid nucleation of microtubules from centrosomes. Here we report that the assembly of the spherical nucleus depends on a critical balance of microtubule dynamics, which is regulated by the chromatin-binding protein Developmental pluripotency-associated 2 (Dppa2). Whereas microtubules normally promote sperm pronuclear expansion, in Dppa2-depleted Xenopus egg extracts excess microtubules cause pronuclear assembly defects, leading to abnormal morphology and disorganized DNA replication. Dppa2 inhibits microtubule polymerization in vitro, and Dppa2 activity is needed at a precise time and location during nascent pronuclear formation. This demonstrates a strict spatiotemporal requirement for local suppression of microtubules during nuclear formation, fulfilled by chromatin-bound microtubule regulators.

Organisation of the endosperm and endosperm-placenta syncytia in bladderworts (Utricularia, Lentibulariaceae) with emphasis on the microtubule arrangement

Multinucleate cells play an important role in higher plants, especially during reproduction; however, the configurations of their cytoskeletons, which are formed as a result of mitosis without cytokinesis, have mainly been studied in coenocytes. Previous authors have proposed that in spite of their developmental origin (cell fusion or mitosis without cytokinesis), in multinucleate plant cells, radiating microtubules determine the regular spacing of individual nuclei. However, with the exception of specific syncytia induced by parasitic nematodes, there is no information about the microtubular cytoskeleton in plant heterokaryotic syncytia, i.e. when the nuclei of fused cells come from different cell pools. In this paper, we describe the arrangement of microtubules in the endosperm and special endosperm-placenta syncytia in two Utricularia species. These syncytia arise from different progenitor cells, i.e. cells of the maternal sporophytic nutritive tissue and the micropylar endosperm haustorium (both maternal and paternal genetic material). The development of the endosperm in the two species studied was very similar. We describe microtubule configurations in the three functional endosperm domains: the micropylar syncytium, the endosperm proper and the chalazal haustorium. In contrast to plant syncytia that are induced by parasitic nematodes, the syncytia of Utricularia had an extensive microtubular network. Within each syncytium, two giant nuclei, coming from endosperm cells, were surrounded by a three-dimensional cage of microtubules, which formed a huge cytoplasmic domain. At the periphery of the syncytium, where new protoplasts of the nutritive cells join the syncytium, the microtubules formed a network which surrounded small nuclei from nutritive tissue cells and were also distributed through the cytoplasm. Thus, in the Utricularia syncytium, there were different sized cytoplasmic domains, whose architecture depended on the source and size of the nuclei. The endosperm proper was isolated from maternal (ovule) tissues by a cuticle layer, so the syncytium and chalazal haustorium were the only way for nutrients to be transported from the maternal tissue towards the developing embryo.

Cell geometric constraints induce modular gene-expression patterns via redistribution of HDAC3 regulated by actomyosin contractility

Physical forces in the form of substrate rigidity or geometrical constraints have been shown to alter gene expression profile and differentiation programs. However, the underlying mechanism of gene regulation by these mechanical cues is largely unknown. In this work, we use micropatterned substrates to alter cellular geometry (shape, aspect ratio, and size) and study the nuclear mechanotransduction to regulate gene expression. Genome-wide transcriptome analysis revealed cell geometry-dependent alterations in actin-related gene expression. Increase in cell size reinforced expression of matrix-related genes, whereas reduced cell-substrate contact resulted in up-regulation of genes involved in cellular homeostasis. We also show that large-scale changes in gene-expression profile mapped onto differential modulation of nuclear morphology, actomyosin contractility and histone acetylation. Interestingly, cytoplasmic-to-nuclear redistribution of histone deacetylase 3 modulated histone acetylation in an actomyosin-dependent manner. In addition, we show that geometric constraints altered the nuclear fraction of myocardin-related transcription factor. These fractions exhibited hindered diffusion time scale within the nucleus, correlated with enhanced serum-response element promoter activity. Furthermore, nuclear accumulation of myocardin-related transcription factor also modulated NF-κB activity. Taken together, our work provides modularity in switching gene-expression patterns by cell geometric constraints via actomyosin contractility.

