The Role of Mitotic Cell-Substrate Adhesion Re-modeling in Animal Cell Division

Human tRNAs with inosine 34 are essential to efficiently translate eukarya-specific low-complexity proteins

The modification of adenosine to inosine at the wobble position (I34) of tRNA anticodons is an abundant and essential feature of eukaryotic tRNAs. The expansion of inosine-containing tRNAs in eukaryotes followed the transformation of the homodimeric bacterial enzyme TadA, which generates I34 in tRNAArg and tRNALeu, into the heterodimeric eukaryotic enzyme ADAT, which modifies up to eight different tRNAs. The emergence of ADAT and its larger set of substrates, strongly influenced the tRNA composition and codon usage of eukaryotic genomes. However, the selective advantages that drove the expansion of I34-tRNAs remain unknown. Here we investigate the functional relevance of I34-tRNAs in human cells and show that a full complement of these tRNAs is necessary for the translation of low-complexity protein domains enriched in amino acids cognate for I34-tRNAs. The coding sequences for these domains require codons translated by I34-tRNAs, in detriment of synonymous codons that use other tRNAs. I34-tRNA-dependent low-complexity proteins are enriched in functional categories related to cell adhesion, and depletion in I34-tRNAs leads to cellular phenotypes consistent with these roles. We show that the distribution of these low-complexity proteins mirrors the distribution of I34-tRNAs in the phylogenetic tree.

The nature of cell division forces in epithelial monolayers

Epithelial cells undergo striking morphological changes during division to ensure proper segregation of genetic and cytoplasmic materials. These morphological changes occur despite dividing cells being mechanically restricted by neighboring cells, indicating the need for extracellular force generation. Beyond driving cell division itself, forces associated with division have been implicated in tissue-scale processes, including development, tissue growth, migration, and epidermal stratification. While forces generated by mitotic rounding are well understood, forces generated after rounding remain unknown. Here, we identify two distinct stages of division force generation that follow rounding: (1) Protrusive forces along the division axis that drive division elongation, and (2) outward forces that facilitate postdivision spreading. Cytokinetic ring contraction of the dividing cell, but not activity of neighboring cells, generates extracellular forces that propel division elongation and contribute to chromosome segregation. Forces from division elongation are observed in epithelia across many model organisms. Thus, division elongation forces represent a universal mechanism that powers cell division in confining epithelia.

From the cytoskeleton to the nucleus: An integrated view on early spindle assembly

Accurate chromosome segregation requires a complete restructuring of cellular organization. Microtubules remodel to assemble a mitotic spindle and the actin cytoskeleton rearranges to form a stiff actomyosin cortex. These cytoplasmic events must be spatially and temporally coordinated with mitotic chromosome condensation and nuclear envelope permeabilization, in order to ensure mitotic timing and fidelity. Here, we discuss the main cytoskeletal and nuclear events that occur during mitotic entry in proliferating animal cells, focusing on their coordinated contribution for early mitotic spindle assembly. We will also explore recent progress in understanding their regulatory biochemical and mechanical pathways.

Epithelial cell transforming 2 is regulated by Yes-associated protein 1 and mediates pancreatic cancer progression and metastasis

Pancreatic ductal adenocarcinoma (PDAC) is highly metastatic and represents one of the deadliest forms of human cancers. Previous studies showed that activation of Yes-associated protein 1 (YAP1) plays a key role in malignant transformation in the pancreas. In this study, we found that YAP1 regulates the expression of epithelial cell transforming 2 (ECT2), a guanine nucleotide exchange factor for Rho-like GTPases. By immunohistochemistry analysis of human tissues, we show that ECT2 is highly expressed in primary PDAC and liver metastasis but not in normal pancreas. These correlations were also observed in a mouse model of PDAC, where pancreatic transformation is driven by mutants of Kras and Trp53. Notably, nuclear ECT2 is upregulated in the transition from preneoplastic lesions to PDAC. High levels of YAP1 or ECT2 expression correlates with the poor overall survival rate of patients with PDAC. We further demonstrate that ECT2 is required for pancreatic cancer cell proliferation and migration in vitro. Finally, using a syngeneic orthotopic xenograft mouse model for pancreatic cancer, we found that ablation of ECT2 expression reduces pancreatic cancer growth and dissemination to the liver. These findings highlight the critical role of ECT2 in promoting pancreatic cancer growth and metastasis and provides insights into the development of novel methods for early detection and treatment.NEW & NOTEWORTHY Pancreatic ductal adenocarcinoma is one of the deadliest forms of human cancers. In this study, we identified a novel signaling mechanism involved in PDAC progression and metastasis. Yes-associated protein 1 mediates the expression of epithelial cell transforming 2, which is elevated in PDAC and correlates with poor survival. Epithelial cell transforming 2 is required for PDAC growth and metastasis. This study provides insights into the development of novel methods for early detection and treatment of PDAC.

