The Drosophila TMEM184B ortholog Tmep ensures proper locomotion by restraining ectopic firing at the neuromuscular junction

TMEM184B is a putative seven-pass membrane protein that promotes axon degeneration after injury. TMEM184B mutation causes aberrant neuromuscular architecture and sensory and motor behavioral defects in mice. The mechanism through which TMEM184B causes neuromuscular defects is unknown. We employed Drosophila melanogaster to investigate the function of the TMEM184B ortholog, Tmep (CG12004) at the neuromuscular junction. We show that Tmep is required for full adult viability and efficient larval locomotion. Tmep mutant larvae have a reduced body contraction rate compared to controls, with stronger deficits in females. Surviving adult Tmep mutant females show "bang sensitivity," a phenotype associated with epileptic seizures. In recordings from body wall muscles, Tmep mutants show substantial hyperexcitability, with many post-synaptic potentials fired in response to a single stimulation, consistent with a role for Tmep in restraining synaptic excitability. Neuromuscular junctions in Tmep mutants show modest structural defects and satellite boutons, which could also contribute to poor locomotor performance. Tmep is expressed in endosomes and synaptic vesicles within motor neurons, suggesting a possible role in synaptic membrane trafficking. Using RNAi knockdown, we show that Tmep is required in motor neurons for proper larval locomotion and excitability. Locomotor defects can be rescued by presynaptic knock-down of endoplasmic reticulum calcium channels or by reducing evoked release probability, suggesting that excess synaptic activity drives behavioral deficiencies. Our work establishes a critical function for the TMEM184B ortholog Tmep in the regulation of synaptic transmission and locomotor behavior.