Sequential degradation of proteins from the nuclear envelope during apoptosis

Nuclear pore protein POM121 regulates subcellular localization and transcriptional activity of PPARγ

Manipulation of the subcellular localization of transcription factors by preventing their shuttling via the nuclear pore complex (NPC) emerges as a novel therapeutic strategy against cancer. One transmembrane component of the NPC is POM121, encoded by a tandem gene locus POM121A/C on chromosome 7. Overexpression of POM121 is associated with metabolic diseases (e.g., diabetes) and unfavorable clinical outcome in patients with colorectal cancer (CRC). Peroxisome proliferator-activated receptor-gamma (PPARγ) is a transcription factor with anti-diabetic and anti-tumoral efficacy. It is inhibited by export from the nucleus to the cytosol via the RAS-RAF-MEK1/2-ERK1/2 signaling pathway, a major oncogenic driver of CRC. We therefore hypothesized that POM121 participates in the transport of PPARγ across the NPC to regulate its transcriptional activity on genes involved in metabolic and tumor control. We found that POM121A/C mRNA was enriched and POM121 protein co-expressed with PPARγ in tissues from CRC patients conferring poor prognosis. Its interactome was predicted to include proteins responsible for tumor metabolism and immunity, and in-silico modeling provided insights into potential 3D structures of POM121. A peptide region downstream of the nuclear localization sequence (NLS) of POM121 was identified as a cytoplasmic interactor of PPARγ. POM121 positivity correlated with the cytoplasmic localization of PPARγ in patients with KRAS mutant CRC. In contrast, POM121A/C silencing by CRISPR/Cas9 sgRNA or siRNA enforced nuclear accumulation of PPARγ and activated PPARγ target genes promoting lipid metabolism and cell cycle arrest resulting in reduced proliferation of human CRC cells. Our data suggest the POM121-PPARγ axis as a potential drugable target in CRC.

Caspase-mediated nuclear pore complex trimming in cell differentiation and endoplasmic reticulum stress

During apoptosis, caspases degrade 8 out of ~30 nucleoporins to irreversibly demolish the nuclear pore complex. However, for poorly understood reasons, caspases are also activated during cell differentiation. Here, we show that sublethal activation of caspases during myogenesis results in the transient proteolysis of four peripheral Nups and one transmembrane Nup. 'Trimmed' NPCs become nuclear export-defective, and we identified in an unbiased manner several classes of cytoplasmic, plasma membrane, and mitochondrial proteins that rapidly accumulate in the nucleus. NPC trimming by non-apoptotic caspases was also observed in neurogenesis and endoplasmic reticulum stress. Our results suggest that caspases can reversibly modulate nuclear transport activity, which allows them to function as agents of cell differentiation and adaptation at sublethal levels.

G2/M checkpoint regulation and apoptosis facilitate the nuclear egress of parvoviral capsids

The nuclear export factor CRM1-mediated pathway is known to be important for the nuclear egress of progeny parvovirus capsids in the host cells with virus-mediated cell cycle arrest at G2/M. However, it is still unclear whether this is the only pathway by which capsids exit the nucleus. Our studies show that the nuclear egress of DNA-containing full canine parvovirus. capsids was reduced but not fully inhibited when CRM1-mediated nuclear export was prevented by leptomycin B. This suggests that canine parvovirus capsids might use additional routes for nuclear escape. This hypothesis was further supported by our findings that nuclear envelope (NE) permeability was increased at the late stages of infection. Inhibitors of cell cycle regulatory protein cyclin-dependent kinase 1 (Cdk1) and pro-apoptotic caspase 3 prevented the NE leakage. The change in NE permeability could be explained by the regulation of the G2/M checkpoint which is accompanied by early mitotic and apoptotic events. The model of G2/M checkpoint activation was supported by infection-induced nuclear accumulation of cyclin B1 and Cdk1. Both NE permeability and nuclear egress of capsids were reduced by the inhibition of Cdk1. Additional proof of checkpoint function regulation and promotion of apoptotic events was the nucleocytoplasmic redistribution of nuclear transport factors, importins, and Ran, in late infection. Consistent with our findings, post-translational histone acetylation that promotes the regulation of several genes related to cell cycle transition and arrest was detected. In conclusion, the model we propose implies that parvoviral capsid egress partially depends on infection-induced G2/M checkpoint regulation involving early mitotic and apoptotic events.

The Nuclear Pore Complex: Birth, Life, and Death of a Cellular Behemoth

Nuclear pore complexes (NPCs) are the only transport channels that cross the nuclear envelope. Constructed from ~500-1000 nucleoporin proteins each, they are among the largest macromolecular assemblies in eukaryotic cells. Thanks to advances in structural analysis approaches, the construction principles and architecture of the NPC have recently been revealed at submolecular resolution. Although the overall structure and inventory of nucleoporins are conserved, NPCs exhibit significant compositional and functional plasticity even within single cells and surprising variability in their assembly pathways. Once assembled, NPCs remain seemingly unexchangeable in post-mitotic cells. There are a number of as yet unresolved questions about how the versatility of NPC assembly and composition is established, how cells monitor the functional state of NPCs or how they could be renewed. Here, we review current progress in our understanding of the key aspects of NPC architecture and lifecycle.

BH3-Only Proteins Noxa and Puma Are Key Regulators of Induced Apoptosis

Apoptosis is an evolutionarily conserved and tightly regulated cell death pathway. Physiological cell death is important for maintaining homeostasis and optimal biological conditions by continuous elimination of undesired or superfluous cells. The BH3-only pro-apoptotic members are strong inducers of apoptosis. The pro-apoptotic BH3-only protein Noxa activates multiple death pathways by inhibiting the anti-apoptotic Bcl-2 family protein, Mcl-1, and other protein members leading to Bax and Bak activation and MOMP. On the other hand, Puma is induced by p53-dependent and p53-independent apoptotic stimuli in several cancer cell lines. Moreover, this protein is involved in several physiological and pathological processes, such as immunity, cancer, and neurodegenerative diseases. Future heat shock research could disclose the effect of hyperthermia on both Noxa and BH3-only proteins. This suggests post-transcriptional mechanisms controlling the translation of both Puma and Noxa mRNA in heat-shocked cells. This study was also the chance to recapitulate the different reactional mechanisms investigated for caspases.

