The necessary role of mTORC1 in central nervous system axon regeneration

The change of microglia numbers within the mice retina, optic nerve and chiasm following intravitreal AAV2-GFP injection

PURPOSE

To explore the optimized concentration of AAV2-GFP for sparse transfection of retinal ganglion cells (RGCs) and optic nerve (ON), and to examine the changes of microglial morphology and distribution in the retina, optic nerve and chiasm after injection.

METHODS

We defined the optimal concentration of AAV2-GFP for sparse labeling of RGCs and axons in WT mice. We further explored the changes of microglial morphology and distribution in the retina, optic nerve and chiasm after intravitreal injection in CX3CR1+/GFP mice.

RESULTS

14 days after intravitreal injection of AAV2-GFP, live imaging of the retina showed that fundus fluorescence was very strong and dense at 2.16 × 10¹¹ VG/retina, 2.16 × 10¹⁰ VG/retina, 2.16 × 10⁹ VG/retina. RGCs were sparsely marked at a concentration 1:1000 (2.16 × 10⁸ VG/retina) and fundus fluorescence was weak. The transfected RGCs and axons were unevenly distributed in the retina and significantly more RGCs were transfected near the injection site of AAV2-GFP compared to the other sites of the flat-mounted retina. Microglia density increased significantly in the retina and part of optic nerve, but not in the optic chiasm. The morphology of microglia was largely unchanged.

CONCLUSIONS

AAV2-GFP was highly efficient and the optimal concentration of sparsely labeled RGCs was 1:1000 (2.16 × 10⁸ VG/retina). After intravitreal injection of AAV2-GFP, the number of microglia increased partly. The morphology of microglia was comparable.

Lab review: Molecular dissection of the signal transduction pathways associated with PTEN deletion-induced optic nerve regeneration

BACKGROUND

Permanent loss of vital functions after central nervous system (CNS) injury occurs in part because axons in the adult mammalian CNS do not regenerate after injury. PTEN was identified as a prominent intrinsic inhibitor of CNS axon regeneration about 10 years ago. The PTEN negatively regulated PI3K-AKT-mTOR pathway, which has been intensively explored in diverse models of axon injury and diseases and its mechanism for axon regeneration is becoming clearer.

OBJECTIVE

It is timely to summarize current knowledge about the PTEN/AKT/mTOR pathway and discuss future directions of translational regenerative research for neural injury and neurodegenerative diseases.

METHODS

Using mouse optic nerve crush as an in vivo retinal ganglion cell axon injury model, we have conducted an extensive molecular dissection of the PI3K-AKT-mTORC1/mTORC2 pathway to illuminate the cross-regulating mechanisms in axon regeneration.

RESULTS

AKT is the nodal point that coordinates both positive (PI3K-PDK1-pAKT-T308) and negative (PI3K-mTORC2-pAKT-S473) signals to regulate adult CNS axon regeneration through two parallel pathways, activating mTORC1 and inhibiting GSK3β. However, mTORC1/S6K1-mediated feedback inhibition after PTEN deletion prevents potent AKT activation.

CONCLUSIONS

A key permissive signal from an unidentified AKT-independent pathway is required for stimulating the neuron-intrinsic growth machinery. Future studies into this complex neuron-intrinsic balancing mechanism involving necessary and permissive signals for axon regeneration is likely to lead to safe and effective regenerative strategies for CNS repair.

AKT-dependent and -independent pathways mediate PTEN deletion-induced CNS axon regeneration

Phosphatase and tensin homolog (PTEN) acts as a brake for the phosphatidylinositol 3-kinase–AKT–mTOR complex 1 (mTORC1) pathway, the deletion of which promotes potent central nervous system (CNS) axon regeneration. Previously, we demonstrated that AKT activation is sufficient to promote CNS axon regeneration to a lesser extent than PTEN deletion. It is still questionable whether AKT is entirely responsible for the regenerative effect of PTEN deletion on CNS axons. Here, we show that blocking AKT or its downstream effectors, mTORC1 and GSK3β, significantly reduces PTEN deletion-induced mouse optic nerve regeneration, indicating the necessary role of AKT-dependent signaling. However, AKT is only marginally activated in PTEN-null mice due to mTORC1-mediated feedback inhibition. That combining PTEN deletion with AKT overexpression or GSK3β deletion achieves significantly more potent axonal regeneration suggests an AKT-independent pathway for axon regeneration. Elucidating the AKT-independent pathway is required to develop effective strategies for CNS axon regeneration.

