Visualization of secretory cargo transport within the Golgi apparatus

Making progress

Mapping proteins in and associated with the Golgi apparatus reveals how this cellular compartment emerges in budding yeast and progresses over time.

Morphomics via next-generation electron microscopy

The living body is composed of innumerable fine and complex structures. Although these structures have been studied in the past, a vast amount of information pertaining to them still remains unknown. When attempting to observe these ultra-structures, the use of electron microscopy (EM) has become indispensable. However, conventional EM settings are limited to a narrow tissue area, which can bias observations. Recently, new trends in EM research have emerged, enabling coverage of far broader, nano-scale fields of view for two-dimensional wide areas and three-dimensional large volumes. Moreover, cutting-edge bioimage informatics conducted via deep learning has accelerated the quantification of complex morphological bioimages. Taken together, these technological and analytical advances have led to the comprehensive acquisition and quantification of cellular morphology, which now arises as a new omics science termed 'morphomics'.

Spatiotemporal dissection of the Golgi apparatus and the ER-Golgi intermediate compartment in budding yeast

Cargo traffic through the Golgi apparatus is mediated by cisternal maturation, but it remains largely unclear how the cis-cisternae, the earliest Golgi sub-compartment, is generated and how the Golgi matures into the trans-Golgi network (TGN). Here, we use high-speed and high-resolution confocal microscopy to analyze the spatiotemporal dynamics of a diverse set of proteins that reside in and around the Golgi in budding yeast. We find many mobile punctate structures that harbor yeast counterparts of mammalian endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC) proteins, which we term 'yeast ERGIC'. It occasionally exhibits approach and contact behavior toward the ER exit sites and gradually matures into the cis-Golgi. Upon treatment with the Golgi-disrupting agent brefeldin A, the ERGIC proteins form larger aggregates corresponding to the Golgi entry core compartment in plants, while cis- and medial-Golgi proteins are absorbed into the ER. We further analyze the dynamics of several late Golgi proteins to better understand the Golgi-TGN transition. Together with our previous studies, we demonstrate a detailed spatiotemporal profile of the entire cisternal maturation process from the ERGIC to the Golgi and further to the TGN.

Protein sorting upon exit from the endoplasmic reticulum dominates Golgi biogenesis in budding yeast

Cells sense and control the number and quality of their organelles, but the underlying mechanisms of this regulation are not understood. Our recent research in the yeast Saccharomyces cerevisiae has shown that long acyl chain ceramides in the endoplasmic reticulum (ER) membrane and the lipid moiety of glycosylphosphatidylinositol (GPI) anchor determine the sorting of GPI-anchored proteins in the ER. Here, we show that a mutant strain, which produces shorter ceramides than the wild-type strain, displays a different count of Golgi cisternae. Moreover, deletions of proteins that remodel the lipid portion of GPI anchors resulted in an abnormal number of Golgi cisternae. Thus, our study reveals that protein sorting in the ER plays a critical role in maintaining Golgi biogenesis.

Golgi retention and oncogenic KIT signaling via PLCγ2-PKD2-PI4KIIIβ activation in gastrointestinal stromal tumor cells

Most gastrointestinal stromal tumors (GISTs) develop due to gain-of-function mutations in the tyrosine kinase gene, KIT. We recently showed that mutant KIT mislocalizes to the Golgi area and initiates uncontrolled signaling. However, the molecular mechanisms underlying its Golgi retention remain unknown. Here, we show that protein kinase D2 (PKD2) is activated by the mutant, which causes Golgi retention of KIT. In PKD2-inhibited cells, KIT migrates from the Golgi region to lysosomes and subsequently undergoes degradation. Importantly, delocalized KIT cannot trigger downstream activation. In the Golgi/trans-Golgi network (TGN), KIT activates the PKD2-phosphatidylinositol 4-kinase IIIβ (PKD2-PI4KIIIβ) pathway through phospholipase Cγ2 (PLCγ2) to generate a PI4P-rich membrane domain, where the AP1-GGA1 complex is aberrantly recruited. Disruption of any factors in this cascade results in the release of KIT from the Golgi/TGN. Our findings show the molecular mechanisms underlying KIT mislocalization and provide evidence for a strategy for inhibition of oncogenic signaling.

Expression of GnT-III decreases chemoresistance via negatively regulating P-glycoprotein expression: Involvement of the TNFR2-NF-κB signaling pathway

The phenomenon of multidrug resistance (MDR) is called chemoresistance with respect to the treatment of cancer, and it continues to be a major challenge. The role of N-glycosylation in chemoresistance, however, remains poorly understood. Here, we established a traditional model for adriamycin resistance in K562 cells, which are also known as K562/adriamycin-resistant (ADR) cells. Lectin blot, mass spectrometry, and RT-PCR analysis showed that the expression levels of N-acetylglucosaminyltransferase III (GnT-III) mRNA and its products, bisected N-glycans, are significantly decreased in K562/ADR cells, compared with the levels in parent K562 cells. By contrast, the expression levels of both P-glycoprotein (P-gp) and its intracellular key regulator, NF-κB signaling, are significantly increased in K562/ADR cells. These upregulations were sufficiently suppressed by the overexpression of GnT-III in K562/ADR cells. We found that the expression of GnT-III consistently decreased chemoresistance for doxorubicin and dasatinib, as well as activation of the NF-κB pathway by tumor necrosis factor (TNF) α, which binds to two structurally distinct glycoproteins, TNF receptor 1 (TNFR1) and TNF receptor 2 (TNFR2), on the cell surface. Interestingly, our immunoprecipitation analysis revealed that only TNFR2, but not TNFR1, contains bisected N-glycans. The lack of GnT-III strongly induced TNFR2's autotrimerization without ligand stimulation, which was rescued by the overexpression of GnT-III in K562/ADR cells. Furthermore, the deficiency of TNFR2 suppressed P-gp expression while it increased GnT-III expression. Taken together, these results clearly show that GnT-III negatively regulates chemoresistance via the suppression of P-gp expression, which is regulated by the TNFR2-NF/κB signaling pathway.

