Reduction in Golgi apparatus dimension in the absence of a residential protein, N-acetylglucosaminyltransferase V

N-glycan antennal modifications are altered in Caenorhabditis elegans lacking the HEX-4 N-acetylgalactosamine-specific hexosaminidase

Simple organisms are often considered to have simple glycomes, but plentiful paucimannosidic and oligomannosidic glycans overshadow the less abundant N-glycans with highly variable core and antennal modifications; Caenorhabditis elegans is no exception. By use of optimized fractionation and assessing wildtype in comparison to mutant strains lacking either the HEX-4 or HEX-5 β-N-acetylgalactosaminidases, we conclude that the model nematode has a total N-glycomic potential of 300 verified isomers. Three pools of glycans were analyzed for each strain: either PNGase F released and eluted from a reversed-phase C18 resin with either water or 15% methanol or PNGase Ar released. While the water-eluted fractions were dominated by typical paucimannosidic and oligomannosidic glycans and the PNGase Ar-released pools by glycans with various core modifications, the methanol-eluted fractions contained a huge range of phosphorylcholine-modified structures with up to three antennae, sometimes with four N-acetylhexosamine residues in series. There were no major differences between the C. elegans wildtype and hex-5 mutant strains, but the hex-4 mutant strains displayed altered sets of methanol-eluted and PNGase Ar-released pools. In keeping with the specificity of HEX-4, there were more glycans capped with N-acetylgalactosamine in the hex-4 mutants, as compared with isomeric chito-oligomer motifs in the wildtype. Considering that fluorescence microscopy showed that a HEX-4::enhanced GFP fusion protein colocalizes with a Golgi tracker, we conclude that HEX-4 plays a significant role in late-stage Golgi processing of N-glycans in C. elegans. Furthermore, finding more "parasite-like" structures in the model worm may facilitate discovery of glycan-processing enzymes occurring in other nematodes.

N-Glycosylation of IgG and IgG-Like Recombinant Therapeutic Proteins: Why Is It Important and How Can We Control It?

Regulatory bodies worldwide consider N-glycosylation to be a critical quality attribute for immunoglobulin G (IgG) and IgG-like therapeutics. This consideration is due to the importance of posttranslational modifications in determining the efficacy, safety, and pharmacokinetic properties of biologics. Given its critical role in protein therapeutic production, we review N-glycosylation beginning with an overview of the myriad interactions of N-glycans with other biological factors. We examine the mechanism and drivers for N-glycosylation during biotherapeutic production and the several competing factors that impact glycan formation, including the abundance of precursor nucleotide sugars, transporters, glycosidases, glycosyltransferases, and process conditions. We explore the role of these factors with a focus on the analytical approaches used to characterize glycosylation and associated processes, followed by the current state of advanced glycosylation modeling techniques. This combination of disciplines allows for a deeper understanding of N-glycosylation and will lead to more rational glycan control.

Post-ER Stress Biogenesis of Golgi Is Governed by Giantin

BACKGROUND

The Golgi apparatus undergoes disorganization in response to stress, but it is able to restore compact and perinuclear structure under recovery. This self-organization mechanism is significant for cellular homeostasis, but remains mostly elusive, as does the role of giantin, the largest Golgi matrix dimeric protein.

METHODS

In HeLa and different prostate cancer cells, we used the model of cellular stress induced by Brefeldin A (BFA). The conformational structure of giantin was assessed by proximity ligation assay and atomic force microscopy. The post-BFA distribution of Golgi resident enzymes was examined by 3D SIM high-resolution microscopy.

RESULTS

We detected that giantin is rather flexible than an extended coiled-coil dimer and BFA-induced Golgi disassembly was associated with giantin monomerization. A fusion of the nascent Golgi membranes after BFA washout is forced by giantin re-dimerization via disulfide bond in its luminal domain and assisted by Rab6a GTPase. GM130-GRASP65-dependent enzymes are able to reach the nascent Golgi membranes, while giantin-sensitive enzymes appeared at the Golgi after its complete recovery via direct interaction of their cytoplasmic tail with N-terminus of giantin.

CONCLUSION

Post-stress recovery of Golgi is conducted by giantin dimer and Golgi proteins refill membranes according to their docking affiliation rather than their intra-Golgi location.

High expression of GMⅡ is associated with poor prognosis of gastric cancer patients

Background: Being an important N-glycosylation enzyme in eukaryotic cells, Golgi α-mannosidaseⅡ (GMⅡ) has been suggested to function as a target for cancer treatment based on the inhibitory effect on cancer growth and metastasis by the swainsonine, an inhibitor of GMⅡ. This study aims to investigate GMⅡ expression and its prognostic value in primary gastric cancer.

Methods: The GMⅡ expression was examined by using the quantitative PCR and Western blotting in 37 paired gastric cancer and noncancerous tissues. We analyzed the relationship between its expression and the clinicopathological parameters by immunohistochemistry in 185 paraffin-embedded gastric cancer tissue specimens. Furthermore, we detected the GMⅡ expression in cultured gastric cancer cell lines and the normal gastric cell line and observed the changes of proliferative and invasive capacities of gastric cell lines after GMⅡ scilencing and overexpressing in vitro.