A Correlation Between Electric Fields That Target the Cell Membrane Potential and Dividing HeLa Cancer Cell Growth Inhibition

OBJECTIVE

Clinical studies show that low intensity (single V/cm), intermediate-frequency (100 kHz-300 kHz) electric fields inhibit the growth of cancer cells, while the mechanism is not yet understood. We examine the hypothesis that electric fields modify the cell membrane potential of dividing cancer cells in a way that correlates with cells growth inhibition.

METHODS

A Schwan based mathematical model calculates the changes in HeLa cells membrane potential due to single V/cm electric fields and frequencies from 0.1 to 1 MHz. An experimental study examines the effect of these electric fields on the inhibition of HeLa cells growth in an incubator.

RESULTS

The theoretical calculation shows that the effects of these electric fields on cell membrane potential decrease with an increase in frequency. The HeLa cells experiments verified the inhibitory effect of these fields on cell growth. The inhibitory effect is decreasing with an increase in frequency, in a way that is similar to the frequency dependent effect of these fields on the cell membrane potential.

CONCLUSIONS

The superposition of the theoretical results and the experimental results suggest a correlation between the effect of these fields on the cell membrane potential and inhibition of cancer cell growth. It should be emphasized that correlations do not prove causality, however, they suggest an area for future research.

SIGNIFICANCE

These findings have value for the understanding of the mechanisms of cancer cells growth inhibition with electric fields and suggest an interesting area of research on the interaction between electromagnetic fields and cancer cells.

Detection of large extracellular silver nanoparticle rings observed during mitosis using darkfield microscopy

During studies on the absorption and interactions between silver nanoparticles and mammalian cells grown in vitro it was observed that large extracellular rings of silver nanoparticles were deposited on the microscope slide, many located near post-mitotic cells. Silver nanoparticles (AgNP, 80nm), coated with citrate, were incubated at concentrations of 0.3 to 30 μg/ml with a human-derived culture of retinal pigment epithelial cells (ARPE-19) and observed using darkfield and fluorescent microscopy, 24 h after treatment. Approximately cell-sized extracellular rings of deposited AgNP were observed on the slides among a field of dispersed individual AgNP. The mean diameter of 45 nanoparticles circles was 62.5 +/-12 microns. Ring structures were frequently observed near what appeared to be post-mitotic daughter cells, giving rise to the possibility that cell membrane fragments were deposited on the slide during mitosis, and those fragments selectively attracted and retained silver nanoparticles from suspension in the cell culture medium. These circular structures were observable for the following technical reasons: 1) darkfield microscope could observe single nanoparticles below 100 nm in size, 2) a large concentration (108 and 109) of nanoparticles was used in these experiments 3) negatively charged nanoparticles were attracted to adhesion membrane proteins remaining on the slide from mitosis. The observation of silver nanoparticles attracted to apparent remnants of cellular mitosis could be a useful tool for the study of normal and abnormal mitosis.

Inhibition of cell membrane ingression at the division site by cell walls in fission yeast

Eukaryotic cells assemble actomyosin rings during cytokinesis to function as force-generating machines to drive membrane invagination and to counteract the intracellular pressure and the cell surface tension. How the extracellular matrix affects actomyosin ring contraction has not been fully explored. While studying the Schizosaccharomyces pombe 1,3-β-glucan-synthase mutant cps1-191, which is defective in division septum synthesis and arrests with a stable actomyosin ring, we found that weakening of the extracellular glycan matrix caused the generated spheroplasts to divide under the nonpermissive condition. This nonmedial slow division was dependent on a functional actomyosin ring and vesicular trafficking, but independent of normal septum synthesis. Interestingly, the high intracellular turgor pressure appears to play a minimal role in inhibiting ring contraction in the absence of cell wall remodeling in cps1-191 mutants, as decreasing the turgor pressure alone did not enable spheroplast division. We propose that during cytokinesis, the extracellular glycan matrix restricts actomyosin ring contraction and membrane ingression, and remodeling of the extracellular components through division septum synthesis relieves the inhibition and facilitates actomyosin ring contraction.

