Tubulovesicular processes emerge from trans-Golgi cisternae, extend along microtubules, and interlink adjacent trans-golgi elements into a reticulum

Photoperiodic-dependent histomorphological changes in the excurrent duct system of helmeted guinea fowl subjected to short day (8L:16D), long-day (16L:8D) light/dark cycles and exogenous melatonin

In the present study, the influence of varying photoperiods [short day light 8L:16D, long day light 16L;8D] and exogeneous melatonin on the excurrent duct system of male helmeted guinea fowl was investigated using histo-morphological and histometric approaches. A total of twenty-eight (28) guinea fowl birds were randomly divided into Group I: Short daylight (SD; 8 HL), Group II: (SD +1mg/kg melatonin; 8 HL+ Mel), Group III: Long daylight (LD; 16 HL) and Group IV: (LD +1mg/kg; 16 HL + Mel)] and comprises of seven birds (n=7) per group. At the end of the 8 weeks of experimentation, the excurrent ducts were excised and processed for routine histological examination and the variations in histo-morphometrical parameters were determined using the GIMP2 software. Histologically, apart from the moderate cellular degeneration observed in efferent duct epithelia of the SD subgroups: (8 HL and 8 HL + Mel), there was remarkable spermatozoa presence in the lumens of the epididymal duct and ductus deferens of both 16 HL and 16 HL + Mel groups. The histo-morphometric data (luminal, ductal diameters and epithelial heights) were significantly increased (p <0.05) in the excurrent ducts of guinea fowl exposed to 16 HL and 16 HL + Mel, as compared to other groups. There was significant decrease (p <0.05) in stereocilia height (SH) in 16 HL compared to 8 HL sub-groups of lower segments. Although, a non-significant (p >0.05) increase in SH was observed in melatonin-treated groups, regardless of photoperiod. Taken together, these sets of data from this study indicate the importance of artificial light and exogenous melatonin in the control of seasonality of reproduction and which could be used to influence the reproductive cycle of the guinea fowl.

Hybrid labeling system for dSTORM imaging of endoplasmic reticulum for uncovering ultrastructural transformations under stress conditions

The endoplasmic reticulum (ER) transforms its morphology to fit versatile cellular functions especially under stress conditions. Since various ER stresses are critical pathophysiological factors, the precise observations of ER can provide insights into disease diagnoses and biological researches. Live-cell super-resolution imaging is highly expected for uncovering microstructures of ER. However, to achieve this, there remains a big challenge in how to efficiently label ER with advanced fluorophores. Herein, we report a new SNAP-tag fluorescent probe, namely, CLP-TMR, for specific and high-density labeling of the newly constructed dual ER-signal (targeting and retention) peptides fused-SNAP proteins. This hybrid labeling system integrating chemical probes with genetically encoded techniques enables molecular position reconstructions of ER morphologies through direct stochastic optical reconstruction microscopy (dSTORM) imaging. The super-resolution imaging reveals several never-known ultrastructural changes responding to different ER stresses, i.e. the formation of peripheral ER sheets to restore the immunogenicity, and the long flattened ER tubules under inflammation.

Golgi Outposts Nucleate Microtubules in Cells with Specialized Shapes

Classically, animal cells nucleate or form new microtubules off the perinuclear centrosome. In recent years, the Golgi outpost has emerged as a satellite organelle that can function as an acentrosomal microtubule-organizing center (MTOC), nucleating new microtubules at distances far from the nucleus or cell body. Golgi outposts can nucleate new microtubules in specialized cells with unique cytoarchitectures, including Drosophila neurons, mouse muscle cells, and rodent oligodendrocytes. This review compares and contrasts topics of functional relevance, including Golgi outpost heterogeneity, formation and transport, as well as regulation of microtubule polarity and branching. Golgi outposts have also been implicated in the pathology of diseases including muscular dystrophy, and neurodegenerative diseases, such as Parkinson's disease (PD). Since Golgi outposts are relatively understudied, many outstanding questions regarding their function and roles in disease remain.

Sphingomyelin is sorted at the trans Golgi network into a distinct class of secretory vesicle

One of the principal functions of the trans Golgi network (TGN) is the sorting of proteins into distinct vesicular transport carriers that mediate secretion and interorganelle trafficking. Are lipids also sorted into distinct TGN-derived carriers? The Golgi is the principal site of the synthesis of sphingomyelin (SM), an abundant sphingolipid that is transported. To address the specificity of SM transport to the plasma membrane, we engineered a natural SM-binding pore-forming toxin, equinatoxin II (Eqt), into a nontoxic reporter termed Eqt-SM and used it to monitor intracellular trafficking of SM. Using quantitative live cell imaging, we found that Eqt-SM is enriched in a subset of TGN-derived secretory vesicles that are also enriched in a glycophosphatidylinositol-anchored protein. In contrast, an integral membrane secretory protein (CD8α) is not enriched in these carriers. Our results demonstrate the sorting of native SM at the TGN and its transport to the plasma membrane by specific carriers.

The Directional Observation of Highly Dynamic Membrane Tubule Formation Induced by Engulfed Liposomes

Highly dynamic tubular structures in cells are responsible for exchanges between organelles. Compared with bacterial invasion, the most affordable and least toxic lipids were found in this study to be gentle and safe exogenous stimuli for the triggering of membrane tubules. A specific lipid system was internalized by NIH3T3 cells. Following cellular uptake, the constructed liposomes traveled towards the nucleus in aggregations and were gradually distributed into moving vesicles and tubules in the cytosol. The triggered tubules proceeded, retreated or fluctuated along the cytoskeleton and were highly dynamic, moving quickly (up to several microns per second), and breaking and fusing frequently. These elongated tubules could also fuse with one another, giving rise to polygonal membrane networks. These lipid systems, with the novel property of accelerating intracellular transport, provide a new paradigm for investigating cellular dynamics.

