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* Center for Neuronal Survival and In this report we examine the biological and
molecular basis of the control of sympathetic neuron
differentiation and survival by NGF and neurotrophin-3
(NT-3). NT-3 is as efficient as NGF in mediating neuritogenesis and expression of growth-associated genes in
NGF-dependent sympathetic neurons, but it is 20-40fold less efficient in supporting their survival. Both
NT-3 and NGF induce similar sustained, long-term activation of TrkA, while NGF is 10-fold more efficient
than NT-3 in mediating acute, short-term TrkA activity. At similar acute levels of TrkA activation, NT-3 still
mediates neuronal survival two- to threefold less well
than NGF. However, a mutant NT-3 that activates
TrkC, but not TrkA, is unable to support sympathetic
neuron survival or neuritogenesis, indicating that NT3-mediated TrkA activation is necessary for both of
these responses. On the basis of these data, we suggest that NGF and NT-3 differentially regulate the TrkA receptor both with regard to activation time course and
downstream targets, leading to selective regulation of
neuritogenesis and survival. Such differential responsiveness to two ligands acting through the same Trk receptor has important implications for neurotrophin function throughout the nervous system.
Brain Tumor Research Center, Montreal Neurological Institute, McGill University,
Montreal, PQ, Canada H3A 2B4; and § The Open University, Mylton Keynes, United Kingdom
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