| In response to nerve crush, axotomized neurons undergo dramatic changes in gene expression. Two principle phases of injury signaling induce the transcription of specific mRNAs to address first, neuronal survival, and second, functional regeneration. The mRNAs synthesized during the second phase are maintained at elevated levels throughout regeneration and therefore likely encode proteins needed for long-term processes, including growth. The identity of these mRNAs is important for understanding axon regeneration; yet, few have been identified. In this thesis study, two such mRNAs were identified in neurons of Aplysia californica. One encodes the novel membrane-bound fas-I homologue, Aplysia fasciclin-like protein (apFasP), and the other encodes the Aplysia homologue of beta-thymosin (apbetaT). This study demonstrated that in response to nerve injury both mRNAs are upregulated in neurons in vivo, but only after a delay. Following induction, however, the mRNA levels remain elevated throughout regeneration. It was further demonstrated that the proteins encoded by these mRNAs are induced in neurons after injury, and they localize to regenerating processes. These findings are consistent with the regulation of these mRNAs by retrogradely transported injury signals of the second phase and are highly suggestive that the protein products of these mRNAs have important functions during regeneration.; The study also addresses the strategies used by neurons to make proteins available to regenerating axons. Membrane-bound apFasP is made in the soma and rapidly transported to growth cones. In contrast, apbetaT is a soluble protein, and because soluble proteins are transported slowly, it would take weeks for it to reach the area of growth. One solution would be to rapidly transport the mRNAs that encode soluble proteins to nerve endings. This study found that the mRNA for apbetaT, but not the mRNA for apFasP, is localized to regenerating axons in vivo. This suggests that regenerating neurons translate the extrasomatic mRNA for apbetaT, and likely the mRNAs for other soluble proteins, in order to rapidly supply the growing distal axon with necessary protein. |
