Dynamics of endosomal sorting complex required for transport (ESCRT) machinery during cytokinesis and its role in abscission

A Model for Primary Cilium Biogenesis by Polarized Epithelial Cells: Role of the Midbody Remnant and Associated Specialized Membranes

Primary cilia are solitary, microtubule-based protrusions surrounded by a ciliary membrane equipped with selected receptors that orchestrate important signaling pathways that control cell growth, differentiation, development and homeostasis. Depending on the cell type, primary cilium assembly takes place intracellularly or at the cell surface. The intracellular route has been the focus of research on primary cilium biogenesis, whereas the route that occurs at the cell surface, which we call the "alternative" route, has been much less thoroughly characterized. In this review, based on recent experimental evidence, we present a model of primary ciliogenesis by the alternative route in which the remnant of the midbody generated upon cytokinesis acquires compact membranes, that are involved in compartmentalization of biological membranes. The midbody remnant delivers part of those membranes to the centrosome in order to assemble the ciliary membrane, thereby licensing primary cilium formation. The midbody remnant's involvement in primary cilium formation, the regulation of its inheritance by the ESCRT machinery, and the assembly of the ciliary membrane from the membranes originally associated with the remnant are discussed in the context of the literature concerning the ciliary membrane, the emerging roles of the midbody remnant, the regulation of cytokinesis, and the role of membrane compartmentalization. We also present a model of cilium emergence during evolution, and summarize the directions for future research.

MicroRNA-488-3p Regulates Neuronal Cell Death in Cerebral Ischemic Stroke Through Vacuolar Protein Sorting 4B (VPS4B)

BACKGROUND

Ischemic stroke, which often occurs with high morbidity, disability, and mortality, is a main cause of brain disease. In various types of human diseases, it is found that microRNAs (miRNAs) are considered as gene regulators. Increasing studies have proved that fluctuation of miRNAs, in the pathologies of ischemic stroke, plays a vital role. However, the accurate regulatory mechanism of cerebral ischemic stroke by miRNAs is still unclear. In this research, we investigated the inhibition mechanism of miR-488-3p on neuronal death through targeting vacuolar protein sorting 4B (VPS4B) in cerebral ischemia/reperfusion (I/R) injury.

METHODS

Western blot and qRT-PCR were utilized to detect the miR-488-3p level and VPS4B expression. The cell counting kit-8 (CCK-8) assay was utilized to measure the function of miR-488-3p in cell death induced by oxygen glucose deprivation/reoxygenation (OGD/R). After middle cerebral artery occlusion/reperfusion (MCAO/R), the impact of miR-488-3p on infarct volume in mouse brain was assessed. The targets of miR-488-3p were confirmed by luciferase analysis and bioinformatics software.

RESULTS

The miR-488-3p level remarkably reduced in primary neuronal cells administrated with OGD/R. Similarly, it also decreased in the mouse brain administrated with MCAO/R. Additionally, the up-regulation of miR-488-3p expression suppressed the death of neuronal cells and restrained ischemic brain infarction in ischemia-stroked mice. Besides, the results showed that VPS4B, which could be inhibited by miR-488-3p, was a direct target of miR-488-3p. This research revealed that the inhibition of VPS4B protected the neuronal cells in ischemic stroke both in vitro as well as in vivo. Meanwhile, this inhibition strengthened positive impact generated by miR-488-3p on ischemic injury.

CONCLUSION

Overall, miR-488-3p played a critical role on neuroprotective function via reducing VPS4B protein level. These results performed a new underlying curative target for the treatment of cerebral ischemic stroke.

Interplay of the nuclear envelope with chromatin in physiology and pathology

The nuclear envelope compartmentalizes chromatin in eukaryotic cells. The main nuclear envelope components are lamins that associate with a panoply of factors, including the LEM domain proteins. The nuclear envelope of mammalian cells opens up during cell division. It is reassembled and associated with chromatin at the end of mitosis when telomeres tether to the nuclear periphery. Lamins, LEM domain proteins, and DNA binding factors, as BAF, contribute to the reorganization of chromatin. In this context, an emerging role is that of the ESCRT complex, a machinery operating in multiple membrane assembly pathways, including nuclear envelope reformation. Research in this area is unraveling how, mechanistically, ESCRTs link to nuclear envelope associated factors as LEM domain proteins. Importantly, ESCRTs work also during interphase for repairing nuclear envelope ruptures. Altogether the advances in this field are giving new clues for the interpretation of diseases implicating nuclear envelope fragility, as laminopathies and cancer.

ABBREVIATIONS

na, not analyzed; ko, knockout; kd, knockdown; NE, nuclear envelope; LEM, LAP2-emerin-MAN1 (LEM)-domain containing proteins; LINC, linker of nucleoskeleton and cytoskeleton complexes; Cyt, cytoplasm; Chr, chromatin; MB, midbody; End, endosomes; Tel, telomeres; INM, inner nuclear membrane; NP, nucleoplasm; NPC, Nuclear Pore Complex; ER, Endoplasmic Reticulum; SPB, spindle pole body.

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.

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.

Protein crowding mediates membrane remodeling in upstream ESCRT-induced formation of intraluminal vesicles

As part of the lysosomal degradation pathway, the endosomal sorting complexes required for transport (ESCRT-0 to -III/VPS4) sequester receptors at the endosome and simultaneously deform the membrane to generate intraluminal vesicles (ILVs). Whereas ESCRT-III/VPS4 have an established function in ILV formation, the role of upstream ESCRTs (0 to II) in membrane shape remodeling is not understood. Combining experimental measurements and electron microscopy analysis of ESCRT-III-depleted cells with a mathematical model, we show that upstream ESCRT-induced alteration of the Gaussian bending rigidity and their crowding in concert with the transmembrane cargo on the membrane induce membrane deformation and facilitate ILV formation: Upstream ESCRT-driven budding does not require ATP consumption as only a small energy barrier needs to be overcome. Our model predicts that ESCRTs do not become part of the ILV, but localize with a high density at the membrane neck, where the steep decline in the Gaussian curvature likely triggers ESCRT-III/VPS4 assembly to enable neck constriction and scission.

