Capacity of the golgi apparatus for biogenesis from the endoplasmic reticulum

ALS/FTD-associated mutation in cyclin F inhibits ER-Golgi trafficking, inducing ER stress, ERAD and Golgi fragmentation

Amyotrophic lateral sclerosis (ALS) is a severely debilitating neurodegenerative condition that is part of the same disease spectrum as frontotemporal dementia (FTD). Mutations in the CCNF gene, encoding cyclin F, are present in both sporadic and familial ALS and FTD. However, the pathophysiological mechanisms underlying neurodegeneration remain unclear. Proper functioning of the endoplasmic reticulum (ER) and Golgi apparatus compartments is essential for normal physiological activities and to maintain cellular viability. Here, we demonstrate that ALS/FTD-associated variant cyclin FS621G inhibits secretory protein transport from the ER to Golgi apparatus, by a mechanism involving dysregulation of COPII vesicles at ER exit sites. Consistent with this finding, cyclin FS621G also induces fragmentation of the Golgi apparatus and activates ER stress, ER-associated degradation, and apoptosis. Induction of Golgi fragmentation and ER stress were confirmed with a second ALS/FTD variant cyclin FS195R, and in cortical primary neurons. Hence, this study provides novel insights into pathogenic mechanisms associated with ALS/FTD-variant cyclin F, involving perturbations to both secretory protein trafficking and ER-Golgi homeostasis.

Breaking Spatiotemporal Barriers of Immunogenic Chemotherapy via an Endoplasmic Reticulum Membrane-Assisted Liposomal Drug Delivery

Immunogenic chemotherapy is a promising approach in cancer treatment, but the number of drugs capable of inducing immunogenic cell death is limited, and chronic immunogenic exposure can delay antitumor immune response and be counteracted by immunosuppressive factors. In this study, we used single-cell and multilevel analyses to highlight the critical importance of the first exposure to calreticulin (CRT) in eliciting immunogenicity. We then developed the ERASION (endoplasmic reticulum (ER) membrane to assist (AS) the presentation of intrinsic onco-immunogenicity (ION)) strategy, leveraging the high expression of functional proteins, including CRT, on the ER membrane. ER membrane-coated liposome (ER@PLip) was able to target the tumor and immune effectors and promoted dendritic cell maturation and T cell infiltration. This enabled eliciting an immunogenic effect from a nonimmunogenic chemotherapeutic drug. By utilizing the ER membrane-associated STING protein, ERASION enabled activating the STING pathway and the generation of adaptive antitumor immunity. This study presents a potential universal platform for integrating traditional chemotherapy and therapeutic modalities.

Caveolae-Mediated Endocytosis Drives Robust siRNA Delivery of Polymeric Nanoparticles to Macrophages

Cytosolic delivery of small interfering RNA (siRNA) remains challenging, and a profound understanding of the cellular uptake and intracellular processing of siRNA delivery systems could greatly improve the development of siRNA-based therapeutics. Here, we show that caveolae-mediated endocytosis (CvME) accounts for the robust siRNA delivery of mannose-modified trimethyl chitosan-cysteine/tripolyphosphate nanoparticles (MTC/TPP NPs) to macrophages by circumventing lysosomes. We show that the Golgi complex and ER are key organelles required for the efficient delivery of siRNA to macrophages in which the siRNA accumulation positively correlates with its silencing efficiency (r = 0.94). We also identify syntaxin6 and Niemann-Pick type C1 (NPC1) as indispensable regulators for MTC/TPP NPs-delivered siRNA into macrophages both in vitro and in vivo. Syntaxin6 and NPC1 knockout substantially decrease the cellular uptake and gene silencing of the siRNA delivered in MTC/TPP NPs in macrophages, which result in poor therapeutic outcomes for mice bearing acute hepatic injury. Our results suggest that highly efficient siRNA delivery can be achieved via CvME, which would give ideas for designing optimal delivery vectors to facilitate the clinical translation of siRNA drugs.

Post-ER Stress Biogenesis of Golgi Is Governed by Giantin

BACKGROUND

The Golgi apparatus undergoes disorganization in response to stress, but it is able to restore compact and perinuclear structure under recovery. This self-organization mechanism is significant for cellular homeostasis, but remains mostly elusive, as does the role of giantin, the largest Golgi matrix dimeric protein.

METHODS

In HeLa and different prostate cancer cells, we used the model of cellular stress induced by Brefeldin A (BFA). The conformational structure of giantin was assessed by proximity ligation assay and atomic force microscopy. The post-BFA distribution of Golgi resident enzymes was examined by 3D SIM high-resolution microscopy.

RESULTS

We detected that giantin is rather flexible than an extended coiled-coil dimer and BFA-induced Golgi disassembly was associated with giantin monomerization. A fusion of the nascent Golgi membranes after BFA washout is forced by giantin re-dimerization via disulfide bond in its luminal domain and assisted by Rab6a GTPase. GM130-GRASP65-dependent enzymes are able to reach the nascent Golgi membranes, while giantin-sensitive enzymes appeared at the Golgi after its complete recovery via direct interaction of their cytoplasmic tail with N-terminus of giantin.

CONCLUSION

Post-stress recovery of Golgi is conducted by giantin dimer and Golgi proteins refill membranes according to their docking affiliation rather than their intra-Golgi location.

Regulating intracellular fate of siRNA by endoplasmic reticulum membrane-decorated hybrid nanoplexes

Most cationic vectors are difficult to avoid the fate of small interfering RNA (siRNA) degradation following the endosome-lysosome pathway during siRNA transfection. In this study, the endoplasmic reticulum (ER) membrane isolated from cancer cells was used to fabricate an integrative hybrid nanoplexes (EhCv/siRNA NPs) for improving siRNA transfection. Compared to the undecorated Cv/siEGFR NPs, the ER membrane-decorated EhCv/siRNA NPs exhibits a significantly higher gene silencing effect of siRNA in vitro and a better antitumor activity in nude mice bearing MCF-7 human breast tumor in vivo. Further mechanistic studies demonstrate that functional proteins on the ER membrane plays important roles on improving cellular uptake and altering intracellular trafficking pathway of siRNA. It is worth to believe that the ER membrane decoration on nanoplexes can effectively transport siRNA through the endosome-Golgi-ER pathway to evade lysosomal degradation and enhance the silencing effects of siRNA.

Stochastic Model of Vesicular Sorting in Cellular Organelles

The proper sorting of membrane components by regulated exchange between cellular organelles is crucial to intracellular organization. This process relies on the budding and fusion of transport vesicles, and should be strongly influenced by stochastic fluctuations, considering the relatively small size of many organelles. We identify the perfect sorting of two membrane components initially mixed in a single compartment as a first passage process, and we show that the mean sorting time exhibits two distinct regimes as a function of the ratio of vesicle fusion to budding rates. Low ratio values lead to fast sorting but result in a broad size distribution of sorted compartments dominated by small entities. High ratio values result in two well-defined sorted compartments but sorting is exponentially slow. Our results suggest an optimal balance between vesicle budding and fusion for the rapid and efficient sorting of membrane components and highlight the importance of stochastic effects for the steady-state organization of intracellular compartments.