[Lack of GAGA protein in Trl mutants causes massive cell death in Drosophila spermatogenesis and oogenesis]

Белок дрозофилы GAGA (GAF) является фактором эпигенетической регуляции транскрипции большой группы генов с широким разнообразием клеточных функций. GAF кодируется геном Trithorax-like (Trl), который экспрессируется в различных органах и тканях на всех стадиях онтогенеза дрозофилы. Мутации этого гена вызывают множественные нарушения развития. В предыдущих работах мы показали, что этот белок необходим для развития половой системы как самцов, так и самок дрозофилы. Снижение экспрессии гена Trl приводило ко множественным нарушениям спермато- и оогенеза. Одно из значительных нарушений было связано с массовой деградацией и потерей клеток зародышевого пути, что позволило предположить, что этот белок вовлечен в регуляцию клеточной гибели. В представленной работе мы провели более детальное цитологическое исследование, чтобы определить, какой тип гибели клеток зародышевого пути характерен для Trl-мутантов, и происходят ли нарушения или изменения этого процесса по сравнению с нормой. Полученные результаты показали, что недостаток белка GAF вызывает массовую гибель клеток зародышевого пути как у самок, так и самцов дрозофилы, но проявляется эта гибель в зависимости от пола по-разному. У самок, мутантных по гену Trl, фенотипически этот процесс не отличается от нормы и в гибнущих яйцевых камерах выявлены признаки апоптоза и аутофагии клеток зародышевого пути. У самцов, мутантных по гену Trl, в отличие от самок, не обнаружены признаки апоптоза. У самцов мутации Trl индуцируют массовую гибель клеток через аутофагию, что не характерно для сперматогенеза дрозофилы и не описано ранее ни в норме, ни у мутаций по другим генам. Таким образом, недостаток GAF у мутантов Trl приводит к усилению апоптотической и аутофагической гибели клеток зародышевого пути. Эктопическая клеточная гибель и атрофия зародышевой линии, вероятно, связаны с нарушением экспрессии генов-мишеней GAGAфактора, среди которых есть гены, регулирующие как апоптоз, так и аутофагию.

Zn2+-Dependent Nuclease Is Involved in Nuclear Degradation during the Programmed Cell Death of Secretory Cavity Formation in Citrus grandis 'Tomentosa' Fruits

Zn2+- and Ca2+-dependent nucleases exhibit activity toward dsDNA in the four classes of cation-dependent nucleases in plants. Programmed cell death (PCD) is involved in the degradation of cells during schizolysigenous secretory cavity formation in Citrus fruits. Recently, the Ca2+-dependent DNase CgCAN was proven to play a key role in nuclear DNA degradation during the PCD of secretory cavity formation in Citrus grandis ‘Tomentosa’ fruits. However, whether Zn2+-dependent nuclease plays a role in the PCD of secretory cells remains poorly understood. Here, we identified a Zn2+-dependent nuclease gene, CgENDO1, from Citrus grandis ‘Tomentosa’, the function of which was studied using Zn2+ ions cytochemical localization, DNase activity assays, in situ hybridization, and protein immunolocalization. The full-length cDNA of CgENDO1 contains an open reading frame of 906 bp that encodes a protein 301 amino acids in length with a S1/P1-like functional domain. CgENDO1 degrades linear double-stranded DNA at acidic and neutral pH. CgENDO1 is mainly expressed in the late stage of nuclear degradation of secretory cells. Further spatiotemporal expression patterns of CgENDO1 showed that CgENDO1 is initially located on the endoplasmic reticulum and then moves into intracellular vesicles and nuclei. During the late stage of nuclear degradation, it was concentrated in the area of nuclear degradation involved in nuclear DNA degradation. Our results suggest that the Zn2+-dependent nuclease CgENDO1 plays a direct role in the late degradation stage of the nuclear DNA in the PCD of secretory cavity cells of Citrus grandis ‘Tomentosa’ fruits.

Progesterone depletion results in Lamin B1 loss and induction of cell death in mouse trophoblast giant cells

Trophoblast giant cells (TGCs), a mouse trophoblast subtype, have large amounts of cytoplasm and high ploidy levels via endocycles. The diverse functions and gene expression profiles of TGCs have been studied well, but their nuclear structures remain unknown. In this study, we focus on Lamin B1, a nuclear lamina, and clarify its expression dynamics, regulation and roles in TGC functions. TGCs that differentiated from trophoblast stem cells were used. From days 0 to 9 after differentiation, the number of TGCs gradually increased, but the amount of LMNB1 peaked at day 3 and then slightly decreased. An immunostaining experiment showed that LMNB1-depleted TGCs increased after day 6 of differentiation. These LMNB1-depleted TGCs diffused peripheral localization of the heterochromatin marker H3K9me2 in the nuclei. However, LMINB1-knock down was not affected TGCs specific gene expression. We found that the death of TGCs also increased after day 6 of differentiation. Moreover, Lamin B1 loss and the cell death in TGCs were protected by 10-6 M progesterone. Our results conclude that progesterone protects against Lamin B1 loss and prolongs the life and function of TGCs.

An engineered pancreatic cancer model with intra-tumoral heterogeneity of driver mutations

Pancreatic ductal adenocarcinoma (PDAC) is a complex disease with significant intra-tumoral heterogeneity (ITH). Currently, no reliable PDAC tumor model is available that can present ITH profiles in a controlled manner. We develop an in vitro microfluidic tumor model mimicking the heterogeneous accumulation of key driver mutations of human PDAC using cancer cells derived from genetically engineered mouse models. These murine pancreatic cancer cell lines have KPC (Kras and Trp53 mutations) and KIC genotypes (Kras mutation and Cdkn2a deletion). Also, the KIC genotypes have two distinct phenotypes - mesenchymal or epithelial. The tumor model mimics the ITH of human PDAC to study the effects of ITH on the gemcitabine response. The results show gemcitabine resistance induced by ITH. Remarkably, it shows that cancer cell-cell interactions induce the gemcitabine resistance potentially through epithelial-mesenchymal-transition. The tumor model can provide a useful testbed to study interaction mechanisms between heterogeneous cancer cell subpopulations.

The nuclear envelope: target and mediator of the apoptotic process

Apoptosis is characterized by the destruction of essential cell organelles, including the cell nucleus. The nuclear envelope (NE) separates the nuclear interior from the cytosol. During apoptosis, the apoptotic machinery, in particular caspases, increases NE permeability by cleaving its proteins, such as those of the nuclear pore complex (NPC) and the nuclear lamina. This in turns leads to passive diffusion of cytosolic apoptogenic proteins, such as caspases and nucleases, through NPCs into the nucleus and the subsequent breakdown of the NE and destruction of the nucleus. However, NE leakiness at early stages of the apoptotic process can also occur in a caspase-independent manner, where Bax, by a non-canonical action, promotes transient and repetitive localized generation and subsequent rupture of nuclear protein-filled nuclear bubbles. This NE rupture leads to discharge of apoptogenic nuclear proteins from the nucleus to the cytosol, a process that can contribute to the death process. Therefore, the NE may play a role as mediator of cell death at early stages of apoptosis. The NE can also serve as a platform for assembly of complexes that regulate the death process. Thus, the NE should be viewed as both a mediator of the cell death process and a target.

