TDP-43, an ALS/FTD causal gene, regulates SREBF2-mediated cholesterol metabolism in the CNS

Maintaining cholesterol homeostasis is essential for health of all animal cells. Because of blood-brain barrier, de novo cholesetrol biosynthesis and intercellular cholesterol transport is thought to maintain cholesterol homeostasis within the central nervous system (CNS). Here, we showed that TDP-43, the pathological signature protein for amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), regulates SREBF2-mediated cholesterol metabolism in the central nervous system (CNS). Unbiased transcriptomic analysis of mice with oligodendroglial TDP-43 deletion revealed a progressive and pathway-wide disruption in the cholesterol metabolism correlating with reduced myelination and cholesterol level. Molecularly, TDP-43 binds directly to mRNA of SREBF2, the master transcription regulator for cholesterol metabolism, and multiple mRNAs encoding proteins in the cholesterol biosynthesis and uptake, including HMGCR, HMGCS1, and LDLR. Depletion of TDP-43 leads to reduced SREBF2 and cholesterol level in vitro and in vivo. The cholesterol reduction can be rescued by reintroducing either the nuclear portion of SREBF2 or LDLR, the latter of which is the receptor for cholesterol-containing low-density lipoproteins (LDLs). Furthermore, LDLR are observed to co-aggregate with pathological TDP-43 in oligodendrocytes of FTD patients and motor neurons of sporadic ALS patients. Taken together, our data indicates that TDP-43 is required to maintain SREBF2-dependent cholesterol homeostasis in the CNS, and disturbance of cholesterol metabolism may be involved in ALS, FTD and TDP-43 proteinopathies-related disease. Differential gene expression analysis in spinal cord sections of mice with TDP-43 deletion in mature oligodendrocytes at P21 and P60 days维持胆固醇稳态对所有动物细胞的健康至关重要。由于血脑屏障（blood-brain barrier）的存在，学界普遍认为中枢神经系统（central nervous system, CNS）内的胆固醇从头合成与细胞间胆固醇转运共同维持其内部的胆固醇稳态。本研究证实，TDP-43——作为肌萎缩侧索硬化（amyotrophic lateral sclerosis, ALS）与额颞叶痴呆（frontotemporal dementia, FTD）的病理性特征蛋白——可调控中枢神经系统内依赖固醇调节元件结合蛋白2（SREBF2）的胆固醇代谢过程。 对少突胶质细胞TDP-43缺失小鼠开展的无偏转录组分析显示，胆固醇代谢通路出现进行性、全通路紊乱，该紊乱与髓鞘形成减少及胆固醇水平降低显著相关。分子机制层面，TDP-43可直接结合胆固醇代谢核心转录调控因子SREBF2的mRNA，同时结合胆固醇合成与摄取通路中多种编码蛋白的mRNA，包括3-羟基-3-甲基戊二酰辅酶A还原酶（HMGCR）、3-羟基-3-甲基戊二酰辅酶A合酶1（HMGCS1）及低密度脂蛋白受体（low-density lipoprotein receptor, LDLR）。体外（in vitro）与体内（in vivo）实验均证实，TDP-43缺失会降低SREBF2的表达水平与细胞内胆固醇含量。该胆固醇水平降低的表型可通过两种方式挽救：一是重新导入SREBF2的核功能区段，二是重新导入LDLR——后者是携带胆固醇的低密度脂蛋白（low-density lipoproteins, LDLs）的受体。此外，研究人员在额颞叶痴呆患者的少突胶质细胞与散发性肌萎缩侧索硬化患者的运动神经元中，观察到LDLR与病理性TDP-43发生共聚集。 综上，本研究数据表明，TDP-43是维持中枢神经系统内依赖SREBF2的胆固醇稳态所必需的，而胆固醇代谢紊乱可能参与肌萎缩侧索硬化、额颞叶痴呆及TDP-43蛋白病相关疾病的发病过程。本研究还对出生后第21天（P21）与第60天（P60）的成熟少突胶质细胞TDP-43缺失小鼠的脊髓组织切片进行了差异基因表达分析。