Dihydroceramide Desaturase Regulates the Compartmentalization of Rac1 for Neuronal Oxidative Stress

Disruption of sphingolipid homeostasis has been shown to cause neurological disorders. How specific sphingolipid species modulate the pathogenesis remains unknown. The last step of sphingolipid de novo synthesis is the conversion of dihydroceramide to ceramide catalyzed by dihydroceramide desaturase (human DEGS1; Drosophila Ifc). Loss of ifc leads to dihydroceramide accumulation and oxidative stress, resulting in photoreceptors degeneration, while DEGS1 variants were associated with leukodystrophy and neuropathy. Here, we demonstrated that ifc regulates Rac1 compartmentalization in fly photoreceptors and further showed that dihydroceramide alters the association of active Rac1 to membranes mimicking specific organelles. We also revealed that the major source of ROS originated from Rac1 and NADPH oxidase (NOX) in the cytoplasm, as the NOX inhibitor apocynin ameliorated the oxidative stress and functional defects in both fly ifc-KO photoreceptors and human neuronal cells with disease-associated variant DEGS1H132R. Therefore, DEGS1/ifc deficiency causes dihydroceramide accumulation, resulting in Rac1 translocation and NOX-dependent neurodegeneration.