Synthesis of Carum Carvi essential oil nanoemulsion, the cytotoxic effect, and expression of caspase 3 gene

Scientists are attempting to find novel methods to overcome cancers. Nanoemulsion systems as the novel drug delivery tools have been widely used in cancer therapy. In this study, the Carum Carvi oil nanoemulsions (CCONE) were prepared and its cytotoxic activity was studied on human colon cancer HT-29 cells using MTT assay. Flow cytometry and Real-time qPCR were triggered to evaluate the nanoemulsions' apoptotic properties. The results showed a significant negative association between the HT-29 cancer cell viability and CCONE doses of treatments compared with Huvec normal cells (p value < 0.001). The IC₅₀ values were estimated 12.5 µg/ml and 50 µg/ml for HT-29 and Huvec, respectively. Moreover, we observed that increasing concentrations of nanoemulsions significantly upregulate Caspase-3 gene expression. The results showed the CCONE is an efficient novel apoptosis inducer for human colon cancer cells without any undesirable side effects. However, further in vitro and in vivo researches are required. PRACTICAL APPLICATIONS: Cancer is a complex and usually untreatable disorder. Several types of cancer therapy strategies have been applied widely to overcome cancers. Chemotherapy has been used in various types of cancers. In most cases, not only it had not been effective on cancer cells but also been distractive within normal tissues. According to results, Carum Carvi essential oil nanoemulsions have apoptotic and cytotoxic effects on colon cancer cells (HT-29). When it comes to cancer of any kind, it's important to realize that no dietary supplement can fully treat, cure, or prevent cancer. However, there are some supplements that can potentially decrease the risk of cancer. Nanoemulsions present several advantages including the ability to incorporate hydrophilic, amphiphilic, and lipophilic excipient ingredients, high physical stability, and rapid gastrointestinal digestibility. The Carum Carvi essential oil nanoemulsion can also be applied as an effective food supplement due to its potent apoptotic activity.

In silico studies, synthesis and anticancer activity of novel diphenyl ether-based pyridine derivatives

A series of novel 2-amino-4-(3-hydroxy-4-phenoxyphenyl)-6-(4-substituted phenyl) nicotinonitriles were synthesized and evaluated against HepG2, A-549 and Vero cell lines. Compounds 3b (IC₅₀ 16.74 ± 0.45 µM) and 3p (IC₅₀ 10.57 ± 0.54 µM) were found to be the most active compounds against A-549 cell line among the evaluated compounds. Further 3b- and 3p-induced apoptosis was characterized by AO/EB (acridine orange/ethidium bromide) nuclear staining method and also by DNA fragmentation study. A decrease in cell viability and initiation of apoptosis was clearly evident through the morphological changes in the A-549 cells treated with 3b and 3p when stained with this method. Fragmentation of DNA into nucleosomes was observed which further confirmed the cell apoptosis in cells treated with compound 3b. Flow cytometry studies confirmed the cell cycle arrest at G2/M phase in A549 cells treated with compound 3b. Further in silico studies performed supported the in vitro anticancer activity of these compounds as depicted by dock score and binding energy values.

Nuclear apoptotic volume decrease in individual cells: Confocal microscopy imaging and kinetic modeling

The dynamics of nuclear morphology changes during apoptosis remains poorly investigated and understood. Using 3D time-lapse confocal microscopy we performed a study of early-stage apoptotic nuclear morphological changes induced by etoposide in single living HepG2 cells. These observations provide a definitive evidence that nuclear apoptotic volume decrease (AVD) is occurring simultaneously with peripheral chromatin condensation (so called "apoptotic ring"). In order to describe quantitatively the dynamics of nuclear morphological changes in the early stage of apoptosis we suggest a general molecular kinetic model, which fits well the obtained experimental data in our study. Results of this work may clarify molecular mechanisms of nuclear morphology changes during apoptosis.

Nitrogen-doped carbon nanodots for bioimaging and delivery of paclitaxel

Carbon nanodots (CNDs) hold great potential in imaging and drug delivery applications. In this study, nitrogen-doped CNDs (NCNDs) were coupled to the anticancer agent paclitaxel (PTX) through a labile ester bond. NCNDs showed excellent cell viability and endowed the NCND-PTX conjugate with good water solubility. The hybrid integrates the optical properties of the nanodots with the anticancer function of the drug into a single unit. Cytotoxicity was evaluated in breast, cervix, lung, and prostate cancer cell lines by the MTT assay while the cellular uptake was monitored using confocal microscopy. NCND-PTX induced apoptosis in cancer cells exhibiting slightly better anticancer activity compared to the drug alone. Moreover, the course of the NCND-PTX interaction with cancer cells was monitored using an xCELLigence system. The NCND-based conjugate represents a promising platform for bioimaging and drug delivery.

Cell death cascade and molecular therapy in ADAR2-deficient motor neurons of ALS

TAR DNA-binding protein (TDP-43) pathology in the motor neurons is the most reliable pathological hallmark of amyotrophic lateral sclerosis (ALS), and motor neurons bearing TDP-43 pathology invariably exhibit failure in RNA editing at the GluA2 glutamine/arginine (Q/R) site due to down-regulation of adenosine deaminase acting on RNA 2 (ADAR2). Conditional ADAR2 knockout (AR2) mice display ALS-like phenotype, including progressive motor dysfunction due to loss of motor neurons. Motor neurons devoid of ADAR2 express Q/R site-unedited GluA2, and AMPA receptors with unedited GluA2 in their subunit assembly are abnormally permeable to Ca²⁺, which results in progressive neuronal death. Moreover, analysis of AR2 mice has demonstrated that exaggerated Ca²⁺ influx through the abnormal AMPA receptors overactivates calpain, a Ca²⁺-dependent protease, that cleaves TDP-43 into aggregation-prone fragments, which serve as seeds for TDP-43 pathology. Activated calpain also disrupts nucleo-cytoplasmic transport and gene expression by cleaving molecules involved in nucleocytoplasmic transport, including nucleoporins. These lines of evidence prompted us to develop molecular targeting therapy for ALS by normalization of disrupted intracellular environment due to ADAR2 down-regulation. In this review, we have summarized the work from our group on the cell death cascade in sporadic ALS and discussed a potential therapeutic strategy for ALS.

