Golgi-dependent Pathway Research Articles

Endomembrane-mediated storage protein trafficking in plants: Golgi-dependent or Golgi-independent?

Seed storage proteins (SSPs) accumulated within plant seeds constitute the major protein nutrition sources for human and livestock. SSPs are synthesized on the endoplasmic reticulum and are then deposited in plant-specific protein bodies, including endoplasmic reticulum-derived protein bodies and protein storage vacuoles. Plant seeds have evolved a distinct endomembrane system to accomplish SSP transport. There are two distinct types of trafficking pathways contributing to SSP delivery to protein storage vacuoles: one is Golgi-dependent and the other is Golgi-independent. In recent years, molecular, genetic, and biochemical studies have shed light on the complex network controlling SSP trafficking, to which both evolutionarily conserved molecular machineries and plant-unique regulators contribute. In this review, we discuss current knowledge of protein body biogenesis and endomembrane-mediated SSP transport, focusing on endoplasmic reticulum export and post-Golgi traffic. This knowledge supports a dominant role for the Golgi-dependent pathways in SSP transport in Arabidopsis and rice. In addition, we describe cutting-edge strategies for dissecting the endomembrane trafficking system in plant seeds to advance the field.

Dually localised proteins found in both the apicoplast and mitochondrion utilize the Golgi-dependent pathway for apicoplast targeting in Toxoplasma gondii.

Like other apicomplexan parasites, Toxoplasma gondii harbours a four-membraned endosymbiotic organelle- the apicoplast. Apicoplast proteins are nuclear encoded and trafficked to the organelle through the endoplasmic reticulum (ER). From the ER to the apicoplast, two distinct protein trafficking pathways can be used. One such pathway is the cell's secretory pathway involving the Golgi, whereas the other is a unique Golgi-independent pathway. Using different experimental approaches, many apicoplast proteins have been shown to utilize the Golgi-independent pathway, whereas a handful of reports show that a few proteins use the Golgi-dependent pathway. This has led to an emphasis towards the unique Golgi-independent pathway when apicoplast protein trafficking is discussed in the literature. Additionally, the molecular features that drive proteins to each pathway are not known. In this report, we systematically test eight apicoplast proteins, using a C-terminal HDEL sequence to assess the role of the Golgi in their transport. We demonstrate that dually localised proteins of the apicoplast and mitochondrion (TgSOD2, TgTPx1/2 and TgACN/IRP) are trafficked through the Golgi, whereas proteins localised exclusively to the apicoplast are trafficked independent of the Golgi. Mutants of the dually localised proteins that localised exclusively to the apicoplast also showed trafficking through the Golgi. Phylogenetic analysis of TgSOD2, TgTPx1/2 and TgACN/IRP suggested that the evolutionary origins of TgSOD2 and TgTPx1/2 lie in the mitochondrion, whereas TgACN/IRP appears to have originated from the apicoplast. Collectively, with these results, for the first time, we establish that the driver of the Golgi-dependent trafficking route to the apicoplast is the dual localisation of the protein to the apicoplast and the mitochondrion.

Sunflower storage proteins are transported in dense vesicles that contain proteins homologous to the pumpkin vacuolar sorting receptor PV 72

Storage proteins are transported to a special storage compartments in seeds by Golgi dependent or independent pathways depending on the plant species. The aim of this work was to study the sunflower storage protein transport pathway and identified component of the sorting machinery. Immature sunflower seeds were analyzed by subcellular fractionation (using percoll and sucrose gradients) and electron microscopy. The vesicles isolated with percoll, have precursors of 11S globulins, α-TIP, δ-TIP, BiP, and two proteins that have homology to the pumpkin vacuolar sorting receptor PV72. Sucrose isolated vesicles have the same composition than percoll ones, except for the lack of BiP and the presence of only one protein that has reactivity with pea VSR BP80. Electronic micrographies of developing seeds show that the formation of electron dense aggregates starts in the endoplasmic reticulum, and that these aggregates are very abundant in the trans-Golgi apparatus, where release of dense vesicles happens. These vesicles contain a homolog of PV72 in their membranes. Storage proteins are also detected in multivesicular bodies whose membranes have reactivity with PV72 serum. All these results indicated that sunflower storage proteins are transported to protein storage vacuoles by a Golgi dependent pathway in a process in which homologous of PV72 are involved.