The Golgi entry core compartment functions as a COPII-independent scaffold for ER-to-Golgi transport in plant cells

Many questions remain about how the stacked structure of the Golgi is formed and maintained. In our previous study, we challenged this question using tobacco BY-2 cells and revealed that, upon Brefeldin A (BFA) treatment, previously undescribed small punctate structures containing a particular subset of cis-Golgi proteins are formed adjacent to the ER-exit sites and act as scaffolds for Golgi regeneration after BFA removal. In this study, we analyzed these structures further. The proteins that localize to these punctate structures originate from the cis-most cisternae. 3D time-lapse observations show that the trans-Golgi marker is transported through these structures during Golgi regeneration. These data indicate that the cis-most cisternae have a specialized region that receives cargo from the ER, which becomes obvious upon BFA treatment. Expression of a dominant mutant form of SAR1 does not affect the formation of the punctate structures. We propose to call these punctate structures the 'Golgi entry core compartment' (GECCO). They act as receivers for the rest of the Golgi materials and are formed independently of the COPII machinery.This article has an associated First Person interview with the first author of the paper.

Nonequilibrium description of de novo biogenesis and transport through Golgi-like cisternae

A central issue in cell biology is the physico-chemical basis of organelle biogenesis in intracellular trafficking pathways, its most impressive manifestation being the biogenesis of Golgi cisternae. At a basic level, such morphologically and chemically distinct compartments should arise from an interplay between the molecular transport and chemical maturation. Here, we formulate analytically tractable, minimalist models, that incorporate this interplay between transport and chemical progression in physical space, and explore the conditions for de novo biogenesis of distinct cisternae. We propose new quantitative measures that can discriminate between the various models of transport in a qualitative manner–this includes measures of the dynamics in steady state and the dynamical response to perturbations of the kind amenable to live-cell imaging.

Microtubule-independent secretion requires functional maturation of Golgi elements

The Golgi apparatus is responsible for processing and sorting of secretory cargos. Microtubules are known to accelerate the transport of proteins from the endoplasmic reticulum to the Golgi apparatus and from the Golgi to the plasma membrane. However, whether post-Golgi transport strictly requires microtubules is still unclear. Using the retention using selective hooks (RUSH) system to synchronize the trafficking of cargos, we show that anterograde transport of tumor necrosis factor (TNF) is strongly reduced without microtubules. We show that two populations of Golgi elements co-exist in these cells. A centrally located and giantin-positive Golgi complex sustains trafficking while newly formed peripheral Golgi mini-stacks accumulate cargos in cells without microtubules. Using a genome-edited GFP-giantin cell line, we observe that the trafficking-competent Golgi population corresponds to the pre-existing one that was present before removal of microtubules. All Golgi elements support trafficking after long-term microtubules depletion or after relocation of Golgi proteins in the endoplasmic reticulum using Brefeldin A. Our results demonstrate that functional maturation of Golgi elements is needed to ensure post-Golgi trafficking and that microtubule-driven post-Golgi transport is not strictly required.

ER trapping reveals Golgi enzymes continually revisit the ER through a recycling pathway that controls Golgi organization

Whether Golgi enzymes remain localized within the Golgi or constitutively cycle through the endoplasmic reticulum (ER) is unclear, yet is important for understanding Golgi dependence on the ER. Here, we demonstrate that the previously reported inefficient ER trapping of Golgi enzymes in a rapamycin-based assay results from an artifact involving an endogenous ER-localized 13-kD FK506 binding protein (FKBP13) competing with the FKBP12-tagged Golgi enzyme for binding to an FKBP-rapamycin binding domain (FRB)-tagged ER trap. When we express an FKBP12-tagged ER trap and FRB-tagged Golgi enzymes, conditions precluding such competition, the Golgi enzymes completely redistribute to the ER upon rapamycin treatment. A photoactivatable FRB-Golgi enzyme, highlighted only in the Golgi, likewise redistributes to the ER. These data establish Golgi enzymes constitutively cycle through the ER. Using our trapping scheme, we identify roles of rab6a and calcium-independent phospholipase A2 (iPLA2) in Golgi enzyme recycling, and show that retrograde transport of Golgi membrane underlies Golgi dispersal during microtubule depolymerization and mitosis.

Golgi fragmentation in amyotrophic lateral sclerosis, an overview of possible triggers and consequences

Amyotrophic Lateral Sclerosis (ALS) is an invariably fatal neurodegenerative disorder, which specifically targets motor neurons in the brain, brain stem and spinal cord. Whilst the etiology of ALS remains unknown, fragmentation of the Golgi apparatus is detected in ALS patient motor neurons and in animal/cellular disease models. The Golgi is a highly dynamic organelle that acts as a dispatching station for the vesicular transport of secretory/transmembrane proteins. It also mediates autophagy and maintains endoplasmic reticulum (ER) and axonal homeostasis. Both the trigger for Golgi fragmentation and the functional consequences of a fragmented Golgi apparatus in ALS remain unclear. However, recent evidence has highlighted defects in vesicular trafficking as a pathogenic mechanism in ALS. This review summarizes the evidence describing Golgi fragmentation in ALS, with possible links to other disease processes including cellular trafficking, ER stress, defective autophagy, and axonal degeneration.

Formation and maintenance of the Golgi apparatus in plant cells

The Golgi apparatus plays essential roles in intracellular trafficking, protein and lipid modification, and polysaccharide synthesis in eukaryotic cells. It is well known for its unique stacked structure, which is conserved among most eukaryotes. However, the mechanisms of biogenesis and maintenance of the structure, which are deeply related to ER-Golgi and intra-Golgi transport systems, have long been mysterious. Now having extremely powerful microscopic technologies developed for live-cell imaging, the plant Golgi apparatus provides an ideal system to resolve the question. The plant Golgi apparatus has unique features that are not conserved in other kingdoms, which will also give new insights into the Golgi functions in plant life. In this review, we will summarize the features of the plant Golgi apparatus and transport mechanisms around it, with a focus on recent advances in Golgi biogenesis by live imaging of plants cells.

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.

