Mammalian cells change volume during mitosis

SKCa- and Kv1-type potassium channels and cancer: Promising therapeutic targets?

Ion channels are transmembrane structures that allow the passage of ions across cell membranes such as the plasma membrane or the membranes of various organelles like the nucleus, endoplasmic reticulum, Golgi apparatus or mitochondria. Aberrant expression of various ion channels has been demonstrated in several tumor cells, leading to the promotion of key functions in tumor development, such as cell proliferation, resistance to apoptosis, angiogenesis, invasion and metastasis. The link between ion channels and these key biological functions that promote tumor development has led to the classification of cancers as oncochannelopathies. Among all ion channels, the most varied and numerous, forming the largest family, are the potassium channels, with over 70 genes encoding them in humans. In this context, this review will provide a non-exhaustive overview of the role of plasma membrane potassium channels in cancer, describing 1) the nomenclature and structure of potassium channels, 2) the role of these channels in the control of biological functions that promotes tumor development such as proliferation, migration and cell death, and 3) the role of two particular classes of potassium channels, the SKCa- and Kv1- type potassium channels in cancer progression.

CLIC1 regulation of cancer stem cells in glioblastoma

Chloride intracellular channel 1 (CLIC1) has emerged as a therapeutic target in various cancers. CLIC1 promotes cell cycle progression and cancer stem cell (CSC) self-renewal. Furthermore, CLIC1 is shown to play diverse roles in proliferation, cell volume regulation, tumour invasion, migration, and angiogenesis. In glioblastoma (GB), CLIC1 facilitates the G1/S phase transition and tightly regulates glioma stem-like cells (GSCs), a rare population of self-renewing CSCs with central roles in tumour resistance to therapy and tumour recurrence. CLIC1 is found as either a monomeric soluble protein or as a non-covalent dimeric protein that can form an ion channel. The ratio of dimeric to monomeric protein is altered in GSCs and depends on the cell redox state. Elucidating the mechanisms underlying the alterations in CLIC1 expression and structural transitions will further our understanding of its role in GSC biology. This review will highlight the role of CLIC1 in GSCs and its significance in facilitating different hallmarks of cancer.

Metaphase Cells Enrichment for Efficient Use in the Dicentric Chromosome Assay

The dicentric chromosome assay (DCA), is considered the 'gold standard' for radiation biodosimetry. Yet, DCA, as currently implemented, may be impractical for emergency response applications, especially when time is of the essence, owing to its labor-intensive and time-consuming nature. The growth of a primary lymphocyte culture for 48 h in vitro is required for DCA, and manual scoring of dicentric chromosomes (DCs) requires an additional 24-48 h, resulting in an overall processing time of 72-96 h for dose estimation. In order to improve this timing. we introduce a protocol that will detect the metaphase cells in a population of cells, and then will harvest only those metaphase cells. Our metaphase enrichment approach is based on fixed human lymphocytes incubated with monoclonal, anti-phosphorylated H3 histone (ser 10). Antibodies against this histone have been shown to be specific for mitotic cells. Colcemid is used to arrest the mitotic cells in metaphase. Following that, a flow-cytometric sorting apparatus isolates the mitotic fraction from a large population of cells, in a few minutes. These mitotic cells are then spread onto a slide and treated with our C-Banding procedure [Gonen et al. 2022], to visualize the centromeres with DAPI. This reduces the chemical processing time to ~2 h. This reduces the time required for the DCA and makes it practical for a much wider set of applications, such as emergency response following exposure of a large population to ionizing radiation.

Protein and lipid mass concentration measurement in tissues by stimulated Raman scattering microscopy

Cell mass and chemical composition are important aggregate cellular properties that are especially relevant to physiological processes, such as growth control and tissue homeostasis. Despite their importance, it has been difficult to measure these features quantitatively at the individual cell level in intact tissue. Here, we introduce normalized Raman imaging (NoRI), a stimulated Raman scattering (SRS) microscopy method that provides the local concentrations of protein, lipid, and water from live or fixed tissue samples with high spatial resolution. Using NoRI, we demonstrate that protein, lipid, and water concentrations at the single cell are maintained in a tight range in cells under the same physiological conditions and are altered in different physiological states, such as cell cycle stages, attachment to substrates of different stiffness, or by entering senescence. In animal tissues, protein and lipid concentration varies with cell types, yet an unexpected cell-to-cell heterogeneity was found in cerebellar Purkinje cells. The protein and lipid concentration profile provides means to quantitatively compare disease-related pathology, as demonstrated using models of Alzheimer’s disease. This demonstration shows that NoRI is a broadly applicable technique for probing the biological regulation of protein mass, lipid mass, and water mass for studies of cellular and tissue growth, homeostasis, and disease.

Mechanics of 3D Cell-Hydrogel Interactions: Experiments, Models, and Mechanisms

Hydrogels are highly water-swollen molecular networks that are ideal platforms to create tissue mimetics owing to their vast and tunable properties. As such, hydrogels are promising cell-delivery vehicles for applications in tissue engineering and have also emerged as an important base for ex vivo models to study healthy and pathophysiological events in a carefully controlled three-dimensional environment. Cells are readily encapsulated in hydrogels resulting in a plethora of biochemical and mechanical communication mechanisms, which recapitulates the natural cell and extracellular matrix interaction in tissues. These interactions are complex, with multiple events that are invariably coupled and spanning multiple length and time scales. To study and identify the underlying mechanisms involved, an integrated experimental and computational approach is ideally needed. This review discusses the state of our knowledge on cell-hydrogel interactions, with a focus on mechanics and transport, and in this context, highlights recent advancements in experiments, mathematical and computational modeling. The review begins with a background on the thermodynamics and physics fundamentals that govern hydrogel mechanics and transport. The review focuses on two main classes of hydrogels, described as semiflexible polymer networks that represent physically cross-linked fibrous hydrogels and flexible polymer networks representing the chemically cross-linked synthetic and natural hydrogels. In this review, we highlight five main cell-hydrogel interactions that involve key cellular functions related to communication, mechanosensing, migration, growth, and tissue deposition and elaboration. For each of these cellular functions, recent experiments and the most up to date modeling strategies are discussed and then followed by a summary of how to tune hydrogel properties to achieve a desired functional cellular outcome. We conclude with a summary linking these advancements and make the case for the need to integrate experiments and modeling to advance our fundamental understanding of cell-matrix interactions that will ultimately help identify new therapeutic approaches and enable successful tissue engineering.

