A cycling cis-Golgi protein mediates endosome-to-Golgi traffic

Inhibition of OSBP blocks retrograde trafficking by inducing partial Golgi degradation

Sterol-binding proteins are important regulators of lipid homeostasis and membrane integrity; however, the discovery of selective modulators can be challenging due to structural similarities in the sterol-binding domains. We report the discovery of potent and selective inhibitors of oxysterol-binding protein (OSBP), which we term oxybipins. Sterol-containing chemical chimeras aimed at identifying new sterol-binding proteins by targeted degradation, led to a significant reduction in levels of Golgi-associated proteins. The degradation occurred in lysosomes, concomitant with changes in protein glycosylation, indicating that the degradation of Golgi proteins was a downstream effect. By establishing a sterol transport protein biophysical assay panel, we discovered that the oxybipins potently inhibited OSBP, resulting in blockage of retrograde trafficking and attenuating Shiga toxin toxicity. As the oxybipins do not target other sterol transporters and only stabilized OSBP in intact cells, we advocate their use as tools to study OSBP function and therapeutic relevance.

Chromosomal 3q amplicon encodes essential regulators of secretory vesicles that drive secretory addiction in cancer

Cancer cells exhibit heightened secretory states that drive tumor progression. Here, we identified a chromosome 3q amplicon that serves as a platform for secretory regulation in cancer. The 3q amplicon encodes multiple Golgi-resident proteins, including the scaffold Golgi integral membrane protein 4 (GOLIM4) and the ion channel ATPase secretory pathway Ca2+ transporting 1 (ATP2C1). We show that GOLIM4 recruited ATP2C1 and Golgi phosphoprotein 3 (GOLPH3) to coordinate Ca2+-dependent cargo loading, Golgi membrane bending, and vesicle scission. GOLIM4 depletion disrupted the protein complex, resulting in a secretory blockade that inhibited the progression of 3q-amplified malignancies. In addition to its role as a scaffold, GOLIM4 maintained intracellular manganese (Mn) homeostasis by binding excess Mn in the Golgi lumen, which initiated the routing of Mn-bound GOLIM4 to lysosomes for degradation. We show that Mn treatment inhibited the progression of multiple types of 3q-amplified malignancies by degrading GOLIM4, resulting in a secretory blockade that interrupted prosurvival autocrine loops and attenuated prometastatic processes in the tumor microenvironment. As it potentially underlies the selective activity of Mn against 3q-amplified malignancies, ATP2C1 coamplification increased Mn influx into the Golgi lumen, resulting in a more rapid degradation of GOLIM4. These findings show that functional cooperativity between coamplified genes underlies heightened secretion and a targetable secretory addiction in 3q-amplified malignancies.

A SURF4-to-proteoglycan relay mechanism that mediates the sorting and secretion of a tagged variant of sonic hedgehog

SignificanceSonic Hedgehog (Shh) is a key signaling molecule that plays important roles in embryonic patterning, cell differentiation, and organ development. Although fundamentally important, the molecular mechanisms that regulate secretion of newly synthesized Shh are still unclear. Our study reveals a role for the cargo receptor, SURF4, in facilitating export of Shh from the endoplasmic reticulum (ER) via a ER export signal. In addition, our study provides evidence suggesting that proteoglycans promote the dissociation of SURF4 from Shh at the Golgi, suggesting a SURF4-to-proteoglycan relay mechanism. These analyses provide insight into an important question in cell biology: how do cargo receptors capture their clients in one compartment, then disengage at their destination?

Single-Cell RNA-Seq Reveals Heterogeneous lncRNA Expression in Xenografted Triple-Negative Breast Cancer Cells

Breast cancer is the most commonly diagnosed cancer in the world, with triple-negative breast cancer (TNBC) making up 12% of these diagnoses. TNBC tumours are highly heterogeneous in both inter-tumour and intra-tumour gene expression profiles, where they form subclonal populations of varying levels of aggressiveness. These aspects make it difficult to study and treat TNBC, requiring further research into tumour heterogeneity as well as potential therapeutic targets and biomarkers. Recently, it was discovered that the majority of the transcribed genome comprises non-coding RNAs, in particular long non-coding RNAs (lncRNAs). LncRNAs are transcripts of >200 nucleotides in length that do not encode a protein. They have been characterised as regulatory molecules and their expression can be associated with a malignant phenotype. We set out to explore TNBC tumour heterogeneity in vivo at a single cell level to investigate whether lncRNA expression varies across different cells within the tumour, even if cells are coming from the same cell line, and whether lncRNA expression is sufficient to define cellular subpopulations. We applied single-cell expression profiling due to its ability to capture expression signals of lncRNAs expressed in small subpopulations of cells. Overall, we observed most lncRNAs to be expressed at low, but detectable levels in TNBC xenografts, with a median of 25 lncRNAs detected per cell. LncRNA expression alone was insufficient to define a subpopulation of cells, and lncRNAs showed highly heterogeneous expression patterns, including ubiquitous expression, subpopulation-specific expression, and a hybrid pattern of lncRNAs expressed in several, but not all subpopulations. These findings reinforce that transcriptionally defined tumour cell subpopulations can be identified in cell-line derived xenografts, and uses single-cell RNA-seq (scRNA-seq) to detect and characterise lncRNA expression across these subpopulations in xenografted tumours. Future studies will aim to investigate the spatial distribution of lncRNAs within xenografts and patient tissues, and study the potential of subclone-specific lncRNAs as new therapeutic targets and/or biomarkers.

Establishment of an Endocytosis-Related Prognostic Signature for Patients With Low-Grade Glioma

Background

Low-grade glioma (LGG) is a heterogeneous tumor that might develop into high-grade malignant glioma, which markedly reduces patient survival time. Endocytosis is a cellular process responsible for the internalization of cell surface proteins or external materials into the cytosol. Dysregulated endocytic pathways have been linked to all steps of oncogenesis, from initial transformation to late invasion and metastasis. However, endocytosis-related gene (ERG) signatures have not been used to study the correlations between endocytosis and prognosis in cancer. Therefore, it is essential to develop a prognostic model for LGG based on the expression profiles of ERGs.

Methods

The Cancer Genome Atlas and the Genotype-Tissue Expression database were used to identify differentially expressed ERGs in LGG patients. Gene ontology, Kyoto Encyclopedia of Genes and Genomes, and Gene set enrichment analysis methodologies were adopted for functional analysis. A protein-protein interaction (PPI) network was constructed and hub genes were identified based on the Search Tool for the Retrieval of Interacting Proteins database. Univariate and multivariate Cox regression analyses were used to develop an ERG signature to predict the overall survival (OS) of LGG patients. Finally, the association between the ERG signature and gene mutation status was further analyzed.

Results

Sixty-two ERGs showed distinct mRNA expression patterns between normal brain tissues and LGG tissues. Functional analysis indicated that these ERGs were strikingly enriched in endosomal trafficking pathways. The PPI network indicated that EGFR was the most central protein. We then built a 29-gene signature, dividing patients into high-risk and low-risk groups with significantly different OS times. The prognostic performance of the 29-gene signature was validated in another LGG cohort. Additionally, we found that the mutation scores calculated based on the TTN, PIK3CA, NF1, and IDH1 mutation status were significantly correlated with the endocytosis-related prognostic signature. Finally, a clinical nomogram with a concordance index of 0.881 predicted the survival probability of LGG patients by integrating clinicopathologic features and ERG signatures.

Conclusion

Our ERG-based prediction models could serve as an independent prognostic tool to accurately predict the outcomes of LGG.

ACBD3 modulates KDEL receptor interaction with PKA for its trafficking via tubulovesicular carrier

Background

KDEL receptor helps establish cellular equilibrium in the early secretory pathway by recycling leaked ER-chaperones to the ER during secretion of newly synthesized proteins. Studies have also shown that KDEL receptor may function as a signaling protein that orchestrates membrane flux through the secretory pathway. We have recently shown that KDEL receptor is also a cell surface receptor, which undergoes highly complex itinerary between trans-Golgi network and the plasma membranes via clathrin-mediated transport carriers. Ironically, however, it is still largely unknown how KDEL receptor is distributed to the Golgi at steady state, since its initial discovery in late 1980s.

