Protein kinase A activity is necessary for fission and fusion of Golgi to endoplasmic reticulum retrograde tubules

Simpson J. Rab6-mediated retrograde trafficking from the Golgi: the trouble with tubules

Next year marks one-quarter of a century since the discovery of the so-called COPI-independent pathway, which operates between the Golgi apparatus and the endoplasmic reticulum (ER) in eukaryotic cells. Unlike almost all other intracellular trafficking pathways, this pathway is not regulated by the physical accumulation of multisubunit proteinaceous coat molecules, but instead by the small GTPase Rab6. What also sets it apart from other pathways is that the transport carriers themselves often take the form of tubules, rather than conventional vesicles. In this review, we assess the relevant literature that has accumulated to date, in an attempt to provide a concerted description of how this pathway is regulated. We discuss the possible cargo molecules that are carried in this pathway, and the likely mechanism of Rab6 tubule biogenesis, including how the cargo itself may play a critical role. We also provide perspective surrounding the various molecular motors of the kinesin, myosin and dynein families that have been implicated in driving Rab6-coated tubular membranes long distances through the cell prior to delivering their cargo to the ER. Finally, we also raise several important questions that require resolution, if we are to ultimately provide a comprehensive molecular description of how the COPI-independent pathway is controlled.

Branched Actin Maintains Acetylated Microtubule Network in the Early Secretory Pathway

In the early secretory pathway, the delivery of anterograde cargoes from the endoplasmic reticulum (ER) exit sites (ERES) to the Golgi apparatus is a multi-step transport process occurring via the ER-Golgi intermediate compartment (IC, also called ERGIC). While the role microtubules in ER-to-Golgi transport has been well established, how the actin cytoskeleton contributes to this process remains poorly understood. Here, we report that Arp2/3 inhibition affects the network of acetylated microtubules around the Golgi and induces the accumulation of unusually long RAB1/GM130-positive carriers around the centrosome. These long carriers are less prone to reach the Golgi apparatus, and arrival of anterograde cargoes to the Golgi is decreased upon Arp2/3 inhibition. Our data suggest that Arp2/3-dependent actin polymerization maintains a stable network of acetylated microtubules, which ensures efficient cargo trafficking at the late stage of ER to Golgi transport.

2D-GolgiTrack-a semi-automated tracking system to quantify morphological changes and dynamics of the Golgi apparatus and Golgi-derived membrane tubules

