Dynamin and cytokinesis

Isoform and splice-variant specific functions of dynamin-2 revealed by analysis of conditional knock-out cells

Dynamin (Dyn) is a multifunctional GTPase implicated in several cellular events, including endocytosis, intracellular trafficking, cell signaling, and cytokinesis. The mammalian genome encodes three isoforms, Dyn1, Dyn2, and Dyn3, and several splice variants of each, leading to the suggestion that distinct isoforms and/or distinct splice variants might mediate distinct cellular functions. We generated a conditional Dyn2 KO cell line and performed knockout and reconstitution experiments to explore the isoform- and splice variant specific cellular functions of ubiquitously expressed Dyn2. We find that Dyn2 is required for clathrin-mediated endocytosis (CME), p75 export from the Golgi, and PDGF-stimulated macropinocytosis and cytokinesis, but not for other endocytic pathways. Surprisingly, CME and p75 exocytosis were efficiently rescued by reintroduction of Dyn2, but not Dyn1, suggesting that these two isoforms function differentially in vesicular trafficking in nonneuronal cells. Both isoforms rescued macropinocytosis and cytokinesis, suggesting that dynamin function in these processes might be mechanistically distinct from its role in CME. Although all four Dyn2 splice variants could equally restore CME, Dyn2ba and -bb were more effective at restoring p75 exocytosis. This splice variant specificity correlated with their differential targeting to the Golgi. These studies reveal isoform and splice-variant specific functions for Dyn2.

Cell division screens and dynamin

Cell division is the most fundamental process in the development of all living organisms. The generation of cell diversity throughout development, the multiplication of cells during wound repair and the maintenance of stem cells in several tissues and organs all rely on proper progress through cell division. Historically, biochemical studies of cell division proved to be difficult, since mitosis is a moving target. The rapid and dynamic nature of mitosis means necessary proteins often exist in multiple isoforms and some for only brief moments during a particular stage in the cell cycle. The advent of proteomics and the introduction of stage-specific inhibitors have enabled the field to identify numerous factors required at distinct steps in the cell cycle. One such factor identified in many of these screens was the highly conserved protein dynamin. Dynamin, long known for its role in endocytosis, is also necessary for co-ordinating actin assembly at membranes. Our knowledge of its precise cell cycle function and upstream/downstream targets, however, is unclear. Our review will describe current knowledge regarding the impacts of several cell division screens and the multiple roles that dynamin may play during mitosis.

Arabidopsis mutants and the network of microtubule-associated functions

In early eukaryotes, the microtubule system was engaged in mitosis, intracellular transport, and flagellum-based motility. In the plant lineage, the evolution of a multicellular body involved the conservation of some core functions, the loss of others, and the elaboration of new microtubule functions associated with the multicellular plant habit. This diversification is reflected by the presence of both conserved (animal/fungi-like) and novel (plant-like) sequences encoding microtubule-related functions in the Arabidopsis genome. The collection of microtubule mutants has grown rapidly over recent years. These mutants present a wide range of phenotypes, consistent with the hypothesis of a functional diversification of the microtubule system. In this review, we focus on mutant analysis and, in particular, discuss double mutant analysis as a valuable tool for pinpointing pathways of gene function. A future challenge will be to define the complete network of genetic and physical interactions of microtubule function in plants. In addition to reviewing recent progress in the functional analysis of the 'MAPome', we present an online database of Arabidopsis mutants impaired in microtubule functions.

Integrin trafficking regulated by Rab21 is necessary for cytokinesis

Adherent cells undergo remarkable changes in shape during cell division. However, the functional interplay between cell adhesion turnover and the mitotic machinery is poorly understood. The endo/exocytic trafficking of integrins is regulated by the small GTPase Rab21, which associates with several integrin alpha subunits. Here, we show that targeted trafficking of integrins to and from the cleavage furrow is required for successful cytokinesis, and that this is regulated by Rab21. Rab21 activity, integrin-Rab21 association, and integrin endocytosis are all necessary for normal cytokinesis, which becomes impaired when integrin-mediated adhesion at the cleavage furrow fails. We also describe a chromosomal deletion and loss of Rab21 gene expression in human cancer, which leads to the accumulation of multinucleate cells. Importantly, reintroduction of Rab21 rescued this phenotype. In conclusion, Rab21-regulated integrin trafficking is essential for normal cell division, and its defects may contribute to multinucleation and genomic instability, which are hallmarks of cancer.