Results

We used a proximity-based in vivo tagging strategy to further dissect mechanisms of KDEL receptor trafficking. Our new results reveal that ACBD3 may be a key protein that regulates KDEL receptor trafficking via modulation of Arf1-dependent tubule formation. We demonstrate that ACBD3 directly interact with KDEL receptor and form a functionally distinct protein complex in ArfGAPs-independent manner. Depletion of ACBD3 results in re-localization of KDEL receptor to the ER by inducing accelerated retrograde trafficking of KDEL receptor. Importantly, this is caused by specifically altering KDEL receptor interaction with Protein Kinase A and Arf1/ArfGAP1, eventually leading to increased Arf1-GTP-dependent tubular carrier formation at the Golgi.

Conclusions

These results suggest that ACBD3 may function as a negative regulator of PKA activity on KDEL receptor, thereby restricting its retrograde trafficking in the absence of KDEL ligand binding. Since ACBD3 was originally identified as PAP7, a PBR/PKA-interacting protein at the Golgi/mitochondria, we propose that Golgi-localization of KDEL receptor is likely to be controlled by its interaction with ACBD3/PKA complex at steady state, providing a novel insight for establishment of cellular homeostasis in the early secretory pathway.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01137-7.

The Golgi-associated retrograde protein (GARP) complex plays an essential role in the maintenance of the Golgi glycosylation machinery

The Golgi complex is a central hub for intracellular protein trafficking and glycosylation. Steady-state localization of glycosylation enzymes is achieved by a combination of mechanisms involving retention and recycling, but the machinery governing these mechanisms is poorly understood. Herein we show that the Golgi-associated retrograde protein (GARP) complex is a critical component of this machinery. Using multiple human cell lines, we show that depletion of GARP subunits impairs Golgi modification of N- and O-glycans and reduces the stability of glycoproteins and Golgi enzymes. Moreover, GARP-knockout (KO) cells exhibit reduced retention of glycosylation enzymes in the Golgi. A RUSH assay shows that, in GARP-KO cells, the enzyme beta-1,4-galactosyltransferase 1 is not retained at the Golgi complex but instead is missorted to the endolysosomal system. We propose that the endosomal system is part of the trafficking itinerary of Golgi enzymes or their recycling adaptors and that the GARP complex is essential for recycling and stabilization of the Golgi glycosylation machinery. [Media: see text].

Insertion variants missing in the human reference genome are widespread among human populations

Background

Structural variants comprise diverse genomic arrangements including deletions, insertions, inversions, and translocations, which can generally be detected in humans through sequence comparison to the reference genome. Among structural variants, insertions are the least frequently identified variants, mainly due to ascertainment bias in the reference genome, lack of previous sequence knowledge, and low complexity of typical insertion sequences. Though recent developments in long-read sequencing deliver promise in annotating individual non-reference insertions, population-level catalogues on non-reference insertion variants have not been identified and the possible functional roles of these hidden variants remain elusive.

Results

To detect non-reference insertion variants, we developed a pipeline, InserTag, which generates non-reference contigs by local de novo assembly and then infers the full-sequence of insertion variants by tracing contigs from non-human primates and other human genome assemblies. Application of the pipeline to data from 2535 individuals of the 1000 Genomes Project helped identify 1696 non-reference insertion variants and re-classify the variants as retention of ancestral sequences or novel sequence insertions based on the ancestral state. Genotyping of the variants showed that individuals had, on average, 0.92-Mbp sequences missing from the reference genome, 92% of the variants were common (allele frequency > 5%) among human populations, and more than half of the variants were major alleles. Among human populations, African populations were the most divergent and had the most non-reference sequences, which was attributed to the greater prevalence of high-frequency insertion variants. The subsets of insertion variants were in high linkage disequilibrium with phenotype-associated SNPs and showed signals of recent continent-specific selection.

Conclusions

Non-reference insertion variants represent an important type of genetic variation in the human population, and our developed pipeline, InserTag, provides the frameworks for the detection and genotyping of non-reference sequences missing from human populations.

Supplementary information

Supplementary information accompanies this paper at 10.1186/s12915-020-00894-1.

Targeting the Early Endosome-to-Golgi Transport of Shiga Toxins as a Therapeutic Strategy

Shiga toxin (STx) produced by Shigella and closely related Shiga toxin 1 and 2 (STx1 and STx2) synthesized by Shiga toxin-producing Escherichia coli (STEC) are bacterial AB₅ toxins. All three toxins target kidney cells and may cause life-threatening renal disease. While Shigella infections can be treated with antibiotics, resistance is increasing. Moreover, antibiotic therapy is contraindicated for STEC, and there are no definitive treatments for STEC-induced disease. To exert cellular toxicity, STx, STx1, and STx2 must undergo retrograde trafficking to reach their cytosolic target, ribosomes. Direct transport from early endosomes to the Golgi apparatus is an essential step that allows the toxins to bypass degradative late endosomes and lysosomes. The essentiality of this transport step also makes it an ideal target for the development of small-molecule inhibitors of toxin trafficking as potential therapeutics. Here, we review the recent advances in understanding the molecular mechanisms of the early endosome-to-Golgi transport of STx, STx1, and STx2, as well as the development of small-molecule inhibitors of toxin trafficking that act at the endosome/Golgi interface.

Functional dissection of the retrograde Shiga toxin trafficking inhibitor Retro-2

The retrograde transport inhibitor Retro-2 has a protective effect on cells and in mice against Shiga-like toxins and ricin. Retro-2 causes toxin accumulation in early endosomes, and relocalization of the Golgi SNARE protein syntaxin-5 to the endoplasmic reticulum. The molecular mechanisms by which this is achieved remain unknown. Here, we show that Retro-2 targets the endoplasmic reticulum exit site component Sec16A, affecting anterograde transport of syntaxin-5 from the endoplasmic reticulum to the Golgi. The formation of canonical SNARE complexes involving syntaxin-5 is not affected in Retro-2-treated cells. In contrast, the interaction of syntaxin-5 with a newly discovered binding partner, the retrograde trafficking chaperone GPP130, is abolished, and we show that GPP130 must indeed bind to syntaxin-5 to drive Shiga toxin transport from endosomes to the Golgi. We thereby identify Sec16A as a druggable target, and provide evidence for a non-SNARE function for syntaxin-5 in interaction with the GPP130.

Unlocking Golgi: Why Does Morphology Matter?

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

Protein Toxins That Utilize Gangliosides as Host Receptors

Subsets of protein toxins utilize gangliosides as host receptors. Gangliosides are preferred receptors due to their extracellular localization on the eukaryotic cell and due to their essential nature in host physiology. Glycosphingolipids, including gangliosides, are mediators of signal transduction within and between eukaryotic cells. Protein toxins possess AB structure-function organization, where the A domain encodes a catalytic function for the posttranslational modification of a host macromolecule, including proteins and nucleic acids, and a B domain, which encodes host receptor recognition, including proteins and glycosphingolipids, alone or in combination. Protein toxins use similar strategies to bind glycans by pockets and loops, generally employing hydrogen bonding and aromatic stacking to stabilize interactions with sugars. In some cases, glycan binding facilitates uptake, while in other cases, cross-linking or a second receptor is necessary to stimulate entry. The affinity that protein toxins have for host glycans is necessary for tissue targeting, but not always sufficient to cause disease. In addition to affinity for binding the glycan, the lipid moiety also plays an important role in productive uptake and tissue tropism. Upon endocytosis, the protein toxin must escape to another intracellular compartment or into cytosol to modify a host substrate, modulating host signaling, often resulting in cytotoxic or apoptotic events in the cell, and a unique morbidity for the organism. The study of protein toxins that utilize gangliosides as host receptors has illuminated numerous eukaryotic cellular processes, identified the basis for developing interventions to prevent disease through vaccines and control bacterial diseases through therapies. In addition, subsets of these protein toxins have been utilized as therapeutic agents to treat numerous human inflictions.

