Association of the nuclear matrix component NuMA with the Cajal body and nuclear speckle compartments during transitions in transcriptional activity in lens cell differentiation

Nuclear High Mobility Group A2 (HMGA2) Interactome Revealed by Biotin Proximity Labeling

The non-histone chromatin binding protein High Mobility Group AT-hook protein 2 (HMGA2) has important functions in chromatin remodeling, and genome maintenance and protection. Expression of HMGA2 is highest in embryonic stem cells, declines during cell differentiation and cell aging, but it is re-expressed in some cancers, where high HMGA2 expression frequently coincides with a poor prognosis. The nuclear functions of HMGA2 cannot be explained by binding to chromatin alone but involve complex interactions with other proteins that are incompletely understood. The present study used biotin proximity labeling, followed by proteomic analysis, to identify the nuclear interaction partners of HMGA2. We tested two different biotin ligase HMGA2 constructs (BioID2 and miniTurbo) with similar results, and identified known and new HMGA2 interaction partners, with functionalities mainly in chromatin biology. These HMGA2 biotin ligase fusion constructs offer exciting new possibilities for interactome discovery research, enabling the monitoring of nuclear HMGA2 interactomes during drug treatments.

Condensation properties of stress granules and processing bodies are compromised in Myotonic Dystrophy Type 1

RNA regulation in mammalian cells requires complex physical compartmentalisation, using structures thought to be formed by liquid-liquid phase separation. Disruption of these structures is implicated in numerous degenerative diseases. Myotonic dystrophy type 1 (DM1) is a multi-systemic trinucleotide repeat disorder resulting from an expansion of nucleotides CTG (CTGexp) in the DNA encoding DM1 protein kinase (DMPK). The cellular hallmark of DM1 is the formation of nuclear foci that contain expanded DMPK RNA (CUGexp) (with thymine instead of uracil). We report here the deregulation of stress granules (SGs) and processing bodies (P-bodies), two cytoplasmic structures key for mRNA regulation, in cell culture models of DM1. Alterations to the rates of formation and dispersal of SGs suggest an altered ability of cells to respond to stress associated with DM1, while changes to the structure and dynamics of SGs and P-bodies suggest that a widespread alteration to the biophysical properties of cellular structures is a consequence of the presence of CUGexp RNA.

Summary: Validation of an inducible model of myotonic dystrophy type 1 that shows altered cellular stress responses. These involve phase-separated cellular structures also implicated in other degenerative conditions.

Crystallin gene expression: Insights from studies of transcriptional bursting

Cellular differentiation is marked by temporally and spatially regulated gene expression. The ocular lens is one of the most powerful mammalian model system since it is composed from only two cell subtypes, called lens epithelial and fiber cells. Lens epithelial cells differentiate into fiber cells through a series of spatially and temporally orchestrated processes, including massive production of crystallins, cellular elongation and the coordinated degradation of nuclei and other organelles. Studies of transcriptional and posttranscriptional gene regulatory mechanisms in lens provide a wide range of opportunities to understand global molecular mechanisms of gene expression as steady-state levels of crystallin mRNAs reach very high levels comparable to globin genes in erythrocytes. Importantly, dysregulation of crystallin gene expression results in lens structural abnormalities and cataracts. The mRNA life cycle is comprised of multiple stages, including transcription, splicing, nuclear export into cytoplasm, stabilization, localization, translation and ultimate decay. In recent years, development of modern mRNA detection methods with single molecule and single cell resolution enabled transformative studies to visualize the mRNA life cycle to generate novel insights into the sequential regulatory mechanisms of gene expression during embryogenesis. This review is focused on recent major advancements in studies of transcriptional bursting in differentiating lens fiber cells, analysis of nascent mRNA expression from bi-directional promoters, transient nuclear accumulation of specific mRNAs, condensation of chromatin prior lens fiber cell denucleation, and outlines future studies to probe the interactions of individual mRNAs with specific RNA-binding proteins (RBPs) in the cytoplasm and regulation of translation and mRNA decay.

