Functional Analysis of the Motor Circuit of Juvenile Caenorhabditis elegans by Yangning Lu A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Graduate Department of Physiology University of Toronto © Copyright by Yangning Lu 2020 Functional Analysis of the Motor Circuit of Juvenile Caenorhabditis elegans Yangning Lu Doctor of Philosophy Department of Physiology University of Toronto 2020 Abstract Caenorhabditis elegans generates alternating dorsal-ventral bending waves across developmental stages. The overall level of dorsal and ventral muscle output is symmetric. In adults, this behavioral symmetry correlates with anatomy symmetry: distinct pools of cholinergic motor neurons activate dorsal and ventral body wall muscles, whereas distinct pools of GABAergic motor neurons contra-laterally inhibit dorsal and ventral muscles, respectively. However, in the first stage larva (L1), cholinergic motor neurons only make neuromuscular junctions to the dorsal muscle, and GABAergic motor neurons to the ventral muscle. How does an asymmetric motor circuit produce symmetric motor output? In the past five years, my colleagues and I undertook anatomical and functional analyses of the L1 larva motor circuit to address this question. First, my colleagues fully reconstructed the connectivity of a complete L1 larva, from which we identified all candidate cellular components for muscle activity. Next, to elucidate the functional contribution of these candidates, I developed a novel all-optical manipulation methodology that allows fast and systemic probing of functional connections of neural circuits. Third, I led the effort to combine free-moving calcium ii imaging, cellular ablation, chemogenetics, optogenetics, and all-optical interrogation to pinpoint mechanisms for the symmetric L1 muscle output. Collectively, results from these studies show that all cells that make anatomic synaptic connections are insufficient to explain the symmetry of body wall muscle excitability in L1 C. elegans; instead, extra-synaptic transmissions play a crucial role in underling muscle activity regulation. In the L1 motor circuit, the anatomically asymmetric circuit is functionally compensated by extra-synaptic transmissions from both excitatory premotor interneurons and inhibitory motor neurons to operate as a symmetric circuit. Excitatory premotor interneurons can bypass the motor neurons layer to directly regulate excitability of muscle cells, thus assuming dual roles. Animals with distinct wirings can produce similar motor patterns. Our findings in the juvenile C. elegans motor circuit show that structurally divergent neural circuits can functionally converge on the functional output, and that neurons can multiplex. This study further demonstrates a larger role of functional communications devoid of ultrastructural features in a developing neural circuit. iii Acknowledgments I thank my PhD supervisor Dr. Mei Zhen who provided the most supporting academic environment during my training. She accepted me to her lab even knowing my hiccup during the previous stint at another graduate school. She offered insightful, critical, and helpful suggestions on every aspect of my PhD studies, and my future development. She gave encouragement whenever I needed it. Without her, it would not be possible for me to finish the PhD program. I owe everything to her for my scientific career. I thank Dr. Aravinthan Samuel at Harvard University who essentially functioned as my co- supervisor. He gave full support and suggestions in almost every step of my PhD program, and was heavily involved in guiding me with writing the thesis. He also provided excellent experimental environment for the collaboration project. I thank Dr. Yue Xiong at University of North Carolina, Chapel Hill who helped me going through one of the most difficult times. He offered unconditional support during my stint in his lab. His help made it possible for me to re-apply for graduate school. I thank my thesis committee Dr. Sheena Josselyn and Dr. Kenichi Okamoto for their incredible patience and help in guiding me through the graduate school adventure. I thank Dr. Zhong-Ping Feng, Dr. Douglas Tweed, and Rosalie Pang for their guidance and incredible help. I thank Asuka Sihui Guan, Daniel Witvliet, Dr. Ben Mulcahy, Dr. Taizo Kawano, Jin Meng, Jun Meng, Dr. Ying Wang, Dr. Wesley Hung, Dr. Shangbang Gao, Dr. Quan Wen, Dr. Vivek Venkatachalam, Anson Sathaseevan, Ben Sorscher, and all other past and present members in Zhen lab to their invaluable contributions to the work presented here and future publications. I also thank Dr. Zhijun Li, Dr. Lei Lv, Dr. Bryce Seifert, Dr. Beezly Groh, Dr. Pu Wang, Matthew Smith, Dr. Feng Cao, Dr. Xianke Dong, among others, for their enthusiastic support. I thank Chao Wang, Jiaxing Jiang, Chen Wu, Xing Su, Weifeng Cao, Peng Zhang, Ao Xu, Yichun Qiu, Siyang Wang, among others, for their amazing friendship that ever lasts. iv I thank my wife Ning Shen who rescued me from dropping out of science, and has been always of utmost support. Without her, I would not be who I am. I owe everything to her. At last, I thank my dear parents, to whom I owe everything. v Table of Contents Contents Acknowledgments.......................................................................................................................... iv Table of Contents ........................................................................................................................... vi List of Tables ............................................................................................................................... xiii List of Figures .............................................................................................................................. xiv Figure contributions ..................................................................................................................... xvi 1 Introduction: The neural circuit underlying of locomotor behaviors and technological advances in neurophysiology ............................................................................1 1.1 Control of locomotion ........................................................................................................1 1.1.1 Locomotion in various forms ...................................................................................1 1.1.1.1 Limbless undulatory movement ................................................................1 1.1.1.2 Limbed movement .....................................................................................2 1.1.2 Properties of motor circuits ......................................................................................3 1.1.2.1 Central pattern generators ..........................................................................3 1.1.2.2 Mechanisms that underlie rhythm generation by CPGs ............................6 1.1.2.3 The organization of locomotory CPGs ......................................................7 1.1.2.4 Symmetry of motor circuits at different behavioral paradigms ...............10 1.1.3 Neuromodulation of CPGs .....................................................................................11 1.1.4 Sensory feedback regulation of motor circuits ......................................................12 1.1.5 Descending control of motor circuits .....................................................................12 1.1.6 Different circuit configurations can generate similar locomotor patterns .............14 1.2 C. elegans locomotion and motor circuit........................................................................14 1.2.1 C. elegans model for motor circuit studies ............................................................14 1.2.2 Anatomy of the C. elegans motor circuit ...............................................................15 1.2.2.1 Reconstruction of neural connectivity by electron microscopy ..............15 vi 1.2.2.2 Symmetry of the adult C. elegans motor circuit ......................................16 1.2.2.3 Asymmetry of motor circuit structure in juvenile (L1) C. elegans .........17 1.3 Tools for neural circuit studies .......................................................................................20 1.3.1 Functional imaging of neuronal activities by calcium reporters ............................20 1.3.2 Establishing causal relationship in neuroscience by optogenetics .........................23 1.3.2.1 The advent of optogenetics ......................................................................23 1.3.2.2 An explosion of next generation optogenetic tools .................................26 1.3.3 Combining optogenetic manipulation and calcium imaging .................................27 1.4 Extrasynaptic transmission .............................................................................................28 1.4.1 The discovery of extrasynaptic transmission for fast neurotransmitters ...............28 1.4.2 Functional imaging of neurotransmission ..............................................................30 2 Results: Hidden functional connectivities that underlie the symmetric locomotor behavior of the juvenile L1 C. elegans ...................................................................................32 2.1 Abstract .............................................................................................................................32 2.2 Introduction ......................................................................................................................32