Difference in phosphorylation of neurofilament proteins in motor neurons and spinal ganglion cells

The composition of the neurofilament proteins (NFPs) in neuronal perikarya was examined by two-dimensional (2-D) gel electrophoresis of isolated perikarya of bovine spinal motor neurons. The extent of phosphorylation of the high molecular weight subunit of NFP (NFP-H) was compared between motor and sensory neuronal perikarya in spinal cord and spinal ganglion by immunocytochemistry with monoclonal antibodies (MAbs) to NFP. Of the 23 MAbs used in this study, one MAb (82E10) was specific to the highly phosphorylated component of NFP-H examined by 2-D immunoblot whereas another MAb (3A8) was specific to NFP-H irrespective of its level of phosphorylation. Immunocytochemically, 82E10 did not stain the perikarya of bovine and rabbit spinal motor neurons but 3A8 stained the perikarya in both animal species. These findings are consistent with 2-D immunoblot of neuronal perikarya of bovine motor neurons isolated in bulk. As for the spinal ganglia, 82E10 stained many, but not all, perikarya of sensory neurons of both animal species. These results indicate that the extent of phosphorylation of NFP-H in the perikarya of most spinal ganglion cells is higher than that of motor neurons. These findings suggest that the rate of phosphorylation of NFP-H in perikarya or the axonal transport of NFP from perikarya to proximal axons is uniform in spinal motor neurons but variable in spinal ganglion cells.