Cu/Zn superoxide dismutase (SOD1) is a cytoplasmic antioxidant enzyme, which, when mutant in humans, is linked to familial cases of the motor neurodegenerative disease amyotrophic lateral sclerosis (ALS). The Drosophila SOD1 gene (Sod) shares a highly conserved sequence with the human homolog, and this study includes examinations of the established hypomorphic n108 allele (Sodn108), alongside a knock-in construct of the G85R allele found in human ALS patients (SodG85R). In addition to previously documented decreased adult lifespan and attenuated motor function, we show that Sod mutant Drosophila display high mortality rates during larval and pupal development. Immunostaining of neuronal membrane at neuromuscular synapses in Sod mutant larvae revealed presynaptic terminals of abnormal, swollen morphology. In SodG85R larvae, in vivo mitochondrial staining demonstrated the presence of aggregated mitochondria inside the swollen synaptic terminals, and a genetically encoded GFP construct targeted to mitochondria revealed aggregates of mitochondria inside the axon bundles of SodG85R larvae. In whole-cell recordings of neuromuscular transmission, both Sodn108 and SodG85R exhibited lower muscle input resistance and smaller miniature excitatory junction potentials (mEJPs) compared to WT. However, evoked EJPs were similar to those of WT. Focal electrophysiological recording showed that both Sodn108 and SodG85R NMJ terminals displayed slightly higher release probability than WT terminals. Treatment of SodG85R with the Shaker channel (Kv1) blocker 4-aminopyridine (4-AP) and the broad-spectrum K+ channel blocker tetraethylammonium (TEA) induced prolonged plateau-like potentials at the larval NMJ upon electrical stimulation. Altogether, this study provides a snapshot of the alterations in mitochondrial distribution, synaptic morphology, and neurotransmission that characterize the motor neurons of Sod mutants prior to neurodegeneration and death of the organism.
bioRxiv Subject Collection: Neuroscience