Congenital disorders of glycosylation (CDG) are severe inherited diseases in which aberrant protein glycosylation is a hallmark. From this genetically and clinically heterogenous group, a significant subgroup due to Golgi homeostasis defects is emerging. We previously identified TMEM165 as a Golgi protein involved in CDG. Extremely conserved in the eukaryotic reign, the molecular mechanism by which TMEM165 deficiencies lead to Golgi glycosylation abnormalities is enigmatic. AsGDT1 is the ortholog of TMEM165 in yeast, both gdt1Δ null mutant yeasts and TMEM165 depleted cells were used. We highlighted that the observed Golgi glycosylation defects due to Gdt1p/TMEM165 deficiency result from Golgi manganese homeostasis defect. We discovered that in both yeasts and mammalian Gdt1p/TMEM165-deficient cells, Mn(2+) supplementation could restore a normal glycosylation. We also showed that the GPP130 Mn(2+) sensitivity was altered in TMEM165 depleted cells. This study not only provides novel insights into the molecular causes of glycosylation defects observed in TMEM165-deficient cells but also suggest that TMEM165 is a key determinant for the regulation of Golgi Mn(2+) homeostasis.

To our knowledge, this is the first description of abnormal glycosylation of lipids in the TMEM165 defect and the first report of successful dietary treatment in TMEM165 deficiency. We recommend the use of oral d-galactose therapy in TMEM165-CDG.

Recent methodological developments in metabolic oligosaccharide engineering (MOE) pave the way for tremendous advances in glycobiology. Herein, we propose a Sequential Bioorthogonal Dual Strategy (SBDS) combining the use of two unprotected alkyne-tagged monosaccharide reporters (ManNAl and SiaNAl) with the bioligation of fluorescent probes by copper-catalysed azide-alkyne cycloaddition (CuAAC). With SBDS, we are able to shed light on trafficking and cellular uptake mechanisms of sialic acid. Using their corresponding analogues, we visualized that SiaNAl enters via endocytosis, whereas its biosynthetic intermediate ManNAl uptake is mediated by a yet unknown but specific plasma membrane transporter. Sialin, a lysosomal protein, is shown to be crucial for the export of exogenous sialic acid from lysosomes to the cytosol. Metabolic labeling with alkyne-tagged derivatives of N-acetylneuraminic acid (Neu5Ac) or N-acetylmannosamine (ManNAc) could thus be used to follow endocytosis in physiological vs. pathological conditions.