Centre for Molecular Medicine and Therapeutics, Department of Medical Genetics, University of British Columbia, and Children's and Women's Hospital, 980 West 28th Avenue, Vancouver, BC, V5Z 4H4, Canada.
Division of Neurology, Department of Medicine, University of British Columbia Hospital, S 192 - 2211 Wesbrook Mall, Vancouver, BC, V6T 2B5, Canada.
J Neuroinflammation. 2017 Nov 17;14(1):225. doi: 10.1186/s12974-017-1000-9.
Progranulin deficiency due to heterozygous null mutations in the GRN gene are a common cause of familial frontotemporal lobar degeneration (FTLD), while homozygous loss-of-function GRN mutations are thought to be a rare cause of neuronal ceroid lipofuscinosis (NCL). Aged progranulin-knockout (Grn-null) mice display highly exaggerated lipofuscinosis, microgliosis, and astrogliosis, as well as mild cell loss in specific brain regions. In the brain, progranulin is predominantly expressed in neurons and microglia, and previously, we demonstrated that neuronal-specific depletion of progranulin does not recapitulate the neuropathological phenotype of Grn-null mice. In this study, we evaluated whether selective depletion of progranulin expression in myeloid-lineage cells, including microglia, causes NCL-like neuropathology or neuroinflammation in mice.
We generated mice with progranulin depleted in myeloid-lineage cells by crossing mice homozygous for a floxed progranulin allele to mice expressing Cre recombinase under control of the LyzM promotor (Lyz-cKO).
Progranulin expression was reduced by approximately 50-70% in isolated microglia compared to WT levels. Lyz-cKO mice aged to 12 months did not display any increase in lipofuscin deposition, microgliosis, or astrogliosis in the four brain regions examined, though increases were observed for many of these measures in Grn-null animals. To evaluate the functional effect of reduced progranulin expression in isolated microglia, primary cultures were stimulated with controlled standard endotoxin and cytokine release was measured. While Grn-null microglia display a hyper-inflammatory phenotype, Lyz-cKO and WT microglia secreted similar levels of inflammatory cytokines.
We conclude that progranulin expression from either microglia or neurons is sufficient to prevent the development of NCL-like neuropathology in mice. Furthermore, microglia that are deficient for progranulin expression but isolated from a progranulin-rich environment have a normal inflammatory profile. Our results suggest that progranulin acts, at least partly, in a non-cell autonomous manner in the brain.
由于 GRN 基因的杂合性无效突变导致的颗粒蛋白缺乏是家族性额颞叶痴呆（FTLD）的常见原因，而同源性缺失功能 GRN 突变被认为是神经元蜡样质脂褐质沉积症（NCL）的罕见原因。年老的颗粒蛋白敲除（Grn-null）小鼠表现出高度夸张的脂褐质沉积、小胶质细胞增生和星形胶质细胞增生，以及特定脑区的轻度细胞丢失。在大脑中，颗粒蛋白主要在神经元和小胶质细胞中表达，此前我们证明神经元特异性颗粒蛋白耗竭不能重现 Grn-null 小鼠的神经病理学表型。在这项研究中，我们评估了髓系细胞（包括小胶质细胞）中颗粒蛋白表达的选择性耗竭是否会导致小鼠出现类似 NCL 的神经病理学或神经炎症。
我们通过将 Grn 基因的 floxed 等位基因与在 LyzM 启动子控制下表达 Cre 重组酶的小鼠杂交，生成了髓系细胞中颗粒蛋白耗竭的小鼠（Lyz-cKO）。
与 WT 水平相比，分离的小胶质细胞中的颗粒蛋白表达降低了约 50-70%。在四个检查的脑区中，Lyz-cKO 小鼠在 12 个月龄时没有显示脂褐质沉积、小胶质细胞增生或星形胶质细胞增生的任何增加，而在 Grn-null 动物中观察到这些指标的许多增加。为了评估分离的小胶质细胞中颗粒蛋白表达减少的功能影响，我们用受控的标准内毒素刺激原代培养物，并测量细胞因子的释放。虽然 Grn-null 小胶质细胞表现出炎症表型，但 Lyz-cKO 和 WT 小胶质细胞分泌的炎症细胞因子水平相似。
我们得出结论，来自小胶质细胞或神经元的颗粒蛋白表达足以防止 NCL 样神经病理学在小鼠中的发展。此外，从小胶质细胞中缺失颗粒蛋白但分离自富含颗粒蛋白的环境中的小胶质细胞具有正常的炎症特征。我们的结果表明，颗粒蛋白在大脑中至少部分以非细胞自主的方式发挥作用。
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