Truncations of the C-terminal cytoplasmic domain of MG160, a medial Golgi sialoglycoprotein, result in its partial transport to the plasma membrane and filopodia

Morphometric alterations of Golgi apparatus in Alzheimer's disease are related to tau hyperphosphorylation

The Golgi apparatus (GA) is a highly dynamic organelle, which is mainly involved in the post-translational processing and targeting of cellular proteins and which undergoes significant morphological changes in response to different physiological and pathological conditions. In the present study, we have analyzed the possible alterations of GA in neurons from the temporal neocortex and hippocampus of Alzheimer's disease (AD) patients, using double immunofluorescence techniques, confocal microscopy and 3D quantification techniques. We found that in AD patients, the percentage of temporal neocortical and CA1 hippocampal pyramidal neurons with a highly altered GA is much higher (approximately 65%) in neurons with neurofibrillary tangles (NFT) than in NFT-free neurons (approximately 6%). Quantitative analysis of the surface area and volume of GA elements in neurons revealed that, compared with NFT-free neurons, NFT-bearing neurons had a reduction of approximately one half in neocortical neurons and one third in CA1 neurons. In both regions, neurons with a pre-tangle stage of phospho-tau accumulation had surface area and GA volume values that were intermediate, that is, between those of NFT-free and NFT-bearing neurons. These findings support the idea that the progressive accumulation of phospho-tau is associated with structural alterations of the GA including fragmentation and a decrease in the surface area and volume of GA elements. These alterations likely impact the processing and trafficking of proteins, which might contribute to neuronal dysfunction in AD.

Changes in the Golgi Apparatus of Neocortical and Hippocampal Neurons in the Hibernating Hamster

Hibernating animals have been used as models to study several aspects of the plastic changes that occur in the metabolism and physiology of neurons. These models are also of interest in the study of Alzheimer's disease because the microtubule-associated protein tau is hyperphosphorylated during the hibernation state known as torpor, similar to the pretangle stage of Alzheimer's disease. Hibernating animals undergo torpor periods with drops in body temperature and metabolic rate, and a virtual cessation of neural activity. These processes are accompanied by morphological and neurochemical changes in neurons, which reverse a few hours after coming out of the torpor state. Since tau has been implicated in the structural regulation of the neuronal Golgi apparatus (GA) we have used Western Blot and immunocytochemistry to analyze whether the GA is modified in cortical neurons of the Syrian hamster at different hibernation stages. The results show that, during the hibernation cycle, the GA undergo important structural changes along with differential modifications in expression levels and distribution patterns of Golgi structural proteins. These changes were accompanied by significant transitory reductions in the volume and surface area of the GA elements during torpor and arousal stages as compared with euthermic animals.

Selective E-selectin ligands

In this issue of Blood, Sreeramkumar and colleagues report that E-selectin ligand-1 (ESL-1) is a highly selective ligand for E-selectin on hematopoietic progenitors with unexpected important contributions to their trafficking.

Cisterna-specific localization of glycosylation-related proteins to the Golgi apparatus

The Golgi apparatus is an intracellular organelle playing central roles in post-translational modification and in the secretion of membrane and secretory proteins. These proteins are synthesized in the endoplasmic reticulum (ER) and transported to the cis-, medial-and trans-cisternae of the Golgi. While trafficking through the Golgi, proteins are sequentially modified with glycan moieties by different glycosyltransferases. Therefore, it is important to analyze the glycosylation function of the Golgi at the level of cisternae. Markers widely used for cis-, medial- and trans-cisternae/trans Golgi network (TGN) in Drosophila are GM130, 120 kDa and Syntaxin16 (Syx16); however the anti-120 kDa antibody is no longer available. In the present study, Drosophila Golgi complex-localized glycoprotein-1 (dGLG1) was identified as an antigen recognized by the anti-120 kDa antibody. A monoclonal anti-dGLG1 antibody suitable for immunohistochemistry was raised in rat. Using these markers, the localization of glycosyltransferases and nucleotide-sugar transporters (NSTs) was studied at the cisternal level. Results showed that glycosyltransferases and NSTs involved in the same sugar modification are localized to the same cisternae. Furthermore, valuable functional information was obtained on the localization of novel NSTs with as yet incompletely characterized biochemical properties.