Role of the AP-5 adaptor protein complex in late endosome-to-Golgi retrieval

The AP-5 adaptor protein complex is presumed to function in membrane traffic, but so far nothing is known about its pathway or its cargo. We have used CRISPR-Cas9 to knock out the AP-5 ζ subunit gene, AP5Z1, in HeLa cells, and then analysed the phenotype by subcellular fractionation profiling and quantitative mass spectrometry. The retromer complex had an altered steady-state distribution in the knockout cells, and several Golgi proteins, including GOLIM4 and GOLM1, were depleted from vesicle-enriched fractions. Immunolocalisation showed that loss of AP-5 led to impaired retrieval of the cation-independent mannose 6-phosphate receptor (CIMPR), GOLIM4, and GOLM1 from endosomes back to the Golgi region. Knocking down the retromer complex exacerbated this phenotype. Both the CIMPR and sortilin interacted with the AP-5–associated protein SPG15 in pull-down assays, and we propose that sortilin may act as a link between Golgi proteins and the AP-5/SPG11/SPG15 complex. Together, our findings suggest that AP-5 functions in a novel sorting step out of late endosomes, acting as a backup pathway for retromer. This provides a mechanistic explanation for why mutations in AP-5/SPG11/SPG15 cause cells to accumulate aberrant endolysosomes, and highlights the role of endosome/lysosome dysfunction in the pathology of hereditary spastic paraplegia and other neurodegenerative disorders.

Eukaryotic cells contain multiple membrane-bound compartments, each with a distinct function and molecular composition. Proteins are transported from one compartment to another by vesicular carriers. Formation of these carriers requires coat proteins, which both shape the membrane into a vesicle and select the proteins that are to be included as cargo. In many cases, cargo selection is facilitated by an adaptor protein (AP) complex, of which 5 have been identified. The most recently identified complex, AP-5, localises to a late endosomal/lysosomal compartment, and patients with mutations in AP-5 have a form of hereditary spastic paraplegia characterised by aberrant lysosomes. However, the precise function of AP-5, including its cargo and its pathway, has until now been unclear. In the present study, we have used unbiased subcellular proteomics to look for changes in the localisation of thousands of different proteins in cells from which AP-5 has been deleted by gene editing. We found that there are defects in the retrieval of several proteins from late endosomes back to the Golgi apparatus. Thus, we propose that AP-5 facilitates a novel late-acting retrieval pathway, which contributes to normal lysosomal homeostasis.

Genome-wide siRNA screen identifies UNC50 as a regulator of Shiga toxin 2 trafficking

Toxins produced by Shigella bacteria undergo endosome-to-Golgi retrograde trafficking to evade degradation in the lysosome and reach the cytosol. Selyunin et al. performed a genome-wide siRNA screen and identify host factors required for the transport and toxicity of Shiga toxins.

Shiga toxins 1 and 2 (STx1 and STx2) undergo retrograde trafficking to reach the cytosol. Early endosome-to-Golgi transport allows the toxins to evade degradation in lysosomes. Targeting this trafficking step has therapeutic promise, but the mechanism of trafficking for the more potent toxin STx2 is unclear. To identify host factors required for early endosome-to-Golgi trafficking of STx2, we performed a viability-based genome-wide siRNA screen in HeLa cells. 564, 535, and 196 genes were found to be required for toxicity induced by STx1 only, STx2 only, and both toxins, respectively. We focused on validating endosome/Golgi-localized hits specific for STx2 and found that depletion of UNC50 blocked early endosome-to-Golgi trafficking and induced lysosomal degradation of STx2. UNC50 acted by recruiting GBF1, an ADP ribosylation factor–guanine nucleotide exchange factor (ARF-GEF), to the Golgi. These results provide new information about STx2 trafficking mechanisms and may advance efforts to generate therapeutically viable toxin-trafficking inhibitors.

GP73 represses host innate immune response to promote virus replication by facilitating MAVS and TRAF6 degradation

Hepatitis C virus (HCV) infection is a leading cause of chronic liver diseases and hepatocellular carcinoma (HCC) and Golgi protein 73 (GP73) is a serum biomarker for liver diseases and HCC. However, the mechanism underlying GP73 regulates HCV infection is largely unknown. Here, we revealed that GP73 acts as a novel negative regulator of host innate immunity to facilitate HCV infection. GP73 expression is activated and correlated with interferon-beta (IFN-β) production during HCV infection in patients’ serum, primary human hepatocytes (PHHs) and human hepatoma cells through mitochondrial antiviral signaling protein (MAVS), TNF receptor-associated factor 6 (TRAF6) and mitogen-activated protein kinase kinase/extracellular regulated protein kinase (MEK/ERK) pathway. Detailed studies revealed that HCV infection activates MAVS that in turn recruits TRAF6 via TRAF-interacting-motifs (TIMs), and TRAF6 subsequently directly recruits GP73 to MAVS via coiled-coil domain. After binding with MAVS and TRAF6, GP73 promotes MAVS and TRAF6 degradation through proteasome-dependent pathway. Moreover, GP73 attenuates IFN-β promoter, IFN-stimulated response element (ISRE) and nuclear factor κB (NF-κB) promoter and down-regulates IFN-β, IFN-λ1, interleukin-6 (IL-6) and IFN-stimulated gene 56 (ISG56), leading to the repression of host innate immunity. Finally, knock-down of GP73 down-regulates HCV infection and replication in Huh7-MAVSR cells and primary human hepatocytes (PHHs), but such repression is rescued by GP73m4 (a mutant GP73 resists to GP73-shRNA#4) in Huh7-MAVSR cells, suggesting that GP73 facilitates HCV infection. Taken together, we demonstrated that GP73 acts as a negative regulator of innate immunity to facilitate HCV infection by interacting with MAVS/TRAF6 and promoting MAVS/TRAF6 degradation. This study provides new insights into the mechanism of HCV infection and pathogenesis, and suggests that GP73 is a new potential antiviral target in the prevention and treatment of HCV associated diseases.

Golgi protein 73 (GP73) is a serum biomarker for liver diseases and hepatocellular carcinoma (HCC). In this study, the authors reveal that GP73 acts as a novel negative regulator of host innate immunity to facilitate hepatitis C virus (HCV) infection. GP73 expression is activated and correlated with IFN-β production during HCV infection in patients’ serum, primary human hepatocytes (PHHs) and human hepatoma cells through mitochondrial antiviral signaling protein (MAVS), TNF receptor-associated factor 6 (TRAF6) and MEK/ERK pathway. They further demonstrate that during viral infection, MAVS recruits TRAF6 that subsequently directly binds with GP73. After binding with MAVS and TRAF6, GP73 promotes MAVS and TRAF6 degradation. Moreover, GP73 attenuates IFN-β promoter, IFN-stimulated response element (ISRE) and NF-κB promoter and down-regulates IFN-β, IFN-λ1, interleukin-6 (IL-6) and IFN-stimulated gene 56 (ISG56), leading to the repression of host innate immunity and the facilitation of virus infection. These results reveal a novel mechanism by which GP73 acts as a novel negative regulator of host innate immunity to facilitate virus infection and also provide new insights into the therapeutic design of anti-HCV drugs.

A Conserved Structural Motif Mediates Retrograde Trafficking of Shiga Toxin Types 1 and 2

Shiga toxin-producing Escherichia coli (STEC) produce two types of Shiga toxin (STx): STx1 and STx2. The toxin A-subunits block protein synthesis, while the B-subunits mediate retrograde trafficking. STEC infections do not have definitive treatments, and there is growing interest in generating toxin transport inhibitors for therapy. However, a comprehensive understanding of the mechanisms of toxin trafficking is essential for drug development. While STx2 is more toxic in vivo, prior studies focused on STx1 B-subunit (STx1B) trafficking. Here, we show that, compared with STx1B, trafficking of the B-subunit of STx2 (STx2B) to the Golgi occurs with slower kinetics. Despite this difference, similar to STx1B, endosome-to-Golgi transport of STx2B does not involve transit through degradative late endosomes and is dependent on dynamin II, epsinR, retromer and syntaxin5. Importantly, additional experiments show that a surface-exposed loop in STx2B (β4-β5 loop) is required for its endosome-to-Golgi trafficking. We previously demonstrated that residues in the corresponding β4-β5 loop of STx1B are required for interaction with GPP130, the STx1B-specific endosomal receptor, and for endosome-to-Golgi transport. Overall, STx1B and STx2B share a common pathway and use a similar structural motif to traffic to the Golgi, suggesting that the underlying mechanisms of endosomal sorting may be evolutionarily conserved.

