Cell death induced by nicotine in human neuroblastoma SH-SY5Y cells is mainly attributed to cytoplasmic vacuolation originating from the trans-Golgi network

Humans are usually exposed to nicotine through the use of tobacco products. Although it is generally believed that nicotine is relatively harmless in tobacco consumption, it is, in fact, a toxic substance that warrants careful consideration of its potential toxicity. However, the current understanding of the neurotoxicity of nicotine is still very limited. In this study, we aim to reveal the toxic risk of nicotine to key target neuronal cells and its potential toxic mechanisms. The results showed that nicotine induced cell death, ROS increase, mitochondrial membrane potential decrease, and DNA damage in SH-SY5Y human neuroblastoma cells at millimolar concentrations, but did not cause toxic effects at the physiological concentration. These toxic effects were accompanied by cytoplasmic vacuolation. The inhibition of cytoplasmic vacuolation by bafilomycin A1 greatly reduced nicotine-induced cell death, indicating that cytoplasmic vacuolation is the key driving factor of cell death. These cytoplasmic vacuoles originated from the trans-Golgi network (TGN) and expressed microtubule-associated protein 1 light chain 3-II (LC3-II) and lysosomal associated membrane protein 1(LAMP1). The presence of LC3-II and LAMP1 within these vacuoles serves as evidence of compromised TGN structure and function. These findings provide valuable new insights into the potential neurotoxic risk and mechanisms of nicotine.

Copper-independent lysosomal localisation of the Wilson disease protein ATP7B

In hepatocytes, the Wilson disease protein ATP7B resides on the trans-Golgi network (TGN) and traffics to peripheral lysosomes to export excess intracellular copper through lysosomal exocytosis. We found that in basal copper or even upon copper chelation, a significant amount of ATP7B persists in the endolysosomal compartment of hepatocytes but not in non-hepatic cells. These ATP7B-harbouring lysosomes lie in close proximity of ~10 nm to the TGN. ATP7B constitutively distributes itself between the sub-domain of the TGN with a lower pH and the TGN-proximal lysosomal compartments. The presence of ATP7B on TGN-lysosome colocalising sites upon Golgi disruption suggested a possible exchange of ATP7B directly between the TGN and its proximal lysosomes. Manipulating lysosomal positioning significantly alters the localisation of ATP7B in the cell. Contrary to previous understanding, we found that upon copper chelation in a copper-replete hepatocyte, ATP7B is not retrieved back to TGN from peripheral lysosomes; rather, ATP7B recycles to these TGN-proximal lysosomes to initiate the next cycle of copper transport. We report a hitherto unknown copper-independent lysosomal localisation of ATP7B and the importance of TGN-proximal lysosomes but not TGN as the terminal acceptor organelle of ATP7B in its retrograde pathway.

Membrane trafficking in breast cancer progression: protein kinase D comes into play

Protein kinase D (PKD) is a serine/threonine kinase family that controls important cellular functions, most notably playing a key role in the secretory pathway at the trans-Golgi network. Aberrant expression of PKD isoforms has been found mainly in breast cancer, where it promotes various cellular processes such as growth, invasion, survival and stem cell maintenance. In this review, we discuss the isoform-specific functions of PKD in breast cancer progression, with a particular focus on how the PKD controlled cellular processes might be linked to deregulated membrane trafficking and secretion. We further highlight the challenges of a therapeutic approach targeting PKD to prevent breast cancer progression.

The clathrin adaptor complex-1 and Rab12 regulate post-golgi trafficking of WT epidermal growth factor receptor (EGFR)

The epidermal growth factor receptor (EGFR) plays important roles in cancer progression and is one of the major drug targets for targeted cancer therapy. Although fundamentally important, how newly synthesized EGFR is delivered to the cell surface to perform its cellular functions remains to be further investigated. In this study, we found using the approaches of gene knockout, siRNA knockdown, streptavidin pull-down, and co-immunoprecipitation assays that the clathrin adaptor complex-1 (AP-1) and Rab12 interact with EGFR and regulate the export of EGFR out of the trans-Golgi network (TGN). In addition, the tyrosine residue at the 998 position on human EGFR is critical to bind to AP-1, and this residue is important for TGN export of EGFR. We demonstrate that AP-1 and Rab12 are important for epidermal growth factor-induced phosphorylation of EGFR, cell elongation, and proliferation, suggesting that AP-1-mediated and Rab12-mediated post-Golgi trafficking is important for EGFR signaling. Moreover, TGN export of the constitutively activated mutant form of EGFR (EGFR^L858R) is independent of AP-1 and Rab12. Our results reveal insights into the molecular mechanisms that mediate the TGN-to-cell surface delivery of EGFR and indicate that TGN export of WT EGFR and EGFR^L858R depends on different cellular factors.

The sorting of cargo proteins in the plant trans-Golgi network

Membrane trafficking contributes to distinct protein compositions of organelles and is essential for proper organellar maintenance and functions. The trans-Golgi network (TGN) acts as a sorting station where various cargo proteins are sorted and directed to post-Golgi compartments, such as the multivesicular body or pre-vacuolar compartment, vacuoles, and plasma membrane. The spatial and temporal segregation of cargo proteins within the TGN, which is mediated with different sets of regulators including small GTPases and cargo adaptors, is a fundamental process in the sorting machinery. Recent studies with powerful imaging technologies have suggested that the TGN possesses spatially distinct subdomains or zones for different trafficking pathways. In this review, we will summarize the spatially and dynamically characteristic features of the plant TGN and their relation to cargo protein trafficking.

Human Cytomegalovirus Manipulates Syntaxin 6 for Successful Trafficking and Subsequent Infection of Monocytes

Human cytomegalovirus (HCMV) exhibits a complex host-pathogen interaction with peripheral blood monocytes. We have identified a unique, cell-type specific retrograde-like intracellular trafficking pattern that HCMV utilizes to gain access to the monocyte nucleus and for productive infection. We show that infection of primary human monocytes, epithelial cells, and fibroblasts leads to an increase in the amount of the trafficking protein Syntaxin 6 (Stx6). However, only knockdown (KD) of Stx6 in monocytes inhibited viral trafficking to the trans-Golgi network (TGN), a requisite step for nuclear translocation in monocytes. Conversely, KD of Stx6 in epithelial cells and fibroblasts did not change the kinetics of nuclear translocation and productive infection. Stx6 predominantly functions at the level of the TGN where it facilitates retrograde transport, a trafficking pathway used by only a few cellular proteins and seldom by pathogens. We also newly identify that in monocytes, Stx6 exhibits an irregular vesicular localization rather than being concentrated at the TGN as seen in other cell-types. Lastly, we implicate that viral particles that associate with both Stx6 and EEA1 early in infection are the viral population that successfully traffics to the TGN at later time points and undergo nuclear translocation. Additionally, we show for the first time that HCMV enters the TGN, and that lack of Stx6 prevents viral trafficking to this organelle. We argue that we have identified an essential cell-type specific regulator that controls early steps in efficient productive infection of a cell-type required for viral persistence and disease. IMPORTANCE Human cytomegalovirus (HCMV) infection causes severe and often fatal disease in the immunocompromised. It is one of the leading infectious causes of birth defects and causes severe complications in transplant recipients. By uncovering the unique pathways used by the virus to infect key cells, such as monocytes, responsible for dissemination and persistence, we provide new potential targets for therapeutic intervention.

Plant cytokinesis and the construction of new cell wall

Cytokinesis in plants is fundamentally different from that in animals and fungi. In plant cells, a cell plate forms through the fusion of cytokinetic vesicles and then develops into the new cell wall, partitioning the cytoplasm of the dividing cell. The formation of the cell plate entails multiple stages that involve highly orchestrated vesicle accumulation, fusion, and membrane maturation, which occur concurrently with the timely deposition of polysaccharides such as callose, cellulose, and cross-linking glycans. This review summarizes the major stages in cytokinesis, endomembrane components involved in cell plate assembly and its transition to a new cell wall. An animation that can be widely used for educational purposes further summarizes the process.

