Endogenous tagging using split mNeonGreen in human iPSCs for live imaging studies

Endogenous tags have become invaluable tools to visualize and study native proteins in live cells. However, generating human cell lines carrying endogenous tags is difficult due to the low efficiency of homology-directed repair. Recently, an engineered split mNeonGreen protein was used to generate a large-scale endogenous tag library in HEK293 cells. Using split mNeonGreen for large-scale endogenous tagging in human iPSCs would open the door to studying protein function in healthy cells and across differentiated cell types. We engineered an iPS cell line to express the large fragment of the split mNeonGreen protein (mNG21-10) and showed that it enables fast and efficient endogenous tagging of proteins with the short fragment (mNG211). We also demonstrate that neural network-based image restoration enables live imaging studies of highly dynamic cellular processes such as cytokinesis in iPSCs. This work represents the first step towards a genome-wide endogenous tag library in human stem cells.

Advances in the design and use of carbon dots for analytical and biomedical applications

Carbon dots (CDs) have garnered significant interest for their potential use in multiple applications due to their size, fluorescent properties, high photostability, low toxicity and biocompatibility. CDs can be tailored for specific needs, as they can be synthesized with diverse precursors and techniques for functionalization. Since the applications of CDs are rapidly expanding, this review highlights recent developments in this burgeoning field. Specifically, we describe advances in CD synthesis tailored for applications that include pH and temperature sensing, biochemical analysis, and bioimaging. We also discuss various challenges and practical solutions that will drive CD-based research forward. Challenges include the lack of standardized synthesis and purification methods for CDs, the lack of clarity regarding their mechanism of action, and procedural flaws in their applications. In conclusion, we provide recommendations for collaboration among disciplines to bridge existing knowledge gaps and address the key challenges required for CDs to be fully commercialized.

Ratiometric Sensing of Glyphosate in Water Using Dual Fluorescent Carbon Dots

Glyphosate is a broad-spectrum pesticide used in crops and is found in many products used by industry and consumers. Unfortunately, glyphosate has been shown to have some toxicity toward many organisms found in our ecosystems and has been reported to have carcinogenic effects on humans. Hence, there is a need to develop novel nanosensors that are more sensitive and facile and permit rapid detection. Current optical-based assays are limited as they rely on changes in signal intensity, which can be affected by multiple factors in the sample. Herein, we report the development of a dual emissive carbon dot (CD) system that can be used to optically detect glyphosate pesticides in water at different pH levels. The fluorescent CDs emit blue and red fluorescence, which we exploit as a ratiometric self-referencing assay. We observe red fluorescence quenching with increasing concentrations of glyphosate in the solution, ascribed to the interaction of the glyphosate pesticide with the CD surface. The blue fluorescence remains unaffected and serves as a reference in this ratiometric approach. Using fluorescence quenching assays, a ratiometric response is observed in the ppm range with detection limits as low as 0.03 ppm. Our CDs can be used to detect other pesticides and contaminants in water, as cost-effective and simple environmental nanosensors.

Diversity is the spice of life: An overview of how cytokinesis regulation varies with cell type

Cytokinesis is required to physically cleave a cell into two daughters at the end of mitosis. Decades of research have led to a comprehensive understanding of the core cytokinesis machinery and how it is regulated in animal cells, however this knowledge was generated using single cells cultured in vitro, or in early embryos before tissues develop. This raises the question of how cytokinesis is regulated in diverse animal cell types and developmental contexts. Recent studies of distinct cell types in the same organism or in similar cell types from different organisms have revealed striking differences in how cytokinesis is regulated, which includes different threshold requirements for the structural components and the mechanisms that regulate them. In this review, we highlight these differences with an emphasis on pathways that are independent of the mitotic spindle, and operate through signals associated with the cortex, kinetochores, or chromatin.

