The Golgi protein p115 associates with gamma-tubulin and plays a role in Golgi structure and mitosis progression

Characterization and functional analysis of Toxoplasma Golgi-associated proteins identified by proximity labelling

Toxoplasma gondii possesses a highly polarized secretory pathway that contains both broadly conserved eukaryotic organelles and unique apicomplexan organelles which play essential roles in the parasite's lytic cycle. As in other eukaryotes, the T. gondii Golgi apparatus sorts and modifies proteins prior to their distribution to downstream organelles. Many of the typical trafficking factors found involved in these processes are missing from apicomplexan genomes, suggesting that these parasites have evolved unique proteins to fill these roles. Here we identify a novel Golgi-localizing protein (ULP1) which contains structural homology to the eukaryotic trafficking factor p115/Uso1. We demonstrate that depletion of ULP1 leads to a dramatic reduction in parasite fitness and replicative ability. Using ULP1 as bait for TurboID proximity labelling and immunoprecipitation, we identify eleven more novel Golgi-associated proteins and demonstrate that ULP1 interacts with the T. gondii COG complex. These proteins include both conserved trafficking factors and parasite-specific proteins. Using a conditional knockdown approach, we assess the effect of each of these eleven proteins on parasite fitness. Together, this work reveals a diverse set of novel T. gondii Golgi-associated proteins that play distinct roles in the secretory pathway. As several of these proteins are absent outside of the Apicomplexa, they represent potential targets for the development of novel therapeutics against these parasites.

Importance

Apicomplexan parasites such as Toxoplasma gondii infect a large percentage of the world's population and cause substantial human disease. These widespread pathogens use specialized secretory organelles to infect their host cells, modulate host cell functions, and cause disease. While the functions of the secretory organelles are now better understood, the Golgi apparatus of the parasite remains largely unexplored, particularly regarding parasite-specific innovations that may help direct traffic intracellularly. In this work, we characterize ULP1, a protein that is unique to parasites but shares structural similarity to the eukaryotic trafficking factor p115/Uso1. We show that ULP1 plays an important role in parasite replication and demonstrate that it interacts with the conserved oligomeric Golgi (COG) complex. We then use ULP1 proximity labelling to identify eleven additional Golgi-associated proteins which we functionally analyze via conditional knockdown. This work expands our knowledge of the Toxoplasma Golgi apparatus and identifies potential targets for therapeutic intervention.

A feedforward loop between JAK/STAT downstream target p115 and STAT in germline stem cells

The maintenance of germline stem cells (GSCs) is essential for tissue homeostasis. JAK/STAT signaling maintains GSC fate in Drosophila testis. However, how JAK/STAT signaling maintains male GSC fate through its downstream targets remains poorly understood. Here, we identify p115, a tER/cis-Golgi golgin protein, as a putative downstream target of JAK/STAT signaling. p115 maintains GSC fate independent of GM130 and GRASP65. p115 localizes in cytosol, the ER and Golgi apparatus in germline cells and is required for the morphology of the ER and Golgi apparatus. Furthermore, depletion of p115 in GSCs results in aberrant spindle orientation. Mechanistically, p115 associates with and stabilizes STAT. Finally, ectopic expression of STAT completely restores GSC loss caused by p115 depletion. Collectively, JAK/STAT signaling and p115 form a feedforward loop to maintain male GSC fate. Our work provides new insights into the regulatory mechanism of how stem cell maintenance is properly controlled by JAK/STAT signaling.

RAB11A and RAB11B control mitotic spindle function in intestinal epithelial progenitor cells

RAB11 small GTPases and associated recycling endosome have been localized to mitotic spindles and implicated in regulating mitosis. However, the physiological significance of such regulation has not been observed in mammalian tissues. We have used newly engineered mouse models to investigate intestinal epithelial renewal in the absence of single or double isoforms of RAB11 family members: Rab11a and Rab11b. Comparing with single knockouts, mice with compound ablation demonstrate a defective cell cycle entry and robust mitotic arrest followed by apoptosis, leading to a total penetrance of lethality within 3 days of gene ablation. Upon Rab11 deletion ex vivo, enteroids show abnormal mitotic spindle formation and cell death. Untargeted proteomic profiling of Rab11a and Rab11b immunoprecipitates has uncovered a shared interactome containing mitotic spindle microtubule regulators. Disrupting Rab11 alters kinesin motor KIF11 function and impairs bipolar spindle formation and cell division. These data demonstrate that RAB11A and RAB11B redundantly control mitotic spindle function and intestinal progenitor cell division, a mechanism that may be utilized to govern the homeostasis and renewal of other mammalian tissues.

