Regeneration of lesioned rat optic nerve fibers is improved after neutralization of myelin-associated neurite growth inhibitors

Abstract

Optic nerve axons do not regenerate after lesions in postnatal mammals, except if peripheral nerve transplants are offered as a substrate [20,36,42]. In the present study, regeneration was assessed after intracranial freeze-crush lesions of the optic nerve, which completely interrupted all axons. In rats lesioned at the age of 8–9 days and surviving for additional 5–6 days, regenerating retinal fibers were seen to enter and partially cross the lesion site, reaching a maximum distance of 0.8 mm (x ±S.E.M.= 0.62 ± 0.07 mm) in the presence of brain-derived neurotrophic factor (BDNF). Without BDNF, almost all the axons were lost due to axonal die-back. This regeneration distance could be significantly enhanced, up to 1.9 mm, by application of a monoclonal antibody (mAB IN-1 [6]) directed against oligodendrocyte- and myelin-associated neurite growth inhibitory proteins. Similar results were obtained in rats lesioned at 16–18 days and surviving for 2 weeks: whereas fibroblast growth factor (FGF) stimulated sprouting did not exceed distances of 0.5 mm (x= 0.38 ± 0.07 mm), FGF and IN-1 antibody treated rats showed regenerations up to 2.3 mm (x= 1.53 ± 0.15 mm). The specificity of this effect was confirmed by lesions of myelin- and oligodendrocyte-free optic nerves: optic nerves were locally X-irradiated at birth, day 2, 4 and 6, a procedure which kills the dividing oligodendrocyte precursor cells [12,30]. When these myelin-free nerves were lesioned at 3 weeks of age, regeneration distances between 2.5 and 3.2 mm were observed 3 weeks later. Our results demonstrate the ability of postnatal rat optic nerve fibers to regenerate within the CNS microenvironment under two conditions: axonal die-back has to be prevented with appropriate neurotrophic factors, and neurite growth inhibitory components, in particular those associated with CNS oligodendrocytes and myelin, have to be inactivated.