Retention in the Golgi apparatus and expression on the cell surface of Cfr/Esl-1/Glg-1/MG-160 are regulated by two distinct mechanisms

Cfr (cysteine-rich fibroblast growth factor receptor) is an Fgf (fibroblast growth factor)-binding protein without a tyrosine kinase. We have shown previously that Cfr is involved in Fgf18 signalling via Fgf receptor 3c. However, as Cfr is also known as Glg (Golgi apparatus protein)-1 or MG-160 and occurs in the Golgi apparatus, it remains unknown how the distribution of Cfr is regulated. In the present study, we performed a mutagenic analysis of Cfr to show that two distinct regions contribute to its distribution and stability. First, the C-terminal region retains Cfr in the Golgi apparatus. Secondly, the Cfr repeats in the extracellular juxtamembrane region destabilizes Cfr passed through the Golgi apparatus. This destabilization does not depend on the cleavage and secretion of the extracellular domain of Cfr. Furthermore, we found that Cfr with a GPI (glycosylphosphatidylinositol) anchor was predominantly expressed on the cell surface in Ba/F3 cells and affected Fgf18 signalling in a similar manner to the full-length Cfr, indicating that the interaction of Cfr with Fgfs on the cell surface is important for its function in Fgf signalling. These results suggest that the expression of Cfr in the Golgi apparatus and on the plasma membrane is finely tuned through two distinct mechanisms for exhibiting different functions.

Leukocyte ligands for endothelial selectins: specialized glycoconjugates that mediate rolling and signaling under flow

Reversible interactions of glycoconjugates on leukocytes with P- and E-selectin on endothelial cells mediate tethering and rolling of leukocytes in inflamed vascular beds, the first step in their recruitment to sites of injury. Although selectin ligands on hematopoietic precursors have been identified, here we review evidence that PSGL-1, CD44, and ESL-1 on mature leukocytes are physiologic glycoprotein ligands for endothelial selectins. Each ligand has specialized adhesive functions during tethering and rolling. Furthermore, PSGL-1 and CD44 induce signals that activate the β2 integrin LFA-1 and promote slow rolling, whereas ESL-1 induces signals that activate the β2 integrin Mac-1 in adherent neutrophils. We also review evidence for glycolipids, CD43, L-selectin, and other glycoconjugates as potential physiologic ligands for endothelial selectins on neutrophils or lymphocytes. Although the physiologic characterization of these ligands has been obtained in mice, we also note reported similarities and differences with human selectin ligands.

The cytoplasmic and transmembrane domains of secretor type alpha1,2fucosyltransferase confer atypical cellular localisation

Carbohydrate structures influence many aspects of cell biology. Manipulating the glycosyltransferase enzymes, that sequentially add carbohydrate moieties to proteins and lipids as they pass through the Golgi and secretory pathway, can alter these carbohydrate epitopes. We previously demonstrated that the eight amino acid cytoplasmic tail of alpha1,2fucosyltransferase (FT) contained a sequence for Golgi localisation. In this study, we examined the localisation of the closely related secretor type alpha1,2fucosyltransferase (Sec) which has a smaller, yet apparently unrelated, five amino acid cytoplasmic tail. In contrast to the Golgi localisation of FT, Sec displayed atypical cytoplasmic vesicular-like staining. However, replacing just the five amino acid tail of Sec with FT was sufficient to relocalise the enzyme to a perinuclear region with Golgi-like staining. The biological significance of this relocalisation was this chimaeric enzyme was more effective than FT at competing for N-Acetyl-lactosamine and thus was superior in reducing expression of the Galalpha(1,3)Gal xenoepitope.