Overcoming challenges of HERG potassium channel liability through rational design: Eag1 inhibitors for cancer treatment

Two decades of research have proven the relevance of ion channel expression for tumor progression in virtually every indication, and it has become clear that inhibition of specific ion channels will eventually become part of the oncology therapeutic arsenal. However, ion channels play relevant roles in all aspects of physiology, and specificity for the tumor tissue remains a challenge to avoid undesired effects. Eag1 (K_V 10.1) is a voltage-gated potassium channel whose expression is very restricted in healthy tissues outside of the brain, while it is overexpressed in 70% of human tumors. Inhibition of Eag1 reduces tumor growth, but the search for potent inhibitors for tumor therapy suffers from the structural similarities with the cardiac HERG channel, a major off-target. Existing inhibitors show low specificity between the two channels, and screenings for Eag1 binders are prone to enrichment in compounds that also bind HERG. Rational drug design requires knowledge of the structure of the target and the understanding of structure-function relationships. Recent studies have shown subtle structural differences between Eag1 and HERG channels with profound functional impact. Thus, although both targets' structure is likely too similar to identify leads that exclusively bind to one of the channels, the structural information combined with the new knowledge of the functional relevance of particular residues or areas suggests the possibility of selective targeting of Eag1 in cancer therapies. Further development of selective Eag1 inhibitors can lead to first-in-class compounds for the treatment of different cancers.

Chloride intracellular channel 1 cooperates with potassium channel EAG2 to promote medulloblastoma growth

Ion channels represent a large class of drug targets, but their role in brain cancer is underexplored. Here, we identify that chloride intracellular channel 1 (CLIC1) is overexpressed in human central nervous system malignancies, including medulloblastoma, a common pediatric brain cancer. While global knockout does not overtly affect mouse development, genetic deletion of CLIC1 suppresses medulloblastoma growth in xenograft and genetically engineered mouse models. Mechanistically, CLIC1 enriches to the plasma membrane during mitosis and cooperates with potassium channel EAG2 at lipid rafts to regulate cell volume homeostasis. CLIC1 deficiency is associated with elevation of cell/nuclear volume ratio, uncoupling between RNA biosynthesis and cell size increase, and activation of the p38 MAPK pathway that suppresses proliferation. Concurrent knockdown of CLIC1/EAG2 and their evolutionarily conserved channels synergistically suppressed the growth of human medulloblastoma cells and Drosophila melanogaster brain tumors, respectively. These findings establish CLIC1 as a molecular dependency in rapidly dividing medulloblastoma cells, provide insights into the mechanism by which CLIC1 regulates tumorigenesis, and reveal that targeting CLIC1 and its functionally cooperative potassium channel is a disease-intervention strategy.

Quantitative Imaging of B1 Cyclin Expression Across the Cell Cycle Using Green Fluorescent Protein Tagging and Epifluorescence

In this article, we report the number of cyclin B1 proteins tagged with enhanced green fluorescent protein (eGFP) in fixed U-2 OS cells across the cell cycle. We use a quantitative analysis of epifluorescence to determine the number of eGFP molecules in a nondestructive way, and integrated over the cell we find 10⁴ to 10⁵ molecules. Based on the measured number of eGFP tagged cyclin B1 proteins, knowledge of cyclin B1 dynamics through the cell cycle, and the cell morphology, we identify the stages of cells in the cell cycle. © 2020 The Authors. Cytometry Part A published by Wiley Periodicals LLC. on behalf of International Society for Advancement of Cytometry.

Limitation in Controlling the Morphology of Mammalian Vero Cells Induced by Cell Division on Asymmetric Tungsten-Silicon Oxide Nanocomposite

Engineered nanomaterials are often used in tissue engineering applications to influence and manipulate the behavior of cells. Recently, a number of tungsten-silicon oxide nanocomposite devices containing equal width (symmetric) tungsten and silicon oxide parallel line comb structures were developed and used by our group. The devices induced over 90% of seeded cells (Vero) to align within ±20° of the axes of 10 µm wide tungsten lines. Furthermore, a mathematical model was successfully developed to predict this alignment behavior and forecast the minimum width of isolated tungsten lines required to induce such behavior. However, the mechanism by which the widths of the symmetrical tungsten and silicon oxide lines induce the alignment behavior is still unknown. Furthermore, the model was never tested on more complex asymmetrical structures. Herewith, experiments were conducted with mammalian cells on complex asymmetrical structures with unequal tungsten and silicon oxide line widths. Results showed that the model could be extended to more complex pattern structures. In addition, cell morphology on the patterned structures reset during cell division because of mitotic rounding, which reduced the population of cells that elongated and aligned on the tungsten lines. Ultimately, we concluded that it was impossible to achieve a 100% alignment with cells having unsynchronized cell cycles because cell rounding during mitosis took precedence over cell alignment; in other words, internal chemical cues had a stronger role in cell morphology than external cues.

Image-based analysis of living mammalian cells using label-free 3D refractive index maps reveals new organelle dynamics and dry mass flux

Holo-tomographic microscopy (HTM) is a label-free microscopy method reporting the fine changes of a cell's refractive indices (RIs) in three dimensions at high spatial and temporal resolution. By combining HTM with epifluorescence, we demonstrate that mammalian cellular organelles such as lipid droplets (LDs) and mitochondria show specific RI 3D patterns. To go further, we developed a computer-vision strategy using FIJI, CellProfiler3 (CP3), and custom code that allows us to use the fine images obtained by HTM in quantitative approaches. We could observe the shape and dry mass dynamics of LDs, endocytic structures, and entire cells' division that have so far, to the best of our knowledge, been out of reach. We finally took advantage of the capacity of HTM to capture the motion of many organelles at the same time to report a multiorganelle spinning phenomenon and study its dynamic properties using pattern matching and homography analysis. This work demonstrates that HTM gives access to an uncharted field of biological dynamics and describes a unique set of simple computer-vision strategies that can be broadly used to quantify HTM images.

The observation of high hypotonicity manipulating cell division

We report a morphological manipulation of cell division which was achieved by changing the environment from isotonic to highly hypotonic. Cells at telophase were observed to undergo a morphological reversal to anaphase, with the contractile ring being reopened and the cell shape reversing from dumb-bell back to spherical. Once restored to isosmotic environment, the reversed cells would either continue to divide or instead to form binuclear cells that further proliferated in runaway fashions. The immunofluorescent staining of tubulins and myosin II indicated that the hypotonic stress affected the accumulation of tubulins and myosin II at the contractile ring. Distinct from previous studies using specific chemical reagents, the present study provides a simple method to manipulate cell division. The morphological reversal is the adaption of dividing cells to the environmental change. The observation opens a new window to understand cell division mechanisms and runaways.

