The Golgi stack reassembles during telophase before arrival of proteins transported from the endoplasmic reticulum

Golgin-84-associated Golgi fragmentation triggers tau hyperphosphorylation by activation of cyclin-dependent kinase-5 and extracellular signal-regulated kinase

Tau hyperphosphorylation is a critical event in Alzheimer's disease, in which the neuronal Golgi fragmentation occurs earlier than tau hyperphosphorylation. However, the intrinsic link between Golgi impairment and tau pathology is missing. By electron microscopy and western blotting, we observed in the present study that the neuronal Golgi fragmentation was increased age-dependently with a correlated tau hyperphosphorylation in the brains of C57BL/6 mice aged from 4 to 16 months. Simultaneously, golgin-84 and Golgi reassembly stacking protein 65, 2 important Golgi matrix proteins, were decreased in the brains of elder mice. Further studies in HEK293/tau cells showed that Golgi-disturbing agents, brefeldin A and nocodazole induced tau hyperphosphorylation. Knockdown of golgin-84, not Golgi reassembly stacking protein 65, by small interfering RNA was sufficient to induce tau hyperphosphorylation, while over-expressing golgin-84 arrested the brefeldin A-induced Golgi fragmentation and tau hyperphosphorylation. Finally, we demonstrated that cyclin-dependent kinase-5 and extracellular signal-regulated kinase were activated after golgin-84 knockdown, and simultaneous inhibition of these kinases abolished the golgin-84 deficit-induced tau hyperphosphorylation. These data suggest Golgi fragmentation could be an upstream event triggering tau hyperphosphorylation through golgin-84 deficit-induced activation of cyclin-dependent kinase-5 and extracellular signal-regulated kinase.

Multi-step down-regulation of the secretory pathway in mitosis: a fresh perspective on protein trafficking

The secretory pathway delivers proteins synthesized at the rough endoplasmic reticulum (RER) to various subcellular locations via the Golgi apparatus. Currently, efforts are focused on understanding the molecular machineries driving individual processes at the RER and Golgi that package, modify and transport proteins. However, studies are routinely performed using non-dividing cells. This obscures the critical issue of how the secretory pathway is affected by cell division. Indeed, several studies have indicated that protein trafficking is down-regulated during mitosis. Moreover, the RER and Golgi apparatus exhibit gross reorganization in mitosis. Here I provide a relatively neglected perspective of how the mitotic cyclin-dependent kinase (CDK1) could regulate various stages of the secretory pathway. I highlight several aspects of the mitotic control of protein trafficking that remain unresolved and suggest that further studies on how the mitotic CDK1 influences the secretory pathway are necessary to obtain a deeper understanding of protein transport.

Sec16A defines the site for vesicle budding from the endoplasmic reticulum on exit from mitosis

Mitotic inhibition of COPII-dependent export of proteins from the endoplasmic reticulum results in disassembly of the Golgi complex. This ensures ordered inheritance of organelles by the two daughter cells. Reassembly of the Golgi is intimately linked to the re-initiation of ER export on exit from mitosis. Here, we show that unlike all other COPII components, which are cytosolic during metaphase, Sec16A remains associated with ER exit sites throughout mitosis, and thereby could provide a template for the rapid assembly of functional export domains in anaphase. Full assembly of COPII at exit sites precedes reassembly of the Golgi in telophase.

Isolation of a point-mutated p47 lacking binding affinity to p97ATPase

p47, a p97-binding protein, functions in Golgi membrane fusion together with p97 and VCIP135, another p97-binding protein. We have succeeded in creating p47 with a point mutation, F253S, which lacks p97-binding affinity. p47 mapping experiments revealed that p47 had two p97-binding regions and the F253S mutation occurred in the first p97-binding site. p47(F253S) could not form a complex with p97 and did not caused any cisternal regrowth in an in vitro Golgi reassembly assay. In addition, mutation corresponding to the p47 F253S mutation in p37 and ufd1 also abolished their binding ability to p97.

Phosphorylation and membrane dissociation of the ARF exchange factor GBF1 in mitosis

Secretory protein trafficking is arrested and the Golgi apparatus fragmented when mammalian cells enter mitosis. These changes are thought to facilitate cell-cycle progression and Golgi inheritance, and are brought about through the actions of mitotically active protein kinases. To better understand how the Golgi apparatus undergoes mitotic fragmentation we have sought to identify novel Golgi targets for mitotic kinases. We report in the present paper the identification of the ARF (ADP-ribosylation factor) exchange factor GBF1 (Golgi-specific brefeldin A-resistant guanine nucleotide-exchange factor 1) as a Golgi phosphoprotein. GBF1 is phosphorylated by CDK1 (cyclin-dependent kinase 1)-cyclin B in mitosis, which results in its dissociation from Golgi membranes. Consistent with a reduced level of GBF1 activity at the Golgi membrane there is a reduction in levels of membrane-associated GTP-bound ARF in mitotic cells. Despite the reduced levels of membrane-bound GBF1 and ARF, COPI (coat protein I) binding to the Golgi membrane appears unaffected in mitotic cells. Surprisingly, this pool of COPI is dependent upon GBF1 for its recruitment to the membrane, suggesting that a low level of GBF1 activity persists in mitosis. We propose that the phosphorylation and membrane dissociation of GBF1 and the consequent reduction in ARF-GTP levels in mitosis are important for changes in Golgi dynamics and possibly other mitotic events mediated through effectors other than the COPI vesicle coat.