Large-scale image-based profiling of single-cell phenotypes in arrayed CRISPR-Cas9 gene perturbation screens

High‐content imaging using automated microscopy and computer vision allows multivariate profiling of single‐cell phenotypes. Here, we present methods for the application of the CISPR‐Cas9 system in large‐scale, image‐based, gene perturbation experiments. We show that CRISPR‐Cas9‐mediated gene perturbation can be achieved in human tissue culture cells in a timeframe that is compatible with image‐based phenotyping. We developed a pipeline to construct a large‐scale arrayed library of 2,281 sequence‐verified CRISPR‐Cas9 targeting plasmids and profiled this library for genes affecting cellular morphology and the subcellular localization of components of the nuclear pore complex (NPC). We conceived a machine‐learning method that harnesses genetic heterogeneity to score gene perturbations and identify phenotypically perturbed cells for in‐depth characterization of gene perturbation effects. This approach enables genome‐scale image‐based multivariate gene perturbation profiling using CRISPR‐Cas9.

Regulation of stressed-induced cell death by the Bcl-2 family of apoptotic proteins

Apoptosis is often deregulated in a number of human diseases. Heat-induced apoptosis is a model system for studying the consequences of protein misfolding and is mediated by the Bcl-2 family of proteins. This family consists of both pro-apoptotic and anti-apoptotic members that control mitochondrial integrity. The BH3-only pro-apoptotic members are strong inducers of apoptotic cell death. Protein damaging stress can activate a process of cellular destruction known as apoptosis. The pro-apoptotic BH3-only proteins and transcription factors activate this death pathway by inhibiting the anti-apoptotic Bcl-2 family proteins eliminating cancer cells in a short period of time.

Calpain-dependent disruption of nucleo-cytoplasmic transport in ALS motor neurons

Nuclear dysfunction in motor neurons has been hypothesized to be a principal cause of amyotrophic lateral sclerosis (ALS) pathogenesis. Here, we investigated the mechanism by which the nuclear pore complex (NPC) is disrupted in dying motor neurons in a mechanistic ALS mouse model (adenosine deaminase acting on RNA 2 (ADAR2) conditional knockout (AR2) mice) and in ALS patients. We showed that nucleoporins (Nups) that constituted the NPC were cleaved by activated calpain via a Ca²⁺-permeable AMPA receptor-mediated mechanism in dying motor neurons lacking ADAR2 expression in AR2 mice. In these neurons, nucleo-cytoplasmic transport was disrupted, and the level of the transcript elongation enzyme RNA polymerase II phosphorylated at Ser2 was significantly decreased. Analogous changes were observed in motor neurons lacking ADAR2 immunoreactivity in sporadic ALS patients. Therefore, calpain-dependent NPC disruption may participate in ALS pathogenesis, and inhibiting Ca²⁺-mediated cell death signals may be a therapeutic strategy for ALS.

DNA damage signalling in D. melanogaster requires non-apoptotic function of initiator caspase Dronc

ϒH2Av response constitutes an important signalling event in DNA damage sensing ensuring effective repair by recruiting DNA repair machinery. In contrast, the occurrence of ϒH2Av response has also been reported in dying cells where it is shown to require activation of CAD (caspase activated DNase). Moreover, caspases are known to be required downstream of DNA damage for cell death execution. We show, for the first time, that initiator caspase Dronc, independent of executioner caspases, acts as an upstream regulator of DNA Damage Response (DDR) by facilitating ϒH2Av signalling perhaps involving non-apoptotic function. Such ϒH2Av response is mediated by ATM rather than ATR, suggesting that Dronc function is required upstream of ATM. In contrast, ϒH2Av appearance during cell death requires effector caspase and is associated with fragmented nuclei. Our study uncovers a novel function of Dronc in response to DNA damage aimed at promoting DDR via ϒH2Av signalling in intact nuclei. We propose that Dronc plays a dual role that can either initiate DDR or apoptosis depending upon the level and the required threshold of its activation in damaged cells.

Proaggregant nuclear factor(s) trigger rapid formation of α-synuclein aggregates in apoptotic neurons

Cell-to-cell transmission of α-synuclein (αS) aggregates has been proposed to be responsible for progressive αS pathology in Parkinson disease (PD) and related disorders, including dementia with Lewy bodies. In support of this concept, a growing body of in vitro and in vivo experimental evidence shows that exogenously introduced αS aggregates can spread into surrounding cells and trigger PD-like pathology. It remains to be determined what factor(s) lead to initiation of αS aggregation that is capable of seeding subsequent propagation. In this study we demonstrate that filamentous αS aggregates form in neurons in response to apoptosis induced by staurosporine or other toxins-6-hydroxy-dopamine and 1-methyl-4-phenylpyridinium (MPP+). Interaction between αS and proaggregant nuclear factor(s) is associated with disruption of nuclear envelope integrity. Knocking down a key nuclear envelop constituent protein, lamin B1, enhances αS aggregation. Moreover, in vitro and in vivo experimental models demonstrate that aggregates released upon cell breakdown can be taken up by surrounding cells. Accordingly, we suggest that at least some αS aggregation might be related to neuronal apoptosis or loss of nuclear membrane integrity, exposing cytosolic α-synuclein to proaggregant nuclear factors. These findings provide new clues to the pathogenesis of PD and related disorders that can lead to novel treatments of these disorders. Specifically, finding ways to limit the effects of apoptosis on αS aggregation, deposition, local uptake and subsequent propagation might significantly impact progression of disease.

Apolipoprotein D subcellular distribution pattern in neuronal cells during oxidative stress

Apolipoprotein D (Apo D) is a secreted glycoprotein, member of the lipocalin superfamily, with a related beneficial role in metabolism and lipid transport due to the presence of a binding pocket that allows its interaction with several lipids. Nowadays, it has been clearly demonstrated that Apo D expression is induced and its subcellular location undergoes modifications in stressful and pathological conditions that characterize aging processes and neurodegenerative diseases. The aim of the present work was to study in detail the effect of H2O2 on the subcellular location of Apo D, in the hippocampal cell line HT22, by structural, ultrastructural, immunocytochemical, and molecular techniques in order to characterize the Apo D distribution pattern in neurons during oxidative stress. Our results indicate that Apo D is located in the cytoplasm under physiological conditions but treatment with H2O2 induces apoptosis and causes a displacement of Apo D location to the nucleus, coinciding with DNA fragmentation. In addition, we demonstrated that Apo D tends to accumulate around the nuclear envelope in neurons and glial cells of different brain areas in some neurodegenerative diseases and during human aging, but never inside the nucleus. These data suggest that the presence of Apo D in the nucleus, which some authors related with a specific transport, is a consequence of structural and functional alterations during oxidative stress and not the result of a specific role in the regulation of nuclear processes.

