Phosphate dependent and independent neurofilament epitopes in the axonal swellings of patients with motor neuron disease and controls

Abstract

Axonal accumulations of neurofilaments (NFs) may result from abnormalities intrinsic to NF subunits (e.g., proteolytic cleavage, altered phosphorylation), or from abnormalities extrinsic to NFs that retard the transport of these neuron-specific intermediate filaments. To evaluate this hypothesis, we probed the NF-rich axonal swellings seen in normal spinal cords, "globules", and the larger ones, "spheroids", seen in the spinal cords of patients with motor neuron disease by using a library of monoclonal antibodies that recognize human NF proteins. These monoclonal antibodies discriminate different phosphorylation states of individual NF subunits. The NF protein determinants of globules and spheroids were similar to each other and to the determinants that predominate in normal spinal cord axons. NF protein-positive inclusion bodies were only seen in the anterior horn cells of one patient with motor neuron disease, and they contained NF protein determinants similar to those normally expressed in perikarya. Thus, the NFs in globules and spheroids appear to be derived from axonal NF proteins, and both kinds of axonal swellings may arise by similar mechanisms. Although our data do not exclude a structural defect in NF proteins to account for the accumulation of NFs in these axonal swellings, the hypotheses being advanced to explain the formation of NF-rich globules and spheroids based on intrinsic alterations of NF proteins must consider that the immunological integrity of disparate NF protein epitopes in different states of phosphorylation is retained in both globules and spheroids.