Selective export of human GPI-anchored proteins from the endoplasmic reticulum

CD59 incorporation protects hepatitis C virus against complement-mediated destruction

Several enveloped viruses including human immunodeficiency virus type 1 (HIV-1), cytomegalovirus (CMV), herpes simplex virus 1 (HSV-1), Ebola virus, vaccinia virus, and influenza virus have been found to incorporate host regulators of complement activation (RCA) into their viral envelopes and, as a result, escape antibody-dependent complement-mediated lysis (ADCML). Hepatitis C virus (HCV) is an enveloped virus of the family Flaviviridae and incorporates more than 10 host lipoproteins. Patients chronically infected with HCV develop high-titer and crossreactive neutralizing antibodies (nAbs), yet fail to clear the virus, raising the possibility that HCV may also use the similar strategy of RCA incorporation to escape ADCML. The current study was therefore undertaken to determine whether HCV virions incorporate biologically functional CD59, a key member of RCA. Our experiments provided several lines of evidence demonstrating that CD59 was associated with the external membrane of HCV particles derived from either Huh7.5.1 cells or plasma samples from HCV-infected patients. First, HCV particles were captured by CD59-specific Abs. Second, CD59 was detected in purified HCV particles by immunoblot analysis and in the cell-free supernatant from HCV-infected Huh7.5.1 cells, but not from uninfected or adenovirus serotype 5 (Ad5) (a nonenveloped cytolytic virus)-infected Huh7.5.1 cells by enzyme-linked immunosorbent assay. Last, abrogation of CD59 function with its blockers increased the sensitivity of HCV virions to ADCML, resulting in a significant reduction of HCV infectivity. Additionally, direct addition of CD59 blockers into plasma samples from HCV-infected patients increased autologous virolysis.

CONCLUSION

Our study, for the first time, demonstrates that CD59 is incorporated into both cell line-derived and plasma primary HCV virions at levels that protect against ADCML. This is also the first report to show that direct addition of RCA blockers into plasma from HCV-infected patients renders endogenous plasma virions sensitive to ADCML.

Vesicle-mediated ER export of proteins and lipids

In eukaryotic cells, the endoplasmic reticulum (ER) is a major site of synthesis of both lipids and proteins, many of which must be transported to other organelles. The COPII coat-comprising Sar1, Sec23/24, Sec13/31-generates transport vesicles that mediate the bulk of protein/lipid export from the ER. The coat exhibits remarkable flexibility in its ability to specifically select and accommodate a large number of cargoes with diverse properties. In this review, we discuss the fundamentals of COPII vesicle production and describe recent advances that further our understanding of just how flexible COPII cargo recruitment and vesicle formation may be. Large or bulky cargo molecules (e.g. collagen rods and lipoprotein particles) exceed the canonical size for COPII vesicles and seem to rely on the additional action of recently identified accessory molecules. Although the bulk of the research has focused on the fate of protein cargo, the mechanisms and regulation of lipid transport are equally critical to cellular survival. From their site of synthesis in the ER, phospholipids, sphingolipids and sterols exit the ER, either accompanying cargo in vesicles or directly across the cytoplasm shielded by lipid-transfer proteins. Finally, we highlight the current challenges to the field in addressing the physiological regulation of COPII vesicle production and the molecular details of how diverse cargoes, both proteins and lipids, are accommodated. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

TMED2/p24β1 is expressed in all gestational stages of human placentas and in choriocarcinoma cell lines

Members of the transmembrane emp24 domain (Tmed)/p24 family of proteins are required for transport of proteins between the endoplasmic reticulum and the Golgi. One member of this family, Tmed2/p24β1, is expressed during placental development in mice and its expression is required for normal development of the labyrinth layer. Although TMED2 is conserved in humans, little is known about its expression and function in human placenta. We examined TMED2 expression in human placenta between 5.5 and 40 weeks of gestation and showed that TMED2 is expressed in syncytiotrophoblast, cytotrophoblast, and stromal cells. We also found high levels of TMED2 expression in BeWo but not in JEG-3 choriocarcinoma cell line. We used the BeWo cell line to determine TMED2 subcellular localization in placental cells and show its co-localization with the endoplasmic reticulum Golgi intermediate compartment. Our findings show conservation of TMED2 expression in human placenta and suggest that this protein may also play a role during placental development in humans.

