Altering the balance of prickle isoforms changes NMJ bouton morphology and predisposes flies to seizures by lowering the seizure threshold. Salleh Ehaideb¹, Katie Cranston¹, Atulya Iyengar¹, Atsushi Ueda¹, Alexander G. Bassuk², David Gubb³, Chun-Fang Wu¹, J. Robert Manak^1,2. 1) Dept of Biology, Univ of Iowa, Iowa City, IA; 2) Dept of Pediatrics, Univ of Iowa, Iowa City, IA; 3) CIC bioGUNE, Biscay Technology Park, Derio, Spain.

   prickle participates in the non-canonical WNT signaling/planar cell polarity (PCP) pathway. We previously reported that fly prickle mutants are seizure-prone, and that mutations in Prickle orthologues are associated with seizures in flies, mice and humans. prickle encodes two adult isoforms, pk^pk and pk^sple. Flies heterozygous for pk^sple mutations display pronounced seizures even though no planar cell polarity defects are visible, suggesting that the PCP and seizure phenotypes can be genetically separated. Remarkably, the pk^pk mutants are actually less seizure-prone than controls, which suggests that pk^pk and pk^sple act antagonistically. Consistent with this hypothesis, overexpression of the pk^pk isoform in brain, motor neurons and muscles (which recapitulates the imbalance of pk^pk and pk^sple isoforms seen in the pk^sple heterozygote) strongly induces fly seizures in an otherwise wild-type fly. Both pk^sple and pk^pk mutants show an increase in terminal bouton numbers at the larval neuromuscular junction (NMJ), with pk^sple mutants showing an increase in large boutons compared to controls, and pk^pk mutants showing an increase in small boutons compared to controls. Using the ElectroConvulsive Seizure (ECS) stimulation paradigm, we show that the pk^sple flies have a lowered seizure threshold compared to controls, similar to observations made for other seizure-prone mutants in flies and mice. Finally, we show that protein encoded by the pk^pk transcript co-localizes with proteins involved in synaptic vesicle fusion.