Super resolution live imaging: The key for unveiling the true dynamics of membrane traffic around the Golgi apparatus in plant cells

In contrast to the relatively static image of the plants, the world inside each cell is surprisingly dynamic. Membrane-bounded organelles move actively on the cytoskeletons and exchange materials by vesicles, tubules, or direct contact between each other. In order to understand what is happening during those events, it is essential to visualize the working components in vivo. After the breakthrough made by the application of fluorescent proteins, the development of light microscopy enabled many discoveries in cell biology, including those about the membrane traffic in plant cells. Especially, super-resolution microscopy, which is becoming more and more accessible, is now one of the most powerful techniques. However, although the spatial resolution has improved a lot, there are still some difficulties in terms of the temporal resolution, which is also a crucial parameter for the visualization of the living nature of the intracellular structures. In this review, we will introduce the super resolution microscopy developed especially for live-cell imaging with high temporal resolution, and show some examples that were made by this tool in plant membrane research.

Super-Resolution Live Imaging of Cargo Traffic Through the Golgi Apparatus in Mammalian Cells

Super-resolution confocal live imaging microscopy (SCLIM) we developed provides high-speed, high-resolution, three- and four-dimensional, and multicolor simultaneous imaging. Using this technology, we are now able to observe the fine details of various dynamic events going on in living cells, such as membrane traffic and organelle dynamics. The retention using selective hooks (RUSH) system is a powerful tool to control synchronous release of natural cargo proteins of interest from the endoplasmic reticulum in mammalian cells. In this chapter, we describe a method for visualizing secretory cargo traffic within and around the Golgi apparatus in HeLa cells using SCLIM in combination with the RUSH assay.

Ubiquitination drives COPI priming and Golgi SNARE localization

Deciphering mechanisms controlling SNARE localization within the Golgi complex is crucial to understanding protein trafficking patterns within the secretory pathway. SNAREs are also thought to prime coatomer protein I (COPI) assembly to ensure incorporation of these essential cargoes into vesicles, but the regulation of these events is poorly understood. Here, we report roles for ubiquitin recognition by COPI in SNARE trafficking and in stabilizing interactions between Arf, COPI, and Golgi SNAREs in Saccharomyces cerevisiae. The ability of COPI to bind ubiquitin, but not the dilysine motif, through its N-terminal WD repeat domain of β'-COP or through an unrelated ubiquitin-binding domain is essential for the proper localization of Golgi SNAREs Bet1 and Gos1. We find that COPI, the ArfGAP Glo3, and multiple Golgi SNAREs are ubiquitinated. Notably, the binding of Arf and COPI to Gos1 is markedly enhanced by ubiquitination of these components. Glo3 is proposed to prime COPI-SNARE interactions; however, Glo3 is not enriched in the ubiquitin-stabilized SNARE-Arf-COPI complex but is instead enriched with COPI complexes that lack SNAREs. These results support a new model for how posttranslational modifications drive COPI priming events crucial for Golgi SNARE localization.

An Update on the Key Factors Required for Plant Golgi Structure Maintenance

Plant Golgi apparatus serves as the central station of the secretory pathway and is the site where protein modification and cell wall matrix polysaccharides synthesis occur. The polarized and stacked cisternal structure is a prerequisite for Golgi function. Our understanding of Golgi structure maintenance and trafficking are largely obtained from mammals and yeast, yet, plant Golgi has many different aspects. In this review, we summarize the key players in Golgi maintenance demonstrated by genetic studies in plants, which function in ER-Golgi, intra-Golgi and post-Golgi transport pathways. Among these, we emphasize on players in intra-Golgi trafficking.

Pleiotropic roles of N-glycans for enzyme activities and stabilities of MIPC synthases, Csh1 and Sur1/Csg1, in Saccharomyces cerevisiae

Mannosyl phosphorylceramide (MIPC) is a membrane lipid classified as a complex sphingolipid in Saccharomyces cerevisiae. MIPC is synthesized by two redundant enzymes, Sur1/Csg1 and Csh1, in the Golgi lumen. MIPC consists of five subtypes (A, B', B, C, and D-type) according to the position and number of hydroxyl groups on the ceramide moiety. Sur1 exerts higher impact on synthesis of MIPC-B and MIPC-C than Csh1. In this study, we elucidated the roles played by N-glycans attached to Sur1 and Csh1, and dissected the mechanisms underlying substrate recognition by these two enzymes. Sur1 carries an N-glycan on Asn-224, while Csh1 has N-glycans on Asn-51 and Asn-247. Although intracellular proteins usually harbor core-type N-glycans, the N-glycan on Asn-51 of Csh1 exhibited a unique mannan-like structure containing a long backbone of mannose. Sur1 N224Q and Csh1 N51Q mutants exhibited a decrease in the activity to synthesize specific MIPC subtypes for each enzyme, suggesting that these N-glycans play a role in substrate recognition through their catalytic domains. Moreover, ectopic insertion of an N-glycosylation consensus sequence (NST) at codon 51 of Sur1 (Sur1-NST51) resulted in an artificial modification with mannan, which markedly decreased protein stability. Our results suggest that the diminished stability of the Sur1-NST51 mutant protein could be attributable to potential structural alterations by the mannan. Collectively, the present study reveals essential luminal domains of Sur1 and Csh1 that dictate substrate specificity and/or the protein stabilities via mannan modification.