Functional involvement of TMF/ARA160 in Rab6-dependent retrograde membrane traffic

The small GTPase Rab6 regulates retrograde membrane traffic from endosomes to the Golgi apparatus and from the Golgi to the endoplasmic reticulum (ER). We examined the role of a Rab6-binding protein, TMF/ARA160 (TATA element modulatory factor/androgen receptor-coactivator of 160 kDa), in this process. High-resolution immunofluorescence imaging revealed that TMF signal surrounded Rab6-positive Golgi structures and immunoelectron microscopy revealed that TMF is concentrated at the budding structures localized at the tips of cisternae. The knockdown of either TMF or Rab6 by RNA interference blocked retrograde transport of endocytosed Shiga toxin from early/recycling endosomes to the trans-Golgi network, causing missorting of the toxin to late endosomes/lysosomes. However, the TMF knockdown caused Rab6-dependent displacement of N-acetylgalactosaminyltransferase-2 (GalNAc-T2), but not beta1,4-galactosyltransferase (GalT), from the Golgi. Analyses using chimeric proteins, in which the cytoplasmic regions of GalNAc-T2 and GalT were exchanged, revealed that the cytoplasmic region of GalNAc-T2 plays a crucial role in its TMF-dependent Golgi retention. These observations suggest critical roles for TMF in two Rab6-dependent retrograde transport processes: one from endosomes to the Golgi and the other from the Golgi to the ER.

Complete identification of E-selectin ligands on neutrophils reveals distinct functions of PSGL-1, ESL-1, and CD44

The selectins and their ligands are required for leukocyte extravasation during inflammation. Several glycoproteins have been suggested to bind to E-selectin in vitro, but the complete identification of its physiological ligands has remained elusive. Here, we showed that E-selectin ligand-1 (ESL-1), P-selectin glycoprotein ligand-1 (PSGL-1), and CD44 encompassed all endothelial-selectin ligand activity on neutrophils by using gene- and RNA-targeted loss of function. PSGL-1 played a major role in the initial leukocyte capture, whereas ESL-1 was critical for converting initial tethers into steady slow rolling. CD44 controlled rolling velocity and mediated E-selectin-dependent redistribution of PSGL-1 and L-selectin to a major pole on slowly rolling leukocytes through p38 signaling. These results suggest distinct and dynamic contributions of these three glycoproteins in selectin-mediated neutrophil adhesion and signaling.

Tau interacts with Golgi membranes and mediates their association with microtubules

Tau, a microtubule-associated protein enriched in the axon, is known to stabilize and promote the formation of microtubules during axonal outgrowth. Several studies have reported that tau was associated with membranes. In the present study, we further characterized the interaction of tau with membranous elements by examining its distribution in subfractions enriched in either Golgi or endoplasmic reticulum membranes isolated from rat brain. A subfraction enriched with markers of the medial Golgi compartment, MG160 and mannosidase II, presented a high tau content indicating that tau was associated with these membranes. Electron microscope morphometry confirmed the enrichment of this subfraction with Golgi membranes. Double-immunogold labeling experiments conducted on this subfraction showed the direct association of tau with vesicles labeled with either an antibody directed against MG160 or TGN38. The association of tau with the Golgi membranes was further confirmed by immunoisolating Golgi membranes with an anti-tau antibody. Immunogold labeling confirmed the presence of tau on the Golgi membranes in neurons in vivo. Overexpression of human tau in primary hippocampal neurons induced the formation of large Golgi vesicles that were found in close vicinity to tau-containing microtubules. This suggested that tau could serve as a link between Golgi membranes and microtubules. Such role for tau was demonstrated in an in vitro reconstitution assay. Finally, our results showed that some tau isoforms present in the Golgi subfraction were phosphorylated at the sites recognized by the phosphorylation-dependent antibodies PHF-1 and AT-8.

