Non-genomic mechanisms stabilizing the abundance of SNAT2 a nutrient transceptor protein in response to diverse catabolic signals

细胞生长是对包括氨基酸在内的营养物质的可用性做出反应的，氨基酸是新蛋白质的前体，也是一种代谢燃料。这种基本的生长反应在酵母等原始生物中最为明显，但这种反应的基础系统保留在哺乳动物细胞中，并越来越被认为是控制人体细胞功能的重要机制。不幸的是，这种反应的敏感度随着年龄的增长而减弱，并导致肌肉质量逐渐减少(称为年龄相关性石棺减少症)，从而限制了活动能力，从而限制了整体健康和生活质量。营养反应系统的重要组成部分是营养可获得性的生物传感器，我们正在研究其中之一(称为SNAT2)，以确定是否可以对其进行操纵，以抵消石棺减少的潜在过程。SNAT2是一种存在于大多数类型的人类细胞表面膜上的蛋白质，我们已经发现，它作为外部氨基酸的传感器(或受体)，以及众所周知的作为这些氨基酸进入细胞内以合成蛋白质(从而促进细胞生长)的运输者的角色。这种转运蛋白/受体蛋白的双重生命体使其被命名为SNAT2‘Transceptor’。与其他蛋白质一样，SNAT2经历了一个连续的合成和降解周期，这种周转确保SNAT2保持在适当的水平，并确保个别SNAT2分子定期被替换，作为细胞“家务”的一部分。SNAT2蛋白在细胞中的含量受到高度调控，当细胞受到氨基酸饥饿或类固醇(地塞米松)处理的压力时，SNAT2蛋白的含量会增加。这种上调可能是细胞对应激反应的重要组成部分，应该有助于调节SNAT2受体系统在不同环境下的敏感性。我们最近的研究表明，SNAT2的翻转速度相对较快，至少在骨骼肌细胞中，其丰度在很大程度上是通过降低蛋白质的降解速度来调节的。SNAT2蛋白被指定为通过顺序连接泛素分子链来降解，确定它们是蛋白酶体的目标，蛋白酶体是分解细胞蛋白质的生化机制的主要组成部分。我们有初步证据表明，SNAT2的泛素靶向系统在不同的应激条件下会受到损害，从而通过降低SNAT2蛋白的降解速度来稳定它们。由于新的SNAT2蛋白不断被合成，最终的结果是增加了它们的总体丰度，从而增加了它们为细胞生长提供信号的能力。该项目旨在找出SNAT2蛋白如何在细胞应激期间逃避降解，这在多大程度上有助于将应激影响降至最低(例如，通过保持某些生长途径的活性)，以及所涉及的机制是否可能是为了促进细胞生长(例如，抵消与年龄相关的肌肉损耗)或如果开发出抑制物，以减少细胞生长(例如，在癌症化疗中)。这类靶向治疗可能包括对蛋白质营养的具体修改以及影响SNAT2活性或其降解机制的药物，这两者可能在改善健康和生活质量以及降低整体医疗成本方面对老年人口特别有益。这项研究还可能有额外的副产品应用和好处，这与泛素-蛋白酶体系统作为其他几个治疗感兴趣的基因(例如，治疗囊性纤维化的靶标enac)的调节器相关的持续兴趣有关。

Cells grow in response to the availability of nutrients including amino acids, which are required as precursors of new protein and also used as a metabolic fuel. This fundamental growth response is most obvious in primitive organisms such as yeast, but the systems underlying the response are retained in mammalian cells and are increasingly recognised as a vital mechanism for control of cell function in the human body. Unfortunately, the sensitivity of this response diminishes as we age and contributes to a gradual loss in muscle mass (known as 'age-related sarcopenia'), which limits mobility and hence overall health and quality of life. Important components of the nutrient-response system are biological sensors of nutrient availability, one of which (known as SNAT2) we are studying to see whether it can be manipulated so as to counteract the processes underlying development of sarcopenia. SNAT2 is a protein in the surface membrane of most types of human cell which, we have discovered, acts as a sensor (or receptor) for external amino acids in tandem with a well-known role as a transporter of these amino acids into the cell for protein synthesis (and hence cell growth). The double-life of this transporter / receptor protein has led to it being christened the SNAT2 'transceptor'. In common with other proteins, SNAT2 undergoes a continual cycle of synthesis and degradation, a turnover which ensures that SNAT2 is maintained at appropriate levels and that individual SNAT2 molecules are replaced regularly as part of cellular 'housekeeping'. The amount of SNAT2 protein in cells is highly-regulated and is increased when cells are stressed either by amino acid starvation or steroid (dexamethasone) treatment. This up-regulation may be an important part of the cellular response to stress and should help modulate the sensitivity of the SNAT2 transceptor system under different circumstances. Our recent research has shown that SNAT2 turns over relatively quickly and, at least in skeletal muscle cells, its abundance is regulated largely by reducing the rate at which the protein is degraded. SNAT2 proteins are earmarked for degradation by the sequential attachment of a chain of ubiquitin molecules, identifying them as targets for the proteasome, a major component of the biochemical machinery which breaks down cellular proteins. We have preliminary evidence that this ubiquitin-targeting system for SNAT2 is compromised during various stresses, thus stabilizing SNAT2 proteins by reducing their rate of degradation. Because new SNAT2 proteins are continually being synthesized, the net effect is to increase their overall abundance and hence their capacity to signal for cell growth. This project aims to work out how SNAT2 proteins evade degradation during cellular stresses, how much this helps to minimize stress effects (e.g. by maintaining activity of certain growth pathways) and whether the mechanisms involved may be targeted in order to promote cell growth (e.g. to counteract age-related muscle wasting) or, if an inhibitor is developed, to reduce cell growth (e.g. in cancer chemotherapy). Targeted therapies of this type might include specific modifications to protein nutrition as well as drugs which affect activity of SNAT2 or its degradation mechanism, both of which might be of particular benefit to the elderly population in terms improving health and quality of life, as well as reducing overall healthcare costs. There may also be additional spin-off applications and benefits to this research related to the ongoing interest in the ubiquitin - proteasome system as a regulator of several other genes of therapeutic interest (e.g. ENac, a target for treatment of cystic fibrosis).