The Insulin-Like Growth Factor 1 Receptor Is Essential for Axonal Regeneration in Adult Central Nervous System Neurons

Axonal regeneration is an essential condition to re-establish functional neuronal connections in the injured adult central nervous system (CNS) but efficient regrowth of severed axons has proven to be very difficult to achieve. Although significant progress has been made in identifying the intrinsic and extrinsic mechanisms involved many aspects remain unresolved. Axonal development in embryonic CNS (hippocampus) requires the obligate activation of the insulin-like growth factor 1 receptor (IGF-1R). Based on known similarities between axonal growth in fetal compared to mature CNS we decided to examine the expression of the IGF-1R using an antibody to the beta gc subunit or a polyclonal anti-peptide antibody directed to the IGF-R (C20) in an in vitro model of adult CNS axonal regeneration namely retinal ganglion cells (RGC) derived from adult rat retinas. Expression of both beta gc and the beta subunit recognized by C20 antibody were low in freshly isolated adult RGC but increased significantly after 4 days in vitro. As in embryonic axons beta gc was localised to distal regions and leading growth cones in RGC. IGF-1R-beta gc co-localised with activated p85 involved in the hosphatidylinositol-3 kinase (PI3K) signaling pathway upon stimulation with IGF-1. Blocking experiments using either an antibody which neutralises IGF-1R activation shRNA designed against the IGF-1R sequence or the PI3K pathway inhibitor LY294002 all significantly reduced axon regeneration from adult RGC in vitro (similar to 40% RGC possessed axons in controls vs 2-8% in the different blocking studies). Finally co-transfection of RGC with shRNA to silence IGF-1R together with a vector containing a constitutively active form of downstream PI3K (p110) fully restored axonal outgrowth in vitro. Hence these data demonstrate that axonal regeneration in adult CNS neurons requires re-expression and activation of IGF-1R and targeting this system may offer new therapeutic approaches to enhancing axonal regeneration following trauma.