Plant endosomes as protein sorting hubs

Endocytosis, secretion, and endosomal trafficking are key cellular processes that control the composition of the plasma membrane. Through the coordination of these trafficking pathways, cells can adjust the composition, localization, and turnover of proteins and lipids in response to developmental or environmental cues. Upon being incorporated into vesicles and internalized through endocytosis, plant plasma membrane proteins are delivered to the trans-Golgi network (TGN). At the TGN, plasma membrane proteins are recycled back to the plasma membrane or transferred to multivesicular endosomes (MVEs), where they are further sorted into intralumenal vesicles for degradation in the vacuole. Both types of plant endosomes, TGN and MVEs, act as sorting organelles for multiple endocytic, recycling, and secretory pathways. Molecular assemblies such as retromer, ESCRT (endosomal sorting complex required for transport) machinery, small GTPases, adaptor proteins, and SNAREs associate with specific domains of endosomal membranes to mediate different sorting and membrane-budding events. In this review, we discuss the mechanisms underlying the recognition and sorting of proteins at endosomes, membrane remodeling and budding, and their implications for cellular trafficking and physiological responses in plants.

AtFTCD-L, a trans-Golgi network localized protein, modulates root growth of Arabidopsis in high-concentration agar culture medium

AtFTCD-L protein is localized on the TGN vesicles in Arabidopsis root cap cells. AtFTCD-L mutation resulted in slow root growth of Arabidopsis in high-concentration agar culture medium. Arabidopsis formiminotransferase cyclodeaminase-like protein (AtFTCD-L) in Arabidopsis is homologous to the formiminotransferase cyclodeaminase (FTCD) protein in animal cells. However, the localization and function of AtFTCD-L remain unknown in Arabidopsis. In this study, we generated and analyzed a deletion mutant of AtFTCD-L with a T-DNA insertion. We found that the growth of Arabidopsis roots with the T-DNA insertion mutation in AtFTCD-L was slower than that of wild-type roots when grown in high-concentration 1/2 MS agar culture medium. AtFTCD-L-GFP could restore the ftcd-l mutant phenotype. In addition, the AtFTCD-L protein was localized on the trans-Golgi network (TGN) vesicles in Arabidopsis root cap cells. Fluorescence recovery after photobleaching (FRAP) experiment using Arabidopsis pollen-specific receptor-like kinase-GFP (AtPRK1-GFP) stably transformed plants showed that the deficiency of AtFTCD-L protein in Arabidopsis led to slower secretion in the root cap peripheral cells. The AtFTCD-L protein deficiency also resulted in a significantly reduced monosaccharides content in the culture medium. Based on the above results, we speculate that the AtFTCD-L protein may be involved in sorting and/or transportation of TGN vesicles in root cap peripheral cells, thereby regulating the extracellular secretion of mucilage components in the root cap.

Influenza A Virus Infection Activates NLRP3 Inflammasome through Trans-Golgi Network Dispersion

The NLRP3 inflammasome consists of NLRP3, ASC, and pro-caspase-1 and is an important arm of the innate immune response against influenza A virus (IAV) infection. Upon infection, the inflammasome is activated, resulting in the production of IL-1β and IL-18, which recruits other immune cells to the site of infection. It has been suggested that in the presence of stress molecules such as nigericin, the trans-Golgi network (TGN) disperses into small puncta-like structures where NLRP3 is recruited and activated. Here, we investigated whether IAV infection could lead to TGN dispersion, whether dispersed TGN (dTGN) is responsible for NLRP3 inflammasome activation, and which viral protein is involved in this process. We showed that the IAV causes dTGN formation, which serves as one of the mechanisms of NLRP3 inflammasome activation in response to IAV infection. Furthermore, we generated a series of mutant IAVs that carry mutations in the M2 protein. We demonstrated the M2 proton channel activity, specifically His37 and Trp41 are pivotal for the dispersion of TGN, NLRP3 conformational change, and IL-1β induction. The results revealed a novel mechanism behind the activation and regulation of the NLRP3 inflammasome in IAV infection.

Live-cell Imaging of Biosynthetic Protein Transport in Hepatocytes

Here, we describe a strategy to analyze the exit of apical and basolateral cargo from the trans-Golgi network in primary hepatocytes. The method is based on recombinant adenovirus-mediated infection combined with a pulse-chase regimen and live-cell imaging analysis of fluorescent protein-tagged dipeptidyl peptidase IV (DPPIV) and vesicular stomatitis virus G (VSVG) cargo proteins, coexpressed and accumulated in the endoplasmic reticulum via DPPIV aggregation through an engineered conditional aggregation domain and VSVG by exploiting the aggregation of the ts045 mutant at its non-permissive temperature of 40 °C.

NLRP3 cages revealed by full-length mouse NLRP3 structure control pathway activation

The NACHT-, leucine-rich-repeat- (LRR), and pyrin domain-containing protein 3 (NLRP3) is emerging to be a critical intracellular inflammasome sensor of membrane integrity and a highly important clinical target against chronic inflammation. Here, we report that an endogenous, stimulus-responsive form of full-length mouse NLRP3 is a 12- to 16-mer double-ring cage held together by LRR-LRR interactions with the pyrin domains shielded within the assembly to avoid premature activation. Surprisingly, this NLRP3 form is predominantly membrane localized, which is consistent with previously noted localization of NLRP3 at various membrane organelles. Structure-guided mutagenesis reveals that trans-Golgi network dispersion into vesicles, an early event observed for many NLRP3-activating stimuli, requires the double-ring cages of NLRP3. Double-ring-defective NLRP3 mutants abolish inflammasome punctum formation, caspase-1 processing, and cell death. Thus, our data uncover a physiological NLRP3 oligomer on the membrane that is poised to sense diverse signals to induce inflammasome activation.

Two NPF transporters mediate iron long-distance transport and homeostasis in Arabidopsis

Iron (Fe) transport and reallocation are essential to Fe homeostasis in plants, but it is unclear how Fe homeostasis is regulated, especially under stress. Here we report that NPF5.9 and its close homolog NPF5.8 redundantly regulate Fe transport and reallocation in Arabidopsis. NPF5.9 is highly upregulated in response to Fe deficiency. NPF5.9 expresses preferentially in vasculature tissues and localizes to the trans-Golgi network, and NPF5.8 showed a similar expression pattern. Long-distance Fe transport and allocation into aerial parts was significantly increased in NPF5.9-overexpressing lines. In the double mutant npf5.8 npf5.9, Fe loading in aerial parts and plant growth were decreased, which were partially rescued by Fe supplementation. Further analysis showed that expression of PYE, the negative regulator for Fe homeostasis, and its downstream target NAS4 were significantly altered in the double mutant. NPF5.9 and NPF5.8 were shown to also mediate nitrate uptake and transport, although nitrate and Fe application did not reciprocally affect each other. Our findings uncovered the novel function of NPF5.9 and NPF5.8 in long-distance Fe transport and homeostasis, and further indicated that they possibly mediate nitrate transport and Fe homeostasis independently in Arabidopsis.

SNARE proteins VAMP721 and VAMP722 mediate the post-Golgi trafficking required for auxin-mediated development in Arabidopsis

The plant hormone auxin controls many aspects of plant development. Membrane trafficking processes, such as secretion, endocytosis and recycling, regulate the polar localization of auxin transporters in order to establish an auxin concentration gradient. Here, we investigate the function of the Arabidopsis thaliana R-SNAREs VESICLE-ASSOCIATED MEMBRANE PROTEIN 721 (VAMP721) and VAMP722 in the post-Golgi trafficking required for proper auxin distribution and seedling growth. We show that multiple growth phenotypes, such as cotyledon development, vein patterning and lateral root growth, were defective in the double homozygous vamp721 vamp722 mutant. Abnormal auxin distribution and root patterning were also observed in the mutant seedlings. Fluorescence imaging revealed that three auxin transporters, PIN-FORMED 1 (PIN1), PIN2 and AUXIN RESISTANT 1 (AUX1), aberrantly accumulate within the cytoplasm of the double mutant, impairing the polar localization at the plasma membrane (PM). Analysis of intracellular trafficking demonstrated the involvement of VAMP721 and VAMP722 in the endocytosis of FM4-64 and the secretion and recycling of the PIN2 transporter protein to the PM, but not its trafficking to the vacuole. Furthermore, vamp721 vamp722 mutant roots display enlarged trans-Golgi network (TGN) structures, as indicated by the subcellular localization of a variety of marker proteins and the ultrastructure observed using transmission electron microscopy. Thus, our results suggest that the R-SNAREs VAMP721 and VAMP722 mediate the post-Golgi trafficking of auxin transporters to the PM from the TGN subdomains, substantially contributing to plant growth.

