Fariñas I, Jones K R, Tessarollo L, Vigers A J, Huang E, Kirstein M, de Caprona D C, Coppola V, Backus C, Reichardt L F, Fritzsch B
Program in Neuroscience, Department of Physiology and Howard Hughes Medical Institute, University of California, San Francisco, California 94143-0724, USA.
J Neurosci. 2001 Aug 15;21(16):6170-80. doi: 10.1523/JNEUROSCI.21-16-06170.2001.
Previous work suggested qualitatively different effects of neurotrophin 3 (NT-3) in cochlear innervation patterning in different null mutants. We now show that all NT-3 null mutants have a similar phenotype and lose all neurons in the basal turn of the cochlea. To understand these longitudinal deficits in neurotrophin mutants, we have compared the development of the deficit in the NT-3 mutant to the spatial-temporal expression patterns of brain-derived neurotrophic factor (BDNF) and NT-3, using lacZ reporters in each gene and with expression of the specific neurotrophin receptors, trkB and trkC. In the NT-3 mutant, almost normal numbers of spiral ganglion neurons form, but fiber outgrowth to the basal turn is eliminated by embryonic day (E) 13.5. Most neurons are lost between E13.5 and E15.5. During the period preceding apoptosis, NT-3 is expressed in supporting cells, whereas BDNF is expressed mainly in hair cells, which become postmitotic in an apical to basal temporal gradient. During the period of neuronal loss, BDNF is absent from the basal cochlea, accounting for the complete loss of basal turn neurons in the NT-3 mutant. The spatial gradients of neuronal loss in these two mutants appear attributable to spatial-temporal gradients of neurotrophin expression. Our immunocytochemical data show equal expression of their receptors, TrkB and TrkC, in spiral sensory neurons and thus do not relate to the basal turn loss. Mice in which NT-3 was replaced by BDNF show a qualitative normal pattern of innervation at E13.5. This suggests that the pattern of expression of neurotrophins rather than their receptors is essential for the spatial loss of spiral sensory neurons in NT-3 null mutants.
先前的研究表明，神经营养因子3（NT-3）在不同基因敲除突变体的耳蜗神经支配模式中具有质的差异。我们现在发现，所有NT-3基因敲除突变体都具有相似的表型，耳蜗基底转的所有神经元均缺失。为了理解神经营养因子突变体中的这些纵向缺陷，我们将NT-3突变体中缺陷的发展与脑源性神经营养因子（BDNF）和NT-3的时空表达模式进行了比较，利用每个基因中的lacZ报告基因以及特定神经营养因子受体trkB和trkC的表达。在NT-3突变体中，形成的螺旋神经节神经元数量几乎正常，但到胚胎第13.5天（E13.5）时，向基底转的纤维长出被消除。大多数神经元在E13.5和E15.5之间丢失。在细胞凋亡之前的时期，NT-3在支持细胞中表达，而BDNF主要在毛细胞中表达，毛细胞以从顶端到基底的时间梯度进入有丝分裂后状态。在神经元丢失期间，基底耳蜗中不存在BDNF，这解释了NT-3突变体中基底转神经元的完全缺失。这两个突变体中神经元丢失的空间梯度似乎归因于神经营养因子表达的时空梯度。我们的免疫细胞化学数据显示，它们的受体TrkB和TrkC在螺旋感觉神经元中表达相同，因此与基底转的丢失无关。用BDNF替代NT-3的小鼠在E13.5时显示出定性正常的神经支配模式。这表明神经营养因子的表达模式而非其受体对于NT-3基因敲除突变体中螺旋感觉神经元的空间丢失至关重要。