Transgenic neuronal overexpression reveals that stringently regulated p23 expression is critical for coordinated movement in mice

Background

p23 belongs to the highly conserved p24 family of type I transmembrane proteins, which participate in the bidirectional protein transport between the endoplasmic reticulum and Golgi apparatus. Mammalian p23 has been shown to interact with γ-secretase complex, and modulate secretory trafficking as well as intramembranous processing of amyloid precursor protein in cultured cells. Negative modulation of β-amyloid production by p23 in cultured cell lines suggested that elevation of p23 expression in neurons might mitigate cerebral amyloid burden.

Results

We generated several lines of transgenic mice expressing human p23 in neurons under the control of Thy-1.2 promoter. We found that even a 50% increase in p23 levels in the central nervous system of mice causes post-natal growth retardation, severe neurological problems characterized by tremors, seizure, ataxia, and uncoordinated movements, and premature death. The severity of the phenotype closely correlated with the level of p23 overexpression in multiple transgenic lines. While the number and general morphology of neurons in Hup23 mice appeared to be normal throughout the brain, abnormal non-Golgi p23 localization was observed in a subset of neurons with high transgene expression in brainstem. Moreover, detailed immunofluorescence analysis revealed marked proliferation of astrocytes, activation of microglia, and thinning of myelinated bundles in brainstem of Hup23 mice.

Conclusions

These results demonstrate that proper level of p23 expression is critical for neuronal function, and perturbing p23 function by overexpression initiates a cascade of cellular reactions in brainstem that leads to severe motor deficits and other neurological problems, which culminate in premature death. The neurological phenotype observed in Hup23 mice highlights significant adverse effects associated with manipulating neuronal expression of p23, a previously described negative modulator of γ-secretase activity and β-amyloid production. Moreover, our report has broader relevance to molecular mechanisms in several neurodegenerative diseases as it highlights the inherent vulnerability of the early secretory pathway mechanisms that ensure proteostasis in neurons.

COPII and the regulation of protein sorting in mammals

Secretory proteins are transported to the Golgi complex in vesicles that bud from the endoplasmic reticulum. The cytoplasmic coat protein complex II (COPII) is responsible for cargo sorting and vesicle morphogenesis. COPII was first described in Saccharomyces cerevisiae, but its basic function is conserved throughout all eukaryotes. Nevertheless, the COPII coat has adapted to the higher complexity of mammalian physiology, achieving more sophisticated levels of secretory regulation. In this review we cover aspects of mammalian COPII-mediated regulation of secretion, in particular related to the function of COPII paralogues, the spatial organization of cargo export and the role of accessory proteins.

A genome-wide RNA interference screen uncovers two p24 proteins as regulators of Wingless secretion

Wnt proteins are secreted, lipid-modified glycoproteins that control animal development and adult tissue homeostasis. Secretion of Wnt proteins is at least partly regulated by a dedicated machinery. Here, we report a genome-wide RNA interference screen for genes involved in the secretion of Wingless (Wg), a Drosophila Wnt. We identify three new genes required for Wg secretion. Of these, Emp24 and Eclair are required for proper export of Wg from the endoplasmic reticulum (ER). We propose that Emp24 and Eca act as specific cargo receptors for Wg to concentrate it in forming vesicles at sites of ER export.

