Cell-cycle-specific Golgi fragmentation: how and why?

Mutant p53-Mediated Tumor Secretome: Bridging Tumor Cells and Stromal Cells

The tumor secretome comprises the totality of protein factors secreted by various cell components within the tumor microenvironment, serving as the primary medium for signal transduction between tumor cells and between tumor cells and stromal cells. The deletion or mutation of the p53 gene leads to alterations in cellular secretion characteristics, contributing to the construction of the tumor microenvironment in a cell non-autonomous manner. This review discusses the critical roles of mutant p53 in regulating the tumor secretome to remodel the tumor microenvironment, drive tumor progression, and influence the plasticity of cancer-associated fibroblasts (CAFs) as well as the dynamics of tumor immunity by focusing on both secreted protein expression and secretion pathways. The aim is to provide new insights for targeted cancer therapies.

Diclofenac: A Nonsteroidal Anti-Inflammatory Drug Inducing Cancer Cell Death by Inhibiting Microtubule Polymerization and Autophagy Flux

Diclofenac, a nonsteroidal anti-inflammatory drug (NSAID) used to treat inflammatory diseases induces cellular toxicity by increasing the production of reactive oxygen species (ROS) and impairing autophagic flux. In this study, we investigated whether diclofenac induces cancer cell death and the mechanism by which diclofenac causes cell death. We observed that diclofenac induces mitotic arrest with a half-maximal effective concentration of 170 μM and cell death with a half-maximal lethal dose of 200 µM during 18-h incubation in HeLa cells. Cellular microtubule imaging and in vitro tubulin polymerization assays demonstrated that treatment with diclofenac elicits microtubule destabilization. Autophagy relies on microtubule-mediated transport and the fusion of autophagic vesicles. We observed that diclofenac inhibits both phagophore movement, an early step of autophagy, and the fusion of autophagosomes and lysosomes, a late step of autophagy. Diclofenac also induces the fragmentation of mitochondria and the Golgi during cell death. We found that diclofenac induces cell death further in combination with 5-fuorouracil, a DNA replication inhibitor than in single treatment in cancer cells. Pancreatic cancer cells, which have high basal autophagy, are particularly sensitive to cell death by diclofenac. Our study suggests that microtubule destabilization by diclofenac induces cancer cell death via compromised spindle assembly checkpoints and increased ROS through impaired autophagy flux. Diclofenac may be a candidate therapeutic drug in certain type of cancers by inhibiting microtubule-mediated cellular events in combination with clinically utilized nucleoside metabolic inhibitors, including 5-fluorouracil, to block cancer cell proliferation.

Mitotic HOOK3 phosphorylation by ERK1c drives microtubule-dependent Golgi destabilization and fragmentation

ERK1c is an alternatively spliced isoform of ERK1 that specifically regulates mitotic Golgi fragmentation, which allows division of the Golgi during mitosis. We have previously shown that ERK1c translocates to the Golgi during mitosis where it is activated by a resident MEK1b to induce Golgi fragmentation. However, the mechanism of ERK1c functions in the Golgi remained obscure. Here, we searched for ERK1c substrates and identified HOOK3 as a mediator of ERK1c-induced mitotic Golgi fragmentation, which requires a second phosphorylation by AuroraA for its function. In cycling cells, HOOK3 interacts with microtubules (MTs) and links them to the Golgi. Early in mitosis, HOOK3 is phosphorylated by ERK1c and later by AuroraA, resulting in HOOK3 detachment from the MTs, and elevated interaction with GM130. This detachment modulates Golgi stability and allows fragmentation of the Golgi. This study demonstrates a novel mechanism of Golgi apparatus destabilization early in mitosis to allow mitotic progression.

Cell Cycle-Dependent Dynamics of the Golgi-Centrosome Association in Motile Cells

Here, we characterize spatial distribution of the Golgi complex in human cells. In contrast to the prevailing view that the Golgi compactly surrounds the centrosome throughout interphase, we observe characteristic differences in the morphology of Golgi ribbons and their association with the centrosome during various periods of the cell cycle. The compact Golgi complex is typical in G1; during S-phase, Golgi ribbons lose their association with the centrosome and extend along the nuclear envelope to largely encircle the nucleus in G2. Interestingly, pre-mitotic separation of duplicated centrosomes always occurs after dissociation from the Golgi. Shortly before the nuclear envelope breakdown, scattered Golgi ribbons reassociate with the separated centrosomes restoring two compact Golgi complexes. Transitions between the compact and distributed Golgi morphologies are microtubule-dependent. However, they occur even in the absence of centrosomes, which implies that Golgi reorganization is not driven by the centrosomal microtubule asters. Cells with different Golgi morphology exhibit distinct differences in the directional persistence and velocity of migration. These data suggest that changes in the radial distribution of the Golgi around the nucleus define the extent of cell polarization and regulate cell motility in a cell cycle-dependent manner.

Golgi organization is regulated by proteasomal degradation

The Golgi is a dynamic organelle whose correct assembly is crucial for cellular homeostasis. Perturbations in Golgi structure are associated with numerous disorders from neurodegeneration to cancer. However, whether and how dispersal of the Golgi apparatus is actively regulated under stress, and the consequences of Golgi dispersal, remain unknown. Here we demonstrate that 26S proteasomes are associated with the cytosolic surface of Golgi membranes to facilitate Golgi Apparatus-Related Degradation (GARD) and degradation of GM130 in response to Golgi stress. The degradation of GM130 is dependent on p97/VCP and 26S proteasomes, and required for Golgi dispersal. Finally, we show that perturbation of Golgi homeostasis induces cell death of multiple myeloma in vitro and in vivo, offering a therapeutic strategy for this malignancy. Taken together, this work reveals a mechanism of Golgi-localized proteasomal degradation, providing a functional link between proteostasis control and Golgi architecture, which may be critical in various secretion-related pathologies.

Correct Golgi assembly is important to cellular homeostasis but regulation of its structure under stress remains unclear. Here, the authors identify stress-induced degradation of GM130 by Golgi-localized 26S proteasomes, leading to Golgi dispersal.

Semi-Intact Cell System for Reconstituting and Analyzing Cellular Golgi Dynamics

Morphology of Golgi apparatus changes frequently and diversely depending on various cellular conditions and these changes correlate with the balance between membrane inflow and outflow at the Golgi via vesicular transports. In a previous study, we introduced a semi-intact cell system suitable for the reconstitution of morphological changes that organelles undergo as well as the vesicular transport between them. Semi-intact cells are cells that have undergone plasma membrane permeabilization by the cholesterol-dependent pore-forming cytolysin, streptolysin O (SLO). Permeabilization enables the introduction of various molecules into the cells, as well as the substitution of the original cytosol with an exogenously made cytosol prepared from cells in various stages of cell cycle, differentiation, and disease progression. Coupled with a green fluorescent protein(GFP)-visualization technique, this cell-based system enables the analysis of the molecular mechanisms underlying biological processes that are highly dependent on the integrity of the intracellular architecture. In this chapter, we present a variety of reconstitution assays concerning biological reactions pertaining to the Golgi apparatus.