Gold Nano-Bio-Interaction to Modulate Mechanobiological Responses for Cancer Therapy Applications

In the present study, we investigate the mechanobiological responses of human lung cancer that may occur through their interactions with two different types of gold nanoparticles: nanostars and nanospheres. Hyperspectral images of nanoparticle-treated cells revealed different spatial distributions of nanoparticles in cells depending on their morphology, with nanospheres being more uniformly distributed in cells than nanostars. Gold nanospheres were also found to be more effective in mechanobiological modulations. They significantly suppressed the migratory ability of cells under different incubation times while lowering the bulk stiffness and adhesion of cells. This in vitro study suggests the potential applications of gold nanoparticles to manage cell migration. Nano-bio-interactions appeared to impact the cytoskeletal organization of cells and consequently alter the mechanical properties of cells, which could influence the cellular functions of cells. According to the results and migratory index model, it is thought that nanoparticle-treated cells experience mechanical changes in their body, which largely reduces their migratory potentials. These findings provide a better understanding of nano-bio-interaction in terms of cell mechanics and highlight the importance of mechanobiological responses in designing gold nanoparticles for cancer therapy.

Diverse mechanisms regulate contractile ring assembly for cytokinesis in the two-cell Caenorhabditis elegans embryo

Cytokinesis occurs at the end of mitosis as a result of the ingression of a contractile ring that cleaves the daughter cells. The core machinery regulating this crucial process is conserved among metazoans. Multiple pathways control ring assembly, but their contribution in different cell types is not known. We found that in the Caenorhabditis elegans embryo, AB and P1 cells fated to be somatic tissue and germline, respectively, have different cytokinesis kinetics supported by distinct myosin levels and organization. Through perturbation of RhoA or polarity regulators and the generation of tetraploid strains, we found that ring assembly is controlled by multiple fate-dependent factors that include myosin levels, and mechanisms that respond to cell size. Active Ran coordinates ring position with the segregating chromatids in HeLa cells by forming an inverse gradient with importins that control the cortical recruitment of anillin. We found that the Ran pathway regulates anillin in AB cells but functions differently in P1 cells. We propose that ring assembly delays in P1 cells caused by low myosin and Ran signaling coordinate the timing of ring closure with their somatic neighbors. This article has an associated First Person interview with the first author of the paper.

Seeing is believing: tools to study the role of Rho GTPases during cytokinesis

Cytokinesis is required to cleave the daughter cells at the end of mitosis and relies on the spatiotemporal control of RhoA GTPase. Cytokinesis failure can lead to changes in cell fate or aneuploidy, which can be detrimental during development and/or can lead to cancer. However, our knowledge of the pathways that regulate RhoA during cytokinesis is limited, and the role of other Rho family GTPases is not clear. This is largely because the study of Rho GTPases presents unique challenges using traditional cell biological and biochemical methods, and they have pleiotropic functions making genetic studies difficult to interpret. The recent generation of optogenetic tools and biosensors that control and detect active Rho has overcome some of these challenges and is helping to elucidate the role of RhoA in cytokinesis. However, improvements are needed to reveal the role of other Rho GTPases in cytokinesis, and to identify the molecular mechanisms that control Rho activity. This review examines some of the outstanding questions in cytokinesis, and explores tools for the imaging and control of Rho GTPases.

Characterization of a recently synthesized microtubule-targeting compound that disrupts mitotic spindle poles in human cells

We reveal the effects of a new microtubule-destabilizing compound in human cells. C75 has a core thienoisoquinoline scaffold with several functional groups amenable to modification. Previously we found that sub micromolar concentrations of C75 caused cytotoxicity. We also found that C75 inhibited microtubule polymerization and competed with colchicine for tubulin-binding in vitro. However, here we found that the two compounds synergized suggesting differences in their mechanism of action. Indeed, live imaging revealed that C75 causes different spindle phenotypes compared to colchicine. Spindles remained bipolar and collapsed after colchicine treatment, while C75 caused bipolar spindles to become multipolar. Importantly, microtubules rapidly disappeared after C75-treatment, but then grew back unevenly and from multiple poles. The C75 spindle phenotype is reminiscent of phenotypes caused by depletion of ch-TOG, a microtubule polymerase, suggesting that C75 blocks microtubule polymerization in metaphase cells. C75 also caused an increase in the number of spindle poles in paclitaxel-treated cells, and combining low amounts of C75 and paclitaxel caused greater regression of multicellular tumour spheroids compared to each compound on their own. These findings warrant further exploration of C75’s anti-cancer potential.