Cyclin-dependent kinase-mediated phosphorylation of the exocyst subunit Exo84 in late G1 phase suppresses exocytic secretion and cell growth in yeast

In eukaryotic cells, the growth rate is strictly regulated for proper progression of the cell cycle. In the budding yeast Saccharomyces cerevisiae, it was previously shown that cell growth dramatically slows down when the cells start budding at the G₁/S transition. However, the molecular mechanism for this G₁/S-associated growth arrest is unclear. In this study, using exocytic secretion, cyclin-dependent kinase (CDK) assay, immunoprecipitation, and microscopy, we demonstrate that the exocyst subunit Exo84, which is known to be phosphorylated in mitosis, can also be phosphorylated directly by Cdk1 in the late G₁ phase. Of note, we found that the Cdk1-mediated Exo84 phosphorylation impairs exocytic secretion in the late G₁ phase. Using conditional cdc mutants and phosphodeficient and phosphomimetic exo84 mutants, we further observed that Cdk1-phosphoryated Exo84 inhibits the exocyst complex assembly, exocytic secretion, and cell growth, which may be important for proper execution of the G₁/S-phase transition before commitment to a complete cell cycle. Our results suggest that the direct Cdk1-mediated regulation of the exocyst complex critically contributes to the coordination of cell growth and cell cycle progression.

Label-Free Volumetric Quantitative Imaging of the Human Somatic Cell Division by Hyperspectral Coherent Anti-Stokes Raman Scattering

Quantifying the chemical composition of unstained intact tissue and cellular samples with high spatio-temporal resolution in three dimensions would provide a step change in cell and tissue analytics critical to progress the field of cell biology. Label-free optical microscopy offers the required resolution and noninvasiveness, yet quantitative imaging with chemical specificity is a challenging endeavor. In this work, we show that hyperspectral coherent anti-Stokes Raman scattering (CARS) microscopy can be used to provide quantitative volumetric imaging of human osteosarcoma cells at various stages through cell division, a fundamental component of the cell cycle progress resulting in the segregation of cellular content to produce two progeny. We have developed and applied a quantitative data analysis method to produce volumetric three-dimensional images of the chemical composition of the dividing cell in terms of water, proteins, DNAP (a mixture of proteins and DNA, similar to chromatin), and lipids. We then used these images to determine the dry masses of the corresponding organic components. The attribution of proteins and DNAP components was validated using specific well-characterized fluorescent probes, by comparison with correlative two-photon fluorescence microscopy of DNA and mitochondria. Furthermore, we map the same chemical components under perturbed conditions, employing a drug that interferes directly with cell division (Taxol), showing its influence on cell organization and the masses of proteins, DNAP, and lipids.

Targeting Taurine Transporter (TauT) for Cancer Immunotherapy of p53 Mutation Mediated Cancers - Molecular Basis and Preclinical Implication

Taurine transporter (TauT) has been identified as a target gene of p53 tumor suppressor. TauT is also found to be overexpressed in variety type of human cancers, such as leukemia. This study showed that expression of TauT was upregulated by c-Myc and c-Jun oncogenes. To explore whether blocking of TauT inhibits tumor development, the RNA interference (RNAi) and immune targeting approaches were tested in tumor cells in vitro and in p53 mutant mice in vivo. Knockdown of TauT expression by RNAi resulted in cell cycle G2 arrest and suppressed human breast cancer MCF-7 cells proliferation determined by colonies production and cell migration assays. Knockdown of TauT also rendered MCF-7 cells more susceptible to chemotherapeutic drug-induced apoptosis. An antibody specifically against TauT blocked taurine uptake and induced cell cycle G2 arrest leading to cell death of variety type of tumor cells without affecting the viability of normal mammalian cells. TauT peptide vaccination significantly increased median lifespan (1.5-fold) of the p53 null mice and rescued p53+/- mice by extending the median lifespan from 315 days to 621 days. Furthermore, single dose treatment of tumor-bearing (thymic lymphoma) p53 null mice with TauT peptide reduced tumor size by about 50% and significantly prolonged survival of these mice from average 7 days (after observing the thymic lymphoma) to 21 days. This finding demonstrates that a novel TauT peptide vaccine can delay, inhibit, and/or treat p53 mutation related spontaneous tumorigenesis in vivo. Therefore, TauT peptide may be used as a universal cancer vaccine to prevent and/or treat patients with p53 mutation-mediated cancers.

Regulation of EGFR Endocytosis by CBL During Mitosis

The overactivation of epidermal growth factor (EGF) receptor (EGFR) is implicated in various cancers. Endocytosis plays an important role in EGFR-mediated cell signaling. We previously found that EGFR endocytosis during mitosis is mediated differently from interphase. While the regulation of EGFR endocytosis in interphase is well understood, little is known regarding the regulation of EGFR endocytosis during mitosis. Here, we found that contrary to interphase cells, mitotic EGFR endocytosis is more reliant on the activation of the E3 ligase CBL. By transfecting HeLa, MCF-7, and 293T cells with CBL siRNA or dominant-negative 70z-CBL, we found that at high EGF doses, CBL is required for EGFR endocytosis in mitotic cells, but not in interphase cells. In addition, the endocytosis of mutant EGFR Y1045F-YFP (mutation at the direct CBL binding site) is strongly delayed. The endocytosis of truncated EGFR Δ1044-YFP that does not bind to CBL is completely inhibited in mitosis. Moreover, EGF induces stronger ubiquitination of mitotic EGFR than interphase EGFR, and mitotic EGFR is trafficked to lysosomes for degradation. Furthermore, we showed that, different from interphase, low doses of EGF still stimulate EGFR endocytosis by non-clathrin mediated endocytosis (NCE) in mitosis. Contrary to interphase, CBL and the CBL-binding regions of EGFR are required for mitotic EGFR endocytosis at low doses. This is due to the mitotic ubiquitination of the EGFR even at low EGF doses. We conclude that mitotic EGFR endocytosis exclusively proceeds through CBL-mediated NCE.

Cell Volume Controls Protein Stability and Compactness of the Unfolded State

Macromolecular crowding is widely accepted as one of the factors that can alter protein stability, structure, and function inside cells. Less often considered is that crowding can be dynamic: as cell volume changes, either as a result of external duress or in the course of the cell cycle, water moves in or out through membrane channels, and crowding changes in tune. Both theory and in vitro experiments predict that protein stability will be altered as a result of crowding changes. However, it is unclear how much the structural ensemble is altered as crowding changes in the cell. To test this, we look at the response of a FRET-labeled kinase to osmotically induced volume changes in live cells. We examine both the folded and unfolded states of the kinase by changing the temperature of the media surrounding the cell. Our data reveals that crowding compacts the structure of its unfolded ensemble but stabilizes the folded protein. We propose that the structure of proteins lacking a rigid, well-defined tertiary structure could be highly sensitive to both increases and decreases in cell volume. Our findings present a possible mechanism for disordered proteins to act as sensors and actuators of cell cycle or external stress events that coincide with a change in macromolecular crowding.