CDP-diacylglycerol phospholipid synthesis in detergent-soluble, non-raft, membrane microdomains of the endoplasmic reticulum

Phosphatidylinositol (PI) is essential for numerous cell functions and is generated by consecutive reactions catalyzed by CDP-diacylglycerol synthase (CDS) and PI synthase. In this study, we investigated the membrane organization of CDP-diacylglycerol synthesis. Separation of mildly disrupted A431 cell membranes on sucrose density gradients revealed cofractionation of CDS and PI synthase activities with cholesterol-poor, endoplasmic reticulum (ER) membranes and partial overlap with plasma membrane caveolae. Cofractionation of CDS activity with caveolae was also observed when low-buoyant density caveolin-enriched membranes were prepared using a carbonate-based method. However, immunoisolation studies determined that CDS activity localized to ER membrane fragments containing calnexin and type III inositol (1,4,5)-trisphosphate receptors but not to caveolae. Membrane fragmentation in neutral pH buffer established that CDP-diacylglycerol and PI syntheses were restricted to a subfraction of the calnexin-positive ER. In contrast to lipid rafts enriched for caveolin, cholesterol, and GM1 glycosphingolipids, the CDS-containing ER membranes were detergent soluble. In cell imaging studies, CDS and calnexin colocalized in microdomain-sized patches of the ER and also unexpectedly at the plasma membrane. These results demonstrate that key components of the PI pathway localize to nonraft, phospholipid-synthesizing microdomains of the ER that are also enriched for calnexin.

Differential selection of Golgi proteins by COPII Sec24 isoforms in procyclic Trypanosoma brucei

The Sec24 subunit of the coat protein complex II (COPII) has been implicated in selecting newly synthesized cargo from the endoplasmic reticulum (ER) for delivery to the Golgi. The protozoan parasite, Trypanosoma brucei, contains two paralogs, TbSec24.1 and TbSec24.2, which were depleted using RNA interference in the insect form of the parasite. Depletion of either TbSec24.1 or TbSec24.2 resulted in growth arrest and modest inhibition of anterograde transport of the putative Golgi enzyme, TbGntB, and the secretory marker, BiPNAVRG-HA9. In contrast, depletion of TbSec24.1, but not TbSec24.2, led to reversible mislocalization of the Golgi stack proteins, TbGRASP and TbGolgin63. The latter accumulated in the ER. The localization of the COPI coatomer subunit, TbεCOP, and the trans Golgi network (TGN) protein, TbGRIP70, was largely unaffected, although the latter was preferentially lost from those Golgi that were not associated with the bilobe, a structure previously implicated in Golgi biogenesis. Together, these data suggest that TbSec24 paralogs can differentiate among proteins destined for the Golgi.

Golgi glycosylation and human inherited diseases

The Golgi factory receives custom glycosylates and dispatches its cargo to the correct cellular locations. The process requires importing donor substrates, moving the cargo, and recycling machinery. Correctly glycosylated cargo reflects the Golgi's quality and efficiency. Genetic disorders in the specific equipment (enzymes), donors (nucleotide sugar transporters), or equipment recycling/reorganization components (COG, SEC, golgins) can all affect glycosylation. Dozens of human glycosylation disorders fit these categories. Many other genes, with or without familiar names, well-annotated pedigrees, or likely homologies will join the ranks of glycosylation disorders. Their broad and unpredictable case-by-case phenotypes cross the traditional medical specialty boundaries. The gene functions in patients may be elusive, but their common feature may include altered glycosylation that provide clues to Golgi function. This article focuses on a group of human disorders that affect protein or lipid glycosylation. Readers may find it useful to generalize some of these patient-based, translational observations to their own research.

Sorting of GPI-anchored proteins into ER exit sites by p24 proteins is dependent on remodeled GPI

p24 complexes act as cargo receptors for sorting GPI-anchored proteins into COPII vesicles.