Golgi tubules: their structure, formation and role in intra-Golgi transport

Tubules are common Golgi elements that can form extensive networks associated with the cis-, lateral and trans-Golgi sides, but despite this, they have almost been forgotten for decades. The molecular mechanisms involved in their formation, elongation and fission are only just beginning to be understood. However, the role of these membranes is not well understood. In the present review, we analyze the mechanisms that induce Golgi tubulation or, conversely, disrupt tubules in order to throw some lights on the nature of these elements. The putative role of these elements in the framework of current models for intra-Golgi transport is also discussed.

Intra-Golgi Transport: Roles for Vesicles, Tubules, and Cisternae

The Golgi complex is considered the central station of the secretory pathway where cargo proteins and lipids are properly modified, classified, packed into specific carriers and delivered to their final destinations. Early electron microscope studies showed the extraordinary structural complexity of this organelle. However, despite the large volume of incoming and outgoing traffic, it is able to maintain its architecture, although it is also flexible enough to adapt to the functional status of the cell. Many components of the molecular machinery involved in membrane traffic and other Golgi functions have been identified. However, some basic aspects of Golgi functioning remain unsolved. For instance, how cargo moves through the stack remains controversial and two classical models have been proposed: vesicular transport and cisternal maturation. Since neither of these models explains all the experimental data, a combination of these models as well as new models have been proposed. In this context, the specific role of the cisternae, vesicles and tubules needs to be clarified. In this review, we summarize our current knowledge of the Golgi organization and function, focusing on the mechanisms of intra-Golgi transport.

Golgi pH, its regulation and roles in human disease

Most organelles within the exocytic and endocytic pathways typically acidify their interiors, a phenomenon that is known to be crucial for their optimal functioning in eukaryotic cells. This review highlights recent advances in our understanding of how Golgi acidity is maintained and regulated, and how its misregulation contributes to organelle dysfunction and disease. Both its biosynthetic products (glycans) and protein-sorting events are highly sensitive to changes in Golgi luminal pH and are affected in certain human disease states such as cancers and cutis laxa. Other potential disease states that are caused by, or are associated with, Golgi pH misregulation will also be discussed.

Spatial filter based 3D resolution improvement and polarization properties of multiphoton multiple-excitation-spot-optical microscopy

Three-dimensional (3D) resolution improvement in multi-photon multiple-excitation-spot-optical microscopy is proposed. Specially designed spatial filter is employed for improving the overall 3D resolution of the imaging system. An improvement up to a factor of 14.5 and sub-femto liter volume excitation is achieved. The system shows substantial sidelobe reduction (<4%) due to the non-linear intensity dependence of multiphoton process. Polarization effect on x-oriented and freely rotating dipoles shows dramatic change in the field distribution at the focal-plane. The resulting point-spread function has the ability to produce several strongly localized polarization dependent field patterns which may find applications in optical engineering and bioimaging.

GOLPH3 bridges phosphatidylinositol-4- phosphate and actomyosin to stretch and shape the Golgi to promote budding

Golgi membranes, from yeast to humans, are uniquely enriched in phosphatidylinositol-4-phosphate (PtdIns(4)P), although the role of this lipid remains poorly understood. Using a proteomic lipid-binding screen, we identify the Golgi protein GOLPH3 (also called GPP34, GMx33, MIDAS, or yeast Vps74p) as a PtdIns(4)P-binding protein that depends on PtdIns(4)P for its Golgi localization. We further show that GOLPH3 binds the unconventional myosin MYO18A, thus connecting the Golgi to F-actin. We demonstrate that this linkage is necessary for normal Golgi trafficking and morphology. The evidence suggests that GOLPH3 binds to PtdIns(4)P-rich trans-Golgi membranes and MYO18A conveying a tensile force required for efficient tubule and vesicle formation. Consequently, this tensile force stretches the Golgi into the extended ribbon observed by fluorescence microscopy and the familiar flattened form observed by electron microscopy.

A single fixation protocol for proteome-wide immunofluorescence localization studies

Immunofluorescence microscopy is a valuable tool for analyzing protein expression and localization at a subcellular level thus providing information regarding protein function, interaction partners and its role in cellular processes. When performing sample fixation, parameters such as difference in accessibility of proteins present in various cellular compartments as well as the chemical composition of the protein to be studied, needs to be taken into account. However, in systematic and proteome-wide efforts, a need exists for standard fixation protocol(s) that works well for the majority of all proteins independent of subcellular localization. Here, we report on a study with the goal to find a standardized protocol based on the analysis of 18 human proteins localized in 11 different organelles and subcellular structures. Six fixation protocols were tested based on either dehydration by alcohols (methanol, ethanol or iso-propanol) or cross-linking by paraformaldehyde followed by detergent permeabilization (Triton X-100 or saponin) in three human cell lines. Our results show that cross-linking is essential for proteome-wide localization studies and that cross-linking using paraformaldehyde followed by Triton X-100 permeabilization successfully can be used as a single fixation protocol for systematic studies.

The role of kinesin, dynein and microtubules in pancreatic secretion

The regulated secretion of pancreatic zymogens depends on a functional cytoskeleton and intracellular vesicle transport. To study the dynamics of tubulin and its motor proteins dynein and kinesin during secretion in pancreatic acinar cells, we infused rats with 0.1 mug/kg/h caerulein. Electron and fluorescence microscopy detected neither dynein nor kinesin at the apical secretory pole, nor on the surface of mature zymogen granules. After 30 min of secretagogue stimulation, kinesin and the Golgi marker protein 58 K were reallocated towards the apical plasma membrane and association of kinesin with tubulin was enhanced. Disruption of acinar cell microtubules had no effect on initial caerulein-induced amylase release but completely blocked secretion during a second stimulus. Our results suggest that mature zymogen granule exocytosis is independent of intact microtubules, kinesin and dynein. However, microtubule-dependent mechanisms seem to be important for the replenishment of secretory vesicles by redistribution of Golgi elements towards the apical cell pole.

