The Arf-GEF GBF1 undergoes multi-domain structural shifts to activate Arf at the Golgi

Golgi homeostasis require the activation of Arf GTPases by the guanine-nucleotide exchange factor requires GBF1, whose recruitment to the Golgi represents a rate limiting step in the process. GBF1 contains a conserved, catalytic, Sec7 domain (Sec7d) and five additional (DCB, HUS, HDS1-3) domains. Herein, we identify the HDS3 domain as essential for GBF1 membrane association in mammalian cells and document the critical role of HDS3 during the development of Drosophila melanogaster. We show that upon binding to Golgi membranes, GBF1 undergoes conformational changes in regions bracketing the catalytic Sec7d. We illuminate GBF1 interdomain arrangements by negative staining electron microscopy of full-length human GBF1 to show that GBF1 forms an anti-parallel dimer held together by the paired central DCB-HUS core, with two sets of HDS1-3 arms extending outward in opposite directions. The catalytic Sec7d protrudes from the central core as a largely independent domain, but is closely opposed to a previously unassigned α-helix from the HDS1 domain. Based on our data, we propose models of GBF1 engagement on the membrane to provide a paradigm for understanding GBF1-mediated Arf activation required for cellular and organismal function.

Modeling the dynamic behaviors of the COPI vesicle formation regulators, the small GTPase Arf1 and its activating Sec7 guanine nucleotide exchange factor GBF1 on Golgi membranes

The components and subprocesses underlying the formation of COPI-coated vesicles at the Golgi are well understood. The coating cascade is initiated after the small GTPase Arf1 is activated by the Sec7 domain–containing guanine nucleotide exchange factor GBF1 (Golgi brefeldin A resistant guanine nucleotide exchange factor 1). This causes a conformational shift within Arf1 that facilitates stable association of Arf1 with the membrane, a process required for subsequent recruitment of the COPI coat. Although we have atomic-level knowledge of Arf1 activation by Sec7 domain–containing GEFs, our understanding of the biophysical processes regulating Arf1 and GBF1 dynamics is limited. We used fluorescence recovery after photobleaching data and kinetic Monte Carlo simulation to assess the behavior of Arf1 and GBF1 during COPI vesicle formation in live cells. Our analyses suggest that Arf1 and GBF1 associate with Golgi membranes independently, with an excess of GBF1 relative to Arf1. Furthermore, the GBF1-mediated Arf1 activation is much faster than GBF1 cycling on/off the membrane, suggesting that GBF1 is regulated by processes other than its interactions Arf1. Interestingly, modeling the behavior of the catalytically inactive GBF1/E794K mutant stabilized on the membrane is inconsistent with the formation of a stable complex between it and an endogenous Arf1 and suggests that GBF1/E794K is stabilized on the membrane independently of complex formation.

Wnt signaling modulator DKK1 as an immunotherapeutic target in ovarian cancer

OBJECTIVES

Wnt pathway mutations are a hallmark of endometrioid and clear cell subtypes of epithelial ovarian carcinoma (EOC). However, no drugs targeting the Wnt pathway in EOC are FDA-approved. Dickkopf-related protein 1 (DKK1), a modulator of the Wnt pathway, has emerged as a promising therapeutic target. We aimed to examine the role of DKK1 and the effects of a monoclonal antibody against DKK1 (DKN-01) in vivo and in a murine model of ovarian cancer.

METHODS

We examined in vitro the role of DKK1 and the effects of DKK1 inhibition in EOC cell lines. We then studied in vivo the role of DKN-01 and DKK1 overexpression on tumor burden and anti-tumor immune cell populations using the ID8 syngeneic mouse model.

RESULTS

DKN-01 did not phenotypically alter ES2 cells in vitro; however, DKK1 inhibition promoted Wnt signaling. Tumor burden and immune populations were unchanged in ID8 challenged mice treated with mDKN01. Mice challenged with ID8 cells overexpressing DKK1 had tumor burden similar to controls (p = 0.175). However, the overexpression of DKK1 decreased CD45⁺ leukocyte infiltration into the peritoneum (p = 0.008) and omentum (p = 0.032), reducing both natural killer (NK) and CD8 T cells, and reducing interferon-gamma (IFNγ) expression on activated CD8 T cells.

