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

O-GlcNAc Dynamics: The Sweet Side of Protein Trafficking Regulation in Mammalian Cells

The transport of proteins between the different cellular compartments and the cell surface is governed by the secretory pathway. Alternatively, unconventional secretion pathways have been described in mammalian cells, especially through multivesicular bodies and exosomes. These highly sophisticated biological processes rely on a wide variety of signaling and regulatory proteins that act sequentially and in a well-orchestrated manner to ensure the proper delivery of cargoes to their final destination. By modifying numerous proteins involved in the regulation of vesicular trafficking, post-translational modifications (PTMs) participate in the tight regulation of cargo transport in response to extracellular stimuli such as nutrient availability and stress. Among the PTMs, O-GlcNAcylation is the reversible addition of a single N-acetylglucosamine monosaccharide (GlcNAc) on serine or threonine residues of cytosolic, nuclear, and mitochondrial proteins. O-GlcNAc cycling is mediated by a single couple of enzymes: the O-GlcNAc transferase (OGT) which catalyzes the addition of O-GlcNAc onto proteins, and the O-GlcNAcase (OGA) which hydrolyses it. Here, we review the current knowledge on the emerging role of O-GlcNAc modification in the regulation of protein trafficking in mammalian cells, in classical and unconventional secretory pathways.

Fine-tuning cell organelle dynamics during mitosis by small GTPases

During mitosis, the allocation of genetic material concurs with organelle transformation and distribution. The coordination of genetic material inheritance with organelle dynamics directs accurate mitotic progression, cell fate determination, and organismal homeostasis. Small GTPases belonging to the Ras superfamily regulate various cell organelles during division. Being the key regulators of membrane dynamics, the dysregulation of small GTPases is widely associated with cell organelle disruption in neoplastic and non-neoplastic diseases, such as cancer and Alzheimer's disease. Recent discoveries shed light on the molecular properties of small GTPases as sophisticated modulators of a remarkably complex and perfect adaptors for rapid structure reformation. This review collects current knowledge on small GTPases in the regulation of cell organelles during mitosis and highlights the mediator role of small GTPase in transducing cell cycle signaling to organelle dynamics during mitosis.

Export Control: Post-transcriptional Regulation of the COPII Trafficking Pathway

The coat protein complex II (COPII) mediates forward trafficking of protein and lipid cargoes from the endoplasmic reticulum. COPII is an ancient and essential pathway in all eukaryotes and COPII dysfunction underlies a range of human diseases. Despite this broad significance, major aspects of COPII trafficking remain incompletely understood. For example, while the biochemical features of COPII vesicle formation are relatively well characterized, much less is known about how the COPII system dynamically adjusts its activity to changing physiologic cues or stresses. Recently, post-transcriptional mechanisms have emerged as a major mode of COPII regulation. Here, we review the current literature on how post-transcriptional events, and especially post-translational modifications, govern the COPII pathway.

Attenuation of cGAS/STING activity during mitosis

The innate immune system recognizes cytosolic DNA associated with microbial infections and cellular stress via the cGAS/STING pathway, leading to activation of phospho-IRF3 and downstream IFN-I and senescence responses. To prevent hyperactivation, cGAS/STING is presumed to be nonresponsive to chromosomal self-DNA during open mitosis, although specific regulatory mechanisms are lacking. Given a role for the Golgi in STING activation, we investigated the state of the cGAS/STING pathway in interphase cells with artificially vesiculated Golgi and in cells arrested in mitosis. We find that whereas cGAS activity is impaired through interaction with mitotic chromosomes, Golgi integrity has little effect on the enzyme's production of cGAMP. In contrast, STING activation in response to either foreign DNA (cGAS-dependent) or exogenous cGAMP is impaired by a vesiculated Golgi. Overall, our data suggest a secondary means for cells to limit potentially harmful cGAS/STING responses during open mitosis via natural Golgi vesiculation.

Mechanisms and Regulation of the Mitotic Inheritance of the Golgi Complex

In mammalian cells, the Golgi complex is structured in the form of a continuous membranous system composed of stacks connected by tubular bridges: the "Golgi ribbon." At the onset of mitosis, the Golgi complex undergoes a multi-step fragmentation process that is required for its correct partition into the dividing cells. Importantly, inhibition of Golgi disassembly results in cell-cycle arrest at the G2 stage, which indicates that accurate inheritance of the Golgi complex is monitored by a "Golgi mitotic checkpoint." Moreover, mitotic Golgi disassembly correlates with the release of a set of Golgi-localized proteins that acquire specific functions during mitosis, such as mitotic spindle formation and regulation of the spindle checkpoint. Most of these events are regulated by small GTPases of the Arf and Rab families. Here, we review recent studies that are revealing the fundamental mechanisms, the molecular players, and the biological significance of mitotic inheritance of the Golgi complex in mammalian cells. We also briefly comment on how Golgi partitioning is coordinated with mitotic progression.

