Rit promotes MEK-independent neurite branching in human neuroblastoma cells.
Nerve growth factor (NGF) is a well-established trophic factor of sympathetic and sensory neurons during development. NGF is, however, little known to be required for the maintenance or regulation of differentiated phenotypes of matured peripheral neurons. Since trophic factors, including NGF, are currently known to be secreted by non-neuronal cells, like Schwann cells and fibroblasts, a highly pure-neuron culture is required to assess the direct action of trophic factors on neurons. We have developed a single-neuron culture from neonatal and adult rat dorsal root ganglia in serum-free conditions, and estimated the primary effect of NGF on the morphological geometry of sensory neurons. We found that NGF promoted the neurite length of neonatal sensory neurons, rather than promoting arborization (branching of neurites), while in adult matured neurons NGF significantly enhanced neurite arborizations, rather than the maximal neurite extension, distance from the cell soma to the maximum margin of the territory of neurite extension. Total neurite length, the summed length of all neurites per neuron was significantly increased by NGF in both neonatal and adult neurons. NGF also increased the size of neuronal soma independent of neuronal maturation. Neonatal sensory neurons tended to die in 1 week despite the presence of NGF. In contrast, some adult sensory neurons were alive for more than 2 weeks in the absence of NGF. These results indicate that NGF more than simply accelerates a pre-existing developmental program in the matured stage, and that the promotion of neurite arborization by NGF in adult sensory neurons suggests that NGF may have some role in peripheral nerve regeneration via promotion of axonal sprouting.