Atypical Legionella GTPase effector hijacks host vesicular transport factor p115 to regulate host lipid droplet

The intracellular bacterial pathogen Legionella pneumophila uses hundreds of effector proteins to manipulate multiple processes of the host cells to establish a replicative niche known as Legionella-containing vacuole (LCV). Biogenesis of the LCV has been known to depend on host small guanosine triphosphatases (GTPases), but whether bacterial effector GTPases are also involved remains unknown. Here, we show that an ankyrin repeat containing effector LegA15 localizes directly in host lipid droplets (LDs), leading to Golgi apparatus fragmentation of the host cells by hijacking the host vesicular transport factor p115. LegA15 is a GTPase with a unique catalytic mechanism, unlike any eukaryotic small GTPases. Moreover, the effector LegA15 co-opts p115 to modulate homeostasis of the host LDs in its GTPase-dependent manner. Together, our data reveal that an atypical GTPase effector regulates the host LDs through impeding the vesicle secretion system of the host cells for intracellular life cycle of Legionella.

Structural Organization and Function of the Golgi Ribbon During Cell Division

The Golgi complex has a central role in the secretory traffic. In vertebrate cells it is generally organized in polarized stacks of cisternae that are laterally connected by membranous tubules, forming a structure known as Golgi ribbon. The steady state ribbon arrangement results from a dynamic equilibrium between formation and cleavage of the membrane tubules connecting the stacks. This balance is of great physiological relevance as the unlinking of the ribbon during G2 is required for mitotic entry. A block of this process induces a potent G2 arrest of the cell cycle, indicating that a mitotic “Golgi checkpoint” controls the correct pre-mitotic segregation of the Golgi ribbon. Then, after mitosis onset, the Golgi stacks undergo an extensive disassembly, which is necessary for proper spindle formation. Notably, several Golgi-associated proteins acquire new roles in spindle formation and mitotic progression during mitosis. Here we summarize the current knowledge about the basic principle of the Golgi architecture and its functional relationship with cell division to highlight crucial aspects that need to be addressed to help us understand the physiological significance of the ribbon and the pathological implications of alterations of this organization.

Unconventional Protein Secretion in Brain Tumors Biology: Enlightening the Mechanisms for Tumor Survival and Progression

Non-canonical secretion pathways, collectively known as unconventional protein secretion (UPS), are alternative secretory mechanisms usually associated with stress-inducing conditions. UPS allows proteins that lack a signal peptide to be secreted, avoiding the conventional endoplasmic reticulum-Golgi complex secretory pathway. Molecules that generally rely on the canonical pathway to be secreted may also use the Golgi bypass, one of the unconventional routes, to reach the extracellular space. UPS studies have been increasingly growing in the literature, including its implication in the biology of several diseases. Intercellular communication between brain tumor cells and the tumor microenvironment is orchestrated by various molecules, including canonical and non-canonical secreted proteins that modulate tumor growth, proliferation, and invasion. Adult brain tumors such as gliomas, which are aggressive and fatal cancers with a dismal prognosis, could exploit UPS mechanisms to communicate with their microenvironment. Herein, we provide functional insights into the UPS machinery in the context of tumor biology, with a particular focus on the secreted proteins by alternative routes as key regulators in the maintenance of brain tumors.

Functional Coordination among the Golgi Complex, the Centrosome and the Microtubule Cytoskeleton during the Cell Cycle

The Golgi complex of mammalian cells is organized in a ribbon-like structure often closely associated with the centrosome during interphase. Conversely, the Golgi complex assumes a fragmented and dispersed configuration away from the centrosome during mitosis. The structure of the Golgi complex and the relative position to the centrosome are dynamically regulated by microtubules. Many pieces of evidence reveal that this microtubule-mediated dynamic association between the Golgi complex and centrosome is of functional significance in cell polarization and division. Here, we summarize findings indicating how the Golgi complex and the centrosome cooperate in organizing the microtubule network for the directional protein transport and centrosome positioning required for cell polarization and regulating fundamental cell division processes.

