神经营养因子NT-3与其受体p75^（NTR）对称复合物的晶体结构

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。

Crystal Structure of the Neurotrophin-3 and P75NTR Symmetrical Complex

Neurotrophins （NTs） are important regulators for the survival, differentiation and maintenance of different peripheral and central neurons. NTs bind to two distinct classes of glycosylated receptor： the p75 neurotrophin receptor （p75^NTR） and tyrosine kinase receptors （Trks）. Whereas p75^NTR binds to all NTs, the Trk subtypes are specific for each NT. The question of whether NTs stimulate p75^NTR by inducing receptor homodimerization is still under debate. Here we reported the 2.6A resolution crystal structure of neurotrophin-3 （ NT-3 ） complexed to the ectodomain of glycosylated p75^NTR.In contrast to the previously reported asymmetric complex structure, which contained a dimer of nerve growth factor （NGF） bound to a single ectodomain of deglycosylated p75^NTR, we showed that NT-3 formed a central homodimer around which two glycosylated p75^NTR molecules bind symmetrically. Symmetrical binding occurs along the NT-3 interfaces, resulting in a 2：2 ligand-receptor cluster. A comparison of the symmetrical and asymmetric structures reveals significant differences in ligand-receptor interactions and p75^NTR conformations.Biochemical experiments indicate that both NT-3 and NGF bind to p75^NTR with 2：2 stoichiometry in solution, whereas the 2 ： 1 complexes are the result of artificial deglycosylation. We therefore proposed that the symmetrical 2：2 complex reflected a native state of p75^NTR activation at the cell surface. These results provide a model for NTs-p75^NTR recognition and signal generation, as well as insights into coordination between p75^NTR and Trks.