ALIX- and ESCRT-III-dependent sorting of tetraspanins to exosomes

The intraluminal vesicles (ILVs) of endosomes mediate the delivery of activated signaling receptors and other proteins to lysosomes for degradation, but they also modulate intercellular communication when secreted as exosomes. The formation of ILVs requires four complexes, ESCRT-0, -I, -II, and -III, with ESCRT-0, -I, and -II presumably involved in cargo sorting and ESCRT-III in membrane deformation and fission. Here, we report that an active form of the ESCRT-associated protein ALIX efficiently recruits ESCRT-III proteins to endosomes. This recruitment occurs independently of other ESCRTs but requires lysobisphosphatidic acid (LBPA) in vivo, and can be reconstituted on supported bilayers in vitro. Our data indicate that this ALIX- and ESCRT-III-dependent pathway promotes the sorting and delivery of tetraspanins to exosomes. We conclude that ALIX provides an additional pathway of ILV formation, secondary to the canonical pathway, and that this pathway controls the targeting of exosomal proteins.

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.

The ESCRTs - converging on mechanism

The endosomal sorting complexes required for transport (ESCRTs) I, -II and -III, and their associated factors are a collection of ∼20 proteins in yeast and ∼30 in mammals, responsible for severing membrane necks in processes that range from multivesicular body formation, HIV release and cytokinesis, to plasma and lysosomal membrane repair. ESCRTs are best known for 'reverse-topology' membrane scission, where they act on the inner surface of membrane necks, often when membranes are budded away from the cytosol. These events are driven by membrane-associated assemblies of dozens to hundreds of ESCRT molecules. ESCRT-III proteins form filaments with a variety of geometries and ESCRT-I has now been shown to also form helical structures. The complex nature of the system and the unusual topology of its action has made progress challenging, and led to controversies with regard to its underlying mechanism. This Review will focus on recent advances obtained by structural in vitro reconstitution and in silico mechanistic studies, and places them in their biological context. The field is converging towards a consensus on the broad outlines of a mechanism that is driven by a progressive ATP-dependent treadmilling exchange of ESCRT subunits, as well as compositional change and geometric transitions in ESCRT filaments.

Polarized human cholangiocytes release distinct populations of apical and basolateral small extracellular vesicles

Intercellular communication is critical for organismal homeostasis, and defects can contribute to human disease states. Polarized epithelial cells execute distinct signaling agendas via apical and basolateral surfaces to communicate with different cell types. Small extracellular vesicles (sEVs), including exosomes and small microvesicles, represent an understudied form of intercellular communication in polarized cells. Human cholangiocytes, epithelial cells lining bile ducts, were cultured as polarized epithelia in a Transwell system as a model with which to study polarized sEV communication. Characterization of isolated apically and basolaterally released EVs revealed enrichment in sEVs. However, differences in apical and basolateral sEV composition and numbers were observed. Genetic or pharmacological perturbation of cellular machinery involved in the biogenesis of intralumenal vesicles at endosomes (the source of exosomes) revealed general and domain-specific effects on sEV biogenesis/release. Additionally, analyses of signaling revealed distinct profiles of activation depending on sEV population, target cell, and the function of the endosomal sorting complex required for transport (ESCRT)-associated factor ALG-2–interacting protein X (ALIX) within the donor cells. These results support the conclusion that polarized cholangiocytes release distinct sEV pools to mediate communication via their apical and basolateral domains and suggest that defective ESCRT function may contribute to disease states through altered sEV signaling.

Midbody Remnant Inheritance Is Regulated by the ESCRT Subunit CHMP4C

The inheritance of the midbody remnant (MBR) breaks the symmetry of the two daughter cells, with functional consequences for lumen and primary cilium formation by polarized epithelial cells, and also for development and differentiation. However, despite its importance, neither the relationship between the plasma membrane and the inherited MBR nor the mechanism of MBR inheritance is well known. Here, the analysis by correlative light and ultra-high-resolution scanning electron microscopy reveals a membranous stalk that physically connects the MBR to the apical membrane of epithelial cells. The stalk, which derives from the uncleaved side of the midbody, concentrates the ESCRT machinery. The ESCRT CHMP4C subunit enables MBR inheritance, and its depletion dramatically reduces the percentage of ciliated cells. We demonstrate (1) that MBRs are physically connected to the plasma membrane, (2) how CHMP4C helps maintain the integrity of the connection, and (3) the functional importance of the connection.

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Most midbody remnants of MDCK cells are physically connected to the apical membrane

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The connection derives from the uncleaved arm of the midbody

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CHMP4C distributes asymmetrically in the connection and maintains its integrity

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A connected midbody remnant is necessary for primary cilium formation by these cells

Human ESCRT-III polymers assemble on positively curved membranes and induce helical membrane tube formation

Endosomal sorting complexes for transport-III (ESCRT-III) assemble in vivo onto membranes with negative Gaussian curvature. How membrane shape influences ESCRT-III polymerization and how ESCRT-III shapes membranes is yet unclear. Human core ESCRT-III proteins, CHMP4B, CHMP2A, CHMP2B and CHMP3 are used to address this issue in vitro by combining membrane nanotube pulling experiments, cryo-electron tomography and AFM. We show that CHMP4B filaments preferentially bind to flat membranes or to tubes with positive mean curvature. Both CHMP2B and CHMP2A/CHMP3 assemble on positively curved membrane tubes. Combinations of CHMP4B/CHMP2B and CHMP4B/CHMP2A/CHMP3 are recruited to the neck of pulled membrane tubes and reshape vesicles into helical "corkscrew-like" membrane tubes. Sub-tomogram averaging reveals that the ESCRT-III filaments assemble parallel and locally perpendicular to the tube axis, highlighting the mechanical stresses imposed by ESCRT-III. Our results underline the versatile membrane remodeling activity of ESCRT-III that may be a general feature required for cellular membrane remodeling processes.