Design, structure-activity relationship study and biological evaluation of the thieno[3,2-c]isoquinoline scaffold as a potential anti-cancer agent

Several derivatives of a series that share a thienoisoquinoline scaffold have demonstrated potent activity against cancer cell lines A549, HeLa, HCT-116, and MDA-MB-231 in the submicromolar concentration range. Structure-activity relationship (SAR) studies on a range of derivatives aided in identifying key pharmacophores in the lead compound. A series of compounds have been identified as the most promising with submicromolar IC₅₀ values against a lung cancer cell line (A549). Microscopy studies of cancer cells treated with the lead compound revealed that it causes mitotic arrest and disrupts microtubules. Further evaluation via an in vitro microtubule polymerization assay and competition studies indicate that the lead compound binds to tubulin via the colchicine site.

CRISPR-Cas tools to study gene function in cytokinesis

Cytokinesis is the process that separates a cell into two daughter cells at the end of mitosis. Most of our knowledge of cytokinesis comes from overexpression studies, which affects our interpretation of protein function. Gene editing can circumvent this issue by introducing functional mutations or fluorescent probes directly into a gene locus. However, despite its potential, gene editing is just starting to be used in the field of cytokinesis. Here, we discuss the benefits of using gene editing tools for the study of cytokinesis and highlight recent studies that successfully used CRISPR-Cas (clustered regularly interspaced short palindromic repeats-CRISPR-associated proteins) technology to answer critical questions regarding the function of cytokinesis proteins. We also present methodologies for editing essential genes and discuss how CRISPR interference (CRISPRi) and activation (CRISPRa) can enable precise control of gene expression to answer important questions in the field. Finally, we address the need for gene editing to study cytokinesis in more physiologically relevant contexts. Therefore, this Review provides a roadmap for gene editing to be used in the study of cytokinesis and other cellular processes.

Multi-tissue patterning drives anterior morphogenesis of the C. elegans embryo

Complex structures derived from multiple tissue types are challenging to study in vivo, and our knowledge of how cells from different tissues are coordinated is limited. Model organisms have proven invaluable for improving our understanding of how chemical and mechanical cues between cells from two different tissues can govern specific morphogenetic events. Here we used Caenorhabditis elegans as a model system to show how cells from three different tissues are coordinated to give rise to the anterior lumen. While some aspects of pharyngeal morphogenesis have been well-described, it is less clear how cells from the pharynx, epidermis and neuroblasts coordinate to define the location of the anterior lumen and supporting structures. Using various microscopy and software approaches, we define the movements and patterns of these cells during anterior morphogenesis. Projections from the anterior-most pharyngeal cells (arcade cells) provide the first visible markers for the location of the future lumen, and facilitate patterning of the surrounding neuroblasts. These neuroblast patterns control the rate of migration of the anterior epidermal cells, whereas the epidermal cells ultimately reinforce and control the position of the future lumen, as they must join with the pharyngeal cells for their epithelialization. Our studies are the first to characterize anterior morphogenesis in C. elegans in detail and should lay the framework for identifying how these different patterns are controlled at the molecular level.

