Biophysical Mechanisms of Cholesterol Homeostasis

摘要 了解细胞用于维持胆固醇稳态的机制在细胞生物学中至关重要， 疾病为了达到这个目的，必须测量细胞质膜中胆固醇的化学活性 因为活性控制胆固醇对细胞过程的影响。迄今为止，质膜胆固醇 浓度已被用于量化胆固醇活性。但胆固醇的活性是由其 化学势;浓度有助于但不能准确反映膜活性。因为 测量胆固醇化学势的方法还没有，不可能正确地 评估胆固醇的许多影响，包括对细胞信号的影响。我们现在已经开发出了 这样做.这些方法和一个新的灌注荧光仪器，我们已经设计，使我们能够遵循 真实的细胞膜胆固醇的化学势。我们发现细胞很快 通过调节其血浆胆固醇化学势来响应细胞外胆固醇的变化 细胞膜中胆固醇的含量没有变化。这一发现揭示了一个以前未知的 维持胆固醇稳态的机制：快速调节质膜化学势 来控制胆固醇的流入和流出我们已经确定了蛋白质支架结构域，以小窝为代表， 作为细胞感知并快速响应外部胆固醇的部位。的丰度和总量 存在于小窝中的胆固醇的水平取决于 小窝蛋白-1，该结构域的基础蛋白。胆固醇在支架结构域之间的穿梭 而Ser 80磷酸化后必然产生的周围环境改变胆固醇的化学势。我们 因此，假设在支架结构域内启动的信号级联负责 当细胞受到外部胆固醇变化和生长时，维持胆固醇稳态 因素我们进一步证实，这些激活的信号级联反应反馈到质膜，以维持细胞内的蛋白质水平。 化学势细胞将被生长因子刺激，相关的信号级联将被激活。 鉴定小窝的丰度将通过测量FRET（荧光共振能量）来评估 传递）信号。我们的初步证据强烈暗示，生长因子和/或 外部胆固醇水平的变化刺激PI 3 K/Akt/mTOR信号通路，该信号通路反馈到 达到胆固醇平衡。将使用光遗传学技术来确定它和/或其他生物是否 实际上负责控制胆固醇。使用相同策略的平行实验将确定 类似于小窝蛋白的flotillins也用作胆固醇化学势的传感器/调节器。

Abstract Understanding mechanisms cells use to maintain cholesterol homeostasis are critical in cell biology and many diseases. To achieve this, the chemical activity of cholesterol in cell plasma membranes must be measured because activity controls cholesterol’s effects on cellular processes. To date, plasma membrane cholesterol concentration has been used to quantify cholesterol activity. But the activity of cholesterol is determined by its chemical potential; concentration contributes to, but does not accurately reflect membrane activity. Because a method to measure cholesterol chemical potential had not been available, it was not possible to properly evaluate many of cholesterol’s effects, including those on cellular signaling. We have now developed methods to do so. These methods and a new perfusion fluorimetry apparatus we have devised allow us to follow the chemical potential of cholesterol of plasma membranes in real time. We have discovered that cells quickly respond to changes in extracellular cholesterol by adjusting the cholesterol chemical potential of their plasma membranes without changing the total content of cellular cholesterol. This finding reveals a previously unknown mechanism to maintain cholesterol homeostasis: quick adjustment of plasma membrane chemical potentials to control cholesterol influx and efflux. We have identified protein scaffolded domains, as typified by caveolae, as sites at which cells sense and rapidly respond to external cholesterol. The abundance and total amount of cholesterol that resides in caveolae are determined by the extent of phosphorylation at position Ser80 of caveolin-1, the foundational protein of the domain. The shuttling of cholesterol between scaffolded domains and the surround which must result upon Ser80 phosphorylation alters cholesterol chemical potential. We therefore hypothesize that signaling cascades initiated within scaffolded domains are responsible for maintaining cholesterol homeostasis when cells are subjected to changes in external cholesterol and to growth factors. We further posit that these activated signaling cascades feed back to the plasma membrane to maintain chemical potentials. Cells will be stimulated with growth factors and relevant signaling cascades will be identified. The abundance of caveolae will be assessed by measuring the FRET (fluorescence resonance energy transfer) signals between caveolins. Our preliminary evidence strongly implicates that growth factors and/or changes in the level of external cholesterol stimulate the PI3K/Akt/mTOR signaling pathway that feeds back to achieve cholesterol homeostasis. Optogenetic techniques will be used to determine whether it and/or others are indeed responsible for control of cholesterol. Parallel experiments using the same strategies will determine if flotillins, analogous to caveolin, also serve as sensors/regulators of cholesterol chemical potentials.