Golgi Fragmentation in ALS Motor Neurons. New Mechanisms Targeting Microtubules, Tethers, and Transport Vesicles

Pathological alterations of the Golgi apparatus, such as its fragmentation represent an early pre-clinical feature of many neurodegenerative diseases and have been widely studied in the motor neuron disease amyotrophic lateral sclerosis (ALS). Yet, the underlying molecular mechanisms have remained cryptic. In principle, Golgi fragmentation may result from defects in three major classes of proteins: structural Golgi proteins, cytoskeletal proteins and molecular motors, as well as proteins mediating transport to and through the Golgi. Here, we present the different mechanisms that may underlie Golgi fragmentation in animal and cellular models of ALS linked to mutations in SOD1, TARDBP (TDP-43), VAPB, and C9Orf72 and we propose a novel one based on findings in progressive motor neuronopathy (pmn) mice. These mice are mutated in the TBCE gene encoding the cis-Golgi localized tubulin-binding cofactor E, one of five chaperones that assist in tubulin folding and microtubule polymerization. Loss of TBCE leads to alterations in Golgi microtubules, which in turn impedes on the maintenance of the Golgi architecture. This is due to down-regulation of COPI coat components, dispersion of Golgi tethers and strong accumulation of ER-Golgi SNAREs. These effects are partially rescued by the GTPase ARF1 through recruitment of TBCE to the Golgi. We hypothesize that defects in COPI vesicles, microtubules and their interaction may also underlie Golgi fragmentation in human ALS linked to other mutations, spinal muscular atrophy (SMA), and related motor neuron diseases. We also discuss the functional relevance of pathological Golgi alterations, in particular their potential causative, contributory, or compensatory role in the degeneration of motor neuron cell bodies, axons and synapses.

Systems biology of the secretory pathway: what have we learned so far?

Several RNAi screens were performed in search for regulators of the secretory pathway. These screens were performed in different organisms and cell lines and relied on different readouts. Therefore, they have only little overlap among their hits, leading to the question of what we have learned from this approach so far and how these screens contributed towards an integrative understanding of the endomembrane system. The aim of this review is to revisit these screens and discuss their strengths and weaknesses as well as potential reasons for their failure to overlap with each other. As with secretory trafficking, RNAi screens were also performed on other cellular processes such as cell migration and autophagy, both of which were shown to be intimately linked to secretion. Another aim of this review is to compare the outcome of the RNAi screens on secretion, autophagy and cell migration and ask whether the functional genomic approaches have uncovered potential mechanistic insights into the links between these processes.

Self-organization of core Golgi material is independent of COPII-mediated endoplasmic reticulum export

The Golgi is a highly organized and dynamic organelle that receives and distributes material from and to the endoplasmic reticulum (ER) and the endocytic pathway. One open question about Golgi organization is whether it is solely based on ER-to-Golgi transport. Here, we analyzed the kinetics of Golgi breakdown in the absence of COPII-dependent ER export with high temporal and spatial resolution using quantitative fluorescence microscopy. We found that Golgi breakdown occurred in two phases. While Golgi enzymes continuously redistributed to the ER, we consistently observed extensive Golgi fragmentation at the beginning of the breakdown, followed by microtubule-dependent formation of a Golgi remnant structure (phase 1). Further Golgi disintegration occurred less uniformly (phase 2). Remarkably, cisternal Golgi morphology was lost early in phase 1 and Golgi fragments instead corresponded to variably sized vesicle clusters. These breakdown intermediates were devoid of COPI-dependent recycling material, but contained typical 'core' Golgi components. Furthermore, Golgi breakdown intermediates were able to disassemble and reassemble following cell division, indicating that they retained important regulatory capabilities. Taken together, these findings support the view that Golgi self-organization exists independently of ER-to-Golgi transport.

On the move: organelle dynamics during mitosis

A cell constitutes the minimal self-replicating unit of all organisms, programmed to propagate its genome as it proceeds through mitotic cell division. The molecular processes entrusted with ensuring high fidelity of DNA replication and subsequent segregation of chromosomes between daughter cells have therefore been studied extensively. However, to process the information encoded in its genome a cell must also pass on its non-genomic identity to future generations. To achieve productive sharing of intracellular organelles, cells have evolved complex mechanisms of organelle inheritance. Many membranous compartments undergo vast spatiotemporal rearrangements throughout mitosis. These controlled organizational changes are crucial to enabling completion of the division cycle and ensuring successful progeny. Herein we review current understanding of intracellular organelle segregation during mitotic division in mammalian cells, with a focus on compartment organization and integrity throughout the inheritance process.

