mRNAs encoding the Aplysia homologues of fasciclin-I and β-thymosin are expressed only in the second phase of nerve injury and are differentially segregated in axons regenerating in vitro and in vivo

Studies using Aplysia californica have demonstrated that transcription after nerve injury occurs during a rapid, transient first phase and a delayed, prolonged second phase. Although the second phase is especially important for regeneration, the mRNAs produced during this phase have not been identified. We characterized two such mRNAs following axotomy. One encodes a novel fasciclin-1 homologue, Aplysia fasciclin-like protein (apFasP), and the other encodes Aplysia β-thymosin (apβT). In addition to mRNA synthesis, proteins required for regeneration must be available at the site of growth, and the transport and local translation of certain extrasomatic mRNAs aids in this process. We found apβT and apFasP proteins and mRNA at growth cones in vitro. However, only the mRNA for apβT was present in regenerating axons in vivo. This implies that the membrane protein apFasP is supplied by rapid transport from the soma, whereas the soluble apβT is synthesized locally.