Impaired catabolism of free oligosaccharides due to MAN2C1 variants causes a neurodevelopmental disorder.

Free oligosaccharides (fOSs) are soluble oligosaccharide species generated during N-glycosylation of proteins. Although little is known about fOS metabolism, the recent identification of NGLY1 deficiency, a congenital disorder of deglycosylation (CDDG) caused by loss of function of an enzyme involved in fOS metabolism, has elicited increased interest in fOS processing. The catabolism of fOSs has been linked to the activity of a specific cytosolic mannosidase, MAN2C1, which cleaves α1,2-, α1,3-, and α1,6-mannose residues. In this study, we report the clinical, biochemical, and molecular features of six individuals, including two fetuses, with bi-allelic pathogenic variants in MAN2C1; the individuals are from four different families. These individuals exhibit dysmorphic facial features, congenital anomalies such as tongue hamartoma, variable degrees of intellectual disability, and brain anomalies including polymicrogyria, interhemispheric cysts, hypothalamic hamartoma, callosal anomalies, and hypoplasia of brainstem and cerebellar vermis. Complementation experiments with isogenic MAN2C1-KO HAP1 cells confirm the pathogenicity of three of the identified MAN2C1 variants. We further demonstrate that MAN2C1 variants lead to accumulation and delay in the processing of fOSs in proband-derived cells. These results emphasize the involvement of MAN2C1 in human neurodevelopmental disease and the importance of fOS catabolism.

Impaired glucose-1,6-biphosphate production due to bi-allelic PGM2L1 mutations is associated with a neurodevelopmental disorder.

We describe a genetic syndrome due to PGM2L1 deficiency. PGM2 and PGM2L1 make hexose-bisphosphates, like glucose-1,6-bisphosphate, which are indispensable cofactors for sugar phosphomutases. These enzymes form the hexose-1-phosphates crucial for NDP-sugars synthesis and ensuing glycosylation reactions. While PGM2 has a wide tissue distribution, PGM2L1 is highly expressed in the brain, accounting for the elevated concentrations of glucose-1,6-bisphosphate found there. Four individuals (three females and one male aged between 2 and 7.5 years) with bi-allelic inactivating mutations of PGM2L1 were identified by exome sequencing. All four had severe developmental and speech delay, dysmorphic facial features, ear anomalies, high arched palate, strabismus, hypotonia, and keratosis pilaris. Early obesity and seizures were present in three individuals. Analysis of the children's fibroblasts showed that glucose-1,6-bisphosphate and other sugar bisphosphates were markedly reduced but still present at concentrations able to stimulate phosphomutases maximally. Hence, the concentrations of NDP-sugars and glycosylation of the heavily glycosylated protein LAMP2 were normal. Consistent with this, serum transferrin was normally glycosylated in affected individuals. PGM2L1 deficiency does not appear to be a glycosylation defect, but the clinical features observed in this neurodevelopmental disorder point toward an important but still unknown role of glucose-1,6-bisphosphate or other sugar bisphosphates in brain metabolism.

The promiscuous binding pocket of SLC35A1 ensures redundant transport of CDP-ribitol to the Golgi.

The glycoprotein α-dystroglycan helps to link the intracellular cytoskeleton to the extracellular matrix. A unique glycan structure attached to this protein is required for its interaction with extracellular matrix proteins such as laminin. Up to now, this is the only mammalian glycan known to contain ribitol phosphate groups. Enzymes in the Golgi apparatus use CDP-ribitol to incorporate ribitol phosphate into the glycan chain of α-dystroglycan. Since CDP-ribitol is synthesized in the cytoplasm, we hypothesized that an unknown transporter must be required for its import into the Golgi apparatus. We discovered that CDP-ribitol transport relies on the CMP-sialic acid transporter SLC35A1 and the transporter SLC35A4 in a redundant manner. These two transporters are closely related, but bulky residues in the predicted binding pocket of SLC35A4 limit its size. We hypothesized that the large binding pocket SLC35A1 might accommodate the bulky CMP-sialic acid and the smaller CDP-ribitol, whereas SLC35A4 might only accept CDP-ribitol. To test this, we expressed SLC35A1 with mutations in its binding pocket in SLC35A1 KO cell lines. When we restricted the binding site of SLC35A1 by introducing the bulky residues present in SLC35A4, the mutant transporter was unable to support sialylation of proteins in cells but still supported ribitol phosphorylation. This demonstrates that the size of the binding pocket determines the substrate specificity of SLC35A1, allowing a variety of cytosine nucleotide conjugates to be transported. The redundancy with SLC35A4 also explains why patients with SLC35A1 mutations do not show symptoms of α-dystroglycan deficiency.