Positive feedback between Golgi membranes, microtubules and ER exit sites directs de novo biogenesis of the Golgi

The Golgi complex is the central organelle of the secretory pathway. It undergoes dynamic changes during the cell cycle, but how it acquires and maintains its complex structure is unclear. To address this question, we have used laser nanosurgery to deplete BSC1 cells of the Golgi complex and have monitored its biogenesis by quantitative time-lapse microscopy and correlative electron microscopy. After Golgi depletion, endoplasmic reticulum (ER) export is inhibited and the number of ER exit sites (ERES) is reduced and does not increase for several hours. Occasional fusion of small post-ER carriers to form the first larger structures triggers a rapid and drastic growth of Golgi precursors, due to the capacity of these structures to attract more carriers by microtubule nucleation and to stimulate ERES biogenesis. Increasing the chances of post-ER carrier fusion close to ERES by depolymerizing microtubules results in the acceleration of Golgi and ERES biogenesis. Taken together, on the basis of our results, we propose a self-organizing principle of the early secretory pathway that integrates Golgi biogenesis, ERES biogenesis and the organization of the microtubule network by positive-feedback loops.

The Aspergillus nidulans peripheral ER: disorganization by ER stress and persistence during mitosis

The genetically amenable fungus Aspergillus nidulans is well suited for cell biology studies involving the secretory pathway and its relationship with hyphal tip growth by apical extension. We exploited live-cell epifluorescence microscopy of the ER labeled with the translocon component Sec63, endogenously tagged with GFP, to study the organization of ‘secretory’ ER domains. The Sec63 A. nidulans ER network includes brightly fluorescent peripheral strands and more faintly labeled nuclear envelopes. In hyphae, the most abundant peripheral ER structures correspond to plasma membrane-associated strands that are polarized, but do not invade the hyphal tip dome, at least in part because the subapical collar of endocytic actin patches constrict the cortical strands in this region. Thus the subapical endocytic ring might provide an attachment for ER strands, thereby ensuring that the growing tip remains ‘loaded’ with secretory ER. Acute disruption of secretory ER function by reductive stress-mediated induction of the unfolded protein response results in the reversible aggregation of ER strands, cessation of exocytosis and swelling of the hyphal tips. The secretory ER is insensitive to brefeldin A treatment and does not undergo changes during mitosis, in agreement with the reports that apical extension continues at normal rates during this period.

Silencing of mammalian Sar1 isoforms reveals COPII-independent protein sorting and transport

The Sar1 GTPase coordinates the assembly of coat protein complex-II (COPII) at specific sites of the endoplasmic reticulum (ER). COPII is required for ER-to-Golgi transport, as it provides a structural and functional framework to ship out protein cargoes produced in the ER. To investigate the requirement of COPII-mediated transport in mammalian cells, we used small interfering RNA (siRNA)-mediated depletion of Sar1A and Sar1B. We report that depletion of these two mammalian forms of Sar1 disrupts COPII assembly and the cells fail to organize transitional elements that coordinate classical ER-to-Golgi protein transfer. Under these conditions, minimal Golgi stacks are seen in proximity to juxtanuclear ER membranes that contain elements of the intermediate compartment, and from which these stacks coordinate biosynthetic transport of protein cargo, such as the vesicular stomatitis virus G protein and albumin. Here, transport of procollagen-I is inhibited. These data provide proof-of-principle for the contribution of alternative mechanisms that support biosynthetic trafficking in mammalian cells, providing evidence of a functional boundary associated with a bypass of COPII.

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.

Division of the intermediate compartment at the onset of mitosis provides a mechanism for Golgi inheritance

As mammalian cells prepare for mitosis, the Golgi ribbon is first unlinked into its constituent stacks and then transformed into spindle-associated, pleiomorphic membrane clusters in a process that remains enigmatic. Also, it remains unclear whether Golgi inheritance involves the incorporation of Golgi enzymes into a pool of coat protein I (COPI) vesicles, or their COPI-independent transfer to the endoplasmic reticulum (ER). Based on the observation that the intermediate compartment (IC) at the ER-Golgi boundary is connected to the centrosome, we examined its mitotic fate and possible role in Golgi breakdown. The use of multiple imaging techniques and markers revealed that the IC elements persist during the M phase, maintain their compositional and structural properties and remain associated with the mitotic spindle, forming circular arrays at the spindle poles. At G2/M transition, the movement of the pericentrosomal domain of the IC (pcIC) to the cell centre and its expansion coincide with the unlinking of the Golgi ribbon. At prophase, coupled to centrosome separation, the pcIC divides together with recycling endosomes, providing novel landmarks for mitotic entry. We provide evidence that the permanent IC elements function as way stations during the COPI-dependent dispersal of Golgi components at prometa- and metaphase, indicating that they correspond to the previously described Golgi clusters. In addition, they continue to communicate with the vesicular 'Golgi haze' and thus are likely to provide templates for Golgi reassembly. These results implicate the IC in mitotic Golgi inheritance, resulting in a model that integrates key features of the two previously proposed pathways.