Mechanochemical Crosstalk Produces Cell-Intrinsic Patterning of the Cortex to Orient the Mitotic Spindle

Proliferating animal cells are able to orient their mitotic spindles along their interphase cell axis, setting up the axis of cell division, despite rounding up as they enter mitosis. This has previously been attributed to molecular memory and, more specifically, to the maintenance of adhesions and retraction fibers in mitosis [1-6], which are thought to act as local cues that pattern cortical Gαi, LGN, and nuclear mitotic apparatus protein (NuMA) [3, 7-18]. This cortical machinery then recruits and activates Dynein motors, which pull on astral microtubules to position the mitotic spindle. Here, we reveal a dynamic two-way crosstalk between the spindle and cortical motor complexes that depends on a Ran-guanosine triphosphate (GTP) signal [12], which is sufficient to drive continuous monopolar spindle motion independently of adhesive cues in flattened human cells in culture. Building on previous work [1, 12, 19-23], we implemented a physical model of the system that recapitulates the observed spindle-cortex interactions. Strikingly, when this model was used to study spindle dynamics in cells entering mitosis, the chromatin-based signal was found to preferentially clear force generators from the short cell axis, so that cortical motors pulling on astral microtubules align bipolar spindles with the interphase long cell axis, without requiring a fixed cue or a physical memory of interphase shape. Thus, our analysis shows that the ability of chromatin to pattern the cortex during the process of mitotic rounding is sufficient to translate interphase shape into a cortical pattern that can be read by the spindle, which then guides the axis of cell division.

Turning Cell Adhesions ON or OFF with High Spatiotemporal Precision Using the Green Light Responsive Protein CarH

Spatiotemporal control of integrin-mediated cell adhesions to extracellular matrix regulates cell behavior with has numerous implications for biotechnological applications. In this work, two approaches for regulating cell adhesions in space and time with high precision are reported, both of which utilize green light. In the first design, CarH, which is a tetramer in the dark, is used to mask cRGD adhesion-peptides on a surface. Upon green light illumination, the CarH tetramer dissociates into its monomers, revealing the adhesion peptide so that cells can adhere. In the second design, the RGD motif is incorporated into the CarH protein tetramer such that cells can adhere to surfaces functionalized with this protein. The cell adhesions can be disrupted with green light, due to the disassembly of the CarH-RGD protein. Both designs allow for photoregulation with noninvasive visible light and open new possibilities to investigate the dynamical regulation of cell adhesions in cell biology.

The Mechanics of Mitotic Cell Rounding

When animal cells enter mitosis, they round up to become spherical. This shape change is accompanied by changes in mechanical properties. Multiple studies using different measurement methods have revealed that cell surface tension, intracellular pressure and cortical stiffness increase upon entry into mitosis. These cell-scale, biophysical changes are driven by alterations in the composition and architecture of the contractile acto-myosin cortex together with osmotic swelling and enable a mitotic cell to exert force against the environment. When the ability of cells to round is limited, for example by physical confinement, cells suffer severe defects in spindle assembly and cell division. The requirement to push against the environment to create space for spindle formation is especially important for cells dividing in tissues. Here we summarize the evidence and the tools used to show that cells exert rounding forces in mitosis in vitro and in vivo, review the molecular basis for this force generation and discuss its function for ensuring successful cell division in single cells and for cells dividing in normal or diseased tissues.

Centrosome-nuclear axis repositioning drives the assembly of a bipolar spindle scaffold to ensure mitotic fidelity

During the initial stages of cell division, the cytoskeleton is extensively reorganized so that a bipolar mitotic spindle can be correctly assembled. This process occurs through the action of molecular motors, cytoskeletal networks, and the nucleus. How the combined activity of these different components is spatiotemporally regulated to ensure efficient spindle assembly remains unclear. To investigate how cell shape, cytoskeletal organization, and molecular motors cross-talk to regulate initial spindle assembly, we use a combination of micropatterning with high-resolution imaging and 3D cellular reconstruction. We show that during prophase, centrosomes and nucleus reorient so that centrosomes are positioned on the shortest nuclear axis at nuclear envelope (NE) breakdown. We also find that this orientation depends on a combination of centrosome movement controlled by Arp2/3-mediated regulation of microtubule dynamics and Dynein-generated forces on the NE that regulate nuclear reorientation. Finally, we observe this centrosome configuration favors the establishment of an initial bipolar spindle scaffold, facilitating chromosome capture and accurate segregation, without compromising division plane orientation.