Induced oligomerization targets Golgi proteins for degradation in lysosomes

Oligomerization or homotypic clustering diverts Golgi membrane proteins into the canonical GGA1/clathrin-dependent Golgi-to-lysosome pathway revealing the presence of cellular quality control that could be useful for therapies designed to down-regulate specific proteins in the secretory pathway.

Manganese protects cells against forms of Shiga toxin by down-regulating the cycling Golgi protein GPP130. Down-regulation occurs when Mn binding causes GPP130 to oligomerize and traffic to lysosomes. To determine how GPP130 is redirected to lysosomes, we tested the role of GGA1 and clathrin, which mediate sorting in the canonical Golgi-to-lysosome pathway. GPP130 oligomerization was induced using either Mn or a self-interacting version of the FKBP domain. Inhibition of GGA1 or clathrin specifically blocked GPP130 redistribution, suggesting recognition of the aggregated GPP130 by the GGA1/clathrin-sorting complex. Unexpectedly, however, GPP130’s cytoplasmic domain was not required, and redistribution also occurred after removal of GPP130 sequences needed for its normal cycling. Therefore, to test whether aggregate recognition might be a general phenomenon rather than one involving a specific GPP130 determinant, we induced homo-oligomerization of two unrelated Golgi-targeted constructs using the FKBP strategy. These were targeted to the cis- and trans-Golgi, respectively, using domains from mannosidase-1 and galactosyltransferase. Significantly, upon oligomerization, each redistributed to peripheral punctae and was degraded. This occurred in the absence of detectable UPR activation. These findings suggest the unexpected presence of quality control in the Golgi that recognizes aggregated Golgi proteins and targets them for degradation in lysosomes.

Chaperone-Mediated Autophagy after Traumatic Brain Injury

Chaperone-mediated autophagy (CMA) and the ubiquitin-proteasomal system (UPS) are two major protein degradation systems responsible for maintaining cellular homeostasis, but how these two systems are regulated after traumatic brain injury (TBI) remains unknown. TBI produces primary mechanical damage that must be repaired to maintain neuronal homeostasis. The level of lysosomal-associated membrane protein type 2A (LAMP2A) is the hallmark of CMA activity. The level of polyubiquitinated proteins (ubi-proteins) reflects UPS activity. This study utilized a moderate fluid percussion injury model in rats to investigate the changes in CMA and the UPS after TBI. Induction of CMA was manifested by significant upregulation of LAMP2A and secondary lysosomes during the periods of 1-15 days of recovery after TBI. In comparison, the levels of ubi-proteins were increased only moderately after TBI. The increases in the levels of LAMP2A and 70 kDa heat-shock protein for CMA after TBI were seen mainly in the secondary lysosome-containing fractions. Confocal and electron microscopy further showed that increased LAMP2A or lysosomes were found mainly in neurons and proliferated microglia. Because CMA and the UPS are two major routes for elimination of different types of cellular aberrant proteins, the consecutive activation of these two pathways may serve as a protective mechanism for maintaining cellular homeostasis after TBI.

Genetic predictors of cervical dysplasia in African American HIV-infected women: ACTG DACS 268

OBJECTIVE

To examine genome-wide associations in HIV-infected women with a history of cervical dysplasia compared with HIV-infected women with no history of abnormal Papanicolaou (Pap) tests.

DESIGN

Case-control study using data from women analyzed for the HIV Controllers Study and enrolled in HIV treatment-naïve studies in the AIDS Clinical Trials Group (ACTG).

METHODS

Genotyping utilized Illumina HumanHap 650 Y or 1MDuo platforms. After quality control and principal component analysis, ~610,000 significant single nucleotide polymorphisms (SNPs) were tested for association. Threshold for significance was P < 5 × 10(-8) for genome-wide associations.

RESULTS

No significant genomic association was observed between women with low-grade dysplasia and controls. The genome-wide association study (GWAS) analysis between women with high-grade dysplasia or invasive cervical cancer and normal controls identified significant SNPs. In the analyses limited to African American women, 11 SNPs were significantly associated with the development of high-grade dysplasia or cancer after correcting for multiple comparisons. The model using significant SNPs alone had improved accuracy in predicting high-grade dysplasia in African American women compared to the use of clinical data (area under the receiver operating characteristic curve for genetic and clinical model = 0.9 and 0.747, respectively).

CONCLUSIONS

These preliminary data serve as proof of concept that there may be a genetic predisposition to developing high-grade cervical dysplasia in African American HIV-infected women. Given the small sample size, the results need to be validated in a separate cohort.

Regulation of traffic and organelle architecture of the ER-Golgi interface by signal transduction

The components that control trafficking between organelles of the secretory pathway as well as their architecture were uncovered to a reasonable extent in the past decades. However, only recently did we begin to explore the regulation of the secretory pathway by cellular signaling. In the current review, we focus on trafficking between the endoplasmic reticulum and the Golgi apparatus. We highlight recent advances that have been made toward a better understanding of how the secretory pathway is regulated by signaling and discuss how this knowledge is important to obtain an integrative view of secretion in the context of other homeostatic processes such as growth and proliferation.

Retrograde trafficking of AB₅ toxins: mechanisms to therapeutics

Bacterial AB5 toxins are a clinically relevant class of exotoxins that include several well-known members such as Shiga, cholera, and pertussis toxins. Infections with toxin-producing bacteria cause devastating human diseases that affect millions of individuals each year and have no definitive medical treatment. The molecular targets of AB5 toxins reside in the cytosol of infected cells, and the toxins reach the cytosol by trafficking through the retrograde membrane transport pathway that avoids degradative late endosomes and lysosomes. Focusing on Shiga toxin as the archetype member, we review recent advances in understanding the molecular mechanisms involved in the retrograde trafficking of AB5 toxins and highlight how these basic science advances are leading to the development of a promising new therapeutic approach based on inhibiting toxin transport.

Golgi phosphoprotein 4 (GPP130) is a sensitive and selective cellular target of manganese exposure

Chronic elevated exposure to manganese (Mn) is associated with neurocognitive and fine motor deficits in children. However, relatively little is understood about cellular responses to Mn spanning the transition between physiologic to toxic levels of exposure. Here, we investigated the specificity, sensitivity, and time course of the Golgi Phosphoprotein 4 (GPP130) response to Mn exposure in AF5 GABAergic neuronal cells, and we determined the extent to which GPP130 degradation occurs in brain cells in vivo in rats subchronically exposed to Mn. Our results show that GPP130 degradation in AF5 cells was specific to Mn, and did not occur following exposure to cobalt, copper, iron, nickel, or zinc. GPP130 degradation occurred without measurable increases in intracellular Mn levels and at Mn exposures as low as 0.54 µM. GPP130 protein was detectable by immunofluorescence in only ∼15-30% of cells in striatal and cortical rat brain slices, and Mn-exposed animals exhibited a significant reduction in both the number of GPP130-positive cells, and the overall levels of GPP130 protein, demonstrating the in vivo relevance of this Mn-specific response within the primary target organ of Mn toxicity. These results provide insight into specific mechanism(s) of cellular Mn regulation and toxicity within the brain, including the selective susceptibility of cells to Mn cytotoxicity.

