Retrograde Regulation of Growth-Associated Gene Expression in Adult Rat Purkinje Cells by Myelin-Associated Neurite Growth Inhibitory Proteins

Abstract

Axon regeneration requires that injured neurons reinitiate long-distance growth and upregulate specific genes. To address the question of whether inhibitory environmental cues along the axon could exert a negative, tonic downregulation of growth-associated genes, we have examined adult rat Purkinje cells, which are endowed with poor regenerative capabilities. First we have compared their response to axotomy with that of neurons of the inferior olive, lateral reticular nucleus, and deep cerebellar nuclei, all of which vigorously regenerate into growth-permissive transplants. These injured neurons upregulate the transcription factors c-Jun and JunD, GAP-43, and NADPH diaphorase. In contrast, most axotomized Purkinje cells fail to express any of these markers, showing that the strength of this response parallels the regenerative potential of the examined neuron populations. However, strong upregulation of the same genes can be induced in Purkinje cells after colchicine injection into the uninjured adult cerebellum, indicating that their expression could be controlled by retrograde signals. To assess whether myelin-associated neurite growth inhibitory proteins contribute to this regulation, we applied the neutralizing antibodies IN-1 against one of the main inhibitory components of central myelin (NI-250) either in vivo or in vitro to organotypic cerebellar cultures. Application of IN-1 antibodies induces the upregulation of c-Jun, JunD, and NADPH diaphorase in Purkinje cells, showing that their expression is suppressed constitutively by myelin-associated neurite growth inhibitors. Thus, the inhibitory activity of the IN-1 antigen on axon growth is not restricted to the control of growth cone motility but also involves a retrograde regulation of gene expression in adult central neurons.