Spastic paraplegia 35 (SPG35) (OMIM: 612319) or
fatty acid hydroxylase-associated neurodegeneration (FAHN) is caused by deficiency of
fatty acid 2-hydroxylase (FA2H). This
enzyme synthesizes
sphingolipids containing 2-hydroxylated
fatty acids, which are particularly abundant in myelin. Fa2h-deficient (Fa2h-/-) mice develop symptoms reminiscent of the human disease and therefore serve as animal model of SPG35. In order to understand further the pathogenesis of SPG35, we compared the
proteome of purified CNS myelin isolated from wild type and Fa2h-/- mice at different time points of
disease progression using tandem mass tag labeling. Data analysis with a focus on myelin
membrane proteins revealed a significant increase of the oligodendrocytic myelin paranodal and inner loop
protein (Opalin) in Fa2h-/- mice, whereas the concentration of other major
myelin proteins was not significantly changed. Western blot analysis revealed an almost 6-fold increase of Opalin in myelin of Fa2h-/- mice aged 21-23 months. A concurrent unaltered Opalin gene expression suggested a decreased turnover of the Opalin
protein in Fa2h-/- mice. Supporting this hypothesis, Opalin
protein half-life was reduced significantly when expressed in CHO cells synthesizing 2-hydroxylated
sulfatide, compared to cells synthesizing only non-hydroxylated
sulfatide. Degradation of Opalin was inhibited by inhibitors of lysosomal degradation but unaffected by
proteasome inhibitors. Taken together, these results reveal a new function of 2-hydroxylated
sphingolipids namely affecting the turnover of a myelin
membrane protein. This may play a role in the pathogenesis of SPG35.