Rydén M, Murray-Rust J, Glass D, Ilag L L, Trupp M, Yancopoulos G D, McDonald N Q, Ibáñez C F
Department of Medical Biochemistry and Biophysics, Karolinska Institute, Stockholm, Sweden.
EMBO J. 1995 May 1;14(9):1979-90. doi: 10.1002/j.1460-2075.1995.tb07190.x.
The neurotrophins mediate their effects through binding to two classes of receptors, a tyrosine kinase receptor, member of the Trk family, and the low-affinity neurotrophin receptor, p75LNGFR, of as yet undefined signalling capacity. The need for a two-component receptor system in neurotrophin signalling is still not understood. Using site-directed mutagenesis, we have identified positively charged surfaces in BDNF, NT-3 and NT-4 that mediate binding to p75LNGFR. Arg31 and His33 in NT-3, and Arg34 and Arg36 in NT-4, located in an exposed hairpin loop, were found to be essential for binding to p75LNGFR. In BDNF, however, positively charged residues critical for p75LNGFR binding (Lys95, Lys96 and Arg97) were found in a spatially close but distinct loop region. Models of each neurotrophin were built using the coordinates of NGF. Analysis of their respective electrostatic surface potentials revealed similar clusters of positively charged residues in each neurotrophin but with differences in their precise spatial locations. Disruption of this positively charged interface abolished binding to p75LNGFR but not activation of cognate Trk receptors or biological activity in Trk-expressing fibroblasts. Unexpectedly, loss of low-affinity binding in NT-4, but not in BDNF or NT-3, affected receptor activation and biological activity in neuronal cells co-expressing p75LNGFR and TrkB, suggesting a role for p75LNGFR in regulating biological responsiveness to NT-4. These findings reveal a possible mechanism of ligand discrimination by p75LNGFR and suggest this receptor may selectively modulate the biological actions of specific neurotrophin family members.
神经营养因子通过与两类受体结合来介导其效应，一类是酪氨酸激酶受体（Trk家族成员），另一类是低亲和力神经营养因子受体p75LNGFR，其信号传导能力尚不清楚。神经营养因子信号传导中双组分受体系统的必要性仍不明确。我们利用定点诱变技术，在脑源性神经营养因子（BDNF）、神经营养因子-3（NT-3）和神经营养因子-4（NT-4）中鉴定出介导与p75LNGFR结合的带正电荷表面。位于暴露发夹环中的NT-3中的精氨酸31和组氨酸33以及NT-4中的精氨酸34和精氨酸36被发现对于与p75LNGFR结合至关重要。然而，在BDNF中，对p75LNGFR结合至关重要的带正电荷残基（赖氨酸95、赖氨酸96和精氨酸97）位于空间上接近但不同的环区域。利用神经生长因子（NGF）的坐标构建了每个神经营养因子的模型。对它们各自的静电表面电位分析显示，每个神经营养因子中都有类似的带正电荷残基簇，但它们的确切空间位置存在差异。破坏这个带正电荷的界面消除了与p75LNGFR的结合，但不影响同源Trk受体激活或表达Trk的成纤维细胞中的生物活性。出乎意料的是，NT-4中低亲和力结合的丧失（而非BDNF或NT-3中的）影响了共表达p75LNGFR和TrkB的神经元细胞中的受体激活和生物活性，提示p75LNGFR在调节对NT-4的生物反应性中起作用。这些发现揭示了p75LNGFR区分配体的一种可能机制，并表明该受体可能选择性地调节特定神经营养因子家族成员的生物学作用。