Centrosomal protein 55: A new paradigm in tumorigenesis

Centrosomal Protein 55 (Cep55), also known as c10orf3 and FLJ10540, was initially discovered as a major player in abscission, the final stage of cytokinesis. Subsequent studies have described its role in regulating the PI3K/AKT pathway, increasing cancer cell stemness, and promoting tumor formation. Clinically, Cep55 has been found to be overexpressed in many cancer types. Cep55 overexpression has been notably associated with tumor stage, tumor aggressiveness, poor prognosis, and metastasis. The present review discusses the role of Cep55 as a crucial biomarker and model in tumorigenesis.

HIV-1 viral cores enter the nucleus collectively through the nuclear endocytosis-like pathway

It is recognized that HIV-1 capsid cores are disassembled in the cytoplasm, releasing their genomes into the nucleus through nuclear pores, but there is also evidence showing the capsid (CA) exists in the nucleus. Whether HIV-1 enters the nucleus and how it enters the nucleus through the undersized nuclear pore remains mysterious. Based on multicolor labeling and real-time imaging of the viral and cellular components, our observations via light and electron microscopy suggest that HIV-1 selectively gathered at the microtubule organization center (MTOC), leading the nearby nuclear envelope (NE) to undergo deformation, invagination and restoration to form a nuclear vesicle in which the viral particles were wrapped; then, the inner membrane of the nuclear vesicle ruptured to release HIV-1 into the nucleus. This unexpected discovery expands our understanding of the complexity of HIV-1 nuclear entry, which may provide new insights to HIV-1 virology.

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.

The Flemmingsome reveals an ESCRT-to-membrane coupling via ALIX/syntenin/syndecan-4 required for completion of cytokinesis

Cytokinesis requires the constriction of ESCRT-III filaments on the side of the midbody, where abscission occurs. After ESCRT recruitment at the midbody, it is not known how the ESCRT-III machinery localizes to the abscission site. To reveal actors involved in abscission, we obtained the proteome of intact, post-abscission midbodies (Flemmingsome) and identified 489 proteins enriched in this organelle. Among these proteins, we further characterized a plasma membrane-to-ESCRT module composed of the transmembrane proteoglycan syndecan-4, ALIX and syntenin, a protein that bridges ESCRT-III/ALIX to syndecans. The three proteins are highly recruited first at the midbody then at the abscission site, and their depletion delays abscission. Mechanistically, direct interactions between ALIX, syntenin and syndecan-4 are essential for proper enrichment of the ESCRT-III machinery at the abscission site, but not at the midbody. We propose that the ESCRT-III machinery must be physically coupled to a membrane protein at the cytokinetic abscission site for efficient scission, uncovering common requirements in cytokinesis, exosome formation and HIV budding.

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.

ESCRT Machinery Mediates Cytokinetic Abscission in the Unicellular Red Alga Cyanidioschyzon merolae

In many eukaryotes, cytokinesis proceeds in two successive steps: first, ingression of the cleavage furrow and second, abscission of the intercellular bridge. In animal cells, the actomyosin contractile ring is involved in the first step, while the endosomal sorting complex required for transport (ESCRT), which participates in various membrane fusion/fission events, mediates the second step. Intriguingly, in archaea, ESCRT is involved in cytokinesis, raising the hypothesis that the function of ESCRT in eukaryotic cytokinesis descended from the archaeal ancestor. In eukaryotes other than in animals, the roles of ESCRT in cytokinesis are poorly understood. To explore the primordial core mechanisms for eukaryotic cytokinesis, we investigated ESCRT functions in the unicellular red alga Cyanidioschyzon merolae that diverged early in eukaryotic evolution. C. merolae provides an excellent experimental system. The cell has a simple organelle composition. The genome (16.5 Mb, 5335 genes) has been completely sequenced, transformation methods are established, and the cell cycle is synchronized by a light and dark cycle. Similar to animal and fungal cells, C. merolae cells divide by furrowing at the division site followed by abscission of the intercellular bridge. However, they lack an actomyosin contractile ring. The proteins that comprise ESCRT-I-IV, the four subcomplexes of ESCRT, are partially conserved in C. merolae. Immunofluorescence of native or tagged proteins localized the homologs of the five ESCRT-III components [charged multivesicular body protein (CHMP) 1, 2, and 4-6], apoptosis-linked gene-2-interacting protein X (ALIX), the ESCRT-III adapter, and the main ESCRT-IV player vacuolar protein sorting (VPS) 4, to the intercellular bridge. In addition, ALIX was enriched around the cleavage furrow early in cytokinesis. When the ESCRT function was perturbed by expressing dominant-negative VPS4, cells with an elongated intercellular bridge accumulated-a phenotype resulting from abscission failure. Our results show that ESCRT mediates cytokinetic abscission in C. merolae. The fact that ESCRT plays a role in cytokinesis in archaea, animals, and early diverged alga C. merolae supports the hypothesis that the function of ESCRT in cytokinesis descended from archaea to a common ancestor of eukaryotes.

Cytokinesis in Eukaryotic Cells: The Furrow Complexity at a Glance

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

Feeding Delivery of dsHvSnf7 Is a Promising Method for Management of the Pest Henosepilachna vigintioctopunctata (Coleoptera: Coccinellidae)

RNA interference (RNAi) techniques have emerged as powerful tools in the development of novel management strategies for the control of insect pests, such as Henosepilachna vigintioctopunctata, which is a major solanaceous pest in Asia. Our results showed that levels of HvSnf7 expression were greater in larval midguts than in other tissues. Silencing of HvSnf7 led to greater H. vigintioctopunctata mortality rates and appeared to be time- and partially dose-dependent. Bacterially expressed dsHvSnf7 that was applied to detached plant leaves caused 98, 88, and 60% mortality in 1st and 3rd instars, and adults after 10, 12, and 14 d, respectively; when applied to living plants, bacterially expressed dsHvSnf7 led to mortality in 1st and 3rd instars, with no effect on adults. Bacterially expressed dsHvSnf7 led to improved plant protection against H. vigintioctopunctata. Ultrastructural changes caused by HvSnf7-RNAi in larval midguts showed extensive loss of cellular contents that indicate loss of membrane integrity. This study indicate that HvSnf7 potentially can be used as RNAi target gene for controlling of H. vigintioctopunctata.

