Chan, W.; Justin, E.; Roenspies, T.; Rumley, J. D.; Murray, J. I.; Colavita, A.

Motor neuron positioning and organization along the neuroaxis is crucial for proper nervous system connectivity and function. In newly hatched C. elegans, the ventral nerve cord contains 22 motor neurons, divided into three classes (DD, DA, and DB), with their cell bodies showing a largely stereotypical positioning and sequential arrangement. However, the mechanisms controlling this precise positioning are not fully understood. Here, we uncover a left-right asymmetry in {beta}-catenin-TCF complex activity that controls motor neuron positioning. Loss of BAR-1/{beta}-catenin or POP-1/TCF causes a shift of motor neuron cell bodies toward the anterior, while loss of PRY-1/Axin shifts them toward the posterior. During embryonic ventral cord morphogenesis, BAR-1 expression is restricted to right-side motor neuron precursors through asymmetric Notch signaling, which promotes PRY-1 expression on the left to degrade BAR-1. Our findings highlight an atypical Notch-mediated regulation of Axin expression and reveal that left-right asymmetry during neuroaxis formation specifies anteroposterior motor neuron placement in the central nerve cord.