A novel all-trans retinoic acid derivative regulates cell cycle and differentiation of myelodysplastic syndrome cells by USO1

4-Amino-2-Trifluoromethyl-Phenyl Retinate (ATPR), a novel all-trans retinoic acid (ATRA) derivative, has been demonstrated that it had a variety of anti-tumor effects by exerting regulation on cellular proliferation, apoptosis and differentiation. Here, we found that ATPR is critical for alleviating myelodysplastic syndrome (MDS) and acute myelogenous leukemia. USO1, vesicle transport factor, belongs to tether protein family and involved in endoplasmic reticulum to Golgi trafficking of protein which is important to tumorigenesis. How USO1 contribute to MDS remain elusive. USO1 is aberrantly activated in MDS and AML in vivo and vitro, aberration of which might be a dominant mechanism for MDS cell survival. During the ATPR-induced remission of MDS, in vitro, USO1 presents a time and concentration-dependent decrease. Silencing of USO1 promotes myeloid differentiation of MDS cells and inhibits MDS cellular proliferation while USO1 over-expression has the opposite effect, suggesting that reduction of USO1 enhances the sensitivity of SKM-1 cells to ATPR treatment. Mechanistically, USO1 exerts its oncogenic role by inactivating Raf/ERK signaling, while ATPR is access to revise it. Notably, the activity of Raf/ERK pathway is required for the development and maintenance of MDS cell proliferation. Collectively, our results demonstrate the USO1- Raf/ERK signaling axis in MDS and highlight the negative role of USO1 in ATPR-regulated remission of MDS.

Centrioles and Ciliary Structures during Male Gametogenesis in Hexapoda: Discovery of New Models

Centrioles are-widely conserved barrel-shaped organelles present in most organisms. They are indirectly involved in the organization of the cytoplasmic microtubules both in interphase and during the cell division by recruiting the molecules needed for microtubule nucleation. Moreover, the centrioles are required to assemble cilia and flagella by the direct elongation of their microtubule wall. Due to the importance of the cytoplasmic microtubules in several aspects of the cell life, any defect in centriole structure can lead to cell abnormalities that in humans may result in significant diseases. Many aspects of the centriole dynamics and function have been clarified in the last years, but little attention has been paid to the exceptions in centriole structure that occasionally appeared within the animal kingdom. Here, we focused our attention on non-canonical aspects of centriole architecture within the Hexapoda. The Hexapoda is one of the major animal groups and represents a good laboratory in which to examine the evolution and the organization of the centrioles. Although these findings represent obvious exceptions to the established rules of centriole organization, they may contribute to advance our understanding of the formation and the function of these organelles.

Fragmentation of the Golgi Apparatus in Neuroblastoma Cells Is Associated with Tau-Induced Ring-Shaped Microtubule Bundles

Abnormal fibrillary aggregation of tau protein is a pathological condition observed in Alzheimer's disease and other tauopathies; however, the presence and pathological significance of early non-fibrillary aggregates of tau remain under investigation. In cell and animal models expressing normal or modified tau, toxic effects altering the structure and function of several membranous organelles have also been reported in the absence of fibrillary structures; however, how these abnormalities are produced is an issue yet to be addressed. In order to obtain more insights into the mechanisms by which tau may disturb intracellular membranous elements, we transiently overexpressed human full-length tau and several truncated tau variants in cultured neuroblastoma cells. After 48 h of transfection, either full-length or truncated tau forms produced significant fragmentation of the Golgi apparatus (GA) with no changes in cell viability. Noteworthy is that in the majority of cells exhibiting dispersion of the GA, a ring-shaped array of cortical or perinuclear microtubule (Mt) bundles was also generated under the expression of either variant of tau. In contrast, Taxol treatment of non-transfected cells increased the amount of Mt bundles but not sufficiently to produce fragmentation of the GA. Tau-induced ring-shaped Mt bundles appeared to be well-organized and stable structures because they were resistant to Nocodazole post-treatment and displayed a high level of tubulin acetylation. These results further indicate that a mechanical force generated by tau-induced Mt-bundling may be responsible for Golgi fragmentation and that the repeated domain region of tau may be the main promoter of this effect.

