A p75/Ret receptor complex as an integrator of survival and death

在整个发育中的神经系统中，产生了多余的神经元，这些神经元是不必要的，或者 不适当的连接，并通过程序性细胞死亡(PCD)消除。在PNS中，范围 细胞凋亡由两种有限的促进存活的神经营养因子的供应所控制 神经支配的靶标，以及由“获胜神经元”分泌的诱导凋亡的竞争因子 已经成功地竞争了这些神经营养因子。神经胶质细胞系来源的神经营养 生长因子(GDNF)家族配体(GFL)是一类强大的生长因子家族，支持肿瘤的生存 自主神经、躯体感觉和脊髓运动神经元。GFL通过以下途径促进生存和增长 一种常见的信号传导受体酪氨酸激酶，Ret。在这个资助期内，我们发现 RET与死亡受体家族中的一员p75相互作用，p75促进GDNF介导的Ret活化和存活。当p75在感觉神经元中被特别删除时， 大约20%在P14和成年期之间丢失，这些丢失选择性地发生在Ret+非肽能伤害性感受器中。这些结果表明，p75是发育所必需的。 微调Ret介导的营养支持的非肽能伤害性感受器谱系。我们还发现， 在PCD期间，颈上神经节(SCG)的交感神经元Ret仅限于一个亚群 迅速经历细胞凋亡的退化神经元。促凋亡的条件会诱导 Ret与p75的结合，从而增强调节的膜内蛋白分解(RIP)切割 P75的表达和下游凋亡效应因子的激活。Ret+神经元中p75的缺失和缺失 Ret，特别是在PCD期间，在体外和体内都能抑制细胞凋亡。这些结果表明， RET以非经典的方式增强p75介导的细胞凋亡。其背后的分子机制 P75增强Ret信号的能力，以及Ret增强p75介导的死亡的能力并不是很好 我们最近还发现信号素3A(Sema3A)， 分泌的排斥性轴突引导分子，诱导原代SCG神经元凋亡，并 其受体成分Neuropilin-1(Npn-1)和PlexinA3(PA3)的缺失可显著降低PCD 在SCG里。Sema3A通过外在途径诱导细胞凋亡，需要caspase-8，而不是 需要caspase-9的交感神经元中由NGF撤退所触发的内源性通路。这个 神经营养因子剥夺诱导的细胞凋亡与死亡受体激活的联合作用 发展中的SCG提出了caspase-8和caspase-9在多大程度上对 PCD过程中的细胞凋亡，以及哪些死亡受体推动了这一过程。这些问题将是 目标2的主题。总而言之，这里提出的实验将定义分子机制和 死亡受体通路，如Npn-1/PA3和p75在PCD中所起作用的大小。

Throughout the developing nervous system excess neurons are generated which are nonessential, or inappropriately connected, and are eliminated by programmed cell death (PCD). In the PNS, the extent of apoptosis is governed by both a limited supply of survival-promoting neurotrophic factors provided by targets of innervation, and by apoptosis-inducing competition factors secreted by “winning neurons” that have successfully competed for these neurotrophic factors. The glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) are a family of potent growth factors that support the survival of autonomic, somatosensory and spinal motor neurons. The GFLs promote survival and growth through a common signal-transducing receptor tyrosine kinase, Ret. During this grant period we discovered that Ret interacts with p75, a member of the TNF family of death receptors, and p75 enhances GDNF-mediated Ret activation and survival. When p75 is deleted specifically in sensory neurons, approximately 20% are lost between P14 and adulthood, and these losses selectively occur in Ret+ nonpeptidergic nociceptors. These results indicate that p75 is required for the development of the nonpeptidergic nociceptor lineage by fine-tuning Ret-mediated trophic support. We also found that during PCD in sympathetic neurons of the superior cervical ganglion (SCG) Ret is restricted to a subset of degenerating neurons that rapidly undergo apoptosis. Pro-apoptotic conditions induce the association of Ret with p75, thereby enhancing the regulated intramembrane proteolysis (RIP) cleavage of p75 and activation of downstream apoptotic effectors. Deletion of p75 in Ret+ neurons, and deletion of Ret, specifically during PCD, inhibits apoptosis both in vitro and in vivo. These results indicate that Ret acts non-canonically to augment p75-mediated apoptosis. The molecular mechanisms that underlie the ability of p75 to enhance Ret signaling, and for Ret to enhance p75 mediated death, are not well understood, and are the subject of Aim 1. We also discovered recently that semaphorin 3A (Sema3A), a secreted repulsive axon guidance molecule, induces apoptosis of primary SCG neurons, and that deletion of its receptor components, Neuropilin-1 (Npn-1) and PlexinA3 (PA3), significantly reduce PCD in the SCG. Sema3A induces apoptosis via the extrinsic pathway, requiring caspase-8, as opposed to the intrinsic pathway triggered by NGF withdrawal in sympathetic neurons that requires caspase-9. The combination of apoptosis induced by neurotrophic factor deprivation and death receptor activation in the developing SCG raises the question of the extent to which caspase-8 and caspase-9 contribute to apoptosis during PCD, and which death receptors are driving this process. These questions will be the subject of Aim 2. Collectively the experiments proposed here will define the molecular mechanisms and magnitude of the role played by death receptor pathways, such as Npn-1/PA3 and p75, in PCD.