Fluorescent staining of subcellular organelles: ER, Golgi complex, and mitochondria

The ability to distinguish and identify specific subcellular compartments is essential to understanding organelle function, biogenesis, and maintenance within cells and to defining protein trafficking pathways. Fluorescent dyes and/or fluorescently labeled lipid derivatives can be used to identify ER, Golgi complex, and mitochondria. Specific conditions for labeling each of these compartments are described.

Microtubule/organelle motility assays

This unit describes an in vitro assay that uses video-enhanced differential interference contrast (VE-DIC) microscopy to examine the motile interactions between isolated organelle fractions and microtubules (MTs). The method can be used to dissect the molecular requirements for organelle movement and membrane trafficking. A field of axoneme-nucleated MTs, growing and shortening as they would in a living cell (dynamic MTs), is generated in a simple microscope perfusion chamber. Various combinations of isolated endoplasmic reticulum (ER) and Golgi apparatus organelles, cytosol containing motor proteins and other soluble factors, nucleotides, and specific pharmacological reagents are then added to the dynamic MT, and the motile interactions between the organelles and MTs are observed by VE-DIC microscopy.

Characterization of Rab21-positive tubular endosomes induced by PI3K inhibitors

We found that wortmannin, a potent phosphoinositide 3-kinase (PI3K) inhibitor, markedly induced the formation of Rab21-positive tubular compartments in A431 cells. By time-lapse fluorescence microscopy of live cells co-expressing fluorescent protein-fused Rab21 and other marker proteins, it was shown that the Rab21-positive tubules in wortmannin-treated cells were derived from Rab5-positive early endosomes, but not from late endosomes, recycling endosomes, lysosomes or the trans-Golgi network. The formation of Rab21-positive tubules was very dynamic and required microtubules. Rab21-positive tubules were also formed by the treatment of cells with 3-methyladenine (3-MA), which inhibits class III PI3K rather than class I PI3K. Furthermore, the loss of PI(3)P correlated with the tubulation of Rab21-positive endosomes in cells co-expressing fluorescent protein-fused Rab21 and a tandem FYVE domain. These results suggest that the lowering of PI(3)P as a result of class III PI3K inhibition may be an important cue for the morphological change of Rab21-positive early endosomes from vesicular to tubular form.

Both sphingomyelin synthases SMS1 and SMS2 are required for sphingomyelin homeostasis and growth in human HeLa cells

Sphingomyelin (SM) is a vital component of cellular membranes in organisms ranging from mammals to protozoa. Its production involves the transfer of phosphocholine from phosphatidylcholine to ceramide, yielding diacylglycerol in the process. The mammalian genome encodes two known SM synthase (SMS) isoforms, SMS1 and SMS2. However, the relative contributions of these enzymes to SM production in mammalian cells remained to be established. Here we show that SMS1 and SMS2 are co-expressed in a variety of cell types and function as the key Golgi- and plasma membrane-associated SM synthases in human cervical carcinoma HeLa cells, respectively. RNA interference-mediated depletion of either SMS1 or SMS2 caused a substantial decrease in SM production levels, an accumulation of ceramides, and a block in cell growth. Although SMS-depleted cells displayed a reduced SM content, external addition of SM did not restore growth. These results indicate that the biological role of SM synthases goes beyond formation of SM.

The formation of TGN-to-plasma-membrane transport carriers

In the trans-Golgi network (TGN), proteins are sorted for transport to the endosomes, plasma membrane, preceding Golgi cisternae, and endoplasmic reticulum. The formation of clathrin-coated vesicles for transport to the endosomes and of COP-I-coated vesicles for retrograde trafficking is fairly well characterized at the molecular level. We describe our current understanding of the TGN-to-cell-surface carriers, with a specific focus on the components involved in membrane fission. Inhibiting the fission machinery promotes growth of transport carriers into large tubules that remain attached to the TGN. Overactivating this machinery, on the other hand, vesiculates the TGN. To understand how membrane fission is regulated by cargo to form transport carriers yet prevents complete vesiculation of the TGN remains a daunting challenge. We discuss these issues with regard to TGN-to-cell-surface transport carriers.

The morphology of the efferent ducts of the testis of the ostrich, a primitive bird

The efferent duct of the ostrich consists of two segments, the proximal efferent duct (PED) and the distal efferent duct (DED) that are continuous, as in some other birds. Both segments of the duct possess an epithelium comprising non-ciliated and ciliated cells in varying proportions between the two segments. The non-ciliated cell (type I) of the PED contains a well-developed, subapical endocytic apparatus of apical tubules and endocytic vacuoles, a solitary, large, heterogeneous lipid droplet, and numerous, oval, dense bodies in the supranuclear region of the cell. Mitochondria tend to concentrate in the basal part of the cell. Intercellular spaces between the non-ciliated cells are enlarged, especially in the basal half of the epithelium. Together, these morphological features confer on the PED an efficient fluid absorption capability. The DED epithelium displays the type II non-ciliated cell whose poorly developed subapical endocytic apparatus as well as the absence of dilated basal intercellular spaces indicate its limited fluid absorptive capacity.

Golgins and GTPases, giving identity and structure to the Golgi apparatus

In this review we will focus on the recent advances in how coiled-coil proteins of the golgin family give identity and structure to the Golgi apparatus in animal cells. A number of recent studies reveal a common theme for the targeting of golgins containing the ARL-binding GRIP domain, and the related ARF-binding GRAB domain. Similarly, other golgins such as the vesicle tethering factor p115 and Bicaudal-D are targeted by the Rab GTPases, Rab1 and Rab6, respectively. Together golgins and their regulatory GTPases form a complex network, commonly known as the Golgi matrix, which organizes Golgi membranes and regulates membrane trafficking.