A New Look at the Functional Organization of the Golgi Ribbon

A characteristic feature of vertebrate cells is a Golgi ribbon consisting of multiple cisternal stacks connected into a single-copy organelle next to the centrosome. Despite numerous studies, the mechanisms that link the stacks together and the functional significance of ribbon formation remain poorly understood. Nevertheless, these questions are of considerable interest, since there is increasing evidence that Golgi fragmentation – the unlinking of the stacks in the ribbon – is intimately connected not only to normal physiological processes, such as cell division and migration, but also to pathological states, including neurodegeneration and cancer. Challenging a commonly held view that ribbon architecture involves the formation of homotypic tubular bridges between the Golgi stacks, we present an alternative model, based on direct interaction between the biosynthetic (pre-Golgi) and endocytic (post-Golgi) membrane networks and their connection with the centrosome. We propose that the central domains of these permanent pre- and post-Golgi networks function together in the biogenesis and maintenance of the more transient Golgi stacks, and thereby establish “linker compartments” that dynamically join the stacks together. This model provides insight into the reversible fragmentation of the Golgi ribbon that takes place in dividing and migrating cells and its regulation along a cell surface – Golgi – centrosome axis. Moreover, it helps to understand transport pathways that either traverse or bypass the Golgi stacks and the positioning of the Golgi apparatus in differentiated neuronal, epithelial, and muscle cells.

Organelle Inheritance Control of Mitotic Entry and Progression: Implications for Tissue Homeostasis and Disease

The Golgi complex (GC), in addition to its well-known role in membrane traffic, is also actively involved in the regulation of mitotic entry and progression. In particular, during the G2 phase of the cell cycle, the Golgi ribbon is unlinked into isolated stacks. Importantly, this ribbon cleavage is required for G2/M transition, indicating that a “Golgi mitotic checkpoint” controls the correct segregation of this organelle. Then, during mitosis, the isolated Golgi stacks are disassembled, and this process is required for spindle formation. Moreover, recent evidence indicates that also proper mitotic segregation of other organelles, such as mitochondria, endosomes, and peroxisomes, is required for correct mitotic progression and/or spindle formation. Collectively, these observations imply that in addition to the control of chromosomes segregation, which is required to preserve the genetic information, the cells actively monitor the disassembly and redistribution of subcellular organelles in mitosis. Here, we provide an overview of the major structural reorganization of the GC and other organelles during G2/M transition and of their regulatory mechanisms, focusing on novel findings that have shed light on the basic processes that link organelle inheritance to mitotic progression and spindle formation, and discussing their implications for tissue homeostasis and diseases.

HDAC6 Inhibition Protects against OGDR-Induced Golgi Fragmentation and Apoptosis

The Golgi apparatus (GA) is a pivotal organelle, and its fragmentation is an essential process in the development of apoptosis. GA is a potential target in the treatment of cerebral ischemia-reperfusion injury. Histone deacetylase 6 (HDAC6) catalyzes the removal of functional acetyl groups from proteins and plays an important role in cell homeostasis. In this study, the neuroprotective effects and the underlying mechanisms of HDAC6 inhibition were assessed in an ischemia-reperfusion injury model. Mouse neuroblastoma N2a cells and cultured neurons were subjected to oxygen-glucose deprivation/reperfusion (OGDR) insult. OGDR induces Golgi fragmentation and reduces tubulin acetylation in N2a cells and cultured neurons. Golgi fragmentation is prior to nuclear chromatin condensation after OGDR injury. Overexpression of GBF1 not only protects against OGDR-induced Golgi fragmentation but also protects against OGDR-induced apoptosis, suggesting that Golgi fragmentation is not secondary to apoptosis but plays a causal role for subsequent apoptosis. HDAC6 inhibition suppresses OGDR-induced tubulin deacetylation, p115 cleavage, and caspase 3 activation and protects against OGDR-induced Golgi fragmentation and apoptosis. This work opens a new avenue for potential clinical application of HDAC6 inhibitors for cerebral ischemia-reperfusion-related disorders.

trans-Golgi network-bound cargo traffic

Cargo following the retrograde trafficking are sorted at endosomes to be targeted the trans-Golgi network (TGN), a central receiving organelle. Though molecular requirements and their interaction networks have been somewhat established, the complete understanding of the intricate nature of their action mechanisms in every step of the retrograde traffic pathway remains unachieved. This review focuses on elucidating known functions of key regulators, including scission factors at the endosome and tethering/fusion mediators at the receiving dock, TGN, as well as a diverse range of cargo.