Dissection of TMEM165 function in Golgi glycosylation and its Mn2+ sensitivity.

Since 2012, the interest for TMEM165 increased due to its implication in a rare genetic human disease named TMEM165-CDG (Congenital Disorder(s) of Glycosylation). TMEM165 is a Golgi localized protein, highly conserved through evolution and belonging to the uncharacterized protein family 0016 (UPF0016). Although the precise function of TMEM165 in glycosylation is still controversial, our results highly suggest that TMEM165 would act as a Golgi Ca2+/Mn2+ transporter regulating both Ca2+ and Mn2+ Golgi homeostasis, the latter is required as a major cofactor of many Golgi glycosylation enzymes. Strikingly, we recently demonstrated that besides its role in regulating Golgi Mn2+ homeostasis and consequently Golgi glycosylation, TMEM165 is sensitive to high manganese exposure. Members of the UPF0016 family contain two particularly highly conserved consensus motifs E-φ-G-D-[KR]-[TS] predicted to be involved in the ion transport function of UPF0016 members. We investigate the contribution of these two specific motifs in the function of TMEM165 in Golgi glycosylation and in its Mn2+ sensitivity. Our results show the crucial importance of these two conserved motifs and underline the contribution of some specific amino acids in both Golgi glycosylation and Mn2+ sensitivity.

Involvement of thapsigargin- and cyclopiazonic acid-sensitive pumps in the rescue of TMEM165-associated glycosylation defects by Mn2.

Congenital disorders of glycosylation are severe inherited diseases in which aberrant protein glycosylation is a hallmark. Transmembrane protein 165 (TMEM165) is a novel Golgi transmembrane protein involved in type II congenital disorders of glycosylation. Although its biologic function is still a controversial issue, we have demonstrated that the Golgi glycosylation defect due to TMEM165 deficiency resulted from a Golgi Mn2+ homeostasis defect. The goal of this study was to delineate the cellular pathway by which extracellular Mn2+ rescues N-glycosylation in TMEM165 knockout (KO) cells. We first demonstrated that after extracellular exposure, Mn2+ uptake by HEK293 cells at the plasma membrane did not rely on endocytosis but was likely done by plasma membrane transporters. Second, we showed that the secretory pathway Ca2+-ATPase 1, also known to mediate the influx of cytosolic Mn2+ into the lumen of the Golgi apparatus, is not crucial for the Mn2+-induced rescue glycosylation of lysosomal-associated membrane protein 2 (LAMP2). In contrast, our results demonstrate the involvement of cyclopiazonic acid- and thapsigargin (Tg)-sensitive pumps in the rescue of TMEM165-associated glycosylation defects by Mn2+. Interestingly, overexpression of sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) 2b isoform in TMEM165 KO cells partially rescues the observed LAMP2 glycosylation defect. Overall, this study indicates that the rescue of Golgi N-glycosylation defects in TMEM165 KO cells by extracellular Mn2+ involves the activity of Tg and cyclopiazonic acid-sensitive pumps, probably the SERCA pumps.-Houdou, M., Lebredonchel, E., Garat, A., Duvet, S., Legrand, D., Decool, V., Klein, A., Ouzzine, M., Gasnier, B., Potelle, S., Foulquier, F. Involvement of thapsigargin- and cyclopiazonic acid-sensitive pumps in the rescue of TMEM165-associated glycosylation defects by Mn2+.

Investigating the function of Gdt1p in yeast Golgi glycosylation.