Inhibition of Very Long Chain Fatty Acids Synthesis Mediates PI3P Homeostasis at Endosomal Compartments

A main characteristic of sphingolipids is the presence of a very long chain fatty acid (VLCFA) whose function in cellular processes is not yet fully understood. VLCFAs of sphingolipids are involved in the intracellular traffic to the vacuole and the maturation of early endosomes into late endosomes is one of the major pathways for vacuolar traffic. Additionally, the anionic phospholipid phosphatidylinositol-3-phosphate (PtdIns (3)P or PI3P) is involved in protein sorting and recruitment of small GTPase effectors at late endosomes/multivesicular bodies (MVBs) during vacuolar trafficking. In contrast to animal cells, PI3P mainly localizes to late endosomes in plant cells and to a minor extent to a discrete sub-domain of the plant's early endosome (EE)/trans-Golgi network (TGN) where the endosomal maturation occurs. However, the mechanisms that control the relative levels of PI3P between TGN and MVBs are unknown. Using metazachlor, an inhibitor of VLCFA synthesis, we found that VLCFAs are involved in the TGN/MVB distribution of PI3P. This effect is independent from either synthesis of PI3P by PI3-kinase or degradation of PI(3,5)P₂ into PI3P by the SUPPRESSOR OF ACTIN1 (SAC1) phosphatase. Using high-resolution live cell imaging microscopy, we detected transient associations between TGNs and MVBs but VLCFAs are not involved in those interactions. Nonetheless, our results suggest that PI3P might be transferable from TGN to MVBs and that VLCFAs act in this process.

Differential contributions of choline phosphotransferases CPT1 and CEPT1 to the biosynthesis of choline phospholipid

Choline phospholipids (PLs) such as phosphatidylcholine (PC) and 1-alkyl-2-acyl-sn-glycerophosphocholine are important components for cell membranes and also serve as a source of several lipid mediators. These lipids are biosynthesized in mammals in the final step of the CDP-choline pathway by the choline phosphotransferases choline phosphotransferase 1 (CPT1) and choline/ethanolamine phosphotransferase 1 (CEPT1). However, the contributions of these enzymes to the de novo biosynthesis of lipids remain unknown. Here, we established and characterized CPT1- and CEPT1-deficient human embryonic kidney 293 cells. Immunohistochemical analyses revealed that CPT1 localizes to the trans-Golgi network and CEPT1 to the endoplasmic reticulum. Enzyme assays and metabolic labeling with radiolabeled choline demonstrated that loss of CEPT1 dramatically decreases choline PL biosynthesis. Quantitative PCR and reintroduction of CPT1 and CEPT1 revealed that the specific activity of CEPT1 was much higher than that of CPT1. LC-MS/MS analysis of newly synthesized lipid molecular species from deuterium-labeled choline also showed that these enzymes have similar preference for the synthesis of PC molecular species, but that CPT1 had higher preference for 1-alkyl-2-acyl-sn-glycerophosphocholine with PUFA than did CEPT1. The endogenous level of PC was not reduced by the loss of these enzymes. However, several 1-alkyl-2-acyl-sn-glycerophosphocholine molecular species were reduced in CPT1-deficient cells and increased in CEPT1-deficient cells when cultured in 0.1% FBS medium. These results suggest that CEPT1 accounts for most choline PL biosynthesis activity, and that both enzymes are responsible for the production of different lipid molecular species in distinct organelles.

Recombinant SARS-CoV-2 envelope protein traffics to the trans-Golgi network following amphipol-mediated delivery into human cells

The severe acute respiratory syndrome coronavirus 2 envelope protein (S2-E) is a conserved membrane protein that is important for coronavirus (CoV) assembly and budding. Here, we describe the recombinant expression and purification of S2-E in amphipol-class amphipathic polymer solutions, which solubilize and stabilize membrane proteins, but do not disrupt membranes. We found that amphipol delivery of S2-E to preformed planar bilayers results in spontaneous membrane integration and formation of viroporin cation channels. Amphipol delivery of the S2-E protein to human cells results in plasma membrane integration, followed by retrograde trafficking to the trans-Golgi network and accumulation in swollen perinuclear lysosomal-associated membrane protein 1-positive vesicles, likely lysosomes. CoV envelope proteins have previously been proposed to manipulate the luminal pH of the trans-Golgi network, which serves as an accumulation station for progeny CoV particles prior to cellular egress via lysosomes. Delivery of S2-E to cells will enable chemical biological approaches for future studies of severe acute respiratory syndrome coronavirus 2 pathogenesis and possibly even development of "Trojan horse" antiviral therapies. Finally, this work also establishes a paradigm for amphipol-mediated delivery of membrane proteins to cells.

Regulation of mTORC1 activity by the Golgi apparatus

Mechanistic (or mammalian) target of rapamycin complex 1 (mTORC1) is a major signalling kinase in cells that regulates proliferation and metabolism and is controlled by extrinsic and intrinsic signals. The lysosome has received considerable attention as a major hub of mTORC1 activation. However, mTOR has also been located to a variety of other intracellular sites, indicating the possibility of spatial regulation of mTORC1 signalling within cells. In particular, there have been numerous recent reports of mTORC1 activation associated with the Golgi apparatus. Here, we review the evidence for the regulation of mTORC1 signalling at the Golgi in mammalian cells. mTORC1 signalling is closely linked to the morphology of the Golgi architecture; a number of Golgi membrane tethers/scaffolds that influence Golgi architecture in mammalian cells that directly or indirectly regulate mTORC1 activation have been identified. Perturbation of the Golgi mTORC1 pathway arising from fragmentation of the Golgi has been shown to promote oncogenesis. Here, we highlight the potential mechanisms for the activation mTORC1 at the Golgi, which is emerging as a major site for mTORC1 signalling.

STING Operation at the ER/Golgi Interface

DNA is present in the nucleus and mitochondria of eukaryotic cells. There are, however, certain instances in which DNA emerges in the cytosol. The two major sources of cytosolic DNA are self DNA that is leaked out from the nucleus or mitochondria, and non-self DNA from DNA viruses. The cytosolic DNA triggers the host immune response. Recent studies have identified two key molecules, cyclic GMP-AMP (cGAMP) synthase (cGAS) and stimulator of interferon genes (STING) in this immune response. STING is an endoplasmic reticulum (ER) protein. After STING binding to cGAMP, STING exits the ER and translocates to the Golgi, where STING triggers the type I interferon- and proinflammatory responses through the activation of interferon regulatory factor 3 (IRF3) and nuclear factor-kappa B (NF-κB). STING also activates other cellular responses including cell senescence, autophagy, and cell death. In this review, we focus on emerging issues regarding the regulation of STING by membrane traffic, with a particular focus on the retrograde membrane traffic from the Golgi to the ER. The retrograde membrane traffic is recently shown by us and others to be critical for silencing the STING signaling pathway and the defect in this traffic underlies the pathogenesis of the COPA syndrome, a monogenic autoinflammatory disease caused by missense mutations of coatomer protein complex subunit α (COP-α).

Subcellular coordination of plant cell wall synthesis

Organelles of the plant cell cooperate to synthesize and secrete a strong yet flexible polysaccharide-based extracellular matrix: the cell wall. Cell wall composition varies among plant species, across cell types within a plant, within different regions of a single cell wall, and in response to intrinsic or extrinsic signals. This diversity in cell wall makeup is underpinned by common cellular mechanisms for cell wall production. Cellulose synthase complexes function at the plasma membrane and deposit their product into the cell wall. Matrix polysaccharides are synthesized by a multitude of glycosyltransferases in hundreds of mobile Golgi stacks, and an extensive set of vesicle trafficking proteins govern secretion to the cell wall. In this review, we discuss the different subcellular locations at which cell wall synthesis occurs, review the molecular mechanisms that control cell wall biosynthesis, and examine how these are regulated in response to different perturbations to maintain cell wall homeostasis.

