The BRCT domains of ECT2 have distinct functions during cytokinesis

During cell division, the guanine nucleotide exchange factor (GEF) ECT2 activates RhoA in a narrow zone at the cell equator in anaphase. ECT2 consists of three BRCT domains (BRCT0, 1, and 2), a catalytic GEF, and a pleckstrin homology (PH) domain. How the conserved BRCT domains spatially and temporally control ECT2 activity remains unclear. We reveal that each BRCT domain makes distinct contributions to the ECT2 function. We find that BRCT0 contributes to, and BRCT1 is essential for, ECT2 activation in anaphase. BRCT2 integrates two functions: GEF inhibition and RACGAP1 binding, which together limit ECT2 activity to a narrow zone at the cell equator. BRCT2-dependent control of active RhoA zone dimension functions in addition to the inhibitory signal of the astral microtubules. Our analysis provides detailed mechanistic insights into how ECT2 activity is regulated and how that regulation ensures, together with other signaling pathways, successful cell division.

Distribution of amoebal endosymbiotic environmental chlamydia Neochlamydia S13 via amoebal cytokinesis

We previously isolated a symbiotic environmental amoeba, harboring an environmental chlamydia, Neochlamydia S13. Interestingly, this bacterium failed to survive outside of host cells and was immediately digested inside other amoebae, indicating bacterial distribution via cytokinesis. This may provide a model for understanding organelle development and chlamydial pathogenesis and evolution; therefore, we assessed our hypothesis of Neochlamydia S13 distribution via cytokinesis by comparative analysis with other environmental Chlamydiae (Protochlamydia R18 and Parachlamydia Bn₉ ). Dual staining with 4',6-diamidino-2-phenylindole and phalloidin revealed that the progeny of Neochlamydia S13 and Protochlamydia R18 existed in both daughter cells with a contractile ring on the verge of separation. However, in contrast to other environmental Chlamydiae, little Neochlamydia S13 16S ribosomal DNA was amplified from the culture supernatant. Interestingly, Neochlamydia S13 failed to infect aposymbiotic amoebae, indicating an intimate interaction with the host cells. Furthermore, its infectious rates in cultures expanded from a single amoeba were always maintained at 100%, indicating distribution via cytokinesis. We concluded that unlike other environmental Chlamydiae, Neochlamydia S13 has a unique ability to divide its progeny only via host amoebal cytokinesis. This may be a suitable model to elucidate the mechanism of cell organelle distribution and of chlamydial pathogenesis and evolution.

An ATM-CHK2-INCENP pathway prevents chromatin breakage by regulating the abscission checkpoint

In response to chromatin bridges, the chromosomal passenger complex (CPC) delays completion of cytokinesis (abscission) to prevent chromosome breakage. Here, we discuss recent findings from our lab showing that an ATM-CHK2-INCENP pathway imposes the abscission checkpoint in human cells by regulating CPC midbody-localization.

Cellular Pushing Forces during Mitosis Drive Mitotic Elongation in Collagen Gels

Cell elongation along the division axis, or mitotic elongation, mediates proper segregation of chromosomes and other intracellular materials, and is required for completion of cell division. In three-dimensionally confining extracellular matrices, such as dense collagen gels, dividing cells must generate space to allow mitotic elongation to occur. In principle, cells can generate space for mitotic elongation during cell spreading, prior to mitosis, or via extracellular force generation or matrix degradation during mitosis. However, the processes by which cells drive mitotic elongation in collagen-rich extracellular matrices remains unclear. Here, it is shown that single cancer cells generate substantial pushing forces on the surrounding collagen extracellular matrix to drive cell division in confining collagen gels and allow mitotic elongation to proceed. Neither cell spreading, prior to mitosis, nor matrix degradation, during spreading or mitotic elongation, are found to be required for mitotic elongation. Mechanistically, laser ablation studies, pharmacological inhibition studies, and computational modeling establish that pushing forces generated during mitosis in collagen gels arise from a combination of interpolar spindle elongation and cytokinetic ring contraction. These results reveal a fundamental mechanism mediating cell division in confining extracellular matrices, providing insight into how tumor cells are able to proliferate in dense collagen-rich tissues.

Cytokinesis in fra2 Arabidopsis thaliana p60-Katanin Mutant: Defects in Cell Plate/Daughter Wall Formation

Cytokinesis is accomplished in higher plants by the phragmoplast, creating and conducting the cell plate to separate daughter nuclei by a new cell wall. The microtubule-severing enzyme p60-katanin plays an important role in the centrifugal expansion and timely disappearance of phragmoplast microtubules. Consequently, aberrant structure and delayed expansion rate of the phragmoplast have been reported to occur in p60-katanin mutants. Here, the consequences of p60-katanin malfunction in cell plate/daughter wall formation were investigated by transmission electron microscopy (TEM), in root cells of the fra2Arabidopsis thaliana loss-of-function mutant. In addition, deviations in the chemical composition of cell plate/new cell wall were identified by immunolabeling and confocal microscopy. It was found that, apart from defective phragmoplast microtubule organization, cell plates/new cell walls also appeared faulty in structure, being unevenly thick and perforated by large gaps. In addition, demethylesterified homogalacturonans were prematurely present in fra2 cell plates, while callose content was significantly lower than in the wild type. Furthermore, KNOLLE syntaxin disappeared from newly formed cell walls in fra2 earlier than in the wild type. Taken together, these observations indicate that delayed cytokinesis, due to faulty phragmoplast organization and expansion, results in a loss of synchronization between cell plate growth and its chemical maturation.

Tools for computational analysis of moving boundary problems in cellular mechanobiology

A cell's ability to change shape is one of the most fundamental biological processes and is essential for maintaining healthy organisms. When the ability to control shape goes awry, it often results in a diseased system. As such, it is important to understand the mechanisms that allow a cell to sense and respond to its environment so as to maintain cellular shape homeostasis. Because of the inherent complexity of the system, computational models that are based on sound theoretical understanding of the biochemistry and biomechanics and that use experimentally measured parameters are an essential tool. These models involve an inherent feedback, whereby shape is determined by the action of regulatory signals whose spatial distribution depends on the shape. To carry out computational simulations of these moving boundary problems requires special computational techniques. A variety of alternative approaches, depending on the type and scale of question being asked, have been used to simulate various biological processes, including cell motility, division, mechanosensation, and cell engulfment. In general, these models consider the forces that act on the system (both internally generated, or externally imposed) and the mechanical properties of the cell that resist these forces. Moving forward, making these techniques more accessible to the non-expert will help improve interdisciplinary research thereby providing new insight into important biological processes that affect human health. This article is categorized under: Cancer > Cancer>Computational Models Cancer > Cancer>Molecular and Cellular Physiology.

VPS4A Mutations in Humans Cause Syndromic Congenital Dyserythropoietic Anemia due to Cytokinesis and Trafficking Defects

The Congenital Dyserythropoietic Anemia (CDA) Registry was established with the goal to facilitate investigations of natural history, biology, and molecular pathogenetic mechanisms of CDA. Three unrelated individuals enrolled in the registry had a syndrome characterized by CDA and severe neurodevelopmental delay. They were found to have missense mutations in VPS4A, a gene coding for an ATPase that regulates the ESCRT-III machinery in a variety of cellular processes including cell division, endosomal vesicle trafficking, and viral budding. Bone marrow studies showed binucleated erythroblasts and erythroblasts with cytoplasmic bridges indicating abnormal cytokinesis and abscission. Circulating red blood cells were found to retain transferrin receptor (CD71) in their membrane, demonstrating that VPS4A is critical for normal reticulocyte maturation. Using proband-derived induced pluripotent stem cells (iPSCs), we have successfully modeled the hematologic aspects of this syndrome in vitro, recapitulating their dyserythropoietic phenotype. Our findings demonstrate that VPS4A mutations cause cytokinesis and trafficking defects leading to a human disease with detrimental effects to erythropoiesis and neurodevelopment.

