Dynamic regulation of middle neurofilament RNA pools during optic nerve regeneration

Abstract

Stereotypical changes in neurofilament subunit expression are highly correlated with the regenerative success of lower vertebrate CNS axons. The phylogenetically conserved binding of ribonucleoproteins to the 3'-untranslated region of the middle neurofilament subunit (NF-M) mRNA suggests that post-transcriptional mechanisms play an important role in the control of NF-M expression. To assess their contribution to the regulated changes in NF-M expression that occur during Xenopus laevis optic axon regeneration, we followed changes in intracellular NF-M RNA pools. Within 3 days after axotomy, when NF-M mRNA levels decrease in the injured retinal ganglion cells, heterogenous nuclear RNA levels increased more than 15-fold, but did so in both the operated and the contralateral unoperated eyes as compared with the eyes of surgically naive frogs. Increased nuclear RNA levels persisted throughout regeneration but never correlated directly with changes in mRNA expression, indicating that such changes most likely arose from alterations in nuclear-cytoplasmic RNA export and turnover. The early phase of optic nerve regeneration also exhibited an increase in the efficiency of translation of NF-M mRNA relative to surgically naive animals. This increase was only transient in unoperated control eye, but persisted through the peak of regeneration in the operated eye. Thus, post-transcriptional control of NF-M expression plays a significant role in regulating the cytoskeletal composition of injured neurons. These findings indicate that changes in protein expression during successful regeneration of CNS axons involve a complex interplay of transcriptional and translational regulation that is controlled by the operation of functional neuronal pathways. These findings also raise the additional possibility that factors regulating post-transcriptional changes in cytoskeletal gene expression may be as important as transcription factors for the successful regeneration of CNS axons.