Emerging roles for the nucleus during neutrophil signal relay and NETosis

The nucleus houses and protects genomic DNA, which is surrounded by the nuclear envelope. Owing to its size and stiffness, the nucleus is often a barrier to migration through confined spaces. Neutrophils are terminally differentiated, short-lived cells that migrate through tissues in response to injury and infections. The neutrophil nucleus is soft, multilobular, and exhibits altered levels of key nuclear envelope proteins. These alterations result in a multifunctional organelle that serves as a signaling hub during migration and NETosis, a process by which neutrophils release decondensed chromatin decorated with granular enzymes that entrap pathogens. In this review, we present emerging evidence suggesting that a unique, ambiguous cell-cycle state is critical for NETosis and migration. Finally, we discuss how the mechanisms underlying migration and NETosis are evolutionarily conserved.

ESCRT subunit CHMP4B localizes to primary cilia and is required for the structural integrity of the ciliary membrane

Proteins specialized in the detection, generation, or stabilization of membrane curvature play important roles in establishing various morphologies of cells and cellular organelles. Primary cilia are cellular organelles that protrude from the cell surface using a microtubule-based cytoskeleton called the axoneme as a structural support. It is unclear whether the integrity of the high curvature of the ciliary membrane depends on membrane curvature-related proteins. Charged Multivesicular Body Protein 4B (CHMP4B), a subunit of the endosomal sorting complexes required for transport (ESCRT), can stabilize membrane curvature. Here we show that CHMP4B is involved in the assembly and maintenance of primary cilia. CHMP4B was localized to primary cilia in mammalian cells. Knockdown of CHMP4B interfered with cilium assembly and also caused fragmentation of preexisting cilia. By contrast, cilium formation was unaffected by the interruption of the ESCRT-dependent endocytic degradation pathway. Morpholino (MO)-mediated CHMP4B depletion in zebrafish embryos induced characteristic phenotypes of ciliary defects such as curved body axis, hydrocephalus, otolith malformation, and kidney cyst. Our study reveals a new role for the multifunctional protein CHMP4B as a key factor in maintaining the structural integrity of primary cilia.

Downregulation of lumican enhanced mitotic defects and aneuploidy in lung cancer cells

Lumican is overexpressed in lung cancer cells and has been implicated in the pathogenesis of tumorigenesis and regulation of cancer cell invasion. Lumican is robustly associated with the binding of p120-catenin protein to modulate cell metastasis. However, its role in cancer cell proliferation is still unclear. This study investigated the effect of lumican on the cell division including mitosis and cytokinesis in non-small lung cancer cells. We found that the downregulation of lumican prolonged the doubling time of cells and retarded the cell growth in H460 and A549 cells. Along with tubulin, lumican localized to the mitotic spindle and centrosome during the metaphase-anaphase stage. The cell cycle was retained in the G2/M phase after the downregulation of lumican. Interestingly, lumican was found to play important roles in central spindle and midbody formation during cytokinesis. Lumican interacted with the midbody-associated proteins such as MKLP1, Aurora B, and ECT2. Notably, the downregulation of lumican decreased the level of MKLP1 accompanied by the retention of midbody-residual that resulted in multi-nucleated cells. Downregulation of lumican promoted the chromosome missegregation and the increment of the bi-/multinucleated cells. The results of this study indicated that lumican associated with tubulin is crucial for spindle fiber formation and midbody assembly in cell division. Downregulation of lumican displayed the defects in mitotic spindle assembly/dynamics and improper kinetochore-microtubules attachment that led to increase aneuploidy. This emerging property of lumican is suggested to tightly control chromosome segregation during cell division in lung cancer cells.Abbreviations: ESCRT: endosomal sorting complex required for transport; PRC1: protein regulator of cytokinesis 1; Nci: negative control siRNA; Lumi: lumican siRNAs; MKLP1: mitotic kinesin-like protein 1; H460LD and A549LD: H460 and A549 cell lines with less expressed lumican.

The many functions of ESCRTs

Cellular membranes can form two principally different involutions, which either exclude or contain cytosol. The 'classical' budding reactions, such as those occurring during endocytosis or formation of exocytic vesicles, involve proteins that assemble on the cytosol-excluding face of the bud neck. Inverse membrane involution occurs in a wide range of cellular processes, supporting cytokinesis, endosome maturation, autophagy, membrane repair and many other processes. Such inverse membrane remodelling is mediated by a heteromultimeric protein machinery known as endosomal sorting complex required for transport (ESCRT). ESCRT proteins assemble on the cytosolic (or nucleoplasmic) face of the neck of the forming involution and cooperate with the ATPase VPS4 to drive membrane scission or sealing. Here, we review similarities and differences of various ESCRT-dependent processes, with special emphasis on mechanisms of ESCRT recruitment.

Extrachromosomal Histone H2B Contributes to the Formation of the Abscission Site for Cell Division

Histones are constitutive components of nucleosomes and key regulators of chromatin structure. We previously observed that an extrachromosomal histone H2B (ecH2B) localizes at the intercellular bridge (ICB) connecting the two daughter cells during cytokinesis independently of DNA and RNA. Here, we show that ecH2B binds and colocalizes with CHMP4B, a key component of the ESCRT-III machinery responsible for abscission, the final step of cell division. Abscission requires the formation of an abscission site at the ICB where the ESCRT-III complex organizes into narrowing cortical helices that drive the physical separation of sibling cells. ecH2B depletion does not prevent membrane cleavage rather results in abscission delay and accumulation of abnormally long and thin ICBs. In the absence of ecH2B, CHMP4B and other components of the fission machinery, such as IST1 and Spastin, are recruited to the ICB and localize at the midbody. However, in the late stage of abscission, these fission factors fail to re-localize at the periphery of the midbody and the abscission site fails to form. These results show that extrachromosomal activity of histone H2B is required in the formation of the abscission site and the proper localization of the fission machinery.

The postmitotic midbody: Regulating polarity, stemness, and proliferation

Peterman and Prekeris review abscission and discuss the diverse roles for the postmitotic midbody in regulating polarity, tumorigenesis, and stemness.