Using intracellular plasmonics to characterize nanomorphology in human cells

Determining the characteristics and localization of nanoparticles inside cells is crucial for nanomedicine design for cancer therapy. Hyperspectral imaging is a fast, straightforward, reliable, and accurate method to study the interactions of nanoparticles and intracellular components. With a hyperspectral image, we could collect spectral information consisting of thousands of pixels in a short time. Using hyperspectral images, in this work, we developed a label-free technique to detect nanoparticles in different regions of the cell. This technique is based on plasmonic shifts taking place during the interaction of nanoparticles with the surrounding medium. The unique optical properties of gold nanoparticles, localized surface plasmon resonance bands, are influenced by their microenvironment. The LSPR properties of nanoparticles, hence, could provide information on regions in which nanoparticles are distributed. To examine the potential of this technique for intracellular detection, we used three different types of gold nanoparticles: nanospheres, nanostars and Swarna Bhasma (SB), an Indian Ayurvedic/Sidha medicine, in A549 (human non-small cell lung cancer) and HepG2 (human hepatocellular carcinoma) cells. All three types of particles exhibited broader and longer bands once they were inside cells; however, their plasmonic shifts could change depending on the size and morphology of particles. This technique, along with dark-field images, revealed the uniform distribution of nanospheres in cells and could provide more accurate information on their intracellular microenvironment compared to the other particles. The region-dependent optical responses of nanoparticles in cells highlight the potential application of this technique for subcellular diagnosis when particles with proper size and morphology are chosen to reflect the microenvironment effects properly.

Importin binding mediates the intramolecular regulation of anillin during cytokinesis

Cytokinesis occurs by the ingression of an actomyosin ring that cleaves a cell into two daughters. This process is tightly controlled to avoid aneuploidy, and we previously showed that active Ran coordinates ring positioning with chromatin. Active Ran is high around chromatin, and forms an inverse gradient to cargo-bound importins. We found that the ring component anillin contains a nuclear localization signal (NLS) that binds to importin and is required for its function during cytokinesis. Here we reveal the mechanism whereby importin binding favors a conformation required for anillin's recruitment to the equatorial cortex. Active RhoA binds to the RhoA-binding domain causing an increase in accessibility of the nearby C2 domain containing the NLS. Importin binding subsequently stabilizes a conformation that favors interactions for cortical recruitment. In addition to revealing a novel mechanism for the importin-mediated regulation of a cortical protein, we also show how importin binding positively regulates protein function.

Complementary functions for the Ran gradient during division

The Ran pathway has a well-described function in nucleocytoplasmic transport, where active Ran dissociates importin/karyopherin-bound cargo containing a nuclear localization signal (NLS) in the nucleus. As cells enter mitosis, the nuclear envelope breaks down and a gradient of active Ran forms where levels are highest near chromatin. This gradient plays a crucial role in regulating mitotic spindle assembly, where active Ran binds to and releases importins from NLS-containing spindle assembly factors. An emerging theme is that the Ran gradient also regulates the actomyosin cortex for processes including polar body extrusion during meiosis, and cytokinesis. For these events, active Ran could play an inhibitory role, where importin-binding may help promote or stabilize a conformation or interaction that favours the recruitment and function of cortical regulators. For either spindle assembly or cortical polarity, the gradient of active Ran determines the extent of importin-binding, the effects of which could vary for different proteins.

Intracellular ratiometric temperature sensing using fluorescent carbon dots

Highly sensitive non-invasive temperature sensing is critical for studying fundamental biological processes and applications in medical diagnostics. Nanoscale-based thermometers are promising non-invasive probes for precise temperature sensing with subcellular resolution. However, many of these systems have limitations as they rely on fluorescence intensity changes, deconvolution of peaks, or the use of hybrid systems to measure thermal events. To address this, we developed a fluorescence-based ratiometric temperature sensing approach using carbon dots prepared via microwave synthesis. These dots possess dual fluorescence signatures in the blue and red regions of the spectrum. We observed a linear response as a function of temperature in the range of 5-60 °C with a thermal resolution of 0.048 K-1 and thermal sensitivity of 1.97% C-1. Temperature-dependent fluorescence was also observed in HeLa cancer cells over a range of 32-42 °C by monitoring changes in the red-to-blue fluorescence signatures. We demonstrate that the ratiometric approach is superior to intensity-based thermal sensing because it is independent of the intracellular concentration of the optical probe. These findings suggest that dual-emitting carbon dots can be an effective tool for in vitro and possibly in vivo fluorescence nanothermometry.