A novel isoform of human Golgi complex-localized glycoprotein-1 (also known as E-selectin ligand-1, MG-160 and cysteine-rich fibroblast growth factor receptor) targets differential subcellular localization

The initial step in trafficking of leukocytes through the vascular endothelium is mediated by an adhesive interaction between molecules of the selectin family and their cognate receptors. Previously, a putative murine E-selectin ligand-1 (ESL-1) was identified and found to be identical to Golgi complex-localized glycoprotein-1 (GLG1), also known as MG-160, and to a previously identified basic fibroblast growth factor (bFGF)-binding protein known as cysteine-rich FGF receptor (CFR). We report here a novel variant of the human GLG1 gene product that we call GLG2, cloned from a human monocyte cDNA library. GLG2 encodes a polypeptide identical to GLG1 except with a unique 24-amino-acid extension at the C-terminus of its cytoplasmic domain. Transfection of chimeric constructs into human embryonic kidney epithelial 293 cells revealed that the cytoplasmic domains of GLG1 and GLG2 targeted the expression of each chimeric protein differentially, GLG1 to the cell surface and GLG2 to the Golgi. Genetic analysis suggests that GLG1 and GLG2 are the products of a single gene, the mRNA of which can be processed by alternative splicing to generate two different transcripts encoding either GLG1 or GLG2. Northern blot analysis showed that the relative amounts of the mRNAs for either isoform differ in a cell- and species-specific manner. These data suggest that alternative splicing of the GLG1 gene transcript might regulate the function of its product.

Dysregulation of stathmin, a microtubule-destabilizing protein, and up-regulation of Hsp25, Hsp27, and the antioxidant peroxiredoxin 6 in a mouse model of familial amyotrophic lateral sclerosis

Gain-of-function mutations of the Cu/Zn superoxide dismutase (SOD1) gene cause dominantly inherited familial amyotrophic lateral sclerosis. The identification of differentially regulated proteins in spinal cords of paralyzed mice expressing SOD1(G93A) may contribute to understanding mechanisms of toxicity by mutant SOD1. Protein profiling showed dysregulation of Stathmin with a marked decrease of its most acidic and phosphorylated isoform, and up-regulation of heat shock proteins 25 and 27, peroxiredoxin 6, phosphatidylinositol transfer protein-alpha, apolipoprotein E, and ferritin heavy chain. Stathmin accumulated in the cytoplasm of 30% of spinal cord motor neurons with fragmented Golgi apparatus. Overexpression of Stathmin in HeLa cells was associated with collapse of microtubule networks and Golgi fragmentation. These results, together with the decrease of one Stathmin isoform, suggest a role of the protein in Golgi fragmentation. Mutant SOD1 co-precipitated and co-localized with Hsp25 in neurons and astrocytes. Mutant SOD1 may thus deprive cells of the anti-apoptotic and other protective activities of Hsp25. Astrocytes contained peroxiredoxin 6, a unique nonredundant antioxidant. The up-regulation of peroxiredoxin 6 probably constitutes a defense to oxidative stress induced by SOD1(G93A). Direct effects of SOD1(G93A) or sequential reactions triggered by the mutant may cause the protein changes.

Disruption of the structure of the Golgi apparatus and the function of the secretory pathway by mutants G93A and G85R of Cu, Zn superoxide dismutase (SOD1) of familial amyotrophic lateral sclerosis