GP73 expression and its significance in the diagnosis of hepatocellular carcinoma: a review

Hepatocellular carcinoma (HCC) is one of the most common malignant tumors, and its incidence has been increasing worldwide. Serum alpha-fetoprotein (AFP) levels and abdominal ultrasound have been widely used for diagnosis as well as surveillance of HCC. However, the sensitivity and specificity of both AFP levels and ultrasound for HCC surveillance have some shortcomings, particularly in the early stages of the disease. Golgi protein-73 (GP73) is a type II Golgi-localized integral membrane protein that is normally expressed in epithelial cells of many human tissues. It is essential for human survival, and might have multiple roles for GP73 in epithelial cell function such as in the kidney and liver. However, details of its biochemical function and regulation of GP73 expression are unknown at present. GP73 expression is upregulated in serum samples from patients with liver disease, with expression being highest in HCC. Therefore, it may be useful as a new serum marker for detection of HCC in at high-risk population. But, this hypothesis needs to be proven in large cohorts.

Shiga toxins

Shiga toxins are virulence factors produced by the bacteria Shigella dysenteriae and certain strains of Escherichia coli. There is currently no available treatment for disease caused by these toxin-producing bacteria, and understanding the biology of the Shiga toxins might be instrumental in addressing this issue. In target cells, the toxins efficiently inhibit protein synthesis by inactivating ribosomes, and they may induce signaling leading to apoptosis. To reach their cytoplasmic target, Shiga toxins are endocytosed and transported by a retrograde pathway to the endoplasmic reticulum, before the enzymatically active moiety is translocated to the cytosol. The toxins thereby serve as powerful tools to investigate mechanisms of intracellular transport. Although Shiga toxins are a serious threat to human health, the toxins may be exploited for medical purposes such as cancer therapy or imaging.

Retromer guides STxB and CD8-M6PR from early to recycling endosomes, EHD1 guides STxB from recycling endosome to Golgi

Retrograde trafficking transports proteins, lipids and toxins from the plasma membrane to the Golgi and endoplasmic reticulum (ER). To reach the Golgi, these cargos must transit the endosomal system, consisting of early endosomes (EE), recycling endosomes, late endosomes and lysosomes. All cargos pass through EE, but may take different routes to the Golgi. Retromer-dependent cargos bypass the late endosomes to reach the Golgi. We compared how two very different retromer-dependent cargos negotiate the endosomal sorting system. Shiga toxin B, bound to the external layer of the plasma membrane, and chimeric CD8-mannose-6-phosphate receptor (CI-M6PR), which is anchored via a transmembrane domain. Both appear to pass through the recycling endosome. Ablation of the recycling endosome diverted both of these cargos to an aberrant compartment and prevented them from reaching the Golgi. Once in the recycling endosome, Shiga toxin required EHD1 to traffic to the TGN, while the CI-M6PR was not significantly dependent on EHD1. Knockdown of retromer components left cargo in the EE, suggesting that it is required for retrograde exit from this compartment. This work establishes the recycling endosome as a required step in retrograde traffic of at least these two retromer-dependent cargos. Along this pathway, retromer is associated with EE to recycling endosome traffic, while EHD1 is associated with recycling endosome to TGN traffic of STxB.

The VP1 subunit of JC polyomavirus recapitulates early events in viral trafficking and is a novel tool to study polyomavirus entry

JC polyomavirus (JCV) is an important human pathogen that causes the fatal demyelinating disease progressive multifocal leukoencephalopathy (PML). In this study we further delineate the early events of JCV entry in human glial cells and demonstrate that a pentameric subunit of the viral capsid is able to recapitulate early events in viral trafficking. We show that JCV traffics to the endoplasmic reticulum (ER) by 6h post infection, and that VP1 pentamers arrive at the ER with similar kinetics. Further, this JCV localization to the ER is critical for infection, as treatment of cells with agents that prevent ER trafficking, ER function, or ER quality control reduce JCV infectivity. These pentamers represent a new tool to study polyomavirus entry, and will be particularly useful in studying recently identified polyomaviruses that are difficult to propagate.

Manganese blocks intracellular trafficking of Shiga toxin and protects against Shiga toxicosis

Infections with Shiga toxin (STx)-producing bacteria cause more than a million deaths each year and have no definitive treatment. To exert its cytotoxic effect, STx invades cells through retrograde membrane trafficking, escaping the lysosomal degradative pathway. We found that the widely available metal manganese (Mn(2+)) blocked endosome-to-Golgi trafficking of STx and caused its degradation in lysosomes. Mn(2+) targeted the cycling Golgi protein GPP130, which STx bound in control cells during sorting into Golgi-directed endosomal tubules that bypass lysosomes. In tissue culture cells, treatment with Mn(2+) yielded a protection factor of 3800 against STx-induced cell death. Furthermore, mice injected with nontoxic doses of Mn(2+) were completely resistant to a lethal STx challenge. Thus, Mn(2+) may represent a low-cost therapeutic agent for the treatment of STx infections.

Advances in understanding the relationship between GP73 and hepatic diseases

Golgi protein-73 (GP73), a recently discovered Golgi glycoprotein localized on the membrane of the Golgi complex, is expressed in many types of human epithelial cells. In normal human liver, GP73 is highly expressed in biliary epithelial cells, but barely detectable in hepatocytes. However, GP73 expression has been found to be strongly up-regulated in hepatocytes and elevated in the serum in patients with liver diseases, especially those with hepatocellular carcinoma (HCC). Thus, GP73 is a candidate serum marker for the early detection of HCC.

Shiga toxins: intracellular trafficking to the ER leading to activation of host cell stress responses

Despite efforts to improve hygenic conditions and regulate food and drinking water safety, the enteric pathogens, Shiga toxin-producing Escherichia coli (STEC) and Shigella dysenteriae serotype 1 remain major public health concerns due to widespread outbreaks and the severity of extra-intestinal diseases they cause, including acute renal failure and central nervous system complications. Shiga toxins are the key virulence factors expressed by these pathogens mediating extra-intestinal disease. Delivery of the toxins to the endoplasmic reticulum (ER) results in host cell protein synthesis inhibition, activation of the ribotoxic stress response, the ER stress response, and in some cases, the induction of apoptosis. Intrinsic and/or extrinsic apoptosis inducing pathways are involved in executing cell death following intoxication. In this review we provide an overview of the current understanding Shiga toxin intracellular trafficking, host cellular responses to the toxin and ER stress-induced apoptosis with an emphasis on recent findings.

Rab33b and Rab6 are functionally overlapping regulators of Golgi homeostasis and trafficking

We used multiple approaches to investigate the coordination of trans and medial Rab proteins in the regulation of intra-Golgi retrograde trafficking. We reasoned that medially located Rab33b might act downstream of the trans Golgi Rab, Rab6, in regulating intra-Golgi retrograde trafficking. We found that knockdown of Rab33b, like Rab6, suppressed conserved oligomeric Golgi (COG) complex- or Zeste White 10 (ZW10)-depletion induced disruption of the Golgi ribbon in HeLa cells. Moreover, efficient GTP-restricted Rab6 induced relocation of Golgi enzymes to the endoplasmic reticulum (ER) was Rab33b-dependent, but not vice versa, suggesting that the two Rabs act sequentially in an intra-Golgi Rab cascade. In support of this hypothesis, we found that overexpression of GTP-Rab33b induced the dissociation of Rab6 from Golgi membranes in vivo. In addition, the transport of Shiga-like toxin B fragment (SLTB) from the trans to cis Golgi and ER required Rab33b. Surprisingly, depletion of Rab33b had little, if any, immediate effect on cell growth and multiplication. Furthermore, anterograde trafficking of tsO45G protein through the Golgi apparatus was normal. We suggest that the Rab33b/Rab6 regulated intra-Golgi retrograde trafficking pathway must coexist with other Golgi trafficking pathways. In conclusion, we provide the first evidence that Rab33b and Rab6 act to coordinate a major intra-Golgi retrograde trafficking pathway. This coordination may have parallels with Rab conversion/cascade events that regulate endosome, phagosome and exocytic processes.