Dexamethasone enhances necrosis-like neuronal death in ischemic rat hippocampus involving μ-calpain activation

Transient forebrain ischemia (TFI) leads to hippocampal CA1 pyramidal cell death which is aggravated by glucocorticoids (GC). It is unknown how GC affect apoptosis and necrosis in cerebral ischemia. We therefore investigated the co-localization of activated caspase-3 (casp-3) with apoptosis- and necrosis-like cell death morphologies in CA1 of rats treated with dexamethasone prior to TFI (DPTI). In addition, apoptosis- (casp-9, casp-3, casp-3-cleaved PARP and cleaved α-spectrin 145/150 and 120kDa) and necrosis-related (calpain-specific casp-9 cleavage, μ-calpain upregulation and cleaved α-spectrin 145/150kDa) cell death mechanisms were investigated by Western blot analysis. DPTI expedited CA1 neuronal death from day 4 to day 1 and increased the magnitude of CA1 neuronal death from 66.2% to 91.3% at day 7. Furthermore, DPTI decreased the overall (days 1-7) percentage of dying neurons displaying apoptosis-like morphology from 4.7% to 0.3% and, conversely, increased the percentage of neurons with necrosis-like morphology from 95.3% to 99.7%. In animals subjected to TFI without dexamethasone (ischemia-only), 7.4% of all dying CA1 neurons were casp-3-immunoreactive (IR), of which 3.1% co-localized with apoptosis-like and 4.3% with necrosis-like changes. By contrast, DPTI decreased the percentage of dying neurons with casp-3 IR to 1.4%, of which 0.3% co-localized with apoptosis-like changes and 1.1% with necrosis-like changes. Western blot analysis from DPTI animals showed a significant elevation of μ-calpain, a calpain-produced necrosis-related casp-9 fragment (25kDa) and cleavage of α-spectrin into 145/150kDa fragments at day 4, whereas in ischemia-only animals a significant increase of casp-3-cleaved PARP, cleavage of α-spectrin into 145/150 and 120kDa fragments was detected at day 7. We conclude that DPTI, in addition to augmenting and expediting CA1 neuronal death, causes a shift from apoptosis-like cell death to necrosis involving μ-calpain activation.

Cellular stress induces Bax-regulated nuclear bubble budding and rupture followed by nuclear protein release

Cellular stress triggers many pathways including nuclear protein redistribution. We previously discovered that this process is regulated by Bax but the underlying mechanism has not yet been studied. Here we define this mechanism by showing that apoptotic stimuli cause Bax-regulated disturbances in lamin A/C and nuclear envelope (NE)-associated proteins which results in the generation and subsequent rupture of nuclear protein-containing bubbles. The bubbles do not contain DNA and are encapsulated by impaired nuclear pore-depleted NE. Stress-induced generation and rupture of nuclear bubbles ultimately leads to the discharge of nuclear proteins into the cytoplasm. This process precedes morphological changes of apoptosis and occurs independently of caspases. Rescue experiments revealed that this Bax effect is non-canonical, i.e. it requires the BH3 domain and α-helices 5 and 6 but it is not inhibited by Bcl(-)xL. Targeting Bax to the NE by the Klarsicht/ANC-1/Syne-1 homology (KASH) domain effectively triggers the generation and rupture of nuclear bubbles. Overall, our findings provide evidence for a novel stress-response, which is regulated by a non-canonical action of Bax on the NE.

High content analysis of differentiation and cell death in human adipocytes

Understanding adipocyte biology and its homeostasis is in the focus of current obesity research. We aimed to introduce a high-content analysis procedure for directly visualizing and quantifying adipogenesis and adipoapoptosis by laser scanning cytometry (LSC) in a large population of cell. Slide-based image cytometry and image processing algorithms were used and optimized for high-throughput analysis of differentiating cells and apoptotic processes in cell culture at high confluence. Both preadipocytes and adipocytes were simultaneously scrutinized for lipid accumulation, texture properties, nuclear condensation, and DNA fragmentation. Adipocyte commitment was found after incubation in adipogenic medium for 3 days identified by lipid droplet formation and increased light absorption, while terminal differentiation of adipocytes occurred throughout day 9-14 with characteristic nuclear shrinkage, eccentric nuclei localization, chromatin condensation, and massive lipid deposition. Preadipocytes were shown to be more prone to tumor necrosis factor alpha (TNFα)-induced apoptosis compared to mature adipocytes. Importantly, spontaneous DNA fragmentation was observed at early stage when adipocyte commitment occurs. This DNA damage was independent from either spontaneous or induced apoptosis and probably was part of the differentiation program. © 2013 International Society for Advancement of Cytometry.

A comparison between nuclear dismantling during plant and animal programmed cell death

Programmed cell death (PCD) is a process of organized destruction of cells, essential for the development and maintenance of cellular homeostasis of multicellular organisms. Cells undergoing PCD begin a degenerative process in response to internal or external signals, whereby the nucleus becomes one of the targets. The process of nuclear dismantling includes events affecting the nuclear envelope, such as formation of lobes at the nuclear surface, selective proteolysis of nucleoporins and nuclear pore complex clustering. In addition, chromatin condensation increases in coordination with DNA fragmentation. These processes have been largely studied in animals, but remain poorly understood in plants. The overall process of cell death has different morphological and biochemical features in plants and animals. However, recent advances suggest that nuclear dismantling in plant cells progresses with morphological and biochemical characteristics similar to those in apoptotic animal cells. In this review, we summarize nuclear dismantling in plant PCD, focusing on the similarities and differences with their animal counterparts.

Regulation of nuclear envelope permeability in cell death and survival

The nuclear pore complex (NPC) mediates macromolecular exchange between nucleus and cytoplasm. It is a regulated channel whose functional properties are modulated in response to the physiological status of the cell. Identifying the factors responsible for regulating NPC activity is crucial to understand how intracellular signaling cues are integrated at the level of this channel to control nucleocytoplasmic trafficking. For proteins lacking active translocation signals the NPC acts as a molecular sieve limiting passage across the nuclear envelope (NE) to proteins with a MW below ~40 kD. Here, we investigate how this permeability barrier is altered in paradigms of cell death and cell survival, i.e., apoptosis induction via staurosporine, and enhanced viability via overexpression of Bcl-2. We monitor dynamic changes of the NPC's size-exclusion limit for passive diffusion by confocal time-lapse microscopy of cells undergoing apoptosis, and use different diffusion markers to determine how Bcl-2 expression affects steady-state NE permeability. We show that staurosporine triggers an immediate and gradual leakiness of the NE preceding the appearance of apoptotic hallmarks. Bcl-2 expression leads to a constitutive increase in NE permeability, and its localization at the NE is sufficient for the effect, evincing a functional role for Bcl-2 at the nuclear membrane. In both settings, NPC leakiness correlates with reduced Ca²⁺ in internal stores, as demonstrated by fluorometric measurements of ER/NE Ca²⁺ levels. By comparing two cellular models with opposite outcome these data pinpoint ER/NE Ca²⁺ as a general and physiologically relevant regulator of the permeability barrier function of the NPC.