Evolutionary linkage between eukaryotic cytokinesis and chloroplast division by dynamin proteins

Chloroplasts have evolved from a cyanobacterial endosymbiont and been retained for more than 1 billion years by coordinated chloroplast division in multiplying eukaryotic cells. Chloroplast division is performed by ring structures at the division site, encompassing both the inside and the outside of the two envelopes. A part of the division machinery is derived from the cyanobacterial cytokinetic activity based on the FtsZ protein. In contrast, other parts of the division machinery involve proteins specific to eukaryotes, including a member of the dynamin family. Each member of the dynamin family is involved in the division or fusion of a distinct eukaryotic membrane system. To gain insight into the kind of ancestral dynamin protein and eukaryotic membrane activity that evolved to regulate chloroplast division, we investigated the functions of the dynamin proteins that are most closely related to chloroplast division proteins. These proteins in the amoeba Dictyostelium discoideum and Arabidopsis thaliana localize at the sites of cell division, where they are involved in cytokinesis. Our results suggest that the dynamin for chloroplast division is derived from that involved in eukaryotic cytokinesis. Therefore, the chloroplast division machinery is a mixture of bacterial and eukaryotic cytokinesis components, with the latter a key factor in the synchronization of endosymbiotic cell division with host cell division, thus helping to establish the permanent endosymbiotic relationship.

SCAMPs highlight the developing cell plate during cytokinesis in tobacco BY-2 cells

We previously demonstrated that rice (Oryza sativa) SECRETORY CARRIER MEMBRANE PROTEIN1 (OsSCAMP1)-yellow fluorescent protein in transgenic tobacco (Nicotiana tabacum) Bright Yellow-2 cells locates to the plasma membrane and to motile punctate structures, which represent the trans-Golgi network/early endosome and are tubular-vesicular in nature. Here, we now show that SCAMPs are diverted to the cell plate during cytokinesis dividing Bright Yellow-2 cells. As cells progress from metaphase to cytokinesis, punctate OsSCAMP1-labeled structures begin to collect in the future division plane. Together with the internalized endosomal marker FM4-64, they then become incorporated into the cell plate as it forms and expands. This was confirmed by immunogold electron microscopy. We also monitored for the Golgi apparatus and the prevacuolar compartment (PVC)/multivesicular body. Golgi stacks tend to accumulate in the vicinity of the division plane, but the signals are clearly separate to the cell plate. The situation with the PVC (labeled by green fluorescent protein-BP-80) is not so clear. Punctate BP-80 signals are seen at the advancing periphery of the cell plate, which was confirmed by immunogold electron microscopy. Specific but weak labeling was observed in the cell plate, but no evidence for a fusion of the PVC/multivesicular body with the cell plate could be obtained. Our data, therefore, support the notion that cell plate formation is mainly a secretory process involving mass incorporation of domains of the trans-Golgi network/early endosome membrane. We regard the involvement of multivesicular late endosomes in this process to be equivocal.

Both daughter cells traffic and exocytose membrane at the cleavage furrow during mammalian cytokinesis

Membrane trafficking during cytokinesis is not well understood. We used advanced live cell imaging techniques to track exocytosis of single vesicles to determine whether constitutively exocytosed membrane is focally delivered to the cleavage furrow. Ultrasensitive three-dimensional confocal time-lapse imaging of the temperature-sensitive membrane cargo protein vesicular stomatitis virus protein–yellow fluorescent protein revealed that vesicles from both daughter cells traffic out of the Golgi and into the furrow, following curvilinear paths. Immunolocalization and photobleaching experiments indicate that individual vesicles accumulate at the midbody and generate a reserve vesicle pool that is distinct from endosomal and lysosomal compartments. Total internal reflection fluorescence microscopy imaging provided direct evidence that Golgi-derived vesicles from both daughter cells not only traffic to the furrow region but dock and fuse there, supporting a symmetrically polarized exocytic delivery model. In contrast, quantitative analysis of midbody abscission showed inheritance of the midbody remnant by one daughter cell, indicating that cytokinesis is composed of both symmetrical and asymmetrical stages.