The Golgi ion homeostasis is tightly regulated to ensure essential cellular processes such as glycosylation, yet our understanding of this regulation remains incomplete. Gdt1p is a member of the conserved Uncharacterized Protein Family (UPF0016). Our previous work suggested that Gdt1p may function in the Golgi by regulating Golgi Ca2+/Mn2+ homeostasis. NMR structural analysis of the polymannan chains isolated from yeasts showed that the gdt1Δ mutant cultured in presence of high Ca2+ concentration, as well as the pmr1Δ and gdt1Δ/pmr1Δ strains presented strong late Golgi glycosylation defects with a lack of α-1,2 mannoses substitution and α-1,3 mannoses termination. The addition of Mn2+ confirmed the rescue of these defects. Interestingly, our structural data confirmed that the glycosylation defect in pmr1Δ could also completely be suppressed by the addition of Ca2+. The use of Pmr1p mutants either defective for Ca2+ or Mn2+ transport or both revealed that the suppression of the observed glycosylation defect in pmr1Δ strains by the intraluminal Golgi Ca2+ requires the activity of Gdt1p. These data support the hypothesis that Gdt1p, in order to sustain the Golgi glycosylation process, imports Mn2+ inside the Golgi lumen when Pmr1p exclusively transports Ca2+. Our results also reinforce the functional link between Gdt1p and Pmr1p as we highlighted that Gdt1p was a Mn2+ sensitive protein whose abundance was directly dependent on the nature of the ion transported by Pmr1p. Finally, this study demonstrated that the aspartic residues of the two conserved motifs E-x-G-D-[KR], likely constituting the cation binding sites of Gdt1p, play a crucial role in Golgi glycosylation and hence in Mn2+/Ca2+transport.

Manganese-induced turnover of TMEM165.

TMEM165 deficiencies lead to one of the congenital disorders of glycosylation (CDG), a group of inherited diseases where the glycosylation process is altered. We recently demonstrated that the Golgi glycosylation defect due to TMEM165 deficiency resulted from a Golgi manganese homeostasis defect and that Mn2+ supplementation was sufficient to rescue normal glycosylation. In the present paper, we highlight TMEM165 as a novel Golgi protein sensitive to manganese. When cells were exposed to high Mn2+ concentrations, TMEM165 was degraded in lysosomes. Remarkably, while the variant R126H was sensitive upon manganese exposure, the variant E108G, recently identified in a novel TMEM165-CDG patient, was found to be insensitive. We also showed that the E108G mutation did not abolish the function of TMEM165 in Golgi glycosylation. Altogether, the present study identified the Golgi protein TMEM165 as a novel Mn2+-sensitive protein in mammalian cells and pointed to the crucial importance of the glutamic acid (E108) in the cytosolic ELGDK motif in Mn2+-induced degradation of TMEM165.

Galactose Supplementation in Patients With TMEM165-CDG Rescues the Glycosylation Defects.

CONTEXT: TMEM165 deficiency is a severe multisystem disease that manifests with metabolic, endocrine, and skeletal involvement. It leads to one type of congenital disorders of glycosylation (CDG), a rapidly growing group of inherited diseases in which the glycosylation process is altered. Patients have decreased galactosylation by serum glycan analysis. There are >100 CDGs, but only specific types are treatable. OBJECTIVE: Galactose has been shown to be beneficial in other CDG types with abnormal galactosylation. The aim of this study was to characterize the effects of galactose supplementation on Golgi glycosylation in TMEM165-depleted HEK293 cells, as well as in 2 patients with TMEM165-CDG and in their cultured skin fibroblast cells. DESIGN AND SETTING: Glycosylation was assessed by mass spectrometry, western blot analysis, and transferrin isoelectrofocusing. PATIENTS AND INTERVENTIONS: Both unrelated patients with TMEM165-CDG with the same deep intronic homozygous mutation (c.792+182G>A) were allocated to receive d-galactose in a daily dose of 1 g/kg. RESULTS: We analyzed N-linked glycans and glycolipids in knockout TMEM165 HEK293 cells, revealing severe hypogalactosylation and GalNAc transfer defects. Although these defects were completely corrected by the addition of Mn2+, we demonstrated that the observed N-glycosylation defect could also be overcome by galactose supplementation. We then demonstrated that oral galactose supplementation in patients with TMEM165-deficient CDG improved biochemical and clinical parameters, including a substantial increase in the negatively charged transferrin isoforms, and a decrease in hypogalactosylated total N-glycan structures, endocrine function, and coagulation parameters. CONCLUSION: 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.