Opposite Surfaces of the Cdc15 F-BAR Domain Create a Membrane Platform That Coordinates Cytoskeletal and Signaling Components for Cytokinesis

Many eukaryotes assemble an actin- and myosin-based cytokinetic ring (CR) on the plasma membrane (PM) for cell division, but how it is anchored there remains unclear. In Schizosaccharomyces pombe, the F-BAR protein Cdc15 links the PM via its F-BAR domain to proteins in the CR’s interior via its SH3 domain. However, Cdc15’s F-BAR domain also directly binds formin Cdc12, suggesting that Cdc15 may polymerize a protein network directly adjacent to the membrane. Here, we determine that the F-BAR domain binds Cdc12 using residues on the face opposite its membrane-binding surface. These residues also bind paxillin-like Pxl1, promoting its recruitment with calcineurin to the CR. Mutation of these F-BAR domain residues results in a shallower CR, with components localizing ~35% closer to the PM than in wild type, and aberrant CR constriction. Thus, F-BAR domains serve as oligomeric membrane-bound platforms that can modulate the architecture of an entire actin structure.

Multiple F-BAR domains link actin structures to membrane. Snider et al. show that the flat Cdc15 F-BAR domain utilizes opposite surfaces to bind the plasma membrane and cytokinetic ring proteins simultaneously. Disrupting Cdc15 F-BAR domain’s interaction with proteins results in an overall compression of the entire cytokinetic ring architecture.

PRC1 is a critical regulator for anaphase spindle midzone assembly and cytokinesis in mouse oocyte meiosis

Protein regulator of cytokinesis 1 (PRC1) is a microtubule bundling protein that is involved in the regulation of the central spindle bundle and spindle orientation during mitosis. However, the functions of PRC1 during meiosis have rarely been studied. In this study, we explored the roles of PRC1 during meiosis using an oocyte model. Our results found that PRC1 was expressed at all stages of mouse oocyte meiosis, and PRC1 accumulated in the midzone/midbody during anaphase/telophase I. Moreover, depleting PRC1 caused defects in polar body extrusion during mouse oocyte maturation. Further analysis found that PRC1 knockdown did not affect meiotic spindle formation or chromosome segregation; however, deleting PRC1 prevented formation of the midzone and midbody at the anaphase/telophase stage of meiosis I, which caused cytokinesis defects and further induced the formation of two spindles in the oocytes. PRC1 knockdown increased the level of tubulin acetylation, indicating that microtubule stability was affected. Furthermore, KIF4A and PRC1 showed similar localization in the midzone/midbody of oocytes at anaphase/telophase I, while the depletion of KIF4A affected the expression and localization of PRC1. The PRC1 mRNA injection rescued the defects caused by PRC1 knockdown in oocytes. In summary, our results suggest that PRC1 is critical for midzone/midbody formation and cytokinesis under regulation of KIF4A in mouse oocytes.

MOB: Pivotal Conserved Proteins in Cytokinesis, Cell Architecture and Tissue Homeostasis

The MOB family proteins are constituted by highly conserved eukaryote kinase signal adaptors that are often essential both for cell and organism survival. Historically, MOB family proteins have been described as kinase activators participating in Hippo and Mitotic Exit Network/ Septation Initiation Network (MEN/SIN) signaling pathways that have central roles in regulating cytokinesis, cell polarity, cell proliferation and cell fate to control organ growth and regeneration. In metazoans, MOB proteins act as central signal adaptors of the core kinase module MST1/2, LATS1/2, and NDR1/2 kinases that phosphorylate the YAP/TAZ transcriptional co-activators, effectors of the Hippo signaling pathway. More recently, MOBs have been shown to also have non-kinase partners and to be involved in cilia biology, indicating that its activity and regulation is more diverse than expected. In this review, we explore the possible ancestral role of MEN/SIN pathways on the built-in nature of a more complex and functionally expanded Hippo pathway, by focusing on the most conserved components of these pathways, the MOB proteins. We discuss the current knowledge of MOBs-regulated signaling, with emphasis on its evolutionary history and role in morphogenesis, cytokinesis, and cell polarity from unicellular to multicellular organisms.

Comparative Analysis of the Roles of Non-muscle Myosin-IIs in Cytokinesis in Budding Yeast, Fission Yeast, and Mammalian Cells

The contractile ring, which plays critical roles in cytokinesis in fungal and animal cells, has fascinated biologists for decades. However, the basic question of how the non-muscle myosin-II and actin filaments are assembled into a ring structure to drive cytokinesis remains poorly understood. It is even more mysterious why and how the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe, and humans construct the ring structure with one, two, and three myosin-II isoforms, respectively. Here, we provide a comparative analysis of the roles of the non-muscle myosin-IIs in cytokinesis in these three model systems, with the goal of defining the common and unique features and highlighting the major questions regarding this family of proteins.

Rac and Arp2/3-Nucleated Actin Networks Antagonize Rho During Mitotic and Meiotic Cleavages

In motile cells, the activities of the different Rho family GTPases are spatially segregated within the cell, and during cytokinesis there is evidence that this may also be the case. But while Rho’s role as the central organizer for contractile ring assembly is well established, the role of Rac and the branched actin networks it promotes is less well understood. To characterize the contributions of these proteins during cytokinesis, we manipulated Rac and Arp2/3 activity during mitosis and meiosis in sea urchin embryos and sea star oocytes. While neither Rac nor Arp2/3 were essential for early embryonic divisions, loss of either Rac or Arp2/3 activity resulted in polar body defects. Expression of activated Rac resulted in cytokinesis failure as early as the first division, and in oocytes, activated Rac suppressed both the Rho wave that traverses the oocyte prior to polar body extrusion as well as polar body formation itself. However, the inhibitory effect of Rac on cytokinesis, polar body formation and the Rho wave could be suppressed by effector-binding mutations or direct inhibition of Arp2/3. Together, these results suggest that Rac- and Arp2/3 mediated actin networks may directly antagonize Rho signaling, thus providing a potential mechanism to explain why Arp2/3-nucleated branched actin networks must be suppressed at the cell equator for successful cytokinesis.

The Cockayne syndrome group A and B proteins are part of a ubiquitin-proteasome degradation complex regulating cell division

Cytokinesis is monitored by a molecular machinery that promotes the degradation of the intercellular bridge, a transient protein structure connecting the two daughter cells. Here, we found that CSA and CSB, primarily defined as DNA repair factors, are located at the midbody, a transient structure in the middle of the intercellular bridge, where they recruit CUL4 and MDM2 ubiquitin ligases and the proteasome. As a part of this molecular machinery, CSA and CSB contribute to the ubiquitination and the degradation of proteins such as PRC1, the Protein Regulator of Cytokinesis, to ensure the correct separation of the two daughter cells. Defects in CSA or CSB result in perturbation of the abscission leading to the formation of long intercellular bridges and multinucleated cells, which might explain part of the Cockayne syndrome phenotypes. Our results enlighten the role played by CSA and CSB as part of a ubiquitin/proteasome degradation process involved in transcription, DNA repair, and cell division.

A Structural View on ESCRT-Mediated Abscission

The endosomal sorting complex required for transport (ESCRT) mediates cellular processes that are related to membrane remodeling, such as multivesicular body (MVB) formation, viral budding and cytokinesis. Abscission is the final stage of cytokinesis that results in the physical separation of the newly formed two daughter cells. Although abscission has been investigated for decades, there are still fundamental open questions related to the spatio-temporal organization of the molecular machinery involved in this process. Reviewing knowledge obtained from in vitro as well as in vivo experiments, we give a brief overview on the role of ESCRT components in abscission mainly focussing on mammalian cells.

DrpB (YedR) Is a Nonessential Cell Division Protein in Escherichia coli

A thorough understanding of bacterial cell division requires identifying and characterizing all of the proteins that participate in this process. Our discovery of DrpB brings us one step closer to this goal in E. coli.