Mitotic inheritance of the Golgi complex and its role in cell division

The Golgi apparatus plays essential roles in the processing and sorting of proteins and lipids, but it can also act as a signalling hub and a microtubule-nucleation centre. The Golgi complex (GC) of mammalian cells is composed of stacks connected by tubular bridges to form a continuous membranous system. In spite of this structural complexity, the GC is highly dynamic, and this feature becomes particularly evident during mitosis, when the GC undergoes a multi-step disassembly process that allows its correct partitioning and inheritance by daughter cells. Strikingly, different steps of Golgi disassembly control mitotic entry and progression, indicating that cells actively monitor Golgi integrity during cell division. Here, we summarise the basic mechanisms and the molecular players that are involved in Golgi disassembly, focussing in particular on recent studies that have revealed the fundamental signalling pathways that connect Golgi inheritance to mitotic entry and progression.

Drosophila p115 is required for Cdk1 activation and G2/M cell cycle transition

Golgi complex inheritance and its relationship with the cell cycle are central in cell biology. Golgi matrix proteins, known as golgins, are one of the components that underlie the shape and functionality of this organelle. In mammalian cells, golgins are phosphorylated during mitosis to allow fragmentation of the Golgi ribbon and they also participate in spindle dynamics; both processes are required for cell cycle progression. Little is known about the function of golgins during mitosis in metazoans in vivo. This is particularly significant in Drosophila, in which the Golgi architecture is distributed in numerous units scattered throughout the cytoplasm, in contrast with mammalian cells. We examined the function of the ER/cis-Golgi golgin p115 during the proliferative phase of the Drosophila wing imaginal disc. Knockdown of p115 decreased tissue size. This phenotype was not caused by programmed cell death or cell size reductions, but by a reduction in the final cell number due to an accumulation of cells at the G2/M transition. This phenomenon frequently allows mitotic bypass and re-replication of DNA. These outcomes are similar to those observed following the partial loss of function of positive regulators of Cdk1 in Drosophila. In agreement with this, Cdk1 activation was reduced upon p115 knockdown. Interestingly, these phenotypes were fully rescued by Cdk1 overexpression and partially rescued by Myt1 depletion, but not by String (also known as Cdc25) overexpression. Additionally, we confirmed the physical interaction between p115 and Cdk1, suggesting that the formation of a complex where both proteins are present is essential for the full activation of Cdk1 and thus the correct progression of mitosis in proliferating tissues.

Modular transcriptional repertoire and MicroRNA target analyses characterize genomic dysregulation in the thymus of Down syndrome infants

Trisomy 21-driven transcriptional alterations in human thymus were characterized through gene coexpression network (GCN) and miRNA-target analyses. We used whole thymic tissue--obtained at heart surgery from Down syndrome (DS) and karyotipically normal subjects (CT)--and a network-based approach for GCN analysis that allows the identification of modular transcriptional repertoires (communities) and the interactions between all the system's constituents through community detection. Changes in the degree of connections observed for hierarchically important hubs/genes in CT and DS networks corresponded to community changes. Distinct communities of highly interconnected genes were topologically identified in these networks. The role of miRNAs in modulating the expression of highly connected genes in CT and DS was revealed through miRNA-target analysis. Trisomy 21 gene dysregulation in thymus may be depicted as the breakdown and altered reorganization of transcriptional modules. Leading networks acting in normal or disease states were identified. CT networks would depict the "canonical" way of thymus functioning. Conversely, DS networks represent a "non-canonical" way, i.e., thymic tissue adaptation under trisomy 21 genomic dysregulation. This adaptation is probably driven by epigenetic mechanisms acting at chromatin level and through the miRNA control of transcriptional programs involving the networks' high-hierarchy genes.

Spatial and Functional Aspects of ER-Golgi Rabs and Tethers

Two conserved Rab GTPases, Rab1 and Rab2, play important roles in biosynthetic-secretory trafficking between the endoplasmic reticulum (ER) and the Golgi apparatus in mammalian cells. Both are expressed as two isoforms that regulate anterograde transport via the intermediate compartment (IC) to the Golgi, but are also required for transport in the retrograde direction. Moreover, Rab1 has been implicated in the formation of autophagosomes. Rab1 and Rab2 have numerous effectors or partners that function in membrane tethering, but also have other roles. These include the coiled-coil proteins p115, GM130, giantin, golgin-84, and GMAP-210, as well as the multisubunit COG (conserved oligomeric Golgi) and TRAPP (transport protein particle) tethering complexes. TRAPP also acts as the GTP exchange factor (GEF) in the activation of Rab1. According to the traditional view of the IC elements as motile, transient structures, the functions of the Rabs could take place at the two ends of the ER-Golgi itinerary, i.e., at ER exit sites (ERES) and/or cis-Golgi. However, there is considerable evidence for their specific association with the IC, including its recently identified pericentrosomal domain (pcIC), where many of the effectors turn out to be present, thus being able to exert their functions at the pre-Golgi level. The IC localization of these proteins is of particular interest based on the imaging of Rab1 dynamics, indicating that the IC is a stable organelle that bidirectionally communicates with the ER and Golgi, and is functionally linked to the endosomal system via the pcIC.

