Cholesterol signaling in single cells: lessons from STAR and sm-FISH.

Cholesterol is an important regulator of cell signaling, both through direct impacts on cell membranes and through oxy-metabolites that activate specific receptors (steroids,hydroxy-cholesterols, bile acids). Cholesterol moves slowly through and between cell membranes with the assistance of specific binding proteins and transfer processes. The prototype cholesterol regulator is the Steroidogenesis Acute Regulatory (STAR), which moves cholesterol into mitochondria, where steroid synthesis is initiated by cytochrome P450 11A1 in multiple endocrine cell types. CYP27A1 generates hydroxyl cholesterol metabolites that activate LXR nuclear receptors to control cholesterol homeostatic and transport mechanisms. LXR regulation of cholesterol transport and storage as cholesterol ester droplets is shared by both steroid-producing cells and macrophage. This cholesterol signaling which is crucial to brain neuron regulation by astrocytes and microglial macrophage, is mediated by ApoE and is sensitive to disruption by β-amyloid plaques. sm-FΙSH delivers appreciable insights into signaling in single cells, by resolving single RNA molecules as mRNA and by quantifying pre-mRNA at gene loci. sm-FΙSH has been applied to problems in physiology, embryo development and cancer biology, where single cell features have critical impacts. sm-FΙSH identifies novel features of STAR transcription in adrenal and testis cells, including asymmetric expression at individual gene loci, delayed splicing and 1:1 association of mRNA with mitochondria. This may represent a functional unit for the translation-dependent cholesterol transfer directed by STAR, which integrates into mitochondrial fusion dynamics. Similar cholesterol dynamics repeat with different players in the cycling of cholesterol between astrocytes and neurons in the brain, which may be abnormal in neurodegenerative diseases.

胆固醇是细胞信号传导的重要调节剂，既通过对细胞膜的直接影响，也通过激活特定受体（类固醇、羟基胆固醇、胆汁酸）的氧化代谢产物来发挥作用。在特定结合蛋白和转运过程的协助下，胆固醇在细胞膜内和细胞膜之间缓慢移动。类固醇生成急性调节蛋白（STAR）是胆固醇调节的原型，它将胆固醇转运到线粒体中，在多种内分泌细胞类型中，细胞色素P450 11A1在线粒体中启动类固醇合成。CYP27A1产生羟基胆固醇代谢产物，激活LXR核受体以控制胆固醇的稳态和转运机制。产生类固醇的细胞和巨噬细胞都存在LXR对胆固醇以胆固醇酯滴形式转运和储存的调节。这种对星形胶质细胞和小胶质细胞巨噬细胞调节脑神经元至关重要的胆固醇信号传导由载脂蛋白E介导，并且对β - 淀粉样斑块的破坏敏感。单分子荧光原位杂交（sm - FISH）通过将单个RNA分子解析为mRNA并对基因位点的前体mRNA进行定量，为单细胞信号传导提供了重要见解。sm - FISH已应用于生理学、胚胎发育和癌症生物学中的问题，其中单细胞特征具有关键影响。sm - FISH识别出肾上腺和睾丸细胞中STAR转录的新特征，包括在单个基因位点的不对称表达、延迟剪接以及mRNA与线粒体1:1的关联。这可能代表了由STAR引导的依赖翻译的胆固醇转运的一个功能单位，它整合到线粒体融合动力学中。在大脑中星形胶质细胞和神经元之间的胆固醇循环中，类似的胆固醇动力学随着不同的参与者重复出现，在神经退行性疾病中可能出现异常。