Heterozygous, loss-of-function mutations in the granulin gene (GRN) encoding progranulin (PGRN) are a common cause of frontotemporal dementia (FTD). Homozygous GRN mutations cause neuronal ceroid lipofuscinosis-11 (CLN11), a lysosome storage disease. PGRN is a secreted glycoprotein that can be proteolytically cleaved into seven bioactive 6 kDa granulins. However, it is unclear how deficiency of PGRN and granulins causes neurodegeneration. To gain insight into the mechanisms of FTD pathogenesis, we utilized Tandem Mass Tag isobaric labeling mass spectrometry to perform an unbiased quantitative proteomic analysis of whole-brain tissue from wild type (Grn+/+) and Grn knockout (Grn−/−) mice at 3- and 19-months of age. At 3-months lysosomal proteins (i.e. Gns, Scarb2, Hexb) are selectively increased indicating lysosomal dysfunction is an early consequence of PGRN deficiency. Additionally, proteins involved in lipid metabolism (Acly, Apoc3, Asah1, Gpld1, Ppt1, and Naaa) are decreased; suggesting lysosomal degradation of lipids may be impaired in the Grn−/− brain. Systems biology using weighted correlation network analysis (WGCNA) of the Grn−/− brain proteome identified 26 modules of highly co-expressed proteins. Three modules strongly correlated to Grn deficiency and were enriched with lysosomal proteins (Gpnmb, CtsD, CtsZ, and Tpp1) and inflammatory proteins (Lgals3, GFAP, CD44, S100a, and C1qa). We find that lysosomal dysregulation is exacerbated with age in the Grn−/− mouse brain leading to neuroinflammation, synaptic loss, and decreased markers of oligodendrocytes, myelin, and neurons. In particular, GPNMB and LGALS3 (galectin-3) were upregulated by microglia and elevated in FTD-GRN brain samples, indicating common pathogenic pathways are dysregulated in human FTD cases and Grn−/− mice. GPNMB levels were significantly increased in the cerebrospinal fluid of FTD-GRN patients, but not in MAPT or C9orf72 carriers, suggesting GPNMB could be a biomarker specific to FTD-GRN to monitor disease onset, progression, and drug response. Our findings support the idea that insufficiency of PGRN and granulins in humans causes neurodegeneration through lysosomal dysfunction, defects in autophagy, and neuroinflammation, which could be targeted to develop effective therapies.

中文翻译：

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前颗粒蛋白缺陷小鼠脑蛋白质组的网络分析揭示了由 GRN 突变引起的人类额颞叶痴呆的共同致病机制

编码颗粒蛋白前体 (PGRN) 的颗粒蛋白基因 (GRN) 中的杂合、功能丧失突变是额颞叶痴呆 (FTD) 的常见原因。纯合 GRN 突变导致神经元蜡样脂褐质沉着症 11 (CLN11)，一种溶酶体贮积病。PGRN 是一种分泌型糖蛋白，可被蛋白水解成七种具有生物活性的 6 kDa 颗粒蛋白。然而，尚不清楚 PGRN 和颗粒蛋白的缺乏如何导致神经变性。为了深入了解 FTD 发病机制，我们利用 Tandem Mass Tag 等压标记质谱法对野生型 (Grn+/+) 和 Grn 敲除 (Grn-/-) 小鼠的全脑组织进行了无偏的定量蛋白质组学分析。 3 个月和 19 个月大。在 3 个月时，溶酶体蛋白（即 Gns、Scarb2、Hexb) 选择性增加，表明溶酶体功能障碍是 PGRN 缺乏的早期后果。此外，参与脂质代谢的蛋白质（Acly、Apoc3、Asah1、Gpld1、Ppt1 和 Naaa）减少；表明 Grn-/- 大脑中脂质的溶酶体降解可能受损。使用 Grn-/- 脑蛋白质组的加权相关网络分析 (WGCNA) 的系统生物学确定了 26 个高度共表达的蛋白质模块。三个模块与 Grn 缺乏密切相关，富含溶酶体蛋白（Gpnmb、CtsD、CtsZ 和 Tpp1）和炎症蛋白（Lgals3、GFAP、CD44、S100a 和 C1qa）。我们发现溶酶体失调在 Grn-/- 小鼠大脑中随着年龄的增长而加剧，导致神经炎症、突触丧失以及少突胶质细胞、髓鞘和神经元的标志物减少。尤其是，GPNMB 和 LGALS3 (galectin-3) 被小胶质细胞上调并在 FTD-GRN 脑样本中升高，表明常见的致病途径在人类 FTD 病例和 Grn-/- 小鼠中失调。GPNMB 水平在 FTD-GRN 患者的脑脊液中显着增加，但在 MAPT 或 C9orf72 携带者中没有，这表明 GPNMB 可能是 FTD-GRN 特异性的生物标志物，用于监测疾病的发作、进展和药物反应。我们的研究结果支持这样一种观点，即人类中 PGRN 和颗粒蛋白的不足会通过溶酶体功能障碍、自噬缺陷和神经炎症导致神经退行性变，这可以作为开发有效疗法的目标。GPNMB 水平在 FTD-GRN 患者的脑脊液中显着增加，但在 MAPT 或 C9orf72 携带者中没有，这表明 GPNMB 可能是 FTD-GRN 特异性的生物标志物，用于监测疾病的发作、进展和药物反应。我们的研究结果支持这样一种观点，即人类中 PGRN 和颗粒蛋白的不足会通过溶酶体功能障碍、自噬缺陷和神经炎症导致神经退行性变，这可以作为开发有效疗法的目标。GPNMB 水平在 FTD-GRN 患者的脑脊液中显着增加，但在 MAPT 或 C9orf72 携带者中没有，这表明 GPNMB 可能是 FTD-GRN 特异性的生物标志物，用于监测疾病的发作、进展和药物反应。我们的研究结果支持这样一种观点，即人类中 PGRN 和颗粒蛋白的不足会通过溶酶体功能障碍、自噬缺陷和神经炎症导致神经退行性变，这可以作为开发有效疗法的目标。