Modulation of Endosome Function, Vesicle Trafficking and Autophagy by Human Herpesviruses

Human herpesviruses are a ubiquitous family of viruses that infect individuals of all ages and are present at a high prevalence worldwide. Herpesviruses are responsible for a broad spectrum of diseases, ranging from skin and mucosal lesions to blindness and life-threatening encephalitis, and some of them, such as Kaposi’s sarcoma-associated herpesvirus (KSHV) and Epstein–Barr virus (EBV), are known to be oncogenic. Furthermore, recent studies suggest that some herpesviruses may be associated with developing neurodegenerative diseases. These viruses can establish lifelong infections in the host and remain in a latent state with periodic reactivations. To achieve infection and yield new infectious viral particles, these viruses require and interact with molecular host determinants for supporting their replication and spread. Important sets of cellular factors involved in the lifecycle of herpesviruses are those participating in intracellular membrane trafficking pathways, as well as autophagic-based organelle recycling processes. These cellular processes are required by these viruses for cell entry and exit steps. Here, we review and discuss recent findings related to how herpesviruses exploit vesicular trafficking and autophagy components by using both host and viral gene products to promote the import and export of infectious viral particles from and to the extracellular environment. Understanding how herpesviruses modulate autophagy, endolysosomal and secretory pathways, as well as other prominent trafficking vesicles within the cell, could enable the engineering of novel antiviral therapies to treat these viruses and counteract their negative health effects.

Expression of a Brassica napus metal transport protein (BnMTP3) in Arabidopsis thaliana confers tolerance to Zn and Mn

The essential micronutrient elements zinc (Zn) and manganese (Mn) are crucial for plant growth and development. As an important oil crop, the yield and quality of rapeseed are affected by Zn and Mn toxicity. The cation diffusion facilitator (CDF) family of proteins play significant roles in maintaining intracellular ionic homeostasis and tolerance in plants. However, research on CDF proteins in rapeseed is lacking. In this study, the function of a Brassica napus cation diffusion facilitator/ metal tolerance protein (CDF/MTP) was investigated. The protein, abbreviated BnMTP3 is homologous to the Arabidopsis thaliana MTP3 (AtMTP3). Heterologous expression of BnMTP3 in yeast enhanced tolerance and intracellular sequestration of Zn and Mn. Expression of BnMTP3 in A. thaliana increased Zn and Mn tolerance and markedly increased Zn accumulation in roots. Quantitative RT-PCR analysis showed that BnMTP3 is primarily expressed in roots, and subcellular localization suggested that BnMTP3 is localized in the trans-Golgi network (TGN) and the prevacuolar compartment (PVC) in Arabidopsis and rape protoplast. After treatment with Zn and Mn, BnMTP3 was observed on the vacuolar membrane in transgenic Arabidopsis lines. These findings suggest that BnMTP3 confers Zn and Mn tolerance by sequestering Zn and/or Mn into the vacuole.

A role for Rab30 in retrograde trafficking and maintenance of endosome-TGN organization

Rab30 is a poorly characterized small GTPase. Here we show that Rab30 is localised primarily to the TGN and recycling endosomes in a range of cell types, including primary neurons; minor levels of Rab30 were also detected throughout the Golgi stack and early endosomes. Silencing of Rab30 resulted in the dispersal of both early and recycling endosomes and TGN compartments in HeLa cells. By analyzing cargo trafficking in Rab30-silenced and Rab30-overexpressing HeLa cells, we demonstrate that Rab30 plays a role in retrograde trafficking of TGN38 from endosomes to the Golgi, but has no apparent role in the endocytic recycling of the transferrin receptor to the plasma membrane. Five interactive partners with Rab30 were identified by pull-down and MS analysis using GFP-tagged Rab30 mutant, Rab30(Q68L). Two of the interactive partners identified were Arf1 and Arf4, known regulators of endosome to TGN retrograde transport. Knockdown of Arf1 and Arf4 results in GFP-Rab30 decorated tubules arising from the recycling endosomes, suggesting association of Rab30 with tubular carriers. Overall our data demonstrates a role for Rab30 in regulating retrograde transport to the TGN and maintenance of endosomal-TGN organization.

Rab6A as a Pan-Astrocytic Marker in Mouse and Human Brain, and Comparison with Other Glial Markers (GFAP, GS, Aldh1L1, SOX9)

Astrocytes contribute to many higher brain functions. A key mechanism in glia-to-neuron signalling is vesicular exocytosis; however, the identity of exocytosis organelles remains a matter of debate. Since vesicles derived from the trans-Golgi network (TGN) are not considered in this context, we studied the astrocyte TGN by immunocytochemistry applying anti-Rab6A. In mouse brain, Rab6A immunostaining is found to be unexpectedly massive, diffuse in all regions, and is detected preferentially and abundantly in the peripheral astrocyte processes, which is hardly evident without glial fibrillary acid protein (GFAP) co-staining. All cells positive for the astrocytic markers glutamine synthetase (GS), GFAP, aldehyde dehydrogenase 1 family member L1 (Aldh1L1), or SRY (sex determining region Y)-box 9 (SOX9) were Rab6A+. Rab6A is excluded from microglia, oligodendrocytes, and NG2 cells using cell type-specific markers. In human cortex, Rab6A labelling is very similar and associated with GFAP+ astrocytes. The mouse data also confirm the specific astrocytic labelling by Aldh1L1 or SOX9; the astrocyte-specific labelling by GS sometimes debated is replicated again. In mouse and human brain, individual astrocytes display high variability in Rab6A+ structures, suggesting dynamic regulation of the glial TGN. In summary, Rab6A expression is an additional, global descriptor of astrocyte identity. Rab6A might constitute an organelle system with a potential role of Rab6A in neuropathological and physiological processes.

Micromanipulation of amyloplasts with optical tweezers in Arabidopsis stems

Intracellular sedimentation of highly dense, starch-filled amyloplasts toward the gravity vector is likely a key initial step for gravity sensing in plants. However, recent live-cell imaging technology revealed that most amyloplasts continuously exhibit dynamic, saltatory movements in the endodermal cells of Arabidopsis stems. These complicated movements led to questions about what type of amyloplast movement triggers gravity sensing. Here we show that a confocal microscope equipped with optical tweezers can be a powerful tool to trap and manipulate amyloplasts noninvasively, while simultaneously observing cellular responses such as vacuolar dynamics in living cells. A near-infrared (λ=1064 nm) laser that was focused into the endodermal cells at 1 mW of laser power attracted and captured amyloplasts at the laser focus. The optical force exerted on the amyloplasts was theoretically estimated to be up to 1 pN. Interestingly, endosomes and trans-Golgi network were trapped at 30 mW but not at 1 mW, which is probably due to lower refractive indices of these organelles than that of the amyloplasts. Because amyloplasts are in close proximity to vacuolar membranes in endodermal cells, their physical interaction could be visualized in real time. The vacuolar membranes drastically stretched and deformed in response to the manipulated movements of amyloplasts by optical tweezers. Our new method provides deep insights into the biophysical properties of plant organelles in vivo and opens a new avenue for studying gravity-sensing mechanisms in plants.

The ARFRP1-dependent Golgi scaffolding protein GOPC is required for insulin secretion from pancreatic β-cells

Objective

Hormone secretion from metabolically active tissues, such as pancreatic islets, is governed by specific and highly regulated signaling pathways. Defects in insulin secretion are among the major causes of diabetes. The molecular mechanisms underlying regulated insulin secretion are, however, not yet completely understood. In this work, we studied the role of the GTPase ARFRP1 on insulin secretion from pancreatic β-cells.

Methods

A β-cell-specific Arfrp1 knockout mouse was phenotypically characterized. Pulldown experiments and mass spectrometry analysis were employed to screen for new ARFRP1-interacting proteins. Co-immunoprecipitation assays as well as super-resolution microscopy were applied for validation.

Results

The GTPase ARFRP1 interacts with the Golgi-associated PDZ and coiled-coil motif-containing protein (GOPC). Both proteins are co-localized at the trans-Golgi network and regulate the first and second phase of insulin secretion by controlling the plasma membrane localization of the SNARE protein SNAP25. Downregulation of both GOPC and ARFRP1 in Min6 cells interferes with the plasma membrane localization of SNAP25 and enhances its degradation, thereby impairing glucose-stimulated insulin release from β-cells. In turn, overexpression of SNAP25 as well as GOPC restores insulin secretion in islets from β-cell-specific Arfrp1 knockout mice.

