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.

Rho-dependent control of the Citron kinase, Sticky, drives midbody ring maturation

Rho-dependent proteins control assembly of the cytokinetic contractile ring, yet it remains unclear how those proteins guide ring closure and how they promote subsequent formation of a stable midbody ring. Citron kinase is one important component required for midbody ring formation but its mechanisms of action and relationship with Rho are controversial. Here, we conduct a structure-function analysis of the Drosophila Citron kinase, Sticky, in Schneider's S2 cells. We define two separable and redundant RhoGEF/Pebble-dependent inputs into Sticky recruitment to the nascent midbody ring and show that each input is subsequently required for retention at, and for the integrity of, the mature midbody ring. The first input is via an actomyosin-independent interaction between Sticky and Anillin, a key scaffold also required for midbody ring formation. The second input requires the Rho-binding domain of Sticky, whose boundaries we have defined. Collectively, these results show how midbody ring biogenesis depends on the coordinated actions of Sticky, Anillin, and Rho.

CLIC5: a novel ETV6 target gene in childhood acute lymphoblastic leukemia

The most common rearrangement in childhood precursor B-cell acute lymphoblastic leukemia is the t(12;21)(p13;q22) translocation resulting in the ETV6-AML1 fusion gene. A frequent concomitant event is the loss of the residual ETV6 allele suggesting a critical role for the ETV6 transcriptional repressor in the etiology of this cancer. However, the precise mechanism through which loss of functional ETV6 contributes to disease pathogenesis is still unclear. To investigate the impact of ETV6 loss on the transcriptional network and to identify new transcriptional targets of ETV6, we used whole transcriptome analysis of both pre-B leukemic cell lines and patients combined with chromatin immunoprecipitation. Using this integrative approach, we identified 4 novel direct ETV6 target genes: CLIC5, BIRC7, ANGPTL2 and WBP1L To further evaluate the role of chloride intracellular channel protein CLIC5 in leukemogenesis, we generated cell lines overexpressing CLIC5 and demonstrated an increased resistance to hydrogen peroxide-induced apoptosis. We further described the implications of CLIC5's ion channel activity in lysosomal-mediated cell death, possibly by modulating the function of the transferrin receptor with which it colocalizes intracellularly. For the first time, we showed that loss of ETV6 leads to significant overexpression of CLIC5, which in turn leads to decreased lysosome-mediated apoptosis. Our data suggest that heightened CLIC5 activity could promote a permissive environment for oxidative stress-induced DNA damage accumulation, and thereby contribute to leukemogenesis.

Kinetochore-localized PP1-Sds22 couples chromosome segregation to polar relaxation

Cell division requires the precise coordination of chromosome segregation and cytokinesis. This coordination is achieved by the recruitment of an actomyosin regulator, Ect2, to overlapping microtubules at the centre of the elongating anaphase spindle. Ect2 then signals to the overlying cortex to promote the assembly and constriction of an actomyosin ring between segregating chromosomes. Here, by studying division in proliferating Drosophila and human cells, we demonstrate the existence of a second, parallel signalling pathway, which triggers the relaxation of the polar cell cortex at mid anaphase. This is independent of furrow formation, centrosomes and microtubules and, instead, depends on PP1 phosphatase and its regulatory subunit Sds22 (refs 2, 3). As separating chromosomes move towards the polar cortex at mid anaphase, kinetochore-localized PP1-Sds22 helps to break cortical symmetry by inducing the dephosphorylation and inactivation of ezrin/radixin/moesin proteins at cell poles. This promotes local softening of the cortex, facilitating anaphase elongation and orderly cell division. In summary, this identifies a conserved kinetochore-based phosphatase signal and substrate, which function together to link anaphase chromosome movements to cortical polarization, thereby coupling chromosome segregation to cell division.