The nuclear mitotic apparatus protein NuMA controls rDNA transcription and mediates the nucleolar stress response in a p53-independent manner

The nuclear mitotic apparatus protein, NuMA, is involved in major cellular events such as DNA damage response, apoptosis and p53-mediated growth-arrest, all of which are under the control of the nucleolus upon stress. Proteomic investigation has identified NuMA among hundreds of nucleolar proteins. Yet, the precise link between NuMA and nucleolar function remains undetermined. We confirm that NuMA is present in the nucleolus and reveal redistribution of NuMA upon actinomycin D or doxorubicin-induced nucleolar stress. NuMA coimmunoprecipitates with RNA polymerase I, with ribosomal proteins RPL26 and RPL24, and with components of B-WICH, an ATP-dependent chromatin remodeling complex associated with rDNA transcription. NuMA also binds to 18S and 28S rRNAs and localizes to rDNA promoter regions. Downregulation of NuMA expression triggers nucleolar stress, as shown by decreased nascent pre-rRNA synthesis, fibrillarin perinucleolar cap formation and upregulation of p27kip1, but not p53. Physiologically relevant nucleolar stress induction with reactive oxygen species reaffirms a p53-independent p27kip1 response pathway and leads to nascent pre-rRNA reduction. It also promotes the decrease in the amount of NuMA. This previously uncharacterized function of NuMA in rDNA transcription and p53-independent nucleolar stress response supports a central role for this nuclear structural protein in cellular homeostasis.

High-content image informatics of the structural nuclear protein NuMA parses trajectories for stem/progenitor cell lineages and oncogenic transformation

Stem and progenitor cells that exhibit significant regenerative potential and critical roles in cancer initiation and progression remain difficult to characterize. Cell fates are determined by reciprocal signaling between the cell microenvironment and the nucleus; hence parameters derived from nuclear remodeling are ideal candidates for stem/progenitor cell characterization. Here we applied high-content, single cell analysis of nuclear shape and organization to examine stem and progenitor cells destined to distinct differentiation endpoints, yet undistinguishable by conventional methods. Nuclear descriptors defined through image informatics classified mesenchymal stem cells poised to either adipogenic or osteogenic differentiation, and oligodendrocyte precursors isolated from different regions of the brain and destined to distinct astrocyte subtypes. Nuclear descriptors also revealed early changes in stem cells after chemical oncogenesis, allowing the identification of a class of cancer-mitigating biomaterials. To capture the metrology of nuclear changes, we developed a simple and quantitative "imaging-derived" parsing index, which reflects the dynamic evolution of the high-dimensional space of nuclear organizational features. A comparative analysis of parsing outcomes via either nuclear shape or textural metrics of the nuclear structural protein NuMA indicates the nuclear shape alone is a weak phenotypic predictor. In contrast, variations in the NuMA organization parsed emergent cell phenotypes and discerned emergent stages of stem cell transformation, supporting a prognosticating role for this protein in the outcomes of nuclear functions.

Novel Nuclear Protein Complexes of Dystrophin 71 Isoforms in Rat Cultured Hippocampal GABAergic and Glutamatergic Neurons

The precise functional role of the dystrophin 71 in neurons is still elusive. Previously, we reported that dystrophin 71d and dystrophin 71f are present in nuclei from cultured neurons. In the present work, we performed a detailed analysis of the intranuclear distribution of dystrophin 71 isoforms (Dp71d and Dp71f), during the temporal course of 7-day postnatal rats hippocampal neurons culture for 1h, 2, 4, 10, 15 and 21 days in vitro (DIV). By immunofluorescence assays, we detected the highest level of nuclear expression of both dystrophin Dp71 isoforms at 10 DIV, during the temporal course of primary culture. Dp71d and Dp71f were detected mainly in bipolar GABAergic (≥60%) and multipolar Glutamatergic (≤40%) neurons, respectively. We also characterized the existence of two nuclear dystrophin-associated protein complexes (DAPC): dystrophin 71d or dystrophin 71f bound to β-dystroglycan, α1-, β-, α2-dystrobrevins, α-syntrophin, and syntrophin-associated protein nNOS (Dp71d-DAPC or Dp71f-DAPC, respectively), in the hippocampal neurons. Furthermore, both complexes were localized in interchromatin granule cluster structures (nuclear speckles) of neuronal nucleoskeleton preparations. The present study evinces that each Dp71's complexes differ slightly in dystrobrevins composition. The results demonstrated that Dp71d-DAPC was mainly localized in bipolar GABAergic and Dp71f-DAPC in multipolar Glutamatergic hippocampal neurons. Taken together, our results show that dystrophin 71d, dystrophin 71f and DAP integrate protein complexes, and both complexes were associated to nuclear speckles structures.