We report that the small Escherichia coli membrane protein DrpB (formerly YedR) is involved in cell division. We discovered DrpB in a screen for multicopy suppressors of a ΔftsEX mutation that prevents divisome assembly when cells are plated on low ionic strength medium, such as lysogeny broth without NaCl. Characterization of DrpB revealed that (i) translation initiates at an ATG annotated as codon 22 rather than the GTG annotated as codon 1, (ii) DrpB localizes to the septal ring when cells are grown in medium of low ionic strength but localization is greatly reduced in medium of high ionic strength, (iii) overproduction of DrpB in a ΔftsEX mutant background improves recruitment of the septal peptidoglycan synthase FtsI, implying multicopy suppression works by rescuing septal ring assembly, (iv) a ΔdrpB mutant divides quite normally, but a ΔdrpB ΔdedD double mutant has a strong division and viability defect, albeit only in medium of high ionic strength, and (v) DrpB homologs are found in E. coli and a few closely related enteric bacteria, but not outside this group. In sum, DrpB is a poorly conserved nonessential division protein that improves the efficiency of cytokinesis under suboptimal conditions. Proteins like DrpB are likely to be a widespread feature of the bacterial cell division apparatus, but they are easily overlooked because mutants lack obvious shape defects.

IMPORTANCE A thorough understanding of bacterial cell division requires identifying and characterizing all of the proteins that participate in this process. Our discovery of DrpB brings us one step closer to this goal in E. coli.

A novel checkpoint pathway controls actomyosin ring constriction trigger in fission yeast

In fission yeast, the septation initiation network (SIN) ensures temporal coordination between actomyosin ring (CAR) constriction with membrane ingression and septum synthesis. However, questions remain about CAR regulation under stress conditions. We show that Rgf1p (Rho1p GEF), participates in a delay of cytokinesis under cell wall stress (blankophor, BP). BP did not interfere with CAR assembly or the rate of CAR constriction, but did delay the onset of constriction in the wild type cells but not in the rgf1Δ cells. This delay was also abolished in the absence of Pmk1p, the MAPK of the cell integrity pathway (CIP), leading to premature abscission and a multi-septated phenotype. Moreover, cytokinesis delay correlates with maintained SIN signaling and depends on the SIN to be achieved. Thus, we propose that the CIP participates in a checkpoint, capable of triggering a CAR constriction delay through the SIN pathway to ensure that cytokinesis terminates successfully.

Cell-Cycle-Associated Expression Patterns Predict Gene Function in Mycobacteria

Although the major events in prokaryotic cell cycle progression are likely to be coordinated with transcriptional and metabolic changes, these processes remain poorly characterized. Unlike many rapidly growing bacteria, DNA replication and cell division are temporally resolved in mycobacteria, making these slow-growing organisms a potentially useful system to investigate the prokaryotic cell cycle. To determine whether cell-cycle-dependent gene regulation occurs in mycobacteria, we characterized the temporal changes in the transcriptome of synchronously replicating populations of Mycobacterium tuberculosis (Mtb). By enriching for genes that display a sinusoidal expression pattern, we discover 485 genes that oscillate with a period consistent with the cell cycle. During cytokinesis, the timing of gene induction could be used to predict the timing of gene function, as mRNA abundance was found to correlate with the order in which proteins were recruited to the developing septum. Similarly, the expression pattern of primary metabolic genes could be used to predict the relative importance of these pathways for different cell cycle processes. Pyrimidine synthetic genes peaked during DNA replication, and their depletion caused a filamentation phenotype that phenocopied defects in this process. In contrast, the inosine monophasphate dehydrogenase dedicated to guanosine synthesis, GuaB2, displayed the opposite expression pattern and its depletion perturbed septation. Together, these data imply obligate coordination between primary metabolism and cell division and identify periodically regulated genes that can be related to specific cell biological functions.

Overproduction of a Dominant Mutant of the Conserved Era GTPase Inhibits Cell Division in Escherichia coli

Cell growth and division are coordinated, ensuring homeostasis under any given growth condition, with division occurring as cell mass doubles. The signals and controlling circuit(s) between growth and division are not well understood; however, it is known in Escherichia coli that the essential GTPase Era, which is growth rate regulated, coordinates the two functions and may be a checkpoint regulator of both. We have isolated a mutant of Era that separates its effect on growth and division. When overproduced, the mutant protein Era647 is dominant to wild-type Era and blocks division, causing cells to filament. Multicopy suppressors that prevent the filamentation phenotype of Era647 either increase the expression of FtsZ or decrease the expression of the Era647 protein. Excess Era647 induces complete delocalization of Z rings, providing an explanation for why Era647 induces filamentation, but this effect is probably not due to direct interaction between Era647 and FtsZ. The hypermorphic ftsZ* allele at the native locus can suppress the effects of Era647 overproduction, indicating that extra FtsZ is not required for the suppression, but another hypermorphic allele that accelerates cell division through periplasmic signaling, ftsL*, cannot. Together, these results suggest that Era647 blocks cell division by destabilizing the Z ring.IMPORTANCE All cells need to coordinate their growth and division, and small GTPases that are conserved throughout life play a key role in this regulation. One of these, Era, provides an essential function in the assembly of the 30S ribosomal subunit in Escherichia coli, but its role in regulating E. coli cell division is much less well understood. Here, we characterize a novel dominant negative mutant of Era (Era647) that uncouples these two activities when overproduced; it inhibits cell division by disrupting assembly of the Z ring, without significantly affecting ribosome production. The unique properties of this mutant should help to elucidate how Era regulates cell division and coordinates this process with ribosome biogenesis.

Animal Cell Cytokinesis: The Rho-Dependent Actomyosin-Anilloseptin Contractile Ring as a Membrane Microdomain Gathering, Compressing, and Sorting Machine

Cytokinesis is the last step of cell division that partitions the cellular organelles and cytoplasm of one cell into two. In animal cells, cytokinesis requires Rho-GTPase-dependent assembly of F-actin and myosin II (actomyosin) to form an equatorial contractile ring (CR) that bisects the cell. Despite 50 years of research, the precise mechanisms of CR assembly, tension generation and closure remain elusive. This hypothesis article considers a holistic view of the CR that, in addition to actomyosin, includes another Rho-dependent cytoskeletal sub-network containing the scaffold protein, Anillin, and septin filaments (collectively termed anillo-septin). We synthesize evidence from our prior work in Drosophila S2 cells that actomyosin and anillo-septin form separable networks that are independently anchored to the plasma membrane. This latter realization leads to a simple conceptual model in which CR assembly and closure depend upon the micro-management of the membrane microdomains to which actomyosin and anillo-septin sub-networks are attached. During CR assembly, actomyosin contractility gathers and compresses its underlying membrane microdomain attachment sites. These microdomains resist this compression, which builds tension. During CR closure, membrane microdomains are transferred from the actomyosin sub-network to the anillo-septin sub-network, with which they flow out of the CR as it advances. This relative outflow of membrane microdomains regulates tension, reduces the circumference of the CR and promotes actomyosin disassembly all at the same time. According to this hypothesis, the metazoan CR can be viewed as a membrane microdomain gathering, compressing and sorting machine that intrinsically buffers its own tension through coordination of actomyosin contractility and anillo-septin-membrane relative outflow, all controlled by Rho. Central to this model is the abandonment of the dogmatic view that the plasma membrane is always readily deformable by the underlying cytoskeleton. Rather, the membrane resists compression to build tension. The notion that the CR might generate tension through resistance to compression of its own membrane microdomain attachment sites, can account for numerous otherwise puzzling observations and warrants further investigation using multiple systems and methods.

Polo-like kinase in trypanosomes: an odd member out of the Polo family

Polo-like kinases (Plks) are evolutionarily conserved serine/threonine protein kinases playing crucial roles during multiple stages of mitosis and cytokinesis in yeast and animals. Plks are characterized by a unique Polo-box domain, which plays regulatory roles in controlling Plk activation, interacting with substrates and targeting Plk to specific subcellular locations. Plk activity and protein abundance are subject to temporal and spatial control through transcription, phosphorylation and proteolysis. In the early branching protists, Plk orthologues are present in some taxa, such as kinetoplastids and Giardia, but are lost in apicomplexans, such as Plasmodium. Works from characterizing a Plk orthologue in Trypanosoma brucei, a kinetoplastid protozoan, discover its essential roles in regulating the inheritance of flagellum-associated cytoskeleton and the initiation of cytokinesis, but not any stage of mitosis. These studies reveal evolutionarily conserved and species-specific features in the control of Plk activation, substrate recognition and protein abundance, and suggest the divergence of Plk function and regulation for specialized needs in this flagellated unicellular eukaryote.