CONCLUSIONS

Our results suggest that DKK1 inhibition does not affect tumor growth in the ID8 ovarian cancer model. DKK1 overexpression alters anti-tumor immune populations within the tumor microenvironment. Thus, our findings confirm DKK1 as a new therapeutic target in EOC and suggest that DKK1 inhibition may function best in a combinatorial, immune-modulatory therapy.

Biomarkers in ovarian cancer: To be or not to be

Biomarkers are becoming increasingly important in the treatment of epithelial ovarian cancer. Recent work from many laboratories has begun to provide clinically meaningful biomarkers. This review summarizes the state of the science regarding biomarkers for stratifying early-stage patients into those who benefit from adjuvant treatment, primary debulking versus interval debulking, and specific targeted therapy. In addition, new molecular imaging technologies have been developed to allow the surgeon to resect subvisible tumor deposits. These efforts should increase clinical effectiveness while minimizing toxicities for patients.

Proline-Glycine-Proline Peptides Are Critical in the Development of Smoke-induced Emphysema

Chronic obstructive pulmonary disease (COPD) is a major cause of mortality worldwide and is characterized by an excessive airway neutrophilic response. The neutrophil chemoattractant proline-glycine-proline (PGP) and its more potent acetylated form (acPGP) have been found to be elevated in patients with COPD and act via CXCR2. Here, we investigated the impact of neutralizing PGP peptides in a murine model for emphysema. The PGP-neutralizing peptide l-arginine-threonine-arginine (RTR) was used first in a 6-week model of cigarette smoke exposure, where it attenuated lung inflammation. Then, in a model of chronic smoke exposure, mice were exposed to cigarette smoke and RTR treatment was initiated after 10 weeks of smoke exposure. This treatment was continued together with smoke exposure for another 13 weeks, for a total of 23 weeks of smoke exposure. RTR significantly inhibited neutrophil and macrophage influx into the lungs in the 6-week model of exposure. RTR also attenuated the development of emphysema, normalized lung volumes, and reduced right ventricular hypertrophy in the chronic exposure model. Murine epithelia expressed CXCR2, and this expression was increased after smoke exposure. In vitro, human bronchial epithelial cells also demonstrated robust expression of CXCR2, and stimulation of primary human bronchial epithelial cells with acPGP led to increased release of MMP-9 and IL-8. Overall, these results provide evidence that acPGP plays a critical role during the development of emphysema in cigarette smoke-induced injury, and highlight a new epithelial mechanism by which acPGP augments neutrophilic inflammation.

Activated PMN Exosomes: Pathogenic Entities Causing Matrix Destruction and Disease in the Lung

Here, we describe a novel pathogenic entity, the activated PMN (polymorphonuclear leukocyte, i.e., neutrophil)-derived exosome. These CD63⁺/CD66b⁺ nanovesicles acquire surface-bound neutrophil elastase (NE) during PMN degranulation, NE being oriented in a configuration resistant to α1-antitrypsin (α1AT). These exosomes bind and degrade extracellular matrix (ECM) via the integrin Mac-1 and NE, respectively, causing the hallmarks of chronic obstructive pulmonary disease (COPD). Due to both ECM targeting and α1AT resistance, exosomal NE is far more potent than free NE. Importantly, such PMN-derived exosomes exist in clinical specimens from subjects with COPD but not healthy controls and are capable of transferring a COPD-like phenotype from humans to mice in an NE-driven manner. Similar findings were observed for another neutrophil-driven disease of ECM remodeling (bronchopulmonary dysplasia [BPD]). These findings reveal an unappreciated role for exosomes in the pathogenesis of disorders of ECM homeostasis such as COPD and BPD, providing a critical mechanism for proteolytic damage.

Exosomal microRNA predicts and protects against severe bronchopulmonary dysplasia in extremely premature infants

