l-Fucose is a candidate monosaccharide neuromodulator and mitigates Alzheimer's synaptic deficits.

Fucosylation, a major glycan modification, has been shown to influence neuronal and microglial mechanisms, but whether unconjugated free l-fucose can affect brain function is unknown. l-Fucose can be transported into cells and metabolized by fucokinase (FCSK) via the poorly understood salvage pathway. Using mouse hippocampal slices, we showed that l-fucose enhanced excitatory neurotransmission and long-term potentiation (LTP) through regulation of presynaptic release. Such effects required l-fucose be metabolized through the FCSK-driven salvage pathway, suggesting a metabolic-signaling mechanism. Human Alzheimer's disease (AD) and 5xFAD mouse brains showed signs of fucose hypometabolism with impaired l-fucose signaling. Such abnormalities were corrected by exogenous l-fucose, exemplified by rectification of LTP deficits in 5xFAD hippocampus. A dietary l-fucose supplement, which increased cerebral free l-fucose levels and up-regulated FCSK to drive the salvage pathway, mitigated synaptic and behavioral deficits of 5xFAD mice. Our data suggest an unrecognized neuromodulatory function of free l-fucose and reveals its therapeutic potential for AD.