Nuclear removal during terminal lens fiber cell differentiation requires CDK1 activity: appropriating mitosis-related nuclear disassembly

Lens epithelial cells and early lens fiber cells contain the typical complement of intracellular organelles. However, as lens fiber cells mature they must destroy their organelles, including nuclei, in a process that has remained enigmatic for over a century, but which is crucial for the formation of the organelle-free zone in the center of the lens that assures clarity and function to transmit light. Nuclear degradation in lens fiber cells requires the nuclease DNase IIβ (DLAD) but the mechanism by which DLAD gains access to nuclear DNA remains unknown. In eukaryotic cells, cyclin-dependent kinase 1 (CDK1), in combination with either activator cyclins A or B, stimulates mitotic entry, in part, by phosphorylating the nuclear lamin proteins leading to the disassembly of the nuclear lamina and subsequent nuclear envelope breakdown. Although most post-mitotic cells lack CDK1 and cyclins, lens fiber cells maintain these proteins. Here, we show that loss of CDK1 from the lens inhibited the phosphorylation of nuclear lamins A and C, prevented the entry of DLAD into the nucleus, and resulted in abnormal retention of nuclei. In the presence of CDK1, a single focus of the phosphonuclear mitotic apparatus is observed, but it is not focused in CDK1-deficient lenses. CDK1 deficiency inhibited mitosis, but did not prevent DNA replication, resulting in an overall reduction of lens epithelial cells, with the remaining cells possessing an abnormally large nucleus. These observations suggest that CDK1-dependent phosphorylations required for the initiation of nuclear membrane disassembly during mitosis are adapted for removal of nuclei during fiber cell differentiation.

The elastin peptide (VGVAPG)3 induces the 3D reorganisation of PML-NBs and SC35 speckles architecture, and accelerates proliferation of fibroblasts and melanoma cells

During melanoma tumour growth, cancerous cells are exposed to the immediate surrounding the micro- and macro environment, which is largely modified through the degradation of the extracellular matrix by fibroblast-derived metalloproteinases. Among the degradation products, (VGVAPG)3, an elastin peptide is known to stimulate the proliferation of both fibroblasts and cancerous cells by binding to the elastin-binding receptor and activating the MEK/ERK signal transduction pathway. As this process strongly modifies mRNA synthesis, we investigated its effect on the relative three-dimensional organisation of the major partners of the mRNA splicing machinery: promyelocytic nuclear bodies (PML-NBs ) and splicing component 35 speckles (SC35) of normal fibroblasts and melanoma SK-MEL-28 cells. SC35 and PML-NBs proteins were immunolabeled and imaged by confocal microscopy within these cells cultured with (VGVAPG)3. Three-dimensional reconstruction was performed to elucidate the organisation of PML-NBs and SC35 speckles and their spatial relationship. In G0 cells, SC35 speckles were sequestered in PML-NBs. Shortly after (VGVAPG)3 stimulation, the three-dimensional organisation of PML-NBs and SC35 speckles changed markedly. In particular, SC35 speckles gradually enlarged and adopted a heterogeneous organisation, intermingled with PML-NBs. Conversely, inhibition of the elastin-binding protein or MEK/ERK pathway induced a remarkable early sequestration of condensed SC35 speckles in PML-NBs, the hallmark of splicing inhibition. The 3D architecture of speckles/PML-NBs highlights the modulation in their spatial relationship, the multiple roles of PML-NBs in activation, inhibition and sequestration, and provides the first demonstration of the dependence of PML-NBs and SC35 speckles on the elastin peptide for these functions.

