Department of Biochemistry, Children’s Hospital, University Medical Center Hamburg-Eppendorf, 20246 Hamburg, Germany.
Brain. 2012 Sep;135(Pt 9):2661-75. doi: 10.1093/brain/aws209.
Mucolipidosis II is a neurometabolic lysosomal trafficking disorder of infancy caused by loss of mannose 6-phosphate targeting signals on lysosomal proteins, leading to lysosomal dysfunction and accumulation of non-degraded material. However, the identity of storage material and mechanisms of neurodegeneration in mucolipidosis II are unknown. We have generated 'knock-in' mice with a common mucolipidosis II patient mutation that show growth retardation, progressive brain atrophy, skeletal abnormalities, elevated lysosomal enzyme activities in serum, lysosomal storage in fibroblasts and brain and premature death, closely mimicking the mucolipidosis II disease in humans. The examination of affected mouse brains at different ages by immunohistochemistry, ultrastructural analysis, immunoblotting and mass spectrometric analyses of glycans and anionic lipids revealed that the expression and proteolytic processing of distinct lysosomal proteins such as α-l-fucosidase, β-hexosaminidase, α-mannosidase or Niemann-Pick C2 protein are more significantly impacted by the loss of mannose 6-phosphate residues than enzymes reaching lysosomes independently of this targeting mechanism. As a consequence, fucosylated N-glycans, GM2 and GM3 gangliosides, cholesterol and bis(monoacylglycero)phosphate accumulate progressively in the brain of mucolipidosis II mice. Prominent astrogliosis and the accumulation of organelles and storage material in focally swollen axons were observed in the cerebellum and were accompanied by a loss of Purkinje cells. Moreover, an increased neuronal level of the microtubule-associated protein 1 light chain 3 and the formation of p62-positive neuronal aggregates indicate an impairment of constitutive autophagy in the mucolipidosis II brain. Our findings demonstrate the essential role of mannose 6-phosphate for selected lysosomal proteins to maintain the capability for degradation of sequestered components in lysosomes and autophagolysosomes and prevent neurodegeneration. These lysosomal proteins might be a potential target for a valid therapeutic approach for mucolipidosis II disease.
黏脂贮积症 II 型是一种婴儿期的神经代谢溶酶体运输障碍，由溶酶体蛋白上甘露糖 6-磷酸靶向信号的缺失引起，导致溶酶体功能障碍和未降解物质的积累。然而，黏脂贮积症 II 型中储存物质的性质和神经退行性变的机制尚不清楚。我们已经产生了带有常见黏脂贮积症 II 型患者突变的“敲入”小鼠，这些小鼠表现出生长迟缓、进行性脑萎缩、骨骼异常、血清中溶酶体酶活性升高、成纤维细胞和脑内溶酶体储存以及早逝，这些表现与人类的黏脂贮积症 II 型疾病非常相似。通过免疫组织化学、超微结构分析、免疫印迹和糖和阴离子脂质的质谱分析，研究不同年龄的受影响小鼠大脑，发现特定溶酶体蛋白（如α-L-岩藻糖苷酶、β-己糖胺酶、α-甘露糖苷酶或尼曼-匹克 C2 蛋白）的表达和蛋白水解加工受到的影响比通过这种靶向机制到达溶酶体的酶更为显著。因此，黏脂贮积症 II 型小鼠的大脑中，岩藻糖基化 N-糖链、GM2 和 GM3 神经节苷脂、胆固醇和双（单酰基甘油）磷酸逐渐积累。在小脑观察到明显的星形胶质增生以及细胞器和储存物质在局灶性肿胀轴突中的积累，并伴有浦肯野细胞的丧失。此外，微管相关蛋白 1 轻链 3 的神经元水平升高和 p62 阳性神经元聚集体的形成表明黏脂贮积症 II 型大脑中组成性自噬受损。我们的研究结果表明，甘露糖 6-磷酸对选定的溶酶体蛋白至关重要，以维持溶酶体和自噬溶酶体中隔离成分的降解能力，并防止神经退行性变。这些溶酶体蛋白可能是黏脂贮积症 II 型疾病有效治疗方法的潜在靶点。
