Neuronal growth factor signaling via cAMP

描述（由申请人提供）：在神经元中，普遍存在的第二信使环AMP （cAMP）与许多生长因子信号通路有关。例如，神经营养因子，如神经生长因子（NGF），通过cAMP与神经元分化、神经新生和轴突再生有关。然而，如何将生长因子或它们的受体与cAMP的变化联系起来仍不清楚。轴突的引导信号，如netrin-1，也通过cAMP发出信号，它们与第二代信使的联系同样不清楚。在此之前，唯一已知的cAMP来源是G蛋白调控的跨膜腺苷酸环化酶（tmac）家族；tmac通过G蛋白偶联的7种跨膜受体介导神经递质对cAMP的反应。但是，所有将tmac与神经元生长因子信号或轴突引导信号联系起来的尝试都失败了，或者充其量被证明是有争议的。我们的实验室在哺乳动物细胞中发现了一种独特的cAMP来源，即“可溶性”腺苷酸环化酶（sAC）。与tmac相反，sAC不受异三聚体G蛋白的调节，而是由碳酸氢盐和钙离子调节。在该应用的初步结果中，我们证明sAC在神经营养因子NGF和轴突引导提示网络-1的作用下负责cAMP的生成。我们已经确定了许多与先前克隆的同种异构体不同的sAC脑同种异构体，并且在本次拨款申请中，我们建议表征这些新的脑sAC蛋白。我们还建议通过实验来阐明sAC在NGF和netrin-1作用下的激活机制，并确定该信号级联是否被其他神经营养因子和其他神经元反应所利用。在这项拨款申请中测试的假设将揭示生长信号和引导线索如何将其信息传递到神经元活动中。这些研究对大脑发育、退行性疾病以及学习和记忆具有重要意义。

DESCRIPTION (provided by applicant): In neurons, the ubiquitous second messenger cyclic AMP (cAMP) has been implicated in a number of growth factor signaling pathways. For example, neurotrophins, such as Nerve Growth Factor (NGF), are linked via cAMP to neuronal differentiation, neuritogenesis, and axonal regeneration. However, it has remained unclear how to link the growth factors, or their receptors, to cAMP changes. Axonal guidance cues, such as netrin-1, also signal via cAMP, and their link to second messenger generation is similarly unclear. Previously, the only known source of cAMP had been the family of G protein regulated, transmembrane adenylyl cyclases (tmACs); tmACs mediate the cAMP changes in response to neurotransmitters which act via G protein coupled, seven transmembrane spanning receptors. But all attempts to link tmACs to neuronal growth factor signals or axonal guidance cues have failed, or at best proven controversial. Our laboratory characterized a distinct source of cAMP in mammalian cells, 'soluble' adenylyl cyclase (sAC). In contrast to tmACs, sAC is not modulated by heterotrimeric G proteins but is, instead, regulated by bicarbonate and calcium ions. In preliminary results for this application, we demonstrate that sAC is responsible for cAMP generation in response to both the neurotrophin NGF and the axonal guidance cue netrin-1. We have identified a number of brain isoforms of sAC which differ from the previously cloned isoforms, and in this grant application, we propose to characterize these novel brain sAC proteins. We also propose experiments to elucidate the mechanism of sAC activation in response to NGF and netrin-1 and to determine whether this signaling cascade is utilized by other neurotrophins and in other neuronal responses. The hypotheses tested in this grant application will reveal how growth signals and guidance cues relay their information into neuronal activity. These studies have important implications for brain development, degenerative diseases and learning and memory.