bims-meglyc 2024-07-21 papers

Issue of 2024–07–21
three papers selected by
Silvia Radenkovic, UMC Utrecht

J Biol Chem. 2024 Jul 16. pii: S0021-9258(24)02085-4. [Epub ahead of print] 107584

Molecular characterization of Rft1, an ER membrane protein associated with congenital disorder of glycosylation RFT1-CDG.

Eri Hirata, Ken-Taro Sakata, Grace I Dearden, Faria Noor, Indu Menon, George N Chiduza, Anant K Menon.

The oligosaccharide needed for protein N-glycosylation is assembled on a lipid carrier via a multi-step pathway. Synthesis is initiated on the cytoplasmic face of the endoplasmic reticulum (ER) and completed on the luminal side after transbilayer translocation of a heptasaccharide lipid intermediate. More than 30 Congenital Disorders of Glycosylation (CDGs) are associated with this pathway, including RFT1-CDG which results from defects in the membrane protein Rft1. Rft1 is essential for the viability of yeast and mammalian cells and was proposed as the transporter needed to flip the heptasaccharide lipid intermediate across the ER membrane. However, other studies indicated that Rft1 is not required for heptasaccharide lipid flipping in microsomes or unilamellar vesicles reconstituted with ER membrane proteins, nor is it required for the viability of at least one eukaryote. It is therefore not known what essential role Rft1 plays in N-glycosylation. Here, we present a molecular characterization of human Rft1, using yeast cells as a reporter system. We show that it is a multi-spanning membrane protein located in the ER, with its N and C-termini facing the cytoplasm. It is not N-glycosylated. The majority of RFT1-CDG mutations map to highly conserved regions of the protein. We identify key residues that are important for Rft1's ability to support N-glycosylation and cell viability. Our results provide a necessary platform for future work on this enigmatic protein.

Keywords: CDG; Ist2; MurJ; N-glycosylation; Nvj1; dolichol-linked oligosaccharide; endoplasmic reticulum; lipid droplet; scramblase; yeast

DOI: https://doi.org/10.1016/j.jbc.2024.107584

bioRxiv. 2024 Jul 02. pii: 2024.06.28.601207. [Epub ahead of print]

Aberrant N-glycosylation is a therapeutic target in carriers of a common and highly pleiotropic mutation in the manganese transporter ZIP8.

Vartika Tomar, John Kang, Ruxian Lin, Steven R Brant, Mark Lazarev, Caitlin Tressler, Kristine Glunde, Natasha Zachara, Joanna Melia.

The treatment of defective glycosylation in clinical practice has been limited to patients with rare and severe phenotypes associated with congenital disorders of glycosylation (CDG). Carried by approximately 5% of the human population, the discovery of the highly pleiotropic, missense mutation in a manganese transporter ZIP8 has exposed under-appreciated roles for Mn homeostasis and aberrant Mn-dependent glycosyltransferases activity leading to defective N-glycosylation in complex human diseases. Here, we test the hypothesis that aberrant N-glycosylation contributes to disease pathogenesis of ZIP8 A391T-associated Crohn's disease. Analysis of N-glycan branching in intestinal biopsies demonstrates perturbation in active Crohn's disease and a genotype-dependent effect characterized by increased truncated N-glycans. A mouse model of ZIP8 391-Thr recapitulates the intestinal glycophenotype of patients carrying mutations in ZIP8. Borrowing from therapeutic strategies employed in the treatment of patients with CDGs, oral monosaccharide therapy with N-acetylglucosamine ameliorates the epithelial N-glycan defect, bile acid dyshomeostasis, intestinal permeability, and susceptibility to chemical-induced colitis in a mouse model of ZIP8 391-Thr. Together, these data support ZIP8 391-Thr alters N-glycosylation to contribute to disease pathogenesis, challenging the clinical paradigm that CDGs are limited to patients with rare diseases. Critically, the defect in glycosylation can be targeted with monosaccharide supplementation, providing an opportunity for genotype-driven, personalized medicine.

DOI: https://doi.org/10.1101/2024.06.28.601207

J Biol Chem. 2024 Jul 11. pii: S0021-9258(24)02068-4. [Epub ahead of print] 107567

Insights into Molecular and Cellular Functions of the Golgi Calcium/Manganese-Proton Antiporter TMEM165.

Stanislovas S Jankauskas, Fahimeh Varzideh, Urna Kansakar, Ghaith Al Tibi, Esther Densu Agyapong, Jessica Gambardella, Gaetano Santulli.

The Golgi compartment performs a number of crucial roles in the cell. However, the exact molecular mechanisms underlying these actions are not fully defined. Pathogenic mutations in genes encoding Golgi proteins may serve as an important source for expanding our knowledge. For instance, mutations in the gene encoding Transmembrane protein 165 (TMEM165) were discovered as a cause of a new type of congenital disorder of glycosylation (CDG). Comprehensive studies of TMEM165 in different model systems, including mammals, yeast, and fish uncovered the new realm of Mn2+ homeostasis regulation. TMEM165 was shown to act as a Ca2+/Mn2+:H+ antiporter in medial- and trans-Golgi network, pumping the metal ions into the Golgi lumen and protons outside. Disruption of TMEM165 antiporter activity results in defects in N- and O-glycosylation of proteins and glycosylation of lipids. An impaired glycosylation of TMEM165-CDG arises from lack of Mn2+ within the Golgi. Nevertheless, Mn2+ insufficiency in the Golgi is compensated by the activity of the ATPase SERCA2. TMEM165 turnover has also been found to be regulated by Mn2+ cytosolic concentration. Besides causing CDG, recent investigations have demonstrated the functional involvement of TMEM165 in several other pathologies including cancer and mental health disorders. This systematic review summarizes the available information on TMEM165 molecular structure, cellular function, and its roles in health and disease.

Keywords: CDG; Glycosylation; Golgi; Lysosomes; Mn(2+); SERCA; SPCA1; TMEM

DOI: https://doi.org/10.1016/j.jbc.2024.107567