Early and selective pathology of light chain neurofilament in the spinal cord and sciatic nerve of G86R mutant superoxide dismutase transgenic mice

Pathologic accumulation of neurofilament protein (NF), both within spheroids of the proximal axon and within inclusions of motor neuron somata, is a hallmark of neurodegeneration in amyotrophic lateral sclerosis (ALS). Transgenic mice that express mutations in superoxide dismutase (SOD-1), which were genetically linked to familial ALS, develop symptomatology and pathology that strongly resemble ALS and therefore provide a useful model for studying the disease. Examining NF in the G86R mutant SOD-1 transgenic mice, we previously demonstrated that phosphorylated NF accumulates in motor neuron somata of symptomatic transgenic mice. In the present study, we expand these results by examining the immunocytochemical distribution of the three subunits of NF (i.e., light, medium, and heavy chains) as well as tubulin in presymptomatic and symptomatic SOD-1 transgenic mice. Although all NF subunits, but not tubulin, accumulate along with phosphorylated NF in the spinal cord inclusions of symptomatic mice, numerous inclusions containing only light chain NF are found in the spinal cord of presymptomatic SOD-1 transgenic mice. In addition to these results in the spinal cord, intensely immunoreactive aggregates of NF-L, but not the other NF subunits or tubulin, were observed in the sciatic nerve of both symptomatic and presymptomatic mutant SOD-1 transgenic mice. These results suggest that the mechanism of NF alteration in SOD-1 transgenic mice, and also perhaps in ALS patients, originates with the disruption of NF-L, only later involving the other subunits.