Conclusion

Our results identify a hitherto unrecognized pathway required for insulin secretion at the level of trans-Golgi sorting.

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β-cell specific deletion of the trans-Golgi residing small GTPase ARFRP1 leads to elevated blood glucose levels in mice.

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GOPC is a newly identified ARFRP1 dependent scaffolding protein.

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ARFRP1 and GOPC are required for glucose-stimulated insulin secretion from pancreatic β-cells.

LRRK2 and the Endolysosomal System in Parkinson's Disease

Mutations in leucine-rich repeat kinase 2 (LRRK2) cause autosomal dominant familial Parkinson’s disease (PD), with pathogenic mutations enhancing LRRK2 kinase activity. There is a growing body of evidence indicating that LRRK2 contributes to neuronal damage and pathology both in familial and sporadic PD, making it of particular interest for understanding the molecular pathways that underlie PD. Although LRRK2 has been extensively studied to date, our understanding of the seemingly diverse functions of LRRK2 throughout the cell remains incomplete. In this review, we discuss the functions of LRRK2 within the endolysosomal pathway. Endocytosis, vesicle trafficking pathways, and lysosomal degradation are commonly disrupted in many neurodegenerative diseases, including PD. Additionally, many PD-linked gene products function in these intersecting pathways, suggesting an important role for the endolysosomal system in maintaining protein homeostasis and neuronal health in PD. LRRK2 activity can regulate synaptic vesicle endocytosis, lysosomal function, Golgi network maintenance and sorting, vesicular trafficking and autophagy, with alterations in LRRK2 kinase activity serving to disrupt or regulate these pathways depending on the distinct cell type or model system. LRRK2 is critically regulated by at least two proteins in the endolysosomal pathway, Rab29 and VPS35, which may serve as master regulators of LRRK2 kinase activity. Investigating the function and regulation of LRRK2 in the endolysosomal pathway in diverse PD models, especially in vivo models, will provide critical insight into the cellular and molecular pathophysiological mechanisms driving PD and whether LRRK2 represents a viable drug target for disease-modification in familial and sporadic PD.

The Vps52 subunit of the GARP and EARP complexes is a novel Arf6-interacting protein that negatively regulates neurite outgrowth of hippocampal neurons

ADP ribosylation factor 6 (Arf6) is a small GTP-binding protein implicated in neuronal morphogenesis through endosomal trafficking and actin remodeling. In this study, we identified Vps52, a core subunit of the Golgi-associated retrograde protein (GARP) and endosome-associated recycling protein (EARP) complexes, as a novel Arf6-binding protein by yeast two-hybrid screening. Vps52 interacted specifically with GTP-bound Arf6 among the Arf family. Immunohistochemical analyses of hippocampal pyramidal cells revealed that fine punctate immunolabeling for Vps52 was distributed throughout neuronal compartments, most densely in the cell body and dendritic shafts, and was largely associated with trans-Golgi network and vesicular endomembranes. In cultured hippocampal neurons, knockdown of Vps52 increased total length of axons and dendrites; these phenotypes were completely restored by co-expression of shRNA-resistant full-length Vps52. However, co-expression of a Vps52 mutant lacking the ability to interact with Arf6 restored only the Vps52-knockdown phenotype of the dendritic length. The present findings suggest that Vps52 is a novel Arf6-interacting protein that regulates neurite outgrowth in hippocampal neurons.

EPSIN1 and MTV1 define functionally overlapping but molecularly distinct trans-Golgi network subdomains in Arabidopsis

The plant trans-Golgi network (TGN) is a central trafficking hub where secretory, vacuolar, recycling, and endocytic pathways merge. Among currently known molecular players involved in TGN transport, three different adaptor protein (AP) complexes promote vesicle generation at the TGN with different cargo specificity and destination. Yet, it remains unresolved how sorting into diverging vesicular routes is spatially organized. Here, we study the family of Arabidopsis thaliana Epsin-like proteins, which are accessory proteins to APs facilitating vesicle biogenesis. By comprehensive molecular, cellular, and genetic analysis of the EPSIN gene family, we identify EPSIN1 and MODIFIED TRANSPORT TO THE VACUOLE1 (MTV1) as its only TGN-associated members. Despite their large phylogenetic distance, they perform overlapping functions in vacuolar and secretory transport. By probing their relationship with AP complexes, we find that they define two molecularly independent pathways: While EPSIN1 associates with AP-1, MTV1 interacts with AP-4, whose function is required for MTV1 recruitment. Although both EPSIN1/AP-1 and MTV1/AP-4 pairs reside at the TGN, high-resolution microscopy reveals them as spatially separate entities. Our results strongly support the hypothesis of molecularly, functionally, and spatially distinct subdomains of the plant TGN and suggest that functional redundancy can be achieved through parallelization of molecularly distinct but functionally overlapping pathways.

Early Endosomal Trafficking Component BEN2/VPS45 Plays a Crucial Role in Internal Tissues in Regulating Root Growth and Meristem Size in Arabidopsis

Polar auxin transport is involved in multiple aspects of plant development, including root growth, lateral root branching, embryogenesis, and vasculature development. PIN-FORMED (PIN) auxin efflux proteins exhibit asymmetric distribution at the plasma membrane (PM) and collectively play pivotal roles in generating local auxin accumulation, which underlies various auxin-dependent developmental processes. In previous research, it has been revealed that endosomal trafficking components BEN1/BIG5 (ARF GEF) and BEN2/VPS45 (Sec1/Munc 18 protein) function in intracellular trafficking of PIN proteins in Arabidopsis. Mutations in both BEN1 and BEN2 resulted in defects in polar PIN localization, auxin response gradients, and in root architecture. In this study, we have attempted to gain insight into the developmental roles of these trafficking components. We showed that while genetic or pharmacological disturbances of auxin distribution reduced dividing cells in the root tips and resulted in reduced root growth, the same manipulations had only moderate impact on ben1; ben2 double mutants. In addition, we established transgenic lines in which BEN2/VPS45 is expressed under control of tissue-specific promoters and demonstrated that BEN2/VPS45 regulates the intracellular traffic of PIN proteins in cell-autonomous manner, at least in stele and epidermal cells. Furthermore, BEN2/VPS45 rescued the root architecture defects when expressed in internal tissues of ben1; ben2 double mutants. These results corroborate the roles of the endosomal trafficking component BEN2/VPS45 in regulation of auxin-dependent developmental processes, and suggest that BEN2/VPS45 is required for sustainable root growth, most likely through regulation of tip-ward auxin transport through the internal tissues of root.

An update on cellular and molecular determinants of Parkinson's disease with emphasis on the role of the retromer complex

Parkinson's disease (PD) is a highly prevalent neurodegenerative condition. The disease involves the progressive degeneration of dopaminergic neurons located in the substantia nigra pars compacta. Among late-onset, familial forms of Parkinson are cases with mutations in the PARK17 locus encoding the vacuolar protein sorting 35 (Vps35), a subunit of the retromer complex. The retromer complex is composed of a heterotrimeric protein core (Vps26-Vps35-Vps29). The best-known role of retromer is the retrieval of cargoes from endosomes to the Golgi complex or the plasma membrane. However, recent literature indicates that retromer performs roles associated with lysosomal and mitochondrial functions and degradative pathways such as autophagy. A common point mutation affecting the retromer subunit Vps35 is D620N, which has been linked to the alterations in the aforementioned cellular processes as well as with neurodegeneration. Here, we review the main aspects of the malfunction of the retromer complex and its implications for PD pathology. Besides, we highlight several controversies still awaiting clarification.