Architecture and Dynamics of the Neuronal Secretory Network

Regulated synthesis and movement of proteins between cellular organelles are central to diverse forms of biological adaptation and plasticity. In neurons, the repertoire of channel, receptor, and adhesion proteins displayed on the cell surface directly impacts cellular development, morphology, excitability, and synapse function. The immensity of the neuronal surface membrane and its division into distinct functional domains present a challenging landscape over which proteins must navigate to reach their appropriate functional domains. This problem becomes more complex considering that neuronal protein synthesis is continuously refined in space and time by neural activity. Here we review our current understanding of how integral membrane and secreted proteins important for neuronal function travel from their sites of synthesis to their functional destinations. We discuss how unique adaptations to the function and distribution of neuronal secretory organelles may facilitate local protein trafficking at remote sites in neuronal dendrites to support diverse forms of synaptic plasticity.

Golgi Oncoprotein GOLPH3 Gene Expression Is Regulated by Functional E2F and CREB/ATF Promoter Elements

The Golgi organelle duplicates its protein and lipid content to segregate evenly between two daughter cells after mitosis. However, how Golgi biogenesis is regulated during interphase remains largely unknown. Here we show that messenger RNA (mRNA) expression of GOLPH3 and GOLGA2, two genes encoding Golgi proteins, is induced specifically in G1 phase, suggesting a link between cell cycle regulation and Golgi growth. We have examined the role of E2F transcription factors, critical regulators of G1 to S progression of the cell cycle, in the expression of Golgi proteins during interphase. We show that promoter activity for GOLPH3, a Golgi protein that is also oncogenic, is induced by E2F1-3 and repressed by E2F7. Mutation of the E2F motifs present in the GOLPH3 promoter region abrogates E2F1-mediated induction of a GOLPH3 luciferase reporter construct. Furthermore, we identify a critical CREB/ATF element in the GOLPH3 promoter that is required for its steady state and ATF2-induced expression. Interestingly, depletion of GOLPH3 with small interfering RNA (siRNA) delays the G1 to S transition in synchronized U2OS cells. Taken together, our results reveal a link between cell cycle regulation and Golgi function, and suggest that E2F-mediated regulation of Golgi genes is required for the timely progression of the cell cycle.

Intersectin-1 interacts with the golgin GCC88 to couple the actin network and Golgi architecture

The maintenance of the Golgi ribbon relies on a dynamic balance between the actin and microtubule networks; however, the pathways controlling actin networks remain poorly defined. Previously, we showed that the trans-Golgi network (TGN) membrane tether/golgin, GCC88, modulates the Golgi ribbon architecture. Here, we show that dispersal of the Golgi ribbon by GCC88 is dependent on actin and the involvement of nonmuscle myosin IIA. We have identified the long isoform of intersectin-1 (ITSN-1), a guanine nucleotide exchange factor for Cdc42, as a novel Golgi component and an interaction partner of GCC88 responsible for mediating the actin-dependent dispersal of the Golgi ribbon. We show that perturbation of Golgi morphology by changes in membrane flux, mediated by silencing the retromer subunit Vps26, or in a model of neurodegeneration, induced by Tau overexpression, are also dependent on the ITSN-1-GCC88 interaction. Overall, our study reveals a role for a TGN golgin and ITSN-1 in linking to the actin cytoskeleton and regulating the balance between a compact Golgi ribbon and a dispersed Golgi, a pathway with relevance to pathophysiological conditions.

RhoA Pathway and Actin Regulation of the Golgi/Centriole Complex

In vertebrate cells, the Golgi apparatus is located in close proximity to the centriole. The architecture of the Golgi/centriole complex depends on a multitude of factors, including the actin filament cytoskeleton. In turn, both the Golgi and centriole act as the actin nucleation centers. Actin organization and polymerization also depend on the small GTPase RhoA pathway. In this chapter, we summarize the most current knowledge on how the genetic, magnetic, or pharmacologic interference with RhoA pathway and actin cytoskeleton directly or indirectly affects architecture, structure, and function of the Golgi/centriole complex.

An integrated approach to infer cross-talks between intracellular protein transport and signaling pathways

Background

The endomembrane system, known as secretory pathway, is responsible for the synthesis and transport of protein molecules in cells. Therefore, genes involved in the secretory pathway are essential for the cellular development and function. Recent scientific investigations show that ER and Golgi apparatus may provide a convenient drug target for cancer therapy. On the other hand, it is known that abundantly expressed genes in different cellular organelles share interconnected pathways and co-regulate each other activities. The cross-talks among these genes play an important role in signaling pathways, associated to the regulation of intracellular protein transport.

Results

In the present study, we device an integrated approach to understand these complex interactions. We analyze gene perturbation expression profiles, reconstruct a directed gene interaction network and decipher the regulatory interactions among genes involved in protein transport signaling. In particular, we focus on expression signatures of genes involved in the secretory pathway of MCF7 breast cancer cell line. Furthermore, network biology analysis delineates these gene-centric cross-talks at the level of specific modules/sub-networks, corresponding to different signaling pathways.

Conclusions

We elucidate the regulatory connections between genes constituting signaling pathways such as PI3K-Akt, Ras, Rap1, calcium, JAK-STAT, EFGR and FGFR signaling. Interestingly, we determine some key regulatory cross-talks between signaling pathways (PI3K-Akt signaling and Ras signaling pathway) and intracellular protein transport.

Electronic supplementary material

The online version of this article (10.1186/s12859-018-2036-2) contains supplementary material, which is available to authorized users.

The Function of the Golgi Ribbon Structure - An Enduring Mystery Unfolds!

The Golgi apparatus in vertebrate cells consists of individual Golgi stacks fused together in a continuous ribbon structure. The ribbon structure per se is not required to mediate the classical functions of this organelle and the relevance of the "ribbon" structure has been a mystery since first identified ultrastructurally in the 1950s. Recent advances recognize a role for the Golgi apparatus in a range of cellular processes, some mediated by signaling networks which are regulated at the Golgi. Here we review the cellular processes and signaling events regulated by the Golgi apparatus and, in particular, explore an emerging theme that the ribbon structure of the Golgi contributes directly to the regulation of these higher order functions.