Premature infants are at high risk for developing bronchopulmonary dysplasia (BPD), characterized by chronic inflammation and inhibition of lung development, which we have recently identified as being modulated by microRNAs (miRNAs) and alterations in the airway microbiome. Exosomes and exosomal miRNAs may regulate cell differentiation and tissue and organ development. We discovered that tracheal aspirates from infants with severe BPD had increased numbers of, but smaller, exosomes compared with term controls. Similarly, bronchoalveolar lavage fluid from hyperoxia-exposed mice (an animal model of BPD) and supernatants from hyperoxia-exposed human bronchial epithelial cells (in vitro model of BPD) had increased exosomes compared with air controls. Next, in a prospective cohort study of tracheal aspirates obtained at birth from extremely preterm infants, utilizing independent discovery and validation cohorts, we identified unbiased exosomal miRNA signatures predictive of severe BPD. The strongest signal of reduced miR-876-3p in BPD-susceptible compared with BPD-resistant infants was confirmed in the animal model and in vitro models of BPD. In addition, based on our recent discovery of increased Proteobacteria in the airway microbiome being associated with BPD, we developed potentially novel in vivo and in vitro models for BPD combining Proteobacterial LPS and hyperoxia exposure. Addition of LPS led to a larger reduction in exosomal miR 876-3p in both hyperoxia and normoxia compared with hyperoxia alone, thus indicating a potential mechanism by which alterations in microbiota can suppress miR 876-3p. Gain of function of miR 876-3p improved the alveolar architecture in the in vivo BPD model, demonstrating a causal link between miR 876-3p and BPD. In summary, we provide evidence for the strong predictive biomarker potential of miR 876-3p in severe BPD. We also provide insights on the pathogenesis of neonatal lung disease, as modulated by hyperoxia and microbial product-induced changes in exosomal miRNA 876-3p, which could be targeted for future therapeutic development.

MicroRNA-145 Antagonism Reverses TGF-β Inhibition of F508del CFTR Correction in Airway Epithelia

RATIONALE

MicroRNAs (miRNAs) destabilize mRNA transcripts and inhibit protein translation. miR-145 is of particular interest in cystic fibrosis (CF) as it has a direct binding site in the 3'-untranslated region of CFTR (cystic fibrosis transmembrane conductance regulator) and is upregulated by the CF genetic modifier TGF (transforming growth factor)-β.

OBJECTIVES

To demonstrate that miR-145 mediates TGF-β inhibition of CFTR synthesis and function in airway epithelia.

METHODS

Primary human CF (F508del homozygous) and non-CF airway epithelial cells were grown to terminal differentiation at the air-liquid interface on permeable supports. TGF-β (5 ng/ml), a miR-145 mimic (20 nM), and a miR-145 antagonist (20 nM) were used to manipulate CFTR function. In CF cells, lumacaftor (3 μM) and ivacaftor (10 μM) corrected mutant F508del CFTR. Quantification of CFTR mRNA, protein, and function was done by standard techniques.

MEASUREMENTS AND MAIN RESULTS

miR-145 is increased fourfold in CF BAL fluid compared with non-CF (P < 0.01) and increased 10-fold in CF primary airway epithelial cells (P < 0.01). Exogenous TGF-β doubles miR-145 expression (P < 0.05), halves wild-type CFTR mRNA and protein levels (P < 0.01), and nullifies lumacaftor/ivacaftor F508del CFTR correction. miR-145 overexpression similarly decreases wild-type CFTR protein synthesis (P < 0.01) and function (P < 0.05), and eliminates F508del corrector benefit. miR-145 antagonism blocks TGF-β suppression of CFTR and enhances lumacaftor correction of F508del CFTR.

CONCLUSIONS

miR-145 mediates TGF-β inhibition of CFTR synthesis and function in airway epithelia. Specific antagonists to miR-145 interrupt TGF-β signaling to restore F508del CFTR modulation. miR-145 antagonism may offer a novel therapeutic opportunity to enhance therapeutic benefit of F508del CFTR correction in CF epithelia.

Analysis of heat source selection for residential buildings in rural areas

The research aiming to check whether the output of currently installed boilers matches the use requirements together with estimation of their energy efficiency was carried out on a group of 84 single-family residential buildings located in rural areas. Heating and hot water energy needs were calculated for each building in order to determine the use requirements. This enabled verification whether the currently installed boilers match the actual use requirements in the buildings. Based on the calculations it was determined that the designed average boiler output in the group of buildings subject to analysis is 15.7 kW, whereas the mean rated output capacity of boilers installed therein is 25.4 kW. On average, the output capacity of the installed boilers exceeds the use requirements for the buildings by 60%. To calculate the energy efficiency of boilers, the mean annual boiler output capacity use coefficient was determined. For boilers selected on the basis of standard calculations, the mean coefficient is 0.47. For boilers currently in use it is 0.31, less than the above figure. The above calculations show that if boilers were correctly selected in compliance to the building needs, then the average estimated seasonal efficiency of 65% would be feasible. However, in the current state the achievable efficiency is approx. 55%.

