[PAST EVENT] Andrew Kottick, Applied Science - Ph.D. Dissertation Defense
Abstract: Breathing is a rhythmic motor behavior with obvious physiological importance: breathing movements are essential for respiration, which sustains homeostasis and life itself in a wide array of animals including humans and all mammals. The breathing rhythm is produced by interneurons of the brainstem preB?tzinger complex (preB?tC) whose progenitors express the transcription factor Dbx1. However, the cellular and synaptic neural mechanisms underlying respiratory rhythmogenesis remain unclear. The first chapter of this dissertation examines a Dbx1 transgenic mouse line often exploited to study the neural control of breathing. It emphasizes the cellular fate of progenitors that express Dbx1 at different times during development. I couple tamoxifen-inducible Dbx1 Cre-driver mice with Cre-dependent reporters, then show that Dbx1-expressing progenitors give rise to preB?tC neurons and glia. Further, I quantify the temporal assemblage of Dbx1 neurons and glia in the preB?tC and provide practical guidance on breeding and tamoxifen administration strategies to bias reporter protein expression toward neurons (or glia), which can aid researchers in targeting studies to unravel their functions in respiratory neurobiology. The second chapter of this dissertation exploits the mouse model characterized in the first chapter and then focuses on mechanisms of respiratory rhythmogenesis. The breathing cycle consists of inspiratory and expiratory phases. Inspiratory burst-initiation and burst-sustaining mechanisms have been investigated by many groups. Here, I specifically investigate the role of short-term synaptic depression in burst termination and the inspiratory-expiratory phase transition using rhythmically active medullary slice preparations from Dbx1 Cre-driver mice coupled with channelrhodopsin reporters. I demonstrate the existence of a post-inspiratory refractory period that precludes light-evoked bursts in channelrhodopsin-expressing Dbx1-derived preB?tC neurons. I show that postsynaptic factors cannot account for the refractory period, and that presynaptic vesicle depletion most likely underlies the refractory period. The third chapter of this dissertation focuses on transcriptomic analysis of Dbx1 preB?tC neurons, and differences in gene expression between Dbx1-derived and non-Dbx1-derived preB?tC neurons. I analyze and quantify the expression of over 20,000 genes, and make the raw data publicly available for further analysis. I argue that this full transcriptome approach will enable our research group (and others) to devise physiological studies that target specific subunits of ion channels and integral membrane proteins to examine the role(s) of Dxb1-derived neurons and glia at the molecular level of breathing behavior. In addition to predictable gene candidates (such as ion channels, etc) this transcriptome analysis delivers unanticipated novel gene candidates that can be investigated in future respiratory physiology experiments. Knowing the site (preB?tC) and cell class (Dbx1) at the point of origin of respiration, this dissertation provides tools and specific investigations that advance understanding of the neural mechanisms of breathing.
Bio: Andrew Kottick earned a B.S. in Neuroscience from Laurentian University in his hometown of Sudbury, Ontario, Canada in 2008. After studying respiratory rhythmogenic mechanisms in the American Bullfrog and earning an M.S. in Neuroscience from the University of Calgary, Andrew began his Ph.D. studies in the Systems Neuroscience laboratory of Professor Christopher A. Del Negro at William & Mary in 2011. His primary research focus is on synaptic mechanisms underlying the neural control of breathing. Andrew has published work in the Journal of Neuroscience, Physiological Reports, PLoS One, and ELife. Upon successful completion of his Ph.D., Andrew will begin postdoctoral research in the School of Medicine at Yale University, investigating neurodegenerative disease, with a particular focus on Alzheimer?s disease.