The Golgi Apparatus and its Next-Door Neighbors

The Golgi apparatus represents a central compartment of membrane traffic. Its apparent architecture, however, differs considerably among species, from unstacked and scattered cisternae in the budding yeast Saccharomyces cerevisiae to beautiful ministacks in plants and further to gigantic ribbon structures typically seen in mammals. Considering the well-conserved functions of the Golgi, its fundamental structure must have been optimized despite seemingly different architectures. In addition to the core layers of cisternae, the Golgi is usually accompanied by next-door compartments on its cis and trans sides. The trans-Golgi network (TGN) can be now considered as a compartment independent from the Golgi stack. On the cis side, the intermediate compartment between the ER and the Golgi (ERGIC) has been known in mammalian cells, and its functional equivalent is now suggested for yeast and plant cells. High-resolution live imaging is extremely powerful for elucidating the dynamics of these compartments and has revealed amazing similarities in their behaviors, indicating common mechanisms conserved along the long course of evolution. From these new findings, I would like to propose reconsideration of compartments and suggest a new concept to describe their roles comprehensively around the Golgi and in the post-Golgi trafficking.

Comparison of the Cisterna Maturation-Progression Model with the Kiss-and-Run Model of Intra-Golgi Transport: Role of Cisternal Pores and Cargo Domains

The Golgi complex is the central station of the secretory pathway. Knowledge about the mechanisms of intra-Golgi transport is inconsistent. Here, we compared the explanatory power of the cisterna maturation-progression model and the kiss-and-run model. During intra-Golgi transport, conventional cargoes undergo concentration and form cisternal distensions or distinct membrane domains that contain only one membrane cargo. These domains and distension are separated from the rest of the Golgi cisternae by rows of pores. After the arrival of any membrane cargo or a large cargo aggregate at the Golgi complex, the cis-Golgi SNAREs become enriched within the membrane of cargo-containing domains and then replaced by the trans-Golgi SNAREs. During the passage of these domains, the number of cisternal pores decreases. Restoration of the cisternal pores is COPI-dependent. Our observations are more in line with the kiss-and-run model.

Clathrin adaptors mediate two sequential pathways of intra-Golgi recycling

The pathways of membrane traffic within the Golgi apparatus are not fully known. This question was addressed using the yeast Saccharomyces cerevisiae, in which the maturation of individual Golgi cisternae can be visualized. We recently proposed that the AP-1 clathrin adaptor mediates intra-Golgi recycling late in the process of cisternal maturation. Here, we demonstrate that AP-1 cooperates with the Ent5 clathrin adaptor to recycle a set of Golgi transmembrane proteins, including some that were previously thought to pass through endosomes. This recycling can be detected by removing AP-1 and Ent5, thereby diverting the AP-1/Ent5-dependent Golgi proteins into an alternative recycling loop that involves traffic to the plasma membrane followed by endocytosis. Unexpectedly, various AP-1/Ent5-dependent Golgi proteins show either intermediate or late kinetics of residence in maturing cisternae. We infer that the AP-1/Ent5 pair mediates two sequential intra-Golgi recycling pathways that define two classes of Golgi proteins. This insight can explain the polarized distribution of transmembrane proteins in the Golgi.

Monitoring Intra-Golgi Transport with Acute Spatiotemporal Control of a Synthetic Cargo

We hereby describe a method to image cargo trafficking from the cis- to the trans-face of the Golgi apparatus. Briefly, we combine nocodazole treatment that breaks down the Golgi ribbon, temperature blocks that slow down cargo transport, and a drug-controlled aggregation system that controls the size of the cargo and its retention at different stages of the secretory pathway. Using this method, we first position the cargo within the cis-face of the Golgi. When traffic resumes upon temperature block release, kinetics of transport can be assessed by confocal microscopy through colocalization of the cargo with cis- and trans-Golgi markers. This method allows for testing various modes of intra-Golgi transports and can be adapted to investigate other steps of the secretory pathway.

Mechanisms of membrane traffic in plant cells

The organelles of the secretory pathway are characterized by specific organization and function but they communicate in different ways with intense functional crosstalk. The best known membrane-bound transport carriers are known as protein-coated vesicles. Other traffic mechanisms, despite the intense investigations, still show incongruences. The review intends to provide a general view of the mechanisms involved in membrane traffic. We evidence that organelles' biogenesis involves mechanisms that actively operate during the entire cell cycle and the persistent interconnections between the Endoplasmic reticulum (ER), Golgi apparatus, trans-Golgi network (TGN) and endosomes, the vacuolar complex and the plasma membrane (PM) may be seen as a very dynamic membrane network in which vesicular traffic is part of a general maturation process.

Assay for dual cargo sorting into endoplasmic reticulum exit sites imaged by 3D Super-resolution Confocal Live Imaging Microscopy (SCLIM)

Understanding how in eukaryotic cells thousands of proteins are sorted from each other through the secretory pathway and delivered to their correct destinations is a central issue of cell biology. We have further investigated in yeast how two distinct types of cargo proteins are sorted into different endoplasmic reticulum (ER) exit sites (ERES) for their differential ER export to the Golgi apparatus. We used an optimized protocol that combines a live cell dual-cargo ER export system with a 3D simultaneous multi-color high-resolution live cell microscopy called Super-resolution Confocal Live Imaging Microscopy (SCLIM). Here, we describe this protocol, which is based on the reversible ER retention of two de novo co-expressed cargos by blocking COPII function upon incubation of the thermo-sensitive COPII allele sec31-1 at restrictive temperature (37°C). ER export is restored by shifting down to permissive temperature (24°C) and progressive incorporation of the two different types of cargos into the fluorescently labelled ERES can be then simultaneously captured at 3D high spatial resolution by SCLIM microscopy. By using this protocol, we have shown that newly synthesized glycosylphosphatidylinositol (GPI)-anchored proteins having a very long chain ceramide lipid moiety are clustered and sorted into specialized ERES that are distinct from those used by transmembrane secretory proteins. Furthermore, we showed that the chain length of the ceramide present in the ER membrane is critical for this sorting selectivity. Therefore, thanks to the presented method we could obtain the first direct in vivo evidence for lipid chain length-based protein cargo sorting into selective ERES.

