Efficient induction of motor neuron disease in transgenic G93A SOD1 mice by prion-like seeding

Mutations in superoxide dismutase 1 (SOD1) cause paralysis in familial amyotrophic lateral sclerosis and promote its misfolding into neurotoxic aggregates. Previous studies have shown that mice expressing the ALS-causing G85R variant of SOD1 develop paralysis much faster after intraspinal injection of spinal homogenates from paralysed G85R SOD1 mice. These findings, and other studies in cell models, established the prionoid templating properties of misfolded mutant SOD1. Previously, however, we noted that the widely used Gur1-G93A SOD1 mice, which express at high levels and develop paralysis by 6 months of age, were resistant to seeding by homogenates from paralysed G93A mice. A line of G93A mice that expresses at very low levels (VLE-G93A) was responsive to seeding but at low efficiency. The poor susceptibility of G93A-SOD1 mice to seeding was not what we expected if prion-like propagation is essential to SOD1 ALS pathogenesis. In our prior studies, seeding homogenates from paralysed G93A-SOD1 mice were injected into the spine of newborn mice, leading us to question whether older G93A SOD1 mice might be more susceptible to seeding. Here, we establish that adult VLE G93A SOD1 mice (up to 12 months of age) injected intrathecally with seeding homogenates containing misfolded G93A or G85R SOD1 developed accelerated motor neuron disease efficiently. Thus, we demonstrate that both the route and age of inoculation can influence the efficiency of SOD1 seeding to induce motor neuron disease in VLE G93A-SOD1 mice. These data, together with our earlier reports, suggest that prion-like templating contributes to disease progression in SOD1-ALS.