The effects of neurotrophins on neurite growth in cultured adult sensory neurons

The compartmented culture method was used to study the regulation of adult sensory neurite growth by neurotrophins. We examined the effects of the neurotrophins nerve growth factor (NGF), neurotrophin-3 (NT3), and brain derived neurotrophic factor (BDNF) on neurite elongation from adult rat dorsal root ganglion (DRG) neurons. Initial proximal neurite growth (within center compartments) did not require neurotrophins. Subsequent elongation into distal compartments resulted from NGF but not NT3 or BDNF treatment. After neurites had extended into distal compartments, treatment with anti-NGF resulted in the cessation of growth with no significant neurite retraction. After axotomy of growing neurites in distal compartments, NGF was necessary to support regrowth. In the same paradigm, both NT3 and BDNF did not result regrowth. The results demonstrated that unlike in vivo nerve regeneration, in vitro regrowth does require NGF. -- Given the above results, the next focus of study was to determine the individual contributions ofthe NGF receptors to the growth response. The requirement of both TrkA and the p75 neurotrophin receptor in neurite growth was examined using several experimental interventions. As expected, inhibition of TrkA activation using K252a totally blocked distal neurite extension into NGF containing compartments. Results showed using BDNF to interfere with NGF binding to p75, found that the addition of BDNF to NGF containing distal compartments reduced distal neurite extension. In contrast MC192 which alters the interaction of NGF with p75, completely inhibited NGF dependent neurite growth. Both compounds were found to partially attenuate NGF induced TrkA phosphorylation. However only MC192 appeared to activate p75 based on immunocytochemical evidence showing nuclear localization of NFk-B. -- These results demonstrate that both TrkA and p75 play a role in neurite growth response to NGF. Furthermore our evidence suggests that any alteration in optimal TrkA-p75 interactions, or direct activation of p75 at the expense of TrkA, results in an inhibition of NGF-dependent neurite growth. -- NGF/TrkA signaling has a number of common pathways in which to elicit various physiological responses. Many of the same pathways are used by other growth factors that also are capable of producing neurite growth in adult sensory neurons. To investigate the influence of other growth factors in addition to NGF, the effects of IGF-1 EGF and FGF on neurite growth from adult rat dorsal root ganglion (DRG) neurons were examined. As expected, NGF elicited robust neuritic growth in both the dissociated and compartmented cultures. The growth response to IGF-1 was similar. There was minimal neurite growth in response to EGF or FGF. In addition, IGF-1 (but neither FGF nor EGF), when applied to cell bodies in compartmented cultures, potentiated the distal neurite growth into NGF- containing side compartments. -- In order to determine the contribution of signaling intermediates downstream of receptor activation, we used pharmacological inhibitors and western blotting. The PI 3- kinase inhibitor, LY294002 attenuated neurite growth evoked by NGF, IGF and EGF in dissociated cultures, while the MEK inhibitor PD98059 only diminished growth in IGF treated cultures. Immunoprecipitation and western blotting results demonstrated differential activation of MAPK, PI 3-kinase, PLCyl and SNT by the different factors. Activation of PI 3-kinase and SNT by both NGF and IGF-1 correlated with their effects on neurite growth. These results support the hypothesis that the PI 3-kinase pathway, and the SNT protein play an important role in neurogenesis. -- In conclusion neurite growth of adult DRG neurons is mediated by NGF in vitro and required TrkA activation. In comparison these results indicate that inhibition of NGF binding to p75 partially inhibits growth while activation of the receptor abolishes the response. Further assessment of NGF-TrkA signaling pathways, using other growth factors for comparison (i.e. IGF-1 and EGF) indicate that PI 3-kinase and SNT signaling intermediates are important contributors to the growth process. These experiments are a part of a larger focus on NGF and its role in neurite growth in neurons (adult DRG neurons) that do not require this factor for survival. Furthermore these studies will hopefully contribute to elucidating the processes involved in neurite growth.