Enhanced Internalization of Indian Ayurvedic Swarna Bhasma (Gold Nanopowder) for Effective Interaction with Human Cells

In the ancient traditional Indian Ayurvedic system of natural healing, gold nanoparticles (Swarna Bhasma, gold ash) have been used for its therapeutic benefits as far back as 2500 B.C. Ayurvedic medicinal preparations are complex mixtures that include many plant-derived products and metals. Bhasmas date as far back as the 8th century and are made by samskaras (processings), such as shodhana (purification and potentiation), jarana (roasting), and marana (incineration, trituration) in the presence of plant products, including juices and concoctions. Previous studies characterized the physical properties of gold ash, and the mechanisms of its entry into human cells, but only preliminary data exist on its toxicity. Before using nanoparticles for therapeutic application, it is extremely important to study their toxicity and cellular internalization. In the present study, various imaging techniques were used to investigate Swarna Bhasma's (gold nanopowder) toxicity in both cancerous and noncancerous cells (HeLa and HFF-1) and to characterize its spectral properties. The results showed that gold ash particles had no impact on the cellular viability of both HeLa and HFF-1 cells, even at high concentrations or long incubation times. Moreover, it was found that the internalization level of Swarna Bhasma to cells may be improved by mechanical breaking of the large aggregates into smaller agglomerates. Hyperspectral images revealed that after breaking, the small agglomerates have different spectral properties in cells, compared to the original aggregates, suggesting that size of particles is instrumental for the subcellular interaction with human cells.

Active Ran regulates anillin function during cytokinesis

We describe a novel mechanism by which active Ran regulates anillin during cytokinesis. Anillin is highly conserved and coordinates RhoA, actomyosin, microtubules, and the membrane for cytokinesis in mammalian cells. This study implicates Ran-GTP in influencing cortical contractility during anaphase by regulating anillin function.

Cytokinesis cleaves a cell into two daughters at the end of mitosis, and must be spatially coordinated with chromosome segregation to prevent aneuploidy. The dogma is that the mitotic spindle governs the assembly and constriction of an actomyosin ring. Here, we reveal a function for active Ran in spatially restricting the ring. Our model is that during anaphase, “free” importins, whose gradient inversely correlates with active Ran and chromatin position, function as a molecular ruler for the recruitment and localization of anillin, a contractile protein and a crucial regulator of cytokinesis. We found that decreasing Ran-GTP levels or tethering active Ran to the equatorial membrane affects anillin’s localization and causes cytokinesis phenotypes. Anillin contains a conserved nuclear localization signal (NLS) at its C-terminus that binds to importin-β and is required for cortical polarity and cytokinesis. Mutating the NLS decreases anillin’s cortical affinity, causing it to be more dominantly regulated by microtubules. Anillin contains a RhoA-GTP binding domain, which autoinhibits the NLS and the neighboring microtubule-binding domain, and RhoA-GTP binding may relieve this inhibition during mitosis. Retention of the C-terminal NLS in anillin homologues suggests that this is a conserved mechanism for controlling anillin function.

Abstract 3247: Discovery of a novel drug that affects centrosome clustering

Our goal is to identify molecular regulators of a mechanism that occurs uniquely in cancer cells, and to develop a selective anti-cancer drug. Most chemotherapies are non-selective, causing severe side-effects. In additions, cancers often develop resistance to some of the more commonly used chemotherapies. To expand the repertoire of available drugs, and to design drugs that are selective, we need to identify molecules that regulate the physiological changes that occur primarily in cancer cells. For example, cancer cells in many hard-to-treat cancers have aberrant centrosomes, which may be supernumerary or fragmented. During mitosis, these aberrant centrosomes must cluster to form bipolar spindles for successful division. Thus, targeting a process like centrosome clustering is ideal, since it is not necessary in healthy cells. We synthesized a small, stable scaffold with amenability to structure-activity-relationship studies, and found several analogues with IC50 values < 50 nM, depending on the cancer cell line. Preliminary tests showed that these compounds prevent tumors from forming and/or cause their regression in vitro. We performed cell biological studies to characterize their mechanism of action. In several different types of cancer cells, these compounds cause mitotic arrest and centrosome declustering at concentrations where they have little affect on non-cancerous cells. Live imaging revealed that within minutes of adding the compounds to HeLa cells expressing GFP-tagged tubulin, we observed rapid microtubule depolymerization and centrosome declustering. After washing out the compound, microtubule polymerization recovered, but the mitotic spindles were multipolar. Adding similar concentrations of Nocodazole, a microtubule-depolymerizing drug, also caused rapid microtubule depolymerization, but after washing out the drug, the spindles were bipolar. We are in the process of identifying the molecular target of these compounds to provide crucial insight to the mechanism governing centrosome clustering, and are continuing to perform SAR studies to obtain compounds with higher efficacy and selectivity.