Architecture of the mammalian Golgi

Since its first visualization in 1898, the Golgi has been a topic of intense morphological research. A typical mammalian Golgi consists of a pile of stapled cisternae, the Golgi stack, which is a key station for modification of newly synthesized proteins and lipids. Distinct stacks are interconnected by tubules to form the Golgi ribbon. At the entrance site of the Golgi, the cis-Golgi, vesicular tubular clusters (VTCs) form the intermediate between the endoplasmic reticulum and the Golgi stack. At the exit site of the Golgi, the trans-Golgi, the trans-Golgi network (TGN) is the major site of sorting proteins to distinct cellular locations. Golgi functioning can only be understood in light of its complex architecture, as was revealed by a range of distinct electron microscopy (EM) approaches. In this article, a general concept of mammalian Golgi architecture, including VTCs and the TGN, is described.

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.

ER exit sites--localization and control of COPII vesicle formation

The first membrane trafficking step in the biosynthetic secretory pathway, the export of proteins and lipids from the endoplasmic reticulum (ER), is mediated by COPII-coated vesicles. In mammalian cells, COPII vesicle budding occurs at specialized sites on the ER, the so-called transitional ER (tER). Here, we discuss aspects of the formation and maintenance of these sites, the mechanisms by which cargo becomes segregated within them, and the propagation of ER exit sites (ERES) during cell division. All of these features are inherently linked to the formation, maintenance and function of the Golgi apparatus underlining the importance of ERES to Golgi function and more widely in terms of intracellular organization and cellular function.

Peripheral ER structure and function

The endoplasmic reticulum (ER) is a single continuous membrane-enclosed organelle made up of functionally and structurally distinct domains. The ER domains include the nuclear envelope (NE) and the peripheral ER, which is a network of tubules and sheets spread throughout the cytoplasm. The structural organization of the ER is related to its many different cellular functions. Here we will discuss how the various functional domains of the peripheral ER are organized into structurally distinct domains that exist within the continuous membrane bilayer throughout the cell cycle. In addition, we will summarize our current knowledge on how peripheral ER membranes contact various other regions of the cytoplasm including the cytoskeleton, mitochondria, Golgi, and the plasma membrane and what is known about the functions of these interactions.

CDC2/SPDY transiently associates with endoplasmic reticulum exit sites during oocyte maturation

BACKGROUND

Mammalian oocytes acquire competence to be fertilized during meiotic maturation. The protein kinase CDC2 plays a pivotal role in several key maturation events, in part through controlled changes in CDC2 localization. Although CDC2 is involved in initiation of maturation, a detailed analysis of CDC2 localization at the onset of maturation is lacking. In this study, the subcellular distribution of CDC2 and its regulatory proteins cyclin B and SPDY in combination with several organelle markers at the onset of pig oocyte maturation has been investigated.

RESULTS

Our results demonstrate that CDC2 transiently associates with a single domain, identified as a cluster of endoplasmic reticulum (ER) exit sites (ERES) by the presence of SEC23, in the cortex of maturing porcine oocytes prior to germinal vesicle break down. Inhibition of meiosis resumption by forskolin treatment prevented translocation of CDC2 to this ERES cluster. Phosphorylated GM130 (P-GM130), which is a marker for fragmented Golgi, localized to ERES in almost all immature oocytes and was not affected by forskolin treatment. After removal of forskolin from the culture media, the transient translocation of CDC2 to ERES was accompanied by a transient dispersion of P-GM130 into the ER suggesting a role for CDC2 in redistributing Golgi components that have collapsed into ERES further into the ER during meiosis. Finally, we show that SPDY, rather than cyclin B, colocalizes with CDC2 at ERES, suggesting a role for the CDC2/SPDY complex in regulating the secretory pathway during oocyte maturation.

CONCLUSION

Our data demonstrate the presence of a novel structure in the cortex of porcine oocytes that comprises ERES and transiently accumulates CDC2 prior to germinal vesicle breakdown. In addition, we show that SPDY, but not cyclin B, localizes to this ERES cluster together with CDC2.

Drosophila Sec16 mediates the biogenesis of tER sites upstream of Sar1 through an arginine-rich motif

tER sites are specialized cup-shaped ER subdomains characterized by the focused budding of COPII vesicles. Sec16 has been proposed to be involved in the biogenesis of tER sites by binding to COPII coat components and clustering nascent-coated vesicles. Here, we show that Drosophila Sec16 (dSec16) acts instead as a tER scaffold upstream of the COPII machinery, including Sar1. We show that dSec16 is required for Sar1-GTP concentration to the tER sites where it recruits in turn the components of the COPII machinery to initiate coat assembly. Last, we show that the dSec16 domain required for its localization maps to an arginine-rich motif located in a nonconserved region. We propose a model in which dSec16 binds ER cups via its arginine-rich domain, interacts with Sar1-GTP that is generated on ER membrane by Sec12 and concentrates it in the ER cups where it initiates the formation of COPII vesicles, thus acting as a tER scaffold.

p38 MAPK regulates COPII recruitment

Here, we investigate regulation of coat protein complex II (COPII) recruitment onto ER export sites in permeabilized cells. In cytosols from nocodazole treated HeLa cells we find COPII loading is inhibited. The stress kinase p38 MAPK is activated in these cytosols and COPII loading can be rescued by depletion of p38 MAPK alpha or by the p38 MAPK inhibitor (SB203580) but not by inhibition/depletion of cdc2. These observations indicate regulation of the early secretory pathway by p38 MAPK.