Polyisoprenylated cysteinyl amide inhibitors disrupt actin cytoskeleton organization, induce cell rounding and block migration of non-small cell lung cancer

The malignant potential of Non-Small Cell Lung Cancer (NSCLC) is dependent on cellular processes that promote metastasis. F-actin organization is central to cell migration, invasion, adhesion and angiogenesis, processes involved in metastasis. F-actin remodeling is enhanced by the overexpression and/or hyper-activation of some members of the Rho family of small GTPases. Therefore, agents that mitigate hyperactive Rho proteins may be relevant for controlling metastasis. We previously reported the role of polyisoprenylated cysteinyl amide inhibitors (PCAIs) as potential inhibitors of cancers with hyperactive small GTPases. In this report, we investigate the potential role of PCAIs against NSCLC cells and show that as low as 0.5 μM PCAIs significantly inhibit 2D and 3D NCI-H1299 cell migration by 48% and 45%, respectively. PCAIs at 1 μM inhibited 2D and 3D NCI-H1299 cell invasion through Matrigel by 50% and 85%, respectively. Additionally, exposure to 5 μM of the PCAIs for 24 h caused at least a 66% drop in the levels of Rac1, Cdc42, and RhoA and a 38% drop in F-actin intensity at the cell membrane. This drop in F-actin was accompanied by a 73% reduction in the number of filopodia per cell. Interestingly, the polyisoprenyl group of the PCAIs is essential for these effects, as NSL-100, a non-farnesylated analog, does not elicit similar effects on F-actin assembly and organization. Our findings indicate that PCAIs disrupt F-actin assembly and organization to suppress cell motility and invasion. The PCAIs may be an effective therapy option for NSCLC metastasis and invasion control.

Polarized Exocytosis

Polarized exocytosis is generally considered as the multistep vesicular trafficking process in which membrane-bounded carriers are transported from the Golgi or endosomal compartments to specific sites of the plasma membrane. Polarized exocytosis in cells is achieved through the coordinated actions of membrane trafficking machinery and cytoskeleton orchestrated by signaling molecules such as the Rho family of small GTPases. Elucidating the molecular mechanisms of polarized exocytosis is essential to our understanding of a wide range of pathophysiological processes from neuronal development to tumor invasion.

An Overview of Sub-Cellular Mechanisms Involved in the Action of TTFields

Long-standing research on electric and electromagnetic field interactions with biological cells and their subcellular structures has mainly focused on the low- and high-frequency regimes. Biological effects at intermediate frequencies between 100 and 300 kHz have been recently discovered and applied to cancer cells as a therapeutic modality called Tumor Treating Fields (TTFields). TTFields are clinically applied to disrupt cell division, primarily for the treatment of glioblastoma multiforme (GBM). In this review, we provide an assessment of possible physical interactions between 100 kHz range alternating electric fields and biological cells in general and their nano-scale subcellular structures in particular. This is intended to mechanistically elucidate the observed strong disruptive effects in cancer cells. Computational models of isolated cells subject to TTFields predict that for intermediate frequencies the intracellular electric field strength significantly increases and that peak dielectrophoretic forces develop in dividing cells. These findings are in agreement with in vitro observations of TTFields' disruptive effects on cellular function. We conclude that the most likely candidates to provide a quantitative explanation of these effects are ionic condensation waves around microtubules as well as dielectrophoretic effects on the dipole moments of microtubules. A less likely possibility is the involvement of actin filaments or ion channels.

Modeling Tumor Treating Fields (TTFields) application in single cells during metaphase and telophase

Effects of electric fields on biological cells have been extensively studied but primarily in the low and high frequency regimes. Low frequency AC fields have been investigated for applications to nerve and muscle stimulation or to examine possible environmental effects of 60 Hz excitation. High frequency fields have been studied to understand tissue heating and tumor ablation. Biological effects at intermediate frequencies (in the 100-500 kHz regime) have only recently been discovered and are now being used clinically to disrupt cell division, primarily for the treatment of recurrent glioblastoma multiforme. In this study, we develop a computational framework to investigate the mechanisms of action of these Tumor Treating Fields (TTFields) and to understand in vitro findings observed in cell culture. Using Finite Element Method models of isolated cells we show that the intermediate frequency range is unique because it constitutes a transition region in which the intracellular electric field, shielded at low frequencies, increases significantly. We also show that the threshold at which this increase occurs depends on the dielectric properties of the cell membrane. Furthermore, our models of different stages of the cell cycle and of the morphological changes associated with cytokinesis show that peak dielectrophoretic forces develop within dividing cells exposed to TTFields. These findings are in agreement with in vitro observations, and enhance our understanding of how TTFields disrupt cellular function.

Mitotic cells form actin-based bridges with adjacent cells to provide intercellular communication during rounding

In order to achieve accurate chromosome segregation, eukaryotic cells undergo a dramatic change in morphology to obtain a spherical shape during mitosis. Interphase cells communicate directly with each other by exchanging ions and small molecules via gap junctions, which have important roles in controlling cell growth and differentiation. As cells round up during mitosis, the gap junctional communication between mitotic cells and adjacent interphase cells ceases. Whether mitotic cells use alternative mechanisms for mediating direct cell-cell communication during rounding is currently unknown. Here, we have studied the mechanisms involved in the remodeling of gap junctions during mitosis. We further demonstrate that mitotic cells are able to form actin-based plasma membrane bridges with adjacent cells during rounding. These structures, termed “mitotic nanotubes,” were found to be involved in mediating the transport of cytoplasm, including Rab11-positive vesicles, between mitotic cells and adjacent cells. Moreover, a subpool of the gap-junction channel protein connexin43 localized in these intercellular bridges during mitosis. Collectively, the data provide new insights into the mechanisms involved in the remodeling of gap junctions during mitosis and identify actin-based plasma membrane bridges as a novel means of communication between mitotic cells and adjacent cells during rounding.

Membrane dynamics of dividing cells imaged by lattice light-sheet microscopy

Lattice light-sheet microscopy is used to examine two problems in membrane dynamics—molecular events in clathrin-coated pit formation and changes in cell shape during cell division. This methodology sets a new standard for imaging membrane dynamics in single cells and multicellular assemblies.