The focal adhesion protein kindlin-2 controls mitotic spindle assembly by inhibiting histone deacetylase 6 and maintaining α-tubulin acetylation

Kindlins are focal adhesion proteins that regulate integrin activation and outside-in signaling. The kindlin family consists of three members, kindlin-1, -2, and -3. Kindlin-2 is widely expressed in multiple cell types, except those from the hematopoietic lineage. A previous study has reported that the Drosophila Fit1 protein (an ortholog of kindlin-2) prevents abnormal spindle assembly; however, the mechanism remains unknown. Here, we show that kindlin-2 maintains spindle integrity in mitotic human cells. The human neuroblastoma SH-SY5Y cell line expresses only kindlin-2, and we found that when SH-SY5Y cells are depleted of kindlin-2, they exhibit pronounced spindle abnormalities and delayed mitosis. Of note, acetylation of α-tubulin, which maintains microtubule flexibility and stability, was diminished in the kindlin-2-depleted cells. Mechanistically, we found that kindlin-2 maintains α-tubulin acetylation by inhibiting the microtubule-associated deacetylase histone deacetylase 6 (HDAC6) via a signaling pathway involving AKT Ser/Thr kinase (AKT)/glycogen synthase kinase 3β (GSK3β) or paxillin. We also provide evidence that prolonged hypoxia down-regulates kindlin-2 expression, leading to spindle abnormalities not only in the SH-SY5Y cell line, but also cell lines derived from colon and breast tissues. The findings of our study highlight that kindlin-2 regulates mitotic spindle assembly and that this process is perturbed in cancer cells in a hypoxic environment.

Oncogenic Signaling Alters Cell Shape and Mechanics to Facilitate Cell Division under Confinement

To divide in a tissue, both normal and cancer cells become spherical and mechanically stiffen as they enter mitosis. We investigated the effect of oncogene activation on this process in normal epithelial cells. We found that short-term induction of oncogenic RasV12 activates downstream mitogen-activated protein kinase (MEK-ERK) signaling to alter cell mechanics and enhance mitotic rounding, so that RasV12-expressing cells are softer in interphase but stiffen more upon entry into mitosis. These RasV12-dependent changes allow cells to round up and divide faithfully when confined underneath a stiff hydrogel, conditions in which normal cells and cells with reduced levels of Ras-ERK signaling suffer multiple spindle assembly and chromosome segregation errors. Thus, by promoting cell rounding and stiffening in mitosis, oncogenic RasV12 enables cells to proliferate under conditions of mechanical confinement like those experienced by cells in crowded tumors.

The crosstalk between microtubules, actin and membranes shapes cell division

Mitotic progression is orchestrated by morphological and mechanical changes promoted by the coordinated activities of the microtubule (MT) cytoskeleton, the actin cytoskeleton and the plasma membrane (PM). MTs assemble the mitotic spindle, which assists sister chromatid separation, and contact the rigid and tensile actomyosin cortex rounded-up underneath the PM. Here, we highlight the dynamic crosstalk between MTs, actin and cell membranes during mitosis, and discuss the molecular connections between them. We also summarize recent views on how MT traction forces, the actomyosin cortex and membrane trafficking contribute to spindle positioning in isolated cells in culture and in epithelial sheets. Finally, we describe the emerging role of membrane trafficking in synchronizing actomyosin tension and cell shape changes with cell-substrate adhesion, cell-cell contacts and extracellular signalling events regulating proliferation.

Cytokinesis in Eukaryotic Cells: The Furrow Complexity at a Glance

The duplication cycle is the fascinating process that, starting from a cell, results in the formation of two daughter cells and it is essential for life. Cytokinesis is the final step of the cell cycle, it is a very complex phase, and is a concert of forces, remodeling, trafficking, and cell signaling. All of the steps of cell division must be properly coordinated with each other to faithfully segregate the genetic material and this task is fundamental for generating viable cells. Given the importance of this process, molecular pathways and proteins that are involved in cytokinesis are conserved from yeast to humans. In this review, we describe symmetric and asymmetric cell division in animal cell and in a model organism, budding yeast. In addition, we illustrate the surveillance mechanisms that ensure a proper cell division and discuss the connections with normal cell proliferation and organs development and with the occurrence of human diseases.

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.

Correlation of Capacitance and Microscopy Measurements Using Image Processing for a Lab-on-CMOS Microsystem

We present a capacitance sensor chip developed in a 0.35-μm complementary metal-oxide-semiconductor process for monitoring biological cell viability and proliferation. The chip measures the cell-to-substrate binding through capacitance-to-frequency conversion with a sensitivity of 590 kHz/fF. In vitro experiments with two human ovarian cancer cell lines (CP70 and A2780) were performed and showed the ability to track cell viability in realtime over three days. An imaging platform was developed to provide time-lapse images of the sensor surface, which allowed for concurrent visual and capacitance observation of the cells. The results showed the ability to detect single-cell binding events and changes in cell morphology. Image processing was performed to estimate the cell coverage of sensor electrodes, showing good linear correlation and providing a sensor gain of 1.28 ± 0.29 aF/μm², which agrees with values reported in the literature. The device is designed for unsupervised operation with minimal packaging requirements. Only a microcontroller is required for readout, making it suitable for applications outside the traditional laboratory setting.

