Crosstalk between neurotrophin and Wnt signaling pathways in neuronal development

描述(申请人提供)：神经系统的正常运作依赖于建立精确的神经元回路。神经营养因子是由神经元靶点分泌的细胞外指示信号，通过调节生存、生长和突触可塑性来影响神经系统的连接。尽管在了解靶向神经营养因子的功能方面取得了相当大的进展，但一个基本的问题仍然存在：有限的神经营养因子及其受体如何协调建立和维持精确的神经元回路所需的各种事件？我们推测，靶源性神经营养因子影响神经元功能的一个机制是通过调节神经元表达和其他分泌生长因子的信号。神经营养因子和其他生长因子家族之间的这种串扰机制可能会扩大神经营养因子在神经系统中的作用范围。我们最近发现了神经营养素和Wnt信号通路在发育过程中的相互作用。WNTs是一个富含半胱氨酸的分泌型蛋白大家族，最初在胚胎发育过程中是形态发生因子。然而，最近的研究发现，WNTS在神经元发育中的作用令人惊讶地多样化。这一提议的总体目标是检验一种假设，即神经元中表达的WNT与其靶组织分泌的神经营养因子之间的相互作用调节神经元的发育和精确神经元回路的建立。因此，我们的具体目标是：(1)检验WNTs在神经元中的表达受靶源性神经营养因子调控的假设：我们发现交感神经元中有几个WNTs及其受体的表达，这是一个研究神经元与靶相互作用的易操作的模型系统。其中，WNT-5a在交感神经轴突到达最终靶点的发育时期强烈表达。这一目的的目的是证明WNT-5a的神经元表达受与靶组织和靶源神经营养因子的相互作用的调节。(2)验证神经性WNT是交感神经元中神经营养因子功能的介体的假设：我们发现WNT-5a足以概括神经营养因子神经生长因子(NGF)在培养交感神经元中的促生长作用。这一目标的目的是证明WNT-5a在NGF下游发挥作用，以调节靶组织神经支配所需的轴突生长。(3)确定交感神经元中Wnt信号的机制：我们将描述交感神经元内的信号转导机制，该机制将细胞外的Wnt-5a信号耦合到轴突生长和分支所需的细胞骨架机制的变化中。我们的研究将为神经元发育的分子机制提供新的见解，并为促进损伤或疾病后神经元的生长和再生提供新的策略。

DESCRIPTION (provided by applicant): The proper functioning of the nervous system relies on the establishment of precise neuronal circuits. Neurotrophins are extracellular instructive cues secreted by neuronal targets that influence wiring of the nervous system by regulating survival, growth and synaptic plasticity. Despite considerable progress in understanding functions of target-derived neurotrophins, a fundamental question still remains: How do a limited repertoire of neurotrophins and their receptors orchestrate the diverse events required to establish and maintain precise neuronal circuits? We hypothesize that one mechanism by which target-derived neurotrophins influence neuronal function is by regulating neuronal expression and signaling of other secreted growth factors. Such a mechanism of crosstalk between neurotrophins and other growth factor families could potentially broaden the repertoire of neurotrophin actions in the nervous system. We have recently identified an interaction between the neurotrophin and Wnt signaling pathways during development. Wnts are a large family of secreted cysteine-rich proteins, initially characterized as morphogens during embryonic patterning. However, recent studies identify surprisingly versatile roles for Wnts in neuronal development. The overall goal of this proposal is to test the hypothesis that interactions between Wnts expressed in neurons and neurotrophins secreted by their target tissues regulate neuronal development and establishment of precise neuronal circuits. Thus, our specific aims are; (1) To test the hypothesis that neuronal expression of Wnts is regulated by target-derived neurotrophins: We found that several Wnts and their receptors are expressed in sympathetic neurons, a tractable model system for studying neuron-target interactions. Of these, Wnt-5a is robustly expressed at a developmental time when sympathetic axons are reaching final targets. The goal of this aim is to demonstrate that neuronal expression of Wnt-5a is regulated by interactions with target tissues and target-derived neurotrophins. (2) To test the hypothesis that neuronal Wnts are mediators of neurotrophin function in sympathetic neurons: We found that Wnt-5a is sufficient to recapitulate the growth-promoting effects of the neurotrophin, Nerve Growth Factor (NGF), in cultured sympathetic neurons. The goal of this aim is to demonstrate that Wnt-5a functions downstream of NGF, to mediate axonal growth required for innervation of target tissues. (3) To identify mechanisms of Wnt signaling in sympathetic neurons: We will characterize the signal transduction machinery within sympathetic neurons that couples an extracellular Wnt-5a signal to changes in the cytoskeletal machinery necessary for axonal growth and branching. Our study will provide new insight into molecular mechanisms underlying neuronal development and allow for new strategies for promoting growth and regeneration of neurons following injury or disease.