SLC10A7 regulates O-GalNAc glycosylation and Ca2+ homeostasis in the secretory pathway: insights into SLC10A7-CDG.

Glycans are known to be fundamental for many cellular and physiological functions. Congenital disorders of glycosylation (CDG) currently encompassing over 160 subtypes, are characterized by glycan synthesis and/or processing defects. Despite the increasing number of CDG patients, therapeutic options remain very limited as our knowledge on glycan synthesis is fragmented. The emergence of CDG resulting from defects in ER/ Golgi homeostasis makes this even more difficult. SLC10A7 belongs to the SLC10 protein family, known as bile acid and steroid transport family, exhibiting a unique structure. It shows a ubiquitous expression and is linked to negative calcium regulation in cells. The mechanisms by which SLC10A7 deficiency leads to Golgi glycosylation abnormalities are unknown. The present study identifies major O-glycosylation defects in both SLC10A7 KO HAP1 cells and SLC10A7-CDG patient fibroblasts and reveals an increased ER and Golgi calcium contents. We also show that the abundance of COSMC and C1GALT1 is altered in SLC10A7-CDG patient cells, as well as the subcellular Golgi localization of the Ca²⁺-binding Cab45 protein. Finally, we demonstrate that supraphysiological manganese supplementation suppresses the deficient electrophoretic mobility of TGN46 by an aberrant transfer of GalNAc residues, and reveal COSMC Mn²⁺ sensitivity. These findings provide novel insights into the mechanisms of Golgi glycosylation defects in SLC10A7-deficient cells. They show that SLC10A7 is a key Golgi transmembrane protein maintaining the tight regulation of Ca²⁺ homeostasis in the ER and Golgi compartments, both essential for glycosylation.