Crosslinking activity of non-muscle myosin II is not sufficient for embryonic cytokinesis in C. elegans

Cytokinesis in animal cells requires the assembly and constriction of a contractile actomyosin ring. Non-muscle myosin II is essential for cytokinesis, but the role of its motor activity remains unclear. Here, we examine cytokinesis in C. elegans embryos expressing non-muscle myosin motor mutants generated by genome editing. Two non-muscle motor-dead myosins capable of binding F-actin do not support cytokinesis in the one-cell embryo, and two partially motor-impaired myosins delay cytokinesis and render rings more sensitive to reduced myosin levels. Further analysis of myosin mutants suggests that it is myosin motor activity, and not the ability of myosin to crosslink F-actin, that drives the alignment and compaction of F-actin bundles during contractile ring assembly, and that myosin motor activity sets the pace of contractile ring constriction. We conclude that myosin motor activity is required at all stages of cytokinesis. Finally, characterization of the corresponding motor mutations in C. elegans major muscle myosin shows that motor activity is required for muscle contraction but is dispensable for F-actin organization in adult muscles.

This article has an associated ‘The people behind the papers’ interview.

Highlighted Article: The motor activity of non-muscle myosin II is essential for cytokinesis and contributes to all stages of the process in C. elegans embryos.

Persistent and polarized global actin flow is essential for directionality during cell migration

Cell migration is hypothesized to involve a cycle of behaviours beginning with leading edge extension. However, recent evidence suggests that the leading edge may be dispensable for migration, raising the question of what actually controls cell directionality. Here, we exploit the embryonic migration of Drosophila macrophages to bridge the different temporal scales of the behaviours controlling motility. This approach reveals that edge fluctuations during random motility are not persistent and are weakly correlated with motion. In contrast, flow of the actin network behind the leading edge is highly persistent. Quantification of actin flow structure during migration reveals a stable organization and asymmetry in the cell-wide flowfield that strongly correlates with cell directionality. This organization is regulated by a gradient of actin network compression and destruction, which is controlled by myosin contraction and cofilin-mediated disassembly. It is this stable actin-flow polarity, which integrates rapid fluctuations of the leading edge, that controls inherent cellular persistence.

Traction forces at the cytokinetic ring regulate cell division and polyploidy in the migrating zebrafish epicardium

Epithelial repair and regeneration are driven by collective cell migration and division. Both cellular functions involve tightly controlled mechanical events, but how physical forces regulate cell division in migrating epithelia is largely unknown. Here we show that cells dividing in the migrating zebrafish epicardium exert large cell-extracellular matrix (ECM) forces during cytokinesis. These forces point towards the division axis and are exerted through focal adhesions that connect the cytokinetic ring to the underlying ECM. When subjected to high loading rates, these cytokinetic focal adhesions prevent closure of the contractile ring, leading to multi-nucleation through cytokinetic failure. By combining a clutch model with experiments on substrates of different rigidity, ECM composition and ligand density, we show that failed cytokinesis is triggered by adhesion reinforcement downstream of increased myosin density. The mechanical interaction between the cytokinetic ring and the ECM thus provides a mechanism for the regulation of cell division and polyploidy that may have implications in regeneration and cancer.

Connections between the cell cycle, cell adhesion and the cytoskeleton

Cell division, the purpose of which is to enable cell replication, and in particular to distribute complete, accurate copies of genetic material to daughter cells, is essential for the propagation of life. At a morphological level, division not only necessitates duplication of cellular structures, but it also relies on polar segregation of this material followed by physical scission of the parent cell. For these fundamental changes in cell shape and positioning to be achieved, mechanisms are required to link the cell cycle to the modulation of cytoarchitecture. Outside of mitosis, the three main cytoskeletal networks not only endow cells with a physical cytoplasmic skeleton, but they also provide a mechanism for spatio-temporal sensing via integrin-associated adhesion complexes and site-directed delivery of cargoes. During mitosis, some interphase functions are retained, but the architecture of the cytoskeleton changes dramatically, and there is a need to generate a mitotic spindle for chromosome segregation. An economical solution is to re-use existing cytoskeletal molecules: transcellular actin stress fibres remodel to create a rigid cortex and a cytokinetic furrow, while unipolar radial microtubules become the primary components of the bipolar spindle. This remodelling implies the existence of specific mechanisms that link the cell-cycle machinery to the control of adhesion and the cytoskeleton. In this article, we review the intimate three-way connection between microenvironmental sensing, adhesion signalling and cell proliferation, particularly in the contexts of normal growth control and aberrant tumour progression. As the morphological changes that occur during mitosis are ancient, the mechanisms linking the cell cycle to the cytoskeleton/adhesion signalling network are likely to be primordial in nature and we discuss recent advances that have elucidated elements of this link. A particular focus is the connection between CDK1 and cell adhesion. This article is part of a discussion meeting issue 'Forces in cancer: interdisciplinary approaches in tumour mechanobiology'.

