Recent studies have demonstrated the existence of a discrete pool of cholesterol in the plasma membranes (PM) of mammalian cells—referred to as the accessible cholesterol pool—that can be detected by the binding of modified versions of bacterial cytolysins (e.g., anthrolysin O). When the amount of accessible cholesterol in the PM exceeds a threshold level, the excess cholesterol moves to the endoplasmic reticulum (ER), where it regulates the SREBP2 pathway and undergoes esterification. We reported previously that the Aster/Gramd1 family of sterol transporters mediates nonvesicular movement of cholesterol from the PM to the ER in multiple mammalian cell types. Here, we investigated the PM pool of accessible cholesterol in cholesterol-loaded fibroblasts with a knockdown of Aster-A and in mouse macrophages from Aster-B and Aster-A/B-deficient mice. Nanoscale secondary ion mass spectrometry (NanoSIMS) analyses revealed expansion of the accessible cholesterol pool in cells lacking Aster expression. The increased accessible cholesterol pool in the PM was accompanied by reduced cholesterol movement to the ER, evidenced by increased expression of SREBP2-regulated genes. Cosedimentation experiments with liposomes revealed that the Aster-B GRAM domain binds to membranes in a cholesterol concentration-dependent manner and that the binding is facilitated by the presence of phosphatidylserine. These studies revealed that the Aster-mediated nonvesicular cholesterol transport pathway controls levels of accessible cholesterol in the PM, as well as the activity of the SREBP pathway.

中文翻译：

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紫苑蛋白调节血浆膜中可及的胆固醇库。

最近的研究表明，在哺乳动物细胞的质膜（PM）中存在离散的胆固醇池（称为可及的胆固醇池），可以通过修饰形式的细菌溶细胞素（例如，溶血素O）的结合来检测胆固醇。当PM中可及的胆固醇含量超过阈值水平时，多余的胆固醇将转移至内质网（ER），在此处调节SREBP2途径并进行酯化。我们以前曾报道过，固醇转运蛋白的Aster / Gramd1家族介导了多种哺乳动物细胞类型中胆固醇从PM到ER的非囊泡运动。在这里，我们调查了载有Aster-A的胆固醇装载的成纤维细胞以及Aster-B和Aster-A / B缺陷小鼠的小鼠巨噬细胞中可及胆固醇的PM池。纳米级二次离子质谱（NanoSIMS）分析显示，缺乏Aster表达的细胞中可及胆固醇池的扩展。PM中可及的胆固醇库增加，同时胆固醇向ER的移动减少，这由SREBP2调控基因的表达增加所证明。用脂质体进行的沉淀试验表明，Aster-B GRAM结构域以胆固醇浓度依赖性的方式与膜结合，并且磷脂酰丝氨酸的存在促进了结合。这些研究表明，Aster介导的非囊泡胆固醇转运途径控制着PM中可及胆固醇的水平以及SREBP途径的活性。