胰高血糖素诱导α-细胞补偿性增生的分子机制

Blood amino acid levels are maintained at a narrow range in vertebrates and do not fluctuate much after feeding or during fasting. The mechanism is not very clear, particularly about the prevention of hyperaminoacidemia. We have recently discovered a conserved liver-α-cell feedback loop mediated by blood amino acids and glucagon. As such, chronic impairment of glucagon action causes blood amino acids accumulation, which specifically induces α-cell proliferation and hyperglucagonemia. This discovery reveals α-cells as the sensor for amino acids. This application focuses on elucidating how hyperaminoacidemia induces α-cell proliferation. Our published work demonstrated that high amino acids are both necessary and sufficient to cause mTORC1 dependent α-cell proliferation ex vivo. Using a glucagon receptor (gcgr) deficient zebrafish model that exhibits mTORC1-dependent α-cell hyperplasia starting at 4-day of age, we now demonstrate that mTORC1 activation is insufficient to cause α-cell proliferation, and that the AA sensitive Calcium sensing receptor CaSR is necessary for α-cell proliferation. We also found that the α-cell hyperplasia is sensitive to several EGFR antagonists. These results support a hypothesis that hyperaminoacidemia from gcgr-deficiency activates mTORC1 and CaSR. The latter transactivates EGFR. Together, these activities cooperate to promote α-cell hyperplasia. We will combine genetic manipulation in zebrafish and pharmacological manipulation in mouse islets ex vivo to test our hypothesis. Specifically, we will determine whether 1) CaSR acts in α-cells for α-cell proliferation; 2) EGFR activation is necessary for α-cell hyperplasia; 3) the mediators (ADAM10/17) of EGFR transactivation are necessary for α-cell hyperplasia; and 4) co-activation of CaSR, EGFR and mTORC1 is sufficient to cause α-cell hyperplasia in the absence of hyperaminoacidemia. Since relative or absolute hyperglucagonemia underlies much of the metabolic symptoms of diabetes. Our studies will provide insights into regulation of α-cell function. Since this feedback loop hinders a promising diabetes treatment by suppression of glucagon function, understanding of the molecular mechanisms of hyperaminoacidemia-induced α-cell hyperplasia may illuminate avenues to mitigate the undesired consequence of glucagon suppression therapy for diabetes.

控制血氨基酸稳态的机制不甚了解。我们最近发现胰岛α细胞对血氨基酸稳态起关键作用。当胰高血糖素的功能长期受阻时，因肝脏代谢降低而存积累的氨基酸反馈刺激α细胞的分泌和增值，导致高胰高血糖素血症。我们证明高氨基酸足以引起依赖于mTORC1 的α细胞增殖。我们用gcgr缺失斑马鱼发现单纯mTORC1激活本身不足以使α细胞增生，并鉴定氨基酸受体CaSR和EGFR可能也参与α细胞增殖。我们设想，gcgr缺失所致的高氨基酸血症活化mTORC1和CaSR，后者转活化EGFR，三者共同促进α细胞增殖。本研究拟通过基因和药物调控手段，确定 CaSR是否在α细胞里通过ADAM10/17转活化EGFR从而引起α细胞增殖。我们将验证CaSR，EGFR，mTORC1三者对于α细胞增殖是充分的。因为 大多数糖尿病患者的α细胞功能紊乱，深入了解α细胞数目和功能的调节这一基础问题有望给糖尿病治疗提示新途径。

胰高血糖素已成为细胞外氨基酸 (AA) 稳态的关键调节剂。胰高血糖素信号通路的缺陷会导致机体高氨基酸血症，从而负反馈驱动胰岛α 细胞的增殖以产生更多胰高血糖素。既往报道中除了雷帕霉素复合物 1 (mTORC1) 以外，尚无研究关于其他的 AA 传感器在哺乳动物 α 细胞增殖中的作用。本课题中，我们使用胰高血糖素受体 (gcgr) 缺陷斑马鱼 模型(Danio rerio)，刻画了α 细胞增殖的分子机制，发现了HAA 需要通过钙感应受体（CaSR）激活细胞外信号调节蛋白激酶 (ERK1/2)进而导致胰岛α 细胞的增殖。 CaSR的表达性缺失或者化学性抑制减少了 α 细胞增殖，而通过重新表达 CaSR 或者是激活其下游Gq而非Gi通路，恢复了HAA引起的 α 细胞增殖。这些结果揭示了另一种 AA 敏感介质CaSR，并确定了高氨基酸血症诱导的 α 细胞增殖的两个必要且充分的信号传导途径。这一研究成果有利于进一步了解肝-α细胞循环轴，并且为治疗α细胞过度增殖和内分泌肿瘤提供了潜在靶点，可能成为新的糖尿病治疗手段。