Congratulations Ryan Cloyd, PhD
On Friday, May 28, 2021 Ryan Cloyd successfully defended his dissertation and earned his doctoral degree. Congratulations, Dr. Cloyd!
THE ROLE OF MICROTUBULE-ASSOCIATED PROTEIN TAU IN NEURONAL EXCITABILITY AND EPILEPTOGENESIS
Doctoral Committee
Dr. Bret Smith, Department of Neuroscience, Mentor
Dr. Steve Estus, Department of Physiology
Dr. Hiroshi Saito, Department of Physiology
Dr. Joe Abisambra, Department of Physiology
Dr. Brandon Miller, Department of Neuroscience
Dr. Younsoo Bae, Department of Pharmaceutical Sciences, Outside Examiner
Abstract
Tauopathies, including Alzheimer’s disease (AD), are devastating diseases with an immense burden on society which is predicted to increase in coming decades. In addition to progressive loss of memory and cognitive function, patients with tauopathies have a 6-10 fold increase in lifetime risk for seizures, and many are diagnosed with epilepsy. The presence of epileptiform activity on electroencephalogram (EEG) recordings from patients with AD predicts faster cognitive decline compared to patients without abnormal EEG readings. Electrophysiological measurements in murine models of AD have identified neuronal hyperexcitability. Furthermore, reducing tau phosphorylation or expression confers seizure resistance in animal epilepsy models. Although evidence suggests the presence of common mechanisms contributing to both tauopathy and epilepsy, more work is needed to understand how this interaction works and whether tau can be effectively targeted to improve patients’ lives. This study investigated the relationship between tauopathy using transgenic mice that expressed no tau protein (tau-/-) or expressed non-mutant, human tau protein without expressing murine tau (htau). The htau mice develop progressive tauopathy with age. Non-transgenic C57BL/6j mice were used as controls. Whole-cell patch-clamp electrophysiology was used to define tau’s role in neuronal excitability in vivo in dentate gyrus granule cells. Both transgenic mouse strains exhibited a lower frequency of evoked action potentials and reduced likelihood of neurotransmitter release from perforant pathway inputs as measured by the paired pulse ratio compared to control at 1.5 months of age, but these differences were lost with age. The similarities between the tau-/- and htau mice suggest that hyperexcitability is related to the amount of normally functioning tau rather than the presence of pathological tau, and that the presence of normal murine tau may influence the results of other studies involving models of tauopathy. Furthermore, tau’s role in epileptogenesis was studied using intrahippocampal injection of kainate (i.ie., IHK) to induce status epilepticus, a model that induces temporal lobe epileptogenesis, in tau-/-, htau, and C57BL/6j mice. The process of epileptogenesis appeared to be modified compared to control in both transgenic strains, but did not appear to be prevented. Compared to either tau-/- or C57Bl/6J mice, htau mice experienced significantly greater mortality after IHK. Modifications in tau expression, wither deletion or humanization, partially abrogated synaptic excitability that developed following IHK. In conclusion, this study showed that neuronal excitability is affected similarly by either deletion or humanization of tau, with the notable exception of survival after IHK. This study provides clearer understanding of tau’s role in acquired epilepsy and suggests novel therapeutics targeting tau may be effective for the treatment of epilepsy.
Acknowledgements
Many people have contributed in one way or another to this dissertation. First and foremost I must thank my dissertation mentor, Bret N. Smith, PhD, for always pushing me to put out the best work I can produce. I may not have always appreciated every push along the way, but in hindsight I know it made me a better scientist. I will always be grateful for your dedication to my training. I also must recognize Joe F. Abisambra, PhD, where I started my graduate school journey. You may not be a neuropathologist, but you’ve been instrumental in putting me on my current trajectory and I appreciate your continued interest in my training. I would also like to thank the other members of my dissertation committee, Steve Estus, PhD, Hiroshi Saito, PhD, and Brandon Miller, MD/PhD, for providing thoughtful insights and constructive criticism along the way. Your contributions have helped me to think more broadly about my work and science in general. I have been fortunate to find immense support within the Department of Physiology that has made this work possible. Thank you to Andrew Hernandez and Tanya Graf, the universal problem solvers, for all the assistance, big and small, along the way. To Tanya Seward, thank you for all of your guidance, scientific and otherwise. I also have to give thanks to the UK MD/PhD program and directors Wally Whiteheart, PhD, Richard King, MD/PhD, and Brandon Miller, MD/PhD. And of course, I have to thank Therese Stearns. Being the best program administrator imaginable would already be more than anyone could ask, but you somehow manage to always go well beyond that lofty goal and my time here wouldn’t have been the same without you. I would also like to thank the rest of the Smith lab for guidance along the way. To Sanghun Lee, PhD, and Jinny Kang, PhD, thank you both for providing invaluable handson guidance in the electrophysiology and epilepsy experiments that made up the majority of this work. To Kati Smith, PhD, thank you for all the help you’ve provided and for generally being a source of positivity. To Soledad Pitra, PhD, Jordan Wean, and Anna Juras, thank you each for every question you’ve answered and every laugh you’ve provided. Both were essential to make it through the long hours at the rig, day after day. And to Rafael Roberts, thank you for all the assistance you’ve provided in the course of this work. You helped make an impossible amount of work feasible, and I hope this is the beginning of a successful career in science and medicine for you. I’m also incredibly grateful for all of the other people I’ve met along the way that have helped get me to this point. To my graduate school crew, Brooke Ahern, Beth Oates, Ben Shaw, and Brandon Farmer, the many lunches, bourbon clubs, and other events have been welcome and much needed distractions along the way. I never know what the conversations will cover but I’m never disappointed. To my first graduate school crew, Shelby Meier, Grant Nation, Shon Koren, Blaine Weiss, and Emad Chisti, it’s been weird not working with you all every day but I will always cherish the time we spent in lab together (even if you never remembered to invite me to lunch). And to Jessica Waldmann and Robert Dorfman (and of course Lily), thank you for being there from the very beginning through every twist and turn. It’s hard to imagine what the last 6 years would have been like without the two (now three) of you there to share it all. Finally, I have to attempt the impossible task of thanking the people that helped me to even get to the start of this dissertation. Thank you to Laura Wysocki, PhD, for igniting my love of research. Thank you to Rudy, Nathan, Blake, Jake, Clarke, Nick, Jorge, and all the other Kappa Sigs (too many to list unfortunately) for being there to mock me and, on rare occasions, provide support. Thank you to my brothers, Paul, Chris, and Kevin, for supporting me and shaping me into the person I am today. Thank you to the Case/ Clevenger/Hicks family for all of the support over the years. I couldn’t have married into a better family. And thank you to my mother, Tonda Neal, for everything you’ve given me in my life. Through every hardship life has dealt us you’ve always managed to make the best of things and I wouldn’t have anything without you. Finally, to my amazing, supportive wife Amanda. Even though I’ve been the one in the lab, at the rig, and on the computer to build this dissertation, you’ve been behind me at each step. Without you there to look out for me and keep me going I never could have gotten here. I can’t thank you enough, except to say I love you and can’t wait to see what tomorrow brings for us.