Transcriptionally correlated subcellular dynamics of MBNL1 during lens development and their implication for the molecular pathology of myotonic dystrophy type 1

DM1 (myotonic dystrophy type 1) is caused by elongation of a CTG repeat in the DMPK (dystrophia myotonica-protein kinase) gene. mRNA transcripts containing these CUGexp (CUG expansion) repeats form accumulations, or foci, in the nucleus of the cell. The pathogenesis of DM1 is proposed to result from inappropriate patterns of alternative splicing caused by sequestration of the developmentally regulated alternative splicing factor MBNL1 (muscleblind-like 1) by these foci. Since eye lens cataract is a common feature of DM1 we have examined the distribution and dynamics of MBNL1 in lens epithelial cell lines derived from patients with DM1. The results of the present study demonstrate that only a small proportion of nuclear MBNL1 accumulates in CUGexp pre-mRNA foci. MBNL1 is, however, highly mobile and changes localization in response to altered transcription and splicing activity. Moreover, immunolocalization studies in lens sections suggest that a change in MBNL1 distribution is important during lens growth and differentiation. Although these data suggest that the loss of MBNL1 function due to accumulation in foci is an unlikely explanation for DM1 symptoms in the lens, they do demonstrate a strong relationship between the subcellular MBNL1 localization and pathways of cellular differentiation, providing an insight into the sensitivity of the lens to changes in MBNL1 distribution.

NuMA is required for the selective induction of p53 target genes

The p53 tumor suppressor protein is a transcription factor controlling various outcomes, such as growth arrest and apoptosis, through the regulation of different sets of target genes. The nuclear mitotic apparatus protein (NuMA) plays important roles in spindle pole organization during mitosis and in chromatin regulation in the nucleus during interphase. Although NuMA has been shown to colocalize with several nuclear proteins, including high-mobility-group proteins I and Y and GAS41, the role of NuMA during interphase remains unclear. Here we report that NuMA binds to p53 to modulate p53-mediated transcription. Acute and partial ablation of NuMA attenuates the induction of the proarrested p21 gene following DNA damage, subsequently causing impaired cell cycle arrest. Interestingly, NuMA knockdown had little effect on the induction of the p53-dependent proapoptotic PUMA gene. Furthermore, NuMA is required for the recruitment of cyclin-dependent kinase 8 (Cdk8), a component of the Mediator complex and a promoter of p53-mediated p21 gene function. These data demonstrate that NuMA is critical for the target selectivity of p53-mediated transcription.

An intranucleolar body associated with rDNA

The nucleolus is the subnuclear organelle responsible for ribosome subunit biogenesis and can also act as a stress sensor. It forms around clusters of ribosomal DNA (rDNA) and is mainly organised in three subcompartments, i.e. fibrillar centre, dense fibrillar component and granular component. Here, we describe the localisation of 21 protein factors to an intranucleolar region different to these main subcompartments, called the intranucleolar body (INB). These factors include proteins involved in DNA maintenance, protein turnover, RNA metabolism, chromatin organisation and the post-translational modifiers SUMO1 and SUMO2/3. Increase in the size and number of INBs is promoted by specific types of DNA damage and depends on the functional integrity of the nucleolus. INBs are abundant in nucleoli of unstressed cells during S phase and localise in close proximity to rDNA with heterochromatic features. The data suggest the INB is linked with regulation of rDNA transcription and/or maintenance of rDNA.

Contributions of extracellular matrix signaling and tissue architecture to nuclear mechanisms and spatial organization of gene expression control

Post-translational modification of histones, ATP-dependent chromatin remodeling, and DNA methylation are interconnected nuclear mechanisms that ultimately lead to the changes in chromatin structure necessary to carry out epigenetic gene expression control. Tissue differentiation is characterized by a specific gene expression profile in association with the acquisition of a defined tissue architecture and function. Elements critical for tissue differentiation, like extracellular stimuli, adhesion and cell shape properties, and transcription factors all contribute to the modulation of gene expression and thus, are likely to impinge on the nuclear mechanisms of epigenetic gene expression control. In this review, we analyze how these elements modify chromatin structure in a hierarchical manner by acting on the nuclear machinery. We discuss how mechanotransduction via the structural continuum of the cell and biochemical signaling to the cell nucleus integrate to provide a comprehensive control of gene expression. The role of nuclear organization in this control is highlighted, with a presentation of differentiation-induced nuclear structure and the concept of nuclear organization as a modulator of the response to incoming signals.

