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神经营养因子(NTs)是一类对神经元发育、存活以及凋亡起重要作用的蛋白质,包括神经生长因子(NGF)和神经营养因子-3(NT-3)等。神经营养因子可结合两类不同的糖基化膜受体——p75神经营养受体(p75NTR)和酪氨酸激酶受体(Trk),其中p75NTR可与所有NTs结合,而Trk则通过不同亚型与不同NTs特异性结合。神经营养因子能否通过诱导p75NTR形成同源二聚体来激活受体一直存在争议。本研究利用X-射线晶体学方法获得了NT-3与p75NTR胞外区复合物的2.6分辨率的三维精细结构。结果发现,与以往报道的NGF与p75NTR形成非对称结构不同,NT-3以2:2的比例同两个糖基化p75NTR分子发生对称结合形成同源二聚体。对称性和不对称结构的对比分析显示,二者在配体-受体作用和p75NTR构象上有显著不同。生化实验研究显示,溶液中NT-3和NGF都是以2:2的比例与p75NTR结合,而2:1的结合是人为去糖基化的非天然结果。这显示,2:2对称结合是神经营养因子在细胞表面激活p75NTR的本来状态。这些研究结果为神经营养因子与p75NTR信号转导的分子机制提供了更加深入与全面的认识。同时,为治疗人类神经系统退行性疾病的药物设计与开发提供了精确可靠的三维结构数据。
Neurotrophic factors (NTs) are a class of proteins that play an important role in neuronal development, survival and apoptosis, including nerve growth factor (NGF) and neurotrophin-3 (NT-3). Neurotrophic factors can bind to two different types of glycosylated membrane receptors, the p75 neurotrophic receptor (p75NTR) and the tyrosine kinase receptor (Trk), of which p75NTR binds to all NTs while Trks pass through different subtypes Type specific binding to different NTs. Whether neurotrophic factors activate receptors by inducing p75NTR to form homodimers remains controversial. In this study, a 2.6-resolution three-dimensional fine structure of the extracellular region of NT-3 and p75NTR was obtained by X-ray crystallography. As a result, it was found that, unlike the previously reported asymmetric structure of NGF and p75NTR, NT-3 symmetrically binds to two glycosylated p75NTR molecules in a ratio of 2: 2 to form homodimers. A comparative analysis of the symmetry and asymmetry structures revealed a significant difference between the ligand-receptor and p75NTR conformations. Biochemical experimental studies show that both NT-3 and NGF are in solution in a ratio of 2: 2 to p75NTR whereas 2: 1 binding is a non-natural result of human deglycosylation. This shows that 2: 2 symmetric binding is a neurotrophic factor that activates the native state of p75NTR on the cell surface. These findings provide a more in-depth and comprehensive understanding of the molecular mechanisms of neurotrophic factors and p75NTR signaling. At the same time, accurate and reliable three-dimensional structural data are provided for the design and development of drugs for treating the degenerative diseases of human nervous system.