Endosomal recycling controls plasma membrane area during mitosis

The shape and total surface of a cell and its daughters change during mitosis. Many cells round up during prophase and metaphase and reacquire their extended and flattened shape during cytokinesis. How does the total area of plasma membrane change to accommodate these morphological changes and by what mechanism is control of total membrane area achieved? Using single-cell imaging methods, we have found that the amount of plasma membrane in attached cells in culture decreases at the beginning of mitosis and recovers rapidly by the end. Clathrin-based endocytosis is normal throughout all phases of cell division, whereas recycling of internalized membranes back to the cell surface slows considerably during the rounding up period and resumes at the time at which recovery of cell membrane begins. Interference with either one of these processes by genetic or chemical means impairs cell division. The total cell-membrane area recovers even in the absence of a functional Golgi apparatus, which would be needed for export of newly synthesized membrane lipids and proteins. We propose a mechanism by which modulation of endosomal recycling controls cell area and surface expression of membrane-bound proteins during cell division.

Capacity of the Golgi apparatus for cargo transport prior to complete assembly

In yeast, particular emphasis has been given to endoplasmic reticulum (ER)-derived, cisternal maturation models of Golgi assembly while in mammalian cells more emphasis has been given to golgins as a potentially stable assembly framework. In the case of de novo Golgi formation from the ER after brefeldin A/H89 washout in HeLa cells, we found that scattered, golgin-enriched, structures formed early and contained golgins including giantin, ranging across the entire cis to trans spectrum of the Golgi apparatus. These structures were incompetent in VSV-G cargo transport. Second, we compared Golgi competence in cargo transport to the kinetics of addition of various glycosyltransferases and glycosidases into nascent, golgin-enriched structures after drug washout. Enzyme accumulation was sequential with trans and then medial glycosyltransferases/glycosidases found in the scattered, nascent Golgi. Involvement in cargo transport preceded full accumulation of enzymes or GPP130 into nascent Golgi. Third, during mitosis, we found that the formation of a golgin-positive acceptor compartment in early telophase preceded the accumulation of a Golgi glycosyltransferase in nascent Golgi structures. We conclude that during mammalian Golgi assembly components fit into a dynamic, first-formed, multigolgin-enriched framework that is initially cargo transport incompetent. Resumption of cargo transport precedes full Golgi assembly.

Mapping the functional domains of the Golgi stacking factor GRASP65

The Golgi reassembly stacking protein (GRASP) family has been implicated in the stacking of Golgi cisternae and the regulation of Golgi disassembly/reassembly during mitosis in mammalian cells. GRASP65 is a dimer that can directly link adjacent surfaces through trans-oligomerization in a mitotically regulated manner. Here we show that the N-terminal GRASP domain (amino acids 1-201) is both necessary and sufficient for dimerization and trans-oligomerization but is not mitotically regulated. The C-terminal serine/proline-rich domain (amino acids 202-446) cannot dimerize nor can it link adjacent surfaces. It does, however, confer mitotic regulation on the GRASP domain through multiple sites phosphorylated by the mitotic kinases, cdc2/B1, and the polo-like kinase. Transient expression corroborated these results by showing that the GRASP domain alone inhibited mitotic fragmentation of the Golgi apparatus.

Secretory trafficking in neuronal dendrites

The neuronal secretory pathway represents the intracellular route for proteins involved in synaptic transmission and plasticity, as well as lipids required for outgrowth and remodelling of dendrites and axons. Although neurons use the same secretory compartments as other eukaryotic cells, the enormous distances involved, as well as the unique morphology of the neuron and its signalling requirements, challenge canonical models of secretory pathway organization. Here, we review evidence for a distributed secretory pathway in neurons, suggest mechanisms that may regulate secretory compartment distribution, and discuss the implications of a distributed secretory pathway for neuronal morphogenesis and neural-circuit plasticity.

Golgi Inheritance Under a Block of Anterograde and Retrograde Traffic

In mitosis, the Golgi complex is inherited following its dispersion, equal partitioning and reformation in each daughter cell. The state of Golgi membranes during mitosis is controversial, and the role of Golgi-intersecting traffic in Golgi inheritance is unclear. We have used brefeldin A (BFA) to perturb Golgi-intersecting membrane traffic at different stages of the cell cycle and followed by live cell imaging the fate of Golgi membranes in those conditions. We observed that addition of the drug on cells in prometaphase prevents mitotic Golgi dispersion. Under continuous treatment, Golgi fragments persist throughout mitosis and accumulate in a Golgi-like structure at the end of mitosis. This structure localizes at microtubule minus ends and contains all classes of Golgi markers, but is not accessible to cargo from the endoplasmic reticulum or the plasma membrane because of the continuous BFA traffic block. However, it contains preaccumulated cargo, and intermixes with the reforming Golgi upon BFA washout. This structure also forms when BFA is added during metaphase, when the Golgi is not discernible by light microscopy. Together the data indicate that independent Golgi fragments that contain all classes of Golgi markers (and that can be isolated from other organelles by blocking anterograde and retrograde Golgi-intersecting traffic) persist throughout mitosis.