Citation Format: Dilan B. Jaunky, Kevin Larocque, Javier Porro Suardiaz, Dan Yang, Emma J. Furze, Pat Forgione, Alisa Piekny. Discovery of a novel drug that affects centrosome clustering [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3247. doi:10.1158/1538-7445.AM2017-3247

Abstract 3808: Novel compound conferring selectivity for cancer cells

There is a need to develop novel compounds that can effectively treat a broad range of cancers on their own, or in combination with approved therapies. As personalized medicine is developed, combinatorial approaches will become more common making it crucial to increase the repertoire of available drugs. In the past, drugs often were developed to target a biologically-relevant molecule, but structural limitations, stability and solubility issues, or lack of selectivity have hindered the clinical use of many of these drugs. Our approach was to first find a ‘high-quality’ compound that is selective for cancer cells, then characterize its mechanism of action and identify its target. High throughput screening (HTS) helped to rapidly identify a subset of compounds with selective toxicity toward MCF-7 (breast cancer) cells. Some of these compounds were further tested for their efficacy in HeLa (cervical cancer) cells, and we found one that selectively causes mitotic arrest at 250 nM in comparison to non-cancerous HFF-1 (foreskin fibroblast) cells. At 200 nM, this compound synergizes with drugs known to affect microtubule dynamics and cause mitotic arrest including Nocodazole and Paclitaxel (currently in use as an anti-cancer drug), causing them to be more effective at lower concentrations. Excitingly, this compound also provides a shielding effect for HFF-1 cells treated with Paclitaxel. To learn the mechanism of action for this compound, we performed immunofluorescence microscopy on HeLa and HFF-1 cells treated with a range of concentrations. We found that the mitotic spindle is improperly organized in HeLa cells at 250 nM, but not in HFF-1 cells. Interestingly, microtubules are completely gone in mitotic HeLa cells and are reduced in mitotic HFF-1 cells treated with >500 nM. Given that this compound synergizes with drugs that directly bind to tubulin subunits to modify their dynamics of assembly and disassembly, and differently affects cancerous vs. healthy cells, we hypothesize that it has a unique mechanism of action and may affect microtubule nucleation. We are continuing to characterize the compound, and will identify its molecular target. In addition, we are generating further iterations to explore the Structure-Activity Relationship, and optimize its efficacy. Our in vitro data shows that our approach has the potential to identify novel compounds with the potential for therapeutic use.

Citation Format: Dilan B. Jaunky, Pat Forgione, Alisa Piekny. Novel compound conferring selectivity for cancer cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3808.

Mechanical forces drive neuroblast morphogenesis and are required for epidermal closure

Tissue morphogenesis requires myosin-dependent events such as cell shape changes and migration to be coordinated between cells within a tissue, and/or with cells from other tissues. However, few studies have investigated the simultaneous morphogenesis of multiple tissues in vivo. We found that during Caenorhabditis elegans ventral enclosure, when epidermal cells collectively migrate to cover the ventral surface of the embryo, the underlying neuroblasts (neuronal precursor cells) also undergo morphogenesis. We found that myosin accumulates as foci along the junction-free edges of the ventral epidermal cells to form a ring, whose closure is myosin-dependent. We also observed the accumulation of myosin foci and the adhesion junction proteins E-cadherin and α-catenin in the underlying neuroblasts. Myosin may help to reorganize a subset of neuroblasts into a rosette-like pattern, and decrease their surface area as the overlying epidermal cells constrict. Since myosin is required in the neuroblasts for ventral enclosure, we propose that mechanical forces in the neuroblasts influence constriction of the overlying epidermal cells. In support of this model, disrupting neuroblast cell division or altering their fate influences myosin localization in the overlying epidermal cells. The coordination of myosin-dependent events and forces between cells in different tissues could be a common theme for coordinating morphogenetic events during metazoan development.