Depletion of the human N-terminal acetyltransferase hNaa30 disrupts Golgi integrity and ARFRP1 localization

The organization of the Golgi apparatus (GA) is tightly regulated. Golgi stack scattering is observed in cellular processes such as apoptosis and mitosis, and has also been associated with disruption of cellular lipid metabolism and neurodegenerative diseases. Our studies show that depletion of the human N-α-acetyltransferase 30 (hNaa30) induces fragmentation of the Golgi stack in HeLa and CAL-62 cell lines. The GA associated GTPase ADP ribosylation factor related protein 1 (ARFRP1) was previously shown to require N-terminal acetylation for membrane association and based on its N-terminal sequence, it is likely to be a substrate of hNaa30. ARFRP1 is involved in endosome-to-trans-Golgi network (TGN) traffic. We observed that ARFRP1 shifted from a predominantly cis-Golgi and TGN localization to localizing both Golgi and non-Golgi vesicular structures in hNaa30-depleted cells. However, we did not observe loss of membrane association of ARFRP1. We conclude that hNaa30 depletion induces Golgi scattering and induces aberrant ARFRP1 Golgi localization.

High Throughput Analysis of Golgi Structure by Imaging Flow Cytometry

The Golgi apparatus is a dynamic organelle, which regulates the vesicular trafficking. While cellular trafficking requires active changes of the Golgi membranes, these are not accompanied by changes in the general Golgi’s structure. However, cellular processes such as mitosis, apoptosis and migration require fragmentation of the Golgi complex. Currently, these changes are most commonly studied by basic immunofluorescence and quantified by manual and subjective classification of the Golgi structure in 100–500 stained cells. Several other high-throughput methods exist as well, but those are either complicated or do not provide enough morphological information. Therefore, a simple and informative high content methodology should be beneficial for the study of Golgi architecture. Here we describe the use of high-throughput imaging flow cytometry for quantification of Golgi fragmentation, which provides a simple way to analyze the changes in an automated, quantitative and non-biased manner. Furthermore, it provides a rapid and accurate way to analyze more than 50,000 cells per sample. Our results demonstrate that this method is robust and statistically powerful, thus, providing a much-needed analytical tool for future studies on Golgi dynamics, and can be adapted to other experimental systems.

Growth of the Mammalian Golgi Apparatus during Interphase

During the cell cycle, genetic materials and organelles are duplicated to ensure that there is sufficient cellular content for daughter cells. While Golgi growth in interphase has been observed in lower eukaryotes, the elaborate ribbon structure of the mammalian Golgi apparatus has made it challenging to monitor. Here we demonstrate the growth of the mammalian Golgi apparatus in its protein content and volume during interphase. Through ultrastructural analyses, physical growth of the Golgi apparatus was revealed to occur by cisternal elongation of the individual Golgi stacks. By examining the timing and regulation of Golgi growth, we established that Golgi growth starts after passage through the cell growth checkpoint at late G1 phase and continues in a manner highly correlated with cell size growth. Finally, by identifying S6 kinase 1 as a major player in Golgi growth, we revealed the coordination between cell size and Golgi growth via activation of the protein synthesis machinery in early interphase.

Specialization of biosynthetic membrane trafficking for neuronal form and function

Neuronal growth and synaptic transmission require the continuous production of adhesion molecules, neurotransmitter receptors, ion-channels, and secreted trophic factors, and thus critically relies on the secretory pathway-the series of intracellular organelles including the endoplasmic reticulum (ER) and the Golgi apparatus (GA), where membrane lipids and proteins are synthesized. Commensurate with the gigantic size of the neuronal membrane and its compartmentalization by thousands of synapses with distinct compositions and activities, the neuronal secretory pathway has evolved to both traffic synaptic components over very long distances, and locally control the composition of specified segments of dendrites. Here we review new insights into the distribution and dynamics of dendritic secretory organelles and their impact on postsynaptic compartments.

Non-canonical features of the Golgi apparatus in bipolar epithelial neural stem cells

Apical radial glia (aRG), the stem cells in developing neocortex, are unique bipolar epithelial cells, extending an apical process to the ventricle and a basal process to the basal lamina. Here, we report novel features of the Golgi apparatus, a central organelle for cell polarity, in mouse aRGs. The Golgi was confined to the apical process but not associated with apical centrosome(s). In contrast, in aRG-derived, delaminating basal progenitors that lose apical polarity, the Golgi became pericentrosomal. The aRG Golgi underwent evolutionarily conserved, accordion-like compression and extension concomitant with cell cycle-dependent nuclear migration. Importantly, in line with endoplasmic reticulum but not Golgi being present in the aRG basal process, its plasma membrane contained glycans lacking Golgi processing, consistent with direct ER-to-cell surface membrane traffic. Our study reveals hitherto unknown complexity of neural stem cell polarity, differential Golgi contribution to their specific architecture, and fundamental Golgi re-organization upon cell fate change.

The dynamic subcellular localization of ERK: mechanisms of translocation and role in various organelles

The dynamic subcellular localization of ERK in resting and stimulated cells plays an important role in its regulation. In resting cells, ERK localizes in the cytoplasm, and upon stimulation, it translocates to its target substrates and organelles. ERK signaling initiated from different places in resting cells has distinct outcomes. In this review, we summarize the mechanisms of ERK1/2 translocation to the nucleus and mitochondria, and of ERK1c to the Golgi. We also show that ERK1/2 translocation to the nucleus is a useful anti cancer target. Unraveling the complex subcellular localization of ERK and its dynamic changes upon stimulation provides a better understanding of the regulation of ERK signaling and may result in the development of new strategies to combat ERK-related diseases.

Assays to Study the Fragmentation of the Golgi Complex During the G2-M Transition of the Cell Cycle

The Golgi complex of mammalian cells is composed of stacks of flattened cisternae that are connected by tubules to form a continuous membrane system, also known as the Golgi ribbon. At the onset of mitosis, the Golgi ribbon is progressively fragmented into small tubular-vesicular clusters and it is reconstituted before completion of cytokinesis. The investigation of the mechanisms behind this reversible cycle of disassembly and reassembly has led to the identification of structural Golgi proteins and regulators. Moreover, these studies allowed to discover that disassembly of the ribbon is necessary for cell entry into mitosis. Here, we describe an in vitro assay that reproduces the mitotic Golgi fragmentation and that has been successfully employed to identify many important mechanisms and proteins involved in the mitotic Golgi reorganization.