A direct role for GRASP65 as a mitotically regulated Golgi stacking factor

Cell-free assays that mimic the disassembly and reassembly cycle of the Golgi apparatus during mitosis implicated GRASP65 as a mitotically regulated stacking factor. We now present evidence that GRASP65 is directly involved in stacking Golgi cisternae. GRASP65 is the major phosphorylation target in rat liver Golgi membranes of two mitotic kinases, cdc2-cyclin B and polo-like kinases, which alone will unstack Golgi membranes, generating single cisternae. Mitotic cells microinjected with antibodies to GRASP65 fail to form proper Golgi stacks after cell division. Beads coated with GRASP65 homodimers form extensive aggregates consistent with the formation of trans oligomers. These can be disaggregated using purified cdc2-cyclin B1 and polo-like kinases, and re-aggregated after dephosphorylation of GRASP65. Together, these data demonstrate that GRASP65 has the properties required to bind surfaces together in a mitotically regulated manner.

Golgi architecture and inheritance

Golgi inheritance proceeds via sequential biogenesis and partitioning phases. Although little is known about Golgi growth and replication (biogenesis), ultrastructural and fluorescence analyses have provided a detailed, though still controversial, perspective of Golgi partitioning during mitosis in mammalian cells. Partitioning requires the fragmentation of the juxtanuclear ribbon of interconnected Golgi stacks into a multitude of tubulovesicular clusters. This process is choreographed by a cohort of mitotic kinases and an inhibition of heterotypic and homotypic Golgi membrane-fusion events. Our model posits that accurate partitioning occurs early in mitosis by the equilibration of Golgi components on either side of the metaphase plate. Disseminated Golgi components then coalesce to regenerate Golgi stacks during telophase. Semi-intact cell and cell-free assays have accurately recreated these processes and allowed their molecular dissection. This review attempts to integrate recent findings to depict a more coherent, synthetic molecular picture of mitotic Golgi fragmentation and reassembly. Of particular importance is the emerging concept of a highly regulated and dynamic Golgi structural matrix or template that interfaces with cargo receptors, Golgi enzymes, Rab-GTPases, and SNAREs to tightly couple biosynthetic transport to Golgi architecture. This structural framework may be instructive for Golgi biogenesis and may encode sufficient information to ensure accurate Golgi inheritance, thereby helping to resolve some of the current discrepancies between different workers.

Nir2, a human homolog of Drosophila melanogaster retinal degeneration B protein, is essential for cytokinesis

Cytokinesis, the final stage of eukaryotic cell division, ensures the production of two daughter cells. It requires fine coordination between the plasma membrane and cytoskeletal networks, and it is known to be regulated by several intracellular proteins, including the small GTPase Rho and its effectors. In this study we provide evidence that the protein Nir2 is essential for cytokinesis. Microinjection of anti-Nir2 antibodies into interphase cells blocks cytokinesis, as it results in the production of multinucleate cells. Immunolocalization studies revealed that Nir2 is mainly localized in the Golgi apparatus in interphase cells, but it is recruited to the cleavage furrow and the midbody during cytokinesis. Nir2 colocalizes with the small GTPase RhoA in the cleavage furrow and the midbody, and it associates with RhoA in mitotic cells. Its N-terminal region, which contains a phosphatidylinositol transfer domain and a novel Rho-inhibitory domain (Rid), is required for normal cytokinesis, as overexpression of an N-terminal-truncated mutant blocks cytokinesis completion. Time-lapse videomicroscopy revealed that this mutant normally initiates cytokinesis but fails to complete it, due to cleavage furrow regression, while Rid markedly affects cytokinesis due to abnormal contractility. Rid-expressing cells exhibit aberrant ingression and ectopic cleavage sites; the cells fail to segregate into daughter cells and they form a long unseparated bridge-like cytoplasmic structure. These results provide new insight into the cellular functions of Nir2 and introduce it as a novel regulator of cytokinesis.

Deconstructing Golgi inheritance

Eukaryotic cells use a variety of strategies to inherit the Golgi apparatus. During vertebrate mitosis, the Golgi reorganizes dramatically in a process that seems to be driven by the reversible fragmentation of existing Golgi structures and the temporary redistribution of Golgi components to the endoplasmic reticulum. Several proteins that participate in vertebrate Golgi inheritance have been identified, but their detailed functions remain unknown. A comparison between vertebrates and other eukaryotes reveals common mechanisms of Golgi inheritance. In many cell types, Golgi stacks undergo fission early in mitosis. Some cells exhibit a further Golgi breakdown that is probably due to a mitotic inhibition of membrane traffic. In all eukaryotes examined, Golgi inheritance involves either the partitioning of pre-existing Golgi elements between the daughter cells or the emergence of new Golgi structures from the endoplasmic reticulum, or some combination of these two pathways.

