The role of EphA-ephrin-A signaling in glucose-inhibition of glucagon secretion

描述(申请人提供)：朗格汉斯胰岛通过葡萄糖调节剂量依赖的激素分泌，在血糖稳态中发挥核心作用。胰岛素治疗的成功使大多数糖尿病研究都集中在细胞上。然而，最近研究表明，胰升糖素(由β-细胞分泌)在糖尿病的病理生理过程中起着重要作用。在1型和2型糖尿病中，胰高血糖素的分泌都不适当地升高和失调，进一步加剧了高血糖。因此，了解β-细胞功能和胰升糖素分泌的机制是开发新的糖尿病治疗策略的重要途径。与目前的治疗方法相结合，基于新的细胞靶点的疗法将通过解决引起高血糖的多种原因而使1型和2型糖尿病患者受益。尽管胰高血糖素具有重要的生理作用，但对其分泌的调节机制仍然知之甚少。活塞实验室最近公布的数据表明，GIG不是β-细胞固有的，而且对胰升糖素分泌的抑制不依赖于钙离子的内流。我们的数据显示，与糖尿病患者相似，分选细胞的纯群体在低糖时显示高血糖素分泌增加，而失去了葡萄糖抑制高血糖素分泌(GIC)。此外，细胞内[Ca~(2+)]升高是对抑制胰升糖素分泌的葡萄糖增加的反应。胰岛内环境对适当的胰升糖素分泌的必要性不能仅仅通过旁分泌信号来解释，因为分离的细胞对拟议的旁分泌抑制剂(胰岛素和生长抑素)不再敏感。 在这里，我们提出了一种新的GIG旁分泌信号依赖模型，该模型调节钙离子内流下游的胰高血糖素分泌。通过EphA-ePhin-A信号通路的辅助性他克林信号已被证明在胰岛细胞的基础激素和葡萄糖依赖激素的分泌中发挥重要作用。由于？-细胞与？-细胞密切相关，我们推测，类似的机制在基础胰高血糖素分泌和糖基化终末糖基化终产物中起作用。有趣的是，人类和小鼠细胞都只表达一种Eph受体，EphA4。在初步研究中，通过药物抑制EphA4受体信号，在基础葡萄糖时，胰高血糖素的分泌增加，并且对葡萄糖的反应不适当地刺激了分泌。这两个发现反映了在分选细胞和糖尿病患者中观察到的胰高血糖素分泌模式，表明EphA4受体信号在调节基础和高血糖时胰高血糖素分泌方面的重要性。我们假设EphA4受体信号在β-细胞中是需要的，以适当地抑制基础葡萄糖和糖基化终产物中的胰升糖素的分泌。此外，我们假设EphA4受体信号允许额外的旁分泌抑制信号(胰岛素和/或生长抑素)。

DESCRIPTION (provided by applicant): The islets of Langerhans play a central role in blood glucose homeostasis through glucose regulated dose-dependent hormone secretion. The therapeutic success of insulin has led most diabetes research to be focused on ?-cells. Recently however, glucagon (secreted by ?-cells) has been shown to significantly contribute to the pathophysiology of diabetes. In both type 1 and type 2 diabetes, glucagon secretion is inappropriately elevated and dysregulated, further exacerbating hyperglycemia. Thus, understanding the mechanisms underlying ?-cell function and glucagon secretion represents an important avenue in the development of new therapeutic strategies for diabetes management. In combination with current treatments, therapeutics based on novel ? -cell targets will benefit patients with type 1 and type 2 diabetes by addressing multiple causes of hyperglycemia. Despite the important physiologic role of glucagon, the mechanisms regulating its secretion remain poorly understood. The Piston lab has recently published data suggesting that GIGS is not intrinsic to ? -cells, and that the inhibition of glucagon secretion is independent of Ca2+ influx. Our data show that similar to diabetic patients, pure populations of sorted ? -cells displa an increase in glucagon secretion at low glucose and a loss of glucose-inhibition of glucagon secretion (GIGS). Additionally, intracellular [Ca2+] is elevated in ¿-cells in response to increase in glucose that inhibit glucagon secretion. The necessity of the intraislet environment for appropriate glucagon secretion cannot be explained solely through paracrine signaling, as sorted ¿-cells are no longer sensitive to proposed paracrine inhibitors (insulin and somatostatin). Here, we propose a new juxtacrine signaling dependent model of GIGS that regulates glucagon secretion downstream of Ca2+ influx. Juxtacrine signaling through the EphA-ephrin-A signaling pathways has been shown to play an important role in both the basal and glucose dependent hormone secretion from islet ?-cells. Since the ?-cell is closely related to the ?-cell, we speculate that a similar mechanism plays a role in basal glucagon secretion and GIGS. Interestingly, both human and mouse ?-cells only express a single Eph receptor, EphA4. In preliminary studies pharmacologically inhibiting EphA4 receptor signaling, glucagon secretion was elevated at basal glucose and secretion was inappropriately stimulated response to glucose. These two findings mirror the glucagon secretion patterns observed in both sorted ?-cells and diabetic patients, indicating the importance of EphA4 receptor signaling in mediating glucagon secretion at basal and elevated glucose. We hypothesize that EphA4 receptor signaling in ?-cells is required for the appropriate suppression of glucagon secretion at basal glucose and for GIGS. Additionally, we hypothesize EphA4 receptor signaling is permissive for additional paracrine inhibitory signals (insulin and/or somatostatin).