Emerging Insights into the Roles of Membrane Tethers from Analysis of Whole Organisms: The Tip of an Iceberg?

Membrane tethers have been identified throughout different compartments of the endomembrane system. It is now well established that a number of membrane tethers mediate docking of membrane carriers in anterograde and retrograde transport and in regulating the organization of membrane compartments. Much of our information on membrane tethers have been obtained from the analysis of individual membrane tethers in cultured cells. In the future it will be important to better appreciate the network of interactions mediated by tethers and the potential co-ordination of their collective functions in vivo. There are now a number of studies which have analyzed membrane tethers in tissues and organisms which are providing new insights into the role of this class of membrane protein at the physiological level. Here we review recent advances in the understanding of the function of membrane tethers from knock outs (or knock downs) in whole organisms and from mutations in tethers associated with disease.

Identification of important long non-coding RNAs and highly recurrent aberrant alternative splicing events in hepatocellular carcinoma through integrative analysis of multiple RNA-Seq datasets

Hepatocellular carcinoma (HCC) is an aggressive and deadly cancer. The molecular pathogenesis of the disease remains poorly understood. To better understand HCC biology and explore potential biomarkers and therapeutic targets, we investigated the whole transcriptome of HCC. Considering the genetic heterogeneity of HCC, four datasets from four studies consisting of 15 pairs of HCC and adjacent normal samples were analyzed. We observed that the number of lncRNAs expressed in each HCC sample was consistently greater than the adjacent normal sample. Moreover, 15 lncRNAs were identified expressed in five to seven HCC tissues but were not detected in any adjacent normal tissue. Differential expression analysis detected 35 up- and 80 down-regulated lncRNAs in HCC samples compared with adjacent normal samples. In addition, five differentially expressed lncRNAs were predicted to play a role in oxidation and reduction process. With regard to splicing alterations, we identified nine highly recurrent differential splicing events belonging to eight genes USO1, RPS24, CCDC50, THNSL2, NUMB, FN1 (two events), SLC39A14 and NR1I3. Of them, splicing alterations of SLC39A14 and NR1I3 were reported for the association with HCC for the first time. The splicing dysregulation in HCC may be influenced by three splicing factors ESRP2, CELF2 and SRSF5 which were significantly down-regulated in HCC samples. This study revealed uncharacterized aspects of HCC transcriptome and identified important lncRNAs and splicing isoforms with the potential to serve as biomarkers and therapeutic targets for the disease.

Mechanisms and Regulation of the Mitotic Inheritance of the Golgi Complex

In mammalian cells, the Golgi complex is structured in the form of a continuous membranous system composed of stacks connected by tubular bridges: the "Golgi ribbon." At the onset of mitosis, the Golgi complex undergoes a multi-step fragmentation process that is required for its correct partition into the dividing cells. Importantly, inhibition of Golgi disassembly results in cell-cycle arrest at the G2 stage, which indicates that accurate inheritance of the Golgi complex is monitored by a "Golgi mitotic checkpoint." Moreover, mitotic Golgi disassembly correlates with the release of a set of Golgi-localized proteins that acquire specific functions during mitosis, such as mitotic spindle formation and regulation of the spindle checkpoint. Most of these events are regulated by small GTPases of the Arf and Rab families. Here, we review recent studies that are revealing the fundamental mechanisms, the molecular players, and the biological significance of mitotic inheritance of the Golgi complex in mammalian cells. We also briefly comment on how Golgi partitioning is coordinated with mitotic progression.

