Regulation of Glucagon Secretion from Pancreatic Islets

朗格汉斯岛通过调节激素分泌在葡萄糖稳态中发挥关键作用。胰岛研究主要集中在分泌胰岛素的 β 细胞上，尽管 α 细胞异常分泌另一种胰岛激素（胰高血糖素）会加剧糖尿病的病理。通过胰高血糖素受体拮抗作用使胰高血糖素作用正常化可以帮助糖尿病患者维持血糖正常并避免低血糖发作，但这种方法的进展因众多副作用而减慢。另一种方法是减少胰高血糖素分泌，但控制其胞吐作用的机制仍然存在争议。我们实验室和其他实验室的最新数据表明，长期以来对 α 细胞 Ca2 信号传导的关注可能具有误导性，胰高血糖素分泌的调节需要通过多种途径进行信号传导。这种复杂性推动了创新的研究策略，我们整合了针对各个组件的机械实验的数据，同时还测试了通路之间可能存在的串扰。 I 型糖尿病中 β 细胞被破坏后，体内胰高血糖素分泌的葡萄糖调节 (GRGS) 丧失，这表明胰岛细胞类型之间的相互作用对于 α 细胞功能至关重要。这导致了以下模型 旁分泌信号传导，其中胰岛 β 和 δ 细胞分泌的因子限制 α 细胞功能。我们已经证明，胰岛素和生长抑素协同作用可降低 cAMP 和 PKA 活性，从而降低胰高血糖素分泌，但其本身并不能解释 GRGS。初步数据表明复合蛋白 2 在 PKA 活性与分泌的联系中发挥着关键作用。我们还表明，一种新的近分泌途径，即 EphA4/7 正向信号传导，是由 β 细胞表面的 ephrinA5 配体激活的。这种效应导致 F-肌动蛋白聚合并减少胰高血糖素分泌（推测是通过 RhoA 激活），并且它似乎具有葡萄糖调节成分。因此，旁分泌和近分泌途径均驱动胰岛的 GRGS，但用肝配蛋白 A5 处理的分散的 α 细胞还表现出额外的 GRGS 机制，这似乎是 α 细胞固有的。基于这些数据，我们假设 GRGS 需要旁分泌、近分泌和细胞内在信号通路的协同组合。这一假设将通过三个具体目标进行检验：1）确定旁分泌介导的 PKA 激活的复合蛋白 2 磷酸化在胰岛素和生长抑素介导的胰高血糖素分泌抑制中的作用； 2)确定RhoA激活在近分泌EphA4/7前向中的作用 导致胰高血糖素分泌抑制的信号传导； 3) 确定 EphA4/7 正向信号在 α 细胞对葡萄糖的内在反应中的作用。我们正在揭示的多种细胞内和细胞间信号传导机制将通过允许精确观察活细胞和胰岛相关动态的方法来阐明。该研究计划还将利用我们的发现，即小鼠和人类 α 细胞中 GRGS 的机制相似，并且我们将尽可能进行跨物种的平行实验。这些实验将进一步加深我们对α细胞功能的理解，这是发现胰高血糖素调节和糖尿病治疗新潜在靶标的关键一步。

The islet of Langerhans plays a key role in glucose homeostasis through regulated hormone secretion. Islet research has focused on the insulin-secreting β-cells, even though aberrant secretion of another islet hormone, glucagon from α-cells, exacerbates the pathology of diabetes. Normalization of glucagon action by glucagon receptor antagonism can help diabetic patients maintain euglycemia and avoid hypoglycemic episodes, but progress on this approach has been slowed by numerous side-effects. An alternate approach would be to reduce glucagon secretion, but the mechanism controlling its exocytosis remains controversial. Recent data from our lab and others suggest that the long-standing focus on α-cell Ca2+ signaling may have been misleading, and that regulation of glucagon secretion requires signaling through multiple pathways. This complexity drives an innovative research strategy where we integrate data from mechanistic experiments focused on individual components, while also testing for possible cross-talk between pathways. The loss of glucose-regulation of glucagon secretion (GRGS) in vivo after β-cells are destroyed in Type I diabetes suggests that interactions between islet cell types are critical to α-cell function. This has led to models of paracrine signaling where secreted factors from islet β- and δ-cells constrain α-cell function. We have shown that insulin and somatostatin work in concert to reduce cAMP and PKA activity, which lowers glucagon secretion, but does not by itself explain GRGS. Preliminary data point to a key role for complexin 2 in linking PKA activity to secretion. We have also shown that a novel juxtacrine pathway, EphA4/7 forward signaling, is activated by ephrinA5 ligands on the β-cell surface. This effect leads to F-actin polymerization and decreased glucagon secretion, putatively via RhoA activation, and it appears to have a glucose-regulated component. Thus, both paracrine and juxtracrine pathways drive GRGS from islets, but dispersed α-cells treated with ephrinA5 also exhibit an additional GRGS mechanism, which appears to be intrinsic to the α-cell. Based on these data, we hypothesize that GRGS requires a synergistic combination of paracrine, juxtacrine, and cell-intrinsic signaling pathways. This hypothesis will be tested via three specific aims: 1) Determine the role of paracrine-mediated PKA-activated phosphorylation of complexin 2 in insulin- and somatostatin-mediated inhibition of glucagon secretion; 2) Determine the role of RhoA activation in the juxtacrine EphA4/7 forward signaling that leads to inhibition of glucagon secretion; 3) Determine the role of EphA4/7 forward signaling in intrinsic α-cell response to glucose. The multiple intracellular and intercellular signaling mechanisms that we are uncovering will be elucidated by methods that allow precise observation of the pertinent dynamics in living cells and islets. The research plan will also leverage our findings that the mechanisms of GRGS are similar in mouse and human α-cells, and we will perform parallel experiments across species to the extent possible. These experiments will further our understanding of α-cell function, which is a critical step towards discovering new potential targets for the regulation of glucagon and treatment of diabetes.