Dicumarol, an inhibitor of ADP-ribosylation of CtBP3/BARS, fragments golgi non-compact tubular zones and inhibits intra-golgi transport

Dicumarol (3,3'-methylenebis[4-hydroxycoumarin]) is an inhibitor of brefeldin-A-dependent ADP-ribosylation that antagonises brefeldin-A-dependent Golgi tubulation and redistribution to the endoplasmic reticulum. We have investigated whether dicumarol can directly affect the morphology of the Golgi apparatus. Here we show that dicumarol induces the breakdown of the tubular reticular networks that interconnect adjacent Golgi stacks and that contain either soluble or membrane-associated cargo proteins. This results in the formation of 65-120-nm vesicles that are sometimes invaginated. In contrast, smaller vesicles (45-65 nm in diameter, a size consistent with that of coat-protein-I-dependent vesicles) that excluded cargo proteins from their lumen are not affected by dicumarol. All other endomembranes are largely unaffected by dicumarol, including Golgi stacks, the ER, multivesicular bodies and the trans-Golgi network. In permeabilized cells, dicumarol activity depends on the function of CtBP3/BARS protein and pre-ADP-ribosylation of cytosol inhibits the breakdown of Golgi tubules by dicumarol. In functional experiments, dicumarol markedly slows down intra-Golgi traffic of VSV-G transport from the endoplasmic reticulum to the medial Golgi, and inhibits the diffusional mobility of both galactosyl transferase and VSV-G tagged with green fluorescent protein. However, it does not affect: transport from the trans-Golgi network to the cell surface; Golgi-to-endoplasmic reticulum traffic of ERGIC58; coat-protein-I-dependent Golgi vesiculation by AlF4 or ADP-ribosylation factor; or ADP-ribosylation factor and beta-coat protein binding to Golgi membranes. Thus the ADP-ribosylation inhibitor dicumarol induces the selective breakdown of the tubular components of the Golgi complex and inhibition of intra-Golgi transport. This suggests that lateral diffusion between adjacent stacks has a role in protein transport through the Golgi complex.

Probing the mobility of membrane proteins inside the cell

Studies using a variety of microscopy-based approaches have led to a consensus that most cell-surface proteins are highly mobile and diffuse rapidly within fenced microdomains. Little attention, however, has so far been given to the analysis of the mobility of intracellular membrane proteins because of their comparative inaccessibility. Recent advances in microinjection, confocal microscopy and the construction of epitope-tagged proteins or of hybrids with an intrinsically fluorescent protein have allowed intracellular membrane proteins to be studied using approaches previously applied to characterize the mobility of cell-surface proteins. Confocal fluorescence recovery after photobleaching (c-FRAP) experiments show that intracellular membrane proteins may also be highly mobile.

Involvement of beta-COP in membrane traffic through the Golgi complex

Non-clathrin-coated vesicles mediate membrane traffic through the Golgi complex. The proteins that constitute the coats of these vesicles have similar molecular weights to the clathrin coat proteins. A major component of the coat of non-clathrin-coated vesicles, beta-COP, has significant homology with the clathrin coat protein beta-adaptin, indicating that the coats of the two different classes of vesicles may be structurally and functionally homologous.

Inhibition of a Golgi complex lysophospholipid acyltransferase induces membrane tubule formation and retrograde trafficking

Recent studies have suggested that formation of Golgi membrane tubules involves the generation of membrane-associated lysophospholipids by a cytoplasmic Ca2+-independent phospholipase A2 (PLA2). Herein, we provide additional support for this idea by showing that inhibition of lysophospholipid reacylation by a novel Golgi-associated lysophosphatidylcholine acyltransferase (LPAT) induces the rapid tubulation of Golgi membranes, leading in their retrograde movement to the endoplasmic reticulum. Inhibition of the Golgi LPAT was achieved by 2,2-dimethyl-N-(2,4,6-trimethoxyphenyl)dodecanamide (CI-976), a previously characterized antagonist of acyl-CoA cholesterol acyltransferase. The effect of CI-976 was similar to that of brefeldin A, except that the coatomer subunit beta-COP remained on Golgi-derived membrane tubules. CI-976 also enhanced the cytosol-dependent formation of tubules from Golgi complexes in vitro and increased the levels of lysophosphatidylcholine in Golgi membranes. Moreover, preincubation of cells with PLA2 antagonists inhibited the ability of CI-976 to induce tubules. These results suggest that Golgi membrane tubule formation can result from increasing the content of lysophospholipids in membranes, either by stimulation of a PLA2 or by inhibition of an LPAT. These two opposing enzyme activities may help to coordinately regulate Golgi membrane shape and tubule formation.

Phospholipase A2 (PLA2) enzymes in membrane trafficking: mediators of membrane shape and function

Since the mid-1990s, there have been tremendous advances in our understanding of the roles that lipid-modifying enzymes play in various intracellular membrane trafficking events. Phospholipases represent the largest group of lipid-modifying enzymes and accordingly display a wide range of functions. The largest class of phospholipases are the phospholipase A(2) (PLA2) enzymes, and these have been most extensively studied for their roles in the generation lipid signaling molecules, e.g. arachidonic acid. In recent years, however, cytoplasmic PLA2 enzymes have also become increasingly associated with various intracellular trafficking events, such as the formation of membrane tubules from the Golgi complex and endosomes, and membrane fusion events in the secretory and endocytic pathways. Moreover, the ability of cytoplasmic PLA2 enzymes to directly affect the structure and function of membranes by altering membrane curvature suggests novel functional roles for these enzymes. This review will focus on the role of cytoplasmic PLA2 enzymes in intracellular membrane trafficking and the mechanisms by which they influence membrane structure and function.

GAIP participates in budding of membrane carriers at the trans-Golgi network

Galpha interacting protein (GAIP) is a regulator of G protein signaling protein that associates dynamically with vesicles and has been implicated in membrane trafficking, although its specific role is not yet known. Using an in vitro budding assay, we show that GAIP is recruited to a specific population of trans-Golgi network-derived vesicles and that these are distinct from coatomer or clathrin-coated vesicles. A truncation mutant (NT-GAIP) encoding only the N-terminal half of GAIP is recruited to trans-Golgi network membranes during the formation of vesicle carriers. Overexpression of NT-GAIP induces the formation of long, coated tubules, which are stabilized by microtubules. Results from the budding assay and from imaging in live cells show that these tubules remain attached to the Golgi stack rather than being released as carrier vesicles. NT-GAIP expression blocks membrane budding and results in the accumulation of tubular carrier intermediates. NT-GAIP-decorated tubules are competent to load vesicular stomatitis virus protein G-green fluorescent protein as post-Golgi, exocytic cargo and in cells expressing NT-GAIP there is reduced surface delivery of vesicular stomatitis virus protein G-green fluorescent protein. We conclude that GAIP functions as an essential part of the membrane budding machinery for a subset of post-Golgi exocytic carriers derived from the trans-Golgi network.

