Modulation of voltage gated sodium channels is thought to play a major role in the pharmacologically induced state of general anesthesia; indeed, several members of this class of channels show a significant response to general anesthetics. However, the detailed mechanism of inhibition or potentiation of these channels is completely unknown. Recently, the structures of several bacterial orthologs became available offering the opportunity to shed some light on this issue. In collaboration with the groups of Manuel Covarrubias from Jefferson University (electrophysiology) and Roderic Eckenhoff from University of Pennsylvania (anesthesiology), we have identified binding sites and access pathways for the volatile general anesthetics isoflurane and sevoflurane for one of these bacterial voltage gated sodium channels, NaChBac. Isoflurane and sevoflurane binds the channel at physiologically relevant concentrations in three distinct locations. One site is in the pore of the channel, suggesting that general anesthetics may hinder permeation of sodium ions, while the other two are distant from the conduction pathway and affect conduction via allosteric modulation. We are currently exploring the effect of drug binding on the stability of the non-conductive (inactivated) structural states of the channel to obtain a quantitative thermodynamic model of ion current modulation by general anesthetics. The exciting perspective is to translate this information to the entire class of pharmacologically relevant mammalian channels present in the axons of neurons and generate a physically based mesoscopic model to study the action of general anesthetics in relevant neural networks.
M. Covarrubias, A. F Barber, V. Carnevale, W. Treptow, and R. G Eckenhoff. Mechanistic insights into the modulation of voltage-gated ion channels by inhaled anesthetics. BIOPHYSICAL JOURNAL, 109:10, 2003-2011, 2015.
A. Barber, V. Carnevale, M. L. Klein, R. G. Eckenhoff, M. Covarrubias Modulation of a voltage-gated Na+channel by sevoflurane involves multiple sites and distinct mechanisms. PROC. NATL. ACAD. SCI. USA, doi: 10.1073/pnas.1405768111.
S. G. Raju, A. B. Barber, D. LeBard, M. L. Klein, V. Carnevale Exploring Volatile General Anesthetic Binding to a Closed Membrane-Bound Bacterial Voltage-Gated Sodium Channel via Computation PLOS COMPUTATIONAL BIOLOGY doi:10.1371/journal.pcbi.1003090 (2013).
One of the most fascinating aspects of ion channels and transporters is selective permeability: subtle structural differences between the hydration shells of, for instance Na+ and K+, often give rise to enormous differences in macroscopic currents. In the recent past, we have investigated the conduction mechanism of a bacterial voltage-gated sodium channel with the ultimate goal of understanding the physico-chemical determinants of sodium selectivity in the mammalian counterparts. Based on the recently determined x-ray structure of NavAb, we characterized the hydration of the ions in the selectivity filter, provided a quantitative description of the Na+binding sites and shed light on the intricacies of the conduction mechanism. This research has evolved in a research project in collaboration with Brad Rothberg (Temple University Medical School) to study the conduction mechanism of the K+-selective channel MthK. More recently, we have started to investigate the Na,K-ATPase. This protein is responsible for the electro-osmotic gradients that enable propagation of the electric signal in neuronal axons. Intriguingly, during the active transport cycle, an ion binding site becomes alternatively accessible to the cytosolic or extracellular compartments. The different selectivity profile shown in the two alternate conformations is the major factor contributing to the active transport of Na+and K+against their concentration gradients. We study the molecular mechanism of this dynamically controlled selectivity in collaboration with the groups of Vincent Voelz (Temple University) and Joshua Berlin (Rutgers New Jersey Medical School).
A. M. Razavi, L. Delemotte, J. R. Berlin, V. Carnevale, V. A. Voelz Free energy calculations suggest a mechanism for Na,K-ATPase ion selectivity, Journal of Biological Chemistry, 292(30), 12412-12423.
H. Dong, G. Fiorin, V. Carnevale, W. Treptow, M. L. Klein Pore waters regulate ion permeation in a calcium release-activated calcium channelPROC. NATL. ACAD. SCI. USA 110(43), 17332-17337 (2013).
L. Stock, L. Delemotte, V. Carnevale, W. Treptow, M. L. Klein Conduction in a biological sodium channel JOURNAL OF PHYSICAL CHEMISTRY B 117(14), 3782–3789 (2013).
V. Carnevale, W. Treptow, M.L. Klein Sodium Ion Binding Sites and Hydration in the Lumen of a Bacterial Ion Channel from Molecular Dynamics Simulations. JOURNAL OF PHYSICAL CHEMISTRY LETTERS, 2(19), 2504–2508 (2011).