Clathrin-containing adhesion complexes

An understanding of the mechanisms whereby cell adhesion complexes (ACs) relay signals bidirectionally across the plasma membrane is necessary to interpret the role of adhesion in regulating migration, differentiation, and growth. A range of AC types has been defined, but to date all have similar compositions and are dependent on a connection to the actin cytoskeleton. Recently, a new class of AC has been reported that normally lacks association with both the cytoskeleton and integrin-associated adhesome components, but is rich in components of the clathrin-mediated endocytosis machinery. The characterization of this new type of adhesion structure, which is emphasized by mitotic cells and cells in long-term culture, identifies a hitherto underappreciated link between the adhesion machinery and clathrin structures at the plasma membrane. While this discovery has implications for how ACs are assembled and disassembled, it raises many other issues. Consequently, to increase awareness within the field, and stimulate research, we explore a number of the most significant questions below.

Molecular form and function of the cytokinetic ring

Animal cells, amoebas and yeast divide using a force-generating, actin- and myosin-based contractile ring or 'cytokinetic ring' (CR). Despite intensive research, questions remain about the spatial organization of CR components, the mechanism by which the CR generates force, and how other cellular processes are coordinated with the CR for successful membrane ingression and ultimate cell separation. This Review highlights new findings about the spatial relationship of the CR to the plasma membrane and the arrangement of molecules within the CR from studies using advanced microscopy techniques, as well as mechanistic information obtained from in vitro approaches. We also consider advances in understanding coordinated cellular processes that impact the architecture and function of the CR.

Local actin nucleation tunes centrosomal microtubule nucleation during passage through mitosis

Cells going through mitosis undergo precisely timed changes in cell shape and organisation, which serve to ensure the fair partitioning of cellular components into the two daughter cells. These structural changes are driven by changes in actin filament and microtubule dynamics and organisation. While most evidence suggests that the two cytoskeletal systems are remodelled in parallel during mitosis, recent work in interphase cells has implicated the centrosome in both microtubule and actin nucleation, suggesting the potential for regulatory crosstalk between the two systems. Here, by using both in vitro and in vivo assays to study centrosomal actin nucleation as cells pass through mitosis, we show that mitotic exit is accompanied by a burst in cytoplasmic actin filament formation that depends on WASH and the Arp2/3 complex. This leads to the accumulation of actin around centrosomes as cells enter anaphase and to a corresponding reduction in the density of centrosomal microtubules. Taken together, these data suggest that the mitotic regulation of centrosomal WASH and the Arp2/3 complex controls local actin nucleation, which may function to tune the levels of centrosomal microtubules during passage through mitosis.

Network Contractility During Cytokinesis-from Molecular to Global Views

Cytokinesis is the last stage of cell division, which partitions the mother cell into two daughter cells. It requires the assembly and constriction of a contractile ring that consists of a filamentous contractile network of actin and myosin. Network contractility depends on network architecture, level of connectivity and myosin motor activity, but how exactly is the contractile ring network organized or interconnected and how much it depends on motor activity remains unclear. Moreover, the contractile ring is not an isolated entity; rather, it is integrated into the surrounding cortex. Therefore, the mechanical properties of the cell cortex and cortical behaviors are expected to impact contractile ring functioning. Due to the complexity of the process, experimental approaches have been coupled to theoretical modeling in order to advance its global understanding. While earlier coarse-grained descriptions attempted to provide an integrated view of the process, recent models have mostly focused on understanding the behavior of an isolated contractile ring. Here we provide an overview of the organization and dynamics of the actomyosin network during cytokinesis and discuss existing theoretical models in light of cortical behaviors and experimental evidence from several systems. Our view on what is missing in current models and should be tested in the future is provided.

Integrins: Moonlighting Proteins in Invadosome Formation

Invadopodia are actin-rich protrusions developed by transformed cells in 2D/3D environments that are implicated in extracellular matrix (ECM) remodeling and degradation. These structures have an undoubted association with cancer invasion and metastasis because invadopodium formation in vivo is a key step for intra/extravasation of tumor cells. Invadopodia are closely related to other actin-rich structures known as podosomes, which are typical structures of normal cells necessary for different physiological processes during development and organogenesis. Invadopodia and podosomes are included in the general term 'invadosomes,' as they both appear as actin puncta on plasma membranes next to extracellular matrix metalloproteinases, although organization, regulation, and function are slightly different. Integrins are transmembrane proteins implicated in cell-cell and cell-matrix interactions and other important processes such as molecular signaling, mechano-transduction, and cell functions, e.g., adhesion, migration, or invasion. It is noteworthy that integrin expression is altered in many tumors, and other pathologies such as cardiovascular or immune dysfunctions. Over the last few years, growing evidence has suggested a role of integrins in the formation of invadopodia. However, their implication in invadopodia formation and adhesion to the ECM is still not well known. This review focuses on the role of integrins in invadopodium formation and provides a general overview of the involvement of these proteins in the mechanisms of metastasis, taking into account classic research through to the latest and most advanced work in the field.

