Modeling the progression of SOD1-linked motor neuron disease

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-arching goal is to
determine the contribution of intrinsic strain-like attributes in SOD1 and extrinsic non-cell autonomous
processes to the prion-like propagation properties of disease-causing SOD1 conformations.

Grant Number: 5R01NS092788-09
NIH Institute/Center: NIH
Principal Investigator: PARAMITA CHAKRABARTY