Control of cell proliferation, endoreduplication, cell size, and cell death by the retinoblastoma-related pathway in maize endosperm

The endosperm of cereal grains is one of the most valuable products of modern agriculture. Cereal endosperm development comprises different phases characterized by mitotic cell proliferation, endoreduplication, the accumulation of storage compounds, and programmed cell death. Although manipulation of these processes could maximize grain yield, how they are regulated and integrated is poorly understood. We show that the Retinoblastoma-related (RBR) pathway controls key aspects of endosperm development in maize. Down-regulation of RBR1 by RNAi resulted in up-regulation of RBR3-type genes, as well as the MINICHROMOSOME MAINTENANCE 2-7 gene family and PROLIFERATING CELL NUCLEAR ANTIGEN, which encode essential DNA replication factors. Both the mitotic and endoreduplication cell cycles were stimulated. Developing transgenic endosperm contained 42-58% more cells and ∼70% more DNA than wild type, whereas there was a reduction in cell and nuclear sizes. In addition, cell death was enhanced. The DNA content of mature endosperm increased 43% upon RBR1 down-regulation, whereas storage protein content and kernel weight were essentially not affected. Down-regulation of both RBR1 and CYCLIN DEPENDENT KINASE A (CDKA);1 indicated that CDKA;1 is epistatic to RBR1 and controls endoreduplication through an RBR1-dependent pathway. However, the repressive activity of RBR1 on downstream targets was independent from CDKA;1, suggesting diversification of RBR1 activities. Furthermore, RBR1 negatively regulated CDK activity, suggesting the presence of a feedback loop. These results indicate that the RBR1 pathway plays a major role in regulation of different processes during maize endosperm development and suggest the presence of tissue/organ-level regulation of endosperm/seed homeostasis.

Nuclear shape changes are induced by knockdown of the SWI/SNF ATPase BRG1 and are independent of cytoskeletal connections

Changes in nuclear morphology occur during normal development and have been observed during the progression of several diseases. The shape of a nucleus is governed by the balance of forces exerted by nuclear-cytoskeletal contacts and internal forces created by the structure of the chromatin and nuclear envelope. However, factors that regulate the balance of these forces and determine nuclear shape are poorly understood. The SWI/SNF chromatin remodeling enzyme ATPase, BRG1, has been shown to contribute to the regulation of overall cell size and shape. Here we document that immortalized mammary epithelial cells show BRG1-dependent nuclear shape changes. Specifically, knockdown of BRG1 induced grooves in the nuclear periphery that could be documented by cytological and ultrastructural methods. To test the hypothesis that the observed changes in nuclear morphology resulted from altered tension exerted by the cytoskeleton, we disrupted the major cytoskeletal networks and quantified the frequency of BRG1-dependent changes in nuclear morphology. The results demonstrated that disruption of cytoskeletal networks did not change the frequency of BRG1-induced nuclear shape changes. These findings suggest that BRG1 mediates control of nuclear shape by internal nuclear mechanisms that likely control chromatin dynamics.

Biomimetic calcium carbonate concentric microgrooves with tunable widths for promoting MC3T3-E1 cell functions

Biomimetic, self-assembled calcium carbonate (CaCO(3) ) concentric microgrooves with groove widths of 5.0 and 10 μm were fabricated through simply controlling incubation temperature. Mouse pre-osteoblastic MC3T3-E1 cells were cultured on flat and microgrooved substrates of CaCO(3) and their adhesion, spreading, proliferation, alkaline phosphatase activity, and calcium content were remarkably enhanced by the microgrooves, in particular, the narrower ones. Furthermore, focal adhesions and actin filaments of MC3T3-E1 cells could be aligned on both 5.0-μm and 10-μm-wide CaCO(3) grooves. Compared with the original round nuclei on the flat substrates and expanded round nuclei on the narrower microgrooves, the MC3T3-E1 cell nuclei on 10-μm-wide CaCO(3) grooves demonstrated preferred entrapment in the grooves and significant alignment with a smaller area after two-day culture.

Nuclear size regulation: from single cells to development and disease

Cell size varies greatly among different cell types and organisms, especially during early development when cell division is rapid with little overall growth. A fundamental question is how organelle size is regulated relative to cell size. The nucleus exhibits exquisite size scaling during development and between species, and nuclear size is often altered in cancer cells. Recent studies have elucidated mechanisms of nuclear size regulation in a variety of experimental systems, opening the door to future research on how nuclear size impacts upon cell and nuclear function and subnuclear organization. In this review we discuss studies that have clarified nuclear size control mechanisms and how these results have or will contribute to our understanding of the functional significance of nuclear size.