Recycling endosomes associate with Golgi stacks in sea urchin embryos

The trans-Golgi network (TGN) and recycling endosome (RE) have been recognized as sorting centers, the former for newly synthesized and the latter for endocytosed proteins. However, recent findings have revealed that TGN also receives endocytosed materials and RE accepts newly synthesized proteins destined to the plasma membrane. Recently, we reported that in both Drosophila and microtubule-disrupted HeLa cells, REs are associated with the trans-side of Golgi stacks. REs are highly dynamic: their separation from and association with Golgi stacks are often observed. Importantly, a newly synthesized cargo, glycosylphosphatidylinositol-anchored-GFP was found to be concentrated in Golgi-associated REs (GA-REs), while another cargo VSVG-GFP was excluded from GA-REs before post-Golgi trafficking to the plasma membrane. This suggested that the sorting of cargos takes place at the interface of Golgi stacks and GA-REs. In this study, we demonstrated that REs could associate with Golgi stacks in sea urchin embryos, further indicating that the association of REs with Golgi stacks is a well-conserved phenomenon in the animal kingdom.

Different localization of lysosomal-associated membrane protein 1 (LAMP1) in mammalian cultured cell lines

Lysosomal-associated membrane protein 1 (LAMP1) mainly localizes to lysosomes and late endosomes. We herein investigated the intracellular localization of lysosomal membrane proteins in five mammalian cultured cell lines. Rat LAMP1 fused to enhanced green fluorescent protein (EGFP) mostly accumulated at a particular cytoplasmic area and barely co-localized with LysoTracker® Red DND-99 in golden hamster kidney BHK-21 cells and Chinese hamster ovary CHO-K1 cells. Golden hamster, Chinese hamster, and human LAMP1-EGFP showed a similar intracellular distribution to rat LAMP1-EGFP in BHK-21 cells. Endogenous LAMP1 was also detected in a perinuclear area in BHK-21 cells and CHO-K1 cells, and co-localized with rat CD63-EGFP in BHK-21 cells. Moreover, rat LAMP1-DsRed-Monomer co-localized well with the human trans-Golgi network protein 2-EGFP in BHK-21 cells. These results reveal that LAMP1 predominantly localizes to the trans-Golgi network in BHK-21 cells.

Differentiation of Trafficking Pathways at Golgi Entry Core Compartments and Post-Golgi Subdomains

Eukaryotic cells have developed specialized membrane structures called organelles, which compartmentalize cellular functions and chemical reactions. Recent improvements in microscopy and membrane compartment isolation techniques are now sophisticating our view. Emerging evidences support that there are distinct sub-populations or subdomains, which are spatially and/or temporally segregated within one type of organelle, contributing to specify differential sorting of various cargos to distinct destinations of the cell. In plant cells, the Golgi apparatus represents a main trafficking hub in which entry occurs through a Golgi Entry Core Compartment (GECCO), that remains to be further characterized, and sorting of cargos is mediated through multiple transport pathways with different sets of regulator proteins at the post-Golgi compartment trans-Golgi network (TGN). Both GECCO and TGN are differentiated sub-populations as compared to the rest of Golgi, and moreover, further subdomain formation within TGN is suggested to play a key role for cargo sorting. In this review, we will summarize recent findings obtained on organelle subdomains, and their relationship with cargo entry at and exit from the Golgi apparatus.

Chromogranin A in the early steps of the neurosecretory pathway

Chromogranin A (CgA) is a soluble glycoprotein stored with hormones and neuropeptides in secretory granules (SG) of most (neuro)endocrine cells and neurons. Since its discovery in 1967, many studies have reported its structural characteristics, biological roles, and mechanisms of action. Indeed, CgA is both a precursor of various biologically active peptides and a granulogenic protein regulating the storage and secretion of hormones and neuropeptides. This review emphasizes the findings and theoretical concepts around the CgA-linked molecular machinery controlling hormone/neuropeptide aggregation and the interaction of CgA-hormone/neuropeptide aggregates with the trans-Golgi membrane to allow hormone/neuropeptide targeting and SG biogenesis. We will also discuss the intriguing alteration of CgA expression and secretion in various neurological disorders, which could provide insights to elucidate the molecular mechanisms underlying these pathophysiological conditions.

Targeting γ-secretase triggers the selective enrichment of oligomeric APP-CTFs in brain extracellular vesicles from Alzheimer cell and mouse models

Background

We recently demonstrated an endolysosomal accumulation of the β-secretase-derived APP C-terminal fragment (CTF) C99 in brains of Alzheimer disease (AD) mouse models. Moreover, we showed that the treatment with the γ-secretase inhibitor (D6) led to further increased endolysosomal APP-CTF levels, but also revealed extracellular APP-CTF-associated immunostaining. We here hypothesized that this latter staining could reflect extracellular vesicle (EV)-associated APP-CTFs and aimed to characterize these γ-secretase inhibitor-induced APP-CTFs.

Methods

EVs were purified from cell media or mouse brains from vehicle- or D6-treated C99 or APP_swedish expressing cells/mice and analyzed for APP-CTFs by immunoblot. Combined pharmacological, immunological and genetic approaches (presenilin invalidation and C99 dimerization mutants (GXXXG)) were used to characterize vesicle-containing APP-CTFs. Subcellular APP-CTF localization was determined by immunocytochemistry.

Results

Purified EVs from both AD cell or mouse models were enriched in APP-CTFs as compared to EVs from control cells/brains. Surprisingly, EVs from D6-treated cells not only displayed increased C99 and C99-derived C83 levels but also higher molecular weight (HMW) APP-CTF-immunoreactivities that were hardly detectable in whole cell extracts. Accordingly, the intracellular levels of HMW APP-CTFs were amplified by the exosomal inhibitor GW4869. By combined pharmacological, immunological and genetic approaches, we established that these HMW APP-CTFs correspond to oligomeric APP-CTFs composed of C99 and/or C83. Immunocytochemical analysis showed that monomers were localized mainly to the trans-Golgi network, whereas oligomers were confined to endosomes and lysosomes, thus providing an anatomical support for the selective recovery of HMW APP-CTFs in EVs. The D6-induced APP-CTF oligomerization and subcellular mislocalization was indeed due to γ-secretase blockade, since it similarly occurred in presenilin-deficient fibroblasts. Further, our data proposed that besides favoring APP-CTF oligomerization by preventing C99 proteolysis, γ-secretase inhibiton also led to a defective SorLA-mediated retrograde transport of HMW APP-CTFs from endosomal compartments to the TGN.

Conclusions

This is the first study to demonstrate the presence of oligomeric APP-CTFs in AD mouse models, the levels of which are selectively enriched in endolysosomal compartments including exosomes and amplified by γ-secretase inhibition. Future studies should evaluate the putative contribution of these exosome-associated APP-CTFs in AD onset, progression and spreading.

Illuminating the membrane contact sites between the endoplasmic reticulum and the trans-Golgi network

Membrane contact sites (MCSs) between different organelles have been identified and extensively studied over the last decade. Several classes of MCSs have now well-established roles, although the contacts between the endoplasmic reticulum (ER) and the trans-side of the Golgi network (TGN) have long remained elusive. Until recently, the study of ER-TGN contact sites has represented a major challenge in the field, as a result of the lack of suitable visualization and isolation techniques. Only in the last 5 years has the combination of advanced technologies and innovative approaches permitted the identification of new molecular players and the functions of ER-TGN MCSs that couple lipid metabolism and anterograde transport. Although much has yet to be discovered, it is now established that ER-TGN MCSs control phosphatidyl-4-phosphate homeostasis by coupling the cis and the trans activity of the ER-resident 4-phosphatase Sac1. In this review, we focus on recent advances on the composition and function of ER-TGN MCSs.

Visualization of Protein Sorting at the Trans-Golgi Network and Endosomes Through Super-Resolution Imaging

The trans-Golgi network (TGN) and endosomes are essential protein sorting stations in the secretory transport pathway. Protein sorting is fundamentally a process of spatial segregation, but the spatial relationships among the proteins that constitute the sorting machinery have not been systematically analyzed at high resolution in mammalian cells. Here, using two-color STORM imaging, we show that the TGN/endosome-localized cargo adaptors, AP-1, GGA2 and epsinR, form elongated structures of over 250 nm in length at the juxta-nuclear Golgi area. Many of these structures are associated with clathrin. We found that AP-1 is spatially segregated from AP-3 and GGA2, whereas a fraction of AP-1 and GGA2 punctae are associated with epsinR. Moreover, we observed that the planar cell polarity cargo proteins, Vangl2 and Frizzled6 associate with different cargo adaptors—AP-1 and GGA2 or epsinR, respectively—when exiting the TGN. Knockdown analysis confirms the functional significance of this segregation. Our data indicates that TGN/endosome-localized cargo adaptors have distinct spatial relationships. The spatially segregated cargo adaptors GGA2 and AP-1 regulate sorting of Frizzled6 and Vangl2, respectively and spatially associated cargo adaptors can cooperatively regulate a specific sorting process.

