Neurite growth promotion by nerve growth factor and insulin‐like growth factor‐1 in cultured adult sensory neurons: Role of phosphoinositide 3‐kinase and mitogen activated protein kinase

Abstract

Although neurons of the PNS no longer require neurotrophins such as Nerve Growth Factor (NGF) for their survival, such factors are involved in regulating axonal sprouting and regeneration after injury. In addition to the neurotrophin receptors, sensory neurons are reported to express IGF-1, EGF and FGF receptors. To investigate the influence of growth factors in addition to NGF, we examined the effects of IGF-1 EGF and FGF on neurite growth from adult rat dorsal root ganglion sensory neurons in both dissociated cultures and in compartmented cultures. As expected, NGF elicited robust neuritic growth in both the dissociated and compartmented cultures. The growth response to IGF-1 was similar, although 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. This potentiation was not seen when these factors were provided along with NGF in the side compartments of compartmented cultures, or in the dissociated cultures. To determine the contribution of signaling intermediates downstream of receptor activation, we used inhibitors of the potential effectors and Western blotting. The PI 3-kinase inhibitor, LY294002 attenuated neurite growth evoked by NGF, IGF and EGF in dissociated cultures, although the MAP kinase kinase (MEK) inhibitor PD098059 diminished the growth in only IGF. Immunoprecipitation and Western blotting results demonstrated differential activation of MAPK, PI 3-kinase, PLCgamma1 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 plays an important role in neuritogenesis.

Acute neuroprotective synergy of erythropoietin and insulin-like growth factor I

“…The catalytic portion of PI3-K is liberated when the p85 subunit is phosphorylated, and then the catalytic portion associates with a variety of growth factor receptors or receptor-associated kinases (33,40). Previous studies have shown that IGF-I leads to p85 phosphorylation in neurons (51), which is likely mediated by the IGF-I receptor (52). Similarly, we show here in neurons that EPO͞EPO-R binding promotes association of the EPO-R with p85 and Jak2.…”

Large-scale in vivo femtosecond laser neurosurgery screen reveals small-molecule enhancer of regeneration

“…We also showed that other kinase inhibitors of PI3K, PKA, MAPKKK, and ROCK did not affect regeneration in a neuron type that is otherwise strongly affected by staurosporine. However, these kinase inhibitors affect neurite growth in other experimental models (45)(46)(47)(48)(49). The lack of response to these other types of kinase inhibitors could be due to the differences among neuronal types, the ineffectiveness of these chemicals in our experimental system, or the absence of inhibitory myelin sheath in C. elegans.…”

Remote neurotrophic support of epidermal nerve fibres in experimental diabetes

“…PI 3-K and Akt may be more important for sensory neurite extension and branching than MAPKdependent signalling, which may, in fact, be inhibitory to sensory neuritic branching [36], although this is controversial [37]. Elegant compartmentalised culture studies have demonstrated that IGF-1 (but neither fibroblast growth factor nor endothelial growth factor) potentiates distal neurite growth into side compartments only when IGF-1 is applied to cell bodies [37]. Clearly, insulin and IGF-1 stabilise the sensory neuron and potentiate its well being following insult or injury.…”