Protein transport between crayfish lateral giant axons

Segmental lateral giant axons (SLGAs) in crayfish were used to determine whether functionally intact proteins can move between axons under physiological conditions. Horseradish peroxidase (HRP) was chosen as the tracer protein because its localization requires intact enzymatic activity and because it can be localized in living cells using a non-cytotoxic procedure. Following iontophoretic injection of HRP in a single SLGA, HRP often transferred to adjacent SLGAs. HRP transferred from an injected SLGA to a caudal SLGA with greater frequency than HRP transferred to a rostral SLGA. When HRP transferred between SLGAs, it was ultrastructurally associated with vesicles on both sides of septate junctions between adjacent SLGAs and was also seen in the perijunctional extracellular space. There was no difference between the electrical resistance of synapses at which HRP transferred and those synapses where HRP did not transfer. HRP transfer was significantly reduced when axons were bathed in reduced calcium saline. These and other observations indicate that axon-to-axon transport in this system is accomplished by exocytosis of HRP from injected axons followed by its endocytotic uptake by adjacent, non-injected axons. Similar transfer of endogenous proteins may contribute to the long-term survival for months to years of distal stumps of severed SLGAs.