Breaking up is hard to do - membrane traffic in cytokinesis

Throughout normal development, and in aberrant conditions such as cancer, cells divide by a process called cytokinesis. Most textbooks suggest that animal cells execute cytokinesis using an actomyosin-containing contractile ring, whereas plant cells generate a new cell wall by the assembly of a novel membrane compartment using vesicle-trafficking machinery in an apparently distinct manner. Recent studies have shown that cytokinesis in animal and plant cells may not be as distinct as these models imply - both have an absolute requirement for vesicle traffic. Moreover, some of the key molecular components of cytokinesis have been identified, many of which are proteins that function to control membrane traffic. Here, we review recent advances in this area.

Dynamics of Arabidopsis dynamin-related protein 1C and a clathrin light chain at the plasma membrane

Plant morphogenesis depends on polarized exocytic and endocytic membrane trafficking. Members of the Arabidopsis thaliana dynamin-related protein 1 (DRP1) subfamily are required for polarized cell expansion and cytokinesis. Using a combination of live-cell imaging techniques, we show that a functional DRP1C green fluorescent fusion protein (DRP1C-GFP) was localized at the division plane in dividing cells and to the plasma membrane in expanding interphase cells. In both tip growing root hairs and diffuse-polar expanding epidermal cells, DRP1C-GFP organized into dynamic foci at the cell cortex, which colocalized with a clathrin light chain fluorescent fusion protein (CLC-FFP), suggesting that DRP1C may participate in clathrin-mediated membrane dynamics. DRP1C-GFP and CLC-GFP foci dynamics are dependent on cytoskeleton organization, cytoplasmic streaming, and functional clathrin-mediated endocytic traffic. Our studies provide insight into DRP1 and clathrin dynamics in the plant cell cortex and indicate that the clathrin endocytic machinery in plants has both similarities and striking differences to that in mammalian cells and yeast.

The endocytic adaptor protein ARH associates with motor and centrosomal proteins and is involved in centrosome assembly and cytokinesis

Numerous proteins involved in endocytosis at the plasma membrane have been shown to be present at novel intracellular locations and to have previously unrecognized functions. ARH (autosomal recessive hypercholesterolemia) is an endocytic clathrin-associated adaptor protein that sorts members of the LDL receptor superfamily (LDLR, megalin, LRP). We report here that ARH also associates with centrosomes in several cell types. ARH interacts with centrosomal (gamma-tubulin and GPC2 and GPC3) and motor (dynein heavy and intermediate chains) proteins. ARH cofractionates with gamma-tubulin on isolated centrosomes, and gamma-tubulin and ARH interact on isolated membrane vesicles. During mitosis, ARH sequentially localizes to the nuclear membrane, kinetochores, spindle poles and the midbody. Arh(-/-) embryonic fibroblasts (MEFs) show smaller or absent centrosomes suggesting ARH plays a role in centrosome assembly. Rat-1 fibroblasts depleted of ARH by siRNA and Arh(-/-) MEFs exhibit a slower rate of growth and prolonged cytokinesis. Taken together the data suggest that the defects in centrosome assembly in ARH depleted cells may give rise to cell cycle and mitotic/cytokinesis defects. We propose that ARH participates in centrosomal and mitotic dynamics by interacting with centrosomal proteins. Whether the centrosomal and mitotic functions of ARH are related to its endocytic role remains to be established.

Two independent forms of endocytosis maintain embryonic cell surface homeostasis during early development

Eukaryotic cells have multiple forms of endocytosis which maintain cell surface homeostasis. One explanation for this apparent redundancy is to allow independent retrieval of surface membranes derived from different types of vesicles. Consistent with this hypothesis we find that sea urchin eggs have at least two types of compensatory endocytosis. One is associated with retrieving cortical vesicle membranes, and formed large endosomes by a mechanism that was inhibited by agatoxin, cadmium, staurosporine and FK506. The second type is thought to compensate for constitutive exocytosis, and formed small endosomes using a mechanism that was insensitive to the above mentioned reagents, but was inhibited by phenylarsine oxide (PAO), and by microinjection of mRNA encoding Src kinase. Both mechanisms could act concurrently, and account for all of the endocytosis occurring during early development. Inhibition of either form did not trigger compensation by the other form, and phorbol ester treatment rescued the endocytotic activity blocked by agatoxin, but not the retrieval blocked by PAO.