Sending out molecules from the TGN

The sorting of secreted cargo proteins and their export from the trans-Golgi network (TGN) remains an enigma in the field of membrane trafficking; although the sorting mechanisms of many transmembrane proteins have been well described. The sorting of secreted proteins at the TGN is crucial for the release of signaling factors, as well as extracellular matrix proteins. These proteins are required for cell-cell communication and integrity of an organism. Missecretion of these factors can cause diseases such as neurological disorders, autoimmune disease, or cancer. The major open question is how soluble proteins that are not associated with the membrane are packed into TGN derived transport carriers to facilitate their transport to the plasma membrane. Recent investigations have identified novel types of protein and lipid machinery that facilitate the packing of these molecules into a TGN derived vesicle. In addition, novel research has uncovered an exciting link between cargo sorting and export in which TGN structure and dynamics, as well as TGN/endoplasmic reticulum contact sites, play a significant role. Here, we have reviewed the progress made in our understanding of these processes.

Should I stay or should I go? Golgi membrane spatial organization for protein sorting and retention

The Golgi complex is the membrane-bound organelle that lies at the center of the secretory pathway. Its main functions are to maintain cellular lipid homeostasis, to orchestrate protein processing and maturation, and to mediate protein sorting and export. These functions are not independent of one another, and they all require that the membranes of the Golgi complex have a well-defined biochemical composition. Importantly, a finely-regulated spatiotemporal organization of the Golgi membrane components is essential for the correct performance of the organelle. In here, we review our current mechanistic and molecular understanding of how Golgi membranes are spatially organized in the lateral and axial directions to fulfill their functions. In particular, we highlight the current evidence and proposed models of intra-Golgi transport, as well as the known mechanisms for the retention of Golgi residents and for the sorting and export of transmembrane cargo proteins. Despite the controversies, conflicting evidence, clashes between models, and technical limitations, the field has moved forward and we have gained extensive knowledge in this fascinating topic. However, there are still many important questions that remain to be completely answered. We hope that this review will help boost future investigations on these issues.

Identification of SYS1 as a Host Factor Required for Shiga Toxin-Mediated Cytotoxicity in Vero Cells

Shiga toxin (STx) or Vero toxin is a virulence factor produced by enterohemorrhagic Escherichia coli. The toxin binds to the glycosphingolipid globotriaosylceramide (Gb3) for its entry, and causes cell death by inhibiting ribosome function. Previously, we performed a loss-of-function screen in HeLa cells using a human CRISPR knockout (KO) library and identified various host genes required for STx-induced cell death. To determine whether this library targeted to the human genome is applicable to non-human primate cells and to identify previously unrecognized factors crucial for STx-induced cell death, we herein performed a similar screen in the African green monkey kidney-derived Vero C1008 subline. Many genes relevant to metabolic enzymes and membrane trafficking were enriched, although the number of enriched genes was less than that obtained in the screening for HeLa cells. Of note, several genes that had not been enriched in the previous screening were enriched: one of these genes was SYS1, which encodes a multi-spanning membrane protein in the Golgi apparatus. In SYS1 KO Vero cells, expression of Gb3 and sphingomyelin was decreased, while that of glucosylceramide and lactosylceramide was increased. In addition, loss of SYS1 inhibited the biosynthesis of protein glycans, deformed the Golgi apparatus, and perturbed the localization of trans-Golgi network protein (TGN) 46. These results indicate that the human CRISPR KO library is applicable to Vero cell lines, and SYS1 has a widespread effect on glycan biosynthesis via regulation of intra-Golgi and endosome–TGN retrograde transports.

Cargo sorting zones in the trans-Golgi network visualized by super-resolution confocal live imaging microscopy in plants

The trans-Golgi network (TGN) has been known as a key platform to sort and transport proteins to their final destinations in post-Golgi membrane trafficking. However, how the TGN sorts proteins with different destinies still remains elusive. Here, we examined 3D localization and 4D dynamics of TGN-localized proteins of Arabidopsis thaliana that are involved in either secretory or vacuolar trafficking from the TGN, by a multicolor high-speed and high-resolution spinning-disk confocal microscopy approach that we developed. We demonstrate that TGN-localized proteins exhibit spatially and temporally distinct distribution. VAMP721 (R-SNARE), AP (adaptor protein complex)-1, and clathrin which are involved in secretory trafficking compose an exclusive subregion, whereas VAMP727 (R-SNARE) and AP-4 involved in vacuolar trafficking compose another subregion on the same TGN. Based on these findings, we propose that the single TGN has at least two subregions, or "zones", responsible for distinct cargo sorting: the secretory-trafficking zone and the vacuolar-trafficking zone.

Nucleobindin-1 regulates ECM degradation by promoting intra-Golgi trafficking of MMPs

Matrix metalloproteinases (MMPs) degrade several ECM components and are crucial modulators of cell invasion and tissue organization. Although much has been reported about their function in remodeling ECM in health and disease, their trafficking across the Golgi apparatus remains poorly understood. Here we report that the cis-Golgi protein nucleobindin-1 (NUCB1) is critical for MMP2 and MT1-MMP trafficking along the Golgi apparatus. This process is Ca2+-dependent and is required for invasive MDA-MB-231 cell migration as well as for gelatin degradation in primary human macrophages. Our findings emphasize the importance of NUCB1 as an essential component of MMP transport and its overall impact on ECM remodeling.