Evidence for prebudding arrest of ER export in animal cell mitosis and its role in generating Golgi partitioning intermediates

During mitosis the interconnected Golgi complex of animal cells breaks down to produce both finely dispersed elements and discrete vesiculotubular structures. The endoplasmic reticulum (ER) plays a controversial role in generating these partitioning intermediates and here we highlight the importance of mitotic ER export arrest in this process. We show that experimental inhibition of ER export (by microinjecting dominant negative Sar1 mutant proteins) is sufficient to induce and maintain transformation of Golgi cisternae to vesiculotubular remnants during interphase and telophase, respectively. We also show that buds on the ER, ER exit sites and COPII vesicles are markedly depleted in mitotic cells and COPII components Sec23p, Sec24p, Sec13p and Sec31p redistribute into the cytosol, indicating ER export is inhibited at an early stage. Finally, we find a markedly uneven distribution of Golgi residents over residual exit sites of metaphase cells, consistent with tubulovesicular Golgi remnants arising by fragmentation rather than redistribution via the ER. Together, these results suggest selective recycling of Golgi residents, combined with prebudding cessation of ER export, induces transformation of Golgi cisternae to vesiculotubular remnants in mitotic cells. The vesiculotubular Golgi remnants, containing populations of slow or nonrecycling Golgi components, arise by fragmentation of a depleted Golgi ribbon independently from the ER.

Golgi clusters and vesicles mediate mitotic inheritance independently of the endoplasmic reticulum

We have examined the fate of Golgi membranes during mitotic inheritance in animal cells using four-dimensional fluorescence microscopy, serial section reconstruction of electron micrographs, and peroxidase cytochemistry to track the fate of a Golgi enzyme fused to horseradish peroxidase. All three approaches show that partitioning of Golgi membranes is mediated by Golgi clusters that persist throughout mitosis, together with shed vesicles that are often found associated with spindle microtubules. We have been unable to find evidence that Golgi membranes fuse during the later phases of mitosis with the endoplasmic reticulum (ER) as a strategy for Golgi partitioning (Zaal, K.J., C.L. Smith, R.S. Polishchuk, N. Altan, N.B. Cole, J. Ellenberg, K. Hirschberg, J.F. Presley, T.H. Roberts, E. Siggia, et al. 1999. Cell. 99:589-601) and suggest that these results, in part, are the consequence of slow or abortive folding of GFP-Golgi chimeras in the ER. Furthermore, we show that accurate partitioning is accomplished early in mitosis, by a process of cytoplasmic redistribution of Golgi fragments and vesicles yielding a balance of Golgi membranes on either side of the metaphase plate before cell division.

The mitotic phosphorylation cycle of the cis-Golgi matrix protein GM130

The cis-Golgi matrix protein GM130 is phosphorylated in mitosis on serine 25. Phosphorylation inhibits binding to p115, a vesicle-tethering protein, and has been implicated as an important step in the mitotic Golgi fragmentation process. We have generated an antibody that specifically recognizes GM130 phosphorylated on serine 25, and used this antibody to study the temporal regulation of phosphorylation in vivo. GM130 is phosphorylated in prophase as the Golgi complex starts to break down, and remains phosphorylated during further breakdown and partitioning of the Golgi fragments in metaphase and anaphase. In telophase, GM130 is dephosphorylated as the Golgi fragments start to reassemble. The timing of phosphorylation and dephosphorylation correlates with the dissociation and reassociation of p115 with Golgi membranes. GM130 phosphorylation and p115 dissociation appear specific to mitosis, since they are not induced by several drugs that trigger nonmitotic Golgi fragmentation. The phosphatase responsible for dephosphorylation of mitotic GM130 was identified as PP2A. The active species was identified as heterotrimeric phosphatase containing the Balpha regulatory subunit, suggesting a role for this isoform in the reassembly of mitotic Golgi membranes at the end of mitosis.

Golgi membranes are absorbed into and reemerge from the ER during mitosis

Quantitative imaging and photobleaching were used to measure ER/Golgi recycling of GFP-tagged Golgi proteins in interphase cells and to monitor the dissolution and reformation of the Golgi during mitosis. In interphase, recycling occurred every 1.5 hr, and blocking ER egress trapped cycling Golgi enzymes in the ER with loss of Golgi structure. In mitosis, when ER export stops, Golgi proteins redistributed into the ER as shown by quantitative imaging in vivo and immuno-EM. Comparison of the mobilities of Golgi proteins and lipids ruled out the persistence of a separate mitotic Golgi vesicle population and supported the idea that all Golgi components are absorbed into the ER. Moreover, reassembly of the Golgi complex after mitosis failed to occur when ER export was blocked. These results demonstrate that in mitosis the Golgi disperses and reforms through the intermediary of the ER, exploiting constitutive recycling pathways. They thus define a novel paradigm for Golgi genesis and inheritance.

