Oral 3 - 3.4
1,3Yangning Lu, 1,3Asuka Guan, 2,3Daniel Witvliet, 3Ben Mulcahy, 3Jin Meng, 1,3Jun Meng, 4Quan Wen, 5Aravinthan Samuel, 1,2,3Mei Zhen
1 Dept. of Physiology, University of Toronto; 2 Dept. of Molecular Genetics, University of Toronto; 3 Lunenfeld-Tanenbaum Research Institute, Mount Sinai Hospital; 4 School of Life Sciences, University of Science and Technology, China; 5 Center for Brain Science, Harvard University, USA
Motor behavior is essential in almost all aspects of animal life. Neural circuits that govern motor output, or motor circuits, are evolutionarily conserved for the core structures and functions. During animal development, motor circuits could experience significant changes; yet, for animals that do not undergo metamorphosis, they might largely maintain their stereotypical locomotor patterns throughout development. How distinct motor circuits generate similar motor output during development is poorly understood. We describe studies here to begin addressing this question using the simple nervous system of C. elegans. C. elegans maintains serpentine-like dorsal/ventral bending waves for locomotion in all developmental stages post hatching. In adult, cholinergic A- and B-type motor neurons innervate and stimulate dorsal and ventral body wall muscles, whereas GABAergic D-type motor neurons innervate and inhibit muscles on both sides. Together, they constitute symmetric motor circuit input onto dorsal and ventral body wall muscles that generates balanced muscle activity in adult worms. The cellular components of motor circuit differ drastically in younger C. elegans. All ventral-innervating cholinergic motor neurons are not born until the end of first larval (L1) stage. A partial electron microscopy (EM) reconstruction study suggests that, A- and B-type motor neurons only innervate dorsal muscle, and D-type motor neurons only innervate ventral muscle in L1 stage. With this layout, L1 motor circuit should generate asymmetric input to muscles, biasing towards dorsal excitation and ventral inhibition. However, L1 larva outputs balanced dorsal/ventral bending waves. How does an asymmetric circuit produce symmetric output? To answer this question, we have analyzed the structure and function of L1 motor circuit. We fully reconstructed the connectivity of a complete L1 larva by EM, and identified previously uncharacterized candidate cellular components for muscle activity in addition to A-, B-, and D-type motor neurons. To elucidate the functional contribution of candidate cells to L1 muscle activity, we developed a novel all-optical manipulation scheme that allows non-invasive systematic probing of neural circuits with a simple setup and unrestricted opsin/calcium-indicator combinations. We then combined calcium imaging, cell ablation, and all-optical manipulation to identify cellular substrates for symmetric L1 muscle output. We unraveled a non-canonical circuit mechanism that drives ventral excitation and dorsal inhibition, hence balancing the asymmetry of A-B-D motor neurons. By utilizing cutting-edge techniques for anatomical and functional studies, our work sheds light on the adaptability of a developing motor circuit, and addresses the basis for functional resemblance of different circuit assemblies, which could be extrapolated to other organisms.