Martínez A, Alcántara S, Borrell V, Del Río J A, Blasi J, Otal R, Campos N, Boronat A, Barbacid M, Silos-Santiago I, Soriano E
Department of Animal and Plant Cell Biology, University of Barcelona, Barcelona 08028, Spain.
J Neurosci. 1998 Sep 15;18(18):7336-50. doi: 10.1523/JNEUROSCI.18-18-07336.1998.
Recent studies have suggested a role for neurotrophins in the growth and refinement of neural connections, in dendritic growth, and in activity-dependent adult plasticity. To unravel the role of endogenous neurotrophins in the development of neural connections in the CNS, we studied the ontogeny of hippocampal afferents in trkB (-/-) and trkC (-/-) mice. Injections of lipophilic tracers in the entorhinal cortex and hippocampus of newborn mutant mice showed that the ingrowth of entorhinal and commissural/associational afferents to the hippocampus was not affected by these mutations. Similarly, injections of biocytin in postnatal mutant mice (P10-P16) did not reveal major differences in the topographic patterns of hippocampal connections. In contrast, quantification of biocytin-filled axons showed that commissural and entorhinal afferents have a reduced number of axon collaterals (21-49%) and decreased densities of axonal varicosities (8-17%) in both trkB (-/-) and trkC (-/-) mice. In addition, electron microscopic analyses showed that trkB (-/-) and trkC (-/-) mice have lower densities of synaptic contacts and important structural alterations of presynaptic boutons, such as decreased density of synaptic vesicles. Finally, immunocytochemical studies revealed a reduced expression of the synaptic-associated proteins responsible for synaptic vesicle exocytosis and neurotransmitter release (v-SNAREs and t-SNAREs), especially in trkB (-/-) mice. We conclude that neither trkB nor trkC genes are essential for the ingrowth or layer-specific targeting of hippocampal connections, although the lack of these receptors results in reduced axonal arborization and synaptic density, which indicates a role for TrkB and TrkC receptors in the developmental regulation of synaptic inputs in the CNS in vivo. The data also suggest that the genes encoding for synaptic proteins may be targets of TrkB and TrkC signaling pathways.
最近的研究表明，神经营养因子在神经连接的生长和精细化、树突生长以及成年期活动依赖性可塑性方面发挥作用。为了阐明内源性神经营养因子在中枢神经系统神经连接发育中的作用，我们研究了trkB（-/-）和trkC（-/-）小鼠海马传入神经的个体发生。在新生突变小鼠的内嗅皮质和海马中注射亲脂性示踪剂表明，内嗅和连合/联合传入神经向海马的长入不受这些突变的影响。同样，在出生后突变小鼠（P10 - P16）中注射生物素，并未揭示海马连接的拓扑模式存在重大差异。相比之下，对生物素填充轴突的定量分析表明，在trkB（-/-）和trkC（-/-）小鼠中，连合和内嗅传入神经的轴突侧支数量减少了21 - 49%，轴突膨体密度降低了8 - 17%。此外，电子显微镜分析表明，trkB（-/-）和trkC（-/-）小鼠的突触接触密度较低，突触前终末存在重要的结构改变，如突触小泡密度降低。最后，免疫细胞化学研究显示，负责突触小泡胞吐和神经递质释放的突触相关蛋白（v - SNAREs和t - SNAREs）表达减少，尤其是在trkB（-/-）小鼠中。我们得出结论，trkB和trkC基因对于海马连接的长入或层特异性靶向并非必不可少，尽管缺乏这些受体会导致轴突分支和突触密度降低，这表明TrkB和TrkC受体在体内中枢神经系统突触输入的发育调节中发挥作用。数据还表明，编码突触蛋白的基因可能是TrkB和TrkC信号通路的靶点。
