Long-distance axonal regeneration induced by CNTF gene transfer is impaired by axonal misguidance in the injured adult optic nerve.

The optic nerve crush injury is a well-accepted model to study the mechanisms of axonal regeneration after trauma in the CNS. The infection of retinal ganglion cells (RGCs) with an adeno-associated virus serotype 2 - ciliary neurotrophic factor (AAV2.CNTF) was previously shown to stimulate axonal regeneration. However, the transfection of axotomized neurons themselves may not be optimal to promote full axonal regeneration in the visual system. Here, we show that the release of CNTF by glial cells is a very powerful stimulus for optic fiber regeneration and RGC survival after optic nerve crush. After 8 weeks, long-distance regeneration of severed optic axons was induced by CNTF until and beyond the optic chiasm. Regenerated axons stayed for at least 6 months in the damaged optic nerve. Strikingly, however, many regenerated axons showed one or several sharp U-turns along their course, suggesting that guidance cues are missing and that long-distance axonal regeneration is limited by the return of the growing axons toward the retina. Even more surprisingly, massive axonal sprouting was observed within the eye, forming a dense plexus of neurites at the inner surface of the retina. These results indicate that massive stimulation of the neuronal growth program can lead to aberrant growth; the absence of local regulatory and guidance factors in the adult, injured optic nerve may therefore represent a major, so far underestimated obstacle to successful axon regeneration.