Conventional and unconventional secretory proteins expressed with silkworm bombyxin signal peptide display functional fidelity

Growth factors are signaling molecules which orchestrate cell growth, proliferation and differentiation. The majority are secreted proteins, exported through the classical endoplasmic reticulum (ER)/Golgi-dependent pathway, but a few are released by unconventional ER/Golgi-independent means. Human fibroblast growth factor 2 (FGF2) and insulin-like growth factor 1 (IGF1), are canonical prototypes secreted by the unconventional and conventional pathway, respectively. We herein examined whether expression of these two growth factors in the Bombyx mori nucleopolyhedrovirus (BmNPV)-based silkworm expression system with its innate signal peptide, bombyxin, secures structural homogeneity at the signal peptide cleavage site regardless of the native secretory route. Proteomic analysis mapped structural microheterogeneity of signal peptide cleavage at the amino terminus of FGF2, whereas IGF1 displayed homogeneous amino-terminal cleavage with complete removal of the bombyxin signal peptide. A cell proliferation assay revealed potent functional activity of both FGF2 and IGF1, suggesting that FGF2 amino-terminal microheterogeneity does not alter mitogenic activity. These findings demonstrate that the occurrence of amino-terminal structural homogeneity may be associated with the original secretion mechanism of a particular growth factor. Furthermore, our results highlight the bombyxin signal peptide as a reliable secretion sequence applicable to mass production of functionally active secretory proteins in a silkworm-based expression platform.

The unconventional secretion of PepN is independent of a functional autophagy machinery in the filamentous fungus Aspergillus niger.

During unconventional protein secretion (UPS), proteins do not pass through the classical endoplasmic reticulum (ER)-Golgi-dependent pathway, but are transported to the cell membrane via alternative routes. One type of UPS is dependent on several autophagy-related (Atg) proteins in yeast and mammalian cells, but mechanisms for unconventional secretion are largely unknown for filamentous fungi. In this study, we investigated whether the autophagy machinery is used for UPS in the filamentous fungus Aspergillus niger An aspartic protease, which we called PepN, was identified as being likely to be secreted unconventionally, as this protein is highly abundant in culture filtrates during carbon starvation while it lacks a conventional N-terminal secretion sequence. We analysed the presence of PepN in the culture filtrates of carbon starved wild-type, atg1 and atg8 deletion mutant strains by Western blot analysis and by secretome analysis using nanoLC-ESI-MS/MS (wild-type and atg8 deletion mutant). Besides the presence of carbohydrate-active enzymes and other types of proteases, PepN was abundantly found in culture filtrates of both wild-type and atg deletion strains, indicating that the secretion of PepN is independent of the autophagy machinery in A. niger and hence most likely occurs via a different mechanism.

Fibrillin-1 mgΔlpn Marfan syndrome mutation associates with preserved proteostasis and bypass of a protein disulfide isomerase-dependent quality checkpoint

Fibrillin-1 mutations promote Marfan syndrome (MFS) via complex yet unclear pathways. The roles of endoplasmic reticulum (ER) and the major ER redox chaperone protein disulfide isomerase-A1 in the processing of normal and mutated fibrillin-1 and ensuing protein secretion and/or intracellular retention are unclear. Our results in mouse embryonic fibroblasts bearing the exon-skipping mgΔlox-P-neo (mgΔlpn) mutation, which associates in vivo with MFS and in vitro with disrupted microfibrils, indicate a preserved ER-dependent proteostasis or redox homeostasis. Rather, mutated fibrillin-1 is secreted normally through Golgi-dependent pathways and is not intracellularly retained. Similar results occurred for the C1039G point mutation. In parallel, we provide evidence that PDIA1 physically interacts with fibrillin-1 in the ER. Moreover, siRNA against PDIA1 augmented fibrillin-1 secretion rates in wild-type cells. However, fibrillin-1 with the mgΔlpn mutation bypassed PDI checkpoint delay, while the C1039G mutation did not. This heretofore undisclosed PDIA1-mediated mechanism may be important to control the extracellular availability of function-competent fibrillin-1, an important determinant of disease phenotype. Moreover, our results may reveal a novel, holdase-like, PDI function associated with ER protein quality control.

Secretion of SerpinB2 from endothelial cells activated with inflammatory stimuli

Due to the lack of an N-terminal signal peptide, SerpinB2 (plasminogen activator inhibitor type 2) accumulates in cells and only a small percentage of it is secreted. The extracellular concentration of SerpinB2 significantly increases during inflammation. In the present study we investigated the mechanism with which SerpinB2 can be secreted from endothelial cells activated with LPS. We evaluated the intracellular distribution of SerpinB2 by double immunogold labeling followed by a high resolution electron microscopy analysis. We found that SerpinB2 gathers in the vesicular structures and in the endothelial cell periphery. These vesicles stained positive for the trans-Golgi network marker TGN46, which is consistent with their formation by the endoplasmatic reticulum (ER) and Golgi-dependent pathways. SerpinB2 was delivered to the plasma membrane, apparently together with TGN46 in the same vesicles, which after fusion with the membranes released cargo. Secretion of SerpinB2 was partially inhibited by brefeldin A. The secreted SerpinB2 was predominantly in its nonglycosylated 43kDa form as evaluated by Western immunoblotting. Our data suggest that increased expression of SerpinB2 by an inflammatory stimulus is sufficient to generate structures that resemble secretory vesicles. These vesicles may represent the mechanism by which high local concentrations of SerpinB2 are released at inflammation sites from endothelial cells.