The Golgi apparatus and membrane tubules derived from this organelle play essential roles in membrane trafficking in eukaryotic cells. High-resolution live cell imaging is one highly suitable method for studying the molecular mechanisms of dynamics of organelles during membrane trafficking events. Due to the complex morphological changes and dynamic movements of the Golgi apparatus and associated membrane tubules during membrane trafficking, it is challenging to accurately quantify them. In this study, a semi-automated 2D tracking system, 2D-GolgiTrack, has been established for quantifying morphological changes and movements of Golgi elements, specifically encompassing the Golgi apparatus and its associated tubules, the fission and fusion of Golgi tubules, and the kinetics of formation of Golgi tubules and redistribution of the Golgi-associated protein Rab6A to the endoplasmic reticulum. The Golgi apparatus and associated tubules are segmented by a combination of Otsu's method and adaptive local normalization thresholding. Curvilinear skeletons and tips of skeletons of segmented tubules are used for calculating tubule length by the Geodesic method. The k-nearest neighbor is applied to search the possible candidate objects in the next frame and link the correct objects of adjacent frames by a tracking algorithm to calculate changes in morphological features of each Golgi object or tubule, e.g., number, length, shape, branch point and position, and fission or fusion events. Tracked objects are classified into morphological subtypes, and the Track-Map function of morphological evolution visualizes events of fission and fusion. Our 2D-GolgiTrack not only provides tracking results with 95% accuracy, but also maps morphological evolution for fast visual interpretation of the fission and fusion events. Our tracking system is able to characterize key morphological and dynamic features of the Golgi apparatus and associated tubules, enabling biologists to gain a greater understanding of the molecular mechanisms of membrane traffic involving this essential organelle. Graphical Abstract Overview of the semi-automated 2D tracking system. There are two main parts to the system, namely detection and tracking. The workflow process requires a raw sequence of images (a), which is filtered by the Gaussian filter method (c), and threshold intensity (b) to segment elements of Golgi cisternae (d) and tubules (e). Post-processing outputs are binary images of the cisternae area and tubule skeletons. The tubules are classified into three lengths, namely short, medium, and long tubules (f). Outputs of segmentation are calculated as morphological features (g). The tracking processing starts by loading the segmented outputs (h) and key-inputs of direction reference (i; (DR)) and interval setting of the start ((S)) and end ((E)) frame numbers (j). A tubule of interest is selected by the user (k; (GT_{interest, S}) as the tubule input ((GT_IN)) at the current frame ((i = S)). The tracking algorithm tracks and links the correct tubules at each subsequent frame ((i = i + 1)). The locations of tubule tips are determined for detecting tubule branches using the (DR) to identify the direction of tubule growth (l: (1); (GT_{tipBr, i}); Golgi cisternae: white area; Golgi tubule: white skeleton; tubule tips: green dots; branched tubules: two branches due to the (DR) of growth of the simulated tubule moving from left-to-right away from the Golgi cisternae location). According to the position of the (GT_IN), five candidates ((GT_{candidates, i})) are searched using the k-nearest neighbor method (l: (2)). Matching of tubules between the (GT_IN) and those (GT_{candidates, i}) uses the bounding box technique to check the amount of tubule-overlap based on the tracking conditions (l: (3)). If there is tubule-overlap, the system collects that tubule as the final output ((GT_{OUT, i})). By contrast, shape (see the Extent feature in Table reftab:1) and distance features are used to generate the tracked output, which has a priority of a minimum of both of these features ((Min_{DIST, EXTENT})); otherwise, it is from the minimum of the distance ((Min_DIST)). Once a loop of the interval track to the last frame is finished ((i = E + 1)), a Track-Map is generated allowing visualization of the morphological pattern of tubule formation and movement, including identification of fission and fusion events (m). Dynamic features are calculated (n). Related outputs are saved, and all features obtained from the detection and tracking processing are exported as MS Excel files (o).

GOLPH3 Regulates EGFR in T98G Glioblastoma Cells by Modulating Its Glycosylation and Ubiquitylation

Protein trafficking is altered when normal cells acquire a tumor phenotype. A key subcellular compartment in regulating protein trafficking is the Golgi apparatus, but its role in carcinogenesis is still not well defined. Golgi phosphoprotein 3 (GOLPH3), a peripheral membrane protein mostly localized at the trans-Golgi network, is overexpressed in several tumor types including glioblastoma multiforme (GBM), the most lethal primary brain tumor. Moreover, GOLPH3 is currently considered an oncoprotein, however its precise function in GBM is not fully understood. Here, we analyzed in T98G cells of GBM, which express high levels of epidermal growth factor receptor (EGFR), the effect of stable RNAi-mediated knockdown of GOLPH3. We found that silencing GOLPH3 caused a significant reduction in the proliferation of T98G cells and an unexpected increase in total EGFR levels, even at the cell surface, which was however less prone to ligand-induced autophosphorylation. Furthermore, silencing GOLPH3 decreased EGFR sialylation and fucosylation, which correlated with delayed ligand-induced EGFR downregulation and its accumulation at endo-lysosomal compartments. Finally, we found that EGF failed at promoting EGFR ubiquitylation when the levels of GOLPH3 were reduced. Altogether, our results show that GOLPH3 in T98G cells regulates the endocytic trafficking and activation of EGFR likely by affecting its extent of glycosylation and ubiquitylation.

The Proteasomal Deubiquitinating Enzyme PSMD14 Regulates Macroautophagy by Controlling Golgi-to-ER Retrograde Transport

