Abstract
Purpose :
Injuries of optic nerve (ON) in optic neuropathies often result in visual function deficits due to the failure of retinal ganglion cells (RGCs) axons to regenerate. Deletion of phosphatase and tensin homolog (PTEN), the negative regulator of phosphatidylinositol 3-kinase (PI3K), induces RGC axon/ON regeneration, potentially through activation of PI3K. AKT is the major effector of the PI3K pathway; it is phosphorylated by PDK1 and mTOR complex 2 (mTORC2) at two different amino acids. AKT is also the upstream regulator of mTOR complex 1 (mTORC1) and GSK3β. Here we elucidate how AKT coordinates the signaling of these two mTOR complexes and GSK3β in adult mouse RGCs to influence ON regeneration in vivo
Methods :
By exploiting the anatomical and technical advantages of adeno-associated virus (AAV)-mediated gene manipulation in RGCs and crushed ON as an in vivo mouse model, we conducted an extensive molecular dissection of the PI3K-PDK1-AKT-mTORC1/GSK3β pathway and the PI3K-mTORC2-AKT pathway specifically in RGCs. 5-7 week old male mice with C57BL/6 background were used in this study.
Results :
We found that the predominant AKT isoform in brain and retina, AKT3, induces much more robust axon regeneration than AKT1. We determined definitively by deletion of the key component of mTORC1, RAPTOR, that mTORC1 is necessary for AKT3-induced axon regeneration. We also identified GSK3β as a key molecule for inhibiting axon regeneration, and found that deletion of GSK3β is sufficient to induce axon regeneration. Surprisingly, phosphorylation of T305 and S472 of AKT3 play opposite roles in axon regeneration; mTORC2 inhibits axon regeneration through phosphorylation of AKT3-S472.
Conclusions :
Our study revealed a neuron-intrinsic balancing mechanism involving AKT as the nodal point of PI3K, mTORC1/2 and GSK3β signaling, which coordinates both positive and negative cues to regulate adult RGC axon regeneration.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.