Molecular Mechanisms of Postnatal Islet alpha-cell Proliferation

高胰高血糖素血症有助于2型糖尿病（T2D）的高血糖。因此，拮抗胰高血糖素的作用有很大的希望作为治疗干预T2D。然而，多种途径阻断胰高血糖素信号通路（IGS）可导致α-细胞增殖和增生。使用多学科方法，我们最近发现血液氨基酸（高胺酸血症或AAHi）的积累，特别是谷氨酰胺和精氨酸，由于肝脏中氨基酸分解代谢减少而驱动α-细胞增殖。这些研究还揭示了一个以前未被认识和保存的（鱼到人）肝-α细胞轴，其中肝胰高血糖素信号调节血清氨基酸水平和增加的AA，特别是谷氨酰胺(Q)，调节胰高血糖素分泌和α-细胞增殖和质量。AAHi以mtorc1依赖的方式引起α-细胞增殖是必要和充分的。然而，仅仅激活mTORC1是不够的。我们假设高胺酸血症激活哺乳动物雷帕霉素靶蛋白1 （mTORC1）和钙敏感受体（CaSR），导致α-细胞特异性增殖，这是由于高表达一组独特的AA转运体和催化酶。在这个多pi提案的合作努力中，我们的团队将采用一种实验策略，利用斑马鱼和小鼠模型的优势，快速识别途径和定义机制，同时并行测试我们的发现在初级人类胰岛的应用和转化。此外，我们将利用一种新的胰岛α-细胞增殖体外实验来补充在鱼、小鼠和移植的人胰岛上的体内研究。我们将在鱼、小鼠和人类胰岛上验证以下假设：1)α细胞中特异性质膜AA转运体的高表达通过允许有效的AA摄取来驱动mTORC1激活；2)谷氨酰胺酶是高胺酸血症通过谷氨酰胺水解激活mTORC1所需的酶；3)高胺酸血症也激活CaSR，与mTORC1协同诱导α-细胞增殖。此外，由于高氨基酸酸血症刺激α-细胞增殖和胰高血糖素分泌，这些研究也应该为氨基酸如何调节胰高血糖素分泌提供新的信息。这些研究将扩大我们对控制α-细胞生物学、功能、增殖和质量的分子机制的理解，并为如何减轻T2D中α-细胞功能障碍提供见解。

Hyperglucagonemia contributes to the hyperglycemia of type 2 diabetes (T2D). As such, antagonism of glucagon action has great promise as a therapeutic intervention for T2D. However, interrupted glucagon signaling (IGS) by multiple approaches leads to α-cell proliferation and hyperplasia. Using a multidisciplinary approach, we recently discovered that the accumulation of blood amino acids (hyperaminoacidemia or AAHi), particularly glutamine and arginine, due to decreased amino acid catabolism in the liver drives α-cell proliferation. These studies also revealed a previously unappreciated and conserved (fish to man) hepatic-α cell axis where hepatic glucagon signaling regulates serum amino acid levels and increased AA, especially glutamine (Q), regulate glucagon secretion and α-cell proliferation and mass. AAHi is necessary and sufficient to cause α-cell proliferation in an mTORC1-dependent manner. However, mTORC1 activation alone is insufficient. We hypothesize that hyperaminoacidemia activates both mammalian target of rapamycin complex 1 (mTORC1) and Calcium Sensing Receptor (CaSR) to cause α-cell specific proliferation due to its high expression of a unique set of AA transporters and catalytic enzymes. In collaborative efforts in this multi-PI proposal, our groups will pursue an experimental strategy that leverages the advantages of zebrafish and mouse models for rapidly identifying pathways and defining mechanisms while in parallel testing the application and translation of our findings into primary human islets. Plus, we will utilize a new in vitro assay for islet α-cell proliferation to complement in vivo studies in fish, mouse, and transplanted human islets. We will test the hypotheses in fish, mouse, and human islets that 1) high expression of specific plasma membrane AA transporters in α cells drives mTORC1 activation by allowing efficient AA uptake; 2) glutaminase is required for hyperaminoacidemia activation of mTORC1 through glutaminolysis; and 3) hyperaminoacidemia also activates CaSR, which synergizes with mTORC1 to induce α-cell proliferation. Furthermore, since hyperaminoacidemia stimulates both α-cell proliferation and glucagon secretion, these studies should also provide new information about how amino acids regulate glucagon secretion. These studies will expand our understanding of the molecular mechanisms controlling α-cell biology, function, proliferation and mass and provide insight into how the α-cell dysfunction in T2D could be mitigated.