Golgi fragmentation in Alzheimer's disease

The Golgi apparatus is an essential cellular organelle for post-translational modifications, sorting, and trafficking of membrane and secretory proteins. Proper functionality of the Golgi requires the formation of its unique cisternal-stacking morphology. The Golgi structure is disrupted in a variety of neurodegenerative diseases, suggesting a common mechanism and contribution of Golgi defects in neurodegenerative disorders. A recent study on Alzheimer's disease (AD) revealed that phosphorylation of the Golgi stacking protein GRASP65 disrupts its function in Golgi structure formation, resulting in Golgi fragmentation. Inhibiting GRASP65 phosphorylation restores the Golgi morphology from Aβ-induced fragmentation and reduces Aβ production. Perturbing Golgi structure and function in neurons may directly impact trafficking, processing, and sorting of a variety of proteins essential for synaptic and dendritic integrity. Therefore, Golgi defects may ultimately promote the development of AD. In the current review, we focus on the cellular impact of impaired Golgi morphology and its potential relationship to AD disease development.

Hsp20 Protects against Oxygen-Glucose Deprivation/Reperfusion-Induced Golgi Fragmentation and Apoptosis through Fas/FasL Pathway

Cerebral ischemia-reperfusion injury plays an important role in the development of tissue injury after acute ischemic stroke. Finding effective neuroprotective agents has become a priority in the treatment of ischemic stroke. The Golgi apparatus (GA) is a pivotal organelle and its protection is an attractive target in the treatment of cerebral ischemia-reperfusion injury. Protective effects of Hsp20, a potential cytoprotective agent due to its chaperone-like activity and involvement in regulation of many vital processes, on GA were assessed in an ischemia-reperfusion injury model. Mouse neuroblastoma Neuro2a (N2a) cells were subjected to oxygen-glucose deprivation/reperfusion (OGDR) insult. OGDR induces Golgi fragmentation, apoptosis, and p115 cleavage in N2a cells. However, transfection with Hsp20 significantly attenuates OGDR-induced Golgi fragmentation and apoptosis. Hsp20 interacts with Bax, decreases FasL and Bax expression, and inhibits caspases 3 and p115 cleavage in N2a cells exposed to OGDR. Our data demonstrate that increased Hsp20 expression protects against OGDR-induced Golgi fragmentation and apoptosis, likely through interaction with Bax and subsequent amelioration of the OGDR-induced elevation in p115 cleavage via the Fas/FasL signaling pathway. This neuroprotective potential of Hsp20 against OGDR insult and the underlying mechanism will pave the way for its potential clinical application for cerebral ischemia-reperfusion related disorders.

Aurora B-dependent phosphorylation of Ataxin-10 promotes the interaction between Ataxin-10 and Plk1 in cytokinesis

Spinocerebellar ataxia type 10 (SCA10) is an autosomal dominant neurologic disorder caused by ATTCT expansion in the ATXN10 gene. Previous investigations have identified that depletion of Ataxin-10, the gene product, leads to cellular apoptosis and cytokinesis failure. Herein we identify the mitotic kinase Aurora B as an Ataxin-10 interacting partner. Aurora B interacts with and phosphorylates Ataxin-10 at S12, as evidenced by in vitro kinase and mass spectrometry analysis. Both endogenous and S12-phosphorylated Ataxin-10 localizes to the midbody during cytokinesis, and cytokinetic defects induced by inhibition of ATXN10 expression is not rescued by the S12A mutant. Inhibition of Aurora B or expression of the S12A mutant renders reduced interaction between Ataxin-10 and polo-like kinase 1 (Plk1), a kinase previously identified to regulate Ataxin-10 in cytokinesis. Taken together, we propose a model that Aurora B phosphorylates Ataxin-10 at S12 to promote the interaction between Ataxin-10 and Plk1 in cytokinesis. These findings identify an Aurora B-dependent mechanism that implicates Ataxin-10 in cytokinesis.

Reduction in Golgi apparatus dimension in the absence of a residential protein, N-acetylglucosaminyltransferase V

Various proteins are involved in the generation and maintenance of the membrane complex known as the Golgi apparatus. We have used mutant Chinese hamster ovary (CHO) cell lines Lec4 and Lec4A lacking N-acetylglucosaminyltransferase V (GlcNAcT-V, MGAT5) activity and protein in the Golgi apparatus to study the effects of the absence of a single glycosyltransferase on the Golgi apparatus dimension. Quantification of immunofluorescence in serial confocal sections for Golgi α-mannosidase II and electron microscopic morphometry revealed a reduction in Golgi volume density up to 49 % in CHO Lec4 and CHO Lec4A cells compared to parental CHO cells. This reduction in Golgi volume density could be reversed by stable transfection of Lec4 cells with a cDNA encoding Mgat5. Inhibition of the synthesis of β1,6-branched N-glycans by swainsonine had no effect on Golgi volume density. In addition, no effect on Golgi volume density was observed in CHO Lec1 cells that contain enzymatically active GlcNAcT-V, but cannot synthesize β1,6-branched glycans due to an inactive GlcNAcT-I in their Golgi apparatus. These results indicate that it may be the absence of the GlcNAcT-V protein that is the determining factor in reducing Golgi volume density. No dimensional differences existed in cross-sectioned cisternal stacks between Lec4 and control CHO cells, but significantly reduced Golgi stack hits were observed in cross-sectioned Lec4 cells. Therefore, the Golgi apparatus dimensional change in Lec4 and Lec4A cells may be due to a compaction of the organelle.

