Human Senataxin Modulates Structural Plasticity of the Neuromuscular Junction in <b><i>Drosophila</i></b> through a Neuronally Conserved TGFβ Signalling Pathway

Mutations in the human Senataxin (hSETX) gene have been shown to cause two forms of neurodegenerative disorders - a dominant form called amyotrophic lateral sclerosis type 4 (ALS4) and a recessive form called ataxia with oculomotor apraxia type 2 (AOA2). SETX is a putative DNA/RNA helicase involved in RNA metabolism. Although several dominant mutations linked with ALS4 have been identified in SETX, their contribution towards ALS4 pathophysiology is still elusive.In order to model ALS4 in Drosophila and to elucidate the morphological, physiological and signalling consequences, we overexpressed the wild-type and pathological forms of hSETX in Drosophila.The pan-neuronal expression of wild-type or mutant forms of hSETX induced morphological plasticity at neuromuscular junction (NMJ) synapses. Surprisingly, we found that while the NMJ synapses were increased in number, the neuronal function was normal. Analysis of signalling pathways revealed that hSETX modulates the Highwire (Hiw; a conserved neuronal E3 ubiquitin ligase)-dependent bone morphogenetic protein/TGFβ pathway. Thus, our study could pave the way for a better understanding of ALS4 progression by SETX through the regulation of neuronal E3 ubiquitin pathways.