Membrane remodeling is an essential part of transferring components to and from the cell surface and membrane-bound organelles and for changes in cell shape, which are particularly critical during cell division. Earlier analyses, based on classical optical live-cell imaging and mostly restricted by technical necessity to the attached bottom surface, showed persistent formation of endocytic clathrin pits and vesicles during mitosis. Taking advantage of the resolution, speed, and noninvasive illumination of the newly developed lattice light-sheet fluorescence microscope, we reexamined their assembly dynamics over the entire cell surface and found that clathrin pits form at a lower rate during late mitosis. Full-cell imaging measurements of cell surface area and volume throughout the cell cycle of single cells in culture and in zebrafish embryos showed that the total surface increased rapidly during the transition from telophase to cytokinesis, whereas cell volume increased slightly in metaphase and was relatively constant during cytokinesis. These applications demonstrate the advantage of lattice light-sheet microscopy and enable a new standard for imaging membrane dynamics in single cells and multicellular assemblies.

Resonant microchannel volume and mass measurements show that suspended cells swell during mitosis

Suspended cells transiently increase their volume during mitosis because of ion exchange through the plasma membrane.

Osmotic regulation of intracellular water during mitosis is poorly understood because methods for monitoring relevant cellular physical properties with sufficient precision have been limited. Here we use a suspended microchannel resonator to monitor the volume and density of single cells in suspension with a precision of 1% and 0.03%, respectively. We find that for transformed murine lymphocytic leukemia and mouse pro–B cell lymphoid cell lines, mitotic cells reversibly increase their volume by more than 10% and decrease their density by 0.4% over a 20-min period. This response is correlated with the mitotic cell cycle but is not coupled to nuclear osmolytes released by nuclear envelope breakdown, chromatin condensation, or cytokinesis and does not result from endocytosis of the surrounding fluid. Inhibiting Na-H exchange eliminates the response. Although mitotic rounding of adherent cells is necessary for proper cell division, our observations that suspended cells undergo reversible swelling during mitosis suggest that regulation of intracellular water may be a more general component of mitosis than previously appreciated.

Optical volume and mass measurements show that mammalian cells swell during mitosis

The extent, mechanism, and function of cell volume changes during specific cellular events, such as cell migration and cell division, have been poorly studied, mostly because of a lack of adequate techniques. Here we unambiguously report that a large range of mammalian cell types display a significant increase in volume during mitosis (up to 30%). We further show that this increase in volume is tightly linked to the mitotic state of the cell and not to its spread or rounded shape and is independent of the presence of an intact actomyosin cortex. Importantly, this volume increase is not accompanied by an increase in dry mass and thus corresponds to a decrease in cell density. This mitotic swelling might have important consequences for mitotic progression: it might contribute to produce strong pushing forces, allowing mitotic cells to round up; it might also, by lowering cytoplasmic density, contribute to the large change of physicochemical properties observed in mitotic cells.

Single-cell analysis and stochastic modelling unveil large cell-to-cell variability in influenza A virus infection

Biochemical reactions are subject to stochastic fluctuations that can give rise to cell-to-cell variability. Yet, how this variability affects viral infections, which themselves involve noisy reactions, remains largely elusive. Here we present single-cell experiments and stochastic simulations that reveal a large heterogeneity between influenza A virus (IAV)-infected cells. In particular, experimental data show that progeny virus titres range from 1 to 970 plaque-forming units and intracellular viral RNA (vRNA) levels span three orders of magnitude. Moreover, the segmentation of IAV genomes seems to increase the susceptibility of their replication to noise, since the level of different genome segments can vary substantially within a cell. In addition, simulations suggest that the abortion of virus entry and random degradation of vRNAs can result in a large fraction of non-productive cells after single-hit infection. These results challenge current beliefs that cell population measurements and deterministic simulations are an accurate representation of viral infections.

EAG2 potassium channel with evolutionarily conserved function as a brain tumor target

Over 20% of the drugs for treating human diseases target ion channels, but no cancer drug approved by the US Food and Drug Administration (FDA) is intended to target an ion channel. We found that the EAG2 (Ether-a-go-go 2) potassium channel has an evolutionarily conserved function for promoting brain tumor growth and metastasis, delineate downstream pathways, and uncover a mechanism for different potassium channels to functionally cooperate and regulate mitotic cell volume and tumor progression. EAG2 potassium channel was enriched at the trailing edge of migrating medulloblastoma (MB) cells to regulate local cell volume dynamics, thereby facilitating cell motility. We identified the FDA-approved antipsychotic drug thioridazine as an EAG2 channel blocker that reduces xenografted MB growth and metastasis, and present a case report of repurposing thioridazine for treating a human patient. Our findings illustrate the potential of targeting ion channels in cancer treatment.

Chloride intracellular channel 1 participates in migration and invasion of hepatocellular carcinoma by targeting maspin

BACKGROUND AND AIM

Our previous proteomic research found that chloride intracellular channel 1 (CLIC1) was upregulated in hepatocellular carcinoma (HCC) tissues with portal vein tumor thrombus. The present study aimed to determine the role of CLIC1 in HCC invasion.

METHODS

Immunohistochemistry was used to explore protein expression of CLIC1 in 15 cirrhotic tissues and 69 pairs of HCC and paracarcinoma tissues. Small interfering RNA (siRNA) and plasmids were transfected into HepG2 and SMMC7721 cells, and the in vitro function of CLIC1 in these cells were assessed with cell counting kit-8 assays, cell apoptosis assays, scratch assays, and transwell assays. Microarray analysis was also performed to further explore the candidate genes related to CLIC1.

RESULTS

Our results confirmed that upregulated CLIC1 expression was significantly correlated with vascular invasion (P = 0.034) in HCC tissues. Knockdown of CLIC1 decreased cell viability and the invasive potency of HepG2 cells, whereas CLIC1 overexpression resulted in an opposite effect in SMMC7721 cells. Microarray analysis identified 618 genes that were differentially expressed (fold change ≥ 2, P < 0.05) between HepG2 cells transfected with CLIC1 siRNA and the negative control. Further studies indicate that knockdown of CLIC1 increased maspin expression and reduced vascular endothelial growth factor (VEGF), matrixmetalloproteinase-2 (MMP2), MMP9, MMP11, and MMP12 expression. In contrast, overexpression of CLIC1 decreased maspin expression and increased VEGF, MMP2, MMP12, and MMP13 expression.

CONCLUSIONS

CLIC1 protein expression is significantly correlated with vascular invasion, and the present study suggests a previously unknown mechanism of CLIC1-mediated control of HCC invasiveness by targeting maspin.

