The extracellular domain of p75(NTR) is necessary to inhibit neurotrophin-3 signaling through TrkA

The TrkA receptor is activated primarily by nerve growth factor (NGF), but it can also be activated by high concentrations of neurotrophin 3 (NT-3). T he pan-neurotrophin receptor p75(NTR) strongly inhibits activation of TrbA by NT-3 but not by NGF. To examine the role of p75(NTR) in regulating the s pecificity of TrkA signaling, we expressed both receptors in Xenopus oocyte s. Application of NGF or NT-3 to oocytes expressing TrkA alone resulted in efflux of Ca-45(2+) by a phospholipase C-gamma -dependent pathway. Coexpres sion of p75(NTR) with TrkA inhibited Ca-45(2+) efflux in response to NT-3 b ut not NGF. The inhibitory effect on NT-3 activation of TrkA increased with increasing expression of p75(NTR). Coexpression of a truncated p75(NTR) re ceptor lacking all but the first 9 amino acids of the cytoplasmic domain in hibited NT-3 stimulation of Ca-45(2+) efflux, whereas coexpression of an ep idermal growth factor receptor/p75(NTR) chimera (extracellular domain of ep idermal growth factor receptor with transmembrane and cytoplasmic domains o f p75(NTR)) did not inhibit NT-3 signaling through TrkA. These studies demo nstrated that the extracellular domain of p75(NTR) was necessary to inhibit NT-3 signaling through TrkA. Remarkably, p75(NTR) binding to NT-3 was not required to prevent signaling through TrkA, since occupying p75(NTR) with b rain-derived neurotrophic factor or anti-p75 antibody (REX) did not rescue the ability of NT-3 to activate Ca-45(2+) efflux. These data suggested a ph ysical association between TrkA and p75(NTR). Documenting this physical int eraction, we showed that p75(NTR) and TrkA could be coimmunoprecipitated fr om Xenopus oocytes. Our results suggest that the interaction of these two r eceptors on the cell surface mediated the inhibition of NT-3-activated sign aling through TrkA.