Neurofilament phosphorylation in peripheral nerve: changes with axonal length and growth state.

We have previously reported that the intrinsic ability of motor axons to sprout can vary considerably from nerve to nerve with an inverse correlation to axonal length. In this study we asked whether this variation might be associated with differences in one axonal component, neurofilaments, near the site of outgrowth. The phosphorylation of epitopes on the heavy subunit of neurofilaments (NF-H) was compared in normal and regenerating axons from long and short nerves in the rat. Quantitative determination of phosphorylation states on NF-H epitopes was made by measuring immunoreactivity to monoclonal antibodies using an enzyme-linked immunosorbent assay system. Our results showed a much higher degree of phosphorylation of epitopes on NF-H in terminal portions of short compared to long axons. There was a significant inverse correlation between phosphorylation of NF-H epitopes and axonal length. In newly formed sprouts NF-H phosphorylation was reduced compared to normal. However, the absolute levels were related to the degree of NF-H phosphorylation in the parent axons. The ability to phosphorylate axonal proteins near the site of outgrowth may correlate with the potential for plastic changes in the axon such as sprouting.