Targeting potassium channels in cancer

Potassium channels are pore-forming transmembrane proteins that regulate a multitude of biological processes by controlling potassium flow across cell membranes. Aberrant potassium channel functions contribute to diseases such as epilepsy, cardiac arrhythmia, and neuromuscular symptoms collectively known as channelopathies. Increasing evidence suggests that cancer constitutes another category of channelopathies associated with dysregulated channel expression. Indeed, potassium channel–modulating agents have demonstrated antitumor efficacy. Potassium channels regulate cancer cell behaviors such as proliferation and migration through both canonical ion permeation–dependent and noncanonical ion permeation–independent functions. Given their cell surface localization and well-known pharmacology, pharmacological strategies to target potassium channel could prove to be promising cancer therapeutics.

Quantification of surface tension and internal pressure generated by single mitotic cells

During mitosis, adherent cells round up, by increasing the tension of the contractile actomyosin cortex while increasing the internal hydrostatic pressure. In the simple scenario of a liquid cell interior, the surface tension is related to the local curvature and the hydrostatic pressure difference by Laplace's law. However, verification of this scenario for cells requires accurate measurements of cell shape. Here, we use wedged micro-cantilevers to uniaxially confine single cells and determine confinement forces while concurrently determining cell shape using confocal microscopy. We fit experimentally measured confined cell shapes to shapes obeying Laplace's law with uniform surface tension and find quantitative agreement. Geometrical parameters derived from fitting the cell shape, and the measured force were used to calculate hydrostatic pressure excess and surface tension of cells. We find that HeLa cells increase their internal hydrostatic pressure excess and surface tension from ≈ 40 Pa and 0.2 mNm(-1) during interphase to ≈ 400 Pa and 1.6 mNm(-1) during metaphase. The method introduced provides a means to determine internal pressure excess and surface tension of rounded cells accurately and with minimal cellular perturbation, and should be applicable to characterize the mechanical properties of various cellular systems.

Persistent mitochondrial hyperfusion promotes G2/M accumulation and caspase-dependent cell death

Cancer cells have several hallmarks that define their neoplastic behavior. One is their unabated replicative potential that allows cells to continually proliferate, and thereby contribute to increasing tumor burden. The progression of a cell through the cell cycle is regulated by a series of checkpoints that ensures successful transmission of genetic information, as well as various cellular components, including organelles and protein complexes to the two resulting daughter cells. The mitochondrial reticulum undergoes coordinated changes in shape to correspond with specific stages of the cell cycle, the most dramatic being complete mitochondrial fragmentation prior to cytokinesis. To determine whether mitochondrial fission is a required step to ensure proper mitochondrial segregation into two daughter cells, we investigated the importance of mitochondrial dynamics to cell cycle progression. We found that mitochondrial hyperfusion promotes a defect in cell cycle progression characterized by an inability for cells to exit G2/M. Additionally, extended periods of persistent mitochondrial fusion led to robust caspase-dependent cell death. The cell death signals were coordinated through activation and cleavage of caspase-8, promoting a potent death response. These results demonstrate the importance of mitochondrial dynamics in cell cycle progression, and that inhibiting mitochondrial fission regulators may provide a therapeutic strategy to target the replicative potential of cancer cells.

Size homeostasis in adherent cells studied by synthetic phase microscopy

The coupling of the rate of cell growth to the rate of cell division determines cell size, a defining characteristic that is central to cell function and, ultimately, to tissue architecture. The physiology of size homeostasis has fascinated generations of biologists, but the mechanism, challenged by experimental limitations, remains largely unknown. In this paper, we propose a unique optical method that can measure the dry mass of thick live cells as accurately as that for thin cells with high computational efficiency. With this technique, we quantify, with unprecedented accuracy, the asymmetry of division in lymphoblasts and epithelial cells. We can then use the Collins-Richmond model of conservation to compute the relationship between growth rate and cell mass. In attached epithelial cells, we find that due to the asymmetry in cell division and size-dependent growth rate, there is active regulation of cell size. Thus, like nonadherent cells, size homeostasis requires feedback control.

Mitotic phosphorylation of Exo84 disrupts exocyst assembly and arrests cell growth

Mitotic phosphorylation of Exo84 disrupts the assembly of the exocyst complex, thereby inhibiting exocytosis of select secreted cargoes and cell surface expansion.

The rate of eukaryotic cell growth is tightly controlled for proper progression through each cell cycle stage and is important for cell size homeostasis. It was previously shown that cell growth is inhibited during mitosis when cells are preparing for division. However, the mechanism for growth arrest at this stage is unknown. Here we demonstrate that exocytosis of a select group of cargoes was inhibited before the metaphase–anaphase transition in the budding yeast Saccharomyces cerevisiae. The cyclin-dependent kinase, Cdk1, when bound to the mitotic cyclin Clb2, directly phosphorylated Exo84, a component of the exocyst complex essential for exocytosis. Mitotic phosphorylation of Exo84 disrupted the assembly of the exocyst complex, thereby affecting exocytosis and cell surface expansion. Our study demonstrates the coordination between membrane trafficking and cell cycle progression and provides a molecular mechanism by which cell growth is controlled during the cell division cycle.

Molecular crosstalk between apoptosis and autophagy induced by a novel 2-methoxyestradiol analogue in cervical adenocarcinoma cells

Background

2-Methoxyestradiol has been shown to induce both autophagy and apoptosis in various carcinogenic cell lines. Although a promising anti-cancer agent, it has poor bioavailability and rapid in vivo metabolism which decreases its efficiency. In order to improve 2-methoxyestradiol’s anti-proliferative properties, a novel 2-methoxyestradiol analogue, 2-ethyl-3-O-sulphamoyl-estra-1,3,5 (10)16-tetraene (ESE-16), was previously in silico-designed in our laboratory. This study investigated ESE-16 for its anti-proliferative potential on a cervical adenocarcinoma cell (HeLa) cell line. Additionally, the possible intracellular crosstalk mechanisms between the two types of cell death were investigated.

Methods and results

HeLa cells exposed to 0.5 μM ESE-16 for 24 hours showed morphological evidence of both apoptotic and autophagic death pathways as assessed by polarization-optical transmitted light differential interference contrast microscopy, fluorescent microscopy and transmission electron microscopy. Flow cytometric cyclin B1 quantification revealed induction of programmed cell death after halting cell cycle progression in metaphase. Confocal microscopy demonstrated that ESE-16 caused microtubule fragmentation. Flow cytometric analysis of cell cycle progression and phosphatidylserine flip determination confirmed induction of apoptosis. Moreover, an increase in aggresome formation and microtubule-associated protein light chain, LC3, was demonstrated indicative of autophagy. Both caspase 8 and 3 were upregulated in a spectrophotometric analysis, indicating the involvement of the extrinsic pathway of apoptotic induction.

