Role of Neurotrophins in the Development of the Mammalian Nervous System

TrkB和TrkC编码许多异构体，包括那些缺乏催化酪氨酸激酶结构域的异构体。对这些激酶缺陷亚型在Trk信号传导中的功能知之甚少。体外研究和我们自己的体内研究表明，截断的Trk受体可以以显性阴性方式或通过配体隔离抑制激酶活性受体亚型的功能。然而，这种活动的生理相关性尚不清楚。截断受体胞内结构域的高度序列保守表明其可能与细胞质蛋白和信号传导能力有特定的相互作用。事实上，最近有报道称BDNF通过截断的TrkB T1受体诱导星形胶质细胞钙波的产生。然而，TrkB T1受体与钙动员的分子机制及其生理作用尚不清楚。有趣的是，TrkB T1在16三体（Ts16）唐氏综合征小鼠模型的大脑中过度表达50%，Ts16海马神经元在培养中过早死亡。神经退行性变通常与人类唐氏综合征有关，TrkB T1在阿尔茨海默病患者中也过表达。为了进一步研究TrkBT1在神经元存活中的作用，我们制造了一只特异性缺乏TrkBT1激酶缺陷受体亚型的小鼠。该突变未引起总体表型，可用于纠正Ts16小鼠体内TrkB T1的水平。重要的是，海马神经元来自TrkB T1 -/+；Ts16小鼠在体外可避免Ts16神经元细胞过早死亡（Dorsey et al. 2006）。这是一个非常令人兴奋的结果，因为它与早期的假设形成了对比，即神经变性是由于神经营养因子供应不足而发生的。相反，我们的研究表明，细胞死亡和存活的调节可以发生在Trk受体水平上。我们现在正在研究导致TrkB失调的分子机制。T1水平和TrkB T1表达升高的潜在有害影响。具体来说，我们正在研究TrkBT1对全长TrkB受体活性和细胞内ca2 ++水平调节的影响。在这方面，我们发现TrkB。T1缺陷小鼠发育正常，但焦虑增加与杏仁核基底外侧神经元的神经突长度和复杂性的形态学异常有关。在体内，通过去除一个BDNF等位基因而减少的TrkB信号传导可以被TrkB部分挽救。T1缺失，这是通过改善与BDNF单倍不足相关的攻击性增强和体重增加而揭示的。因此，我们的结果提供了证据，在生理水平上，TrkB。T1受体是TrkB的重要调节因子。体内FL信号。此外，我们最近发现Rbfox1（一种与智力残疾、癫痫和自闭症相关的RNA结合蛋白）的上调会选择性地增加海马TrkB。T1异构体表达。生理上，Rbfox1的增加会损害bdnf依赖性的LTP，而LTP可以通过基因恢复TrkB来挽救。T1的水平。Rbfox1上调与TrkB特异性升高的海马RNA-seq分析。T1亚型的表达也表明受Rbfox1功能获得影响的基因与受Rbfox1缺失影响的基因惊人地不同。这些发现不仅确定了TrkB是Rbfox1病理生理的主要靶点，而且表明Rbfox1功能的获得或丧失调节着不同的遗传景观。

TrkB and TrkC encode a number of isoforms, including those that lack the catalytic tyrosine kinase domain. Little is known about the function of these kinase deficient isoforms in Trk signaling. In vitro studies, and our own in vivo studies, have shown that truncated Trk receptors can inhibit the function of kinase-active receptor isoforms in a dominant-negative manner or by ligand sequestration. The physiological relevance of this activity is, however, still unclear. The high degree of sequence conservation of the intracellular domains of truncated receptors suggests the potential for specific interactions with cytoplasmic proteins and signaling capabilities. Indeed, it has been reported recently that BDNF induces the production of calcium waves in astroglia through the truncated TrkB T1 receptor. However, the molecular mechanism(s) linking the TrkB T1 receptor to calcium mobilization and its physiological role is still unknown. Interestingly, TrkB T1 is 50% overexpressed in the brain of the trisomy 16 (Ts16) mouse model of Down syndrome and Ts16 hippocampal neurons die prematurely in culture. Neurodegeneration is commonly associated with Down syndrome in humans and TrkB T1 is also overexpressed in Alzheimer's patients. To further investigate the role of TrkB T1 in neuronal survival, we generated a mouse lacking specifically the TrkBT1 kinase-deficient receptor isoform. This mutation caused no gross phenotype and could be used to correct the levels of TrkB T1 in Ts16 mice in vivo. Importantly, hippocampal neurons from TrkB T1 -/+; Ts16 mice escaped the premature cell death of Ts16 neurons in vitro (Dorsey et al. 2006). This is a very exciting result because it contrasts with earlier hypotheses that neurodegeneration occurs due to insufficient supply of neurotrophic factors. Rather, our studies suggest that modulation of cell death and survival can occur at the level of the Trk receptor. We are now investigating the molecular mechanisms leading to the dysregulation of TrkB.T1 levels and underlying the detrimental effect of elevated TrkB T1 expression. Specifically, we are addressing both the effects of TrkBT1 on the activity of the full-length TrkB receptor and on the intracellular regulation of Ca++ levels. In this respect we have found that TrkB.T1 deficient mice develop normally but show increased anxiety in association with morphological abnormalities in the length and complexity of neurites of neurons in the basolateral amygdala. In vivo reduction of TrkB signaling by removal of one BDNF allele could be partially rescued by TrkB.T1 deletion, which was revealed by an amelioration of the enhanced aggression and weight gain associated to BDNF haploinsufficiency. Thus, our results provide evidence that at the physiological level, TrkB.T1 receptors are important regulators of TrkB.FL signaling in vivo. In addition, we have recently found that upregulation of Rbfox1, an RNA binding protein associated with intellectual disability, epilepsy and autism, increases selectively hippocampal TrkB.T1 isoform expression. Physiologically, increased Rbfox1 impairs BDNF-dependent LTP which can be rescued by genetically restoring TrkB.T1 levels. RNA-seq analysis of hippocampi with upregulation of Rbfox1 in conjunction with the specific increase of TrkB.T1 isoform expression also shows that the genes affected by Rbfox1 gain of function are surprisingly different from those influenced by Rbfox1 deletion. These findings not only identify TrkB as a major target of Rbfox1 pathophysiology but also suggest that gain or loss of function of Rbfox1 regulate different genetic landscapes.