On the mechanism of organelle degradation in the vertebrate lens

The programmed elimination of cytoplasmic organelles occurs during terminal differentiation of erythrocytes, keratinocytes and lens fiber cells. In each case, the process is relatively well understood phenomenologically, but the underlying molecular mechanisms have been surprisingly slow to emerge. This brief review considers the particular case of the lens where, in addition to their specialized physiological roles, organelles represent potential sources of light scattering. The article describes how the elimination of organelles from lens cells located on the visual axis contributes to the transparency of lens tissue. Classic anatomical studies of lens organelle degradation are discussed, along with more contemporary work utilizing confocal microscopy and other imaging modalities. Finally, recent data on the biochemistry of organelle degradation are reviewed. Several review articles on lens organelle degradation are available [Wride, M.A., 1996. Cellular and molecular features of lens differentiation: a review of recent advances. Differentiation 61, 77-93; Wride, M.A., 2000. Minireview: apoptosis as seen through a lens. Apoptosis 5, 203-209; Bassnett, S., 2002. Lens organelle degradation. Exp. Eye Res. 74, 1-6; Dahm, R., 2004. Dying to see. Sci. Am. 291, 82-89] and readers are directed to these for a comprehensive discussion of the earlier literature on this topic.

Intranuclear mobility of estrogen receptor alpha and progesterone receptors in association with nuclear matrix dynamics

We analyzed the intranuclear dynamics of estrogen receptor alpha (ER alpha) and progesterone receptor (PR)-A/B labeled with different spectral variants of green fluorescent protein (GFP) in living cells. The distribution of ER alpha and PR-A/B were changed from a diffuse to discrete pattern after the addition of both ligands, but the extent of discrete cluster formation of PR-A/B was lower than that of ER alpha. The nuclear areas where PR-A/B were accumulated were colocalized with the cluster of ER alpha, suggesting that cross-talk in the transcriptional regulation occurred in the loci. Fluorescence recovery after photobleaching (FRAP) analysis revealed that the mobility of PR-A/B was hastened by the coexistence of ER alpha, while the mobility of ER alpha was not changed by the coexistence of PR-A/B. Cluster formation was correlated with the nuclear matrix binding, because nuclear matrix binding capacity was also lower in PR-A/B than ER alpha. By ATP-depletion from the cells, most of ER alpha and PR-A/B were bound to the nuclear matrix and their mobilities were extinguished both in the absence and presence of ligand. Fluorescent protein (FP) tagged nuclear matrix component protein (NuMA), which was colocalized with ER alpha and PR-A/B, showed ATP-dependent rapid exchange in the nucleus. These results indicate that the mobility of ER alpha and PR-A/B is associated with the dynamics of the nuclear matrix.

Reorganization of centrosomal marker proteins coincides with epithelial cell differentiation in the vertebrate lens

The differentiation of epithelial cells in the vertebrate lens involves a series of changes that includes the degradation of all intracellular organelles and a dramatic elongation of the cells. The latter is accompanied by a substantial remodelling of the cytoskeleton and changes in the distribution of the actin, microtubule and intermediate filament cytoskeletons during lens cell differentiation have been well documented. There have, however, been no studies of microtubule organizing centres (MTOCs) and specifically centrosomes during lens cell differentiation. We have investigated the fate of the centrosomal MTOCs during cellular differentiation in the bovine lens using gamma-tubulin, ninein, centrin 2 and centrin 3 as markers. Our studies show that these markers oscillate between a clear centrosome-based association in epithelial cells and a defocused cluster in lens fibre cells. Our data further reveal a transient loss of signal for the typical centrosomal marker gamma-tubulin as the lens epithelial cells begin to differentiate into lens fibre cells. This marker apparently disappears in the most distal epithelial cells at the lens equator, only to reappear in early lens fibre cells. The changes in gamma-tubulin distribution are mirrored by the other centrosomal markers, centrins 2 and 3 and ninein that also show a similar transient loss of their signals and subsequent clustering at the apical ends of differentiating fibre cells. The transient loss of staining for these centrosomal markers in the most posterior epithelial cells is a distinctive feature that precedes lens cell elongation. The dramatic reorganization of MTOC markers coincides with gap junction reorganization as seen by the loss of connexin 43 (alpha1-connexin) in these lens epithelial cells suggesting that these events mark a significant change preceding subsequent cell elongation and differentiation into fibre cells.