Binding of the vesicle docking protein p115 to the GTPase Rab1b regulates membrane recruitment of the COPI vesicle coat

Membrane recruitment of the COPI vesicle coat is fundamental to its function and contributes to compartment identity in the early secretory pathway. COPI recruitment is triggered by guanine nucleotide exchange activating the Arf1 GTPase, but the key exchange factor, GBF1, is a peripheral membrane component whose membrane association is dependent on another GTPase, Rab1. Inactive Rab GTPases are in a soluble complex with guanine nucleotide dissociation inhibitor (GDI) and activation of Rab GTPases by exchange factors can be enhanced by GDI dissociation factors (GDFs). In the present study, we investigated the vesicle docking protein p115 and it's binding to the Rab1 isoform Rab1b. Inhibition of p115 expression induced dissociation of Rab1b from Golgi membranes. Rab1b bound the cc2 domain of p115 and p115 lacking this domain failed to recruit Rab1b. Further, p115 inhibition blocked association of the COPI coat with Golgi membranes and this was suppressed by constitutive activation of Rab1b. These findings show p115 enhancement of Rab1b activation leading to COPI recruitment suggesting a connection between the vesicle docking machinery and the vesicle coat complex during the establishment of post-ER compartment identity.

Golgi depletion from living cells with laser nanosurgery

How Golgi biogenesis occurs in mammalian cells is a controversial problem. Can the Golgi complex (GC) form de novo from ER membranes or does it require a template? The method described in this chapter uses growth of cells on micropatterns to displace the GC from its juxtanuclear position and laser nanosurgery to subsequently deplete it from living cells. Golgi-depleted karyoplasts can be followed by time-lapse microscopy to address if and how the GC can be de novo synthesized from ER membranes. Furthermore, the study of different processes in the absence of the GC can shed light on the role of this organelle in the intracellular signaling and homeostasis.

Isoform-specific tethering links the Golgi ribbon to maintain compartmentalization

Use of photoinactivation, cisternae-specific fluorescence recovery, and high-resolution microscopy shows that the membrane tethers GRASP65 and GRASP55 on early and late Golgi membranes, respectively, are critical to the specific, homotypic fusion of the membranes on which they reside.

Homotypic membrane tethering by the Golgi reassembly and stacking proteins (GRASPs) is required for the lateral linkage of mammalian Golgi ministacks into a ribbon-like membrane network. Although GRASP65 and GRASP55 are specifically localized to cis and medial/trans cisternae, respectively, it is unknown whether each GRASP mediates cisternae-specific tethering and whether such specificity is necessary for Golgi compartmentalization. Here each GRASP was tagged with KillerRed (KR), expressed in HeLa cells, and inhibited by 1-min exposure to light. Significantly, inactivation of either GRASP unlinked the Golgi ribbon, and the immediate effect of GRASP65-KR inactivation was a loss of cis- rather than trans-Golgi integrity, whereas inactivation of GRASP55-KR first affected the trans- and not the cis-Golgi. Thus each GRASP appears to play a direct and cisternae-specific role in linking ministacks into a continuous membrane network. To test the consequence of loss of cisternae-specific tethering, we generated Golgi membranes with a single GRASP on all cisternae. Remarkably, the membranes exhibited the full connectivity of wild-type Golgi ribbons but were decompartmentalized and defective in glycan processing. Thus the GRASP isoforms specifically link analogous cisternae to ensure Golgi compartmentalization and proper processing.

The ER-Golgi intermediate compartment is a key membrane source for the LC3 lipidation step of autophagosome biogenesis

Autophagy is a catabolic process for bulk degradation of cytosolic materials mediated by double-membraned autophagosomes. The membrane determinant to initiate the formation of autophagosomes remains elusive. Here, we establish a cell-free assay based on LC3 lipidation to define the organelle membrane supporting early autophagosome formation. In vitro LC3 lipidation requires energy and is subject to regulation by the pathways modulating autophagy in vivo. We developed a systematic membrane isolation scheme to identify the endoplasmic reticulum-Golgi intermediate compartment (ERGIC) as a primary membrane source both necessary and sufficient to trigger LC3 lipidation in vitro. Functional studies demonstrate that the ERGIC is required for autophagosome biogenesis in vivo. Moreover, we find that the ERGIC acts by recruiting the early autophagosome marker ATG14, a critical step for the generation of preautophagosomal membranes. DOI:http://dx.doi.org/10.7554/eLife.00947.001.

cis-Golgi proteins accumulate near the ER exit sites and act as the scaffold for Golgi regeneration after brefeldin A treatment in tobacco BY-2 cells

The Golgi apparatus forms stacks of cisternae in many eukaryotic cells. However, little is known about how such a stacked structure is formed and maintained. To address this question, plant cells provide a system suitable for live-imaging approaches because individual Golgi stacks are well separated in the cytoplasm. We established tobacco BY-2 cell lines expressing multiple Golgi markers tagged by different fluorescent proteins and observed their responses to brefeldin A (BFA) treatment and BFA removal. BFA treatment disrupted cis, medial, and trans cisternae but caused distinct relocalization patterns depending on the proteins examined. Medial- and trans-Golgi proteins, as well as one cis-Golgi protein, were absorbed into the endoplasmic reticulum (ER), but two other cis-Golgi proteins formed small punctate structures. After BFA removal, these puncta coalesced first, and then the Golgi stacks regenerated from them in the cis-to-trans order. We suggest that these structures have a property similar to the ER-Golgi intermediate compartment and function as the scaffold of Golgi regeneration.

Non-muscle myosin IIA transports a Golgi glycosyltransferase to the endoplasmic reticulum by binding to its cytoplasmic tail

The mechanism of the Golgi-to-ER transport of Golgi glycosyltransferases is not clear. We utilize a cell line expressing the core 2 N-acetylglucosaminyltransferase-M (C2GnT-M) tagged with c-Myc to explore this mechanism. By immunoprecipitation using anti-c-Myc antibodies coupled with proteomics analysis, we have identified several proteins including non-muscle myosin IIA (NMIIA), heat shock protein (HSP)-70 and ubiquitin activating enzyme E1 in the immunoprecipitate. Employing yeast-two-hybrid analysis and pulldown experiments, we show that the C-terminal region of the NMIIA heavy chain binds to the 1-6 amino acids in the cytoplasmic tail of C2GnT-M. We have found that NMIIA co-localizes with C2GnT-M at the periphery of the Golgi. In addition, inhibition or knockdown of NMIIA prevents the brefeldin A-induced collapse of the Golgi as shown by the inhibition of the migration of both Giantin, a Golgi matrix protein, and C2GnT-M, a Golgi non-matrix protein, to the ER. In contrast, knockdown of HSP70 retains Giantin in the Golgi but moves C2GnT-M to the ER, a process also blocked by inhibition or knockdown of NMIIA. Also, the intracellular distribution of C2GnT-M is not affected by knockdown of β-coatomer protein with or without inhibition of HSPs, suggesting that the Golgi-to-ER trafficking of C2GnT-M does not depend on coat protein complex-I. Further, inhibition of proteasome results in accumulation of ubiquitinated C2GnT-M, suggesting its degradation by proteasome. Therefore, NMIIA and not coat protein complex-I is responsible for transporting the Golgi glycosyltransferase to the ER for proteasomal degradation. The data suggest that NMIIA is involved in the Golgi remodeling.