Equatorial Non-muscle Myosin II and Plastin Cooperate to Align and Compact F-actin Bundles in the Cytokinetic Ring

Cytokinesis is the last step of cell division that physically partitions the mother cell into two daughter cells. Cytokinesis requires the assembly and constriction of a contractile ring, a circumferential array of filamentous actin (F-actin), non-muscle myosin II motors (myosin), and actin-binding proteins that forms at the cell equator. Cytokinesis is accompanied by long-range cortical flows from regions of relaxation toward regions of compression. In the C. elegans one-cell embryo, it has been suggested that anterior-directed cortical flows are the main driver of contractile ring assembly. Here, we use embryos co-expressing motor-dead and wild-type myosin to show that cortical flows can be severely reduced without major effects on contractile ring assembly and timely completion of cytokinesis. Fluorescence recovery after photobleaching in the ingressing furrow reveals that myosin recruitment kinetics are also unaffected by the absence of cortical flows. We find that myosin cooperates with the F-actin crosslinker plastin to align and compact F-actin bundles at the cell equator, and that this cross-talk is essential for cytokinesis. Our results thus argue against the idea that cortical flows are a major determinant of contractile ring assembly. Instead, we propose that contractile ring assembly requires localized concerted action of motor-competent myosin and plastin at the cell equator.

Klotho Exerts an Emerging Role in Cytokinesis

The Klotho gene functions as an anti-aging gene. A previous klotho-knockout mice study indicated that neither male nor female gametocytes could accomplish the first meiotic division. It suggested that Klotho might regulate cell division. In this study, we determined the roles of Klotho in cytokinesis in cultural human cells (HEK293 and HeLa) and in zebrafish embryos. Immunoprecipitation, mass spectrometry analysis, and a zebrafish model were used in this study. The results showed that Klotho is located in the midbody, which correlated with cytokinesis related kinases, Aurora kinase B and citron kinases, in the late stage of cytokinesis. There was a spatial correlation between the abscission site and the location of Klotho in the cytokinesis bridge. A three-dimensional structural reconstruction study demonstrated there was a spatial correlation among Klotho, Aurora kinase B, and citron kinases in the midbody. In addition, Klotho depletion inactivated Aurora kinases; it was also indicated that Klotho depletion caused aberrant cell cycle and delayed cytokinesis in a cell model. The study with zebrafish embryos suggested that klotho knockdown caused early embryo development abnormality due to dysregulated cytokinesis. In conclusion, Klotho might have a critical role in cytokinesis regulation by interacting with the cytokinesis related kinases.

Critical Role for Cold Shock Protein YB-1 in Cytokinesis

Simple Summary

Y-box-binding protein-1, YB-1, plays an important role in regulating the cell cycle, although precisely how it does the is unknown. Using live cell imaging, we show that YB-1 is essential for initiating the last step of cell division (cytokinesis), required for creation of two daughter cells. Using confocal microscopy we showed that YB-1 regulates the spatial distribution of key proteins essential for cytokinesis to occur and that this required YB-1 to be phosphorylated on several residues. In-silico modeling demonstrated that modifications at these residues resulted in conformational changes in YB-1 protein allowing it to interact with proteins essential for cytokinesis. As many cancers have high levels YB-1 and these are associated with poor prognosis, our data suggest developing small molecule inhibitors to block YB-1 phosphorylation could be a novel approach to cancer therapy.

Abstract

High levels of the cold shock protein Y-box-binding protein-1, YB-1, are tightly correlated with increased cell proliferation and progression. However, the precise mechanism by which YB-1 regulates proliferation is unknown. Here, we found that YB-1 depletion in several cancer cell lines and in immortalized fibroblasts resulted in cytokinesis failure and consequent multinucleation. Rescue experiments indicated that YB-1 was required for completion of cytokinesis. Using confocal imaging we found that YB-1 was essential for orchestrating the spatio-temporal distribution of the microtubules, β-actin and the chromosome passenger complex (CPC) to define the cleavage plane. We show that phosphorylation at six serine residues was essential for cytokinesis, of which novel sites were identified using mass spectrometry. Using atomistic modelling we show how phosphorylation at multiple sites alters YB-1 conformation, allowing it to interact with protein partners. Our results establish phosphorylated YB-1 as a critical regulator of cytokinesis, defining precisely how YB-1 regulates cell division.

Cell division is antagonized by the activity of peptidoglycan endopeptidases that promote cell elongation

A peptidoglycan (PG) cell wall composed of glycans crosslinked by short peptides surrounds most bacteria and protects them against osmotic rupture. In Escherichia coli, cell elongation requires crosslink cleavage by PG endopeptidases to make space for the incorporation of new PG material throughout the cell cylinder. Cell division, on the contrary, requires the localized synthesis and remodeling of new PG at midcell by the divisome. Little is known about the factors that modulate transitions between these two modes of PG biogenesis. In a transposon-insertion sequencing screen to identify mutants synthetically lethal with a defect in the division protein FtsP, we discovered that mutants impaired for cell division are sensitive to elevated activity of the endopeptidases. Increased endopeptidase activity in these cells was shown to interfere with the assembly of mature divisomes, and conversely, inactivation of MepS was found to suppress the lethality of mutations in essential division genes. Overall, our results are consistent with a model in which the cell elongation and division systems are in competition with one another and that control of PG endopeptidase activity represents an important point of regulation influencing the transition from elongation to the division mode of PG biogenesis.

A Non-canonical BRCT-Phosphopeptide Recognition Mechanism Underlies RhoA Activation in Cytokinesis

Cytokinesis partitions the cell contents to complete mitosis. During cytokinesis, polo-like kinase 1 (PLK1) activates the small GTPase RhoA to assemble a contractile actomyosin ring. PLK1 is proposed to pattern RhoA activation by creating a docking site on the central spindle that concentrates the RhoA guanine nucleotide exchange factor ECT2. However, ECT2 targeting to the central spindle is dispensable for cytokinesis, indicating that how PLK1 controls RhoA activation remains unresolved. To address this question, we employed an unbiased approach targeting ∼100 predicted PLK1 sites in two RhoA regulators: ECT2 and the centralspindlin complex, composed of CYK4 and kinesin-6. This comprehensive approach suggested that the only functionally critical PLK1 target sites are in a single cluster in the CYK4 N terminus. Phosphorylation of this cluster promoted direct interaction of CYK4 with the BRCT repeat module of ECT2. However, mutational analysis in vitro and in vivo led to the surprising finding that the interaction was independent of the conserved "canonical" residues in ECT2's BRCT repeat module that, based on structurally characterized BRCT-phosphopeptide interactions, were presumed critical for binding. Instead, we show that the ECT2 BRCT module binds phosphorylated CYK4 via a distinct conserved basic surface. Basic surface mutations mimic the effects on cytokinesis of loss of CYK4 cluster phosphorylation or inhibition of PLK1 activity. Together with evidence for ECT2 autoinhibition limiting interaction with CYK4 in the cytoplasm, these results suggest that a spatial gradient of phosphorylated CYK4 around the central spindle patterns RhoA activation by interacting with ECT2 on the adjacent plasma membrane.

Enhancement of Migration and Invasion of Gastric Cancer Cells by IQGAP3

Although gastric cancer is one of the most common causes of cancer death in the world, mechanisms underlying this type of tumor have not been fully understood. In this study, we found that IQGAP3, a member of the IQGAP gene family, was significantly up-regulated in human gastric cancer starting from the early stages of tumor progression. Overexpression of IQGAP3 in 293T and NIH3T3 cells, which have no endogenous IQGAP3 expression, resulted in morphological change with multiple dendritic-like protrusions and enhanced migration. Overexpression of IQGAP3 also led to reduced cell–cell adhesion in 293T cells, likely as a result of its interactions with e-cadherin or β-catenin proteins. Additionally, IQGAP3 accumulated along the leading edge of migrating cells and at the cleavage furrow of dividing cells. In contrast, suppression of IQGAP3 by short-interfering RNA (siRNA) markedly reduced invasion and anchorage-independent growth of MKN1 and TMK-1 gastric cancer cells. We further confirmed that IQGAP3 interacted with Rho family GTPases, and had an important role in cytokinesis. Taken together, we demonstrated that IQGAP3 plays critical roles in migration and invasion of human gastric cancer cells, and regulates cytoskeletal remodeling, cell migration and adhesion. These findings may open a new avenue for the diagnosis and treatment of gastric cancer.