The mammalian Golgi--complex debates

Since the first description of the Golgi in 1898, key issues regarding this organelle have remained contentious among cell biologists. Resolving these complex debates, which revolve around Golgi structure-function relationships, is prerequisite to understanding how the Golgi fulfils its role as the central organelle and sorting station of the mammalian secretory pathway.

Motoring around the Golgi

The Golgi apparatus is a dynamic organelle through which nascent secretory and transmembrane proteins are transported, post-translationally modified and finally packaged into carrier vesicles for transport along the cytoskeleton to a variety of destinations. In the past decade, studies have shown that a number of 'molecular motors' are involved in maintaining the proper structure and function of the Golgi apparatus. Here, we review just some of the many functions performed by these mechanochemical enzymes - dyneins, kinesins, myosins and dynamin - in relation to the Golgi apparatus.

GBF1, a guanine nucleotide exchange factor for ADP-ribosylation factors, is localized to the cis-Golgi and involved in membrane association of the COPI coat

Formation of coated carrier vesicles, such as COPI-coated vesicles from the cis-Golgi, is triggered by membrane binding of the GTP-bound form of ADP-ribosylation factors. This process is blocked by brefeldin A, which is an inhibitor of guanine nucleotide exchange factors for ADP-ribosylation factor. GBF1 is one of the guanine nucleotide-exchange factors for ADP-ribosylation factor and is localized in the Golgi region. In the present study, we have determined the detailed subcellular localization of GBF1. Immunofluorescence microscopy of cells treated with nocodazole or incubated at 15 degrees C has suggested that GBF1 behaves similarly to proteins recycling between the cis-Golgi and the endoplasmic reticulum. Immunoelectron microscopy has revealed that GBF1 localizes primarily to vesicular and tubular structures apposed to the cis-face of Golgi stacks and minor fractions to the Golgi stacks. GBF1 overexpressed in cells causes recruitment of class I and class II ADP-ribosylation factors onto Golgi membranes. Furthermore, overexpressed GBF1 antagonizes various effects of brefeldin A, such as inhibition of membrane recruitment of ADP-ribosylation factors and the COPI coat, and redistribution of Golgi-resident and itinerant proteins. These observations indicate that GBF1 is involved in the formation of COPI-coated vesicles from the cis-Golgi or the pre-Golgi intermediate compartment through activating ADP-ribosylation factors.

Overexpression of an ADP-ribosylation factor-guanine nucleotide exchange factor, BIG2, uncouples brefeldin A-induced adaptor protein-1 coat dissociation and membrane tubulation

BIG2 is a guanine nucleotide exchange factor (GEF) for the ADP-ribosylation factor (ARF) family of small GTPases, which regulate membrane association of COPI and adaptor protein (AP)-1 coat protein complexes. A fungal metabolite, brefeldin A (BFA), inhibits ARF-GEFs and leads to redistribution of coat proteins from membranes to the cytoplasm and membrane tubulation of the Golgi complex and the trans-Golgi network (TGN). To investigate the function of BIG2, we examined the effects of BIG2-overexpression on the BFA-induced redistribution of ARF, coat proteins, and organelle markers. The BIG2 overexpression blocked BFA-induced redistribution from membranes of ARF1 and the AP-1 complex but not that of the COPI complex. These observations indicate that BIG2 is implicated in membrane association of AP-1, but not that of COPI, through activating ARF. Furthermore, not only BIG2 but also ARF1 and AP-1 were found as queues of spherical swellings along the BFA-induced membrane tubules emanating from the TGN. These observations indicate that BFA-induced AP-1 dissociation from TGN membranes and tubulation of TGN membranes are not coupled events and suggest that a BFA target other than ARF-GEFs exists in the cell.

Regulators of GTP-binding proteins cause morphological changes in the vacuole system of the filamentous fungus, Pisolithus tinctorius

Tubule formation is a widespread feature of the endomembrane system of eukaryotic cells, serving as an alternative to the better-known transport process of vesicular shuttling. In filamentous fungi, tubule formation by vacuoles is particularly pronounced, but little is known of its regulation. Using the hyphae of the basidiomycete Pisolithus tinctorius as our test system, we have investigated the effects of four drugs whose modulation, in animal cells, of the tubule/vesicle equilibrium is believed to be due to the altered activity of a GTP-binding protein (GTP gamma S, GDP beta S, aluminium fluoride, and Brefeldin A). In Pisolithus tinctorius, GTP gamma S, a non-hydrolysable form of GTP, strongly promoted vacuolar tubule formation in the tip cell and next four cells. The effects of GTP gamma S could be antagonised by pre-treatment of hyphae with GDP beta S, a non-phosphorylatable form of GDP. These results support the idea that a GTP-binding protein plays a regulatory role in vacuolar tubule formation. This could be a dynamin-like GTP-ase, since GTP gamma S-stimulated tubule formation has only been reported previously in cases where a dynamin is involved. Treatment with aluminium fluoride stimulated vacuolar tubule formation at a distance from the tip cell, but NaF controls indicated that this was not a GTP-binding-protein specific effect. Brefeldin A antagonised GTP gamma S, and inhibited tubule formation in the tip cell. Given that Brefeldin A also affects the ER and Golgi bodies of Pisolithus tinctorius, as shown previously, it is not clear yet whether the effects of Brefeldin A on the vacuole system are direct or indirect.

