Function of Ret-GFL signaling in sensory neurons

描述（由申请人提供）：感知疼痛的小直径感觉神经元被归类为伤害感受器。在胚胎发育过程中，几乎所有的伤害感受器都表达神经营养因子NGF的受体TrkA，并依赖于NGF的生存。在胚胎晚期和出生后早期，小直径无髓鞘伤害感受器（c纤维伤害感受器）分化为两个数量大致相等的不同亚群。一个群体继续表达TrkA。另一个群体下调TrkA表达，表达Ret，属于GDNF家族的神经营养因子受体。除了营养因子受体的表达外，这两个c纤维伤害感受器亚群还可以根据肽表达、它们与植物凝集素、分离素B4 （IB4）结合的能力以及它们的中枢和外周投射模式来区分。表达TrkA的痛觉感受器也表达肽CGRP和P物质，不结合IB4，并投射到背角的I和IIo层。表达Ret的痛感受体通常不表达这些神经肽，但会结合凝集素IB4，并投射到背角的LIIi。有证据表明，神经营养因子信号调节了产后和成熟期感觉神经元的结构和功能特性。肾发育引起的Ret缺失小鼠围产期死亡限制了Ret在出生后和成熟感觉神经元中的功能研究。为了克服这一限制，我们通过将一只携带Ret等位基因的小鼠（Ret条件小鼠）与一只在伤害感受器特异性启动子（钠通道亚基1.8启动子）下表达Cre重组酶的小鼠杂交，创造了一只在ib4阳性伤害感受器中Ret完全缺失的小鼠。我们将使用解剖学和行为学方法来确定Ret缺失对该小鼠IB4伤害感受器结构和功能的影响。对这只小鼠的分析将使我们能够确定Ret信号在伤害感觉神经元存活和维持伤害感受器结构和功能中的作用。此外，这只小鼠可能为疼痛感受器的IB4亚群对疼痛转导的特定贡献提供见解。慢性疼痛的治疗是一个主要的未满足的医疗需求。我们已经创造了一种新的小鼠，在这种小鼠中，GDNF家族神经营养因子配体（GFLs）的受体Ret在伤害感受器（感知疼痛的神经元）中被删除。这种新型小鼠将使我们能够确定GFLs在伤害感受器存活和功能中的作用，并研究GFLs在治疗慢性疼痛综合征中的有效机制。

DESCRIPTION (provided by applicant): Small-diameter sensory neurons that sense pain are classified as nociceptors. During embryonic development virtually all nociceptors express TrkA, the receptor for the neurotrophic factor NGF, and depend on NGF for survival. During late embryonic and early postnatal life, small-diameter unmyelinated nociceptors (C-fiber nociceptors) diverge into two distinct subpopulations of roughly equal number. One population continues to express TrkA. The other population down-regulates TrkA expression and expresses Ret, the receptor for the neurotrophic factors belonging to the GDNF family. In addition to trophic factor receptor expression, these two subpopulations of C-fiber nociceptors can be distinguished based on peptide expression, their ability to bind the plant lectin, isolectin B4 (IB4), and the pattern of their central and peripheral projections. Nociceptors that express TrkA also express the peptides CGRP and substance P, do not bind IB4, and project to lamina I and IIo of the dorsal horn. Nociceptors that express Ret do not typically express these neuropeptides, do bind the lectin IB4, and project to LIIi of the dorsal horn. Evidence suggests that neurotrophic factor signaling modulates structural and functional properties of sensory neurons in the postnatal period and in maturity. Perinatal death of Ret-null mice because of kidney agenesis has limited the study of Ret function in postnatal and mature sensory neurons. To overcome this limitation, we have created a mouse in which Ret is deleted exclusively in IB4-positive nociceptors by crossing a mouse with floxed Ret alleles (Ret-conditional mouse) with a mouse expressing Cre recombinase under a nociceptor specific promoter (the sodium channel a subunit 1.8 promoter). We will use anatomical and behavioral methods to determine the effect of Ret deletion on the structure and function of IB4 nociceptors in this mouse. Analysis of this mouse will enable us to determine the role of Ret signaling in nociceptive sensory-neuron survival and in the maintenance of nociceptor structure and function. In addition, this mouse may provide insights into the specific contribution of the IB4 subpopulation of nociceptors to pain transduction. The treatment of chronic pain is a major unmet medical need. We have created a novel mouse in which Ret, the receptor for the GDNF family neurotrophic factor ligands (GFLs) is deleted in nociceptors (neurons that sense pain). This novel mouse will allow us to determine the role of the GFLs in nociceptor survival and function and to study mechanisms by which GFLs are effective in the treatment of chronic pain syndromes.