Detachment of esophageal carcinoma cells from extracellular matrix causes relocalization of death receptor 5 and apoptosis

To investigate the effect of detachment of esophageal cancer cells from extracellular matrix on the localization of death receptor 5 (DR5) and apoptosis. Anchorage-dependent EC9706 cells of esophageal squamous cell carcinoma were pretreated or not treated with brefeldin A. Detached cells were harvested by ethylenediaminetetraacetic acid digestion. Expression and localization of DR5 in these cells were determined by immunocytochemical and immunofluorescence assays, as well as flow cytometry analysis. Apoptosis of EC9706 cells was detected by flow cytometry after stained with fluorescein isothiocyanate-labeled annexin V/propidium iodide. Activation of caspase 8 was detected by Western blot analysis. Immunocytochemical assay indicated that DR5 was predominantly perinuclear in adherent cells but was mainly localized in cell membrane in detached cells. In addition, immunofluorescence assay also confirmed the above-mentioned results, and further demonstrated that DR5 was present in the form of coarse granules in detached cells, but in the form of fine granules in adherent cells. Cytometry analysis revealed higher levels of DR5 expression on the surfaces of brefeldin-A-untreated cells than on the surfaces of brefeldin-A-treated cells, but brefeldin A treatment did not affect the total DR5 expression levels. Moreover, nocodazole did not influence the extracelluar DR5 expression levels in EC9706 cells. Apoptosis assay revealed that detached cells were more sensitive to DR5 antibody-induced apoptosis than adherent cells. Western blotting showed that caspase 8 was activated in temporarily detached cells 4 h earlier than in adherent cells. Progress from adhesion to detachment of EC9706 cells causes DR5 relocalization, and promotes cytoplasmic translocation of DR5 to cell surfaces via a Golgi-dependent pathway. Moreover, it might also result in DR5 aggregation to render apoptosis of detached cells.

Vesicular trafficking and secretion of matrix metalloproteinases-2, -9 and tissue inhibitor of metalloproteinases-1 in neuronal cells

Matrix metalloproteinases (MMPs) are endopeptidases that cleave matrix, soluble and membrane-bound proteins and are regulated by their endogenous inhibitors the tissue inhibitors of MMPs (TIMPs). Nothing is known about MMP/TIMP trafficking and secretion in neuronal cells. We focussed our attention on the gelatinases MMP-2 and MMP-9, and their inhibitor TIMP-1. MMPs and TIMP-1 fused to GFP were expressed in N2a neuroblastoma and primary neuronal cells to study trafficking and secretion using real time video-microscopy, imaging, electron microscopy and biochemical approaches. We show that MMPs and TIMP-1 are secreted in 160–200 nm vesicles in a Golgi-dependent pathway. These vesicles distribute along microtubules and microfilaments, co-localise differentially with the molecular motors kinesin and myosin Va and undergo both anterograde and retrograde trafficking. MMP-9 retrograde transport involves the dynein/dynactin molecular motor. In hippocampal neurons, MMP-2 and MMP-9 vesicles are preferentially distributed in the somato-dendritic compartment and are found in dendritic spines. Non-transfected hippocampal neurons also demonstrate vesicular secretion of MMP-2 in both its pro- and active forms and gelatinolytic activity localised within dendritic spines. Our results show differential trafficking of MMP and TIMP-1-containing vesicles in neuronal cells and suggest that these vesicles could play a role in neuronal and synaptic plasticity.

