Pancreatic Islets Dynamics Regulating Glucagon Secretion

描述（由申请人提供）：胰岛通过调节激素分泌在血糖稳态中发挥重要作用。胰岛素在治疗上的成功使得大多数胰岛研究集中在<$-细胞上，尽管其他胰岛激素包括胰高血糖素（由<$-细胞分泌）和生长抑素（由<$-细胞分泌）具有重要的生理作用。胰高血糖素在糖尿病的病理学中起着关键作用，但调节胰高血糖素分泌的机制仍然知之甚少。与其他胰岛细胞类型的相互作用可能涉及胰高血糖素分泌的葡萄糖抑制（GIGS），因为在I型糖尿病中胰岛细胞被破坏或在晚期2型糖尿病中失败后，体内抑制作用丧失。当从胰岛中分离出β-细胞时，GIGS也在体外丢失。它的构图，建筑， 胰岛细胞类型的蛋白质表达模式在种属之间不同，但GIGS的幅度和时间模式几乎是相同的，无论是种属。因此，我们建议利用物种的相似性，以揭示潜在的GIGS的分子机制。的分泌产物。- 细胞，生长抑素，是这种相互作用的一种有前途的分子介质，因为GIGS在具有生长抑素基因缺失的胰岛中丢失，并且生长抑素受体亚型2在人和小鼠-细胞中主导其信号传导。然而，单独的生长抑素不能抑制分离的胰岛细胞的胰高血糖素分泌，这表明生长抑素必须与胰岛中的其他细胞-细胞相互作用联合收割机结合以进行适当的GIGS。一种可以抑制胞吐作用的潜在的Ephacrine信号传导机制是ephrinA-EphA正向信号传导，并且转录组研究表明EphA 4是在人和小鼠细胞中表达的唯一Eph受体。我们已经表明，即使胰岛细胞Ca 2+水平升高，胰岛胰高血糖素分泌也会对葡萄糖产生抑制作用。这种作用与在缺乏细胞间隙连接的胰岛中观察到的作用相似，其中即使当Ca 2+水平升高时，胰岛素分泌也可以被ephrinA-EphA信号传导介导的肾上腺素通讯抑制。自从由于胰岛β-细胞仅含有一种Eph受体亚型，且与胰岛β-细胞的Eph受体亚型密切相关，我们推测胰岛β-细胞GIGS可能也有类似的机制。因此，我们假设GIGS需要来自胰岛细胞的生长抑素的旁分泌信号传导和来自胰岛细胞的通过EphA 4正向信号传导的促生长素分泌通信。将通过三个具体目的来测试该假设：1）确定促生长素EphA 4正向信号传导对胰岛胰高血糖素分泌的作用; 2）确定生长抑素介导的旁分泌信号传导与EphA促生长素信号传导组合对来自完整胰岛中的胰岛细胞的GIGS的作用; 3）确定这些旁分泌和旁分泌信号传导途径对来自健康和II型糖尿病供体的人胰岛的GIGS的作用。这些实验结果将进一步加深我们对小鼠和人类胰岛中胰岛细胞功能的理解。这种机制信息将为胰高血糖素的调节提供新的治疗靶点，这是预防糖尿病患者低血糖发作和血糖正常化的新机会。

DESCRIPTION (provided by applicant): The islet of Langerhans plays an important role in blood glucose homeostasis through regulated hormone secretion. The therapeutic success of insulin has led most islet research to focus on ¿-cells, although other islet hormones including glucagon (secreted by ¿-cells) and somatostatin (secreted by ¿-cells) have important physiological actions. Glucagon plays a critical role in the pathology of diabetes, but the mechanisms that regulate glucagon secretion remain poorly understood. Interactions with other islet cell types are likely involved in glucose-inhibition of glucagon secretion (GIGS), since inhibition is lost in vivo after ¿-cells are destroyed in Type I or fail in advanced Type 2 diabete. GIGS is also lost in vitro when ¿-cells are isolated from the islet. The composition, architecture, and protein expression patterns of islet cell types vary between species, but the amplitude and temporal pattern of GIGS is nearly identical regardless of species. Thus, we propose to leverage species similarities towards uncovering molecular mechanisms underlying GIGS. The secretory product of ¿-cells, somatostatin, is a promising molecular mediator of this interaction because GIGS is lost in islets with a genetic deletion of somatostatin, and the somatostatin receptor subtype 2 dominates its signaling in both human and mouse ¿-cells. However, somatostatin alone does not inhibit glucagon secretion from isolated ¿-cells, which suggests that somatostatin must combine with other cell-cell interactions in the islet for proper GIGS. One potential juxtacrine signaling mechanism that can inhibit exocytosis is ephrinA-EphA forward signaling, and transcriptome studies show that EphA4 is the only Eph receptor expressed in both human and mouse ¿-cells. We have shown that islet glucagon secretion is suppressed in response to glucose even though ¿-cell Ca2+ levels are elevated. This effect parallels that seen in islets lacking ¿-cell gap junctions, where insulin secretion can be inhibited by juxtacrine communication mediated by ephrinA-EphA signaling even when Ca2+ levels are elevated. Since the ¿-cell contains only a single Eph receptor sub-type and it is closely related to the ¿-cell, we speculate that a similar mechanism may play a role in GIGS from islet ¿-cells. Thus, we hypothesize that GIGS requires both paracrine signaling by somatostatin from ¿-cells in the islet and juxtacrine communication from ¿-cells via EphA4 forward signaling. This hypothesis will be tested via three specific aims: 1) determine the role of juxtacrine EphA4 forward signaling on islet glucagon secretion; 2) determine the role of somatostatin-mediated paracrine signaling combined with EphA juxtacrine signaling on GIGS from ¿-cells in intact islets; 3) determine the roles of these paracrine and juxtacrine signaling pathways on GIGS from human islets of healthy and Type II diabetic donors. These experimental results will further our understanding of ¿-cell function in mouse and human islets. This mechanistic information will provide novel therapeutic targets for the regulation of glucagon, which is an emerging opportunity for preventing hypoglycemic episodes and normalizing blood glucose in diabetic patients.