Intracellular trafficking and secretion of inflammatory cytokines

The secretion of cytokines by immune cells plays a significant role in determining the course of an inflammatory response. The levels and timing of each cytokine released are critical for mounting an effective but confined response, whereas excessive or dysregulated inflammation contributes to many diseases. Cytokines are both culprits and targets for effective treatments in some diseases. The multiple points and mechanisms that have evolved for cellular control of cytokine secretion highlight the potency of these mediators and the fine tuning required to manage inflammation. Cytokine production in cells is regulated by cell signaling, and at mRNA and protein synthesis levels. Thereafter, the intracellular transport pathways and molecular trafficking machinery have intricate and essential roles in dictating the release and activity of cytokines. The trafficking machinery and secretory (exocytic) pathways are complex and highly regulated in many cells, involving specialized membranes, molecules and organelles that enable these cells to deliver cytokines to often-distinct areas of the cell surface, in a timely manner. This review provides an overview of secretory pathways - both conventional and unconventional - and key families of trafficking machinery. The prevailing knowledge about the trafficking and secretion of a number of individual cytokines is also summarized. In conclusion, we present emerging concepts about the functional plasticity of secretory pathways and their modulation for controlling cytokines and inflammation.

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.

Golgi disruption and early embryonic lethality in mice lacking USO1

Golgins are a family of long rod-like proteins characterized by the presence of central coiled-coil domains. Members of the golgin family have important roles in membrane trafficking, where they function as tethering factors that capture transport vesicles and facilitate membrane fusion. Golgin family members also have essential roles in maintaining the organization of the Golgi apparatus. Knockdown of individual golgins in cultured cells resulted in the disruption of the Golgi structure and the dispersal of Golgi marker proteins throughout the cytoplasm. However, these cellular phenotypes have not always been recapitulated in vivo. For example, embryonic development proceeds much further than expected and Golgi disruption was observed in only a subset of cell types in mice lacking the ubiquitously expressed golgin GMAP-210. Cell-type specific functional compensation among golgins may explain the absence of global cell lethality when a ubiquitously expressed golgin is missing. In this study we show that functional compensation does not occur for the golgin USO1. Mice lacking this ubiquitously expressed protein exhibit disruption of Golgi structure and early embryonic lethality, indicating that USO1 is indispensable for early embryonic development.

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.

Proteomic analysis of hepatitis C virus (HCV) core protein transfection and host regulator PA28γ knockout in HCV pathogenesis: a network-based study

Hepatitis C virus (HCV) causes chronic liver disease worldwide. HCV Core protein (Core) forms the viral capsid and is crucial for HCV pathogenesis and HCV-induced hepatocellular carcinoma, through its interaction with the host factor proteasome activator PA28γ. Here, using BD-PowerBlot high-throughput Western array, we attempt to further investigate HCV pathogenesis by comparing the protein levels in liver samples from Core-transgenic mice with or without the knockout of PA28γ expression (abbreviated PA28γ(-/-)CoreTG and CoreTG, respectively) against the wild-type (WT). The differentially expressed proteins integrated into the human interactome were shown to participate in compact and well-connected cellular networks. Functional analysis of the interaction networks using a newly developed data warehouse system highlighted cellular pathways associated with vesicular transport, immune system, cellular adhesion, and cell growth and death among others that were prominently influenced by Core and PA28γ in HCV infection. Follow-up assays with in vitro HCV cell culture systems validated VTI1A, a vesicular transport associated factor, which was upregulated in CoreTG but not in PA28γ(-/-)CoreTG, as a novel regulator of HCV release but not replication. Our analysis provided novel insights into the Core-PA28γ interplay in HCV pathogenesis and identified potential targets for better anti-HCV therapy and potentially novel biomarkers of HCV infection.