In the past 5 years, it has become clear that the protein pathology of many human neurodegenerative diseases exhibits characteristics of prions, including transmissibility, strain variation, and the ability to spread from a focal site of introduction. Amyotrophic lateral sclerosis (ALS) stands out as an example where the hallmarks of prion-like spreading is evident as weakness spreads along anatomically connected pathways. In familial ALS caused by mutations in superoxide dismutase 1 (SOD1-ALS), patients inheriting the A4V variant of SOD1 weakness spreads rapidly (average survival <1.5 years after the onset), whereas in patients inheriting the G37R variant weakness spreads slowly (average survival ~17 years). In the initial funding period of this award, our laboratory has uncovered evidence that this defining feature of SOD1-ALS may be explained by prion-like characteristics of mutant SOD1. Transgenic mice that express low levels of ALS mutant SOD1 develop disease late in life if at all. We have shown that paralysis can be accelerated in these mice by injecting spinal cord homogenates prepared from paralyzed mutant SOD1 transgenic mice or from human patients. We have also shown that we can inject these homogenates into the sciatic nerve of vulnerable mice to initiate a disease process that closely mimics the unilateral spread of weakness from one limb to another limb that is seen in humans. We have also successfully used purified recombinant SOD1 fibrilized in vitro to seed early onset paralysis in host mice, proving that SOD1 is capable of acting like a prion. Because we can propagate disease- causing conformations of SOD1 to naïve SOD1 “host proteins”, from hereafter we will refer to the misfolded conformation associated with disease-causing mutant SOD1 as an ALS prion. Building on the success of our initial work, we now propose four Specific Aims that are designed to improve our understanding of the biological role of prion-like spread in the pathogenesis of SOD1-ALS. In Aim 1, we seek to investigate whether the disease-causing mutations encrypt unique strain-like characteristics in misfolded SOD1 that influences the rate of prion-like spread in animal to animal transmission studies. In Aim 2, we seek to determine how the route of transmission and age of the host recipient mouse influence the propagation of SOD1-ALS prions. In our third and fourth Aims, using our novel model system we will turn our attention towards determining the extrinsic factors that determine how misfolded protein conformations may spread in the CNS and whether inflammatory signaling may influence such spread. In Aim 3, we will use newly generated loxp G85R-SOD1:YFP mice to determine the contribution of astrocytes in propagating SOD1-ALS prions. In Aim 4, we will use adeno- associated vectors to express pro- and anti-inflammatory cytokines as a means to assess the role of activated astrocytes and microglia in the propagation and spreading of SOD1-ALS prions. Our over-arch...