Investigations into modifications of neuromuscular physiology by axonal transport disruptions in Drosophila SOD1 mutants

The reactive oxygen species (ROS)-scavenging enzyme Cu/Zn superoxide dismutase (SOD1) is an evolutionarily conserved mechanism for the maintenance of oxidative homeostasis, and missense mutations in the human SOD1 gene are associated with the motor neuron degenerative disease amyotrophic lateral sclerosis (ALS). Mutations in the Drosophila melanogaster SOD1 gene (Sod) shorten fly lifespan, attenuate motor function, and induce developmental mortality. We have previously found morphological and physiological defects at the neuromuscular junctions (NMJs) of hypomorphic Sodn108 and ALS model SodG85R mutant larvae. Here, we report genetic interactions causing striking modifications of Sod mutant phenotypes by mutations in the gene Prickle (Pk), which are linked to planar cell polarity, epilepsy, and axonal transport disruptions. Pk is expressed as two isoforms prickle-spiny-legs (sple) and prickle-prickle (pk), which are each suppressed by specific hypomorphic mutations (sple1 and pk1). Interestingly, Sod phenotypes are distinctly modified depending on both the Pk isoform suppressed, and whether said suppression is heterozygous or homozygous. Heterozygous sple1 and pk1 improved the developmental survival of Sod mutants, whereas homozygous sple1 and pk1 drastically increased mortality. Further, only heterozygous sple1 and pk1 clearly ameliorated morphological defects at the neuromuscular junctions of Sod mutant larvae. Pharmacological treatment of Sod mutants reveals allele-specific motor neuron terminal hyperexcitability, characterized by synaptic transmissions of extended duration and abnormal presynaptic Ca2+ transients. Heterozygous sple1 mutation suppresses this Sod mutant terminal hyperexcitability, but pk1 does not. We reversed this suppressive effect of sple1 by pharmacological blockade of Ca2+-activated K+ channel slowpoke. Altogether, this study builds on our prior knowledge of Sod mutant development and physiology to show that axonal transport-linked gene mutations strikingly modify Sod mutant phenotypes, providing strong evidence for a role of intracellular transport in alterations of neuromuscular morphology and physiology by SOD1 mutations.