Role of microtubules in the organization of the Golgi complex

The Golgi complex of mammalian cells is composed of cisternal stacks that function in processing and sorting of membrane and luminal proteins during transport from the site of synthesis in the endoplasmic reticulum to lysosomes, secretory vacuoles, and the cell surface. Even though exceptions are found, the Golgi stacks are usually arranged as an interconnected network in the region around the centrosome, the major organizing center for cytoplasmic microtubules. A close relation thus exists between Golgi elements and microtubules (especially the stable subpopulation enriched in detyrosinated and acetylated tubulin). After drug-induced disruption of microtubules, the Golgi stacks are disconnected from each other, partly broken up, dispersed in the cytoplasm, and redistributed to endoplasmic reticulum exit sites. Despite this, intracellular protein traffic is only moderately disturbed. Following removal of the drugs, scattered Golgi elements move along reassembling microtubules back to the centrosomal region and reunite into a continuous system. The microtubule-dependent motor proteins cytoplasmic dynein and kinesin bind to Golgi membranes and have been implicated in vesicular transport to and from the Golgi complex. Microinjection of dynein heavy chain antibodies causes dispersal of the Golgi complex, and the Golgi complex of cells lacking cytoplasmic dynein is likewise spread throughout the cytoplasm. In a similar manner, kinesin antibodies have been found to inhibit Golgi-to-endoplasmic reticulum transport in brefeldin A-treated cells and scattering of Golgi elements along remaining microtubules in cells exposed to a low concentration of nocodazole. The molecular mechanisms in the interaction between microtubules and membranes are, however, incompletely understood. During mitosis, the Golgi complex is extensively reorganized in order to ensure an equal partitioning of this single-copy organelle between the daughter cells. Mitosis-promoting factor, a complex of cdc2 kinase and cyclin B, is a key regulator of this and other events in the induction of cell division. Cytoplasmic microtubules depolymerize in prophase and as a result thereof, the Golgi stacks become smaller, disengage from each other, and take up a perinuclear distribution. The mitotic spindle is thereafter put together, aligns the chromosomes in the metaphase plate, and eventually pulls the sister chromatids apart in anaphase. In parallel, the Golgi stacks are broken down into clusters of vesicles and tubules and movement of protein along the exocytic and endocytic pathways is inhibited. Using a cell-free system, it has been established that the fragmentation of the Golgi stacks is due to a continued budding of transport vesicles and a concomitant inhibition of the fusion of the vesicles with their target membranes. In telophase and after cytokinesis, a Golgi complex made up of interconnected cisternal stacks is recreated in each daughter cell and intracellular protein traffic is resumed. This restoration of a normal interphase morphology and function is dependent on reassembly of a radiating array of cytoplasmic microtubules along which vesicles can be carried and on reactivation of the machinery for membrane fusion.

Inheritance of the mammalian Golgi apparatus during the cell cycle

The creation and propagation of the intricate Golgi architecture during the cell cycle poses a fascinating problem for biologists. Similar to the inheritance process for nuclear DNA, the inheritance of the Golgi apparatus consists of biogenesis (replication) and partitioning (mitosis/meiosis) phases, in which Golgi components must double in unit mass, then be appropriately divided between nascent daughter cells during cytokinesis. In this article we focus discussion on the recent advances in the area of Golgi inheritance, first outlining our current understanding of the behaviour of the Golgi apparatus during cell division, then concluding with a more conceptual discussion of the Golgi biogenesis problem. Throughout, we attempt to integrate ultrastructural and biochemical findings with more recent information obtained using live cell microscopy and morphological techniques.

Cell cycle regulation of organelle transport

Microtubule- and actin-based motors play a wide range of vital roles in the organisation and function of cells during both interphase and mitosis, all of which are likely to be under strict control. Here, we describe how one of these roles--the movement of membranes--is regulated through the cell cycle. Organelle movement in many species is greatly reduced in mitosis as compared to interphase, and this change occurs concomitantly with an inhibition of most membrane traffic functions. Data from in vitro studies is shedding light on how microtubule motor regulation may be achieved.

The mammalian Golgi apparatus during M-phase

The Golgi apparatus in mammalian cells disassembles into several thousand vesicles as cells enter M-phase. Disassembly is dependent on the action of cdc2-kinase and at least two pathways contribute to the fragmentation: One involves the budding of COP-coated vesicles from Golgi cisternae with concomitant inhibition of fusion with their target membranes, the other is a less well characterised COP-independent pathway. During telophase, the Golgi fragments reassemble and fuse into a fully functional Golgi stack, using at least two distinct fusion pathways. The morphological changes of the Golgi apparatus during M-phase offer an ideal system to study how cellular organelles are generated and how their structure is maintained during interphase.