Retention Using Selective Hooks (RUSH) Cargo Sorting Assay for Live-cell Vesicle Tracking in the Secretory Pathway Using HeLa Cells

More than 30% of the total amount of proteins synthesized in mammalian cells follow the secretory pathway in order to mature and be properly sorted to their final destinations. Among several methodologies that describe live-cell monitoring of vesicles, the Retention Using Selective Hooks (RUSH) system is a powerful one that allows to visualize cargo trafficking under physiological conditions. The present protocol describes a method to use the RUSH system in live-cell microscopy and a subsequent quantitative analysis of cargo vesicles to dissect protein trafficking. In brief, HeLa cells are transiently transfected with an MMP2-RUSH construct and vesicle trafficking is evaluated by wide-field microscopy, recording videos in 1-min time frames for 45 min. We also present a quantitative approach that can be used to identify kinetics of uncharacterized protein cargo, as well as to evaluate with more detail processes such as ER-to-Golgi vesicle trafficking. Graphic abstract: Live-cell RUSH: a tool to monitor real-time protein trafficking in the secretory pathway.

The Fast and the Furious: Golgi Contact Sites

Contact sites are areas of close apposition between two membranes that coordinate nonvesicular communication between organelles. Such interactions serve a wide range of cellular functions from regulating metabolic pathways to executing stress responses and coordinating organelle inheritance. The past decade has seen a dramatic increase in information on certain contact sites, mostly those involving the endoplasmic reticulum. However, despite its central role in the secretory pathway, the Golgi apparatus and its contact sites remain largely unexplored. In this review, we discuss the current knowledge of Golgi contact sites and share our thoughts as to why Golgi contact sites are understudied. We also highlight what exciting future directions may exist in this emerging field.

Sec71 separates Golgi stacks in Drosophila S2 cells

Golgi stacks are the basic structural units of the Golgi. Golgi stacks are separated from each other and scattered in the cytoplasm of Drosophila cells. Here, we report that the ARF-GEF inhibitor Brefeldin A (BFA) induces the formation of BFA bodies, which are aggregates of Golgi stacks, trans-Golgi networks and recycling endosomes. Recycling endosomes are located in the centers of BFA bodies, while Golgi stacks surround them on their trans sides. Live imaging of S2 cells revealed that Golgi stacks repeatedly merged and separated on their trans sides, and BFA caused successive merger by inhibiting separation, forming BFA bodies. S2 cells carrying genome-edited BFA-resistant mutant Sec71M717L did not form BFA bodies at high concentrations of BFA; S2 cells carrying genome-edited BFA-hypersensitive mutant Sec71F713Y produced BFA bodies at low concentrations of BFA. These results indicate that Sec71 is the sole BFA target for BFA body formation and controls Golgi stack separation. Finally, we showed that impairment of Sec71 in fly photoreceptors induces BFA body formation, with accumulation of both apical and basolateral cargoes, resulting in inhibition of polarized transport.

From cartoons to quantitative models in Golgi transport

BACKGROUND

Cell biology is evolving to become a more formal and quantitative science. In particular, several mathematical models have been proposed to address Golgi self-organisation and protein and lipid transport. However, most scientific articles about the Golgi apparatus are still using static cartoons that miss the dynamism of this organelle.

RESULTS

In this report, we show that schematic drawings of Golgi trafficking can be easily translated into an agent-based model using the Repast platform. The simulations generate an active interplay among cisternae and vesicles rendering quantitative predictions about Golgi stability and transport of soluble and membrane-associated cargoes. The models can incorporate complex networks of molecular interactions and chemical reactions by association with COPASI, a software that handles ordinary differential equations.

CONCLUSIONS

The strategy described provides a simple, flexible and multiscale support to analyse Golgi transport. The simulations can be used to address issues directly linked to the mechanism of transport or as a way to incorporate the complexity of trafficking to other cellular processes that occur in dynamic organelles.

SIGNIFICANCE

We show that the rules implicitly present in most schematic representations of intracellular trafficking can be used to build dynamic models with quantitative outputs that can be compared with experimental results.

ESCargo: a regulatable fluorescent secretory cargo for diverse model organisms

Membrane traffic can be studied by imaging a cargo protein as it transits the secretory pathway. The best tools for this purpose initially block export of the secretory cargo from the endoplasmic reticulum (ER) and then release the block to generate a cargo wave. However, previously developed regulatable secretory cargoes are often tricky to use or specific for a single model organism. To overcome these hurdles for budding yeast, we recently optimized an artificial fluorescent secretory protein that exits the ER with the aid of the Erv29 cargo receptor, which is homologous to mammalian Surf4. The fluorescent secretory protein forms aggregates in the ER lumen and can be rapidly disaggregated by addition of a ligand to generate a nearly synchronized cargo wave. Here we term this regulatable secretory protein ESCargo (Erv29/Surf4-dependent secretory cargo) and demonstrate its utility not only in yeast cells, but also in cultured mammalian cells, Drosophila cells, and the ciliate Tetrahymena thermophila. Kinetic studies indicate that rapid export from the ER requires recognition by Erv29/Surf4. By choosing an appropriate ER signal sequence and expression vector, this simple technology can likely be used with many model organisms.