Toll-Like Receptor 4 Engagement Mediates Prolyl Endopeptidase Release from Airway Epithelia via Exosomes

Proteases are important regulators of pulmonary remodeling and airway inflammation. Recently, we have characterized the enzyme prolyl endopeptidase (PE), a serine peptidase, as a critical protease in the generation of the neutrophil chemoattractant tripeptide Pro-Gly-Pro (PGP) from collagen. However, PE has been characterized as a cytosolic enzyme, and the mechanism mediating PE release extracellularly remains unknown. We examined the role of exosomes derived from airway epithelia as a mechanism for PE release and the potential extracellular signals that regulate the release of these exosomes. We demonstrate a specific regulatory pathway of exosome release from airway epithelia and identify PE as novel exosome cargo. LPS stimulation of airway epithelial cells induces release of PE-containing exosomes, which is significantly attenuated by small interfering RNA depletion of Toll-like receptor 4 (TLR4). These differences were recapitulated upon intratracheal LPS administration in mice competent versus deficient for TLR4 signaling. Finally, sputum samples from subjects with cystic fibrosis colonized with Pseudomonas aeruginosa demonstrate elevated exosome content and increased PE levels. This TLR4-based mechanism highlights the first report of nonstochastic release of exosomes in the lung and couples TLR4 activation with matrikine generation. The increased quantity of these proteolytic exosomes in the airways of subjects with chronic lung disease highlights a new mechanism of injury and inflammation in the pathogenesis of pulmonary disorders.

Genetic regulation of expression of leukotriene A4 hydrolase

In chronic inflammatory lung disorders such as chronic obstructive pulmonary disease (COPD), the concurrent organ-specific and systemic inflammatory responses lead to airway remodelling and vascular dysfunction. Although a major common risk factor for COPD, cigarette smoke alone cannot explain the progression of this disease; there is increasing evidence that genetic predisposition also plays a role in COPD susceptibility and progression. A key enzyme in chronic lung inflammation is leukotriene A4 hydrolase (LTA4H). With its aminopeptidase activity, LTA4H degrades the neutrophil chemoattractant tripeptide PGP. In this study, we used the luciferase reporter gene analysis system and quantitative trait locus analysis to explore the impact of single-nucleotide polymorphisms (SNPs) in the putative promoter region of LTA4H on LTA4H expression. We show that not only is the putative promoter of LTA4H larger than previously reported but also that SNPs in the expanded promoter region regulate expression of LTA4H both in cell-based systems and in peripheral blood samples from human subjects. These findings provide significant evidence for an active region upstream of the previously reported LTA4H promoter, which may have implications related to ongoing inflammatory processes in chronic lung disease.

The Garz Sec7 domain guanine nucleotide exchange factor for Arf regulates salivary gland development in Drosophila

Surface delivery of proteins involved in cell-cell and cell-matrix interactions in cultured mammalian cells requires the GBF1 guanine nucleotide exchange factor. However, the role of GBF1 in delivery of adhesion proteins during organogenesis in intact animals has not been characterized. Here, we report the function of the fly GBF1 homolog, Gartenzwerg (Garz) in the development of the salivary gland in Drosophila melanogaster. We used the GAL4/UAS system to selectively deplete Garz from salivary gland cells. We show that depletion of Garz disrupts the secretory pathway as evidenced by the collapse of Golgi-localized Lava lamp (Lva) and the TGN-localized γ subunit of the clathrin-adaptor protein complex (AP-1). Additionally, Garz depletion inhibits trafficking of cell-cell adhesion proteins cadherin (DE-cad) and Flamingo to the cell surface. Disregulation of trafficking correlates with mistargeting of the tumor suppressor protein Discs large involved in epithelial polarity determination. Garz-depleted salivary cells are smaller and lack well-defined plasma membrane domains. Garz depletion also inhibits normal elongation and positioning of epithelial cells, resulting in a disorganized salivary gland that lacks a well defined luminal duct. Our findings suggest that Garz is essential for establishment of epithelial structures and demonstrate an absolute requirement for Garz during Drosophila development.