A cytoskeletal protein complex is essential for division of intracellular amastigotes of Leishmania mexicana

Previous studies in Leishmania mexicana have identified the cytoskeletal protein KHARON as being important for both flagellar trafficking of the glucose transporter GT1 and for successful cytokinesis and survival of infectious amastigote forms inside mammalian macrophages. KHARON is located in three distinct regions of the cytoskeleton: the base of the flagellum, the subpellicular microtubules, and the mitotic spindle. To deconvolve the different functions for KHARON, we have identified two partner proteins, KHAP1 and KHAP2, which associate with KHARON. KHAP1 is located only in the subpellicular microtubules, whereas KHAP2 is located at the subpellicular microtubules and the base of the flagellum. Both KHAP1 and KHAP2 null mutants are unable to execute cytokinesis but are able to traffic GT1 to the flagellum. These results confirm that KHARON assembles into distinct functional complexes and that the subpellicular complex is essential for cytokinesis and viability of disease-causing amastigotes but not for flagellar membrane trafficking.

Cleavage-furrow formation without F-actin in Chlamydomonas

It is widely believed that cleavage-furrow formation during cytokinesis is driven by the contraction of a ring containing F-actin and type-II myosin. However, even in cells that have such rings, they are not always essential for furrow formation. Moreover, many taxonomically diverse eukaryotic cells divide by furrowing but have no type-II myosin, making it unlikely that an actomyosin ring drives furrowing. To explore this issue further, we have used one such organism, the green alga Chlamydomonas reinhardtii We found that although F-actin is associated with the furrow region, none of the three myosins (of types VIII and XI) is localized there. Moreover, when F-actin was eliminated through a combination of a mutation and a drug, furrows still formed and the cells divided, although somewhat less efficiently than normal. Unexpectedly, division of the large Chlamydomonas chloroplast was delayed in the cells lacking F-actin; as this organelle lies directly in the path of the cleavage furrow, this delay may explain, at least in part, the delay in cytokinesis itself. Earlier studies had shown an association of microtubules with the cleavage furrow, and we used a fluorescently tagged EB1 protein to show that microtubules are still associated with the furrows in the absence of F-actin, consistent with the possibility that the microtubules are important for furrow formation. We suggest that the actomyosin ring evolved as one way to improve the efficiency of a core process for furrow formation that was already present in ancestral eukaryotes.

Cell lineage-dependent chiral actomyosin flows drive cellular rearrangements in early Caenorhabditis elegans development

Proper positioning of cells is essential for many aspects of development. Daughter cell positions can be specified via orienting the cell division axis during cytokinesis. Rotatory actomyosin flows during division have been implied in specifying and reorienting the cell division axis, but how general such reorientation events are, and how they are controlled, remains unclear. We followed the first nine divisions of Caenorhabditis elegans embryo development and demonstrate that chiral counter-rotating flows arise systematically in early AB lineage, but not in early P/EMS lineage cell divisions. Combining our experiments with thin film active chiral fluid theory we identify a mechanism by which chiral counter-rotating actomyosin flows arise in the AB lineage only, and show that they drive lineage-specific spindle skew and cell reorientation events. In conclusion, our work sheds light on the physical processes that underlie chiral morphogenesis in early development.

Cell Division Protein FtsZ Is Unfolded for N-Terminal Degradation by Antibiotic-Activated ClpP

Acyldepsipeptide (ADEP) antibiotics effectively kill multidrug-resistant Gram-positive pathogens, including vancomycin-resistant enterococcus, penicillin-resistant Streptococcus pneumoniae (PRSP), and methicillin-resistant Staphylococcus aureus (MRSA). The antibacterial activity of ADEP depends on a new mechanism of action, i.e., the deregulation of bacterial protease ClpP that leads to bacterial self-digestion. Our data allow new insights into the mode of ADEP action by providing a molecular explanation for the distinct bacterial phenotypes observed at low versus high ADEP concentrations. In addition, we show that ClpP alone, in the absence of any unfoldase or energy-consuming system, and only activated by the small molecule antibiotic ADEP, leads to the unfolding of the cell division protein FtsZ.

Antibiotic acyldepsipeptides (ADEPs) deregulate ClpP, the proteolytic core of the bacterial Clp protease, thereby inhibiting its native functions and concomitantly activating it for uncontrolled proteolysis of nonnative substrates. Importantly, although ADEP-activated ClpP is assumed to target multiple polypeptide and protein substrates in the bacterial cell, not all proteins seem equally susceptible. In Bacillus subtilis, the cell division protein FtsZ emerged to be particularly sensitive to degradation by ADEP-activated ClpP at low inhibitory ADEP concentrations. In fact, FtsZ is the only bacterial protein that has been confirmed to be degraded in vitro as well as within bacterial cells so far. However, the molecular reason for this preferred degradation remained elusive. Here, we report the unexpected finding that ADEP-activated ClpP alone, in the absence of any Clp-ATPase, leads to an unfolding and subsequent degradation of the N-terminal domain of FtsZ, which can be prevented by the stabilization of the FtsZ fold via nucleotide binding. At elevated antibiotic concentrations, importantly, the C terminus of FtsZ is notably targeted for degradation in addition to the N terminus. Our results show that different target structures are more or less accessible to ClpP, depending on the ADEP level present. Moreover, our data assign a Clp-ATPase-independent protein unfolding capability to the ClpP core of the bacterial Clp protease and suggest that the protein fold of FtsZ may be more flexible than previously anticipated.

The LKB1-like Kinase Elm1 Controls Septin Hourglass Assembly and Stability by Regulating Filament Pairing

Septins form rod-shaped hetero-oligomeric complexes that assemble into filaments and other higher-order structures, such as rings or hourglasses, at the cell division site in fungal and animal cells [1-4] to carry out a wide range of functions, including cytokinesis and cell morphogenesis. However, the architecture of septin higher-order assemblies and their control mechanisms, including regulation by conserved kinases [5, 6], remain largely unknown. In the budding yeast Saccharomyces cerevisiae, the five mitotic septins (Cdc3, Cdc10, Cdc11, Cdc12, and Shs1) localize to the bud neck and form an hourglass before cytokinesis that acts as a scaffold for proteins involved in multiple processes as well as a membrane-diffusible barrier between the mother and developing bud [7-9]. The hourglass is remodeled into a double ring that sandwiches the actomyosin ring at the onset of cytokinesis [10-13]. How septins are assembled into a highly ordered hourglass structure at the division site [13] is largely unexplored. Here we show that the LKB1-like kinase Elm1, which has been implicated in septin organization [14], cell morphogenesis [15], and mitotic exit [16, 17], specifically associates with the septin hourglass during the cell cycle and controls hourglass assembly and stability, especially for the daughter half, by regulating filament pairing and the functionality of its substrate, the septin-binding protein Bni5. This study illustrates how a protein kinase regulates septin architecture at the filament level and suggests that filament pairing is a highly regulated process during septin assembly and remodeling in vivo.

Modulation of Mammalian Cardiomyocyte Cytokinesis by the Extracellular Matrix

RATIONALE

After birth, cycling mammalian CMs (cardiomyocytes) progressively lose the ability to undergo cytokinesis and hence they become binucleated, which leads to cell cycle exit and loss of regenerative capacity. During late embryonic and early postnatal heart growth, CM development is accompanied by an expansion of the cardiac fibroblast (cFb) population and compositional changes in the ECM (extracellular matrix). Whether and how these changes influence cardiomyocyte cytokinesis is currently unknown.

OBJECTIVE

To elucidate the role of postnatal cFbs and the ECM in cardiomyocyte cytokinesis and identify ECM proteins that promote cardiomyocyte cytokinesis.