Conclusions

We conclude that the novel in silico-designed compound, ESE-16, exerts its anti-proliferative effect on the tumorigenic human epithelial cervical (HeLa) cells by sequentially targeting microtubule integrity, resulting in a metaphase block, causing induction of both autophagic and apoptotic cell death via a crosstalk mechanism that involves the extrinsic pathway. Future investigations will expand on signal transduction pathways involved in both apoptosis and autophagy for assessment of ESE-16 effects on microtubule dynamic instability parameters.

K(+) channels and cell cycle progression in tumor cells

K⁺ ions play a major role in many cellular processes. The deregulation of K⁺ signaling is associated with a variety of diseases such as hypertension, atherosclerosis, or diabetes. K⁺ ions are important for setting the membrane potential, the driving force for Ca²⁺ influx, and regulate volume of growing cells. Moreover, it is increasingly recognized that K⁺ channels control cell proliferation through a novel signaling mechanisms triggered and modulated independently of ion fluxes. In cancer, aberrant expression, regulation and/or sublocalization of K⁺ channels can alter the downstream signals that converge on the cell cycle machinery. Various K⁺ channels are involved in cell cycle progression and are needed only at particular stages of the cell cycle. Consistent with this idea, the expression of Eag1 and HERG channels fluctuate along the cell cycle. Despite of acquired knowledge, our understanding of K⁺ channels functioning in cancer cells requires further studies. These include identifying the molecular mechanisms controlling the cell cycle machinery. By understanding how K⁺ channels regulate cell cycle progression in cancer cells, we will gain insights into how cancer cells subvert the need for K⁺ signal and its downstream targets to proliferate.

Mitotic inhibition of clathrin-mediated endocytosis

Endocytosis and mitosis are fundamental processes in a cell's life. Nearly 50 years of research suggest that these processes are linked and that endocytosis is shut down as cells undergo the early stages of mitosis. Precisely how this occurs at the molecular level is an open question. In this review, we summarize the early work characterizing the inhibition of clathrin-mediated endocytosis and discuss recent challenges to this established concept. We also set out four proposed mechanisms for the inhibition: mitotic phosphorylation of endocytic proteins, altered membrane tension, moonlighting of endocytic proteins, and a mitotic spindle-dependent mechanism. Finally, we speculate on the functional consequences of endocytic shutdown during mitosis and where an understanding of the mechanism of inhibition will lead us in the future.

Common mechanisms regulating cell cortex properties during cell division and cell migration

Single cell morphogenesis results from a balance of forces involving internal pressure (also called turgor pressure in plants and fungi) and the plastic and dynamic outer shell of the cell. Dominated by the cell wall in plants and fungi, mechanical properties of the outer shell of animal cells arise from the cell cortex, which is mostly composed of the plasma membrane (and membrane proteins) and the underlying meshwork of actin filaments and myosin motors (and associated proteins). In this review, following Bray and White [1988; Science 239:883-889], we draw a parallel between the regulation of the cell cortex during cell division and cell migration in animal cells. Starting from the similarities in shape changes and underlying mechanical properties, we further propose that the analogy between cell division and cell migration might run deeper, down to the basic molecular mechanisms driving cell cortex remodeling. We focus our attention on how an heterogeneous and dynamic cortex can be generated to allow cell shape changes while preserving cell integrity.

Differential regulation of Smad3 and of the type II transforming growth factor-β receptor in mitosis: implications for signaling

The response to transforming growth factor-β (TGF-β) depends on cellular context. This context is changed in mitosis through selective inhibition of vesicle trafficking, reduction in cell volume and the activation of mitotic kinases. We hypothesized that these alterations in cell context may induce a differential regulation of Smads and TGF-β receptors. We tested this hypothesis in mesenchymal-like ovarian cancer cells, arrested (or not) in mitosis with 2-methoxyestradiol (2ME2). In mitosis, without TGF-β stimulation, Smad3 was phosphorylated at the C-terminus and linker regions and localized to the mitotic spindle. Phosphorylated Smad3 interacted with the negative regulators of Smad signaling, Smurf2 and Ski, and failed to induce a transcriptional response. Moreover, in cells arrested in mitosis, Smad3 levels were progressively reduced. These phosphorylations and reduction in the levels of Smad3 depended on ERK activation and Mps1 kinase activity, and were abrogated by increasing the volume of cells arrested in mitosis with hypotonic medium. Furthermore, an Mps1-dependent phosphorylation of GFP-Smad3 was also observed upon its over-expression in interphase cells, suggesting a mechanism of negative regulation which counters increases in Smad3 concentration. Arrest in mitosis also induced a block in the clathrin-mediated endocytosis of the type II TGF-β receptor (TβRII). Moreover, following the stimulation of mitotic cells with TGF-β, the proteasome-mediated attenuation of TGF-β receptor activity, the degradation and clearance of TβRII from the plasma membrane, and the clearance of the TGF-β ligand from the medium were compromised, and the C-terminus phosphorylation of Smad3 was prolonged. We propose that the reduction in Smad3 levels, its linker phosphorylation, and its association with negative regulators (observed in mitosis prior to ligand stimulation) represent a signal attenuating mechanism. This mechanism is balanced by the retention of active TGF-β receptors at the plasma membrane. Together, both mechanisms allow for a regulated cellular response to TGF-β stimuli in mitosis.

Voltage-gated potassium channel EAG2 controls mitotic entry and tumor growth in medulloblastoma via regulating cell volume dynamics

Medulloblastoma (MB) is the most common pediatric CNS malignancy. We identify EAG2 as an overexpressed potassium channel in MBs across different molecular and histological subgroups. EAG2 knockdown not only impairs MB cell growth in vitro, but also reduces tumor burden in vivo and enhances survival in xenograft studies. Mechanistically, we demonstrate that EAG2 protein is confined intracellularly during interphase but is enriched in the plasma membrane during late G2 phase and mitosis. Disruption of EAG2 expression results in G2 arrest and mitotic catastrophe associated with failure of premitotic cytoplasmic condensation. While the tumor suppression function of EAG2 knockdown is independent of p53 activation, DNA damage checkpoint activation, or changes in the AKT pathway, this defective cell volume control is specifically associated with hyperactivation of the p38 MAPK pathway. Inhibition of the p38 pathway significantly rescues the growth defect and G2 arrest. Strikingly, ectopic membrane expression of EAG2 in cells at interphase results in cell volume reduction and mitotic-like morphology. Our study establishes the functional significance of EAG2 in promoting MB tumor progression via regulating cell volume dynamics, the perturbation of which activates the tumor suppressor p38 MAPK pathway, and provides clinical relevance for targeting this ion channel in human MBs.