Trafficking of yellow-fluorescent-protein-tagged mu1 subunit of clathrin adaptor AP-1 complex in living cells

Clathrin adaptor protein AP-1 complex is thought to function in forming clathrin-coated vesicles at the trans-Golgi network (TGN) and mediating transport of cargo between the TGN and endosomes. To study trafficking of AP-1 in living cells, yellow fluorescent protein (YFP) was inserted in the middle of mu1 A subunit of AP-1. When expressed in a tetracycline-dependent manner in HeLa cells, YFP-mu1 was efficiently incorporated into the AP-1 complex, replacing endogenous mu1 in most of cellular AP-1. Time-lapse imaging revealed that YFP-mu1/AP-1 departs from TGN as isolated vesicles and spherical structures, or varicosities, associated with fine tubular processes. Typically, several vesicles or varicosities were seen moving sequentially along the same 'tracks' from TGN to cell periphery. These data suggest that AP-1 may function after formation of Golgi transport intermediates in facilitating their intracellular movement. Mutagenesis of YFP-mu1 determined that the structural requirements for its binding to tyrosine-containing sequence motifs are similar to those previously defined in mu2 subunit of AP-2. Moreover, the carboxyl-terminal half of mu2 could replace the corresponding fragment of mu1 without loss of the ability of the resulting mu1-YFP-mu2 chimeric protein to incorporate into AP-1 and bind tyrosine-containing motifs. Mutations that abolish binding capacity for tyrosine motifs did not mistarget AP-1 in the cell, suggesting that AP-1 interactions with this type of sorting signals are not essential for membrane docking of AP-1 at the TGN. Altogether, this study demonstrates that YFP-tagged mu1 protein can serve as a useful tool for visualizing the dynamics of AP-1 in living cells and for the structure-function analysis of mu1-cargo interactions.

Protein kinase D regulates the fission of cell surface destined transport carriers from the trans-Golgi network

When a kinase inactive form of Protein Kinase D (PKD-K618N) was expressed in HeLa cells, it localized to the trans-Golgi network (TGN) and caused extensive tubulation. Cargo that was destined for the plasma membrane was found in PKD-K618N-containing tubes but the tubes did not detach from the TGN. As a result, the transfer of cargo from TGN to the plasma membrane was inhibited. We have also demonstrated the formation and subsequent detachment of cargo-containing tubes from the TGN in cells stably expressing low levels of PKD-K618N. Our results suggest that PKD regulates the fission from the TGN of transport carriers that are en route to the cell surface.

Microtubules, but not actin microfilaments, regulate vacuole motility and morphology in hyphae of Pisolithus tinctorius

While it is now recognised that transport within the endomembrane system may occur via membranous tubules, spatial regulation of this process is poorly understood. We have investigated the role of the cytoskeleton in regulating the motility and morphology of the motile vacuole system in hyphae of the fungus Pisolithus tinctorius by studying (1) the effects of anti-microtubule (oryzalin, nocodazole) and anti-actin drugs (cytochalasins, latrunculin) on vacuolar activity, monitored by fluorescence microscopy of living cells; and (2) the ultrastructural relationship of microtubules, actin microfilaments, and vacuoles in hyphae prepared by rapid-freezing and freeze-substitution. Anti-microtubule drugs reduced the tubular component of the vacuole system in a dose-dependent and reversible manner, the extent of which correlated strongly with the degree of disruption of the microtubule network (monitored by immunofluorescence microscopy). The highest doses of anti-microtubule drugs completely eliminated tubular vacuoles, and only spherical vacuoles were observed. In contrast, anti-actin drugs did not reduce the frequency of tubular vacuoles or the motility of these vacuoles, even though immunofluorescence microscopy confirmed perturbation of microfilament organisation. Electron microscopy showed that vacuoles were always accompanied by microtubules. Bundles of microtubules were found running in parallel along the length of tubular vacuoles and individual microtubules were often within one microtubule diameter of a vacuole membrane. Our results strongly support a role for microtubules, but not actin microfilaments, in the spatial regulation of vacuole motility and morphology in fungal hyphae.

Aspects of the functional morphology of the ductus epididymidis in domestic anseriform and galliform birds

The structure of the ductus epididymidis, and its capacity to take up luminal particulate material (India ink) were studied, using scanning and transmission electron microscopy, in the domestic fowl, turkey and Japanese quail (Order Galliformes) and drake (Order Anseriformes). The epithelium appeared contractile and stable structurally, especially in the drake in which, among other cytoskeletal features, abundant perinuclear intermediate filaments radiated peripherally in the principal, nonciliated (Type III) cell. The Golgi complex was well developed, except in the turkey in which it was extremely inconspicuous. Sparsely granulated profiles were the main component of the elaborate endoplasmic reticulum, which was best developed in the drake and least in the turkey. The cell in the turkey was uniquely laden with large aggregates of lipid droplets. Generally, characteristics of a cell active in the synthesis and merocrine secretion of proteinaceous material were evident. There was no evidence of apocrine-like secretion in well-fixed tissues. The cell lacked the capacity to take up luminal India ink particles, and displayed very poorly developed endocytic structural features, which are also probably only selectively and minimally absorptive. Ultrastructural features varied between and within the orders studied, necessitating further studies.

