Endomannosidase undergoes phosphorylation in the Golgi apparatus

LncRNA and mRNA expression associated with myasthenia gravis in patients with thymoma

Objective

Pathological alterations of the thymus are observed in the majority of patients with myasthenia gravis (MG). To explore the potential mechanisms of these alterations, we performed a transcriptome analysis and measured co‐expression of aberrant long non‐coding RNAs (lncRNAs) and messenger RNAs (mRNAs).

Methods

RNA was extracted from eight patients with thymoma, five of whom had MG. Transcriptome profiles were acquired through mRNA and lncRNA microarray analysis. Quantitative reverse transcription polymerase chain reaction was used to verify the results of the microarray analysis. LncRNAs co‐expressed with mRNA were analyzed with Pearson's coefficient. Next, cis‐regulated and trans‐regulated target genes were predicted. The functions of aberrant lncRNAs were explored on the basis of Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses of target mRNAs.

Results

The comparative microarray analysis identified 4360 lncRNAs and 2545 mRNAs with significant differential expression. The most significant GO enrichment terms were phosphoric ester hydrolase activity, phosphatase activity, and hydrolase activity, which were assigned as molecular functions. Regulation of endosome size was the most significant GO enrichment term assigned as a biological process, and Golgi apparatus was the most significant GO enrichment term assigned as cellular component. The reliability prediction terms of KEGG included calcium signaling pathway, glycosphingolipid biosynthesis, and caffeine metabolism.

Conclusion

MG‐positive thymoma is associated with overactive biological processes and molecular functions, especially dephosphorylation and hydrolysis, which may affect thymocyte survival during selection in the thymus.

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.

Demonstration that endoplasmic reticulum-associated degradation of glycoproteins can occur downstream of processing by endomannosidase

During quality control in the ER (endoplasmic reticulum), nascent glycoproteins are deglucosylated by ER glucosidases I and II. In the post-ER compartments, glycoprotein endo-α-mannosidase provides an alternative route for deglucosylation. Previous evidence suggests that endomannosidase non-selectively deglucosylates glycoproteins that escape quality control in the ER, facilitating secretion of aberrantly folded as well as normal glycoproteins. In the present study, we employed FOS (free oligosaccharides) released from degrading glycoproteins as biomarkers of ERAD (ER-associated degradation), allowing us to gain a global rather than single protein-centred view of ERAD. Glucosidase inhibition was used to discriminate between glucosidase- and endomannosidase-mediated ERAD pathways. Endomannosidase expression was manipulated in CHO (Chinese-hamster ovary)-K1 cells, naturally lacking a functional version of the enzyme, and HEK (human embryonic kidney)-293T cells. Endomannosidase was shown to decrease the levels of total FOS, suggesting decreased rates of ERAD. However, following pharmacological inhibition of ER glucosidases I and II, endomannosidase expression resulted in a partial switch between glucosylated FOS, released from ER-confined glycoproteins, to deglucosylated FOS, released from endomannosidase-processed glycoproteins transported from the Golgi/ERGIC (ER/Golgi intermediate compartment) to the ER. Using this approach, we have identified a previously unknown pathway of glycoprotein flow, undetectable by the commonly employed methods, in which secretory cargo is targeted back to the ER after being processed by endomannosidase.