An aberrant leukotriene A4 hydrolase-proline-glycine-proline pathway in the pathogenesis of chronic obstructive pulmonary disease

RATIONALE

Chronic neutrophilic inflammation is a hallmark in the pathogenesis of chronic obstructive pulmonary disease (COPD) and persists after cigarette smoking has stopped. Mechanisms involved in this ongoing inflammatory response have not been delineated.

OBJECTIVES

We investigated changes to the leukotriene A4 hydrolase (LTA4H)-proline-glycine-proline (PGP) pathway and chronic inflammation in the development of COPD.

METHODS

A/J mice were exposed to air or cigarette smoke for 22 weeks followed by bronchoalveolar lavage and lung and cardiac tissue analysis. Two human cohorts were used to analyze changes to the LTA4H-PGP pathway in never smokers, control smokers, COPD smokers, and COPD former smokers. PGP/AcPGP and LTA4H aminopeptidase activity were detected by mass spectroscopy, LTA4H amounts were detected by ELISA, and acrolein was detected by Western blot.

MEASUREMENTS AND MAIN RESULTS

Mice exposed to cigarette smoke developed emphysema with increased PGP, neutrophilic inflammation, and selective inhibition of LTA4H aminopeptidase, which ordinarily degrades PGP. We recapitulated these findings in smokers with and without COPD. PGP and AcPGP are closely associated with cigarette smoke use. Once chronic inflammation is established, changes to LTA4H aminopeptidase remain, even in the absence of ongoing cigarette use. Acrolein modifies LTA4H and inhibits aminopeptidase activity to the same extent as cigarette smoke.

CONCLUSIONS

These results demonstrate a novel pathway of aberrant regulation of PGP/AcPGP, suggesting this inflammatory pathway may be intimately involved in disease progression in the absence of ongoing cigarette smoke exposure. We highlight a mechanism by which acrolein potentiates neutrophilic inflammation through selective inhibition of LTA4H aminopeptidase activity. Clinical trial registered with www.clinicaltrials.gov (NCT 00292552).

The Sec7 guanine nucleotide exchange factor GBF1 regulates membrane recruitment of BIG1 and BIG2 guanine nucleotide exchange factors to the trans-Golgi network (TGN)

Three Sec7 guanine nucleotide exchange factors (GEFs) activate ADP-ribosylation factors (ARFs) to facilitate coating of transport vesicles within the secretory and endosomal pathways. GBF1 recruits COPI to pre-Golgi and Golgi compartments, whereas BIG1 and BIG2 recruit AP1 and GGA clathrin adaptors to the trans-Golgi network (TGN) and endosomes. Here, we report a functional cascade between these GEFs by showing that GBF1-activated ARFs (ARF4 and ARF5, but not ARF3) facilitate BIG1 and BIG2 recruitment to the TGN. We localize GBF1 ultrastructurally to the pre-Golgi, the Golgi, and also the TGN. Our findings suggest a model in which GBF1 localized within pre-Golgi and Golgi compartments mediates ARF activation to facilitate recruitment of COPI to membranes, whereas GBF1 localized at the TGN mediates ARF activation that leads to the recruitment of BIG1 and BIG2 to the TGN. Membrane-associated BIG1/2 then activates ARFs that recruit clathrin adaptors. In this cascade, an early acting GEF (GBF1) activates ARFs that mediate recruitment of late acting GEFs (BIG1/2) to coordinate coating events within the pre-Golgi/Golgi/TGN continuum. Such coordination may optimize the efficiency and/or selectivity of cargo trafficking through the compartments of the secretory pathway.

COPII and COPI traffic at the ER-Golgi interface

Protein traffic is necessary to maintain homeostasis in all eukaryotic organisms. All newly synthesized secretory proteins destined to the secretory and endolysosmal systems are transported from the endoplasmic reticulum to the Golgi before delivery to their final destinations. Here, we describe the COPII and COPI coating machineries that generate carrier vesicles and the tethers and SNAREs that mediate COPII and COPI vesicle fusion at the ER-Golgi interface.