Mitosis controls the Golgi and the Golgi controls mitosis

In mammals, the Golgi complex is structured in the form of a continuous membranous system composed of up to 100 stacks connected by tubular bridges, the 'Golgi ribbon'. During mitosis, the Golgi undergoes extensive fragmentation through a multistage process that allows its correct partitioning and inheritance by daughter cells. Strikingly, this Golgi fragmentation is required not only for inheritance but also for mitotic entrance itself, since its block results in the arrest of the cell cycle in G2. This is called the 'Golgi mitotic checkpoint'. Recent studies have identified the severing of the ribbon into its constituent stacks during early G2 as the precise stage of Golgi fragmentation that controls mitotic entry. This opens new ways to elucidate the mechanism of the Golgi checkpoint.

Inheritance and biogenesis of organelles in the secretory pathway

In eukaryotic cells, cellular functions are compartmentalized into membrane-bound organelles. This has many advantages, as shown by the success of the eukaryotic lineage, but creates many problems for cells, such as the need to build and partition these organelles during cell growth and division. Diverse mechanisms for biogenesis of the endoplasmic reticulum and Golgi apparatus have evolved, ranging from de novo synthesis to the copying of a template organelle. The different mechanisms by which organelles are inherited in yeasts, protozoa and metazoans probably reflect the differences in the structure and copy number of these organelles.

Golgi inheritance in mammalian cells is mediated through endoplasmic reticulum export activities

Golgi inheritance during mammalian cell division occurs through the disassembly, partitioning, and reassembly of Golgi membranes. The mechanisms responsible for these processes are poorly understood. To address these mechanisms, we have examined the identity and dynamics of Golgi proteins within mitotic membranes using live cell imaging and electron microscopy techniques. Mitotic Golgi fragments, seen in prometaphase and telophase, were found to localize adjacent to endoplasmic reticulum (ER) export domains, and resident Golgi transmembrane proteins cycled rapidly into and out of these fragments. Golgi proteins within mitotic Golgi haze-seen during metaphase-were found to redistribute with ER markers into fragments when the ER was fragmented by ionomycin treatment. The temperature-sensitive misfolding mutant ts045VSVG protein, when localized to the Golgi at the start of mitosis, became trapped in the ER at the end of mitosis in cells shifted to 40 degrees C. Finally, reporters for Arf1 and Sar1 activity revealed that Arf1 and Sar1 undergo sequential inactivation during mitotic Golgi breakdown and sequential reactivation upon Golgi reassembly at the end of mitosis. Together, these findings support a model of mitotic Golgi inheritance that involves inhibition and subsequent reactivation of cellular activities controlling the cycling of Golgi components into and out of the ER.

Endoplasmic reticulum positioning and partitioning in mitotic HeLa cells

Cell division serves to distribute chromosomes and organelles into two daughter cells, but the mechanism of rough endoplasmic reticulum (RER) segregation in animal cell mitosis is poorly understood. Here we study the distribution of RER in mitotic HeLa cells and its relation to the cytoskeleton. At metaphase, the RER was located in the cell cortex and was most concentrated in two locations. Close to the plasma membrane the RER was closely associated with cortical actin, and after treatment with Latrunculin A RER elements retracted to the deep cortex and became more tubular. Positioning was therefore dependent on cortical F-actin. Deeper in the cortex cisternae were wrapped tightly around the contours of the spindle body and orientated along microtubules close the spindle poles. Stereology revealed a close correlation between RER volume and cell volume in telophase daughter cells. These results suggest that the RER is positioned at the outer and inner regions of metaphase cortex by association with cytoskeleton. This arrangement combined with a disposition in concentric layers, deep to the plasma membrane, appears to distribute the RER evenly in the cortex and may help to couple quantities of RER and cell constituents.

Dynamics of COPII vesicles and the Golgi apparatus in cultured Nicotiana tabacum BY-2 cells provides evidence for transient association of Golgi stacks with endoplasmic reticulum exit sites

