Regulatory Role of Growth Hormone Secretagogue Receptor

描述（由申请人提供）：我们的目标是提供治疗干预，以维持老年人的高生活质量。我们开发了小分子，恢复了老年人的生长激素（GH）释放的生理特征，然后克隆了孤儿G蛋白偶联受体（GPCR）介体，将其命名为生长激素促分泌素受体（GHS-R）。随后，Kojima鉴定了内源性激动剂ghrelin;因此，小分子GHS-R激动剂被称为ghrelin模拟物。在体弱的老年人中，长期给予生长激素释放肽模拟物MK 677以恢复GH轴，伴随着骨密度、瘦体重和力量的增加。因此，我们得出结论，MK 677挽救了内源性ghrelin信号传导中的年龄依赖性缺陷。除了生长激素轴，这种缺陷可能在衰老过程中产生更深远的后果，因为GHS-R（生长激素释放肽受体）定位于调节情绪，记忆和学习的神经元。多巴胺受体亚型-1（D1 R）在这些相同的神经元中共表达，暗示ghrelin是多巴胺信号的调节剂。事实上，在共表达GHS-R和D1 R的细胞中，ghrelin放大了多巴胺诱导的cAMP积累。为了研究生长激素释放肽的神经调节的广泛意义，将比较D1 R、生长激素释放激素（GHRH）受体和生长抑素受体（sst 5和sst 2）信号传导的GHS-R调节。这些GPCR伴侣在体内与GHS-R内源性共表达并调节cAMP信号传导。机制研究将调查：腺苷酸环化酶（AC）活性;鉴定涉及的AC同工酶; Ga亚基的修饰; γ-亚基清除; GHS-R/GPCR异源二聚化。具体目标：确定由生长素释放肽受体（GHS-R）的共激活引起的多巴胺D1 R诱导的cAMP积累的放大的分子机制，并在野生型、生长素释放肽-/-和生长素释放肽受体敲除小鼠（Ghsr-/-）中测试生长素释放肽介导的放大在体内发生以增强D1 R和GHS-R表达神经元中的信号传导从而导致行为变化的假设;确定Ghrelin激活GHS-R如何放大GHRH受体介导的cAMP积累的分子机制，并与D1 R诱导的cAMP积累的放大进行对比;确定ghrelin激活GHS-R如何减弱sst 5信号传导对cAMP积累的抑制作用的分子机制，与sst 2比较，确定sst 5和sst 2是否与脑内GHS-R共表达。由于生长激素释放肽对大脑中信号传导的广泛调节作用，挽救内源性生长激素释放肽信号传导中与年龄相关的缺陷具有广泛的人类健康意义。在老年人中，生长激素释放肽对多巴胺信号的增强与衰老和帕金森病直接相关，因为学习和记忆通过增加神经元中cAMP的积累而得到改善。我们推测，临床使用生长激素释放肽模拟物将转化为改善老年人的认知功能。

DESCRIPTION (provided by applicant): Our objective is to provide therapeutic intervention that will maintain high quality of life during aging. We developed small molecules that restore the physiological profile of episodic growth hormone (GH) release in the elderly to that of young adults and then cloned the orphan G-protein coupled receptor (GPCR) mediator involved, naming it the growth hormone secretagogue receptor (GHS-R). Subsequently, Kojima identified the endogenous agonist, ghrelin; hence, the small molecule GHS-R agonists are referred to as ghrelin mimetics. In the frail elderly, chronic administration of a ghrelin mimetic, MK677, to rejuvenate the GH axis, was accompanied by increased bone density, lean mass and strength. Therefore, we concluded that MK677 rescued an age-dependent deficit in endogenous ghrelin signaling. Aside from the GH axis, this deficit could have more profound consequences during aging because the GHS-R (ghrelin receptor) localizes to neurons regulating mood, memory and learning. The dopamine receptor subtype-1 (D1R) is coexpressed in these same neurons, implicating ghrelin as a modulator of dopamine signaling. Indeed, in cells coexpressing GHS-R and D1R, ghrelin amplifies dopamine-induced accumulation of cAMP. To investigate the broad significance of neuromodulation by ghrelin, GHS-R modulation of D1R, GH-releasing hormone (GHRH) receptor, and somatostatin receptors (sst5 and sst2) signaling will be compared. These GPCR partners are endogenously coexpressed with the GHS-R in vivo and regulate cAMP signaling. Mechanistic studies will investigate: adenylyl cyclase (AC) activity; identifying AC-isozymes involved; modification of Ga subunits; ¿y-subunit scavenging; GHS-R/GPCR heterodimerization. Specific Aims: define the molecular mechanism of amplification of dopamine D1R-induced cAMP accumulation caused by co-activation of the ghrelin receptor (GHS-R), and test the hypothesis in wildtype, ghrelin-/- and ghrelin receptor knockout mice (Ghsr-/-) that ghrelin-mediated amplification occurs in vivo to enhance signaling in D1R and GHS-R expressing neurons resulting in behavioral changes; determine the molecular mechanism of how ghrelin activation of GHS-R amplifies GHRH receptor-mediated cAMP accumulation and contrasts with amplification of D1R-induced cAMP accumulation; determine the molecular mechanism of how ghrelin activation of GHS-R attenuates the inhibitory effects sst5-signaling on cAMP accumulation, compare with sst2, and determine whether sst5 and sst2 are coexpressed with the GHS-R in brain. Because of ghrelin's broad modulatory role on signaling in the brain, rescuing age-related deficits in endogenous ghrelin signaling has broad human health implications. Augmentation of dopamine signaling by ghrelin in the elderly is directly relevant to aging and Parkinson's disease, because learning and memory are improved by increasing cAMP accumulation in neurons. We hypothesize that clinical use of ghrelin mimetics will translate to improved cognitive function in the elderly.