Mitotic Golgi translocation of ERK1c is mediated by a PI4KIIIβ-14-3-3γ shuttling complex

Golgi fragmentation is a highly regulated process that allows division of the Golgi complex between the two daughter cells. The mitotic reorganization of the Golgi is accompanied by a temporary block in Golgi functioning, as protein transport in and out of the Golgi stops. Our group has previously demonstrated the involvement of the alternatively spliced variants ERK1c and MEK1b (ERK1 is also known as MAPK3, and MEK1 as MAP2K1) in mitotic Golgi fragmentation. We had also found that ERK1c translocates to the Golgi at the G2 to M phase transition, but the molecular mechanism underlying this recruitment remains unknown. In this study, we narrowed the translocation timing to prophase and prometaphase, and elucidated its molecular mechanism. We found that CDK1 phosphorylates Ser343 of ERK1c, thereby allowing the binding of phosphorylated ERK1c to a complex that consists of PI4KIIIβ (also known as PI4KB) and the 14-3-3γ dimer (encoded by YWHAB). The stability of the complex is regulated by protein kinase D (PKD)-mediated phosphorylation of PI4KIIIβ. The complex assembly induces the Golgi shuttling of ERK1c, where it is activated by MEK1b, and induces Golgi fragmentation. Our work shows that protein shuttling to the Golgi is not completely abolished at the G2 to M phase transition, thus integrating several independent Golgi-regulating processes into one coherent pathway.

Oncolytic reovirus induces intracellular redistribution of Ras to promote apoptosis and progeny virus release

Reovirus is a naturally oncolytic virus that preferentially replicates in Ras-transformed cells and is currently undergoing clinical trials as a cancer therapeutic. Ras transformation promotes reovirus oncolysis by enhancing virion disassembly during entry, viral progeny production, and virus release through apoptosis; however, the mechanism behind the latter is not well understood. Here, we show that reovirus alters the intracellular location of oncogenic Ras to induce apoptosis of H-RasV12-transformed fibroblasts. Reovirus infection decreases Ras palmitoylation levels and causes accumulation of Ras in the Golgi through Golgi fragmentation. With the Golgi being the site of Ras palmitoylation, treatment of target cells with the palmitoylation inhibitor, 2-bromopalmitate (2BP), prompts a greater accumulation of H-RasV12 in the Golgi, and a dose-dependent increase in progeny virus release and subsequent spread. Conversely, tethering H-RasV12 to the plasma membrane (thereby preventing its movement to the Golgi) allows for efficient virus production, but results in basal levels of reovirus-induced cell death. Analysis of Ras downstream signaling reveals that cells expressing cycling H-RasV12 have elevated levels of phosphorylated JNK (c-Jun N-terminal kinase), and that Ras retained at the Golgi body by 2BP increases activation of the MEKK1/MKK4/JNK signaling pathway to promote cell death. Collectively, our data suggest that reovirus induces Golgi fragmentation of target cells, and the subsequent accumulation of oncogenic Ras in the Golgi body initiates apoptotic signaling events required for virus release and spread.

Interactome analysis reveals that FAM161A, deficient in recessive retinitis pigmentosa, is a component of the Golgi-centrosomal network

Defects in FAM161A, a protein of unknown function localized at the cilium of retinal photoreceptor cells, cause retinitis pigmentosa, a form of hereditary blindness. By using different fragments of this protein as baits to screen cDNA libraries of human and bovine retinas, we defined a yeast two-hybrid-based FAM161A interactome, identifying 53 bona fide partners. In addition to statistically significant enrichment in ciliary proteins, as expected, this interactome revealed a substantial bias towards proteins from the Golgi apparatus, the centrosome and the microtubule network. Validation of interaction with key partners by co-immunoprecipitation and proximity ligation assay confirmed that FAM161A is a member of the recently recognized Golgi-centrosomal interactome, a network of proteins interconnecting Golgi maintenance, intracellular transport and centrosome organization. Notable FAM161A interactors included AKAP9, FIP3, GOLGA3, KIFC3, KLC2, PDE4DIP, NIN and TRIP11. Furthermore, analysis of FAM161A localization during the cell cycle revealed that this protein followed the centrosome during all stages of mitosis, likely reflecting a specific compartmentalization related to its role at the ciliary basal body during the G0 phase. Altogether, these findings suggest that FAM161A's activities are probably not limited to ciliary tasks but also extend to more general cellular functions, highlighting possible novel mechanisms for the molecular pathology of retinal disease.

Dynamin-mediated lipid acquisition is essential for Chlamydia trachomatis development

Chlamydia trachomatis is an obligate intracellular pathogen responsible for a high burden of human disease. Here, a loss-of-function screen using a set of lentivirally transduced shRNAs identified 14 human host cell factors that modulate C. trachomatis infectivity. Notably, knockdown of dynamin, a host GTPase, decreased C. trachomatis infectivity. Dynamin functions in multiple cytoplasmic locations, including vesicle formation at the plasma membrane and the trans-Golgi network. However, its role in C. trachomatis infection remains unclear. Here we report that dynamin is essential for homotypic fusion of C. trachomatis inclusions but not for C. trachomatis internalization into the host cell. Further, dynamin activity is necessary for lipid transport into C. trachomatis inclusions and for normal re-differentiation from reticulate to elementary bodies. Fragmentation of the Golgi apparatus is proposed to be an important strategy used by C. trachomatis for efficient lipid acquisition and replication within the host. Here we show that a subset of C. trachomatis-infected cells displayed Golgi fragmentation, which was concurrent with increased mitotic accumulation. Golgi fragmentation was dispensable for dynamin-mediated lipid acquisition into C. trachomatis inclusions, irrespective of the cell cycle phase. Thus, our study reveals a critical role of dynamin in host-derived lipid acquisition for C. trachomatis development.

Golgi-associated RhoBTB3 targets cyclin E for ubiquitylation and promotes cell cycle progression

The Golgi protein RhoBTB3 in complex with CUL3 and RBX1 promotes Cyclin E ubiquitylation to allow its turnover during S phase and progression through the cell cycle.

Cyclin E regulates the cell cycle transition from G1 to S phase and is degraded before entry into G2 phase. Here we show that RhoBTB3, a Golgi-associated, Rho-related ATPase, regulates the S/G2 transition of the cell cycle by targeting Cyclin E for ubiquitylation. Depletion of RhoBTB3 arrested cells in S phase, triggered Golgi fragmentation, and elevated Cyclin E levels. On the Golgi, RhoBTB3 bound Cyclin E as part of a Cullin3 (CUL3)-dependent RING–E3 ubiquitin ligase complex comprised of RhoBTB3, CUL3, and RBX1. Golgi association of this complex was required for its ability to catalyze Cyclin E ubiquitylation and allow normal cell cycle progression. These experiments reveal a novel role for a Ras superfamily member in catalyzing Cyclin E turnover during S phase, as well as an unexpected, essential role for the Golgi as a ubiquitylation platform for cell cycle control.