The Golgi apparatus of motor neurons (GA) is fragmented in sporadic amyotrophic lateral sclerosis (ALS), in familial ALS with SOD1 mutations, and in mice that express SOD1G93A of familial ALS, in which it was detected months before paralysis. In paralyzed transgenic mice expressing SOD1G93A or SOD1G85R, mutant proteins aggregated not only in the cytoplasm of motor neurons, but also in astrocytes and oligodendrocytes. Furthermore, aggregation of the G85R protein damaged astrocytes and was associated with rapidly progressing disease. In order to gain insight into the functional state of the fragmented GA, we examined the effects of S0D1 mutants G93A and G85R in Chinese Hamster Ovary Cells (CHO). In contrast to cells expressing the wt and G93A, the G85R expressers had no SOD1 activity. However, cells expressing both mutants, and to a lesser degree the wt, showed decreased survival, fragmentation of the GA, and dysfunction of the secretory pathway, which was assessed by measuring the amount of cell surface co-expressed CD4, a glycoprotein processed through the GA. The G93A and wt proteins were partially recovered in detergent insoluble fractions; while the recovery of G85R was minimal. Both mutants showed equal reductions of cell survival and function of the secretory pathway, in comparison to the wt and cells expressing mutant alsin, a protein found in rare cases of fALS. These results are consistent with the conclusion that the two SOD1 mutants, by an unknown mechanism, promote the dispersion of the GA and the dysfunction of the secretory pathway. This and other in vitro models of mutant SOD1 toxicity may prove useful in the elucidation of pathogenetic mechanisms.

The cytoplasmic tail of alpha 1,3-galactosyltransferase inhibits Golgi localization of the full-length enzyme

It is currently under debate whether the mechanism of Golgi retention of different glycosyltransferases is determined by sequences in the transmembrane, luminal, or cytoplasmic domains or a combination of these domains. We have shown that the cytoplasmic domains of alpha1,3-galactosyltransferase (GT) and alpha1,2-fucosyltransferase (FT) are involved in Golgi localization. Here we show that the cytoplasmic tails of GT and FT are sufficient to confer specific Golgi localization. Further, we show that the expression of only the cytoplasmic tail of GT can lead to displacement or inhibition of binding of the whole transferase and that cells expressing the cytoplasmic tail of GT were not able to express full-length GT or its product, Galalpha1,3Gal. Thus, the presence of the cytoplasmic tail prevented the localization and function of full-length GT, suggesting a possible specific Golgi binding site for GT. The effect was not altered by the inclusion of the transmembrane domain. Although the transmembrane domain may act as an anchor, these data show that, for GT, only the cytoplasmic tail is involved in specific localization to the Golgi.

A novel proliferation-associated variant of CFR-1 defined by a human monoclonal antibody

The germline coded human monoclonal IgM antibody 103/51 was isolated from a gastric carcinoma patient. This antibody binds to a 130-kd membrane molecule and has a mitotic effect on tumor cells in vitro. To characterize the target, we sequenced the protein and showed that the antibody binds to the cysteine-rich fibroblast growth factor receptor (CFR)-1, which is highly homologous to MG-160 and the E-selectin-ligand (ESL)-1. The epitope was determined by glycosidase-digestion experiments to be an N-linked carbohydrate side chain. Immunohistochemistry was used to investigate the tissue distribution of CFR-1. Different healthy tissues were tested and only the collecting tubes of the kidney, the Golgi apparatus, and the glomerular and fascicular zones of the adrenal gland stained positive. However, on malignant tissue the receptor is overexpressed in nearly all tested stomach cancers (12 of 15) and other tested carcinomas (13 of 15). Most interestingly, the receptor is also present in Helicobacter pylori gastritis and gastric dysplasia, but absent on uninflamed stomach mucosa. This restricted tissue pattern indicates that antibody 103/51 reacts with a membrane-bound variant of CFR-1, which is mainly expressed on transformed cells and precursor lesions and is essential for proliferation processes. The possible activity of antibody 103/51 as an activating ligand in these proliferative changes of gastric epithelial mucosa is discussed.