Ubiquitination regulates several biological processes, however the role of specific members of the ubiquitinome on intracellular membrane trafficking is not yet fully understood. Here, we search for ubiquitin-related genes implicated in protein membrane trafficking performing a High-Content siRNA Screening including 1187 genes of the human "ubiquitinome" using amyloid precursor protein (APP) as a reporter. We identified the deubiquitinating enzyme PSMD14, a subunit of the 19S regulatory particle of the proteasome, specific for K63-Ub chains in cells, as a novel regulator of Golgi-to-endoplasmic reticulum (ER) retrograde transport. Silencing or pharmacological inhibition of PSMD14 with Capzimin (CZM) caused a robust increase in APP levels at the Golgi apparatus and the swelling of this organelle. We showed that this phenotype is the result of rapid inhibition of Golgi-to-ER retrograde transport, a pathway implicated in the early steps of the autophagosomal formation. Indeed, we observed that inhibition of PSMD14 with CZM acts as a potent blocker of macroautophagy by a mechanism related to the retention of Atg9A and Rab1A at the Golgi apparatus. As pharmacological inhibition of the proteolytic core of the 20S proteasome did not recapitulate these effects, we concluded that PSMD14, and the K63-Ub chains, act as a crucial regulatory factor for macroautophagy by controlling Golgi-to-ER retrograde transport.

Unlocking Golgi: Why Does Morphology Matter?

The mammalian Golgi apparatus is a highly dynamic organelle, which is normally localized in the juxtanuclear space and plays an essential role in the regulation of cellular homeostasis. While posttranslational modification of cargo is mediated by the resident enzymes (glycosyltransferases, glycosidases, and kinases), the ribbon structure of Golgi and its cisternal stacking mostly rely on the cooperation of coiled-coil matrix golgins. Among them, giantin, GM130, and GRASPs are unique, because they form a tripartite complex and serve as Golgi docking sites for cargo delivered from the endoplasmic reticulum (ER). Golgi undergoes significant disorganization in many pathologies associated with a block of the ER-to-Golgi or intra-Golgi transport, including cancer, different neurological diseases, alcoholic liver damage, ischemic stress, viral infections, etc. In addition, Golgi fragments during apoptosis and mitosis. Here, we summarize and analyze clinically relevant observations indicating that Golgi fragmentation is associated with the selective loss of Golgi residency for some enzymes and, conversely, with the relocation of some cytoplasmic proteins to the Golgi. The central concept is that ER and Golgi stresses impair giantin docking site but have no impact on the GM130-GRASP65 complex, thus inducing mislocalization of giantin-sensitive enzymes only. This cardinally changes the processing of proteins by eliminating the pathways controlled by the missing enzymes and by activating the processes now driven by the GM130-GRASP65-dependent proteins. This type of Golgi disorganization is different from the one induced by the cytoskeleton alteration, which despite Golgi de-centralization, neither impairs function of golgins nor alters trafficking.

Caveolin-1 impairs PKA-DRP1-mediated remodelling of ER-mitochondria communication during the early phase of ER stress

Close contacts between endoplasmic reticulum and mitochondria enable reciprocal Ca²⁺ exchange, a key mechanism in the regulation of mitochondrial bioenergetics. During the early phase of endoplasmic reticulum stress, this inter-organellar communication increases as an adaptive mechanism to ensure cell survival. The signalling pathways governing this response, however, have not been characterized. Here we show that caveolin-1 localizes to the endoplasmic reticulum–mitochondria interface, where it impairs the remodelling of endoplasmic reticulum–mitochondria contacts, quenching Ca²⁺ transfer and rendering mitochondrial bioenergetics unresponsive to endoplasmic reticulum stress. Protein kinase A, in contrast, promotes endoplasmic reticulum and mitochondria remodelling and communication during endoplasmic reticulum stress to promote organelle dynamics and Ca²⁺ transfer as well as enhance mitochondrial bioenergetics during the adaptive response. Importantly, caveolin-1 expression reduces protein kinase A signalling, as evidenced by impaired phosphorylation and alterations in organelle distribution of the GTPase dynamin-related protein 1, thereby enhancing cell death in response to endoplasmic reticulum stress. In conclusion, caveolin-1 precludes stress-induced protein kinase A-dependent remodelling of endoplasmic reticulum–mitochondria communication.