Cargo trafficking between endosomes and the trans-Golgi network

The retrograde membrane transport pathways from endosomes to the trans-Golgi network (TGN) are now recognized as critical intracellular pathways to recycle and shuttle a selective subgroup of membrane proteins, including sorting receptors, membrane-bound enzymes, transporters, as well as providing an avenue for the intracellular transport of various bacterial toxins. Multiple pathways from endosomes to the TGN have now been defined which differ between the cargo transported and the machinery used. Here, we review advances in these pathways and the requirement for TGN organization, and also discuss the development of unbiased analytical approaches to quantitatively track cargo that use these endosome-to-TGN pathways.

The UBXN-2/p37/p47 adaptors of CDC-48/p97 regulate mitosis by limiting the centrosomal recruitment of Aurora A

UBXN-2, a substrate adaptor of the AAA ATPase CDC-48/p97, is required to coordinate centrosome maturation timing with mitosis.

Coordination of cell cycle events in space and time is crucial to achieve a successful cell division. Here, we demonstrate that UBXN-2, a substrate adaptor of the AAA ATPase Cdc48/p97, is required to coordinate centrosome maturation timing with mitosis. In UBXN-2–depleted Caenorhabditis elegans embryos, centrosomes recruited more AIR-1 (Aurora A), matured precociously, and alignment of the mitotic spindle with the axis of polarity was impaired. UBXN-2 and CDC-48 coimmunoprecipitated with AIR-1 and the spindle alignment defect was partially rescued by co-depleting AIR-1, indicating that UBXN-2 controls these processes via AIR-1. Similarly, depletion in human cells of the UBXN-2 orthologues p37/p47 resulted in an accumulation of Aurora A at centrosomes and a delay in centrosome separation. The latter defect was also rescued by inhibiting Aurora A. We therefore postulate that the role of this adaptor in cell cycle regulation is conserved.

Identification of genes affecting vacuole membrane fragmentation in Saccharomyces cerevisiae

The equilibrium of membrane fusion and fission influences the volume and copy number of organelles. Fusion of yeast vacuoles has been well characterized but their fission and the mechanisms determining vacuole size and abundance remain poorly understood. We therefore attempted to systematically characterize factors necessary for vacuole fission. Here, we present results of an in vivo screening for deficiencies in vacuolar fragmentation activity of an ordered collection deletion mutants, representing 4881 non-essential genes of the yeast Saccharomyces cerevisiae. The screen identified 133 mutants with strong defects in vacuole fragmentation. These comprise numerous known fragmentation factors, such as the Fab1p complex, Tor1p, Sit4p and the V-ATPase, thus validating the approach. The screen identified many novel factors promoting vacuole fragmentation. Among those are 22 open reading frames of unknown function and three conspicuous clusters of proteins with known function. The clusters concern the ESCRT machinery, adaptins, and lipases, which influence the production of diacylglycerol and phosphatidic acid. A common feature of these factors of known function is their capacity to change membrane curvature, suggesting that they might promote vacuole fragmentation via this property.

Mitosis-coupled, microtubule-dependent clustering of endosomal vesicles around centrosomes

Upon cell division, not only cells themselves but also their organelles undergo drastic shape changes, although the behaviors of organelles other than the Golgi apparatus remain poorly understood. We followed the spatiotemporal changes in the localization of transferrin receptor (TfnR) and other proteins. In early mitotic phases, a population of proteins cycling through the endocytic recycling compartment (ERC) exhibits a distinct spatiotemporal change from that of Golgi proteins. In prophase/prometaphase, when the cell surface-to-volume ratio is reaching its minimum, the ERC proteins are transiently assembled around the centrated centrosome in a microtubule- and dynein-dependent manner, and soon separated polewards into two clusters concomitant with separation of duplicated centrosomes. Electron microscopic analysis revealed that endosomal vesicles containing endocytosed transferrin cluster tightly around centrosomes without fusing with one another. As cytokinesis proceeds, the clusters gradually collapse, and the ERC proteins reassemble around the furrowing equatorial region. FRAP (fluorescence recovery after photobleaching) analyses of EGFP-TfnR-expressing cells revealed minimal membrane exchange between the endosomal clusters and other cellular compartments until anaphase/telophase, when membrane traffic resumes. Our observations indicate that ERC clustering around centrosomes plays a fundamental role in restricting membrane delivery to the plasma membrane during early mitotic phases, when the cell surface-to-volume ratio reaches its minimum.

Signaling at the Golgi during mitosis

The Golgi complex of mammalian cells is composed of interconnected stacks of flattened cisternae that form a continuous membrane system in the pericentriolar region of the cell. At the onset of mitosis, this so-called Golgi ribbon is converted into small tubular-vesicular clusters in a tightly regulated fragmentation process, which leads to a temporary loss of the physical Golgi-centrosome proximity. Mitotic Golgi breakdown is required for Golgi partitioning into the two daughter cells, cell cycle progression and may contribute to the dispersal of Golgi-associated signaling molecules. Here, we review our current understanding of the mechanisms that control mitotic Golgi reorganization, its biological significance, and assays that are used to study this process.

PKD controls mitotic Golgi complex fragmentation through a Raf-MEK1 pathway

Before entering mitosis, the stacks of the Golgi cisternae are separated from each other, and inhibiting this process delays entry of mammalian cells into mitosis. Protein kinase D (PKD) is known to be involved in Golgi-to-cell surface transport by controlling the biogenesis of specific transport carriers. Here we show that depletion of PKD1 and PKD2 proteins from HeLa cells by small interfering RNA leads to the accumulation of cells in the G2 phase of the cell cycle and prevents cells from entering mitosis. We further provide evidence that inhibition of PKD blocks mitotic Raf-1 and mitogen-activated protein kinase kinase (MEK) activation, and, as a consequence, mitotic Golgi fragmentation, which could be rescued by expression of active MEK1. Finally, Golgi fluorescence recovery after photobleaching analyses demonstrate that PKD is crucial for the cleavage of the noncompact zones of Golgi membranes in G2 phase. Our findings suggest that PKD controls interstack Golgi connections in a Raf-1/MEK1-dependent manner, a process required for entry of the cells into mitosis.

Division of the intermediate compartment at the onset of mitosis provides a mechanism for Golgi inheritance

As mammalian cells prepare for mitosis, the Golgi ribbon is first unlinked into its constituent stacks and then transformed into spindle-associated, pleiomorphic membrane clusters in a process that remains enigmatic. Also, it remains unclear whether Golgi inheritance involves the incorporation of Golgi enzymes into a pool of coat protein I (COPI) vesicles, or their COPI-independent transfer to the endoplasmic reticulum (ER). Based on the observation that the intermediate compartment (IC) at the ER-Golgi boundary is connected to the centrosome, we examined its mitotic fate and possible role in Golgi breakdown. The use of multiple imaging techniques and markers revealed that the IC elements persist during the M phase, maintain their compositional and structural properties and remain associated with the mitotic spindle, forming circular arrays at the spindle poles. At G2/M transition, the movement of the pericentrosomal domain of the IC (pcIC) to the cell centre and its expansion coincide with the unlinking of the Golgi ribbon. At prophase, coupled to centrosome separation, the pcIC divides together with recycling endosomes, providing novel landmarks for mitotic entry. We provide evidence that the permanent IC elements function as way stations during the COPI-dependent dispersal of Golgi components at prometa- and metaphase, indicating that they correspond to the previously described Golgi clusters. In addition, they continue to communicate with the vesicular 'Golgi haze' and thus are likely to provide templates for Golgi reassembly. These results implicate the IC in mitotic Golgi inheritance, resulting in a model that integrates key features of the two previously proposed pathways.

