Amino Acids-Rab1A Nutrient Signaling in the Regulation of Glucose Homeostasis

氨基酸(AAs)是生命的基本组成部分和燃料。它们也越来越多地被认为是调节生长和代谢过程的化学信号。越来越多的证据清楚地表明，AA感知和信号在健康和疾病中发挥着关键作用。MTOR是AA传感和信令的主调节器。最近的进展使人们对AA如何激活mTOR有了一些基本的了解。RAG是一种锚定在溶酶体上的异源二聚体小GTP酶，可在AA刺激下激活mTOR。令人惊讶的是，我们和其他人发现AAs仍然可以在Rag基因敲除酵母和小鼠中激活mTOR，这表明存在另一种AA信号机制。在最近发表的一项研究中，我们进行了酵母遗传筛选，并确定了Rab1A，一个位于ER/Golgi上的小GTP酶，代表了酵母和哺乳动物中mTOR上游AA信号的一种新的、保守的介体(Thomas等人，癌细胞26：754)。到目前为止，AA-Rab1A信号的生理功能尚不清楚。因此，我们在幼年成年小鼠中产生了他莫昔芬诱导的全身Rab1a基因敲除。Rab1A型基因敲除动物除表现出高血糖和葡萄糖耐量异常外，其余均为正常。进一步的分析表明，突变动物的β细胞团明显较小，并且它们的β细胞经历了向α细胞的转分化。此外，在Rab1a基因敲除的胰腺β-细胞中，胰岛素基因和蛋白的表达均显著降低。我们进一步证明，AA-Rab1a信号调节β细胞系中的胰岛素转录。基于这些初步结果，我们建议验证β细胞自主的AA-Rab1a信号通过调节哺乳动物的胰岛素转录、β细胞的增殖和维持来控制血糖稳态的假说。我们将使用细胞和基因工程小鼠模型来剖析正常生理和病理条件下的基本机制和意义。如果成功，该项目将增强有关营养物质如何控制全身葡萄糖动态平衡的基础知识。它还可能为糖尿病、癌症和其他相关疾病的营养和/或治疗干预带来新的机会。

Amino acids (AAs) are basic building blocks and fuels of life. They are also increasingly appreciated as chemical signals that regulate growth and metabolic processes. Accumulating evidence clearly show that AA sensing and signaling play a critical role in health and diseases. mTOR is a master regulator of AA sensing and signaling. Recent progress has led to some basic understanding of how AA activates mTOR. Rag, a heterodimeric small GTPase anchored on the lysosomes, was shown to activate mTOR in response to AA stimulation. Surprisingly, we and others found that AAs can still activate mTOR in Rag-knockout yeast and mice, indicating that an alternative AA signaling mechanism exists. In a recently published study, we carried out a yeast genetic screening and identified Rab1A, a small GTPase localized on the ER/Golgi, represents a novel, conserved mediator of AA signaling upstream of mTOR in yeast and mammals (Thomas et al., Cancer Cell 26:754). To date, the physiological functions of AA-Rab1A signaling are not known. Therefore, we generated tamoxifen- induced whole body Rab1A knockout in young adult mice. Rab1A knockout animals were relatively normal except exhibiting hyperglycemia and glucose intolerance. Further analysis showed that the mutant animals had significantly smaller β-cell mass and their β-cells underwent trans-differentiation to α-cells. Moreover, both insulin mRNA and protein expression were markedly reduced in Rab1A knockout pancreatic β-cells. Consistently, we further showed that AA-Rab1A signaling regulated insulin transcription in β-cell lines. Based on these preliminary results, we propose to test the hypothesis that β-cell-autonomous AA-Rab1A signaling controls glucose homeostasis by regulating insulin transcription, β-cell proliferation and maintenance in mammals. We will dissect the basic mechanisms and significance under normal physiological and pathological conditions using cell and genetically engineered mouse models. If successful, this project will enhance the basic knowledge on how nutrients control whole body’s glucose homeostasis. It could also lead to new opportunities for nutritional and/or therapeutic intervention for diabetes, cancer and other related diseases.