Coordination of Necessary and Permissive Signals by PTEN Inhibition for CNS Axon Regeneration

In the nearly 10 years since PTEN was identified as a prominent intrinsic inhibitor of CNS axon regeneration, the PTEN negatively regulated PI3K-AKT-mTOR pathway has been intensively explored in diverse models of axon injury and diseases and its mechanism for axon regeneration is becoming clearer. It is therefore timely to summarize current knowledge and discuss future directions of translational regenerative research for neural injury and neurodegenerative diseases. Using mouse optic nerve crush as an in vivo retinal ganglion cell axon injury model, we have conducted an extensive molecular dissection of the PI3K-AKT pathway to illuminate the cross-regulating mechanisms in axon regeneration. AKT is the nodal point that coordinates both positive and negative signals to regulate adult CNS axon regeneration through two parallel pathways, activating mTORC1 and inhibiting GSK3ββ. Activation of mTORC1 or its effector S6K1 alone can only slightly promote axon regeneration, whereas blocking mTORC1 significantly prevent axon regeneration, suggesting the necessary role of mTORC1 in axon regeneration. However, mTORC1/S6K1-mediated feedback inhibition prevents potent AKT activation, which suggests a key permissive signal from an unidentified AKT-independent pathway is required for stimulating the neuron-intrinsic growth machinery. Future studies into this complex neuron-intrinsic balancing mechanism involving necessary and permissive signals for axon regeneration is likely to lead eventually to safe and effective regenerative strategies for CNS repair.

mTORC1 is necessary but mTORC2 and GSK3β are inhibitory for AKT3-induced axon regeneration in the central nervous system

Injured mature CNS axons do not regenerate in mammals. Deletion of PTEN, the negative regulator of PI3K, induces CNS axon regeneration through the activation of PI3K-mTOR signaling. We have conducted an extensive molecular dissection of the cross-regulating mechanisms in axon regeneration that involve the downstream effectors of PI3K, AKT and the two mTOR complexes (mTORC1 and mTORC2). We found that the predominant AKT isoform in CNS, AKT3, induces much more robust axon regeneration than AKT1 and that activation of mTORC1 and inhibition of GSK3β are two critical parallel pathways for AKT-induced axon regeneration. Surprisingly, phosphorylation of T308 and S473 of AKT play opposite roles in GSK3β phosphorylation and inhibition, by which mTORC2 and pAKT-S473 negatively regulate axon regeneration. Thus, our study revealed a complex neuron-intrinsic balancing mechanism involving AKT as the nodal point of PI3K, mTORC1/2 and GSK3β that coordinates both positive and negative cues to regulate adult CNS axon regeneration.

DOI:http://dx.doi.org/10.7554/eLife.14908.001

The central nervous system consists of the neurons that make up the brain and spinal cord. An important part of a neuron is the long, slender projection along which electrical signals travel, called the axon. In the central nervous system of mammals, damaged axons cannot regrow, which is why spinal injuries or optic nerve injuries can result in life-long neuronal deficits.

Recent studies have found that activating a particular signaling pathway in central nervous system neurons causes their axons to regenerate. A key protein in this pathway is called AKT. Several signaling cascades are triggered by AKT to regulate cell survival and growth, but it was not known how the different branches of the AKT pathway are involved in axon regeneration.

Miao, Yang et al. have now investigated AKT’s role in axon regeneration using a range of approaches to manipulate signaling in damaged mouse neurons. This revealed that a particular form of AKT (called AKT3) causes damaged axons to regenerate to a greater extent than other forms of this protein. This response depends on two parallel pathways: one in which AKT3 activates a protein complex called mTORC1, and one where AKT3 inhibits a protein called GSK3β. In addition, another protein complex called mTORC2, which is closely related to mTORC1, helps to inhibit the activity of AKT3 on GSK3β and hence inhibits axon regeneration.

These findings reveal that a complex balancing mechanism, with AKT at its center, coordinates the many signals that regulate axon regeneration. Future studies into this system could ultimately help to develop new treatments for brain and spinal injuries.

DOI:http://dx.doi.org/10.7554/eLife.14908.002