NanoJ: a high-performance open-source super-resolution microscopy toolbox

Super-resolution microscopy (SRM) has become essential for the study of nanoscale biological processes. This type of imaging often requires the use of specialised image analysis tools to process a large volume of recorded data and extract quantitative information. In recent years, our team has built an open-source image analysis framework for SRM designed to combine high performance and ease of use. We named it NanoJ-a reference to the popular ImageJ software it was developed for. In this paper, we highlight the current capabilities of NanoJ for several essential processing steps: spatio-temporal alignment of raw data (NanoJ-Core), super-resolution image reconstruction (NanoJ-SRRF), image quality assessment (NanoJ-SQUIRREL), structural modelling (NanoJ-VirusMapper) and control of the sample environment (NanoJ-Fluidics). We expect to expand NanoJ in the future through the development of new tools designed to improve quantitative data analysis and measure the reliability of fluorescent microscopy studies.

Fix Your Membrane Receptor Imaging: Actin Cytoskeleton and CD4 Membrane Organization Disruption by Chemical Fixation

Single-molecule localization microscopy (SMLM) techniques allow near molecular scale resolution (~ 20 nm) as well as precise and robust analysis of protein organization at different scales. SMLM hardware, analytics and probes have been the focus of a variety of studies and are now commonly used in laboratories across the world. Protocol reliability and artifact identification are increasingly seen as important aspects of super-resolution microscopy. The reliability of these approaches thus requires in-depth evaluation so that biological findings are based on solid foundations. Here we explore how different fixation approaches that disrupt or preserve the actin cytoskeleton affect membrane protein organization. Using CD4 as a model, we show that fixation-mediated disruption of the actin cytoskeleton correlates with changes in CD4 membrane organization. We highlight how these artifacts are easy to overlook and how careful sample preparation is essential for extracting meaningful results from super-resolution microscopy.

Automating multimodal microscopy with NanoJ-Fluidics

Combining and multiplexing microscopy approaches is crucial to understand cellular events, but requires elaborate workflows. Here, we present a robust, open-source approach for treating, labelling and imaging live or fixed cells in automated sequences. NanoJ-Fluidics is based on low-cost Lego hardware controlled by ImageJ-based software, making high-content, multimodal imaging easy to implement on any microscope with high reproducibility. We demonstrate its capacity on event-driven, super-resolved live-to-fixed and multiplexed STORM/DNA-PAINT experiments.

Automating multimodal microscopy with NanoJ-Fluidics

Combining and multiplexing microscopy approaches is crucial to understand cellular events, but requires elaborate workflows. Here, we present a robust, open-source approach for treating, labelling and imaging live or fixed cells in automated sequences. NanoJ-Fluidics is based on low-cost Lego hardware controlled by ImageJ-based software, making high-content, multimodal imaging easy to implement on any microscope with high reproducibility. We demonstrate its capacity on event-driven, super-resolved live-to-fixed and multiplexed STORM/DNA-PAINT experiments.

Signal transduction via integrin adhesion complexes

Integrin adhesion complexes (IACs) have evolved over millions of years to integrate metazoan cells physically with their microenvironment. It is presumed that the simultaneous interaction of thousands of integrin receptors to binding sites in anisotropic extracellular matrix (ECM) networks enables cells to assemble a topological description of the chemical and mechanical properties of their surroundings. This information is then converted into intracellular signals that influence cell positioning, differentiation and growth, but may also influence other fundamental processes, such as protein synthesis and energy regulation. In this way, changes in the microenvironment can influence all aspects of cell phenotype. Current concepts envisage cell fate decisions being controlled by the integrated signalling output of myriad receptor clusters, but the mechanisms are not understood. Analyses of the adhesome, the complement of proteins attracted to the vicinity of IACs, are now providing insights into some of the primordial links connecting these processes. This article reviews recent advances in our understanding of the composition of IACs, the mechanisms used to transduce signals through these junctions, and the links between IACs and cell phenotype.

