Function and gene expression of the high-affinity NGF receptor

采用定量RT-PCR方法，我们研究了trkA mRNA在无血清培养的17天胎鼠基底前神经元中的表达调控。除了增加ChAT活性外，NGF还以时间和NGF浓度依赖的方式显著诱导trkA基因表达。因此，NGF可能在基底前脑胆碱能神经元发育过程中对trkA基因表达起关键作用。此外，为了研究这种上调是否与基底前脑胆碱能神经元的营养作用有关，我们检测了其他一些神经营养剂（BDNF、NT-3、bFGF、CNTF和40 mM KC1）对ChAT活性和trkA基因表达的影响。一些神经营养因子增加ChAT活性的程度与NGF相同，而它们对trkA mRNA表达的刺激没有那么强。NT-3 + K252b可促进PC12细胞中TrkA的酪氨酸磷酸化，并增加培养的基底前脑胆碱能神经元如NGF的ChAT活性。我们发现NT-3 + K252b上调trkA mRNA水平。这些结果表明，trkA mRNA的表达受其特异性配体NGF的直接调节，而不是受基底前脑胆碱能神经元的神经营养作用的影响，并且这种上调与NGF与trkA受体结合引发的分子事件有关。

Using a quantitative RT-PCR,we studied the regulation of trkA mRNA expression in serum-free, cultured basal forebain neurons from 17-day fetal rats. Besides increasing ChAT activities, NGF strikingly induced trkA gene expression in a time- and NGF concentration-dependent manner. Therefore, NGF might play a critical role in trkA gene expression during the development of basal forebrain cholinergic neurons. Furthermore, to investigate whether this up-regulation is connected with the trophic effects on basal forebrain cholinergic neurons, we examined the effects of some other neurotrophic agents (BDNF,NT-3, bFGF,CNTF and 40 mM KC1) upon ChAT activity and trkA gene expression. Some neurotrophic factors increased ChAT activities to the same degree as NGF,whereas they did not stimulate trkA mRNA expression so potently. NT-3 plus K252b promotes the tyrosine phosphorylation of TrkA in PC12 cells and increases ChAT activity in cultured basal forebrain cholinergic neurons like NGF.We found that NT-3 plus K252b up-regulated the level of trkA mRNA.These results suggested that the expression of trkA mRNA is regulated directly by its specific ligand NGF,rather than by neurotrophic effects upon basal forebrain cholinergic neurons and that the up-regulation is connected to a molecular event initiated by the binding of NGF to the TrkA receptor.