Modular organization of the mammalian Golgi apparatus

The Golgi apparatus is essential for post-translational modifications and sorting of proteins in the secretory pathway. In addition, it further performs a broad range of specialized functions. This functional diversity is achieved by combining basic morphological modules of cisternae into higher ordered structures. Linking cisternae into stacks that are further connected through tubules into a continuous Golgi ribbon greatly increases its efficiency and expands its repertoire of functions. During cell division, the different modules of the Golgi are inherited by different mechanisms to maintain its functional and morphological composition.

Sar1 translocation onto the ER-membrane for vesicle budding has different pathways for promotion and suppression of ER-to-Golgi transport mediated through H89-sensitive kinase and ER-resident G protein

ER-to-Golgi protein transport involves transport vesicles of which formation is initiated by assembly of Sar1. The assembly of Sar1 is suppressed by protein kinase inhibitor H89, suggesting that ER-to-Golgi transport is regulated progressively by H89 sensitive kinase. ER-resident G(i2) protein suppresses vesicle formation with inhibition of Sar1 assembly. This study examined whether these promotion and suppression of vesicle transport share the same signal pathway, by examining the effects of G(i/o) protein activator mastoparan 7 (Mp-7) and H89 on Sar1 and Sec23 recruitment onto microsomes. In a cell-free system for Sar1 translocation assay, GTPγS addition induced the translocation of Sar1 onto microsomes. Mp-7 and H89 decreased the Sar1 translocation. Double treatment of Mp-7 and H89 strongly decreased Sar1 translocation. In single and double treatments, however, G(i/o) protein inactivator pertussis toxin (IAP) partially restored the suppressive effect of Mp-7, but had not any effect on H89-induced effect. Then, the assembly of Sec23 onto the microsome was also increased by the addition of GTPγS. Sec23 translocation was decreased by Mp-7 and/or H89 treatment and recovered by IAP pretreatment except for H89 single treatment, similarly to Sar1 translocation in each treatment. Inhibitory effects of H89 and Mp-7on ER-to-Golgi vesicle transport by H89 or Mp-7 were also confirmed in a cell culture system by BFA-dispersion and BFA-reconstruction experiments. These findings indicate that promotion and suppression of ER-to-Golgi vesicle transport are modulated through separate signal pathways.

Live-cell assays to identify regulators of ER-to-Golgi trafficking

We applied fluorescence microscopy-based quantitative assays to living cells to identify regulators of endoplasmic reticulum (ER)-to-Golgi trafficking and/or Golgi complex maintenance. We first validated an automated procedure to identify factors which influence Golgi-to-ER relocalization of GalT-CFP (β1,4-galactosyltransferase I-cyan fluorescent protein) after brefeldin A (BFA) addition and/or wash-out. We then tested 14 proteins that localize to the ER and/or Golgi complex when overexpressed for a role in ER-to-Golgi trafficking. Nine of them interfered with the rate of BFA-induced redistribution of GalT-CFP from the Golgi complex to the ER, six of them interfered with GalT-CFP redistribution from the ER to a juxtanuclear region (i.e. the Golgi complex) after BFA wash-out and six of them were positive effectors in both assays. Notably, our live-cell approach captures regulator function in ER-to-Golgi trafficking, which was missed in previous fixed cell assays, as well as assigns putative roles for other less characterized proteins. Moreover, we show that our assays can be extended to RNAi and chemical screens.

A novel laser nanosurgery approach supports de novo Golgi biogenesis in mammalian cells

The Golgi complex has a central role in the secretory pathway of all higher organisms. To explain the synthesis of its unique stacked structure in mammalian cells, two major models have been proposed. One suggests that it is synthesized de novo from the endoplasmic reticulum. The second model postulates a pre-existing Golgi template that serves as a scaffold for its biogenesis. To test these hypotheses directly, we have developed an approach in which we deplete the Golgi complex from living cells by laser nanosurgery, and subsequently analyze the 'Golgi-depleted' karyoplast using time-lapse and electron microscopy. We show that biosynthetic transport is blocked after Golgi depletion, but is restored 12 hours later. This recovery of secretory transport coincides with an ordered assembly of stacked Golgi structures, and we also observe the appearance of matrix proteins before that of Golgi enzymes. Functional experiments using RNA interference-mediated knockdown of GM130 further demonstrate the importance of the matrix during Golgi biogenesis. By contrast, the centrosome, which can also be removed by laser nanosurgery and is not reformed within the considered time frame, is not required for this process. Altogether, our data provide evidence that de novo Golgi biogenesis can occur in mammalian cells.

Unraveling the Golgi ribbon

The Golgi apparatus lies at the heart of the secretory pathway where it receives, modifies and sorts protein cargo to the proper intracellular or extracellular location. Although this secretory function is highly conserved throughout the eukaryotic kingdom, the structure of the Golgi complex is arranged very differently among species. In particular, Golgi membranes in vertebrate cells are integrated into a single compact entity termed the Golgi ribbon that is normally localized in the perinuclear area and in close vicinity to the centrosomes. This organization poses a challenge for cell division when the single Golgi ribbon needs to be partitioned into the two daughter cells. To ensure faithful inheritance in the progeny, the Golgi ribbon is divided in three consecutive steps in mitosis, namely disassembly, partitioning and reassembly. However, the structure of the Golgi ribbon is only present in higher animals and Golgi disassembly during mitosis is not ubiquitous in all organisms. Therefore, there must be unique reasons to build up the Golgi in this particular conformation and to preserve it over generations. In this review, we first highlight the diversity of the Golgi architecture in different organisms and revisit the concept of the Golgi ribbon. Following on, we discuss why the ribbon is needed and how it forms in vertebrate cells. Lastly, we conclude with likely purposes of mitotic ribbon disassembly and further propose mechanisms by which it regulates mitosis.

Sequential depletion and acquisition of proteins during Golgi stack disassembly and reformation

Herein, we report the stepwise transport of multiple plant Golgi membrane markers during disassembly of the Golgi apparatus in tobacco leaf epidermal cells in response to the induced expression of the GTP-locked Sar1p or Brefeldin A (BFA), and reassembly on BFA washout. The distribution of fluorescent Golgi-resident N-glycan processing enzymes and matrix proteins (golgins) with specific cis-trans-Golgi sub-locations was followed by confocal microscopy during disassembly and reassembly. The first event during Golgi disassembly was the loss of trans-Golgi enzymes and golgins from Golgi membranes, followed by a sequential redistribution of medial and cis-Golgi enzymes into the endoplasmic reticulum (ER), whilst golgins were relocated to the ER or cytoplasm. This event was confirmed by fractionation and immuno-blotting. The sequential redistribution of Golgi components in a trans-cis sequence may highlight a novel retrograde trafficking pathway between the trans-Golgi and the ER in plants. Release of Golgi markers from the ER upon BFA washout occurred in the opposite sequence, with cis-matrix proteins labelling Golgi-like structures before cis/medial enzymes. Trans-enzyme location was preceded by trans-matrix proteins being recruited back to Golgi membranes. Our results show that Golgi disassembly and reassembly occur in a highly ordered fashion in plants.