Functional disruption of the Golgi apparatus protein ARF1 sensitizes MDA-MB-231 breast cancer cells to the antitumor drugs Actinomycin D and Vinblastine through ERK and AKT signaling

Increasing evidence indicates that the Golgi apparatus plays active roles in cancer, but a comprehensive understanding of its functions in the oncogenic transformation has not yet emerged. At the same time, the Golgi is becoming well recognized as a hub that integrates its functions of protein and lipid biosynthesis to signal transduction for cell proliferation and migration in cancer cells. Nevertheless, the active function of the Golgi apparatus in cancer cells has not been fully evaluated as a target for combined treatment. Here, we analyzed the effect of perturbing the Golgi apparatus on the sensitivity of the MDA-MB-231 breast cancer cell line to the drugs Actinomycin D and Vinblastine. We disrupted the function of ARF1, a protein necessary for the homeostasis of the Golgi apparatus. We found that the expression of the ARF1-Q71L mutant increased the sensitivity of MDA-MB-231 cells to both Actinomycin D and Vinblastine, resulting in decreased cell proliferation and cell migration, as well as in increased apoptosis. Likewise, the combined treatment of cells with Actinomycin D or Vinblastine and Brefeldin A or Golgicide A, two disrupting agents of the ARF1 function, resulted in similar effects on cell proliferation, cell migration and apoptosis. Interestingly, each combined treatment had distinct effects on ERK1/2 and AKT signaling, as indicated by the decreased levels of either phospho-ERK1/2 or phospho-AKT. Our results suggest that disruption of Golgi function could be used as a strategy for the sensitization of cancer cells to chemotherapy.

Spatial-Temporal Study of Rab1b Dynamics and Function at the ER-Golgi Interface

The GTPase Rab1b is involved in ER to Golgi transport, with multiple Rab1b effectors (located at ERES, VTCs and the Golgi complex) being required for its function. In this study, we performed live-cell dual-expression studies to analyze the dynamics of Rab1b and some effectors located at the ERES-Golgi interface. Rab1b occupied widely distributed mobile punctate and tubular structures, displaying a transient overlaps with its effectors and showing that these overlaps occurred at the same time in spatially distinct steps of ER to Golgi transport. In addition, we assessed Rab1b dynamics during cargo sorting by analyzing the concentration at ERES of a Golgi protein (SialT2-CFP) during Brefeldin A washout (BFA WO). Rab1b was associated to most of the ERES structures, but at different times during BFA WO, and recurrently SialT2-CFP was sorted in the ERES-Rab1b positive structures. Furthermore, we reveal for first time that Rab1b localization time at ERES depended on GBF1, a Rab1b effector that acts as the guanine nucleotide exchange factor of Arf1, and that Rab1b membrane association/dissociation dynamics at ERES was dependent on the GBF1 membrane association and activity, which strongly suggests that GBF1 activity modulates Rab1b membrane cycling dynamic.

Distinct Biochemical Pools of Golgi Phosphoprotein 3 in the Human Breast Cancer Cell Lines MCF7 and MDA-MB-231

Golgi phosphoprotein 3 (GOLPH3) has been implicated in the development of carcinomas in many human tissues, and is currently considered a bona fide oncoprotein. Importantly, several tumor types show overexpression of GOLPH3, which is associated with tumor progress and poor prognosis. However, the underlying molecular mechanisms that connect GOLPH3 function with tumorigenicity are poorly understood. Experimental evidence shows that depletion of GOLPH3 abolishes transformation and proliferation of tumor cells in GOLPH3-overexpressing cell lines. Conversely, GOLPH3 overexpression drives transformation of primary cell lines and enhances mouse xenograft tumor growth in vivo. This evidence suggests that overexpression of GOLPH3 could result in distinct features of GOLPH3 in tumor cells compared to that of non-tumorigenic cells. GOLPH3 is a peripheral membrane protein mostly localized at the trans-Golgi network, and its association with Golgi membranes depends on binding to phosphatidylinositol-4-phosphate. GOLPH3 is also contained in a large cytosolic pool that rapidly exchanges with Golgi-associated pools. GOLPH3 has also been observed associated with vesicles and tubules arising from the Golgi, as well as other cellular compartments, and hence it has been implicated in several membrane trafficking events. Whether these and other features are typical to all different types of cells is unknown. Moreover, it remains undetermined how GOLPH3 acts as an oncoprotein at the Golgi. Therefore, to better understand the roles of GOLPH3 in cancer cells, we sought to compare some of its biochemical and cellular properties in the human breast cancer cell lines MCF7 and MDA-MB-231 with that of the non-tumorigenic breast human cell line MCF 10A. We found unexpected differences that support the notion that in different cancer cells, overexpression of GOLPH3 functions in diverse fashions, which may influence specific tumorigenic phenotypes.