The balance between adhesion and contraction during cell division

The ability to divide is a fundamental property of a living cell. The 3D orientation of cell division is essential for embryogenesis, maintenance of tissue organization and architecture, as well as controlling cell fate. Much attention has been placed on the mitotic spindle's role in placing itself along the cell's longest axis, where a shape sensing mechanism between a population of microtubules extending from mitotic centrosomes to the cell cortex occurs. However, contractile forces at the cell cortex also likely play a decisive role in determining the final placement of daughter cells following division. In this review, we discuss recent literature that describes the role of these contractile forces and how these forces could be balanced by mitotic adhesion complexes.

Identification of myosin II as a cripto binding protein and regulator of cripto function in stem cells and tissue regeneration

Cripto regulates stem cell function in normal and disease contexts via TGFbeta/activin/nodal, PI3K/Akt, MAPK and Wnt signaling. Still, the molecular mechanisms that govern these pleiotropic functions of Cripto remain poorly understood. We performed an unbiased screen for novel Cripto binding proteins using proteomics-based methods, and identified novel proteins including members of myosin II complexes, the actin cytoskeleton, the cellular stress response, and extracellular exosomes. We report that myosin II, and upstream ROCK1/2 activities are required for localization of Cripto to cytoplasm/membrane domains and its subsequent release into the conditioned media fraction of cultured cells. Functionally, we demonstrate that soluble Cripto (one-eyed pinhead in zebrafish) promotes proliferation in mesenchymal stem cells (MSCs) and stem cell-mediated wound healing in the zebrafish caudal fin model of regeneration. Notably, we demonstrate that both Cripto and myosin II inhibitors attenuated regeneration to a similar degree and in a non-additive manner. Taken together, our data present a novel role for myosin II function in regulating subcellular Cripto localization and function in stem cells and an important regulatory mechanism of tissue regeneration. Importantly, these insights may further the development of context-dependent Cripto agonists and antagonists for therapeutic benefit.

Cell-Cycle-Dependent Regulation of Cell Adhesions: Adhering to the Schedule: Three papers reveal unexpected properties of adhesion structures as cells progress through the cell cycle

Focal adhesions disassemble during mitosis, but surprisingly little is known about how these structures respond to other phases of the cell cycle. Three recent papers reveal unexpected results as they examine adhesions through the cell cycle. A biphasic response is detected where focal adhesions grow during S phase before disassembly begins early in G2. In M phase, activated integrins at the tips of retraction fibers anchor mitotic cells, but these adhesions lack the defining components of focal adhesions, such as talin, paxillin, and zyxin. Re-examining cell-matrix adhesion reveals reticular adhesions, a new class of adhesion. These αVβ5 integrin-mediated adhesions also lack conventional focal adhesion components and anchor mitotic cells to the extracellular matrix. As reviewed here, these studies present insight into how adhesion complexes vary through the cell cycle, and how unconventional adhesions maintain attachment during mitosis while providing spatial memory to guide daughter cell re-spreading after cell division.

Reticular adhesions are a distinct class of cell-matrix adhesions that mediate attachment during mitosis

Adhesion to the extracellular matrix persists during mitosis in most cell types. However, while classical adhesion complexes, such as focal adhesions, do and must disassemble to enable mitotic rounding, the mechanisms of residual mitotic cell-extracellular matrix adhesion remain undefined. Here, we identify 'reticular adhesions', a class of adhesion complex that is mediated by integrin αvβ5, formed during interphase, and preserved at cell-extracellular matrix attachment sites throughout cell division. Consistent with this role, integrin β5 depletion perturbs mitosis and disrupts spatial memory transmission between cell generations. Reticular adhesions are morphologically and dynamically distinct from classical focal adhesions. Mass spectrometry defines their unique composition, enriched in phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2)-binding proteins but lacking virtually all consensus adhesome components. Indeed, reticular adhesions are promoted by PtdIns(4,5)P2, and form independently of talin and F-actin. The distinct characteristics of reticular adhesions provide a solution to the problem of maintaining cell-extracellular matrix attachment during mitotic rounding and division.

Cell cycle pacemaker keeps adhesion in step with division

Adherent cells round up before division but it is unclear how detachment is regulated by the cell cycle. In this issue, Jones et al. (2018. J. Cell Biol. https://doi.org/10.1083/jcb.201802088) find the kinase CDK1 maintains adhesion during interphase by phosphorylating integrin adhesome proteins, including the formin FMNL2, and loss of this function of CDK1 activity in G2 triggers adhesion disassembly.

Mitosis-Resistant Adhesions Provide Molecular Memory to Dividing Cells

During mitosis, animal cells disassemble focal adhesions and round up while remaining attached to the substrata via actin cables and unknown adhesive structures. In this issue of Developmental Cell, Dix et al. (2018) describe integrin-positive adhesions, devoid of classical focal adhesion components, that persist throughout mitosis to contribute to re-spreading.