Involvement of the penta-EF-hand protein Pef1p in the Ca2+-dependent regulation of COPII subunit assembly in Saccharomyces cerevisiae

Although it is well established that the coat protein complex II (COPII) mediates the transport of proteins and lipids from the endoplasmic reticulum (ER) to the Golgi apparatus, the regulation of the vesicular transport event and the mechanisms that act to counterbalance the vesicle flow between the ER and Golgi are poorly understood. In this study, we present data indicating that the penta-EF-hand Ca²⁺-binding protein Pef1p directly interacts with the COPII coat subunit Sec31p and regulates COPII assembly in Saccharomyces cerevisiae. ALG-2, a mammalian homolog of Pef1p, has been shown to interact with Sec31A in a Ca²⁺-dependent manner and to have a role in stabilizing the association of the Sec13/31 complex with the membrane. However, Pef1p displayed reversed Ca²⁺ dependence for Sec13/31p association; only the Ca²⁺-free form of Pef1p bound to the Sec13/31p complex. In addition, the influence on COPII coat assembly also appeared to be reversed; Pef1p binding acted as a kinetic inhibitor to delay Sec13/31p recruitment. Our results provide further evidence for a linkage between Ca²⁺-dependent signaling and ER-to-Golgi trafficking, but its mechanism of action in yeast seems to be different from the mechanism reported for its mammalian homolog ALG-2.

The Golgi apparatus in the endomembrane-rich gastric parietal cells exist as functional stable mini-stacks dispersed throughout the cytoplasm

Background information. Acid-secreting gastric parietal cells are polarized epithelial cells that harbour highly abundant and specialized, H⁺,K⁺ ATPase-containing, tubulovesicular membranes in the apical cytoplasm. The Golgi apparatus has been implicated in the biogenesis of the tubulovesicular membranes; however, an unanswered question is how a typical Golgi organization could regulate normal membrane transport within the membrane-dense cytoplasm of parietal cells.

Results. Here, we demonstrate that the Golgi apparatus of parietal cells is not the typical juxta-nuclear ribbon of stacks, but rather individual Golgi units are scattered throughout the cytoplasm. The Golgi membrane structures labelled with markers of both cis- and trans-Golgi membrane, indicating the presence of intact Golgi stacks. The parietal cell Golgi stacks were closely aligned with the microtubule network and were shown to participate in both anterograde and retrograde transport pathways. Dispersed Golgi stacks were also observed in parietal cells from H⁺,K⁺ ATPase-deficient mice that lack tubulovesicular membranes.

Conclusions. These results indicate that the unusual organization of individual Golgi stacks dispersed throughout the cytoplasm of these terminally differentiated cells is likely to be a developmentally regulated event.

Cell-free reconstitution of vacuole membrane fragmentation reveals regulation of vacuole size and number by TORC1

The size and copy number of an organelle depend on an equilibrium of membrane fusion and fission. In vitro reconstitution of yeast vacuole fission and fusion shows that TORC1 selectively stimulates fission but does not change fusion activity. This explains the morphological transitions of yeast vacuoles in response to nutrient availability.

Size and copy number of organelles are influenced by an equilibrium of membrane fusion and fission. We studied this equilibrium on vacuoles—the lysosomes of yeast. Vacuole fusion can readily be reconstituted and quantified in vitro, but it had not been possible to study fission of the organelle in a similar way. Here we present a cell-free system that reconstitutes fragmentation of purified yeast vacuoles (lysosomes) into smaller vesicles. Fragmentation in vitro reproduces physiological aspects. It requires the dynamin-like GTPase Vps1p, V-ATPase pump activity, cytosolic proteins, and ATP and GTP hydrolysis. We used the in vitro system to show that the vacuole-associated TOR complex 1 (TORC1) stimulates vacuole fragmentation but not the opposing reaction of vacuole fusion. Under nutrient restriction, TORC1 is inactivated, and the continuing fusion activity then dominates the fusion/fission equilibrium, decreasing the copy number and increasing the volume of the vacuolar compartment. This result can explain why nutrient restriction not only induces autophagy and a massive buildup of vacuolar/lysosomal hydrolases, but also leads to a concomitant increase in volume of the vacuolar compartment by coalescence of the organelles into a single large compartment.

Golgi during development

The Golgi is essential for processing proteins and sorting them, as well as plasma membrane components, to their final destinations. Not surprisingly, this organelle, a major compartment of the secretory pathway, is an important venue for regulating many aspects of development in both invertebrates and vertebrates. Through its role as a site for protein cleavage and glycosylation as well as through changes in its spatial organization and secretory trafficking, the Golgi exerts highly specific effects on cellular differentiation and morphogenesis by spatially and temporally constraining developmental pathways.

Golgi partitioning controls mitotic entry through Aurora-A kinase

At the onset of mitosis, the Golgi complex undergoes a multistep fragmentation process that is required for its correct partitioning into the daughter cells. Inhibition of this Golgi fragmentation results in cell cycle arrest at the G2 stage, suggesting that correct inheritance of the Golgi complex is monitored by a "Golgi mitotic checkpoint." However, the molecular basis of this G2 block is not known. Here, we show that the G2-specific Golgi fragmentation stage is concomitant with centrosome recruitment and activation of the mitotic kinase Aurora-A, an essential regulator for entry into mitosis. We show that a block of Golgi partitioning impairs centrosome recruitment and activation of Aurora-A, which results in the G2 block of cell cycle progression. Overexpression of Aurora-A overrides this cell cycle block, indicating that Aurora-A is a major effector of the Golgi checkpoint. Our findings provide the basis for further understanding of the signaling pathways that coordinate organelle inheritance and cell duplication.