A gene-networked gel matrix-supported lipid bilayer as a synthetic nucleus system

A spheroidal transgene-networked gel matrix was designed as a synthetic nucleus system. It was spheroidically manufactured using both advanced lithography and DNA nanotechnology. Stable Aqueorea coerulescens green fluorescent protein (AcGFP)-encoding gene cross-networks have been optimized in various parameters: the number of gene-networked gel (G-net-gel) spheroids, the concentration of a AcGFP plasmid in the scaffold, and the molar ratio between the X-DNA building blocks and the gene. It was then assessed that 800 units of the gene networked gel matrix at a 4000:1 molar ratio of X-DNA blocks and AcGFP gene components accomplished 20-fold enhanced in vitro protein expression efficiency for 36 h. Furthermore, once with lipid capping, it reproduced the natural nucleus system, demonstrating the 2-fold increased levels of messenger RNAs (mRNAs) relative to solution phase vectors.

MYC gene delivery to adult mouse utricles stimulates proliferation of postmitotic supporting cells in vitro

The inner ears of adult humans and other mammals possess a limited capacity for regenerating sensory hair cells, which can lead to permanent auditory and vestibular deficits. During development and regeneration, undifferentiated supporting cells within inner ear sensory epithelia can self-renew and give rise to new hair cells; however, these otic progenitors become depleted postnatally. Therefore, reprogramming differentiated supporting cells into otic progenitors is a potential strategy for restoring regenerative potential to the ear. Transient expression of the induced pluripotency transcription factors, Oct3/4, Klf4, Sox2, and c-Myc reprograms fibroblasts into neural progenitors under neural-promoting culture conditions, so as a first step, we explored whether ectopic expression of these factors can reverse supporting cell quiescence in whole organ cultures of adult mouse utricles. Co-infection of utricles with adenoviral vectors separately encoding Oct3/4, Klf4, Sox2, and the degradation-resistant T58A mutant of c-Myc (c-MycT58A) triggered significant levels of supporting cell S-phase entry as assessed by continuous BrdU labeling. Of the four factors, c-MycT58A alone was both necessary and sufficient for the proliferative response. The number of BrdU-labeled cells plateaued between 5–7 days after infection, and then decreased ∼60% by 3 weeks, as many cycling cells appeared to enter apoptosis. Switching to differentiation-promoting culture medium at 5 days after ectopic expression of c-MycT58A temporarily attenuated the loss of BrdU-labeled cells and accompanied a very modest but significant expansion of the sensory epithelium. A small number of the proliferating cells in these cultures labeled for the hair cell marker, myosin VIIA, suggesting they had begun differentiating towards a hair cell fate. The results indicate that ectopic expression of c-MycT58A in combination with methods for promoting cell survival and differentiation may restore regenerative potential to supporting cells within the adult mammalian inner ear.

Nesprin interchain associations control nuclear size

Nesprins-1/-2/-3/-4 are nuclear envelope proteins, which connect nuclei to the cytoskeleton. The largest nesprin-1/-2 isoforms (termed giant) tether F-actin through their N-terminal actin binding domain (ABD). Nesprin-3, however, lacks an ABD and associates instead to plectin, which binds intermediate filaments. Nesprins are integrated into the outer nuclear membrane via their C-terminal KASH-domain. Here, we show that nesprin-1/-2 ABDs physically and functionally interact with nesprin-3. Thus, both ends of nesprin-1/-2 giant are integrated at the nuclear surface: via the C-terminal KASH-domain and the N-terminal ABD-nesprin-3 association. Interestingly, nesprin-2 ABD or KASH-domain overexpression leads to increased nuclear areas. Conversely, nesprin-2 mini (contains the ABD and KASH-domain but lacks the massive nesprin-2 giant rod segment) expression yields smaller nuclei. Nuclear shrinkage is further enhanced upon nesprin-3 co-expression or microfilament depolymerization. Our findings suggest that multivariate intermolecular nesprin interactions with the cytoskeleton form a lattice-like filamentous network covering the outer nuclear membrane, which determines nuclear size.

Nuclear and cellular alignment of primary corneal epithelial cells on topography

The basement membrane of the corneal epithelium presents biophysical cues in the form of topography and compliance that can modulate cytoskeletal dynamics, which, in turn, can result in altering cellular and nuclear morphology and alignment. In this study, the effect of topographic patterns of alternating ridges and grooves on nuclear and cellular shape and alignment was determined. Primary corneal epithelial cells were cultured on either planar or topographically patterned (400-4000 nm pitch) substrates. Alignment of individual cell body was correlated with respective nucleus for the analysis of orientation and elongation. A biphasic response in alignment was observed. Cell bodies preferentially aligned perpendicular to the 800 nm pitch; and with increasing pitch, cells increasingly aligned parallel to the substratum. Nuclear orientation largely followed this trend with the exception of those on 400 nm. On this biomimetic size scale, some nuclei oriented perpendicular to the topography while their cytoskeleton elements aligned parallel. Both nuclei and cell bodies were elongated on topography compared to those on flat surfaces. Our data demonstrate that nuclear orientation and shape are differentially altered by topographic features that are not mandated by alignment of the cell body. This novel finding suggests that nuanced differences in alignment of the nucleus versus the cell body exist and that these differences could have consequences on gene and protein regulation that ultimately regulate cell behaviors. A full understanding of these mechanisms could disclose novel pathways that would better inform evolving strategies in cell, stem cell, and tissue engineering as well as the design and fabrication of improved prosthetic devices.

