Reciprocal modulation of TrkA and p75^NTR affinity states is mediated by direct receptor interactions

Abstract

Equilibrium binding of 125I-nerve growth factor (125I-NGF) to cells coexpressing the tyrosine kinase receptor A (TrkA) and common neurotrophin receptor (p75NTR), cells coexpressing both receptors where p75NTR is occupied, and cells expressing only p75NTR, 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 PC12nnr5 cells suggested that liganded and unliganded p75NTR induce a higher affinity state within TrkA, while TrkA induces a lower affinity state within p75NTR. These data are consistent with receptor allosterism, and prompted a search for TrkA/p75NTR complexes in the absence of NGF. Chemical crosslinking studies revealed high molecular weight receptor complexes that specifically bound 125I-NGF, and were immunoprecipitated by antibodies to both receptors. The heteroreceptor complex of TrkA and p75NTR 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.

Zinc inhibits p75NTR-mediated apoptosis in chick neural retina

“…When co-expressed with TrkA, p75 NTR serves a secondary role of promoting the binding of NGF to TrkA by influencing the allosteric properties of this receptor, possibly through the formation of a heteroreceptor complex. 17 This concept is supported by studies which demonstrated that both receptors can be co-immunoprecipitated with antibodies to both p75 NTR and TrkA when PC12 cells are incubated with EDC/SNHS prior to the addition of 125 I-NGF. 17 In ED5.5 chick retinal cells, we found that p75 NTR displayed sub-nanomolar binding affinity measured by dissociation equilibrium analysis and showed that p75 NTR binding in retinal cells displayed higher affinity than in PC12 cells by displacement studies.…”

“…17 This concept is supported by studies which demonstrated that both receptors can be co-immunoprecipitated with antibodies to both p75 NTR and TrkA when PC12 cells are incubated with EDC/SNHS prior to the addition of 125 I-NGF. 17 In ED5.5 chick retinal cells, we found that p75 NTR displayed sub-nanomolar binding affinity measured by dissociation equilibrium analysis and showed that p75 NTR binding in retinal cells displayed higher affinity than in PC12 cells by displacement studies. Therefore, apoptosis mediated by the interaction of NGF and p75 NTR occurs when this receptor is in its high affinity conformation.…”

“…To confirm this observation we also determined the IC 50 value using various concentrations of unlabeled NGF (0.04 ± 40 nM) to inhibit 125 I-NGF (0.15 nM) from chemically cross-linking p75 NTR in retinal and in PC12 cells which express both p75 NTR and TrkA. 17 Results were evaluated with SDS ± PAGE and autoradiography (Figure 1b,c), followed by excision of bands and counting of radioactivity incorporated into both monomers and dimers of p75 NTR (Figure 1d). In both cell types, the displacement of 125 I-NGF from monomers and dimers did not differ statistically.…”

“…17 However, binding analysis of 125 I-NGF to p75 NTR has not been reported in cells that undergo cell death following this binding. In PC12 cells that express p75 NTR and TrkA, p75 NTR adopts a lower affinity state (K d &4.0 nM) than when p75 NTR is expressed alone (PC12 nnr5 cells; K d &0.2 nM).…”

“…15,16 When expressed in the absence of TrkA, p75 NTR binds NGF with a higher affinity than when expressed alone, however this parameter has not been tested in cells which undergo p75 NTR mediated apoptosis. 17 The signal transduction pathways of p75 NTR -mediated apoptosis have yet to be fully characterized, but are thought to involve the production of ceramide, 18 activation of c-jun kinase, 7 the accumulation of p53, 19 and activation of various caspases. 20 NGF interaction with p75 NTR also leads to the translocation of nuclear factor kB (NF-kB), but this effect is thought to be anti-apoptotic.…”