Opposing actions of septins and Sticky on Anillin promote the transition from contractile to midbody ring

During cytokinesis, closure of the actomyosin contractile ring (CR) is coupled to the formation of a midbody ring (MR), through poorly understood mechanisms. Using time-lapse microscopy of Drosophila melanogaster S2 cells, we show that the transition from the CR to the MR proceeds via a previously uncharacterized maturation process that requires opposing mechanisms of removal and retention of the scaffold protein Anillin. The septin cytoskeleton acts on the C terminus of Anillin to locally trim away excess membrane from the late CR/nascent MR via internalization, extrusion, and shedding, whereas the citron kinase Sticky acts on the N terminus of Anillin to retain it at the mature MR. Simultaneous depletion of septins and Sticky not only disrupted MR formation but also caused earlier CR oscillations, uncovering redundant mechanisms of CR stability that can partly explain the essential role of Anillin in this process. Our findings highlight the relatedness of the CR and MR and suggest that membrane removal is coordinated with CR disassembly.

Anillin acts as a bifunctional linker coordinating midbody ring biogenesis during cytokinesis

Animal cell cytokinesis proceeds via constriction of an actomyosin-based contractile ring (CR) [1, 2]. Upon reaching a diameter of ~1 μm [3], a midbody ring (MR) forms to stabilize the intercellular bridge until abscission [4-6]. How MR formation is coupled to CR closure and how plasma membrane anchoring is maintained at this key transition is unknown. Time-lapse microscopy of Drosophila S2 cells depleted of the scaffold protein Anillin [7-9] revealed that Anillin is required for complete closure of the CR and formation of the MR. Truncation analysis revealed that Anillin N termini connected with the actomyosin CR and supported formation of stable MR-like structures, but these could not maintain anchoring of the plasma membrane. Conversely, Anillin C termini failed to connect with the CR or MR but recruited the septin Peanut to ectopic structures at the equatorial cortex. Peanut depletion mimicked truncation of the Anillin C terminus, resulting in MR-like structures that failed to anchor the membrane. These data demonstrate that Anillin coordinates the transition from CR to MR and that it does so by linking two distinct cortical cytoskeletal elements. One apparently acts as the core structural template for MR assembly, while the other ensures stable anchoring of the plasma membrane beyond the CR stage.

Rho-dependent control of anillin behavior during cytokinesis

Anillin is a conserved protein required for cytokinesis but its molecular function is unclear. Anillin accumulation at the cleavage furrow is Rho guanine nucleotide exchange factor (GEF)(Pbl)-dependent but may also be mediated by known anillin interactions with F-actin and myosin II, which are under RhoGEF(Pbl)-dependent control themselves. Microscopy of Drosophila melanogaster S2 cells reveal here that although myosin II and F-actin do contribute, equatorial anillin localization persists in their absence. Using latrunculin A, the inhibitor of F-actin assembly, we uncovered a separate RhoGEF(Pbl)-dependent pathway that, at the normal time of furrowing, allows stable filamentous structures containing anillin, Rho1, and septins to form directly at the equatorial plasma membrane. These structures associate with microtubule (MT) ends and can still form after MT depolymerization, although they are delocalized under such conditions. Thus, a novel RhoGEF(Pbl)-dependent input promotes the simultaneous association of anillin with the plasma membrane, septins, and MTs, independently of F-actin. We propose that such interactions occur dynamically and transiently to promote furrow stability.

Rho-kinase controls cell shape changes during cytokinesis

BACKGROUND

Animal cell cytokinesis is characterized by a sequence of dramatic cortical rearrangements. How these are coordinated and coupled with mitosis is largely unknown. To explore the initiation of cytokinesis, we focused on the earliest cell shape change, cell elongation, which occurs during anaphase B and prior to cytokinetic furrowing.

