Objective: To determine whether overexpression of Bcl-2 is sufficient to support optic nerve regeneration in mouse in the early neonatal stage, when the brain environment remains permissive for axonal growth, and whether regenerating axons follow the guidance cues to reach the appropriate targets in the brain.
Methods: Optic nerve crush was performed in wild-type (C57BL/6J) and Bcl-2 transgenic mouse pups at 3 days after birth (P3). To assess optic nerve regeneration, mouse pups were allowed to survive for 1-4 days post-surgery. An anterograde tracer-cholera toxin B subunit (CTB) and anti-GAP-43 immunofluorescence were applied to reveal regenerating axons.
Results: In wild-type mice, severed optic nerves failed to regenerate, and most severed axons retracted within 24 hour post-surgery. In contrast, in all of the Bcl-2 transgenic mice examined, optic nerves regenerated robustly over long distances and reached their brain targets in 4 days after optic nerve crush. However, the majority of regenerating axons, appearing to follow the existing optic tract and visual pathways, entered the visual targets that were ipsilateral to the nerve injury. Much less number of regenerating axons innervated their contralateral visual targets.
Conclusion: Overexpression of Bcl-2 is sufficient to support the intrinsic growth mechanisms of retinal ganglion cell axons and enables robust optic nerve regeneration over long distances in vivo if the injury is incurred at 3 days after birth. However, the regenerating axons appear to follow the existing CNS pathways and innervate uninjured brain areas.