Ceramide chain length-dependent protein sorting into selective endoplasmic reticulum exit sites

Protein sorting in the secretory pathway is crucial to maintain cellular compartmentalization and homeostasis. In addition to coat-mediated sorting, the role of lipids in driving protein sorting during secretory transport is a longstanding fundamental question that still remains unanswered. Here, we conduct 3D simultaneous multicolor high-resolution live imaging to demonstrate in vivo that newly synthesized glycosylphosphatidylinositol-anchored proteins having a very long chain ceramide lipid moiety are clustered and sorted into specialized endoplasmic reticulum exit sites that are distinct from those used by transmembrane proteins. Furthermore, we show that the chain length of ceramide in the endoplasmic reticulum membrane is critical for this sorting selectivity. Our study provides the first direct in vivo evidence for lipid chain length-based protein cargo sorting into selective export sites of the secretory pathway.

Phosphatidylserine biosynthesis pathways in lipid homeostasis: Toward resolution of the pending central issue for decades

Enzymatic control of lipid homeostasis in the cell is a vital element in the complex organization of life. Phosphatidylserine (PS) is an essential anionic phospholipid of cell membranes, and conducts numerous roles for their structural and functional integrity. In mammalian cells, two distinct enzymes phosphatidylserine synthases-1 (PSS1) and -2 (PSS2) in the mitochondria-associated membrane (MAM) in the ER perform de novo synthesis of PS. It is based on base-exchange reactions of the preexisting dominant phospholipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE). While PSS2 specifically catalyzes the reaction "PE → PS," whether or not PSS1 is responsible for the same reaction along with the reaction "PC → PS" remains unsettled despite its fundamental impact on the major stoichiometry. We propose here that a key but the only report that appeared to have put scientists on hold for decades in answering to this issue may be viewed consistently with other available research reports; PSS1 utilizes the two dominant phospholipid classes at a similar intrinsic rate. In this review, we discuss the issue in view of the current information for the enzyme machineries, membrane structure and dynamics, intracellular network of lipid transport, and PS synthesis in health and disease. Resolution of the pending issue is thus critical in advancing our understanding of roles of the essential anionic lipid in biology, health, and disease.

Live-cell Imaging by Super-resolution Confocal Live Imaging Microscopy (SCLIM): Simultaneous Three-color and Four-dimensional Live Cell Imaging with High Space and Time Resolution

Many questions in cell biology can be solved by state-of-the-art technology of live cell imaging. One good example is the mechanism of membrane traffic, in which small membrane carriers are rapidly moving around in the cytoplasm to deliver cargo proteins between organelles. For directly visualizing the events in membrane trafficking system, researchers have long awaited the technology that enables simultaneous multi-color and four-dimensional observation at high space and time resolution. Super-resolution microscopy methods, for example STED, PALM/STORM, and SIM, provide greater spatial resolution, however, these methods are not enough in temporal resolution. The super-resolution confocal live imaging microscopy (SCLIM) that we developed has now achieved the performance required. By using SCLIM, we have conducted high spatiotemporal visualization of secretory cargo together with early and late Golgi resident proteins tagged with three different fluorescence proteins. We have demonstrated that secretory cargo is indeed delivered within the Golgi by cisternal maturation. In addition, we have visualized details of secretory cargo trafficking in the Golgi, including formation of zones within a maturing cisterna, in which Golgi resident proteins are segregated, and movement of cargo between these zones. This protocol can be used for simultaneous three-color and four-dimensional observation of various phenomena in living cells, from yeast to higher plants and animals, at high spatiotemporal resolution.

A microscopy-based kinetic analysis of yeast vacuolar protein sorting

Saccharomyces cerevisiae is amenable to studying membrane traffic by live-cell fluorescence microscopy. We used this system to explore two aspects of cargo protein traffic through prevacuolar endosome (PVE) compartments to the vacuole. First, at what point during Golgi maturation does a biosynthetic vacuolar cargo depart from the maturing cisternae? To address this question, we modified a regulatable fluorescent secretory cargo by adding a vacuolar targeting signal. Traffic of the vacuolar cargo requires the GGA clathrin adaptors, which arrive during the early-to-late Golgi transition. Accordingly, the vacuolar cargo begins to exit the Golgi near the midpoint of maturation, significantly before exit of a secretory cargo. Second, how are cargoes delivered from PVE compartments to the vacuole? To address this question, we tracked biosynthetic and endocytic cargoes after they had accumulated in PVE compartments. The results suggest that stable PVE compartments repeatedly deliver material to the vacuole by a kiss-and-run mechanism.

Recycling endosomes associate with Golgi stacks in sea urchin embryos

The trans-Golgi network (TGN) and recycling endosome (RE) have been recognized as sorting centers, the former for newly synthesized and the latter for endocytosed proteins. However, recent findings have revealed that TGN also receives endocytosed materials and RE accepts newly synthesized proteins destined to the plasma membrane. Recently, we reported that in both Drosophila and microtubule-disrupted HeLa cells, REs are associated with the trans-side of Golgi stacks. REs are highly dynamic: their separation from and association with Golgi stacks are often observed. Importantly, a newly synthesized cargo, glycosylphosphatidylinositol-anchored-GFP was found to be concentrated in Golgi-associated REs (GA-REs), while another cargo VSVG-GFP was excluded from GA-REs before post-Golgi trafficking to the plasma membrane. This suggested that the sorting of cargos takes place at the interface of Golgi stacks and GA-REs. In this study, we demonstrated that REs could associate with Golgi stacks in sea urchin embryos, further indicating that the association of REs with Golgi stacks is a well-conserved phenomenon in the animal kingdom.

