Study on the mechanism of demyelination in hereditary leukodystrophy

在几种遗传性脑白质营养不良中，Krabbe病(GLD)是独一无二的，因为缺乏缺陷酶的自然底物半乳糖神经酰胺的积累。我们最近证明了半乳糖神经酰胺的水解酶是由两种不同基因的酸性β-半乳糖苷酶(半乳糖神经酰胺酶I和II)催化的，其中一种酶(I)缺乏GLD。半乳糖基鞘氨基精氨酸是半乳糖基神经酰胺酶I的良好底物，但不是半乳糖基神经酰胺酶II的好底物。Svennerholm等人报道了GLD患者脑内脂质的积聚，但测定方法过于复杂和耗时。我们建立了一种简单而灵敏的半乳糖基鞘氨基葡萄糖苷的高效液相色谱检测方法，并证明了脂类物质不仅在大脑中异常积聚，而且在人和小鼠的GLD病例的躯体器官中也出现了异常积聚。半乳糖鞘氨醇蓄积的组织分布和细胞毒性强烈提示在GLD病例中半乳糖鞘氨醇的蓄积与脱髓鞘密切相关。在异染性脑白质营养不良症(MLD)中，我们发现溶酶硫脂的异常积聚。脑白质、脊髓和周围神经中蓄积量较高。这种脂类还有很强的细胞毒性，累积的溶酶硫脂可能会导致MLD患者脱髓鞘，就像GLD的情况一样。关于这些溶质化合物对髓鞘形成细胞(雪旺细胞和少突胶质细胞)的毒性作用，将有必要进行进一步的研究。

Among several hereditary leukodystrophies, krabbe disease (GLD) is unique because of the absence of accumulation of a natural substrate of the deficient enzyme, galactosylceramide. We have recently demonstrated that the hydrolysis of galactosylceramide is catalyzed by two genetically distinct acid beta-galactosidases (galactosylceramidase I and II) and one of them (I) is deficient in GLD. Galactosylspingosine (psychosine) is a good substrate of galactosylceramidase I but not galactosylceramidase II. Svennerholm et al reported an accumulation of the lipid in the brain of GLD patients but the assay method was too complex and time-consuming. We developed a simple and sensitive assay method of galactosylspingosine using high-performance liquid chromatography, and demonstrated an abnormal accumulation of the lipid not only in the brain but also in somatic organs of human and murine cases of GLD. The tissue distribution of the accumulation and the cytotoxicity of galactosylspingosine strongly suggested the close relationship between the accumulation of galactosylsphingosine and demyelination seen in cases of GLD. Galactosylsphingosine was found to be synthesized from free spingoid bases by the catalysis of galactosyltransferase.In metachromatic leukodystrophy (MLD), we found an abnormal accumulation of lysosulfatide. The accumulated amount was high in the cerebral white matter, spinal cord and peripheral nerve. The lipid was also so cytotoxic that accumulated lysosulfatide may cause demyelination in patients with MLD as in the case of GLD.Further studies concerning toxic effects of these lyso-compounds to myelinforming cells (Schwann cells and oligodendrocytes) will be necessary.