Storage vesicles in neurons are related to Golgi complex alterations in mucopolysaccharidosis IIIB

The accumulation of intracellular storage vesicles is a hallmark of lysosomal storage diseases. Neither the identity nor origin of these implicated storage vesicles have yet been established. The vesicles are often considered as lysosomes, endosomes, and/or autophagosomes that are engorged with undigested materials. Our studies in the mouse model of mucopolysaccharidosis type IIIB, a lysosomal storage disease that induces neurodegeneration, showed that large storage vesicles in cortical neurons did not receive material from either the endocytic or autophagy pathway, which functioned normally. Storage vesicles expressed GM130, a Golgi matrix protein, which mediates vesicle tethering in both pre- and cis-Golgi compartments. However, other components of the tethering/fusion complex were not associated with GM130 on storage vesicles, likely accounting for both the resistance of the vesicles to brefeldin A and the alteration of Golgi ribbon architecture, which comprised distended cisterna connected to LAMP1-positive storage vesicles. We propose that alteration in the GM130-mediated control of vesicle trafficking in pre-Golgi and Golgi compartments affects Golgi biogenesis and gives rise to a dead-end storage compartment. Vesicle accumulation, Golgi disorganization, and alterations of other GM130 functions may account for neuron dysfunction and death.

A modeling approach to the self-assembly of the Golgi apparatus

The dynamic compartmentalization of eukaryotic cells is a fascinating phenomenon that is not yet understood. A prominent example of this challenge is the Golgi apparatus, the central hub for protein sorting and lipid metabolism in the secretory pathway. Despite major advances in elucidating its molecular biology, the fundamental question of how the morphogenesis of this organelle is organized on a system level has remained elusive. Here, we have formulated a coarse-grained computational model that captures key features of the dynamic morphogenesis of a Golgi apparatus. In particular, our model relates the experimentally observed Golgi phenotypes, the typical turnover times, and the size and number of cisternae to three basic, experimentally accessible quantities: the rates for material influx from the endoplasmic reticulum, and the anterograde and retrograde transport rates. Based on these results, we propose which molecular factors should be mutated to alter the organelle's phenotype and dynamics.

p28, a novel ERGIC/cis Golgi protein, required for Golgi ribbon formation

The mammalian Golgi apparatus consists of individual cisternae that are stacked in a polarized manner to form the compact zones of the Golgi. Several stacks are linked to form a ribbon via dynamic lateral bridges. The determinants required for maintaining the characteristic Golgi structure are incompletely understood. Here, we have characterized p28, a new gamma-subfamily member of p24 membrane proteins. p28 localized to endoplasmic reticulum-Golgi intermediate compartment (ERGIC) and cis Golgi and accumulated in the ERGIC upon Brefeldin A treatment, typical for a protein cycling in the early secretory pathway. p28 interacted with a subset of p24 proteins. Its depletion by small interfering RNA (siRNA) led to fragmentation of the Golgi without affecting the overall organization of microtubules but considerably reducing the amount of acetylated tubulin. The distribution of COPI and tethers, including GM130, was not affected. At the ultrastructural level, the Golgi fragments appeared as mini-stacks with apparently unchanged cis-trans topology. Golgi fragmentation did not impair anterograde or retrograde traffic. Fluorescence recovery after photobleaching (FRAP) experiments revealed that silencing p28 prevents protein exchange between Golgi stacks during reassembly after Brefeldin A-induced Golgi breakdown. These results show that the formation of a Golgi ribbon requires the structural membrane protein p28 in addition to previously identified SNAREs, coat proteins and tethers.

Membrane traffic within the Golgi apparatus

Newly synthesized secretory cargo molecules pass through the Golgi apparatus while resident Golgi proteins remain in the organelle. However, the pathways of membrane traffic within the Golgi are still uncertain. Most of the available data can be accommodated by the cisternal maturation model, which postulates that Golgi cisternae form de novo, carry secretory cargoes forward and ultimately disappear. The entry face of the Golgi receives material that has been exported from transitional endoplasmic reticulum sites, and the exit face of the Golgi is intimately connected with endocytic compartments. These conserved features are enhanced by cell-type-specific elaborations such as tubular connections between mammalian Golgi cisternae. Key mechanistic questions remain about the formation and maturation of Golgi cisternae, the recycling of resident Golgi proteins, the origins of Golgi compartmental identity, the establishment of Golgi architecture, and the roles of Golgi structural elements in membrane traffic.

Acyl-coenzyme A binding domain containing 3 (ACBD3; PAP7; GCP60): an emerging signaling molecule

Golgi body-mediated signaling has been linked to its fragmentation and regeneration during the mitotic cycle of the cell. During this process, Golgi-resident proteins are released to the cytosol and interact with other signaling molecules to regulate various cellular processes. Acyl-coenzyme A binding domain containing 3 protein (ACBD3) is a Golgi protein involved in several signaling events. ACBD3 protein was previously known as peripheral-type benzodiazepine receptor and cAMP-dependent protein kinase associated protein 7 (PAP7), Golgi complex-associated protein of 60kDa (GCP60), Golgi complex-associated protein 1 (GOCAP1), and Golgi phosphoprotein 1 (GOLPH1). In this review, we present the gene ontology of ACBD3, its relations to other Acyl-coenzyme A binding domain containing (ACBD) proteins, and its biological function in steroidogenesis, apoptosis, neurogenesis, and embryogenesis. We also discuss the role of ACBD3 in asymmetric cell division and cancer. New findings about ACBD3 may help understand this newly characterized signaling molecule and stimulate further research into its role in molecular endocrinology, neurology, and stem cell biology.

