Growth-associated proteins and regeneration-induced gene expression in the aging neuron

Axonal elongation and sprouting during regeneration are retarded with aging but the etiology of this is unclear. We investigated whether this age-associated decline is related to a decline in expression of three different growth-associated proteins (GAPs): alpha(1)-tubulin, neurofilament (NF) light subunit (NF-L) and GAP-43. Northern analysis was performed on L4-L5 dorsal root ganglia (DRG) of young (3 months) and aged (23 months) rats following a sciatic nerve crush and compared to their age-matched controls. The results show that initial mRNA levels of alpha(1)-tubulin and NF-L in the control aged rat DRG were half those of the control young adults, whereas expression of GAP-43 was unchanged. Two weeks after axotomy, the expression of alpha(1)-tubulin and GAP-43 in the aged DRG was induced to the same levels as in the axotomized young adult, and the expression of NF-L decreased proportionately in both age groups. These results indicate that certain neuronal mRNAs, such as alpha(1)-tubulin and NF-L may be maintained at lower levels in aging DRG neurons, whereas others, such as GAP-43 appear to be unaltered. However, during regeneration, the aging DRG neuron appears capable of inducing alpha(1)-tubulin, NF-L and GAP-43 as well as the young adult.