Extensive GTPase crosstalk regulates Golgi trafficking and maturation

Virtually all transport events at the Golgi complex are regulated by Arf and Rab family GTPases. Recent work has advanced our knowledge regarding the mechanisms controlling GTPase activity, and it has become clear that GTPases do not act in isolation but rather function in complex networks of crosstalk and feedback. Together with earlier findings, these recent studies indicate that communication between GTPases, their regulatory proteins, effectors, and lipids plays a pivotal role in Golgi transport and cisternal maturation.

Recycling endosomes attach to the trans-side of Golgi stacks in Drosophila and mammalian cells

Historically, the trans-Golgi network (TGN) has been recognized as a sorting center of newly synthesized proteins, whereas the recycling endosome (RE) is a compartment where endocytosed materials transit before being recycled to the plasma membrane. However, recent findings revealed that both the TGN and RE connect endocytosis and exocytosis and, thus, are functionally overlapping. Here we report, in both Drosophila and microtubule-disrupted HeLa cells, that REs are interconvertible between two distinct states, namely Golgi-associated REs and free REs. Detachment and reattachment of REs and Golgi stacks are often observed, and newly synthesized glycosylphosphatidylinositol-anchored cargo protein but not vesicular stomatitis virus G protein is transported through these two types of RE. In plants, there are two types of TGN - Golgi-associated TGN and Golgi-independent TGN. We show that dynamics of REs in both Drosophila and mammalian cells are very similar compared with those of plant TGNs. And, together with the similarity on the molecular level, our results indicate that fly and mammalian REs are organelles that are equivalent to TGNs in plants. This suggests that the identities and functional relationships between REs and TGNs should be reconsidered.

ST3GAL3, ST3GAL4, and ST3GAL6 differ in their regulation of biological functions via the specificities for the α2,3-sialylation of target proteins

The α2,3-sialylation of N-glycans is considered important but complicated because the functions of the three β-galactoside α2,3-sialyltransferases, ST3GAL3, ST3GAL4, and ST3GAL6, could be compensating for one another. To distinguish their specific functions, we established each individual knockout (KO) cell line. Loss of either the ST3GAL3 or ST3GAL6 genes decreased cell proliferation and colony formation, as opposed to the effect in the ST3GAL4 KO cells. The phosphorylation levels of ERK and AKT were significantly suppressed in the ST3GAL6 KO and ST3GAL3 KO cells, respectively. The cell aggregations were clearly observed in the KO cells, particularly the ST3GAL3 KO and ST3GAL6 KO cells, and the expression levels of E-cadherin and claudin-1 were enhanced in both those cell lines, but were suppressed in the ST3GAL4 KO cells. Those alterations were reversed with an overexpression of each corresponding gene in rescued cells. Of particular interest, the α2,3-sialylation levels of β1 integrin were clearly suppressed in the ST3GAL4 KO cells, but these were increased in the ST3GAL3 KO and ST3GAL6 KO cells, whereas the α2,3-sialylation levels of EGFR were significantly decreased in the ST3GAL6 KO cells. The decrease in α2,3-sialylation increased the α2,6-sialylation on β1, but not EGFR. Furthermore, a cross-restoration of each of the three genes in ST3GAL6 KO cells showed that overexpression of ST3GAL6 sufficiently rescued the total α2,3-sialylation levels, cell morphology, and α2,3-sialylation of EGFR, whereas the α2,3-sialylation levels of β1 were greatly enhanced by an overexpression of ST3GAL4. These results clearly demonstrate that the three α2,3-sialyltransferases modify characteristic target proteins and regulate cell biological functions in different ways.

Models of Intracellular Transport: Pros and Cons

Intracellular transport is one of the most confusing issues in the field of cell biology. Many different models and their combinations have been proposed to explain the experimental data on intracellular transport. Here, we analyse the data related to the mechanisms of endoplasmic reticulum-to-Golgi and intra-Golgi transport from the point of view of the main models of intracellular transport; namely: the vesicular model, the diffusion model, the compartment maturation–progression model, and the kiss-and-run model. This review initially describes our current understanding of Golgi function, while highlighting the recent progress that has been made. It then continues to discuss the outstanding questions and potential avenues for future research with regard to the models of these transport steps. To compare the power of these models, we have applied the method proposed by K. Popper; namely, the formulation of prohibitive observations according to, and the consecutive evaluation of, previous data, on the basis on the new models. The levels to which the different models can explain the experimental observations are different, and to date, the most powerful has been the kiss-and-run model, whereas the least powerful has been the diffusion model.

A Kinetic View of Membrane Traffic Pathways Can Transcend the Classical View of Golgi Compartments

A long-standing assumption is that the cisternae of the Golgi apparatus can be grouped into functionally distinct compartments, yet the molecular identities of those compartments have not been clearly described. The concept of a compartmentalized Golgi is challenged by the cisternal maturation model, which postulates that cisternae form de novo and then undergo progressive biochemical changes. Cisternal maturation can potentially be reconciled with Golgi compartmentation by defining compartments as discrete kinetic stages in the maturation process. These kinetic stages are distinguished by the traffic pathways that are operating. For example, a major transition occurs when a cisterna stops producing COPI vesicles and begins producing clathrin-coated vesicles. This transition separates one kinetic stage, the "early Golgi," from a subsequent kinetic stage, the "late Golgi" or "trans-Golgi network (TGN)." But multiple traffic pathways drive Golgi maturation, and the periods of operation for different traffic pathways can partially overlap, so there is no simple way to define a full set of Golgi compartments in terms of kinetic stages. Instead, we propose that the focus should be on the series of transitions experienced by a Golgi cisterna as various traffic pathways are switched on and off. These traffic pathways drive changes in resident transmembrane protein composition. Transitions in traffic pathways seem to be the fundamental, conserved determinants of Golgi organization. According to this view, the initial goal is to identify the relevant traffic pathways and place them on the kinetic map of Golgi maturation, and the ultimate goal is to elucidate the logic circuit that switches individual traffic pathways on and off as a cisterna matures.

