Encephalopathy Caused by Ablation of Very Long Acyl Chain Ceramide Synthesis May Be Largely Due to Reduced Galactosylceramide Levels

Abstract

Sphingolipids (SLs) act as signaling molecules and as structural components in both neuronal cells and myelin. We now characterize the biochemical, histological, and behavioral abnormalities in the brain of a mouse lacking very long acyl (C22-C24) chain SLs. This mouse, which is defective in the ability to synthesize C22-C24-SLs due to ablation of ceramide synthase 2, has reduced levels of galactosylceramide (GalCer), a major component of myelin, and in particular reduced levels of non-hydroxy-C22-C24-GalCer and 2-hydroxy-C22-C24-GalCer. Noteworthy brain lesions develop with a time course consistent with a vital role for C22-C24-GalCer in myelin stability. Myelin degeneration and detachment was observed as was abnormal motor behavior originating from a subcortical region. Additional abnormalities included bilateral and symmetrical vacuolization and gliosis in specific brain areas, which corresponded to some extent to the pattern of ceramide synthase 2 expression, with astrogliosis considerably more pronounced than microglial activation. Unexpectedly, unidentified storage materials were detected in lysosomes of astrocytes, reminiscent of the accumulation that occurs in lysosomal storage disorders. Together, our data demonstrate a key role in the brain for SLs containing very long acyl chains and in particular GalCer with a reduction in their levels leading to distinctive morphological abnormalities in defined brain regions.

Ablation of ceramide synthase 2 strongly affects biophysical properties of membranes

“…In older in increased membrane order, which can be attributed to depletion of unsaturated SLs. The decrease in VLC-SLs in brain membranes was not as large as in liver because brain is enriched in LC-SLs ( 8 ), such that the length of the acyl chains contributes less to the increased membrane fl uidity. Despite the lower fl uidity of CerS2 null brain membranes, altered morphology was also observed.…”

“…and composition of CerS2 null mice compromise membrane lipid packing, promoting changes in membrane curvature and enhancing processes such as fusion and budding. Because there are no signifi cant variations in phospholipid and cholesterol content between CerS2 null and control mice ( 6,8 ), we conclude that the changes in biophysical properties are a direct consequence of changes in the SL composition.…”

“…We previously demonstrated that CerS2 null mouse liver and brain exhibit a signifi cantly altered SL composition but no major alterations in phospholipid and cholesterol composition ( 6,8 ). We now characterize the SL profi le of microsomal lipids obtained from liver and brain of control and CerS2 null mice of different ages.…”

“…S1A) compared with whole brain where SM and hexosylceramide (HexCer), which consists of both glucosylceramide (GlcCer) and galactosylceramide (GalCer), predominate ( 8 ). Because HexCer, particularly GalCer ( 8 ), is the major SL in myelin, the enrichment of Cer and the low levels of HexCer in microsomal brain lipids demonstrate that the extracts have little or no myelin lipid contamination. This was further confi rmed by Western blotting, in which myelin basic protein was barely detectable in the microsomal fractions, which were highly enriched in the ER marker, BiP (data not shown).…”

Ablation of Ceramide Synthase 2 Causes Chronic Oxidative Stress Due to Disruption of the Mitochondrial Respiratory Chain

“…This mouse displays severe hepatopathy (11) and hepatic insulin resistance (12), defects in myelin in the central nervous system (13), and a number of pronounced biophysical changes in the properties of membrane lipids (14,15).…”