Manganese-induced trafficking and turnover of the cis-Golgi glycoprotein GPP130

Manganese is an essential element that is also neurotoxic at elevated exposure. However, mechanisms regulating Mn homeostasis in mammalian cells are largely unknown. Because increases in cytosolic Mn induce rapid changes in the localization of proteins involved in regulating intracellular Mn concentrations in yeast, we were intrigued to discover that low concentrations of extracellular Mn induced rapid redistribution of the mammalian cis-Golgi glycoprotein Golgi phosphoprotein of 130 kDa (GPP130) to multivesicular bodies. GPP130 was subsequently degraded in lysosomes. The Mn-induced trafficking of GPP130 occurred from the Golgi via a Rab-7-dependent pathway and did not require its transit through the plasma membrane or early endosomes. Although the cytoplasmic domain of GPP130 was dispensable for its ability to respond to Mn, its lumenal stem domain was required and it had to be targeted to the cis-Golgi for the Mn response to occur. Remarkably, the stem domain was sufficient to confer Mn sensitivity to another cis-Golgi protein. Our results identify the stem domain of GPP130 as a novel Mn sensor in the Golgi lumen of mammalian cells.

Identification of beta-secretase (BACE1) substrates using quantitative proteomics

β-site APP cleaving enzyme 1 (BACE1) is a transmembrane aspartyl protease with a lumenal active site that sheds the ectodomains of membrane proteins through juxtamembrane proteolysis. BACE1 has been studied principally for its role in Alzheimer's disease as the β-secretase responsible for generating the amyloid-β protein. Emerging evidence from mouse models has identified the importance of BACE1 in myelination and cognitive performance. However, the substrates that BACE1 processes to regulate these functions are unknown, and to date only a few β-secretase substrates have been identified through candidate-based studies. Using an unbiased approach to substrate identification, we performed quantitative proteomic analysis of two human epithelial cell lines stably expressing BACE1 and identified 68 putative β-secretase substrates, a number of which we validated in a cell culture system. The vast majority were of type I transmembrane topology, although one was type II and three were GPI-linked proteins. Intriguingly, a preponderance of these proteins are involved in contact-dependent intercellular communication or serve as receptors and have recognized roles in the nervous system and other organs. No consistent sequence motif predicting BACE1 cleavage was identified in substrates versus non-substrates. These findings expand our understanding of the proteins and cellular processes that BACE1 may regulate, and suggest possible mechanisms of toxicity arising from chronic BACE1 inhibition.

EHD3 regulates early-endosome-to-Golgi transport and preserves Golgi morphology

Depletion of EHD3 affects sorting in endosomes by altering the kinetics and route of receptor recycling to the plasma membrane. Here we demonstrate that siRNA knockdown of EHD3, or its interaction partner rabenosyn-5, causes redistribution of sorting nexin 1 (SNX1) to enlarged early endosomes and disrupts transport of internalized Shiga toxin B subunit (STxB) to the Golgi. Moreover, under these conditions, Golgi morphology appears as a series of highly dispersed and fragmented stacks that maintain characteristics of cis-, medial- and trans-Golgi membranes. Although Arf1 still assembled onto these dispersed Golgi membranes, the level of AP-1 gamma-adaptin recruited to the Golgi was diminished. Whereas VSV-G-secretion from the dispersed Golgi remained largely unaffected, the distribution of mannose 6-phosphate receptor (M6PR) was altered: it remained in peripheral endosomes and did not return to the Golgi. Cathepsin D, a hydrolase that is normally transported to lysosomes via an M6PR-dependent pathway, remained trapped at the Golgi. Our findings support a role for EHD3 in regulating endosome-to-Golgi transport, and as a consequence, lysosomal biosynthetic, but not secretory, transport pathways are also affected. These data also suggest that impaired endosome-to-Golgi transport and the resulting lack of recruitment of AP-1 gamma-adaptin to Golgi membranes affect Golgi morphology.

Ancestry and evolution of a secretory pathway serpin

BACKGROUND

The serpin (serine protease inhibitor) superfamily constitutes a class of functionally highly diverse proteins usually encompassing several dozens of paralogs in mammals. Though phylogenetic classification of vertebrate serpins into six groups based on gene organisation is well established, the evolutionary roots beyond the fish/tetrapod split are unresolved. The aim of this study was to elucidate the phylogenetic relationships of serpins involved in surveying the secretory pathway routes against uncontrolled proteolytic activity.

RESULTS

Here, rare genomic characters are used to show that orthologs of neuroserpin, a prominent representative of vertebrate group 3 serpin genes, exist in early diverging deuterostomes and probably also in cnidarians, indicating that the origin of a mammalian serpin can be traced back far in the history of eumetazoans. A C-terminal address code assigning association with secretory pathway organelles is present in all neuroserpin orthologs, suggesting that supervision of cellular export/import routes by antiproteolytic serpins is an ancient trait, though subtle functional and compartmental specialisations have developed during their evolution. The results also suggest that massive changes in the exon-intron organisation of serpin genes have occurred along the lineage leading to vertebrate neuroserpin, in contrast with the immediately adjacent PDCD10 gene that is linked to its neighbour at least since divergence of echinoderms. The intron distribution pattern of closely adjacent and co-regulated genes thus may experience quite different fates during evolution of metazoans.

CONCLUSION

This study demonstrates that the analysis of microsynteny and other rare characters can provide insight into the intricate family history of metazoan serpins. Serpins with the capacity to defend the main cellular export/import routes against uncontrolled endogenous and/or foreign proteolytic activity represent an ancient trait in eukaryotes that has been maintained continuously in metazoans though subtle changes affecting function and subcellular location have evolved. It is shown that the intron distribution pattern of neuroserpin gene orthologs has undergone substantial rearrangements during metazoan evolution.

Measuring retrograde transport to the trans-Golgi network

The recently described retrograde transport route is a highly selective pathway that allows some internalized molecules to reach the trans-Golgi network from early/recycling endosomes, bypassing the recycling route to the plasma membrane and the late endocytic pathway. The non-toxic receptor-binding B-subunit of bacterial Shiga toxin has played an important role in the discovery and molecular dissection of membrane trafficking at the early/recycling endosomes-TGN interface. This unit describes several recent methods for quantitative biochemical and morphological analysis of retrograde transport. The sulfation assay permits the detection and quantification of cargo protein transport from endosomes to the TGN, describing how sulfation-site peptides can be chemically coupled to cargo proteins. Furthermore, a variant of the sulfation assay on permeabilized cells is presented. The chemical crosslinking theme is extended to horseradish peroxidase for the ultrastructural study of the Shiga toxin-containing early/recycling endosomes by whole mount analysis. Finally, an endocytosis assay describes concomitant analysis of cellular uptake of Shiga toxin and transferrin.

Intracellular neutralization of shiga toxin 2 by an a subunit-specific human monoclonal antibody

Infection of children with Shiga toxin (Stx)-producing Escherichia coli (STEC) is the leading cause of hemolytic-uremic syndrome (HUS). Stx2, one of two toxins liberated by the bacteria, is directly linked with HUS. We have previously shown that Stx2-specific human monoclonal antibodies (HuMAbs) protect mice and piglets from fatal systemic complications of Stx2. The present study investigates the mechanisms by which our most efficacious A- and B-subunit-specific HuMAbs neutralize the cytotoxic effects of Stx2 in vitro. Whereas the B-subunit-specific HuMAb 5H8 blocked binding of Stx2 to its receptor on the cell surface, the A-subunit-specific HuMAb 5C12 did not interfere with the toxin-receptor binding. Further investigations revealed that 5C12 did not block endocytosis of Stx2 by HeLa cells as both Stx2 and 5C12 colocalized with early endosomes. However, 5C12 blocked the retrograde transport of the toxin into the Golgi and the endoplasmic reticulum, preventing the toxin from entering the cytosol where the toxin exerts its cytotoxic effect. The endocytosed 5C12/Stx2 complexes appear to be rapidly transported to the plasma membrane and/or to the slow recycling perinuclear compartments, followed by their slow recycling to the plasma membrane, and release into the extracellular environment.

