Development of Survival Responsiveness to Brain-Derived Neurotrophic Factor, Neurotrophin 3 and Neurotrophin 4/5, But Not to Nerve Growth Factor, in Cultured Motoneurons from Chick Embryo Spinal Cord

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Abstract
During embryonic development, most neuronal populations undergo a process usually referred to as naturally occurring neuronal death. For motoneurons (MTNs) of the lumbar spinal cord of chick embryos, this process takes place in a well defined period of time, between embryonic days 6 and 10 (E6–E10). Neurotrophins (NTs) are the best characterized family of neurotrophic factors and exert their effects through activation of their specific Trk receptors. In vitro andin vivo studies have demonstrated that rodent motoneurons survive in response to BDNF, NT3, and NT4/5. In contrast, the trophic dependencies of chicken motoneurons have been difficult to elucidate, and various apparently conflicting reports have been published. In the present study, we describe how freshly isolated motoneurons from E5.5 chick embryos did not respond to any neurotrophinin vitro. Yet, because motoneurons were maintained alive in culture in the presence of muscle extract, they developed a delayed specific survival response to BDNF, NT3, and NT4/5 that is clearly dose-dependent, reaching saturation at doses of 100 pg/ml. This trophic response correlated with increasing expression of the corresponding functional receptors TrkB and TrkC. Moreover, TrkB receptor is able to become autophosphorylated and to activate classical intracellular signaling pathways such as the extracellular signal-regulated protein kinase when it is stimulated with its cognate ligand BDNF. Therefore, our results reconcile the reported differences betweenin vivo and in vitro studies on the ability of chicken MTNs to respond to some members of the neurotrophin family of trophic factors.