Accessory Subunits in the Regulation and Dysregulation of Myocardial Sodium Channels
Mentor: Jeanne M. Nerbonne, PhD, Alumni Endowed Professor of Molecular Biology and Pharmacology, Departments of Medicine, Cardiovascular Division, and Developmental Biology
Lab description: A major goal of the research program in the Nerbonne lab is to define the physiological mechanisms that control the expression, properties, and functioning of myocardial Na+ (Nav) and K+ (Kv) channels, which are key determinants of myocardial membrane excitability, affecting the waveforms of action potentials in individual myocytes and the propagation of electrical activity through the heart. In addition, we are exploring the pathophysiological mechanisms contributing to the dysregulation of Nav and Kv channels in inherited and acquired cardiovascular diseases. A major focus of ongoing research in the lab is identifying the components of native myocardial Nav channels and defining the mechanisms involved in the regulation and modulation of these channels by accessory proteins. The following research project is presently accepting interested undergraduates to work under the direct oversight of a graduate student or postdoctoral fellow and with the guidance and mentorship of Dr. Nerbonne.
Project: Mechanisms linking voltage-gated sodium (Nav) channel accessory subunits to the regulation and dysregulation of membrane excitability in the mammalian heart: Considerable evidence suggests that native cardiac Nav channels function in macromolecular complexes of the Nav1.5 α subunit and multiple accessory proteins. The Nav1.5 α subunit, which forms the Na+ selective channel pore, has four homologous domains (DI-DIV), each with six transmembrane segments (S1-S6), connected by cytoplasmic linkers. The S1-S4 segments form the channel voltage-sensing domains, and the S5-S6 segments form the ion-selective pore. On depolarization, the voltage-sensing domains move and coupled to the pore through intracellular S4-S5 linkers, cause channel opening. Hydrophobic amino acids in the DIII-DIV linker, the IFM motif, are essential for inactivation. Native Nav channel function, however, is determined not only by the α subunit but also by the associations with intracellular and transmembrane accessory subunits. In collaboration with the laboratory of Jon Silva, Associate Professor of Biomedical Engineering, ongoing studies are detailing the molecular mechanisms underlying the regulation of myocardial Nav1.5-encoded channels by intracellular fibroblast growth factor 12 (iFGF12), the predominant iFGF expressed in the human heart. We are also testing the hypothesis that the iFGF12A variant, which we have found is upregulated in failing human hearts, has functional effects on the biophysical and pharmacological properties of myocardial Nav1.5-encoded channels distinct from the effects of iFGF12B. An additional goal is to test the hypothesis that another intracellular Nav channel auxiliary subunit, calmodulin (CaM), which binds to the C-terminus of Nav1.5 very near to the site of iFGF12 binding, modulates the effects of iFGF12B and/or iFGF12A on Nav1.5 channel gating. We are combining cellular electrophysiological and molecular genetic approaches to accomplish these goals. We are also developing a mathematical model of the Nav currents in native (mouse and human) myocytes and will use these in dynamic clamp experiments to manipulate the properties or the amplitudes of the Nav currents computationally in situ and in real-time during current-clamp recordings. In addition, biochemical and mass spectrometry-based proteomic approaches are being employed to identify the molecular components of native (mouse and human) myocardial Nav channel macromolecular protein complexes.