Beta-catenin mediates the apoptosis induction effect of celastrol in HT29 cells

AIM

We evaluated the apoptosis induction effects of celastrol in human colorectal cancer cell line HT29 in WNT/beta-catenin pathway.

MAIN METHODS

HT29 cells were treated with various concentrations (10-100μM) for 24h, MTT assay was performed to examine the effect of celastrol on growth inhibition of HT29 cells. Beta-catenin siRNA was used for transfection of cells. Cell apoptosis was detected through both DNA laddering analysis and Tdt-mediated dUTP nick end labeling (TUNEL) assay. Western blot analysis and real-time reverse transcription polymerase chain reaction technologies were applied to assess the expression level of c-Myc, Bax, and Bcl-2 in HT29 cells.

KEY FINDINGS

Treatment of HT29 cells with celastrol resulted in a growth inhibition effect, and the IC(50) value was 56μM. Celastrol induced HT29 cells apoptosis, and increased the nuclear translocation of beta-catenin. Apoptosis induction effects of celastrol were significantly attenuated by beta-catenin siRNA transfection. Beta-catenin siRNA markedly increased mRNA and protein levels of c-Myc compared with control siRNA. Beta-catenin siRNA significantly inhibited the expression of Bax and Bcl-2 in celastrol-treated HT29 cells.

SIGNIFICANCE

Beta-catenin mediates the apoptosis induction effects of celastrol in HT29 cells.

Gray level co-occurrence matrix texture analysis of germinal center light zone lymphocyte nuclei: physiology viewpoint with focus on apoptosis

In our study we investigated the relationship between conventional morphometric indicators of nuclear size and shape (area and circularity) and the parameters of gray level co-occurrence matrix texture analysis (entropy, homogeneity, and angular second moment) in cells committed to apoptosis. A total of 432 lymphocyte nuclei images from the spleen germinal center light zones (cells in early stages of apoptosis) were obtained from eight healthy male guinea pigs previously immunized with sheep red blood cells (antigen). For each nucleus, area, circularity, entropy, homogeneity, and angular second moment were determined. All measured parameters of gray level co-occurrence matrix (GLCM) were significantly correlated with morphometric indicators of nuclear size and shape. The strongest correlation was observed between GLCM homogeneity and nuclear area (p < 0.0001, r(s) = 0.61). Angular second moment values were also highly significantly correlated with nuclear area (r(s)= 0.39, p < 0.0001). These results indicate that the GLCM method may be a powerful tool in evaluation of ultrastructural nuclear changes during early stages of the apoptotic process.

Case study on live cell apoptosis-assay using lamin-chromobody cell-lines for high-content analysis

The understanding of cellular processes and their physiopathological alterations requires comprehensive data on the abundance, distribution, modification and interaction of cellular components. On the one hand, artificially introduced fluorescent fusion proteins provide information about their distribution and dynamics in living cells but not on endogenous factors. On the other hand, antibodies can detect endogenous proteins, posttranslational modifications and other cellular components but mostly in fixed and permeabilized cells. Here we highlight a new technology based on the antigen-binding domain of heavy-chain antibodies (VHH) from Camelidae. We have demonstrated that these VHH domains can be fused with fluorescent proteins and expressed in living cells. Those fluorescent antigen-binding proteins-called chromobodies-can be used to detect and trace proteins and other cellular components in vivo. In principle chromobodies can detect any antigenic structure including posttranslational modifications or nonprotein components and thereby dramatically expand the quality and quantity of information that can be gathered in high-content analyses. Here we demonstrate the suitability of this technology to follow apoptosis in living cells in real time.

Effect of viral infection on the nuclear envelope and nuclear pore complex

The nuclear envelope (NE) is a vital structure that separates the nucleus from the cytoplasm. Because the NE is such a critical cellular barrier, many viral pathogens have evolved to modulate its permeability. They do this either by breaching the NE or by disrupting the integrity and functionality of the nuclear pore complex (NPC). Viruses modulate NE permeability for different reasons. Some viruses disrupt NE to deliver the viral genome into the nucleus for replication, while others cause NE disruption during nuclear egress of newly assembled capsids. Yet, other viruses modulate NE permeability and affect the compartmentalization of host proteins or block the nuclear transport of host proteins involved in the host antiviral response. Recent scientific advances demonstrated that other viruses use proteins of the NPC for viral assembly or disassembly. Here we review the ways in which various viruses affect NE and NPC during infection.

A time-lapse imaging assay to study nuclear envelope breakdown

Real-time imaging coupled with a permeabilized cell system presents a very versatile platform to visualize the dynamic and intricate nature of nuclear envelope breakdown, one of the major morphological changes of mitosis. Here, we describe such a strategy in which the plasma membrane of cells expressing fluorescently tagged nucleoporin POM121 and Histone H2B is permeabilized with digitonin. These cells are then incubated with mitotic Xenopus egg extract to create conditions that recapitulate the major events of mitotic nuclear remodeling seen in live-cell imaging, providing the opportunity to probe mechanisms and pathways that coordinate nuclear disassembly.

Nuclear proteins acting on mitochondria

An important mechanism in apoptotic regulation is changes in the subcellular distribution of pro- and anti-apoptotic proteins. Among the proteins that change in their localization and may promote apoptosis are nuclear proteins. Several of these nuclear proteins such as p53, Nur77, histone H1.2, and nucleophosmin were reported to accumulate in the cytosol and/or mitochondria and to promote the mitochondrial apoptotic pathway in response to apoptotic stressors. In this review, we will discuss the functions of these and other nuclear proteins in promoting the mitochondrial apoptotic pathway, the mechanisms that regulate their accumulation in the cytosol and/or mitochondria and the potential role of Bax and Bak in this process. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.

Defects in nuclear pore assembly lead to activation of an Aurora B-mediated abscission checkpoint

Correct assembly of nuclear pore complexes (NPCs), which directly and indirectly control nuclear environment and architecture, is vital to genomic regulation. We previously found that nucleoporin 153 (Nup 153) is required for timely progression through late mitosis. In this study, we report that disruption of Nup 153 function by either small interfering RNA-mediated depletion or expression of a dominant-interfering Nup 153 fragment results in dramatic mistargeting of the pore basket components Tpr and Nup 50 in midbody-stage cells. We find a concomitant appearance of aberrantly localized active Aurora B and an Aurora B-dependent delay in abscission. Depletion of Nup 50 is also sufficient to increase the number of midbody-stage cells and, likewise, triggers distinctive mislocalization of Aurora B. Together, our results suggest that defects in nuclear pore assembly, and specifically the basket structure, at this time of the cell cycle activate an Aurora B-mediated abscission checkpoint, thereby ensuring that daughter cells are generated only when fully formed NPCs are present.