Generalized cell morphological parameters based on interferometric phase microscopy and their application to cell life cycle characterization

We present analysis tools which are formulated using wide-field interferometric phase microscopy measurements, and show their ability to uniquely quantify the life cycle of live cancer cells. These parameters are based directly on the optical path delay profile of the sample and do not necessitate decoupling the refractive index and the thickness in the cell interferometric phase profile, and thus can be calculated using a single-frame acquisition. To demonstrate the use of these parameters, we have constructed a wide-field interferometric phase microscopy setup and closely traced the full lifecycle of HeLa cancer cells. These initial results show the potential of the parameters to distinguish between the different phases of the cell lifecycle, as well others biological phenomena.

Clathrin-mediated endocytosis is inhibited during mitosis

A long-standing paradigm in cell biology is the shutdown of endocytosis during mitosis. There is consensus that transferrin uptake is inhibited after entry into prophase and that it resumes in telophase. A recent study proposed that endocytosis is continuous throughout the cell cycle and that the observed inhibition of transferrin uptake is due to a decrease in available transferrin receptor at the cell surface, and not to a shutdown of endocytosis. This challenge to the established view is gradually becoming accepted. Because of this controversy, we revisited the question of endocytic activity during mitosis. Using an antibody uptake assay and controlling for potential changes in surface receptor density, we demonstrate the strong inhibition of endocytosis in mitosis of CD8 chimeras containing any of the three major internalization motifs for clathrin-mediated endocytosis (YXXΦ, [DE]XXXL[LI], or FXNPXY) or a CD8 protein with the cytoplasmic tail of the cation-independent mannose 6-phosphate receptor. The shutdown is not gradual: We describe a binary switch from endocytosis being "on" in interphase to "off" in mitosis as cells traverse the G(2)/M checkpoint. In addition, we show that the inhibition of transferrin uptake in mitosis occurs despite abundant transferrin receptor at the surface of HeLa cells. Our study finds no support for the recent idea that endocytosis continues during mitosis, and we conclude that endocytosis is temporarily shutdown during early mitosis.

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

Cortical rigidity of round cells in mitotic phase and suspended state

This paper describes the results of the analysis of cortical rigidity in two round cell states: mitotic round cells and detached round cells after trypsinization using atomic force microscopy (AFM). These two states are primary cell events with dynamic morphological alterations in vitro. The trypsinized detached cells were fixed on the substrate of membrane anchoring oleyl surface. Fluorescent images taken by confocal laser scanning microscopy revealed diverse cell surface protrusions and cortical actin development in the round cells under different conditions. Although the cortical actin of these cells seemed to develop similarly, cortical rigidity of the trypsinized round cells showed greater stiffness than that of mitotic round cells. The elasticity measurements by AFM may detect invisible information about the maturation or strength of F-actin structures and such measurements may indicate that the strength of the actomyosin cortex would be higher in trypsinized round cells compared to mitotic cells. The mechanical properties can help provide better insights into the characteristics of the actin cytoskeleton network in vicinity of cell surface during dynamic morphological alterations.

A semi-Markov model for mitosis segmentation in time-lapse phase contrast microscopy image sequences of stem cell populations

We propose a semi-Markov model trained in a max-margin learning framework for mitosis event segmentation in large-scale time-lapse phase contrast microscopy image sequences of stem cell populations. Our method consists of three steps. First, we apply a constrained optimization based microscopy image segmentation method that exploits phase contrast optics to extract candidate subsequences in the input image sequence that contains mitosis events. Then, we apply a max-margin hidden conditional random field (MM-HCRF) classifier learned from human-annotated mitotic and nonmitotic sequences to classify each candidate subsequence as a mitosis or not. Finally, a max-margin semi-Markov model (MM-SMM) trained on manually-segmented mitotic sequences is utilized to reinforce the mitosis classification results, and to further segment each mitosis into four predefined temporal stages. The proposed method outperforms the event-detection CRF model recently reported by Huh as well as several other competing methods in very challenging image sequences of multipolar-shaped C3H10T1/2 mesenchymal stem cells. For mitosis detection, an overall precision of 95.8% and a recall of 88.1% were achieved. For mitosis segmentation, the mean and standard deviation for the localization errors of the start and end points of all mitosis stages were well below 1 and 2 frames, respectively. In particular, an overall temporal location error of 0.73 ± 1.29 frames was achieved for locating daughter cell birth events.

The regulation of abscission by multi-protein complexes

The terminal stage of cytokinesis – a process termed abscission – is the severing of the thin intercellular bridge that connects the two daughter cells. Recent work provides new insight into the mechanism by which this microtubule-dense membrane bridge is resolved, and highlights important roles for multi-protein assemblies in different facets of abscission. These include the endosomal sorting complex required for transport (ESCRT), which appears to have a decisive role in the final scission event, and vesicle tethering complexes, which potentially act at an earlier stage, and might serve to prepare the abscission site. Here, we review recent studies of the structure, function and regulation of these complexes as related to abscission. We focus largely on studies of cytokinesis in mammalian cells. However, cell division in other systems, such as plants and Archae, is also considered, reflecting the mechanistic conservation of membrane-scission processes during cell division.

Vesicle trafficking and membrane remodelling in cytokinesis

All cells complete cell division by the process of cytokinesis. At the end of mitosis, eukaryotic cells accurately mark the site of division between the replicated genetic material and assemble a contractile ring comprised of myosin II, actin filaments and other proteins, which is attached to the plasma membrane. The myosin-actin interaction drives constriction of the contractile ring, forming a cleavage furrow (the so-called 'purse-string' model of cytokinesis). After furrowing is completed, the cells remain attached by a thin cytoplasmic bridge, filled with two anti-parallel arrays of microtubules with their plus-ends interdigitating in the midbody region. The cell then assembles the abscission machinery required for cleavage of the intercellular bridge, and so forms two genetically identical daughter cells. We now know much of the molecular detail of cytokinesis, including a list of potential genes/proteins involved, analysis of the function of some of these proteins, and the temporal order of their arrival at the cleavage site. Such studies reveal that membrane trafficking and/or remodelling appears to play crucial roles in both furrowing and abscission. In the present review, we assess studies of vesicular trafficking during cytokinesis, discuss the role of the lipid components of the plasma membrane and endosomes and their role in cytokinesis, and describe some novel molecules implicated in cytokinesis. The present review covers experiments performed mainly on tissue culture cells. We will end by considering how this mechanistic insight may be related to cytokinesis in other systems, and how other forms of cytokinesis may utilize similar aspects of the same machinery.