METHODS AND RESULTS

Using primary rat cardiomyocyte cultures, we found that a proportion of postnatal, but not embryonic, cycling cardiomyocytes fail to progress through cytokinesis and subsequently binucleate, consistent with published reports of in vitro and in vivo observations. Direct coculture with postnatal cFbs increased cardiomyocyte binucleation, which could be inhibited by RGD peptide treatment. In contrast, cFb-conditioned medium or transwell coculture did not significantly increase cardiomyocyte binucleation, suggesting that cFbs inhibit cardiomyocyte cytokinesis through ECM modulation rather than by secreting diffusible factors. Furthermore, we found that both embryonic and postnatal CMs binucleate at a significantly higher rate when cultured on postnatal cFb-derived ECM compared with embryonic cFb-derived ECM. These cytokinetic defects correlate with cardiomyocyte inefficiency in mitotic rounding, a process which is key to successful cytokinesis. To identify ECM proteins that modulate cardiomyocyte cytokinesis, we compared the composition of embryonic and postnatal cFb-derived ECM by mass spectrometry followed by functional assessment. We found that 2 embryonically enriched ECM proteins, SLIT2 and NPNT (nephronectin), promote cytokinesis of postnatal CMs in vitro and in vivo.

CONCLUSIONS

We identified the postnatal cardiac ECM as a nonpermissive environment for cardiomyocyte cytokinesis and uncovered novel functions for the embryonic ECM proteins SLIT2 and NPNT (nephronectin) in promoting postnatal cardiomyocyte cytokinesis. Graphic Abstract: A graphic abstract is available for this article.

Unilateral Cleavage Furrows in Multinucleate Cells

Multinucleate cells can be produced in Dictyostelium by electric pulse-induced fusion. In these cells, unilateral cleavage furrows are formed at spaces between areas that are controlled by aster microtubules. A peculiarity of unilateral cleavage furrows is their propensity to join laterally with other furrows into rings to form constrictions. This means cytokinesis is biphasic in multinucleate cells, the final abscission of daughter cells being independent of the initial direction of furrow progression. Myosin-II and the actin filament cross-linking protein cortexillin accumulate in unilateral furrows, as they do in the normal cleavage furrows of mononucleate cells. In a myosin-II-null background, multinucleate or mononucleate cells were produced by cultivation either in suspension or on an adhesive substrate. Myosin-II is not essential for cytokinesis either in mononucleate or in multinucleate cells but stabilizes and confines the position of the cleavage furrows. In fused wild-type cells, unilateral furrows ingress with an average velocity of 1.7 µm × min-1, with no appreciable decrease of velocity in the course of ingression. In multinucleate myosin-II-null cells, some of the furrows stop growing, thus leaving space for the extensive broadening of the few remaining furrows.

Role of Anillin in Tumour: From a Prognostic Biomarker to a Novel Target

Anillin (ANLN), an actin-binding protein, reportedly plays a vital role in cell proliferation and migration, particularly in cytokinesis. Although there have been findings pointing to a contribution of ANLN to the development of cancer, the association of ANLN to cancer remains not fully understood. Here, we gather evidence to determine the applicability of ANLN as a prognostic tool for some types of cancer, and the impact that ANLN has on the hallmarks of cancer. We searched academic repositories including PubMed and Google Scholar to find and review studies related to cancer and ANLN. The conclusion is that ANLN could be a potent target for cancer treatment, but the roles ANLN, other than in cytokinesis and its influence on tumour microenvironment remodeling in cancer development, must be further elucidated, and specific ANLN inhibitors should be found.

Overexpression of the response regulator rpaA causes an impaired cell division in the Cyanobacterium Synechocystis sp. PCC 6803

In photosynthetic microorganisms, cell cycle progression depends on day and night cycles; however, how cell division is regulated in response to these environmental changes is poorly understood. RpaA has been implicated in the signal output from both circadian clocks and light/dark conditions in the unicellular spherical-celled cyanobacterium Synechocystis sp. PCC 6803. In the present study, we investigated the involvement of a two-component response regulator RpaA in cell division regulation. Firstly, we examined the effects of rpaA overexpression on cell morphology and the expression levels of cell division genes. We observed an increase in the volume of non-dividing cells and a high proportion of dividing cells in rpaA-overexpressing strains by light microscopy. The expression levels of selected cell division-related genes were higher in the rpaA-overexpressing strain than in the wild type, including minD of the Min system; cdv3 and zipN, which encode two divisome components; and murB, murC, and pbp2, which are involved in peptidoglycan (PG) synthesis. Moreover, in the rpaA-overexpressing strain, the outer membrane and cell wall PG layer were not smooth, and the outer membrane was not clearly visible by transmission electron microscopy. These results demonstrated that rpaA overexpression causes an impaired cell division, which is accompanied by transcriptional activation of cell division genes and morphological changes in the PG layer and outer membrane.

The F-BAR Domain of Rga7 Relies on a Cooperative Mechanism of Membrane Binding with a Partner Protein during Fission Yeast Cytokinesis

F-BAR proteins bind the plasma membrane (PM) to scaffold and organize the actin cytoskeleton. To understand how F-BAR proteins achieve their PM association, we studied the localization of a Schizosaccharomyces pombe F-BAR protein Rga7, which requires the coiled-coil protein Rng10 for targeting to the division site during cytokinesis. We find that the Rga7 F-BAR domain directly binds a motif in Rng10 simultaneously with the PM, and that an adjacent Rng10 motif independently binds the PM. Together, these multivalent interactions significantly enhance Rga7 F-BAR avidity for membranes at physiological protein concentrations, ensuring the division site localization of Rga7. Moreover, the requirement for the F-BAR domain in Rga7 localization and function in cytokinesis is bypassed by tethering an Rga7 construct lacking its F-BAR to Rng10, indicating that at least some F-BAR domains are necessary but not sufficient for PM targeting and are stably localized to specific cortical positions through adaptor proteins.

Liu et al. show that the Rga7 F-BAR domain binds an adaptor protein Rng10, which contains a second membrane-binding module, to enhance Rga7 membrane avidity and stabilize its membrane association. The authors reveal a mechanism by which F-BAR domains can achieve high-avidity binding with the plasma membrane.

Extracellular Vesicles of Human Periodontal Ligament Stem Cells Contain MicroRNAs Associated to Proto-Oncogenes: Implications in Cytokinesis

The human Periodontal Ligament Stem Cells (hPDLSCs) exhibit self-renewal capacity and clonogenicity potential. The Extracellular Vesicles (EVs) secreted by hPDLSCs are particles containing lipids, proteins, mRNAs, and non-coding RNAs, among which microRNAs, that are important in intercellular communication. The purpose of this study was the analysis of the non-coding RNAs contained in the EVs derived from hPDLSCs using Next Generation Sequencing. Moreover, our data were enriched using bioinformatic tools. The analysis highlighted the presence of non-coding RNAs and five microRNAs: MIR24-2, MIR142, MIR335, MIR490, and MIR296. Our results show that these miRNAs target the genes classified in two terms of the Gene Ontology: "Ras protein signal transduction" and "Actin/microtubule cytoskeleton organization." Noteworthy, the in-deep analysis of our EVs highlights that the miRNAs could be implicated in the silencing of proto-oncogenes involved in 12 different types of tumors.

The Scaffold Proteins Paxillin B and α-Actinin Regulate Septation in Aspergillus nidulans via Control of Actin Ring Contraction

Cytokinesis, as the final step of cell division, plays an important role in fungal growth and proliferation. In the filamentous fungus Aspergillus nidulans, defective cytokinesis is able to induce abnormal multinuclear or nonnucleated cells and then result in reduced hyphal growth and abolished sporulation. Previous studies have reported that a conserved contractile actin ring (CAR) protein complex and the septation initiation network (SIN) signaling kinase cascade are required for cytokinesis and septation; however, little is known about the role(s) of scaffold proteins involved in these two important cellular processes. In this study, we show that a septum-localized scaffold protein paxillin B (PaxB) is essential for cytokinesis/septation in A. nidulans The septation defects observed in a paxB deletion strain resemble those caused by the absence of another identified scaffold protein, α-actinin (AcnA). Deletion of α-actinin (AcnA) leads to undetectable PaxB at the septation site, whereas deletion of paxB does not affect the localization of α-actinin at septa. However, deletion of either α-actinin (acnA) or paxB causes the actin ring to disappear at septation sites during cytokinesis. Notably, overexpression of α-actinin acnA partially rescues the septum defects of the paxB mutant but not vice versa, suggesting AcnA may play a dominant role over that of PaxB for cytokinesis and septation. In addition, PaxB and α-actinin affect the septal dynamic localization of MobA, a conserved component of the SIN pathway, suggesting they may affect the SIN protein complex function at septa. Protein pull-down assays combined with liquid chromatography-mass spectrometry identification indicate that α-actinin AcnA and PaxB likely do not directly interact, but presumably belong to an actin cytoskeleton protein network that is required for the assembly and contraction of the CAR. Taken together, findings in this study provide novel insights into the roles of conserved scaffold proteins during fungal septation in A. nidulans.

