Neurofilaments and Orthograde Transport Are Reduced in Ventral Root Axons of Transgenic Mice that Express Human SOD1 with a G93A Mutation

Abstract

Mice engineered to express a transgene encoding a human Cu/Zn superoxide dismutase (SOD1) with a Gly93 → Ala (G93A) mutation found in patients who succumb to familial amyotrophic lateral sclerosis (FALS) develop a rapidly progressive and fatal motor neuron disease (MND) similar to amyotrophic lateral sclerosis (ALS). Hallmark ALS lesions such as fragmentation of the Golgi apparatus and neurofilament (NF)-rich inclusions in surviving spinal cord motor neurons as well as the selective degeneration of this population of neurons were also observed in these animals. Since the mechanism whereby mutations in SOD1 lead to MND remains enigmatic, we asked whether NF inclusions in motor neurons compromise axonal transport during the onset and progression of MND in a line of mice that contained ∼30% fewer copies of the transgene than the original G93A (Gurney et al., 1994). The onset of MND was delayed in these mice compared to the original G93A mice, but they developed the same neuropathologic abnormalities seen in the original G93A mice, albeit at a later time point with fewer vacuoles and more NF inclusions. Quantitative Western blot analyses showed a progressive decrease in the level of NF proteins in the L5 ventral roots of G93A mice and a concomitant reduction in axon caliber with the onset of motor weakness. By ∼200 d, both fast and slow axonal transports were impaired in the ventral roots of these mice coincidental with the appearance of NF inclusions and vacuoles in the axons and perikarya of vulnerable motor neurons. This is the first demonstration of impaired axonal transport in a mouse model of ALS, and we infer that similar impairments occur in authentic ALS. Based on the temporal correlation of these impairments with the onset of motor weakness and the appearance of NF inclusions and vacuoles in vulnerable motor neurons, the latter lesions may be the proximal cause of motor neuron dysfunction and degeneration in the G93A mice and in FALS patients with SOD1 mutations.

A mouse forward genetics screen identifies LISTERIN as an E3 ubiquitin ligase involved in neurodegeneration

“…However, the L5 axonal loss occurs late in disease. This is different from motor neuron degeneration in SOD1 mouse model where L5 axonal degeneration occurs at symptom onset (20). Thus, although axonal degeneration may contribute to clinical symptoms of lister mice, it is likely not the primary cause of the motor dysfunction phenotype.…”

“…3 H and J). Despite severe motor dysfunction in lister mice, the reduction of motor axons from the L5 root was not as significant as seen in SOD1 mouse model (20). We therefore asked whether distal axons showed signs of degeneration.…”

Alsin, the Product of ALS2 Gene, Suppresses SOD1 Mutant Neurotoxicity through RhoGEF Domain by Interacting with SOD1 Mutants

“…Expression of SOD1 mutants increased oxidative stress; the production of the reactive oxygen species is elevated by expression of SOD1 mutants (6), and hydroxyl radical generation is elevated in G93A-SOD1 transgenic mice (7). SOD1 mutants inhibit slow axonal transport; axonal transport is reduced in G93A-SOD1 transgenic mice (8), and slowing axonal transport occurs as a very early manifestation of toxicity by SOD1 mutants (9). Also, expression of SOD1 mutants causes vacuolar degeneration of mitochondria (10) and dysfunction of oxidative phosphorylation in mitochondria (11).…”

Molecular chaperone Hsp110 rescues a vesicle transport defect produced by an ALS-associated mutant SOD1 protein in squid axoplasm

“…A large number of different amino acid substitutions in human SOD1 have been associated with ALS, and many have been shown to destabilize the protein, disposing it to misfolding and aggregation (10,11). The exact mechanisms of toxicity remain unclear, but development of disease is associated with alterations in motor neurons, including electrophysiological changes (12)(13)(14), mitochondrial defects (15), axonal transport deficiency (16)(17)(18)(19)(20), and neuromuscular junction dysfunction/retraction (21,22). In addition, glia, which also express the mutant protein, play a significant role in progression of disease (23)(24)(25)(26).…”