Mitogen-activated protein/extracellular signal-regulated kinase kinase 1act/tubulin interaction is an important determinant of mitotic stability in cultured HT1080 human fibrosarcoma cells

Activation of the mitogen-activated protein kinase (MAPK) pathway plays a major role in neoplastic cell transformation. Using a proteomics approach, we identified alpha tubulin and beta tubulin as proteins that interact with activated MAP/extracellular signal-regulated kinase kinase 1 (MEK1), a central MAPK regulatory kinase. Confocal analysis revealed spatiotemporal control of MEK1-tubulin colocalization that was most prominent in the mitotic spindle apparatus in variant HT1080 human fibrosarcoma cells. Peptide arrays identified the critical role of positively charged amino acids R108, R113, R160, and K157 on the surface of MEK1 for tubulin interaction. Overexpression of activated MEK1 caused defects in spindle arrangement, chromosome segregation, and ploidy. In contrast, chromosome polyploidy was reduced in the presence of an activated MEK1 mutant (R108A, R113A) that disrupted interactions with tubulin. Our findings indicate the importance of signaling by activated MEK1-tubulin in spindle organization and chromosomal instability.

MAPK signaling to the early secretory pathway revealed by kinase/phosphatase functional screening

To what extent the secretory pathway is regulated by cellular signaling is unknown. In this study, we used RNA interference to explore the function of human kinases and phosphatases in controlling the organization of and trafficking within the secretory pathway. We identified 122 kinases/phosphatases that affect endoplasmic reticulum (ER) export, ER exit sites (ERESs), and/or the Golgi apparatus. Numerous kinases/phosphatases regulate the number of ERESs and ER to Golgi protein trafficking. Among the pathways identified, the Raf-MEK (MAPK/ERK [extracellular signal-regulated kinase] kinase)-ERK cascade, including its regulatory proteins CNK1 (connector enhancer of the kinase suppressor of Ras-1) and neurofibromin, controls the number of ERESs via ERK2, which targets Sec16, a key regulator of ERESs and COPII (coat protein II) vesicle biogenesis. Our analysis reveals an unanticipated complexity of kinase/phosphatase-mediated regulation of the secretory pathway, uncovering a link between growth factor signaling and ER export.

PRMT5 regulates Golgi apparatus structure through methylation of the golgin GM130

Maintenance of the Golgi apparatus (GA) structure and function depends on Golgi matrix proteins. The posttranslational modification of Golgi proteins such as phosphorylation of members of the golgin and GRASP families is important for determining Golgi architecture. Some Golgi proteins including golgin-84 are also known to be methylated, but the function of golgin methylation remains unclear. Here, we show that the protein arginine methyltransferase 5 (PRMT5) localizes to the GA and forms complexes with several components involved in GA ribbon formation and vesicle tethering. PRMT5 interacts with the golgin GM130, and depletion of PRMT5 causes defects in Golgi ribbon formation. Furthermore, PRMT5 methylates N-terminal arginines in GM130, and such arginine methylation appears critical for GA ribbon formation. Our findings reveal a molecular mechanism by which PRMT5-dependent arginine methylation of GM130 controls the maintenance of GA architecture.

The Golgi and the centrosome: building a functional partnership

The mammalian Golgi apparatus is characterized by a ribbon-like organization adjacent to the centrosome during interphase and extensive fragmentation and dispersal away from the centrosome during mitosis. It is not clear whether this dynamic association between the Golgi and centrosome is of functional significance. We discuss recent findings indicating that the Golgi–centrosome relationship may be important for directional protein transport and centrosome positioning, which are both required for cell polarization. We also summarize our current knowledge of the link between Golgi organization and cell cycle progression.

Src kinase regulates the integrity and function of the Golgi apparatus via activation of dynamin 2

The size and integrity of the Golgi apparatus is maintained via a tightly controlled regulation of membrane traffic using a variety of different signaling and cytoskeletal proteins. We have recently observed that activation of c-Src has profound effects on Golgi structure, leading to dramatically vesiculated cisternae in a variety of cell types. As the large GTPase dynamin (Dyn2) has been implicated in Golgi vesiculation during secretion, we tested whether inhibiting Dyn2 activity by expression of a Dyn2K44A mutant or siRNA knockdown could attenuate active Src-induced Golgi fragmentation. Indeed, these perturbations attenuated fragmentation, and expression of a Dyn2Y(231/597)F mutant protein that cannot be phosphorylated by Src kinase had a similar effect . Finally, we find that Dyn2 is markedly phosphorylated during the transit of VSV-G protein through the TGN whereas expression of the Dyn2Y(231/597)F mutant significantly reduces exit of the nascent protein from this compartment. These findings demonstrate that activation of Dyn2 by Src kinase regulates Golgi integrity and vesiculation during the secretory process.

Trk activation in the secretory pathway promotes Golgi fragmentation

Activation of nascent receptor tyrosine kinases within the secretory pathway has been reported, yet the consequences of intracellular activation are largely unexplored. We report that overexpression of the Trk neurotrophin receptors causes accumulation of autoactivated receptors in the ER-Golgi intermediate compartment. Autoactivated receptors exhibit inhibited Golgi-mediated processing and they inhibit Golgi-mediated processing of other co-expressed transmembrane proteins, apparently by inducing fragmentation of the Golgi apparatus. Signaling from G protein-coupled receptors is known to induce Trk transactivation. Transactivation of nascent TrkB in hippocampal neurons resulting from exposure to the neuropeptide PACAP caused Golgi fragmentation, whereas BDNF-dependent activation of TrkB did not. TrkB-mediated Golgi fragmentation employs a MEK-dependent signaling pathway resembling that implicated in regulation of Golgi fragmentation in mitotic cells. Neuronal Golgi fragments, in the form of dendritically localized Golgi outposts, are important determinants of dendritic growth and branching. The capacity of transactivated TrkB to enhance neuronal Golgi fragmentation may represent a novel mechanism regulating neural plasticity.

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

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

Silencing the SPCA1 (secretory pathway Ca2+-ATPase isoform 1) impairs Ca2+ homeostasis in the Golgi and disturbs neural polarity

Neural cell differentiation involves a complex regulatory signal transduction network in which Ca(2+) ions and the secretory pathway play pivotal roles. The secretory pathway Ca(2+)-ATPase isoform 1 (SPCA1) is found in the Golgi apparatus where it is actively involved in the transport of Ca(2+) or Mn(2+) from the cytosol to the Golgi lumen. Its expression during brain development in different types of neurons has been documented recently, which raises the possibility that SPCA1 contributes to neuronal differentiation. In the present study, we investigated the potential impact of SPCA1 on neuronal polarization both in a cell line and in primary neuronal culture. In N2a neuroblastoma cells, SPCA1 was immunocytochemically localized in the juxtanuclear Golgi. Knockdown of SPCA1 by RNA interference markedly delayed the differentiation in these cells. The cells retarded in differentiation showed increased numbers of neurites of reduced length compared with control cells. Ca(2+) imaging assays showed that the lack of SPCA1 impaired Golgi Ca(2+) homeostasis and resulted in disturbed trafficking of different classes of proteins including normally Golgi-localized cameleon GT-YC3.3, bearing a Golgi-specific galactosyltransferase N terminus, and a normally plasma membrane-targeted, glycosyl phosphatidyl inositol-anchored cyan fluorescent protein construct. Also in hippocampal primary neurons, which showed a differential distribution of SPCA1 expression in Golgi stacks depending on differentiation stage, partial silencing of SPCA1 resulted in delayed differentiation, whereas total suppression drastically affected the cell survival. The disturbed overall cellular Ca(2+) homeostasis and/or the altered targeting of organellar proteins under conditions of SPCA1 knockdown highlight the importance of SPCA1 function for normal neural differentiation.