Abscission, the final stage of cell division, requires well-orchestrated changes in endocytic trafficking, microtubule severing, actin clearance, and the physical sealing of the daughter cell membranes. These processes are highly regulated, and any missteps in localized membrane and cytoskeleton dynamics often lead to a delay or a failure in cell division. The midbody, a microtubule-rich structure that forms during cytokinesis, is a key regulator of abscission and appears to function as a signaling platform coordinating cytoskeleton and endosomal dynamics during the terminal stages of cell division. It was long thought that immediately following abscission and the conclusion of cell division, the midbody is either released or rapidly degraded by one of the daughter cells. Recently, the midbody has gained prominence for exerting postmitotic functions. In this review, we detail the role of the midbody in orchestrating abscission, as well as discuss the relatively new field of postabscission midbody biology, particularly focusing on how it may act to regulate cell polarity and its potential to regulate cell tumorigenicity or stemness.

Building bridges between chromosomes: novel insights into the abscission checkpoint

In the presence of chromatin bridges, mammalian cells delay completion of cytokinesis (abscission) to prevent chromatin breakage or tetraploidization by regression of the cleavage furrow. This abscission delay is called "the abscission checkpoint" and is dependent on Aurora B kinase. Furthermore, cells stabilize the narrow cytoplasmic canal between the two daughter cells until the DNA bridges are resolved. Impaired abscission checkpoint signaling or unstable intercellular canals can lead to accumulation of DNA damage, aneuploidy, or generation of polyploid cells which are associated with tumourigenesis. However, the molecular mechanisms involved have only recently started to emerge. In this review, we focus on the molecular pathways of the abscission checkpoint and describe newly identified triggers, Aurora B-regulators and effector proteins in abscission checkpoint signaling. We also describe mechanisms that control intercellular bridge stabilization, DNA bridge resolution, or abscission checkpoint silencing upon satisfaction, and discuss how abscission checkpoint proteins can be targeted to potentially improve cancer therapy.

CDK11p58-cyclin L1β regulates abscission site assembly

Rigorous spatiotemporal regulation of cell division is required to maintain genome stability. The final stage in cell division, when the cells physically separate (abscission), is tightly regulated to ensure that it occurs after cytokinetic events such as chromosome segregation. A key regulator of abscission timing is Aurora B kinase activity, which inhibits abscission and forms the major activity of the abscission checkpoint. This checkpoint prevents abscission until chromosomes have been cleared from the cytokinetic machinery. Here we demonstrate that the mitosis-specific CDK11^p58 kinase specifically forms a complex with cyclin L1β that, in late cytokinesis, localizes to the stem body, a structure in the middle of the intercellular bridge that forms between two dividing cells. Depletion of CDK11 inhibits abscission, and rescue of this phenotype requires CDK11^p58 kinase activity or inhibition of Aurora B kinase activity. Furthermore, CDK11^p58 kinase activity is required for formation of endosomal sorting complex required for transport III filaments at the site of abscission. Combined, these data suggest that CDK11^p58 kinase activity opposes Aurora B activity to enable abscission to proceed and result in successful completion of cytokinesis.

The post-abscission midbody is an intracellular signaling organelle that regulates cell proliferation

Once thought to be a remnant of cell division, the midbody (MB) has recently been shown to have roles beyond its primary function of orchestrating abscission. Despite the emerging roles of post-abscission MBs, how MBs accumulate in the cytoplasm and signal to regulate cellular functions remains unknown. Here, we show that extracellular post-abscission MBs can be internalized by interphase cells, where they reside in the cytoplasm as a membrane-bound signaling structure that we have named the MBsome. We demonstrate that MBsomes stimulate cell proliferation and that MBsome formation is a phagocytosis-like process that depends on a phosphatidylserine/integrin complex, driven by actin-rich membrane protrusions. Finally, we show that MBsomes rely on dynamic actin coats to slow lysosomal degradation and propagate their signaling function. In summary, MBsomes may sometimes serve as intracellular organelles that signal via integrin and EGFR-dependent pathways to promote cell proliferation and anchorage-independent growth and survival.

The exocyst complex and Rab5 are required for abscission by localizing ESCRT III subunits to the cytokinetic bridge

Cytokinesis is the final step of cell division following chromosome segregation that generates two daughter cells. The conserved exocyst complex is required for scission of the intercellular cytokinetic bridge, although the molecular mechanisms it employs in this process are unclear. We identify and validate the early endocytic GTPase Rab5 as interacting with the exocyst complex in mammalian cells. Rab5 localizes in the cytokinetic bridge and on the midbody ring in a manner similar to the exocyst complex. Depletion of Rab5 led to delayed abscission. Caenorhabditis elegans orthologs of both exocyst complex subunits and Rab5 localize along the cleavage furrow and are required for cytokinesis in early embryos. Cytokinetic cells depleted of either Rab5 or the exocyst subunits Exoc3 and Exoc4 showed impaired deposition of the endosomal sorting complexes required for transport (ESCRT) III subunits CHMP2B and/or CHMP4B near the midbody ring. The study reveals an evolutionarily conserved role for the early endocytic marker Rab5 in cytokinetic abscission. In addition, it uncovers a key requirement of the exocyst and Rab5 for the delivery of components of the membrane-severing ESCRT III machinery to complete cytokinesis.

Conceptual Evolution of Cell Signaling

During the last 100 years, cell signaling has evolved into a common mechanism for most physiological processes across systems. Although the majority of cell signaling principles were initially derived from hormonal studies, its exponential growth has been supported by interdisciplinary inputs, e.g., from physics, chemistry, mathematics, statistics, and computational fields. As a result, cell signaling has grown out of scope for any general review. Here, we review how the messages are transferred from the first messenger (the ligand) to the receptor, and then decoded with the help of cascades of second messengers (kinases, phosphatases, GTPases, ions, and small molecules such as cAMP, cGMP, diacylglycerol, etc.). The message is thus relayed from the membrane to the nucleus where gene expression ns, subsequent translations, and protein targeting to the cell membrane and other organelles are triggered. Although there are limited numbers of intracellular messengers, the specificity of the response profiles to the ligands is generated by the involvement of a combination of selected intracellular signaling intermediates. Other crucial parameters in cell signaling are its directionality and distribution of signaling strengths in different pathways that may crosstalk to adjust the amplitude and quality of the final effector output. Finally, we have reflected upon its possible developments during the coming years.