Partitioning of cytoplasmic organelles during mitosis with special reference to the Golgi complex

During mitosis, not only the genetic material stored in the nucleus but also the constituents of the cytoplasm should be equally partitioned between the daughter cells. For this sake, the dividing cell goes through an extensive structural reorganization and transport along the endocytic and exocytic pathways is temporarily arrested. Early in prophase, the radiating array of cytoplasmic microtubules disassembles and the membrane systems of the secretory apparatus start to split up. In metaphase, the nuclear envelope fragments and the condensing chromosomes associate with the forming mitotic spindle. The cisternal and tubular elements of the endoplasmic reticulum and the Golgi complex break down into small vesicles, presumably as the result of an imbalance between vesicle budding and fusion. In anaphase, the two sets of chromosomes are pulled apart and a cleavage furrow forms halfway between the spindle poles. Since most organelles occur in multiple and widely dispersed copies at this stage, they will be evenly distributed between the daughter cells. During telophase and cytokinesis, the preceding fragmentation process is reversed. A nuclear envelope reappears around the chromosomes and cytoplasmic microtubules reassemble. The endoplasmic reticulum is rebuilt as a continuous system of flattened cisternae and tubules. Stacks of Golgi cisternae arise from small vesicles and are rearranged in an interconnected network. In parallel, the biosynthetic functions of the cell are normalized and intracellular membrane traffic is resumed.

Golgi division and membrane traffic

The Golgi complex has a distinctive morphology in mammalian cells, comprising a ribbon of closely apposed, stacked cisternae located adjacent to the nucleus and often the centrioles. Observations since the turn of the century have revealed dramatic changes in Golgi structure as cells undergo mitosis, and more recent microscopic analyses have confirmed that the Golgi ribbon in converted to clusters of vesicles and tubules dispersed throughout the mitotic cell. We have long been interested in this fragmentation since it offers a unique opportunity to study organelle division at the molecular level. Here, we describe the way in which our understanding has developed through another dramatic change to membrane function in mitosis, namely the inhibition of membrane traffic.

Role of NAD+ and ADP-ribosylation in the maintenance of the Golgi structure

We have investigated the role of the ADP- ribosylation induced by brefeldin A (BFA) in the mechanisms controlling the architecture of the Golgi complex. BFA causes the rapid disassembly of this organelle into a network of tubules, prevents the association of coatomer and other proteins to Golgi membranes, and stimulates the ADP-ribosylation of two cytosolic proteins of 38 and 50 kD (GAPDH and BARS-50; De Matteis, M.A., M. DiGirolamo, A. Colanzi, M. Pallas, G. Di Tullio, L.J. McDonald, J. Moss, G. Santini, S. Bannykh, D. Corda, and A. Luini. 1994. Proc. Natl. Acad. Sci. USA. 91:1114-1118; Di Girolamo, M., M.G. Silletta, M.A. De Matteis, A. Braca, A. Colanzi, D. Pawlak, M.M. Rasenick, A. Luini, and D. Corda. 1995. Proc. Natl. Acad. Sci. USA. 92:7065-7069). To study the role of ADP-ribosylation, this reaction was inhibited by depletion of NAD+ (the ADP-ribose donor) or by using selective pharmacological blockers in permeabilized cells. In NAD+-depleted cells and in the presence of dialized cytosol, BFA detached coat proteins from Golgi membranes with normal potency but failed to alter the organelle's structure. Readdition of NAD+ triggered Golgi disassembly by BFA. This effect of NAD+ was mimicked by the use of pre-ADP- ribosylated cytosol. The further addition of extracts enriched in native BARS-50 abolished the ability of ADP-ribosylated cytosol to support the effect of BFA. Pharmacological blockers of the BFA-dependent ADP-ribosylation (Weigert, R., A. Colanzi, A. Mironov, R. Buccione, C. Cericola, M.G. Sciulli, G. Santini, S. Flati, A. Fusella, J. Donaldson, M. DiGirolamo, D. Corda, M.A. De Matteis, and A. Luini. 1997. J. Biol. Chem. 272:14200-14207) prevented Golgi disassembly by BFA in permeabilized cells. These inhibitors became inactive in the presence of pre-ADP-ribosylated cytosol, and their activity was rescued by supplementing the cytosol with a native BARS-50-enriched fraction. These results indicate that ADP-ribosylation plays a role in the Golgi disassembling activity of BFA, and suggest that the ADP-ribosylated substrates are components of the machinery controlling the structure of the Golgi apparatus.

Partitioning of the Golgi apparatus during mitosis in living HeLa cells

The Golgi apparatus of HeLa cells was fluorescently tagged with a green fluorescent protein (GFP), localized by attachment to the NH2-terminal retention signal of N-acetylglucosaminyltransferase I (NAGT I). The location was confirmed by immunogold and immunofluorescence microscopy using a variety of Golgi markers. The behavior of the fluorescent Golgi marker was observed in fixed and living mitotic cells using confocal microscopy. By metaphase, cells contained a constant number of Golgi fragments dispersed throughout the cytoplasm. Conventional and cryoimmunoelectron microscopy showed that the NAGT I-GFP chimera (NAGFP)-positive fragments were tubulo-vesicular mitotic Golgi clusters. Mitotic conversion of Golgi stacks into mitotic clusters had surprisingly little effect on the polarity of Golgi membrane markers at the level of fluorescence microscopy. In living cells, there was little self-directed movement of the clusters in the period from metaphase to early telophase. In late telophase, the Golgi ribbon began to be reformed by a dynamic process of congregation and tubulation of the newly inherited Golgi fragments. The accuracy of partitioning the NAGFP-tagged Golgi was found to exceed that expected for a stochastic partitioning process. The results provide direct evidence for mitotic clusters as the unit of partitioning and suggest that precise regulation of the number, position, and compartmentation of mitotic membranes is a critical feature for the ordered inheritance of the Golgi apparatus.