Despite the ubiquitous presence of the COPI, COPII, and clathrin vesicle budding machineries in all eukaryotes, the organization of the secretory pathway in plants differs significantly from that in yeast and mammalian cells. Mobile Golgi stacks and the lack of both transitional endoplasmic reticulum (ER) and a distinct ER-to-Golgi intermediate compartment are the most prominent distinguishing morphological features of the early secretory pathway in plants. Although the formation of COPI vesicles at periphery of Golgi cisternae has been demonstrated in plants, exit from the ER has been difficult to visualize, and the spatial relationship of this event is now a matter of controversy. Using tobacco (Nicotiana tabacum) BY-2 cells, which represent a highly active secretory system, we have used two approaches to investigate the location and dynamics of COPII binding to the ER and the relationship of these ER exit sites (ERES) to the Golgi apparatus. On the one hand, we have identified endogenous COPII using affinity purified antisera generated against selected COPII-coat proteins (Sar1, Sec13, and Sec23); on the other hand, we have prepared a BY-2 cell line expressing Sec13:green fluorescent protein (GFP) to perform live cell imaging with red fluorescent protein-labeled ER or Golgi stacks. COPII binding to the ER in BY-2 cells is visualized as fluorescent punctate structures uniformly distributed over the surface of the ER, both after antibody staining as well as by Sec13:GFP expression. These structures are smaller and greatly outnumber the Golgi stacks. They are stationary, but have an extremely short half-life (<10 s). Without correlative imaging data on the export of membrane or lumenal ER cargo it was not possible to equate unequivocally these COPII binding loci with ERES. When a GDP-fixed Sar1 mutant is expressed, ER export is blocked and the visualization of COPII binding is perturbed. On the other hand, when secretion is inhibited by brefeldin A, COPII binding sites on the ER remain visible even after the Golgi apparatus has been lost. Live cell imaging in a confocal laser scanning microscope equipped with spinning disk optics allowed us to investigate the relationship between mobile Golgi stacks and COPII binding sites. As they move, Golgi stacks temporarily associated with COPII binding sites at their rims. Golgi stacks were visualized with their peripheries partially or fully occupied with COPII. In the latter case, Golgi stacks had the appearance of a COPII halo. Slow moving Golgi stacks tended to have more peripheral COPII than faster moving ones. However, some stationary Golgi stacks entirely lacking COPII were also observed. Our results indicate that, in a cell type with highly mobile Golgi stacks like tobacco BY-2, the Golgi apparatus is not continually linked to a single ERES. By contrast, Golgi stacks associate intermittently and sometimes concurrently with several ERES as they move.

Golgi inheritance in small buds of Saccharomyces cerevisiae is linked to endoplasmic reticulum inheritance

According to the cisternal maturation hypothesis, endoplasmic reticulum (ER)-derived membranes nucleate new Golgi cisternae. The yeast Saccharomyces cerevisiae offers a unique opportunity to test this idea because small buds contain both ER and Golgi structures early in the cell cycle. We previously predicted that mutants defective in ER inheritance also would show defects in Golgi inheritance. Surprisingly, studies of S. cerevisiae have not revealed the expected link between ER and Golgi inheritance. Here, we revisit this issue by generating mutant strains in which many of the small buds are devoid of detectable ER. These strains also show defects in the inheritance of both early and late Golgi cisternae. Strikingly, virtually all of the buds that lack ER also lack early Golgi cisternae. Our results fit with the idea that membranes exported from the ER coalesce with vesicles derived from existing Golgi compartments to generate new Golgi cisternae. This basic mechanism of Golgi inheritance may be conserved from yeast to vertebrate cells.

Golgi inheritance: shaken but not stirred

Our view of what happens to the Golgi and ER during mitosis in mammalian cells has been shaken once more. Rather than the Golgi contents being recycled through, or mixed with the ER, two recent studies taking complementary approaches, find that the contents of these organelles remain separate throughout mitosis.

Regulation of a COPII component by cytosolic O-glycosylation during mitosis

Endoplasmic reticulum (ER)-to-Golgi transport is blocked in mammalian cells during mitosis; however, the mechanism underlying this blockade remains unknown. Since COPII proteins are involved in this transport pathway, we investigated at the biochemical level post-translational modifications of COPII components during the course of mitosis that could be linked to inhibition of ER-to-Golgi transport. By comparing biochemical properties of cytosolic COPII components during interphase and mitosis, we found that Sec24p isoforms underwent post-translational modifications resulting in an increase in their apparent molecular weight. No such modification was observed for the other COPII components Sec23p, Sec13p, Sec31p or Sar1p. Analyzing in more details Sec24p isoforms in interphase and mitotic conditions, we found that the interphase form of Sec24p was O-N-acetylglucosamine modified, a feature lost upon entering into mitosis. This mitotic deglycosylation was coupled to Sec24p phosphorylation, a feature likely responsible for the increase in apparent molecular weight of these molecules. These modifications correlated with an alteration in the membrane binding properties of Sec24p. These data suggest that when entering into mitosis, the COPII component Sec24p is simultaneously deglycosylated and phosphorylated, a process which may contribute to the observed mitotic ER-to-Golgi traffic block.

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.

Morphodynamics of the secretory pathway

A careful scrutiny of the dynamics of secretory compartments in the entire eukaryotic world reveals many common themes. The most fundamental theme is that the Golgi apparatus and related structures appear as compartments formed by the act of transporting cargo. The second common theme is the pivotal importance for endomembrane dynamics of shifting back and forth the equilibrium between full and perforated cisternae along the pathway. The third theme is the role of a continuous membrane flow in anterograde transfer of molecules from the endoplasmic reticulum through the Golgi apparatus. The last common theme is the self-regulatory balance between anatomical continuities and discontinuities of the endomembrane system. As this balance depends on secretory activity, it provides a source of morphological variability among cell types or, for a given cell type, according to environmental conditions. Beyond this first source of variability, it appears that divergent strategies pave the evolutionary routes in different eukaryotic kingdoms. These divergent strategies primarily affect the levels of stacking, of stabilization, and of clustering of the Golgi apparatus. They presumably underscore a trade-off between versatility and stability to adapt the secretory function to the degree of environmental variability. Nonequilibrium secretory structures would provide yeasts, and plants to a lesser extent, with the required versatility to cope with ever changing environments, by contrast to the stabler milieu intérieur of homeothermic animals.

