Axonal regeneration from CNS neurons in the cerebellum and brainstem of adult rats: Correlation with the patterns of expression and distribution of messenger RNAs for L1, CHL1, c-jun and growth-associated protein-43

Some neurons in the brain and spinal cord will regenerate axons into a livi ng peripheral nerve graft inserted at the site of injury, others will not. We have examined the patterns of expression of four molecules thought to be involved in developmental and regenerative axonal growth, in the cerebellu m and brainstem of adult rats, following the implantation into the cerebell um of peripheral nerve grafts. We also determined how the expression patter ns observed correlate with the abilities of neurons in these regions to reg enerate axons. Three days to 16 weeks after insertion of living tibial nerv e autografts, neurons which had regenerated axons into the graft were retro gradely labelled from the distal extremity of the graft with cholera toxin conjugated to horseradish peroxidase, and sections through the cerebellum a nd brainstem were processed for visualization of transported tracer and/or hybridized with riboprobes to detect messenger RNAs for the cell recognitio n molecules L1 and CHL1 (close homologue of L1), growth-associated protein- 43 and the cellular oncogene c-jun. Retrogradely labelled neurons were pres ent in cerebellar deep nuclei close to the graft and in brainstem nuclei kn own to project to the cerebellum. Neurons in these same nuclei were found t o have up-regulated expression of all four messenger RNAs. Individual retro gradely labelled neurons also expressed high levels of L1, CHL1, c-jun or g rowth-associated protein-43 messenger RNAs (and vice versa), and every mess enger RNA investigated was co-localized with at least one other messenger R NA. Purkinje cells did not regenerate axons into the graft or up-regulate L 1, CHL1 or growth-associated protein-43 messenger RNAs, but there was incre ased expression of c-jun messenger RNA in some Purkinje cells close to the graft. Freeze-killed grafts produced no retrograde labelling of neurons, an d resulted in only transient and low levels of up-regulation of the tested molecules, mainly L1 and CHL1. These findings show that cerebellar deep nucleus neurons and precerebellar brainstem neurons, but not Purkinje cells, have a high propensity for axon regeneration, and that axonal regeneration by these neurons is accompanied by increased expression of L1, CHL1, c-jun and growth-associated protein-43 . Furthermore, although the patterns of expression of the four molecules in vestigated are not identical in regenerating neuronal populations, it is pr obable that all four are up-regulated in all neurons whose axons regenerate into the grafts and that their up-regulation may be required for axon rege neration to occur. Finally, because c-jun upregulation is seen in Purkinje cells close to the graft, unaccompanied by up-regulation of the other molec ules investigated, c-jun upregulation alone cannot be taken to reliably sig nify a regenerative response to axotomy. (C) 2000 IBRO. Published by Elsevi er Science Ltd. All rights reserved.