Regulation of inositol biosynthesis and consequences of inositol depletion

肌醇是真核细胞生存所必需的。肌醇是所有肌醇化合物的前体， 它们在细胞信号和新陈代谢中起着关键作用。与其重要性一致的是，肌醇的扰动 动态平衡与各种病理有关，如神经和精神疾病，肌病， 癌症和糖尿病。此外，肌醇耗竭是药物作用的一种假想的治疗机制。 用于治疗双相情感障碍，这是一种毁灭性的精神疾病，影响到约2%的人口。从这个角度来看，它 令人惊讶的是，人们对肌醇在人类细胞中的稳态调节知之甚少。我们的研究 寻求确定肌醇合成是如何被调节的，以及肌醇缺乏如何影响必要的 人类细胞的细胞功能。为此，我们鉴定了第一个负转录调控因子 哺乳动物细胞中肌醇的合成--肌醇六磷酸激酶1(IP6K1)。IP6K1抑制表达 在ISYNA1中，编码肌醇合成限速酶的基因，肌醇-3-P合成酶(MIPS)。 进一步，我们证明了IP6K1与磷脂酸(PA)的结合是进行核定位所必需的。 IP6K1和抑制MIPS的表达。我们的第一个项目将严谨地剖析 IP6K1对ISYNA1的调控。为了阐明肌醇缺乏的后果，我们构建了一个 ISYNA1基因敲除(ISYNA1-KO)人细胞系，不能合成肌醇。去肌醇的ISYNA1- KO细胞表现出严重的脂质稳态改变和应激信号相关基因的表达。 具体地说，我们观察到神经酰胺的增加和介导未折叠的基因表达的增加。 蛋白质反应(UPR)，一种由神经酰胺激活的应激反应。我们的第二个项目将测试 假设肌醇剥夺通过上调神经酰胺而导致UPR途径的激活。 有趣的是，肌醇缺乏的细胞表现出单溶心磷脂(MLCL)水平的增加，这是 巴特综合征线粒体紊乱的生化特征。与线粒体功能障碍相一致， 我们观察到，在肌醇饥饿的细胞中，线粒体分裂的启动子ERK的激活增加。第三 我们研究的重点将检验肌醇剥夺扰乱线粒体的工作假说 功能。成功完成拟议的研究将导致第一个分子模型的调节 在哺乳动物细胞中肌醇的合成，并展示肌醇剥夺如何影响细胞应激 反应和线粒体功能。这一认识将对理解肌醇具有重要意义。 动态平衡以及肌醇耗竭的情绪稳定药物的治疗机制。

Inositol is essential for the viability of eukaryotic cells. Myo-inositol is the precursor of all inositol compounds, which play pivotal roles in cell signaling and metabolism. Consistent with its importance, perturbation of inositol homeostasis is associated with pathologies as diverse as neurological and psychiatric illnesses, myopathies, cancer, and diabetes. Furthermore, inositol depletion is a hypothesized therapeutic mechanism of action of drugs used to treat bipolar disorder, a devastating psychiatric illness that affects ~2% of the population. In this light, it is striking that very little is known about the regulation of inositol homeostasis in human cells. Our research seeks to determine how inositol synthesis is regulated and how inositol deprivation affects essential cellular functions in human cells. Toward this end, we identified the first negative transcriptional regulator of inositol synthesis in mammalian cells – inositol hexakisphosphate kinase 1 (IP6K1). IP6K1 represses expression of ISYNA1, the gene coding for the rate-limiting enzyme of inositol synthesis, myo-inositol-3-P synthase (MIPS). Further, we demonstrated that binding of IP6K1 to phosphatidic acid (PA) is required for nuclear localization of IP6K1 and repression of MIPS expression. Our first project will rigorously dissect the mechanism of regulation of ISYNA1 by IP6K1. To elucidate the consequences of inositol deprivation, we constructed an ISYNA1 knock-out (ISYNA1-KO) human cell line, which cannot synthesize inositol. Inositol-deprived ISYNA1- KO cells exhibit profound alteration of lipid homeostasis and expression of genes involved in stress signaling. Specifically, we observed an increase in ceramides and increased expression of genes that mediate the unfolded protein response (UPR), a stress response that is activated by ceramides. Our second project will test the hypothesis that inositol deprivation results in activation of the UPR pathway by upregulating ceramides. Intriguingly, inositol-deprived cells exhibited increased levels of monolysocardiolipin (MLCL), which is the biochemical hallmark of the mitochondrial disorder Barth syndrome. Consistent with mitochondrial dysfunction, we observed increased activation of ERK, a promoter of mitochondrial fission, in inositol-starved cells. The third focus of our studies will test the working hypothesis that inositol deprivation perturbs mitochondrial function. Successful completion of the proposed studies will lead to the first molecular model of regulation of inositol synthesis in mammalian cells and demonstrate how inositol deprivation affects the cellular stress response and mitochondrial function. This knowledge will have important implications for understanding inositol homeostasis as well as the therapeutic mechanisms of inositol-depleting mood stabilizing drugs.