Beta 2 integrins are involved in cytokine responses to whole Gram-positive bacteria

Proinflammatory cytokines have an important pathophysiologic role in septic shock. CD14 is involved in cytokine responses to a number of purified bacterial products, including LPS. However, little is known of monocyte receptors involved in cytokine responses to whole bacteria. To identify these receptors, human monocytes were pretreated with different mAbs and TNF-alpha was measured in culture supernatants after stimulation with whole heat-killed bacteria. Human serum and anti-CD14 Abs significantly increased and decreased, respectively, TNF-alpha responses to the Gram-negative Escherichia coli. However, neither treatment influenced responses to any of the Gram-positive bacteria tested, including group A and B streptococci, Listeria monocytogenes, and Staphylococcus aureus. Complement receptor type III (CR3 or CD18/CD11b) Abs prevented TNF-alpha release induced by heat-killed group A or B streptococci. In contrast, the same Abs had no effects when monocytes were stimulated with L. monocytogenes or S. aureus. Using either of the latter bacteria, significant inhibition of TNF-alpha release was produced by Abs to CD11c, one of the subunits of CR4. To confirm these blocking Ab data, IL-6 release was measured in CR3-, CR4-, or CD14-transfected Chinese hamster ovary cells after bacterial stimulation. Accordingly, streptococci triggered moderate IL-6 production (p < 0.05) in CR3 but not CD14 or CR4 transfectants. In contrast, L. monocytogenes and S. aureus induced IL-6 release in CR4 but not CR3 or CD14 transfectants. Collectively our data indicate that beta 2 integrins, such as CR3 and CR4, may be involved in cytokine responses to Gram-positive bacteria. Moreover, CD14 may play a more important role in responses to whole Gram-negative bacteria relative to Gram-positive ones.

In vitro formation of the endoplasmic reticulum occurs independently of microtubules by a controlled fusion reaction

We have established an in vitro system for the formation of the endoplasmic reticulum (ER). Starting from small membrane vesicles prepared from Xenopus laevis eggs, an elaborate network of membrane tubules is formed in the presence of cytosol. In the absence of cytosol, the vesicles only fuse to form large spheres. Network formation requires a ubiquitous cytosolic protein and nucleoside triphosphates, is sensitive to N-ethylmaleimide and high cytosolic Ca(2+) concentrations, and proceeds via an intermediate stage in which vesicles appear to be clustered. Microtubules are not required for membrane tubule and network formation. Formation of the ER network shares significant similarities with formation of the nuclear envelope. Our results suggest that the ER network forms in a process in which cytosolic factors modify and regulate a basic reaction of membrane vesicle fusion.

Coat proteins regulating membrane traffic

This review focuses on the roles of coat proteins in regulating the membrane traffic of eukaryotic cells. Coat proteins are recruited to the donor organelle membrane from a cytosolic pool by specific small GTP-binding proteins and are required for the budding of coated vesicles. This review first describes the four types of coat complexes that have been characterized so far: clathrin and its adaptors, the adaptor-related AP-3 complex, COPI, and COPII. It then discusses the ascribed functions of coat proteins in vesicular transport, including the physical deformation of the membrane into a bud, the selection of cargo, and the targeting of the budded vesicle. It also mentions how the coat proteins may function in an alternative model for transport, namely via tubular connections, and how traffic is regulated. Finally, this review outlines the evidence that related coat proteins may regulate other steps of membrane traffic.

Dynamic association of cytoplasmic dynein heavy chain 1a with the Golgi apparatus and intermediate compartment

Microtubule motors, such as the minus end-directed motor, cytoplasmic dynein, play an important role in maintaining the integrity, intracellular location, and function of the Golgi apparatus, as well as in the translocation of membrane between the endoplasmic reticulum and Golgi apparatus. We have immunolocalised conventional cytoplasmic dynein heavy chain to the Golgi apparatus in cultured vertebrate cells. In addition, we present evidence that cytoplasmic dynein heavy chain cycles constitutively between the endoplasmic reticulum and Golgi apparatus: it colocalises partially with the intermediate compartment, it is found on nocodazole-induced peripheral Golgi elements and, most strikingly, on Brefeldin A-induced tubules that are moving towards microtubule plus ends. The direction of movement of membrane between the endoplasmic reticulum and Golgi apparatus is therefore unlikely to be regulated by controlling motor-membrane interactions: rather, the motors probably remain bound throughout the whole cycle, with their activity being modulated instead. We also report that the overexpression of p50/dynamitin results in the loss of cytoplasmic dynein heavy chain from the membrane of peripheral Golgi elements. These results explain how dynamitin overexpression causes the inhibition of endoplasmic reticulum-to-Golgi transport complex movement towards the centrosomal region, and support the general model that an intact dynactin complex is required for cytoplasmic dynein binding to all cargoes.

Membrane tubule-mediated reassembly and maintenance of the Golgi complex is disrupted by phospholipase A2 antagonists

Although membrane tubules can be found extending from, and associated with, the Golgi complex of eukaryotic cells, their physiological function has remained unclear. To gain insight into the biological significance of membrane tubules, we have developed methods for selectively preventing their formation. We show here that a broad range of phospholipase A2 (PLA2) antagonists not only arrest membrane tubule-mediated events that occur late in the assembly of the Golgi complex but also perturb its normal steady-state tubulovesicular architecture by inducing a reversible fragmentation into separate "mini-stacks." In addition, we show that these same compounds prevent the formation of membrane tubules from Golgi stacks in an in vitro reconstitution system. This in vitro assay was further used to demonstrate that the relevant PLA2 activity originates from the cytoplasm. Taken together, these results demonstrate that Golgi membrane tubules, sensitive to potent and selective PLA2 antagonists, mediate both late events in the reassembly of the Golgi complex and the dynamic maintenance of its steady-state architecture. In addition, they implicate a role for cytoplasmic PLA2 enzymes in mediating these membrane trafficking events.

Further studies of the glandular tissue of the Sauromatum guttatum (Araceae) appendix

Electron microscopic studies showed that the trans-Golgi network (trans indicates the polarity of cisternae within the Golgi apparatus; it is opposite to the cis-face that is adjacent to the rough endoplasmic reticulum) was involved in the processing of the osmiophilic material present in the appendix of the inflorescence of Sauromatum guttatum. This material accumulated in the rough endoplasmic reticulum and in special pockets of the plasma membrane prior to heat production. Associations between the endoplasmic reticulum and trans-Golgi network were observed. The Golgi apparatus was composed of 5-6 dictyosomes on one side and one or two somewhat detached cisternae on the other side. Various nonosmiophilic Golgi-derived vesicles were observed: small ones covered with spike-like material, large ones with a smooth surface, and irregularly shaped ones. These electron-translucent vesicles seemed to accumulate in specific localities at the plasma membrane surface in the vicinity of the osmiophilic material; they were not found when the aroma was released. During heat production, the Golgi structures shrank and the activity of the trans-Golgi network seemed to be reduced. At the same time, coated pits were seen at the plasma membrane surface. In some cells, hypertrophic Golgi apparatuses were seen with only 2-3 dictyosomes that contained granulated material in their lumens. Finally, the osmiophilic material was also found in the plasmodesmata.