Specific phosphorylation and activation of ERK1c by MEK1b: a unique route in the ERK cascade

Extracellular signal-regulated kinases (ERKs) are key signaling molecules that regulate a large number of cellular processes, including mitosis. We showed previously that ERK1c, an alternatively spliced form of ERK1, facilitates mitotic Golgi fragmentation without the involvement of ERK1 and ERK2. Here we demonstrate that activation of ERK1c is mainly mediated by mitogen-activated protein kinase (MAPK)/ERK kinase 1b (MEK1b), which is an alternatively spliced form of MEK1 that was previously considered an inactive kinase. MEK1b phosphorylation and activity are preferentially stimulated by nocodazole, to induce its specific activity toward ERK1c. MEK1/2, on the other hand, preferentially target ERK1/2 in response to growth factors, such as EGF. As previously demonstrated for ERK1c, also MEK1b expression and activity are elevated during mitosis, and thereby enhance Golgi fragmentation and mitotic rate. MEK1 activity is also increased during mitosis, but this isoform facilitates mitotic progression without affecting the Golgi architecture. These results illustrate that the ERK cascade is divided into two routes: the classic MEK1/2-ERK1/2 and the splice-variant MEK1b-ERK1c, each of which regulates distinct cellular processes and thus extends the cascade specificity.

A non-canonical MEK/ERK signaling pathway regulates autophagy via regulating Beclin 1

Autophagy-essential proteins are the molecular basis of protective or destructive autophagy machinery. However, little is known about the signaling mechanisms governing these proteins and the opposing consequences of autophagy in mammals. Here we report that a non-canonical MEK/ERK module, which is positioned downstream of AMP-activated protein kinase (AMPK) and upstream of tuberous sclerosis complex (TSC), regulates autophagy by regulating Beclin 1. Depletion of ERK partially inhibited autophagy, whereas specific inhibition on MEK completely inhibited autophagy. MEK could bypass ERK to promote autophagy. Basal MEK/ERK activity conferred basal Beclin 1 by preventing disassembly of mammalian target of rapamycin complex 1 (mTORC1) and mTORC2. Activation of MEK/ERK by AMPK upon autophagy stimuli disassembled mTORC1 via binding to and activating TSC but disassembled mTORC2 independently of TSC. Inhibition of mTORC1 or mTORC2 by transiently or moderately activated MEK/ERK caused moderately enhanced Beclin 1 resulting in cytoprotective autophagy, whereas inhibition of both mTORC1 and mTORC2 by sustained MEK/ERK activation caused strongly pronounced Beclin 1 leading to cytodestructive autophagy. Our findings thus propose that the AMPK-MEK/ERK-TSC-mTOR pathway regulation of Beclin 1 represents different thresholds responsible for a protective or destructive autophagy.

CDC2/SPDY transiently associates with endoplasmic reticulum exit sites during oocyte maturation

BACKGROUND

Mammalian oocytes acquire competence to be fertilized during meiotic maturation. The protein kinase CDC2 plays a pivotal role in several key maturation events, in part through controlled changes in CDC2 localization. Although CDC2 is involved in initiation of maturation, a detailed analysis of CDC2 localization at the onset of maturation is lacking. In this study, the subcellular distribution of CDC2 and its regulatory proteins cyclin B and SPDY in combination with several organelle markers at the onset of pig oocyte maturation has been investigated.

RESULTS

Our results demonstrate that CDC2 transiently associates with a single domain, identified as a cluster of endoplasmic reticulum (ER) exit sites (ERES) by the presence of SEC23, in the cortex of maturing porcine oocytes prior to germinal vesicle break down. Inhibition of meiosis resumption by forskolin treatment prevented translocation of CDC2 to this ERES cluster. Phosphorylated GM130 (P-GM130), which is a marker for fragmented Golgi, localized to ERES in almost all immature oocytes and was not affected by forskolin treatment. After removal of forskolin from the culture media, the transient translocation of CDC2 to ERES was accompanied by a transient dispersion of P-GM130 into the ER suggesting a role for CDC2 in redistributing Golgi components that have collapsed into ERES further into the ER during meiosis. Finally, we show that SPDY, rather than cyclin B, colocalizes with CDC2 at ERES, suggesting a role for the CDC2/SPDY complex in regulating the secretory pathway during oocyte maturation.

CONCLUSION

Our data demonstrate the presence of a novel structure in the cortex of porcine oocytes that comprises ERES and transiently accumulates CDC2 prior to germinal vesicle breakdown. In addition, we show that SPDY, but not cyclin B, localizes to this ERES cluster together with CDC2.

GOLPH2 and MYO6: putative prostate cancer markers localized to the Golgi apparatus

BACKGROUND

Malignant transformation is often accompanied by morphological and functional alterations in subcellular organelles. The Golgi apparatus is a subcellular structure primarily involved in modification and sorting of macromolecules for secretion and transport to other cellular destinations. Molecular alterations associated with the Golgi apparatus may take place during prostate carcinogenesis but such alterations have not been documented.

METHODS

To demonstrate that the Golgi apparatus undergoes alterations during prostate carcinogenesis, we examined the expression and localization of two candidate molecules, Golgi phosphoprotein 2 (GOLPH2) and myosin VI (MYO6), both overexpressed in prostate cancer as initially identified by expression microarray analysis.

RESULTS

Elevated GOLPH2 expression in prostate cancers was validated through real-time RT-PCR, Western blot, and tissue microarray analysis, and its Golgi localization in surgical prostate cancer tissues confirmed using two-color immunofluorescence. In addition, distinctive juxtanuclear MYO6 staining pattern consistent with Golgi localization was observed in surgical prostate cancer tissues. Two-color immunofluorescence revealed intensive Golgi-specific staining for both GOLPH2 and myosin VI in prostate cancer cells but not in the adjacent normal prostate epithelium.

CONCLUSIONS

We show that the Golgi apparatus in prostate cancer cells differs from the normal Golgi by elevated levels of two molecules, GOLPH2 and MYO6. These results for the first time demonstrated consistent cancer cell-specific alterations in the molecular composition of the Golgi apparatus. Such alterations can be explored for discovery of novel prostate cancer biomarkers through targeted organellar approaches.