Both post-Golgi and intra-Golgi cycling affect the distribution of the Golgi phosphoprotein GPP130

Golgi phosphoprotein, GPP130, a cis Golgi protein, is representative of proteins cycling between the Golgi apparatus and endosomes in a pH-sensitive manner. The present qualitative data are insufficient to distinguish the relative contributions of Golgi and endosomal processes in regulating the cycling of such proteins. We have taken a quantitative approach to analyze GPP130 distribution in response to pH perturbation. We have used Shiga-like toxin B fragment, a protein that traffics from the cell surface and Golgi apparatus by the late endosomal bypass pathway, as a probe to highlight one aspect of GPP130 cycling and similarly the trafficking of tsO45-green fluorescent protein (GFP) between the Golgi apparatus and the plasma membrane to treat that aspect of GPP130 cycling in isolation. Overall, we conclude from quantitative analysis and simulations that treatment of HeLa cells with the pH perturbant, monensin, affects GPP130 cycling at several stages with effects on (i) intra-Golgi cycling, (ii) trans Golgi to endosome transport and (iii) endosome to Golgi transport. Our analysis indicates that the effect is greatest at the trans Golgi, the most acidic portion of the Golgi apparatus. In sum, multiple, regulated steps affect the trafficking of GPP130.

Alternate routes for drug delivery to the cell interior: pathways to the Golgi apparatus and endoplasmic reticulum

The targeted delivery of drugs to the cell interior can be accomplished by taking advantage of the various receptor-mediated endocytic pathways operating in a particular cell. Among these pathways, the retrograde trafficking pathway from endosomes to the Golgi apparatus, and endoplasmic reticulum is of special importance since it provides a route to deliver drugs bypassing the acid pH, hydrolytic environment of the lysosome. The existence of pathways for drug or antigen delivery to the endoplasmic reticulum and Golgi apparatus has been to a large extent an outcome of research on the trafficking of A/B type-bacterial or plant toxins such as Shiga toxin within the cell. The targeting properties of these toxins reside in their B subunit. In this article we present an overview of the multiplicity of pathways to deliver drugs intracellularly. We highlight the retrograde trafficking pathway illustrated by Shiga toxin and Shiga-like toxin, and the potential role of the B subunit of these toxins as carriers of drugs, antigens and imaging agents.

The trans-Golgi Network Golgin, GCC185, is Required for Endosome-to-Golgi Transport and Maintenance of Golgi Structure

Four mammalian golgins are specifically targeted to the trans-Golgi network (TGN) membranes via their C-terminal GRIP domains. The TGN golgins, p230/golgin-245 and golgin-97, are recruited via the GTPase Arl1, whereas the TGN golgin GCC185 is recruited independently of Arl1. Here we show that GCC185 is localized to a region of the TGN distinct from Arl1 and plays an essential role in maintaining the organization of the Golgi apparatus. Using both small interfering RNA (siRNA) and microRNA (miRNA), we show that depletion of GCC185 in HeLa cells frequently resulted in fragmentation of the Golgi apparatus. Golgi apparatus fragments were dispersed throughout the cytoplasm and contained both cis and trans markers. Trafficking of anterograde and retrograde cargo was analysed over an extended period following GCC185 depletion. Early effects of GCC185 depletion included a perturbation in the distribution of the mannose-6-phosphate receptor and a block in shiga toxin trafficking to the Golgi apparatus, which occurred in parallel with the fragmentation of the Golgi ribbon. Internalized shiga toxin accumulated in Rab11-positive endosomes, indicating GCC185 is essential for transport between the recycling endosome and the TGN. In contrast, the plasma membrane-TGN recycling protein TGN38 was efficiently transported into GCC185-depleted Golgi apparatus fragments throughout a 96-h period, and anterograde transport of E-cadherin was functional until a late stage of GCC185 depletion. This study demonstrated (i) a more effective long-term depletion of GCC185 using miRNA than siRNA and (ii) a dual role for the GCC185 golgin in the regulation of endosome-to-TGN membrane transport and in the organization of the Golgi apparatus.

Syntaxin 16 and syntaxin 5 are required for efficient retrograde transport of several exogenous and endogenous cargo proteins

Retrograde transport allows proteins and lipids to leave the endocytic pathway to reach other intracellular compartments, such as trans-Golgi network (TGN)/Golgi membranes, the endoplasmic reticulum and, in some instances, the cytosol. Here, we have used RNA interference against the SNARE proteins syntaxin 5 and syntaxin 16, combined with recently developed quantitative trafficking assays, morphological approaches and cell intoxication analysis to show that these SNARE proteins are not only required for efficient retrograde transport of Shiga toxin, but also for that of an endogenous cargo protein - the mannose 6-phosphate receptor - and for the productive trafficking into cells of cholera toxin and ricin. We have found that the function of syntaxin 16 was specifically required for, and restricted to, the retrograde pathway. Strikingly, syntaxin 5 RNA interference protected cells particularly strongly against Shiga toxin. Since our trafficking analysis showed that apart from inhibiting retrograde endosome-to-TGN transport, the silencing of syntaxin 5 had no additional effect on Shiga toxin endocytosis or trafficking from TGN/Golgi membranes to the endoplasmic reticulum, we hypothesize that syntaxin 5 also has trafficking-independent functions. In summary, our data demonstrate that several cellular and exogenous cargo proteins use elements of the same SNARE machinery for efficient retrograde transport between early/recycling endosomes and TGN/Golgi membranes.

Functionally different pools of Shiga toxin receptor, globotriaosyl ceramide, in HeLa cells

Many studies have investigated the intracellular trafficking of Shiga toxin, but very little is known about the underlying dynamics of its cellular receptor, the glycosphingolipid globotriaosyl ceramide. In this study, we show that globotriaosyl ceramide is required not only for Shiga toxin binding to cells, but also for its intracellular trafficking. Shiga toxin induces globotriaosyl ceramide recruitment to detergent-resistant membranes, and subsequent internalization of the lipid. The globotriaosyl ceramide pool at the plasma membrane is then replenished from internal stores. Whereas endocytosis is not affected in the recovery condition, retrograde transport of Shiga toxin to the Golgi apparatus and the endoplasmic reticulum is strongly inhibited. This effect is specific, as cholera toxin trafficking on GM(1) and protein biosynthesis are not impaired. The differential behavior of both toxins is also paralleled by the selective loss of Shiga toxin association with detergent-resistant membranes in the recovery condition, and comparison of the molecular species composition of plasma membrane globotriaosyl ceramide indicates subtle changes in favor of unsaturated fatty acids. In conclusion, this study demonstrates the dynamic behavior of globotriaosyl ceramide at the plasma membrane and suggests that globotriaosyl ceramide-specific determinants, possibly its molecular species composition, are selectively required for efficient retrograde sorting on endosomes, but not for endocytosis.

Precursor B cell receptor signaling activity can be uncoupled from surface expression

Signals from the precursor BCR (preBCR) cause proliferation and differentiation of progenitor (pro-) B cells into pre-B cells. Given the very low amounts of surface preBCRs and the demonstrated cell autonomy of preBCR signaling, we examined the possible occurrence of preBCR signal propagation from intracellular membranes such as the endoplasmic reticulum (ER) and the trans-Golgi network (TGN) in transformed and primary pro-B cells. PreBCRs composed of normal Ig mu or truncated Dmu heavy chains (HCs) were redirected to intracellular sites via localization sequences appended to the HC cytoplasmic tail. PreBCR complexes retained in the TGN or shunted from the TGN to lysosomes were as or 50% as active as the corresponding wild-type preBCRs in directing preBCR-dependent events, including CD2 and CD22 expression and proliferation in primary pro-B cells. This occurred despite their low to undetectable surface expression in transformed cells, which otherwise allowed significant surface accumulation of wild-type preBCRs. In contrast, ER-retained preBCRs were inactive. These results suggest that preBCR signaling is remarkably tolerant of dramatic changes in its subcellular distribution within post-ER compartments and support the possibility that the preBCR can activate signaling pathways in the TGN as well as the plasma membrane.