Spatiotemporal dissection of the trans-Golgi network in budding yeast

The trans-Golgi network (TGN) acts as a sorting hub for membrane traffic. It receives newly synthesized and recycled proteins, and sorts and delivers them to specific targets such as the plasma membrane, endosomes and lysosomes/vacuoles. Accumulating evidence suggests that the TGN is generated from the trans-most cisterna of the Golgi by maturation, but the detailed transition processes remain obscure. Here, we examine spatiotemporal assembly dynamics of various Golgi/TGN-resident proteins in budding yeast by high-speed and high-resolution spinning-disk confocal microscopy. The Golgi–TGN transition gradually proceeds via at least three successive stages: the ‘Golgi stage’ where glycosylation occurs; the ‘early TGN stage’, which receives retrograde traffic; and the ‘late TGN stage’, where transport carriers are produced. During the stage transition periods, earlier and later markers are often compartmentalized within a cisterna. Furthermore, for the late TGN stage, various types of coat/adaptor proteins exhibit distinct assembly patterns. Taken together, our findings characterize the identity of the TGN as a membrane compartment that is structurally and functionally distinguishable from the Golgi.

This article has an associated First Person interview with the first author of the paper.

Highlighted Article: The TGN displays two sub-stages of maturation: ‘early TGN’, when retrograde traffic is received, and ‘late TGN’, when transport carriers are produced. At the late TGN, various coat/adaptor proteins exhibit distinct assembly dynamics.

Lipid-dependent coupling of secretory cargo sorting and trafficking at the trans-Golgi network

In eukaryotic cells, the trans-Golgi network (TGN) serves as a platform for secretory cargo sorting and trafficking. In recent years, it has become evident that a complex network of lipid–lipid and lipid–protein interactions contributes to these key functions. This review addresses the role of lipids at the TGN with a particular emphasis on sphingolipids and diacylglycerol. We further highlight how these lipids couple secretory cargo sorting and trafficking for spatiotemporal coordination of protein transport to the plasma membrane.

Examination of Mitotic and Meiotic Fission Yeast Nuclear Dynamics by Fluorescence Live-cell Microscopy

Live-cell imaging is a microscopy technique used to examine cell and protein dynamics in living cells. This imaging method is not toxic, generally does not interfere with cell physiology, and requires minimal experimental handling. The low levels of technical interference enable researchers to study cells across multiple cycles of mitosis and to observe meiosis from beginning to end. Using fluorescent tags such as Green Fluorescent Protein (GFP) and Red Fluorescent Protein (RFP), researchers can analyze different factors whose functions are important for processes like transcription, DNA replication, cohesion, and segregation. Coupled with data analysis using Fiji (a free, optimized ImageJ version), live-cell imaging offers various ways of assessing protein movement, localization, stability, and timing, as well as nuclear dynamics and chromosome segregation. However, as is the case with other microscopy methods, live-cell imaging is limited by the intrinsic properties of light, which put a limit to the resolution power at high magnifications, and is also sensitive to photobleaching or phototoxicity at high wavelength frequencies. However, with some care, investigators can bypass these physical limitations by carefully choosing the right conditions, strains, and fluorescent markers to allow for the appropriate visualization of mitotic and meiotic events.

Acylation - A New Means to Control Traffic Through the Golgi

The Golgi is well known to act as center for modification and sorting of proteins for secretion and delivery to other organelles. A key sorting step occurs at the trans-Golgi network and is mediated by protein adapters. However, recent data indicate that sorting also occurs much earlier, at the cis-Golgi, and uses lipid acylation as a novel means to regulate anterograde flux. Here, we examine an emerging role of S-palmitoylation/acylation as a mechanism to regulate anterograde routing. We discuss the critical Golgi-localized DHHC S-palmitoyltransferase enzymes that orchestrate this lipid modification, as well as their diverse protein clients (e.g., MAP6, SNAP25, CSP, LAT, β-adrenergic receptors, GABA receptors, and GLUT4 glucose transporters). Critically, for integral membrane proteins, S-acylation can act as new a “self-sorting” signal to concentrate these cargoes in rims of Golgi cisternae, and to promote their rapid traffic through the Golgi or, potentially, to bypass the Golgi. We discuss this mechanism and examine its potential relevance to human physiology and disease, including diabetes and neurodegenerative diseases.

Maturation-driven transport and AP-1-dependent recycling of a secretory cargo in the Golgi

The Golgi cisternal maturation model predicts that secretory cargo proteins should be continuously present within the cisternae while resident Golgi proteins come and go. Casler et al. verify this prediction by tracking the passage of a fluorescent secretory cargo through the yeast Golgi.

Golgi cisternal maturation has been visualized by fluorescence imaging of individual cisternae in the yeast Saccharomyces cerevisiae, but those experiments did not track passage of a secretory cargo. The expectation is that a secretory cargo will be continuously present within maturing cisternae as resident Golgi proteins arrive and depart. We tested this idea using a regulatable fluorescent secretory cargo that forms ER-localized aggregates, which dissociate into tetramers upon addition of a ligand. The solubilized tetramers rapidly exit the ER and then transit through early and late Golgi compartments before being secreted. Early Golgi cisternae form near the ER and become loaded with the secretory cargo. As predicted, cisternae contain the secretory cargo throughout the maturation process. An unexpected finding is that a burst of intra-Golgi recycling delivers additional secretory cargo molecules to cisternae during the early-to-late Golgi transition. This recycling requires the AP-1 adaptor, suggesting that AP-1 can recycle secretory cargo proteins as well as resident Golgi proteins.