Classic galactosemia (CG) is an autosomal recessive disorder resulting from loss of
galactose-1-phosphate uridyltransferase (GALT), which catalyzes conversion of
galactose-1-phosphate and
uridine diphosphate (
UDP)-glucose to
glucose-1-phosphate and
UDP-galactose, immediately upstream of
UDP-N-
acetylgalactosamine and
UDP-
N-acetylglucosamine synthesis. These four
UDP-sugars are essential donors for driving the synthesis of
glycoproteins and
glycolipids, which heavily decorate cell surfaces and extracellular spaces. In addition to acute, potentially lethal neonatal symptoms, maturing individuals with CG develop striking neurodevelopmental, motor and
cognitive impairments. Previous studies suggest that neurological symptoms are associated with glycosylation defects, with CG recently being described as a
congenital disorder of glycosylation (CDG), showing defects in both N- and O-linked
glycans. Here, we characterize behavioral traits, synaptic development and glycosylated synaptomatrix formation in a GALT-deficient Drosophila disease model. Loss of Drosophila GALT (dGALT) greatly impairs coordinated movement and results in structural overelaboration and architectural abnormalities at the neuromuscular junction (NMJ). Dietary
galactose and mutation of
galactokinase (dGALK) or
UDP-glucose dehydrogenase (sugarless) genes are identified, respectively, as critical environmental and genetic modifiers of behavioral and cellular defects. Assaying the NMJ extracellular synaptomatrix with a broad panel of
lectin probes reveals profound alterations in dGALT mutants, including depletion of galactosyl, N-
acetylgalactosamine and fucosylated
horseradish peroxidase (HRP) moieties, which are differentially corrected by dGALK co-removal and sugarless overexpression. Synaptogenesis relies on trans-synaptic signals modulated by this synaptomatrix
carbohydrate environment, and dGALT-null NMJs display striking changes in
heparan sulfate proteoglycan (
HSPG) co-receptor and Wnt
ligand levels, which are also corrected by dGALK co-removal and sugarless overexpression. These results reveal synaptomatrix glycosylation losses, altered trans-synaptic signaling pathway components, defective synaptogenesis and impaired coordinated movement in a CG neurological disease model.