Discrete, continuous, and stochastic models of protein sorting in the Golgi apparatus

The Golgi apparatus plays a central role in processing and sorting proteins and lipids in eukaryotic cells. Golgi compartments constantly exchange material with each other and with other cellular components, allowing them to maintain and reform distinct identities despite dramatic changes in structure and size during cell division, development, and osmotic stress. We have developed three minimal models of membrane and protein exchange in the Golgi-a discrete, stochastic model, a continuous ordinary differential equation model, and a continuous stochastic differential equation model-each based on two fundamental mechanisms: vesicle-coat-mediated selective concentration of cargoes and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins during vesicle formation and SNARE-mediated selective fusion of vesicles. By exploring where the models differ, we hope to discover whether the discrete, stochastic nature of vesicle-mediated transport is likely to have appreciable functional consequences for the Golgi. All three models show similar ability to restore and maintain distinct identities over broad parameter ranges. They diverge, however, in conditions corresponding to collapse and reassembly of the Golgi. The results suggest that a continuum model provides a good description of Golgi maintenance but that considering the discrete nature of vesicle-based traffic is important to understanding assembly and disassembly of the Golgi. Experimental analysis validates a prediction of the models that altering guanine nucleotide exchange factor expression levels will modulate Golgi size.

Journeys through the Golgi--taking stock in a new era

The Golgi apparatus is essential for protein sorting and transport. Many researchers have long been fascinated with the form and function of this organelle. Yet, despite decades of scrutiny, the mechanisms by which proteins are transported across the Golgi remain controversial. At a recent meeting, many prominent Golgi researchers assembled to critically evaluate the core issues in the field. This report presents the outcome of their discussions and highlights the key open questions that will help guide the field into a new era.

The yeast Golgi apparatus: insights and mysteries

The Golgi apparatus is known to modify and sort newly synthesized secretory proteins. However, fundamental mysteries remain about the structure, operation, and dynamics of this organelle. Important insights have emerged from studying the Golgi in yeasts. For example, yeasts have provided direct evidence for Golgi cisternal maturation, a mechanism that is likely to be broadly conserved. Here, we highlight features of the yeast Golgi as well as challenges that lie ahead.

Golgi membrane dynamics after induction of a dominant-negative mutant Sar1 GTPase in tobacco

An inducible system has been established in Nicotiana tabacum plants allowing controlled expression of Sar1-GTP and thus the investigation of protein dynamics after inhibition of endoplasmic reticulum (ER) to Golgi transport. Complete Golgi disassembly and redistribution of Golgi markers into the ER was observed within 18-24h after induction. At the ultrastructural level Sar1-GTP expression led to a decrease in Golgi stack size followed by Golgi fragmentation and accumulation of vesicle remnants. Induction of Sar1-GTP resulted in redistribution of the green fluorescent protein (GFP)-tagged Arabidopsis golgins AtCASP and GC1 (golgin candidate 1, an Arabidopsis golgin 84 isoform) into the ER or cytoplasm, respectively. Additionally, both fusion proteins were observed in punctate structures, which co-located with a yellow fluorescent protein (YFP)-tagged version of Sar1-GTP. The Sar1-GTP-inducible system is compared with constitutive Sar1-GTP expression and brefeldin A treatment, and its potential for the study of the composition of ER exit sites and early cis-Golgi structures is discussed.

The Golgi apparatus: lessons from Drosophila

Historically, Drosophila has been a model organism for studying molecular and developmental biology leading to many important discoveries in this field. More recently, the fruit fly has started to be used to address cell biology issues including studies of the secretory pathway, and more specifically on the functional integrity of the Golgi apparatus. A number of advances have been made that are reviewed below. Furthermore, with the development of RNAi technology, Drosophila tissue culture cells have been used to perform genome-wide screens addressing similar issues. Last, the Golgi function has been involved in specific developmental processes, thus shedding new light on the functions of a number of Golgi proteins.

Chronic stress elicits prolonged activation of alpha-MSH secretion and subsequent degeneration of melanotroph

Prolonged stress affects homeostasis in various organs and induces stress-associated disorders. We examined the cellular changes of pituitary gland under the continuous stress condition using a rat model in which rats were kept in a cage filled with water to a height of 1.5 cm for up to 5 days. Among the pituitary hormone mRNAs, proopiomelanocortin mRNA was up-regulated specifically in the intermediate lobe (IL) of this rat model. Additionally, the peripheral blood levels of alpha-melanocyte stimulating hormone (alpha-MSH), a major product of proopiomelanocortin in IL were increased. The alpha-MSH secreting cells, melanotrophs, showed a markedly developed endoplasmic reticulum and Golgi apparatus in the early phase of the experiment. Subsequent continuous stress caused remarkable dilation of the endoplasmic reticulum, disruption of the Golgi structure, and the degeneration of some melanotrophs. In addition the dopaminergic nerve fibers from hypothalamus were markedly decreased in IL. A dopamine antagonist elicited the similar morphologic changes of melanotroph in normal rat. These findings suggest that prolonged stress suppressed hypothalamus-derived dopamine release in IL, which elicited over-secretion of alpha-MSH from the melanotrophs. The present study also suggests that prolonged hyperactivation of endocrine cells could lead to disorder of secretion mechanisms and eventual degeneration.

Golgicide A reveals essential roles for GBF1 in Golgi assembly and function

ADP-ribosylation factor 1 (Arf1) plays a critical role in regulating secretory traffic and membrane transport within the Golgi of eukaryotic cells. Arf1 is activated by guanine nucleotide exchange factors (ArfGEFs) which confer spatial and temporal specificity to vesicular transport. We describe here the discovery and characterization of Golgicide A (GCA), a potent, highly specific, and reversible inhibitor of the cis-Golgi ArfGEF, GBF1. Inhibition of GBF1 function resulted in rapid dissociation of COPI vesicle coat from Golgi membranes and subsequent disassembly of the Golgi and trans-Golgi network (TGN). Secretion of soluble and membrane-associated proteins was arrested at the ER-Golgi intermediate compartment, whereas endocytosis and recycling of transferrin was unaffected by GBF1 inhibition. Internalized shiga toxin was arrested within the endocytic compartment and was unable to reach the dispersed TGN. Collectively, these results highlight the central role for GBF1 in coordinating bidirectional transport and maintaining structural integrity of the Golgi.

A novel small molecule regulator of guanine nucleotide exchange activity of the ADP-ribosylation factor and golgi membrane trafficking

An image-based phenotypic screen was developed to identify small molecule regulators of intracellular traffic. Using this screen we found that AG1478, a previously known inhibitor of epidermal growth factor receptor, had epidermal growth factor receptor-independent activity in inducing the disassembly of the Golgi in human cells. Similar to brefeldin A (BFA), a known disrupter of the Golgi, AG1478 inhibits the activity of small GTPase ADP-ribosylation factor. Unlike BFA, AG1478 exhibits low cytotoxicity and selectively targets the cis-Golgi without affecting endosomal compartment. We show that AG1478 inhibits GBF1, a large nucleotide exchange factor for the ADP-ribosylation factor, in a Sec7 domain-dependent manner and mimics the phenotype of a GBF1 mutant that has an inactive mutation. The treatment with AG1478 leads to the recruitment of GBF1 to the vesicular-tubular clusters adjacent to the endoplasmic reticulum exit sites, a step only transiently observed previously in the presence of BFA. We propose that the treatment with AG1478 delineates a membrane trafficking intermediate step that depends upon the Sec7 domain.

