Reciprocal modulation of TrkA and p75NTR affinity states is mediated by direct receptor interactions

Equilibrium binding of I-125-nerve growth factor (I-125-NGF) to cells coexpressing the tyrosine kinase receptor A (TrkA) and common neurotrophin receptor (p75(NTR)), cells coexpressing both receptors where p75(NTR) is occupied, and cells expressing only p75(NTR), revealed reciprocal modulation of receptor affinity states, Analysis of receptor affinity states in PC12 cells, PC12 cells in the presence of brain-derived neurotrophic factor (BDNF), and PC12(nnr5) cells suggested that liganded and unliganded p75(NTR) induce a higher affinity state within TrkA, while TrkA induces a lower affinity state within p75(NTR). These data are consistent with receptor allosterism, and prompted a search for TrkA/p75(NTR) complexes in the absence of NGF. Chemical crosslinking studies revealed high molecular weight receptor complexes that specifically bound I-125-NGF, and were immunoprecipitated by antibodies to both receptors. The heteroreceptor complex of TrkA and p75(NTR) alters conformation and/or dissociates in the presence of NGF, as indicated by the ability of low concentrations of NGF to prevent heteroreceptor crosslinking. These data suggest a new model of receptor interaction, whereby structural changes within a heteroreceptor complex are induced by ligand binding.