Evidence for splice transcript variants of TMEM165, a gene involved in CDG.

BACKGROUND: Defects in TMEM165 gene cause a type-II Congenital Disorder of Glycosylation affecting Golgi glycosylation processes. TMEM165 patients exhibit psychomotor retardation, important osteoporosis, scoliosis, irregular epiphyses and thin bone cortex. TMEM165 protein is highly conserved in evolution and belongs to the family of UPF0016 membrane proteins which could be an unique group of Ca2+/H+ antiporters regulating Ca2+ and pH homeostasis and mainly localized in the Golgi apparatus. METHODS: RT-PCR from human brain tissues revealed TMEM165 splice-transcript variants. mRNA expression was analyzed by RT-Q-PCR. Expression plasmids allowed us to visualize isoform proteins and their subcellular localization. Their functions on glycosylation were achieved by looking at the gel mobility of highly glycosylated proteins in cells overexpressing isoforms. RESULTS: In this study, we highlight, as previously shown for other ion channels, the existence of TMEM165 splice-transcripts isoforms, in particular the Short-Form (SF) and the Long-Form (LF) transcripts, leading to a 129 aa and 259 aa protein isoform, respectively. These proteins both localize in the endoplasmic reticulum and have different effects on glycosylation compared to the wild-type protein (324 aa). We also point out that the SF is expressed at low levels in all human cells and tissues checked, excepted in brain, and forms homodimer. The LF was only expressed in the temporal lobe of human brain. GENERAL SIGNIFICANCE: The finding of numerous splice variants could lead to a family of TMEM165 isoforms. This family of TMEM165 splice transcripts could participate in the fine regulation of TMEM165 isoforms' functions and localizations.

TMEM165 deficiencies in Congenital Disorders of Glycosylation type II (CDG-II): Clues and evidences for roles of the protein in Golgi functions and ion homeostasis.

Congenital Disorders of Glycosylation (CDG) are rare inherited diseases causing glycosylation defects responsible for severe growth and psychomotor retardations in patients. Whereas most genetic defects affect enzymes directly involved in the glycosylation process, like glycosyltransferases or sugar transporters, recent findings revealed the impact of gene mutations on proteins implicated in both Golgi vesicular trafficking and ion homeostasis. TMEM165 is one of these deficient Golgi proteins found in CDG patients whose function in the secretory pathway has been deduced from several recent studies using TMEM165 deficient mammalian cells or yeast cells deficient in Gtd1p, the yeast TMEM165 ortholog. These studies actually confirm previous observations based on both sequence and predicted topology of this transmembrane protein and the phenotypes of human and yeast cells, namely that TMEM165 is very probably a transporter involved in ion homeostasis. Whereas the exact function of TMEM165 remains to be fully characterized, several studies hypothesize that TMEM165 could be a Golgi localized Ca2+/H+ antiporter. However, recent data also support the role of TMEM165 in Golgi Mn2+ homeostasis then arguing for a putative role of Mn2+ transporter for TMEM165 essential to achieve the correct N-glycosylation process of proteins in the secretory pathway. This manuscript is a review of the current state of knowledge on TMEM165 deficiencies in Congenital Disorders of Glycosylation as well as new data on function of TMEM165 and some speculative models on TMEM165/Golgi functions are discussed.

Glycosylation abnormalities in Gdt1p/TMEM165 deficient cells result from a defect in Golgi manganese homeostasis.

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.

Golgi post-translational modifications and associated diseases.

For non specialists, Golgi is a very well known subcellular compartment involved in secretion and correct targeting of soluble and transmembrane proteins. Nevertheless, Golgi is also specifically involved in many different and diverse post-translational modifications. Through its diverse functions, Golgi is not only able to modify secreted and transmembrane proteins but also cytoplasmic proteins. The Golgi apparatus research field is so broad that an exhaustive review of this organelle is not doable here. The goal of this review is to cover the main post-translational modifications occurring at the Golgi level and present the identified associated diseases.