DNA sequence-dependent compartmentalization and silencing of chromatin at the nuclear lamina

A large fraction of the mammalian genome is organized into inactive chromosomal domains along the nuclear lamina. The mechanism by which these lamina associated domains (LADs) are established remains to be elucidated. Using genomic repositioning assays, we show that LADs, spanning the developmentally regulated IgH and Cyp3a loci contain discrete DNA regions that associate chromatin with the nuclear lamina and repress gene activity in fibroblasts. Lamina interaction is established during mitosis and likely involves the localized recruitment of Lamin B during late anaphase. Fine-scale mapping of LADs reveals numerous lamina-associating sequences (LASs), which are enriched for a GAGA motif. This repeated motif directs lamina association and is bound by the transcriptional repressor cKrox, in a complex with HDAC3 and Lap2β. Knockdown of cKrox or HDAC3 results in dissociation of LASs/LADs from the nuclear lamina. These results reveal a mechanism that couples nuclear compartmentalization of chromatin domains with the control of gene activity.

Shaping the nucleus: factors and forces

Take a look at a textbook illustration of a cell and you will immediately be able to locate the nucleus, which is often drawn as a spherical or ovoid shaped structure. But not all cells have such nuclei. In fact, some disease states are diagnosed by the presence of nuclei that have an abnormal shape or size. What defines nuclear shape and nuclear size, and how does nuclear geometry affect nuclear function? While the answer to the latter question remains largely unknown, significant progress has been made towards understanding the former. In this review, we provide an overview of the factors and forces that affect nuclear shape and size, discuss the relationship between ER structure and nuclear morphology, and speculate on the possible connection between nuclear size and its shape. We also note the many interesting questions that remain to be explored.

The budding yeast nuclear envelope adjacent to the nucleolus serves as a membrane sink during mitotic delay

The mechanisms that dictate nuclear shape are largely unknown. Here we screened the budding yeast deletion collection for mutants with abnormal nuclear shape. A common phenotype was the appearance of a nuclear extension, particularly in mutants in DNA repair and chromosome segregation genes. Our data suggest that these mutations led to the abnormal nuclear morphology indirectly, by causing a checkpoint-induced cell-cycle delay. Indeed, delaying cells in mitosis by other means also led to the appearance of nuclear extensions, whereas inactivating the DNA damage checkpoint pathway in a DNA repair mutant reduced the fraction of cells with nuclear extensions. Formation of a nuclear extension was specific to a mitotic delay, because cells arrested in S or G2 had round nuclei. Moreover, the nuclear extension always coincided with the nucleolus, while the morphology of the DNA mass remained largely unchanged. Finally, we found that phospholipid synthesis continued unperturbed when cells delayed in mitosis, and inhibiting phospholipid synthesis abolished the formation of nuclear extensions. Our data suggest a mechanism that promotes nuclear envelope expansion during mitosis. When mitotic progression is delayed, cells sequester the added membrane to the nuclear envelope associated with the nucleolus, possibly to avoid disruption of intranuclear organization.

Divergence of mitotic strategies in fission yeasts

The aim of mitosis is to produce two daughter nuclei, each containing a chromosome complement identical to that of the mother nucleus. This can be accomplished through a variety of strategies, with "open" and "closed" modes of mitosis positioned at the opposite ends of the spectrum and a range of intermediate patterns in between. In the "closed" mitosis, the nuclear envelope remains intact throughout the nuclear division. In the "open" division type, the envelope of the original nucleus breaks down early in mitosis and reassembles around the segregated daughter genomes. In any case, the nuclear membrane has to remodel to accommodate the mitotic spindle assembly, chromosome segregation and formation of the daughter nuclei. We have recently shown that within the fission yeast clade, the mitotic control of the nuclear surface area may determine the choice between the nuclear envelope breakdown and a fully "closed" division. Here we discuss our data and argue that comparative cell biology studies using two fission yeast species, Schizosaccharomyces pombe and Schizosaccharomyces japonicus, could provide unprecedented insights into physiology and evolution of mitosis.