A Septin Double Ring Controls the Spatiotemporal Organization of the ESCRT Machinery in Cytokinetic Abscission

Abscission is the terminal step of mitosis that physically separates two daughter cells [1, 2]. Abscission requires the endocytic sorting complex required for transport (ESCRT), a molecular machinery of multiple subcomplexes (ESCRT-I/II/III) that promotes membrane remodeling and scission [3-5]. Recruitment of ESCRT-I/II complexes to the midbody of telophase cells initiates ESCRT-III assembly into two rings, which subsequently expand into helices and spirals that narrow down to the incipient site of abscission [6-8]. ESCRT-III assembly is highly dynamic and spatiotemporally ordered, but the underlying mechanisms are poorly understood. Here, we report that, after cleavage furrow closure, septins form a membrane-bound double ring that controls the organization and function of ESCRT-III. The septin double ring demarcates the sites of ESCRT-III assembly into rings and disassembles before ESCRT-III rings expand into helices and spirals. We show that septin 9 (SEPT9) depletion, which abrogates abscission, impairs recruitment of VPS25 (ESCRT-II) and CHMP6 (ESCRT-III). Strikingly, ESCRT-III subunits (CHMP4B and CHMP2A/B) accumulate to the midbody, but they are highly disorganized, failing to form symmetric rings and to expand laterally into the cone-shaped helices and spirals of abscission. We found that SEPT9 interacts directly with the ubiquitin E2 variant (UEV) domain of ESCRT-I protein TSG101 through two N-terminal PTAP motifs, which are required for the recruitment of VPS25 and CHMP6, and the spatial organization of ESCRT-III (CHMP4B and CHMP2B) into functional rings. These results reveal that septins function in the ESCRT-I-ESCRT-II-CHMP6 pathway of ESCRT-III assembly and provide a framework for the spatiotemporal control of the ESCRT machinery of cytokinetic abscission.

The Chlamydia trachomatis Extrusion Exit Mechanism Is Regulated by Host Abscission Proteins

The cellular exit strategies of intracellular pathogens have a direct impact on microbial dissemination, transmission, and engagement of immune responses of the host. Chlamydia exit their host via a budding mechanism called extrusion, which offers protective benefits to Chlamydia as they navigate their extracellular environment. Many intracellular pathogens co-opt cellular abscission machinery to facilitate cell exit, which is utilized to perform scission of two newly formed daughter cells following mitosis. Similar to viral budding exit strategies, we hypothesize that an abscission-like mechanism is required to physically sever the chlamydial extrusion from the host cell, co-opting the membrane fission activities of the endosomal sorting complex required for transport (ESCRT) family of proteins that are necessary for cellular scission events, including abscission. To test this, C. trachomatis L2-infected HeLa cells were depleted of key abscission machinery proteins charged multivesicle body protein 4b (CHMP4B), ALIX, centrosome protein 55 (CEP55), or vacuolar protein sorting-associated protein 4A (VPS4A), using RNA interference (RNAi). Over 50% reduction in extrusion formation was achieved by depletion of CHMP4B, VPS4A, and ALIX, but no effect on extrusion was observed with CEP55 depletion. These results demonstrate a role for abscission machinery in C. trachomatis extrusion from the host cell, with ALIX, VPS4A and CHMP4B playing key functional roles in optimal extrusion release.

VPS4 triggers constriction and cleavage of ESCRT-III helical filaments

Many cellular processes such as endosomal vesicle budding, virus budding, and cytokinesis require extensive membrane remodeling by the endosomal sorting complex required for transport III (ESCRT-III). ESCRT-III protein family members form spirals with variable diameters in vitro and in vivo inside tubular membrane structures, which need to be constricted to proceed to membrane fission. Here, we show, using high-speed atomic force microscopy and electron microscopy, that the AAA-type adenosine triphosphatase VPS4 constricts and cleaves ESCRT-III CHMP2A-CHMP3 helical filaments in vitro. Constriction starts asymmetrically and progressively decreases the diameter of CHMP2A-CHMP3 tubular structure, thereby coiling up the CHMP2A-CHMP3 filaments into dome-like end caps. Our results demonstrate that VPS4 actively constricts ESCRT-III filaments and cleaves them before their complete disassembly. We propose that the formation of ESCRT-III dome-like end caps by VPS4 within a membrane neck structure constricts the membrane to set the stage for membrane fission.

Growth factor stimulation promotes multivesicular endosome biogenesis by prolonging recruitment of the late-acting ESCRT machinery

The formation of multivesicular endosomes (MVEs) mediates the turnover of numerous integral membrane proteins and has been implicated in the down-regulation of growth factor signaling, thereby exhibiting properties of a tumor suppressor. The endosomal sorting complex required for transport (ESCRT) machinery plays a key role in MVE biogenesis, enabling cargo selection and intralumenal vesicle (ILV) budding. However, the spatiotemporal pattern of endogenous ESCRT complex assembly and disassembly in mammalian cells remains poorly defined. By combining CRISPR/Cas9-mediated genome editing and live cell imaging using lattice light sheet microscopy (LLSM), we determined the native dynamics of both early- and late-acting ESCRT components at MVEs under multiple growth conditions. Specifically, our data indicate that ESCRT-0 accumulates quickly on endosomes, typically in less than 30 seconds, and its levels oscillate in a manner dependent on the downstream recruitment of ESCRT-I. Similarly, levels of the ESCRT-I complex also fluctuate on endosomes, but its average residency time is more than fivefold shorter compared with ESCRT-0. Vps4 accumulation is the most transient, however, suggesting that the completion of ILV formation occurs rapidly. Upon addition of epidermal growth factor (EGF), both ESCRT-I and Vps4 are retained at endosomes for dramatically extended periods of time, while ESCRT-0 dynamics are only modestly affected. Our findings are consistent with a model in which growth factor stimulation stabilizes late-acting components of the ESCRT machinery at endosomes to accelerate the rate of ILV biogenesis and attenuate signal transduction initiated by receptor activation.