Glycosylphosphatidylinositol (GPI) anchoring of proteins is a posttranslational modification occurring in the endoplasmic reticulum (ER). After GPI attachment, proteins are transported by coat protein complex II (COPII)-coated vesicles from the ER. Because GPI-anchored proteins (GPI-APs) are localized in the lumen, they cannot interact with cytosolic COPII components directly. Receptors that link GPI-APs to COPII are thought to be involved in efficient packaging of GPI-APs into vesicles; however, mechanisms of GPI-AP sorting are not well understood. Here we describe two remodeling reactions for GPI anchors, mediated by PGAP1 and PGAP5, which were required for sorting of GPI-APs to ER exit sites. The p24 family of proteins recognized the remodeled GPI-APs and sorted them into COPII vesicles. Association of p24 proteins with GPI-APs was pH dependent, which suggests that they bind in the ER and dissociate in post-ER acidic compartments. Our results indicate that p24 complexes act as cargo receptors for correctly remodeled GPI-APs to be sorted into COPII vesicles.

The yeast p24 complex regulates GPI-anchored protein transport and quality control by monitoring anchor remodeling

Two functions of the p24 complex are described: one connects GPI-anchored proteins to COPII proteins at ER exit sites to facilitate their incorporation into ER-derived vesicles, and the other serves in quality control of GPI-anchored proteins to retrieve unremodeled GPI-anchored proteins from the Golgi back to the ER.

Glycosylphosphatidylinositol (GPI)-anchored proteins are secretory proteins that are attached to the cell surface of eukaryotic cells by a glycolipid moiety. Once GPI anchoring has occurred in the lumen of the endoplasmic reticulum (ER), the structure of the lipid part on the GPI anchor undergoes a remodeling process prior to ER exit. In this study, we provide evidence suggesting that the yeast p24 complex, through binding specifically to GPI-anchored proteins in an anchor-dependent manner, plays a dual role in their selective trafficking. First, the p24 complex promotes efficient ER exit of remodeled GPI-anchored proteins after concentration by connecting them with the COPII coat and thus facilitates their incorporation into vesicles. Second, it retrieves escaped, unremodeled GPI-anchored proteins from the Golgi to the ER in COPI vesicles. Therefore the p24 complex, by sensing the status of the GPI anchor, regulates GPI-anchored protein intracellular transport and coordinates this with correct anchor remodeling.

Structural remodeling, trafficking and functions of glycosylphosphatidylinositol-anchored proteins

Glycosylphosphatidylinositol (GPI) is a glycolipid that is covalently attached to proteins as a post-translational modification. Such modification leads to the anchoring of the protein to the outer leaflet of the plasma membrane. Proteins that are decorated with GPIs have unique properties in terms of their physical nature. In particular, these proteins tend to accumulate in lipid rafts, which are critical for the functions and trafficking of GPI-anchored proteins (GPI-APs). Recent studies mainly using mutant cells revealed that various structural remodeling reactions occur to GPIs present in GPI-APs as they are transported from the endoplasmic reticulum to the cell surface. This review examines the recent progress describing the mechanisms of structural remodeling of mammalian GPI-anchors, such as inositol deacylation, glycan remodeling and fatty acid remodeling, with particular focus on their trafficking and functions, as well as the pathogenesis involving GPI-APs and their deficiency.

The serotonin transporter is an exclusive client of the coat protein complex II (COPII) component SEC24C