Identification of a functional domain within the p115 tethering factor that is required for Golgi ribbon assembly and membrane trafficking

The tethering factor p115 (known as Uso1p in yeast) has been shown to facilitate Golgi biogenesis and membrane traffic in cells in culture. However, the role of p115 within an intact animal is largely unknown. Here, we document that depletion of p115 by using RNA interference (RNAi) in C. elegans causes accumulation of the 170 kD soluble yolk protein (YP170) in the body cavity and retention of the yolk receptor RME-2 in the ER and the Golgi within oocytes. Structure-function analyses of p115 have identified two homology regions (H1 and H2) within the N-terminal globular head and the coiled-coil 1 (CC1) domain as essential for p115 function. We identify a new C-terminal domain of p115 as necessary for Golgi ribbon formation and cargo trafficking. We show that p115 mutants that lack the fourth CC domain (CC4) act in a dominant-negative manner to disrupt Golgi and prevent cargo trafficking in cells containing endogenous p115. Furthermore, using RNAi of p115 and the subsequent transfection with p115 deletion mutants, we show that CC4 is necessary for Golgi ribbon formation and membrane trafficking in cells depleted of endogenous p115. p115 has been shown to bind a subset of ER-Golgi SNAREs through CC1 and CC4 domains (Shorter et al., 2002). Our findings show that CC4 is required for p115 function, and suggest that both the CC1 and the CC4 SNARE-binding motifs participate in p115-mediated membrane tethering.

Novel C-terminal motif within Sec7 domain of guanine nucleotide exchange factors regulates ADP-ribosylation factor (ARF) binding and activation

ADP-ribosylation factors (ARFs) and their activating guanine nucleotide exchange factors (GEFs) play key roles in membrane traffic and signaling. All ARF GEFs share a ∼200-residue Sec7 domain (Sec7d) that alone catalyzes the GDP to GTP exchange that activates ARF. We determined the crystal structure of human BIG2 Sec7d. A C-terminal loop immediately following helix J (loop>J) was predicted to form contacts with helix H and the switch I region of the cognate ARF, suggesting that loop>J may participate in the catalytic reaction. Indeed, we identified multiple alanine substitutions within loop>J of the full length and/or Sec7d of two large brefeldin A-sensitive GEFs (GBF1 and BIG2) and one small brefeldin A-resistant GEF (ARNO) that abrogated binding of ARF and a single alanine substitution that allowed ARF binding but inhibited GDP to GTP exchange. Loop>J sequences are highly conserved, suggesting that loop>J plays a crucial role in the catalytic activity of all ARF GEFs. Using GEF mutants unable to bind ARF, we showed that GEFs associate with membranes independently of ARF and catalyze ARF activation in vivo only when membrane-associated. Our structural, cell biological, and biochemical findings identify loop>J as a key regulatory motif essential for ARF binding and GDP to GTP exchange by GEFs and provide evidence for the requirement of membrane association during GEF activity.

ArfGAP1 promotes COPI vesicle formation by facilitating coatomer polymerization

The role of ArfGAP1 in COPI vesicle biogenesis has been controversial. In work using isolated Golgi membranes, ArfGAP1 was found to promote COPI vesicle formation. In contrast, in studies using large unilamellar vesicles (LUVs) as model membranes, ArfGAP1 functioned as an uncoating factor inhibiting COPI vesicle formation. We set out to discriminate between these models. First, we reexamined the effect of ArfGAP1 on LUVs. We found that ArfGAP1 increased the efficiency of coatomer-induced deformation of LUVs. Second, ArfGAP1 and peptides from cargo facilitated self-assembly of coatomer into spherical structures in the absence of membranes, reminiscent of clathrin self-assembly. Third, in vivo, ArfGAP1 overexpression induced the accumulation of vesicles and allowed normal trafficking of a COPI cargo. Taken together, these data support the model in which ArfGAP1 promotes COPI vesicle formation and membrane traffic and identify a function for ArfGAP1 in the assembly of coatomer into COPI.

Dissecting the role of the ARF guanine nucleotide exchange factor GBF1 in Golgi biogenesis and protein trafficking