RESULTS

Using RNAi and live video microscopy in Drosophila S2 cells, we implicate Rho-kinase (Rok) and myosin II in anaphase cell elongation. rok RNAi decreased equatorial myosin II recruitment, prevented cell elongation, and caused a remarkable spindle defect where the spindle poles collided with an unyielding cell cortex and the interpolar microtubules buckled outward as they continued to extend. Disruption of the actin cytoskeleton with Latrunculin A, which abolishes cortical rigidity, suppressed the spindle defect. rok RNAi also affected furrowing, which was delayed and slowed, sometimes distorted, and in severe cases blocked altogether. Codepletion of the myosin binding subunit (Mbs) of myosin phosphatase, an antagonist of myosin II activation, only partially suppressed the cell-elongation defect and the furrowing delay, but prevented cytokinesis failures induced by prolonged rok RNAi. The marked sensitivity of cell elongation to Rok depletion was highlighted by RNAi to other genes in the Rho pathway, such as pebble, racGAP50C, and diaphanous, which had profound effects on furrowing but lesser effects on elongation.

CONCLUSIONS

We show that cortical changes underlying cell elongation are more sensitive to depletion of Rok and myosin II, in comparison to other regulators of cytokinesis, and suggest that a distinct regulatory pathway promotes cell elongation.

Rab11-FIP3 and FIP4 interact with Arf6 and the exocyst to control membrane traffic in cytokinesis

The dual Rab11/Arf binding proteins, family of Rab11-interacting proteins FIP3 and FIP4 function in the delivery of recycling endosomes to the cleavage furrow and are, together with Rab11, essential for completion of abscission, the terminal step of cytokinesis. Here, we report that both FIP3 and FIP4 bind Arf6 in a nucleotide-dependent manner but exhibit differential affinities for Rab11 and Arf6. Both FIP3 and FIP4 can form ternary complexes with Rab11 and Arf6. Arf6 is localised to the furrow and midbody and we show that Arf6-GTP functions to localise FIP3 and FIP4 to midbodies during cytokinesis. Exo70p, a component of the Exocyst complex, also localises to the furrow of dividing cells and interacts with Arf6. We show that depletion of Exo70p leads to cytokinesis failure and an impairment of FIP3 and Rab11 localisation to the furrow and midbody. Moreover, Exo70p co-immunoprecipitates FIP3 and FIP4. Hence, we propose that FIP3 and FIP4 serve to couple Rab11-positive vesicle traffic from recycling endosomes to the cleavage furrow/midbody where they are tethered prior to fusion events via interactions with Arf6 and the Exocyst.

Terminal cytokinesis events uncovered after an RNAi screen

Much of our understanding of animal cell cytokinesis centers on the regulation of the equatorial acto-myosin contractile ring that drives the rapid ingression of a deep cleavage furrow. However, the central part of the mitotic spindle collapses to a dense structure that impedes the furrow and keeps the daughter cells connected via an intercellular bridge. Factors involved in the formation, maintenance, and resolution of this bridge are largely unknown. Using a library of 7,216 double-stranded RNAs (dsRNAs) representing the conserved genes of Drosophila, we performed an RNA interference (RNAi) screen for cytokinesis genes in Schneider's S2 cells. We identified both familiar and novel genes whose inactivation induced a multi-nucleate phenotype. Using live video microscopy, we show that three genes: anillin, citron-kinase (CG10522), and soluble N-ethylmaleimide sensitive factor (NSF) attachment protein (alpha-SNAP), are essential for the terminal (post-furrowing) events of cytokinesis. anillin RNAi caused gradual disruption of the intercellular bridge after furrowing; citron-kinase RNAi destabilized the bridge at a later stage; alpha-SNAP RNAi caused sister cells to fuse many hours later and by a different mechanism. We have shown that the stability of the intercellular bridge is essential for successful cytokinesis and have defined genes contributing to this stability.

