The anti-p75 antibody, MC192, and brain-derived neurotrophic factor inhibit nerve growth factor-dependent neurite growth from adult sensory neurons.

We have investigated nerve growth factor-dependent neurite growth from adult sensory neurons using the compartmented culture system. The requirement of both TrkA and the p75 neurotrophin receptors in neurite growth was examined using several experimental interventions. Inhibition of TrkA activation using K252a resulted in a total block of distal neurite extension into nerve growth factor-containing compartments. Brain-derived neurotrophic factor and the anti-p75 monoclonal antibody MC192 have been shown to interfere with the binding of nerve growth factor to p75. Brain-derived neurotrophic factor, which binds p75 but not TrkA, competes with nerve growth factorforp75, while the anti-p75 antibody MC192 has been shown to decrease the interaction of nerve growth factor with TrkA. The addition of brain-derived neurotophic factor to nerve growth factor-containing distal compartments inhibited, but did not totally block, distal neurite extension. MC192, on the other hand, totally inhibited nerve growth factor-dependent neurite growth. To test whether MC192 and brain-derived neurotrophic factor might be influencing Trk activation, TrkA phosphorylation was examined biochemically. Both compounds were found to attenuate nerve growth factor-induced Trk phosphorylation, although neither inhibited the activation completely. The possibility that MC192 or brain-derived neurotrophic factor might activate p75 signaling directly (and potentially antagonize TrkA signaling) was also investigated. This was assessed by quantitating the activation and nuclear translocation of the transcription factor NFkB using immunocytochemistry. Only treatment with the anti-p75 antibody MC192 resulted in prolonged and significant increase in the number of neurons displaying nuclear staining for NFkB. Our results demonstrate that both TrkA and p75 play a role in neurite growth response to nerve growth factor, and further suggest that any alteration in optimal TrkA-p75 interactions, or direct activation of p75 at the expense of TrkA, results in an inhibition of nerve growth factor-dependent neurite growth in adult sensory neurons.