Beta Human Papillomavirus 8E6 Attenuates LATS Phosphorylation after Failed Cytokinesis

Beta genus human papillomaviruses (β-HPVs) cause cutaneous squamous cell carcinomas (cSCCs) in a subset of immunocompromised patients. However, β-HPVs are not necessary for tumor maintenance in the general population. Instead, they may destabilize the genome in the early stages of cancer development. Supporting this idea, β-HPV's 8E6 protein attenuates p53 accumulation after failed cytokinesis. This paper offers mechanistic insight into how β-HPV E6 causes this change in cell signaling. An in silico screen and characterization of HCT 116 cells lacking p300 suggested that the histone acetyltransferase is a negative regulator of Hippo pathway (HP) gene expression. HP activation restricts growth in response to stimuli, including failed cytokinesis. Loss of p300 resulted in increased HP gene expression, including proproliferative genes associated with HP inactivation. β-HPV 8E6 expression recapitulates some of these phenotypes. We used a chemical inhibitor of cytokinesis (dihydrocytochalasin B [H2CB]) to induce failed cytokinesis. This system allowed us to show that β-HPV 8E6 reduced activation of large tumor suppressor kinase (LATS), an HP kinase. LATS is required for p53 accumulation following failed cytokinesis. These phenotypes were dependent on β-HPV 8E6 destabilizing p300 and did not completely attenuate the HP. It did not alter H2CB-induced nuclear exclusion of the transcription factor YAP. β-HPV 8E6 also did not decrease HP activation in cells grown to a high density. Although our group and others have previously described inhibition of DNA repair, to the best of our knowledge, this marks the first time that a β-HPV E6 protein has been shown to hinder HP signaling.IMPORTANCE β-HPVs contribute to cSCC development in immunocompromised populations. However, it is unclear if these common cutaneous viruses are tumorigenic in the general population. Thus, a more thorough investigation of β-HPV biology is warranted. If β-HPV infections do promote cSCCs, they are hypothesized to destabilize the cellular genome. In vitro data support this idea by demonstrating the ability of the β-HPV E6 protein to disrupt DNA repair signaling events following UV exposure. We show that β-HPV E6 more broadly impairs cellular signaling, indicating that the viral protein dysregulates the HP. The HP protects genome fidelity by regulating cell growth and apoptosis in response to a myriad of deleterious stimuli, including failed cytokinesis. After failed cytokinesis, β-HPV 8E6 attenuates phosphorylation of the HP kinase (LATS). This decreases some, but not all, HP signaling events. Notably, β-HPV 8E6 does not limit senescence associated with failed cytokinesis.

Transcriptional Activity and Stability of CD39+CD103+CD8+ T Cells in Human High-Grade Endometrial Cancer

Tumor-infiltrating CD8+ T cells (TIL) are of the utmost importance in anti-tumor immunity. CD103 defines tumor-resident memory T cells (TRM cells) associated with improved survival and response to immune checkpoint blockade (ICB) across human tumors. Co-expression of CD39 and CD103 marks tumor-specific TRM with enhanced cytolytic potential, suggesting that CD39+CD103+ TRM could be a suitable biomarker for immunotherapy. However, little is known about the transcriptional activity of TRM cells in situ. We analyzed CD39+CD103+ TRM cells sorted from human high-grade endometrial cancers (n = 3) using mRNA sequencing. Cells remained untreated or were incubated with PMA/ionomycin (activation), actinomycin D (a platinum-like chemotherapeutic that inhibits transcription), or a combination of the two. Resting CD39+CD103+ TRM cells were transcriptionally active and expressed a characteristic TRM signature. Activated CD39+CD103+ TRM cells differentially expressed PLEK, TWNK, and FOS, and cytokine genes IFNG, TNF, IL2, CSF2 (GM-CSF), and IL21. Findings were confirmed using qPCR and cytokine production was validated by flow cytometry of cytotoxic TIL. We studied transcript stability and found that PMA-responsive genes and mitochondrial genes were particularly stable. In conclusion, CD39+CD103+ TRM cells are transcriptionally active TRM cells with a polyfunctional, reactivation-responsive repertoire. Secondly, we hypothesize that differential regulation of transcript stability potentiates rapid responses upon TRM reactivation in tumors.

Integrative Cluster Analysis of Whole Hearts Reveals Proliferative Cardiomyocytes in Adult Mice

The recent development and broad application of sequencing techniques at the single-cell level is generating an unprecedented amount of data. The different techniques have their individual limits, but the datasets also offer unexpected possibilities when utilized collectively. Here, we applied snRNA-seq in whole adult murine hearts from an inbred (C57BL/6NRj) and an outbred (Fzt:DU) mouse strain to directly compare the data with the publicly available scRNA-seq data of the tabula muris project. Explicitly choosing a single-nucleus approach allowed us to pin down the typical heart tissue-specific technical bias, coming up with novel insights on the mammalian heart cell composition. For our integrated dataset, cardiomyocytes, fibroblasts, and endothelial cells constituted the three main cell populations accounting for about 75% of all cells. However, their numbers severely differed between the individual datasets, with cardiomyocyte proportions ranging from about 9% in the tabula muris data to around 23% for our BL6 data, representing the prime example for cell capture technique related bias when using a conventional single-cell approach for these large cells. Most strikingly in our comparison was the discovery of a minor population of cardiomyocytes characterized by proliferation markers that could not be identified by analyzing the datasets individually. It is now widely accepted that the heart has an, albeit very restricted, regenerative potential. However there is still an ongoing debate where new cardiomyocytes arise from. Our findings support the idea that the renewal of the cardiomyocyte pool is driven by cytokinesis of resident cardiomyocytes rather than differentiation of progenitor cells. We thus provide data that can contribute to an understanding of heart cell regeneration, which is a prerequisite for future applications to enhance the process of heart repair.

A method for studying cell division in Nicotiana benthamiana pavement cells based on Agrobacterium infiltration

This article is a Commentary on Xu et al., 226: 1213–1221.

Cholesterol-binding protein TSPO2 coordinates maturation and proliferation of terminally differentiating erythroblasts

TSPO2 (translocator protein 2) is a transmembrane protein specifically expressed in late erythroblasts and has been postulated to mediate intracellular redistribution of cholesterol. We identified TSPO2 as the causative gene for the HK (high-K⁺) trait with immature red cell phenotypes in dogs and investigated the effects of the TSPO2 defects on erythropoiesis in HK dogs with the TSPO2 mutation and Tspo2 knockout (Tspo2^−/−) mouse models. Bone marrow–derived erythroblasts from HK dogs showed increased binucleated and apoptotic cells at various stages of maturation and shed large nuclei with incomplete condensation when cultured in the presence of erythropoietin, indicating impaired maturation and cytokinesis. The canine TSPO2 induces cholesterol accumulation in the endoplasmic reticulum and could thereby regulate cholesterol availability by changing intracellular cholesterol distribution in erythroblasts. Tspo2^−/− mice consistently showed impaired cytokinesis with increased binucleated erythroblasts, resulting in compensated anemia, and their red cell membranes had increased Na,K-ATPase, resembling the HK phenotype in dogs. Tspo2-deficient mouse embryonic stem cell–derived erythroid progenitor (MEDEP) cells exhibited similar morphological defects associated with a cell-cycle arrest at the G₂/M phase, resulting in decreased cell proliferation and had a depletion in intracellular unesterified and esterified cholesterol. When the terminal maturation was induced, Tspo2^−/− MEDEP cells showed delays in hemoglobinization; maturation-associated phenotypic changes in CD44, CD71, and TER119 expression; and cell-cycle progression. Taken together, these findings imply that TSPO2 is essential for coordination of maturation and proliferation of erythroblasts during normal erythropoiesis.