The protease-mediated nucleus shuttles of subnanometer gold quantum dots for real-time monitoring of apoptotic cell death

Subnanometer photoluminescent gold quantum dots (GQDs) are functionalized with a peptide moiety that contains both nuclear export signal (NES) and nuclear localization signal (NLS) sequences. By taking advantage of its small size and great photostability, the functionalized GQDs are used to mimic the actions of nucleus shuttle proteins, especially of those activated during cell apoptotic death, to work as protease-mediated cytoplasm-nucleus shuttles for dynamic monitoring of apoptosis. The resulting construct demonstrates activation of the nuclear pore complex (NPC) of cells, for bidirectional transport between nucleus and cytoplasm. A caspase-3 recognition sequence (DEVD), placed within the NLS/NES peptide, serves as a proteolytic site for activated caspase-3. Upon the induction of apoptosis, the activated caspase-3 cleaves the functional peptide on GQDs resulting in changes of subcellular distribution of GQDs. Such changes can be quantified as a function of time, by the ratios of GQDs photoluminescence in nucleus to that in cytoplasm. As such, the NES-linker-DEVD-linker-NLS peptide enables the GQDs to function as molecular probes for the real-time monitoring of cellular apoptosis.

Survivin withdrawal by nuclear export failure as a physiological switch to commit cells to apoptosis

Apoptosis is a tightly controlled process regulated by many signaling pathways; however, the mechanisms and cellular events that decide whether a cell lives or dies remain poorly understood. Here we showed that when a cell is under apoptotic stress, the prosurvival protein Survivin redistributes from the cytoplasm to the nucleus, thus acting as a physiological switch to commit the cell to apoptosis. The nuclear relocalization of Survivin is a result of inefficient assembly of functional RanGTP-CRM1-Survivin export complex due to apoptotic RanGTP gradient collapse. Subsequently, Survivin undergoes ubiquitination, which not only physically prevents its diffusion back to the cytoplasm but also facilitates its degradation. Together, this spatial and functional regulation of Survivin abolishes its cytoprotective effect toward the apoptotic executors and thus commits a cell to apoptosis. Our data indicate that the withdrawal of Survivin is a novel and active physiological regulatory mechanism that tilts the survival balance and promotes the progression of apoptosis.

Review: Lamin A/C, caspase-6, and chromatin configuration during meiosis resumption in the mouse oocyte

After in vitro maturation (IVM), isolation of the healthiest oocytes is essential for successful in vitro fertilization. As germinal vesicle (GV) oocytes resume meiosis through healthy or apoptotic pathways without discernable morphological criteria, we checked for an apoptotic element acting at the nucleus level. We hypothesized that caspase-6 with its corresponding substrate, lamin A/C, could be a potential target candidate, because caspase-6 is the only functional caspase for lamin A/C. We used immunohistochemistry methods, Western blots, and a specific caspase-6 inhibitor to determine the presence of lamin A/C and caspase-6 during oogenesis and in isolated oocytes. Our results demonstrated that these proteins were always present and that their distributions were related to oocyte maturity, determined by chromatin configuration and oocyte diameter. Caspase-6 inhibition slowed meiosis resumption suggesting the involvement of caspase-6 in the oocyte apoptotic pathway. Lamin A/C and caspase-6 could be valuable tools in the knowledge of oocyte in vitro destiny.

Nuclear contour irregularity and abnormal transporter protein distribution in anterior horn cells in amyotrophic lateral sclerosis

The nucleocytoplasmic transport system is essential for maintaining cell viability; transport of proteins and nucleic acids between the nucleus and the cytoplasm occurs through nuclear pore complexes (NPCs). In this study, we examined the immunohistochemical distribution of the major protein components of NPCs, Nup62, Nup88, and Nup153, in spinal cords from controls and patients with sporadic or familial amyotrophic lateral sclerosis (SALS or FALS) and its mouse model. In control subjects, immunolabeling on the nuclear envelopes of anterior horn cells (AHCs) was invariably smooth and continuous, whereas in SALS and FALS patients, the AHCs predominantly showed irregular nuclear contours. Double immunofluorescence staining demonstrated that in SALS patients, importin-beta immunoreactivity was absent in the nuclei in a subset of AHCs; in these cells, Nup62 immunolabeling of nuclear membrane was invariably irregular, suggesting that there was dysfunctional nucleocytoplasmic transport in those AHCs. In the mouse model, Nup62-immunolabeled AHCs with irregular nuclear contours were predominant as early as the presymptomatic stage and the contours became progressively discontinuous along with disease development. Together, these observations suggest that dysfunctional nucleocytoplasmic transport may underlie the pathogenesis of ALS.

Influence of heart failure on nucleocytoplasmic transport in human cardiomyocytes

AIMS

The role of the cell nucleus in the development of heart failure (HF) is unknown, so the objectives of this study were to analyse the effect of HF on nucleocytoplasmic transport and density of the nuclear pore complex (NPC).

METHODS AND RESULTS

A total of 51 human heart samples from ischaemic (ICM, n = 30) and dilated (DCM, n = 16) patients undergoing heart transplantation and control donors (CNT, n = 5) were analysed by western blotting. Subcellular distribution of proteins and NPC were analysed by fluorescence and electron microscopy, respectively. When we compared nucleocytoplasmic machinery protein levels according to aetiology of HF, ICM showed higher levels of importins [(IMP-beta3) (150%, P < 0.0001), IMP-alpha2 (69%, P = 0.001)] and exportins [EXP-1 (178%, P < 0.0001), EXP-4 (81%, P = 0.006)] than those of the CNT group. Furthermore, DCM also showed significant differences for IMP-beta3 (192%, P < 0.0001), IMP-alpha2 (52%, P = 0.025), and EXP-1 (228%, P < 0.0001). RanGTPase-activating proteins (RanGAP1 and RaGAP1u) were increased in ICM (76%, P = 0.005; 51%, P = 0.012) and DCM (41%, P = 0.042; 50%, P = 0.029). Furthermore, subcellular distribution of nucleocytoplasmic machinery was not altered in pathological hearts. Finally, nucleoporin (Nup) p62 was increased in ICM (80%) and DCM (109%) (P < 0.001 and P = 0.024). Nuclear pore density was comparable in pathological and CNT hearts, and ICM showed a low diameter (P = 0.005) and different structural configuration of NPC.