Binding of the vesicle docking protein p115 to Golgi membranes is inhibited under mitotic conditions

The vesicle docking protein p115 showed saturable, high affinity binding to interphase Golgi membranes. The affinity of binding was up to 20-fold lower using membranes preincubated with mitotic cytosol. In contrast, binding was not affected by mitotic pretreatment of p115. The reduction in p115 binding was mediated by phosphorylation, could be induced by a cyclin-dependent kinase, and was fully reversible. A shift of p115 from membranes to cytosol was also found after fractionating mitotic cells. The functional significance of the decreased binding was addressed by in vitro mitotic incubations which disassemble Golgi cisternae, predominantly producing transport vesicles. The addition of excess p115 decreased loss of membrane from cisternae, indicating that p115's action is limiting while transport vesicles accumulate. The cessation of intra-Golgi traffic in mitosis has been hypothesized to result from an inhibition of membrane fusion while budding of transport vesicles continues. This process also contributes to mitotic Golgi disassembly. Our results imply that there is a mitotic modification to Golgi membranes leading to a reduction in the affinity of the p115 receptor. Reduced p115 binding may play a part in the inhibition of membrane fusion by preventing prior vesicle docking.

The role of the membrane-spanning domain and stalk region of N-acetylglucosaminyltransferase I in retention, kin recognition and structural maintenance of the Golgi apparatus in HeLa cells

Using a series of chimeric and truncated N-acetylglucosaminyltransferase I (NAGT I) molecules we have shown that part of the lumenal stalk region is both necessary and sufficient for kin recognition of mannosidase II and retention in the Golgi stack. The membrane-spanning domain was not required for retention, but replacing part or all of this domain with leucine residues did have a dramatic effect on Golgi morphology. In stable cell lines, stacked cisternae were replaced by tubulo-vesicular clusters containing the mutated NAGT I. The loss of stacked cisternae was proportional to the number of leucines used to replace the membrane-spanning domain.

Molecular mechanisms in the disassembly and reassembly of the mammalian Golgi apparatus during M-phase

The mitotic disassembly and reassembly of the mammalian Golgi apparatus is an ideal system to study the molecular mechanisms involved in biogenesis and maintenance of membranous organelles. As cells enter M-phase, Golgi stacks are converted into Golgi clusters of small membrane fragments, which are dispersed throughout the cytoplasmic space during metaphase. Disassembly is dependent on the action of cdc2-kinase and at least two distinct pathways contribute to the fragmentation: one involves the budding of COP I-coated vesicles from Golgi cisternae, the other is a less well characterised COP I-independent pathway. During telophase, the Golgi fragments reassemble and fuse into a fully functional Golgi stack, using at least two distinct ATPase-mediated fusion pathways.

Intracellular membrane morphology

The membrane-bound organelles on the exocytic and endocytic pathways are linked by vesicles which bud from one membrane compartment, carrying selected cargo, and fuse with the next on the pathway. The principles underlying this vesicle-mediated traffic go a long way towards explaining the morphology of these intracellular organelles and their behaviour during mitosis.

An NSF-like ATPase, p97, and NSF mediate cisternal regrowth from mitotic Golgi fragments

Golgi cisternae regrew in a cell-free system from mitotic Golgi fragments incubated with buffer alone. Pretreatment with NEM or salt washing inhibited regrowth, but this could be restored either by p97, an NSF-like ATPase, or by NSF together with SNAPs and p115, a vesicle docking protein. The morphology of cisternae regrown with p97 and NSF-SNAPs-p115 differed, suggesting that they play distinct roles in rebuilding Golgi cisternae after mitosis.

Mitotic disassembly of the Golgi apparatus in vivo

Populations enriched in prophase cells were obtained either by using a cell line with a temperature-sensitive mutation in the mitotic kinase, p34cdc2, or by treating cells with olomoucine, an inhibitor of this kinase. Both methods resulted in efficient and reversible block of the cells at the G2/M boundary. After cells were released from the cell cycle block, the morphological changes to the Golgi apparatus were characterised using both quantitative conventional electron microscopy and immuno-gold microscopy. The early mitotic phases were divided into six stages (G2 to pro-metaphase) based on the morphology of the nucleus. During prophase the cross-sectional length of Golgi stacks decreased prior to unstacking. At the same time, small vesicular profiles, typically 50–70 nm in diameter, accumulated in the vicinity of the stacks. The disappearance of Golgi stacks was accompanied by the transient appearance of tubular networks. By the time cells entered prometaphase, the stacks had completely disassembled and only clusters consisting of Golgi vesicles and short tubular elements were left. When cells were released from the G2/M boundary and pulsed briefly with [AlF4]- to prevent uncoating of transport vesicles, vesicular profiles with a morphology reminiscent of COP-coated vesicles appeared. These vesicular profiles were either associated with Golgi stacks or, at later stages, with clusters, but were formed at all stages of disassembly. Together these results provide further support for our model that continued budding of vesicles from the rims of Golgi cisternae is at least partly responsible for the disassembly of the Golgi apparatus.