Cortical division zone establishment in plant cells

Plant cell division is spatially organized to maintain a critical cell volume and to control growth directionality. The correct orientation of the separating cell wall is secured by means of specialized cytoskeletal structures that guide the newly formed cell plate toward a predefined cortical position. A ring of microtubules called preprophase band defines a cortical zone that corresponds to the future division plane. Coincident with the disappearance of the preprophase band microtubules, cortical actin is removed at the corresponding position, leaving an actin-depleted zone that persists throughout mitosis. Here, we review the spatial and structural organization of the cortical division zone and discuss evidence that implicate the plasma membrane in division plane establishment.

The Cbl-interacting protein TULA inhibits dynamin-dependent endocytosis

The Cbl- and ubiquitin-interacting protein T-cell ubiquitin ligand (TULA) has been demonstrated to inhibit endocytosis and downregulation of ligand-activated EGF receptor (EGFR) by impairing Cbl-induced ubiquitination. We presently report that TULA additionally inhibited clathrin-dependent endocytosis in general, as both uptake of transferrin (Tf) and low-density lipoprotein (LDL) was inhibited. Additionally, endocytosis of the raft proteins CD59 and major histocompatibility complex class I (MHC-I), which we demonstrate were mainly endocytosed clathrin-independently, but dynamin-dependently, was blocked in cells overexpressing TULA. By contrast, the uptake of ricin, which is mainly endocytosed clathrin- and dynamin-independently, was not affected by overexpressed TULA. Consistently, TULA and dynamin co-immunoprecipitated and colocalized intracellularly, and upon overexpression of dynamin the TULA-mediated inhibitory effect on endocytosis of Tf, LDL, CD59 and MHC-I was counteracted. Overexpressed dynamin did not restore ubiquitination of the EGFR, and consistently dynamin did not rescue endocytosis of the EGFR in cells overexpressing TULA. We conclude that TULA inhibits both clathrin-dependent and clathrin-independent endocytic pathways by functionally sequestering dynamin via the SH3 domain of TULA binding proline-rich sequences in dynamin.

Proteomic analysis of spinal protein expression in rats exposed to repeated intrathecal morphine injection

Repeated administration of morphine for treating severe chronic pain may lead to neuroadaptive changes in the spinal cord that are thought to underlie molecular mechanisms of the development of morphine tolerance and physical dependence. Here, we employed a 2-D gel-based proteomic technique to detect the global changes of the spinal cord protein expression in rats that had developed morphine tolerance. Morphine tolerance at the spinal cord level was induced by repeated intrathecal injections of morphine (20 microg/10 microL) twice daily for 5 days and evaluated by measurements of paw withdrawal latencies and maximal possible analgesic effect at day 5. After behavioral tests, the lumbar enlargement segments of spinal cord were harvested and proteins resolved by 2-DE. We found that eight proteins were significantly up-regulated or down-regulated in spinal cord after morphine tolerance development, including proteins involved in targeting and trafficking of the glutamate receptors and opioid receptors, proteins involved in oxidative stress, and cytoskeletal proteins, some of which were confirmed by Western blot analysis. Morphine-induced expressional changes of these proteins in the spinal cord might be involved in the central mechanisms that underlie the development of morphine tolerance. It is very likely that these identified proteins may serve as potential molecular targets for prevention of the development of morphine tolerance and physical dependence.