CONCLUSION

This study shows the effect of HF on nucleocytoplasmic trafficking machinery, evidenced by higher levels of importins, exportins, Ran regulators and Nup p62 in ischaemic and dilated human hearts than those in the controls, with NPCs acquiring a different configuration and morphology in ICM.

Peptidoglycan induces loss of a nuclear peptidoglycan recognition protein during host tissue development in a beneficial animal-bacterial symbiosis

Peptidoglycan recognition proteins (PGRPs) are mediators of innate immunity and recently have been implicated in developmental regulation. To explore the interplay between these two roles, we characterized a PGRP in the host squid Euprymna scolopes (EsPGRP1) during colonization by the mutualistic bacterium Vibrio fischeri. Previous research on the squid-vibrio symbiosis had shown that, upon colonization of deep epithelium-lined crypts of the host light organ, symbiont-derived peptidoglycan monomers induce apoptosis-mediated regression of remote epithelial fields involved in the inoculation process. In this study, immunofluorescence microscopy revealed that EsPGRP1 localizes to the nuclei of epithelial cells, and symbiont colonization induces the loss of EsPGRP1 from apoptotic nuclei. The loss of nuclear EsPGRP1 occurred prior to DNA cleavage and breakdown of the nuclear membrane, but followed chromatin condensation, suggesting that it occurs during late-stage apoptosis. Experiments with purified peptidoglycan monomers and with V. fischeri mutants defective in peptidoglycan-monomer release provided evidence that these molecules trigger nuclear loss of EsPGRP1 and apoptosis. The demonstration of a nuclear PGRP is unprecedented, and the dynamics of EsPGRP1 during apoptosis provide a striking example of a connection between microbial recognition and developmental responses in the establishment of symbiosis.

Apoptotic histone modification inhibits nuclear transport by regulating RCC1

A number of signalling pathways have been identified that regulate apoptosis, but the mechanism that initiates apoptosis remains incompletely understood. We have found that the nuclear RanGTP level is diminished during the early stages of apoptosis, which correlates with immobilization of RCC1 on the chromosomes. Furthermore, the expression of phosphomimetic histone H2B or caspase-activated Mst1 immobilizes RCC1 and causes reduction of nuclear RanGTP levels, which leads to inactivation of the nuclear transport machinery. As a consequence, nuclear localization signal (NLS)-containing proteins, including NF-kappaB-p65, remain bound to importins alpha and beta in the cytoplasm. Knocking down Mst1 allows resumption of nuclear transport and the nuclear entry of NF-kappaB-p65, which have important roles in rescuing cells from apoptosis. Therefore, we propose that RCC1 reads the histone code created by caspase-activated Mst1 to initiate apoptosis by reducing the level of RanGTP in the nucleus.

Fas death pathway in sarcomas correlates with epidermal growth factor transcription

Modulation of apoptosis may influence sarcoma pathogenesis and/or aggressiveness. The Fas death pathway, mediated by FasL or TGFbeta, is one of two apoptotic pathways. Recent studies report that EGF can modulate TGFbeta and/or FasL expression/activity; thus, EGF has the potential to influence activation of the Fas pathway. EGF is not always detectable in mesenchymal tumors; therefore, we hypothesized EGF would define which Fas ligand predominates. We assayed 57 surgically removed human sarcomas for 10 genes involved in the Fas pathway. Skeletal muscle biopsies from eight patients served as controls. Sample transcripts were detected by real-time RT-PCR. We attempted to identify relevant predictor variables. The 57 sarcomas were segregated into two categories defined by EGF mRNA content: (1) 23 tumors with EGF concentrations that approximated muscle EGF transcript levels (high-EGF tumors); and (2) 34 tumors that either lacked EGF mRNA, or whose mRNA levels were very low and frequently undetected by PCR (low-EGF tumors). TGFbeta1 expression best predicted Fas transcript concentrations in the 34 low-EGF sarcomas, while FasL predicted Fas mRNA levels in the remaining 23 high-EGF sarcomas. The results suggest ligand activity in the Fas death pathway correlates with EGF transcription in sarcomas.

Apoptosis leads to a degradation of vital components of active nuclear transport and a dissociation of the nuclear lamina

Apoptosis, a physiologically critical process, is characterized by a destruction of the cell after sequential degradation of key cellular components. Here, we set out to explore the fate of the physiologically indispensable nuclear envelope (NE) in this process. The NE mediates the critical nucleocytoplasmic transport through nuclear pore complexes (NPCs). In addition, the NE is involved in gene expression and contributes significantly to the overall structure and mechanical stability of the cell nucleus through the nuclear lamina, which underlies the entire nucleoplasmic face of the NE and thereby interconnects the NPCs, the NE, and the genomic material. Using the nano-imaging and mechanical probing approach atomic force microscopy (AFM) and biochemical methods, we unveiled the fate of the NE during apoptosis. The doomed NE sustains a degradation of both the mediators of the critical selective nucleocytoplasmic transport, namely NPC cytoplasmic filaments and basket, and the nuclear lamina. These observations are paralleled by marked softening and destabilization of the NE and the detection of vesicle-like nuclear fragments. We conclude that destruction of the cell nucleus during apoptosis proceeds in a strategic fashion. Degradation of NPC cytoplasmic filaments and basket shuts down the critical selective nucleocytoplasmic cross-talk. Degradation of the nuclear lamina disrupts the pivotal connection between the NE and the chromatin, breaks up the overall nuclear architecture, and softens the NE, thereby enabling the formation of nuclear fragments at later stages of apoptosis.

BAF as a caspase-dependent mediator of nuclear apoptosis in Drosophila

BAF is a double-stranded DNA binding protein required for proper nuclear morphology and function in Drosophila development. Imaginal discs of Drosophila baf-null mutants were found to exist only in younger larvae as small degenerative tissues. Immunohistochemical analyses showed diffuse lamin distribution, DNA fragmentation, and activation of caspase drICE in these tissues, suggesting that apoptotic events can be induced by the loss of baf. We therefore investigated the fate of BAF after induction of the pro-apoptotic hid transgene, and found that the loss of DNA binding forms of BAF preceded that of non-DNA binding forms of BAF. Furthermore, the DNA binding forms of BAF disappeared from nuclei before DNA fragmentation and NPC clustering were detected, showing that the loss of BAF occurs at the initial stages of nuclear apoptosis. This BAF loss was not detected before drICE activation and was inhibited by Ac-DEVD-CHO caspase inhibitors. In summary, BAF disappears at an early stage due to caspase activity when apoptosis is induced by hid, and its depletion in mutants is sufficient in itself to induce cell death, suggesting it is an apoptotic mediator.