Development and adult morphology of the eye lens in the zebrafish

The zebrafish has become an important vertebrate model organism to study the development of the visual system. Mutagenesis projects have resulted in the identification of hundreds of eye mutants. Analysis of the phenotypes of these mutants relies on in depth knowledge of the embryogenesis in wild-type animals. While the morphological events leading to the formation of the retina and its connections to the central nervous system have been described in great detail, the characterization of the development of the eye lens is still incomplete. In the present study, we provide a morphological description of embryonic and larval lens development as well as adult lens morphology in the zebrafish. Our analyses show that, in contrast to other vertebrate species, the zebrafish lens delaminates from the surface ectoderm as a solid cluster of cells. Detachment of the prospective lens from the surface ectoderm is facilitated by apoptosis. Primary fibre cell elongation occurs in a circular fashion resulting in an embryonic lens nucleus with concentric shells of fibres. After formation of a monolayer of lens epithelial cells, differentiation and elongation of secondary lens fibres result in a final lens morphology similar to that of other vertebrate species. As in other vertebrates, secondary fibre cell differentiation includes the programmed degradation of nuclei, the interconnection of adjacent fibres via protrusions at the fibre cells' edges and the establishment of gap junctions between lens fibre cells. The very close spacing of the nuclei of the differentiating secondary fibres in a narrow zone close to the equatorial epithelium, however, suggests that secondary fibre cell differentiation deviates from that described for mammalian or avian lenses. In summary, while there are similarities in the development and final morphology of the zebrafish lens with mammalian and avian lenses, there are also significant differences, suggesting caution when extrapolating findings on the zebrafish to, for example, human lens development or function.

The control of tissue architecture over nuclear organization is crucial for epithelial cell fate

The remodeling of nuclear organization during differentiation and the dramatic alteration of nuclear organization associated with cancer development are well documented. However, the importance of tissue architecture in the control of nuclear organization remains to be determined. Differentiation of mammary epithelial cells into functional tissue structures, in three-dimensional culture, is characterized by a specific tissue architecture (i.e. a basoapical polarity axis), cell cycle exit and maintenance of cell survival. Here we show that induction of partial differentiation (i.e. basal polarity only, cell cycle exit and cell survival) by epigenetic mechanisms in malignant breast cells is sufficient to restore features of differentiation-specific nuclear organization, including perinucleolar heterochromatin, large splicing factor speckles, and distinct nuclear mitotic apparatus protein (NuMA) foci. Upon alteration of nuclear organization using an antibody against NuMA, differentiated non-neoplastic cells undergo apoptosis, whereas partially differentiated malignant cells enter the cell cycle. Non-neoplastic cells cultured under conditions that prevent the establishment of apical polarity also enter the cell cycle upon NuMA antibody treatment. These findings demonstrate that the differentiation status rather than the non-neoplastic or neoplastic origin of cells controls nuclear organization and suggest a link between nuclear organization and epigenetic mechanisms dictated by tissue architecture for the control of cell behavior.