Non-muscle Myosin-II Is Required for the Generation of a Constriction Site for Subsequent Abscission

It remains unknown when, where, and how the site of abscission is generated during cytokinesis. Here, we show that the sites of constriction, i.e., the sites of future abscission, are initially formed at the ends of the intercellular bridge during early midbody stage, and that these sites are associated with the non-muscle myosin-IIB (not myosin-IIA), actin filaments, and septin 9 until abscission. The ESCRT-III component CHMP4B localizes to the midbody and "spreads" to the site of abscission only during late midbody stage. Strikingly, inhibition of myosin-II motor activity by a low dose of Blebbistatin completely abolishes the formation of the constriction sites, resulting in the localization of all the above-mentioned components to the midbody region. These data strongly suggest that a secondary actomyosin ring provides the primary driving force for the thinning of the intercellular bridge to allow ESCRT-mediated membrane fission.

ATP-dependent force generation and membrane scission by ESCRT-III and Vps4

The endosomal sorting complexes required for transport (ESCRTs) catalyze reverse-topology scission from the inner face of membrane necks in HIV budding, multivesicular endosome biogenesis, cytokinesis, and other pathways. We encapsulated ESCRT-III subunits Snf7, Vps24, and Vps2 and the AAA+ ATPase (adenosine triphosphatase) Vps4 in giant vesicles from which membrane nanotubes reflecting the correct topology of scission could be pulled. Upon ATP release by photo-uncaging, this system generated forces within the nanotubes that led to membrane scission in a manner dependent upon Vps4 catalytic activity and Vps4 coupling to the ESCRT-III proteins. Imaging of scission revealed Snf7 and Vps4 puncta within nanotubes whose presence followed ATP release, correlated with force generation and nanotube constriction, and preceded scission. These observations directly verify long-standing predictions that ATP-hydrolyzing assemblies of ESCRT-III and Vps4 sever membranes.

Purification of Plant ESCRT Proteins for Polyclonal Antibody Production

Most endosomal sorting complex required for transport (ESCRT)-III proteins are not fully functional when expressed as fusion of fluorescent or epitope tags, frequently making the use of specific antibodies the only available method for their detection. Heterologous expression of ESCRT-III proteins in bacteria often results in the formation of insoluble aggregates or inclusion bodies that interfere with their purification. However, inclusion bodies are usually pure protein aggregates with high antigenicity. In addition, since proteins within inclusion bodies are presented in a range of folding states, immunization with inclusion bodies can potentially result in antibodies with specificity for different folding states of the protein under study. We describe here a protocol to isolate bacterial inclusion bodies of plant ESCRT-III proteins for production of polyclonal antibodies. We also describe a nitrocellulose-based immunoaffinity purification method that allows the immobilization of ESCRT-III proteins and the subsequent isolation of specific antibodies from a crude serum.

Purification of Recombinant ESCRT-III Proteins and Their Use in Atomic Force Microscopy and In Vitro Binding and Phosphorylation Assays

The endosomal sorting complex required for transport (ESCRT)-III proteins are known to assemble into filaments that mediate membrane remodeling and fission in various biological processes, including the formation of endosomal multivesicular bodies, viral budding, cytokinesis, plasma membrane repair, nuclear pore quality control, nuclear envelope reformation, and neuron pruning. The study of the regulation and function of ESCRT-III proteins is therefore crucial to understand these events and requires a combination of in vivo and in vitro experimental techniques. Here we describe two protocols for the purification of human and Drosophila ESCRT-III proteins from bacteria and their use in in vitro phosphorylation assays and atomic force microscopy experiments on membrane lipid bilayers. These protocols can also be applied for the purification of other proteins that are insoluble when expressed in bacteria.

Studying the Spatial Organization of ESCRTs in Cytokinetic Abscission Using the High-Resolution Imaging Techniques SIM and Cryo-SXT

The ESCRT machinery mediates scission of the intercellular bridge that connects two daughter cells at the end of cytokinesis. Structured illumination microscopy (SIM) and cryo-soft-X-ray tomography (cryo-SXT) have been used in recent years to study the topology of ESCRT-driven cytokinetic abscission. These studies revealed that the intercellular bridge is occupied by cortical rings and spiral-like filaments and that ESCRTs form ring-like structures in this region during abscission. In this chapter, we provide two protocols: a protocol for determining the spatial organization of specific ESCRT components at the intercellular bridge using SIM and a protocol for resolving the ultrastructural organization of cortical filaments at the intercellular bridge using cryo-SXT.

Regulating a key mitotic regulator, polo-like kinase 1 (PLK1)

During cell division, duplicated genetic material is separated into two distinct daughter cells. This process is essential for initial tissue formation during development and to maintain tissue integrity throughout an organism's lifetime. To ensure the efficacy and efficiency of this process, the cell employs a variety of regulatory and signaling proteins that function as mitotic regulators and checkpoint proteins. One vital mitotic regulator is polo-like kinase 1 (PLK1), a highly conserved member of the polo-like kinase family. Unique from its paralogues, it functions specifically during mitosis as a regulator of cell division. PLK1 is spatially and temporally enriched at three distinct subcellular locales; the mitotic centrosomes, kinetochores, and the cytokinetic midbody. These localization patterns allow PLK1 to phosphorylate specific downstream targets to regulate mitosis. In this review, we will explore how polo-like kinases were originally discovered and diverged into the five paralogues (PLK1-5) in mammals. We will then focus specifically on the most conserved, PLK1, where we will discuss what is known about how its activity is modulated, its role during the cell cycle, and new, innovative tools that have been developed to examine its function and interactions in cells.

Structures, Functions, and Dynamics of ESCRT-III/Vps4 Membrane Remodeling and Fission Complexes

The endosomal sorting complexes required for transport (ESCRT) pathway mediates cellular membrane remodeling and fission reactions. The pathway comprises five core complexes: ALIX, ESCRT-I, ESCRT-II, ESCRT-III, and Vps4. These soluble complexes are typically recruited to target membranes by site-specific adaptors that bind one or both of the early-acting ESCRT factors: ALIX and ESCRT-I/ESCRT-II. These factors, in turn, nucleate assembly of ESCRT-III subunits into membrane-bound filaments that recruit the AAA ATPase Vps4. Together, ESCRT-III filaments and Vps4 remodel and sever membranes. Here, we review recent advances in our understanding of the structures, activities, and mechanisms of the ESCRT-III and Vps4 machinery, including the first high-resolution structures of ESCRT-III filaments, the assembled Vps4 enzyme in complex with an ESCRT-III substrate, the discovery that ESCRT-III/Vps4 complexes can promote both inside-out and outside-in membrane fission reactions, and emerging mechanistic models for ESCRT-mediated membrane fission.