COPI recruitment to membranes appears to be essential for the biogenesis of the Golgi and for secretory trafficking. Preventing COPI recruitment by expressing inactive forms of the ADP-ribosylation factor (ARF) or the ARF-activating guanine nucleotide exchange factor GBF1, or by treating cells with brefeldin A (BFA), causes the collapse of the Golgi into the endoplasmic reticulum (ER) and arrests trafficking of soluble and transmembrane proteins at the ER. Here, we assess COPI function in Golgi biogenesis and protein trafficking by preventing COPI recruitment to membranes by removing GBF1. We report that siRNA-mediated depletion of GBF1 causes COPI dispersal but does not lead to collapse of the Golgi. Instead, it causes extensive tubulation of the cis-Golgi. The Golgi-derived tubules target to peripheral ER-Golgi intermediate compartment (ERGIC) sites and create dynamic continuities between the ERGIC and the cis-Golgi compartment. COPI dispersal in GBF1-depleted cells causes dramatic inhibition of the trafficking of transmembrane proteins. Unexpectedly, soluble proteins continue to be secreted from GBF1-depleted cells. Our findings suggest that a secretory pathway capable of trafficking soluble proteins can be maintained in cells in which COPI recruitment is compromised by GBF1 depletion. However, the trafficking of transmembrane proteins through the existing pathway requires GBF1-mediated ARF activation and COPI recruitment.

Activation of ADP-ribosylation factor regulates biogenesis of the ATP7A-containing trans-Golgi network compartment and its Cu-induced trafficking

ATP7A (MNK) regulates copper homeostasis by translocating from a compartment localized within the trans-Golgi network to the plasma membrane (PM) in response to increased copper load. The mechanisms that regulate the biogenesis of the MNK compartment and the trafficking of MNK are unclear. Here we show that the architecture of the MNK compartment is linked to the structure of the Golgi ribbon. Depletion of p115 tethering factor, which causes fragmentation of the Golgi ribbon, also disrupts the MNK compartment. In p115-depleted cells, MNK localizes to punctate structures that pattern on Golgi ministacks dispersed throughout the cell. Despite altered localization MNK trafficking still occurs, and MNK relocates from and returns to the fragmented compartment in response to copper. We further show that the biogenesis of the MNK compartment requires activation of ADP-ribosylation factor (Arf)1 GTPase, shown previously to facilitate the biogenesis of the Golgi ribbon. Activation of cellular Arf1 is prevented by 1) expressing an inactive "empty" form of Arf (Arf1/N126I), 2) expressing an inactive form of GBF1 (GBF1/E794K), guanine nucleotide exchange factor for Arf1, or 3) treating cells with brefeldin A, an inhibitor of GBF1 that disrupts MNK into a diffuse pattern. Importantly, preventing Arf activation inhibits copper-responsive trafficking of MNK to the PM. Our findings support a model in which active Arf is essential for the generation of the MNK compartment and for copper-responsive trafficking of MNK from there to the PM. Our findings provide an exciting foundation for identifying Arf1 effectors that facilitate the biogenesis of the MNK compartment and MNK traffic.

On and off membrane dynamics of the endoplasmic reticulum-golgi tethering factor p115 in vivo

The mechanisms regulating membrane recruitment of the p115 tethering factor in vivo are unknown. Here, we describe cycling of p115 between membranes and cytosol and document the effects of Golgi matrix proteins, Rab1, and soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptors (SNAREs) on this process. Rapid membrane/cytosol exchange is shown by swift (t1/2 approximately 20 s) loss of Golgi-localized p115-green fluorescent protein (GFP) after repeated photobleaching of cell periphery and rapid (t1/2 approximately 13 s) fluorescence recovery after photobleaching Golgi-localized p115-GFP. p115 mutant missing the GM130/giantin binding site exhibits analogous fluorescence recovery after photobleaching (FRAP) (t1/2 approximately 13 s), suggesting that GM130 and giantin are not major determinants of p115 membrane dynamics. In contrast, p115-GFP exchanges more rapidly (t1/2 approximately 8 s) in cells expressing the inactive Rab1/N121I mutant, indicating that p115 cycling is influenced by Rab1. p115-GFP dynamics is also influenced by the assembly status of SNAREs. In cells expressing an ATPase-deficient NSF/E329Q mutant that inhibits SNARE complex disassembly, the cycling kinetics of p115-GFP are significantly slower (t1/2 approximately 21 s). In contrast, in cells incubated at reduced temperature (10 degrees C) that inhibits vesicular traffic, the cycling kinetics of p115-GFP are faster (t1/2 approximately 7 s). These data suggest that p115-binding sites on the membrane are provided by unassembled SNAREs. In agreement, biochemical studies show increased p115 recruitment to membranes in the presence of NSF and alpha-SNAP. Our data support a model in which recruitment of tethers is directly regulated by the assembly status of SNAREs.