Actin cytoskeleton remodeling during early Drosophila furrow formation requires recycling endosomal components Nuclear-fallout and Rab11

Cytokinesis requires a dramatic remodeling of the cortical cytoskeleton as well as membrane addition. The Drosophila pericentrosomal protein, Nuclear-fallout (Nuf), provides a link between these two processes. In nuf-derived embryos, actin remodeling and membrane recruitment during the initial stages of metaphase and cellular furrow formation are disrupted. Nuf is a homologue of arfophilin-2, an ADP ribosylation factor effector that binds Rab11 and influences recycling endosome (RE) organization. Here, we show that Nuf is an important component of the RE, and that these phenotypes are a consequence of Nuf activities at the RE. Nuf exhibits extensive colocalization with Rab11, a key RE component. GST pull-downs and the presence of a conserved Rab11-binding domain in Nuf demonstrate that Nuf and Rab11 physically associate. In addition, Nuf and Rab11 are mutually required for their localization to the RE. Embryos with reduced levels of Rab11 produce membrane recruitment and actin remodeling defects strikingly similar to nuf-derived embryos. These analyses support a common role for Nuf and Rab11 at the RE in membrane trafficking and actin remodeling during the initial stages of furrow formation.

Arfophilins are dual Arf/Rab 11 binding proteins that regulate recycling endosome distribution and are related to Drosophila nuclear fallout

Arfophilin is an ADP ribosylation factor (Arf) binding protein of unknown function. It is identical to the Rab11 binding protein eferin/Rab11-FIP3, and we show it binds both Arf5 and Rab11. We describe a related protein, arfophilin-2, that interacts with Arf5 in a nucleotide-dependent manner, but not Arf1, 4, or 6 and also binds Rab11. Arfophilin-2 localized to a perinuclear compartment, the centrosomal area, and focal adhesions. The localization of arfophilin-2 to the perinuclear compartment was selectively blocked by overexpression of Arf5-T31N. In contrast, a green fluorescent protein-arfophilin-2 chimera or arfophilin-2 deletions were localized around the centrosome in a region that was also enriched for transferrin receptors and Rab11 but not early endosome markers, suggesting that the distribution of the endosomal recycling compartment was altered. The arfophilins belong to a conserved family that includes Drosophila melanogaster nuclear fallout, a centrosomal protein required for cellularization. Expression of green fluorescent protein-nuclear fallout in HeLa cells resulted in a similar phenotype, indicative of functional homology and thus implicating the arfophilins in mitosis/cytokinesis. We suggest that the novel dual GTPase-binding capacity of the arfophilins could serve as an interface of signals from Rab and Arf GTPases to regulate membrane traffic and integrate distinct signals in the late endosomal recycling compartment.

Cyclin B destruction triggers changes in kinetochore behavior essential for successful anaphase

Successful mitosis requires that anaphase chromosomes sustain a commitment to move to their assigned spindle poles. This requires stable spindle attachment of anaphase kinetochores. Prior to anaphase, stable spindle attachment depends on tension created by opposing forces on sister kinetochores [1]. Because tension is lost when kinetochores disjoin, stable attachment in anaphase must have a different basis. After expression of nondegradable cyclin B (CYC-B(S)) in Drosophila embryos, sister chromosomes disjoined normally but their anaphase behavior was abnormal [2]. Chromosomes exhibited cycles of reorientation from one pole to the other. Additionally, the unpaired kinetochores accumulated attachments to both poles (merotelic attachments), congressed (again) to a pseudometaphase plate, and reacquired associations with checkpoint proteins more characteristic of prometaphase kinetochores. Unpaired prometaphase kinetochores, which occurred in a mutant entering mitosis with unreplicated (unpaired) chromosomes, behaved just like the anaphase kinetochores at the CYC-B(S) arrest. Finally, the normal anaphase release of AuroraB/INCENP from kinetochores was blocked by CYC-B(S) expression and, reciprocally, was advanced in a CycB mutant. Given its established role in destabilizing kinetochore-microtubule interactions [3], Aurora B dissociation is likely to be key to the change in kinetochore behavior. These findings show that, in addition to loss of sister chromosome cohesion, successful anaphase requires a kinetochore behavioral transition triggered by CYC-B destruction.