A cancer-associated polymorphism in ESCRT-III disrupts the abscission checkpoint and promotes genome instability

Cytokinetic abscission facilitates the irreversible separation of daughter cells. This process requires the endosomal-sorting complexes required for transport (ESCRT) machinery and is tightly regulated by charged multivesicular body protein 4C (CHMP4C), an ESCRT-III subunit that engages the abscission checkpoint (NoCut) in response to mitotic problems such as persisting chromatin bridges within the midbody. Importantly, a human polymorphism in CHMP4C (rs35094336, CHMP4CT232) increases cancer susceptibility. Here, we explain the structural and functional basis for this cancer association: The CHMP4CT232 allele unwinds the C-terminal helix of CHMP4C, impairs binding to the early-acting ESCRT factor ALIX, and disrupts the abscission checkpoint. Cells expressing CHMP4CT232 exhibit increased levels of DNA damage and are sensitized to several conditions that increase chromosome missegregation, including DNA replication stress, inhibition of the mitotic checkpoint, and loss of p53. Our data demonstrate the biological importance of the abscission checkpoint and suggest that dysregulation of abscission by CHMP4CT232 may synergize with oncogene-induced mitotic stress to promote genomic instability and tumorigenesis.

Domes and cones: Adhesion-induced fission of membranes by ESCRT proteins

ESCRT proteins participate in the fission step of exocytic membrane budding, by assisting in the closure and scission of the membrane neck that connects the nascent bud to the plasma membrane. However, the precise mechanism by which the proteins achieve this so-called reverse-topology membrane scission remains to be elucidated. One mechanism is described by the dome model, which postulates that ESCRT-III proteins assemble in the shape of a hemispherical dome at the location of the neck, and guide the closure of this neck via membrane–protein adhesion. A different mechanism is described by the flattening cone model, in which the ESCRT-III complex first assembles at the neck in the shape of a cone, which then flattens leading to neck closure. Here, we use the theoretical framework of curvature elasticity and membrane–protein adhesion to quantitatively describe and compare both mechanisms. This comparison shows that the minimal adhesive strength of the membrane–protein interactions required for scission is much lower for cones than for domes, and that the geometric constraints on the shape of the assembly required to induce scission are more stringent for domes than for cones. Finally, we compute for the first time the adhesion-induced constriction forces exerted by the ESCRT assemblies onto the membrane necks. These forces are higher for cones and of the order of 100 pN.

Membrane fission is a crucial step in many biological processes ranging from cell division to viral budding. During fission, the membrane forms a narrow neck that is subsequently cleaved by proteins. ESCRT proteins initiate this process by forming supramolecular assemblies that adhere to the cytosolic face of the membranes. However, how these assemblies achieve the closure and scission of membrane necks is still a matter of controversy. Here, we elucidate and compare the two most prominent mechanisms that have been proposed for ESCRT-induced membrane fission. In both mechanisms, the ESCRT molecules form assemblies that adhere to the open neck of the membrane bud. In the first mechanism, these assemblies grow in a dome-like shape, whereas in the second mechanism the assemblies take a cone-like shape and flatten as they grow. Our computational study shows that the minimal strength of membrane–protein adhesion necessary for cone-induced fission is lower than for dome-induced fission, and that cones generate higher constriction forces of the order of 100 pN onto the membrane necks.

Resolving ESCRT-III Spirals at the Intercellular Bridge of Dividing Cells Using 3D STORM

The ESCRT machinery mediates membrane fission in a variety of processes in cells. According to current models, ESCRT-III proteins drive membrane fission by assembling into helical filaments on membranes. Here, we used 3D STORM imaging of endogenous ESCRT-III component IST1 to reveal the evolution of the structural organization of ESCRT-III in mammalian cytokinetic abscission. Using this approach, ESCRT-III ring and spiral assemblies were resolved and characterized at different stages of abscission. Visualization of IST1 structures in cells lacking the microtubule-severing enzyme spastin and in cells depleted of specific ESCRT-III components or the ATPase VPS4 demonstrated the contribution of these components to the organization and function of ESCRTs in cells. This work provides direct evidence that ESCRT-III proteins form helical filaments to mediate their function in cells and raises new mechanistic scenarios for ESCRT-driven cytokinetic abscission.

Concerted ESCRT and clathrin recruitment waves define the timing and morphology of intraluminal vesicle formation

The endosomal sorting complex required for transport (ESCRT) machinery mediates cargo sorting, membrane deformation and membrane scission on the surface of endosomes, generating intraluminal vesicles (ILVs) to degrade signaling receptors. By live-cell imaging of individual endosomes in human cells, we find that ESCRT proteins are recruited in a repetitive pattern: ESCRT-0 and -I show a gradual and linear recruitment and dissociation, whereas ESCRT-III and its regulatory ATPase VPS4 display fast and transient dynamics. Electron microscopy shows that ILVs are formed consecutively, starting immediately after endocytic uptake of cargo proteins and correlating with the repeated ESCRT recruitment waves, unraveling the timing of ILV formation. Clathrin, recruited by ESCRT-0, is required for timely ESCRT-0 dissociation, efficient ILV formation, correct ILV size and cargo degradation. Thus, cargo sorting and ILV formation occur by concerted, coordinated and repetitive recruitment waves of individual ESCRT subcomplexes and are controlled by clathrin.

Intraluminal vesicles are formed by the endosomal sorting complex required for transport (ESCRT) machinery. Here, the authors unravel the timing of vesicle budding, and that endosomal clathrin regulates concerted recruitment of ESCRT subcomplexes, required for efficient membrane remodeling.