Simulated de novo assembly of golgi compartments by selective cargo capture during vesicle budding and targeted vesicle fusion

The Golgi apparatus is comprised of stacked cisternal membranes forming subcompartments specialized for posttranslational processing of newly synthesized secretory cargo. Recent experimental evidence indicates that the Golgi apparatus can undergo de novo biogenesis from the endoplasmic reticulum, but the mechanism by which the membranes self assemble into compartmentalized structures remains unknown. We developed a discrete-event computer simulation model to test whether two fundamental mechanisms-vesicle-coat-mediated selective concentration of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins during vesicle formation, and SNARE-mediated selective fusion of vesicles-suffice to generate and maintain compartments. Simulations verified that this minimal model is adequate for homeostasis of preestablished compartments, even in response to small perturbations, and for de novo formation of stable compartments. The model led to a novel prediction that Golgi size is in part dependent on target SNARE expression level. This prediction was supported by a demonstration that exogenous expression of the Golgi target SNARE syntaxin-5 alters Golgi size in living cells.

GRASP55 regulates Golgi ribbon formation

Recent work indicates that mitogen-activated protein kinase kinase (MEK)1 signaling at the G2/M cell cycle transition unlinks the contiguous mammalian Golgi apparatus and that this regulates cell cycle progression. Here, we sought to determine the role in this pathway of Golgi reassembly protein (GRASP)55, a Golgi-localized target of MEK/extracellular signal-regulated kinase (ERK) phosphorylation at mitosis. In support of the hypothesis that GRASP55 is inhibited in late G2 phase, causing unlinking of the Golgi ribbon, we found that HeLa cells depleted of GRASP55 show a fragmented Golgi similar to control cells arrested in G2 phase. In the absence of GRASP55, Golgi stack length is shortened but Golgi stacking, compartmentalization, and transport seem normal. Absence of GRASP55 was also sufficient to suppress the requirement for MEK1 in the G2/M transition, a requirement that we previously found depends on an intact Golgi ribbon. Furthermore, mimicking mitotic phosphorylation of GRASP55 by using aspartic acid substitutions is sufficient to unlink the Golgi apparatus in a gene replacement assay. Our results implicate MEK1/ERK regulation of GRASP55-mediated Golgi linking as a control point in cell cycle progression.

Coat-tether interaction in Golgi organization

Biogenesis of the Golgi apparatus is likely mediated by the COPI vesicle coat complex, but the mechanism is poorly understood. Modeling of the COPI subunit betaCOP based on the clathrin adaptor AP2 suggested that the betaCOP C terminus forms an appendage domain with a conserved FW binding pocket motif. On gene replacement after knockdown, versions of betaCOP with a mutated FW motif or flanking basic residues yielded a defect in Golgi organization reminiscent of that occurring in the absence of the vesicle tether p115. Indeed, betaCOP bound p115, and this depended on the betaCOP FW motif. Furthermore, the interaction depended on E(19)E(21) in the p115 head domain and inverse charge substitution blocked Golgi biogenesis in intact cells. Finally, Golgi assembly in permeabilized cells was significantly reduced by inhibitors containing intact, but not mutated, betaCOP FW or p115 EE motifs. Thus, Golgi organization depends on mutually interacting domains in betaCOP and p115, suggesting that vesicle tethering at the Golgi involves p115 binding to the COPI coat.

Essential role of cyclin-G-associated kinase (Auxilin-2) in developing and mature mice

Hsc70 with its cochaperone, either auxilin or GAK, not only uncoats clathrin-coated vesicles but also acts as a chaperone during clathrin-mediated endocytosis. However, because synaptojanin is also involved in uncoating, it is not clear whether GAK is an essential gene. To answer this question, GAK conditional knockout mice were generated and then mated to mice expressing Cre recombinase under the control of the nestin, albumin, or keratin-14 promoters, all of which turn on during embryonic development. Deletion of GAK from brain, liver, or skin dramatically altered the histology of these tissues, causing the mice to die shortly after birth. Furthermore, by expressing a tamoxifen-inducible promoter to express Cre recombinase we showed that deletion of GAK caused lethality in adult mice. Mouse embryonic fibroblasts in which the GAK was disrupted showed a lack of clathrin-coated pits and a complete block in clathrin-mediated endocytosis. We conclude that GAK deletion blocks development and causes lethality in adult animals by disrupting clathrin-mediated endocytosis.

Bap31 is an itinerant protein that moves between the peripheral endoplasmic reticulum (ER) and a juxtanuclear compartment related to ER-associated Degradation

Certain endoplasmic reticulum (ER)-associated degradation (ERAD) substrates with transmembrane domains are segregated from other ER proteins and sorted into a juxtanuclear subcompartment, known as the ER quality control compartment. Bap31 is an ER protein with three transmembrane domains, and it is assumed to be a cargo receptor for ER export of some transmembrane proteins, especially those prone to ERAD. Here, we show that Bap31 is a component of the ER quality control compartment and that it moves between the peripheral ER and a juxtanuclear ER or ER-related compartment distinct from the conventional ER-Golgi intermediate compartment. The third and second transmembrane domains of Bap31 are principally responsible for the movement to and recycling from the juxtanuclear region, respectively. This cycling was blocked by depolymerization of microtubules and disruption of dynein-dynactin function. Overexpression of Sar1p and Arf1 mutants affected Bap31 cycling, suggesting that this cycling pathway is related to the conventional vesicular transport pathways.

Golgi regeneration after brefeldin A treatment in BY-2 cells entails stack enlargement and cisternal growth followed by division

Brefeldin A (BFA) treatment stops secretion and leads to the resorption of much of the Golgi apparatus into the endoplasmic reticulum. This effect is reversible upon washing out the drug, providing a situation for studying Golgi biogenesis. In this investigation Golgi regeneration in synchronized tobacco BY-2 cells was followed by electron microscopy and by the immunofluorescence detection of ARF1, which localizes to the rims of Golgi cisternae and serves as an indicator of COPI vesiculation. Beginning as clusters of vesicles that are COPI positive, mini-Golgi stacks first become recognizable 60 min after BFA washout. They continue to increase in terms of numbers and length of cisternae for a further 90 min before overshooting the size of control Golgi stacks. As a result, increasing numbers of dividing Golgi stacks were observed 120 min after BFA washout. BFA-regeneration experiments performed on cells treated with BFA (10 microg mL(-1)) for only short periods (30-45 min) showed that the formation of ER-Golgi hybrid structures, once initiated by BFA treatment, is an irreversible process, the further incorporation of Golgi membranes into the ER continuing during a subsequent drug washout. Application of the protein kinase A inhibitor H-89, which effectively blocks the reassembly of the Golgi apparatus in mammalian cells, also prevented stack regeneration in BY-2 cells, but only at very high, almost toxic concentrations (>200 microm). Our data suggest that under normal conditions mitosis-related Golgi stack duplication may likely occur via cisternal growth followed by fission.