Results: The GMⅡ mRNA (P<0.0001) and protein (P<0.01) expression of 37 tumor tissues were increased compared with those of the matched adjacent normal tissues. Human gastric cancer cell lines also showed higher GMⅡ expression (P<0.001) compared with normal gastric cell lines. The immunohistochemical analysis revealed that GMⅡ was an independent predictor of the overall survival of patients. In addition, GMⅡ overexpression in the normal gastric cell line GES-1 significantly promoted the cell proliferation and invasion, while GMⅡ knockdown in gastric cancer cell line BGC-823 significantly inhibited the cell proliferation and invasion.

Conclusion: GMⅡ may become an indicator for monitoring the prognosis of primary gastric cancer and it may provide a new direction for precise treatment.

Ablation of N-acetylglucosaminyltransferases in Caenorhabditis induces expression of unusual intersected and bisected N-glycans

The modification in the Golgi of N-glycans by N-acetylglucosaminyltransferase I (GlcNAc-TI, MGAT1) can be considered to be a hallmark of multicellular eukaryotes as it is found in all metazoans and plants, but rarely in unicellular organisms. The enzyme is key for the normal processing of N-glycans to either complex or paucimannosidic forms, both of which are found in the model nematode Caenorhabditis elegans. Unusually, this organism has three different GlcNAc-TI genes (gly-12, gly-13 and gly-14); therefore, a complete abolition of GlcNAc-TI activity required the generation of a triple knock-out strain. Previously, the compositions of N-glycans from this mutant were described, but no detailed structures. Using an off-line HPLC-MALDI-TOF-MS approach combined with exoglycosidase digestions and MS/MS, we reveal that the multiple hexose residues of the N-glycans of the gly-12;gly-13;gly-14 triple mutant are not just mannose, but include galactoses in three different positions (β-intersecting, β-bisecting and α-terminal) on isomeric forms of Hex_4-8HexNAc₂ structures; some of these structures are fucosylated and/or methylated. Thus, the N-glycomic repertoire of Caenorhabditis is even wider than expected and exhibits a large degree of plasticity even in the absence of key glycan processing enzymes from the Golgi apparatus.

The contributions of individual galactosyltransferases to protein specific N-glycan processing in Chinese Hamster Ovary cells

Galactosylation as part of N-glycan processing is conducted by a set of beta-1,4-galactosyltransferases (B4GALTs), with B4GALT1 as the dominant isoenzyme for this reaction. Nevertheless, the exact contributions of this key-player as well as of the other isoenzymes involved in N-glycosylation, B4GALT2, B4GALT3 and B4GALT4, have not been studied in-depth. To increase the understanding of the protein- and site-specific activities of individual galactosyltransferases in Chinese Hamster Ovary cells, a panel of triple deletion cell lines was generated that expressed only one isoform of B4GALT each. Two model proteins were selected for this study to cover a large spectrum of possible N-glycan structures: erythropoietin and deamine-oxidase. They were expressed as Fc-fusion constructs (EPO-Fc and Fc-DAO) and their N-glycan processing status was analyzed by site-specific mass spectrometry. The sole activity of B4GALT1 resulted in a decrease of 15-21 % of fully galactosylated structures for erythropoietin, emphasizing the involvement of other isoenzymes. Interestingly, the contributions of B4GALT2 and B4GALT3 differed for the two model proteins. Unexpectedly, removal of galactosyltransferases influenced the overall process of N-glycan maturation, with the result of a higher occurrence of poorly processed oligosaccharides. In the context of high productivity cell lines, which can push N-glycan maturation towards incomplete galactosylation, galactosyltransferases are potential targets to ensure stable product quality. In view of our results, specifically engineered "designer" cell lines may be required for different proteins.

Golgi apparatus dis- and reorganizations studied with the aid of 2-deoxy-D-glucose and visualized by 3D-electron tomography

We studied Golgi apparatus disorganizations and reorganizations in human HepG2 hepatoblastoma cells by using the nonmetabolizable glucose analogue 2-deoxy-d-glucose (2DG) and analyzing the changes in Golgi stack architectures by 3D-electron tomography. Golgi stacks remodel in response to 2DG-treatment and are replaced by tubulo-glomerular Golgi bodies, from which mini-Golgi stacks emerge again after removal of 2DG. The Golgi stack changes correlate with the measured ATP-values. Our findings indicate that the classic Golgi stack architecture is impeded, while cells are under the influence of 2DG at constantly low ATP-levels, but the Golgi apparatus is maintained in forms of the Golgi bodies and Golgi stacks can be rebuilt as soon as 2DG is removed. The 3D-electron microscopic results highlight connecting regions that interlink membrane compartments in all phases of Golgi stack reorganizations and show that the compact Golgi bodies mainly consist of continuous intertwined tubules. Connections and continuities point to possible new transport pathways that could substitute for other modes of traffic. The changing architectures visualized in this work reflect Golgi stack dynamics that may be essential for basic cell physiologic and pathologic processes and help to learn, how cells respond to conditions of stress.

The Histochemistry and Cell Biology pandect: the year 2014 in review

This review encompasses a brief synopsis of the articles published in 2014 in Histochemistry and Cell Biology. Out of the total of 12 issues published in 2014, two special issues were devoted to "Single-Molecule Super-Resolution Microscopy." The present review is divided into 11 categories, providing an easy format for readers to quickly peruse topics of particular interest to them.