Fusogenic domains of golgi membranes are sequestered into specialized regions of the stack that can be released by mechanical fragmentation

A well-characterized cell-free assay that reconstitutes Golgi transport is shown to require physically fragmented Golgi fractions for maximal activity. A Golgi fraction containing large, highly stacked flattened cisternae associated with coatomer-rich components was inactive in the intra-Golgi transport assay. In contrast, more fragmented hepatic Golgi fractions of lower purity were highly active in this assay. Control experiments ruled out defects in glycosylation, the presence of excess coatomer or inhibitory factors, as well as the lack or consumption of limiting diffusible factors as responsible for the lower activity of intact Golgi fractions. Neither Brefeldin A treatment, preincubation with KCl (that completely removed associated coatomer) or preincubation with imidazole buffers that caused unstacking, activated stacked fractions for transport. Only physical fragmentation promoted recovery of Golgi fractions active for transport in vitro. Rate-zonal centrifugation partially separated smaller transport-active Golgi fragments with a unique v-SNARE pattern, away from the bulk of Golgi-derived elements identified by their morphology and content of Golgi marker enzymes (N-acetyl glucosaminyl and galactosyl transferase activities). These fragments released during activation likely represent intra-Golgi continuities involved in maintaining the dynamic redistribution of resident enzymes during rapid anterograde transport of secretory cargo through the Golgi in vivo.

Golgi structure in three dimensions: functional insights from the normal rat kidney cell

Three-dimensional reconstructions of portions of the Golgi complex from cryofixed, freeze-substituted normal rat kidney cells have been made by dual-axis, high-voltage EM tomography at approximately 7-nm resolution. The reconstruction shown here ( approximately 1 x 1 x 4 microm3) contains two stacks of seven cisternae separated by a noncompact region across which bridges connect some cisternae at equivalent levels, but none at nonequivalent levels. The rest of the noncompact region is filled with both vesicles and polymorphic membranous elements. All cisternae are fenestrated and display coated buds. They all have about the same surface area, but they differ in volume by as much as 50%. The trans-most cisterna produces exclusively clathrin-coated buds, whereas the others display only nonclathrin coated buds. This finding challenges traditional views of where sorting occurs within the Golgi complex. Tubules with budding profiles extend from the margins of both cis and trans cisternae. They pass beyond neighboring cisternae, suggesting that these tubules contribute to traffic to and/or from the Golgi. Vesicle-filled "wells" open to both the cis and lateral sides of the stacks. The stacks of cisternae are positioned between two types of ER, cis and trans. The cis ER lies adjacent to the ER-Golgi intermediate compartment, which consists of discrete polymorphic membranous elements layered in front of the cis-most Golgi cisterna. The extensive trans ER forms close contacts with the two trans-most cisternae; this apposition may permit direct transfer of lipids between ER and Golgi membranes. Within 0.2 microm of the cisternae studied, there are 394 vesicles (8 clathrin coated, 190 nonclathrin coated, and 196 noncoated), indicating considerable vesicular traffic in this Golgi region. Our data place structural constraints on models of trafficking to, through, and from the Golgi complex.

SNARE membrane trafficking dynamics in vivo

The ER/Golgi soluble NSF attachment protein receptor (SNARE) membrin, rsec22b, and rbet1 are enriched in approximately 1-micrometer cytoplasmic structures that lie very close to the ER. These appear to be ER exit sites since secretory cargo concentrates in and exits from these structures. rsec22b and rbet1 fused to fluorescent proteins are enriched at approximately 1-micrometer ER exit sites that remained more or less stationary, but periodically emitted streaks of fluorescence that traveled generally in the direction of the Golgi complex. These exit sites were reused and subsequent tubules or streams of vesicles followed similar trajectories. Fluorescent membrin- enriched approximately 1-micrometer peripheral structures were more mobile and appeared to translocate through the cytoplasm back and forth, between the periphery and the Golgi area. These mobile structures could serve to collect secretory cargo by fusing with ER-derived vesicles and ferrying the cargo to the Golgi. The post-Golgi SNAREs, syntaxin 6 and syntaxin 13, when fused to fluorescent proteins each displayed characteristic patterns of movement. However, syntaxin 13 was the only SNARE whose life cycle appeared to involve interactions with the plasma membrane. These studies reveal the in vivo spatiotemporal dynamics of SNARE proteins and provide new insight into their roles in membrane trafficking.

Coalescence of Golgi fragments in microtubule-deprived living cells

The process of stack coalescence, an important mechanism of Golgi recovery from mitosis, was examined using novel experimental paradigms. In living cells with disrupted (by nocodazole) microtubules, galactosyl transferase-GFP-labelled Golgi fragments constantly appeared, grew, sometimes moved with a speed of 1-2 microns/min, coalesced or gradually diminished and disappeared. The rate of Golgi fragment turnover and coalescence was highly balanced to maintain a constant number of Golgi units per cell. Moreover some Golgi islands appear and some received new GalTase-GFP after photobleaching of cell cytoplasm. Short tubules extending from the rims of scattered Golgi fragments frequently formed bridges between ministacks, inducing their coalescence. The frequency of coalescence could also be inhibited by disruption of actin microfilaments. After the Golgi redistribution into endoplasmic reticulum induced by brefeldin A, either the growth of small Golgi fragments or their coalescence leads to compartmentalized stack formation without the participation of microtubules. These results demonstrate that this coalescence between isolated Golgi stacks is microtubule-independent and could thus be mediated by membranous tubules.