Reassembly of Golgi stacks from mitotic Golgi fragments in a cell-free system

Rat liver Golgi stacks were incubated with mitotic cytosol for 30 min at 37 degrees C to generate mitotic Golgi fragments comprising vesicles, tubules, and cisternal remnants. These were isolated and further incubated with rat liver cytosol for 60 min. The earliest intermediate observed by electron microscopy was a single, curved cisterna with tubular networks fused to the cisternal rims. Elongation of this cisterna was accompanied by stacking and further growth at the cisternal rims. Stacks also fused laterally so that the typical end product was a highly curved stack of 2-3 cisternae mostly enclosing an electron-lucent space. Reassembly occurred in the presence of nocodazole or cytochalasin B but not at 4 degrees C or in the absence of energy supplied in the form of ATP and GTP. Pretreatment of the mitotic fragments and cytosol with N-ethylmaleimide (NEM) also prevented reassembly. GTP gamma S and A1F prevented reassembly when added during fragmentation but not when added to the reassembly mixture. In fact, GTP gamma S stimulated reassembly such that all cisternae were stacked at the end of the incubation and comprised 40% of the total membrane. In contrast, microcystin inhibited stacking so that only single cisternae accumulated. Together these results provide a detailed picture of the reassembly process and open up the study of the architecture of the Golgi apparatus to a combined morphological and biochemical analysis.

Orientation of spindle axis and distribution of plasma membrane proteins during cell division in polarized MDCKII cells

MDCKII cells differentiate into a simple columnar epithelium when grown on a permeable support; the monolayer is polarized for transport and secretion. Individual cells within the monolayer continue to divide at a low rate without disturbing the function of the epithelium as a barrier to solutes. This presents an interesting model for the study of mitosis in a differentiated epithelium which we have investigated by confocal immunofluorescence microscopy. We monitored the distribution of microtubules, centrioles, nucleus, tight junctions, and plasma membrane proteins that are specifically targeted to the apical and basolateral domains. The stable interphase microtubule cytoskeleton was rapidly disassembled at prophase onset and reassembled at cytokinesis. As the interphase microtubules disassembled at prophase, the centrioles moved from their interphase position at the apical membrane to the nucleus and acquired the ability to organize microtubule asters. Orientation of the spindle parallel to the plane of the monolayer occurred between late prophase and metaphase and persisted through cytokinesis. The cleavage furrow formed asymmetrically perpendicular to the plane of the monolayer initiating at the basolateral side and proceeding to the apical domain. The interphase microtubule network reformed after the centrioles migrated from the spindle poles to resume their interphase apical position. Tight junctions (ZO-1), which separate the apical from the basolateral domains, remained assembled throughout all phases of mitosis. E-cadherin and a 58-kD antigen maintained their basolateral plasma membrane distributions, and a 114-kD antigen remained polarized to the apical domain. These proteins were useful for monitoring the changes in shape of the mitotic cells relative to neighboring cells, especially during telophase when the cell shape changes dramatically. We discuss the changes in centriole position during the cell cycle, mechanisms of spindle orientation, and how the maintenance of polarized plasma membrane domains through mitosis may facilitate the rapid reformation of the polarized interphase cytoplasm.

COP-coated vesicles are involved in the mitotic fragmentation of Golgi stacks in a cell-free system

Rat liver Golgi stacks fragmented when incubated with mitotic but not interphase cytosol in a process dependent on time, temperature, energy (added in the form of ATP) and cdc2 kinase. The cross-sectional length of Golgi stacks fell in the presence of mitotic cytosol by approximately 50% over 30 min without a corresponding decrease in the number of cisternae in the stack. The loss of membrane from stacked and single cisternae occurred with a half-time of approximately 20 min, and was matched by the appearance of both small (50-100 nm in diameter) and large (100-200 nm in diameter) vesicular profiles. Small vesicular profiles constituted more than 50% of the total membrane after 60 min of incubation and they were shown to be vesicles or very short tubules by serial sectioning. In the presence of GTP gamma S all of the small vesicles were COP-coated and both the extent and the rate at which they formed were sufficient to account for the production of small vesicles during mitotic incubation. The involvement of the COP-mediated budding mechanism was confirmed by immunodepletion of one of the subunits of COP coats (the coatomer) from mitotic cytosol. Vesicles were no longer formed but highly fenestrated networks appeared, an effect reversed by the readdition of purified coatomer. Together these experiments provide strong support for our hypothesis that the observed vesiculation of the Golgi apparatus during mitosis in animal cells is caused by continued budding of COP-coated transport vesicles but an inhibition of their fusion with their target membranes.

Implications of the SNARE hypothesis for intracellular membrane topology and dynamics

The SNARE hypothesis provides a mechanism for the specific docking and fusion of transport vesicles with their target membranes. A simple extension of the hypothesis can explain many cellular processes, including the stacking of Golgi cisternae, retrograde transport and homotypic fusion; it can also explain the morphology of intracellular membranes and their dynamics during mitosis.