The cytoplasmic tail of alpha 1,2-fucosyltransferase contains a sequence for golgi localization

The Golgi apparatus has a central role in the glycosylation of proteins and lipids. There is a sequential addition of carbohydrates by glycosyltransferases that are distributed within the Golgi in the order in which the glycosylation occurs. The mechanism of glycosyltransferase retention is considered to involve their transmembrane domains and flanking regions, although we have shown that the cytoplasmic tail of alpha1,2-fucosyltransferase is important for its Golgi localization. Here we show that the removal of the alpha1,2-fucosyltransferase cytoplasmic tail altered its function of fucosylation and its localization site. When the tail was removed, the enzyme moved from the Golgi to the trans Golgi network, suggesting that the transmembrane is responsible for retention and that the cytoplasmic tail is responsible for localization. The cytoplasmic tail of alpha1,2-fucosyltransferase contains 8 amino acids (MWVPSRRH), and mutating these to alanine indicated a role for amino acids 3 to 7 in localization with a particular role of Ser(5). Mutagenesis of Ser(5) to amino acids containing an hydroxyl (Tyr and Thr) demonstrated that the hydroxyl at position 5 is important. Thus, the cytoplasmic tail, and especially a single amino acid, has a predominant role in the localization and thus the function of alpha1,2-fucosyltransferase.

Proteomics characterization of abundant Golgi membrane proteins

A mass spectrometric analysis of proteins partitioning into Triton X-114 from purified hepatic Golgi apparatus (84% purity by morphometry, 122-fold enrichment over the homogenate for the Golgi marker galactosyl transferase) led to the unambiguous identification of 81 proteins including a novel Golgi-associated protein of 34 kDa (GPP34). The membrane protein complement was resolved by SDS-polyacrylamide gel electrophoresis and subjected to a hierarchical approach using delayed extraction matrix-assisted laser desorption ionization mass spectrometry characterization by peptide mass fingerprinting, tandem mass spectrometry to generate sequence tags, and Edman sequencing of proteins. Major membrane proteins corresponded to known Golgi residents, a Golgi lectin, anterograde cargo, and an abundance of trafficking proteins including KDEL receptors, p24 family members, SNAREs, Rabs, a single ARF-guanine nucleotide exchange factor, and two SCAMPs. Analytical fractionation and gold immunolabeling of proteins in the purified Golgi fraction were used to assess the intra-Golgi and total cellular distribution of GPP34, two SNAREs, SCAMPs, and the trafficking proteins GBF1, BAP31, and alpha(2)P24 identified by the proteomics approach as well as the endoplasmic reticulum contaminant calnexin. Although GPP34 has never previously been identified as a protein, the localization of GPP34 to the Golgi complex, the conservation of GPP34 from yeast to humans, and the cytosolically exposed location of GPP34 predict a role for a novel coat protein in Golgi trafficking.

The golgi apparatus is present in perisynaptic, subependymal and perivascular processes of astrocytes and in processes of retinal Müller glia

The Golgi apparatus (GA) of innervated rat and chicken skeletal muscle is present in a typical perinuclear location, and in subsynaptic areas where it disperses after denervation. It was suggested that the subsynaptic segments of the GA are linked with functions involved in the maturation and targeting of synaptic proteins. Similarly, the GA of rat myocardium is found in a perinuclear location and between myofibrils, adjacent to the T system of tubules. These findings raise the question whether the GA of polarized cells is present in a typical perinuclear location, for the performance of general "housekeeping" functions, and in distal areas, for the mediation of specialized functions. Astrocytes may contain GA within their long cytoplasmic processes which are difficult to identify in thin sections. To ensure the astrocytic origin of GA in otherwise unidentifiable small processes, we used transgenic mice expressing the rat MG160 medial Golgi sialoglycoprotein only in the GA of astrocytes, and visualized the GA with monoclonal antibody 10A8 (mAb10A8) which reacts only with rat MG160. Thus, we identified cisternae of the GA in distal perisynaptic and subependymal processes, in perivascular foot plates of cerebral astrocytes, and in processes of the Müller glia in the retina. A similar strategy may be adopted in future investigations aiming at the detection of elements of the GA in distal processes of neurons and oligodendrocytes. The functional implications of GA in perisynaptic astrocytic processes and other processes are unknown. However, the isolation and molecular characterization of the perisynaptic subset of astrocytic Golgi may be feasible, since others have purified the astrocytic glutamate transporter 1 (GLT1) from crude synaptosomal fractions in which astrocytic processes are probably unavoidable contaminants.