Progeny Varicella-Zoster Virus Capsids Exit the Nucleus but Never Undergo Secondary Envelopment during Autophagic Flux Inhibition by Bafilomycin A1

Varicella-zoster virus (VZV) is an alphaherpesvirus that lacks the herpesviral neurovirulence protein ICP34.5. The underlying hypothesis of this project was that inhibitors of autophagy reduce VZV infectivity. We selected the vacuolar proton ATPase inhibitor bafilomycin A1 for analysis because of its well-known antiautophagy property of impeding acidification during the late stage of autophagic flux. We documented that bafilomycin treatment from 48 to 72 h postinfection lowered VZV titers substantially (P ≤ 0.008). Because we were unable to define the site of the block in the infectious cycle by confocal microscopy, we turned to electron microscopy. Capsids were observed in the nucleus, in the perinuclear space, and in the cytoplasm adjacent to Golgi apparatus vesicles. Many of the capsids had an aberrant appearance, as has been observed previously in infections not treated with bafilomycin. In contrast to prior untreated infections, however, secondary envelopment of capsids was not seen in the trans-Golgi network, nor were prototypical enveloped particles with capsids (virions) seen in cytoplasmic vesicles after bafilomycin treatment. Instead, multiple particles with varying diameters without capsids (light particles) were seen in large virus assembly compartments near the disorganized Golgi apparatus. Bafilomycin treatment also led to increased numbers of multivesicular bodies in the cytoplasm, some of which contained remnants of the Golgi apparatus. In summary, we have defined a previously unrecognized property of bafilomycin whereby it disrupted the site of secondary envelopment of VZV capsids by altering the pH of the trans-Golgi network and thereby preventing the correct formation of virus assembly compartments.IMPORTANCE This study of VZV assembly in the presence of bafilomycin A1 emphasizes the importance of the Golgi apparatus/trans-Golgi network as a platform in the alphaherpesvirus life cycle. We have previously shown that VZV induces levels of autophagy far above the basal levels of autophagy in human skin, a major site of VZV assembly. The current study documented that bafilomycin treatment led to impaired assembly of VZV capsids after primary envelopment/de-envelopment but before secondary reenvelopment. This VZV study also complemented prior herpes simplex virus 1 and pseudorabies virus studies investigating two other inhibitors of endoplasmic reticulum (ER)/Golgi apparatus function: brefeldin A and monensin. Studies with porcine herpesvirus demonstrated that primary enveloped particles accumulated in the perinuclear space in the presence of brefeldin A, while studies with herpes simplex virus 1 documented an impaired secondary assembly of enveloped viral particles in the presence of monensin.

Overexpression of trans-Golgi network t-SNAREs rescues vacuolar trafficking and TGN morphology defects in a putative tethering factor mutant

The trans-Golgi network (TGN) is a major site for sorting of cargo to either the vacuole or apoplast. The TGN-localized coiled-coil protein TNO1 is a putative tethering factor that interacts with the TGN t-SNARE SYP41 and is required for correct localization of the SYP61 t-SNARE. An Arabidopsis thaliana tno1 mutant is hypersensitive to salt stress and partially mislocalizes vacuolar proteins to the apoplast, indicating a role in vacuolar trafficking. Here, we show that overexpression of SYP41 or SYP61 significantly increases SYP41-SYP61 complex formation in a tno1 mutant, and rescues the salt sensitivity and defective vacuolar trafficking of the tno1 mutant. The TGN is disrupted and vesicle budding from Golgi cisternae is reduced in the tno1 mutant, and these defects are also rescued by overexpression of SYP41 or SYP61. Our results suggest that the trafficking and Golgi morphology defects caused by loss of TNO1 can be rescued by increasing SYP41-SYP61 t-SNARE complex formation, implicating TNO1 as a tethering factor mediating efficient vesicle fusion at the TGN.

Lipid-dependent coupling of secretory cargo sorting and trafficking at the trans-Golgi network

In eukaryotic cells, the trans-Golgi network (TGN) serves as a platform for secretory cargo sorting and trafficking. In recent years, it has become evident that a complex network of lipid–lipid and lipid–protein interactions contributes to these key functions. This review addresses the role of lipids at the TGN with a particular emphasis on sphingolipids and diacylglycerol. We further highlight how these lipids couple secretory cargo sorting and trafficking for spatiotemporal coordination of protein transport to the plasma membrane.

Cargo Sorting at the trans-Golgi Network for Shunting into Specific Transport Routes: Role of Arf Small G Proteins and Adaptor Complexes

The trans-Golgi network (TGN) is responsible for selectively recruiting newly synthesized cargo into transport carriers for delivery to their appropriate destination. In addition, the TGN is responsible for receiving and recycling cargo from endosomes. The membrane organization of the TGN facilitates the sorting of cargoes into distinct populations of transport vesicles. There have been significant advances in defining the molecular mechanism involved in the recognition of membrane cargoes for recruitment into different populations of transport carriers. This machinery includes cargo adaptors of the adaptor protein (AP) complex family, and monomeric Golgi-localized γ ear-containing Arf-binding protein (GGA) family, small G proteins, coat proteins, as well as accessory factors to promote budding and fission of transport vesicles. Here, we review this literature with a particular focus on the transport pathway(s) mediated by the individual cargo adaptors and the cargo motifs recognized by these adaptors. Defects in these cargo adaptors lead to a wide variety of diseases.

Yeast Dop1 is required for glycosyltransferase retrieval from the trans-Golgi network

BACKGROUND

Glycosyltransferases are type II membrane proteins that are responsible for glycan modification of proteins and lipids, and localize to distinct cisternae in the Golgi apparatus. During cisternal maturation, retrograde trafficking helps maintain the steady-state localization of these enzymes in the sub-compartments of the Golgi.

METHODS

To understand how glycosyltransferases are recycled in the late Golgi complex, we searched for genes that are essential for budding yeast cell growth and that encode proteins localized in endosomes and in the Golgi. We specifically analyzed the roles of Dop1 and its binding partner Neo1 in retaining Golgi-resident glycosyltransferases, in the late Golgi complex.

RESULTS

Dop1 primarily localized to younger compartments of the trans-Golgi network (TGN) and seemed to cycle within the TGN. In contrast, Neo1, a P4-ATPase that interacts with Dop1, localized to the TGN. Abolition of DOP1 expression led to defects in the FM4-64 endocytic pathway. Dop1 and Neo1 were required for correct glycosylation of invertase, a secretory protein, at the Golgi. In DOP1-shutdown cells, Och1, a mannosyltransferase that is typically located in the cis-Golgi, mislocalized to the TGN. In addition, the function of multiple glycosyltransferases required for N- and O-glycosylation were impaired in DOP1-shutdown cells.

CONCLUSIONS

Our results indicate that Dop1 is involved in vesicular transport at the TGN, and is critical for retrieving glycosyltransferases from the TGN to the Golgi in yeast.

GENERAL SIGNIFICANCE

Golgi-resident glycosyltransferases recycling from the TGN to the Golgi is dependent on Dop1 and the P4-ATPase Neo1.

