Structural and molecular basis of drug-induced IKACh reduction
Principal Investigator: SAMI FOUAD NOUJAIM
Abstract: SUMMARY: This application for NIH support is aimed at facilitating my transition from the current mentored stage of my career toward independence. It will give me the opportunity to learn new concepts and techniques in structural and molecular biology, which I will add to my background in cardiac electrophysiology. My long term career objective is to be an independent scientist, and to investigate structural, functional and trafficking aspects of drug-ion channels interactions. Therefore, I foresee that my laboratory will use novel approaches geared towards improving existing or generating new pharmacological therapies. I obtained my PhD from the Department of Pharmacology at SUNY Syracuse in 2007. My thesis focused on ionic and body size determinants of ventricular fibrillation (VF) initiation and maintenance. I elucidated the roles of sarcolemmal inward rectifier (Kir2.x) potassium channel proteins in the maintenance of VF, and of the ryanodine receptor type 2 in the initiation of ventricular tachyarrhythmias at the level of the His- Purkinje system. Additionally, I demonstrated that rotors are the mechanism of VF across mammalian species. Since 2008, I have been a postdoctoral fellow at the University of Michigan (U of M) Center for Arrhythmia Research. I also received an American Heart Association Postdoctoral Fellowship. Here I collaborate with U of M investigators towards elucidating, from the molecule to the organ, the interactions between chloroquine and inward rectifier channels using optical mapping, patch clamping and molecular modeling. Such interactions result in the reduction of inward rectifier currents, and lead to the termination of atrial fibrillation (AF) and VF. I propose to take advantage of opportunities readily available at U of M to combine my background in cardiac electrophysiology with new methodologies and skills that I hope to acquire through this proposal, to develop a scientific niche for myself. That niche will be dissimilar from, yet complimentary to, my past scientific endeavors, and will provide a solid basis of my work as an independent investigator. My proposal stems from the premise that antiarrhythmic drug-ion channel interactions remain poorly understood, and that incomplete knowledge and poor drug design may underlie the inefficacy of currently available antiarrhythmics. The Kir3.1 and Kir3.4 proteins that form the channels responsible for the acetylcholine-activated potassium current (IKAch) are important in perpetuating the rotors that underlie AF. Recently, the crystal structure of the Kir3.1 cytoplasmic domain was solved and the main features of Kir3.1 and Kir3.4 trafficking have been described. This offers an exciting opportunity to provide novel mechanistic insight into putative drug-channel interactions that result in AF termination through IKACh reduction. My hypothesis is that pharmacological reduction of IKACh can be achieved through two mechanisms: (1) direct channel blockade involving specific amino acids in the cytoplasmic domain of the channel;and (2) internalization of Kir3.1/Kir3.4 heteromers through the Arf-6 GTPase dependent pathway. I will utilize chloroquine, an antimalarial quinoline that blocks IKACh, and has been shown to terminate AF in some patients, as a model agent to study the structural and molecular basis of drug-induced IKACh reduction. My preliminary data indicate that chloroquine: 1- terminates cholinergic AF in the isolated sheep heart;2- impedes ion movement through the channel's vestibule by interacting with specific amino acid residues as suggested by molecular modeling;3- causes the internalization of Kir3.1/Kir3.4 in neonatal rat atrial myocytes, possibly through a direct interaction with the carboxyl terminus acidic cluster of Kir3.4, as suggested by nuclear magnetic resonance (NMR) experiments. These preliminary data support the feasibility of the experiments I propose to test my hypothesis. To achieve my aims, I will use a multidisciplinary approach, involving fluorescence microscopy, chemiluminescence, NMR spectroscopy, X-ray crystallography and electrophysiology. These integrative studies represent a novel step that can set the stage for the rational design of atrial-specific antifibrillatory agents. The outstanding environment at the U of M is ideal for attaining expertise in structural biology and ion channel trafficking. I will make use of the stellar facilities and investigators to become proficient in these new fields. The detailed mentoring plan laid out by my mentor, Dr. Jose Jalife, and co-mentors will ensure that I will acquire the necessary expertise in 1- X-ray crystallography under the guidance of Dr. Jeanne Stuckey, managing director of the Center for Structural biology at U of M, where I propose to crystallize and solve a high resolution 3-D structure of Kir3.1 in complex with chloroquine, and 2- microscopy and biochemistry of trafficking of Kir3.1/Kir3.4 proteins, and their chloroquine-induced internalization under the mentorship of Dr. Jeffery Martens, Associate Professor of Pharmacology at U of M, and Dr. Stephane Hatem, Director of Research at the INSERM, and Professor at the Faculty of Medicine Piti[unreadable]-Salp[unreadable]tri[unreadable]re of the Pierre Marie Curie University in Paris, France. Through the combination of the new techniques and concepts I will learn, and the relevant courses and seminars in crystallography and proteonomics I will attend, my mentors will ensure my transition to independence. I will be equipped with the wherewithal and skill to create a laboratory focused on structure/function relations and trafficking of ion channels, which will help to ensure the successful attainment of my ultimate goal of contributing to the improvement of the antifibrillatory armamentarium, and/or the discovery of new more effective antiarrhythmic drugs.
Funding Period: 2011-01-01 - 2012-07-31
more information: NIH RePORT