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Jian Zhong, Kaijie Ma, Zhigang He, Kevin J O’Donovan; B-RAF signaling promotes optic nerve regeneration. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1439.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the role of intracellular B-RAF signaling in promoting regeneration in the optic nerve after injury.
We genetically engineered mice to study gain of B-RAF function in promoting axon regeneration. These mice bear in the endogenous B-RAF locus a construct that expresses a kinase-activated B-RAF (kaB-RAF) upon Cre recombination. Adult mice were injected with AAV2-Cre into the vitreous of the eye to activate B-RAF signaling in the RGCs, and 14 days later were subjected to proximal crush lesion of the optic nerve. Outgrowing axons were labeled by CTB-Alexa anterograde tracers. Optic nerves were dissected, sectioned and imaged two weeks after the injury. The same procedures were performed with mice lacking the canonical B-RAF effectors MEK1 and MEK2, as well as with mice homozygous for the conditional deletion of PTEN (Park et al., 2008).
In mice expressing kaB-RAF in RGCs we observed robust axon regeneration up to 3.5 mm past the lesion site. Comparing to the PTEN loss-of-function model, the number of fibers growing past the lesion was almost 4fold higher in the kaB-RAF model. Average length of regeneration was similar in both models. Blocking of the canonical B-RAF signaling pathway by conditional elimination of MEK1 and MEK2 completely abolished axon regeneration induced by kaB-RAF. Concomitant activation of both B-RAF and the PI3 kinase-mTOR pathway (by removal of PTEN) drove axon growth beyond a simple additive effect.
We conclude that activation of B-RAF drives substantial axon regeneration in the crushed optic nerve via canonical B-RAF - MEK signaling.
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