The trans-Golgi network is a major site for α-secretase processing of amyloid precursor protein in primary neurons

Amyloid precursor protein (APP) is processed along the amyloidogenic pathway by the β-secretase, BACE1, generating β-amyloid (Aβ), or along the nonamyloidogenic pathway by α-secretase, precluding Aβ production. The plasma membrane is considered the major site for α-secretase-mediated APP cleavage, but other cellular locations have not been rigorously investigated. Here, we report that APP is processed by endogenous α-secretase at the trans-Golgi network (TGN) of both transfected HeLa cells and mouse primary neurons. We have previously shown the adaptor protein complex, AP-4, and small G protein ADP-ribosylation factor-like GTPase 5b (Arl5b) are required for efficient post-Golgi transport of APP to endosomes. We found here that AP-4 or Arl5b depletion results in Golgi accumulation of APP and increased secretion of the soluble α-secretase cleavage product sAPPα. Moreover, inhibition of γ-secretase following APP accumulation in the TGN increases the levels of the membrane-bound C-terminal fragments of APP from both α-secretase cleavage (α-CTF, named C83 according to its band size) and BACE1 cleavage (β-CTF/C99). The level of C83 was ∼4 times higher than that of C99, indicating that α-secretase processing is the major pathway and that BACE1 processing is the minor pathway in the TGN. AP-4 silencing in mouse primary neurons also resulted in the accumulation of endogenous APP in the TGN and enhanced α-secretase processing. These findings identify the TGN as a major site for α-secretase processing in HeLa cells and primary neurons and indicate that both APP processing pathways can occur within the TGN compartment along the secretory pathway.

The Intracellular Transporter AtNRAMP6 Is Involved in Fe Homeostasis in Arabidopsis

Natural resistance-associated macrophage proteins (NRAMPs) have been shown to transport a wide range of divalent metal ions, such as manganese (Mn), cadmium (Cd), and Iron (Fe). Iron is an essential micronutrient for plants and Fe deficiency can lead to chlorosis or decreased biomass. AtNRAMP6 has demonstrated the capability to transport Cd, but its physiological function is currently unclear. This study demonstrates that AtNRAMP6 is localized to the Golgi/trans-Golgi network and plays an important role in intracellular Fe homeostasis in the flowering plant genus Arabidopsis. GUS tissue-specific expression revealed that AtNRAMP6 is highly expressed in the lateral roots and young leaves (three to four top leaves) of Arabidopsis. Moreover, knocking out AtNRAMP6 was shown to impair lateral root growth without having a differential effect on the main root under Fe-deficient conditions. Lastly, the expression of AtNRAMP6 was found to exacerbate the sensitivity of the yeast mutant Δccc1 to an excessive amount of Fe. These findings indicate that AtNRAMP6 plays an important role in the growth of Arabidopsis in Fe-deficient conditions.

RAB30 regulates PI4KB (phosphatidylinositol 4-kinase beta)-dependent autophagy against group A Streptococcus

Macroautophagy/autophagy plays an important role in the immune response to invasion by intracellular pathogens such as group A Streptococcus (GAS; Streptococcus pyogenes). We previously identified RAB30, a Golgi-resident GTPase, as a novel anti-bacterial autophagic regulator in the formation of GAS-containing autophagosome-like vacuoles (GcAVs); however, the precise mechanism underlying this process remains elusive. Here, we elucidate a novel property of RAB30: the ability to recruit PI4KB (phosphatidylinositol 4-kinase beta) to the Golgi apparatus and GcAVs. We found that trans-Golgi network (TGN) vesicles were incorporated into GcAVs via RAB30 to promote GcAV formation. Moreover, depletion of phosphatidylinositol-4-phosphate (PtdIns4P), a phosphatidylinositol enriched in the TGN, by wortmannin and phenylarsine oxide, followed by subsequent repletion with exogenous PtdIns4P revealed that PtdIns4P is crucial for GcAV formation. Furthermore, we identify an interaction between RAB30 and PI4KB, in which the knockdown of RAB30 decreased the localization of PI4KB to the TGN and GcAVs. Finally, PI4KB knockout suppressed autophagy by inhibiting GcAV formation, resulting in the increased survival of GAS. Our results demonstrate a novel autophagosomal formation mechanism involving coordinative functions of RAB30 and PI4KB distinct from those utilized in canonical autophagy. Abbreviations: GAS: group A Streptococcus; GcAVs: GAS-containing autophagosome-like vacuoles; PI4KB: phosphatidylinositol 4-kinase beta; PtdIns: phosphatidylinositol; PtdIns3P: phosphatidylinositol-3-phosphate; PtdIns4P: phosphatidylinositol-4-phosphate; PtdIns5P: phosphatidylinositol-5-phosphate; SLO: streptolysin O; TGN: trans-Golgi network; TGOLN2: trans-golgi network protein 2; PH: plekstrin homology; OSBP: oxysterol binding protein.

Diverse Viruses Require the Calcium Transporter SPCA1 for Maturation and Spread

Respiratory and arthropod-borne viral infections are a global threat due to the lack of effective antivirals and vaccines. A potential strategy is to target host proteins required for viruses but non-essential for the host. To identify such proteins, we performed a genome-wide knockout screen in human haploid cells and identified the calcium pump SPCA1. SPCA1 is required by viruses from the Paramyxoviridae, Flaviviridae, and Togaviridae families, including measles, dengue, West Nile, Zika, and chikungunya viruses. Calcium transport activity is required for SPCA1 to promote virus spread. SPCA1 regulates proteases within the trans-Golgi network that require calcium for their activity and are critical for virus glycoprotein maturation. Consistent with these findings, viral glycoproteins fail to mature in SPCA1-deficient cells preventing viral spread, which is evident even in cells with partial loss of SPCA1. Thus, SPCA1 is an attractive antiviral host target for a broad spectrum of established and emerging viral infections.

Retrograde transport of γ-secretase from endosomes to the trans-Golgi network regulates Aβ42 production

The aberrant metabolism of amyloid-β protein (Aβ) in the human brain has been implicated in the etiology of Alzheimer disease (AD). γ-Secretase is the enzyme that generates various forms of Aβ, such as Aβ40 and Aβ42, the latter being an aggregation-prone toxic peptide that is involved in the pathogenesis of AD. Recently, we found that clathrin-mediated endocytosis of γ-secretase affects the production and deposition of Aβ42 in vivo, suggesting that the membrane trafficking of γ-secretase affects its enzymatic activity. However, the detailed intracellular trafficking pathway of γ-secretase and its contribution to Aβ42 generation remain unclear. Here, we show that Retro-2, which inhibits the retrograde transport, elevated the Aβ42-generating activity both in cultured cells and mice brain. However, the result of in vitro γ-secretase assay using a recombinant substrate suggested that Retro-2 did not elevate the intrinsic Aβ42-production activity of γ-secretase. Immunocytochemistry and cell-surface biotinylation experiments revealed that γ-secretase is recycled via the endosome-to-trans-Golgi network transport. In addition, γ-secretase is retrogradely transported by syntaxin 5/6, known as targets of Retro-2, independent pathway. Conversely, TPT-260, which enhances the trafficking function of retromers, lowered Aβ42 levels and the Aβ42/(Aβ40 + Aβ42) ratio in secreted Aβ from cultured cells. Our results strongly suggest that the endosome-to-trans-Golgi network trafficking of γ-secretase regulates its Aβ42 production activity. Modulation of this trafficking pathway might be a potential target for the development of Aβ42-lowering AD therapeutics.

OPEN PRACTICES

Open Science: This manuscript was awarded with the Open Materials Badge. For more information see: https://cos.io/our-services/open-science-badges/.

The tumor suppressor protein DLC1 maintains protein kinase D activity and Golgi secretory function

Many newly synthesized cellular proteins pass through the Golgi complex from where secretory transport carriers sort them to the plasma membrane and the extracellular environment. The formation of these secretory carriers at the trans-Golgi network is promoted by the protein kinase D (PKD) family of serine/threonine kinases. Here, using mathematical modeling and experimental validation of the PKD activation and substrate phosphorylation kinetics, we reveal that the expression level of the PKD substrate deleted in liver cancer 1 (DLC1), a Rho GTPase-activating protein that is inhibited by PKD-mediated phosphorylation, determines PKD activity at the Golgi membranes. RNAi-mediated depletion of DLC1 reduced PKD activity in a Rho-Rho-associated protein kinase (ROCK)-dependent manner, impaired the exocytosis of the cargo protein horseradish peroxidase, and was associated with the accumulation of the small GTPase RAB6 on Golgi membranes, indicating a protein-trafficking defect. In summary, our findings reveal that DLC1 maintains basal activation of PKD at the Golgi and Golgi secretory activity, in part by down-regulating Rho-ROCK signaling. We propose that PKD senses cytoskeletal changes downstream of DLC1 to coordinate Rho signaling with Golgi secretory function.