NuMA influences higher order chromatin organization in human mammary epithelium

The coiled-coil protein NuMA is an important contributor to mitotic spindle formation and stabilization. A potential role for NuMA in nuclear organization or gene regulation is suggested by the observations that its pattern of nuclear distribution depends upon cell phenotype and that it interacts and/or colocalizes with transcription factors. To date, the precise contribution of NuMA to nuclear function remains unclear. Previously, we observed that antibody-induced alteration of NuMA distribution in growth-arrested and differentiated mammary epithelial structures (acini) in three-dimensional culture triggers the loss of acinar differentiation. Here, we show that in mammary epithelial cells, NuMA is present in both the nuclear matrix and chromatin compartments. Expression of a portion of the C terminus of NuMA that shares sequence similarity with the chromatin regulator HPC2 is sufficient to inhibit acinar differentiation and results in the redistribution of NuMA, chromatin markers acetyl-H4 and H4K20m, and regions of deoxyribonuclease I-sensitive chromatin compared with control cells. Short-term alteration of NuMA distribution with anti-NuMA C-terminus antibodies in live acinar cells indicates that changes in NuMA and chromatin organization precede loss of acinar differentiation. These findings suggest that NuMA has a role in mammary epithelial differentiation by influencing the organization of chromatin.

Automated local bright feature image analysis of nuclear protein distribution identifies changes in tissue phenotype

The organization of nuclear proteins is linked to cell and tissue phenotypes. When cells arrest proliferation, undergo apoptosis, or differentiate, distribution of nuclear proteins changes. Conversely, forced alteration of the distribution of nuclear proteins modifies cell phenotype. Immunostaining and fluorescence microscopy have been critical for such findings. However, there is increasing need for quantitative analysis of nuclear protein distribution to decipher epigenetic relationships between nuclear structure and cell phenotype and to unravel the mechanisms linking nuclear structure and function. We have developed imaging methods to quantify the distribution of fluorescently stained nuclear protein NuMA in different mammary phenotypes obtained using 3D cell culture. Automated image segmentation of DAPI-stained nuclei was generated to isolate thousands of nuclei from 3D confocal images. Prominent features of fluorescently stained NuMA were detected by using a previously undescribed local bright feature analysis technique, and their normalized spatial density was calculated as a function of the distance from the nuclear perimeter to its center. The results revealed marked changes in the distribution of the density of NuMA bright features when nonneoplastic cells underwent phenotypically normal acinar morphogenesis. Conversely, we did not detect any reorganization of NuMA during formation of tumor nodules by malignant cells. Importantly, the analysis also discriminated proliferating nonneoplastic from proliferating malignant cells, suggesting that these imaging methods are capable of identifying alterations linked not only to the proliferation status but also to the malignant character of cells. We believe that this quantitative analysis will have additional applications for classifying normal and pathological tissues.

Architectural defects in pronuclei of mouse nuclear transplant embryos

Reprogramming somatic nuclear function by transplantation of nuclei into recipient oocytes is associated with a morphological remodeling of the somatic nucleus. Successful cloning of animals by nuclear transplantation (NT) demonstrates that reprogramming somatic cell function is possible. However, low pregnancy rates and high frequencies of lethal abnormalities in animals born suggest that reprogramming is rarely complete. To address this issue, we tested the hypothesis that nuclear transplantation leads to nuclear remodeling deficiencies. We report the identification of several markers of morphological remodeling, or lack thereof, of mouse cumulus cell nuclei after transplantation into oocytes. Notably, nuclear transplant mouse embryos exhibit nuclear assembly of the differentiated cell-specific A-type lamins at the one-cell stage, as a result of misregulation of lamin A gene expression. The transplanted nuclei also display enhanced concentration of the nuclear matrix-associated protein NuMA as a result of translation from maternal mRNA and de novo transcription. The A-kinase anchoring protein 95 (AKAP95), a marker of the nuclear envelope-chromatin interface, is of somatic origin. Furthermore, greater resistance of AKAP95 and DNA to in situ extractions of one-cell stage NT embryos with non-ionic detergent, DNase, RNase and NaCl reflects an enhanced proportion of heterochromatin in these embryos. Passage through first embryonic mitosis does not rescue the defects detected in one-cell stage embryos. We propose that somatic nuclear reprogramming deficiencies by NT might emanate from, at least in part, failure to remodel the somatic nucleus morphologically into a functional embryonic nucleus.