Visualizing neuroblast cytokinesis during C. elegans embryogenesis

This protocol describes the use of fluorescence microscopy to image dividing cells within developing Caenorhabditis elegans embryos. In particular, this protocol focuses on how to image dividing neuroblasts, which are found underneath the epidermal cells and may be important for epidermal morphogenesis. Tissue formation is crucial for metazoan development and relies on external cues from neighboring tissues. C. elegans is an excellent model organism to study tissue morphogenesis in vivo due to its transparency and simple organization, making its tissues easy to study via microscopy. Ventral enclosure is the process where the ventral surface of the embryo is covered by a single layer of epithelial cells. This event is thought to be facilitated by the underlying neuroblasts, which provide chemical guidance cues to mediate migration of the overlying epithelial cells. However, the neuroblasts are highly proliferative and also may act as a mechanical substrate for the ventral epidermal cells. Studies using this experimental protocol could uncover the importance of intercellular communication during tissue formation, and could be used to reveal the roles of genes involved in cell division within developing tissues.

Anillin interacts with microtubules and is part of the astral pathway that defines cortical domains

Cytokinesis occurs by the ingression of an actomyosin ring that separates the cell into two daughter cells. The mitotic spindle, comprised of astral and central spindle microtubules, couples contractile ring ingression with DNA segregation. Cues from the central spindle activate RhoA, the upstream regulator of the contractile ring. However, additional cues from the astral microtubules also reinforce the localization of active RhoA. Using human cells, we show that astral and central spindle microtubules independently control the localization of contractile proteins during cytokinesis. Astral microtubules restrict the accumulation and localization of contractile proteins during mitosis, while the central spindle forms a discrete ring by directing RhoA activation in the equatorial plane. Anillin stabilizes the contractile ring during cytokinesis. We show that human anillin interacts with astral microtubules, which is competed by its cortical recruitment by active RhoA. Anillin restricts the localization of myosin at the equatorial cortex, and NuMA (part of the microtubule-tethering complex that regulates spindle position) at the polar cortex. The sequestration of anillin by astral microtubules may alter the organization of cortical proteins to polarize cells for cytokinesis.

A conserved RhoGAP limits M phase contractility and coordinates with microtubule asters to confine RhoA during cytokinesis

During animal cell cytokinesis, the spindle directs contractile ring assembly by activating RhoA in a narrow equatorial zone. Rapid GTPase activating protein (GAP)-mediated inactivation (RhoA flux) is proposed to limit RhoA zone dimensions. Testing the significance of RhoA flux has been hampered by the fact that the GAP targeting RhoA is not known. Here, we identify M phase GAP (MP-GAP) as the primary GAP targeting RhoA during mitosis and cytokinesis. MP-GAP inhibition caused excessive RhoA activation in M phase, leading to the uncontrolled formation of large cortical protrusions and late cytokinesis failure. RhoA zone width was broadened by attenuation of the centrosomal asters but was not affected by MP-GAP inhibition alone. Simultaneous aster attenuation and MP-GAP inhibition led to RhoA accumulation around the entire cell periphery. These results identify the major GAP restraining RhoA during cell division and delineate the relative contributions of RhoA flux and centrosomal asters in controlling RhoA zone dimensions.

