Zirrgiebel U, Lindholm D
Max Planck Institute for Psychiatry, Department of Neurochemistry, Martinsried/Munich, Germany.
Neurochem Res. 1996 Jul;21(7):851-9. doi: 10.1007/BF02532309.
The neurotrophins brain derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) are both expressed in developing cerebellum in addition to their tyrosine kinase receptors. TrkB and TrkC. In contrast to BDNF.NT-3 has only a negligible or a transient survival activity on cultured cerebellar granule neurons. The granule neurons however, express both TrkC and Trk B receptors which suggests a basic difference in signaling between BDNF and NT-3 in these neurons. Here we have studied whether this difference can be attributed to the presence of alternative TrkC receptor variants on the granule neurons and which signaling pathway is specifically activated by BDNF but not by NT-3 in these neurons. Using RT-PCR it was shown that the cerebellar granule neurons express the full length TrkC receptor, in addition to variant receptors containing small inserts in the receptor tyrosine kinase domain. There was no dramatic change in the relative amounts of different TrkC receptors during development. However, we found the TrkC receptor constitutively phosphorylated even in the absence of added ligand suggesting an interaction of TrkC with endogenously produced NT-3. In addition, NT-3 was able to phosphorylate the BDNF receptor, TrkB but only at higher concentration (50 ng/ml). There were also distinct differences in the activation of intracellular molecules by BDNF and NT-3. Thus, p21 Ras and PLC gamma were activated by BDNF but not by NT-3 whereas both BDNF and NT-3 increased calcium and c-fos mRNA in the granule neurons. These results show that differential activation of specific intracellular pathways such as that of p21 Ras determines the specific effects of BDNF and NT-3 on granule neuron survival. In addition, since calcium is increased by NT-3 in the cerebellar granule neurons, this neurotrophin might have some unknown important effects on these neurons.
神经营养因子脑源性神经营养因子（BDNF）和神经营养因子-3（NT-3）除了它们的酪氨酸激酶受体TrkB和TrkC外，在发育中的小脑中均有表达。与BDNF相反，NT-3对培养的小脑颗粒神经元只有微不足道或短暂的存活活性。然而，颗粒神经元同时表达TrkC和TrkB受体，这表明BDNF和NT-3在这些神经元中的信号传导存在根本差异。在这里，我们研究了这种差异是否可归因于颗粒神经元上存在TrkC受体变体，以及在这些神经元中BDNF而非NT-专门激活的是哪种信号通路。通过逆转录聚合酶链反应（RT-PCR）表明，小脑颗粒神经元除了在受体酪氨酸激酶结构域含有小插入片段的变体受体外，还表达全长TrkC受体。在发育过程中，不同TrkC受体的相对含量没有显著变化。然而，我们发现即使在没有添加配体的情况下，TrkC受体也组成性磷酸化，这表明TrkC与内源性产生的NT-3相互作用。此外，NT-3能够磷酸化BDNF受体TrkB，但仅在较高浓度（）时。BDNF和NT-3对细胞内分子的激活也存在明显差异。因此，p21 Ras和磷脂酶Cγ（PLCγ）被BDNF激活而不被NT-3激活，而BDNF和NT-3都能增加颗粒神经元中的钙和c-fos信使核糖核酸（mRNA）。这些结果表明，特定细胞内通路（如p21 Ras通路）的差异激活决定了BDNF和NT-3对颗粒神经元存活的特定影响。此外，由于NT-3在小脑颗粒神经元中增加了钙，这种神经营养因子可能对这些神经元有一些未知的重要影响。