Dissection of membrane dynamics of the ARF-guanine nucleotide exchange factor GBF1

ADP-ribosylation factor (ARF)-facilitated recruitment of COP I to membranes is required for secretory traffic. The guanine nucleotide exchange factor GBF1 activates ARF and regulates ARF/COP I dynamics at the endoplasmic reticulum (ER)-Golgi interface. Like ARF and coatomer, GBF1 peripherally associates with membranes. ADP-ribosylation factor and coatomer have been shown to rapidly cycle between membranes and cytosol, but the membrane dynamics of GBF1 are unknown. Here, we used fluorescence recovery after photobleaching to characterize the behavior of GFP-tagged GBF1. We report that GBF1 rapidly cycles between membranes and the cytosol (t1/2 is approximately 17 +/- 1 seconds). GBF1 cycles faster than GFP-tagged ARF, suggesting that in each round of association/dissociation, GBF1 catalyzes a single event of ARF activation, and that the activated ARF remains on membrane after GBF1 dissociation. Using three different approaches [expression of an inactive (E794K) GBF1 mutant, expression of the ARF1 (T31N) mutant with decreased affinity for GTP and Brefeldin A treatment], we show that GBF1 is stabilized on membranes when in a complex with ARF-GDP. GBF1 dissociation from ARF and membranes is triggered by its catalytic activity, i.e. the displacement of GDP and the subsequent binding of GTP to ARF. Our findings imply that continuous cycles of recruitment and dissociation of GBF1 to membranes are required for sustained ARF activation and COP I recruitment that underlies ER-Golgi traffic.

Mutations in COCH that result in non-syndromic autosomal dominant deafness (DFNA9) affect matrix deposition of cochlin

The COCH gene mutated in autosomal dominant sensorineural deafness (DFNA9) encodes cochlin, a major constituent of the inner ear extracellular matrix. Sequence analysis of cochlin from DFNA9 patients identified five distinct single-amino-acid mutations within a conserved region (the LCCL domain) of cochlin. To define the molecular basis of DFNA9, we have generated myc-tagged wild-type and mutant cochlins and explored their behavior in transient transfection systems. Western blotting of cell lysates and culture media indicates that wild-type and mutant cochlins are synthesized and secreted in similar amounts. Immunofluorescent staining confirms that all are detected within the endoplasmic reticulum and the Golgi complex of transfected cells. Our findings suggest that COCH mutations are unlikely to cause abnormalities in secretion and suggest that extracellular events might cause DFNA9 pathology. In agreement, we show that wild-type cochlin accumulates in extracellular deposits that closely parallel the matrix component fibronectin, whereas mutant cochlins vary in the amount and pattern of extracellular material. Whereas some mutants exhibit an almost normal deposition pattern, some show complete lack of deposition. Our results suggest that DFNA9 results from gene products that fail to integrate correctly into the extracellular matrix. The partial or complete penetrance of integration defects suggests that DFNA9 pathology may be caused by multiple molecular mechanisms, including compromised ability of cochlin to self-assemble or to form appropriate complexes with other matrix components.

ADP-ribosylation factor/COPI-dependent events at the endoplasmic reticulum-Golgi interface are regulated by the guanine nucleotide exchange factor GBF1

ADP-ribosylation factor (ARF) mediated recruitment of COPI to membranes plays a central role in transport between the endoplasmic reticulum (ER) and the Golgi. The activation of ARFs is mediated by guanine nucleotide exchange factors (GEFs). Although several ARF-GEFs have been identified, the transport steps in which they function are still poorly understood. Here we report that GBF1, a member of the Sec7-domain family of GEFs, is responsible for the regulation of COPI-mediated events at the ER-Golgi interface. We show that GBF1 is essential for the formation, differentiation, and translocation of pre-Golgi intermediates and for the maintenance of Golgi integrity. We also show that the formation of transport-competent ER-to-Golgi intermediates proceeds in two stages: first, a COPI-independent event leads to the formation of an unstable compartment, which is rapidly reabsorbed in the absence of GBF1 activity. Second, the association of GBF1 with this compartment allows COPI recruitment and leads to its maturation into transport intermediates. The recruitment of GBF1 to this compartment is specifically inhibited by brefeldin A. Our findings imply that the continuous recruitment of GBF1 to spatially differentiated membrane domains is required for sustained membrane remodeling that underlies membrane traffic and Golgi biogenesis.