An anillin-Ect2 complex stabilizes central spindle microtubules at the cortex during cytokinesis

Cytokinesis occurs due to the RhoA-dependent ingression of an actomyosin ring. During anaphase, the Rho GEF (guanine nucleotide exchange factor) Ect2 is recruited to the central spindle via its interaction with MgcRacGAP/Cyk-4, and activates RhoA in the central plane of the cell. Ect2 also localizes to the cortex, where it has access to RhoA. The N-terminus of Ect2 binds to Cyk-4, and the C-terminus contains conserved DH (Dbl homologous) and PH (Pleckstrin Homology) domains with GEF activity. The PH domain is required for Ect2's cortical localization, but its molecular function is not known. In cultured human cells, we found that the PH domain interacts with anillin, a contractile ring protein that scaffolds actin and myosin and interacts with RhoA. The anillin-Ect2 interaction may require Ect2's association with lipids, since a novel mutation in the PH domain, which disrupts phospholipid association, weakens their interaction. An anillin-RacGAP50C (homologue of Cyk-4) complex was previously described in Drosophila, which may crosslink the central spindle to the cortex to stabilize the position of the contractile ring. Our data supports an analogous function for the anillin-Ect2 complex in human cells and one hypothesis is that this complex has functionally replaced the Drosophila anillin-RacGAP50C complex. Complexes between central spindle proteins and cortical proteins could regulate the position of the contractile ring by stabilizing microtubule-cortical interactions at the division plane to ensure the generation of active RhoA in a discrete zone.

Anillin promotes astral microtubule-directed cortical myosin polarization

Assembly of a cytokinetic contractile ring is a form of cell polarization in which the equatorial cell cortex becomes differentiated from the polar regions. Microtubules direct cytokinetic polarization via the central spindle and astral microtubules. The mechanism of central spindle-directed furrow formation is reasonably well understood, but the aster-directed pathway is not. In aster-directed furrowing, cytoskeletal factors accumulate to high levels at sites distal to the asters and at reduced levels at cortical sites near the asters. In this paper, we demonstrate that the cytoskeletal organizing protein anillin (ANI-1) promotes the formation of an aster-directed furrow in Caenorhabditis elegans embryos. Microtubule-directed nonmuscle myosin II polarization is aberrant in embryos depleted of ANI-1. In contrast, microtubule-directed polarized ANI-1 localization is largely unaffected by myosin II depletion. Consistent with a role in the induction of cortical asymmetry, ANI-1 also contributes to the polarization of arrested oocytes. Anillin has an evolutionarily conserved capacity to associate with microtubules, possibly providing an inhibitory mechanism to promote polarization of the cell cortex.

An ECT2-centralspindlin complex regulates the localization and function of RhoA

In anaphase, the spindle dictates the site of contractile ring assembly. Assembly and ingression of the contractile ring involves activation of myosin-II and actin polymerization, which are triggered by the GTPase RhoA. In many cells, the central spindle affects division plane positioning via unknown molecular mechanisms. Here, we dissect furrow formation in human cells and show that the RhoGEF ECT2 is required for cortical localization of RhoA and contractile ring assembly. ECT2 concentrates on the central spindle by binding to centralspindlin. Depletion of the centralspindlin component MKLP1 prevents central spindle localization of ECT2; however, RhoA, F-actin, and myosin still accumulate on the equatorial cell cortex. Depletion of the other centralspindlin component, CYK-4/MgcRacGAP, prevents cortical accumulation of RhoA, F-actin, and myosin. CYK-4 and ECT2 interact, and this interaction is cell cycle regulated via ECT2 phosphorylation. Thus, central spindle localization of ECT2 assists division plane positioning and the CYK-4 subunit of centralspindlin acts upstream of RhoA to promote furrow assembly.

Cytokinesis: welcome to the Rho zone

Cytokinesis follows nuclear division and generates two distinct daughter cells, each replete with a full complement of the genome and cytoplasmic organelles. Members of the Rho family of GTPases are crucial regulators of this process in a wide variety of species. In many cell types, cytokinesis is mediated by a discretely localized contractile ring that is rich in actin and myosin. In this article (which is part of the Cytokinesis series), we review recent studies in animal cells that have shown that local assembly of the contractile ring is mediated by a discrete pool of GTP-bound, active RhoA. Advances in detecting the active pool of RhoA have allowed insights into the mechanisms and the molecules that promote the accumulation of active RhoA at the correct time and place in the cell.