The transporters for serotonin (SERT), dopamine, and noradrenaline have a conserved hydrophobic core but divergent N and C termini. The C terminus harbors the binding site for the coat protein complex II (COPII) cargo-binding protein SEC24. Here we explored which SEC24 isoform was required for export of SERT from the endoplasmic reticulum (ER). Three lines of evidence argue that SERT can only exit the ER by recruiting SEC24C: (i) Mass spectrometry showed that a peptide corresponding to the C terminus of SERT recruited SEC24C-containing COPII complexes from mouse brain lysates. (ii) Depletion of individual SEC24 isoforms by siRNAs revealed that SERT was trapped in the ER only if SEC24C was down-regulated, in both, cells that expressed SERT endogenously or after transfection. The combination of all siRNAs was not more effective than that directed against SEC24C. A SERT mutant in which the SEC24C-binding motif ((607)RI(608)) was replaced by alanine was insensitive to down-regulation of SEC24C levels. (iii) Overexpression of a SEC24C variant with a mutation in the candidate cargo-binding motif (SEC24C-D796V/D797N) but not of the corresponding mutant SEC24D-D733V/D734N reduced SERT surface levels. In contrast, noradrenaline and dopamine transporters and the more distantly related GABA transporter 1 relied on SEC24D for ER export. These observations demonstrate that closely related transporters are exclusive client cargo proteins for different SEC24 isoforms. The short promoter polymorphism results in reduced SERT cell surface levels and renders affected individuals more susceptible to depression. By inference, variations in the Sec24C gene may also affect SERT cell surface levels and thus be linked to mood disorders.

Growth arrest-specific 1 binds to and controls the maturation and processing of the amyloid-beta precursor protein

Alzheimer's disease (AD), the most common neurodegenerative disorder, is characterized by cerebral deposition of amyloid-β (Aβ), a series of peptides derived from the processing of the amyloid-β precursor protein (APP). To identify new candidate genes for AD, we recently performed a transcriptome analysis to screen for genes preferentially expressed in the hippocampus and located in AD linkage regions. This strategy identified CALHM1 (calcium homeostasis modulator 1), a gene modulating AD age at onset and Aβ metabolism. Here, we focused our attention on another candidate identified using this screen, growth arrest-specific 1 (Gas1), a gene involved in the central nervous system development. We found that Gas1 formed a complex with APP and controlled APP maturation and processing. Gas1 expression inhibited APP full glycosylation and routing to the cell surface by leading to a trafficking blockade of APP between the endoplasmic reticulum and the Golgi. Gas1 expression also resulted in a robust inhibition of APP transport into multivesicular bodies, further demonstrating that Gas1 negatively regulated APP intracellular trafficking. Consequently, Gas1 overexpression led to a reduction in Aβ production, and conversely, Gas1 silencing in cells expressing endogenously Gas1 increased Aβ levels. These results suggest that Gas1 is a novel APP-interacting protein involved in the control of APP maturation and processing.

Akt phosphorylates Sec24: new clues into the regulation of ER-to-Golgi trafficking

Regulation of protein transport within the early secretory pathway is a relatively unexplored area. Here, we propose a new player in the control of protein transport from the endoplasmic reticulum (ER) to the Golgi. Akt is an important signaling kinase whose functioning is perturbed in diseases such as cancer and diabetes. We discovered that Akt phosphorylates Sec24, an essential coat protein II (COPII) component involved in mediating cargo selection for ER-to-Golgi trafficking. We discuss how this finding may provide new insights into the regulation of protein transport.

Cargo loading at the ER

Trafficking of newly synthesized cargo through the early secretory pathway defines and maintains the intracellular organization of eukaryotic cells as well as the organization of tissues and organs. The importance of this pathway is underlined by the increasing number of mutations in key components of the ER export machinery that are causative of a diversity of human diseases. Here we discuss the molecular mechanisms that dictate cargo selection during vesicle budding. While, in vitro reconstitution assays, unicellular organisms such as budding yeast, and mammalian cell culture still have much to offer in terms of gaining a full understanding of the molecular basis for secretory cargo export, such assays have to date been limited to analysis of smaller, freely diffusible cargoes. The export of large macromolecular complexes from the ER such as collagens (up to 300 nm) or lipoproteins (~500 nm) presents a clear problem in terms of maintaining both selectivity and efficiency of export. It has also become clear that in order to translate our knowledge of the molecular basis for ER export to a full understanding of the implications for normal development and disease progression, the use of metazoan models is essential. Combined, these approaches are now starting to shed light not only on the mechanisms of macromolecular cargo export from the ER but also reveal the implications of failure of this process to human development and disease.