Rebalancing the motor circuit restores movement in a Caenorhabditis elegans model for TDP-43-toxicity

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia are caused by the abnormal accumulation of TAR DNA-binding protein 43 (TDP-43) in the cytoplasm of neurons. How TDP-43 accumulation leads to disease symptoms is not well-characterized. Here, we use a C. elegans model for TDP-43-induced toxicity to identify the biological mechanisms that lead to disease-related phenotypes. By applying deep behavioral phenotyping, we established a phenotypic fingerprint of TDP-43 worms. This fingerprint was compared to that of 294 C. elegans mutants, in which genes were mutated that are important for nervous system and muscle functioning. By using a computational clustering approach, we found that the release of acetylcholine and GABA was the primary defect in TDP-43 worms. We then functionally dissected the neuromuscular circuit to show that GABA transmission was more severely diminished compared to acetylcholine. Whereas the loss of GABA transmission was caused by a profound loss of GABA synapses, acetylcholine neurons appeared to be functionally silenced. Enhancing functional output of repressed acetylcholine neurons at the level of G-protein coupled receptors or through optogenetic stimulation restored neurotransmission, but inefficiently rescued locomotion. Surprisingly, rebalancing the excitatory and inhibitory input by simultaneous stimulation of GABA and acetylcholine input into muscles not only synergized the effects of boosting individual neurotransmitter systems, but instantaneously improved movement. Our results suggest that interventions accounting for the altered connectome may be more efficient in restoring motor function than those solely focusing on diseased neuron populations.