Rotavirus Nonstructural Glycoprotein NSP4 Is Secreted from the Apical Surfaces of Polarized Epithelial Cells

NSP4, a nonstructural glycoprotein encoded by rotavirus, is involved in the morphogenesis of virus particles in the endoplasmic reticulum of infected cells. NSP4 is also implicated in the pathophysiology of rotavirus-induced diarrhea by acting as an enterotoxin. To mediate enterotoxic effects in vivo, NSP4 must be secreted or released from rotavirus-infected cells in a soluble form; however, previous studies have indicated that NSP4 is a transmembrane glycoprotein localized within endomembrane compartments in infected cells. In this study, we examined the fate of NSP4 synthesized in Caco-2 cells infected with bovine rotavirus. Our studies reveal that NSP4 is actively secreted into the culture medium, preferentially from the infected-cell apical surface. The secretion of NSP4 is dramatically inhibited by brefeldin A and monensin, suggesting that a Golgi-dependent pathway is involved in release of the protein. In agreement with the proposed involvement of the Golgi apparatus during secretion, secreted NSP4 appears to undergo additional posttranslational modification compared to its cell-associated counterpart and is partially resistant to deglycosylation by endoglycosidase H. Our experiments identify a novel, soluble form of NSP4 secreted from virus-infected cells with the potential to carry out the enterotoxigenic role previously attributed to recombinant forms of the protein.

Pulmonary surfactant transport in alveolar type II cells

Pulmonary surfactant (PS) is a mixture of several lipids (mainly phosphatidylcholine; PC) and four apoproteins (A, B, C and D). The classical hypothesis of PS transport suggests that PS is synthesized in the endoplasmic reticulum and transported to the lamellar body (LB) via the Golgi apparatus. However, recent studies have raised questions regarding this single route. This study examined, independently, the intracellular trafficking route of three different components of PS, that is, PC, SP-A and SP-B. Alveolar type II cells were isolated from Sprague-Dawley rats or Japanese white rabbits. The cells were cultured with either [3H]choline or [35S]methionine/cysteine with or without brefeldin A, which disassembles the Golgi apparatus. LB was purified from disintegrated cells with sucrose density gradient centrifugation. [3H]PC was extracted from radiolabeled media, cells, and the LB fraction with Bligh-Dyer's method. [35S]SP-A or [35S]SP-B was immunoprecipitated from each sample with a specific antibody. [3H]PC was transported and stored to the LB via a Golgi-independent pathway. [35S]SP-A was transported to the Golgi apparatus, underwent glycosylation, and was then constitutively secreted. The secreted [35S]SP-A was re-uptaken into the LB. [35S]SP-B was transported and stored to the LB via the Golgi-dependent pathway. These results indicate that, rather than a single route, surfactant components take different pathways to reside in the LB. These different pathways may reflect the different nature and role of each surfactant component such as surface tension-lowering activity and innate host defense.

Golgi-dependent transport of cholesterol to the Chlamydia trachomatis inclusion.

Cholesterol, a lipid not normally found in prokaryotes, was identified in purified Chlamydia trachomatis elementary bodies and in the chlamydial parasitophorous vacuole (inclusion) membrane of infected HeLa cells. Chlamydiae obtained eukaryotic host cell cholesterol both from de novo synthesis or low-density lipoprotein. Acquisition of either de novo-synthesized cholesterol or low-density lipoprotein-derived cholesterol was microtubule-dependent and brefeldin A-sensitive, indicating a requirement for the Golgi apparatus. Transport also required chlamydial protein synthesis, indicative of a pathogen-directed process. The cholesterol trafficking pathway appears to coincide with a previously characterized delivery of sphingomyelin to the inclusion in that similar pharmacological treatments inhibited transport of both sphingomyelin and cholesterol. These results support the hypothesis that sphingomyelin and cholesterol may be cotransported via a Golgi-dependent pathway and that the chlamydial inclusion receives cholesterol preferentially from a brefeldin A-sensitive pathway of cholesterol trafficking from the Golgi apparatus to the plasma membrane.

Vitamin E and thyroid disease: a potential link that kindles hope.

Vitamin E was discovered in 1922 and is the best known lipid-soluble antioxidant [1]. Natural vitamin E represents a complex of 8 tocopherol and tocotrienol isomers of which α-tocopherol (atoc) is predominant in most species and significantly more potent than any other naturally occurring tocopherol [2]. Vitamin E is considered the prototype of phenolic based chain-breaking antioxidants, its primary role consisting in preventing free radical initiated lipid peroxidation damage and thus in protecting the integrity of tissues [3]. Vitamin E is mainly distributed in adipose tissue and in the subcellular membrane fractions, the intracellular transport and release of α-tocopherol into the plasma involves via a Golgi-dependent pathway a specific protein, the α-tocopherol transfer protein [4]. Oxidative stress is defined as the overproduction of oxygen free radicals (OFR) that can no longer be compensated by the antioxidant defence system [5]. Oxidative stress appears to play a pivotal role in various diseases such as atherosclerosis, cancer and Alzheimer’s and is associated with diminution of antioxidant molecules. α-tocopherol decreases, in a specific and concentration dependent way, the proliferation of vascular smooth cells by interacting with the protein kinase C (PKC) [6]. The interaction with the cytosolic PKC results in inhibition of its membrane translocation and activation [7]. The PKC specific activity is diminished due to dephosphorylation following the activation of protein phosphatase 2A1 activity by α-tocopherol at a cellular level [8]. Thus, the diminution of a-toc in smooth muscle cells may consequently augment the PKC activity and lead to an increase in cell proliferation also by a non-antioxidant mechanism [8].

