Regulation of Aberrant Neurofilament Phosphorylation in Neuronal Perikarya. I. Production Following Colchicine Application to the Sciatic Nerve

Abstract

Neurofilament (NF) triplet proteins are normally poorly phosphorylated in neuronal perikarya, the two high molecular weight polypeptides becoming extensively phosphorylated once the NF enters the axon. Abnormal expression of phosphorylated NF (pNF) epitopes in neuronal perikarya has been revealed using monoclonal antibodies in a variety of human and experimental conditions. In the present study, we asked whether pNF epitopes are expressed in sensory neurons in the L4 and L5 dorsal root ganglia (DRG) following blockade of fast axonal transport in a model producing few (less than 1%) degenerating fibers. Colchicine (5 mM) was briefly (45 minutes) applied to the sciatic nerve at mid-thigh twice (once weekly) and the animals studied two weeks following the first colchicine application; contralateral nerves were either treated with saline or crushed. Modest to intense immunoreactivity was found with antibody 07-05 (directed against pNF epitopes on the two high molecular weight NF polypeptides) in 30.4% and 45.1% of DRG neurons from colchicine-treated and crushed nerves, respectively; only a rare cell body demonstrated immunostaining from the contralateral saline-treated nerves. Immunoreactivity was not observed with antibody 07-05 at two and five days following single colchicine application. In a separate study, colchicine or saline was applied (as above) 1-2 cm proximal to a nerve crush. Colchicine application did not influence the extent of DRG neurons expressing pNF epitopes; immunostaining with antibody 07-05 was present in 44.7% and 43.8% of DRG neurons from saline-treated and colchicine-treated crushed nerves, respectively. The results indicate that structural interruption of nerve-target contact is not necessary to induce aberrant NF phosphorylation in neuronal perikarya. It is suggested that loss of a retrogradely transported "trophic" signal(s) triggers this response.