Cell-cycle-specific Golgi fragmentation: how and why?

The Golgi membranes, in the form of stacks of cisternae, are contained in the pericentriolar region of mammalian cells. During mitosis, these membranes are fragmented and dispersed throughout the cell. Recent studies are revealing the significance of pericentriolar position of the Golgi apparatus and mechanism by which these membranes are fragmented during mitosis.

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.

De novo formation, fusion and fission of mammalian COPII-coated endoplasmic reticulum exit sites

Transport between the endoplasmic reticulum (ER) and Golgi is mediated by the sequential action of the COPII and COPI coat complexes. COPII subunits are recruited to the ER membrane where they mediate the selection of cargo for transport to the Golgi, and also membrane deformation and vesicle formation. New ER exit sites can be generated by lateral growth and medial fission (in Pythium sp.) or by de novo formation (in Pichia pastoris) but it is not known how mammalian ER exit sites form. Here, time-lapse imaging of COPII-coated structures in live mammalian cells reveals that the number of ER export sites increases greatly during interphase by de novo formation. These results show the fusion of pre-existing ER export sites and the fission of larger structures. These three mechanisms of de novo formation, fusion and fission probably cooperate to regulate the size of these sites in mammalian cells.

Inhibition of autophagy in mitotic animal cells

In nutrient-deprived cells autophagy recycles cytoplasmic constituents by engulfing and degrading them in membrane-bound autophagic vacuoles. The regulation of autophagic vacuole formation is poorly understood, but here we show this process is under strict cell-cycle control in cultured animal cells. We found strong inhibition of autophagic vacuole accumulation in nocodazole-arrested pseudo-prometaphase cells, and also in metaphase and anaphase cells generated on release from the nocodazole arrest. Autophagic vacuoles reappeared after closure of the nuclear envelope in telophase/G1. Treatment with phosphoinositide 3(PI3)-kinase inhibitors wortmannin, LY294002 and 3-methyladenine (known to inhibit the autophagic response in interphase cells) rescued autophagy in mitotic cells without inducing reassembly of vesiculated ER and Golgi compartments. The autophagy induced in mitotic cells was inhibited by amino acids, and the resulting autophagosomes contained proteins LC3 and Lamp1, known to be associated with autophagosomes in interphase cells. The mitotic inhibition of autophagy was not relieved by rapamycin treatment or in PDK1-/- embryonic stem cells, by microinjection of inhibitory antibodies against the class III PI3 kinase VPS34, or in cell lines lacking the p85 regulatory subunits of class IA PI3 kinases. Our results show that autophagy is under strict mitotic control and indicate a novel role for phosphoinositide 3-kinases or other wortmannin/LY294002-sensitive kinases in mitotic membrane traffic regulation.

The overexpression of GMAP-210 blocks anterograde and retrograde transport between the ER and the Golgi apparatus

Golgi Microtubule-Associated Protein (GMAP)-210 is a peripheral coiled-coil protein associated with the cis-Golgi network that interacts with microtubule minus ends. GMAP-210 overexpression has previously been shown to perturb the microtubule network and to induce a dramatic enlargement and fragmentation of the Golgi apparatus (Infante C, Ramos-Morales F, Fedriani C, Bornens M, Rios RM. J Cell Biol 1999; 145: 83-98). We now report that overexpressing GMAP-210 blocks the anterograde transport of both a soluble form of alkaline phosphatase and the hemagglutinin protein of influenza virus, an integral membrane protein, between the endoplasmic reticulum and the cis/medial (mannosidase II-positive) Golgi compartment. Retrograde transport of the Shiga toxin B-subunit is also blocked between the Golgi apparatus and the endoplasmic reticulum. As a consequence, the B-subunit accumulates in compartments positive for GMAP-210. Ultrastructural analysis revealed that, under these conditions, the Golgi complex is totally disassembled and Golgi proteins as well as proteins of the intermediate compartment are found in vesicle clusters distributed throughout the cell. The role of GMAP-210 on membrane processes at the interface between the endoplasmic reticulum and the Golgi apparatus is discussed in the light of the property of this protein to bind CGN membranes and microtubules.