Vesicle trafficking in plant immune responses

In plants, perception of pathogen-associated molecular patterns at the surface is the first line of defence in cellular immunity. This review summarizes recent evidence of the involvement of vesicle trafficking in the plant's immune response against pathogens. I first discuss aspects of ligand-stimulated receptor endocytosis. The best-characterized pattern-recognition receptor (PRR), FLS2, is a transmembrane leucine-rich repeat receptor kinase that recognizes bacterial flagellin. FLS2 was recently shown to undergo internalization upon activation with its cognate ligand. An animal PRR, TLR4 that mediates perception of bacterial-derived lipopolysaccharides, similarly exhibits ligand-stimulated endocytosis. The second focus is N-ethylmaleimide-sensitive factor adaptor protein receptor (SNARE)-mediated immunity involving syntaxins and their cognate partners. One of the genes involved in basal immunity in Arabidopsis, PEN1, encodes a syntaxin that focally accumulates at fungal penetration sites, raising the possibility that induced exocytosis is important for active defence. Pathogen-triggered endocytic and exocytic processes have to be balanced to ensure host cell homeostasis. Thus, understanding how phytopathogens have evolved strategies to exploit host cell vesicle trafficking to manipulate immune responses is currently an area of intense study.

Asymmetric Cell Division in Plant Development

Plant embryogenesis creates a seedling with a basic body plan. Post-embryonically the seedling elaborates with a lifelong ability to develop new tissues and organs. As a result asymmetric cell divisions serve essential roles during embryonic and postembryonic development to generate cell diversity. This review highlights selective cases of asymmetric division in the model plant Arabidopsis thaliana and describes the current knowledge on fate determinants and mechanisms involved. Common themes that emerge are: 1. role of the plant hormone auxin and its polar transport machinery; 2. a MAP kinase signaling cascade and; 3. asymmetric segregating transcription factors that are involved in several asymmetric cell divisions.

Domain requirements for an endocytosis-independent, isoform-specific function of dynamin-2

Endocytosis is inhibited by overexpression of either dynamin-1 or dynamin-2 mutants because both isoforms form heterotetramers with endogenous dynamin-2 and interfere with its function. By contrast, other phenotypes, which are specifically triggered by overexpression of dynamin-2, but not dynamin-1 are likely to reflect endocytosis-independent, dynamin-2-specific functions and/or interactions. Using Dyn2/Dyn1 chimeras, we explored the structural requirements for a readily quantifiable, isoform-specific function of dynamin-2, the activation of caspase-3 to trigger apoptosis. Strikingly, swapping the highly homologous GTPase domain of dynamin-2 into dynamin-1 was sufficient to confer caspase-3 activation. Moreover, assembly-defective mutations in GED, dynamin's GAP/assembly domain, that inhibit endocytosis enhance caspase-3 activation. Thus, this dynamin-2-specific function is mechanistically distinct from and independent of its role in endocytosis. These findings have important implications for interpreting dynamin-2 dependent phenotypes in overexpression studies.

Cytokinesis in plant and animal cells: endosomes 'shut the door'

For many years, cytokinesis in eukaryotic cells was considered to be a process that took a variety of forms. This is rather surprising in the face of an apparently conservative mitosis. Animal cytokinesis was described as a process based on an actomyosin-based contractile ring, assembling, and acting at the cell periphery. In contrast, cytokinesis of plant cells was viewed as the centrifugal generation of a new cell wall by fusion of Golgi apparatus-derived vesicles. However, recent advances in animal and plant cell biology have revealed that many features formerly considered as plant-specific are, in fact, valid also for cytokinetic animal cells. For example, vesicular trafficking has turned out to be important not only for plant but also for animal cytokinesis. Moreover, the terminal phase of animal cytokinesis based on midbody microtubule activity resembles plant cytokinesis in that interdigitating microtubules play a decisive role in the recruitment of cytokinetic vesicles and directing them towards the cytokinetic spaces which need to be plugged by fusing endosomes. Presently, we are approaching another turning point which brings cytokinesis in plant and animal cells even closer. As an unexpected twist, new studies reveal that both plant and animal cytokinesis is driven not so much by Golgi-derived vesicles but rather by homotypically and heterotypically fusing endosomes. These are generated from cytokinetic cortical sites defined by preprophase microtubules and contractile actomyosin ring, which induce local endocytosis of both the plasma membrane and cell wall material. Finally, plant and animal cytokinesis meet together at the physical separation of daughter cells despite obvious differences in their preparatory events.