Neuromuscular Defects in a Drosophila Model of the Congenital Disorder of Glycosylation SLC35A2-CDG

SLC35A2-CDG is a congenital disorder of glycosylation caused by mutations in the <i>SLC35A2</i> gene encoding a Golgi-localized UDP-galactose transporter. This transporter plays an essential role in glycan synthesis by transporting UDP-galactose from the cytoplasm into the Golgi lumen. Its dysfunction leads to impaired galactose-containing glycans and various neurological symptoms, although the underlying mechanisms remain largely unknown. We identified a novel SLC35A2-CDG patient carrying a pathogenic variant (c.617_620del, p.(Gln206ArgfsTer45)) who exhibited neurological abnormalities including bilateral ventriculomegaly. To investigate the disease mechanism, we established the first <i>Drosophila</i> model of SLC35A2-CDG. Knockout of <i>Ugalt</i>, the fly ortholog of <i>SLC35A2</i>, resulted in embryonic lethality, indicating its essential role. Knockdown of <i>Ugalt</i> reduced mucin-type <i>O</i>-glycans on muscles and neuromuscular junctions (NMJs), without affecting <i>N</i>-glycans. <i>Ugalt</i> knockdown larvae exhibited mislocalized NMJ boutons accompanied by a deficiency in basement membrane components on muscles. This phenotype resembles that of mutants of <i>dC1GalT1</i> and <i>dGlcAT-P</i>, both involved in mucin-type <i>O</i>-glycosylation. Genetic interaction between <i>Ugalt</i> and <i>dC1GalT1</i> was confirmed through double knockdown and double heterozygous analyses. Given that <i>Drosophila</i> NMJs are widely used as a model for mammalian central synapses, our findings suggest that Ugalt regulates NMJ architecture via mucin-type <i>O</i>-glycosylation and provide insights into the molecular basis of neurological abnormalities in SLC35A2-CDG.