Analysis of de novo Golgi complex formation after enzyme-based inactivation

The Golgi complex is characterized by its unique morphology of closely apposed flattened cisternae that persists despite the large quantity of lipids and proteins that transit bidirectionally. Whether such a structure is maintained through endoplasmic reticulum (ER)-based recycling and auto-organization or whether it depends on a permanent Golgi structure is strongly debated. To further study Golgi maintenance in interphase cells, we developed a method allowing for a drug-free inactivation of Golgi dynamics and function in living cells. After Golgi inactivation, a new Golgi-like structure, containing only certain Golgi markers and newly synthesized cargoes, was produced. However, this structure did not acquire a normal Golgi architecture and was unable to ensure a normal trafficking activity. This suggests an integrative model for Golgi maintenance in interphase where the ER is able to autonomously produce Golgi-like structures that need pre-existing Golgi complexes to be organized as morphologically normal and active Golgi elements.

Alternate routes for drug delivery to the cell interior: pathways to the Golgi apparatus and endoplasmic reticulum

The targeted delivery of drugs to the cell interior can be accomplished by taking advantage of the various receptor-mediated endocytic pathways operating in a particular cell. Among these pathways, the retrograde trafficking pathway from endosomes to the Golgi apparatus, and endoplasmic reticulum is of special importance since it provides a route to deliver drugs bypassing the acid pH, hydrolytic environment of the lysosome. The existence of pathways for drug or antigen delivery to the endoplasmic reticulum and Golgi apparatus has been to a large extent an outcome of research on the trafficking of A/B type-bacterial or plant toxins such as Shiga toxin within the cell. The targeting properties of these toxins reside in their B subunit. In this article we present an overview of the multiplicity of pathways to deliver drugs intracellularly. We highlight the retrograde trafficking pathway illustrated by Shiga toxin and Shiga-like toxin, and the potential role of the B subunit of these toxins as carriers of drugs, antigens and imaging agents.

Membrane trafficking of the human organic anion-transporting polypeptide C (hOATPC)

INTRODUCTION

The human organic anion transporting polypeptide C (OATPC) is one of the major transport proteins involved in the enterohepatic circulation of bile salts and plays an important role in vectorial transport of organic anions and drugs across hepatocytes.

MATERIALS AND METHODS

In this study, the effects of biological reagents on the membrane localization of OATPC were investigated by confocal microscopy and estrone-3-sulfate transport.

RESULTS

Our results demonstrated that the functional membrane expression of fluorescent chimera OATPC-GFP was achieved in non-polarized (COS7 and HEK293) and polarized (MDCK) cells. Both brefeldin A (a Golgi complex disruptor) and bafilomycin A1 (an inhibitor of vacuolar H+-ATPase) treatment significantly decreased the polarized membrane trafficking and markedly reduced the uptake of estrone-3-sulfate ( approximately 40-90%) in OATPC-GFP transfected cells, suggesting that membrane sorting of hOATPC-GFP was mediated by Golgi complex and vacuolar H+-ATPase-related vesicle transport pathways. Treatment with 8-Br-cAMP (a cAMP analog) stimulated OATPC-GFP membrane localization and enhanced estrone-3-sulfate uptake by approximately 20%. The protein kinase A (PKA) inhibitors (H89 and KT5720), but not a PKG inhibitor, blocked the polarized membrane expression of OATPC-GFP and reduced estrone-3-sulfate transport activity. The simultaneous treatment of cells with PKA activator/inhibitor and bafilomycin A1 demonstrated that bafilomycin A1 did not change the effects of 8-Br-cAMP and H89 on the membrane localization of OATPC-GFP compared with the use of 8-Br-cAMP and H89 alone.

DISCUSSION

These data suggest that a cAMP-PKA sensitive membrane sorting pathway for OATPC-GFP is independent of the vacuolar H+-ATPase associated (bafilomycin A1 sensitive) vesicle mediated membrane sorting pathway. In contrast, with combined treatment with brefeldin A, neither the PKA-activator (8-Br-cAMP) nor the inhibitor (H89) further altered the plasma membrane expression and transport activity of OATPC-GFP compared with brefeldin A treatment alone. These data suggest that the cAMP-PKA regulation of OATPC membrane expression involves the Golgi complex. When the Golgi apparatus was disrupted by brefeldin A treatment, the effects of cAMP-PKA on the Golgi-to-basolateral surface sorting process of OATPC was also diminished. In summary, the plasma membrane localization of human OATPC is mediated by Golgi complex and vacuolar H+-ATPase vesicle mediated membrane sorting pathways. cAMP-PKA regulates sorting process through the Golgi complex but not the vacuolar H+-ATPase associated vesicular pathway.

COPII under the microscope

Transport through the secretory pathway begins with COPII regulation of ER export. Driven by the Sar1 GTPase cycle, cytosolic COPII proteins exchange on and off the membrane at specific sites on the ER to regulate cargo exit. Here recent developments in COPII research are discussed, particularly the use of live-cell imaging, which has revealed surprising insights into the coat's role. The seemingly static ER exit sites are in fact highly dynamic, and the ability to visualise trafficking processes in intact living cells has highlighted the adaptable nature of COPII in cargo transport and the emerging roles of auxiliary factors.

A role for clathrin in reassembly of the Golgi apparatus

The Golgi apparatus is a highly dynamic organelle whose organization is maintained by a proteinaceous matrix, cytoskeletal components, and inositol phospholipids. In mammalian cells, disassembly of the organelle occurs reversibly at the onset of mitosis and irreversibly during apoptosis. Several pharmacological agents including nocodazole, brefeldin A (BFA), and primary alcohols (1-butanol) induce reversible fragmentation of the Golgi apparatus. To dissect the mechanism of Golgi reassembly, rat NRK and GH3 cells were treated with 1-butanol, BFA, or nocodazole. During washout of 1-butanol, clathrin, a ubiquitous coat protein implicated in vesicle traffic at the trans-Golgi network and plasma membrane, and abundant clathrin coated vesicles were recruited to the region of nascent Golgi cisternae. Knockdown of endogenous clathrin heavy chain showed that the Golgi apparatus failed to reform efficiently after BFA or 1-butanol removal. Instead, upon 1-butanol washout, it maintained a compact, tight morphology. Our results suggest that clathrin is required to reassemble fragmented Golgi elements. In addition, we show that after butanol treatment the Golgi apparatus reforms via an initial compact intermediate structure that is subsequently remodeled into the characteristic interphase lace-like morphology and that reassembly requires clathrin.