Functions of the Plant Qbc SNARE SNAP25 in Cytokinesis and Biotic and Abiotic Stress Responses

Eukaryotes transport biomolecules between intracellular organelles and between cells and the environment via vesicle trafficking. Soluble N -ethylmaleimide-sensitive factor attachment protein receptors (SNARE proteins) play pivotal roles in vesicle and membrane trafficking. These proteins are categorized as Qa, Qb, Qc, and R SNAREs and form a complex that induces vesicle fusion for targeting of vesicle cargos. As the core components of the SNARE complex, the SNAP25 Qbc SNAREs perform various functions related to cellular homeostasis. The Arabidopsis thaliana SNAP25 homolog AtSNAP33 interacts with Qa and R SNAREs and plays a key role in cytokinesis and in triggering innate immune responses. However, other Arabidopsis SNAP25 homologs, such as AtSNAP29 and AtSNAP30, are not well studied; this includes their localization, interactions, structures, and functions. Here, we discuss three biological functions of plant SNAP25 orthologs in the context of AtSNAP33 and highlight recent findings on SNAP25 orthologs in various plants. We propose future directions for determining the roles of the less well-characterized AtSNAP29 and AtSNAP30 proteins.

Expression of South East Asian Ovalocytic Band 3 Disrupts Erythroblast Cytokinesis and Reticulocyte Maturation

Southeast Asian Ovalocytosis results from a heterozygous deletion of 9 amino acids in the erythrocyte anion exchange protein AE1 (band 3). The report of the first successful birth of an individual homozygous for this mutation showed an association with severe dyserythropoietic anemia. Imaging of the proband’s erythrocytes revealed the presence of band 3 at their surface, a reduction in Wr(b) antigen expression, and increases in glycophorin C, CD44, and CD147 immunoreactivity. Immunoblotting of membranes from heterozygous Southeast Asian Ovalocytosis red cells showed a quantitative increase in CD44, CD147, and calreticulin suggesting a defect in reticulocyte maturation, as well as an increase in phosphorylation at residue Tyr359 of band 3, and peroxiredoxin-2 at the membrane, suggesting altered band 3 trafficking and oxidative stress, respectively. In vitro culture of homozygous and heterozygous Southeast Asian Ovalocytosis erythroid progenitor cells produced bi- and multi-nucleated cells. Enucleation was severely impaired in the homozygous cells and reduced in the heterozygous cells. Large internal vesicular accumulations of band 3 formed, which co-localized with other plasma membrane proteins and with the autophagosome marker, LC3, but not with ER, Golgi or recycling endosome markers. Immunoprecipitation of band 3 from erythroblast cell lysates at the orthochromatic stage showed increased interaction of the mutant band 3 with heat shock proteins, ubiquitin and cytoskeleton proteins, ankyrin, spectrin and actin. We also found that the mutant band 3 forms a strong interaction with non-muscle myosins IIA and IIB, while this interaction could not be detected in wild type erythroblasts. Consistent with this, the localization of non-muscle myosin IIA and actin was perturbed in some Southeast Asian Ovalocytosis erythroblasts. These findings provide new insights toward understanding in vivo dyserythropoiesis caused by the expression of mutant membrane proteins.

Cytokinesis and postabscission midbody remnants are regulated during mammalian brain development

Building a brain of the proper size and structure requires neural stem cells (NSCs) to divide with tight temporal and spatial control to produce different daughter cell types in proper numbers and sequence. Mammalian NSCs in the embryonic cortex must maintain their polarized epithelial structure as they undergo both early proliferative divisions and later neurogenic divisions. To do this, they undergo a polarized form of cytokinesis at the apical membrane that is not well understood. Here, we investigate whether polarized furrowing and abscission in mouse NSCs are regulated differently at earlier and later stages and in a cytokinesis mutant, Kif20b This mutant was previously shown to have microcephaly and elevated apoptosis of NSCs. We developed methods to live image furrow ingression and midbody abscission in NSCs within cortical explants. We find that polarized furrow ingression occurs at a steady rate and completes in ∼15 min at two different ages. However, ingression is slower in a subset of Kif20b mutant NSCs. Abscission is usually observed on both sides of the midbody and takes 65 to 75 min to complete. Surprisingly, abscission is accelerated in the Kif20b mutant NSCs. Postabscission midbody remnants are observed at the apical membranes of daughter cells and are much more abundant in early-stage cortices. After NSC divisions in vitro, midbody remnants are more often retained on the daughter cells of early proliferative divisions. Altogether, these results suggest that regulation of abscission timing and midbody remnants in embryonic NSCs may influence proper brain growth and structure.

Mutations in a Single Signaling Pathway Allow Cell Growth in Heavy Water

Life is completely dependent on water. To analyze the role of water as a solvent in biology, we replaced water with heavy water (D₂O) and investigated the biological effects by a wide range of techniques, using Schizosaccharomyces pombe as model organism. We show that high concentrations of D₂O lead to altered glucose metabolism and growth retardation. After prolonged incubation in D₂O, cells displayed gross morphological changes, thickened cell walls, and aberrant cytoskeletal organization. By transcriptomics and genetic screens, we show that the solvent replacement activates two signaling pathways: (1) the heat-shock response pathway and (2) the cell integrity pathway. Although the heat-shock response system upregulates various chaperones and other stress-relieving enzymes, we find that the activation of this pathway does not offer any fitness advantage to the cells under the solvent-replaced conditions. However, limiting the D₂O-triggered activation of the cell integrity pathway allows cell growth when H₂O is completely replaced with D₂O. The isolated D₂O-tolerant strains may aid biological production of deuterated biomolecules.

Regulation of the Total Cell Surface Area in Dividing Dictyostelium Cells

When a cell divides into two daughter cells, the total cell surface area should increase. There are two models for membrane supply to support cell division: (1) unfolding of small surface membrane reservoirs such as microvilli or wrinkles and (2) exocytosis of intracellular vesicles. Here, we precisely measured the total cell surface area in dividing Dictyostelium cells, flattened by the agar overlay that eliminated the complexity of unfolding surface membrane reservoirs. Because the cells divided normally under the agar overlay, unfolding of surface membrane reservoirs was not required for cell division. Under the agar overlay, the total cell surface area slightly decreased from the interphase to the metaphase and then increased about 20% during cytokinesis. Both endocytosis and exocytosis were suppressed in the early mitotic phase but recovered during cytokinesis. The imbalance of endocytosis and exocytosis could contribute to the changes observed in the cell surface area. Clathrin-dependent endocytosis was also substantially suppressed during cytokinesis, but contrary to previous reports in cultured animal cells, it did not significantly contribute to the regulation of the cell surface area. Furrowing during cytokinesis was indispensable for the cell membrane increase, and vice versa.

Cep55 promotes cytokinesis of neural progenitors but is dispensable for most mammalian cell divisions

In mammalian cell lines, the endosomal sorting complex required for transport (ESCRT)-III mediates abscission, the process that physically separates daughter cells and completes cell division. Cep55 protein is regarded as the master regulator of abscission, because it recruits ESCRT-III to the midbody (MB), the site of abscission. However, the importance of this mechanism in a mammalian organism has never been tested. Here we show that Cep55 is dispensable for mouse embryonic development and adult tissue homeostasis. Cep55-knockout offspring show microcephaly and primary neural progenitors require Cep55 and ESCRT for survival and abscission. However, Cep55 is dispensable for cell division in embryonic or adult tissues. In vitro, division of primary fibroblasts occurs without Cep55 and ESCRT-III at the midbody and is not affected by ESCRT depletion. Our work defines Cep55 as an abscission regulator only in specific tissue contexts and necessitates the re-evaluation of an alternative ESCRT-independent cell division mechanism.