Inheritance of the endoplasmic reticulum and Golgi apparatus

Yeast and mammalian cells use a variety of different mechanisms to ensure that the endoplasmic reticulum and Golgi apparatus are inherited by both daughter cells on cell division. In yeast, endoplasmic reticulum inheritance involves both active microtubule and passive actin-based mechanisms, while the Golgi is transported into the forming daughter cell by an active actin-based mechanism. Animal cells actively partition the endoplasmic reticulum and Golgi apparatus, but association with the mitotic spindle-rather than the actin cytoskeleton-appears to be the mechanism

COPII-dependent transport from the endoplasmic reticulum

The coat protein complex II (COPII) forms transport vesicles from the endoplasmic reticulum and segregates biosynthetic cargo from ER-resident proteins. Recent high-resolution structural studies on individual COPII subunits and on the polymerized coat reveal the molecular architecture of COPII vesicles. Other advances have shown that integral membrane accessory proteins act with the COPII coat to collect specific cargo molecules into ER-derived transport vesicles.

Golgi complex: biogenesis de novo?

Whether Golgi biogenesis occurs by self-assembly or around a pre-existing template is currently a matter of debate. Recent studies have shown that Golgi structural proteins are more dynamic than previously thought, suggesting that self-assembly of the Golgi complex may be possible.

Partitioning of the matrix fraction of the Golgi apparatus during mitosis in animal cells

The Golgi apparatus is partitioned during mitosis in animal cells by a process of fragmentation, dispersal, and reassembly in each daughter cell. We fractionated the Golgi apparatus in vivo using the drug brefeldin A or a dominant-negative mutant of the Sar1p protein. After these treatments, Golgi enzymes moved back to the endoplasmic reticulum, leaving behind a matrix of Golgi structural proteins. Under these conditions, cells still entered and exited mitosis normally, and their Golgi matrix partitioned in a manner very similar to that of the complete organelle. Thus, the matrix may be the partitioning unit of the Golgi apparatus and may carry the Golgi enzyme-containing membranes into the daughter cells.

Mitotic Golgi is in a dynamic equilibrium between clustered and free vesicles independent of the ER

Golgi inheritance during cell division involves Golgi disassembly but it remains unclear whether the breakdown product is dispersed vesicles, clusters of vesicles or a fused ER/Golgi network. Evidence against the fused ER/Golgi hypothesis was previously obtained from subcellular fractionation studies, but left concerns about the means used to obtain and disrupt mitotic cells. Here, we performed velocity gradient analysis on otherwise untreated cells shaken from plates 9 h after release from an S-phase block. In addition, we used digitonin and freeze/thaw permeabilization as alternatives to mechanical homogenization. Under each of these conditions, approximately 75% of the Golgi was recovered in a population of small vesicles that lacked detectable ER. We also used multilabel fluorescent microscopy with optical sectioning by deconvolution to compare the 3D metaphase staining pattern of endogenous Golgi and ER markers. Although both ER and Golgi staining were primarily diffuse, only the ER was excluded from the mitotic spindle region. Surprisingly, only 2% of the Golgi fluorescence was present as resolvable structures previously characterized as vesicle clusters. These were not present in the ER pattern. Significantly, a portion of the diffuse Golgi fluorescence, presumably representing dispersed 60-nm vesicles, underwent an apparent rapid aggregation with the larger Golgi structures upon treatments that impaired microtubule integrity. Therefore, mitotic Golgi appears to be in a dynamic equilibrium between clustered and free vesicles, and accurate partitioning may be facilitated by microtubule-based motors acting on the clusters to insure random and uniform distribution of the vesicles.

Maintenance of Golgi structure and function depends on the integrity of ER export

The Golgi apparatus comprises an enormous array of components that generate its unique architecture and function within cells. Here, we use quantitative fluorescence imaging techniques and ultrastructural analysis to address whether the Golgi apparatus is a steady-state or a stable organelle. We found that all classes of Golgi components are dynamically associated with this organelle, contrary to the prediction of the stable organelle model. Enzymes and recycling components are continuously exiting and reentering the Golgi apparatus by membrane trafficking pathways to and from the ER, whereas Golgi matrix proteins and coatomer undergo constant, rapid exchange between membrane and cytoplasm. When ER to Golgi transport is inhibited without disrupting COPII-dependent ER export machinery (by brefeldin A treatment or expression of Arf1[T31N]), the Golgi structure disassembles, leaving no residual Golgi membranes. Rather, all Golgi components redistribute into the ER, the cytoplasm, or to ER exit sites still active for recruitment of selective membrane-bound and peripherally associated cargos. A similar phenomenon is induced by the constitutively active Sar1[H79G] mutant, which has the additional effect of causing COPII-associated membranes to cluster to a juxtanuclear region. In cells expressing Sar1[T39N], a constitutively inactive form of Sar1 that completely disrupts ER exit sites, Golgi glycosylation enzymes, matrix, and itinerant proteins all redistribute to the ER. These results argue against the hypothesis that the Golgi apparatus contains stable components that can serve as a template for its biogenesis. Instead, they suggest that the Golgi complex is a dynamic, steady-state system, whose membranes can be nucleated and are maintained by the activities of the Sar1-COPII and Arf1-coatomer systems.

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.