Glutamate Signaling in Human Islets of Langerhans

描述（由申请人提供）： 关于啮齿动物胰岛的生理学有大量的信息，但人类胰岛的生物学仍然知之甚少。由于越来越多的证据表明人类胰岛在结构和细胞组成方面存在显著差异，因此需要重新评估人类胰岛生物学。我们的长期目标是为人类胰岛生物学的理解做出贡献。该提案解决了人类胰岛功能的一个方面：胰高血糖素释放的调节。尽管胰高血糖素对预防低血糖症并因此对存活率很重要，但胰岛调节胰高血糖素分泌的机制仍存在争议。胰高血糖素分泌通过降低血浆葡萄糖水平来刺激。然而，当分离时，高葡萄糖水平刺激胰高血糖素分泌α细胞。为了调和这些相互矛盾的观察结果，已经提出旁分泌或自分泌信号传导是控制α细胞活性和胰高血糖素释放的主要调节机制。 谷氨酸是大脑中的一种主要神经递质，最近被认为是一种旁分泌信号分子，对啮齿动物胰岛中的胰高血糖素分泌提供负反馈。对啮齿动物胰岛的研究表明，α细胞释放谷氨酸，β细胞和α细胞分别表达离子型谷氨酸受体（iGluRs）和代谢型谷氨酸受体。与这些结果形成鲜明对比的是，我们对人类和猴子胰岛的初步研究表明，谷氨酸只在α细胞中诱导Ca2+反应。谷氨酸强烈刺激胰高血糖素释放，但不刺激胰岛素释放。这些对谷氨酸的反应由AMPA型iGluR介导。当AMPA受体被阻断时，响应于降低的葡萄糖水平的胰高血糖素释放强烈减少。基于这些结果，我们假设谷氨酸信号提供了一个自分泌回路，这是α细胞对低血糖反应所必需的。这种正反馈允许胰高血糖素反应在整个胰岛中传播。三个具体目标将检验这一创新理念： 1)确定灵长类动物胰岛中α细胞释放谷氨酸 2)确定灵长类动物α细胞表达功能性谷氨酸受体 3)确定灵长类动物胰岛中谷氨酸自分泌环路对胰高血糖素反应是必要的 如果得到验证，这项研究将指导临床医生考虑激活AMPA受体的疗法，以防止接受胰岛素治疗的1型糖尿病患者复发性低血糖。

DESCRIPTION (provided by applicant): There is a wealth of information about the physiology of rodent islets, but the biology of human islets of Langerhans remains poorly understood. Because there is growing evidence that human islets are strikingly different in terms of architecture and cellular composition, human islet biology needs to be reevaluated. Our long-term goal is to contribute to the understanding of human islet biology. This proposal addresses one aspect of human islet function: the regulation of glucagon release. Despite its importance to prevent hypoglycemia and therefore for survival, the mechanisms for the regulation of glucagon secretion by pancreatic islets are still controversial. Glucagon secretion is stimulated by decreasing plasma glucose levels. However, when isolated, glucagon-secreting alpha cells are stimulated by high glucose levels. To reconcile these contradictory observations, it has been suggested that paracrine or autocrine signaling is the primary regulatory mechanism governing alpha cell activity and glucagon release. Glutamate is a major neurotransmitter in the brain that has been recently proposed as a paracrine signaling molecule providing negative feedback on glucagon secretion in rodent islets. Studies on rodent islets have shown that alpha cells release glutamate and that beta cells and alpha cells express ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors, respectively. In striking contrast to these results, our preliminary studies on human and monkey islets show that glutamate induces Ca2+ responses exclusively in alpha cells. Glutamate strongly stimulates glucagon release but not insulin release. These responses to glutamate are mediated by iGluRs of the AMPA type. Glucagon release in response to decreasing glucose levels is strongly reduced when the AMPA receptors are blocked. Based on these results, we hypothesize that glutamate signaling provides an autocrine loop that is necessary for alpha cell responses to hypoglycemia. This positive feedback allows for propagation of glucagon responses throughout the islet. Three Specific Aims will test this innovative idea: 1) Determine that glutamate is released by alpha cells in primate islets 2) Determine that primate alpha cells express functional glutamate receptors 3) Determine that glutamate autocrine loops in primate islets are necessary for glucagon responses If validated, this research will direct clinicians to consider therapies that activate AMPA receptors to prevent recurrent hypoglycemia in type 1 diabetic patients treated with insulin.