Department of Pathophysiology and Metabolism, Kawasaki Medical School, Okayama, Japan.
Department of Pharmacology, Kawasaki Medical School, Okayama, Japan.
J Neurochem. 2023 Aug;166(4):720-746. doi: 10.1111/jnc.15876. Epub 2023 Jun 20.
Krabbe disease is an inherited demyelinating disease caused by a genetic deficiency of the lysosomal enzyme galactosylceramide (GalCer) β-galactosidase (GALC). The Twitcher (Twi) mouse is a naturally occurring, genetically and enzymatically authentic mouse model that mimics infantile-onset Krabbe disease. The major substrate for GALC is the myelin lipid GalCer. However, the pathogenesis of Krabbe disease has long been explained by the accumulation of psychosine, a lyso-derivative of GalCer. Two metabolic pathways have been proposed for the accumulation of psychosine: a synthetic pathway in which galactose is transferred to sphingosine and a degradation pathway in which GalCer is deacylated by acid ceramidase (ACDase). Saposin-D (Sap-D) is essential for the degradation of ceramide by ACDase in lysosome. In this study, we generated Twi mice with a Sap-D deficiency (Twi/Sap-D KO), which are genetically deficient in both GALC and Sap-D and found that very little psychosine accumulated in the CNS or PNS of the mouse. As expected, demyelination with the infiltration of multinucleated macrophages (globoid cells) characteristic of Krabbe disease was milder in Twi/Sap-D KO mice than in Twi mice both in the CNS and PNS during the early disease stage. However, at the later disease stage, qualitatively and quantitatively comparable demyelination occurred in Twi/Sap-D KO mice, particularly in the PNS, and the lifespans of Twi/Sap-D KO mice were even shorter than that of Twi mice. Bone marrow-derived macrophages from both Twi and Twi/Sap-D KO mice produced significant amounts of TNF-α upon exposure to GalCer and were transformed into globoid cells. These results indicate that psychosine in Krabbe disease is mainly produced via the deacylation of GalCer by ACDase. The demyelination observed in Twi/Sap-D KO mice may be mediated by a psychosine-independent, Sap-D-dependent mechanism. GalCer-induced activation of Sap-D-deficient macrophages/microglia may play an important role in the neuroinflammation and demyelination in Twi/Sap-D KO mice.
克拉伯病是一种遗传性脱髓鞘疾病，由溶酶体酶半乳糖脑苷脂（GalCer）β-半乳糖苷酶（GALC）的遗传缺陷引起。抽搐（Twi）小鼠是一种自然发生的、遗传和酶学上真实的小鼠模型，模拟婴儿期发作的克拉伯病。GALC 的主要底物是髓鞘脂质半乳糖脑苷脂。然而，克拉伯病的发病机制长期以来一直被神经肌醇的积累所解释，神经肌醇是半乳糖脑苷脂的溶酶体衍生物。已经提出了两种神经肌醇积累的代谢途径：一种是半乳糖转移到神经鞘氨醇的合成途径，另一种是半乳糖脑苷脂被酸性神经酰胺酶（ACDase）脱酰化的降解途径。SAP 蛋白 D（Sap-D）对于 ACDase 在溶酶体中降解神经酰胺是必不可少的。在这项研究中，我们生成了 SAP 蛋白 D 缺陷（Twi/Sap-D KO）的 Twi 小鼠，该小鼠在 GALC 和 SAP 蛋白 D 两个基因上均有缺陷，并且发现神经肌醇在小鼠的中枢神经系统或周围神经系统中几乎没有积累。正如预期的那样，与 Twi 小鼠相比，Twi/Sap-D KO 小鼠在中枢神经系统和周围神经系统中的脱髓鞘程度较轻，在疾病早期阶段，有特征性的多核巨噬细胞（类球体细胞）浸润。然而，在疾病后期阶段，Twi/Sap-D KO 小鼠出现了可定性和定量比较的脱髓鞘，特别是在周围神经系统，并且 Twi/Sap-D KO 小鼠的寿命甚至比 Twi 小鼠更短。来自 Twi 和 Twi/Sap-D KO 小鼠的骨髓来源的巨噬细胞在暴露于半乳糖脑苷脂后会产生大量的 TNF-α，并转化为类球体细胞。这些结果表明，克拉伯病中的神经肌醇主要是通过 ACDase 对半乳糖脑苷脂的脱酰化产生的。在 Twi/Sap-D KO 小鼠中观察到的脱髓鞘可能是通过一种与神经肌醇无关、依赖 SAP 蛋白 D 的机制介导的。GalCer 诱导的缺乏 Sap-D 的巨噬细胞/小胶质细胞的激活可能在 Twi/Sap-D KO 小鼠的神经炎症和脱髓鞘中发挥重要作用。