Transport of Ricin from Endosomes to the Golgi Apparatus is Regulated by Rab6A and Rab6A′

Ricin is transported from early endosomes and/or the recycling compartment to the trans-Golgi network (TGN) and subsequently to the endoplasmic recticulum (ER) before it enters the cytosol and intoxicates cells. We have investigated the role of the Rab6 isoforms in retrograde transport of ricin using both oligo- and vector-based RNAi assays. Ricin transport to the TGN was inhibited by the depletion of Rab6A when the Rab6A messenger RNA (mRNA) levels were reduced by more than 40% and less than 75%. However, when Rab6A mRNA was reduced by more than 75% and Rab6A' mRNA was simultaneously up-regulated, the inhibition of ricin sulfation was abolished, indicating that the up-regulation of Rab6A' may compensate for the loss of Rab6A function. In addition, we found that a near complete depletion of Rab6A' gave approximately 40% reduction in ricin sulfation. The up-regulation of Rab6A mRNA levels did not seem to compensate for the loss of Rab6A' function. The depletion of both Rab6A and Rab6A' gave a stronger inhibition of ricin sulfation than what was observed knocking down the two isoforms separately. In conclusion, both Rab6A and Rab6A' seem to be involved in the transport of ricin from endosomes to the Golgi apparatus.

COG complex-mediated recycling of Golgi glycosyltransferases is essential for normal protein glycosylation

Defects in conserved oligomeric Golgi (COG) complex result in multiple deficiencies in protein glycosylation. On the other hand, acute knock-down (KD) of Cog3p (COG3 KD) causes accumulation of intra-Golgi COG complex-dependent (CCD) vesicles. Here, we analyzed cellular phenotypes at different stages of COG3 KD to uncover the molecular link between COG function and glycosylation disorders. For the first time, we demonstrated that medial-Golgi enzymes are transiently relocated into CCD vesicles in COG3 KD cells. As a result, Golgi modifications of both plasma membrane (CD44) and lysosomal (Lamp2) glycoproteins are distorted. Localization of these proteins is not altered, indicating that the COG complex is not required for anterograde trafficking and accurate sorting. COG7 KD and double COG3/COG7 KD caused similar defects with respect to both Golgi traffic and glycosylation, suggesting that the entire COG complex orchestrates recycling of medial-Golgi-resident proteins. COG complex-dependent docking of isolated CCD vesicles was reconstituted in vitro, supporting their role as functional trafficking intermediates. Altogether, the data suggest that constantly cycling medial-Golgi enzymes are transported from distal compartments in CCD vesicles. Dysfunction of COG complex leads to separation of glycosyltransferases from anterograde cargo molecules passing along secretory pathway, thus affecting normal protein glycosylation.

GM130 and GRASP65-dependent lateral cisternal fusion allows uniform Golgi-enzyme distribution

The mammalian Golgi apparatus exists as stacks of cisternae that are laterally linked to form a continuous membrane ribbon, but neither the molecular requirements for, nor the purpose of, Golgi ribbon formation are known. Here, we demonstrate that ribbon formation is mediated by specific membrane-fusion events that occur during Golgi assembly, and require the Golgi proteins GM130 and GRASP65. Furthermore, these GM130 and GRASP65-dependent lateral cisternal-fusion reactions are necessary to achieve uniform distribution of enzymes in the Golgi ribbon. The membrane continuity created by ribbon formation facilitates optimal processing conditions in the biosynthetic pathway.

Protein toxins: intracellular trafficking for targeted therapy

The immunotoxin approach is based on the use of tumor-targeting ligands or antibodies that are linked to the catalytic (toxic) moieties of bacterial or plant protein toxins. In this review, we first discuss the current state of clinical development of immunotoxin approaches describing the results obtained with the two toxins most frequently used: diphtheria and Pseudomonas toxin-derived proteins. In the second part of the review, a novel concept will be presented in which the roles are inverted: nontoxic receptor-binding toxin moieties are used for the targeting of therapeutic and diagnostic compounds to cancer or immune cells. The cell biological basis of these novel types of toxin-based therapeutics will be discussed, and we will summarize ongoing preclinical and clinical testing.

Stability of the homopentameric B subunits of shiga toxins 1 and 2 in solution and the gas phase as revealed by nanoelectrospray fourier transform ion cyclotron resonance mass spectrometry

The assembly of the B subunits of Shiga toxins (Stx) 1 and 2 and the influence of solution conditions (protein concentration, temperature, pH, and ionic strength) on it are investigated using temperature-controlled nanoflow electrospray (nano-ES) ionization and Fourier-transform ion cyclotron resonance mass spectrometry. Despite the similar higher order structure predicted by X-ray crystallography analysis, the B(5) homopentamers of Stx1 and Stx2 exhibit differences in stability under the solution conditions investigated. At solution temperatures ranging from 0 to 60 degrees C and subunit concentrations ranging from 5 to 85 microM, the Stx1 B subunit exists almost entirely as the homopentamer in aqueous solutions, independent of the ionic strength. In contrast, the degree of assembly of Stx2 B subunit is strongly dependent on temperature, subunit concentration, and ionic strength. At subunit concentrations of more than 50 microM, the Stx2 B subunit exists predominantly as a pentamer, although smaller multimers (dimer, trimer, and tetramer) are also evident. At lower concentrations, the Stx2 B subunit exists predominantly as monomer and dimer. The relative abundance of multimeric species of the Stx2 B subunit was insensitive to the ion source conditions, suggesting that gas-phase dissociation of the pentamer ions in the source does not influence the mass spectrum. Blackbody infrared radiative dissociation of the protonated B(5) ions of Stx2 at the +12 and +13 charge states proceeds, at reaction temperatures of 120 to 180 degrees C, predominantly by the ejection of a single subunit from the complex. Dissociation into dimer and trimer ions constitutes a minor pathway. It follows that the dimer and trimer ions and, likely, the monomer ions observed in the nano-ES mass spectra of Stx2 B subunit originated in solution and not from gas-phase reactions. It is concluded that, under the solution conditions investigated, the homopentamer of Stx2 B subunit is thermodynamically less stable than that of Stx1 B subunit. Arrhenius activation parameters determined for the protonated Stx2 B(5) ions at the +12 and +13 charge states were compared with values reported for the corresponding B(5) ions of Stx1 B subunit. In contrast to the differential stability of the Stx1 and Stx2 B pentamers in solution, the dissociation activation energies (E(a)) determined for the gaseous complexes are indistinguishable at a given charge state. The similarity in the E(a) values suggests that the protonated pentamer ions of both toxins are stabilized by similar intersubunit interactions in the gas phase, a result that is in agreement with the X-ray crystal structures of the holotoxins.

tGolgin-1 (p230, golgin-245) modulates Shiga-toxin transport to the Golgi and Golgi motility towards the microtubule-organizing centre

tGolgin-1 (trans-Golgi p230, golgin-245) is a member of a family of large peripheral membrane proteins that associate with the trans-Golgi network (TGN) via a C-terminal GRIP domain. Some GRIP-domain proteins have been implicated in endosome-to-TGN transport but no function for tGolgin-1 has been described. Here, we show that tGolgin-1 production is required for efficient retrograde distribution of Shiga toxin from endosomes to the Golgi. Surprisingly, we also found an indirect requirement for tGolgin-1 in Golgi positioning. In HeLa cells depleted of tGolgin-1, the normally centralized Golgi and TGN membranes were displaced to the periphery, forming `mini stacks'. These stacks resembled those in cells with disrupted microtubules or dynein-dynactin motor, in that they localized to endoplasmic-reticulum exit sites, maintained their secretory capacity and cis-trans polarity, and were relatively immobile by video microscopy. The mini stacks formed concomitant with a failure of pre-Golgi elements to migrate along microtubules towards the microtubule-organizing centre. The requirement for tGolgin-1 in Golgi positioning did not appear to reflect direct binding of tGolgin-1 to motile pre-Golgi membranes, because distinct Golgi and tGolgin-1-containing TGN elements that formed after recovery of HeLa cells from brefeldin-A treatment moved independently toward the microtubule-organizing centre. These data demonstrate that tGolgin-1 functions in Golgi positioning indirectly, probably by regulating retrograde movement of cargo required for recruitment or activation of dynein-dynactin complexes on newly formed Golgi elements.