The Protein-tyrosine Phosphatase CD45 Reaches the Cell Surface via Golgi-dependent and -independent Pathways

CD45 is a receptor protein-tyrosine phosphatase essential for T cell development and lymphocyte activation. It is highly glycosylated, with multiple isoforms and glycoforms expressed on the cell surface depending on the cell type and stage of differentiation. Interestingly, we found two pools of newly synthesized CD45 expressed on plasma membrane, one of which arrived by 5 min after synthesis. The remaining pool of CD45 was fully glycosylated and began to arrive at the cell surface at approximately 15 min. The rapidly expressed population of CD45 possessed exclusively endoglycosidase H-sensitive N-linked carbohydrate. Additionally, this rapidly expressed pool of CD45 appeared on the cell surface in a brefeldin A (BFA)-insensitive manner, suggesting that it reached the cell surface independent of the Golgi complex. The remaining CD45 trafficked through the Golgi complex, and transport proceeded via a BFA-sensitive mechanism. These data suggest that CD45 is able to reach the cell surface via two distinct routes. The first is a conventional Golgi-dependent pathway that allows fully processed CD45 to be expressed. The second utilizes an ill defined mechanism that is independent of the Golgi, is BFA-resistant, and allows for the expression of CD45 with immature carbohydrate on the cell surface.

Toxoplasma gondii Rab5 enhances cholesterol acquisition from host cells.

The role of endocytosis in nutrient uptake by Toxoplasma gondii is unknown. To explore this issue, we characterized an endosomal compartment by identifying a T. gondii Rab5 homologue, a molecular marker for early endosomes in eukaryotic cells. The deduced amino acid sequence of the T. gondii Rab5 gene encodes a protein of 240 amino acids, which we termed TgRab51. TgRab51 was epitope-tagged at the N-terminus, expressed in the parasite, and localized by immunofluorescence and immunoelectron microscopy to tubulovesicular structures anterior to the parasite nucleus and adjacent to, but distinct from the Golgi. By immunofluorescence analysis, TgRab51wt-HA staining partially overlapped with Golgi/TGN markers, but not with the T. gondii secretory organelles. A dominant positive mutant, TgRab51Q103L-HA, enhanced uptake of exogenous cholesterol analogues in intracellular parasites, augmented formation of lipid droplets and accelerated parasite growth. Brefeldin A disrupted the TgRab51 compartment, and altered the distribution of fluorescent exogenous cholesterol in cells expressing TgRab51Q103L-HA. These results suggest that TgRab51 facilitates sterol uptake, possibly through a Golgi-dependent pathway.

Molecular and cellular mechanisms of adipose secretion: Comparison of leptin and angiotensinogen

Besides their function of lipid storage, the adipose cells secrete a number of proteins of physiopathological importance. To get further insights into this function, which remains poorly characterized, we sought to compare the mechanisms and regulation of secretion of two individual proteins in the same cells. Leptin and angiotensinogen were chosen and assessed by radioimmunoassay and quantitative immunoblotting, respectively, in primary culture of epididymal adipose cells from young obese Zucker rats. Leptin was secreted at a steady rate of 4 ng/10(6) cells/h over 2-6 h. Despite secretion, leptin cellular content remained stable at 3 ng/10(6) cells. In contrast, the rate of angiotensinogen secretion decreased regularly from 45 arbitrary units/10(6) cells/h at 2 h, to half this value at 6 h, although cell content remained constant at 100 arbitrary units/10(6) cells. Inhibition of protein synthesis by cycloheximide depleted the cells from leptin, but not from angiotensinogen for up to 6 h. Insulin increased leptin secretion (+75%) and cell content (+70 %), without affecting angiotensinogen. Secretion of both proteins was inhibited by Golgi-disturbing agents, brefeldin A and monensin. The presence of brefeldin A led to a specific rise in leptin cell content, an effect inhibited by cycloheximide and enhanced by insulin (+80%). These data show that leptin and angiotensinogen are both secreted through Golgi-dependent pathways and that their respective intracellular pool exhibit distinct turn-over rate and insulin sensitivity. These characteristics might account for the differential response of these adipose proteins to variations in the systemic environment.